{"full_text": "Data will be made available on request.Fossil fuels are essential building blocks in the petrochemical industries for producing materials such as plastics, synthetic fibres, rubbers, lubricants, detergents, and solvents (Keim, 2010; Speight, 2011). However, their non-renewable nature poses a sustainability risk, prompting the search for sustainable alternatives based on renewable biomass sources (Ozturk et al., 2017). Oil produced from pyrolysis of lignocellulosic biomass, such as oil palm biomass, can be a sustainable alternative to fossil fuels, given its carbon-neutral properties with low sulfur and nitrogen content (Mart\u00ednez et al., 2014; Palamanit et al., 2019). During pyrolysis, the thermal decomposition of oil palm biomass may produce more than 300 chemical compounds in the oil, which can be used as precursors for synthesizing petrochemical products (Keim, 2010; Machado et al., 2022). Oil palm biomass-derived pyrolysis oil mainly consists of oxygenated compounds due to its high oxygen content in raw biomass. Such oil requires modification to improve the hydrocarbon content (Palamanit et al., 2019). Co-pyrolysis of oil palm biomass with plastics like polypropylene (PP) is a promising method for increasing hydrocarbon content (Al-Maari et al., 2021). PP, rich in carbon and hydrogen, provides the hydrocarbon pool required for the deoxygenation reaction of oxygenated compounds from biomass to form hydrocarbons such as aliphatic and aromatic hydrocarbons in oil.Solid acidic catalysts can further promote the deoxygenation reactions (i.e., dehydration, decarbonylation, and decarboxylation) of pyrolytic volatiles to improve the hydrocarbon content in pyrolysis oil (Hassan et al., 2019; Shafaghat et al., 2019; Zhang et al., 2016). Due to its excellent catalytic performance for deoxygenation, which generates hydrocarbons such as olefins, aliphatic, and aromatic hydrocarbons, high-acidity zeolites have been widely used in several studies (Balasundram et al., 2018; Wang et al., 2020). In such a reaction, oxygen is typically removed by releasing by-products such as water, carbon dioxide, and carbon monoxide (Hassan et al., 2019). Zeolites, nonetheless, are microporous catalysts. Thus, micropore-related flow restriction can affect their deoxygenation catalytic performance, especially if relatively large molecules such as lignin-derived compounds are involved (Shafaghat et al., 2019). Such flow restriction can also cause coke formation, create pore blockage, catalyst deactivation, and catalyst poisoning, thereby reducing the performance of catalysts (Hassan et al., 2019; Shafaghat et al., 2019). To address this, mesoporous acidic catalysts such as titania (TiO2) and alumina (Al2O3) based catalysts with larger pore sizes were introduced, allowing large molecules to diffuse and reducing pore blockage and catalyst deactivation (Lu et al., 2010; Zhou et al., 2019). The high chemical and thermal stabilities of TiO2 and/or Al2O3-based catalysts have also sparked interest (Bagheri et al., 2014; Paranjpe, 2017). It has been proposed that doping of metals such as nickel, copper, molybdenum, cobalt, palladium, and cerium into TiO2 and/or Al2O3-based catalysts can improve deoxygenation (Bagheri et al., 2014; Lu et al., 2010; Zhou et al., 2019).Several works have evaluated TiO2 and/or Al2O3-based catalysts in oil upgrading through catalytic pyrolysis and co-pyrolysis in a nitrogen atmosphere. Dong et al. (2019) compared the catalytic performances of titania-based catalysts doped with different metals, including copper (10% Cu/TiO2), iron (10% Fe/TiO2), and molybdenum (10% Mo/TiO2) on the phenol conversion during the catalytic pyrolysis of corn straw lignin at 450\u00a0\u00b0C. They reported that the highest total phenol conversion was attained using 10% Mo/TiO2, followed by 10% Cu/TiO2, TiO2, and then 10% Fe/TiO2. Lu et al. (2010) studied the catalytic upgrading of oil from pyrolysis of poplar wood using the titania, zirconia, and titania-zirconia-based catalysts doped with cerium, ruthenium, and palladium at 600\u00a0\u00b0C. In general, all the catalysts reduced the sugars (i.e., levoglucosan) in the oil, while titania-zirconia-based catalysts yielded a high amount of hydrocarbons and ketones. TiO2-based catalysts promoted the formation of phenols. Mysore Prabhakara et al. (2021) investigated the catalytic performance of \u03b3-Al2O3, dolomite, and hydrotalcite (HTC) MG70 with the addition of 20\u00a0wt.% Na2CO3 into the catalysts during the catalytic pyrolysis of beechwood at 500\u00a0\u00b0C. All these catalysts significantly reduced the oxygenated compounds and enhanced the formation of aliphatic, monoaromatic, and polyaromatic hydrocarbons. Zhou et al. (2019) investigated the utilization of NiO/\u03b3-Al2O3 catalyst on the dehydration reaction mechanism during the pyrolysis of rice husks. Weak acid sites on Al2O3 were discovered to facilitate the dehydration reaction the most throughout the process. In addition, Imran et al. (2014) reported that the alumina-supported sodium carbonate (Na2CO3/\u03b3-Al2O3) catalyst improved the quality of oil from the pyrolysis of wood fibers.No studies have used titania and alumina-based catalysts in the co-pyrolysis of OPT and PP to improve the targeted oil composition. The mesopores in these catalysts may facilitate the diffusion rate of large molecules (i.e., compounds derived from the thermal degradation of OPT and PP) through the pores of the catalysts and promote the conversion into hydrocarbons during catalytic co-pyrolysis. This study investigated the catalytic performance of a titania-based catalyst doped with nickel-molybdenum (Ni\u2013Mo/TiO2) and an alumina-based catalyst with nickel (Ni/Al2O3) for the upgrade of oil generated from co-pyrolysis of OPT and PP. The effect of the catalysts on the oil composition was evaluated.OPT was collected from an oil palm plantation in Saratok, Sarawak. OPT was pre-dried in the oven at 105\u00a0\u00b0C for 24\u00a0h, ground (Fritsch rotary mill, PULVERISETTE 14), and sieved (Fritsch sieve shaker, ANALYSETTE 3 PRO) to obtain the samples with a particle size of 500\u00a0\u03bcm and below. Locally sourced PP food containers were cut into smaller sizes and sieved using a sieve shaker (Fritsch, ANALYSETTE 3 PRO) to obtain samples with a particle size of 500\u00a0\u03bcm and below. The sieved PP was stored under ambient conditions before use. Two catalysts used in this study, Ni\u2013Mo/TiO2 and Ni/Al2O3, were synthesized based on the impregnation method reported by Aqsha et al. (2015).The catalysts\u2019 specific surface area, average pore diameter, and pore volume were determined via nitrogen adsorption-desorption isotherm analysis (Brunauer\u2013Emmett\u2013Teller (BET) surface area and pore size analyzer, Quantachrome Nova 4200e). Before the analysis, the samples were degassed at 200\u00a0\u00b0C for 12\u00a0h to remove any surface-adsorbed residual moisture.The crystallinity of the catalysts was investigated using powder X-ray diffraction (XRD) (X-ray Diffractometer, Rigaku SmartLab). Cu-K\u03b1 radiation (\u03bb\u00a0=\u00a00.154\u00a0nm) was used to measure the diffraction patterns in the range of 2\u03b8 from 5 to 100\u00b0.XRF was used to analyze the composition of the catalysts with an accelerating voltage of 15\u00a0kV and a current of 30\u00a0\u03bcA (Bruker S2 PUMA).The acidity of the catalysts was determined through NH3-temperature programmed desorption (TPD) analysis (Micromeritics Chemisorb 2750). The sample was pre-treated by heating it from room temperature to 200\u00a0\u00b0C in helium gas flow for 120\u00a0min. Adsorption of NH3 was carried out at 100\u00a0\u00b0C for 60\u00a0min (5% in He, v/v), followed by helium purging at the same temperature for another 60\u00a0min. Following that, NH3 desorption was carried out by heating from 50 to 800\u00a0\u00b0C at a ramping rate of 10\u00a0\u00b0C min-1 and holding at the final temperature of 800\u00a0\u00b0C for 15\u00a0min.The catalytic co-pyrolysis was carried out in a horizontal tube furnace (MTI, GSL-1100X) with a 400\u00a0mL\u00a0min-1 nitrogen flow rate to form an inert condition in the tube furnace. 3\u00a0g of OPT and PP mixture sample (weight ratio of OPT: PP of 1:1) with 0.3\u00a0g of catalyst were loaded into the reactor and nitrogen purged for 5\u00a0min. The reactor was heated to the desired operating temperature (i.e., 500, 600, and 700\u00a0\u00b0C) at a heating rate of 10\u00a0\u00b0C min-1, with a holding time of 40\u00a0min. Afterwards, the reactor was cooled down to 200\u00a0\u00b0C while continuously purged with nitrogen gas. A cold trap in an ice bath (2\u20133\u00a0\u00b0C) was connected to the tube reactor outlet to collect the liquid product (oil) from the experiment. The collected oil was stored at 2\u20137\u00a0\u00b0C until further analysis. The non-condensable gases were released into the environment. The product yield obtained from the experiments was calculated using \nEquations (1)\u2013(3)\n.\n\n(1)\n\n\nP\ny\nr\no\nl\ny\ns\ni\ns\n\no\ni\nl\n\ny\ni\ne\nl\nd\n\n\n(\nw\nt\n.\n%\n)\n\n=\n\n\nM\na\ns\ns\n\no\nf\n\np\ny\nr\no\nl\ny\ns\ni\ns\n\no\ni\nl\n\no\nb\nt\na\ni\nn\ne\nd\n\n\n(\ng\n)\n\n\n\nM\na\ns\ns\n\no\nf\n\ns\na\nm\np\nl\ne\n\n\n(\ng\n)\n\n\n\n\nx\n\n100\n%\n\n\n\n\n\n\n(2)\n\n\nS\no\nl\ni\nd\n\n\n\ny\ni\ne\nl\nd\n\n*\n\n\n\n(\nw\nt\n.\n%\n)\n\n=\n\n\nM\na\ns\ns\n\no\nf\n\ns\no\nl\ni\nd\n\no\nb\nt\na\ni\nn\ne\nd\n\n\n(\ng\n)\n\n\n\nM\na\ns\ns\n\no\nf\n\ns\na\nm\np\nl\ne\n\n\n(\ng\n)\n\n\n\n\nx\n\n100\n%\n\n\n\n\n*Solid yield refers to all solid residues collected from the experiments, including feedstock residue, catalysts, and coke.\n\n(3)\n\n\nG\na\ns\n\ny\ni\ne\nl\nd\n\n\n(\nw\nt\n.\n%\n)\n\n=\n100\n\nw\nt\n.\n%\n\n\u2013\n\np\ny\nr\no\nl\ny\ns\ni\ns\n\no\ni\nl\n\ny\ni\ne\nl\nd\n\n\n(\nw\nt\n.\n%\n)\n\n\n\u2013\n\ns\no\nl\ni\nd\n\ny\ni\ne\nl\nd\n\n\n(\nw\nt\n.\n%\n)\n\n\n\n\n\nThe composition of pyrolysis oil was determined using a gas chromatography-mass spectrometer (GC-MS) with an HP-5MS column (Agilent, 30\u00a0m length x 0.25\u00a0mm inner diameter x 0.25\u00a0m film thickness) (Agilent, 6890\u00a0N). The column oven was programmed to operate at 40\u00a0\u00b0C for 3\u00a0min. Afterwards, it was heated from 40 to 200\u00a0\u00b0C at the rate of 8\u00a0\u00b0C min-1 with a holding time of 10\u00a0min. The temperature was then ramped from 200 to 220\u00a0\u00b0C at a rate of 10\u00a0\u00b0C min-1 and held for 10\u00a0min. The column was kept at a pressure of 7.04 psi and a flow rate of 1\u00a0mL\u00a0min-1 of helium. The split ratio of 50:1 was used in the analysis. Before the analysis, 0.2\u00a0g of pyrolysis oil was diluted in 10\u00a0mL of acetone. A syringe filter was used to filter the diluted oil sample before it was transferred to the GC sample vial and injected into the equipment via auto-injection mode for analysis. Compounds were identified by comparing the NIST08 mass spectral data library entries.\nTable 1\n presents the textural properties (i.e., specific surface area, pore volume, and average pore diameter) of Ni\u2013Mo/TiO2 and Ni/Al2O3. The lower specific surface area of Ni\u2013Mo/TiO2 compared to Ni/Al2O3 could be attributed to the accumulation of two types of metal particles on the catalyst's surface or within its pores (Kumar et al., 2019). Both catalysts are categorized as mesoporous since their average pore diameter sizes are between 2 and 50\u00a0nm (Thommes et al., 2015). The large pores allow large molecules, such as lignin-derived compounds, to flow in and out of the catalysts' pores for higher conversion of the compounds during the catalytic co-pyrolysis (Lu et al., 2010).\nFig. 1\n depicts the acidities of the catalysts analyzed with NH3-TPD, which reveals the acid site distribution. The temperature region where the ammonia desorption peak has located indicates the types of acid sites (i.e., weak, medium, and strong acid sites) on the surface of both catalysts. The weak acid sites correspond to the ammonia desorption peak at temperatures less than 250\u00a0\u00b0C. In comparison, the medium acid sites appear in the temperature region between 250 and 500\u00a0\u00b0C. The ammonia desorption peak, which appears at temperatures above 500\u00a0\u00b0C, represents strong acid sites (Phan et al., 2020). Strong acid sites with higher acid strength likely provide higher catalytic cracking activity for converting the compounds into desirable products through the catalyst (Li et al., 2020). Fig. 1(a) shows that most ammonia desorption peaks are between 250 and 500\u00a0\u00b0C, indicating the presence of medium acid sites for Ni\u2013Mo/TiO2. On the other hand, weak, medium, and strong acid sites are present on Ni/Al2O3 catalyst surface as the ammonia desorption peaks are detected in all three temperature regions (Fig. 1(b)). A higher peak intensity value in Ni\u2013Mo/TiO2 relative to that in Ni/Al2O3 contributes to the higher acidity in the former catalyst (Table 1).\nFig. 2\n shows the powder XRD patterns of Ni\u2013Mo/TiO2 (upper) and Ni/Al2O3 (bottom) catalysts, respectively. Numerous peaks appear on the pattern of Ni\u2013Mo/TiO2, indicating the presence of a mix of phases. Indexing reveals three major oxide phases, i.e., anatase (TiO2), molybdenum oxide (Mo9O26), and nickel oxide (NiO2). An intense peak at 2\u03b8 of 25.3\u00b0 is detected for TiO2 phase, along with weak peaks at 2\u03b8 of 37.8\u00b0, 48.0\u00b0, 53.9\u00b0, 55.1\u00b0, and 62.7\u00b0 (COD#96-720-6076). For Mo9O26 phase, intense peaks are observed at 2\u03b8 of 24.9\u00b0 and 25.3\u00b0, while weak peaks are present at 2\u03b8 of 27.3\u00b0, 32.2\u00b0, and 33.0\u00b0 (ICSD#98-002-7510). NiO2 has a weak characteristic peak at 2\u03b8 of 37.1\u00b0 (ICSD#98-007-8698). Ni/Al2O3 has two-phase components, i.e., nickel oxide (NiO) and alumina (Al2O3). The intense peaks of NiO are observed at 2\u03b8 of 37.2\u00b0 and 43.3\u00b0 while the weak peak is detected at 2\u03b8 of 62.93\u00b0 (ICDD#03-065-6920). On the other hand, the characteristic peaks of Al2O3 are observed at 2\u03b8 of 46.0\u00b0 and 66.8\u00b0 (ICDD#00-004-0858). The catalyst's composition from XRF analyses is presented in Table S1 in Supplementary Information.\nFig. 3\n depicts the product yield obtained from non-catalytic and catalytic co-pyrolysis of OPT and PP with Ni\u2013Mo/TiO2 and Ni/Al2O3 at temperatures ranging from 500 to 700\u00a0\u00b0C. The solid yield in Fig. 3 refers to all solid residues collected from the experiments, including feedstock residue, catalysts, and coke. When the temperature rises from 500 to 700\u00a0\u00b0C, the solid yield decreases for non-catalytic and catalytic conditions due to the decomposition of char present in the solid fraction into the oil and gas with the rising temperature. According to Zhou et al. (2013), char formation is more favorable at a lower temperature (450\u00a0\u00b0C) due to the lower decomposition rate of the feedstocks. At temperatures above 450\u00a0\u00b0C, the decomposition of feedstocks into condensable volatiles and non-condensable gases improves while char formation decreases. The pyrolysis oil yield in non-catalytic co-pyrolysis is maintained at 16\u00a0wt.% from 500 to 600\u00a0\u00b0C and drops to 11.50\u00a0wt.% when the temperature rises to 700\u00a0\u00b0C. The pyrolysis oil yield in catalytic co-pyrolysis with Ni\u2013Mo/TiO2 increases from 12.67 to 19.50\u00a0wt.% with the rise in temperature from 500 to 600\u00a0\u00b0C. Further increase of temperature to 700\u00a0\u00b0C reduces the oil yield to 17\u00a0wt.% due to the enhancement of the secondary reactions of the primary volatiles into the gaseous products at higher temperatures (>600\u00a0\u00b0C) (Fan et al., 2017; Zhou et al., 2013). The highest pyrolysis oil yield obtained from catalytic co-pyrolysis of Ni/Al2O3 is 17.17\u00a0wt.% at 500\u00a0\u00b0C, followed by a reduction to 12.33\u00a0wt.% at 600\u00a0\u00b0C. Such oil yield reduction is likely due to the increase of gas yield by 10\u00a0wt.% at this temperature. Ni/Al2O3 has been shown to improve the formation of gaseous hydrocarbons rather than liquid hydrocarbons during catalytic cracking of OPT and PP (Lin et al., 2020; Singh et al., 2019; Xue et al., 2017). This finding is consistent with the lower amount of liquid hydrocarbons obtained at 600\u00a0\u00b0C, as shown in Fig. 4\n. On the other hand, the gas yield increases with rising temperatures from 500 to 700\u00a0\u00b0C for three cases (Fig. 3). The secondary reaction of primary volatiles into lighter compounds at higher temperatures results in the formation of non-condensable gases, increasing gas yield with temperature (Hassan et al., 2019).\nFig. 4 shows the oil composition obtained from the non-catalytic and catalytic co-pyrolysis of OPT and PP with Ni\u2013Mo/TiO2 and Ni/Al2O3 in the temperature range of 500\u2013700\u00a0\u00b0C. The oil from non-catalytic co-pyrolysis consists mainly of oxygenated (39.74\u201352.10%) and phenolic compounds (34.01\u201341.85%). The oil contains a small amount of hydrocarbons (5.19\u201310.22%), as evidenced by the relatively low GC-MS relative area for these components. During non-catalytic co-pyrolysis, the oxygenated and phenolic compounds are generated from the thermal decomposition of OPT (i.e., hemicellulose, cellulose, and lignin) (Palamanit et al., 2019; Stefanidis et al., 2014). The thermal degradation of PP produces hydrocarbons via a series of reactions that include random chain scission, mid-chain \u03b2-scission, end chain \u03b2-scission, radical recombination, and hydrogen transfer reactions (Singh et al., 2019; Xue et al., 2017).When Ni\u2013Mo/TiO2 and Ni/Al2O3 are used as the catalysts in the co-pyrolysis of OPT and PP, the hydrocarbons contained in the oil are significantly increased, as shown by an increase in the GC-MS relative area of up to 54.07\u201358.18% and 37.28\u201368.77%, respectively (Fig. 4). The amount of phenolic compounds is reduced, with the reduction in the GC-MS relative area for Ni\u2013Mo/TiO2 (down to 8.46\u201320.16%) and Ni/Al2O3 (down to 2.93\u201314.56%). The presence of catalyst generally reduces the amount of oxygenated compounds, although no clear trend can be drawn concerning the parametric effect of temperature and catalyst type. Fig. 5\n illustrates the proposed reaction mechanism for the hydrocarbon formation from the analyses based on relevant previous works (Dai et al., 2020; Lin et al., 2020; Singh et al., 2019; Xue et al., 2017). The increase of the hydrocarbon content in the catalytic co-pyrolysis is due to the catalytic cracking of PP and deoxygenation of oxygenated and phenolic compounds promoted by Ni\u2013Mo/TiO2 and Ni/Al2O3 catalyst in addition to the thermal decomposition of PP (Fig. 5).The two catalysts used here rely on the presence of both metal (Ni and Ni\u2013Mo) and acidic (TiO2 and Al2O3) sites to provide high deoxygenation ability and thus improve hydrocarbon production. The oxygenated and phenolic compounds undergo deoxygenation reactions via dehydration, decarbonylation, and decarboxylation to form hydrocarbons (Fig. 5) (Dai et al., 2020). The oxygen in the oil is removed during deoxygenation reactions with water, carbon dioxide, and carbon monoxide released as by-products. The acidic sites in the two catalysts, TiO2 and Al2O3, tend to the occurrence of dehydration reaction over decarbonylation and decarboxylation reactions, resulting in the removal of oxygen from the oil and its subsequent combination with hydrogen to form water as a by-product (Ding et al., 2020). This reaction pathway nonetheless consumes the hydrogen in the oil, which is required to produce hydrocarbon. The presence of metal sites, namely Ni and Ni\u2013Mo, in the two catalysts is expected to partially counteract this pathway, resulting in a more dominant occurrence of decarbonylation and decarboxylation reactions in Ni\u2013Mo/TiO2 and Ni/Al2O3-catalyzed co-pyrolysis of OPT and PP (Balasundram et al., 2018; Dai et al., 2020). Higher acidity of Ni\u2013Mo/TiO2 relative to Ni/Al2O3 (Table 1) due to more abundant acidic sites and synergy between Ni and Mo leads to the formation of a higher amount of hydrocarbons from the catalytic co-pyrolysis of OPT and PP (Fig. 4).During the catalytic co-pyrolysis, the hemicellulose, cellulose, and lignin present in OPT undergo thermal decomposition to produce primary products or intermediates. Afterwards, these products and intermediates diffuse through the pores of Ni\u2013Mo/TiO2 and Ni/Al2O3 and undergo catalytic cracking and deoxygenation reactions to produce secondary products (Balasundram et al., 2018; Lin et al., 2020). The thermal decomposition of hemicellulose primarily yields ketones, furans, and acids, which are then catalytically cracked into smaller oxygenates (i.e., acetic acid, acetone, and simple furans) and olefins on the acidic sites of the catalysts (Dai et al., 2020). Conversely, cellulose is degraded to form anhydrosugars as primary products (Lin et al., 2009). The acidic sites in the two catalysts aid in the dehydration of anhydrosugars to produce more furans. Likewise, the catalytic cracking and deoxygenation of furans form smaller oxygenates and olefins (Dai et al., 2020; Praveen Kumar and Srinivas, 2020). Table 2\n shows the decrease of acids and furans in pyrolysis oil after adding Ni\u2013Mo/TiO2 and Ni/Al2O3 catalysts. The result suggests their conversion into olefins, which are the important precursors for the formation of hydrocarbons (Peng et al., 2022). The presence of Ni and Mo in Ni\u2013Mo/TiO2 promotes the decarbonylation and decarboxylation of oxygenated compounds (i.e., ketones, acids, and furans), producing olefins for the subsequent production of hydrocarbons (Balasundram et al., 2018; Xue et al., 2021). Despite this, the amount of ketones in the oil increases after adding these two catalysts (Table 2). This is likely due to catalyst-promoted radical interactions between OPT and PP (Lin et al., 2020).Compared to hemicellulose and cellulose, lignin has a more complex structure, thus producing larger molecules of oligomers during thermal decomposition (Jiang et al., 2010; Lu et al., 2010; Stefanidis et al., 2014). The mesoporous structure of Ni\u2013Mo/TiO2 and Ni/Al2O3 catalysts with wide channels allow for higher diffusion of these lignin-derived oligomers, resulting in high conversion into simple phenols (Lu et al., 2010), which are then converted into olefins via deoxygenation (Hassan et al., 2019; Xue et al., 2017). During the thermal decomposition of PP, olefins can be produced via radical recombination and hydrogen transfer reactions of PP-derived radicals (Singh et al., 2019; Xue et al., 2017). These olefins would produce cyclic hydrocarbons via isomerization and oligomerization. The acidic sites in the catalysts have previously been reported to aid in the isomerization and oligomerization reactions resulting in the formation of cyclic hydrocarbons. Fig. 6\n shows a higher amount of cyclic hydrocarbons in the oil derived from the catalytic co-pyrolysis than that from the non-catalytic co-pyrolysis (Peng et al., 2022).Aliphatic hydrocarbons, on the other hand, are produced during PP decomposition through random chain scission, \u03b2-scission, radical recombination, and hydrogen transfer reactions (Singh et al., 2019; Xue et al., 2017). Fig. 6 depicts an increase in aliphatic hydrocarbons in the oil produced by catalytic co-pyrolysis compared to non-catalytic co-pyrolysis. The metal sites (i.e., Ni and Ni\u2013Mo) in the two catalysts promote the hydrogen transfer reactions (Peng et al., 2022). The presence of Mo in Ni\u2013Mo/TiO2 promotes the transfer of electrons from Mo to Ni, which enhances the catalyst's electron density and thus improves the hydrogen transfer reaction (Maluf and Assaf, 2009). The lower relative amount of aliphatic hydrocarbons observed in Fig. 6 compared to cyclic hydrocarbons is consistent with the nature of aliphatic hydrocarbons as intermediates. Furthermore, some aliphatic hydrocarbons may go through additional isomerization and oligomerization reactions to become cyclic hydrocarbons, facilitated by the acidic sites of the catalysts (Xue et al., 2017). Table 3\n compares the catalytic performances of the catalysts used in this work with other works (Imran et al., 2014; Lu et al., 2010; Mysore Prabhakara et al., 2021). Significantly higher content of hydrocarbons is obtained with the use of Ni\u2013Mo/TiO2 and Ni/Al2O3 as compared to the other TiO2 and Al2O3-based catalysts. However, this is also contributed by the addition of PP as the co-feeding material that provides a sufficient hydrogen source. High oxygenated compounds in the oil reported in the other works are expected, mainly from the decomposition of the wood biomass in the presence of TiO2 and Al2O3-based catalysts.Ni\u2013Mo/TiO2 and Ni/Al2O3 are mesoporous acidic catalysts based on nitrogen adsorption-desorption isotherm and NH3-TPD analyses. Between 500 and 700\u00a0\u00b0C, the pyrolysis oil yields from the catalytic co-pyrolysis of OPT and PP using Ni\u2013Mo/TiO2 and Ni/Al2O3 were 12.67\u201319.50\u00a0wt.% and 12.33\u201317.17\u00a0wt.%, respectively. The acidic properties of both catalysts enhanced the production of hydrocarbon in oil by facilitating the deoxygenation of oxygenated and phenolic compounds and the catalytic cracking of PP. By adding transition metals (Ni and Mo) into the acidic TiO2 and Al2O3-based catalysts, the deoxygenation mechanism was shifted towards decarbonylation and decarboxylation, removing oxygen from oil as carbon dioxide and carbon monoxide gases, which can conserve hydrogen for hydrocarbon formation. Compared to the non-catalytic co-pyrolysis case, the high amount of cyclic hydrocarbons in oil from catalytic co-pyrolysis with Ni\u2013Mo/TiO2 and Ni/Al2O3 catalysts indicates their high catalytic ability in promoting the isomerization and oligomerization reactions of olefins and aliphatic hydrocarbons.Liza Melia Terry: Methodology, Validation, Formal analysis, Investigation, Visualization, Writing \u2013 original draft. Melvin Xin Jie Wee: Methodology, Resources. Jiuan Jing Chew: Supervision, Resources, Writing \u2013 review & editing. Deni Shidqi Khaerudini: Resources, Writing \u2013 review & editing. Nono Darsono: Resources, Writing \u2013 review & editing. Aqsha Aqsha: Conceptualization, Resources, Funding acquisition, Writing \u2013 review & editing. Agus Saptoro: Resources, Writing \u2013 review & editing. Jaka Sunarso: Supervision, Resources, Writing \u2013 review & editing, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Liza Melia Terry gratefully acknowledges the Tun Taib Scholarship from Sarawak Foundation. The authors acknowledge the facilities, scientific, and technical support from Advanced Characterization Laboratories Serpong, National Research and Innovation Agency through E-Layanan Sains, Badan Riset dan Inovasi Nasional. The authors also acknowledge the facilities for GC-MS analysis and funding support from Curtin University Malaysia through Strategic Research Incentives (SRI).The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.envres.2023.115550.", "descript": "\n                  Pyrolysis oil from oil palm biomass can be a sustainable alternative to fossil fuels and the precursor for synthesizing petrochemical products due to its carbon-neutral properties and low sulfur and nitrogen content. This work investigated the effect of applying mesoporous acidic catalysts, Ni\u2013Mo/TiO2 and Ni/Al2O3, in a catalytic co-pyrolysis of oil palm trunk (OPT) and polypropylene (PP) from 500 to 700\u00a0\u00b0C. The obtained oil yields varied between 12.67 and 19.50\u00a0wt.% and 12.33\u201317.17\u00a0wt.% for Ni\u2013Mo/TiO2 and Ni/Al2O3, respectively. The hydrocarbon content in oil significantly increased up to 54.07\u201358.18% and 37.28\u201368.77% after adding Ni\u2013Mo/TiO2 and Ni/Al2O3, respectively. The phenolic compounds content was substantially reduced to 8.46\u201320.16% for Ni\u2013Mo/TiO2 and 2.93\u201314.56% for Ni/Al2O3. Minor reduction in oxygenated compounds was noticed from catalytic co-pyrolysis, though the parametric effects of temperature and catalyst type remain unclear. The enhanced deoxygenation and cracking of phenolic and oxygenated compounds and the PP decomposition resulted in increased hydrocarbon production in oil during catalytic co-pyrolysis. Catalyst addition also promoted the isomerization and oligomerization reactions, enhancing the formation of cyclic relative to aliphatic hydrocarbon.\n               "}
{"full_text": "Biomass represents a sustainable alternative carbon source compared to fossil resources like oil, gas and coal [1\u20133]. Considering the limited reserves of these fossil resources, growing research efforts are being devoted to the development of efficient catalytic systems for biomass valorisation into biofuels and biobased chemicals [1,3,4]. For the upgrading of biobased compounds into valuable chemicals, metallic catalysts are often required for one or more step(s) in a multi-step reaction that may involve hydrogenation, oxidation and/or hydrogenolysis [3,4]. On one hand, noble metal catalysts, such as Au, Pt, Pd and Ru nanoparticles, often exhibit excellent catalytic performance in specific reactions [3]; on the other hand, their high cost limits the extension of their application from lab-scale to the industry. Moreover, these catalysts often suffer from stability issues since the nanoparticles tend to aggregate and thus decrease their activity under hydrothermal reaction conditions [4,5]. As such, there is a strong need for developing noble-metal-free catalysts, which ideally should have comparable performance and better stability compared to those noble metal catalysts [3]. Among the biobased compounds that typically require the use of noble metal catalysts for their oxidation, glycerol is an attractive platform molecule [6,7]. It is produced in large amounts (above 1 million tons crude glycerol in 2016) as the major side product from the biodiesel industry by transesterification of vegetable oils with methanol [4,8]. This led to an oversupply of glycerol and, therefore, has prompted both academia and industry to develop efficient catalytic routes to convert it into several valuable chemical products [9\u201311]. Lactic acid and alkyl lactates can be produced from glycerol through a dehydrogenation-rearrangement pathway (Scheme 1\n) [8,12\u201314]. Lactic acid has a wide range of applications, including that as monomer of poly-lactic acid, a biodegradable bio-polymer with various applications in the food, pharmaceutical and packaging industry [12]. Currently, lactic acid is produced by fermentation of carbohydrates, which generates large amounts of salts in the product work-up section and has a relatively low volumetric production rate [15,16]. The chemocatalytic route involving the dehydrogenation of glycerol and consecutive rearrangement of the triose intermediates (Scheme 1) is considered a viable, sustainable alternative to the fermentation process [12]. This chemocatalytic route implies a nominal formation of H2 and in this sense can be correlated to the use of glycerol as feedstock for the sustainable production of H2 through aqueous-phase reforming (APR) [7,11,17]. Hydrogen is widely used in current chemical industry (e.g. ammonia synthesis, Fischer-Tropsch process, steel industry and various hydrogenation reactions) and in the power fuel cell systems as a clean power source [2,11,18]. Clearly, routes that allow producing H2 from a renewable source such as biomass represent a sustainable alternative to the current production through methane steam reforming, which is based on a fossil resource and requires extremely harsh conditions [2,19].The conversion of glycerol into lactic acid requires metallic sites for the first step, i.e. the dehydrogenative oxidation, and a base or a combination of Br\u00f8nsted and Lewis acid sites for the second step (Scheme 1). Most studies used noble metal catalysts for the first step, such as Pt, Pd, Au and their alloys [12,20\u201322]. Pt/C was used for the hydrogenolysis of glycerol under He atmosphere and gave 55% selectivity to lactic acid at 95% conversion of glycerol [23,24]. Supported Au and its alloy catalysts (AuPt/TiO2) were firstly used with O2 as the oxidant, reaching 30% glycerol conversion and 86% selectivity to lactic acid at 90\u202f\u00b0C [21]. The first report of a bifunctional catalyst for the conversion of glycerol into lactic acid without adding a base employed Pt supported on a zeolite (Sn-MFI) and achieved an excellent 81% selectivity towards lactic acid at 90% conversion of glycerol under O2 (6\u202fbar) at a relatively mild temperature (90\u202f\u00b0C) [14]. Catalysts based on non-noble transition metals, such as Ni, Co and Cu, were also found to be active in converting glycerol to lactic acid under inert atmosphere in the presence of a base [20,25\u201329]. A Ni/graphite catalyst tested at 250\u202f\u00b0C for 2\u202fh yielded 89% lactic acid at full glycerol conversion [20]. A series of 30%CuO/ZrO2 catalysts were also developed and reached 95% yield of lactic acid at 200\u202f\u00b0C [29]. A recent study reported a 20%Co3O4/CeO2 catalyst that achieved 80% selectivity to lactic acid with 85% glycerol conversion at 250\u202f\u00b0C for 8\u202fh [27]. All these non-noble metal catalysts were employed in the presence of a homogeneous base (NaOH) and at relatively high reaction temperatures (200\u2013250\u202f\u00b0C), under which conditions the base alone would display a significant activity in the conversion of glycerol to lactic acid [30,31]. An additional drawback of the Ni, Cu and Co-based systems is the high metal-to-glycerol ratio that was needed to achieve acceptable reaction rates. Moreover, the Cu and Co-based catalysts suffered remarkable loss of activity upon reuse, probably due to leaching of metal species under the hydrothermal conditions [27,29]. If the conversion of glycerol to lactic acid (salt) is carried out under inert atmosphere, the initial dehydrogenative oxidation step (Scheme 1) nominally liberates one molecule of H2 per molecule of glycerol [14,25]. However, the hydrogen generated in such system is highly diluted by N2 in most cases and is thus difficult to collect. In this context, it is more attractive to utilise in-situ the hydrogen removed from glycerol in the reduction of relevant target compounds. Here, we report a bimetallic Ni-Co catalyst supported on CeO2 with remarkably high activity in the transfer hydrogenation between glycerol and several H2 acceptors, under relatively mild hydrothermal conditions (160\u202f\u00b0C) and in the presence of NaOH as promotor. The choice of investigating a Ni-based catalyst was inspired by the above-mentioned activity of this metal in converting glycerol to lactic acid, combined with its well-known activity in catalysing hydrogenation reactions as significantly cheaper alternative to noble metals (e.g. Pt and Pd) [3,32]. The idea of using Ni in a bimetallic system was justified by previous reports that showed that the catalytic performance of Ni could be enhanced by incorporating another component, such as Co or Cu, which led to stronger metal-support interaction with consequent smaller metal particle size [3,4,33]. Namely, bimetallic Ni-based catalysts supported on ZrO2 showed much better performance in the dry reforming of methane (Ni-Co) or in the oxidative steam reforming of methanol (Ni-Cu) compared to their monometallic counterparts [34\u201336]. In this work, different oxides were tested as support for the Ni-based catalysts, with CeO2 leading to the highest activity in glycerol conversion. Our bimetallic Ni-Co catalytic system was also compared to its monometallic counterparts, showing higher activity and allowing to reach very high conversion of glycerol with excellent selectivity towards lactic acid, and to combine this reaction with the efficient hydrogenation of several unsaturated compounds in a one-pot process.Glycerol (99%), 1,3-dihydroxyacetone dimer (97%), glyceraldehyde (90%), glycolic acid (99%), lactic acid (98%), pyruvic aldehyde (40\u202fwt% in H2O), cyclohexene (99%), cyclohexane (99.5%), sodium hydroxide (98%), benzene (99.9%), levulinic acid (99%), 4-hydroxypentanoic acid, \u03b3-valerolactone (99%), nickel(II) nitrate hexahydrate (98.5%), cobalt(II) nitrate hexahydrate (98%), copper(II) nitrate hemi(pentahydrate) (98%), titanium oxide (P25), magnesium oxide (99%) cerium oxide (nanopowder, nominally < 25\u202fnm, though some large particles were observed by TEM; this compound is denoted as CeO2 for the sake of simplicity, though it contains both CeIV and CeIII and it is thus actually CeO2-x), zirconium oxide (nanopowder, < 100\u202fnm) were purchased from Sigma Aldrich. Glyceric acid (20\u202fwt% in H2O), nitrobenzene (99.5%), aniline (98%), azobenzene (98%), azoxybenzene (98%) were purchased from TCI Chemicals. Active carbon Norit SX1G was purchased from Cabot. The H2O used in this work was always of MilliQ grade. All chemicals were used without further purification.A wet impregnation method was used for the preparation of catalysts based on Ni, Co, Cu, NiCo, NiCu supported on CeO2 and ZrO2. Typically, CeO2 (2\u202fg) was mixed with an aqueous solution of Ni(NO3)2 or Co(NO3)2 or Cu(NO3)2 or the combination of two of them (2\u202fM, with the volume of the solution being defined by the target loading of Ni, Co and Cu). The slurry was stirred at room temperature until the water evaporated. The solid mixture was then dried at 100\u202f\u00b0C overnight. The resulting solids were milled to fine powder and then calcined at 550\u202f\u00b0C in the oven under static air (heating rate 3\u202f\u00b0C/min). The calcined catalysts were further reduced in a tube oven under H2 flow (99.9% and 200\u202fmL/min) at 400\u202f\u00b0C (heating rate 3\u202f\u00b0C/min) for 2\u202fh. The gas flow was switched to N2 for 1\u202fh to wipe away the adsorbed H2 on the catalyst surface before taking the catalyst out from the tube oven. A typical reduced catalyst prepared by this method was named as 10NiCo/CeO2, in which 10 stands for the total loading of Ni and Co (wt%), in which the mass ratio between Ni and Co is always kept as 1:1. In addition, as a reference, the catalyst was also used directly after calcination at 550\u202f\u00b0C without further reduction in H2, which was named as 10NiCo/CeO2-C.The catalytic experiments were carried out in a 100\u202fmL Parr stainless steel autoclave reactor equipped with a Teflon liner and an overhead stirrer. In a typical test, a predetermined amount of the catalyst together with a mixture of aqueous solution of glycerol (0.5\u202fM in 20\u202fmL), NaOH (0.15\u202fmol) and the selected hydrogen acceptor (0.2\u202fmol, as organic phase) were loaded into the reactor. The reaction was performed under N2 (20\u202fbar) for 4.5\u202fh at 160 \u1d52C (extra heating time 0.5\u202fh) at a stirring speed of 800\u202frpm. Next, the reactor was depressurised and the reaction content (in two phases) was taken separately and filtered to remove the catalyst. The organic phase was analysed by gas chromatography using a Thermo Trace GC equipped with a Restek Stabilwax-DA column (30\u202fm\u202f\u00d7\u202f0.32\u202fmm \u00d71\u202f\u03bcm) and a FID detector. The aqueous phase was first neutralised and diluted by H2SO4 (1\u202fM), then analysed by high performance liquid chromatography (HPLC, Agilent Technologies 1200 series, Bio-Rad AminexHPX-87H 300\u202f\u00d7\u202f7.8\u202fmm column) at T = 60\u202f\u00b0C, with 0.5\u202fmM H2SO4 as eluent (flow rate: 0.55\u202fmL/min) using a combination of refractive index detector and ultra-violet detector. For the analysis of nitrobenzene and its products, conversion and selectivity were determined by GC analysis using an Agilent Technologies 7980B GC equipped with an Agilent DB-5#6 (5%-Phenyl)-methylpolysiloxane column (15\u202fm, 320\u202f\u03bcm ID). The identification of the products was performed by GC-mass spectrometry (GC\u2013MS) on an HP 6890 Series GC equipped with a Restek Rxi-5Si MS fused silica column (30\u202fm, 250\u202f\u03bcm ID) coupled to an HP 5973 Mass Selective Detector. Each component was calibrated using solutions of the individual components at 4 different concentrations.For the catalyst recycle tests, a small amount of the reaction mixture was collected for analysis, the remaining mixture was filtered and the catalyst was recovered. The catalyst was washed first with H2O (20\u202fmL), then with ethanol (20\u202fmL), and this procedure was repeated 3 times, after which the solid was dried overnight at 100 \u1d52C. This solid was used for another batch experiment.Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping measurements were performed on a FEI Tecnai T20 electron microscope operating at 200\u202fkeV with an Oxford Xmax 80\u202fT detector. The samples were prepared by ultra-sonication in ethanol followed by drop-casting of the material on a copper grid.Nitrogen physisorption isotherms were measured at \u2212196\u202f\u00b0C using a Micromeritics ASAP 2420 apparatus. The Brunauer-Emmet-Teller (BET) method was used to calculate the specific surface area. The Barrett-Joyner-Halenda (BJH) method was used to calculate the pore volume.Inductively-coupled plasma optical emission spectrometry (ICP-OES) was performed using a Perkin Elmer Optima 7000 DV instrument in order to obtain the actual metal loadings on the supports.X-ray photoelectron spectroscopy (XPS) was measured by mounting the catalysts on a conductive tape adhered to the XPS sample holder. No further treatment was carried out prior to the XPS measurement. Then, the sample was loaded into the load lock and the pressure was reduced below 1\u00b710\u22127 mbar. The XPS measurements were performed using a Surface Science SSX-100 ESCA instrument equipped with a monochromatic Al K\u03b1 X-ray source (h\u03bd =1486.6\u202feV). During the measurement, the pressure was kept below 2\u00b710-9 mbar in the analysis chamber. For acquiring the data, a spot size with a 600\u202f\u03bcm diameter was used. The neutraliser was on to avoid charging effects. All XPS spectra were analysed using the Winspec software package developed by LISE laboratory, University of Namur, Belgium, including Shirley background subtraction and peak deconvolution.Hydrogen-temperature programmed reduction (H2-TPR) measurements were performed on an Autochem II 2920 from Micromeritics. In a typical experiment, 80\u202fmg of sample was pre-treated at 500\u202f\u00b0C (heating rate 10\u202f\u00b0C/min) for 1\u202fh in a flow of He (30\u202fmL/min). Subsequently, the sample was cooled down to 50\u202f\u00b0C under the same flow of He. The reduction analysis was performed from 50 to 900\u202f\u00b0C (10\u202f\u00b0C/min) in a 30\u202fmL/min flow of 5\u202fvol.% H2 in He.X-ray diffraction (XRD) measurements were performed on a D8 Advance Bruker diffractometer with a CuK\u03b1 1 radiation (\u03bb\u202f=\u202f1.5418\u202f\u00c5). The XRD patterns were collected under 40\u202fkV and 40\u202fmA in the range of 10-80\u00b0.Definitions:The glycerol conversion (Conv./%) is defined by Eq. (1):\n\n(1)\n\nConv\n.\n=\n\n\n\nC\n\n\ng\n,\n0\n\n\n\n-\n\nC\n\n(\ng\n)\n\n\n\n\nC\n\n(\ng\n,\n0\n)\n\n\n\n\u00d7\n100\n%\n\n\n\nin which C(g) is the molar concentration of glycerol after a certain reaction time and C(g,0) is the initial concentration of glycerol.Product selectivity for a compound P is defined by Eq. (2):\n\n(2)\n\n\nS\np\n\n\u2009\n=\n\n\nC\n\n(\np\n)\n\n\n\n\nC\n\n(\ng\n,\n0\n)\n\n\n-\n\nC\n\n(\ng\n)\n\n\n\n\n\u00d7\n100\n%\n\n\n\nin which C(p) is the molar concentration of a product after a certain reaction time.The yield of transfer hydrogenation is defined by Eq. (3):\n\n(3)\n\n\nY\n\ntrans\n-\nH\u2009\n\n\n=\n\n\n\u2211\n\n(\nx\n*\n\nn\n\n\np\n1\n\n\n\n)\n\n\n\n\nn\n\n(\ng\n,\n0\n)\n\n\n-\n\nn\n\n(\ng\n)\n\n\n\n\n\u00d7\n100\n%\n\n\n\nin which x is the number of hydrogen atoms needed for the reduction of product 1, n(p1)\n is the molar amount of product 1, n(g)\n is the molar amount of glycerol after a certain reaction time and n(g,0)\n is the initial molar amount of glycerol.The term \u201clactic acid\u201d is used in this article to describe the product obtained from the reaction mixture, which is actually sodium lactate (mixed with a small portion of lactic acid from hydrolysis).Our study of the conversion of glycerol into lactic acid coupled with the transfer hydrogenation to an unsaturated compound started with the investigation of the catalytic behaviour of Ni catalysts (10\u202fwt%) as a function of the type of the material (activated carbon (AC) and various metal oxides) on which the metal particles were supported by wet impregnation. The five catalysts were tested at 160\u202f\u00b0C in the presence of NaOH as promotor and using a model compound as cyclohexene as the H2 acceptor (Table 1\n). Ni supported on AC, MgO and TiO2 showed relatively low activity (entries 1\u20133, Table 1), whereas the activity was significantly higher when nanosized CeO2 and nanosized ZrO2 were used as support for Ni (glycerol conversion 53% and 63%, respectively; entry 4\u20135, Table 1), in line with previous reports on other (de)hydrogenation reactions [4,33]. In all cases, high selectivity towards lactic acid (> 91%) was observed. This is attributed to the presence of NaOH, which effectively promotes the deprotonation of glycerol and catalyses the successive isomerisation of the intermediates (glyceraldehyde and dihydroxyacetone) into the lactic acid salt (Scheme 1), thus granting very high selectivity towards the desired product [21,22,28,37]. Small amounts of glyceric acid, glycolic acid and propanediol were detected as side products (Table 1). Glyceric acid is formed through the further dehydrogenation of glyceraldehyde and glycolic acid probably originates from oxidative CC bond cleavage of glyceric acid [13]. Propanediol (as a mixture of 1,2- and 1,3-isomers) probably forms via the hydrogenolysis of glycerol [38\u201340]. In addition, for all reactions, very minor amounts of glyceraldehyde and propanoic acid were observed as side products, with selectivity below 0.2% for each of them.Based on this preliminary study, CeO2 and ZrO2 were selected as supports for further study of Ni-based catalysts. Then, we aimed at improving the catalytic performance by incorporating another metallic component, i.e. Co or Cu [3,4,33]. The activity of the bimetallic catalysts was compared to the monometallic counterparts (Table 2\n), while keeping the total loading of metal at 10\u202fwt% (and with 1:1 mass ratio for the bimetallic systems). The incorporation of Co into the catalyst formulation was highly beneficial when CeO2 was used as support (10NiCo/CeO2), leading to 91% glycerol conversion (entry 1, Table 2) compared to 53% conversion obtained over 10Ni/CeO2 and 46% conversion over 10Co/CeO2 (entry 5, Table 2). Also the incorporation of Cu enhanced the activity compared to the monometallic counterparts, though the effect was less marked (compare entry 2 in Table 2 to entry 5 in Table 1 and entry 6 in Table 2). On the other hand, the 10NiCo/ZrO2 catalyst showed almost the same activity as the monometallic 10Ni/ZrO2, (compare entry 3 in Table 2 with entry 4 in Table 1), whereas the incorporation of Cu proved more beneficial when ZrO2 was the support, reaching 80% glycerol conversion (entry 4, Table 2). These results indicate a complex interplay between the type of metals and the supports. The benefit brought about by the bimetallic formulation will be elucidated further in the case of the optimum catalyst, i.e. 10NiCo/CeO2 (vide infra). In all these tests, the selectivity towards lactic acid remained very high (94\u201396%). Glyceric acid, glycolic acid and propanediol were detected as the main side products, with selectivity < 6% in total. Though the incorporation of Cu enhanced the activity of the Ni-based catalysts, leaching of metal species was observed in the basic medium under hydrothermal conditions, with significant amount of brown Cu-containing precipitate deposition on the stirring bar and reactor walls [28,33,41\u201343]. Therefore, 10NiCo/CeO2 was selected for further investigation aimed both at a deeper evaluation of the catalytic performance and at understanding the relationship between structure and catalytic behaviour.The catalysts presented in this work were prepared by wet impregnation, followed by calcination and finally reduction by H2. The actual loading of Ni and/or Co determined by ICP-OES (Table 3\n) was found to be very close to the nominal 10\u202fwt% loading. In the bimetallic Ni-Co catalyst, the actual loading of Ni and Co is 5.6\u202fwt% for both metals, which is slightly higher than the theoretical 5\u202fwt%. The BET surface area was measured before and after loading Ni and Co, showing only a slight decrease (from 32 to 28 m2/g) compared to the fresh CeO2 support.To investigate the possible organisation of Ni, Co and Ni-Co species in crystalline phases on the CeO2 support, the catalysts were further characterised by XRD before and after reduction (Fig. 1\n). The materials before reduction (Fig. 1A) display the characteristic peaks of the CeO2 support together with the typical peaks of NiO (in 10Ni/CeO2-C) or Co3O4 (in 10Co/CeO2-C) [34,35,44]. The bimetallic 10NiCo/CeO2-C shows a broad peak at 36.7\u00b0, which is slightly shifted compared to the Co3O4 peak (37\u00b0) and has been attributed to the mixed oxide NiCo2O4 [34,45\u201347]. After reduction at 400\u202f\u00b0C in H2 flow, besides the peaks of the CeO2 support, only one peak at 44.7\u00b0 belonging to metallic Ni can be seen in the pattern of 10Ni/CeO2 (Fig. 1B). On the other hand, no signals stemming from Co and/or Ni phases were observed in 10Co/CeO2 and 10NiCo/CeO2. These results suggest that relatively large crystalline Ni particles formed upon reduction in 10Ni/CeO2, while the Co or Ni-Co species obtained after reduction were highly dispersed in the other two catalysts [45\u201348].To achieve deeper insight on the dispersion of Ni, Co and bimetallic Ni-Co catalysts supported on nanosized CeO2, TEM and STEM-EDX-mapping were used to investigate the average size of these metallic domains (Figure S1, 2 and 3). Since the atomic mass of cerium is much higher than that of nickel or cobalt, it is hard to determine the particle size of Ni, Co or Ni-Co alloy on the CeO2 support based on TEM pictures (Figure S1), as the darker zones are not necessarily corresponding to Ni or Co domains.Analysis by STEM coupled with EDX mapping was more informative as it allows identifying the elemental composition within the image (Fig. 2\n). The large green domains in Fig. 2A and 3B indicate the presence of Ni-containing nanoparticles on CeO2. Based on the XRD data (Fig. 1A), these domains are identified as large NiO nanoparticles (mainly around 100\u202fnm, with some smaller particles, see Fig. 2A) in the sample before reduction (10Ni/CeO2-C), and to large domains of metallic Ni (around 75\u202fnm, Fig. 2B) after the sample was reduced (10Ni/CeO2). For the monometallic material prepared by supporting Co on CeO2 and prior to reduction (10Co/CeO2-C), the Co3O4 identified by XRD (Fig. 1A) was found to be better dispersed on the CeO2 support (Fig. 2C) compared to NiO on CeO2. The 10Co/CeO2 material obtained upon reduction showed nearly homogeneously dispersed Co species (Fig. 2D), which indicates that the particle size of Co is lower than the detection limit of EDX-mapping (around 30\u202fnm). The relatively small size of the Co nanoparticles is also in agreement with the absence of any signal due to metallic Co in the XRD pattern of 10Co/CeO2 (Fig. 1B), which suggests a strong metal-support interaction between Co and CeO2 [4,33,35,45,46,48].STEM and EDX-mapping of the Ni-Co bimetallic material prior to reduction (10NiCo/CeO2-C), showed that both Ni and Co are nearly homogeneously dispersed on the CeO2 surface (Fig. 4 A\u2013D). This demonstrates that the presence of Co prevents the aggregation of Ni species, in contrast to the large domains observed in 10Ni/CeO2-C. After reduction at 400\u202f\u00b0C under H2, Ni and Co still preserve very good dispersion, with no large metal particles (i.e.\u202f>\u202f30\u202fnm) being visible (Fig. 3\n\nH). The strong interaction between Co and the CeO2 support, which promotes the observed high dispersion of both Co and Ni on the surface, has been shown to be related to the formation of a thin layer of reduced CeOx at the interface with the metallic Co [35]. Based on our results, we infer that this feature prevents Ni from forming large particles in the process of calcination and reduction [33,35,46].The reducibility of Ni, Co and Ni-Co supported on CeO2 was further investigated by H2-TPR (Fig. 4). The support, CeO2, exhibited two dominant peaks centred at 490\u202f\u00b0C (from 300 to 550\u202f\u00b0C) and 880\u202f\u00b0C (from 700 to above 900\u202f\u00b0C), which are attributed to the reduction of surface ceria and bulk ceria, respectively [35,49]. Besides the reduction peaks of CeO2 at 420 and 815\u202f\u00b0C, which are slightly shifted to lower temperature, the monometallic 10Ni/CeO2-C displays two peaks at 213\u202f\u00b0C (minor) and 320\u202f\u00b0C (dominant), which are attributed to the reduction of adsorbed oxygen and NiO, respectively [35,50]. The monometallic 10Co/CeO2-C showed two main peaks at 260 and 315\u202f\u00b0C, which are attributed to the two-step reduction Co3O4\u2192CoO\u2192Co [51,52]. The large and broad shoulder extending from 350 to 500\u202f\u00b0C is probably due the reduction of surface CeO2. Compared to the monometallic Ni catalyst, the significant increase of the intensity of the reduction peak of surface CeO2 in the monometallic Co catalyst supports the existence of a strong metal-support interaction between Co and CeO2, which is in agreement with the formation of a thin layer of reduced support on the metallic Co surface reported in the literature [35,48]. The 10NiCo/CeO2-C material showed almost identical profile as the one of 10Co/CeO2-C, with all the peaks shifted by ca. 5\u202f\u00b0C to lower temperature. This suggests that, in the bimetallic Ni-Co catalyst, the reduction behaviour is mainly dictated by the presence of Co, including the strong metal-support interaction indicated by the broad shoulder between 350 and 500\u202f\u00b0C. This result explains the observed much better dispersion of the metal species in the bimetallic Ni-Co catalyst compared to the monometallic Ni catalyst (Figs. 2 and 4) [35].The characterisation by EDX-mapping and H2-TPR indicates a geometrical effect of the presence of Co on the dispersion of Ni on the CeO2 support. To investigate further the interaction between Co, Ni and the support, selected catalysts were analysed by XPS (Figure S2-4). The XPS signal of the Ni 2p3/2 core level region of the unreduced 5Ni/CeO2-C catalyst was deconvoluted into 3 main peaks: at 853.6\u202feV, assigned to NiO; at 855.6\u202feV, attributed to Ni(OH)2 and/or NiO(OH); and a satellite peak at 860.6\u202feV [53\u201356]. Similar peaks were identified by deconvoluting the Ni 2p3/2 signal of the unreduced 10NiCo/CeO2-C catalyst (Figure S2.A and B). After reduction (Figure S2.C and D), in addition to the 3 peaks mentioned above, the deconvolution allowed identifying a peak ascribed to Ni0 (at 852.3\u202feV) in catalysts 5Ni/CeO2 and 10 NiCo/CeO2 [54,55,57]. These data confirm the successful reduction to metallic Ni. The fact that the majority of the XPS signal stems from oxidised Ni species can be explained considering that XPS is a surface technique (information from the top 1\u201310\u202fnm of the material) and that the surface of the particles is expected to tend to oxidise in contact with air and moisture [58,59]. The XPS signal of the Co 2p3/2 core level region of the unreduced 5Co/CeO2-C catalysts was deconvoluted into 3 main peaks: at 779.5\u202feV, assigned to cobalt oxides (CoO and/or Co3O4); at 781.5\u202feV, ascribed to Co(OH)2; and a satellite peak at 785.5\u202feV [55,60,61]. Analogous peaks were identified by deconvoluting the Co 2p3/2 signal of the unreduced 10NiCo/CeO2-C catalyst (Figure S3.A and B). After reduction (Figure S3.C and D), in addition to the 3 peaks mentioned above, the deconvolution showed a peak assigned to Co0 (at 778.0\u202feV) in the catalysts 5Co/CeO2 and 10 NiCo/CeO2 [55,62]. Similarly to what discussed in the case of the supported Ni particles, the presence of oxidised Co species in the reduced samples is attributed to the formation of a layer of oxides and hydroxides at the surface of the particles, generated by contact with air and moisture. The features of the XPS signal of the Ce 3d core level region support the anticipated strong interaction between Co and CeO2 (Figure S4). This is indicated by the surface reduction of Ce4+ and the increase in Ce3+ observed in the XPS spectra of the Co-containing catalysts (whereas this effect is absent in the spectra of the catalysts containing Ni but no Co). This matches well with the literature and with our H2-TPR results [35,52,54]. The XPS data are not conclusive on possible synergistic electronic effects between Ni and Co. Therefore, we infer that the main reason for the improved catalytic performance of bimetallic 10NiCo/CeO2 is the smaller size and better dispersion of the Ni-containing particles.Based on this characterisation study, the optimum activity observed with the bimetallic 10NiCo/CeO2 catalyst is attributed to presence of the more active Ni compared to the monometallic 10Co/CeO2, and to the better dispersion of the active metallic species compared to the monometallic 10Ni/CeO2 catalyst. To further confirm the nature of the active sites, unreduced Ni, Co and bimetallic Ni-Co catalysts were tested under the same conditions employed for the reduced catalysts (Table S1). In the unreduced materials, the metal oxides (NiO, Co3O4 and NiCo2O4) would be the catalytic sites rather than the metallic sites. All the unreduced catalysts had significantly lower activity compared to the reduced ones (Table 1 and 2), with the conversion of glycerol being < 16% in all cases. These results confirm that the metallic sites are the active site in this transfer hydrogenation reaction between glycerol and cyclohexene, in agreement with what shown in the literature [27\u201329].The Ni, Co and Ni-Co catalysts with different loading (2, 5 and 10\u202fwt%) supported on CeO2 were tested to gain better understanding on the effect of the Ni and Co composition (Fig. 5\n). With the Ni/CeO2 catalysts, the conversion of glycerol increased with the metal loading up to 5\u202fwt% Ni, at which it reached 55%, whereas it remained nearly constant upon further increase to 10\u202fwt % of Ni. This trend is completely different from the one observed with the Co/CeO2 and NiCo/CeO2 catalysts, for which the glycerol conversion and the lactic acid yield exhibited an increasing trend with the increase in metal loading (Fig. 5A). The performance of these catalysts can be analysed also in terms of turnover number (TON) (Fig. 5B). These data show that the TON is nearly constant as a function of metal loading for the monometallic Co-catalysts, whereas an increasing loading of Ni causes a gradual decrease in TON, which is more marked for the monometallic Ni-catalysts compared to the bimetallic Ni-Co materials. These trends are in agreement with the tendency of Ni to form large particles at high loading (see Fig. 3.A\u2013B), which implies that a smaller fraction of the metal is available to act as active site, thus leading to the observed lower TON. On the other hand, Co maintains small metallic domains on the CeO2 surface also at 10\u202fwt% metal loading (Fig. 2.C\u2013D), thus enabling to have a nearly constant TON as a function of metal loading. The highest TON was observed for 2Ni/CeO2 and 2NiCo/CeO2, whereas among the catalysts with 10\u202fwt% metal loading, the highest TON was found for 10NiCo/CeO2, despite the decrease compared to the 2\u202fwt% material. This confirms the higher intrinsic activity of Ni compared to Co in catalysing the dehydrogenative oxidation of glycerol. Non-noble metal catalysts are generally used with high loading to give high productivity. Indeed, when the catalytic performance is compared in terms of productivity (Fig. 5C) the highest value among the tested catalysts is obtained with the material with the highest TON among those with 10\u202fwt% metal loading, i.e. 10NiCo/CeO2. This underlines the benefit of the presence of Co in combination with Ni on the catalytic performance [33\u201335,44,48].The 5NiCo/CeO2 catalyst, which achieved intermediate glycerol conversion at 160\u202f\u00b0C, was selected for investigating the effect of the reaction temperature (in the range 140 to 200\u202f\u00b0C, Figure S2). The conversion of glycerol increased with higher reaction temperature, from 11% (at 140\u202f\u00b0C) to 99% (at 200\u202f\u00b0C), while the selectivity to lactic acid remained > 98%. The selectivity towards the transfer hydrogenation was steady at around 25% in all range of temperatures. It should be noted that, when only NaOH was used in the reaction system, the conversion of glycerol was rather low, though it increased from 1.6 to 16% (from 140 to 200\u202f\u00b0C, Figure S5). This demonstrates the need for a heterogeneous catalyst to carry out the dehydrogenation reaction in this range of relatively mild temperatures [30,31].To further investigate the effects of the catalyst amount on this reaction, different weights of catalyst (from 0.025 to 0.15\u202fg) were used, while all other parameters were kept constant. The results show a gradual increase in the conversion of glycerol from 29% to > 99.9% upon increase of the loading of the 10NiCo/CeO2 catalyst (Figure S6).The role of NaOH was studied in more detail by varying the molar ratio between NaOH and glycerol (from 0 to 2, Figure S7). Without the addition of NaOH, both the conversion of glycerol and the selectivity to lactic acid were very low (conversion of glycerol\u202f=\u202f3.5%). If the molar ratio between NaOH and glycerol was increased, the conversion of glycerol gradually increased reaching 91% with 85% yield of lactic acid salt at NaOH/glycerol\u202f=\u202f1.5. However, a further increase in the NaOH/glycerol molar ratio to 2 caused a decrease in the conversion of glycerol to 81%, thus indicating that the employed ratio (1.5) is the optimum value. These results confirm that the presence of a base like NaOH in the reaction mixture is critical to induce the deprotonation of one of the hydroxyl groups of glycerol, thus promoting the dehydrogenation of glycerol [21,28]. Moreover, NaOH can catalyse the isomerisation of glyceraldehyde and dihydroxyacetone and lead to the formation of sodium lactate with very high selectivity.The reaction profile as a function of the reaction time was studied with the 10NiCo/CeO2 catalyst (Figure S8). The conversion of glycerol increased almost linearly within the first 4.5\u202fh, corresponding to a productivity of lactic acid of 17.4\u202fg(LA)/(g(metal)h). After 6.5 h of reaction, almost complete glycerol conversion (97%) was achieved, with 93% lactic acid (salt) yield. The selectivity towards lactic acid stayed always above 90% and the total selectivity towards by-products (glyceric acid, glycolic acid and propanediol) was around 4%. The selectivity towards the transfer hydrogenation slightly decreased with the reaction time, from 31% to 26%. These results suggest that under the employed reaction conditions the dehydrogenation of glycerol is the rate-determining step, and that once the dihydroxyacetone and/or glyceraldehyde formed, they would be transformed into lactic acid (salt) in a very fast and selective way.Catalyst 10NiCo/CeO2 was also selected for a reusability test (Fig. 6\n). The fresh catalyst showed 91% conversion of glycerol and 85% yield to lactic acid, while recycling after straightforward washing and drying led to a slight, gradual decrease in activity. After 5 runs, the conversion of glycerol decreased to 73%, while the selectivity towards lactic acid remained unaltered (> 94%). Meanwhile, the selectivity in the transfer hydrogenation gradually increased from 24 to 28% between the first and the fifth run. The gradual loss of activity is probably caused by the leaching of a small fraction of the active components in the alkaline hydrothermal reaction system, since the loading of Ni and Co decreased from 5.6\u202fwt% (each) in the fresh catalyst to 4.4\u202fwt% (each) after 5 runs (entry 5, Table 3).During the optimisation of the Ni-based catalyst presented above, cyclohexene was employed as hydrogen acceptor in the transfer hydrogenation reaction from glycerol. To expand the scope of applicability of the transfer hydrogenation, we tested a set of H2 acceptors with different features (a biobased compound as levulinic acid, an aromatic compound as benzene, a compound containing both an aromatic ring and another reducible group as nitrobenzene and a linear, terminal alkene as 1-decene). While cyclohexene and 1-decene were selected as model compounds, the hydrogenation of benzene, nitrobenzene and levulinic acid is of potential industrial relevance [63\u201369]. The tests were carried out with a 1:1 molar ratio between glycerol and the hydrogen acceptor, at 160\u202f\u00b0C under N2 atmosphere (Scheme 2\n and Table 4\n).When levulinic acid was employed as the H2 acceptor, two main products were observed: 4-hydroxypentanoic acid (27% yield), obtained by hydrogenation of the carbonyl group of levulinic acid, and \u03b3-valerolactone (48% yield), obtained by subsequent dehydration (Scheme 2 and entry 1 in Table 4). \u03b3-Valerolactone can be used as food additive, solvent and precursor for polymers [6,68,70,71]. This reaction also gave an 86% yield of lactic acid at 87% glycerol conversion with a very good 88% selectivity in the transfer hydrogenation.When benzene was tested as H2 acceptor, a very high selectivity (97%) in the transfer hydrogenation from glycerol was observed, with cyclohexane being the only product (corresponding to complete reduction of benzene). The reduction of benzene is the industrial route for the production of cyclohexane, which is employed as precursor in the synthesis of adipic acid used in the manufacturing of nylon [72,73]. The yield achieved here (25%) is promising considering that under the employed reaction conditions (1:1 molar ratio between glycerol and benzene), the maximum theoretical yield of cyclohexane is 33%. These results were coupled with 79% conversion of glycerol and 77% yield of lactic acid (entry 2, Table 4).When nitrobenzene was employed as hydrogen acceptor, the reduction of the nitro group is expected to be favoured over the reaction of the aromatic ring. Indeed, the observed products (azoxybenzene with 59% yield, azobenzene with 18% yield and aniline with 7.5% yield) all originate from the reduction of the nitro group (Scheme 2) [63,74\u201376]. These are all industrially valuable products, with azoxybenzene being utilised in dyes, reducing agents and polymerisation inhibitors; azobenzene being used in dyes, indicators and as additive in polymers; and aniline finding application in producing pesticides, dyes and as the precursor to polyurethane [77\u201379]. For this reaction, the selectivity in the transfer hydrogenation from glycerol was > 100%. This can be explained considering the strong oxidative ability of nitrobenzene, which led to the further oxidation of the triose intermediates to glyceric acid and glycolic acid (entry 3, Table 4), similarly to what is generally observed in the oxidation of glycerol in the presence of O2 [25,80\u201382]. Therefore, glyceric acid (52% yield) becomes the major product under these conditions, with lactic acid being obtained in much lower yield (23%).When 1-decene was selected as a linear H2 acceptor with a primary CC bond, 92% conversion of glycerol and 91% yield of lactic acid was achieved after reaction, while 85% of decene was hydrogenated to decane, corresponding to a remarkably high 94% selectivity in the transfer hydrogenation (entry 4, Table 4). This is much higher than what was found when using cyclohexene as the H2 acceptor (entry 5, Table 4). This result is probably due to the higher accessibility of the CC bond in a linear alkene with a terminal double bond as 1-decene compared to the more sterically-hindered cyclohexene.The study of substrate scope for the transfer hydrogenation reaction from glycerol demonstrated that our catalytic system based on 10NiCo/CeO2 is able to efficiently promote the conversion glycerol to lactic acid while exploiting the liberated hydrogen in the reduction of different unsaturated compounds to achieve the synthesis of useful target products without requiring an external H2 source.Bimetallic Ni-Co catalysts supported on CeO2 were prepared and tested for the transfer hydrogenation from glycerol to various unsaturated compounds, in which lactic acid and the corresponding hydrogenated products were obtained in a one-pot batch reaction. Introducing Co into the formulation of the Ni-based catalysts was crucial to prevent the aggregation of Ni into large particles. This was proven by the higher activity of the bimetallic 10NiCo/CeO2 catalyst compared the Ni- or Co-based counterparts, and by characterisation of the catalytic materials by EDX-mapping and H2-TPR, which demonstrated the high dispersion of Ni-Co sites on the CeO2 support. The bimetallic 10NiCo/CeO2 catalyst exhibited very high activity (91% glycerol conversion) and selectivity to lactic acid (94%) at 160\u202f\u00b0C, 4.5\u202fh under N2 atmosphere in the presence of NaOH as promoter. This result demonstrates that excellent conversion and selectivity can be achieved using a catalyst with a relatively low loading of Ni and Co and that operates at milder reaction conditions compared to other non-noble metal catalysts for glycerol dehydrogenation reactions [20,25\u201329]. Moreover, various H2 acceptors (levulinic acid, benzene, nitrobenzene, 1-decene, cyclohexene) were tested in the transfer hydrogenation from glycerol, exploiting in-situ the hydrogen liberated in the dehydrogenative oxidation of glycerol to generate several useful products.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We would like to thank the financial support from the China Scholarship Council for the Ph.D. grant of Zhenchen Tang, the technical support from Leon Rohrbach, Jan Henk Marsman, Erwin Wilbers, Anne Appeldoorn and Marcel de Vries, the TEM-EDX support from Dr. Marc Stuart and the ICP-OES support from Johannes van der Velde. We also acknowledge Dr. Matteo Miola for useful scientific discussion of the XPS data.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118273.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Bimetallic Ni-Co catalysts supported on nanosized CeO2 were prepared and investigated as heterogeneous catalysts for the transfer hydrogenation between glycerol and various H2 acceptors (levulinic acid, benzene, nitrobenzene, 1-decene, cyclohexene) to selectively produce lactic acid (salt) and the target hydrogenated compound. The bimetallic NiCo/CeO2 catalyst showed much higher activity than the monometallic Ni or Co counterparts (with equal total metal mass), thus indicating strong synergetic effects. The interaction between the metallic sites and the CeO2 support was thoroughly characterised by means of transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX) mapping, X-ray photoelectron spectroscopy (XPS), hydrogen-temperature programmed reduction (H2-TPR) and X-ray diffraction (XRD). Combining characterisation and catalytic results proved that the Ni species are intrinsically more active than Co species, but that incorporating Co into the catalyst formulation prevented the formation of large Ni particles and led to highly dispersed metal nanoparticles on CeO2, thus leading to the observed enhanced activity for the bimetallic system. The highest yield of lactic acid (salt) achieved in this work was 93% at 97% glycerol conversion (160\u202f\u00b0C, 6.5\u202fh at 20\u202fbar N2, NaOH: glycerol\u202f=\u202f1.5). The NiCo/CeO2 catalyst also exhibited high activity and selectivity towards the target hydrogenated products in the transfer hydrogenation reactions between glycerol and various H2 acceptors. Batch recycle experiments showed good reusability, with retention of 80% of the original activity after 5 runs.\n               "}
{"full_text": "No data was used for the research described in the article.The efficient emission control of unburned methane in power plants and vehicles that use natural gas as a potential bridge fuel in the transition toward renewable energy is of vital importance [1,2], given that this pollutant is strongly involved in the greenhouse effect. Its potent greenhouse effect is around 25 times higher than that of CO2. Presently, the most adequate approach to minimize the negative impact of the release of residual methane to the atmosphere is catalytic oxidation, which allows the direct conversion of the hydrocarbon to carbon dioxide and water. Probably there is no doubt that noble metal-based catalysts, particularly Pd catalysts, are the systems with the highest intrinsic oxidation power for this abatement strategy [3\u20137]. However, although large efforts are continuously being made to increase its thermal and chemical stability under operating conditions, its wide use is fundamentally penalized by economic reasons [8]. Thus, the proposal of cheaper, highly efficient, alternative catalysts is a challenge of relevant interest. Most studies have been focused on the use of transition metal oxides, namely nickel [9,10], manganese [11,12], copper [13] or iron [14]. However, it is widely accepted that spinel cobalt oxide (Co3O4) is the most attractive oxide phase for the lean methane catalytic combustion owing to the presence of variable valance states (Co3+/Co2+), its lower bonding energy of Co-O bonds and the high mobility of active oxygen species capable of activating the C-H bond [15\u201317]. Nevertheless, bulk cobalt oxide, as well as other bulk transition oxides, usually exhibit very poor textural and structural properties, especially when synthesized by simple methodology routes [18,19]. Thus, their good behavior is mainly assigned to their high metallic content (>70 %wt.), thereby resulting in a markedly low intrinsic activity. For this reason, several strategies have been proposed in order to enhance the performance of Co3O4-based catalysts with the ultimate goal of maximizing the population of active sites. The selection of the support for depositing the active phase is the first obvious approach to take into consideration. Furthermore, advances in the optimized design of supported catalysts are highly relevant since the final configuration of a commercial catalytic unit will be a structured catalyst operating with large gas flows [20]. These catalysts will be surely prepared by washcoating a thin catalytic layer (metal oxide/support) onto the surface a monolithic/foam substrate.In this sense, it must be stated that, owing to the high affinity of cobalt for most typical inorganic supports (\u03b3-Al2O3, SiO2 or MgO), a certain fixation of cobalt as less active CoAl2O4, CoSiO3 or Co-Mg mixed oxides must be assumed [21,22]. This unavoidably involves the use of relatively high Co loadings (20\u201340 %wt.) to compensate partially the useless presence of a fraction of deposited Co. The use of alternative supports such ceria or alpha-alumina prevented this strong undesired interaction but their relatively low intrinsic surface area do not usually lead to a substantially improvement in behavior of the resultant composite catalysts [23,24]. A complementary option to adjust the amount and/or reactivity of oxygen species is the addition of a promoter that could improve the reducibility of the resultant catalyst at low temperatures. Based on its comparable ionic radius and coordination and oxidation states to cobalt, nickel is the most preferred promoter. The incorporation of nickel is mainly justified by the notable activity shown by the NiCo2O4 spinel that can be formed from the interaction between cobalt and nickel. This approach is quite interesting for methane oxidative abatement [25\u201328], but requires a large amount of nickel (around 50 % of the Co content for a Ni/Co molar ratio of 0.5). In addition, the synthesis of stoichiometric nickel cobaltite is largely dependent on very well controlled synthesis conditions in terms of calcination temperature and selected preparation route, usually oriented to the synthesis of the mixed oxide in its bulk form. In other words, it would be of interest to explore alternatives for taking advantage of the known beneficial effects of nickel promoter, without the need of large amounts of this additive and using a relatively simple route for obtaining an active Ni-promoted cobalt catalyst.Therefore, the objective of this work is the study of Ni/Co-Al2O3 catalysts for the oxidation of methane under conditions similar to those found in the exhaust of vehicular natural gas engines (relatively low residence times, and presence of water and carbon dioxide). Thus, for a total metal loading of 30 % by weight, the effect of the addition of 5 % and 10 %wt.Ni on cobalt catalysts with a content of 25 % and 20 %wt.Co, respectively, was investigated. These samples were prepared by sequential precipitation of cobalt and nickel, with an intermediate calcination step. Along with these catalysts, monometallic cobalt (20 %, 25 % and 30 %wt.) and nickel (30 %wt.) catalysts with a content of 30 %wt. (30Co and 30Ni, respectively) were synthesized as well for comparative purposes.All oxide catalysts were synthesized following a precipitation route over a thermally-stabilized (calcined at 850\u00a0C for 8\u00a0h) \u03b3-Al2O3 (Saint Gobain), which selected as the support. Three cobalt oxide catalysts, namely 20Co, 25Co and 30Co samples, were prepared by precipitation of aqueous solution of cobalt nitrate hexahydrate with an adjusted concentration to obtain the desired nominal Co loading (20, 25 and 30\u00a0wt. %, respectively), at 80\u00a0\u00b0C using an aqueous solution of sodium carbonate (1.2\u00a0M) until reaching a pH of 8.5. After precipitation, the precursors were dried at 110\u00a0\u00b0C overnight.Then, the catalyst precursors were calcined at 600\u00a0C for 8\u00a0h in static air. In the case of the reference nickel catalyst (30Ni, with a nominal Ni content of 30 %wt.), the starting salt was nickel nitrate hexahydrate. This sample was also submitted to the same aforementioned thermal treatment.Two bimetallic Ni-Co catalysts were obtained by sequential precipitation using the same metallic salts and precipitating conditions (pH = 8.5, 80 \u00baC). Thus, nickel was added to the previously prepared 20Co and 25Co samples with a nominal content of 10 % and 5 %wt., respectively. Thus, the total metallic loading of these samples was fixed at 30 %wt. Finally, the samples were again calcined at 600\u00a0C for additional 4\u00a0h. In this way, both monometallic and bimetallic catalysts were activated under identical thermal conditions. The resulting Ni-Co catalysts were designated as 10Ni/20Co and 5Ni/25Co.The supported catalysts were characterized by a wide number of analytical techniques, including scanning electron microscopy (SEM) coupled to energy dispersive X-ray spectroscopy (EDX), scanning transmission electron microscopy - high angle annular dark field (STEM-HAADF) coupled to EDX mapping, wavelength dispersive X-ray fluorescence (WDXRF), N2 physisorption, X-Ray diffraction (XRD), Raman spectroscopy, X-Ray photoelectron spectroscopy (XPS), temperature-programmed reduction with hydrogen (H2-TPR) and temperature programmed reaction with methane (CH4-TPRe). Experimental details on each of these techniques are included in the Supplementary Material.The activity of the synthesized catalysts for the oxidation of residual methane was determined in a fixed bed reactor (Microactivity by PID Eng&Tech S.L.) between 200 and 600\u00a0\u00b0C. The reaction products were quantified with an on-line gas chromatograph (Agilent Technologies 7890\u00a0N) equipped with a thermal conductivity detector. In each reaction test one gram of catalyst (particle size 0.25\u20130.30\u00a0mm) diluted with 1\u00a0g of inert quartz (particle size 0.5\u20130.8\u00a0mm) was used. A reaction mixture of composition 1 %CH4/10 %O2/89 %N2 was used with a total flow rate of 500\u00a0mL\u00a0min\u22121, which represents an approximate space velocity of 60,000\u00a0h\u22121. To ensure that the mass and heat transfer effects were not affecting the kinetic results, the inter- and intraphase concentration and temperature gradients (Table S1, Supplementary Material) were verify to be negligible according to the criteria proposed by Eurokin [29]. The absence of mass and heat transfer limitations within the reactor was evaluated not only under differential conditions (X\u00a0<\u00a020 %) but also under the least favorable conditions (450\u2013600 \u00baC). Additionally, the stability of the most promising catalyst with time on stream was evaluated at constant temperature (575\u00a0\u00b0C) for a total reaction interval of 150\u00a0h under alternate dry, humid (10 %) or CO2-rich (10 %) conditions while maintaining the O2/CH4 molar ratio at 10.Prior to the discussion of the characterization results of the prepared catalysts, it is highly relevant to remark that the variety of oxide phases that can be present in Co- and/or Ni-containing gamma-alumina supported catalysts thermally activated at moderate temperatures (600\u00a0\u00b0C) is wide. In addition to the expected Co3O4 and NiO oxides, and obviously the \u03b3-Al2O3 support, the presence of mixed spinels such as CoAl2O4 and NiAl2O4 is normally unavoidable. These new metallic oxides are formed due to the strong interaction between the Co and Ni species and the support that results in the partial insertion of Co or Ni atoms into the lattice of the gamma alumina. Moreover, it is commonly accepted that the morphology of these spinels will be essentially amorphous since its transformation into a crystalline structure requires calcination temperatures as high as 800\u2013850\u00a0\u00b0C [30,31]. Besides, the formation of Ni/Co mixed oxides can occur. Thus, based on these considerations both monometallic (20Co, 25Co, 30Co and 30Ni) and bimetallic (5Ni/25Co and 10Ni/20Co) catalysts were thoroughly investigated by a wide number of analytical techniques including N2-physisorption, SEM coupled to EDX, XRF, STEM-HAADF coupled to EELS or EDX, Raman spectroscopy, XPS, H2-TPR and CH4-TPRe.\n\nTable 1 include the textural properties of the metal oxide catalysts. The corresponding pore size distribution are included in Fig. S1, Supplementary Material. The thermally-stabilized (calcined at 850\u00a0\u00b0C for 8\u00a0h) blank alumina support showed a surface area of around 140 m2 g-1 and a pore volume of 0.56\u00a0cm3 g\u22121. Its pore size distribution was bimodal with maxima located at 110 and 150\u00a0\u00c5. After the addition of increasing amounts of cobalt (20Co, 25Co and 30Co samples), the surface area appreciably decreased to 120\u2013108\u00a0m2 g\u22121 due to pore blocking. Accordingly, their pore volume was notably affected since it decreased to 0.35\u20130.29\u00a0cm3 g\u22121. The resultant narrower average pore size was in the 94\u201398\u00a0\u00c5 range. It was then evident that cobalt species preferentially deposited on the larger pores of the support (150\u00a0\u00c5). In the case of the nickel catalyst (30Ni sample), the addition of the metal affected the textural properties to lesser extent when compared with its cobalt counterpart with the same loading (30Co sample). Thus, a surface area close to 130\u00a0m2 g\u22121 was observed. This was probably connected to a trade-off effect between the pore blocking of the support by nickel and the newly formed NiAl2O4 phase with a high intrinsic surface area. This rationale was supported by the notable surface area (170\u00a0m2 g\u22121) of an as-prepared bulk NiAl2O4, which was prepared by precipitation and calcined at 600\u00a0\u00b0C.Regarding the bimetallic catalysts, the addition of nickel (5\u201310 %wt.) to the Co/Al2O3 samples produced a loss of specific surface area0 around 7\u20138 % with respect to the corresponding monometallic sample with the same Co content (25Co and 20Co samples). Furthermore, while the pore volume remained almost constant, a slight increase in the mean pore size (from 94 to 98\u2013107\u00a0\u00c5) was found. In view of these results, it could be concluded that the deposition of the promoter had no marked effect on the textural properties, as relatively similar surface areas, pore volumes, and pore diameters were obtained independently from the Ni/Co ratio of the bimetallic samples.The microstructural morphology of the four monometallic samples (20Co, 25Co, 30Co and 30Ni) and the two bimetallic samples (5Ni/25Co and 10Ni/20Co) was examined by SEM. Irrespective of the composition of the catalysts, the micrographs (Fig. S2, Supplementary Material) revealed a heterogeneous surface on which irregular particles with sizes ranging from 5 to 20\u00a0\u00b5m are arranged with an aggregated morphology. Elemental identification and quantitative compositional information could be obtained by an energy dispersive X-Ray analyzer. Thus, the average surface composition of various defined regions (40\u00a0\u00d740\u00a0\u00b5m with a sampling depth of about 1\u00a0\u00b5m) for each catalyst was determined. \nTable 2 compares the bulk and surface composition as analyzed by XRF and EDX. As for the monometallic samples, an expected surface enrichment was found as revealed by their comparatively higher metal (Co or Ni)/Al molar ratios in relation to the respective bulk molar ratios. Particularly, this ratio at the surface as determined by EDX increased by a factor of 1.6\u20132.1 in the case of the Co-containing catalysts, and a factor of 1.1 in the case of the 30Ni sample.The monometallic 30Co and 30 Ni samples were also examined by scanning transmission electron microscopy\u2013high-angle annular dark field (STEM\u2013HAADF). Additionally, EELS elemental maps (Fig. S3, Supplementary Material) were obtained for certain regions in each sample to examine the spatial distribution of these metals in the catalysts. It was revealed that both Co and Ni were homogeneously distributed over the surface and no large uncoated support regions were apparently observed. This suggested a relative good metallic coverage of the alumina surface. The samples were characterized by the presence of polycrystallites (in some cases formed by the apparent attachment of smaller crystallites) with sizes ranging from 10 to 40\u00a0nm. It is worth pointing out the detection of crystalline phases on the surface of the 30Ni catalyst was comparatively less frequent, thereby suggesting the deposited metallic species on this sample exhibited a more amorphous nature.As for the bimetallic Ni-Co catalysts, it must be pointed that, although the Ni/Co molar ratio at the surface was higher than the corresponding bulk ratio, this increase was not very marked, from 0.23 to 0.25 over the 5Ni/25Co sample and from 0.58 to 0.60 over the 10Ni/20Co sample. This suggested a partial Ni diffusion into the cobalt catalytic layer. Likewise, surface chemical mapping, in this case carried out by STEM-HAADF coupled to EDX, was carried out to study the distribution of both metals on the surface of the bimetallic catalysts. As seen in the compositional maps included in \nFigs. 1 and \n2, both cobalt and nickel were relatively well dispersed over the surface, with no visible clustering or agglomeration of either metal. Seemingly, the mixing between cobalt and nickel seemed to be equally intimate for both Ni-Co catalysts.X-ray diffraction analysis was used to identify the crystalline phases present in each oxide catalyst. The corresponding patterns are included in \nFig. 3. The monometallic cobalt catalysts (20Co, 25Co and 30Co) showed the characteristic signals of a cubic spinelic phase (2\u03b8 = 19.2, 31.4, 37.1, 45.1, 59.6 and 65.5\u00b0) that would be in agreement with the formation of Co3O4 (ICDD 00\u2013042\u20131467) and/or CoAl2O4 (ICDD 00\u2013044\u20130160). Certainly, as will be evidenced later by both Raman spectroscopy and H2-TPR analysis, these samples consisted of a mixture of these cobalt oxides. However, while assuming the present cobalt aluminate will be preferentially amorphous under mild calcination at 600\u2009\u00b0C, the visible diffraction signals in these patterns could be exclusively assigned to highly crystalline Co3O4. On the other hand, the reference 30Ni catalyst evidenced the typical signals of a cubic phase at 2\u03b8 =\u200937.2; 43.1; 62.9 and 75.4\u00b0 corresponding to the presence of nickel oxide NiO (ICDD 00\u2013089\u20137131). It must pointed out that although the formation of nickel aluminate is highly likely, this could not be detected probably due to its poor crystallinity. Recall that no clear signals attributable to crystalline NiAl2O4 (ICDD 00\u2013078\u20131601) were observed. However, and similar to the results found for the Co catalysts, the existence of this spinel will be verified by redox and structural studies. Finally, a weak signal attributable to \u03b3-alumina (ICDD 01\u2013074\u20132206) support was also observed at 2\u03b8 =\u200967.2\u00b0 over these four monometallic samples. The diffractograms of the bimetallic Ni-Co catalysts did not reveal the presence of segregated NiO, which suggested that this oxide was finely dispersed on the Co/Al2O3 matrix. Thus, only diffraction signals related to Co3O4 were noted. The crystallite size of this oxide (Table 1) was determined from the full width half maximum of the characteristic signal at 37.1\u00b0, using the Bragg equation. It thus ranged from 19 to 21\u2009nm for the 20Co and 25Co samples to 35\u2009nm for the 30Co catalyst. Interestingly, the addition of nickel to the Co/Al2O3 samples did not significantly alter the crystallite size (17\u201319\u2009nm). On the other hand, the crystallite size of the NiO phase in the 30Ni catalyst was 14\u2009nm.The examination of the structure of the oxide catalysts was carried out by Raman spectroscopy (\nFig. 4). The Raman spectra of the 20Co, 25Co and 30Co samples displayed the five typical vibration modes of Co3O4 at 196, 480, 520, 619 and 687\u2009cm\u22121\n[32]. The presence of CoAl2O4 was also evidenced by the two shoulders located at 706 and 725\u2009cm\u22121\n[33]. Apparently the contribution of these two additional signals was more marked in the case of the 20Co and 25Co, thus suggesting that the formation of cobalt aluminate would be favored with lower loadings of cobalt. Therefore, the cobalt phases present in the studied Co/Al2O3 catalysts would be a mixture of Co3O4 and CoAl2O4, with a higher relative abundance of the aluminate phase when the total Co content of the sample was lower. Lastly, the Raman spectra of the 30Ni catalyst was dominated by a wide signal located at 545\u2009cm\u22121, which would be coherent with the presence of a mixture of NiO (its main Raman mode is located at 510\u2009cm\u22121) and NiAl2O4 (its main Raman mode is located at 574\u2009cm\u22121). The existence of the nickel spinel was further evidenced by the weaker signals at 746\u2009cm\u22121 and 835\u2009cm\u22121\n[34]. On the other hand, the addition of nickel to the Co-Al2O3 catalysts did not substantially modify the spectra of the resulting samples (5Ni/25Co and 10Ni/20Co). Thus, the only observable Raman modes coincided with those corresponding to the parent cobalt catalyst, namely a mixed contribution of Co3O4 and CoAl2O4 phases. The marked presence of NiO and/or NiAl2O4 could be ruled out in these bimetallic catalysts.The surface composition of the samples and, more importantly, the distribution of the various metallic (cobalt and nickel) and oxygen species was investigated by analyzing the Co2p3/2 (777\u2013792\u2009eV), Ni2p3/2 (850\u2013870\u2009eV) and O1s (526\u2013538\u2009eV) XPS spectra of the samples, as shown in \nFig. 5. Prior to the analysis in the XPS chamber, the as-calcined oxide catalysts were stored in airtight polyethylene containers in order to limit their exposure to ambient air. The Co2p3/2 spectra were deconvoluted into three main and two satellite contributions. The main contributions were located at 779.5, 780.7 and 782.4\u2009eV, and were tentatively attributed to the presence of Co3+(Co3O4), Co2+(Co3O4 and/or CoAl2O4) and Co2+(CoO) species, respectively [35]. For all oxide catalysts, the relative abundance of the signal related to CoO was lower than 10 % of the total Co2p3/2 signal. This species was assumed to be formed by reduction under the vacuum conditions in the XPS chamber. The two signals located at 785.5 and 789.5\u2009eV were assigned to the shake-up satellite peaks from Co2+ and Co3+ ions.Following a similar procedure, the Ni2p3/2 spectra were deconvoluted into five signals. The three main signals were centered at around 853.9, 855.4 and 856.9\u2009eV and were associated with the presence of Ni2+(NiO), Ni2+(nickel belonging to a spinelic phase) and Ni3+(Ni2O3) species, respectively [36]. The satellite contribution of the spectra was dominated by an intense signal located at 861.0\u2009eV, characteristic of the presence of Ni2+, and a small shoulder at 865.3\u2009eV, which was a consequence of the relatively reduced presence of Ni3+ ions in these samples. Finally, the O1s spectra of the samples was characterized by three signals located at 529.3, 531.3 and 532.6\u2009eV, which were attributed to oxygen species from the crystalline lattice (Olatt), superficially adsorbed oxygen species (Oads), and carbonate and hydroxyl species, respectively [37]. From the quantification of the aforementioned spectra, the elemental surface composition and the distribution of ionic species could be determined, as summarized in Table 2.As for the monometallic cobalt catalysts, it was observed that the Co3+/Co2+ molar ratio was in the 0.60\u20130.69 range, which was markedly lower than that expected for the exclusive presence of Co3O4. These moderate ratios suggested the presence of Co2+-rich oxides such as CoAl2O4, as previously pointed out by Raman spectroscopy. In fact, it could be inferred that cobalt aluminate was preferentially formed for low Co loadings, since the 20Co sample showed the lowest Co3+/Co2+ ratio (0.60). On the other hand, the Ni2p3/2 spectrum of the pure nickel catalyst (30Ni) clearly evidenced the presence of comparable amounts of nickel oxide and nickel aluminate. Hence, in addition to traces of Ni3+ species, the observed nickel was in the form of NiO (36 %) and Ni2AlO4 (49 %). The incorporation of nickel markedly affected the distribution of cobalt species on the 5Ni/25Co and 10Ni/20Co catalysts. Interestingly, the addition of this promoter favored the presence of Co3+ cations. This enrichment was a priori related to the partial insertion of Ni2+ ions into the structure of the Co3O4, which would imply the generation of the mixed NiCo2O4 spinel to some extent. In this sense, since the increased population of Co3+ ions was more noticeable for the 5Ni/25Co catalyst (with a Co3+/Co2+ molar ratio of 0.80) in comparison with the 10Ni/20Co sample (with a Co3+/Co2+ molar ratio of 0.64), a more extensive formation of nickel cobaltite was likely for low concentrations of the promoter. Accordingly, these samples showed a high population of Ni2+ species related a spinel-like phase (NiCo2O4) at the cost of Ni2+ as NiO. On the other hand, the presence at the surface of Ni3+ species (as Ni2O3) was also observed over the bimetallic Ni-Co and 30Ni samples, which was favored for low Ni loadings. Finally, it must be remarked that all these structural changes induced by nickel addition on the surface of the alumina supported cobalt catalysts led to an increase of lattice oxygen species. These are widely accepted to play a key role in methane oxidation [38]. Hence, the Olatt/Oads molar ratios were between 0.50 (10Ni/20Co) and 0.63 (5Ni/25Co), apparently higher than those of the respective Ni-free counterparts (0.25 for 20Co and 0.43 for 25Co).The analysis of the metallic catalysts by temperature-programmed reduction with hydrogen (H2-TPR) could be also helpful in identifying the nature of the oxide species present in each sample. The corresponding profiles are compared in \nFig. 6. The redox behavior of the monometallic catalysts (20Co, 25Co, 30Co and 30Ni) was initially discussed in order to facilitate the subsequent interpretation of the results corresponding to the bimetallic Ni-Co samples. As for the Co-containing catalysts, two reduction events were clearly observable. Above 800\u2009\u00b0C no measurable H2 consumption was noticed. Thus, the low-temperature uptake at 250\u2013500\u2009\u00b0C was assigned to the reduction of free Co3O4, according to the two-stage Co3+ \u2192 Co2+ \u2192 Co0 process [39]. The stoichiometric H2:Co molar ratio of this step is 1.33. The high-temperature consumption in the 550\u2013750\u2009\u00b0C corresponded to the reduction of the present cobalt aluminate [40]. The H2:Co stoichiometry for full reduction of this oxide is 1. Note that the presence of this highly stable oxide was in agreement with the results derived from both Raman and XPS spectroscopies. \nTable 3 includes the total H2 uptake of each monometallic sample, which increased from 3.5 to 5.2\u2009mmol\u2009H2 g\u22121. The comparison of these values with the theoretical consumption expected when assuming that all cobalt was exclusively present as Co3O4 (which would vary from 4.2 to 6.1\u2009mmol\u2009H2 g\u22121) resulted in reducibility degrees around 84\u201385 %. From these values, the relative distribution of Co atoms as Co3O4 or CoAl2O4 could be estimated. Hence, the abundance of cobalt as cobalt oxide gradually increased with the Co loading from 35 % to 37 % and 39 % over the 20Co, 25Co and 30Co samples, respectively.On the other hand, the fixation of the deposited metal as aluminate due to the strong metal-support interaction was observed for the 30Ni nickel catalyst as well. Therefore, its reduction trace also revealed two distinct H2 uptakes at moderate (400\u2009\u00b0C) and high (700\u2009\u00b0C) temperatures, which were associated with the presence of NiO and NiAl2O4, respectively [41], in consonance with the Raman and XPS results. It is worth pointing out that the stability of nickel aluminate was significantly higher than that of cobalt aluminate since its full reduction needed temperatures higher than 800\u2009\u00b0C. Both oxides present a H2:Ni stoichiometry of 1. A relative good agreement was found between the experimental (4.5\u2009mmol\u2009H2 g\u22121) and theoretical (4.6\u2009mmol\u2009H2 g\u22121) consumptions. Consequently, a reducibility close to 100 % was evidenced. An estimation of the relative contribution of each nickel species suggested a roughly similar population of both oxide phases (43 %Ni as NiO and 57 %Ni as NiAl2O4).The incorporation of nickel to the 20Co and 25Co samples did not significantly altered the shape of the corresponding redox patterns since these also showed two reduction uptakes at low (250\u2013475\u2009\u00b0C) and high temperatures (550\u2013750\u2009\u00b0C). In view of the reduction pattern of the 30Ni sample, it was reasonable to expect that the reduction of the Ni2+ species present in the bimetallic samples (preferentially as free NiO) would occur mainly at the low temperature window, thus simultaneously coinciding with the reduction of Co3O4 species. Likewise, a small uptake at around 170\u2009\u00b0C, which was not observed in the monometallic Co-Al2O3 counterparts, was visible. This consumption was assigned to the reduction of finely dispersed NiO nickel species [42,43], and was comparatively more noticeable for the 5Ni/25Co catalyst. In addition, the Ni-Co samples exhibited an appreciable shoulder at around 800\u2009\u00b0C that was related to the reduction of nickel aluminate, probably due the strong interaction of added nickel with trace amounts of uncovered alumina.As shown in Table 3, owing to their higher total metallic loading the quantitative analysis of the reduction profiles expectedly evidenced a higher H2 uptake for the bimetallic samples (5Ni/25Co and 10Ni/20Co) in comparison with the respective Ni-free cobalt counterparts (25Co and 20Co, respectively). Thus, the overall reducibility increased from 4.4 to 5.2\u2009mmol\u2009H2 g\u22121 in the case of 25Co and 5Ni/25Co catalysts, and from 3.5 to 5.2\u2009mmol\u2009H2 g\u22121 in the case of the 20Co and 10Ni/20Co catalysts. Note that the total uptake of the Ni/Co samples (5.2\u2009mmol\u2009H2 g\u22121) was virtually identical to that of the 30Co catalyst (5.2\u2009mmol\u2009H2 g\u22121). Also relevant was the fact the reducibility, within the experimental error, of the bimetallic samples was promoted after the addition of nickel. Thus, it increased from 84 % over the 25Co sample to 90 % over the 5Ni/25Co sample, and from 84 % over the 20Co sample to 92 % over the 10Ni/20Co sample. This suggested that the incorporation of nickel promoted the presence of Co3+ cations with a higher H2 consumption per Co (1.5). As revealed by XPS, the simultaneous presence of nickel and cobalt could result in the formation of NiCo2O4-like spinel that ultimately increased the catalyst overall reducibility. Moreover, keeping in mind that the catalytic activity in the methane oxidation is expected to be mainly controlled by oxygen species consumed in the low-temperature range, it was found that the introduction of nickel was efficient for achieving this purpose. Hence, this uptake increased from 1.2 (20Co) to 1.6\u2009mmol\u2009H2 g\u22121 (10Ni/20Co), and from 1.7 (25Co) to 2.0\u2009mmol\u2009H2 g\u22121 (5Ni/25Co). In this latter case, a comparable uptake was found with respect to the 30Co catalyst.The reactivity of the available oxygen species present in the synthesized catalysts was complementary investigated by monitoring the conversion of methane in the absence of oxygen at increasing temperature (CH4-TPRe). The explored temperature range was 50\u2013600\u2009\u00b0C with a heating ramp of 10\u2009\u00b0C\u2009min\u22121. The samples were then kept at 600\u2009\u00b0C for 15\u2009min. The composition of the product stream was followed by mass spectrometry (m/z\u2009=\u200944 (CO2), 28 (CO) and 2 (H2) signals). The resulting profiles of the bimetallic Ni-Co and monometallic (30Co and 30Ni) catalysts are shown in \nFig. 7. Theoretically, methane is expected to be oxidized to carbon oxides at relatively low temperatures by active oxygen species at the catalyst surface. This will result in a progressive reduction of the metallic oxides, and a concomitant high-temperature conversion of methane into reforming products including CO, H2 and CO2, and/or cracking products (H2 and carbonaceous deposits) that will be catalyzed by partially reduced or metallic cobalt and/or nickel. Following this rationale, which is schematically depicted in Fig. S4 (Supplementary Material), the most relevant findings derived by this characterization technique were essentially those corresponding to the low temperature range, at which the complete oxidation of methane would be favorably occurring.The CH4-TPRe profiles revealed the formation of substantial amounts of CO2 at two relatively well-discernible temperature windows. On the one hand, the signal detected at lower temperatures (400\u2013450\u2009\u00b0C) was attributed to the gradual complete oxidation of methane by oxygen species. Note that no CO or H2 were detected in this temperature range. On the other hand, when the total oxidation process was no longer possible, the progressive reduction of the catalyst by methane then activated the transformation of the feed into CO, CO2 and H2, as can be evidenced by the co-existence of these three products at higher temperatures (500\u2013550\u2009\u00b0C). Moreover, the XRD analysis of the spent samples evidenced the presence of metallic cobalt (ICDD 00\u2013015\u20130806) and nickel (ICDD 00\u2013001\u20131258), and crystalline coke (ICDD 01\u2013075\u20131621) (Fig. S5, Supplementary Material).As aforementioned, only the oxygen species involved in the low-temperature CO2 formation signal will be assumed to be highly active in the catalytic combustion reaction. After a proper quantification of the amount of formed CO2, the corresponding amount of consumed oxygen species could be estimated. In this sense, the 5Ni/25Co bimetallic catalyst showed the largest consumption (0.16\u2009mmol\u2009O2 g\u22121) followed by the 10Ni/20Co sample (0.09\u2009mmol\u2009O2 g\u22121) and the Co and Ni monometallic catalysts (0.08 and 0.04\u2009mmol\u2009O2 g\u22121, respectively). In addition, it is worth pointing out that the 5Ni/25Co the oxidation reaction also started at significantly lower temperatures (200\u2009\u00b0C) in comparison with the other samples (300\u2013500\u2009\u00b0C).The efficiency in the oxidation of methane into carbon dioxide of the four samples having the same nominal metallic content (30 %wt. %), namely 5Ni/25Co, 10Ni/20Co, 30Co and 30Ni catalysts, was analyzed operating at 300\u2009mL CH4 g\u22121 h\u22121 between 200 and 600\u2009\u00b0C. Three consecutive light-off tests were conducted over each catalyst. After the first test, which could be understood as an equilibration step of the catalyst under reaction conditions, a certain decrease in conversion was observed. Interestingly, no significant differences in conversion were found between the second and third tests resulting in a virtually identical light-off curve. Thus, the conversion profiles shown in \nFig. 8 correspond to the third catalytic reaction run. All Co-based samples exhibited 100 % CO2 selectivity in the whole temperature range. Nevertheless, substantial amounts of carbon monoxide were formed over the 30Ni sample, leading to CO2 selectivity of only 90 % even at the highest reaction temperatures (600 \u00baC). It was observed that bimetallic catalysts exhibited a considerably better performance compared with the monometallic samples. The T50 values, listed in \nTable 4, were similar for the two monometallic catalysts (550\u2009\u00b0C) and higher than those shown by the bimetallic counterparts (535\u2009\u00b0C for 10Ni/20Co and 525\u2009\u00b0C for 5Ni/25Co). \nTable 5.The specific reaction rates, calculated using the differential method (for conversions less than 20 %) at 450\u2009\u00b0C, revealed a higher intrinsic activity of the 5Ni/25Co catalyst (0.80\u2009mmol CH4 h\u22121 g\u22121), compared with the monometallic 30Co and 30Ni samples (Table 4). The other investigated bimetallic sample (10Ni/20Co) showed an intermediate behavior (0.63\u2009mmol CH4 h\u22121 g\u22121). When referred to the total metallic loading, the best intrinsic activity of the 5Ni/25Co sample was also evidenced. From the correlations depicted in \nFig. 9, the observed catalytic activity trend was coherent with the abundance of Co3+ species in the samples. Thus, the 5Ni/25Co catalyst presented the highest Co3+/Co2+ molar ratio due to the more efficient insertion of Ni2+ ions in the structure of the Co3O4 spinel leading to the generation of the nickel cobaltite-like species. As shown in Fig. S6 (Supplementary Material), this dependence was also valid when referred to the reaction rate normalized per gram of metal. The excellent behavior of this mixed oxide as oxidation catalyst for a variety of hydrocarbons [44,45], carbon monoxide [46], carbonaceous particulate matter [47] and methane [48] as well has been previously reported. On the other hand, it must be pointed out that both NiAl2O4 and CoAl2O4 spinel are not particularly active for the complete oxidation of methane [49,50], owing to their relatively low reducibility and highly stable oxygen species that penalized methane oxidative conversion by the Mars\u2013van Krevelen mechanism. Besides, their formation could be detrimental for the generation of NiCo2O4 due to the decrease in the amount of available Co3O4 and Ni for their mutual interaction. In our study, the formation of this highly active mixed spinel was apparently enhanced with adding small amounts of nickel, since a Ni content as high as 10 %wt. did not lead to a better efficiency than the 30Co catalyst. This was probably owing to the fact that the incorporated Ni was more efficiently dispersed over the 5Ni/25Co catalyst in comparison with the 10Ni/20Co counterpart, as evidenced by its lower NiO/Ni molar ratio. This favored the interaction between Co3O4 and the deposited Ni to form NiCo2O4 to a larger extent as suggested by its large amount of Co3+. This increased presence of easily reducible Co3+ was accompanied by a concomitant higher presence of active lattice oxygen species that were able to activate the oxidation of methane at relatively low temperatures. This was also evidenced by the strong dependence of the intrinsic activity with the Olatt/Oads molar ratio and the amount of consumed oxygen at low temperatures in the CH4-TPRe runs (Fig. 9). On the other hand, it was found that the intrinsic activity of Olatt species present in the 30Ni catalyst was significantly lower than that exhibited by the Olatt species in the Co-containing catalysts.The Mars-van Krevelen mechanism, also known as the redox mechanism, has been widely used for kinetics modeling of methane oxidation over metal oxides. This is based on the assumption of a constant oxygen surface concentration on the catalyst, with reaction occurring by interaction between a molecule of reactant and an oxidized portion of the catalyst. Thus, the model assumes that the oxidation of the hydrocarbon occurs in two steps. In the first step, the compound react with the lattice oxygen resulting in its reduction and the corresponding formation of oxygen vacant site. In the second step, the reduced metal oxide is reoxidized by the gas phase oxygen present in the feed. In the steady state, the rates of the reduction and oxidation steps must be equal. Then, the kinetic equation (Eq. 1) can be expressed as:\n\n(1)\n\n\n(\n\u2212\nr\n)\n=\n\n\n\n\nk\n\n\nred\n\n\n\n\nk\n\n\nox\n\n\n\n\nP\n\n\nCH\n4\n\n\n\n\nP\n\n\nO\n2\n\n\n\n\n\n\nk\n\n\nox\n\n\n\n\nP\n\n\nO\n2\n\n\n+\n\u03b3\n\n\nk\n\n\nred\n\n\n\n\nP\n\n\nCH\n4\n\n\n\n\n\n\n\nwhere kred is the rate constant of the oxidation of the hydrocarbon by the lattice oxygen, kox the rate constant of the lattice re-oxidation and \u03b3 is the overall stoichiometry of the reaction. For conditions with oxygen excess (in our case, a PO2/PCH4 ratio of 10 at the inlet of the reactor), the term koxPO2 is considerably larger than \u03b3kredPCH4. Consequently, the rate equation simplifies to a power rate law equation (Eq. 2).\n\n(2)\n\n\n(\n\u2212\nr\n)\n\u2245\n\n\nk\n\n\nred\n\n\n\n\nP\n\n\nCH\n4\n\n\n\n\n\n\nAccordingly, the integral method was applied to estimate the apparent activation energy when assuming a first pseudo-order for methane and a zeroth pseudo-order for oxygen [38,51]. Conversions between 10 % and 90 % were fit to the following linearized equation for the integral reactor (Eq. 3) where X is the fractional conversion of methane, k0 is the pre-exponential factor of the Arrhenius equation and FCH40/W is the weight hourly space velocity. The goodness of the numerical fit is depicted in Fig. S7 (Supplementary Material). It was observed that the apparent activation energy of the 30Ni catalyst (128\u2009kJ\u2009mol\u22121) was markedly higher than that of the cobalt catalysts, in line with the lower activity of this catalyst for complete oxidation. The bimetallic catalysts and the 30Co catalyst showed a relatively similar value between 90 and 103\u2009kJ\u2009mol\u22121. It is worth pointing out that this range of values was appreciably higher than that found for this reaction catalyzed by bulk Co3O4 (70\u201375\u2009kJ\u2009mol\u22121) [52\u201354], thereby suggesting that the intrinsic activity of the examined cobalt catalysts was negatively affected by the presence of cobalt aluminate.\n\n(3)\n\n\nln\n\n\n\n\u2212\nln\n\n\n\n1\n\u2212\nX\n\n\n\n\n\n\n=\nln\n\n\n\n\n\nk\n\n\n0\n\n\n\n\nC\n\n\n\n\nCH\n4\n\n\n0\n\n\n\n\n\n\n\n\n\nW\n\n\n\n\nF\n\n\n\n\nCH\n4\n\n\n0\n\n\n\n\n\n\n\n\n\n\n\n\n\u2212\n\n\n\n\nE\n\n\na\n\n\n\n\nRT\n\n\n\n\n\n\nFinally, given the presence of notable amounts of water vapor and carbon dioxide in the real exhaust gases of a natural gas engine, an attempt to evaluate the stability of the most efficient catalyst, namely the 5Ni/25Co sample, with time on stream was made. Thus, the evolution of conversion at 575\u2009\u00b0C was examined when the composition was alternated following this sequence: 1 %CH4/10 %O2/N2, 1 %CH4/10 %CO2 /10 %O2/N2, 1 %CH4/10 %O2/N2, 1%CH4/10 %\u2009H2O 10 %O2/N2, 1 %CH4/10 %O2/N2, and 1 %CH4/10 %\u2009H2O/10 %CO2/10 %O2/N2. For each composition, a reaction time interval of 25\u2009h was analyzed, with an accumulated time on stream of 150\u2009h (\nFig. 10). During the first 15\u201320\u2009h under base conditions (absence of water and CO2) a slight decrease in conversion from 80 % to 70 % was noticed. Then this conversion was stable, and was not affected by the addition of carbon dioxide for additional 25\u2009h. Therefore, after an initial equilibration of the catalyst under reaction conditions, a relatively good thermal stability and resistance to the presence of CO2 was evidenced (75\u2009h time on stream). However, after the admission of water into the reactor during additional 25\u2009h, conversion dropped to a stable value of 40 % due to water adsorption on the surface [55]. Interestingly, when water was subsequently cut off, the methane conversion was almost fully recovered (65 %) upon returning to dry conditions. Thus, it was evidenced that this temporary inhibiting effect of water did not result in a remarkable irreversible deactivation of the sample. The catalyst was submitted to a further analysis under humid conditions but combined with the addition of carbon dioxide as well (25\u2009h). Again, a decrease in conversion to 35 % was appreciated due to competitive effects caused by water.The state of the used catalyst in this long-term run was carried out by N2 physisorption, XRD and CH4-TPRe. The textural analysis revealed a slight decrease in surface area to 101\u2009m2 g\u22121 (107\u2009m2 g\u22121 for the fresh counterpart), thus suggesting the sintering of the active Co3O4 phase as in parallel confirmed by XRD. It is worth highlighting that irreversible poisoning was ruled out in view of the composition of the gas flow at the reactor inlet (CH4/O2/H2O/CO2). Besides, the formation of carbonaceous deposits (coke) was not observed given the net oxidizing character of the feedstream (PO2/PCH4=10 at the inlet of the reactor) that inhibited the eventual decomposition/cracking of methane. Hence, an enlargement of the crystallite size (25\u2009nm, 19\u2009nm for the fresh sample) was verified. These structural changes led in turn to a poorer oxidation ability at low temperatures judging from the results by CH4-TPRe analysis (Fig. S8, Supplementary Material). A shift of around 10\u2009\u00b0C was noted for the peak oxidation temperature, from 410\u2009\u00b0C (fresh sample) to 420\u2009\u00b0C (used catalyst). However, it must be pointed out that the total amount of active oxygen species was not substantially modified (0.16\u2009mmol\u2009O2 g\u22121).From a structural point of view, the monometallic samples consisted of a mixture of crystalline Co3O4 and amorphous cobalt aluminate in the case of the Co-containing catalysts (20Co, 25Co and 30Co), and a mixture of crystalline NiO and nickel aluminate in the case of the 30Ni sample. The formation of these undesired Al-based spinels due to the unavoidable strong interaction between the transition metal and gamma alumina was appreciable since around 40\u201365 % of the deposited metal was fixed as a metal-Al mixed oxide. It is worth pointing out that the generation of these aluminates was unfavored with the metallic loading.As revealed by STEM-HAADF coupled to chemical mapping the added nickel was homogeneously deposited on the surface of the corresponding cobalt catalyst as no clusters or visible agglomerates were distinguished. Thus, a relative good dispersion of the promoter could be inferred. As a result, the overall redox properties of the bimetallic catalysts were enhanced, which was essentially attributed to the formation of a new NiCo2O4-like spinel that increased the relative population of Co3+ species in the resulting Ni-Co samples. Hence, these structural changes induced by nickel led to an increase in the amount and mobility/reactivity of lattice oxygen species at lower temperatures with respect with the reference pure Co counterparts, which eventually resulted in a higher intrinsic activity and lower ignition temperatures for methane abatement. The optimal catalyst composition, which globally enhanced the abundance of Co3+ by a proper combination of highly active Co3O4 and NiCo2O4 phases, was that of the 5Ni/25Co sample. The 10Ni/20Co and the 30Co catalysts exhibited a similar efficiency. Therefore, this study demonstrated that the synergistic effect between the two metal sites is an efficient strategy to activate lattice oxygen species, which can affect the catalytic oxidation activity significantly.\nAndoni Choya: Investigation, Writing - original draft. Beatriz de Rivas: Methodology, Formal analysis, Validation. Jose Ignacio Guti\u00e9rrez-Ortiz: Methodology, Formal analysis, Funding acquisition. Rub\u00e9n L\u00f3pez-Fonseca: Conceptualization, Writing - review & editing, Supervision, Funding acquisition, Project administration.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the Spanish Ministry of Science and Innovation (PID2019-107105RB-I00 AEI/FEDER, UE), Basque Government (IT1509-22) and the University of The Basque Country UPV/EHU (DOCREC21/23). The authors wish to thank the technical and human support provided by SGIker (UPV/EHU). In addition, authors acknowledge the use of instrumentation as well as the technical advice provided by the National Facility ELECMI ICTS, node \u2018Advanced Microscopy Laboratory\u2019 at University of Zaragoza.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2022.108816.\n\n\n\nSupplementary material.\n\n\n\n.", "descript": "\n                  In this work bimetallic Ni catalysts supported over Co-Al2O3 and monometallic Co-Al2O3 and Ni-Al2O3 catalysts were examined for the complete oxidation of methane. With a 30 % total metallic loading, the samples were synthesized by a sequential precipitation route. All samples were characterized by nitrogen physisorption, X-ray fluorescence, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, scanning-transmission electron microscopy, X-Ray photoelectron spectroscopy, and temperature-programmed reduction with hydrogen and methane. Their catalytic performance was investigated in the temperature range of 200\u2013600\u00a0\u00b0C with a space velocity of 60.000\u00a0h\u22121. The bimetallic catalysts showed a better behavior in the oxidation reaction than the monometallic counterparts, mainly due to the good dispersion of Ni on the surface of the Co-Al2O3 samples. This has enabled the insertion of Ni2+ ions into the cobalt spinel lattice, which in turn provoked an increase in the amount of Co3+ species, and a subsequent enhanced mobility of oxygen species in the spinel. In this sense, the 5Ni/25Co catalyst showed the best performance, thus reducing the value of the T50 by 25\u00a0\u00b0C with respect to the monometallic catalysts.\n               "}
{"full_text": "Brunauer\u2013Emmett\u2013TellerBarret\u2013Joyner\u2013HalendaCold gas efficiencyCassava rhizomeEnergy dispersive spectroscopyEquivalence ratioHigher heating valueInternational Union of Pure and Applied ChemistryLower heating valueMobil Composition of Matter No. 41Relative pressureScanning electron microscopeSimulated flue gasX-ray powder diffractionX-ray FluorescenceBiomass is one of the potential renewable energy resources\u00a0[1]. Gasification is a promising thermochemical conversion, the main driver for converting biomass composition into useful gases and chemicals. Gaseous fuels from biomass gasification can be sources of producer gas (CO, \n\n\nH\n\n\n2\n\n\n, CH4, CO2), and syngas (CO, \n\n\nH\n\n\n2\n\n\n). However, the inherent drawbacks of biomass are low energy density, hydrophilic materials, bulky volume and short time storage. Torrefaction is a pre-treatment process at a temperature of 200\u2013300\u00a0\u00b0C in an inert atmosphere to increase the volumetric energy density, which can enhance the biomass conversion efficiency\u00a0[2]. During torrefaction, the original component in biomass such as volatile compounds, lignocellulosic materials, inter and intra-molecular hydrogen and CO, CH bonds are destructed at different temperatures\u00a0[3\u20135]. Generally, conventional torrefaction is carried out in a nitrogen atmosphere, which leads to a higher operating cost, stemming from the requirement of separation of \n\n\nN\n\n\n2\n\n\n from air. Oxidative torrefaction is another special torrefaction process, in which biomass is torrefied in an oxidative environment (containing 3\u201310 vol.% \n\n\nO\n\n\n2\n\n\n). The study of Wang et\u00a0al. [6] indicated that torrefied sawdust\u2019s properties and its pellets in oxidative exposure, such as density and higher heating value, were close to those in inert atmospheres. From the work of Chen et\u00a0al. [7], it was reported that higher heating value of liquid product derived from the torrefied palm oil fiber pellets in inert and oxidative exposure was in range of 10.10\u201313.20MJ/kg, which could increase to 23.20\u201328.70MJ/kg after dehydration. From research by Li et\u00a0al. [8] which torrefied pine and poplar under CO2 carrier gas at temperature ranging from 220\u2013340\u00a0\u00b0C. They concluded the higher temperature plays the reaction of deacetylation and dehydration while the mainly reaction decarboxylation occurs at the low temperature torrefaction. It can thus be concluded that using combustion flue gases as carrier gases for the torrefaction of biomass is feasible.Cassava rhizome (CR) biomass, agricultural residues in Thailand, can be converted into a gaseous product by gasification or pyrolysis. Many studies have evaluated CR behavior in thermo-chemical processes such as combustion, gasification, and pyrolysis. Previous studies have investigated cassava residues during later process such as fast pyrolysis of stalk and rhizome of cassava plants by a pyrolysis GC/MS\u00a0[9]. There were some slow pyrolysis researches of palm kernel cake and cassava pulp residue in a fixed-bed reactor\u00a0[10]. Homchat et\u00a0al. [11] conducted slow pyrolysis of fresh and dried CR in a large scale metal kiln which resulted in less charcoal than fresh CR, due to the effect of the moisture content. Most of previous researches investigated the type of reactor; however, high oxygen component in CR (38-57\u00a0wt%) caused low heating value and oxygenated compound emission in the bio-oil product. In a few recent studies, it has been reported that torrefied biomass can significantly affect the efficiency of biomass gasification. Phanphanich and Mani [12] investigated the fuel characteristics and grindability of pine chips and logging residues torrefied at temperatures ranging from 225\u00a0\u00b0C to 300\u00a0\u00b0C and 30 min residence time. They found that high hemicellulose and lignin in the biomass produce more tar during the gasification. Tremel et\u00a0al. [13] found that the overall gasification efficiency and carbon conversion efficiency of the entrained flow gasifier was observed to be superior for the smaller (\n\n160\n\n\u03bc\nm\n\n) particles torrefied biomass compared to that of the larger (\n\n250\n\n\u03bc\nm\n\n) particles.Various zeolite catalysts such as dolomite, olivine, and metal oxide have been introduced in biomass gasification or pyrolysis in order to improve the quality of the product. Several catalysts have been tested, either for coal or biomass gasification i.e.,\u00a0dolomite, fluid catalytic cracking catalysts (FCC)\u00a0[14], and metal based catalysts\u00a0[15]. Particularly, zeolites are widely applied in more than 90% of petrochemical and refining industries. During the thermo-chemical process, zeolite catalyzes to upgrade biomass (i.e.,\u00a0cellulose, cellobiose, D-glucose and xylitol) at moderate temperatures of 400\u2013600\u00a0\u00b0C and enhances the yields of aromatic and aliphatic hydrocarbons. MCM-41 zeolite properties are a regular array of uniform and one-dimension mesopores. The extremely high surface area of ca. 900\u20131000 m2/g makes these materials promising candidates as catalysts or as catalysts support. Generally, metal such as nickel (Ni), shows excellent catalytic activity. Previous works have recorded improvement of catalytic activity and stability in steam gasification of biomass through Ni/MCM-41\u00a0[16], partial oxidation of CH4\u00a0[17], and CO\n\n\n\n2\n\n\nreforming of CH4\u00a0[18,19]. Many supporters of Ni catalyst, such as MgO, Al2O3, ZrO2, and CeO2 were tested in the activities. Moreover, porous structure of MCM-41 interaction with Ni metal are important for catalytic process during steam reforming of hydrocarbons into light products (\n\n\nC\n\n\n1\n\n\n\n\n\n\n\nC\n\n\n5\n\n\n) or the gasoline (\n\n\nC\n\n\n5\n\n\n\n\n\n\n\nC\n\n\n12\n\n\n)\u00a0[16,20].MCM-41 can be synthesized from waste, such as cold fly ash or rice husk. Because abundance of silicon source composed in the solid waste. Research by Li et\u00a0al. [8] found that the BET surface area and average pore diameter of MCM-41 synthesized from coal fly ash, were 1347 m2/g and 3.80\u00a0nm, respectively. Illite is a raw material in many industrial applications particularly in ceramics and refractories. Almost all the illite clay waste in Thailand was disposed of in the mining area after mining and dressing illite clay, which caused an environmental problem. To date there have been no systematic studies of the recovery of illite waste for MCM-41 synthesization. Illite waste can be major a silica (Si) source for MCM-41 zeolite synthesis. Illite waste was treated with an alkaline solution or silica, which make alumina to be the first extracted from clay with hot alkaline solution and consequently this process resulted in the supernatants. Then, the supernatants were applied as a starting material for MCM-41 zeolite synthesis by hydrothermal processing.According to, new trend of the renewable energy and zero waste and circular economy, the utilization of illite waste as the raw material for zeolite synthesis was focused on this work. The objective of this synergy study is to propose the value-added pathway on solving of the illite mining waste, flue gas emission and drawback of CR fuel. The transition metal Ni can be loaded on MCM-41 by impregnation or post-synthesis which is generally a desirable method. The focus of this work is to study MCM-41 synthesized by illite waste (Ni/MCM-41) for catalytic gasification of torrefied CR at 700\u00a0\u00b0C for 30 min for generation of high-quality gas products.Torrefaction of CR particle size of 0.425\u20130.850\u00a0mm and 0.850\u20132\u00a0mm was conducted at temperature of 260\u00a0\u00b0C for 60 min in nitrogen gas atmosphere and simulated flue gas (SFG) and used as a raw material for gasification. In case of SFG mixed, CO2 (15 vol.%) and \n\n\nO\n\n\n2\n\n\n (5 vol.%) in \n\n\nN\n\n\n2\n\n\n balance was applied in this work. The picture of the CR samples is displayed in Fig.\u00a01. The element of initial CR before torrefaction such as carbon, hydrogen, nitrogen, and oxygen were 37.60, 5.41, 0.37, and 55.93, respectively. The properties of CR sample are listed in Table\u00a01.\n\n\nThe chemical compositions and phase analysis of illite waste were characterized by X-ray Fluorescence (XRF) and X-ray powder diffraction (XRD) techniques. The elements that are found in the highest quantities are O, Si, Al, Fe, K, and Na. These are also the major elements found in illite waste. The chemical composition of illite sample mainly consisted of 73.01wt.% SiO2, 16.52\u00a0wt% Al2O3, 5.28\u00a0wt% \n\n\nK\n\n\n2\n\n\nO and 2.38wt.% Fe2O3 and low contents of MgO, TiO2, Na2O, and SO3. Phase analysis of illite powders was determined by XRD (PANalytical, model X\u2019 Pert Pro) with 40\u00a0kV, Cu K\n\u03b1\n radiation. The scanning ranges from 10-60\u00b0 with a step size of 0.02 are shown in Fig.\u00a02(a). Microstructure of illite sample was measured by scanning electron microscope (SEM) (Hitachi, model SU-5000) as shown in Fig.\u00a02(b).\n\nIllite was fused with NaOH with the weight ratio of NaOH-Illite at 1.2:1. Calcined temperature of sample was 550\u00a0\u00b0C for 1 h. The illite fusion was slowly dissolved in 22.50\u00a0ml of deionized water for 24 h. The supernatant was obtained after filtration of suspension. 3.45 g of cetyltrimethylammonium bromide (99%, Aldrich Chem Co) was dissolved in 45\u00a0ml of deionized water (D.I.), mixed with 5.4\u00a0ml of ammonium hydroxide and continuously stirred at 25\u00a0\u00b0C for 30 min. After addition of 8.33\u201313\u00a0ml of tetraethyl orthosilicate (98%, Aldrich Chem Co) stirring continued until a homogeneous mixture emerged, with pH \n=\n 10.5\u201311.5 adjustment by acetic acid. The mixed liquid was transferred to a Teflon-lined stainless-steel autoclave and heated at 110\u00a0\u00b0C for 72 h. The precipitated powder of MCM-41 was filtered and washed with deionized water. MCM-41 was dried at 105\u00a0\u00b0C in oven and then calcined in air at 540\u00a0\u00b0C for 4 h. 5Ni/MCM-41 catalyst was prepared by impregnation and evaporation. A certain amount nickel (II) nitrate hexahydrate (Ni (NO3)2\n\n\u22c5\n6H2O, 98.5%, Aldrich Chem Co) loading (5\u00a0wt%) was dissolved in ethanol. MCM-41 powder was added to the mixture and stirred for 3 h followed by evaporation of the mixture at 50\u00a0\u00b0C. The solids obtained were calcined in a muffle furnace at 550\u00a0\u00b0C for 4 h with a heating rate of 1\u00a0\u00b0C/min in the presence of air.A downflow gasifier system consists of five main parts: (1) biomass feeder (2) carrier gas unit, (3) stainless steel reactor and catalyst holder, (4) condenser, and (5) gas filters and collection unit. Gasification zone and catalytic section temperatures were set at 700\u00a0\u00b0C, and 500\u00a0\u00b0C, respectively. The 5Ni/MCM-41 catalyst was mixed with silicon carbide (SiC) in the ratio of 1:55 and placed in a top holder section. Three thermocouples were fitted at the top and bottom part of the reactor, and at catalyst holder. The reactor was purged with \n\n\nN\n\n\n2\n\n\n to avoid combustion before operating. The carrier gas (\n\n\nN\n\n\n2\n\n\n) entered the reactor along with the gasifying agent (\n\n\nO\n\n\n2\n\n\n) which was fed into the bottom of the reactor. The ratio of \n\n\nN\n\n\n2\n\n\n and \n\n\nO\n\n\n2\n\n\n was adjusted to a target equivalence ratio (ER) of 0.4. The CR was continuously fed into the system of 1.0 g/min for a 30 min. The condensate products, such as tar were retained in condensers and gas washers. Gas produced was measured by means of volumetric gas meter after separation of condensate before conveyed into the main gas line by a vacuum pump at a flow rate of 0.5 L/h. The gaseous products such as CO2, CO, H2, CH4 and other hydrocarbons as C\n\n\n\nx\n\n\nH\n\n\n\ny\n\n\n were measured by Gasboard-3100p instrument. The solid portion was later collected for further analysis.X-ray diffraction pattern of synthesized catalyst was derived by XRD (Rigaku TTRAX III) with low angle range of 0.5\u20135\u00b0 and XRD (PANalytical, X\u2019 Pert Pro) for wide range of 20\u201370\u00b0. Ni metal dispersion of catalysts was analyzed by Energy dispersive X-ray spectroscope (EDS). Surface area, average pore size, and total pore volume of the fresh catalysts were determined by \n\n\nN\n\n\n2\n\n\n adsorption and desorption Brunauer\u2013Emmett\u2013Teller (BET) isotherms. Pore size, pore size distribution and pore volume were obtained by the Barret\u2013Joyner\u2013Halenda (BJH) pore analysis.As expected, at any torrefaction conditions, the oxygen component was decreased in the sample while more carbon was retained in the torrefied CR. For torrefaction in \n\n\nN\n\n\n2\n\n\n atmosphere, the carbon content for CR size 0.425\u20130.850\u00a0mm and 0.850\u20132\u00a0mm were 53.40\u00a0wt% and 51.49\u00a0wt%, respectively. Carbon contents in torrefied CR under SFG atmosphere drastically improved when compared to the original CR as noticed in Table\u00a01. This effect of SFG is prominent for carbon and oxygen components in all torrefied CR particle sizes. The decrease of hydrogen and oxygen in torrefaction of CR process because of the breakage of OC and CC bonds\u00a0[20]. After undergoing SFG torrefaction, OC atomic ratio (O/C) and HC atomic ratio (H/C) were reduced to become less than those under \n\n\nN\n\n\n2\n\n\n carrier gas. Proximate analysis of these samples was shown in Table\u00a01, the loss of some of the organics affected the loss of the volatiles while ash content increased. Additionally, the heating value of CR was originally 15.6\u201315.9\u00a0MJ/kg and after torrefaction with \n\n\nN\n\n\n2\n\n\n and SFG, heating value was increased to 20.20\u201322.07\u00a0MJ/kg and 22.07\u201324.37\u00a0MJ/kg, respectively. These results can be expressed by the low-energy bond of HC and OC reduction and high energy bond of CC. Nitrogen and ash content increased in torrefied CR. This is simply attributed to the fact that all the components containing nitrogen and other minerals (in ash) retain in the biomass solid phase, whereas C, H, and O leave the solid.The XRD pattern of MCM-41 after calcination is illustrated in Fig.\u00a03(a). The MCM-41 catalyst samples display the high intensity peak in 2\n\u03b8\n of 2.16\u00b0, 3.74\u00b0, 4.30\u00b0, and 5.72\u00b0 and sharp diffraction peak (\n\n\nd\n\n\n100\n\n\n), (\n\n\nd\n\n\n110\n\n\n), (\n\n\nd\n\n\n200\n\n\n) and (\n\n\nd\n\n\n210\n\n\n), respectively. These diffraction peaks indicate a long-range ordered hexagonal mesoporous structure of MCM-41 synthesis from illite waste. The hexagonal diameter and pore wall thickness can be calculated by equation of \n\n\na\n\n\n0\n\n\n\n\n=\n 2d\n\n\n\n\n100\n\n\n\u2215\n\n\n3\n\n\n\n\u00a0[21]. The \n\n\na\n\n\n0\n\n\n of MCM-41 was 45.71 \u00c5.The XRD pattern of fresh 5Ni/MCM-41 is exhibited in Fig.\u00a03(b). The diffraction peaks of Ni phase at 42\u00b0 and 50\u00b0 occurred at the NiO crystalline phase at 37\u00b0, 43\u00b0 and 51\u00b0. The other crystalline such as Ni2SiO4 (nickel silicate) and Al2SiO5 (aluminum silicate) were observed at 26\u00b0 and 61\u00b0, respectively. It can be concluded that Ni atoms displayed good dispersion in the support porous.\nThe nitrogen adsorption\u2013desorption isotherm and pore size distributions of catalyst sample are illustrated in Fig.\u00a04(a) and 4(b). All the samples are isotherm type IV according to IUPAC classification. In Fig.\u00a04, the immediate increase in the region of 0.25 < P/P0 < 0.40 is related to the capillary condensation inside the mesoporous wall\u00a0[17,19]. In general, the long range at higher relative pressure suggested that the adsorption continued on the surfaces of MCM-41 sample at P/P0 > 0.45 due to an increase in pore size. The isotherm of MCM-41 sample presented mesoporous filling steps with pore size larger than 40 \u00c5. In Fig.\u00a04b, the isotherm shows an identical shape, although the adsorption capacity decreased with Ni loading on MCM-41 support because the particles of nickel oxide finely dispersed inside the MCM-41 supported porous by impregnation with ethanol.\nThe textural properties of synthesized catalysts are presented in Table\u00a02. The surface area of 5Ni/MCM-41 catalysts was slightly decreased from 804.03 m2/g to 737.88 m2/g. The 5Ni/MCM-41 pore diameter was close to MCM-41 supported sample, because of the dispersion of Ni metal particles in MCM-41 and less blockage of MCM-41 supported pore by the impregnation method\u00a0[22]. It can be noted that the porosity of the catalyst is not significantly changed in this work.\nSEM technique was applied for analysis of the surface topology and to assess the dispersion of Ni components covered on supporter. SEM image of illite waste, MCM-41 support and 5Ni/MCM-41 can be seen in Fig.\u00a05. All the samples contained irregular shapes and grain sizes\u00a0[23]. The surface morphology of MCM-41 synthesized at pH 10.5\u201311.5 gave well-order with diameter size around 100\u00a0nm. The small amount of Ni particles can be observed on MCM-41 surface, whereas some Ni particles were found inside the MCM-41 pores\u00a0[24,25]. Ni metal dispersion was obtained for 3.6 and 5.6\u00a0\u00b1\u00a00.2\u00a0wt% by EDS for analysis.\n\nIn this work, the torrefied CR was used in catalytic gasification. The presence of nickel enhanced the gas fraction of product yield. Liquid yield from CR size 0.425\u20130.850\u00a0mm was lower than the larger size in gasification with a catalyst. Carbon and hydrogen conversion were calculated as the molar of CO and \n\n\nH\n\n\n2\n\n\nproduced. The syngas composition from no catalyst gasification experiment consisted of CO (3.32\u20137.90 vol.%), \n\n\nH\n\n\n2\n\n\n (2.48\u20133.02 vol.%), CH4 (2.23\u20133.29 vol.%), \n\n\nC\n\n\nx\n\n\nH\n\n\n\ny\n\n\n (0.07\u20130.16 vol.%) and CO2 (9.30\u201316.52 vol.%). Torrefied CR had lower O/C ratio, and when it was gasified, the torrefied CR produced lower CO2. Gasification with 5Ni/MCM-41 showed higher \n\n\nH\n\n\n2\n\n\n and CO for torrefied CR with lower CO\n\n\n\n2\n\n\nconcentration, in tune with the findings of Tapasavi et\u00a0al.\u00a0[26]. Tar was effectively removed by 5Ni/MCM-41 catalyst. Typically, Ni based catalysts exhibit high tar cracking. This metal property along with the ongoing Boudouard and water gas shift reaction activities allow favorable composition adjustment of \n\n\nH\n\n\n2\n\n\n and CO more than that of CO2 in the product gas. The CO and \n\n\nH\n\n\n2\n\n\n concentration obviously increased with the use of a catalyst, as can be seen in Table\u00a03.\nIn the downflow reactor, the gasification of torrefied CR reactions can be explained the following main equations (Eqs.\u00a0(1)\u2013(6)). Devolatilization in gasification of torrefied CR occurs more than others since volatiles react with themselves in gas\u2013gas phase. In addition, the water-gas shift reaction can increase the CO2 amount in syngas. These are explained in gas-phase phenomena of volatile gases released during gasification. In gas phase reactions, the water gas shift reaction is important for increasing the \n\n\nH\n\n\n2\n\n\n in syngas, while the methanation influences the CH4 product. The Boudouard reaction converts CO2 into CO. Because temperatures are below 1000\u00a0\u00b0C, this reaction is in equilibrium and the CO remains in the synthesized gas. \n\n\n(1)\n\n\nBoudouard\u00a0reaction\u00a0\nC\n+\n\n\nCO\n\n\n2\n\n\n\u2192\n2\nCO\n\n\n\n\n(2)\n\n\nWater-gas\u00a0shift\u00a0reaction\u00a0\n\n\nCO\n\n\n2\n\n\n+\n\n\nH\n\n\n2\n\n\n\u2194\nCO\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n(3)\n\n\nMethanation\u00a0reaction\u00a0\nC\n+\n2\n\n\nH\n\n\n2\n\n\n\u2192\n\n\nCH\n\n\n4\n\n\n\n\n\n\n(4)\n\n\nC\n+\n3\n\n\nH\n\n\n2\n\n\n\u2192\n\n\nCH\n\n\n4\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\nThe nitrogen compounds contained in materials enhanced the direct release of isocyanic acid as part of volatile matter during degradation of CR at low temperature in downflow reactor. HNCO can react with steam from water gas shift reaction, yielding ammonia (NH3) and CO2. The moisture in CR could participate in the reaction involving hydrogen cyanide (HCN) and give rise to NH3 and CO as per equation below\u00a0[27]. This results in a comparatively steady amount of CO. \n\n\n(5)\n\n\nHNCO\n+\n\n\nH\n\n\n2\n\n\nO\n\u2192\n\n\nNH\n\n\n3\n\n\n+\n\n\nCO\n\n\n2\n\n\n\n\n\n\n(6)\n\n\nHCN\n+\n\n\nH\n\n\n2\n\n\nO\n\u2192\n\n\nNH\n\n\n3\n\n\n+\nCO\n\n\n\n\n\nThe effect of 5Ni/MCM-41 catalyst on torrefied CR gasification was studied. The catalyst holder was placed on the top of reactor. The operating temperature of catalyst was set at 500\u00a0\u00b0C, which is generally the range for tar cracking in the gasification, similar to the use of Ni catalyst in gasification temperature of 500\u00a0\u00b0C and 600\u00a0\u00b0C of cedar wood and sunflower stalk, respectively\u00a0[16,25]. Surface acidity active sites of 5Ni/MCM-41 can improved the cracking or reforming reaction. In this work, the gas production and carbon conversion slightly increased from 66 to 73\u00a0wt% and 75 to 80%, respectively. H conversion obviously increased from 18.47 to 27.39% as illustrated in Fig.\u00a06. Having surface acidity active sites on 5Ni/MCM-41 can improve the cracking or reforming reactions. All researches reported over 60% conversion of reforming biomass tar over Ni-based catalysts\u00a0[28\u201330]. Moreover, synthesized 5Ni/MCM-41 can crack the vapor fraction in the system which increase the product gas volume due to the decomposition of gaseous components of the synthesized gas obtained during cracking of liquid products\u00a0[31]. 5Ni/MCM-41 will drive Sabatier reaction forward and yield more CH4 as described in equation below\u00a0[32]. \n\n(7)\n\n\n\n\nCO\n\n\n2\n\n\n+\n4\n\n\nH\n\n\n2\n\n\n\u2192\n\n\nCH\n\n\n4\n\n\n+\n2\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\nCombined with the characterization results, the trend of the catalytic performance of the active Ni surface areas and dispersion can be clearly observed. Ni/MCM-41 enhanced the higher H2 production from CR gasification, suggesting that some small Ni particle sized of 3\u00a0nm maybe inside the MCM-41 porous which promoted water gas shift and reforming reactions of C\n\n\n\nx\n\n\nH\n\n\n\ny\n\n\n and CH4. This is because of the reactants\u2019 longer residence time inside of the MCM-41 pores\u00a0[20].Cold gas efficiency (CGE) is the fraction of energy output over the energy input. The heating value of the gas products obtained from the gasification of torrefied CR is presented in Fig.\u00a07. A lower O/C ratio in torrefied CR indicate the increasing amount of C and the calorific value which implies higher gasification efficiency. The ranges of CGE varies with heating value of gas products.\nThe minimum of gas heating value (lower heating value) was 7.78\u20138.37\u00a0MJ/kg without catalyst gasification, while the maximum range was 9.38\u201310.03\u00a0MJ/kg. These results indicated a high CO, \n\n\nH\n\n\n2\n\n\n and hydrocarbon gaseous mixture in the gas product. Gasification efficiency in gasification without catalyst has some issues worth mentioning as per following. Utilizing CR size 0.425\u20130.850\u00a0mm will yield slightly more CGE than larger CR while smaller CR obtaining from torrefied under SFG will yield considerably 10% less than larger size. CGE is fundamentally calculated from heating value of produced gas and CR biomass sample, yet there is another parameter namely gas-volumetric which induced CGE to become disproportional and not corresponding with other results. For example, gasification of CR size 0.850\u20132\u00a0mm dropped for 3%, which is statistically insignificant. It can be concluded that addition of Ni/MCM-41 enhanced overall efficiency. Catalytic gasification of CR size 0.85\u20132\u00a0mm obtaining from torrefied under SFG showed outstanding result in term of CH conversion and \n\n\nH\n\n\n2\n\n\n/CO data.Gasification of torrefied CR with 5Ni/MCM-41 catalyst was investigated in this work under partial oxidative atmosphere such as simulated flue gas from typical power plant. Results from the experiments confirmed the benefits of torrefaction of CR prior to gasification under described atmosphere. Illite waste can be utilized as a precursor to MCM-41 synthesis and added as a catalyst in gasification of torrefied CR. This work has revealed the synergy of utilizing torrefaction and catalyst from waste for energy generation considering from its efficiency in various factors such as gasification of torrefied CR under SFG atmosphere and \n\n\nN\n\n\n2\n\n\n with catalyst would yield 10% less unfavorable liquid. MCM-41 supported Ni metal was prepared by ethanol assisted impregnation method. NiO dispersed throughout on MCM-41 supported and partially formed into Ni2SiO4 which showed high surface area and pore volume. 5Ni/MCM-41 plays an important role on total gas yields and CO and \n\n\nH\n\n\n2\n\n\n conversions with its excellent properties in gasification and tar cracking. In summary, applying SFG for CR torrefaction is a promising technique to produce quality green fuel at economical cost for further utilization as renewable energy via catalytic gasification process employing illite waste as precursor for synthesizing effective catalyst.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by Royal Golden Jubilee Ph.D. Programme, Thailand [grant no. PHD/0212/2557]. Additional research scholarships were provided by Overseas Academic Presentation Scholarship for Graduate Students, Thailand, and the 90th Anniversary of Chulalongkorn University Fund (Ratchadaphiseksomphot Endowment Fund), Thailand\n . The authors would like to thank the National Metal and Materials Technology Center (MTEC) and Interdisciplinary Program in Environmental Science, Graduate School, Chulalongkorn University.", "descript": "\n                  In this work, torrefaction of cassava rhizome (CR) under nitrogen gas (\n                        \n                           \n                              N\n                           \n                           \n                              2\n                           \n                        \n                     ) and a simulated flue gas (CO2 (15 vol.%) and \n                        \n                           \n                              O\n                           \n                           \n                              2\n                           \n                        \n                      (5 vol.%) in \n                        \n                           \n                              N\n                           \n                           \n                              2\n                           \n                        \n                      balance) atmosphere was examined in a downflow reactor at 260\u00baC for a residence time of 60\u00a0min to produce a superior solid fuel for subsequent 5Ni/MCM-41 catalytic gasification of CR utilization. Mesoporous molecular sieves (MCM-41 zeolite) was synthesized from illite waste as a silica source. The MCM-41 synthesis was carried out by hydrothermal and post-synthesis for Ni loading. Various characterization techniques, such as XRD, SEM, and BET were employed to thoroughly characterize catalyst. High surface area (737.88 m2/g) and a typical type IV pattern of hysteresis loop (0.25 \u20130.40) obtained 5Ni/MCM-41 catalyst is calculated by \n                        \n                           \n                              N\n                           \n                           \n                              2\n                           \n                        \n                      adsorption\u2013desorption technique. Catalyst characterization and discussion of results are presented in this work. 5Ni/MCM-41 catalyst strongly enhances the \n                        \n                           \n                              H\n                           \n                           \n                              2\n                           \n                        \n                      and CO production from gasification of torrefied CR at a temperature of 700\u00baC. Carbon and hydrogen conversions were 80.17% and 27.39%, respectively while liquid yield was lower than 10\u00a0wt%. The syngas from the conversion maintained \n                        \n                           \n                              H\n                           \n                           \n                              2\n                           \n                        \n                     /CO ratio of 0.55 with the highest gaseous efficiency of 49.35%. Obviously, synergy of synthesized 5Ni/MCM-41 catalyst and torrefied CR with gasification is valuable useful as potential renewable energy generation process.\n               "}
{"full_text": "Utilization of CO2 is currently a hot topic in catalysis due to the chance to decrease anthropogenic CO2 emissions on the one hand and to recycle it as a C1 source in exchange to fossil fuels on the other hand. So called power-to-gas (PtG) and power-to-liquid (PtL) technologies enable chemical storage of surplus energy from regenerative sources by reaction of renewable H2 with CO2 to energy carrier such as methane (PtG) or liquid fuels (PtL) [1\u20133]. Especially the PtG technology has high potential as a chemical energy storage technology since infrastructure for fast energy generation as well as a natural gas grid based on fossil natural gas is already well established and a state-of-the-art-technology. Hence, renewably produced CH4 via PtG can be easily feed into the existing gas grid and in a future perspective completely replace fossil natural gas.Ni is the state of the art catalyst for CO2 methanation (Eq. (1)) already since its discovery by Paul Sabatier in 1902 [4] and has been center of several studies on various supports, whereat reviews can be found elsewhere [1\u20133,5\u201314].\n\n(1)\nCO2\u202f+\u202f4\u202fH2\u202f\u2192\u202fCH4\u202f+\u202f2\u202fH2O\n\n\nBesides Ni, also other metals are active in CO2 methanation [15,16]. Mills and Steffgen classified the important metals for methanation catalysts by its activity (Ru\u202f>\u202fFe\u202f>\u202fNi\u202f>\u202fCo\u202f>\u202fMo) and selectivity to methane (Ni\u202f>\u202fCo\u202f>\u202fFe\u202f>\u202fRu) [17].Ni shows high activity with a very good selectivity to CH4. Nevertheless, traditional Ni-catalysts suffer from deactivation by sintering of the Ni particles upon heat evolution from the highly exothermic methanation reaction [18]. Deposition of coke and formation of volatile nickel carbonyls contribute to additional catalyst deactivation [19,20]. Besides, Ni is of toxicological concern. The sequences of Mills and Steffgen point out, that Fe has a very high activity for CO2 activation but suffers from low selectivity. In contrast to Ni, iron is not toxic, is much more abundant and hence around 180 times cheaper than nickel.Surprisingly, only a few studies focus on optimization of Fe based catalysts for CO2 methanation. Kirchner et al. investigated bare iron oxide samples in the CO2 methanation and obtained best activity for nano-sized \u03b3-Fe2O3 with maximum CH4 yield of 60 % at 400\u202f\u00b0C and ambient pressure [21]. In addition, pure \u03b1-Fe2O3 based catalysts can be promoted with 2\u202fwt % Mg in order to increase the basicity and hence interaction of CO2 with the catalysts. This promotion leads to improved CH4 yield up to 32 % at 8\u202fbar and a GHSV of 10,000\u202fh\u22121 [22]. The results emphasize that the methanation takes place predominantly on surface carbon and iron carbide species on promoted bulk Fe2O3 catalysts [22]. In general, the high activity of Fe for CO2 activation results from the high reverse-water-gas-shift (RWGS) activity (Eq. (2)) and especially at elevated pressure its further capability of CO hydrogenation via Fischer\u2013Tropsch-Reaction (FTR) (Eq. (3)).\n\n(2)\nCO2\u202f+\u202fH2\u202f\u2192\u202fCO\u202f+\u202fH2O\n\n\n\n\n(3)\nnCO\u202f+\u202fm/2n\u202f+\u202f1\u202fH2\u202f\u2192\u202f1/n\u202fCnHm\u202f+\u202fn\u202fH2O\n\n\nLee et al. investigated the CO2 hydrogenation via FTR on Fe catalysts at 1\u201325\u202fbar and in various H2/CO2 ratios [23]. They found that metallic Fe transforms into mixtures of magnetite and carbides under reaction conditions. Especially in the pressure range of 1\u201310\u202fbar the increase of pressure leads to an increase of the chain length and higher temperature increases the CO2 conversion as well as CO and CH4 yield. In contrast, the produced H2O from FTR contributes to the equilibrium of the RWGS-reaction that limits the CO2 conversion [23]. In line, on K and S promoted Fe-based catalysts it was shown that the CO2 methanation activity is strongly influenced by the H2/H2O ratio effluent from the reactor [24,25]. It was claimed that conversions increase with increasing H2/CO2 ratio and cannot be further improved than their maximum CO2 conversion of 44 % obtained at 20\u202fbar and a H2/CO2 ratio of 8 [24].With the aim of tailoring Fe-based materials as CO2 methanation catalysts, studies on increasing the C2\u2013C4 fraction in CO-FTR, with CH4 as an undesirable product, provide information on the direction of necessary properties for high CH4 yields: In general, iron carbides are considered as the active phase in FTR and active carbon sites contribute to the chain growth mechanism [26]. In addition, the activity and selectivity is closely related to the particle size of the Fe-based catalysts [26]. Smaller Fe nanoparticles (<7\u20139\u202fnm) lead to higher CH4 selectivity [27\u201329]. It was concluded, that low coordinatively unsaturated corner and edge sites are important for CH4 formation, while terrace sites of the bigger Fe particles are responsible for olefin generation [29,30]. Hence, the selectivity of Fe based catalysts for CO2 methanation could be improved by decreasing the Fe particle size. This stands in contrast to the particle size dependency of Ni based CO2 methanation catalysts, which decrease in selectivity if the particle size decrease below 2\u202fnm [31,32].Supporting Fe on zeolites enables a way to produce stable and highly dispersed Fe species. This has been proven by their use as highly stable selective catalytic reduction (SCR) catalysts [33\u201335]. Despite the high Fe dispersion, zeolites offer additional tailoring possibilities and have shown to positively influence the CO2 methanation performance of Ni-based catalysts [36]. Namely by their compensating cation [37], Si/Al ratio [38] and zeolite framework type such as FAU, BEA, MFI and MOR [39]. Due to the high affinity of zeolites to adsorb water they allow further improvement of catalytic activity by applying so called sorption enhanced conditions whereat H2O is adsorbed by the zeolite and in that way pulled away from the reaction center [40\u201344]. To the best or our knowledge it was not investigated yet how the combination of Fe supported on zeolites perform in CO2 methanation. In the present study a series of differently loaded Fe on zeolite catalysts are investigated at ambient and elevated pressure up to 15\u202fbar with the aim to increase the CO2 methanation performance. In order to avoid restricting the product spectrum resulting from pore size effects within the zeolite, 13X was selected as zeolite support. On the one hand due to its relatively large and three dimensional pore structure where molecules up to a kinetic diameter of 7.35\u202f\u00c5 can form and diffuse freely along all axis including CO, CH4, CH3OH as well as C\u2013C coupled products up to at least C6 compounds. On the other hand a large range of Fe loadings can be theoretically ion exchanged due to the high aluminium content of 13X. As main focus the trends in activity and selectivity with increasing pressure as well as iron loading are carefully analysed and correlated with the properties of the catalysts. This leads to a justification if CO2 methanation on Fe-based catalyst will become feasible as an attractive alternative.1, 5 and 10\u202fwt % Fe/13X catalysts were synthesised by wet impregnation with a 0.05\u202fM Fe(NO3)3 \u00b7 9H2O (99 % Sigma-Aldrich) solution in ethanol on commercial Na-13X zeolite (ZEOCHEM, Si/Al\u202f=\u202f2.5; Faujasite structure). After ion exchange for 24\u202fh at room temperature under intense stirring ethanol was evaporated in a rotary evaporator. The resulting solids were dried at 80\u202f\u00b0C for 12\u202fh and calcined at 400\u202f\u00b0C (heating ramp\u202f=\u202f5\u202fK/min) for 5\u202fh in a continuous flow of air. The 5\u202fwt % catalyst with collapsed zeolite structure was synthesised by wet impregnation for 30\u202fmin with a 0.05\u202fM Fe(NO3)3 \u2219 9H2O aqueous solution on commercial Na-13X. Water was evaporated in a rotary evaporator, the resulting solid dried at 80\u202f\u00b0C for 12\u202fh and calcined at 500\u202f\u00b0C (heating ramp 5\u202fK/min) for 5\u202fh in a flow of air.The weight loadings of iron of all samples were analysed via Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) on an Agilent 720 ES. The X-ray powder diffraction pattern were measured on a Bruker D8 Advance diffractometer with Ni filtered Cu K\u03b1 radiation (\u03bb\u202f=\u202f1.5406\u202f\u00c5) and a step size of 0.2\u00b0 from 2\u00b0\u03b8\u202f=\u202f20\u201390. Crystallite sizes of the Fe-particles were calculated according the Debeye\u2013Scherrer equation using the half width of the reflex at 44.7\u00b0. UV/vis spectra were collected on a UVVISNIR Lambda 950 spectrometer from Perkin Elmer equipped with a 150\u202fmm integration sphere to analyse the diffuse reflectance of the Fe-zeolites. The spectra where recorded in reflexion mode in a wavelength region of 800\u2013200\u202fnm and a step size of 5\u202fnm. Specific surface area, pore diameter and pore diameter dispersion were analysed by N2 physisorption at 77\u202fK in a Quantachrome Autosorb IQ TPX. All samples were degassed for 12\u202fh in vacuum at 200\u202f\u00b0C. The pore diameter and dispersion were analysed according the BJH method from the desorption branch and specific surface area (SSA) by using the BET method. The pressure range for analysis was defined by rouquerol analysis in order to stay in the linear regime of the BET analysis [45]. The microporous surface area was distinguished from the external and mesoporous surface area by the t-plot method. Temperature controlled analysis were performed in the same Quantachrome Autosorb IQ TPX in dynamic mode and with a thermal conductivity detector. For temperature programmed reduction (H2-TPR) all samples were degassed at 400\u202f\u00b0C in a flow of N2 for 30\u202fmin. Subsequent to the cooling down procedure to 40\u202f\u00b0C, TPR was started in a flow of 5\u202fvol% H2 in N2, with a total flow rate of 25\u202fmL/min and a heating ramp of 5\u202fK/min up to 850\u202f\u00b0C and isothermally treated at the end temperature for additional 30\u202fmin. NH3 was used in order to analyse the acidic properties of the zeolite in the temperature controlled desorption (TPD) experiments. Prior to the analysis all samples were reduced in a flow of 50 % H2 in N2 at 400\u202f\u00b0C for 30\u202fmin, accordingly to the pre-treatment of the catalytic tests. Residual adsorbed hydrogen was flushed-off from the samples by additional 2\u202fh treatment in N2 at 400\u202f\u00b0C. Subsequently, adsorption of 10 % NH3 in N2 was performed at 100\u202f\u00b0C and physisorbed NH3 was purged in a flow of N2 at 100\u202f\u00b0C for 30\u202fmin. TPD was performed in a flow of 25\u202fmL/min N2 and a heating ramp of 10\u202fK/min up to 800\u202f\u00b0C. Scanning electron microscopy (SEM) analysis was performed in a Thermo Scientific Phenom XL equipped with a back scattered detector. Concurrent elemental mapping was carried out by using the integrated EDX detector.Methanation tests were performed in fixed bed flow reactor system with an inner diameter of 6\u202fmm at ambient and elevated pressure (5, 10, 15\u202fbar) at a GHSV\u202f=\u202f4186\u202fh\u22121. Prior to the catalytic tests all catalysts were reduced within the reactor in a flow of 50 % H2 in N2 for 30\u202fmin at 400\u202f\u00b0C and ambient pressure. In a typical run 25\u202fmL/min CO2, 100\u202fmL/min H2 and 12\u202fmL/min N2 as internal standard were supplied by mass flow controller (Bronkhorst, El Flow). During the methanation tests the temperature was raised from 200 to 400\u202f\u00b0C in steps of 50\u202f\u00b0C and kept constant at reaction temperature for 30\u202fmin. The composition of effluent gases from the reactor was monitored by online raman spectroscopy (Kaiser Raman RXN2 spectrometer equipped with AirHead probes). The conversion X, selectivity S and reaction rate of CO2 conversion (r(CO2)) were calculated according Eqs. (4)\u2013(6):\n\n(4)\n\nX\n\n\nC\n\nO\n2\n\n\n\n=\n\n\n\nn\n\u02d9\n\n\n\nC\n\nO\n\n2\n\u2009\ni\nn\n\n\n\n\n-\n\u2009\n\nn\n\u02d9\n\n(\nC\n\nO\n\n2\n\u2009\no\nu\nt\n\n\n)\n\n\n\n\nn\n\u2009\n\n\u02d9\n\n(\nC\n\nO\n\n2\ni\nn\n\n\n)\n\n\n\n\n\n\n\n(5)\n\nS\n\n\n\n\nC\nH\n\n4\n\n\n\n=\n\u2009\n\n\n\nn\n\u02d9\n\n(\n\n\nC\nH\n\n\n4\n\u2009\no\nu\nt\n\n\n)\n\n\n\u2211\n\n\nn\n\u02d9\n\n\u2009\n(\np\nr\no\nd\nu\nc\nt\ns\n)\n\n\n\n\n\n\n\n\n(6)\n\nr\n\n\n\n\nC\nO\n\n2\n\n\n\n=\n\u2009\n\n\nX\n\n\n\n\nC\nO\n\n2\n\n\n\n\u2009\n\u00d7\n\u2009\n\nn\n\u02d9\n\n(\n\n\nC\nO\n\n2\n\n)\n\u2009\n\n\n\u2009\nn\n\n\n\n\nF\ne\n\n\nc\na\nt\n\n\n\n\n\u2009\n\n\n\n\nWith \n\n\n\nn\n\u02d9\n\n\ni\n\n as the molar flow of component i, and n(Fecat) as the molar amount of Fe in the catalyst bed within the reactor.Catalysts with three different weight loadings (1, 5, 10\u202fwt %) of Fe on 13X were prepared via impregnation. Elemental analysis via ICP-OES confirms the presence of Fe on 13X close to the aimed amounts of Fe on the samples (Table 1\n).Since the zeolite framework is prone to destruction by iron, the integrity of the structure was validated via XRD analysis.The impregnation procedure and calcination temperature strongly influences the stability of the iron impregnated zeolites. Hence, a synthesis optimization was conducted: the zeolite structure stays intact only by avoiding H2O as a solvent and using ethanol as well as decreasing the calcination temperature to 400\u202f\u00b0C (Fig. 1\n). Nevertheless, with higher Fe-loading the decrease of intensity of reflexes shows the incipient destruction of the framework even by applying the optimized procedure. Compared to the pure 13X, 1\u202fwt % Fe/13X shows nearly no changes in intensity and all catalysts show reasonable stability. In contrast, the zeolite structure of 5\u202fwt % Fe/13X impregnated in H2O and calcined at 500\u202f\u00b0C completely vanishes. For this reason, all catalysts were prepared in ethanol and by calcination at 400\u202f\u00b0C and it was avoided to exceed this temperature at any time. As a pretreatment in the catalytic test a reduction of the catalysts in 50 % H2 in N2 at 400\u202f\u00b0C for 30\u202fmin was performed. Comparison of XRD of ex situ reduced and as calcined catalysts (Fig. 1) ensure that the zeolite structure stays intact for all Fe loadings during the pre-reduction and confirm the Fe-reduction by the raise of the specific reflex of metallic Fe at 2\u00b0\u03b8 of 44.7\u00b0 (inset in Fig. 1). According to the Debeye\u2013Scherrer equation extracted Fe crystallite sizes from this reflex are 33 and 23\u202fnm for 5\u202fwt % and 10\u202fwt % Fe/13X, respectively. Solely the reduced 1\u202fwt % Fe/13X does not show this specific reflex. This could be due to two reasons, or a combination thereof: Either the Fe loading is too low for the sensitivity of XRD or the Fe species are highly dispersed within the framework of the zeolite.N2 physisorption analysis confirms the presence of microporosity of all Fe/13X catalysts calcined at 400\u202f\u00b0C. Nevertheless, the specific surface area decreases from 612 to 161\u202fm2/g with increasing Fe loading. In line with the decreasing reflex intensity of the zeolite lattice from XRD analysis the micropore area extracted from t-plot analysis decreases from 573 down to 32\u202fm2/g (see Table 1).The dispersion of Fe within the zeolite framework after calcination was analysed with UV/vis spectroscopy. The line shape of the spectra arising from O\u202f\u2192\u202fFe3+ charge transfer are rather similar (Fig. 2\n). In all spectra, four distinct peaks are separated by deconvolution (Figs. S1\u2013S3). Two strong bands are found below 300\u202fnm that are assigned to isolated Fe3+ ions. Whereat the band centered at 205\u202fnm attributes to charge transfer from tetrahedral coordinated Fe3+ and the band at 250\u202fnm relates to Fe3+ in higher coordination [34]. The two bands above 300\u202fnm arise from agglomerated Fe-species. Whereby the band from octahedral Fe3+ species in small oligomeric FexOy cluster appears at 350\u202fnm and from large Fe oxide particles as a very broad band at 436\u202fnm. Quantitative analysis of the deconvoluted bands shows that all samples have the same relative amount of Fe3+ in tetrahedral sites. Contrary to this, 1\u202fwt % Fe 13X shows with 55 % of all Fe3+ ions relatively more Fe ions in dispersed and oligomeric octahedral sites. Solely 30 % of the Fe ions agglomerate to particles. In comparison to this, the two higher loaded samples have comparable factional amounts of Fe in all sites and more than 55 % of Fe agglomerate into particles.The reducibility of the Fe/13X catalysts was investigated by H2TPR experiments (Fig. 3\n). In line with the Fe loading of catalyst the intensity of the signals increases and the features of 5 and 10\u202fwt % Fe/13X samples are rather similar. These two samples show a very intense and broad signal between 200\u2013550\u202f\u00b0C with a peak maximum that shifts to lower temperatures from 442 to 405\u202f\u00b0C with increasing Fe loading from 5 to 10\u202fwt %. In agreement with literature these signals correspond to the reduction of Fe of the agglomerated FeOx particles and dominate the TPR [46]. In addition, two more signals appear at temperatures higher than 550\u202f\u00b0C that go in line with the collapse of the zeolite structure.In the TPR of 1\u202fwt % Fe/13X three distinct peaks appear in the temperature region of zeolite\u2019s thermal stability with peak maxima at 375, 424 and 498\u202f\u00b0C. According to literature, reduction of Fe3+ within the zeolite structures as well as reduction of Fe2O3 to Fe3O4 from oligomeric and small cluster takes place at lower temperature [47]. The visibility of the fine structure of reduction under the same measurement conditions shows on the one hand that agglomerated FeOx-species are not the main species, and on the other hand, that Fe species coordinated on different sites of the zeolite framework are present in this sample.Temperature programmed desorption of NH3 was performed in order to analyse the influence of the Fe loading on the zeolites acidity (Fig. S4). In line with the decrease of reflex intensity of the zeolite framework in the XRD with increasing Fe loading the total number of acid sites decreases. The main signal in the TPD appear at the same temperature region. Hence, even though the number of acid sites decreases with increasing Fe load, the acid strength as well as nature of acid sites remain constant in all samples. Therefore, it can be excluded to significantly influence the selectivity of the catalysts.All prepared materials were investigated in a temperature region of 200\u2013400\u202f\u00b0C and at pressures of 1, 5, 10 and 15\u202fbar.In a first step the two 5\u202fwt % Fe/13X with a collapsed (prepared in H2O and calcined at 500\u202f\u00b0C, broken lines in Fig. 4\n) and intact zeolite structure (calcined at 400\u202f\u00b0C & exclusion of H2O from the synthesis, solid lines in Fig. 4) were compared by their catalytic performances. In case of the catalysts with a collapsed framework after synthesis a rather low CO2 conversion of 10 % was observed by increasing the temperature up to 400\u202f\u00b0C, even at 15\u202fbar. For this reason, the temperature range of the catalytic test was expanded to 550\u202f\u00b0C for this catalyst. At 1\u202fbar no significant CO2 conversion was observed up to 550\u202f\u00b0C. Likewise all other investigated catalysts, the CO2 conversion increases with temperature and increasing pressure of the catalytic tests. With 5\u202fwt % Fe on collapsed 13X reasonable CO2 conversion was achieved up to 74 % at 550\u202f\u00b0C and 15\u202fbar. On this catalyst, CO is the main product at low pressure. With increasing pressure, selectivity towards CH4 increases up to 85 % at 15\u202fbar and 550\u202f\u00b0C. No Fischer\u2013Tropsch products were observed under any conditions.On the contrary, 5\u202fwt % Fe/13X with an intact zeolite framework shows already reasonable CO2 conversion of 33 % at 1\u202fbar and 400\u202f\u00b0C. CO2 conversion increases with temperature and pressure up to 88 % at 400\u202f\u00b0C and 15\u202fbar. Incipient activity is already obtained at 250\u202f\u00b0C. Hence, the comparison of these two catalysts clearly demonstrates that an intact zeolite framework is essential to obtain and support high catalytic performances at reasonable temperatures.In order to stay in the kinetic regime the Fe-normalized reaction rates at 300\u202f\u00b0C are used to compare the activity of produced catalysts with intact zeolite structure and different Fe loading at pressures from 1 to 15\u202fbar (Fig. 5\n). The catalyst masses included in the reactor and corresponding Fe-content from ICP analysis were used to calculate the Fe molar reaction rates. The two catalysts with 5 and 10\u202fwt % Fe/13X show similar and increasing reaction rates at increasing pressure of up to 12 and 8\u202fmmol(CO2)/(mol(Fe)\u2219s), respectively. This points out that the main active sites are the same in these two catalysts. In opposition to these results, 1\u202fwt % Fe/13X shows much higher reaction rates at all investigated pressures up to 42\u202fmmol(CO2)/(mol(Fe)\u2219s) at 10\u202fbar. These trends show in correlation to the UV/vis analysis that finely dispersed Fe-species, which are the main species in 1\u202fwt % Fe/13X, have a much higher catalytic activity than the agglomerated Fe-species, that are the main species of the 5 and 10\u202fwt % loaded Fe/13X catalysts. Nevertheless, the reaction rate decreases upon further increase of the pressure from 10 to 15\u202fbar against the principle of Le Chatelier. This might be either due to hampered desorption of one of the products from the catalyst surface or reconstruction of the Fe-species or zeolite framework at elevated pressure.The catalytic performances at 350\u202f\u00b0C are used to compare the variation of product selectivity of different Fe loading and at varying pressures (Fig. 6\n).At 1\u202fbar the 10\u202fwt % Fe/13X catalyst shows a very high selectivity towards CO (S(CO)\u202f=\u202f97 %) and minor selectivity to CH4 (Fig. 6a). With increasing pressure to 15\u202fbar the selectivity towards C\u2013C coupled products and CH4 increases monotonously. CH4 becomes the main product at 10\u202fbar and reaches its maximum selectivity of 61 % at 15\u202fbar. The selectivity to C\u2013C-coupled products increases up to 22 %, while the selectivity towards CO decreases to 15 %.Likewise, 5\u202fwt % Fe/13X catalyst shows the same trend with increasing pressure (Fig. 6b). At high pressures it increases its selectivity towards the desired product CH4 up to 68 %, while the selectivity towards CO (S(CO)\u202f=\u202f14 %) and CC-coupled products (S(CC)\u202f=\u202f17 %) stays relatively low.In contrast to the behavior of the two higher loaded Fe catalysts 1\u202fwt % Fe/13X shows at 1\u202fbar already significant CH4 selectivity of 22 % (Fig. 6c). The CO selectivity of 21 % at 350\u202f\u00b0C and 1\u202fbar is relatively low and selectivity towards C\u2013C coupled products is at 56 % and therefore surprisingly high in that sequence. In opposition to the trend of 5 and 10\u202fwt % Fe/13X as well as literature [26] on Fe-based Fischer\u2013Tropsch catalysts, the selectivity towards C\u2013C-coupled products decreases with increasing pressure on 1\u202fwt % Fe/13X. The main product is CH4 from 5 to 15\u202fbar with a selectivity up to 76 % at 10\u202fbar and 350\u202f\u00b0C. Comparable product selectivites of S(CO)\u202f=\u202f11 % and S(CC)\u202f=\u202f14 % are observed at 10 and 15\u202fbar. This trend of decreasing selectivity towards C\u2013C coupled products and increasing CH4 selectivity with increasing pressure is opposed to the general trend of Fe-based Fisher\u2013Tropsch catalysts reported in literature [26], in which Fe3C is regarded as active species. But it stands in line, that more coordinative unsaturated Fe species have a higher tendency to produce CH4 [29,30].XRD analysis of the catalyst after the catalytic tests shows a decrease of the reflexes from the zeolite framework for all samples (Fig. 7\n). Nevertheless, 1\u202fwt % Fe/13X shows considerably high intensities. Hence, the integrity of the framework is still given in the mayor fraction of the sample even though a small fraction of the zeolite framework collapses. In the XRD of this sample no other reflexes from other phases than the 13X framework are visible. In contrast to the results of 1\u202fwt % Fe catalysts 5 and 10\u202fwt % Fe catalyst show significant decrease and a full depletion of the reflexes from the zeolite. Additionally, reflexes from a Fe3C phase appear in the diffractograms of both catalysts after the methanation experiments. Given by the sharp shape of metallic Fe reflex in the 5 and 10\u202fwt % Fe/13X catalysts the crystallite size of Fe increases during the catalysis. On the low loaded 1\u202fwt % Fe/13X still no reflexes origin from metallic Fe or Fe3C, respectively, under reaction conditions. Hence, the included Fe is very stable within the framework of the zeolite.The collapse of the zeolite with high Fe loading is visible in the SEM micrographs as well. The spherical shape of the zeolite crystallites is still visible in the used 5\u202fwt % Fe/13X catalysts (Fig. S5). This shape nearly vanishes completely on the 10\u202fwt % Fe/13X catalyst after operation. Larger fragments with different morphology, consisting of Al and Si, become obvious instead (Fig. 8\n bottom, middle & right). In addition to this, the formation of larger Fe particles is visible, too. In comparison to SEM images of the reduced catalysts prior to the catalytic testing it seems that Fe migrates out of the zeolite particles and forms, together with deposited carbon, an outer shell around the support (Fig. 8 bottom). In the case of the used 10\u202fwt % Fe/13X catalyst the EDX mapping indicates that residual zeolite particles do not contain Fe at all, while the concentration of Fe is comparably high on the amorphous fragments (Fig. 8). As opposed to this the spherical shape of the 1\u202fwt % Fe/13X zeolite catalyst particles appears to be unchanged after the reaction in the SEM micrographs (Fig. 8 top). In addition, EDX analysis shows a homogeneous dispersion of Fe over the sample and no larger particle agglomerations of Fe or creation of a common Fe\u2013C shell/layer is visible. Hence, the SEM micrographs confirm together with XRD analysis the destruction of the higher loaded zeolite during the catalytic run under formation of a Fe3C shell at the outer layer of the catalysts.The hydrogenation of CO2 towards CH4 on differently loaded Fe/13X catalysts was investigated at ambient and elevated pressure (5\u201315\u202fbar). Comparison of the catalytic performances with a catalyst with collapsed zeolite framework shows, that an intact zeolite structure and hence high dispersion of Fe within the catalyst is essential for high CO2 conversion at temperatures below 400\u202f\u00b0C and all investigated pressures.Catalytic tests on 10, 5 and 1\u202fwt % Fe/13X catalysts with intact zeolite structure revealed a different reactivity of the two higher loaded catalysts compared to the 1\u202fwt % Fe/13X catalyst. Higher Fe-loading leads to relatively low reaction rates of up to 12\u202fmmol(CO2)/(mol(Fe)\u2219s) at 15\u202fbar. CO is the main reaction product at low pressures of 1 and 5\u202fbar. With increasing pressure the selectivity towards CH4 as well as C\u2013C-coupled products increases. The low Fe-loading of 1\u202fwt % leads to a significant increase of the molar reaction rate at all investigated pressures up to 42\u202fmmol(CO2)/(mol(Fe)\u2219s) at 300\u202f\u00b0C and 10\u202fbar. In contrast to both higher Fe-loadings, the lower Fe-loading leads to high selectivity for C\u2013C-coupled products of 56 % at 1\u202fbar. The selectivity towards desired CH4 increases up to 76 % with increasing pressure at the expenses of the formation of CO and C\u2013C-coupled products.Physico-chemical characterization before and after the catalytic run show on the one hand that in 5 and 10\u202fwt % Fe catalysts, Fe is mainly present as agglomerated particles. This leads to a destabilization of the zeolite and further agglomeration of Fe under reaction conditions with simultaneous formation of Fe particles embedded in a Fe3C-phase as an outer shell layer. On the other hand, in 1\u202fwt % Fe/13X, Fe is mainly present as octahedrally coordinated dispersed and oligomeric species. This leads to a higher hydrothermal stability of the catalysts and neither formation of larger Fe agglomerates nor Fe3C-phase formation under operation. The high dispersion of Fe within the material suppresses CC coupling reactions at higher pressure due to confined neighboring Fe sites and this in turn supports the hydrogenation of CO2 to methane. At 15\u202fbar the selectivity towards CO is limited down to 8 %. Even though the performance is not yet fully optimized, the presented results show, that the utilization of Fe-based catalysts as alternative to more expensive and especially hazardous Ni-catalysts for e.g. biogas upgrading and feed into the natural gas grid becomes considerable and provides essential prerequisites for the direction of further catalyst optimization.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\nTanja Franken: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization, Project administration, Funding acquisition. Andre Heel: Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration, Funding acquisition.The authors kindly acknowledge funding of the SmartHiFe Project by the Swiss Federal Office of Energy (grant number: SI/501754-01). Additionally the authors kindly thank Michal Gorbar and Dr. Roman Kontic for their support during the SEM and XRD analyses.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2020.101175.The following are Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  The raise of regenerative but unsteadily produced energy demands a highly flexible way to store the energy for time periods when less energy is produced than consumed. In the current study, it is investigated if catalysts based on environmentally more attractive and less hazardous to health Fe might be able to be considered as an alternative to Ni catalysts in the CO2 methanation at elevated pressure. For this a set of catalysts with 1\u201310\u202fwt % Fe supported on the zeolite 13X is analysed in CO2 methanation at 1\u201315\u202fbar. The trends of activity as well as selectivity with varying Fe loading and pressure are presented. Correlation with thorough characterization of the materials shows that a very high dispersion of Fe in octahedral sites within the zeolite is necessary to generate CH4 as the main reaction product and suppress the Fischer\u2013Tropsch activity towards CC coupling reactions at elevated pressure. Especially with low Fe loading such as 1\u202fwt % high reaction rates of 42\u202fmmol(CO2)/(mol(Fe)\u2219s) with a CH4 selectivity of 76 % at 300\u202f\u00b0C and 10\u202fbar are obtained. In contrast to that, highly Fe loaded catalysts tend to form increasing amounts of Fischer\u2013Tropsch products at increasing pressure. In addition, highly Fe-loaded catalysts are much more susceptible to destruction of the zeolite under reaction conditions. At the same time, highly loaded catalysts form a Fe3C shell around the remaining support. Hence, avoiding the formation of a Fe3C phase is crucial for high CH4 selectivity. The results presented here therefore show that catalysts with a very high Fe-dispersion in particular can gain considerably in importance as alternatives to Ni-methanation catalysts at elevated pressure.\n               "}
{"full_text": "Data will be made available on request.Due to the growing concern of the greenhouse gas reduction and the discovery of surplus shale gas reservation, the dry reforming of methane (DRM) reaction has attracted widespread attention [1\u20137]. The dry reforming of methane (DRM) is a prospective process to convert two major greenhouse gases, carbon dioxide (CO2) and methane (CH4), into syngas. The methane consumption in DRM is half of the steam reforming and partial oxidation reforming of methane [5]. It could potentially alleviate the adverse influence of these pollutants while supplying widely consumed chemicals [8\u201310]. The syngas produced in the reaction has a stoichiometric ratio of molecular hydrogen (H2) to carbon monoxide (CO) of 1:1, so to obtain a more favorable ratio, such as that for Fischer\u2013Tropsch synthesis (2:1), we must cope by implementing such processes as steam reforming (3:1) or autothermal reforming (2.5:1) [11]. The problem is that these two processes have a very negative carbon footprint, and it can be more efficient to trap carbon and raise the H2:CO ratio by taking advantage of the reactivity of CO [12,13]. Moreover, the ratio can be lower with a subsequent reverse water-gas shift reaction. The rest of the reactions involved in the process are the Boudouard and CH4 decomposition [14]. One of the critical elements in this reaction is the catalyst stability, which depends on various factors, including coking [15,16]. The stability of the catalyst plays a role that could be more important than the activity per se [17].Among the long list of catalysts used and tested for this process, nickel (Ni)-supported catalysts stand out for their balanced cheap price and high activity from an industrial point view [18]. However, these catalysts greatly suffer from severe coking and thermal sintering due to their low Tammann temperature [19\u201322]. Hence, the development of Ni-based catalysts with superior activity and stability sparked numerous catalysis studies. Many factors impact the sintering and coking resistance of the Ni-based catalysts, such as particle size, the strong metal support interactions (SMSI), surface oxygen species and lattice oxygen in the support, surface carbonate species and the formation of alloy. Reducing the particle size will provide more active sites and suppress the carbon deposition [23]. The SMSI highly improves the dispersion of Ni particles and alleviate the possibility of sintering, leading to the enhancement of the activity and preventing coking [24\u201326]. Significant studies by Kawi\u2019s group have demonstrated that both surface and lattice oxygen species were investigated to play a key role in activating the C-H bond of CH4 molecule and carbon suppression [27\u201329]. The lattice oxygen could react with CO2, forming monodentate and bidentate carbonate, which alleviates the carbon deposition by oxidation of surface carbon and exhaust as CO [30]. In addition, alloying with a second metal, altering the geometric and electronic structure of Ni active site, will achieve an optimized synergetic effect, enhancing the activity, selectivity, and stability [31]. Due to the inherent scientific significance and crucial role in technology view, bimetallic alloys have obtained widespread appeal [32]. In this case, the goal is to change the electron density of the Ni atoms by inducting additional metal, affecting the typically considered rate-determining step: CH4 dissociation [33]. The alloy formed between the Ni and second metal can improve the stability, alleviating the coke deposition (e.g., noble metals are known to enhance the stability of Ni-based catalysts) [34]. A promising approach to enhance the performance of Ni-supported catalysts without affecting the price that much is to use a secondary metal, such as iron (Fe), cobalt (Co), copper (Cu), or molybdenum (Mo) [31,35]. Moreover, Fe is very interesting due to its low cost and synergistic effect upon intimate interaction with Ni [36,37].The support and its integration with the metal also play a crucial role in catalyst activity, selectivity, and stability [38]. Conventionally, the synthesis, such as wet impregnation or vapor deposition, lacks control of particle size, dispersion, morphology, and metal-support interaction, leading to faster sintering and coking [39\u201341]. The in situ exsolution on perovskites overcomes these problems, improving the metal-support interactions and stabilizing the Ni exsolved particles [42\u201346]. In this method, the transition metal cations partially substitute the perovskite oxide (ABO3) B-site cations, then migrate (exsolve) from the host lattice and agglomerate in the form of nanoparticles under reduction conditions [47,48].Among the investigated perovskites in the exsolution concept, A-site ordered double-layer perovskite system PrBaMn2O5+\u03b4 has drawn significant attention in solid oxide fuel cells due to its thermal and chemical stability, high oxygen diffusion rate, appreciable catalytic activity in hydrocarbon oxidation and the high flexibility to regulate first-row transition metal in the B sites as the active sites [49,50]. All these features benefit the performance of the catalyst under harsh DRM conditions. In DRM, several studies have investigated this type of perovskite with Ni with an 11 % CH4 conversion at 800\u00a0\u00b0C and 5\u00a0h on stream with little coke deposition (0.017\u00a0gcoke gcat\n\u20111 h\u20111) [51]; or with Co-Mo with a CO2 conversion higher than 95 % at 800\u00a0\u00b0C and 24\u00a0h on stream [52]. To avoid most metal cations remaining embedded in the host bulk, resulting in low metal utilization, Joo et al. [53] performed systematic research on the perovskite by adopting the topotactic exsolution approaches in which guest cation was first supported on the matrix perovskite and then ion exchanged with the exsolved cation. The multi-step strategy improved the B-site transition metal exsolved fractions compared to typical conditions, which was applied to catalyst systems such as Ni-Fe [54], Co-Fe [55], and Co-Ni-Fe [56], to facilitate more exsolved nanoparticles, all of which displayed enhanced catalytic activity. However, novel strategies need to be explored and developed aiming a larger fraction of exsolved metal in a facile way. In addition, the enhancements of the stability and fundamental understanding of the deactivation mechanisms over these exsolved catalyst systems need to be clarified too.Firstly, we aim to explore and develop new methods to exsolve a high fraction of Ni and Ni-Fe alloy and make uniform-sized metal nanoparticles anchored into a stoichiometric PrBaMn1.6Ni0.4\u20132xFe2xO5+\u03b4 double-layer perovskite. Secondly and given the intimate interaction between the nanoparticles and the support, our goal is to understand the reasons behind the potential higher stability (slower coking and sintering) in worst case scenarios: long term reactions and at high pressure. To this aim, we will combine several catalyst formulations with variable proportions of Ni and Fe in PrBaMn1.6Ni0.4\u20132xFe2xO5+\u03b4 matrix with fixed ratios of Pr, Ba, and Mn, preparation methods, and characterization techniques, including x-ray absorption spectroscopy, dry reforming reactions and ab initio calculations. We will determine the fine structure of the catalyst responsible for the potential higher stability of the catalyst.A series of Pr0.5Ba0.5Mn0.8Ni0.2\u2212xFexO3 (x\u00a0=\u00a00, 0.05, 0.1, 0.2), defined as P-Ni0.2\u2212xFex was synthesized using the improved sol-gel method. Stoichiometric Pr(NO3)3\u00b76\u00a0H2O (Aldrich, 99.9 %, metal basis), Ba(NO3)2 (Aldrich, 99 %), Mn(NO3)2\u00b74\u00a0H2O (Aldrich, 98 %), Ni(NO3)2\u00b76\u00a0H2O (Aldrich, 98.5 %), and Fe(NO3)3\u00b79\u00a0H2O (Aldrich, 98 %) were dissolved in distilled water. The appropriate amounts of citric acid (Aldrich 99.5 %) and ethylene glycol (Aldrich) were added into the solutions as complexation agents, adjusting the mole ratio of metal ion to citric acid to ethylene glycol as 1:3:1.5. The pH value of the solution was maintained at around 8 by adding ammonium hydroxide. The resulting aqueous solution was continuously stirred at 85\u00a0\u00b0C forming a uniform gel, which was heated at 350\u00a0\u00b0C to decompose slowly and completely. Then, the precursor powder was ground and calcined at 950\u00a0\u00b0C for 4\u00a0h in air. After reduction pretreatment, the single layer perovskite transformed to double layered PrBaMn1.6Ni0.4\u20132xFe2xO5+\u03c3, defined as E-Ni0.2-\nxFex. A sequence of Pr0.5Ba0.5Mn1\u2212xNixO3 (x\u00a0=\u00a00, 0.1, 0.3), defined as E-Nix, was also prepared with the same procedure to investigate the effect of the Fe promoter. In contrast, the corresponding Ni-impregnated Pr0.5Ba0.5MnO3 was prepared via the wetness impregnation method. Regarding the wetness impregnation method, a Pr0.5Ba0.5MnO3 perovskite support, prepared with the same process mentioned above, was impregnated with a proper amount of Ni(NO3)2\u00b76\u00a0H2O (Aldrich, 98.5 %) solution. And then the slurry was dried overnight at 80\u00a0\u00b0C, calcined at 350\u00a0\u00b0C for 2\u00a0h and at 950\u00a0\u00b0C for 4\u00a0h, respectively to achieve the Pr0.5Ba0.5MnO3/Imp Nix (x\u00a0=\u00a00.1, 0.2, 0.3) samples, defined as Syn-I-Nix. After reduction pretreatment, the perovskite support in Pr0.5Ba0.5MnO3/Imp Nix transformed to double layered perovskite in PrBaMn2O5+\u03c3/Imp Nix, defined as I-Nix. The chemical composition of the prepared materials and their abbreviations are presented in Table S1. The nominal and actual loading of different catalysts are presented in Table S2.The XRD was performed using a Bruker D8 Advanced A25 diffractometer in Bragg\u2013Brentano geometry equipped with a Cu K\u03b1 target (\u03bb\u00a0=\u00a01.54056\u00a0\u00c5) at 40\u00a0kV and 40\u00a0mA in the range of 10\u00ba to 80\u00b0 under continuous scanning mode. The N2 adsorption-desorption measurement was performed to analyze the specific surface area with a Micromeritics ASAP 2020 surface area and porosity analyzer by collecting the nitrogen sorption isotherms at 77\u00a0K, from which the specific surface area was calculated according to the Brunauer\u2013Emmett\u2013Teller equation.Thermogravimetric analysis and differential thermogravimetric measurements of the fresh samples were conducted in 5 % H2/N2 from 25\u00a0\u00b0C to 800\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C\u00a0min\u20131 using the Mettler\u2013Toledo Star system. The thermogravimetric analysis mass spectrometry was conducted under temperature-programmed oxidation to study the amount and type of C deposited on the catalysts. The used catalysts were heated to 800\u00a0\u00b0C at 10\u00a0\u00b0C\u00a0min\u20111 in 10 % molecular oxygen (O2)/argon (Ar).The reducibility of the perovskites was investigated by the H2 temperature-programmed reduction using a Micromeritics AutoChem II 2920 chemisorption analyzer. Before the measurement to remove the absorbed impurities, 100\u00a0mg sample was pretreated at 300\u00a0\u00b0C for 1\u00a0h with Ar and then cooled to 50\u00a0\u00b0C. After switching the gas flow to 10 % H2/Ar, the H2 consumption was monitored with a thermal conductivity detector during heating the temperature from 150\u00a0\u00b0C to 850\u00a0\u00b0C at 10\u00a0\u00b0C\u00a0min\u20131. Moreover, a cold trap made of ice was set between the sample and detector to remove the water formed during the process.The oxygen temperature-programmed desorption (O2-TPD) was also conducted with the Micromeritics AutoChem II 2920 chemisorption analyzer. The samples were first pre-reduced at 800\u00a0oC with 10 % H2/Ar for 6\u00a0h and then in-situ reduced for 30\u00a0min in AutoChem. After flushing the sample with He at the sample temperature for 30\u00a0min, the system cooled down to 50\u00a0\u00b0C in He. Then the sample was treated with 20 % O2/N2 at 50\u00a0\u00b0C for 30\u00a0min. After flushing the sample with He for 1\u00a0h, the O2-TPD was performed by increasing the temperature from 50\u00a0\u00b0C to 800\u00a0\u00b0C in 40\u00a0mL\u00a0min\u20131 He with a ramp of 10\u00a0\u00b0C\u00a0min\u20131.The morphology of the samples was observed using scanning electron microscopy (SEM; FEI Teneo VS) at an accelerated voltage of 5\u00a0kV. The sample was deposited on graphite and sputtered with a gold (Au) conductive layer. The sample TEM was performed using a Cs-Probe corrected Titan microscope from Thermo Fisher Scientific. It operated at the accelerating voltage of 300\u00a0kV and with a 0.5\u20130.8\u00a0nA beam current. Dark-field imaging was performed by STEM coupled with a HAADF detector. The STEM-HAADF data were acquired with a convergence angle of 21.4 mrad and a HAADF inner angle of 49 mrad.Furthermore, an x-ray energy-dispersive spectrometer (FEI SuperX, \u22480.7 sR collection angle) was also employed with dark-field STEM imaging to acquire STEM-EDS spectrum-imaging datasets (dwell time 2.5 \u00b5s). A corresponding EDS spectrum was obtained during the acquisition of these datasets to generate the elemental maps at every image pixel. After background subtraction, the elemental maps for Ni, Fe, Mn, Pr, Ba, and O atoms were computed using the extracted intensity of their respective K\u03b1 or L\u03b1 lines. The generated maps were slightly post-filtered by applying a Gaussian filter (sigma = 0.5). The STEM electron energy loss spectroscopy (EELS) analysis was performed by operating the microscope at the accelerating voltage of 300\u00a0kV using a convergence angle of 17 mrad and an effective collection angle of 36 mrad. The spectrum-imaging dataset included the simultaneous acquisition of zero-loss and core-loss spectra (DualEELS) using a 0.5\u00a0eV/channel dispersion. It was recorded using a beam current of 0.2\u00a0nA and a 5\u00a0ms pixel\u20131 dwell time. The Fe L2,3-edge, Ni L2,3-edge, and O K-edge were selected to build the chemical maps. Plural scattering was removed from the Fe and Ni L-edges using Fourier-ratio deconvolution with prior energy shift correction and background subtraction (power-law model). The contribution of transitions from the 2p3/2 and 2p1/2 initial states to the continuum states must be considered to acquire the white-line intensities of the L3 and L2 edges. The latter was conducted through a classical normalization with the Athena softwar [57]. Then, the L-edge spectra were modeled with a double arctangent step function and two split Lorentzian functions to account for the peak asymmetry [58]. The white-line intensities were finally computed from the area of the Lorentzian peaks.The x-ray absorption spectroscopy of the prepared and reduced catalysts was performed at the 1W1B beamline at the Beijing Synchrotron Radiation Facility. The catalysts were ex-situ reduced for XAS analysis. The fresh catalysts were first pressed into pellets and reduced under the corresponding gas conditions. And then, the reduced pellets were collected and sealed in a glove box. Afterward, the sealed pellets were tested. The data were collected in the transmission mode via a Si (111) double crystal monochromator, detuned to reject higher harmonics. In addition, the Ni (8333\u00a0eV) and Fe (7113\u00a0eV) standard foils were applied for energy calibration. The Ni foil, NiO, Fe foil, FeO, and Fe2O3 were used as references to analyze the XANES and EXAFS. The as-obtained XAFS data were processed using the Athena software program.The XPS was conducted to investigate the chemical surface composition using a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic aluminum K\u03b1 x-ray source (h\u03bd = 1486.6\u00a0eV). The XPS spectra were referenced to the C1s binding energy of 284.6\u00a0eV. The fitting of the XPS peaks was processed using XPSPEAK software.The Raman analysis of the used catalysts was performed on an RXN1 Raman spectrometer (Kaiser Optical Systems) fitted with a 532\u00a0nm laser operating at 40\u00a0mW. In addition, inductively coupled plasma-optical emission spectrometry was performed to analyze the exact elemental content using an Agilent 5100 instrument. Before the measurement, the materials were digested in an ETHOS1 microwave digestion milestone.In-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) was conducted in a Nicolet 6700 IR spectrophotometer (Thermo Scientific) equipped with a Harrick Praying Mantis DRIFTS gas cell. Before the measurements, the catalysts were pre-reduced at 800\u00a0\u00b0C for 6\u00a0h in a quartz tube and then reduced in situ in the DRIFTS cell at 450\u00a0\u00b0C for 1\u00a0h, followed by flushing with He flow. The background was collected under a He flow at 450\u00a0\u00b0C. Gas-switching experiments (step 1 (He flow after CH4) \u2192 step 2 (CO2 flow) \u2192 step 3 (mix gas flow: CH4/CO2/N2 =33/34/33) were carried out at 450\u00a0\u00b0C to unravel the evolution characteristics of surface species on the catalysts. The time-resolution IR spectra were recorded in a range of 400\u20134000\u00a0cm\u22121 at 32 scans per spectrum and 4\u00a0cm\u22121 resolution with an interval of 30\u00a0sCatalytic tests were performed in a four-channel Flowrence XD platform from Avantium. The reactors are 300-mm-long quartz tubes, of which the outside and inside diameters are 3 and 2\u00a0mm, respectively. One of the reactors was adopted as the blank without a loading catalyst among the four channels. Typically, 10\u00a0mg catalyst was loaded in a quartz reactor, and the gas flow was 5\u00a0mL\u00a0min\u20131 for each reactor. The catalysts were pelletized and sieved to achieve a powder with sizes between 150 and 250\u00a0\u00b5m. The proper amount of catalyst and reactant mixture gas flow was used to maintain the gas hourly space velocity (GHSV) per channel at 30,000\u00a0mL\u00a0gcat\n\u20131 h\u20131 under atmospheric pressure and 12,000\u00a0mL\u00a0gcat\n\u20131 h\u20131 under high pressure at 14\u00a0bar, respectively. The composition of the reactant mixture gas is CH4:CO2:N2 =\u00a033 %:34 %:33 %. Prior to feeding the reactant gas, the catalysts were reduced in situ in a 10 % H2/Ar atmosphere for 6\u00a0h at 800\u00a0\u00b0C. The reactants and products were continuously monitored with an online micro gas chromatograph (Agilent 7890B).The conversions of i (CH4 or CO2) were calculated as:\n\n(1)\nX\ni\n = F\ni\n\ninlet \u2013 F\ni\n\noutlet / F\ni\n\ninlet\n\n\nWhere F\ni\n\ninlet and F\ni\n\noutlet, denote the inlet and outlet molar flow rate of i. The apparent rates of reaction are calculated as follows:\n\n(2)\n-r\ni\n|app = F\ni\n\ninlet X\ni\n/(WNi)\n\nWhere W is the catalyst loading and WNi is the nickel loading. The H2/CO ratio is defined as:\n\n(3)\nH2/CO = FH2\noutlet / FCO\noutlet\n\n\n\nThe apparent coke formation rate (rcoke|app in mmol gcat\n\u22121 s\u22121) was evaluated via temperature-programmed oxidation (TPO) combined with MS experiments. The coke formation rate was calculated based on Eq. (4).\n\n(4)\nrcoke|app = fcoke/t\n\nWhere fcoke represents the amount of coke formed on the catalyst in mmol gcat\n\u22121 considering coke as pure carbon, at a given time on stream (t).The thermodynamic equilibrium conversions of CH4 and CO2 and the H2/CO ratio were determined by Aspen Plus software.Adsorption energy calculations were performed with the first-principles DFT using the Vienna Ab Initio Simulation Package (VASP). The electron exchange and correlation interactions were modeled using the generalized gradient approximation with the Perdew\u2013Burke\u2013Ernzerhof functional. The electron-ion interactions were defined using the projector-augmented wave method. A plane-wave basis set was used to describe the valence electrons with an energy cut-off of 400\u00a0eV. The Brillouin zone, sampled at the Monkhorst-Pack 3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a01 k-point grid, was used as the Ni (111) and Ni4Fe1 (111) model. The Ni (111) and Ni4Fe1 (111) surfaces were modeled as a four-layer slab using a 5\u00a0\u00d7\u00a05 supercell with 15\u00a0\u00c5 of vacuum between the slabs. All geometries were optimized until the convergence reached 1.0\u00b710\u22126 eV, and the atomic forces were smaller than 0.05\u00a0eV\u00a0\u00c5\u22121. For the gas phase molecule, a cubic box of 15\u00a0\u00d7\u00a015\u00a0\u00d7\u00a015\u00a0\u00c53 was used. The climbing image nudged-elastic band method was used to identify the transition state structures of the elementary reactions involved in the reaction mechanisms. The following equation was used to calculate the binding energy of species present in the reaction media:\n\n(5)\nEBind = Eadsorbate+surface -Eadsorbate -Esurface,\n\nwhere E\n\nadsorbate+surface\n is the total energy of the adsorbate on the metal surface, and E\n\nadsorbate\n and E\n\nsurface\n denote the total energy of adsorbate in the gas phase and the bare metal surface, respectively. A more negative binding energy value refers to the species adsorbed stronger on the metal surface (or a stronger interaction between the adsorbate and the metal site of the surface), and vice versa.The crystalline structures of the parent PrBaMn2O5+\u03b4 perovskite (P) and the corresponding exsolved counterparts P-Ni0.2\u2212xFex (x\u00a0=\u00a00, 0.05, 0.1, 0.2) with different Ni/Fe loading were analyzed using x-ray diffraction (XRD) before and after reduction (\nFig. 1). Moreover, the crystalline structures of varying Ni substitution amounts for Mn have also been investigated (Fig. S1). The parent and exsolved materials comprise a perovskite structure with a mixture of hexagonal and cubic phases (Fig. 1a). However, the reduction induced the transformation of the original perovskite into a layered perovskite in the tetragonal phase (Fig. 1b), which might increase the specific surface area due to the formation of metal (Ni or Ni-Fe alloy) nanoparticles on the support surface and defects in the lattice (Fig. S2) [49].For P-Ni0.2, the diffraction peak located at 44.5\u00b0 is attributed to metallic Ni due the exsolution [59,60]. Concerning P-Ni0.15Fe0.05, the similar diffraction peak shifts slightly to a lower diffraction degree, assigned to the diffraction peak of Ni-Fe alloy (Fig. 1c), deriving from Fe dissolving into the Ni lattice, revealing that the exsolved metal forms a binary Ni-Fe alloy [54]. Upon reduction, the exsolved nanoparticles in B sites are MnO for P, metallic Ni for P-Ni0.2, and Ni-Fe alloy for P-Ni0.15Fe0.05 and P\u2011Ni0.1Fe0.1. The composition of the exsolved Ni-Fe alloy varies according to the Fe substitution since the increase of the Fe content in the matrix relatively decreases the exsolved Ni amount. Combined with the following TEM analysis, from E-Ni0.15Fe0.05 to E-Ni0.1Fe0.1, the exsolved Ni-Fe alloy composition varied from Ni4Fe1 to Ni3Fe1. In contrast, no exsolved Fe phase is evident for P-Fe0.2 (Fig. 1c). The same trend of transition metal exsolution in the layered perovskite was previously observed, indicating that Ni exsolves more efficiently to the surface than Mn and Fe [48]. The results demonstrate that, although Fe alone hardly exsolves, the existence of Ni in the B site promotes the exsolution of Fe, forming a Ni-rich Ni-Fe alloy nanoparticle.The dynamic exsolution process was evaluated by analyzing the weighted loss profiles (through thermogravimetry, \nFig. 2a and b), H2 consumption (through the thermal conductivity detector, Fig. 2c) and SEM (Fig. S3) during the reduction and exsolution processes. For the P-Ni0.2\u2212xFex materials, three peaks are observed at (i) 250\u2013450\u2009\u00b0C, attributed to the loss of oxygen in the PrOx plane; (ii) 450\u2013650\u2009\u00b0C, attributed to the partial escape of the intensely bonded lattice oxygen atoms from the perovskite [61]; and (iii) above 650\u2009\u00b0C, attributed to the reduction of metal cations located at the B site and the exsolved process of Ni or Ni-Fe to the double-layer perovskite surface. A higher Ni content in the perovskite triggered a lower reduction temperature of the first peak due to the higher electronegativity of Ni (1.75) compared to Mn (1.69) [62]. As Ni is more reducible than Fe, from P-Ni0.2 to P-Fe0.2, the reaction peak shifts to a higher temperature, and the hydrogen consumption decreases in sequence with the increase in the Fe.The exsolution of the nanoparticles was observed from the morphology of the P-Ni0.15Fe0.05 material. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and energy-dispersive x-ray spectroscopy (EDS) elemental mapping were performed to confirm the composition of the exsolved nanoparticles and support. The pristine P-Ni0.15Fe0.05 material has a homogeneous porous surface (\nFig. 3a), whereas numerous nanoparticles emerged on the surface after H2 treatment (Fig. 3b). A detail of an exsolved nanoparticle that was partially socketed in the support is shown Fig. 3c. The results shown in Fig. 3d indicate the spherical shape of the Ni-Fe particles anchored to the surface of the double-layer perovskite. The interplanar spacing of the nanoparticles is 2.05\u2009\u00c5, which is consistent with the d\u2011spacing of the (111) plane of the Ni-Fe alloy phase (Fig. S4) [37]. The particle size analysis of the E-Ni0.15Fe0.05 catalyst indicates an average of 21\u2009\u00b1\u20095.6\u2009nm. The EDS elemental mappings highlight that only Ni and Fe atoms were found in the emergent nanoparticles. The quantitative analysis of the EDS data operated on more than 30 nanoparticles indicated a molar Ni/Fe ratio of 3.6\u2009\u00b1\u20091.0. The chemical elements Pr, Ba, Mn, and O are homogeneously distributed across the support. Only traces of Ni atoms remained in the perovskite, whereas a substantial number of Fe atoms were still present, in agreement with the calculations by Kwon et al.[48] The same exsolution phenomenon of Ni-Fe and Ni nanoparticles was also observed for E- Ni0.1Fe0.1 (Figs. S5 and S6) and E-Ni0.2 (Fig. S7), respectively.The local coordination environment and chemical state of the exsolved nanoparticles were further analyzed using x-ray absorption fine structure spectroscopy (XAFS) at the Ni and Fe K-edges of E-Ni0.2 and E-Ni0.15Fe0.05 catalysts. Even though the reduced pellets were handled carefully during the transfer and test process, it could not be excluded that slight surface oxidation could occur. In the normalized Ni K-edge x-ray absorption near-edge structure (XANES) spectra (\nFig. 4a), the Ni K-edge spectra almost fit with the Ni foil reference concerning the energy position and pattern, indicating that Ni is prevailingly reduced to Ni0 in the two catalysts. In addition, the pre-edge peak slightly shifts to higher energy from E-Ni0.2 to E-Ni0.15Fe0.05 (an enlarged region in Fig. 4a), demonstrating the minor increase in the Ni average oxidation state, indicating the formation of the Ni-Fe alloy [56].Moreover, the white line (7131\u2009eV) of the Fe K-edge XANES spectrum of the E-Ni0.15Fe0.05 catalyst is more intense than that of Fe foil in Fig. 4b, indicating that part of the Fe is oxidized [63]. The intensity of the pre-edge peak of E-Ni0.15Fe0.05 is stronger than that of the Fe2O3 reference. Thus, it is deduced that part of the Fe is reduced to metallic Fe0, whereas another part remains oxidized in the double-layer perovskite support. The wavelet transform signals of the Ni-metal bond were observed around 8\u2009\u00c5\u22121 in the contour plots of the E-Ni0.15Fe0.05, E-Ni0.2, NiO reference, and Ni foil. In contrast, the Ni-O bond signals were absent except for the NiO standard (Fig. 4c and Fig. S8). In addition, Fig. 4d and e exhibit the extended XAFS (EXAFS) spectra in the k-space and the corresponding Fourier transform in the R space at the Ni K-edges. The monometallic E-Ni0.2 reveals the same oscillations as those of the Ni foil, whereas in terms of the bimetallic E-Ni0.15Fe0.05, the changes minor shift to a smaller k-value. The slight k-value shifted for the bimetallic Ni-Fe system is reported by previous work [36]. The EXAFS spectra of both E-Ni0.2 and E\u2011Ni0.15Fe0.05 have similar characteristics to those of Ni foils, with the Ni-Ni coordination at \u223c2.18\u2009\u00c5 (Fig. 4e). This result confirms the existence of Ni in the metallic state [64].The linear combination fitting (LCF) method was adopted based on the identifiable features of each reference in the XANES spectra to quantify the distribution of different Ni and Fe oxidation states. The LCF analysis of the Ni K-edge XANES spectrum reveals that about 94.0 % of Ni is reduced to the metallic state, which is almost double the other reported results (58 % Ni exsolved) [48], and only 6.0 % of Ni remains in the oxidation state in E-Ni0.15Fe0.05 (Fig. 4f). However, the distribution of Fe in E\u2011Ni0.15Fe0.05 is approximately 25.1 % Fe0 and 74.9 % in the Fe oxidation state (Fig. 4g).The x-ray photoelectron (XPS) spectra of the fresh catalyst (\nFig. 5a) indicate that only divalent Ni2+ (\u223c854.5\u2009eV, 856.6\u2009eV) and satellite peaks (\u223c860.8\u2009eV) are detected in P\u2011Ni0.15Fe0.05\n[65]. The metallic nickel Ni0 peaks (\u223c852.3\u2009eV) appeared after reduction (Fig. 5b), accounting for only a small fraction of the Ni elements, most likely due to oxidation during ex situ movement. Similar to Ni, the Fe element exhibits mixed oxidation states consisting of Fe2+ (\u223c709.6\u2009eV) and Fe3+ (\u223c710.8\u2009eV) before reduction (Fig. 5d), a small part of which is reduced to Fe0 (706.7\u2009eV) after reduction treatment (Fig. 5e) [66]. The XPS spectra indicate that metallic Ni0 and Fe0 coexist in the E\u2011Ni0.15Fe0.05 catalyst, further implying the formation of Ni-Fe alloy. The oxygen species consists of lattice oxygen (\u223c528.5\u2009eV) and adsorbed oxygen species (\u223c531\u2009eV) as shown in Fig. S9a and b. The peak ratio of the adsorbed/lattice oxygen increases on the E-Ni0.15Fe0.05 surface with respect to that on the P-Ni0.15Fe0.05 surface [67], which is ascribed to the formation of oxygen vacancies along with the Ni-Fe alloy exsolution process [68].Thus, combined with the XRD (Fig. 1), TEM (Fig. 3), and XAFS analysis (Fig. 4), we infer that Fe was partially reduced to the metallic phase and exsolved to the surface of the perovskite matrix, forming an alloy with Ni [59].The catalytic activity and stability of the preceding impregnated (I-) and exsolved (E-) catalysts were evaluated in dry reforming at 800\u2009\u00b0C under atmospheric pressure and high pressure (14\u2009bar). The activity of the catalysts is compared based on the apparent reaction rates of CH4 and CO2. The amount of Ni in the normalized activity is obtained by ICP. This term is considered \u201capparent\u201d because it is derived from any value of conversion, including the ones of the integral reactor (X\u2009>\u200910 %). Although it is not convenient to refer to it as an \u201cintrinsic reaction rate\u201d, it provides us with a parameter that can be used to compare activity across our catalysts and those of the literature.The initial apparent reaction rate of CH4 for exsolved catalysts (E\u2011Nix, x\u2009=\u20090.1, 0.2, and 0.3) are much higher than those of their impregnated counterparts (I-Nix; \nFig. 6a). Moreover, unlike the fast deactivation of the impregnated catalyst, even the most stable I-Ni0.2 catalyst in the impregnated catalysts dropped by 11.5 % within 12\u2009h (Fig. 6a), the E-Nix catalysts remain stable for the CH4 reaction rate throughout 40\u2009h on stream, mirroring the same stability as the H2/CO ratio (Fig. S10). As for the effect of the Ni loading, the apparent reaction rate of E-Nix catalysts decreased as the amount of Ni increases, despite the conversion delivered opposite trend (Fig. S10).Based on the results, E-Ni0.2 is further modified with the dopant of Fe in the B site (E-Ni0.2\u2212xFex (x\u2009=\u20090.05, 0.1, 0.2)). The CH4 apparent reaction rate of the E-Ni0.15Fe0.05 catalyst is slightly lower than that of the E-Ni0.2 (Fig. 6a and b). The CH4 apparent reaction rate of the E-Ni0.1Fe0.1 catalyst is not stable during 40\u2009h on stream. For E-Fe0.2, its catalytic performance results show negligible CH4 and CO2 apparent reaction rate, indicating that both the perovskite substrate and Fe nanoparticles are not active site for the conversion of CH4 (Figs. 6b and S11), in agreement with the previous reports [69]. The CO2 conversions are slightly higher than the corresponding CH4 conversions for all catalysts, independent of the metal loading and the Ni/Fe ratio (\nFigs. 6 and 7), probably originating from the reverse water-gas shift reaction (RWGS) [56,59]. RWGS usually occurs as a side reaction in DRM, which results in higher CO2 conversion than CH4 conversion and makes the H2/CO ratio lower than 1 since the reaction produces more CO and consumes H2\n[70]. The RWGS is observed in our catalysts in the results presented in Figs. S10\u201315 and corroborated by previous works [32,70]. Besides, compared to the exsolved catalyst, the unreduced E-Ni0.15Fe0.05 catalyst without the generation of exsolved metal species synthesized by the same method exhibited relatively low activity and deactivated fast within 15\u2009h (Fig. S12). Similarly, the E-Ni0.15Fe0.05 catalyst reduced under pure H2 at 800\u2009\u00b0C also displayed low performance, probably due to the perovskite decomposition under pure H2 (Fig. S13).The long-term stability test for the E-Ni0.15Fe0.05 and E-Ni0.2 catalysts is presented in Fig. 7a and Fig. S14. After around 135\u2009h on stream, the stability test of E-Ni0.2 ceases because the catalyst bed is congested with coke (the pressure drop rises and the flow rate decreases). The E-Ni0.15Fe0.05 catalyst displays a stable apparent reaction rate with no noticeable deactivation for 260\u2009h on stream at 800\u2009\u00b0C. This result indicates that Fe plays a critical role in stabilizing the catalyst. Besides, the E-Ni0.15Fe0.05 catalyst shows 100\u2009h stability under undilute gas conditions with CH4:CO2 =\u20091:1 as feed gas without deactivation (Fig. S15). Industrial syngas must be compressed, such as compression from 1 to 10 bars, for utilization, which costs more than 85 % of the total capital investment and 60 % of the operational costs [71]. Additionally, the high-pressure operation increases the production capacity, as well. However, coking is highly favored in high-pressure conditions, which is the central dilemma to address. Both E-Ni0.15Fe0.05 and E-Ni0.2 exhibited 40\u2009h stability in reaction conditions at 14\u2009bar (Fig. 7b). Compared to a previous study [71], E-Ni0.15Fe0.05 exhibits similar CH4 conversion, lower CO2 conversion, and a much higher H2/CO ratio (Fig. S16), even without 10 % H2O feeding, indicating that E-Ni0.15Fe0.05 is also a promising catalyst for the high-pressure dry reforming of CH4.To elucidate the role of Fe on the carbon resistance of Ni-based catalysts under reaction conditions, thermogravimetric techniques were applied to measure the carbon deposition on the used catalyst. The CO2 signal was analyzed in a mass spectrometer during combustion. As depicted in Fig. 7c, the apparent coke formation rate on the E-Ni0.2 catalyst is 2.57\u00b710\u20135 mmol gcat\n\u22121 s\u22121 during the 135\u2009h on stream, whereas the apparent coke formation rate significantly drops to 4.71\u00b710\u20138 mmol gcat\n\u22121 s\u22121 for the E-Ni0.15Fe0.05 catalyst during the 260\u2009h on stream. Compared with approximately 140 catalysts from the state-of-the-art references, the proposed E-Ni0.15Fe0.05 catalyst has the slowest apparent coke formation rates of the relatively extensive catalyst portfolio (Fig. 7e and Table S3). Many studies in the literature do not assess coke fouling on the catalysts for the following reasons: (i) insufficient time on stream, (ii) unrealistically mild reaction conditions, or (iii) inappropriate excess of catalyst and the subsequent underestimation of coke formation. Per the thermogravimetric results, contrary to the severe coking on the used E-Ni0.2 catalyst, the Raman spectra indicated no coke accumulation on the used E-Ni0.15Fe0.05 catalyst (Fig. S17). The minor coking deposition convincingly demonstrates the coking-resistant effect of Fe, coinciding with previous reports that Fe substitution improves the coking resistance of Ni-based catalysts in reaction conditions [72]. In addition, the used catalyst after high-pressure conditions at 14\u2009bar is also characterized and shown in Fig. S18. Generally, the high pressure favored the coking formation. The weight loss of the used E-Ni0.15Fe0.05 is only 2 % after 40\u2009h on stream. Compared to the used E-Ni0.2, the D and G band intensity of the used E-Ni0.15Fe0.05 is lower, further indicating an improvement of coking resistance with the introduction Fe even at high pressure.The particle size distribution analysis revealed that the used E-Ni0.15Fe0.05 catalyst possessed exsolved Ni-Fe nanoparticles with an average size of 30.1\u2009nm after 260\u2009h on stream, slightly larger than the nanoparticles of the E-Ni0.15Fe0.05 before the reaction (21.0\u2009nm; Fig. 7d). The exsolved nanoparticles of the used E-Ni0.15Fe0.05 catalyst anchor partially in the support without any observable coke with a morphology similar to the pristine one (Fig. S19a and c). However, the counterpart used I-Ni0.2 catalyst exhibited severe sintering of Ni particles from 35.2 to 56.6\u2009nm only after 12\u2009h on stream with apparent filamentous coke on the catalyst surface (Fig. 7d, S19b and d). Hence, the exsolved E-Ni0.15Fe0.05 catalyst exhibited improved sintering resistance compared with the I-Ni0.2 catalyst, which is closely associated with their stability (Figs. 6 and 7).Structural changes in the E-Ni0.15Fe0.05 catalyst between the reduced state and after the reaction were further investigated using STEM coupled with electron energy loss spectroscopy (EELS). Spatially resolved EELS spectra at the L-edges were used to analyze the metal oxidation state in the core and particle surfaces. For 3d metals, a typical L-edge EELS spectrum includes a pair of strong white lines corresponding to 2p3/2 \u2192 3d (L3-edge) and 2p1/2 \u2192 3d (L2\u2013edge) transitions and two edge jumps corresponding to 2p \u2192 continuum transitions. The two white lines are separated by the spin\u2013orbit interaction of the 2p core states. A one-electron excitation theory usually fails to interpret the spectral fingerprint (e.g., branching ratio and multiple interactions) because the 2p- and 3d-hole have radial wave functions overlapping significantly [73].In this work, we restrict the analysis of the L2,3-edges by considering only the total intensity of the white lines. Previous authors have demonstrated that the total number of 3d holes is proportional to the integrated L2,3-edge peaks (see experimental method section), which is a useful feature to determine the metal oxidation state [74\u201377]. \nFig. 8 presents the Ni and Fe L-edge spectra of the metal and metal monoxide standards, with spectra taken in the core and shell of the Ni-Fe nanoparticles before and after the catalytic reaction. The comparison of the total intensities of the white lines with the Fe and Ni standards (I(Fe) =\u200921 and I(Ni) =\u200910) indicated that the nanoparticle core remains in a metallic state throughout the reaction (I(Fe) =\u200920 and I(Ni) =\u200910\u201311). This result is in agreement with the XPS spectra of the used catalyst (Fig. 5c and f). For the oxidized shell of the used catalyst, the Ni atoms were in a mixture of Ni0 and Ni2+ states (I(Ni) =\u200916 vs. I(Ni) =\u200923 in NiO), whereas the Fe atoms were predominantly in the Fe2+ state with a probable minor presence of the Fe3+ state (I(Fe) =\u200932 vs. I(Fe) =\u200929 in FeO).The same observations were made for the oxidized shell of the reduced catalyst. Thus, the presence of the latter oxide shell on metal nanoparticles was essentially due to handling the catalyst in the air prior to the TEM analysis. To eliminate these effects, the quasi in-situ TEM is performed in reaction conditions and displayed in Fig. S20. This result shows the partial redistribution of Fe and a relatively low concentration of oxygen layer surrounding the Ni-Fe nanoparticles in the E-Ni0.15Fe0.05 catalyst. Thus, we can consider that as the initial status of the catalyst (before the reaction). In contrast, FeOx species are formed on the outer layer of the Ni-Fe alloy nanoparticles during the dry reforming conditions [78]. Besides, the average Ni/Fe molar ratio of 3.6 significantly increased to 9.2 after the reaction with a much larger standard deviation (from 1.0 to 5.3, measured on 38 particles). This evidence that part of the metallic iron was redistributed on the support during the reaction agrees with previous observations of Coperet et al. [63].The DFT calculations were performed to compare the adsorption energy of the key intermediates on Ni4Fe1 (111) models to that on monometallic Ni (111) models to elucidate the improved coking resistance of the Ni-Fe bimetallic catalyst at the atomic resolution level. These surfaces were considered as they were identified experimentally using XRD (Fig. 1c), high-resolution TEM (Figs. S4 and S7), EDX analysis (Fig. 3d), EELS analysis (Fig. 8). Even though the oxygen vacancy defects in the perovskite matrix will assist increasing ratios of singlet oxygen species on the surfaces for removing carbon species, the TGA and TPR results in a reduction atmosphere (as shown in Fig. 2) indicated that the amount of the oxygen vacancy originating from metal exsolution of P-Ni0.2 is slightly larger than that of P-Ni0.15Fe0.05. However, the coking resistance of E-Ni0.15Fe0.05 is much better than that of E-Ni0.2, indicating that in these catalysts, the oxygen vacancy facilities carbon removal, but it is not the main factor. Therefore, the effect of the perovskite support is not considered in the DFT calculations, which focused on elucidating the improvement of the coking resistance of E-Ni0.15Fe0.05. Thus, only the monometallic Ni and bimetallic Ni-Fe alloy were considered for the slab model (Figs. S21-S23). By calculating the effective barriers for the C and CH oxidation pathways on Ni and Ni4Fe1 (111) surface (Fig. S24), it is considered that the O*\u2009originating from the CO2 dissociation directly oxidizes the intermediate CH*\u2009is the dominant oxidation pathway on both Ni and Ni4Fe1 (111) surfaces, which is consistent with other reports [36,81]. Although the slab model cannot conclude the fine structure of the Ni-Fe binary alloy, a reasonable understanding of the reaction mechanism was gained throughout the DFT.The binding energies of the critical intermediates on Ni4Fe1 (111) were compared with those on Ni (111), as presented in Table S4. Despite the same binding energy of CH4 between Ni (111) and Ni4Fe1 (111), the binding strength of carbon-containing intermediates, including CH3 *\u2009, CH2 *\u2009, CH*\u2009, C*\u2009, and CO*\u2009, are all weaker on the Ni4Fe1 (111) surface than that on the Ni (111) surface, on which coke is likely prone to form due to the higher C*\u2009binding energy [82]. In addition, due to the stronger Fe-O bond, the adsorption of the oxygen-containing species, such as O*\u2009and CHO*\u2009, on the Ni4Fe1 (111) surface is stronger than that on the Ni (111) surface, in agreement with the time-resolution DRIFTS spectra under switching gas conditions (Figs. S25 and S26) and previously reported results [36]. The O*\u2009and CHO*\u2009adsorption strengths on Ni4Fe1 (111) surface are 0.15 and 0.08\u2009eV higher than the pure Ni (111) surface. To observe the effects of Fe on the catalyst, the chemisorption energy of O, CO and C on pure Ni (111), Ni4Fe1 (111) and pure Fe (111) are plotted in \nFig. 9a, b and c, respectively. The adsorption energy of O increases along with the increase of Fe composition, implying the oxophilic nature of Fe (Fig. 9a), consistent with the O2-TPD results (Fig. S27). In contrast, the adsorption energy of CO has no obvious correlation with the Fe composition (Fig. 9b). These trends indicate that the difference of CO2 dissociation energy for various catalysts mainly originates from the adsorption capability of O*\u2009species, instead of that of CO*\u2009species (Fig. 9a and b). Besides, as shown in Fig. 9c, the adsorption energy of C closely connected with the Ni composition on the active site, further implying that coke prefers to form on pure Ni site other than Fe site.In DFT calculations, we are mainly focusing on the coke formation reactions. The energy barriers of some key elementary steps of the reaction on Ni (111) and Ni4Fe1 (111) surfaces are displayed in Figs. S24 and S28 and Table S5. The dissociative adsorption energies of CH4 and CO2 are presented in Fig. 9d and e. The CH4 dissociation energy of Ni4Fe1 is 0.04\u2009eV higher than that of Ni (111), leading to less CHx *\u2009(x\u2009=\u20090, 1, 2, 3) intermediate species on the Ni4Fe1 (111) sites. Moreover, the CO2 dissociation energy of Ni4Fe1 (111) is 0.06\u2009eV lower than that of pure Ni (111). Due to the oxophilic nature of Fe, adding Fe to Ni-based catalyst enhances the adsorption of O*\u2009species and reduces the CO2 dissociation energy (exhibiting a slightly inferior CH4 dissociation energy compared to Ni). The higher concertation of O*\u2009species on the surface of the Ni-Fe binary alloy catalyst reacts with C*\u2009species and lower the coking rate, contributing to the atypical coking resistance of this catalyst.The direct exsolution of Ni and Ni-Fe in a single reduction step leads to well dispersed, anchored and alloyed (in the case of the bimetallic sample) nanoparticles on PrBaMn1.6Ni0.4\u20132xFe2xO5+\u03b4 double-layer perovskite. We synthesized these catalysts together with counterparts prepared by impregnation, characterized and tested them in the dry reforming of methane. The exsolved Ni-based catalyst has a significantly superior performance and longer stability due to enhanced metal-support interaction. The exsolved Ni-Fe alloy catalyst shows slightly slower reaction rates but a significantly longer lifetime: with negligible coke depositions at 800\u2009\u00b0C during 260\u2009h on stream under 1\u2009bar or 40\u2009h on stream under 14\u2009bar (more relevant for industrial implementation). Our main objective has been to understand the reasons behind the higher stability of this Ni-Fe catalyst by characterization, ab initio calculations, and dry reforming reactions. Our results show that Fe (in the exsolved Ni-Fe catalyst) stabilizes O*\u2009species, helps in the CO2 dissociation, and facilitates the reactions of C*\u2009species as its adsorption is weakened. At the same time, the stronger metal-support interaction in this catalyst leads to slower sintering. These combined effects are the reasons behind the atypical more extended stability of the exsolved Ni-Fe alloy catalyst.\nXueli Yao: Conceptualization, Investigation, Methodology, Data curation, Writing \u2013 original draft, Writing \u2013 review & editing, Visualization. Qingpeng Cheng: Conceptualization, Methodology, Data curation, Writing \u2013 review & editing. Yerrayya Attada: DFT calculations and Formal analysis. Samy Ould-Chikh: Data curation, Formal analysis and Writing \u2013 review. Adrian Ram\u00edrez: Data curation and Formal analysis. Xueqin Bai: Investigation. Hend Omar Mohamed: Formal analysis. Guanxing Li: Data curation. Genrikh Shterk: Formal analysis. Lirong Zheng: Data curation and Formal analysis. Jorge Gascon: Formal analysis and review. Yu Han: Formal analysis and review. Osman M. Bakr: Formal analysis and review. Pedro Casta\u00f1o: Funding acquisition, Project administration, Resources, Supervision, Formal analysis, Writing \u2013 review & editing.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Pedro Castano reports financial support was provided by King Abdullah University of Science and Technology. Pedro Castano has patent pending to King Abdullah University of Science and Technology (KAUST).This work was conducted thanks to the financial support of the King Abdullah University of Science and Technology (KAUST, BAS/1/1403).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2023.122479.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Dry reforming of methane simultaneously achieves several sustainability goals: valorizing methane-activating carbon dioxide while producing syngas. The catalyst has an enormous influence on the process viability by controlling activity, selectivity, and stability. A catalyst with uniform-sized Ni-Fe alloy nanoparticles anchored into PrBaMn1.6Ni0.3Fe0.1O5+\u03b4 double-layered perovskite is assembled via a facile one-step reduction strategy. Our method attains more exsolved Ni nanoparticles (94 %) than the common conditions. The exsolved Ni0.15Fe0.05 catalyst shows exceptional stability in 260\u00a0h tests at 800\u00a0\u00b0C, with one of the slowest coke formation rates compared with the state-of-the-art catalysts. Besides, no deactivation was observed during 40\u00a0h operation at more demanding and coking conditions (14\u00a0bar) where this process is more likely to operate industrially. Via experimental characterizations and computational calculations, the stability of the robust exsolved Ni-Fe catalyst is demonstrated by its unique balance of adsorbed species, which inhibits coking.\n               "}
{"full_text": "Ethylene (C2H4) is regarded as the most important petrochemical platform molecule to produce diverse commodity chemicals such as polyethylene, ethylene oxide, vinyl chloride, and polystyrene, with global demand of 153 million tons in 2016 and net added demand of about 5.2 million tons every year (Xu, 2017). Nowadays, its universal production in industry is based on the steam cracking of oil-based naphtha. However, the oil resource is increasingly dwindling, and thus it has turned out to be a hotspot in modern industries to pave the way for efficient and ecofriendly utilization of the nonoil resources (e.g., natural gas, coal, and renewable biomass) to produce ethylene with the aid of effective catalytic processes. Ethane (C2H6) is abundant in natural gas, and in particular, the shale gas revolution in recent years greatly enriches ethane resources (Sattler et\u00a0al., 2014). Therefore, ethane-to-ethylene conversion (in terms of oxidative dehydrogenation of ethane [ODE], catalytic dehydrogenation, and steam cracking) tantalizes global enthusiasm. The latter two suffer from their thermodynamic constraints and high operation temperature (>700\u00b0C), and the ODE is thus more competitive, benefitting from its oxidative feature that can cast off the thermodynamic limitation and allow lower operation temperature (350\u00b0C\u2013550\u00b0C) (Heynderickx et\u00a0al., 2005).However, controlling the ethylene selectivity for ODE reaction represents the grandest challenge because the excessive oxidation of ethylene to carbon dioxide is thermodynamically and kinetically favorable. Therefore, developing a qualified catalyst with high activity and selectivity is the goal of most efforts for this reaction. To date, various catalysts have been explored (such as alkaline-/rare-earth metal oxides, Mulla et\u00a0al., 2001, Gaab et\u00a0al., 2003; noble metals, Fu et\u00a0al., 2013; and transition metal oxides, Liu et\u00a0al., 2003; Nakamura et\u00a0al., 2006), and NiO-based catalysts are the most attractive owing to its low operation temperature, simple preparation, and low cost (Heracleous and Lemonidou, 2006, 2010; Savova et\u00a0al., 2010; Zhu et\u00a0al., 2012). However, NiO alone mainly yields carbon dioxide due to the large amount of electrophilic (unselective) oxygen species (Heracleous and Lemonidou, 2006, 2010; Savova et\u00a0al., 2010; Zhu et\u00a0al., 2012). Many kinds of oxides were doped into NiO to tune the oxidative properties of oxygen species. Lemonidou et\u00a0al. explored a series of alter-valent cations such as Li, Mg, Al, Ga, Ti, Ta, and Nb (Heracleous and Lemonidou, 2010), and the unselective oxygen amount on NiO surface declines along with the increase in dopant cations' valence. Accordingly, the Nb2O5-doped catalyst offers the highest ODE performance such as 78% ethylene selectivity and 33% ethane conversion at 350\u00b0C (Savova et\u00a0al., 2010). They further proposed that Nb doping into NiO lattice by filling the cationic vacancies on defective non-stoichiometric NiO surface and/or substituting Ni atoms reduces the amount of unselective oxygen (Zhu et\u00a0al., 2012; Heracleous and Lemonidou, 2006). However, such Nb2O5-NiO catalysts suffer from poor stability due to their sintering deactivation (Heracleous and Lemonidou, 2006, 2010; Savova et\u00a0al., 2010; Zhu et\u00a0al., 2012).Despite the above-mentioned interesting advances, the real-world use of these catalysts still remains a challenge as their poor thermal conductivity is detrimental to rapid dissipation of reaction heat released in this strongly exothermic ODE reaction (\u0394H\u00a0= \u2212104\u00a0kJ mol\u22121), which causes severe hotspots in the catalyst bed and therefore leads to the ethylene excessive oxidation while releasing more heat. Recently, the development of structured catalyst based on the monolithic metal-foam has been attracting great interest in heterogeneous catalysis because of the intensified heat transfer, which is favorable to tailor catalysts for strongly exothermic reactions (Chen et\u00a0al., 2019; Zhao et\u00a0al., 2016; Zhang et\u00a0al., 2018a, 2018b). However, the main issue is how to make these promising metal-foam qualified catalysts, or more concretely, how to fabricate the highly active and selective NiO-based nanocomposites onto foam surface.Herein, we demonstrate the remarkable improvement of the Nb2O5-NiO/Ni-foam catalyst performance for ODE reaction, by finely tuning the Nb2O5-NiO interaction by morphology-controllable growth of NiO-precursors onto Ni-foam.First, three kinds of NiO-precursors with different morphologies (i.e., clump for Ni(OH)2, rod for NiC2O4, nanosheet for nickel terephthalate (Ni-Tp), identified by X-ray diffracxtion [XRD] in Figure\u00a0S1) were controllably and endogenously grown onto a Ni-foam (100 pores per inch). Against the smooth surface of Ni-foam (Figures 1A\u20131C), clearly, the in situ growth of three morphology-different NiO-precursor layers on the foam struts succeeds clump with dense stacking for Ni(OH)2 layer by ammonia evaporation method (Figures 1D, 1G, and 1J), rod with diameter of about 450\u00a0nm for NiC2O4 layer by hydrothermal method (Figures 1E, 1H, and 1K), and nanosheet of thickness 30\u00a0nm for Ni-Tp layer by solvothermal method (Figures 1F, 1I, and 1L). Moreover, unlike the dense layer feature of the Ni(OH)2 clump and NiC2O4 rod, the Ni-Tp nanosheets stand upright and irregularly cross-link each other to form honeycomb-like porous layer. Not surprisingly, the Ni-Tp/Ni-foam delivers a specific surface area (SSA) of 6.3 m2 g\u22121 much higher than 1\u20132 m2 g\u22121 for the Ni(OH)2/Ni-foam and NiC2O4/Ni-foam (Table 1\n).Subsequently, niobium ammonium oxalate was wet-impregnated onto the above-obtained Ni(OH)2/Ni-foam, NiC2O4/Ni-foam, and Ni-Tp/Ni-foam at a Nb2O5 content of 5 wt. % (including the Ni-foam mass), followed by drying overnight and calcining in air at 450\u00b0C, to form Ni-foam-structured Nb2O5-NiO catalysts (Figures 1M\u20131U). These catalysts are denoted as Nb2O5-NiO/Ni-foam-C (clump), Nb2O5-NiO/Ni-foam-R (rod), and Nb2O5-NiO/Ni-foam-NS (nanosheet), which all possess equivalent NiO content (\u223c21 wt. %, including Ni-foam mass; Table 1). The NiO and Ni (from Ni-foam) phases are clearly detected by XRD for all three catalysts, whereas no Nb2O5 diffraction peaks are observed, indicating its high dispersion or amorphous structure (Figure\u00a0S2) (Liu et\u00a0al., 2016). Notably, the Ni(OH)2-, NiC2O4-, and Ni-Tp-derived nano-NiO aggregations show well-preserved clump-, rod- and nanosheet-morphologies regardless of Nb2O5 introduction (Figures 1M\u20131O). In addition, the Nb2O5-NiO ensembles show porous feature in association with the thermolysis of their precursors (Figures 1P\u20131R) thereby leading to a visible increase in their SSA (Table 1).Interestingly, the Nb2O5-NiO/Ni-foam-NS achieves an SSA of 20.8 m2 g\u22121, much higher than 12\u201313 m2 g\u22121 seen with the other two catalysts (Table 1). The enhanced surface area can be related to the fact that\u00a0the\u00a0nanosheet-like morphology of Ni-Tp/Ni-foam not only favors the formation of catalyst with high\u00a0SSA (see NiO/Ni-foam-NS, Table 1) but also is helpful for highly dispersing Nb2O5-precursor onto the Ni-Tp nanosheet to hinder the crystallization of NiO during the calcination process (Solsona et\u00a0al., 2011, 2012) (Table 1). Not surprisingly, the Nb2O5-NiO/Ni-foam-NS catalyst provides an average NiO size of 13.5\u00a0nm, smaller than that of \u223c20\u00a0nm for the Nb2O5-NiO/Ni-foam-C and Nb2O5-NiO/Ni-foam-R (Table\u00a01). Nevertheless, the NiO/Ni-foam-NS obtained by calcining the Ni-Tp/Ni-foam in air at 450\u00b0C offers\u00a0an\u00a0average NiO size of \u223c20\u00a0nm, being compatible to that seen with the ones derived from Ni(OH)2/Ni-foam and NiC2O4/Ni-foam. This observation reveals that Nb2O5 introduction favors the decomposition of Ni-Tp nanosheets, rather than Ni(OH)2-clump and NiC2O4-rod, to form smaller NiO nanoparticles. Moreover, the Nb2O5-NiO/Ni-foam-NS achieves more homogeneous NiO-Nb2O5 composites than the other two catalysts (Figures 1S\u20131U).The Nb2O5 modification dramatically improves the ethylene selectivity and slightly the ethane conversion while leading to a remarkable increase in the turnover frequency (TOF) for ethylene formation from \u223c0.62\u00a0h\u22121 for the Nb2O5-free samples to 0.91\u20130.96 h\u22121 at 300\u00b0C (Table 1 and Table S1; the detailed calculation method in the Supplemental Information). As shown in Figure\u00a02\n, three Nb2O5-free samples all achieve almost identical ethane conversion and ethylene selectivity in the whole temperature range studied. In contrast, the Nb2O5-NiO/Ni-foam catalysts exhibit different ODE performance under identical reaction conditions, showing the NiO-precursor morphology dependence; the Nb2O5-NiO/Ni-foam-NS is obviously superior to the Nb2O5-NiO/Ni-foam-C and Nb2O5-NiO/Ni-foam-R catalysts (Figure\u00a02), achieving a 58.4% ethane conversion and 75.4% ethylene selectivity at 425\u00b0C. In addition, compared with the very low productivity of only 0.18 gethylene gcat.\n\u22121 h\u22121 over the Nb2O5-free NiO/Ni-foam catalysts, Nb2O5 modification gets the ethylene productivity doubled even more. The Nb2O5-NiO/Ni-foam-NS achieves the highest ethylene productivity of 0.46 gethylene gcat.\n\u22121 h\u22121 (Figure\u00a0S3).To reveal the underlying origin of the NiO-precursor morphology-dependent ODE catalysis on the above Nb2O5-NiO/Ni-foam catalysts, the amount and type of oxygen species were collaboratively probed by H2-temperature-programmed reduction (H2-TPR) and O2-temperature-programmed desorption (O2-TPD) (Zhu et\u00a0al., 2012; Zhang et\u00a0al., 2018a, 2018b). Clearly, whereas the Nb2O5-free NiO/Ni-foam samples show quite different NiO morphologies (Figure\u00a0S4), they all possess identical reducibility (by H2-TPR) and properties of surface oxygen species (by O2-TPD), solidly evidenced by their almost same H2-TPR and O2-TPD profiles (shape, peak area, and peak temperature; Figures 3A and 3B, profiles 1\u20133). It is thus not surprising that they achieve NiO-precursor morphology-independent ODE performance (Figure\u00a02). In combining this information with the observation of NiO-precursor morphology dependences of distinct ODE performance after Nb2O5 modification, it is safe to say that the NiO-Nb2O5 interaction is sensitive to NiO-precursor morphology, which in nature is responsible for the distinct ODE performance for the Nb2O5-NiO/Ni-foam catalysts.Indeed, the reducibility and properties of the surface oxygen species of the Nb2O5-NiO/Ni-foam catalysts show strong NiO-precursor morphology dependence (Figures 3A and 3B, profiles 4\u20136). The Nb2O5-NiO/Ni-foam-C offers a single H2-TPR peak at 340\u00b0C with an 8\u00b0C delay compared with the NiO/Ni-foam, likely due to the weak Nb2O5-NiO interaction. The Nb2O5-NiO/Ni-foam-R delivers a main peak at 332\u00b0C and a weak shoulder at 358\u00b0C, suggesting the very limited local occurrence of moderate NiO-Nb2O5 interaction. In contrast, the Nb2O5-NiO/Ni-foam-NS provides a main peak at 371\u00b0C and a very weak one at only 297\u00b0C. It should be noted that the H2 consumption is attributed exclusively to the NiO reduction because Nb2O5 reduction cannot occur under such conditions (Zhang et\u00a0al., 2018a, 2018b). Particularly, the NiO size of the Nb2O5-NiO/Ni-foam-NS is 13.5\u00a0nm, smaller than 20\u00a0nm for the others. In general, the lattice oxygen of the smaller NiO nanocrystallites diffuses more efficiently than the larger ones (Zhu et\u00a0al., 2012). So, the weak peak at 297\u00b0C is assignable to the small NiO species that interacted weakly with Nb2O5, whereas the main peak at 371\u00b0C is ascribable to the comprehensive occurrence of strong NiO-Nb2O5 interaction.All catalysts with and without Nb2O5 modification deliver dual-peak O2-TPD profiles, in which the peak at 342\u00b0C is assigned to O2\n- and the one at 543\u00b0C is assigned to O\u2212 - (Figure\u00a03B) (Wu et\u00a0al., 2012; Iwamoto et\u00a0al., 1976). The O2\n- species have strong oxidizing electrophilicity and thus are considered to be non-selective oxygen species that favor the deep oxidation of product (Wu et\u00a0al., 2012; Iwamoto et\u00a0al., 1976). The amount and desorption behavior of O2\n- and O\u2212 - species are tuned markedly by Nb2O5 modification, showing clear NiO-precursor morphology dependence (Table S2 and Figure\u00a03B). The desorbability of such two types of surface oxygen species is almost unchanged for the Nb2O5-NiO/Ni-foam-C and Nb2O5-NiO/Ni-foam-R, whereas their non-selective O2\n- amounts are markedly reduced in association with a slight decline of the O\u2212 amount, when compared with the Nb2O5-free samples (Table S2 and Figure\u00a03B). For Nb2O5-NiO/Ni-foam-NS, most notably, the Nb2O5 modification makes the non-selective O2\n- species almost disappear, but slightly decreases the selective O\u2212 - species, whereas lowers the desorption temperature of O\u2212 - species to 520\u00b0C by 23\u00b0C (Table S2 and Figure\u00a03B). According to the Mars van Krevelen mechanism (Figure\u00a0S5) (Zhu et\u00a0al., 2012), the types of oxygen species determines the further reaction of ethyl radical to form ethylene (\u03b2-elimination) or CO2 (C-C bond cleavage). It is not surprising that Nb2O5 modification and thinning the NiO-precursor thickness are inclined to reduce the non-selective O2\n- species amount and form the ethylene via \u03b2-elimination.In nature, tuning the NiO-precursor morphology from dense Ni(OH)2 clump and NiC2O4 rod (450\u00a0nm) to Ni-Tp nanosheet (30\u00a0nm thickness) strengthens the NiO-Nb2O5 interaction thereby leading to almost elimination of the non-selective O2\n- species and meanwhile improving the mobility of the highly selective O\u2212 - species. Improved mobility of the O\u2212 - species (lowered desorption temperature, Figure\u00a03B) (Wu et\u00a0al., 2012; Skoufa et\u00a0al., 2014) makes it more active than the other two catalysts, which in turn compensates the activity loss caused by the reduction of non-selective O2\n- and selective O\u2212 - species (Zhu et\u00a0al., 2016). That is the reason why the Nb2O5-NiO/Ni-foam-NS catalyst always achieves higher conversion especially above 375\u00b0C (Figure\u00a02A).To further gain insight into the O2\n- reduction caused by Nb2O5-NiO interaction, the surfaces of the NiO/Ni-foam and Nb2O5-NiO/Ni-foam catalysts were probed by X-ray photoelectron spectroscopy (XPS). Figure\u00a03C shows the Ni2p spectra of the catalyst samples. Three peaks are detected: main peak at binding energy (BE) of 853.8 eV for Ni2+ in NiO; satellite peak at 855.8 eV S(I) for Ni3+ in Ni2O3, Ni2+-OH species, and Ni2+ vacancies; and the other satellite peak at 861.3 S(II), involving a ligand-metal charge transfer (Salagre et\u00a0al., 1996; Veenendaal and Sawatzky, 1993). The intensity ratio of S(I) to the main peak at 853.8 eV has been used to present the surface and/or structural density of defect sites (Solsona et\u00a0al., 2012; Zhu et\u00a0al., 2015), offering the information about the non-stoichiometric (or non-selective) property of NiO. Notably, this ratio declines from 4.0 for the NiO/Ni-foam-NS to 1.9 for the Nb2O5-NiO/Ni-foam-C, to 1.7 for the Nb2O5-NiO/Ni-foam-R, and further to 1.1 for the Nb2O5-NiO/Ni-foam-NS (Table S3). Clearly, Nb2O5 modification provides the ability to markedly reduce the non-stoichiometric Ni3+ (responsible for the non-selective O2\n- species), whereas the nanosheet NiO-precursor morphology synergistically promoted such Nb2O5 modification effect. This observation is in good agreement with the O2-TPD results (Figure\u00a03B). Figure\u00a03D shows the XPS spectra in Nb3d region for the Nb2O5-NiO/Ni-foam catalysts. Taking the Nb5+ in pure Nb2O5 (207.4 eV) as reference (Liu et\u00a0al., 2016), the BE of Nb5+ shifts to 207.2 eV for the Nb2O5-NiO/Ni-foam-C, 207.1 eV for the Nb2O5-NiO/Ni-foam-R, and then 206.9 eV for the Nb2O5-NiO/Ni-foam-NS. This trend is consistent with the increasingly stronger NiO-Nb2O5 interaction (Zhu et\u00a0al., 2012).As aforementioned, the nanosheet Ni-Tp precursor is much thinner than Ni(OH)2 clump and NiC2O4 rod and is irregularly aligned to form a porous layer (Figures 1F, 1I, and 1L). This morphology undoubtedly gives higher SSA, which is helpful for highly dispersing Nb2O5 into the NiO matrix (Figures 1S, 1T, and 1U), leading to the lower Ni/Nb ratio in catalyst surface (Table S3); furthermore, as indicated by the high-angle annular dark-field scanning transmission electron microscopy images and elemental maps in Figures 4A\u20134F, the Nb2O5-NiO/Ni-foam-NS achieves the contacting of NiO with Nb2O5 more sufficient than the Nb2O5-NiO/Ni-foam-C and Nb2O5-NiO/Ni-foam-R. On the other hand, the thinner nanosheet feature of Ni-Tp facilitates the incorporation of Nb ions into NiO during calcination treatment. Indeed, the lattice constant obtained by XRD (Solsona et\u00a0al., 2012) reveals that the NiO lattice constant in the Nb2O5-NiO/Ni-foam-NS is 4.1724\u00a0\u00c5, smaller than 4.1767\u00a0\u00c5 for the NiO/Ni-foam and 4.1752\u00a0\u00c5 for both the Nb2O5-NiO/Ni-foam-C and Nb2O5-NiO/Ni-foam-R (Table 1). This observation evidences that Nb ions are, at least partially, incorporated into NiO to the most extent for the Nb2O5-NiO/Ni-foam-NS (Solsona et\u00a0al., 2012; Zhu et\u00a0al., 2012).Last but not the least, according to the foregoing findings, we are confident that the ODE performance of Nb2O5-NiO/Ni-foam catalyst can be improved further if the NiO-precursor nanosheet is able to be thinned further. Indeed, the Ni(OH)2 nanosheet (\u223c20\u00a0nm) is successfully structured onto the Ni-foam by hydrothermal treatment in an aqueous solution of NH4F (denoted as Ni(OH)2/Ni-foam-F, Figures 5\nA\u20135C and S6), and therefore, a Nb2O5-NiO/Ni-foam-F catalyst was obtained by subsequent Nb2O5 modification. As expected, such catalyst shows much higher activity and selectivity than the Nb2O5-NiO/Ni-foam-C; when compared with the Nb2O5-NiO/Ni-foam-NS it achieves comparable activity but markedly improved selectivity (Figure\u00a0S7). Notably, our Nb2O5-NiO/Ni-foam-F catalyst yields better performance (especially the selectivity, stability, and TOF) than the NiO-based catalysts (Tables 2\n and S4) and powdered Nb2O5/NiO (5/21, w/w) catalyst literature (Table S5). Moreover, the ethylene yield (ethane conversion times ethylene selectivity) for such catalyst is comparable to the costly MoVTeNbO catalyst when it is tested at\u00a02,120\u00a0cm3 g\u22121 h\u22121, but our catalyst runs at much higher reactor capacity (GHSV) of 9,000\u00a0cm3 g\u22121 h\u22121 (Table 2).In addition, it is not surprising that the Nb2O5-NiO/Ni-foam-F exhibits highly enhanced Nb2O5-NiO interaction (Figures 5D\u20135F) by further thinning the NiO-precursor, which results in a further reduction of the NiO lattice constant (Table 1), the NiO nanoparticle size (Table 1 and Figure\u00a0S6), and especially the non-selective O2\n- amount as well as the NiO reducibility (Figure\u00a0S8), compared with the ones using Ni(OH)2/Ni-foam-C (dense clump of Ni(OH)2) and Ni-Tp/Ni-foam-NS (\u223c30\u00a0nm Ni-Tp nanosheet). This is undoubtedly responsible for the further catalytic performance improvement observed on the Nb2O5-NiO/Ni-foam-F catalyst. Most notably, this catalyst exhibits favorable stability, being stable for at least 240\u00a0h at 400\u00b0C with \u223c44% ethane conversion and \u223c82% ethylene selectivity (Figure\u00a05G), which shows great superiority when compared with the previously reported Nb2O5-NiO catalysts (Table S4). This is benefited from the high Nb2O5-NiO sintering resistance (evidenced by the well-preserved SSA and particle size of NiO for the used catalyst, Table 1), as a result of the strong interaction between NiO and Nb2O5 (Solsona et\u00a0al., 2011, 2012) in combination with the enhanced heat transfer of the Ni-foam-structured designing that could rapidly dissipate the large quantity of reaction heat from the ODE reaction (Table S5) (Li et\u00a0al., 2015; Zhao et\u00a0al., 2016; Zhang et\u00a0al., 2018a, 2018b).In summary, a low-temperature active, highly selective, and highly stable Nb2O5-NiO/Ni-foam catalyst has been developed for the ODE reaction, by carefully tuning the NiO-precursor morphology-dependent Nb2O5-NiO interaction. The Nb2O5-NiO interaction can be markedly improved by thinning the NiO-precursors endogenously grown onto the Ni-foam substrate, especially leading to significant elimination of\u00a0the nonselective O2\n- species and, meanwhile, remarkable improvement of the mobility of selective O\u2212 species. This work provides an interesting clue to tailor high-performance ODE catalyst via morphology modulation strategy.The ammonium niobium oxalate is a little bit costly.All methods can be found in the accompanying Transparent Methods supplemental file.We acknowledge the financial supports from the Key Basic Research Project (grant 18JC1412100) form the Shanghai Municipal Science and Technology Commission, the National Natural Science Foundation of China (grants 21773069, 21703069, 21703137, 21473057, U1462129, 21273075), and the National Key Basic Research Program (grant 2011CB201403) from the Ministry of Science and Technology of the People's Republic of China.Y. Lu, Z.Z., and G.Z. conceived the idea for the project and designed the experiments. Z.Z., G.Z., Y. Liu, and Y. Lu carried out the interpretation and wrote the manuscript. Z.Z. conducted the material synthesis, characterizations, and catalytic tests. W.S. drew the structure of Ni-foam in Figure\u00a01. All authors discussed and commented on the manuscript. Y. Lu directed the research.Y. Lu, Z.Z., G.Z., and Y. Liu have a patent application related to this work filed with the Chinese Patent Office on October 15, 2017 (201710956118.5). The authors declare that they have no competing interests.Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2019.09.021.\n\n\nDocument S1. Transparent Methods, Figures S1\u2013S8, and Tables S1\u2013S5\n\n\n\n", "descript": "\n                  Large-scale shale gas exploitation greatly enriches ethane resources, making the oxidative dehydrogenation of ethane to ethylene quite fascinating, but the qualified catalyst with unique combination of enhanced activity/selectivity, enhanced heat transfer, and low pressure drop presents a grand challenge. Herein, a high-performance Nb2O5-NiO/Ni-foam catalyst engineered from nano- to macroscale for this reaction is tailored by finely tuning the performance-relevant Nb2O5-NiO interaction that is strongly dependent on NiO-precursor morphology. Three NiO-precursors of different morphologies (clump, rod, and nanosheet) were directly grown onto Ni-foam followed by Nb2O5 modification to obtain the catalyst products. Notably, the one from the NiO-precursor of nanosheet achieves the highest ethylene yield, in nature, because of markedly diminished unselective oxygen species due to enhanced interaction between Nb2O5 and NiO nanosheet. An advanced catalyst is developed by further thinning the NiO-precursor nanosheet, which achieves 60% conversion with 80% selectivity and is stable for at least 240 h.\n               "}
{"full_text": "An increase in the consumption of fossil-based resources has initiated the exploration of biomass conversion to produce high-value chemicals and fuels [1,2]. Furan-based chemicals, owing to the abundant biomass sources, have received considerable attention as valuable alternatives to chemicals obtained from fossil resources.[3\u20135]. 5-hydroxymethylfurfural (HMF) is considered a key molecule for sustainable development [6,7] because it is widely used as a platform chemical in biorefineries. HMF is hydrogenated to 2,5-dimethylfuran, 5-methylfurfural (MF), 2,5-bis(hydroxymethyl)furan (BHMF), and 5-methyl-2-furfuryl alcohol depending on the catalyst and reaction conditions [8,9].Among HMF hydrogenation products, BHMF is significant for the synthesis of several foams, polyethers, and crown ethers owing to the presence of a symmetrical diol functional group [10\u201314]. The key to selective hydrogenation of HMF to BHMF is to saturate the CO bond while avoiding cleavage of the CO bond. Various noble metal-based heterogeneous catalysts have shown high selectivity in BHMF formation [15\u201319]. Pt/C was the first catalyst for the synthesis of BHMF in 2012, achieving 82\u00a0% yield after 18\u00a0h [15]. Zhang et al. developed an Ir/TiO2 catalyst and achieved 94\u00a0% yield under harsh conditions and an H2 pressure of 6\u00a0MPa [16]. A layered double oxide Ru/ZnAlZr prepared by Gao et al. delivered nearly 94\u00a0% yield at 473\u00a0K [17]. Ohyama et al. reported an aluminum oxide-supported gold catalyst for BHMF synthesis with an 80\u00a0% yield after 2\u00a0h at an H2 pressure of 3.8\u00a0MPa without significant furan ring hydrogenation [18]. Recently, Nishimura et al. reported the selective hydroconversion of HMF over a Pd/Al2O3 catalyst under ambient conditions using sodium hypophosphite to form hydrogen atoms and tetrahydrofuran/water as the solvent with a low 60\u00a0% yield [19]. Although many significant results have been obtained using noble metal-based catalysts, their high cost, low abundance, and status as a strategic resource limits their applications. Consequently, the design of novel catalysts based on non-noble metals has attracted considerable attention.Transitionmetals have shown excellent performance as catalysts for the selective hydrogen reactions of various functional groups [20]. Co/C was used in the selective hydroconversion of HMF under a H2 pressure of 2\u00a0MPa for 6\u00a0h to furnish 93\u00a0% of the product[21]. Recently, Rao et al. synthesized a Cu/Al2O3 catalyst using solvent-free solid-state grinding to produce 92\u00a0% of BHMF under a hydrogen pressure of 3\u00a0MPa [22]. Elsayed et al. prepared CuO-Fe3O4/AC for the selective hydroconversion of HMF via catalytic transfer hydrogenation with 92\u00a0% yield at 413\u00a0K for 5\u00a0h [23]. Poor selectivity of Ni-based catalysts is a possible reason that they are rarely applied in BHMF synthesis; for example, the furan ring of BHMF was reduced over Raney Ni [24]. The recent development of nanoscience has made it feasible to regulate the catalyst function using special nanomaterials [25\u201327]. Carbon nanotubes (CNTs) have proven to be excellent supports for catalysts owing to their thermal conductivity, specific surface areas, and porous structures. This has led to increasing investigations on metal\u2013carbon catalytic systems [28\u201330]. It is known that the surface of oxygen-functionalized Ni/CNTs bears free carboxyl groups (\u2013COOH) that are mainly grafted onto the CNT surface and promote electron transfer from Ni atoms to the CNT support [31].In this study, a Ni/CNTs catalyst was synthesized and used for the selective hydrogenation of HMF to BHMF. Catalyst samples with different Ni/CNTs ratios were prepared using the impregnation synthesis method and characterized using various techniques. Factors affecting the hydrogenation process, such as the H2 pressure, reaction temperature, catalyst loading, and reaction time, were optimized to achieve a relatively high yield of BHMF.2\u00a0g of raw 10\u201320\u00a0nm multiwalled carbon nanotubes (CNTs) (purchased from XFNANO Co., ltd.) were oxidized with 200\u00a0mL of concentrated HNO3 (purchased from Sinopharm Chemical Reagent Co., ltd.) at 348\u00a0K for 24\u00a0h. Ni/CNTs samples were subsequently prepared by an impregnation synthesis method as reported by Lee et al [27] with slight modifications. Considering the target Ni:CNTs weight ratio of 3:17, 0.2\u00a0g of CNTs and 0.1765\u00a0g of nickel nitrate hexahydrate (purchased from Sinopharm Chemical Reagent Co., ltd.) were mixed in a beaker. Subsequently, 10\u00a0mL of ultrapure water was added with continuous stirring. The precursor was calcined in an H2/Ar atmosphere at 673\u00a0K for 2\u00a0h after drying at 393\u00a0K for 10\u00a0h. The applied heating rate from 323 to 673\u00a0K was 5\u00a0K/min. The prepared catalyst, denoted as 15\u00a0wt% Ni/CNTs, is a highly magnetic material. The catalysts with different Ni contents were prepared through the same procedure.X-ray diffraction (XRD) patterns of the prepared catalyst samples were recorded using a Ultima IV powder X-ray diffractometer (Rigaku, Japan) with a Cu K-\u03b1 radiation source and a tube pressure of 40\u00a0kV for a diffraction angle (2\u03b8) ranging from 10\u00b0 to 90\u00b0. X-ray photoelectron spectroscopy (XPS) was performed using a K-\u03b1 spectrometer (Thermo Scientific, USA) under vacuum conditions, and spectra were corrected based on the C1s line at 284.80\u00a0eV. Nitrogen adsorption measurements were performed on the samples using an ASAP 2460 sorption analyzer (Micromeritics, USA). The samples were outgassed at 473\u00a0K for 4\u00a0h before measurements. Transmission electron microscopy (TEM) images were obtained using a TF20 TEM (FEI, USA) equipped with a super\u00a0X\u00a0field emission gun.Typically, HMF (1\u00a0mmol, 126\u00a0mg), catalyst (50\u00a0mg), and tetrahydrofuran (10.0\u00a0mL) were added to a 50\u00a0mL autoclave which was sealed and purged with hydrogen four times. Hydrogenation of HMF was performed at a certain reaction temperature and hydrogen pressure with magnetic stirring. After the reaction was completed, the reactor was cooled using ice water. The solid product was removed, and the solution was analyzed using a gas chromatograph (GC; Nexis GC-2030, Shimadzu, Japan) equipped with a flame ionization detector (FID) and a capillary column (SH-Rtx-1701). Structural characteristics of the samples were analyzed by gas chromatography\u2013mass spectrometry (GC\u2013MS) (GC-2010, Shimadzu, Japan).The temperature procedure for GC was as follows:313\u00a0Kfor2\u00a0min, 313 to 373\u00a0K (at 20\u00a0K/min),3\u00a0min, 373 to 473\u00a0K (at 20\u00a0K/min), and 473\u00a0Kfor2\u00a0min. The equations for the HMF conversion, BHMF selectivity, and MF selectivity are shown in Eqs. (1)\u2013(3).\n\n(1)\n\n\nH\nM\nF\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\n\n\n%\n\n\n\n=\n\n\n1\n-\n\n\nMoleofHMF\n\n\nInitialmoleofHMF\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(2)\n\n\nB\nH\nM\nF\n\nY\ni\ne\nl\nd\n\n\n\n%\n\n\n=\n\n\n\nMoleofBHMF\n\n\nInitalmoleofHMF\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(3)\n\n\nM\nF\n\nY\ni\ne\nl\nd\n\n\n%\n\n\n=\n\n\n\nMoleofMF\n\n\nInitalmoleofHMF\n\n\n\u00d7\n100\n%\n\n\n\n\nAll the analyzed catalyst samples (5, 10, 15, 20, and 25\u00a0wt% Ni/CNTs) exhibited peaks characteristic of graphite-2H and the face-centered cubic crystal structure of metallic Ni as shown in Fig. 1\n. The CNT structure was not destroyed during the synthesis process as demonstrated by peaks characterized at 2\u03b8\u00a0=\u00a026.3\u00b0 and 42.2\u00b0 for all samples. Peaks at 2\u03b8\u00a0=\u00a044.5\u00b0, 51.8\u00b0, and 76.4\u00b0 correspond to (111), (200), and (220) crystal faces of the nickel face-centered cubic structure, respectively [27]. No peaks representing NiO are observed establishing the fact that no metal oxidation occurred during the synthesis. The impregnation method resulted in good loading of the metal onto the surface of CNTs as demonstrated by these results. Remarkably, the observed peak intensity of Ni increased as the Ni loading in the catalyst increased to 15\u00a0wt% in Ni/CNTs.\nFig. 2\na shows the particle size distribution of 15\u00a0wt% Ni/CNTs. The distribution revealed an average particle size of 9.31\u00a0nm which is not visible in the raw TEM image. Fig. 2b shows the high-angle annular dark-field (HAADF)\u2013TEM image indicating that Ni nanoparticles are strongly attached to the surface of the CNT support. Elemental maps of the catalysts are shown in Fig. 2c\u2013f demonstrating that Ni is evenly distributed on the support surface with partial metal agglomeration. Functionalized carbon nanotube supports can effectively disperse elemental Ni, increase the number of active sites, and reduce the amount of metal. This result, in particular, encourages the investigation of low-loading CNTs as effective catalysts.The XPS full elemental survey shown in Fig. 2g confirms the presence of oxygen, carbon, and nickel in the catalyst. The presence of oxygen is attributed to the \u2013COOH group on the surface of CNTs as well as to some adsorbed oxygen. In accordance with the published data [27,32], XPS spectra of the analyzed catalysts displayed peaks corresponding to Ni2+ (or NiO) and Ni0. Peaks at 853.1 and 856.1\u00a0eV correspond to Ni0 2p1/2 and Ni2+ 2p1/2, respectively. Binding energies at 871.1 and 874.5\u00a0eV correspond to Ni0 2p3/2 and Ni0 2p3/2, respectively. Two additional satellite peaks of Ni are detected at 861.6 and 880.1\u00a0eV. This is because the grafted carboxyl groups strengthen the Ni\u00a0\u2212\u00a0CNT interaction resulting in the reduction of Ni2+ to Ni0 as observed in an amorphous form [31]. XPS peaks of the carboxyl group in Fig. 2i and XRD peaks of metallic Ni confirm this hypothesis.\nTable 1\n shows the physical properties of the synthesized Ni/CNTs catalysts.The Brunauer\u2013Emmett\u2013Teller (BET) surface areas of 5\u201320\u00a0wt% Ni/CNTs) range from 132 to 140 m2g\u22121. For CNTs coated with Ni, the surface area of samples decreased gradually with increasing Ni loading. As these values are nearly equal to those of CNT carriers (143.41 m2g\u22121), it indicated that Ni nanoparticles were deposited on CNTs without collapsing the nanostructure. Fig. 3\n indicates isotherms of all catalysts are typical type-IV isotherms with a hysteresis loop of type H1. These isotherms strongly prove the mesoporous structure of the prepared samples. Despite the mesoporous structures, the proportion of macropores gradually increased with the addition of Ni as observed from the pore size distribution curves. Fig. 3g indicates a large number of microporous in 25\u00a0wt% Ni/CNTs. The destruction of the mesoporous structure is the cause of catalytic activity degradation with increasing Ni content.Reaction conditions: 1\u00a0mmol HMF, 40\u00a0% catalyst/HMF, 10\u00a0mL tetrahydrofuran, 0.5\u00a0MPa hydrogen pressure, 3\u00a0h, 393\u00a0K.Ni/CNTs catalysts with different nickel loadings were used for selective hydrogenation of HMF at a temperature of 393\u00a0K and H2 pressure of 0.5\u00a0MPa (Table 2\n). MF and a few other substances identified by GC\u2013MS were formed as reaction byproducts under these conditions. The BHMF yield increased with an increasing Ni content from 5 to 15\u00a0wt%. An HMF conversion of 52\u00a0% with 95.1\u00a0% of BHMF selectivity was achieved using 15\u00a0wt% Ni/CNTs. A further increase in nickel loading reduced the BHMF yield and the BET surface area of the catalysts to a small extent. This is because an excess Ni loading interferes with the surface structure of CNTs leading to a decreased catalytic activity. Thus, 15\u00a0wt% Ni/CNTs was considered the optimum loading to catalyze hydrogenation of HMF to a diol.15\u00a0wt% Ni/CNTs was selected for the next part of the study. The dependence of the product distribution on these parameters was explored by HMF hydrogenation at different reaction times ranging from 3 to 12\u00a0h. Fig. 4\n shows that the BHMF yield is not affected by the reaction time (in the explored range). It was constant at 75\u00a0% after a reaction time of 6\u00a0h. BHMF was the main reaction product, and the selectivity of MF slightly increased with the reaction time. HMF conversion was not increased with time because of the substrate adsorption\u2013desorption equilibrium on the catalyst. Thus, it was difficult to improve the BHMF yield.The conversion of HMF significantly improved with an increase in the catalyst ratio, whereas the selectivity of BHMF formation was maintained at approximately 95\u00a0% as shown in Fig. 5\n. Notably, 99.8\u00a0% of HMF was converted to form 94.8\u00a0% of BHMF with the catalyst/HMF ratio of 100\u00a0%. Combined with the previous investigation of the dependence on the reaction time, this effect can be explained by the fact that more active sites are available for catalyzing HMF hydrogenation upon increasing the catalyst amount. This resulted in shifting the reaction equilibrium to form products.Hydrogen pressure had a significant effect on the conversion of HMF but a slight effect on the selectivity of BHMF as shown in Fig. 6\n. The conversion was relatively low in a reactor in a low-pressure atmosphere (0.25\u00a0MPa). Increasing the hydrogen pressure resulted in a gradual increase in the conversion with a maximum conversion of 72.6\u00a0% at 1\u00a0MPa. A further increase in the hydrogen pressure to 1.25\u00a0MPa or higher did not significantly increase the HMF conversion. The yield of MF as a byproduct was very low (<1.9\u00a0%).The catalyst was used for the conversion of HMF to BHMF at different temperatures in the range of 353\u2013453\u00a0K to investigate the effect of the reaction temperature. Hydrogenation of HMF rarely occurs at 353\u00a0K because the energy acquired at this temperature is insufficient to overcome the energy barrier. With an increase in the reaction temperature, BHMF selectivity decreased from approximately 95.0\u00a0% to 84.3\u00a0% at 453\u00a0K as shown in Fig. 7\n. This behavior is attributed to the conversion of BHMF to MF and MFA at higher temperatures. The direct conversion of HMF to MF was excluded because it requires higher activation energy than that required for the conversion of BHMF. This result will be discussed in more detail in Section 3.3. In summary, our results suggest that a lower reaction temperature is beneficial for the selectivity of BHMF although it results in lower conversion.In previous reports, hydrogen under high pressure was necessary to produce BHMF from HMF (more than 3\u00a0MPa) [21,33,34]. In this work, the high dispersion of active sites due to the high specific surface area and defect sites formed due to the oxidation treatment of CNTs resulted in hydrogenation under the pressure of 0.5\u00a0MPa H2. In addition, Ni/CNTs is a strong magnetic material that is easier to recycle than nonmagnetic materials.Aldehyde and hydroxyl groups present on opposite sides of the HMF furan ring can react with a suitable hydrogen donor to saturate the CO bond and break CO bond to form different products. Therefore, MF was expected to be formed as a byproduct (Scheme 1\n). Hydroconversion of HMF was performed at three different temperatures to determine the kinetic parameters: 393, 413, and 433\u00a0K. Note that in all experiments, an excess amount of hydrogen as compared to the amount of the substrate (1\u00a0mmol HMF) was used under 0.5\u00a0MPa pressure for assuming a constant amount of hydrogen during the entire hydrogenation process. Under these conditions, kinetic rate constants and activation energies of the reactions were calculated using a Pseudo first order model. Equations (4)\u2013(6) were used to determine the kinetic parameters for the hydrogenation of HMF.\n\n(4)\n\n\nd\n\n[\nH\nM\nF\n]\n\n/\nd\nt\n=\n-\n\nk\n1\n\n\u00d7\n\n[\nH\nM\nF\n]\n\n-\n\nk\n1\n\n\u00d7\n\n[\nH\nM\nF\n]\n\n\n\n\n\n\n\n(5)\n\n\nD\n\n[\nB\nH\nM\nF\n]\n\n/\nd\nt\n=\n\nk\n1\n\n\u00d7\n\n[\nH\nM\nF\n]\n\n\n\n\n\n\n\n(6)\n\n\nd\n\n[\nM\nF\n]\n\n/\nd\nt\n=\n\nk\n2\n\n\u00d7\n\n[\nH\nM\nF\n]\n\n\n\n\n\nwhere k1 and k2 are the Pseudo first order rate constants for formation of BHMF and MF, respectively, at a specific reaction temperature, and t is the reaction time (h).Equations (4)\u2013(6) are integrated under the initial conditions corresponding to \n\nt\n\n=\n\n0\n\n and \n\n\n[\nH\nM\nF\n]\n\n\n=\n\n\n\n[\nH\nM\nF\n]\n\n0\n\n\n. The concentrations and reaction times are expressed using Eqs. (7)\u2013(9).\n\n(7)\n\n\n\n\nH\nM\nF\n\n\n=\n\n\n[\nH\nM\nF\n]\n\n0\n\n\u00d7\ne\nx\np\n\n(\n-\n\n(\n\nk\n1\n\n+\n\nk\n2\n\n)\n\n\u00d7\nt\n)\n\n\n\n\n\n\n\n(8)\n\n\n\n\nB\nH\nM\nF\n\n\n=\n\n\n[\nH\nM\nF\n]\n\n0\n\n\u00d7\n\nk\n1\n\n/\n\n(\n\nk\n1\n\n+\n\nk\n2\n\n)\n\n\n\u00d7\n\n(\n1\n-\nexp\n\n\n-\n\n\n\nk\n1\n\n+\n\nk\n2\n\n\n\n\u00d7\nt\n\n\n)\n\n\n\n\n\n\n\n(9)\n\n\n\n\nM\nF\n\n\n=\n\n\n[\nH\nM\nF\n]\n\n0\n\n\u00d7\n\nk\n2\n\n/\n\n(\n\nk\n1\n\n+\n\nk\n2\n\n)\n\n\n\u00d7\n\n(\n1\n-\nexp\n\n\n-\n\n\n\nk\n1\n\n+\n\nk\n2\n\n\n\n\u00d7\nt\n\n\n)\n\n\n\n\n\nAll parameters were estimated using OriginPro Learning Edition software by performing a nonlinear curve fit to the selected data with the corresponding rate equation. Curve fits of the experimental data at the investigated reaction temperatures are shown in Fig. 8\n. Table 3\n lists the activation energies determined from the Arrhenius equation and reaction rate constants corresponding to the investigated temperatures. The similarity between experimental data and fitting curves confirms that the Pseudo first order reaction model accurately describes the hydrogenation process. As expected from the experimental data, the rate constant (k1) of BHMF formation was significantly larger than that of MF (k2). Similarly, the activation energy of BHMF formation (21.12\u00a0kJ/mol) was almost half as large as that of MF formation (51.46\u00a0kJ/mol). The results from the kinetic investigation explain the high selectivity of BHMF. Larger rate constants are always associated with higher temperatures which explain the higher MF yields observed at elevated temperatures.Various HMF derivatives can be produced by reducing the different functional groups present in HMF, thus making it a complex reaction. In this study, 15\u00a0wt% Ni/CNTs exhibited high selectivity for reducing CO among the other functional groups. The XPS results revealed that Ni0 and Ni2+ ions are present on the 15\u00a0wt% Ni/CNTs surface. These act as active sites for the conversion of HMF substrates in a hydrogen atmosphere. Additional experiments were performed to explain the role of both Ni0 and Ni2+ in the selective HMF hydrogenation. We observed a decrease in the Ni0/Ni2+ ratio on the 15\u00a0wt% Ni/CNTs surface with an increasing reaction temperature that was in agreement with previously reported data [27]. The smaller the size of Ni nanoparticles, a larger surface area, and larger NiO film area are expected.To understand the impact of nickel ions on the reaction, 15\u00a0wt% Ni/CNTs was prepared at different reduction temperatures, namely 573, 673, and 773\u00a0K. The catalyst was denoted as 15\u00a0wt% Ni/CNTsx, where\u00a0\u00d7\u00a0denotes the temperature. The samples were characterized by XPS (Fig. 9\n and Table 4\n). The results clearly show that the lowest Ni0 content on the surface was associated with the slowest reaction rate. This relationship may be due to the slowing down of dissociative adsorption of hydrogen which requires Ni0. However, increasing the Ni0 content from 24.1\u00a0% to 37.2\u00a0% did not increase the catalytic activity. Based on this evidence, we believe that there is a synergistic effect between Ni0 and Ni2+ ions. Ni2+ adsorbs nucleophilic unsaturated functional groups and Ni0 provides activated hydrogen. Consequently, the presence of excessive Ni2+ on the surface adsorbs nucleophilic groups more efficiently and boosts the hydrolysis reaction to form MFA and DMF as products.The condensed local nucleophilicity index was obtained using the Multiwfn software [35] by calculating the molecular index of the optimized HMF molecule. According to Table 5\n, 16(O) is more nucleophilic than 14(O) with a condensed local nucleophilicity index almost three times larger (Fig. 10\n shows the numbering pattern). This evidence suggests that a Lewis acid, such as Ni2+, prefers to adsorb HMF through the aldehyde group instead of the hydroxyl group.Feng et al. proposed the coexistence of both metallic and electrophilic metal species as a prerequisite for selective hydrogenation of HMF [36]. We proposed a plausible reaction mechanism to explain the synergistic effect between Ni0 and Ni2+ as shown in Scheme 2\n based on the above experimental results and literature reports [22,37\u201339]. Initially, the carbonyl group in HMF is adsorbed onto the electrophilic Ni2+ species on the catalyst surface and is activated. Simultaneously, hydrogen dissociates at the metallic nickel (Ni0) site. The electron lone pair of H\u2212 attacks the C atom of the activated carbonyl group, whereas the same on the CO bond is transferred to the O atom. HMF is converted to BHMF after the activated O atom and H+ form a CO bond.BHMF yields gradually decreased as the number of cycles increased as observed in Fig. 11\n. The leaching test results shown in Fig. 12\n indicate that the yield increases continuously for 3\u00a0h after which it became constant on removing 15\u00a0wt% Ni/CNTs from the reaction solution. This result demonstrates that active sites were not leached and 15\u00a0wt% Ni/CNTs is heterogeneous. To investigate the reason for the decreased catalytic activity, XRD tests of the fresh and spent catalysts were carried out as shown in Fig. 13\n. XRD results excluded the presence of any additional peaks, demonstrating that the crystalline structure of the catalyst was not changed even after its use. However, the intensity of metallic nickel peaks slightly decreased along with XRD peaks at 2\u03b8\u00a0=\u00a026.3\u00b0 and 42.2\u00b0. It is possible that the substrate or other amorphous material was attached to the catalyst after the reaction resulting in a broad peak in the range of 15\u00b0\u201340\u00b0. The above result explains the decline in catalytic activity as shown in Fig. 11. Amorphous substances covering the active sites reduces the active sites with successive catalytic cycles. As previously reported, the spent catalyst partly recovers its catalytic activity after recalcination in a mixture of 20\u00a0% H2/Ar at 670\u00a0K [39]. Even so, repeated high-temperature treatments lead to metal agglomeration making the catalyst unable to recover completely.A highly efficient carbon nanotube-supported nickel catalyst (Ni/CNTs) was prepared using an impregnation method. The catalyst exhibits excellent activity and selectivity and is substantially less expensive. The high selectivity of the catalyst results from the optimal Ni0:Ni2+ ratio and the small size of nanoparticles. The reaction temperature and catalyst amount are crucial parameters for achieving a high BHMF yield. Under the optimal reaction conditions, a 93.1\u00a0% yield of BHMF was achieved. The kinetic study revealed that the conversion of HMF to BHMF is associated with the lowest activation energy (21.12\u00a0kJ/mol) which is half of that required to form MF (51.46\u00a0kJ/mol). The difference between the activation energies of BHMF and MF explains the high selectivity toward BHMF. These results provide a novel method for the selective hydrogenation of HMF to BHMF and promote research on biomass energy.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 22278121 and 21975070), the China Postdoctoral Science Foundation (2019\u00a0M662787), and the Science and Technology Planning Project of Hunan Province (2021GK5083).", "descript": "\n                  2,5-Bis(hydroxymethyl)furan (BHMF) is a high-value, bio-based, rigid diol that resembles aromatic monomers for the production of different polyesters. In this work, a carbonnanotubes (CNTs)-supported nickel catalyst (Ni/CNTs)was prepared and used for the selective hydrogenation of 5-hydroxymethylfurfural (HMF) to BHMF at low hydrogen pressure. The prepared catalyst was analyzed by nitrogen adsorption\u2013desorption isotherms, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). According to kinetic studies, the rate constant for BHMF formation is significantly larger than that for the formation of the byproduct, 5-methyl furfural (MF). At optimal reaction conditions, conversion and selectivity rates of HMF and BHMF were 99.8\u00a0% and 95.0\u00a0%, respectively. The mechanistic study indicated the coexistence of Ni0 and Ni2+ species on the catalyst surface affects the catalytic performance. A possible mechanism was proposed to describe the synergetic effects of Ni0 and Ni2+. Furthermore, the catalyst can be easily separated from the reaction mixture for recycling.\n               "}
{"full_text": "The catalytic hydroconversion of n-paraffins is an important reaction to improve the quality of diesel and gasoline in the oil-refining industry (Zhou et al., 2022). Hydroisomerization of light alkane can produce high-octane fractions for gasoline blending with non-aromatic hydrocarbons to meet increasingly stringent environmental protection regulations (Zhan et al., 2022). Environmentally friendly solid superacid catalysts, especially for sulfated zirconia (SZ)-based samples, have been regarded as the most promising candidates for preparation of isomerization catalysts with high catalytic activity at low reaction temperature (Wang et al., 2020). However, the catalytic performance of pristine SZ is known to be unacceptable caused by the rapid deactivation in practical application. Therefore, the dopants of noble metals (Pt, Pd) and/or various transition metals (Fe, Ni, Mn, and Cu) have been adopted to modify the catalyst and results in much higher activity than that of raw SZ (Lyu et al., 2021; Song et al., 2015). Typically, the Ni belongs to the same family of Pd and Pt, more and more attentions have been paid to design low-cost Ni-modified SZ catalyst for replacement of noble metal. Though, the Ni-SZ catalyst exhibits acceptable isomerization activity at low temperature, the deactivation is also needed to be taken into consideration (Song et al., 2016a). Reasons for deactivation of SZ-based catalysts have been reported to be complex (Liu et al., 2020; Wang et al., 2016; Kim et al., 2000; Li and Stair, 1996), such as coke deposition (Li et al., 2006), leach of sulfate species (Ng and Horv\u00e1t, 1995), change in surface acidity (Gonz\u00e1lez et al., 1997) and phase transition from tetragonal to monoclinic zirconia (Li and Stair, 1996). Therefore, it is of great of interest to design non-noble metal modified SZ catalysts with high catalytic activity and stability.Recently, many efforts have been made to alleviate the deactivation and the introduction of alumina into SZ are found to improve the catalytic activity and stability for isomerization performance (Hua et al., 2000; Gao et al., 1998). The modification of alumina contributed to the enhanced concentration of active sites and acid sites. Furthermore, the addition of alumina can retard the crystal phase transformation of ZrO2 from tetragonal to monoclinic phase (Zhou et al., 2021; Wang et al., 2022). As compared to intrinsic Pt\u2013SO4/ZrO2 catalyst, the alumina-modified sample exhibited higher pore volume and specific surface area, more importantly, the alumina resulted in enhanced stability of tetragonal zirconia, which contributed to an excellent stability and activity for light naphtha isomerization (Zhou et al., 2022). Our previous research (Song et al., 2014) also found that the addition of an appropriate amount of Al (2.5 wt% of Al) can increase the amount of acid sites and the surface area, suppressing the phase transformation of tetragonal ZrO2 to monoclinic ZrO2. But an excessive amount of Al would decrease the number of surface tetragonal ZrO2 particles and led to a decrease in the formation of acid sites, which was generated by sulfate species adsorbed on the stepped edges of tetragonal ZrO2, and thus resulted in a significant decrease in activity. Therefore, the complete utilization of the skeleton structure and acid nature of \u03b3-Al2O3 support is limited due to the low Al content of <5 wt%. Considering the disadvantages, the as-prepared catalyst often exhibited poor stability caused by the deactivation of SZ. (Song et al., 2016b).Generally, the pore structure and acidity of the support played a vital role on the coke deposition during n-alkane isomerization. For example, the pore with large size can facilitate the mass transfer, which reduces the residence time of hydrocarbon compounds on the catalyst surface and suppresses the deposition of carbon. Recently, core-shell structure materials have attracted worldwide attention due to their unique physical and chemical properties (Das et al., 2021). The core-shell nanoparticles exhibit many advantages, such as tunable surface modification, improved functionality, enhanced stability by protecting the active phase from contact with poisoning substances, lower consumption of precious materials, and so on (Gao et al., 2021). However, to the best of our knowledge, few studies have been reported concerning the use of \u03b3-Al2O3 as a core support material for preparation of superacid SZ catalysts.Herein, a method for preparing highly active and highly stable non-noble-nickel-modified persulfated Al2O3@ZrO2 core-shell catalyst (Ni\u2013S2O8\n2\u2212/Al2O3@ZrO2) was proposed to make full use of the respective advantages of Al2O3 and ZrO2. The Al2O3 core can impart the core-shell structure materials with high internal surface area and high mechanical strength for the support, which contributes to the formation of external shell with more and smaller tetragonal ZrO2 particles. As a result, the formation of superacid sites is accelerated due to the intimate contact between Zr and S species. Besides, the Al2O3 endows additional acid sites for the core-shell support and stabilizes the active tetragonal phase of ZrO2, which is also responsible for the improved catalytic performance. In the case of n-pentane isomerization, the core-shell Ni\u2013S2O8\n2\u2212/Al2O3@ZrO2 catalyst showed a high isopentane yield (63%) with little or no deactivation within 5000 min. To the best of our knowledge, such a non-noble superacid catalyst with high isopentane yield and excellent stability at a low pressure (2.0 MPa) is extremely unusual.The Al2O3@ZrO2 (core@shell, A@Z) supports were synthesized by deposition of zirconia on the \u03b3-Al2O3. In a typical procedure, a certain amount of \u03b3-Al2O3 and butanol were mixed at room temperature, and deionized water was added dropwise to the suspension under vigorous stirring for further dispersion. Then, the calculated amount of zirconium (IV) butoxide was dissolved into the resulting suspension with different Zr/Al mass ratio, and stirred for another 30 min. Subsequently, the suspension was transferred into autoclaves for hydrothermal reaction at 443 K for 24 h. After cooling down to room temperature, the obtained products were separated by centrifugation, and then dried at 353 K for 24 h to obtain the A@Z-x samples, where x represented the percentage of Al content. Then the product was re-dispersed into a 0.75 M (NH4)2S2O8 solution and stirred for 15 min. After aging for 6 h, the samples were separated by centrifugation and dried at 353 K for 24 h to obtain the SA@Z-x samples.The supported core@shell nickel catalysts (Ni-SA@Z-x) were prepared by the incipient wetness impregnation method (Song et al., 2015). Typically, calculated amount of the SA@Z-x material and Ni(NO3)2\u00b76H2O were added into 10 mL deionized water. Then the obtained samples were dried at 373 K for 12 h and calcined at 923 K for 3 h to obtain the Ni-SA@Z-x with Ni loading of 1.0 wt%.According to our previous study (Song et al., 2014), Pd-SZA catalyst made from Al content of 2.5 wt% exhibited the best performance. Therefore, for comparison the common SZA with Al2O3 content of 2.5 wt% was chosen to synthesize the supported Ni catalyst with Ni loading of 1.0 wt%. And the obtained catalyst was designed as Ni-SZA-2.5.X-ray powder diffraction (XRD) patterns were recorded on a D/max-2200PC X-ray diffractometer (40 kV, 40 mA) fitted with Cu K\u03b1 radiation (0.15404 nm). N2-adsorption was measured at 77 K using Micromeritics ASAP 2460 analyzer to obtain the microporous and mesoporous porosities, respectively. Transmission electron microscope (TEM) examinations were performed using the JEM-2010 instrument supplied by JEOL. Scanning electron microscope (SEM) with an acceleration voltage of 10 kV was conducted using Zeiss SIGMA equipment. Thermogravimetric analysis (TG) was performed on the samples (10 mg) after reaction using a Perkin-Elmer Diamond instrument under air with a flow rate of 100 mL min\u22121, from room temperature to 1123 K, and with a heating rate of 10 K min\u22121. Fourier transform infrared spectroscopy (FT-IR) measurements were carried out with a Bruker Tensor 27 FT-IR spectrometer. Fourier transform infrared spectroscopy of pyridine adsorption (Py-IR) was recorded on a Spectrum GX Fourier by adding 64 scans for the sample at a resolution of 4 cm\u22121. The metal loadings of the samples were determined by X-ray fluorescence (XRF) with a spectrometer XRF-1800. XPS were acquired with a PHI-1600 spectrometer equipped with a hemispherical electron analyzer and a Mg K\u03b1 (1253.6 eV) X-ray source.The isomerization reaction of n-pentane was chosen to evaluate the catalytic activity of the prepared catalysts. The reactions were performed in a fixed-bed flow reactor. Prior to reaction, 2 g of the catalyst was activated with flowing H2 stream (20 mL min\u22121) at 573 K for 3 h, and then cooled to the reaction temperature. The reaction conditions were set to a weight hourly space velocity (WHSV) of 1 h\u22121, an H2/n-pentane mole ratio of 4.0, a total pressure of 2.0 MPa and a temperature ranging from 433 to 533 K. The reaction products were analyzed by an online FL9790 gas chromatograph equipped with a FID detector.The XRD patterns of fresh and spent samples were shown in Fig. 1\n. As depicted in Fig. 1a, all samples showed the diffraction peaks at 2\u03b8\u202f=\u202f30.3\u00b0, 35.3\u00b0, 50.4\u00b0 and 60.4\u00b0, which were related to the (101), (110), (112) and (211) planes of tetragonal ZrO2, respectively (Reddy et al., 2018). The crystalline of tetragonal ZrO2 was affected by the dopant of Al, the diffraction peak of tetragonal ZrO2 was broadened with the increased Al content from 2.5 wt% to 50 wt%, indicating the decreased crystalline size of zirconia particles with incremental Al content. The absence of Al2O3 peaks in Ni-SA@Z-x with the high Al content of 30\u201350 wt% proved that Al2O3 core was totally coated by ZrO2 shell (Yang et al., 2013), suggesting the successful preparation of core-shell material. Besides, no crystalline phase of nickel oxide was detected due to the low content or high dispersion of nickel. Compared with the pattern of traditional Ni-SZA-2.5, the peak intensity of tetragonal ZrO2 decreased remarkably for all Ni-SA@Z-x catalysts, indicating that the core-shell structure can effectively suppress the growth of crystalline zirconia particles and result in much smaller particles size of zirconia even at the same Al content of 2.5 wt%.As shown in Fig. 1b, the diffraction peaks of monoclinic ZrO2 was detected for spent Ni-SZA-2.5 and Ni-SA@Z-2.5, which indicated the transformation of ZrO2 from the metastable tetragonal to the monoclinic phase during the isomerization reaction. Generally, the binary ZrO2/Al2O3 composite was often prepared by traditional sol-gel method and resulted in the uniform dispersion of Al and Zr species on the surface of binary nanocomposite, which led to the higher crystallizing temperature of tetragonal ZrO2 caused by the addition of Al2O3 (Zhao et al., 2007; Liu et al., 2012). Alternatively, the core-shell structure SA@Z showed advantages than ZrO2/Al2O3 composite, since the Al2O3 was totally covered by active tetragonal ZrO2 phase. What's more, the tetragonal ZrO2 phase has been reported to be necessary for isomerization performance (Liu et al., 2012). Besides, the monoclinic phase peaks of the spent Ni-SA@Z-x (x\u202f=\u202f30\u201350) were very weak, suggesting the more stable tetragonal structure as compared to Ni-SA@Z-2.5 and higher catalytic stability in the case of n-pentane isomerization.The crystal sizes of tetragonal zirconia for all samples were calculated by the Debye-Scherrer equation and listed in Table 1\n. Compared to traditional Ni-SZA-2.5 (9.7 nm), all Ni-SA@Z-x samples showed smaller crystallite size with increased Al addition. In detail, the tetragonal ZrO2 crystallite size of Ni-SA@Z-x decreased from 8.2 to 4.9 nm (decreased by 40.2%) with the increased Al content from 2.5 wt% to 50 wt%, indicating the positive effect of Al species on the formation of tetragonal ZrO2 crystallite with smaller size. Similar result has been reported by Zarubica et al. (2021), an increase in Al content promoted the stabilization of smaller tetragonal ZrO2 particles on the surface. As mentioned, the core-shell structure was beneficial to the formation of smaller tetragonal ZrO2 particles, which also accelerated the contact between Zr and S species to form Zr\u2013S bonds and deduced the formation of a superacid structure and dispersion of active sites and acid sites. This will be further discussed in Sections 3.5 and 3.6. Besides, the ZrO2 crystallite size of Ni-SA@Z-x samples with 2.5 wt%\u223c50 wt% Al content increased about 1.6\u20130.7 nm after reaction, which was still much lower than that of Ni-SZA-2.5. This further confirmed that the core-shell structure could restrain sintering of the tetragonal ZrO2 phase and remain the integrity of its microscopic structure.The N2 adsorption-desorption isotherms and pore size distributions of the catalysts were shown in Fig. S1. Accordingly, all the isotherms showed a type IV characteristic feature of isotherm, indicating the presence of some mesopores (Thommes et al., 2015). Ni-SZA-2.5 showed a narrow pore size distribution centered at around 3 nm. The Ni-SA@Z-2.5 showed a broader pore size distribution from 5 to 10 nm, and the main peak was close to that of \u03b3-Al2O3. With increasing Al content, the pore size increased remarkably owing to the abundant Al provided more mesopores, and some of the micropores gradually merged into mesopores.In comparison with Ni-SZA-2.5 (95.1 m2 g\u22121), the Ni-SA@Z-2.5 showed a slightly higher specific surface area (S\nBET) of 99.5 m2 g\u22121 (Table 1). In addition, the pore size (D\np) increased remarkably from 3.7 to 5.6 nm, and the pore volume (V\nTotal) increased from 0.089 to 0.103 cm3 g\u22121. This showed that the pore structures of these two catalysts were entirely different, even though the Al content and the compositions of the individual components are the same. The large D\np and V\nTotal of Ni-SA@Z-2.5 would enhance the diffusion rates of reactant and products. In particular, branched or large-sized products could pass through the pores more easily, suppressing carbon deposition on the surface of the catalyst. This would effectively arrest catalyst deactivation since carbon deposition is one of the main reasons for deactivation of catalysts of this kind (Song et al., 2016b). In addition, the large D\np and V\nTotal are also beneficial to the isomerization reaction. With increasing the Al content, the S\nBET, V\nTotal and D\np of Ni-SA@Z-x increased remarkably. It is worth noting that the D\np and V\nTotal of Ni-SA@Z-50 were 2.1 and 4.2 times higher than those of Ni-SA@Z-2.5, respectively.For Ni-SZA-2.5, S\nBET, V\nTotal and D\np was dramatically decreased after reaction. The narrowed D\np indicated that carbon deposition mainly occurred inside the pores during the reaction. The deposited carbon would have coated the active metal sites and acid sites on the surface of the catalyst, leading to its deactivation. However, a slight decline in textural parameter of Ni-SA@Z-x catalysts was observed after reaction.\nFig. 2\n exhibited the TEM images of Ni-SZA-2.5, Ni-SA@Z-2.5 and Ni-SA@Z-50 catalysts. Mokari et al. (2005) proposed that Zr particles may be easily identified by their dark contrast in TEM, as a result of the electron density contrast between Al and Zr. Moreover, because of low content and high dispersion, Ni particles could not be observed (Nichele et al., 2012). It can be seen from Fig. 2a, c) that the ZrO2 particle size in the Ni-SZA-2.5 catalyst was approximately 8.4 nm with interplanar distances of 0.295 nm for ZrO2 (101) plane (Bang et al., 2020). In Fig. 2b, a light-color core surrounded by a dark shell can be clearly discerned. This indicated that a core-shell structure had been successfully synthesized. Besides, the core-shell structure of SA@Z-30 materials was also detected in SEM images (Fig. S2). The ZrO2 particle size in the core-shell Ni-SA@Z-2.5 catalyst was about 6.7 nm (Fig. 2d), smaller than that of Ni-SZA-2.5, implying that the core-shell structure was beneficial to the formation of smaller tetragonal ZrO2 particles. This observation was consistent with the XRD results (Table 1). For Ni-SA@Z-50, the Zr particle size decreased to about 3.9 nm with further increased Al content, indicating the positive effect of Al on dispersion of Zr species.The TG results of samples were shown in Fig. 3\n. All samples display weight loss in the range of room temperature to 938 K, which is attributed to desorption of physically and chemically adsorbed water molecules and the dihydroxylation process on the surface of ZrO2 (Arkatova, 2010; Joo et al., 2013). Significant weight loss was clearly started at 938 K, which could be attributed to the decomposition of persulfate species with the evolution of sulfur dioxide, similar results have been reported elsewhere (Kim et al., 2006; Satam and Jayaram, 2008). Compared with Ni-SZA-2.5, the decomposition of persulfate species of Ni-SA@Z-x was shifted to higher temperatures of 963 K. These observations suggest that persulfate anions on the surface of Ni-SA@Z-x were bonded more strongly to dehydrated zirconia, leading to the increased thermal stability of superacid. This will be further discussed in Section 3.9.The FTIR spectrum of fresh catalysts was depicted in Fig. 4\n. All the samples showed similar peaks, the band at 3422 cm\u22121 and 1630 cm\u22121 was assigned to the physically adsorbed water molecules and the bending mode (\u03b4\nHOH) of coordinated molecular water associated with the persulfate group, respectively (Sarkar et al., 2007). The bands at 1156 cm\u22121 and 1077 cm\u22121 were assigned to the symmetric O\u2013S\u2013O stretching mode of bidentate persulfate ions coordinated to the metal ion, which was responsible for the Lewis acid sites in persulfated zirconia samples. The band at 1255 cm\u22121 corresponded to the antisymmetric OSO stretching frequency of persulfate ions bonded to ZrO2, which was responsible for the Br\u00f8nsted acid sites in persulfated zirconia samples (Mishra et al., 2003). These three bands appearing at about 1077, 1156 and 1255 cm\u22121 were assigned to bidentate S ions coordinated to ZrO2 in C\n2\u03c5 symmetry with a \u03c53 vibration, indicating the formation of a strongly superacid structure (Yadav and Murkute, 2004). The intensity and degree of splitting of the persulfate bands reflect the proportion of acid sites of the catalyst. The Ni-SA@Z-2.5 catalyst exhibited three vibration bands corresponding to SO and S\u2013O bond, which showed higher intensity and degree of splitting than those of Ni-SZA-2.5. This indicated that the Ni-SA@Z-2.5 provided more acid sites and stronger acidity, as further confirmed by Py-IR results (see Section 3.6). With increasing Al content, the intensity and the degree of splitting of the vibrational bands corresponding to SO (1255 cm\u22121) and S\u2013O (1077 cm\u22121 and 1156 cm\u22121) of Ni-SA@Z-x increased, and the Ni-SA@Z-30 possessed the highest intensity and the splitting degree among Ni-SA@Z-x.The Py-IR results of fresh and spent catalysts were listed in Tables S1 and S2. All of the catalysts possessed more Lewis acid sites than Br\u00f8nsted acid sites, and both of them decreased with increasing desorption temperature. Compared with Ni-SZA-2.5, distinct increases in the amount of Br\u00f8nsted and Lewis acid sites could be observed for Ni-SA@Z-2.5, which possessed smaller (Table 1) and more uniformly dispersed ZrO2 particles (Fig. 2 TEM) on the surface of mesoporous Al2O3 nanoparticles, facilitating interaction with S2O8\n2\u2212 anions to generate acid sites. With increasing Al content, all the amounts of Lewis acid sites and Br\u00f8nsted acid sites for Ni-SA@Z-x increased remarkably. These results indicated that the addition of Al improved the stability of the persulfate loaded on the surface to form stronger acid sites. Foo et al. (2015) proposed that the Br\u00f8nsted acidity was associated with persulfuric acid clusters on zirconia. With increasing Al content, the persulfate anions were bonded more strongly to dehydrated zirconia (as shown in TG analysis) and thus formed more superacid sites.For all of the spent catalysts (Table S2), the amount of Br\u00f8nsted acid sites and Lewis acid sites were both decreased as compared to the corresponding fresh one. However, the amount of Br\u00f8nsted acid sites decreased more significantly than that of Lewis acid sites, implying that the former were the main active acid sites for isomerization (Yang and Weng, 2010). For Ni-SZA-2.5, the strong acid sites had completely disappeared after reaction. However, the spent Ni-SA@Z-2.5 still possessed the strong acid sites. In addition, the amounts of weak, moderate, and strong acid sites on Ni-SA@Z-x (x\u202f<\u202f50) were still maintained at high levels after reaction. This can be attributed to a stabilizing effect of Al on S species on the catalyst surface and some suppression of the loss of acid sites (Hou et al., 2017). Analysis of the bulk sulfur content also confirmed it (Table2, Section 3.8).The surficial chemical composition of the catalysts was investigated by XPS analysis. As shown in Fig. 5\na, the full-scan XPS spectrum shows that the Ni-SZA-2.5 and Ni-SA@Z-x contains Ni, Zr, Al, S and O species, respectively. In Fig. 5b, the high-resolution S 2p spectrum consists of two contributions for all the samples. The peak centered at 169.1 eV can be assigned to S6+ species of peroxydisulfate (Shanthi et al., 2019). Sulfur with an oxidation state of +6 is known to be the most active and essential for the formation of solid superacid sites. While the peak appeared at 170.4\u2013169.9 eV can be attributed to S\u2013O\u2013Zr bond. As compared to the Ni-SZA-2.5 and Ni-SA@Z-2.5, the binding energy of Ni-SA@Z-30 shifted to lower value, indicating that the electronic environment of S has changed when the Al content raised to 30 wt% (Wang et al., 2018).\nTable S3 showed the atomic contents obtained from XRF, XPS and carbon-sulfur analysis. The Ni contents of all of the catalysts were roughly equal to the stoichiometric content. In addition, compared to the fresh catalysts, no significant change was observed after reaction, showing that deactivation of the catalyst was not caused by the Ni leaching loss. Compared with Ni-SZA-2.5 (1.74 wt%), the sulfur content of the Ni-SA@Z-2.5 increased to a slightly higher value of 2.36 wt%, indicating that Ni-SA@Z-2.5 could stabilize more S species, as discussed in Section 3.7. With incremental Al content, the sulfur content further increased and the sulfur content of Ni-SA@Z-50 reached up to 3.41 wt%. After reaction, both Ni-SZA-2.5 and Ni-SA@Z-x catalysts underwent an overt sulfur loss. Many researchers (Yang and Weng, 2009; Saha and Sengupta, 2015) have found that the loss of loosely bound S species during reaction resulted in catalyst deactivation. Besides, for Ni-SZA-2.5, significant carbon deposition occurred during isomerization (0.45 wt%). The amount of carbon deposition on the spent Ni-SA@Z-x samples was improved as compared to that of the spent Ni-SZA-2.5, which supported the view that the larger mesopore volume of the core-shell catalysts effectively enhanced the diffusion rate and inhibited carbon deposition. Thus, it can be speculated that the Ni-SA@Z-x catalysts would exhibit excellent thermal stability (Kuznetsov et al., 2017) (see Scheme 1).The Ni-SZA-2.5 and Ni-SA@Z-x catalysts have been tested in the isomerization of n-pentane at a pressure of 2.0 MPa, an H2/n-pentane molar ratio of 4.0, and a WHSV of 1.0 h\u22121 and results were illustrated in Fig. 6\n and Fig. S3. The catalytic activities of all of the catalysts first increased and reached a maximum, and then decreased with increasing temperature. The raw Ni-SZA-2.5 showed a maximum isopentane yield of 60.3% at optimized temperature (Fig. S3). For Ni-SA@Z-2.5, the isopentane yield of 65.6% was reached at 473 K. With increasing the Al content, the optimum temperature decreased and then increased. The Ni-SA@Z-30 possessed the lowest optimum temperature at 453 K with the high isopentane yield of 64.7%. Possible reasons to explain the high isopentane yield of Ni-SA@Z-30 catalyst at lower temperature may be as follows (Scheme 2\n\n). (i) More and stronger superacid sites are formed featured by the FTIR and Py-IR analysis (Table S1 and Fig. 5). In particular, the amount of strong Br\u00f8nsted acid sites of Ni-SA@Z-30 was 6.0 and 54.3 times higher than that of Ni-SA@Z-2.5 and Ni-SZA-2.5, respectively. (ii) Better dispersion of active acid and metal sites was achieved due to the high surface area (Table 1). However, further increased Al content resulted in an adverse activity, which led to higher reaction temperature and downtrend in isopentane yield. According to Kamoun et al. (2015), addition of excessive Al to Ni/ZrO2\u2013SO4\n2\u2212 have the negative effect of the Al on isomerization activity at low temperature. Similar results have been reported in our previous study (Song et al., 2014), which showed that the isopentane yield over Pd-SZA-2.5 (Al content of 2.5%) was 64.3% at 511 K, however, when the amount of Al content increased to 5 wt%, the optimum temperature increased to 553 K with a sharp decline in isopentane yield. This can be attributed to the decrease in the tetragonal phase and its crystallinity at a higher Al content.(Reaction condition: p\u202f=\u202f2.0 MPa, H2/n-pentane molar ratio\u202f=\u202f4.0, WHSV\u202f=\u202f1.0 h\u22121).\nFig. 7\n showed the stability results for Ni-SZA-2.5 and Ni-SA@Z-x over a period of 5000 min at their corresponding optimum reaction temperatures with other conditions maintained the same. The isopentane yield of the Ni-SZA-2.5 catalyst showed an obvious decline during isomerization, which decreased dramatically from 60.3% to 20.0% (decreased by 66.8%) after 1500 min. Compared to traditional Ni-SZA-2.5 catalysts, the Ni-SA@Z-2.5 exhibited much better stability, and the isopentane yield showed a slight decreased from 65.4% to 60.2% (decreased by 7.7%) after 1500 min and to 50.1% (decreased by 23.1%) after 5000 min. The Ni-SA@Z-30 catalyst exhibited the most promising catalytic performance and showed a high isopentane yield of approximately 63.1% with no or tiny deactivation after 5000 min. Possible reasons may be proposed to explain the great stability of the Ni-SA@Z-x catalyst for n-pentane isomerization (Scheme 2). (i) The pore sizes and volumes of the catalysts increased in the order: Ni-SZA-2.5 (3.7 nm, 0.089 cm3 g\u22121)\u202f<\u202fNi-SA@Z-2.5 (5.6 nm, 0.103 cm3 g\u22121)\u202f<\u202fNi-SA@Z-30 (6.9 nm, 0.214 cm3 g\u22121) (Table 1). The large pore size and pore volume enhanced the diffusion rates of the reactant and products and largely suppressed carbon deposition. This was confirmed by analysis of carbon deposition on the spent samples. The amounts of carbon deposited on the spent Ni-SA@Z-2.5 and Ni-SA@Z-30 were only 0.07 wt% and 0.05 wt%, respectively, much lower than that of spent Ni-SZA-2.5 (0.45 wt%, Table S3). The color changes of the catalysts after reaction also supported this (Fig. 7). (ii) The loss of sulfur entities can be suppressed for Ni-SA@Z-x. The ZrO2 shell, which consists of more and smaller tetragonal ZrO2 particles because of the large surface area of the Al2O3 core (Table 1), ensured intimate contact between Zr and S. Therefore, the superacid became more stable in thermally (See TG analysis). Elemental analysis showed that the sulfur content of Ni-SA@Z-2.5 and Ni-SA@Z-30 catalysts decreased by 16.1 and 18.1% after reaction for 5000 min on stream, respectively, whereas the Ni-SZA-2.5 underwent a higher sulfur loss of 27.6% after 1500 min (Table S3). Therefore, the deactivation of the catalysts caused by sulfur removal was somewhat suppressed for Ni-SA@Z-x. (iii) For the Ni-SZA-2.5 catalyst, the strong Br\u00f8nsted acid sites, which played an important role in isomerization (Li et al., 2020), completely disappeared after reaction (Table S2). As contrast, the content of Br\u00f8nsted acid for the spent Ni-SA@Z-2.5 and Ni-SA@Z-30 was 0.6 \u03bcmol g\u22121 and 6.5 \u03bcmol g\u22121, respectively. In addition, the contents of weak, moderate, and strong acid sites of Ni-SA@Z-30 still maintained to a great extent after reaction.(Reaction condition: p\u202f=\u202f2.0 MPa, H2/n-pentane molar ratio\u202f=\u202f4:1, WHSV\u202f=\u202f1.0 h\u22121).This study paved a new path for the synthesis of highly active and highly stable non-noble Ni-SA@Z-x catalysts for n-pentane isomerization. The Ni-SA@Z-30 provided a sustained high isopentane yield (64.7%) with little or no deactivation within 5000 min at a low temperature of 453 K. The high isopentane yield of Ni-SA@Z-30 can be attributed to the formation of more and stronger superacid sites due to numerous small tetragonal ZrO2 particles derived from ZrO2 shell and better dispersion of active acid and metal sites. The excellent stability can be attributed to the following factors: (i) carbon deposition was greatly suppressed by the large pore size and huge pore volume; (ii) the loss of sulfur entities was suppressed due to the stronger interaction between small tetragonal ZrO2 particles and S species; (iii) the loss of strong Br\u00f8nsted acid sites was improved during the isomerization reaction. To the best of our knowledge, such a non-noble superacid catalyst with high isopentane yield and excellent stability at a low pressure (2.0 MPa) is extremely unusual and being reported for the first time.The following is the Supplementary data to this article.\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.petsci.2023.02.027.", "descript": "\n                  The non-noble metal modified sulfated zirconia was found easy to deactivate. Herein, highly active and highly stable non-noble core-shell Ni\u2013S2O8\n                     2\u2212/Al2O3@ZrO2 catalysts (Ni-SA@Z-x, x\u202f=\u202fAl content in wt%) have been successfully prepared and investigated for n-pentane isomerization. The results showed that the core-shell Ni-SA@Z-30 provided a sustained high isopentane yield (63.1%) with little or no deactivation within 5000 min at a mild reaction pressure of 2.0 MPa, which can be attributed to the following factors: (i) carbon deposition was greatly suppressed by the large pore size and huge pore volume; (ii) the loss of sulfur entities was suppressed because the small and highly dispersed tetragonal ZrO2 particles can bond with the S species strongly; (iii) strong Br\u00f8nsted acidity can be maintained well after the isomerization. The pore structures and acid nature of the core-shell Ni-SA@Z-x are entirely different from those of the normal structure Ni\u2013S2O8\n                     2\u2212/ZrO2\u2013Al2O3, even though the Al content and the compositions of the individual components are the same. The Al2O3 cores endow the catalysts with a high surface area, large pore size, huge pore volume, and high mechanical strength. Meanwhile, the ZrO2 shell, which consists of more and smaller tetragonal ZrO2 particles because of the large surface area of the Al2O3 core, promotes the formation of more stable sulfur species and stronger binding sites.\n               "}
{"full_text": "Data will be made available on request.The combination of green hydrogen with biogenic carbon dioxide feedstocks generates synthetic fuel with low carbon footprint [1]. So far, two main synthetic fuel routes have been extensively proposed: Power-to-Gas (Sabatier), which produces almost pure CH4; and Power-to-Liquid (Fischer-Tropsch), which aims to mimic the composition of current fossil liquid hydrocarbons (C5\n+), as gasoline, kerosene, light and heavy diesel. In contrast, there is not a well-established low-carbon fuel route to produce light alkanes (C2-C4), which are now present in the fossil-based natural gas (1\u201310\u00a0%) and in liquefied petroleum gas (LPG) [2]. Indeed, C2-C4 hydrocarbons are vastly used (>300 MMT annually [3]) as fuel in heating appliances, cooking equipment and vehicle transport. Therefore, a novel catalytic route favouring CC coupling for the generation of a high-calorie synthetic gas (HC-SG) is of special interest for several applications and different locations.A mixture of CH4 and C2-C4 hydrocarbons composes the so-called HC-SG, which exhibits a higher heating value (\u223c57.72\u00a0MJ/Nm3) [4] than fossil natural gas (42\u201346\u00a0MJ/Nm3) and much higher than from the product of Sabatier synthesis (37.74\u00a0MJ/Nm3) [5]. As emerging fuel, there are no well-defined standards of HC-SG properties. As reference, a mixture exceeding 40\u00a0MJ/Nm3 can be considered a high-calorie gas, as it can be comparable with current natural gas specifications. To satisfy this standard, HC-SG should contain at least 5\u201315\u00a0vol% of C2\u2013C4 paraffin hydrocarbons [6].In principle, HC-SG could be produced alternatively from CO and CO2 feedstocks. In this sense, the main reactions involved in the HC-SG synthesis depend on the carbon source. The direct pathway for the production of hydrocarbons is through the so-called modified Fischer-Tropsch reaction (m-FT, Eq. (1)). Nevertheless, an indirect pathway occurs when the CO2 molecule is converted to CO by means of the reverse Water Gas Shift reaction (rWGS, Eq. (2)), which generates the intermediate for the production C1-C4 hydrocarbons, similar to the Fischer-Tropsch reaction (FT, Eq. (3)). The relative extension of the abovementioned reactions (Eqs. (1) - (3)) depends on (i) the nature of the catalytic material and (ii) the reaction conditions, which need to be controlled to achieve the desired selectivity [7].\n\n(1)\n\n\nnC\n\nO\n2\n\n\n+\n\n3\nn\n\nH\n2\n\n\n\u21cb\n\nC\nn\n\n\nH\n\n2\nn\n\n\n\n+\n\n2\nn\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\u0394\nH\n\n\u00b0\n\n298\nK\n\n\n=\n-\n128\n\nKJ\nmol\n\n\n\n\n\n\n\n\n(2)\n\n\nC\n\nO\n2\n\n\n+\n\nH\n2\n\n\n\u21cb\nCO\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n\n\n\n\n\u0394\n\n\u00b0\n\n298\nK\n\n\n=\n+\n41\n\nKJ\nmol\n\n\n\n\n\n\n\n(3)\n\n\nnCO\n\n+\n\n\n\n\n\n2\nn\n+\n1\n\n\n\n\n\nH\n2\n\n\u21cb\n\nC\nn\n\n\nH\n\n2\nn\n+\n2\n\n\n\n+\nn\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\n\n\n\n\n\u0394\nH\n\n\u00b0\n\n298\nK\n\n\n=\n-\n166\n\n\nKJ\nmol\n\n\n\n\n\nCobalt (Co) [8] and iron (Fe) [910] are suitable catalysts for the production of HC-SG. Cobalt-based catalysts have the best compromise between performance and cost for the synthesis of hydrocarbons from H2/CO mixtures [1112]. Qi et al. indicated that the synthesis of highly dispersed Co catalysts requires the initial formation of very small CoO or Co3O4 crystallites [13]. The formation of these small oxide clusters, in turn, requires strong interactions between the support and the Co precursor. Besides, Lee et al. reported that Co-based catalyst performance towards the production of C2-C4 hydrocarbons can be enhanced by the incorporation of a second metal. Recently, the effect of Mn and Ru on Co-based catalysts was evaluated [14]. They found that Mn is able to modify the surface acidity, and promote carbon-rich environment on the surface, which resulted in an increase of the C2-C4 yield. Concerning Ru, they claimed that this metal phase is able to increase the reducibility of catalysts, resulting in a high activity at a lower temperature. In other works, the combination of Co and Fe was also reported. Co-Fe/Al2O3 catalysts were more selective to light hydrocarbons (C2\u2013C4), with respect to monometallic Co-based catalysts [15]. Furthermore, it was observed that the formation of FeCo alloy can destabilize the iron carbide phase and suppress the carbon chain growth [16].In addition to Co-Fe, other bimetallic catalysts have been proposed for the production of HC-SG from syngas, such as Fe-Ni [2], Fe-Zn [4], Fe-Cu [17] and Fe-Pd [18]. In the case of bimetallic Fe-Zn catalyst, Zn exhibited hydrogen spillover ability, which increases CO hydrogenation. Most recently, catalytic systems based on CeO2\u2212Pt@mSiO2\u2212Co[19], Ni3xCoxO4\n[20], as well as tri-metallic Co-Fe-Ni catalysts [21] have been studied for the production of HC-SG. In this latter, Kim et al. concluded that the metal dispersion and reducibility were enhanced in the presence of nickel, leading to an improved catalytic activity.Those studies reported that the incorporation of a second metal increases the fraction of reduced metal and, consequently, its activity to HC-SG. Despite efforts to elucidate the effect of the second metal on the HC-SG reaction, some major key issues related to hydrocarbon C2-C4 promotion remained elusive. The adsorption trend of COx over cobalt catalyst should play a significant role in the product distribution in HC-SG and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies can give valuable information about adsorption trends and formation of intermediates during the thermocatalytic reaction. In this direction, the beneficial role of lanthanide metal oxides on Al2O3 supports for CO2 methanation and Fischer-Tropsch reaction using nickel [22] and cobalt [23] catalysts was recently reported. In this aspect, lanthanide promotion on bimetallic cobalt catalyst would be able to incorporate moderate basic sites, and thus, facilitate COx adsorption, which arises as an interesting strategy to increase the production of HC-SG.The main goal of this work is to propose a La2O3 promoted bimetallic catalytic system able to increase HC-SG production from CO and CO2 carbon sources under moderate pressure. As well, to identify the most favourable reaction temperature conditions for each catalyst and carbon source. To the best of our knowledge, this catalytic system has not reported for HC-SG synthesis in the open literature. With this aim, we developed a series of catalysts based on Co-X/La2O3-Al2O3, using Ni, Pt and Fe as promising second active metal phases (X). The promoted support, monometallic and bimetallic catalysts were evaluated at 200\u2013300\u00a0\u00b0C, 10\u00a0bar\u00b7g and relatively high gas flowrates. In-situ DRIFTS experiments were used to elucidate the role of the second metal when exposed to the different carbon sources. The heating values of the obtained gas mixtures and potential reaction engineering design for HC-SG production is hereby discussed.A series of micro-catalysts with particle sizes between 200 and 300\u00a0\u03bcm, were prepared by a melting infiltration method previously proposed by our group [24]. Catalyst samples were composed by 80\u00a0wt% of the promoted support (65\u00a0wt% of \u03b3-Al2O3 and 15\u00a0wt% of La2O3) and 20\u00a0wt% of metal active phase (10\u00a0wt% Co\u00a0+\u00a010\u00a0wt% second metal Ni, Pt or Fe for bimetallic and Co for monometallic). The bimetallic catalysts were denoted as Co-Ni, Co-Pt and Co-Fe. The content of the promoter phase (15\u00a0wt%) was selected according to a previous work [22].For the impregnation of a 5\u00a0g-batch, the salt precursors (cobalt\u00a0+\u00a0second metal\u00a0+\u00a0promoter) were added to the alumina support, mixed and dissolved on a rotary evaporator at 120\u00a0\u00b0C for 1\u00a0h. In the case of Co-Pt catalyst, 3.9\u00a0mL of water were added to guarantee the dissolution of the PtCl4 metal precursor. Then, the temperature was reduced to 90\u00a0\u00b0C and vacuum was applied until complete evaporation, 4\u00a0h approximately. The impregnated material was kept at 110\u00a0\u00b0C in an atmospheric oven overnight. Subsequently, the catalysts were calcined at 450\u00a0\u00b0C for 30\u00a0min, with a heating ramp of 1\u00a0\u00b0C/min.Chemicals used for catalyst synthesis were \u03b3-Al2O3 in shape of microspheres with particle diameters dp\u00a0=\u00a0200\u2013300\u00a0\u00b5m (Puralox) as support, salt precursor of lanthanum (III) nitrate hexahydrate [La(NO3)3\u00b76H2O] (99.99\u00a0% purity, Aldrich) as promoter, and salt precursors of cobalt (II) nitrate hexahydrate [Co(NO3)2\u00b76H2O] (100\u00a0% purity, Emsure), nickel (II) nitrate hexahydrate [Ni(NO3)2\u00b76H2O] (98\u00a0% purity, Alfa Aesar), tetra platinum (IV) chloride [PtCl4] (99.99\u00a0% purity, Alfa Aesar), iron (III) nitrate nonahydrate [Fe(NO3)3\u00b79H2O] (98\u00a0% purity, Sigma-Aldrich) as active phases.The microstructure morphology and elemental composition analysis of the catalysts were studied using scanning electron microscopy (Zeiss Auriga 60) equipped with an energy dispersive X-rays spectroscopy detector (EDX, Oxford Instruments), respectively. SEM images were recorded using the SE2 detector at a power beam range of 3\u00a0kV, working distance (WD) of 5.2\u00a0mm and a magnification of 100 X. In the case of SEM-EDX analysis, these were conducted at 20\u00a0kV using a copper standard for the system calibration. The chemical composition analysis was restricted to Co, Ni, Pt, Fe, Al, La and O, and it was calculated as the average over five measurements (standard deviation \u03c3\u00a0\u00b1\u00a01) on different regions for each sample.N2-physisorption (adsorption/desorption) measurements were determined at liquid nitrogen temperature using an automated TriStar II 3020-Micromeritics analyzer. Samples were degassed at 90\u00a0\u00b0C for 1\u00a0h, and then at 250\u00a0\u00b0C for 4\u00a0h in a FlowPrep 060-Micromeritics. Brunauer-Emmett-Teller (BET) method was used to calculate the BET surface area for a relative pressure (P/Po) range of 0.05\u20130.30. Barrett-Joyner-Halenda (BJH) method was applied to desorption branch of the isotherms to determine the average pore size and the total pore volume, which was calculated from the maximum adsorption value at P/Po\u00a0=\u00a00.999.The true densities of catalysts were studied using a helium pycnometer (Ultrapyc pycnometer 1200e, Quantachrome Instruments). Experiments were carried out on a large sample cell that was filled only the 75\u00a0% of its volume to ensure accuracy (\u00b10.02). Prior to measurements, the cell loaded with catalyst was transferred to the sample chamber. True density values were estimated by the average of collected data points from three runs measured at 20 psi.Micrometrics Autochem II equipment was used to study the reducibility of the catalysts in the programmed temperature range from 25 to 800\u00a0\u00b0C. For the analysis, 0.1\u00a0g of each sample was placed in a U-shaped quartz reactor and supported on quartz wool. A mixture of 10\u00a0vol% H2/Argon (50NmL/min) was used as a reducing gas in the tests, while the temperature was raised from 25\u00a0\u00b0C to 800\u00a0\u00b0C with a ramp of 10\u00a0\u00b0C/min. The signal of H2 consumption was detected by a thermal conductivity detector (TCD). The amount of reduced metal oxides to metal species was calculated by integrating the reduction peaks in the H2-TPR profiles and expressed as a percentage of consumption to reduce the metal species in the catalysts.XRD patterns were collected within the 2\u03f4 range 20-80\u00b0 in a Bruker type XRD D8 Advance A25 diffractometer using a Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5), a voltage of 40\u00a0kV, a current of 40\u00a0mA and a step size of 0.05\u00b0 (with 3\u00a0s duration at each step). Dataset was normalized to guarantee a proper interpretation of the results. For calcined sample, the average crystallite size of Co3O4 was estimated using the Scherrer\u2019s equation at 2\u0398\u00a0=\u00a036.9\u00b0. D=(K\u03bb/\u03b2Cos\u03f4), where \u03bb is the X-ray wavelength, \u03b2 is the full width of the diffraction line at half maximum (FWHM), and \u03f4 is the Bragg angle. On the other hand, the average crystallite sizes of the metallic Co (Co0) and alloys (CoX0) were estimated at 2\u0398\u00a0=\u00a044.21\u00b0 for Co [111], 44.50\u00b0 for CoNi [111], 41.55\u00b0 for CoPt [111], and 44.83\u00b0 for CoFe [110].The metal dispersion (D) was calculated from the average metal crystallite size (M\u00a0=\u00a0Co0 and CoX0), by using Eq. (4). It is important to mentioned that the applicability of this equation is viable only if we assumed that the promoter phase and/or second metal phase is not present in the catalytic composition. In other words, all the catalysts should be considered monometallic Co-based catalysts with spherical uniform metal crystallite with a site density of 14.6 atoms/nm2.\n\n(4)\n\n\nD\n\n(\n%\n)\n\n=\n96\n/\nd\n\n(\n\n\nM\n\n0\n\n)\n\n\n\n\n\nThe reactions for study of catalytic activity were conducted on a laboratory fixed-bed rector with a diameter of 13\u00a0mm and a length of 305\u00a0mm (Microactivity Reference, PID Eng&Tech). The tubular stainless-steel reactor was placed inside a ceramic chamber, which was heated by an electrical resistance. The reaction temperature was monitored using a K-type thermocouple placed in the middle of the catalytic bed. Experiments were carried out using 300\u00a0mg of catalyst, which was diluted with 3\u00a0g of silicon carbide of similar particle size (355\u00a0\u03bcm) to guarantee an isothermal catalytic bed. The mixture reactants (H2 (99.999\u00a0%, Linde), CO2 (99.999\u00a0%, Linde) and CO (99.999\u00a0%, Linde)) were supplied by mass flow controllers (MFC, Bronkshorst) at 200\u00a0N\u00a0mL/min. H2:CO2\u00a0=\u00a03 and H2:CO\u00a0=\u00a03\u00a0molar ratio was set. Thus, experiments were carried at 40.000\u00a0N\u00a0mL/gcat\u00b7h of gas hourly space velocity. Pressure was set at the reactor outlet by an automatic valve at 10\u00a0bar\u00b7g.After reaction, the products passed through a cold liquid\u2013gas separator (5\u00a0\u00b0C), where water was trapped, and then the dry flow was measured by a mass flow meter (MF, Bronkshorst). The composition of the dry gas was analysed by a micro-chromatograph Aglient Technologies 490 Micro GC Biogas Analyzer model. It was equipped with three channels, the first channel (CP-Sil 5 CB) analysed C3H6, C3H8, C4H10 and C5+; the second channel (CP-PoralPLOT U) analysed CO2, C2H4 and C2H6; and the third channel (CP-Molsieve 5A) analysed H2, CH4 and CO.Prior to reaction, catalysts were in-situ reduced under H2 flow (100\u00a0N\u00a0mL/min) at 500\u00a0\u00b0C for 3\u00a0h using a heating ramp of 1\u00a0\u00b0C/min, and then cooled to 50\u00a0\u00b0C with the same ramp rate. The catalytic activity was evaluated in a range of temperature from 200 to 300\u00a0\u00b0C, with an interval of 50\u00a0\u00b0C.The conversion of COx and C1-balance selectivity toward the hydrocarbon products were calculated using Eqs. (5) - (7):\n\n(5)\n\n\nConversion\n\no\nf\n\n\nC\n\nO\nx\n\n\n\n\n\n%\n\n\n\n=\n\n\n\n\n1\n-\n\n\nC\n\nO\n\nx\n,\no\nu\nt\n\n\n\n\nC\n\nO\n\nx\n,\ni\nn\n\n\n\n\n\n\n\n\n\u00b7\n100\n\n\n\n\nwhere CoX (x\u00a0=\u00a01 for CO and 2 for CO2) represents the molar flow rate of the species in the inlet and outlet gas.\n\n(6)\n\n\nSelectivity\n\n\n\nC\nn\n\n\nH\nm\n\n\n(\n%\n)\n\n=\n\n\n\n\n\nn\n\u00b7\n\nC\nn\n\n\nH\nm\n\n\n\n\u2211\n\n\n(\nn\n\u00b7\n\nC\nn\n\n\nH\nm\n\n)\n\n\nout\n\n\n+\nC\n\nO\n\nx\n,\no\nu\nt\n\n\n\n\n\n\n\n\u00b7\n100\n\n\n\n\nwhere CnHm is the hydrocarbon of carbon n and hydrogen m (m\u00a0=\u00a02n\u00a0+\u00a02 for paraffins y m\u00a0=\u00a02n for olefins).\n\n(7)\n\n\nSelectivity\n\n\nt\no\n\n\nC\n\nO\nx\n\n\n(\n%\n)\n\n=\n\n\n\n\n\nC\n\nO\n\nx\n,\no\nu\nt\n\n\n\n\n\u2211\n\n\n(\nn\n\u00b7\n\nC\nn\n\n\nH\nm\n\n)\n\n\nout\n\n\n+\nC\n\nO\n\nx\n,\no\nu\nt\n\n\n\n\n\n\n\n\u00b7\n100\n\n\n\n\nAt the outlet, products such as CO, CO2 and H2 species were not considered as part of the HC-SG. Therefore, the heating values (Eq. (8)) for HC-SG were only calculated based on the NIST Chemistry WebBook data for heat of combustion of the methane and C2-C4 hydrocarbons (CH4:891\u00a0MJ/mol; C2H4:1411\u00a0MJ/mol; C2H6:1561\u00a0MJ/mol; C3H6:2058\u00a0MJ/mol; C3H8: 2220\u00a0MJ/mol; C4H10: 2878\u00a0MJ/mol) [5].\n\n(8)\n\n\nHeating\n\nV\na\nl\nu\ne\n\n\n\n\n\n\nMJ\n\n\nN\n\nm\n3\n\n\n\n\n\n\n=\n\n\u2211\n\nn\n=\n1\n\n4\n\n\n\n\n\n\nVolume\n\nf\nr\na\nc\nt\ni\no\nn\n\u00b7\n\nH\ne\na\nt\n\no\nf\n\nc\no\nm\nb\nu\ns\nt\ni\no\nn\n\n\n\n\n\n\nMJ\n\n\nmol\n\n\n\n\n\n\n\nSpecific\n\nv\no\nl\nu\nm\ne\n\n\n\n\n\n\n\nN\n\nm\n3\n\n\n\nmol\n\n\n\n\n\n\n\n\n\n\n\no\nf\n\nt\nh\ne\n\nh\ny\nd\nr\no\nc\na\nr\nb\no\nn\n\no\nf\n\nc\na\nr\nb\no\nn\n\nn\nu\nm\nb\ne\nr\n\nn\n\n\n\n\nDRIFTS measurements were performed on a Bruker-Vertex70 spectrophotometer equipped with a MCT detector and a high temperature reaction cell (Harrick Praying Mantis) with two ZnSe windows. Prior to the experiments, the samples were reduced at 500\u00a0\u00b0C in the reaction cell under an Ar/H2 flux. A flux of 20\u00a0mL/min with an Ar:H2:CO2 ratio of 12:3:1 was applied for the reaction with CO2 and a flux of 40\u00a0mL/min was applied for the reaction with CO with an Ar:H2:CO ratio of 12:3:1. The reactions were studied in the temperature range of 50\u2013300\u00a0\u00b0C, at intervals of 50\u00a0\u00b0C. Background spectra were recorded under Ar at each temperature.A series of catalysts based on Co-X/La2O3-Al2O3 were prepared, characterized and evaluated. The physicochemical properties of the \u03b3-Al2O3 support, the promoted La2O3 support, the monometallic Co catalyst and the bimetallic Co-Ni, Co-Pt and Co-Fe catalysts are described as follows.SEM-EDX analysis of Co-X/La2O3-Al2O3 revealed the presence and distribution of the metals related to the active phases (Co-X; X\u00a0=\u00a0Ni, Pt, Fe) and promoter phase (La), thus confirming metal impregnation. As a representative example, SEM-EDX mapping of bimetallic Co-Ni catalyst is shown in Fig. 1\n. As it can be observed, Ni, Co and La elements were distributed uniformly over the support. Furthermore, the SEM image of the Ni-Co indicated that the topological characteristics (size and shape) of the bimetallic catalyst were analogous to those of the \u03b3-Al2O3 support, i.e. micro-spherical catalysts with particle diameters between 200 and 300\u00a0\u03bcm. An approximatio of the elemental composition of the series of catalysts is presented in Table 1\n. The percentage in weight of the bimetallic active phase (Co-X\u00a0=\u00a017\u201321\u00a0wt%) and the metal oxide promoter (La2O3\u00a0=\u00a012\u201316\u00a0wt%) phase were fairly close to the nominal ones. Therefore, EDX data suggest a good and consistent impregnation of whole series of catalysts.The nitrogen adsorption/desorption isotherms of the catalysts were type IV classification (see Figure SI1) [25]. As expected, the \u03b3-Al2O3 support presented larger BET surface area, pore volume and pore diameter than the promoted support and the rest of catalysts. The addition of La2O3 promoter together with the active phases Co or Co-Ni, Co-Pt and Co-Fe in \u03b3-Al2O3 support resulted in a generalized reduction in their textural properties of the catalysts caused by the incorporation of non-porous metal-oxides on a porous support. Fig. 2\n suggests that between Co and Co-X samples a narrow distribution in the mesoporous range was achieved, peaking higher than 6.71\u00a0nm. The true density of the catalysts was always increased after metal loading to the support.The H2-TPR profiles are displayed in Fig. 3\n. At the studied reduction temperature range (T\u00a0=\u00a025\u2013800\u00a0\u00b0C), the promoted support composed by La2O3-\u03b3-Al2O3 showed one characteristic peak at around 450\u00a0\u00b0C, which was related to the reduction of the La2O3 promoter, whereas the reduction of \u03b3-Al2O3 support was not identified at this temperature range. The main broad peaks for monometallic Co catalyst were categorized in two zones: a low temperature zone (250\u2013375\u00a0\u00b0C) related to the reduction of Co3O4 to CoO and a high temperature zone (420\u2013600\u00a0\u00b0C) related to the final reduction of CoO to Co0\n[26].Bimetallic catalystsexhibited a different reduction behaviour than monometallic Co. H2-TPR profiles of bimetallic catalysts presented a deviation to lower temperatures and new reduction peaks appeared. As for Co-Ni, a shoulder located around 200\u2013275\u00a0\u00b0C was detected and assigned to the reduction of NiO to Ni0\n[27]. Compared to other bimetallic catalysts, the reduction of the PtOx species over Co-Pt catalyst was identified at much lower temperature, <200\u00a0\u00b0C [28]. Regarding to Fe-Co catalyst, the reduction peaks located around 280\u00a0\u00b0C and 400\u00a0\u00b0C were assigned to the reduction of FexOy species [29]. According to these results, it can be inferred that cobalt oxide particles have a different interaction degree with the promoted La2O3-Al2O3 support and strong Co-X bonds benefit Co reduction. It was well reported that La2O3 on Co/Al2O3 increased catalyst reducibility [30]. On the other hand, the total percentage of catalyst reduction is presented in Table 2\n. At the selected reduction temperature of 500\u00a0\u00b0C, all bimetallic catalysts, except Co-Fe, showed a high reducibility (\u226574\u00a0%) compared to the monometallic Co analogue (\u224869\u00a0%). Furthermore, as the total reduction was only achieved for Co-Pt, it was inferred that the reduced catalyst structure of Co, Co-Ni and Co-Pt were composed by a mixture of metallic oxide particles (CoO, La2O3-Al2O3) and active metal sites in a single form (Co) for the monometallic Co and alloy form (CoNi, CoFe, CoPt) for bimetallic catalysts.The X-ray diffraction patterns of the series of catalysts in their calcined states, are reported in Figure SI2. The addition of La2O3 did not give rise to crystalline phases and only contributed to the reduction in the intensity of the \u03b3-Al2O3 reflections. The [220], [311], [222], [400], [511] and [440] crystal planes corresponding to \u03b3-Al2O3 phase (JCPDS:00\u2013010-0425) were identified at 2\u03b8\u00a0=\u00a032.35, 37.90, 39.11, 46.15, 61.25 and 67.25\u00b0, respectively. In addition to \u03b3-Al2O3, Co3O4 phase was detected in all the Co-based catalysts. The reflections of the Co3O4 phase (JCPDS:00\u2013043-1003) were recognized at 2\u03b8\u00a0=\u00a031.24, 36.96, 44.83, 59.17 and 65.18\u00b0, corresponding to the [220], [311], [400], [400] and [440] crystal planes. In the bimetallic catalysts, the Co3O4 phase was shifted to the left (see Figure SI3), indicating a change in the lattice parameter of this phase. The lattice CO3O4 deviation can be caused by its interaction with the NiOx, PtOx, and FexOy atoms of the second metal phase. Furthermore, no well-defined reflections linked to the oxide phase of the second metal were detected in the bimetallic catalysts. The absence of these reflections indicates that the metal oxide species could be present in an amorphous phase, in a highly dispersed crystalline phase or in the formation of a mixed oxide. Therefore, in order to confirm the reduction of metal oxides and the formation of CoX alloys, the structural properties of all catalysts in their reduced state were also evaluated.XRD patterns of the reduced catalysts are shown in Fig. 4\n. After the reduction of the samples, Al2O3 and La2O3 phases related to the support and promoter were respectively detected. The new reflection of La2O3 phase (JCPDS:00\u2013050-0602) was located at 2\u03b8\u00a0=\u00a028.59\u00b0. Besides Al2O3 and La2O3 phases, it was expected the presence of CoO, as most of the catalysts were not totally reduced at 500\u00a0\u00b0C, according to TPR results. However, this metal oxide phase cannot be identified over the reduced samples. The absence of this reflection was attributed to its highly dispersed crystalline phase. In contrast, metallic Co was identified in both mono and bimetallic catalysts. The [111] and [200] crystal planes of the Co phase (JCPDS:00\u2013015-0806) were detected at 2\u03b8\u00a0=\u00a044.21 and 51.52\u00b0, respectively. Interestingly, in the reduced bimetallic catalysts, new reflections attributed to the formation of CoX alloys were identified. The main characteristic reflections appearing at 44.50\u00b0, 41.66\u00b0 and 44.83\u00b0 correspond to CoNi [111] (JCPDS:00\u2013010-8308), CoPt [101] (JCPDS:00\u2013043-1358) and CoFe [110] (JCPDS:00\u2013044-1483), respectively. In particular, in the reduced Co-Pt, three reflections were additionally located at 25.81, 30.59 and 34.06\u00b0 and assigned to PtCl4 [131], [240] and [241] crystal planes (JCPDS:00\u2013030-0886); indicating that the chemical precursor was still present in Co-Pt catalyst.The metallic crystallite sizes of Co and alloys (CoNi, CoPt and CoFe) were calculated from XRD patterns using the Scherrer\u2019s equation. A crystallite size of 9.05\u00a0nm was estimated for the reduced Co catalyst. For the reduced bimetallic catalysts, the interaction of Co and the second metal (X: Ni, Pt and Fe) over promoted La2O3-Al2O3 support led to the formation of CoX crystallites with sizes higher than 10\u00a0nm, suggesting that the structure of the bimetallic Co-X phases were preferentially conformed by CoX alloys. As it is shown in Table 2, CoPt crystallite size (16.42\u00a0nm) was much higher than that estimated for CoNi (14.12\u00a0nm) and CoFe (10.43\u00a0nm), causing an inferior metal dispersion over the bimetallic catalysts. In particular, the low Pt dispersion identified over Co-Pt can be also influenced by the presence of PtCl4. This compose has measurable vapor pressure and is mobile, and therefore susceptible to segregation [31]. On the other hand, the active metal content (>8\u00b710-6 mol/g) estimated from SEM-EDX, XRD, and TPR data, suggested that the percentage of reduction of the catalysts is a key point for their performances.The catalytic performance of the different catalyst formulations, the support and the promoted support was evaluated on the synthesis of HC-SG from both CO2 and CO as carbon sources at different reaction temperatures.All the catalysts, Co, Co-Ni, Co-Pt and Co-Fe, were active at the selected conditions and CO2 conversions always increased with temperature (see Fig. 5\n). Overall, the catalytic activity followed this order: Co-Ni\u00a0>\u00a0Co\u00a0>\u00a0Co-Fe\u00a0>\u00a0Co-Pt \u226b promoted support. Co-Ni was the most active, achieving a maximum CO2 conversion of 49.31\u00a0%. Therefore, the strategy of adding a second active phase only seems to be beneficial in the case of Ni, in the view of the CO2 conversion results.The main product species measured were CO, CH4, and C2-C4, whereas large C5+ hydrocarbons were not detected from CO2 hydrogenation[323232]. In contrast, the La2O3 promoted support was not able to form hydrocarbons. Fig. 6\n shows the product distribution of the different catalysts and temperature conditions, and it reveals that low temperatures were preferred to produce C2-C4 hydrocarbons. It can be observed that the monometallic Co catalyst was the less selective towards C2-C4 hydrocarbons. Therefore, this catalytic behaviour revealed that the incorporation of the second metal was a positive strategy in terms of selectivity to C2-C4 hydrocarbons. At the other end, a very different mixture, which was composed by CO and C2-C4 hydrocarbons species were formed over Co-Pt catalyst. In the case of Co-Ni, the most active catalyst, it was preferentially selective to form CH4. A similar behaviour was identified for bimetallic Co-Fe, which displayed a drop in C2-C4 hydrocarbon selectivity as temperature increased.\nFig. 7\n shows the DRIFTS spectra recorded over the Co-based catalysts at a temperature of 250\u00a0\u00b0C using CO2 as carbon source. At this temperature, methane is the main product of the hydrogenation reaction, confirmed by its characteristic peaks at 3015 and 1314\u00a0cm\u22121 present in all the spectra, which is well aligned with the results obtained in the catalytic experiments. There are, however, different species adsorbed at the surface of the catalysts at every temperature that account for the different reactivity observed in the catalytic experiments. Over monometallic Co (see Figure SI4), besides generation of methane above 200\u00a0\u00b0C, carbonate species (1700\u20131340\u00a0cm\u22121) and accumulation of physisorbed water (3240\u00a0cm\u22121) were also observed on the surface of the catalysts. At 250\u00a0\u00b0C, a new peak was identified at 1340\u00a0cm\u22121 and assigned to monodentate carbonate species. This characteristic peak was also observed, although less intense, over bimetallic catalysts. It should be noted that in Co-Ni, release of methane is observed from 150\u00a0\u00b0C (see Figure SI5), proving the high activity of this catalyst towards the methanation of CO2. At the same time, the ill-defined bands between 1700 and 1400\u00a0cm\u22121 are attributed to the presence of carbonate and carboxylates species adsorbed on the support [33]. A comparable behaviour is observed for the Fe-Co catalyst, which displays similar and less intense peaks (see Figure SI6). On the other hand, when using the Co-Pt catalyst, coordination of CO on Pt sites is indicated by the presence of a peak at 2070 and a shoulder at 1990\u00a0cm\u22121\n[34] (see Figure SI7). This observation is in line with the catalytic experiments, as the Co-Pt catalyst is the only one that significantly yielded CO as product at all the temperatures studied. It can be therefore inferred that the Co-Pt bimetallic catalyst facilitates the rWGS reaction [35], which explains the lower production of methane of this catalyst. Two broad bands centred at 1560 and 1375\u00a0cm\u22121, that decrease in intensity at higher temperatures, are attributed to the adsorption of formate species on the promoted support [36].Catalytic performance using CO as carbon source is displayed in Fig. 8\n. In comparison to CO2, the use of CO as a carbon source was very advantageous in terms of gas reactivity. The achieved CO conversion was very dependent on the temperature and ranged between 0.89 and 90.65\u00a0%, much higher values with respect to CO2 conversion (<50\u00a0%). In the present reaction system, monometallic Co was more active than the bimetallic Co-X catalysts, implying that Ni, Pt and Fe are less active when CO is used as carbon source. These results are in correlation with the literature since cobalt-based catalysts are usually found as an active catalyst for mixtures H2/CO in the FTS process [37]. CO conversion on the studied catalysts at all the used temperatures complies with the following order: Co\u00a0>\u00a0Co-Ni\u00a0>\u00a0Co-Fe\u00a0>\u00a0Co-Pt \u226b promoted support.Selectivity from COhydrogenation is presented in\nFig. 9\n. Besides conversion, selectivity to C2-C4 hydrocarbons was also enhanced by the utilization of CO as a carbon source. Species such as CO2, CH4, C2-C4 and even C5 were detected in the evaluated temperature range of 200\u2013300\u00a0\u00b0C. In this case, the promoted support was preferentially selective to form small amounts of CO2. The best results of selectivity to C2-C4 hydrocarbons were achieved over bimetallic catalysts at 250\u00a0\u00b0C, being the Co-Ni the most promising compared to Co-Pt and even more than Co-Fe. However, its important to note that in terms of hydrocarbon selectivity, the Co-Fe shows competitive values at the higher tested temperature of 300\u00a0\u00b0C, implying that Fe was beneficial to form C2+ hydrocarbons and Ni was also favourable to form CH4. Therefore, the addition of a second metal as a catalyst design strategy was proved to improve the selectivity towards the formation of C2-C4 hydrocarbons. Similar to CO2 hydrogenation, low temperatures are preferred to favour C2-C4 hydrocarbon production.\nFig. 10\n shows the DRIFTS spectra collected for the Co-based catalysts at a reaction temperature of 250\u00a0\u00b0C. In the hydrogenation of CO, peaks related to hydroxyl groups (400\u20133500\u00a0cm\u22121) and CO species adsorbed on Lewis acid sites (1606 and 1573\u00a0cm\u22121) and Br\u00f8nsted acid sites (1651\u00a0cm\u22121) of the La2O3-Al2O3\n[3839] support were identified. Furthermore, release of hydrocarbons is observed by the characteristic \u03bd(CH) modes of methyl (CH3) and methylene (CH2) groups at 2958, 2924 and 2850\u00a0cm\u22121\n[40 41], respectively, as well as methane at 3015 and 1305\u00a0cm\u22121 (see Figure SI8). Compared to monometallic Co, new peaks attributed to CO species adsorbed on Lewis acid sites (1629, 1620, 1610, 1492 and 1450) and strong Br\u00f8nsted acid sites (1639\u00a0cm\u22121) were identified over Co-Ni [3839]. The production of CH4 and hydrocarbons (2990 and 2968\u00a0cm\u22121 (methyl), 2896, 2873 and 2862\u00a0cm\u22121 (methylene)) was mainly visible at temperatures above 200\u00a0\u00b0C [42] (see Figure SI9). Formate species detected 1585\u00a0cm\u22121 were related to the formation of methane as it exhibited an analogous behaviour to the methane band. In addition, the signal at 2360\u00a0cm\u22121 detected at all the temperatures studied is attributed to formation of gaseous CO2, which indicates that the water gas shift (WGS) reaction takes place from very low temperatures. At 300\u00a0\u00b0C, the signal of CO2 significantly increases in intensity while that of methane was maintained, and those of methyl and methylene groups even slightly decrease, which could suggest a strong competition between the WGS reaction and the FT reaction at this temperature. When the Co-Fe catalyst is used, formation of CO2 and water is observed by the broad band centred at 3250\u00a0cm\u22121. The series of multiple peaks between 1700 and 1200\u00a0cm\u22121 can be assigned to carbonate species adsorbed on the surface of the promoted support (see Figure SI10). For the Co-Pt catalyst, adsorption of linear CO species on Pt sites of different natures is detected by the presence of a peak at 2080\u00a0cm\u22121 and a shoulder at 2057\u00a0cm\u22121\n[43], appearing at higher temperatures (see Figure SI11). Generation of methane and longer hydrocarbons is observed at temperatures above 200\u00a0\u00b0C by the appearance of peaks at 3015, 2960, 2930 and 2870\u00a0cm\u22121, along with a broad band centred at 3240\u00a0cm\u22121 and attributed to water, which is product of the C2-C4 formation reactions. Again, gaseous CO2 is observed by the peaks at 2354 and 2320\u00a0cm\u22121, as a result of the water gas shift reaction.According to these results, the promising production of CH4 and C2-C4 hydrocarbons over bimetallic Co-X can be attributed to CO adsorbed on Lewis and Br\u00f8nsted active sites. For the promoted support (See Figure SI12), the peaks associated with the CO adsorption on La2O3-AL2O3 surface around 1700\u20131400\u00a0cm\u22121 was enhanced as temperature increased from 200 to 300\u00a0\u00b0C. However, with the addition of the second metal phase, the peaks of Lewis and Br\u00f8nsted were different, indicating that the acid strengths differed between monometallic and bimetallic samples. Between Co and Co-Ni, the presence of new peaks and the difference in intensities indicates a difference in the amount of acid sites between, and thus in the formation of C2-C4 hydrocarbons (see Figure SI13).In summary, the main highlightsof thecatalytic resultsobtained at the selected conditions are the following:\n\ni)\nIn both cases (CO2 or CO), competitive C2-C4 hydrocarbons selectivities were achieved using as low as possible temperatures at the expense of the conversion.\n\n\nii)\nCO as carbon source was beneficial in terms of activity and C2-C4 hydrocarbons selectivity.\n\n\niii)\nCo-Ni was identified as the most promising catalyst as led to an enhanced production of CH4 and C2-C4 hydrocarbon species, compared to the rest of Co-X bimetallic and monometallic Co catalyst.\n\n\niv)\nDRIFTS experiments revealed that the chemical properties of promoted support have close relationships with the COx activation as different carbon species adsorbed on Lewis and Br\u00f8nsted active sites can be identified over the La2O3 promoted Co-X based catalysts. Furthermore, it was confirmed by DRIFTS that the addition of the second metal promoted the formation of species CH4 and C2-C4 hydrocarbons. The most promising HC-SG production detected over bimetallic Ni-Co can be attributed to the long-chain hydrocarbons typically formed on Co and effectively hydrocracked by Ni, which is known to be active in C\u00a0\u00a0C bond cleavage [44].\n\n\nIn both cases (CO2 or CO), competitive C2-C4 hydrocarbons selectivities were achieved using as low as possible temperatures at the expense of the conversion.CO as carbon source was beneficial in terms of activity and C2-C4 hydrocarbons selectivity.Co-Ni was identified as the most promising catalyst as led to an enhanced production of CH4 and C2-C4 hydrocarbon species, compared to the rest of Co-X bimetallic and monometallic Co catalyst.DRIFTS experiments revealed that the chemical properties of promoted support have close relationships with the COx activation as different carbon species adsorbed on Lewis and Br\u00f8nsted active sites can be identified over the La2O3 promoted Co-X based catalysts. Furthermore, it was confirmed by DRIFTS that the addition of the second metal promoted the formation of species CH4 and C2-C4 hydrocarbons. The most promising HC-SG production detected over bimetallic Ni-Co can be attributed to the long-chain hydrocarbons typically formed on Co and effectively hydrocracked by Ni, which is known to be active in C\u00a0\u00a0C bond cleavage [44].Therefore, the characterization of the materials indicated that the addition of Ni and Pt on Co-based catalyst improved its reducibility, while the addition of Fe was noticed to enhance its metal dispersion. Furthermore, the modification of the Al2O3 support with La2O3 promotes the formation of CoX alloys, favouring the hydrogenation reaction at low temperatures and controlling methane and hydrocarbon selectivity production. Finally, it can be claimed that the high catalytic activity and preferential selectivity to C4 and C2-C4 hydrocarbons of the Co-Ni was due to the formation of CoX alloy, high reducibility (73.82\u00a0%) and suitable active metal content (9.65x10-6mmol/g).A summary of the product distribution over the series of catalyst at the most promising reaction temperature is presented in Table 3\n. Product distribution at the rest of temperatures can be found in supporting information, Table SI1. Competitive HC-SG mixtures were successfully achieved during the hydrogenation of CO. In particular, a gas product with a HHV of 57.90\u00a0MJ/Nm3 was achieved under CO hydrogenation and using the bimetallic Ni-Co as catalytic material at 250\u00a0\u00b0C. The HHV of the generated HC-SG was in the range of the reported ones (<57.72\u00a0MJ/Nm3), which operated at very low gas hourly space velocities (GHSV\u00a0=\u00a06,000 NmL/gcat\u00b7h) and using non-promoted bimetallic Fe-Zn/Al2O3\n[4] and tri-metallic Ni-Co-Fe/Al2O3 systems [21]. Therefore, the addition of La2O3 to the traditional bimetallic system based on Co-X/Al2O3 was found to be positive, since higher GHSVs can be used during the hydrogenation of CO [23].In contrast, the use of CO2 as carbon source seems more challenging. In this case, the maximum HHV was also achieved over bimetallic Ni-Co (39.73\u00a0MJ/Nm3) at the lowest temperature, 200\u00a0\u00b0C, being significantly lower than the use of CO as carbon source. These results also reveal the lower CO2 conversion values compared to CO as carbon source. As previously described, Co-Pt catalyst favors CO and C2-C4 formation. However, experiments on CO indicated that part of the generated CO would be converted back to CO2, therefore reducing the global CO2 conversion. According to these results, it seems that the utilization of a single catalyst seems not feasible for CO2 hydrogenation as the selectivity to C2-C4 is limited or rWGS reaction to CO is favoured, restricting the HHV obtained.The implementation of a dual catalytic bed configured by two different Co-X catalysts can be an interesting strategy when using CO2 as carbon source, as reported by Gao et al. [45]. In the present HC-SG reactor engineering concept, the catalytic bed would be composed by two zones, which will work to different reaction conditions. A schematic representation is shown in Figure SI14. The first one is denoted as the CO2 decomposition zone and designed to favour the conversion of CO2 to CO and CHx (x\u00a0=\u00a01,2,3) species. In this zone, the bimetallic catalyst based on Co-Pt can be used to guarantee the reactive mixture composition. As the temperature is a key reaction condition to achieve high CO selectivities, the temperature of the catalytic bed in this zone can be fixed at 200\u00a0\u00b0C. After the first zone, in the same catalytic bed, a second zone denoted as the HC-SG formation zone is designed to favour the conversion of CO to HC-SG (CH4 and C2-C4 hydrocarbons). As Ni-Co exhibited the most promising HC-SG production, this can be the bimetallic catalyst implemented in the second zone. Compared to the previous one, the hydrogenation of CO to CH4 and C2-C4 hydrocarbons over Co-Ni should be performed at a higher temperature 250\u00a0\u00b0C.Unfortunately, the inefficient catalytic performance under the H2/CO2 mixture has been also identified by other reported HC-SG catalysts, see Table SI2. Literature suggested that reaction temperatures higher than 250\u00a0\u00b0C and pressures of 30\u00a0bar\u00b7g should be used to achieve relatively high conversions (<42\u00a0%). A comparison of HC-SNG productivity of the estate-of-the-art of catalysts is shown in Fig. 11\n. For both cases, CO2 or CO as a carbon source, the relationship between GHSV and HC-SG selectivity positioned the Co-Ni as a rentable material since significant productivity can be achieved by the implementation of technically feasible reaction conditions, leading to an HC-SG process economically profitable to be scaled-up at industrial levels. The productivity of the Co-Ni was around 8.08x102 mL/gcat\u00b7h using CO2 and 5.15x103 mL/gcat\u00b7h using CO.In this work, HC-SG synthesis was performed over a series of bimetallic Co-X (X\u00a0=\u00a0Ni, Pt and Fe) catalysts for the selective production of CH4 and C2\u00a0\u2212\u00a0C4 hydrocarbons from CO2 and CO as carbon sources. Catalytic results indicated that the utilization of CO as carbon source is very positive in both conversion and C2-C4 hydrocarbon selectivities. Among catalysts, Co-Ni was the most promising catalyst for production of HC-SG. Therefore, the strategy of adding a second metal proved to the positive. At H2/CO\u00a0=\u00a03, T\u00a0=\u00a0250\u00a0\u00b0C, and P\u00a0=\u00a010\u00a0bar\u00b7g, very interesting selectivities to CH4 (40.01\u00a0%) and C2\u2013C4\nhydrocarbons (50.04\u00a0%) were obtained, with a reduced selectivity to CO2 (5.05\u00a0%) and C5+ (4.89\u00a0%) formation. In this direction, a competitive HC-SG with a heating value of 57.90\u00a0MJ/Nm3 was achieved using Co-Ni bimetallic catalysts.The successful catalytic performance was attributed to the acid-basic sites formed on the catalyst surface by the synergic effects caused by the presence of La2O3 and CoNi alloy phases, which favours in the production of CH4 and C2-C4 hydrocarbons under lower temperatures. Besides, the bimetallic Ni-Co catalyst showed higher reducibility (73.82\u00a0%) and active metal content (9.65x10-6mmol/g). The findings from this study contribute to our understanding of the low temperature CO2\nand CO hydrogenation activities ofLa2O3 promoted Co-X/Al2O3\nbased catalysts and provide insights for the design of materials for HC-SG production.Bimetallic Co-Ni catalyst can be used as a benchmark to optimize or design novel reactor approaches for the HC-SG process intensification. In the case of using CO2 as carbon source, an adapted HC-SG reactor concept, configured by two Co-X catalytic zones is proposed to promote the use of CO2 as carbon source. A first Co-Pt catalytic zone operated at 200\u00a0\u00b0C to favour the conversion CO2 to CO and CHx (x\u00a0=\u00a01,2,3) species, followed by a second Co-Ni catalytic zone operated at 250\u00a0\u00b0C to achieve the conversion CO to HC-SG (CH4 and C2-C4 hydrocarbons). However, further studies should be carried out for the validation of this reactor engineering concept in a full-scale reactor. In any case, the use of bimetallic catalysts is interesting to divert selectivity towards the most desired products on each occasion.\nAndreina Alarc\u00f3n: Writing \u2013 original draft, Investigation, Formal analysis, Visualization. Olatz Palma: Investigation, Validation. Elena Mart\u00edn Morales: Investigation, Formal analysis, Writing \u2013 review & editing. Mart\u00ed Biset-Peir\u00f3: Methodology, Resources. Teresa Andreu: Conceptualization, Validation, Supervision, Funding acquisition, Writing \u2013 review & editing. Jordi Guilera: Writing \u2013 original draft, Conceptualization, Methodology, Resources, Supervision, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by project TED2021-132365B-I00, funded by MCIN/AEI/10.13039/ 501100011033 and by the European Union \u201cNextGenerationEU\u201d/PRTR and PID2019-108136RB-C33 (MCIN/AEI/10.13039/501100011033). Andreina is grateful for support by the Margarita Sala Grant funded by the University of Barcelona (UNI/551/2021). The authors thank SASOL for kindly providing alumina support material (Puralox). Authors kindly thank Dr. Albert Llorente for assistance withthe characterization of materials.Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2023.127726.The following are the Supplementary data to this article:\n\nSupplementary Data 1\n\n\n\n", "descript": "\n                  A new catalytic route for the production of a high-calorie synthetic gas (40\u201360\u00a0MJ/Nm3), composed by C1-C4 hydrocarbons, has industrial interest for gas applications and locations with high heating requirements. In this work, a series of bimetallic Co-X (X\u00a0=\u00a0Ni, Pt and Fe) catalysts supported on La2O3 promoted Al2O3 micro-spheres were evaluated using both CO2 and CO carbon sources under mild temperature (T\u00a0=\u00a0200\u2013300\u00a0\u00b0C), moderate pressure (P\u00a0=\u00a010\u00a0bar\u00b7g) and relatively high gas hourly space velocity (40,000\u00a0N\u00a0mL/gcat\u00b7h). Experimental results proved that the incorporation of nickel as a second metal is beneficial for high-calorie gas application. Besides, catalytic results showed that the utilization of CO as carbon source is beneficial in both conversion and C1-C4 hydrocarbon selectivities. Co-Ni presented the most interesting results, leading to a heating value of 57.9\u00a0MJ/Nm3 (40.01\u00a0% CH4 and 50.04\u00a0% C2-C4 hydrocarbon) at 250\u00a0\u00b0C through CO hydrogenation. The enhanced catalytic performance achieved over bimetallic Co-Ni was attributed to CoNi alloy catalytic activity, high reducibility (73.82\u00a0%), active metal content (9.65x10-4 mmol/g) and appropriate acid-basic sites for COx activation. In contrast, the conversion of CO2 to high-calorie gas was found to be more challenging and lower gas heating values were achieved (39.73\u00a0MJ/Nm3). In this case, an adapted reactor concept using a dual bimetallic catalyst and different reaction conditions is hereby proposed to shift selectivity towards the targeted products. This findings represent a step forwards in catalytic engineering for the development of high-calorie synthetic gas reactors.\n               "}
{"full_text": "Conversion of low-cost biomass-derived oxygenates, such as glycerol (C3H6O8), to hydrogen (H2) is a promising route for making eco-friendly renewable H2 fuels, being able to increase the share and availability of clean energy while lowering the greenhouse gas (GHG) emissions [1]. Glycerol is commonly produced as an organic waste from biodiesel production processes over homogeneous alkaline catalysts (via transesterification of lipids such as plants oils and/or animal fats with alcohols), and hence conversion of glycerol to H2 can undoubtedly improve the economics of biodiesel production [2,3]. Steam reforming of glycerol (SRG) is a promising and industrially important reaction, in which renewable H2 or synthesis gas can be produced to valorise glycerol [4]. Importantly, H2 produced from SRG could not contribute to global warming based on the assumption of using crude glycerol generated as a biowaste of biodiesel mass-production. According to the overall reaction of SRG (Eq. 1), the relative proportion of hydrogen in crude glycerol (theoretically, 7\u00a0mol of hydrogen which can be produced from every 1\u00a0mol of crude glycerol) makes it not only an advantageous option for producing renewable H2, but also more economically and environmentally competitive compared to fossil fuels, e.g., methane [5]. Additionally, steam reforming is a well-established technology, indicating that the feasibility of shifting the current fossil feedstock to glycerol without significant modification of the current infrastructure.\n\n(1)\n\n\nC\n3\n\n\nH\n8\n\n\nO\n3\n\n+\n3\n\n\nH\n2\n\nO\n\u2192\n3\n\n\nCO\n2\n\n+\n7\n\n\nH\n2\n\n\n\u0394\n\nH\n298\n\n=\n127.7\n\nkJ\n/\nmol\n\n\n\n\nSRG is endothermic, which is favoured at high reaction temperatures (>600\u00a0\u00b0C [4,6]), and hence is inevitably associated with catalyst sintering and coking issues. Also in comparison with fossil hydrocarbons the reaction network of SRG is rather complex (as shown in Table S1 in the Supporting Information, SI [4,5]), with a higher susceptibility to undesired products formation through many side reactions, which could affects the overall selectivity and yield of H2 [7].Ni-based catalysts are common reforming catalysts because of their good ability to cleave CC, OH and CH bonds in chemical reactions and low cost. However, the performances of Ni-based catalysts frequently depletes from rapid deactivation caused by Ni aggregation (due to sintering) and carbon deposition (induced by undesired reactions such as thermal decomposition of methane and CO reduction, Table S1) in reforming catalysis at high temperatures (typically >800\u00a0\u00b0C) [8,9] such as SRG and methane dry reforming. Therefore, various strategies have been exploited to develop anti-coking and anti-sintering reforming catalysts. Stabilisation of Ni nanoparticles (NPs) and regulation of the particle sizes can be achieved via (i) appropriate selection of promoters [10\u201312], (ii) use of porous supports [13], (iii) doping with the second metallic phase (i.e., bimetallic catalysts) [14\u201316] and (iv) design of unique catalysts structures such as core-shell to improve Ni dispersion and reduce coke deposition on the Ni-based catalysts in various reforming reactions [17\u201321].In SRG, modification of the Ni@\u03b3-Al2O3 catalyst was achieved by exploring the benefits of different preparation procedures and employing promoters to improve the catalyst surface and prevent carbon formation [22,23]. For example, CeO2 was used to dope the conventional Ni@Al2O3 (i.e. Ni@12Ce-Al2O3) catalyst, which enhanced Ni dispersion on \u03b3-Al2O3 support with relatively smaller Ni NPs and showed the improved performance in SRG, and high ability to resist coking compared to the benchmark Ni@\u03b3-Al2O3 [23]. Recently, encapsulation strategies based on porous materials are shown to be promising to prepare highly dispersed yet segregated metal particles, which can prevent metal particles sintering and carbon deposition in high-temperature catalytic reactions effectively including reforming reactions [24], which were exemplified by silicalite-1 zeolite encapsulated Ni catalyst for dry reforming of methane with CO2 [25].Zeolites as a class of porous materials are suitable supports to confine metal NPs, mainly owing to their high porosities and large surface areas [26,27]. However, due to the intrinsic microporosity of zeolites and the associated accessibility/diffusion issues, metal-supported zeolite catalysts prepared using the conventional impregnation procedures often have distribution of large metal-particles on the external surface of the zeolite with poor dispersion of metal particles, which are not ideal for catalysis. Conversely, metal precursors inclusion can also be integrated with the zeolite synthesis cleverly to ensure the encapsulation of metal NPs after reduction. In such encapsulated metal NPs catalysts, segregation of metal NPs using the inorganic crystalline framework can mitigate sintering and improve stability effectively during catalysis. Additionally, if the dimension of the encapsulated metal NPs could be managed, the activity and selectivity of the catalysis can be tuned as well. Although there are many benefits, most of the metal NPs are confined in space with the protective and microporous zeolitic shell, and hence diffusion limitation in these zeolite encapsulated metal catalysts can potentially jeopardise their performance in catalysis [28].Diffusion issues in such zeolite encapsulated catalysts can be addressed by introducing mesoporous structures in them, which can reduce the average diffusion length and enhance the accessibility to the encapsulated metal NPs, being beneficial for catalytic performance and reaction kinetics [29]. Shiwen Li et al. [30] prepared Ni, Co and Cu encapsulated in mesoporous MFI zeolites with hollow structures for catalytic reduction of hydrocarbons particularly toluene and mesitylene with kinetic diameters of 0.58 and 0.87\u00a0nm, respectively. The findings confirmed that the catalytic performance is strongly linked to the characteristic diffusion behaviour of molecules over the MFI zeolite layer, with the silicalite-1 encapsulated Ni NPs being a highly active phase for toluene reduction. In catalytic SRG, the reactant of glycerol has a kinetic diameter of ~0.60\u00a0nm, therefore, diffusion resistance through a zeolite-based catalyst, including the encapsulated ones, can be expected, and introduction of mesoporous structure into the catalysts can be beneficial to the catalysis.Herein, a strategy of developing Ni NPs encapsulated in mesoporous hollow silicalite-1 zeolite catalyst (i.e., Ni@HolSi-1) was explored. In detail, the conventional encapsulated Ni catalyst (i.e., Ni@Si-1) was prepared via one-pot hydrothermal synthesis of silicalite-1 in presence of the Ni precursor, and the mesoporous hollow structure was achieved by treating the prepared Ni@Si-1 catalyst with tetrapropylammonium hydroxide (TPAOH) solution. The encapsulation strategy enabled the formation of highly dispersed ultra-small Ni NPs, and the post-treatment rendered the formation of mesoporous hollow structure, which were highly beneficial to catalytic SRG. The physiochemical features of the investigated catalysts were determined comprehensively by employing various methods and techniques. Comparative and systematic catalytic SRG was performed over the developed catalysts to assess their performance regarding conversion of glycerol, H2 yield/selectivity, and distribution of CO2, CO and CH4, respectively. The findings show that the developed Ni@HolSi-1 catalyst was catalytically active and stable in SRG even after 100\u00a0h on stream, as well as being coke resistant.Pristine Si-1 zeolite was prepared via crystallisation of the synthesis solution, consisting of tetraethyl orthosilicate (TEOS, Sigma-Aldrich, \u226599.0%), tetrapropylammonium hydroxide solution (TPAOH, Sigma-Aldrich, 25\u00a0wt% in H2O) and deionised water, according to a previously reported method [31]. Typically, a mixture, containing TPAOH (~13\u00a0g) and deionised water was prepared under continuous stirring at room temperature (RT). Then 8.32\u00a0g of TEOS was gradually introduced into the aforementioned mixture under continuous stirring in a Teflon beaker (to hydrolyse TEOS fully). The mixture was then transferred and heated in a stainless-steel autoclave lined with Teflon, under the constant hydrothermal conditions (at 170\u00a0\u00b0C and 96\u00a0h). After crystallisation, the final product was recovered through centrifugation, and rinsed with distilled water and ethanol several times before being dried overnight and air-calcined for 8\u00a0h at 550\u00a0\u00b0C.A control catalyst of Ni/Si-1 was synthesised using the conventional method of incipient wetness impregnation (IWI). To impregnate 2\u00a0g of calcined Si-1 zeolite containing 5\u00a0wt% of theoretical Ni loading, the prepared aqueous solution of Ni precursor salt (i.e. nickel nitrate hexahydrate (Ni[NO3]2\u00b76H2O\u00a0\u2265\u00a099%, Sigma-Aldrich) was used to develop a solid product, which was then oven-dried overnight and calcined in air for 8\u00a0h at 550\u00a0\u00b0C.The encapsulated Ni catalyst was synthesised in a single pot by hydrothermal procedure under the same condition using the same protocol for preparing Si-1 zeolite except the addition of the pre-prepared [Ni(NH2CH2CH2NH2)3](NO3)2 solution which was employed as the precursor to achieve the encapsulation of 5\u00a0wt% theoretical Ni loading during the synthesis of Si-1 crystals [25,31,32]. Specifically, 0.95\u00a0g of Ni precursor salt (Ni(NO3)2\u00b76H2O, \u226599%) was dissolved into 10\u00a0mL of aqueous solution containing 2\u00a0ml of ethylenediamine (NH2CH2CH2NH2) until it was fully completed under continuous agitation at RT. Finally, the prepared gel was obtained with a molar proportion of 1 SiO2: 0.4 TPAOH: 35 H2O: 0.045 [Ni(NH2CH2CH2NH2)3]2+).Ni@HolSi-1 was prepared using a post-synthetic treatment method with Ni@Si-1 as the parent material and TPAOH solution [33]. In detail, 20\u00a0mL of 0.3\u00a0M TPAOH aqueous solution was used to treat the as-synthesised Ni@Si-l inside an autoclave at 170\u00a0\u00b0C for 24\u00a0h. After the treatment, the obtained solid product was separated from the solution at RT by centrifugation, rinsed multiple times with deionised water and ethanol, and oven-dried overnight at 80\u00a0\u00b0C. Then, the Ni@HolSi-1 catalyst was achieved following similar drying and calcination conditions as described previously.The crystal patterns of the calcined and reduced catalysts were measured by Powder X-ray diffraction (XRD) on a Philips X'Pert X-ray diffractometer by employing a CuK\u03b11 X-ray source radiation. The analysis of all XRD patterns of the catalysts were matched and compared with the known materials data available in the database (ICDD, JADE 6 software, Materials Data Inc., Livermore, CA). Quantachrome Quadrasorb instrument was employed to detect the N2 adsorption-desorption of the catalysts at \u2212196.15\u00a0\u00b0C, and their textural properties covering specific surface areas (S\nBET) and pore information of the catalysts. Before N2 physisorption, all the catalysts were pre-treated in a vacuum at 350\u00a0\u00b0C for 24\u00a0h. The Brunaur-Emmett-Teller (BET) was used to calculate the S\nBET, and total specific pore volume (V\ntotal) was determined using the adsorbed quantity of N2 at the relative pressure of p/p\n\n0\n\u00a0=\u00a00.99. Similarly, t-plot method was used to calculate the specific micropore volume (V\nmicro), whereas the V\nmicro is taken off the V\ntotal to determine the specific mesoporous volume (V\nmeso). Hydrogen temperature programmed reduction (H2-TPR) profiles of the calcined catalysts were examined on a ChemBET Pulsar TPR/TPD equipment (ChemBET-3000), and all the catalysts were initially degassed as reported elsewhere [23]. Comparative characterisation of the surface acidity of fresh catalysts was carried out using NH3 temperature-programmed desorption (NH3-TPD) experiments in the same instrument employed for H2-TPR measurements, as previously described elsewhere [23]. For each catalyst under investigation, the surface areas and dispersions of Ni were determined using the same H2 pulse chemisorption methodology, as previously reported elsewhere [23]. Inductively coupled plasma optical emission spectrometry (ICP-OES, Plasma Quant PQ 9000) was used to measure the actual quantity of Ni available in the calcined catalysts. Typically, the catalysts were microwave-digested in a mixture of acidic solution prior to ICP analysis. Bruker Vertex 7.0 fourier transform infrared (FT-IR) spectrometer was employed to collect the spectroscopies of the calcined catalysts, with a scanning wavenumber (ranging from 400 through 4000\u00a0cm\u22121) and a spectral resolution within the interval of 4\u00a0cm\u22121. Electron microscopes such as scanning electron microscope (SEM, Tescan Mira FEG operating at 5\u00a0kV accelerating voltage) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F20 electron microscope at 300\u00a0kV) were used to examine the morphologies of the catalysts. Prior to SEM measurement, all catalysts were coated using platinum (Pt) metal and the corresponding EDX spectrum (energy-dispersive X-ray spectroscopy) were detected with an Oxford Ultim\u00ae Max system. ESCALAB\u2122 250Xi electron spectrometer (Thermo Scientific) was used to analysed the X-ray photoelectron spectroscopy (XPS) spectra of the catalysts using Al-K\u03b1 (hv\u00a0=\u00a01486.6\u00a0eV) as the source of X-ray radiation during the measurements. C1s peak was used as the XPS calibration standard with a fixed binding energy (BE, at ~284.6\u00a0eV). Thermogravimetric analysis (TGA) experiments were conducted on a thermal system (TGA 550) in air atmosphere (at 40\u00a0mL\u00a0min\u22121) with a heating rate of 10\u00a0\u00b0C\u00a0min\u22121 from RT to 900\u00a0\u00b0C. Before the analysis, all species including adsorbed water and other physisorbed molecules were removed from the spent catalysts, and the amount of carbon deposition was determined based on the derivative weight reduction at temperatures above 500\u00a0\u00b0C.Catalytic SRG over different catalysts was carried out in a continuous-flow quartz tubular-reactor (12\u00a0mm I.D. \u00d7 450\u00a0mm length) and atmospheric pressure, as described previously elsewhere [23]. Specifically, the temperature of the packed bed was observed using a K-type thermocouple (OMEGA\u00ae), and the bed was supported by quartz wool in a tubular furnace (Carbolite, EVT-12). Typically, ~200\u00a0mg of each catalyst was crushed and packed in form of pellets (~250\u2013425\u00a0\u03bcm), and then treated in situ from RT to 800\u00a0\u00b0C under H2 flow (at 100\u00a0mL\u00a0min\u22121 STP). Then the carrier gas (N2 at 50\u00a0mL (STP) min\u22121) was employed to sweep the H2 gas until the temperature of the bed was completely set to the required reaction temperature. Following that, a mixture of steam and glycerol (SGFR\u00a0=\u00a010:1) was injected into a vaporiser that was held at 320\u00a0\u00b0C at a continuous feed flow rate of 6\u00a0mL\u00a0h\u22121, using a syringe pump (Harvard Apparatus, PHD ULTRA). During SRG, the flowrates of gases (with a total gas hourly space velocity (GHSV) of 3120\u00a0h\u22121 (STP)) were maintained using mass flow controllers (MKS instruments). The outlet stream of the reaction was analysed one hour after the reaction condition was changed, in the temperature interval 500\u2013750\u00a0\u00b0C, using 50\u00a0\u00b0C increments) under steady-state conditions. Gas chromatography (PerkinElmer Clarus\u00ae 580) equipped with HayeSep DB 100/120 mesh and Shin Carbon ST 100/120 mesh columns, thermal conductivity (TCD) and flame ionisation (FID) detectors, was used to examine the gas composition of the outlet stream. The condensable products from SRG (for example, unreacted glycerol) were continuously collected at the reactor's outlet stream by a water trap cooled by a circulating bath. Subsequently, the condensed unreacted glycerol was analysed off-line by GC (Agilent 7820A, in a 30\u00a0m long, 0.32\u00a0mm I.D. Stabil-wax column, operated with the program: 5\u00a0min at 100\u00a0\u00b0C, heating to 10\u00a0\u00b0C\u00a0min\u22121 up to 180\u00a0\u00b0C, keeping this temperature for 15\u00a0min, using FID). The performance of catalytic SRG was evaluated by measuring the dry gas flowrate with a bubble flowmeter, and the glycerol conversion (X\n\nG\n), selectivity (S) and yield (Y) of the gaseous products were specified in Table 1\n. Kinetic experiments were performed to establish the specific reaction rate and activation energy for SRG systems employing different catalysts over temperatures ranging from 300 to 450\u00a0\u00b0C. However, in order to avoid the diffusion limitation during SRG reaction and obtain relevant information close to intrinsic kinetic, a tubular reactor containing a small amount of the catalysts under investigation (i.e., 12\u00a0mm I.D. \u00d7 12.5\u00a0mm length, with 100\u00a0mg of the pelletized Ni/Si-1, Ni@Si-1 and Ni@HolSi-1 catalysts and bed volume of about 1.4\u00a0cm3) was used for kinetic studies (with a flow rate\u00a0=\u00a012\u00a0ml (STP) h\u22121, GHSV\u00a0=\u00a010,360 (STP) h\u22121) at low conversions below 20% to preclude the effect of mass and heat transfer limitations.XRD technique was used to characterise the crystal structures of the Si-1, the calcined and reduced catalysts, and their corresponding diffraction patterns are illustrated in Fig. 1\n. In the case of all calcined samples, the diffraction phases associated with the MFI-type zeolite framework was well-recognised at 2\u03b8\u00a0<\u00a040\u00b0 (JCPDS 44\u20130696), proving that the retained silicalite-1 phase after the thermal treatments (Fig. 1a). The diffraction peak associated with NiO species (at 2\u03b8\u00a0=\u00a0~43.4\u00b0) was detected clearly in the structures of the calcined Ni/Si-1 catalyst developed by the impregnation method (shaded by the orange rectangle in Fig. 1b), whilst it was not found in Ni@Si-1 and Ni@HolSi-1, which were prepared via encapsulation. This suggests the presence of homogeneously distributed NiO species within the Si-1 framework of the two catalysts. After reduction, the crystalline structure of Si-1 support was maintained in the catalysts, as shown in Fig. 1c. Same findings regarding the structural integrity of Si-1 were also obtained by FT-IR analysis (Fig. S1). Fig. 1d shows that the corresponding diffractions phases of metallic Ni (at 2\u03b8\u00a0=\u00a0~44.8\u00b0, shaded by the orange rectangle) was measured in the reduced Ni/Si-1 rather than the encapsulated Ni@Si-1 and Ni@HolSi-1 catalysts, showing the possible formation of widely dispersed small Ni NPs in Si-1 zeolite.\nFig. 2\n and Fig. S2 shows the morphologies and the associated EDX mapping assessment of the catalysts under investigation. The morphology of Si-1 zeolite was very spherical in shape with an average crystal sizes of ~0.28\u00a0\u03bcm (Fig. S2). After Ni impregnation, the impregnated Ni/Si-1 catalyst has lower particle sizes of about ~0.18\u00a0\u03bcm to that of Si-1 (Fig. 2a), and a large amount of Ni species was found on Ni/Si-1, as observed in the EDX mapping (Fig. 2b). The encapsulated Ni@Si-1 particles via the one-pot synthesis have larger sizes (~1.9\u00a0\u03bcm, Fig. 2c) in comparison to the particle sizes in Ni/Si-1. Comparatively, the particle size of the Ni@HolSi-1 catalyst after the post-synthetic treatment (of Ni@Si-1) was reduced at ~0.30\u00a0\u03bcm, as shown in Fig. 2e. EDX analysis of Ni@Si-1 and Ni@HolSi-1 (Fig. 2d and Fig. 2f) show that the surface Ni species in the two catalysts are less dense than the one of Ni/Si-1, showing that proportion of Ni species were likely encapsulated within the Si-1 framework which cannot be detected by surface EDX elemental mapping.\nFig. 3a presented the N2 adsorption-desorption isotherms, and the associated textural and structural characteristics of the materials are summarised in Table 2\n. Physisorption isotherms of Si-1, Ni/Si-1 and Ni@Si-1 exhibit the shape close to Type-1 isotherm for microporous materials. Conversely, a H2 hysteresis curve (closes at p/p\n\n0\n\u00a0=\u00a00.45 in the desorption branch) was found in the isotherm of Ni@HolSi-1 (type IV), which proves the presence of mesoporous structure in the catalyst. The Ni/Si-1 with values of the S\nBET and V\ntotal (S\nBET\u00a0=\u00a0522\u00a0m2\u00a0g\u22121 and V\ntotal\u00a0=\u00a00.56\u00a0cm3\u00a0g\u22121) are slightly lower than that of Si-1 (S\nBET\u00a0=\u00a0698\u00a0m2\u00a0g\u22121 and V\ntotal\u00a0=\u00a00.61\u00a0cm3\u00a0g\u22121, Table 2) which could be due to pore clogging caused (by Ni deposition, as shown by the reduced V\nmicro, values from 0.23 to 0.16\u00a0cm3\u00a0g\u22121). Synthesis of Si-1 in the presence of Ni precursor caused the decrease in the BET specific surface area, i.e., S\nBET\u00a0=\u00a0347\u00a0m2\u00a0g\u22121 for Ni@Si-1, as well as the reduced micropore volume (V\nmicro\u00a0=\u00a00.10\u00a0cm3\u00a0g\u22121) in comparison with that of Si-1 (Table 2). Compared to Si-1, the micropore volume in Ni@Si-1 was decreased by ~57%, indicating that after the one-pot hydrothermal synthesis, the encapsulated Ni clusters could preoccupy some spaces inside the Si-1 zeolite (e.g., species of metal confined within zeolite crystals and/or encapsulated within hollow cavities in zeolite crystals [34]), and hence affecting the porous structure. After the post-treatment (of Ni@Si-1) using TPAOH, the resulting Ni@HolSi-1 showed significant mesoporous features, as evidenced by the increased mesopore volume (of 0.5\u00a0cm3\u00a0g\u22121), being much more significant that the parent Ni@Si-1 (V\nmeso\u00a0=\u00a00.24\u00a0cm3\u00a0g\u22121). Also, the well-developed mesopores features in Ni@HolSi-1 was also reflected based on the comparison of pore size distributions (PSD) of the materials under study (as illustrated in Fig. 3b), in which Ni@HolSi-1 shows the PSD of mesopores centred at about 5\u00a0nm.TEM measurement was undertaken to explore the microscopic feature of Ni NPs in the developed catalysts. The TEM images of the Ni/Si-1 catalyst (Fig. 4a and b) show the presence of Ni particles with an average size of 2.9\u00a0\u00b1\u00a00.9\u00a0nm, suggesting the possible Ni NPs location on the surface of the Si-1 crystals (Figs. 4b). When compared with Ni/Si-1, the existence of Ni NPs in the encapsulated Ni@Si-1 catalyst (Fig. 4c and d) and Ni@HolSi-1 (Fig. 4e and f) catalyst could not be clearly identified by the current TEM analyses. This might be due to the dispersion of Ni NPs that are encapsulated within the Si-1 crystals. Based on the TEM micrograph of Ni@HolSi-1, clearly, after the post-treatment of the encapsulated Ni-based catalyst (i.e., Ni@Si-1) with TPAOH solution, large void structures were formed in the interior crystals of Si-1 zeolite due to silicon extraction under alkaline conditions and subsequent recrystallisation in presence of TPA+. The TEM results correspond well to the findings by N2 physisorption analysis discussed above.All the catalysts in this work were prepared with the 5\u00a0wt% theoretical Ni loading, and the quantified Ni content of the catalysts (by ICP-OE) are lower than the value as shown in Table 3\n. H2 pulse chemisorption analysis was used to determine the Ni dispersion and metallic surface area of the catalysts, and the corresponding findings are summarised in Table 3. Regarding the measured metallic Ni surface area and Ni dispersion, the impregnated Ni/Si-1 and encapsulated Ni@Si-1 catalysts are rather comparable. Conversely, the Ni@HolSi-1 catalyst with the encapsulated Ni and mesoporous hollow structures demonstrated the highest Ni dispersion and metallic surface area at 1.4% and 9.3\u00a0m2 gNi\n\u22121, respectively, as presented in Table 3. Specially, in comparison with the parent Ni@Si-1 1 (with 0.30% Ni dispersion and 1.7\u00a0m2 gNi\n\u22121 metallic Ni surface area), the increase in the metallic Ni dispersion and surface area of Ni@HolSi-1 suggests that the post-synthetic TPAOH treatment can improve the exposure of Ni phases significantly, which can potentially benefit catalysis.The acidic characteristic of the materials was investigated by NH3-TPD analysis, as presented in Fig. 5a, Fig. S3 and Table S2. According to results indicated in Fig. 5a and Fig. S3, all materials show the presence of weak suface acidity, which can bind NH3 and leads to the assocaited NH3 desoption at 100\u2013350\u00a0\u00b0C during NH3-TPD analyses. Comapratively, the measured total surface acidity of the Si-1 support (~3401\u00a0\u03bcmol gcat\n\u22121), the impregnated Ni/Si-1 (~5623\u00a0\u03bcmol gcat\n\u22121), and the encapsulated Ni@Si-1 (~4112\u00a0\u03bcmol gcat\n\u22121) are rather similar, whilst that of Ni@HolSi-1 is much lower at ~1491\u00a0\u03bcmol gcat\n\u22121. The reduced surface acdity of Ni@HolSi-1 could be due to the post-treatment, which can potentially benefit reforming reactions since the presence of acidity in the catalysts tends to encourage coking. The reducing behaviour of Ni species in the synthesised catalysts was probed by H2-TPR analysis, and the relevant peaks are shown in Fig. 5b. For the impregnated Ni/Si-1 catalyst, the major reduction peak of the Ni species in it was at about 465\u00a0\u00b0C, and it can be attributed to the reduction of bulk NiO crystallites situated on the external surface of the Si-1 zeolite support with weak interaction [35]. For the encapsulated Ni@Si-1 and Ni@HolSi-1 catalysts, the major reduction behaviour of the Ni species in them occurred at about 790\u00a0\u00b0C, which are much higher than that of Ni/Si-1, suggesting an improved interaction between the encapsulated Ni species and Si-1 framework [36,37]. Based on the findings above, the NiO phases in the encapsulated catalysts are more resistant to thermal reduction (under H2) in comparison with the large NiO crystallites deposited on the outer surface of Si-1 material, due to the stabilisation of the encapsulated NiO species within the Si-1 framework [16,38]. Also, based on the findings from the H2-TPR analysis, a temperature of 800\u00a0\u00b0C was set to reduce the Ni catalysts under investigation for catalytic SRG.The chemical surface states and the position of Ni phases in the calcined catalysts were examined using XPS, and the XPS survey scans of the relevant catalysts are illustrated in Fig. 6\n. As depicted in Fig. 6a, the band associated with Ni in the impregnated Ni/Si-1 was much stronger than that of the encapsulated Ni@Si-1 and Ni@HolSi-1, respectively. The finding confirms that the Ni phases in Ni/Si-1 are mostly on the outer layer of the Si-1 material since XPS is the technique for probing relevant surface properties in the outermost 2\u201310\u00a0nm of a solid surface. Similarly, high-resolution XPS spectra of the Ni phases (as presented in Fig. 6b), show that all the developed catalysts exhibit a Ni 2p3/2 as the main peak (at the binding energy, B.E., of about 852\u2013859\u00a0eV) along with the associated shake-up satellite peak at the B.E. of 859\u2013871\u00a0eV and a Ni 2p1/2 peak (at the B.E. of about 871\u2013876\u00a0eV) along with the satellite peak at the B.E. of 876\u2013888\u00a0eV, respectively. Meanwhile, in all the developed catalysts, the characteristics peaks at a lower B.E. (i.e., ~855\u2013857\u00a0eV and\u00a0~\u00a0872\u2013874\u00a0eV) representing the proportion of NiO species. To compare the developed catalysts in this work with the literature data, the B.E. values (Ni2+ 2P3/2) of the developed catalysts are in between that corresponding to the B.E. of the pure NiO phase (i.e., Ni2+ 2p3/2, at about ~854.4\u00a0eV) and that of pure NiAl2O4 phase (i.e., Ni2+ 2p3/2, at about ~857.3\u00a0eV) [39]. Furthermore, the Ni2+ B.E. of Ni@HolSi-1 (at about 55.6\u00a0eV) was rather comparable to that of the reference NiO phase, suggesting a higher proportion of Ni2+ in the form of NiO oxide than on the surface of the catalyst in spinel structure. The developed Ni/Si-1 catalyst prepared by impregnation exhibits relatively high B.E. values which are close to that of the spinel NiAl2O4 (i.e., 856.7\u00a0eV versus 857.3\u00a0eV). Comparatively, the intensity of the Ni 2p regions observed in Ni/Si-1 is much higher than that of the encapsulated Ni@Si-1 and Ni@HolSi-1 catalysts, which confirms that most of the Ni species in Ni/Si-1 are positioned on the outer surface of the Si-1 support. Considering Ni@Si-1, the Ni species are likely dispersed uniformly within Si-1 framework including the top layer of the catalyst (within 10\u00a0nm), which is proved by the relatively low signal intensity (note that all three catalysts have comparable actual Ni loadings, Table 3). Regarding Ni@HolSi-1, the intensity of its Ni2p regions is the lowest, indicating that bulk of the Ni species are situated within the hollow Si-1 structures (Fig. 4f) due to the dissolution-recrystallization mechanism of the post-treatment of Si-1 zeolite using TPAOH aqueous solutions.The performance of catalytic SRG over the catalysts under investigation was comparatively evaluated with respect to glycerol conversion, product selectivity and yield towards the desired H2 versus CH4 and CO2 versus CO, under steady-state conditions at 500\u2013750\u00a0\u00b0C and atmospheric pressure. The gaseous products of H2, CO2, CO and CH4 were detected at the end of each run during the catalyst measurement. Based on these observations, the conversion of glycerol could be attributed to the transformation of the glycerol molecule into H2 formation via SRG (Eq. 1). The promotion of H2 generation and suppression of the formation of CH4 and CO was accompanied by several side reactions (as shown in Table S1) such as CH4 reforming into (Eq. S2), production of CO and H2 through thermal decomposition of glycerol (Eq. S3), water-gas shift reaction (WGSR) of converting CO into CO2 along with H2 (Eq. S4) and CO and CO2 methanation reactions as shown in Eqns. S5(a) and 5(b), respectively. Fig. 7\n showed the glycerol conversions and selectivites/yields of H2, CO2, CO and CH4 as a function of the reaction temperature in the stream. Noticeably, glycerol conversion increases with reaction temperature in all the developed catalysts displayed in Fig. 7, which is in accordance with the temperature dependence of the reforming reactions. However, as shown in Fig. 7a and b, the pristine Si-1 showed insignificant glycerol conversion of <10% and hydrogen yield of <2%, respectively, which could be attributed to the non-catalytic gas-phase reactions under thermal conditions. The product distribution of gaseous products (under consideration in this work) were determined for the non-catalytic thermal system, as depicted in Fig. 7c\u20137f, which shows that the selectivity to CO was the highest at ~60% over the temperature range due to thermal decomposition of glycerol (Eq. S3). Production of H2 was enabled at T\u00a0>\u00a0500\u00a0\u00b0C with the selectivity of ~20%. Considering the stoichiometry of Eq. S3, selectivity to H2 is relatively low. Since the selectivity to CO2 was <5%, the desired SRG reaction (Eq. S1) is unlikely. CH4 was also produced with the selectivity of 15\u201320%, suggesting the presence of CO methanation (Eq. S5a).In catalytic SRG, compared to the thermal case, the conversion of glycerol and yield of H2 over the encapsulated catalysts (of Ni@Si-1 and Ni@HolSi-1) increased as a result of the reaction temperature rise from 500 to 700\u00a0\u00b0C, as shown in Fig. 7a and b. The selectivity to various gaseous products in the two systems over the two encapsulated catalysts was rather comparable, but Ni@HolSi-1 showed better performance than Ni@Si-1, especially at high temperatures. At temperatures of 600\u2013650\u00a0\u00b0C, the Ni@HolSi-1 catalyst (with glycerol conversions of >90% and hydrogen yield of >40%) outperformed the Ni@Si-1 (with glycerol conversions of <85% and hydrogen yield of <40%). This can be attributed to (i) the highly dispersed Ni NPs due to the encapsulation strategy (Fig. 4 and Table 3) and (ii) the presence of mesoporous hollow structures in the Ni@HolSi-1 catalyst (as indicated in Figs. 3 and 4), which could improve the molecular diffusion through the catalyst structure. Conversely, the impregnated Ni/Si-1 catalyst showed the comparatively lowest glycerol conversion of <75% and insignificant hydrogen yield of <30%, respectively. The gaseous product distribution of catalytic SRG (H2, CO2, CO and CH4) over different catalyst were investigated, as shown in Fig. 7c\u20137f. By comparing the selectivity to different gaseous products at 750\u00a0\u00b0C, the impregnated Ni/Si-1 catalyst presents the lowest selectivity to H2, CO2, CO and CH4 at ~48%, ~20%, ~24% and\u00a0~\u00a05%, respectively. Comparatively, the amount of H2 and CO2 increased significantly over the Ni@HolSi-1 catalyst with the relative proportion of the selectivity of H2 at ~65%, CO2 at ~23% and CO at ~14%, respectively. In accordance with the previous results, it was found that the use of the Ni@HolSi-1 catalyst could prevent the production of the unwanted side products of CO and CH4 (Fig. 7e and f), which demonstrates the benefits of the mesoporous hollow framework of Si-1 zeolite crystals to promote the diffusion of intermediates in SRG and convert CO to CO2 and H2 via water gas shift reaction (Eq. S4). Similarly, the product distribution of H2, CO2, CO and CH4 were clearly less affected by the reaction temperatures under study, which might be attributed to the endothermic and exothermic nature of different side reactions (from Eqs. (S2) to (S10)), which favours the formation of H2, CO2, CO and CH4 at varying temperatures.As shown in Fig. S4a, among all the catalysts under study, the Ni@HolSi-1 shows a stable molar ratio (of H2/CO2\u00a0\u2265\u00a02.33) in the reformed mixture at temperatures of 500\u2013750\u00a0\u00b0C, being close to the theoretical ratio based on the stoichiometry of the SRG reaction (Eq. 1). Under the conditions used, in accordance with SRG (Eq. 1), the existence of other reactions (Table S1) such as WGSR, Eq. S4 [40] was also likely since CO and CH4 were also detected in the catalytic SRG systems. It is essential to highlight the catalyst (i.e. Ni@HolSi-1) that showed the lowest CO/CO2 molar ratio of <1 at temperatures investigated, as shown in Fig. S4b, with the lowest yet insignificant selectivity to CH4 (Fig. 7f). In comparison, over the impregnated Ni/Si-1 catalyst, the highest CO/CO2 molar ratios of >1 were observed, suggesting that the large and aggregated Ni particles promoted side reactions. Interesting, the CO/CO2 molar ratio in the impregnated Ni/Si-1 catalyst shows a volcano shape as a result of rising the reaction temperature, and the values are significantly high at 600\u2013650\u00a0\u00b0C. This phenomenon could be attributed to the carbon deposition on large particles of Ni according to the Boudouard reaction (Eq. S8), whilst the gradual decrease of the ratio could be attributed to the reduction of the generated CO into the carbon species (Eq. S9) [41]. The possible carbon formation on the catalyst's surface, as shown through the carbon balance (Fig. S4c) was calculated using Eq. S14 without the consideration of solid products. On the other hand, in all of the catalyst systems under study, the average total mass balance was mostly determined in the range of ~90\u201396\u00a0wt%, with the loss of about ~4\u20136\u00a0wt%, which could be attributed to the uncondensed gaseous products and unavoidable retention of some condensed liquids by the inner wall of the reactor and the lines in the experimental rig. An example of mass balance calculation over different catalytic systems was shown in Table S4, using Eq. S15.Specific reaction rate of glycerol conversion (rX\n\nG\n) and hydrogen formation (rH\n\n2\n) on the developed catalysts were calculated using the results from kinetic experiments, which are presented in Fig. 9 and Table S3, respectively. Specifically, kinetic experiments were performed at 300\u2013450\u00a0\u00b0C, atmospheric pressure and GHSV\u00a0=\u00a010,360 (STP) h\u22121 to ensure that the influence of mass and heat transfer boundaries are avoided. The summary of kinetic measurements of SRG over the catalysts are presented in Table S3, and the Ni@HolSi-1 catalyst demonstrated the higher specific reaction rates for glycerol conversion (i.e. ~1.8\u00a0\u00d7\u00a010\u22125\u00a0mol\u00a0s\u22121\u00a0g\u22121 at 400\u00a0\u00b0C) compared to that of the impregnated Ni/Si-1 catalyst (i.e. ~0.8\u00a0\u00d7\u00a010\u22125\u00a0mol\u00a0s\u22121\u00a0g\u22121 at 400\u00a0\u00b0C), perhaps due to the well-dispersed small Ni NPs of the Ni@HolSi-1 catalyst, suggesting higher contact efficiency, and hence the high glycerol conversions. Comparatively, the specific reaction rate of glycerol conversion over Ni@S-1 at 400\u00a0\u00b0C was about 1.5\u00a0\u00d7\u00a010\u22125\u00a0mol\u00a0s\u22121\u00a0g\u22121, being lower than that of Ni@HolSi-1 which could be due to the lack of mesoporous structure in it. Similar phenomena were found as well for the production of hydrogen with improved specific rate. For example, Ni@HolSi-1 demonstrated an excellent hydrogen generation with specific rate of about 2.7\u00a0\u00d7\u00a010\u22125\u00a0mol\u00a0s\u22121\u00a0g\u22121 at 400\u00a0\u00b0C, surpassing that of Ni/Si-1 (at about 0.38\u00a0\u00d7\u00a010\u221210\u00a0mol\u00a0s\u22121\u00a0g\u22121) under the same conditions. Arrhenius plots were obtained based on the results from the kinetic experiments to determine the activation energy (Ea) for the glycerol conversion using Eq. S13, and all the catalysts demonstrated common Arrhenius behaviour, as shown in Fig. 8\n. According to the highlighted results shown in Table S3, the Ni@HolSi-1 catalyst presents the lowest E\n\na\n value of 19\u00a0kJ\u00a0mol\u22121, not as much as that of the Ni/Si-1 catalyst (i.e., 46\u00a0kJ\u00b7mol\u22121), as well as the Ni@Si-1 catalyst (~27\u00a0kJ\u00b7mol\u22121), which demonstrates the advantage of the Ni@HolSi-1 catalyst for promoting SRG.Stability of the Ni/Si-1, Ni@Si-1 and Ni@HolSi-1 catalysts under study was evaluated during the SRG. The longivity experiments were performed by running the freshly reduced catalysts continuously in a stream for 100\u00a0h. The catalytst testings were run under the same experimental conditions including the reaction temperature at 750\u00a0\u00b0C and GHSV\u00a0=\u00a03120\u00a0h\u22121 (STP). Glycerol conversion and hydrogen yield for the catalysts in stream of 100\u00a0h are displayed in Fig. 9\n. As indicated in Fig. 9a, for the impregnated Ni/Si-1 catalyst, continuous deactivation was measured. In detail, the initial conversion of glycerol and hydrogen yield were 73% and 45%, respectively. However, the performance of the Ni/Si-1 catalyst kept decresing gradually as a function of TOS, which confirms the significant deactivation of Ni/Si-1 during catalytic SRG. The continuous deactivation of the impregnated Ni/Si-1 catalyst could be attributed to the poorly dispersed and large Ni particles which are prone to coking during the SRG reaction. Regarding the catalysts prepared by the encapsulation strategy, i.e., Ni@Si-1 and Ni@HolSi-1, they showed very stable performance over the 100-h longevity tests. Specifically, glycerol conversions of 99\u00a0\u00b1\u00a01% and H2 yield of 60\u00a0\u00b1\u00a01% were achieved over the Ni@Si-1 catalyst, as indicated in Fig. 9c. The post-synthetic TPAOH treatment resulting in the creation of mesoporous void structure was beneficial to improve hydrogen production, as shown in Fig. 9d, in which (i) the glycerol conversion over Ni@HolSi-1 is comparable to that of Ni@Si-1, and (ii) the H2 yield is higher at 70\u00a0\u00b1\u00a01%. The findings also suggest that the post-treatment was able to form the mesoporous structure without jeopodising the highly dispersed Ni NPs from the in situ encapsulation strategy. The selectivity to H2, CO, CO2 and CH4 gases in different catalytic SRG systems over different catalysts is shown in Fig. 9b, d and f. As indicated in Fig. 9b, over the Ni/Si-1 catalyst, H2 selectivity was stable from 0 to 50\u00a0h, then decreased from 50 to 100\u00a0h, as a result of reduction of CO and CO2 with H2 (Eq. S9 and Eq. S10) with the associated carbon deposition on the Ni/Si-1 catalyst. Comparatively, as shown in Fig. 9d and f, the encapsulated Ni catalysts maintained stable selectivities of H2, CO, CO2 and CH4 over the ToS of 100\u00a0h. The relatively stable performance of the encapsulated Ni catalysts could be associated to the presence of highly dispersed small Ni NPs, which are most active for reforming reaction than large Ni particles [42].In Fig. S5, the results of H2/CO2 and CO/CO2 ratios as determined for all the respective catalysts under study over 100\u00a0h on stream was highlighted. The H2/CO2 molar ratio of the Ni/Si-1 is observably changing over time on stream, until the catalyst was gradually deactivated (Fig. S5a). In the cases of encapsulated Ni catalysts (Figs. S5b and S5c), the H2/CO2 molar ratios are rather stable, especially the Ni@HolSi-1 catalyst, suggesting that the encapsulated yet highly dispersed Ni NPs combining the mesoporous hollow structure could mitigate catalyst deactivation considerably. Regarding the molar ratio of CO/CO2, the Ni@HolSi-1 catalyst exhibited a steady performance at <1 over 100\u00a0h in the reaction stream at 750\u00a0\u00b0C (Fig. S5c). The encapsulated Ni@Si-1 catalyst (Fig. S5b) produced slightly more carbon monoxide and showed the molar ratios of CO/CO2\u00a0>\u00a01. Conversely, the impregnated Ni/Si-1 catalyst presented the molar ratio of CO/CO2 at ~1.5\u20135.3 over the 100\u00a0h, which could be due to the fact that carbon deposits are formed on the catalyst during the longevity test of catalytic SRG.Post-reaction characterisation of the used catalysts was performed to assess the effect of longevity SRG experiments on them. Comparative TGA analysis of the fresh (black solid lines) and used (black dot dash lines) catalysts was shown in Fig. 10\n. All the fresh and calcined catalysts (i.e., Ni/Si-1, Ni@Si-1 and N@HolSi-1 catalysts, respectively) were very stable during TGA tests, showing no weight loss. Mass loss at temperatures below 200\u00a0\u00b0C was measured for the used encapsulated catalysts of Ni@Si-1 and Ni@HolSi-1, as shown in Fig. 10b and c, which was associated with the dehydration of all physisorbed water molecules within the frameworks of Si-1 zeolite supported catalysts. Regarding the weight loss which occurs at 550\u2013800\u00a0\u00b0C, as shown by the derivated weight loss (solid red lines in Fig. 10a), the used Ni/Si-1 catalyst demonstrated a large weight loss of ~15\u00a0wt% (after 100\u00a0h SRG on stream at 750\u00a0\u00b0C), suggesting significant effects of carbon formed on the impregnated Ni/Si-1 catalyst during the SRG [43]. Similarly, a substantial weight loss of ~8\u00a0wt% at 550\u2013800\u00a0\u00b0C was formed on the used encapsulated Ni@Si-1 catalyst (Fig. 10b). Conversely, the TGA profile of the used Ni@HolSi-1 showed insignificant weigh loss of <0.5\u00a0wt% in the high temperature range, demonstrating the anti-coking ability, which can be attributed to the formed mesoporous hollow structure from the post-synthetic TPAOH treatment of the encapsulated Ni catalyst (i.e., Ni@Si-1). Comparatively, post-analysis of XRD structures in the used catalysts were conducted at the end of the longevity tests of catalytic SRG at 750\u00a0\u00b0C (Fig. S6). The Si-1 structures were maintained after the 100-h test in all the tested catalysts, showing that the siliceous silicalite-1 zeolite is the suitable stable support for developing relevant reforming catalysts for applications under harsh conditions. The diffraction peak associated with the Ni phase was clearly observed for the used Ni/Si-1 catalyst, being comparable with that of the fresh catalyst (Fig. 1). For the encapsulated catalysts, the small Ni peaks were measured, as highlighted in Fig. 1b, especially Ni@HolSi-1, suggesting that the post treatment could render some encapsulated Ni phases exposed, which is subject to sintering at high temperatures. This was further evidenced by Fig. S7, in which some carbon nanostructures were formed on the used Ni@HolSi-1 catalyst. However, based on the catalytic SRG and comparative TGA results (Figs. 9 and 10), the developed encapsulation strategy and the post-synthetic treatment were effective for promoting catalytic SRG processes.Reforming reactions are class of important catalysis for producing many chemicals/fuels such as hydrogen, and they commonly require harsh thermal conditions, and hence the strategies of mitigating catalyst deactivation are needed to make the reforming processes more sustainable. In principle, catalyst design with small metal nanoparticles and segregated metal dispersion is beneficial to reduce deactivation because such catalysts can afford high activity with less chances of metal sintering and coke formation. Herein, towards H2 production via catalytic steam reforming of glycerol (SRG), an encapsulation strategy was employed to develop the encapsulated ultra-small Ni catalysts in silicalite-1 zeolite (Ni@Si-1) with high Ni dispersion, which showed much better performance than the conventional impregnated Ni catalyst (Ni/Si-1) regarding the activity and anti-coking ability. More importantly, the encapsulated Ni@Si-1 can be treated further using a post-synthetic method employing TPAOH solution under hydrothermal condition, which resulted in the encapsulated Ni catalysts with the mesoporous hollow structure, i.e., Ni@HolSi-1. The obtained Ni@HolSi-1 catalyst preserves the highly dispersed small Ni nanoparticles, being responsible for the measured high activity in SRG, e.g., a stable glycerol conversion of 99\u00a0\u00b1\u00a01% and H2 yield of 70\u00a0\u00b1\u00a01% over 100\u00a0h on stream together with the H2/CO2 molar ratio of >2.33 and CO/CO2 molar ratio of <1 at 750\u00a0\u00b0C. Compared to its parent of Ni@Si-1, the mesoporous Ni@HolSi-1demonstrated a very good anti-coking ability with insignificant coke deposition after a 100-h longevity test, whilst the encapsulated Ni@Si-1 showed ~8\u00a0wt% coke deposition under the same condition. The findings of this work show that (i) encapsulation is a very effective strategy for making highly dispersed metal catalysts for catalysis under harsh conditions, (ii) siliceous silicalite-1 zeolite is a good support candidate for preparing highly stable catalyst, and (iii) the presence of mesoporous feature in the encapsulated catalyst benefits local mass transfer, which is highly desired for reducing coke deposition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 872102. A.I. thanks the financial support of Petroleum Technology Development Fund (PTDF) in Nigeria for merit PhD scholarship (PTDF/ED/OSS/PHD/IA/1209/17). H.C. thanks the financial support from the European Commission under the Marie Sk\u0142odowska-Curie Individual Fellowship (H2020-MSCA-IF-NTPleasure-748196), the funding support from the Jiangsu Specially-Appointed Professors Program, the Natural Science Foundation of Jiangsu Province (BK20200704), and the State Key Laboratory of Materials-Oriented Chemical Engineering (No. ZK202001). Acknowledgements are extended to Dr. Shaojun Xu (Cardiff University) and Dr. Shaoliang Guan (Cardiff University) for their help with the TEM and XPS measurements, respectively.\n\n\n\nSupplementary material.\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.fuproc.2022.107306.", "descript": "\n                  Valorisation of crude glycerol via steam reforming, i.e., SRG, is a promising method to produce sustainable hydrogen. However, catalyst deactivation under harsh SRG conditions is still a main challenge which hinders the further development of practical SRG. In this work, the encapsulated Ni catalyst in siliceous silicalite-1 zeolite (Ni@Si-1) were developed to show the improved performance and enhanced anti-deactivation potentials in catalytic SRG as compared with the conventional impregnated Ni catalysts (i.e., Ni/Si-1). Importantly, the post-synthetic treatment of Ni@Si-1 using TPAOH solution formed the encapsulated Ni catalyst with the mesoporous hollow structure (i.e., Ni@HolSi-1), which demonstrate even better performance in SRG with glycerol conversion of >95%, H2 yield of ~70%, H2/CO2 molar ratio of >2.33 and CO/CO2 molar ratio of <1 at 750\u00a0\u00b0C. Specifically, highly dispersed ultrasmall encapsulated Ni particles were retained within the hollow crystals of siliceous silicalite-1, as confirmed by XPS and HRTEM characterisation. The activation energy for glycerol conversion over Ni@HolSi-1 (i.e., Ea\u00a0=\u00a0~ 19\u00a0kJ\u00a0mol\u22121) was much lower than that of Ni/Si-1 and Ni@Si-1. 100-h longevity tests over the three catalysts were investigated at 750\u00a0\u00b0C, and the Ni@HolSi-1 catalyst exhibited an excellent stability and activity, as well as insignificant coke deposition, which could be due to the enhancement of highly dispersed yet accessible Ni NPs within the hollow Si-1 crystals. The findings of the work show the promise of the encapsulation strategy and mesoporous zeolites for developing the future reforming catalysts.\n               "}
{"full_text": "Data will be made available on request.In heterogeneous catalysis, the interaction between (metal) nanoparticles and a support is a crucial factor for the catalytic performance. Stabilizing the nanoparticles is the main reason to use a support, resulting in a high particle dispersion during synthesis [1] and preventing them from sintering during catalysis. In addition, supports can affect the catalytic activity and selectivity, for instance via the absorption of reactants or intermediates, by influencing the particle size or shape, or by altering reaction pathways [2,3].All mentioned factors can affect the performance of catalysts in the Power-to-Gas process, where CO2 is hydrogenated to methane. This is a highly interesting reaction to allow the storage of renewable hydrogen in synthetic natural gas [4]. A wide range of metals, for example Ru, Rh and in particular Ni have been investigated for this reaction [5,6]. Compared to noble metals, Ni is relatively low-priced, active and abundant. Typical supports used for this reaction are SiO2 and Al2O3\n[6,7], with a recent switch to reducible oxides, such as CeO2, TiO2 or ZrO2 because of their increased CO2 adsorption activity [8,9]. In this reaction, factors such as metal particle size, support and promoter effects are important to understand, but can at the same time be very challenging to disentangle.Recently the use of carbon, especially carbon nanotubes (CNTs) as support for CO2 hydrogenation catalysts has gained more attention for fundamental studies [10\u201312]. Carbon materials are interesting model supports, because of their relatively high surface area and tunable surface chemistry [13]. Furthermore, carbon supports can be used to diminish the formation of species that strongly interaction with the support, for example metal silicates or aluminates [14\u201316], or enhance the interaction between active metal and promoters [17].During methanation, catalyst deactivation is an important factor to consider. This can be caused by the formation of nickel carbonyl species at low temperatures, whereas particle growth usually occurs at high temperatures [2,18,19]. Another challenge is the formation of carbon deposits, blocking the active metal surface, although this can be prevented by working at elevated pressures [20]. Carbon offers a high heat conductivity [10], which is crucial to prevent the formation of local hot spots during the exothermic methanation reaction (\u0394H\u2070 = \u2212165\u00a0kJ\u00a0mol\u22121) [21]. Modifying the surface chemistry of a support can help to stabilize nanoparticles.Typical support surface groups introduced to carbon supports are oxygen and nitrogen containing groups, changing the chemical properties of the carbon surface without changing its structural properties [22\u201324]. As a result, it is possible to vary the point of zero charge (PZC) and consequently the acidity or basicity over a wide range. A reflux treatment of pristine carbon (in this case graphite nanoplatelets, GNP) in HNO3 typically results in the incorporation of carboxylic, lactone and anhydride surface groups [25\u201327], increasing the acidic character of the material. An amination treatment of the oxidized carbon (GNP-O) converts the oxygen- into nitrogen-containing surface groups (GNP-N), which increase the surface basicity [22,23,28]. Support surface groups are often found to influence the final metal particle size of fresh catalysts. They can enhance the wetting of the precursor solution or can anchor the metal precursor more strongly [29]. Both could result in smaller nanoparticles [30\u201335], or even single atoms or clusters [12,36].Functionalization of carbon supports can improve the catalyst stability, by preventing nanoparticle growth [37\u201339]. Besides, the catalytic activity can be modulated, for instance by introducing N-containing species, to increase the basicity of the support [10,21,40\u201342], allowing enhanced CO2 adsorption [23]. Gon\u04abalves et al. performed a systematic study on the effect of support surface treatment of nickel on active carbon for low pressure CO2 hydrogenation and found that the use of the most basic carbons resulted in the highest catalytic activity [40]. However, the interference of differences in nanoparticle size on catalysis and the effect of support modification on catalyst stability were not addressed in full detail.In this paper, we discuss the effect of support functionalization for high pressure CO2 hydrogenation using graphite carbon nanoplatelets (GNP) as model support for Ni nanoparticles. Both oxygen and nitrogen containing surface groups were introduced to the carbon support surface before deposition of the nickel. We kept other parameters, such as the initial Ni particle size, the same and discuss the effect of the support treatment on catalytic performance during CO2 hydrogenation at 300\u00a0\u00b0C and 30\u00a0bar, with main focus on catalyst stability.Nickel nitrate hexahydrate (Ni(NO3)2\u00b76H2O, Sigma Aldrich, \u2265\u00a097.0%), nitric acid (HNO3, Merck, 65%) and Silicon Carbide, (SIKA ABR I, F70) were used as received. Graphite nanoplatelets (GNP-500, XG Sciences, grade C \u223c500\u00a0m2 g\u22121 surface area) were either used as received, referred to as GNP, functionalized with oxygen-containing support surface groups (GNP-O) or nitrogen-containing support groups (GNP-N) or washed (GNP-W). To prepare oxidized carbon, approximately 10\u00a0g of the pristine GNP-500 was heated in 400\u00a0mL 65% HNO3 to 80\u00a0\u00b0C for 2\u00a0h while stirring. Afterwards, the suspension was washed several times with 5\u00a0L demi water each time until a pH of 6 was reached. After the last washing step, the support was dried at 120\u00a0\u00b0C for at least 24\u00a0h and subsequently crushed. Nitrogen functionalities were introduced to the support by substitution of oxygen functionalities [43]. Typically, \u223c3\u00a0g GNP-O was loaded into a tubular oven, purged for 15\u00a0min with N2 gas at room temperature (200\u00a0mL\u00a0min\u22121) and subsequently exposed to a flow of NH3 gas at 600\u00a0\u00b0C (220\u00a0mL\u00a0min\u22121, 5\u00a0\u00b0C\u00a0min\u22121, 4\u00a0h). To prepare GNP-W, approximately 2\u00a0g GNP was washed in 50\u00a0mL 1\u00a0M HNO3 at room temperature while stirring for 2\u00a0h. Afterwards the suspension was washed several times with 100\u00a0mL demi water each time until a pH of 6 was reached and dried in the same way as GNP-O.Nickel was deposited on either GNP, GNP-O, GNP-N or GNP-W using incipient wetness impregnation. Typically, 1.0\u00a0g of carbon support was dried in a round-bottom flask for 120\u00a0min at 170\u00a0\u00b0C, while stirring under dynamic vacuum to remove water and air from the pores. Aqueous nickel nitrate solutions were prepared by dissolving 2.0\u00a0M Ni(NO3)2 in mili Q water. The solution was acidified with 0.10\u00a0M HNO3 to ensure a pH around 1. The dried carbon support was impregnated with 0.73\u00a0mL\u00a0gsupport\n\u22121 (90% of the pore volume of pristine GNP, determined using N2 physisorption) under static vacuum while stirring, to ensure that the solution was homogeneously spread over the support. Subsequently the sample was dried overnight at room temperature under dynamic vacuum. To decompose the precursor, 1\u00a0g of the sample was transferred to a plug-flow reactor and heated to 350\u00a0\u00b0C in 200\u00a0mL\u00a0min\u22121 N2 (3\u00a0\u00b0C\u00a0min\u22121, 90\u00a0min) to decompose the nitrate. The reactor was then cooled down and the gas was switched to 5% H2/N2 (200\u00a0mL\u00a0min\u22121), which was the gas atmosphere for the subsequent reduction at 350\u00a0\u00b0C (2\u00a0\u00b0C\u00a0min\u22121, 90\u00a0min). After cooling down, the catalyst was slowly exposed to air to passivate the nickel nanoparticles. The catalysts are denoted as Ni/GNP-X, where GNP-X is the type of carbon used (GNP, GNP-O, GNP-N or GNP-W).The pore volume and surface area of carbon supports were analyzed using N2-physisorption. Isotherms were measured at \u2212\u00a0196\u00a0\u00b0C on a Micromeritics TriStar II Plus apparatus. The samples were dried overnight under vacuum at 170\u00a0\u00b0C before the measurement. The specific surface area of the support was calculated using the BET equation (0.05\u00a0< p/p\n\n0\n < 0.25) and the total pore volume was derived from the amount of N2 adsorbed at p/p\n\n0\n =\u00a00.995.The density of acidic and basic surface groups was determined by potentiometric titration using a TIM 880 Titralab Titration Manager. The carbon materials were suspended in 65\u00a0mL 0.1\u00a0M KCl solution and degassed under N2 flow and vigorous stirring. For both acid and base titrations \u223c25\u00a0mg of carbon material was used. The titrations were performed with either a 0.01\u00a0M NaOH or 0.01\u00a0M HCl solution, both in 0.1\u00a0M KCl solution. The amount of surface groups per gram carbon material was calculated based on the equivalence points of the titration data. Combined with the BET surface area obtained from physisorption, the density of surface groups (# groups nm\u22122) was determined for the different supports. The point of zero charge (PZC) of the support was determined through mass titration of the carbon material. Increasing amounts of carbon material were suspended in 10\u00a0g of 0.1 KCl solution, increasing the weight percentage of the support in the liquid, while measuring the pH. It is assumed that the amphoteric behavior of the surface groups will lead to a system pH equal to the PZC.[44].The supports were imaged with scanning electron microscopy (SEM) on a Helios G3 UC at 2 or 5\u00a0kV. The images were measured in field-free mode with a current of 0.40\u00a0nA. EDX analysis was performed using an Oxford silicon drift detector and Aztec software.The catalysts were imaged with transmission electron microscopy (TEM) on a Thermo Fisher Talos L120C operated at 120\u00a0kV or a Thermo Fisher Talos F200X microscope operated at 200\u00a0kV. The catalyst sample was dispersed as a dry catalyst powder onto a Cu sample grid coated with holey carbon (Agar 300 mesh Cu). Because of the nature of the carbon, consisting of thin graphitic sheets, dispersion of the catalyst powder in a solution and subsequent sonication was not necessary during the sample preparation. At least 400 nickel nanoparticles were manually counted per catalyst sample on at least 8 different catalyst locations using ImageJ analysis software. The determination of the Ni particle sizes is described in Supporting Information\nSection 1.High resolution TEM imaging was performed on a Thermo Fisher Spectra 300 monochromated, double-aberration corrected microscope operated at 300\u00a0kV. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast (iDPC) images were acquired in parallel. The screen current was ca. 0.05\u00a0nA and the camera length 145\u00a0mm.Powder X-ray diffraction (XRD) was performed on a Bruker D2 X-ray diffractometer, equipped with a Co-Ka1,2 radiation source (\u03bb\u00a0=\u00a01.790\u00a0\u00c5) and a Lynxeye detector. All catalysts were measured with diffraction angles varying between 10\u00b0 and 95\u00b0 2\u03b8 with a step size of 0.05\u00b0 2\u03b8/step while the sample was rotated at a rate of 15\u00a0rpm. All diffractograms were normalized to the carbon (002) peak at 30.9\u00b0. The crystallite sizes were calculated by applying the Scherrer equation to the NiO (111) peak at 43\u00b0 or the Ni0 (200) peak at 61\u00b0.Thermogravimetric analysis was performed on a Perkin Elmer TGA800 coupled to an Hiden Analytical HPR-20 MS system. For the bare supports, the weight of 4\u20138\u00a0mg sample was determined while heating in Ar (10\u00a0\u00b0C\u00a0min\u22121) to identify the weight % of functional groups on the supports. This technique was also used to determine the Ni weight-loading of the catalysts before and after catalysis as reported before [45] and described in detail in the Supporting Information\nSection 1.A Kratos Axis Ultra DLD system was used to collect XPS spectra using a monochromatic Al K\u03b1 X-ray source operating at 168\u00a0W (12\u00a0mA x 14\u00a0kV). Data was collected with pass energies of 160\u00a0eV for survey spectra, and 20\u00a0eV for the high-resolution scans with step sizes of 1\u00a0eV and 0.1\u00a0eV respectively. The system was operated in the Hybrid mode, using a combination of magnetic immersion and electrostatic lenses, and acquired over an area of approximately 300\u00a0\u00d7\u00a0700\u00a0\u00b5m2. A magnetically confined charge compensation system using low energy electrons was used to minimize charging of the sample surface and all spectra were taken with a 90\u00b0 take of angle. A pressure of ca. 5\u00a0\u00d7\u00a010\u22129 Torr was maintained during collection of the spectra. All samples were mounted into recesses of a modified Kratos Axis Ultra standard sample bar and gently pressed flat with iso-propyl alcohol cleaned glass slides before insertion into the spectrometer. All data was analyzed using CasaXPS (v2.3.24) [46] after subtraction of a Shirley background and using modified Wagner sensitivity factors as supplied by the instrument manufacturer. Cure fits were performed using an asymmetric Lorentzian form (LA line shape in CasaXPS), whereas the line shape for graphitic, sp2 carbon, was based on a cleaved, oxygen free HOPG sample.Temperature-programmed desorption (TPD) was performed on a Micromeritics AutoChem II 2920 apparatus. For the bare supports, 80\u00a0mg support was dried at 120\u00a0\u00b0C in Ar for 15\u00a0min. The sample was cooled down to 40\u00a0\u00b0C and subsequently heated in Ar (15\u00a0mL\u00a0min\u22121) with 5\u00a0\u00b0C\u00a0min\u22121 to 900\u00a0\u00b0C. H2O was captured with a dry ice/isopropanol cold trap. The outgoing gas was analyzed using a mass spectrometer (MS) of Hiden Analytical equipped with a QGA Professional software package.H2 chemisorption was measured on a Micromeritics ASAP 2020\u00a0C apparatus using \u223c100\u00a0mg of sample. Prior to the measurement, the sample was reduced in pure H2 (6.0, Linde) at 300\u00a0\u00b0C for 2\u00a0h (5\u00a0\u00b0C\u00a0min\u22121), after which full reduction was assumed, based on TPR analysis. The sample was then evacuated and cooled to 35\u00a0\u00b0C, and H2 chemisorption was measured at that temperature. The Ni surface area was obtained from extrapolation of the linear range of the adsorption isotherm of H2 to a pressure of 0 kPa, giving the H2 uptake (\u03bcmol gcat\n\u22121). The determination of the experimental and theoretical Ni surface areas is described in Supporting Information\nSection 1.The CO2 methanation catalysis was performed in a high throughput gas-phase 16-parallel fixed bed reactor system (Avantium Flowrence). Prior to the catalytic test, the catalyst powders were pelletized using a hydraulic press and subsequently sieved into a fraction of 75\u2013150\u00a0\u00b5m. 60\u00a0mg catalyst was diluted with 240\u00a0mg SiC (>150\u00a0\u00b5m) to prevent the formation of hotspots. The mixture of catalyst granules and SiC were loaded in stainless steel reactor tubes (2.6\u00a0mm inner diameter) on top of \u223c0.5\u00a0cm SiC granules. This was topped off with SiC.The Ni/GNP-X catalysts were in situ reduced prior to the reaction in a flow of 10% H2/N2 at 300\u00a0\u00b0C (2\u00a0\u00b0C\u00a0min\u22121) for 3\u00a0h. Subsequently the reactors were cooled down to 120\u00a0\u00b0C before the reaction mixture was added. The reaction mixture consisted of CO2:H2:He =\u00a019:76:5, 120\u00a0mL\u00a0min\u22121, and was divided over 16 reactors. The resulting GHSV was 7500\u00a0mL\u00a0gcat\n\u22121 h\u22121\n. The reactor was gradually pressurized to 30\u00a0bar and subsequently heated to 300\u00a0\u00b0C with 2\u00a0\u00b0C\u00a0min\u22121. This temperature was determined with TPR, see Fig. S1. The catalysts were tested up to 100\u00a0h to study both the activity and stability. The products were analyzed directly with online gas chromatography (GC, Agilent 7890B) with a sampling time of 14\u00a0min. Thus when all 16 reactors were in use, each sample was analyzed every \u223c4\u00a0h. For each catalyst, three reactors were loaded and tested. After confirming the reproducibility, the catalytic results were averaged.To test the selectivity at different conversions, after 100\u00a0h the GHSV was varied. The total flow over the 16 reactors was adapted (50, 75 and 150\u00a0mL\u00a0min\u22121 total flow) while keeping the gas mixture the same. Each new flow was equilibrated for 1\u00a0h and at least 2 datapoints per catalyst were taken (with 4\u00a0h difference). After the reaction, the catalysts were flushed with He and left to cool down to 60\u00a0\u00b0C before exposing them to air. This resulted in controlled passivation for post-catalytic characterization, for which the contents of the three reactors were combined. The formulas to determine the conversion, selectivity and turnover frequency are described in Supporting Information\nSection 1.\n\nTable 1 shows the structural characteristics of the graphite nanoplatelets that were used as-received (pristine, GNP), after the oxidation treatment (GNP-O) and after the amination treatment (GNP-N). The BET surface area and total pore volume of pristine carbon were 456\u00a0m2 g\u22121 and 0.81\u00a0mL\u00a0g\u22121, respectively. After surface modification, GNP-O and GNP-N exhibited a surface area of to 415 and 308\u00a0m2 g\u22121 and pore volume of 0.72 and 0.62\u00a0mL\u00a0g\u22121, respectively. The N2 physisorption isotherms are shown in Fig. S2. A decrease in surface area and pore volume is common for this relatively harsh oxidation treatment [22,47,48] and is probably due to the removal of an amorphous carbon fraction (with high specific surface area) as well as some collapse of the ordered graphite pore structure.The pristine GNP contained oxygen and its overall surface chemistry was slightly acidic (Table 1, Fig. S3). With the introduction of more oxygen-containing surface groups, the acidity increased, as evidenced by a decrease of the point of zero charge (PZC) from 4.0 to 3.0 (Table 1, Fig. S4). With the introduction of nitrogen functionalities, the PZC was increased to 9.0 and only basic groups were detected with titration (Table 1).Support treatment did not lead to significant changes in the X-ray diffractograms between 2\u03b8 =\u00a020 and 95\u00b0 (Fig. S5). At lower angles an extra peak was present for GNP, which mostly disappeared upon surface treatment. This likely indicated that the treatment influenced the stacking of the carbon platelets. The D-parameter represents the ratio between sp2 and sp3 carbon and is derived from the differential of the carbon x-ray induced Auger peak in the XPS spectrum [49,50]. This value was similar for all carbons (between 21.5 and 22.5\u00a0\u00b1\u00a01.0\u00a0eV), corresponding to a sp3 carbon content of ca. 10% [49]. This value was in good agreement with the C1s fitting (Table S1). No significant differences in morphology between the highly graphitic GNP and GNP-O were identified with scanning electron microscopy (SEM) (Fig. S6).The nature of the oxygen functionalities was investigated by following the gas release of the supports with temperature programmed reduction coupled with mass spectrometry (TPD-MS) up to 900\u00a0\u00b0C in argon (\nFig. 1). We first consider the pristine and oxidized carbon supports. In both cases CO2 and CO were released, due to the presence of oxygen-containing surface groups. In the case of GNP-O about double the amount was released compared to GNP, in line with the differences in acidity (Table 1). In addition thermographic analysis (TGA) showed a larger weight-loss of GNP-O (9.6%) than GNP (5.4%) at 800\u00a0\u00b0C in Ar (Fig. S7), confirming the presence of more oxygen in the carbon.The formation of CO2 (Fig. 1A) was attributed to the decomposition of carboxylic acids (100 and 400\u2009\u00b0C), anhydrides (200 \u2013 600\u2009\u00b0C) and lactone groups (400 \u2013 900\u2009\u00b0C) [25\u201327,51]. CO formation (Fig. 1B) at low temperatures (< 300\u2009\u00b0C) was caused by to the decomposition of aldehyde or ketone groups [25,52]. At higher temperatures, the peaks are typically ascribed to the decomposition of anhydrides (350 \u2013 600\u2009\u00b0C), phenols (500 \u2013 750\u2009\u00b0C) and carbonyl or quinone groups (650 \u2013 950\u2009\u00b0C) [22]. Altogether, TPD-MS analysis showed that the oxidation treatment had resulted in the incorporation of a range of oxygen containing surface groups, that could be carboxylic acids, anhydrides and phenols.Interestingly, for GNP-N, Fig. 1A and B show that only minor amounts of CO2 and CO were released; only above 600\u2009\u00b0C a peak was observed for m/z\u2009=\u200928. This peak could either represent the formation CO from relatively stable oxygen containing surface groups (carbonyls or quinones) or the formation of N2 from nitrogen containing surface groups. The absence of CO2 and CO release at lower temperatures indicates that with the amination treatment, (most) oxygen containing surface groups were successfully removed. The release of some NO (m/z\u2009=\u200930, Fig. 1C) implies that nitrogen-containing groups had been successfully introduced to the support and that (part of) the nitrogen functional groups also contained oxygen, in agreement with results reported by Arrigo et al. [53].XPS analysis (Table 1) showed the increase in oxygen content for GNP-O (7.7\u2009at%) with respect to the GNP (4.6\u2009at%). The amination treatment was observed to cause significant loss of oxygen (<1at% left in GNP-N), whilst there was a concomitant increase in nitrogen (2\u2009at%). High resolution spectra analysis of both C1s and N1s regions was performed to understand the chemical functionality. Fitting of the O1s spectra of the GNP materials (\nFig. 2A) identified contributions of two major peaks located at ca. 531.5\u2009eV and 533\u2009eV, corresponding to oxygen doubly or singly bound to carbon, respectively,[6,40] whilst the peaks between 535 and 540\u2009eV are characteristic of a shake-up structure for carbonyl containing species.The oxidation treatment doubled the CO content (1.4\u2009at% in GNP to 3.0\u2009at% in GNP-O), and also exhibited a corresponding increase in the C\u2212O functionality from 2.3 to 3.5\u2009at% (Table S1), whilst amination caused low levels of these species to remain (0.5 and 0.3\u2009at% for CO and C\u2212O respectively). The C1s spectra were more complicated to fit given the similar binding energies of some oxygen and nitrogen containing functions, together with the asymmetry of the graphitic carbon and the uncertainty in the shape of the photoelectron background [49]. Nevertheless, XPS confirmed for GNP-O a high amount of several types of oxygen functional groups, with the groups comprising of either a C\u2212O or a CO bond being dominant over the COO\u2212 groups (Fig. S8, Table S1).The main peak in the XP N1s spectrum of GNP-N is located at ca. 398\u2009eV and is attributed to pyridinic-type groups (Fig. 2B, Table S2), whilst the peak at ca. 400\u2009eV corresponds to pyrrolic- or pyridonic-type nitrogen species [26,53] or absorbed NHx. The smaller peaks between 402 and 405\u2009eV could originate from graphitic N (\u223c403\u2009eV) and oxidized nitrogen (\u223c405\u2009eV) [54], however given the signal at 398\u2009eV and these higher energy signals, these are likely to be attributed to loss structure from nitrogen in conformations such as that found in g-C3N4\n[55]. In short, our findings from the XPS analysis and the TPD-MS analysis are in agreement and confirm the presence of oxygen or nitrogen-containing surface groups on the carbon support. Hence with the oxidation and subsequent amination treatment of GNP, three supports were prepared with different acidity/basicity and different types and amounts of support surface groups; C\u2212O and CO groups for GNP and GNP-O and mainly pyridinic N for GNP-N.The main goal was to deposit Ni nanoparticles on the different supports without changing other parameters such as Ni particle size or loading. Indeed, all catalysts contained a Ni loading of \u223c8\u2009wt% and Ni nanoparticles of about 5\u2009nm in diameter (\nTable 2). The nickel deposition lowered the BET surface areas, but to a similar extent for all catalysts (249, 238 and 218\u2009m2 g\u22121 for Ni/GNP, Ni/GNP-O and Ni/GNP-N, respectively, Table S3). The properties for the used catalysts, also displayed in Table 2, will be discussed in detail in Section 3.5.The D-parameter (Table S4) of the fresh Ni/GNP and Ni/GNP-O catalysts, determined from the Auger peak in XPS, was 22 \u2013 23\u2009\u00b1\u20091.0\u2009eV, in agreement with \u223c10% sp3 carbon determined from the fitting of the C1s spectra (Table S4). Thus, the nickel deposition yielded negligible difference in the graphitic nature of the support. At the elevated temperatures used during Ni deposition (350\u2009\u00b0C), carboxylic acid groups were not stable. As a consequence, the ratio between the C\u2212O and CO groups decreased upon Ni deposition (Fig. S9, Table S5). Hence CO functionalities were preferentially retained after Ni deposition. This could either mean that these are more stable than C-O containing groups or, less likely, that the Ni nanoparticles bind preferably to C\u2212O surface groups. For the Ni/GNP-N, the presence of nitrogen was evidenced by TGA-MS analysis, as NOx was released between 300 and 700\u2009\u00b0C while heating this catalyst in oxygen atmosphere, which was done to determine the Ni weight loading (Fig. S10).\n\nFig. 3A-C show transmission electron microscopy (TEM) images of three catalysts prepared using pristine (A, blue), oxidized (B, orange) and aminated (C, green) carbon with corresponding particle size distributions (insets in A, B and C). Independent of the support used, the surface averaged particle diameter (d\n\ns\n) was 5\u2009nm (Table 2). Although the TEM particle size was similar for all catalysts, the metallic surface area, determined using H2 chemisorption (Table 2, Table S6 and Fig. S11) was higher for Ni/GNP-N than for Ni/GNP and Ni/GNP-O. The experimental metal surface area of Ni/GNP-N was in agreement with the theoretical surface area calculated from the TEM particle size for a spherical nanoparticle, whereas for the other catalysts clearly lower specific metal surface areas were measured.The NiO(111) crystallite sizes determined from the peak at 2\u03b8 =\u200943\u00b0 were 4.2, 5.2 and 3.2\u2009nm for Ni/GNP, Ni/GNP-O and Ni/GNP-N respectively (Fig. 3D, Table 2), roughly matching the TEM results. The peak at low angles and the increased background of the bare GNP had disappeared, indicating that the nickel deposition had caused changes in the morphology of the GNP. No crystalline Ni3C was observed with XRD. Whilst this is not definitive proof of the absence of Ni3C, because of the relatively small peak shift compared to Ni, XPS supports this finding. Carbides typically give a distinct and narrow peak or shoulder in the lower binding energy side of the C1s peak (between 282.5 and 283.5\u2009eV) [56], which was not observed for our catalysts (Fig. S9). Thus, XPS analysis of Ni/GNP-O and Ni/GNP catalysts also indicated that the presence of Ni3C was unlikely. Hence, we prepared nickel on carbon catalysts with different support surface groups, but similar Ni particle sizes and loadings and specific metal surface areas.The effect of the catalytic properties of the Ni-based catalysts was investigated under industrially relevant CO2 hydrogenation pressure and temperatures (30\u2009bar, 300\u2009\u00b0C). \nFig. 4 shows the CO2 conversion of the catalysts, which were tested at relatively low conversion (10\u201320%) to allow examination of their performance far from equilibrium conversion (close to 100% at 30\u2009bar and 300\u2009\u00b0C). All bare supports were inactive for CO2 hydrogenation under these conditions. Ni/GNP-N showed the highest weight based activity, e.g. normalized to Ni content (22.6% CO2 conversion) (Fig. 4). The initial conversion of Ni/GNP-O and Ni/GNP were 15.6% and 12.9%, respectively. Thus the trend in initial conversion was Ni/GNP-N\u2009>\u2009Ni/GNP-O >\u2009Ni/GNP. This trend was reproducible throughout different catalytic tests and for various batches of catalysts (Fig. S12).The turnover frequency (TOF) based on this active metal surface area of the fresh catalysts, as determined by H2 chemisorption, was similar for all catalysts at the start of catalysis (1.3 \u2013 1.6\u2009*10\u22122 s\u22121, Table 2). Hence the differences in weight-based activity might be explained by a different metal-support interaction and/or specific accessible Ni surface area for the different supports. Alternatively Gon\u04abalves et al. reported for activated carbon and carbon nanotubes [12,40], that the amination treatment increased the catalytic activity during CO2 hydrogenation, due to enhanced adsorption of CO2\n[23,40,57]. The latter was ascribed to the increased basicity of the support, where the adsorbed reaction intermediates can spill over from the basic groups onto the metal.\n\nFig. 5A compares the selectivity towards CH4 of the Ni catalysts on different supports. Ni/GNP-N gave the highest CH4 selectivity (initially 92%). The initial selectivity was slightly lower for Ni/GNP-O (86%) and lowest for Ni/GNP (82%). Over the course of 100\u2009h on stream, the CH4 selectivity of Ni/GNP-O and Ni/GNP-N were relatively stable, while a substantial decrease in selectivity was observed for Ni/GNP (to 65% after 100\u2009h CO2 hydrogenation). In all cases CO was the main side product.One must take into account that in the low conversion range the selectivity to CH4 increases with conversion [45]. After the 100\u2009h stability test, the reactant flow (and as a result GHSV) was changed to vary the CO2 conversion (Fig. S13). This allowed the study of CH4 selectivity versus CO2 conversion (Fig. 5B). Interestingly, the curves for Ni/GNP-N and Ni/GNP-O completely overlap. For Ni/GNP the CH4 selectivity versus CO2 conversion was substantially lower than for Ni/GNP-N and Ni/GNP-O. Thus, the support surface treatments have a positive effect on the CH4 selectivity although it does not seem to matter which kind of surface groups are introduced.Nanoparticle size and/or the formation of nickel carbide or a carbon layer around the Ni particles might influence the selectivity. However, a slight increase in selectivity is expected for larger particles [45], hence particle size effects cannot explain the differences. Besides, particle growth was severe for Ni/GNP-N, without a great change in selectivity. Similar to the fresh catalysts, XPS showed no indication of the presence of nickel carbide in the used Ni/GNP-O and Ni/GNP (Fig. S9).For pristine GNP, traces of support with different morphology might have affected the catalysis. Small amounts of, for instance, alkali elements might act as promoter or poison for supported metal catalysts [16,58\u201360]. With SEM-EDX analysis, no impurities were detected except some SiO2 in both GNP and GNP-O (Fig. S5). Because promoters might be active in low concentrations [61], impurities with concentrations below the detection limit of SEM-EDX might still have affected the selectivity. Hence we gave the pristine carbon a mild treatment not to introduce any surface groups, but nevertheless mimicking the treatment for GNP-O and GNP-N by washing in 1\u2009M HNO3 at room temperature. The surface area of GNP-W was similar to GNP (488 vs 456\u2009m2 g\u22121 respectively) and the TPD-MS profile was barely affected (Fig. S14). The selectivity of Ni/GNP-W was greatly enhanced as a result of the washing, and now similar to the selectivity of Ni on functionalized carbon (see Fig. 7). At the same time, the washing did not affect the activity as the CO2 conversion was still similar to the conversion Ni/GNP (Fig. S15). This shows that most likely the CH4 selectivity is very sensitive to low concentrations of contaminants. The exact influence of small concentrations of contaminants is interesting for further study. In conclusion, support treatment had a positive effect on the CH4 selectivity, most likely explained by the removal of traces of impurities.The activity evolution, normalized to the activity at t\u2009=\u20090, is depicted in \nFig. 6A. The activity loss of Ni/GNP-O was 19\u2009\u00b1\u20095% during 100\u2009h on stream, whereas for Ni/GNP this was 28\u2009\u00b1\u20095%. The most severe deactivation occurred for Ni/GNP-N, which lost 37\u2009\u00b1\u20095% activity during 100\u2009h CO2 hydrogenation under these conditions.The most likely explanation for the activity loss is the loss of Ni active surface, as TEM analysis after CO2 hydrogenation (Fig. 6B-D) revealed particle growth in all catalysts. However, the extent of the particle growth was distinctly different for the different catalysts. The least growth occurred for the Ni nanoparticles in the Ni/GNP-O catalysts (from d\n\ns\n = 5.1\u2009\u00b1\u20091.2 to 6.0\u2009\u00b1\u20092.0\u2009nm). This was followed by Ni/GNP (from d\n\ns\n = 5.3\u2009\u00b1\u20091.4 to 7.6\u2009\u00b1\u20092.7\u2009nm) while most severe particle growth had occurred for Ni/GNP-N (from d\n\ns\n = 5.0\u2009\u00b1\u20091.7 to 10.7\u2009\u00b1\u20095.6\u2009nm. This trend was confirmed by XRD (Fig. S16) and H2 chemisorption (Table 2). Overall the turnover frequencies were quite similar for all three catalysts, both before and after catalysis (Table 2). Although the main peak of the histograms was located around 5\u20136\u2009nm for all catalysts, the size distributions of Ni/GNP and Ni/GNP-N were more broad than for Ni/GNP-O, especially a longer tail of large particles was visible in the histograms. For Ni/GNP-N, 12% of the nanoparticles counted had a diameter >\u200910\u2009nm, whereas this value decreased to 5% for Ni/GNP and only 1.4% for Ni/GNP-O. A modest increase of intrinsic activity is expected with increasing particle sizes up to \u223c8\u2009nm, at least for Ni/GNP-O [45]. However, if there is an optimum above this size, as for example is the case for Co in Fischer Tropsch catalysis [62,63], one would expect the nanoparticles that had grown substantially (> 10\u2009nm) to be less active in catalysis due to the lower surface area.\n\nFig. 7 shows high resolution STEM images of the catalysts after CO2 hydrogenation, acquired in both High Angle Annular Dark Field (HAADF-STEM) and integrated differential phase contrast (iDPC) mode. iDPC analysis allows the visibility of the light carbon in the same image as the heavier nickel nanoparticles [64,65], The HAADF-STEM images revealed the appearance of core-shell nanoparticles, with a metallic Ni core surrounded by a 1\u20132\u2009nm NiO shell, as expected from passivation in air. Some small (<5\u2009nm) nanoparticles were fully oxidized. Especially for GNP-N, the Ni nanoparticles on the edge of the carbon sheets appeared less embedded in the carbon compared to the other supports (Fig. 7E and F). These observations might illustrate a weaker interaction of the nickel metal with specifically the GNP-N support, which has a low density of functional groups and could explain both the initially higher CO2 conversion and the poorer stability of Ni/GNP-N.The HAADF-STEM images and the iDPC images of Fig. 7 further showed that, independent of the support used, several nanoparticles were partially covered by carbon. The combination of the two imaging modes allows identification of these thin carbon layers. Although no statistical information can be derived from the 2D TEM images, we did not observe clear indications that aminated carbon in particular prevented Ni surface coverage as suggested by Wang et al. [21] At the same time, for none of the supports it was observed that these layers of (graphitic) carbon fully covered the nickel surface. This is in line with the accessible metal surface area as measured by H2 chemisorption as well as the fact that upon exposure to air the nickel nanoparticles were oxidized. The latter would be prevented when they would be fully encapsulated in carbon [66].Besides particle growth, we checked whether changes in the support might have contributed to the activity loss. The support surface areas remained between 200 and 220\u2009m2 gNi\n\u22121 (Table S3). The D-parameter of both Ni/GNP-O and Ni/GNP was 22\u2009\u00b1\u20091.0\u2009eV, within error the same as before CO2 hydrogenation (23\u2009\u00b1\u20091.0\u2009eV) ((Table S4). Also, the ratio between C\u2212O and CO surface groups remained the same for both Ni/GNP and Ni/GNP-O (Table S5). Upon heating, the used Ni/GNP-N catalysts still released NOx in oxygen atmosphere (Fig. S7). Finally, for none of the catalysts, the Ni weight loading was affected (Table 2). This shows that under the catalytic conditions used, the catalyst supports were fairly stable, and there was no significant Ni leaching. At the same time, compared to Ni on oxidic supports [7,67] these catalysts were neither the most stable, nor the most active catalysts for CO2 hydrogenation. However, it was not our aim to improve existing industrial catalyst but rather to present a series of model catalysts, allowing fundamental studies on support effects. This could be extended to, for instance the addition of another metal, such as Fe [6,40], or metal oxide promoters [35,67\u201369] improve catalyst activity and/or stability.All data support the conclusion that the catalyst deactivation was related to a loss of Ni active surface area, due to particle growth, which was influenced by the support properties. Ni/GNP-O was clearly the most stable catalyst, followed by Ni/GNP and finally Ni/GNP-N. It is most likely that the Ni particles remain smallest, and hence the most active in the GNP-O support due the remaining surface groups on this support. The treatment of the carbon to introduce functional groups might also have created defects [38]. However, if these were responsible for anchoring the nanoparticles, these must have been removed during the amination treatment. It is interesting that the interaction of the Ni nanoparticles was stronger with GNP-O than with GNP or GNP-N, despite the fact that nitrogen-containing surface groups are reported to stabilize nanoparticles [21,39,40,70]. Carboxylic surface groups are unstable during the heat treatments and thus are least likely to be present during high pressure CO2 hydrogenation. Nevertheless, their presence during Ni deposition could have resulted in a higher degree of nanoparticle embedding in the carbon support. Altogether, the support surface groups that are stable up to higher temperatures (> 350\u2009\u00b0C), such as anhydrides, phenols, lactones or quinones are most probable to have contributed to the higher stability of the Ni nanoparticles, either by anchoring them, or by blocking their movement over the support.We have demonstrated the effect of carbon support functionalization with introduction of both oxygen and nitrogen-containing surface groups for Ni supported catalysts for CO2 hydrogenation. The surface modifications did not severely affect the Ni nanoparticle size of the fresh catalysts. The treatment to introduce nitrogen-containing surface groups resulted in the initially most active, but also least stable catalyst. Both phenomena were likely caused by the weak interaction between the Ni particles and the support, caused by the low amount of surface groups present. A higher available metal surface area benefited the activity, without affecting the TOF. The introduction of oxygen-containing surface groups significantly enhanced the catalyst stability. The oxygen-containing surface groups that were stable above 350\u2009\u00b0C, either anchored the nanoparticles or prevented them from moving over the support. Finally, we showed that the type of support surface groups did not affect the CH4 selectivity significantly, but it was important to remove trace contaminants. With this work we showed that initial improvement in activity is not always optimal for long term catalysis. When using carbon as a support, the introduction of oxygen-containing support surface groups is advised for the severe conditions needed for synthesis of Ni-based catalysts (at least 350\u2009\u00b0C) and high pressure CO2 hydrogenation.\nNienke L. Visser: Conceptualization, Methodology, Investigation, Formal analysis, Validation, Writing \u2013 original draft, Visualization, Project administration. Juliette C. Verschoor: Methodology, Investigation, Formal analysis, Validation. Luc C.J. Smulders: Investigation. Francesco Mattarozzi: Formal analysis. David J. Morgan: Investigation, Formal analysis. Johannes D. Meeldijk: Investigation. Jessi E.S. van der Hoeven: Supervision. Joseph A. Stewart: Conceptualization. Bart D. Vandegehuchte: Conceptualization. Petra E. de Jongh: Conceptualization, Resources, Supervision, Project administration, Funding acquisition. All authors: Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank Jan Willem de Rijk and Remco Dalebout for their support in the catalytic experiments. Suzan Schoemaker, Kristiaan Helfferich and Laura Barberis are thanked for performing the N2-physisorption experiments. Dennie Wezendonk is kindly acknowledged for performing the TGA-MS experiments. We thank Ali Kosari for his input on the HRTEM measurements and Claudia Keijzer for performing the SEM measurements. This project is part of the Consortium on Metal Nanocatalysis funded by TotalEnergies OneTech Belgium, under TOTB Contract Ref IPA-5443.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cattod.2023.114071.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  The interaction between metal nanoparticles and a support is of key importance in catalysis. In this study, we demonstrate that the introduction of oxygen- or nitrogen-containing surface groups on a graphite nanoplatelet support influences the performance of nickel supported catalysts during CO2 hydrogenation. By careful design of the synthesis conditions, the Ni nanoparticle size of the fresh catalysts was not affected by the type of support surface groups. A combination of H2 chemisorption and high resolution TEM demonstrates that the available metal surface depends on the interaction with the carbon support. The amination treatment to introduce nitrogen-containing groups results in the weakest interaction between the Ni and the support, showing the highest initial Ni weight-based activity, although at the expense of nanoparticle stability. Hence initial enhancement in activity is not always optimal for long term catalysis. The use of carbon with a higher density of oxygen functional groups that are stable above 350\u00a0\u00b0C, is beneficial for preventing deactivation due to particle growth. Furthermore, small amounts of contaminants can have a substantial influence on the CH4 selectivity at low conversions.\n               "}
{"full_text": "Water splitting by electrolysis (2H2O\u00a0\u2192\u00a0O2 +\u00a02H2) provides a possible path for the conversion of clean, renewable energy to H2 fuel to power human civilization [1,2]. The efficiency of water electrolysis is partially limited by the high kinetic overpotential associated with driving the oxygen evolution reaction (OER) [1,3,4]. Therefore, development of efficient catalysts is indispensable to facilitate fast kinetics (i.e. low overpotential). An ideal OER catalyst would be composed of nontoxic earth-abundant elements, economical to manufacture, chemically and mechanically stable, and sufficiently electrically conductive [5\u20137].Development of improved catalysts can be accelerated by an enhanced understanding of the underlying electrocatalytic mechanism and its dependence on catalyst composition and structure. The paradigm for understanding heterogeneous OER catalysis that has emerged over a century of research is based on the application of the Sabatier principle. The OER occurs on the catalyst surface sites, M, via a series of intermediates (e.g. M-OH, M-O, M-OOH, M-OO) [3]. If all of the intermediates are bound by an M-O bond, plotting activity versus the M-O bond strength should, in principle, result in a volcano-shaped graph, i.e a so-called \u201cvolcano plot\u201d. At either side of the apex of the volcano the bond strength is sub-optimal; surfaces with either too large or too small M-O bond strengths are poor catalysts, as both lead to rate-determining steps with free energies that are larger than the average free energy for the steps in the mechanism [3]. OER catalysts based on earth-abundant first-row transition metals is of particular interest, as these catalysts might be used in water electrolysis or photoelectrolysis systems at a scale commensurate with global energy use. Consequently, there have been many experimental and computational efforts to correlate OER activity to chemical or material parameters. Mn [8], Fe [9], Co [10], and Ni-based [11] metal oxides and (oxy)hydroxides have been broadly studied and benchmarked for OER catalysis.Ni and its bimetallic oxides, particularly with Fe, are state-of-the-art catalysts in alkaline medium [12\u201315]; a Ni0.9Fe0.1O\nx\n OER activity was reported to surpass that of IrO2\n[11]. Early studies by Corrigan and co-workers [16,17] and more recent ones by Boettcher et al. [18] show that Fe impurities from the electrolyte are readily incorporated into the Ni(OH)2 and significantly enhance the activity, but the role of Fe is still being debated. While various types of NiFe with alloys have been reported [14], Cui et al. [19] reported that a porous monolithic NiFe structure prepared by dealloying NiFeAl alloy exhibited much higher OER activity than the NiFe alloy itself. The improved performance was attributed to a large number of active sites and fast electron/mass transfer induced by the porous structure.Despite the high activity of some metal oxides, reported so far, most metal oxides possess insufficient electric conductivity for electrocatalytic purposes, as a low conductivity impedes the electron transport inside the bulk of the catalysts and between neighbouring catalyst nanoparticles (NPs), compromising kinetics [20]. However, Stevens et al. [21] concluded that in case of electrodeposited thin layers, conductivity enhancements does not necessarily enhance the electrocatalytic activity. Moreover, transition-metal-oxide/transition-metal nanocomposites such as NiO/Ni, FeO/Fe, and CoO/Co are inherently magnetic, the magnetic properties varying with size, crystal structure, and morphology, thus showing a wide variety of intriguing phenomena [22,23]. In the present context, the main issue concerning magnetism is that it may adversely affect colloidal stability and lead to particle agglomeration. This, in turn, decreases the active surface area and consequently leads to lower catalytic activity [24,25]. However, it should be noted that the decrease in activity due to magnetism might not be palpable when the catalysts are prepared via electrodeposition or formed on a porous support [26] as the agglomeration happens just in the powder form, and not an issue when they are prepared on a substrate. Therefore, optimizing the electrical behavior of the transition metal oxides or hydroxides and enhancing their colloidal stability to maintain the desired high specific surface area are two main properties that need to be considered in designing efficient catalysts. In order to overcome the aforementioned issues, conductive additives, such as carbon, have been extensively used to support transition metals and semiconducting or insulating metal oxide nanoparticles [27,28]. However, corrosion of carbon materials under OER conditions is under debate, and the absence of a solution to this problem prevents the industry from considering them as additives or supports for anodes in water electrolysis systems. Moreover, anodic degradation of carbon materials may not only decrease the extent of metal oxide utilization during the OER, but also leads to an uncertainty in the determination of the OER activity if the corrosion contribution to the oxidation current is not considered explicitly [29\u201331].In this context, transition metal phosphides (TMPs) [32] and carbon-encapsulated materials [33,34] have been reported as promising candidates for efficient electrocatalysis with enhanced activity compared with transition metal or metal oxides, which can be ascribed to both their nonmagnetic nature [35] (which translates to higher active surface area), and optimizing the electron transport inside the bulk of the electrocatalyst [36]. Among, all the tested transition metal-based catalysts, TMPs have the lowest overpotentials demonstrated to date [37]. A number of studies show that TMPs undergo an in situ electrochemical transformation under anodic oxidation conditions, being irreversibly converted to transition metal (oxy)hydroxides (TMOHs). These TMHOs have been proposed to be the true catalytically active species for the OER [38]. On the other hand, no such transformation was observed in TMP electrocatalysts after the OER by Liu et al. [39] and Liang et al. [40]. In this respect, TMPs are mainly considered as \u201cpre-catalysts\u201d, i.e. a catalyst that transforms into the actual catalytic material under and as a consequence of the operating conditions, rather than \u201ccatalysts\u201d that maintains its nascent structure under any relevant conditions [32,38].Interestingly, there are many reports showing that the electrocatalytic activity of TMPs is enhanced by in situ formation of TMHOs on the surface. In other words, TMHOs-TMP composites formed in situ exhibit a better apparent OER performance than the corresponding pristine TMOs or TMHOs synthesized directly [41,42]. Although the underlying mechanisms are not fully understood, many studies have provided clues that the electrochemical oxidation of TMPs would enable the exposure of high density catalytically active sites. Moreover, any TMP with superior conductivity underneath a TMHO surface layers would facilitate electron transfer at the interface as well as electron transport inside the bulk component [43,44]. In the past few years, large research efforts have been devoted towards developing various TMP pre-catalysts for use in catalyzing the OER.The concept of encapsulating nanoparticles of non-precious 3d TMs and their alloys in various carbon matrices as an alternative towards efficient catalysts for the OER, ORR, and HER has recently attracted substantial attention [34,45,46]. Depending on the purpose, the carbon shell in carbon encapsulated nanoparticles plays different roles or provide multi-functionality. For instance, carbon encapsulated Pt nanoparticles in which Pt is electrochemically active show high durability as a result of a protection provided by the carbon shell. On the other hand, the electronic properties of the carbon shell can be modulated by the metallic nanoparticle cores, allowing for the binding energies of reaction intermediates on the carbon surface to be tuned. In some cases, carbon encapsulated metal nanoparticles exhibit high activity simultaneously against a variety of electrochemical reactions (e.g., HER and OER), demonstrating a bi-functional catalyst [47\u201349].Different methods, such as chemical vapor deposition (CVD), the polymer coating method, the solvothermal method, and the high-temperature pyrolytic method, have been utilized to form a thin carbon shell to encapsulate metal nanoparticles. Among all these, the solvothermal method has been given the most interest due to several advantages including a low temperature process (\u00a0<\u00a0300 \u00b0C), morphology tuning, time-efficient, possible scale-up, possibility of engineering the carbon shell, and so forth [47].For the first time, Carenco et al.\n[50] reported synthesis of carbon-encapsulated Ni2P nanoparticles via a solvothermal method, in which amorphous Ni2P nanoparticles were synthesized with excess amount of trioctylphosphine (TOP) at 220 \u00b0C and then subsequently converted to carbon-encapsulated nanoparticles by heating in a Schlenk tube for 30\u2009min at 400 \u00b0C, under N2. The carbon layer was formed due to the decomposition of an excess amount of TOP during an annealing procedure.Recently, Jung and co-workers [47,51] have reported the synthesis of various transition metal nanoparticles encapsulated by carbon shell through the solvothermal method, which involves decomposing metal acetylacetonates precursors in organic solvents with surfactants under inert atmospheres at temperatures below 300 \u00b0C, after which the products are processed and subjected to annealing under different gas conditions to yield different carbon encapsulated metal structures. The carbon layer formed through the annealing step, in which the carbon atoms absorbed inside the lattice of the metal alloys diffuse to the nanoparticle surface, producing a mono or bilayer-level uniform carbon shell at the sub-nm scale.In the present work, we report the fabrication of ternary Ni12\u2212x\nFe\nx\nP5 nanoparticles (x\u2009=\u20090,\u00a01.2,\u00a02.4,\u00a03.6) via a colloidal synthesis route. By introducing Fe precursors to the synthetic solution, a self-generated carbon layer surrounds the particles as the native ligand covering the nanoparticles is decomposed and lead to the formation of a carbon layer. This is contingent on the decomposition of the precursors happening at a high enough temperature, 300 \u00b0C in this work, and which is possibly catalyzed by the Ni-Fe bimetallic system. The key aspect of this catalyst design is that the carbon layer can provide a large specific area and interconnected electrically conducting networks which promotes the electrocatalytic activity of NiFeP nanoparticles significantly. Moreover, the stability of the carbon layer and NiFeP catalyst after being subjected to OER conditions were evaluated by TEM and Raman spectroscopy.Oleylamine (OAm; technical grade, 70 %), tri-n-octylphosphine (TOP; 97 %), nickel(II) acetylacetonate (Ni(acac)2; 97 %), iron(III) acetylacetonate (Fe(acac)3; anhydrous, 95 %), toluene (anhydrous, 99.8 %), acetone (99.5 %), isopropanol (IPA; 99.5 %), potassium hydroxide (99.99 %), and (5\u2009wt%) Nafion 117 solution. All chemicals were purchased from Sigma-Aldrich and used as received, without further purification.Deionized water (DI-water), generated by a Milli-Q water system 18.2\u2009M\u03a9\u2009cm\u22121, was used for all measurements.For all the catalysts in this work, the entire synthesis was completed in a single reactor in a dry, oxygen-free, Ar atmosphere (99.9999 %) by the use of Schlenk lines and a glove box. The protocol developed to synthesize Ni12\u2212x\nFe\nx\nP5 nanoparticles is based on the method refined by Muthuswamy et al. [52] to synthesize discrete Ni12P5 phase-pure nanoparticles.Formation of Ni12P5 and nickel-iron phosphide nanoparticles was achieved by reaction of Ni(acac)2 or mixtures of Ni(acac)2 and Fe(acac)3, respectively, with TOP as the P source in the presence of oleylamine via a two-step process. The two-step procedure is comprised of the generation of Ni and Ni\nx\nFe1\u2212x\n precursor particles at 220 \u00b0C followed by further reaction and crystallization at 300 \u00b0C. In a typical synthesis, 50\u2009mL of OAm (156\u2009mmol) was added to a 250\u2009mL three-neck round bottom flask and evacuated for 10\u2009min at room temperature. In the next step the corresponding amount (overall 15.6\u2009mmol) of the two metal precursors (Ni(acac)2 and Fe(acac)3) (Fe:Ni molar ratios were 0.1, 0.2, or 0.3), and 14\u2009mL TOP (31.2\u2009mmol) were added to the solution and kept at 50 \u00b0C (ramp rate of \n\n\n\n3\n\n\n\u00b0\n\n\nC\n\n\nmin\n\n\n\u2212\n1\n\n\n\n) for 5\u2009min under Ar atmosphere (99.9999 %). Then the temperature was ramped to 220 \u00b0C at rate of \n\n\n\n8\n\n\n\u00b0\n\n\nC\n\n\nmin\n\n\n\u2212\n1\n\n\n\n and kept at this temperature for 2\u2009h. In the second step the flask was heated further until 300 \u00b0C and kept for 30\u2009min at this temperature. Once the reaction had finished, the flask was left to cool to room temperature either gradually while it was kept inside the heating mantle or with the heating mantle removed immediately after synthesis. The nanoparticles were isolated and washed at least three times using a mixture of isopropanol, toluene, and acetone to remove the remaining reagents and organic matter. Black powder (1.2\u2009g) was obtained, which corresponds to a 100 % yield of Ni12\u2212x\nFe\nx\nP5 nanoparticles.We will designate the Ni12\u2212x\nFe\nx\nP5 compositions as Ni10.8Fe1.2P5 for x\u2009=\u20091.2,\u00a0Ni9.6Fe2.4P5 for x\u2009=\u20092.4, and Ni8.4Fe3.6P5 for x\u2009=\u20093.6 below.Powder X-ray diffraction (PXRD) was carried out on a Bruker D8 DaVinci X-ray Diffractometer with Cu K\u03b1 radiation (Billerica, Massachusetts, USA). Samples were deposited onto zero background silicon sample holders and analyzed in the 2\u03b8 range between 20\u00b0 and 80\u00b0 with a step size of 0.04\u00b0 and a collection time of 6\u2009s. Identification of phases was made by comparison to the powder diffraction files (PDFs) of the International Center of Diffraction Data (ICDD) using Eva 5.1 software. The background was subtracted using EVA software for easier phase identification.Rietveld analysis was carried out using the Bruker TOPAS version 6.0, using a pseudo-Voigt function model. Refinements of diffraction patterns were performed within space groups Fd-3\u2009m:1, I4/m. The occupancies were set to nominal values and were not refined.Scanning transmission electron microscopy (S(T)EM) was carried out on a Hitachi S-5500 FESEM (Krefeld, Germany) equipped with an INCA 350 energy-dispersion X-ray (EDS) analysis unit. Acceleration voltages of 30\u2009kV and 20\u2009kV were used for the images and the analyses, respectively. All samples were prepared by dropping a toluene suspension containing uniformly dispersed nanoparticles on a carbon film supported on a 300-mesh copper grid.TEM bright-field, TEM high-angle annular dark-field imaging (HAADF), and TEM-EDS were performed using a spherical aberration-corrected field emission JEOL 2100F TEM operating at 200\u2009kV. EDS mapping was performed using a JEOL Silicon Drift Detector.Raman spectroscopy was carried out using a WITec alpha 300 R Confocal Raman device equipped with a 532\u2009nm laser. Raman spectra were obtained after 20 accumulations for 20\u2009s from 100 to 1250\u2009cm\u22121.Spectra were collected on an Axis Ultra (Kratos Analytical) equipped with a Mg K\u03b1 X-ray source operating at 280\u2009W Physical Electronics radiation source. The samples were analyzed under ultra-high-vacuum conditions (2.5\u2009\u00d7 10\u221210 Torr base pressure). After recording a broad range spectrum (pass energy, 100\u2009eV), high-resolution spectra were recorded for the C 1s, Ni 2p, Fe 2p and P 2p core XPS levels (pass energy, 200\u2009eV). The binding energies were calibrated with respect to the C 1s peak at 284.8\u2009eV. Spectrum processing was carried out using the Casa XPS software package.Electrochemical characterization was carried out in a standard three-electrode rotating disc electrode (RDE) setup from Pine Instruments. Polished glassy carbon (GC) electrodes were used as working electrodes (A = 0.196\u2009cm2, Pine Instruments) and a Pt mesh was used as a counter electrode. The working electrode potentials were measured versus a Hg\u2223HgO reference electrode filled with 4.2\u2009mol dm\u22123 KOH from Pine Instruments. Polytetrafluoroethylene (PTFE) containers were used both for electrochemical experiments and electrolyte preparation. All measurements were controlled using a Bio-Logic Potentiostat/Galvanostat (Model VMP3) in 1\u2009mol dm\u22123 KOH (Fe-free electrolyte, 99.99 % and 85 % trace metal basis).Cyclic and linear sweep voltammograms were collected at a rotation frequency of 1600\u2009rpm. Polarization curves were collected using chronoamperometry with Eappl (applied potential) stepped from 1.4 to 1.7\u2009V vs. RHE in 20\u2009mV increments. At each potential step, steady-state data were collected at angular velocities (\u03c9) corresponding to rotational frequencies of 2000 and 600\u2009rpm. Data were also collected in the absence of disk rotation. Catalyst inks were prepared by dispersing 2.5\u2009mg of the catalyst powders in a mixture of 750\u2009\u03bcL of milli-Q water, 250\u2009\u03bcL of 2-propanol, and 50\u2009\u03bcL of Nafion (5\u2009wt%). The inks were homogeneously dispersed by ultrasonication for 20\u2009min and then 10\u2009\u03bcL was drop-cast on the GC electrode to make up a final metal loading of 0.12\u2009mg\u2009cm\u22122. All electrochemical data were corrected for uncompensated series resistance after data collection. The uncompensated resistance of the cell was measured with a single-point high-frequency impedance measurement, and IR drop was compensated at 85 % through positive feedback using the Bio-Logic EC-Lab software. Our electrochemical cell typically had R\n\nu\n ~ 4\u2009\u03a9 in 1\u2009mol dm\u22123 KOH. Electrochemical impedance spectroscopy measurements were carried out at five different overpotentials (0.6, 0.61, 0.615, 0.62, 0.625\u2009V vs. Hg/HgO from 10\u2009mHz to 1\u2009MHz with an amplitude of 10\u2009mV.Prior to all catalytic tests, the electrode was first subjected to continuous potential cycling at 50\u2009mV\u22121s in the potential range of 1.0 through 1.6\u2009V vs.(RHE) until reproducible voltammograms were obtained.The electrochemically active surface area (ECSA) was estimated from the double layer capacitance [53]. The double-layer capacitance, in turn, was estimated by cyclic voltammetry (CV) in a potential region in which faradaic currents can be assumed absent. The CV measurements were conducted in a quiescent solution by sweeping the potential across this non-faradaic region from the more positive to negative potential and back at 7 different scan rates: 10, 30, 50, 70, 100, 200, and 300\u2009mV\u22121s. The working electrode was held at each potential vertex for 10\u2009s before beginning the next sweep [54,55]. The double\u2013layer capacitance was estimated from the slope of the plots of the charging current i\n\nc\n vs. the scan rate \u03bd as dictated by the equation\n\n(1)\n\n\n\n\ni\n\n\nc\n\n\n=\n\n\nC\n\n\ndl\n\n\n\u00d7\n\u03bd\n\n\n\nin which C\ndl is the double-layer capacitance [55].For rotating ring-disk electrode (RRDE) experiments, electrodes with various loadings (12\u201348\u2009\u03bcg\u2009cm\u22122) were employed. 500\u2009\u03bcL of the ink described above was diluted with 500\u2009\u03bcL of milli-Q water. An amount of the ink corresponding to the desired loading was drop-cast on to a working electrode. The working electrode was a RRDE with a GC disk (5\u2009mm diameter) and a gold ring (7.5\u2009mm outer diameter and 6.5\u2009mm inner diameter) equipped with an MSR rotator system, both from Pine Research Instruments. The counter electrode was a smooth Pt wire and the reference electrode was a Hg/HgO electrode filled with 4.2\u2009mol dm\u22123 KOH. All cyclic voltammograms (CVs) of the disk electrode were recorded at a sweep rate of 10\u2009mV\u22121s. The RRDE collection efficiency (24.1 % at 900\u2009rpm) was determined from the ring and disk current ratios in 1\u2009mol dm\u22123 KOH +\u200910\u2009mol dm\u22123\n\n\n\n\nK\n\n\n3\n\n\n\n[\n\nFe\n\n\n\n(\n\nCN\n\n)\n\n\n\n6\n\n\n\n]\n\n\n solution. The ring potential (+0.3\u2009V vs. RHE) for RRDE studies of the OER was chosen based on previous reports for oxygen reduction reaction on a gold electrode [29,56]. Before each RRDE measurement, the gold surface of the ring electrode was cleaned by applying 100 potential cycles in the interval from 0.03 to 1.53\u2009V at 100\u2009mV\u2009s\u22121.The Faradaic efficiency was also calculated using an eudiometer set-up (Figure S.10) based on collecting the generated oxygen gas bubbles by applying 10\u2009mA (51\u2009mA\u2009cm\u22122) constant current. The amount of the generated O2 was calculated from the volume of gas evolved corrected for the water vapour pressure and relating the amount of oxygen to the measured volume through the ideal gas equation. The theoretical amount of O2 expected to be produced by applying 10\u2009mA (51\u2009mA\u2009cm\u22122) was calculated from the electrical charge passed through the electrode using the Faraday equation:\n\n(2)\n\n\nn\n\n(\n\n\nmoles of produced\n\n\n\n\nO\n\n\n2\n\n\n\n)\n\n=\n\n\nI\nt\n\n\n4\nF\n\n\n\n\n\n\nThe calibration of the Hg/HgO electrode was performed in a standard three-electrode system with polished Pt foil as the working and counter electrodes, and the Hg/HgO electrode as the reference electrode. Electrolytes were pre-purged and saturated with 99.999 % H2. Linear sweep voltammetry (LSV) was then performed at a scan rate of 0.5\u2009mV\u2009s\u22121, and the potential at which the current crossed zero was considered to be the thermodynamic potential for the hydrogen electrode reaction [57]. For example, in 1\u2009mol dm\u22123 KOH, the zero current point appeared at \u2212\u20090.900\u2009V, and so the potential with respect to the reversible hydrogen electrode (RHE) is given by E(RHE)\u00a0=\u2009E(Hg\u2215HgO)\u00a0+\u20090.900\u2009V.\n\nFig. 1 shows the XRD patterns for the different target compositions. The XRD diffractogram for the nickel phosphide composition without any Fe unambiguously matches that of the pure Ni12P5-phase (tetragonal) structure (PDF 03-065-1623). For all iron-containing compositions, the diffraction peaks were shifted to larger angles compared to the corresponding peaks in the Ni12P5 diffractogram. As shown in Fig. 1, the Ni10.8Fe1.2P5 nanoparticles crystallized in the same tetragonal phase as Ni12P5 nanoparticles, suggesting the formation of homogeneous Ni-Fe-P compositions with no detectable crystalline impurities.However, clear changes in the diffractograms can be discerned upon further increase in the Fe content to above x\u2009=\u20091.2. At an Fe content to above x\u2009=\u20091.2, the peak at 49.2\u00b0, corresponding to the (312) plane of Ni12P5, dwindled while the intensity of the peak at 47.1\u00b0, corresponding to the (420) plane of Ni12P5, increased. Also, while the diffractogram for the composition with x\u2009=\u20091.2 contained the same peaks as the Ni12P5 catalyst, new peaks have emerged for the compositions with x\u2009>\u20091.2. This suggests the development of a second phase for x\u2009=\u20092.4, i.e. when the Fe content is increased beyond x\u2009=\u20091.2. This second phase is most likely an Fe3O4 phase; the new peaks at 35.16\u00b0 and 31.7\u00b0 agree well with the (101) and (211) planes of Fe3O4, respectively. The peak corresponding to the (420) plane in Ni12P5 overlaps with the peak corresponding to the (202) plane in Fe3O4. Therefore, the increase in the intensity at 47.1\u00b0 with increasing Fe content can be attributed to a growing Fe3O4 phase. The peaks at 57\u00b0 and 62.6\u00b0 in Ni8.4Fe3.6P5 belong to the (115) and (044) planes in Fe3O4. For Ni8.4Fe3.6P5 (x\u2009=\u20093.6), Fe2O3 is formed as a third phase, and the reason for the higher intensity of the 49.2\u00b0 peak than in NiFeP@Fe3O4(x\u2009=\u20092.4) is likely to be due to its overlap with the (024) plane of Fe2O3. The emergence of the new peak at 32.7\u00b0 is also attributed to the Fe2O3, viz. its (104) plane, which substantiates the suggested presence of an Fe2O3 phase.Based on the Vegard\u2019s law for alloys, we would expect a linear relation between lattice parameters and the composition. However, such a linear behavior was not observed. This deviation from Vegard\u2019s law has been previously reported for Fe\nx\nNi2\u2212x\nP bulk solid solutions and nanoparticles, and has been attributed to an unequal distribution of the two different metals in sites of different size in the lattice [58,59].The Ni12P5 tetragonal structure type has two metal coordination sites. The Ni atoms in the first site are surrounded by the four nearest P atoms at distances 2.194\u20132.467\u2009\u00c5 and eight Ni atoms located at 2.526\u20132.725\u2009\u00c5. There are 11 atoms forming coordination polyhedra around the second Ni site, viz. two nearest P atoms at 2.260\u2009\u00c5 \u2013 2.283\u2009\u00c5, five Ni atoms at 2.517\u20132.575\u2009\u00c5, two P atoms at 2.619\u2009\u00c5, and two Ni atoms at 2.725\u2009\u00c5 [60]. The atomic radius of Fe is slightly larger than that of Ni 125\u2009pm vs. 121\u2009pm. Among the two available sites, we would normally expect Fe to occupy the larger one in Ni12\u2212x\nFe\nx\nP5. This is the case for high Fe fractions. However, studies of bulk hexagonal structures, which also have two different metal sites, suggest that occupancy is dependent on composition; at low Fe metal fractions, the smaller site is preferentially occupied by Fe. Goodenough [61] has suggested that the preference of Ni for the larger site in Fe-poor compositions is due to electron transfer from Fe to Ni. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) investigations of the Fe\nx\nNi2\u2212x\nP system [62] have revealed that the electron density of Ni atoms has been increased, presumably due to electron transfer from Fe to the more electronegative Ni atoms, consistent with this hypothesis.Bright-field TEM images of Ni12P5 show that the catalyst consists of quasi-spherical nanoparticles with average diameter of 15.20\u2009\u00b1\u20092.25\u2009nm. An example is given in Fig. S.1 in the Supporting Information. Analysis of the high-resolution TEM (HR-TEM) image (Fig. S.1(b)) gives lattice-fringe spacings of about 2.1\u2009\u00c5, corresponding to the (400) lattice plane of tetragonal Ni12P5. Energy-dispersive spectroscopy (EDS) in TEM indicate uniform distributions of Ni and P across the Ni12P5 nanoparticles (Fig. 1.2). Based on the EDS maps performed in the TEM, the apparent ratio of P:Ni was estimated to 0.38.TEM images of Ni12\u2212x\nFe\nx\nP5 nanoparticles (Fig. S.2, \n2, and S.3) show that the quasi-spherical Ni12P5 nanoparticles were converted to highly faceted nanoparticles with pentagonal cross-sections in TEM upon addition of Fe. The size distribution became broader with increasing Fe content.Analysis of the HR-TEM image of Ni10.8Fe1.2P5 nanoparticles (Fig. S.2(c)) revealed a d-spacing of 2.1\u2009\u00c5 corresponding to the (400) plane of the Ni12P5 tetragonal crystal structure, which is consistent with the XRD results. Moreover, the EDS mapping confirmed the uniform distribution of Ni, Fe, P, and O across the particles. The EDS composition was in relatively good agreement with the targeted stoichiometry.Upon increasing the Fe content to x\u2009=\u20092.4 and 3.6, the particles became more faceted and irregular in shape. Results of energy-dispersive spectroscopy (EDS) indicate a core-shell structure for the Ni9.6Fe2.4P5 and Ni8.4Fe3.6P5 nanoparticles (Fig. 2 and S.3) where Ni and P are evenly distributed in the core while Fe and O that reside in the shell dominate over that in the bulk. This partial segregation is compatible with the XRD patterns, which indicates the evolution of Fe3O4 as the second phase. The TEM images demonstrate that the Fe3O4 phase forms a shell surrounding a Ni12\u2212x\nFe\nx\nP5 core, in which 1.2\u2009< x (stoichiometry of Fe) <\u20093.6. The core is rich in Ni and P, while the shell is rich in Fe and O. For simplicity, we will refer below to these particles as NiFeP@Fe3O4(x\u2009=\u20092.4) for the sample of nominal composition x\u2009=\u20092.4 or NiFeP@Fe3O4(x\u2009=\u20093.6) for the sample of nominal composition x\u2009=\u20093.6, while referring to the compositions in the general sense as Ni12\u2212x\nFe\nx\nP5 as before when the catalyst architecture is not important.Close inspection of the TEM images of Ni12\u2212x\nFe\nx\nP5 nanoparticles reveals the existence of a relatively regular coating at least partly covering the NiFeP@Fe3O4 particles (Fig. 2(b) and (c)). The thickness of this layer varies from catalyst to catalyst and it is more developed (thicker) for NiFeP@Fe3O4(x\u2009=\u20092.4), and NiFeP@Fe3O4(x\u2009=\u20093.6) in comparison with Ni10.8Fe1.2P5.Raman spectra of the synthesized Ni12\u2212x\nFe\nx\nP5 nanoparticles are shown in \nFig. 3. The peak positions are listed in \nTable 1. All the recorded spectra were subjected to a Voight-based deconvolution analysis.The data in Table 1 show that when the Fe content increases all peaks below 350\u2009cm\u22121 are blue-shifted and those with wavenumbers higher than 1105\u2009cm\u22121 are red-shifted. As indicated in Fig. S.4 some of the observed peaks were attributed to NiO and FeO\nx\n species [63,64]. In all Raman spectra of Ni12\u2212x\nFe\nx\nP5 nanoparticles, Fig. 3, two peaks at around 1582 and 1360\u2009cm\u22121 can be clearly seen. For comparison, similar peaks were also observed at the glassy-carbon electrode used for the electrochemical measurements, see Section 3.3 below \nTable 2.XPS survey spectra recorded for Ni12\u2212x\nFe\nx\nP5 (see Figure S.5 in the Supporting Information) show clear peaks corresponding to Fe which are not present in the spectrum for the pure Ni12P5 phase. This indicates the successful incorporation of Fe in the former samples.\n\nFig. 4 (a) shows the Ni 2p XPS core-level spectra of the synthesized nanoparticles. The Ni 2p spectrum contains two main peaks, resulting from the spin-orbit splitting of the p orbital that are assigned as Ni 2p\n3\u22152 (850\u2013865\u2009eV) and Ni 2p\n1\u22152 (865\u2013885\u2009eV). The Ni 2p\n3\u22152 region was further deconvoluted into three peaks for Ni12P5, NiFeP@Fe3O4(x\u2009=\u20092.4), and NiFeP@Fe3O4(x\u2009=\u20093.64). However, since the satellite and oxidized Ni was quite well-separated for Ni10.8Fe1.2P, the Ni 2p\n3\u22152 region was therefore deconvoluted into four peaks. The peak at 853\u2009eV can be related to both Ni and Ni-P [13]. Unfortunately, an unambiguous separation of the contributions from these two species through XPS is challenging. A previous study by Li et al. attributed both Ni and Ni-P to the same BE of 853.1\u2009eV, [65] while others have tabulated Ni(0) at 852.7\u2009eV and Ni2P at 852.9\u2009eV, only 0.2\u2009eV apart [66]. We therefore made no attempt at separating the two contributions here. However, for NiFeP@Fe3O4(x\u2009=\u20093.6) the peak at 854.4\u2009eV can be exclusively assigned to Ni-P [67].Regarding the shift in Ni 2p peaks with addition of Fe, there is evidence in the literature [61,62] showing that electron transfer from Fe to Ni will take place in nickel iron phosphide compounds, which in turn increases the electron density of Ni atoms. Considering the fact that Ni atoms have higher electron density upon addition of Fe, we would expect a shift to lower binding energies in Ni. This is in accordance with our experimental results.The XPS spectra for the as-synthesized Ni12\u2212x\nFe\nx\nP5 nanoparticles (Fig. 4) could be fitted to an Fe 2p\n3\u22152 peak at 712.15\u2009eV and an Fe 2p\n1\u22152 peak at 724.18\u2009eV. This indicates that two distinct Fe species are present in the samples. The Fe 2p\n3\u22152 peak can, in turn, be decomposed into two peaks approximately at 706 and 713\u2009eV, respectively originating from the iron(0) and oxidized iron [68].For all the catalysts except Ni10.8Fe1.2P5, the P 2p region was deconvoluted into four peaks. For the Ni12P5 catalyst, components at 129.4, 130.4, 132.6 and 133.3\u2009eV, corresponding to phosphide, P(0), P(III) and P(V) species, respectively [69,70], proved to fit the spectra well. The values were in good agreement with the corresponding values reported for Ni12P5 in the literature [71]. The P(V) and P(III) components have been interpreted as surface phosphate and phosphite [70], respectively. These may have formed as a result of the exposure of the nanoparticles to air while being stored at the ambient conditions. Upon addition of Fe, a noticeable shift is observed in all the components, possibly due to the interaction of P with Fe. It is also worth noting that the fraction of oxidized phosphorous is larger in Fe-containing nanoparticles than in Ni12P5, which indicates that the addition of Fe makes particles more vulnerable to oxidation. The ratio of oxidized to non-oxidized phosphorous species increases in the order of Ni10.8Fe1.2P5 >\u2009NiFeP@Fe3O4(x\u2009=\u20093.6) >\u2009NiFeP@Fe3O4(x\u2009=\u20092.4), which is opposite of the order in terms of the thickness of the self-generated carbon layer. Therefore, it is reasonable to conclude that the carbon layer to some extent protects the particles from oxidation. For the Ni10.8Fe1.2P5 nanoparticles with the thinnest carbon layer, the metal-P (phosphide) and P(0) species were barely detectable, indicating negligible carbon-layer protection and extensive surface oxidation of nanoparticles. All the parameters obtained from fits to the XPS data are presented in Table S.1.\n\nFig. 5 shows cyclic voltammograms of Ni12\u2212x\nFe\nx\nP5 catalysts. For comparison, an Fe-free Ni12P5 catalyst was also tested as a benchmark compound to explore the effect of the addition of Fe on the electrocatalytic activity. The CVs for all the tested catalysts contained redox peaks at potentials below the onset of the oxygen evolution reaction, attributed to Ni3+\u2215Ni2+. However, the peak position differs depending on the composition of the catalyst.For the Ni12P5, the anodic redox peak appears at E\nanodic =\u20091.36\u2009V in the CV and the cathodic peak at E\ncathodic =\u20091.28\u2009V. Interestingly, upon addition of Fe the anodic redox peak is shifted towards positive potentials. The anodic peak in Ni10.8Fe1.2P5 is split into two peaks (\u00a0~ 1.34 and 1.40\u2009V) whereas the cathodic peak is observed at the same potential as Ni12P5.Splitting of the anodic peak is also observed for the NiFeP@Fe3O4(x\u2009=\u20093.6) catalyst. A wider separation of the peaks was observed in this case, however, with peak positions at E\nanodic =\u20091.34 and 1.42\u2009V. The splitting of the anodic peak suggests two types of Ni sites in the particles, one corresponding to Ni sites in Ni12P5 and another at which Ni interacts with Fe. The absence of any cathodic split maybe related to sluggish kinetics. The cathodic peak shifts to the more positive potential of 1.34\u2009V. Finally, the cyclic voltammogram of the NiFeP@Fe3O4(x\u2009=\u20092.4) catalyst shows a redox peak without any splitting at E\nanodic =\u20091.41\u2009V and E\ncathodic =\u20091.34\u2009V.The shift in the Ni3+\u2215Ni2+ redox peak to more positive potentials upon addition of Fe is well documented, and has generally been attributed to the stabilization of the Ni2+ state in the presence of Fe [72\u201374]. The larger peak current in the case of NiFeP@Fe3O4(x\u2009=\u20092.4) is opposite of what is normally reported in the literature [75,76], and the effect of Fe is usually that of reducing the peak current density.\n\nFig. 6 shows the linear sweep voltammograms for the Ni12\u2212x\nFe\nx\nP5 catalysts. Fig. 6 also includes polarization curves recorded by chronoamperometry, which are in excellent agreement with those recorded by linear sweep voltammetry. The overpotential needed for all tested catalysts to deliver 10 and 50\u2009mA\u2009cm\u22122 (i.e. \u03b7\n10 and \u03b7\n50) are tabulated in \nTable 3. To reach the benchmark current density \u03b7\n10 at the Ni12P5, an overpotential of 301\u2009mV is needed, while NiFeP@Fe3O4(x\u2009=\u20092.4) merely requires an overpotential of 220\u2009mV, showing a significant improvement in the OER activity. The apparent OER activity per mass for all the tested catalysts follows the order: Ni12P5 <\u2009Ni10.8Fe1.2P5 <\u2009NiFeP@Fe3O4(x\u2009=\u20093.6)\u00a0<\u2009NiFeP@Fe3O4(x\u2009=\u20092.4).The polarization curves presented in Fig. 6 all show an up-turn at high overpotentials, which is a common feature of plots of electrode potential vs. the logarithm of current for the OER as presented in the literature [77\u201380,81,82]. Such changes in the slope \n\nd\nE\n\u2215\nd\nlog\ni\n\n with increasing potential are most often attributed to either a change in the rate-determining step (rds) within a given pathway [79] or to saturation or depletion of intermediates at the surface [77]. The degree of consistency between the data recorded by LSV and CA, suggests that the dual-slope behavior is mechanistically significant and not due to electrode blocking, mass-transport limitations or ohmic effects. Kinetic parameters, including Tafel slopes (i.e. \n\nd\nE\n\u2215\nd\nlog\ni\n\n), determined from the lower overpotential region (below the up-turn) are presented in Table 3.The results of EIS measurements, plotted as Tafel impedance (Z\n\nt\n), at different overpotentials for Ni12\u2212x\nFe\nx\nP5 with (x\u2009=\u20090,\u00a01.2,\u00a02.4,\u00a03.6) are shown in \nFig. 7. Z\n\nt\n was computed from the impedance by multiplication of the latter with the steady-state current density as [83],\n\n(3)\n\n\n\n\nZ\n\n\nt\n\n\n=\n\n\n\n\nE\n\n\n\u02dc\n\n\n\n\n\n\ni\n\n\n\u02dc\n\n\n\n\n\n\ni\n\n\ns\ns\n\n\n\n\n\nwhere \n\n\nE\n\n\n\u02dc\n\n\n is the potential amplitude, \n\n\ni\n\n\n\u02dc\n\n\n the current-density amplitude, and i\n\nss\n is the steady-state current density. (The ohmic resistance, as assessed from the high-frequency intercept of the impedance-plane plot with the real axis, was subtracted from all data prior to the conversion to Tafel impedance). As can be seen, the low-frequency intercept increases slightly as the overpotential is increasing. In these plots, the dE\u2215dlogi slope can be read off as the value of the low-frequency intercept with the real axis [84,85]. For all samples the diameter of the arc in the Tafel-impedance plane plot are in reasonable agreement with the slopes from the steady-state curves, Fig. 6. However, due to some ambiguity in determining the appropriate region to use for fitting the steady-state data, we consider the Tafel slopes obtained through impedance to represent the more accurate of the two sets of values. The Tafel slopes from the impedance data cluster around 40\u2009mV for all the iron-containing samples (Ni10.8Fe1.2P5, NiFeP@Fe3O4, and NiFeP@Fe3O4), whereas the Tafel slope for Ni12P5 is significantly higher, 60\u2009mV).Data from which the double-layer capacitances (C\ndl\n) were evaluated and the ECSA were estimated, are given in the Supporting Information(Fig. S.6). C\ndl\n values of 3.35, 3.56, 2.85 and 2.26\u2009mF\u2009cm\u22122 were obtained for NiFeP@Fe3O4(x\u2009=\u20092.4), NiFeP@Fe3O4(x\u2009=\u20093.6), Ni10.8Fe1.2P5, Ni12P5 respectively. In general, the double layer capacitances and hence the ECSA for the iron-containing Ni12\u2212x\nFe\nx\nP5 catalysts were found to be larger than those for the Ni12P5 catalyst. In effect, the C\ndl\n value is increasing along with the thickness of the carbon shell.\n\nFig. 8 compares the mass activity and overpotential of the NiFeP@Fe3O4(x\u2009=\u20092.4) catalyst in this work with data for other catalysts based on non-precious metals as collected by Kibsgaard and Chorkendorff [86]. As can be seen from the plot, the NiFeP@Fe3O4(x\u2009=\u20092.4) catalyst is among the best catalysts reported so far, displaying a mass activity of 0.1\u2009A\u2009mg\u22121 and an overpotential of 220\u2009mV at 10\u2009mA\u2009cm\n\n\n\ngeo\n\n\n2\n\n\n. Faradaic efficiencies of ~ 95 % and ~\u00a097 % (see the Supporting Information) were estimated from measurements of the volume of the collected gas and by use of a ring-disc electrode (see Section 2.9).In addition to the high OER catalytic activity, the NiFeP@Fe3O4(x\u2009=\u20092.4) catalyst also showed a high stability under OER conditions, as measured by 500 potential cycles between 1.1 and 1.7\u2009V at a scan rate of 10\u2009mVs\u22121 (\nFig. 9(a)). From cycle 10 to cycle 500 the current at 1.525\u2009V decreased from 140\u2009mA\u2009cm\u22122 to 100\u2009mA\u2009cm\u22122. There is no noticeable decrease in the charge associated with the anodic redox peak at 1.43\u2009V and the corresponding cathodic peak at 1.35\u2009V. There is, however, a slight shift to lower potentials with increasing number of scans. It is well-known that the addition of Fe to Ni catalysts will shift the redox peak to higher potentials. Therefore, we associate the shift in the peaks to lower overpotentials to a slight change in the surface composition and a concomitant change (8 %) in the catalytic activity also visible in the figure. The chronoamperometric measurement involved applying a constant current of 50\u2009mAcm\u22122) for 10\u2009h (Fig. 9(b)) in 1\u2009mol dm\u22123. No noticeable increase in the potential was observed after 10\u2009h, indicating that NiFeP@Fe3O4(x\u2009=\u20092.4) is very stable.\n\nFig. 10 and Fig. S.7 exhibit TEM images of the semi-spherical NiFeP@Fe3O4(x\u2009=\u20092.4) nanoparticles and the corresponding EDS mappings after they had been subjected to a constant 10\u2009mA current for 5\u2009h. The TEM-EDS mapping of NiFeP@Fe3O4(x\u2009=\u20092.4) nanoparticles (Fig. S.7) shows that phosphorus remains a part of the catalyst after exposure to the OER conditions. The bulk Ni:P ratio was 2.5, essentially similar to that of the as-prepared nanoparticles, with the Ni:P ratio of 2.7 prior to the test.\nFig. 10(b) shows the HR-TEM images of two adjacent NiFeP@Fe3O4(x\u2009=\u20092.4) nanoparticles. The images are similar to those in Fig. 2(b) and (c), and the coating covering the particles that is visible in Fig. 2(b) and (c) is still intact after exposure to the electrolyte and high electrode potentials associated with the OER. The HR-TEM image of the particles also shows crystalline domains at their center, but somewhat less crystalline domains at their periphery.\nFig. 3 shows the Raman spectra of as-prepared NiFeP@Fe3O4(x\u2009=\u20092.4)/GC (glassy carbon) electrodes and the same sample after immersion for 10\u2009min, and after the sample had been subjected to 100 cycles between 1 and 1.7\u2009V and a constant current of 10\u2009mA (50\u2009mA\u2009cm\u22122) for 2\u2009h. A large peak at 1100\u2009cm\u22121 in the NiFeP@Fe3O4(x\u2009=\u20092.4) powder in the Raman spectra prior to mixing the ink (Fig. 3) is no longer present in the spectra of the same catalyst on the GC electrode, i.e. post mortem (Fig. 3). We associate this with dissolution of phosphate/phosphite species during the ink preparation. Apart from that, no other change was observed related to the changing/reorganization of the NiFeP@Fe3O4(x\u2009=\u20092.4) catalyst after OER.Apart from their high mass activity and current efficiency for the OER, the most prominent feature of the Ni12\u2212x\nFe\nx\nP5 catalysts is the presence of a coating both in the pristine catalysts as in Fig. 10(b), and post mortem as in Fig. 2(b) and (c). Regarding the fact that nanoparticles were synthesized in the presence of organic compounds (i.e. oleylamine and TOP), it is likely that the layer consists of carbon which has been generated upon decomposition of organic moieties adhered on the nanoparticles during the synthesis [69]. However, according to Jung et al. [51] and considering the fact that carbon atoms can be absorbed inside the lattice of the metal nanoparticles where metal acetylacetonate is used as a metal precursor [87], another possibility for the formation of the carbon layer could be the diffusion of carbon atoms from the interior of the metal nanoparticles to their surfaces in the phosphidation step at 300 \u00b0C. Moreover, the fact that the layer is invisible in the high-angle annular dark-field image of NiFeP@Fe3O4(x\u2009=\u20092.4) (Fig. 2(d)) is consistent with the layer being composed of a lighter element, such as carbon, than those of the catalyst particle itself.The bands peaking at wavenumbers 1582c\u22121 and 1360\u2009cm\u22121 in Fig. 3 are consistent with the G- and D-bands, respectively, for carbon samples [88], and supports the suggestion that a carbon layer has been formed at the particle surfaces. The G-band is associated with an ordered graphite structure and the D-band with defects, respectively. The peak height ratios I\nD:I\nG are 0.98 for NiFeP@Fe3O4(x\u2009=\u20092.4) (Fig. 3), 0.96 for NiFeP@Fe3O4(x\u2009=\u20093.6), and 0.95 for the Ni10.8Fe1.2P5. The higher ratio found for NiFeP@Fe3O4(x\u2009=\u20092.4) indicate a more defective nature and porous structure of the carbon layer [88] for this sample.The fact that the particle size is quite narrow, Fig. S5, makes it likely that at least the majority of the particles have been coated by carbon. A narrow particle size distribution indicates that the oleylamine and TOP were efficient in preventing particle growth, and therefore organic residues will coat all particles within the dominating size range. It is these residues that would be converted to carbon in the heating step, hence coating all particles within the size range indicated in Fig. S5.We therefore conclude that the coating covering the catalyst particles is a self-generated layer of carbon coming from ligand decomposition, with some possible doping by nitrogen or phosphorus from the ligands or even Fe [89] or Ni from the metal precursor. This layer only forms with iron present in the nanoparticles. Therefore, it is probably catalyzed by iron and therefore only present in the bimetallic system. The process of formation is therefore somewhat analogous to that suggested for the growth of carbon nanotubes on NiP amorphous nanoparticles, in the absence of any Fe, by annealing at the substantially higher temperature of 400 \u00b0C in an inert atmosphere [69,90].The differences in the ECSA (and in the peak heights in the voltammograms in Fig. 5 between the different catalysts are a likely manifestation of the carbon coating. This is because a carbon layer may help keeping catalyst particles apart and prevent agglomeration. As has been reported previously, one of the main advantages of carbon encapsulation of nanoparticles is the increase of the active surface area as a consequence of reduction in the agglomeration of nanoparticles [33,91,92]. The NiFeP@Fe3O4(x\u2009=\u20092.4) catalyst showed the highest C\ndl and also has the thickest self-generated carbon layer, whereas the Ni12P5 nanoparticles showed the lowest C\ndl among all tested catalysts and which have negligible carbon coverage. Presumably, carbon layers separate particles from each other and provide more area due to an increased access to some \u201cinner surfaces\u201d and leads to higher C\ndl. These observations suggest that the presence and thickness of the carbon coverage display a prominent role in the obtained value for C\ndl and consequently in the electrochemical active surface area.Oxygen evolution at catalysts covered by a carbon layer would require transport of reaction products and reactants either directly through the layer itself or through pinholes in the layer. Reaction through pinholes is not likely due to the very high activity of these catalysts; the catalytic activity would have to be rather extreme to explain this, and this is not compatible with the stability measurements indicating that the carbon layer does protect the catalysts. According to the Pourbaix diagrams, most transition metal phosphides will not be stable under OER conditions [32]. The fact that our catalysts are stable, indicates that the layer does keep the phosphides from disintegrating.) A direct influence of the metal on the carbon as suggested by Cui et al. for carbon monolayers [34] is not likely in view of the thickness of the carbon layers in this work. Also, the observation of pronounced pre-catalytic redox peaks attributed to Ni3+\u2215Ni2+ (Fig. 5) rules out carbon as being the only electrochemically active site. Other options are diffusion of iron into the carbon creating a carbon iron catalyst [89] or exfoliation of the carbon layer, providing electrolyte access to the metal sites underneath [45]. However, our post-mortem TEM images clearly shows that the carbon layer is completely preserved after being exposed to the OER conditions, excluding exfoliation of the carbon layer as a possibility.The quite intense Ni3+\u2215Ni2+ redox peak shows that the surface of the Ni12\u2212x\nFe\nx\nP5 particles caged inside the carbon layer is electrochemically active. Therefore, this Ni12\u2212x\nFe\nx\nP5 surface is likely to contribute significantly to the catalytic activity in the OER potential region as well. We tentatively propose a mechanism in which the hydroxide anions are transported through the carbon layer, possibly in a fashion similar to intercalation. Slow reaction steps are catalyzed at the Ni12\u2212x\nFe\nx\nP5 surface, and reaction intermediates formed in steps downstream of the rate-determining step are transported by diffusion in the graphitic or disordered carbon layers, again in a similar fashion to intercalation. The exact details of theses processes will, however, have to await further investigation beyond the scope here.A solution phase synthetic method for discrete Ni12\u2212x\nFe\nx\nP5 (x\u2009=\u20090,\u00a01.2,\u00a02.4,\u00a03.6) nanoparticles was developed. The ternary Ni10.8Fe1.2P5 nanoparticles have a tetragonal crystal structure corresponding to that of Ni12P5, indicating the formation of a uniform Ni-Fe-P alloy. However, the XRD results showed that for the x\u2009>\u20091.2, particles with a core-shell structure were formed, in which a NiFeP alloy forms the core and Fe3O4 the shell (NiFeP@Fe3O4). A detailed inspection of the TEM images revealed that in effect a self-generated porous carbon layer covers Ni12\u2212x\nFe\nx\nP5 nanoparticles. This carbon layer is formed as a result of the decomposition of organic precursors during synthesis. We observed no such carbon layer for Ni12P5 nanoparticles synthesized under the same conditions, i.e with Ni12P5 catalysts not containing iron. This suggests that the decomposition of organic compounds are catalyzed by the bimetallic system (i.e. NiFe). Encapsulation of the particles with carbon was further substantiated with Raman spectroscopy, in which the two characteristic peaks of carbon at 1395 and 1520\u2009cm\u22121 were clearly observed and are attributed to the carbon D and G-band, respectively. Based on the TEM images, the self-generated porous carbon layer was thickest (~ 5\u2009nm) for NiFeP@Fe3O4(x\u2009=\u20092.4) nanoparticles and thinnest (~ 1\u2009nm) for Ni10.8Fe1.2P5 nanoparticles. All the as-synthesized nanoparticles were applied as electrocatalysts for the OER. The activity for the OER increases in the order Ni12P5 <\u2009Ni10.8Fe1.2P5 <\u2009NiFeP@Fe3O4(x\u2009=\u20093.6) <\u2009NiFeP@Fe3O4(x\u2009=\u20092.4). NiFeP@Fe3O4(x\u2009=\u20092.4) nanoparticles showed an extraordinary electrocatalytic activity by achieving 10\u2009mA\u2009cm\u22122 at 220\u2009mV. A difference in the Tafel slopes between catalysts containing iron and Ni12P5 indicates that the reaction mechanism for the OER changes as iron is included in the composition. Post-mortem TEM characterization of NiFeP@Fe3O4(x\u2009=\u20092.4) showed that the carbon layer is very stable and is preserved after OER, consistent with in-situ Raman spectra which did not show any significant structural change upon exposure to the potentials at which the OER proceeds (1.6\u2009V\u221551\u2009mA\u2009cm\u22122) for 5\u2009h.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the financial support from the Norwegian University of Science and Technology (NTNU) (project no. 81771154).\nFatemeh Poureshghi: Conceptualization, Investigation, Methodology, Writing \u2013 original draft, Writing \u2013 review & editing. Frode Seland: Supervision, review, Methodology. Jens Oluf Jensen: Funding acquisition, Supervision, review, Methodology. Svein Sunde: Funding acquisition, Conceptualization, Supervision (investigation), Methodology, Writing \u2013 review & editing, Project administration.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcata.2022.118786.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Rational design of efficient, earth-abundant, and durable electrocatalysts to accelerate the oxygen evolution reaction (OER) is critical for hydrogen ion by water electrolysis. In the present work, nanostructured Ni12\u2212x\n                     Fe\n                        x\n                     P5 (x\u00a0=\u00a01.2,\u00a02.4,\u00a03.6) OER electrocatalysts synthesized by a colloidal method is reported. For x\u00a0=\u00a01.2, an alloy of Ni, Fe, and P is formed. For x\u00a0=\u00a02.4 or x\u00a0=\u00a03.6, a core-shell NiFeP@Fe3O4 structure is formed. The nanoparticles are encapsulated in a self-generated carbon layer. The carbon layer is formed during synthesis from synthesis residues. The carbon-encapsulated Ni9.6Fe2.4P5 catalyst offers the outstanding mass activity of 0.1\u202fA\u202fmg\u22121 and overpotential of 220 mV at 10\u202fmA\u202fcm\u22122, assigned to a combination of enhanced electrical conductivity provided by the carbon shell, a large surface area, and a high specific catalytic activity. Post-mortem characterization indicates that the carbon encapsulation remains intact under conditions of the OER.\n               "}
{"full_text": "\nn-Hexane from petroleum and gasoline industries is a typical volatile organic compound (VOC), which has been widely used as polymerization reaction media, cleaning agent, and a solvent in industries, with the high volatility and high toxicity at low concentrations. The emissions of n-hexane can affect human health and contribute to air pollution [1,2]. Among various removal technologies, catalytic combustion is thought to be one of the predominant strategies for VOCs elimination because of its low cost, simple treatment, no secondary pollution, and maximum efficacy [3]. Supported precious metal-based catalysts (e.g., Pt, Au, Pd, Ru, and Rh) show the outstanding performance for VOCs oxidation, but their scalable industrial applications are limited owing to the expensiveness and easy sintering of precious metals [4,5]. Thus, it is necessary to develop the promising catalysts with both low cost and high efficiency for VOCs removal.Considering the alternative to noble metals, transition-metal oxides have been focused in recent years because of their earth abundance, inexpensive cost, and high activity [3]. Among them, chromium oxide attracts a great attention for catalytic VOCs oxidation due to its strong oxidizing ability, insolubility, and chemical stability [6]. For example, Xing et\u00a0al. generated the CrO\nx\n/\u03b3-Al2O3 catalysts, and found that the surface Cr6+ species were the active sites, and chromia with monolayer dispersion presented the optimum catalytic activity for benzene oxidation [7]. Tian et\u00a0al. used the sol\u2013gel method to synthesize the chromium oxides (CrO\nx\n) catalysts, and claimed that Cr-300 showed the best performance for the oxidative dehydrogenation of propane (ODP) to propene owning to the smallest crystallite grain size and the highest Cr6+/Cr3+ and Olatt/Oads atomic ratios. In addition, the DFT calculations reveal that the Cr\u2013O site is the leading active site in the ODP reaction [8]. Working on the ODP over the Cr-MSU-x catalysts, Baek et\u00a0al. pointed out that the initial composition of the soft Cr(VI) in the total Cr(VI) was a major dominant factor governing the catalytic performance [9]. Actually, in order to enhance catalytic activity, polymetallic oxides usually exhibit better performance than individual metal due to the synergistic effect between the different metals of the former. Specially, the cobalt and nickel species with relatively low cost and high activity have been applied in various industries. For instance, Greluk et\u00a0al. prepared the CeO2-supported Co and Ni catalysts for the steam reforming of ethanol, and thought that the cobalt/nickel terrace was the preferential reaction sites but the edge/steps sites favored the cleavage of the C\u2013C bond; moreover, good dispersion and strong metal\u2212oxide interactions between Co or Ni and CeO2 could modify chemical properties of the catalyst [10]. After loading bimetallic Co\u2013Ni on alumina for methane combustion, Choya et\u00a0al. found that simultaneous loading of Co and Ni improved redox property of the catalyst due to partial inhibition of the interaction between alumina and Co3O4 and favorable generation of NiCo2O4 [11]. Li et\u00a0al. designed the atomic Co/Ni and Co\u2013Ni alloy nanoparticles (NPs) in N-ZIF-67 for bifunctional oxygen electrocatalysis, and claimed that the atom-level Co/Ni dual active sites exhibited a high electrocatalytic activity than single noble-metal-free catalyst because of the synergistic impact of the atomic Co/Ni\u2013N\u2013C bonds and microstructure in the sample [12]. Nowadays, single-atom catalysts (SACs) have been deemed as a promising material in various fields due to its maximum atom utilization efficiency, excellent performance, and strong metal\u2212support interaction [13,14]. To the best of our knowledge, nevertheless, preparing bimetallic cobalt\u2212nickel single-atom catalysts for VOCs combustion have been rarely reported in the literature. Herein, we developed a facile approach to prepare the mesoporous chromic oxide (meso-Cr2O3)-supported bimetallic cobalt\u2212nickel single-atom (Co1Ni1/meso-Cr2O3) and bimetallic Co and Ni nanoparticle (CoNPNiNP/meso-Cr2O3) catalysts, measured their physicochemical properties, evaluated their catalytic performance for n-hexane combustion, and clarified the involved catalytic mechanisms.Mesoporous silica (KIT-6) template was fabricated according to the procedures stated in the literature [15]. Three-dimensionally (3D) ordered mesoporous Cr2O3 (denoted as meso-Cr2O3) was synthesized with KIT-6 as hard template. The synthesis steps are as follows: 1.0\u00a0g of KIT-6 was added to 20\u00a0mL of ethanol solution, followed by the ultrasonic treatment for 0.5\u00a0h. 2.0\u00a0g of Cr(NO3)3\u22c59H2O was then added to the above KIT-6- and ethanol-containing mixture solution, followed by the ultrasonic treatment until ethanol was completely volatilized to obtain the precursor@KIT-6 composite. The above obtained composite was dried in an oven at 60\u00a0\u00b0C for 10\u00a0h. After that, the precursor@KIT-6 composite was calcined at a ramp of 2\u00a0\u00b0C min\u22121 in a muffle furnace from room temperature (RT) to 400\u00a0\u00b0C and maintained at 400\u00a0\u00b0C for 5\u00a0h. Thus, the meso-Cr2O3 support was generated after KIT-6 template removal by washing with a sodium hydroxide aqueous solution (2.00\u00a0mol/L) at 80\u00a0\u00b0C for 2\u00a0h three times and drying at 60\u00a0\u00b0C for 10\u00a0h.The meso-Cr2O3-supported Co and Ni NPs (CoNPNiNP/meso-Cr2O3) catalyst was fabricated by the one-pot polyvinyl alcohol (PVA)-protecting method. Typically, 13.4\u00a0mg of Co(NO3)2\u22c56H2O and 13.7\u00a0mg of Ni(NO3)2\u22c56H2O were added to the ethanol\u2212water mixed solution (ethanol/water volumetric ratio\u00a0=\u00a02:3) with 0.01\u00a0g of PVA under stirring for 10\u00a0min, then 0.08\u00a0g of NaBH4 was added to the above mixed solution and stirred for 10\u00a0min. After that, 0.5\u00a0g of meso-Cr2O3 was added to the above mixture and stirred for 12\u00a0h. Finally, the CoNPNiNP/meso-Cr2O3 catalyst was obtained after being filtered and dried in an oven at 60\u00a0\u00b0C for 12\u00a0h, and calcined at a ramp of 2\u00a0\u00b0C min\u22121 in the muffle furnace from RT to 400\u00a0\u00b0C and maintained at 400\u00a0\u00b0C for 5\u00a0h.The Co1/meso-Cr2O3 or Ni1/meso-Cr2O3 catalyst was prepared using the one-pot polyvinyl pyrrolidone (PVP)-protecting method with vitamin C as reducing agent. Typically, 26.7\u00a0mg of Co(NO3)2\u22c56H2O or 29.7\u00a0mg of Ni(NO3)2\u22c56H2O was added to the ethanol\u2212water mixed solution (ethanol/water volumetric ratio\u00a0=\u00a02:3) with 0.01\u00a0g of PVP under stirring for 30\u00a0min, then 0.17\u00a0g of vitamin C was added and stirred for 2\u00a0h. After that, 0.5\u00a0g of meso-Cr2O3 was added to the above mixture and stirred for 6\u00a0h. Finally, the Co1/meso-Cr2O3 or Ni1/meso-Cr2O3 catalyst was generated after filtration and drying in an oven at 60\u00a0\u00b0C for 12\u00a0h, and calcination at a ramp of 2\u00a0\u00b0C min\u22121 in the muffle furnace from RT to 400\u00a0\u00b0C and maintaining at 400\u00a0\u00b0C for 5\u00a0h.The Co1Ni1/meso-Cr2O3 catalyst was prepared adopting the one-pot PVP-protecting method with vitamin C as reducing agent. Typically, 6.2\u00a0mg of Co(NO3)2\u22c56H2O, 5.3\u00a0mg of Ni(NO3)2\u22c56H2O, and 0.01\u00a0g of PVP were added to an ethanol\u2212water mixed solution (ethanol/water volumetric ratio\u00a0=\u00a02:3) under stirring for 30\u00a0min. Then, 0.15\u00a0g of vitamin C was added to the above mixed solution and stirred for 2\u00a0h. After that, the mixture containing Co(NO3)2\u22c56H2O and Ni(NO3)2\u22c56H2O were mixed, and 0.5\u00a0g of meso-Cr2O3 was afterwards added to the above mixture and stirred for 6\u00a0h. Finally, the Co1Ni1/meso-Cr2O3 catalyst was obtained after filtration and drying in an oven at 60\u00a0\u00b0C for 12\u00a0h, followed by calcining at a ramp of 2\u00a0\u00b0C min\u22121 in the muffle furnace from RT to 400\u00a0\u00b0C and keeping at 400\u00a0\u00b0C for 5\u00a0h.Physicochemical properties of all of the as-prepared catalysts were measured using the techniques as follows: inductively coupled plasma\u2212atomic emission spectroscopy (ICP\u2212AES), small- and wide-angle X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and elemental mapping, high-angle annular dark field\u2212scanning transmission electron microscopy (HAADF-STEM), and X-ray absorption spectroscopy (XAS), N2 adsorption\u2212desorption (BET), X-ray photoelectron spectroscopy (XPS), oxygen temperature-programmed desorption (O2-TPD), hydrogen temperature-programmed reduction (H2-TPR), n-hexane temperature-programmed desorption (n-hexane-TPD), n-hexane temperature-programmed surface reaction (n-hexane-TPSR), density functional theory (DFT) calculations, gas chromatography\u2212mass spectrometry (GC\u2013MS), and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The detailed measurement procedures are presented in the Supplementary material.Catalytic activities of the samples for n-hexane combustion were evaluated in a continuous flow fixed-bed quartz tubular microreactor (i.d.\u00a0=\u00a06.0\u00a0mm) at 1 atm. 50 mg of the catalyst was mixed with 0.25\u00a0g of quartz sand to be loaded in the microreactor. The total flow rate of the (1000\u00a0ppm n-hexane\u00a0+\u00a020\u00a0vol% O2\u00a0+\u00a0N2 (balance)) gas mixture was 33.3\u00a0mL\u00a0min\u22121, giving a space velocity (SV) of 40,000 mL g\u22121 h\u22121. 5.0 vol% CO2 and/or 10.0 vol% H2O were introduced to the reaction system, so that their effects on catalytic activity were examined. The reactants and products were detected online by a gas chromatograph. The n-hexane conversion was defined as (C\ninlet\u00a0\u2212\u00a0C\noutlet)/C\ninlet\u00a0\u00d7\u00a0100%, where the C\ninlet and C\noutlet are the inlet and outlet n-hexane concentrations in the feed stream, respectively.The actual metal contents in the as-prepared samples are listed in Table 1\n. The actual Co contents in Co1/meso-Cr2O3, CoNPNiNP/meso-Cr2O3, and Co1Ni1/meso-Cr2O3 were 0.96, 0.45, and 0.17\u00a0wt%, respectively. The actual Ni contents in Ni1/meso-Cr2O3, CoNPNiNP/meso-Cr2O3, and Co1Ni1/meso-Cr2O3 were 1.04, 0.47, and 0.14\u00a0wt%, respectively. The XRD analysis was performed to identify the crystal phases of the samples. There were diffraction peaks of each sample at 2\u03b8\u00a0=\u00a024.5\u00b0, 33.6\u00b0, 36.2\u00b0, 39.7\u00b0, 41.5\u00b0, 44.2\u00b0, 50.2\u00b0, 54.9\u00b0, 57.1\u00b0, 58.4\u00b0, 63.4\u00b0, 65.1\u00b0, 72.9\u00b0, 73.3\u00b0, 76.8\u00b0, and 79.1\u00b0 (Fig.\u00a01\n), matching well with the (012) (104) (110) (006) (113) (202) (024) (116) (211) (122) (214) (300) (1010) (119) (220), and (306) lattice planes, respectively, which were due to the rhombohedral Cr2O3 phase (JCPDS PDF no. 38\u20131479). The results indicate that each sample exhibits a rhombohedral Cr2O3 crystal structure. In the meanwhile, no reflections of the Co and/or Ni phases were detected, probably owing to lower Co and Ni loadings in Ni1/meso-Cr2O3, CoNPNiNP/meso-Cr2O3, Co1/meso-Cr2O3, and Co1Ni1/meso-Cr2O3. Crystallite sizes (D\nc) of all of the samples were calculated using the (116) crystal plane of Cr2O3 according to the Scherrer equation, and their results are summarized in Table 1. The D\nc value (17.8\u00a0nm) of meso-Cr2O3 was bigger than those (15.4\u201317.7\u00a0nm) of the other samples. In addition, diffraction peak intensity of the other samples was weaker compared with that of meso-Cr2O3, indicating the decrease in crystallite size after loading of Co and/or Ni.\nFig.\u00a02\n presents TEM images in various regions of meso-Cr2O3 and Co1Ni1/meso-Cr2O3. The meso-Cr2O3 sample exhibited a high-quality 3D ordered mesoporous (3DOM) architecture with a mesopore diameter of ca. 11\u00a0nm. As expected, the well-ordered mesoporous structure remained perfectly after the loading of bimetallic Co and Ni single atoms, and lattice spacing of the Cr2O3 (012) crystal plane was ca. 0.36\u00a0nm. This result indicates that reduction treatment did not induce a remarkable change in 3DOM structure. There was a diffraction peak at 2\u03b8\u00a0=\u00a00.85\u20131.00\u00b0 in low-angle XRD pattern of each sample (Fig.\u00a0S1), and a H2-typed hysteresis loop in the relative pressure (p/p\n0) range of 0.70\u20131.00 was observed in the isotherm of each sample, together with a peak in pore-size distribution of each sample (Fig.\u00a0S4). These results demonstrate that the ordered mesopores were generated in each sample. Furthermore, surface areas of these samples were 75\u201392\u00a0m2/g (Table 1). The meso-Cr2O3 with a regularly porous channel structure and a larger surface area can be regarded as an ideal support, which was beneficial for the adsorption, diffusion, and activation of reactant molecules, and could effectively prevent the migration and aggregation of Co and/or Ni atoms, expose more active sites, and enhance the interface effect between meso-Cr2O3 and Co or Ni atoms.\nFig.\u00a03\nA\u2212C displays TEM images of the Co1/meso-Cr2O3, Ni1/meso-Cr2O3, and Co1Ni1/meso-Cr2O3 samples, from which the Co or Ni NPs and/or nanoclusters were hard to be observed on the meso-Cr2O3 surface, Moreover, the energy-dispersive spectroscopic (EDS) element mappings analysis (Fig.\u00a03a\u2212c) reveals that the Cr, O, Co and/or Ni elements were homogenously distributed in each of the samples. More importantly, the high-angle annular dark field\u2212 STEM (HAADF\u2212STEM) technique was used to disclose the dispersion of single Co and Ni atoms (Fig.\u00a03D, E, and S2(A-C)). As a result, a number of small isolated bright spots were clearly observed, assignable to the Co and Ni single atoms due to their Z-contrasts higher than those of the Cr and O atoms. Furthermore, the single-atom EEL spectroscopy (Fig.\u00a0S2(E)) was used to better distinguish the Co and Ni single atoms, but no significant signals were observed in the sample. On the one hand, the atomic numbers of Co, Ni, and Cr are similar; on the other hand, although the extranuclear electrons of Co and Ni atoms were excited, the meso-Cr2O3 support had a certain thickness. The excited extranuclear electrons might be extinguished in the substrate and could not penetrate the substrate, which was not received by the detector. In addition, we also made the statistics of the distance between two adjacent single atoms in blue square region in Fig.\u00a0S2(C), and the results are presented in Fig.\u00a0S2(D). The XAS measurements were conducted to further reveal the coordination environment in the Co1Ni1/meso-Cr2O3 sample. However, the absorption edge energies of Co, Ni, and Cr were about 7709, 8333, and 5989\u00a0eV, respectively. On the one hand, when the cobalt and nickel were co-existed in the sample, the edge collision absorption phenomenon could occur during the test. On the other hand, the support material possessed a high content of Cr, which resulted in the self-absorption phenomenon after light irradiation. Hence, as shown in Fig.\u00a0S3, the above-mentioned factors led to the inability to measure the results of Ni signals. Furthermore, it was also difficult to determine the results of Co signals.The XPS experiments were done to identify elemental compositions and chemical states of the samples, and their XPS spectra and quantitative analysis results are shown in Fig.\u00a04\nA\u2212D and Table 1, respectively. The Cr 2p3/2 spectrum (Fig.\u00a04A) of each sample was deconvoluted into four components that were associated with the surface Cr2+ (binding energy (BE)\u00a0=\u00a0575.1\u2013575.3\u00a0eV), Cr3+ (BE\u00a0=\u00a0576.3\u2013576.6\u00a0eV), Cr5+ (BE\u00a0=\u00a0577.6\u2013577.9\u00a0eV), and Cr6+ (BE\u00a0=\u00a0578.9\u00a0eV) species [16\u201318]. The (Cr5+\u00a0+\u00a0Cr6+)/(Cr2+ +Cr3+) molar ratio increased in the order of meso-Cr2O3 (0.27)\u00a0<\u00a0Ni1/meso-Cr2O3 (0.37)\u00a0<\u00a0CoNPNiNP/Cr2O3 (0.41)\u00a0<\u00a0Co1/meso-Cr2O3 (0.43)\u00a0<\u00a0Co1Ni1/meso-Cr2O3 (0.48). It was reported that the Cr species with a higher chemical state (Cr5+ or Cr6+) (i.e., higher electronegativity) was beneficial for the redox reaction [19]. The O1s spectrum of each sample was divided into three components at BE\u00a0=\u00a0529.5\u2013529.9, 531.1\u2013531.6, and 532.7\u2013532.8\u00a0eV (Fig.\u00a04B), which was related to the surface lattice oxygen (Olatt) in the form of M\u2212O (M\u00a0=\u00a0Cr, Ni or Co) bond, adsorbed oxygen (Oads), and adsorbed molecular water (\n\n\nO\n\n\nH\n2\n\nO\n\n\n\n) or carbonate species [20], respectively. The components of Co 2p3/2 spectrum at BE\u00a0=\u00a0782.2 and 786.6\u00a0eV corresponded to the surface Co2+ species and the satellite signal (Fig.\u00a04C), respectively. For CoNPNiNP/meso-Cr2O3, the component at BE\u00a0=\u00a0778.4\u00a0eV was related to the surface metallic Co0 species, the one at BE\u00a0=\u00a0780.2\u00a0eV of Co1/meso-Cr2O3 in Co 2p3/2 spectrum belonged to the surface Co3+ species, and the one at BE\u00a0=\u00a0779.1\u00a0eV of Co1Ni1/meso-Cr2O3 in Co 2p3/2 spectrum was associated with the surface Co\n\u03b4+ (3 > \u03b4\u00a0>\u00a02) species [21]. In the Ni 2p3/2 spectra (Fig.\u00a04D) of the Ni1/meso-Cr2O3, CoNPNiNP/meso-Cr2O3, and Co1Ni1/meso-Cr2O3 samples, there were four components at BE\u00a0=\u00a0853.7, 855.7\u2013855.9, 859.2\u2013859.9, and 861.8\u2013864.3\u00a0eV, ascribable to the surface metallic Ni0, Ni2+, and Ni3+ species and the satellite signal [22], respectively. The results indicate that the Co1 or/and Ni1 in Ni1/meso-Cr2O3, Co1/meso-Cr2O3 or Co1Ni1/meso-Cr2O3 existed in an oxidized valence state, whereas the CoNP and NiNP in CoNPNiNP/meso-Cr2O3 were present in the form of combined oxidized with metallic valence states. Compared with meso-Cr2O3, Cr 2p3/2 peaks of the Ni1/meso-Cr2O3, CoNPNiNP/meso-Cr2O3, Co1/meso-Cr2O3, and Co1Ni1/meso-Cr2O3 samples were shifted to the positions with higher BEs, suggesting a strong synergistic effect between Ni and Co atoms or NPs and meso-Cr2O3, and their corresponding O 1s peaks also exhibited higher BE values in contrast to those on meso-Cr2O3. It can be inferred that electron transfer may proceed along a route of meso-Cr2O3 \u2192 Ni and/or Co atoms or NPs in those samples, where meso-Cr2O3 was regarded as electron acceptor while Ni and/or Co atoms or NPs as electron donor. Furthermore, the component of O 1s spectrum on the Co1Ni1/meso-Cr2O3 sample displayed a higher BE than that on the Ni1/meso-Cr2O3, CoNPNiNP/meso-Cr2O3, and Co1/meso-Cr2O3 samples, and the component of Ni 2p3/2 spectrum on Co1Ni1/meso-Cr2O3 also showed a higher BE than that on Ni1/meso-Cr2O3 or CoNPNiNP/meso-Cr2O3. This result indicates that loading bimetallic Co and Ni single atoms was more conducive to accumulating electrons on the surface of the double active sites to optimize the \u0394G\nH\u2217, thus improving the catalytic performance [23].The O2-TPD experiments were conducted to measure the types and mobility of oxygen species of the samples, and their profiles are shown in Fig.\u00a05\n. The peaks at < 220\u00a0\u00b0C, 220\u2013420, and above 420\u00a0\u00b0C were assigned to desorption of the surface adsorbed oxygen (Oads), surface lattice oxygen, and bulk lattice oxygen (Olatt) [20], respectively. The loading of the highly dispersed Co or Ni atoms might generate the lattice defects and facilitate mobility of the Olatt species on/in the meso-Cr2O3 support. The surface Olatt desorption temperature (377\u00a0\u00b0C) and bulk Olatt desorption temperature (803\u00a0\u00b0C) from the CoNPNiNP/meso-Cr2O3 sample were higher than those (373 and 581\u2013703\u00a0\u00b0C) from the Co1/meso-Cr2O3 sample and those (363 and 596\u2013716\u00a0\u00b0C) from the Ni1/meso-Cr2O3 sample, respectively, implying a lower lattice oxygen mobility in/on the former. Desorption peaks of the lattice oxygen species from the Co1Ni1/meso-Cr2O3 sample were observed at lower temperatures than those from the other samples, demonstrating that loading of the highly dispersed bimetallic Co and Ni active sites can significantly activate the bond between surface metal sites and surface Olatt to promote the mobility of lattice oxygen. It is generally accepted that surface Oads species plays a vital role in catalytic reactions governed by a suprafacial catalytic process [24]. The Co1Ni1/meso-Cr2O3 sample with the lowest desorption temperature of the surface Oads species exhibited the highest catalytic activity, indicating the importance of the surface Oads species in n-hexane combustion. In addition, mobility of the Olatt species was in favor of a redox reaction [25], thus promoting the enhancement in catalytic n-hexane combustion activity.The H2 temperature-programmed reduction (TPR) technique was applied to assess reducibility of all of the samples, and their H2-TPR curves are depicted in Fig.\u00a06\nA. The meso-Cr2O3 support exhibited a four-stage reduction feature with a weak reduction peak at 320\u00a0\u00b0C, a strong shoulder at 391\u00a0\u00b0C, a distinct reduction peak at 640\u00a0\u00b0C, and a weak reduction peak at 858\u00a0\u00b0C. The first peak was attributed to the reduction of Cr6+ to Cr5+ (or Cr3+) [26\u201328], the second one was assigned to the reduction of Cr5+ to Cr3+, the third one was attributed to the reduction of Cr3+ to Cr2+ [29,30], and the last one was associated with the direct reduction of bulk chromia to Cr2+ [30], accompanied by the removal of the surface Oads, surface Olatt, and bulk Olatt species, respectively. After loading of Ni single atoms, there were a sharp peak centered at 197\u00a0\u00b0C, a weak shoulder between 280 and 337\u00a0\u00b0C, a broad peak at 459\u00a0\u00b0C, a weak peak around 694\u00a0\u00b0C, and a peak at 851\u00a0\u00b0C, which corresponded to the reduction of Cr6+ to Cr5+ (or Cr3+) with removal of the Oads species, Cr5+ to Cr3+ with removal of the weak surface Olatt species, Cr3+ to Cr2+ and Ni3+ to Ni2+ with removal of the partial bulk Olatt species [31], Ni2+ to Ni0 with removal of the partial bulk Olatt species [32], and bulk chromia to Cr2+ with removal of the deep bulk Olatt species, respectively. As for the Co1/meso-Cr2O3 sample, there were an obvious peak located at 233\u00a0\u00b0C, a weak and broad peak at 396\u00a0\u00b0C, a wide peak at 637\u00a0\u00b0C, and an extremely weak peak at 867\u00a0\u00b0C, which represented the reduction of Cr6+ to Cr5+ (or Cr3+) with removal of the Oads species, Cr5+ to Cr3+ and Co3+ to Co2+ [33] with removal of the weak surface Olatt species, Cr3+ to Cr2+ and Co2+ to Co0 [34] with removal of the partial bulk Olatt species, and bulk chromia to Cr2+ with removal of the deep bulk Olatt species, respectively. The CoNPNiNP/meso-Cr2O3 sample exhibited a distinct peak at 231\u00a0\u00b0C, a band at 293\u00a0\u00b0C, a weak shoulder between 390 and 490\u00a0\u00b0C, a broad peak at 593\u00a0\u00b0C, and a peak at 823\u00a0\u00b0C, which were ascribed to the reduction of Cr6+ to Cr5+ (or Cr3+) with removal of the weakly adsorbed Oads species, Cr5+ to Cr3+ with removal of the partial surface Olatt species, Ni3+ to Ni2+ with removal of the partial bulk Olatt species, Cr3+ to Cr2+ and Ni2+ to Ni0 or Co2+ to Co0 with removal of the partial bulk Olatt species, and bulk chromia to Cr2+ with removal of the deep bulk Olatt species, respectively. The Co1Ni1/meso-Cr2O3 sample showed a distinct peak at 213\u00a0\u00b0C, a tiny peak at 392\u00a0\u00b0C, a wide peak at 490\u00a0\u00b0C, and an especially faint peak at 860\u00a0\u00b0C, attributable to the reduction of Cr6+ to Cr5+ (or Cr3+), Cr5+ to Cr3+ and Co\n\u03b4+ (3 > \u03b4\u00a0>\u00a02) and/or Ni3+ to Co2+ and/or Ni2+, Cr3+ to Cr2+ and Co2+ and/or Ni2+ to Co0 and/or Ni0, and bulk chromia to Cr2+, respectively, which was also accompanied by the consumption of the corresponding oxygen species. Additionally, H2 consumption of the samples are summarized in Table 2\n. Obviously, the total hydrogen consumption followed a declined sequence of meso-Cr2O3 (1.14\u00a0mmol gcat\n\u22121)\u00a0>\u00a0Co1/meso-Cr2O3 (0.99\u00a0mmol gcat\n\u22121)\u00a0>\u00a0Co1Ni1/meso-Cr2O3 (0.92\u00a0mmol gcat\n\u22121)\u00a0>\u00a0Ni1/meso-Cr2O3 (0.88\u00a0mmol gcat\n\u22121)\u00a0>\u00a0CoNPNiNP/meso-Cr2O3 (0.86\u00a0mmol gcat\n\u22121). It should be mentioned that hydrogen consumption of the first peak obeyed a decreased order of Co1/meso-Cr2O3 (0.59\u00a0mmol gcat\n\u22121)\u00a0>\u00a0CoNPNiNP/meso-Cr2O3 (0.44\u00a0mmol gcat\n\u22121)\u00a0>\u00a0Co1Ni1/meso-Cr2O3 (0.39\u00a0mmol gcat\n\u22121)\u00a0>\u00a0Ni1/meso-Cr2O3 (0.31\u00a0mmol gcat\n\u22121)\u00a0>\u00a0meso-Cr2O3 (0.30\u00a0mmol gcat\n\u22121). The first reduction signal was associated with the reduction of chromia with the high-valence states as well as the removal of the Oads species, and the higher H2 consumption and the shift to lower temperatures would be beneficial for the redox reaction. It can be seen that the strong interaction between the highly dispersed Ni atoms and meso-Cr2O3 was more conducive to the improvements in mobility of the surface Olatt species and redox ability (from Cr6+ to Cr5+ (or Cr3+) and Cr5+ to Cr3+). Loading Co atoms tended to generate higher contents of chromium with the high-valence states and surface oxygen species. Apparently, the low-temperature reducibility declined in the order of Ni-meso/Cr2O3\u00a0>\u00a0Co1Ni1/meso-Cr2O3\u00a0\u2248\u00a0Co1/meso-Cr2O3\u00a0>\u00a0CoNPNiNP/meso-Cr2O3\u00a0>\u00a0meso-Cr2O3. Besides, the initial H2 consumption rate (at which less than 25% oxygen in the sample is consumed for the first reduction peak) presented in Fig.\u00a06B also possessed such a changing trend.The n-hexane-TPD technique were used to probe n-hexane adsorption behaviors of the samples. The desorption signals of C6H14 (m/z\u00a0=\u00a057), CO2 (m/z\u00a0=\u00a044), H2O (m/z\u00a0=\u00a018), CO (m/z\u00a0=\u00a028), acrylic acid (m/z\u00a0=\u00a015), and 2-methyloxirane (m/z\u00a0=\u00a031) were recorded in n-hexane-TPD profiles of the samples. From Fig.\u00a07\nA, we can observe that there is a sharp n-hexane desorption peak at 82\u00a0\u00b0C for Co1Ni1/meso-Cr2O3 and an obvious broad peak at about 276\u00a0\u00b0C for CoNPNiNP/meso-Cr2O3, which are related to desorption of the physically or weakly chemically adsorbed n-hexane and the strongly chemically adsorbed n-hexane, respectively. Extremely weak desorption peaks were observed for the Co1/meso-Cr2O3 and Ni1/meso-Cr2O3 samples. Besides, the n-hexane adsorption capacity followed an increased order of Co1/meso-Cr2O3 (1.3\u00a0\u00d7\u00a010\u22127\u00a0\u03bcmol gcat\n\u22121)\u00a0<\u00a0Ni1/meso-Cr2O3 (2.1\u00a0\u00d7\u00a010\u22127\u00a0\u03bcmol gcat\n\u22121)\u00a0<\u00a0Co1Ni1/meso-Cr2O3 (59.2\u00a0\u00d7\u00a010\u22127\u00a0\u03bcmol gcat\n\u22121)< CoNPNiNP/meso-Cr2O3 (89.6\u00a0\u00d7\u00a010\u22127\u00a0\u03bcmol gcat\n\u22121) (Table 2). Significantly, Co1Ni1/meso-Cr2O3 showed a n-hexane desorption peak at the lowest temperature among all of the samples and a larger n-hexane adsorption capacity than the Co1/meso-Cr2O3 and Ni1/meso-Cr2O3 samples, suggesting that this sample possesses a stronger capability to adsorb n-hexane than the other samples. Since no gaseous oxygen was present in the experiments, the generated CO2 (Fig.\u00a07B), H2O (Fig.\u00a07C), CO (Fig.\u00a07D), acrylic acid (Fig.\u00a07E), and 2-methyloxirane (Fig.\u00a07F) were attributed to the products due to interaction of the Oads and Olatt species with the adsorbed n-hexane on the sample surface. Additionally, the desorption peaks at < 220, 220\u2013420, and >420\u00a0\u00b0C were associated with the reactions of the Oads, surface Olatt, and bulk Olatt species with the adsorbed n-hexane, respectively. It can be seen that the adsorbed n-hexane reacted with the surface Oads (and possibly a small amount of active surface Olatt species) to generate the intermediates, such as CO and acrylic acid. The large amount of CO2 and H2O generation suggests that the Olatt species mainly participate in the oxidation of adsorbed n-hexane and intermediates. Apparently, a lower desorption peak temperature was observed for the Co1Ni1/meso-Cr2O3 sample, furthermore a larger amount of CO2 was generated over this sample, indicating that the Co1Ni1/meso-Cr2O3 sample possesses a stronger ability to oxidize n-hexane than the other samples.The subsequent n-hexane-TPSR technique was used to investigate the relevance of the active oxygen species and n-hexane dissociation, and their profiles are depicted in Fig.\u00a08\n. Interestingly, when gaseous oxygen was introduced into the system, a n-hexane desorption peak appeared at 83\u00a0\u00b0C for the Co1/meso-Cr2O3 sample (Fig.\u00a08A), which might be due to the fact that gaseous oxygen can more easily supply the surface Oads species to enhance the generation of oxygen vacancies, thus improving the ability to adsorb n-hexane on the surface of Co1/meso-Cr2O3. Additionally, the n-hexane desorption peak of Co1Ni1/meso-Cr2O3 decreased in intensity, and almost disappeared for CoNPNiNP/meso-Cr2O3 compared with that shown in Fig.\u00a07A, demonstrating that there was a competitive adsorption between gaseous oxygen and n-hexane. Obviously, as presented in Fig.\u00a08B\u2212F, desorption peaks of the generated CO2, H2O, CO, acrylic acid, and 2-methyloxirane were shifted to lower temperatures, and increased in intensity as compared with the results of n-hexane-TPD characterization. The results indicate that gaseous oxygen can supplement the active oxygen species, thus accelerating the n-hexane oxidation rate [35,36]. It can be also viewed that the meso-Cr2O3 sample exhibits the higher desorption peak temperatures of MS signals due to the involved intermediates and products than the other samples, suggesting that loading Co1 and/or Ni1 as well as CoNPNiNP can effectively promote the active oxygen migration, thus increasing the catalytic performance of n-hexane combustion, in good accordance with the O2-TPD characterization results. Furthermore, the amounts of CO2 and other by-products generated over the Co1Ni1/meso-Cr2O3 sample were more than those formed during the n-hexane-TPSR process (consistent with the n-hexane-TPD process), indicating a strong ability of Co1Ni1/meso-Cr2O3 to adsorb and activate n-hexane in the presence of oxygen.To better clearly elucidate the adsorption of n-hexane on the sample surface, the projector augmented wave (PAW) method with the Vienna Ab Initio Simulation Package (VASP) based on the density functional theory (DFT) was applied to do the theoretical calculations. The DFT-optimized structures of model and related energies are presented in Fig.\u00a09\n and Table 4. The adsorption energies of n-hexane on the surface of the CoNPNiNP/meso-Cr2O3 and Co1Ni1/meso-Cr2O3 catalysts were compared. Besides, due to the fact that the particle size of nanoparticles is much larger than that of clusters, the stable sections of metal (Co (0001) or Ni (111)) are usually used for calculations in microscopic view. The adsorption energy (\u0394E\nads) was calculated according to the equation: \u0394E\nads\u00a0=\u00a0E\ntotal\u00a0\u2212\u00a0E\nslab\u00a0\u2212\u00a0E\n\nn-hexane, where E\ntotal is the total energy of the surface slab with n-hexane adsorption, E\nslab is the energy of the Co1Ni1/meso-Cr2O3 (116), Co (0001) or Ni (111) surface, and E\n\nn-hexane represents the energy of the isolated n-hexane molecule in the gas phase. It can be seen that the model of Co (0001) or Ni (111) exhibits a higher adsorption energy (\u22120.895\u00a0eV or \u22121.060\u00a0eV) than Co1Ni1/meso-Cr2O3 (116) (\u22121.371\u00a0eV), indicating that Co1Ni1/meso-Cr2O3 possessed a stronger n-hexane adsorption ability than CoNPNiNP/meso-Cr2O3.Catalytic activities of the as-prepared samples for n-hexane combustion are presented in Fig.\u00a010\nA and Table 3\n\n. Apparently, n-hexane conversion over each sample increased with increasing the temperature, and catalytic activity below 220\u00a0\u00b0C dropped in the order of Co1Ni1/meso-Cr2O3\u00a0>\u00a0Co1/meso-Cr2O3\u00a0>\u00a0Ni1/meso-Cr2O3\u00a0>\u00a0CoNPNiNP/meso-Cr2O3\u00a0>\u00a0meso-Cr2O3, while that above 220\u00a0\u00b0C decreased in the sequence of Co1Ni1/meso-Cr2O3\u00a0>\u00a0Co1/meso-Cr2O3\u00a0>\u00a0CoNPNiNP/meso-Cr2O3\u00a0>\u00a0Ni1/meso-Cr2O3\u00a0>\u00a0meso-Cr2O3. Moreover, the temperatures (T\n10%, T\n50%, and T\n90%) reaching 10, 50, and 90% n-hexane conversions are also used to compare catalytic performance of the samples, respectively. Obviously, the Co1Ni1/meso-Cr2O3 sample displayed the best catalytic activity: the T\n10%, T\n50%, and T\n90% were 200, 239, and 263\u00a0\u00b0C at SV\u00a0=\u00a040,000\u00a0mL\u00a0g\u22121\u00a0h\u22121, respectively, which were lower than those (207, 249, and 281\u00a0\u00b0C) over Co1/meso-Cr2O3, those (214, 260, and 289\u00a0\u00b0C) over CoNPNiNP/meso-Cr2O3, those (208, 270, and 303\u00a0\u00b0C) over Ni1/meso-Cr2O3, and those (276, 383, and 470\u00a0\u00b0C) over meso-Cr2O3, respectively. Such a result may be due to the fact that the strong synergistic interaction between bimetallic Co and Ni single atoms and meso-Cr2O3 support is more beneficial for generating the larger amount of higher-valence chromium ions and easier mobile active Olatt species to efficiently activate the C\u2013H bonds in n-hexane; in the meanwhile, the stronger n-hexane adsorption and activation ability of the Co1Ni1/meso-Cr2O3 sample can accelerate the n-hexane combustion process. In addition, the CoNPNiNP/meso-Cr2O3 sample displayed a better catalytic activity than the Ni1/meso-Cr2O3 sample at higher temperatures due to the stronger chemisorption of n-hexane on the former, but a worse catalytic activity at lower temperatures owing to the fact that the Ni1/meso-Cr2O3 sample possesses better low-temperature reducibility than the CoNPNiNP/meso-Cr2O3 sample.Apparent activation energies (E\na) were obtained from the Arrhenius plots of ln k versus inverse temperature of the samples, and their results are shown in Fig.\u00a010B and Table 3. The sequence in E\na value increased according to Co1Ni1/meso-Cr2O3 (54.7\u00a0kJ\u00a0mol\u22121)\u00a0<\u00a0Co1/meso-Cr2O3 (58.4\u00a0kJ\u00a0mol\u22121) <\u00a0Ni1/meso-Cr2O3 (62.2\u00a0kJ\u00a0mol\u22121)\u00a0<\u00a0CoNPNiNP/meso-Cr2O3 (93.5\u00a0kJ\u00a0mol\u22121)\u00a0<\u00a0meso-Cr2O3 (100.6\u00a0kJ\u00a0mol\u22121), which was in consistency with that in n-hexane conversion above 220\u00a0\u00b0C. Moreover, the calculated specific reaction rate at 260\u00a0\u00b0C (4.3\u00a0\u00d7\u00a010\u22127\u00a0mol gcat\n\u22121\u00a0s\u22121) over Co1Ni1/meso-Cr2O3 was higher than that (3.4\u00a0\u00d7\u00a010\u22127\u00a0mol gcat\n\u22121\u00a0s\u22121) over Co1/meso-Cr2O3, that (2.5\u00a0\u00d7\u00a010\u22127\u00a0mol gcat\n\u22121\u00a0s\u22121) over CoNPNiNP/meso-Cr2O3, that (1.9\u00a0\u00d7\u00a010\u22127\u00a0mol gcat\n\u22121\u00a0s\u22121) over Ni1/meso-Cr2O3, and that (0.2\u00a0\u00d7\u00a010\u22127\u00a0mol gcat\n\u22121\u00a0s\u22121) over meso-Cr2O3 (Table 3). Furthermore, the specific reaction rate at 260\u00a0\u00b0C for n-hexane combustion over Co1Ni1/meso-Cr2O3 was much higher than those over 0.4 Mn/Pt-1 nm (D) [37] and SS/ZrO2/Pt (SP) [38], but inferior to those over Mn0\u00b77Ce0\u00b73/Al2O3 [39] and 0.12Pt/0.4Mn/Al2O3 [40] reported in the literature (Table S1).Effect of the SV on catalytic performance of Co1Ni1/meso-Cr2O3 was investigated in the range of 5000\u201380,000\u00a0mL\u00a0g\u22121\u00a0h\u22121. As shown in Fig.\u00a0S5, catalytic activity dropped with the increased SV due to the reduction in residence time of the reactant gases on the sample surface [41]. It can be obviously observed that Co1Ni1/meso-Cr2O3 also exhibited a better catalytic activity (T\n90%\u00a0=\u00a0276\u00a0\u00b0C) even at a higher SV of 80,000\u00a0mL\u00a0g\u22121\u00a0h\u22121. To better examine thermal stability of the sample under the kinetically controlled reaction conditions [42], 20-h on-stream n-hexane combustion was performed over the Co1Ni1/meso-Cr2O3 sample at two different temperatures (239 and 263\u00a0\u00b0C) and SV\u00a0=\u00a040,000\u00a0mL\u00a0g\u22121\u00a0h\u22121. As illustrated in Fig.\u00a010C, there was no remarkable loss in catalytic activity during the durability test process, demonstrating good thermal stability of Co1Ni1/meso-Cr2O3 under the adopted reaction conditions. Considering the possibility of CO2 and/or H2O presence in industrial VOCs emissions, we examined the influence of 5.0\u00a0vol% CO2 and/or 10.0\u00a0vol% H2O on catalytic activity of Co1Ni1/meso-Cr2O3 for n-hexane combustion, as shown in Fig.\u00a010D. Apparently, n-hexane conversion was not distinctly changed after 5.0\u00a0vol% CO2 was added to the reaction feedstock, indicating the good resistance to CO2 of the Co1Ni1/meso-Cr2O3 sample. The introduction of 10\u00a0vol% H2O to the reaction system exerted a minor negative effect on activity, with the conversion of n-hexane being dropped by ca. 4% because of the competitive adsorption of H2O and reactants molecules [43]. After both 5.0\u00a0vol% CO2 and 10\u00a0vol% H2O were introduced into the reaction system at a T\n90% for 8\u00a0h, there was no obvious changes in catalytic activity. Furthermore, crystalline phase and surface topography of the Co1Ni1/meso-Cr2O3 sample exhibited almost no differences. The sample (Co1Ni1/meso-Cr2O3-(H2O\u00a0+\u00a0CO2)) treated in both 5.0\u00a0vol% CO2 and 10.0\u00a0vol% H2O at 263\u00a0\u00b0C for 8\u00a0h, as illustrated in Figs. 1 and 3(F, G) showed the good stability and CO2- and H2O-resistant performance. Consequently, the Co1Ni1/meso-Cr2O3 sample with good CO2 and H2O resistance can be regarded as a potential catalyst for n-hexane combustion.In situ DRIFTS spectra of n-hexane adsorption with the increased temperatures were collected on the Co1/meso-Cr2O3, Ni1/meso-Cr2O3, and Co1Ni1/meso-Cr2O3 samples to compare reactivity of the Olatt species on/in the samples, as illustrated in Fig.\u00a011\nA, C, and E. The adsorption process was as follows: The samples were treated in a (1000\u00a0ppm n-hexane\u00a0+\u00a0N2 (balance)) mixture flow of 33.3\u00a0mL\u00a0min\u22121\u00a0at 180\u00a0\u00b0C for 0.5\u00a0h (denoted as ad-saturation) after being pretreated in an O2 flow of 20\u00a0mL\u00a0min\u22121\u00a0at 250\u00a0\u00b0C for 1\u00a0h and purging in a N2 flow of 20\u00a0mL\u00a0min\u22121\u00a0at 250\u00a0\u00b0C for 1\u00a0h. Noticeably, the absorption bands at 2850\u20132980 (2966, 2934, and 2877\u00a0cm\u22121), 1466, and 1379\u00a0cm\u22121 were recorded on all of the samples, which were ascribable to the \u03bd(C\u2013H), \u03b4as (CH3), and \u03b4s (CH3) in adsorbed n-hexane [44,45], respectively. Besides, the bands in the range of 3100\u20133600 and 2300\u22122400\u00a0cm\u22121 were due to the \u03bd(O\u2013H) and gas CO2 [44,46], respectively. For the Co1Ni1/meso-Cr2O3 sample, there were three new bands at 1717, 1626, and 1287\u00a0cm\u22121 owing to the vibration modes of \u03bd(CO), \u03bd(CC), and \u03b4(C\u2013H), respectively, which indicates the generation of 3-hexanone, 2-hexanone, and olefins (which were deduced after taking into consideration of the GC\u2013MS results shown in Fig.\u00a0S6). This result suggests that n-hexane was partially preferentially oxidized by surface labile lattice oxygen species. Obviously, several new characteristic bands were observed and the accumulation of CO2 in all of the spectra increased with a rise in temperature. The bands at 1637, 1626, 1539, 1532, and 1521\u00a0cm\u22121 were due to the vibration of \u03bd(CC) in olefins; the ones at 1419 and 1326, 1383, and 1290\u00a0cm\u22121 were owing to the vibration of \u03b4(O\u2013H), \u03b4s (CH3), and \u03b4(C\u2013H) [47], respectively; and the ones at 1206 and 1217, 1086 and 1074, 1139, and 1246\u00a0cm\u22121 were ascribable to the vibration of \u03bd(C\u2013O) in alcohols, \u03bd(C\u2013O) in five-membered cyclic ether, \u03bd(C\u2013C), and \u03bd(C\u2013O) in epoxide ethers or alcohols, respectively. Additionally, the bands at 968, 920, 891, 771, 751, and 744\u00a0cm\u22121 were due to the vibration of the \u03b3(C\u2013H). Combining the results of GC\u2013MS, it is evident that 2,5-dimethyltetrahydrofuran, 3-hexyl hydroperoxide, and olefins may be regarded as the main intermediates on the surface of Co1/meso-Cr2O3 or Ni1/meso-Cr2O3 with increasing temperature. In the case of the Co1Ni1/meso-Cr2O3 sample, there were not only the abovementioned intermediates but also 2-methyloxirane or 2-ethyl-oxetane. This result indicates that the adsorbed n-hexane reacts with the surface labile lattice oxygen species on each sample, in which loading bimetallic Co and Ni single atoms can effectively promote the activation of lattice oxygen to accelerate the n-hexane oxidation process.To further reveal the n-hexane combustion mechanism over the as-prepared samples, 20\u00a0vol% O2 was first introduced for 30\u00a0min (defined as O2-30\u00a0min) after the saturated adsorption of n-hexane, and in situ DRIFTS spectra (Fig.\u00a011B, D, and F) were then recorded with the increased temperatures. When O2 was introduced to the Co1/meso-Cr2O3 or Ni1/meso-Cr2O3 sample with a rise in temperature, new adsorption bands at 1722 and 945\u00a0cm\u22121 due to the vibration of \u03bd(CO) and \u03b3(C\u2013H) were recorded as compared with the feed gas without O2, respectively, and the ones at 1641, 1599, and 1612\u00a0cm\u22121 were owing to the \u03bd(CC) of olefins, demonstrating generation of 3-hexanone and 2-hexanone (also according to the GC\u2013MS results). As for the Co1Ni1/meso-Cr2O3 sample, the bands at 1626, 1539, 1419, 1246, 968, 891, and 744\u00a0cm\u22121 disappeared, and new bands at 1717, 1546, 1483, and 948\u00a0cm\u22121 might be associated with the vibration of \u03bd(CO), \u03bd\nas (COO), \u03bd\ns (COO), and \u03b3(C\u2013H) [48], respectively, which implies that 2-methyloxirane, 2-ethyl-oxetane, and 3-hexyl hydroperoxide decrease in amount and the acrylic acid intermediate appears, while the band intensity of 3-hexanone, 2-hexanone, 2,5-hexanedione, and CO2 enhanced gradually (connecting with the GC\u2013MS results). On the basis of the above results, the possible n-hexane combustion pathways over the as-obtained samples can be described as follows: n-hexane \u2192 olefins or 3-hexyl hydroperoxide \u2192 3-hexanone, 2-hexanone or 2,5-dimethyltetrahydrofuran \u2192 2-methyloxirane or 2-ethyl-oxetane \u2192 acrylic acid \u2192 CO\nx\n \u2192 CO2 and H2O. To sum up, n-hexane could be oxidized in the absence of gas-phase O2, demonstrating that the labile lattice oxygen species participate in the reaction. Gaseous oxygen could be activated to the active oxygen species adsorbed at oxygen vacancies caused by the consumption of labile lattice oxygen, suggesting the Mars\u2212van Krevelen (MvK) mechanism. Significantly, loading bimetallic Co and Ni single atoms can markedly accelerate this process due to the facilely activated lattice oxygen species, resulting in the predominant catalytic performance.The 3DOM chromium oxide-supported Co and/or Ni single-atom (Co1/meso-Cr2O3, Ni1/meso-Cr2O3, and Co1Ni1/meso-Cr2O3) and CoNPNiNP/meso-Cr2O3 catalysts were prepared using the one-pot PVP- and PVA-protecting methods, respectively. As a result, catalytic activity below 220\u00a0\u00b0C decreased in an order of Co1Ni1/meso-Cr2O3\u00a0>\u00a0Co1/meso-Cr2O3\u00a0>\u00a0Ni1/meso-Cr2O3\u00a0>\u00a0CoNPNiNP/meso-Cr2O3\u00a0>\u00a0meso-Cr2O3, and that above 220\u00a0\u00b0C decreased in a sequence of Co1Ni1/meso-Cr2O3\u00a0>\u00a0Co1/meso-Cr2O3\u00a0>\u00a0CoNPNiNP/meso-Cr2O3\u00a0>\u00a0Ni1/meso-Cr2O3\u00a0>\u00a0meso-Cr2O3, which was due to the fact that CoNPNiNP/meso-Cr2O3 shows the strong chemisorption of n-hexane at higher temperatures but Ni1/meso-Cr2O3 possesses the better low-temperature reducibility. The Co1Ni1/meso-Cr2O3 catalyst showed the best activity (T\n50% and T\n90% were 239 and 263\u00a0\u00b0C at SV\u00a0=\u00a040,000\u00a0mL\u00a0g\u22121\u00a0h\u22121, respectively), the lowest E\na (54.7\u00a0kJ\u00a0mol\u22121), and the highest specific reaction rate at 260\u00a0\u00b0C (4.3\u00a0\u00d7\u00a010\u22127\u00a0mol gcat\n\u22121\u00a0s\u22121). Long-term stability and CO2 or H2O resistance tests over the Co1Ni1/meso-Cr2O3 sample can also be considered as a promising catalyst for VOCs combustion. The good catalytic performance was associated with the fact that the strong synergistic effect between Co1 and Ni1 and meso-Cr2O3 makes Co1Ni1/meso-Cr2O3 possess a larger amount of higher-valence chromium ions (Cr5+and Cr6+) and easily activated lattice oxygen species, which can efficiently promote the enhancement in n-hexane adsorption and activation ability and the breaking of C\u2013H bonds in n-hexane. The combustion of n-hexane occurs via the MvK mechanism, and its possible pathways are as follows: n-hexane \u2192 olefins or 3-hexyl hydroperoxide \u2192 3-hexanone, 2-hexanone or 2,5-dimethyltetrahydrofuran \u2192 2-methyloxirane or 2-ethyl-oxetane \u2192 acrylic acid \u2192 CO\nx\n \u2192 CO2 and H2O.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Committee of China\u2212Liaoning Provincial People's Government Joint Fund (U1908204), National Natural Science Foundation of China (21876006, 21976009, and 21961160743), Foundation on the Creative Research Team Construction Promotion Project of Beijing Municipal Institutions (IDHT20190503), Natural Science Foundation of Beijing Municipal Commission of Education (KM201710005004), and Development Program for the Youth Outstanding\u2212Notch Talent of Beijing Municipal Commission of Education (CIT&TCD201904019).The following is the Supplementary data to this article.\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2022.12.008.", "descript": "\n                  Developing the alternative supported noble metal catalysts with low cost, high catalytic efficiency, and good resistance toward carbon dioxide and water vapor is critically demanded for the oxidative removal of volatile organic compounds (VOCs). In this work, we prepared the mesoporous chromia-supported bimetallic Co and Ni single-atom (Co1Ni1/meso-Cr2O3) and bimetallic Co and Ni nanoparticle (CoNPNiNP/meso-Cr2O3) catalysts adopting the one-pot polyvinyl pyrrolidone (PVP)- and polyvinyl alcohol (PVA)-protecting approaches, respectively. The results indicate that the Co1Ni1/meso-Cr2O3 catalyst exhibited the best catalytic activity for n-hexane (C6H14) combustion (T\n                     50% and T\n                     90% were 239 and 263\u00a0\u00b0C at a space velocity of 40,000\u00a0mL\u00a0g\u22121\u00a0h\u22121; apparent activation energy and specific reaction rate at 260\u00a0\u00b0C were 54.7\u00a0kJ\u00a0mol\u22121 and 4.3\u00a0\u00d7\u00a010\u22127\u00a0mol gcat\n                     \u22121\u00a0s\u22121, respectively), which was associated with its higher (Cr5+\u00a0+\u00a0Cr6+) amount, large n-hexane adsorption capacity, and good lattice oxygen mobility that could enhance the deep oxidation of n-hexane, in which Ni1 was beneficial for the enhancements in surface lattice oxygen mobility and low-temperature reducibility, while Co1 preferred to generate higher contents of the high-valence states of chromium and surface oxygen species as well as adsorption and activation of n-hexane. n-Hexane combustion takes place via the Mars\u2212van Krevelen (MvK) mechanism, and its reaction pathways are as follows: n-hexane \u2192 olefins or 3-hexyl hydroperoxide \u2192 3-hexanone, 2-hexanone or 2,5-dimethyltetrahydrofuran \u2192 2-methyloxirane or 2-ethyl-oxetane \u2192 acrylic acid \u2192 CO\n                        x\n                      \u2192 CO2 and H2O.\n               "}
{"full_text": "Biomass is a valuable source to produce liquid fuels; however, the bio-oils obtained from it have high oxygen and water contents and require further upgrading [1]. Hydrodeoxygenation (HDO) is a preferred process to decrease the oxygen content for bio-oil upgrading [2]. HDO reactions involve the presence of a catalyst under a hydrogen atmosphere at temperatures between 200\u00a0\u00b0C and 400\u00a0\u00b0C and high pressure [3], in which oxygen is eliminated as water or carbon oxide(s) [4]. A diversity of materials such as metal sulfides [5], oxynitrides [6], phosphides [7], metal oxides [8], molecular sieve-supported metal catalysts [9], bifunctional catalysts [10], and carbides [11], have been used for bio-oil upgrading via HDO reactions [12]. In particular, molybdenum-based catalysts (MoS2, Mo2C, and MoO3) have shown good activity [13], with molybdenum sulfide being the most active one; however, it has low stability at the reaction conditions and could present sulfur loss. Hence, the preparation of stable MoS2 or alternative catalysts for HDO are highly desired [14]. Molybdenum carbides seem to be an excellent alternative to unstable molybdenum sulfide-based catalysts.Molybdenum carbides are attractive active phases due to their low cost, corrosion resistance, high melting point, and catalytic activity [11,15,16]. Nowadays, there is a particular interest in the control of different parameters as morphology, particle size, and the phase of molybdenum carbides [17]. Their catalytic activity is attributed to the permeation of carbon atoms into the molybdenum metal lattice, which lengthens the metal-metal distance, increasing the d-band electron density at the Fermi level of molybdenum [18].Recent studies have demonstrated that molybdenum carbides are active catalysts for a range of oxygenated compounds found in biomass-derived bio-oils [2,15]. When oxygen is eliminated as water during HDO, this could induce the deactivation of the catalysts by the formation of molybdenum oxides [19]. However, it has been found that molybdenum carbides could be stable at the HDO reaction conditions; i.e., in the presence of water [20].Ni-modified Mo2C catalysts show superior activity than Mo2C, Ni has high hydrogenation activity [21,22], but low electrophilicity in comparison with molybdenum, making it less favorable for the activation and direct scission of C=O and C\u2013O bonds [23]. Mo2C catalysts have high selectivity to deoxygenation without the hydrogenation of furanic or aromatic rings, as it specifically facilitates the \u03b72(C, O) adsorption of oxygenated compounds, for this reason, a direct scission of C=O or C\u2013O bond is possible [16,24\u201326]. Wang et al. [27] studied Ni\u2013Mo\u2013C catalysts deposited on various supports in the hydroprocessing of soybean oil. The authors found that Ni\u2013Mo\u2013C active phase supported on mesoporous supports such as \u03b3-Al2O3 and Al-SBA-15 showed an increased yield of hydrocarbons containing mainly C15\u2013C18 associated with decarboxylation, decarbonylation, and hydrodeoxygenation reactions. On the other hand, the carbon nanotubes used as support could also serve as the carbon source for the Mo2C formation [28]. Mai et al. [20] reported that \u03b2-Mo2C supported on carbon nanotubes was an efficient catalyst for the selective conversion of levulinic acid into \u03b3-valerolactone in the aqueous phase. In a turnover frequency (TOF) basis, the activity of the catalyst was similar to that obtained for a ruthenium catalyst evaluated under the same conditions.According to the literature, differentiated catalytic performance is observed when metal particles are located inside or outside of CNTs [29]. Moreover, the preparation of MoxC carbide supported inside multi-wall carbon CNTs and its catalytic performance in hydrodeoxygenation are yet not well known. This work proposed a controllable synthesis of CNTs-supported Ni-Mo carbide catalysts. Additionally, the Ni/Mo molar ratio was evaluated for HDO using benzofuran as a model compound for some bio-oil fractions; indeed, benzofuran is a good model [7,29\u201331] because the majority of organo-oxo compounds have either a phenolic or a furanic structure [32].Multi-walled carbon nanotubes (CNTs) with 10\u201320\u00a0\u03bcm length, external diameter 10\u201320\u00a0nm, and purity >98\u00a0wt%, were obtained from Timesnano Company (Chengdu Organic Chemicals Co. Ltd), ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24\u22194H2O, 99%) was obtained from Sigma-Aldrich, and nickel nitrate hexahydrate (Ni(NO3)2\u22196H2O, 98%) was obtained from Alfa Aesar.In a typical procedure, 1.0\u00a0g of CNTs was refluxed in HNO3 (65\u00a0wt%, 20\u00a0mL per gram of CNTs) using an oil bath for 10\u00a0h, then the CNTs were washed with deionized water until pH\u00a0~\u00a07, and then dried at 100\u00a0\u00b0C for 12\u00a0h. The catalysts were prepared by wet impregnation of CNTs with an aqueous solution obtained by dissolving (NH4)6Mo7O24\u22194H2O and Ni(NO3)2\u22196H2O with variable proportions (namely, Ni/Mo\u00a0=\u00a00.2, 0.3, 1.0, 3.0\u00a0M ratio) to get 1.3\u00a0mmol of metal (Ni\u00a0+\u00a0Mo) per gram of CNTs. During impregnation, the samples were sonicated for 1\u00a0h, and then the water was slowly evaporated at 25\u00a0\u00b0C and kept at 200\u00a0rpm using magnetic stirring, and then dried at 100\u00a0\u00b0C for 12\u00a0h. The precursors were then submitted to temperature-programmed carburization (TPC) under a stream of 20% CH4/H2 (100\u00a0mL\u00a0min\u22121) using a heating ramp of 2\u00a0\u00b0C\u00a0min\u22121 and kept at 700\u00a0\u00b0C for 2\u00a0h. The system was then cooled down under inert atmosphere (Ar), and passivated with an air/Ar mixture (10/90) at room temperature. Catalysts containing only Ni or Mo were prepared following the same procedure.Structure features of the catalysts were determined by X-ray diffraction (XRD) using a PANalytical X'pert PRO MPD diffractometer with Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5), in the 2\u03b8 angle range of 20\u00b0 to 70\u00b0 with a step size of 0.026\u00b0 and period of 50\u00a0s. TEM micrographs were obtained using a Tecnai F20 Super Twin TMP instrument; samples were dispersed in acetone and sonicated for 15\u00a0min before being dropped on a carbon-coated copper grid. The specific surface area was determined using the N2 adsorption at \u2212196\u00a0\u00b0C performed with an Autosorb-iQ from Quantachrome and using the Brunauer-Emmett-Teller (BET) model to estimate the specific surface areas. X-ray photoelectron spectra (XPS) were performed in an ultrahigh vacuum (UHV) using a SPECS multi-technique analysis instrument equipped with a monochromatic Al-K\u03b1 source (1486.7\u00a0eV, 13\u00a0kV, 100\u00a0W) and an electron analyzer PHOIBOS 150 1D-DLD. The step was 1\u00a0eV and 0.1\u00a0eV for the general and the high-resolution spectra, respectively.Preliminary tests were carried out to evaluate the catalytic performance of the different catalysis. Typically, for each run 300\u00a0\u03bcL (0.33\u00a0g) of benzofuran, 30\u00a0mg of catalyst, and 300\u00a0\u03bcL of tetradecane (as internal standard) in 10\u00a0g of hexadecane, were loaded into a 50\u00a0mL batch autoclave reactor equipped with an electromagnetic stirrer. The reaction was carried out under an H2 environment at 5\u00a0MPa with a stirring speed of 400\u00a0rpm at 280\u00a0\u00b0C for 4\u00a0h. Additionally, the temperature (200\u00a0\u00b0C\u2013320\u00a0\u00b0C) and reaction time (2\u00a0h\u00a0\u2212\u00a08\u00a0h) effect, and catalyst recycling, were evaluated using the catalyst with the best catalytic performance in the preliminary tests.At the end of each experiment, the products were separated from the catalyst by filtration and the identification and quantification were performed in a GC/MS Shimadzu (2010-Plus) equipped with a DB-5 capillary column (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u03bcm) using n-tetradecane as the internal standard. The initial oven temperature was 50\u00a0\u00b0C for 1\u00a0min. The temperature was programmed to increase from 50 to 220\u00a0\u00b0C at 2\u00a0\u00b0C\u00a0min\u22121 and hold for 10\u00a0min. The conversion of benzofuran (XBF), the yield of each product (Yi) and the selectivity (Si) were calculated in molar basis relative to the feed following Eqs. (1\u20133):\n\n(1)\n\n\nX\nBF\n\n=\n\n\n1\n\u2212\n\n\n\nC\nBF\n\n\n\n\n\n\nC\nBF\n\n\n0\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n(2)\n\n\nY\ni\n\n=\n\n\n\n\nC\ni\n\n\n\n\n\n\nC\nBF\n\n\n0\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n(3)\n\n\nS\ni\n\n=\n\n\n\n\nY\ni\n\n\n\n\nX\nBF\n\n\n\n\u00d7\n100\n%\n\n\n\nWhere CBF|0 is the initial concentration of benzofuran in the reactor feed, CBF and Ci is the molar concentration of benzofuran and the product species after reaction, respectively.\nFig. 1a shows the preliminary results of benzofuran conversion and product distribution. Monometallic Ni/CNTs catalyst exhibited a higher conversion of 21.0% in comparison with 0.7% for Mo/CNTs. On the other hand, the products over Ni/CNTs were hydrogenated products, 2,3-dihydrobenzofuran (2,3-DHBF, yield: 18.1%), and octahydrobenzofuran (OHBF, yield: 2.6%), this is related to the high hydrogenation activity attributed to metallic Ni particles dispersed on the surface of CNTs [1,33]. The lowest conversion of Mo/CNTs, when molybdenum is found mainly as amorphous carbides (MoxC) according to XRD results, may be related to the smaller surface area determined by BET (Table S1), the lower activity of MoxC carbide, and the reaction conditions (reaction temperature of 280\u00a0\u00b0C). This results is consistent with literature reports which indicate that Mo2C-based catalyst showed activity for HDO reactions at temperatures higher than 300\u00a0\u00b0C for guaiacol and 350\u00a0\u00b0C for stearic acid and for the catalytic upgrading of residual biomass derived bio-oil [34,35].Ni and Mo have been used to prepare catalysts for a high number of reactions, in which the Ni/Mo ratio is a key parameter for the catalysts performance [36\u201339]. The results for NiMo-x/CNTs catalysts show an important effect of Ni/Mo molar ratio on the conversion and products distribution in the benzofuran HDO. As previously mentioned, without Ni, the conversion on Mo/CNTs was only 0.7%. After introducing both Ni and Mo, conversion, hydrogenation, and deoxygenation degree were increased. Reduced Ni acted as hydrogen activation sites supplying reactive hydrogen species for the hydrogenation reaction [40]. For example, 73.2% conversion was obtained on NiMo-0.2/CNTs, with 67.6% yield to 2,3-DHBF. For NiMo-0.3/CNTs, the BF conversion was nearly 100%, though the yield to 2,3-DHBF decreased to 64.6%; this catalyst showed an increase in the formation of OHBF with a yield of 19.9%, where OHBF is the product of the hydrogenation of 2,3-DHBF (see Fig. 1b). Additionally, the yield for HDO products, ethylcyclohexane (ECH) and methylcyclohexane (MCH), increased to 5.1%, which suggests that the reaction path begins with the hydrogenation of BF followed by the hydrogenation 2,3-DHBF. In contrast, for the NiMo-3/CNTs catalyst, the BF conversion decreased to 43.9%, with a 39.4% yield to 2,3-DHBF.On the other hand, reactions carried out in the presence of unsupported Ni-Mo2C-0.3 showed a BF conversion lower than 1.0% (Fig. S1). The low conversion of BF over unsupported catalyst clearly indicated a positive effect of CNTs as catalyst support, and the results showed in Fig. 1 suggest a synergistic effect when Ni and Mo are deposited in CNTs. Although the Ni/Mo ratio of 0.3 has a higher activity to OHBF and HDO products, the optimum Ni/Mo ratio can be different for other reactions. Recently, Smirnov et al. [41] prepared carbide catalysts with different Ni/Mo ratios (0, 0.5, 1, 2, and 6) using a method based on the Pechini process. The catalysts were evaluated in the hydrodeoxygenation of anisole and ethyl caprate when the Ni2MoC (Ni/Mo\u00a0=\u00a02) catalyst showed the highest activity, which is attributed to the presence of Ni-Mo\u2013C active sites. Under adopted reaction conditions (280\u00a0\u00b0C), the main products were 2,3-DHBF and OHBF; thereby, high hydrogenating properties for these catalysts are inferred. The high activity of Ni/CNTs in comparison to Mo/CNT is noteworthy; some properties such as particle size and the evaluation of other reaction conditions could lead to obtaining valuable products, however, this is not further considered in this work.\nFig. 1c shows the XRD patterns of the catalysts. The peaks at 2\u03b8\u00a0=\u00a044.5\u00b0, 2\u03b8\u00a0=\u00a051.8\u00b0 for Ni/CNTs corresponds to (111, 002) planes for metallic Ni (ICSD 64989). In the case of the Mo/CNTs, diffraction peaks are roughly observed with the peak at 2\u03b8\u00a0=\u00a037\u00b0 and 2\u03b8\u00a0=\u00a039\u00b0 showing low intensity. On the other hand, NiMo/CNTs catalysts show peaks at 2\u03b8\u00a0=\u00a034.5\u00b0, 2\u03b8\u00a0=\u00a037.9\u00b0, and 2\u03b8\u00a0=\u00a039.5\u00b0 corresponding to (010, 002, 011) planes of \u03b2-Mo2C (ICSD # 77158). The results show that the Ni addition improves the \u03b2-Mo2C phase formation, which can be attributed to methane dissociation on metallic Ni particles, generating chemisorbed carbon species suitable for the formation of molybdenum carbide [42,43]. Samples prepared with molar ratios Ni/Mo in the range 0.2\u20131.0 show representative signals of the \u03b2-Mo2C hexagonal phase, namely, 2\u03b8: 34.5\u00b0 (010), 37.9\u00b0 (002), 39.5\u00b0 (011), and 52.2\u00b0 (012). However, for the sample with a Ni/Mo\u00a0=\u00a03 (Ni-Mo/CNTs-3) any diffraction peak corresponding to \u03b2-Mo2C phase was detected, possibly due to the lower loading of Mo (about 3\u00a0wt%), as compared to other samples. On the other hand, all the samples showed peaks corresponding to metallic nickel phase 2\u03b8: 44.5\u00b0 (111) and 51.8\u00b0 (002). For comparison, an unsupported NiMo-0.3 solid was prepared under similar conditions (i.e., 700\u00a0\u00b0C and 20% CH4/H2), and the XRD results corroborate the formation of \u03b2-Mo2C (Fig. S1), which is consistent with the results reported by Jin et al. [44]. The formation of bimetallic Mo3Ni2C or NixMo1-x phases were not observed in the prepared catalysts, which may be due to the different preparation methods employed in this work. Other parameters can also influence the formation of \u03b2-Mo2C; for example, Liang et al. [40] prepared \u03b2-Mo2C/CNTs at a lower temperature, using a more complex procedure. Namely, a calcination in air at 500\u00a0\u00b0C was performed for 3\u00a0h, followed by reduction under H2 atmosphere at 700\u00a0\u00b0C for 4\u00a0h; however, it was found that \u03b2-Mo2C particles were mainly located outside of CNTs.In this work transmission electron microscopy was employed to identify the position of the particles, as well as their size and morphologic features. Representative micrographs are presented in Fig. 2\n for NiMo-0.3/CNTs. STEM results (Fig. 2a-c) indicate that most of the particles are inside of CNTs, as expected according to the methodology for the preparation of the catalysts. Atomic quantification of one of these particles showed the presence of Ni of 43.9% and Mo 56.1% free of C and O (Fig. 2c). Fig. 2d-f show TEM images with particles inside and some outside of CNTs, the particles observed outside of CNTs present an interplanar distance of 2.13\u00a0\u00c5 (Fig. 2g), which can be assigned to the (002) plane of MoC phase. The particle inside of the CNTs with interplanar distance of 2.36\u00a0\u00c5 (Fig. 2h) corresponding to the (002) plane of \u03b2-Mo2C phase. The diameter of CNTs limited the diameter of formed Ni-Mo2C particles; as is shown in Fig. 2i, most particles are less than 12\u00a0nm, and most of particles with an approximate size of 5\u00a0nm contrast with the particle size of those present outside the CNTs, measuring about 28\u00a0nm.The effect of reaction temperature on HDO performance over the catalyst NiMo-0.3/CNTs has also been investigated. As shown in Fig. 3\n, BF conversion was 4.7% at 200\u00a0\u00b0C. The conversion increased to ~100% at temperatures higher than 280\u00a0\u00b0C, where hydrogenated products (i.e., 2,3-DHBF and OHBF) were predominant up to 280\u00a0\u00b0C (yield up to 64.2%). The yield to hydrogenated products (2,3-DHBF, ECHOH) decreased up to 6.2% at 320\u00a0\u00b0C, and the hydrodeoxygenated products EB ECH, and MCH increase approximately to 3.9%, 74.4% and 15.6%, respectively (93.8% of deoxygenated products at 320\u00a0\u00b0C), which indicated that the hydrodeoxygenation selectivity was improved with the increase in reaction temperature. Other products as ethylphenol (2-EtPh) were detected at all temperatures, which should be formed by the rupture of the C\u2013O bond of 2,3-DHBF before the ring hydrogenation; however, its yield was low throughout the temperature range in comparison with OHBF.The present results from HDO of benzofuran over NiMo-x/CNTs catalysts are consistent with the reaction network proposed in the literature [45,46], where the initial step is the hydrogenation of BF to 2,3-DHBF. As expected, higher temperatures (320\u00a0\u00b0C) caused an increase in BF conversion and led to higher yield to HDO products, particularly saturated products, such as ECH and MCH.The influence of reaction time on deoxygenation of benzofuran was studied and the results are shown in Fig. 4\n. Once the reactor reached the reaction conditions (i.e., 300\u00a0\u00b0C) the measured conversion was 17.5%, with hydrogenation being the main reaction taking place by forming exclusively 2,3-DHBF; after one hour the conversion reached 100% and the consecutive hydrogenation reactions occurred with 2,3-DHBF and OHBF as the main products. Prolonged reaction time favored hydrodeoxygenation reactions and then at 6\u00a0h of reaction the total deoxygenated product yield was 69.6%, with ECH (56%) as the main product, indicating that incorporation of both Ni and Mo could not only increase the HDO conversion over NiMo-x/CNTs catalysts but also promote the desired HDO reaction over NiMo-0.3/CNTs.The detected deoxygenated products yield over NiMo-0.3/CNTs catalyst after 6\u00a0h decreased in the order: ECH\u00a0>\u00a0MCH\u00a0>\u00a0EB\u00a0>\u00a0ECHE. In addition, the yield toward O-containing intermediates decreased in the order: OHBF >2,3-DHBF > CHEOH. The presence of OHBF indicated that the NiMo-0.3/CNTs catalyst had a high hydrogenation properties, even higher than that reported over Ni2P/Al-SBA-15 catalyst where 2,3-DHBF is transformed into 2-EtPh [47].Based on the results concerning the effect of temperature and the formed products with time, the plausible reaction route is shown in Fig. 5\n. The first step involves hydrogenation of the furan ring, which led to the formation of 2,3-DHBF, which is further converted into OHBF by hydrogenation of the benzene ring. Then, ECHOH is obtained from OHBF by a C\u2013O bond cleavage of the heterocyclic ring through hydrogenolysis. Finally, ECHOH is transformed into ECH by dehydration, followed by demethylation of ECH to yield MCH.The catalyst follows mainly the route BF\u00a0\u2192\u00a02,3-DHBF\u2192OHBF\u2192ECHOH\u2192ECH, which is different of that reported for Nd-Ni2P or Y-Ni2P [46], Ni-Cu/\u03b3-Al2O3 [48], Ni2P/Al-SBA-15 [7] catalysts, where OHBF is not detected and the reaction follow the pathways: BF\u00a0\u2192\u00a02,3-DHBF\u21922-EtPh \u2192EB\u00a0\u2192\u00a0ECH. The present results suggest that the catalyst NiMo-0.3/CNTs have higher hydrogenation properties, which are similar to those reported for reduced Mo and Ni\u2013Mo/ \u03b3-Al2O3 [49] and silica-alumina-supported Pt, Pd, and Pt\u00a0\u2212\u00a0Pd catalysts [50,51], where OHBF is detected.\nFig. 6\n shows the catalyst recycling of NiMo-0.3/CNTs in the BF hydrodeoxygenation. After each reaction, the catalyst was washed with dichloromethane, dried at 100\u00a0\u00b0C overnight, and reused for the next reaction. The conversion decreased from 97% in the first cycle to 51% in the fifth cycle. In general, the yield decreased in the order 2,3-DHBF>2-EtPh>OHBF>ECH\u00a0>\u00a0MCH. Upon reuse, the amount of 2-EtPh was higher with respect to OHBF, which suggests that the hydrogenating properties of the catalyst decreased.The surface characterization of NiMo-0.3/CNTs catalyst was further probed by XPS. From the survey spectrum displayed in Fig. S2, elements of C, O, Ni, and Mo can be identified. Fig. 7\n shows that the high-resolution spectra of Mo 3d can be deconvoluted into three doublets for Mo6+ (89.6%), Mo4+(5.8%) and Mo2+(4.6%). The peaks at binding energies 233.0\u00a0eV and 236.1\u00a0eV (Mo 3d5/2 and 3d3/2) for Mo6+ and 230.9\u00a0eV and 232.6\u00a0eV (Mo 3d5/2 and 3d3/2) for Mo4+, in molybdenum oxidized phases [11,52,53], which was probably caused by surface oxidation of Mo2C due to air contact during the passivation process. The small peaks centered at 228.7\u00a0eV and 231.9\u00a0eV (Mo 3d5/2 and 3d3/2) were assigned to Mo2+, which can be attributed to the carbide phase, indicating the presence of Mo2C [54]. Additionally, a small Ni 2p XPS signal was detected (Fig. 7d), which can be assigned to Ni in Ni-MoxC particles. The deconvolution of the C 1\u00a0s peak is shown in Fig. 7b, peaks at 284.8\u00a0eV, 286.3\u00a0eV, and 291.1\u00a0eV can be assigned to C=C/C\u2013C/C\u2013H, C\u00a0\u2212\u00a0OH/C\u2013O\u2013C and O\u2013C=O bonds respectively [55]. Additionally, the oxygen region (Fig. 7c) shows peaks at 530.5\u00a0eV for M\u2013O and 532.1\u00a0eV for C-O bonds.It should be noted that the thickness of more than 10\u00a0nm for CNTs makes it difficult to observe the actual surface of the Ni-Mo2C particles inside of CNTs. However, this analysis allows us to observe the accumulation of organic material, likely coke precursor on the CNTs, as has been reported for Ni-Mo based catalysts [56]. After five reaction cycles, the XPS signals change significantly (Fig. 7e-h). The most significant change is observed for C 1\u00a0s and O 1\u00a0s signals, the increase in 286.0\u00a0eV and 532.7\u00a0eV and 534.1\u00a0eV signals indicate the presence of bonds C\u2013OH, C\u2013O\u2013C, and C=O. The increased formation of oxidized species (C\u2013O, C=O, C\u2013OH) changes from 6.49% to 58.61%, which could be due to the presence of hydrocarbon residues or coke precursors deposited on the catalyst surface during the reaction [57]. Several studies reported the formation of coke and the deactivation of the catalysts [58\u201360], however further experiments are required to determine the true reason for the deactivation of NiMo/CNTs catalysts. According to high-resolution XPS, Mo6+ decreased after five reaction cycles from 89.6% to 52.7%, whereas the signal for Mo carbide increased from 4.64% to 9.70%.The results showed that the proposed methodology improved the deposition of the Ni-Mo2C particles inside of CNTs. Ni/CNTs plays higher activity than Mo/CNTs, however Ni improves the formation \u03b2-Mo2C, possibly by the C\u2013H bond dissociation of CH4, forming C species during the activation process. All Ni- Mo2C/CNTs catalysts showed higher conversion and yield toward hydrogenation and hydrodeoxygenation products than mono-metallic Ni/CNTs and Mo/CNTs. Nickel-modified molybdenum carbide particles inside CNTs with Ni/Mo molar ratio of 0.3 showed the highest catalytic performance, much better than unsupported NiMo-0.3 catalyst. The results suggest that carbon nanotubes limit the size of the formed Ni-Mo2C particles inside CNTs, exposing a higher number of active sites for the reaction. The NiMo-x/CNTs catalysts showed high hydrogenation properties, and the main pathway for benzofuran HDO was BF\u00a0\u2192\u00a02,3-DHBF \u2192 OHBF \u2192 ECHOH \u2192 ECH\u00a0\u2192\u00a0MCE. This reaction route is favored at high temperatures (320\u00a0\u00b0C), where 93.8% yield to deoxygenated products were obtained after 4\u00a0h of reaction, with ECH (74% of yield) as the main product.Nevertheless, it must be mentioned that a progressive lost in activity was observed upon catalyst reusing for 5\u00a0cycles (after solvent washing). The decrease of the catalyst activity is likely due to the accumulation of oxygenated species, possibly coke precursors on the catalyst surface; therefore, further work is required to improve stability for their reuse.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank the financial support from the National Key R&D Program of China (2018YFB1501403), and the National Natural Science Foundation of China (22078220 and 51776134). S.P would like to express his gratitude to the 2016 China-LAC Young Scientist Exchange Program for the financial support to perform studies at the Key Laboratory of Coal Science and Technology (Taiyuan University of Technology). D.L and A.M thank the Universidad de Antioquia UdeA (Colombia).\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.fuproc.2022.107416.", "descript": "\n                  This work proposed a controllable synthesis of Ni-Mo catalyst supported inside multi-wall carbon nanotubes (CNTs). The results indicate that Ni improved the \u03b2-Mo2C formation and markedly promoted the benzofuran (BF) hydrogenation and hydrodeoxygenation activity of the catalysts. The synergistic interaction between Ni and Mo reached the maximum at a Ni/Mo molar ratio of 0.3, which could be favored by the proximity between the Ni and \u03b2-Mo2C particles inside the CNTs reaching a 99.5% of BF conversion to hydrogenated and deoxygenated products as 2,3-dihydrobenzofuran, octahydrobenzofuran, and ethylcyclohexane; in contrast, BF conversion on unsupported Ni-Mo2C-0.3 was only 0.7%. Deoxygenated products are favored under different conditions, such as the time, and mainly with the temperature achieving 93.8% of yield toward deoxygenated products with 100% of BF conversion at 320\u00a0\u00b0C. However, the catalyst activity is lost through reuse cycles, likely due to the deposition of high molecular weight compounds (coke) on the catalyst surface.\n               "}
{"full_text": "\n\n\nData will be made available on request.\n\n\nData will be made available on request.The classical Hirao reaction involves the P\u2013C coupling between vinyl- or arylbromides and dialkyl phosphites performed in the presence of palladium-tetrakistriphenylphosphine (Pd(PPh3)4) as the catalyst [1,2]. Shortly after the invention of this elegant method for the synthesis of vinyl- or arylphosphonates, Pd- or Ni-salts (e.g. Pd(OAc)2 or NiCl2, respectively) were applied with different mono- and diphosphines as the P-ligands, and the reaction was extended to different aryl- and heteroaryl derivatives, and alkyl H-phosphinates along with secondary phosphine oxides [3\u201310]. Our research group developed a microwave (MW)-promoted method, in which Pd(OAc)2 or NiCl2 was the catalyst precursor, and the excess of the >P(O)H reagent provided the P-ligand via its trivalent tautomeric form (>P-OH) [11,12]. The fine mechanism was explored experimentally, and by high level quantum chemical calculations [13,14]. As regards the Ni-catalyzed P\u2013C coupling reactions, surprisingly a Ni(II)\u00a0\u2192\u00a0Ni(IV) transition was found instead of the generally believed Ni(0)\u00a0\u2192\u00a0Ni(II) formula [15\u201317].The application of Cu(I) salts, this occasion, together with N-ligands is another option. A series of N-containing compounds, such as DMEDA [18\u201321], 2,2\u2032-bipyridine, 1,10-phenanthroline, TMEDA, 1-methyl-1H-imidazole [22], N-methylpyrrolidine-2-carboxamide [23], proline and pipecolic acid [24], (S)-\u03b1-phenylethylamine [25,26], or 1-pyrrodinylphosphonic acid monophenyl ester [27] were described. Due to the lower activity of the Cu-catalysts, in most cases iodoarenes were the starting materials in reaction with P-reagents. It was also a possibility that aryl bromides were prereacted with potassium iodide [24,27]. Interestingly, \u201cligand-free\u201d Cu-catalyzed protocols were also proposed [28,29]. We were the first, who investigated the mechanism of the Cu(I)-salt catalyzed P\u2013C couplings. Moreover, in this study no conventional ligands were added, only a diarylphosphine oxide and triethylamine was present as the reactant and as the base, respectively [30].In this article, we overview our new results including the use of Cu(II)-salts as catalyst precursors. Cu(II) salts have a few advantages against Cu(I), such as better solubility, chemical stability, and a lower commercial price.As it was shown, a number of methods were described for the Hirao reaction. We wished to find the best P\u2013C coupling methods for the synthesis of diethyl phenylphosphonate ((EtO)2PhP(O) (1a)) and triphenylphosphine oxide (Ph3P(O) (1b)) as the product of the simplest model reactions. However, the syntheses were carried out in different laboratories and by different hands, not speaking about the MW reactors, whose activity may have not been the same, as it decreases with aging. For this, we decided to reproduce the relevant experiments in a single MW reactor in our laboratory.First of all, the model reaction of bromobenzene (PhBr) with diethyl phosphite ((EtO)2P(O)H) was investigated under different conditions. In all cases triethylamine (NEt3) was used as the base. We were not successful in reproducing the original method using Pd(PPh3)4 as the catalyst at 90 \u00b0C for 2.5\u00a0h in the absence of any solvent. Phosphonate 1a was obtained in a yield of 55% that was in contrast with the outcome of 92% reported earlier [31] (Table\u00a01\n/entry 1). On the one hand, this catalyst is a sensitive species, we could identify Ph3P(O) as a by-product. The reproduction of the experiment using Pd(OAc)2/PPh3 in ethanol (EtOH) at 80 \u00b0C for 16\u00a0h was more successful. Our yield of 89% was not far from the reported 94% [32] (Table\u00a01/entry 2). Here again, Ph3P(O) was a contaminant. In both previous cases, the Ph3P(O) may have come from the PPh3 ligand. The next case studied was the MW-assisted protocol involving Pd(OAc)2 as the catalyst precursor and some excess of the P-reagent applied as the ligand. Using EtOH at 120 \u00b0C for 0.5\u00a0h, the 71% yield could be somewhat exceeded [33] (Table\u00a01/entry 3). However, when acetonitrile (MeCN) was the solvent, there was need for a longer reaction time of 1\u00a0h instead of 0.5\u00a0h. In this way, the 61% outcome could be exceeded by a better yield of 83% [13] (Table\u00a01/entries 4 and 5). In the third, solvent-free variation at 150 \u00b0C, the 93% yield could not be reproduced applying a 5 min\u2019 irradiation time, but prolonging the time of exposure, after 0.5\u00a0h, we could reach an outcome of 80% [34] (Table\u00a01/ entries 6 and 7).As regards the P\u2013C coupling of PhBr and diphenylphosphine oxide (Ph2P(O)H), using the previous approach comprising EtOH as the solvent at 120 \u00b0C, the 83% yield of Ph3P(O) (1b) could be reproduced [14] (Table\u00a01/entry 8). At the same time, performing the synthesis in MeCN at 150 \u00b0C, there was need for prolonged irradiation of 1\u00a0h (instead of 0.5\u00a0h) to achieve a better yield of 75%, as compared to the reported 67% [13] (Table\u00a01/entries 9 and 10).In summary, the best general method for the P\u2013C coupling of PhBr with (EtO)2P(O)H and Ph2P(O)H is the MW-promoted accomplishment involving 5\u201310% of Pd(OAc)2 as the catalyst precursor and 1.15\u20131.30 equiv. of the >P(O)H reagent as the P-ligand in EtOH using NEt3 at 120 \u00b0C.As regards the use of NiCl2 as the catalyst precursor, the coupling of PhBr and (EtO)2P(O)H led to similar results, no matter if it was performed using K2CO3 in MeCN, or NEt3 without any solvent. After an irradiation at 150 \u00b0C for 1\u00a0h and 2\u00a0h, respectively, the outcomes were similar. The literature procedure reported a yield of 70% and 67%, respectively, while our own reproductions provided yields of 62% and 58%, respectively (Table\u00a02\n/entries 1 and 2).Reproduction of the reaction of Ph2P(O)H with PhBr at 150 \u00b0C applying Cs2CO3 or K2CO3 in MeCN gave closer results. Our yields were 74% and 86%, as compared to the literature values of 79% and 91%, respectively (Table\u00a02/entries 3 and 4).One may conclude that, in general, the Pd-catalyzed P\u2013C couplings are somewhat more efficient than the Ni-promoted ones.After clarifying the best variations with the Pd(II)- and Ni(II)-salts, we wished to test the applicability of the cheaper and more practical Cu(II)-salts in the P\u2013C coupling reactions of Ph2P(O)H with halobenzene. In these cases, PhBr was not expected to reveal a suitable reactivity, for this, iodobenzene (PhI) was applied [30]. Using 20% of anhydrous CuSO4, 1 equiv. of Ph2P(O)H and the same amount of NEt3 at 165 \u00b0C in EtOH, the conversion was not complete, and the proportion of Ph3P(O) (1b) was only 57% (Table\u00a03\n/entry 1). Increasing the quantity of Ph2P(O)H to 1.4 equiv., almost a similar result was obtained (Table\u00a03/entry 2). At the same time, when 1 equiv. of Ph2P(O)H was applied together with 2 equiv. of NEt3, the formation of the phosphine oxide (1b) was clear-cut, having manifested in a quantitative conversion, and in a preparative yield of 85% (Table\u00a03/entry 3). Changing for Cu(OAc)2\u00a0\u00b7\u00a0H2O as the precursor, the trend remained the same, and the overall yields were somewhat higher (50/59%, Table\u00a03/entries 4 and 5), or slightly lower (75%, Table\u00a03/entry 6). One could conclude that applying CuSO4 or Cu(OAc)2\u00a0\u00b7\u00a0H2O, NEt3 must have a role in the ligation of the center Cu.Applying CuCl2\u00a0\u00b7\u00a02H2O and CuSO4\u00a0\u00b7\u00a05H2O as the metal salt component of the catalyst, and Ph2P(O)H and NEt3 in a 1:2 ratio, the useful conversions and yields were lower (78/72% and 48/56%, respectively, Table\u00a03/entries 7 and 8) than in the previous cases.Then the P\u2013C coupling reaction was extended to diarylphosphine oxides. Applying bis(4-methylphenyl)phosphine oxide and either CuSO4 or Cu(OAc)2\u00a0\u00b7\u00a0H2O as the catalyst precursor, there was a smaller difference between the experiments using Ar2P(O)H and NEt3 in a ratio of 1:1 or 1:2. However, in all cases the latter ratio was more favorable (Table\u00a04\n/entries 1\u20134). The coupling with bis(3,5-dimethylphenyl)phosphine oxide was also quantitative with two equivalents of NEt3, no matter if CuSO4 or Cu(OAc)2\u00a0\u00b7\u00a0H2O was the precursor (Table\u00a04/entries 5 and 6).According to the experimental findings, catalytic amount of a Cu(II) salt can effectively promote the Hirao reaction of iodobenzenes with Ph2P(O)H in protic solvents. Earlier, a plausible reaction mechanism was explored for the Cu(I)-catalyzed Hirao reactions, where the metal is complexed by both the P-reactant and the excess of the NEt3 base [30].An analogous reaction mechanism was assumed using Cu(II) (Fig\u00a01\n). Hypothetically, the starting species of the reaction sequence is the complexed form of Cu(II), which may be easily deprotonated by the base (NEt3) present in the reaction mixture. This is followed by the P-Cu-P\u00a0\u2192\u00a0P-Cu-O isomerization. Analogously to the Cu(I) mechanism, a PhBr molecule (selected instead of PhI to simplify the calculations) was complexed via the pi-system of its aromatic ring. In the next step, the C\u2013Br bond would be cleaved, resulting in a covalent Cu\u2013C bond, meanwhile the Cu2+ would be oxidized formally to Cu4+, leading to intermediate-II. However, according to a careful theoretical investigation, this step did not prove to be feasible, as there was no real TS on the potential energy surface, which could link the two sides, intermediate-I and intermediate-II with a continuous pathway. It was confirmed by a stepwise bond scanning and IRC investigation. Due to the strong electron deficient central Cu(II) ion, intermediate-II represents a rather distorted structure. A similar situation was observed in an earlier study for the Ni2+-catalyzed Hirao reaction [16]. Formally, the \u201cremaining part\u201d of the mechanism could lead to the desired product. In this case, the second TS represents a rather low, almost negligible (ca. 5\u00a0kJ mol\u20131) enthalpy barrier, so the C\u2013C bond formation could happen.The invalid catalytic cycle is shown in Fig\u00a02\n.In order to explain the failure of the oxidative addition with Cu(II), the mechanism assumed should be compared with that computed for Cu(I) earlier [30]. The process may be simplified to two subprocesses described by the change in the oxidation state of Cu. In the course of the oxidative addition step, the oxidation number of Cu is increased by two. In the case of Cu(I), the oxidation leads formally to Cu(III) that step is connected with the reduction of the \u03b1 carbon atom of the phenyl group affording Ph\u2013. The activation enthalpy of this step was found quite low (+45.7\u00a0kJ mol\u20131). In contrast to this, the analogous oxidation of Cu(II) to Cu(IV) is not feasible, as the formal negative charge in the phenyl ring of the Cu(IV)\u2013Ph complex would be greater than in the previous Cu(I) complex. As a consequence of this, Cu(II) cannot be oxidized to Cu(IV). In other words, in the case under discussion, there is no continuous pathway leading to intermediate-II.The reductive elimination (that assumes +88.0\u00a0kJ mol\u20131 for the Cu(III)\u00a0\u2192\u00a0Cu(I) case) would practically have no barrier for the Cu(IV)\u00a0\u2192\u00a0Cu(II) process. See Table\u00a05\n and Fig\u00a03\n.In search for a valid mechanism with the Cu(II) precursor, we looked for an alternative explanation. Here, we propose a process, where Cu(II) is reduced to Cu(I) by the secondary phosphine oxide added to the reaction in a slight excess, meanwhile the P(III) atom is oxidized to P(V). A plausible red-ox reaction is suggested in Scheme\u00a01\n, where Cu(II) is transformed to Cu(I) in the presence of NEt3 as the base and in ethanol as the solvent. The reaction enthalpy is exothermic exhibiting 811\u00a0kJ mol\u20131 meaning a reasonable driving force.Once the Cu(I) ion was formed, the reaction sequence may proceed further as it was proposed earlier [30]. It is assumed that the in situ formed Cu(I) species may be advantageous, as the on site formation provides a better solubility, and consequently a faster rate.The reactions were carried out in a CEM\u00ae Discover (300\u00a0W) focused microwave reactor equipped with a stirrer and a pressure controller using 80\u2013100\u00a0W irradiation under isothermal conditions. The reaction mixtures were irradiated in sealed borosilicate glass vessels (with a volume of 10\u00a0mL) available from the supplier of CEM\u00ae. The reaction temperature was monitored by an external IR sensor.The 31P, 13C and 1H NMR spectra were taken in CDCl3 solution on a Bruker AV-300 spectrometer operating at 121.5, 75.5 and 300\u00a0MHz, respectively. The 31P chemical shifts are referred to H3PO4, while the 13C and 1H chemical shifts are referred to TMS. The couplings are given in Hz. The exact mass measurements were performed using an Agilent 6545 Q-TOF mass spectrometer in high resolution, positive electrospray mode.To 0.047\u00a0mmol (0.011\u00a0g) or 0.094\u00a0mmol (0.021\u00a0g) of the Pd(OAc)2 catalyst in 1\u00a0mL of ethanol or acetonitrile or without any solvent were added 0.95\u00a0mmol (0.10\u00a0mL) of bromobenzene, 1.09\u00a0mmol (0.14\u00a0mL) or 1.42\u00a0mmol (0.18\u00a0mL) or 1.23\u00a0mmol (0.15\u00a0mL) of diethyl phosphite and 1.04\u00a0mmol (0.15\u00a0mL) of triethylamine. Then, the mixture was irradiated in a closed vial in the microwave reactor at 120\u00a0\u00b0C or 150 \u00b0C for the times (0.5 or 1\u00a0h) shown in Table\u00a01. The reaction mixture was diluted with 3\u00a0mL of the corresponding solvent or EtOH, filtrated, and the residue obtained after evaporation was passed through a thin (2\u20133\u00a0cm) layer of silica gel using ethyl acetate as the eluent. The crude mixture was analyzed by 31P NMR spectroscopy, then it was purified by column chromatography (silica gel and ethyl acetate as the eluent). For the results see Table\u00a01/entries 3, 5 and 7.To 0.022\u00a0mmol (0.0049\u00a0g) of the Pd(OAc)2 in 1\u00a0mL of ethanol were added 0.43\u00a0mmol (0.045\u00a0mL) of bromobenzene, 0.49\u00a0mmol (0.10\u00a0g) of diphenylphosphine oxide and 0.47\u00a0mmol (0.066\u00a0mL) of triethylamine. Then, the mixture was irradiated in a closed vial in the microwave reactor at 120\u00a0\u00b0C for 1\u00a0h. The reaction mixture was diluted with 3\u00a0mL of EtOH, filtrated, and the residue obtained after evaporation was passed through a thin (2\u20133\u00a0cm) layer of silica gel using ethyl acetate as the eluent. The crude mixture was analyzed by 31P NMR spectroscopy, then it was purified by column chromatography (silica gel and dichloromethane-methanol 97:3 as the eluent) to provide 84% (0.11\u00a0g) of phosphine oxide 1b.To 0.049\u00a0mmol (0.0064\u00a0g) of the NiCl2 in 1\u00a0mL of acetonitrile were added 0.49\u00a0mmol (0.055\u00a0mL) of bromobenzene, 0.64\u00a0mmol of >P(O)H-reagent [diethyl phosphite: 0.082\u00a0mL or diphenylphosphine oxide: 0.13\u00a0g] and 0.49\u00a0mmol (0.068\u00a0g) or 0.59\u00a0mmol (0.082\u00a0g) of potassium carbonate. Then, the mixture was irradiated in a closed vial in the microwave reactor at 150\u00a0\u00b0C for 60 or 45\u00a0min. The reaction mixture was diluted with 3\u00a0mL of MeCN, filtrated, and the residue obtained after evaporation was passed through a thin (2\u20133\u00a0cm) layer of silica gel using dichloromethane-methanol 97:3 as the eluent. The crude mixture was analyzed by 31P NMR spectroscopy, then it was purified by column chromatography (silica gel and ethyl acetate or dichloromethane-methanol 97:3 as the eluent) to give 62% (0.065\u00a0g) of phosphonate 1a and 86% (0.12\u00a0g) of phosphine oxide 1b, respectively.To 0.099\u00a0mmol of the catalyst (CuSO4: 0.016\u00a0g, Cu(OAc)2\u00a0\u00b7\u00a0H2O: 0.018\u00a0g) in 1\u00a0mL of ethanol were added 0.49\u00a0mmol (0.055\u00a0mL) of iodobenzene, 0.49\u00a0mmol (0.10\u00a0g) of diphenylphosphine oxide and 0.99\u00a0mmol (0.14\u00a0mL) of triethylamine. Then, the mixture was irradiated in a closed vial in the microwave reactor at 165\u00a0\u00b0C for 3\u00a0h. The reaction mixture was diluted with 3\u00a0mL of EtOH, filtrated, and the residue obtained after evaporation was passed through a thin (2\u20133\u00a0cm) layer of silica gel using dichloromethane-methanol 97:3 as the eluent. The crude mixture was analyzed by 31P NMR spectroscopy, then it was purified by column chromatography (silica gel and dichloromethane-methanol 97:3 as the eluent) to furnish phosphine oxide 1b in a yield of 85% (0.11\u00a0g) and 75% (0.10\u00a0g), respectively.To 0.087\u00a0mmol of the catalyst (CuSO4: 0.0138\u00a0g, Cu(OAc)2\u00a0\u00b7\u00a0H2O: 0.016\u00a0g) in 1\u00a0mL of ethanol were added 0.048\u00a0mL (0.43\u00a0mmol) of iodobenzene, 0.43\u00a0mmol of diarylphosphine oxide [bis(4-methylphenyl)phosphine oxide: 0.10\u00a0g or bis(3,5-dimethylphenyl)phosphine oxide: 0.11\u00a0g] and 0.87\u00a0mmol (0.12\u00a0mL) of triethylamine. Then, the mixture was irradiated in a closed vial in the microwave reactor at 165\u00a0\u00b0C for 3\u00a0h. The reaction mixtures were diluted with 3\u00a0mL of EtOH, filtrated, and the residue obtained after evaporation was passed through a thin (2\u20133\u00a0cm) layer of silica gel using dichloromethane-methanol 97:3 as the eluent. The crude product was analyzed by 31P NMR spectroscopy, then it was purified by column chromatography (silica gel, and dichloromethane-methanol 97:3 as the eluent). For the results see Table\u00a04/entries 2, 4, 5 and 6.Appearance: colorless oil; 31P NMR (CDCl3, 121.5\u00a0MHz) \u03b4 18.8, \u03b4P\n[12] (CDCl3, 121.5\u00a0MHz) 19.7, \u03b4P\n[24] (CDCl3, 121\u00a0MHz) 19.4; 13C NMR (CDCl3, 75.5\u00a0MHz) \u03b4 16.2 (d, J\u00a0=\u00a06.5\u00a0Hz, CH3), 62.0 (d, J\u00a0=\u00a05.4\u00a0Hz, CH2), 128.3 (d, J\u00a0=\u00a0187.9\u00a0Hz, C1), 128.4 (d, J\u00a0=\u00a015.0\u00a0Hz, C2)a, 131.7 (d, J\u00a0=\u00a09.9\u00a0Hz, C3)a, 132.3 (d, J\u00a0=\u00a03.0\u00a0Hz, C4), amay be reversed, \u03b4C\n[35] (CDCl3, 75.5\u00a0MHz) 16.3 (d, J\u00a0=\u00a06.5\u00a0Hz), 62.0 (d, J\u00a0=\u00a05.3\u00a0Hz), 128.3 (d, J\u00a0=\u00a0187.8\u00a0Hz), 128.4 (d, J\u00a0=\u00a014.9\u00a0Hz), 131.7 (d, J\u00a0=\u00a09.8\u00a0Hz), 132.3 (d, J\u00a0=\u00a03.1\u00a0Hz); 1H NMR (CDCl3, 300\u00a0MHz) \u03b4 1.29 (t, 6H, J\u00a0=\u00a07.1\u00a0Hz, CH3) 3.97\u20134.21 (m, 4H, OCH2), 7.37\u20137.56 (m, 3H, ArH), 7.71\u20137.84 (m, 2H, ArH), \u03b4H\n[35] (CDCl3, 300\u00a0MHz) \u03b4 1.40 (t, 6H, J\u00a0=\u00a07.1\u00a0Hz), 4.10 (m, 4H), 7.42\u20137.48 (m, 2H), 7.54 (m, 1H), 7.77\u20137.84 (m, 2H), \u03b4H\n[24] (CDCl3, 300\u00a0MHz) \u03b4 1.32 (t, 6H, J\u00a0=\u00a06.87\u00a0Hz), 4.10\u20134.14 (m, 4H), 7.42\u20137.48 (m, 3H), 7.81\u20137.85 (q, 2H); [M\u00a0+\u00a0H]+\u00a0=\u00a0215.0838 C10H16O3P requires 215.0837.Appearance: white crystals, mp 156\u2013157 \u00b0C, mp [36] 156.6\u2013157.4\u00a0\u00b0C; 31P NMR (CDCl3, 121.5\u00a0MHz) \u03b4 29.1, \u03b4P\n[36] (CDCl3, 162\u00a0MHz) 29.5, \u03b4P\n[12] (CDCl3, 121.5\u00a0MHz) 30.3; 13C NMR (CDCl3, 75.5\u00a0MHz) \u03b4 128.6 (d, J\u00a0=\u00a012.1\u00a0Hz, C2)a, 132.0 (d, J\u00a0=\u00a02.8\u00a0Hz, C4), 132.2 (d, J\u00a0=\u00a09.9\u00a0Hz, C3)a, 132.7 (d, J\u00a0=\u00a0103.8\u00a0Hz, C1), amay be reversed, \u03b4C\n[36] (CDCl3, 100\u00a0MHz) 128.4 (d, J\u00a0=\u00a012.1\u00a0Hz), 131.9 (d, J\u00a0=\u00a02.2\u00a0Hz), 132.5 (d, J\u00a0=\u00a09.9\u00a0Hz), 132.8 (d, J\u00a0=\u00a0104.6\u00a0Hz); 1H NMR (CDCl3, 300\u00a0MHz) \u03b4 7.38\u20137.48 (m, 6H, ArH), 7.48\u20137.56 (m, 3H, ArH), 7.59\u20137.72 (m, 6H, ArH), \u03b4H\n[36] (CDCl3, 400\u00a0MHz) \u03b4 7.43\u20137.48 (m, 6H), 7.52\u20137.56 (m, 3H), 7.64\u20137.70 (m, 6H); [M+H]+\u00a0=\u00a0279.0934 C18H16OP requires 279.0939.Appearance: white crystals, 31P NMR (CDCl3, 121.5\u00a0MHz) \u03b4 27.8, \u03b4P\n[12] (CDCl3, 162\u00a0MHz) 29.4, \u03b4P\n[37] (CDCl3, 162\u00a0MHz) 30.5; 13C NMR (CDCl3, 75.5\u00a0MHz) \u03b4 21.6 (CH3), 128.5 (d, J\u00a0=\u00a012.1\u00a0Hz, C2\u2032)a, 129.3 (d, J\u00a0=\u00a012.5\u00a0Hz, C2)b, 129.4 (d, J\u00a0=\u00a0106.6\u00a0Hz, C1), 131.8 (d, J\u00a0=\u00a02.7\u00a0Hz, C4\u2032), 132.1 (d, J\u00a0=\u00a09.8\u00a0Hz, C3\u2032)a, 132.1 (d, J\u00a0=\u00a010.3\u00a0Hz, C3)b, 133.1 (d, J\u00a0=\u00a0104.1\u00a0Hz, C1\u2032), 142.4 (d, J\u00a0=\u00a02.8\u00a0Hz, C4\u2032), a,bmay be reversed, \u03b4C\n[37] (CDCl3, 100\u00a0MHz) 21.7, 128.6 (d, J\u00a0=\u00a011.8\u00a0Hz), 129.4 (d, J\u00a0=\u00a012.6\u00a0Hz), 129.4 (d, J\u00a0=\u00a0106.9\u00a0Hz), 131.9 (d, J\u00a0=\u00a03.2\u00a0Hz), 132.0 (d, J\u00a0=\u00a08.7\u00a0Hz), 132.2 (d, J\u00a0=\u00a010.2\u00a0Hz), 133.0 (d, J\u00a0=\u00a0102.5\u00a0Hz), 142.6 (d, J\u00a0=\u00a02.9\u00a0Hz); 1H NMR (CDCl3, 300\u00a0MHz): \u03b4 2.39 (s, 6H, CH3), 7.18\u20137.32 (m, 4H, ArH), 7.37\u20137.47 (m, 2H, ArH), 7.47\u20137.61 (m, 5H, ArH), 7.61\u20137.73 (m, 2H, ArH); \u03b4H\n[37] (CDCl3, 400\u00a0MHz) 2.38 (s, 6H), 7.24 (dd, J\u00a0=\u00a08.4\u00a0Hz, 2.4\u00a0Hz, 4H), 7.48 (m, 1H), 7.53 (dd, J\u00a0=\u00a011.8\u00a0Hz, 8.0\u00a0Hz, 4H), 7.62\u20137.68 (m, 2H);, [M+H]+\u00a0=\u00a0307.1252 C20H19OP requires 307.1252.Appearance: white crystals, 31P NMR (CDCl3, 121.5\u00a0MHz) \u03b4 29.6, \u03b4P\n[37] (CDCl3, 162\u00a0MHz) 30.9, 13C NMR (CDCl3, 300\u00a0MHz) \u03b4 21.4 (CH3), 128.4 (d, J\u00a0=\u00a012.0\u00a0Hz, C2\u2032)a, 129.7 (d, J\u00a0=\u00a09.8\u00a0Hz, C2), 131.7 (C4\u2032), 132.1 (d, J\u00a0=\u00a09.9\u00a0Hz, C3\u2032)a, 132.4 (d, J\u00a0=\u00a0105.3\u00a0Hz, C1), 133.1 (d, J\u00a0=\u00a0103.1\u00a0Hz, C1\u2032), 133.7 (d, J\u00a0=\u00a02.8\u00a0Hz, C4), 138.1 (d, J\u00a0=\u00a012.7\u00a0Hz, C3), amay be reversed, \u03b4C\n[37] (CDCl3, 100\u00a0MHz) 21.56, 128.6 (d, J\u00a0=\u00a011.7\u00a0Hz), 129.8 (d, J\u00a0=\u00a010.0\u00a0Hz), 131.9 (d, J\u00a0=\u00a02.2\u00a0Hz), 132.3 (d, J\u00a0=\u00a09.7\u00a0Hz), 132.4 (d, J\u00a0=\u00a0102.6\u00a0Hz), 133.1 (d, J\u00a0=\u00a0102.7\u00a0Hz), 133.9 (d, J\u00a0=\u00a02.3\u00a0Hz), 138.3 (d, J\u00a0=\u00a012.2\u00a0Hz); 1H NMR (CDCl3, 300\u00a0MHz) 2.31 (s, 12H, CH3), 7.15 (s, 2H, ArH), 7.28 (d, J\u00a0=\u00a012.2\u00a0Hz, 4H, ArH), 7.39\u20137.55 (m, 3H, ArH), 7.62\u20137.73 (m, 2H, ArH), \u03b4H\n[37] (CDCl3, 400\u00a0MHz) 2.31 (s, 12H), 7.15 (s, 2H), 7.26 (d, J\u00a0=\u00a012.4\u00a0Hz, 4H), 7.42\u20137.47 (m, 2H), 7.51\u20137.55 (m, 1H), 7.63\u20137.68 (m, 2H); [M+H]+\u00a0=\u00a0335.1566 C22H23OP requires 335.1565.All computations were carried out with the Gaussian16 program package (G16) [38], using standard convergence criteria for the gradients of the root mean square (RMS) Force, Maximum Force, RMS displacement and maximum displacement vectors (3.0\u00a0\u00d7\u00a010\u20134, 4.5\u00a0\u00d7\u00a010\u20134, 1.2\u00a0\u00d7\u00a010\u20133 and 1.8\u00a0\u00d7\u00a010\u20133). Computations were carried out at M06\u20132X level of theory [39]. The basis set of 6\u201331G(d,p) was applied for C, H, O, P, N, Cl, Br, Cu and SDD/MWB46 for iodine [40]. The vibrational frequencies were computed at the same levels of theory, in order to confirm properly all structures as residing at minima on their potential energy hypersurfaces (PESs). Thermodynamic functions U, H, G and S were computed at 398.15\u00a0K. Beside the vacuum calculations, the IEFPCM method was also applied to model the solvent effect, by using the default settings of G16, setting the \u03b5\u00a0=\u00a024.852 [41]. See the Supporting Information for details.While the Pd- and Ni-catalyzed Hirao reactions have been studied in detail, Cu-catalysis is a somewhat neglected field. After a short summary of the Pd- and Ni-promoted P\u2013C couplings, the Cu(II)-salt (CuSO4 and Cu(OAc)2\u00a0\u00b7\u00a0H2O) catalyzed reaction of iodobenzene with diethyl phosphite and secondary phosphine oxides (diarylphosphine oxides) was investigated under microwave irradiation. The P\u2013C couplings were the most efficient, when the >P(O)H reagent and triethylamine used as a base were applied in a 1:2 molar ratio suggesting that the amine is also involved in the intermediate complexes as a ligand. A detailed study on the mechanism revealed that, as a matter of fact, the in situ formed Cu(I) species was the primary species in the oxidative addition step towards the P\u2013C couplings.The whole research was supervised and directed by GK. The manuscript was written by GK, ZM and BH. The synthetic work was performed by BH and RSz. The theoretical calculations were carried out by ZM. The fund was collected by GK. All authors have approved the final version of the manuscript.NMR spetra of the products prepared and details of the quantum chemical calculations.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This project was supported by the National Research, Development and Innovation Office (K134318) (G.K.) and Bolyai Research Scholarship (BO/799/21/7) (Z.M).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jorganchem.2022.122526.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  After surveying the most important results described on the Pd(0)/Pd(II)- and Ni(II)-catalyzed P\u2013C couplings of bromobenzene and diethyl phosphite or diphenylphosphine oxide, the simple Hirao reactions catalyzed by different Cu(II) salts were investigated. In these instances, the more reactive iodobenzene had to be used. CuSO4 and Cu(OAc)2\u00a0\u00b7\u00a0H2O were found suitable catalyst precursors in case they were used together with two equiv. of triethylamine suggesting the role of the amine in the complexation. Theoretical calculations suggested that the corresponding Cu(I) species formed from the initial Cu(II) salt is involved in the catalytic cycle.\n               "}
{"full_text": "Coal tar, as a main by-product in the process of coal pyrolysis, contains a large number of aromatic hydrocarbons (mainly bicyclic and tricyclic aromatic hydrocarbons) (Ardakani and Smith, 2011; Hodoshima et\u00a0al., 2003; Liang et\u00a0al., 2009; Zhao et\u00a0al., 2010). The aromatics in low temperature coal tar can be converted into hydrogenated aromatics or cycloalkanes through catalytic hydrogenation, which can be used as the ideal component of jet fuel (Kim et\u00a0al., 2017; Martin et\u00a0al., 2020). Therefore, how to obtain high performance aromatic hydrogenation catalyst is the key to produce environmental-friendly fuels.At present, transition metal sulfide catalysts are the main commercial catalysts for aromatic hydrogenation. Although they possess certain anti-poisoning ability, its hydrogenation activity is relatively low (Hart et\u00a0al., 2020; Jing et\u00a0al., 2020; Ohta et\u00a0al., 1999). Noble metal catalysts show excellent hydrogenation saturation ability, but the high cost and easily poisoned by heteroatomic compounds containing S and N have restricted their industrial application (Tao et\u00a0al., 2013; Zeng et\u00a0al., 2018). In recently researches, transition metal carbides, transition metal nitrides and transition metal phosphides show great potential in the field of aromatic hydrogenation (Dongil, 2019; Prats et\u00a0al., 2019; Zhang et\u00a0al., 2019). More importantly, transition metal phosphide catalysts not only possess high hydrogenation activity, but also exhibit anti-toxicity ability (Usman et\u00a0al., 2015). Among all phosphide catalysts, Ni2P catalysts show high intrinsic hydrogenation activity due to its unique crystal morphology and electronic structure, which is expected to become a new generation of efficient aromatic hydrogenation catalyst (Oyama, 2003; Oyama and Lee, 2008; Zhao et\u00a0al., 2020). However, the unsupported Ni2P catalysts have the problems of small specific surface area, poor dispersion of active phase, low mechanical strength and poor heat dissipation, which are unfavorable to its hydrogenation activity. Therefore, researchers usually load the Ni2P active components on various supports so as to expose more active sites and enhance hydrogenating activity.As to the supported Ni2P catalysts, the properties of different supports have an important effect on the dispersion of Ni2P particles (Oyama and Lee, 2008). In recent years, compared with the traditional mesoporous silica nanoparticles, wrinkle silica nanospheres (WSNs) has attracted rapid attention due to its unique three-dimensional central-radial pore channels (Polshettiwar et\u00a0al., 2011; Thankamony et\u00a0al., 2015). Up to now, WSNs has a wide range of applications, such as catalysis, biotherapy delivery, water treatment, dye-sensitized solar cells, supercapacitors, fluorescent probes, titanium dioxide capture, and biological imaging, photonics, composite materials, etc. Singh and Polshettiwar found that WSNs materials showed better textural stability, thermal stability and CO2 capture capacity than the traditional mesoporous MCM-41 due to its unique fibrous morphology (Singh and Polshettiwar, 2016).In order to improve the activity of the catalyst, scientific researchers focus on the introduction of organic complexing agents in the preparation of hydrogenation catalysts. Thomson R firstly introduced nitrilotriacetic acid (NTA) into NiMo/SiO2 catalyst during impregnation process (Thompson, 1986). The hydrogenation denitrification (HDN) activity of NTA modified NiMo/SiO2 catalyst was found to be 6 times higher than that of the traditional NiMo/SiO2 catalyst without complexing agents. After that, many complexing agents, such as citric acid (CA), ethylene glycol (EG), nitrilotriacetic acid (NTA), ethylenediamine tetraacetic acid (EDTA), cyclohexylenediamine tetraacetic acid (CYDTA) and ethylenediamine (EN), were widely applied to the preparation of hydrogenation catalysts (Ding et\u00a0al., 2017; Garcia-Ortiz et\u00a0al., 2020; Jiang et\u00a0al., 2020; Li et\u00a0al., 2021; Santolalla-Vargas et\u00a0al., 2020). Furthermore, Oyama and his groups found that small sizes of Ni2P particles could expose more Ni(2) sites, thus enhancing the hydrogenation activity (Zhao et\u00a0al., 2015; Shu et\u00a0al., 2005). However, researches on the synthesis of small Ni2P particles over the supported catalysts, especially less than 5\u00a0nm, are still limited. Therefore, the goal of this research is to synthesize small sizes of Ni2P particles supported on the wrinkle silica nanoparticles.In this study, the series of Ni2P/WSNs catalysts were prepared by introducing chelators EDTA or NTA during the impregnation process. And the corresponding effects of chelators NTA and EDTA to Ni2P/WSNs catalysts were also studied. The reaction of naphthalene hydrogenation was used to evaluate the catalytic activity and stability of chelators modified Ni2P catalysts.The WSNs material was synthesized as mentioned in the previous papers (Hu et\u00a0al., 2019). 10 g CTAB and 6\u00a0g urea were dissolved in 300\u00a0mL distilled water and 300\u00a0mL cyclohexane. After complete dissolution, 40\u00a0mL TEOS was added dropwise. Then the mixture was continuously stirred for 24\u00a0h under 70\u00a0\u00b0C. The WSNs products were obtained by centrifugation, desiccation and calcination at 550\u00a0\u00b0C for 6\u00a0h.The supported Ni2P/WSNs catalysts were prepared by impregnation process and temperature-programmed reduction (TPR). The oxidic Ni2P/WSNs catalyst precursors were obtained via a two-step incipient wetness method impregnated with aqueous solutions of nickel nitrate (Ni(NO3)2\u00b76H2O) and ammonium hypophosphite (NH4H2PO4). After each impregnation, the catalyst was dried at 100\u00a0\u00b0C for 12\u00a0h and then calcined at 550\u00a0\u00b0C kept for another 3\u00a0h. The precursors were reduced at continuous H2 flow rate of 150\u00a0mL\u00a0min\u22121 and temperature of 440\u00a0\u00b0C kept for 1\u00a0h, then rising to 550\u00a0\u00b0C.The EDTA modified Ni2PE/WSNs catalysts were prepared similar to the above process. Firstly, WSNs sample was impregnated with the aqueous mixed solutions of nickel nitrate (Ni(NO3)2\u00b76H2O) and EDTA. After dried at 100\u00a0\u00b0C for 12\u00a0h and then calcined at 550\u00a0\u00b0C for 3\u00a0h, the oxidic catalyst precursors were obtained by impregnating with ammonium hypophosphite (NH4H2PO4). The molar ratios of Ni:P:EDTA were 1:1:0.5, 1:1:1 and 1:1:1.5, then the corresponding catalysts were named as Ni2PE(0.5)/WSNs, Ni2PE(1.0)/WSNs, and Ni2PE(1.5)/WSNs respectively. Finally, the Ni2PE/WSNs catalysts were collected by the same TPR progress.The NTA modified Ni2PN/WSNs catalysts were prepared similar to the above Ni2PE/WSNs process. The oxidic Ni2PN/WSNs catalyst precursors were obtained by a two-step incipient wetness method with the mixed solution of nickel nitrate (Ni(NO3)2\u00b76H2O) and NTA, and ammonium hypophosphite (NH4H2PO4). After each impregnation, the catalyst was dried at 100\u00a0\u00b0C for 12\u00a0h and then calcined at 550\u00a0\u00b0C kept for another 3\u00a0h. The synthesized catalysts were named as Ni2PN(0.5)/WSNs, Ni2PN(1.0)/WSNs, and Ni2PN(1.5)/WSNs with different molar ratios of Ni:P:NTA (1:1:0.5, 1:1:1, 1:1:2). Finally, the Ni2PN/WSNs catalysts were collected by the same TPR progress.Wide-angle X-ray diffraction patterns were collected on a Japan Shimadzu X-6000 system (Cu Ka radiation, 40\u00a0kV, 30\u00a0mA, \u03bb\u00a0=\u00a00.1540598\u00a0nm). X-ray photoelectron spectroscopy (XPS) analysis was conducted using a PerkinElmer PHI-1600 ESCA spectrometer. N2 adsorption-desorption experiments were performed at 77\u00a0K after degassing samples in flowing N2 at 350\u00a0\u00b0C for 4\u00a0h (using a Q Micromeritics Tristar 3020). The morphological features of the samples were characterized by scanning electron microscopy (SEM, Hitachi SU-8010, 5.0\u00a0kV). Transmission electron microscopy (TEM, Philips Tecnai G2 F20\u00a0S-TWIN, 300\u00a0kV). The size distributions of Ni2P phases were counted by more than 300 Ni2P particles from different images through high-resolution transmission electron microscopy. The average Ni2P particles were calculated according to Eq. (1).\n\n(1)\n\n\n\nD\naver\n\n=\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\nd\ni\n\n\n\nn\ni\n\n\n\n\n\nwhere d\n\ni\n represents the size of each Ni2P particle and n\n\ni\n is the total number of Ni2P particles.The naphthalene hydrogenation was carried out with a fixed bed reactor. In general, 1.0\u00a0g of Ni2P supported catalysts were loaded in the middle of a stainless steel tube reactor with both ends filled with mesh quartz sands. Before the reaction, the synthesized oxidation precursor was reduced to the active nickel phosphide in H2 (4\u00a0MPa) at a flow rate of 150\u00a0mL\u00a0min\u22121 by heating from room temperature at a heating rate of 5\u00a0\u00b0C min\u22121 to a temperature range as maintaining at 120\u00a0\u00b0C for 1\u00a0h, then rising up to 440\u00a0\u00b0C for 1\u00a0h and 550\u00a0\u00b0C for another 3\u00a0h, finally followed by cooling to room temperature in the continued H2 flow. The 5% naphthalene in cyclohexane was used as the model compound to access aromatic hydrogenation activity. The system was pressurized under the condition of 4\u00a0MPa, H2/Oil volumetric ratio of 500 (v/v), liquid hourly space velocity (WHSV) of 10\u00a0h\u22121. The catalysts were evaluated at 300\u2013380\u00a0\u00b0C with an interval of 20\u00a0\u00b0C. The outlet stream was analyzed using gas chromatography-mass spectrometry (GC-MS).The conversion of naphthalene can be expressed by Eq. (2):\n\n(2)\n\n\nNaphthalene\n\n(\n%\n)\n\n=\n\n\n\nN\nf\n\n\u2212\n\nN\nP\n\n\n\nN\nP\n\n\n\n\n\nwhere the N\nf is the mass fraction of naphthalene in the feedstock and N\np is the mass fraction of naphthalene in the products.The peaks at 40.8o, 44.6o, 47.3o, 54.2o and 72.7o are tracked in Fig.\u00a01\n, which are attributed to the characteristic responses of Ni2P (PDF 3\u2013953) (Tang et\u00a0al., 2017). It is found that the peak intensities of Ni2P particles become gradually weak with the addition of chelating agents, which maybe be ascribed that Ni2P particles on the catalyst surface are too small to be detected by XRD characterization (Pullan et\u00a0al., 2016; Yang et\u00a0al., 2006).The XRD patterns of the spent catalysts are also characterized to test their stability. As shown in Fig.\u00a01b, there is almost no change of the Ni2P characteristic peaks and no other unrelated peaks are detected in XRD, which confirms the high stability of the series Ni2P/WSNs catalysts after reaction.The N2 adsorption-desorption patterns of the synthesized catalysts after the addition of chelators are shown in Fig.\u00a02\na. It can be seen that all catalysts showed type IV isotherms, indicating the good mesoporous structure. Fig.\u00a02b shows the pore size distribution of the modified Ni2P/WSNs catalysts, and the results are summarized in Table\u00a01\n. The surface area and average pore size of the catalysts modified with chelators EDTA and NTA increase to some extent compared to the unmodified catalysts, which indicates that chelators can effectively inhibit the aggregation of Ni2P active phases.It can be seen in TEM images, all the catalysts still retain their original morphologies, which demonstrate that the addition of chelators has little damage to the morphology of the WSNs support. In addition, the morphologies of the spent catalysts after reaction are nearly the same as the fresh catalysts, as displayed in Fig.\u00a03 and Fig.\u00a04\n\n, further confirming the high stabilities of the chelators modified Ni2P/WSNs catalysts.The average particle sizes of Ni2P can be statistically calculated from high resolution TEM images (Figs.\u00a0S1 and S2), as summarized in Table\u00a02\n. It can be found that the average Ni2P particle sizes of Ni2PE/WSNs and Ni2PN/WSNs catalysts are both smaller than those of unmodified Ni2P/WSNs catalysts. Moreover, Ni2PE/WSNs catalysts modified by EDTA possess smaller sizes of Ni2P particles compared with Ni2PN/WSNs catalysts with the addition of NTA, confirming that the chelator EDTA plays a greater role in preventing Ni2P aggregation than the chelator NTA.It can be seen from Fig.\u00a05\n that H2-TPR patterns show several H2 consumption peaks in the vicinity of 400\u00a0\u00b0C and 700\u00a0\u00b0C. The H2 consumption peak near 400\u00a0\u00b0C is attributed to the reduction of NiO species (Louis et\u00a0al., 1993), while the H2 consumption peak near 700\u00a0\u00b0C is derived from the P species due to the high P\u2014O bond energy (Zuzaniuk and Prins, 2003).The reduction peak of Ni species is shifted to the higher temperature for the samples obtained with the addition of chelators, indicating that the interaction between Ni and support become stronger as the chelator EDTA or NTA being added. Meanwhile, the reduction temperature of P species over EDTA modified catalysts is shifted to slightly lower temperature, leading to the small reduction-peak interval of Ni and P species, thus promoting the formation of Ni2P active phase.XPS was used to investigate the differences in the chemical states of Ni and P on the surface of Ni2P supported catalysts. As can be seen from Fig.\u00a0S3 and Table\u00a03\n, there are two XPS peaks of Ni 2p at around 853.1\u00a0eV and 856.9 eV, which are caused by the Ni\u03b4+ (0\u00a0<\u00a0\u03b4\u00a0<\u00a01) species in Ni2P and Ni2+ interacting with PO4\n3\u2212, respectively.Three P 2p XPS peaks appeared at the binding energy of about 129.5\u00a0eV, 133.5\u00a0eV and 134.5\u00a0eV (Fig.\u00a0S4), which are assigned to P\u03b4\u2212 (0\u00a0<\u00a0\u03b4\u00a0<\u00a01) in Ni2P, P3+ in H2PO3\n\u2212 and P5+ in PO4\n3\u2212, respectively. It can be found from Table\u00a02, the supported Ni2P catalysts modified with the chelators of ETDA and NTA exhibit much higher proportions of Ni2P than the traditional Ni2P/WSNs catalysts without chelator addition.Naphthalene is chosen as the model compound to investigate the aromatic hydrogenation activity of synthesized catalysts, and the reaction pathway of naphthalene hydrogenation is displayed in Fig.\u00a0S5. As shown in Fig.\u00a06\n, the hydrogenation conversion of the EDTA and NTA modified catalysts are higher than that of supported Ni2P catalysts without chelators. The decalin selectivity graph is shown in Fig.\u00a07\n. In the experimental temperature range, EDTA-modified catalysts show high hydrogenation ability, among which Ni2PE(1.5)/WSNs exhibits the highest decalin selectivity (almost reaching 100%). In the case of the NTA modified catalysts, high decalin selectivity of naphthalene is shown in temperature of 300\u00a0\u00b0C, but then decreases a lot when the reaction temperature enhances.The Ni2P supported series catalysts are successfully prepared on wrinkle silica nanoparticles (WSNs) support through temperature programmed reduction, which can be confirmed by the Ni2P characteristic peaks as shown on the XRD pattern. More importantly, the peaks of Ni2P become broader and weaker as the addition of chelators, indicating that the corresponding sizes of Ni2P also become smaller according to the Scherrer formula (Song et\u00a0al., 2018). These results are in agreement with the TEM results, as shown in Table\u00a01. The average sizes of Ni2P particles decrease as the increasing addition of chelators. Among all the catalysts, Ni2PE(1.5)/WSNs catalyst possesses the smallest Ni2P average particle size of only 2.6\u00a0nm. The Ni2P particle size obtained by the addition of EDTA and NTA in this work is smaller than the published works, realizing the formation of utral-small Ni2P particles (Table\u00a0S1). Previous reports (Rui and Smith, 2010; Zhang et\u00a0al., 2017) have been reported that the chelators can form a metal complex with the active metals, so as to restrict the aggregation of metal specials on the supports. Therefore, the chelators have a positive effect on the formation of smaller Ni2P particles on the supported catalysts.The addition of chelator can contribute to the formation of Ni2P active phase. As seen in XPS results, the Ni2P proportion of the supported Ni2P catalysts with the addition of EDTA and NTA is higher than that of Ni2P/WSNs catalysts without chelators. More importantly, when the content of chelators is enhanced, the corresponding percentage of Ni2P is also increased. Therefore, it can be speculated that EDTA and NTA chelators are beneficial to the reduction of Ni2P, which can effectively avoid the generation of other impurity phase. Moreover, H2-TPR results showed that the reduction-peak interval between Ni and P species becomes smaller as the addition of chelators, which further confirms the positive effect on the formation of Ni2P active phase. During the H2-TPR process, nickel species were first reduced and then H2 dissociated from metallic Ni sites to form hydrogen spillover effects, which contributes to the reduction of the P species (Chen et\u00a0al., 2010; Yang et\u00a0al., 2013). The high dispersion of nickel species influenced by the chelator EDTA and NTA could have a chain effect on the later reduction of the P\u2014O bond. Therefore, the better Ni dispersion of supported Ni2P catalysts resulted from chelator additions are beneficial to the formation of nickel phosphide phases.On close observation of the structure of the chelating agent, it can be seen from Fig.\u00a08\n that there are four carboxyl groups and two nitrogen atoms in the case of EDTA, while there are only three carboxyl groups and one nitrogen atom with regard to NTA (Rufus et\u00a0al., 2004). It is well known that Ni ion has a coordination number of six. When the Ni ion is bound to the chelator EDTA, all six ligand groups of the EDTA fill all the available coordination sites of Ni ion (Wang et\u00a0al., 2002). However, four of the six coordination sites of Ni ion are occupied by the chelator NTA, leaving two free binding sites on the Ni ion (Lauer and Nolan, 2002). Therefore, Ni-EDTA complex is more stable compared with Ni-NTA complex since all coordination sites are involved in the formation of Ni-EDTA complex. In this case, the addition of EDTA can significantly reduce the interaction between Ni2P and the support, thus forming relatively small Ni2P particles with good dispersion.The result of naphthalene hydrogenation can also reflect the different stabilities of the chelators EDTA and NTA, as shown in Fig.\u00a07. The NTA modified catalysts exhibit high decalin selectivity under low temperature of 300\u00a0\u00b0C, which may be ascribed to the two free binding sites available on the Ni metal ion. However, the hydrogenation conversion of NTA modified catalyst decreases dramatically when the reaction temperature raises higher, while EDTA tailored catalysts still remain high stability under high temperature owing to all the coordination sites involved in the formation of complexation. The long-period (100\u00a0h) naphthalene hydrogenation experiments (Fig.\u00a0S6) and XPS results of the spent Ni2PE(1.5)/WSNs catalyst (Fig.\u00a0S7 and Table\u00a0S2) also shows that Ni2PE(1.5)/WSNs catalyst possesses outstanding catalytic stabilities.A facile strategy was developed for the preparation of the supported Ni2P catalysts with small Ni2P particles through the addition of chelators in this research. The reaction evaluation of naphthalene hydrogenation has shown that Ni2P supported catalysts with the addition of EDTA and NTA chelators display higher catalytic activity than the traditional catalysts, among which Ni2PE(1.5)/WSNs catalyst with the smallest Ni2P particles nearly reaches 100% naphthalene conversion. Moreover, EDTA modified catalysts display high catalytic stability under high temperature due to the chelator EDTA completely bound to all coordination sites of Ni ions. On the contrary, NTA modified catalysts show high decalin selectivity on the temperature of 300\u00a0\u00b0C, but decrease a lot when the reaction temperature becomes higher due to the two free binding sites on the Ni ion.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research is financially supported by the National Natural Science Foundation of China (No. 21878330), Key Research and Development Program of Ministry of Science and Technology of China (No. 2019YFC1907602) and Scientific Research and Technology Development Program of China National Petroleum Corporation (2020B-2116).The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.petsci.2022.11.017.", "descript": "\n                  Ni2P supported catalysts exhibit high catalytic activities in hydrogenation reaction, of which the particle sizes of Ni2P active phases are the key influential factor. This research focus on the effect of chelators on the size of Ni2P particles over wrinkle silica nanoparticles (WSNs) by introducing chelating agents EDTA and NTA during impregnation process. The characterization results show that chelators modified catalysts possess smaller size of Ni2P particles than the unmodified Ni2P catalysts. Among all the synthesized catalysts, the EDTA modified Ni2PE(1.5)/WSNs catalyst possesses smallest average particle size of Ni2P, only 2.6\u00a0nm. Moreover, the Ni2P catalysts with the assistance of EDTA exhibits better catalytic activity than that of NTA under high reaction temperature, which can be ascribed to the strong bonding between EDTA and Ni. And the EDTA modified Ni2PE(1.5)/WSNs catalyst shows highest hydrogenation ability, almost reaching 100% decalin selectivity.\n               "}
{"full_text": "Data will be made available on request.Carbon nanofibers (CNFs) have attracted considerable attention in a wide variety of applications, including biosensing [1], energy storage [2], water purification [3] and catalysis [4]. CNFs are cylindrical nanostructures that consist of stacked graphene sheets of various sizes and orientations. Their length may vary from tens to hundreds of nanometers, while their diameter may reach up to tens of micrometers. CNFs possess a large surface area which can be functionalized with a variety of chemical species. In addition, CNFs have high electrical conductivity, which makes them ideal candidates for electrochemical biosensing [5\u20137].CNFs can be grown using a variety of techniques, including electrospinning [8], chemical vapor deposition (CVD) [9] and hot filament assisted sputtering [10]. Plasma enhanced CVD (PECVD) is a widely used method for growing vertically aligned CNFs (VACNFs) at relatively low temperatures. A PECVD growth process for CNFs typically involves a carbonaceous gas (e.g. C2H2, C2H4) and an etchant gas (e.g. NH3 or H2), which are activated in a glow discharge. The growth of CNFs occurs through the nucleation of a nanoscale metal catalyst layer (e.g. Ni, Fe, Pd or Pt) [11]. An adhesive layer (e.g. Ti, Cr, W) is often deposited between the substrate and the catalyst layer, in order to prevent the intermixing of the catalyst and substrate materials.The microstructure and macroscale morphology of CNFs depend on the process parameters, e.g. temperature, time, gas flow, reactor pressure and plasma power. Moreover, the composition and thickness of adhesive and catalyst layers also affect these properties. These effects have been reported in various studies. For example, Melechko et al. demonstrated that CNF growth mode changes from tip-type to base-type by changing the flow ratio of carbonaceous and etchant gases in the PECVD process [12]. R\u00f6thlisberger et al. documented the effects of Ni layer thickness on bidirectional growth of CNFs (i.e. both tip-type and base-type simultaneously) [13]. Merkulov et al. reported the growth of individual VACNFs by controlling the size of catalyst layer dots, using Ni as the catalyst layer and Ti as the adhesive layer [14].An aspect of the reaction that is often overlooked is the interaction between the substrate, catalyst and adhesive layers. To the best of our knowledge, this phenomenon has not been systematically investigated. Furthermore, many studies on electrochemical biosensing of electrodes do not address the effects of well controlled macroscale geometrical parameters \u2013 length, diameter, population density, etc. \u2013 on the performance of the electrode.In this paper, we report PECVD growth of CNFs on two types of substrates \u2013 80\u00a0nm Cr\u00a0+\u00a020\u00a0nm Ni and 20\u00a0nm Ti\u00a0+\u00a020\u00a0nm Ni (hereafter referred to as Cr-Ni and Ti-Ni, respectively). For each substrate type, we prepared four sets of CNF samples by varying the growth time (1, 5, 10 and 30\u00a0min), while the other growth parameters were kept constant. We studied the differences in macroscale geometry between the CNFs prepared under these conditions using scanning electron microscopy (SEM). We used phase diagrams and thermodynamics combined with detailed transmission electron microscopy (TEM) study to rationalize the differences between CNFs grown on the two types of substrates. Finally, we investigated basic electrochemical properties of each type of CNF, and rationalized the effects of macroscale morphology on pseudocapacitance and electrochemical windows of the CNFs.p-type Silicon wafers (Siegert Wafers, Germany) were used as substrates for all the samples. First, the wafers were coated with adhesive and catalyst metal layers \u2013 20\u00a0nm Ti followed by 20\u00a0nm Ni for Ti-Ni-CNFs, and 80\u00a0nm Cr followed by 20\u00a0nm Ni for Cr-Ni-CNFs. An electron beam evaporator (MASA IM-9912) was used for depositing metal layers. The chamber pressure was approximately 2\u00a0\u00d7\u00a010\u22127\u00a0mbar during evaporation. Subsequently, the wafers were cleaved into smaller pieces, measuring approximately 7\u00a0mm\u00a0\u00d7\u00a07\u00a0mm. Finally, CNF growth was carried out using a PECVD reactor (Aixtron Black Magic).The PECVD process was carried out as follows: First, the chamber was pumped down to 0.1\u00a0mbar. Then, the chamber was heated to 400\u00a0\u00b0C with a ramp speed of approximately 250\u00a0\u00b0C per minute. When the temperature reached 395\u00a0\u00b0C, the chamber was injected with a 100 sccm NH3 buffer. The ramp rate was then increased to 300\u00a0\u00b0C per minute and the chamber was heated up to 600\u00a0\u00b0C. When the temperature reached 575\u00a0\u00b0C, 230\u00a0W\u00a0DC plasma was ignited. 30 sccm C2H2 was simultaneously injected into the chamber, while the flow rate of NH3 was increased to 125 sccm. These parameters were maintained for 1, 5, 10 or 30\u00a0min, in order to prepare CNFs of four different lengths. The chamber pressure was approximately 3\u00a0mbar during the growth process.CNF morphology and geometry were studied using SEM (Zeiss Supra 40 and Zeiss Sigma VP). Length, diameter and area analyses were carried out using imageJ. We estimated the average length and diameter by measuring 20 CNFs from the cross-sectional SEM images (Fig. 1\n). Area analysis was carried out as follows: Area covered by Ni was highlighted by applying brightness/contrast and threshold settings to the top-view SEM images of 1\u00a0min grown CNF samples (Fig. S2). The percentage of white pixels, which corresponds to Ni, was then estimated using imageJ software. An area of 7000\u00a0nm\u00a0\u00d7\u00a04800\u00a0nm was used in this analysis. While a more detailed analysis would be required to obtain precise quantitative values, our analysis is sufficient to demonstrate that there is a significant difference between the two substrates.TEM imaging was performed on a Jeol JEM 2200FS TEM equipped with an X-ray energy dispersive spectrometer (EDS). Cross-sectional TEM samples were prepared by EAG Laboratories (USA) using a focused ion beam (FIB). Sputtered carbon was used as the filler material. The sample was coated locally at the cross-section site with two additional layers of carbon.Binary phase diagrams (Figs. 4, S6) were generated using FactSage Education 8.1 Package. FactSage is a fully integrated database computing system in chemical thermodynamics and consists of a variety of information, database, calculation and manipulation modules that access various pure substances and solution databases [15].Cyclic voltammetry was performed with a Gamry Reference 600 potentiostat and Gamry Framework software in a three-electrode setup with a Ag/AgCl as reference electrode and a Pt wire as the counter electrode. The solutions were purged with nitrogen gas for at least 15\u00a0min. The potential window measurements were done in 0.15\u00a0M H2SO4 and PBS pH\u00a07.4 (NaCl (137\u00a0mM), KCl (2.7\u00a0mM), Na2HPO4 (10\u00a0mM), and KH2PO4 (1.8\u00a0mM)). The outer-sphere redox (OSR) probe was 1\u00a0mM Ru(NH3)6\n2+/3+ prepared from hexaammineruthenium(III) chloride (Sigma-Aldrich) dissolved in 1\u00a0M KCl (Merck Suprapur).SEM images presented in Fig. 1 demonstrate the effect of growth duration on the length of CNFs on Cr-Ni and Ti-Ni substrates. Measured values are presented in Fig. 2\n and Table 1\n. There is a significant difference between the evolution of length in both types of substrates. Samples grown for 1\u00a0min do not contain nanofibers \u2013 instead, we observe the initial stages of CNF nucleation. There are notable differences between Cr-Ni and Ti-Ni substrates already at this stage (Fig. 3\n), which we elaborate below.At 5\u00a0min, CNFs are clearly visible in both the substrates. On Cr-Ni substrates, the average length of CNFs is approximately 221\u00a0nm, while on Ti-Ni substrates, the average length is considerably larger - 361\u00a0nm. After 10\u00a0min of growth, the corresponding values are 415\u00a0nm and 774\u00a0nm, respectively. After 30\u00a0min, however, the CNF lengths on both substrates are similar - 915\u00a0nm and 873\u00a0nm, respectively. Thus, the rate of CNF growth is lower in Cr-Ni substrates compared to Ti-Ni substrates up to 10\u00a0min. Interestingly, the length seems to saturate at <1\u00a0\u03bcm for both substrates. Moreover, CNFs grown on Cr-Ni substrates have a narrower distribution of length, i.e., they are more uniform in length. The average diameters of CNFs do not differ significantly between the two substrates. The average diameters also remain similar for different growth duration (approx. 70\u00a0nm). However, Ti-Ni-CNFs have a wider distribution of diameters.It should be noted that measuring the average dimensions of CNFs is not straightforward. The longer CNFs are curved, and their diameters are not strictly identical along the entire length. Moreover, the smaller CNFs are likely to be obstructed by larger ones in the SEM micrographs. However, we believe that the values presented in this work provide a qualitative comparison between different batches.SEM images presented in Fig. 3 demonstrate the difference between the population density of fibers grown on Cr-Ni and Ti-Ni substrates. The nucleation of Ni film occurs differently in both types of substrates. On the Cr-Ni substrate, we observe that the Ni film breaks down into particles of more uniform size. Moreover, a larger proportion of the surface is covered with Ni nanoparticles. On the other hand, on the Ti-Ni substrate, we see a less even distribution of Ni nanoparticles. Moreover, a smaller proportion of the surface is covered with Ni. In the Cr-Ni substrate, 50.04\u00a0% of the visible surface is covered with Ni, whereas the corresponding value for Ti-Ni substrate is only 37.13\u00a0% (Fig. S2, Table 1). Fig. 3 and Table 1 support our observations from Fig. 1, that CNFs grown on Cr-Ni substrates have a narrower distribution of length and diameter. A similar analysis could not be carried out for the 5-, 10- and 30-min grown samples because longer CNFs partially obstruct the underlying surface.We can safely assume that the entire Ni film is nucleated in the early stages of the reaction, therefore, the population density of CNFs remains the same across the four growth times (1, 5, 10 and 30\u00a0min). Thus, even though lengths of CNFs grown for 30\u00a0min are similar for both substrates, we observe a greater population density of CNFs in Cr-Ni substates. This results in a larger surface area, and hence a larger pseudocapacitance as discussed below.In addition to the reaction between the carbon source and the catalyst layer that has been extensively studied [14,16\u201318], the much less investigated interaction between the catalyst and adhesive layers also has a significant role in the CNF growth process. To rationalize interfacial effects, we use binary and ternary phase diagrams that are available in the literature (see below) and the concept of local equilibrium at the interfaces between the phases. Local equilibrium is defined so that the equilibrium exists only at the interfaces between the different phases present in the system. This means that the thermodynamic functions are continuous across the interface, and the compositions of the phases right at the interface are very close to those indicated by the equilibrium phase diagram. This also indicates that there are activity gradients in the adjoining phases. These gradients, together with the diffusivities, determine the diffusion of components in the various phases of a joint region. This concept is different from the assumption of global equilibrium, where it is assumed that the system's Gibbs free energy (G) function has reached its global minimum value and then, the system is in mechanical, thermal, and chemical equilibrium with its surroundings. Consequently, there are no gradients inside the individual phases, and no changes in the macroscopic properties of the system are to be expected.Binary phase diagrams of the Ni-Ti and Ni-Cr systems (Fig. 4\n) provide us information about the equilibrium phases present in these systems at different temperatures. Phase diagrams do not provide any information about the kinetics of these reactions or the spatial distribution of the phases. However, they provide a useful framework for comparing the two systems, and especially for ruling out thermodynamically impossible phases [19].In the Cr-Ni phase diagram (Fig. 4(a)), there are no intermetallic compounds that are stable at 600\u00a0\u00b0C. Based on the assumption that local equilibrium is established in the system, we can see that at this temperature, stable phases are BCC_A2 (i.e. BCC Cr), BCC_A2\u00a0+\u00a0FCC_A1, and FCC_A1 (i.e. FCC Ni). In addition, whereas the solubility of BCC Cr is very high to FCC Ni, the solubility of FCC Ni to BCC Cr is orders of magnitude smaller. Therefore we expect that most of the Ni would remain unreacted and available for CNF catalysis, at the growth temperature (600\u00a0\u00b0C). This is consistent with the observation of the relatively high population density of fibers grown on Cr-Ni substrate compared to Ti-Ni substrate (Fig. 3). However, at lower temperatures, CrNi2 is stable as well. Hence, if the ramp rate is low, it is possible that intermetallic compound CrNi2 would start to form during ramping, which would reduce the amount of Ni that is available for CNF growth. This explains the significantly lower population density of fibers on Cr-Ni substrate at a low ramp rate (Fig. 5\n).TEM micrograph shown in Fig. 6\n as well as the associated EDX analyses (Fig. S3) provide strong evidence for the above reasoning. We can see that the interface after the CNF growth consists of Cr with some dissolved graphite and/or chromium carbide, and there is practically no Ni left at the interface. There are also no intermetallic layers visible at the interfacial area consistent with the binary phase diagram.On the other hand, the Ti-Ni system contains several intermediate phases at 600\u00a0\u00b0C (Fig. 4(b)). Thus, at equilibrium, we expect the formation of Ti2Ni, TiNi3, \u03b1-Ti and Ni. Even though our system is not at equilibrium (precursor gases and plasma are injected into the system after a few minutes of ramping), we can reasonably assume that a significant part of Ni reacts with Ti or diffuses through the Ti layer towards the Si substrate. The latter is driven by the fact that Ni has a high affinity for forming silicide [20] (Fig. S6). Thus, the more thermal energy we provide to the system, the less Ni will be available for CNF formation. The following observations support this hypothesis: (1) When we decrease the ramp rate, there is no CNF growth on Ti-Ni substrates. (2) If we add a pre-annealing step in the recipe, we get no CNF growth (some samples still result in growth under these conditions, but the vast majority do not). (3) CNF growth on Ti-Ni substrate is not very repeatable. On the other hand, Cr-Ni substrates resulted in uniform CNF growth across multiple batches. Finally, (4) the TEM cross-section (Fig. 6) clearly shows the formation of an intermediate reaction layer at the Ti/Si interface, which contains significant amounts of Ni.\nFig. 2 shows that 5\u00a0min and 10\u00a0min Cr-Ni-CNFs are considerably shorter than their Ti-Ni counterparts, while they reach similar lengths after 30\u00a0min. Can we somehow rationalize this observation based on the thermodynamics and kinetics of the system? One important factor is the dissolution of carbon into the underlying metal layers. Our EDS scans indicate the presence of carbon in both Cr and Ti underlayers (Fig. S3). But what are the differences? Firstly, the 80\u00a0nm thick Cr layer is likely to dissolve significantly more carbon than the 20\u00a0nm thick Ti layer simply owing to its higher overall volume. Secondly, based on the diffusion coefficients calculated from the diffusion couple experiments [Cr,Ti] the intrinsic diffusion coefficient of carbon in Cr is slightly smaller than that of Ti, indicating that it will likely take considerably more time for carbon to reach the Cr/Si than the Ti/Si interface. In fact, there is a clear carbon peak at approximately the midpoint of the Cr layer (Fig. S3(c)), while there is no carbon peak at the Cr-Si interface (Fig. S3(d)). This clearly indicates that carbon has not yet reached the bottom of the Cr layer. On the other hand, there is a clear carbon peak at Ti layer as well as the Ni-silicide layer below it (Fig. S3(e, f)), which indicates that carbon has diffused throughout the underlayer already after 10\u00a0min of growth. Hence, as the Cr layer is not fully saturated with carbon after 10\u00a0min, some of the available carbon will continue to dissolve into Cr instead of forming CNFs, and this is reflected in the observed lengths of the growing CNFs.The presence of oxygen is expected to play a significant role in the evolution and distribution of phases at the Si-Ti and Si-Cr interfaces [21]. In our experiments, both Cr-Ni and Ti-Ni substrates were exposed to air between our processing steps. Hence we expect the substrates to be saturated with oxygen at room temperature. It should be noted that Ti has a higher affinity for oxygen than Cr [21]. From the ternary Ti-Si-O and Cr-Si-O phase diagrams, one can readily see that the thermodynamic stability of the interfaces is very different. Based on Fig. 7(a) and (b), local thermodynamic equilibrium is possible between Cr and SiO2, whereas it is not the case with Ti and SiO2 as in the former case there is a tie-line connecting the two phases directly, a feature that is missing in the Ti-Si-O phase diagram. This naturally means that from the thermodynamic point of view, Cr-SiO2 interface is far more stable than Ti-SiO2 where the formation of several additional phases is expected.Richter et al. reported that when Ti starts to form TiSi2, the oxygen that was initially contained in the consumed Ti region accumulates into the Ti film near the Ti-TiSi2 interface. This phenomenon, commonly referred to as the \u201csnowplow effect\u201d, slows down and ultimately prevents the formation of further TiSi2. In experiments done on a pure Si-Ti system (i.e. no Ni overlayer), it has been reported that oxygen accumulation leads to the formation of a Ti5Si3 interlayer between TiSi2 and Ti [24]. This can be readily rationalized by the fact that out of the Ti-silicides only Ti5Si3 exhibits significant ternary oxygen solubility as well as can at the same time exist at local equilibrium with SiO2. As our annealing time is rather short, we do not see the sequential formation predicted by the stable phase diagram in Fig. 7(b) but instead are dealing with local thermodynamic equilibrium [25]. Nevertheless, we can conclude that the inherently unstable Ti-SiO2 interface will undergo interdiffusion and redistribution of species, as also shown in the TEM micrographs. As the solubility of oxygen to Ti is extremely high, and the driving force for the dissolution is high [26], it is likely that SiO2 at the interface will also be mostly reduced and the resulting oxygen incorporated into the Ti-phase.On the other hand, no redistribution of oxygen was reported during Cr-silicide formation under similar experimental conditions [21]. This behavior can again be rationalized based on the available ternary phase diagram (Fig. 7(a)), which shows, as discussed above, that local equilibrium exists between Cr and SiO2. Thus, there is no need, from the thermodynamics of the system, to rearrange the phase at the interphasial area and therefore also the SiO2 at the interface can be expected to stay intact. Note also that the solubility of oxygen in Cr is much smaller than that in Ti.What are the consequences of the above discussion for our system? We can see that in the Cr-Ni layer structure, there is hardly any Ni at the interface (it is located exclusively at the tips of the fibers) whereas in Ti-Ni layer structure, there is a Ni-silicide phase between the Ti and S as shown by the TEM micrographs. It has been reported that the formation of NiSi is suppressed in the presence of a thin native oxide film (which is present in our samples) [27]. On the other hand, Lee et al. reported that the stability of NiSi is enhanced if Ti is incorporated in Ni thin films [28]. In their study, Ni silicidation reaction was observed at a significantly lower temperature due to the incorporation of Ti, and it was proposed that Ti reacts with interfacial SiO2, resulting in the formation of a Ni-permeable diffusion membrane [28]. Again, this behavior is evident based on the phase diagrams in Fig. 7, which shows that because of the absence of local equilibrium between Ti and SiO2 additional phases will form, and oxygen redistribution (partial or complete reduction of SiO2) is inevitable as discussed above. This will then provide Ni feasible access to Si substrate and result in formation of NixSi1-x phase(s).We used cyclic voltammetry (CV) to investigate the electrochemical properties of CNF samples. In this work, we focus on two important electrochemical parameters - analytical potential window and pseudocapacitance. Sulphuric acid is among the most widely used electrolytes in electroanalytical chemistry, while Phosphate Buffered Saline (PBS) is frequently used as an electrolyte in biosensing applications. Therefore, we determined the pseudocapacitance and analytical potential window of the CNFs in these two electrolytes. The effects of the dimensions of CNFs on the electron transfer kinetics were studied using an OSR probe Ru(NH3)6Cl3 (in PBS), which is known to be insensitive to the surface chemistry of the electrodes [29].The solvent window of an electrolyte is defined as the potential range between the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which occur at the cathodic and anodic ends, respectively. From the electroanalytical point of view, it is more useful to define an analytical potential window where the analyte signal can be precisely measured, even though the exponential increase in current due to HER and OER is not yet seen. The analytical potential window is defined using a self-chosen threshold current value and is, by definition, narrower than the solvent window. We selected a threshold current density of \u00b1150\u00a0\u03bcA/cm2 for this purpose.The analytical potential windows (Table 2\n) were determined with cyclic voltammetry in H2SO4 and in PBS. The cyclic voltammograms are presented in Fig. S5. Ti-Ni-CNFs possess larger potential windows than Cr-Ni-CNFs in both electrolytes for all CNF lengths. However, the difference between the potential windows of Ti-Ni-CNFs and Cr-Ni-CNFs was smaller in PBS in comparison to H2SO4, where Cr-Ni-CNFs and Ti-Ni-CNFs with varied lengths showed potential windows ranging from 1.02\u20131.42 to 1.23\u20131.54\u00a0V respectively. Cr-Ni-CNFs demonstrated a larger potential window in PBS compared to H2SO4, indicating better stability of the Cr-Ni-CNFs in a physiological saline environment. Contrary to Cr-Ni-CNFs, the potential windows of Ti-Ni-CNFs are approximately similar in both PBS and in H2SO4. Moreover, the width of the potential windows for Cr-Ni CNF decreased with an increase in fiber length.Pseudocapacitance (C\n\ndl\n) is a faradaic property that arises on electrode surfaces during electrochemical reactions, wherein due to thermodynamic reasons, charge q depends on potential, resulting in pseudocapacitance C\u00a0=\u00a0d(\u0394q)/dV\n[30]. Several electrochemical processes contribute to pseudocapacitance, including adsorption, intercalation and surface redox reactions. We expect the pseudocapacitance to increase with the increase in CNF length as well as population density, since both these parameters increase the available surface area.The pseudocapacitance of the electrical double layers of all electrodes was calculated from cyclic voltammograms recorded in blank PBS (pH\u00a07.4) and in H2SO4 (pH\u00a00.8) at different scan rates (10\u00a0mv/s\u2013400\u00a0mV/s). The difference between anodic and cathodic current densities (defined from the measured current dividing it by the geometric area of the electrode) at different scan rates and the equation \u0394i\u00a0=\u00a02\u00a0\u00d7\u00a0C\u00a0\u00d7\u00a0v was used to determine the numerical value of the pseudocapacitance.Cr-Ni-CNFs showed higher C\n\ndl\n in H2SO4 than in PBS electrolyte, while C\n\ndl\n for Ti-Ni-CNF electrodes is approximately similar in both electrolytes (Table 2). As expected, the C\n\ndl\n of the electrodes increases with the increase in the length in both Cr-Ni and Ti-Ni systems, indicating the increase in the surface area. C\n\ndl\n of 5- and 10-minute grown Ti-Ni-CNFs deviate from the trend. Nonetheless, C\n\ndl\n of the longest CNFs is significantly larger than the shortest CNFs - about two times larger for Ti-Ni-CNFs and three times larger for Cr-Ni-CNFs. However, C\n\ndl\n of the electrodes in Cr-Ni system is higher, indicating that the overall surface area, which is contributed by the population density, diameter and lengths of the fibers, is higher in comparison to Ti-Ni-CNFs. These results correlate with our analysis above, where we demonstrated that Cr-Ni-CNFs have greater population density, and therefore, greater surface area in comparison to Ti-Ni-CNFs. Overall, Cr-Ni-CNFs showed enhanced capacitance with a decrease in the potential window with respect to the increase in the fiber length. While length of the fibers did not influence the potential window of Ti-Ni-CNFs in any systematic way, C\n\ndl\n\n, however, increased with the increase in fiber length.Peak potential separation (\u0394E\n\np\n) and the ratio of the oxidation to the reduction peak current (I\n\np,a\n\n/I\n\np,c\n) values at 1\u00a0V/s scan rate are shown in Table 2 for electrodes with the shortest and longest CNF lengths studied for both Cr-Ni and Ti-Ni interfaces. Based on these results, the electron transfer kinetics appear nearly reversible for all the electrodes. However, I\n\np,a\n\n/I\n\np,c\n is closer to 1 for the CNFs with 1-minute growth in comparison to CNF with 30\u00a0min growth on both types of substrates. This indicates higher reversibility of the OSR redox reaction on electrodes with shorter fibers. Moreover, according to the \u0394E\n\np\n values, the electron transfer kinetics appear slightly faster with the longest CNF in comparison to the shorter CNF on both interfaces. However, this may be caused by thin liquid layer formation, which will have a similar effect of reducing the \u0394E\n\np\n values than higher heterogeneous electron transfer (HET) rates have. This phenomenon occurs at a certain ratio of the diffusion layer to surface feature thickness and is therefore dependent on both the fiber length and the scan rate. As the nanofiber lengths reach hundreds of nanometers, thin liquid layer formation is a feasible option in this system, especially in the case of the longest CNFs within the scan rates used in this study. A detailed study of the thin-liquid layer formation on these electrodes is presented in a separate work [29].We have demonstrated that the interaction between the catalyst and adhesive layers plays a notable role in the growth behavior, the macroscale morphology of PECVD-grown CNFs and their electrochemical performance. Our results show that (1) Cr-Ni-CNFs have a larger population density than Ti-Ni-CNFs, (2) Ti-Ni-CNFs grow faster for the first 10\u00a0min of the growth process, however, both types of CNFs saturate to similar lengths after 30\u00a0min, and (3) the macroscale morphology of the CNFs can be used to tune their electrochemical properties. All these features can be rationalized by considering the interfacial interactions in the two systems. Owing to the inherent instability of the Ti-Ni interface at our process temperature, a portion of the Ni in the catalyst layer diffuses through Ti and forms a silicide. It is likely that oxygen redistribution also plays a role in the formation of NixSi1-x phases. As a result, a smaller amount of Ni is available for CNF nucleation, and therefore, Ti-Ni-CNFs have a smaller population density than their Cr-Ni counterparts. The stability of the Cr-Ni interface at our process temperature, on the other hand, results in a higher availability of Ni for CNF nucleation, and therefore, a larger population density of fibers. It is likely that the difference in the rate of growth between the two types of substrates is caused by the gradual dissolution of carbon into the thicker Cr layer. Further, we show that the macroscale geometry of fibers influences, for instance, the pseudocapacitance of CNF electrodes without significantly affecting the electron transfer kinetics. Thus, this study paves the way towards designing application-specific CNF electrodes by precisely controlling their macroscale morphology.\nIshan Pande: Conceptualization, Methodology, Writing - Original draft preparation, Writing - Reviewing and Editing, Investigation, Resources, Visualization. Laura Ferrer Pascual: Investigation, Writing - Original draft preparation. Ayesha Kousar: Writing - Original draft preparation. Emilia Peltola: Funding acquisition, Writing - Review & Editing. Hua Jiang: Investigation. Tomi Laurila: Conceptualization, Methodology, Writing - Original draft preparation, Writing - Reviewing and Editing, Resources, Funding acquisition, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by funding from the Academy of Finland (#321996 and #328854) and Jane and Aatos Erkko Foundation. The authors acknowledge the provision of facilities and technical support by Aalto University at OtaNano - Nanomicroscopy Center (Aalto-NMC) and at Micronova Nanofabrication Centre. I.P. would like to thank Elli Lepp\u00e4nen and Petri Mustonen for discussions regarding the PECVD process, and Dr. Jani Sainio for help with sample characterization.\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.diamond.2022.109566.", "descript": "\n                  The effect of catalyst materials and different process parameters on the growth of carbon nanofibers (CNFs) has been widely investigated. Typically, an adhesion metallization is required together with the catalyst to secure adequate attachment to the surface. The interactions within this multilayer structure and their effect on CNF growth and morphology has, however, not been thoroughly assessed. Thus, this work presents the growth behavior, the macroscale morphology, and the basic electrochemical characteristics of CNFs grown on two types of substrates - (1) Si\u00a0+\u00a080\u00a0nm Cr\u00a0+\u00a020\u00a0nm Ni, and (2) Si\u00a0+\u00a020\u00a0nm Ti\u00a0+\u00a020\u00a0nm Ni. Our results show that the macroscale geometric parameters of CNFs can be readily altered by using different adhesive layers. The inherently unstable Ti-Ni interface results in diffusion of Ni towards the silicon wafer to form silicide, which reduces the amount of available Ni for CNF nucleation, and therefore, the population density of fibers is reduced. On the other hand, the Cr-Ni interface results in a larger population density, but the rate of growth is reduced due to diffusion of carbon into the thicker Cr layer. The results are rationalized by using relevant binary and ternary phase diagrams. Further, cyclic voltammetry experiments show that the pseudocapacitance of CNFs shows a correlation with the length and population density of fibers, while the electron transfer kinetics appear nearly reversible for all the electrodes. This simple approach can be used for tailoring CNFs for specific applications by controlling their macroscale geometrical parameters.\n               "}
{"full_text": "Data will be made available on request.Within the renewable energy portfolio, biomass is a key resource for mitigating climate change and reducing reliance on fossil fuels [1]. Lignin is a major component of biomass and has the potential to be converted into hydrocarbon fuels and value-added compounds [1]. Fast pyrolysis technique may be used to extract desired compounds from biomass, and the liquid produced is referred to as bio-oil [1]. Bio-oils produced from the rapid pyrolysis of lignin often include a high concentration of oxygenated compounds. Due to the high oxygen content of bio-oil, it has significant disadvantages such as high viscosity, low solubility in other hydrocarbons, low volatility, corrosiveness, and low heating value, which prevents its direct use as a transportation fuel [2]. Additionally, the reactive compounds have extremely low stability during the storage process. As a result, in order to create carbon neutral fuels from bio-oil, it must be upgraded to meet these fuel specifications.Catalytic hydrodeoxygenation (HDO) is one of the rapidly developing technologies for upgrading bio-oils to produce transportation fuel or value-added chemicals [1]. This process is generally performed by using a hydrotreating process under high pressure [3]. It is essential to deoxygenate bio-oils to eliminate the high viscosity and chemical instability and also increase heating value [2]. Ideally, oxygen is removed in the presence of catalyst with minimal saturation of the aromatic rings, which also reduces the hydrogen consumption [1].In order to make the HDO process more economically appealing, H donors other than hydrogen gas have been considered as alternatives [4]. Water is a cheap and available source of hydrogen that near its critical point can also be an excellent solvent for liquid phase HDO, making the process sustainable and economically more feasible. In this alternative process, catalysts must be designed to facilitate water dissociation on the surface, which will provide hydrogen to the catalytic HDO reaction. Noble metal and non-noble metal catalyst supported on carbon have been studied by our research group before, demonstrating the feasibility of the viability of \u201cH2-free\u201d HDO reactions using water as reaction media [4\u20137]. Generally speaking, noble metals present higher activity compared to transition metal catalysts. Ru/C displays the highest activity among studied catalysts (i.e. Au/C, Pd/C, Rh/C and Ru/C). The high activity is attributed to the smaller mental particle size, greater dispersion of metal particles and its intrinsic activity for this reaction [4].Guaiacol has been selected as lignin model compound for the HDO reaction because the molecule is composed of two typical functional groups, i.e. hydroxy and methoxy groups [2]. The targeted products of guaiacol HDO reaction are ideally hydrocarbons or hydrogenated aromatic compounds. Transition metal-based catalysts, noble metal catalysts and zeolites-based catalysts have been widely investigated to produce hydrodeoxygenated aromatic compounds from guaiacol. To assist HDO reactions two functions are required for the catalyst, including the activation of the oxygen containing groups on the reactant (water activation) and the hydrogen donation (hydrodeoxygenation reaction) [4,6,7]. For noble metal catalysts such as Pd/C, Rh/C, Au/C, Ru/C and Pt/NC [4,6,7], the activation of the O-containing groups on the reactant take place on the metal sites or the metal-support- interface, and the hydrodeoxygenation reaction occurs on the surface of noble metals [1]. Despite their high activity and stability in the HDO reaction, noble metals constitute critical raw materials with limited availability, and it is necessary to identify earth abundant, low-cost alternatives which are more sustainable. Ni catalysts have been extensively investigated in HDO process considering the cheap price and similar performance in comparison to the noble metal catalysts [8]. Ni-based catalysts have good capability towards C-C bond rupture, and high activity in hydrogenation. However, Ni catalysts are susceptible to coke deposition, leading to the deactivation of the catalyst and hence poor stability [5]. The selection of support can significantly influence coking resistance of Ni-based catalysts [9]. It is reported that the performance of the conventional catalysts can be improved by using Zr2O as promoter in order to prevent coke deposition [10]. Therefore, Zr2O was explored as a promoter for this research effort.Carbon materials are ideal candidates for catalysts supports for HDO reaction, since they are inert, with limited interactions with the active phase. Activated carbon is also a viable support considering its hydrophobicity, which could decrease the possibility of metal deactivation in water-existing reaction systems [11]. Graphene and its derivates deserve more attention due to their large surface area, and unusual electronic, mechanical, and thermal properties [12]. Graphene consists of a monolayer of carbon atoms arranged in a hexagonal structure. It is reported that graphene supported catalysts present the highest activity in deoxygenation of vegetable oil among other carbon materials including glassy spherical carbon, activated carbon and mesoporous carbon. The superior activity can be attributed to its large pore size, which facilitates the transportation of reactant and fine dispersion of metal particles on the surface of graphene support [12]. To optimise metal-graphene interaction, further actions could be done such as reducing the nanoparticle sizes, improving the homogeneous distributions of the nanoparticles and increasing the number of defect sites on graphene surfaces [13]. Various methods have been proposed to engineer the electronic structure of graphene, including preparing carbon sheets with different layers and graphene with and without defects, chemical functionalities of graphene, and chemical doping [14,15]. Chemical doping is the introduction of a heteroatom substituting a carbon atom in the graphitic structure [16]. Nitrogen substitution are considered magnificent candidates owing to the fact that they have comparable atomic size and strong valence bonds with carbon atoms [15,16]. The increased deoxygenation capacity of the N-doped samples is attributed to increased activity of the N-support and N-metal interfaces. Such interfaces are envisioned as electron-rich regions capable of activating C-O bonds [17].In our work, different Ni-based graphene supported catalysts with/without nitrogen doping have been synthesised, characterised, and studied in guaiacol HDO reaction using water as hydrogen supplier. Consequently, in this study, the effect of synthesis methods, nitrogen doping and the addition of ZrO2 as a promoter were explored.Reduced graphene oxide (Gr) and N-doped reduced graphene oxide (Gr-n) were employed as supports in this investigation. As a precursor, graphite oxide (GO325) was required for the production of both Gr and Gr-n. GO325 was made using commercial natural flake graphite (G, 99.9% purity) obtained from Alfa Aesar and a modified Brodie process [18]. The reduced graphene oxides were created by thermally reducing GO325 in a vertical cylindrical packed bed reactor. The reactor was loaded (350\u00a0mg GO325) and purged with N2 for one hour at room temperature at a flow rate of 100 sccm. Following that, the N2 flow was lowered to 87 sccm, and a flow of 3 and 10 sccm H2 and NH3 respectively, were introduced into the reactor for the N-doped sample. Then, several temperature treatment programs were used. The initial ramp was from room temperature to 100\u00a0\u00b0C at a rate of 5\u00a0\u00b0C/min. After that, it was raised to 700\u00a0\u00b0C at a rate of 5\u00a0\u00b0C/min and held at that temperature for 5\u00a0min. After the furnace heating was completed, the reactor was allowed to cool to 400\u00a0\u00b0C, the H2/NH3 flows were turned off, and the system was allowed to cool in N2 atmosphere. This graphene was given the name Gr-n. The reduction of the undoped sample was carried out in the same manner as stated above, but without the addition of NH3 to the reactor; this graphene was designated as Gr.Four types of catalysts, labelled as Ni/Gr, Ni/Gr-n, NiZrO2/Gr and NiZrO2/Gr-n were synthesised.Wet impregnation synthesis method it is utilised for supported catalysts. Firstly, the necessary amounts of metal precursor (Ni(NO3)2\u00b76\u00a0H2O) were dissolved in deionised water and added to the support that was prior synthesised. After that, in order to obtain homogeneity of the suspensions, they were stirred at room temperature. Secondly, the excess water was removed in a rotary evaporator under reduced pressure and the materials were dried in an oven at 80\u00a0\u00b0C for 12\u00a0h. The last step of the method was the calcination at 500\u00a0\u00b0C (5\u00a0\u00b0C/min ramp) for 3\u00a0h under an inert atmosphere.10\u00a0wt% ZrO2 was impregnated on Gr and Gr-n. 0.0989\u00a0g Zr(NO)36\u00a0H2O was dissolved in 50\u00a0mL of acetone, and 350\u00a0mg of the appropriate support was added while stirring for 4\u00a0h. The solvent was then evaporated, and the resultant sample was dried in an oven at 100\u00a0\u00b0C overnight. The samples were then calcined at 350\u00a0\u00b0C for 5\u00a0h with a temperature ramp of 1\u00a0\u00b0C/min. Then, by using Ni(NO3)2\u00b76\u00a0H2O as a precursor salt and the same process as described above, impregnation of 15\u00a0wt% Ni was carried out. Ni loading was the same for all synthesized catalysts. Sigma-Aldrich supplied all of the reactants.The catalysts have been characterised by means of XRD, H2-TPR, TEM and XPS.\nXRD. X-ray diffraction (XRD) analysis was conducted on fresh, reduced and used catalysts using an X\u2032Pert Pro Powder Diffractometer by PANalytical. The 2\u03b8 angle was increased by 0.05\u00b0 every 240\u00a0s over the range of 10\u201380\u00a0\u00b0. Diffraction patterns were recorded at 40\u00a0mA and 45\u00a0kV, using Cu K\u03b1 radiation (\u03bb\u00a0=\u00a00.154\u00a0nm).\nH\n\n2\n\n-TPR. Temperature programmed reduction with hydrogen (TPR) analysis was carried out on the calcined catalysts in a quartz tube reactor. 50\u00a0mg of sample was heated to 900\u00a0\u00b0C at a rate of 10\u00a0\u00b0C/min with a total flow of 50\u00a0mL\u00a0min\u22121 of 5% H2 in N2. A CO2-ethanol trap was used to condense the gaseous products, mostly water, before the on-stream thermal conductivity detector (TCD). The H2 uptake was quantified by comparison with the hydrogen consumption of a CuO reference sample.\nTEM. Information about the supported metal particles was acquired by TEM (Transmission electron microscopy) in a JEOL 2100\u00a0F field emission gun electron microscope operated at 200\u00a0kV and equipped with an Energy-Dispersive X-Ray detector, XEDS. The sample was ground until powder and a small amount was suspended in acetone solution using an ultrasonic bath. Some drops were added to the copper grid (Aname, Lacey carbon 200 mesh) and the solvent was evaporated at room temperature before introduction in the microscope. XEDS-mapping analysis was performed in STEM mode with a probe size of 1\u00a0nm using the INCA x-sight (Oxford Instruments) detector.\nXPS. The XPS spectra were obtained by using non-monochromatic Al radiation (200\u00a0W, 1486,61\u00a0eV) through a SPECS GmbH with UHV system and with an energy analyser (PHOIBOS 150 9MCD). The samples were pre-treated at 500\u00baC for an hour in H2 and subsequently, for another hour in He at room temperature. After that, the samples were placed in the sample holder using a double-sided copper tape and transferred to the analysis chamber. The survey spectra were obtained with a 50-eV pass energy and region spectra were obtained at 20\u00a0eV pass energy. The binding energy (BE) was finally measured taking as a reference the C1s peak at 284.6\u00a0eV and the equipment error was considered as less than 0.01\u00a0eV for the determination of energies.The HDO reactions were conducted in a batch reactor (Parr Series 5500 HPCL Reactor with a 4848 Reactor Controller) using 300\u00a0mL PTFE gaskets. Catalysts were pre-treated ex-situ in a continuous flow quartz reactor, at 550\u00a0\u00b0C for 1\u00a0h in a 100\u00a0mL/min gas flow (H2:Ar=1:4) before being used in the HDO reaction. A quantity of 0.5\u00a0g of guaiacol, 49.5\u00a0g of water and 0.05\u00a0g of catalyst were loaded in a glass-lined steel vessel. To avoid any air contamination, N2 was bubbled through the solution for 5\u00a0min under a stirring speed of 100\u00a0rpm before closing the reaction vessel. Then, the reactor was heated to the desired temperature (250\u00a0\u00b0C/300\u00a0\u00b0C) and held at this temperature for 4\u00a0h under a stirring speed of 300\u00a0rpm. The pressure of the vessel was fixed according to the natural pressure generated by the solvent (water) at 50/100\u00a0bar during the reactions respectively. After the reaction, the spent catalyst was recovered from the liquid by filtration, followed by drying treatment. The organic products were dissolved and recovered with ethyl acetate extraction. The organic compounds products were identified by a gas chromatography-mass spectrometry (GC-MS). Quantitative analysis was performed with a gas chromatograph-flame ionisation detector (GC/FID). The GC injector temperature was 280\u00a0\u00b0C. The GC separation was performed by using a Carboxen Packed Analytical Column (30\u00a0m\u00d7320\u00a0\u00b5m\u00d70.25\u00a0\u00b5m). A split ratio of 8:1 was held. The column was firstly held at 50\u00a0\u00b0C for 1\u00a0min, then increased to 240\u00a0\u00b0C at a heating rate of 5\u00a0\u00b0C/min and held at 240\u00a0\u00b0C for 10\u00a0minThe conversion of guaiacol and selectivity (based on C mol) of the products was calculated using Equation 1 and 2, respectively.\n\n(1)\n\n\nConversion\n\nof\n\nGuaiacol\n\n(\nmol\n\n%\n)\n=\n\n\n\n\n(\n\n\nm\n\n\nGin\n\n\n\u2212\n\n\nm\n\n\nGout\n\n\n)\n*\n\n\nN\n\n\nG\n\n\n\n\n\n\nM\n\n\nG\n\n\n\n\n\n\n\n\nm\n\n\nGin\n\n\n/\n\n\nM\n\n\nG\n\n\n*\n\n\nN\n\n\nG\n\n\n\n\n*\n100\n\n\n\n\n\n\n(2)\n\n\nCarbon\n\nweighted\n\nselectivity\n\nof\n\nproduct\n\nx\n\n(\nmol\n\n%\n)\n=\n\n\n\n\n\n\n\nm\n\n\nx\n\n\n*\nNx\n\n\n\n\nM\n\n\nx\n\n\n\n\n\n\n\n\n\n(\n\n\n\nm\n\n\nGin\n\n\n\u2212\n\n\nm\n\n\nGout\n\n\n\n)\n\n*\n\n\nN\n\n\nG\n\n\n\n\n\n\nM\n\n\nG\n\n\n\n\n\n\n\n*\n100\n\n\n\nmGin: Initial mass of guaiacol [gr]; mGout: Detected mass of guaiacol in the organic phase [gr]; mx: Mass of product x [gr]. MG: Molar mass of guaiacol[mol/gr]; Mx: Molar mass of product x[mol/gr]. NG: Number of Carbon in guaiacol; Nx: Number of Carbon in product x.The catalytic behaviour of the reduced catalysts in the guaiacol HDO process was studied at 250\u2009\u00b0C and 300\u2009\u00b0C for 4\u2009h. Activity results are displayed in \n\nFigs. 1 and 2. Numerical data can be found in the supporting information ( \n\nTables 1 and 2 respectively).The reaction results of our Ni-based catalysts at 250\u2009\u00b0C are presented in Fig. 1. Guaiacol conversion of all the catalysts varied between 15% and 20%. Such conversion ranges might look modest however we shall emphasise that in our study the HDO process is conducted using water as hydrogen source and suppressing completely external H2 input our reaction system. Hence these are interesting results given the significant process savings. A clear effect of the N-doping on the catalytic activity was seen since the Ni/Gr-n exhibited the highest guaiacol conversion (20%) compared to 17% of the undoped catalyst. However, no promotion effect on the conversion of guaiacol was observed when ZrO2 was added as a promoter.Three mono-aromatic compounds including phenol, cresol and catechol were detected on the organic phase. Such products are associated with a potential guaiacol HDO reaction pathway proposed in \nFig. 3. The formation of catechol is the most preferred process because the C(sp3)-O bond is most likely to be broken owning to its low bond energy [19]. Despite retaining two oxygens, catechol is one of the intermediates that ultimately leads to more advanced (more deoxygenated) products such as phenol (1 oxygen), benzene, or cyclohexane (fully deoxygenated products) [5]. The production of partial deoxygenated compounds phenol and catechol were improved over Ni/Gr catalyst in comparison to all other samples indicating the superior ability of demethoxylation and/or dihydroxylation [5]. When N was present, the results showed slight variations between the catalysts. The N-doped catalysts produced some phenol, showing that the C-O cleavage was preferred in N-doped systems. The latter is consistent with previous studies of palm oil HDO utilising N-doped activated carbons catalysts, in which the increased deoxygenation capacity of the N-doped samples is attributed to increased activity of the N-support and N-metal interfaces. Such interfaces are envisioned as electron-rich regions capable of activating C-O bonds [17]. Overall, all three samples have similar selectivity towards phenol, cresol and catechol being phenol the most advanced deoxygenation product since it represents just the last step prior to benzene, the final product in the deoxygenation route according as depicted in Fig. 3. Indeed phenol presence in our liquid products mixtures is an encouraging results since despite the absence of external hydrogen source our catalysts can trigger the reaction and get very close to full deoxygenation.To enhance conversion levels, the reaction temperature was raised to 300\u2009\u00b0C. Catalytic behaviour of Ni-based catalysts and Gr-supports at 300\u2009\u00b0C are presented in Fig. 2, where differences in activity can be observed. Guaiacol conversion of all the catalysts varied between 40% and 55%. This guaiacol conversion was almost doubled in comparison with the catalytic activity presented at 250\u2009\u00b0C in Fig. 1 showcasing that upon tunning the reaction parameters remarkable catalytic performance boosting can be attained. Although a full reaction parameters optimisation is beyond the scope of this proof-of-concept paper this result suggests there is big room for overall process improvement reinforcing the potential of \u201cH2-free\u201d HDO strategies.The conversion increased at 300\u2009\u00b0C compared to that obtained at 250\u2009\u00b0C is a general trend for all the studied catalysts. For example, the conversion of guaiacol obtained over calcined Ni/Gr (17%) at 250\u2009\u00b0C was 26% lower compared to that obtained over reduced Ni/Gr (43%) catalyst at 300\u2009\u00b0C. Also, temperature and pressure had a greatest influence on the activity of Ni-Gr sample, since 20% of guaiacol conversion was obtained at 250\u2009\u00b0C and 45% of guaiacol conversion at 300\u2009\u00b0C. By comparing both Figs. (1 and 2), we can determine that temperature and pressure had great influence on the catalytic performance. Therefore, we can conclude that during the HDO process an increase of 50\u2009\u00b0C in temperature improved notably the conversion of guaiacol. More remarkably, both the addition of the catalyst and the rise of temperature and pressure, had a positive effect on the selectivity of the products and conversion of the guaiacol. Furthermore, promotion effect on the conversion of guaiacol was observed when adding ZrO2 as a promoter at 300\u2009\u00b0C, due to the enhanced oxygen mobility provided by ZrO2 which allowed the activation of C-O bonds [10]. The presence of nitrogen modifies the electrical density and acid/base characteristics of carbon, hence influencing its overall reactivity. As previously reported by W. Jin and co-workers, the increased activity of N-doped supports is due to the beneficial effect of nitrogen, which might aid to stabilize metal particles and prevent their re-oxidation [17]. In addition, nitrogen sites inserted into the carbon network are envisioned as electron-rich reaction sites. Regardless of the reaction routes, there is no question that N as a dopant has a favourable influence on the upgrading reaction [17]. To conclude, the NiZr2O/Gr-n catalyst showed the best result according to the objective, since the production of cresol has been enhanced as a reaction product and is the catalyst that shows best catalytic activity. By nitrogen-doping our carbon support, we may possibly increase the activity [17]. In view of these results, Ni-based catalysts are also an advisable choice considering their relatively low price.It is pointed out that the products analysed were the dominant products in the organic liquid phase. The rest of compounds, up to 100% of selectivity were other aromatics hydrocarbons in addition to some secondaries products derivates of reactions like decarboxylation cracking, and hydrocracking can be found. In terms of selectivity results, three partially deoxygenated mono-aromatic compounds, namely phenol, cresol and catechol were detected in the organic phase as per observed also at 250\u2009\u00b0C. The production of the partial deoxygenated products phenol and cresol was improved over Ni/Gr catalyst, indicating its superior ability of demethoxylation and/or dihydroxylation at 300\u2009\u00b0C.A schematic representation of the potential HDO pathways of guaiacol is proposed in Fig. 3\n[20]. The high selectivity of catechol in all the product distribution indicated a preferential HDO pathway. The formation of catechol was the most preferred process because the C(sp3)- O bond was most likely to be broken, since it presented the lowest bond energy [19]. Unfortunately, benzene was not produced in our reaction system. This result should not be considered unfavourable since the main challenge in an \u201cH2-free\u201d HDO process is to incorporate the hydrogen into the organic molecules without an external H2 supplier. It is important to remember that because we are engaging in an HDO process in which there is no addition of external hydrogen, only locally produced hydrogen during the reaction may combine with the oxygenated molecules. However, the process economic viability and safety issues attributed to hydrogen manipulation and transport make this pathway desirable for oxygenated hydrocarbon upgrading despite the generally low conversion values reported.A quantitative analysis of the nitrogen species presents in the samples was obtained by deconvolution of the XPS spectra of the N1s core level. In Table 1, the data of the N1s region XPS analyses are shown for selected catalysts. The main nitrogen component on the fresh and reduced catalyst is pyridinic nitrogen followed by pyrrolic and quaternary which have been created with similar ratio. The similar ratios and binding energies obtained on both the fresh and spent catalysts seem to indicate that the nitrogen species are stable under reaction conditions.The XPS spectra of Ni 2p3/2 in the reduced-passivated catalyst is shown in \nFig. 4. The corresponding binding energies (B.E), atomic percentages and relative proportions are provided in Table 2. Ni/Gr and, NiZr2O/Gr catalysts exhibited a peak at 852.8\u2009eV and NiGr -n showed a peak at 853.0\u2009eV, corresponding these peaks to Ni0. In these catalysts, peaks at higher B.E associated to Ni2+ are also observed. Ni/Gr catalyst could be deconvoluted into contributions at 861.2\u2009eV (Ni2+ satellite peak), 858.1\u2009eV and 855.3\u2009eV (Ni2+) and 852.8\u2009eV (Ni0). NiZr2O/Gr, Ni/Gr-n and NiZr2O/Gr-n exhibited similar species. This suggests the presence of metallic Ni and Ni2+ species on the support\u2019s surface of the catalysts, in accordance with the XRD data obtained (\nFig. 5). In Table 2 it is observed that the atomic percentages of Ni0 are higher in the Ni/Gr-n and NiZr2O/Gr-n catalyst than in the Ni/Gr and NiZr2O/Gr catalyst, suggesting a possible stabilising effect of nitrogen on the metallic Ni in good agreement with the H2-TPR data described below.\nFig. 5 shows the different XRD pattern of Ni/Gr, Ni/Gr-n, NiZr2O/Gr and NiZr2O/Gr-n samples for fresh, reduced and post reaction (at 250\u2009\u00b0C and 300\u2009\u00b0C) catalyst forms.A weak and broad diffraction peak at around 2\u03b8 =\u200926.5\u00b0 can be observed in the XRD patterns of Ni/Gr and NiZrO2/Gr catalysts (Fig. 5 (A) and (C) respectively), assigned to the (002) planes of the graphitic carbon frame (JCPDS 41\u20131487) [21,22] with more amorphous structure and lower order in crystallinity. In contrast, this peak for n-doped samples was stronger and sharper, indicating a higher order of graphic structure. Moreover, these materials recovered the graphitic structure to a higher extent [23]. A second diffraction peak at 44.0\u25e6 corresponding (100) plane of graphite indicated the reduction of the GO matrix [24,25]. However, this peak overlapped with the Ni metallic peak at 2\u03b8 =\u200944.5\u00b0 (JCPDS 87\u20130712).An interesting finding was that metallic Ni (JCPDS 87\u20130712) [5] was the main phase instead of NiO for fresh catalysts (except Ni/Gr-n), but NiO presence could not be discarded as the main peak could overlapped with the Ni metallic peak. This was probably due to the partial reduction of NiO during calcination process assisted by the support [26]. No diffraction peak of NiO (JCPS 04\u20130835) appeared in the XRD patterns of all reduced samples, indicating the success of reduction pre-treatment. Moreover, NiO reduction zones have been observed in the TPR profiles, which are discussed below.The characteristic diffraction peaks at 2\u03b8 =\u200930.2\u00b0, 34.5\u00b0 and 50.2\u00b0 corresponding to the (101), (110) and (200) refection of ZrO2 phase (JCPDS 70\u20131769) [27,28] can be clearly observed in the XRD patterns of ZrO2 containing catalysts (Fig. 5 C) and D)). The diffraction peaks of tetragonal phase of ZrO2 were stronger and sharper in n-doped catalyst compared to that of non-doped sample. Results indicated that there was a higher extent of crystallinity of ZrO2 in n-doped catalyst. However, the characteristic peaks of monoclinic phase ZrO2 were not observed in our case. It is reported that the t-ZrO2 phase is formed at 400\u2009\u00b0C, since the synthesis temperature utilised was below this temperature mixed phase ZrO2 (both t-ZrO2 and m-ZrO2) were not expected consistently with the XRD data [29].Redox properties and information concerning metal-support interactions were studied by H2-temperature-programmed-reduction (TPR) analysis. The H2-TPR profile of Ni/Gr catalyst is shown in \nFig. 6. Three reduction zones can be observed, at around 280\u2009\u00b0C, 330\u2009\u00b0C and 530\u2009\u00b0C. They all corresponded to the reduction of finely dispersed NiO on the support. Normally, smaller particle size presents reduction zone at lower temperatures [30].In case of Ni/Gr-n, it is hypothesised that when the Ni atom is placed onto the doped support, there is a repulsion between the Ni and the nitrogen dopant, causing the Ni atom to establish an association [14]. The dopant alters the local surface binding configuration, increasing the binding energy, and hence altering the Ni-carbon interaction. This implies that doping might boost the catalyst's durability [14].In the NiZrO2/Gr sample TPR profile, two reduction regions can be observed at 200 \u00b0C-400 \u00b0C and 550 \u00b0C-850 \u00b0C. The first peak centred at 250\u2009\u00b0C is formed as a consequence of the NiO reduction on the support [30]. NiZr2O/Gr sample has a lower reduction temperature, indicating it is easier to reduce. This might be due to lower particle size of Ni compared to the other samples due to the lower reduction temperatures in the TPR.The second peak was attributed to the reduction of oxygen superficial groups on the support [30]. In general, this peak is formed due to the reduction of finely dispersed NiO on the support, as when ZrO2 is used in Ni-based catalysts it stabilises the cubic structure at high temperatures and improves oxygen storage capacity [31]. Moreover, it is important to mention that hydrogenation of carbon atoms in graphite [32] was taken into consideration at temperatures higher than 800 \u00b0C, since methane production has been observed in this temperature range. Two reduction peaks were present in the H2-TPR profile of NiZrO2/Gr-n sample, one at around 300\u2009\u00b0C and around 500\u2009\u00b0C. They all corresponded to the reduction of finely dispersed NiO on the support [30].As observed in the TPR profiles, the highest reduction temperature of the metal active phase of the catalyst was around 550\u2009\u00b0C. Therefore, this was the temperature selected to reduce the catalyst prior to the reaction, since it has been demonstrated in previous publications that the reduction of the active phase of the samples improve the catalytic performance [14]. The success of the reduction prior to the reaction has been demonstrated by the XRD patterns above.Transmission Electron Microscopy (TEM) was used to study the composition and distribution of the different elements in the synthesised reduced samples. The TEM images of fresh Ni/Gr, Ni/ZrO2Gr and Ni/ZrO2Gr-n are presented in \nFig. 7. The micrographs clearly showed the exfoliated graphene layers along with zirconia and some nickel particles.In general, a better metal dispersion in the N-doped sample can be observed in the TEM images, corroborating that the presence of nitrogen in the sample can help to obtain a better dispersion of active phases, as discussed in the TPR analysis. Some areas were further analyzed by the corresponding Fast-Fourier transform (FFT) pattern, and they are shown in \nFig. 8 A) and B). In general, a relevant search on several zones of the material, showed that the undoped sample barely displayed zones where Ni and ZrO2 were collocated. In contrast, the catalyst NiZrO2-Gr-n did present this Ni-Zr interaction. Moreover, the analysis of the FFT pattern confirmed that ZrO2 was mainly present in its tetragonal phase.Coking and metal sintering of active phase may happen under liquid phase reactions with high pressure [9]. Hence, the XRD patterns of spent catalysts were analysed. As shown in Fig. 5 (A, B, and C), part or all of the metallic nickel were oxidized into NiO during the HDO reaction for Gr supported catalysts. In comparison to the calcined samples, some new diffractions peaks have been detected for the Ni/Gr, Ni/Gr-n, and NiZrO2/Gr catalyst. Diffractions peaks at 37.3\u25e6, 43.3\u25e6 and 63\u25e6 were observed, corresponding to the (111), (200) and (220) planes of the NiO fcc phase respectively, in agreement with JCPDS no. 00\u2013047\u20131049. This points out oxidation of the Ni metallic phase, which is partly expected given the selected reaction media (H2O). On the other hand, such oxidation phenomenon was not observed for the NiZrO2/Gr-n samples, hence reflecting higher stability due to the nitrogen doping and the ability of Zr to act as a promoter [10,16]. The different width of this peak is related to the different interlayer distance generated during the thermal reducing treatment. Therefore, it can be appreciated that at a higher the reaction temperature, the particle size increases and becomes more amorphous due to sintering of the metal phase.TEM images of the samples (Ni/Gr, Ni/Gr-n, NiZrO2/Gr and NiZrO2/Gr-n) after the catalytic reaction at 300\u2009\u00b0C are presented in \nFig. 9. By comparing all figures, few particles agglomeration can be seen over NiZrO2/Gr (Fig. 9 C)), but the particle distribution still remained homogenous for all samples. In comparison to the calcined samples, the sintering of the metal particles marginally increased the particle size. This was a result of the reduction procedure and reaction conditions used. Despite the very demanding reaction conditions, this finding demonstrated the structural and morphological stability of the NiZrO2/Gr sample for the hydrothermal upgrading of lignin model compounds.Based on a novel \"H2-free\" HDO strategy, this research showcases a promising catalytic route for biomass upgrading. More specifically, we show that hydrothermal deoxygenation of guaiacol as a lignin model compound can be performed without external hydrogen input. For the in-situ production of hydrogen coupled to HDO, we propose multicomponent catalysts capable of activating water and facilitating the subsequent HDO reaction. Our catalysts based on Ni nanoparticles supported on N-doped and non-doped graphene decorated with zirconia particles were able to partially deoxygenate the original feedstock effectively. Furthermore, the samples were stable under the reaction conditions (high pressure, high temperature in a hydrothermal medium), as indicated by post-reaction XRD and TEM examination at 250\u2009\u00b0C and 300 \u00b0C. Our results indicate that the NiZr2O/Gr-n catalyst led to the best results, as the synthesis of cresol as a reaction product was increased, and it exhibited the highest catalytic activity. Cresol can be used as sources for high-value chemical products [33]. Nitrogen doping of the support was shown to improve conversion in all cases and is an effective strategy for promoting activity. In light of these findings, Ni-based catalysts are also a viable option for the HDO reaction of guaiacol due to their comparatively cheap cost and comparable catalytic performance compared to noble metal catalysts.Overall, the catalytic performance of the designed catalysts may be considered moderate in contrast to existing catalysts in the standard HDO when high pressure hydrogen is supplied. Generally, it is necessary to increase overall deoxygenation efficiency by fine-tuning the catalyst composition for the water assisted HDO process. Also performing HDO of chemical intermediates (such as catechol) will aid to gather knowledge of the hydrogen transfer route and reaction pathways in the water assisted HDO process and further guide the catalysts design. Nonetheless, the distinctive benefit of our approach is the absence of external hydrogen input. Hence, despite still on its early development stages, our concept should drive future research efforts to enhance the catalytic formulation and improve performance. In this approach, we demonstrate the crucial role of heterogeneous catalysis in bio-resources upgrading to aid in the transition to a low-carbon economy.\nS. Parrilla-Lahoz: Writing \u2013 original draft, Conceptualization, Visualization, Investigation. W.Jin: Writing \u2013 original draft, Conceptualization, Visualization, Investigation. L. Pastor-P\u00e9rez: Funding acquisition, Conceptualization, Project administration, Supervision. M.S. Duyar: Funding acquisition, Conceptualization, Project administration, Supervision. L.Mart\u00ednez Quintana: Experimental, Data curation. A.B. Dongil: Funding acquisition, Conceptualization, Project administration, Supervision. T.R. Reina: Funding acquisition, Conceptualization, Project administration, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper: Tomas Ramirez Reina reports financial support was provided by Junta de Andaluc\u00eda Consejer\u00eda de Educaci\u00f3n. Tomas Ramirez Reina reports financial support was provided by Spain Ministry of Science and Innovation.Financial support for this work has been obtained from Junta de Andaluc\u00eda project P20-00667, co-funded by the European Union FEDER. This work is also sponsored by the Spanish Ministry of Science and Innovation through the projects PID2019-108502RJ-I00 and grant IJC2019- 040560-I both funded by MCIN/AEI/10.13039/501100011033 as well as RYC2018-024387-I funded by MCIN/AEI/10.13039/501100011033 and by ESF Investing in your future. Financial support from the European Commission through the H2020-MSCA-RISE-2020\nBIOALL project (Grant Agreement: 101008058) and RYC2020-030626-I (MCIN) and project 20228AT002 (CSIC) is also acknowledged.", "descript": "\n                  Catalytic hydrodeoxygenation (HDO) is a critical technique for upgrading biomass derivatives to deoxygenated fuels or other high-value compounds. Phenol, guaiacol, anisole, p-cresol, m-cresol and vanillin are all monomeric phenolics produced from lignin. Guaiacol is often utilised as a model lignin compound to deduce mechanistic information about the bio-oil upgrading process. Typically, a source of H2 is supplied as reactant for the HDO reaction. However, the H2 supply, due to the high cost of production and additional safety precautions needed for storage and transportation, imposes significant economic infeasibilities on the HDO process's scaling up. We investigated a novel H2-free hydrodeoxygenation (HDO) reaction of guaiacol at low temperatures and pressures, using water as both a reaction medium and hydrogen source. A variety of Ni catalysts supported on zirconia/graphene/with/without nitrogen doping were synthesised and evaluated at 250\u00a0\u00b0C and 300\u00a0\u00b0C in a batch reactor, with the goal of performing a multi-step tandem reaction including water splitting followed by HDO. The catalysts were characterised using H2-TPR, XRD, TEM and XPS to better understand the physicochemical properties and their correlation with catalytic performance of the samples in the HDO process. Indeed, our NiZr2O/Gr-n present the best activity/selectivity balance and it is deemed as a promising catalyst to conduct the H2-free HDO reaction. The catalyst reached commendable conversion levels and selectivity to mono-oxygenated compounds considering the very challenging reaction conditions. This innovative HDO approach provides a new avenue for cost-effective biomass upgrading.\n               "}
{"full_text": "The utilization of natural gas has become more challenging as many resources contain high amounts of CO2. The Natuna Sea is the largest natural gas resource in Indonesia, with 46 TSCF proven reserves consisting of 71\u00a0mol% CO2 and 28\u00a0mol% of CH4 [1]. To date, the natural gas reserve in the Natuna Sea has not been utilized because of its higher CO2 content compared to other natural gas reserves [2,3]. Dry reforming of methane (DRM) is considered a promising technology for converting natural gas reserves into synthesis gas (syngas), as it utilizes methane and CO2 [4]. In the chemical industry, syngas is an important intermediate because it is a source of hydrogen and raw material required to produce various chemical compounds [5\u201310]. Syngas produced from the DRM is supposed to have a H2/CO ratio close to 1, which is suitable for producing oxygenated chemicals and hydrocarbons through the Fischer-Tropsch process [11\u201314]. The syngas can then be converted into various products such as wax, olefin, alcohol, and dimethyl ether [15,16]. The main reaction in the DRM is as follows:\n\n(1)\n\n\n\nCH\n4\n\n+\n\nCO\n2\n\n\u21cc\n2\nCO\n+\n2\n\nH\n2\n\n\n\u0394\n\n\n\nH\no\n\n\n298\n\n=\n247\n\n\nk\nJ\n\n\nm\no\nl\n\n\n\n\n\n\nAs shown in equation (1), DRM is an endothermic reaction; thus, a high operating temperature is required to achieve high equilibrium conversion [17]. According to Wang et\u00a0al. (1996), 870\u20131,040\u00a0\u00b0C is the optimum temperature range to minimize catalyst deactivation caused by carbon formation [18].However, one of the major challenges in commercializing the DRM is severe catalyst deactivation due to sintering and carbon deposition [19]. Sintering occurs in the active phase of the catalyst because of the high operating temperature required to achieve high conversion in the DRM [20]. Meanwhile, solid carbon deposition on the catalyst surface is caused by methane cracking (Eq. (5)) and the Boudouard reaction (Eq. (6)) [21,22]. Thus, it is important to develop a catalyst formulation with good activity and less carbon formation.\n\n(2)\n\n\n\nCH\n4\n\n\u2192\n\u00a0C\n+\n2\n\nH\n2\n\n\n\u0394\n\n\n\nH\no\n\n\n298\n\n=\n75\n\n\nk\nJ\n\n\nm\no\nl\n\n\n\n\n\n\n\n\n(3)\n\n\n2\nCO\n\u2192\n\u00a0C\n+\n\nCO\n2\n\n\n\u0394\n\n\n\nH\no\n\n\n298\n\n=\n\u2212\n172,4\n\n\nk\nJ\n\n\nm\no\nl\n\n\n\n\n\n\nOther side reactions that affect the H2/CO ratio and reactant conversion are steam reforming (4), reverse water gas shift (5), and carbon gasification (6) [22,23].\n\n(4)\n\n\n\nCH\n4\n\n+\n\nH\n2\n\nO\n\u21cc\n2\nCO\n+\n3\n\nH\n2\n\n\n\u0394\n\n\n\nH\no\n\n\n298\n\n=\n228\n\n\nk\nJ\n\n\nm\no\nl\n\n\n\n\n\n\n\n\n(5)\n\n\n\nCO\n2\n\n+\n\nH\n2\n\n\u21cc\n\u00a0CO\n+\n\nH\n2\n\nO\n\n\u0394\n\n\n\nH\no\n\n\n298\n\n=\n41\n\n\nk\nJ\n\n\nm\no\nl\n\n\n\n\n\n\n\n\n(6)\n\n\nC\n+\n\nH\n2\n\nO\n\u21cc\n\u00a0CO\n+\n\nH\n2\n\n\n\u0394\n\n\n\nH\no\n\n\n298\n\n=\n131\n\n\nk\nJ\n\n\nm\no\nl\n\n\n\n\n\n\nThe reverse water gas shift reaction, which simultaneously occurs with the DRM, causes the H2/CO ratio to decrease, and the CO2 conversion becomes higher than the CH4 conversion [24].Previous researchers have investigated the utilization of several noble and non-noble metals such as Pd, Pt, Ru, Co, and Ni for use as active phase in catalysts [25\u201331]. However, the use of noble metals as the active phase is not economically attractive because of their scarcity and high price [32]. Nickel-based catalysts are more appealing because nickel is more abundant in nature and is cheaper [33]. However, nickel is rapidly deactivated because of the coking and sintering phenomena [34,35].The support material also plays a vital role in catalyst activity and carbon resistance [36]. Gamma-alumina (\u03b3-Al2O3) is commonly examined as a catalyst support, but it can deteriorate at high temperatures and can undergo a phase change to \u03b1-Al2O3 [37]. Zeolite and other mesoporous materials such as KIT-6, SBA-15, and MCM-41 have also been developed as catalyst supports because of their high surface area and good thermal stability [38\u201341]. MCM-41 is a mesoporous material with a pore diameter of 2\u20134\u00a0nm, uniform 2-dimensional hexagonal structure, and surface area of \u223c1000\u00a0m2/g to allow for high metal dispersion [27,42]. Several studies have shown that a nickel-based catalyst supported by MCM-41 needs further improvement to enhance its activity and suppress coke formation. Fakeeha et\u00a0al. (2019) reported that an MCM-41-supported catalyst yielded a CH4 conversion of 48%. Furthermore, CO2-Temperature Programmed Desorption (CO2-TPD) showed that the number of base sites in the Ni/MCM-41 catalyst was smaller than that in other materials (Al2O3, SiO2, and SBA-15). Thus, it can be concluded that the adsorption of CO2 during the reaction is inhibited because of the fewer base sites. Hence, modification is needed to add some base sites to the Ni/MCM-41 catalyst so that its activity and stability could be enhanced. The number of base sites of the catalyst can be increased by adding a promoter [43].Ibrahim et\u00a0al. (2018) compared the effects of adding Cs, Ce, Gd, Sc, and Ga promoters to the Ni/MCM-41 catalyst. The experiment showed that catalysts with Ga, Gd, and Ce promoters yielded higher CH4 and CO2 conversions due to the additional metal active sites from these promoters. Meanwhile, Thermogravimetric Analysis (TGA) revealed that the Ni/MCM-41 catalyst with a promoter also showed less carbon formation than did the catalyst without any promoter [30]. Finally, Al-fatesh et\u00a0al. (2019) examined the effect of adding a Gd promoter to the Ni/MCM-41 catalyst. Research has shown that Gd-promoted catalysts yield 80.6% CH4 conversion and 87.9% CO2 conversion, with negligible carbon formation [44].Another potential material that can be used to enhance catalyst activity and stability is the base promoter [45]. According to Nikolaos and Thessaloniki (2018), the base promoter can enhance the adsorption of CO2 and suppress the sintering of the catalyst [46]. Jeong et\u00a0al. (2006) examined the effect of adding Mg, Mn, K, and Ca as promoters to Ni/HY catalysts. The Ni\u2013Mg/HY catalyst activity test resulted in more than 85% CH4 conversion without catalyst deactivation for 72\u00a0h. This was because MgOx generated from Mg covered the nickel surface to prevent the agglomeration of nickel [47]. Horiuchi et\u00a0al. (1996) found that the addition of Mg, K, Ca, and Na to a Ni/\u03b3-Al2O3 catalyst enhanced the adsorption of CO2, thereby reducing the amount of carbon formation [48]. The novelty of this research is the combination of MCM-41 as catalyst support and Mg, Ca, Na, and K as catalyst promoter for conducting DRM. This study aims to examine the effect of several base promoters (Mg, Ca, Na, and K) on the activity of MCM-41-supported nickel catalysts for the DRM and on the amount of carbon deposition.MCM-41 powder was purchased from XFNano Material, China (surface area: 1014.9\u00a0m2/g). Ni(NO3)2.6H2O (Merck, \u223c99%) was dissolved in water and then mixed with either Mg(NO3)2\u00b76H2O (Merck, 99.9%), Ca(NO3)2\u00b74H2O (Merck, 99.9%), or KNO3 (Merck, 99.9%), or NaNO3 (Merck, 99.9%) depending on the type of catalyst promoter. The amount of metal promoter loading was 1\u00a0wt%. For instance, to prepare 3\u00a0g of 5\u00a0wt% Ni-1 wt% Mg/MCM-41 catalyst, 0.74\u00a0g Ni(NO3)2\u00b76H2O was dissolved in 2.93\u00a0mL of water and mixed with 0.32\u00a0g\u00a0Mg(NO3)2\u00b76H2O. After impregnation, the catalysts were dried at room temperature and calcined in air at 700\u00a0\u00b0C (heating rate 5\u00a0\u00a0\u00b0C/min) for 4\u00a0h.X-ray diffraction (XRD) patterns were acquired using a Bruker D8 Advance with Cu K\u03b11 (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5) radiation at 40\u00a0kV and 35\u00a0mA, meanwhile low angle XRD patterns were obtained using X-Ray Scattering Shimadzu SAG-6 (\u03bb\u00a0=\u00a01.54\u00a0\u00c5). N2 physisorption was conducted using a Micromeritics Tristar II instrument. Before the measurement, the samples were degassed at 250\u00a0\u00b0C for 3\u00a0h. A Micromeritics Chemisorb 2750 instrument was used to carry out CO2-TPD analysis. For TPD measurements, 150\u00a0mg of the sample was heated to 200\u00a0\u00b0C and then flushed with helium for 1\u00a0h. CO2 adsorption was conducted at 50\u00a0\u00b0C for 30\u00a0min using a 40\u00a0mL/min mixture gas of 5% CO2/95% He. H2-Temperature Programmed Reduction (H2-TPR) was conducted using Micromeritics Chemisorb 1750. Prior to the measurement, the samples were heated to 150\u00a0\u00b0C for 1\u00a0h using helium at a flow rate of 40\u00a0mL/min. Afterward, the samples were cooled to room temperature. The line system purging was performed for 30\u00a0min, and then the samples were reduced at 800\u00a0\u00b0C (heating rate 10\u00a0\u00a0\u00b0C/min) with 40\u00a0mL/min of 5% H2/95% Ar. TGA analysis was carried out using a TA-60WS Shimadzu instrument to measure the amount of carbon deposition. For this measurement, 15\u00a0mg of the sample was heated to 900\u00a0\u00b0C at a heating rate of 20\u00a0\u00a0\u00b0C/min.The activity test was conducted in a fixed-bed reactor at atmospheric pressure and 700\u00a0\u00b0C. First, 60\u00a0mg of the catalyst was loaded into the reactor, and the system was purged with nitrogen at 400\u00a0\u00b0C for 1\u00a0h. Afterward, the catalyst was reduced in situ at 700\u00a0\u00b0C and at atmospheric pressure for 1\u00a0h, and then, the system was flushed with nitrogen. Feed consisting of mixed gas was introduced at a flow rate of 60\u00a0mL/min (Gas Hourly Space Velocity\u00a0=\u00a060,000 mL/g-cat.h) and at a CH4: CO2: N2 ratio of 1:1:1. The product from the reaction was analyzed by Shimadzu Gas Chromatography with PQ and MS columns. The catalyst activity and stability were investigated at 700\u00a0\u00b0C, atmospheric pressure, and time on stream (TOS) of 240\u00a0min. A schematic of the equipment used for activity testing is shown in Fig.\u00a01\n.The conversion of CH4 and CO2, as well as selectivity of H2 and CO, and H2/CO ratio were calculated based on the following formulas.\n\n(7)\n\n\n\nCH\n4\n\n\u00a0Conversion\n=\n\n\n\nF\n\n\nCH\n\n4\n,\n\n\nin\n\n\n\u2212\n\nF\n\n\nCH\n4\n\n,\n\u00a0out\n\n\n\n\nF\n\n\nCH\n\n4\n,\n\n\nin\n\n\n\n\nx\u00a0\n100\n%\n\n\n\n\n\n\n(8)\n\n\n\nCO\n2\n\n\u00a0Conversion\n=\n\n\n\nF\n\n\nCO\n\n2\n,\n\n\nin\n\n\n\u2212\n\nF\n\n\nCO\n2\n\n,\n\u00a0out\n\n\n\n\nF\n\n\nCO\n\n2\n,\n\n\nin\n\n\n\n\nx\u00a0\n100\n%\n\n\n\n\n\n\n(9)\n\n\n\nYield\u00a0H\n2\n\n=\n\n\nF\n\n\nH\n2\n\n,\n\u00a0out\n\n\n\n2\n\nF\n\n\nCH\n\n4\n,\n\n\nin\n\n\n\n\n\nx\u00a0\n100\n%\n\n\n\n\n\n\n(10)\n\n\nYield\u00a0CO\n=\n\n\nF\n\nCO\n,\n\u00a0out\n\n\n\n\nF\n\n\nCH\n\n4\n,\n\n\nin\n\n\n+\n\nF\n\n\nCO\n\n2\n,\n\n\nin\n\n\n\n\n\nx\u00a0\n100\n%\n\n\n\n\n\n\n(11)\n\n\n\n\nH\n2\n\nCO\n\n\u00a0Ratio\n=\n\n\n\nF\n\n\nH\n2\n\n,\n\u00a0out\n\n\n\nF\n\nCO\n,\n\u00a0out\n\n\n\n\n\n\n\nThe synthesized catalysts were characterized by XRD analysis to determine their crystalline phase. Fig.\u00a02\n shows the XRD pattern of the catalysts.It can be seen from Fig.\u00a02a that diffraction peaks appear at 2\u03b8 of 37.3\u00b0, 43.3\u00b0, 62.8\u00b0, 75.4\u00b0, and 79.4\u00b0. According to JCPDS no. 47-1049, those peaks correspond to the crystalline NiO phase. The NiO phases in these catalysts are (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) [49]. The crystallite size of NiO was determined using the Scherrer equation. The crystallite sizes for 5\u00a0wt% Ni/MCM-41, 5\u00a0wt% Ni\u2013Mg/MCM-41, 5\u00a0wt% Ni\u2013Ca/MCM-41, 5\u00a0wt% Ni\u2013Na/MCM- 41, and 5\u00a0wt% Ni\u2013K/MCM-41 were 5.507, 6.288, 6.645, 4.190, and 4.901\u00a0nm, respectively. Furthermore, peaks at 2\u03b8\u00a0=\u00a023\u00b0 showed the amorphous phase of MCM-41 [50,51]. The addition of a base promoter other than Na and K does not result in a significant difference in the crystalline structure, which means the promoter oxide is well-dispersed in the catalyst [51]. However, as shown in Fig.\u00a02, there was peak broadening in the case of Ni\u2013Na/MCM-41 and Ni\u2013K/MCM-41 because of the formation of small crystalline particles in the catalysts, as described by Contreras and Fuentes (2012) [52].Moreover, it can be inferred from Fig.\u00a02b that in calcined MCM-41 sample, there is peak ranging from 2\u03b8\u00a0=\u00a02\u20133\u00b0 that indicates the diffraction patterns of typical MCM-41 material [57]. However, in other catalysts, mainly in Ni\u2013Na/MCM-41 and Ni\u2013K/MCM-41, the diffraction peak is not clearly seen. Since the diffraction peak of MCM-41 is not observed, it is indicated that the structure of MCM-41 is collapsed.N2 physisorption was performed to determine surface area and pore size. As shown in Fig.\u00a03\n, the catalysts showed type IV isotherms, and capillary condensation occurred in the high P/P0 region [13]. In addition, the catalysts displayed an H1-type hysteresis loop and exhibited cylindrical geometry with uniform pore size [44]. As shown in Fig.\u00a03, there was a significant amount of nitrogen uptake in the relative pressure range of 0.3\u20130.4 in MCM-41 support. This presented an indication of micropores and mesopores in MCM-41 [53]. The surface area and pore volume of the catalysts are shown in Table 1\n. Additionally, Table 1 shows there was a significant decrease in the surface area, particularly for the Ni\u2013Na/MCM-41 and Ni\u2013K/MCM-41 catalysts. This phenomenon was mainly caused by sintering, which causes the catalyst support to collapse, thereby causing a dramatic decrease in the surface area [54]. Sintering can be caused by the presence of water inside the pores of the support [54].Furthermore, pore size distribution of the MCM-41-based catalysts is shown in Fig.\u00a04\n. It can be seen that Ni/MCM-41 has pore diameter between 1.5 and 8.6\u00a0nm, with average pore diameter of 2.64\u00a0nm. Moreover, Ni\u2013Mg/MCM-41 has pore diameter between 1.5 and 58.76\u00a0nm average pore diameter: 5.98\u00a0nm and Ni\u2013Ca/MCM-41 has pore diameter between 2.7 and 74\u00a0nm (average pore diameter: 9.65\u00a0nm). On the other hand, Ni\u2013Na/MCM-41 has pore diameter between 14.76 and 147.6\u00a0nm (average pore diameter: 13.4\u00a0nm) and Ni\u2013K/MCM-41 has pore diameter between 12.6 and 79.9\u00a0nm (average pore diameter: 6.69\u00a0nm).H2-TPR analysis was conducted to observe the reducibility of the catalyst. As shown in Fig.\u00a05\n, all catalysts show peaks in the high-temperature regions. Peaks that appeared at 350\u2013500\u00a0\u00b0C were related to the reduction of Ni2+ to Ni0, while peaks at 500\u2013800\u00a0\u00b0C, indicate a strong interaction between NiO and the catalyst support [13]. The addition of the base promoter affected the reducibility of the catalyst. Base promoter replenishment increases total H2 consumption, as shown in Table 2\n. The addition of the base promoter affected the reducibility of the catalyst. Base promoter replenishment increases total H2 consumption, as shown in Table 2. The base promoter formed metal oxide on the catalyst surface, such as MgOx, that made the active phase better dispersed on the catalyst support. Thus, it made the amount of H2 consumption to reduce the catalysts is higher than the catalyst without promoter.The CO2-TPD profiles depicting the basicity measurements of the catalysts are shown in Fig.\u00a06\n. From CO2-TPD analysis, the capacity of CO2 adsorption at the catalyst surface can be measured. Strong basic site is indicated by peaks at 50oC\u2013128\u00a0\u00b0C, while medium basic site is showed by peaks at 220oC\u2013360\u00a0\u00b0C, and peaks strong basic site is declared at 580.7\u2013780\u00a0\u00b0C [51]. As can be seen from Fig.\u00a06, all catalysts showed peaks in the temperature range 100\u2013150\u00a0\u00b0C, which indicating weak base sites on the MCM-41-based catalyst. Furthermore, the addition of Mg and Ca promoters produced higher peaks in regions 100\u2013150\u00a0\u00b0C and 250\u2013400\u00a0\u00b0C; thus, it is indicated that the addition of Mg and Ca promoters leads to higher CO2 adsorption capacity of the catalysts. The higher CO2 conversion of Ni\u2013Mg/MCM-41 and Ni\u2013Ca/MCM-41 during activity testing is due to the basicity aspect. Furthermore, it also can be seen that Ni\u2013Na/MCM-41 catalyst does not show any peak in temperature range 350\u2013800\u00a0\u00b0C. This finding is similar to that reported by Lovell et\u00a0al. (2014), for incorporating Na into Ni/MCM-41 catalyst does not promote significant increase of basicity [49]. On the other hand, Ni\u2013K/MCM-41 showed the same trend of CO2-TPD to other catalyst, but with different intensity.An activity test was conducted to observe the impact of catalyst type on conversion, selectivity, and yield of dry methane reforming. The activity test was conducted using a fixed bed reactor, with a time on stream of 240\u00a0min and at a temperature of 700\u00a0\u00b0C. The feed was a gas mixture consisting of CH4, CO2, and N2 at a ratio of 1:1:1. In this experiment, nitrogen was used as the internal standard and diluent. The activities of the synthesized catalysts were compared to those of commercial catalysts (methanation and steam reforming catalysts from fertilizer plants) in the DRM. The methanation catalyst consisted of 27.95\u00a0wt% nickel, 58.86\u00a0wt% alumina, 12.10\u00a0wt% calcium, 0.99\u00a0wt% silica, and 0.10\u00a0wt% ferrous. Meanwhile, the steam reforming catalyst is consisted of nickel and alumina. The results of the activity tests of the catalysts are shown in Fig.\u00a07\n.The methanation catalyst showed conversion decline during the first 30\u00a0min, and then there was no conversion of CH4 and CO2 in the 150th min. Therefore, catalyst deactivation, as shown by the methanation catalyst, is due to methane decomposition and Boudouard reaction. Fig.\u00a08\n and Table 3\n shows the TGA results for the amount of carbon deposition that leads to catalyst deactivation. Based on Fig.\u00a08, the use of a methanation catalyst for the DRM resulted in carbon deposition as much as 41\u00a0wt%. Furthermore, it can be concluded that the methanation catalyst leads to the reaction towards carbon formation. On the other hand, the MCM-41-based catalysts had less carbon deposition than the other catalysts.Moreover, steam reforming catalysts yielded CH4 and CO2 conversions of as much as 85% and 64%, respectively. Although the catalyst generated high reactant conversion, it also yields 30\u00a0wt% carbon deposition, and as can be seen from Fig.\u00a07, the H2 yield constantly decreased. Therefore, it is indicated that the steam reforming catalyst also directs the reaction toward carbon formation.From Fig.\u00a07, it can also be seen that all synthesized catalysts generated good stability for 240\u00a0min. No significant decrease in reactant conversion was observed during the reaction. Based on the thermodynamic analysis, the maximum conversion that can be obtained at 700\u00a0\u00b0C and atmospheric pressure for CH4 is 91.5%; meanwhile, the maximum conversion of CO2 is 66.3% [55].The catalyst that showed the best performance in converting CH4 was 5\u00a0wt% Ni\u2013Mg/MCM-41, which yielded 72% conversion, followed by 5\u00a0wt% Ni\u2013Ca/MCM-41, which yielded 69% conversion. These catalysts gave higher CH4 conversions than the unpromoted catalyst (62%). The good activity of the 5\u00a0wt% Ni\u2013Mg/MCM-41 catalyst can be explained by H2-TPR characterization results, where the 5\u00a0wt% Ni\u2013Mg/MCM-41 catalyst generates 3.146\u00a0mmol/g H2 consumption. According to Ibrahim et\u00a0al. (2018), high total hydrogen consumption is related to the number of active sites in the catalyst for the DRM. The more active sites in the catalyst, the higher is the conversion [51].The order of the synthesized catalysts that yielded the highest conversion of CO2 was 5\u00a0wt% Ni\u2013Ca/MCM-41 (55%)\u00a0>\u00a05\u00a0wt% Ni\u2013Mg/MCM-41 (54%)\u00a0>\u00a05\u00a0wt% Ni/MCM-41 (52%)\u00a0>\u00a05\u00a0wt% Ni\u2013K/MCM-41 (44%)\u00a0>\u00a05\u00a0wt% Ni\u2013Na/MCM-41 (35%). The CO2 conversion capability of the catalyst is related to the basicity of the catalyst, as a catalyst with stronger basicity can adsorb more CO2, resulting in higher CO2 conversion. Based on Fig.\u00a06, both Ni\u2013K/MCM-41 and Ni\u2013Na/MCM-41 has weak and medium base sites, and thus both catalysts yield lower CO2 conversion than the other synthesized catalysts. Furthermore, sintering phenomena on the catalysts that is indicated from N2 Physisorption analysis (mainly Ni\u2013K/MCM-41 and Ni\u2013Na/MCM-41) reduces the number of active sites in the catalysts and makes the support structure to collapse; hence, the activity of the catalyst is low. It can also be concluded that catalysts with Mg and Ca addition as promoters give a higher conversion of CH4 and CO2 than the 5\u00a0wt% Ni/MCM-41 catalyst due to its CO2 adsorption capacity that is indicated from CO2-TPD and also due to its well-dispersed active phase that is indicated from H2-TPR analysis. Moreover, in this study, the conversion of CO2 is higher than CH4 conversion. It is predicted that the presence of water that is produced from reverse water gas shift reaction (CO2+H2\n\n\n\n\u21cc\n\n CO\u00a0+\u00a0H2O) inside MCM-41 pores shifts the reaction towards steam reforming reaction (CH4+H2O \n\n\n\u21cc\n\n 2CO+3H2), so that the CH4 conversion is higher than the CO2 conversion. Furthermore, CH4 decomposition reaction occurs more dominantly at higher temperature, so that the amount of CH4 conversion is getting higher [30].\nFig.\u00a09\n displays the product yield and H2/CO ratio. From Fig.\u00a09a, it can be seen that 5\u00a0wt% Ni\u2013Mg/MCM-41 and 5\u00a0wt% Ni\u2013Ca/MCM-41 generated H2 yields of 45% and 40%, respectively. Those results are related to CH4 conversion obtained by both catalysts, as the higher CH4 conversion, the more H2 production. Furthermore, from Fig.\u00a09b it can be concluded that 5\u00a0wt% Ni\u2013Mg/MCM-41 and 5\u00a0wt% Ni\u2013Ca/MCM-41 also produced high, stable CO yield. Combination of H2 and CO yield resulting in H2/CO ratio, which can be seen in Fig.\u00a09c. From Fig.\u00a09c, 5\u00a0wt% Ni\u2013Mg/MCM-41 and 5\u00a0wt% Ni\u2013Ca/MCM-41 generated H2/CO ratios of 0.83 and 0.78, respectively [56]. On the other hand, the unpromoted 5\u00a0wt% Ni/MCM-41 catalyst generated an H2/CO ratio of 0.88. The result is almost the same as that obtained by Amin et\u00a0al. (2013), where the H2/CO value obtained was 0.83. Thus, all synthesized catalysts had a ratio of H2/CO less than 1. This indicates that the reverse water gas shift reaction (CO2+H2\u21ccCO\u00a0+\u00a0H2O) occurred dominantly [56]. If the reverse water gas shift reaction is dominant, H2 is consumed as a reactant; meanwhile, CO is generated as a product; therefore, CO is higher than H2, causing an H2/CO ratio of less than 1.The activity testing result in this study showed almost the same trend with Jeong et\u00a0al. (2006) that investigating the effect of Mg, Mn, K, and Ca as promoter to Ni/HY catalyst. The result was Ni\u2013Mg/HY gave the best performance (with CH4 conversion more than 85%, H2/CO ratio 0.97, and carbon deposition 18\u00a0wt%), followed by Ni\u2013Mn/HY, Ni\u2013Ca/HY, Ni/HY, and the last was Ni\u2013K/HY. The superior performance of Ni\u2013Mg/HY was caused by the presence of MgOx species covering the nickel surface, so that it prevented the nickel to agglomerate and the conversion of CH4 could be maximized [47].In this experiment, 5\u00a0wt% Ni\u2013Mg/MCM-41 was tested at two different values of WHSV: 60,000 mL/g-hour and 72,000 mL/g-hour. The results of the catalyst testing are shown in Fig.\u00a010\n. The catalyst tested at WHSV\u00a0=\u00a060.000 mL/g-h yielded higher values for both CH4 and CO2 conversions. Because of the lower WHSV, the contact time between the reactants and catalyst increases, so more reactants are converted. The experiment conducted by Ibrahim et\u00a0al. (2018) also generated the same result, as 5\u00a0wt% Ni+1\u00a0wt% Ga/MCM-41 tested at WHSV\u00a0=\u00a039,000\u00a0mL\u00a0g\u22121\u00a0h\u22121 yields higher conversion than the catalyst tested at WHSV\u00a0=\u00a078,000\u00a0mL\u00a0g\u22121\u00a0h\u22121 [51].Ni/MCM-41, Ni\u2013Mg/MCM-41, Ni\u2013Ca/MCM-41, Ni\u2013Na/MCM-41, and Ni\u2013K/MCM-41 catalysts were successfully synthesized. XRD characterization showed that the size of NiO crystallite is 4,19 nm\u20136,65\u00a0nm. N2 physisorption results showed a decrease in the surface area and pore volume due to pore blockage. The sharp drop that occurred in Ni\u2013Na/MCM-41 and Ni\u2013K/MCM-41 was caused by severe sintering, so that the support was collapsed. H2-TPR results showed that the addition of base promoters strengthened the interaction between NiO and MCM-41. CO2-TPD results showed that the addition of Mg and Ca promoters increased the CO2 adsorption capacity of the catalyst. Furthermore, activity tests on Ni/MCM-41, Ni\u2013Mg/MCM-41, Ni\u2013Ca/MCM-41, Ni\u2013Na/MCM-41, and Ni\u2013K/MCM-41 yielded CH4 conversions of 62%, 72%, 69%, 36%, and 46%, respectively, with corresponding CO2 conversion rates of 52%, 54%, 55%, 35%, and 44%. Ni\u2013Mg/MCM-41 and Ni\u2013Ca/MCM-41 produced the highest H2 and CO yield. All catalysts yielded good stability for 240\u00a0min. On the other hand, the commercial catalyst for methanation showed activity for only 140\u00a0min, with carbon deposition as much as 41\u00a0wt%. This phenomenon indicates that the methanation catalyst shifts the reaction toward carbon formation. Steam reforming commercial catalysts yield CH4 and CO2 conversions of as much as 85% and 65%, respectively; however, there was a decrease in the H2 yield with time, and the carbon deposition is 31\u00a0wt%. This indicates that the steam reforming catalyst also shifts the reaction toward carbon formation. In this study, the effect of WHSV on the activity of the 5% Ni\u2013Mg/MCM-41 catalyst was investigated. The catalyst tested at WHSV\u00a0=\u00a060.000\u00a0mL\u00a0g\u22121\u00a0h\u22121 showed higher activity than the catalyst tested at WHSV\u00a0=\u00a072.000\u00a0mL\u00a0g\u22121. h\u22121. A catalyst with a lower WHSV has more space time, and thus, it leads to higher conversion.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the Ministry of Education, Culture, Research, and Technology of the Republic of Indonesia through the research grant of Konsorsium Riset Unggulan Perguruan Tinggi 2021 [grant number 304/IT1.B07.1/SPP-LPPM/VII/2021]; and partially supported by Indonesia endowment fund for education (LPDP), the Ministry of Finance of Indonesia [grant number RISPRO/KI/BI/KOM/II/16507/I/2020].", "descript": "\n                  Dry reforming of methane (DRM) is considered a promising reforming technology that converts natural gas in the Natuna Sea into synthesis gas, which can be further utilized to produce beneficial chemicals such as olefins, alcohols, and liquid hydrocarbons. However, the challenges in commercializing the DRM process are carbon deposition and sintering of the catalyst at high temperatures, because of which the catalyst is easily deactivated. This study aimed to test the activity and stability of MCM-41-based catalysts for the DRM; determine the effect of promoter type on the activity and stability of MCM-41-based catalysts; and determine the effect of base promoter addition on the amount of carbon deposition. MCM-41-based catalysts were synthesized using incipient wetness impregnation method. XRD, N2 Physisorption, H2-TPR, CO2-TPD, and TGA analysis were conducted to determine the physicochemical properties of the catalysts. The catalysts activity was tested in a fixed-bed reactor, under atmospheric pressure at 700\u00a0\u00b0C. Overall, all catalysts exhibited good stability for 240\u00a0min. Moreover, catalysts with Mg and Ca promoters showed the highest CH4 and CO2 conversion among all catalysts. Ni\u2013Mg/MCM-41 catalyst yielded 72% CH4 conversion and 54% CO2 conversion, meanwhile Ni\u2013Ca/MCM-41 yielded 69% CH4 conversion and 55% CO2 conversion. Furthermore, MCM-41-based catalysts with base promoter produced small amount of carbon deposition.\n               "}
{"full_text": "\nReducing greenhouse gas emissions and the consumption of fossil fuels are necessary to limit the ongoing climate change. Hence, renewable energy systems such as wind or solar power plants are a suitable solution to provide sustainable energy. One drawback of these technologies is the weather dependency. To overcome the weather dependency, energy storages and high efficient on demand power supply is needed. Hydrogen seems to be a promising energy storage but higher volumetric energy densities are often advantageous. Therefore, hydrogen based energy storage media with high volumetric energy density such as ammonia and hydrocarbons are used. These fuels can be later reformed into hydrogen rich gas compositions. Today, most reforming reactors are used to gain hydrogen from different fuels or hydrogen carriers. For example, the reforming of carbon-based fuels now accounts for more than three quarters of global H2 production\u00a0[1]. However, the reformed hydrogen rich gases have to be converted into heat or electric energy. One possibility for high efficient on demand heat and electric energy supply are solid oxide fuel cell (SOFC) systems\u00a0[2]. In previous research it was found, that SOFC systems are able to use a variety of hydrogen based storage media such as (i) ammonia\u00a0[3], (ii) methane\u00a0[4,5], (iii) carbon monoxide\u00a0[6], (iv) gasified biomass\u00a0[7\u20139] or (v) synthetic liquid hydrocarbons\u00a0[10\u201312]. These fuels can be produced by supplying solid oxide electrolysis cells and fuel post processing methods with green energy, water and CO2\u00a0[13,14]. Further, the reversible operation of solid oxide cells, operation in electrolysis and fuel cell mode, were successfully applied\u00a0[15,16]. However, if SOFC-systems are fuelled with carbonaceous fuels, a fuel reformer upstream of the SOFC can be useful. Reforming of methane upstream of the SOFC for example, reduces thermal stresses within SOFCs if compared to direct internal reformed methane\u00a0[17]. Direct internal reforming refers to steam reforming taking place at the SOFCs anode. The reduction of thermal stresses along SOFCs can prevent leakages and breaking of the few hundred \n\n\u03bc\nm\n\n thick cells. Thermal stresses are induced by endothermic steam reforming of hydrocarbons at the SOFC gas inlet and by exothermic oxidation reactions along the SOFC\u00a0[17]. Further, direct internal reforming of other fuels, especially long chain hydrocarbons can lead to rapid degradation and failure of the SOFC\u00a0[12,18]. Due to thermal stresses and rapid degradation, external reforming of hydrocarbons upstream of the SOFC is often necessary. Most used external reforming processes are, (i) steam reforming\u00a0[19], (ii) partial oxidation\u00a0[20], (iii) auto-thermal reforming\u00a0[19,21] and (iv) dry reforming\u00a0[22]. However, inadequate operating conditions or local effects caused by reaction kinetics, non-uniform temperature distribution and non-uniform fuel mixing can lead to degradation of catalysts even at safe operating conditions\u00a0[23\u201325]. The degradation is caused by reducing chemical reactive surfaces. Latter leads to a reduction of the reactivity of the catalyst and lower fuel conversion rates. Degradation can even lead to a total system failure and catalyst destruction. Degradation mechanisms of Ni catalysts are coking\u00a0[26,27], oxidation of Ni\u00a0[26], poisoning due to sulphur\u00a0[28\u201330], chlorine and other impurities\u00a0[9,29,30]. If such degradation effects are detected at an early stage, countermeasures can be applied. These countermeasures can limit further damage of the catalyst or even reverse degradation. Still, no direct degradation online monitoring of Ni-based catalysts based on electrochemical impedance spectroscopy (EIS) measurements is known in the field of reforming carbonaceous gases. One study used a radio frequency-based-method for in situ coke detection\u00a0[31]. However, established practices to detect degradation are measuring temperature profiles and gas compositions along fixed bed reforming reactors\u00a0[28,32]. Several studies are available that address the field of sensor development to detect coking of reforming catalysts. The sensors which are based on the impedance measurement method consist of a catalytic active material which changes its conductivity due to the amount of carbon loaded on them. The catalytic active material is either especially manufactured as sensor\u00a0[33,34] or commercial catalysts are used as active sensor material\u00a0[35,36]. Both types of sensors, however, measure coking of the catalytic active sensor material instead coking of the used catalysts. Using sensors instead of direct monitoring of the catalyst does not allow to detect changes within the catalyst directly. In addition, the placement of the sensor is very important and often difficult. Especially for reforming processes like auto-thermal reforming, where different reactions (partial oxidation, steam reforming and water gas shift reaction) and degradation mechanisms can occur within short ranges. In this study, we applied EIS based online monitoring on operating commercial Ni catalysts. The EIS measurements are applied to monitor the catalysts during heat up, reduction and operation. Within this study, the catalysts are contacted in three different ways. This is done to determine influences and characteristics of the contacting method on the measurement results. To validate the EIS measurement results, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy and X-ray diffraction (XRD) measurements are conducted post mortem. Considering this, the main goal of this study is to find an additional online monitoring method which expands the knowledge of the behaviour of catalysts. Results achieved, such as observation of NiO reduction or carbon deposition, are first steps towards EIS based online monitoring of the state of health of catalysts. Furthermore, additional information about the processes occurring in the microstructure of the catalyst could be obtained in situ. Using EIS based measurements to monitor catalysts is also advantageous for commercial SOFC-reformer applications since SOFCs are already often monitored using EIS measurements\u00a0[16,37\u201339]. As a result, only one device would be necessary to monitor the chemical active parts of SOFC-reformer systems. The results of the impedance measurements could be implemented in diagnostic algorithms as proposed in\u00a0[40].In this section, the experimental setup, test preparation and the testing procedure are explained. The first part of this section is about preparing the commercial Ni catalysts to apply electrical measurement methods. Within the second part, the testing procedure is described and the third part is about the used test rig.Commercial available Ni based catalysts for steam reforming are used within this work. The catalyst consists of K promoted NiO on a Calcium-Aluminate support and is commercially available. To conduct electrical measurements, the catalysts have to be contacted to wires. We used silver wires with a diameter of \n\n1\n\nmm\n\n to connect the measurement device and the contacted catalyst. To contact the catalyst with silver wires, three different methods are used to investigate the influence of contacting on the measurement results, see Fig.\u00a01.The first contacting method, contacting with bare silver wires, is based on application of silver wires with a diameter of 3\u00a0mm. The ends of the silver wires are sanded to fit tight in the catalyst holes. After sanding, the prepared silver wires are stuck in opposite holes of the catalyst, see Fig.\u00a01(a). It was expected, that the different thermal expansion coefficients result in a sufficient contacting at reforming temperature.For the second contacting method, silver wires with silver ink, silver wires with a diameter of \n\n2\n.\n5\n\nmm\n\n are used. In addition, \n\n50\n\nmg\n\n of silver ink is applied to contact each silver wire with the catalyst. The silver ink is applied inside the holes, where the silver wires are placed, see Fig.\u00a01(b). After assembling the catalyst contacting, the silver ink is dried for \n\n1\n\nh\u00a0at\u00a0\n200\n\n\n\n\n\u2218\n\n\nC\n\n in air. The silver ink is purchased as Silver SOFC Ink from FuelCellStore\u00a9 and consists of \n\n70\n\nwt%\n\n Silver and \n\n30\n\nwt%\n\n Diethyl Glycol Ether Acetate.Connecting silver wires with Ni mesh and silver ink to the catalyst is the third contacting method. The silver ink is applied in the central hole of the catalyst, on its outer cylindrical surface and between the silver wires and Ni meshes, see Fig.\u00a01(c). The silver wire, which is fixed on the outer Ni mesh has a diameter of \n\n1\n\nmm\n\n. One end of this wire is flattened and laid between the catalyst and the Ni mesh. The silver wire placed inside the central hole has a diameter of \n\n3\n\nmm\n\n. The contacted catalysts are dried for \n\n1\n\nh\u00a0at\u00a0\n200\n\n\n\n\n\u2218\n\n\nC\n\n in air.In this work, the measurements of four test specimens (same Ni catalysts but different electrical contacting) are presented. They are chosen to give an impression of the impact on the measurement results and the scattering within a contacting method. The four test specimens include one specimen contacted with bare silver wires, two specimens contacted with silver wires and silver ink and one test specimen contacted with silver wires with Ni mesh and silver ink, see Table\u00a01.\n\nAfter preparing the catalysts as described in Section\u00a02.1, the catalysts are built into the reformer test rig. The catalysts are then heated up to target temperature (700\u00a0\u00b0C or 750\u00a0\u00b0C) under a constant N2 flow rate of \n\n2\n\nslpm\n\n (standard litre per minute). When the target temperature is reached, the reduction process is started. The catalysts are reduced in \n\n0\n.\n5\n\nslpm\n\n H2 and \n\n2\n\nslpm\n\n N2 for \n\n12\n\nh\n\n. After the reduction, the H2 flow is turned off. The catalysts are then loaded with coke by applying an automated \n\n0\n.\n4\n\nslpm\n\n CH4 pulse for \n\n20\n\ns\n\n, see Fig.\u00a02. The described CH4 pulse is repeated several times and between each CH4 pulse, an EIS measurement is conducted in N2 atmosphere. To avoid oxidation, the catalysts are supplied with \n\n2\n\nslpm\n\n N2 during the whole testing phase.\n\nThe used catalyst test rig for testing EIS measurements of Ni based catalysts consists of five main parts: a gas mixing unit, a tube furnace, a catalyst within a reformer, an impedance measurement device and a continuous off-gas analyser, see Fig.\u00a03.\n\nThe gas mixing unit is used to supply the catalyst with the required fuel mixture. For the tests within this work, only methane, nitrogen and hydrogen are used. The volume flow rates of each gas are controlled by mass flow controllers (MFCs), which are purchased by Voegtlin Instruments GmbH\u00a9\u00a0[41].The reformer is placed downstream of the gas mixing unit within a tube furnace. The tube furnace is purchased from Carbolite Gero GmbH & Co. KG\u00a9 (Carbolite Gero CTF 12/75/700)\u00a0[42]. The reformer consists of a stainless steel tube with a flange at the outlet. An alumina tube is placed within the reformer to isolate the catalyst and wires against the metal parts and prevent short circuits. The silver wires to contact the catalyst are led out radial between two flanges at the outlet of the reformer. To avoid an electrical contact between the silver wires and the flange, isolation material is used. The wires are isolated using glass fabric adhesive tape purchased from HORST\u00a9. In addition, isolating and slightly flexible sealing made out of mica or aramid is placed between the flanges. Upstream of the catalyst, a thermocouple type K (TCat) is placed to monitor the gas temperature.A gas analyser from ABB\u00a9 is located downstream of the reformer to monitor the off-gas composition. The gas analyser consists of a sample gas cooling unit (SCC-C), a sample gas feed unit (SCC-F) and a gas analyser module (AO2020)\u00a0[43]. The impedance was measured using either an impedance analyser from BioLogic\u00a9 (SP-150)\u00a0[44] or Gamry\u00a9 (Reference 3000)\u00a0[45]. The impedance measurements were carried out in potentiostatic mode with the parameters listed in Table\u00a02. The parameters were adapted for each of the three phases of the test procedure according to the expected rate of change. Due to fast changes during the heat up phase, the impedance is only measured at one frequency.All microscopic investigations were conducted by the Austrian Centre for Electron Microscopy and Nanoanalysis using a Zeiss Ultra 55.Within this section, the results observed during the catalysts monitoring, as well as post mortem analysis results are shown. Since the (i) heat up, (ii) reduction and (iii) carbon deposition of catalysts might have an impact on the catalyst performance, all of these processes were investigated. The post mortem analysis is shown in the first part to interpret the EIS measurement results. The results of the EIS measurements during the heat up process are discussed in the second part of this section. The reduction process is analysed in the third part and online monitoring of catalyst degradation due to carbon loading is presented in the fourth part.To gain further information and be able to interpret the EIS measurement results in a correct manner, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy and X-ray diffraction (XRD) is used. SEM in combination with EDS is used to investigate microstructural changes in the cross section of the catalyst. Raman spectroscopy identified the structure and relative quantity of carbon. XRD was used to gain information about the relative quantity of NiO and Ni on the catalysts surface.The applied test procedure which is described in Section\u00a02.2 leads to microstructural changes of the used Ni based catalysts. These changes are proofed by post mortem analysis and are used to interpret the EIS measurement results in the following sections. A sketch of the microstructural changes which are observed during the heat up, reduction and carbon deposition process is shown in Fig.\u00a04.\nThe catalyst, in its original state, consists of NiO on an alumina carrier. Additional, nano crystalline carbon and calcium aluminate is found, see Fig.\u00a05(a). In Fig.\u00a05, peaks P1 and P2 indicate calcium aluminate. Peaks P3, P4, P5 and P6 indicate graphite and peak P7 indicates O\u2013H groups. The Raman shift measurements revealed, that carbon is present on the original catalyst and the present carbon has a nano crystalline appearance. This nano crystalline carbon is not present on catalysts which were heated up in N2 atmosphere. Hence, it is assumed that nano crystalline carbon from the original state mainly reacts with NiO to Ni, CO and CO2, see Fig.\u00a04(a).\nTo determine the reduction state of the catalyst after the heat up and reduction process, see Figs.\u00a04(a) and 4(b), the relative surface quantities of NiO and Ni are measured with XRD. The XRD results of four different reduction states are shown in Fig.\u00a06. In Fig.\u00a06, peaks P1, P2, P5 and P6 indicate NiO and peaks P3, P4 and P7 indicate Ni. Fig.\u00a06a shows the original catalyst where all Ni is mainly present in form of NiO. Fig.\u00a06b shows a mixture of Ni and NiO although no reducing medium was used. The catalyst was only heated up and cooled down in N2 atmosphere. The presence of Ni on this catalyst indicates the self reducing effect caused by the reaction of nano crystalline carbon with NiO. Fig.\u00a06c shows a completely reduced catalyst without the presence of NiO at its surface. The reduction procedure used for the catalyst in Fig.\u00a06c is the standard reduction procedure within this work as described in Section\u00a02.2. Another reduction procedure with a lower H2 flow rate (\n\n0\n.\n2\n\nslpm\n\n H2) was tested and is shown in Fig.\u00a06d. This reduction procedure shows a lower ratio of NiO to Ni than in Fig.\u00a06b but a higher NiO to Ni ratio than in Fig.\u00a06c. It is visible, that the reduction process is not completed using \n\n0\n.\n2\n\nslpm\n\n H2 in \n\n2\n\nslpm\n\n N2 for \n\n12\n\nh\n\n, see Fig.\u00a06d.\nThe last step of the test procedure is the carbon deposition process, as described in Section\u00a02.2. The presence of carbon after the carbon deposition process and the absence of carbon after the reduction process are proven by Raman shift spectra, see Fig.\u00a05. In Fig.\u00a07, SEM and EDS of the original state of the catalyst and the state after the carbon deposition process are shown. The brighter parts of the EDS image (Fig.\u00a07(d)) correlate with the brighter parts of the SEM image (Fig.\u00a07(c)). The brighter parts of the SEM image are Ni and the brighter parts of the EDS image are carbon depositions, compare Figs.\u00a07(c) and 7(d). Hence, carbon seems to form at the catalytic active parts of the catalyst. Formation of solid carbon on the catalytic active parts of the catalyst leads to performance deterioration due to less catalytic active surface area.\nThe knowledge of the microstructure of catalysts completes the post mortem analysis. Hence, the cross-sections of catalysts at different states are compared in Fig.\u00a08. The original state of the catalyst shows a layer of NiO (bright part in Fig.\u00a08(a)) at the surface of the catalyst. The occurrence of NiO is confirmed by EDS and XRD. NiO is also found as a thin layer on the surface of the alumina support throughout the catalyst cross section (bright edges in Fig.\u00a08(a)). The appearance of NiO on the surface and the grey colour of the original catalyst leads to the assumption, that non-stoichiometric NiO with excess oxygen is present. This type of NiO has a higher conductivity than stoichiometric NiO. Although both types of NiO have a semiconductive behaviour\u00a0[46,47].\nThe cross section of a catalyst which was held at \n\n750\n\n\n\n\n\u2218\n\n\nC\n\n for \n\n60\n\nh\n\n is shown in Fig.\u00a08(b). The comparison of the SEM image in Figs.\u00a08(b) and 8(a) reveals a shrinking of Ni/NiO grains which could be explained by a transition of NiO grains to Ni grains. This reduction is observed in XRD, see Fig.\u00a06.The SEM of the catalyst, which is reduced for \n\n12\n\nh\n\n at \n\n0\n.\n5\n\nslpm\n\n H2, Fig.\u00a08(c), shows no clear differences compared to Fig.\u00a08(b). Only the XRD revealed, that nearly all NiO is reduced to Ni, see Fig.\u00a06. Fig.\u00a08(d) shows clear differences compared to Figs. 8(a)\u20138(c). The catalyst shown in this figure was loaded with carbon by applying \n\n16\n\n CH4 pulses as described in Section\u00a02.2. Fig.\u00a08(d) shows that the Ni surface layer of the catalyst diffused in the alumina carrier structure or eroded since there is no Ni layer visible on the surface as it is in Fig.\u00a08(a) to 8(c).EIS based online monitoring of catalysts depends on knowledge of the initial or not degraded state. Different absolute impedance values were measured after reducing the catalyst and before starting the carbon deposition tests. This made it difficult to compare the state of degradation of the catalysts. To overcome this problem and get a better understanding of factors influencing the EIS results, measurements are also conducted during the heat up and the reduction phase. Within this section, we show that contacting methods have a significant impact on the EIS results after and during the heat up process.\nFig.\u00a09 shows the absolute impedances and phase shifts at an AC frequency of \n\n1\n\n\n0\n\n\n5\n\n\n\nHz\n\n for the three contacting methods introduced in Section\u00a02.1. Contacting using bare silver wires is shown in Fig.\u00a09(a). A decreasing absolute impedance and increasing phase shift can be observed up to \n\n600\n\n\n\n\n\u2218\n\n\nC\n\n. A further increase of the temperature led to an increase of the absolute impedance and a decrease of the phase shift. This behaviour is only observed for contacting with bare silver wires. Hence it is assumed, that the resistance of the contacting is responsible for the shown characteristic. Figs.\u00a09(b) and 9(c) show the absolute impedances and phase shifts for contacting with silver wires and silver ink, applied to two catalysts. A steady decrease of the impedance and a steady increase of the phase shift is observed except the temperature range between \n\n400\n\n\n\n\n\u2218\n\n\nC\n\n and \n\n500\n\n\n\n\n\u2218\n\n\nC\n\n. Comparing both measurements with each other reveals relatively noisy and high absolute impedances in Fig.\u00a09(c). We assume, that although the contacting procedure was the same in Figs.\u00a09(b) and 9(c), slight differences due to the manual contacting can be seen, resulting in a higher contacting resistance in Fig.\u00a09(c). Finally, the measurement results for contacting with silver wires, Ni mesh and silver ink are shown in Fig.\u00a09(d). This contacting method shows the lowest absolute impedance values compared to other contacting methods. However, the tendencies of the absolute impedance and phase shift are similar to these of Fig.\u00a09(b). This leads to the conclusion, that trends can be observed even with higher contacting resistances. Nonetheless, the heat up procedure should be monitored to identify the initial state of the catalyst independent of the used contacting method. The lower absolute impedance observed with the silver wires, Ni mesh and silver ink combination could be attributed to a reduction of the measured distance through the catalyst and an increase in contact area (nickel mesh).\nAt the beginning of the heat up procedure, all Ni of the catalyst is mainly present in form of NiO, see Section\u00a03.1. These NiO grains have a semiconductive behaviour as long as the molar fraction of Ni in Ni\u2013NiO grains is lower \n\n20\n\nmol%\n\n. For higher yields of Ni in Ni\u2013NiO grains, a metallic conductive behaviour of the Ni\u2013NiO grains was observed by Tare et\u00a0al.\u00a0[47]. This change of semiconductive to metallic conductive behaviour during the reduction of NiO grains to Ni grains seems to explain the absolute impedance drop in Figs.\u00a09(b) and 9(d). Moreover, as mentioned above the catalyst used are partially reduced during the heat up process. This was proven by post mortem analysis carried out at both, the initial stage and after the heat up process, see Section\u00a03.1.The reduction process is the last step to prepare catalysts for the reforming process. During the reduction process, H2 is added to the N2 flow to reduce NiO to Ni. The reduction should cause a decrease in the catalysts resistance since Ni has a lower specific resistance than NiO. However, morphological changes of the catalyst can occur if temperature and gas composition changes. Hence, effects such as the shrinkage of the Ni\u2013NiO grains when they are reduced to Ni grains have to be taken into account. The shrinkage causes wider gaps between the Ni grains, see Fig.\u00a04. Due to this gap, the Ni grains act as capacitors which increases the overall absolute impedance of the catalyst. Further, a reduction process, which has already lead to a reduced resistance is observed during the heat up phase.During the reduction process of the catalysts, impedance spectra between \n\n1\n\n\n0\n\n\n\u2212\n1\n\n\n\nHz\n\n and \n\n1\n\n\n0\n\n\n6\n\n\n\nHz\n\n are measured, see Figs. 10(b), 10(d), 10(f) and 10(h). The absolute impedances and phase shifts at \n\n1\n\n\n0\n\n\n5\n\n\n\nHz\n\n over time are shown in Figs. 10(a), 10(c), 10(e) and 10(g). All test specimens show a slight decrease of the absolute impedance within the first one to two hours of the reduction process. The highest reduction of the impedance is observed for contacting with bare silver wires. After the initial reduction of the absolute impedance, a slight increase of the absolute impedance is observed. This increase seems to be related with the absolute impedance value at the beginning of the reduction process (\n\n0\n\nh\n\n). Smaller absolute impedance values show a relatively higher increase of the absolute impedance.\nIn Figs. 10(a), 10(c), 10(e) and 10(g) it is also observed, that the absolute impedance does not seem to be settled at a certain value, although no NiO is detected by the XRD after the reduction process. The steady increase of the impedance is especially visible at low absolute impedance values, see Figs.\u00a010(c) and 10(g). Hence, other hydration or reduction processes or Ni agglomeration might be observed in Figs. 10(a), 10(c), 10(e) and 10(g), as it is for SOFC anodes\u00a0[48]. Summarizing this part shows, that lower contacting resistances seems to increase the observability of microstructural changes within the catalyst or the electrical contacting of the catalyst.The measured impedance spectra between \n\n1\n\n\n0\n\n\n\u2212\n1\n\n\n\nHz\n\n and \n\n1\n\n\n0\n\n\n6\n\n\n\nHz\n\n are shown in Figs. 10(b), 10(d), 10(f) and 10(h). Within these figures, Bode plots of the EIS measurement results are shown for \n\n0\n\nh\n\n, \n\n6\n\nh\n\n and \n\n12\n\nh\n\n after the start of the reduction process. Catalysts contacted with bare silver wires, Fig.\u00a010(b), show a capacitive behaviour in the high frequency range with phase shifts up to \n\n\u2212\n90\n\u00b0\n\n. Catalysts contacted with silver wires and silver ink show different characteristics in the high frequency range comparing Figs.\u00a010(d) and 10(f). Fig.\u00a010(d) shows a characteristic close to an ohmic resistance but with a slight capacitive behaviour. The capacitive behaviour is especially visible for the measurement after \n\n6\n\nh\n\n, whereas a capacitive characteristic is visible in Fig.\u00a010(f) in the high frequency range. The catalyst contacted with silver wires, Ni mesh and silver ink is shown in Fig.\u00a010(h). The characteristic of this contacting method is clearly different to the others. Instead of a capacitive behaviour at high frequency ranges, an inductive behaviour is visible. Contacting with silver wires, Ni mesh and silver ink also shows a decreasing inductive behaviour along the reduction time. The summarized results from Figs. 10(b), 10(d), 10(f) and 10(h) show, that test specimen with a lower absolute impedances tend to a less capacitive behaviour. This leads to the conclusion, that the contacting resistance might not only reflect an ohmic resistance but a combination of a resistor and a capacitor. The influence of different contacting methods is visible within all measurements done during the heat up and reduction process. Therefore, this influence has to be taken into account for further measurements and monitoring during carbon deposition.The main objective of online monitoring is to detect degradation effects at early stages. Carbon deposition is one major degradation effect for catalysts used in syngas production. The performance deterioration of Ni catalysts through carbon deposition is caused by carbon covering catalytic active surfaces, blocking gas channels and causing Ni dusting. Carbon deposition on and between Ni grains should be detectable through decreasing electrical resistances, since carbon has a higher conductivity than Ni and is bridging gaps between the Ni grains.During the carbon deposition process, the impedance spectra are measured after each CH4 pulse, see Section\u00a02.2. This is done to measure the changes of the impedance caused by possible carbon deposition. Fig.\u00a011(a) shows the absolute impedance and phase shift at \n\n1\n\n\n0\n\n\n5\n\n\n\nHz\n\n over time for the test specimen \u201cCat. silver wires with silver ink, low resistance\u201d. It is observed, that the impedance decreases and the phase shift increases after every CH4 pulse. Such behaviour is linked with carbon deposition on the catalyst since carbon was found on the test specimen after the carbon deposition process, see Section\u00a03.1. The degradation of the catalyst is identified as a decrease of the methane cracking reaction (Eq.\u00a0(1)). Latter is caused by deposited carbon which leads to less catalytic active surface area of the catalyst. Due to less catalytic active surface area, a lower amount of CH4 is converted and the measured volumetric percentage of methane increases. \n\n(1)\n\n\n\n\nCH\n\n\n4\n\n\n\u2192\n2\n\n\n\nH\n\n\n2\n\n\n+\nC\n\n\n\n\nIn Fig.\u00a011(a) the decrease in methane cracking is observed through decreasing H2 and increasing CH4 peaks over time. The first methane pulse shows nearly the same volume fraction of CH4 and H2 in the dry off-gas. However, the following CH4 pulses show a decreasing volume fraction of H2 and an increasing volume fraction of CH4. The decrease of CH4 conversion behaves like a degressive function, which is indicated by the decrease of H2 peaks measured in the off-gas in Fig.\u00a011(a). The decreasing conversion rate of CH4 is caused by coking of the catalyst. The same degressive function of CH4 conversion caused by coking of the catalyst is observed in other studies. Franz et\u00a0al. and Li et\u00a0al. observed a decrease of CH4 conversion due to coking under dry-reforming conditions (\n\n\n\nCO\n\n\n2\n\n\n\n/\n\n\n\nCH\n\n\n4\n\n\n\n=\n\n1\n\n)\u00a0[22,49]. Their observed decrease of CH4 conversion also behaves like a degressive function. The same behaviour for the decrease of CH4 conversion is found under bi-reforming conditions and summarized by Mohanty et\u00a0al.\u00a0[50].\nIn Fig.\u00a011(a), the absolute impedance at \n\n1\n\n\n0\n\n\n5\n\n\n\nHz\n\n shows a decreasing rate of change for an increasing number of CH4 pulses which are interpreted as an increasing amount of deposited carbon. The absolute impedance value changed by \n\n80\n\n%\n\n after the first CH4 pulse, whereas the absolute impedance after the second CH4 pulse decreased by \n\n90\n\n%\n\n compared to the initial state or \n\n47\n\n%\n\n compared to the absolute impedance value after the first CH4 pulse. The measurements reveal, that even slight carbon deposition (the first CH4 pulses) has a significant impact on the impedance values as seen by the decrease of the absolute impedance value in Fig.\u00a011(a). The decrease of the absolute impedance is explained by higher conductivity of carbon compared to Ni and carbon or graphite particles bridging gaps between Ni-grains, see Fig.\u00a04(c). If carbon deposition is detected at early stages, countermeasures such as increasing the operating temperature, the amount of steam or oxygen could be applied. These countermeasures could lead to a prolonged lifetime of the catalyst.The impedance spectra of the test specimen \u201cCat. silver wires with silver ink, low resistance\u201d are shown as Bode plots in Fig.\u00a011(b). The Bode plots show a decreasing phase shift for frequencies above \n\n1\n\n\n0\n\n\n4\n\n\n\nHz\n\n at increasing carbon loading. Below \n\n1\n\n\n0\n\n\n4\n\n\n\nHz\n\n, the phase shift does not show changes for an increased carbon load. The absolute impedance decreases with increasing carbon load along the whole frequency spectra. A change of the absolute impedance characteristic is also observed at frequencies above \n\n1\n\n\n0\n\n\n5\n\n\n\nHz\n\n for increasing carbon loading. However, the decrease of the absolute impedance is the dominating aspect for monitoring carbon depositions. The same trends for the absolute impedance values and phase shift caused by carbon deposition on a Ni based impedance sensor were found by M\u00fcller et\u00a0al.\u00a0[51].The same evaluation as in Fig.\u00a011 is done in Fig.\u00a012 for the test specimen \u201cCat. silver wires with silver ink, high resistance\u201d to identify the influence of the contacting resistance. The absolute impedance at \n\n1\n\n\n0\n\n\n5\n\n\n\nHz\n\n over time shows higher changes for the test specimen \u201cCat. silver wires with silver ink, high resistance\u201d compared to \u201cCat. silver wires with silver ink, low resistance\u201d. The change of the absolute impedance after the first CH4 pulse is \n\n94\n\n%\n\n in Fig.\u00a012(a) and \n\n80\n\n%\n\n in Fig.\u00a011(a) compared to the respective initial state. Hence, it seems that higher contacting resistances are favourable for online monitoring of carbon deposition on Ni catalysts. Though, the higher sensitivity could be caused by local phenomenon such as graphite deposition between the silver wires and the catalyst. Thus, the measurement might not be representative for the whole catalyst and its state of health. However, the Bode plots of the measured absolute impedances show the same trends and characteristics for both contacting resistances, compare Figs.\u00a011(b) and 12(b). Even the absolute impedance after the 16th CH4 pulse is nearly identical for the test specimen in Figs.\u00a011 and 12.\nDuring the measurements conducted to test specimen \u201cCat. silver wires with silver ink, high resistance\u201d the 7th and 8th CH4 pulse were skipped, see Fig.\u00a012(a). The impedance spectra, however, were measured at the same time steps as previous measurements. This was done to prove that a steady state is reached between CH4 pulses. These two additional measurements between two CH4 pulses show nearly the same absolute impedances and phase shifts. The maximum deviation of the absolute impedance value of these three measurements is within a range of \n\n\u00b1\n\n1\n.\n3\n\n\n% of their mean value. This suggests, that carbon deposition and no other phenomenon is observed since carbon deposition increases the conductivity of the catalyst due to its high conductivity. Further, the deposited carbon creates electrical connections of Ni-grains which also increases the conductivity of the catalyst. It should be mentioned, that the first CH4 pulse is not visible in the dry off gas composition of Fig.\u00a012(a) since an error at the gas pump occurred.This study demonstrated applicability of EIS-based online monitoring tools to identify degradation mechanisms that occur in commercial Ni-based reforming catalysts, which was presented for the first time. We investigated this measurement method to expand the knowledge of online monitoring methods for commercial Ni-based catalysts since no EIS based online monitoring method for commercial Ni based catalysts were available in literature.By applying EIS measurements during the heat up and reduction of the catalyst, it was possible to determine microstructural changes such as NiO reduction by dropping absolute impedance values and set the impedance value for the not degraded condition of each test specimen. Further, three different contacting methods were tested and their influence on the measurement results is presented. Differences of the ohmic resistance of more than \n\n1\n\n\n0\n\n\n2\n\n\n\n\u03a9\n\n between the contacting methods were observed after the heat up procedure. Nevertheless, it was possible to observe catalyst degradation due to carbon deposition even with higher contacting resistances. Even after a short period of carbon loading, changes in the absolute impedance values up to \n\n94\n\n\n% of the initial values were measured. However, to detect other microstructural changes such as NiO reduction, it might be necessary to choose a contacting method with a lower contacting resistance, e.g.\u00a0contacting with silver wires with Ni mesh and silver ink.These initial tests and findings lay the foundation for online monitoring of catalysts, especially but not only for SOFC-reformer systems. This measurement methodology could further be used to gain more insights in the reforming processes at laboratory scale and help finding safe and high efficient operating conditions. Within our future research, we are going to apply the measurement methodology to real small scale reformer applications and even to small scale SOFC-reformer systems.\nThe following abbreviations are used in this manuscript:\n\n\n\n\n\n\n\n\nAC\nAlternating current\n\n\nDC\nDirect current\n\n\nEDS\nEnergy dispersive X-ray spectroscopy\n\n\nEIS\nElectrochemical impedance spectroscopy\n\n\nMFC\nMass flow controller\n\n\nSEM\nScanning electron microscopy\n\n\nslpm\nstandard litre per minute\n\n\nSOFC\nSolid oxide fuel cell\n\n\nXRD\nX-ray diffraction\n\n\nZ\nImpedance\n\n\n\n\n\n\nMichael H\u00f6ber: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Data curation, Writing \u2013 original draft, Visualization, Project administration. Philipp Wachter: Methodology, Investigation, Resources. Benjamin K\u00f6nigshofer: Investigation, Resources. Felix M\u00fctter: Investigation, Visualization. Hartmuth Schr\u00f6ttner: Investigation, Data curation. Christoph Hochenauer: Supervision, Project administration, Funding acquisition. Vanja Suboti\u0107: Conceptualization, Methodology, Resources, Writing \u2013 review & editing, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This project has been funded by partners of the ERA-Net SES 2018 joint call RegSys (www.eranet-smartenergysystems.eu) - a network of 30 national and regional RTD funding agencies of 23 European countries. As such, this project has received funding from the European Union\u2019s Horizon 2020 research and innovation programme under grant agreement no. 775970. The authors gratefully acknowledge the funding of this project entitled \u201cAGRO-SOFC\u201d (Grant No. 872299) by The Austrian Research Promotion Agency (FFG)\n . We also want to mention that this work is done within the research initiative \u201cNachhaltige Personen- und G\u00fctermobilit\u00e4t\u201d.", "descript": "\n                  More than \n                        \n                           75\n                           \n                           %\n                        \n                      of today\u2019s H2 production is based on reforming processes using heterogeneous catalysts. In addition, catalysts are needed for hydrogen generation from renewable resources such as biomass or biogas. However, no direct online monitoring of commercial Ni based catalysts is established. Catalysts are only monitored indirectly by measuring gas compositions, temperature profiles or using coke sensors, although direct online monitoring could detect degradation mechanisms at early stages. We demonstrate the methodology for electrochemical impedance spectroscopy based online monitoring of commercial Ni catalysts. Furthermore, we studied the impact of three different contacting methods of Ni catalysts with ohmic resistances between \n                        \n                           10\n                           \n                           \u03a9\n                        \n                      and \n                        \n                           1\n                           \n                              \n                                 0\n                              \n                              \n                                 5\n                              \n                           \n                           \n                           \u03a9\n                        \n                      after the heat up procedure on the measurement results. Monitoring of the heat up phase revealed, that choosing the right contacting method is essential to observe processes such as NiO reduction, whereas monitoring of degradation due to carbon loading was observed with every tested contacting method. The demonstrated online monitoring of catalysts could be used to find and maintain more efficient and stable reforming conditions. In addition, the gained knowledge could even be used to prolong the lifetime of catalysts by in situ adapting of operating conditions.\n               "}
{"full_text": "Data will be made available on request.The increase in crude oil prices and its negative impact on the environment such as air pollution, ozone depletion, and climate change has led to the growing interest in the use of renewable and less-pollutant resources [1,2]. Synthetic gas (or syngas), a mixture of CO and H2 is recognized as an environmentally friendly alternative energy source in recent years and it can be directly used as a fuel source for electricity generation and transport fuel [3,4]. Commonly, syngas is produced through partial oxidation of methane [5], methane steam or dry reforming [6,7], oxidative methane steam or dry reforming [8,9], ethanol steam or dry reforming [10,11] and oxidative ethanol steam reforming [12]. However, methane from natural gas is not a renewable source and thus its availability is limited. There is a growing interest in the use of ethanol among biomass-derived feedstocks [13]. Compared to other feedstocks such as, glycerol, ethanol offers low toxicity, ease of production in large quantities, relatively high hydrogen content and it is free from sulfur-containing compounds [14]. Ethanol can be produced either by fermenting sugar or starch (first generation) or hydrolysing lignocellulose and fermenting it (second generation) [15]. There have been many studies conducted on reforming processes using both non-noble (Ni-based catalysts) and noble metal (Pt and Rh) catalysts to produce syngas. Osaze et al. studied the effect of temperature from 923 to 1023\u2009K over 10 %Ni/SBA-15 catalyst on the performance of methane dry reforming and found that when temperature increased both CH4 and CO2 conversions raised about 83.4 % and 59 %, respectively due to endothermic nature of methane dry reforming [16]. However, Ni catalysts are currently faced with the challenge of early deactivation caused by the coke formation at lower temperatures [17]. In addition, cobalt-based catalysts are also used to produce syngas from oxidative ethanol steam reforming due to their high activity, stability, and low-cost alternative to noble metals [18,19]. Pereira et al. investigated the catalytic behavior and regeneration processes of oxidative ethanol steam reforming over Co/SiO2, Co\u2013Rh/SiO2, and Co\u2013Ru/SiO2 catalysts. By using oxidative treatment, CoRh/SiO2 and CoRu/SiO2 catalysts were activated, resulting in higher ethanol conversion and hydrogen selectivity after regeneration [20]. Sukri et al. also studied the effect of cobalt loading (Co=10 %, 15 %, 20 % and 25 %) over Co/MgO catalysts in methane dry reforming and found that the 10 %Co catalyst showed good activity, stability, the highest CH4 and CO2 conversions, and the lowest rate of carbon deposition at 750\u2009\u00b0C [21]. Thus, a new, and environmentally more positive approach is oxidative ethanol dry reforming (OEDR) (cf. Eq. (1)), which converts CO2 greenhouse gas and produces value-added synthesis gas.\n\n(1)\n\n\n3\n\n\nC\n\n\n2\n\n\n\n\nH\n\n\n5\n\n\nOH\n+\nC\n\n\nO\n\n\n2\n\n\n+\n\n\nO\n\n\n2\n\n\n\u2192\n7\nCO \n+\n9\n\n\nH\n\n\n2\n\n\n\n\n\n\u0394\n\n\nH\n\n\n298\nK\n\n\n0\n\n\n=\n325.3\nkJ\nmo\n\n\nl\n\n\n\u2212\n1\n\n\n\n\n\n\n\n\n\nTo the best of our knowledge, none of the available studies have explored oxidative ethanol dry reforming over Co/Al2O3 catalyst. Therefore, the objective of this was the chemical and physical characteristics of 10 %Co/Al2O3 catalyst in addition to investigating the effect of reaction temperature on the activity and selectivity of OEDR reaction.The wet impregnation method was used to impregnate 10 % (by weight, metallic) cobalt on alumina [21]. To ensure thermal stability, an adequate amount of puralox alumina (SCCa\u2010150/200 procured from Sasol, Hamburg, Germany) was calcined for 5\u2009h at 1023\u2009K in a Carbolite (Bemaford, Sheffield, UK) furnace with air and a heating rate of 5\u2009K\u2009min\u22121. An aqueous solution of Co(NO3)3.6\u2009H2O was supplied and magnetically stirred for 3\u2009h with pretreated \u03b3-Al2O3 support in am ambient environment (Sigma\u2010 Aldrich, St. Louis, Missouri). The mixture was dried at 383\u2009K for 24\u2009h. Moreover, it was calcined in air with a heating rate of 5\u2009K\u2009min\u22121 and kept at constant temperature of 773\u2009K for 5\u2009h. Post crushing and sieving, the catalyst was introduced into a fixed-bed reactor with a particle size between 125 and 160\u2009\u00b5m.Micromeritics ASAP-2020 (Norcross, Georgia) at 77\u2009K was used to measure Brunauer-Emmett-Teller (BET) surface areas for 10 %Co/Al2O3 catalyst and \u03b3-Al2O3 support. During BET measurement, the example was degassed for 1\u2009h at 573\u2009K in N2 flow to remove moisture and volatile contaminants. Rigaku Miniflex II (Akishima\u2010shi, Tokyo, Japan) X-ray diffraction system was utilized to study the crystal structure of \u03b3-Al2O3 support and 10 %Co/Al2O3 catalyst at 30\u2009kV and 15\u2009mA and Cu target was used as a source of radiation (wavelength, \u03bb of 1.5418\u2009\u00c5). Diffraction patterns were scanned from 3\u00b0 to 80\u00b0 with an imaging speed of 1\u00b0 min\u22121 and a step size of 0.02\u00b0 to obtain high-resolution X-ray diffractograms. A software tool (Match! version 2.3.3) was used to measure all X-ray patterns. A micromeritics AutoChem II-2920 apparatus was used for both alumina and 10 %Co/Al2O3 catalyst to conduct the H2-TPR experiment. The U-tube of quartz was loaded with 0.1\u2009g of sample and sandwiched with quartz wool. As an initial treatment, the sample was heated to 373\u2009K under 50\u2009ml\u2009min\u22121 in He flow for 30\u2009min to remove volatile compounds from the sample. Following this, the temperature of the sample was increased to 1173\u2009K and kept at the constant temperature for 30\u2009min under 50\u2009ml\u2009min\u22121 10 %H2/Ar mixture. The amount of carbon accumulated on the spent specimen surface after OEDR, temperature-programmed oxidation (TPO) was measured using a thermogravimetric analyzer (TGA Q500, TA Instruments, New Castle, Delaware). During TPO, the catalyst was preheated to 373\u2009K (heating rate 10\u2009K\u2009min\u22121) for 30\u2009min under N2 (100\u2009ml\u2009min\u22121) atmosphere. Thereafter, the temperature was increased from 373 to 1023\u2009K (10\u2009K\u2009min\u22121 ramping rate) under 3\u2009N2:1\u2009O2 flow. Under N2 atmosphere, the sample was cooled to ambient temperature and was isothermally heated. Isothermal heating of the sample was carried out for 30\u2009min and the sample had to be cooled with N2 to reach ambient temperature. Micromeritics AutoChem II-2920 chemisorption system was utilized to determine both catalyst and support acidic properties. Before each measurement, approximately 0.1\u2009g of the sample was pretreated at 773\u2009K for 1\u2009h at 50\u2009ml\u2009min\u22121 under He flow to eliminate moisture and physisorbed compounds. The sample was cooled to 423\u2009K under inert atmosphere after reduction in situ. Thereafter, adsorption was performed for 30\u2009min at the same temperature in 50\u2009ml\u2009min\u22121 of 10 %H2/Ar. The NH3 molecules in the gas phase were removed by purging with He gas for 30\u2009min at 423\u2009K after 1\u2009h of adsorption using 5 % NH3 in He balance. As part of the purging process at the same temperature with He gas for 30\u2009min, NH3 molecules were removed from the gas phase by heating at 1073\u2009K (heating rate 10\u2009K\u2009min\u22121) for 10\u2009min. Thermal conductivity detectors (TCD) were used to measure the quantity of desorbed NH3 gas entering the U-tube from the outlet.A quartz tube reactor having an outer diameter of 3/8\u2009in. and length of 17\u2009in. was used to conduct OEDR experiments. This reactor was placed vertically within a split tubular furnace (LT furnace) during the experiments with stoichiometrically set to 3:1:1 for C2H5OH: CO2:O2 and temperatures between 773 and 973\u2009K under atmospheric pressure. OEDR was performed on the catalyst by reducing it to 973\u2009K with 50 % H2/N2 (60\u2009ml\u2009min\u22121) with heating at a rate of 10 Kmin\u22121 for 2\u2009h before the reaction. The quartz tube reactor was filled with approximately 0.1\u2009gcat of the catalyst surrounded by a layer of quartz wool. In this experiment, KellyMed KL602 syringe pump (Beijing, China) and Alicat mass flow controller (Tucson, Arizona) were employed to ensure that ethanol and gas (viz, CO2, O2 reactant and N2 diluent) were accurately fed to the top of the reactor. The gas hourly space velocity (GHSV) was calculated as 42\u2009L\u2009gcat\n\u22121 h\u22121 for each reaction. To obtain the intrinsic catalytic activity, high GHSV, small catalyst loadings, and tiny particle sizes were selected in order to ensure negligible mass and heat transfer resistances. The detailed calculation is included in the supplementary information for avoiding the mass and heat transfer intrusions. To maintain the 70\u2009ml\u2009min\u22121 flow rate, N2 was used as a tie component. As part of the analysis, a gas chromatograph (GC) from the Agilent 6890 Series (Agilent, Santa Clara, California) fitted with FID and TCD detectors to determine the composition of the gaseous effluent. The carbon balance is calculated by dividing the total moles of carbon in the products with the total moles of carbon reacted. The carbon mass balance was carried out for each run of the reaction, and it was greater than 91.3 %\u221298.8 %, confirming their remarkable resilience toward coke deposition during the OEDR.The \u03b3-Al2O3 support and 10 %Co/Al2O3 catalyst were examined for their textural characteristics, such as BET surface area, average pore volume, and pore diameter. It was observed that the \u03b3-Al2O3 support had a relatively BET area of 175.2\u2009m2 g\u22121, an average pore volume of 0.46\u2009cm3 g\u22121, and a pore diameter of 10.7\u2009nm. However, the surface area, pore-volume, and pore size of the 10 %Co/Al2O3 catalyst were smaller having values of 143.1\u2009m2 g\u22121, 0.36\u2009cm3 g\u22121 and 10.6\u2009nm, respectively. This could possibly be due to the introduction of Co oxides onto the \u03b3-Al2O3 support surface.\n\nFig. 1 displays the comparison of fresh and spent XRD profiles of 10 %Co/Al2O3 catalyst and the calcined \u03b3-Al2O3 support. The Joint Committee on Powder Diffraction Standards database was utilized to obtain a qualitative interpretation of the crystalline phase present in all specimens [22]. The \u03b3-Al2O3 phase peaks at 2\u03b8 of 18.92\u00ba, 32.88\u00ba, 37.10\u00ba, 45.61\u00ba, and 67.17\u00ba was detected on fresh 10 %Co/Al2O3 catalyst (JCPDS card number: 04\u20130858) see Fig. 1(a). Furthermore, the spinel CoAl2O4 phase was observed at 2\u03b8 of 59.51\u00ba and 65.38\u00ba (JCPDS card number: 82\u20132246) over 10 %Co/Al2O3 catalyst. This was due to strong metal support interaction between Al2O3 and CoO, resulting in the formation of CoAl2O4 (see Fig. 1(b) and (c)) [23]. However, CoAl2O4 form was also observed on spent specimens (see Fig. 1(c)). As a result, it would be expected that the low peak intensity and absence of 2\u03b8 =\u200965.38\u00b0 would indicate that the lower amount of CoAl2O4 phase on the spent catalyst than the fresh catalyst could be due to the reduction of H2 to Co0 during activation. The XRD patterns of spent 10 %Co/Al2O3 catalyst after the OEDR at \n\n\nP\n\n\nC\n\n\nO\n\n\n2\n\n\n\n\n=\n\n\nP\n\n\n\n\nO\n\n\n2\n\n\n\n\n=\u20095 kPa,\n\n\nP\n\n\n\n\nC\n\n\n2\n\n\n\n\nH\n\n\n5\n\n\nO\nH\n\n\n=\u200915, and 973\u2009K is shown in Fig. 1(c). In both fresh and spent samples, Co3O4 phase was detected at 2\u03b8 of 31.45\u00ba, 37.10\u00ba, and 44.79\u00ba (JCPDS card number: 74\u20132120) see Fig. 1(b) and (c). However, the presence of the Co3O4 phase on the spent catalyst indicates that the Co0 metallic phase was unavoidably re-oxidized during the OEDR process due to the catalyst being sufficiently reduced in H2. Based on a diffractogram of the spent catalyst, the first broad peak centered around 2\u03b8 of 26.38\u00ba can be attributed to graphitic carbon (JCPDS card number: 75\u20130444) that is likely to have formed during the decomposition of ethanol and cracking of CH4 intermediate at a high temperature [24]. Additionally, a new peak was observed on spent catalyst at 2\u03b8 of 51.50\u00ba (JCPDS card number: 15\u20130806) can be attributed to the Co phase [25,26]. Consequently, the stability of the catalytic performance can be attributed to the maintenance of the active metal phase after the OEDR process.The H2-TPR method was performed to investigate the reducibility of catalyst and support. According to \nFig. 2(a), the H2-TPR analysis of calcined \u03b3-Al2O3 did not indicate any reduction peaks and it was stable and did not reduce in response to H2. Furthermore, three significant peaks (P1, P2, and P3) were observed on 10 %Co/Al2O3 catalyst surface (Fig. 2(b)). P1 at temperatures between 458 and 720\u2009K was due to the reduction of Co3O4 into intermediate CoO (cf. Eq. 2), while P2 at temperatures between 743 and 765\u2009K corresponds to the reduction of CoO into metallic Co0 (cf. Eq. 3) [27]. Moreover, another shoulder peak (P3) was observed at temperatures between 766 and 1014\u2009K. This is attributed to the reduction of the spinel CoAl2O4 phase into the metallic Co0 phase [28] (see Eq. 4).\n\n(2)\n\n\n\n\nCo\n\n\n3\n\n\n\n\nO\n\n\n4\n\n\n+\n\n\nH\n\n\n2\n\n\n\u2192\n3\nCoO\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n(3)\n\n\nCoO\n+\n\n\nH\n\n\n2\n\n\n\u2192\nCo\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n(4)\n\n\nCo\n\n\nAl\n\n\n2\n\n\n\n\nO\n\n\n4\n\n\n+\n\n\nH\n\n\n2\n\n\n\u2192\nCo\n+\n\n\nAl\n\n\n2\n\n\n\n\nO\n\n\n3\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\nIn addition, Papageridis et al. [29] have also revealed that, due to high calcination temperatures, Co2+ ions migrate into the lattice of Al2O3 support and persist in tetrahedral positions in spinel CoAl2O4. As a result, CoO and Al2O3 interact strongly in CoAl2O4 species, which can produce a strong resistance to H2 reduction.\n\nFig. 3 shows a measurement of the NH3-TPD over \u03b3-Al2O3 support and 10 %Co/Al2O3 catalyst. The \u03b3-Al2O3 support and 10 %Co/Al2O3 catalyst exhibit weak, medium, and strong acid sites for different desorption temperatures ranging from 423 to 570\u2009K, 571\u2013710\u2009K, and 721\u20131026\u2009K, respectively [30,31]. Consequently, the strong acid sites possess a higher NH3 desorption temperature than 713\u2009K and is likely that they correspond to Br\u00f8nsted acid sites. However, while the weak and medium acid sites possess a lower NH3 desorption temperature, indicating the presence of Lewis and/or Br\u00f8nsted acids sites [32]. According to Fig. 3, the \u03b3-Al2O3 support contains three different acid centres, resulting in an overall NH3 uptake of 4.77\u2009mmol NH3 gcat\n\u22121. Adding Co metal to \u03b3-Al2O3 significantly improved the NH3 uptake from 4.77 to 6.89\u2009mmol NH3 gcat\n\u22121 (about 44.4 %). Based on this observation, it is possible that an extra acid site is formed at the interface between the Co metal and \u03b3-Al2O3 support. Cheng et al. [33] reported that the adding Co to the calcined support increased acid site concentration and increased strong acid site concentration. According to this observation, some weak acid sites were replaced during thermal activation by impregnating Co species, resulting in strong acid sites. Thus, the catalytically active site may be protonated and likely located at the interface between the metal and alumina support.In terms of carbon formation on a surface, it is well known that the acidity of the surface is a significant factor, whether the surface is the catalyst or the support. The formation of carbon is accelerated by positively charged acidic sites on a surface due to acidic sites catalyzing the cracking reaction. Gamma alumina is generally used as a support material during the reforming process, and its acidic properties facilitate carbon formation [34,35].This study examined the effect of reaction temperature over 10 %Co/Al2O3 catalyst with stoichiometric amounts of \n\n\nP\n\n\nC\n\n\nO\n\n\n2\n\n\n\n\n=\n\n\nP\n\n\n\n\nO\n\n\n2\n\n\n\n\n=\u20095 kPa, and\n\n\nP\n\n\n\n\nC\n\n\n2\n\n\n\n\nH\n\n\n5\n\n\nO\nH\n\n\n=\u200915 kPa. The study was conducted within a temperature range of 773 and 973\u2009K under atmospheric pressure. As illustrated in \nFig. 4, temperature increase from 773 to 973\u2009K resulted in increased conversions of C2H5OH and CO2 by 22.5\u201393.6 % and 16.9\u201352.8 %, respectively. This observation can be attributed to the ethanol decomposition reaction (see Eq. (5)) [36].\n\n(5)\n\n\n\n\nC\n\n\n2\n\n\n\n\nH\n\n\n5\n\n\nOH\n\n\u2192\nCO\n+\n\n\n\nH\n\n\n2\n\n\n\n+\n\n\n\nCH\n\n\n4\n\n\n\n\n\n\n(\n\n\n\u0394\nH\n\n\n298K\n\n\n0\n\n\n\n=\n\n50.1\n\n\n\nkJmol\n\n\n-\n1\n\n\n)\n\n\n\n\nThe reason for the enhanced performance of C2H5OH conversion rather than CO2 conversion is the presence of side reactions with reasonable decomposition of ethanol and dehydrogenation. The significant conversion of C2H5OH over CO2 was due to the numerous dehydrogenation and ethanol decomposition side reactions [37]. Furthermore, the addition of O2 during the reforming reaction suppresses carbon formation and decreases the required heat, resulting in an exothermic reaction [38].\n\nFig. 5 illustrates the yields of CO, H2 and CH4 as a function of temperature at \n\n\nP\n\n\nC\n\n\nO\n\n\n2\n\n\n\n\n =\u2009\n\n\nP\n\n\n\n\nO\n\n\n2\n\n\n\n\n =\u20095 kPa, and\n\n\nP\n\n\n\n\nC\n\n\n2\n\n\n\n\nH\n\n\n5\n\n\nO\nH\n\n\n =\u200915 kPa. With an increase in temperature from 773\u2009K to 973\u2009K, the yield of both products (H2 and CO) increased from 16.0 % to 68.1 % and 13.5\u201358.3 %, respectively. Increasing the temperature resulted in an increase in both H2 and CO, which is consistent with the endothermic nature of Eq. (1). On the other hand, CH4 yield also increased with rising reaction temperature (see Fig. 5). This indicates that during the C2H5OH decomposition (see Eq. (5)), CH4 production rate was higher than the CH4 reforming rate (reforming of CH4 by CO2 to produce syngas). Besides, this may indicate the successful conversion of ethanol into syngas [39]. As Bartholomew previously reported, the increase in CH4 yield with reaction temperature may be due to lower carbon deposition (methane dehydrogenation) [40]. Moreover, O2 as a reactant decreased the amount of carbon deposition during the OEDR reaction while improving the stability of the catalytic reaction for a long period of time.The CH4/CO and H2/CO ratios are determined by varying the reaction temperature at \n\n\nP\n\n\nC\n\n\nO\n\n\n2\n\n\n\n\n=\u2009\n\n\nP\n\n\n\n\nO\n\n\n2\n\n\n\n\n=\u20095 kPa and =\u200915 kPa in \nFig. 6. Increasing reaction temperature resulted in a linear increase of H2/CO ratio from 1.2 to 1.5, indicating an improved C2H5OH dehydrogenation reaction [41]. As the reaction temperature increased, CH4/CO ratio improved. It indicates that the rate of dry reforming of CH4 was lower than the rate of C2H5OH decomposition. Alongside, the preferred CO/H2 ratio is less than 2 and can be used as feedstocks in Fischer-Tropsch synthesis to produce green fuels [42].\n\nTable 1 shows the summary of the evaluation of the 10 %Co/Al2O3 catalyst for OEDR, as well as other catalysts recently used in the oxidative steam reforming (OSR) reaction. Based on the results shown in Table 1, the 10 %Co/Al2O3 catalyst exhibited relatively comparable conversion of C2H5OH and H2 selectivity during the OEDR runs when compared with other Co-based and noble-based catalysts in the literature. Even though the 10 %Co/Al2O3 catalyst in this study has a slightly lower activity than noble metal catalysts, from a practical and economic standpoint, it would be a useful catalyst for large-scale syngas production via OEDR.TPO measurements were used to determine the amount of carbon deposition on the surface of the spent 10 %Co/Al2O3 catalyst. \nFig. 7 shows the TPO results for the weight percentage of the spent sample. The spent 10 %Co/Al2O3 catalyst deposited the least amount of carbon (28,92 %) at 973\u2009K. Nevertheless, the reaction temperature decreased from 973 to 773\u2009K, and the amount of carbon deposition improved by 41.48 %. This demonstrates quicker deposition of carbon on the catalyst surface. As shown in Figs. 4 and 7, the trend of carbon weight vs. temperature curve is opposite to that of CO2 and C2H5OH conversions, further indicating that the catalytic activity improved via the oxidization of carbonaceous deposition. On the other hand, XRD analysis also showed that graphitic carbon was present on the surface of the spent catalyst (see Fig. 1(c)). Ruckenstein and Wang also reported that the stability of Co/\u03b3-Al2O3 catalysts with several Co loadings and calcination temperature (6\u2009wt. % for Tc =500\u2009\u00b0C and 9\u2009wt. % for Tc =1000\u2009\u00b0C) exhibited stable activity. However, catalysts with high Co loadings (above 12\u2009wt. %) accumulated significant amounts of carbon during reforming and demonstrated deactivation [51]. Thus, the reduction of carbon deposited on the catalyst surface resulted in a higher conversion of C2H5OH and CO2.The present study describes the OEDR for syngas production over Co/Al2O3 catalyst at various reaction temperatures. The catalyst design consists of 10\u2009wt % Co and \u03b3-Al2O3 support with a high specific surface area, which can prevent the sintering impact. OEDR allows the active metal phase of the catalyst to be maintained during the catalytic process, which contributes to a stable catalytic performance. The interaction between CoO and Al2O3 can produce CoAl2O4 species, and these compounds exhibit strong resistance to H2 reduction. The level of NH3 uptake was increased significantly from 4.77 to 6.89\u2009mmol NH3 gcat\n\u22121, resulting in the formation of extra acid sites at the interface of the Co metal and \u03b3-Al2O3 support. The catalyst displays high performance for oxidative ethanol dry reforming to generate synthesis gas. Thus, it is a suitable candidate to be used as a fuel for internal combustion engines and as a chemical feedstock for the production of ammonia and methanol. According to tests conducted under various reaction temperatures, the conversion of C2H5OH and CO2 increased with an increase in reaction temperature and decreased with a decrease in reaction temperature. Further, the addition of oxygen to the feed gas enhances the production of H2, CO, and CH4 while at the same time limiting the accumulation of carbon.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the financial support from the Universiti Malaysia Pahang (UMP) Research Grant Scheme (RDU130376). Fahim Fayaz would like to thank Dr. Dai-Viet N. Vo for his invaluable guidance and support during the research work. Fahim Fayaz is also grateful for the funds received from the Institute of International Education\u2019s Scholar Rescue Fund (IIE-SRF) and the Finnish National Agency for Education (EDUFI) for supporting his postdoctoral fellowship at Tampere University.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mtcomm.2023.105671.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Till date, oxidative ethanol steam reforming use Ni-based catalysts to produce syngas. However, Ni catalysts suffer from easy deactivation due to the coke formation at low temperatures. Therefore, oxidative ethanol dry reforming is a promising method and was investigated over 10 %Co/Al2O3 catalyst due to their high activity and stability to produce high-quality syngas. More importantly, the syngas can be upgraded to produce liquid biofuels and chemicals. The catalyst was evaluated in a quartz fixed-bed reactor under atmospheric pressure at \n                        \n                           \n                              P\n                           \n                           \n                              C\n                              \n                                 \n                                    O\n                                 \n                                 \n                                    2\n                                 \n                              \n                           \n                        \n                      =\n                        \n                           \n                              P\n                           \n                           \n                              \n                                 \n                                    O\n                                 \n                                 \n                                    2\n                                 \n                              \n                           \n                        \n                     =\u20095 kPa, \n                        \n                           \n                              P\n                           \n                           \n                              \n                                 \n                                    C\n                                 \n                                 \n                                    2\n                                 \n                              \n                              \n                                 \n                                    H\n                                 \n                                 \n                                    5\n                                 \n                              \n                              O\n                              H\n                           \n                        \n                      =\u200915 kPa, with reaction temperature ranging between 773 and 973\u2009K. The \u03b3-Al2O3 support and 10 %Co/Al2O3 catalyst had BET surface areas of 175.2 m2 g\u22121 and 143.1\u2009m2 g\u22121, respectively. Co3O4 and spinel CoAl2O4 phases were detected through X-ray diffraction measurements on the 10 %Co/Al2O3 catalyst surface. H2-TPR measurements indicate that the 10 %Co/Al2O3 catalyst was completely reduced at a temperature beyond 1000\u2009K. NH3-TPD measurements indicated the presence of the weak, medium, and strong acid sites on the \u03b3-Al2O3 support and 10 %Co/Al2O3 catalyst. Due to increased reaction temperature from 773 to 973\u2009K, C2H5OH and CO2 conversions improved from 22.5 % to 93.6 % and 16.9\u201352.8 %, respectively. Additionally, the optimal yield of H2 and CO obtained at 68.1 % and 58.3 %, respectively. Temperature-programmed oxidation experiments indicated that the amount of carbon deposition was the lowest (28,92 %) at 973\u2009K and increased by 41.48 % at 773\u2009K.\n               "}
{"full_text": "Brunauer\u2013Emmett\u2013TellerBarret\u2013Joyner\u2013Halendaaverage pore diameterFourier transform infraredspecific surface area obtained by BET methodspecific surface area of microporesThermal conductivity detectorThermogravimetric AnalysisTemperature programmed desorptionpore volumeMicropore volumeBiomass as a renewable energy source provides decreasing of the emitted CO2 amount. Based on statics, globally, 170 billion metric tons of biomass source is available from which significant waste is left behind [1]. The products resulted in pyrolysis and gasification of different biomass can be used as an alternative fuel, blending component, besides its value-added conversation the products can reach the appropriate quality. In 2020, the emitted carbon dioxide was 31.5\u00a0Gt, which - based on the statics - will reach 37\u00a0Gt in 2023. It is important to note, that the predicted value can be reduced to 28.5\u201330\u00a0Gt by the use of sustainable and renewable materials [2\u20135].The synthesis gas, sourced by biomass gasification is one of the most important intermediary components in the chemical industry. Based on the origin of the biomass, the gas product contain different concentrations of hydrogen, carbon monoxide, carbon dioxide, methane, hydrocarbons with low molecular weight, sulphur, nitrogen and chlorine-containing compounds. Henceforward, purifying of the synthesis gas is an important issue, mainly for CO2 capture [6\u20138].During biomass pyrolysis-gasification, numerous reactions take place, therefore the composition of the gaseous product are mainly affected by them. E.g. the equilibrium reactions can be shifted to the appropriate composition with the use of steam and catalysts. Nickel-containing catalysts are commonly used owing to their high efficiency in cracking of the C\u2013H, C\u2013C and C\u2013O bonds, their low price and their regeneration availability. Besides nickel, the ruthenium and the rhenium-containing catalysts can be listed, however, their price cannot be considered economic friendly, especially in scale-up. Regarding the catalyst support, the ZSM-5, HZSM-5, Al2O3, Al2O3\u2013SiO2 are the most commonly used. Henceforward, the CaO and the natural zeolites should be perspective materials, owing to their particle size and efficiency in not only gasification processes, but in-situ carbon capture. Based on the literature, the CaO, as well as the natural zeolites, can be impregnated promoting the carbon capture processes and increasing the yield of the gaseous product [9\u201312].Wang et\u00a0al. investigated the effects of the Ni/Al2O3 and Ni/CaO during high-temperature biomass gasification [9]. They demonstrated, that the catalyst with CaO carrier can increased the gas yield, the hydrogen yield, however, decreased the amount of hydrocarbons with low molecular weight [9]. W.X. Peng et\u00a0al. concluded, that the residue of high-temperature gasification of wood residue was significantly decreased in the presence of Ni/Al2O3 and Ni/Ce/Al2O3 catalysts, but higher gas yield was found using Ni/Ce/Al2O3 catalyst [13]. It is important to mention that the H2/CO ratio was not significantly affected by the different amounts of the catalysts during the measurement [13]. Higher gas and hydrogen yield was also reported by Yue Chai et\u00a0al. with the nickel content of Ni\u2013CaO\u2013C catalyst during biomass and plastic waste co-pyrolysis [14]. It is important to note that there was no significant change in the quality of the gaseous products over nickel content of 10%. Afterwards, the regenerated catalysts can be reused, but with decreased efficiency [15]. Shang et\u00a0al. investigated the regeneration and cycle stability of Ni modified Zr-MOF catalyst during biomass gasification, where the agglomeration of nickel was indicated after several reuse. It was observed, that the catalytic performance of regenerated catalyst was slightly increased [16]. It was also concluded, that the zeolite catalysts had partially lost their activity due to several regeneration cycles using pinewood sample [17].This work aims to investigate the pyrolysis-gasification of biomass with maximizing of the gaseous product and syngas, and simultaneously the carbon dioxide capture. The main goal of the experiments was to compare the effect of the CaO with zeolites in biomass gasification and CO2 adsorption. Since the regeneration and the cycling reuse of the catalyst are proved to be a research gap, the efficiency of the catalysts during ten regeneration cycles was also investigated from an economic point of view.Crashed (<3\u00a0mm) and dried form of maize biomass waste (roots, leaves, stems, corn stalk) was used as raw material. Biomass sample has 36.5% carbon, 5.2% hydrogen, 0.9% nitrogen and 57.4% oxygen content.During the experiments four different types of catalysts were used, namely are Ni/ZSM-5, Ni/Al2O3, Ni/CaO, Ni/Clinoptilolite. The support had been chosen based on their acidity, the efficiency in the cracking reactions and the in-situ carbon capture effect in the relevant cases. The preparation method was the same in all cases. The catalyst supporters had been impregnated by the using of Ni(NO3)2\u00b76H2O solution at 80\u00a0\u00b0C till 3\u00a0h, then the slurry was filtered and dried at 110\u00a0\u00b0C till 10\u00a0h. Finally the treated catalysts were calcined at 600\u00a0\u00b0C till 3\u00a0h.The main surface properties of the catalysts were determined by a Micromeritics 3Flex 3500 instrument using the BET (Brunauer\u2013Emmett\u2013Teller) method. The pore-size distribution and pore volumes were calculated using the BJH (Barret\u2013Joyner\u2013Halenda) model. The temperature programmed desorption of ammonia (NH3-TPD) was used to measure the number and acid strengths of sites found on solid catalysts using an analyser Micromeritics AutoChem-2920 precision chemisorption analyzer equipped with a heat-conductivity detector. The prepared modified catalysts were also investigated by an Apreo S LoVac instrument (FEI/ThermoFischer) coupled with an energy-dispersive X-ray spectrometer (AMETEK, Octane Elect Plus), operated at 2.0 for secondary electron imaging, and 25.0\u00a0kV for elemental analysis. The main properties of neat catalysts are summered in Table\u00a01\n.The Ni/ZSM-5, Ni/Al2O3 and Ni/CaO catalysts were in the form of fine powder with average diameter under 20\u00a0\u03bcm. Contrary, the Ni/Clinoptilolite catalyst has higher drain size. Another significant difference was found in the BET surface, because the Ni/ZSM-5 catalyst has 335\u00a0m2/g surface area, while the others less than 60\u00a0m2/g. Similar notable difference was reported regarding the \u201cSmicro\u201c value. It is important to mentioned, that Ni/ZSM-5 catalyst has the smallest average pore diameter, while Ni/Al2O3 has the highest. Regarding the C\u2013C chemical bond scission effect of the catalysts, not only their surface area and Si/Al ratio, but also the acidity should be also a crucial property. The acidity follows the order of Ni/Al2O3\u00a0<\u00a0Ni/CaO\u00a0<\u00a0Ni/ZSM-5\u00a0<\u00a0Ni/Clinoptilolite. EDAX result well demonstrated, that due to the natural source of the clinoptilolite, it has 8.0% other elements (Fe, Ca, K, Na) in addition to those listed. It is also worth to mentioned, that the nickel content of the catalysts changes significantly, which could be explained by the difference surface properties of the applied supporters.The desorption of ammonia is often used in order to determine the strength and amount of acid centres. The strength of the centres correlates with the desorption temperature. Since strongly bound probe molecules have high binding energies, increases temperatures are necessary to desorb these adsorbates [18]. The NH3-TPD was carried out on four samples (fresh catalysts) under same condition. Tmax values of TPD plots of ammonia desorbed from fresh catalysts decreases the in the following order: Ni/CaO\u00a0>\u00a0Ni/Al2O3 > Ni/Clinoptilolite\u00a0>\u00a0Ni/ZSM-5. However, the number of acid sites of fresh catalysts decreases as follows: Ni/Clinoptilolite\u00a0>\u00a0Ni/CaO\u00a0\u2248\u00a0Ni/ZSM-5 > Ni/Al2O3. However, it is important to be mentioned, that in case of CaO the ammonia TPD results are not concrete, due to the fact that CaO is a basic oxide. The high acidity of Ni/CaO can be explained by the measurement conditions. During the ammonia TPD, the number of the acidic sites are calculated by the consumed ammonia, which in case of CaO, resulted in the following chemical reaction:\n\n(1)\n3CaO\u00a0+ 2NH3 \u2192 3Ca\u00a0+\u00a0N2\u00a0+\u00a0H2O\n\n\nBasically, the mentioned reaction requires higher temperatures (\u223c500\u2013700\u00a0\u00b0C), but owing to the nickel content, lower temperature was adequate (308\u00a0\u00b0C).\nFig.\u00a01\n shows the morphology of prepared catalysts impregnated with nickel. Based on their structure, rings and channels of the catalysts, their nickel content evolved differently. The CaO had the highest nickel content (9.2%) owing to its wide channel openings and active sites, while clinoptilolite had the smallest (1.9%) due to its lower surface area and pore volume. Besides, as it can be seen, the distribution of nickel is not unified, which also can be explained by the different pore sizes, average pore diameter of catalysts and the wet impregnation method. Afterwards, it is important to be mentioned, that the surface area of Ni/Al2O3 is significantly lower comparing the Al2O3, due to the impregnation, which could clogged its pores and covered the surface of catalyst.The experiments were carried out in a two zone tubular reactor (Fig.\u00a02\n), aiming the in-situ carbon dioxide capture with increased synthesis gas yield. In the first reactor zone 5g of the raw material was placed, while 2.5g catalyst (Ni/ZSM-5, Ni/Al2O3, Ni/CaO, Ni/Clinoptilolite) was used in the second zone. Firstly, the temperature of the 1st reactor zone was determined, then that of the 2nd was investigated between 500 and 700\u00a0\u00b0C. Regarding the final temperature of the 2nd reactor zone, it was chosen with the consideration of the regeneration temperature. It is important to mention, that higher temperature than 800\u00a0\u00b0C for regeneration could cause structure deformation in case of catalysts. Afterwards the determination of the temperatures in the reactor zones, the regeneration cycles (during 10 cycles) of each catalyst was investigated for economic reasons. In case of preserving the structure of catalysts and its decoking, the regeneration was carried out at 800\u00a0\u00b0C for 1\u00a0h. The measurements were performed under inert conditions (nitrogen flow, 42\u00a0ml/min), for 20\u00a0min. In order to capture the moisture of the gas silica gel, while for the gaseous product a Tedlar type gas bag was used, with only one sampling at the end of the measurement. At the end of the measurements, the product yield was calculated based on their weight balance (2).\n\n(2)\n\n\n100\n\u2212\n\n\nR\ne\ns\ni\nd\nu\ne\n\n(\ng\n)\n\n+\nG\ne\nn\ne\nr\na\nt\ne\nd\n\nw\na\nt\ne\nr\n\n\n(\ng\n)\n\n\n\nR\na\nw\n\nm\na\nt\ne\nr\ni\na\nl\n\n\n(\ng\n)\n\n\n\n\u2217\n100\n=\nP\nr\no\nd\nu\nc\ne\nd\n\ng\na\ns\n\n\n(\n%\n)\n\n\n\n\n\nThe composition of the gas products was investigated using a DANI type gas chromatograph equipped with a programed injector and a flame ionization detector. Rtx-1 PONA type 100\u00a0m long column with an internal diameter of 0.25\u00a0mm and film thickness of 0.5\u00a0\u03bcm was placed in the chromatograph. The analysis was performed at 35\u00a0\u00b0C isothermal condition. The detector and injector temperatures were 230\u00a0\u00b0C. The chromatograms were evaluated using Clarity software.The hydrogen content of the gaseous products was also determined by gas chromatography using a DANI type gas chromatograph (with TCD detector) equipped with a CarboxenTM 1006 PLOT (30\u00a0m\u00a0\u00d7\u00a00.53\u00a0mm) column. During the experiments the following temperature program was used, the column space temperature at 30\u00a0\u00b0C for 18\u00a0min was kept, then it was raised to 120\u00a0\u00b0C with a heating rate of 15\u00a0\u00b0C/min, then the temperature was maintained at 120\u00a0\u00b0C for 2\u00a0min.To investigate the weight loss of raw material, a thermogravimetric analysis was performed by a Netzsch thermogravimetric analyser. During the measurement nitrogen atmosphere was used, with 20\u00a0\u00b0C/min heating rate until 800\u00a0\u00b0C. The arisen gases were analysed by a Bruker type FTIR connected to the TGA. The weight loss and dm/dt result of biomass raw material is shown in Fig.\u00a03\n.The decomposition of the raw material took place in several stages, resulting in a residue of 45.73%. As Fig.\u00a01 shows, the degradation of the biomass took place in three main steps. Up to 135\u00a0\u00b0C, the physically bound moisture of the sample was removed (4.06%). The first decomposition step took place between 135\u00a0\u00b0C and 245\u00a0\u00b0C with a maximum of 225\u00a0\u00b0C. The weight loss rate was 6.36%. The second step was observed between 245\u00a0\u00b0C and 340\u00a0\u00b0C with a maximum of 325\u00a0\u00b0C, while the third decomposition step was observed between 325\u00a0\u00b0C and 405\u00a0\u00b0C. The last decomposition step was observed at a maximum of 405\u00a0\u00b0C. The weight loss of the sample was 19.62% in the second decomposition step and 15.11% in the third. In the first degradation step, mainly the lighter molecular weight volatiles was removed, while in case of the third the hemicellulose units started to degrade, while that of the second degradation step both the hemicellulose and cellulose units. At the last step, mainly the lignin units started to decompose, due to its phenolic polymer structure.The TG-FTIR provides information about the functional groups of volatiles including non-condensable gases, (CO, CO2, CH4) and condensable volatiles (H2O, methanol, acids, phenols) as a function of temperature. As it can be seen in Fig.\u00a03, the raw material degraded as it was found during pyrolysis of lignocellulose materials [15]. The carbon monoxide mainly arises at low temperature (\u223c350\u2013450\u00a0\u00b0C) from the cracked ether and carbonyl groups, while at higher temperature (>600\u00a0\u00b0C), it can mainly be originated from the secondary reactions [15]. At low temperature carbon dioxide arises from hemicellulose, while that of at higher temperature from lignin units. The mentioned CO2 peaks can be observed at 2356\u00a0cm\u22121 and between 668 and 500\u00a0cm\u22121. The different stretching and bonds were revealed in the first 1000\u00a0s, except the characteristic peaks of CO2. Regarding the H2O, the peaks are caused by the evaporation of moisture content and dehydroxylation of carbohydrates [15]. O\u2013H stretching of H2O between 4000 and 3468\u00a0cm\u22121, while at 1645\u00a0cm\u22121\u00a0H\u2013O\u2013H bending can be detected. The same conclusion can be noted regarding the C=O aldehyde and ketone stretching and C\u2013O\u2013C stretching which appeared at 1830-1650\u00a0cm\u22121 and at 1100\u00a0cm\u22121, respectively. Methane tetrahedral \u03c54 vibration appeared at 1306\u00a0cm\u22121 with low absorbance, while asymmetrical C\u2013H bending and stretching can be observed at 1500\u00a0cm\u22121, which can be originated from the lignin content. Over time, the mentioned groups cannot be detected excepting the characteristic peaks of CO2.In order to get the appropriate temperatures during the experiments, preliminary experiments are needed. At first, the temperature of the 1st reactor zone should be chosen, considering the yields (Fig.\u00a02). As Fig.\u00a03 shows, with the increase of the temperature the yield of gas was escalated while that of the residue was decreased. Besides, in the range of 400\u2013700\u00a0\u00b0C the yields was not significantly changed. That phenomenon can be explained by the degradation of the lignocellulosic units. As it is widely known, the biomass is built up from cellulose (40\u201360%), hemicellulose (15\u201330%) and lignin (10\u201325%), having different degradation temperature ranges regarding its component [19]. A significant difference cannot be remarked between 500 and 700\u00a0\u00b0C, however, at higher temperatures, the yield of gas was increased by 10.6% due to the decomposition of lignin units [19]. Nevertheless, during biomass degradation, water formation takes place, which in case of the mentioned experiments was under 1%. Based on the mentioned low value, the amount of water is not depicted on Fig.\u00a04\n.Regarding the gas products, as Fig.\u00a05\n depicts, with increasing temperature, the yield of hydrogen, carbon monoxide and methane was escalated mainly owing to the following reactions [20]:\n\n(3)\nCnHm\u2192(m/4)CH4+(n-m/4)C\n\n\n\n\n(4)\nC\u00a0+\u00a0CO2\u21c42CO\n\n\n\n\n(5)\nCO\u00a0+\u00a0H2O\u21c4CO2+H2\n\n\n\n\n\n(6)\nCO+3H2\u21c4CH4+H2O\n\n\n\n\n(7)\nCnHm\u00a0+\u00a0nH2O\u2192(m/2+n)H2+nCO2\n\n\n\n\n\n(8)\nCnHm\u00a0+\u00a0nCO2\u21922nCO2+(m/2)H2\n\n\n\nBesides, the amount of carbon dioxide and lighter hydrocarbons was decreased, due to the thermal cracking (3), the steam reforming (water has arisen from the moisture of biomass) (7) and dry reforming reactions (8) (carbon dioxide surplus from biomass, even at higher temperatures) [20]. Regarding the amount of carbon monoxide and lighter hydrocarbons, the raise of the temperature was not resulted in significant change, while that of hydrogen, methane and carbon dioxide was remarkable with 1.1\u201319%, 2.2\u20138.5% and 5.8\u201328.8%, respectively. Since our work is mainly focusing on the regeneration cycles of the used catalyst and its properties in carbon dioxide conversation, in the 1st reactor zone lower temperature, 400\u00a0\u00b0C was used in the following measurements, where the carbon dioxide yield is \u223c55%. Besides, the process has lower energy requirement, also, it can provide a possibility for further investigations on the effect of the temperature.After the determination of the temperature in the 1st reactor zone, the temperature of the 2nd was also investigated with and without catalysts between 500 and 700\u00a0\u00b0C (Fig.\u00a02). As Fig.\u00a06\n depicts, the gas yield was increased by the enhanced temperature and with the presence of catalysts. However, it should be mentioned that the differences between the yields are lower than 5%, which can occurred by the diverse particle size of raw material (<3\u00a0mm).Also, it is important to note, that remarkable difference could not be observed among catalyst free and thermos-catalytic cases (only 1.2\u20137.5% of change). Besides the yield, the composition of the gas (Fig.\u00a07\n) was also investigated in the presence of the four different catalysts. In the absence of catalyst the yield of hydrogen and carbon monoxide were increased by the escalated temperature in the 2nd reaction zone, while that of lighter hydrocarbons was decreased by 2.6%. Regarding the results, mainly the same tendency can be observed at all temperatures, while significantly the amount of the components changed.\nFig.\u00a07 shows the change in the amounts of components compared to the non-catalytic measurement (Fig.\u00a07(a)) at 500\u00a0\u00b0C (Fig.\u00a07(b)), 600\u00a0\u00b0C (Fig.\u00a07(c)) and 700\u00a0\u00b0C (Fig.\u00a07(d)). In case of hydrogen, carbon monoxide and carbon dioxide yield remarkable changes were observed at both temperatures. The hydrogen yield was increased by 0.3\u20136.9\u00a0mmol/g due to the higher temperature and the nickel content on the catalysts surface. In the presence of catalysts the carbon monoxide yields were enhanced by 3.8\u201313.8\u00a0mmol/g, which can be explained by the shifting in Boudouard reaction (4), water gas reaction (5) and reforming reaction (7). Latter can be observed with the lower methane and higher hydrogen yields in almost each case. Nevertheless, the mentioned Boudouard reaction (3) was only observed in the presence of catalysts, where coke deposition was appeared on its surface promoting the solid-gas reaction.The effect of carbon dioxide capturing and decreasing was detected in case of Ni/Al2O3, Ni/CaO and Ni/Clinoptilolite with 0.3\u201323.7\u00a0mmol/g values. Based on the literature, in case of Ni/Al2O3 the reduction in the yield of CO2 can be described with its specific surface area and crystallite size [21,22]. However, as it was mentioned before, during nickel impregnation, the specific surface area of the Al2O3 has decreased significantly, therefore, the CO2 reduction besides the properties of alumina-oxide, was mainly occurred by the higher temperature and its Ni content which can also enhance the CO2 adsorption capacity, as well as the adsorption activity. Besides, as it also can be mentioned in case of Ni/CaO, chemical adsorption can occur due to the hydroxyl and oxide groups on the surface resulting in carbonate or bicarbonate species [21,22]. Regarding the Ni/Clinoptilolite, the high acidity can be mentioned which can evolve the CO2 capturing effect, therefore CO2 can be easily adsorbed owing to its beneficial properties (high quadruple moment) next to the hydrogen, methane and lighter hydrocarbons [23]. As Fig.\u00a07 depicts the amount of the lighter hydrocarbons was decreased by 1.1 and 10.9\u00a0mmol/g, from which the highest value belongs to the Ni/ZSM-5 (8.1\u201310.9\u00a0mmol/g) owing to its high Si/Al ratio, its pore structure (narrow zig-zag pattern, limited diffusion) and better cracking function.In terms of the generated syngas the presence of different catalysts were also investigated. As it can be seen in Fig.\u00a08\n, with increasing temperature, the sum of hydrogen and carbon monoxide also increased. Besides, not only the nickel content on the catalyst surface but the carbon capturing/conversion had a positive effect as well as temperature on syngas yield. As it was mentioned before, the highest hydrogen yield (8.9 and 11.1\u00a0mmol/g respectively) was generated in the presence of Ni/CaO and Ni/Clinoptilolite, while the highest carbon monoxide was obtained in the presence of Ni/Clinoptilolite. Referred to the results, Ni/Clinoptilolite provided the highest syngas yield at all temperatures (37.4, 38.6 and 48.3\u00a0mmol/g). Furthermore, it should be mentioned, that in syngas yield, compared to the non-catalytic points, a significant difference was observed (4.1\u201320.2\u00a0mmol/g). Based on the results, hereinafter 700\u00a0\u00b0C was used in the 2nd reactor zone, with 400\u00a0\u00b0C in the 1st reactor zone.\nFig.\u00a09\n shows the difference in the amount of each component compared to the first measurement point. As it is depicted, the amount of the lighter molecular weight hydrocarbons was increased by each catalyst in a wide range (0.7\u201311.4\u00a0mmol/g) from which Ni/ZSM-5 had the strongest influence in each regeneration cycle. This phenomena can be explained by coking and continuous pore clogging despite the regeneration cycles. Nevertheless, it should be mentioned that Ni/ZSM-5 and Ni/Al2O3 decreased the carbon monoxide and carbon dioxide amount with a significant value (0.5\u20138.1 and 4.5\u201311.8\u00a0mmol/g, respectively) as the cycles were progressed.Regarding the Ni/CaO, the amount of carbon dioxide until the third regeneration cycle was decreased by 0.2\u20131\u00a0mmol/g, thereafter it was increased by 1.4\u20134.9\u00a0mmol/g. On the basis of earlier work, it can be explained by escalated specific surface area owing to the catalyst sintering effect caused by the severally used high temperature [26]. Besides, the amount of hydrogen and carbon monoxide was reduced by 2.1\u20137.1\u00a0mmol/g and 3.8\u20138.3\u00a0mmol/g, respectively. However, with the decreasing of hydrogen, the methane was increased by 0.1\u20132.3\u00a0mmol/g, which can be explained by the methanation reaction (6). In the presence of Ni/Clinoptilolite the hydrogen and carbon dioxide content was decreased at each regeneration cycle by 1.0\u20137.1\u00a0mmol/g and 0.3\u20134.4\u00a0mmol/g, while carbon monoxide and methane content was increased (1.4\u20133.7 and 0.3\u20132.2\u00a0mmol/g). Concerning the amount of carbon monoxide, its value from the seventh regeneration cycle was significantly reduced by 4.5\u20139.2\u00a0mmol/g. The introduced phenomena can be explained by the favourable acidic properties of Clinoptilolite in carbon capturing, while the remarkable changes in composition from the seventh regeneration cycle may occurred by the coke deposition in the tetrahedral structure and on the surface of catalyst [23,24].\nFig.\u00a010\n shows the yield of the generated syngas at each regeneration cycle, where its yield was changed between the range 24.9\u201350.5\u00a0mmol/g. In the presence of Ni/ZSM-5, Ni/Al2O3 and Ni/CaO a slight decrease can be observed along with the regeneration cycles. However, in case of Ni/Clinoptilolite the synthesis gas yield was escalated until the sixth cycle (43\u201350.5\u00a0mmol/g), despite that it had the highest value from 7th-10th regeneration cycle compared with the used catalysts. Based on the results, it can be concluded, that the Ni/ZSM-5 and Ni/Al2O3 decreased the amount of carbon dioxide until the 5th regeneration cycle, while in case of Ni/CaO its value was reduced until the 3rd cycle. In the presence of Ni/Clinoptilolite the carbon dioxide content showed decreasing even at the last regeneration cycle. Due to the represented data, the Ni/Clinoptilolite resulted in the highest synthesis gas and the lowest carbon dioxide yield, while in case of Ni/Al2O3 and Ni/CaO the generated carbon dioxide has remained under 30\u00a0mmol/g for ten regeneration cycles.3.5 Morphology of used catalysts.Afterwards the regeneration cycles, the main properties of the catalysts was investigated, which are summarized in Table\u00a02\n. During catalytic pyrolysis-gasification, the deactivation of catalysts strongly depends on the secondary reactions, as well as coke deposition. It should be mentioned, that through the thermochemical degradation of biomass oxygenates such as phenols, alcohols, ketones and aldehydes could formed, which are at higher temperatures and in the presence of catalysts start to decompose with the formation of coke. The generated coke clogs the pores and covers the specific surface area, causing activity decreasing. Therefore, the regeneration of catalysts is necessary.As it can be seen the surface areas (except Ni/Al2O3) was decreased by the coke deposition, which caused clogging in the micropores of Ni/ZSM-5 and Ni/Clinoptilolite and pores at about 4\u00a0nm of Ni/CaO. Comparing the fresh catalysts (Section 2.2, Table\u00a01), with the coked and regenerated ones, it can be said, that the highest and lowest decrease of SBET values were observed for Ni/CaO coked (54%) and Ni/ZSM-5 coked (17%) respectively. Also, slight reduction was observed in the Si/Al ratio and Vmicro in case of Ni/ZSM-5 and Ni/Clinoptilolite which can be explained by the partly blocked and inaccessible acidic centres. Besides, in case of Ni/CaO it can be mentioned, that the fresh catalyst did not have micropores, while that of the coked and regenerated were occurred, which also can be clarified with the increased average pore diameter values. This phenomenon can be explained with sintering effect which was caused by the numerous use at 700\u00a0\u00b0C, as well as with the several regeneration cycles at 800\u00a0\u00b0C. However, the mentioned effect was occurred until the 6th-7th regeneration cycles. In addition, the regeneration was considered effective due to the lower amount of coke deposition, which was removed almost totally in case of Ni/Al2O3, Ni/ZSM-5 and Ni/Clinoptilolite. Meanwhile no significant regeneration effect was observed after 10th cycle at 800\u00a0\u00b0C on surface area, pore volume, pore size distribution. The morphological parameters showed almost the same values for \u201ccoked\u201d and \u201cregenerated\u201d samples.In this work, agricultural biomass was pyrolysed-gasified in a two-zone tubular reactor. As a catalyst, nickel impregnated ZSM-5, Al2O3, CaO and Clinoptilolite were used. The main aim was the reduction of carbon dioxide with the increase of syngas yield. During the measurements, not only the effect of the temperature and catalysts but the regeneration cycles of catalysts was investigated. At first, the temperature of the 1st and the 2nd reactor zone was determined, where 400\u00a0\u00b0C and 700\u00a0\u00b0C, while for the temperature of regeneration 800\u00a0\u00b0C was chosen. Regarding the 1st zone, in case of great amount of carbon dioxide production, as well as to investigate the CO2 reduction and conversion effect of catalysts, low temperature was chosen. Besides, with the use of low temperature, the process is more energy efficient.In case of all catalysts, the amount of lighter hydrocarbons was increased especially in the presence of Ni/ZSM-5 which was caused by coking and pore-clogging. In the presence of Ni/Al2O3 mostly the temperature had an effect on the product yield owing to the low SBET surface of the catalyst which was mostly caused by the nickel impregnation. Regarding the Ni/CaO it can be mentioned, that due to the 10 regeneration cycles a sintering effect was noticed where a small amount of micropores was generated increasing the SBET of Ni/CaO. Concerning the results obtained in the presence of Ni/Clinoptilolite, it can be mentioned that the highest synthesis gas yield was observed until the 6th regeneration cycle, due to its great quadrupole moment which helps in carbon dioxide capturing, with the increasing of carbon monoxide. Based on the results, it can be noted that the amount of carbon dioxide was decreased until the 5th and 3rd regeneration cycle (in case of Ni/ZSM-5, Ni/CaO respectively), while with the presence of Ni/Clinoptilolite the decreasing can be observed even at the last regeneration cycle. Therefore, the Ni/ZSM-5, Ni/CaO can be used for 10 regeneration cycles with low activity change, however, not for syngas yield increasing in these measurement series.Afterwards, it was stated, that the Ni/Clinoptilolite is suitable for CO2 capturing with syngas yield enhancing for 10 regeneration cycles. However, in case of Ni/Clinoptilolite, longer regeneration cycles should be investigated in case of more information of activity changing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sk\u0142odowska-Curie grant agreement No 823745.", "descript": "\n                  In this work, agricultural biomass waste (maize) was used as raw material in a pyrolysis-gasification process with the presence of nickel-loaded zeolites, CaO and Al2O3. Henceforward, the reusability and the deteriorating effect of the catalysts are important to be analysed, therefore the catalysts were reused for ten regeneration cycles. During the measurements, the effect of temperature, catalysts and the regeneration cycles of catalysts were also investigated, as well as the components of the gaseous product. At first, the proper temperature was determined in the 1st (200\u2013800\u00a0\u00b0C) and the 2nd (500\u2013700\u00b0) reactor zone, where 400\u00a0\u00b0C and 700\u00a0\u00b0C, while for the regeneration 800\u00a0\u00b0C was chosen. Throughout the regeneration cycles in case of Ni/ZSM-5 lighter hydrocarbons was increased while that of the carbon monoxide and carbon dioxide was decreased. In the presence of Ni/CaO the amount of carbon dioxide decreased until the 3rd regeneration cycle, while carbon monoxide and hydrogen was decreased till the 10th cycle. Regarding the Ni/Al2O3, the amount of CO2 and CO was significantly decreased at each cycles, while in case of Ni/Clinoptilolite reduction was observed in hydrogen and carbon dioxide, however not only the amount of carbon monoxide but the syngas yield increased remarkably. Due to the mentioned results, the Ni/Clinoptilolite has an effective syngas enhancing and carbon dioxide capturing effect, even with low pyrolysis temperature.\n               "}
{"full_text": "\n\n\nData will be made available on request.\n\n\nData will be made available on request.There are several serious disadvantages to use fossil fuels. These are especially worrying for the environment (global warming) and for the energy independence of nonproducing countries. The transport sector is one of the main consumers of fossil fuels and a key point for the decarbonization strategies of economies. Thus, there is great interest in the production of alternative fuels based on renewable sources and residues.The hydroconversion of vegetable oils, fats and used vegetable oils and Fischer-Tropsch synthesis are two routes for the synthesis of clean fuels. However, both processes yield a mix of linear hydrocarbons that cannot be used directly as fuels; in both cases, a hydroisomerization step is necessary to produce green fuel with physicochemical properties to be used as fuels [1]. Hydroisomerization is a process for converting linear paraffins (n-alkanes) into branched isomers [2]. This process is needed to produce high-quality gasoline (clean gasoline) with improved octane number and jet/diesel fuels as well as lubricant oils with improved low-temperature performance to comply with the cold flow properties of diesel fuel specifications such as the pour point (low-temperature freezing) and viscosity index [2\u20134]. In addition, an increasing proportion of isomerized gasoline in the gasoline pool contributes to environmental protection and product upgrades [5].The hydroisomerization process is generally performed on bifunctional catalysts with metallic sites for hydrogenation/dehydrogenation and acidic sites for isomerization through carbenium ions [3,6,7]. Among several options, WO\n3\n supported in Al2O3 or ZrO\n2\n is a good option for the acidic function, including noble metals in the metal components (platinum, palladium), with Pt being the most active [3,5]. Based on the good previous results we obtained on similar systems, we selected Al2O3 as a support and WO3 as an acidic functional group [1,8]. The proper acid/metal content is decisive in achieving high activity, stability and product selectivity of these catalysts and in obtaining ideal hydroisomerization-cracking behavior of hydrocarbons [4,6,7]. The high cost of noble metals and their limited availability limit their use and applications [3,5]. The hydroisomerization performance (catalyst activity and isomerization selectivity) of bifunctional catalysts is influenced by another important factor: the acidity and pore structure of the support. Hydrocracking is promoted by a strong acidity, whereas high isomerization selectivity is obtained with medium strength acidity [3]. In addition, the hydrocracking of monobranched isomers is more difficult than that of multibranched isomers.Nickel-supported catalysts are used in the oil refining industry to produce high-quality gasoline and diesel fuels and lubricating oils [4] because among the nonnoble metals, nickel has shown the best catalytic activity for hydroisomerization of n-alkanes [2]. These catalysts cost less than noble metal catalysts, are less likely to poison, and have more availability, but the high tendency for hydrocracking reduces the hydroisomerization yield [3,4]. Other disadvantages are the tendency to deactivate by coking and the excess cracking producing a large amount of gasses [4]. The preparation of nonnoble metal catalysts (for instance, nickel) is important to avoid large metal particle formation, however, the high metal loadings are necessary due to the low specific activity of these metals with respect to the noble metals. This problem restricts the optimization of the existing nonnoble catalysts [5]. This means that there is a need for high nickel loading to be active. An increase in the metal particle size is produced by this loading, and pore opening could be restricted [4]. Therefore, it is an appealing task to look for a novel strategy to prepare nonnoble metal-based catalysts with reduced particle size for the hydroisomerization reaction [5]. This means that novel bifunctional catalysts with improved stability and diffusion properties can improve the capacity in the conversion of heavy paraffin fractions [4]. To our knowledge, there have been a few reports about the influence of nickel loading on the hydroisomerization of n-dodecane. Some groups have studied higher nickel content [9,10] than we reported here. In a previous work [8], we studied n-dodecane hydroisomerization with zirconia- and alumina-supported Pt.The present work introduces Ni as a metal to replace Pt in alumina-supported WO3 catalysts for the hydroisomerization of n-dodecane due to the high cost and limited availability of noble metals previously mentioned. However, the low activity of Ni compared to that of Pt, makes that nickel still cannot compete with Pt when the general economy of the process is considered. It seems that the problem of the low catalytic activity could be easily resolved by increasing the Ni loading on the support since nickel is approximately 1000 times cheaper than Pt [2].The employed tungsten precursor was ammonium metatungstate hydrate ((NH4)6(H2W12O40)\u00b7xH2O) (99%) purchased from Honeywell, and the nickel precursor was nickel(II) nitrate hexahydrate (Ni(NO3)2\u00b76H2O) (99%) from Sigma-Aldrich. Alumina (\u03b3-Al2O3) was purchased from Saint Gobain-NORPRO (1.5\u00a0\u00d7\u00a04\u00a0mm trilobes, SA6975) and used as support.The catalysts were prepared by applying the wetness impregnation method on the supports to introduce W and Ni. In order to remove the moisture of the support, it was treated overnight at 120\u00a0\u00b0C before any impregnation.The following procedure was used to incorporate W in the \u03b3-Al2O3 pellets by wet impregnation: a round flask was used to place the pellets in contact with an aqueous solution of ammonium metatungstate hydrate ((NH4)6(H2W12O40)\u00b7xH2O) (20\u00a0ml for 6\u00a0g of Al2O3). The mixture was stirred for 1\u00a0h, followed by the evaporation of the solvent under reduced pressure for 20\u00a0min. Finally, the solid was calcined in an oven at 500\u00a0\u00b0C for 2\u00a0h [8].The following protocol was used to incorporate Ni by wet impregnation: a round flask was used to place the pellets in contact with an aqueous solution of nickel (II) nitrate hexahydrate (Ni(NO3)2\u00b76H2O) (20\u00a0ml for 6\u00a0g Al2O3). Pellets and solution were stirred for 1\u00a0h after that time, the solvent was eliminated by evaporation at reduced pressure for 20\u00a0min. Finally, the solid was calcined at 400\u00a0\u00b0C for 2\u00a0h in an over under static air conditions.Following these procedures, the catalysts were prepared and labeled as Al2O3, WO3-Al2O3, and Ni/WO3/Al2O3= NiW/Al-[wt%] catalysts, where [wt%] indicates the wt% of Ni on the catalysts. The W loading was 15\u00a0wt% in all catalysts and the WO3-Al2O3 sample.The textural properties were determined with a Micromeritics ASAP 2420 from the adsorption\u2013desorption experiments [1,8]. The X-ray diffraction patterns were recorded with a X'Pert Pro PANalytical to identify the crystalline phases in the catalysts. The equipment have a CuK\u03b1 radiation source (\u03bb\u00a0=\u00a00.15418\u00a0nm) and X'Celerator detector based on RTMS (Real Time Multiple Strip) [1,11]. Temperature-programmed desorption and the acidity of the catalysts were performed on a Micromeritics TPD/TPR 2900 apparatus with a thermal conductivity detector (TCD) [8]. The nature of the acidity (Br\u00f8nsted and Lewis acid sites) was determined by in situ IR spectroscopy of chemisorbed pyridine. The measurements of the IR spectra have been done in a DRIFT cell installed in a 6700 Nicolet FTIR spectrometer [1]. XPS spectra were obtained with a SPECS GmbH electron spectroscopy system with a PHOIBOS 150 9MCD energy analyzer, and Mg X-ray source [8,12].The metal contents of the catalysts were determined by elemental analysis via ICP\u2013OES [8].The hydroisomerization performance of n-dodecane was tested with the prepared catalysts. A trickled-bed mode reactor worked in parallel flow and at high pressure to ensure close contact between the gas, the liquid, and the solid. The calcined catalyst pellets (1\u00a0g) diluted in 8\u00a0g of inert support were reduced in the reactor at 425\u00a0\u00b0C and atmospheric pressure. After the reduction, the reaction conditions were set to pressure of 2.0\u00a0MPa, reaction temperature of 300\u00a0\u00b0C, and the reactor was fed by a liquid flow of 0.1\u00a0mL\u00b7min\u22121 of n-dodecane and hydrogen flow of 340\u00a0mLN\u00b7min\u22121. The gas outlet was measured online with a \u00b5-GC and the liquid samples were collected and analyzed offline. The time-on-stream was 24\u00a0h.More detailed information about the catalyst characterization methods and reaction conditions is included in the supplementary information.The chemical compositions of the catalysts were confirmed by ICP analysis, as shown in Table\u00a01\n. The measured chemical composition is very close to the nominal values.The textural properties were studied by N2 adsorption-desorption. Table\u00a02\n shows the BET surface area, average pore volume and average pore diameter of alumina, support, and catalysts. The surface areas decrease firstly when WO3 is incorporated by impregnation onto the support (Al2O3) to obtain WO3-Al2O3. Then surface areas decrease more when nickel is impregnated onto WO3-Al2O3. The support surface is partially covered by the incorporation of WO3 and NiO as particles which block the support pores and reduce nitrogen access and adsorption capacity. But the variation with respect to WO3/Al2O3 is small, indicating that all catalyst samples have similar textural properties.The porosity of the samples was studied with the N2 adsorption-desorption isotherms of the support and catalysts. According to the IUPAC 2015 classification or Brunauer, Deming, Deming, and Teller (BDDT) classification for mesoporous materials all isotherms can be classified mainly by type IV (Fig. 1S) with hysteresis H1 due to mesoporous aggregates and in the high P/Po there is a small contribution of type II character due to unfilled macroporosity, since the isotherms do not level off entirely. Thus, the materials are formed by mesoporous and somewhat macroporous aggregates [13\u201318]. Isotherms are very similar in the support and catalysts indicating a high dispersion of the tungsten and nickel active phases on the support. The pore size distributions (Fig. 2S) were located in the mesopore range from 2 to 50\u00a0nm for the support and catalysts.The crystalline phases present in the catalysts were studied by X-ray diffraction. All PDF cards belong to ICDD database. The XRD profile of Al2O3 presents three main peaks located at 37.1\u00b0 (with a shoulder at 39.4\u00b0), 46.0\u00b0 and 66.7\u00b0, corresponding to the (110), (111) and (211) planes of \u03b3-Al2O3, respectively, of the cubic crystal system (PDF card 00-001-1303). The WO3/Al2O3 sample profile shows the same peak structure as the Al2O3 sample. Nonobvious diffraction peaks were found for WO3 (PDF card 00-005-0388) or other tungsten oxides which indicates a high dispersion of tungsten oxide in the support and/or a domain size smaller than \u223c2\u00a0nm for WO3 crystallites [19]. The structure of tungsten species dispersed on oxide supports depends on both the nature of the support and the concentration of tungsten [20].The X-ray profiles of the Ni catalysts show the same diffraction lines from the support except for the NiW/Al-10 catalysts (Figs.\u00a01\n and 3S), which shows a new peak at 43.3\u00b0 due to NiO associated with the (012) plane of the rhombohedral crystal system (PDF card 00-044-1159). It can be deduced that there is a high dispersion of NiO on the surface forming particles that are too small to diffract except for the catalyst with the highest loading (10% wt Ni).The TPR profiles are compiled on Fig.\u00a02\n(A and B). Fig.\u00a02B is an enlargement of Fig.\u00a02A up to 500\u00a0\u00b0C. As the nickel content increases, the reduction peak due to NiO (approximately 400\u00a0\u00b0C) is shifted to lower temperatures. This is because with low Ni concentrations, NiO particles are smaller, meaning that more species are in close contact with the support (greater interaction) and therefore are more difficult to be reduced. This is also reflected in the XRD diffractograms of the catalysts, which indicate that catalysts with higher Ni loadings present a lower dispersion. In the reduction profile of the NiW/Al-10 catalyst, there is a second peak at approximately 425\u00a0\u00b0C (Fig.\u00a02B) that is due to NiO reduction to metallic Ni when nickel oxide interacts with WO3 sites. There is a second peak at high temperatures (750\u00a0\u00b0C) due to partial reduction of tungstate oxide. In the WO3-Al2O3 support, the tungsten oxide band at high temperatures is not visible. This is because metallic nickel activates hydrogen, and through the spill-over process, it facilitates tungsten oxide reduction. Considering these results, 425\u00a0\u00b0C is the reduction temperature for the activation of the catalysts before the reaction because all NiO will be reduced to metallic Ni.The acidity is an important factor that determines the reactivity of the catalysts used in the hydroisomerization reaction. NH3-TPD was applied to study the acid properties of the catalyst surface, evaluating the total acid site amount, and the distribution of acid sites by strength. Ammonia is a suitable probe molecule for acidity because its small size and basicity allow the interaction with most of the acid sites. The temperature of desorption is indicative of the acid site strength, while the amount of adsorbed ammonia is proportional to the number of acid centers [21].The NH3-TPD profiles for all catalysts are shown in Fig.\u00a03\n. There are three different desorption peaks corresponding to different acid strengths: (a) desorption a lower temperature than 250\u00a0\u00b0C: weak acid sites, (b) desorption temperature from 250 to 400\u00a0\u00b0C: intermediate strength and (c) desorption at higher temperature than 400\u00a0\u00b0C: strong. WO3-Al2O3 exhibits two main desorption peaks at 155 and 555\u00a0\u00b0C corresponding to weak and strong acid sites. WOx species appear to interact strongly with sites on the surface of \u03b3-Al2O3\n[22]. At low nickel loadings (0.5\u00a0wt.% Ni), the catalysts have features similar to those of WO3-Al2O3. When nickel loading increases, the low-acid site peaks are moved to higher temperatures, and medium-acid sites appear [9]. All catalysts show a peak (520\u2013550\u00a0\u00b0C) of strong acid sites, which is divided into two peaks at high nickel loadings (6\u201310\u00a0wt%), appearing as a new peak at approximately 470\u00a0\u00b0C. The acidity was introduced through WOx, so it was expected that all samples show a similar value of total acidity because all samples had a comparable W loading (ca.15% wt). This value was measured and was approximately 1.6\u00a0mmol NH3/g.The nature of the surface acid sites was studied of IR of adsorbed pyridine. IR spectroscopy of adsorbed pyridine facilitates the distinction of different acid sites. The catalysts were reduced at 425\u00a0\u00b0C and then the FTIR pyridine (DRIFT) adsorption spectra (Fig.\u00a04\n) were measured at room temperature.The FTIR spectra show adsorption bands centered at approximately 1610, 1575 and 1448\u00a0cm\u22121, which correspond to adsorbed pyridine on Lewis acid sites (L) [23,24]. The alumina support has three possible Al3+ coordination with Lewis acidity: five, four and three [23]. A small band at 1540\u00a0cm\u22121, which is not visible in some catalysts, can be attributed to vibration modes of pyridinium ions on Br\u00f8nsted acid sites (B) [24]. The nonobvious band at 1540\u00a0cm\u22121 for some catalysts is ascribed to the coverage of Br\u00f8nsted acid sites with NiO at high content [3]. The absorption band at approximately 1490\u00a0cm\u22121 is due to a combination of signals of pyridine absorbed on Lewis or Br\u00f8nsted acid sites. Br\u00f8nsted acid sites are due to the reducible domains that act as redox sites required for the formation of H+species from H2\n[25]. A shoulder at 1622\u00a0cm\u22121 is observed in the samples without nickel or with low nickel content, whose intensity decreases with increasing nickel loading. This signal corresponds to adsorbed Py on very strong Lewis acid sites, which are considered tetrahedral sites with cationic vacancies in close proximity. These species are covered by nickel species [26].The nature and dispersion of nickel and tungsten species on the surface of the catalysts were studied by XPS analysis. Fig.\u00a05\n shows the Ni 2p XPS spectra for the NiW/Al-2 to 10 catalysts, Fig. 4S shows the W 4f XPS spectra for the NiW/Al-0.5 to 10 catalysts and Table\u00a03\n lists the binding energies (eV) (W 4f7/2 and Ni2p) and atomic surface ratios of the present elements. Al 2p signal at 74.5\u00a0eV [8] was the reference for the binding energies to correct for charging effects.In Fig.\u00a05, the signal at approximately 856.7\u00a0eV is attributed to Ni2+species (NiO) and are accompanied by a satellite signal at approximately 862.9\u00a0eV, characteristic of this oxidation state [4,27,28]. The peak due to metallic nickel is not found because metallic nickel is formed in the reduction process before the reaction. Prior to this reduction, nickel is in the oxide form (NiO).The W 4f signal presents two contributions of the typical doublet corresponding to spin-orbital splitting (W4f7/2 and W4f5/2). One component is attributed to the WO3 species at approximately 35\u00a0eV for W4f7/2 [8,25] and 38.2\u00a0eV for W4f5/2. A second component at approximately 36\u00a0eV for W4f7/2 and 37.3\u00a0eV for W4f5/2 can be assigned to Al2(WO4)3, aluminum tungstate [8,29].The surface atomic ratios of Ni/W or Ni/Al generally increase with Ni loading (Table\u00a03), but this increase is lower than the corresponding increase in the nickel bulk concentration. This implies a decrease in the nickel dispersion as the nickel loading is higher. The most evident case is the absence of change in the Ni/Al surface atomic ratio between NiW/Al-8 and NiW/Al-10 catalysts, a clear indication of the formation of bulky crystal structures of nickel species as was detected by XRD. The Ni/W surface atomic ratio is higher than the Ni/W bulk atomic ratio. This effect can be due to the preferential deposition of nickel on tungsten moieties, which reduces the W signal with respect to the expected chemical composition. The lower (Ni/W) xps/(Ni/W) bulk ratio is for the NiW/Al-10 catalyst in which the nickel species start to form bulky structures, implying a lower Ni coverture of W with respect to the other catalysts without the development of bulky structures of nickel species.In conclusion, the characterization techniques applied for studying the catalysts, i.e., tN2 adsorption-desorption, XRD, H2-TPR, FTIR, NH3-TPD and XPS, indicated that there is a high dispersion of NiO and WO3 on the support surface revealed by the absence of NiO and WO3 diffraction peaks except for the NiW/Al-10 catalyst, which shows NiO peaks seen in XRD analysis. The XPS data also showed a decrease in the nickel dispersion with increasing nickel loading, indicating the formation of bulky crystal structures of nickel species, as was detected by XRD. Nickel deposits preferentially on tungsten moieties as the surface Ni/W atomic surface ratio is higher than the Ni/W atomic bulk ratio. The similarity between the support and catalyst textural properties in combination with XRD and XPS results indicates a high dispersion of the tungsten and nickel active phases on the samples. Medium acid sites (moderate acidity) are the main sites responsible for the reaction performance [30\u201332]. FTIR spectra show that more Lewis acid sites are present than Br\u00f8nsted acid sites. It is worth noting the shoulder at 1622\u00a0cm\u22121 observed in the samples without nickel or low nickel loading corresponds to adsorbed Py on very strong Lewis acid sites. These species disappear because they are covered by the presence of nickel species.The hydroisomerization of n-dodecane was tested with the series of NiW/Al-x catalysts (x\u00a0=\u00a00.5, 1, 2, 4, 6, 8, 10). The time on stream was 24\u00a0h and the displayed results (conversion and selectivity) are the average of these 24\u00a0h. The samples were first reduced at 425\u00a0\u00b0C, this reduction temperature was deduced from TPR results since at 425\u00a0\u00b0C NiO will be reduced to metallic Ni. The experimental results show that all catalysts are active for the hydroisomerization of n-dodecane (Treaction\u00a0=\u00a0300\u00a0\u00b0C, P\u00a0=\u00a02.0\u00a0MPa). The GC analyses show that the products contain iso-dodecane (i-C12), branched C12 and cracked products (C6-10) under the applied reaction conditions. Fig.\u00a06\n shows the n-C12 conversions and (i-C12+branched C12) yields for all catalysts studied.The conversion of n-dodecane and selectivity to branched-C12 tends to increase with Ni loading until the catalyst contains 6\u00a0wt% Ni. This catalyst shows the highest conversion (28%) obtained, and the XPS analysis showed that this catalyst has a higher Ni loading, and that the Ni/Al surface ratio increases before stabilizing (Table\u00a03). Above this Ni loading, the conversion and selectivity to branched-C12 started to decrease. The enhanced activity with the presence of nickel is related to the location of Ni centers near the acid centers, which favors a fast desorption rate of the intermediates involved in the isomerization reactions [9] due to the higher amount of surface Ni species. It can also be correlated with the acidity of the catalysts. In the TPD measurements, it can be seen that with an increase in the nickel percentage in the catalysts, the intermediate acidity centers grow, which are directly related to the activity [33\u201335]. Such a catalytic behavior is typical for bifunctional catalysts. The higher accessibility of Ni sites in the proximity of acid sites enables that carbenium ions and olefins dehydrogenate rapidly and desorbs them as alkanes before they undergo cracking reactions. This effect is due to a lower metallic site/acid site ratio than the optimal for isomerization. If the catalysts metal function (in this case nickel) is increased, the formation of isomers via a decrease in the diffusion path between two metallic sites, will be promoted. This is because the proximity of metallic sites reduces the probability of the intermediate species interacting with acid sites and in consequence the crack of these species. For all that, there is an initial increase in isomerization selectivity (isomerized/cracked ratio) with increasing Ni loading from 0.5 to 6% [9,10]. In contrast, in the range of 8\u201310% Ni, metallic sites are enough to form olefins covering all the acid sites, and the increase in metal function (nickel loading) avoids the isomerization reaction and increases the cracking reactions [9,10] due to its low dispersion. The generation of the acid sites and the elimination of ionic intermediates from the surface before \u03b2-scission reactions occur depends on the accessibility of the dissociated hydrogen. Metallic nickel ensures the formation of this dissociated hydrogen needed to generate active acid sites. This avoids polymerization and cracking reactions and increases the isomerization selectivity [9]. The selectivity (Fig.\u00a07\n) also changes as a function of the nickel loading in the catalysts. The maximum selectivity toward iso-C12 and toward branched-C12 was obtained with the NiW/Al-2 catalyst. Higher loads of nickel revealed a change in the selectivity toward cracking products. This trend increases with the quantity of nickel, showing a similar tendency to that observed in the conversion results. Although the NiW/Al-2 catalyst offers the highest selectivity toward the desired products, the NiW/Al-6 catalyst presents the highest yield of the reaction because of the high conversion and selectivity to iso-C12 and to branched-C12 (Fig.\u00a06). This change in the selectivity, observed when a high amount of nickel (>\u00a06\u00a0wt%) is employed, can be related to a mismatch between metal/acid centers that contain the catalysts (hydrogenation ability and acidity) [36]. This can be associated with the growth of strong acidity centers for high metal loading, as can be observed in the TPD analysis, which can favor cracking reactions and at the same time reduce the yield of the reaction owing to a slower desorption rate of intermediates [33,34]. Additionally, this can be explained by the metal distribution. A high nickel content can promote cracking reactions due to the worse metal dispersion caused by sintering or the presence of larger nickel particles, as seen in the XRD analysis for the NiW/Al-10 catalyst [9,10,35,37].\nThe obtained data show a compromise between conversion and selectivity, which is a characteristic behavior of bifunctional catalysts [9,35,37], with NiW/Al-6 being the most active catalyst attributable to an optimum balance between metal and acid centers. XPS data showed a good dispersion, and FTIR of adsorbed pyridine showed that this is the first sample without the 1622\u00a0cm\u22121 signal, a clear indication of a good balance between metal and acid centers.\nTable\u00a04\n compares the results we have obtained with similar reactions published in the bibliography. Hydroisomerization reactions varied in the temperature range of 250\u2013350\u00a0\u00b0C. In general, zeolites are clearly more active in hydroisomerization reactions but with lower selectivity with respect to the catalytic system presented in this work; see Refs. [2,21]. Several reactions with catalysts supported on SiO2-WO3 [9,10,38] were carried out at 250\u00a0\u00b0C, and the results show that these catalysts are less selective than our catalytic system. Our catalytic system reaches a very high selectivity to long chain hydrocarbons with respect to the state of the art (Table\u00a04), with a moderate conversion level, similar to previous works with nickel on amorphous supports. The acid/metal balance of the catalytic system presented in this work seems very efficient for the hydroisomerization of long chain hydrocarbons as a balance between conversion and selectivity results is derired and obtained.The conversion of n-dodecane and selectivity to branched C12 trends to increase with Ni loading in the nickel containing catalysts prepared by the wetness impregnation method. The highest conversion (28%) was achieved with the NiW/Al-6 catalyst (6% wt. Ni). Above 6\u00a0wt% Ni loading, a decrease in conversion and increase in selectivity toward cracking products were observed. This trend is more pronounced when the nickel loading rises. By increasing the nickel content to 6\u00a0wt%, the intermediate acidity was enlarged, which is directly correlated with the activity performance, increasing the yield toward branched-C12. However, above 6\u00a0wt% Ni, the formation of larger metal particles and the strong acidity increased, both factors promoting cracking reactions instead of isomerization reactions. At high nickel loading (10% wt. Ni), XPS and XRD data showed a decrease in nickel dispersion which was deposited preferentially on tungsten moieties forming bulky crystal structures. The obtained data confirm the role of an optimum balance between metal and medium-strength acid centers in the catalysts, which is the bottleneck for maximizing the yield in this kind of reaction.\nD. Garc\u00eda-P\u00e9rez: Investigation, Formal analysis, Writing \u2013 original draft. A. Lopez-Garcia: Investigation, Formal analysis. P. Re\u00f1ones: Investigation, Formal analysis, Writing \u2013 original draft. M.C. Alvarez-Galvan: Supervision, Writing \u2013 review & editing, Funding acquisition. J.M. Campos-Martin: Conceptualization, Supervision, Writing \u2013 review & editing, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The support of MICIN/AEI (Spain) through project ENE2016-74889-C4-3-R is acknowledged. DGP acknowledges MICIN/AEI for her contract (BES-2017-079679) (Spain). This research is part of the CSIC program for the Spanish Recovery, Transformation, and Resilience Plan funded by the Recovery and Resilience Facility of the European Union, established by Regulation (EU) 2020/2094 (TRE2021-03-012). We acknowledge the support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mcat.2022.112556.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Heterogeneous catalysts based on alumina-supported tungsten oxides (15\u00a0wt% W) with different loadings of nickel (0.5, 1, 2, 4, 6, 8 and 10\u00a0wt% Ni) were selected to study the influence of Ni loading on the hydroisomerization of n-dodecane. The catalysts were prepared by applying the wetness impregnation method on the supports to introduce W and Ni. The characterization techniques applied for determining physicochemical properties of the catalysts were N2 adsorption-desorption at 77 K (textural properties), X-ray diffraction (structure and crystalline phases), H2-TPR (redox properties), FTIR, NH3-TPD (acid sites analyses) and XPS (chemical surface analysis). The catalytic properties of such catalysts were found to be crucial in the n-dodecane conversion. The NH3-TPD profiles indicate that the medium acid sites are the main sites responsible for the reaction performance. The formation of bulky crystal structures of nickel species in the high nickel loading catalyst (10\u00a0wt% Ni) was confirmed by XRD and XPS results, resulting in the largest cracking activity. The conversion of n-dodecane and selectivity to i-C12+branched C12 tend to increase with Ni loading until the catalyst contains 6\u00a0wt% Ni (28% n-C12 conversion and 94% of branched C12 selectivity). The lower selectivity at high nickel loading is due to metal-based cracking reaction. An optimum balance between metal and acid centers is needed to achieve a compromise between conversion and selectivity, avoiding or minimizing cracking reactions.\n               "}
{"full_text": "In recent years, hydrogen has gained increasing attention as an alternative to fossil fuels, enabling net zero targets to be realized. As stated in the 2021 UK Hydrogen Strategy [1], the UK is aiming for a total of 10\u00a0GW of low-carbon hydrogen production capacity by 2030 to decarbonize vital industries and provide clean energy across the heat, power, and transport sectors. This requires considerable effort in scaling up and optimizing carbon capture and storage systems as well as new hydrogen production processes, such as sorption-enhanced reforming.Hydrogen can be produced from a variety of renewable and non-renewable sources, and can be divided into three categories depending on its production pathway.\n\n\u2022\nBlack/Grey/Brown hydrogen: from fossil fuel-based production (coal, natural gas, and lignite, respectively) with CO2 released to the atmosphere.\n\n\n\u2022\nBlue hydrogen: from fossil fuel-based production with carbon capture, utilization, and storage (CCUS).\n\n\n\u2022\nGreen hydrogen: from renewable sources, commonly electrolysis-based production.\n\n\nBlack/Grey/Brown hydrogen: from fossil fuel-based production (coal, natural gas, and lignite, respectively) with CO2 released to the atmosphere.Blue hydrogen: from fossil fuel-based production with carbon capture, utilization, and storage (CCUS).Green hydrogen: from renewable sources, commonly electrolysis-based production.Different hydrogen production pathways using renewable sources have been investigated, including water electrolysis [2], thermochemical (pyrolysis and gasification), or biological conversion (fermentation and photolysis) of biomass [3]. However, various techno-economic studies have demonstrated that compared to these processes, fossil fuel reforming with CCUS remains the most cost-competitive option with the highest hydrogen production efficiency [4\u20136]. Steam reforming, partial oxidation, and autothermal reforming are three main fossil fuel reforming technologies for hydrogen production, among which the steam reforming of methane (SMR) is by far the most deployed method. Although SMR is a mature technology, one of the most significant problems is its high CO2 emission. It is estimated that without CCUS, hydrogen from SMR has an emission factor of 222\u2013325\u00a0gCO2eq per kWh of H2 (10 tCO2/tH2) [4,7].Apart from the traditional approach of employing a downstream amine scrubbing process, an alternative option to mitigate carbon emissions is adding a simultaneous carbon capture step to the conventional SMR process. A novel hydrogen production technology, known as sorption-enhanced steam methane reforming (SESMR), combines the conventional SMR process with a simultaneous in-situ absorption of CO2 using a solid sorbent (usually CaO). The main reactions involved in the SESMR process are as follows [8].Steam reforming of methane\n\n(1)\n\n\n\n\n\nCH\n4\n\n+\n\nH\n2\n\nO\n\n\u2194\nCO\n+\n3\n\nH\n2\n\n\n\n\u0394\n\nH\n298\n\u00b0\n\n=\n+\n206\n\nkJ\n/\nmol\n\n\n\n\n\n\nWater-gas shift (WGS) reaction\n\n(2)\n\n\n\n\nCO\n+\n\nH\n2\n\nO\n\n\u2194\n\nCO\n2\n\n+\n\nH\n2\n\n\n\n\u0394\n\nH\n298\n\u00b0\n\n=\n\u2212\n41\n\nkJ\n/\nmol\n\n\n\n\n\n\nCO2 sorption and sorbent regeneration\n\n(3)\n\n\n\n\n\nCO\n2\n\n+\nCaO\n\n\u2194\n\nCaCO\n3\n\n\n\n\u0394\n\nH\n298\n\u00b0\n\n=\n\u2212\n178\n\nkJ\n/\nmol\n\n\n\n\n\n\nOverall equation for SESMR\n\n(4)\n\n\n\n\n\nCH\n4\n\n+\n2\n\nH\n2\n\nO\n+\nCaO\n=\n4\n\nH\n2\n\n+\n\nCaCO\n3\n\n\n\n\u0394\n\nH\n298\n\u00b0\n\n=\n\u2212\n13\n\nkJ\n/\nmol\n\n\n\n\n\n\nIn comparison to the traditional SMR process, SESMR enables the removal of CO2 from the reaction zone, which shifts the equilibrium towards the product side, enhancing the production of hydrogen. The high-purity CO2 stream released from the sorbent regeneration step can also be easily separated from the sorbent, and transported or stored for further use. The sorption enhanced steam reforming process has been applied to other feedstocks as well, including phenol [9], glycerol [10], ethanol [11], and biomass [12]. In general, the CO2 sorbent is combined with active catalytic metal(s) to form a bi-functional material, using alumina, perovskite or mayenite as the structural support. Since both SMR and SESMR require the use of catalysts to proceed, and the types of catalysts used for both processes are in general identical, they will be reviewed holistically in this paper.The main reaction steps of SMR are listed in Table 1\n, including the dissociative adsorption of the reactants, dehydrogenation and bond reformation steps. It is generally agreed that the activation of the first C\u2013H bond of the CH4 decomposition step (step 1) is the rate-determining step of SMR [13\u201315]; but at lower temperatures (T\u00a0<\u00a0500\u00a0\u00b0C), the CO formation step (step 7) becomes dominant [14]. The energy barrier for C\u2013H bond activation over Ni surface is relatively low, and at the same time, the adsorption of C\u2217, H\u2217 and O\u2217 is not so strong that the species cannot react off the surface easily. Nickel is also widely employed commercially due to its low price and high availability. However, Ni-based catalysts are prone to sintering and coke formation [16,17], hydrogen reduction of nickel-based catalysts before the reforming process is also necessary for activating the material. It is, therefore, of interest to investigate the anti-sintering ability, coke resistance, as well as the reducibility and self-activation ability of SMR catalysts.Apart from nickel, noble metals (Rh, Ru, Pd, Pt, and Ir) are also promising candidates for SMR because of their excellent catalytic ability and resistance to carbon formation. Currently, there is no definitive conclusion as to how the noble metals are ordered regarding their catalytic activity for SMR, however, several experimental [18,19] and numerical [20,21] studies reported that the catalytic activity of noble metals follows the order of Rh\u00a0\u223c\u00a0Ru\u00a0>\u00a0Ir\u00a0>\u00a0Pt\u00a0\u223c\u00a0Pd. Despite their advantages, noble metal-based catalysts are limited by their high prices.One way to maintain the excellent performances of noble metals while maintaining a reasonable price is by combining two or more types of metals, using cheap transition metals (usually nickel or cobalt) as the base and noble metals as promoters. Bi/polymetallic catalysts have gained increasing attention in recent years, and the synergistic effect between commonly used metal elements has been investigated experimentally and numerically. Numerical studies focus on the reaction pathway, activation energies of certain reaction steps (in particular the C\u2013H bond cleavage of the CH4 decomposition step), as well as the adsorption energies of atomic or molecular species on the catalyst's surface, which are indicators of the material's catalytic activity and stability. Some materials were also tested experimentally, usually in lab-scale reactors, and evaluated based on their methane conversion ability, hydrogen yield, etc. To the best of the author's knowledge, there has been no literature that systematically summarizes the bi/polymetallic catalysts that have been employed in (SE)SMR. The aim of this review is therefore to provide an overview of the bi/polymetallic catalysts that have been tested for (SE)SMR, to summarize their advantages and limitations, and to identify the gap in current (SE)SMR catalyst development for future studies.Amongst the eight noble metals, platinum group metals (including Rh, Ru, Ir, Pt, and Pd) have the highest SMR activity and are most commonly used as a promoter for Ni-based catalysts. Their catalytic performance arises from the partially filled d-subshells \u2013 electrons can be easily added to or removed from these orbitals, resulting in an optimal interaction between the metal surface and the gas-phase adsorbate. A lower degree of d-band filling leads to the too-strong adsorption of both reactants and reaction products on the metal surface, which easily blocks the active catalytic surface. On the other hand, with a higher degree of filling, the metal surface does not interact strongly enough with the reactants, which is the case for both Ag and Au. This results in a relatively low catalytic activity but a more stable and clean metal surface, which is why Ag and Au are usually added for their coke-resistant property. There is currently no literature available regarding the use of Os as an SMR catalyst, possibly due to its tendency to form a volatile and toxic oxide \u2013 OsO4 [18].Ru\u2013Ni catalysts have been tested under lab-scale experimental SMR conditions by Jeong et\u00a0al. [22] to study the effect of doping Ru over Ni/Al2O3 and Ni/MgAl2O4. They concluded that adding a small amount of Ru (0.5\u00a0wt.%) greatly suppressed coke formation on the catalyst surface and facilitated NiO reduction. The coke-resisting ability of Ru was studied on an atomic scale using Density Functional Theory (DFT) study [23]. It was demonstrated that when the noble metal was added, the activation energy of the CHO\u2217-producing step (step 5 shown in Fig.\u00a01\n) was significantly lower than that of the C\u2217 and H\u2217-producing step (step 4), meaning CO production was favoured over carbon deposition.Results also showed that Ru-promoted Ni catalysts were able to self-activate at a temperature of 700\u00a0\u00b0C without any pre-reduction using hydrogen, which is beneficial from an economic and process operation point of view [24,25]. Ni-based catalysts doped with small (0.5\u00a0wt.%) or even trace amounts (0.05\u00a0wt.%) of Ru showed good self-activation properties without pre-reduction, and achieved a higher CH4 conversion rate compared to monometallic Ru or Ni catalyst. Ru decreased the reduction temperature of Ni by inducing hydrogen spill over on the Ni surface, a process in which H2 molecules disassociate on the noble metal surface to H species and diffuse into Ni via the catalyst support.Similar to Ru, the addition of Rh was also found to facilitate Ni reduction and produce a synergistic effect [26,27]. The bimetallic catalyst (with 0.2\u00a0wt.% Rh) showed higher activity compared to the linear combination of monometallic Rh or Ni catalyst. This was attributed to the enhanced textural properties of the bimetallic catalysts \u2013 the exposed metallic surface area and metal dispersion of the bimetallic catalysts, measured by CO chemisorption, were greatly promoted by the addition of Rh, and the increase was more significant with a higher Rh loading. The same synergistic effect was observed by Morales-Cano et\u00a0al. as well [28], who studied the promoting effect of Ru, Rh, and Ir on Ni/\u03b1-Al2O3 catalyst. The catalysts were also aged for 240\u00a0h at 800\u00a0\u00b0C with an S/C ratio of 6 to induce the sintering of Ni particles. The activity of the aged Ni\u2013Rh and Ni\u2013Ir was found to be significantly higher than the aged monometallic catalysts, proving their ability to resist Ni sintering. This ability was attributed to the migration and diffusion of Ni into the Face Centred Cubic structure of Rh and Ir during the aging process, which enabled the formation of Ni\u2013Rh and Ni\u2013Ir alloys, and retained the high surface area of the materials.The optimal loading of noble metals in Ni-based catalysts was also investigated. Katheria et\u00a0al. [29] tested a series of Ni/MgAl2O4 catalysts with Rh concentration varying from 0.1 to 1\u00a0wt.%. Results showed that 0.1\u00a0wt.% of Rh was sufficient to increase CH4 conversion by 20%, whereas a further increase in Rh concentration did not have a significant effect. A higher metal loading does not necessarily mean a better catalytic ability due to the less evenly distribution of active metal in the support. This observation was also verified by testing a series of Ni/MgAl2O4 catalysts with Pt loading varying from 0.01 to 1\u00a0wt.% [30,31]. Both studies reported that a Pt loading of 0.1\u00a0wt.% resulted in the highest catalytic activity. Further increase in Pt loading led to a decrease in both catalytic activity and stability. Results from the physical characterization of the materials showed that the highest surface area and maximum dispersion of active metal were achieved with 0.1\u00a0wt.% Pt loading, whereas higher Pt concentration, resulted in agglomeration on the material surface.Chaichi et\u00a0al. [32] synthesized a novel supportless Ni\u2013Pd-carbon nanotube material and compared its performance with Ni and Ni\u2013Pd catalysts under SMR conditions. The addition of Pd facilitated the reduction of metallic oxides, whereas both Pd and carbon nanotube increased the specific surface area. The resultant CH4 conversion of the Ni\u2013Pd-carbon nanotube material was 22% higher than the monometallic Ni/MgO catalyst. Reducibility enhancement by Pd was also reported by Batebi et\u00a0al. [33] in a test of Ni\u2013Pd/Al2O3 for combined steam and CO2 reforming of methane. By adding Pd, the reduction degree was increased from 69% to 83%, leading to higher CH4 conversion and H2 yield while reducing coke deposition. Bimetallic Ni\u2013Pd materials have also been tested for the oxidative SMR process [34\u201338]. Results from these studies demonstrated that the addition of Pd promoted the reduction of Ni, Pd\u2013Ni alloy was also found to form preferentially on the material surface, contributing to its high activity and coke resistance.Li and Miyata conducted a series of tests to study the doping effect of Ru [39,40], Rh [41,42], Pt [42\u201344], and Pd [42] on Ni/Mg(Al)O catalysts in a daily start-up and shut-down operation of SMR under steam purging. As was presented previously, the addition of all four types of promoters improved the reducibility of the catalyst by decreasing Ni reduction temperature and increasing the amount of hydrogen uptake on Ni. Ru, Rh and Pt were also capable of suppressing the deactivation of the catalyst due to Ni oxidation, which was attributed to the self-regeneration of Ni0 from Ni2+ assisted by hydrogen spill over on the noble metal surface and the reversible reduction-oxidation between Ni0 and Ni2+ in the Mg(Ni, Al)O periclase. Self-activation without any reduction treatment of Rh-, Pt-, and Pd\u2013Ni bimetallic catalysts was also observed during the daily start-up and shut-down operation. Compared to the complete deactivation of the pure Ni catalyst after the first steam purging, the CH4 conversion of the bimetallic catalysts was kept at the value of thermodynamic equilibrium even after 4 cycles of steam purging. The self-activation and self-regeneration properties of these bimetallic materials proved Ru, Rh, Pt, and Pd to be useful additives to the conventional Ni-based catalysts.Although the reactivity of monometallic Ag and Au is relatively low, they have also been tested as promoters for Ni-based SMR catalysts due to their excellent stability. DFT-based studies [45,46] showed that the Ag-doped Ni surface is less prone to carbon deposition \u2013 the threefold Ag\u2013Ni\u2013Ni adsorption site is unstable for carbon atoms. Carbon atoms initially positioned at these sites will therefore move to the adjacent Ni\u2013Ni\u2013Ni site, which has lower adsorption energy. These negative interactions between the carbon atom and the Ag\u2013Ni alloy surface indicate that Ag can be added as a coke-resistant promoter, which was also validated against experimental findings [47\u201349]. Both research teams studied the promoting effect of Ag (0.03\u20131\u00a0wt.%) on Ni/\u03b3-Al2O3 and concluded that even the minimum Ag loading of 0.3\u00a0wt.% could reduce carbon deposition significantly. However, this also compromised the catalytic activity of the material, which decreased by 25% compared to monometallic Ni catalysts. This is due to the fact that Ag atoms are energetically favoured to replace Ni atoms on the step edges, which are the most active sites for methane decomposition, compared to the terrace sites [50]. Similar properties were found in Au-doped Ni catalysts. Both computational [51\u201353] and experimental [54,55] studies suggested that the overall catalytic activity of Au\u2013Ni was affected by the higher energy barrier for C\u2013H bond cleavage in the rate-determining CH4 dissociation step; whereas the stability of the material was enhanced due to the suppression of carbon formation.Ag and Au have also been employed as promoters to Ni electrodes for solid oxide fuel cells under internal SMR conditions. Ag [56] or Au [57] with a loading of 1\u20135\u00a0wt.% doped on Ni/yttria-stabilized zirconia (YSZ), as well as Au with a loading of 1\u20134\u00a0at.% doped on Ni/CeO2-Gd2O3 [58] anode were tested at temperatures ranging from 650 to 800\u00a0\u00b0C. Both exhibited satisfying performance with enhanced tolerance to carbon formation. However, the performance of Ag-doped materials is largely influenced by the reaction temperature. At temperatures higher than 750\u00a0\u00b0C, the Ag\u2013Ni/YSZ cell degraded rapidly due to the low melting point of Ag [56]. At temperatures lower than 600\u00a0\u00b0C, the catalytic activity of the Ag\u2013Ni/Al2O3 catalyst decreased significantly, whereas the Au-doped catalyst still exhibited higher activity than monometallic Ni/Al2O3 [59,60]. Sapountzi et\u00a0al. [61] reported that an Au amount of 2.3\u00a0wt.% promoted the reducibility of Ni catalyst, and the Ni\u2013Au alloy formed on the catalyst surface was able to inhibit the formation of sulphuric compounds, including nickel sulphide.Liu et\u00a0al. [62] reported that Ni\u2013Ir alloy supported on MgAl2O4 was a durable catalyst for steam and CO2 bi-reforming of methane under pressurized conditions. The bimetallic catalyst was composed of small metallic clusters (with a mean size of \u223c2\u00a0nm) and the cluster size was retained for a duration of 434\u00a0h, in contrast to the significant increase in cluster size of the monometallic Ni/MgAl2O4 catalyst, showing the anti-sintering ability of Ir. The coke resistance of the bimetallic material was attributed to the combined effect of small ensemble sizes, increased surface oxophilicity, and higher activation barrier for CH4 dissociation. The number of active sites was evaluated by H2 chemisorption, it was found that an Ir loading of 0.1\u00a0wt.% was able to quadrupole the quantity of active sites of Ni/MgAl2O4, and the promoting effect increased with a higher Ir loading. The bimetallic Ir10Ni90/MgAl2O4 catalyst achieved CH4 and CO2 conversion of 95% and 98%, respectively, at 1\u00a0bar; and was able to maintain a relatively high CH4 and CO2 conversion of \u223c60% when pressurized to 20\u00a0bar.Despite multiple advantages, the use of noble metals is still constrained for economic reasons. Therefore, many researchers have turned to the application of non-noble metals as potential promoters of Ni-based catalysts.Ni\u2013Fe-based catalysts/oxygen carriers have been tested for chemical looping steam methane reforming (CL-SMR). Hu et\u00a0al. [63] used Ni\u2013Fe modified calcite as an oxygen carrier and concluded that a Fe/Ni ratio of 0.67 was optimal for the reaction, while higher Fe concentration led to sintering. The novel material exhibited good catalytic performance with the highest CH4 conversion of 98.9%, and high stability during the long-term reaction process. Garai et\u00a0al. [64] tested Ni-ferrite supported on ZrO2 and CeO2, which showed high H2 and CO selectivity, as well as high CH4 conversion (93%, 98%, and 99%, respectively). No carbon deposition was observed on the used material, due to the ability of iron oxides (FeOx) to remove carbon via a surface redox cycle to produce Fe and CO2 [65]. Djaidja et\u00a0al. [66] reported that although the addition of Fe slightly decreased the CH4 conversion of the (Ni\u2013Mg)2Al catalyst from 93% to 91%, both H2 and CO yield were improved and carbon formation was significantly suppressed.In addition, the Fe-promoted Ni catalyst was also proved to be sulphur-resistant. Tsodikov's team [67,68] tested Ni\u2013Fe/\u03b3-Al2O3 catalysts prepared by epitaxial coating and a novel core-shell type catalyst containing Ni and Fe (Fig.\u00a02\n) under SMR conditions in the presence of up to 30\u00a0ppm H2S. The materials showed good catalytic activity, unlike conventional Ni-based catalysts, which lose activity rapidly when the gas-vapour mixture contains H2S. This property was attributed to the core-shell structure, in which the core containing Ni\u2013Fe nanoparticles provided the catalytic ability while the \u03b3-Fe2O3 shell provided vacancies for H2S to decompose to elemental sulphur following the reactions below:\n\n(5)\n\nFeO\n+\n\nH\n2\n\nS\n\u2192\nFeS\n+\n\nH\n2\n\nO\n\n\n\n\n\n(6)\n\nFeS\n=\nFe\n\n\nvacancy\n\n+\nS\n\n\n\n\n\n(7)\n\nFe\n\nvacancy\n\n+\n\nH\n2\n\nO\n\u2192\nFeO\n+\n\nH\n2\n\n\n\n\nCobalt is also considered a promising SMR catalyst additive because of its good activity for the WGS reaction, which assists in shifting the equilibrium towards higher H2 production. However, one problem related to the usage of Co is its tendency to oxidize when the temperature and steam partial pressure are in the range used for SMR [69]. Alloying it with Ni is a potential solution to this problem while preserving the advantages of both elements. A series of Ni\u2013Co/ZrO2 bimetallic catalysts with different Co loadings were tested and compared to monometallic Ni and Co catalysts by Harshini et\u00a0al. [70]. Their results suggested that a Ni/Co ratio of 1:1 was optimal for limiting both oxidation of Co and carbon formation caused by Ni. The material also exhibited long-term stability with a constant CH4 conversion of 81.8% within 50\u00a0h with no surface carbon formation. You et\u00a0al. [71] tested a series of Ni\u2013Co/\u03b3-Al2O3 catalysts and found that at a temperature of 800\u00a0\u00b0C Co-modified catalysts exhibited the same reforming activity as unmodified ones with enhanced coke resistance. The performance of the bimetallic catalyst was not as good as Ni at low temperatures, possibly due to the formation of Ni\u2013Co alloy, which increased the crystallite and particle size of the material, while decreasing metal dispersion and surface area, and blocking low-coordinate active Ni sites where the rate-determining CH4 dissociation step takes place.Ni\u2013Cu bimetallic catalysts have been used for the conventional SMR process [60] as well as low-temperature SMR [73\u201375]. Results showed that by adding Cu as the promoter, a larger Ni crystallite size, surface area, and a better metal dispersion was obtained [74], and the overall carbon resistance of the material was enhanced [66,72]. TGA results before and after a long-term SMR test (20\u00a0h) showed that carbon formation on Ni\u2013Cu/Al2O3 was great suppressed (8.9%) compared to commercial Ni/Al2O3 (28.3%). The addition of Cu has also been proven to enhance the activity of the WGS reaction [76], which explains the increase in CH4 conversion when using Cu\u2013Ni as the catalyst. However, it should be noted that an upper limit exists in terms of the promoting effect of Cu. A Cu/Ni ratio equal to or higher than 5 in the material will result in reduced catalytic activity, as reported by Huang et\u00a0al. [73].The promoting effect of Zirconium was investigated by Lertwittayanon et\u00a0al. [77] using Ni/\u03b1-Al2O3 catalysts containing CaZrO3 nanoparticles. CaZrO3 loading between 10 and 15\u00a0wt.% showed the best catalytic performance with a CH4 conversion of 67%. Results also showed that unlike conventional SMR catalysts requiring an S/C ratio of approximately 3, an S/C ratio of 1/3 or 1 was most appropriate for the CaZrO3-modified catalyst. This is because a high S/C ratio causes an excessive amount of steam to adsorb on the CaZrO3 surface, which competes with the adsorption of CH4.Boudjeloud et\u00a0al. [78] tested a series of La-promoted Ni/\u03b1-Al2O3 catalysts. The highest CH4 conversion (97%) and H2 yield (94%) were obtained with a Ni/La ratio of 7:3. The improved activity compared to monometallic Ni was credited to the decrease in Ni particle size and enhanced metal dispersion, which prevented the agglomeration and sintering of the bimetallic material. The addition of La also facilitated the reduction of Ni, however, its effect on coke resistance was not significant [27].The effect of doping Molybdenum was studied by Maluf and Assaf [79] using Mo\u2013Ni/Al2O3 catalysts with different Mo concentrations. The addition of Mo decreased the surface area of the catalyst, possibly due to the blockage of active Ni sites by MoOx. However, the specific activity of each active site was increased, which was attributed to the transfer of electrons from MoOx to Ni particles resulting in an increase in electron density in Ni. Molybdenum carbide has also been employed in the methane reforming process and is known to have good catalytic activity and stability at high pressures [80]. However, its stability quickly drops at atmospheric pressure due to the surface oxidation of Mo2C to MoOx by CO2. This problem can be mitigated by combining Mo2C with nickel. Despite being a major reason for the deactivation of traditional Ni catalysts, carbon deposited on the bimetallic Ni\u2013Mo2C surface promotes the carburization of NiMoOx back to its carbide form [81,82]. Ni\u2013Mo2C catalysts have been tested for dry methane reforming [81\u201383], steam reforming of methanol [84], as well as steam-CO2 bi-reforming of methane [85], and have shown more promising results than unpromoted Mo2C catalyst or Ni\u2013Mo catalyst in their reduced form.The promoting effect of the rare earth element, rhenium, was reported by Xu et\u00a0al. [86]. They concluded that by coating a Ni\u2013Re bimetallic layer on the surface of a high cell density Ni monolith catalyst, the reducibility and catalytic performance of the material were enhanced. The low hydrogen adsorption energy of Re atoms also facilitates the adsorption of hydrogen atoms on Re and adjacent Ni atoms, which suppressed the oxidation of Ni and led to enhanced catalyst stability.Silicon is one of the most studied metalloids as it is often employed as the catalyst support for the SMR process, usually in its oxide or carbide form, because of its thermal stability and potentially high surface area. Silica is generally considered an inert material, as it has weak metal-support interaction with the active metal (Ni in most cases) due to its low reducibility [87]. The lack of metal-support interaction in Ni/SiO2 is also a source of filamentous whisker carbon formation [88]. To improve the interaction between Ni and the silica support matrix, Majewski et\u00a0al. [89] synthesized a core-shell typed Ni/SiO2 catalyst using the St\u00f6ber-deposition-precipitation method, and tested it under different SMR conditions. Results showed that the core-shell structure increased the coke resistance of the catalyst, as deposited carbon was only detected at a low s/c ratio (1:1) and temperature (550\u00a0\u00b0C). Other characteristics of the support material, including crystallite size and metal dispersion, are affected by the acidity/basicity of the support and are also known to influence the rate of carbon growth on the catalyst surface. The acidity of the silica support facilitates the decomposition of methane, but at the same time promotes cracking and polymerization leading to catalyst deactivation because of carbon formation [88]. To achieve a better acid-basic balance, basic metal oxides, including ceria [90,91] and magnesia [92,93] are often added to Ni/SiO2 to tune the surface acidity of the support, and it was found that a homogeneous distribution of the basic sites on the acidic silica framework improved the long-term stability of the catalyst.Elements with a similar electronic structure of carbon include tetra- and penta-valent p such as Sn, Sb, As, Ge, Pb, Ag, etc. The addition of these metals was predicted to have a coke-resisting effect, because, similar to the formation of nickel carbide (interaction between 2p electrons of carbon and 3d electrons of Ni), the reaction between these metals with Ni could potentially reduce the chance of Ni\u2013C interaction. A few of the above candidates were tested by D.L.Trimm [94], and their coke-resistant ability followed the order of As\u00a0>\u00a0Ag\u00a0> Sb\u00a0>\u00a0Sn\u00a0>\u00a0Pb.The addition of Sn was also investigated by Nikolla et\u00a0al. [95\u201397] experimentally and numerically. The bimetallic Ni\u2013Sn catalysts showed lower CH4 conversion during the first 30\u00a0min of the reaction, however, its long-term stability was greatly enhanced. The carbon resistance of the Ni\u2013Sn/YSZ catalyst was explained by DFT calculated reaction energy barriers, which showed that the Ni\u2013Sn alloy surface preferentially oxidizes C\u2217 rather than forming C\u2013C bonds. The presence of Sn also lowers the binding of C\u2217 to low-coordinated sites, which is the position for carbon nucleation. The decrease in the catalytic activity of Ni\u2013Sn alloy is possibly due to the blockage of low-coordinate Ni sites by Sn, as these sites are the most active for the rate-determining C\u2013H bond activation step.Similar to Sn, boron-promoted Ni catalyst has also demonstrated good stability due to the reduction in carbon nucleation centres. A boron loading of 1\u00a0wt.% was sufficient to enhance the overall stability of the material without compromising its catalytic activity [98]. Apart from this, Ligthart et\u00a0al. [27] also reported the structural-promoting ability of boron for obtaining small Ni particles. However, one limitation of the bimetallic material is that the addition of boron strongly impeded the reduction of Ni.Apart from the experimental work mentioned above, the SMR activity of metalloid-promoted nickel catalysts was also studied using numerical methods. In the work by Xu et\u00a0al. [21], the catalytic activity of a series of bimetallic alloys was predicted based on a microkinetic model, and DFT-calculated atomic adsorption energies on the bimetallic M1M2 (211) surface. By setting the conditions as 793\u00a0\u00b0C, 12.2\u00a0bar, and with gas composition as 50% to equilibrium, alloys including Ni3M (M\u00a0=\u00a0Sn, Sb, Ge, and As) and Co3Ge were predicted to have the highest activity (Fig.\u00a03\n). Although these elements showed promising results, experimental validation of the in-silico study has not been found. Further study can be carried out on these metalloids (Sb, As, Ge)-based catalysts in search of an optimal balance between activity and stability.Apart from nickel, some other transition metals, such as Co, Cu, and Fe, have been used as catalysts for reforming processes (dry and steam reforming of hydrocarbons, glycerol, or bio-derived material). Shen et\u00a0al. [99] tested a series of monometallic Co/CeO2 and bimetallic Co-M/CeO2 catalysts (M\u00a0=\u00a0Ni, Al, and Cu) under conventional SMR conditions to study the effect of Co loading and different promoters. As mentioned previously, higher active metal loading does not necessarily mean better performance because of the uneven distribution of active compounds in the support. A Co loading of 12% was found to be optimal in terms of CH4 conversion and H2 yield. The addition of both Ni and Al increased CH4 conversion, whereas Cu slightly reduced the overall catalytic activity, possibly due to the sintering of Cu. The combination of Ni\u2013Co was chosen over Al\u2013Co because it exhibited higher CH4 conversion (76.1%), H2 selectivity (58.5%), and H2 yield (44.5%).The cobalt-based catalyst prepared from hydrotalcite precursors using the anion-exchange method was tested by Lucredio and Assaf [100] with low H2O/CH4 feed ratios of 2 and 0.5 to test the stability of the material under extreme conditions. For an H2O/CH4 ratio of 2, CH4 conversion was maintained at 80% during 6\u00a0h of reaction; the carbon amount on the used catalyst was found to be only 2.7\u00a0wt.% after 30\u00a0h of reaction. The catalyst began to show a deactivation tendency due to the deposition of excess carbon when H2O/CH4 ratio is further decreased to 0.5, and CH4 conversion decreased from \u223c60% to 40% during 6\u00a0h of reaction.Although Co is less prone to coke formation compared to Ni, the interaction between Co and the metal oxide support is strong, leading to the formation of cobalt oxides with limited reducibility [101]. As presented in section 2.1, by adding a small amount of noble metal the reducibility of the transition metal-based catalysts can be largely improved because of the hydrogen spill over effect. Profeti et\u00a0al. [102] explored the effect of noble-metal promoters (0.3\u00a0wt.% Pt, Pd, Ru, and Ir) on Co/Al2O3 catalysts. Results showed that the addition of the noble metals significantly decreased the reduction temperature of cobalt species, with their promoting effect following the order of Pd\u00a0>\u00a0Pt\u00a0>\u00a0Ru\u00a0>\u00a0Ir. In terms of their catalytic activity, the Co-based bimetallic catalysts did not show satisfying results. Average CH4 conversion of 50\u201360% was obtained for Pd-, Pt- and Ir\u2013Co, 30% for Ru\u2013Co, and only 7% for Co/Al2O3, which was possibly due to the partial oxidation of cobalt active sites in the presence of water molecules.Akbari-Emadabadi et\u00a0al. tested a Ca\u2013Co bi-functional catalyst/sorbent (with a mass ratio of Ca/Co\u00a0=\u00a09) in the CL-SMR process, and investigated the promoting effect of yttrium [103] and zirconium [104], with a mass ratio of Ca/Y = Ca/Zr\u00a0=\u00a04.5. Both promoted samples remained stable at 700\u00a0\u00b0C for up to 16 redox cycles, whereas the unpromoted one was deactivated after 10 cycles. The catalytic performance of the materials is summarized in the table below (Table 2\n). Both Y and Zr showed promoting effect regarding catalytic activity for SMR and H2 selectivity, and the usage of Zr was more advantageous in comparison to Y. Based on the results from catalyst characterization, the addition of Co reduced the overall surface area of the material by more than 30%, whereas the addition of Zr compensated this negative effect to some extent. Results also showed that Zr prevented the formation of Ca2Co2O5 spinel in the structure of the bimetallic material, which lowered the risk of losing active sites of Co. The study proved Y and Zr to be promising textural promoters of the bi-functional catalyst/sorbent materials employed in CL-SMR.Apart from Co-based bimetallic catalysts, catalysts combining two types of noble metal have also been studied. The research by Roy et\u00a0al. [105] focused on a novel Pt\u2013Rh (1.2\u00a0wt.%) catalyst supported on metal foam. The sample was tested in a multichannel heat exchanger platform reactor to evaluate its potential in solid oxide fuel cell application. The combination of Pt and Rh was proven to enhance the production of hydrogen by SMR, with CH4 conversion, H2 yield, and H2/CO ratio of 97.2%, 3.16\u00a0mol per mol of CH4 input and 6.03, respectively, all of which were higher than commercial monometallic Ni and Ru catalysts. The novel catalyst also showed excellent stability, negligible coke deposition was found after 200\u00a0h of SMR reaction at 800\u00a0\u00b0C. Further research from an economic point of view should be carried out to evaluate the potential of these materials in large-scale applications.Compared to the relatively simple mono and bimetallic system, the application of catalysts containing three or more types of active metals in SMR has not been investigated in detail. Existing literature mainly examined Ni-based material with the addition of two or three commonly used elements, such as Co, Cu, Ru, Pt, etc.The effect of the simultaneous presence of copper and zinc in Ni/Al2O3 catalyst was investigated by Nazari and Alavi [106]. They reported that Cu and Zn affect the Ni-based catalyst in different ways \u2013 Cu enables a better resistance to coke formation while Zn improves the catalyst's activity, stability, and H2 selectivity. The optimal combination of the three metals for SMR was found to be 15%Ni\u20131%Cu\u20135%Zn, which achieved a CH4 conversion of 94% and an H2 yield of 3.12.Jeon et\u00a0al. [107] investigated the performance of a selection of bi- and trimetallic Ni-based catalysts under steam-deficient conditions. A series of bimetallic catalysts containing 5\u00a0wt.% of alkaline earth metal (Mg, Ca, Sr, Ba) or 0.5\u00a0wt.% noble metal (Ru, Rh, Pt, Pd), and trimetallic catalysts containing both alkaline earth metal and Ru were synthesized. Results from the tests demonstrated that adding Mg or Ca enhanced the coke resistance of Ni-based catalysts, whereas the effect of Sr and Ba was not significant. Among the noble metals, Ru was the best candidate for suppressing coke deposition. Based on these conclusions, a catalyst with optimized composition \u2013 0.5%Ru\u20135%Mg\u201310%Ni/\u03b3-Al2O3 \u2013 was selected and tested. A CH4 conversion of 96% was maintained for 250\u00a0h, proving its excellent long-term stability.Bi-functional polymetallic materials combining the catalytic ability of transition metals and CO2 sorbents are commonly employed for the SESMR process [108]. Chen et\u00a0al. [109,110] found that a simple physical mixture of Ni/Al2O3 (20\u00a0wt.%) and CaO was able to improve H2 purity to above 95%, compared to 72% without in-situ CO2 removal. Based on an elemental mapping analysis of the samples, each Ni elemental point was surrounded by several Ca points, which allowed the efficient capture of CO2 produced during the reaction.Dewoolkar and Vaidya [111] synthesized hybrid Ni\u2013CaO/Al2O3 and Ni-hydrotalcite materials by coprecipitation and incipient wet impregnation, respectively. The materials were tested at T\u00a0=\u00a0500\u00a0\u00b0C with an S/C ratio of 9 for 20 cycles for stability evaluation. Results showed that CH4 conversion and H2 yield of the hybrid materials were higher than those obtained using a physical mixture of catalyst and sorbent, due to the more efficient mass and heat transfer. The hybrid materials also exhibited better stability, Ni\u2013CaO/Al2O3 and Ni-hydrotalcite maintained high H2 purity of 90% for up to 11 and 16 cycles, respectively, compared to the sintering and deactivation of the mixed material after only 2 cycles.Di Giuliano et\u00a0al. [112,113] reported the use of mayenite as for bi-functional Ni\u2013CaO material. Results from multicycle sorption/regeneration TGA demonstrated the stable sorption capacity of the material after 20 cycles. This enhanced stability compared to commercial CaO was attributed to the presence of mayenite as an inert binding, preventing CaO from sintering. The same phenomenon was also observed by Dang et\u00a0al. [9], confirming the role of mayenite as a structural stabilizer. Di Giuliano et\u00a0al. also concluded that the nickel precursor used for material synthesis may affect the texture and reducibility properties, and nickel nitrate hexahydrate was found to the most suitable precursor.Kim et\u00a0al. used ruthenium as the reforming catalyst, and tested the performance of Ru\u2013CaO/Ca3Al2O6 under SESMR conditions [114]. As Ru is a highly active catalyst for SESMR, the mass fraction of CaO was able to be increased significantly compared to conventional Ni\u2013CaO-based materials. Ca3Al2O6 acted as the structural stabilizer against sintering, and maintained the surface area of Ru\u2013CaO/Ca3Al2O6 at 18\u00a0m2/g after 10 cycles of SESMR, compared to 5 and 6\u00a0m2/g for Ru/lime and Ru/CaO.Hafizi et\u00a0al. [115] modified conventional calcium-based CO2 sorbent with CeO2, and tested it for CL-SESMR together with Co3O4/SiO2. The addition of CeO2 significantly improved the morphology of CaO by increasing its surface area and uniformly distributed pores in the sorbent structure. Combined with the Co-based catalyst, the material was able to produce high purity H2 (93\u201396%) for 8 redox cycles, and maintain the same CO2 removal efficiency for 3 carbonation/calcination cycles.Ghungrud et\u00a0al. [116] reported a novel trimetallic bi-functional material for SESMR, consisting of Ni and Co (with concentrations varying from 0 to 40%) supported on hydrotalcite and promoted by 2.5\u00a0wt.% cerium. The hybrid material was evaluated in terms of its H2 production ability, sorption capacity, and cyclic stability. Results revealed that CH4 conversion increases with the Co content in the material, which is possibly due to the enhancement of WGS reaction by Co. Under optimal reaction conditions, CH4 conversion of 95.7% and 90.7% were obtained by two Ce-promoted Ni\u2013Co samples (with Ni/Co ratios of 1/3 and 1, respectively). The maximum sorption capacity was found to be 1.74 and 1.51\u00a0mol CO2/kg, respectively. The material was also tested at optimal conditions for 25 cycles, samples showed good stability by maintaining an H2 concentration higher than 90%, and remained stable for 21 and 16 cycles. This property was attributed to the effective metal-support interaction and higher active metal dispersion within the promoted material. The author concluded that the hydrotalcite-supported Ce\u2013Ni\u2013Co (2.5, 10, 30\u00a0wt.%) trimetallic catalyst/sorbent shows good performance and could be a promising candidate for large-scale SESMR application.Similarly, Dewoolkar and Vaidya investigated the promoting effect of Ce and Zr on a Ni/hydrotalcite bi-functional material [117]. Results showed that both Ce and Zr were able to increase the surface area and surface basicity, which inhibited coke formation. Both Ce and Zr promoted materials remained stable for 13 and 17 cycles, respectively, whereas the unpromoted material became unstable after 9 cycles. The addition of Ce was found to be particularly beneficial, as the promoted bi-functional material reached a high CH4 conversion of 96.4% and an adsorption capacity of 1.41\u00a0mol CO2/kg sorbent.Apart from the type of promoter added to Ni-based catalysts, the structure of the promoted catalyst also influences its overall performance. Cho et\u00a0al. [118] synthesized a bimetallic catalyst with a novel egg-shell structure by selectively placing Ni and Ru on the shell of alumina. Three types of catalysts were compared, including (a) egg-shell-type bimetallic catalyst (5\u00a0wt.% Ni\u00a0+\u00a00.7\u00a0wt.% Ru/Al2O3), (b) monometallic 1\u00a0wt.% Ru/Al2O3 catalyst with the same egg-shell structure, and (c) monometallic 1\u00a0wt.% Ru/Al2O3 synthesized by the conventional wet impregnation method. The comparison between (b) and (c) showed that the novel structure was able to improve methane conversion and maintain it at a higher level when gas hourly space velocity was largely increased. This was because the novel structure enables the active metal, ruthenium, to be mainly deposited on the outer region of the alumina pellets and can therefore be utilized more efficiently. With the addition of nickel, (a) achieved an even higher methane conversion than (b) at higher gas hourly space velocity. This proved that the novel structure enables the efficient utilization of active noble metals, reducing the metal loading necessary and, therefore, the overall cost.Obradovic et\u00a0al. [119,120] proposed a novel plate-type catalyst for SESMR, as demonstrated in Fig.\u00a04\n. This material was synthesized by depositing Pt and Al2O3 on a static mixer element made of Ni alloy. During the SESMR process, it acts simultaneously as the catalyst, the distributor for solid sorbent, as well as the gas-phase radial mixer. The CH4 conversion obtained by this novel catalyst was 15 times higher than monometallic Ni under the same conditions. However, its catalytic activity rapidly decreased after 10\u00a0h of reaction, temperatures higher than 590\u00a0\u00b0C also led to activity loss due to carbon accumulation on the material surface. Further investigation on modifying relevant parameters (such as the promoter type, metal loading, and reaction conditions) to increase its stability may be of interest.Based on the above review, it can be concluded that an increase in catalytic activity is usually achieved either by modifying the textural properties of the material (e.g. increasing surface area and metal dispersion) using a second active metal; or by modifying the energetics of the surface reactions, in particular the rate-determining step (e.g. decreasing activation energy of the first C\u2013H bond in CH4 dissociation). Although a variety of metal combinations have been studied for their performance in the SMR reaction, there exist many other combinations of elements that have been predicted to be active or have not been evaluated at all regarding their reforming activity.SMR is currently the most dominant hydrogen production technology, and extensive research on the catalytic aspect of this process has been carried out. Carbon emissions from SMR can be reduced by adding a CO2-sorption step. This review provides insights on recent developments in the use of bi/polymetallic catalysts for (SE)SMR. The performance of the bi/polymetallic catalysts presented in this review is briefly summarized in Table 3\n and is evaluated based on three main factors: stability (resistance to carbon, sulphur, sintering, and oxidation), catalytic activity and selectivity, as well as their physical/chemical properties (reducibility and self-activation ability).The most widely used SMR catalyst to date is ceramic-supported nickel because of its relatively good performance and inexpensive price, but problems such as sintering and coke formation still exist. In search of catalysts with better performance, various elements have been added as promoters to conventional Ni-based catalysts. Noble metal-promoted catalysts generally have superior reactivity, coke resistance, and enhanced reducibility, but their applications are often limited by their prices. Researchers have therefore turned to non-noble metals and metalloids. The addition of iron was found to be coke and sulphur-resistant due to the surface redox reaction between Fe0 and FeOx. Both cobalt and copper can enhance the activity of the WGS reaction, thus shifting the reaction towards more hydrogen production. Elements including zirconium, yttrium, and lanthanum were found to be good textural promoters due to their ability to increase the surface area and metal dispersion of the material. Silicon is often used as the catalyst support in its oxide or carbide form, and the addition of ceria or magnesia was able to tune the surface acidity of silica for better long-term stability. The addition of other metalloids (tin, boron) led to enhanced coke-resisting ability, but often at the expense of losing catalytic activity. A DFT-based study has also predicted germanium, arsenic, and antimony to be effective promoters of nickel-based catalysts, however, further experimental verification is still necessary.Apart from the type of element selected as the promoter, the loading of each component is also a critical parameter that influences the overall performance of alloy material. A higher active metal loading does not necessarily indicate a higher activity due to the restricted distribution of active metal in the material. The influence of material structure on catalytic activity was also investigated. Although novel core-shell type, plate type, and metal foam-support structures were found to be beneficial to the overall catalytic performance, this largely complicates the synthesis process and limits the wide application of the materials. It is, therefore, necessary to find a balance between improvements to the material properties and a viable and efficient material preparation process.Siqi Wang: Writing - Original Draft. Seyed A. Nabavi: Writing - Review & Editing. Peter T. Clough: Writing - Review & Editing, Supervision.PTC and SAN thank BEIS for funding under the H2 BECCS competition \u2013 Bio-HyPER project (H2BECCS107).All data underlying the results are available as part of the article and no additional source data are required.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.", "descript": "\n                  Blue hydrogen production by steam methane reforming (SMR) with carbon capture is by far the most commercialised production method, and with the addition of a simultaneous in-situ CO2 adsorption process, sorption-enhanced steam methane reforming (SESMR) can further decrease the cost of H2 production. Ni-based catalysts have been extensively used for SMR because of their excellent activity and relatively low price, but carbon deposition, sulphation, and sintering can lead to catalyst deactivation. One effective solution is to introduce additional metal element(s) to improve the overall performance. This review summarizes recent developments on bi/polymetallic catalysts for SMR, including promoted nickel-based catalysts and other transition metal-based bi/polymetallic materials. The review mainly focuses on experimental studies, but also includes results from simulations to evaluate the synergistic effects of selected metals from an atomic point of view. An outlook is provided for the future development of bi/polymetallic SMR catalysts.\n               "}
{"full_text": "Sustainable renewable energy devices, such as water splitting, fuel cells, and metal-air batteries, have attracted immense attention.\n1\u20134\n However, their large-scale application, such as in Zn-air batteries, has been severely hindered by the intrinsic sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in rechargeable air electrodes. Cost-effective bifunctional catalysts with performance comparable (or even superior) to that of commercial Pt/C or Ru(Ir)O2 have long been sought after.\n5\u20137\n The precise atomic design of transition-metal sites and their local electron engineering through substrates have emerged as an intriguing yet challenging strategy.\n8\u201313\n Given that studies on the rational construction of single-atom transition-metal sites with high OER activity are scarce, great efforts have been devoted to regulating the reaction pathway to facilitate the rate-determining step (RDS), such as O\u2217 and OOH\u2217 formation, through optimizing the d band electron structure of active transition-metal sites.\n14\u201319\n Among them, the multi-site synergistic effects have shown the promising potential to enhance OER efficiency.\n17\n\n,\n\n18\n Specifically, Duan's group demonstrated a relatively high OER activity of Ni-N-C single-atom catalysts (SACs), in which the OH\u2217 adsorbed on the C site facilitated the Ni site to form OOH\u2217 according to the so-called dual-site mechanism.\n20\n Hu's group observed the crucial importance of the formation of Co\u2013Fe dual sites by the adsorption of a trace amount of Fe3+ in KOH electrolyte on the single-atom Co site\n21\n. These intriguing results have, however, seldom been explored in view of the geometric effects of the configuration matching between dual sites and the reaction intermediates.In addition, the local electron modulation of active sites by substrates is also a promising way to enhance catalytic activity. Among them, C-based substrates have been extensively studied because of their unique advantages for electron-transfer facilitation, tunable molecular structures, and strong tolerance to acid and alkaline electrolysis.\n22\u201324\n Heteroatom doping, such as with N and P, endowing C substrates with more chemically active sites for metal coordination and higher electrochemical activity for local electron structure modulation is a challenging metric for achieving simultaneous enhancements of the catalytic stability and activity.\n25\n\n,\n\n26\n Currently, the most acceptable mechanism for the high activity of the dual-metallic catalysts is the formation of one specific higher-valence site through electron transfer from another promoter site, which has been proved to optimize the adsorption energy of intermediates.\n27\n\n,\n\n28\n In this way, the local electron modulation can also be achieved by engineering the first coordination sphere of active metal sites, such as metal-N and metal-O coordination provided by substrates.\n18\n\n,\n\n29\n In addition, further local electron structure modulation by the rational design of substrates providing a second coordination sphere for active metal sites remains intriguing yet challenging. Bearing these in mind, engineering metallic atomic dual sites with modulated local electron structure on N,P-co-doped C could become a potential strategy for synergistically boosting OER performance through simultaneously achieving electron structure optimization and electron- and/or mass-transfer facilitation.Herein, we report a theory-guided design and fabrication of an Fe\u2013Ni dual-site catalyst supported on microporous C for the OER process. We first employed density functional theory (DFT) calculations to reveal the simultaneous activation of OH\u2217 and O\u2217 on Fe\u2013Ni dual sites, and this simple Fe\u2013Ni adjacent association regulated the RDS of the formation of intermediate OOH\u2217 and significantly reduced its formation energy. Moreover, through electron-rich sp3 hybridization P doping, the M-N-P-C moiety modulated both the electronic and geometric structures of the local environment to improve the catalytic properties of active metal sites. The grand canonical Monte Carlo\u00a0(GCMC) simulation further demonstrated the thermodynamic stability of alternative N-coordinated Fe\u2013Ni configuration in the ZIF-8 cage. Thereafter, the Fe\u2013Ni dual sites on N,P-co-doped C were fabricated in situ by one-step pyrolysis of Fe/Ni-adsorbed\u00a0ZIF-8 with NaH2PO2 co-feeding. Consequently, the asymmetric Fe\u2013Ni dual sites and the local P doping in the obtained Fe-Ni-N-P-C comprehensively enhanced OER catalytic performance (superior to that of all reported mono- and diatomic transition-metal N-based catalysts) and could take the place of commercial RuO2. Finally, Fe-Ni-N-P-C also exhibited an attractive ORR activity; a high-performance rechargeable Zn-air battery with Fe-Ni-N-P-C as the bifunctional catalyst for the air cathode was achieved. This transition-metal dual-site construction approach with configuration matching and local electron modulation is a promising strategy for complex electrocatalytic reactions.We explored the atomic multi-site structure of the most favorable catalyst by comparing the formation energy (\u0394G) profiles of four typical steps during the OER process (Equations S11\u2013S14) on the geometric optimized catalysts of the representative Fe/Ni-based catalysts\n30\n\n,\n\n31\n (Figures S1 and S2). Among them, Fe-Ni-N-P-C, with the heteroatomic Fe\u2013Ni dual sites coordinated by N and P doping in nearby C, exhibited the lowest energy barrier of 1.90 eV for the commonly accepted RDS of the OOH\u2217 formation (Figure\u00a01\nD). It is worth mentioning that the formation energies of OH\u2217 and O\u2217 on the SACs of Fe-N-P-C or Ni-N-P-C (Figure\u00a0S2) were quite different in that the Fe site favored the stable adsorption of OH\u2217 (0.23 eV), whereas the Ni site facilitates\\d the formation of O\u2217 (0.31 eV). This provides the possibility to regulate the OOH\u2217 formation pathway through a simple adjacent association of neighboring Fe-OH\u2217 and Ni-O\u2217, consequently boosting the OER efficiency on the asymmetric Fe\u2013Ni dual sites. This proposed mechanism was vividly verified by the OOH\u2217 adsorption on Fe-Ni-N-P-C, where both O atoms were bonded with Fe\u2013Ni dual sites through bidentate bonding (Figure\u00a01A), whereas only one O atom was bonded with the Fe or Ni SACs through monodentate bonding (Figure\u00a01B), resulting in a high energy barrier of the OOH\u2217 formation. More precisely, the much stronger binding energy of OOH\u2217 on the Fe\u2013Ni dual sites (\u22121,632\u00a0kJ mol\u22121) than on the symmetric Fe\u2013Fe (\u22121,517\u00a0kJ mol\u22121) and Ni\u2013Ni (\u22121,267\u00a0kJ mol\u22121) diatomic sites (Figure\u00a0S3) further demonstrates the importance of the heteroatomic Fe\u2014Ni dual sites on the precious configuration matching. Moreover, Bader analysis of the electron density\n32\u201334\n further revealed the contribution of the local electronic engineering through the electron-rich sp3 hybridization P doping: the electron donation to OOH\u2217 significantly increased from 0.62 e (without P doping) to 1.05 e (with P doping) such that the charge redistribution on Fe\u2013Ni dual sites endowed them with stronger metallic activity (Figures 1A, 1B, and S4 and Table S1). In contrast to the bidentate bonding form on Fe-Ni-N-P-C, the OOH\u2217 adopted the monodentate binding form on Fe-Ni-N-C as a result of its relatively lower electron density without P doping, resulting in an increase in formation energy (1.95 eV). Therefore, the asymmetric Fe\u2013Ni dual sites and the local electron engineering by P doping were able to synergistically achieve an optimized configuration matching with OOH\u2217, reducing the formation energy of the RDS in OER.As a crucial factor in practice, the thermodynamic stability of this promising Fe\u2013Ni dual-site configuration was demonstrated by a GCMC simulation (simulation details can be found in the supplemental information). The full-atomic models with two different initial configurations of alternative and parallel distributions of Fe3+ and Ni2+ in the typical template of a ZIF-8 cage,\n35\n\n,\n\n36\n corresponding to the asymmetric Fe\u2013Ni dual sites and symmetric diatomic Fe\u2013Fe and Ni\u2013Ni sites, respectively, were constructed (Figures 1C and S5). However, ZIF-8 has proved to be an ideal sacrificed template for providing N coordination sites to stabilize Fe\u2013Ni dual sites and the C matrix. When the configuration geometry approaches equilibrium, the alternative distribution of Fe3+ and Ni2+ is much more stable in the ZIF-8 cage, whereas the parallelly distributed Fe3+ and Ni2+ ions repel each other and consequently redistribute themselves in different cages. This provides clear evidence that the formation of asymmetric Fe\u2013Ni dual sites is more thermodynamically favorable than the symmetric diatomic Fe\u2013Fe and Ni\u2013Ni sites. Moreover, an out-of-plane geometric structure caused by P sp3 hybridization is able to reduce the strain of C matrix (Figure\u00a01A) and hence improve the stability.\n37\n\nAccordingly, we precisely fabricated the Fe\u2013Ni dual sites in N,P-co-doped C through a simple in situ adsorption-pyrolysis method\n38\n\n,\n\n39\n by using porous ZIF-8 as the cage for alternative N-coordinated Fe\u2013Ni dual sites and the template of the C matrix, as well as by co-feeding NaH2PO2 for P doping (synthesis details can be found in the supplemental information). X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that the intrinsic highly uniform rhombic dodecahedron crystal structure of ZIF-8 was well preserved after the incorporation of Fe3+ and Ni2+ (Figures S6A and S6B). After the pyrolysis, Fe-Ni-N-P-C still maintained the morphology of the parent ZIF-8 with a slight size shrink (Figures 2A and 2B). The Raman spectra of Fe-Ni-N-P-C possessed two D and G band peaks at 1,362 and 1,587\u00a0cm\u22121, respectively, and the intensity ratio of the D1 to G bands (ID1/IG) for Fe-Ni-N-P-C was calculated to be 3.79, which is higher than that for Fe-Ni-N-C without P doping (3.13), indicating the formation of a defect-rich C matrix due to the heteroatom doping (Figure\u00a0S6D). In addition, the hierarchical porosity with a high Brunauer-Emmett-Teller (BET) surface area of 625 m2 g\u22121 and a pore size of 1\u20134\u00a0nm (Figure\u00a0S8 and Table S3) made the catalytic active sites highly exposed and facilitated the adsorption of OH\u2212 and the diffusion of O2. Only typical graphite peaks at 24\u00b0 and 43\u00b0\n40\n (no characteristic peaks of either Fe or Ni species) were observed in its XRD pattern (Figure\u00a0S6C). The transmission electron microscopy (TEM) image of Fe-Ni-N-P-C showed a hollow structure due to the typical Kirkendall effect\n41\n\n,\n\n42\n without the formation of any metal nanoparticles (Figure\u00a02C). Closer observation in the aberration-corrected high-angle annular dark-field (HAADF) scanning TEM (STEM) revealed many highly dispersed small bright dual dots (marked as red circles) throughout the C matrix (Figures 2J, 2K, and S7E\u2013S7H), indicating the formation of atomic dual sites. Energy-dispersive X-ray (EDX) mapping and inductively coupled plasma (ICP) emission spectrometry showed that Fe, Ni, N, and P were highly uniformly dispersed in the C matrix (Figures 2D\u20132I) with mass contents of 1.3%, 1.1%, 6.75%, and 1.7%, respectively.The bonding configurations of Fe-Ni-N-P-C are illustrated by X-ray photoelectron (XPS) spectroscopy (Figure\u00a0S9), in which the coordinated N-M peak at 399.6 eV can be clearly observed in the high-resolution N 1s spectrum,\n43\n\n,\n\n44\n suggesting the formation of N\u2013Fe and N\u2013Ni sites. The co-existence of Fe2+ (708.8 and 724.6 eV) and Fe3+ (711.8 and 726.1 eV) and the mixed valence (855.3 eV) between Ni0 (825.5 eV) and Ni2+ (855.7 eV) are evident in the high-resolution Fe 2p and Ni 2p spectra,\n45\n and the absence of peaks belonging to Ni3+ species can be ascribed to the partial reduction of Ni2+ by adjacent C and N atoms during the pyrolysis at high temperature. Because the prior stable N-coordinated Fe\u2013Ni sites were in the ZIF-8 cage before the pyrolysis, the P 2p spectrum shows no characteristic peaks of P-M bonds\u2014only P\u2013C (132.8 eV) and P\u2013O (133.9 eV) bonds.\n46\n\n,\n\n47\n Notably, no characteristic peaks of FePx or NiPx species are evident, which could be attributed to the prior formation of metal\u2013N bonds in the Fe/Ni-ZIF-8 precursor during the pyrolysis; thereafter, Fe or Ni species can hardly react with the PH3 generated by NaH2PO2\u22192H2O. Therefore, the designed configuration of Fe-Ni-N-P-C suggests that Fe\u2013Ni dual sites are coordinated with N in the C matrix with P doping nearby (Figure\u00a03\nA).This proposed atomic configuration of Fe-Ni-N-P-C (Figure\u00a03A) was further verified by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) measurements. Fe-Ni-N-P-C showed a Ni K-edge XANES spectrum similar to that of Ni phthalocyanine (NiPc), in which a stronger peak occurs at around 8,334 eV (Figure\u00a03B) as a result of the distorted D4h symmetry of Ni caused by the metal-metal path.\n48\n\n,\n\n49\n Moreover, the EXAFS spectrum of Ni in Fe-Ni-N-P-C (Figure\u00a03C) exhibits two main peaks at 1.58 and 2.44\u00a0\u00c5, suggesting the co-existence of Ni\u2013N and metal\u2013Ni sites, respectively, neither of which exists in the spectrum of SAC Ni-N-C or Fe-N-C\n50\n or is the location of the Ni-Ni or Fe-Fe peak in Ni or Fe foil, respectively; thus, this peak can be attributed to Fe\u2013Ni dual sites. Similar results were also obtained from the Fe K-edge XANES and EXAFS spectra (Figures 3D and S10A), which show distinct Fe\u2013Ni dual sites at 2.48\u00a0\u00c5. It is worth noting that the peaks of Ni\u2013N (1.58\u00a0\u00c5) and Fe\u2013N (1.52\u00a0\u00c5) show significant shifts compared with NiPc (1.30\u00a0\u00c5; Figure\u00a03C) and FePc (1.45\u00a0\u00c5; Figure\u00a03D), and this can be attributed to the metal-metal interaction between Fe\u2013Ni dual sites and the sp3 hybridization P doping with a relatively large atomic radius. It is worth mentioning that the shifted Ni\u2013N and Fe\u2013N peaks might overlap the reported Ni\u2013O (1.6\u00a0\u00c5) and Fe\u2013O (1.5\u00a0\u00c5) peaks,\n18\n\n,\n\n51\n but it is hard to form Ni-O and Fe-O bonds because of the lack of O-rich coordinated sites in the ZIF-8 cage and the much weaker metal-O coordination than metal-N coordination.\n52\n Therefore, we think that Fe3+ and Ni2+ are coordinated with N sites in the ZIF-8 cage and convert to Fe/Ni\u2013N species during the pyrolysis. More specifically, the DFT calculation also confirms this proposed atomic configuration of Fe-Ni-N-P-C. The first shell-fitting results reveal that the coordination number of Fe\u2013N and Ni\u2013N is 3.65 and 3.35, respectively, suggesting the co-existence of N3\u2013Fe\u2013Ni\u2013N3 dual-metallic coordination and Fe\u2013N4 and Ni\u2013N4 single-metallic coordination (Figure\u00a0S10B and Table S4). We also calculated that 66% of metal\u2013N bonds in Fe-Ni-N-P-C are derived from Fe\u2013Ni dual sites, and the rest of the N is coordinated with single Fe or Ni atoms (details in Figure\u00a0S11). More importantly, the calculated Fe\u2013Ni path length of 2.37\u00a0\u00c5 (Figure\u00a0S10C) is in good agreement with the EXAFS spectra of 2.44\u00a0\u00c5 for Ni\u2013M and 2.48\u00a0\u00c5 for Fe\u2013M (Figures 3C, 3D, and S10D). Both second shell-fitting results of Ni and Fe K-edge EXAFS also fit well with the experimental spectra (Figures 3E and 3F).As a proof of concept, determined by the rotating disc electrode (RDE) approach, Fe-Ni-N-P-C with an Fe/Ni mass ratio of 1:1 exhibited the smallest overpotential of 337\u00a0mV at a current density of 10 mA cm\u22122 in 0.1\u00a0M KOH (Figure\u00a04\nA) and further decreased to 250\u00a0mV in 1\u00a0M KOH (Figures S12 and S13A). More significantly, the calculated mass activity was as high as 8,894 A g metal\u22121 at an overpotential of 350\u00a0mV, and the corresponding turnover frequency (TOF) reached 0.66 s\u22121. To the best of our knowledge, this exceeds all the reported transition-metal N-based catalysts (M-N-C) (Table S5). It is worth mentioning that, compared with single-atom Fe-N-C and Ni-N-C, the physical mixture of Fe/Ni-N-C-PM, and N-C with trace Zn (0.015%) counterparts, Fe-Ni-N-C exhibited a much smaller overpotential of 395\u00a0mV to deliver the current density of 10 mA cm\u22122, indicating that excellent OER activity originates in the atomic Fe\u2013Ni dual sites rather than single-atom Fe\u2013N/Ni\u2013N sites or the trace Zn site. In addition, the smaller OER overpotential of Fe-Ni-N-P-C (335\u00a0mV) than of Fe-Ni-N-C (395\u00a0mV) demonstrates the important role of P doping for enhanced OER activity, which is consistent with the results of the DFT calculation.In addition, the slope of the Tafel plot of Fe-Ni-N-P-C was calculated to be only 76\u00a0mV dec\u22121 (Figure\u00a04B), much lower than that of commercial RuO2 (120\u00a0mV dec\u22121); moreover, the larger slope of Fe-Ni-N-C without P doping (100\u00a0mV dec\u22121) further demonstrates the synergistic effect of Fe\u2013Ni dual sites and P doping on facilitating the OER kinetics through configuration matching and local electronic environment modulation. In addition, the intrinsic charge-transfer resistance of Fe-Ni-N-P-C was also measured by electrochemical impedance spectroscopy (EIS) analysis. The Nyquist plot of Fe-Ni-N-P-C showed the smallest semicircle in the low-frequency region (Figure\u00a0S14), indicating the charge-transfer superiority over other controlled samples. Moreover, attributed to the thermodynamically stable Fe\u2013Ni dual sites, Fe-Ni-N-P-C demonstrated excellent durability over 1,000 cyclic voltammetry (CV) tests (Figure\u00a0S13B), and no obvious peak shifts were observed in the high-resolution Fe 2p or Ni 2p spectra of Fe-Ni-N-P-C after the OER cycle (Figure\u00a0S15). A 12 h, the chronoamperometric response remained 86% of the initial current density (Figure\u00a04C), superior to that of commercial RuO2 (66%).More particularly, Fe-Ni-N-P-C also exhibited an attractive ORR activity with a half-wave potential (E1/2) of 0.823\u00a0V (versus reversible hydrogen electrode [RHE]; Figure\u00a0S16A), close to that of commercial 20% Pt/C (0.830 V); its lower Tafel slope of 87\u00a0mV dec\u22121 also demonstrates the favorable reaction kinetics compared with those of Pt/C (90\u00a0mV dec\u22121; Figure\u00a0S16B). Moreover, further analysis of the Koutecky-Levich plot and rotating ring-disk electrode (RRDE) testing (Figures S16D\u2013S16F) both showed the near-four-electron ORR pathway for Fe-Ni-N-P-C with low H2O2 yield (4%). Importantly, no obvious decay was observed in E1/2 after 3,000 continuous potential cycles, indicating superior long-term stability to Pt/C (Figure\u00a0S16C). Attributed to both excellent OER and ORR activities, the reversible overpotential (\u0394E) achieved on Fe-Ni-N-P-C (0.744 V; Figure\u00a04D) was lower than that on commercial Pt/C (1.015 V), RuO2 (0.960 V), and many studied OER and ORR bifunctional electrocatalysts (Table S6). Inspiringly, a rechargeable Zn-air battery was assembled to illuminate the orange light-emitting diode (LED) with an open-circuit voltage of 1.458\u00a0V (battery assembling details can be found in the supplemental information) (Figures 5A, 5B, and S17). Other excellent characteristics include high constant current discharge potential (1.24\u00a0V at a current density of 10 mA cm\u22122), excellent discharge peak power density (120 mW cm\u22122), small charge-discharge potential gap at high current density, and long-term cycle stability with an energy efficiency of 64% after 90 h, which is superior to that of the Pt/C\u00a0+ RuO2 mixture (57%; Figures 5C\u20135E and S17). It is worth mentioning that although the polarization curve of Pt/C\u00a0+ RuO2 coincided with that of Fe-Ni-N-P-C at a lower current density, as the current density increased (>225 mA cm\u22122), the voltage of Fe-Ni-N-P-C stayed relatively high while the voltage of Pt/C\u00a0+ RuO2 cathode dropped dramatically (Figure\u00a0S17). This superiority is consistent with its small charge-transfer resistance and large active surface area. Attributed to its porous hollow structure, Fe-Ni-N-P-C provides facilitating channels for O2 diffusion and electrolyte transportation and ultimately enhances electron transport and the electrode reaction. The assembled battery performance based on the Fe-Ni-N-P-C cathode was superior to that of not only commercial Pt/C\u00a0+ RuO2 but also many other reported cathode catalysts (Table S7). The promising application of the Fe-Ni-N-P-C-based cathode in the rechargeable Zn-air battery provides clear evidence that it is a practical catalyst design and engineering strategy at the atomic level.In summary, we have demonstrated a DFT-guided strategy for OER catalytic active-site design at atomic precision. The constructed Fe\u2013Ni dual sites with P doping are able to facilitate the formation of OOH\u2217 through geometric matching because of their asymmetric affinities with OH\u2217 to O\u2217, as well as the local electron environment engineering for strong electron donation. Confirmed by the GCMC simulation, the thermodynamically stable catalyst of Fe-Ni-N-P-C has been precisely fabricated through the pyrolysis of alternative N-coordinated Fe\u2013Ni dual sites in the ZIF-8 cage by co-feeding NaH2PO2 for P doping. The resultant Fe-Ni-N-P-C has demonstrated a superior OER activity with a very low overpotential of 250\u00a0mV at a current density of 10 mA cm\u20132 and a high TOF of 0.66 s\u20131 at an overpotential of 350\u00a0mV, largely exceeding those of commercial RuO2 and all reported transition-metal N-based catalysts. Moreover, a high-performance rechargeable Zn-air battery has been achieved with attractive ORR activity (half-wave potential of 0.82 V). We believe that this robust strategy of atomic configuration matching with local electron modulation opens a new window for electrocatalyst construction.Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Jun Hu (junhu@ecust.edu.cn).All materials generated in this study are available from the lead contact without restriction.All data needed to support the conclusions of this manuscript are included in the main text or supplemental information.The Fe\u2013Ni dual sites in N,P-co-doped C were synthesized by a double-solvent method. Typically, ZIF-8 was dispersed in n-hexane (10\u00a0mL) under ultrasound for 10\u00a0min at room temperature. Subsequently, a controlled amount of FeCl3\u22196H2O aqueous solution (100\u00a0mg mL\u22121, 30\u00a0\u03bcL) and Ni(NO3)2 aqueous solution (100\u00a0mg mL\u22121, 30\u00a0\u03bcL) was slowly injected into the ZIF-8 n-hexane solution under stirring. Then the whole solution was subjected to ultrasound for another 10\u00a0min. After that, the mixed solution was vigorously stirred for 3\u00a0h at room temperature. Finally, the yellow powder of Fe/Ni-ZIF-8 was centrifuged and dried in vacuum at 65\u00b0C for 6 h. The Fe/Ni-ZIF-8 sample and NaH2PO2 H2O were separately placed into two porcelain boats with NaH2PO2\u2219H2O at the upstream side of the furnace. The mass ratio of Fe/Ni-ZIF-8 to NaH2PO2\u2219H2O was 1:5. The annealing was performed at 1,000\u00b0C for 2\u00a0h at a heating rate of 5\u00b0C min\u20131 in N2 atmosphere. The resultant catalysts were denoted as Fe-Ni-N-P-C. For comparison, by adjusting the mass ratio of Fe and Ni, Fe-N-C, Ni-N-C, and Fe-Ni-N-C, we synthesized FexNiy-N-P-C via a similar procedure without adding some specific species as precursors. We also pyrolyzed ZIF-8 at 1,000\u00b0C for 2\u00a0h in N2 atmosphere to investigate the existence of Zn and its contribution to OER performance.This work was supported by the Natural Science Foundation of China (nos. 91834301, 21676080, and 21878076) and the Science and Technology Commission of Shanghai Municipality (no.19160712100).F.P. conceived the idea, prepared and characterized the catalysts, performed the catalytic measurements, and wrote the manuscript under the supervision of J.H. and H.L. T.J. analyzed the data and revised the manuscript. X.D., Y.C., and X.Z. provided valuable discussions and suggestions for manuscript revision. W.Y. and H.L. helped to perform the DFT calculation and GCMC simulation. All authors contributed to the preparation of the manuscript and gave approval to the final version of the manuscript.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.checat.2021.06.017.\n\n\nDocument S1. Supplemental experimental procedures, Figures S1\u2013S19, and Tables S1\u2013S7\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  The principle of how the active sites of catalysts match the reaction intermediates has long been sought after. Herein, we report a theory-guided atomic design and fabrication strategy of a C-based catalyst with diatomic Fe\u2013Ni and N,P co-doping for the oxygen evolution reaction (OER). The configuration matching (with O\u2217 on the Ni site and OH\u2217 on the adjacent Fe site) and the local electron engineering by P doping significantly facilitate the rate-determining step of OOH\u2217 formation. Such diatomic Fe\u2013Ni is demonstrated to be thermodynamically stable and is precisely constructed through the pyrolysis of Fe3+/Ni2+-adsorbed ZIF-8 under NaH2PO2 co-feeding. The synergistic effects endow the catalyst with a low overpotential and high turnover frequency, exceeding all transition-metal N-based catalysts so far as we know, which provides a deep understanding of the OER mechanism on heteroatomic metal-based catalysts. This strategy will pave the way for novel catalyst design and the replacement of noble-metal-based catalysts.\n               "}
{"full_text": "\n\n\nData will be made available on request.\n\n\nData will be made available on request.Plastic waste pollution is one of the most serious environmental issues worldwide today. A sustainable waste management strategy is necessary to overcome this problem. One of the most investigated methods for plastic waste treatment is pyrolysis. The term \u201cpyrolysis\u201d refers to a variety of thermal and thermo-catalytic conversion processes and technologies aimed at producing (a) liquid and gas products composed mainly of hydrocarbons (HC), oxygenated HC (OHC), and hydrogen (H2), and (b) carbon-rich solids. The pyrolysis-catalysis of plastic waste has already been extensively researched for the production of carbon nanomaterials (CNMs) and H2\n[1]. In a two-stage process, the plastic is pyrolyzed to produce different hydrocarbons that pass through a catalytic bed for cracking and catalytic decomposition. So far, evidence has been obtained that converting waste plastics into valuable products via pyrolysis-catalysis processes could be a promising alternative for plastic waste management [2].CNMs, especially carbon nanotubes (CNTs), have remarkable and valuable properties, including high electric and thermal conductivities [3]. These materials can be used in several applications, such as composites [4], catalysis [5], adsorption for environmental cleaning [6\u20138], field-effect emitters [9], and electrode materials for supercapacitors [10]. Carbon-based light molecules, such as methane, ethylene, and acetylene, are usually used as carbon precursors for synthesizing CNMs. However, the carbon contained in plastic waste is a better alternative as the raw material has a negative value. Its use as feedstock renders the intended processes environmentally friendly and provides a sustainable approach to the mass production of CNMs.As expected, catalysts play an important role in the production of CNMs. Metal-based catalysts are the most commonly used. Accomb et\u00a0al. [11] have investigated the effect of using different transition metals (Fe, Ni, Co, and Cu) supported by alumina on CNT production from low-density polyethylene (LDPE). Fe- and Ni-based catalysts gave the largest yield of CNTs and hydrogen, followed by Co. Cu gave no filamentous carbon because of weak metal\u2013support interaction [11]. Fe-based catalysts have a desirable catalyst\u2013support interaction and large carbon solubility [12]. Moreover, regarding the degree of graphitization, Fe-based catalysts also perform better [13]. Other authors [6,14] have tested bimetallic catalysts to understand the synergistic effect between the metals. Increasing the Ni to Fe molar ratio in an Fe-Ni catalyst enhances the thermal stability and graphitization of the formed carbon. Moreover, the yields of CNM and hydrogen also increased. Ratkovic et\u00a0al. [15] have demonstrated that the yield of CNTs from catalytic decomposition of ethylene over Fe-Ni/Al2O3 is three times higher than over Fe/Al2O3. The performance of the bimetallic catalyst has been attributed to the strong metal\u2013support interaction that leads to well-dispersed small particles.The size and the shape of the metal particles are the most important factors determining the type of CNM produced. When the particles are a few tens of nanometers and well-dispersed, CNTs are produced instead of carbon nanofilaments CNFs [12]. Aboul-Enein and Awadallah [16] have studied the production of CNMs using Fe-Mo/MgO catalysts during catalytic pyrolysis of waste PE. They found that the Fe/Mo ratio has a key effect on the type and morphology of the produced CNMs. With high loads of Fe and Mo, large-diameter carbon nanofilaments (CNFs) and hollow CNTs were formed due to the aggregation of metal particles. In another study [17], the same authors studied the effect of adding Cu to Ni-La2O3 on the decomposition of the non-condensable gases produced from the pyrolysis of polypropylene (PP). The bimetallic Cu-Ni particles were in a quasi-liquid state, which increased the size of the metal particles and led to the formation of large-diameter cap-stack CNFs, while highly dispersed Ni particles were responsible for the growth of multi-walled carbon nanotubes (MWCNTs).The type of plastic has also impacted the production of CNMs. Polyethylene (PE) and PP are usually used because of their abundance and high carbon content. Aboul-Enein et\u00a0al. [18] have demonstrated that the morphology and crystallinity of the CNM depend on the type of plastic waste. LDPE and PP produced MWCNTs with a high degree of graphitization, while high-density polyethylene (HDPE) produced MWCNT with a rugged surface. A small yield with low quality and purity was obtained from polystyrene (PS) and polyethylene terephthalate (PET). Similar results have also been obtained by Veksha et\u00a0al. [19], who also found that the influence of plastic type is more pronounced at lower temperatures. Real-world plastic waste was also investigated [13] and the carbon produced was mainly filamentous with some amorphous carbon. Moreover, contaminants in the feedstock are known to affect the pyrolysis process and products [20]. Wu et\u00a0al. [21] have demonstrated that 0.3 wt% of polyvinyl chloride (PVC) in the feedstock led to a significant reduction in the quality and purity of CNTs.Despite abundant research into CNT production using catalysis-pyrolysis, there is a lack of data collected in continuous feeding mode. To fill this gap, this work investigates the synthesis of CNMs from waste plastic using a new catalyst in a continuous feeding mode. The support used for synthesizing the catalyst is UGSO (UpGraded Slug Oxides), a negative-value mining residue. These oxides contain a significant amount of Fe, making them very attractive for synthesizing CNMs. The combination of Ni with these oxides enhances their catalytic activity, as shown in previous studies [22,23]. The main targets of this study are:\n\n1)\nInvestigate the performance of the Ni-UGSO catalyst during the pyrolysis-catalysis of waste HDPE for the synthesis of CNMs and compare its activity with a catalyst widely used in literature (Fe/Al2O3).\n\n\n2)\nStudy the effect of plastic waste on CNM synthesis by using different plastics (virgin HDPE, used HDPE, and mixed plastics).\n\n\nInvestigate the performance of the Ni-UGSO catalyst during the pyrolysis-catalysis of waste HDPE for the synthesis of CNMs and compare its activity with a catalyst widely used in literature (Fe/Al2O3).Study the effect of plastic waste on CNM synthesis by using different plastics (virgin HDPE, used HDPE, and mixed plastics).This work presents a new concept for synthesizing CNMs from waste plastic. The process has an ecological impact because it valorizes waste plastic. Moreover, the catalyst used is also made from a negative-value mining residue.Virgin HDPE was purchased from McMaster-Carr. The pellets are spherical, with a diameter of 6.35 mm. The post-consumer HDPE and the mixed polyolefin samples were obtained from KWI Solutions Polymers, Inc. The HDPE waste particles have different shapes and dimensions, varying from 3 mm to 1 cm. The post-consumer mix of polyolefins has an average composition of 80 wt% HDPE and LDPE, 15 wt% PP, 4 wt% PC and PET, and 1 wt% metals and wood. A photograph of the different plastics is provided in Appendix A (Fig. A.1). All plastics were used without further grinding or treatments. UGSO, which is a mix of different oxides, was provided by Rio Tinto Iron & Titanium and it is the same material used in previous publications [22\u201324] Nickel and iron nitrates were purchased from Sigma-Aldrich, and \u03b3-Al2O3, with particle size of 210 \u00b5m, was purchased from McMaster-Carr.Both UGSO and \u03b3-Fe2O3 were ground and screened using a sieve with a size of 53 \u00b5m. Ni-UGSO and Fe/Al2O3 were prepared by incipient wetness impregnation. Both catalysts were dried for 3 h at 105\u00b0C. Ni-UGSO was calcined for 16 h at 900\u00b0C, according to a previous study [24], while Al2O3 was loaded with 10 wt% of Fe and calcined for 2 h at 750\u00b0C, as reported in the literature [25]. According to a previous study, the Ni loading of 10 wt% is a concentration that favors carbon formation [23]. After calcination, both catalysts were crushed and sieved to give particles of size below 53 \u00b5m.The pyrolysis-catalysis of the plastic waste was performed in a two-stage fixed-bed quartz reactor. Pyrolysis takes place in the first stage. The second stage consists of a catalytic bed for catalysis of the produced gases. Temperature can be controlled in both stages separately. The experimental setup is described in more detail in previous work [26]. For each experiment, 5 g of catalyst was dispersed in quartz wool positioned homogeneously in the middle of the catalysis zone of the cylindrical reactor. Another quartz wool piece was placed in the middle of the pyrolysis stage to receive the waste plastic which falls inside the reactor. In this process, metals and other contaminants were retained in the pyrolysis step as char residue. For each experiment, the catalyst was activated with high purity H2 at a flow rate of 0.10 SLPM for 3 h at 600\u00b0C. After 3 h, the hydrogen flow was stopped and replaced by N2 at a flow rate of 0.03 SLPM. Following a previous study [22], for all experiments, the temperature of the catalysis stage was fixed at 650\u00b0C, while that of the pyrolysis zone was fixed at 700\u00b0C to favor gas production. When both temperatures reached their set point, the reaction began by feeding the plastic using a two-stage feeder at a rate of 0.33 g\u00b7min\u22121. The duration of each reaction was 2 h; this restriction is due to the fact that the carbon formed accumulated in the reactor and caused a pressure rise beyond the maximum acceptable level. Condensable liquids were recovered in a cold trap immersed in an ice bath. The gas exiting this condenser passed through a charcoal column so that all possible liquid hydrocarbons were retained as a mist. Gas samples were taken every 10 min for gas chromatography (GC) analysis using a SCION 456-GC equipped with flame ionization detector FID and thermal conductivity detector TCD. The experiments were duplicated and the error is below 5%. Average values were taken for all data provided.At the end of each reaction, the solid accumulated on the quartz wool and catalytic bed (catalyst on quartz wool) was weighed to estimate the yield of solid products, Ysolid using Eq.\u00a0(1).\n\n(1)\n\n\n\nY\n\ns\no\nl\ni\nd\n\n\n\n(\n\nw\nt\n%\n\n)\n\n=\n\n\nm\n\ns\no\nl\ni\nd\n\n\n\nm\n\np\nl\na\ns\nt\ni\nc\n\n\n\n\u00d7\n100\n\n\n\n\nThe cold trap and the adsorbent were also weighed to determine the yield of the liquids and wax, Yliquids.\n\n(2)\n\n\n\nY\n\nl\ni\nq\nu\ni\nd\ns\n\n\n\n(\n\nw\nt\n%\n\n)\n\n=\n\n\nm\n\nl\ni\nq\nu\ni\nd\ns\n+\nw\na\nx\n\n\n\nm\n\np\nl\na\ns\nt\ni\nc\n\n\n\n\u00d7\n100\n\n\n\n\nThe gas yield, Ygas, was determined according to the following equation:\n\n(3)\n\n\n\nY\n\ng\na\ns\n\n\n\n(\n\nw\nt\n%\n\n)\n\n=\n100\n\u2212\n\nY\n\nl\ni\nq\nu\ni\nd\ns\n\n\n\u2212\n\nY\n\ns\no\nl\ni\nd\n\n\n\n\n\n\nThe yield of filamentous carbon was determined as:\n\n(4)\n\n\n\nY\n\nf\ni\nl\na\nm\ne\nn\nt\no\nu\ns\n\nc\na\nr\nb\no\nn\n\n\n\n\n(\n\nw\nt\n%\n\n)\n\n=\n\n\nm\n\nf\ni\nl\na\nm\ne\nn\nt\no\nu\ns\n\nc\na\nr\nb\no\nn\n\n\n\nm\n\nc\na\nr\nb\no\nn\n\nc\no\nn\nt\na\ni\nn\ne\nd\n\ni\nn\n\nt\nh\ne\n\np\nl\na\ns\nt\ni\nc\n\n\n\n\n\u00d7\n100\n\n\n\n\nThe yield of hydrogen was determined as:\n\n(5)\n\n\n\nY\n\nh\ny\nd\nr\no\ng\ne\nn\n\n\n\n(\n\nw\nt\n%\n\n)\n\n=\n\n\n\nm\n\nh\ny\nd\nr\no\ng\ne\nn\n\n\n\n\u00d7\n2\n\n\nm\n\nh\ny\nd\nr\no\ng\ne\nn\n\nc\no\nn\nt\na\ni\nn\ne\nd\n\ni\nn\n\nt\nh\ne\n\np\nl\na\ns\nt\ni\nc\n\n\n\n\n\u00d7\n100\n\n\n\n\nX-ray diffraction (XRD) was used to analyze both catalysts before and after the pyrolysis-catalysis experiments; XRD provided information about the different crystalline phases in the fresh and used catalysts. A Philips X'pert PRO diffractometer (PANalytical) with a CuK\u03b1 radiation source producing at 40 kV and 50 mA was used. The diffraction spectra were collected in the range of 15\u00b0\u201380\u00b0 with a step of 0.05\u00b0 per 700 s. The data were analyzed with the MDI JADE 010 software.The Brunauer\u2013Emmett\u2013Teller (BET) multipoint method was used to compare the specific area and pore volume of both fresh catalysts. Samples were exposed to N2 physisorption at 110\u00b0C for 18 h with an accelerated surface area and porosimetry system (ASAP 2020 V4.01).A temperature-programmed reduction (TPR) was used to quantify the amount of reduced oxides in both catalysts. Samples of \u205330 mg of catalyst were deposited in a U-shaped quartz tube. The sample was pretreated with Ar at a flow of 20 mL\u00b7min\u22121 for 1 h at 140\u00b0C. The catalyst was reduced using a gas mixture (10% H2 in Ar), with a flow rate of 20 mL\u00b7min\u22121, while it was heated from room temperature to 1100\u00b0C at 3\u00b0C\u00b7min\u22121. To remove the H2O formed during the reduction period, the gas goes through a cold trap containing 2-propanol on liquid N2. The H2 variation was detected by a cathetometer, and its consumption is proportional to the peak area.In order to examine the morphologies of the CNMs, a Hitachi SU8230 was used in scanning transmission electron microscopy (STEM) mode. The energy dispersive X-ray spectroscopy (EDXS) detector used was a FlatQuad 5060F (Bruker, Germany). SEM images were taken using an in-lens SE detector with an acceleration voltage of 2 kV. Samples were prepared by dispersing the particles into ethanol with ultrasound, then dropping the suspension on a copper grid coated with amorphous carbon.Thermogravimetric analysis (TGA) was used to quantify the amount of carbon deposited on the surface of the catalysts and distinguish filamentous carbon from amorphous carbon. The analyses were performed using a Setaram Setsys 24 analyzer under a stream of 20% O2 and 80% Ar, from room temperature to 1000\u00b0C at a heating rate of 10\u00b0C\u00b7 min\u22121.Raman spectroscopy (SP2500 Acton spectrometer) with a 30-mW 414-nm laser was used to determine the intensity ratio (G/D) and provide information about defects present in the graphene sheets.The physicochemical properties of the fresh catalysts are presented in Table\u00a01\n. Ni-UGSO shows the highest surface area and pore volume. However, both Ni-UGSO and Fe/Al2O3 have similar average pore volumes. The difference in terms of H2 chemisorption is very important. After reduction, Ni-UGSO has three times more metal-active sites than Fe/Al2O3. This is expected, as UGSO contains nearly 30 wt% of iron, according to its elemental analysis [24], plus the added Ni.The XRD results of both catalysts are shown in Fig.\u00a01\n. UGSO is a mix of different oxides, most of which are spinels. When Ni is added to UGSO, the peaks at 36.9, 42.9, 62.4, and 78.7\u00b0 increase in intensity. These angles correspond to the spectra of NiO and MgO. EDXS mapping from a previous study [23] has shown that NiO formed a solid solution with the MgO present in UGSO. The presence of the Ni-Mg-O solid solution offers the highest dispersion of Ni in a basic environment [27]. Thus, the metal\u2013support interaction is very strong. For Fe/Al2O3, alumina (Al2O3) and hematite (Fe2O3) were detected in the XRD spectrum. However, the formation of hercynite (FeAl2O4) is not significant. The most intense peak in the FeAl2O4 spectrum is at 36.5\u00b0, and it also appears in the spectrum of the Fe/Al2O3 catalyst at low intensity. This indicates that a minor quantity of Fe formed FeAl2O4, and the rest is present in the form of Fe2O3. This may be due to the calcination method: a higher temperature and longer calcination times would favor the formation of FeAl2O4, as reported in the literature [28].\nTable\u00a02\n presents the results of the pyrolysis-catalysis of post-consumer HDPE using Ni-UGSO. The yields of filamentous carbon and H2 are 68 wt% and 79.4 wt%, respectively. The conversion is seen to decrease in the gas composition slightly over time, shown in Fig.\u00a02\n. This is expected as the access to the active sites of the catalyst becomes more and more limited as the filamentous carbon forms. These results are quite similar to those obtained during the catalytic cracking of ethylene using the same catalyst [22]. At the beginning of the reaction, the yield of the solid is high, and that of the liquid is low. Over time, the hydrocarbons produced from the pyrolysis of waste HDPE have less access to the metal sites, and the rate of catalytic cracking decreases. As a result, more liquids and waxes are produced from thermal cracking activity. At these conditions, the performance of Fe/Al2O3 is very limited, with a yield of only 3.72 wt% of filamentous carbon and a 29 wt% yield of liquids. Previous studies have shown that Fe/Al2O3 performs better for producing CNMs and H2 during the catalytic pyrolysis of waste plastic [11\u201313]. However, all the experiments reported in these studies were conducted at higher temperatures (700, 800, and 900\u00b0C). Higher temperatures up to 800\u00b0C are known to increase the production of filamentous carbon, as the kinetics of hydrocarbon decomposition and diffusion through the metal particles rates increases [13]. As shown in the TPR results, the differences in composition influence the reducibility and the dispersion of active metals in the tested catalysts.\nTable\u00a03 compares the results of this work with other reported results. Ni-UGSO produces a high yield of carbon and H2 at a lower temperature (650\u00b0C). In the case of this work, average yields during 2 h of reaction are presented. However, the results reported in previous works are from batch regime experiments. Nevertheless, the relatively excellent catalytic performance of Ni-UGSO can be attributed to the presence of Fe in UGSO. In this study, the temperature is significantly lower than the other temperatures reported in previous reports, which makes this process more economical. Moreover, the catalytic temperature affects the quantity and quality of the produced CNMs. An increase in temperature might enhance the yield and the quality of CNMs [13]. The reduction of Ni-UGSO with H2 prior to each experiment leads to the formation of metallic Fe, Ni, and Fe-Ni alloys, as has been proven in previous studies [22,23]. These metals are the active phases that produce atomic carbon, leading to CNMs [12]. This is discussed in more detail in Section\u00a03.2.1.After the reaction, the XRD spectrum of Ni-UGSO in Fig.\u00a03\n shows the presence of metallic Ni, Fe, and Fe-Ni alloys. This result confirms previous claims regarding the presence of these metals at the surface of the catalyst and their major role in decomposing hydrocarbons. The Ni in the Ni-Mg-O solid solution is reduced to metallic Ni, whereas the Fe in hematite (Fe2O3) was reduced to w\u00fcstite (FeO) and then to metallic Fe. The same reduction of Fe occurs in Fe/Al2O3; however, the peak of metallic Fe is not as intense as in the case of Ni-UGSO, confirming the TPR results about the difference in metal content at the surface of both catalysts. The diffraction peak at 2\u03b8= 26\u00b0 corresponds to the d002 of graphitic carbon. In the case of Ni-UGSO, this peak is intense and sharp, indicating the presence of crystalline carbon. This peak is also present in the case of Fe/Al2O3 at a lower intensity.From these results, Ni-UGSO has a metal\u2013support interaction sufficient to avoid catalyst sintering but not high enough to inhibit CNM formation. A strong metal\u2013support interaction reduces the amount of the surface accessible to hydrocarbons [12]. Moreover, a strong metal\u2013support interaction prevents the migration of carbon into the subsurface of the metal layer [29] and disturbs the distribution of carbon atoms over the catalyst particle, leading to the formation of defects in the CNMs.The morphologies of the filamentous carbon formed on Ni-UGSO are presented in Fig.\u00a04\n. The filaments are of different diameters (between 8\u201390 nm). The high-angle annular dark-field (HAADF) image shows some metal particles located at the tip of the filaments and others trapped inside them. The trapped metal particles have a smooth morphology, whereas the metals at the tip of the CNMs have an angular form. The non-uniformity of the filaments is expected, as UGSO is a mining residue containing different particle sizes. Thus, the metal particles formed during the catalyst activation are not uniform in size. Furthermore, the size of these metals can change because of sintering phenomena, especially at high temperatures. It has been proven that the size of CNMs is directly related to the particle size of the metal crystals [30,31].EDXS analysis of a metal particle at the tip of a filament is presented in Fig.\u00a05\n. The peak of Fe is intense; Ni and C are also present. The Cu comes from the support, not the sample. This indicates that the metal particle contains both metals, and from the bright field scanning transmission electron microscopy (BFSTEM) image, it is also covered by filamentous carbon. Ni interacts very efficiently with hydrocarbon molecules and promotes their decomposition, whereas carbon diffusion and nucleation occur on the surface of Fe. In other words, Ni dehydrogenates the adsorbed hydrocarbons more quickly, while Fe solubilizes carbon better than Ni [32]. Yao and Wang [28] have demonstrated the synergetic effect of Fe and Ni during the pyrolysis-catalysis of PP for the production of CNMs. They found that the presence of both metals led to the production of highly graphitized bamboo-like MWCNTs compared to monometallic catalysts.The state of the metal particles in the BFSTEM image in the inset of Fig.\u00a05 is in accordance with the observations of Krivoruchko and Zaikovskii [33]. These authors have demonstrated that the metal\u2013carbon particle is in a quasi-liquid state. A nanometer-scale metal particle has very high surface energy, which leads to weaker bonds between metallic atoms. Consequently, the metallic surface melts and spreads on the carbon [34]. This reshaping of the metal particles explains the different shapes of metallic particles seen in Fig.\u00a04. The mechanistic model suggested for the growth of these filaments is called the vapor\u2013liquid\u2013solid (VLS) model [35]. The process starts with hydrocarbon adsorption and decomposition on the active sites to produce atomic carbon. The latter diffuses inside the metal as liquid metastable carbides until saturation is reached. Finally, carbon precipitates to grow a filament.TGA analysis of both catalysts is presented in Fig.\u00a06\n. The weight loss for Ni-UGSO is estimated at 75 wt%, confirming the yield of filamentous carbon calculated by the mass balance. Meanwhile, the mass loss of Fe/Al2O3 is about 15.3 wt%, which is an overestimate for this small sample because the mass balance gave a smaller carbon yield. The derivative plot shows that the weight loss peak for Ni-UGSO occurs at 620\u00b0C, while for Fe/Al2O3, it occurs at a lower temperature. The higher the degree of structural order of the filaments, the higher the oxidation temperature is. The oxidation of pure graphite occurs at 645\u00b0C [36]. The absence of a peak at a temperature lower than 600\u00b0C indicates that no amorphous carbon was produced during the pyrolysis-catalysis of used HDPE using Ni-UGSO as a catalyst. Consequently, all the deposited carbon on the catalyst surface is crystalline. The STEM results discussed in Section 3.3.1 will help identify the type of CNM produced.\nTable\u00a04 presents the total filamentous carbon and H2 yields for each feedstock. There is no significant difference between the products of virgin and used HDPE. This result indicates that the used HDPE might not be highly contaminated. However, for the mixed plastics, the yield of deposited carbon decreases by 10 wt%. This is attributed to impurities present in mixed plastics. Moreover, The differences in polymers, size, and physical properties of mixed plastics should be considered potential, or at least partial, causes for the observed differences. The average carbon production rate decreases, and the mass of solid residue found in the quartz wool increases. Metal and wood particles are also observed on the quartz wool. Catalyst poisoning occurs because of the different impurities, which might cause a decrease in catalyst activity. A comparison of the detailed molar composition of the gaseous streams is provided in Fig. A.2 of Appendix A.BFSTEM analysis of filamentous carbon produced from different plastics is presented in Fig.\u00a07\n. These filaments have different diameters, as explained previously. Most of the filaments have a tubular shape, where the graphene layers are parallel to the growth axis. These filaments are not smooth and have a large diameter, which indicates that they are tubular CNFs. The bamboo-like CNFs are also observed in experiments using virgin and used HDPE, as shown in Fig.\u00a08\n. Fig.\u00a07(b) shows a torn filament to look like a helical nanofiber. The same types of CNMs have been reported in the literature reporting the pyrolysis-catalysis of plastic waste [2,14,25,37]. The distance between the graphitic sheets is around 0.33 nm for all produced CNFs. A closer observation shows a layer of amorphous carbon covers the CNF in Fig.\u00a07(f). This layer has an average thickness of 3.7 nm near the tip of the CNF, which increases to 4.1 nm on the other side. This phenomenon is not observed in virgin and used HDPE, only for mixed plastics. This suggests that the quality of CNFs produced by mixed plastics is lower than those from a single type of plastic. The lower carbon yield and the presence of amorphous carbon are attributed to different contaminants, such as wood, oxygenated plastics such as PET, and PS. The presence of PET and PS in the feedstock is known to decrease the quality and quantity of CNMs, as reported in other studies [18,19]. PET and PS produce smaller quantities of gases during pyrolysis [38]. In addition, the pyrolysis of PET generates oxygenated compounds, while the pyrolysis of PS produces a high liquid fraction containing mainly styrene and aromatics [39].According to Chen et\u00a0al. [40], a balance between the dissociation rate of hydrocarbons and carbon diffusion rate must be maintained to ensure the continuous growth of CNFs. When the decomposition rate of hydrocarbons is high due to factors such as high metal loading or high temperatures, the balance is not maintained, and this causes some defects in carbon structure, such as carbon onions. According to the results of this work and the absence of carbon onions, Ni-UGSO demonstrates a good balance between the dissociation and diffusion rates, assuring the continuous growth of CNMs.\nFig.\u00a09\n shows the derivative plots of the TGA analysis of the spent catalyst used for different types of plastics. One weight loss peak for virgin and used HDPE appears above 600\u00b0C. This indicates that the carbon produced from these two plastics is crystalline and not amorphous. For the mixed plastics, two peaks are present: one at 554\u00b0C and the other at 600\u00b0C. The first peak is attributed to amorphous carbon, and the second to crystalline carbon. These results are in accordance with the BFSTEM images.The Raman spectroscopy results are presented in Fig.\u00a010\n. Peaks are seen at around 1350 and 1585 cm\u22121 for all samples. The peak at 1350 cm\u22121 (D peak) is the scattering peak of the disordered component, while the peak at 1585 cm\u22121 (G peak) is the resonance peak of graphite. The G\u00b4 peak observed at \u20532700 cm\u22121 is associated with the process of two-photon elastic scattering, indicating the purity of the CNM. The G/D ratio measures the quality and crystallinity of the CNMs, with a higher G/D ratio indicating a higher purity. For the virgin HDPE, the G/D ratio is low (0.94), indicating that the CNFs are of low quality. The CNFs from mixed plastics also have low quality, as the G intensity equals the D intensity. In addition, the G\u00b4/G intensity is the lowest, and the G\u00b4 is broad and not as sharp as the other G\u00b4 peaks. However, this is because of the presence of filamentous carbon seen in the STEM results. The CNFs from used HDPE showed the highest quality compared to the others, with a G/D ratio of 1.13. The low G/D ratio for the CNFs from virgin plastic is not expected as it is supposed to be the purest feedstock, particularly compared to the used HDPE and mixed plastics. The analysis was repeated several times, and the results were consistent, suggesting the presence of defects in the graphitic lattice, such as edge dislocations.To improve the quality of the CNMs, some studies suggest adding steam or other oxygenated compounds, such as CO2, to the pyrolysis gases [21,41,42]. Acomb et\u00a0al. [42] have increased the G/D ratio of CNTs (produced from PS) from 1.08 to 1.43 by adding steam at a rate of 0.25 g\u00b7h\u22121. However, the steam decreases the quality of CNTs produced from PP and LDPE. Wu et\u00a0al. [21] have reported that adding steam reduces the quantity of amorphous carbon. The MWCNT Raman peaks are smoother, but the D peak increases. The authors have observed that the nature of the CNTs changes, becoming more tangled. Azara et\u00a0al. [22] have demonstrated that the presence of CO2 enhances the quality of the CNFs produced from the dry reforming of ethylene using Ni-UGSO as a catalyst. Compared to other results from the literature, the CNFs produced in this work have low Raman indicators, as shown in Table\u00a03.\n\nOptimization is required to enhance the quality of the produced CNFs by either varying different parameters (such as temperature and Ni content) or adding oxygenated precursors (such as steam and CO2).Plastics decompose in the first stage of the reactor to give different hydrocarbon products. The thermal cracking HDPE at 700\u00b0C gives 79 wt% of gas, composed of 37 wt% C2H4 and 32 wt% of CH4, as shown in Table A.5 of Appendix A. For polyolefins, the decomposition mechanism is \u03b2-scission [20]. Then, the gases pass through the catalytic bed at 650\u00b0C, where the macromolecules decompose to lighter components. The yield of liquids for the pyrolysis test without a catalyst is 20.0 wt%, and that of the pyrolysis-catalysis tests is 8.4 wt%. This confirms the further degradation of macromolecules in the catalytic zone. In the presence of Ni-UGSO, the C-C and C-H bonds are cleaved mostly by the Ni, which is present either alone or alloyed with Fe. Carbon atoms form the graphene layers, and hydrogen atoms are released as H2 gas at the outlet of the reactor.A previous study [22] has shown that for Ni-UGSO catalyst, the Ni (111) plane is responsible for the adsorption and dissociation of hydrocarbons, while the diffusion of atomic carbon occurs mainly on the Fe (110) plane. The atomic carbon produced has a greater affinity for dissolving into Fe nanoparticles than Ni nanoparticles. This diffusion process reduces the particle melting point to below the melting point of pure iron (1538\u00b0C), reshaping the particle [43]. The diffusion of atomic carbon continues until saturation is reached and the formation of Fe3C begins. The high concentration leads the carbon to dissolve out as graphene layers, forming CNFs [12]. Therefore, the good performance of Ni-UGSO for carbon diffusion and formation of carbon layers is attributed to the synergistic effect of the Fe-Ni alloy formed from the Fe present in UGSO and the Ni added to the support.CNMs and hydrogen were produced from waste plastic in a two-stage pyrolysis-catalysis reactor operating in continuous mode. The catalyst is made from negative-value mining residues containing a significant amount of Fe. The main conclusions can be listed as follows:\n\n\u2022\nNi-UGSO demonstrated excellent catalytic performance in the synthesis of CNMs and production of H2 with yields of 56.6 and 6.6 g /100 gplastic, respectively.\n\n\n\u2022\nIn the same conditions and catalytic temperature (650\u00b0C), Ni-UGSO showed far better performance than Fe/Al2O3.\n\n\n\u2022\nThe CNMs produced were mostly tubular CNFs with different diameters and irregular shapes.\n\n\n\u2022\nThe type of plastic affected the quantity and quality of the produced CNFs. There was no significant difference in the quantity of CNMs and H2 produced from virgin and used HDPE.\n\n\n\u2022\nWhen mixed plastics were used as a feedstock, the yield of CNMs decreased by \u205310 wt%, and amorphous carbon was produced. This was attributed to the presence of contaminants and non-polyolefenic plastics.\n\n\nNi-UGSO demonstrated excellent catalytic performance in the synthesis of CNMs and production of H2 with yields of 56.6 and 6.6 g /100 gplastic, respectively.In the same conditions and catalytic temperature (650\u00b0C), Ni-UGSO showed far better performance than Fe/Al2O3.The CNMs produced were mostly tubular CNFs with different diameters and irregular shapes.The type of plastic affected the quantity and quality of the produced CNFs. There was no significant difference in the quantity of CNMs and H2 produced from virgin and used HDPE.When mixed plastics were used as a feedstock, the yield of CNMs decreased by \u205310 wt%, and amorphous carbon was produced. This was attributed to the presence of contaminants and non-polyolefenic plastics.In light of these results, it is concluded that Ni-UGSO, which is made from waste, can be used to treat waste plastic at low temperatures for the mass production of CNMs and H2. Some of the future prospects can be listed as follow:\n\n\u2022\nStudy the effect of CO2 and vapor on the quality of CNFs.\n\n\n\u2022\nStudy the effect of pyrolysis and catalysis temperatures on the yield and quality of CNFs.\n\n\n\u2022\nApply the produced CNFs in composite materials and catalysis.\n\n\nStudy the effect of CO2 and vapor on the quality of CNFs.Study the effect of pyrolysis and catalysis temperatures on the yield and quality of CNFs.Apply the produced CNFs in composite materials and catalysis.This work was supported by the National Science & Engineering Research Council of Canada (NSERC) and KWI Polymers.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank M. Nicolas Brodusch from McGill University for the STEM micrographs and the University de Sherbrooke's Platforme de Recherche et d'Analyse des Mat\u00e9riaux (PRAM). We thank Marc Couture, Karen Bechwaty, Mohamed Hossam Eldakamawy, Sabrina Bahia Karakache, and Marc-Alexandre Fortin for their technical support.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2022.100424.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Pyrolysis and in-line catalytic decomposition of plastic waste were performed for the production of carbon nanomaterials (CNMs) and hydrogen. A new catalyst, Ni-UGSO (Nickel-UpGraded Slug Oxide), was used in a continuous mode, and its activity was compared to that of a typical Fe/Al2O3 catalytic formulation. Moreover, the pyrolysis-catalysis of different plastics (virgin high-density polyethylene (HDPE), used HDPE, and mixed plastics) was studied to investigate the effect of the plastic type on the quantity and quality of the produced CNMs. Ni-UGSO exhibited the highest catalytic activity for the production of CNMs and H2 of the two formulations tested, with yields of 56.6 kg/100 kgplastic and 13.2 kmol/100 kgplastic, respectively. The high activity and performance of Ni-UGSO were attributed to the synergistic effect of the Ni and Fe in Ni-UGSO. Scanning transmission electron microscopy (STEM) results revealed that most of the produced carbon was in the form of carbon nanofilaments (CNFs) of different diameters, ranging from 8 to 90 nm. The use of mixed plastics as a feedstock decreased the yield of CNMs by 10 wt%, and a layer of amorphous carbon covered the CNFs. This layer is due to the presence of polystyrene (PS), polyethylene terephthalate (PET), and other contaminants in the feedstock. Raman spectroscopy showed that the CNFs produced from used HDPE had the highest intensity ratio G/D (1.13).\n               "}
{"full_text": "Data will be made available on request.The extensive utilization of fossil fuels for daily life and industrial applications has led to significant emissions of greenhouse gases (GHGs) including carbon dioxide (CO2) and methane (CH4) [1]. CO2 or dry reforming of methane (DRM), which converts both GHGs into syngas (H2 and CO), is an attractive process to produce sustainable fuels with low carbon emissions [2]. However, the activation and dissociation of the CO bond in CO2 and the C-H bond in CH4 both require high energy input (750\u00a0kJ\u00a0mol\u22121 and 439.5\u00a0kJ\u00a0mol\u22121, respectively) [3,4]. DRM can be driven by sustainable solar energy, and efforts have been spent on researching on solar-driven thermochemical process in which solar energy is used to reach the required high temperatures [5]. On the other hand, solar-driven photothermochemical DRM (PTC-DRM) is an emerging approach that can incorporate photocatalysis into thermochemical DRM [6\u20139]\n. Compared to conventional solar thermochemical DRM process, the PTC-DRM process has a couple of additional unique aspects in utilizing light irradiation when a specially designed catalyst is applied: (1) light-driven thermocatalysis due to surface plasmon resonance effect when a plasmonic metal catalyst such as nickel is used, and (2) photo-excited electron-hole pairs from a semiconductor support that actively participate in the redox reaction [1,6,10]. Thus, by combining both thermocatalysis and photocatalysis in one reaction system, PTC-DRM activities are largely enhanced compared with traditional thermochemical DRM [1,7,9,11\u201313].Metal supported on metal oxides has been one of the most widely researched catalyst structures for PTC-DRM process [14\u201329]. Ni is an attractive metal candidate in terms of low cost, abundance, and high PTC-DRM activities [21\u201329]. However, high-temperature DRM environment is prone to cause Ni sintering and carbon formation, leading to deactivation of Ni-based catalysts [30]. As regard to photocatalytic DRM, which was conducted under ultraviolent or visible light irradiation at low temperatures. For example, with ultraviolet light irradiation of 150\u00a0mW/cm2 at 100\u00a0\u00b0C, production rates of CO and H2 can reach 750 \u03bcmol g\u22121 h\u22121 and 1126 \u03bcmol g\u22121 h\u22121, respectively on Ni-montmorillonite/TiO2\n[31]. With visible light irradiation of 790\u00a0mW/cm2 at 400\u00a0\u00b0C, 2Ni/CeO2\u2212x yielded CH4 and CO2 rates of 0.21 and 0.75\u00a0mmol (gcat \u2022\u00a0min)\u22121, respectively [32]. However, these works achieved low reaction rates, far from industrial application requirements, thus exploring photocatalysts for more efficient photothermal reaction under high temperature and full-spectrum solar irradiation is very necessary.On the other hand, adjustable bulk and surface components of metal oxides, such as perovskite oxides of general formula ABO3, is a promising catalyst candidate [33]. In the perovskite oxide framework, the A-site is generally a rare earth or alkaline earth metal, such as La [34\u201337] and Sr [38], while transient metal, such as Ni [34,35,37] and Co [39] can occupy B-site. Compared to a metal oxide-supported metal catalyst system, perovskite oxides have the advantages of uniform dispersion of active metal sites and highly tunable oxygen vacancies concentration, thus can counter deactivation [34,35]. Another beneficial advantage of the perovskite oxide catalyst is its considerable photocatalytic activity due to a desirable narrow band gap (e.g., LaNiO3: \u223c2.2\u00a0eV) [40,41]. This makes perovskite oxides also attractive in the field of photocatalytic organic compounds degradation, water splitting and N2 fixation [42\u201344]. From existing research, the base perovskite structure, LaNiO3, was reported to decompose completely during the DRM reaction, and Ni-La2O3 alone was unable to resist Ni sintering and carbon deposition, thus resulting in inefficient DRM reaction [36,45]. Partial substitution at the A-site of perovskite presents an effective approach, as this modification may remarkably enhance the catalytic activities by altering the electronic state of B-site cations and/or introducing oxygen vacancies [46], thus both average Ni oxidation states and Ni particle size will be reduced, and carbon deposition can be suppressed. Wang et al. reported that Ce substitution at the A-site of LaNi0.5Fe0.5O3 perovskite introduced more oxygen vacancies and activated B-site cations, thus the DRM activity was enhanced [36]. Valderrama et al. also reported that partial substitution of La by Sr at A-site in LaCoO3 structure improved Co metallic phase dispersion, leading to high DRM activities and coke resistance [47]. Therefore, LaNiO3 perovskite catalyst with partial substitution at A-site can be a promising candidate for enhanced PTC-DRM performance due to its photoactivity and enhanced properties to resist metal sintering and coke deposition.This work aims to conduct a systematic exploration of efficient PTC-DRM activities and mechanism studies over efficient La1\u2212xCexNiO3 catalysts (x\u00a0=\u00a00.0 \u2013 1.0). The catalyst morphology, surface chemical states of catalysts before and after PTC-DRM reaction, and optical properties of fresh catalysts were characterized to understand the promoting effects. Then, the in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was performed on the catalysts within the temperature range of 25\u2013600\u00a0\u00b0C under light and dark conditions to understand the intermediate change relation with the catalyst activities. Finally, the roles of each component of PTC-DRM were discussed to understand the PTC-DRM mechanism.La1\u2212xCexNiO3 catalysts were synthesized by the Pechini method [48], and the corresponding metal nitrates were utilized with appropriate stoichiometry. Specifically, to synthesize La0.9Ce0.1NiO3, 79.9\u00a0mg La(NO3)3\u20226\u00a0H2O, 8.9\u00a0mg Ce(NO3)3\u20226\u00a0H2O and 59.6\u00a0mg Ni(NO3)2\u20226\u00a0H2O along with 78.8\u00a0mg citric acid were dissolved in 5\u00a0ml water at a metal cations to citric acid ratio of 1:1, denoted as solution A. Another 78.8\u00a0mg citric acid was dissolved in 2\u00a0ml ethylene glycol and denoted as solution B. Solution B was added dropwise to solution A. The resulting solution was stirred for 15\u00a0min at 400\u00a0rpm and was then heated to 120\u00a0\u00b0C to form a viscous gel and finally yielded a solid precursor. This product was then transferred to an oven to be calcinated under air at 750\u00a0\u00b0C for 5\u00a0h to produce the corresponding catalyst samples.Morphology, structure, and composition of the catalysts were characterized by transmission electron microscopy (TEM, FEI Tecnai G2 F20ST), high-angle angular dark-field scanning transmission electron microscopy (Hitachi 2700\u00a0C), X-ray diffraction (XRD, BRIKER D8), and X-ray photoelectron spectroscopy (XPS, Omicron), Raman spectroscopy (Horiba Jobin-Yvon LabRam HR, 633\u00a0nm laser source). UV\u2013vis diffuse reflectance spectra were collected by a Hitachi U4100 UV\u2013vis\u2013NIR Spectrophotometer with Praying Mantis accessory. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were collected on a Nicolet 6700 infrared spectrometer (Thermo Electron) equipped with a Praying Mantis DRIFTS accessory and a reaction chamber (Harrick Scientific, HVC-DRP). The maximum allowable operating temperature of the chamber is 600\u00a0\u00b0C. Because PTC-DRM activities were evaluated after reducing catalysts in H2/Ar mixture at 700\u00a0\u00b0C for 2\u00a0h, TEM, XRD, XPS, UV\u2013vis characterization was conducted on H2-reduced catalysts. H2 temperature-programmed reduction (H2-TPR, Micromeritics, AutoChem II 2920) was performed on 0.15\u00a0g fresh catalysts under a 10% H2/90% Ar gas flow of 40 standard cubic centimeters per min (sccm) with a heating rate of 10\u00a0\u00b0C/min from 200\u00b0 to 700\u00b0C. The thermogravimetric analysis (METTLER TOLEDO, TGA) was performed on 20\u00a0mg spent catalysts under an air flow of 40 sccm with a heating rate of 10\u00a0\u00b0C/min from 25\u00b0 to 800\u00b0C and kept at 800\u00a0\u00b0C for 3\u00a0h.A similar experimental setup applied in the PTC-DRM performance measurements was reported in our previous works [14,16,49], and the reactor configuration is shown in Fig. S1. The concentrated solar irradiation can be operated as high as 1200\u00a0W, and the corresponding light intensity was measured to be 3.6\u00a0W/cm2 (Fig. S2), which resulted in 420\u00a0\u00b0C on the catalyst surface. Auxiliary heat was supplied from a tube furnace to reach higher temperatures. A thermocouple was in contact with the center of the catalyst surface and connected to the furnace to provide feedback to the heating program, thus ensuring catalyst surface temperature was the same under light and dark conditions once a designated temperature was set. For PTC-DRM experiments, 5\u00a0mg catalyst was dispersed in 5\u00a0ml deionized water and sonicated to form a uniform ink. The ink was then dropped onto a piece of Whatman\u2122 Quartz filter paper and placed on the catalyst holder and transferred into the tube reactor. The reactor was first purged with 150 sccm Ar for 30\u00a0min to remove impurities under room temperature, followed by reducing the catalyst under a mixed flow of 23 sccm H2 and 28 sccm Ar for 1\u00a0h at 700\u00a0\u00b0C. Then, the reactor was purged with 150 sccm Ar to remove the remaining H2. After that, the reactant gases (10% CO2/10% CH4/80% Ar) were introduced into the reactor with a flow rate of 14 sccm. Only CO and H2 were detected as the products by an on-line gas chromatograph (GC 2010, Shimadzu) equipped with a thermal conductivity detector (TCD) and a methanizer-assisted flame ionization detector (FID). The production rates of CO and H2 (n, mol g\u22121 h\u22121) were calculated using the following formula:\n\n\n\nn\n=\n\n\nP\n\u2219\nV\n\u2219\n\n\nv\n\n\ni\n\n\n\n\nm\n\u2219\nR\n\u2219\nT\n\n\n\u2219\n3600\n\n\n\nWhere P is the pressure (1.01\u2009\u00d7105 pa), V is the gas volumetric flow rate (2.3\u2009\u00d7\u200910\u22127 m3 s\u22121), \n\n\nv\n\n\ni\n\n\n is the volume concentration of each gas, which is converted from GC measurements and calibration, T is the temperature (298.15\u2009K), R is the gas constant (8.314\u2009J\u2009mol\u22121 K\u22121), and m is the loaded catalyst weight (5\u2009\u00d7\u200910\u22123 g).The CO2 and CH4 conversion % were calculated using the following formula:\n\n\n\nConversion\n\n%\n=\n\n\n\n\n\n[\nX\n]\n\n\nin\n\n\n\u2212\n\n\n[\nX\n]\n\n\nout\n\n\n\n\n\n\n[\nX\n]\n\n\nin\n\n\n\n\n\n\u00d7\n100\n\n\n\nWhere \n\n\n[\nX\n]\n\n\nin\n\n\n is the concentration of each original reactant gas (CO2, CH4), and \n\n\n[\nX\n]\n\n\nout\n\n\n is the measured concentration of each gas at the outlet.\nIn situ DRIFTS spectra were recorded on a Nicolet 6700 spectrometer (Thermo Electron) equipped with a liquid nitrogen cooled HgCdTe (MCT) detector, a Praying Mantis DRIFTS accessory and a reaction chamber (Harrick Scientific, HVC-DRP) [16]. The reaction cell was equipped with a sample cup to load powder samples and a heater and temperature controller to control the reaction temperature. The maximum operation temperature of the reaction chamber is 600\u2009\u00b0C. The dome of the DRIFTS cell has two ZnSe windows allowing IR transmission and a third (quartz) window allowing transmission of light irradiation. Light was introduced into the DRIFTS cell through an optical fiber connected to the solar simulator operated at 1200\u2009W. The intensity of the light measured at the outlet optical fiber was close to 0.1\u2009W/cm2. After loading 10\u2009mg of catalyst sample on the sample cup, the sample was first reduced with a gas mixture of 23 sccm H2 and 28 sccm Ar for 10\u2009min at 600\u2009\u00b0C, and no spectra change was observed, meaning the complete reduction of the catalyst occurred. After the reduction process, the reaction chamber was cooled down to room temperature and simultaneously purged by Ar. The reaction temperature was then set to targeted temperatures (25\u2013600\u2009\u00b0C) with either light or dark condition, and at the same time, DRM gases were introduced. The DRIFTS data were then taken continuously until no spectra change was observed (10\u2009min). The final DRIFTS spectra were collected and presented.Reducibility of the catalyst determines the active form of the catalyst [46], thus as-prepared fully oxidized catalysts were examined by H2 temperature-programmed reduction (H2-TPR) analysis (\nFig. 1). For LaNiO3, the reduction was observed to happen in 3 steps, as the peaks appeared at 262 and 382\u2009\u00b0C along with a broad peak ranging from 457\u00b0 to 613\u2009\u00b0C. The peaks can be associated with different intermediary species of Ni, and La2O3, as the possible reduction steps of perovskite structures are as follows [50\u201352]:\n\n(1)\n2LaNiO3 + H2 \u2192 La2Ni2O5 + H2O\n\n\n\n\n(2)\nLa2Ni2O5 + 2\u2009H2 \u2192 La2O3 + 2Ni + 2\u2009H2O\n\n\nIn general, the lowest-temperature peak is associated with reduction of Ni3+ to Ni2+, and the peaks at higher temperatures are due to the reduction of Ni2+ into Ni0+ and partial reduction of Ce4+ or La3+. The reduction peaks of La0.9Ce0.1NiO3 shifted to lower temperatures, namely, 357\u2009\u00b0C and 486\u2009\u00b0C, corresponding very well with previous literature findings [50,53]. This result showed that the partial Ce substitution promoted the reaction between H2 molecules and NiO species to occur at lower temperatures. It is likely the partial incorporation of Ce in perovskite lattice resulted in the distortion of the perovskite structure [54], thus making the reduction of perovskite easier. Additionally, the overall H2 consumption was measured to increase from 2327 \u03bcmol/g on LaNiO3 to 3310 \u03bcmol/g on La0.9Ce0.1NiO3, thus indicating surface oxygen species and bulk lattice oxygen amounts being higher on La0.9Ce0.1NiO3, which demonstrated an enhanced oxygen mobility upon Ce substitution [55]. On the other hand, La0.5Ce0.5NiO3 and CeNiO3 have only one reduction peak, which is likely due to the reduction of NiO and partial reduction of ceria-based oxides [56]. By calculating the total oxygen storage capacity (OSC) of each catalyst (Table S1), it was found the existence of Ce increased the total OSC on all La1\u2212xCexNiO3 (x\u2009=\u20090.1, 0.5, 1.0) catalysts compared with LaNiO3. It was also observed that at 700\u2009\u00b0C, both catalysts have been completely reduced. Therefore, 700\u2009\u00b0C was chosen as the reduction temperature to fully reduce Ni.X-ray diffraction (XRD) patterns of reduced catalysts were then characterized and presented in \nFig. 2. For all catalysts, the absence of NiO (JCPDS 89\u20133080) and presence of Ni (JCPDS 04\u20130850) indicated that Ni element has been fully reduced, thus can act as the active metallic sites for DRM [9], which agrees with the H2-TPR profiles. In addition, for La1\u2212xCexNiO3 with x\u2009=\u20090.0, 0.1, 0.5, La2O3 (JCPDS 71\u20135408) peaks are clearly resolved. CeO2 (JCPDS 81\u20139325) was weakly observed on La0.9Ce0.1NiO3, which is likely due to the relatively low concentration and uniform distribution of CeO2. Peaks at 2\u03b8 of 30.2\u00b0, 39.7\u00b0, and 47.4\u00b0, characteristic of LaNiO3 structure (JCPDS 33\u20130711) [57,58], have been only identified on x\u2009=\u20090.0 and 0.1 samples. Wang et al. proposed the \u201cself-regeneration\u201d effect that Ce cations (Ce3+/Ce4+) will reversibly shuttle between CeO2 and perovskite structure depending on the local redox fluctuations [36]. The absence of perovskite structure on La0.5Ce0.5NiO3 is likely due to its transition to CeO2. For CeNiO3 sample, only Ni and CeO2 phases are identified. These results revealed the existence of perovskite structure only on LaNiO3 and La0.9Ce0.1NiO3.Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS) tests were then conducted on the reduced La1\u2212xCexNiO3 samples to investigate the morphology and elemental compositions (\nFig. 3). The EDS elemental mapping evidenced the presence of each element (actual metal atomic fraction listed in Table S2) and demonstrated the uniform distribution of La and Ce elements. It is widely accepted that small particle size highly benefits the coking resistance and light absorption properties of Ni-based catalysts for the DRM process [59]. While most of the particle sizes ranged from 5\u2009nm to 40\u2009nm, with higher Ce substitution, the average Ni particle size gradually increased. Specifically, the average particle sizes increased from 12.3\u2009\u00b1\u20095.7\u2009nm on LaNiO3 to 22.3\u2009\u00b1\u20098.0\u2009nm on CeNiO3. In addition, La0.5Ce0.5NiO3 showed clearly separate and large metallic Ni particles. The appearance of the Ni particles is likely due to the weak interaction with the La2O3 and CeO2 with an excess amount of Ce substitution. Moon et al. also reported on La0.5Ce0.5NiO3, segregated phases of singular NiO, CeO2, and La2O3 were observed, and poor activities of steam CO2 reforming of CH4 were received due to the weak metal-support interaction [50]. These results confirmed that Ni distribution is optimal on La0.9Ce0.1NiO3 among these samples.Perovskite structure materials are reported to desorb part of the lattice oxygen at high temperature in the reducing environment; concurrently oxygen vacancies (VO) will be formed and partial valence change of the B-site ions will happen [60]. The oxygen vacancies have been generally believed to benefit DRM performance in both CO2 adsorption and coke mitigation [12,61]. The dissociation of C-O of CO2 can happen on the VO then produce CO and O, in which O becomes mobile and can thus participate in the removal of deposited coke by oxidizing C into CO [62]. Therefore, X-ray photoelectron spectroscopy (XPS) analyses were conducted on reduced La0.9Ce0.1NiO3 and LaNiO3 catalysts to investigate the surface elemental compositions. XPS analysis of La (830 \u223c 860\u2009eV), Ni (850 \u223c 880\u2009eV) and Ce (880 \u223c 925\u2009eV) were not discussed as the peaks of these three elements overlap, making it impractical to reach meaningful conclusions. O 1\u2009s deconvolution was then performed in \nFig. 4, and two types of oxygen species were located, lattice oxygen (OL) at \u223c530\u2009eV, and chemisorbed oxygen (OA) related to the presence of oxygen vacancies (VO) at \u223c532\u2009eV [63]. The OA concentration was calculated as the peak area ratio of OA, and the values on LaNiO3 and La0.9Ce0.1NiO3 were 42.8% and 82.0%, respectively. Therefore, the partial Ce substitution clearly introduces more VO on the reduced catalyst.The optical properties were then characterized by UV\u2013vis absorption spectra (\nFig. 5). All catalysts showed strong UV light absorption abilities, and the band gaps of LaNiO3, La0.5Ce0.5NiO3, CeNiO3 were calculated to be 2.19\u2009eV, 2.94\u2009eV, and 2.76\u2009eV, respectively, demonstrating their semiconductor properties. However, according to the Tauc plot of La0.9Ce0.1NiO3, it is not plausible to determine the band gap value. LaNiO3 and CeNiO3 showed very similar light absorption across the wavelength range from 200 to 800\u2009nm and a characteristic adsorption peak centered at around 280\u2009nm was identified, which is similar to reported absorption curves [64,65]. La0.9Ce0.1NiO3 expressed strongest light absorption ability, especially across visible light wavelength range. The reason is likely due to the small and well-distributed Ni particles (indicated by TEM and EDS) since black Ni particles have a dominating light absorption ability and hinder the light transmission to other components [22].The PTC-DRM activities on La1\u2212xCexNiO3 (with x\u2009=\u20090.0\u20131.0) were evaluated at 700\u2009\u00b0C under illumination by a 1200\u2009W concentrated solar simulator and were compared at the same temperature under dark conditions. The results are presented in \nFig. 6 and Fig. S3. All catalysts showed DRM performance enhancement under light conditions compared with those under dark conditions, demonstrating the photoactive nature of perovskite catalysts. It was found that with a small amount of Ce substitution (0.1, 0.3), the PTC-DRM activities were largely enhanced. By comparing LaNiO3 and La0.9Ce0.1NiO3, the average CO and H2 production rates increased from 363\u2009mmol\u2009g\u22121 h\u22121 and 258\u2009mmol\u2009g\u22121 h\u22121 to 550 and 545\u2009mmol\u2009g\u22121 h\u22121 in the dark with Ce substitution, while under light illumination, the average CO and H2 production rates even increased from 468\u2009mmol\u2009g\u22121 h\u22121 and 395\u2009mmol\u2009g\u22121 h\u22121 to 616\u2009mmol\u2009g\u22121 h\u22121 and 620\u2009mmol\u2009g\u22121 h\u22121. However, when Ce concentration is rich (x\u2009=\u20090.5 and x\u2009=\u20091.0), the performance of the catalyst did not further increase. Wang et al. [36] reported the similar catalyst performance as the function of the Ce substitution degree on La1\u2212xCexNi0.5Fe0.5O3 and suggested that Ni phase provides the primary catalytic activity, and the (LaCe)(NiFe)O3 perovskite further enhances the catalytic activity, while CeO2 is not responsible for the activity enhancement.It was also observed that CO production rates are more stable than H2 production rates during the reaction time. Specifically, on La0.9Ce0.1NiO3 under light condition, the 10th-h CO production rate was only 2.23% less than initial value, while H2 production rate showed a 15.7% decrease. The CH4 conversion rate also decreased relatively faster than CO2 (Fig. S4). These results may be attributed to the consumption of the Ni active sites for CH4 conversion to produce H2 and the sufficient concentration of Vo active sites for CO2 conversion to produce CO [12].Ultimately, La0.9Ce0.1NiO3 exhibited good catalytic DRM stability in both light and dark conditions. Furthermore, it was found that La0.9Ce0.1NiO3 produced an average H2/CO ratio of 1.01 under light conditions, which is optimal in achieving near-unity syngas production. Overall, these results present the promotional effects of Ce substitution and light illumination to improve both activity and stability for DRM.In our previous studies, we have conducted PTC-DRM on both Pt-based and Ni-based catalysts [7,8,12,49]. Comparing the Pt-based and Ni-based catalysts, the required loading of Ni is generally larger than Pt to achieve optimal DRM activity. However, under photo-illumination when electron-hole pairs are generated, the too large an amount of metal may result in a stronger charge recombination, leading to a lower photocatalytic contribution [22,66,67]. We further compared this work with the state-of-the-art results in the literature, including both PTC-DRM and DRM. In literature, different light sources and formats of results were reported (e.g., H2 and CO production rates, CO2 and CH4 consumption rates, average CO2 and CH4 conversion percentage, etc.), thus making it difficult to compare the performance directly. However, from \nTable 1, it is still clear to see the La0.9Ce0.1NiO3 catalyst ranks among the top ones in terms of average CO2 and CH4 conversion percentages.\nIn situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis was conducted to investigate the intermediates\u2019 change under concentrated solar irradiation or in the dark conditions in the DRM reactant gas atmosphere at temperatures ranging from room temperature (25\u2009\u00b0C) to maximum operating temperature (600\u2009\u00b0C) (Fig. S5). Upon exposure to the CO2 and CH4, several peaks were observed. According to previous reports, the peaks centered at 3016\u2009cm\u22121, 1338\u2009cm\u22121, and 1308\u2009cm\u22121 were identified as gaseous CH4\n[74], and the broad peak centered at 2313\u2009cm\u22121 was identified as gaseous CO2\n[75]. The peak intensities of gaseous CH4 and CO2 were observed to decrease as temperature increased, indicating the endothermic characteristic of DRM. The adsorption bands located at 2178\u2009cm\u22121 and 2111\u2009cm\u22121 are assigned to gaseous CO [76,77], which appeared only after 400\u2009\u00b0C in the dark, but was observed starting from 200\u2009\u00b0C under light, indicating that light irradiation can activate the CO production at lower temperature.To clearly observe the intermediates\u2019 change, the in situ DRIFTS spectra at the wavenumber ranging from 1200 to 2000\u2009cm\u22121 are presented in \nFig. 7. It was observed that the solar irradiation led to an intensity increase in formate, m-CO3\n2-, and b-CO3\n2- bands, which are all active intermediates in CO2 reduction reactions [78,79]. The stronger peak intensities under light are likely due to the generation of VO under light that promoted CO2 adsorption and formation of these peaks [8,49]. This aligns with the promoted reaction rates in the DRM process under light irradiation.It was also noticed that under light condition, the peak at 1753\u2009cm\u22121, which is attributed to La2O2CO3 species [80], only appeared under light irradiation when temperature was between 100 and 600\u2009\u00b0C. As previously reported, two parallel routes for CO2 activation may be occurring on the La-based catalysts: (1) Through direct decomposition on oxygen vacancies and (2) Through the formation and decomposition of La2O2CO3\n[60,81]. Thus, La2O2CO3 has been widely recognized to be an active intermediate formed by the reaction between La2O3 and CO2, further reacting with carbonaceous intermediates on Ni at the metal-support interface to produce CO and regenerate the La2O3. The generation of La2O2CO3 under light irradiation aligns well with the higher DRM activities. Similarly, Akula et al. also observed the formation of La2O2CO3 on La2O3/TiO2 during photocatalytic water splitting reaction, which can highly improve the photo-induced electro-hole pairs separation and speed up the photocatalytic methanol decomposition [82]. Gao et al. also reported the presence of La2O2CO3 during the PTC-DRM process, which can facilitate the coke mitigation on metal surface [83].All the carbonate peaks intensities are also weaker on LaNiO3, and La2O2CO3 was not observed on the Ce free sample (Fig. S6). This result indicated that partial Ce substitution can benefit the adsorption of CO2 and formation of La2O2CO3 upon light irradiation. It is likely that the rich VO density from the partial Ce substitution boosted the generation of intermediates.Characterization of spent catalysts was conducted to reveal information about catalyst stability, as both Ni sintering and coke formation are generally believed to be responsible for deterioration of DRM performance [59]. TEM measurements were first carried out on spent LaNiO3 and La0.9Ce0.1NiO3 after 10\u2009h DRM reaction at 700\u2009\u00b0C under the dark and light conditions (\nFig. 8). By comparing these TEM images with Fig. 3, Ni sintering can be observed on the spent LaNiO3 after DRM reaction at 700\u2009\u00b0C under both dark and light conditions (large Ni particles are circled in Fig. 8), while no obvious Ni sintering was observed on the spent La0.9Ce0.1NiO3. To quantify the extent of Ni sintering, we analyzed the Ni particle size distribution over 100 particles for each spent sample and presented in Fig. S7\n. Ni with much larger particle sizes were clearly observed on spent LaNiO3 (average value of 32.0\u2009\u00b1\u200914.8\u2009nm and 37.6\u2009\u00b1\u200917.1\u2009nm for light and dark, respectively), while La0.9Ce0.1NiO3 showed much smaller values (average value of 23.2\u2009\u00b1\u20096.3\u2009nm and 25.0\u2009\u00b1\u20098.4\u2009nm for light and dark, respectively), which matched the PTC-DRM performance difference presented in Fig. 6. These results suggest that the presence of Ce benefited Ni stabilization, and mitigated Ni sintering, resulting in better PTC-DRM performance on La0.9Ce0.1NiO3. As widely reported, Ce-containing Ni-based catalysts showed strong metal support interactions that prevented Ni from sintering and deactivating [84\u201386]. However, no obvious differences were observed on both catalysts between light and dark conditions, meaning the Ni sintering is mainly the result of thermal stability and independent of light conditions.Large amounts of carbon filaments (e.g., CNTs) were clearly observed on both samples under dark conditions, yet under light conditions, the coke species were mainly active carbon. We conducted further analyses of additional TEM images of spent LaNiO3 and La0.9Ce0.1NiO3 after DRM reaction at 700\u2009\u00b0C under the dark condition (Fig. S8). It was observed that on La0.9Ce0.1NiO3, the majority of Ni particles were still closely attached to the supports, whereas, on LaNiO3, many Ni particles were detached from the support and likely encapsulated in carbon (as circled yellow in Fig. S8a). This indicates severer Ni deactivation on LaNiO3 and agrees with its lower DRM performance compared to La0.9Ce0.1NiO3. Liu et al. also observed that light irradiation can tune the carbon deposition behavior of the Ni-based catalysts during PTC-DRM process [25].Thermogravimetric mass spectrometric (TGA) analysis and Raman spectroscopy was then performed to determine the deposited coke amount on spent catalysts after DRM reactions under the light and dark conditions (Fig. S9, S10). Specifically, a weight loss of 47.0% and 51.6% was observed on LaNiO3 under light and in the dark condition, respectively. While on La0.9Ce0.1NiO3, the values reduced to 20.2% and 27.6%. The difference in coke formation amounts on the two catalysts agrees with the visual observations on TEM images. With regard to Raman analysis, two peaks, with D band at \u223c1330\u2009cm\u22121 and G band at \u223c1580\u2009cm\u22121 were observed on all catalysts, which are assigned to amorphous carbon and graphitic carbon, respectively, where carbon filaments are usually composed of graphitic carbon [25,87]. We calculated the intensity ratios of D- and G-band (ID/IG) on spent catalysts after DRM reaction at 700\u2009\u00b0C under dark and light conditions (Table S3). Clearly, the ID/IG ratio is larger under light than that in the dark, and the ID/IG ratio of spent La0.9Ce0.1NiO3 is larger than that of spent LaNiO3. The ratio of ID/IG is highest on spent La0.9Ce0.1NiO3 under light, while it has the highest DRM performance. The positive correlation of the fraction of amorphous carbon with DRM performance agrees with the literature that amorphous carbon is more reactive and easier to be gasified so that Ni can continuously serve as the active sites for DRM reaction [88,89].The effect of Ni NPs size on the nucleation and growth of coke in DRM is well reported in the literature [60,90,91]. In our case, the average Ni NP size on fresh La0.9Ce0.1NiO3 and LaNiO3 (16.5\u2009\u00b1\u20097.3\u2009nm and 12.3\u2009\u00b1\u20095.7\u2009nm, respectively) are similar, but TEM images indicated the severe agglomeration of Ni NPs on spent LaNiO3. Thus, the partial substitution of Ce can prevent the Ni from aggregation, reducing the coke formation, and thus improving the PTC-DRM activities and stability.Then, XRD analysis was performed on the spent La0.9Ce0.1NiO3 and LaNO3 and compared with the reduced samples to observe the structure change (Fig. S11). It is clearly observed that on La0.9Ce0.1NiO3, the perovskite LaNiO3 structure was still present after the DRM reactions, while on LaNiO3, the structure was destroyed and became Ni and La2O3. Similarly, Das et al. reported that the perovskite structure was maintained in the DRM atmosphere by partially substituting Ni with Fe on La0.9Sr0.1NiO3\n[60]. Wang et al. also reported that the stability of perovskite (LaCe)(NiFe)O3 structure are credited to the preserved perovskite structure during the DRM reaction environment [36].Furthermore, we conducted XPS analysis on spent La0.9Ce0.1NiO3 after DRM reaction at 700\u2009\u00b0C under the dark and light conditions and presented the deconvolution of O 1\u2009S XPS spectra in Fig. S12. Similar to that observed in Fig. 4, two types of oxygen species, lattice oxygen (OL) at \u223c530\u2009eV, and chemisorbed oxygen (OA) related to the presence of oxygen vacancies (VO) at \u223c532\u2009eV, were identified. By calculating the peak area ratio, the OA concentration value was determined to be 58.6% and 73.3% under the dark and light conditions, respectively, both are lower than the value of fresh La0.9Ce0.1NiO3 (82.0%), indicating the consumption of VO during reaction process. More importantly, the higher VO concentration under light can possibly enhance CO2 adsorption and the adsorbed O on VO can oxidize deposited coke, thus improved PTC-DRM activity and stability were achieved under light condition. Several other publications also reported that light illumination can induce the generation of VO on CeO2\n[12,49,92], SrTiO3\n[93], or TiO2\n[94], because photogenerated electrons may weaken and break metal-O bonds.Multiple compositions are responsible for the enhanced PTC-DRM activities on La0.9Ce0.1NiO3. The effects of Ni metallic phase, CeO2, and perovskite structure LaNiO3 are thus discussed.The Ni metallic phase is undoubtedly the primary catalytic site for PTC-DRM reaction, and previous study also confirmed that performance on Ni-free supports was extremely poor [6,7]. Our TEM analysis indicated that Ni NPs were uniformly distributed on La0.9Ce0.1NiO3, and the aggregation of Ni NPs was mitigated during the reaction. It is likely that the Ce promoter reduced the chemical interaction between Ni and support, leading to increased reducibility, evidenced by H2-TPR results (Fig. 1), thus better dispersion of Ni [95]. The tiny Ni NPs enhanced the CH4 conversion and the interaction of Ni-ceria improved CHx oxidization, avoiding complete decomposition of CHx to carbon, thus less carbon was observed on La0.9Ce0.1NiO3\n[96,97]. Ye et al. also reported that Ni LSPR property enhances PTC-DRM activities with smaller particle size, while large Ni particles can lead to weak optical property [22]. The light absorption on La0.9Ce0.1NiO3 extended to light of visible and near infra-red region, therefore it can strongly harvest solar energy and the PTC-DRM activities under light irradiation was significantly boosted.CeO2 was reported to generate electron-hole pairs and oxygen vacancies under light irradiation [7,12]. CO2 adsorption and conversion can happen on oxygen vacancies to produce CO, and CH4 or intermediates (CHx) can be oxidized to produce CO and H2, thus PTC-DRM activities were highly enhanced by CeO2. As indicated from the O 1\u2009s XPS spectra (Fig. 4), oxygen vacancies concentrations are much higher on La0.9Ce0.1NiO3 than LaNiO3, and the light absorption ability is also stronger on La0.9Ce0.1NiO3 (Fig. 5). Therefore, it is likely that the Ce partial substitution retains the perovskite structure in the DRM environment, and the generated oxygen vacancies and electrons can boost CO2 adsorption and activation, thus promoting the generation of active intermediate La2O2CO3 to mitigate carbon formation and boost the DRM activities. However, for Ce-rich catalysts (x\u2009=\u20090.5, 1), although higher values of oxygen storage capacity were obtained, they showed declined activities. Therefore, CeO2 functions as an optimal promoter at a lower concentration (x\u2009=\u20090.1).The perovskite structure can also be the active sites, in which the dominant defects during DRM process are oxygen vacancies, which can act as the active sites for adsorption and activation of the reactants and intermediates [36]. In our study, the observed preservation of perovskite structure on La0.9Ce0.1NiO3 is likely benefited by two factors: (1) replenishment of oxygen vacancies from Ce substitution; (2) uniform distribution of Ni to boost the oxidation process [60,98].For comparison purpose, the La0.9Ce0.1 oxides supports were first synthesized, and same amount of Ni was wet impregnated on the supports to yield Ni/La0.9Ce0.1Ox. By conducting PTC-DRM on La0.9Ce0.1NiO3 and Ni/La0.9Ce0.1NiOx at the same DRM reaction conditions (Fig. S12), it was found that Ni/La0.9Ce0.1NiOx received inefficient and unstable PTC-DRM performance, proving the catalytic properties of perovskite structure. The perovskite catalyst was also widely used in photocatalytic CO2 reduction since it has strong light absorption and can generate electron-hole pairs for CO2 reduction at surface sites [99].Additionally, to verify the potential photocatalysis on La0.9Ce0.1NiO3 in the solar-driven DRM process, a control experiment was conducted at 700\u2009\u00b0C with a 495\u2009nm long-pass filter applied. The comparison of 10\u2009h average DRM performance of La0.9Ce0.1NiO3 under full spectrum, 495\u2009nm long-pass filter, and dark conditions was presented in the \nTable 2. The DRM performance is almost the same under the 495\u2009nm long-pass filter and dark conditions, indicating the existence of photocatalysis that boosted the DRM reaction on La0.9Ce0.1NiO3 under full spectrum irradiation.Based on the above experimental results and discussion, the possible PTC-DRM reaction pathways on La1\u2212xCexNiO3 catalyst are proposed as follows: CH4 dissociation takes place on Ni reaction sites to form C*\u2009and H*\u2009intermediates (Steps 1\u20132) [60]. Two H*\u2009can couple and form H2 (Step 3), and C* can be oxidized by lattice oxygen (OL) to generate CO (Step 4). On the other hand, CO2 can directly dissociate over oxygen vacancies (VO) or hydrogenate yielding CO, O, carbonate, or formate as intermediates (Steps 5\u20137), which can further react to form CO (Steps 8\u20139) [100\u2013102]. CO2 can also be adsorbed on the La2O3 surface and subsequently convert La2O3 into La2O2CO3 intermediate, which can actively react with deposited coke (Steps 10\u201311) [100,103]. Additionally, under light illumination, high-energy electron (e-) and holes (h+) will be generated on the perovskite catalysts [11,31,69]. The e- can generate VO on the catalyst surface and enhance the CO production, while h+ can boost CH4 dissociation towards higher H2 production (Steps 12\u201314).\n\n(1)\n\n\n\n\nCH\n\n\n4\n\n\n\u2192\n\n\nCH\n\n\nx\n\n\n*\n\n\n+\n(\n4\n\u2212\nx\n)\n\n\nH\n\n\n*\n\n\n\n\n\n\n\n\n(2)\nCH \u2192 C* + H*\n\n\n\n\n(3)\n2\u2009H* \u2192 H2\n\n\n\n\n\n(4)\nC* + OL \u2192 CO + VO\n\n\n\n\n\n(5)\nCO2 + VO \u2192 CO + OL\n\n\n\n\n\n(6)\nCO2 + H* \u2192 HCOO*\n\n\n\n\n(7)\nCO2 + OL \u2192 CO3\n2-\n\n\n\n\n\n(8)\nHCOO* \u2192 CO + OH*\n\n\n\n\n(9)\nCO3\n2- + C* \u2192 2CO + OL\n\n\n\n\n\n(10)\nLa2O3 + CO2 \u2192 La2O2CO3\n\n\n\n\n\n(11)\nC*+ La2O2CO3 \u2192 2CO + La2O3\n\n\n\n\n\n(12)\nLaxCe1\u2212xNiO3 + hv \u2192 e- + h+\n\n\n\n\n\n(13)\nCe4+ + e- \u2192 Ce3+ + VO + O\n\n\n\n\n(14)\nCH4 + xh+ \u2192 CH(4\u2212x) + xH+\n\n\n\nIn summary, promoting effects of partial substitution of Ce into LaNiO3 perovskite catalysts on the PTC-DRM activities were presented. Ce, as a promoter, was found to benefit Ni NPs active sites distribution and perovskite structure retention on La0.9Ce0.1NiO3. Therefore, Ni aggregation can be mitigated during PTC-DRM process due to stronger metal-support interaction. In addition, by conducting in situ DRIFTS analysis and control experiment with 495\u2009nm long-pass filter, the light irradiation was found to enhance CO2 adsorption and formation of active intermediate La2O2CO3, induce photocatalytic activities on La0.9Ce0.1NiO3, assisted by generated oxygen vacancies and electron-hole pairs. These advantages led to carbon mitigation and promoted PTC-DRM activities. As a result, at 700\u2009\u00b0C under 30 suns light irradiation, the La0.9Ce0.1NiO3 showed highest PTC-DRM activities with CO and H2 production rates of 616 and 620\u2009mmol\u2009g\u22121 h\u22121, respectively. This work systematically advances the design of cost-effective catalysts and the study of the light contribution mechanism thus promoting efficient solar-powered conversion of greenhouse gases.\nZichen Du: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing \u2013 original draft, Visualization, Writing \u2013 review & editing. Cullen Petru: Investigation, Methodology, Writing \u2013 review & editing. Xiaokun Yang: Validation, Resources, Writing \u2013 review & editing. Fan Chen\n: Validation, Resources. Siyuan Fang: Validation, Resources. Fuping Pan: Validation, Writing\u00a0\u2013 review & editing. Yang Gang: Validation, Writing \u2013 review & editing. Hong-Cai Zhou: Resources, Writing \u2013 review & editing. Yun Hang Hu: Resources, Writing \u2013 review & editing. Ying Li: Conceptualization, Writing \u2013 review & editing, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by U.S. National Science Foundation (Grant No. 1924466). The use of Materials Characterization Facility (MCF) at Texas A&M University is acknowledged.Reactor configuration, irradiation spectrum of concentrated solar, additional PTC-DRM activities comparison, in situ DRIFTS spectra, TGA, XRD, Raman characterization.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2022.102317.\n\n\nFigure S1\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Solar-driven photothermochemical dry reforming of methane (PTC-DRM) is a promising technique to produce syngas using greenhouse gases (CO2 and CH4). In this work, Ce-substituted LaNiO3, i.e., La1\u2212xCexNiO3 perovskite catalysts were synthesized for PTC-DRM reaction under concentrated sunlight. At 700\u00a0\u00b0C under 30 suns light irradiation, the CO and H2 production rates were at 616 and 620\u00a0mmol\u00a0g\u22121 h\u22121, respectively, over the La0.9Ce0.1NiO3 catalyst, notably higher than those obtained in dark at the same reaction temperature and higher than those over LaNiO3 under the same light irradiation condition. The CO2 and CH4 conversion by the La0.9Ce0.1NiO3 catalyst are among the top-performing catalysts reported in the literature. The Ce substitution of La at a small fraction (x\u00a0=\u00a00.1) was found to benefit Ni active sites distribution and retention of the perovskite structure, which led to mitigation of both Ni sintering and carbon formation, thus promoting light absorption and PTC-DRM activities. A higher fraction of Ce substitution (x\u00a0\u2265\u00a00.5), however, did not show any beneficial effects. By conducting in situ DRIFTS at PTC-DRM reaction conditions and control experiment using 495\u00a0nm long-pass filter, light irradiation was found to induce photocatalytic activities on La0.9Ce0.1NiO3 and enhance CO2 adsorption and formation of active lanthanum oxycarbonates intermediates (La2O2CO3), possibly due to the generation of oxygen vacancies and electron-hole pairs. This work reports a new catalyst design and mechanistic studies for PTC-DRM reaction, and the findings are of importance for the application low-carbon fuel generation from sunlight.\n               "}
{"full_text": "No data was used for the research described in the article.Over the last decades, the increasing incidence of Diabetes Mellitus in both developing and affluent countries [1] has further highlighted the need for new approaches to glucose sensing. Currently, most of the commercially available products are still based on the same principle behind the enzymatic biosensor proposed by Clark in 1962 [2,3]. Although enzymatic electrochemical sensors have the potential to be produced at a low cost and be compact, they suffer from a limited functional period which complicates their use for continuous monitoring applications [4]. The poor stability of the signal can chiefly be attributed to fouling by non-target species, and in particular to the use of the enzyme, whose activity progressively reduces over time [5]. Additionally, the enzyme's sensitivity to temperature, pH and humidity changes and the high risk of enzyme leaching increases the complexity of the manufacturing process and imposes more delicate conditions of usage [6]. To overcome these drawbacks, mechanisms of non-enzymatic electrochemical glucose sensing have been studied [5,7,8]. In this regard, nanomaterials play a major part [9]. Different nanostructured electrode materials have been developed over the years and in particular noble metals (Au, Pt, Pd etc.) [10] played a significant role in the advancement towards fourth-generation glucose sensors.Considerable effort has been dedicated in understanding the mechanism of direct electrooxidation at metal electrodes. Accordingly, two models have been proposed: the chemisorption model [11] and the incipient hydrous oxide adatom mediator model [12].Despite the extensive studies on the oxidation process, the use of bulk noble metals for glucose sensing is limited, due to the presence of many shortcomings [13,14]. First, the competition of anions such as halides for glucose adsorption, leading to catalyst poisoning [15,16]. Secondly, the electrode's degradation as a result of fouling by species commonly found in physiological solutions. Lastly, the much greater cost of noble metals compared to other materials, due to their scarcity and use as assets in the financial system [17].For these reasons, the research community has focused its attention on finding alternative materials capable of electrooxidizing glucose without being as affected by poisoning and fouling, while costing a fraction of the noble metals. Examples are metal oxides (NiO, CuO, Co3O4 etc.) [18\u201320], metal sulfides (NiS, FeS2, CuS2, etc.) [21\u201323], metal organic frameworks [24\u201326] and carbon materials (graphene, carbon nanotubes etc.) [27]. Alloying [28], doping [29] and carbon composites [30] are common approaches being explored to fine tune the catalytic activity and/or poisoning resistance.In particular, in the field of glucose sensing the element nickel has shown exceptional potential, finding application in its pure form, in compounds and also recently in conductive metal organic frameworks. Its widespread use can be mainly attributed to the highly oxidizing power of the Ni(II)/Ni(III) couple [31,32] and to its intrinsic resistance to halogen poisoning (both by itself and in combination with noble metals) [33].Furthermore, nickel is a naturally occurring element that can be readily found in the environment, existing in over a 100 different mineral forms [34]. It belongs to the transition series with other group 10 elements such as Pt and Pd. Sometimes considered as the \u201cimpoverished\u201d sibling of Pt, its high activity towards sluggish reactions in homogeneous [35] and heterogenous catalysis [36] proves this characterization to be limited. Additionally, the wider availability of Ni makes it vastly more affordable than common noble metal catalysts (e.g., the price of Ni is presently 2500-fold lower than for Au). As a result, more researchers are turning to nickel-based materials for application as either supports or catalysts due to their outstanding yet highly tunable properties.This review article summarizes the most recent progress in the development of Ni-based non-enzymatic electrochemical glucose sensors. The fundamentals of glucose electrooxidation at Ni electrodes are highlighted, which can in some part be extended to other Ni-based systems. The literature on the chloride poisoning is briefly presented and a possible explanation for its resilience compared to noble metals is proposed. Additionally, the analytical performance of sensors based on Ni compounds, bimetallic nanostructured systems and metal organic frameworks with Ni centers is discussed.The electrochemical oxidation of glucose on Ni electrodes has been reported in numerous studies to occur at the same potential of the Ni (III) oxide formation [37]. Accordingly, the electron transfer appears to be mediated by the \n\nNi\n\n\n(\nOH\n)\n\n2\n\n\n/NiO(OH) redox couple. As a consequence, the overall electrocatalytic activity of Ni towards glucose electrooxidation is positively correlated with pH.In alkaline solutions, the following mechanism has been proposed by Fleischmann and others:\n\n(1)\n\n\nNi\n+\n2\n\nOH\n\u2212\n\n\u2192\nNi\n\n\n(\nOH\n)\n\n2\n\n+\n2\n\ne\n\u2212\n\n\n\n\n\n\n\n(2)\n\n\nNiO\n+\n\nH\n2\n\nO\n\u2192\nNi\n\n\n(\nOH\n)\n\n2\n\n\n\n\n\n\n\n(3)\n\n\nNi\n\n\n(\nOH\n)\n\n2\n\n+\n\nOH\n\u2212\n\n\u2192\nNiO\n\n(\nOH\n)\n\n+\n\nH\n2\n\nO\n+\n\ne\n\u2212\n\n\n\n\n\n\n\n(4)\n\nNiO\n\nOH\n\n+\nglucose\n\u2192\nNi\n\n\nOH\n\n2\n\n+\ngluconolactone\n\n\n\nThe Nickel(III) acts as a strong oxidant towards glucose (and other organic compounds [32]) by participating in the rate limiting step of the electrooxidation: the hydrogen abstraction from the \n\n\nC\n\n\n\u03b1\n\n\n\n to the functional group. The irreversible nature of the oxidation reaction is confirmed by the increase in peak current for increasing glucose concentrations limited only to the anodic scan (Fig. 1\n-a).To achieve reproducible results, a good strategy is to first perform a few scans of cyclic voltammetry between 0V and 0.7V (vs Ag/AgCl/KCl(3\u00a0M) reference electrode), which guarantees the complete conversion of Ni and NiO species to \n\nNi\n\n\n(\nOH\n)\n\n2\n\n\n and NiO(OH) [39].For the sake of comparison, many researchers evaluate their Ni-based electrodes\u2019 performance at pH 13. Nevertheless, an optimization study should always be considered in order to evaluate the hydroxyl concentration that leads to the highest peak current and the lowest background current (current response in the absence of the analyte). In a recent work, Ko et al. [38] proved via chronoamperometry that the concentration of NaOH producing the best signal to noise ratio was 0.5\u00a0M. Clearly, care should be taken not to generalize their results to other systems having different substrates or additional catalysts.Two different forms exist for \n\nNi\n\n\n(\nOH\n)\n\n2\n\n\n (\n\n\u03b1\n\n and \n\n\u03b2\n\n) and for NiO(OH) (\n\n\u03b2\n\nand \n\n\u03b3\n\n ) [40]. Visscher and Barendrecht [41] as well as Hahn et al. [42] observed that \n\n\u03b1\n\n- \n\nNi\n\n\n(\nOH\n)\n\n2\n\n\n slowly transforms in \n\n\u03b2\n\n- \n\nNi\n\n\n(\nOH\n)\n\n2\n\n\n as a result of electrochemical ageing. In particular, the transformation from \n\n\u03b1\n\n- \n\nNi\n\n\n(\nOH\n)\n\n2\n\n\n to \n\n\u03b2\n\n- \n\nNi\n\n\n(\nOH\n)\n\n2\n\n\n is associated with a shift of the anodic peak position to higher potentials. As a result, for a fixed potential the current density for water oxidation will be reduced for aged electrodes. This polymorphism can account for the diverse electrochemical response of different Ni electrodes.One of the main disadvantages of non-enzymatic glucose sensors is the lack of a selective recognition mechanism. Nickel-based catalysts and non-enzymatic glucose sensors as a whole, once polarized can oxidize many other molecules in addition to glucose. This can be attributed to the easily accessible catalytic sites (e.g. NiOOH) which on a flat electrode surface can be reached by small and large molecules alike. As a result, the electrode's current response may poorly correlate with the real serum glucose concentration. In particular, the main interferents are reportedly: ascorbic acid, uric acid, acetaminophen and dopamine [43]. Acetaminophen and dopamine can be especially problematic when their oxidation products have an affinity for the electrode surface (usually carbon-based) because they can effectively block the catalytically active sites [44,45]. Other interferents include mono and disaccharides such as fructose and sucrose, which possess a similar chemical structure to glucose. Disaccharides do not exist in the bloodstream [46], since they get separated into their constituents by enzymes during digestion (e.g. sucrose gets separated by sucrase into glucose and fructose). However, in the case of fructose its blood concentration is usually 10\u20131000 times lower than that of glucose [47], thus posing no particular issue from an analytical standpoint.Overall, nickel-based non enzymatic glucose sensors can achieve a satisfactory level of selectivity and this can be attributed to a number of reasons. First of all, the concentration of interferents in the blood serum is usually more than 10 fold lower than that of glucose (which for healthy individuals is around 4\u00a0mM [48]). Accordingly, even supposing to oxidize the interferents with the same kinetic rate, this would at most cause a 10% uncertainty in the current signal. Second of all, the majority of the proposed electrode configurations in the literature involve some measure of surface nanostructuring [9], which can act as a size control for molecules larger than glucose. Additionally, the high degree of hydroxylation of Ni-based electrodes renders the surface extremely hydrophilic, which naturally repels hydrophobic species such as most fouling agents [49]. Moreover, the electrochemical technique being used also has an impact. Accordingly, good selectivity can be achieved in certain systems with potentiostatic approaches (chronoamperometry) by choosing a potential at which glucose is oxidized, but not the interferents. Lastly, the use of membranes (e.g. Nafion) has been shown to effectively protect the surface from fouling, while maximizing the overall sensor's selectivity mostly through a size selection mechanism [50,51].The chloride ion (Cl\u2212), with a molar mass of 35.45\u00a0g/mol, is the most abundant anion in the human serum with a concentration around 97\u2013107\u00a0mM [52]. It plays a significant role in the fluid homeostasis, electrolyte balance, conservation of electrical neutrality and acid base status [53].Its significant concentration in the extracellular fluid is well known to impair the performance of non-enzymatic glucose sensors in a process referred to as halide poisoning, where the surface-active sites of the catalyst are being blocked. Noble metals, such as Pt and Au, are reported to be particularly affected by halide poisoning for glucose and methanol electrooxidation [54\u201357]. The reason for the loss of catalytic activity has been attributed to the specific adsorption of Cl\u2212. Moreover, the presence of chlorides causes the formation of soluble species instead of an oxide layer from Au [16,58], inevitably leading to corrosion. Similarly, electrochemical cycling in chloride rich solutions is known to cause Pt etching [15].Conversely, the surface of metals such as Ni spontaneously forms a passivation film composed of an inner NiO with an hydroxylated outer layer, which protects it against corrosion [59]. In certain conditions, the interaction with halides can still lead to the breakdown of the passivation film, and the subsequent corrosion by pitting.Nonetheless, NiOx is drastically less susceptible to poisoning compared to noble metals and the reason for this has generally been attributed to a difference in adsorption energy of the halide ion on the different surfaces [60,61]. A deep evaluation of the poisoning resistance of NiOx electrodes was first performed by El-Rafaei and colleagues [33], who recorded the glucose electrooxidation peak in 0.5\u00a0M NaOH before and after the addition of 0.1\u00a0M Cl\u2212. After chloride addition, the peak current decreased to a minimum at the 4th cycle (losing 4% of the initial value), and then it increased again almost to the original value after the 15th cycle. This behavior may be explained by the low absorbability of Cl\u2212 ions, and possibly by the high reversible oxidation potentials of the Cl2/Cl\u2212 couples [62].A deeper understanding of the mechanisms at play is provided by computational approaches. In a pivotal work, Bouzoubaa et al. [63] described the interaction of an hydroxylated defect-free NiO(111) surface with different halides (F\u2212,I\u2212,Cl\u2212,Br\u2212).\nFig. 2\n shows the modeled slab for different OH- substitutions by halide ions X\u2212.In particular, as the chloride coverage increases and the Cl\u2212 progressively substitutes more OH\u2212 groups, the lateral anion-anion repulsions gradually reduce the OH\u2212 substitution energy (becoming endothermic at more than 75% substitution). This result is confirmed by previous studies [64]. Moreover, since the ionic radius of Cl\u2212 is larger than that of OH\u2212, the NiCl bond length is larger than that of NiO, thus leading to a splitting in the mixed topmost plane.\nFig. 3\n may be read as follows: at low chloride concentrations (<10\u22124\u00a0M) the NiOH termination is the most energetically favorable, whereas above 10\u22124\u00a0M the 25% OH\u2212 substitution is the most stable configuration even at high Cl concentrations, due to the strong anionic repulsion between Cl ions. This calculation may explain why NiOx electrodes are intrinsically resilient to chloride poisoning: approximately three quarters of surface hydroxyl groups are maintained, which during polarization in an alkaline electrolytes can still give rise to the highly oxidizing Ni(III).The small difference in electronegativity between Ni (\u03c7\u00a0=\u00a01.91) and Se (\u03c7\u00a0=\u00a02.55) allows the formation of different nickel selenides [65]. A wide range of phases have been reported to be stable at room temperature: NiSe2, NiSe, and Ni3Se2. Based on the prediction of Horn and Goodenough's [66], an increase in the covalency of the metal-oxygen bond significantly influences the binding of oxygen-related intermediate species, which in alkaline solutions are important pathways to glucose electrooxidation.In the field of electrocatalysis of the different nickel selenides, a prominent role is played by Ni3Se2 and NiSe2, which boast a narrow band gap and high conductivity, making them intriguing candidates for non-enzymatic glucose sensors. Moreover, metal selenides possess higher electrical conductivity, compared to the respective oxides and sulfides, due to the strong metallic character of selenium [67,68]. Different selenylation approaches have been proposed, but most are based on techniques such as hydrothermal [69] or solvothermal [70] synthesis, electrodeposition [71], solution chemical process [72] or solid state synthesis [73]. Although the literature on nickel selenides for glucose sensing is still scarce, the current reports are highly promising. The first study on the use of NiSe2 as an electrode modifier for OH\u2212 mediated glucose electrooxidation was done by Mani et al. [74]. Here, the researchers employed a hydrothermal synthesis method and then a drop cast of the nanosheets in an alcoholic dispersion on a glassy carbon electrode. The authors proposed the following redox process for the electrode in the absence of glucose in alkaline solution:\n\n(5)\n\n\n\nNiSe\n2\n\n+\n\nOH\n\u2212\n\n\u2194\nNiSeOH\n+\nSe\n+\n\ne\n\u2212\n\n\n\n\n\nDuring the cyclic voltammogram the Ni(III) and Se(II) species are oxidized to NI(IV) and Se(III), which readily oxidize glucose to glucolactone. The enhancement in the electrocatalytic activity may be attributed to the ability of the Se constituent to increase the charge transport efficiency between the Ni center and the electrode substrate. Although reasonable, in the absence of a Ni electrode control such assertions still need to be verified. The calculated sensitivity for the electrode after amperometric calibration in 0.1\u00a0M NaOH was limited to 5.6\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 with a limit of detection (LOD) of 0.023\u00a0\u03bcM.In order to improve the electron transfer capabilities of nickel selenide-based electrodes, some researchers have combined them with carbon nanostructures. For instance, in a recent work, Xu et al. [75] fabricated a hierarchical electrode composed of carbon nanorods and NiSe2, synthesized though a facile thermal route. After dispersion in a Nafion solution, it was drop cast on a glassy carbon electrode. The reported sensitivity was 3636\u00a0\u03bcA\u00a0mM\u22121 cm\u22122 with a LOD of 0.38\u00a0\u03bcM, after amperometric calibration in 0.1\u00a0M NaOH. Evidently, the mechanical stabilization of the NiSe2 nanosheets with a conductive membrane greatly improves the overall performance of the sensor. Fig. 4\n clarifies the effect of the carbon nanorod incorporation, having a significant and positive influence on the current increment as a result of glucose addition.While electrode fabrication procedures such as drop casting and thin film coating are common in the literature, they are time consuming and require the use of polymeric binders in order to fix the catalyst on the electrode's surface. In these cases, the catalytic active centers can get significantly blocked by the polymer, thus hindering the electron transfer capabilities of the electrode as a whole [76].A more scalable solution, which has been explored not only for Ni selenides, is the synthesis of the active catalyst directly from the corresponding metal (e.g., Ni). In this way, there is no need for a membrane to guarantee the direct catalyst-electrode contact, which, as previously stated, impairs the electrical conductivity.As an example, Ma et al. [77] fabricated through hydrothermal routes Ni3Se2 nanosheets (NS) supported on a Ni foam, with a reported sensitivity of 5962\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 and a LOD of 0.04\u00a0\u03bcM. Fig. 5\n highlights the improvement in the current density (\n\n\u0394\nj\n\n), going from a bare Ni foam to a Ni3Se2/Ni Foam. The authors attributed the increase in sensitivity to the synergistic interaction between the Ni3Se2 nanosheets and the Ni support.\nTable 1\n lists all the non-enzymatic glucose sensors described in this section.Compounds of Ni with chalcogenides for glucose sensing are not solely limited to selenides. Ni sulfides have been closely investigated as well. The main reported advantages of these materials are their high redox ability, good electrical conductivity and thermomechanical stability [80].Electronic and band structure calculation suggest that as the ratio of S to Ni increases, the Ni d-band centers become more negative and the S p-band centers become more positive [81]. The presence of a band gap between Ni d-band and the S p-band explains why nickel sulfides are generally less conductive than pure Ni. It has been shown that the phase of nickel sulfide has a meaningful effect on the catalytic activity for the hydrogen evolution reaction [82]. As of now, no studies have definitively clarified its impact towards glucose electrooxidation.Nickel sulfides can exist in different crystalline structures and stochiometric ratios, such as Ni3S2, NiS, NiS2, Ni3S4. They find application in different fields, ranging from dye-sensitized solar cell [83] to supercapacitors [84] and electro(photo)catalytic oxygen/hydrogen evolution [28,29]. The current scientific research on nickel sulfides for glucose sensing is mainly focused on exploring the catalyst-support interaction which provides the greatest sensitivity, stability and reproducibility. The first work on an electrodeposited NiS film was performed by Kannan et al. directly on indium tin oxide (ITO) electrodes [85]. The chosen synthesis method was straightforward and easily scalable, granting a sensitivity in 0.1\u00a0M NaOH of 7430\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122.In alkaline solution the proposed reversible redox reaction is the following [86]:\n\n(6)\n\n\nNiS\n+\n\nOH\n\u2212\n\n\u2194\nNiSOH\n+\n\ne\n\u2212\n\n\n\n\n\nGiven that the morphology has a strong effect on the catalytic activity, the majority of the published research works report catalyst nanostructuring. Accordingly, hollow spheres of \u03b1-NiS have been studied due to their good electrocatalytic activity and stability, ease of synthesis, and environmental compatibility [87]. Interestingly, the authors observed a significant difference between the \u03b1-NiS and the \u03b2-NiS hollow spheres, with the former leading to a stronger electrocatalytic response. However, the abovementioned sensor had a sensitivity of only 155\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122. This was likely the result of aggregation, which reduced the active sites and the non-exceptional electrical conductivity of NiS. To overcome these obstacles many authors make use of a conductive matrix, such as functionalized carbon black [88]. Relatedly, Ni3S2/carbon composites have also been fabricated, as done by Lin and colleagues [89], by using an hydrothermal method where the carbonaceous matrix consisted of multiwalled carbon nanotubes.With an interesting approach, Meng et al. [90] grew a Ni3S2 nanoworm (NW) network directly on a poly (3,4-ethylenedioxythiophene)-reduced graphene oxide hybrid films (PEDOT-rGO HFs) modified on glassy carbon electrode. A schematic of the fabrication process is illustrated in Fig. 6\n. The sensor showed good sensitivity (2123\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122) and low LOD (0.48\u00a0\u03bcM), that the authors attributed to a combination of high surface area, morphology, hydrophilic nature allowing easy OH\u2212 adsorption and good coupling between the different electrode layers.As of now, one of the most promising technological methods is to directly grow the nickel sulfide catalyst directly on a nickel foam. For instance, Huo et al. [91] fabricated a 3D Ni3S2 nanosheet array supported on a Ni foam by hydrothermal synthesis using nickel nitrate and thiourea. After amperometric calibration in 0.5\u00a0M NaOH, the measured sensitivity was 6148\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122, with a LOD of 1.2\u00a0\u03bcM, mainly ascribed to the open channel structure, combined with a fast electron and ion transport. Alternatively, as illustrated in a similar work by Kim et al., Ni3S2 nanostructures can be hydrothermally grown on a Ni-foam by having it react with thioacetamide in an alcohol and water medium [92]. By adjusting the solvent composition, a hierarchical cauliflower-like structure was obtained (Fig. 7\nb). The sensor showed superior sensitivity (16\u00a0460\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122) and good LOD (0.82\u00a0\u03bcM) in a 0.5\u00a0M NaOH solution.\nTable 2\n describes the detection parameters of non-enzymatic glucose sensors based on nickel sulfides.Nickel nitride (Ni3N) has recently shown to be a promising electrocatalyst for glucose sensing. Ni3N is a low temperature solid state phase at the boundary between the hcp and hcp\u00a0+\u00a0fcc zones, where the nitrogen atoms occupy the octahedral interstitial sites of the nickel lattice in a way that minimizes the N\u2013N interactions [94].Calculated Density of States (DOS) studies indicate that bulk Ni3N is intrinsically metallic and that the carrier concentration can be additionally enhanced when dimensional confinement was applied along with nanoscale structure [95].Accordingly, the nitridation process induces a contraction in the d-band near the Fermi level, thus favorably changing the electronic structure for catalytic purposes.Different nitridation techniques are reported in the literature, such ammonolysis in a NH3 atmosphere [96], reactive physical vapor deposition in N2 [97], direct liquid injection chemical vapor deposition with NH3 as a co-reactant [98], nitrogen ion implantation [99], plasma based nitridation [100] or a solvothermal process with highly reactive azide or hydrazine [101,102]. When going from bulk to nanostructured materials, multiple stoichiometries of nickel nitride have been reported (e.g., NiN, Ni2N, Ni4N, Ni8N), due to the fact that phase boundaries can change when the characteristic size is at the nanoscale [103]. The first investigation of Ni3N as a glucose electrocatalyst was done by Xie et al. [104]. The authors synthesized nickel nitride nanosheets on a Ti mesh by ammonolysis of a previously deposited Ni layer. The sensor after amperometric calibration in 0.1\u00a0M NaOH displayed a very high sensitivity of 7688\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 and a LOD of 0.06\u00a0\u03bcM. However the linear range was only up to 1.5\u00a0mM, which limits its potential application since the usual blood glucose concentration is around 4\u20137\u00a0mM [105].As seen with nickel selenides (Section 4.1) and sulfides (Section 4.2), and many other catalytic systems [106\u2013108] the integration of a catalyst, such as Ni3N, in a conductive carbonaceous matrix can lead to significant enhancements in the sensing capabilities due to the well-known improvements in the electron transfer afforded by carbon materials. In this regard, Liu et al. [109] investigated how a change in the structural parameters of different carbon matrices affected the electrocatalytic activity of a Ni3N nanosheet/carbon electrode as a whole. The authors concluded that a hollow/tubular 3D porous architecture leads to the best sensing performance towards glucose, due to a greater dispersion of the nanosheets in the inner and outer walls of the carbon fibers. The calculated sensitivity in 0.1\u00a0M NaOH was 1620\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 until 1.75\u00a0mM and 856\u00a0\u03bcA\u00a0cm\u22122\u00a0mM\u22121 (from 1.75 to 9.18\u00a0mM), with a LOD of 0.05\u00a0\u03bcM for the lower concentration range.To further increase the sensing performance of composite Ni3N/carbon electrodes, a current trend is to work on improving the synergy between the carbon matrix and the nickel nitride catalyst. Nitrogen doping of carbon materials allows to fix the metal sites and to facilitate the catalytic process by regulating the electronic structure of the carbon matrix [110]. In a recent work, Chen et al. [111] successfully fabricated a sensor based on nickel nitride decorated nitrogen doped carbon spheres (Ni3N/NCS). Fig. 8\n shows the morphological details of the bare and modified N-doped carbon nano spheres, and also the XRD spectrum of the synthesized material. A facile, eco-friendly one pot nitridation process was employed and after amperometric calibration in 0.1\u00a0M NaOH the calculated sensitivity was 2024.18 \u03bcAmM\u22121cm\u22122 (up to 3\u00a0mM) and 1256.98\u00a0\u03bcA\u00a0mM\u22121cm\u22122 (from 3 to 7\u00a0mM) with respective LOD of 0.1\u00a0\u03bcM and 0.35\u00a0\u03bcM.The sensing parameters of the above-described nickel nitride-based sensors are listed in Table 3\n.A commonly employed strategy to maximize the electrode's sensitivity is to introduce nanoparticles in the design, in order to take advantage of their high specific surface area and augment the number of active sites. A fairly recent avenue for glucose sensing is represented by bimetallic alloy nanomaterials, which by definition are comprised of two or more metals. Due to synergistic interactions, bimetallic nanomaterials are considered to be more electroactive than their monometallic counterpart [116,117]. Depending on the metal combination, significant improvements have been reported in terms of sensitivity, stability, biocompatibility and specificity due to biomimetic behavior [118]. The change in reactivity of a metal as a result of alloying can either be due to a change of the electronic structure, increased number of possible bonding geometries for adsorbates, or more indirectly, due to a change in the lattice parameters [119]. In general, a useful, albeit simple, descriptor for a metal's reactivity has been recognized to be the position of d-band center \n\n\n\u03b5\nd\n\n\n . The higher (lower) the d-band center, the stronger (weaker) the affinity of an adsorbate to the metal site [120\u2013122]. As a molecular adsorbate interacts with the metal's d-band it gives rise to bonding and antibonding molecular orbitals. As a consequence of alloying an upward (downward) shift in the metal d-band is produced, which leads to a decreased (increased) filling of the metal-adsorbate anti-bonding orbitals. It should be noted that there are exceptions to this rule [123], and more refined models that take into account the shape of the d-band have been proposed [124].The main synthetic methods for Ni-based bimetallic nanostructures are co-reduction [125,126], thermal decomposition [127], seed mediated growth [128], galvanic replacement reaction [129] and electrodeposition [130\u2013132].The tendency of Ni to oxidize complicates its synthesis in aqueous solutions [133]. An additional hurdle arises due to the magnetic properties of Ni nanoparticles, causing them to cluster together. For this reason there are few reports on the synthesis of monodispersed size distributions for Ni nanoparticles [134,135].Due to the strong electrocatalytic activity of both copper and nickel in alkaline solution towards glucose electrooxidation [136,137], many researchers have tried to explore how these two metals interact at the nanoscale and how their catalytic activity may change as a result. In a recent study, Wei et al. [138] developed a dendritic Cu@Ni on a Ni foam (NF) electrode for glucose sensing through a facile electrodeposition method. A schematic representation of the system is shown in Fig. 9\n.The sensor, after amperometric calibration in 0.1\u00a0M NaOH, showed a very high sensitivity of 11340 \u03bcAmM\u22121cm\u22122 with a LOD of 2\u00a0\u03bcM. NiO(OH) and CuO(OH) both contribute to a single anodic peak, due to the closeness of their respective oxidation potentials, as also observed in a similar work by Bilal et al. [139]. The strong electrochemical response was attributed to the synergistic interaction the between oxide shell and the conductive metal core, combined with the high surface area allowing easy glucose diffusion. The investigation of Lin et al. [140] on electrodeposited Ni and Cu nanoparticles on multiwalled carbon nanotubes shed light on two important aspects. First, that the multiwalled carbon nanotubes provide a large conductive area onto which Ni and Cu ions can electrodeposit without competing, thus leading a more ordered and active structure. Secondly, there is a ratio of Cu:Ni which gives the highest glucose oxidation current increment, which was observed to be 1:1.Analogously, Ammara et al. explored the combination of a Ni\u2013Cu nanocomposite with carbon nanotubes, by sequential electrodeposition on electrophoretically deposited carbon nanotube film [141]. In this way, the oxidation of Cu and Ni is lessened, while guaranteeing good mechanical stability and improved electrical conductivity though the formation of a percolation path. Here, the researchers also noted that the ideal ratio Ni:Cu to be 1:1.In a recent work, Xu and colleagues [142] fabricated through a one-step hydrothermal synthesis method Ni\u2013Cu bimetallic alloy nanoparticles on reduced graphene oxide. Surprisingly, after running an optimization study on the molar ratio of Ni:Cu, the strongest sensitivity was associated with a 4:1 ratio, in contradiction with the results of Lin et al. [140] and Ammara et al. [141]. In fact, a 1:1 ratio caused a decrease in the sensitivity compared to the bare metal surfaces. The difference in the optimum ratio between the studies is not trivial to explain, but may be due to the different synthetic methods being employed. Possibly, co-electrodeposition might cause the blocking of active sites of Cu at lower molar ratios, compared to hydrothermal methods.Bimetallic nanostructured systems composed of Ni and a noble metal are a promising solution to the current limitations of pure noble metal electrodes for glucose sensing: namely, the sensitivity to chlorides, which impairs their long term stability [16], and their high cost. Moreover, the addition of a second metal can modulate the catalytic activity and facilitate the reactant adsorption and product desorption, as implied above.Simultaneously, integrating a noble metal to a Ni electrode produces two main beneficial effects. First, it allows to extend the sensors\u2019 range of activity to neutral pH. This is because noble metals (such as Pt, Au) are able to directly electrooxidize glucose without the need for a high concentration of solution hydroxyls in the initial rate-limiting step of C1 dehydrogenation [43,143]. Secondly, it causes an increase in the electrical conductivity to the bare Ni electrode. In particular, many authors [144\u2013147], have explored the combination of Au and Ni for electrochemical glucose sensors, noting the presence of synergistic interactions between the two metals.As an example, the group of Yang synthesized spherical Au@Ni nanoparticles with a core-shell structure through a seed-mediated growth in oleylamine [148]. The core shell structure of the nanoparticles can be clearly appreciated in the TEM image shown in Fig. 10\nc. The oxidation of glucose on core-shell Au@Ni nanostructures was noted to be analogous to that of a pure Au particle. A similar observation was also done by Guo et al. [149] with a 4\u00a0nm electrodeposited Ni(OH)2 layer on nanoporous Au. The fabricated core-shell nanostructures were able to shield the active sites on the particle surface from Cl\u2212 and intermediates adsorption. At the same time the formed Ni layer allowed the formation of metal-OH sites, similarly to the Au\u2013OH sites at more negative potentials. In this way it was possible to avoid the oxidation of other interfering substances present in the electrolytic solution.With the use of electrodeposition, Zhou et al. [150] constructed a NiPt nanosheet array on carbon paper and concluded that the Pt:Ni ratio which gave the highest anodic peak in alkaline solution was 1:160. For higher amounts of Pt a decrease in current was observed, likely because the benefits in terms of improved electrical conductivity did not counterbalance the substitution of NiO(OH) centers with the less active PtOH.Other Ni-based bimetallic nanoparticles for glucose sensing include Ag-Ni [151] for which the Ag:Ni ratio giving the strongest activity was observed to be 1:4.In the future, Ni-noble metal systems might play a larger role in the field of continuous non enzymatic glucose monitoring devices. However most of the literature still performs the amperometric calibration in highly alkaline conditions, which is far from those of clinical applications requiring continuous glucose monitoring [105]. Therefore, a calibration in phosphate buffer solution at neutral pH should be the end-goal.\nTable 4\n summarizes the sensing characteristics of enzyme-free glucose sensors using bimetallic nickel-based nanomaterials.\nMetal organic frameworks (MOFs), have emerged as a new class of materials that combine unique properties such as microporosity, high apparent surface areas, and exceptional thermal and chemical stability [152]. For all these reasons MOFs are highly attractive as potential materials for the development of sensors. However, their low electrical conductivity and instability in the aqueous media has limited their applications for electrochemical sensing [153,154]. However, enthralling opportunities are provided by the relatively novel field of conductive MOFs, which are characterized by highly conjugated and delocalized \u03c0-bond in the ligand. Such a structure facilitates electron transport and greatly enhances its electrical conductivity with high sensing capability [155].In a recent work, Zeraati and colleagues synthesized a Ni-MOF with an ultrasonic assisted reverse micelle synthetic route, with a sensitivity of 2859.95\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 and a LOD of 0.76\u00a0\u03bcM [156]. Using a facile one pot solution process Xiao et al. [157] demonstrated that the Ni-MOF nanobelt morphology is favorable to glucose oxidation, in particular due to its reduced thickness which maximizes the surface area.An interesting avenue, proposed by Wang et al. [158], consists in the combination of a hierarchical flower-like Ni-MOF with single walled carbon nanotubes (SWCNT) used to enhance the electrical conductivity. The authors noted that the addition of the SWCNT not only led to an increase in faradaic current density but also led to a decrease in the peak-to-peak distance for the Ni(II)/Ni(III) couple suggesting an improvement also in the electrochemical reversibility. Analogously, Zhang et al. [159] achieved an extremely high sensitivity of 13850\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 by combining a Ni-MOF with carbon nanotubes. An even greater sensitivity of 21\u2006744\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 was reported by Qiao et al. [160] for a Ni-MOF synthesized via solvothermal methods on carbon cloth.With an exciting approach, Xue et al. [161] fabricated a 2D Ni@Cu-MOF by simple room temperature stirring. After performing electrochemical impedance spectroscopy analysis, the researchers concluded that the addition of Ni to the Cu-MOF caused an overall decrease in electrical resistivity. The as obtained sensor showed a sensitivity of 1703\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 and a LOD of 1.67\u00a0\u03bcM in 0.1\u00a0M NaOH. In an comparable study, Kim et al. [162] developed a Ni@Cu MOF though a two-step hydrothermal method.Bimetallic MOF based on Co have also been synthesized with different morphologies and supports [163,164], due to the notable performance afforded by the synergistic interactions between the two metals.As an example, in the solution proposed by Xu et al. [165], a nanorod-like bimetallic Ni/Co MOF was grown on a carbon cloth support. The sensor boasted a high sensitivity of 3250\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122 and a low LOD of 0.1\u00a0\u03bcM. The authors attributed the notable performance to the Ni/Co synergy and to the high surface area of the open framework structure. Cao and colleagues [166] investigated the effect of the integration of Ag nanoparticles in a matrix of Ni-MOF nanosheets. Compared to other systems in the literature, their sensor displayed a lower sensitivity (160\u00a0\u03bcA\u00a0mM\u22121\u00a0cm\u22122).A fascinating proposal by Lu et al. [167] consists in the use of core-shell MOF@MOF by internal extended growth of a shell of Ni-MOF on a core UiO-67. A schematic of the novel synthetic process is presented in Fig. 11\n. The researchers compared the electrochemical response of the composite with that of Ni-MOF and noted a decrease in the peak-to-peak distance and an increase in the glucose oxidation peak. This was attributed to the excellent electrical conductivity of UiO-67, which in turn improved the electron transfer rate constant.\nTable 5\n provides a summary of the sensing performance of nickel MOF-based sensors.Ni-based materials are attracting the attention of the scientific community for their outstanding performance towards glucose electrooxidation. Ni selenides, sulfides and nitrates are only recently being studied and have already shown promising results due to their strong redox capabilities and good electrical conductivity. The combination with a conductive carbonaceous matrix to form composite electrodes is a common solution to achieve a stronger electrocatalytic response due to an improved charge transfer constant. As a general rule, the highest sensitivity values for electrodes based on Ni-based compounds are obtained when the catalyst is grown directly on a Ni foam, instead of being drop cast and/or fixed on the surface with a binder.A promising avenue is the use of bimetallic nanosystems where Ni is a component (e.g., Cu/Ni, Co/Ni, Ag/Ni, Au/Ni and Pt/Ni) due to their superior activity, biocompatibility and fouling/poisoning resistance.Conductive MOF with Ni centers results in strong current responses thanks to their open pore structure combined with the oxidative power of Ni(III). The integration of Ni-MOF with nano-carbon based materials significantly improves the electrical conductivity and the combination with Cu and Co engenders synergistic interactions.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.", "descript": "\n                  Nickel-based catalysts are currently the subject of intensive study in the search for novel electrode materials for non-enzymatic glucose sensing. Their strong activity towards glucose electrooxidation and intrinsic resistance to chloride poisoning makes these catalysts ideal candidates for the development of affordable and stable glucose sensors. In this review, the mechanism of glucose electrooxidation at Ni electrodes is described, clarifying the effect of the different phases of Ni on their catalytic activity. Moreover, a brief background on chloride poisoning is provided, supplemented by computational studies. Furthermore, this article details the most intriguing compounds of Ni (selenides, sulfides, nitrates) and the analytical performance of the respective sensors. Additional focus points of this work are multimetallic nanosystems where Ni is a component, and the growing field of conductive metal organic frameworks with Ni centers. This review will be beneficial for researchers who aim at delving deeper into the potential of Ni-based materials for glucose sensing.\n               "}
{"full_text": "\n\n\nData will be made available on request.\n\n\nData will be made available on request.Growing concerns about international dependencies and environmental impacts of fossil fuel sources are driving a national transition towards more sustainable and renewable energy sources, including bio-derived liquid fuels (Schiffer,\u00a02022; O'Riordan\u00a0and Sandford,\u00a02022). Moreover, rising mobility demands are leading to intensified efforts to identify renewable drop-in replacements for both diesel and aviation fuel (Yan\u00a0et\u00a0al., 2021). The aviation industry in particular has high investment for biofuel substitutes considering electrification isn't as viable option for decarbonization as it is in the ground transportation sector. These shifting priorities are reflected in recent federal legislation that incentivizes biofuel production and use through tax credits and grants (Sustainable\u00a0Aviation Fuel Grand Challenge,\u00a02023; Brownley,\u00a02023; Yarmuth,\u00a02022). It follows that many groups are examining the potential of waste streams as reliable carbon sources that can serve a circular economic agenda (Awogbemi\u00a0et\u00a0al., 2021; Al-Muhtaseb\u00a0et\u00a0al., 2021). Among these, waste oils, fats and greases (e.g., used cooking oil) have drawn attention due to the high energy content and chemical similarity of long chain fatty acids to conventional diesel fuels (Borugadda\u00a0and Dalai,\u00a02018; Orsavova\u00a0et\u00a0al., 2015). Direct use of long chain fatty acids, and even their conversion to esterified biodiesel, is challenging due to flash point and viscosity discrepancies that limit mixing with or wholesale replacement of petroleum-derived fuels (Dey\u00a0and Ray,\u00a02020). However, fatty acid conversion to long-chain alkanes through deoxygenation and decarboxylation mechanisms offer potential for improved diesel engine compatibility with preferred ignition quality owing to their high cetane number (Yanowitz\u00a0et\u00a0al., 2017). This provides a compelling reason for identifying strategies for selectively deoxygenating fatty acids to hydrocarbons, especially fatty acids prevalent in food waste streams, e.g., oleic acid.Adding to the challenge in deoxygenating fatty acids with conventional refinery catalysts, waste fatty acid streams are often characterized by high moisture contents (Peng\u00a0et\u00a0al., 2008; Lizhi\u00a0et\u00a0al., 2008; Peterson\u00a0et\u00a0al., 2008), and the hydrodeoxygenation process, itself, generates water as a stoichiometric byproduct. As a result, there has been significant efforts directed towards development and commercialization of aqueous-phase catalytic processes for conversion of fatty acids, most often performed under hydrothermal conditions (250\u2013400\u00a0\u00b0C, 10\u201318\u00a0MPa) (Peterson\u00a0et\u00a0al., 2008; ReadiJet- ARA\u00a02023). Using a suitable hydrogen source, fatty acids like oleic acid can be converted in hydrothermal media to linear alkanes by hydrogenation (Eq.\u00a0(1a)) and decarboxylation (Eq.\u00a0(1b)) mechanisms:\n\n(1a)\n\n\n\n\n\n\n\n\n\n(1b)\n\n\n\n\n\n\n\nWhile there is a growing number of reports on hydrothermal fatty acid-to-alkane conversion, most studies have employed expensive noble metal catalysts, including platinum, palladium, and rhodium (M\u00e4ki-Arvela\u00a0et\u00a0al., 2007; Murata\u00a0et\u00a0al., 2010). Recent costs of these active metals has ranged from $900 to 2400/oz (Daily\u00a0Metal Price:\u00a0Free Metal Price Tables and Charts 2023). Moreover, significant negative environmental impacts are often associated with mining and processing of these rare metals (Burnett\u00a0et\u00a0al., 2021; Amatayakul\u00a0and Ramn\u00e4s,\u00a02001), something that is counter to the broader sustainability goals of biorenewable fuels. Some of the present authors recently reported on successful hydrothermal fatty acid-to-hydrocarbon conversion using supported ruthenium catalysts as a lower cost noble metal substitute, but market prices for this metal have also grown dramatically in recent years, highlighting additional challenges associated with the price volatility of these trace metals.The above discussion has led to renewed interest in identifying more earth-abundant and low-cost metals that might also be effective catalysts for hydrothermal fatty acid conversions. A recent report by Zhang\u00a0et\u00a0al. (2018a) showed hydrothermal conversion of oleic acid to heptadecane can be accomplished using a Ni-Cu bimetal catalyst supported on ZrO2, where hydrogen was supplied by in situ aqueous phase reformation of methanol. Nickel and copper are priced at $0.74/oz and $0.25/oz, respectively, making them an obvious price cognizant replacement for aforementioned noble metals (Daily\u00a0Metal Price:\u00a0Free Metal Price Tables and Charts, 2023). ZrO2 has shown promise as a stable support material for use in harsh hydrothermal environments (Papageridis\u00a0et\u00a0al., 2020; Joshi\u00a0et\u00a0al., 2014). Furthermore, zirconium may have a functional role in reactions since it has been shown to facilitate hydrogen production from water gas shift reactions of liquid hydrogen sources (Stekrova\u00a0et\u00a0al., 2018; Amatayakul\u00a0and Ramn\u00e4s,\u00a02001; Lytkina\u00a0et\u00a0al., 2015). Though Cu itself is not thought to be catalytic on its own, its interactions with Ni have been shown to enhance the latter's activity for a variety of upgrading processes, including gasification, hydrogenation and pyrolysis (Rashidi\u00a0and Tavasoli,\u00a02015; Wang\u00a0et\u00a0al., 2020; Kumar\u00a0et\u00a0al., 2019). For example, Luo et\u00a0al. used core-shell structured Ni-Cu nanocrystals on a carbon support for hydrodeoxygenation of 5-hydroxymethylfurfural and showed that the incorporation of copper resulted in >30% higher selectivity for 2,5-dimethylfuran as compared to the corresponding supported Ni mono-metal catalyst (Stekrova\u00a0et\u00a0al., 2018; Luo\u00a0et\u00a0al., 2017). Modeling of this co-metal has shown its ability to increase the rate and selectivity of the water gas shift reaction, leading to higher H2 production in aqueous systems (Stekrova\u00a0et\u00a0al., 2018; Lytkina\u00a0et\u00a0al., 2015; Gan\u00a0et\u00a0al., 2012) and promote active site clearing through carbon deposit oxidation (Stekrova\u00a0et\u00a0al., 2018; Boualouache\u00a0and Boucenna,\u00a02020).The present report revisits the recent findings by Zhang and coworkers (Rashidi\u00a0and Tavasoli,\u00a02015; Zhang\u00a0et\u00a0al., 2018a) to further examine the reactions of oleic acid and related fatty acids (stearic acid and linoleic acid) over Ni- and Cu- catalysts in hydrothermal media. Surprisingly, initial experiments comparing mono-metal and bimetal catalysts activity show similar oleic acid conversion with Ni/ZrO2 as Ni-Cu/ZrO2, inspiring a deeper investigation of Ni/ZrO2 reactivity. Thus, we conducted the first thorough evaluation of Ni/ZrO2 for deoxygenation of multiple long-chain fatty acids under hydrothermal conditions. Method of catalyst preparation (co-precipitation versus wet impregnation), hydrogen source and concentration, and catalyst deactivation pathways were all studied. Through relation of the highest performing catalysts\u2019 abilities under incrementally changing conditions to their morphological distinctions, we build the foundation for more directed future catalyst design. Findings point to a path forward towards development of lower cost and more environmentally sustainable materials that will be critical to efforts targeting valorization of waste carbon streams like used oils, fats, and greases.Zirconyl(IV) nitrate hydrate (99.5%) was obtained from Acros Organics. Glycerol (ACS grade) was purchased from Merck. Sodium hydroxide (20\u00a0N) and methanol (optima grade) were obtained from Fisher Chemical. Boron trifluoride (20% in methanol solution), hexane (>97.0%), stearic acid (>98.5%), linoleic acid (99%), nickel(II) nitrate hexahydrate (>94.5%), copper(II) nitrate trihydrate (analysis grade), oleic acid (90%), ethylene glycol (>99%), glucose (>99.5%), formic acid solution (1\u00a0M), dichloromethane (>99.8%) and sodium carbonate (>99.5%) were obtained from Sigma-Aldrich.Metal co-precipitation and wet impregnation methods were used to synthesize Ni- and Cu-based mono-metal and bimetal catalysts using tetragonal ZrO2 as a hydrothermally stable support material. Co-precipitation methods were adapted from Zhang\u00a0et\u00a0al.\u00a0(2018). A 0.1\u00a0M solution of metal precursors, with the final Cu, Ni, or Cu-Ni (1:1 molar ratio) loading of 15 wt% relative to Zr salt was mixed with a second solution containing 0.15\u00a0M NaOH and 0.045\u00a0M Na2CO3. The two solutions were mixed in a round bottom flask to obtain a pH of 9.5, and stirred at 1000\u00a0rpm overnight. The resulting solid was collected by vacuum filtration and washed with deionized water before drying in an oven for 12\u00a0h at 110\u00a0\u00b0C. Tetragonal ZrO2 was synthesized in the same manner except that Ni and Cu salts were omitted from the preparation. Catalysts prepared by wet impregnation were synthesized using procedures adapted from previous reports (Freitas\u00a0et\u00a0al., 2018). Cu or Ni nitrate salt solutions (either 6.336\u00a0g Cu or 5.191\u00a0g Ni salts) were immobilized onto 6.667\u00a0g of the pre-synthesized tetragonal ZrO2 support material in 50\u00a0mL of water while sonicating for 30\u00a0min, followed by stirring at room temperature overnight at 250\u00a0rpm. The resulting solid was recovered and washed three times with water and ethanol before drying in an oven at 110\u00a0\u00b0C for 12\u00a0h. Independent of synthesis procedure, dried catalysts were ground with a motor and pestle before calcining in a furnace at 600\u00a0\u00b0C for 4\u00a0h with 1\u00a0h ramp time. Following calcination, individual catalysts were activated in a tube furnace under flowing H2 for 1\u00a0h at 650 \u00b0C after a 1\u00a0h ramp time (\u223c10 \u00b0C/min).Elemental compositions of the catalysts and selected aqueous phase samples following oleic acid conversions were obtained by inductively coupled plasma-atomic emission spectrometry (ICP-AES; Perkin Elmer). Prior to analysis, solids (3.5\u00a0mg) were microwave digested in 3\u00a0ml HCl\u00a0+\u00a09\u00a0ml HNO3 before diluting to 100\u00a0mL in deionized water for analysis. Phase composition was determined by X-ray diffraction (XRD, PANalytical PW3040 X-ray diffractometer) between 10 and 80\u00b0 (2\u03b8) at a scan rate of 37\u00b0 min\u22121. HighScore spectral analysis was used for spectrographic assurance. To supplement ICP-AES and XRD for compositional confirmation, energy dispersive scanning transmission electron microscopy was carried out using a FEI Talos F200X STEM operating at 200\u00a0kV. To test for differences in fresh vs spent catalyst surface chemistry, temperature programmed reduction (TPR) was performed using a Micromeritics AutoChem II 2920 unit. Prior to TPR analysis, 35\u00a0mg of catalyst was pretreated to 550\u00a0\u00b0C under He/O2 gas mixture for 30\u00a0min. After which, the TPR was carried out to 600\u00a0\u00b0C under H2/Ar atmosphere at a gas flow rate of 50\u00a0ml min\u22121, temperature ramp of 10\u00a0\u00b0C min\u22121, and thermal conductivity detection (TCD).Mono- and bimetal catalyst activity was evaluated for hydrothermal conversion of oleic acid to stearic acid (hydrogenation, Eq.\u00a0(1a)) and subsequent conversion to heptadecane (decarboxylation, Eq.\u00a0(1b)) and possibly other hydrocarbon products. Hydrothermal batch reactions were performed in a stainless steel Swagelok microreactors (1.27\u00a0cm outer diameter\u00a0\u00d7\u00a010\u00a0cm length, 0.12\u00a0cm wall thickness) heated by submerging in a fluidized sand bath. Previous tests showed that the microreactors reach setpoint temperatures in <3\u00a0min (Li\u00a0and Strathmann,\u00a02019). Activity of the different catalysts were first compared at baseline conditions where 30\u00a0mg of catalyst was reacted with 100\u00a0mg of oleic acid and 20\u00a0mg methanol (as a source for in situ hydrogen production) in 1\u00a0m water at 350\u00a0\u00b0C for 5\u00a0h. Controls containing no catalyst were also performed at the same conditions. Activity of the most promising catalyst was further examined at a range of reaction conditions, including organic hydrogen source and concentration, temperature, and time. For comparison, reactions were also conducted with stearic (saturated fatty acid analogue) and linoleic acid (polyunsaturated fatty acid analogue) as the starting reactant. Reactions were quenched by removing reactors from the heated sand bath and submerging in a bath of water at room temperature. Once cooled, reactor contents were removed and washed three times with dichloromethane (DCM, 10\u00a0ml total) to extract residual fatty acid and conversion products. All reactions were carried out at least in duplicate.Fatty acids and hydrocarbons were analyzed by gas chromatography using a flame ionization detector (GC-FID; Thermo Scientific TRACE 1310 equipped with an Agilent DB-Wax 30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a025\u00a0\u00b5m capillary column). Hydrocarbon products were directly analyzed after injecting DCM extracts. For fatty acid analysis, samples were first subjected to a fatty acid methyl esterification (FAMES) procedure (Araujo\u00a0et\u00a0al., 2008) before GC-FID analysis. The injection and detection temperatures were 250 and 280\u00a0\u00b0C, respectively. Column temperature was increased from 50 to 200\u00a0\u00b0C at a ramp rate of 25\u00a0\u00b0C min\u22121 and then further to 230\u00a0\u00b0C at a ramp rate of 3\u00a0\u00b0C min\u22121. High purity H2 served was used as a carrier gas.Oleic acid conversions were calculated in molar yield percentages to gage the activity and selectivity of respective catalysts. Eqs. (2-3) were used to determine such activity and are as follows:\n\n(2)\n\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n\n\n(\n\nm\no\nl\n%\n\n)\n\n=\n\n(\n\n1\n\u2212\n\n\nm\no\nl\n\no\nl\ne\ni\nc\n\na\nc\ni\nd\n\nf\ni\nn\na\nl\n\n\nm\no\nl\n\no\nl\ne\ni\nc\n\na\nc\ni\nd\n\ni\nn\ni\nt\ni\na\nl\n\n\n\n)\n\n\u00d7\n\n100\n\n\n\n\n\n\n(3)\n\n\nP\nr\no\nd\nu\nc\nt\n\nY\ni\ne\nl\nd\n\n\n(\n\nm\no\nl\n%\n\n)\n\n=\n\n(\n\n\nm\no\nl\n\np\nr\no\nd\nu\nc\nt\n\n\nm\no\nl\n\ni\nn\ni\nt\ni\na\nl\n\no\nl\ne\ni\nc\n\na\nc\ni\nd\n\n\n)\n\n\u00d7\n\n100\n%\n\n\n\n\n\nFig.\u00a01\n shows results from initial experiments screening reactivity of oleic acid, a representative long-chain mono-unsaturated fatty acid, with mono-metal (Ni and Cu) and bimetal (Ni-Cu) ZrO2-supported catalysts. ZrO2 alone only led to minimal conversion of oleic acid to the corresponding saturated fatty acid, stearic acid, after 5\u00a0h of reaction. ZrO2 co-precipitated with Cu, however, saw more than double the conversion of oleic acid and production of stearic acid. No catalysts saw heptadecane production in the absence of nickel (Fig.\u00a01a, Table\u00a01\n). These findings track with those found by Zhang et\u00a0al. when employing Cu on ZrO2 for hydrodeoxygenation of oleic acid with methanol as a hydrogen source. In comparison, complete conversion of oleic acid was observed during reactions with Ni/ZrO2 and Ni-Cu/ZrO2, with a mix of stearic acid and heptadecane products, the latter being the product of stearic acid decarboxylation. The GC-FID chromatograms (Fig.\u00a02\n) showed minimal formation of other hydrocarbon cracking products, estimated to be <5% of product% yield. These products were relatively irrelevant for alkane production productivity and thus were not included in the final yields reported. It should be noted that, even when considering the cracking products, the quantified products don't lead to mass balance closure. This finding is not atypical for hydrothermal reactions, (Papageridis\u00a0et\u00a0al., 2020; Zhang\u00a0et\u00a0al., 2018c; Miao\u00a0et\u00a0al., 2016; Cai\u00a0et\u00a0al., 2022) particularly those conducted over nickel catalysts given its lack of controllability (Ananikov,\u00a02015). For example, oleic acid is susceptible to aromatization under these conditions (Tian\u00a0et\u00a0al., 2017). Additionally, if acetic acid is generated via hydrocarboxylation of the methanol precursor, which has been shown to be promoted by ruthenium based catalysts, its presence can promote the peracid mechanism for the epoxidation of oleic acid (Qian\u00a0et\u00a0al., 2016; Jalil\u00a0et\u00a0al., 2019). Oleic acid epoxides can then polymerize to form polyesters and other long chain products with low vapor pressures making them difficult to detect by common analytical techniques (Japir\u00a0et\u00a0al., 2021; Borugadda\u00a0and Dalai,\u00a02018; Yeh\u00a0et\u00a0al., 2015). The presence of an unsaturated bond is necessary for these reactions, which is supported by the relatively higher mass balance achieved when starting with a saturated fatty acid (e.g., see Fig.\u00a05). It is assumed that the products of these side reactions are consuming the remainder of the oleic acid, and no mass is being lost between reactor loading and product analysis. Surprisingly, heptadecane yields were slightly higher for the mono-metal Ni/ZrO2 (25.3%) compared to the bimetallic (20.5%) catalyst. This finding contrasts with the recent report by Zhang and co-workers (Zhang\u00a0et\u00a0al., 2018a), where heptadecane yields were markedly higher for Cu-Ni/ZrO2 (32.2%) compared with Ni/ZrO2 (22.7%) at the same reaction conditions used here and 3\u00a0h of reaction. The promotional reaction effects attributed to Cu in bimetallic catalysts are often recognized as electronic manipulation of the active metal catalyst via bimetal alloying (Kim\u00a0et\u00a0al., 2014). If metals are alloyed, their coordination state shifts, which is hypothesized to impact reactivity characteristics like intermediate adsorption time (Jin\u00a0and Choi,\u00a02019). XRD results in Fig.\u00a03\na suggest that Cu and Ni are alloyed. Therefore, alloying cannot be independently claimed as the key to reactive synergy. It is possible that the addition of copper interferes with this interaction, possibly intercepting the support and the nickel at their interface. In future discussion, we acknowledge the significance of support interactions with the active metal, justifying how this interception could inhibit nickel's activity. It should also be noted that the study by Zhang et\u00a0al. yielded higher in-situ pressure generation due to a higher loading to reactor headspace ratio, which could have promoted activity. The requirement of Ni for high overall activity is consistent with the metal's documented behavior in hydrogenation applications (Ananikov,\u00a02015; Wang\u00a0et\u00a0al., 2020; Li\u00a0et\u00a0al., 2022). However, the requirement for decarboxylation is less clear given the non-reductive nature of decarboxylation reactions. Vardon and co-workers (Vardon\u00a0et\u00a0al., 2014) also observed elevated rates of stearic acid decarboxylation by Pt/C and Pt-Re/C catalysts when applying a reducing H2(g) headspace compared to inert N2(g) headspace despite the non-reductive nature of the reaction mechanism. It was hypothesized that the reducing conditions served to maintain the active metals in their catalytic form.Further tests supported the use of co-precipitation as an effective method for catalyst synthesis, as both the Ni and Ni-Cu catalysts prepared by alternative methods of wet impregnation of the ZrO2 support proved to be less active for the decarboxylation step critical to heptadecane formation (Fig.\u00a01b). The high reactivity of co-precipitated Ni-Cu/ZrO2 has been attributed to increased nickel dispersion and metal-support contact, both initially due to nucleation rate differences of the two metals and long term due to stabilization effects by the bimetal (Liang\u00a0et\u00a0al., 2017; Elliott\u00a0et\u00a0al., 2006).\nFig.\u00a03 and Table\u00a02\n summarizes characteristics of the mono- and bimetallic catalysts, including Ni and Cu content from ICP-AES analysis. Catalysts were prepared with theoretical loadings of 15 wt% for each metal on the ZrO2 support, with ICP-AES analysis showing some divergence from these values. Higher than expected values can be attributed to less than theoretical ZrO2(s) recovery from solution. Nonetheless, active metal contents in the most active co-precipitated formulations of Ni/ZrO2 and Ni-Cu/ZrO2 were close to the nominal values.XRD analysis confirmed that all synthesized materials were tetragonal ZrO2 (t-ZrO2). These were used, in part, because commercial sources of ZrO2 are typically the less active monoclinic form of ZrO2 (Samson\u00a0et\u00a0al., 2014). The presence of a t-ZrO2 phase is indicated by XRD diffraction peaks at 2\u03b8 of 30.2\u00b0, 50.4\u00b0, 50.6\u00b0, and 60.0\u00b0 (full scan XRD data provided in Fig. S1 in Supplementary Materials). For m-ZrO2, peaks would have been observed at 2\u03b8\u00a0=\u00a028.7\u00b0 and 34.2\u00b0. Additionally, XRD showed peaks at 43.9\u00b0, 50.6\u00b0, and 74.9\u00b0, which are representative of nickel and copper hybridized face-centered-cubic lattice peaks at signature (001), (200), and (220) planes respectively. The lack of a distinguished Ni peak at 45\u00b0 in Fig.\u00a03a-2 is either due to high dispersion leading to low diffractive definition, or peak overlay by ZrO2 at 45.5\u00b0 (Fig.\u00a03a-4). Regardless, the peak shift to a centralized 44\u00b0 position in Fig.\u00a03a-3 indicates a hybrid diffraction between Ni and Cu which suggests metal-metal interaction. The orientation of such peaks are slightly offset between what would normally be characteristic locations of these lattice dimensions for both nickel and copper, possibly suggesting an alloyed bimetal structure.Dispersion of Ni and Cu on the synthesized materials was variable as shown in the STEM-EDS images provided in Fig.\u00a03b\u2013e. The co-precipitated Ni-Cu bimetallic catalyst shows finite nickel agglomerates embedded in the support matrix with copper more evenly dispersed. In comparison, wet impregnation of the Ni and Cu salts leads to larger agglomerated clusters around the ZrO2 support. These findings are consistent with reports by Zhang et\u00a0al. that co-precipitation of Ni (and Cu in the case of bimetallic formulation) with Zr maximizes active metal-support interfaces that may be important for hydrothermal catalytic activity with oleic acid (Zhang\u00a0et\u00a0al., 2020). Fig.\u00a03d, e show STEM-EDS and SEM of the fresh Ni/ZrO2 (CP) catalyst synthesized via co-precipitation. SEM shows the morphology to be clumpy with slight geometric features and looks of deposits/growth on a larger support (Fig.\u00a03e).Differences in reactivity observed with oleic acid (Fig.\u00a01) go beyond elemental composition. Ni/ZrO2 prepared by co-precipitation showed higher heptadecane yields than the catalyst prepared by wet impregnation of Ni onto ZrO2, despite the latter formulation having significantly higher Ni content. The low activity of the bimetallic catalyst prepared by wet impregnation methods may result from incongruent deposition wherein Cu overlays Ni, limiting interactions with fatty acids or the hydrogen donor. Bimetal deposition is highly sensitive to synthesis conditions and slight deviations of metal collocation has shown to make large differences in the adsorptive behavior of the catalyst (Zhao\u00a0et\u00a0al., 2016; Yang\u00a0and Cheng,\u00a02014). During co-precipitation, the concerted nucleation of all metals promotes mixed metal deposition, which can prevent active site blockage (Tang\u00a0et\u00a0al., 2018). Additionally, coprecipitation of the support in the presence of simultaneously nucleating metals can also promote the formation of a higher concentration of oxygen vacancies in the support ZrO2, since the introduction of structural impurities can induce reverse oxygen spillover. Such vacancies may strengthen support-metal interactions and stabilize the support by lowering the dielectric constant as a result of lattice distortion around the vacancy (Samson\u00a0et\u00a0al., 2014; Chen\u00a0et\u00a0al., 2014; Gao\u00a0et\u00a0al., 2011).While these findings support earlier reports by Zhang and co-workers, the higher than anticipated reactivity observed for co-precipitated Ni/ZrO2 calls for a more in-depth examination of the material's hydrothermal reactivity with oleic acid and related fatty acids. Fig.\u00a04\n shows the effects of reaction temperature (Fig.\u00a04a), reaction time (Fig.\u00a04b), and catalyst loading (Fig.\u00a04c) on oleic acid conversion and yields of heptadecane and stearic acid products. Complete conversion of oleic acid with Ni/ZrO2 within 5\u00a0h was observed at all temperatures >200\u00a0\u00b0C, but decarboxylation to produce heptadecane was not appreciable until temperatures reached 325\u00a0\u00b0C (Fig.\u00a04a), reaching a maximum of 42% at 370\u00a0\u00b0C. Interestingly, side products decreasing stearic acid and heptadecane yields were already significant at a temperature of 200\u00a0\u00b0C, but only increased slightly at higher temperatures reflecting competition among parallel pathways for oleic acid conversion.A reaction temperature of 350\u00a0\u00b0C, intermediate between conditions where heptadecane production was observed, was used when examining other system variables. Complete conversion of oleic acid at this temperature was observed within 2\u00a0h, yielding principally stearic acid, but longer reaction times were required to further convert the saturated fatty acid to heptadecane (Fig.\u00a04b). Further increases in heptadecane yield were limited after 20\u00a0h. Dosing additional methanol at t\u00a0=\u00a020\u00a0h also had limited effect on further conversion, so availability of excess hydrogen was not believed to be responsible for the stalled conversion. This is more likely the result of hydrothermal water-induced catalyst deactivation during the first 20\u00a0h of reaction. This would be consistent with previous reports (Wang\u00a0et\u00a0al., 2014; Elliott,\u00a02008; Champon\u00a0et\u00a0al., 2020). Reasons for reduced activity are explored in Section\u00a03.6.The yields shown in Fig.\u00a04a and Table\u00a01 should be contextualized in the literature of materials used for hydrothermal deoxygenation. Noble metals are the most prominent for these reactions and their higher yields are not unprecedented. Reports by Fu and coworkers (Fu\u00a0et\u00a0al., 2010, 2011) evaluate platinum and palladium on carbon for hydrothermal deoxygenation of fatty acids. At a reaction temperature of 330 \u00b0C, they observed a 9.3% yield of heptadecane over Pt/C from oleic acid after 1.5\u00a0h with no added hydrogen source. Interestingly, when starting with palmitic acid they observed a 55% yield for the decarboxylation product, pentadecane, after just 1\u00a0h. When starting with Pd/C, this yield dropped slightly to 50% pentadecane. Additionally, Vardon et\u00a0al. observed a 37% heptadecane yield after 9\u00a0h of reacting a Pt-Re/C bimetal catalyst at 330 \u00b0C with oleic acid using glycerol as a hydrogen donor (Vardon\u00a0et\u00a0al., 2014). Ni/ZrO2 has also been explored for stearic acid hydrothermal deoxygenation by Miao et\u00a0al. in 2018, which yielded 37% heptadecane after 9\u00a0h at 330 \u00b0C in the absence of an external H2 source (Miao\u00a0et\u00a0al., 2018). This agrees reasonably well with the yield we observed after reacting stearic acid for 5\u00a0h at 350 \u00b0C, albeit in the presence of methanol as a H2 source (Table\u00a01). In 2021, Zeng at al. was able to achieve 60% heptadecane yield from stearic acid using Co3O4 nanoparticles and a carbon matrix shell, underscoring the possibilities that advanced catalyst design can have for the future of non-noble metal deoxygenation catalysis (Zeng\u00a0et\u00a0al., 2022).In the absence of a catalyst, around 18% of oleic acid was converted to other products, 27% was converted to stearic acid and none of it was converted to heptadecane (Fig.\u00a04c). However, when 15\u00a0mg of the catalyst was loaded, rather than the typical 30\u00a0mg, the heptadecane yield dropped about 80% and the stearic acid yield almost doubled from 26% to 40%. Counterintuitively, when doubling the catalyst weight from 30\u00a0mg to 60\u00a0mg, heptadecane and stearic acid yields dropped to 17% and 12%, respectively, with complete oleic acid conversion retained. This optimum concentration suggests parallel pathways (e.g. polymerization, saponification, isomerization, etc.) are promoted by Ni ZrO2 and such are able to overcome decarboxylation in the presence of excess material. It should be noted that products produced from these pathways were not identified or quantified, however, there is a possibility that aromatics make up a portion of these products (Zhang\u00a0et\u00a0al., 2018b). Though these products have been found to be minimal in aqueous solvent systems (Tian\u00a0et\u00a0al., 2017), if produced they could be valorized for their high energy density and low smoke point heavily valued for diesel and jet fuel.Hydrothermal conversion experiments were also performed for the corresponding saturated fatty acid, stearic acid, and polyunsaturated fatty acid, linoleic acid, both with and without Ni/ZrO2. Results shown in Fig.\u00a05\n show that heptadecane production over the 5\u00a0h increases with increasing degree of initial fatty acid saturation. The only fatty acid that didn't see full conversion was stearic acid, further supporting that hydrogenation (Eq.\u00a0(1a)) is not the rate limiting step in the production of heptadecane. It should be noted that, since full hydrogenation occurred in each reaction, no residual linoleic or oleic acid was detected, therefore they are not listed as products in Fig.\u00a05. Cumulative heptadecane product yields also decreased with increasing degree of saturation, suggesting that the double bond(s) promote the rate of parallel side reactions that will deviate reactants from the desired deoxygenation pathway.Following from earlier work (Zhang\u00a0et\u00a0al., 2018a), methanol was added to reactor solutions to serve as a source for in situ hydrogen production in place of pressurized H2 gas addition. H2 is not only necessary for the saturation of oleic acid, but also the continuous reduction of the metal into its active metallic state. In hydrothermal water, methanol and other low molecular weight organic co-constituents can undergo aqueous phase reformation (APR) (Stekrova\u00a0et\u00a0al., 2018; Coronado\u00a0et\u00a0al., 2017). For example, each mole of methanol can react to form up to 3\u00a0mol equivalents of H2 with suitable catalyst and temperature (Eq. 4): (Coronado\u00a0et\u00a0al., 2017)\n\n(4)\n\n\nC\n\nH\n3\n\nO\nH\n\n\n(\n\nm\ne\nt\nh\na\nn\no\nl\n\n)\n\n+\n\n\nH\n2\n\nO\n\u2192\n3\n\nH\n2\n\n+\nC\n\nO\n2\n\n\n\n\n\nIn comparison, each mole of glycerol (C3H8O3) and formic acid (CH2O2) can react to form up to 7 mol and 1\u00a0mol of H2, respectively (Eqs. (5-6)):\n\n(5)\n\n\n\nC\n3\n\n\nH\n8\n\n\nO\n3\n\n\n\n(\n\ng\nl\ny\nc\ne\nr\no\nl\n\n)\n\n+\n\n3\n\nH\n2\n\nO\n\u2192\n7\n\nH\n2\n\n+\n3\nC\n\nO\n2\n\n\n\n\n\n\n\n(6)\n\n\nC\n\nH\n2\n\n\nO\n2\n\n\n\n(\n\nf\no\nr\nm\ni\nc\n\na\nc\ni\nd\n\n)\n\n\u2192\n\nH\n2\n\n+\nC\n\nO\n2\n\n\n\n\n\n\nFig.\u00a06\na shows the effects of adding different hydrogen donor sources in oleic acid conversion by Ni/ZrO2. Added concentrations of individual hydrogen donors were varied according to source to keep total theoretical H2 production constant. Pressure variations resulting from differing hydrogen source loadings were taken to be negligible. Results showed that oleic acid conversion was dependent on hydrogen donor source, with heptadecane yields being greatest for methanol in comparison to glycerol and formic acid, despite the same theoretical hydrogen production potential. These discrepancies in decarboxylation potential can be related to the structures of the hydrogen donors. It has been shown that the binding of acidic functional groups, like that in formic acid, to the basic sites of amphoteric ZrO2 impedes catalytic transfer hydrogenation (CTH) responsible for hydrogenation and cyclization of butyl levulinate to valerolactone (Chia\u00a0and Dumesic,\u00a02011). Similar blockage could be inhibiting H2 donation or blocking sites necessary for reactant adsorption for reaction. Polyalcohols including sugars undergo aqueous phase reforming for H2 production similarly to methanol. However, methanol has been shown to have a greater selectivity for hydrogen when reformed over nickel (Coronado\u00a0et\u00a0al., 2016). Additionally, the methanol and water solvent mixture has been shown to have synergistic effects for biomass liquification, which also contains hydrogenation (Zhang\u00a0et\u00a0al., 2019; Zhao\u00a0et\u00a0al., 2020).\nFig.\u00a06b shows the effect of varying methanol concentration initially added to the reactor. In the absence of an external hydrogen source, oleic acid was still able to be hydrogenated, indicating the ability of Ni ZrO2 to promote the water gas shift reaction and produce H2\nin situ. While the catalyst fully converted oleic acid at all conditions, heptadecane yield and overall product yield was lower when the methanol concentration was cut in half compared to the baseline concentration. More surprisingly, though, we also found that heptadecane yields dropped when higher methanol concentrations were used despite higher stearic acid yields. This suggests that formation of other products of APR, e.g., dissolved carbonate species, may act to inhibit stearic acid decarboxylation if present in too high of concentration. One such product could be CO, formed form either decarbonylation of the fatty acid or the reverse water gas shift reaction mentioned previously. CO has been shown to block nickel active sites, which would slow the production of heptadecane (Loe\u00a0et\u00a0al., 2019; Jackson\u00a0et\u00a0al., 1998). It is possible that some H2 donor precursors such as formic acid or glycerol could also promote the production of CO, which might limit decarboxylation efficiency. Future work is recommended to measure H2 and CO generation directly during reactions, which was precluded by the micro-reactors used in the present study.Finally, experiments were conducted to assess deactivation and potential regeneration of Ni/ZrO2 catalysts. It was already discussed that reactions appeared to stall after 20\u00a0h of reaction initiated with oleic acid, with further decarboxylation of stearic acid to heptadecane being limited between 20 and 30\u00a0h of reaction. Addition of a second spike of methanol had little effect, indicating that the stalled activity was not the result of limiting hydrogen supply (which we estimated to be in significant excess of hydrogenation requirements to begin with). These findings are consistent with previous reports of deactivation of Ni-based catalysts, where loss in activity is often attributed to corrosion, sintering, or leaching of the active metal, or surface coking of organic products (Crawford\u00a0et\u00a0al., 2020; Miao\u00a0et\u00a0al., 2016; Cheng\u00a0et\u00a0al., 2021; Champon\u00a0et\u00a0al., 2020). Analysis of dissolved Ni after the initial 20\u00a0h of reaction (Table\u00a02) indicated minimal leaching, so other deactivation mechanisms were suspected. A separate recycling experiment compared oleic acid conversion using Ni/ZrO2 which had already reacted for 20\u00a0h with oleic acid compared to virgin catalyst. Partial deactivation was observed, where yields for heptadecane were 52% lower for the used catalyst than virgin catalyst (Fig.\u00a07\n). Additional experiments showed that the catalyst deactivation occurs regardless of whether or not oleic acid and methanol are introduced with the catalyst to the hydrothermal media during the 20\u00a0h prior to initiating the reaction.Catalyst exposed to hydrothermal conditions for 20\u00a0h before introducing oleic acid and methanol was similarly deactivated (Fig.\u00a07), excluding potential active site coverage by deposited organic species as the primary mode of deactivation. Catalyst pre-exposed to 20\u00a0h of hydrothermal conditions was also subjected to reactivation through re-calcination and furnace reduction as described in the initial catalyst synthesis procedure. Results showed no recovered activity for heptadecane production (\nFig.\u00a07), suggesting that sintering may be responsible for the observed deactivation. The increase in the temperature of reduction as seen in H2-TPR data (Fig.\u00a08\n) is observed after catalyst deactivation, increasing from 276\u00a0\u00b0C to 313\u00a0\u00b0C after undergoing 20\u00a0h reaction. This increase indicates a less H2-reactive form of Ni after extended exposure to the hydrothermal reaction environment. This may result from loss of active metal / support interaction upon hydrothermal treatment. Further, the shoulder peak observed in the fresh catalyst indicates inner-support nickel particles which have maximized support interface, often responsible for greater activity (Roh\u00a0et\u00a0al., 2002; Zhang\u00a0et\u00a0al., 2020). These results support that metal migration and sintering from the bulk to the surface occurs under hydrothermal conditions. Metal migration also displaces metals from their initial support deposition chemistry, disrupting the electronic effects offered by this interaction and leading to lower reducibility of the nickel (Colorado\u00a0School of Mines,\u00a02023). Catalyst activity recovery post-sintering has not yet been formalized in literature. Therefore, it should be prioritized in future work to synthesize catalysts in a way which limits or slows this morphological transformation in the hydrothermal reaction environment. Bimetals have been shown to prevent metal migration. However, copper did not prove useful for this purpose in similar deactivation experiments conducted here. Another option would be physical anchoring of the metals in the support matrix through ligands (Jenkins\u00a0and Medlin,\u00a02021), complex organic framework caging (Zhao\u00a0et\u00a0al., 2018), or surface locking through synthesis modification (Yang\u00a0et\u00a0al., 2019). Successful material development to circumvent this deactivation could further the competitiveness of this earth abundant catalyst against its noble metal counterparts.Co-precipitated Ni on tetragonal ZrO2 proved to produce the most efficient catalyst for decarboxylation of C18 fatty acids to heptadecane under subcritical hydrothermal conditions (300 \u2013 370\u00a0\u00b0C). Non noble active metals were chosen for evaluation due to their price point relative to other commonly used platinum group transition metals which can cost over 99% more (Daily\u00a0Metal Price:\u00a0Free Metal Price Tables and Charts, 2023, BASF,\u00a02023). Influence of copper as a non-noble bimetal on this reaction was null, contradicting recent reports. Methanol outperformed other organic liquid hydrogen sources. Catalyst and hydrogen donor loading were found to have an optimizable concentrations to encourage the reaction towards decarboxylation and away from fatty acid consuming parallel reactions. Greater heptadecane selectivity was observed when stearic acid was used as the starting reactant, emphasizing that the undesired parallel reaction pathways are more significant for the unsaturated fatty acid than saturated fatty acids. Deoxygenation over the active mono-metal catalyst scaled with temperature and reaction time up until the deactivation threshold, at which point metal sintering appears to deactivate the material. These specifications can inform optimal use of Ni ZrO2 for deoxygenation of fatty acids to liquid alkanes, however, more information regarding preventative deactivation synthesis is necessary to further advance these materials as a reliable alternative to their noble metal counterparts.Financial support for this work was provided by the National Science Foundation (NSF) through the NSF Engineering through the NSF Engineering Research Center for Reinventing the Nation's Urban Water Infrastructure (ReNUWIt; EEC-1028968) and NSF award CBET-1804513.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Financial support for this work was provided by the National Science Foundation (NSF) through the NSF Engineering through the NSF Engineering Research Center for Reinventing the Nation's Urban Water Infrastructure (ReNUWIt; EEC-1028968) and NSF award CBET-1804513. Moises Carreon, Praveen Kumar, Melodie Chen-Glasser, Ryan Richards, Galen Dennis and Matthew Posewitz (CSM) are acknowledged for assistance with analysis and materials characterization.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.hazadv.2023.100273.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  There is growing interest in the substitution of expensive noble metal catalysts with low-cost earth-abundant metals in applications targeting biofuels production from waste organic feedstocks. Here, nickel (Ni) catalysts supported on zirconium dioxide (ZrO2), both with and without copper (Cu) as a co-metal, were tested in hydrothermal reactions of unsaturated and saturated C18 fatty acids as models for waste oil feedstocks. In contrast to recent reports, this study showed no enhancement of nickel's activity for fatty acid conversion to alkane products when Cu was present. Ni/ZrO2 prepared by metal coprecipitation methods converted 100% of oleic acid with 25% selectivity to heptadecane after 5\u00a0h of reaction at 350 \u00b0C using methanol as a hydrogen donor source, increasing to 41% heptadecane after 20\u00a0h. Lower yields were observed with catalysts prepared by wet impregnation and using alternative hydrogen donor sources (glycerol, formic acid). Greater selectivity to heptadecane was also observed at higher temperatures (370 \u00b0C) and when the initial fatty acid had greater saturation. Longer term exposure to hydrothermal media led to metal sintering and catalyst deactivation. Findings support a path forward to the development of earth-abundant metal catalysts for the upgrading of waste organic feedstocks.\n               "}
{"full_text": "As important intermediates, alicyclic amines are widely used in chemical (anticorrosion products) and pharmaceutical industry. They are normally synthesized from the corresponding aromatic amines [1]. Precious metal-based catalysts [1\u201312] were mainly studied, while Co- and Ni-based catalysts were also reported [13\u201316]. Ru-based catalysts are applied in the reactions such as hydrogenation of aniline [1,5\u20137] and p-phenylenediamine [8]. They usually displayed high activity and selectivity to primary amines [1]. Nevertheless, deep understanding of ruthenium catalysts for the reactions at hand is still necessary.We have previously prepared supported Ni catalysts (60\u00a0wt%) and found that they were highly active for the hydrogenation of aromatic rings [17\u201319]. In particular, ethylamines were added so that the effects of -NH2 on the hydrogenation of toluene were studied [19]. We found that ethylamines inhibited the hydrogenation of toluene by restraining the adsorption of reactants on the Ni surface. In addition, the Ni-based catalysts with basic supports were less influenced by the ethylamine. Supported Co catalysts (60\u00a0wt%) were also prepared and it was found that the heat of TEA adsorption was lower on Co than on Ni [20], leading to the higher activity exhibited by Co/MgAlO than Ni/MgAlO for the hydrogenation of toluene in the presence of TEA.Previous work [19,20] demonstrated that the Ni and Co catalysts supported on MgAlO were more active than those with Al2O3 and MgO alone for the hydrogenation of toluene in the presence of TEA.In this work, 2\u20136\u00a0wt% Ru/MgO-Al2O3 catalysts were prepared for the hydrogenation of aromatic amines to alicyclic amines. Hydrogenation of toluene with and without triethylamine (TEA) was used as probe reaction. The heat of adsorption for TEA on Ru was found to be low, and thus the toluene adsorption was less affected by TEA on Ru than on Co and Ni. Thus, the activity was less affected by TEA on Ru than on Ni and Co for the hydrogenation of toluene. The selectivity to primary amines was also found to be high over the Ru catalysts for the hydrogenation of aromatic amines, owing to the weak adsorption of the amine group on the Ru surface.The 2\u20136\u00a0wt% Ru/MgAlO (MgO:Al2O3\u00a0=\u00a07:1, w/w) were prepared by a two-step co-precipitation method similar to the one described previously [21]. Another 2\u00a0wt% Ru/MgAlO (designated as 2%Ru/MgAlO-2) as well as a 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO were prepared by the one-step co-precipitation method [21] for comparison. The detailed procedure of catalysts synthesis is described in the Supporting Information (SI).The composition and structural properties of the catalysts samples were characterized by N2 adsorption-desorption isotherms, powder X-ray diffraction (XRD), and X-ray fluorescence spectroscopy (XRF).Hydrogen temperature programmed reduction (H2-TPR), H2 titration and microcalorimetric adsorption of H2, toluene and TEA, were applied to study the surface chemical properties of catalysts. All the catalysts were reduced and passivated before characterizations were conducted. The detailed procedures are described in SI.The hydrogenation of aromatic amines and toluene was carried out in a stainless steel fixed-bed reactor with 10\u00a0mm inner diameter. The products were collected and analyzed by a GC with a capillary column and a FID detector. The turnover frequency (TOF) of toluene was calculated by dividing the number of toluene molecules converted per second by the number of surface Ru sites obtained from H2 titration. Under the high conversions used, the TOF (s\u22121) estimated represents an average site activity. Detailed description of the above procedures can be found in SI. The absence of mass transport effects (diffusional resistances) could be excluded for the reactions studied in this work as reported in the SI.Fig. S1 displays the N2 adsorption-desorption isotherms and pore size distributions of the reduced Ru/MgAlO catalysts, while Table 1\n summarizes the information obtained. The isotherms indicated the presence of mesoporous structure (type IV with H3 hysteresis loops). The BET surface areas of the 2\u00a0wt% Ru/MgAlO, 4\u00a0wt% Ru/MgAlO and 6\u00a0wt% Ru/MgAlO were 369, 378 and 277 m2/g, respectively, corresponding to pore volumes of 1.37, 1.36 and 0.99\u00a0cm3/g, respectively. It is seen that the 2\u00a0wt% Ru/MgAlO and 4\u00a0wt% Ru/MgAlO possessed similar pore structures and surface areas, while the 6\u00a0wt% Ru/MgAlO displayed remarkably lower pore volume and surface area.The loadings of Ru measured and the H2 uptakes (metal dispersion measurements) are also given in Table 1. The loadings of Ru were close to the nominal values, indicating no loss of Ru during the preparation. When the loading of Ru increased from 2 to 4\u00a0wt%, the H2 uptake increased to 52\u00a0\u03bcmol/g. However, when the loading of Ru further increased to 6\u00a0wt%, the H2 uptake increased only by 30\u00a0\u03bcmol/g, indicating a decreasing dispersion of Ru in the 6\u00a0wt% Ru/MgAlO catalytic system. In fact, the ratio of H2 uptake to Ru loading was the highest for the 4\u00a0wt% Ru/MgAlO, which implies the highest dispersion of Ru among the series of supported Ru catalysts studied. After assuming the complete reduction of supported ruthenium, the dispersions and mean particle sizes of Ru in the catalysts were estimated and presented in Table 1.Fig. S2 shows the powder XRD patterns for the reduced 6\u00a0wt% Ru/MgAlO and MgAlO support alone. Only diffraction peaks belonging to MgO (at 44 and 62\u00b0) could be observed, indicating good dispersion of Ru particles in the 6\u00a0wt% Ru/MgAlO (Ru particles of less than 4\u00a0nm). The major peaks for Al2O3 are overlapped with those for MgO [22,23]. The peaks at 44 and 62\u00b0 were more likely MgO since the mass ratio of MgO and Al2O3 in the Ru/MgAlO was 7:1. The powder XRD patterns for the 2\u00a0wt% Ru/MgAlO and 4\u00a0wt% Ru/MgAlO are not given since they were very similar to that of 6\u00a0wt% Ru/MgAlO.Fig. S3 shows the results of H2-TPR for the fresh Ru/MgAlO samples. For the 2\u00a0wt% Ru/MgAlO, a broad peak at 697\u00a0K and a narrow peak at 786\u00a0K were detected, indicating a strong interaction between Ru species and the support, where part of Ru species was difficult to be reduced. Only one broad peak was observed at 679\u00a0K for the 4\u00a0wt% Ru/MgAlO, which is due to Ru species interacting strongly with the support. This peak decreased to 663\u00a0K for the 6\u00a0wt% Ru/MgAlO, with an additional reduction peak appearing at 600\u00a0K. The latter represents weaker interactions of Ru species with the support and which led to the decreased dispersion of Ru in agreement with the H2 adsorption results.\nFig. 1\n shows the results of the conversion of aniline with temperature for its hydrogenation to cyclohexylamine (CHA) and the selectivity of reaction for the latter product over the 2\u00a0wt% Ru/MgAlO, 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO catalysts. CHA was the main product and dicyclohexylamine (DCHA) was the only by-product. It is observed that the conversion of aniline increased with reaction temperature. The 2\u00a0wt% Ru/MgAlO exhibited significantly higher conversion values of aniline than the 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO at the lower temperature range. The selectivity to CHA decreased significantly (deamination to DCHA) over the 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO, while this was maintained high on the 2\u00a0wt% Ru/MgAlO with increasing reaction temperature. For example, the selectivity to CHA at 453\u00a0K was 88, 39 and 40% over the 2\u00a0wt% Ru/MgAlO, 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO, respectively. Thus, the deamination reactions were significantly less active on Ru than on Ni and Co, leading to a higher selectivity to the primary alicyclic amines.The 4\u00a0wt% Ru/MgAlO, 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO catalysts were also compared for the hydrogenation of m-xylylenediamine (MXDA) to 1,3-cyclohexanedimethanamine (1,3-BAC) and 4,4-diaminodiphenyl methane (MDA) to 4,4\u2032-diaminodicyclohexyl methane (H12MDA). The obtained results are shown in Table 2\n. The by-products resulted mainly from the deamination reactions, except for the 60\u00a0wt% Co/MgAlO over which the main by-product was the 4,4\u2032-diaminomonocyclohexyl monophenyl methane (H6MDA) for the hydrogenation of MDA. The results presented in Table 2 showed that the 4\u00a0wt% Ru/MgAlO was highly active and selective, as opposed to the other two catalytic systems for the the two reactions reported. A stability test for the hydrogenation of MXDA over the 4\u00a0wt% Ru/MgAlO is shown in Fig. S4, which indicated that the catalyst was stable for at least 240\u00a0h of reaction.To explain the high activity and selectivity of the Ru/MgAlO catalysts for the hydrogenation of aromatic amines, the hydrogenation of toluene in the presence of TEA was performed as a probe reaction. Fig. S5 compares the 2\u00a0wt% Ru/MgAlO, 60\u00a0wt% Ni/Al2O3 and 60\u00a0wt% Co/MgAlO catalysts for the hydrogenation of toluene. Methylcyclohexane was the only product; only 0.5% 4-methyl-1-cyclohexene was detected over the Ru/MgAlO at the high WHSV (weight hourly space velocity). It was found that the 2\u00a0wt% Ru/MgAlO displayed high activity for the reaction, similar to the 60\u00a0wt% Ni/Al2O3 and 60\u00a0wt% Co/MgAlO.\nFig. 2\n presents the effect of Ru loading on the conversion of toluene and TOF for the hydrogenation of toluene as a function of WHSV over the Ru/MgAlO catalysts. The conversion of toluene was about 1.4 times larger on the 4\u00a0wt% Ru/MgAlO than the 2\u00a0wt% Ru/MgAlO at WHSV of 80\u00a0h\u22121. However, the toluene conversion over the 6\u00a0wt% Ru/MgAlO is only 1.1 times larger compared to the 4\u00a0wt% Ru/MgAlO catalyst. According to the H2 uptakes, TOF values of toluene conversion over the Ru/MgAlO catalysts were calculated. The TOF increased with increasing WHSV, and a rather constant value was reached in the 80\u2013100\u00a0h\u22121 range. TOF values of 1.3, 0.81 and 0.68\u00a0s\u22121 were estimated for the 2\u00a0wt% Ru/MgAlO, 4\u00a0wt% Ru/MgAlO and 6\u00a0wt% Ru/MgAlO, respectively. However, the average intrinsic activity of a Ru atom decreased, indicating that surface Ru atoms on the smaller particles were more coordinatively unsaturated, and thus more active.Fig. S6 and Table 3\n compare the activities (conversion and TOF) of the Ru/MgAlO catalysts for the hydrogenation of toluene after using different amounts of TEA. It is apparent that the addition of TEA decreased toluene conversion and the activities further decreased with increasing amount of TEA added. However, the loading of Ru did not seem to affect the inhibitory effect of TEA.Table S2 compares the effect of TEA on the conversion and TOF of toluene for its hydrogenation over the 2\u00a0wt% Ru/MgAlO, 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO catalysts. The results showed that the 2\u00a0wt% Ru/MgAlO was much less affected by TEA than the 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO. For example, the TOF decreased by 42, 93 and 89% over the 2\u00a0wt% Ru/MgAlO, 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO, respectively, when 5\u00a0wt% TEA was added. These results agree well with those for the hydrogenation of aromatic amines to alicyclic amines over the Ru, Co and Ni catalysts.Fig. S7 (SI) compares the TEA adsorption on the reduced 2\u00a0wt% Ru/MgAlO, 60\u00a0wt% Ni/MgAlO, 60\u00a0wt% Co/MgAlO and MgAlO support, with the initial heats of 109, 177, 82 and 111\u00a0kJ/mol [19,20], respectively. It is seen that the interaction of TEA with MgAlO is fairly strong (111\u00a0kJ/mol), while the interaction of TEA with Ni is very strong (177\u00a0kJ/mol). In contrast, the interactions of TEA with Ru and Co surfaces were even weaker than with that of support (MgAlO), which might be the reason why the conversion of toluene was less affected by TEA on the Ru/MgAlO and Co/MgAlO. It should be mentioned that the lower initial heat of TEA obtained on the 60\u00a0wt% Co/MgAlO than on the 2\u00a0wt% Ru/MgAlO catalytic surface does not necessarily mean a stronger interaction of TEA with Ru than Co, since the loading of Ru in the 2\u00a0wt% Ru/MgAlO was low, leading to the similar coverages and heats for the TEA adsorption on the 2\u00a0wt% Ru/MgAlO and MgAlO surfaces.The low heat of adsorption of TEA on Ru indicates the weak interaction of -NH2 group with the Ru surface, which might account for the low selectivity to the deamination reactions (including condensation of -NH2 groups), i. e., the high selectivity to the primary amines.\nFig. 3\n shows the effect of surface coverage of toluene on the heat of adsorption of toluene with and without pre-adsorbed TEA on the surface of the 2\u00a0wt% Ru/MgAlO and MgAlO support alone. The initial heats of adsorption were found to be 71 and 22\u00a0kJ/mol on the clean 2\u00a0wt% Ru/MgAlO and MgAlO, respectively, indicating the fairly strong interaction of toluene with Ru and the weak adsorption of toluene on the MgAlO support. With pre-adsorbed TEA, the initial heat of toluene adsorption on the 2\u00a0wt% Ru/MgAlO decreased to 50\u00a0kJ/mol, suggesting the occupation of some surface Ru sites by pre-adsorbed TEA, and the restriction of toluene adsorption on the Ru surface.\nFig. 4\n shows results of the heat of adsorption of hydrogen on its surface coverage following preadsorption of TEA over the 2\u00a0wt% Ru/MgAlO catalyst. The initial heat of H2 adosorption on the 2\u00a0wt% Ru/MgAlO was found to be 69\u00a0kJ/mol. With pre-adsorbed TEA, the initial heat of adsorption (very low surface coverage) decreased to 48\u00a0kJ/mol, while the coverage was practically the same, indicating the inhibition effect of TEA on the H2 adsorption.Table S3 compares the initial heats of adsorption of toluene and H2 on the 2\u00a0wt% Ru/MgAlO, 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO without and with pre-adsorbed TEA. With pre-adsorbed TEA, the initial heat of toluene adsorption on the 2\u00a0wt% Ru/MgAlO decreased by 21\u00a0kJ/mol, significantly lower than that on the 60\u00a0wt% Ni/MgAlO (85\u00a0kJ/mol) and 60\u00a0wt% Co/MgAlO (42\u00a0kJ/mol). This result is suggested to explain well why Ru was less affected by TEA than Ni and Co for the hydrogenation of toluene. Table S3 (SI) also shows that pre-adsorbed TEA greatly decreased the initial heat of H2 adsorption over the 2\u00a0wt% Ru/MgAlO. However, the coverage of H2 did not change much. Our previous work indicated that strongly adsorbed H might be less active than the weakly adsorbed one [24]. Thus, it is suggested that the decreased activity for toluene hydrogenation over the supported Ru, Ni and Co catalytic surfaces might be mainly due to the inhibition of adsorption of toluene (rather than that of adsorption of H2) by TEA.It has been reported that the majority of active Pt metal sites for H2-SCR might be located within a ring around the Pt metal particles [25,26]. Thus, in the present work the metal-support interface might play important role. Unfortunately, the current data do not allow to identify and quantify the role played by the metal-support interface in the various catalytic systems.Highly dispersed Ru (2\u20136\u00a0wt%) supported on MgAlO carrier prepared by a two-step co-precipitation method were found much more active and selective than 60\u00a0wt% Ni/MgAlO and 60\u00a0wt% Co/MgAlO for the hydrogenation of aromatic amines to alicyclic amines, and for the hydrogenation of toluene in the presence of TEA (probe reaction) as well.The activity for toluene hydrogenation over the Ru/MgAlO decreased significantly upon the addition of TEA. However, the Ru/MgAlO catalysts were remarkably less affected by TEA than Ni/MgAlO and Co/MgAlO for the hydrogenation of toluene, which explains well why Ru/MgAlO catalysts were much more active than Ni/MgAlO and Co/MgAlO for the hydrogenation of aromatic amines to alicyclic amines.The pre-adsorption of TEA was found to decrease the heat of adsorption for toluene on the investigated catalysts. However, the heat of adsorption decrease was greatly less on the Ru/MgAlO than the Ni/MgAlO and Co/MgAlO. This result might account for that the Ru/MgAlO was less influenced by the presence of adsorbed TEA than the Ni/MgAlO and Co/MgAlO for toluene hydrogenation.The high selectivity to the primary amines for the hydrogenation of aromatic amines over the supported Ru catalysts can also be attributed to the low heat of adsorption due to the presence of amine group over the Ru surface.\nLi Zuo: Data curation, Writing \u2013 original draft. Jingxuan Cai: Methodology, Supervision, Writing \u2013 review & editing. Zhouyang Guo: Resources, Investigation. Yuchuan Fu: Resources, Visualization, Supervision. Jianyi Shen: Conceptualization, Methodology, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Financial supports from the NSFC (21773108), NSFC-DFG (21761132006) and fundamental research funds for central universities are acknowledged.\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106496.", "descript": "\n                  Ru/MgO-Al2O3 catalysts (2-6\u00a0wt% Ru) were prepared by a two-step co-precipitation method and were found to be highly active and selective for the hydrogenation of aromatic amines to alicyclic amines. Hydrogenation of toluene in the presence of triethylamine (TEA) was used as a probe reaction to explain the obtained results. It was found that adsorption of toluene in the presence of TEA on Ru than on Co and Ni catalysts was significantly less inhibited and thus the conversion of toluene. Ru/MgO-Al2O3 was thus found to be significantly superior to the Ni/MgO-Al2O3 and Co/MgO-Al2O3 for the hydrogenation of aromatic amines to alicyclic amines.\n               "}
{"full_text": "With the decrease of light crude oil supplies, there is rising concern about converting heavy oil into transportation fuels. As a poor-quality heavy oil, fluid catalytic cracking (FCC) slurry oil is mainly used as a blending component for producing marine fuel oil [1,2]. To improve air quality, strict emission regulations for marine fuel oil have been made compulsory by International Maritime Organization. The high sulfur and nitrogen content of FCC slurry oil inevitably affects its market share as the blending component of marine fuel oil, owing to the increasingly stringent ship emission standards. FCC slurry oil is abundant in polycyclic aromatic hydrocarbons and contains a certain amount of nitrogen, sulfur, and heavy metal compounds. Hydroprocessing is one of the most important routes to utilize the heavy oil resources effectively [3\u20139]. During the hydroprocessing of FCC slurry oil, the feedstock impurities such as sulfur and nitrogen can be removed by hydrotreating reactions. Meanwhile, large molecules can be transformed to value-added liquid fractions of light and medium distillates by hydrocracking reactions.The development of hydrogenation catalyst is believed to play a decisive role in the overall hydroprocessing effect [10\u201313]. The supported Mo-based sulfide catalysts are often used for hydroprocessing heavy feeds [11,14\u201317], and Ni is used as a promoter to decorate the edges of MoS2 slabs. Additionally, the support surface disperses the active metals via interaction and facilitates catalyst activation (sulfidation). However, too strong interaction between active metals and the catalyst support can lead to poor reducibility of metal species, which will make it difficult to sulfide and ultimately result in poor catalytic performance. Acidic supports such as alumina [11,15] and mixed oxides (SiO2-Al2O3\n[18,19], TiO2-Al2O3\n[3,20,21], B2O3-Al2O3\n[22,23], etc.) has been reported for the heavy-oil upgrading process. In particular, the introduction of boron and, or phosphorus to alumina mainly affects the acidity and active metal dispersion [24\u201329]. Still, contradictory results have been obtained on the distribution of Mo species. Adding boron into Al2O3 slightly increased the acidity of the B2O3-Al2O3 support and promoted the hydrocracking activity of NiMo/B2O3-Al2O3\n[22]. Moreover, an appropriate amount of B2O3 could improve the dispersion of active nickel and molybdenum species and promote the hydrogenation ability of the NiMo/B2O3-Al2O3 catalyst. In contrast, Morishige et al. [30] showed that boron doping weakened the interaction between \u03b3-Al2O3 and Mo species in the Mo/\u03b3-Al2O3 catalyst and decreased the Mo dispersion. Xiang et al. [31] indicated that phosphorus doping promoted MoS2 dispersing on the NiMo/Al2O3 catalyst surface by reducing the slab length and increasing the stacking number of MoS2 particles. Zhao et al. [26] reported that phosphorus modification enhanced the Mo sulfidation degree as well as the Mo dispersion and thus caused an increase in the denitrification rate. In addition, a decline in asphaltene conversion [11] and a slight increase in activities of hydrodesulfurization(HDS), hydrodenitrogenation(HDN), and hydrodemetallization [25] were found on NiMo/Al2O3 with phosphorus modification for the hydrocracking of vacuum residue. In these above studies, boron and, or phosphorus modification involves boric acid, borane isoproxyl [29], phosphoric acid, or ammonium dihydrogen phosphate [31] as starting materials.Boron phosphate (BPO4) has been applied as an effective catalyst or a catalyst support for some reactions such as oligomerization [32,33], dehydration [34], epoxide activation [35], and oxidative dehydrogenation [36]. However, little attention has been given to the boron phosphate-aluminum (BPO4-Al) composite as catalyst support for catalytic hydrogenation of real oils. In this study, the NiMo/BPO4-Al catalysts with different boron phosphate/aluminum molar ratios were synthesized with a complete liquid-phase method. A sol\u2013gel route was used to prepare the catalyst precursor and then a thermal treatment in liquid phase was performed. The catalytic performance of these catalysts was evaluated with the hydroprocessing of FCC slurry oil in a batch reactor. The impact of boron phosphate as a support modifier on the catalytic performance was discussed in terms of HDS rate, HDN rate, and changes of hydrocarbon groups fractions and distillation fractions distribution of liquid oil product. X-ray diffraction (XRD), nitrogen physisorption, H2 temperature-programmed reduction (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) techniques were employed to characterize these catalysts and discuss their catalytic activities.A set of four oxidized NiMoAl catalysts with boron phosphate modification were prepared through the complete liquid-phase method, with a MoO3 loading of 24.0\u00a0wt% and a Ni/Mo molar ratio of 0.5. The preparation process was based on the description for synthesizing CuZnAl catalyst by Huang et. al. [37,38]. Appropriate amounts of isopropyl alcohol aluminum (AIP) were firstly mixed with isopropyl alcohol, and the resulting mixture was stirred at 85\u00a0\u00b0C for 3\u00a0h. Then deionized water was slowly added, and nitric acid was used to adjust the pH value of the mixture solution. After 2\u00a0h of the hydrolysis reaction of AIP, nickel nitrate, ammonium molybdate, and boron phosphate were introduced into the above mixture successively and this system was maintained at 95\u00a0\u00b0C for 6\u00a0h to get the light green thin sol. After aging, the as-made gel was dispersed in the liquid paraffin system containing span-80 and treated under the nitrogen flow at 300\u00a0\u00b0C for 4\u00a0h. The resulting four oxidized catalysts were labeled as NM-BPA(0), NM-BPA(0.04), NM-BPA(0.28), and NM-BPA(0.55), indicating the molar ratio of boron phosphate/aluminum of 0, 0.04, 0.28, and 0.55 in the BPO4-Al mixture support.Before the measurements, the prepared oxidized NM-BPA(x) catalysts were thoroughly extracted with petroleum ether to remove the surface liquid paraffin and then dried in an oven overnight. XRD patterns were collected on a X-ray diffractometer (Rigaku D/MAX-2500) with a Cu K\u03b1 radiation source at 40\u00a0kV and 100\u00a0mA. Nitrogen adsorption/desorption experiments were carried out with an Autosorb analyzer (Quantachrome) at \u2212196\u00a0\u00b0C. Before each run, 0.20\u00a0g of sample was degassed under vacuum at 200\u00a0\u00b0C for 5\u00a0h to remove impurities. H2-TPR measurements were made using an automatic analyzer (Micromeritics AutoChem II 2920) equipped with a thermal conductivity detector. The sample was heated in a mixture of 5\u00a0% H2 in Ar and the TPR profile was obtained in the temperature range of 50\u2013750\u00a0\u00b0C. NH3-TPD measurements were conducted on a chemisorption analyzer (Xianquan TP-5080) equipped with a mass spectrometry detector. The sample was pretreated with He at 300\u00a0\u00b0C for 30\u00a0min and then adsorbed with ammonia at 100\u00a0\u00b0C. After purging for 30\u00a0min with He, the sample was heated up to 800\u00a0\u00b0C with a heating rate of 10\u00a0\u00b0C/min. The sulfided NiMo samples were characterized with XPS (Thermo Scientific Escalab 250Xi) and TEM (JEOL JEM-2100F) techniques. Detailed experimental conditions could be found in the previous literature [39,40].The catalytic hydroprocessing experiments were performed in a high-pressure autoclave (100\u00a0mL) equipped with a magnetically driven impeller. An FCC slurry oil provided by Shaanxi Yanchang Petroleum Group Co. ltd. of China was used as the feed. In a typical test, 1.2\u00a0g of oxidized catalyst, 0.338\u00a0g of carbon disulfide, and 30\u00a0g of FCC slurry oil were placed into the reactor. After replacing the air with hydrogen completely, the autoclave was pressurized with hydrogen to 6\u00a0MPa at room temperature. The reaction conditions were as follows: reaction temperature of 400\u00a0\u00b0C, reaction pressure of 9\u00a0MPa, reaction time of 6\u00a0h, and stirring rate of 600 r/min.The sulfur and nitrogen concentrations in the feed and the collected liquid product oil were measured with an ultraviolet fluorescence sulfur analyzer (ZDS-2000A, Jiangsu XinGaoke) and a chemical luminescent nitrogen analyzer (ZDN-2000A, Jiangsu XinGaoke), respectively. The boiling range distribution of liquid oil samples was determined with a gas chromatography simulated distillation method (ASTM D2887). Saturates, aromatics, resins, and asphaltenes (SARA) compounds in the feedstock and liquid products were separated and quantified according to NB/SH/T 0509. The H/C atomic ratio of liquid products was determined by elementary analysis (Thermo Scientific FlashSmart Elemental Analyzer).The HDS rate (X\nS) and HDN rate (X\nN) were calculated by using the following formula:\n\n(1)\n\n\n\nX\nS\n\n=\n\n\n\nm\n0\n\n\n\n\u00d7\nS\n\n\nf\ne\ne\nd\n\n\n-\n\nm\n1\n\n\n\n\u00d7\ns\n\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\n\nm\n0\n\n\n\n\u00d7\nS\n\n\nf\ne\ne\nd\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(2)\n\n\n\nX\nN\n\n=\n\n\n\n\n\nm\n0\n\n\u00d7\nN\n\n\nf\ne\ne\nd\n\n\n-\n\nm\n1\n\n\u00d7\n\nN\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\n\nm\n0\n\n\u00d7\n\nN\n\nf\ne\ne\nd\n\n\n\n\n\u00d7\n100\n%\n\n\n\nwhere S\nfeed and S\nproduct denote the sulfur concentrations (\u03bcg/g) in the feedstock and liquid product; N\nfeed and N\nproduct indicate the nitrogen concentrations (\u03bcg/g) in the feedstock and liquid product, respectively; m\n0 and m\n1 are the mass (g) of the feedstock and liquid product, respectively.The prepared oxidized catalysts were characterized by nitrogen physisorption, XRD, H2-TPR, and NH3-TPD. The textural properties of NM-BPA(0) and NM-BPA(0.55) samples are given in Table S1 and Figure S1. The BET surface area and total pore volume of NM-BPA(0) are 281\u00a0m2\u00b7g\u22121 and 0.47\u00a0cm3\u00b7g\u22121. The NM-BPA(0.55) sample with BPO4 modification has a lower specific surface area and a higher pore volume, which is 190\u00a0m2\u00b7g\u22121 and 0.53\u00a0cm3\u00b7g\u22121, respectively. Both two catalysts show a bimodal mesopore distribution. In the smaller pore diameter region, mesopores with a pore size of 3.7\u00a0nm can be seen in these two samples. Besides, secondary mesopores with a wide pore size distribution is centered at 5.8\u00a0nm for NM-BPA(0) and 6.9\u00a0nm for NM-BPA(0.55). Fig. 1\n shows XRD patterns of NM-BPA(x) samples. As for the unmodified NiMoAl catalyst, four weak peaks at 28.2\u00b0, 38.4\u00b0, 49.3\u00b0, and 64.5\u00b0 are observed, and they correspond to AlOOH (JSPDS 49\u20130133). In the three NM-BPA(x\u00a0=\u00a00.04, 0.28, 0.55) samples, we can observe new distinct peaks at 24.5\u00b0, 26.8\u00b0, 29.1\u00b0, 40.0\u00b0, 48.8\u00b0, 50.2\u00b0, and 63.7\u00b0, which are attributed to boron phosphate (JSPDS 34\u20130132). This suggests the presence of BPO4-AlOOH mixed support in these BPO4-modified NiMoAl samples. As for the above four catalysts, no nickel and molybdenum species are noticed, indicating that active metal components are well dispersed on the catalyst support.The H2-TPR profiles of oxidized NM-BPA(x) samples are presented in Fig. 2\n. The unmodified NiMoAl catalyst exhibits two hydrogen consumption peaks. According to the literature [41\u201343], peak \u03b1 at 485\u00a0\u00b0C is related to the partial reduction of polymolybdate octahedral Mo species (Mo6+ to Mo4+), whereas peak \u03b2 at around 583\u00a0\u00b0C is associated with the reduction of Ni species. Note that the NM-BPA(0.55) catalyst shows two reduction peaks at 467\u00a0\u00b0C (peak \u03b1) and 569\u00a0\u00b0C (peak \u03b2), respectively. The incorporation of boron phosphate (BPO4/Al\u00a0=\u00a00.55) into unmodified NiMoAl catalyst shifts peak \u03b1 to lower temperature, suggesting that the polymolybdate species existing on NM-BPA(0.55) is easier to be reduced and sulfided.\nFig. 3\nA displays a set of four NH3-TPD profiles for NM-BPA(x) samples. The NH3 desorption temperature is related to the strength of acid sites. Two distinct NH3 desorption peaks can be observed at the temperature of 100\u2013350\u00a0\u00b0C and 350\u2013800\u00a0\u00b0C, which represent weak-intermediate and strong site acidity, respectively. Similar results were reported for NiMo/\u03b3-Al2O3 catalysts with phosphorus or boron promoters [44,45]. As shown in Fig. 3A, for the samples with BPO4 modification, both of two NH3 desorption peaks shift to a higher temperature compared to the unmodified NM-BPA(0) catalyst, implying the strengthening of weak-intermediate and strong acid sites in the catalysts owing to the addition of boron phosphate. This can result from the strong interaction between boron phosphate and surface acid sites of AlOOH support. To obtain the distribution of weak-intermediate and strong acid amounts of the catalysts, each NH3-TPD profile was cut into two parts and integrated the corresponding peak areas, and the results are presented in Fig. 3B. The total acid amount of the NM-BPA(0) sample was considered to be 100, and those of other samples were calculated on the basis of the peak area ratio of BPO4-modified catalysts to the NM-BPA(0) sample. As can be seen in Fig. 3B, both the weak-intermediate acid amount and total acid amount of NM-BPA(x) samples decrease with increasing the molar ratio (0\u20130.55) of boron phosphate/aluminum in the BPO4-Al mixture support. But the strong acid amount shows a different variation trend, which means rising firstly and then falling. Therefore, BPO4 modification can change the surface acidity of the NiMoAl catalyst. The acidity strength is improved but the total acid amount decreases with incorporating BPO4 into the catalyst.The sulfided NM-BPA(x) catalysts were analyzed by XPS to obtain the surface element compositions and the chemical state. According to the literature [46], the calculated surface atomic ratios of n(Ni)/n(Al) and n(Mo)/n(Al) can be used to obtain information on the dispersion of active metals on the catalyst surface. As listed in Table 1\n, the NM-BPA(0) sample has a n(Ni)/n(Al) ratio of 0.04 and a n(Mo)/n(Al) ratio of 0.10, respectively. It is important to point out that both the surface atomic ratios of n(Ni)/n(Al) and n(Mo)/n(Al) increase gradually with increasing the molar ratio of boron phosphate/aluminum in NM-BPA(x) catalysts. This indicates that adding BPO4 enriches Ni and Mo species on the catalyst surface, which may improve atom utilization and thereby lead to better catalytic activity.\nFig. 4\n shows the measured and curve-fitted Mo 3d XPS spectra of four sulfided NM-BPA(x) catalysts. The fitting principles have been described in previous work [47]. As can be seen in Fig. 4, three Mo species with different oxidation states, i.e., Mo4+ (MoS2), Mo5+ (MoSxOy), and Mo6+ (MoO3), are present on these sulfided NM-BPA(x) samples. It is generally acknowledged that n(Mo4+)/n(Mo4+ + Mo5+ + Mo6+) can be regarded as the sulfidation degree of Mo, and the calculated values for sulfided NM-BPA(x) samples are shown in Table 1. As can be seen from Table 1, NM-BPA(0.04) and NM-BPA(0.28) have similar Mo sulfidation degree with unmodified NM-BPA(0) sample. But the Mo sulfidation degree of NM-BPA(0.55) is 64\u00a0%, which is higher than that of the unmodified catalyst (54\u00a0%). This indicates that adding an appropriate amount of BPO4 into the NiMoAl catalyst promotes the sulfidation of Mo species to active Mo4+ species, which is consistent with the TPR result mentioned above.TEM analysis has been widely used to study the morphology structure of active phase for the sulfided NiMo catalysts [24,48]. Fig. 5\n displays representative TEM images of four sulfided NM-BPA(x) catalysts. The observed black stripes are the (Ni)MoS2 active phase with a typical layered structure. To understand the influence of boron phosphate modification on the active phase structure of the sulfided NiMoAl catalyst, the slab length and stacking number of (Ni)MoS2 slabs have been statistically analyzed on the basis of 400\u2013500 slabs for each sulfided sample, and the results are shown in Table 1 and Fig. 6\n. As can be seen from Table 1, distinct differences in the average slab length and stacking number of (Ni)MoS2 crystallites are found among these four sulfided NM-BPA(x) catalysts. With increasing the molar ratio (0\u20130.55) of boron phosphate/aluminum in the BPO4-Al mixture support, the average stacking layer number increases gradually from 1.3 to 2.1, whereas the average slab length of (Ni)MoS2 firstly decreases from 4.6\u00a0nm to 3.7\u00a0nm and then rises to 4.8\u00a0nm. It is widely accepted that, on the basis of \u201cRim-Edge\u201d model of MoS2 active phase[49,50], multi-layer MoS2 slabs are more conducive to the adsorption of reactant molecules on the edge sites in comparison with single-layer (Ni)MoS2, thereby promoting the catalytic activity in hydroprocessing reactions. Meanwhile, for (Ni)MoS2 slabs with a certain stacking number, a shorter average slab length can lead to a larger dispersion of effective Mo atoms. Although NM-BPA(0), NM-BPA(0.04), and NM-BPA(0.28) have similar Mo sulfidation degree, the NM-BPA(0.28) catalyst has a higher average stacking number and smaller average slab length, indicating a higher dispersion of (Ni)MoS2 slabs.The distributions of (Ni)MoS2 stacking number and slab length of sulfided NM-BPA(x) samples are displayed in Fig. 6. As can be observed from Fig. 6A, the percentage of single-layer active phases gradually decreases, and the percentage of multi-layers (two to five layers) active phases increases when the molar ratio of BPO4/Al in NM-BPA(x) catalysts changes from 0 to 0.55. It is generally believed that two or more stacking layers of (Ni)MoS2 active phases can exhibit higher hydrogenation activity [26]. As displayed in Fig. 6B, the slab length of (Ni)MoS2 active phases in all four catalysts mostly distributes in the range of 2\u20136\u00a0nm. With the incorporation of boron phosphate (BPO4/Al\u00a0=\u00a00.04, 0.28), the percentage of (Ni)MoS2 slabs<4\u00a0nm increases. Although the NM-BPA(0.55) catalyst shows a similar distribution of (Ni)MoS2 slab length with the unmodified NM-BPA(0) catalyst, it has a much higher Mo sulfidation and (Ni)MoS2 stacking number, which could result in better catalytic activity.The catalytic activities of four prepared NM-BPA(x) catalysts were determined for hydroprocessing FCC slurry oil in terms of HDS, HDN, and hydrocracking. The desulfurization and denitrification results for all NM-BPA(x) catalysts are displayed in Fig. 7\n. The feed FCC slurry oil contains 3047\u00a0ppm sulfur and 2212\u00a0ppm nitrogen. As shown in Fig. 7, the NM-BPA(0.04) catalyst has a higher HDS rate (77.3\u00a0%) compared to the unmodified catalyst. A decline in HDS activity can be observed with further increasing the molar ratio of boron phosphate/aluminum in the BPO4-Al mixture support. However, it is noted that the addition of boron phosphate greatly enhances the conversion of nitrogen-containing compounds. For example, the HDN rate increases from 7.6\u00a0% up to 24.2\u00a0% when the molar ratio of boron phosphate/aluminum reaches 0.55.It is known that the HDS and HDN activities of the NiMo catalyst are associated with the Mo sulfidation degree and the dispersion of active metals. The HDN reaction could proceed harder than the HDS reaction, owing to the stronger bond energy of CN bond than that of CS bond. During the HDN process, two types of reactions are involved, i.e., aromatic ring hydro-saturation and following CN bond scission. The coordinatively unsaturated sites (CUS, or \u201csulfur vacancies\u201d), which are mainly located at the edge or corner sites of (Ni)MoS2 slabs, are responsible for the hydro-saturation reaction [20]. But in the HDS process, the sulfur removal of sulfur-containing compounds can occur directly on the CUS sites without associated aromatic ring hydrogenation. Thus, the competitive adsorption and hydrogenation of nitrogen compounds on the same active sites could hinder the adsorption and desulfurization of sulfur compounds.For the NM-BPA(x) catalysts, as discussed above, the incorporation of boron phosphate has a significant influence on the catalyst structures. XPS analysis evidences that the presence of boron phosphate causes the enrichment of Ni and Mo species on the catalyst surface and an increase in the Mo sulfidation degree, which favors promoting the HDN reaction. Besides, as indicated by TEM characterization, the morphology structures of active phases, i.e., the stacking layer number and slab length of (Ni)MoS2, are significantly adjusted because of the addition of boron phosphate into the NiMoAl catalyst. The dispersion of (Ni)MoS2 active phases is considerably improved with increasing the molar ratio (0\u20130.55) of boron phosphate/aluminum in the BPO4-Al mixture support, which results in a significant increase in the HDN activity. Although high HDN activity is often connected to a rise in the amount of intermediate strength acid centers [23,51], the variations in acid strength and acid amount caused by adding boron phosphate into NM-BPA(0) seem not directly correlated to the HDN activity, as indicated by NH3-TPD results. Additionally, due to competitive adsorption and hydro-conversion of nitrogen-containing molecules on the same active sites, the HDS rate of sulfur compounds decreases with increasing the molar ratio (0\u20130.55) of boron phosphate/aluminum in NM-BPA(x) samples. Similar results were also reported by Ferdous et al. [23] and Chen et al. [45]. A decrease in HDS activity with increasing boron content in NiMo/Al2O3 was observed for hydrotreating heavy gas oil [23]. Chen et al. [45] observed that the addition of boron promoted the HDN activity of NiMo/\u03b3-Al2O3 catalyst, whereas it presented a detrimental effect on the HDS activity.To obtain the chemical composition change of the feed before and after hydroprocessing, oil samples can be separated and quantified by four different hydrocarbon groups based on solubility and adsorption characteristics, i.e., saturates, aromatics, resins, and asphaltenes [52\u201354]. The SARA fractions of liquid products obtained during hydroprocessing FCC slurry oil with different NM-BPA(x) catalysts are given in Table 2\n. The FCC slurry oil is composed of 24.2\u00a0wt% of saturates, 44.3\u00a0wt% of aromatics, 26.0\u00a0wt% of resins, and 5.5\u00a0wt% of n-C7 asphaltenes. Using the unmodified NM-BPA(0) catalyst, the liquid product contains 31.6\u00a0wt% of saturates, 47.2\u00a0wt% of aromatics, 20.3\u00a0wt% of resins, and 0.9\u00a0wt% of n-C7 asphaltenes, indicating that the resins and asphaltenes fractions are cracked into lighter fractions. Moreover, the H/C atomic ratio increases from 1.21 to 1.29 through hydro-saturation reactions, in accordance with an increase in saturates fractions. As the molar ratio (0\u20130.55) of boron phosphate/aluminum in NM-BPA(x) catalysts increases, the resins fraction in liquid product falls from 20.3\u00a0wt% to 14.0\u00a0wt%, and the saturates fraction rises from 31.6\u00a0wt% to 37.8\u00a0wt%, implying that the addition of boron phosphate into the NiMoAl catalyst further upgrades the quality of liquid oil product. Although four prepared NM-BPA(x) catalysts show considerable activity in terms of n-C7 asphaltenes conversion, the NM-BPA(0.55) catalyst produces more saturates and fewer resins than other catalysts.The distillation fractions distribution of liquid oil products, as well as the variations of heavy components in SARA fractions, could also be used to evaluate the hydrocracking performance of catalysts [55\u201357]. As listed in Table 2, the liquid products are categorized on the basis of the boiling point as naphtha (<180\u00a0\u00b0C), middle distillate (180\u2013350\u00a0\u00b0C), vacuum gas oil (350\u2013500\u00a0\u00b0C), and residue (>500\u00a0\u00b0C). Among the four prepared catalysts in this study, the NM-BPA(0.55) catalyst gives the lowest residue fraction and the highest naphtha fraction. From these results, the NM-BPA(0.55) catalyst is suggested to be effective for hydrocracking heavy components into lighter oil. The introduction of boron phosphate into the NiMoAl catalyst promotes the hydrocracking activity during the hydroprocessing of FCC slurry oil.The present study showed the impact of BPO4 modification on the structure and heavy oil hydroprocessing performance of the NiMoAl catalyst. In this respect, the BPO4-AlOOH mixed support formed during the complete liquid-phase synthesis of BPO4-modified NiMoAl catalysts. At the same time, these catalysts exhibited a bimodal mesopore distribution. With the addition of BPO4, the surface acidity was strengthened, but the weak-intermediate acid amount and the total acid amount declined. For the sulfided NM-BPA(x) catalysts, both the surface atomic ratios of n(Ni)/n(Al) and n(Mo)/n(Al) increased with increasing the molar ratio of boron phosphate/aluminum, as indicated by XPS analysis. With the proper BPO4/Al molar ratio in NM-BPA(x) catalysts, the dispersion of (Ni)MoS2 active phases was enhanced by lowering the slab length and increasing the stacking layer number.It was noted that the HDN activity of the NiMoAl catalyst was greatly enhanced with increasing the molar ratio of boron phosphate/aluminum in NM-BPA(x) samples. The enrichment of Ni and Mo species on the catalyst surface, higher Mo sulfidation, and better dispersion of active phases were responsible for that significant increase in the HDN activity. However, the desulfurization rate increased first and then decreased with increasing the boron phosphate/aluminum ratio. Additionally, our work also confirmed that boron phosphate modification improved the hydrocracking activity of the NiMoAl catalyst during the hydroprocessing of FCC slurry oil. Compared to other synthesized catalysts, the liquid product obtained by using the NM-BPA(0.55) catalyst showed a considerable n-C7 asphaltenes fraction but more saturates and fewer resins. Meanwhile, lower residue and higher naphtha fraction in the liquid product were reached over the NM-BPA(0.55) catalyst.\nChangwei Liu: Data curation, Writing \u2013 original draft. Chunyan Tu: Methodology, Supervision, Writing \u2013 review & editing. Qi Chen: Data curation. Qian Zhang: Data curation. Wei Huang: Supervision.The authors declare no conflicts of interests.This work was supported by the National Natural Science Foundation of China (21808155) and the National Key R&D Program of China (2018YFB0604600-01).Supplementary data to this article can be found online at https://doi.org/10.1016/j.crcon.2022.07.001.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Mesoporous NiMoAl catalysts with boron phosphate (BPO4) modification were synthesized through the complete liquid-phase method. X-ray diffraction (XRD) analysis evidenced the presence of BPO4-AlOOH mixed support in these BPO4-modified NiMoAl samples. The total amount of acid sites declined, but the surface acidity was strengthened by adding BPO4 into the NiMoAl catalyst. It's worth noting that the incorporation of BPO4 could increase the concentrations of Ni and Mo species on the catalyst surface and greatly improve the dispersion of (Ni)MoS2 active phases, as indicated by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. The catalytic performance of these BPO4-modified NiMoAl catalysts was investigated with the hydroprocessing of fluid catalytic cracking (FCC) slurry oil. The nitrogen-containing compounds removal from the oil was significantly enhanced with increasing the molar ratio of boron phosphate/aluminum. The NM-BPA(0.55) catalyst exhibited the best hydrodenitrogenation (HDN) activity, highlighting the significant impact of Mo sulfidation degree and the dispersion of active metals on HDN performance. The introduction of boron phosphate could also promote the hydrocracking activity of the NiMoAl catalyst, as demonstrated by SARA analysis and simulated distillation of liquid products.\n               "}
{"full_text": "The rapid economic and societal development seen in recent decades has led to a dramatic increase in energy consumption worldwide. Fossil fuels such as coal and oil have been our major energy sources since the first industrial revolution; however, they are not clean energy sources. The combustion of fossil fuels releases a variety of pollutants, such as carbon dioxide (CO2), sulfur dioxide (SO2), oxynitride (NO\nx\n), and other volatile organic compounds (VOCs), which cause a range of environmental problems [1\u20133]. Among them, the greenhouse effect, which is caused by excessive CO2 emissions, has gained wide attention because it causes climate change and endangers human health and mankind's survival [4,5]. Furthermore, human reliance on fossil fuels has led to energy shortages. Therefore, converting CO2 into fuels and/or other valuable commodity chemicals constitutes an attractive strategy for mitigating these problems [6\u20138].Scientists around the world have devoted enormous effort to solving the problem of CO2 reduction. CO2 reduction reactions can be classified into four categories according to the energy source: thermocatalysis, photocatalysis, electrocatalysis, and photothermal catalysis, which is a combination of thermo- and photocatalysis [9\u201311]. Of these, thermocatalysis has some obvious drawbacks, such as high energy cost and relatively low product selectivity, which is hard to regulate. Photocatalysis and electrocatalysis are catalytic methods that have received increasing research attention in recent years because they can be used to convert CO2 into valuable fuels under milder conditions and are more efficient than conventional thermochemical processes [9]. However, the reaction barrier of electrocatalysis is high and the reaction kinetics are sluggish, and photochemical catalysis is subject to limited utilization of solar energy [12]. Recently, scholars have attempted to combine light and heat to perform CO2 reduction by photothermal catalysis. The coupling of solar and thermal energy can effectively regulate the activity and selectivity of this reduction reaction. In addition, it offers a new way to fully utilize solar energy.Photothermal catalytic materials such as metal sulfides, metal oxides, carbon nitrides, and metal\u2013organic frameworks (MOFs) have been widely researched. Metallic materials are commonly used to prepare photothermal catalysts, among which group VIII elements have unique activation capacities and efficient energy utilization over the entire solar spectrum, as experimentally confirmed by Meng et\u00a0al. [13] They reported that group VIII elements show great promise for CO2 reduction. Among these elements, Ni is a particularly good candidate catalyst material because, in addition to its high activity, it is more abundant in nature and thus easier to obtain than noble metals. Although no CO2-to-fuel technology has been industrialized, Ni-based materials are very promising for such applications. Ni is also the first-choice catalytic material for most methanation-manufacturing plants [14]. Furthermore, when Ni is present in the form of metal nanoparticles (NPs), it acts not only as active reaction sites [15], but also as heat-collection centers to increase the local temperature of the reaction system [15,16]. In addition, the local surface plasmon resonance (LSPR) effect is no longer limited to noble metals, and that of Ni NPs has recently gained attention [16,17]. Thus, Ni-based catalysts have strong applicability in photothermal catalytic systems.Herein, the mechanism of photothermal catalysis will be introduced from the perspective of different products. Furthermore, this review provides an overview of recent advances in Ni-based catalysis for photothermal CO2 reduction (Fig.\u00a01\n).At present, scientists still have not reached a unified opinion on the definition of photothermal CO2 catalytic reduction. The source of heat and the substrate of the reaction are the main considerations when defining photothermal catalysis. Here, we adopt the most commonly recognized classification method, and divide photothermal catalytic reactions into three categories; photo-assisted thermal catalysis, thermally assisted photocatalysis, and photothermal co-catalysis. In this section, we will briefly illustrate their differences (Fig.\u00a02\n).Photo-assisted thermocatalysis is essentially a thermocatalytic reaction. In this mode, there are two sources of thermal energy, one is an external heat source that increases the local temperature of the catalyst, and the other is solar energy. In the former case, the elevated temperature lowers the activation energy barrier of traditional thermocatalysis and alleviates catalyst poisoning. In the latter case, light serves as an energy source to drive thermocatalysis (also known as photo-driven thermocatalysis) [18,19]. Catalysts with a broad-spectrum absorption range efficiently convert concentrated solar energy into thermal energy, which then drives the reaction. Photo-driven thermocatalysis greatly improves the utilization of solar energy, and can be carried out under milder catalytic conditions.In thermal-assisted photocatalysis, the catalyst absorbs light or external heat to increase the temperature of the system and then drives the reaction with photo-generated electrons. When catalysts (usual semiconductors) absorb adequate energy from external photons, the electrons (e\u2212) and holes (h+) formed in the conduction and valence bands are separated and transferred to the catalyst surface, where they then participate in redox reactions [19,20]. In contrast to that for thermal catalysis, the temperature for this mode is relatively low and the catalyst has no thermocatalytic activity. The heat energy reduces the apparent activation energy of the photocatalysis, accelerates the thermal molecular movement of reactants (or intermediate substances), and promotes the mobility of charge carriers.In photothermal co-catalysis, photo- and thermocatalysis are coupled and act simultaneously to promote the catalytic process. In this mode, the sources of thermal energy can be photothermal conversion, external resistive heating, and exothermic chemical reactions. The synergistic effects of light and heat energy improve the reactivity and selectivity of the system [19].It is difficult to completely distinguish between the thermochemical and photochemical pathways in photothermal catalysis systems [18]. In photo-assisted thermocatalytic reactions, very high photothermal conversion efficiency of the catalyst is required to reduce the input of other forms of heat, and it is therefore not commonly studied in practice. At present, whether a reaction proceeds by thermal-assisted photocatalysis or photothermal co-catalysis remains controversial, and there is still a lack of accurate judgment indicators [15]. In the case of the Ni-based catalysts of interest in this review, the temperature of the catalytic system increases under light irradiation due to such activity as plasmonic localized heating and the thermal vibration of molecules. Some studies classify such reactions as photothermal co-catalysis because the elevated temperature promotes the conversion of CO2 to some extent [16,21]. However, the contribution of thermal catalysis is relatively small compared with that of photocatalysis. Thus, some researchers believe that such reactions are still essentially photocatalytic reactions or thermal-assisted photocatalytic reactions [15,20]. Regardless of the catalytic mode categorized, Ni-based catalysts do not require external heat sources during CO2 reduction.A wide variety of products, including carbon monoxide (CO), methane (CH4), and methanol (CH3OH), can be obtained because of the different reaction processes of CO2 reduction [13,22\u201324]. Scientists have strived to improve the selectivity for a specific product in CO2 reduction reactions to avoid the need for subsequent separation of different products. Most catalytic techniques for CO2 reduction are based on hydrogenation, such as the reverse water\u2013gas shift (RWGS) and Sabatier reactions. In this section, we will categorize the reactions by products and review the photo/photothermal CO2 reduction mechanism.The RWGS reaction is one of the most common CO2 catalytic conversion reactions (\nEq. (1)\n).\n\n(1)\n\n\n\nCO\n2\n\n+\n\nH\n2\n\n=\nCO\n+\n\nH\n2\n\nO\n\n\u0394\n\nH\n\n298\nK\n\n\n\n=\n\n41.2\n\u00a0\u200bkJ/mol\n\n\n\n\nThe final product of the RWGS reaction is CO, a valuable product that can be used as a reducing agent to smelt metals. CO can also be transformed into other higher-value fuels such as CH3OH or certain liquid hydrocarbons through Fischer-Tropsch (FT) synthesis. Thus, some scientists classify the RWGS reaction as part of the kinetic network in CH3OH production and FT synthesis [25,26].Based on the point at which C\u2013O bond scission occurs in the process, two possible mechanisms have been proposed for RWGS reactions: 1) a surface redox pathway, in which C\u2013O bond scission occurs first, followed by hydrogenation of the resulting O atom to form H2O (Path 1 in Fig.\u00a03\n); and 2) a formate-mediated pathway, in which CO2 is hydrogenated to form a carboxyl group (COOH) or converted to carbonate (CO3\n2\u2212) or bicarbonate (HCO3\n\u2212) on the surface of the catalyst and then transformed into formate (HCOO) (Path 2 in Fig.\u00a03) [25]. In the second mechanism, CO2 hydrogenation occurs before C\u2013O bond scission [25,27]. The type of catalyst metal plays a crucial role in determining the main reaction pathway. Dietz et\u00a0al. [28] reported that metals with high affinities for oxygen (O2) promote CO2 dissociation (the surface redox pathway). As the interaction between O atoms and metals weakens, CO2 hydrogenation becomes more favorable (the formate-mediated pathway). For instance, Ni catalysts with high affinities for O2 tend to follow the surface-redox mechanism rather than the formate-mediated pathway in RWGS reactions. Conversely, metals such as Pt have poor affinity for O2, which is not conducive to lowering the CO2 dissociation barrier, and therefore the formate-mediated pathway is more advantageous.The RWGS reaction is essentially a thermal catalytic reaction that needs to be carried out under relatively stringent conditions. Its appropriate reaction temperature is 400\u2013700\u00a0\u200b\u00b0C, which places high demands on energy supply [26,29]. In recent years, researchers have attempted to introduce light energy to RWGS reaction systems to develop suitable photothermal catalytic materials that enable the RWGS to proceed under a milder conditions while maintaining selectivity toward CO. For example, Jia and coworkers [30] reported Ni-NPs/N-doped CeO2 (Ni/N\u2013CeO2) as a novel photothermal catalyst, and achieved nearly 100% selectivity for CO at around 340\u00a0\u200b\u00b0C due to the photothermal effect.CH4, which can be generated from CO2 through the Sabatier reaction, is widely used as fuel for heating and lighting. Despite extensive research, the mechanism of CO2 methanation is not yet fully understood. A general description is shown in \nEq. (2)\n.\n\n(2)\n\n\nCO\n2\n\n+\n4\n\nH\n2\n\n=\n\nCH\n4\n\n+\n2\n\nH\n2\n\nO\n\n\u0394\n\nH\n\n298\nK\n\n\n\n=\n\n-\n164\n\nkJ/mol\n\n\n\nUsually, the formation of CO is an essential step in the Sabatier reaction. According to the ways in which methyl (\u2013CH3) groups form, three possible theories for CO2 methanation have been proposed: 1) the C atomic hydrogenation pathway, 2) the HCO hydrogenation pathway, and 3) the COH hydrogenation pathway. In the C atomic hydrogenation pathway, CO breaks down to C and O atoms, and then the C atom is further hydrogenated to \u2013CH3 (Path 3 in Fig.\u00a03). The HCO hydrogenation pathway starts with HCOO, which is generated as an intermediate in RWGS reactions. The first step is the dissociation of HCOO to form HCO and an O atom. In the next step, HCO is hydrogenated to CH3O and then dissociates to form \u2013CH3 (Path 4 in Fig.\u00a03). In the COH hydrogenation pathway, hydrogenation of COH occurs after COOH dissociation, resulting in the formation of CH3OH. CH3OH further dissociates to produce \u2013CH3 [31] (Path 5 in Fig.\u00a04\n).In recent years, considerable effort has been dedicated to improving the selectivity for CH4 products in CO2 reduction. Mateo and coworkers [16] reported a composite catalyst consisting of barium titanate (Ni-BTO)-supported Ni NPs, which can hydrogenate CO2 to CH4 at nearly 100% selectivity under optimal reaction conditions. Methanation catalysts in photothermal catalysis, including Ni-BTO, will be discussed in more detail in the next section.CH3OH is a basic raw material of many organic chemical processes and has significant commercial value. It is considered not only as an alternative hydrogen carrier, but also as a future energy source with a higher energy density than those of Li-ion batteries and liquefied H2 [32]. Transforming CO2 into CH3OH has become an active area of research in recent years (\nEq. (3)\n).\n\n(3)\nCO2 + 3H2 \u2192 CH3OH\u00a0\u200b+\u00a0\u200bH2O\n\n\nSinghal and coworkers [33] reported Ni-loaded InTaO4 that achieved a CH3OH yield of 200\u00a0\u200b\u03bcmol/g, which is 1.9 times higher than that of bare InTaO4, without an external heat source. However, as well as the general challenges of photothermal CO2 reduction, the CH3OH synthesis reaction suffers from relatively low product selectivity [23,32]. For example, Wu et\u00a0al. [23] found out that upon introducing light irradiation to Pd/ZnO-catalyzed CO2 reduction, although the space-time yield (STY) of CH3OH was improved 1.8 fold at 250\u00a0\u200b\u00b0C, the CO STY was promoted more (4.2 fold). Clearly, there is still a long way to go in the development of light-driven CO2-to-CH3OH conversion.Compared with C1 products, organic compounds with two or more carbons (C2+) have higher added value because of their wider usage. According to some techno-economic analyses, currently multiple-electron transfer products, including ethanol (C2H5OH), ethylene (C2H4), and other C2+ products, have low preparation selectivity but high market prices [34].In addition to improving catalyst activity, some researchers have aimed to improve product selectivity toward C2+ hydrocarbons. Albero et\u00a0al. [35] summarized the factors affecting selectivity for C2+ products. CO2-to-C2+-product conversion is a multi-electron transfer process, and materials or reaction conditions that favor e\u2212 photogeneration and storage are more favorable for C2+ product formation. Therefore, two different co-catalysts are usually used to manage the transfer of e\u2212 and h+ separately and alleviate the carrier recombination problem. Materials with good electron conductivity, such as carbon nanomaterials, also facilitate electron delocalization and prolong charge lifetime. Surface defects such as oxygen vacancies (VO) and mid-gap states can act as buffer sites of abundant local e\u2212 and act as reaction active sites. The surface plasmon resonance effect (SPR) of metal NPs can generate hot e\u2212 and promote the synthesis of C2+ products. Scientists have reported some catalysts with outstanding selectivity for C2+ products under specific conditions. For example, in 2017, Billo and coworkers [36] reported a study on black-TiO2-supported Ni-nanoclusters (Ni/TiO2[Vo]). The Ni nanoclusters and intentionally introduced VO create dual active sites for adsorption and dissociation of CO2 molecules with mixed carbon-oxygen coordination. This catalyst showed 100% selectivity toward CH3CHO under moist CO2 and a 300\u00a0\u200bW halogen lamp.Present knowledge of C2+ products is empirical and remains based on experimental results. Accordingly, theoretical and mechanistic understanding of the factors that influence the formation of C2+ products is poor [35]. The vast majority studies still focus on the RWGS and Sabatier reactions.Owing to the complexity of the CO2 reduction process, multiple reaction processes and various products typically coexist in such reaction systems. To more accurately understand the mechanism of photothermal CO2 reduction, infrared (IR) spectral absorption method is often used to identify the intermediate substances and products in the reaction process. In situ Fourier-transform IR (FTIR) spectroscopy is widely used in the mechanistic study of photothermal CO2 catalysis because it provides real-time information on product evolution without damaging sample structure [37]. Using in situ FTIR spectroscopy and spectroscopic analysis, Zhang et\u00a0al. [15] identified intermediates such as carbonate species, formate, CO, and methyl (\u2013CH3) in the catalytic process, which led to elucidation of the CO2 methanation pathway. Table\u00a01\n summarizes the characteristic groups and associated characteristic IR frequencies of some common substances. It should be noted that under different reaction conditions, the characteristic peaks of these substances and functional groups may slightly change due to intermolecular forces, induction effects, conjugation effects, and other factors, so the table is for reference only.Density functional theory (DFT) is used to study the electronic structure of multi-electron systems. In the field of CO2 reduction, DFT can be used to identify optimal reaction paths and determine their rate-determining steps by calculating the Gibbs free energy change (\u0394G) for each reaction step [38]. For example, in many catalytic systems, the generation of adsorbed COOH is the rate-determining step, and by rational design of catalyst structure, it is possible to reduce the energy barrier for this step (or even change the rate-determining step) to facilitate CO2 reduction. Furthermore, DFT provides a mature theoretical basis for efficient catalyst design in terms of morphology and crystal plane design, which saves significant time and energy in catalyst synthesis [38,39]. For example, in a study by Yang et\u00a0al. [39] Ni-MOL-100, which has abundant (100) crystal faces, was found to form stronger interactions with CO2 and be more favorable for the CO2-to-CO reduction process than Ni-MOL-010, which has (010) as the main exposed crystal face.In addition to IR spectroscopy and DFT theoretical calculations, many qualitative analytical techniques have been used for the mechanistic study of photothermal CO2 reduction. For example, temperature-programmed desorption (TPD) is used to quantitatively measure the number of active sites [40]. X-ray photoelectron spectroscopy (XPS) can be used to detect changes in the valence states of metal elements during chemical reactions [36,39]. Isotope labeling is also commonly used [15]. Thus, to realize the mechanistic study of photothermal CO2 reduction, researchers need to choose appropriate technical tools according to the actual situation.Several important parameters are involved in evaluating the photothermal CO2 reduction process, including catalytic conversion efficiency, distribution and selectivity of products, quantum yield (QY), solar energy conversion efficiency, and confirmation of the carbon source. In this section, a brief introduction of each is presented.The CO2-conversion rate is an important issue that must be considered in catalytic systems. Turnover number (TON) and turnover frequency (TOF) reflect CO2-conversion efficiency and the activity of catalytic centers. TON and TOF are typically calculated as follows (\nEqs. 4-5\n):\n\n(4)\n\nTON\n=\n\n\nn\n\np\nr\no\nd\nu\nc\nt\n\n\n\nn\n\nc\na\nt\na\nl\ny\ns\nt\n\n\n\n\n\n\n\n\n(5)\n\nTOF\n=\n\n\nn\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\nn\n\nc\na\nt\na\nl\ny\ns\nt\n\n\n\u00d7\nt\n\n\n\n\nwhere \n\n\nn\n\np\nr\no\nd\nu\nc\nt\n\n\n\n and \n\n\nn\n\nc\na\nt\na\nl\ny\ns\nt\n\n\n\n represent the molar numbers of total or desired products and catalysts, respectively, and \n\nt\n\n is the reaction time [45,46]. The two parameters TON and TOF are widely used to evaluate the performances of catalysts, especially for metal NPs and homogeneous metal complex catalysts [46]. For instance, Chen et\u00a0al. [47] prepared gigantic coordination molecules using a large number of metal ions and organic ligands, including Ni36Gd102, which exhibited good photocatalytic activity in CO2-to-CO conversion reactions. In this study, owing to the complex composition of the catalyst, the authors chose Ni2+ to represent the catalyst and calculate its TON and TOF. Based on initial CO productivity, the TON and TOF of Ni36Gd102 were calculated to be 29,700 and 1.2 s\u22121, respectively.As previously described, CO2 reduction can generate various products, including C1 products, such as CO and CH4, and multi-carbon products, such as C2H5OH. Accordingly, it is necessary to regulate the distribution of products in the photothermal catalysis process to improve its selectivity for specific products. Product selectivity is calculated as follows (\nEq. (6)\n):\n\n(6)\n\nProduct\n\nselectivity\n=\n\n\nn\n\nt\na\nr\ng\ne\nt\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n\nn\n\nt\no\nt\na\nl\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n\n\n\n\nThe C1 products are dominated by CO and CH4. Currently, 80% or even 95% selectivities for each can be achieved. There are many ways to tune product selectivity, including:\n\n1)\nSelecting a suitable support. For example, under the same reaction conditions, the main product obtained using a Ni catalyst supported on TiO2 or Al2O3 is CO, while that obtained using a BTO support is CH4 [16].\n\n\n2)\nAdjusting the loading amount and size of the metal NPs. It has been reported that Ni NPs in the particle-size range 2\u20133\u00a0\u200bnm provide the highest CH4 yields [27].\n\n\n3)\nControlling metal valence. The metal valence state is a critical factor in the conversion of CO to CH4. Under the same experimental conditions (UV/Visible light@142\u00a0\u200bmW/cm2), NiII and Ni0 favor the production of CO and CH4, respectively [15].\n\n\n4)\nAdjusting the position of metal NPs in sheet materials (discussed later) [48].\n\n\n5)\nRegulating the temperature of the reaction. According to \nEqs. 1-2\n, the Sabatier reaction is highly exothermic, while the RWGS reaction is endothermic. Thus, the low temperature region is thermodynamically favorable for CH4 production. As the temperature increases, the reaction will gradually favor the production of CO [40].\n\n\nSelecting a suitable support. For example, under the same reaction conditions, the main product obtained using a Ni catalyst supported on TiO2 or Al2O3 is CO, while that obtained using a BTO support is CH4 [16].Adjusting the loading amount and size of the metal NPs. It has been reported that Ni NPs in the particle-size range 2\u20133\u00a0\u200bnm provide the highest CH4 yields [27].Controlling metal valence. The metal valence state is a critical factor in the conversion of CO to CH4. Under the same experimental conditions (UV/Visible light@142\u00a0\u200bmW/cm2), NiII and Ni0 favor the production of CO and CH4, respectively [15].Adjusting the position of metal NPs in sheet materials (discussed later) [48].Regulating the temperature of the reaction. According to \nEqs. 1-2\n, the Sabatier reaction is highly exothermic, while the RWGS reaction is endothermic. Thus, the low temperature region is thermodynamically favorable for CH4 production. As the temperature increases, the reaction will gradually favor the production of CO [40].It should be noted that, with few exceptions, most current catalysts exhibit poor selectivity for multi-carbon products under mild conditions, largely because the reaction steps and mechanisms that determine selectivity for C2+ products are not fully understood. For example, the selectivity of multi-walled carbon nanotubes supported on TiO2 for C2H5OH is 69.7% (reaction conditions: H2O/CO2, 5:1 (mol:mol), 15\u00a0\u200bW UV lamp @ 365\u00a0\u200bnm, 5\u00a0\u200bh) [49,50]. The selectivity of Au/TiO2 for ethane is only 27% (reaction conditions: moist CO2, Hg lamp @ 254\u00a0\u200bnm, 20\u00a0\u200bmW/cm2) [50]. Some researchers speculate that reaction conditions or materials that favor electron photogeneration and storage should be more conducive to the synthesis of C2+ products [35]. Not surprisingly, the synthesis of C2+ products requires higher electron concentrations at active sites, which makes it more challenging than the synthesis of C1 products.QY can be used to evaluate the performance of a photocatalytic or photothermal catalytic system. The overall quantum yield (OQY) and apparent quantum yield (AQY) can be calculated using (\nEqs. 7-8\n) [45,46,51]:\n\n(7)\n\nOQY\u00a0\u200b\n\n(\n%\n)\n\n=\n\n\n\u03b1\n\u00d7\n\nn\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\nn\n\np\nh\no\nt\no\nn\n\n\n\n\u00d7\n100\n\n\n\n\n\n(8)\n\nAQY\u00a0\u200b\n\n(\n%\n)\n\n=\n\n\n\u03b1\n\u00d7\n\nn\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\nn\n\np\nh\no\nt\no\nn\n\n'\n\n\n\u00d7\n100\n\n\nwhere \n\n\u03b1\n\n and \n\n\nn\n\np\nr\no\nd\nu\nc\nt\n\n\n\n represent the number of electrons needed for product evolution and the number of desired products. Thus, \n\n\u03b1\n\u00d7\n\nn\n\np\nr\no\nd\nu\nc\nt\n\n\n\n is the number of electrons that participate in the reaction. For example, if the product is CH4, \u03b1 is equal to 8. For the denominator, \n\n\nn\n\np\nh\no\nt\no\nn\n\n\n\n and \n\n\nn\n\np\nh\no\nt\no\nn\n\n\u2032\n\n\n represent the number of absorbed and incident photons, respectively. Because only some of the incident photons can be absorbed by the catalyst, \n\n\nn\n\np\nh\no\nt\no\nn\n\n\n\n is always smaller than \n\n\nn\n\np\nh\no\nt\no\nn\n\n\u2032\n\n\n, and OQY is higher than AQY [46,51].Solar energy conversion efficiency (\u03b7) is another evaluation index that should be considered in photocatalytic and photothermal catalytic systems. It can be calculated as (\nEq. (9)\n) [45,51]:\n\n(9)\n\n\u03b7\n\n\n(\n%\n)\n\n=\n\n\nO\nu\nt\np\nu\nt\n\nc\nh\ne\nm\ni\nc\na\nl\n\ne\nn\ne\nr\ng\ny\n\n\nE\nn\ne\nr\ng\ny\n\no\n\ni\nn\nc\ni\nd\ne\nn\nt\n\ns\no\nl\na\nr\n\nl\ni\ng\nh\nt\n\n\n\u00d7\n100\n\n\n\nSimilar to that of general photocatalytic reactions, the efficiency of photothermal CO2 reduction largely depends on the absorption ability of the catalyst to the light source and the selectivity to the wavelength. Most current photocatalysts absorb light mainly in the ultraviolet band. For instance, Zhang and coworkers [15] investigated the effect of excitation light wavelength on the performance of Ni (10\u00a0\u200bwt%)\u2013ZrO2-Reduced photocatalyst, and found that light with a wavelength lower than 320\u00a0\u200bnm contributed the most (79%) to CO2 conversion. Expanding the spectral utilization range of catalysts and improving the absorption and conversion ability of catalytic systems in the visible and near-IR regions are highly active areas of research [15,16,24,36]. As previously described, photothermal catalysts efficiently collect energy from the solar spectrum. Furthermore, group VIII metals show great light-harvesting abilities and can efficiently convert CO2 to CH4 [13]. Mateo et\u00a0al. [16] demonstrated that Ni NPs elevate the light absorption of BTO by absorbing light in the entire visible region.In summary, it is extremely important to consider solar-energy-conversion efficiency in photothermal catalysis.Determining the source of carbon products generated during the CO2 reduction process is necessary to assess these catalytic systems. This is particularly important when using catalysts based on semiconductor carriers (especially carbon-based materials such as graphene and carbon nanotubes), where surface contaminants may play a role [45]. Thus, isotopic labeling experiments using 13CO2 under identical catalytic conditions and subsequent spectroscopic/spectrometric analyses are usually required. For example, Wu et\u00a0al. [52] reported N-doped graphene quantum dots (NGQDs), which showed excellent distribution and selectivity for C2+ products. They confirmed that the carbon in the products originated from CO2 rather than the decomposition of NGQDs through isotopic 13CO2 labeling experiments. Furthermore, isotope tracing experiments can also be used to identify intermediates and reaction mechanisms during the catalytic CO2 reduction process [15].Ni is one of the most commonly used catalyst materials because of its high catalytic activity, economic feasibility, and abundance in nature [25]. Ni NPs come in various crystalline forms, among which Ni(111) has been widely studied as the most stable extended Ni surface [25,31,39,53]. Thus, the catalytic effect of Ni(111) on CO2 hydrogenation is presented here as an example.Under suitable conditions, Ni(111) can effectively catalyze RWGS reactions via the surface redox route and the formate-mediated pathway [25,31,53]. This has been confirmed from both theoretical calculations (usually DFT calculations) and experimental study. Heine et\u00a0al. [53] observed the Ni(111)-catalyzed formation of CO by ambient-pressure XPS, confirming that the RWGS reaction occurred accompanied by the formation of C atoms. However, the catalytic effect of Ni(111) on CO2 methanation remains controversial. Certainly, CH4 can be formed over supported Ni catalysts. For example, Vogt et\u00a0al. [54] reported the formation of CH4 over Ni catalysts supported on different metal oxides, such as TiO2, Al2O3, and ZrO2. By assessing the activity of catalysts with different supports at different temperatures, they found that the apparent activation energy from CO2 to CH4 is almost independent of the support, indicating that the reaction occurs mainly on the Ni NPs. In the case of Ni itself, most researchers support its catalytic effect on the methanation of CO2. They believe that the disproportionation step (\nEq. (10)\n) is the critical process, after which atomic carbon can be further hydrogenated to CH4 [55].\n\n(10)\n2CO\u2192C\u00a0\u200b+\u00a0\u200bCO2 (g)\n\n\nIt should be noted that Lozano-Reis et\u00a0al. [31] recently reported that CH4 does not form on Ni(111) under any operating conditions. This statement was supported by accurate kinetic Monte Carlo simulations. The authors suggested that some previous DFT and microkinetic studies were based on unreliable assumptions and did not provide direct evidence for CH4 formation.Rather than using Ni alone as a catalyst, increasing research attention is being paid to combining Ni particles with specific supports or other substances, such as metal oxides and carbon-based materials, to catalyze the photothermal reduction of CO2 through the synergistic effects between substances.Most metal oxide semiconductors exhibit good photo-oxidation activity, stability, and recyclability. However, owing to their large bandgaps (\n\n\nE\ng\n\n>\n3\n\neV\n\n), most metal oxide photocatalysts can only operate under UV irradiation (wavelengths shorter than 400\u00a0\u200bnm) and thus achieve limited use of the solar spectrum [45]. To address this issue, bandgap engineering and coloration have been proposed as steps to improve the photo/photothermal catalytic performances of metal oxides, where Ni doping with photocatalyst pre-hydrogenation is a common strategy (Fig.\u00a04) [7,36].TiO2 was one of the first semiconductor photocatalysts to be studied and applied to water splitting and CO2 reduction. Because of its chemical stability, non-toxicity, and low cost, it has been used extensively to prepare heterogeneous photocatalysts. Although TiO2 photocatalysis research has made great progress, its inherent disadvantages, such as wide bandgap, poor light absorption, and short lifetime of photogenerated charge carriers, still limit its use [36,56]. Billo et\u00a0al. [36] first introduced metallic Ni to pure TiO2, generating a disordered TiO2 surface. Owing to its altered photophysical properties, the catalyst developed a significant mid-gap state at 1.3\u00a0\u200beV, which is the strongest region in the solar spectrum, significantly broadening the absorption spectrum of the catalyst (Fig.\u00a05\na). Moreover, subsequent hydrogenation of Ni/TiO2 introduced abundant VO, which also serve as active catalytic sites. Through experimental studies as well as theoretical calculations such as DFT analysis, researchers have confirmed that the formation of Ni nanoclusters and VO in Ni/TiO2[Vo] provides more CO2 adsorption and dissociation sites. The light-harvesting ability of the catalyst is enhanced by reducing the optical bandgap and creating mid-gap states. In addition, photoluminescence (PL) and corresponding time-resolved PL measurements were performed (Fig.\u00a05b). They revealed that hydrogenation of the Ni/TiO2 surface leads to the formation of metallic Ni0. Photoexcited electrons are transferred from TiO2 to the Ni0 surface, which inhibits the recombination of electron\u2013hole pairs, thereby prolonging carrier lifetime and improving photocatalytic CO2 reduction efficiency. Ni/TiO2[Vo] exhibits a high product selectivity for CH3CHO, and its product yield (10 \u03bcmol/gcat), although not high, was still more than 18 times higher than the solar fuel yield of commercial TiO2 (P-25) [36].Similarly, Zhang et\u00a0al. [15] pretreated Ni/ZrO2 samples under H2, showing that the valence state of the metal has a significant influence on product selectivity. As shown in Fig.\u00a05c and d, smaller spheres/spheroids with an average particle size of 2.2\u00a0\u200b\u00b1\u00a0\u200b0.8\u00a0\u200bnm (Fig.\u00a05e) attached on the ZrO2 crystals can be observed, demonstrating the successful loading of Ni NPs. According to their study, the formation rate of CH4 on Ni/ZrO2 without H2 pretreatment is only 0.94% of that after pre-reduction treatment. This difference is caused by the transformation of Ni2+ into Ni0, which is consistent with other reports. Another noteworthy point is that controlled experiments to confirm the effects of light and heat on the whole catalytic process were performed. The authors found that photoexcitation is necessary for CO2 activation on the ZrO2 surface and the formation of CH4, while Ni NPs can act as heat-collecting centers to convert light energy into heat energy and increase the local temperature of the metal particles, thus providing a synergistic photothermal catalytic effect without an external heat source (Fig.\u00a05f).Perovskites are a class of complex metal oxides with ABO3 structures. BTO, a typical perovskite material, has been shown to have photocatalytic activity for water-splitting reactions, but it is less used for photocatalytic CO2 conversion reactions. Mateo et\u00a0al. [16] used a wet impregnation method to prepare Ni-BTO catalysts. The same H2/Ar reduction treatment was performed to obtain Ni NPs. During the catalytic reaction, there was no external heat source. The temperature increase was mainly due to the light radiation, the photothermal effect exhibited by the loaded Ni NPs, and the heat released by the reaction. The absorption spectrum of Ni-BTO under UV\u2013visible radiation conditions was greatly broadened (Fig.\u00a05g) and the CH4 yield reached 103.7 mmol/(g\u00b7h), which is higher than those of most currently reported photothermal catalysts. However, the stability of the Ni-BTO catalysts was poor. The CO2 conversion of the Ni-BTO catalyst dropped to 47% of its original value at the third use. To investigate the cause of this catalyst deactivation, the used Ni-BTO samples were subjected to X-ray photoelectron spectroscopy (XPS) analysis. The authors claimed that the reaction by-product H2O adsorbs on the active sites and reacts with Ni to form Ni(OH)2, which inhibits further methanation reactions. Thus, surface oxidation of Ni greatly reduces the catalytic performance of the reaction. To retain catalyst activity, a clean Ni surface needs to be regenerated before use. To address this issue, the catalyst was reactivated with H2 and light after each catalytic cycle, and this treatment allowed five consecutive cycles of photothermal CO2 reduction to be completed without any significant deactivation (Fig.\u00a05h). Table\u00a02\n summarizes the performances of some recently reported Ni/metal oxide photothermal catalytic systems. It can be seen that this type of catalyst is relatively well developed in terms of selectivity for desired products.As well as metal oxides, sulfides have also garnered attention in photocatalysis research. Xu et\u00a0al. [24] introduced metallic Ni into CoS2 and used the partially filled conduction band of the metal photocatalysts as an intermediate band, allowing Ni\u2013CoS2 to satisfy the redox potential of IR-light-driven CO2 reduction. Meanwhile, the carrier CoS2 was designed to be ultrathin, thereby reducing the diffusion length of carriers and the electron\u2013hole recombination rate. The yields of CH4 and CO in the CO2 reduction reaction catalyzed by Ni\u2013CoS2 nanosheets were 101.8 and 37.5 \u03bcmol/(g\u00b7h), respectively, which is the highest recorded for IR-light-driven CO2 reduction among all reported single-component photocatalysts. In their experiment, a quartz tray was used instead of a liquid solvent to effectively retain the heat generated by photoinduction. No external heat source was used in their experiments. According to their report, the catalyst surface temperature increased by 26\u00a0\u200b\u00b0C during the CO2 reduction process, which also had a promotional effect on the catalytic process.Clearly, CO and CH4 still dominate the products of CO2 photo/photothermal catalytic reduction, and the yields of C2+ products are low. Furthermore, the photothermal effect in current metal-complex-catalyzed photothermal CO2 reductions mainly comes from light radiation and heat collection by metal NPs, rather than from an external heat source, constituting a thermally assisted photocatalytic reaction. The summary of the Ni-based catalysts presented above reveals that the following strategies are often adopted to improve the photothermal catalytic properties of Ni/metal complexes.\n\n1)\nConstructing complexes. Combining Ni with a suitable support can promote the directional migration of carriers and prolong the lifetime of photogenerated e\u2212. Ni combined with other semiconductors can also narrow the photocatalyst bandgap and improve its spectral absorption properties. The synergistic effect of Ni with supports is significant.\n\n\n2)\nIntroduction of surface defects such as VO and mid-gap states. Defects can improve the light absorption of photothermal catalysts and, in some cases, these defects can also act as reactive sites [36].\n\n\n3)\nHydrogenation treatment. Hydrogenation pretreatment can adjust the valence state of Ni, which has an impact on product selectivity [15]. NiO or Ni(OH)2 may be generated in the process of CO2 reduction, decreasing the catalytic activity of the photothermal catalytic system, and the catalyst after the reaction can be treated with H2 to enhance its cyclic stability [16].\n\n\n4)\nAdaptation of experimental equipment to facilitate the application of the photothermal effect, such as the use of quartz disks to retain the heat generated by light induction, as described above [24].\n\n\nConstructing complexes. Combining Ni with a suitable support can promote the directional migration of carriers and prolong the lifetime of photogenerated e\u2212. Ni combined with other semiconductors can also narrow the photocatalyst bandgap and improve its spectral absorption properties. The synergistic effect of Ni with supports is significant.Introduction of surface defects such as VO and mid-gap states. Defects can improve the light absorption of photothermal catalysts and, in some cases, these defects can also act as reactive sites [36].Hydrogenation treatment. Hydrogenation pretreatment can adjust the valence state of Ni, which has an impact on product selectivity [15]. NiO or Ni(OH)2 may be generated in the process of CO2 reduction, decreasing the catalytic activity of the photothermal catalytic system, and the catalyst after the reaction can be treated with H2 to enhance its cyclic stability [16].Adaptation of experimental equipment to facilitate the application of the photothermal effect, such as the use of quartz disks to retain the heat generated by light induction, as described above [24].Because metals exist in various forms and their morphologies are very malleable, there is still much room for research on photothermal CO2 catalytic reduction using metal complexes. Thus, their application prospects are expanding.MOFs are a class of porous crystalline materials that have been developed in recent years. Their combination with Ni is advantageous for the improvement of catalysis efficiency [39]. MOFs are a class of hybrid compounds formed by assembling metal ions or metal clusters with bifunctional organic ligands. Because of their high specific surface areas and porosities, tunable pore sizes, good thermochemical stabilities, diverse topologies, and ease of functionalization, they show excellent promise in environment pollutant removal and remediation, including applications for CO2 reduction [7,60,61].During the photocatalytic reduction of CO2, photoexcited electrons in the MOFs are transferred from the highest occupied molecular orbital (HOMO) of the organic linkers to the lowest unoccupied molecular orbital (LUMO) of the metal nodes, achieving a charge-separated state that promotes the photocatalytic reaction [60]. The introduction of metal NPs to MOFs can generate hot electrons through the LSPR effect under light irradiation, resulting in local temperature increase at the metal particles and a photothermal effect, which actives the absorbed reactants and significantly enhances catalytic activity [16,17,62]. Noble-metal NPs like Pd and Ru NPs have been widely studied. Recently, more attention has been paid to transition metals such as Ni, Co, and Fe [17].Several factors influence the catalytic performance of MOFs, including:\n\n1)\nChoice of Ligand. To expand the light absorption region of the catalyst and to enhance the interaction between CO2 and MOFs, polar functionalized ligands and metal ligands can be used to replace unmodified organic ligands [60,63]. For example, the ligands with polar \u2013NH2 groups facilitate ligand-to-metal charge transfer, which alleviates photogenerated electron\u2013hole recombination by prolonging charge separation, thereby further enhancing the efficiency of the photocatalytic reaction [60]. Furthermore, \u2013COOH and \u2013OH are considered to be polar groups favorable for CO2 capture [64].\n\n\n2)\nChoice of metal moiety. Introducing certain metals to MOF materials to lower the redox potential and promote charge transfer can further optimize the photothermal CO2 reduction process [60]. Compared with most MOF-based catalysts with single-metal active centers, Ni-MOFs exhibit high CO2-to-CO catalytic selectivity (97.7%) [65]. Furthermore, the construction of bimetallic systems by partial metal substitution opens up new directions for efficient photocatalytic CO2 reduction. Chen et\u00a0al. [66] introduced Ni with high electron affinity into a Ti oxo cluster, and obtained an extended charge separation state in the Ni/Ti bimetallic MOF. This catalyst shows a larger spectral absorption range and much higher photocatalytic efficiency than those of Ti-MOFs and Ni-MOFs.\n\n\n3)\nSurface area of the MOF catalyst. Reducing the size of MOFs catalysts and converting bulky MOFs to low-dimensional MOFs are strategies that are often employed to increase the surface areas of catalysts. As shown in Fig.\u00a06\nb\u2013g, Yang and coworkers [39] found that ultrathin Ni-MOFs exposing abundant (010) and (100) planes both showed much higher photocatalytic CO2-to-CO activities than those of bulky Ni-MOFs (Fig.\u00a06a). The CO yield of the (100)-rich Ni-MOFs was 11.89\u00a0\u200b\u00b1\u00a0\u200b0.65 mmol/gcat under 4\u00a0\u200bh of photoexcitation, which was \u223c4.5 times higher than that of the bulky MOFs (Fig.\u00a06h). This is mainly due to its larger surface area and more metal sites that can bind CO2. Adjacent Ni catalytic sites also produce synergistic catalysis in the CO2 reduction process.\n\n\n4)\nPore properties of MOFs. High porosity means a high surface area, ensuring adequate contact between the catalytic centers and CO2 [60]. However, larger pore volumes are not always better. CO2 adsorption ability is the result of multiple factors. For example, Tran et\u00a0al. [67] prepared Ni-MOF-184 and Zn-184 and compared the pore volumes and CO2 adsorption capacities of the two catalysts. They found that, although Ni-MOF has a smaller pore volume of 1.10\u00a0\u200bcm3/g, its CO2 uptake was 71\u00a0\u200bcm3/g, i.e., 1.65 times that of Zn-MOF-184, which has a larger pore volume (1.12\u00a0\u200bcm3/g).\n\n\nChoice of Ligand. To expand the light absorption region of the catalyst and to enhance the interaction between CO2 and MOFs, polar functionalized ligands and metal ligands can be used to replace unmodified organic ligands [60,63]. For example, the ligands with polar \u2013NH2 groups facilitate ligand-to-metal charge transfer, which alleviates photogenerated electron\u2013hole recombination by prolonging charge separation, thereby further enhancing the efficiency of the photocatalytic reaction [60]. Furthermore, \u2013COOH and \u2013OH are considered to be polar groups favorable for CO2 capture [64].Choice of metal moiety. Introducing certain metals to MOF materials to lower the redox potential and promote charge transfer can further optimize the photothermal CO2 reduction process [60]. Compared with most MOF-based catalysts with single-metal active centers, Ni-MOFs exhibit high CO2-to-CO catalytic selectivity (97.7%) [65]. Furthermore, the construction of bimetallic systems by partial metal substitution opens up new directions for efficient photocatalytic CO2 reduction. Chen et\u00a0al. [66] introduced Ni with high electron affinity into a Ti oxo cluster, and obtained an extended charge separation state in the Ni/Ti bimetallic MOF. This catalyst shows a larger spectral absorption range and much higher photocatalytic efficiency than those of Ti-MOFs and Ni-MOFs.Surface area of the MOF catalyst. Reducing the size of MOFs catalysts and converting bulky MOFs to low-dimensional MOFs are strategies that are often employed to increase the surface areas of catalysts. As shown in Fig.\u00a06\nb\u2013g, Yang and coworkers [39] found that ultrathin Ni-MOFs exposing abundant (010) and (100) planes both showed much higher photocatalytic CO2-to-CO activities than those of bulky Ni-MOFs (Fig.\u00a06a). The CO yield of the (100)-rich Ni-MOFs was 11.89\u00a0\u200b\u00b1\u00a0\u200b0.65 mmol/gcat under 4\u00a0\u200bh of photoexcitation, which was \u223c4.5 times higher than that of the bulky MOFs (Fig.\u00a06h). This is mainly due to its larger surface area and more metal sites that can bind CO2. Adjacent Ni catalytic sites also produce synergistic catalysis in the CO2 reduction process.Pore properties of MOFs. High porosity means a high surface area, ensuring adequate contact between the catalytic centers and CO2 [60]. However, larger pore volumes are not always better. CO2 adsorption ability is the result of multiple factors. For example, Tran et\u00a0al. [67] prepared Ni-MOF-184 and Zn-184 and compared the pore volumes and CO2 adsorption capacities of the two catalysts. They found that, although Ni-MOF has a smaller pore volume of 1.10\u00a0\u200bcm3/g, its CO2 uptake was 71\u00a0\u200bcm3/g, i.e., 1.65 times that of Zn-MOF-184, which has a larger pore volume (1.12\u00a0\u200bcm3/g).In addition to pure MOFs catalysts, many MOF-derived materials have been investigated for catalytic CO2 reduction. For instance, Khan et\u00a0al. [17] reported an efficient MOF-derived Ni-based catalyst for photothermal CO2-to-CH4 conversion (Fig.\u00a06i). They first synthesized Ni-MOF-74, and then pyrolyzed this MOF material at different temperatures under continuous N2 flow to modulate the properties of the resulting carbonaceous species. Fig.\u00a06j shows the homogeneous distribution of Ni NPs throughout the carbon matrix. Because higher pyrolysis temperatures led to a greater degree of graphitization of the catalytic material, the MOF-derived carbon-based material obtained at a pyrolysis temperature of 600\u00a0\u200b\u00b0C exhibited the highest CH4 production rate (448 mmol/(g\u00b7h)). This production rate is also the highest among all reported CO2 photo-methanation catalysts. The temperature and pressure changes during the reaction are shown in Fig.\u00a06k. Under light irradiation, all the samples exhibit a progressive increase in temperature during CO2 reduction, while the pressure shows the opposite trend due to the consumption of gas reactants, confirming the occurrence of the photothermal reaction.In summary, the excellent properties of Ni-based MOF materials, such as large surface area, good structural controllability, and low photogenerated electron\u2013hole recombination give them high application potential for photo/photothermal CO2 catalytic reduction. Ni-based MOF catalysts will undoubtedly make a greater contribution to CO2 reduction with the advent of rational structural design.Metal-free materials are widely used in aerospace, electronics, engineering machinery, and other fields owing to their low specific gravity and high strength [52,68,69]. In CO2 catalytic reduction, metal-free materials have been extensively researched owing to their abundance in nature, good electrical conductivity, environmental friendliness, and low cost [70]. This section will focus on graphene and graphitic carbon nitride (g-C3N4), which are frequently used in CO2 photo/photothermal catalytic reduction systems. Although graphene and g-C3N4 share the same 2D layered structure, they have very different electrical properties and thus play different roles in CO2 catalytic reduction systems.Graphene is a 2D material with a honeycomb lattice structure connected by sp2 hybridization [70,71]. In the internal structure of graphene, each carbon atom has four valence electrons, three of which form sp2-bonds, leaving one unbonded electron in the Pz orbital. Similar to the electronic configuration of the benzene ring, the Pz orbital of each carbon atom perpendicular to the layer plane can form \u03c0-bonds with multiple atoms throughout the layer. Such a structure enables graphene to have excellent electrical and optical properties as a zero-bandgap semiconductor. It is worth noting that graphene can act as both an acceptor and transporter of electrons, prolonging the separated state of electron\u2013hole pairs and the lifetimes of charge carriers, thereby promoting multi-electron reactions. Furthermore, graphene can improve photostability and promote CO2 adsorption by catalysts [70]. Lin and coworkers [72] prepared a photocatalyst with a magnetic hollow structure consisting of metallic Ni NPs surrounded by few-aof graphene (Ni@GC) (Fig.\u00a07\na\u2013d). Ni@GC are hollow spheres with large surface areas and highly porous structures that accelerate the separation and transport of photoexcited charge carriers. Because of the synergistic contribution of Ni NPs with high electron density and graphene with porous structure, this catalyst exhibited a high CO2 adsorption capacity of \u223c28\u00a0\u200bcm3/g for CO2 at 273\u00a0\u200bK (Fig.\u00a07e and f).g-C3N4 is a typical polymeric semiconductor in which the CN atoms in the structure are sp2 hybridized to form a highly exotic \u03c0-conjugated system. The bandgap of g-C3N4 is \u223c2.7\u00a0\u200beV, which is sufficient to absorb blue-violet light with wavelengths less than 475\u00a0\u200bnm in the solar spectrum. This photoresponse range is already larger than those of most photocatalysts [70]. In the absence of a co-catalyst, g-C3N4 has a good catalytic effect on CO2 reduction. Since 2012, when researchers demonstrated that g-C3N4 could photoreduce CO2 to CO in the presence of water vapor, this material has become very actively researched in photocatalysis [73]. By using an active unsaturated edge confinement strategy, Cheng et\u00a0al. [74] recently synthesized few-layer porous g-C3N4 photocatalysts with single Ni atoms as anchoring points for CO2 reduction (Fig.\u00a07g). The detailed few-layer morphology and porous structure can be seen in the TEM and STEM images of the Ni5\u2013CN sample (Fig.\u00a07h and i). The graphitic \u03c0-conjugated layer structure, anisotropic structure, and defective vacancy self-modification of porous ultra-thin g-C3N4 materials contribute to the performance enhancement of this catalyst. The porous structure provides vacancy ligands for trapping single Ni atoms, realizing a high density of single-atom active sites, which has significant advantages for improving the CO2 adsorption capacity of the catalyst. Because of the strong chemisorption and carrier dynamics, CO2 is converted from the gaseous to the adsorbed state by releasing absorption heat (QP and QC) (Fig.\u00a07 j). Upon the introduction of single-atomic-site Ni, it strongly binds to CO2 molecules as adsorption active sites with the appropriate activation energy (Ea) (Fig.\u00a07 k). Ea is lower than the dissociation energy (Qs), so Ni rapidly binds with CO2 and promotes carrier transfer. The highly unsaturated Ni\u2013N coordination maximizes the formation of photocatalytic sites. According to this study, the CO yield of this catalyst is 7.8 times higher than that of pure g-C3N4 under visible-light excitation.Besides the two metal-free materials mentioned above, many other carbon-containing materials also exist, such as carbon nanotubes (CNTs) and covalent organic frameworks, and they have been widely applied in CO2 photo/photothermal catalysis [35,75]. However, low surface area, unstable carrier dynamics, and other drawbacks of all-metal-free materials hinder their wider application. Thus, to further enhance their catalytic efficiencies, a series of strategies, such as doping, morphology control, and surface modification have been adopted. Combining metal-free materials with metallic Ni has also been extensively studied.1) Elemental doping. In heteroatom doping certain carbon atoms in the graphite structure are replaced with other atoms, thus changing the spin density and charge distribution of the carbon material, modulating its adsorption capacity for the reactants and expanding the light-response range of the catalyst. This method can significantly modulate the optical and electronic properties of the material. The dopants can be classified into nonmetal atoms (e.g., N, O, and S) and metal atoms (e.g., Ni, Co, and Zn) [26]. This review mainly introduces metal-atom doping [71].Doping of metal-free materials with metal atoms can create impurity levels in the semiconductor bandgap that act as electron traps when the semiconductor is excited, inhibiting the recombination of photoexcited electrons and holes [70]. For example, metal-doped CNTs exhibit better adsorption capacities for CO2 than normal CNTs and carbon nanocages. Furthermore, CO2 adsorption capacity and catalytic effect vary with the metal species. The transition metal Ni has been reported to show good performance. Xu et\u00a0al. [75] conducted a density flooding theory study on the reaction process of CO2 reduction to CH3OH on the surface of Ni-doped CNTs. The results showed that the Ni-doped carbon nanotubes exhibit lower overpotential values and higher reaction energies.2) Morphology control. The sizes, shapes, and geometric features of catalytic materials greatly influence their photocatalytic performances. Morphology control can facilitate carrier migration and promote the surface reactivity of such materials [70]. For example, graphene and g-C3N4 are typical 2D materials, which have larger surface areas and extensive \u03c0-conjugation compared with those of 3D materials. Such morphology effectively promotes the flow of charge carriers. The preparation method of the catalyst plays a vital role in morphological control. For instance, solvent and thermal exfoliation are commonly used to increase specific surface area, and template methods are employed to introduce porous structures [76].In conclusion, using strategies such as elemental doping and morphological tuning can improve the light-harvesting capacities of metal-free materials and reduce photogenerated-charge-carrier recombination rates. The combination of Ni with metal-free materials such as g-C3N4 and graphene by doping or loading can make full use of their morphologies to increase their contact areas with reactants and promote light absorption. Furthermore, in some catalytic systems, Ni can act as a heat-collecting center to increase temperature and is also an active site for CO2 adsorption and conversion. Thus, the effect of the combination of Ni and metal-free materials is greater than the summed effects of the two parts.Ni exists in many other forms and can have numerous loading supports. In this section, some other Ni-based materials for CO2 photo/photothermal catalysis will be introduced.Si is the second most abundant element on earth, especially in the form of SiO2, and has been extensively studied as a carrier for photocatalysts. However, Ni/SiO2 lacks specific products selectivity [77]. Yan et\u00a0al. [48] prepared siloxane nanosheets loaded with Ni NPs by employing 2D silicon surface chemistry. They used ethanol and water as dispersants for the Ni source and prepared two samples, labeled Ni@SiXNS-EtOH and Ni@SiXNS-H2O. The two samples were found to have different structures, and this structural difference affects the pathway of CO2 reduction. In Ni@SiXNS-EtOH, most Ni NPs are sandwiched between the SiXNS, and very few Ni NPs are immobilized on the surface (Fig.\u00a08\nb), which makes the formation of bridging \u2217CO difficult and limits the generation of CO. Therefore, CH4 is formed mainly through the HCOO pathway (Fig.\u00a08d). Under the reaction conditions of light irradiation at 300\u00a0\u200b\u00b0C, the CH4 yield reached 100 mmol/(gNi\u00b7h) and the selectivity for CH4 was \u223c90%. However, in Ni@SiXNS-H2O, the Ni NPs are mainly present on the surface of the nanosheets (Fig.\u00a08a), and the main reaction paths were: C\u2013O bond breakage to form O and \u2217CO, \u2217CO transformation from the adsorbed state to the gaseous state as the main product, and further dissociation of some \u2217CO to \u2217C, followed by hydrogenation to form CH4 (Fig.\u00a08c). A comparison between the two and Ni@SiO2 is given in Table\u00a03\n. This study demonstrates once again that the choice of catalyst material and preparation method significantly influences the reaction path and product selectivity for CO2 reduction.In the CO2 reduction reaction, competition with the hydrogen evolution reaction (HER) exists. Thermodynamically, proton reduction occurs more easily than CO2 reduction. Kinetically, CO2 reduction is a multi-electron reaction, while hydrogen precipitation reaction is a two-electron reaction [78]. Therefore, inhibiting hydrogen evolution during CO2 reduction is also a major challenge. Transition-metal complexes are considered good candidates for CO2 reduction because of their multiple redox states, which facilitates electron/proton transfer. Lin et\u00a0al. [79] designed a simple heterogeneous photocatalytic system consisting of a Ni bipyridine complex (Ni(bpy)3Cl2) and cadmium sulfide (CdS). CdS readily catalyzes the generation of H2 on the hydrogenated surface. The introduction of Ni(bpy)3Cl2 promotes the transfer of photogenerated electrons from CdS to CO2, further improving selectivity for the CO2 reduction reaction. Under the same reaction conditions, the CO yield for the complex system containing Ni(bpy)3Cl2 was \u223c6 times that for the CdS-catalyzed system.More Ni-based catalysts are listed in Table\u00a04\n [80\u201387]. The products of the catalytic system using H2 as the proton source are more diverse, and all are carbon-containing substances at various selectivities. In contrast, CH4 can also be used as a proton source. This type of reaction is called CO2 reduction with methane (CRM), and the products are mainly CO and H2. Selectivity for the highly utilizable product H2 can reach \u223c45%. The catalysts for CRM are often prepared by constructing bimetallic alloys on suitable carriers, and Ni is one of the most promising metals here. CRM has gained much attention owing to the possibility of achieving both high fuel productivity and light-to-fuel efficiency (\u03b7) [87].This chapter briefly introduces and lists some Ni-based catalysts combined with metal complexes, metal-free materials, and MOFs. The Ni-based catalysts do not generate CO2 directly during the synthesis, but generate carbon emissions indirectly from the electrical energy consumption when using related equipment. Therefore, photothermal catalysts with good catalytic activities and long service lives are required to reduce the energy consumption and carbon emissions generated during preparation. However, photothermal CO2 catalytic reaction systems are subject to low solar energy utilization, severe photogenerated e\u2212/h+ pair recombination, competition from the HER, and low product selectivity [36,45,77,78]. In this section, strategies to improve the catalytic performances of photothermal catalysts, such as the selection of suitable carriers or complexes, morphology modification, and elemental doping, are presented. In the practical application of Ni-based catalysts, cost and cycling stability are influencing factors. Therefore, a composite materials\u2019 cost and availability should be considered. The cycle stability of photothermal catalysts can be enhanced by operations such as hydrogenation treatment and reasonable light illumination [16], as well as ensuring a sufficient supply of reaction feedstock (e.g., CO2, H2, or H2O).Photo/photothermal catalytic reduction of CO2 is a potentially viable strategy to solve global warming and the energy crisis. In this review, three modes of photothermal catalysis were introduced: photo-assisted thermocatalysis, thermal-assisted photocatalysis, and photothermal co-catalysis. CO2 reduction is a complex process, and parameters such as CO2 conversion, product yield, product selectivity, and QY are often used to judge the effectiveness of a particular photo/photothermal catalytic system. However, although the products of CO2 reduction as a multi-electron transfer process are very diverse, the most common are C1 products, including CO and CH4. Currently reported photothermal catalysts are still less selective for other products.Ni is a potential candidate for CO2 photo/photothermal catalysis owing to its availability, low cost, and, more importantly, its proven photocatalytic activity. When Ni is present in the form of NPs, the local temperature around the metal particles increases under light illumination owing to the surface plasmon resonance effect, achieving the effect of synergistic photothermal catalysis. Combining Ni with suitable supports or materials to take full advantage of the synergistic effect can significantly improve catalytic CO2 reduction performance. Combination with metal oxides and MOFs results in good photothermal catalytic performance, with product yields reaching the mmol/(g\u00b7h) level under optimal conditions, e.g., 103.7 and 448 mmol/(g\u00b7h) for CH4 in BTO and Ni\u2013C-600 photothermal catalytic systems, respectively. In addition, strategies such as elemental doping, morphology control, and pore adjustment can effectively solve some common problems in photothermal CO2 reduction. Medium-sized, uniformly distributed NPs and sheet-like materials with large surface areas always lead to better CO2 reduction performance. Although photo/photothermal CO2 reduction has been extensively researched, corresponding catalyst research is still in the developmental stage, and the following challenges remain to be overcome. They are listed here as a reference for researchers to conduct follow-up studies (Fig.\u00a09\n).First, although Ni-based catalysts play an important role in the photothermal reduction of CO2, their development is still limited by various factors, such as: how to choose a suitable support, how to achieve a stable connection between the support and Ni NPs, the reduction method of Ni precursor solution and the catalyst deactivation brought about by Ni0 oxidation during the catalytic process. Solutions to the above problems will greatly advance the application of Ni-based catalysts in the field of CO2 reduction.Second, to efficiently and rationally design Ni-based catalysts, it is essential to thoroughly understand the mechanisms of the different reaction pathways possible for CO2 reduction. Although this review presents several widely accepted reaction pathways, a fully unified mechanism supported by experimental evidence and theory remains elusive. Using traditional characterization techniques with photothermal catalytic systems, isotope tracing techniques, and theoretical calculation (e.g., DFT) to thoroughly explore these catalytic reaction pathways will provide valuable information required for the modification and development of photothermal catalysts.Third, achieving C\u2013C coupling to obtain high-value-added C2+ products (e.g., C2H5OH and olefins) will surely emerge as a focus of future research. Current studies on the formation of C2+ products are based on experimental results, and there is a lack of systematic theoretical studies on their formation factors. Thus, improving the selectivity for C2+ products remain a challenge. Meanwhile, various in situ characterization techniques should also be fully utilized to provide directions for the rational structural design of catalysts. Furthermore, the sensitivity of C2+-product detection should be enhanced by the development of related instruments.Fourth, the practical application and large-scale production of photothermal catalysts for catalytic CO2 reduction must be considered, not just that at the laboratory stage. The photothermal catalysts currently reported show good catalytic performances under ideal conditions, such as high CO2 concentration and artificial-light excitation. However, the application of such catalysts is still limited in practical situations with low atmospheric CO2 concentrations or for industrial waste gases with complex compositions [89]. Therefore, designing cost-effective catalysts with strong adsorption and high selectivity for CO2 must also be a major endeavor. Strategies that can be used here include adjusting pore structure, expanding specific surface area, and making full use of the synergistic effect between catalyst materials.Finally, carbon capture, utilization, and storage (CCUS) technology has received widespread attention worldwide as a means to combat severe global climate change. CO2 reduction is one method of carbon utilization. It is worth considering how this process might be combined with CCUS, i.e., capturing and purifying the CO2 emitted during the production process and then carrying out the reduction operation.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (No. 21906056), Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. ESK202104), the Science and Technology Commission of Shanghai Municipality (22ZR1418600), Shanghai Municipal Science and Technology (No. 20DZ2250400).", "descript": "\n                  Converting CO2 to fuel is a promising strategy to mitigate the greenhouse effect and achieve \u2018carbon neutrality\u2019. Photothermal catalysis has been widely used for CO2 reduction because it effectively reduces the apparent activation energy of the reaction and provides milder catalytic conditions as well as higher catalytic efficiency than conventional catalytic methods. In this review, the basic principles of photothermal catalytic CO2 reduction and the factors used to evaluate photothermal catalytic conversion efficiency are introduced. Then, the common types of Ni-based catalysts and their design strategies are summarized and discussed. Among these catalysts, metal oxides have been extensively studied and developed. Accordingly, they currently achieve product yields up to the mmol/(g\u00b7h) level. Strategies such as elemental doping and morphology control are often adopted for the modification of photothermal catalysts as a means to improve catalytic performance. Finally, future trends in the field of photothermal catalytic CO2 reduction are proposed, including mechanistic studies, practical applications, and coupling with other carbon-neutral technologies.\n               "}
{"full_text": "No data was used for the research described in the article.Biopolymers are gaining traction in plastics because of their inherent biodegradability and unique characteristics for particular applications. Biopolymers may be taken from nature, biosynthesized by live organisms, or chemically synthesized from biological components. Biopolymer waste disposal is now one of the most significant worldwide issues confronting humankind and ecological balance. As the world's population continues to rise, so does the need for the polymer to meet the rising consumption levels, resulting in a severe environmental problem caused by the buildup of biopolymer waste. Biopolymer pyrolysis, a well-known technique for producing usable liquid fuels from low-value polymeric wastes, has fewer greenhouse gas emissions than other technologies, such as incineration and gasification. Bio-based polymers may be made from a variety of renewable sources. Plant-based precursors have made bio-based polymers, including lignocellulose fibers, cellulose esters, polylactic acid, and polyhydroxyalkanoates [1]. One of the abundant biopolymers is cellulose, produced from many living organisms, including plants, animals, bacteria, and certain amoebas. Organic materials/precursors with high amounts of cellulose and other fibers are chosen because they improve their mechanical intensity [2]. Food and agriculture waste are an appealing source of cellulose for industrial applications since it does not endanger the food supply and boosts the local economy [3,4].An interesting alternative to the direct treatment of cellulose is the cracking of solutions where the cellulose is dissolved in a solvent. The influence of the solvent during the thermal degradation of polymers is significant. A proper solvent medium must be used to break the plastic waste into low molecular-weight products. Solvents that may donate hydrogen, in particular, participate in the thermal breakdown of polymers, which impacts the generation and dispersion of hydrocarbons [5]. One of The polar functional groups allows plastics to be solvated by polar solvents like carbolic acid (phenol). The phenol molecule comprises two hydroxyl (\u2212OH) groups attached to a phenyl group (\u2212C6H5), making it an aromatic organic chemical that is volatile. Although most microorganisms are poisonous to phenol, it is often found in many industrial effluents and is frequently utilized as broad-spectrum disinfection [6,7]. Therefore, the presence of phenolic compounds in aquatic environments is unpleasant and unwanted and dangerous to animal and human health [8]. Phenolic constituents often result from the production of petrochemical by-products [9] and makeup around 38% of the unwanted pyrolysis oil ingredient [10]. Phenolic compounds can be extracted from bio-oil by a method such as the liquid-liquid extraction technique because its presence reduces the bio-oil quality and causes high viscosity, high acidity, corrosiveness, low heating value, and faulty product that harms machinery [11]. Thus, it is environmentally favorable to utilize the phenolic compounds as a cellulose solvent using an appropriate method not only for liquid fuel generation but also for carbon-free gas fuel (hydrogen) production due to hydrogen bonds.Polymer pyrolysis is a well-known process for producing valuable liquid fuels and has fewer greenhouse gas net emissions than other contemporary technologies like incineration and gasification and is a valuable method for chemical recycling, which lowers the carbon footprint of polymers. It can mitigate the adverse environmental effects of current management practices via landfilling and incineration and partially reduce carbon emissions while manufacturing virgin polymers. As we used a hydrocarbon solvent with six hydrogen atoms, hydrogen can also be produced, which significantly enhances the yield of generated H2 gas during the reaction. A few technologies have also been employed for H2 generation from bio-oil derivatives, such as dry reformation, partial oxidation, and auto-thermal reformation. Steam reforming is the most efficient and practical technique for producing hydrogen from hydrocarbons [12,13]. Compared to traditional reforming processes, the steam reforming reaction might well be conducted at significantly lower reaction temperatures, lowering the risk of catalyst carbonization and sintering and capital and operating costs. Additionally, most of the heat needed for the endothermic reforming processes is provided by the heat emitted by the exothermic carbonation reaction. Therefore, we conducted the in-situ catalytic steam reforming of phenol coupled with cellulose thermal cracking (or pyrolysis). Phenol has been used in many of previous research as a source for hydrogen production [9,14,15]. However, a significant obstacle to phenol steam reforming is the endothermic nature of the process, which has a complex of numerous side reactions, including phenol breakdown, which produces carbon dioxide, carbon monoxide, and most significantly and negatively, coke [16]. This issue can be solved by developing suitable, active, and stable nano-sized catalytic materials for the pyrolysis-catalytic steam reforming reaction.A catalyst is any chemical compound that reduces the activation energy to speed up chemical reactions like steam reforming and thermal cracking without being wasted during the reaction. Various transition metals such as nickel [17], cobalt [18], lanthanum [19], molybdenum [20] and tungsten [21] and noble metals like rhodium [22], platinum [23], ruthenium [24], palladium [25] and supports such as alumina [26], titanium [27], calcium [28] and etc have been studied to produce hydrogen from reforming reaction of various feedstock. The availability, affordability, chemical safety, and stability of a reducible metal oxide like titanium dioxide (TiO2) make it exceptional support [29,30]. However, it experiences coke formation, which has a negative impact on its long-term H2 generation sustainability [14]. The CaO may be put on the appropriate support to get around this restriction and improve stability [31]. Additionally, the CaO alone suffers from cyclic instability and severe attrition loss in the reaction due to its weak mechanical strength. The mechanical strength and cyclic stability of CaO materials can also be improved by introducing TiO2 to ensure the sustainability of H2 production. Due to their superior physicochemical characteristics, we discovered in our earlier study [31] that equal ratios of Ti and Ca showed bifunctional capabilities, had both basic and acid phases, and had a variety of impacts on the catalyst activity in the transesterification process. The carrying capabilities of both heterogeneous and homogeneous catalysts allow transition metal catalysts to be employed as hybrid catalysts [32]. Because of their low price especially in comparison to catalysts like Rh, Ru, or Pt, as well as their effectiveness for C\u2013O, C\u2013H, and C\u2013C bond breakage and water gas shift reaction, which was caused by the high Lewis acidity intensity of nickel metal, nickel-based catalysts have been explored more in-depth for the removal of tar [33,34]. However, the supported mono-metallic nickel (Ni) catalyst often experiences quick deactivations brought on by the sintering of Ni nanoparticles (NPs) and the accumulation of coke [35]. Adding a second active metal to a bimetallic catalyst uses the synergy between the active metals, often transition metals, to increase coke resistance and active phase dispersion. We have illustrated that this issue of nickel can be solved by introducing another transition metal in the catalyst with great resistance to carbon [16,36]. The enhanced coking resistance of the bimetallic and trimetallic catalysts may be attributed to changing the electronic structure of the catalyst. The advantages of lanthanum (La2O3) as promoters were determined by steam reforming of the bio-aqueous oil's component [18]. La2O3 is intended to increase the distribution of active metal particles on the support and decrease the agglomeration of such materials throughout reforming. Additionally, because of the increased mobility of lattice oxygen anions, it may reduce the formation of coke [19]. La2O3 enhances the catalyst's thermal stability and modifies the acidic and basic characteristics of compounds [37]. Ni and La are examples of transition bimetallic NPs with large surface area and energy, making them effective catalysts. In a bimetallic Ni-La catalyst, the strong oxygen affinity of La promotes carbon oxidation and minimizes coking, while hydrogen overflow from Ni to La limits its oxidation. Despite the encouraging results for bimetallic catalysts [10,25,27,35], the catalyst's stability for extended periods of time in the stream during cellulose cracking at high temperatures has to be clarified. Additionally, the overall surface area and activity of these catalysts are significantly impacted by the coking and sintering of relatively large metal particles [38], particularly when it comes to the heat breaking of polymer bonds. Therefore, it is thought to be desirable to promote the bimetallic active transition metals by a little quantity of noble metal in order to profit from their higher coking resistance and stability while reducing the problem of their high cost and scarcity [39]. The hydrogen spillover mechanism, which accounts for the lowering of the reduction temperature by noble metal promotion, states that as hydrogen adsorbs and dissociates on noble metals, the adsorbed hydrogen atoms disperse on the support surface to reach non-noble metal species, boosting their reduction. Additionally, morphology and composition of the noble metals catalysts are crucial factors that affect their catalytic activity and stability. Since they have the capacity to dramatically alter the catalytical structure and effect performance, their partial application is economically feasible even at low concentrations (\u223c1\u00a0wt%) [40]. When Pd NPs had rough surfaces, dendritic topologies, or porous architectures, for instance, they outperformed their compact counterparts in terms of catalytic efficacy [41]. To improve the catalytic performance, the porous structure may provide a significant specific area, many exposed active sites, and an effective diffusion channel for molecules and electrons. As Pd promoters have attracted great interest, we also targeted the in situ hydrogen production reaction by employing a small amount of Pd in our previous work with remarkable increase in the catalytic activity [25,42]. Pd promoters are good substrate components for customized multimetallic catalysts for hydrogen generation provided the control mechanisms learned, and their increased thermal stability is boosted by a strong metal-support interactions.Despite the evidence disclosed in previous research regarding trimetallic catalysts, there is still a lack of investigations on developing trimetallic nanosized catalysts to crack and reform the cellulose bonds and phenol compound to liquid fuels and hydrogen gas in the scientific literature. To increase the feasibility of this study and increase the ratio of the cellulose to solvent to 2:8, which is much higher compared to our previous works [43\u201345], we modified the experimental setup and connected a Parr Benchtop Reactor (PBR) (see \nFig. 1). This modification allows the easily liquefying and increasing of the amount of polymer and plastic waste in the reaction without causing line blockage. The novelty of the work also lies in developing an understanding of the role of the trimetallic Ni-La-Pd catalyst supported on TiCa for hydrogen production and liquid fuel generation from cellulose dissolved in phenol in the unique process conditions of the in-situ pyrolysis-catalytic steam reforming. Four catalysts were synthesized by hydrothermal treatment methods followed by conventional impregnations method and named as TiCa (ratio 1:1), N/TiCa (Ni to TiCa ratio is 1:9), NL/TiCa (Ni:0.7, La:0.3, TiCa:9), and NLP/TiCa (Ni:0.6, La:0.25, Pd:0.15, TiCa:9) nanocatalysts. The physicochemical characteristics of the fresh catalyst were examined XRD, BET, N2 adsorption-desorption isotherm, NH3-TPD, IR-Pyridine, IR-Pyrrole, H2-TPR, CO2-TPD, FTIR-KBr, TEM, EDX, HRTEM, SAED, Elemental mapping analysis, and ICP test. Catalysts were screened at 600\u00a0\u00b0C, the optimum catalyst was tested at 500\u2013800\u00a0\u00b0C, and stability was studied for 72\u00a0h on stream. GC-TCD characterized gaseous products, and liquid fuels were also examined by GC/MS, GC-FID, and FTIR. The catalysts were analyzed by TGA-DTG, BET, N2 adsorption-desorption isotherm, TEM, FTIR-KBr, and CHNS.The nanosized TiCa support hydrothermal followed by impregnation route and followed our published works [31,44]. A particular amount of CaO and TiO2 were combined with 100\u2009mL of deionized water for dilution purposes and with the 1:10 mass ratio and agitated for a couple of hours, separately in two separate beakers, after 5\u2009M of sodium hydroxide (NaOH) had been dissolved and mixed for an hour. The NaOH was employed to improve the nucleation and growth rates of the NPs [46]. A 100\u2009mL Teflon container was used to contain the fluid. Then, an autoclave made of stainless steel was firmly sealed and housed two Teflon bottles filled with these solutions. The autoclave was put into a temperature-controlled electric oven and hydrothermally treated for 48\u2009h at 160\u2009\u00b0C. The treated sample was filtered and washed with distilled water several times and dried at 110\u2009\u00b0C overnight. The prepared TiCa sample was calcined in an oven (Model Ney Vulcan D-130) at 800\u2009\u00b0C for 3\u2009h. The synthesis steps for each catalyst are depicted in Fig. S1.The monometallic N/TiCa nanocatalyst was prepared in the same steps as the TiCa nanocatalyst. Firstly, a specific amount of nickel nitrate hexahydrate and then 5\u2009M NaOH were dissolved in 100\u2009mL deionized water at room temperature for an hour until a homogenous and clear solution appeared. Subsequently, the solution was transferred into an autoclave reactor equipped with a 100-mL Teflon cylinder and kept in the oven for 48\u2009h at 160\u2009\u00b0C. The powder containing Ni NPs was then washed via filter paper on a Buchner funnel that was sealed with a rubber bung on the top of a side arm conical flask. The side arm of the flask was connected with a vacuum pump to speed the filtration and washing process of the samples, followed by drying at 110\u2009\u00b0C overnight and then calcination for three hours at 800\u2009\u00b0C. The Ni NPs supported on TiCa nanosized support with a volume ratio of 1\u20139 were prepared via impregnation. Briefly, after gently adding the prepared TiCa material into 150\u2009mL of a beaker filled with deionized water, the Ni powder was introduced to the solution and kept stirring on a hot plate stirrer at 90\u2009\u00b0C until water vaporized. The slurry was dried at 110\u2009\u00b0C overnight and then calcination for three hours at 800\u2009\u00b0C. The same procedure was employed for synthesizing La NPs by hydrothermal treatment method and then impregnating La into N/TiCa to produce a bimetallic NL/TiCa nano catalyst. Trimetallic NLP/TiCa nanocatalyst (Ni:La:Pd:TiCa ratio is 0.6:0.25:0.15:9) was prepared in the same way as the monometallic and bimetallic ones, replacing the TiCa support by a bimetallic NL/TiCa nanocatalyst.The crystalline structure of the catalysts was characterized by X-ray diffraction (XRD) conducted on a D8 ADVANCE Bruker diffractometer equipped with Cu K\u03b1 radiation (\u03bb\u2009=\u20090.154\u2009nm, Philip), 40\u2009kV and 30\u2009mA. The Fourier-transform infrared (FTIR) spectra (from 4000 to 400\u2009cm\u22121) were collected on a Shimadzu IR-Prestige-21 spectrometer to examine functional groups in the synthesized and used catalysts. Before measurement, the samples were diluted with potassium bromide (KBr) and pressed into pellets. The KBr pellet was prepared by mixing KBr and catalyst (1 (mg):100 (mg)), and the excellently designed combination was pressed to procedure a 13\u2009mm diameter pellet. N2 adsorption-desorption isotherms of the fresh and used samples were obtained at \u2212\u2009196.1 \u00baC over the whole range of relative pressures using a Beckman Coulter SA3100\u2122 instrument. Before N2 adsorption-desorption measurements, samples were degassed at 180\u2009\u00b0C in a vacuum for 12\u2009h. Specific surface areas (SBET) of the fresh and used samples were calculated by the Brunauer-Emmet-Teller (BET) equation, considering the range of relative pressures 0.1\u2009<\u2009P/Po <\u20090.3. Barrett-Jouner-Halenda (BJH) technique was used to compute pore size and volume from the desorption branch of the isotherm, and the BET method was used to estimate surface area. The elemental composition of the catalysts was detected using inductively coupled plasma (ICP-test) on the Agilent ICPOES720. A JEOL JEM-ARM200F apparatus operating at 200\u2009kV was used to capture the samples' transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM) micrographs. The same instrument was employed for the Energy Dispersive X-Ray (EDX) elemental analysis and crystallographic experimental analysis by the selected area diffraction (SEAD) technique. Temperature-programmed reduction (TPR) scans were performed to study the catalyst's reducibility in a Micromeritics Chemisorb 2720 apparatus. Approximately 100\u2009mg of the precursor material and a flow of 20\u2009mL/min of pure hydrogen, with a 20\u2009\u00b0C/min heating rate, were used for the tests. Using the Micromeritics Chemisorb 2720 apparatus, temperature-programmed desorption of ammonia (NH3-TPD) was carried out to investigate acidity. The material was pre-treated in He flow at 200\u2009\u00b0C for 30\u2009min before being admitted with ammonia. Following cooling to room temperature, the sample was exposed to a stream of pure NH3 (20\u2009mL/min) for 30\u2009min. The sample was purged in flowing He (20\u2009mL/min), and the temperature of the catalytic sample was then raised to 900\u2009\u00b0C (T\u2009=\u200920\u2009\u00b0C/min), removing the physically adsorbed ammonia. The surface acidity and the evaluation of the catalysts' protonic and Lewis acid sites and the supports were also investigated by means of an FTIR spectroscopic study of adsorbed pyridine as a probe molecule. Pyridine (2\u2009Torr) was first adsorbed for 30\u2009min at 423\u2009K, then released for the same amount of time at 500\u2009\u00b0C. The analysis was done using a Cary 640 FTIR spectrometer (Agilent, Selangor, Malaysia) with CaF2 windows and a stainless steel cell that can withstand heat. The pelletized material was activated for one hour at 400\u2009\u00b0C before pyridine adsorption. After that, the sample was heated to 150\u2009\u00b0C while being exposed to pyridine (4\u2009Torr), and the spectra were then gathered at room temperature. To investigate catalysts' basicity, temperature-programmed desorption of carbon monoxide (CO2-TPD) experiments were conducted using the same apparatus and procedures of NH3-TPD analysis except with the replacement of NH3 by CO2 flow. The fundamental characteristics of the catalyst were further characterized using pyrrole-probed IR spectroscopy. On an Agilent Cary 640 FTIR spectrometer with a high-temperature stainless steel cell and CaF2 windows, in situ FTIR was used to accomplish the experiments. All samples underwent a 1-hour activation period at 500\u2009\u00b0C before the measurements. The activated catalyst was then outgassed at room temperature for 15\u2009min after being exposed to 4\u2009Torr of pyrrole for 15\u2009min. Three scans were used to capture each spectrum at ambient temperature with an aspect ratio of 8\u2009cm\u22121.The amount and type of coke formation on the catalysts after being used in the in-situ pyrolysis-catalytic steam reforming conditions were determined by thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG), TEM, and CHNS (carbon, hydrogen, nitrogen, and sulfur) elemental analyzer. The TEM images of the used catalyst were conducted with a JEOL JEM-1011 microscope that functioned at 80\u2009kV. TEM specimens were equipped by dispersing the catalyst powder in acetone with sonication and dropping it onto an ultrathin carbon-coated copper grid. The TGA analysis was performed using the Shimadzu TG-50 instrument in a nitrogen flow at a heating rate of 20\u2009\u00b0C/min.The experimental setup for the in-situ pyrolysis-catalytic steam reforming reactions is mainly comprised of two reactors, and the setup is shown schematically in Fig. 1. The first is a Parr Benchtop Reactor (PBR) equipped with a stir-shaft, a pressure gauge, an autoclave body, a sampling tube, a safety valve, a heating jacket, and a thermocouple thermometer. The PBR is installed to homogenize the liquid phase of the high volume of cellulose in the phenol (2:8) at 70\u2009\u00b0C and pressurize the solution with the N2 gas into the fixed bed reactor. The exit line of the PBR is swathed with glass fiber heating tape to vaporize the liquid before entering into the second reactor that is responsible for catalytic testing. The water line before the reactor was also preheated to 200\u2009\u00b0C so that the water could first vaporize before being mixed with the gas phase of the cellulose and phenol. A vertical tube reactor with an inner diameter of 8\u2009mm and a length of 300\u2009mm was used for the catalytic testing, and it was situated within a furnace with a heating zone. Before the reaction, 0.2\u2009g of catalyst powder was in-situ reduced at 600\u2009\u00b0C for 1\u2009h in pure hydrogen 30\u2009mL/min flow after being fixed in the reactor's center using layers of quartz cotton at atmospheric pressure. After reduction, the reactor was purge-gassed with pure nitrogen for a time to clear out any excess reducing gas. A mass-flow controller system was used in each test to regulate the feeding stream. The thermocouples were positioned in the middle of the inflow area of the fixed bed reactor and were used to monitor the pressure, flow rates, and temperature continually. After the reaction, condensable molecules were liquefied by a glass coil heat exchanger equipped with a chiller at 10\u2009\u00b0C. The gas reactor effluent was analyzed online employing a GC-TCD (Agilent 6890\u2009N), and the liquid product was analyzed using a GC-FID (HP 5890 Series II) equipped with a 0.53\u2009mm x 30\u2009m CP-Wax capillary column and GC/MS (Agilent 7890B). Each run was repeated at least six times to ensure accuracy and reproducibility. The result analyses, such as phenol conversion (based on a calibration curve from GC-FID results), and produced gas composition in yield, were calculated following our previous research [25] and as shown in Eqs. (1), (2), (3), and (4).\n\n(1)\n\n\nPhenol\nconversion\n(\n%\n)\n=\n\n\n\n\n[\nPhenol\n]\n\n\nin\n\n\n\u2212\n\n\n[\nPhenol\n]\n\n\nout\n\n\n\n\n\n\n[\nPhenol\n]\n\n\nin\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(2)\n\n\n\n\nH\n\n\n2\n\n\nyield\n(\n%\n)\n=\n\n\nmoles\nof\n\n\nH\n\n\n2\n\n\nobtained\n\n\nmoles\nof\n\n\nH\n\n\n2\n\n\nstoichiometric\n\n\n\u00d7\n100\n\n\n\n\n\n\n(3)\n\n\nCO\nyield\n\n\n\n%\n\n\n\n=\n\n\nmoles\nof\nCO\nobtained\n\n\nmoles\nof\nCO\nstoichiometric\n\n\n\u00d7\n100\n\n\n\n\n\n\n(4)\n\n\n\n\nCO\n\n\n2\n\n\nyield\n\n\n\n%\n\n\n\n=\n\n\nmoles\nof\n\n\nCO\n\n\n2\n\n\nobtained\n\n\nmoles\nof\n\n\nCO\n\n\n2\n\n\nstoichiometric\n\n\n\u00d7\n100\n\n\n\n\nThe quantity of each chemical that must react for the reaction to be fully catalyzed is known as the stoichiometric moles. So, for example, we have Eq. 5's representation of the balancing steam reforming equation.\n\n(5)\n\n\n\n\n\n\nC\n\n\n6\n\n\nH\n\n\n5\n\n\nOH\n+\n11\n\n\nH\n\n\n2\n\n\nO\n\u2194\n6\nC\n\n\nO\n\n\n2\n\n\n+\n14\n\n\nH\n\n\n2\n\n\n\n\n\u0394\nH\n\n\no\n\n\n=\n463.65\n\n\nkJ\n\n/\n\nmol\n\n\n\n\n\n\n\n\n(6)\n\n\n\n\n\n\nC\n\n\n6\n\n\nH\n\n\n5\n\n\nOH\n+\n5\n\n\nH\n\n\n2\n\n\nO\n\u27f6\n6\nCO\n+\n8\n\n\nH\n\n\n2\n\n\n\n\n\u0394\nH\n\n\no\n\n\n=\n710.91\n\n\nkJ\n\n/\n\nmol\n\n\n\n\n\n\n\n\n(7)\n\n\nCO\n+\n\n\nH\n\n\n2\n\n\nO\n\u2194\nC\n\n\nO\n\n\n2\n\n\n+\n\n\nH\n\n\n2\n\n\n\n\n\u0394\nH\n\n\no\n\n\n=\n\u2212\n41.15\n\n\nkJ\n\n/\n\nmol\n\n\n\n\n\n\nThe structural properties of calcined catalysts determined from nitrogen adsorption isotherms at \u2212\u2009196.1 \u00baC are shown in \nTable 1, while the adsorption-desorption isotherms curves of nitrogen at \u2212\u2009196.1 \u00baC and pore size distribution profiles of the fresh samples are presented in \nFig. 2. Table 1 shows the textural properties of fresh catalysts, which were specific surface area, total pore volume, and average pore diameters. The BET surface area of the TiCa catalyst is 6.26\u2009m2/g. After the nickel material, the surface area increased to 11.89\u2009m2/g, the pore volume also rose from 0.0374 to 0.0803\u2009cm3g\u22121, and the average pore size reduced from 23.89 to 27.02\u2009nm. After introducing lanthanum, the surface area increased to 18.01\u2009m2/g, and pore volume and average pore diameters were reduced to 0.0685\u2009cm3/g and 15.22\u2009nm. The addition of transition metals significantly increased the surface areas, most probably due to the increasing metal and support interactions. When palladium was added, the surface area increased significantly (to 28.17\u2009m2/g), and the pore volume rose (to 0.0995\u2009cm3/g). It is important to note that the textural characteristics of the catalysts enhanced somewhat with the addition of the noble metals, but just a modest improvement was seen for the transition materials. The increase in the surface area and the total pore volume could also be obtained through these ways. The deposition of metal nanoparticles on the external surface leads to new adsorptive sites, which increase the adsorption of N2 on the surface. TiCa mixed with metal nanoparticles could form a porous coordinated complex composition wherein metal NPs could be inserted between TiCa large particles ( in the form of a sandwich structure). The low surface area of bare TiCa calcined at the same temperature confirmed this statement. The larger pore volume and surface area of the NLP/TiCa nanocatalyst compare to TiCa, N/TiCa, and NL/TiCa are beneficial for mass transfer, which often results in the high catalytic activity of the catalysts.Various pore morphologies have often been linked to the geometries of hysteresis loops. As presented in Fig. 2, the N2 adsorption-desorption isotherms of TiCa support belong to Type III (without a hysteresis loop), while the type of hysteresis loops for N/TiCa, NL/TiCa, and NLP/TiCa catalysts are Type H4 (with a significant increase in the adsorbed amount at P/Po>0.7) according to IUPAC classification [47,48]. This shows that it has a micro/mesoporous structure with a variety and abundance of mesopores. Adsorption was somewhat constrained at high P/Po, which may have been brought on by the presence of non-rigid aggregates of plate-like particles or collections of slit-shaped pores [49]. According to the adsorption isotherms, monolayer adsorption forms primarily at low relative pressure, but at high relative pressure, mesopore adsorption causes the production of many layers up to capillary condensation, which results in a significant rise in adsorption volume. Finally, the isotherm reaches a plateau, and the adsorption terminates in the mesopores. Only the TiCa and N/TiCa catalysts (approximately 11\u2009nm and 30\u2009nm, respectively) exhibit big pores, according to Fig. 2 of the BJH pore size distribution, but the pore diameters of the NL/TiCa and NLP/TiCa catalysts are between 5 and 9\u2009nm.\nFig. 2 shows that the bottom portion of the hysteresis loop area for this isotherm (up to 25\u2009cm3.g\u22121 (STP)) overlaps the same region of the isotherm obtained for the TiCa nanocatalyst when the acquired isotherm is modified upward by \u223c2.5\u2009cm3.g\u22121 (STP). In contrast, the top portion of the hysteresis loop area (above 60\u2009cm3.g\u22121 (STP) for this isotherm) overlaps the same region of the isotherm for the N/TiCa catalyst when the isotherm obtained following the introduction of nickel is modified higher by 3.3\u2009cm3.g\u22121 (STP). Also, the amount of adsorbed nitrogen at higher relative pressures (P/Po) decreased with La doping, indicating a decrease in the mesoporous and improved in the specific surface area for the NL/TiCa nanocatalyst. This result shows that La has filled pores where capillary condensation occurs at intermediate relative pressures and that La has not interfered with the capillary condensation processes happening inside pores filling in either the higher or lower parts of the hysteresis loop. Such effects can also be attributed to the partial loading of pores and the formation of La crystallites on the external surface of N/TiCa particles. Compared to other samples, the NLP/Ti sample had the most N2 uptake in the 0.6\u20130.9 (P/Po) range, which suggests a larger mesopore volume. Increasing Pd and La modifiers loading seems to narrow the pore size distribution. The existence of Pd causes to block the pores of the catalysts, which leads to a decrease in the internal surface area. The deposition of Pd NPs on the external surface of the catalyst results in the generation of new adsorptive sites. Based on these results, we can tentatively presume that the catalyst surface area increased after loading Pd metal, which is considered beneficial for the in-situ pyrolysis-catalytic steam reforming conditions of cellulose dissolved in phenol.\n\nFig. 3(a) and Table 1 show the XRD pattern and quantitative data of total crystal sizes, which were obtained through the analysis of the structure of crystalline materials and the identification of the crystalline phases present in a material to reveal chemical composition information based on their diffraction pattern. Diffraction data and JCPDS (Joint Committee on Powder Diffraction Standards) were analyzed using the X\u2032pert Highscore software. The XRD curves of all catalysts showed characteristics peaks at 2\u03b8 angles of 23.05\u00b0, 34.33\u00b0, 37.32\u00b0, 47.45\u00b0, 53.82\u00b0, 59.28\u00b0, 69.84\u00b0, and 79.45\u00b0 that were signed by red hearts corresponding to 101, 210, 102, 202, 103, 042, 242, and 161 diffractions of orthorhombic phase, which are in parallel with the standard JCPDS card number 96\u2013231\u20130619 for Ca(TiO3) alloy and 92.8\u2009nm of crystal size. The XRD pattern obtained for the TiCa catalyst shows individual peak characteristics of crystallized Ca(TiO3) and equals 93.4\u2009nm of crystal size at 2\u03b8 angles of 68.84\u00b0 and corresponding to 402 crystal structure. The two diffraction peaks at 50.81\u00b0 and 72.34\u00b0 for the TiCa catalyst (marked with green trefoil shapes) are ascribed to 211 and 123 monoclinic structural phase of Baddeleyite (Ti4O8; JCPDS 96\u2013901\u20135356). The intensity of the characteristic peak of the N/TiCa catalyst is weaker than TiCa; probably, it may be due to the entry of Ni into the lattice of TiCa, which could cause a formation of a new solid solution structure. The prepared samples all displayed the characteristic diffraction peaks of Ti6O11 and were marked with blue diamonds (JCPDS 96\u2013152\u20131096) with four prominent diffraction peaks (133.3\u2009nm of crystal size) appeared at 30.29\u00b0, 32.17\u00b0, 64.09\u00b0 and 67.34\u00b0, which could be ascribed to the 114, 206, 423, and 609 crystal planes of monoclinic Ti6O11, respectively. The diffraction peak with 2\u03b8\u00b0 values of 39.86 and marked with a purple triangle corresponding to La3Ni2O6.84 crystal plane of 312 and 43.6\u2009nm of crystal size, confirming La3Ni2O6.84 orthorhombic structure (JCPDS 96\u2013153\u20132218). After introducing La and Pd materials, multiple peaks were detected at 23.2\u00b0, 25.28\u00b0, 31.07\u00b0, 40.71\u00b0, 55.06\u00b0, and 75.04\u00b0. Clearly visible LaPd5 alloy (marked by blue circles with 26.6\u2009nm of crystal size) was confirmed by the diffraction peaks of hexagonal at 40.71\u00b0, with the corresponding 002 crystal facets (JCPDS 96\u2013152\u20132598). Similar to NL/TiCa catalyst, peaks were again observed at 75.04\u00b0 with green circles, which is attributed to the metallic LaNi5 alloy with hexagonal phase structure of 211 and 259.6\u2009nm of crystal structure (JCPDS 96\u2013153\u20137852). The additional peak observed at 23.2\u00b0 and marked with blue stars with a crystal size of 73.2\u2009nm represents the 101 crystalline planes of La2.32O12Ti4 orthorhombic structure (JCPDS 96\u2013412\u20134542). The anatase phase structure of TiO2 was seen at 25.258\u00b0 (101) and ascribed by a red star with 89.3\u2009nm crystal size and a JCPDS of 96\u2013900\u20138215. The (102) La plane diffraction peak (with green star) observed for NLP/TiCa appeared at 31.07\u00b0 with 58.9\u2009nm (JCPDS 96\u2013900\u20138526). Peaks for Ti6O11 crystal (marked with blue hearts) were detected at 55.06\u00b0 and 62.75\u00b0, ascribed to the 514 and 517 crystal planes of monoclinic structures (175.8\u2009nm, JCPDS 96\u2013152\u20131096), respectively. For the NLP/TiCa and NL/TiCa samples, the diffraction peak intensity of the Ca(TiO3) alloy crystal phase is enhanced. When calcination at high temperatures, the spinel NLP/TiCa and NL/TiCa might completely decompose into Ca and Ti alloy. The excellent crystallinity and the largest surface area of NLP/TiCa nanocatalyst could be exhibited in the presents of LaPd5, LaNi5, La2.32O12Ti4, and Ca(TiO3) alloys resulting from high reducibility and metal support interaction as depicted in Fig. 3(b). It is expected that the NLP/TiCa can perform an excellent catalytic activity that can be directly associated with the catalyst's physical characteristics, such as crystallinity and surface area.The catalytic characteristics of transition and noble metal NPs deposited on TiCa may sometimes also be described in terms of the chemical contact, even though this impact of chemical interaction on catalytic activity is more often seen in Ni [25] and TiCa [31]. As in the case of the catalysts created by the deposition of Ni, La, and Pd on TiCa, strong bonding of deposited metals with the TiCa may promote efficient active center formation. In this regard, we conducted the H2-TPR analysis to study the redox properties of as-prepared catalysts, and the quantitative and profile results are depicted in Table 1 and Fig. 3(b), respectively. H2-TPR profiles of TiCa reveal that all the titanium-based oxides possess three reduction peaks at 268\u2009\u00b0C, 411\u2009\u00b0C, and 556\u2009\u00b0C. These results differ from the H2-TPR profiles of our previous research [31], most probably because we used organic ash as the source of calcium material. The low-temperature peak at about 268\u2009\u00b0C was assigned to the reduction of surface oxygen species. TPR profile at 411\u2009\u00b0C correlated with a partial reduction of TiO2. The TPR curves for each sample show peaks over 500\u2009\u00b0C, which are attributed to CaO reduction with significant TiO2 interaction, leading to the creation of Ca(TiO3) alloy, as demonstrated by XRD analysis. For the Ca sample, the peak seen at around 556\u2009\u00b0C was connected to the process of CaCO3, which is created by CaO carbonation, and decomposing. The peak at 556\u2009\u00b0C for a mixed TiCa support might be due to a decrease in the oxygen covering the surface of CaO. With a wide shoulder up to 654\u2009\u00b0C, the N/TiCa catalyst begins to reduce into Ni\u00b0 species at 390\u2009\u00b0C, showing a low degree of reducibility and strong metal-support interaction with the TiCa. The reduction shoulder at 390\u2009\u00b0C is associated with the reduction of nickel oxide (NiO\u2009+\u2009H2 \u2192\u2009Ni\u00b0\u2009+\u2009H2O), which has poor interaction with the TiCa. This might be the reason for the H2 consumption decreasing from 12.09\u2009mmol/g to 9.53\u2009mmol/g when Ni is introduced to the TiCa support. The second peak at 654\u2009\u00b0C belongs to the reduction of the nickel aluminate (Ni2+ \u2192\u2009Ni\u00b0) due to the significant interaction between the nickel and the TiCa. The two identified reduction processes have been labeled Hw (weakly adsorbed hydrogen) and Hs since they are characteristics of transition metals (strongly adsorbed hydrogen). For particles between 0.9 and 2.2\u2009nm, Sayari et al. [50] 's correlation of a greater quantity of Hw is in excellent accord with the current results for N/TiCa, NL/TiCa, and NLP/TiCa samples. This can be due to the non-dissociative nature of the H2 adsorption/desorption. N/TiCa, NL/TiCa, and NLP/TiCa samples, in contrast, have a high concentration of Hs species and metallic particles larger than 2.2\u2009nm. This implies that Pd and La may facilitate H2's dissociative adsorption. The former peak (418\u2009\u00b0C) is ascribed to the reduction of La2+ to metallic Lanthanum (La\u00b0), and the latter peak at 677\u2009\u00b0C is accredited to the reduction of Ni2+ to metallic nickel (Ni\u00b0), and the 677\u2009\u00b0C peak consumes more H2 than the 418\u2009\u00b0C peak. Briefly, the low H2 consumption peaks at 176 \u00b0C and 314\u2009\u00b0C for the NLP/TiCa curve, corresponding to the total reduction of Pd2+ to Pd\u00b0. The addition of Pd retards the reducibility of the NL/TiCa, as it is demonstrated by the move of the maximum of the peak from 677\u2009\u00b0C to 705\u2009\u00b0C, therefore, that a Pd*La, La*Ni, La*Ti and Ca*Ti interaction exist as proven by XRD result for LaPd5, LaNi5, La2.32O12Ti4, and Ca(TiO3) alloys, repectively.The study of form, which includes shape, size, and structure, is known as morphology. Morphology is significant for studying nanostructured materials because, in this context, the form determines the physical and chemical characteristics. The morphologies of fresh catalyst were analyzed by TEM, HRTEM, and SEAD, as shown in \nFig. 4, and the elemental composition of materials was identified by EDX and elemental mapping analysis, as shown in \nFig. 5. The Gatan microscopy suite software version 2.11 was employed to analyze the materials' TEM and HRTEM images and lattice d-spacing. The morphology of the prepared samples is verified by TEM analysis. As revealed in Fig. 4(a), the precursor TiCa nanoparticles synthesized by the hydrothermal method are interconnected and overlapped, nano-sized irregular structures, and exhibit two different shapes of particles. The overlapping and interconnection of Ti and Ca elements might be because of the formation of TiCa alloys as confirmed by XRD analysis (Ca(TiO3) alloy) and discussed in the reducibility study. Ti is illustrated mainly in spherical nanoparticles with approximate sizes of 70\u2009nm, while Ca particles are in irregular cubic and rectangular shapes with an average diameter of 300\u2009nm with a lattice d-spacing of 0.195 and 0.242\u2009nm, respectively. The HRTEM images also show lattice edges with a spacing of about 0.195\u2009nm, matching the 101 crystal plane of anatase-type TiO2 in the catalyst. As shown in the representative TEM images in Fig. 4(b) and (c), mesoporous nickel spheres with almost similar sizes and uniform spherical morphology were successfully synthesized. In the HRTEM image of Fig. 4(g) and (h), La has noticeable lattice spaces of 0.279\u2009nm belonging to the (312) plane of La3Ni2O6.84 orthorhombic structure, as confirmed by XRD. The lattice edges with spacing at 0.211\u2009nm can be assigned to the (002) plane of LaPd5 alloy, and the lattice edges value of 0.157\u2009nm corresponds with the (211) plane of LaNi5 alloy. The SEAD pattern further confirms this structure (Fig. 4(i) and Fig. 4(j)).The elemental composition and the presence of Ni in N/TiCa (Fig. 5(a)) and Ni, La, and Pd in NLP/TiCa (Fig. 5(b)) were confirmed from the EDX spectrum, which shows the presence of all the expected elements without having any external impurities. As seen in Fig. 5(a), metallic Ni particles are in close proximity to an amorphous TiCa phase. The EDX mapping of NLP/TiCa nanocatalysts (Fig. 5(b)) indicates despite its complexities with the existence of trace elements such as Pd, La, and Ni, it is not difficult to note that the dispersion effect of Ni, La, and Pd elements on the NLP/TiCa catalyst is better and more uniform. Pd metal particles have small particle sizes, do not aggregate and are typically evenly scattered. Compared with the N/TiCa monometallic catalyst, areas in which the colors of trimetallic catalyst mixed indicate the interface of the active metal due to their overlapping EDX signals. The elemental line scanning analysis of the NLP/TiCa further verified the strong interaction of metal support and the formation of alloys, which agrees with the XRD, TPR, and TEM analysis.The catalytic activity of the catalyst surface and its resistance to carbon deposition in reforming and cracking processes are significantly influenced by its acid-base characteristics. The distribution of weak, intermediate, and strong basic sites and the total basicity of materials significantly impact the adsorption and dissociation capacity of the phenol and polymer molecules. This could speed up the removal of carbonaceous deposition from the catalyst surface, improving the catalytic performance and stability. Thus, CO2-TPD and pyrrole probed IR spectroscopy was carried out on fresh catalysts to understand the influence of transition and noble metals NPs content on the basicity of TiCa catalyst. The results of CO2 adsorption capacity and accessibility data of CO2 uptake for the fresh catalysts are shown in \nFig. 6. By measuring and fitting the CO2 desorption peak, as shown in Table 1, it is possible to determine the catalyst's CO2 desorption quantity. Fig. 6(a) shows that all catalysts display a broad desorption peak at various temperatures, demonstrating the presence of several types of basic sites in the catalysts, including weak basic sites (100\u2013230\u2009\u00b0C), moderate basic sites (230\u2013500\u2009\u00b0C), and strong basic sites (above 500\u2009\u00b0C) [51]. While the C atom in CO2 is the electron-deficient core and the CO2 molecules have vacant orbitals at low energy levels, TiCa may readily shed its outside electrons due to its relatively low initial ionization energy [52]. Many basic sites and adsorbed O2 on the surface were attributed to the diminished catalysts' capacity to absorb CO2. The medium desorption peak at 260\u2009\u00b0C for the N/TiCa nanocatalyst is because of the under-coordinated O2\u2212. The NL/TiCa, and NLP/TiCa nanocatalysts detected small shoulders at around 169, 206, and 219\u2009\u00b0C are assigned to surface \u2013OH [53] indicating the presence of small weak basic sites resulting in less desorption and activation of CO2 in their structure. Those desorption peaks corresponded to the interaction of CO2 with weakly basic hydroxyl groups on NL/TiCa, and NLP/TiCa nanocatalysts. Furthermore, adding La and Pd materials produced higher peaks in regions 637\u2009\u00b0C and 850\u2009\u00b0C; thus, it is indicated that adding La and Pd materials leads to the higher CO2 adsorption capacity of the catalysts. The peak position given to the strong basic increased further after Pd loading, indicating its strongest binding affinity to CO2 that enhanced the catalyst\u2019s basicity. This inference was made because Pd and La interactions with the N/TiCa nanocatalysts are compatible with the observed increase in the quantity of CO2 desorbed. Since CO2 molecules may be converted into reactive CO2\n\u03b4- species as a result of the transition and noble metals utilized in this study, it is possible to efficiently boost CO2 molecule absorption by the N/TiCa, NL/TiCa, and NLP/TiCa nanocatalysts. As increased basicity may decrease the production of coke [54] during the reaction, it is anticipated that the change of basicity with La and Pd concentration in the catalysts may have some impact on the catalytic activity. The H2-TPD, in which NLP/TiCa exhibits substantial H2 adsorption, and the CO2-TPD exhibit strong correlations.The catalyst's basic sites engage with the pyrrole's N\u2009\u2212\u2009H group via the development of an H\u2212bond interaction, which the stretching can see in the N\u2009\u2212\u2009H group. Fig. 6(b) shows the measurement of adsorbed pyrrole on the calcined catalysts using N\u2009\u2212\u2009H stretching and the wide peak in the range of 3200\u20133700\u2009cm\u22121. Additionally, it has been shown that pyrrole chemisorbs dissociatively over powerful basic metal oxides and that deprotonation of pyrrole on the most powerful basic oxygens results in the generation of pyrrolate anions that are stabilized by surface cations [55]. In N/TiCa and NLP/TiCa nanocatalysts, a strong peak was found at \u223c3544\u2009cm\u22121, indicative of stretching vibrations between the H atom of pyrrole and the (Ni\u2212O\u2212 and Pd\u2212O\u2212) group of basic oxygen present in the catalysts' framework. Interestingly, the NLP/TiCa catalyst had a larger peak intensity than the other catalysts, indicating that the catalyst was constructed with many basic sites. The development of intra- and inter-particle porosity was one potential factor that increased the basicity of the NLP/TiCa catalyst [56]. The peak above 3544\u2009cm\u22121 also represented the N\u2009\u2212\u2009H group of pyrrole molecules in the environment, and the N\u2009\u2212\u2009H band (physisorbed pyrrole in a liquid-like state) interacts with the \u03c0-system of nearby pyrrole molecules.Metal-acid bifunctional compositions are often used in industrial catalysts for cracking and reforming processes, where metallic sites catalyze hydrogenation/dehydrogenation reactions, and acidic sites catalyze cracking. One of the crucial variables affecting the catalytic performance in the n-situ pyrolysis-catalytic steam reforming processes is the catalyst's acidity. The findings of NH3-TPD's analysis of surface acidity in terms of the quantity and strength of acid sites are shown in \nFig. 7(a) and Table 1. All samples have strong acid sites because they showed the presence of peaks above 400\u2009\u00b0C. The highest quantity and strength of acid sites for the TiCa sample proposes that the superior acidity of the TiCa nano-catalyst may be due to the Br\u00f8nsted acid sites on the catalyst [31]. A higher acidity may cause to produce better activities, as it is known to break C\u2212C and CO\u2212\u2009binding, but it may also favor higher coke formation during the reforming reactions. Therefore, the bare TiCa might face catalytic deactivation by coke deposition. As can be seen in the quantitative data, N/TiCa and NL/TiCa had basically similar desorption of NH3, indicating that each sample's total acid site density was the same. Ni and La were added to the TiCa structure, which reduced the number of acid sites while simultaneously shifting the high-temperature peak to lower temperatures. With Ni and La in porous TiCa support, Ni and La and support atoms are bonded to form Lewis acid sites due to the different electronegativity between the transition metals and the TiCa atoms. Thus, the acidic sites in N/TiCa and NL/TiCa may be formed by the sites containing electron holes in porous TiCa. These two catalysts had mild acid and basic properties compared to the bare TiCa with the strongest acidity and NLP/TiCa with the strongest basicity. The low acido-basicity properties of N/TiCa and NL/TiCa might also be attributed to the low amount of alkali, which was insufficient to form strong interactions between the alkali and the TiCa support during the preparation of catalysts. Since the \u2013OH groups were lost when Pd2+ ions interacted with the support surface after the addition of Pd metal, it is possible that the catalyst's acidic quantity was dramatically reduced with the insertion of Pd atoms [57]. Consistent with the results of Pd-modified NL/TiCa, the introduction of Pd leads to catalysts of higher basicity and lower acid site having bifunctional properties, which are significant in the in-situ catalytic steam reforming of phenol coupled with thermal cracking (or pyrolysis) of cellulose.The distribution and nature of acidic sites and their effect on selectivity must also be considered. As a consequence, the acidity of the catalysts was further investigated using FTIR spectroscopy with adsorbed pyridine as a probe molecule, and the results are shown in Fig. 7(b). For coordinatively bound (Lewis acid sites, \"L\") and protonated bonded (Br\u00f8nsted acid sites, \"B\") pyridines, the bands at 1540 and 1440\u2009cm\u20131 were used as measures, respectively [58]. Higher wavenumbers also demonstrated better surface adhesion, increasing Br\u00f8nsted acid's strength. It is noteworthy that the TiCa has many Br\u00f8nsted and Lewis acid sites compared to other catalysts with transition and noble metals. All samples had bands at around 1540\u2009cm\u22121 caused by pyridine adsorbed on Br\u00f8nsted acid sites; however, the N/TiCa and NL/TiCa catalysts did not show any bands at 1440\u2009cm\u22121 that were caused by the pyridinium ion (PyH+), which is responsible for Lewis acid sites. This does not imply that there are no Lewis acid sites; it only indicates that the Lewis acid sites were insufficiently powerful to create the pyridinium ion. However, it is simple to induce carbon deposition and deactivate the catalyst at the Br\u00f8nsted acid and strong acid sites [59]. The Pd NPs inserted into an NL/TiCa have jointly disturbed the catalyst's Br\u00f8nsted and Lewis acid phases. As Pd addition increases, the catalysts' pyridine adsorption peaks become less intense, indicating a weaker acid. To conclude, reducing the strength of Br\u00f8nsted and Lewis acid phases is vital to prevent the undesired reaction, especially for the reactions that deal with polymer molecule thermal cracking. Therefore, it is expected that NLP/TiCa will show high catalytic activity and selectivity in the in-situ catalytic steam reforming of phenol coupled with the pyrolysis of cellulose.The spectroscopic method known as FTIR is highly effective and widely used for identifying functional groups in compounds and complex substances. In this work, KBr pellets are utilized as a carrier for the sample in the IR spectrum since they are optically transparent to light in the IR measurement range. We choose the KBr pellet approach for FTIR spectroscopy because it is straightforward and enables us to run the whole mid-IR region down to 400 wavenumbers without running split mulls [60]. All catalysts were subjected to FTIR-KBr spectroscopy, and the spectra in the 4000\u2013400\u2009cm\u20131 region are shown in \nFig. 8. The TiCa sample exhibited an absorption peak at 957\u2009cm\u20131, ascribed to C\u2013O symmetrical structure ether groups. These lines are connected to chromophore vibrations (CC stretches and hydrogen out-of-plane vibrations, respectively), so it is more likely that exchangeable protons will influence them in the chromophore than exchangeable protons in other regions of the molecules. This peak shifted to 833\u2009cm\u20131 after adding Ni, La, and Pd metals. The stretching vibrations of metal oxide in octahedral group complex Ni(III)\u2013O2\u2212, La(III)\u2013O2\u2212, and Pd(III)\u2013O2\u2212 tetrahedral group complex formation is proved by the bands at 594\u2009cm\u20131. The FTIR spectra at 1165\u2009cm\u22121 and 1774\u2009cm\u22121 for the N/TiCa, NL/TiCa, and NLP/TiCa nanocatalysts correspond to the stretching and bending vibration of C\u2212H, C\u2212O\u2212H, C\u2212O\u2212C bonds and \u2013CO absorption vibration, respectively. There is a highly characteristic weak band at 2376\u2009cm\u22121 for all samples that correspond to vibrations of the \u028b(C\u2212H) mode related to CH2 \u2212CO groups; while the peak at 2932\u2009cm\u22121 indicates the existence of C\u2212H vibrations in methyl (\u2013CH3) and methylene (\u2212CH2 \u2212) group. The \u03bd(OH) band around 3642\u2009cm\u22121, which is assigned to non-hydrogen-bonded water molecules coadsorbed with CO, exhibits a gradual decrease by adding Ni, La, and Pd metals. Besides, FTIR bands observed in the hydroxyl region at 3765\u2009cm\u22121 are ascribed to the vibration of OH\u2013 groups adsorbed along the support surface and correspond to terminal hydroxyl groups. The biggest bands centered at 1443\u2009cm\u22121 can be attributed to the surface complex of \u2013CH2 bending (methylene group), while the 1463\u2009cm\u22121 peak can correspond to the presence of metal carbonates (stretching vibration of CC). Notably, the intensity of FTIR peaks ascribed to the methylene groups and metal carbonates of the NLP/TiCa, NL/TiCa, and N/TiCa catalysts were more intense than the TiCa catalyst. This suggests that the differences in the surface methylene groups are caused by the Ni, La, and Pd crystal structure.The catalyst's performance on hydrogen generation and its stability in the system have been investigated using catalytic activity parameters regarding phenol conversion, product yield, temperature (for the best catalyst), and time on stream performance. The screening of reduced catalysts was performed at 600\u2009\u00b0C. The activity was investigated and repeated for six cycles with an experimental duration lasting 60\u2009min for each run; the results are illustrated in \nFig. 9. Analysis of produced gas composition shows that the most significant changes in concentration occur for H2 yield, CO2 yield, and phenol conversion. It was confirmed that the addition of transmission and noble metals did not significantly affect CO yield but slightly decreased. At TiCa nanocatalyst, the phenol conversion and H2 yields were 34.3% and 40.5%, respectively. Phenol conversion was increased to 49.3% and 70.2%, and H2 yield enhanced to 45% and 57.2% for the N/TiCa and NL/TiCa, respectively. The highest conversions of phenol at 82.6% and H2 yield at 82.2% were achieved with NLP/TiCa catalyst. The increased catalytic activity under the NLP/TiCa nanocatalyst can be attributed to the increased surface area, metal support interaction, basicity, and active metal distribution of the synthesized catalyst as characterized by BET surface area, CO2-TPD, H2-TPR, and EDX. A reactant's contact with a higher surface area affects the number of collisions and the reaction rate. The NLP/TiCa nano catalyst has more porosity and is more active than those other samples because it has more surface area to form the active sites, leading to greater activities in this work. As a result of the large dispersion of active sites and the accessibility of reactants to active sites, we can assert that the high BET surface area and pore volume of Pd-containing catalysts increase the selectivity to hydrogen. Furthermore, the maximum hydrogen consumption of the NLP/TiCa nanocatalyst in the H2-TPR process also proves the total reduction of Pd2+ to Pd\u00b0. The addition of Pd in the NL/TiCa increased the reducibility properties of the catalyst; therefore, a Ca*Ti, Pd*La, La*Ti, and La*Ni interaction exist as proven by XRD result for LaPd5, LaNi5, La2.32O12Ti4, and Ca(TiO3) alloys. From the particle size distribution of the active metal in the EDX images, it can also be found that the NLP/TiCa nanocatalyst has a smaller particle size and a more uniform distribution. Therefore, it shows the most excellent catalytic activity in the reaction. According to the CO2-TPD and Pyrrole adsorption FTIR spectra findings in Fig. 6, the NLP/TiCa nanocatalyst's more excellent catalytic activity may be caused by the nature of Pd and the existence of a significant number of basic sites in the structure [61]. These findings suggest that the catalyst's basicity facilitates the considerable adsorption of CO2 molecules and increases H2 production. For instance, Pizzolitto et al. [62] observed that the NiLa/ZrO2 catalyst's basicity increase significantly reduced the dehydration of ethanol. We also found the same positive effect of basic catalyst sites on the catalytic pyrolysis steam reforming reaction PET-phenol for H2 generation [25,27,35,45]. Less crystal size of NLP/TiCa nanocatalyst (as seen in XRD analysis) plays a substantial part in minimizing the coke production and deposition and enhancing the catalyst lifetime throughout the in-situ pyrolysis-catalytic steam reforming reaction process. Given its good catalytic performance for phenol steam reforming (in terms of phenol conversion and H2 selectivity), the NLP/TiCa nanocatalyst was selected for further evaluation based on temperature effect and deactivation check studies.The reforming temperature significantly influences the concentration of reforming products; the results are shown in \nFig. 10. Conversely, using only 500\u2009\u00b0C resulted in 67.6% of phenol conversion and 59.2% of H2 yield. Due to the accelerated phenol steam reforming reaction (\n\n\n\n\n\nC\n\n\n6\n\n\nH\n\n\n5\n\n\nOH\n+\n11\n\n\nH\n\n\n2\n\n\nO\n\u2194\n6\nC\n\n\nO\n\n\n2\n\n\n+\n14\n\n\nH\n\n\n2\n\n\n\n) and water gas shift reaction (\n\nCO\n+\n\n\nH\n\n\n2\n\n\nO\n\u2194\nC\n\n\nO\n\n\n2\n\n\n+\n\n\nH\n\n\n2\n\n\n\n), an increase in temperature regulates the enhancement of phenol conversion into gaseous products and H2 yield. Higher gas yields compared to 500\u2009\u00b0C were seen along with the high conversion at the maximum temperature, which is anticipated given that the gas-forming cracking processes are thermally regulated. Almost complete conversion of phenol (98.7%) and maximum H2 yield (99.6%) were achieved at 800\u2009\u00b0C. At the same time, the CO2 yield is enhanced from 16.7% at 500\u2009\u00b0C to 27.4% at 800\u2009\u00b0C, while the CO yield decreases from 18.9% to 8.8% due to improving reaction conditions for water gas shift reaction. In addition, it was also observed that the increase in acid sites, decrease of the surface area, and metal dispersion of the TiCa nanocatalyst were sufficient to justify the low phenol conversion and H2 yield. This made it feasible to conclude that the major pathway for the deactivation of the TiCa catalyst and likely has an impact on how well the TiCa catalyst performs in terms of producing H2 is the development of amorphous coke. This statement is verified in the characterization of used catalysts.The phenol conversion and H2 production over the NLP/TiCa nano catalyst were evaluated at 600\u2009\u00b0C for 72\u2009h of time-on-steam, and the stability results are displayed in \nFig. 11. With a slight decrease (\u223c4% loss in yield), the H2 yield was maintained constantly at approximately all reaction times. During the reaction, the CO yield decreased from 14.9% to 8.6%, while the CO2 yield fraction increased from 19.7% to 23.7%. After 32\u2009h, it was noticeable that phenol conversion was about 74% and remained almost stable (with a negligible decrease) for the rest of the time (72.3% at 72\u2009h). This is almost in agreement with the total basic sites, Pd and La dispersion, high metal-support interactions, and different alloys determined by CO2-TPD, H2-TPR, and XRD, respectively.To understand the component presented in the produced liquid during the thermal reaction of cellulose and steam reforming reaction of phenol, the liquid product at 600\u2009\u00b0C was analyzed by GCMS technique, and the identified compounds are summarized in \nTable 2. The chemical components in the liquid product are analyzed in accordance with the thermal breakdown of the feedstock's cellulose and phenol. Due to their volatility and complicated structure, several chemicals were undetected. The aromatics and alkanes occupied mostly the liquid product, mainly from cellulose decomposition. The liquid was mainly composed of C2-C12 aliphatic hydrocarbon, C6-C9 aromatic hydrocarbon, C13-C18 aliphatic hydrocarbon, and C19-C72 aromatic hydrocarbon. At a retention time of \u223c59\u2009min, the liquid was composed of a C70 aliphatic hydrocarbon for the NL/TiCa nanocatalyst. It is evident that despite the variable catalytic activity of different catalysts, the relative product profile remains largely the same, with 1-propanol, ethanol, toluene, and phenol (incoverted reactant) being the major product for all the catalysts except 2,4-dimethyl-benzo[h]quinoline, and hexadecane components for the N/TiCa catalyst. The aromatic byproducts of the cracking process, which resulted from breaking the main chain in cellulose and dehydration of the \u2013OH bond on its alkyl chains, may also be the source of phenolic compounds. The chemicals contain oxygen such as C8H10O5PW+, C40H48O4, C29H23NO, C4H7OH, C14H42O7Si7, C16H32, C17H34O2, C23H39NO2S, C28H43NO2, C28H45NO2, C40H73NO5Si4, and C21H46O2Si produced for the TiCa nanocatalyst could lead to the instability of catalyst. The oxygen compounds in produced fuel decreased with the addition of transition and noble metals NPs deposited on TiCa, indicating that Ni, La, and Pd metals promoted deoxygenation, and more oxygen compounds were decomposed into low molecular-weight substances. The occurrence of 2,4-dimethyl-benzo[h]quinoline is the unique structure of petroleum triaromatic azaarenes that could be more firmly established after the identification of individual compounds. It has been possible to radiolabel exosomes, hydrogels, and other biological materials since 1982 with the help of the generated hexadecane chemical. This substance is also helpful for positron emission tomography. Catalytic cracking activity of the cellulose and phenol under NLP-TiCa catalyst produced unique compounds such as (E)\u2212\u20092-bromobutyloxychalcone, beta,epsilon-Carotene-3,3\u2032,8,19-tetrol, 7,8-dihydro-, 1,5-benzodiazocin-6(1\u2009H)-one, 8,10-bis(dodecylsulfonyl)\u2212\u20092,3,4,5-tetrahydro-5-methyl-, 2,4,6,8,10,12-Tridecahexaenoic acid, 13-(3-chloro-4-methoxyphenyl)-, 2-decyl-3-methoxy-5-pentylphenyl ester and Lanost-9(11)-en-18-oic acid, 23-(acetyloxy)\u2212\u20093-[(4-bromobenzoyl)oxy]\u2212\u200920-hydroxy-,.gamma.-lactone, (3.beta.,20.xi.). The Pd metal induced stronger cracking activity than other catalysts by significantly increasing the proportion of aromatics while significantly decreasing the oxygenated products (including phenols). Furthermore, it has been shown that the H2 yield and phenol conversion during the catalytic process seem to have increased in the presence of the NLP/TiCa catalyst (see Fig. 9). The liquid products were further analyzed using the FTIR technique.The FTIR method was used to identify the functional groups present in the liquid fuel, and the FTIR curves with the intensities of each band are displayed in \nFig. 12 (a) and Fig. 12 (b). The absorption band present in the wave number range 632\u2009cm\u22121 can be linked to the OH\u2013 vibration [63]. At band detection 879\u2009cm\u22121 (HNO3), the carbonate bending mode can be assigned to the out-of-plane deformation band [64]. There is a substantial boost in intensity of the bands at about 1033\u2009cm\u22121 wavelengths, signifying the chemical functional groups of \u2013CH2\u2013 bending vibration of the aliphatic hydrocarbon [65]. Andrea et al. [66] stated that the bands at 1033\u2009cm\u20131 could also be attributed to the C\u2013H in-plane deformation vibration of 1,4-disubstituted or 1,2,4-trisubstituted benzene rings. This peak might also correspond to the C\u2013O stretching and C\u2013O bending of the C\u2013O\u2013H carbohydrates [67]. The band at 1404\u2009cm\u22121 signified carboxylate CO stretching weakened, demonstrating that the non-conjugate CO structure in lignin has been decomposed [68]. This peak may also be ascribed to the aliphatic C\u2212H deformation of CH2 and CH3 bending and C\u2212OH deformation of COOH, COO\u2212 symmetric stretch [69]. The characteristic absorption peak at 2345\u2009cm\u22121 corresponds to CO2 based on its vibrational occurrence and reflection as a 1,3,5-triamino-2,4,6-trinitrobenzene thermal decomposition product [70] and the O\u2013C\u2013O anti-symmetric stretching mode [71]. The vibration peaks of \u2212CH3 (\u03bdas(CH3)) in 2,5-dimethylfuran were observed at 2962\u2009cm\u20131\n[72,73], and symmetric and asymmetric stretching vibration of N\u2009\u2212\u2009H (\u03bd(NH) bound) [74,75] and the presence of urethane groups [76,77] were observed at 3333\u2009cm\u20131.Characterization of the spent catalyst is critical for stabilizing stable catalysts against coke formation and long-term usage. The transient deactivation of the catalyst caused by the buildup of carbonaceous deposits (coke) during catalysis affects throughput, necessitates regeneration procedures, and results in a partial permanent loss of catalytic efficiency. This part analyzed the spent catalysts by TGA-DTG, CHNS, BET FTIR-KBr, and TEM. By observing the weight change that occurs while a sample is heated at a consistent rate, the TGA analytical method may be used to evaluate a material's thermal stability and the percentage of volatile components. The rates at which these volatile components are removed in %/min are determined by DTG, and results are shown in Fig. 12 (c) and Fig. 12 (d) and \nTable 3. The TGA curves of the used catalysts may be roughly separated into three sections, as observed. Weight loss in the first stage, denoted by WL1, is evident in the temperature range of 25\u2013200\u2009\u00b0C and results from removing adsorbed water and unreacted molecules. The chemical adsorbed on the sample's surface or found in the mesopores is responsible for the WL1\n[78]. The second part (WL2), located in the medium temperature range (200\u2013600\u2009\u00b0C) shows an increasing tendency in weight percent and can be attributed to the burning of deposited coke. Here as \u201ccoke,\u201d we considered the carbon deposited on the catalyst and all the condensed hydrocarbons determined by the material balance. The inadequate breakdown of nitrate during the creation of metal oxides should also contribute to the WL2. The WL2 area is ascribed to the overlaps of metal oxidation and removal of the amorphous carbon, except for the weight increase of the TiCa catalyst, which may be driven by the oxidation of the metallic Ti and Ca [10,16]. Amorphous carbon burns at temperatures lower than 400\u2009\u00b0C [79\u201381]. Amorphous carbon may be readily removed by oxidizing at low temperatures, but it often encircles the catalyst's active metal particles and renders it inactive. The third phase (WL3) above about 600\u2009\u00b0C comprised the decomposition of remaining residues and heavy carbonaceous species, most probably by graphitic coke. The shell structure is composed of graphitic carbon with some defect sites, as confirmed by TEM analysis (Fig. 15). The weight loss between 80 and 120\u2009\u00b0C is ascribed to the loss of surface hydroxyls and physically or chemically linked water from all samples, which was supported by their DTG endothermic peaks [82]. The first derivative of TGA curves (DTG) in Fig. 12 (c) presented the highest weight reduction rate for all catalysts with higher intensities for catalysts with metals occurred at below 100\u2009\u00b0C, whereas it was significantly higher for TiCa at \u223c750\u2009\u00b0C\u2009min and N/TiCa at \u223c450 \u00b0C. Primary pyrolysis processes occur on cellulose at low-temperature ranges. In this stage, monomeric phenols undergo side-chain reactions, ether bond cleavage, and evaporation. Methoxy group bonds are broken down, and aromatic rings are broken down and condensed during the secondary pyrolysis events, which take place above 400\u2009\u00b0C [83,84]. However, DTG curves show that the peaks of TiCa and N/TiCa became more considerable than that of Pd and La, indicating that the introduction of Pd and La into the catalyst can considerably affect the reforming and pyrolysis reaction behavior of phenol and cellulose. The catalyst sintering and carbon deposition behaviors are low in NLP/TiCa and NL/TiCa, whereas the TiCa catalyst deactivation is caused by carbon deposition rather than the metal sintering, but N/TiCa displays severe sintering and carbon formation performances. Generally speaking, the total weight losses of TiCa, N/TiCa, NL/TiCa, and NLP/TiCa were 72.31%, 61.36%, 61.36%, and 51.38%, respectively; which are in line with the carbon content from CHNS analysis (13.8%, 9.6%, 4.1%, and 3.8%, respectively). Most of the carbon combustion happens at the WL2 region, indicating an exothermic process consistent with oxidation. The breakdown of CO groups on the surface of carbon-based catalysts may also be responsible for the weight loss over 700\u2009\u00b0C [85]. The carbon deposition side reaction of the TiCa and N/TiCa catalysts is also related to the higher acidity of those samples, as described in NH3-TPD and Pyridine-FTIR spectra in Fig. 7.The spent catalysts were also examined by BET surface area and N2 adsorption-desorption isotherm, and the results are depicted in Table 3 and \nFig. 13. As displayed in Table 3, the surface area of the spent catalysts employed in this work was considerably decreased compared to that of the fresh samples, most probably because of thermal sintering and carbon deposition. The SBET of spent TiCa, N/TiCa, NL/TiCa, and NLP/TiCa nanocatalysts were 0.131, 1.39, 3.89, and 10.6\u2009m2/g, respectively. However, the highest surface area remains for the NLP/TiCa nanocatalyst with more porosity and active sites than those other samples. This sample with Pd NPs showed the most increased catalytic activity (Fig. 9) and lowest carbon deposition (). These results also indicate that the catalysts with Pd NPs are more competitive for recovering the textural characteristics of the spent samples. The results revealed that the H4 type isotherms loop remained unchanged for all catalysts with transition and noble metals NPs with pore size distribution in the range of 2\u201325\u2009nm after the reaction, demonstrating the collapse of the mesoporous framework was not pronounced. These results indicated that adding Pd is beneficial to keeping the catalyst's activity and improving the catalyst's anti-coking performance.Furthermore, a qualitative analysis of coke structures on the spent catalysts was conducted using FTIR spectroscopy. The two key bond vibration areas in the FTIR spectra of complete coke deposited on TiCa, N/TiCa, NL/TiCa, and NLP/TiCa nano-catalysts are shown in \nFig. 14. While the vibrations in the area of 1650\u20131350\u2009cm\u22121 belong to aromatics and specific bending modes of aliphatics, the vibrations in the 3200\u20132700\u2009cm\u22121 essentially correspond to olefins (asymmetric and symmetric stretching) and monocyclic aromatics (olefins). Specifically, the IR bands at 586\u2009cm\u22121, 864\u2009cm\u20131, and 957\u2009cm\u20131 are designated to functional groups of amide VI species, =CH bending out of the plane, and C\u2013O symmetrical structure in aliphatic nature of coke, respectively. The single shoulder for the N/TiCa nanocatalyst at around 1265\u2009cm\u22121 is ascribed to the \u2212C\u2013O single bond vibration of \u2212C\u2013OH group. The parent peaks at 1443\u2009cm\u22121, which can approve the presence of skeleton vibration of the pyrrole ring and CC stretching [86] clearly indicating it is strengthened in this trend TiCa<Ni/TiCa<NL/TiCa<NLP/TiCA. Thus, pyrrole ring and CC stretching are assigned to the catalyst with higher weight loss and lower catalytic performance. These peaks might also correspond to the asymmetric stretch vibration of CO3\n2- molecules [87]. The characteristic peaks at 1635\u2009cm\u22121 correspond to the adsorbed H2O molecules at the spent catalysts, whereas the 1805\u2009cm\u22121 single bans for the NLP/TiCa sample are ascribed to the C=\u2009\u039f stretching modes. The FTIR peaks at 2399\u2009cm\u22121, 2924\u2009cm\u22121, and 3449\u2009cm\u22121 wavenumbers for the spent catalysts with transition and noble metals were equivalent to the linearly adsorbed OCO species [88], asymmetric stretching vibration of C\u2212H [89], at ascribed to the O\u2013H stretching vibration [90,91], respectively. However, the intensities of those peaks were increased in the spent catalysts with transition and noble metals, which is evidence for decreasing monomeric phenols via successive C\u2013O bond cleavage.TEM images of the spent NLP/TiCa sample after 72\u2009h time on stream are revealed in \nFig. 15. After the in-situ pyrolysis-catalytic steam reforming reaction, the basic morphologies of the NLP/TiCa catalyst are retained, agreeing with the preferable performance and stability during the reaction; along with some smaller microparticles placed on the surface of the larger cuboid particles. The substances may identify Coke deposits with non-uniformly shaped particles shown in the photos. The coke deposits' disorganized structure matches that of amorphous coke exactly. The deposited carbon appears as the core-shell structure on the catalyst particles can be ascribed to the encapsulated graphitic carbon with a shell of 15\u2009nm. The two factors responsible for this carbon production are carbon development on the surface of nickel and lanthanum catalysts and carbon diffusion via transition metal particle surfaces. This sort of carbon causes transition metal active sites to have weaker coke covering, which results in active metallic particles still being in contact with the gas flow and delaying the catalyst's deactivation [92].This research revealed the synergistic influence of Pd, Ni, and TiCa support for hydrogen and valuable liquid generation from cellulose bio-polymer wastes dissolved in phenol. To increase this study's feasibility and the cellulose ratio to solvent ratio, we modified the experimental setup and added a Parr Benchtop Reactor. This modification allows the quickly liquefying and rising of the amount of polymer and plastic waste in the reaction without causing line blockage. Four catalysts were prepared through hydrothermal and impregnation techniques: TiCa, N/TiCa, NL/TiCa, and NLP/TiCa nanocatalysts. The physicochemical characteristics of the fresh catalysts were examined by XRD, BET, N2 adsorption-desorption isotherm, NH3-TPD, IR-Pyridine, IR-Pyrrole, H2-TPR, CO2-TPD, FTIR-KBr, TEM, EDX, HRTEM, SAED, EDX, and ICP test and the used catalysts were characterized by TGA-DTG, BET, TEM, FTIR-KBr and CHNS. The best catalytic interaction between Pd, Ni, La, and TiCa was observed for the trimetallic NLP/TiCa nanocatalyst, which showed the highest catalytic activity at 600\u2009\u00b0C compared to monometallic N/TiCa and bimetallic NL/TiCa catalysts, with high stability for 72\u2009h of time on stream, almost complete phenol conversion (98.7%), and 99.6% of H2 yield at 800\u2009\u00b0C. The increased catalytic performance under the NLP/TiCa nanocatalyst can be ascribed to the increased surface area, metal support interaction, basicity, and active metal distribution of the synthesized catalyst as analyzed by BET surface area, H2-TPR, CO2-TPD, and EDX. The addition of Pd in the NL/TiCa increased the reducibility properties of the catalyst; therefore, a Ca*Ti, Pd*La, La*Ti, and La*Ni interaction exist as proven by XRD result for LaPd5, LaNi5, La2.32O12Ti4, and Ca(TiO3) alloys. Carbon deposition was not considered the only reason for the low performance for TiCa and N/TiCa nanocatalysts. The catalyst sintering and carbon deposition behaviors are low in NLP/TiCa and NL/TiCa, whereas the TiCa catalyst deactivation is caused by carbon deposition rather than the metal sintering, but N/TiCa displays severe sintering and carbon formation performances. The carbon deposition side reaction of the TiCa and N/TiCa catalysts is also related to the higher acidity of those samples, as described in NH3-TPD and Pyridine-FTIR spectra. The material balance also determined all the condensed hydrocarbons. The deposited coke appears as either large amorphous and graphitic with catalyst particles or as a rather uniform carbon coating that covers the catalyst particles. The liquid product compounds produced mainly from the thermal cracking of cellulose in almost all samples were ethanol, toluene, phenol, 2,4-dimethyl-benzo[h]quinoline, and hexadecane. The current study will offer an industrial basis for synthesizing effective trimetallic nanosized catalysts and help in the operative utilization of cellulose biopolymer waste and phenolic compounds for liquid fuel and hydrogen gas production.W. Nabgan: Writing, Experimental and characterization, T.A. Tuan Abdullah: Supervision and Methodology, M. Ikram: Writing and characterization, A.H.K. Owgi: Writing & English editing, A.H. Hatta: Writing tasks of the experimental part, M. Alhassan: Writing tasks of the reaction and stability parts, F.F.A. Aziz: Figures development and English editing, A.A. Jalil: English editing and characterization, T.V. Tran: Writing & English revision, R. Djellabi: Writing & English editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The primary author, Walid Nabgan, is grateful for the support from Universitat Rovira i Virgili under the Maria Zambrano Programme (Reference number: 2021URV-MZ-10). The authors are also grateful for the support given by Universiti Teknologi Malaysia (UTM) allocation budget in Pusat Pengurusan Makmal Universiti (PPMU) laboratory.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2023.109311.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Hydrogen and liquid fuel production from biopolymer waste, such as cellulose dissolved in phenol, was investigated using in-situ pyrolysis-catalytic steam reforming conditions. Developing a sustainable method for the thermal cracking of such biopolymers still faces difficulties due to the catalyst stability primarily impacted by coke deposition. The key to the proposed method is improving a highly active and stable catalytic reforming process in which trimetallic Ni-La-Pd supported on TiCa acts as a primary reforming catalyst. Catalysts were prepared by hydrothermal, and impregnation techniques, and the physicochemical characteristics of the fresh and spent materials were examined. The results showed that the NLP/TiCa catalysts performed effectively due to their comparatively high surface area, strong basicity, evenly distributed Pd particles, and appropriate redox and desorption characteristics. The addition of Pd retards the reducibility of the NL/TiCa; therefore, a Pd*La, La*Ni, La*Ti, and Ca*Ti interaction exist. Almost complete conversion of phenol (98.7%) and maximum H2 yield (99.6%) were achieved at 800\u00a0\u00b0C for the NLP/TiCa. These findings give an insight into industrial-scale development. They have significant potential for enhancing the generation of hydrogen and liquid products from phenol and cellulose waste, such as propanol, ethanol, toluene, etc.\n               "}
{"full_text": "Heterogeneous catalysis systems have created attention due to availability of number of catalysts or the use of diverse substrates such as silica, sepiolites, zeolites, and polymers for metal complex immobilization [1,2] associated with numerous advantages including effective recovery of the catalysts by filtering, reusability, robustness, enhanced air and moisture stability, environmental compatibility, low cost, non-toxicity, non-corrosiveness etc. [3\u20139]. The ability to alter the shape and local environment at the catalytic site in accordance with demands of a specific reaction is the most significant characteristic of porous material for which they exhibit enormous potential for catalytic applications. Zeolites are crystalline structure with pore containing aluminosilicate, which is commonly applied in many demands such as separations, catalysis, ion exchange, and adsorption because of their tunable properties such as high surface area and narrow pore size distribution, acidity, thermal and structural stability with specific functionality [10\u201312]. In comparison to traditional micrometer-sized zeolites, nanocrystalline zeolites have greater outside surface areas and well adsorption property, which causes diffusion of reactant and product molecules easily during a reaction. Na\u2013Y zeolite have a unique structure of Faujasite-type framework (FAU) with three-dimensional channel and pore size of 7.4\u00a0\u200b\u00c5 that can support organic and inorganic compounds to form a hybrid material [13\u201315]. It is a synthetic zeolite carries negative charge in the framework and sodium cation neutralizes the aluminosilicate framework structure [16]. In the era of catalysis, transition metal complexes play a significant role [17] due to their adaptable coordination environment, and they have a wide range of applications in several areas including optical, electrical, magnetic materials, and catalysts [18\u201320]. In addition to enhancing the catalytic performance of the support, incorporation of transition metal complex ions in a silica surface also results in high thermal stability of the hybrid material, consistency and availability of the active sites, ease of separation, recyclability, and an environmentally friendly pathway for chemical reactions, among other benefits [21].There are many reported works on catalysis using Ni complex on functionalized porous materials, F. Havasi et\u00a0al. described the use of Ni complex attached functionalized MCM-41 for the synthesis of 2,3-dihydroquninazolin-4(1H)-ones [22]. L. Tahmasbi et\u00a0al. reported Co(II) and Ni(II) complex immobilized mesoporous silica and enzymes supported metal complex for the investigation of antibacterial activity against gram-positive and gram-negative [23]. 2D and 3D porous clay nanostructure were also synthesized and Ni-\u03b2-diimine organometallic complex was applied in oxidation of ethylene oligomerization [24]. There are various reports on Co(II) and Ni(II) complex anchored functionalized mesoporous materials for the application of oxidation of olefins with different epoxides, sulfides, ethylene oligomerization, epoxidation of styrene, olefins etc [25]. S. Sain and their team reported work on synthesis of zeolite and grafted a metal complex Pd2+, Ni2+ for the synthesis of novel catalyst polycyclic hetrocycle for the application of suzuki\u2014miyaura cross coupling reaction with good yield [26]. Nickel (II), Copper (II) and Oxovanadium (IV), complexes) anchored on the pore of Y-zeolite for the application of oxidation reaction with different reactant like styrene, cyclohexane and methyl phenyl sulfide with H2O2 oxidizing agent and author noticed that with maximum conversion achieved of methyl sulphide and high selectivity sulfoxide [27]. Ni (II), Fe (III), Cr (III), Bi (III), and Zn (II) complexes were tethered on cavity of Y zeolite and prepared catalyst is used in the application of oxidation of phenol with oxidizing agent H2O2 with reasonable selectivity of the product [28] and \u03b2-diimine Ni complex anchored on SBA-15 was also used for the application of ethylene oligomerization with 98% selectivity of the product [29]. Ni complex incorporated in acid functionalized Mesoporous moiety for photocatalytic degradation of methylene blue under visible light are found to show 92% degradation of organic pollutants (MB) in 15\u00a0\u200bmin [30].Epoxides are tiny molecules that react with various nucleophiles, including alcohols, amines, water, and others [31\u201337]. In presence of amines, the bonds of epoxides are opened for the synthesis of \u03b2-amino alcohols. However, they can undergo regioselective ring opening of oxiranes with creation of a large variety of attractive intermediates for the synthesis of significant biological impulsive substances and synthetic products with chiral auxiliaries [38]. \u03b2-amino alcohols have established themselves as important components of medical, pharmaceutical chemistry and biological treatment [39\u201343]. Numerous attempts have been includingmade by researchers to improve the electrophilic nature of epoxides through metallic coordination or H bonding [44]. For this aim, several techniques have been utilized include the employment of transition metal halides, alumina, metal amides, metal triflates, alkali metal perchlorates, silica under high pressure, and montmorillonite clay [45\u201352] etc.Other reports are also available on heterogeneous catalysis of epoxide ring opening reaction. M. Mohammadikish et\u00a0al. prepared MOF NH2-MIL-88B(Fe) (Fe-MOF) and post-synthetic modification was done by sulphuric acid. The prepared catalyst was used in epoxide ring opening reaction with 100% conversion of the reactant [53]. S.S. Mortazvai and their team have reported synthesis of multi-functional acid functionalized porous support anchoring ionic liquid, and the prepared heterogeneous catalyst was used in epoxide ring opening reaction of styrene oxide [54]. M. Magre et\u00a0al. prepared magnesium based catalysts Mg-Bu2 and Mg(NTf2)2 which were applied in ring opening of a range of epoxides with excellent yield in secondary and tertiary alcohols [55]. N. Deshpande et\u00a0al. synthesized a series of heterogeneous Lewis acid catalysts for epoxide ring opening reaction [56]. K. N. Tayade reported a work on zirconium triflate grafted mesoporous SBA-15 (ZrTf/S) and applied in epoxide ring opening reaction of amines and alcohol under ambient condition [57]. Mesoporous TiO2\u2013Fe2O3 with strong catalytic activity was used, according to Roy et\u00a0al. to prepare amino alcohols and benzimidazole derivatives [58].This work was motivated by the extensive use of organic group-modified materials and transition metal complexes immobilized on the surface of porous materials as catalysts in important industrial reactions. Our groups have carried out thorough study on inorganic/organic hybrid catalysts like acid/base functionalized, mono, bi and tetranuclear Zn, Ni based complex anchored mesoporous/layered materials [21,30,59\u201362] on epoxide ring opening of propylene oxide and cyclohexene oxide with piperidine, morpholine, aniline, etc. Using a well-established synthesis protocol, tetranuclear Ni complex has been used to anchor onto the surface of thiol-functionalized Na\u2013Y zeolite. Different physiochemical techniques were used to characterize the prepared hybrid materials to verify their shape, crystallinity, phase purity, and functionality in the material. The catalytic activity of tetranuclear Ni complex immobilized thiol functionalized Na\u2013Y zeolite (Na\u2013Y\u2013S\u2013Ni4) was investigated for ring breaking of cyclohexene oxide. Maximum reactant conversion and product selectivity were achieved by fine-tuning of the reaction parameters.Na\u2013Y zeolites was obtained from Sud-Chemie-India Ltd. The received Na\u2013Y was calcined at 540\u00a0\u200b\u00b0C for 5\u00a0\u200bh to remove moisture content.For the modification process the calcined Na\u2013Y zeolite and the silane reagent 3-mercaptopropyltrimethoxysilane (MPTES) were mixed in 30\u00a0\u200bml of anhydrous dichloromethane. The salination was accomplish for one day at room temperature under stirring condition. The final product was rinsed multiple times with anhydrous dichloromethane and dehydrated at 100\u00a0\u200b\u00b0C [63].The tetranuclear Ni complex was synthesized as per the past reported technique [30].An appropriate amount of thiol-modified materials was activated. After that, thiol modified material and metal complex [Ni4(LH)4]\u00b7CH3CN (I) (LH3 = (E)-2-(hydroxymethyl)-6-(((2-hydroxyphenyl)imino)methyl)phenol)) was mixed in presence of anhydrous toluene solvent at 110\u00a0\u200b\u00b0C for 18\u00a0\u200bh. The hybrid catalyst was separated, washed thoroughly and dehydrated at 80\u00a0\u200b\u00b0C and depicted in Scheme 1\n [21].The catalytic performance was evaluated in a round bottle flask equipped with a condenser under stirring. Na\u2013Y zeolite as a catalyst had been modified with a thiol organic group (Na\u2013Y\u2013SH) and Ni4 complex was heated to 80\u00a0\u200b\u00b0C for an hour before the reaction. Over Na\u2013Y\u2013S\u2013Ni4 materials, various electrophilic ring breaking reactions of cyclohexene oxide with amines were carried out. Gas chromatography (Scientific Thermo Fisher Trace 1310, TG 5 column) was used to analyze the reaction's end product. Multiple reactions were carried out in an effort to improve the reaction conditions for maximizing conversion and selectivity. Pyrrolidine and piperidine were used as nucleophiles for ring opening of cyclohexene oxide shown in Scheme 2\n, over Na\u2013Y\u2013S\u2013Ni4. The optimized reaction conditions are as follows: 20\u00a0\u200bmg catalyst, 6\u00a0\u200bh reaction time and 70\u00a0\u200b\u00b0C for the pyrrolidine system and 30\u00a0\u200bmg catalyst, 6\u00a0\u200bh reaction time and 80\u00a0\u200b\u00b0C for the piperidine system.The prepared material was analyzed by different analytical techniques such as scanning electron microscope, powder XRD, N2 sorption, nuclear mass spectroscopy, and thermogravimetric analysis. The morphology and elemental indexing of elements present in the material were analyzed by using Zeiss Ultra-55 machine. Crystallinity of the materials was examined by powder XRD by using empyrean equipment with graphite monochromatized CuK\u03b1 radiation at 40\u00a0\u200bkV and 30\u00a0\u200bmA. The surface area, pore volume and diameter were determined by equipped Autosorb iQ S/N: 1050013927 Station: 1\u00a0\u200bat 77\u00a0\u200bK. Before analysis, the samples were degassed at 400\u00a0\u200b\u00b0C for 16\u00a0\u200bh with flowing N2. The MAS NMR spectra of 29Si and 13C were measured on a Varian 600\u00a0\u200bPS solid NMR spectrometer with a spinning frequency of 7\u00a0\u200bkHz, a 90\u00b0 pulse length of 5.6\u00a0\u200b\u03bcs, and a cycle delay time of 5\u00a0\u200bs. Hexamethyl benzene and 3-(trimethylsilyl) propionic-2,2,3,3-d4 acid sodium salt was implemented as references for the 13C and 29Si chemical shifts. Using a Hitachi STA 7200 equipment, thermogravimetric analysis (TGA) was measured to examine the thermal constancy. At room temperature, IR spectra of modified materials were recorded using an FT-IR spectrometer (PerkinElmer, Spectrum Two) with a resolution of 4\u00a0\u200bcm\u22121. For elemental analysis, ICP-OES was carried out utilizing the Seiko SPS7000 instrument.X-ray diffraction pattern of Na\u2013Y zeolite, Na-Y-SH zeolite and Na\u2013Y\u2013S\u2013Ni4 was logged up to 2\u03b8 value of 10\u201350\u00b0 and it is found to be well matched from the literature [64] and depicted in Fig.\u00a01\n. The XRD of Pure Ni4 complex was compared with Na\u2013Y\u2013S\u2013Ni4 (Fig.\u00a01 B) and two peaks at 11.2 and 25.6 2\u03b8 value are matching with each other. However, there are clear overlapping of the peaks of Na\u2013Y zeolite with the pure complex. Very less amount of metal complex was incorporated to solid support as well as it is anticipated that the guest moieties are homogeneously dispersed so it is difficult to get distinct peaks for complex in the NaY\u2013S\u2013Ni4 material in X-ray diffraction patterns. The successful immobilization of Ni4 was confirmed by ICP analysis and SEM EDX, FT-IR etc. Peak intensity decreased when the Na\u2013Y zeolite was functionalized with the thiol group and Ni complex, this could be because the metal complex and thiol functionality were tethered to the support's channel.\nFig.\u00a02\n(A and B) illustrates the SEM image of the catalyst. Based on the findings of the SEM study, it is apparent that Na\u2013Y zeolite appears as granules with a cuboid-shaped surface. After the modification of Na\u2013Y zeolite, the morphology sustain.The average particle size of the materials is found to be 0.5\u00a0\u200b\u03bcm approximately. The EDX analysis of a particular area of Na\u2013Y\u2013S\u2013Ni4 is depicted in Fig.\u00a02(C) and the occurrence of C, O, Na, Al, Si and Ni signals in EDX mapping confirm existence of Ni complex in the modified Na\u2013Y zeolite.To determine the functional group present in the material and bond formation between the metal complex to the surface of a solid support, FT-IR was performed. FT-IR spectra Na\u2013Y, Na\u2013Y\u2013S\u2013Ni4 and pure Ni complex were shown in Fig.\u00a03\n(A). The vibrational bands at 3423\u00a0\u200bcm\u22121 and 1620\u00a0\u200bcm\u22121 are responsible for O\u2013H stretching of silanol groups' hydrogen bonds [65]. Two strong bands at 998\u00a0\u200bcm\u22121 and 788\u00a0\u200bcm\u22121 were noted as responsible for symmetric and asymmetric stretching vibration of the Si\u2013O\u2013Si, [66]. Presence of peak at 1375\u00a0\u200bcm\u22121 which is refer to \u2013CH2 comes from the propyl chain that confirms the successful functionalization of thiol group in the material [67]. Another two peaks were observed at 737\u00a0\u200bcm\u22121 and 855\u00a0\u200bcm\u22121 corresponding to Ni\u2013O and Ni\u2013OH [68,69] respectively. The above bands confirm the existence of Ni complex in the hybrid catalyst.To determine the thermal stability of the materials, thermogravimetric analysis was examined. The TGA of Na\u2013Y, Na\u2013Y\u2013S\u2013Ni4 and Ni complex are depicted in Fig.\u00a03(B). In pure sample Na\u2013Y only one stage of the weight (4.5%) was observed at 348\u00a0\u200bK which is related to humidity. There are two stages of the weight loss observed in Na\u2013Y zeolite, the first stage of weight loss at 336\u00a0\u200bK was responsible for the moisture present in the material and the second stage of weight loss at 425\u00a0\u200bK was responsible for the functional group/metal complex present in the material. In case of pure Ni complex, steady weight loss was observed above 700\u00a0\u200bK. The overall weight loss Ni complex incorporated Na\u2013Y zeolite is 26% and Ni complex is 70%. From the analysis it is clear that the thermal stability of the pure Ni-complex enhances after incorporation in porous supportIn order to determine the textural and structural features of Na\u2013Y nonstructural zeolite, N2 adsorption-desorption analysis was evaluated. N2 adsorption-desorption isotherms of Na\u2013Y zeolite and Na\u2013Y\u2013S\u2013Ni4 are shown in Fig.\u00a04\n(A) which exhibits type I isotherm that demonstrates the preservation of a microporous structure of Na\u2013Y zeolite. After the modification, pore volume decreases due to the immobilization of the organic group and metal complex into the channel which confirms the successful functionalization and anchoring of the metal complex into the functionalized surface. The surface area, pore volume and pore diameter of modified and unmodified Na\u2013Y zeolite are given in Table\u00a01\n and pore size distributions curve given in Fig.\u00a04(B).The ICP-OES method was used to determine the metal content, which is summarized in Table\u00a01. The Si/Ni ratio in Na\u2013Y\u2013S\u2013Ni4 is found to be 160, indicating that the tetranuclear Ni complex has been immobilized in the functionalized Na\u2013Y zeolite.\nFig.\u00a05\n(A) displays the 29Si MAS NMR spectroscopy of Ni complex anchored thiol-functionalized Na\u2013Y zeolite materials. In 29Si MAS NMR of Na\u2013Y\u2013S\u2013Ni4, different silaonic environments like Q4 (4Si, 0Al), Q3 (3Si, 1Al), Q2 (2Si, 2Al) and Q1 (1Si, 3Al) chemical shifts at -105, -100, -94 and -89\u00a0\u200bppm respectively were detected [70]. In order to approve the effective functionalization and metal complex incorporation in Na\u2013Y zeolite 13C CP MAS NMR was also examined and shown in Fig.\u00a05(B). Peaks at 15, 21 and 45\u00a0\u200bppm are assigned for C1, C2 and C3 carbon groups from the propyl chain attached in MPTES [71] indicating of successful anchoring of thiol group. Peaks in the range of 120\u2013200\u00a0\u200bppm attributed for the phenyl benzene ring attached with the ligand was also present in the NMR spectra [61].After the confirmation of the presence of organic functionality and Ni4 complex, the catalytic activity of the prepared materials was checked in epoxide ring breaking reaction with various nucleophiles. Pyrrolidine and piperidine are used as a nucleophile for ring opening of cyclohexene oxide. The catalytic activity of the Na\u2013Y\u2013S\u2013Ni4 is explained in Fig.\u00a06\n.\nFig.\u00a06 llustrates that the metal complex-anchored materials are more effective than the thiol-functionalized materials and pure Na\u2013Y zeolite in the epoxide ring breaking reaction with 20\u00a0\u200bmg of catalyst for the pyrrolidine system and 30\u00a0\u200bmg of catalyst for the piperidine system. Catalyst amount, time and temperature are important parameters for optimizing the reaction system. In comparing the activities of unmodified Na\u2013Y zeolite, thiol-functionalized Na\u2013Y zeolite and Na\u2013Y\u2013S\u2013Ni4, it was found that the conversion of pyrrolidine and piperidine increased dramatically and exhibited 89% and 90% respectively. In contrast, the conversion of pyrrolidine and piperidine in blank reactions was only 12% and 24%, and 24% and 33% with thiol-functionalized respectively. There was a clear increase in catalytic activity after Ni complex immobilization with thiol-functionalized Na\u2013Y zeolite, suggesting that Ni complex had outstanding catalytic characteristics. Electrophilic activation, or the sturdy electron deficiency of the Ni+2 ion delocalizing the negatively charged oxygen atom in the intermediate state, intensely boosts the catalytic activity of Na\u2013Y\u2013S\u2013Ni4, making the hybrid material highly efficient.The catalyst dosage was changed from 10 to 20\u00a0\u200bmg to establish the catalytic system using various nucleophiles at temperature of 70\u00a0\u200b\u00b0C for 6\u00a0\u200bh reaction period in the pyrrolidine system and 20\u201330\u00a0\u200bmg at 80\u00a0\u200b\u00b0C for 6\u00a0\u200bh reaction period in the piperidine system to achieve the highest conversion. As the quantity of catalyst increases, the reactant's conversion also rises, as illustrated in Fig.\u00a07\n. The conversion of pyrrolidine and piperidine were shown in Fig.\u00a07(A and B). From 10 to 20\u00a0\u200bmg of catalyst, pyrrolidine conversion improved from 50 to 89% and from 20 to 30\u00a0\u200bmg of catalyst, piperidine conversion improved from 76 to 90%. However, after increase in catalyst amount from 20 to 25\u00a0\u200bmg in pyrrolidine reaction system 10% decrease in conversion of reactant was observed and increase in catalyst amount from 30 to 35\u00a0\u200bmg in piperdine reaction system 3% decrease in conversion was observed.After doing all the reactions with different catalyst amounts in both pyrrolidine and piperidine systems it is noticed that 20\u00a0\u200bmg catalyst is an optimized catalyst for pyrrolidine system and 30\u00a0\u200bmg catalyst for piperidine system as this amount of catalyst gives maximum conversion of reactant.Reaction kinetics is a key determinant of catalytic behavior for a reaction, and the kinetics is investigated for all the reactions. The reaction time was varied from 6 to 10\u00a0\u200bh in both pyrrolidine at 20\u00a0\u200bmg of catalyst, 70\u00a0\u200b\u00b0C and piperidine at 30\u00a0\u200bmg of catalyst and 80\u00a0\u200b\u00b0C, as shown in Fig.\u00a08\n(A and B). Epoxide ring breaking of cyclohexene oxide with pyrrolidine for 6\u00a0\u200bh, 89% conversion was observed, in 8\u00a0\u200bh reaction time, 75% and in 10\u00a0\u200bh reaction time, 70% conversion was observed. The conversion of pyrrolidine falls down as reaction time increases.During epoxide ring breaking of cyclohexene oxide with piperidine for 6\u00a0\u200bh, 90% conversion was noticed, while in 8\u00a0\u200bh 94% and in 10\u00a0\u200bh 70% conversion was observed. With the rise in reaction time from 6 to 8\u00a0\u200bh conversion also increased but with further increase in reaction time from 8 to 10\u00a0\u200bh, decrease in conversion of piperidine was observed. After performing the reaction in both the systems, 6\u00a0\u200bh time is optimized to get maximum conversion.The reaction temperature was varied from 60 to 75\u00a0\u200b\u00b0C for pyrrolidine and 70 to 100\u00a0\u200b\u00b0C for piperidine taking 20\u00a0\u200bmg and 30\u00a0\u200bmg of catalyst and 6\u00a0\u200bh reaction time is shown in Fig.\u00a09\n. At low-temperature, the reaction's conversion is also low; after an increase in temperature, conversion of the reactant also increases and further increase in temperature 90 to 100\u00a0\u200b\u00b0C, conversion of the reactant decreases. At 60\u00a0\u200b\u00b0C, 74% pyrrolidine conversion was observed, at 65\u00a0\u200b\u00b0C, 70\u00a0\u200b\u00b0C and 75\u00a0\u200b\u00b0C, 79%, 89% and 80% conversion of pyrrolidine was observed. After varying the reaction temperature 70\u00a0\u200b\u00b0C was considered as optimized reaction temperature for this reaction system.Similarly, with piperidine, at 70\u00a0\u200b\u00b0C, 83%, 80\u00a0\u200b\u00b0C 90% and 90\u00a0\u200b\u00b0C 99% conversion of piperidine was observed. The optimized reaction temperature for piperidine system is taken as 80\u00a0\u200b\u00b0C because a reasonable conversion was achieved (90%) at that temperature. Reaction at higher temperature was avoided at the cost of conversion.This reaction involves ring opening of cyclohexene oxide using Na\u2013Y\u2013S\u2013Ni4 catalyst. In the first step, the bond breaking of the cyclohexene oxide takes place due to high ring tension. In the subsequent step, the catalyst got attached to oxygen to form an intermediate complex. Later, the lone pair of the nucleophile attacks the carbocation center of the intermediate complex leading to the formation of bond between them. Intermolecular proton transfer takes place from nitrogen to oxygen to form product leaving the catalyst free. The reaction mechanism between cyclohexene oxide with pyrrolidine and piperidine are shown in Scheme 3\n.The reusability of the prepared Na\u2013Y\u2013S\u2013Ni4 catalyst was also investigated as displayed in Fig.\u00a010\n. The catalytic activity of the material was persistent even after 4th run and no drastic change in reactivity was observed.FT-IR and powder XRD, was also performed to check the structural integrity of the spent catalysts after the reaction, and the findings are displayed in Fig.\u00a011\n(A and B). The well resolved peaks were found in XRD after reusing the catalyst. The peak intensity has significantly decreased after repeated use of the catalyst may be due to vigorous washing and activating at high temperature. However, all the signature peaks were present even after repeated use. FT-IR analysis also shows that the functionality of the catalyst was restored, including characteristic peaks of the parent materials.In this work, the synthesis of tetranuclear Ni complex and grafting of the complex in thiol functionalized Na\u2013Y zeolite is described. Powder XRD, FT-IR, SEM-EDX, N2 sorption analysis, thermogravimetric analysis (TGA) 13C CP and 29Si MAS NMR confirm the successful grafting of Ni complex. No change in morphology was observed after modification with the preservation of crystallinity, phase purity. The existence of functional groups and Ni-complex in Na\u2013Y zeolite was confirmed by FT-IR and NMR analyses reveals the successful immobilization of the material. Finally, the modified catalyst was used for the synthesis of \u03b2-amino alcohol by epoxide ring opening of cyclohexene oxide with pyrrolidine and piperidine. The designed hybrid catalyst has found to be extremely active and truly heterogeneous in nature and can be used up to 4 times without losing much of activity.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Madhu Pandey acknowledges financial support to SHODH (Scheme of Developing High quality research). The authors acknowledge Dr. Ashish Unnarkat of Pandit Deendayal Energy University, Gandhinagar and Dr. Dheeraj Kumar Singh of Institute of Infrastructure Technology Research and Management (IITRAM), for assistance in TG analyses and FT-IR, respectively.", "descript": "\n                  In this present work, synthesized tetranuclear Ni complex is anchored on thiol functionalized Na\u2013Y zeolite. Structure, phase integrity, shape, thermal stability and the existence of functional groups are identified by using different analytical techniques like powder XRD, N2 adsorption-desorption, Fourier transformed infrared spectroscopy (FT-IR), Nuclear magnetic resolution (29Si MAS NMR and 13C CP MAS NMR), scanning electron microscope (SEM-EDX), thermogravimetric analysis etc. The prepared hybrid catalyst Na\u2013Y\u2013S\u2013Ni4 shows remarkable catalytic property by promoting the formation of high-yield products like \u03b2-amino alcohol by electrophilic ring opening reaction. 89% and 90% conversion of pyrrolidine and piperidine with high selectivity of the desired product are observed in ring breaking of cyclohexene oxide.\n               "}
{"full_text": "Dyes are mainly used in the plastics, textile, paper, rubber, wood, and food industries [1]. The presence of colorant materials in streams is not only aesthetically unpleasant but can also cause problems for the environment by elevating toxicity, oxygen demand levels, and lagging the photosynthetic phenomena by reducing light penetration [2,3]. Dyes belonging to the groups based on anthraquinone, azo, and triphenylmethane are the main pollutants released from industrial effluents [4,5]. Most of the dye laden-effluents are produced by the textile industry, where 10\u201315% of the dye is wasted during the production process [4,6]. One of the dyes released from the textile is phenol red. Its solubility in water and ethanol is 0.77\u00a0g/L and 2.9\u00a0g/L, respectively [7]. Phenol red dye is water-soluble and can be utilized as a pH indicator in various laboratories [8,9]. This dye is used in industries such as pharmaceuticals, textiles, and chemicals, and the wastewater discharged from these industries must be properly treated [9].Various physical, chemical, and biological methods have been tested to eliminate dyes. Physical and chemical methods can be used on a small scale [10]. Advanced oxidation methods (AOMs) have been considered by researchers for cleaning effluents containing resistant organic compounds [11,12]. AOMs are based on the production of active species for degrading pollutants using solar, chemical, or other forms of energy [13,14]. The photocatalytic oxidation process is a superficial phenomenon and light reactions happen on the catalyst surface like TiO2 [11,15]. Titanium dioxide (TiO2) has so far been considered comparable to other semiconductors for many reasons, including its cheapness, availability, safety, and good reactivity [11,16]. Also, it can degrade organic compounds and as a result, the pollutants are converted to H2O and CO2 [16]. Nickel is also a useful photocatalyst that has low cost and good stability in alkaline solutions [17]. However, it is difficult to use titanium dioxide and nickel alone because their separating from the solution is a time-consuming and costly process. Therefore, the photocatalyst should be stabilized on a support surface. Many supporting surfaces like activated carbon [18], graphene oxide [19], silicate [20], and glass spheres [21] have been used for titanium dioxide and nickel oxide.The FSM-16 powder has outstanding features like high thermal stability, high specific surface area, and high pollutant sorption capacity, which makes it a suitable option for photocatalyst support. Although FSM-16 has been used to support photocatalysts to decompose some pollutants [22,23], its application as support of titanium dioxide and nickel oxide in dye removal is not yet clear.Accordingly, the main aim of this study was to evaluate the photocatalytic activity of TiO2-FSM-16 and Ni-FSM-16 in phenol red degradation. Phenol red decomposition using TiO2-FSM-16 and Ni-FSM-16 was examined in the presence of UV rays and the effect of operating parameters was studied. The characteristics of photocatalysts, reaction kinetics, catalyst reusability, leaching of the photocatalyst components, and synergy of process components have also been investigated. In another part of the work, the dye removal efficiency of TiO2-FSM-16 and Ni-FSM-16 was compared with TiO2-carbon and Ni-carbon.Acetyl trimethylammonium bromide (C19H42BrN, purity >99.9%), hydrochloric acid (HCl, 37\u00a0wt%), sodium hydroxide (NaOH, purity >99.9%), silicon dioxide (SiO2, purity >99.8%), ammonium titanyl oxalate monohydrate ((NH4)2TiO(C2O4)2\u00b7H2O, purity: 98%), ethanol (C2H6O, purity \u226599.9%), nickel nitrate (Ni(NO3)2, purity> 99.8%) were purchased from Merck Company. Phenol red (C19H14O5S, purity: 98%), tert-butanol alcohol (C\u2084H\u2081\u2080O, purity \u226599.5%), p-benzoquinone (C6H4(=O), purity >98%), silver nitrate (AgNO3, purity \u226599%), and ammonium oxalate ((NH4)2C2O4\u00b7H2O, purity \u226599.99%) were provided from Sigma-Aldrich Company. The stock solution of the phenol red dye was prepared daily in doubly distilled water (DDW). All working solutions were prepared by diluting the stock with DDW.Morphology of FSM-16, TiO2-FSM-16, and Ni-FSM-16 was done using the TeScan-Mira III Scanning Field Emission Microscope (FE-SEM, Czech Republic). Nitrogen absorption-desorption test was performed by BET instrument, Micrometric model ASAP2020 (USA). The XRD pattern was taken by the PANalytical manufacturer, Xpert-pro model (USA) at 2\u03b8 equal to 0.5\u201360\u00b0 and CuK\u03b1 of 1.5406\u00a0nm. Phenol red content was determined by UV\u2013vis spectrophotometer, PerkinElmer, Lambda 25 (USA) at \u03bb\nmax 423\u00a0nm. An example of a calibration curve to determine the concentration of phenol red is presented in Fig. S1 in the Supplementary Information. The amount of dye mineralization was measured by the TOC device (Shimadzu, model VCSH, Japan).First, 3.8\u00a0g NaOH was added to 100\u00a0mL of DDW and 6\u00a0g of SiO2 was poured into the solution to make the SiO2/Na2O ratio equal to 2. It was then stirred for 3\u00a0h under normal conditions at ambient temperature (27\u00a0\u00b0C) to obtain a uniform solution. To remove the available solvent, the sample was placed in a rotary evaporator balloon. The evaporator temperature was set to 100\u00a0\u00b0C and the speed was 30\u00a0rpm. Approximately 60\u201370\u00a0mL of water was evaporated and resulted in a jelly-like liquid of sodium silicate. Finally, the sample was poured into a porcelain plate and transferred to a furnace to complete the synthesis and calcination process of layered sodium silicate. The furnace was programmed with a temperature rate of 2\u00a0\u00b0C/min at 700\u00a0\u00b0C for 6\u00a0h. The calcined sample was layered sodium silicate or Kanemite. This material was crushed in a mortar and 4\u00a0g of it was poured into 40\u00a0mL of DDW and agitated for 3\u00a0h at 27\u00a0\u00b0C. After the preparation of Kanemite paste, which was used as a source of silica in the synthesis of regular mesopores of FSM-16. Through a mechanism, the Kanemite silicate plates were folded and cross-linked to form a three-dimensional structure. To make a wet Kanemite paste, the resulting solution was filtered. The Kanemite was dispersed in 88\u00a0mL of n-hexadecyl trimethyl ammonium bromide solution (0.125\u00a0mol/L) and stirred for 3\u00a0h at 70\u00a0\u00b0C. In this step, pH should be adjusted between 11.5 and 12.5. After that, the pH was decreased to 8.5 using 2\u00a0mol/L HCl. The suspension solution was stirred for 3\u00a0h at 70\u00a0\u00b0C. The produced material was then washed at room temperature with a liter of distilled water and air-dried. The synthesized meso-cavity was placed in the Soxule device for washing. Washing was done with 20\u00a0mL of ethanol and 0.5\u00a0mL of HCl per gram of material. The meso-cavity material was placed in a Soxule device for 72\u00a0h. The material was then dried for 24\u00a0h at ambient temperature. Finally, it was placed in the furnace (950\u00a0\u00b0C, 9\u00a0h) to remove the surfactant [24].To deposit titanium dioxide on FSM-16, a solution with a concentration of 0.1\u00a0M of titanium oxalate ammonium salt was prepared. Then, 2\u00a0g of FSM-16 was added to 25\u00a0mL of titanium solution. The sample was stirred at room temperature for 24\u00a0h. After filtering and washing the samples with DDW and air-drying, the samples were calcined at 450\u00a0\u00b0C for 12\u00a0h [25].To make the Ni-FSM-16 catalyst, initially, 1\u00a0g of FSM-16 mesoporous silica was dissolved in 20\u00a0mL of normal hexane and stirred at room temperature for 2\u00a0h (Solution A). The amount of 0.5675\u00a0g of nickel nitrate was added to another vessel with 1.14\u00a0mL of DDW (Solution B). After 2\u00a0h, Solution B was added dropwise to Solution A and the new mixture was stirred for 3\u00a0h. It was then air-dried for 24\u00a0h. The solids were subjected to argon gas for calcination at 550\u00a0\u00b0C for 5\u00a0h. The gradient for increasing and decreasing the temperature at this stage was 2.5\u00a0\u00b0C/min and the flow rate of argon gas injection into the furnace was 2\u00a0L/h [26].To make TiO2-carbon and Ni-carbon, the same methods as TiO2-FSM-16 and Ni-FSM-16 were done. It should be noted that activated carbon in this study was purchased from Merck and according to the supplier's information, it had an active surface of 810\u00a0m2/g.Four factors were analyzed by the response surface method. The effect of four experimental variables, namely TiO2-FSM-16 and Ni-FSM-16 quantity, solution pH, UV irradiation time, and dye concentration on the photocatalytic removal of phenol red was explored. The variables and their upper and lower levels are listed in Table S1 in the Supplementary Information.A total of 30 tests were performed, including 16 experiments at factorial points, 8 experiments at axial points, and 6 replications at central points.The efficiency of dye removal was calculated using the following equation:\n\n(1)\n\n\nD\ny\ne\n\nr\ne\nm\no\nv\na\nl\n\n\n(\n%\n)\n\n=\n\n\n(\n\n\n\n[\n\nD\ny\ne\n\n]\n\n\ni\nn\ni\nt\ni\na\nl\n\n\n\u2212\n\n\n[\n\nD\ny\ne\n\n]\n\n\nr\ne\ns\ni\nd\nu\na\nl\n\n\n\n)\n\n\n\n[\n\nD\ny\ne\n\n]\n\n\ni\nn\ni\nt\ni\na\nl\n\n\n\nx\n\n100\n\n\n\n\nThe photocatalysts (TiO2-FSM-16 and Ni-FSM-16) were recovered after being used in dye removal. The photocatalysts were separated using a centrifuge, washed (with water and ethanol), dehydrated (at 105\u00a0\u00b0C), and then reused for dye degradation. The reusability tests were done 7 times.Compounds of tert-butanol (500\u00a0mM), p-benzoquinone (10\u00a0mM), silver nitrate (10\u00a0mM), and ammonium oxalate (50\u00a0mM) were used as scavengers of active species and radicals in the process of phenol red removal [27]. This test was performed at optimal conditions (pH: 3, dye concentration: 20\u00a0mg/L, catalyst dose: 2\u00a0g/L, time: 120\u00a0min).The micrographs (FESEM) of the FSM-16 are presented in Fig. 1\nA, B. Based on the FESEM graphs, nanoparticles with an approximate size of 200\u00a0nm were observed. In the study conducted by Hashemi et al., the FSM-16 nanoparticles have a spherical shape with a size of 100\u00a0nm [28]. The FESEM images of TiO2-FSM-16 are shown in Fig. 1C,D. As this figure shows, the photocatalyst particles are non-uniform and spherical. TiO2-FSM-16 particles have a similar shape to those reported in other works [22]. This particle size distribution with spherical morphology corresponded to van der Waals forces [29]. To decrease the surface energy, the primary particles tend to condense, with the formation of spherical masses, in the minimum surface-to-volume ratio, the minimum free surface energy can be obtained. The TiO2-FSM-16 composite powder is composed of nanoscale particles, which indicates that the prepared powder has a large specific surface area and volume. Therefore, the TiO2-FSM-16 composite can provide suitable active sites for photocatalysis. Fig. 1C,D shows that TiO2 particles are dispersed on the surface of SiO2 nanoparticles and have good stability.The shape and size of crystals in Ni-FSM-16 are slightly different from TiO2-FSM-16 (Fig. 1E,F). The Ni-FSM-16 particles have a small size and are aggregates of spherical microcrystal particles. Silicate plates are a combination of tens to hundreds of hexagonal cavities composed of Ni-FSM-16 catalyst particles. Adhesion between particles may be due to the small magnetism or the polymer between them [30,31].The XRD image of FSM-16, TiO2-FSM-16, and Ni-FSM-16 is shown in Fig. 2\n. A wideband with an angle equal to 20\u201330\u00b0 can be seen in the FSM-16 spectrum, revealing that the material is amorphous and has no specific crystalline peaks. The disappearance of the higher-order diffraction peaks indicates that the hexagonal arrangement of the channels in the TiO2-FSM-16 and Ni-FSM-16 mesoporous materials is slightly irregular. Therefore, it can be concluded that the addition of metal in the mesoporous structure of FSM-16 has a negative effect on the crystallinity of the material. This is because the use of metal ores weakens the self-assembly process and produces a less regular meso-cavity structure. In the XRD pattern of TiO2-FSM-16 and Ni-FSM-16, in addition to the four distinct peaks of the FSM-16 nanoparticles, the characteristic peaks of TiO2 and NiO are also seen, indicating no noticeable change in the crystal structure of the photocatalysts [32]. The low-angle XRD image of FSM-16 showed that the diffraction plate has a regular hexagonal structure [33]. An anatase-like reference pattern (JCPDS # 21\u20131272) was seen in TiO2-FSM-16 [34]. Five peaks at 37, 43, 48, 52, and 58 in Ni-FSM-16 photocatalyst are related to Miller indices of (111), (200), (220), (311), and (222), respectively, which confirms that the photocatalyst contains NiO [35\u201337]. Based on the literature [38], a main reflection at 2theta of 43.5\u00b0, corresponds to the (200) plane of cubic NiO (PDF-2, 01\u2013071\u20131179).The active area of the photocatalysts was computed using Brunauer-Emmett-Teller (BET) technique and the pore diameter and pore volume were calculated by the BJH technique (Fig. 3\na\u2013c). For the FSM-16 sample, the obtained BET was 1099.08\u00a0m2/g, and the absorption isotherm is almost synchronous with the desorption branch, indicating the mesoporous structure of the sample. Fig. 3b and c shows a similar pattern of BET for TiO2-FSM-16 (844.93\u00a0m2/g) and Ni-FSM-16 (718.63\u00a0m2/g), showing that the mesoporous feature of FSM-16 did not alter after the composition with Ni and TiO2 [39].The specific surface area of Ni-FSM-16 and TiO2-FSM-16 was lower than that of FSM-16, which is linked to the occupation of pores by Ni and TiO2 (Fig. 3). However, the level of synthesized photocatalyst (844.93\u00a0m2/g) is much bigger than the values stated for other photocatalysts in the literature. For example, Fatima and Supia prepared the TiO2-MCM-41 photocatalyst with a surface of 400.7\u00a0m2/g [40]. Sugiyama et al. have incorporated Ni into FSM-16 and MCM-41 structures to produce Ni-FSM-16 and Ni-MCM-41 and reported that the active surface of photocatalysts was lower than that of the base material [41].ANOVA was utilized to determine the relationship between the removal rate and the variables. It is noteworthy that the judgment was based on the F-values and P-values for phenol red removal. The F-values of the model for TiO2-FSM-16 and Ni-FSM-16 were obtained at 16.77 and 12.16, respectively, indicating that the models are significant. The P-value was <0.05 indicating the significance of the model expression. In this case, the \u2018A\u2019 variable in TiO2-FSM-16 and \u2018A, B, and D\u2019 in Ni-FSM-16 were significant. In addition, the validity of the design with values of P and high value of correlation coefficients as well as the non-significance of \u201cLack of Fit\u201d is confirmed [42]. P\u00a0<\u00a00.0001 reveals the importance of the model and the interaction of variables on phenol red elimination. The pH factor played an important role in phenol red removal using both photocatalysts (TiO2-FSM-16 and Ni-FSM-16) with the biggest F-value (60.56 and 133.57, respectively) compared to other variables (Table S2 and S3). To evaluate the quality of the models, the figures for the predicted response vs the actual values and the normal data distribution diagrams are shown in Fig. S2. The normal diagram of the residues is provided in Fig. S2a,c. Such a graph is very useful for optimizing complex systems such as multivariate optimization. The points are in a straight line and no deviations are seen in the distribution of data, indicating a suitable correlation and distribution between the values [43]. Since the specific trend is not related to variance changes (decrease or increase), the variance is fixed, which shows the scatter of points against the given values (Fig. S2b,d).Investigation of the effect of the desired catalysts (TiO2-FSM-16 and Ni-FSM-16) on phenol red degradation efficiency under ultraviolet radiation is depicted in Fig. S3a. In this figure, the positive and negative effects and the magnitude of the effect of each variable on the response are identified. The sharpness of the slope in a factor indicates that it is a vital variable in the reaction [5]. Oppositely, a relatively flat line shows the insensitivity of the response to alteration in the given variable. As can be seen in Fig. S3a, the TiO2-FSM-16 curve shows the time and dye concentration of the slow curvature, showing that these factors have a small effect on the response. Accordingly, the significant sloping curvature at pH indicates that the phenol red removal was sensitive to this variable. Fig. S3b shows that the curvature time and dye concentration have a slow slope, indicating that the mentioned factors have a small impact on the response.In Fig. 4\na\u2013d, the response surface (3D surfaces) and contour diagrams (2D contours) are depicted as a function of pH and photocatalyst dose. For both generated materials (TiO2-FSM-16 and Ni-FSM-16), while the irradiation time (120\u00a0min) and the phenol red concentration (20\u00a0mg/L) were constant, the amount of degradation decreases with increasing pH. In other words, for both catalysts, the highest efficiency occurred at pH 3. At acidic pHs, the phenol red molecule has the zwitterion form with two functional groups (sulfate with a negative charge and ketone with a positive charge) with an additional proton. The FSM-16-TiO2 surface also has protonated functional groups at low pH. The dye molecule has positive and negative charges at low pH. Therefore, the maximum efficiency at the acidic pH of 3 is due to the electrostatic attraction between protonated groups and zwitterion negative charges [44]. If the pH of the solution is higher than the pKa of phenol red (about 7.9), the proton of the ketone group is lost and the molecule takes on a negative charge [45]. The higher the pH, the more negative the catalyst level becomes (pHzpc of both photocatalysts was about 5.4). In such a situation, the dye molecule is rejected from the surface of the catalysts, and as a result, the removal efficiency decreases. Kumar et al. [46] reported that at acidic pHs, the surface of titanium dioxide becomes positive, leading to higher dye removal.With the increasing load of TiO2-FSM-16, a slight change in removal efficiency was observed. This has been linked to the tendency of particles to accumulate, which reduces the BET area of the catalyst and leads to less production of active radicals [47,48]. For the Ni-FSM-16, increasing the photocatalyst mass did not play a role in increasing the efficiency and the graph was smooth. Higher content of TiO2-FSM-16 than Ni-FSM-16 can lead to more radical production and thus higher efficiency [25]. Similar observations have been reported by researchers for degrading cephalexin using NiS and NiS-support Fe3O4@PPY photocatalysts [49].The effect of treatment time and photocatalyst dose on the phenol red elimination is depicted in Fig. 5\na\u2013d. As shown in Fig. 5, increasing the dose of TiO2-FSM-16 photocatalyst has resulted in a slow increase in the removal efficiency, while with increasing Ni-FSM-16 the efficiency has remained almost constant. With more time of exposure of TiO2 nanoparticles to light, the dye removal efficiency increases. Longer exposure to light implies the production of more hydroxyl radicals, which are responsible for the oxidation of the phenol red dye molecule [50].With increasing filtration time, the amount of dye removal by the TiO2-FSM-16 photocatalyst increased due to the sufficient opportunity for radicals to attack the dye. But with increasing time, the removal efficiency by Ni-FSM-16 decreases, which is probably due to the desorption of absorbed dye from the catalyst surface. So, it can be confirmed that the Ni-FSM-16 photocatalyst may not have been able to oxidize the dye but rather remove it by the adsorption process.A recyclable photocatalyst would be economically and environmentally beneficial. The photocatalyst recovery results are shown in Fig. 6\na. Based on Fig. 6a, the reusability behavior of the two produced materials was different. The TiO2-FSM-16 photocatalyst has higher usability and good strength. TiO2-FSM-16 catalyst could be reused up to 3 times, while Ni-FSM-16 had up to 2 times good removal efficiency. The reduction in the efficiency of the recycled catalyst can be due to the leakage of its effective components. In these two photocatalysts, nickel and titanium are important components. The results of Fig. 6b show that these elements have leaked from the photocatalyst. Another significant point is that the amount of nickel leakage is lower than the drinking water standard (100\u00a0\u03bcg/L). Among the two prepared catalysts, the amount of leakage in TiO2-FSM-16 was lower, which indicates its stability and, as a result, its higher efficiency.In Fig. 7\n, the effect of system components on the removal efficiency of phenol red dye is compared with the whole system. As depicted in this figure, the efficiency of the system components and even UV along with nickel and titanium dioxides were much lower than the whole system. FSM-16 has been reported to have a 51% removal of phenol red, possibly due to the adsorption mechanism. The high surface of the FSM-16 material has also provided a suitable platform for dye adsorption. The synergy factor for the TiO2-FSM-16/UV was calculated based on the following formulas [51]:\n\n(2)\n\n\n\n\nS\ny\nn\ne\nr\ng\ny\n\nf\na\nc\nt\no\nr\n\n\nT\ni\nO\n2\n\u2212\nF\nS\nM\n\u2212\n16\n\n\n=\n\n\nk\n\no\nb\ns\n\n\nT\ni\nO\n2\n\u2212\nF\nS\nM\n\u2212\n16\n\n\n\n\nk\n\no\nb\ns\n\n\nT\ni\nO\n2\n\n\n+\n\nk\n\no\nb\ns\n\n\nF\nS\nM\n\u2212\n16\n\n\n\n\n\n\n\n\n\n\n(3)\n\n\n\n\nS\ny\nn\ne\nr\ng\ny\n\nf\na\nc\nt\no\nr\n\n\nT\ni\nO\n2\n\u2212\nF\nS\nM\n\u2212\n16\n/\nU\nV\n\n\n=\n\n\nk\n\no\nb\ns\n\n\nT\ni\nO\n2\n\u2212\nF\nS\nM\n\u2212\n16\n/\nU\nV\n\n\n\n\nk\n\no\nb\ns\n\n\nT\ni\nO\n2\n\n\n+\n\nk\n\no\nb\ns\n\n\nF\nS\nM\n\u2212\n16\n\n\n+\n\nk\n\no\nb\ns\n\n\nU\nV\n\n\n\n\n\n\n\n\nThe synergistic factor for the TiO2-FSM-16 and TiO2-FSM-16/UV processes were computed at 1.55 and 2.12, respectively. These findings reveal the effective interaction of UV, TiO2, and FSM-16 components with each other that lead to elevated degradation of phenol red dye.Components like hydroxyl radicals (OH\u2022), superoxide (O2\n\u2212\u2022), photogenerated holes (h\u03d1+), and electrons (e\u2212) can be effective in the photocatalytic dye removal process [52\u201354]. According to scientific papers [43,47], the compounds of tert-butanol, p-benzoquinone, silver nitrate, and ammonium oxalate are effective in abducting the active components of hydroxyl radicals (OH\u2022), superoxide (O2\n\u2212\u2022), electrons, and photogenerated holes (h\u03d1+), respectively. To clarify the decomposition mechanism, the effect of these scavengers on dye removal efficiency was explored and the results are drawn in Fig. 8\na. According to Fig. 8a, it is clear that the amount of dye removal has decreased with the addition of scavengers. The most severe decrease in phenol red removal efficiency was for ammonium oxalate, followed by silver nitrate, p-benzoquinone, and tert-butanol. This indicates that photogenerated holes (h\u03d1+) were the most effective species in the photocatalytic process of phenol red removal. Similar results have been stated for the catalytic elimination of p-aminophenol and methylene blue dye by TiO2/RGO catalyst [47] and Brilliant green by MgFe2O4 catalyst [43].The kinetics of the phenol red degradation process was evaluated under optimal conditions (see Fig. 8b). The first-order equation was utilized to assess the kinetic behavior of phenol red catalytic decontamination:\n\n(4)\n\n\nLn\n\n\n[\n\nf\ni\nn\na\nl\n\nd\ny\ne\n\nc\no\nn\nc\n.\n\n]\n\n\n[\n\ni\nn\ni\nt\ni\na\nl\n\nd\ny\ne\n\nc\no\nn\nc\n.\n\n]\n\n\n=\n\u2212\nk\n.\nt\n\n\n\n\nIf the graph \n\nLn\n\n\n[\n\nf\ni\nn\na\nl\n\nd\ny\ne\n\nc\no\nn\nc\n.\n\n]\n\n\n[\n\ni\nn\ni\nt\ni\na\nl\n\nd\ny\ne\n\nc\no\nn\nc\n.\n\n]\n\n\n\n\nvs time is drawn, a line is attained, and its slope is the reaction rate constant (k, min\u22121). As shown in Fig. 8b, the data follow first-order kinetics (R2\u00a0>\u00a00.96). The phenol red photodegradation rate constant using TiO2-FSM-16 and Ni-FSM-16 was calculated at 0.028 and 0.018 min\u22121, respectively. The reaction rate with TiO2-FSM-16 photocatalyst was about 1.5 times Ni-FSM-16. The kinetics of dye removal by various photocatalysts have followed the first-order model [43,52,55].The amount of dye mineralization was investigated by two photocatalysts. The results showed that under optimal conditions (pH: 3, dye concentration: 20\u00a0mg/L, photocatalyst dose: 2\u00a0g/L, time: 120\u00a0min), the amount of dye mineralization by TiO2-FSM-16 and Ni-FSM-16 is 46% and 35%, respectively. It shows the effectiveness of the TiO2-FSM-16 photocatalyst in dye removal compared to Ni-FSM-16. The results of this study are in the range of values reported for acid orange removal using BiVO4/TiO2 in the presence of H2O2 and FeSO4 [56].Several methods have been reported so far for removing dyes. Among these methods, researchers emphasize the oxidation and adsorption of dyes. Table 1\n compares the methods mentioned in scientific texts for dye removal with our method. As can be seen, concerning the conditions of the tests, the photocatalysts in this paper are among the satisfactory catalysts for dye removal.As it is clear in Table 1, the laboratory conditions of previously published work are different from our work, and an exact comparison cannot be made. Therefore, activated carbon as the most famous base for the catalyst was made under the same conditions as the catalysts of TiO2-FSM-16 and Ni-FSM-16 of this research and was investigated in the removal of the target pollutant. As shown in this table, the removal efficiency of carbon-based photocatalysts (BET: 810\u00a0m2/g) is lower than that of FSM-based ones. These differences are probably due to the lower surface area that carbon provided for the reaction compared to FSM.In this study, TiO2-FSM-16 and Ni-FSM-16 photocatalysts were produced and used to remove phenol red dye in the presence of UV light. The TiO2-FSM-16 and Ni-FSM-16 photocatalyst had a BET area of 844.93\u00a0m2/g and 718.63\u00a0m2/g, respectively. Maximum phenol red removal using TiO2-FSM-16 and Ni-FSM-16 was obtained at 84% and 66%, respectively under the conditions of pH 3, dye concentration of 20\u00a0mg/L, catalyst dose of 2\u00a0g/L, and irradiation time of 120\u00a0min. The photocatalyst activity of FSM-16 was increased after the corporation with TiO2 and Ni. The synergistic factor of TiO2-FSM-16 and TiO2-FSM-16/UV processes were found to be 1.55 and 2.12, respectively. The TiO2-FSM-16 photocatalyst with phenol red dye removal of 87% (TOC removal: 46%) had better activity than Ni-FSM-16 with 76% removal (TOC removal: 35%). Both photocatalysts, especially TiO2-FSM-16, had good capabilities in removing phenol red dye.Seyed Mohamadsadegh Mousavi: Conceived and designed the experiments; Performed the experiments.Seyed Hamed Meraji: Analyzed and interpreted the data; Wrote the paper.Ali Mohammad Sanati: Contributed reagents, materials, analysis tools or data; Performed the experiments.Bahman Ramavandi: Analyzed and interpreted the data; Wrote the paper.This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.Data will be made available on request.Supplementary content related to this article has been published online at [URL].The authors declare no conflict of interest.We are grateful to Bushehr University of Medical Sciences for access to laboratory equipment.The following is the supplementary data related to this article:\n\nSupplementary information-R1\nSupplementary information-R1\n\n\n\nSupplementary data related to this article can be found at https://doi.org/10.1016/j.heliyon.2023.e14488.", "descript": "\n                  In this study, the performance of Ni-FSM-16 and TiO2-FSM-16 photocatalysts in phenol red removal was explored. The XRD, FE-SEM, and BET tests were used to characterize the catalysts. All experiments were performed at ambient temperature and under UV (20\u00a0W). The parameters including dye concentration (20\u201380\u00a0mg/L), photocatalyst concentration (0\u20138\u00a0g/L), UV exposure duration, and contact time (0\u2013160\u00a0min) were optimized using RSM software. BET values of Ni-FSM-16 and TiO2-FSM-16 were 718.63\u00a0m2/g and 844.93\u00a0m2/g, respectively. TiO2-FSM-16 showed better performance in dye removal than Ni-FSM-16. At pH 3, the maximum dye removal by TiO2-FSM-16/UV and Ni-FSM-16/UV was obtained 87% and 64%, respectively. The positive hole species had the main role in photocatalytic phenol red removal. The reusability study was done for up to 7 cycles, but the catalysts can be reused effectively for up to 3 cycles. The synergistic factor for the TiO2-FSM-16 and TiO2-FSM-16/UV processes were calculated to be 1.55 and 2.12, respectively. The dye removal efficiency by TiO2-carbon and Ni-carbon was slightly lower than those obtained by the FSM-16 ones. The TiO2-FSM-16 and Ni-FSM-16 catalysts had a suitable surface and acceptable efficiency in phenol red removal.\n               "}
{"full_text": "Data will be made available on request.CH4 reforming with CO2 (also known as dry reforming of methane, or DRM) has attracted increasing interest in building a sustainable carbon-neutral society, as it can convert two main greenhouse gases into higher value fuels and chemicals while alleviating the negative effects of greenhouse gas emissions [1,2]. Moreover, since CO2 and CH4 are the main components of biogas, this process is also appealing for the efficient utilization of biogas without CO2 separation. The main product of this process - syngas (CO + H2) is an important feedstock for making chemicals and fuels (e.g., Fischer-Tropsch process and methanol synthesis) [3].The thermal-catalytic route has received the most attention for DRM research, and its feasibility has been demonstrated in small-scale industrial cases [4]. However, this reaction is highly endothermic because of the highly stable reactants (CH4 and CO2), and thus high temperatures (700\u2013900\u00a0\u00b0C) are typically required to achieve reasonable performance. These harsh reaction conditions result in not only high energy input and operational costs, but also coke deposition and the deactivation of catalysts [5]. Therefore, it is of great interest to explore new processes for DRM under mild conditions.Non-thermal plasma (NTP) technology is an emerging approach for conventional chemical reactions (e.g., DRM) due to its unique non-equilibrium property [6,7]. NTP can be generated by gas discharges under mild conditions such as atmospheric pressure and near room temperature. NTP contains a large number of reactive species, including highly energetic electrons, excited ions, molecules, atoms and free radicals, which are the primary driving forces for thermodynamically unfavorable chemical reactions [8\u201311]. NTP-enabled processes can start-up and shut-down very quickly, compared to the long-time heating-up and cooling-down processes in thermal catalytic systems. This allows NTP processes to be easily powered by intermittent renewable energy sources (e.g., wind and solar energy) [12], reducing fossil fuel consumption and greenhouse gas emissions. In addition, a synergistic effect may be generated when catalysts are introduced into the NTP process under suitable conditions [13]. In this case, the catalysts are activated at low temperatures and stabilized by collisions between energetic electrons and other reactive species, thereby promoting reaction performance. These factors may provide a cost advantage for plasma-catalytic DRM over thermal catalytic route.Different NTP systems have been investigated for plasma chemical reactions, including dielectric barrier discharge (DBD), spark discharge, gliding arc, corona discharge and microwave discharge [14]. DBD is the most commonly used NTP in plasma chemical processes due to its simple structure and ease of plasma-catalyst coupling [15]. DBD coupled with catalysis has been extensively explored in a number of areas, including volatile organic compound (VOC) removal, CO2 and CH4 conversion, tar reforming, NH3 synthesis, etc [2,16\u201325]. Extensive research has been conducted on the plasma-catalytic DRM using DBDs, evaluating the influence of process parameters, catalysts, and reactor configurations on reaction performance. Nevertheless, the energy efficiency of this process is still insufficient for commercialization [15].The catalytic material is critical in the plasma-catalytic reforming process. Various packing materials with both non-catalytic and catalytic properties have been used for plasma-catalytic DRM, including glass beads, zeolites, metal oxides, supported catalysts with both transition and noble metals, as well as catalysts with perovskites and spinel structure [15,26\u201328]. Among these packing materials, Ni/\u03b3-Al2O3 is the most commonly used catalyst in the plasma-catalytic DRM [13,29\u201331]. Recently, modified Ni catalysts by promoting the active metal and the support, as well as other supported catalysts (e.g., Cu/\u03b3-Al2O3, Pt/\u03b3-Al2O3, Ag/\u03b3-Al2O3 and Au/\u03b3-Al2O3) have also been investigated to enhance gas conversion, product selectivity and process energy efficiency [32\u201343]. According to previous research, the most important factors influencing the plasm-catalytic reforming performance are catalyst properties, discharge characteristics when coupling catalysts and the influence of plasma on catalysts. It is critical to connect these variables to reforming performance, especially gas conversion and energy efficiency, as well as the selectivity of some specific products. Although significant progress has been made in this field through both the experimental and simulation approaches [40,43,44], there is still much room to further investigate the relationship between reaction performance and plasma-catalyst coupling for DRM.Herein, we developed a coaxial DBD reactor with a water electrode for plasma-catalytic DRM into syngas, hydrocarbons and oxygenates. Based on previous research [13,36,42,45], the most common supported catalyst (Ni/\u03b3-Al2O3) and other two noble metal catalysts (Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3) were selected for this work. The properties of these catalytic materials were examined using a variety of characterization techniques. Electrical signals were used to investigate the discharge characteristics in plasma catalysis when using different catalysts. The reaction performance was evaluated using the conversion of CO2 and CH4, the selectivity of gaseous and liquid products, and energy efficiency for gas conversion and product formation. The relationship between catalyst properties, discharge characteristics and reaction performance was also discussed.The supported catalysts with different active metals (Ni, Ag and Pt) were prepared using the modified impregnation method with plasma treatment. The metal loading in these catalysts was 10\u00a0wt%, 1\u00a0wt% and 1\u00a0wt%, respectively. The metal precursors Ni(NO3)2\u22196H2O, AgNO3 and H2PtCl6 were dissolved in deionized water, respectively, followed by adding the support (\u03b3-Al2O3 beads with a diameter of 1\u00a0mm). The resulting mixtures were kept at room temperature for 12\u00a0h. After that, the solutions were then evaporated to dry at 80\u00a0\u00b0C, before being dried overnight at 110\u00a0\u00b0C. The obtained samples were treated with Ar/H2 DBD for 40\u00a0min. For plasma treatment, the input power and total gas flow rate were 40\u00a0W and 50\u00a0ml/min, respectively, with an Ar/H2 molar ratio of 4:1. The above prepared catalysts were denoted as Ni/\u03b3-Al2O3, Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3.The N2 adsorption-desorption isotherm experiments of \u03b3-Al2O3 and the supported catalysts were carried out using a BELSORP MAX instrument. The specific surface area (S\nBET) and the total pore volumes (V\np) were obtained at a relative pressure (P/P\n0) of 0.99, while the average pore diameter (D\np) was determined by the BJH method. The X-ray diffraction (XRD) profiles were collected by a Rigaku Smartlab diffractometer with a Cu-K\u03b1 radiation source. Transmission electron microscopy (TEM) analysis was carried out using a FEI Tecnai G2 F20 electron microscope. The CO2 temperature-programmed desorption (CO2-TPD) was performed using a Belcat instrument. Each sample was treated with He flow at 200\u00a0\u00b0C for 1\u00a0h prior to testing, followed by decreasing to 50\u00a0\u00b0C. The pure CO2 flow was then introduced for 1\u00a0h after the catalyst surface had been saturated. The CO2-TPD profiles were collected in a temperature range of 50\u2013700\u00a0\u00b0C. Following the evaluation of catalytic performance, the used catalysts were analyzed by thermogravimetric analysis (TGA) in a simultaneous thermal analyzer (NETZSCH STA 449F3). Each sample was heated from 30\u00a0\u00b0C to 700\u00a0\u00b0C at 10\u00a0\u00b0C/min in a synthetic air flow.\n\nFig. 1 shows the schematic diagram of the experimental setup. The main part of the DBD reactor contained two cylindrical quartz tubes and a stainless-steel rod. These two quartz tubes formed a casing tube with water circulating inside. A stainless-steel needle was inserted into the casing tube through a hole sealed with a rubber gasket. The needle made contact with the circulating water, allowing it to function as a low-voltage electrode. A reference capacitor (7800\u00a0pF) was connected between the low-voltage electrode and ground. The discharge length was the same as the length of the casing tube (5\u00a0cm). The flow rate of the circulating water was kept constant at 12\u00a0L/min using a water pump, and its temperature was controlled at 0\u00a0\u00b0C by a water-ice mixture. The inner quartz tube (8\u00a0mm i.d.\u00a0\u00d7\u00a010\u00a0mm o.d.) served as a dielectric layer. A stainless-steel rod (4\u00a0mm o.d.) was used as a high-voltage electrode, and was set coaxially with the inner quartz tube. Therefore, the discharge gap was fixed at 2\u00a0mm in this configuration. The discharge gap was fully packed with the catalyst particles. The DBD reactor was powered by a custom-built AC power source with a discharge frequency of 0\u201320\u00a0kHz and a peak-to-peak voltage of 0\u201330\u00a0kV. The input power of the plasma system can be controlled by changing the applied voltage and was monitored by a power meter. In this work, the input power was maintained at 70\u00a0W, and the discharge frequency was fixed at 10\u00a0kHz. The signals of the applied voltage, the voltage across the reference capacitor and the total current were sampled by a Tektronix digital oscilloscope (TDS-2014B) with a Tektronix high-voltage probe (P6015A), a Pintech differential probe (N1070A) and a Pearson current coil monitor (6585). The number of spikes in the current signals was calculated using the reported method [46,47]. The lifetime and magnitude of the current spikes were determined by the peak value and the full width at half maximum of these spikes [48]. Lissajous figures were used to determine discharge power, effective capacitance, and charge characteristics under different conditions [30,49]. The temperatures in the plasma region were measured by a thermal infrared camera (Fotric 223\u00a0s). These temperatures were lower than 70\u00a0\u00b0C under the experimental conditions in this work. The details of temperature measurement can be found in the Supplementary information (Fig. S1 and S2).A mixture of CH4 and CO2 was used as the reactants, and their individual flow rates were controlled by two mass flow controllers (Seven Star D17\u201309). The flow rate of each gas was 25\u2009ml/min, and they were thoroughly mixed to produce a homogeneous gas mixture before entering into the reactor. After the reaction, the gas stream firstly flowed through a U-shaped tube in a cold trap containing a water-ice mixture to condense the liquid products. These liquid products were dissolved in dichloromethane (CH2Cl2) and were qualitatively and quantitatively analyzed by a GC-MS (GC 7820A and MSD 5975C) and a GC (GC 7820) from Agilent Technologies. The gas products from the U-shaped tube were analyzed online with a PANNA GC (A60). A digital soap film flow meter was used to measure the total flow rate after the reaction. The flow rate of each gas component after the plasma reaction was calculated based on this total gas flow rate and the concentration of each gas component was determined by GC analysis. Then, based on the following equations, the conversion (C) of reactants, the selectivity (S) of main gaseous and liquid products, the carbon balance (CB) as well as the energy efficiency (EE) for gas conversion and product formation can be determined. In these equations, q is the flow rate of each gas component. The subscripts in and out account for the inlet and outlet of the reactor, respectively. P refers to the discharge power of the plasma-catalytic reforming process under different conditions. CxHyOz stands for oxygenates and it represents gaseous hydrocarbons when z equals 0.\n\n(1)\n\n\n\n\nC\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n(\n%\n)\n\n=\n\n\n\n\n\nq\n\n\n\n\nCO\n\n\n2\n,\n\n\nin\n\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\u2212\n\n\n\n\n\nq\n\n\n\n\nCO\n\n\n2\n\n\n,\nout\n\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\n\n\n\nq\n\n\n\n\nCO\n\n\n2\n\n\n,\nin\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(2)\n\n\n\n\nC\n\n\n\n\nCH\n\n\n4\n\n\n\n\n\n(\n%\n)\n\n=\n\n\n\n\nq\n\n\n\n\nCH\n\n\n4\n\n\n,\nin\n\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\u2212\n\n\n\n\n\n\nq\n\n\n\n\nCH\n\n\n4\n,\nout\n\n\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\n\n\nq\n\n\n\n\nCH\n\n\n4\n\n\n,\nin\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\n\n\n\n\u00d7\n\n\n100\n\n\n\n\n\n\n(3)\n\n\n\n\nS\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n(\n%\n)\n\n=\n\n\n\n\nq\n\n\n\n\nH\n\n\n2\n\n\n,\nout\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\n\n\n2\n\u00d7\n\n(\n\n\n\nq\n\n\n\n\nCH\n\n\n4\n\n\n,\nin\n\n\n\u2212\n\n\nq\n\n\n\n\nCH\n\n\n4\n\n\n,\nout\n\n\n\n)\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\n\n\n\n\u00d7\n\n\n100\n\n\n\n\n\n\n(4)\n\n\n\nS\nCO\n\n\n%\n\n=\n\n\n\nq\n\nCO\n,\n\nout\n\n\n\n\nmol\n/\ns\n\n\n\n\n\n\n\n\n\nq\n\n\nCH\n4\n\n,\n\nin\n\n\n\u2212\n\nq\n\n\nCH\n4\n\n,\nout\n\n\n\n\n\n\nmol\n/\ns\n\n\n+\n\n\n\nq\n\n\nCO\n2\n\n,\n\nin\n\n\n\u2212\n\nq\n\n\nCO\n2\n\n,\nout\n\n\n\n\n\n\nmol\n/\ns\n\n\n\n\n\n\n\n\n\n\u00d7\n\n\n100\n\n\n\n\n\n\n(5)\n\n\n\nS\n\n\nC\nx\n\n\nH\ny\n\n\nO\nz\n\n\n\n\n%\n\n=\n\n\nx\n\u00d7\n\nq\n\n\nC\nx\n\n\nH\ny\n\n\nO\nz\n\n,\nout\n\n\n\n\n\nmol\n/\ns\n\n\n\n\n\n\n\n\nq\n\n\nCH\n4\n\n,\nin\n\n\n\u2212\n\nq\n\n\nCH\n4\n\n,\nout\n\n\n\n\n\n\nmol\n/\ns\n\n\n+\n\n\n\nq\n\n\nCO\n2\n\n,\nin\n\n\n\u2212\n\nq\n\n\nCO\n2\n\n,\nout\n\n\n\n\n\n\nmol\n/\ns\n\n\n\n\n\n\n\n\u00d7\n\n\n100\n\n\n\n\n\n\n(6)\n\n\nCB\n\n(\n%\n)\n\n\n\n\n=\n\n\n\n\n\nS\n\n\nCO\n\n\n\n\n\n+\n\n\n\n\u2211\n\n\n\nS\n\n\n\n\nC\n\n\nx\n\n\n\n\nH\n\n\ny\n\n\n\n\nO\n\n\nz\n\n\n\n\n\n\n\n\n\n\n\n(7)\n\n\n\n\nEE\n\n\nreactant\n\n\n\n(\n\n\nmmol\n\n/\n\nkJ\n\n\n)\n\n=\n\n\n\n(\n\n\n\nq\n\n\n\n\nCH\n\n\n4\n\n\n,\nin\n\n\n\u2212\n\n\nq\n\n\n\n\nCH\n\n\n4\n\n\n,\nout\n\n\n\n)\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n+\n\n(\n\n\n\nq\n\n\n\n\nCO\n\n\n2\n\n\n,\nin\n\n\n\u2212\n\n\nq\n\n\n\n\nCO\n\n\n2\n\n\n,\nout\n\n\n\n)\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\nP\n\n(\nW\n)\n\n\n\n\n\n\n\n\n\n(8)\n\n\n\n\nEE\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n(\n\n\nmmol\n\n/\n\nkJ\n\n\n)\n\n=\n\n\n\n\nq\n\n\n\n\nH\n\n\n2\n\n\n,\nout\n,\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\nP\n\n(\nW\n)\n\n\n\n\n\n\n\n\n\n(9)\n\n\n\n\nEE\n\n\nSyngas\n\n\n\n(\n\n\nmmol\n\n/\n\nkJ\n\n\n)\n\n=\n\n\n\n\nq\n\n\n\n\nH\n\n\n2\n\n\n,\nout\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n+\n\n\n\nq\n\n\nCO\n,\nout\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\nP\n\n(\nW\n)\n\n\n\n\n\n\n\n\n\n(10)\n\n\n\n\nEE\n\n\n\n\nC\n\n\nx\n\n\n\n\nH\n\n\ny\n\n\n\n\nO\n\n\nz\n\n\n\n\n\n(\n\n\n\nmmol\n\n/\n\nkJ\n\n\n\n\n)\n\n=\n\n\n\u2211\n\n\n\nq\n\n\n\n\nC\n\n\nx\n\n\n\n\nH\n\n\ny\n\n\n\n\nO\n\n\nz\n\n\n,\n\n\n\n\n(\n\n\nmol\n\n/\ns\n\n)\n\n\n\nP\n\n(\nW\n)\n\n\n\n\n\n\n\n\n\nFig. 2 illustrates the structural properties of the catalytic materials before and after reaction, including \u03b3-Al2O3 and the supported catalysts. As shown in Fig. 2(a), the fresh \u03b3-Al2O3 has a large specific surface area of 288.7\u2009m2/g, a pore volume of 0.378\u2009m3/g and an average pore diameter of 5.574\u2009nm. The impregnation of the \u03b3-Al2O3 beads with active metals (Ni, Ag, or Pt) reduced these three parameters to varying degrees, which can be ascribed to pore blocking in \u03b3-Al2O3 by the penetration of these active metals [45,50]. Among the supported catalysts, these three parameters of Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3 were higher than those of Ni/\u03b3-Al2O3, possibly due to lower active metal loadings. Following the plasma reforming process, the specific surface area, pore volume and average pore diameter of the \u03b3-Al2O3 beads decreased noticeably, which could be attributed to carbon deposition formed on the \u03b3-Al2O3 beads during the plasma reforming reaction. However, after the plasma reaction, the textual parameters of these supported catalysts were barely changed, indicating that their properties can be kept relatively stable and that metal sintering may not occur during the plasma reaction process.The diffraction peaks of \u03b3-Al2O3 phase were observed for all the supported catalysts in the XRD patterns of the catalytic materials before and after reaction (Fig. 2(b)). The diffraction peaks assigned to the active metals were detected on fresh samples after the active metals were loaded. The diffraction peaks at 44.4\u00b0, 51.6\u00b0 and 76.1\u00b0 in the Ni/\u03b3-Al2O3 catalyst correspond to the (111), (200) and (220) phases of the metallic Ni in the enlarged XRD profile images of the fresh catalysts (see Fig. S4 in the Supplementary information). For the Ag/\u03b3-Al2O3 catalyst, the diffraction peaks at 64.5\u00b0 and 77.6\u00b0 are assigned to the (220) and (311) phases of metallic Ag. The observed diffraction peaks in the Pt/\u03b3-Al2O3 catalyst (46.4\u00b0 and 81.3\u00b0) are attributed to the (200) and (311) phases of metallic Pt [51\u201353]. These peaks broadened with low intensity, suggesting that the active metallic species are highly dispersed on the catalyst support. This phenomenon may benefit from the catalyst preparation method used in this work. Instead of thermal calcination and reduction at high temperatures, the metal precursors of these supported catalysts were decomposed and the corresponding metallic oxides were reduced in a subsequent treatment by the Ar/H2 DBD, which could remove un-desired templates from catalysts and provide strong collision by highly reactive species (e.g., energetic electrons) under mild conditions [54]. This can enhance metal dispersion on the catalyst and achieve higher reaction activity by speeding up nucleation and slowing down crystal growth [55]. The TEM images of the fresh catalysts (see Fig. S5 in the Supplementary information) confirm the formation of crystalline and spherical nanoparticles of the active metals. The average diameter of Ni, Ag and Pt nanoparticles in the fresh catalysts was 4.4\u2009nm, 3.0\u2009nm and 2.6\u2009nm, respectively. The existence of Ni, Ag and Pt was demonstrated by the selected area electron diffraction (SAED) and the energy-dispersive X-ray spectrum (EDX) (Fig. S5 and S6 in the Supplementary information). The SAED patterns confirm the crystalline structure of these metallic nanoparticles while the rings in the patterns can be assigned to the corresponding refection plane of each metallic phase in the fresh catalyst, as shown in their XRD patterns. The (111) and (200) reflection planes of the metallic Ag, as well as the (111) and (220) reflection planes of the metallic Pt, were observed in their SAED patterns but not detected in their XRD profiles. This phenomenon might be due to the weak diffraction peaks of these metallic phases, which overlap with the \u03b3-Al2O3 peak. After the plasma reforming process, the diffraction peaks of \u03b3-Al2O3 especially located at 2\u03b8\u2009=\u200938.2\u00b0, 49.2\u00b0, 67.1\u00b0 were intensified, which revealed that the crystallite size was enhanced during the plasma reforming process. No obvious changes were observed in the diffraction peaks for the supported catalysts before and after the reaction, indicating that stable properties were obtained. The increase in crystallite size of \u03b3-Al2O3 after the reaction would decrease its specific surface area. These results are in line with those of the textual characterizations.The basic nature of the catalyst is a crucial influencing factor for plasma-catalytic reforming performance. The CO2-TPD was used to investigate the properties of the basic sites of these catalytic materials, which can be divided into three types based on the desorption temperature of CO2, namely low, medium, and high basic sites within the desorption temperature ranges of 80\u2013140\u2009\u00b0C, 160\u2013240\u2009\u00b0C and over 300\u2009\u00b0C, respectively [56]. The area under their corresponding CO2-TPD curves can be used to calculate the number of these basic sites [45]. Catalysts with more strong basic sites can absorb more CO2 and thus promote gas conversion in the plasma-catalytic DRM [57]. \nFig. 3 illustrates the basic nature of \u03b3-Al2O3 and the supported catalysts. The \u03b3-Al2O3 support exhibited two CO2 desorption peaks: the first one is in the range of 50\u2013180\u2009\u00b0C and peaked at 112.5\u2009\u00b0C, which can be assigned to the low basic sites; the second one featured a broader range (240\u2009\u00b0C to 640\u2009\u00b0C) with a peak value of 484.1\u2009\u00b0C, indicating the existence of medium and high basic sites. Compared with \u03b3-Al2O3, the peaks assigned to the low basic sites were weakened, while those corresponding to the medium and high basic sites showed different trends in the supported catalysts. For the Ni/\u03b3-Al2O3 catalyst, the medium and high basic sites are reflected by three more intensified peaks (312.2\u2009\u00b0C, 458.9\u2009\u00b0C and 563.1\u2009\u00b0C) within the similar temperature range as \u03b3-Al2O3. For the Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3 catalysts, the peaks for medium and high basic sites were shifted to higher temperature ranges, with the highest peak temperatures of 600.5\u2009\u00b0C and 629.0\u2009\u00b0C, respectively. These findings suggest that loading the active metals, especially Ag and Pt enhanced the medium and high basic sites. In addition, the high specific surface areas obtained when Ag and Pt were loaded onto the \u03b3-Al2O3 support may provide more strong basic sites for CO2 adsorption and shift the CO2-TPD peaks to higher temperatures. The basicity of the support and catalysts decreased in the following order, according to the areas under CO2-TPD curves of basic sites with different strengths: Ag/\u03b3-Al2O3 >\u2009Ni/\u03b3-Al2O3 >\u2009Pt/\u03b3-Al2O3 >\u2009\u03b3-Al2O3.Since the performance of the plasma-catalytic DRM reaction is strongly dependent on the mutual interactions between plasma and catalysts, it is important to understand how the packing materials affect the physical properties of DBD plasma and link them to the performance of the plasma-catalytic DRM. \nFig. 4 shows the electrical signals of DBD under different packing conditions. Clearly, both the voltage and current signals are quasi-sinusoidal under all packing conditions, with numerous spikes in the current signals every half-cycle. Compared to the electrical signals of the DBD without packing (Fig. S7 in the Supplementary information), the number and magnitude of the current spikes in the DBD with packing were significantly reduced. This finding can be attributed to a change in discharge mode from the typical filamentary discharge of the DBD without packing to a combination of weak filamentary discharge and surface discharge in the DBD with packing [49]. We found that loading the active metals (Ni, Ag and Pt) onto the \u03b3-Al2O3 support considerably enhanced the density and intensity of the current spikes. Kim et al. reported a similar phenomenon when they compared the voltage-current characteristics of packed-bed DBD for benzene degradation using zeolite MS-13X and supported catalysts Ag/MS-13X as packing materials [58].To evaluate the current properties more qualitatively, the average values of the number, lifetime and magnitude of the current spikes in each cycle were estimated, as shown in \nFig. 5(a). These characteristic parameters are the indicators of the formation of discharge channels and the generation of energetic electrons for plasma chemical reactions [47], and they were all enhanced when using the supported catalysts. The largest number and longest lifetime of the current spikes were achieved in the presence of Pt/\u03b3-Al2O3, whereas using the Ni/\u03b3-Al2O3 catalyst produced the highest average magnitude of current spikes. The current properties were also reflected by the Lissajous figures (see Fig. 5(b)). Loading the active metals on \u03b3-Al2O3 changed the shape of the Lissajous figure from near oval to a parallelogram, following the order of \u03b3-Al2O3, the supported Ag, Ni and Pt catalysts, indicating changes in the discharge characteristics under different packing conditions. In addition, the transferred charge every half cycle, effective capacitance and discharge power were determined using the Lissajous figures (Fig. 5(c) and (d)). These parameters have a strong correlation with the production of reactive species, the spatial distribution of discharge channels and the energy dissipated in plasma reactions [27,59,60]. The transferred charge per half cycle increased from 338.7 nC (\u03b3-Al2O3) to 448.4 nC (Ni/\u03b3-Al2O3), 415.9 nC (Ag/\u03b3-Al2O3) and 509.1 nC (Pt/\u03b3-Al2O3). The variation trends in effective capacitance were similar to those in transferred charge. Loading active metals also increased discharge power, with the highest discharge power of around 45\u2009W (at the same input power of 70\u2009W) achieved in the presence of Ni/\u03b3-Al2O3 and Pt/\u03b3-Al2O3, which correspond to the changing order of the Lissajous figures. Based on the physical properties listed above, the plasma-catalytic systems packed with Ni/\u03b3-Al2O3 and Pt/\u03b3-Al2O3 maybe more capable of generating discharge channels and reactive species, as well as dissipating power into the plasma reforming reactions, than the Ag/\u03b3-Al2O3 catalyst and the \u03b3-Al2O3 support.\n\nFig. 6 depicts the performances of different catalysts. Using the supported catalysts clearly improved the conversions of CO2 and CH4. The highest conversion (21.4 % for CO2 and 27.6 % for CH4) was achieved when the Ag/\u03b3-Al2O3 catalyst was coupled with DBD, which was 25.1 % and 24.9 % higher than when the \u03b3-Al2O3 support was used. The promotion effect of packing supported catalysts on plasma reforming performance was also reported in the previous work. For example, Wang et al. investigated the plasma-catalytic CO2 reforming of CH4 in a DBD reactor and found that the conversions of CO2 and CH4 were enhanced by 6.6 % and 11.0 %, respectively, when packing supported Pt/\u03b3-Al2O3 catalyst [42]. Andersen et al. evaluated the activity of \u03b3-Al2O3 and different \u03b3-Al2O3 supported catalysts in the plasma CO2 reforming of CH4\n[36]. Compared to the reforming using \u03b3-Al2O3, the conversion of CH4 was increased by 20.0 % (from 27.1 % to 32.4 %) in the presence of Pt/\u03b3-Al2O3, while the CO2 conversion was almost unchanged (from 21.7 % to 22.0 %). However, the conversions using Ag/\u03b3-Al2O3 were similar to those using \u03b3-Al2O3 in their work [36]. The results of this work are quite different from this finding, which can be explained as follows. The reaction performance of plasma-catalytic DRM is influenced not only by catalytic materials (e.g., textual properties and basic nature), but also by processing parameters that control plasma properties (e.g., electric field, formation of reactive species, electron energy), which influences the plasma-catalyst interaction and thus the reaction. In the plasma-catalytic reforming process, all of these effects are coupled and interact, resulting in different reaction performances when the reaction conditions are changed [2]. The order of CO2 conversion was consistent with the basicity of these supported catalysts (Fig. 3), indicating that the basic sites on the catalyst surface are the main contributors to activating and converting CO2. The catalyst with higher basicity has a greater CO2 adsorption capacity, promoting CO2 conversion while producing O to enhance CH4 conversion. However, CH4 conversion did not follow the same order as CO2 conversion, implying that the textual properties of catalysts and the physical properties of plasma discharge were more important in CH4 conversion. The highest CH4 conversion was achieved with the combined effect of these factors when using Ag/\u03b3-Al2O3, followed by the other two supported catalysts (Ni/\u03b3-Al2O3 and Pt/\u03b3-Al2O3) and the \u03b3-Al2O3 support.\n\nFig. 7 shows the variations in the selectivities of main products, including gaseous and liquid compounds. Similar to gas conversion, loading the active metals enhanced H2 selectivity. The highest H2 selectivity (around 34.5 %) was achieved when packing the supported noble metal catalysts (Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3), which was 32.7 % higher than that obtained in the presence of \u03b3-Al2O3. Apart from hydrogen, the main carbon-containing gaseous products include CO, C2H6 and C3H6/C3H8, with minor amounts of C2H4, C2H2 and C4H8/C4H10. The selectivity of CO followed the order of the basic nature of these catalysts, with Ag/\u03b3-Al2O3 having the highest CO selectivity at 61.1%. This finding is explained by the fact that CO was mainly generated by the conversion of CO2, whereas the CO2 conversion was significantly influenced by the basic nature of the catalysts. A similar phenomenon was reported by Zeng et al. [33]. For gaseous hydrocarbons, C2H6 showed the highest selectivity (above 10 %) regardless of the catalytic materials, followed by C3H6/C3H8, C2H2, C2H4 and C4H8/C4H10. When compared to the other catalytic materials, coupling the Pt/\u03b3-Al2O3 catalyst with DBD produced the highest selectivity for all of these gaseous hydrocarbons. The total selectivity of these carbon-containing gaseous products was around 80 % in the presence of supported catalysts, which was significantly higher than when the \u03b3-Al2O3 support was packed (63.6 %).The support and catalysts significantly influence the distribution of the liquid products. Methanol was the main oxygenate with the highest selectivity (8.0 %) obtained when using the \u03b3-Al2O3 support. The other two oxygenates were acetic acid and ethanol, and their selectivity was enhanced by loading the active metals onto the \u03b3-Al2O3 support. The production of these oxygenates as the main liquid products was also reported in the plasma-catalytic DRM process in previous work. For example, Li et al. reported a total selectivity (\u223c40 %) of liquid products (mainly methanol and acetic acid) when using Co/SiO2 and Fe/SiO2 in a DBD plasma conversion of CO2 and CH4\n[37]. Wang et al. achieved the highest selectivity of acetic acid (40.2 %), a major liquid product in the plasma-catalytic conversion of CO2 and CH4 using Cu/\u03b3-Al2O3\n[42]. Andersen and his co-workers reported the highest methanol selectivity of 3.6% in the plasma-catalytic DRM over a similar Cu/\u03b3-Al2O3 catalyst [36]. Li et al. reported a total selectivity of more than 30 % for liquid products in the plasma-catalytic DRM using structured Ni-based catalysts [44]. In this work, we also detected formaldehyde as one major liquid product, but it was only formed in the presence of Pt/\u03b3-Al2O3, which was consistent with the findings of Wang et al. [42]. The total selectivity of the liquid products with different catalytic materials was in the following order: Ni/\u03b3-Al2O3 (14.1 %) >\u2009Pt/\u03b3-Al2O3 (13.4 %) >\u2009\u03b3-Al2O3 (11.5 %) >\u2009Ag/\u03b3-Al2O3 (11.3 %). The basic and acidic nature of the catalyst has been found to be critical in influencing the distribution of liquid products [35,40], and acidic sites have been reported to promote the formation of oxygenates [35]. Coupling the Ag/\u03b3-Al2O3 catalyst resulted in the lowest liquid selectivity, which was understandable given that Ag/\u03b3-Al2O3 had the highest basicity of these catalysts. The total selectivity of these liquid products, however, did not strictly follow the order of catalyst basicity, as the structure, surface composition, and reducibility of catalysts all had a significant impact on the distribution of the produced liquid products [44].The gas conversion and product selectivity were stable in the plasma reforming over supported catalysts for 180\u2009min (\nFig. 8). For Ni/\u03b3-Al2O3, the selectivity of the main oxygenates (i.e., methanol, acetic acid and ethanol) remained nearly unchanged during the plasma DRM reaction. When using the support (\u03b3-Al2O3), the gas conversions and product selectivities were relatively stable for the first 60\u2009min, but then gradually decreased as the reaction progressed. The high stable reforming performance of these supported catalysts could be attributed to the stable properties of the plasma-modified catalysts in this study. To confirm this hypothesis, we compared the plasma-catalytic reforming performance of the Ni/\u03b3-Al2O3 catalyst prepared by the plasma-modified impregnation method and the traditional thermal method over a longer reaction time (10\u2009h). Higher and more stable gas conversions were observed when using the Ni/\u03b3-Al2O3 catalyst prepared by the plasma-modified impregnation method, as shown in Fig. S3. The influence of different preparation methods, as well as the underlying mechanism, should be investigated further. For instance, the conversion of CO2 and CH4 was both reduced by approximately 10 % (from 17.2 % and 22.1\u201315.6 % and 20.0 %, respectively) when the reaction kept running for 180\u2009min, compared with that in the initial stage (20\u2009min). This decrease in the reaction performance might be due to the carbon deposition on \u03b3-Al2O3 during the reforming process, as evidenced by TGA results.\n\nFig. 9 shows the performance stability in terms of carbon balance and carbon resistance of catalyst. According to the TGA profiles (Fig. 9(a)), \u03b3-Al2O3 had the highest overall weight loss, followed by Ni/\u03b3-Al2O3, Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3. Clearly, the weight loss occurred mainly in two temperature ranges: 25\u2013150\u2009\u00b0C and 150\u2009\u00b0C\u2013700\u2009\u00b0C, which were ascribed to the desorption of water and the elimination of deposited carbon [9]. The catalysts were heated at 110\u2009\u00b0C for 2\u2009h to remove humidity before the reaction. The presence of water in the catalysts after the reaction indicated that the reverse water gas shift reaction occurred in the reforming process [61]. In the second stage of weight loss due to the removal of carbon deposition, three types of carbonaceous species can form: active, less active, and inactive carbon species [62]. In this work, the weight loss of the used noble metal supported catalysts (Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3) occurred mainly between 200\u2009\u00b0C and 550\u2009\u00b0C, indicating the formation of active carbonaceous species. These active carbonaceous species could be easily oxidized during the reforming process [63], and thus would not have a severe impact on their activity and stability, as evidenced by the time variations of the gas conversion. Part of the weight loss in other two used samples, especially \u03b3-Al2O3, occurred at temperatures greater than 600\u2009\u00b0C, which was attributed to the oxidation of inactive graphite carbon. This type of carbonaceous species was stable and could not be oxidized during the plasma-catalytic DRM at low temperatures, resulting in catalyst deactivation over a long period of reaction time, which could be the main reason for the decrease in gas conversion with reaction time.The carbon balance for all catalytic materials was less than 100 % (Fig. 9(b)), which can be attributed to carbon deposition on both catalysts and the inner electrode surface, as well as the generated liquid oxygenates attached to the inner surface of the quartz tube that were not completely collected [45,47]. Clearly, the carbon balance when using the supported catalysts was higher than 90 %, and the highest value (94.9 %) was achieved when using Pt/\u03b3-Al2O3. However, the carbon balance with \u03b3-Al2O3 was only 75.3 %, which was mainly caused by the relatively severe carbon deposition evidenced by the TGA analysis.\n\nFig. 10 shows the energy efficiencies for both gas conversion and product formation (e.g., syngas, H2, CxHy and CxHyOz) under different packing conditions. Packing \u03b3-Al2O3 and the non-noble catalyst (Ni/\u03b3-Al2O3) exhibited similar energy efficiencies for gas conversion and gas product formation. Higher gas conversions and gas product selectivities were obtained using Ni/\u03b3-Al2O3 compared to \u03b3-Al2O3, (Figs. 6 and 7), but only at higher discharge power (Fig. 5). This was the main reason for their comparable energy efficiency. Packing the noble metal catalysts in the plasma reactor resulted in higher energy efficiencies. For example, the highest energy efficiency for gas conversion (0.22\u2009mmol/kJ), hydrogen (0.079\u2009mmol/kJ) and syngas formation (0.20\u2009mmol/kJ) was attained when using Ag/\u03b3-Al2O3, while Pt/\u03b3-Al2O3 yielded the highest energy efficiency for the production of CxHy (0.020\u2009mmol/kJ) and CxHyOz (0.014\u2009mmol/kJ). The higher energy efficiencies achieved when using these two noble metal catalysts are mainly due to the improved gas conversion and product selectivity even at relatively lower discharge power. This phenomenon suggests that low discharge power may be advantageous in achieving high energy efficiency [36].\n\nTable 1 compares the energy efficiencies for plasma-enabled catalytic reforming of CH4 and CO2 using different DBDs. The highest energy efficiency obtained in this work is comparable to previous work using a similar DBD reactor [33,36]. It should be noted that the highest energy efficiency for gas conversion and product formation is not always achieved simultaneously under the same conditions. In addition, the energy efficiency of DBDs is still lower than that of other plasma technologies such as gliding arc discharge [64,65]. More research is needed to develop a more efficient plasma-catalytic system for promoting the reaction performance in terms of gas conversion, target product selectivity, and energy efficiency. Since high reaction performance was achieved with noble catalysts (e.g., Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3), using the Ni catalyst promoted by noble metals could be a useful approach in terms of both catalytic performance and catalyst cost, as demonstrated in the thermal-catalytic reforming processes [1,66,67]. Similarly, bimetallic catalysts using Ni and other transition metals (e.g., Co, Fe, Cu and Mn) could also be a promising option [1,3]. To make plasma catalysis a promising and economically competitive alternative for DRM, more efforts should be put into rational catalyst design, reactor innovation, optimizing plasma-catalysis configurations and operating parameters, and thoroughly understanding the plasma-catalysis mechanism, particularly plasma-assisted surface reactions [2,15,38,40,43,44].The plasma-catalytic DRM over different supported metal catalysts (Ni/\u03b3-Al2O3, Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3) was carried out in a DBD reactor at low temperatures. The results showed that the active metals were uniformly distributed across the \u03b3-Al2O3 support, and that loading the active metal slightly decreased the textual properties of the catalytic materials, while significantly enhancing their basic nature. The conversion of CO2 followed the order of the basicity of the catalytic materials, suggesting that the catalyst basicity plays a dominant role in CO2 conversion, whereas the conversion of CH4 was determined by the combined effect of catalyst properties and discharge characteristics. The highest CO2 and CH4 conversions (21.4 % and 27.6 %, respectively) were achieved when using Ag/\u03b3-Al2O3. Coupling the noble metal catalysts (Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3) with DBD resulted in the highest selectivity of gas products (34.5 % for H2 and \u223c81.0 for CO and CxHy), while using Ni/\u03b3-Al2O3 gave the highest selectivity of liquid products (14.1 %). The supported catalysts prepared by the modified impregnation method exhibited high stability, evidenced by the time variations in gas conversion and catalyst characterization. In addition, high energy efficiencies were achieved when using the noble metal catalysts. Specifically, coupling the Ag/\u03b3-Al2O3 catalyst with DBD yielded the highest energy efficiency for gas conversion (0.22\u2009mmol/kJ), which was comparable to published results. Future research should concentrate on the development of more efficient catalysts, the optimization of plasma-catalysis configurations and operating parameters, and a thorough understanding of the mechanism of plasma-catalytic reforming.Danhua Mei: Conceptualization, Methodology, Investigation, Formal analysis, Writing \u2013 original draft, Minjie Sun: Investigation, Formal analysis, Data curation, Validation, Shiyun Liu: Methodology, Formal analysis, Validation, Peng Zhang: Methodology, Formal analysis, Visualization, Zhi Fang: Conceptualization, Supervision, Resources, Writing \u2013 reviewing and editing, Project administration, Xin Tu: Conceptualization, Supervision, Writing \u2013 reviewing and editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is financially supported by the National Natural Science Foundation of China (No. 51807087 and No. 52177149), the Natural Science Foundation for Colleges in Jiangsu Province (No. 19KJB470005) and the Project of Six Talent Peak High-Level Talent Team of Jiangsu Province (No. TD-JNHB-006). P. Zhang acknowledges the support from the Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. SJCX22_0424). X. Tu acknowledges funding from the European Union\u2019s Horizon 2020 research and innovation program under the Marie Sklodowska- Curie grant agreement No. 813393.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2022.102307.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Dry reforming of CH4 (DRM) using a plasma-enabled catalytic process is an appealing approach for reducing greenhouse gas emissions while producing fuels and chemicals. However, this is a complex process that is influenced by both catalysts and discharge plasmas, and low energy efficiency remains a challenge for this technology. Here, we developed a water-cooled dielectric barrier discharge (DBD) reactor for plasma DRM reactions over supported catalysts (Ni/\u03b3-Al2O3, Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3) prepared via plasma-modified impregnation. Results show that metal loading on \u03b3-Al2O3 enhanced the basic nature of the catalysts and promoted the formation of discharge channels and reactive species. The maximum conversion of CO2 (21.4 %) and CH4 (27.6 %) was obtained when using Ag/\u03b3-Al2O3. The basic nature of the catalytic materials dominated CO2 conversion, whereas the properties of the catalyst and discharge plasma determined CH4 conversion. The highest selectivity of hydrogen (\u223c34.5 %) and carbon-containing gas products (\u223c81.0 %) were attained when using the noble metal catalysts (Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3), while the highest total selectivity of liquid products (14.1 %) was achieved in the presence of Ni/\u03b3-Al2O3. Compared with \u03b3-Al2O3, the supported catalysts demonstrated higher stability, especially for Ag/\u03b3-Al2O3 and Pt/\u03b3-Al2O3, which also provided higher energy efficiency for gas conversion and product formation.\n               "}
{"full_text": "The sustainable production of hydrogen through electrocatalytic water splitting is a potential pathway for obtaining clean energy due to its environmental friendliness and high energy conversion efficiency [1,2]. Effective electrocatalysts are necessary for high efficient hydrogen evolution reaction (HER). Pt-group catalysts are regarded as high-activity HER electrocatalysts [3]. However, their practical applications are limited by high cost and scarce global resources [4]. In the past few years, cheap, earth-rich, efficient and sustainable catalysts, including transition metal phosphides [5,6], sulfides [7,8], selenides [9,10] and oxides [11,12], are widely studied to alter the noble metal-based electrocatalysts. In addition, these catalysts are usually loaded on carbon or foam supports to form the electrode, which would affect the long-term stability and be difficult for widespread application [13]. Hence, it is very crucial to explore self-supported catalysts with excellent efficiency and good stability for the HER.Many methods, such as template assisted [14], hydrothermal [15], and dealloying [16], were used to synthesize self-supported electrocatalysts. Dealloying is a selective corrosion process involving the dissolution of active atoms and the reorganization of inert atoms [17]. The uniform bicontinuous nanoporous structure produced by dealloying can expose a larger specific surface area and provide more accessible interior active sites [18]. It was reported that nanoporous transition metal compound catalysts exhibited good HER performance in alkaline electrolyte [6]. Nevertheless, the friability of self-supported material is serious problem for its industrialization in flexible electrode devices. Recently, the amorphous alloys with good mechanical properties were suggested to be catalysts for HER. Moreover, amorphous alloys exhibit good catalytic activity due to the unique disorder structure and inherent abundant defects that would generate more active sites [18]. Hu et\u00a0al. prepared PdNiCuP ribbons with stable amorphous structure as a high activity electrocatalyst for water splitting process [19]. Xu et\u00a0al. synthesized an amorphous nanoporous Ni-Fe-P material with good properties for both HER and OER in alkaline conditions [20]. Zhang et\u00a0al. reported that FeCoPC catalyst exhibited a good HER activity [5].In this work, a facile and promising strategy was designed to fabricate self-supported nanoporous Ni-Co-P (np-Ni-Co-P) catalysts via an electrochemical dealloying method. The 3D binder-free amorphous np-Ni-Co-P catalyst exhibits markedly high catalytic activity for HER with overpotential of 114\u00a0\u200bmV\u00a0\u200bat a current density of 10\u00a0\u200bmA\u00a0\u200bcm\u22122, low Tafel slope around 57.3\u00a0\u200bmV dec\u22121 and good stability in alkaline medium.The Ni60Co20P20 master alloy was prepared by melting pure Ni (99.99\u00a0\u200bat.%), pure Co (99.99\u00a0\u200bat.%) and Ni2P (99.7\u00a0\u200bat.%) in a high-frequency induction melting furnace under high purity argon atmosphere. A melt spinning technique was used to prepare amorphous alloy ribbons with the dimensions of ~1\u00a0\u200bmm wide and ~20\u00a0\u200b\u03bcm thick. The precursor ribbons were cut to 30\u00a0\u200bmm\u00a0\u200b\u00d7\u00a0\u200b1\u00a0\u200bmm. The part of ribbon with length of 10\u00a0\u200bmm was served as the working area, and the other part was sealed with silica gel as the clamping part. Thus the area of the working electrode was 0.2\u00a0\u200bcm\u22122. The Ni60Co20P20 ribbons were electrochemically etched under 0.2\u00a0\u200bV vs. saturated calomel electrode (SCE) in 1\u00a0\u200bM HCl solution in a standard three-electrode setup with a SCE as the reference electrode and a graphite sheet as the counter electrode by using an electrochemical workstation. The control samples for nanoporous Ni-P (np-Ni-P) and nanoporous Co-P (np-Co-P) catalyst was prepared by using the same process with Ni60Mn20P20 and Co80P20 as the precursor alloys, respectively. Finally, the dealloyed samples were washed in distilled water and ethyl alcohol for three times and then dried for 12\u00a0\u200bh in a vacuum drying oven.The phases of the as-synthesized samples were analyzed by X-ray diffraction (XRD, BrukerD8) using Cu K\u03b1 radiation. The surface morphologies and elemental composition of the as-synthesized samples were obtained by scanning electron microscopy (SEM, Hitachi S-4800) and energy-dispersive X-ray spectroscopy (EDX, JSM-7800F). Transmission electron microscope (TEM, JEOL 2100\u00a0\u200bM) was used to carried out transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) tests. The surface valence state were collected by X-ray photoelectron spectroscopy (XPS, PHL1600ESCA).The electrochemical property of all samples were measured on a typical three-electrode setup (Gamry Reference 1000) at room temperature, with 1.0\u00a0\u200bM KOH solution as the electrolyte. The as-synthesized catalysts, a graphite sheet, and a saturated calomel electrode (SCE) electrode served as the working electrode, counter electrode, and reference electrode, respectively. The potential conversion was calibrated (from vs. SCE) to the reversible hydrogen electrode (vs. RHE) based on the Nernst equation: E(RHE)\u00a0\u200b=\u00a0\u200bE(SCE)+0.241\u00a0\u200b+\u00a0\u200b0.059\u00a0\u200bpH. Linear-sweep voltammetry (LSV) for the HER was tested with a scanning rate of 5\u00a0\u200bmV\u00a0\u200bs\u22121. Electrochemical impedance spectroscopy (EIS) data were obtained at 150\u00a0\u200bmV (vs. RHE) for the HER with the frequency ranging from 0.1 to 105\u00a0\u200bHz. All polarization curves were corrected for iR compensation by applying the following equation: Ecorr\u00a0\u200b=\u00a0\u200bEmea\u2212iRs, where Ecorr is the iR-corrected potential, Emea is the measured potential and Rs is the resistance of the system. Cyclic voltammetry (CV) was conducted to estimate the electrochemical active surface area (ECSA) within \u00b150\u00a0\u200bmV vs open-circuit potential (OCP) under different scan rates of 10, 20, 40, 60, 80 and 100\u00a0\u200bmV\u00a0\u200bs\u22121. The turnover frequency (TOF) value is calculated from the equation: TOF\u00a0\u200b=\u00a0\u200bJ\u00a0\u200b\u00d7\u00a0\u200bA/(2\u00a0\u200b\u00d7\u00a0\u200bF\u00a0\u200b\u00d7\u00a0\u200bn), where J is the measured current density at the overpotential of 100\u00a0\u200bmV, A is the surface area of the working electrode, F is the Faraday constant (96485 c mol\u22121), n is the number of moles of active materials loaded on the electrodes [21,22]. The durability of the samples were measured by chronoamperometry (CA) and 3000 cycles of CV scans. Moreover, commercial Pt/C catalyst (5\u00a0\u200bwt%, Alfa Aesar) was used as the control electrodes. 4\u00a0\u200bmg of commercial Pt/C was added to a solution involving 50\u00a0\u200b\u03bcL Nafion and 200\u00a0\u200b\u03bcL ethanol to form a homogeneous ink. Finally, 20\u00a0\u200b\u03bcL catalyst ink was coated on to the surface of a glassy carbon electrode (surface area: 0.2826\u00a0\u200bcm\u22122) for three times and the dried at room temperature.\nFig.\u00a01\na shows the X-ray diffraction (XRD) patterns of all the precursor ribbons. The Ni60Co20P20 and Ni60Mn20P20 precursors are amorphous, while the Co80P20 precursorexists two crystalline phases, including orthorhombic Co2P and hexagonal-close-packed (HCP) cobalt [23]. After dealloying, np-NiCoP and np-NiP still maintain amorphous structure, while the HCP cobalt is significantly reduced for the np-Co-P sample (Fig.\u00a01b). Compared with the crystal phase, the surface chemistry of amorphous catalysts can be optimized to enhance the performance on the level of molecular [8,24]. The retained mechanical flexibility after undergoing eletrochemical dealloying demonstrate a superior self-supported capability of np-Ni-Co-P (as shown in Fig.\u00a0S1), which would favor in elimination of the interface overpotential and rapid charge transfer during the electrolysis process [25].\nFig.\u00a02\n shows the SEM images of the as-synthesized np-Ni-Co-P, np-Co-P and np-Ni-P samples. All samples exhibit the analogous bicontinuous nanostructures with both the ligaments and pores on a large scale. The nano-scale ligaments and pores can efficiently provide abundant active sites and ion-diffusion routes, which are conducive to mass transfer and electron mobility. The corresponding energy dispersive X-ray spectroscopy (EDS) analysis indicates that the atomic ratio of the np-Ni-Co-P, np-Co-P and np-Ni-P are close to 1.34: 0.76: 1, 2.04: 1 and 2.60: 1, respectively (Fig.\u00a0S2 and Table\u00a0S1). The phosphorus content in all samples are also similar (about 30\u00a0\u200bat.%). The transmission electron microscopy (TEM) image of the np-Ni-Co-P (Fig.\u00a03\na) further reveals a uniform bicontinuous nanoporous architecture consisting of interlinked metallic ligaments. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) (inset of Fig.\u00a03b) further certify a homogeneous amorphous nature of the np-Ni-Co-P down to nanoscale (Fig.\u00a03b) because diffraction spots and crystalline lattices cannot be seen. Element mapping result in Fig.\u00a03c manifests that Ni, Co and P elements disperse on the np-Ni-Co-P sample surface uniformly, where no phase separation or elemental segregation is observed.X-ray photoelectron spectroscopy (XPS) test was performed to determine the surface chemistry for np-Ni-Co-P, np-Co-P and np-Ni-P in the Ni 2p, Co 2p and P 2p spectra. In the Ni 2p3/2 spectrum (Fig.\u00a04\na), the binding energy at 852.84\u00a0\u200beV can be assigned to Ni\u03b4+ in metal phosphides, while other peaks with higher binding energies of 853.42, 856.00 and 860.5\u00a0\u200beV belong to Ni2+, Ni3+ and the appropriate satellite peak, respectively [26,27]. The Co 2p3/2 XPS spectrum (Fig.\u00a04b) could be divided into three peaks. The peak at 778.33\u00a0\u200beV belongs to Co\u03b4+ in metal phosphides, and the other two peaks located at 780.20 and 782.37\u00a0\u200beV can be attributed to the Co2+ and Co3+, respectively, which should arise from the surface oxidation after long-time exposure to the air [28,29]. Fig.\u00a04c shows the high-resolution P 2p spectra consisting of a doublet of P 2p3/2 component at 129.25\u00a0\u200beV and P 2p1/2 component at 130.13\u00a0\u200beV from metal phosphides. Moreover, the peak at 132.67\u00a0\u200beV is ascribed to oxidized phosphate species [30,31]. Obviously, Ni 2p3/2 and Co 2p3/2 in the np-Ni-Co-P exhibit as lightly positive energy shift, while P 2p shows a slightly negative shift compared to np-Ni-P and np-Co-P, implying the different electronic structures for all samples. The positive shift of Ni 2p3/2 and Co 2p3/2 binding energies of np-Ni-Co-P catalyst reveals the enhanced electron transfer, while the negative shift of P 2p3/2 indicates the improved electron occupation, which would lead to heightening electron donating ability [16]. In the electrocatalysis process, the positive Ni and Co centers serve as hydride-acceptor, while the negative P centers serve as proton-acceptor sites [32]. The diffusion of electrons from metallic centers Ni and Co to P can effectively accelerate absorption/desorption of H atoms on active site for the HER [33].The electrocatalytic HER properties were evaluated by means of a typical three-electrode setup in 1\u00a0\u200bM KOH solution, where the as-synthesized materials were directly employed as binder-free catalytic electrodes. Fig.\u00a05\na shows the linear sweep voltammetry (LSV) curves of all the as-synthesized samples with a scanning rate of 5\u00a0\u200bmV\u00a0\u200bs\u22121. As seen, the commercial Pt/C (20%) requires a lowest overpotential of 26\u00a0\u200bmV to reach the current density of 10\u00a0\u200bmA\u00a0\u200bcm\u22122. The np-Ni-Co-P catalyst exhibits good catalytic activity with overpotential of 114\u00a0\u200bmV to achieve the current density of 10\u00a0\u200bmA\u00a0\u200bcm\u22122, which is 9 and 206\u00a0\u200bmV lower than those of np-Co-P and np-Ni-P. According to the LSV results, bimetallic phosphides (np-Ni-Co-P) display a lower overpotential compared with the single-metal phosphides (np-Co-P and np-Ni-P). This result illustrates that the synergistic interplay of Ni and Co could efficaciously enhance the HER electrocatalytic performance.The HER reaction kinetics is assessed by the Tafel slope. In Fig.\u00a05b, the np-Ni-Co-P exhibits a Tafel slope of 57.3\u00a0\u200bmV dec\u22121, outperforming the np-Co-P (69.9\u00a0\u200bmV dec\u22121) and np-Ni-P (123.1\u00a0\u200bmV dec\u22121), suggesting the markedly accelerated HER kinetics of the np-Ni-Co-P catalyst. The Tafel slope value of np-Ni-Co-P catalyst within the range of 40\u223c120\u00a0\u200bmV dec\u22121 implies that the HER processes via the Volmer-Heyrovsky mechanism [34,35], which can be described as follow:\n\n(1)\nVolmer reaction:H2O\u00a0\u200b+\u00a0\u200bM\u00a0\u200b+\u00a0\u200be\u2212\u2192 M-Had\u00a0\u200b+\u00a0\u200bOH\u2212\n\n\n\n\n\n(2)\nHeyrovsky reaction:H2O\u00a0\u200b+\u00a0\u200bM-Had\u00a0\u200b+\u00a0\u200be\u2212 \u2192 H2+OH\u2212\n\n\nwhere M presents the active site of the catalyst for HER, and Had presents a H atom absorbed at the active site of the catalyst. The low overpotential and Tafel slope values demonstrate that the np-Ni-Co-P catalyst has better electrocatalytic activity than other reported metal-based HER catalysts (Table\u00a0S2 and Table\u00a0S3).The electrochemical active area (ECSA) of the as-synthesized catalysts were estimated by CV curves in the non-Faraday region of \u00b10.05\u00a0\u200bV versus open-circuit potential (OCP) (Fig.\u00a0S3). Noticeably, the number of active sites of np-Co-P is 84 times higher than that of np-Ni-P, suggesting that Co might form the major active sites toward HER. The number of active sites with np-Ni-Co-P catalyst is reduced by 4.7 times compared to the np-Co-P catalyst. Nevertheless, according to the turnover frequency (TOF) result, the TOF of np-Ni-Co-P catalyst is calculated to be 0.13\u00a0\u200bs\u22121\u00a0\u200bat \u03b7\u00a0\u200b=\u00a0\u200b100\u00a0\u200bmV, which is 1.3 and 37 times higher than that of np-Co-P (0.10\u00a0\u200bs\u22121) and np-Ni-P (0.0035\u00a0\u200bs\u22121), respectively (Fig.\u00a05c). The intrinsic catalytic activity of the np-Ni-Co-P catalyst is significantly enhanced with the addition of Co. The enhanced intrinsic activity might be ascribed to the joint action of Ni and Co due to the change of electron structure of P which further optimizes the free energy of H adsorption [36]. The charge-transfer resistance is also an indispensable parameter affecting the performance of electrocatalysts. EIS was carried out to further evaluate the HER reaction kinetics of as-synthesized catalysts. As displayed in Fig.\u00a04d, the charge-transfer resistance (Rct) value of np-Ni-Co-P is 5.9\u00a0\u200b\u03a9, which are much smaller than of np-Co-P (9.2\u00a0\u200b\u03a9) and np-Ni-P (47.7\u00a0\u200b\u03a9), revealing rapid electron transport ability and charge-transfer kinetic during the HER process. This is favorable for HER performance.The durability of the electrocatalysts was assessed by chronoamperometry and cyclic CV tests. Fig.\u00a06\na shows a slight current degradation of the np-Ni-Co-P catalyst under constant overpotential of 120\u00a0\u200bmV for 20\u00a0\u200bh test, indicating good durability. However, the CV curve of np-Ni-Co-P has small decay (~28\u00a0\u200bmV) after 3000 cycles (Fig.\u00a06b). The structure of the np-Ni-Co-P catalyst after the electrochemical durability test for 3000 CV cycles in alkaline medium was also characterized by using XRD, SEM, TEM, HRTEM and XPS. The XRD patterns (Figure. S4) of the np-Ni-Co-P catalyst after 3000 CV cycles is almost same as that of the initial catalyst. The SEM, TEM and HRTEM images (Figure. S5 and Figure. S6) still display similar nanoporous structure and uniform distribution of all elements with those in Figs.\u00a02a and 3. Fig.\u00a0S7 shows the XPS spectrum of np-Ni-Co-P after 3000 CV cycles. The extinction of Ni\u03b4+/Co\u03b4+ species is connected with the decrease of Ni/Co-P bonds, which indicates that metal phosphide is gradually transformed into metal oxide/hydroxide. Meanwhile, the appearance of a new peak at 529.18\u00a0\u200beV in the O 1s spectrum further illustrates the formation of Ni-Co oxides/hydroxides. This should be due to the large polarization during the CV test. The Ni-Co oxides/hydroxides are intrinsically adverse electrocatalytic species for the HER, resulting in initial decay of current density [28]. Nevertheless, the intrinsic active of np-Ni-Co-P could transform the partial surface electronic state of the Ni-Co oxides/hydroxides, thus retaining the outstanding HER catalytic activity inalkaline electrolyte.An amorphous nanoporous Ni-Co-P electrocatalyst has been successfully fabricated via a facile electrochemical delloying process. The np-Ni-Co-P catalyst exhibits outstanding HER catalytic activity with the overpotential of 114\u00a0\u200bmV\u00a0\u200bat a current density of 10\u00a0\u200bmA\u00a0\u200bcm\u22122 and small Tafel slope of 57.3\u00a0\u200bmV dec\u22121 in alkaline solution. The outstanding HER performance is ascribed to the bicontinuous nanoporous structure, the alloying effect and disorder atomic arrangement. The synergetic effect of the Ni and Co elements improves the intrinsic activity of active sites. The facile synthesis of np-Ni-Co-P electrocatalysts with high performance and great durability may open up a new strategy for the application.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We gratefully acknowledge support by the National Natural Science Foundation of China (51771131).The following is the supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.pnsc.2020.12.006.", "descript": "\n                  Hydrogen evolution reaction (HER) through electrocatalysis using cost-efficient and long-term stable bimetallic phosphide as electrocatalyst holds a great promise for sustainable clean energy technologies. In this study, self-supported nanoporous Ni-Co-P (np-Ni-Co-P) catalyst with amorphous structure was synthesized by utilizing a facile electrochemical dealloying strategy. The results showed that due to the nanoporous structure, disorder atomic arrangement and alloying effect, the np-Ni-Co-P exhibited outstanding electrocatalytic performance for HER with low overpotential of 114\u00a0\u200bmV\u00a0\u200bat a current density of 10\u00a0\u200bmA\u00a0\u200bcm\u22122, small Tafel slope (57.3\u00a0\u200bmV dec\u22121) and good long-term durability in 1\u00a0\u200bM KOH. The synergetic effect of Ni and Co elements improves the intrinsic activity of the active sites. This research provides a direction for the exploration of bimetallic phosphides toward HER in the water splitting process.\n               "}
{"full_text": "Hydrogen as a clean and renewable energy carrier has attracted more and more interests owing to the perspective vision of future hydrogen energy society [1]. However, conventional hydrogen production technology based on coal, petroleum or natural gas are still needed to cope with resource exhaustion, environmental pollution and CO2 emission etc. [2,3]. And the technology based on water reserves by electrolysis or photolysis is in its infancy with high energy consumption and low efficiency [4,5]. Therefore in recent years, increasing attentions have been paid on hydrogen production, based on biomass and biomass-derived oxygenates (e.g. ethanol, ethylene glycol and glycerol), due to their sustainable and renewable properties [6\u20139].Ethylene glycol (EG) can be obtained directly and efficiently by thermal cracking and catalytic reforming of biomass [10,11]. It is the simplest polyol, often been used as representative compound to investigate steam reforming (SR) of bio-derived polyols due to the same C/O ratio [12]. Some base and noble metals, such as Ni, Co, Rh and Pt, have been evidenced to be effective for SR of EG [13\u201315]. It has been established that Ni-based catalysts favor C\u2014C rupture and are ideal for steam reforming of biomass-derived oxygenates [16\u201319]. However, they are easily to be deactivated due to the coke formation at low temperature and the sintering of Ni particles at high temperature [20\u201322]. The approaches such as changing character of support, optimizing the catalyst preparation procedures and adding a second metal have been used to improve the stability of Ni catalyst [23\u201329]. It is reported that silica has a large specific surface area favoring the dispersion of the active metal [30]. Ceria has been widely used in SR reaction owing to its excellent redox property, it can also enhance the dispersion of the active metal, increasing reforming capacity and suppressing coke formation [31]. A second metal can be added into SR catalysts to promote the activity and stability by synergistic effect. Tupy et\u00a0al. [18] reported that the catalytic activity and the stability of NiPt catalysts were better than Pt catalysts in steam reforming of EG and the selectivity of by-products depended both on support and metal. Moraes et\u00a0al. [27] investigated steam reforming of ethanol over Ni and PtNi catalysts and proposed that Pt addition increased the hydrogenation rate of carbon species and minimized the formation of nickel carbide, improving catalyst stability.The steam reforming of EG to produce hydrogen on Ni catalysts were mainly investigated at high temperature (usually\u00a0>\u00a0400\u00a0\u00b0C). However, a low temperature reaction is more favored due to the energy saving. The aim of this paper is to enhance the stability and activity of EG steam reforming at low temperature over Ni catalysts via modification with Pt. The catalytic properties of bimetallic catalysts were compared to those of monometallic ones, and the CeO2 supported ones was compared with the SiO2 supported. After characterization with nitrogen adsorption\u2013desorption, X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS), the intrinsic reasons of improved stability and activity over bimetallic catalysts were revealed.Commercial SiO2 (Qingdao Ocean Chemical Co., Ltd.; SSA\u00a0=\u00a0420.7\u00a0m2\u00a0g\u22121; dp\u00a0=\u00a04.88\u00a0nm) was washed with distilled water and dried at 100\u00a0\u00b0C for 12\u00a0h and then calcined at 500\u00a0\u00b0C for 4\u00a0h before use. CeO2 was prepared by precipitation method. Cerium nitrate aqueous solution (0.08\u00a0mol L\u22121) was precipitated by slowly adding KOH solution (6\u00a0mol L\u22121) drop by drop and later calcining at 500\u00a0\u00b0C for 4\u00a0h.The supported monometallic Pt and Ni catalysts and bimetallic Pt and Ni catalysts were prepared via impregnation methods. The preparing processes of catalysts are noted in Table 1. The monometallic catalysts were prepared through impregnating SiO2 or CeO2 support with an aqueous solution of H2PtCl6\u00b76H2O or Ni(NO3)2\u00b76H2O, for 12\u00a0h respectively. Then the samples were dried at 100\u00a0\u00b0C overnight and calcined at 500\u00a0\u00b0C for 4\u00a0h. The Pt/CeO2 sample was further treated at 400\u00a0\u00b0C for 12\u00a0h under hydrogen (50\u00a0mL min\u22121) to remove residual Cl\u2212, and then cooled with a nitrogen flow to room temperature during 6\u00a0h. The catalysts are identified as Ni/SiO2, Ni/CeO2 and Pt/CeO2. And the Ni and Pt loading is 10\u00a0wt% and 3\u00a0wt%, respectively.The preparation of bimetallic catalysts is similar to that reported in the literature [32]. The total metal contents are 10\u00a0wt%, and the subscripts of metals in Table 1\n represents the content of corresponding metals. Pt\u00a0+\u00a0Ni/SiO2 catalyst was prepared by co-impregnation with a mixed aqueous solution of H2PtCl6\u00b76H2O and Ni(NO3)2\u00b76H2O. Ni\u2013Pt/SiO2 catalyst was prepared by consecutive impregnation with Ni first and then Pt, namely the calcined Ni/SiO2 sample was further impregnated with an aqueous solution of H2PtCl6\u00b76H2O. After impregnation, the Pt\u00a0+\u00a0Ni/SiO2 and Ni\u2013Pt/SiO2 samples were dried at 100\u00a0\u00b0C overnight and treated using the same procedure as Pt/CeO2 to remove residual Cl\u2212. For H2-TPR characterization, some of Pt\u00a0+\u00a0Ni/SiO2 catalyst was calcined at 500\u00a0\u00b0C for 4\u00a0h, denoting as Pt\u00a0+\u00a0Ni/SiO2 calcined. The Pt\u2013Ni series catalysts were prepared by consecutive impregnation also but with the inverse sequence, namely the Cl\u22121 removed Pt/SiO2 or Pt/CeO2 sample was impregnated with an aqueous solution of Ni(NO3)2\u00b76H2O and then drying (100\u00a0\u00b0C) and calcining (500\u00a0\u00b0C).Temperature-programmed reduction (TPR) measurement was conducted on a TP-5000 apparatus (Tianjin Xianquan Adsorption Instrument Ltd Co., China). About 50\u00a0mg sample was loaded in a quartz tube reactor each run. The catalyst was heated from room temperature to 800\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C min\u22121 under a flow of 20\u00a0mL min\u22121 of a mixture gas of 10\u00a0vol% H2/Ar. The H2 consumption was monitored by a thermal conductivity detector.The crystal structure and phases of catalysts were determined on an XRD-6100 powder diffractometer (Shimadzu Analytical Instrument Co., Japan) using a Cu K\u03b1 source. It was operated at 50\u00a0kV and 30\u00a0mA with a scanning angle (2\u03b8) of 10\u00b0\u201380\u00b0 and scanning speed of 6\u00b0 min\u22121.Textural properties of catalysts were measured with NOVA 3000e (Quantachrome Instruments, USA) using N2 as the adsorbate at 77\u00a0K. The special surface areas were determined according to BET method.X-ray photoelectron spectroscopy (XPS) analysis was recorded on ESCALAB 250Xi spectrometer (Thermo Fisher Co., USA) using an Al K\u03b1 monochromator with a light spot of 500\u00a0\u03bcm. The binding energy is calibrated with a reference of C1s peak at 284.6\u00a0eV.High Resolution Transmission Electron Microscopy (HRTEM) analysis was performed on a JEOL JEM-2010 microscope operated at an accelerating voltage of 200\u00a0kV.The analysis of carbon content on the spent catalyst was operated on a Vario EL Cube analyzer (Elementar, Germany).The catalytic performance for SR of EG to hydrogen was carried out in a continuous flow fixed-bed micro-reactor at atmospheric pressure and 300\u00a0\u00b0C with 0.25\u00a0g catalyst loading. The catalysts were reduced at 400\u00a0\u00b0C for 1\u00a0h under a flow of 30\u00a0mL min\u22121 of pure H2 before running the reaction. And then the reactor was purged for 30\u00a0min with a flow of pure Ar, and then the system was cooled down to the reaction temperature. An aqueous solution of EG (10%) as the feedstock was pumped continuously via an HPLC pump (Beijing Weixing manufacturer, China) at a flow rate of 0.06\u00a0mL min\u22121 and vaporized by a band heater at 210\u00a0\u00b0C. The outlet gas products were analyzed online using a GC-950 gas chromatograph (Shanghai Haixin group Co., Ltd) with TCD detector and two columns of 5A molecular and TDX-01.In comparison of activity and selectivity over various catalysts, the data were reported by averaging three times analyses at steady state. In stability experiment, the data were directly recorded.The carbon conversion (X\nEG), H2 selectivity (\n\n\nS\n\n\nH\n2\n\n\n\n\n) and H2 yield (\n\n\nS\n\n\nH\n2\n\n\n\n\n) was calculated according to following definitions as described previously in the literature [14]:\n\n(1)\n\n\n\nX\n\nEG\n\n\n=\n\n\n\n(\n\n\nC\n\nCO\n\n\n+\n\nC\n\n\n\nCO\n\n2\n\n\n\n+\n\nC\n\n\n\nCH\n\n4\n\n\n\n\n)\n\n\n/\n\n2\n\nC\n\nEG\n\n\n\u00d7\n100\n\n\n%\n\n\n\n\n\n\n(2)\n\n\n\nS\n\n\nH\n2\n\n\n\n=\n\n\n\nC\n\n\nH\n2\n\n\n\n\n/\n\n\n(\n\n\nC\n\nCO\n\n\n+\n\nC\n\n\n\nCO\n\n2\n\n\n\n+\n\nC\n\n\n\nCH\n\n4\n\n\n\n+\n\nC\n\n\nH\n2\n\n\n\n\n)\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\n(3)\n\n\n\nY\n\n\nH\n2\n\n\n\n=\n\nX\n\nEG\n\n\n\u00d7\n\nS\n\n\nH\n2\n\n\n\n\u00d7\n100\n%\n\n\n\nwhere C\n\nx\n is the content of various product.The SR reaction pathway of EG is quite complex and many side reactions can take place. Extensive studies have evidenced that EG is firstly decomposed to produce H2 and CO, which is followed by water\u2013gas shift (WGS) reaction to convert CO to CO2, leading to more H2 production. In addition, the SR process of EG can be accompanied by side reactions such as methanation reaction, carbon deposition reaction and so on [33].To determine the fitful preparation method, we firstly investigated the influence of metal impregnation sequence of bimetallic catalysts on the performance of EG steam reforming at 300\u00a0\u00b0C during 2\u00a0h on stream. The activity of Pt\u2013Ni bimetallic catalysts supported on SiO2 followed an order of Pt1\u2013Ni9/SiO2\u00a0>\u00a0Ni9\u2013Pt1/SiO2\u00a0>\u00a0Pt1\u00a0+\u00a0Ni9/SiO2 (Table 2\n). The Pt1\u2013Ni9/SiO2 catalyst, prepared by first impregnation of Pt and then Ni, gave the highest H2 yield (60.9%) and EG conversion (88.5%). And this catalyst showed the lowest CO and CH4 contents in effluent gas. Ni9\u2013Pt1/SiO2 catalyst with inverse impregnation sequence presented 83.8% EG conversion and 58.2% H2 yield. Co-impregnating Pt1\u00a0+\u00a0Ni9/SiO2 catalyst has the lowest catalytic activity due to considerable amount of CO intermediate and low EG conversion. Based on these results, the optimized impregnation sequence of first Pt and then Ni is used for the preparation of other bimetallic catalysts.EG steam reforming was conducted to compare catalysts of monometallic Pt and Ni and bimetallic Pt\u2013Ni supported on SiO2 and CeO2. As shown in Table 3\n, all of the catalysts supported on SiO2 had considerable activity, with more than 85% EG conversion during 2\u00a0h on steam. Compared to monometallic Ni/SiO2, addition of Pt improved the catalytic activity. And the H2 yield increased with the increase of Pt/Ni ratio, Pt3\u2013Ni7/SiO2 catalyst exhibited the highest EG conversion (90.0%) and H2 yield (64.9%). By analyzing gas product distribution, a small amount of CH4 was detected over the Pt3\u2013Ni7/SiO2 catalyst, but no CO existed, which indicated that the Pt addition has positive effect on the WGS reaction.The activity and selectivity properties of monometallic catalysts (Pt, Ni) and bimetallic catalysts (Pt\u2013Ni) supported on CeO2 were shown in entries 4\u20139 of Table 3. Compared to Ni/CeO2, Pt\u2013Ni/CeO2 catalyst series also presented higher activity. And the influence of Pt/Ni ratio was consistent with that when SiO2 was used. The Pt3\u2013Ni7/CeO2 catalyst with the highest Pt content showed the highest EG conversion (87.8%) and H2 yield (63.5%) among CeO2 supported catalysts. As seen in entry 8 of Table 3, the 3 wt% Pt/CeO2 catalyst exhibited modest catalytic activity with low CH4 concentrations but with high CO content. In contrast, 10 wt% Ni/CeO2 catalyst (entry 4 in Table 3) generated considerable amount of CH4, but no CO was detected. Interestingly, Pt\u2013Ni bimetallic catalysts showed very high selectivity towards H2 with little CO and CH4 contents, suggesting the synergy effect between Ni and Pt.It was reported that both Ni and Pt single metal catalyst revealed a good ability in the C\u2014C bond rupture of the EG, but Pt has low activity for methanation and WGS, in contrary Ni exhibits high activity for both reactions [17,18]. Here the bimetallic Pt\u2013Ni catalysts promoted only the WGS reaction but inhibited the methanation reaction, resulting in increase of H2 selectivity. Thus the Pt\u2013Ni catalysts combined the advantages of Pt and Ni metals and avoided their disadvantages through the synergy interaction between Pt and Ni. To verify the existence of this synergy effect between Pt and Ni, 3\u00a0wt% Pt/CeO2\u00a0+\u00a07\u00a0wt% Ni/CeO2 hybrid by physical mixing was tested in EG steam reforming (last entry of Table 3). As expected, the catalytic performance was lower, because the metal amount of 3\u00a0wt% Pt/CeO2\u00a0+\u00a07\u00a0wt% Ni/CeO2 is only half of Pt3\u2013Ni7/CeO2. However, an appreciable amount of CO (7.3%) was detected over the mechanic mixing catalyst, far greater than 1.3% of Pt3\u2013Ni7/CeO2. Thus the increased activity and decreased CO selectivity of Pt\u2013Ni bimetallic catalysts should be attributed to the synergy interaction between Pt and Ni [18].The properties of support can influence the catalytic activity apparently. In the previous article [14], the Ni supported on CeO2 was proved favorable for the steam reforming reaction of EG due to the presence of surface oxygen vacancy on CeO2. However, in this study, the SiO2 supported catalysts always exhibited slightly higher activity than corresponding CeO2 supported ones. The reasons can be ascribed to the interaction of CeO2 with active metals which was discussed in detail in the following text.It was observed that the Ni catalysts is sensitive to deactivation in the steam reforming of biomass, especially in low reaction temperature [21]. For investigating the stability, the catalytic activity of monometallic Ni and bimetallic catalysts were tested with elongated reaction time under operating conditions (0.06\u00a0mL min\u22121 of 10% EG in water at 300\u00a0\u00b0C and\u00a01\u00a0atm). Fig.\u00a01\n showed the H2 yield and EG conversion over Ni/SiO2, Pt3\u2013Ni7/SiO2, Ni/CeO2 and Pt3\u2013Ni7/CeO2 catalysts. For the monometallic Ni catalyst, the activity of Ni/SiO2 was significantly higher than Ni/CeO2 at initial stage. But the EG conversion and the H2 yield decreased rapidly during 24\u00a0h on steam. It is well known that the coke is easy to form and deposit on the Ni surface at low reaction temperature, the rapid loss of activity can be considered as that the Ni active sites are blocked by the formation of coke [21]. As for Ni/CeO2, although the yield of H2 and the conversion of EG are lower at beginning, they decreased more slowly with prolonged reaction time, indicating that the stability of Ni/CeO2 was better than Ni/SiO2. It was reported that the rich oxygen vacancies of CeO2 surface are capable removing the carbon deposition and the interaction between NiO and CeO2 inhibits the aggregation of NiO particles [31].For the bimetallic Pt\u2013Ni catalyst, both of the two investigated catalysts showed very good stability and very high activity, as shown in Fig.\u00a01b. The H2 yield of Pt3\u2013Ni7/SiO2 and Pt3\u2013Ni7/CeO2 catalyst kept unchanged basically and the EG conversion declined slightly during 100 h-on-stream in EG steam reforming reaction. This evidenced that the addition of Pt into Ni catalyst not only improves the reaction activity but also enhance the stability.Selectivity of C1 gas products including CO, CH4 and CO2 in EG steam reforming during 24\u00a0h reaction time on Ni and Pt\u2013Ni catalysts was shown in Fig.\u00a02\n. For the supported Ni catalysts (Fig.\u00a02a), an increase of the CO content companied by a decrease of the CO2 content was observed on Ni/SiO2 with the time on stream. This was ascribed to the deactivation of Ni catalyst. However, the concentrations of CO and CO2 hardly changed over Ni/CeO2, evidencing a better stability. Ni supported on CeO2 showed higher selectivity of methanation than on SiO2. And a slight decrease of CH4 content over the two monometallic Ni catalysts was observed during the 24\u00a0h reaction period, which may related with the deactivation of catalysts.For Pt3\u2013Ni7/SiO2 catalyst (Fig.\u00a02b), the CO content gradually increased companied by the decrease of the CO2 content during the 24\u00a0h reaction time, but the rate of change was much slower than that over Ni/SiO2. Meanwhile, CH4 content only showed little drop. As for Pt3\u2013Ni7/CeO2 (Fig.\u00a02b), the selectivity of C1 gas products remained almost stable throughout the reforming process, further indicating the excellent stability. Furthermore, no CO was detected throughout the 24\u00a0h reaction time. Similar to the Ni monometallic catalysts, the bimetallic Pt3\u2013Ni7 supported on CeO2 presents higher selectivity for methane than supported on SiO2. But the content of CH4 was lower over Pt3\u2013Ni7/CeO2 catalyst than over Ni/CeO2 catalyst, suggesting that interaction of Pt with Ni can depress the methanation reaction.Compared Fig.\u00a02a and b, CeO2 as support exhibited higher stability for C1 components than SiO2, which is consistent with the results of EG conversion and H2 yield shown in Fig.\u00a01. But CeO2 lead to higher methane selectivity, a detrimental effect on the H2 production. This can be partly suppressed by the addition of Pt. Both Ni and Pt\u2013Ni supported on CeO2 showed almost zero CO and 25% CO2 selectivity, suggesting the beneficial effect of CeO2 to WGS reaction. The CO augment over Ni/SiO2 and Pt3\u2013Ni7/SiO2 with the extended reaction time can be attributed to the WGS reaction inhibition by the catalyst deactivation. With Pt addition, the deactivation was apparently slowed by comparison of Ni/SiO2 with Pt3\u2013Ni7/SiO2.Further evidence was obtained by measuring the carbon formation rate over the four catalysts after stability test in EG steam reforming, as shown in Table 4\n. For the monometallic Ni catalyst, the carbon content of Ni/SiO2 was significantly higher than Ni/CeO2. The high content of coke covered the active centers, leading to the low stability of Ni/SiO2. For the bimetallic Pt\u2013Ni catalyst, however, only a small amount of carbon deposition was observed after long reaction time (100\u00a0h), especially for Pt3\u2013Ni7/CeO2. This results implied that Pt can effectively inhibit carbon deposition to improve stability. The rate of carbon deposition followed an order of Pt3\u2013Ni7/CeO2\u00a0>\u00a0Pt3\u2013Ni7/SiO2\u00a0>\u00a0Ni/CeO2\u00a0>\u00a0Ni/SiO2, consistent with the stability performance of the catalysts. The main cause of catalyst deactivation can be attributed to the carbon deposition, which blocks or covers the active sites.The reaction equation of EG steam reforming can be expressed simply as:\n\n(4)\n\n\n\nC\n2\n\n\nH\n6\n\n\nO\n2\n\n+\n2\n\nH\n2\n\nO\n=\n5\n\nH\n2\n\n+\n2\n\n\nCO\n\n2\n\n.\n\n\n\n\nAt full conversion of EG, the carbon conversion (X\nEG) is determined as 100%, and H2 selectivity (\n\n\nS\n\n\nH\n2\n\n\n\n\n) is calculated as 71.4% according to Eqs (1) and (2), so the ideal or the highest H2 yield for the EG steaming reform is 71.4% using this valuating method. The stability experiments showed that during 100\u00a0h reaction the H2 yield over the Pt modified catalysts can keep at the extent of 60\u201365%, corresponding to 84\u201391% of the ideal H2 yield. These results are among the best results of EG steam reform observed in literature [12,15,18].\nFig.\u00a03\n shows the H2-TPR profiles of Pt and Ni bimetallic catalysts supported on SiO2 prepared with different impregnation sequence. For Pt1\u00a0+\u00a0Ni9/SiO2 and Ni9\u2013Pt1/SiO2, only a small peak at low temperature (164\u00a0\u00b0C and 143\u00a0\u00b0C, respectively) was observed. The reason is that the two catalysts had been previously reduced to remove the residue Cl\u22121 in catalyst preparation, the small peak was the reduction of surface Ni oxide [24]. This was evidenced by the calcination experiment of Pt1\u00a0+\u00a0Ni9/SiO2, a much larger peak appears at about 290\u00a0\u00b0C for Pt1\u00a0+\u00a0Ni9/SiO2 calcined (Fig.\u00a01c). The Ni/SiO2 shows a single reduction peak at 375\u00a0\u00b0C attributed to the reduction of NiO phase. The addition of Pt results in a shift of the reduction peak to the lower temperature.The H2-TPR result shows that the impregnation sequence can apparently affect the reducibility of bimetal catalysts. As we know, the Pt oxide is much easier to be reduced than Ni oxide. During reduction of Pt\u2013Ni oxide catalysts, the Pt oxide is reduced to metal before the Ni oxide. Thus the hydrogen is firstly activated on Pt surface, then the active hydrogen is transferred to Ni by the hydrogen spillover. Meanwhile, the Ni can also be reduced by the electron transfer from Pt metal. With an addition sequence of Ni first and then Pt, the Pt was exposed on surface, which is favorable for the hydrogen activation and hydrogen spillover, thus this Ni9\u2013Pt1/SiO2 catalyst was reduced at lowest temperature (Fig.\u00a03b). With co-impregnation, Ni and Pt is strongly interacted, the hydrogen activation become relatively difficult owing to less Pt on the surface, thus the reduction temperatures of Pt1\u00a0+\u00a0Ni9/SiO2 and Pt1\u00a0+\u00a0Ni9/SiO2 (calcined) were higher than that of Ni9\u2013Pt1/SiO2. As for Pt1\u2013Ni9/SiO2 with Pt impregnating first and then Ni, Ni was sufficiently exposed on surface, contrarily, the Pt was covered by Ni, leading to that the hydrogen activation and reduction of Ni oxide occurred at higher temperature. But this catalyst still exhibited lower reduction temperature than Ni/SiO2. The interaction between Ni and Pt facilitated the electron transfer from Pt to Ni responsible for the enhanced reducibility.From H2-TPR results of catalysts with different impregnation sequence, it is suggested that the reduction of Ni9\u2013Pt1/SiO2 is mainly through hydrogen spillover, electron transfer plays a less important role. On the contrary, the reducibility of Pt1\u2013Ni9/SiO2 is mainly influenced by the electron transfer from Pt to Ni. And both hydrogen spillover and electron transfer contribute to the reduction of Pt1\u00a0+\u00a0Ni9/SiO2 series.Although the Ni9\u2013Pt1/SiO2 and Pt1\u00a0+\u00a0Ni9/SiO2 showed better reducibility, they had less H2 yield than Pt1\u2013Ni9/SiO2. The order of SR activity of EG is Pt1\u2013Ni9/SiO2\u00a0>\u00a0Ni9\u2013Pt1/SiO2\u00a0>\u00a0Pt1\u00a0+\u00a0Ni9/SiO2\u00a0>\u00a0Ni/SiO2. Thus the reducibility of metals was not necessary to be directly related with the catalytic activity. It is deduced that the surface metal dispersion and synergy interaction of Ni and Pt contribute to the activity and selectivity of SR reaction of EG. The intimated contact and strong Ni\u2013Pt interaction in Pt1\u00a0+\u00a0Ni9/SiO2 owing to the co-impregnation resulted in high CO and CH4 content with low EG conversion and H2 yield. Ni9\u2013Pt1/SiO2 with the sequence of Ni first and then Pt impregnation had proper interaction showed improved catalytic activity. And Pt1\u2013Ni9/SiO2 with inverse impregnating sequence had Ni surface enrichment and proper interaction, giving the best catalysis performance.The influence of support and the Pt/Ni ratio on the reducibility was also studied by the H2-TPR (Fig.\u00a04\n). As expected, the single reduction peak of Ni/SiO2 shifted to a lower temperature with the Pt/Ni ratio increases owing to the low temperature reducibility of Pt. The profile of Ni/CeO2 showed three typical reduction peaks at 217, 276 and 371\u00a0\u00b0C, which reflected the strength of the interaction between NiO particles and CeO2 (no interaction, weak interaction and strong interaction, respectively). The Pt\u2013Ni/CeO2 catalysts show only two reduction peaks, the lower peak temperature is attributed to the reduction of PtO\nx\n. It's worth noting that reduction peak of Ni species at higher temperature and Pt species are obviously shifted to low temperature with the Pt/Ni ratio increases. The shift can also be assigned to hydrogen spillover and the electron transfer from Pt to Ni [27,34]. The peak at lowest temperature, which is attributed to the NiO particles without interaction with CeO2 disappeared, and the peak area at lower temperature increase gradually in the expense of the peak area at higher temperature with the increase of Pt/Ni ratio. These suggested that the strong Ni interaction with CeO2 was weakened with addition of Pt. The TPR data are consistent with that of Palma et\u00a0al. [35].By comparison of SiO2 supported catalysts with CeO2 supported ones, it is observed that Ni and Pt\u2013Ni metals are easier to be reduced on CeO2 support. And the interactions of Ni metal with CeO2 mainly possess two states with different strengths, which is attributed to co-existence of Ce3+ and Ce4+ ions. The electron transfer from the oxygen vacancy can facilitate the reduction of active metal oxide. But the increased reducibility did not improve the catalytic activity. The CeO2 supported catalysts showed lower EG conversions and H2 yields than corresponding SiO2 ones during 2\u00a0h reaction. This can be attributed to the interaction of active metals with the CeO2\n[31]. However, this interaction of CeO2 certainly resulted in improved stability.\nFig.\u00a05\n showed XRD patterns of the SiO2 supported catalysts. For all the catalysts, diffraction peaks appear at approximately 36.9\u00b0, 43.3\u00b0, 62.7\u00b0, which are corresponding to the (111), (200), (220) planes of NiO, respectively. By increasing Pt/Ni ratio, the diffraction peaks of NiO move to the low angle region and became wider, indicating that the existence of interaction between Pt and Ni atoms [29,36]. The shift of NiO peaks to lower angle illustrates the increase of distance between crystal surfaces, which may be ascribed to that some Ni ions were substituted by Pt ions with larger radii. The peak broadening is due to smaller NiO particles, suggesting the better dispersion with Pt addition. No Pt diffraction peaks were found, which indicated that Pt metal is highly dispersed on the catalysts.The XRD patterns of the catalysts supported on CeO2 were shown in Fig.\u00a06\n. Regarding CeO2, the diffraction peaks are located at 28.5\u00b0, 33.3\u00b0, 47.5\u00b0and 56.3\u00b0, corresponding to the cubic fluorite structure of CeO2. Compared with the catalysts supported on SiO2, the peak intensity of NiO was lower, indicating a well-dispersed NiO phase on CeO2\n[14]. And no Pt diffraction peaks are detected. This illustrates that INTERACTION of CeO2 enhance the dispersion of Ni and Pt species. And the Pt modification can further increase the dispersion of NiO, which was evidenced by the decreased diffraction peaks of NiO in Fig.\u00a06b\u2013d.The textural properties of Ni/SiO2, Pt3\u2013Ni7/SiO2, Ni/CeO2 and Pt3\u2013Ni7/CeO2 catalyst were summarized in Table 5\n. Larger specific surface area and pore volume were observed for the catalysts supported on SiO2, and larger pore was obtained with the CeO2 as support. The addition of Pt does not significantly change the textural properties of the catalysts, only the specific surface area is slight decreased, probably due to the twice calcinations process leading to little sinter of oxide support.XPS analysis was used to investigate the valence states of elements and the nature of the surface of reduced catalysts. The XPS spectra of Ce 3d, O 1s, Pt 4f and Ni 2p were illustrated in Fig.\u00a07\n and detailed results of XPS data were summarized in Tables 5 and 6\n. The spectra region of Ce 3d core usually lies within 880 and 920\u00a0eV, complicated spectra are obtained due to the presence of multiple oxidation state and spin orbit coupling [22]. As shown in Fig.\u00a07A, the main features are represented by two sets of peaks (V, V\u2032, V\u2033, V\u2034 and U, U\u2032, U\u2033, U\u2034). The peaks labeled as V, V\u2033, V\u2034 or U, U\u2033, U\u2034 are assigned to 3d3/2 and 3d5/2 of Ce4+ respectively and the peaks labeled as V\u2032 or U\u2032 are attributed to 3d3/2 and 3d5/2 of Ce3+ respectively. It notes that the three catalysts all contain a small percentage of Ce3+ oxide. As reported in early paper [37], the existence of Ce3+ implies the defect structure of CeO2-x\n due to oxygen vacancies. The Ce3+/(Ce3+\u00a0+\u00a0Ce4+) ratios were calculated according to the measured peak areas (Table 6). This ratio showed no apparent difference in three catalysts, suggesting that impregnation of active metals may not change the reduction of CeO2. But the binding energy shifts to higher energy with the impregnation of metals, suggesting interaction of CeO2 support with the active metals.XPS spectra of O 1s region is composed of two obvious peaks, as shown in Fig.\u00a07B. The peak with lower binding energy (labeled as O I) is ascribed to the lattice oxygen in CeO2 and the high binding energy peak (labeled as O II) is assigned to the chemisorbed oxygen species or defect-oxide involving the surface oxygen vacancies [22,25]. The percentage of O II in total oxygen was calculated in the same way as for Ce3+ (Table 6). Unlike the percentage of Ce3+, the O II contents show apparent difference among CeO2 supported catalysts. It is due to that O II species not only relate with the oxygen vacancies but also involve in the oxygen species interacting with active metals. Thus the O II percentage in some extent reflects the interaction of CeO2 support with active metals. It can be seen that Pt3\u2013Ni7/CeO2 possess the highest O II content among the three catalysts. This partly explains its high stability and activity [38].The XPS results of Pt 4f were shown in the Fig.\u00a07C. The two peaks at about 71.5 and 74.8\u00a0eV are attributed to Pt 4f7/2 and Pt 4f5/2. These two peaks can be decomposed further according to the valence state of Pt [27,33]. For 3%Pt/CeO2, the Pt 4f7/2 peak at 71.5\u00a0eV and the Pt 4f5/2 peak at 74.8\u00a0eV are ascribed to Pt0. The Pt 4f7/2 peak at 72.8\u00a0eV and the Pt 4f5/2 peak at 76.0\u00a0eV are assigned to Pt2+. In the case of Pt3\u2013Ni7/SiO2 and Pt3\u2013Ni7/CeO2, an additional peak appeared at low binding energy region at about 68.5\u00a0eV, which is attributed to the Ni 3p species. Table 7\n gives the distribution of different valence states of Pt species on the catalysts. The content of Pt0 in Pt3\u2013Ni7/CeO2 is higher than that in 3%Pt/CeO2. This is due to the interaction of Pt with Ni. And the highest Pt0 (69.8%) is detected on Pt3\u2013Ni7/SiO2 catalyst owing to the weakened interaction of SiO2 with Pt.The Ni 2p core level profiles of all the Ni containing catalysts are shown in Fig.\u00a07D. It can be seen that the Ni 2p3/2 spectra region is broad including multiple valence state (Ni0, NiO and Ni2O3) between 851 and 859\u00a0eV. In addition, the Ni 2p1/2 region partially overlaps with the low binding energy tail of Ce 3d5/2 region [27]. The binding energy of Ni 2p1/2 and Ni 2p3/2 of all Ni containing catalysts are summarized in Table 7. The addition of Pt did not change the binding energy of Ni 2p for the SiO2 supported catalysts. But Ni 2p peak shifts to the higher binding energy region for Pt3\u2013Ni7/CeO2 when compared with Ni/CeO2. This illustrates that the interaction of Pt and Ni suppressed the electron transfer from the oxygen vacancy.\nFig.\u00a08\n shows the HRTEM images of fresh Ni/SiO2, Ni/CeO2, Pt3\u2013Ni7/SiO2 and Pt3\u2013Ni7/CeO2 catalyst. With the amorphous SiO2 supported ones, the metals particles are not even dispersed and the morphology is not regular also. The size of metal particles are several to a dozen nanometers. The weak support interaction of SiO2 give rise to high activity but also lead some sinter [35,39]. On the other hand, the metal and CeO2 particles are dispersed more evenly with the size of about 10\u00a0nm and no obvious aggregates can be observed. Owing to the interaction of active metal with CeO2, the enlargement or sinter of active metals are inhibited. This may have some contribution to the stability of CeO2 based catalysts.From the characterization and catalytic experiments, it was observed that the activity, H2 selectivity and stability of EG SR over Ni-based catalysts were significantly improved due to Pt addition. Using the Pt first and then Ni impregnation sequence, the Ni was enriched on the surface and was interacted with Pt through the electron transfer from Pt to Ni. These facilitate the C\u2014C rupture and WGS reactions, meanwhile inhibit the methanation reaction. However, the Pt\u2013Ni interaction cannot be too strong, which was the situation of co-impregnation Pt\u00a0+\u00a0Ni catalyst, and cannot be too weak, such as the mechanical mixing catalyst. Increasing the Pt content improves the reaction properties. Thus Pt3\u2013Ni7 catalysts with the suitable interaction showed the best catalytic performance in our study.The main reason of deactivation of Ni-based catalysts is the coke formation. It is more serious at low temperature. With addition of Pt, the Ni site is stabilized in the reduced state by the electron transfer and the hydrogen spillover from Pt to Ni [24], thus the formation of coke carbon species on the Ni surface can be efficiently inhibited.The interaction of CeO2 with Ni and Pt stabilize the active metals avoiding sinter at reaction temperature. And surface oxygen vacancy on CeO2 like Pt can provide electron transfer, stabilizing the Ni in reduced status. Thus the usage of CeO2 support further improved the stability of Ni and Pt\u2013Ni catalysts. However, the interaction of CeO2 support with active metals lowered the catalytic activity, resulted in the decreases of EG conversion and H2 yield, especially at initial reaction stage. These comparison investigations between Ni and Pt\u2013Ni, as well as SiO2 and CeO2, provide important clues for the H2 production.As for hydrogen production by EG steam reforming at low temperature (300\u00a0\u00b0C) over Pt\u2013Ni series. The impregnation sequence of active metals influenced the catalytic activity apparently. The Pt\u2013Ni catalysts with Pt first and then Ni impregnating sequence possessed proper Pt\u2013Ni interaction and the Ni enrichment on surface, showing high activity and H2 selectivity. It is due to that the high methanation activity of Ni was suppressed by Pt modification. And the significant role of Ni in WGS reaction was remained. Increasing Pt/Ni ratio further increased activity and H2 selectivity, suggesting the importance of Pt addition. The stability experiment showed that the deactivation rate of Ni/CeO2 is lower than Ni/SiO2 mainly due to the less coke deposition. The existence of surface oxygen vacancy of CeO2 leads to the electron transfer from support to Ni, which can suppress carbon deposition formation. Interaction of CeO2 with Ni metals increase the dispersion avoiding the sinter, but decrease the catalytic activities. Pt stabilizes the Ni0 and suppress the coke formation by the electron transfer and hydrogen spillover. Thus the Pt3\u2013Ni7 bimetallic catalysts exhibited the excellent activity and stability.There is no conflict of interest.The work was supported by Natural Science Foundation of China (Grant 21273193, 21473231 and 20973148). The authors also gratefully thank Mrs. Gao Ling from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, for XPS analysis and Prof. Song Liguo from department of chemistry, University of Tennessee-Knoxville, for improving language level of the manuscript.", "descript": "\n                  Hydrogen production by steam reforming of ethylene glycol (EG) at 300\u00a0\u00b0C was investigated over SiO2 and CeO2 supported Pt\u2013Ni bimetallic catalysts prepared by incipient wetness impregnation methods. It was observed that impregnation sequence of Pt and Ni can affect the performance of catalysts apparently. Catalyst with Pt first and then Ni addition showed higher EG conversion and H2 yield owing to the Ni enrichment on the surface and the proper interaction between Pt and Ni. It was observed that although SiO2 supported catalysts exhibited better activity and H2 selectivity, CeO2 supported ones had better stability. This is attributed to the less coke formation on CeO2. Increasing Pt/Ni ratio enhanced the reaction activity, and Pt3\u2013Ni7 catalysts with 3 wt% Pt and 7 wt% Ni showed the highest activity and stability. Ni surficial enrichment facilitated the C\u2014C bond rupture and water gas shift reactions; and Pt addition inhibited methanation reaction. Electron transfer and hydrogen spillover from Pt to Ni suppressed carbon deposition. These combined effects lead to the excellent performance of Pt3\u2013Ni7 supported catalysts.\n               "}
{"full_text": "The abrupt climate changes caused by global warming and the non-stoppable ever-increasing energy consumption are complex challenges humankind must urgently deal with. The extreme dependence of up-to-date energy production technologies on non-renewable sources had been of negative effect on environmental and energy security issues [1]. Environmentally, in order to maintain the present standard of living, producing energy from non-renewable sources is unsustainable as it is directly or indirectly correlated to large amounts of greenhouse gases [1,2]. Economically, the establishment of a competitive energetic matrix, minimally dependent on foreign oil-based sources, is a matter of national sovereignty and of external political relationships [3,4]. A viable solution for the problem presented above could be the development of the so-called hydrogen economy, in which hydrogen would be inserted in the energetic matrices, diminishing crude-oil participation.Hydrogen is considered an important energetic vector for the future generations and its production techniques have been exhaustively studied in the literature [5\u20139]. This important energy source can be produced by various routes as exemplified by coal and biomass gasification [6,7,10], steam reforming and partial oxidation of ethanol and the consolidated catalytic steam reforming of methane [11\u201315]. An interesting process that has called attention recently is the steam reforming of bio-oil produced by different types of biomass transformation. Bio-oil is a complex mixture of at least 200 different compounds, including acids, aldehydes, ketones, alcohols and lignin oligomers emulsified in aqueous medium [16\u201320]. Biomass derived liquids composition is fully dependent of the biomass origin and the applied technology used on its conversion. Due to the complexity of bio-oil composition and to the fact that the catalytic phenomena are utterly correlated to the substrate/surface interactions, authors have studied the catalytic activation of its major component, acetic acid, which can have a 12\u201314\u202fwt% content in bio-oil [18,21,22].The steam reforming of acetic acid is presented by Eq. (1):\n\n(1)\n\n\n\nCH\n3\n\nCOOH\n+\n2\n\nH\n2\n\nO\n\u2192\n2\n\nCO\n2\n\n+\n4\n\nH\n2\n\n\n\n\n\nSteam reforming of acetic acid has been studied on different catalytic systems, such as supported noble metals [5,23,24]. Supported noble metals catalysts showed themselves quite active and relatively stable against coke formation, but these systems have the disadvantage to be quite expensive when compared to nickel and copper-based materials [16,25]. The advantage in using nickel-containing catalysts is due to its low price and relatively high availability if compared with noble metals. It has been reported that nickel-based catalysts are as active as noble metal systems but deactivation phenomena due to coke formation is still a problem to be deeper understood in those systems. To better understand the effect of the synthesis route on the steam reforming of acetic acid, Xiang and co-workers [26] have synthesized diverse batches of Ni/\u03b3-Al2O3 using nitrate, chlorate, acetate and Sulphur-containing nickel precursors calcined at 600\u202f\u00b0C. The work demonstrated the inadequacy of S-containing synthesis routes due to nickel silicate formation and the interaction of alumina with chlorine revealed to be the main reason to the support\u2019s sintering. The acetate and nitrate routes produced Ni-based catalysts with similar activity towards syngas production, being the coke formation still an issue. Lee and co-workers [27] have evaluated the effect of Ni/\u03b3-Al2O3 catalyst promotion with Mg, La, K and Cu dopants on acetic acid conversion and coke suppression. Mg doping was shown to suppress coke formation circa 60% by comparison with unpromoted Ni/\u03b3-Al2O3 and the presence of lanthanum and potassium seemed to have contributed to an increase in the total catalyst\u2019s basicity. Still on the evaluation of the effect of synthesis route on the final performance of Ni-based catalysts, Lago and co-workers [28] have verified that destruction of ABO3 perovskite-type precursors in a reducing atmosphere, prior to the reaction, could generate a B0/A2O3 catalyst with B0 metallic particles finely dispersed on A2O3 oxide. During the last decade Noronha and co-workers have been studying the application of perovskite-type oxides on syngas production [29\u201333]. Specifically, on steam reforming of acetic acid, a positive effect of La-site substitution by Pr and Sm was observed on the suppression of coke formation, without significative effect on acetic acid conversion. The authors agreed that the tradeoff between rare earth\u2019s elevated cost and their activity respect to acetic acid conversion makes their large-scale utilization not feasible.In the present work, nickel-based catalysts were obtained in situ, by hydrogen reducing treatment of the La1\u2212xCaxNiO3 system and their activity towards syngas and stability due to coking formation were evaluated in the steam reforming of acetic acid during 23\u202fh time on stream (TOS).Perovskites La1\u2212xCaxNiO3 (x\u202f=\u202f0, 0.15, 0.30, 0.50) were prepared by citrate method [34]. This method consists in a simple dissolution of stoichiometric amounts of metallic nitrates, together with an excess of citric acid, that is introduced to guarantee first the formation of the gel-like viscous syrup after water evaporation and finally, the formation of the glassy material that will be calcined in further steps [35]. Stoichiometric quantities of lanthanum, calcium and nickel nitrates were dissolved in water, then citric acid was added to this solution, being the ratio between citric acid molar amount and total molar amount of metallic ions equal to 1.5. The solution was heated for 1\u202fh at 90\u202f\u00b0C until the formation of a gel that immediately was calcined in air in different steps: 100\u202f\u00b0C for 1\u202fh; 300\u202f\u00b0C for 2\u202fh; 800\u202f\u00b0C for 4\u202fh, always with heating rate equal to 10\u202f\u00b0C/min. The non-Ca containing perovskite was named LaNiO3 and the calcium-containing samples were named Ca 15%, Ca 30% and Ca 50% (molar fraction based).X-Ray Powder Diffraction (XRD) analyses of the calcined samples were conducted at room temperature with Cu K\u03b1 radiation (\u03bb\u202f=\u202f1.5418\u202f\u00c5) using a Shimadzu XRD-6000 diffractometer. Data were collected in the 2\u03b8 range of 10\u00b0 to 80\u00b0, with a scan rate of 0.25\u00b0/min. Temperature programmed reduction (TPR) analysis was performed by reducing an amount of 20\u202fmg of perovskite diluted with 20\u202fmg of quartz powder in a flow of 30\u202fmL/min (5% H2/He) from room temperature until 800\u202f\u00b0C at a ramping rate of 10\u202f\u00b0C/min and the hydrogen consumption was monitored by mass spectrometry (Balzer, model QMS 200). In situ diffraction experiments were conducted in a furnace installed into a Huber goniometer operating in Bragg\u2013Brentano geometry at the D10B-XPD beamline at the Laborat\u00f3rio Nacional de Luz S\u00edncrotron at Campinas \u2013 S\u00e3o Paulo, using a radiation with \u03bb\u202f=\u202f1.5500\u202f\u00c5. The analyses were performed during the reduction of the catalyst precursors under a flow of 30\u202fmL/min of 5% H2/He mixture.Steam reforming (SR) of acetic acid was performed in a fixed-bed reactor using a Microactivity Reference equipment (PID Eng & Tech.) previously described in [29,30]. The samples (10\u202fmg of catalyst diluted in 150\u202fmg of SiC) were previously reduced with 30\u202fmL/min pure H2 stream at 800\u202f\u00b0C for 1\u202fh. The reactant mixture containing a water/acetic acid ratio of 3 on molar basis was pumped (0.25\u202fmL/min) into a vaporizer and diluted with He (200\u202fmL/min) and then, it was fed to the reactor. Initial proof experiments and thermodynamic calculations presented in topic 3.3 were performed to determine the range of temperature for the catalytic tests in order to minimize carbon formation. Being so, LaNiO3 catalyst were evaluated from 400 to 700\u202f\u00b0C and the Ca-containing catalysts at 600\u202f\u00b0C. Reactants and products were analyzed by gas chromatography (Agilent 7890A) equipped with a thermal conductivity detector (TCD) and a Porapak Q column.Eqs. (2), 3 and 4 were used to calculate conversion, compositions, molar flow in the entrance (ninlet) and outlet (noutlet) of the reactor.\n\n(2)\n\n\nConversion\n\no\nf\n\nr\ne\na\nc\nt\na\nn\nt\n\ni\n:\n\n\nX\ni\n\n=\n100\n\n\u2217\n\n\n\n\nn\n\ni\n\n\ni\nn\nl\ne\nt\n\n\n-\n\nn\n\ni\n\n\no\nu\nt\nl\ne\nt\n\n\n\n\nn\n\ni\n\n\ni\nn\nl\ne\nt\n\n\n\n\n\n\n\n\n\n(3)\n\n\nComposition\n\no\nf\n\np\nr\no\nd\nu\nc\nt\n\ni\n:\n\n\nY\ni\n\n=\n100\n\n\u2217\n\n\n\nn\n\ni\n\n\no\nu\nt\nl\ne\nt\n\n\n\nn\n\nt\no\nt\na\nl\n\n\no\nu\nt\nl\ne\nt\n\n\n\n\n\n\n\n\n\n(4)\n\n\n\nn\n\nt\no\nt\na\nl\n\n\no\nu\nt\nl\ne\nt\n\n\n=\n\u2211\n\nn\n\ni\n\n\no\nu\nt\nl\ne\nt\n\n\n,\ni\n=\nA\nc\ne\nt\ni\nc\n\na\nc\ni\nd\n,\n\nH\n2\n\nO\n,\n\nH\n2\n\n,\nC\nO\n,\n\n\nCO\n\n2\n\n,\n\n\nCH\n\n4\n\n\na\nn\nd\n\na\nc\ne\nt\no\nn\ne\n\n\n\n\n\nFig. 1\n shows the XRD diffraction lines of the calcined La1\u2212xCaxNiO3 samples.XRD patterns of the calcined LaNiO3 sample revealed the characteristic lines of LaNiO3 rhombohedral phase (PDF 33-0711), as mentioned in literature [36], indicating that the perovskite structure is the main phase obtained after calcination for the Ca-free sample (x\u202f=\u202f0). The substitution of Lanthanum by Calcium resulted in the appearance of segregated La2NiO4 (PDF 34-0314), CaO (PDF 03-1123) and NiO (PDF 44-1159). Lima and co-workers [34] have showed that values of x\u202f\u2265\u202f0.1 on La1\u2212xCaxNiO3 system unfavored the formation of the perovskite-type structures, giving path to the segregation of part of the cations into NiO, CaO and La2NiO4 spinel. Fig. 2\n shows the LaNiO3 TPR-H2 profile and some of the temperatures retained for in situ XRD studies (\u03bb\u202f=\u202f1.5418\u202f\u00c5) presented by the Figs. 3\u20135\n\n\n.The reduction profile for LaNiO3 (Fig. 2) shows four events. The first event starts at 171\u202f\u00b0C followed by a shoulder at 230\u202f\u00b0C that is rapidly superimposed to a peak with its maximum value at 330\u202f\u00b0C. Until 280\u202f\u00b0C as shown in Fig. 3, there is no detectable difference between the corresponding diffractograms, indicating that the initial hydrogen consumption could be correlated to the gradual oxygen loss until the maximum structural limit, with the formation of LaNiO2,7, as proposed by Jia et al. [37]. The transformation presented by the perovskite between 280\u202f\u00b0C and 420\u202f\u00b0C can be verified in those diffractograms presented in Fig. 4 and corresponds to the reduction of Ni3+ to Ni2+, forming La2Ni2O5 (PDF 36-1230). This latter transformation is totally in agreement with the literature [38,39]. The difference between LaNiO3 and La2Ni2O5 (2\u03b8\u202f=\u202f26; 28,8; 29,86; 39,16) XRD profiles are very subtle as already shown by Valderrama et al [38]. The conversion from Ni2+ to Ni0 is favoured at temperatures higher than 420\u202f\u00b0C, as shown in Fig. 5, generating Ni0 (PDF 00-001-1258) supported on lanthanum oxide (PDF 01-073-2141). TPR allied to in situ XRD measurements showed that the destruction of La2Ni2O5 to generate Ni0/La2O3 occurs in the 420\u2013600\u202f\u00b0C temperature range. The TPR profiles of La1\u2212xCaxNiO3 precursors were significantly changed and shifted to higher temperatures as can be seen in Fig. 6\n.These profiles presented a first peak of H2 consumption at (400\u2013440\u202f\u00b0C), followed by a broad peak (450\u2013620\u202f\u00b0C) and a third peak at high temperature (650\u2013670\u202f\u00b0C). Attempting to obtain a better understanding also on the redox properties of those calcium-containing precursors, studies of in situ XRD were performed with Ca 50%. The results can be seen in Fig. 7\n. As a matter of fact, the analysis of Ca 50% can be extended to the other calcium-containing materials as the TPR profiles are quite similar.For the La0.5Ca0.5NiO3 precursor, the diffractogram obtained after reduction at 490\u202f\u00b0C showed that the intensity of the diffraction lines characteristic of La2NiO4 phase decreased whereas weak lines due to La2O3 and metallic Ni were observed. In the TPR profile of this catalyst, the first hydrogen consumption (maximum at 440\u202f\u00b0C) can be attributed to the reduction of segregated NiO, in agreement with the disappearance above 490\u202f\u00b0C of the corresponding diffraction line (2\u0398\u202f=\u202f63\u00b0). In the presence of calcium (50%) La2NiO4 appears more stable at high temperature if compared to the Ca-free system. The TPR reduction peak above 645\u202f\u00b0C is therefore attributed to the complete reduction of La2NiO4\n[40,41]. Fig. 7 exhibits only the lines of La2O3 and metallic Ni after further increase of the reduction temperature up to 740\u202f\u00b0C. Although the average size of La2NiO4 crystallites is apparently not affected by the Ca content, the average crystallite size of reduced nickel crystallites obtained after reduction of the catalysts is affected by the presence of Ca. the size of Ni\u00b0 crystallites increases strongly when adding 15% of Ca in the LaNiO3 perovskite, and then in a more moderate way when going from 15 to 50% of Ca. This increase in Ni\u00b0 crystallite size when increasing Ca content is probably linked to the difficulty to fully reduce the catalysts as evidenced by TPR curves in Fig. 2. The maximum temperature of the final reduction process is 510, 655, 670, 680\u202f\u00b0C for the catalysts containing respectively 0, 15; 30 and 50% Ca. The higher the final reduction temperature, the more difficult reduction, but the higher the Ni\u00b0 diffusion rate, generating therefore larger Ni\u00b0 crystallite sizes. The sintering of Ni crystallites is apparently not directly correlated to the crystallite size of the parent oxide structure, suggesting that the reduction destroy at least in part the spatial organization of the precursors. Then, together with an agglomeration of Ni\u00b0 species during reduction, LaOx species also reorganize to generate La2O3 based compounds associated in an unknown way to Ca species. As no change in metallic nickel phase diagram is observed after reduction, the amount of Ca inside Ni\u00b0 structure must be negligible. Table 1\n shows the average crystallite size of La2NiO4 found on calcium-containing calcined samples and the metallic Ni formed after reduction in situ at 800\u202f\u00b0C, calculated by Debye Scherrer Equation, using the respective most intense diffraction lines (La2NiO4 at 2\u03b8\u202f=\u202f32 and Ni0 at 2 \u03b8\u202f=\u202f44,2).The average size of La2NiO4 crystallites did not change significantly with calcium content, but the size of metallic nickel crystallites formed after \u201cin situ\u201d reduction, is clearly increased as the amount of Ca is increased.In order to check the feasibility of hydrogen production from acetic acid steam reforming on the conditions previously detailed in the experimental section, thermodynamic data were calculated by Gibb\u2019s free energy minimization using a home-made code with Mathematica\u00ae software. It was considered the presence of carbon graphite in the equilibrium and the ideality for all the gaseous species. In the experimental conditions, the steam reforming of acetic acid leads only to hydrogen, acetone, methane, carbon dioxide and monoxide gaseous products and being so, the thermodynamic allowed conversion of acetic acid and water and the products composition (dry basis) are confronted with the homogeneous experimental results. The confrontation is shown in Fig. 8\n(A)\u2013(C).The complete conversion of acetic acid in this range of temperature is not limited by thermodynamics but its steam reforming in gas phase is kinetically unfavoured. The maximum conversion is reached only at 700\u202f\u00b0C and it is below 10%. The conversion of water is even lower as can be seen in Fig. 8(A). In the equilibrium, as can be seen in Fig. 8(B), only traces of acetone and solid carbon were expected in the 400\u2013700\u202f\u00b0C range and the formation of hydrogen and carbon monoxide are favoured as the temperature increases. In the entire range of temperature acetic acid is not found in the equilibrium and at 400\u202f\u00b0C it is fully decomposed in methane and carbon dioxide. The presence of hydrogen at low temperatures accompanies water and methane consumption, being the presence of the later directly correlated to the velocity in which the hydrogen is released. Once methane lacks in the system hydrogen tends to find its maximum yield and the concentration of water does not change anymore. The quantity of carbon expected by thermodynamics is irrelevant.Homogeneous run is presented in Fig. 8(C). Carbon balances shows that the conversion of acetic acid is small but leads to a great formation of coke. The carbon containing substances in gas phase does not justify the quantity of acetic acid converted and only at 700\u202f\u00b0C traces of gaseous products were detected.In order to understand the effect of temperature on catalytic steam reforming of acetic acid, LaNiO3 sample was tested on 23\u202fh TOS in a 400\u2013700\u202f\u00b0C range, with increments of 100\u202f\u00b0C. The results of conversion and compositions are shown in Fig. 9\n.The increment in temperature had a significant effect in acetic acid conversion and on products distribution. At 400\u202f\u00b0C, the low acetic acid conversion leads almost completely to its ketonization product (Eq.8) and coke, and it is completely in agreement with the presence of carbon dioxide and acetone in gas phase.\n\n(8)\n\n\nKetonization\n:\n\n2\nC\n\nH\n3\n\nC\nO\nO\nH\n\u2192\n\n\n(\n\n\nC\nH\n\n3\n\n)\n\n2\n\nC\nO\n+\n\nH\n2\n\nO\n+\n\n\nC\nO\n\n2\n\n\n\n\n\nIn the first hours of reaction the conversion of acetic acid is low but rapidly increases as hydrogen is being released in the system and this fact could be correlated to the oxido-reducing character of the gas composition. The gas composition at the beginning could partially reoxidize the Ni0 species and as hydrogen is been produced, nickel oxidized species tend to return to their original metallic state. At this temperature water conversion was very low and so are the yields of hydrogen and carbon monoxide.When the temperature increases, the acetic acid conversion increases up to 600\u202f\u00b0C. It should be pointed out that up to 600\u202f\u00b0C the conversion of acetic acid has its steady state delayed as the temperature increases and it is achieved exactly when acetone concentration remains constant, also reaching its steady state condition. In all temperatures, excepted 400\u202f\u00b0C, the conversion of acetic acid is at its maximum value at the beginning of the run, but rapidly decreases, due to catalyst deactivation. The deactivation observed, as Takanabe et al. reported [24], could be explained by coke formation that is directly correlated to the presence of acetone in the system. Acetone could suffer aldol-condensation type reactions forming coke deposits [24]. The molar fraction of carbon was calculated by mass balance and is presented in Fig. 10\n.As already presented in Fig. 9, acetic acid conversion has no thermodynamic limitations and due to kinetic effects, it could also generate carbon deposits on the catalyst. The Fig. 11\n shows the potentiality of acetic acid ketonization occurrence in the experimental conditions stablished in this work.\n\n(9)\n\n\nK\n=\n\n\n\np\n\nacetone\n\n\n\u2217\n\np\n\nH\n2\nO\n\n\n\n\n\u2217\np\n\n\nC\nO\n2\n\n\n\n\np\n\nacetic\n\na\nc\ni\nd\n\n2\n\n\n\n\n\n\nThe constant Kexp (experimental) and Keq (equilibrium) are calculated using (Eq. (9)). The former was calculated using the experimental partial pressures verified during the tests and the later using thermodynamic data. In all experiments the Kexp/Keq ratio is lower than 1, meaning that the formation of acetone is favoured by thermodynamics during the entire run, no matter the temperature.The presence of methane at 700\u202f\u00b0C is higher than in any other reaction temperature during the first hours of run and its presence is attributed to acetic acid decomposition [24]. Takanabe et al. proposed a kinetic scheme in which acetic acid decomposes into methane, COx and hydrogen, showing that the contribution in methane formation due to carbon monoxide hydrogenation is irrelevant. A bifunctional mechanism could be retained for the catalytic process, in which water could be activated by the support, generating hydroxyl groups that could be used in steam reforming or WGS reactions (Eq. (10)).\n\n(10)\n\n\nWGS\n:\n\nC\nO\n+\n\nH\n2\n\nO\n\u2192\n\n\n\n\nC\nO\n\n2\n\n+\nH\n\n2\n\n\n\n\n\nThe acetic acid could be decomposed into hydrogen, methane, carbon monoxide and dioxide and CHx species that could oligomerize, blocking catalytic sites and also could recombine to give path to acetone synthesis that is also raw material for oligomerization products. The activation of water on the support surface plays an important role not only in hydrogen formation step (recombination of hydrogen adsorbed atoms) but also in cleaning the active phase by hydrogenating those CHx groups. The effect of support composition is studied in the next section.The effect of calcium content on La1\u2212xCaxNiO3 perovskites in the steam reforming of acetic acid and the activity of the corresponding catalysts is presented in Fig. 12\n.The conversion of acetic acid presented by the calcium-containing catalysts is lower than the one achieved by the reduced LaNiO3. Table 2\n shows the values of the reactant\u2019s conversion achieved after 23\u202fh TOS and the Ni0 average crystalize size calculated by Debye Scherrer\u2019s Equation for all samples.As can be seen in Table 2, a rather good correlation is observed between the conversion of acetic acid and the Ni0 average crystallite size. The average diameter of Ni0 crystallites when LaNiO3 is reduced in situ is less than 50% of those obtained by reduction of calcium-containing catalysts and so, the metallic area accessible when calcium is present is approximately half of that accessible in the reduced LaNiO3. The smaller is the active phase crystallite size, the higher is the presence of steps and kinks on its surfaces, increasing so the turnover rate [42] what could explain the higher acetic acid conversion on Ni/La2O3 obtained from LaNiO3 precursor. The presence of calcium has a positive effect on water conversion, as can be seen in Table 2. It increases with calcium content and when calcium substitutes lanthanum in Ca 50% sample, the conversion of water increases by a factor of 4.As it has already been mentioned, it is believed that steam reforming of acetic acid proceed via bifunctional mechanism, being the water activated on the support generating hydroxyl groups that could be recombined to give hydrogen and to slow down the deactivation by steam reforming of those possible CHx intermediates derived from acetic acid decomposition. In fact, the increase in calcium content showed an increase in water conversion and also a significant increase in hydrogen production. It could be inferred that the increase in calcium content produces a more efficient support for water activation. The presence of calcium seemed to mitigate acetic acid conversion to acetone and the increase in calcium content makes acetone depletion come sooner with time on stream. The system containing reduced LaNiO3 displayed detectable acetone during the entire 23\u202fh TOS but in those systems containing calcium, acetone formation was pulled back and its presence could not be detected after 5\u202fh TOS in presence of Ca50%.The catalysts generated in situ were active when applied to the steam reforming of acetic acid. XRD studies showed that the presence of calcium increased significantly the Ni0 crystallite average size. The presence of calcium seemed to anticipate both production and complete depletion of acetone, an undesired side product that is the precursor of carbon solid structures that could accelerate the catalyst deactivation. It was possible to verify a linear correlation between Ca content and the conversion of water, being the presence of the former beneficial to water activation. Hydrogen and carbon monoxide generation was also directly proportional to Ca content.The authors acknowledge the Brazilian Synchrotron Light Laboratory for the acceptance of missions D10B-XPD 9253 and 10799.", "descript": "\n                  Ni/CaO-La2O3 catalysts generated by in situ reduction of La1\u2212xCaxNiO3 perovskite systems (x\u202f=\u202f0; 0.15; 0.30 and 0.50) were prepared and evaluated in steam reforming of acetic acid under steady state conditions. The objective of this work was to study the effect of calcium content towards activity and syngas formation in such catalytic systems. The catalytic materials were characterized by in situ X-ray diffraction and temperature programmed reduction. The catalytic activity was evaluated in a packed bed reactor in a temperature range from 400 to 700\u202f\u00b0C for LaNiO3 reduced samples and at 600\u202f\u00b0C for the La1\u2212xCaxNiO3 reduced precursors. The tests indicated that the presence of calcium oxide directly promotes hydrogen formation, by permitting a greater amount of water to be converted and limits the occurrence of ketonization.\n               "}
{"full_text": "To tackle the problems of energy crisis and environmental pollution, clean and renewable energy sources are required to replace the fossil fuels widely used to generate electricity. As an alternative energy sources, hydrogen is becoming an important part of the future energy system because of its high energy density and environment-friendliness. In recent years, hydrogen production by hydrolysis has garnered attention worldwide as an economic and feasible method. At present, the development of catalysts with high hydrolysis efficiency is an unresolved issue; various rare-earth-rich non-precious metal catalysts containing transition metal compounds such as sulfides, phosphates, carbides, nitrides, oxides and selenides have been developed (Chang et al., 2016). The addition of non-metallic elements (O, S or N) to the transition metal-based electrocatalyst can also adjust the kinetics of the reaction and improve the catalytic activity (Xu et al., 2017; Hao et al., 2017; Anjum et al., 2018).As yet, nickel-supported catalysts were used for hydrogenation, oxygen reduction and olefin oxidation of nitrobenzene and nitrophenol; these catalysts have attracted interest because of their low cost and excellent catalytic performance. To facilitate catalysts recovery, nickel particles are usually dispersed in a solid matrix. Recently, much efforts were made to prepare heterogeneous nickel catalysts using various materials such as silica (Mitchell et al., 2021), alumina, graphene/amorphous carbon (Sung et al., 2018), zirconia (Wojciech Gac, et al., 2020), titanium dioxide (Jiang, et al., 2017.), magnesia (Yusuf et al., 2021) and carbon. Among them, porous carbon is the most commonly used economic carrier, and carbon-based carriers such as cellulose paper with metallic nickel particles chemically deposited on the surface (Sahasrabudhe et al., 2018), nickel based mesoporous carbons (Yang et al., 2014), in situ prepared of Ru nanoclusters and porous carbon (Ding R et al., 2020b) have many advantages over other carriers because of their chemical inertia and stability. Compared with other carbon materials, the advantages of carbonised fibre obtained from biomass include easier availability, easier regeneration and lower cost (Lai et al., 2019).Meanwhile, the gradual shift of technology towards green synthetic strategy has necessitated the use of nontoxic, renewable and environmentally benign chemicals (Zhou et al., 2018; Kuo et al., 2019). Hence, designing high-value products with long life, reusability, cost-effectiveness and high efficiency has become an urgent need. Unfortunately, the products of agricultural, industrial or forestry wastes are complex and difficult to separate. Therefore, one of the great challenges in the preparation of biological carriers is to transform them into specific products with specific properties and complexity. Other complex factors include excessive accumulation of chemicals during the use of the product, natural ageing, the recycling process itself and the flow of materials and products associated with it (K\u00fcmmerer et al., 2020). However, these biomass catalysts are mostly powders with small particle size. To solve the problem of small particle size of carbon-based solids, nickel-supported catalysts were prepared from poplar with the original skeleton structure. Carbonised wood has the potential to be used as catalyst carrier owing to its good microstructure. Poplar wood has the advantages of low weight, long fibres, high content and easy processing, and hence, it is widely used in housing and as pulp and plywood. Poplar is widely grown because of its fast-growing nature and huge carbon emission reduction potential. It is regarded an important industrial raw material in many countries and its use as an energy crop for biomass or biofuel is gaining interest.Steam explosion is an optional and mature pretreatment technology in the field of biomass conversion. The effect of this technology on hardwood is second only to that on gramineous plants. Since most gramineous plants are a part of crop waste, and the sampling time is greatly affected by seasons, researchers focused on poplar among the perennial broad-leaved trees. The particle sizes of different types of biological raw materials differ after steam blasting. In the process of the steam explosion treatment of biomass raw materials, a large amount of water vapour permeates into raw biomass materials, resulting in the formation of hydrogen bonds with some hydroxyl groups on the cellulose molecular chain. In addition, high temperature and high pressure exacerbate the fracture of the hydrogen bond in cellulose and cause the release of new hydroxyl groups; the specific surface area of cellulose increases and the adsorption capacity of blasting products is improved. Because lignocellulose is renewable and rich in hydroxyl groups, it is an ideal carbon source for preparing carbon carriers. During the preparation process, embedding the nickel nanoparticles directly into carbon materials is crucial for preparing \u2018embedded\u2019 catalysts.Herein, we report a simple and effective method for preparing a non-metallic ion-doped nickel-supported catalyst using economical and recyclable fibre raw materials as carriers. The nickel-supported catalysts were prepared by adsorption and reduction at room temperature; among the catalysts, non-metallic ions and Ni-Fe metal particles are highly dispersed. The nanoparticles dispersed and anchored on a rational support can efficiently inhibit the aggregation and thus enhance the catalytic activity (Fu et al., 2019a). Non-metallic ion-doped nickel-supported catalysts exhibited catalytic activity and durability, and can be used in various catalytic reactions, such as electrochemical reactions, 4-nitrophenol (4-NP) reduction and so on. In general, the reported preparation method of the nickel-supported catalyst is convenient, economic and environment-friendly, which is in line with many green chemistry and sustainable development principles and employs widely available starting materials.Poplar was steam exploded at 213\u00a0\u00b0C for 5\u00a0min. A compositional analysis of steam exploded poplar (SEP) on a dry basis was performed. Analytical grade hydrogen sodium borohydride (NaBH4) and 4-NP were procured from Sigma-Aldrich (Shanghai, China). Nickel nitrate hexahydrate, iron nitrate nonahydrate and ethanol were analytical grade and procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used without further purification.SEP was prepared at 213\u00a0\u00b0C for 5\u00a0min by steam explosion. First, 2\u00a0g SEP and Nickel nitrate hexahydrate (5\u00a0mmol) were dispersed in 100\u00a0mL of deionised water for 15\u00a0min under ultrasonic treatment, and stirred for 40\u00a0min to completely dissolve. After Ni2+ was completely adsorbed by the SEP, 5\u00a0mL of NaBH4 (0.5\u00a0mol/L) solution was added, and the boron-containing metal oxide was grown vertically in situ in the SEP at room temperature. The resulting product was collected by centrifugation and washing with deionised water and ethanol, and then dried overnight in vacuo. The obtained carbon material is hereafter referred to as N-B-Ni/SEP. Thereafter, the obtained product was used to pyrolyse the feedstock as follows: the precursor and 200\u00a0mg NaH2PO2 will be prepared at both ends of the alumina crucible. Temperature was increased at a rate of 5\u00a0\u00b0C/min in a nitrogen atmosphere, and the N-B-Ni/SEP was held for 1.5\u00a0h at 350\u00a0\u00b0C, followed by cooling to room temperature inside the furnace. The carbon material obtained thus is hereinafter referred to as N-B-NiP/SEP. N-B-Ni5Fe5P/SEP was prepared by adding nickel nitrate hexahydrate (5\u00a0mmol) and iron nitrate nonahydrate (5\u00a0mmol), under identical conditions as described in the previous sentences.A Fourier transform infrared (FT-IR) spectrometer (Karlsruebrook, Germany) using KBr pellet technology was employed to measure FT-IR. A Zeiss Merlin instrument was used for scanning electron microscopy (SEM) under 10\u00a0kV voltage. Energy dispersive spectroscopy (EDS) was performed to determine the elemental composition. The crystal structure of the sample was analyzed by using an Ultima IV X-ray diffractometer. The working voltage of the X-ray diffractometer was 40\u00a0kV, and the current density was 30\u00a0mA. X-ray photoelectron spectroscopy (XPS) analysis was carried out by using an ESCALAB 250 analyzer (Thermo Science) and a monochromatic Al Ka X-ray source. Raman spectroscopy (Horiba evolution) measurements were employed to study the physical properties of the samples. The adsorption\u2013desorption isotherms of nitrogen were determined using a BELSORP-mini II instrument and the Brunauer-Emmett-Teller (BET) method. The ultraviolet (UV)- visible (vis) absorption spectra were recorded via a UV-2900 spectrophotometer (Hitachi, Japan). Inductive coupling plasma emission spectroscopy (ICP-OES) was performed using a PerkinElmer 8300 analyzer.The reduction of 4-NP was performed in a quartz cuvette and monitored by performing UV\u2013vis spectroscopy (Hitachi UV-2900) at room temperature. For comparison, an aqueous 4-NP solution (0.01\u00a0M) was prepared and measured prior to monitoring the change in absorption. Then, a total of 25\u00a0\u03bcl of aqueous 4-NP solution was mixed with 2.5\u00a0mL of fresh NaBH4 (0.01\u00a0M) solution. Subsequently, a fixed amount of nickel catalyst was added to start the reaction, and UV spectrometry was employed to monitor the reduction in situ by measuring the absorbance of the solution at 400\u00a0nm over time.The electrochemical measurement was carried out at room temperature using a three-electrode device using CHI760E electrochemical workstation. A glassy carbon electrode was the working electrode (opposite electrode), and the Ag/AgCl electrode was the reference electrode. The linear sweep voltammogram (LSV) was recorded at a scanning rate of 5\u00a0mV/s in 1.0\u00a0M KOH electrolyte for OER, and in 0.5\u00a0M H2SO4 for HER. The scanning range was 1.0\u20131.8 vs. reversible hydrogen electrode (RHE). The LSV curve was obtained at a scanning rate of 5\u00a0mV/s, and the LSV curve was corrected by 90% IR compensation method. Using the Nernst equation (ERHE\u00a0=\u00a0EAg/AgCl\u00a0+\u00a00.059\u00b7pH\u00a0+\u00a00.197), the measured potential was converted into the corresponding RHE potential. The current density (J) was normalized to the geometric surface area, and the measured potential EAppl (vs. Ag/AgCl) was converted into the RHE. The overpotential (\u03b7) of OER when the current density was 10\u00a0mA/cm2 was calculated by using the equation \u03b7\u00a0=\u00a0ERHE-1.23\u00a0V. The overpotential (\u03b7) of the HER when the current density was 10\u00a0mA/cm2 was calculated using the equation (\u03b7\u00a0=\u00a0ERHE). According to the Tafel equation (\u03b7\u00a0=\u00a0a\u00a0+\u00a0b\u00b7log (J) to calculate the Tafel slope (b), the Tafel slope was obtained by fitting the linear part of the Tafel curve (Cao et al., 2020; Lan et al., 2019).The FT-IR analyzer was used to identify the functional groups on the catalyst samples and SEP surface, as shown in Fig. 1\n. The FT-IR spectra showed strong absorption at 3421\u00a0cm\u22121, which is attributed to the stretching of the phenolic and aliphatic hydroxyl groups. The peaks at around 2921\u00a0cm\u22121 that were related to the C-H functional group changed after the nickel-supported catalyst samples were prepared by the SEP. The results showed that chemical interactions and ion changes occurred between OH, C\u2013H, C=O and heavy metal ions in the nickel bio-adsorption process (Foroutan et al., 2019b). The FT-IR spectra of N-B-NiP/SEP, N-B-Ni5Fe5P/SEP and N-B-Ni5Fe5P/SEP-1 confirmed the existence of NO3\n\u2212 and OH\u2212 group in the nickel\u2013iron loaded catalyst (Fig. 2\n). The bands at 1596, 1363 and 777\u00a0cm\u22121 were the characteristic vibrations for H2O, \u2013NO3\n\u2212and Metal-O (M\u2013O) (Lee et al. 2019; Yang et al. 2019), respectively, thereby showing again that Ni-Fe formed on SEP. Compared with the blank SEP, the change in the absorption peak at 777\u00a0cm\u22121 indicated that metal particles were attached to the surface of SEP. In contrast, the weak peaks at 1112\u00a0cm\u22121 are characteristic of the C-N stretching mode (Coates 2006). The absorption peaks of the repeatedly used catalysts at 777\u00a0cm\u22121 did not diminish, indicating that the catalytic process did not affect the transition metal particles on the carrier surface.The morphology and microstructural information of the N-B-Ni5Fe5P/SEP and N, B-NiP/SEP were systematically studied using electron microscopy techniques (Xiao et al., 2016). The closely packed Ni-Fe coating deposited at room temperature did not change the fibre structure of the SEP (Fig. 3\na-b). The nickel-plated iron or nickel SEM contain a large number of voids in the bracket. SEM images (Fig. 3a-b) show that the growth of the Ni-Fe layer with vertically arranged nano-thin sheets, with interconnected macroporous morphology, will not hinder the underlying macroporous structure. This interesting morphology is beneficial for electrocatalysis because it provides a large number of exposed catalytic active sites and enables electrons to travel rapidly along vertical nanoflakes. Energy dispersive X-ray (EDX) spectroscopy was performed to further characterize the elemental composition and distribution of the N-B-Ni5Fe5P/SEP sample by performing EDS surface scans (Fig. 3d-g). The results show that Ni, Fe, P, B and N are uniformly distributed in the sample, and that the atomic ratio is 1.28 (Ni): 1.21 (Fe). Further, B and N atoms were confirmed to have successfully entered the SEP. The above results further prove that the N-B-Ni5Fe5P/SEP was successfully realized by introducing zero-valent N and B atoms.The detailed structural features of the obtained sample were first investigated by an X-ray diffraction (XRD) study. All the diffraction peaks were ascribed to the hexagonal NiP (JCPDS card No. 03-065-1989) without any peaks for impurities, suggesting that the N-B-NiP/SEP precursor was successfully converted into nickel phosphide/SEP. (Pinilla et al., 2016; Sun et al., 2020). The diffraction pattern for PC has a broad peak at 26\u00b0, which is a characteristic of the (002) plane of graphitic carbon (Fig. 4\n).Compared with N-B-NiP/SEP, the four diffraction peaks of NiP in the XRD spectrum of the Fe-doped catalyst (N-B-Ni5Fe5P/SEP) shifted to a larger diffraction angle with Fe doping, indicating that Fe atoms enter the Ni lattice to form an Fe-Ni alloy. The intensity of the diffraction peaks of 111, 201, 210 and 300 of Ni decreases with Fe doping, indicating that Fe doping affects the crystallinity of the alloy particles.The average crystallite size was determined to be about 10.78\u00a0nm for N-B-Ni5Fe5P/SEP, and 17.97\u00a0nm for N-B-NiP/SEP from the (111) reflection by utilizing Scherrer\u2019s equation that relates the coherently scattering domains with Bragg peak widths: D\u00a0=\u00a0k\u03bb/B\u2009cos(\u03b8), where k\u00a0=\u00a00.89 for spherical particles, and B is the full angular width at half-maximum of the peak in radians. Combined with the aforementioned energy spectrum (Fig. 3d-g), it can be seen that the Ni-Fe elements are uniformly distributed on the carrier surface. From these results, we come to the conclusion that the metal particles are well dispersed on the fibre surface, and Fe doping affects the crystallinity of the alloy particles. The above results show that the addition of Ni can effectively promote the miniaturization of Fe grains (Mansouriieh et al., 2016). The XRD pattern of Ni5Fe5/Ni-P electrode (Fig. 4) further confirms the amorphous nature of Ni-Fe catalyst layer as no new peaks are observed besides those corresponding to the catalyst. In fact, it has been proposed that amorphous Ni-Fe electrocatalysts are much more active than their crystalline counterparts because the amorphous electrocatalysts have good structural flexibility and high density of co-ordinatively unsaturated sites that help in the adsorption of oxidized intermediates.The XPS survey scan spectrum (Fig. 5\na) clearly confirmed that Ni, Fe, B, P, N, O, and C elements were present in the samples. According to the XPS analysis, the Ni and Fe contents in N-B-Ni5Fe5P/SEP were 11.14 and 14.84\u00a0wt% (Table 1\n), respectively. The molar ratio and actual total loading content of Fe and Ni in the N-B-Ni5Fe5P/SEP and N-B-NiP/SEP catalysts were further determined by ICP-MS, as listed in Table 1. The results are in good agreement with the theoretical molar ratio, indicating that the Ni and Fe metal particles are uniformly dispersed on the SEP carrier. Note that the Ni and Fe loading of N-B-Ni5Fe5P/SEP from ICP-OES (17.19 and 15.58\u00a0wt%, respectively) analysis were much higher than the outmost surface Ni and Fe content (11.14 and 14.84\u00a0wt%, respectively) as measured by XPS. Hence, we conclude that the tiny Ni and Fe particles are embedded in the carbon fibre instead of being anchored on the surface (Ding et al., 2020). This phenomenon is more obvious in the nickel content of the N-B-NiP/SEP.The high-resolution spectra of the Ni 2p region showed two peaks, 2p3/2 (856.82\u00a0eV) and 2p1/2 (874.47\u00a0eV) corresponding to the Ni2+ derived from the oxidation of the NiP surface (with the corresponding shakeup satellite peaks at 862.26 and 880.03\u00a0eV, respectively) (Ding et al., 2020). The Fe 2p spectrum (Fig. 5h) was fitted into two separate peaks at 711.76 and 724.68\u00a0eV corresponding to the spin\u2013orbit states of Fe 2p3/2and Fe 2p1/2, respectively. This finding also confirms that the Fe predominantly exists in the Fe3+state. As shown in Fig. 5g, compared with the N-B-NiP/SEP, the negative shift of NiP indicates a decrease in the number of electrons at Fe site and the accumulation of electrons around the Ni site (Jiao et al., 2019). These changes in electron accumulation cause changes in the distribution of electrons, thus changing the local electronic structure of the metal position.The XPS spectrum (Fig. 5c) for O 1s of samples can be deconvoluted into two peaks at binding energies of 531.08 and 532.08\u00a0eV, which were attributed to the surface-adsorbed water (\u2013OH) and C-O species (oxygen vacancies), respectively. The oxygen vacancies indicate a defect site with low oxygen coordination, which decrease the barrier for the adsorption of OH\u2212 and promotes OER. In particular, N-B-Ni5Fe5P/SEP and N-B-NiP/SEP show a clear difference in the area of oxygen vacancies because of the presence of Fe metal ions (29.49 %: 51.62 %) (Xu et al., 2018; Kim et al., 2020).As shown in Fig. 5b, the four components of C1s spectrum (284.77, 286.36, 288.49 and 291.54\u00a0eV) were attributed to sp2 C-C, sp3 C-C, C-O and carboxylic groups, respectively. In the high-resolution XPS spectra, P 2p exhibits three contributions, P 2p3/2 and P 2p1/2, located at 129.49 and 130.46\u00a0eV (Fig. 5e), respectively, which are assigned to NiP, and the peak at 133.72\u00a0eV that is attributed to the oxidized P species.The B 1s spectrum (Fig. 5f) clearly evidences the presence of three chemical environments for phosphorus atoms (B-O, B-C, and B-Ni). The existence of B3+ in N-B-Ni5Fe5P/SEP and N-B-NiP/SEP catalyst is evidenced by the peak at 191.60\u00a0eV (Fig. 5f), which can be attributed to the surface oxidation of the borate species. Compared with N-B-NiP/SEP, the peak intensity at 190.59\u00a0eV of N-B-Ni5Fe5P/SEP that corresponds to B-C bonds was higher, indicating that some C atoms in the carbon fibre were replaced by B atoms. Pleasantly, the peak at 187.33\u00a0eV can be attributed to B(0) in the Ni-B bonds, which matches well with the literature. This result suggests that there are abundant zero-valent B atoms in the N-B-Ni5Fe5P/SEP and N-B-NiP/SEP after N2 treatment.In the high-resolution N 1s spectrum, in addition to the characteristics related to pyrrolic-N (402.07\u00a0eV) and pyridinic-N (400.00\u00a0eV), a characteristic peak with a 397.10\u00a0eV binding energy is observed in the N regions. It is attributed to metal-nitrogen bonds, indicating the presence of zero-valent N(0) atoms in the N-B-Ni5Fe5P/SEP and N, B-NiP/ SEP (Fig. 5d). The presence of N dopant in the sample will inherently improve the interaction ability with the reactants and produce a higher positive charge density on its adjacent carbon atoms, which may also contribute to the high activity of the sample (Sun et al., 2020). Therefore, the above results indicate that the zero-valent N and B atoms were successfully doped into N, B-NiP/SEP and N-B-Ni5Fe5P/SEP.The crystallization and graphitization degree of the carbonized SEP support on the Ni-supported catalyst and Ni-Fe bimetallic catalyst were studied by Raman spectroscopy. In general, an ID/IG ratio less than one is ideal. As shown in Fig. 6\n, the carbon fibre carriers have high quality and crystallinity, and the peak intensity ratio (ID/IG) is less than one, and the spectra of carbon samples show two distinct bands. The first band is the well-known D band, located at 1363\u00a0cm\u22121, attributed to the disorder in the carbon structure, such as defects in the carbon structure or amorphous carbon (Msda et al., 2002; Awadallah et al., 2013). The vibration of sp2 carbon atoms in the graphitization region forms the G band located at 1589\u00a0cm\u22121 (Ali et al., 2017; Allaedini et al., 2015). Generally, the ratio of the D-band strength to G-band strength ID/IG is used to reflect the degree of graphitization. The ID/IG ratio of N-B-Ni5Fe5P/SEP is 0.89, which indicates that a large number of defects and irregular structures were introduced into the carbon fiber carrier. The ID/IG ratio further increased to 0.95 for N-B-NiP/SEP, indicating the enhanced number of structural defects, increased localized sp3 defects in sp2 framework and high electrical conductivity.The specific surface area and porosity of the obtained materials were investigated by N2 adsorption\u2013desorption experiments. In the curves of N-B-Ni5Fe5P/SEP and N-B-NiP/SEP (Fig. 7\na), the type IV adsorption branches corresponded to the mesoporous structure. According to IUPAC classification, the isotherms (Fig. 7) of the mixed oxides were classified as type IV with an H3 hysteresis loop, suggesting the existence of mesoporous materials with an incision-like pore geometry. The specific surface area of N-B-Ni5Fe5P/SEP and N-B-NiP/SEP were calculated to be 55.44 and 57.18\u00a0m2/g, respectively. The pore size distributions are shown in Fig. 7b. The average pore size of N-B-Ni5Fe5P/SEP was about 11.86\u00a0nm, while those of N-B-NiP/SEP was around 8.42\u00a0nm. It was clear that N-B-Ni5Fe5P/SEP and N-B-NiP/SEP were mainly composed of micropores and mesopores of size around 10\u00a0nm. As shown in Fig. 7d, the average pore widths of two samples follow the order of N-B-Ni5Fe5P/SEP\u00a0>\u00a0N-B-NiP/SEP and the pore volumes of N-B-Ni5Fe5P/SEP and N-B-NiP/SEP were 0.19 and 0.11\u00a0cm3/g, respectively. The pore structure of materials play an important and even decisive role in determining many material properties. When using carbon materials as carriers, their porous properties are conducive to the diffusion of substrates and products and can lead to the exposure of more active sites, thus improving the overall activity of the catalyst.The electrocatalytic OER performance of N-B-Ni5Fe5P/SEP and N-B-NiP/SEP were studied in O2-saturated 1\u00a0M KOH. The LSV data (Fig. 8\na-b) were recorded with the scan rate of 5\u00a0mV/s. For N-B-NiP/SEP, the Ni2+/Ni3+ was oxidized in the potential range of 1.35\u20131.5\u00a0V (all potentials were versus the RHE) (Fig. 8b). The presence of the oxidation peak indicated that because of insufficient oxidation, a fully protected NiO shell may not be formed outside the Ni nanoparticles, leading to corrosion of metal Ni and the formation of NiOOH during OER in the alkaline solution (Sivanantham et al., 2016). The curves of polarization (Fig. 8a-b) showed that the N-B-Ni5Fe5P/SEP exhibited excellent OER performance with an overpotential of 395\u00a0mV at 10\u00a0mA/cm2 and 488\u00a0mV at 30\u00a0mA/cm2 current density, compared to N-B-NiP/SEP (431 and 579\u00a0mV, respectively).In addition, to investigate the kinetics of these catalysts, the Tafel slopes obtained from the LSV polarization curves are shown in Fig. 8c. The Tafel slope of N-B-Ni5Fe5P/SEP (101\u00a0mV/dec) was considerably smaller than that of N-B-NiP/SEP (151\u00a0mV/dec), confirming the faster OER kinetics of the former. Our research results indicate that the synergetic effect of the Ni-Fe bimetal loading and carbon carrier played an important role in facilitating the OER kinetics (Li et al., 2020; Jiang et al., 2018; Yue et al., 2019).To assess the electrocatalytic HER activity of the N-B-Ni5Fe5P/SEP and N-B-NiP/SEP, the related electrochemical measurements were performed using a three-electrode system. Fig. 8d shows the polarization curves of the N-B-Ni5Fe5P/SEP and N-B-NiP/SEP in N2-saturated 0.5\u00a0M H2SO4 solution. While the N-B-NiP/SEP, which has a \u03b710 value of 397\u00a0mV, the N-B-Ni5Fe5P/SEP requires 392\u00a0mV to reach 10\u00a0mA/cm2, implying that the Fe trace in N-B-Ni5Fe5P/SEP does not contribute to the electrochemical activities and remains a mere spectator species. Tafel slopes were drawn to evaluate HER kinetics (Fig. 8e). The Tafel slope is 122\u00a0mV/dec for N-B-Ni5Fe5P/SEP, which is much smaller than that of the N-B-NiP/SEP (119\u00a0mV/dec). In the study of the mechanism of electrocatalytic hydrogen evolution in acidic media, it is generally believed that the reaction process is divided into the following three steps: the first step is the electrochemical reaction process; the second step is the electrochemical desorption process; the third step is the compound desorption process. The general HER mechanism includes at least an electrochemical process and a desorption process, and hence, it can be divided into the Volmer-Heyrovsky mechanism or Volmer-Tafel mechanism according to the different rate steps. As seen from Fig. 8c and e, the Tafel slopes of the N-B-Ni5Fe5P/SEP and N-B-NiP/SEP are 122 and 119\u00a0mV/dec, respectively. So the hydrogen evolution process of the catalyst in acidic medium is a slow discharge mechanism, and the Volmer reaction process has a rate-control step, which is the Volmer-Heyrovsky mechanism (Conway and Tilak, 2002; Li et al., 2014; Li et al., 2011).The removal of 4-NP from wastewater is of significant importance from the perspective of environment protection as 4-NP is a prevalent contaminant produced in industry and agriculture (Choi and Oh, 2019; Ding et al., 2020). It is known that 4-aminophenol (4-AP) is very useful and important in many applications, and it is used in analgesic and antipyretic drugs, photographic developer, corrosion inhibitors and anticorrosion lubricants. The reduction of 4-NP to 4-AP has been extensively used as a benchmark system to evaluate the catalytic activity of metal NPs (Chang et al., 2012; Yang et al., 2014).Therefore, the reduction of 4-NP toward 4-AP in the presence of NaBH4 was selected as a model reaction to further confirm the generality of the N-B-Ni5Fe5P/SEP and N-B-NiP/SEP. As shown in Fig. 9\na, the adsorption peak of 4-NP was red-shifted from 317 to 400\u00a0nm immediately upon the addition of NaBH4 solution which corresponds to a colour change from light yellow to yellow green because of the formation of the 4-nitrophenolate ion under alkaline conditions. When the catalyst was added, the intensity of the characteristic peak at 400\u00a0nm rapidly declined. The reduction of 4-NP was completed within 10\u00a0min over 10\u00a0mg\u00a0N-B-NiP/SEP and N-B-Ni5Fe5P/SEP (Fig. 9c). Considering that the reductant concentration is much higher than that of 4-NP (CNaBH4/C4-NP\u00a0=\u00a0100) in the reaction mixture, the pseudo-first-order rate kinetics with respect to 4-NP concentration could be used to evaluate the catalytic rate. The reaction kinetics can be described as\u00a0\u2212\u00a0ln(Ct/C0)\u00a0=\u00a0kt, where k is the rate constant at a given temperature and t is the reaction time. C0 and Ct are the 4-NP concentration at the beginning and at time t, respectively. As expected, a good linear correlation of ln(Ct/C0) vs. reaction time t was obtained (Fig. 9b), and the kinetic rate constant k was estimated as 0.19 (R2\u00a0=\u00a00.99) and 0.344 (R2\u00a0=\u00a00.99) min\u22121 for N-B-NiP/SEP and N-B-Ni5Fe5P/SEP, respectively. To compare different catalysts, we calculated the ratio of rate constant K over the total weight of the nickel catalyst, where K\u00a0=\u00a0k/m. Thus the activity factor K was calculated as 19 and 34.4\u00a0min\u22121\u00b7g\u22121 for N-B-NiP/SEP and N-B-Ni5Fe5P/SEP, respectively. It is clear that N-B-Ni5Fe5P/SEP clearly had the largest activity factor, compared with other precious metal catalysts such as Ru/C (0.034\u00a0min\u22121) and Ru/PC-IM (0.198\u00a0min\u22121) (Ding et al., 2020). With an increase in the number of cycles, the conversion of 4-NP decreased slightly, possibly because of the partial loss of active surface area caused by the partial loss of catalyst during recovery.The excellent catalytic performances of N-B-Ni5Fe5P/SEP for 4-NP reduction lead to the following advantages. From the point of view of catalysis, SEP is an ideal substrate for the growth of an active catalyst layer. Because there are abundant coordination hydroxyl groups and epoxy functional groups on the cellulose microfiber, the ultra-fine and clean metal nanoparticles formed in situ are uniformly dispersed on the surface of the carrier rather than being embedded in the carrier. Together, these two functions can lead to stronger binding and faster mass transfer kinetics.In summary, using economical and recyclable fiber raw materials as carriers, nickel-supported catalysts were prepared by adsorption and reduction at room temperature. For the model catalytic hydrogenation of 4-NP by NaBH4, the N-B-NiP/SEP and N-B-Ni5Fe5P/SEP catalysts exhibited much better catalytic performances than the other catalysts recently reported in terms of the catalytic activity (with the proposed catalysts, the reaction was completed within 10\u00a0min) and reaction rate constant (0.19 and 0.344\u00a0min\u22121 for N-B-NiP/SEP and N-B-Ni5Fe5P/SEP catalysts, respectively). The catalyst showed activities for electrocatalytic HER and OER under ambient conditions. In general, the reported preparation method of nickel-supported catalyst is convenient, economical and environment-friendly, which is in agreement with many green chemistry and sustainable development principles, and the method employs widely available starting materials.The authors are grateful for the support of the National Nature Science Foundation of China (NSFC, No. 21978074).", "descript": "\n                  A simple and effective method for preparing a non-metallic ion-doped nickel-supported catalyst is reported. Using economical and recyclable fibre raw materials as carriers, nickel-supported catalysts were prepared by adsorption and reduction at room temperature. The nanoparticles dispersed and anchored on a rational support, efficiently inhibiting their aggregation and thus enhancing the catalytic activity. For the model catalytic hydrogenation of 4-nitrophenol by NaBH4, the N-B-NiP/steam-exploded poplar (SEP) and N-B-Ni5Fe5P/SEP catalysts exhibited much better catalytic performances than the other recently reported catalysts in terms of the catalytic activity (the reaction was completed within 10\u00a0min for both aforementioned catalysts), reaction rate constant (0.19 and 0.344\u00a0min\u22121, respectively) and the activity factor K (19 and 34.4\u00a0min\u22121\u00b7g\u22121, respectively). The catalysts showed activities for electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under ambient conditions. In general, the reported preparation method of nickel-supported catalysts is convenient, economical and environment-friendly, and is agreement with many green chemistry and sustainable development principles; further, it employs widely available starting materials.\n               "}
{"full_text": "The development of sustainable, clean, and environmentally friendly energy is becoming increasingly important to address the growing threat of energy exhaustion and greenhouse gas emissions that contribute to dangerous climate change (K\u0131l\u0131n\u00e7 and Sahin, 2018; Chen et al., 2011). Hydrogen (H2) is a potential alternative to meeting the world's increasing energy demand while also serving as an eco-friendly energy carrier molecule for future applications (K\u0131l\u0131n\u00e7 and Sahin, 2018). From the standpoint of the application, the safe generation, storage, and transportation of H2 are necessary conditions (Cai et al., 2016). Chemical hydrides are considered suitable materials for all of the applications as mentioned above. Chemical hydride hydrolysis is gaining popularity as a potential in-situ H2 supply method for proton exchange membrane fuel cells (PEMFCs) (Crisafulli et al., 2011; Kassem et al., 2019). Amongst chemical hydrides, sodium borohydride (NaBH4) is a preferable material for H2 storage and generation due to its high H2 capacity (\u223c10.9\u00a0wt%) (Lee et al., 2021; Abdelhamid, 2021; Abdelhamid, 2021), high stability in alkaline solution (Ritter, 2003), pure H2 production (Kim, 2004), recycling of the by-products (Calabretta and Davis, 2007; Santos, 2010; Santos and Sequeira, 2011) and non-flammable and less expensive (Chinnappan et al., 2011). Theoretically, one mole of NaBH4 can produce four moles of H2 in water (Eq. (1)) (Yao et al., 2020):\n\n(1)\n\n\nNaB\n\nH\n4\n\n+\n2\n\nH\n2\n\nO\n\n\n\ncatalyst\n\n\u2192\n\n\n4\n\nH\n2\n\n+\n\nN\na\nB\n\nO\n2\n\n\n\n\n\nSelf-hydrolysis of NaBH4 is slow; thus, the addition of an appropriate catalyst can greatly accelerate the hydrolysis reaction (Wechsler et al., 2008). Hitherto, various nano-catalytic materials (e.g., Pt, Ru, Rh, Pd, Ni, Co, Fe, and their alloys) have been used in the hydrolysis reaction of NaBH4 (Dinc et al., 2012; Oh et al., 2015; Shen et al., 2015; Ding et al., 2010; Park et al., 2008; Arzac et al., 2012; Lee et al., 2021; Larichev et al., 2010; Xu et al., 2008; Chen and Kim, 2008). Ni-based catalysts provide objective interest as a catalyst because of their low cost and environmentally friendly construction (Ozay et al., 2011). However, because Ni has a high energy surface and magnetic properties, it must be dispersed and stabilized by appropriate materials to achieve long-term durability without the formation of aggregates (Wang et al., 2021). Furthermore, the formation of NaBO2 as a by-product during NaBH4 hydrolysis leads to deactivation of the catalyst surface (Chinnappan et al., 2011). Because metal-based catalysts are typically used in powder form, they are inconvenient for start-and-stop applications. Separation of the catalyst powder from the reaction media and the possibility of catalyst particle aggregation significant practical issues (Chinnappan and Kim HJIjohe, 2012). The supporting materials significantly impact catalyst activity and durability (Li et al., 2012). Polymer substrates, as we know, have flexible design structures and are easily separated from reactants. As a result, various Ni\u2013polymer hybrids with varying morphologies have been synthesized using various preparation methods (K\u0131l\u0131n\u00e7 and Sahin, 2018; Chen et al., 2011; Cai et al., 2016; Sagbas, 2012; Seven and Sahiner, 2014; Liu et al., 2013; Yan et al., 2009; Chen et al., 2015; Cai et al., 2016; \u00d6zhava et al., 2015; Chen et al., 2009). They demonstrated a high value in H2 generation from NaBH4 while overcoming the mentioned above. As we know, the method of preparation and the morphology of the catalysts directly impact their catalytic activity. Polymer nanofiber membrane (PNFM) have been proposed as supporting materials for various NPs in various chemical reactions. When compared to other supporting materials, PNFM are easily recyclable and reused with high efficiency. Nanofibers are able to form a highly porous mesh therefore their usage is almost endless (Chinnappan et al., 2011). Li et al (Li et al., 2014), They prepared composite nanofibers by immobilizing Cobalt (II) chloride on polyacrylonitrile NFs, which demonstrated excellent catalytic performance and stability in H2 generation from NABH4 solutions. Kim and his group developed hybrid NFs based on polyvinylidene fluoride (PVDF) as a support substrate in the production of H2 from NaBH4 (Chinnappan et al., 2011); Y-zeolite/CoCl2-PVDF (Li et al., 2012), dicationic tetrachloronickelate (II) anion (dicationic ionic salt [C6(mpy)2][NiCl4]2)-PVDF (Chinnappan and Kim HJIjohe, 2012); Ni NPs-PVDF (Sheikh et al., 2011). They found that the prepared hybrid membranes have good catalytic activity and are reusable. Electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) has recently been introduced as a polymer electrolyte and membrane in various applications (e.g., fuel cells, dye-sensitized solar cells, lithium-ion batteries, and water separation) (Raghavan et al., 2008; Mali\u0161 et al., 2013; Vijayakumar et al., 2015; Zhang et al., 2021; Tian and Jiang, 2008). In comparison to other host polymers (e.g. poly(ethylene oxide) (PEO), poly(ethylene glycol), poly(urethane acrylate), (PVdF), poly(methyl methacrylate), and PEO-modified poly(methacrylate)), PVDF-HFP is regarded as the most suitable host polymer for preparing hybrid composites (Raghavan et al., 2008). It has a high affinity to absorb electrolyte solution with good chemical and electrochemical stability (Zhang et al., 2014). In this study, we exploited the hydrophobic properties of PVDF-HFP to deposit NPs on its surface because the salt contains tetrahydrate, which makes deposition of NPs on the surface preferable to embedding NPs supporting on PVDF-HFP NFs. This hypothesis has two consequences: (1) reduce polymer crystallinity and (2) increase solution uptake, which may improve contact between the NaBH4 and catalyst surface (Raghavan et al., 2008). Accordingly, PVDF-HFP is considered an efficient supporting material for NPs which is a chemically stable and easily recyclable polymer. To our knowledge, no research has been conducted on the preparation of Ni NPs@PVDF-HFP membrane NFs for H2 production from NaBH4 hydrolysis as an efficient and easily reusable catalyst material. Metallic Ni nanoparticles embedded in PVDF-HFP NFs were investigated as non-precious catalysts for H2 generation from NaBH4 in this study. The NFs presented here were prepared using an electrospinning technique and a chemical reduction process. Typically, electrospun nanofibers composed of NiAc and PVDF-HFP were dried and then reduced in-situ with NaBH4 to form Ni NPs supporting on PVDF-HFP NFs. The in-situ reduction process has been done in methanol solution. The utilized physicochemical characterizations indicated that the reduction process leads to form Ni supporting on the PVDF-HFP NFs.Nickel (II) acetate tetrahydrate (NiAc, 98\u00a0% assay), poly(vinylidene fluoride-co-hexafluoropropylene) ((PVDF-HFP), 98\u00a0% assay) with a molecular weight of 65,000\u00a0g/mol, and sodium borohydride (NaBH4, 98\u00a0% assay) were purchased from Aldrich Co., USA. N, N-dimethylformamide (DMF, reagent grade, 99\u00a0% assay), and acetone were brought from Fluka. All these chemicals were used without further purification.First, 15\u00a0wt% PVDF-HFP solutions were prepared by dissolved 1.5 gm PVDF-HFP in a mixture of DMF and acetone (4:1\u00a0wt ratio). Four different percentages of NiAc-based PVDF-HFP (10\u00a0% (HFB-10), 20\u00a0% (HFB-20), 30\u00a0% (HFB-30), and 40\u00a0% wt. (HFB-40)), NiAc was dissolved in a determined amount of DMF before added to the PVDF-HFP solution, have been added to PVDF-HFP solution in the separated laboratory glass bottles. The solutions were kept in the magnetic stirrer overnight. The prepared sol-gels were subjected to the lab-scale electro-spinner machine. The sol\u2013gel was placed in a plastic capillary syringe. A copper wire was inserted inside the syringe from one side, and the other side was connected to a high-voltage power supply (positive electrode). The negative electrode was connected with a ground iron drum covered by aluminum foil to collect electrospun NF mats. 20\u00a0kV voltage was applied between syringe and drum. The collected electrospun NF mats were dried at 30\u00a0\u00b0C under a vacuum overnight. The pristine PVDF-HFP-free NiAc membrane has been prepared with the same procedure.Typically, determining amounts from prepared electrospun NF mats were immersed in a 500\u00a0mL beaker containing methanol solution. A determined amount of NaBH4 was added to the previous solution. The molar ratios between metals precursor and NaBH4 were adjusted at 1:5 to obtain a full reduction reaction. As soon as the mat was attached to the NaBH4 solution, the color was changed from green to black, and the membranes looked like a black dying piece of cloth (Fig. 1\n). The mat was left in solution until the gas bubble stopped. The produced mat was washed three times with deionized water and ethanol to remove any residues. Finally, the reduced mat was dried under vacuum at 30\u00a0\u00b0C overnight.The morphology of the prepared catalytic NFs was observed by scanning electron microscope (SEM, Hitachi S-7400, Japan), equipped with energy dispersive X-ray (EDX) before inspection samples were coated with gold. For the investigation of the morphology of PVDF-HFP membrane NFs and their interactions with the Ni NPs, Fourier transform infrared (FTIR), using the smart ATR-FTIR model \u201cNicolet iS 10\u201d (Thermo Fisher Scientific, MA USA) equipped with the specular reflectance. The scanning range was 400\u20133500\u00a0cm\u22121, and the samples were placed on the top of the spectrophotometer. The catalysts' crystalline structure and crystal size were determined by X-ray diffraction (Rigaku Co., Japan) with Cu K\u03b1 (\u03bb\u00a0=\u00a01.54056\u00a0\u00c5). An X-ray photoelectron spectroscopy analysis (XPS, AXIS-NOVA, Kratos Analytical, UK) was conducted with the following conditions: base pressure of 6.5\u00a0\u00d7\u00a010\u22129 Torr, resolution (pass energy) of 20\u00a0eV, and scan step of 0.05\u00a0eV/step. A thermogravimetric analyzer (TGA) was used for the thermal analysis and stability of NFs samples with a heating rate of 10\u00a0\u00b0C/min and nitrogen flow rate of 20\u00a0mL/min. The temperature range for the analysis was set between room temperature and 900\u00a0\u00b0C.The H2 gas produced during the reaction was passed through a tube and collected in an inverted burette using the water displacement method. The volume of hydrogen generated was calculated by measuring the change in the height of the water level in the burettes at different time intervals. The catalytic reaction was carried out in a reactor made up of Pyrex round bottom flask reactors. The volume of H2 produced was calculated using the water displacement method. The reaction vessel was immersed in a temperature-controlled water bath to control reaction temperature. The determined concentration of aqueous NaBH4 solution and amount of catalyst was added to the reaction vessel. The hydrolysis reaction's kinetics investigated by varying the amount of catalyst, NaBH4 amount, and temperature. It also investigated the long-term durability of the introduced membrane NFs via the recycling process.Proposing the electrospinning technique for preparing polymeric nanofibrous membranes could display many distinct features, including increased interconnectivity, flexibility, excellent porosity, and extraordinary surface-to-volume ratios (Gibson et al., 2001; Yousef et al., 2012). Among the commonly used polymeric chemicals for fabricating these films, PVdF-HFP is the most preferred due to its semi-crystalline nature, good thermal stability, increased dielectric constant, and hydrophobicity besides its piezo and pyroelectric characteristics (Shin et al., 2010; Kumar GGJJoMC. , 2011). Fig. 2\n\na and b show the low and high magnifications SEM image of electrospun PVdF-HFP NF mats after drying; as shown in the figure, a good nanofibrous structure without any beads are formed. Furthermore, nano-cracks appeared on the surface of the NFs. This could be due to the rapid evaporation rate of the acetone solvent during the electrospinning process before NFs reach the drum's surface that helps produce this nano-cracks structure as a suitable site for the nucleation of Ni crystals. The water content of the used nickel precursor salt appreciably improved the hydrophilicity of fabricated polymeric membranes with enhanced demixing rate inside the liquid\u2013liquid phases to favor the formation of numerous pores onto their structure (Chen et al., 2015). Afterward, the analyte molecules could be easily trapped in these pores with the lowest diffusion resistance and facilitate the H2 evolution.Moreover, the presence of metals salts has a beneficial role in increasing the electrical conductivity and the gelation of the polymer solution with the generation of maximum elongation of a jet along its axis to produce extremely small-sized polymeric nanofibers (Kang et al., 2014) finally. During the in-situ reduction of Ni(II) ions used NaBH4 as a powerful and effective reducing agent in methanol media, metallic Ni NPs are produced and deposited onto the PVdF-HFP membrane surface. SEM images of electrospun Ni@PVdF-HFP NF membranes NFs were shown in Fig. [2c (HFB-10), d (HF-B20), e (HF-B30), and f (HF-B40)], the inset show the high magnifications SEM images. As shown in the figure, the rough and beads-free NFs are formed. Furthermore, reduced Ni ions are covered the surface of PVdF-HFP NF mats as they built up based on the nano-cracks present on PVdF-HFP NFs surface.During the methanolysis process (Ayd\u0131n et al., 2020), sodium tetra methoxy borate (NaB(OCH3)4) is formed as a by-product (Equation\n\n(2)\n). During washing, sodium tetra methoxy borate reacts with water (Equation\n\n(2)\n) to produce sodium borate and methanol that wash out with excess water.\n\n(2)\n\n\nNaB\n\nH\n4\n\n+\n\n4\nC\n\nH\n3\n\nO\nH\n\u2192\nN\na\nB\n\n\n\n\n\nOC\n\nH\n3\n\n\n\n\n\n4\n\n+\n\n4\n\nH\n2\n\n\n\n\n\n\n\n(3)\n\n\nNaB\n\n\n\n\n\nOC\n\nH\n3\n\n\n\n\n\n4\n\n+\n\n2\n\nH\n2\n\nO\n\u2192\nN\na\nB\n\nO\n2\n\n+\n\n4\nC\n\nH\n3\n\nO\nH\n\n\n\n\nUsing methanol for dehydrogenating NaBH4 has advantages over the use of water. One of them is related to the nature of the by-product. In methanolysis, NaB(OCH3)4 forms. Unlike NaB(OH)4, NaB(OCH3)4 does not have the propensity to polymerize into polyborates. Avoiding the NaB(OH)4 precipitation inhibits catalyst poisoning (Ayd\u0131n et al., 2020; Lo et al., 2007). Hongming et al. (Zhang, 2020), prepared ultrafine Co NPs @ carbon nanospheres using hydrothermal and reduction processes as an efficient catalyst for H2 production from NaBH4 hydrolysis. They indicated that the reduction of Co ions in the ethanol solution media is better than in water media, in which partial NaBO2 was precipitated out with the Co NPs due to the insolubility of NaBO2 in ethanol could separate and further prevent the agglomeration of the Co nanoparticles. At last, the NaBO2 was washed off with DI water. The electrospun Ni2+/PVdF-HFP membranes had white-colored surfaces. The chemical reduction of these metallic ions tends to change the color of their related polymeric membranes into a black-look-like black dying piece of cloth (Fig. 1), which suggests the growth of tiny Ni NPs on the surface of PVdF-HFP membranes. In other words, the complete coverage of PVdF-HFP membranes surface with skin layers of black nickel dots might resemble the shell \u201cNi nanoparticles\u201d-core \u201cpolymeric film\u201d arrangements. An elemental mapping image of the HFP-40 membrane was presented in Fig. (3 a, b, and c)\n. It is clear that the high distribution of Ni NPs around the membrane NFs, is confirmed by the SEM images. EDX chart of HFP-40 membrane was presented in Fig. (4 a and b)\n. The related peaks of carbon, nickel, and fluorine were detected to ensure the successful fabrication of these composite membranes. The XRD study in Figs explored the crystal structure of PVdF-HFP and HFP-40 membranes. (5 a and b), respectively. Three main diffraction planes were observed at 2\u03b8 values of 18.04\u00b0, 20.24\u00b0, and 36.19\u00b0 in the XRD chart of PVdF-HFP membrane corresponding to (100), (020), and (021) crystal indices, respectively (Stephan et al., 2006)[see Fig. 5\n\na]. Besides these defined PVdF-HFP membrane peaks, nickel species were identified through three characteristic planes at 42.77\u00b0 (111), 49.75\u00b0 (200), and 73.60\u00b0 (220) to ascertain the formation of the face-centered cubic crystalline structure of nickel [JCPDS card No. 04\u20130850] (Barakat and Kim, 2009) [see Fig. 5\nb]. The particle size of Ni nanoparticles is calculate used Scherrer equation (Yao et al., 2016), it was found to be 19.5\u00a0nm. It is worth mentioning that the reduced membrane in water media showed the same color as pristine PVDF-HFP spectra. As the membrane kept its green color. The stability of PVdF-HFP and Ni@PVdF-HFP membranes when subjected to elevated temperatures was examined through TGA charts in Fig. 6\n. One weight loss section was observed for bare HFP-10 film at 420\u00a0\u00b0C due to the random scission of its units during the degradation process (Kim et al., 2011; Babu et al., 2015). This step was shown at a lower temperature value [346\u00a0\u00b0C] after nickel species were incorporated into HFP-10 film, besides a small change at 95\u00a0\u00b0C when physisorbed water molecules were evaporated. The presence of Ni NPs in the structure of this polymeric membrane was responsible for weakening the van der Waals\u2032 interacting forces between its chains. This,facilitated the degradation of the metal-supporting polymeric membrane at decreased temperatures when related to the case at bare film (Vijaya\u00e1Kumar, 2001). FTIR spectra of PVdF-HFP and Ni@PVdF-HFP membranes were also described in Fig. 7\n. Common vibrational bands were noticed in both charts that depicted the polymeric membrane. Its \u03b1 and \u03b2 phases were confirmed by their respective peaks at frequency values of 749 and 837\u00a0cm\u22121 (Kumar and Nahm, 2008). Another two specific vibrational bands were centred at 672 and 872\u00a0cm\u22121 that were assigned for C\u00a0\u2212\u00a0F and CH2 wagging of vinylidene units in the amorphous phase of PVdF-HFP film. Furthermore, the symmetric C\u00a0\u2212\u00a0F stretching, CF2 stretching, and deformed vibrations in this membrane were also shown through their corresponding bands at 1071, 1175, and 1400\u00a0cm\u22121 (Mandal et al., 2014). Additional two peaks appeared when nickel precursor salt was introduced during the polymeric film fabrication. The formation of Ni\u00a0\u2212\u00a0O species into this nanomaterial was supporting by its stretching vibration peaks at 1561\u00a0cm\u22121 (Nath, 2014).Their shape influenced the catalytic activity of the catalysts. Shape-anisotropic nanostructures possessing more active sites for catalysis led to improved catalytic performance. NFs have a large surface area compared to other nanostructures, leading to better performance as catalyst support. Nonetheless, it was demonstrated that the nanofibrous morphologies corresponding to the long axial ratio provide a significant performance when compared to other nanostructures. In this regard, NFM outperforms typical powder-like catalysts in terms of catalyst separation and capacity to be reused. Ni@PVDF-HFP MNFs were prepared by electrospinning technique, tested as a catalyst in the hydrolysis of NABH4 and found to be the highly active catalyst to generate H2. The hydrolysis of 1.34\u00a0mmol of alkaline NaBH4 occurred without any catalyst; however, it obtained 28\u00a0mL after 60\u00a0min. The addition of pristine PVDF-HFP MNFs to 1.34\u00a0mmol of alkaline NaBH4 did not significantly affect the NaBH4 hydrolysis. However, the H2 generation is increased with a reduction in the time using electrospun Ni@PVdF-HFP MNFs (Fig. 8\n), as shown by the variation in the catalytic activity using different Ni contents. Experiments were performed by adding 100\u00a0mg of MNFs from all formulations, in the separated glass reactor, into 1.34\u00a0mmol of alkaline NaBH4 at 25\u00a0\u00b0C to determine the best activity of the synthesized membrane NFs. As manifested in the figure the HFP-40 (103\u00a0mL in 60\u00a0min) membrane, NFs have shown the highest H2 generation rate than other ratios [HFB-10 (68\u00a0mL in 60\u00a0min), HFB-20 (81\u00a0mL in 60\u00a0min), and HFB-30 (93\u00a0mL in 60\u00a0min), so it has chosen to be used in further experiments. The fabricated MNFs have shown better catalytic activity, and it produced 103\u00a0mL in 60\u00a0min using 1.34\u00a0mmol NaBH4 and 100\u00a0mg catalyst at 25\u00a0\u00b0C, than PVDF-[C6(mpy)2][NiCl4]2 NFs composite catalyst (Sheikh et al., 2011), ]; it was produced about 140\u00a0mL H2 in 60\u00a0min used 158.72\u00a0mM NaBH4 and 40\u00a0mg from the catalyst contain 2.5\u00a0% Ni, and 40\u00a0% Ni@TiO2\n (D\u00f6nmez and Ayas NJIJoHE. , 2021), it was produced 37.89\u00a0mL H2.g\u22121\ncat min\u22121 used 100\u00a0mg NaBH4, 100\u00a0mg catalyst, 5\u00a0mL 0.25\u00a0M NaOH at 20\u00a0\u00b0C. Jaeyeong et al. (Lee et al., 2019), demonstrated that the Ni powder needed the longest time (280\u00a0min) to generate the same amount of hydrogen (500\u00a0mL) compared with Ni thin film (240\u00a0min), and etched Ni foil (60\u00a0min) used 1.5\u00a0g NaBH4 and 0.01\u00a0g from catalysts at 25\u00a0\u00b0C although Ni powder high surface area. This could be due to the easily contaminated or oxidized. Accordingly, the rougher surface, which was made by in situ difficult reduction reaction of NiAc to metallic Ni on the surface of PVDF-HFP, made the catalyst more efficient. Catalytic hydrolysis of NaBH4 depends on the type of catalyst and other parameters such as NaBH4 concentration, reaction temperature, catalyst amount, and reusability. For this reason, the effect of these parameters on the hydrolysis of NaBH4 was investigated in the presence of the HFB-40. The effect of NaBH4 concentration on the reaction rate has been tested (Fig. 9\n\na) using 100\u00a0mg of HFP-40 MNFs at 25\u00a0\u00b0C. As shown in the figure, the initial H2 generation rate is linearly increased with an increase in NaBH4 concentration. When the amount of NaBH4is increases from 50\u00a0mg to 125\u00a0mg, the hydrogen production rate increases from 255.2\u00a0mL.g\u22121\ncat min\u22121 to 689.9\u00a0mL.g\u22121\ncat min\u22121\n, respectively. The estimated slope of the best-fit line was 0.74 (Fig. 9\nb), which clarifies that the H2 production rate follows pseudo-first-order kinetics concerning NaBH4. This is due to the use of low NaBH4 concentration as the higher concentration follows the pseudo-zero order reaction, in which at higher concentration, the viscosity increases, the reactant diffusion resistance, reaction rate decreases, and the by-product (NaBO2) formed in hydrolysis can adsorb on the catalyst surface and block the active sites (Ozay et al., 2011; Sagbas, 2012; Walter et al., 2008; Saka and Eygi, 2020). Our study was investigated at low concentration compared to the zero-order reaction. To examine the influence of the reaction temperature effect (Fig. 10\n), experiments were executed using 2.67\u00a0mmol of alkaline NaBH4with 100\u00a0mg of HFP-40 MNFs at temperatures ranging from 298\u00a0K to 328\u00a0K to obtain the activation energy (Ea) of NaBH4hydrolysis catalyzed HFB-40 using the Arrhenius equation (Eq.\n\n(4)\n).\n\n(4)\n\n\nln\nk\n=\nln\nA\n-\n\n\nE\na\n\n\nRT\n\n\n\n\n\n\nWhere k is a rate constant, A is a pre-exponential factor, R is a gas constant, and T is the reaction temperature. As expected, the H2 generation increased as the reaction temperature and H2 volume vs reaction time changed linearly (Fig. 10a). As shown in the figure, the reaction time for H2 generation has been reduced at elevated temperature. Furthermore, the H2 generation is increased. As seen in Fig. 10, when the reaction temperature rises from 25\u00a0\u00b0C to 55\u00a0\u00b0C, the hydrogen yield increases from 50\u00a0% to 100\u00a0% in 28\u00a0min. This obtained is compatible with the studies in the literature (Ekinci et al., 2020; Wei et al., 2017; Li et al., 2014). The rate (k) value is obtained from the linear portion of temperature graphs. From Arrhenius plot ln (k) versus 1/T in Fig. 10\nb and Arrhenius equation (Eq.\n\n(2)\n), the Ea was estimated to be 23.52\u00a0kJ\u00a0mol\u22121. Activation energies of non-noble metals were reported in the literature between 16.28 and 42.45\u00a0kJ\u00a0mol-1 (D\u00f6nmez and Ayas NJIJoHE. , 2021; Tamboli et al., 2015; Hua et al., 2003; Soltani, 2020; K\u0131l\u0131n\u00e7 and \u015eahin, 2019). Activation energy values of prepared NFs and Ni-based catalysts are compared (Table1\n), indicating superior catalytic performance of the introduced Ni@PVDF-HFP. The H2 generation gathered over time by employing different catalyst amounts (100 to 250\u00a0mg) of HFP-40 MNFs is given in Fig. 11\n. As expected, the H2 generation increased when the amount of HFP-40 was increased as more catalyst provides more active sites for NaBH4 dehydrogenation. One can note that the H2 generation from the hydrolysis reaction of NaBH4 proceeds very slowly and then stops without catalyst (Fig. 8). The H2 generation increased as the catalyst amount increased (Fig. 11\na), attributed to the fact that hydrolysis reaction of NaBH4 is a catalyst-controlled reaction. This phenomenon can be attributed to the fact that the reaction rate increased due to the active sites of the catalyst increasing in direct proportion to catalyst amount, called structure sensitivity. Thus, it is clear that H2 generation can be determined by controlling catalyst amount. Fig. 11\nb shows the ln H2 generation rate vs ln HFB-40. The determined slope of the best-fit line is 1.29, suggesting that the produced H2 agrees with pseudo-first-order kinetics regarding the amount of catalyst. According to the results obtained from the effect of catalyst concentration, NaBH4 concentration, and reaction temperature, the NaBH4 dehydrogenation kinetic equation catalyzed by the introduced NFs membrane can be written according to Eqs. (5), (6), and (7).\n\n(5)\n\n\nr\n=\n-\nd\n\n\n\n\nSBH\n\n\n\n\nd\nt\n=\nk\n\n\n\n\n\nHFB\n-\n40\n\n\n\n\n\n1.29\n\n\n\n\n\n\n\nSBH\n\n\n\n\n\n0.74\n\n\n\n\n\n\n\n\n(6)\n\n\nk\n=\nA\n\ne\n\n\n\n\n\n-\n\nE\na\n\n\n\nRT\n\n\n\n\n\n\n\u2192\nln\nk\n=\nln\nA\n-\n\n\nE\na\n\n\nRT\n\n\n\n\n\n\n\n\n(7)\n\n\nr\n=\n-\nd\n\n\n\n\nSBH\n\n\n\n\nd\nt\n=\n22337.01\n\ne\n\n\n\n\n\n2828.7\n\nT\n\n\n\n\n\n\n\n\n\n\nHBF\n-\n40\n\n\n\n\n\n1.29\n\n\n\n\n\n\nSBH\n\n\n\n\n0.74\n\n\n\n\nGibbs free energy of activation (\u0394G, (kJ\u00a0mol\u22121)) can be determined using thermodynamic data (activation enthalpy (\u0394H, (kJ\u00a0mol\u22121)) and activation entropy (\u0394S, (J\u00a0mol\n\u2212\n\n1\u00a0K\u22121)) according to Eqs. (8) and (9):\n\n(8)\n\n\nln\n\nk\nD\n\n=\nln\n\n\nk\nB\n\nh\n\n+\n\n\n\u0394\nS\n\nR\n\n-\n\n\n\u0394\nH\n\n\nRT\n\n\n\n\n\n\n\n\n(9)\n\n\n\u0394\nG\n=\n\u0394\nH\n-\nT\n\u0394\nS\n\n\n\n\nWhere kD = (k/T), KB and h are the Boltzmann constant (1.381\u00a0\u00d7\u00a010\u221223\u00a0J\u00a0K\u22121) and the Planck constant (6.626\u00a0\u00d7\u00a010\u221234\u00a0J\u00a0s\u22121), respectively. According to Eq. (9) and Fig. 10\n, \u0394H and \u0394S is estimated to be 20.92\u00a0kJ\u00a0mol\u22121 and 0.0272\u00a0kJ\u00a0mol\u22121, respectively. \u0394G equation can be summarized as follows:\n\n(10)\n\n\n\u0394\nG\n=\n20.92\n-\n0.0272\nT\n\n\n\n\n\u0394G values are estimated to be 12.81 and 11.9\u00a0kJ\u00a0mol\u22121 at 298 and 323\u00a0K, respectively. This main that the reaction's spontaneity is directly increased with the temperature.Reusability and stability are significant factors in determining whether the catalyst is suitable for a practical application. The reusability of the HFB-40 membrane NFs was tested repeatedly ten times to confirm its stability in the presence of 2.67\u00a0mmol of alkaline NaBH4, at each cycle, with 100\u00a0mg of HFP-40 MNFs at 25\u00a0\u00b0C (Fig. 12\n). The membrane NFs was used ten times without makeup, reactivation, or regeneration. As seen in the figure, the catalytic activity of the membrane NFs catalyst is maintained even if the use is repeated for up to 8 cycles as the H2 generation rate remains unchanged. The slight decreases have been demonstrated as the cycle number increases. The same amount of H2 has been generated extended reaction time. This could be due to the precipitate of reaction products on the membrane that inhibited the active metal sites as the membrane is used without cleaning during all cycles, which this accumulation harms the hydrogen generation rate. The slight decrease in catalytic activity after eight cycles may be due to the increase in the number of boron products on the membrane surface as an increase in the solution viscosity (Yang et al., 2017), which decreases the accessibility of active sites or blockage of pores in the membrane NFs. XPS of the HFB-40 catalyst confirms this after reuse for ten cycles (Fig. 13\n). The Ni 2p XPS spectrum of Ni 2p spectra shows two main peaks positioned at 856.1 and 873.6\u00a0eV, which are assigned to Ni 2P3/2 and Ni 2P1/2, together with two corresponding satellite peaks located at 861.7 and 880.6\u00a0eV (Zhu et al., 2017). As depicted in Figure, the Ni 2p3 peak located at 856.1\u00a0eV demonstrated the formation of Ni(OH)2 after reuse ten times, which is similar to the value obtained in literature (Yao et al., 2016). Furthermore, Na 1\u00a0s peak located at 1072.2\u00a0eV and B 1\u00a0s peak located at 180\u00a0eV appeared after reuse, ten cycles because the Na ions are generated from NaBH4 hydrolysis, which mainly constitutes contaminants, are dissolved in the solution during the hydrolysis. The SEM image of HFB-40 catalyst after reuse for ten cycles (Fig. 14\n) indicates that the NFS kept their nanofibrous structure.Metallic Ni NPs @ PVDF-HFP membrane NFs catalysts have been successfully prepared via electrospinning technique followed by in-situ reduction of metal ions to metallic Ni. The prepared metal catalysts have shown an excellent catalytic performance in the H2 generation from NaBH4. The sample was composed of 40\u00a0%wt. NiAc showed the highest catalytic activity compared to the other formulations. Whereas 103\u00a0mL of H2, from the hydrolysis of 1.34\u00a0mmol NaBH4, was produced using 40\u00a0% wt. NiAc tis compared to 68\u00a0mL, 81\u00a0mL, and 93\u00a0mL for 10\u00a0% wt., 20\u00a0% wt., and 30\u00a0% wt. NiAc, respectively, in 60\u00a0min at 25\u00a0\u00b0C. The increase in the temperature lead to an increase in the hydrogen production rate and obtained low activation energy. (23.52\u00a0kJ\u00a0mol\u22121). The kinetics study revealed that the reaction was pseudo-first-order in sodium borohydride concentration and catalyst amount.Furthermore, the catalyst exhibits satisfactory stability in the hydrolysis process for ten cycles. Because of its easy recyclability, the introduced catalyst has a wide range of potential applications in the generation of H2 from sodium borohydride hydrolysis. Considering that the eco-friendly and inexpensive Ni@PVDF-HFP membrane NFs are catalytically effective with superior reusability, they should have potential application in the H2 generation from the sodium borohydride hydrolysis.\nAbdullah M. Al-Enizi: Conceptualization, Investigation, Methodology, Writing \u2013 original draft. Ayman Yousef: Conceptualization, Data curation, Investigation, Methodology, Writing \u2013 original draft, Writing \u2013 review & editing. Shoyebmohamad F. Shaikh: Conceptualization, Investigation, Methodology, Writing \u2013 review & editing. Bidhan Pandit: Methodology, Writing \u2013 review & editing. M.M. El-Halwany: Investigation, Methodology, Validation.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors extend their sincere appreciation to the Researchers Supporting Project number (RSP-2021/370), King Saud University, Riyadh, Saudi Arabia, for the financial support.Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2022.104207.The following are the Supplementary data to this article:\n\nSupplementary video 1\n\n\n\n\n\n\n", "descript": "\n                  Nickel nanoparticles (Ni NPs) supported on Poly(vinylidene fluoride-co-hexafluoropropylene) nanofibers (PVDF-HFP NFs) were successfully synthesized through electrospinning and in-situ reduction of Ni2+ salts into the surface of PVDF-HFP NFs to form metallic Ni NPs@PVDF-HFP NFs. Different percentages of nickel acetate tetrahydrate (NiAc) (10\u00a0%, 20\u00a0%, 30\u00a0%, 40\u00a0% wt.) based PVDF-HFP. The formation of tiny metallic Ni NPs @PVDF-HFP membrane NFs was demonstrated using standard physiochemical techniques. Nanofibers membranes have demonstrated good catalytic activity in H2 production from sodium borohydride (NaBH4). The sample composed of 40\u00a0%wt Ni showed the highest catalytic activity compared to the other formulations. Whereas 103\u00a0mL of H2, from the hydrolysis of 1.34\u00a0mmol NaBH4, was produced using 40\u00a0wt% NiAc compared to 68\u00a0mL, 81\u00a0mL, and 93\u00a0mL for 10\u00a0wt%, 20\u00a0wt%, and 30\u00a0wt% NiAc, respectively, in 60\u00a0min at 25\u00a0\u00b0C. The hydrogen generation has been enhanced with an increase in the Nanofibers membrane amount and reaction temperature. The latter results in a low activation energy (23.52\u00a0kJ\u00a0mol\u22121). The kinetics study revealed that the reaction was pseudo-first-order in sodium borohydride concentration and catalyst amount. Furthermore, the catalyst exhibits satisfactory stability in the hydrolysis process for ten cycles. Because of its easy recyclability, the introduced catalyst has a wide range of potential applications in the generation of H2 from sodium borohydride hydrolysis.\n               "}
{"full_text": "In recent years, interest in lignocellulosic biomass valorization into value-added chemicals and fuels has gained importance due to decreasing fossil raw materials [1\u20133]. Along with hemicellulose and cellulose, lignin is one of the three most important biopolymer components of lignocellulosic biomass and comprises an aromatic structure with ether linkages, methoxy-, and hydroxyl groups. Depending on the species, lignin makes up about one-third of the solids in wood. Lignin, which consists of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, is an amorphous, three-dimensional polymer. Cross-linking occurs via C-O-C (\u03b2-\u039f-4, \u03b1-\u039f-4, 4-\u039f-5) and CC (\u03b2-1, \u03b2-\u03b2, 5\u20135) bonds. The largest proportion of these are the stable C-O-C bonds, with the 4-O-5 bond having the strongest bond dissociation energy at 330\u00a0kJ\u00a0mol\u22121. This makes effective cleavage of such C-O-C bonds the key factor in the depolymerization of lignin [2,4]. A general structure of lignin is shown in Fig. 1\n. A variety of different strategies for the depolymerization of lignin have been investigated, such as hydrocracking [5], hydrogenolysis [6], pyrolysis [7], and oxidation [8,9]. The catalytic pathway is considered to be the key technology for lignin utilization technologies. In current industrial processes, more than 80% use various catalysts for the synthesis of a wide range of chemical, petrochemical, and biochemical products as well as polymers [10\u201312]. Homogeneous catalysis has limited application in the industry compared to heterogeneous catalysis for depolymerization of lignin, due to their inherent disadvantages like the difficult separation from the reaction mixture, and corrosiveness for example in the case of soluble acid catalysts [13]. The reusability and recyclability of heterogeneous catalysts are attracting the interest of the scientific community [14]. Precious metal-based catalysts, such as Pt, Pd, and Ru have high activity in the depolymerization of lignin due to their high hydrodeoxygenation activity [15,16]. To date, many different metal-loaded heterogeneous catalysts have been investigated for lignin depolymerization. The substrates range from model compounds to biomass-derived lignin. Most of these processes are based on reductive depolymerization strategies, which additionally require high pressures of externally supplied hydrogen [14]. Sturgeon et al. used a nickel-loaded HTC catalyst which acts similarly to the base-catalyzed mechanism due to the basic nature of the support material [14]. Metallic nickel has shown particular promise for the formation of phenols from lignin, as it can cleave aryl-aryl C-O-C bonds and C-OH bonds into side chains with CH3 or CH2 functions, usually yielding C1-C3 alkane-substituted guaiacol as the main products [17]. Besse et al. investigated the catalytic conversion of lignin model compounds with a Pt/C catalyst in ethanol/water mixtures, using ethanol as both H-donor and solvent. The study highlights the high selectivity of water/ethanol mixtures at hydrothermal conditions concerning hydrogenolysis products and hydrogenation products. CO bond cleavage of the model compounds could be achieved by H-transfer hydrogenolysis without externally added hydrogen [18]. Huang et al. describe an increase in the rates of Guerbet, esterification, and alkylation reactions when HTC is loaded with up to 20% Cu. A combination of Cu and basic sites also facilitates the dehydrogenation of ethanol, generating the hydrogen needed for the hydrogenolysis and hydrodeoxygenation reactions, resulting in a higher yield of monomers [19]. Chaudharya and Dhepe describe a range of transition metal-free catalysts suitable for base-catalyzed depolymerization of lignin, including zeolites, metal oxides, hydrotalcites, and hydroxyapatite. Although the reaction conditions described allow the preparation of relevant monomer and oligomer building blocks, they do so only in low yield and with high catalyst input [20]. Support materials equipped with transition metals are another class of catalysts suitable for the degradation of lignin. For example, US9631146B2 describes a process using nickel on a double-layered hydroxide as a catalyst. However, studies of such a system showed that the yield of desired products was relatively low due to a high proportion of the coke fraction in the reaction product [21]. The objective of the present study was to provide a process for the catalyzed degradation of lignin with a high yield and high selectivity for phenolic building blocks with minimal formation of the coke fraction and to find a process for degradation under mild reaction conditions, i.e. low temperature, low pressure and without inert gas or hydrogen atmosphere. In addition, part of the present study was to realize a significant reduction in the amount of catalyst required [22]. Due to the great potential of precious metal-containing catalysts, the advantages of HTC as an alkaline support material, and the described advantage of ethanol/water mixtures as solvent and H-donor for the depolymerization of lignin, a catalyst screening followed by optimization of the most promising catalyst system (5% Pt-1% Ni/HTC) via DoE was performed in this study--Organosolv lignin was purchased from Chemical Point. All catalysts used in this work were supplied from Heraeus Deutschland GmbH & Co. KG. Ethanol (EtOH) was obtained from AustrAlco, ethyl acetate (EtOAc), tetrahydrofuran (THF) and sodium sulfate from Roth, and hydrochloric acid (HCl) from VWR. All chemicals in this work were used without further purification.All catalysts were prepared by impregnation. The preparation of the 5%Pt-1%Ni/HTC catalyst is described as an example. 5\u00a0g Pt as platinum(II) nitrate (Pt(NO3)2 solution with 15.2% Pt, Heraeus) and 2\u00a0g Ni as nickel(II) nitrate hexahydrate (Ni(NO3)2*6 H2O with 20% Ni, Merck) were diluted to 30\u00a0mL and homogenized. The solution was added to 93\u00a0g HTC (Sasol, BET surface area 19 m2 g\u22121) and the mixture was homogenized. The mixture was dried overnight at 110\u00a0\u00b0C under a nitrogen atmosphere in a vacuum. This was followed by thermal treatment for 14\u00a0h in an oxygen-containing atmosphere, during which the temperature was gradually increased to 250\u00a0\u00b0C. Finally, the material was treated for 16\u00a0h with forming gas (95\u00a0vol% nitrogen, 5\u00a0vol% hydrogen) at up to 250\u00a0\u00b0C.The characterizations of the catalysts were carried out by Heraeus Precious Metals Company in Hanau, Germany.The determination of the BET surface area of the catalysts was carried out at \u2212195.8\u00a0\u00b0C using the Q-Surf SA-9601 measuring device from Horiba. Each catalyst was analyzed using a 3-point measurement with a 70% helium/30% nitrogen gas mixture at three different points of the adsorption isotherm (i.e. at different relative pressures). To remove gases and vapors adsorbed on the catalyst, especially water, the samples were degassed at 250\u00a0\u00b0C for 60\u00a0min before measurement.The precious metal surface area of the catalysts was determined via Carbon monoxide (CO) adsorption. Each catalyst was first reduced for 20\u00a0min at 400\u00a0\u00b0C under forming gas consisting of 95% argon and 5% hydrogen in a closed container. CO (with helium as carrier gas) was then dosed in pulses into the container. This was done until constant CO peaks were detected behind the catalyst. By determining the peak area of the dosed CO and the peak area of the converted CO, the amount of CO absorbed by the catalyst was determined. From the amount of CO absorbed obtained, it was calculated how much CO was stored per amount of catalytically active composition used. From the measured amount of CO stored at the active centers, the surface area of the active precious metal centers (often referred to as precious metal surface area) could be determined by conversions.The pH of the catalysts was determined with a WTW inoLab pH\u00a07310 instrument. This was done by adding 10\u00a0mL of water to 800\u00a0mg of dry catalyst in a test tube and mixing well. For the ionic equilibrium to be established, the measurement is carried out after one hour.Powder XRD patterns of the catalysts were recorded with an STOE Stadi P Diffractometer in the 2\u03b8 range of 5\u201381\u00b0 using a Cu-K\u03b1 radiation source (k\u00a0=\u00a01.54056\u00a0\u00c5). The operating voltage and current were 40\u00a0kV and 30\u00a0mA, respectively, with a scanning speed of 4\u00a0deg.\u00a0min\u22121 for data acquisition.Focused ion beam scanning electron microscopy (FIB-SEM) images were acquired by using a Field emission SEM of ZEISS, 1540XB. All the samples were sputtered with 5\u201310\u00a0nm of gold.Transmission electron microscopy (TEM) investigations were conducted using an FEI Talos 20\u2013200 transmission microscope at 200kV, The measurements were performed in TEM mode. Energy-dispersive x-ray spectroscopy was used to detect differences in local chemical composition. Carbonized Cu-grid, Plano Mesh 300, was used. In scanning transmission electron microscope (STEM) collected images using Bright Field (BF) and high-angle annular dark-field (HAADF) detector. The pictures from HAADF and BF are in the same folder. BF is almost the same that you see in TEM mode, HAADF shows Z contrast (atomic number), which means that Pt or Pd particle will be very visible on C or Al matrix.The heterogeneously catalyzed depolymerization experiments were carried out in a 450\u00a0mL stainless steel autoclave (PARR, 4871 Process Controller, Software: SpecView3). The scheme of lignin depolymerization and the workup of the fractions was similar to the previous study [2] and is schematically shown in Scheme 1\n. In a typical experiment, 20\u00a0g of lignin and 4\u00a0g of heterogeneous catalyst were used. The catalyst:lignin ratios described in the literature for the use of supported HTC catalysts for the depolymerization of lignin (2:1 wt:wt Strungeon et al.; 1:2 wt:wt Huang et al) are difficult to implement for large-scale use of catalysts for the depolymerization of lignin. Therefore, in this study, we chose to screen for a much lower ratio of 1:5 wt:wt. The reaction parameters for all screening experiments were set to 200\u00a0\u00b0C, 30\u00a0min residence time, and an ethanol/deion. water mixture with an Ethanol concentration of 45.9% (v/v). Lignin was suspended in 200\u00a0mL of the reaction mixture and treated in the Parr reactor at a stirring speed of 5\u00a0s\u22121. The gas produced during the reaction was removed and determined volumetrically. Subsequently, the reaction mixture was separated into the product fractions lignin oil (monomer yield), lignin tar (oligomer yield), lignin coke (coke formation), and water solubles. The reaction mixture was transferred in a beaker with a spatula and 45.9% EtOH (v/v) and acidified with concentrated HCl (37\u00a0wt%) to a pH value of 2 to precipitate the lignin tar, which were separated by vacuum filtration through a suction filter with a porosity of 3. The solid was washed three times with diluted HCl. The aqueous phase was extracted three times with EtOAc. The organic phases were combined, dried with sodium sulfate, and filtered. The EtOAc was evaporated using a rotary evaporator to obtain the EtOAc soluble products (lignin oil). The solids (lignin tar, coke, and catalyst) were suspended in THF to dissolve the THF soluble products. The solid remaining after filtering in the suction filter consisted of lignin coke and heterogeneous catalyst. The liquid phase was evaporated using the rotary evaporator to obtain the THF soluble products (lignin tar).The yields of all fractions were calculated thus:\n\n(1)\n\nyield of lignin oil\n\nwt\n%\n=\n\n\nweight of lignin oil\nweight of lignin\n\n\n\u2219\n100\n\n\n\n\n\n(2)\n\nyield lignin\n\ntar\n\nwt\n%\n=\n\n\n\nweight of lignin\n\ntar\n\nweight of lignin\n\n\n\u2219\n100\n\n\n\n\n\n(3)\n\ncoke formation\n\nwt\n%\n=\n\n\n\nweight of lignin\n\ncok\ne\n\u2212\nweight of catalyst\n\nweight of lignin\n\n\n\u2219\n100\n\n\n\n\n\n(4)\n\n\nwater solubles\n\nwt\n%\n=\n\n\nweight of water solubles\nweight of lignin\n\n\n\u2219\n100\n\n\n\n\nThe most promising catalyst system was optimized using Statistical DoE (Design Expert \u00ae) to maximize the value-added fractions of lignin oil and lignin tar products and minimize the coke fraction. For this purpose, a reduced response surface central composite design (CCD) was planned. In this experimental plan the three numeric variables temperature (T), residence time (t), and the catalyst amount were studied. The stirrer speed was kept constant at 300\u00a0rpm in all experiments. For this design, 13 randomized experiments, including 3 replicates at the center point were performed. Randomization was done to minimize unobserved effects which could influence the results. The influence of each numeric variable and also the interaction between the variables could be studied with this experiment. The effect of the three factors temperature, residence time, and catalyst amount on the depolymerization of organosolv lignin was evaluated using the gravimetric yields of the product fractions lignin oil and lignin tar and the unwanted coke formation as response variables. After analyzing the experiments by ANOVA with a 95% confidence level, an optimization of the reaction parameters was calculated with the software \u201cDesign Expert\u201d and a verification experiment was performed.Organosolv lignin was isolated from bluebells and corn by ChemicalPoint in Germany. The characterization was done by NREL analysis: total solids (d.m.) [23], ash [24], carbohydrates and lignin [25]. The relative ratios of the main linkages were determined by 2D-HSQC NMR techniques following reported procedures [26].For determination of the phenolic OH groups the Folin Ciocalteu method [2] and the 31P NMR method [27] with cholesterol as an internal standard was applied. Experimental details for sample pre-treatment and derivatization reaction can be found in the Supporting information.The analysis of the product fractions was done similarly to S\u00fcss et al. [2]. Molecular weight determination of the lignin oil, lignin tar, and the origin OL were done by GPC (Thermo Scientific, Dionex ICS 5000+) using a PSS MCX analytical 100A\u00a0+\u00a01000A\u00a0+\u00a0100000A column (8\u00a0mm\u00a0\u00d7\u00a0300\u00a0mm, Thermo Fischer). It was performed at 30\u00a0\u00b0C with 0.1\u00a0mol\u00a0L\u22121 NaOH as eluent at a flow rate of 0.5\u00a0mL\u00a0min\u22121. For detection, a wavelength detector set at 280\u00a0nm was used. The system was calibrated using standards from PSS (Polymer Standard Service) with a molecular range of 891\u2013976,000\u00a0g\u00a0mol\u22121 and Vanillin. For determination of the phenolic OH groups of both fractions, lignin oil and lignin tar, and the used OL the Folin Ciocalteu method [2] and the 31P NMR method [27] with cholesterol as internal standard was applied. The relative ratios of the main linkages were determined by 2D-HSQC NMR techniques following reported procedures [26]. Additionally, the lignin oil products were analyzed quantitively and qualitatively by a Shimadzu GC MS QP 2020 instrument with an HP SM5 capillary column (60\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u03bcm) and electron impact ionization (EI). As carrier gas, He was used with a split ration of 10. The oven temperature was increased from 50\u00a0\u00b0C to 300\u00a0\u00b0C with a 10\u00a0\u00b0C\u00a0min\u22121 heating rate. For oven temperatures of 120\u00a0\u00b0C and 280\u00a0\u00b0C, holding times of 5 and 8\u00a0min were set. The injection temperature was set to 250\u00a0\u00b0C. For quantification, the internal standard toluene and an external calibration with 41 possible monomer substances that can be formed during depolymerization were used.In our study, we investigated the heterogeneously catalyzed depolymerization of OL in an ethanol-water mixture (45.9% v/v). The HTC-supported catalysts were prepared in cooperation with Heraeus GmbH, Germany. Because of the high purity and total lignin content of 98.24\u00a0wt%, no further purification before the depolymerization experiments of the purchased OL was necessary. Analysis results for the determination of the dry matter, ash content, lignin content (acid-soluble, acid-insoluble), and sugar contents, are shown in Table 1\n.All prepared catalysts were characterized by Heraeus GmbH using precious metal surface (PM surface) (CO / m2 g\u22121), BET surface area (m2\u00a0g\u22121) analysis, and determination of the pH value as listed in Table 2\n. Fig. 2\n shows the XRD pattern of the 5%Pt-1%Ni/HTC catalyst, which was found to be the most suitable catalyst for depolymerization in ethanol/water after catalyst screening. Typical characteristic bands for HTC were found at 11.4\u00b0, 34.2\u00b0, and 60.0\u00b0. The peaks arising from mixed nickel oxides are found at 22.5\u00b0and 37.8\u00b0. The fact that nickel is mainly present as Ni(OH)2 and other mixed nickel oxides indicate an interaction of nickel with the HTC support. These results are in agreement with results from the literature [14].\nFig. 3\n shows three selected catalysts, 0.5%Pt/HTC, 5%Pt/HTC and 5%Pt-1%Ni/HTC. The amount of noble metal or additional loading of Ni does not influence the optical appearance of the support material, as shown in figures d, h and l. The shell shape, which is typical for this substrate, is retained regardless of the loading. In the TEM images, a significant difference between the individual catalysts is observed. With increasing loading, the particles with an average particle size of 2.51\u00a0nm for the 5%Pt/HTC catalyst (d) and 1.73\u00a0nm for 5%Pt-1%Ni/HTC (g) are significantly more uniform in size and much more homogeneously distributed on the catalyst surface than the 0.5%Pt/HTC (a) catalyst with an average particle size of 7.31\u00a0nm which is consistent with the literature [4]. It can be seen that the additional supporting of nickel leads to a different distribution of the metallic particles than when the substrate is only supported with Pt. This effect needs to be further investigated with additional analytic techniques.On the SI-EDS-HAADF images, the actual active metal particles are visible (Pt red, Ni yellow).The influence of the platinum loading and also the loading with a second metal (Cu or Ni) of the HTC supported catalysts on the product yields were examined in detail (Fig. 4\n).20\u00a0g of OL and 4\u00a0g of catalyst were used for each experiment to compare the different catalysts in terms of yields of the value-added fractions (lignin oil and lignin tar) and reduction of coke formation. The reaction parameters were set at 200\u00a0\u00b0C, 30\u00a0min, and 300\u00a0rpm. After depolymerization, the fractions were separated as shown in Scheme 1.Gravimetrical yields of all screening experiments are shown in Fig. 4. Without a catalyst, only small amounts of the value-added product fractions (3\u00a0wt% lignin oil and 15\u00a0wt% ligni tar) were achieved. The undesired coke formation with 58\u00a0wt% is by far the largest fraction formed without a catalyst under these reaction conditions. The influence of the Pt loading showed an increase of the lignin oil fraction and lignin tar fraction from 13 to 20\u00a0wt% (lignin oil) and 43 to 53\u00a0wt% (lignin tar) and a reduction of the coke formation from 30 to 20\u00a0wt% by increasing the Pt loading from 0.5% to 5%. To evaluate the influence of Cu and Ni, depolymerization experiments with 2% Cu on HTC and 2% Ni on HTC were conducted. Both metals showed a significantly higher coke formation (Cu: 27\u00a0wt%, Ni: 24\u00a0wt%) with approximately 60\u00a0wt% higher lignin tar yield compared to the 5% Pt catalyst. Comparing the pure 5%Pt loading with the combination of 5%Pt on HTC with a second metal at 1% or 2% (Cu or Ni) each, the yield of lignin oil decreased from 20 to 16 and 17\u00a0wt%, respectively. The yield of lignin tar was increased from 53 to 59\u00a0wt%. The positive effect of Cu and Ni on the reduction of coke formation described in the literature could be verified in our study under the tested conditions, compared to the reaction without catalyst [17,19,28]. Comparing the coke formation of the depolymerization reactions catalyzed with 2%Cu/HTC or 2%Ni/HTC with the Pt/HTC catalyzed ones, the coke formation of Cu or Ni is more effectively suppressed, but the yield of lignin oil decreases significantly, whereby loading with Ni achieves both less coke formation and higher yields of lignin oil than that with Cu. To gain the benefits of Pt and the additional loading of Cu or Ni, a combined loading was tested. The combination increased the yields of the value-added fractions lignin oil and lignin tar and further reduced the coke formation when 5%Pt-1%Ni/HTC was used to 15%. Again, Ni showed better performance over Cu in terms of coke avoidance.To compare the molecular mass distribution of the monomeric and oligomeric product fractions and of the purchased lignin GPC was used. GPC data are shown in Table 3\n. A representative GPC chromatogram for the untreated OL and depolymerized lignin fractions after reaction with 5% Pt on HTC support at the standard reaction parameters of 200\u00a0\u00b0C, 30\u00a0min, and 300\u00a0rpm is shown in the supporting information (Fig. S5).It can be seen that higher loading with Pt or additional loading with Ni leads to higher molar masses in the lignin tar fraction. This can be explained by the fact that both Pt and Ni suppress coke formation and thus more, but larger lignin tar fragments are formed. The molecular mass distribution is narrower when only 5%Pt is loaded on the support than when a second metal is additionally loaded on the support. Since the molecular mass distribution of the catalyst 5%Pt-1%Ni/HTC is nevertheless very narrow compared to the OL and the gravimetric yields of the value-added fractions could be increased again with the loading of a second metal and above all the coke formation was reduced, this catalyst was considered worthwhile for optimizing the process parameters.Based on the catalyst system found to be suitable (5%Pt-1%Ni/HTC), a DoE was prepared to further minimize coke formation and maximize the value-added fractions (lignin oil and lignin tar). For this purpose, a randomized response surface CCD was performed. Using the program Design Expert\u00a9, the optimal reaction parameters (catalyst amount, residence time, and temperature) were determined, and a verification experiment was performed. The layout of the design as well as its limitations, surface plots and evaluation can be seen in Tables S1-S5 and Fig. S1.The analysis of the results gave a reduced quadratic model for the lignin oil and lignin tar fractions as described in Eqs. (5) & (6).\n\n(5)\n\nlignin oil fraction\n=\n+\n14.8947\n+\n0.050682\n\u2219\nT\n\u2212\n10.69925\n\u2219\n\nm\ncat\n\n+\n2.60\n\u2219\n\n\nm\ncat\n\n2\n\n\n\n\n\n\n(6)\n\nlignin\n\ntar\n\nfraction\n=\n+\n93.91967\n\u2212\n0.20497\n\u2219\nT\n+\n13.11667\n\u2219\n\nm\ncat\n\n\u2212\n3.29673\n\u2219\n\n\nm\ncat\n\n2\n\n\n\n\nIn the case of coke formation, the analysis led to a reduced linear model as can be described by Eq. (7).\n\n(7)\n\ncoke formation\n=\n\u2212\n0.18865\n+\n4.61728\n\u2219\n\nm\ncat\n\n\n\n\nThese models were used to optimize the process parameters for depolymerization of OL in ethanol-water mixture with the heterogeneous catalyst 5%Pt-1%Ni/HTC. The resulting process parameters, the predicted yields of the response fractions yield of lignin oil, yield of lignin tar, and coke formation as well as the yields of the verification experiment are shown in Table 4\n.As can be seen in the verification experiment, the yield of the lignin oil fraction and coke formation is slightly lower than predicted and the yield of the lignin tar is much higher. These results indicate, that the model is still not perfect to describe all effects of the depolymerization conditions. These yields are comparable to the homogeneously catalyzed depolymerization of organosolv lignin using NaOH, where 9.4% lignin oil, 86.2% lignin tar, and 0.5% coke formation were achieved [2]. Since the overall yields of the valuable products were higher and the coke formation lower than predicted and thus the share of value-added products was increased overall, the model is considered sufficient for us. Queneau and Han describe the challenges of biomass recovery in their study. The use of catalysts capable of accommodating the multifunctional nature of biomolecular substrates as well as preventing undesirable over-converted products such as coke formation is a challenge for the scientific community [29]. In the process conditions determined by DoE, the catalyst used in this study can increase the product fractions suitable for further processing while minimizing the undesirable coke fraction and the required catalyst amount. To verify the influence of the support material or the optimized process parameters on the gravimetric yields, additional tests were carried out with the pure support material (0.237\u00a0g) and tests without catalyst, in each case at 233\u00a0\u00b0C and a residence time of 88\u00a0min. The yields are shown in Table 5\n. The use of 0.237\u00a0g of pure support material instead of 5%Pt-1%Ni/HTC shows a small positive influence on the yields of the lignin oil and lignin tar fractions and a slightly positive influence on the prevention of coke formation compared to the experiment where neither support material nor catalyst was used at the process parameters determined by DoE. These results show that the loading of the support material has a decisive influence on the gravimetric yields of the value-adding fractions (ligin oil and lignin tar) and a significant influence on the prevention of coke formation. Wang et al. report that the presence of Ni on the surface of the support material has a positive effect on the hydrogenation and hydrogenolysis of large lignin fragments [30]. Using NaOH catalysis in association with Ru/C, Long et al. were able to reduce coke formation to 14.03% [31]. A possible reaction mechanism is shown in Fig. 5\n.A possible explanation for the effective prevention of coke under optimized conditions is the hydrogen transfer of ethanol in the solvent, which is supported by Ni in the catalyst. However, it is necessary to investigate the mechanistic background in more detail considering different coke qualities of different catalysts in a follow-up study.2D-HSQC NMR was used to investigate the S/G/H ratios and the number of \u03b2-O-4 bonds of the purchased organosolv lignin and the verification of bond cleavage in the degradation products. The analysis showed an S/G/H ratio of 12/83/5 and 28 \u03b2-O-4 bonds for the initial lignin and no remaining \u03b2-O-4 bonds in the product fractions. The HSQC spectra are shown in Figs. S2-S4.GPC was employed to determine the molecular weight distribution of the used lignin and the lignin oil and lignin tar product fractions. Results are listed in Table 6\n and chromatograms are shown in Fig. S5&S6.These data indicate that the molar mass distributions are very similar for the standard and optimized conditions in both cases, the two fractions have significantly smaller molecular weight distributions than the lignin used. The high yield and especially the significantly reduced coke formation make the optimal conditions more preferable compared to the standard conditions.The determination of the phenolic OH-groups is done by the Folin-Ciocalteu method. Results of the measured OH-groups are listed in Table 7\n.The content of phenolic OH groups already increased significantly at the standard conditions in the case of the lignin oil fraction, compared to the initial lignin. Under the optimized conditions, the OH group content slightly decreased again. Looking at the lignin tar fraction, the content of phenolic OH groups was reduced in both cases compared to the initial lignin.Additionally for the purchased lignin and the depolymerization products of the reaction with the optimized process parameters, the determination of the phenolic OH-groups was done by a 31P NMR method. With this method, aliphatic OH groups, phenolic OH groups, and acid groups can be determined. The phenolic OH groups can be further divided into C5 substituted, guaiacyl, and p-hydroxyphenyl groups. Results are listed in Table 8\n. The spectra are shown in Fig. S7.Comparing the results of the Folin-Ciocalteu method with those of the 31P NMR method, it can be seen that the 31P NMR method can not only split the phenolic OH groups into different subgroups but can also detect the aliphatic OH groups as well as the carboxylic acid groups. The comparison of both methods indicates that the Folin-Ciocalteu method detects the aliphatic and phenolic OH groups as sum parameters. Whether the OH groups increase or decrease in total due to the degradation can be determined with both methods. The Folin-Ciocalteu method is a simple and cost-effective method to determine the trend of phenolic OH groups. Both methods showed an increase in phenolic OH groups in the lignin oil fraction and a decrease in the lignin tar fraction. Using the 31P NMR method, it was shown that the aliphatic OH groups decreased significantly in the case of the lignin oil fraction. The acid groups decrease significantly in both fractions compared to the starting lignin.Chen et al. report that for Pt and Ni catalysts, respectively, direct deoxygenation and hydrogenation are parallel primary reactions, but the selectivity depends not only on the metal used but also on the solvent used and the process conditions. This was confirmed in our experiments, as the selectivities of the obtained products are significantly different between the experiments at standard conditions (200\u00a0\u00b0C, 30\u00a0min, 3\u00a0g catalyst input) and the optimized conditions (233\u00a0\u00b0C, 88\u00a0min, 0.237\u00a0g catalyst input) [33]. It can be seen that the product distribution is different for the two reaction conditions. Under the standard conditions, the main product was 2-methoxy-4-vinylphenol, whereas, under the optimized conditions, the main products were 2-Methoxy-4-vinylphenol, 3,5-Dimehtoxy-4-hydroxy acetophenone as well as 2,6-Dimethoxyphenol. Significantly more Phenol was formed too. The fact that carboxylic acids are found in the GC\u2013MS analyses can be explained via the workup process by separating the fractions using aqueous HCl. Table 9\n shows the qualitatively and quantitatively determined products of both fractions. Chromatogramm can be seen in Fig. S8. To keep the results of the GC\u2013MS arranged, only monoaromatic compounds that account for more than 1\u00a0wt% of the identified compounds were evaluated.Organosolv lignin was depolymerized to lignin oil and lignin tar fractions with the developed catalyst system 5%Pt-1%Ni/HTC and process optimization using DoE, avoiding coke formation. By optimizing the process parameters to 233\u00a0\u00b0C, 88\u00a0min residence time, and a catalyst input of only 0.24\u00a0g per 20\u00a0g lignin, 18\u00a0wt% lignin oil fraction, with significantly more yield of Phenol and 2-Methoxyphenol, and 72\u00a0wt% lignin tar fraction were obtained. The coke formation could be reduced to 0.4\u00a0wt%. The monomer building blocks could be characterized by GC\u2013MS. Both product fractions, lignin oil and lignin tar showed a significant reduction in molar mass distribution. The obtained product fractions have great potential as bio-based substitutes for Phenol in phenol-formaldehyde resins and as antioxidant additives in the plastics industry.\nRaphaela S\u00fcss: Methodology, Validation, Investigation, Writing \u2013 original draft, Writing \u2013 review & editing, Visualization. Gottfried Aufischer: Methodology, Investigation. Lukas Zeilerbauer: Investigation. Birgit Kamm: Conceptualization, Project administration, Funding acquisition. Gisa Meissner: Resources. Hendrik Spod: Resources. Christian Paulik: Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was funded by Heraeus Deutschland GmbH and Co.KG and the Austrian Research Funding Association (FFG), (Wood K plus Comet Funding Period 2019-2022).\n\n\n\nDepolymerisation of organosolv lignin by supported Pt metal catalysts\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106503.", "descript": "\n                  The conversion of lignocellulosic biomass into value-added chemicals and biofuels has been attracting the attention of researchers in recent years. Lignin is an abundant, natural polymer and a major component of lignocellulose comprising an aromatic structure with ether linkages, methoxy-, and hydroxyl groups. Therefore, it has great potential as a sustainable source to produce basic chemical products. In this study, precious metal-loaded hydrotalcite (HTC) catalysts for the depolymerization of organosolv lignin (OL) were investigated concerning minimizing coke formation and maximizing the value-added lignin oil fraction and lignin tar fraction. The influences of the catalyst support, the platinum loading as well as the loading with a second metal (Cu or Ni) were examined. The resulting depolymerization fractions (lignin oil, lignin tar, aqueous fraction, and coke) were determined gravimetrically. To compare the molecular mass distribution of the lignin oil and lignin tar fractions as well as the purchased OL, gel permeation chromatography (GPC) was used. The lignin oil fractions were analyzed quantitatively and qualitatively by gas chromatography-mass spectroscopy (GC\u2013MS). Regarding the most suitable catalyst system (5%Pt-1%Ni/HTC), a design of experiment (DoE) was prepared to further minimize coke formation and maximize the value-added fractions (lignin oil and lignin tar). This optimization led to 18\u00a0wt% lignin oil fraction, 72\u00a0wt% lignin tar fraction, and 0.4\u00a0wt% coke formation.\n               "}
{"full_text": "", "descript": "\n                  The composite Ni/Al2O3-SiC catalyst was designed and synthesized for hydrogen production from propane partial oxidation. Through the composite of high thermal conductivity SiC and porous solid acid Al2O3, it improved the shortcoming of sintering and inactivation of high-temperature reaction for Ni-based catalysts. The samples were characterized by X-ray diffraction, N2 physical adsorption, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis etc. From the results, the SiC doped catalyst exhibited superior catalytic properties for hydrogen production by propane partial oxidation. By doping SiC with high thermal conductivity to Al2O3 support, the catalyst was improved for sintering and carbon deposition resistance and reducing the aggregation of Ni active sites reflected by improving the stability of hydrogen production.\n               "}
{"full_text": "Production of waste is rapidly increasing and World Energy Council expected a waste daily production of more than 6 million tons by the year 2025 [1]. In order to convert the organic fraction of the waste into energy, several techniques can be used like direct combustion, pyrolysis, gasification, transesterification, fermentation, and anaerobic digestion [2]. However, anaerobic digestion is one of the most widely used techniques that result in the production of biogas and biofertilizers [2]. The resulting biogas can be used for the production of heat and electricity. Biogas is mainly composed of carbon dioxide (CO2) and methane (CH4) among other components present in lower amounts (H2, O2, volatile organic compounds, nitrogen, etc.) [3].Biogas has a very low heating value (LHV: 15\u201330\u00a0MJ Kg\u22121) when compared to natural gas (LHV: 50\u00a0MJ Kg\u22121) and hydrogen (LHV: 120\u00a0MJ Kg\u22121). Therefore, thermocatalytic dry reforming of methane is often employed for the up-gradation of biogas into more valuable syngas. However, this method has not yet been implemented commercially [4]. In this process, a mixture of biogas (i.e., methane and carbon dioxide) is converted in the presence of a suitable catalyst into syngas (a mixture of carbon monoxide and hydrogen). This well-studied reaction in the literature [5\u20136] continues to pique the interest and attention of scientists due to its importance in valorizing carbon dioxide, a major greenhouse gas. The reforming of biogas was studied on various nickel-based catalytic materials [7\u201311] and several promoters such as Gd, Sc [7], Ce [8,10,11], Co [9], and Mg [11] were used aiming to improve the stability of these catalysts. It is reported that the addition of ceria and lanthanum in zirconia-supported nickel catalysts favored carbon formation at low reaction temperatures [8]. Huang et al. [11] studied the effect of pressure and revealed that high pressure favored filamentous carbon formation on Ni/MoCeZr/MgAl2O4-MgO catalysts.However, the simultaneous conversion of a biogas mixture into syngas and carbon-derived materials has been rarely reported in the literature [12\u201315]. Generally, this process is known as catalytic decomposition of biogas, a process that yields a syngas mixture and high-quality bio-nanostructured carbon materials through a series of simultaneous reforming, decomposition, and Boudouard reactions between CH4, CO2, and CO [13]. Notably, the carbon materials produced can be used in several applications, including catalyst support [16] and energy storage materials [17]. The production of carbon materials from biogas has the advantage of producing such high added value materials from a renewable source in comparison to other fossil fuel-based methods.In previous studies, the authors used a fusion method to synthesize: i) Ni/Al2O3\n[14], ii) bimetallic Ni-Co/Al2O3\n[15], and Ni, Co, and Fe supported on alumina catalysts [13]. The same team also worked on a more sophisticated fluidized bed reactor configuration to continuously produce carbon nanofibers and syngas [12]. Based on these studies and among monometallic compositions, Ni/Al2O3 catalyst seems to be the best choice for this reaction showing maximum carbon production at a moderate temperature [13]. These findings are consistent with our recent work on iron- and nickel-loaded mesoporous alumina catalysts. We demonstrated the superiority of Ni over Fe in catalysing biogas reforming while producing greater amounts of carbonaceous deposits, classified as carbon nanotubes [18].It has been demonstrated that one-pot nickel-based mesoporous alumina catalysts (5\u00a0wt% Ni) exhibit higher activity and stability in methane reforming reactions than similar catalysts prepared by conventional impregnation [19\u201321]. To the best of our knowledge, while conventional non-porous alumina was used for carbon nanofibers and syngas production [12\u201315], mesoporous alumina materials were rarely used as catalysts for combined biogas reforming and decomposition reactions.This study aims to develop and test ordered mesoporous alumina-supported Ni-based (20, 50\u00a0wt%) catalysts for biogas (CH4 and CO2 mixture) reforming and decomposition reaction. The catalysts were prepared using a facile one-pot evaporation-induced self-assembly method. Unlike previous studies [18\u201321], this work exploits catalysts with high active metal loading to achieve a high yield of the targeted carbon nanomaterials (e.g., CNF) and syngas with the desired H2/CO ratio (\u223c 1). Furthermore, detailed thermodynamic simulations for biogas reforming and decomposition under similar reaction conditions were performed to elucidate the experimental results and gain a better understanding of the reaction pathways and mechanisms.The mesoporous Ni incorporated Al2O3 samples were synthesized following a well-established \u201cone-pot\u201d evaporation-induced self-assembly (EISA) method [22\u201323] with some minor modifications [18\u201321,24]. In a typical synthesis first 1\u00a0g of P123 Pluronic triblock copolymer ((EO)20(PO)70(EO)20, Mn\u00a0=\u00a05800, Sigma Aldrich) is dissolved in 20\u00a0ml of absolute ethanol (CH3CH2OH, Sigma Aldrich) under vigorous stirring at room temperature. After complete dissolution of the structuring agent, 1.6\u00a0ml of nitric acid (65.0\u00a0wt%, Johnson Matthey S.A) is added under stirring, together with A mmol of aluminum isopropoxide (Al (OPri)3, C9H21AlO3, Sigma Aldrich) and B mmol of nickel nitrate hexahydrate (Ni(NO3)2\u00b76H2O, Sigma Aldrich). All chemicals were used as received, without additional purification. For each synthesis, the total molar composition was always kept constant, equal to 10\u00a0mmol (i.e. [A\u00a0+\u00a0B]\u00a0=\u00a010\u00a0mmol). For the Ni20%Al2O3 sample, 0.58\u00a0g of Ni (NO3)2\u00b76H2O and 1.63\u00a0g of C9H21AlO3 were used for its preparation. Regarding the high Ni-loaded sample (Ni50%Al2O3), higher amounts of Ni nitrates and lower amounts of aluminum precursor were used respectively. Typically, 1.45\u00a0g of Ni (NO3)2\u00b76H2O and 1.02\u00a0g of C9H21AlO3 were co-introduced in the same reactor. Ni-free alumina sample was synthesized using the same experimental protocol yet without the addition of Ni precursor. For each preparation, the final mixture was covered with a polyethylene film, continuously stirred at RT for at least 7\u00a0h until complete dissolution then, transferred into a double-layer jacketed beaker supplied by a flow of distilled water regulated at 60\u00a0\u00b0C to undergo slow evaporation (ethanol, acid) for 48\u00a0h straight. The obtained greenish xerogels (the intensity of color augments with Ni content) were slowly calcined (air, thin-bed) at 600\u00a0\u00b0C for 5\u00a0h (heating rate 0.5\u00a0\u00b0C\u00a0min\u22121) to give calcined \u201cone-pot\u201d alumina-based materials.Nitrogen adsorption\u2013desorption isotherms were performed on a Micromeritics Tristar (II) 3020, porosity and surface area analyzer. Prior to measurements, calcined samples were degassed under vacuum for 3\u00a0h at 200\u00a0\u00b0C then cooled down to room temperature before being placed at \u2212196\u00a0\u00b0C (liquid nitrogen temperature). The Brunauer-Emmett-Teller (BET) surface areas were evaluated from the BET equation for a relative pressure (P/P0) range of 0.05\u20130.25, while pore size distribution (PSD) was calculated using the Barrett-Joyner-Halenda (BJH) method for the desorption branch of the isotherm.The reducibility of nickel species in calcined materials was estimated by temperature-programmed reduction (H2-TPR) upon carrying out the experiments on an Autochem 2920 (Micromeritics) apparatus. The unit was equipped with a thermal conductivity detector (TCD) for a continuous recording of the overall H2 consumption. Each sample (70\u00a0mg) was deposited on quartz wool in a U-shaped quartz reactor and degassed using argon at 150\u00a0\u00b0C for 60\u00a0min, followed by cooling to 25\u00a0\u00b0C. Then, the furnace was heated to 1000\u00a0\u00b0C at the rate of 10\u00a0\u00b0C\u00a0min\u22121 under the flow (40\u00a0ml\u00a0min\u22121) of H2/Ar mixture (10\u00a0vol% H2). A cold trap made of ice and salt (NaCl) was used to condense water generated during reduction from the effluent gas before it reached the TCD detector. This assures that the obtained signal (difference in thermal conductivity between reference and analysis gases) is essentially linked with H2 consumption.Structural characteristics of calcined materials were carried out at room temperature by performing powder X-ray diffraction (XRD) at wide angles. Diffractograms were recorded on Rigaku (Mini-flex) diffractometer operating at 40\u00a0kV and 40\u00a0mA and using a Cu K\u03b1 irradiation source (\u03bb\u00a0=\u00a01.5418 \u00c5). The acquisitions were logged for 2\u03b8 values between 10.0 and 90.0\u00b0 and a step size of 0.02\u00b0. A comparison with standard powder XRD files published by the International Center for Diffraction Data (ICDD) helped in the identification of present crystalline phases. The HighScore Plus software was used to examine the data file of the instrument. Crystallite sizes were calculated using the Scherrer equation: D (hkl) = (K\u03bb/\u03b2cos\u03b8), where K\u00a0=\u00a00.9 is the shape factor for spherical particles, \u03bb is the X-ray wavelength (\u03bb\u00a0=\u00a01.5405\u00a0nm for Cu K\u03b1), \u03b2 is the full width at half maximum (FWHM) of the diffraction peak and \u03b8 is the peak position.The morphological aspects of spent catalysts were examined employing scanning electron microscopy (SEM). Micrographs were registered on the JEOL JSM-6360A microscope accompanied by a Li/Si lens for energy dispersive spectroscopy (EDS) analyses.Thermal gravimetric analysis (TGA) was performed to quantify carbon deposition amounts on spent catalysts. Experiments were recorded on a TGA-1515 SHIMADZU thermal analyzer where; 0.010\u20130.015\u00a0g of the spent catalyst was put into a platinum pan deposited on a thermo-balance. During analysis, the temperature was raised from RT to 900\u00a0\u00b0C (heating rate: 20\u00a0\u00b0C\u00a0min\u22121) in flowing air (50\u00a0ml\u00a0min\u22121) and the change in mass was constantly monitored and computed.The biogas reforming and decomposition reaction was carried out at atmospheric pressure in a tubular stainless-steel reactor (internal diameter: 9\u00a0mm and length of 300\u00a0mm). For each experiment, a 100\u00a0mg sample (non-diluted powder), sandwiched between two quartz wool beds, was used. The respective catalysts were first in situ reduced at 600\u00a0\u00b0C for 1\u00a0h in pure H2 (30\u00a0ml\u00a0min\u22121) to ensure complete reduction of NiO to catalytically active Ni0 nanoparticles. Then, the flow was switched to the reactant mixture (CH4:CO2:N2\u00a0=\u00a01:1:0.33), and the total gas hourly space velocity (GHSV) was 42 Lgcat\n-1hr-1. Stability measurements were carried out at 700\u00a0\u00b0C for 5\u00a0h on time-on-stream where the temperature was controlled using a K-type thermocouple placed in the middle of the catalyst bed. The effluent gas was quantified by online gas chromatography using a Micro-GC equipped with a thermal conductivity detector and coupled to two columns placed in parallel for the detection of CH4, H2, and CO (type: Molecular Sieve 5A column) and CO2 (Porapak Q column). Conversions of CO2 (XCO2), CH4 (XCH4), and H2:CO molar ratio were calculated based on Eqs. (1)\u2013(3):\n\n(1)\n\n\n\nX\n\n\nCO\n\n2\n\n\n\n\n\n%\n\n\n\n=\n\n\n\n\n\n\n\n\nCO\n\n2\n\n\n\n\n\nin\n\n\n-\n\n\n\n\n\n\nCO\n\n2\n\n\n\n\n\nout\n\n\n\n\n\n\n\n\n\nCO\n\n2\n\n\n\n\n\nin\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(2)\n\n\n\nX\n\n\nCH\n\n4\n\n\n\n\n\n%\n\n\n\n=\n\n\n\n\n\n\n\n\n\nCH\n\n4\n\n\n\n\n\nin\n\n\n-\n\n\n\n\n\n\nCH\n\n4\n\n\n\n\n\nout\n\n\n\n\n\n\n\n\n\nCH\n\n4\n\n\n\n\n\nin\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(3)\n\n\n\n\nH\n\n\n2\n\n\n:\nC\nO\n\n\n(\nm\no\nl\na\nr\n)\n\n=\n\n\nmol\n\nof\n\n\n\nH\n\n\n2\n\n\n\np\nr\no\nd\nu\nc\ne\nd\n\n\nmol\n\nof\n\nCO\n\nproduced\n\n\n\n\n\n\nThe HSC (versions 7.1 and 10) Chemistry software (where H, S, and C stand for the enthalpy, entropy, and heat capacity, respectively) was used to generate theoretical models imitating experimental conditions. Maximum allowed thermodynamic values, for different inlet feed compositions and fixed pressure of 1\u00a0atm, are plotted as a function of temperature (range: 200\u20131000\u00a0\u00b0C). It is worth noting that simulations allowed for both gaseous and solid species. CH4, CO2, H2, CO, and N2 were included as gaseous components, while C(s) (whenever added) was introduced separately in the solid phase. Two of the simulations were performed with an inlet CH4:CO2:N2 molar ratio of 1:1:0.33 with and without the consideration of C(s) as a potential product of the reaction. The third simulation was conducted for pure methane decomposition (CH4:N2 ratio of 1:0.33, no CO2 was included in the feed) and served for comparison purposes on the effect of CO2 on the amount of C(s) and product distribution. The general principle of each simulation is based on a procedure relying on the minimization of the total Gibbs free energy of the reacting system [25\u201326]. In this case, the system becomes thermodynamically favorable when its total Gibbs free energy value is at a minimum and its differential is essentially zero under specified conditions of temperature, pressure, and feed composition. The equilibrium state is determined without any specification of main and side reactions occurring in the system. Additional details on the overall principle and involved mathematical models can be found in our recent thermodynamic studies on methane reforming [27] and iron oxide reduction in the hydrogen atmosphere [28].All prepared materials (Ni-free and Ni-containing) exhibit type IV N2-sorption isotherms (Fig. 1\n) with H1-shaped hysteresis loops and rises in adsorbed nitrogen amounts within P/P0 values between 0.4 and 0.6. Such observations are classified as typical features of ordered mesoporous materials [29]. Corresponding textural properties are listed in Table 1\n and surface area values are in the range of 177\u2013206 m2g\u22121, as high as in previous reports on mesoporous alumina synthesized under comparable conditions [19\u201324]. The calcined, Ni-free, Al2O3 sample (curve a, Fig. 1) displays a very steep hysteresis loop indicative of the presence of uniformly arranged cylindrical mesopores [22\u201323,29], with a mean average diameter of 7.1\u00a0nm (Table 1). The incorporation of Ni in the course of preparation induces textural changes manifested by a gradual shift of the hysteresis loop to higher relative pressure values accompanied by a decrease in its steepness. The phenomenon is more accentuated with increasing Ni content (curve c, Fig. 1). Such a shift is the result of pore widening, revealing larger yet slightly ordered mesopores [22\u201323,29\u201330]. This can be also illustrated by referring to the larger range of pore sizes, deduced after deriving BJH curves from the desorption branch of N2-sorption isotherms (inset in Fig. 1, Table 1). Such textural changes could not have been derived from thermal treatments since all materials were subjected to the same conditions of evaporation and calcination. In fact, this increase in pore diameter could be attributed to the presence of nitrates-bearing water molecules, specifically, hexahydrates initially present within the composition of the salt precursor. During stirring, nitrates hydrolyze and produce H+ cations (via dissociation of water molecules) acidifying the ethanol solution and creating, therefore, a different atmosphere than that expected in Ni-free solutions. As a result, the hydrogen potential (pH) of the resulting solution changes. Importantly, such pH changes have been reported to be major contributors to influencing the balance at the organic/inorganic interface, causing a disruption in the assembly process. The assembly initiates during stirring at RT and extends to evaporation at 60\u00a0\u00b0C [31]. Such alterations of acidity levels together with possible access restriction to some pores by occluded species could also contribute to the slight loss of surface area seen in Ni20%Al2O3 compared to Ni-free Al2O3 (Table 1). A main noticeable observation is the surface area of Ni50%Al2O3 being close to that of Ni-free Al2O3 (Table 1), in spite of its highest disordered structural arrangement. The Ni50%Al2O3 contains an excess Ni in comparison to the Ni/Al molar ratio characteristic for nickel aluminate (NiAl2O4) [22]. Therefore, some Ni species are mainly present in the form of free Ni, as confirmed by TPR experiments (Section 3.2), outside of the alumina matrix. The external deposition of (some) Ni species creates structural surface defects accompanied by an increase in the amount of adsorbed N2 during monolayer coverage. Besides, a notable evolution in these Ni-containing samples is the increase of pore volume with Ni enrichment, as previously reported on analogous \u201cone-pot\u201d synthesized Ni-alumina materials in our study [20] as well in another for materials calcined up to 400\u00a0\u00b0C [22], but not yet clarified. Regardless of these variances between samples, their isotherms classify them as mesoporous materials with high thermal resistance, as they have been treated up to 600\u00a0\u00b0C, making them suitable for the high-temperature dry reforming of methane reaction.The reducibility of calcined samples was studied by following the H2-consumption behavior of nickel species, under hydrogen flow, from RT up to 800\u00a0\u00b0C. The H2-TPR profiles of freshly calcined samples are displayed in Fig. 2\n. Table 1 shows the overall experimental H2-uptakes (200\u2013800\u00a0\u00b0C) estimated from respective reduction profiles, as well as the relative contribution (in terms of consumed H2) of each peak. The absence of any H2-consumption peak on the profile of the Ni-free mesoporous Al2O3 (pattern a, Fig. 2) is in complete accordance with the fact that this sample contains no reducible species. A major hydrogen reduction peak is seen for Ni20%Al2O3 (pattern b, Fig. 2), with maxima localized at around 570\u00a0\u00b0C. Such type of profile already noticed in similarly prepared \u201cone-pot\u201d Ni-alumina materials [19\u201321,24], is typical for demonstrating the presence of oxidized Ni species strongly interacting with the support matrix, possibly in the form of mixed spinel phases. The reduction of such phases yields small, well-dispersed, Ni0 nanoparticles that are active and stable on time-on-stream [19\u201321,24]. As for Ni50%Al2O3, the larger area of reduction pattern is attributed to the bigger amount of Ni deposited in its course of preparation. As expected, integration of the area under the curve revealed an overall H2 amount of 4432\u00a0\u00b5mol\u00a0g\u22121 for Ni20%Al2O3 and 13459\u00a0\u00b5mol\u00a0g\u22121 for Ni50%Al2O3 (Table 1). The overall experimental H2 uptake at 800\u00a0\u00b0C is consistent with the complete reduction of the targeted amount of Ni species (theoretical H2 amount is 4400\u00a0\u03bcmol\u00a0g\u22121 for 20\u00a0wt% Ni and 11000\u00a0\u03bcmol\u00a0g\u22121 for 50\u00a0wt% Ni) revealing a good accuracy between expected amounts and those recovered after evaporation and calcination steps. Regarding the TPR profile of Ni50%Al2O3 (pattern c, Fig. 2), different types of Ni species are co-present within this sample. Those undergoing reduction at high-temperatures (peak centered at circa 640\u00a0\u00b0C) are similar in nature to nickel species present in Ni20%Al2O3 and are characterized by the establishment of strong metal-support interactions (MSIs). The other type of species reduces at lower temperatures (reduction peak centered at circa 340\u00a0\u00b0C) and is best described as free and easily reducible NiO species with weaker MSI [19,32\u201333]. Based on quantification results (Table 1), around 20% of species are classified as free NiO whereas the majority is constituted of Ni strongly interacting with the support, requiring a higher reduction temperature. Such trends in the TPR profile of the highly loaded Ni sample (Ni50%Al2O3) could result from Ni sintering, in the course of calcination, leading to the deposition of nanoparticles outside the mesoporous framework. Another possible explanation is that these nickel atoms did not chemically interact with aluminum atoms during the early stages of preparation due to the lower availability of aluminum atoms (high Ni/Al ratio) in Ni50%Al2O3 compared to Ni20%Al2O3. It should be recalled that the total molar composition of aluminum and nickel remains fixed across all syntheses. Given the slight temperature differences observed for the main reduction peaks between the two samples (patterns b and c, Fig. 2), it is worth mentioning that TPR experiments are conducted under dynamic conditions, far from equilibrium, which potentially affects the overall reduction signature according to the size and the localization of reducible nickel species.Catalytic tests were performed over in situ reduced Ni20%Al2O3 and Ni50%Al2O3 catalysts at a gas hourly space velocity of 42 L gcat\n-1 hr-1 and in the presence of excess reactants (i.e. CH4 and CO2) as compared to inert gaseous diluent. Such conditions can be addressed as drastic compared to previously adopted experimental protocols applied on similarly prepared materials for running combined steam and dry reforming, biogas reforming, or dry reforming of methane reactions [18\u201321]. Corresponding experimental details on reaction medium and conditions previously selected to run reactions as compared to those chosen for this study are listed in Table 2\n. As shown based on tabulated data, the molar CH4 (or CO2): diluent ratio used in this study is much higher than in any other study (i.e. 3.03 compared to 0.128 and even lower). In fact, diluents are sometimes blended with the reactants mixture for safety reasons in the event of a leak. However, adding a diluent to the feed affects the conversion of reactants by shifting the equilibrium to lower temperatures [27]. Operating \u201cone-pot\u201d Ni-alumina catalysts under slightly diluted conditions adds a realistic outlook to their catalytic performances. Indeed, raw feed streams, for instance, biogas from various biomass resources, to the reformer unit are usually slightly diluted where; the amount of the inert component does not surpass 25\u00a0vol% of the mixture [34]. Regarding the GHSV value of 42 L gcat\n-1 hr-1, it falls between previously used values (Table 2) and depends on the amount of catalyst loaded in the reactor. Lowering that content decreases the residence time and therefore affects the extent of conversion [20\u201321]. In this study, an intermediate contact time is allowed inside the reformer, and the effect of catalyzing an equimolar CH4:CO2 inlet feed stream, much richer in chemically active species is evaluated. As for the choice of the temperature, its value is in line with those adopted for running dry reforming of methane, DRM (Table 2) since it allows the attainment of high methane and carbon dioxide conversion levels, as will be shown based on thermodynamic discussions (Section 3.3.3).Catalytic performances in terms of CH4 and CO2 conversion and H2:CO molar ratios as a function of time-on-stream, at a fixed temperature of 700\u00a0\u00b0C, are displayed in Fig. 3\n. Under our reaction conditions and based on thermodynamic calculations accounting for C(s) deposition (dashed lines, Fig. 3), a higher CH4 conversion (by almost 24%) should be anticipated priori compared to that of CO2. Regarding selectivity outcome, expressed in terms of H2:CO ratio, a value of 1.45 is expected signifying excessive H2 production compared to CO for an equimolar inlet feed of CH4 to CO2. Catalytic data of Ni20%Al2O3 (curve a Fig. 3) show stable performances for 300\u00a0min on stream with CO2 conversion being constantly higher (by about 10%) than that of methane. A behavior that is opposite to thermodynamic data estimated for C(s)-assisted methane reforming (dashed line, Fig. 3A), being instead in line with those projected for a C(s)-free DRM operation (straight line, Fig. 3A). Consequently, the H2:CO ratio is lower than 1.45 (curve a Fig. 3C), being very close to 0.88, the expected thermodynamic molar value from a C(s)-free DRM reaction (straight line, Fig. 3C). Surprisingly, the high-loaded Ni-sample displayed an un-classical behavior in the course of DRM (curves b, Fig. 3). Initially, conversion values were at the same level then, underwent opposite trends where; methane conversion continued increasing, that of carbon dioxide constantly decreasing and the variation of the molar H2:CO ratio was most affected by methane conversion. It is worth noting that Ni50%Al2O3 operated for 120\u00a0min owing to excessive rise in pressure resulting from a blockage due to heavy C(s) deposition. In-depth analysis of such carbonaceous deposits are discussed in upcoming sections. In fact, the pressure reached a value of 8.5\u00a0bar after 120\u00a0min of reaction. The pressure was initially controlled at atmospheric value then rises to exceed this value as the reaction proceeded because of severe accumulation of solid carbon deposits on the catalyst surface as well as in the reactor tubes. Within the reaction time of 120\u00a0min, methane conversion was always higher than that of carbon dioxide attesting to a mechanism involving C(s) development (dashed lines, Fig. 3A). The continuous increment in CH4 conversion accompanied by the incessant reduction in CO2 conversion indicate a preferential reactivity of Ni0 sites toward methane as compared to carbon dioxide. Thus, methane is being preferentially activated into its elementary H2 and C(s) products justifying the rising tendency noted on the molar H2:CO profile (curve b, Fig. 3C).For a better understanding of reactivity data, thermodynamic plots reflecting non-C(s)-developing and C(s)-developing biogas reforming and decomposition operations along with those attributed to pure methane decomposition are presented in Fig. 4\n. Based on these plots, and for a temperature value of 700\u00a0\u00b0C, gaseous CO and H2 products are to be expected for both cases involving CO2 as a reactant (Fig. 4A, B). Their content differs depending on whether C(s) are allowed to be produced in the medium. For example, in simulations involving C(s) generation, the expected amount of CO is around 1 Kmol whereas it is close to 1.5 Kmol in absence of C(s) as a product. This infers that the CO is consumed in C(s)-formation reactions involving CO hydrogenation (CO\u00a0+\u00a0H2\n\n\n\u2194\n\n H2O\u00a0+\u00a0C(s)) and CO disproportionation (known as Boudouard reaction, 2CO \n\n\u2194\n\n CO2\u00a0+\u00a0C(s)), both being thermodynamically favored within low to intermediate temperature ranges, accounting in its lower amounts at 700\u00a0\u00b0C (Fig. 4B). As for H2, close amounts (range: 1.37\u20131.53 Kmol per 1\u00a0mol of CH4) are expected in both cases (Fig. 4A, B). Despite similar contents, it is worth noting that methane conversion is expected to be much higher in the scenario involving C(s) formation than that predicted without C(s) generation (Table 2). This implies that methane will participate in a series of reactions accounting for its consumption through its decomposition, dry reforming as well as reforming with steam. For the latter reaction, it is indeed favored owing to the richness in H2O as compared to a deprived environment for the simulation designed without C(s) (Fig. 4B). Methane is then believed to undergo consumption, solely, via DRM under C(s)-free conditions (Fig. 4A). The similarity in H2 amounts, at 700\u00a0\u00b0C, resulting from different methane conversion values (CH4 being a main H2 source) attests to the fact that the majority of methane is consumed at lower temperatures before reaching 700\u00a0\u00b0C, under C(s)-developing conditions, through steam reforming (CH4\u00a0+\u00a0H2O \n\n\u2194\n\n 3H2\u00a0+\u00a0CO) being characterized by a lower endothermic nature than dry reforming [27].Coming back to experimental data, it can be seen that Ni20%Al2O3 is catalyzing dry reforming of methane in absence of severe C(s) deposition/accumulation (Section 3.3.2), as will be verified based on XRD, SEM, and TGA data (upcoming sections). The performance of this catalyst could have been anticipated as previous \u201cone-pot\u201d synthesized samples displayed very stable reactivity levels in CSDRM, BR, and DRM yet, under more diluted inlet feed compositions (Table 2). Thus, this sample reflects high adaptability to drastic conditions making it efficient for catalyzing realistic biogas compositions. Regarding Ni50%Al2O3, its performance does not really imitate dry reforming in either scenario since for both C(s)-developing and non-C(s)-developing DRM, methane and carbon dioxide should remain stable, under isothermal conditions (Fig. 4A,B). This sample is rather catalyzing, preferentially, methane over carbon dioxide as if it is promoting methane decomposition (Fig. 4C). In fact, it has been recently reported, based on transient CH4 and CO decomposition experiments, that methane decomposition appears to be the main pathway for solid carbon formation (and its subsequent deposition) under typical DRM conditions (atmospheric pressure and temperature close to 750\u00a0\u00b0C) particularly in presence of supported Ni-based catalysts [35]. Indeed, as time proceeds, more methane is being consumed yielding H2 and C(s), as evidenced by the characterization results of spent materials. A much smaller fraction of CO2 still undergoes consumption yet, its amount decreases on stream.The performance of Ni20%Al2O3 appears to be promising when compared to literature data obtained by other research teams over Al2O3-based catalysts, with comparable properties to ours, even for feeds more enriched in diluent gas and reformed at lower temperatures (Table 2). For a 16\u00a0wt%, \u201cone-pot\u201d synthesized mesoporous Ni-Al2O3 sample, lower reactivity levels along with deactivating performances were noted when the catalyst was operated at 600\u00a0\u00b0C for a CH4:CO2:Ar inlet molar ratio of 1:1:1 and for a smaller GHSV value (ref. 36, Table 2). Catalytic data similar to those noted over Ni20%Al2O3 were found over a Ni5%Al2O3 catalyst operated at the same temperature and under a richer CH4-CO2 reactional feed stream (ref. 37, Table 2). The similarity in outcomes possibly originates from the preparation method being based on the EISA approach. The different Ni content seemed to slightly affect reactivity levels, probably due to differences in situ reduction treatments for the samples prior to catalytic testing. In our case, the reduction was conducted at lower temperatures and under pure H2 stream whereas, in the reported work, reduction took place at 750\u00a0\u00b0C under a diluted H2 stream (5\u00a0vol% H2 in N2) [37]. Depending on the richness of hydrogen in the medium, its atomic recombination is believed to be increased in presence of a diluent improving, therefore, H2-spillover and affecting positively the reactivity of nickel [40]. When \u201cone-pot\u201d synthesized Ni-Al2O3 samples were allowed to catalyze undiluted feed streams (refs. 38 and 39, Table 2), a common trend is witnessed. Catalytic deactivation is noted owing to sintering and subsequently to heavy carbon accumulation on active sites since the early stages of reaction. A possible perspective for stable performances could involve an enrichment in Ni content (for wider availability of active sites), upon adopting the EISA \u201cone-pot\u201d methodology, since catalytically active Ni0 nanoparticles were shown to remain encapsulated within the mesoporous matrix, protected from sintering and from the deposition of graphitic, known as deactivating, carbonaceous species [18\u201321,24].In view of Ni50%Al2O3, its performance is quite promising when compared to previously reported data on catalysts designed for methane decomposition (MD). High intrinsic activity values are typically expected over Ni-based catalysts for MD. For instance, a 50\u201360\u00a0wt% Ni-loaded Al2O3 supported catalyst converts methane at a rate of roughly 64% [41]. However, when such samples are tested isothermally at 675\u00a0\u00b0C, they underwent drastic deactivation within just 30\u00a0min on stream. When Ni-doped (12.5\u00a0wt%) hydrotalcite-like Mg-Al (Mg2+/Al3+:0.24) solid-solution materials were tested for MD, a methane conversion of 47% was obtained at 650\u00a0\u00b0C for 30\u00a0min on time-on-stream. However, other hydrotalcite-based catalysts, synthesized using various Mg2+/Al3+ ratios, deactivated dramatically after a few minutes of exposure to reactional medium [42]. Similar behavior has been also reported over Ni-based catalysts supported on mesoporous alumina [43]. Improvement tactics have been already developed, especially for Fe-based materials, and these include (i) the addition of secondary, transition metal, elements (i.e. Ni, Mo, and Co) to tune the characteristics of the material [43\u201345] and, (ii) the incorporation of trace amounts of noble metals for reactivity boosting [46]. Although these solutions improve catalytic reactivity and stability to some extent, they are time-consuming and involve multi-step complicated synthesis procedures, increasing the overall complexity of the process. Moreover, some of these steps are also expensive especially when noble metals are involved, which accounts for additional expenses. The \u201cone-pot\u201d methodology reported in this study offered the direct development of a monometallic Ni-based sample that is inherently porous and capable of catalyzing (preferentially) methane reforming and decomposition while undergoing in situ activation, yielding higher and stable conversion on time-on-stream. Most importantly, developed carbonaceous deposits belong to the family of non-deactivating species, mainly carbon nanotubes, having a wide range of industrial applications and high economic interests (Section 3.4).The spent catalysts were characterized by several techniques to quantify and identify the nature of carbonaceous deposits. XRD was performed over fresh and spent materials for a direct comparison of the evolution of Ni species and the crystallographic state of carbon deposits. These patterns are presented in Fig. 5\n. Depending on the amount of added metal, XRD diffractograms of calcined catalysts presented different signatures. For Ni20%Al2O3 (pattern a, Fig. 5), the XRD signal was mostly amorphous indicative of high dispersion of species in the mesoporous alumina skeleton. Even if the presence of NiO was not detectable in the calcined material, it was confirmed by TPR (discussed previously in Section 3.2). For the Ni-enriched sample, its XRD signature (pattern b, Fig. 5) displayed apparent diffraction peaks characteristic of cubic NiO. Applying Scherrer\u2019s equation at 2\u03b8 of 43.17\u00b0 (attributable to the 200-diffraction plane) revealed an average NiO particle size of 31.6\u00a0nm. For both calcined materials, no diffraction peaks were visible for crystalline alumina attesting to its high thermal stability against dihydroxylation and/or dehydration after calcination at 600\u00a0\u00b0C. As a result of in situ reductions followed successively by catalysis, XRD patterns of Ni-containing catalysts showed some structural modifications. Diffraction peaks corresponding to the phase transition of the alumina skeleton from amorphous (patterns a and b, Fig. 5) to (partially) crystalline \u03b3-Al2O3 became visible (patterns a\u2019 and b\u2019, Fig. 5). Despite their appearance on diffraction patterns, their crystallographic domains are very small (at least compared to other diffraction peaks on the same patterns) attesting the high structural resistance of the ordered alumina framework. Moreover, the peaks attributable to (cubically arranged) metallic nickel were identifiable over both samples. The determination of the exact average Ni0 particle size, based on XRD data, cannot be possible because of the overlapping of Ni0 diffraction peaks with those corresponding to crystalline \u03b3-Al2O3 (patterns a\u2019 and b\u2019, Fig. 5). Nevertheless, the stable catalytic performance of Ni20%Al2O3 toward dry reforming of methane and the un-classical catalytic behavior (described by a progressive rise in methane conversion) of Ni50%Al2O3 toward methane decomposition attest to the continuous presence of accessible and catalytically active Ni0 centers that remained dispersed and resistant to sintering despite the high temperature and the amount of loaded metal. The sole presence of metallic nickel diffraction peaks over spent catalysts indicates that no reoxidation into (catalytically inactive) NiO took place during the reaction. Additional diffraction peaks typical of crystalline C(s) were also present on the diffractograms with varying degrees of peak intensity (patterns a\u2019 and b\u2019, Fig. 5). With correlation to catalytic data (Section 3.3), the amount of deposited carbon is in accordance with reactivity trends. In fact, higher amounts of carbonaceous deposits were found over Ni50%Al2O3 in line with its preferential reactivity for catalyzing methane decomposition yielding thus 1\u00a0mol of C(s) per 1\u00a0mol of methane.Representative SEM images of fresh and spent Ni20%Al2O3 and Ni50%Al2O3 catalysts are shown in Fig. 6\n. Based on Fig. 6A there is no evidence of externally deposited nickel oxide nanoparticles, over calcined Ni20%Al2O3, because of the absence of brilliant colors which normally result from the diffusion of metals having higher electronic densities than aluminum oxide. However, some light spots are visible on the external surface of alumina grains for calcined Ni50%Al2O3 (Fig. 6B). Their presence indicates that (some) nickel particles have been deposited outside of the mesoporous framework. It results in the formation of surface defects (as deduced from N2-sorption data, Section 3.1) and the presence of an easily reducible population of Ni particles (as shown based on TPR profiles, Section 3.2). Such nanoparticles are reported to be more prone to coking, during catalysis, than those stabilized inside the alumina matrix since carbon formation is hindered over internally incorporated species owing to steric constraints imposed by the support structure [18\u201321,24,47]. After catalysis, crystalline carbonaceous deposits detected by XRD were also visualized by SEM. Over Ni20%Al2O3 spent catalyst, some (small) domains of sponge-like and short-carbon filaments were shown on the external surface of alumina grains (Fig. 6A\u2019). Compared to a typical SEM image of spent Ni50%Al2O3 (Fig. 6B\u2019), alumina grains in this sample were almost undetectable owing to complete coverage by solid carbon deposits. Higher-resolution SEM images, taken for a clearer visualization of such carbonaceous materials overspent Ni50%Al2O3 (Fig. 7\n), revealed that the catalyst consisted mostly of hollow and randomly oriented nanotubes of several branches with apparently (i) an uneven surface morphology (Fig. 7A) and (ii) recognizable Y-junctions between nanotubes (Fig. 7B). Additionally, these images showed that developed nanotubes had an open-ended geometry where; Ni0 nanoparticles were found hanging on the tips of nanotubes (as indicated by the dashed circles, Fig. 7). The presence of such Ni0 particles at the end of nanotubes is indicative of their formation through the well-established tip-growth mechanism [48]. Therefore, a part of the Ni surface remained accessible for catalysis, where nanoparticles catalyzed preferentially methane decomposition.The amount of C(s) deposits and their reactivity in the oxidative atmosphere (flowing air) have been studied by TGA and corresponding profiles are displayed in Fig. 8\n. Based on their oxidation temperature(s), carbon species can be classified as weakly stable amorphous C\u03b1 (sp2 C-atoms, graphene-like species, oxidation peak temperature between 300 and 450\u00a0\u00b0C), moderately stable C\u03b2 (C-nanotubes, oxidation peak temperature between 450 and 600\u00a0\u00b0C), and highly stable C\u03b3 (sp3 C-atoms, oxidation peak temperature exceeding 600\u00a0\u00b0C) [49\u201350]. The first significant information provided by TGA is the total C(s) content over spent catalysts. Notably, the amount of carbon produced over Ni20%Al2O3 spent catalyst is much lower (i.e., \u223c5\u00a0wt%) than that formed over Ni50%Al2O3 spent catalyst (i.e., around 98\u00a0wt%). These results are in accordance with the above-discussed XRD and SEM data, highlighting that each catalyst followed a different mechanistic pathway under a similar reactional feed stream. Furthermore, this finding is consistent with recent data on Ni-supported catalysts tested for DRM, which show a proportional relationship between the size of Ni nanoparticles and amounts of accumulated carbon [35]. Authors showcased that depending on tailored catalyst composition preferential formation of C(s) nanotubes could be favored. It is worth mentioning that for spent Ni20%Al2O3, a small rise in its TGA profile is noted (pattern a, Fig. 8) and this could be attributed to a re-oxidation of (small) Ni0 into NiO under flowing air. The second information deduced from TGA profiles is the type of carbonaceous deposits. As evidenced by the carbon oxidation temperature (ca. 550\u00a0\u00b0C) and corresponding weight loss, over Ni50%Al2O3 catalyst a significant amount of carbon nanotubes is produced (pattern b, Fig. 8). This confirms the continuous reactivity of this sample on time-on-stream as the produced nanotubes did not encapsulate active centers rather, they detached the Ni particles that are weakly bonded to the support and grew whilst these particles catalyzed methane into hydrogen and nanotubes. Such a preferential activation of methane could be described based on a recent study by Jiang et al. [51] over bimetallic Ni-Co/CeO2 catalysts in DRM. The authors demonstrated, using activation energy calculations, that the support, irrespective of the transition metal site, is crucial in activating CO2 during DRM. As for methane, it can be activated on metallic sites and such activation remains persistent as long as interfacial metallic sites remain available and accessible for reaction. Likewise, Liang et al. [52] proposed a mechanistic scheme showing the importance of support in CO2 activation and the role it plays in carbon gasification. Authors highlighted the fact that methane undergoes activation on metallic while CO2 activates on the support (CO2\u00a0\u2192\u00a0CO\u00a0+\u00a0O), owing to the richness in oxygen vacancies, yielding CO and O. Then, the as-produced atomic oxygen interferes in the in-situ oxidation of carbon, resulting from CH4 decomposition, through the reaction C(s)\u00a0+\u00a0O\u00a0\u2192\u00a0CO. In view of the results reported in our study, we can tentatively attribute the behavior of Ni50%Al2O3 to the coverage of the alumina surface by carbon nanotubes, preventing the direct contact of CO2 with the slightly basic sites of Al2O3, resulting in decay in CO2 conversion while that of methane remained intact and rising. The progressive increase in methane conversion potentially results from the enhanced electronic properties of Ni0 as they are present on the tips of nanotubes, characterized by high electronic conductivities [53]. In fact, the high thermal and electronic conductivities of nanotubes along with their high mechanical strength are making them of great interest for application in multiple fields including electronics, catalysis, and biosensors [53\u201355].One-pot mesoporous alumina containing 20 and 50\u00a0wt% nickel-based catalysts were successfully prepared using the EISA method. The physicochemical characterizations showed a partial disorder in the porosity and the deposition of some nickel outside the mesoporous alumina grains when the Ni loading was increased to 50\u00a0wt%. However, a high Ni metal loading (50\u00a0wt%) in the catalyst was required for the simultaneous production of syngas and the desired carbon nanofibers. These latter materials, known as bio-nanostructured carbon, were successfully produced without significant deactivation of the catalysts in a continuous and highly concentrated reactants stream (CH4, CO2, and\u00a0<\u00a015% Ar diluent). This work demonstrated the efficacy of the one-pot EISA method for developing highly active and selective catalysts for the simultaneous production of carbon nanofibers and syngas from biogas. These findings establish a foundation for a more rational synthesis of active and selective catalysts for biogas valorisation applications. In the future, we aim to investigate other transition metals and operate the catalytic reaction under high pressure and industrially relevant conditions.\nNissrine El Hassan: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing \u2013 original draft. Karam Jabbour: Methodology, Formal analysis, Investigation, Data curation, Writing \u2013 original draft, Visualization. Anis H. Fakeeha: Validation, Resources, Writing \u2013 review & editing. Yara Nasr: Formal analysis, Investigation. Muhammad A. Naeem: Data curation, Writing \u2013 review & editing. Salwa Bader Alreshaidan: Formal analysis, Investigation. Ahmad Al Fatesh: Formal analysis, Investigation, Resources.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to extend their sincere appreciation to Researchers Supporting Project RSP-2021/368, King Saud University, Riyadh, Saudi Arabia.", "descript": "\n                  Biogas, a renewable energy source, is primarily composed of CH4 and CO2. It is a promising alternative to fossil fuels and can be used directly for electricity production as well as heat generation via combustion. Concerns about climate change and a greater emphasis on renewable energy sources have recently increased interest in biogas utilization. In this context, biogas reforming and decomposition (BRD) into synthesis gas and carbon nanofibers (CNFs) is viewed as a new and attractive way of efficiently valorising biogas. In this study, Ni-loaded (i.e., 20, 50\u00a0wt%) mesoporous alumina materials were prepared using one-pot evaporation-induced self-assembly method for BRD. Synthesized materials were characterized by various techniques: N2-physisorption, X-ray diffraction, temperature-programmed reduction, scanning electron microscopy, and thermal gravimetric analysis. Results showed that textural and structural properties of synthesised materials differed with Ni loading. High Ni-loaded catalyst displayed higher surface area, pore volume, pore size distribution, and average particle size which is the result of deposition of Ni species outside alumina grains creating thus, surface defects. BRD results were greatly influenced by Ni content with Ni50%Al2O3 reflecting catalytic behaviour similar to those expected for pure methane decomposition. Most importantly, this catalyst was also capable of generating, selectively, interesting carbon nanofibers.\n               "}
{"full_text": "Due to the advantageous property and long lifetime, plastics are important materials in our society. The yearly plastics consumption is around 300 million tonnes in worldwide and about 60 million tons in the EU [1]. Most of the waste polymers are landfilled (27%) and valorised with energy recovery (41%) in Europe [1\u20133]. However, there is a great difference among the landfill rate of different countries; some of them has less than 5%, while others have more than 70%. It is a positive tendency, that the amount of the collected, recycled and energy recovered waste plastic are significantly increased; besides the amount of the landfilled plastic waste is decreased in the last ten years [1]. Currently, on average, 30% of the plastic wastes are recycled, inside and outside of the EU by mechanically or chemically. Pyrolysis (so called chemical recycling) may be an efficient option to resume the energy from waste polymers and to obtain valuable hydrocarbons [3,4]. Gas, pyrolysis oil and coke should be the products of pyrolysis process. The arising oil can be used as fuel oil in furnaces or after further upgrading as fuel in vehicles. Whereas different types of the plastics have variance in their chemical structure and physical nature, the yield and composition of the pyrolysis products are significantly different [5\u20137].It is also well known, that high quality, waste derived hydrocarbons could be obtained by the pyrolysis of polyolefin (high density polyethylene (HDPE), low density polyethylene (LDPE) and polypropylene (PP)) and polystyrene (PS). On the other hand, contaminations and the presence of other polymers, such as polyvinyl chloride (PVC), polyamide (PA), paper, biomass or even additives for plastics, containing different elements except for carbon and hydrogen, can significantly deteriorate the advance property of the pyrolysis products. The problem can be eliminating by the efficient type selective collection of waste materials, but it needs relatively high installing and operating costs. Regarding the undesired constituents, chlorine is one of the key components, because significant corrosion can be caused by chlorinated components during storage, transportation, combustion, etc. Another problem is that the chlorinated components can poison the catalysts not only during the pyrolysis, but also in further application of pyrolysis oil [8\u201313]. PVC has a key role in the plastics industry and the occurrence of PVC in streams of municipal plastic wastes is very common. Approximately 10% of the plastic is PVC, used for building and construction, packaging, households, automotive, electric and electronic, etc. sector [1].During the thermal decomposition of PVC toxic and corrosive compounds are generated, especially vast amount of hydrogen chloride in gaseous product and chlorinated organic compounds in pyrolysis oil [8,10,11]. On the other hand, the reduction of chlorinated compounds in pyrolysis product obtained from PVC containing raw materials is an unsolved problem. For this intention the pyrolysis kinetic of both virgin and waste PVC, each alone or mixed with other plastics, have been studied by many authors [10,11,14]. The presence of the other plastics and the PVC concentration can significantly affect the dechlorination reactions [14].The in-situ reduction in the concentration of chlorinated compounds can be divided into different groups: stepwise pyrolysis, catalytic pyrolysis, pyrolysis with adsorbents added to the sample, or even hydrothermal carbonisation are used [3,4,15\u201323]. In stepwise pyrolysis, a prior low temperature step (up to 350\u00a0\u00b0C) is used for the sake of eliminating the chlorine from the original sample in the form of HCl, which leaves the reactor system as gas; then the temperature is elevated and the sample is pyrolyzed in a conventional run between 400 and 800\u00a0\u00b0C [4,12,17\u201319]. In general, the co-pyrolysis of PVC with other plastics and biomass are investigated by stepwise process. E.g. H. Kuramochi et\u00a0al. concluded that the HCl emission could significantly reduce due to the presence of wood, because of the hemicellulose as a strong Cl absorbent [18]. Stepwise pyrolysis often applying different or separated reactor vessels with different temperatures, because more than 95% of the dehydrochlorination reactions take place in the first unit at low temperature (300\u2013350\u00a0\u00b0C), while the chlorine content of the products from the following reactors were less (<0.025%) [19]. By biomass-PVC co-pyrolysis also the reaction kinetic parameter should be modified [20]. Regarding hydrothermal carbonisation, the high temperature favoured to the aromatic hydrocarbons [21]. Nowadays, more and more researcher investigates the co-pyrolysis of plastic waste and different types of biomass. In this case, the initial characterization of feedstock has a great significance to know the suitability of raw material to carry out further experimental sets, because all the biomass could not be suitable, economic, qualitative and high yield bio-oil producer. Thermogravimetric analysis is appropriate to characterize the feedstock. B.B. Uzun and E. Yaman studied the pyrolysis kinetics of walnut shell and waste polyolefins using thermogravimetric analysis. The results provide valuable information on pyrolysis mechanism and based on them, walnut shell have considerable potential for pyrolysis. However, the heating rate has a significant effect on the pyrolysis process [22]. Akancha et\u00a0al. investigated the co-pyrolysis of waste polypropylene and rice bran wax to produce biofuel. The physical properties of the pyrolysis oil were fairly comparable to Diesel and gasoline [23].In catalytic pyrolysis, some catalysts with metal content have been studied in order to demonstrate their positive properties. In general, the used catalysts are metals on inorganic supporter, such as synthetic zeolites, therefore, they have double role, as pyrolysis catalysts and inhibitors for HCl formation [3,18\u201321,24\u201326]. Not only single catalysts, but also metal loaded catalyst and catalyst mixtures/composites are used. E.g. fluid catalytic cracking, hydrocracking catalysts or ZSM-5 and iron oxide composite catalyst can efficiency used for reducing the chlorine content of products of vacuum gas oil and PVC pyrolysis [16].Regarding the adsorbents, especially the HCl emission can be decreased, however, the reduction in chlorinated compounds in pyrolysis oils would be also advantageous. Numerous materials as biomass constituents, petrochemical residues and alkaline adsorbents (NaHCO3, CaO, CaCO3, Na2CO3, Ca(OH)2) have been used as HCl adsorbents [4,16,25,27].Another option for quality improving of the chlorinated pyrolysis products is the upgrading. A. Lopez-Urionabarrenechea et\u00a0al. reported, that chlorinated oils from waste plastic pyrolysis could be upgraded by red mud, because it can promote the cracking and dechlorination reactions [27]. Andrei Veksha et\u00a0al. can upgrade the non-condensable pyrolysis gases using CaCO3 supported catalytic sorbent containing 5% NiO during high temperature (700\u00a0\u00b0C) process [28]. S. Kumagai et\u00a0al. investigated the effect of calcium oxide and calcium hydroxide catalysts on the product distribution of decomposition at 600\u00a0\u00b0C of individual and mixed plastics under a steam atmosphere. CaO increased gas and liquid production from the plastic mixture, which was further enhanced in the presence of Ca(OH)2\n[29].It is worth to say, that owing to the reaction scheme of decomposition reactions, the high olefin content is the characteristic of the pyrolysis products, however it is changing wide range. Generally pyrolysis oils have 30\u201370% unsaturated hydrocarbons, depending on the reaction parameters [30\u201336]. Not only the before mentioned chlorinated compounds in pyrolysis oil, but also the unsaturated hydrocarbons can occur problems during the long term utilization. As it is known, the reactivity of the unsaturated compounds is higher than saturated hydrocarbons due to the distorted electron cloud. The consequence of this fact is that CC bonds can easily react with another CC bond, which lead to oligomerization and polymerization e.g. during long term storage. The high temperature or sunlight radiation redounds to those reactions [30\u201336]. More information is available about the aging of biomass sourced pyrolysis oils (hardwood, softwood, sewage sludge, wheat straw, chicken manure, etc.), but only few result is published regarding the aging of pyrolysis oil obtained by contaminated or PVC containing plastic waste [37\u201339]. Regarding the aging test, an accelerated test at 80\u00a0\u00b0C for 24\u00a0h is the mostly used.In this work, the thermo-catalytic pyrolysis of real waste plastics containing PVC was performed for in-situ improving of pyrolysis oil and the long term utilization of pyrolysis oil was investigated via accelerated aging and corrosion tests.Mixtures of polyolefin rich real municipal waste plastics contained PVC was used as raw material in our current work: 35% LDPE, 32% HDPE, 24% PP, 4% PVC, 3% ethylene-propylene dimer and 2% polystyrene. Raw material was supplied by household collection and contained 1.5% chlorine, while the ash content was 4.7%. Waste plastics had been shredded and then crashed by laboratory device (Dipre).Mixtures of Ni/ZSM-5, Ni/SAPO-11 catalysts, Ca(OH)2 (supplied by VWR) and red mud from Bayer process for alumina production were added to the raw materials for the in-situ product improving. Red mud contains Fe2O3 (74.8%), Al2O3 (9.8%), CaO (4.6%), TiO2 (3.2%), SiO2 (2.2%), MnO (1.2%), Na2O (0.9%), V2O5 (0.4%), SrO2 (0.2%) and 2.6% others. The BET surface of the Ca(OH)2 and red mud was 28.4 and 35.1\u00a0m2/g, respectively. Table 1\n summarizes the composition of catalyst mixtures.The ZSM-5 ([Nan(H2O)16][AlnSi96-nO192]) catalyst supporter is MFI-type 10 membered ring channel synthetic zeolite with 5.3\u00a0\u00d7\u00a05.6\u00a0\u00c5 channels, while AEL-type, crystalline silicoaluminophosphate (SAPO-11) catalyst ([Nan][Al20SinP20-nO80]) supporter is a one-dimensional also 10-membered, but elliptical pore channel synthetic zeolite with 6.3\u00a0\u00d7\u00a03.9\u00a0\u00c5 pore size.Both the ZSM-5 and SAPO-11 catalysts were loaded by nickel using the following procedure. Catalysts were continuously stirred in 1\u00a0M Ni(NO3)2\u00b76H2O (supplied by VWR) at 80\u00a0\u00b0C for 5\u00a0h, then they were washed by deionized water, filtered and dried for 10\u00a0h at 110\u00a0\u00b0C. In the final step, each catalyst was conditioned at 500\u00a0\u00b0C for 5\u00a0h in air.Ni/ZSM-5 had 15.1 Si/Al ratio, 324\u00a0m2/g BET surface and 1.55\u00a0nm average pore diameter. Contrary, Ni/SAPO-11 catalyst had 0.25 Si/Al ratio, 211\u00a0m2/g BET surface and 2.11\u00a0nm average pore diameter. The micro, meso and total pore volumes were 0.081, 0.018 and 0.099\u00a0cm3/g regarding Ni/ZSM-5 and 0.052, 0.063 and 0.15\u00a0cm3/g in case of Ni/SAPO-11, respectively. The nickel content of catalysts was 9%, since the catalyst supporters were able to bind this amount of nickel during the impregnation of catalysts. This amount of nickel content is appropriate according to literature references [40\u201342].Waste plastics were pyrolyzed in a one stage stainless steel batch reactor at 510\u2013520\u00a0\u00b0C in nitrogen atmosphere (Fig.\u00a01\n). The heating rate was 15\u00a0\u00b0C/min. The nitrogen flow was 5\u00a0dm3/h [43]. The reactor external wall was fitted with electric heating system and the temperature was controlled by PID controller. Firstly, 50\u00a0g of the raw material together with 2.5\u00a0g of catalyst was placed in the reactor, then, after the assembling of the rig, the temperature was elevated to the set value.Volatiles from the decomposed hydrocarbons were driven though a heat exchanger, where condensable from hydrocarbon gases had been transformed into liquid product, then the fractions were separated in phase separator into pyrolysis oil and hydrocarbon gases. Hydrocarbon gases were bubbled through a scrubber filled with sodium hydroxide solution. The yields of products were calculated based on the mass balance. The weight of pyrolysis oil and residue were measured after the experiment. Gas yield was calculated in the knowledge of pyrolysis oil and solid residue (Y(gas)\u00a0=\u00a0100-Y(pyrolysis oil)-Y(residue)).A DANI type GC instrument was used fitted with programmed injector, flame ionized detector for analysis of hydrocarbon composition of gases: Rtx PONA column (100\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm, surface thickness of 0.5\u00a0\u03bcm) and Rtx-5 PONA (100\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm, surface thickness of 1\u00a0\u03bcm). Sample analysis was taken using isotherm conditions (T\u00a0=\u00a030\u00a0\u00b0C). The temperature of injector and detector were 240\u00a0\u00b0C.Hydrogen content of the gas products was measured by a Shimadzu GC-2010 gas chromatograph (TCD detector, Carboxen\u2122 1006 PLOT column (30\u00a0m\u00a0\u00d7\u00a00.53\u00a0mm)). The temperature was increased from 35\u00a0\u00b0C (hold time 2\u00a0min) to 250\u00a0\u00b0C at 40\u00a0\u00b0C/min heating rate, then the final temperature was hold till 5\u00a0min.Pyrolysis oil was analysed by DANI type GC (Rtx 1 dimetil-polysiloxan capillary (30\u00a0m\u00a0\u00d7\u00a00.53\u00a0mm, thickens of 0.25\u00a0\u03bcm)) using the following temperature program: 40\u00a0\u00b0C for 5\u00a0min, then the temperature was elevated by 10\u00a0\u00b0C/min till 350\u00a0\u00b0C and it was kept at 350\u00a0\u00b0C till 20\u00a0min. Both the injector and detector temperature was 350\u00a0\u00b0C. Components were identified based on their retention time, while the pyrolysis oil composition was evaluated via peak areas belong to the components.The chlorine content of products was measured by x-ray instruments (PHILIPS MiniPal PW 4025/02 non-polarized EDXRFS). The spectrometer was powered by PW 4051 MiniPal/MiniMate Software V 2.0A, equipped with a rhodium-side window tube anode and Si-PIN detector. The fluorescent x-ray was detected by a Si-PIN detector with a beryllium window. The analysis was taken in helium medium.The long term application of the pyrolysis oils was followed by accelerated aging and corrosion test. Samples were stored in a sample holder at 80\u00a0\u00b0C till 7 days. The density (EN ISO 12185) and the viscosity (EN 16896) were measured at 25\u00a0\u00b0C, moreover the Total Acid Number (TAN) (ISO 6618) and Jodine Number (MSZ 19971-1983) were followed at 0, 1, 3, 5 and 7 days of the test. To investigate the corrosion property, a copper plate was stored in pyrolysis oil at 25\u00a0\u00b0C till 60 days and the weight loss of the copper plate was calculated by the following equation:\n\n(1)\n\n\nw\ne\ni\ng\nh\nt\n\nl\no\ns\ns\n=\n\n\n\n\n\n\nm\n\n\ni\n\n\n\u2212\n\n\nm\n\n\nf\n\n\n\n\n\n\nm\n\n\ni\n\n\n\n\n\u00b7\n100\n\n\nA\n\n\n\n\n\nwhere mi and mf are the initial and final weights of the metal plate, A is the surface area of the plate.The pyrolysis oils were also investigated by Fourier Transformed Infrared spectroscopy (TENSPR 27 FTIR instrument with ATR unit). The resolution was 3\u00a0cm\u22121, the illumination was SiC Globar light, a RT-DLaTGS type detector was used. The change of the olefin content was followed by the integrated peak areas at 890, 910, 950 and 990\u00a0cm\u22121 using the following equation:\n\n(2)\n\n\n\u0394\n\n(\n\nI\nA\n\n)\n\n=\n\n\nI\nA\n\n\n(\nx\n)\n\n\n0\n\n\n\u2212\nI\nA\n\n\n(\nx\n)\n\n\ni\n\n\n\n\nI\nA\n\n\n(\nx\n)\n\n\n0\n\n\n\n\n\u00b7\n100\n\n\n\nwhere IA(x)0 is the integrated peak area at x\u00a0=\u00a0890, 910, 950 and 990\u00a0cm\u22121 without aging, and IA(x)i is the integrated peak area at x\u00a0=\u00a0890, 910, 950 and 990\u00a0cm\u22121 after i\u00a0=\u00a01,3,5,7 days.The oligomers and polymers formed by the aging test was separated from the by pyrolysis oil by filtration using VWR (514-0066) filer, Nylon, 0.2\u00a0\u03bcm, D 25\u00a0mm type device. Then the amount of the separated fraction had been weighted and the polymerized ratio was calculated as following:\n\n(3)\n\n\nP\nR\n=\n\n\n\n\nm\n\n\ni\n\n\n\u2212\n\n\nm\n\n\n0\n\n\n\n\n\n\nm\n\n\n0\n\n\n\n\n\u00b7\n100\n\n\n\nwhere mi is the weight of the separated oligomers-polymers i\u00a0=\u00a01,3,5,7 days and m0 is the weight of the tested sample.The yields of pyrolysis products are summarized in Fig.\u00a02\n. Pyrolysis was taken till no volatiles could evaporate from the reactor. In general, higher yield of gaseous products (18.8\u201326.2%) was observed over ZSM-5 based catalyst mixtures; while more pyrolysis oil was obtain using Ni/SAPO-11:Ca(OH)2:red mud catalyst mixtures (64.2\u201371.9%). As it was before discussed, Ni/ZSM-5 catalysts had larger BET surface area, than Ni/SAPO-11 catalysts, furthermore the micropore volume of Ni/ZSM-5 catalysts were also higher than Ni/SAPO-11. For higher yields of gases especially the larger micropore volume could be blamed. It is also well established that the CC bond cracking properties of Ni/ZSM-5 with a high Si/Al ratio were higher than that of the Ni/SAPO-11 catalyst. This was particularly significant for 1:1:2 red mud:Ca(OH)2:Ni/ZSM-5 catalysts. It is important to mention, that the ratio of pyrolysis oil and gases was higher using SAPO-11 based catalysts, than that of ZSM-5 based. During the pyrolysis, the hydrocarbon macromolecules enter the pores of the catalyst, where they further cracked. Due to the pore size of the catalysts, the decomposed polymer macromolecules could escape from the smaller pore size ZSM-5 supported catalysts only after a larger degradation. The SAPO-11 has a bit larger average pore diameter than ZSM-5; for this reason, the larger molecules can also come out from the pores of the catalyst without further cracking. When these longer carbon-chain molecules reach the active centres of the catalyst they undergo cyclization and polycyclization [44\u201346]. Due to these reactions, coke formation is initiated which leads to deactivation of catalyst. The more coke deposition on the catalyst, the larger coke agglomerates are formed on its surface.Regarding the residue, a bit more solid phase decomposed products was obtained over SAPO-11 based catalysts, than ZSM-5. Residue was investigated by SEM instrument. It was concluded, that catalysts, inorganic content of the plastic and the coke agglomerated into larger particles in case of Ni/SAPO-11 based catalyst mixtures (Fig.\u00a03\n). Whereas, the catalyst grain were well separated from each other and did not covered by thick and hard coke layer.Gases contained hydrogen and C1\nC5 hydrocarbons, dominantly C2, C3 and C4; n-olefin, n-paraffin, branched olefin and branched paraffin. The compositions of hydrocarbon gases are summarized in Table 2\n.Without catalyst, the n-olefin (38.5%) and the n-paraffin (41.2%) hydrocarbons were the main products. However, owing to the isomerization effect of the catalysts, the concentrations of branched hydrocarbons can significantly increase during thermo-catalytic pyrolysis, such as isobutane, isobutene, trans-but-2-ene, cisz-but-2-ene, isopentane, 2,2 dimethyl-butane, methyl-pentane. Both the Ni/ZSM-5 and Ni/SAPO-11 containing catalyst mixtures occurred notable increasing in branched hydrocarbons. Without catalyst the concentrations of non-branched and branched hydrocarbons was 80.5 and 19.5%, respectively. Contrary, the summarized concentration of branched hydrocarbons was 51.2\u201356.1% in pyrolysis oil by Ni/ZSM-5 catalyst mixtures and 47.7\u201358.6% by mixtures of Ni/SAPO-11:Ca(OH)2:red mud catalysts. It is worth to note, that the isomerization effect could only slightly decreased by the adding of Ca(OH)2 and red mud to the catalyst mixture.The n-olefin/n-paraffin ratio was between 0.57 and 1.15 by catalysts. The thermal and catalytic degradation of hydrocarbons occurs primarily by \u03b2-scission, resulting approximately 1:1 ratio of n-olefin and n-paraffin. As it is shown by the results, the amount of olefins can be increased by the using of catalysts. For the isomerization of hydrocarbons obtained by the degradation of the longer molecules, the catalyst must have sufficient acidity for the rearrangement of the hydrocarbon skeletal and dehydrogenation/hydrogenation function. Results well shown, that significantly more isomers of hydrocarbons were obtained using high concentration of both Ni/ZSM-5 (112-Z) and Ni/SAPO-11 (112-S) catalysts. Results show, that the ratio of non-branched/branched hydrocarbons decreased from 3.93 to 0.71\u20131.09 by the use of catalysts; but there were no trend-like, significant differences among the catalysts. However, the using of catalyst mixtures with 50% Ni/ZSM-5 and Ni/SAPO-11 catalysts the highest branched and lowest non-branched hydrocarbons were resulted.It is worth to investigate the ratio of saturated and unsaturated hydrocarbons within the branched molecules. Less branched olefin and more branched paraffin were obtained by ZSM-5 catalysts (111-Z, 112-Z, 121-Z, 211-Z) than SAPO-11 catalysts (111-S, 112-S, 121-S, 211-S). Comparing the effects of the two catalysts to the formation of branched unsaturated hydrocarbons, it can be concluded, that the higher surface areas and Si/Al ratios were the cause for elevated olefin content regarding ZSM-5 catalysts. For example, the ratio of branched olefin/branched paraffin ranged between 1.74 and 2.43 for ZSM-5 catalysts or 3.58\u20135.26 for SAPO-11 catalysts. In both cases, the highest value was achieved by 121-Z and 121-S catalyst, while the lowest by 2:1:1 ratios. The SAPO-11 catalyst is particularly beneficial for the formation of isobutene, trans-but-2-en, cis-but-2-en and isopentene. It is also observed, that more multi-branched isomers was obtained over SAPO-11 catalysts.It is right, that the used pyrolysis temperature was low for significant hydrogen production over transition metal loaded catalysts [47]. The hydrogen content of the gas products was 0.5% without catalysts, which could be increased to 1.1\u20134.5% and 1.5\u20135.9% using Ni/ZSM-5 and Ni/SAPO-11 containing catalyst mixtures, respectively. That result was also supports the higher isoolefin content. The reason for this may be the following. Firstly, the n-paraffin adsorbed and dehydrogenated on the metallic active centres of the catalyst resulting n-olefins, and then n-olefin transformed to carbenium ion at the acidic sites of the zeolite. That non-branched carbenium ion rearrange to branched olefin beside proton loss. It is also known, that the branched olefin is hydrogenated to paraffin and leaves the catalyst using suitable zeolites. The metallic sites are required to maintain the proper olefin concentration and to maintain the propagation reactions. On the SAPO-11 catalysts, in the first step, hydrogen and carbonium ions are formed, which is also isomerised, but the final hydrogenation of the branched olefin into branched paraffin was slightly occurred, which led to higher hydrogen content in gases. In both cases, the high Ni ion-exchanged catalyst (112-Z and 112-S) could mostly increase the hydrogen concentration, while red mud mixed into the catalysts had synergic effect. The relatively low hydrogen content was also supported by previous results. E.g. A. L\u00f3pez et\u00a0al. demonstrated that less than 1.2% hydrogen was obtained from stepwise pyrolysis of the mixtures of PE, PP, PS, PET and PVC, while the gas fraction contained mainly C2, C3 and C4 hydrocarbons [4].The pyrolysis oils were analysed by GC-FID. Results are shown in Fig.\u00a04\n. Based on result, pyrolysis oils were the mixtures of C5\nC30 hydrocarbons: n-olefin, n-paraffin, branched hydrocarbons and aromatics.Firstly, the catalysts can also affect the concentrations of different compounds in pyrolysis oils. The concentrations of n-paraffin, n-olefin, branched hydrocarbons (both saturated and unsaturated) and aromatic were 30.8%, 27.0%, 40.3% and 1.9% without catalyst. It is well shown, that mostly the concentrations of aromatics and branched hydrocarbons can increase with the addition of catalysts.Each of the catalysts showed high activity in isomerization reactions which led to the increasing in branched hydrocarbons, however, aromatization reactions was occurred rather on Ni/ZSM-5 containing catalysts. Regarding aromatic compounds in pyrolysis oil from catalyst free experiment, the polystyrene in raw material was their source. Contrary, the aromatic content was 11.4\u201317.6% over Ni/ZSM-5 containing catalysts, while 8.1\u201311.0% using Ni/SAPO-11 containing catalysts; mostly benzene, toluene, xylenes and other short-chain substituted aromatic hydrocarbons. It is important to remark, that owing to the difference in catalyst channels, the concentration of short-chain substituted aromatic hydrocarbons was higher when Ni/SAPO-11 containing catalysts were used, than that of Ni/ZSM-5 containing. One dimension of Ni/SAPO-11 catalyst was larger, than Ni/ZSM-5 catalyst; therefore the larger hydrocarbons (e.g. short alkyl chain substituted aromatics) can leave the catalyst surface without decomposition. It is also well shown, that Ni/SAPO-11 catalysts had a bit favourable effect for production of n-paraffin, however the concentration of branched hydrocarbons were tendency-like less, than that of using Ni/ZSM-5 catalysts.The n-paraffin/n-olefin ratio were 1.09\u20131.18 using Ni/ZSM-5 containing catalysts, while 1.31\u20131.43 over Ni/SAPO-11 catalysts. The ratio of branched and non-branched aliphatic hydrocarbons in the pyrolysis oil was 0.70 for cracking without catalysts, which increased to 1.31\u20132.17 by Ni/ZSM-5 catalysts and 0.97\u20131.75 using Ni/SAPO-11 catalysts.The ratio of branched and non-branched aliphatic hydrocarbons increased in the order of 211, 121, 111 and 112 signed catalysts using both ZSM-5 and SAPO-11 supporters. This was due to the catalyst skeleton rearranging property. However, more n-paraffin and less n-olefin hydrocarbons were produced over SAPO-11 catalyst. This is due to local hydrogenation reactions. During the decomposition of polymer waste, the source of hydrogen is primarily the cyclization and aromatization reactions of aliphatic hydrocarbons and the forms of isomerization reactions when non-branched hydrocarbons transformed to branched unsaturated hydrocarbons. It is also important to remark, that the hydrogen content of the gas products correlates with the ratio of n-parafin/n-olefin in pyrolysis oils in case of SAPO-11 catalysts. Regarding the amount of hydrogen, for catalysts, primarily the aromatization reactions can be blamed while regarding the SAPO-11 catalysts the dehydrogenation isomerization reactions resulting in unsaturated branched hydrocarbons should be also caused according to the following reaction scheme (Fig.\u00a05\n).The olefin content of pyrolysis oils was followed via their Jodine number (Fig.\u00a08). According to the result, the total olefin content of the products was higher with Ni/SAPO-11 based catalysts than on Ni/ZSM-5 based regarding each sample. Considering, that the n-olefin content of products is decreased in the presence of catalysts and the fact that the n-olefin content of the products was lower using SAPO-11 supported catalysts than that of the ZSM-5, it is easy to see that more branched olefins can be obtained over SAPO-11 catalysts with accordance the before mentioned reaction scheme.The chlorine content of pyrolysis products is important for their future utilization, which is summarized in Fig.\u00a06\n. Catalysts can significantly affect the chlorine content of the products. It is well known that the decomposition of PVC is taken at relatively low temperature (300\u2013350\u00a0\u00b0C), which is lower than the temperature needed to the cracking of the polyethylene CC bonds (400\u2013420\u00a0\u00b0C) [4]. The dechlorination efficiency is increase by the increase of the temperature and pyrolysis time [13]. However higher temperature needs more energy and due to the recombination reaction, pyrolysis oils contain higher concentration of chlorinated compounds. During the pyrolysis of PVC, the CCl bond with high dipole moment begins to break, resulting in chlorine and alkyl radicals. Hydrogen may be also generated by further reactions from the alkyl radicals, which should recombine with chlorine to produce HCl. This will primarily appear in gas products. However, the chlorine radical may also be recombined with alkyl radicals, resulting in organic chlorine compounds which will initially appear in the pyrolysis oil and the residual product of pyrolysis. The problem is mainly with organic chlorine compounds in pyrolysis oil because their removals are difficult and significantly deteriorate the long-term utilization and stability of the products. Inorganic chlorine in the gas product, mostly HCl, can be easily removed with various sorption processes.Regarding pyrolysis products, 102202\u00a0ppm chlorine was in the gases, 4364\u00a0ppm was in the pyrolysis oil and 1950\u00a0ppm was in the residue without catalyst. These values were significantly changed by the using of catalysts. The distribution of chlorine among the different products and the high chlorine content in gases was confirmed by others [27]. Generally, catalysts reduced the amount of chlorine in gas products and pyrolysis oil, while the amount of chlorine in the residue fraction increased. When analysing data, it is also worth considering that chlorine is generated in the first half of the decomposition reactions, that is why, the longer takes the pyrolysis, or the higher the yield of the given fraction, their diluent effect will prevail.Comparing the two zeolites, the chlorine content of the gas products was lower, and that of the pyrolysis oil were higher in all cases produced on the mixtures of Ni/ZSM-5 catalysts containing catalyst, than that of over Ni/SAPO-11 catalysts. This was the consequence probably the fact that, owing to the higher hydrogen content of the product mixture over SAPO-11 catalysts, the chlorine radicals were easily able to combine to HCl and reduce the concentration of organic chlorine compounds in hydrocarbons. This assumption is also supported by the fact that the chlorine content of the gas products correlates with the hydrogen content. It was confirmed by others, who concluded that the chlorine from PVC correlates with hydrogen to generate hydrochloric acid (HCl) at a lower temperature using metal oxide catalysts [14]. It is also worth note that the presence of red mud and Ca(OH)2 was highly preferred in the reduction of the chlorine content of pyrolysis oil. This was observed for both ZSM-5 and SAPO-11 supported catalysts. The advanced property of the red mud for chlorine reducing in gases and pyrolysis oil was also reported by A. Lopez-Urionabarrenechea et\u00a0al. They demonstrated that vast amount of chlorine moved to the solid phase during the catalytic post treating of pyrolysis oil [27].The reason for this was that due to the alkali character of both substances, chlorine-containing compounds could be chemically react with additional compounds of catalysts, resulting in a significant increase in the residual chlorine content, while the chlorine content of the gas products and pyrolysis oils was significantly reduced. The chlorine content of the pyrolysis oil decreased from 4364\u00a0ppm to 284\u00a0ppm while the chlorine content of gases changed from 81762\u00a0ppm to 48128\u00a0ppm using 211-Z, while 228\u00a0ppm chlorine was measured in pyrolysis oil using 211-S catalyst.\nFig.\u00a07\n\n shows the appearances of the pyrolysis oil after 60 days treating and summarize the result of corrosion test. A copper plate was put into the pyrolysis oil at 20\u00a0\u00b0C till 60 days. The change in weight of the copper plate was registers in each 5th day. According to results, increasing tendencies were found in case of both catalyst free and catalyst supported pyrolysis. The change in the weight of copper plate was 0.458%/mm2 without catalysts, which value could be significantly decreased by catalysts. It is also well shown, that lower weight loss was found in case of pyrolysis oils obtained by the using of SAPO-11 supported catalysts. Especially the presence of Ca(OH)2 and red mud rich catalysts showed high efficiency in better corrosion properties. E.g. the weight loss of copper plate was 0.055%/mm2 (121-Z) and 0.039%/mm2 (211-Z) or 0.039%/mm2 (121-S) or 0.025%/mm2 (211-S). These results are supported by the chlorine content of pyrolysis oils shown in Fig.\u00a06. The lower the chlorine content of pyrolysis oil, the less the weight loss of the copper plate. Regarding the appearances of the pyrolysis oil, they were dark after the treating excluded the using of 121-Z, 211-Z, 121-S and 211-S catalysts. In those cases the colours of the oil only slightly become darker, and they kept yellowish transparent liquid.That result was confirmed also by TAN during the aging accelerated test (Fig.\u00a08). The TAN of pyrolysis oil obtained by catalyst free pyrolysis was 5.9\u00a0mg KOH/mg sample. However pyrolysis oil had less acidic components by the using of catalysts, especially SAPO-11 based; 2.7\u20134.3\u00a0mg KOH/mg sample in case of ZSM-5 based catalysts, while 2.1\u20134.2\u00a0mg KOH/mg sample using SAPO-11 catalysts. On the other hand the TAN increased in less degree when catalyst was used for pyrolysis, especially with high Ca(OH)2 and red mud concentration. Without catalyst the change in TAN was 70% (from 5.9\u00a0mg KOH/g to 19.4mgKOH/g). The increasing in TAN was less using catalysts, especially in case of SAPO-11, because the TAN increased with 36\u201357% and 29\u201348% using Ni/ZSM-5 and Ni/SAPO-11 containing catalysts, respectively. Presumably acidic chlorine containing compounds are transformed from the chlorinated organic components during the aging, which can be neutralized by KOH.The change in the main properties of the pyrolysis oil as function of time is key property for their long term application. An accelerated aging test was performed to investigate the longer term utilization of pyrolysis oils and the effect of the catalysts to the properties. Results are summarized in Fig.\u00a08.Regarding the density and viscosity slight increasing tendencies were found both without and with catalysts. However, catalysts had advantageous effect to both properties. The density and viscosity of the pyrolysis oil was 0.851\u00a0g/cm3 and 1.905\u00a0mm2/s without catalysts, respectively. The density could be increased by 1.99%, while the viscosity by 5.49% without catalysts. It is clear, that the catalysts can increase both the density and viscosity increasing of pyrolysis oil at the end of treating. It is also important to note, that lower density and viscosity of pyrolysis oils was measured by the using of catalysts. The density of pyrolysis oil obtained by absence of catalysts increased to 0.868\u00a0g/cm3 at 7th day of the treating at 80\u00a0\u00b0C. Comparing, when catalysts were used, the pyrolysis oil density was 0.773\u2013796\u00a0g/cm3 without treating, which increased to 0.791\u20130.817\u00a0g/cm3 during the accelerated aging. It means, that the density can increased by 2.03\u20132.68% using Ni/ZSM-5 catalysts, while 2.57\u20133.06% using Ni/SAPO-11 catalysts. Similar phenomena was concluded regarding the viscosity, however the increasing ratio was higher: 5.95\u20139.32% in case of Ni/ZSM-5 catalyst mixtures and 9.15\u201311.44% by Ni/SAPO-11 based catalysts. It is important to remark, that same order of catalysts was found regarding the density and viscosity increasing in case of both Ni/ZSM-5 and Ni/SAPO-11 catalysts: 121, 211, 111 and 211. That order is the same, as it was earlier mentioned for increasing in the concentrations of unsaturated hydrocarbons. The higher concentration of unsaturated hydrocarbons is the cause for higher increasing in both density and viscosity using of SAPO-11 catalysts.The change in olefin content was also followed via the FTIR spectra of the products at 990, 955, 910 and 890\u00a0cm\u22121 wavenumbers. Pyrolysis oils show typical hydrocarbon FTIR spectra (Fig.\u00a09\n). There were infrared activity between 2800 and 3000\u00a0cm\u22121 (CH2\n, CH3 symmetric and asymmetric vibration bands), between 1250 and 1500\u00a0cm\u22121 (asymmetric and symmetric deformation stretching of CH3 groups). Peaks with low intensity at 1615\u00a0cm\u22121 was attributed to the aromatic hydrocarbons (CC streching vibration), while around 610 and 690\u00a0cm\u22121 refer to the chlorinated hydrocarbons CCl streching vibration, which was the less in case of catalyst mixtures with high red mud and Ca(OH)2 content. Bands in the range of 850\u20131000\u00a0cm\u22121 refer to the olefins and mowing vibration at 720\u00a0cm\u22121 was caused by CH2\n group. In our current work the 850-1000\u00a0cm\u22121 range was investigated. There are four peaks in that range: at 890\u00a0cm\u22121 (vinylidene), at 910 and 990\u00a0cm\u22121 (vinyl) or 950\u00a0cm\u22121 (vinylene). In general, the infrared signal at 910 and 990\u00a0cm\u22121 decreased, while at 950\u00a0cm\u22121 increased by the using of catalysts, compared to catalyst free case, which refers to the rearrangement of hydrocarbon skeletal.\nFig.\u00a09 summarizes the change in peak areas during the accelerated aging. As it seems, the peak areas of the FTIR spectra of pyrolysis oils only slightly decreased (less than 5%), which is similar result as it was demonstrated regarding density or viscosity. It is important to note, that the change of vinyl type olefins was higher than that of the other two types, furthermore the catalyst use during the pyrolysis can also decrease the change of the integrated peak areas. Results refers, that vinyl type olefins had greater role during the aging than the other two. Glancing the result it can be also concluded, that the relative change in integrated peak areas was typically between 5 and 7 days using catalyst mixture with Ni/ZSM-5, whereas between 1 and 3 days in case of SAPO-11 supported catalysts. It means that the aging of pyrolysis oils obtained by ZSM-5 supported catalyst takes later than that of SAPO-11 catalysts.The concentration of unsaturated hydrocarbons during the aging was followed via Jodine number, which refers to the amount of the Jodine needed for the saturation of CC bonds in mass unit of the sample. It is important observation, that pyrolysis oils by higher unsaturated branched hydrocarbons resulted faster aging. However it is also worth to note, that only slight and not significant decreased was found, which compliant result with changing in density or viscosity is. Jodine numbers are slightly decreased as function of treating days. E.g. it changed from 149 to 144 regarding pyrolysis oil obtained from catalyst free pyrolysis, which means 3.6% decreasing after the 7th day. Ni/ZSM-5 based catalysts could not affected the Jodine number of pyrolysis oils (3.3\u20133.9% decreasing), however the concentration of unsaturated hydrocarbons decreased a bit greater extent using Ni/SAPO-11 based catalysts (4.3\u20136.1%). Results well demonstrate that the highest decreasing in unsaturated hydrocarbons was found by the using of 1:1:2 catalyst ratio both in case of Ni/ZSM-5 and Ni/SAPO-11 catalysts.\nFig.\u00a010\n summarized the oligomer-polymer phase separated from the pyrolysis oils during the aging test, which well supported the infrared results. Without catalyst nearly linear relationship was found regarding the polymerized olefins as function of treating days. At the 7th day 0.15% of the pyrolysis oil could be separated by filtration. That value was significantly lower by the using of catalysts: 0.05\u20130.69% in case of Ni/ZSM-5 based catalysts, and 0.06\u20130.078% using Ni/SAPO-11 based catalysts. It means that the catalysts can reduced the amount of the separated fraction. Presumably, it could be explained by the difference in the distribution of the hydrocarbon structure of unsaturated compounds. In accordance with the FTIR result, in case of pyrolysis oils by ZSM-5 catalyst showed faster change during the end of the treating time (after 4th days), while the polymerization reactions took place rather during the first half of the accelerated aging (before 3rd day). It is important to remark, that the polymerized fraction is accumulated in the bottom of sample holder, and the pyrolysis oil kept transparent above that part.In this work the thermo-catalytic pyrolysis of real municipal plastic waste, the longer term stability and corrosion properties of pyrolysis products were investigated. It was found, that the PVC containing raw material could be converted mainly into pyrolysis oil, however the composition of catalysts had a notable effect to the gas:pyrolysis oil ratio. High synthetic zeolite containing catalyst mixtures can drastically increase the gas yield. Comparing the Ni/ZSM-5 and Ni/SAPO-11 catalyst mixtures, due to the larger pore areas and higher Si/Al ratio, the ZSM-5 containing compositions resulted higher yield in gases, than SAPO-11. SEM result well demonstrated that larger agglomerates were found in residues obtained from thermo-catalytic pyrolysis using SAPO-11 based catalyst, than ZSM-5. Regarding gases, catalysts can isomerize the main carbon frame (especially using catalysts with high concentration of Ni/ZSM-5 and Ni/SAPO-11), and promote the production of unsaturated hydrocarbons; however the n-olefin and n-paraffin ratio slightly increased. The synergetic property of Ni ion-exchanged catalyst and red mud to hydrogen production was also demonstrated. High concentration of red mud led to the highest proportion of hydrogen in gases: 121-Z and 121-S; furthermore more hydrogen was obtained using SAPO-11 based catalysts. In accordance with hydrogen content, the concentration of unsaturated branched hydrocarbons, especially isobutene, trans-but-2-en and cis-but-2-en were also high in case of SAPO-11 containing catalyst. Pyrolysis oils contain C5\nC30 hydrocarbons, while the aromatic, branched and unsaturated hydrocarbon content increased by the use of catalysts. ZSM-5 based catalysts show high efficiency in aromatization reaction. For conclusion the stability tests, the density and viscosity are only slightly increased, while the olefin content (especially vinyl type) slightly decreased during the accelerated aging at 80\u00a0\u00b0C till 7 days, demonstrating, that the high vinylene olefin content of given fractions is not crucial regarding the stability. Catalysts can decrease the ratios of changing comparing to the catalyst free thermal pyrolysis. However, the aging of pyrolysis oils obtained by Ni/ZSM-5 containing catalyst takes later than that of Ni/SAPO-11. Contrary, acidic components can cause more significant change in both TAN or even during the corrosion test. Both the TAN and relative change in copper plate weight loss during the corrosion test were lower using catalysts during the pyrolysis, especially SAPO-11 based. Regarding the chlorinated compounds, however a single step and not stepwise process was used, the chlorine could be transformed mainly into gaseous products and the chlorine concentration in pyrolysis oils can further decreased by the using of catalysts. Especially the presence of red mud and Ca(OH)2 was highly preferred for the reduction of the chlorine content of pyrolysis oil, and catalyst mixtures with Ni/SAPO-11 showed better properties for chlorine reduction than Ni/ZSM-5 based.The authors acknowledge the Horizon 2020, Marie Curie Research and Innovation Staff Exchange (RISE) (MSCA-RISE-2014 (Flexi-pyrocat, No.: 643322)). The authors also acknowledge the financial support of Sz\u00e9chenyi 2020 under the EFOP-3.6.1-16-2016-00015 and Economic Development and Innovation Operative Program of Hungary, GINOP-2.3.2-15-2016-00053: Development of liquid fuels having high hydrogen content in the molecule (contribution to sustainable mobility).", "descript": "\n                  This work is focussing to the thermo-catalytic batch pyrolysis of contaminated real municipal plastic waste using different catalyst mixtures in their different ratios: Ni/ZSM-5, red mud, Ca(OH)2 and Ni/SAPO-11, red mud, Ca(OH)2. The effect of the catalysts to the pyrolysis oil properties and the in-situ upgrading (especially the storage, transportation and corrosion stability) of pyrolysis oil was investigated. High concentration of Ni/ZSM-5 and Ni/SAPO-11 zeolites in catalyst mixtures can increase the yield of gases and pyrolysis oil, the concentration of aromatics or the hydrogen content in gases; however the presence of red mud in higher content can further increase the hydrogen concentration. ZSM-5 based catalysts showed higher efficiency in aromatization reactions. An accelerated aging test at 80\u00a0\u00b0C till 1 week was performed to investigate the storage and transportation stability of pyrolysis oils. Only slight increase was found in the density and viscosity, on the other hand there was a bit greater increase using SAPO-11 based catalysts than ZSM-5. The change in the olefin content was followed via bromine number and FTIR spectra of pyrolysis oil, which resulted \u223c3% and \u223c4% decreasing using Ni/ZSM-5 and Ni/SAPO-11 containing catalyst mixtures. Regarding acidic components, they significantly increased by aging time, while the high red mud and/or Ca(OH)2 in catalyst mixtures had notable benefit, because they can drastically decrease the concentration of chlorinated compounds, which led to less weight loss during corrosion test using copper plate till 60 days at 20\u00a0\u00b0C.\n               "}
{"full_text": "Hydrogen (H2) has significant advantages as an energy vector compared to petroleum or other conventional fossil fuels, although currently there are problems that must be solved, associated with its production, storage, and transportation [1]. Formic acid (FA) is a non-toxic renewable biomass material that can be used as an ideal liquid hydrogen carrier and achieve efficient hydrogen production. The development and utilization of FA fuel cells, using it as a hydrogen storage medium, is an effective method to solve the problem of energy depletion and environmental degradation [1,2]. However, the side reaction in the dehydration process of FA causes the catalyst to be poisoned resulting in a decrease in catalytic performance [3\u20135]. The development of high-performance FA dehydrogenation catalysts is of great significance for promoting the commercial application of these fuel cells.FA is produced by chemical methods such as the hydrolysis of methyl formate, but it is also obtained in equimolar proportions together with levulinic acid, by hydrolysis of cellulose raw materials derived from biomass. Currently, with the increased interest in the production of levulinic acid and other valuable chemicals from biomass, it is important to develop processes to use the derived formic acid, since otherwise, it constitutes a waste material [1]. In this direction, the interest in the use of the decomposition reaction of FA to produce H2 has increased remarkably. Therefore, the challenge is to produce pure H2, with minimum CO content, at the lowest possible temperature. This demand can be achieved through the careful choice of the catalyst and the reaction conditions, which is why many research efforts are currently being devoted to this line.The production of H2 from FA using homogeneous and heterogeneous catalysts has been studied in aqueous [3] and vapor phases [2,3], but in most cases, formulations based on noble metals, such as Rh, Pt, Ru, Au, Ag, and Pd supported on C, Al2O3 and SiO2 have been investigated [1\u20133]. For the vapor phase reaction, Solymosi et al. [2] found the following order of activity on a set of carbon-supported noble metals: Ir > Pt > Rh > Pd > Ru.On the other hand, in the case of non-noble metals, molybdenum carbide has attracted great interest but it showed substantially lower activity even at high temperatures [6]. In an earlier study, we reported the catalytic performance of molybdenum carbide supported on carbon with promising results [7], though high temperatures are required for the catalyst synthesis. Moreover, the catalytic activities of supported Cu and Ni catalysts have been measured in the FA decomposition reaction, proving to be active at relatively higher temperatures (> 220\u00a0\u00b0C) than noble metal catalysts [8\u201310]. These authors study also the catalytic decomposition of FA on Ni, and Ni-Cu alloy powders and report an improved selectivity toward dehydrogenation reaction although lower rate when Cu is added to Ni. More recently, Pechenkin et al. described 100% conversion and 98% yield to H2 using 10%CuO-5%CeO2/\u03b3-Al2O3 at 200\u00a0\u00b0C [11]. In this line, we have shown a Ni/SiO2 catalyst doped with 19.3\u00a0wt% of Ca which gives 100% conversion of formic acid at 160\u00a0\u00b0C, with a 92% selectivity to hydrogen [12]. In addition, we recently reported that the bimetallic Ni-Cu system supported on carbon has better catalytic performance than the monometallic Ni or Cu catalysts [13]. Bulushev and coworkers [14] reported an improved catalytic performance for the hydrogen production from formic acid over Ni catalysts supported on carbon doped with nitrogen. These authors also suggested the Ni single atoms stabilized on the pyridinic nitrogen sites are responsible of the improved behavior of these Ni catalysts [15]. In addition, we have demonstrated that the catalytic activity and selectivity of the bimetallic Ni-Cu system is enhanced if the carbon support is doped with N-pyrrolic heteroatoms [16].In supported Pd catalysts, the temperature required for vapor phase FA decomposition can be reduced to less than 80\u00a0\u00b0C by the addition of K2CO3\n[1]. The difference lies in the initial stages of the reaction since FA reacts with potassium ions to give formate species dissolved in the formic acid/condensed water solution in the catalyst pores [5] increasing the overall activity of the process. In a similar way, studies of adsorption and decomposition of FA on potassium modified Cu(110) reveal that the modification of the copper surface with potassium is accompanied by a decrease in the temperatures of HCOOH decomposition [17]. In a similar way, added cesium on Cu(110) increases formate species production during FA adsorption and accelerates its decomposition [18].Hence, in this work, the synthesis of Ni-Cu bimetallic catalysts supported on a non-porous high surface area graphite to be used in the decomposition reaction of FA in the vapor phase was carried out. The effect of doping on support with alkali metals (Li, K and Na) is studied, using in all cases an atomic ratio of alkali metal to active metal (Ni, Cu or Ni-Cu) equal to unity. The catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). In addition, the programmed temperature surface reaction (TPSR) of FA was studied, analyzing the gases released at the outlet of the reactor employing the mass spectrometer, and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy of the used catalysts was also performed.Nickel and copper monometallic catalysts and also bimetallic catalysts were synthesized. The technique employed to impregnate the metals was incipient wetness impregnation. Commercial high surface area graphite (H400; SBET = 399\u00a0m2/g) was obtained from Timcal Graphite. In all cases, the total metal concentration was 5% by weight, with 2.5% of each metal in bimetallic catalysts. The salts Ni(NO3)2\u00b76H2O (Alpha Aesar) and Cu(NO3)2\u00b73H2O (Sigma Aldrich) were used as precursors. The samples were dried at 100\u00a0\u00b0C. The following salts were used for doping with alkaline elements the graphite support: Na2CO3\u00b7H2O (Alpha Aesar), KNO3 (Panreac) and LiNO3 (Sigma Aldrich). The alkaline loadings, Li (0.6% w / w), Na (2% w / w) and K (3.4% w / w) were added so that the resulting catalysts contain the same atomic charge of alkaline metal and active metal. To incorporate the dopants, the incipient wetness impregnation method was also used. These materials were dried in an oven for 12\u00a0h at 100\u00a0\u00b0C and treated in He at 500\u00a0\u00b0C for 3\u00a0h. Subsequently, the metallic charge of Ni and Cu with 2.5% of each metal, was incorporated as previously indicated.The crystalline structure of the samples was examined by X-ray diffraction (XRD) using an X \u0301Pert Pro PANalytical. The temperature-programmed reduction (TPR) experiments were carried out in a conventional fixed-bed flow reactor and the effluent gases were continuously monitored by a thermal conductivity detector (Konik TCD); the samples were heated up in a 5% H2/Ar stream with a rate of 10\u00a0\u00b0C/min up to 675\u00a0\u00b0C. Transmission electron microscopy (TEM) images of the reduced catalysts were acquired using a JEOL 2100F field emission gun electron microscope equipped with an energy dispersive X-ray (EDX) detector. The fresh sample was reduced at 300 or 400\u00a0\u00b0C for 1\u00a0h in a pure H2 stream. The catalysts were reduced ex-situ and a He flow passivation procedure was carried out at room temperature. The used samples were measured after the catalytic test and a passivation procedure to room temperature in an inert atmosphere. The samples were dispersed in ethanol and mounted on the appropriate grid for the TEM microscope. The particle size was determined by counting at least 300 particles.The catalytic activity measurements for the FA decomposition in the vapor phase were carried out in a conventional fixed-bed flow reactor. The copper and bimetallic catalysts were pretreated in H2 flux at 300\u00a0\u00b0C for 1\u00a0h and then cooled in N2 flux at the reaction temperature. The nickel catalyst was pretreated at 400\u00a0\u00b0C for 1\u00a0h in H2 flux and then cooled in N2 flux at the reaction temperature. A mixture of FA diluted with N2 was fed to the reactor using a saturator-condenser at 15\u00a0\u00b0C (HCOOH concentration equal to 6%, with a flow of 25\u00a0ml\u00a0min-1). For all the experiments, 75\u00a0mg of catalyst were charged to obtain a ratio of W (weight of catalyst)/ F (total flux) equal to 5 10-5 g\u00a0h\u00a0ml-1. The reactants and products were analyzed by gas chromatography (Varian 3400) fitted with a 60/80 Carboxen TM 1000 column and a thermal conductivity detector (TCD). At each temperature, a few measurements were performed in order to ensure that steady-state activity was reached. During the test, the unique products determined were CO, CO2 and H2. The total conversion of formic acid was determined as the sum of CO and CO2 concentrations related to the initial concentration of FA. In addition, the CO2 selectivity was calculated as the CO2 concentration related to the sum of CO and CO2 concentrations. The catalysts were studied in two heating cycles. The stability of the catalyst was evaluated over 14\u00a0h at a selected temperature. Catalytic tests with the supports proved that conversion was negligible.The temperature-programmed surface reaction (TPSR) measurements were carried out in conventional dynamic vacuum equipment coupled to a quadrupole mass spectrometer (SRS RGA-200). The catalysts were reduced before experiments in hydrogen flow at 300 or 400\u00a0\u00b0C and were degassed under high vacuum at the same temperature. The adsorption was then carried out using a 40\u00a0Torr pulse of HCOOH at 40\u00a0\u00b0C. Once the gas phase was evacuated, the desorption step was carried out at a programmed temperature, analyzing the gases released employing the mass spectrometer. The evolution of signals assigned at H2, N2, H2O, CO, HCOOH, CH2O, O2, and CO2 (m/e = 2, 14, 17/18, 28, 29/46, 30, 32 and 44, respectively) was followed as a function of temperature. Calibration of the relative intensity of the H2 and CO2 signals, m/e equal to 2 and 44, respectively, was performed. In a conventional vacuum equipment system, a certain number of moles of H2 and CO2 was admitted and a recirculation pump was employing for the mixture of the gases and then this stream was analyzed by the mass spectrometer.Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were acquired using a Bruker Vector 22 spectrometer equipped with a germanium crystal. For the ATR experiments, the SiC was carefully separated from the solid catalyst after the catalytic tests (stability tests for 14\u00a0h on stream) and then, was placed the sample on the crystal and collected the spectrum. A total of 256 scans with a resolution of 4\u00a0cm-1 were collected to measure each used catalyst.Several articles have been widely investigated for CO2 and H2 reactions supported by noble metals (Rh, Ru, Ir, Pt and Pd) and non-noble metals (Ni, Co, Cu and Fe) [1\u20136,19\u201331]. The metal and promoters, redox properties, acid/base features and surface area of the support, the metal particle size and metal-support interactions are the key factors for obtaining a good activity/stability balance on the catalysts.The hydrogen production from FA has been studied fundamentally on catalysts based on noble metals [1\u201323]. The strength of this work is to achieve high conversion and selectivity employing non-noble metals such as Ni and Cu. A graphitic material with high specific surface was used as a support and it was modified by adding alkali metals to change its surface basicity and thus favor the adsorption of HCOOH and its decomposition. The supports were modified with Na, Li and K; the percentages used were such that the number of dopant atoms is equal to the amount of active phase present in the catalyst.The nomenclature of the catalysts only indicates the metal, Ni or Cu, in the monometallic and Ni-Cu in the bimetallic, because in all cases the support was H400 graphite material, and those in which an alkaline was added as /A (where A = Na, Li or K). The loadings were indicated in the experimental session.The reducibility of the supported catalysts was studied by temperature-programmed reduction (TPR). This technique is a powerful tool for the study of the behavior of metal precursors and obtaining the strength of the oxide-support interaction. \nFig. 1 shows the TPR profiles of the catalysts in the temperature range of 40\u2013675\u00a0\u00b0C. The copper monometallic catalyst profile shows a single peak at 226\u00a0\u00b0C associated with the reduction of highly dispersed Cu2+\n[27,32].The nickel monometallic catalyst profile presents a reduction peak at 263\u2009\u00b0C associated with easily reducible Ni2+ species and another peak at 313\u2009\u00b0C associated with the reduction of Ni2+ bulk. Furthermore, a high-temperature peak, at about 416\u2009\u00b0C, is asymmetric and wide is attributed to remaining Ni2+ particles and also to gasification of support carbon atoms in the vicinity of the nickel particles, which catalyze this reaction producing methane [28,33\u201336].The profile for the bimetallic catalyst shows peaks in intermediate temperatures to the monometallic ones. The main peak is located at 254\u2009\u00b0C, one of less intensity at 318\u2009\u00b0C and another broad peak in the region between 400 and 600\u2009\u00b0C with a center at 522\u2009\u00b0C. The lower temperature peak would be associated with the reduction of Cu2+ and Ni2+ particles while the other peaks could indicate the existence of small Ni2+ particles which reduce at higher temperature and metal catalyzed methanation of carbon atoms around metallic particles [30,37].On the other hand, doping with alkaline oxides slows down the reduction process of the Ni and Cu particles, causing a displacement of the TPR profiles to higher temperatures.The catalyst doped with Li has a profile similar to the bimetallic without dopant shifted to higher temperatures. The main peak is located at 298\u2009\u00b0C, one of less intensity at 375\u2009\u00b0C and another broad peak in the region between 400 and 600\u2009\u00b0C with a center at 525\u2009\u00b0C. The catalyst doped with Na has a similar profile, too; however, a high-temperature peak less marked. The Na doped catalyst profile shifted to higher temperatures, a peak at 340, another with less intensity at 407 and the third at 506\u2009\u00b0C completing the profile at 600\u2009\u00b0C.The catalyst doped with K has a different TPR profile, has a broad peak in the region between 200 and 425\u2009\u00b0C with three positions marked at 292, 317 and 350\u2009\u00b0C. In this catalyst, the proximity of the maximums could indicate a narrower particle sizes distribution. According to the results of the TPR experiments, it can be observed that when modifying the supports with alkali metals, the metal-support interactions were modified, displacing the reduction profiles at higher temperatures. The decrease in the reducibility of the samples which causes a shift to higher reduction temperature was observed on Ni-based catalyst doped with different contents of Na [38], on Cu catalysts modified with different metal oxides (MgO, BaO, ZnO and MnO) [39] and on Cu-Ni system doped with MnO [30].Considering these TPR results, before the catalytic test and analysis with characterization techniques, the catalysts were pre-treated in hydrogen flow at 300\u2009\u00b0C, except for the Ni monometallic solid, which was pre-treated in hydrogen flow at 400\u2009\u00b0C for 1\u2009h.The catalysts were measured by X-ray diffraction to observe the presence of crystalline phases. The H400 support diffractogram (\nFig. 2) exhibits the characteristic diffraction pattern of graphitic materials, with a pronounced peak at 2\u03b8 =\u200926\u00b0 due to reflection from the basal plane (002). The characteristic peaks of (100), (101), (004) and (110) crystal planes are also present [32,40].In the diffractograms of the reduced catalysts, it is observed that there are no changes in the graphitic crystalline structure after the incorporation of the metallic phases by incipient wetness impregnation. In reduced Cu monometallic catalyst, three peaks at 43.0\u00b0, 50.5\u00b0 and 74.3\u00b0 assigned to the characteristic diffraction peaks of metallic copper phase (JCPDS 65-9743) [28], corresponding to (111), (200), (220) plane phase, respectively. Furthermore, a broad peak at 2\u03b8 =\u200936.3\u00b0, 42.3\u00b0 (over a graphite peak) and another at approximately 2\u03b8 =\u200961\u00b0 are observed, indicative of the Cu2O crystal structure (JCPDS 01-078-2076) probably due to the atmosphere exposure between the pretreatment and the XRD experiment. In reduced Ni monometallic catalyst, three broad peaks at 37.2\u00b0, 43.3\u00b0 and 62.9\u00b0 assigned to the characteristic diffraction peaks of NiO phase (JCPDS 71-1179) [41].In the diffractogram of the reduced bimetallic Ni-Cu catalyst, no characteristic signals of Ni or Cu are detected (Fig. 2), probably due to the smaller particle sizes obtained in this solid.When the catalysts were doped with alkali metals, three peaks at 43.0\u00b0, 50.5\u00b0 and 74.3\u00b0 assigned to the characteristic diffraction peaks of metallic copper corresponding to (111), (200), (220) plane phase, respectively (JCPDS 65-9743) are observed [28]. The relative intensity of these signals concerning those of the support is greater for the catalyst with Li > K > Na. In the diffractogram of the catalyst doped with Na, signals are corresponding to sodium carbonate that remained without decomposing in the treatment at 500\u2009\u00b0C with He flux (JCPDS 37-0451) [42]. No signals corresponding to Ni-Cu bimetallic nanoparticles were detected in the catalysts evidencing that these have sizes of 4.5\u2009nm or less, which is the detection threshold of the XRD technique.The distribution, shape and size of the particles were examined by transmission electronic microscopy. TEM images were taken with different magnifications of the samples reduced in H2 at 300\u2009\u00b0C (400\u2009\u00b0C for the Ni monometallic catalyst). In \nFig. 3, TEM images of the Cu and Ni monometallic, undoped, Na and K-doped bimetallic catalysts are shown. It can be seen that the particles of both metals are evenly distributed on the support. To estimate the mean size, 300 particles were measured, being 4.7, 4.9, 4.2, 4.3 and 5.3\u2009nm for the Cu, Ni, Ni-Cu and Ni-Cu/Na and Ni-Cu/K catalyst, respectively. The doping with Na and K did not considerably modify the average size of the Ni-Cu bimetallic particles, but it did slightly modify the distribution of the particles (see histograms in Fig. 3).\n\nFig. 4 shows the images obtained for the reduced Ni-Cu/K catalyst in the STEM mode for the select area for the EDX mapping of copper (yellow), nickel (light blue), potassium (magenta) and oxygen (red). These images showed Cu particles of heterogeneous size, while Ni and K appears well dispersed. The observation of larger copper agglomerates is in agreement with the XRD results (Fig. 2). Nevertheless, the EDX analysis revealed that Ni, Cu and K particles are evenly distributed on graphitic material and coincide in occupying the same space on the support, evidencing the formation of nickel-copper particles dispersed over the K doped HSAG.The decomposition of formic acid is via dual-path mechanisms:\n\n\n\nSelective\n\n\ndehydrogenation:\n\n\n\nHCOOH\n\n\n(\n\ng\n\n)\n\n\u2192\n\n\nH\n\n\n2\n\n\n+\n\n\nCO\n\n\n2\n\n\n\n\n\u2206\n\n\nG\n\n\n298\n\n\n=\n\u2212\n48.4\n\n\nkJ\n\n\n\n\nmol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n\n\n\nUndesirable\n\n\ndehydration:\n\n\n\u2009\n\nHCOOH\n\n\n\n(\n\ng\n\n)\n\n\u2192\n\n\nH\n\n\n2\n\n\nO\n+\n\nCO\n\n\n\n\u2206\n\n\nG\n\n\n298\n\n\n=\n\u2212\n28.5\n\n\nkJ\n\n\n\n\nmol\n\n\n\u2212\n1\n\n\n\n\n\n\nTherefore, high activity and selective catalysts are desirable for the generation of hydrogen from the decomposition of HCOOH. The catalytic activity of all catalysts was evaluated in a fix bed reactor with a mass/flow ratio (W/F) equal to 5 10-5 g\u2009h\u2009ml-1 in the temperature range of 60\u2013200\u2009\u00b0C to build light-off curves.\n\nFig. 5 shows the conversion of formic acid as a function of the reaction temperature for the series of mono and bimetallic Ni and Cu catalysts. Previous to the catalytic test, the catalysts were pre-treated in hydrogen flow at 300\u2009\u00b0C, except for the Ni monometallic solid, which was pre-treated in hydrogen flow at 400\u2009\u00b0C for 1\u2009h, considering the TPR results.It is important to note that the comparison of the catalytic activity was measured when the Ni-Cu/Na catalyst was reduced at 300 and 400\u2009\u00b0C and no marked difference was found (Fig. S1-supplementary information). The conversion values as a function of time for the two experiments were also included, observing that they are almost coincident in the 14\u2009h tested at 120\u2009\u00b0C (Fig. S2-supplementary information). Therefore, it is concluded that the choice of the pretreatment temperature was adequate.The light-off curves were performed following the same procedure in all the samples. After reducing the catalysts in H2 flux, it was cooled in a stream of N2 to 60\u2009\u00b0C, and then the reaction mixture was fed with a concentration of HCOOH of 6% in N2. After completing the curve from 0% to 100% (1st evaluation-Fig. 5), the temperature was lowered to leave it isothermal and measure the stability of the samples (\nFig. 6). After 14\u2009h on stream, the temperature was decreased and the complete light-off curve (2nd evaluation-Fig. 5) was again measured.\n\nTable 1 compares the reaction temperature values for which the catalysts reach 50% conversion of FA. It can be seen that comparing monometallic catalysts, Ni reached the conversion value at a lower temperature (138\u2009\u00b0C) than the Cu catalyst. However, the high selectivity to hydrogen production of the Cu catalyst is notable. Moreover, comparison with previous results obtained over a Ni/SiO2 catalyst [13], which gives 50% conversion at 180\u2009\u00b0C with selectivities to H2 of 91%, demonstrates the superior performance of Ni catalyst supported on graphite for the hydrogen production from formic acid. A composition equal to 2.5% Ni and 2.5% Cu was chosen for the bimetallic catalysts, due to the catalytic performance of the monometallic catalysts supported on HSAG-400, seeking that the solids have the highest Ni activity and at the same time the high selectivity towards H2 provided by Cu. These results are in agreement with the DFT study of Herron et al. [43] which predicts the formation of CO2 and H2 on Cu(111) and Cu(100) while on Ni(111) and Ni(100) dehydration products CO and H2O are expected. Moreover, experiments have shown the most preferable pathway for HCOOH dissociation on the stepped Ni surface was HCOOH dehydrogenation to give COOH followed by dehydroxylation to form CO [44].In the case of the Ni-Cu bimetallic catalyst, it is observed that the activity and selectivity were intermediate, reaching 50% conversion at 145\u2009\u00b0C, but with a selectivity of 97% towards H2. It is worth noting the low concentration of CO (3%) in the reactor outlet stream, which is associated with a parallel dehydration reaction of HCOOH.This work has also addressed the study of doping with alkali metals (Li, Na and K) with an atomic ratio of metal/alkaline equal to 1 to promote the basicity and the catalytic activity. Doping with K and Na shows a marked effect on the temperature at which it reaches 50% conversion of HCOOH, which in both cases was significantly lower than that of the undoped catalyst, which reveals a positive effect on the catalytic activity in these multi-component solids that make them competitive with noble metals [5]. However, the doping with Li worsened the behavior of the material, since only at the temperature of 160\u2009\u00b0C is the 50% conversion reached, this temperature being even higher than that of the less active monometallic (Cu catalyst). Therefore, among the alkali metals studied only potassium and sodium gave promotion to the bimetallic Ni-Cu catalyst, the order of the activities measured for samples being K > Na > undoped > Li (Table 1, Fig. 5). This finding is somewhat different to that found over Pd/C catalyst for what all the alkali metal species gave promotion [1]. The doped Ni-Cu catalysts have shown high selectivity values towards hydrogen (95\u201397%) and the selectivities were maintained at high conversions.The specific catalyst-mass based reaction rates obtained at 100\u2009\u00b0C and the turnover frequency (TOF) calculated per surface metal atom are compiled in \nTable 2. Also, the apparent activation energies calculated from the Arrhenius plots for all the catalysts studied are given.In Table 2 it is seen that the reaction rate and TOF value at 100\u2009\u00b0C for the Cu catalyst are lower than (approximately half of) those obtained for the supported Ni, while the Cu-Ni bimetallic gives intermediate values. This order in catalytic activity agrees with that found in an earlier work for Ni, Cu and Ni-Cu alloy powders [10]. It is important to note that the TOF of the Ni-Cu catalyst is significantly increased when Na and specially K is added, since the TOF of the Ni-Cu is doubled for Ni-Cu/K catalyst. Thus, the TOF for the Ni-Cu/K catalyst is 0.0113\u2009s-1 at 100\u2009\u00b0C, value of reaction rate considerably higher than those reported for Ni [16] and Ni-Cu [17] supported on nitrogen doped carbon materials. However, it is very close to those reported in the literature for noble metals [1,45]. Jia et al. report a TOF value of 0.013\u2009s-1 at 80\u2009\u00b0C for a Pd/C catalyst, although this is in a great extent increased when K is added to the catalyst.Interesting that the values of apparent activation energies for the two monometallic catalysts were similar, and also in the case of the undoped bimetallic. For catalysts modified with Li, Na and K, the apparent activation energy values were lower, the lowest value being obtained for the K-doped catalyst. It should be note that in this work the apparent activation energy values were lower than those reported in an earlier work for the decomposition of formic acid over Ni powder without support and Ni catalysts supported on silica and alumina, as well as for the decomposition of Ni formate [11,46]. Also, for similar catalysts, Ni supported on carbon, the obtained values were higher in the range of 100\u2009\u00b1\u200910\u2009kJ\u2009mol-1 than those calculated for the present catalysts [15,16].On the other hand, the catalysts were relatively stable under the conditions tested, although it can be seen in Fig. 5 that the points corresponding to the 2nd evaluation are below those obtained in the first, probably due to a restructuring of the material at the reached temperature (130\u2013190\u2009\u00b0C) and with conversion levels close to 100%.In the case of catalyst doping with Li (Fig. 6) a lower activity is observed in the first hours on stream followed by a slight decay of the conversion for the temperature (145\u2009\u00b0C) at which the stability test was carried out for this catalyst. It should be noted the marked positive effect on the conversions achieved doping the catalysts with K and Na in the whole range of temperature. The doping of K in Pd catalysts supported over SiO2, Al2O3 and activated carbon has been previously reported [5]. These authors observed a significant effect of improvement in the catalytic behavior of noble metal for the formic acid decomposition. As a reaction mechanism they proposed, as a first step, the formation of a phase containing liquid formic acid condensed in the pores of the catalyst and this phase provides a reservoir for the formation of formate ions with the participation of K+ ions; that later decompose to form CO2 and H2. In our materials, since the support is a non-porous material, condensation of formic acid is not likely to occur in pores, however, formates or oxyhydroxide phase could form in the alkali metal in the doped catalysts, these species being the reaction intermediates. In addition, the promotion effect of alkali metals increases down in the group (Li < Na < K) when the formation of these species is boosted [47].Temperature programmed surface reaction (TPSR) is a powerful technique to determine the surface chemistry of bulk metal, supported metal and bulk metal oxide on supported catalysts. It can provide both qualitative and quantitative analysis of the surface active sites present on the catalyst surface, the reaction mechanisms, and kinetics occurring on the catalyst surface by using chemical probe molecules such as alcohols, carboxylates, or specific acidic-basic reacting gases. In the present paper, the experiments were carried out by adsorbing the HCOOH molecule (reactive under study) and monitoring the gas outlet flux with a mass spectrometer during desorption at programmed temperature experiments.The TPSR experiments were carried out to understand the differences in the catalytic performance. The catalysts were reduced before experiments in hydrogen flow at 300 or 400\u2009\u00b0C and were degassed in a high vacuum at the same temperature for 1\u2009h. The adsorption was carried out using a pulse of 40\u2009Torr of HCOOH at 40\u2009\u00b0C. The TPSR experiments for the Ni and Cu based catalysts are shown in \n\nFigs. 7 and 8. As above stated the acid formic decomposition may proceed through either dehydrogenation giving CO2 and H2 and dehydration producing H2O and CO. So, the evolution of signals assigned to H2, N2, H2O, CO, HCOOH, CH2O, O2, and CO2 (m/e = 2, 14, 17/18, 28, 29/46, 30, 32 and 44, respectively) was followed as a function of temperature. The evolution of the desorbed masses of H2, CO2 and HCOOH was plotted as a function of temperature to make the comparison clearer in Figures. The small amount of CO (m/e = 28) desorption is not plotted because the mass 28 is also secondary of CO2 which is the major product of decomposition and the determination of real CO production is prone to considerable error.The TPSR experiments for the Ni, Cu and Ni-Cu catalysts are shown in Fig. 7. At lower temperatures (< 100\u2009\u00b0C) the desorption of the unreacted HCOOH is observed and above 50\u2009\u00b0C the decomposition process begins to produce H2 and CO2. This behavior was similar for the three samples compared in this figure.For the Ni catalyst, in Fig. 7a the H2 and CO2 profiles show in addition to the 80\u2009\u00b0C peak a maximum at 125\u2009\u00b0C, then an increase mainly in CO2 signal with another maximum at 200\u2009\u00b0C. After that, both signals remain at zero until the region of 300\u2013400\u2009\u00b0C where the decomposition of the fraction of retained FA in the catalyst occurs. Previous studies using spectroscopic techniques and temperature programmed desorption (TPD) of FA adsorption over Ni(111) surfaces [48] reported the formation of bidentate formates at low temperature which transformed to monodentate formates and decomposed in the 25\u2013225\u2009\u00b0C temperature range, producing CO2 at 100\u2009\u00b0C and also CO at 157\u2009\u00b0C.For the Cu catalyst, Fig. 7b, the HCOOH signal had two steps marked of desorption at 80 and 140\u2009\u00b0C. In addition, at that temperature of 140\u2009\u00b0C, a maximum of CO2 production is observed and also in the H2 signal at 150\u2009\u00b0C. The adsorption and decomposition of formic acid on clean Cu(110) has been previously studied by means of thermal desorption mass spectroscopy [14,49]. On the Cu(110) surface the formate species formed is stable up to 127\u2009\u00b0C, but decomposes to simultaneously evolve H2 and CO2 in TDS peaks at 190\u2013200\u2009\u00b0C. However, the temperature for HCOOH decomposition on Cu metallic powder has been shown to be lower than on Cu(110) surface [50]. In addition, the reaction is structure sensitive on Cu catalysts since Cu(100) and Cu(211) bind HCOO much more strongly than Cu(111) and have varied barriers for the likely rate determining step, formate species dehydrogenation [51].For the bimetallic catalyst, Fig. 7c, the H2 and CO2 profiles are not the results of adding the profiles obtained for the monometallic catalysts. The low-temperature region is similar to that mentioned for monometallics, however, the CO2 and H2 signals show a maximum at 145\u2009\u00b0C and another increase in the region of 160 and 250\u2009\u00b0C. For this catalyst, a remarkable coincidence is observed in the profiles of both gaseous products.In Fig. 8, the undoped bimetallic catalyst and the catalysts doped with K, Li and Na are compared. The objective of adding an alkali metal to the Ni-Cu bimetallic catalyst was to promote the catalytic activity by increasing its basicity. These alkali metals could also favor the formation of stable carbonates at high temperatures [14].In the three cases, the alkali/active metal ratio was maintained to compare the effect caused by each one. It is possible to observe that the profiles have similar shapes although with different relative intensities (Fig. 8b\u2013d). In addition, for all the catalysts was observed that at a lower temperature (< 120\u2009\u00b0C) desorption of the unreacted HCOOH is observed and above 50\u2009\u00b0C the decomposition process begins to produce H2 and CO2. Relative to the non-alkali modified Ni-Cu catalyst the intensity of the CO2 and H2 profiles for the alkali modified catalysts have considerably increased which is related with the promotion by the alkali of the formate species formation [13].The potassium-doped catalyst shows a first maximum of the product signals at 105\u2009\u00b0C, this being the lowest temperature observed for all the catalysts under study. This could explain the higher activity observed for this catalyst.The sodium-doped catalyst shows the first maximum at 135\u2009\u00b0C where a greater intensity of the CO2 signal and retention of H2 at that temperature was observed. This behavior could be related to the sodium precursor salt (sodium carbonate) employed in this solid.The lithium-doped catalyst shows a first signal at 145\u2009\u00b0C of both products. It can be seen, that the Ni and Cu monometallic, undoped bimetallic and Li-doped catalysts had lower adsorption of HCOOH and subsequent lower production of H2 and CO2 (Figs. 7a\u2013c and 8b). This could be related to the lower catalytic activity observed for these catalysts.The H2/CO2 ratio was determined from the HCOOH TPD profiles (Table 1) using a calibration of the relative intensity of the H2 and CO2 signals, m/e equal to 2 and 44, respectively. The monometallic catalysts present values of the H2/CO2 intensity ratio less than 1 (0.43 and 0.49), this could indicate that the fraction of hydrogen not released after decomposition is retained, forming hydride or formate species. For the bimetallic non-doped catalyst, a desorbed equimolecular ratio is observed, indicating that neither H2 nor CO2 are retained on the catalyst. The catalysts doped with K, Na and Li present values of H2/CO2 ratios greater than 1, indicating stronger adsorption of CO2, probably forming carbonate species [14]. If the samples doped with Na and K are compared, it can be seen that in the first one, a greater amount of CO2 (H2/CO2 = 0.5) was desorbed by the decomposition of formic acid at a lower temperature (100\u2013150\u2009\u00b0C range, Fig. 8c). For the other catalyst, Ni-Cu/K, the production of H2 and CO2 was equimolar as corresponds to the formic acid decomposition reaction at a lower temperature (100\u2013150\u2009\u00b0C range, Fig. 8d). The sample doped with Na presents greater desorption of H2 at a higher temperature. This indicates in the case of Na greater stability of species, for example, formate or bicarbonate type, that store hydrogen in this solid. The presence of formate, bicarbonate and carbonate species in the catalysts used on reaction was confirmed by attenuated total reflectance (ATR) (\nFig. 9).The spectra of the used catalysts show signals assigned to bridged carbonates (\u03bd(C\n\nO): 1730\u20131640\u2009cm-1; \u03bdas(COO): 1285\u20131280\u2009cm-1; \u03bds(COO): 1020\u20131000\u2009cm-1), bidentate carbonates (\u03bd(C\n\nO): 1670\u20131530\u2009cm-1; \u03bdas(COO): 1270\u20131220\u2009cm-1; \u03bds(COO): 1030\u2013980\u2009cm-1), monodentate carbonates (\u03bdas(\n\n\nCO\n\n\n3\n\n\n2\n\u2212\n\n\n): 1530\u20131470\u2009cm-1; \u03bds(\n\n\nCO\n\n\n3\n\n\n2\n\u2212\n\n\n): 1370\u20131300\u2009cm-1; \u03bd(C-O): 1080\u20131040\u2009cm-1), carbonites (\u03bdas(CO2): 1495\u20131478\u2009cm-1; \u03bds(CO2): 890\u2013717\u2009cm-1) and formates species (\u03bdas(COO-): 1605\u20131540\u2009cm-1; \u03bds(COO-): 1370\u20131345\u2009cm-1) [44,52\u201354]. It is important to note that the signals associated with the formate species present a higher relative intensity for the Na-doped catalyst, being consistent with what was observed in the formic acid TPSR experiments. Probably the lower stability of these species in the catalyst doped with potassium is the factor that determines their greater capacity to promote the catalytic activity.The synthesis of Ni-Cu bimetallic catalysts supported on a non-porous high surface area graphite was performed. The catalysts were employed in the decomposition reaction of formic acid in the vapor phase. The effect of doping on support with alkali metals (Li, K and Na) was studied, using in all cases an atomic ratio of alkali metal to active metal (Ni, Cu or Ni-Cu) equal to unity.The bimetallic Ni-Cu catalyst has an intermediate behavior of monometallic, with high catalytic activity for the decomposition of formic acid and high selectivity for the production of hydrogen (97%). The comparative study of the promotion of the Ni-Cu catalyst with the alkali metals Li, Na and K shows that the catalyst doped with K has the best behavior in the formic acid decomposition reaction. TPRS experiments show that formate, bicarbonate or carbonate species decompose at different temperatures depending on the alkali metal present in the catalyst and the formation/decomposition of these species turns out to be an important factor in promoting catalytic activity. The sample doped with Na presents greater desorption of H2 at a higher temperature and a higher relative intensity of the formate species was observed in the used catalyst by ATR. The potassium-doped catalyst shows the maximum production of H2 and CO2 equimolar at 105\u2009\u00b0C, being the lowest temperature observed for all the catalysts under study. This could explain the higher activity observed for this catalyst.The bimetallic catalyst doped with K showed 100% conversion of formic acid at 130\u2009\u00b0C with a 95% of selectivity to hydrogen. Also, all the tested materials were promising for their application since they showed catalytic behaviors close to those of noble metals reported in the literature.\nB.M. Faroldi: Conceived and designed the experiments, Investigation, Writing\u00a0\u2013 original draft. J.M. Conesa: Investigation, collaborated with the catalytic and TPRS experiments. A. Guerrero-Ruiz: Writing \u2013 review & editing. I. Rodr\u00edguez-Ramos: conceived and designed the experiments, Project administration, Writing \u2013 review & editing, Funding acquisition, B.F. and I.R.R. conceived and designed the experiments. All authors discussed the results and contributed to the manuscript.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors wish to acknowledge the financial support received from the Spanish Agencia Estatal de Investigaci\u00f3n (AEI) and EU (FEDER) (projects CTQ2017-89443-C3-1-R, CTQ2017-89443-C3-3-R, PID2020-119160RB-C21 and PID2020-119160RB-C22). B. Faroldi thanks CONICET for Postdoctoral External Fellowship Program.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcata.2021.118419.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Ni, Cu and Ni-Cu catalysts supported on high surface area graphite were synthesized by incipient wet impregnation. Also, the effect of doping the graphite support with alkali oxides (Li, Na and K) was studied. The catalysts were tested in the formic acid decomposition reaction to produce hydrogen. The bimetallic Ni-Cu catalyst doped with K showed the best catalytic performance with 100% conversion of formic acid at 130\u00a0\u00b0C and a 95% of selectivity to hydrogen. The turnover frequency (TOF) of the catalysts follows the order: Ni-Cu/K > NiCu/Na > Ni-Cu > Ni-Cu/Li. While the order for the apparent activation energy values is: Ni-Cu > Ni-Cu/Li > Ni-Cu/Na > Ni-Cu/K. The mechanism of the reaction is approached by programmed temperature surface reaction (TPSR) experiments and attenuated total reflectance (ATR). The greater catalytic activity of the Ni-Cu catalyst doped with potassium is ascribed to the lower stability of the formate, bicarbonate and carbonate species on its surface.\n               "}
{"full_text": "World production of waste plastic grows year by year, as a consequence of the huge demand for plastic materials in every commercial field [1]. However, a significant proportion of waste plastics end up into the waste stream leading to many environmental problems. Plastics in the ocean are of increasing concern due to their persistence and effects on the oceans, wildlife and potentially, humans [2]. The cumulative quantity of waste plastic is predicted to be nearly 250 million tonnes per year by 2025. Therefore, there is an urgent need to develop more effective methods to process waste plastics and improve its utilization efficiency.Chemical recycling processes such as pyrolysis are an effective option to recover energy from waste plastic. A wide distribution of products including gas, chemicals, chars and other products can be obtained from pyrolysis of plastics [3,4]. Furthermore, pyrolysis with a subsequent catalytic steam reforming process enables the conversion of plastic into more valuable gases such as hydrogen [5]. A 60\u2009g/h scale continuous tank reactor for plastic pyrolysis followed by the catalytic packed-bed reactor for steam reforming was designed by Park [6] and Namioka [7] for the hydrogen-rich gas production from waste polypropylene and polystyrene, while the optimum operating conditions were also studied. Erkiaga et al. [8] compared the products from pyrolysis (500\u2009\u00b0C)-steam reforming of HDPE with those from a gasification (900\u2009\u00b0C)-steam reforming system by the same authors [9]. Results show that the former one produced a high H2 yield of 81.5% of the maximum stoichiometric value, which was a little bit lower than the later one (83%) but enabling a more energy-efficient technology for plastics utilization. The pyrolysis and in-line steam reforming of waste plastics has been reviewed by Lopez et al. [10], and they reported that more than 30\u2009wt.% of H2 yield with up to 70\u2009vol.% of concentration could be obtained.Catalysts can assist in chain-scission reactions and the breakage of chemical bonds during the pyrolysis-steam reforming process, allowing the decomposition of plastics to occur at a lower temperature and shorten the reaction time. Different types of catalyst such as olivine [11], Ru [6], Fe [12], and Ni [13,14] catalysts have been investigated for gaseous products from the pyrolysis-reforming of waste plastics. Because of the high activation ability of CC and CH bonds on the Ni metal surface as well as the relatively low cost, Ni based catalysts have been a preferred choice in the process [15]. There has been much reported work in the literature devoted to the selection of the optimum loading content, promoters and preparation method of Ni based catalysts for the pyrolysis-reforming of plastics. By varying the flow rate of reduction gas and metal addition to the Ni catalyst, Mazumder et al. [16] found that the acid\u2013base properties, metal dispersion and crystal size of catalyst can be greatly improved. Wu and Williams [17] suggested that the increase in Ni loading could improve hydrogen production from polypropylene, and Mg modified Ni catalyst showed better coke resistance than the non-modified catalysts. Other promoters such as Ce and Zr were also explored, and the improvement in catalyst intrinsic activity was ascribed to the enhancement of water adsorption/dissociation [18]. In order to obtain a higher Ni dispersion, some novel assisted methods were developed to relieve the diffusion resistance of Ni into the inner structure of catalyst. For example, ethylene glycol [19,20] and ethylenediaminetetraacetic acid [21] assisted impregnation methods were used to prepare Ni catalysts with good stability and activity for hydrocarbons reforming.Catalyst synthesis methods, in particular, the metal loading method, is a curial factor to be considered for catalyst activity. The physical structure and chemical characterizations of the catalyst, including the porosity, reducibility and stability would be closely related to the preparation process [22,23]. Impregnation (or incipient wetness) is the most common method for catalyst preparation, because of the simple procedure and the flexibility to include different catalyst promoters. Co-precipitated Ni catalyst was designed to minimize catalyst deactivation and promote hydrogen production from waste hydrocarbons [24,25]. Meanwhile, sol-gel prepared catalysts have attracted more attention recently [26]. The reinforced impact on Ni dispersion with average size of 20\u201324\u2009nm was found by using a sol-gel method, leading to superior catalyst activity towards methane reforming [27]. Some reports have compared different catalyst preparation methods, for example, Bibela et al. [28] used a Ni-Ce/Mg-Al catalyst for steam reforming of bio-oil, and found that the wetness impregnated catalyst showed higher carbon conversion than a catalyst prepared via co-precipitation at increasing pH. A sol-gel prepared and promoted Ni/Al2O3 catalyst was reported to benefit the metal-support interaction with better particle size uniformity than an impregnated catalyst [27,29]. Around twice the hydrogen yield was produced from steam reforming of ethanol with a Ni/SiO2 prepared catalyst using a sol-gel method compared with that by an impregnation method [30].It is known that the co-precipitation, impregnation and sol-gel methods have been adopted as suitable metal loading alternatives for catalyst synthesis. However, the published literatures concerning the comparison of Ni catalyst made by these three methods for catalytic thermal processing of waste plastics are limited. Considering this, the aim of this present work was to investigate Ni/Al2O3 catalysts prepared via co-precipitation, impregnation and sol-gel methods for the pyrolysis-steam reforming of waste plastics. The catalyst activity was evaluated in terms of the hydrogen and carbon monoxide production, as well as the catalyst coke formation. In addition, it has been shown that different plastics show different pyrolysis behaviour, producing different product hydrocarbons, which may effect the catalytic steam reforming process and catalyst coke formation and product distributions [31,32]. Therefore, the influence of the type of plastic feedstock on the product selectivity and catalyst activity was also investigated.This work follows on from our previous reports [4,12,33] which investigated the influence of different types of catalyst and process parameters on the pyrolysis catalytic steam reforming of waste plastics in relation to hydrogen productionThree different waste plastics, high density polyethylene (HDPE), polypropylene (PP) and polystyrene (PS), which are the most common plastic wastes worldwide, were supplied by Regain Polymers Limited, Castleford, UK. Plastics were collected from real-word waste plastics and mechanically recycled to produce 2\u20133\u2009mm spheres. The ultimate analysis of the plastic wastes were determined using a Vario Micro Element Analyser, and the results are shown in Table 1\n. The proximate analyses including moisture, volatiles and ash content of waste plastics were conducted according to ASTM standards E790, E897 and E830, respectively. Briefly, the moisture content was determined by placing 1\u2009g of plastic uniformly in a sample boat in an oven at 105\u2009\u00b0C for 1\u2009h. The measurement of volatiles content was operated by using a sealed crucible containing 1\u2009g of plastic in an electric furnace at 950\u2009\u00b0C for 7\u2009min, while the ash content was obtained by placing 1\u2009g of plastic in a sample boat in air at 550\u2009\u00b0C for 1\u2009h. Results were summarized in Table 1. As the plastics used in this work were from real-world applications instead of the pure polymers, some additives may have been present in the samples. For example, oxygen was detected for the elemental analysis, whereas it would not be present in the pure polymer. Waste HDPE was observed to have the highest ash content of 4.98\u2009wt.%, while the other two plastics show little ash content.The Ni/Al2O3 catalysts prepared using co-precipitation method (Ni/Al-Co), impregnation method (Ni/Al-Im) and sol-gel method (Ni/Al-Sg) were tested to catalyse the pyrolysis-reforming of waste plastics. Ni/Al-Im was obtained by a conventional wet impregnation method. 10\u2009g \u03b3-Al2O3 and 5.503\u2009g Ni(NO3)2\u00b76H2O (corresponding to Ni loading of 10\u2009wt.%) were mixed in deionized water. The mixture was then stirred using a magnetic stirring apparatus at 100\u2009\u00b0C until it turned into slurry. The precursor was dried overnight and calcined at 750\u2009\u00b0C for 3\u2009h. The prepared Ni/Al-Co catalyst involved mixing the metallic nitrates of 7.43\u2009g Ni(NO3)2\u00b76H2O and 99.34\u2009g Al(NO3)3\u00b79H2O (Sigma-Aldrich) together with 150\u2009ml deionized water, so that a 10\u2009wt.% Ni loading was obtained. The solution was kept at 40\u2009\u00b0C with moderate stirring, then the precursor was precipitated with NH4(OH) dropwise until the final pH of around 8 was achieved. The precipitates were filtered and washed with deionized water and then dried at 105\u2009\u00b0C overnight, followed by calcination at 750\u2009\u00b0C in air for 3\u2009h. The Ni/Al-Sg catalyst with the same Ni loading of 10\u2009wt.% was prepared by a simple sol-gel method. 20\u2009g of Aluminium tri-sec-butoxide (ATB, Sigma-Aldrich, 97%) was firstly dissolved into 150\u2009ml absolute ethanol (>99.5%, Merck) and stirred for 2.5\u2009h at 50\u2009\u00b0C. 2.210\u2009g of Ni(NO3)2\u00b76H2O was dissolved in 8\u2009ml deionised water separately to form the Ni precursor. Then the Ni solution was pipetted into the support solution while maintaining stirring at 75\u2009\u00b0C for 0.5\u2009h. 1\u2009M HNO3 was added into above solution until the pH of 4.8 was obtained. After drying at 105\u2009\u00b0C overnight, the precursors were calcined at 450\u2009\u00b0C in air for 3\u2009h.All of the catalysts were ground and sieved with a size range between 50 and 212\u2009\u03bcm. The catalysts used in this work were reduced in 5\u2009vol.% H2 (balanced with N2) atmosphere at 800\u2009\u00b0C for 1\u2009h before each experiment.A schematic diagram of the pyrolysis-catalytic steam reforming reactor system for waste plastics is shown in Fig. 1\n [33]. The experimental system consisted essentially of a continuous steam injection system using a water syringe pump, a nitrogen gas supply system, a two-stage stainless tube reactor, a gaseous product condensing system using dry ice, and gas measurement system. The reactor has two separate heating zones, i.e. first stage plastic pyrolysis reactor of 200\u2009mm height and 40\u2009mm i.d; second stage catalytic reactor of 300\u2009mm height and 22\u2009mm i.d. The real temperatures of two zones were monitored by thermocouples placing in the middle of each reactor and controlled separately. The calibration of the reactor temperature was performed before this set of experiments, and the temperature described in this paper was given as the real one. For each experiment, 0.5\u2009g of catalyst was loaded into the second stage where the temperature was maintained at 800\u2009\u00b0C. High purity nitrogen was supplied as the inert carrier gas. 1\u2009g of plastics were placed in the first stage and then heated from room temperature to 500\u2009\u00b0C at 40\u2009\u00b0C min\u22121, and the evolved volatiles passed into the catalyst reactor for reforming. Water was injected into the second stage with a flow rate of 6\u2009g h\u22121. After the reforming process, the condensable liquids were collected into condensers while the non-condensable gases were collected into a 25\u2009l Tedlar\u2122 gas sample bag off-line gas chromatography (GC) measurement. Each experiment was repeated to ensure the reliability of the results.The gas products were separated and quantified by packed column GCs. A Varian 3380 GC packed with 60\u201380 mesh molecular sieve, coupled with thermal conductivity detector (TCD) was used to analyse permanent gases (H2, O2, N2, CO). CO2 was determined by another Varian 3380 GC/TCD. Argon was used as the carrier gas for both GCs. Hydrocarbons (C1 to C4) were analysed using a different Varian 3380 GC/FID coupled with a HayeSep 80\u2013100 mesh molecular sieve column and using nitrogen as carrier gas. Each gas compound mass yield was calculated combining the flow rate of nitrogen and its composition obtained from the GC.The yield of non-reacted pyrolysis oil was calculated as the mass difference between fresh and used condenser system in relation to the total weight of plastic and steam input. Coke yield was determined from the temperature programmed oxidation analysis of the spent catalyst. Residue yield was measured as the mass difference between fresh and the used whole reactor system in relation to the total weight of plastic and steam input. Mass balance was therefore calculated as the sum of gas, liquid and residue obtained in relation to the total plastic and steam input.X-ray diffraction (XRD) analysis of the fresh catalysts was carried out using a Bruker D8 instrument with Cu K\u03b1 radiation operated at 40\u2009kV and 40\u2009mA. In order to explore the distribution of active sites on the catalysts, the Debye-Scherrer equation was used to obtain the average crystal size from the XRD results. The porous properties of the fresh catalysts were determined using a Nova 2200e instrument. Around 0.2\u2009g of each sample was degassed at 300\u2009\u00b0C for 2\u2009h prior to the analysis. The specific surface area was calculated using Brunauer, Emmett and Teller (BET) method. The total pore volume was determined at a relative pressure P/P0 of 0.99, and the pore distribution was obtained from the desorption isotherms via the BJH method. In order to determine the actual loading of nickel in the catalyst, a Optima 5300DV (Perkin Elmer Inc.) inductively coupled plasma optical emission spectrometer(ICP-OES)was used. About 25\u2009mg of catalyst was previously dissolved in acidic solution, followed by diluting with deionized water to 50\u2009ml in preparation for analysis.The morphologies of the fresh prepared catalysts and the coke deposited on the used catalysts were investigated using a Hitachi SU8230 scanning electron microscope (SEM), which was operated at 2\u2009kV and working distance of 3\u2009mm. An energy dispersive X-ray spectroscope (EDXS) was connected to the SEM to study the elemental distribution. A FEI Helios G4 CX Dual Beam SEM with precise focused ion beam (FIB) was used to analyse the cross-section of the prepared catalysts. Before the analysis, the catalyst was coated with platinum in order to protect the sample during the sectioning process. Fresh catalysts were further examined at a higher magnification by a high-resolution transmission electron microscope (TEM, FEI Tecnai TF20) coupled with a connected EDXS for microstructure and elemental distribution. For the TEM analysis preparation, samples were initially dispersed well in methanol using an ultrasonic apparatus, and were pipetted on to a carbon film coated copper grid. The coke deposited on the surface of catalyst was characterized by temperature programmed oxidation (TPO) with a Shimadzu TGA 50. For each TPO analysis, around 25\u2009mg of spent catalyst was heated from room temperature to 800\u2009\u00b0C in an air atmosphere (100\u2009ml min\u22121) at a heating rate of 15\u2009\u00b0C\u2009min\u22121 and a holding time of 10\u2009min at 800\u2009\u00b0C.The XRD patterns of the fresh catalysts are shown in Fig. 2\n. The Ni/Al2O3 catalyst prepared by the impregnation method produced sharp peaks compared to the other fresh catalysts. The easily identified peaks centred at 2\u03b8\u2009=\u200944.5, 51.9 and 76.4\u00b0 corresponding to the (111), (200) and (220) plane respectively, confirmed the presence of Ni (JCPDS: 01-087-0712) in the cubic form. The aluminium oxides at 37.6, 45.8, 66.8\u00b0 were also determined (00-029-0063). As there was no NiO detected from the XRD results, it demonstrates that the nickel catalyst precursors had been completely reduced into active compounds (Ni) before each experiment. According to the Scherrer equation, the average crystallite size of Ni based on the main peak at around 2\u03b8 at 44.5\u00b0 was determined to be 26.17, 52.28 and 19.69\u2009nm for the Ni/Al-Co, Ni/Al-Im and Ni/Al-Sg catalysts, respectively. This indicates that a higher Ni dispersion and smaller Ni particles were found for the catalyst prepared by the sol-gel method compared with impregnation and co-precipitation.\nTable 2\n summarizes the BET surface areas and pore size properties of the fresh nickel Ni/Al2O3 catalysts. The Ni/Al-Co and Ni/Al-Im catalyst showed surface areas of 192.24 and 146.41\u2009m2\u2009g\u22121, respectively. The Ni catalyst produced via the sol-gel method showed a higher surface area of 305.21\u2009m2 \u2009g\u22121 compared to the catalysts obtained by impregnation or co-precipitation. The Ni/Al-Sg catalyst also gave the highest pore volume of 0.915\u2009ml g\u22121 while Ni/Al-Im generated the lowest. However, the average pore size of these three catalysts were similar, at around 6.6\u2009nm. Therefore, it indicates that the Ni catalyst prepared by the sol-gel method gives a more porous structure compared to the other two methods. The adsorption/desorption isotherms and pore size distribution of fresh catalysts are shown in Fig. 3\n. All of the physisorption isotherm types for the three catalysts appear to be type IV according to the IUPAC classification [34]. From the pore size distributions, the Ni/Al2O3 catalyst prepared by the sol-gel method shows a quite narrow pore size distribution, while the impregnated prepared catalyst shows a broad distribution. This indicates that compared with Ni/Al-Co and Ni/Al-Im catalyst, the Ni/Al-Sg catalyst produces a more uniform porous structure, and most pores are with a size of around 6.64\u2009nm. Therefore, it may be concluded that a mesostructured Ni/Al2O3 catalyst can be obtained by the sol-gel preparation method. The results of the real nickel loading from ICP-OES analysis was also listed in Table 2. It can be seen the real content of Ni in the co-precipitated and sol-gel catalyst was a little lower than the designed value, while it was excellent agreement for the impregnated Ni/Al-Im catalyst. In summary, the active Ni sites were successfully loaded into the catalyst by different preparation method.The morphologies and the distribution of active metallic Ni for the fresh catalysts were determined by SEM-EDX analysis, as shown in Fig. 4\n. Compared with the Ni/Al-Co catalyst shown in Fig. 4, which shows a flat surface, the catalyst particles of Ni/Al-Im observed were irregular. The nickel catalyst prepared by the sol-gel method seems to be composed of many small particles in a loose structure. The Ni EDX mapping showed a uniform distribution of Ni particles in the catalysts. In order to investigate the inner structure of the fresh catalysts, the cross-sectional morphologies of catalyst particles were examined by FIB/SEM. From Fig. 5\n, the Ni/Al-Co catalyst which has a relatively low surface area (Table 2) shows a tight structure, whereas the Ni/Al-Sg catalyst shows a porous inner structure. The observations agree well with the porosity results that show the Ni/Al-Sg catalyst generates a higher surface area and higher pore volume compared with the other catalysts. This type of structure was reported to benefit Ni penetration inside the catalyst particles, and further promote the catalyst activity [33].The fresh catalysts were further examined under high magnification by TEM and the results are shown in Fig. 6\n. The images show obvious dark spots, which were ascribed to the presence of metallic Ni. As can be seen, all the Ni particles were well dispersed, and hardly any agglomeration was seen. Statistical analysis of the Ni particle size distributions of the three TEM images was carried out by ImageJ software, and the results are shown in Fig. 6(d)\u2013(f). More than 95 percent of the Ni particles present were of a size less than 50\u2009nm. The Ni/Al catalyst prepared by the sol-gel method showed the narrowest size distribution, with the smallest average particle size of 15.40\u2009nm. Both Ni/Al-Co and Ni/Al-Im catalysts have ta size distribution concentrated at 15\u223c30\u2009nm, but they show larger average particle size of 28.91 and 29.60\u2009nm, respectively. Therefore, the sol-gel prepared catalyst exhibited the highest homogeneity and smallest active metal size among the three catalysts, which is in good agreement with the results from XRD analysis. The EDX mappings of the sol-gel synthesized Ni/Al catalyst shown in Fig. 6(g) also demonstrate that both the Ni and Al were uniformly distributed inside the catalyst.The use of different nickel catalysts prepared via co-precipitation, impregnation and sol-gel methods for the pyrolysis-catalytic steam reforming of waste polyethylene was investigated in this section. The results of syngas production and gas composition are summarized in Table 3\n. The mass balance of all the experiments in this paper were calculated to be in the range of 92 to 98\u2009wt.%. In addition, results from the repeated trials show that the standard deviations of the hydrogen and carbon monoxide yield were 0.26 and 0.29\u2009mmol g\u22121\nplastic respectively. For the volumetric gas concentrations, the standard deviation was 0.26% for H2 and 0.08% for CO respectively. These data indicates the reliability of the experimental procedure. From Table 3, the highest hydrogen yield of 60.26\u2009mmol g-1\nplastic was obtained with the Ni/Al catalyst prepared by the sol-gel method, followed by that prepared by the impregnation method. The lowest hydrogen yield of 43.07\u2009mmol -1\nplastic was obtained with the Ni/Al-Co catalyst. The production of carbon monoxide has the same trend as that of hydrogen yield. Syngas production achieved its maximum with the Ni/Al-Sg catalyst, that is, per unit mass of the polyethylene can yield 83.28\u2009mmol of syngas. The gas composition is also shown in Table 3. It can be observed that the concentration of H2 and CO2 were steadily increased with the catalyst order: Ni/Al-Co\u2009<\u2009Ni/Al-Im\u2009<\u2009Ni/Al-Sg, while the content of CH4, CO and C2-C4 were decreased correspondingly. During the pyrolysis-reforming of waste plastic, the thermal decomposition of plastic occurs in the pyrolysis stage as Eq. (1). The pyrolysis volatiles were then steam reformed by the catalyst to produce more valuable gases like hydrogen and carbon monoxide (Eqs. (2) and (3)). As the CH4 and C2\u2013C4 concentrations from Ni/Al-Sg were rather lower, while H2 and CO yields were significantly higher than those from the other two catalysts, it can be concluded that the steam reforming of hydrocarbons (Eq. (2)) was greatly promoted in the presence of the Ni/Al-Sg catalyst. In addition, the ratio of H2 to CO, which can reveal the degree of waster gas shift reaction Eq. (3), achieved its maximum of 2.62 with the Ni/Al-Sg catalyst. Therefore, the nickel catalyst prepared by sol-gel method displayed the highest activity to both hydrocarbons reforming and water gas shift reactions among the three catalysts investigated.\n\n(1)\nCxHyOz \u2192 (CH4 + H2 + C2-4 + CO + \u2026) + Tar\u2009+\u2009Char\n\n\n\n\n(2)\nCxHy + H2O \u2192 CO + H2\n\n\n\n\n\n(3)\nCO + H2O \u21d4 CO2 + H2\n\n\n\nTemperature programmed oxidation was used to investigate the coke deposition on the used catalyst. As shown in Fig. 7\n, the oxidation process involved three main stages: the removal of water in the range of 100\u223c300\u2009\u00b0C, the oxidation of Ni from 300 to 450\u2009\u00b0C, and carbonaceous coke combustion from 450\u2009\u00b0C onwards, which were also observed in our previous studies [35]. The amount of coke was calculated based on the weight loss of spent catalyst from 450\u2009\u00b0C (when Ni had finished oxidization and coke started to combust) to 800\u2009\u00b0C, and the results are shown in Table 3. It can be observed that, during the pyrolysis-catalytic steam reforming of waste polyethylene, the Ni/Al-Sg catalyst produced the highest coke yield of 7.41\u2009wt.% among the three catalysts, but displayed the highest catalyst activity for syngas production. In addition, the Ni/Al-Co catalyst which generated the lowest hydrogen yield produced the least coke formation. It should be noted that the catalytic volatiles thermal cracking Eq. (4) may also be involved during this process. The results regarding syngas production and coke yield with the three catalysts indicate that the Ni/Al-Sg catalyst showed high catalytic activity for both reforming reactions and volatiles thermal cracking reactions. The derivative weight loss thermograms in Fig. 7 showed two distinct peaks at temperatures around 530 and 650\u2009\u00b0C. It has been reported that the oxidation peak at lower temperature was related to amorphous coke, while the peak at higher temperature is linked to graphitic filamentous coke oxidation [12,36]. The coke deposited on the Ni/Al-Sg catalyst appears to be mainly in the form of filamentous carbon, which was also confirmed in the SEM morphology analysis shown in Fig. 8\n(c). While for the Ni/Al-Co catalyst SEM results shown in Fig. 8(a), more coke deposits without any regular shapes were observed. The larger production of amorphous coke on the spent Ni/Al-Co catalyst compared with the other two catalysts could also be responsible for the lower syngas production, since the amorphous coke was considered to be more detrimental to catalyst activity than the filamentous carbons. In addition, compared with the SEM results of the fresh catalysts shown in Fig. 4, the morphologies of the three nickel catalysts did not change significantly. For example, the catalyst prepared by the sol-gel method, maintained its loose structure after the reforming process, indicating the good thermal stability of the catalyst.\n\n(4)\nCxHy \u2192 C + H2\n\n\n\nPolypropylene was also investigated for the pyrolysis-catalytic steam reforming process for hydrogen production in the presence of the three different Ni/Al catalysts to produce more gases. The gas productions and concentrations are shown in Table 4\n. The Ni/Al-Sg catalyst displayed the most efficient catalytic activity in terms of the steam reforming of the polypropylene, as the gas yield was 144.03\u2009wt.% which was much higher than the other two catalysts. In addition, much higher hydrogen yield (67.00\u2009mmol g\u22121\nplastic) and carbon monoxide yield (29.98\u2009mmol g\u22121\nplastic) were obtained by using the Ni/Al-Sg catalyst. The syngas production from PP with the Ni/Al-Sg catalyst was slightly higher than was observed from HDPE, and this phenomenon can also be found with the Ni/Al-Co and Ni/Al-Im catalysts. This may be due to the higher hydrogen and carbon content and lower ash content of PP compared with HDPE (Table 1), and which suggests more effective hydrocarbons participation in the reforming reactions to obtain more syngas. The nickel catalyst prepared by the co-precipitation method showed the least activity for the reforming process, producing 46.05\u2009mmol H2\u2009g\u22121\nplastic and 20.39\u2009mmol CO g\u22121\nplastic. The composition of the gases from waste polypropylene were mainly composed of H2, CH4, CO, C2-4 hydrocarbons and CO2. The concentration of H2 and CO with Ni/Al-Sg achieved 59.38 and 26.57\u2009vol.%, respectively. The CH4 and C2-4 gases content with the Ni/Al-Sg catalyst were lower than the case with the other two catalysts, which also indicates the higher catalytic activity of the catalyst made by the sol-gel method.The amount and the type of coke deposition on the three catalysts from the pyrolysis-steam reforming of polypropylene were determined by TPO analysis, as shown in Fig. 9\n. The amount of carbonaceous coke was calculated and the results are shown in Table 4. The Ni/Al-Co and Ni/Al-Im catalysts produced around 5 to 6\u2009wt.% of coke, lower than the case of the Ni/Al-Sg catalyst which showed a 8.49\u2009wt.% coke yield. From the derivative weight loss results, the peak associated with amorphous carbon was much larger than that of the filamentous carbon with the Ni/Al-Co catalyst. However, for the Ni/Al-Im and Ni/Al-Sg catalysts, produced more filamentous carbons. This phenomenon was also observed with HDPE. It can be deduced that the nickel catalyst prepared by impregnation and sol-gel methods favour the production of filamentous carbonaceous coke from the pyrolysis-steam reforming of waste plastics. The presence of both amorphous carbon and filamentous carbon with the Ni/Al-Co and Ni/Al-Im catalyst were further confirmed by the SEM images shown in Fig. 10\n(a) and (b). Fig. 10(c) shows that the deposits on the used Ni/Al-Sg catalyst were predominantly filamentous carbon. Furthermore, there was a dense covering of carbon on the catalyst no matter which type of catalyst was used. The amount of carbon deposits from SEM images seems to be larger for PP than the case for HDPE, which was consistent with the TPO results.Wu and Williams [13] used an incipient wetness method (similar to the impregnation method in this work) prepared Ni/Al2O3 for steam gasification of PP. A potential H2 yield of 26.7\u2009wt.% was obtained, with the gaseous product containing 56.3\u2009vol.% of H2 and 20.0\u2009vol.% of CO. The syngas production can be calculated as 77.62\u2009mmol g\u22121\nplastic, which was close to the yield obtained in this study (75.77\u2009mmol g\u22121\nplastic). However, the coke deposition (11.2\u2009wt.%) was higher it from this study (5.34\u2009wt.%), which might be due to the lower surface area (90 m2/g) compared with Ni/Al-Im used here (146.41\u2009m2/g). High hydrogen yields of 21.9\u2009g g\u22121\nPP and 52\u2009wt.% (potential value) were obtained by the same authors from polypropylene with co-precipitation prepared Ni-Mg-Al (Ni:Mg:Al ratio\u2009=\u20091:1:1, 800\u2009\u00b0C, 4.74\u2009g h\u22121 steam) [37] and co-impregnated Ni/CeO2/Al2O3 catalyst (Ni 10\u2009wt.%, CeO2 20\u2009wt.%, 900\u2009\u00b0C) [14], respectively. It should be noted that the Ni catalysts in these literatures were prepared either at a higher loading or with promoter added, otherwise using at higher catalysis temperature. It also suggests that hydrogen production can be promoted by the use of effective catalyst promoters or by regulation of operational parameters. Czernik and French [38] concluded that many common plastics can be converted into hydrogen by thermo-catalytic process with a microscale reactor interfaced with molecular beam mass spectrometer. A bench-scale plastic pyrolysis-reforming system was also carried out by them using PP as a representative polymer, while 20.5\u2009g/h H2 was generated with a 60\u2009g/h of PP feeding rate.The product distributions in terms of gas yield and composition from the pyrolysis-catalytic steam reforming of waste polystyrene with the different catalysts are displayed in Table 5\n. The Ni/Al-Co catalyst produced a H2 yield of 51.31\u2009mmol g\u22121\nplastic which was a little lower than the yield of 55.04\u2009mmol H2\u2009g\u22121\nplastic with the Ni/Al-Im catalyst. Among the three catalysts, the Ni/Al-Sg catalyst produced the maximum H2 yield and CO yield per mass of plastic feedstock, which was also observed with HDPE and PP. However, compared with HDPE and PP, PS shows a comparatively higher yield of CO, with values up to 36.10\u2009mmol g\u22121\nplastic with the sol-gel prepared catalyst. Most of the concentrations of CO and CO2 obtained from PS were also larger than the corresponding data from PP or HDPE. It may due to the higher content of elemental carbon in the feedstock. In addition, as the coke yield produced using PS was lower than from PP or HDPE except from those with Ni/Al-Co, it suggests most of the carbon in PS was converted into gas product by participating in catalytic steam reforming reactions Eq. (2) or the water gas shift reaction Eq. (3). The hydrogen content of the product gases fluctuated slightly in the range of 57.90 and 59.32\u2009vol.% depending on the different catalyst applied. The hydrocarbons in the final gas product were relatively low for PS whichever type of catalyst was used, and the concentration of C2-C4 was less than 1.10\u2009vol.%. The water gas shift reaction Eq. (3) was promoted by the Ni/Al-Co catalyst as the H2/CO ratio was higher than with the other catalysts.TPO analysis of the used catalysts from the pyrolysis-catalytic steam reforming of polystyrene was also carried out to characterize the carbonaceous coke deposition on the catalyst, as shown in Fig. 11\n. The results of the calculated amount of coke produced are shown in Table 5. The Ni/Al-Sg catalyst produced the highest coke yield of 6.14\u2009wt.% even though it produced the largest hydrogen production amongst the three catalysts. It suggests that both steam reforming and decomposition of hydrocarbons Eq. (3) and Eq. (4) were significantly facilitated with the Ni/Al-Sg catalyst during the pyrolysis-steam reforming of waste polystyrene. As for the type of carbon deposits, overlapping derivative weight loss peaks were observed with the Ni/Al-Im and Ni/Al-Sg catalysts, indicating that both amorphous and filamentous coke were produced. This is in agreement with the morphologies observed by SEM images shown in Fig. 12\n. The Ni/Al catalyst prepared by co-precipitation displayed the deposits in a great proportion of amorphous form, and the derivative TPO peak at lower temperature was more significant than that at higher oxidation temperature.The yield of hydrogen and carbon monoxide from pyrolysis-steam reforming of waste plastics varied with the catalyst preparation method used. Overall, despite the difference in the feedstock, the sol-gel prepared nickel catalyst produced the highest syngas production, while the co-precipitation prepared catalyst produced the lowest syngas production among the three catalysts investigated. In addition, the maximum carbonaceous coke deposition on the catalyst was also obtained with the Ni/Al-Sg catalyst. This suggests that both the hydrocarbon reforming reactions and the hydrocarbon thermal decomposition reactions were promoted more in the presence of the sol-gel prepared catalyst. For example, the largest production of H2 and CO was obtained with the Ni/Al-Sg catalyst with waste polypropylene at 67.00\u2009mmol H2\u2009g\u22121\ncatalyst and 29.98\u2009mmol CO g\u22121\ncatalyst and also the highest coke yield of 8.49\u2009wt.%. This is in agreement with previous results from Efika et al. [39] that a sol\u2013gel prepared NiO/SiO2 catalyst generated higher syngas yield than the catalyst made by an incipient wetness method, and the former one also appeared to have more carbon formation on its surface. Although the syngas production achieved the maximum at the presence of Ni/Al-Sg, it should still be noted that the CO content was relatively high. It may be related to the high reforming temperature which was unfavourable to the Reaction (3) due to the exothermic nature of the reaction [40]. Furthermore, a dual functional Ni catalyst with both catalysis and CO2 sorption, for example, a sol-gel prepared Ni/Al catalyst coupled with CaO, was suggested for further study, in order to promote the WGS reaction for higher H2 yield [37,41].The catalytic performance in terms of hydrogen yield and CO production was also influenced by physicochemical characteristics e.g. the porosity, and the type of coke deposited. In particular, the increase in the surface area and pore volume could not only improve the dispersion of metal ions, but also facilitate the interaction of reactant molecules with the catalyst internal surface [42]. In addition, the catalyst was generally deactivated by two types of carbonaceous coke, amorphous (or monoatomic) and filamentous carbon. The filamentous carbon was found to have little influence on the catalytic activities, while the amorphous carbon has been reported to be more detrimental to catalyst activity [13]. Furthermore, these two factors are associated with each other, as Li et al. [43] have suggested that the catalyst activity can be improved by uniform Ni dispersion, while uneven distribution and large Ni particles are the main reason for the formation of non-filamentous coke which leads to the loss of catalyst activity.In this work, the sol-gel prepared Ni catalyst showed a high surface area and uniform Ni dispersion, as evidenced from the BET and TEM results. Furthermore, the coke obtained is in the filamentous form (from TPO and SEM results). Therefore, the Ni/Al-Sg prepared catalyst presents an excellent catalytic performance towards syngas production for the pyrolysis-catalytic steam reforming of waste plastics. However, for the co-precipitation prepared Ni catalyst, the catalyst coke deposits were found to be of the monoatomic or amorphous type, though it showed a higher surface area than the Ni/Al-Im catalyst. It suggests that a high activity at the initial reaction stage may occur, but it experienced a rapid deactivation by detrimental coke deposition.The hydrogen and carbon monoxide yield from the pyrolysis-catalytic steam reforming of PP was higher than that observed for HDPE, no matter which catalyst was applied, indicating more syngas production can be obtained per mass of PP compared to HDPE in this work. This may be due to the fact that PP had higher H and C elemental contents compared to HDPE (Table 1), while the ash content of HDPE was relatively higher. In addition, PS was found to produce the highest CO and syngas yield among the three plastics. Barbarias et al. [44] investigated the valorisation of PP, PE, PS and PET for hydrogen production by pyrolysis-catalytic steam reforming. The pyrolysis volatiles at 500\u2009\u00b0C were identified, and results show that nearly 100\u2009wt.% of PS was converted into volatiles with 70.6\u2009wt.% of styrene, while more than 65\u2009wt.% of wax were obtained from polyolefins. Therefore, the higher syngas production from PS in this study may due to the fact that more styrene from PS pyrolysis instead of the wax from polyolefins were introduced into the steam reforming stage. They concluded that the H2 yields from PS was lower than those from polyolefins, while the H2 production from PS in this study was comparative even higher than those from HDPE and PP. Around 38, 35 and 30\u2009wt.% of hydrogen yield were achieved by the same authors [44,45] at 16.7 gcat min g\u22121\nplastic of space time, 700\u2009\u00b0C from HDPE, PP and PS respectively. The difference between those values and the yield in this work were attributed to the different reactor system as well as the operational parameters.However, in this study, it is still difficult to evaluate the ability of each plastic for H2 and CO production in relation to C or H elemental content. Therefore, CO conversion Eq. (5) and H2 conversion Eq. (6) were calculated, to reveal the degree of C or H in the gas product. These two indicators essentially reflect the reforming ability of each plastic by catalyst towards H2 or CO. In addition, the coke conversion was also calculated as Eq. (7).\n\n(5)\nCO conversion (wt.%) = (C content in CO gas) / (C content in raw plastic)\n\n\n\n\n(6)\nH2 conversion (wt.%) = (H content in H2 gas) / (H content in raw plastic)\n\n\n\n\n(7)\nCoke conversion (wt.%) = (Coke yield per unit mass of plastic)/ (C content in raw plastic)\n\n\nThe results of these indicators with the Ni/Al-Sg catalyst was taken as an example in relation to the different plastics and the results presented in Fig. 13\n (the results with the other two catalysts are not presented here, but they show a similar trend). From Fig. 13, the ability for H2 production of HDPE and PP was rather close, as the H2 conversion obtained was around 95\u2009wt.%. However, the H2 conversion was significantly increased in the presence of PS, with the highest conversion of 145.11\u2009wt.%. The conversion of over 100 percent was due to the production of H2 from H2O. The CO conversion was gradually increased with the order: HDPE\u2009<\u2009PP\u2009<\u2009PS and suggests that the steam reforming reaction of hydrocarbons (Eq. (2)) was more favourable with PS, generating more H2 and CO. The maximum syngas production of 98.36\u2009mmol g\u22121\nplastic was obtained using PS with the Ni/Al-Sg catalyst. In regard to the gas compositions from different plastics, the molar ratio of H2/CO achieved was in the range of 1.72 to 2.62, and it was relatively higher from the polyolefin plastics. Therefore, there should be potential in industrial applications in that the H2 to CO ratio can be tuned to meet the desired ratio by adjusting the mixed proportion of different plastics.From the TPO results related to catalyst carbon coke deposition, PP generated the highest coke yield of 8.48\u2009wt.%, but from Fig. 13, it can be seen that the calculated carbon conversion was in order of PP>HDPE\u2009>\u2009PS, even though PS has more C content in the feedstock. The results suggest that the coke formation by decomposition of hydrocarbons Eq. (4) was more favourable in the presence of PP. Wu and Williams [37] also found that PP generated the highest coke deposition on used Ni catalysts when the catalyst temperature was 800\u2009\u00b0C with a water flow rate of 4.74\u2009g h\u22121, compared with HDPE and PS. Also, PS produced relatively lower coke yield among the three plastics under variable process conditions. A similar trend was also reported by Acomb et al. [46], when exploring the pyrolysis-gasification of LDPE, PP and PS, as higher residue yields were obtained from LDPE and PP. Furthermore, the reforming temperature in this work was 800\u2009\u00b0C, and Namioka et al. [7] also found that the coke deposition of PP was more apparent than that of PS at higher reforming temperatures (>903\u2009K).In relation to the type of carbon deposition on the catalyst, the results show that the carbon was mainly in the form of the filamentous type from waste HDPE and PP (Fig. 7) and Fig. 9), while more amorphous carbon was produced from PS (Fig. 11). This phenomenon was especially evident for the Ni/Al-Im and Ni/Al-Sg catalysts, which generated both types of carbon. This can be explained by the difference in the gas composition, as Angeli et al. [47] suggested that the increase in the C-number in the mixed gases favoured the formation of filamentous carbon, and Ochoa et al. [48] reported that the carbonization of adsorbed coke to form multi-walled filamentous carbon can be promoted by the reaction of CH4 dehydrogenation. In this work, HDPE and PP produced a higher content of C1-C4 hydrocarbon gases compared with PS (comparing Tables 3\u20135), resulting in the two polyolefin plastics producing more filamentous carbon on the used catalyst.The Ni/Al catalyst prepared by the sol-gel method generated higher H2 and CO yields from waste plastics than the catalysts prepared by co-precipitation and impregnation, due to the higher surface area and fine nickel particle size with uniform dispersion.The Ni/Al-Co catalyst prepared by co-precipitation produced the least syngas yield among three catalyst preparation methods investigated. From the TPO results, the type of carbon deposited on the Ni/Al-Co catalyst was mainly amorphous type carbon while it was in filamentous form for the impregnation (Ni/Al-Im) and sol-gel (Ni/Al-Sg) prepared catalysts.Thermal decomposition reactions were more favoured with olefin type plastics (HDPE and PP) to produce higher hydrogen and coke, whereas the steam reforming reactions were more significant with polystyrene. The maximum H2 yield of 67.00\u2009mmol g1\nplastic was obtained from pyrolysis-catalytic steam reforming of waste polypropylene with more hydrocarbons in the product gases, while waste polystyrene generated the highest syngas yield of 98.36\u2009mmol g1\nplastic with more oxygen-containing gases in the produced gases.The authors wish to express their sincere thanks for the financial support from the National Natural Science Foundation of China (51622604) and the Foundation of State Key Laboratory of Coal Combustion (FSKLCCB1805). The experiment was also assisted by the Analysis Laboratory in the School of Chemical and Process Engineering at the University of Leeds and Analytical and Testing Center in Huazhong University of Science & Technology (Wuhan, China). This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowski-Curie grant agreement No. 643322 (FLEXI-PYROCAT).", "descript": "\n                  Three Ni/Al2O3 catalysts prepared by co-precipitation, impregnation and sol-gel methods were investigated for the pyrolysis-steam reforming of waste plastics. The influence of Ni loading method on the physicochemical properties and the catalytic activity towards hydrogen and carbon monoxide production were studied. Three different plastic feedstocks were used, high density polyethylene (HDPE), polypropylene (PP) and polystyrene (PS), and compared in relation to syngas production. Results showed that the overall performance of the Ni catalyst prepared by different synthesis method was found to be correlated with the porosity, metal dispersion and the type of coke deposits on the catalyst. The porosity of the catalyst and Ni dispersion were significantly improved using the sol-gel method, producing a catalyst surface area of 305.21\u2009m2/g and average Ni particle size of 15.40\u2009nm, leading to the highest activity among the three catalysts investigated. The least effective catalytic performance was found with the co-precipitation prepared catalyst which was due to the uniform Ni dispersion and the amorphous coke deposits on the catalyst. In regarding to the type of plastic, polypropylene experienced more decomposition reactions at the conditions investigated, resulting in higher hydrogen and coke yield. However, the catalytic steam reforming ability was more evident with polystyrene, producing more hydrogen from the feedstock and converting more carbon into carbon monoxide gases. Overall the maximum syngas production was achieved from polystyrene in the presence of the sol-gel prepared Ni/Al2O3 catalyst, with production of 62.26\u2009mmol H2\u2009g\u22121\n                     plastic and 36.10\u2009mmol CO g\u22121\n                     plastic.\n               "}
{"full_text": "No data was used for the research described in the article.Greenhouse gases such as CO2 and CH4 absorb and emit radiant energy, causing global warming [1]. The concentration of CO2 in the atmosphere has been rising despite nature\u2019s effort to curb it via the carbon cycle. Human activities have substantially contributed to this, with a 45\u00a0% increase since the age of the industrial revolution, from 280\u00a0ppm to 419\u00a0ppm in 2022 [2]. In addition, CH4 emissions resulting from processing oil and gas extraction and agriculture increase. It will become increasingly imperative to convert or eliminate these greenhouse gases to lower their atmospheric concentrations.The catalytic dry reforming of methane is a promising technology that could utilize these greenhouse gases. It combines these two molecules with a catalyst to produce synthesis gas (H2 and CO):\n\n(1)\n\n\nC\n\n\nH\n\n\n4\n\n\n+\nC\n\n\nO\n\n\n2\n\n\n\u2194\n2\n\n\nH\n\n\n2\n\n\n+\n2\nC\nO\n\n\u0394\n\n\nH\n\n\n298\nK\n\n\n\n=\n\n247\n\nk\nJ\n/\nm\no\nl\n\n\n\n\nThus, CH4 and CO2 emissions could potentially be reduced to some extent [3\u20136]. Alternatively, synthesis gas is a crucial feedstock for producing chemicals and fuels via Fischer-Tropsch synthesis, methanol, and dimethyl ether [7,8].DRM is accompanied by secondary reactions such as the reverse water\u2013gas shift reaction, methane decomposition and CO disproportionation:\n\n(2)\n\n\n\n\nCO\n\n\n2\n\n\n+\n\n\nH\n\n\n2\n\n\n\u2194\n\n\nH\n\n\n2\n\n\nO\n+\nC\nO\n\n\u0394\n\n\nH\n\n\n298\nK\n\n\n\n=\n\n41\n\nk\nJ\n/\nm\no\nl\n\n\n\n\n\n\n(3)\n\n\nC\n\n\nH\n\n\n4\n\n\n\u2194\nC\n+\n2\n\n\nH\n\n\n2\n\n\n\n\u0394\n\n\nH\n\n\n298\nK\n\n\n\n=\n\n75\n\nk\nJ\n/\nm\no\nl\n\n\n\n\n\n\n(4)\n\n\n2\nC\nO\n\u2194\nC\n+\nC\n\n\nO\n\n\n2\n\n\n\n\u0394\n\n\nH\n\n\n298\nK\n\n\n\n=\n\n-\n172\n\nk\nJ\n/\nm\no\nl\n\n\n\n\nCatalyst selection must consider the endothermic nature of the reaction, which necessitates a high temperature and may result in the formation of carbon deposits according to equations 3 and 4 [9,10]. Many researchers tried to improve the reaction parameters such as temperature, WHSV, CH4 and CO2 ratio to enhance the DRM efficiency. Although high temperature is favorable for endothermic reactions, it may cause metal agglomeration and sintering, while carbon deposits block active metal sites, resulting in the deactivation of the catalyst [11]. Appropriate choice of the active metal, support, promoter, structure and methods for preparation and activation are considered as tools to enhance the activity of DRM [12]. Strong metal-support interaction is considered one of the key properties to provide a stable and active catalyst by proposing high surface area, highly dispersed small active metal particles [12,13]. Several nobel and transition metals (e.g., Pt, Ru, Rh, Co and Ni) have been evaluated as active phases in DRM [14,15]. Except for Ni, which is inexpensive and has excellent catalytic performance [16], their promising results are constrained by their high costs. However, Ni is also rendered inactive by sintering and coking. Researchers have proposed numerous methods for improving the performance of Ni-based catalysts. Synthesis methods were shown to improve acidity and alkalinity and redox character properties of Ni active metal sites for DRM [17]. Ni supported Ce catalysts prepared using three different methods: microemulsion, sol\u2013gel and auto-combustion. The last showed the best catalytic performance in DRM due to the presence of small monoclinic domains of NiO dispersed and strongly stabilized on the partially reduced ceria surface [18\u201320]. On the other hand, preparing Ni on CeO2 with the precipitation method enhanced the gasification of coke due to the contribution of oxygen in CeO2 support and prevented Ni sintering due to metal-support interaction (MSI) [21]. Furthermore, other factors can play a role: i) improving the active metal dispersion by suitable high-surface-area supports [22\u201324] or ii) using supports such as SiO2-Al2O3, ZrO2-Al2O3, MgAl2O4\n[25], iii) the use of promoters to increase the MSI effect [26], and iv) the introduction of active sites capable of activating CO2 for carbon removal via the reverse Boudouard reaction (back reaction to eq. (4)) [27].It is well known that the rate of CO2 reforming of methane depends on the dispersion and nature of metal clusters. Regardless of thermodynamic and transport effects, the carbon formation rate increases as Ni crystal diameter increases [28]. Herein, we will focus on the use of mesoporous supports as interesting results have been reported with their application [29\u201331]. Taking advantage of confinement in such well-defined pore systems could be an alternative strategy for stabilizing small Ni particles.Mesoporous silica supports, such as SBA-15 and SBA-16, are excellent candidates for grafting Ni particles due to their uniform pore diameters, which could inhibit Ni particle agglomeration. Despite being used at elevated temperatures, the thick walls and well-arranged 2D hexagonal pore structures of these supports remain unchanged [7,32]. Furthermore, Ni can be easily supported on mesoporous silica by immobilizing Ni nanoparticles within the pores using the conventional impregnation method [33].Despite the excellent dispersion of Ni particles on the support achieved by this approach, Ni particle sintering can still occur in DRM at around 600\u00a0\u00b0C [34,35]. To improve the confinement of Ni particles, the addition of other active metals such as Co could be a solution [36]. Ni-Co-based catalysts are well known in the literature that their alloy and proper ratio of Co and Ni can affect the stability of the catalyst against coke formation. It was found that only Co supported on TiO2 and/or ZrO2 deactivates due to the oxidation of Co [36,37]. XAS showed the presence of NiCo alloy, which produced a negligible amount of deposited carbon, likely due to balancing the oxidative (CO2, H2O) and reductive (CH4, CO, H2) molecules leading to a long-life catalyst [38]. Another method was to support Ni on mesoporous materials such as SBA-15 and to modify them with Mg, Sc, and La oxides as promoters [33]. The evaluation of these catalysts produced intriguing findings. The CH4 conversion was increased by 28 and 26\u00a0%, respectively, for the Mg and Sc promoted catalysts compared to the parent catalyst, which reached about 53\u00a0% over 6\u00a0h on stream at 700\u00a0\u00b0C. The performance of these catalysts was attributed to the good dispersion of the active metals on the support enhanced by the added promoters. Also, La3+ and Sm3+ were added to Cu-doped to be used as supports for 5\u00a0wt.% Ni. Although Sm3+ showed a higher number of basic sites compared to La3+, it was found that La3+ modified Ni/Cu-CeO2 catalyst produced 13 times less carbon deposits due to the formation of small particles of Ni and NiCu alloy. Furthermore, the larger mobility of surface/subsurface lattice oxygen on La3+modified catalyst facilitated carbon removal [39]. The type and the amount of these surface/subsurface oxygen species depend on the nature of the dopants used, as has been reported in the literature [33,40]. The effects of promotors were studied, and the addition of Co to Ni showed higher activity compared to Ca addition, which was also not stable [41]. Furthermore, the method of preparation has an impact on the activity of the catalyst. Ni supported on Ce-Zr using surfactant was not active in DRM compared to coprecipitated material due to the covering the Ni active sites with Ce-Zr. However, the addition of Ce to Ni-ZrO2 was beneficial in terms of decreasing the amount of filamentous coke [42]. In the current work, we have examined the role of Sc in the catalyst system when using single active metal Ni over Sc-SBA-15 support, thereby avoiding the use of expensive Co. Consequently, mesoporous support SBA-15 was doped with different lower scandium loadings (0.5, 1, and 3\u00a0wt.%) for optimization. Thereafter, the catalysts were tested in DRM with various feed compositions at different temperatures and evaluated based on CO2, CH4 conversion, H2/CO ratio, and analysis of carbon deposits.SBA-15 was prepared by a method described in one of our previous publications [43]. Both promoter (0.5, 1, 3\u00a0wt.% of Sc) and active metal (5\u00a0wt.% of Ni) were incorporated into the structure of the support by sequential wet impregnation. At first, calculated amounts of Sc(NO3)3 to set Sc loadings of 0.5, 1, and 3\u00a0wt.% were dissolved in water (50\u00a0mg Sc/g solution). This was impregnated onto the support, dried overnight (120\u00a0\u00b0C) and calcined in air at 600\u00a0\u00b0C for 3\u00a0h. The specified amount of Ni(NO3)2\u00b76H2O, equivalent to 5\u00a0wt.% of Ni, was dissolved in 30\u00a0ml of water. This solution was subsequently impregnated onto the Sc-modified support. The resulting samples were then dried overnight and calcined in the air as done before. The catalyst precursors were designated as Ni-xSc/SBA-15 (x\u00a0=\u00a00.5, 1, 3\u00a0wt.%).The catalysts were tested in DRM in a stainless steel fixed-bed tubular reactor with an internal diameter of 0.94\u00a0cm and length of 30\u00a0cm (PID Eng & Tech micro activity reactor, Madrid, Spain). A load of 0.1\u00a0g of the catalyst powder (not diluted with inert) was placed over glass wool inside the reactor together with a thermocouple that was in contact with the catalyst bed to monitor the temperature. Before starting the reaction, the catalyst was activated in H2 atmosphere (30\u00a0ml/min) at 700\u00a0\u00b0C for one hour. Thereafter, N2 with a flow of 20\u00a0ml/min was used to purge the reactor of any residual H2. To ensure no remnant of H2 was present, GC runs were taken while the temperature was increased to 750\u00a0\u00b0C under flowing of N2. The reaction was performed at 1\u00a0bar, CH4/CO2/N2 feed ratio of 3:3:1 (respectively corresponding to 30, 30 and 10\u00a0ml/min volumetric flow rate), and 70\u00a0ml/min total flow rate and a space velocity of 42,000\u00a0ml/(gcat\u00b7h). The feed and product were analyzed by an online gas chromatograph (Shimadzu GC-2014) equipped with a combination of molecular sieve and Porapak Q columns and thermal conductivity detector (TCD). The calculation of conversion is based on the following relations:\n\n(5)\n\n\nC\n\n\nH\n\n\n4\n\n\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\n=\n\n\nm\no\nl\ne\n\no\nf\n\nC\n\n\nH\n\n\n4\n,\n\ni\nn\n\n\n-\nm\no\nl\ne\n\no\nf\n\nC\n\n\nH\n\n\n4\n,\n\no\nu\nt\n\n\n\n\nm\no\nl\ne\n\no\nf\n\nC\n\n\nH\n\n\n4\n,\n\ni\nn\n\n\n\n\nX\n100\n\n\n\n\n\n\n(6)\n\n\nC\n\n\nO\n\n\n2\n\n\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\n=\n\n\nm\no\nl\ne\n\no\nf\n\nC\n\n\nO\n\n\n2\n,\n\ni\nn\n\n\n-\nm\no\nl\ne\n\no\nf\n\nC\n\n\nO\n\n\n2\n,\n\no\nu\nt\n\n\n\n\nm\no\nl\ne\n\no\nf\n\nC\n\n\nO\n\n\n2\n,\n\ni\nn\n\n\n\n\nX\n100\n\n\n\n\n\n\n(7)\n\n\n\n\n\n\nH\n\n\n2\n\n\n\n\nC\nO\n\n\n=\n\n\nm\no\nl\ne\n\no\nf\n\n\n\nH\n\n\n2\n\n\n\np\nr\no\nd\nu\nc\ne\nd\n\n\nm\no\nl\ne\n\no\nf\n\nC\nO\n\np\nr\no\nd\nu\nc\ne\nd\n\n\n\n\n\n\nThe N2 adsorption\u2013desorption technique was used to determine the specific surface area of the samples using Tristar II 3020 (Micromeritics, Norcross, GA, USA). For the analysis, 0.2\u20130.3\u00a0g of the sample was taken and outgassed at 200\u00a0\u00b0C for 3\u00a0h. Adsorption-desorption isotherms were recorded at liquid N2 temperature of \u2212196\u00a0\u00b0C.X-ray powder diffraction (XRD) was done with a Miniflex diffractometer, (Rigaku Corporation, The Woodlands, TX, USA) with a Cu K radiation source and a nickel filter. The device was operated at 40\u00a0kV and 40\u00a0mA at a step size of 0.01\u00b0. The 2\u03b8 scanning range adopted for recording the diffraction patterns was 1\u20133\u00b0 for low-angle analysis and 10\u201380\u00b0 for wide-angle. The Joint Committee on Powder Diffraction Standards (JCPDS) database was used to identify the different phases from the diffractogram.The reducibility of the fresh catalysts was determined with AutoChem II (Micromeritics), where 0.075\u00a0g of sample was loaded into the sample tube holder. Samples were heated under pure Ar at 150\u00a0\u00b0C for 30\u00a0min, followed by cooling to room temperature. Thereafter, the temperature was raised to 900\u00a0\u00b0C at 10\u00a0K/min under a flow of 10\u00a0%H2/Ar at 40\u00a0ml/min. A TCD was used to monitor the H2 content at the outlet. Temperature-programmed CO2 desorption (CO2-TPD) was measured using automatic chemisorption equipment (Micromeritics AutoChem II 2920) with a TCD. A 70\u00a0mg sample was heated at 200\u00a0\u00b0C for 1\u00a0h under helium (He) flow to remove adsorbed components and then cooled. Then, CO2 adsorption was carried out at 50\u00a0\u00b0C for 60\u00a0min in a He/CO2 gas mixture (90:10\u00a0vol ratio) at 30\u00a0ml/min. Afterwards, the temperature was raised to 800\u00a0\u00b0C at 10\u00a0K/min while the TCD recorded the CO2 desorption signals.X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ELS5000 spectrometer (Omicron Nanotechnology) with a monochromatic artificial intelligence source. A beam of X-rays was released from a flood gun having about 400\u00a0\u03bcm radius. The samples were scanned at different ranges of 395\u2013415\u00a0eV, 526\u2013540\u00a0eV and 840\u2013900\u00a0eV at energy steps of 5, 2, and 10\u00a0eV, respectively. The pass energy used was 200\u00a0eV.Thermogravimetric analysis (TGA) was performed on a TGA-51 device (Shimadzu, Kyoto, Japan). The spent catalysts (about 10\u00a0mg) were recovered after stopping the reaction at ambient temperature and heated from room temperature to 1000\u00a0\u00b0C with 20\u00a0K/min under airflow at 50\u00a0ml/min.A high-performance Transmission Electron Microscope (TEM) analysis of the catalysts was made with a JEOL JEM-2100F operated at 120\u00a0kV accelerating voltage to study the morphology of the carbon deposits.\nFig. 1\n demonstrates that the bare support and the fresh samples have very similar N2 adsorption\u2013desorption isotherms of type IV with H1 type hysteresis loops due to capillary condensation at relative pressure above 0.5, according to IUPAC classification [44]. This is typical of mesoporous materials. At a relative pressure above 0.7, a sharp rise in the adsorption isotherm is observed for all samples, indicating that the well-ordered hexagonal framework of the support was maintained after metal impregnation [45]. When Sc and Ni were added to the SBA-15 support, the surface area and pore volume decreased because of pore blockage (Table 1\n). It is not surprising that the surface area decreased as the wt.% of promoter increased. All samples have an average pore diameter of approximately 6.5\u00a0nm, classifying them as mesoporous materials.Small-angle XRD analysis (Fig. 2\nA) shows three reflections appearing at 0.84, 1.45, and 1.68\u00b0 in the diffractograms, representing the (100), (110), and (200) planes. These reflections are typical for the 2-D hexagonal symmetry (space group p6mm) of the support SBA-15 [46]. The addition of Ni reduced the reflection intensity of the (100) plane, which completely vanished on adding 1% Sc.For the wide-angle diffraction patterns depicted in Figs. 2B-D, a diffraction line corresponding to amorphous silica from the support common appeared at around 22.7\u00b0 [47]. Fig. 2B compares the diffraction patterns of SBA-15 support with those of fresh 5Ni/SBA-15 catalyst. This clearly shows the presence of the crystalline NiO phase at the typical diffraction angles 37.2, 43.2, 62.8, 75.3, and 79.3\u00b0 (JCPDS 78\u20130643). These reflections belong to the corresponding crystallographic planes (111), (200), (220), (311), and (222) of NiO [48]. No additional peak was observed for samples containing Sc in addition to Ni (Fig. 2C, 2D). The only noticeable difference is a slight reduction in signal intensity representing the silica phase. These findings indicate that the support structure is highly stable.The reduction behavior of the support and the fresh calcined catalysts is shown in Fig. 3\n. The support exhibited no reaction with H2. The small peaks that appeared between 150 and 200\u00a0\u00b0C could be assigned to impurities present in the support as this is common to all the samples. The temperature at which the reduction peaks for Ni2+ to Ni0 appear is dependent on the interaction strength between NiO species and the support. The peaks below 400\u00a0\u00b0C are associated with the reduction of easily accessible or weakly bound NiO species, whereas the peaks above 500\u00a0\u00b0C represent NiO species having medium-strength interaction with the support [49]. Increasing the Sc loading reinforces the interaction between NiO species and the support by influencing their distribution and strength. Initially, the intensity of the weakly bound NiO species decreases with increasing Sc loading, while the intensity of the moderately bound NiO species becomes prominent. Secondly, there are shifts in the reduction temperature towards higher values with Sc content for both types of NiO species. The peaks above 700\u00a0\u00b0C represent the NiO species with a strong interaction with the support but are only noticeable at the highest Sc loading. A similar observation has been reported in studies that utilized Sc as a promoter [50,51].The CO2-TPD was performed with fresh 5Ni-xSc/SBA-15 samples to study their basic properties (Fig. 4\n). The curves in Fig. 4 (CO2-TPD) can be classified into weak, medium, and strong basicity regions, attributed to surface hydroxyl, surface oxygen anion and bulk oxygen anion/oxygen vacancy sites, respectively. The strength of the basic sites was classified as weak (less than 300\u00a0\u00b0C), medium (300 \u2013 400\u00a0\u00b0C), strong (400 \u2013 650\u00a0\u00b0C), and very strong (> 650\u00a0\u00b0C) depending on the CO2 desorption temperature [52]. These catalysts are characterized by medium basic sites as the CO2 desorption temperature maxima appear at around 270\u00a0\u00b0C. Compared to 5Ni/SBA-15, the incorporation of Sc increased the number of basic sites as the intensity of the peaks increased on increasing the amount of Sc.The XPS analysis of the fresh catalysts is shown in Fig. 5\n. The Ni 2p region, shown in Fig. 5a, exhibited a complex structure of the NiO doublet, with Ni 2p3/2\n peak at 855\u00a0eV, Ni 2p1/2\n at 873\u00a0eV, and pronounced satellite features at about 862 and 880\u00a0eV for the scandium-free sample [53]. The entire Ni 2p spectrum was slightly shifted to higher binding energies (by\u00a0\u223c\u00a01.5\u00a0eV) when adding 0.5\u00a0wt.% scandium. This shift, in turn, decreased with higher Sc loading. In the Sc 2p1/2\n region, a characteristic doublet at 403.3\u2013407.8\u00a0eV with a splitting of the two peaks by\u00a0\u223c\u00a04.5\u00a0eV was observed (Fig. 5b), assigned to oxides and hydroxides of scandium, respectively [54]. The O 1\u00a0s region (Fig. 5c) showed a main peak at about 532.8\u00a0eV, generated by SiO2 structures of the SBA-15 support. Because of Si 2p (not shown here), O 1\u00a0s and Sc 2p spectra exhibited rather stable binding energies in contrast to Ni 2p. A shift in the BE of O 1\u00a0s upon increasing the Sc content was observed, supposing that the added scandium seems to interact mainly with the nickel.Prior to the activity measurements with catalysts, blank tests were performed without active Ni metal using the same operating conditions. Table S1 displays the CH4 and CO2 conversions of the catalysts in the absence of Ni. The CH4 and CO2 conversions are below 2\u00a0% for the bare SBA-15 and in the presence of different Sc wt.%.The catalysts 5Ni-xSc/SBA-15 (x\u00a0=\u00a00, 0.5, 1, 3 wt.%) were tested at 750\u00a0\u00b0C, with a total feed flow rate of 70\u00a0ml/min and a space velocity of 42,000\u00a0ml/(gcat\u00b7h). The initial CH4 conversion was 76% without Sc, while it increased to 78% upon adding 0.5\u00a0wt.% Sc (Fig. 6\nA, 6B). Increasing the Sc loading above 0.5\u00a0wt.% lowered the CH4 conversion to approximately 70% for both 1 and 3\u00a0wt.% of Sc. As for the H2/CO ratio in Table 2\n, all the catalysts show high values (\u2265 0.94) and the sample with 0.5\u00a0wt.% Sc loading led to the highest value (0.99). The deviation from equity is explained by the competing reverse water\u2013gas shift reaction, which converts CO2 with the H2 produced via DRM to form CO and H2O. Obviously, the higher loading of Sc did not enhance the performance of the pristine catalyst 5Ni/SBA-15 [2,33]. On increasing the amount of Sc content above 0.5%, it is probable that the Sc covers the Ni active sites decreasing activity.All the catalysts showed acceptable stability with a low relative performance loss over 8\u00a0h on stream, as revealed in Table 2. 5Ni-0.5Sc/SBA-15 was the least deactivated catalyst, indicating that 0.5% Sc undoubtedly was the optimum loading.The H2-TPR profiles (section 3.3) show that the amount of NiO species decreases at low temperatures, while additional peaks appear at higher temperatures upon adding Sc. This may suggest that more NiO particles become strongly bonded to the support, making them difficult to reduce but, at the same time, more resistant to agglomeration. Besides, the XPS results confirm the increase in the binding energy (Ni 2p signal) with the Sc loading.For all the catalysts, CO2 conversion was higher than that of CH4. This suggests that CO2 was partly involved in the reverse water gas shift (RWGS) reaction, which converts CO2 with the H2 produced via DRM to form CO and H2O.The above-discussed tests were performed at the stoichiometric ratio of CH4 and CO2 (eq. 1), whereas in many technically relevant cases, the ratio is higher (CH4/CO2\u00a0=\u00a02:1), e.g., in biogas or natural gas. Fig. 7\n depicts the results obtained with the best-performing catalyst herein, 5Ni-0.5Sc/SBA-15, at a CH4/CO2 feed ratio\u00a0=\u00a02 (750\u00a0\u00b0C, space velocity\u00a0=\u00a042,000\u00a0ml/(gcat\u00b7h)).The initial CO2 conversion was around 69% and gradually decreased to 66% after 8\u00a0h on stream. Due to CH4 excess and the stoichiometry of DRM, CO2 is acting as the limiting reactant, and CH4 conversion in DRM cannot exceed 50% of the CO2 conversion unless other side reactions occur. With ongoing experiment, the CH4 conversion stabilized at around 32%. Additionally, carbon analysis of the used sample was performed to determine whether a side reaction had occurred. The corresponding TGA plot is available in the supplementary file (Fig. S3). The resulting weight loss assigned to the amount of carbon deposit is around 8\u00a0%. In addition, the measured H2/CO ratio is shown in Fig. 7B, where the values obtained during the reaction are close to unity, confirming that the contribution of side reactions is negligible.To get more insight into coke formation, an additional run was made using 5Ni-0.5Sc/SBA-15 with only CH4 and N2 as the feed at comparable conditions. The feed flow rate was set to 30 and 10\u00a0ml/min for CH4 and N2, respectively. The reaction was performed at 750\u00a0\u00b0C and 1\u00a0bar. The results are presented in the supplementary file (Fig. S1). No significant decomposition of CH4 was observed for this catalyst, as CH4 conversion was less than 0.5% over 4\u00a0h on stream.At the end of the 8\u00a0h runs with 5Ni-0.5Sc/SBA-15, the spent catalyst samples were recovered and were then subjected to thermal analysis in the air to quantify the amount of carbon deposits. The weight-loss curves are shown in Fig. 8\n. The minute weight gain recorded at the beginning of the analysis could be because of the time-delayed initialization of the precision balance. After that, the sample remained quasistatic until around 450\u00a0\u00b0C, where a hump was observed for all spent catalysts.\nFig. 8 reveals that Ni/SBA-15 has produced the highest amount of carbon deposits of about 8.0%, while the Ni-3Sc/SBA-15 catalyst formed the least amount of 2.5%. As the loading of Sc increases, the weight loss decreases. The catalysts promoted with 0.5 and 1\u00a0wt.% Sc demonstrated intermediate weight losses (4.0% and 3.5%, respectively). The basicity was more pronounced in 5Ni-3Sc/SBA-15, as evidenced by CO2-TPD characterization (section 3.4). This sample adsorbed more CO2, which consequently might enhance the gasification of deposited carbon at DRM conditions.A long-term test over 80\u00a0h was carried out using 5Ni-0.5Sc/SBA-15 catalyst (Fig. 9\n) at 1\u00a0bar, CH4/CO2/N2 feed ratio of 3:3:1, and a total flow rate of 70\u00a0ml/min. The initial conversions of CH4 and CO2 reached 78 and 84\u00a0%, respectively. The catalyst suffered only 14% activity loss over the complete time on stream to reach final conversions of 67.2 and 74% for CH4 and CO2, respectively. The loss in activity might be due to the partial oxidation of the catalyst in the presence of CO2. As for the H2/CO ratio, a value of about 0.97 was recorded at the start and 0.91 at the end (Fig. 9). The ratio below unity suggests the occurrence of reverse water gas shift reaction. This agrees with the observed higher CO2 conversion relative to that of CH4.This catalyst has good stability for many reasons, possibly due to the well-balanced reduction-CO2 adsorption cycle, which plays a role in the coke gasification, as discussed in section 3.6. It is expected that the catalyst will still be active if the rate of carbon deposition does not outweigh the rate of carbon gasification. On increasing, the amount of Sc content above 0.5%, it is probably that the Sc is covering the Ni active sites leading to the decrease in the activity [33]. This can be seen in the TPR profiles of the samples by observing the first peak, which decreased as the Sc loadings increased.\nFig. 10\n presents the TEM images of both fresh and spent samples of 5Ni/SBA-15, and 5Ni-0.5Sc/SBA-15 used to investigate the sintering of the Ni metal particles as well as the location and appearance of carbon deposits on the catalysts.TEM images confirm the hexagonal structure of the SBA-15 pores in the fresh Sc-free catalyst, as highlighted in the zoomed area in Fig. 10A. This agrees with the XRD analysis. For 5Ni/SBA-15 (Fig. 10A), the Ni particles appear to be partly confined within the pores of the support and partly localized at the mouth of the pores. As for the fresh 0.5% Sc promoted catalyst in Fig. 10C, the Ni particles are mainly dispersed within the pores except for some sparse particles appearing at the entrance of the pores. In addition to that, the SBA-15 support obviously retained its structure upon adding Sc and Ni. Thus, the addition of Sc facilitates the dispersion of Ni particles within the pores of the support.The particle size distributions as recorded by TEM for fresh and spent samples of 5Ni/SBA-15 and 5Ni-0.5Sc/SBA-15 are shown in Fig. 11\n. In the fresh 5Ni/SBA-15, the Ni particle size ranges from 2 to 20\u00a0nm, while the spent sample shows much larger Ni particles up to 40\u00a0nm. This is accompanied by a slight shift in mean particle size from 6.5\u00a0nm to 9.6\u00a0nm at the end of the reaction (Fig. 11 A, B). Carbon deposits are present in two morphologies: large filaments based on carbon nanotubes and larger Ni particles encapsulated in carbon spheres are visible (inset in Fig. 10B). Fig. 10D shows that the carbon deposits found on spent 5Ni-0.5Sc/SBA-15 are mostly multi-walled carbon nanotubes of varying diameters. Some encapsulated carbon could also be spotted, like for the Sc-free catalyst. The addition of Sc obviously assisted in preserving the structure of the support.As for the fresh and spent 5Ni-0.5Sc/SBA-15 catalyst (Fig. 11C, D), the mean particle size (2.5\u201313.5\u00a0nm) remained almost the same after reaction (2.5\u201315\u00a0nm). Compared to unpromoted samples, the presence of 0.5% Sc loading inhibits the agglomeration of active Ni metal particles.DRM was performed using Ni-xSc/SBA-15 catalysts (x\u00a0=\u00a00.5, 1, 3\u00a0wt.%). 5Ni-0.5Sc/SBA-15 catalysts outperformed and showed the highest CH4 and CO2 conversions of 78 and 86%, respectively, and the highest stability compared to the other catalysts. This can be explained based on the H2-TPR analysis, which showed that adding Sc strengthened the bonding between Ni and the support as found by shifting the reduction temperature on increasing the Sc loading leading to the stability of the catalyst. The XRD analysis revealed the presence of NiO on the as-prepared calcined catalysts, whereas the absence of reflections for Sc-containing phases indicated its high dispersion even at a 3\u00a0wt.% loading. The XPS studies displayed that the binding energy of Ni with the support increased by about 1.5\u00a0eV upon adding only 0.5\u00a0wt.% Sc and O 1s also showed that on increasing Sc loading above 0.5%, the interaction of Ni and Sc increases which might lead to the deactivation of the catalyst. Additionally, the CO2-TPD indicated that all catalysts have medium basic sites, and increasing the amount of Sc increases the basicity with an optimum of 0.5% Sc. The catalyst was stable over 80\u00a0h showing a loss of 8%.One of the reasons for catalyst deactivation besides coking is its partial oxidation with CO2. This was particularly observed for the sample with a 3\u00a0wt.% Sc load, which formed the least amount of carbon at 2.8%, despite exhibiting more pronounced deactivation. The TEM images of the spent catalyst 5Ni-0.5Sc/SBA-15 displayed that the Ni particle size remained unchanged after the reaction. Therefore, a catalytic test of 5Ni-0.5Sc/SBA-15 catalyst using a CH4/CO2 feed ratio\u00a0=\u00a02 to simulate biogas feed was performed. The results indicated that dry reforming reaction was predominant under these conditions. However, this study was limited as the catalytic tests were measured only at one set point reaction temperature. It would be interesting to perform the catalytic tests at different measurements to calculate kinetic energy. Additionally, no investigation of the pressure effect. Also, no variation of contact time, e.g., the effect of conversion on coking. It is recommended to apply catalytic tests soon to investigate the previous issues.The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU\u00a0Programs\u00a0Body (SEUPB).\nAhmed S. Al-Fatesh: Conceptualization, Methodology, Writing \u2013 review & editing, Supervision. Samsudeen O. Kasim: Conceptualization, Methodology, Writing \u2013 review & editing, Supervision. Ahmed A. Ibrahim: Conceptualization, Methodology, Writing \u2013 original draft. Ahmed I. Osman: Conceptualization, Methodology, Writing \u2013 review & editing, Supervision. Ahmed E. Abasaeed: Conceptualization, Methodology, Writing \u2013 original draft. Hanan Atia: Writing \u2013 reviewing, Supervision, Editing. Udo Armbruster: Writing \u2013 reviewing, Supervision, Editing. Leone Frusteri: Data curation, Formal analysis. Abdulrahman bin Jumah: Writing \u2013 original draft. Yousef Mohammed Alanazi: Writing \u2013 original draft. Anis H. Fakeeha: Writing \u2013 reviewing, Supervision, Editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to sincerely appreciate to Researchers Supporting Project number (RSP-2021/368), King Saud University, Riyadh, Saudi Arabia. Dr Ahmed I. Osman wishes to acknowledge the support of The Bryden Centre project (Project ID VA5048). The Bryden Centre project is supported by the European Union\u2019s INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB).Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2022.125523.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  This study investigated the performance of supported Ni catalysts in the utilization of greenhouse gases like CO2 and CH4 via dry reforming. The support SBA-15 was impregnated first with Sc at different loadings (0.5, 1, and 3\u00a0wt.%) and then with Ni (5\u00a0wt.%). The catalysts were first tested up to 8\u00a0h on stream with stoichiometric feed as well as methane in excess. The as-prepared catalysts were characterized using BET, XRD, TPR, CO2-TPD, XPS, TGA, and TEM.\n                  This is in accordance with the surface area measurement, XRD, and TEM data. The Ni added to Sc-SBA-15 appeared to interact with both the support and Sc as the intensity and reduction temperature of the Sc promoted catalysts increased relatively to the unpromoted sample depending on Sc content.\n                  The catalyst with 0.5\u00a0wt.% Sc loading led to the highest conversion and the lowest relative activity loss. The CH4 and CO2 conversions, on average, were 78 and 86\u00a0%, respectively, at the end of the runs at 750\u00a0\u00b0C. The final H2/CO ratio was 0.99, which is a good value compared to many literature catalysts. This catalyst also showed relatively constant CH4 and CO2 conversions over 80\u00a0h on stream. Increasing the Sc loading above 0.5\u00a0wt.% was not beneficial in terms of activity.\n               "}
{"full_text": "No data was used for the research described in the article.Extensive use of fossil fuels along has conducted to a rapid and continuous increase of CO2 concentration in the atmosphere [1], up to 420\u00a0ppm in April 2022, while preindustrial levels were of 278\u00a0ppm (in ca. 1750) [2]. Therefore, there is an increasing interest in CO2 capture and conversion processes that can contribute to revert this trend.CO2 capture consists of selectively removing the CO2 from ambient air or industrial process streams to produce a concentrated stream of CO2 that can be transported to the storage site [3]. Among the technologies developed for CO2 capture, chemical and physical absorption stand out, since they are the most mature and used at commercial scale [4]. Usually, CO2 is absorbed by aqueous solutions of amines or NaOH [5,6]. However, CO2 capture and storage presents high costs due to the desorption and compression steps prior to transportation [5]. The utilization of this captured CO2 to produce value-added chemicals may compensate the costs associated to its capture [7]. Nevertheless, the industrial use of CO2 as a raw material is still limited due to its high chemical stability [8].There are different alternatives to produce chemicals and fuels from CO2, including hydrogenation [9], electrochemical reactions [10] and photochemical reactions [11]. However, these technologies still show low yields and high cost and, therefore, further development is necessary [12]. Hydrogen is the main reductant used in these processes. Although nowadays most of the hydrogen is still produced by energy intensive processes, such as the endothermic steam methane reforming (SMR) [13], it is expected that in the near future abundant green hydrogen, produced by water hydrolysis, will be available, since pilot to commercial-scale plants are rapidly developing [14]. In-situ hydrogen production methods that can yield a more reactive reductant at a lower cost also are of high interest.High-temperature water (HTW) has emerged as an alternative hydrogen donor and reaction media due to the fact that it presents fewer and weaker hydrogen bonds, lower dielectric constant and a higher isothermal compressibility than water at room temperature. Moreover, its use is preferred over organic solvents because it is an environmentally friendly solvent [15]. The dissociation of water with a metal under hydrothermal conditions is an alternative to direct use of hydrogen in the reduction of CO2. Studies on the abiotic synthesis of organics indicate the feasibility of H2O dissociation and production of organics by CO2 reduction using metals under hydrothermal conditions [16\u201318]. The formation of long-chain hydrocarbons by hydrothermal reduction has also been recently demonstrated [19]. Moreover, organics, such as CH4\n[20] and other hydrocarbons [21] have been found in hydrothermal oceanic vents which may indicate the leading role that reactions such as serpentinization of magnesium-and iron-rich rocks to produce H2 may have had in the origin of life on the Earth.For the industrial development of this technology, to find an active catalyst for CO2 conversion under mild conditions is crucial for using CO2 as a raw material in the production of chemical and fuels. Among other types, such as homogeneous and biological catalysts, heterogeneous catalysts show some advantages such as high stability and easy separation from reactants and products. Thus, they are usually preferred for their use in industrial applications [22].Different products can be synthesized from CO2 using metal reductants and catalysts in hydrothermal conditions. For example, CH4 was produced with a yield of 98\u00a0% from NaHCO3 using Raney Ni nanoparticles as catalyst [23], and acetate was obtained with yields in the range of 10\u00a0% using cobalt-based catalysts [24]. In addition to CH4, methanol can also be formed using CO2 as the raw material [25,26]. The production of formic acid by the reduction of CO2 captured as NaHCO3 under hydrothermal conditions has been previously studied to optimize the reaction parameters, particularly the reductant:catalyst:NaHCO3 molar ratio, the reaction temperature and time and the amount of water employed. For instance, using a combination of Ni as catalyst and Fe as the reductant with a ratio 1:1, a yield to formic acid of 15.6\u00a0% was reached after 2\u00a0h of reaction at 300 \u00baC [27,28]. Higher yields to formic acid of 63.6\u00a0% were found when using a combination of Fe and Cu with a molar ratio 6:6:1 of Fe/Cu/NaHCO3 at the same reaction time and temperature [29]. The performance of Fe reductant without catalyst was also investigated, yielding 92\u00a0% of formic acid when employing high proportions of Fe [30]. Zn can also be used as reductant for the hydrothermal reduction of NaHCO3, with yields between 64\u00a0% and 78\u00a0% at 300 and 325 \u00baC respectively [31,32], values that could be increased with Ni catalysts [33]. In the case of Al, the yield to formic acid obtained after 2\u00a0h of reaction at 300 \u00baC was also 64\u00a0% [34]. Besides the high yields obtained in the hydrothermal reduction of CO2 using metals, it presents solutions to two of the challenges of CO2 reduction processes presented above: the reactivity of CO2 captured in basic solutions, such as HCO3\n-, is higher than that of gaseous CO2, and the reaction can take place in the same aqueous media where CO2 is captured by NaOH, without intermediate separation or purification processes, thus avoiding the related energy consumption and processing costs.The main product obtained in most of these studies is formic acid. Formic acid can be used as preservative and insecticide, as a reducing agent, or as carbon source in synthetic chemical industries [35]. The dehydrogenation of formic acid to produce hydrogen is a fast and easily controllable process and therefore, in the past years, formic acid has also gained great attention as a hydrogen storage vector.The hydrothermal reduction of CO2 therefore presents promising advantages in terms of integration with capture processes, selectivity and yield, but the harsh pressure and, particularly, temperature conditions required to carry it out still are a concern, since these conditions have a direct impact on the cost of the process and on the stability of the base (e.g. amine) used to capture CO2 and, therefore, the possibility to recycle it. It is therefore of great interest to reduce the required operating temperature, while maintaining the performance of the process. With this purpose, in this work a large number of combinations of metal reductants and catalysts are tested systematically. In comparison with previous studies, in which tested temperatures were above 250 \u00baC, in this work operating temperatures are reduced down to 200 \u00baC. Moreover, a kinetic model under the optimum reaction conditions is provided.NaHCO3 (100\u00a0%) was purchased from COFARCAS (Spain). The reductants employed included Zn powder (< 150\u00a0\u00b5m, 99.995\u00a0% metal basis) and Fe powder (\u2265 99\u00a0%), both provided by Sigma Aldrich (Spain), and granular Al (< 1\u00a0mm, 99.7\u00a0%) from Panreac (Spain). The catalysts used encompassed Cu powder (<425\u00a0\u00b5m, 99.5\u00a0% metal basis), Ni powder (< 150\u00a0\u00b5m, 99.99\u00a0%), Fe3O4 powder (50\u2013100\u00a0nm, 97\u00a0% metal basis) and Pd/C (5\u00a0% Pd content) acquired from Sigma Aldrich (Spain), as well as Fe2O3 powder (< 5\u00a0\u00b5m, \u2265 96\u00a0%) from Panreac (Spain). A standard reagent formic acid (puriss. \u223c98\u00a0%) from Sigma Aldrich was used for obtaining the calibration curves. All reductants and catalysts were used without further treatment or purification.Aqueous solutions of NaHCO3 were used as the CO2 source. NaHCO3 solutions were prepared with MilliQ water at a concentration of 0.5\u00a0M. Experiments were conducted in batch reactors (length: 16\u00a0cm, o.d.: \u00bd\u201d, wall thickness: 0.083\u2033) made of SS 316 stainless steel tubing with an internal volume of 9\u00a0mL.Each reactor was loaded with the selected reductant in a molar ratio reductant:CO2 of 5:1 and the catalyst in a molar ratio catalyst:CO2 of 2:1, except in the case of Pd/C catalyst, were due to limitations with respect to the volume of solids that could be loaded in the reactor while maintaining an efficient stirring, a catalyst:CO2 mass ratio of 0.25:1, corresponding to a Pd: CO2 molar ratio of 0.005:1, was used. Thereafter, the reactor was filled with NaHCO3 solution up to approximately 40\u00a0% of the volume of the reactor. The closed reactors were placed in an Al2O3 sand fluidized heating bed preheated at the target temperature (200, 250 or 300 \u00baC) to ensure a rapid heating, which required between 3 and 5\u00a0min at the temperature range of 200\u2013300 \u00baC. After the reaction time was completed, reactors were introduced in a cold water bath to quench the reaction. Liquid samples were collected and the solid reductants and catalysts were separated by vacuum filtration and dried at 105 \u00baC overnight. To ensure reproducibility, each reaction was conducted at least twice and the standard deviation between the results of the repeated experiments was calculated.Liquid samples were analyzed by HPLC (Waters, Alliance separation module e2695) attached to a RI detector (Waters, 2414 module) using a Rezex-ROA-Organic Acid (8\u00a0%, pore size 300\u00a0\u00d77.8\u00a0mm) purchased from Phenomenex. Prior to analysis, all the samples were filtered through a 0.22\u00a0\u00b5m filter. The HPLC method consisted of passing a mobile phase of 25\u00a0mM of H2SO4 with a flow rate of 0.5\u00a0mL/min during 30\u00a0min. The temperatures of the column and the detector were set at 40 and 30 \u00baC, respectively. Each sample was analyzed twice to ensure reproducibility of the HPLC operation.The yield of formate was calculated according to Eq. 1:\n\n(1)\n\n\n\n\nY\n\n\nFormate\n\n\n=\n\n\n\n\nC\n\n\nFormate\n,\nf\n\n\n\n\n\n\nC\n\n\nNaHCO\n3\n,\ni\n\n\n\n\n\u00d7\n100\n\n\n\nwhere C\n\nFormate,f\n is the molar concentration of formate obtained at the end of the reaction calculated by calibration curves in HPLC analysis, and C\n\nNaHCO3,i\n is the initial concentration of the NaHCO3 aqueous solution, fixed at 0.5\u2009M.XRD patterns were recorded using a Bruker D8 Discover A25 diffractometer attached to a LynxEye detector operated at a voltage of 40\u2009kV and a current of 30\u2009mA. Data were collected at room temperature in the 2\u03b8 range from 5 to 70\u00ba with a step size of 0.020\u00ba using Cu K\u03b1 radiation (\u03bb\u2009=\u20091.5418 \u00c5). Database PDF-2-ICDD 2020 was used to analyze the XRD patterns collected.N2 gas adsorption was used to determine the surface area of the fresh Pd/C catalyst. The catalyst surface area was measured according to BET method using a ASAP\u2122 2420 Micromeritics Accelerated Surface Area and Porosimetry System. 0.1133\u2009g of fresh Pd/C was introduced in the sampling tube and after degassing it under vacuum overnight, and the N2 isotherms were recorded at \u2212\u2009196 \u00baC.In a previous work of this research group [35], it was stated that in this type of reactions the equilibrium formic acid/formate is mostly shifted to formate due to the reaction pH, which is alkaline. Therefore, the reactions considered are presented in R1-R4.\n\n(R1)\n\n\n2\n\n\nH\n\n\n2\n\n\nO\n\n\n\nl\n\n\n\n+\nAl\n\n\n\ns\n\n\n\n\u2192\n1.5\n\n\nH\n\n\n2\n\n\n\n\n\naq\n\n\n\n+\nAlO\n(\nOH\n)\n(\ns\n)\n\n\n\n\n\n\n(R2)\n\n\nNaHC\n\n\nO\n\n\n3\n\n\n+\n\n\nH\n\n\n2\n\n\n\u2192\n\n\nNa\n\n\n+\n\n\n+\n\n\nHCOO\n\n\n\u2212\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n(R3)\n\n\nHCO\n\n\nO\n\n\n\u2212\n\n\n\n\n\naq\n\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\nl\n\n\n\n\u2192\nHC\n\n\nO\n\n\n3\n\n\n\u2212\n\n\n\n\n\naq\n\n\n\n+\n\n\nH\n\n\n2\n\n\n(\naq\n)\n\n\n\n\n\n\n(R4)\n\n\nHCO\n\n\nO\n\n\n\u2212\n\n\n\n\n\naq\n\n\n\n\u2192\nCO\n\n\n\ng\n\n\n\n+\nO\n\n\nH\n\n\n\u2212\n\n\n(\naq\n)\n\n\n\n\nIn this previous work, it was observed that the reaction R1 of production of hydrogen was much faster than bicarbonate reduction [35]. Thus, this first step was not considered in the simplified model, assuming that H2 is instantaneously formed. With R2, bicarbonate is reduced to formate using hydrogen. This formate is decomposed to CO2 (R3) and to CO (R4) [36]. As the conversion of the decomposition of HCOO- to CO (R4) is at least an order of magnitude lower than the conversions of R3, the formation of CO was also not taken into account for simplification purposes [36]. As equilibrium calculation presented in a formed work [35] resulted that most CO2 was dissolved in aqueous solutions as bicarbonate, in R3 the product is HCO3\n- instead of gaseous CO2.Having into account these simplifications, the global reaction taken account in the model is R5, presented as a pseudo-equilibrium reaction.\n\n(R5)\n\n\nNaHC\n\n\nO\n\n\n3\n\n\n+\n\n\nH\n\n\n2\n\n\n\u2194\n\n\nNa\n\n\n+\n\n\n+\n\n\nHCOO\n\n\n\u2212\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\nWhose kinetics follow Eq. 1:\n\n(1)\n\n\n\n\n\n\ndC\n\n\nNaHC\n\n\nO\n\n\n3\n\n\n\n\n\n\ndt\n\n\n=\n\n\n1\n\n\n\n\nS\n\n\ncat\n\n\n\n\n\n\n\n\n\n\u2212\nk\n\n\n1\n\n\n\u2219\n\n\nC\n\n\nNaHC\n\n\nO\n\n\n3\n\n\n\n\nm\n\n\n\u2219\n\n\nC\n\n\n\n\nH\n\n\n2\n\n\n\n\nn\n\n\n+\n\n\nk\n\n\n2\n\n\n\u2219\n\n\nC\n\n\n\n\nHCOO\n\n\n\u2212\n\n\n\n\np\n\n\n\n\n\n\n\n\nWhere \n\n\nS\n\n\ncat\n\n\n is the surface area of the catalyst, \n\n\nk\n\n\n1\n\n\n and \n\n\nk\n\n\n2\n\n\n are the kinetic constants of bicarbonate and formate decompositions, direct and inverse reactions, respectively; \nm\n, \nn\n and \np\n the order of the reaction respect to each compound, \n\n\nC\n\n\nNaHC\n\n\nO\n\n\n3\n\n\n\n\nis the concentration of bicarbonate, \n\n\nC\n\n\nHCO\n\n\nO\n\n\n\u2212\n\n\n\n\n is the concentration of formate and \n\n\nC\n\n\n\n\nH\n\n\n2\n\n\n\n\nis the concentration of H2.To study the kinetics of the reaction, experiments were conducted at different times, specifically 15, 30, 60, 90 and 120\u2009min. At these times, the concentration of NaHCO3 and HCOO- were quantified by HPLC analysis. It is important to highlight that the concentration of H2 (\n\n\nC\n\n\n\n\nH\n\n\n2\n\n\n\n\n) is considered constant due to the fact that it is assumed that the metal reductant is completely oxidized and H2 is released very quickly, according to the results obtained in a previous work [35]. As the mole ratio of reductant to NaHCO3 employed is 5:1, the amount of H2 formed is highly in excess in comparison to NaHCO3. Moreover, H2 is present in both gas and liquid phase. The concentration of H2 in the liquid phase is determined by its solubility at the pressure and temperature of the reaction media. To calculate the solubility of H2 the next steps were followed:\n\n(1)\nAs aforementioned, it is assumed that the reductant is completely oxidized. Therefore, the amount of H2 formed depends on the redox reaction of the reductant and water. In the case of Al reductant, 1.5\u2009mol of H2 is formed per each mol of Al, according to Reaction R1. Considering this, it can be assumed that the concentration of H2 remains constant during the reaction since the molar ratio H2:CO2 is 7.5, and therefore, NaHCO3 is the limiting reactant.\n\n\n(2)\nThe volume that the H2 occupies is 5.4\u2009mL, this is, the total volume of the reactor (9\u2009mL) minus the volume of the solution of NaHCO3 added (3.6\u2009mL).\n\n\n(3)\nUsing the ideal gas equation, the pressure of H2 is calculated in that volume at the reaction temperature.\n\n\n(4)\nUsing data generated with the Predictive Soave Redlich Kwong (PSRK) equation [37], a model to calculate the molar fraction of H2 dissolved in H2O is developed. The model is valid from pressures from 50 to 150\u2009atm. For simplification of the calculations, the results of this thermodynamic model were correlated according to Eqs. 2\u20134 for 200, 250 and 300 \u00baC respectively. Model fit showed a \n\n\n\nR\n\n\n2\n\n\n=\n1.00\n\n,\n\n\n\nR\n\n\n2\n\n\n=\n0.998\n\n and \n\n\n\nR\n\n\n2\n\n\n=\n0.988\n\n for the temperatures 200, 250 and 300 \u00baC respectively.\n\n(2)\n\n\n\n\nx\n\n\nH\n2\n\n\n=\n5.834\n\u2219\n\n\n10\n\n\n\u2212\n8\n\n\n\n\nP\n\n\n2\n\n\n+\n2.004\n\u2219\n\n\n10\n\n\n\u2212\n5\n\n\nP\n\u2212\n3.0689\n\u2219\n\n\n10\n\n\n\u2212\n4\n\n\n\n\n\n\n\n\n(3)\n\n\n\n\nx\n\n\nH\n2\n\n\n=\n2.375\n\u2219\n\n\n10\n\n\n\u2212\n7\n\n\n\n\nP\n\n\n2\n\n\n\u2212\n1.084\n\u2219\n\n\n10\n\n\n\u2212\n5\n\n\nP\n+\n1.154\n\u2219\n\n\n10\n\n\n\u2212\n3\n\n\n\n\n\n\n\n\n(4)\n\n\n\n\nx\n\n\nH\n2\n\n\n=\n5.748\n\u2219\n\n\n10\n\n\n\u2212\n7\n\n\n\n\nP\n\n\n2\n\n\n\u2212\n7.5346\n\u2219\n\n\n10\n\n\n\u2212\n5\n\n\nP\n+\n2.906\n\u2219\n\n\n10\n\n\n\u2212\n3\n\n\n\n\n\nwhere \n\n\nx\n\n\nH\n2\n\n\n is the molar fraction of H2 in H2O and \nP\n the pressure in atm.\n\n\n(5)\nThe number of mole of H2O at reaction conditions is calculated taken into the account its density at the reaction temperature and pressure. The density of the H2O was calculated with the MS Excel Add-In Water97v13.xla [38].\n\n\n(6)\nThe solubility of H2, and thus, the amount of H2 in water, can be calculated using the model developed in step 4 and the amount of H2O calculated in step 5.\n\n\n(7)\nOnce the amount of H2 in H2O is known, the H2 remaining in the gas phase is recalculated.\n\n\n(8)\nWith this new value of the H2 in gas phase, its pressure is calculated according to step 3. Steps 4, 5, 6 and 7 are iterated until the pressure calculated in step 7 converges to the one used in step 3.\n\n\n(9)\nOnce the values of the H2 pressure calculated in steps 3 and 7 are equal, the amount of H2 in H2O is given by steps 4 and 5.\n\n\nAs aforementioned, it is assumed that the reductant is completely oxidized. Therefore, the amount of H2 formed depends on the redox reaction of the reductant and water. In the case of Al reductant, 1.5\u2009mol of H2 is formed per each mol of Al, according to Reaction R1. Considering this, it can be assumed that the concentration of H2 remains constant during the reaction since the molar ratio H2:CO2 is 7.5, and therefore, NaHCO3 is the limiting reactant.The volume that the H2 occupies is 5.4\u2009mL, this is, the total volume of the reactor (9\u2009mL) minus the volume of the solution of NaHCO3 added (3.6\u2009mL).Using the ideal gas equation, the pressure of H2 is calculated in that volume at the reaction temperature.Using data generated with the Predictive Soave Redlich Kwong (PSRK) equation [37], a model to calculate the molar fraction of H2 dissolved in H2O is developed. The model is valid from pressures from 50 to 150\u2009atm. For simplification of the calculations, the results of this thermodynamic model were correlated according to Eqs. 2\u20134 for 200, 250 and 300 \u00baC respectively. Model fit showed a \n\n\n\nR\n\n\n2\n\n\n=\n1.00\n\n,\n\n\n\nR\n\n\n2\n\n\n=\n0.998\n\n and \n\n\n\nR\n\n\n2\n\n\n=\n0.988\n\n for the temperatures 200, 250 and 300 \u00baC respectively.\n\n(2)\n\n\n\n\nx\n\n\nH\n2\n\n\n=\n5.834\n\u2219\n\n\n10\n\n\n\u2212\n8\n\n\n\n\nP\n\n\n2\n\n\n+\n2.004\n\u2219\n\n\n10\n\n\n\u2212\n5\n\n\nP\n\u2212\n3.0689\n\u2219\n\n\n10\n\n\n\u2212\n4\n\n\n\n\n\n\n\n\n(3)\n\n\n\n\nx\n\n\nH\n2\n\n\n=\n2.375\n\u2219\n\n\n10\n\n\n\u2212\n7\n\n\n\n\nP\n\n\n2\n\n\n\u2212\n1.084\n\u2219\n\n\n10\n\n\n\u2212\n5\n\n\nP\n+\n1.154\n\u2219\n\n\n10\n\n\n\u2212\n3\n\n\n\n\n\n\n\n\n(4)\n\n\n\n\nx\n\n\nH\n2\n\n\n=\n5.748\n\u2219\n\n\n10\n\n\n\u2212\n7\n\n\n\n\nP\n\n\n2\n\n\n\u2212\n7.5346\n\u2219\n\n\n10\n\n\n\u2212\n5\n\n\nP\n+\n2.906\n\u2219\n\n\n10\n\n\n\u2212\n3\n\n\n\n\n\nwhere \n\n\nx\n\n\nH\n2\n\n\n is the molar fraction of H2 in H2O and \nP\n the pressure in atm.The number of mole of H2O at reaction conditions is calculated taken into the account its density at the reaction temperature and pressure. The density of the H2O was calculated with the MS Excel Add-In Water97v13.xla [38].The solubility of H2, and thus, the amount of H2 in water, can be calculated using the model developed in step 4 and the amount of H2O calculated in step 5.Once the amount of H2 in H2O is known, the H2 remaining in the gas phase is recalculated.With this new value of the H2 in gas phase, its pressure is calculated according to step 3. Steps 4, 5, 6 and 7 are iterated until the pressure calculated in step 7 converges to the one used in step 3.Once the values of the H2 pressure calculated in steps 3 and 7 are equal, the amount of H2 in H2O is given by steps 4 and 5.Once the initial concentrations of NaHCO3, H2 and HCOO- are known, the system is modeled considered it as a discontinuous stirred tank reactor, solving the mass balances to formic acid, hydrogen and bicarbonate. In addition to Eq. 1, Eqs. 5 and 6 were also solved, using the Euler numerical method:\n\n(5)\n\n\n\n\nC\n\n\nNaHCO\n3\n\n\n\n\n\nt\n\n\n\n=\n\n\nC\n\n\nNaHCO\n3\n\n\n\n\n\nt\n\u2212\n1\n\n\n\n+\n\n\nd\n\n\nC\n\n\nNaHCO\n3\n\n\n(\nt\n\u2212\n1\n)\n\n\ndt\n\n\n\u2219\ndt\n\n\n\n\n\n\n(6)\n\n\n\n\nC\n\n\nHCO\n\n\nO\n\n\n\u2212\n\n\n\n\n\n\n\nt\n\n\n\n=\n\n\nC\n\n\nHCO\n\n\nO\n\n\n\u2212\n\n\n\n\n\n\n\nt\n\u2212\n1\n\n\n\n\u2212\n\n\nd\n\n\nC\n\n\nHCO\n\n\nO\n\n\n\u2212\n\n\n\n\n(\nt\n\u2212\n1\n)\n\n\ndt\n\n\n\u2219\ndt\n\n\n\n\n\n\n(1)\nThe differential equations are solved using the Euler method with a time step of 10\u2009s. The values of the orders of the reaction, this is m, n and p simulated were 1, 2 and 0.5. All the combinations of these values were simulated in order to optimize.\n\n\n(2)\nWith the fixed reactions orders, the values k1 and k2 were optimized in order to minimize the objective Eq. 7 using the function Solver of Excel:\n\n(7)\n\n\nAVERAGE\n\n\u2211\n\nt\n=\n15\nmin\n\n\nt\n=\n120\nmin\n\n\n\nABS\n\n\n\n\n\n\n\nC\n\n\nNaHCO\n3\n_\nMODEL\n\n\n\u2212\n\n\nC\n\n\nNaHCO\n3\n_\nEXPERIMENTAL\n\n\n\n\n\n\nC\n\n\nNaHCO\n3\n_\nEXPERIMENTAL\n\n\n\n\n\n\n\n\n=\n0\n\n\n\nwhere \n\n\nC\n\n\nNaHCO\n3\n_MODEL\n\n\n is the concentrration of NaHCO3 obtained with the model and \n\n\nC\n\n\nNaHCO\n3\n_EXPERIMENTAL\n\n\n is the value of NaHCO3 experimentally obtained.\n\n\n(3)\nThe combination of reaction orders selected was the one where the sum of the average absolute error for the concentration of NaHCO3 plus the average absolute error of the concentration of formate was smaller.\n\n\nThe differential equations are solved using the Euler method with a time step of 10\u2009s. The values of the orders of the reaction, this is m, n and p simulated were 1, 2 and 0.5. All the combinations of these values were simulated in order to optimize.With the fixed reactions orders, the values k1 and k2 were optimized in order to minimize the objective Eq. 7 using the function Solver of Excel:\n\n(7)\n\n\nAVERAGE\n\n\u2211\n\nt\n=\n15\nmin\n\n\nt\n=\n120\nmin\n\n\n\nABS\n\n\n\n\n\n\n\nC\n\n\nNaHCO\n3\n_\nMODEL\n\n\n\u2212\n\n\nC\n\n\nNaHCO\n3\n_\nEXPERIMENTAL\n\n\n\n\n\n\nC\n\n\nNaHCO\n3\n_\nEXPERIMENTAL\n\n\n\n\n\n\n\n\n=\n0\n\n\n\nwhere \n\n\nC\n\n\nNaHCO\n3\n_MODEL\n\n\n is the concentrration of NaHCO3 obtained with the model and \n\n\nC\n\n\nNaHCO\n3\n_EXPERIMENTAL\n\n\n is the value of NaHCO3 experimentally obtained.The combination of reaction orders selected was the one where the sum of the average absolute error for the concentration of NaHCO3 plus the average absolute error of the concentration of formate was smaller.The kinetic constants were correlated at three different temperatures: 200, 250 and 300 \u00baC. The values of the constant were adjusted to Arrhenius equation to easily calculate the effect of the temperature (Eq. 8).\n\n(8)\n\n\nk\n\n\n\nT\n\n\n\n=\nA\n\u2219\n\n\ne\n\n\n\u2212\n\n\n\n\nE\n\n\na\n\n\n\n\nRT\n\n\n\n\n\n\n\nwhere \n\nk\n(\nT\n)\n\n is the rate constant as a function of the temperature, \nA\n is the pre-exponential factor, \n\n\nE\n\n\na\n\n\n the activation energy, \nR\n is the gas constant and \nT\n is the absolute the reaction temperature.This work studied the influence of different combinations of reductants and catalysts to reduce CO2 to formate under hydrothermal conditions. As aforementioned, NaHCO3 was used as the carbon source, being it the product resulting from the capture of CO2 with basic NaOH solutions. The reductants employed included Zn, Al, and Fe and the catalysts were Cu, Ni, Fe2O3, Fe3O4 and Pd/C. Three different reaction temperatures were also explored, specifically 200, 250 and 300 \u00baC. Temperatures above 300 \u00baC were not tested since the decomposition of formic acid into CO2 and H2 at high temperatures under hydrothermal conditions is high [36].When using Al or Zn as reductants, the selectivity of liquid-phase products towards formate was 100\u00a0%. In the case of using iron as reductant, a peak corresponding to another unidentified product was found apart form the peak of formate. Sample chromatograms are provided as Supplementary Information (Fig. S1). Formate yields are plotted in \nFig. 1 together with the error bars calculated from the results of repeated experiments. The standard deviation of the two replicates was lower than 1.5\u00a0% in most of the cases and, therefore, some of the error bars cannot be appreciated in Fig. 1 due to the axis scale. \nTable 1 compiles the yields obtained with the different combinations of metal reductants, catalysts and temperature.It is clear from Fig. 1 that the temperature has a positive effect on the yield to formate. The higher the temperature, the higher the yield and, in general, the change observed in the yield is greater from 250 to 300 \u00baC than from 200 to 250 \u00baC, except in the case of Pd/C and Ni catalysts (Fig. 1b a 1c respectively), where the yield at 300 \u00baC was only slightly higher than that at 250 \u00baC when using Zn as the reductant. In contrast, with Al and Fe reductants, the yield to formate decreased moderately from 250 to 300 \u00baC.The highest formate yield reached a value of 57\u00a0%. It was observed at 250 \u00baC when the reductant was Al and the catalyst Pd/C. A similar yield of 55\u00a0% was obtained at 300 \u00baC with Cu catalyst, again with Al reductant. Interestingly, a comparable yield of 52\u00a0% was detected at 300 \u00baC with Zn reductant in the absence of catalyst. Indeed, in the absence of catalyst, Zn showed the best performance at the three temperatures evaluated, yielding 5.4\u00a0% and 12\u00a0% of formate at 200 and 250\u00baC respectively. In contrast, at 200 \u00baC, the yields obtained with Al and Fe and without catalyst were negligible and at 250 \u00baC the yield with Al was of 5.6\u00a0% while in the case of Fe was only 1\u00a0%. The feasibility of using Zn reductant to produce formic acid from NaHCO3 under hydrothermal conditions was previously demonstrated [32], where the intermediate Zn-H, obtained by the oxidation of Zn by HTW, may have a leading role by acting as the active hydrogen source in CO2 hydrogenation [39].With Fe reductant, the performance of Fe2O3 catalyst at 200 and 250 \u00baC is practically constant, yielding 3\u00a0% and 4\u00a0% of formate, respectively. The same trend can be observed with Fe3O4 catalyst, where the yield at both temperatures was lower than 1\u00a0%. However, at 300 \u00baC, Fe3O4 showed a better performance than Fe2O3 catalyst for Fe reductant yielding 30\u00a0% of formate, while the yield with Fe2O3 was 24\u00a0%. In general, in the temperature range studied, Fe reductant showed the lowest yields, while Al reductant exhibited the best performance. However, in the absence of catalyst with Fe reductant at 300 \u00baC, the yield obtained was higher than in the case of Al powder reductant, reaching a value of 45\u00a0%. This yield is comparable to that observed by Duo et al. [30], who determined a yield of approximately 50\u00a0% with Fe, although they used a low NaHCO3 concentration of 2\u2009mmol/L. Duo et al. [30] significantly increased the yield of formic acid to more than 90\u00a0% by increasing the amount of Fe powder employed. The best performance of Fe reductant detected by Duo et al. [30] can be explained by the particle size of the reductant and the more diluted reaction conditions, along with the horizontally shaken of the reactor which may have enhanced mixing and heat transfer, favoring the reaction. However, for practical applications, higher reactant concentrations are desirable to increase throughput.It is very well-known that Cu catalyst is very selective to methanol when reacting with CO2\n[25]. However, methanol was not observed in this work, probably due to the alkaline pH of the reaction media, since Huo et al. [25] used HCl to acidify the media to produce methanol.\n\nFig. 2 presents SEM micrographs of selected solid samples after reaction experiments, while \n\n\nFigs. 3\u20135 presents the corresponding XRD patterns. XRD was employed to investigate changes in the phases present in both the reductants and selected catalysts after reaction. Fig. 3 shows the changes in the phases of the three reductants employed after 120\u2009min of reaction at 300 \u00baC. Reference diffractograms of the original, unoxidized metals can be retrieved from [40].As it is clear from Fig. 3, the only reductant completely oxidized after reacting during 120\u2009min at 300 \u00baC was Zn. This result is in agreement with the works of Jin et al. and Roman-Gonzalez et al. [32,34] who demonstrated that under hydrothermal conditions, Zn was almost completely oxidized to ZnO after 10\u2009min of reaction time. In the case of Al reductant, both crystal phases are present, Al and AlO(OH). However, Yao et al. [33] concluded that the oxidation of Al under hydrothermal conditions in the presence on NaHCO3 was completed after 30 and 90\u2009min of reaction time. This disagreement in the results may be explained by the different particle size of Al powder employed. In Fig. 2b it is shown that Fe was oxidized to Fe3O4 under the reaction conditions, but not completely, since typical peaks of Fe crystal phases, specifically at 2\u03b8 of 44.8 and 65\u00ba, are still present.XRD analysis of Fe reductant combined with Fe2O3 and Fe3O4 catalysts was also conducted. The XRD patterns obtained are shown in Fig. 4. It can be seen in Fig. 4 that Fe2O3 is only present when it was used as catalyst (Fig. 4a) and therefore, under reaction conditions Fe reductant is only oxidized to Fe3O4 (Fig. 4b). The same conclusion can be reached by looking at Fig. 2b. Duo et al. [30] also stated that under hydrothermal conditions, Fe may oxidized to Fe3O4 rather than Fe2O3. Interestingly, it seems that the presence of Fe3O4 in the media promotes in some extent the oxidation of Fe, because after reaction only one a small characteristic peak of Fe phase appeared at 44.5\u00ba (Fig. 4b), while in the case of using Fe2O3 as catalyst (Fig. 4a) or just Fe as reductant (Fig. 3b), two characteristic phase peaks of Fe are detected, specifically at 44.5\u00ba and 65\u00ba and with high intensities.The evolution of the crystal phases of Pd/C catalyst and Al reductant at different reaction times was also investigated. The results are shown in Fig. 4. The XRD patterns at different times shown no apparent differences, as can be seen from Fig. 5. No matter long or short reaction times, Al reductant was not completely oxidized and typical Al crystal phase at 2\u03b8 of 38\u00ba, 44\u00ba and 65\u00ba are still detected after 120\u2009min of reaction time at 300 \u00baC. On the other hand, Pd crystal phase could not be detected in the diffractogram, as a consequence of the low concentration of Pd in the Pd/C catalyst (5\u00a0%wt) and the low proportion of the catalyst in the total sample (0.018\u2009g of catalyst vs 0.24\u2009g of Al).The kinetic behavior of the reaction was investigated at three different temperatures according to the method explained in Section 2.5. The best adjustment to the experimental data at 250 \u00baC and 300 \u00baC was for a first order reaction respect to all components. Therefore, the units of the kinetic constants obtained are expressed in m\u22122s\u22121. Figures from 5 to 7 show the model for a pseudo-first order reaction respect to all components and the experimental data for temperatures of 200, 250 and 300 \u00baC respectively, while \nTable 2 presents the average and maximum deviations between experiments and calculations (Eq. 7) obtained at each temperature.As it can be seen from \n\n\nFigs. 6 to 8, the model correctly describes the experimental results at the three tested temperatures, with average deviations in the concentration of NaHCO3 ranging from 1.5\u00a0% to 3.6\u00a0%, slightly increasing with temperature. At 250 \u00baC it appears that the experimental point at 120\u2009min differs significantly from the model prediction. Errors in the calculation of the concentration of formate are slightly higher, ranging from 4.9\u00a0% to 5.9\u00a0%. Again, the experimental point at 120\u2009min is the one which shows more variation respect to the model. These deviations of the model with respect to experiments are satisfactory since they are comparable to uncertainties in experimental results, reported in section3.1; indeed, inspection of Figs. 6 to 8 indicate that deviations can be attributed to a large extent to scatter of some experimental data, with the model correctly reproducing the global trends of variation of the results. Figs. 6 to 8 indicate that at least 95\u00a0% of the equilibrium concentration for both bicarbonate conversion and formate production are reached in the first 30\u2009min of the reaction. Therefore, the reaction can be stopped after 30\u2009min, reaching comparable yields with respect to 120\u2009min reaction for this specific set of reaction parameters, this is, Pd/C catalyst and Al reductant at 250 \u00baC.\n\nTable 3 summarizes the values of the kinetic constants, both for bicarbonate decomposition (k1) and formate formation (k2) at 200, 250 and 300 \u00baC. As one might expect, both kinetic constants increased at higher temperatures. The improvement observed from 250 to 300 \u00baC was more significant than from 200 to 250 \u00baC. \nFig. 9.The values of the activation energy (\n\n\n\nE\n\n\na\n\n\n/\nR\n\n) and the pre-exponential factor (\nA\n) calculated by Eq. 3 using the data obtained in Fig. 8 are shown in \nTable 4. The value of R2 are also included.The \n\n\nE\n\n\na\n\n\n for the formation of HCOO- from NaHCO3 in hydrothermal media is 49.8\u2009kJ/mol. The activation energy of the hydrogenation of CO2 into formic acid over a Cu/ZnO/Al2O3 catalyst has been previously calculated resulting in a value of 21.4\u2009kJ/mol [41]. Higher values of the activation energy indicate that the formation of the formate requires more energy input. Therefore, the formation of formate from NaHCO3 under hydrothermal conditions needs more energy than when the production takes place by the hydrogenation of CO2. The isothermal decomposition of NaHCO3 into Na2CO3, CO2 and H2O presented an \n\n\nE\n\n\na\n\n\n of 94.3\u2009kJ/mol [42] under nitrogen atmosphere which is a higher value in comparison to the decomposition of NaHCO3 under hydrothermal calculated in this work which was 82.6\u2009kJ/mol.In this work, the hydrothermal conversion of CO2 dissolved in aqueous solutions as NaHCO3 was optimized at lower temperatures, with imply lower costs and milder conditions for the reuse of bases used for CO2 capture, considering a number of combinations of reductants and catalysts, demonstrating the possibility of enhancing the yield to formate at lower temperatures by selecting the appropriate combination of reductant and catalyst. Moreover, the reductants and catalysts tested are in general abundant and commercially available materials. The highest yield to formate observed was 57\u00a0% using Al as the reductant and Pd/C as the catalyst at 250 \u00baC. Using Al as reductant allowed to reach yields to formate higher than 50\u00a0% at 250 \u00baC. In the case of the other reductants tested (Zn and Fe), yields higher than 50\u00a0% were only observed when the temperature was 300 \u00baC. This improvement caused by the temperature was greater in the case of Fe reductant and Fe oxides catalysts, where the yields increased from 4\u00a0% at 250 \u00baC to 24\u00a0% and 31\u00a0% at 300 \u00baC for Fe2O3 and Fe3O4 respectively. Pd/C catalyst also showed higher yields to formate in comparison with the other catalysts tested at low temperatures with both Al and Zn reductants. In the case of Fe reductant, the yield with Pd/C was practically constant in the temperature range investigated.Furthermore, a kinetic model was developed to describe the reduction of bicarbonate using Al as reductant and Pd as Catalyst. The simplified model model presents the system as a pseudo-equilibrium between formate formation and destruction. The model is able to reproduce the resulting concentrations, with average deviations with respect to experimental data ranging from 1.5\u00a0% to 5.9\u00a0%, and a maximum deviation lower than 10\u00a0%, and correctly predicts the variation of the performance of the reaction with the operating temperature.The present study shows not only the potential of reducing CO2 emissions by using it as a C1 building block, but also a sustainable alternative to produce value-added chemicals such as formic acid.\nLaura Quintana-G\u00f3mez: Methodology, Investigation, Writing \u2013 original draft. Pablo Mart\u00ednez-\u00c1lvarez: Investigation. Jos\u00e9 J. Segovia: Conceptualization, Methodology. \u00c1ngel Mart\u00edn: Conceptualization. Methodology. Resources. Writing \u2013 review & editing. Visualization. Supervision. Writing \u2013 review & editing. Mar\u00eda Dolores Bermejo: Conceptualization, Methodology, Resources, Writing \u2013 Review & Editing, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This project has been funded by the Ministry of Science and Universities through project RTI2018-097456-B-I00 and by the Junta de Castilla y Le\u00f3n through project by FEDER FUNDS under the BioEcoUVa Strategic Program (CLU-2019-04). Authors also thank Jes\u00fas Salvador Azpeleta Izquierdo (Universidad de Valladolid) for his support and assistance in XRD analysis and Mar\u00eda Dolores Marqu\u00e9s Guti\u00e9rrez (Laboratorio de S\u00f3lidos Porosos Servicios Centrales de Apoyo a la Investigaci\u00f3n \u2013Universidad de M\u00e1laga) for her assistance in BET surface area analysis.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2022.102369.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  The hydrothermal reduction of CO2 captured in aqueous solutions using metal reductants is a promising novel approach that achieves high yields of conversion and high selectivity, but it presents the limitation of the high temperatures needed for the reaction to take place. In this work, experiments combining several reductant metals (Zn, Al and Fe), catalysts (Pd/C, Ni, Cu, Fe2O3 and Fe3O4) and temperatures (200, 250 and 300 \u00baC) were performed to optimize the process at milder temperatures. Using Al as reductant and Pd/C as catalyst, yields as high as 38\u00a0% were obtained at 200\u00baC, compared with the highest yield (57\u00a0%) observed at 250 \u00baC. Thus a significant temperature reduction can be achieved using a suitable combination of reductant and catalyst. Using this reaction system, Pd/C as catalyst and Al as reductant, an extensive set of experiments at different times and temperatures were performed in order to determine the kinetics of the process and correlate them to a mathematical model of the process. The model correctly reproduces the experimental data with average errors lower than 5.9\u00a0%. These results demonstrate the feasibility of lower the operating temperature while maintaining the performance, when using an adequate combination of catalyst and reductant.\n               "}
{"full_text": "The hydroxides and oxyhydroxides of Ni are among the most extensively studied electrocatalysts because of their high activity in alkaline media [1,2], for instance, in the electrochemical oxidation of alcohols [3,4], aldehydes [5\u201310], glucose [11\u201314], hydrogen peroxide [15], water splitting [16\u201320], etc. There have been various expressions about their electrocatalytic mechanism. Sometimes, the electrocatalysis towards small organic/inorganic compounds occurred by the direct electrocatalysis of Ni(OH)2\n[21\u201324] or the direct electrocatalysis of NiOOH [25\u201327]. However, most of the literature describes the electrocatalytic oxidation via a mediated mechanism [2,22,28] that is based on the Ni(OH)2/NiOOH redox transition in alkaline media, where the NiOOH formed during the charging acts as an oxidant for many organic/inorganic compounds [11,40]:\n\n(1.1)\n\n\nNi\n\n\n\nO\nH\n\n\n2\n\n+\n\n\nOH\n\n-\n\n-\n\n\ne\n\n-\n\n\u2194\nN\ni\nO\nO\nH\n+\n\nH\n2\n\nO\n\n\nc\nh\na\nr\ng\ni\nn\ng\n\u2194\nd\ni\ns\nc\nh\na\nr\ng\ni\nn\ng\n\n\n\n\n\n\n\n\n(1.2)\n\n\nNiOOH\n+\no\nr\ng\na\nn\ni\nc\n/\ni\nn\no\nr\ng\na\nn\ni\nc\n\nc\no\nm\np\no\nu\nn\nd\n\u2192\nN\ni\n\n\n\nO\nH\n\n\n2\n\n+\np\nr\no\nd\nu\nc\nt\n\n\n\n\nHowever, the catalytic behaviour of Ni(OH)2/NiOOH in alkaline solutions is known to differ depending on the fine structural details, e.g., crystal structure, composition, etc. This is the reason why so much effort has been devoted to characterize the crystal phases present on catalyst surfaces during charging and discharging [29\u201331]. According to Bode\u2019s diagram [29,32], Ni(OH)2 can exist in two crystal structures, in the literature denoted as \u201cpoorly-crystallised\u201d \u03b1-phase or \u201cwell-crystallised\u201d \u03b2-phase [29,33]. In the \u03b2-phase, the constituents are arranged in a hexagonal close-packed (hcp) layered structure of OH\u2212 ions with Ni occupying the octahedral interstices, with individual layers bonded by weak Van der Waals forces [29,33]. The structure of \u03b1-Ni(OH)2 is still considered to be Ni(II) hydroxide, but containing a variable excess of intersheet water [29,33]. Depending on the experimental method and the experimental conditions, either the \u03b1- or \u03b2-phases can be prepared via several chemical or electrochemical approaches [22,33]. Usually, \u03b1-Ni(OH)2 is isolated as a primary precipitation product, from which \u03b2-Ni(OH)2 might be obtained by ageing or potential cycling in alkaline solutions. However, \u03b2-Ni(OH)2 obtained from \u03b1-phase always contains adsorbed foreign ions or water [29]. Furthermore, the scheme presented by Bode et al. [34] also involves two phases of oxidized materials, \u03b2- or \u03b3-NiOOH, that can be obtained through either the chemical or electrochemical oxidation of Ni(OH)2. \u03b2-NiOOH is a relatively well-defined material that crystallises in the hexagonal system and can be regarded as being derived from \u03b2-Ni(OH)2. \u03b3-NiOOH crystallises in the rhombohedral system and represents a whole family of compounds exhibiting large intersheet distances where H2O or alkali ions (K+ or Na+) are intercalated. The \u03b3-NiOOH system can be regarded as being derived from either \u03b1-Ni(OH)2 or \u03b2-NiOOH, once overcharge happens [22,29].Furthermore, numerous additional phases, i.e., structurally disordered phases, have been proposed and can be ascribed as structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH with various possible structural irregularities, e.g., stacking fault disorder, \u03b1/\u03b2-interstratification, internal mechanical stress or ionic substitution, etc. [30,31,33]. All the disordered structures can be obtained from the \u03b2-Ni(OH)2 by an intercalation process and slab gliding [35] or by electrochemical potential cycling through the Ni(OH)2/NiOOH redox peaks in KOH solutions from free Ni or NiO2 surface sites [3,22 2]:\n\n(1.3)\n\n\nNi\n+\n2\n\n\nOH\n\n-\n\n-\n\n\ne\n\n-\n\n\n\u2192\nN\ni\n(\nO\n\n\n\nH\n)\n\n2\n\n,\n\no\nr\n\n\n\n\n\n\n(1.4)\n\n\nNi\n\nO\n2\n\n+\n\nH\n2\n\n\nO\n\u2192\nN\ni\n(\nO\n\n\n\nH\n)\n\n2\n\n\n\n\n\nContrary to \u03b1-/\u03b2-Ni(OH)2, they exhibit an X-ray diffraction pattern with very broad peaks, which means that its structure cannot be accurately determined [35].Several studies have linked structural changes in Ni(OH)2/NiOOH-based electrodes to their electrocatalytic performance (i.e., catalytic rate and input energy/onset potential) [36]. As presented by Bode [37], the phase transformation among different Ni(OH)2 and NiOOH species could proceed during charging and discharging in alkaline media, i.e., \u03b1-/\u03b3- or \u03b2-/\u03b2- transformations. It is generally believed that the \u03b1-/\u03b3-phases have been considered as the less catalytically active phases since the transformation involves more than one electron transfer, i.e., 1.6\u20131.67, suggesting that \u03b3-NiOOH might include Ni+4 ions [38\u201340]. While still uncertain, the \u03b2-/\u03b2-phases are generally considered as more active since the transformation involves a 1-electron transfer (from Ni2+ to Ni3+) [1,41,42]. However, the transformations in between crystalline active phases (\u03b1\u00a0\u2194\u00a0\u03b3 or \u03b2\u00a0\u2194\u00a0\u03b2) possess some intrinsic disadvantages, such as low stability during the electrochemical reactions [2,20,43]. Recently, structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH-based electrocatalysts have attracted attention because of their long cycle life, fast catalytic rate, and the low input energy needed for charging and discharging in alkaline media [44\u201346]. Their remarkable catalytic performance in electrochemical oxidation reactions can be associated with their disordered structure, i.e., \u03b2-Ni(OH)2/\u03b2-NiOOH with the lattice distortion or surface defects, etc. [47,33].The described redox transition between Ni(OH)2/NiOOH also plays a pivotal role in the electrochemical catalytic oxidation of formaldehyde (HCHO), as the latter is produced by the partial oxidation of methanol, which has technological significance in industrial catalytic processes [48], e.g., fuel-cell technology. The literature shows that interest has been focused on Ni-based electrocatalysts for HCHO oxidation as they possess a low overpotential and good durability to promote the HCHO oxidation reaction kinetics (i.e., catalytic rate) [5,7,8,10,49]. Also, they represent a good replacement for noble-metals-based electrocatalysts (Pt [50], Ag [51], Au [38,52], etc.) that were found to suffer from poor repeatability, reproducibility and the low sensitivity generated by surface poisoning from the adsorbed intermediates, i.e., carbon monoxide [17,53,54]. Recently, Ru\u2013Ni\u2013Ni(OH)2/NiO multi-metallic systems have been selected to be efficient non-precious catalysts [55,56] as their synergistic effects can significantly enhance the catalytic activity and stability. However, improving the catalytic activity of single metallic systems is of great interest for developing high-performance catalysts.Herein, we tailored the amount of highly active structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species via a CV-KOH modification process on two different electrodeposited Ni-based thin films; one with the surface NiO2\u2013\u03b1-Ni(OH) for deposition performed at pH\u00a0=\u00a02.5, and one with the surface NiO2 for deposition performed at pH\u00a0=\u00a05.5. Based on the calculations of the amount of \u03b2-Ni(OH)2/\u03b2-NiOOH it was found that the KOH-modified Ni film (pH\u00a0=\u00a05.5) contains a larger amount of structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species (55% more) than the KOH-modified Ni film (pH\u00a0=\u00a02.5) where the surface is already covered by electrodeposited \u03b1-Ni(OH)2. A higher HCHO electrocatalytic activity, i.e., the lowest onset overpotential, increased the catalytic rate exhibited by the KOH-modified Ni film (pH\u00a0=\u00a05.5) due to the presence of a larger amount of structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species, as per the requirement for only 1 electron transfer, and with this connected low energy input. The KOH-modified Ni film (pH\u00a0=\u00a02.5) also performed as an efficient HCHO catalyst but less active than the KOH-modified Ni film (pH\u00a0=\u00a05.5) due to the intrinsically different surface composition consisting of some \u03b1-Ni(OH)2 after the Ni plating. A KOH-modified Ni film (pH\u00a0=\u00a02.5) transforms to both the active \u03b1-Ni(OH)2/\u03b3-NiOOH (that requires the transfer of 1.67 electrons) and the highly active \u03b2-Ni(OH)2/\u03b2-NiOOH (that requires a transfer of only 1 electron) redox pairs.Ni-based electrodes (i.e., Ni thin films) were electrodeposited on the surface of a conductive SiO2/Au substrate that was prepared by the Physical Vapour Deposition (PVD) of the Au (600-nm-thick layer) on the glass. Before the PVD of Au, a 30-nm-thick layer of Cr was sputtered for better adhesion of the Au. The solutions used for the cyclic voltammetry and the cathodic electrodeposition were performed in 1-mol L\u22121 NiSO4\u00b76H2O (98+%, Sigma-Aldrich). The pH was adjusted in the range of 2.5\u20135.5 by the addition of H2SO4 (99.9%, Sigma-Aldrich). The CV measurements were carried out in the potential range from \u22121.2\u00a0V to +0.4\u00a0V vs. Ag/AgCl at a scan rate of 50\u00a0mV\u00a0s\u22121. The electrodeposition of the Ni thin films was carried out by applying a constant potential of \u22121.0\u00a0V for 300\u00a0s. Thin films were electrodeposited from the electrolyte with a pH\u00a0=\u00a02.5 and a pH\u00a0=\u00a05.5.Electrochemical measurements were performed with a Gamry Reference 600 potentiostat/galvanostat equipped PHE 200 software. The measurements were conducted in a Teflon electrolytic cell using a standard three-electrode system at room temperature. The working electrodes were \u201cas-deposited\u201d Ni thin films, the reference electrode was an Ag/AgCl/3.5-mol L\u22121 KCl (HANA Instruments GmbH-type HI5311), and a circular platinum mesh served as the counter electrode. The obtained output currents were normalized to the electrochemically active surface area (Aecsa) determined by the oxalate method (Supplementary data). The Aecsa of the Ni thin film (pH\u00a0=\u00a02.5) and Ni thin film (pH\u00a0=\u00a05.5) were determined to be 1.72\u00a0\u00b1\u00a00.05\u00a0cm2 and 2.18\u00a0\u00b1\u00a00.02\u00a0cm2, respectively.The KOH modification of the Ni thin films (pH\u00a0=\u00a02.5 and 5.5) was performed in an electrolyte composed of 0.5-mol L\u22121 KOH (pellets, Sigma-Aldrich) by cyclic voltammetry (CV). The cyclic voltammetric (CV) profiles were recorded from \u22121.0\u00a0V to +0.6\u00a0V in the 1st cycle and from 0\u00a0V to 0.6\u00a0V in the 2nd\u201360th cycle at a scan rate of 200\u00a0mV\u00a0s\u22121. The electrochemical studies of HCHO oxidation were carried out in solutions with HCHO (37% w/v, Carbo Erba) concentrations ranging from 0.1-\u03bcmol L\u22121 to 20-mmol L\u22121 by cyclic voltammetry (CV) and chrono-amperometry (CA). The CVs were performed from 0\u00a0V to +1.0\u00a0V at a scan rate of 100\u00a0mV\u00a0s\u22121. The pH of 13.7 was adjusted with the addition of NaOH pellets (Sigma-Aldrich). The electrolytes for the KOH-modification process and HCHO-detection experiments were de-oxygenated with pure N2 (for 15\u00a0min) and used immediately for every set of CV or CA measurements.The morphology of Ni thin films (pH\u00a0=\u00a02.5 and 5.5) was observed with a field-emission-gun scanning electron microscope (FEG-SEM, JEOL-JSM 7600F equipped with EDXS analysis) operating at 10\u00a0keV. The crystallinity and phase composition of the Ni thin films (pH\u00a0=\u00a02.5 and 5.5) were determined by analysing powder X-ray diffraction patterns (XRD, Bruker, D8 ADVANCE) with Cu-K\u03b1 radiation (\u03bb\u00a0=\u00a01.5418\u00a0\u00c5), with a step size of 0.01\u00a0s/\u00b0 and a time per step of 600\u00a0s. Fourier-transform infrared spectroscopy (FT-IR) was performed to evaluate the presence of the \u2013O-H chemical bonds on the surface of the dried Ni thin-film samples. The FT-IR spectra of the samples were taken using a Spectrum 100 spectrometer (Perkin Elmer, USA) in the wavenumber range from 4000 to 400\u00a0cm\u22121.In order to select the electrodeposition potential for the Ni-based thin films, cyclic voltammetry (CV) measurements were performed. The CV curves (Fig. 1\n) on the Au/Cr/SiO2 substrate, in 1-mol L\u22121 NiSO4 electrolyte were recorded in the potential range from the open-circuit potential (OCP, ~ 0\u00a0V) to \u22121.2\u00a0V in the forward (cathodic) scan and from \u22121.2\u00a0V to +0.4\u00a0V in the reverse (anodic) scan at a scan rate of 50\u00a0mV\u00a0s\u22121. From Fig. 1A and B, it is evident that the voltammogram curves depend on the electrolyte pH. In the solution with the pH of 2.5 (Fig. 1A), the first anodic current increase at \u22120.4\u00a0V is attributed to the reduction of H3O+ (2H3O+ + 2e\u2212 \u2192H2\u00a0+\u00a02H2O), cH1, due to the low pH value. This reduction reaction is followed by a plateau at E \n\u2248\n \u22120.8\u00a0V that indicates the reduction of nickel (Ni2+ + 2e\u2212 \u2192Ni0), cNi, and the hydrogen-evolution region via the water reduction at E\u00a0>\u00a0\u22121.0\u00a0V (2H2O\u00a0+\u00a02e\u2212 \u2192H2\u00a0+\u00a02OH\u2212), cH2.The CV profile observed in the solution with the pH of 5.5 (Fig. 1B) was only characterized with a cathodic peak (cNi) at E \n\u2248\n \u22120.8\u00a0V, which indicates the reduction of nickel Ni2+ to Ni0 (Ni2+ + 2e\u2212 \u2192 Ni0). Also, from E\u00a0>\u00a0\u22121.0\u00a0V onwards, the evolution of hydrogen from water reduction (cH2) is taking place according to the reaction 2H2O\u00a0+\u00a02e\u2212 \u2192 H \u00a0+\u00a02OH\u2212. From the CV profiles (Fig. 1A and B) it is also evident that the output current is strongly influenced by the electrolyte pH. The current density of the cathodic peak cNi increases (6x) with a pH decrease (from 5.5 to 2.5), which indicates a change in the reaction mechanism for the electrodeposition of Ni [57]. Double the maximum current of the cathodic peak cH2 was observed at a pH of 2.5 (i\u00a0=\u00a018\u00a0mA at E\u00a0=\u00a0\u22121.2\u00a0V), which indicates a more intensive hydrogen-evolution reaction [57]. In agreement with the excessive 2H2O\u00a0+\u00a02e\u2212 \u2192 H2\u00a0+\u00a02OH\u2212 reaction at pH\u00a0=\u00a02.5 (as seen from Fig. 1, A) there is also an increase of OH\u2212 ions in the vicinity of the electrode. Taking this as a fact and due to the presence of Ni2+ ions in the electrolyte, an (electro)chemical precipitation of the poorly soluble Ni(OH)2 might occur at the electrode surface, as was explained by Hall et al. [33]. Furthermore, from Fig. 1A, the fluctuations of the anodic output current were observed at E\u00a0>\u00a0\u22121.0\u00a0V which are induced by the pronounced evolution of H2 bubbles that affect the mass-transport as they block the surface of cathode [58\u201360].In the reverse scan, the anodic current peak aH at E\u00a0=\u00a0\u22120.015\u00a0V (Fig. 1A and B) was observed, which is attributed to the oxidation of the adsorbed hydrogen (formed during the forward scan at cH). However, the anodic peak aNi (at E = +0.15\u00a0V) that corresponds to the oxidation of Ni0\u00a0+\u00a02OH\u2212 \u2192 Ni(OH)2\u00a0+\u00a02e\u2212 was observed only in Fig. 1B. The absence of the anodic peak aNi on the CV presented in Fig. 1A was expected since the formation of Ni(OH)2 via the (electro)chemical precipitation takes place in the forward scan (as described above). From the CV measurements it was concluded that different surface compositions and morphologies of the Ni thin films can be achieved by applying negative potentials\u00a0\u2264\u00a0\u22121V. For this reason, the Ni thin films were fabricated from electrolytes with a pH of 2.5 and 5.5 when applying a constant potential of \u22121.0\u00a0V for 300\u00a0s.\nFig. 2\nA\u2013D, shows the FEG-SEM images of the Ni thin films electrodeposited from two pH electrolytes (2.5 and 5.5) when applying a constant potential of \u22121.0\u00a0V. Fig. 2A and B, shows a typical morphology of the Ni thin film obtained by electrodeposition from the electrolyte with a pH of 2.5. As seen in Fig. 2A, the Ni thin film electrodeposited at lower pH (pH\u00a0=\u00a02.5), i.e., as-deposited Ni thin film (pH\u00a0=\u00a02.5), is composed of small crystalline nanoparticles (50\u2013100\u00a0nm). The lines are present due to the topography of the underlying Au substrate. The image of the as-deposited Ni thin film (pH\u00a0=\u00a02.5) obtained at lower magnification (Fig. 2B) reveals that the Au substrate could not be entirely covered by Ni (Ni2+ + 2e\u2212\n\n\u2192\n 2Ni0) due to the extensive hydrogen formation at low (highly acidic) pH. The chemical composition of electrodeposited Ni thin film (pH\u00a0=\u00a02.5) was examined by EDS and XRD (explained below). The main detected element was Ni (91\u201395 at. %). Also, the residual amounts of oxygen (1\u20135 at. %) and gold (1\u20135 at. %) from the underlying substrate were detected which implies that the film is thin (<1 \u03bcm), as the interaction volume with using accelerating voltage of 10\u00a0keV is below 1\u00a0\u03bcm3. Fig. 2C and D, shows the typical morphology of the Ni thin film obtained by electrodeposition from the electrolyte with a pH of 5.5. As seen in Fig. 2C, the Ni thin film electrodeposited at higher pH (pH\u00a0=\u00a05.5) is composed of larger crystallites expanded to the size of 100\u2013300\u00a0nm, i.e., as-deposited Ni thin film (pH\u00a0=\u00a05.5). The image of the as-deposited Ni thin film (pH\u00a0=\u00a05.5) obtained at lower magnification (Fig. 2D) indicates that the Ni deposit is homogeneously covering the Au substrate since hydrogen formation is less effective due to the higher pH. The chemical composition of electrodeposited Ni thin film (pH\u00a0=\u00a05.5) was also determined by EDS and XRD (explained below). The main detected element was Ni (97\u201399 at. %). Also, the residual amount of oxygen was detected (1\u20133 at. %). In comparison to Ni thin film (pH\u00a0=\u00a02.5), the deposit is thicker as the EDS did not show the presence of underlying Au substrate (the interaction volume with using accelerating voltage of 10\u00a0keV is below 1\u00a0\u03bcm3). The larger thickness might be the reason for irregular cracks in the Ni thin film, pH\u00a0=\u00a05.5 (see, for example, Fig. 2C and D). As stated in the literature [30,31,61,62], the cracked nature of the thicker Ni films is a common problem of wet chemical deposition methods, attributed to the drying contraction caused due to tensile stress. These findings are in agreement with those presented by Boubatra et al. [57]. They explained that the intensive hydrogen formation (favoured at lower pH) influences the electrochemical conditions in the vicinity of the cathode, changes the growth/nucleation processes and thus affects the final thickness and size distribution of the crystallites, e.g., a lower pH (2.5) decreases the grain size of the Ni electrodeposit by increasing the nucleation rate, or a higher pH (5.5) increases the grain size of the Ni by decreasing the nucleation rate [57,63\u201365].The influence of the electrolyte pH on the surface crystal structure of as-deposited Ni thin films (pH\u00a0=\u00a02.5 and 5.5) was analysed by XRD. Fig. 3\n is an XRD pattern of the A) Au/Cr/SiO2 substrate, i.e., background, B) as-deposited Ni thin film (pH\u00a0=\u00a02.5) and C) Ni thin film (pH\u00a0=\u00a05.5) observed in the 2-theta range 10\u201390\u00b0 with the insets showing the narrow 2-theta region 10\u201326\u00b0, 32\u201343\u00b0 and 70\u201390\u00b0 for better clarity. Rough indexing of the XRD pattern of the Au/Cr/SiO2 substrate displays the presence of fcc Au with three characteristics peaks at 38.2\u00b0, 77.5\u00b0 and 81.7\u00b0 corresponding to the standard Bragg reflections (111), (311) and (222) (ICDD 01-089-3697) [66]. The intense peak at 38.2\u00b0represents preferential growth in the (111) direction. The presence of bcc Cr was observed at 36.5\u00b0 and 81.7\u00b0 (ICDD 00-006-0694) [67] and a broad hump at 2-theta 20-25\u00b0 indicates the presence of SiO2 (ICDD 00-033-1161) [68,69]. The representative characteristic peaks for the substrate were also identified from the XRD patterns of the as-deposited Ni thin film: pH\u00a0=\u00a02.5 (Fig. 3B) and the Ni thin film: pH\u00a0=\u00a05.5 (Fig. 3C). Furthermore, in the XRD diffractogram of the Ni thin films (pH\u00a0=\u00a02.5 and 5.5), two characteristic peaks for nickel were observed at 44.5\u00b0 and 76.5\u00b0 and were assigned to the standard Bragg reflections (111) and (220) of the fcc lattice (ICDD 00-001-1258) [57]. The intense peak at 44.5\u00b0 represents preferential crystallographic growth in the (111) direction. Furthermore, significant differences in the crystal structure of the Ni thin films (pH\u00a0=\u00a02.5 and 5.5) were observed at 2-theta equal to 12\u00b0, 12.5\u00b0, 34\u00b0, 34,6\u00b0 and 42.3\u00b0. These reflections indicate the presence of hexagonal \u2018\u2019quasi\u2019\u2019 close-packed [29] \u03b1-Ni(OH)2 (ICDD 00-038-0715 and ICDD 00-022-0444) only in the case of the Ni thin film (pH\u00a0=\u00a02.5). The presence of \u03b1-Ni(OH)2 is a result of the electrochemical deposition conditions (pH of electrolyte). As already described, during the cathodic electrodeposition of the Ni thin film (pH\u00a0=\u00a02.5) from a low-pH electrolyte, some of the current is consumed by hydrogen formation. This side reaction affects the local pH due to the formation of hydrogen and water at the cathode surface. Since the cathode is held at a very negative potential (E\u00a0=\u00a0\u22121.0\u00a0V), the intensive reduction of water occurs (2H2O\u00a0+\u00a02e\u2212 \u2192 H2\u00a0+\u00a02OH\u2212) and thus leads to the massive production of the OH\u2212. Due to the low solubility of Ni(OH)2, (electro)chemical precipitation immediately occurs on the surface of the cathode [33,57].However, when using the XRD, we were not able to identify the peak at 2-theta equals 19\u00b0 (Fig. 3B and C, insets) that can be attributed to the rhombohedral NiO2 (ICDD 04-012-0153) or hexagonal close-packed (hcp) \u03b2-Ni(OH)2 (ICDD 04-015-5276). It was assumed that the characteristic peak at 19\u00b0 in the case of the Ni thin film: pH\u00a0=\u00a02.5 indicates the presence of rhombohedral NiO2, rather than the presence of \u03b2-Ni(OH), as it was prepared via the (electro)chemical precipitation process. The literature states [29,33] that hydroxides do not tend to form \u201cwell-crystallized\u201d structures (i.e., \u03b2-Ni(OH)2) during (electro)chemical precipitation from aqueous salt (acidic) solutions. In the case the Ni thin film (pH\u00a0=\u00a05.5), we were not able to attribute any crystal phase to the characteristic peak at 19\u00b0 from the XRD (the other representative characteristic peaks for NiO2 and Ni(OH)2 are overlapping with the Au or Ni). In addition, FT-IR analysis was performed.\nFig. 4\n has the FT-IR spectra of the (A) as-deposited Ni thin film (pH\u00a0=\u00a02.5) and (B) as-deposited Ni thin film (pH\u00a0=\u00a05.5). In order to examine the presence of NiO2 or Ni(OH)2, the FT-IR spectra were observed in the wavelength range from 400 to 4000\u00a0cm\u22121. Fig. 4A shows typical FT-IR spectra of the as-deposited Ni thin film (pH\u00a0=\u00a02.5). The inset spectrum, which represents the wavelength region from 400 to 700\u00a0cm\u22121, indicates the presence of the Ni\u2013O\u2013H vibration peak at 610\u00a0cm\u22121. Furthermore, the band located at 3000\u20133600\u00a0cm\u22121 indicates the presence of the symmetric \u2013O\u2013H vibrations and the absorption peak for the \u2013OH (hydroxyl) functional group at 1720\u00a0cm\u22121\n[31,70]. From these results, the presence of Ni(OH)2 on the Ni thin film (pH\u00a0=\u00a02.5) was additionally confirmed. Fig. 4B shows the FT-IR spectra of the as-deposited Ni thin film (pH\u00a0=\u00a05.5) that was measured following the same procedure as the as-deposited Ni thin film: pH\u00a0=\u00a02.5 (Fig. 4A). The FT-IR analysis did not reveal any vibration bands that would be typical for Ni(OH)2 being present on the Ni thin film: pH\u00a0=\u00a05.5) (i.e., the absence of two of the most representative \u2013O\u2013H vibration peaks at 3000\u20133600\u00a0cm\u22121 and 1720\u00a0cm\u22121). The findings obtained from the FT-IR measurements (Fig. 4B) confirmed the absence of Ni(OH)2. By this observation, we concluded that the surface of Ni thin film (pH\u00a0=\u00a05.5) is covered with the NiO2 (an XRD characteristic peak at 19\u00b0, Fig. 3C) formed by surface passivation once it is exposed to air and/or water molecules [2,22,46].In order to remove the native NiO2\n[2,46] and build up the catalytically more active structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species [34], potential cycling in an alkaline electrolyte (i.e., the KOH-modification process) was introduced to the Ni thin films (pH\u00a0=\u00a02.5 and 5.5). This experimental step is mandatory in the case of the Ni thin film (pH\u00a0=\u00a05.5) because a system consisting of native NiO2 without the presence of Ni(OH)2 on the surface does not show catalytic activity towards the HCHO oxidation [12,46]. Fig. 5\n presents the CV profiles of (A) the as-deposited Ni thin film (pH\u00a0=\u00a02.5) and (B) the as-deposited Ni thin film (pH\u00a0=\u00a05.5) observed in 0.5-mol L\u22121 KOH at a scan rate 200\u00a0mV\u00a0s\u22121. The scan rate of 200\u00a0mV\u00a0s\u22121 was selected based on our previous research [43,46,71,72]. Cycles 1\u201360 are included in order to show the evolution of CVs behaviour with the increasing number of cycles.\nFig. 5, A-1 and B-1, demonstrate CV profiles in the 1st cycle where the potential window was chosen from \u22121.0\u00a0V to +0.7\u00a0V in the anodic region (forward scan) and from 0.7\u00a0V to 0.2\u00a0V in the cathodic region (reverse scan). The first increase in the anodic current density (a1) was observed at the potential \u22120.7\u00a0V (1-A)/-0.5\u00a0V (1-B) and corresponded to the formation of \u03b1-Ni(OH)2. The \u03b1-Ni(OH)2 was formed according to the following reactions: Ni\u00a0+\u00a02 OH\u2212\n\n\u2192\n \u03b1-Ni(OH)2\n[3,22] or NiO2\u00a0+\u00a0H2O \n\u2192\n \u03b1-Ni(OH)2\n[22]. The CV profiles in the 1st cycle demonstrate that the values of the anodic peak potential (Ea1) vary between the Ni thin films (pH\u00a0=\u00a02.5 and 5.5). The shift of the a1 peak potential is a result of an electro-crystallisation process, where the electrochemical reaction (electron transfer) is accompanied by atomic rearrangements (e.g., from fcc Ni0 or rhombohedral NiO2 structures to a brucite type of \u2018\u2019quasi\u2019\u2019-hcp \u03b1-Ni(OH)2 structure [73]). As the chemical composition and crystal phase of the surface species vary between the as-deposited Ni thin films: pH\u00a0=\u00a02.5 and 5.5 (Figs. 3 and 4), each of these electrodes needs a certain driving force (overpotential) for electro-crystallisation [74,75]. Thus, the electro-crystallisation process is probably the main reason for a shift of the a1 peak potential in between the Ni thin films (pH\u00a0=\u00a02.5 and 5.5). Furthermore, the CV plots (Fig. 5, 1-A and 1-B) reveal a difference in the observed a1 current densities: j (a1, Ni thin film: pH\u00a0=\u00a02.5) \n\u2248\n 0.01\u00a0mA\u00a0cm\u22122 (Fig. 5, 1-A) and j (a1, Ni thin film: pH\u00a0=\u00a05.5) \n\u2248\n 0.5\u00a0mA\u00a0cm\u22122(Fig. 5, 1-B). This observation indicates that only a small amount of \u03b1-Ni(OH)2 is formed at a1 on the surface of the as-deposited Ni thin film (pH\u00a0=\u00a02.5), since the surface is already occupied with \u03b1-Ni(OH)2 after electrodeposition (proved by the XRD and FT-IR, Figs. 3 and 4). On the other hand, the larger increase in the anodic current peak a1 was observed in the case of Ni thin film (pH\u00a0=\u00a05.5), demonstrating the conversion of the surface Ni/NiO2 (Figs. 3 and 4) to \u03b1-Ni(OH)2.With the continuation of the potential cycling to the more positive potentials (from \u22120.4\u00a0V to +0.4\u00a0V) the chemical dehydration of \u03b1-Ni(OH)2 to \u03b2-Ni(OH)2 (a2) occurs [34]. Once the \u03b2-Ni(OH)2 is formed, it cannot be electrochemically reduced back to \u03b1-Ni(OH)2/NiO/Ni(metallic) as large atomic rearrangements are no longer expected due to the high electrochemical stability of the \u03b2-phase [9].After the potential cycling to the more positive potentials, the next increase in the current densities was observed at a3 (E \u2248 0.4\u00a0V for Ni thin film: pH\u00a0=\u00a02.5 and E \u2248 0.45\u00a0V for Ni thin film: pH\u00a0=\u00a05.5). The a3 represents the oxidation of the \u03b2-Ni(OH)2 (formed at a2) to \u03b2-NiOOH according to the reaction: \u03b2-Ni(OH)2\u00a0+\u00a0OH\u2212\n\n\u2192\n \u03b2-NiOOH\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212. As seen from Fig. 5, A-1 (Ni thin film: pH\u00a0=\u00a02.5), the j(a3) reaches higher values than the j(a1). This means that the formation of surface \u03b2-NiOOH (i.e., \u03b1-Ni(OH)2 (formed at a1) \n\u2192\n \u03b2-Ni(OH)2 (formed at a2)\u00a0+\u00a0OH\u2212\n\n\u2192\n \u03b2-NiOOH (formed at a3)\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212) can also occur through the small amount of electrodeposited \u03b1-Ni(OH)2 if dehydrated. However, in the case of the Ni thin film: pH\u00a0=\u00a05.5 (Fig. 5, 1-B), the j(a3)\u00a0<\u00a0j(a1) indicating the incomplete transformation of \u03b1-Ni(OH)2 (formed at a1) to \u03b2-Ni(OH)2 (formed at a2). However, once the a3 current peak (Fig. 5, A-1 and B-1) was observed, the scans were immediately reversed back to +0.0\u00a0V in order to avoid the production of molecular oxygen (4OH\u2212\n\n\u2192\n O2\u00a0+\u00a0H2O\u00a0+\u00a04e\u2212). The formation of O2 is undesirable due to the possible adsorption on the surface of the Ni thin films [4,46] and thus blocking the surface-active Ni sites [4,72]. In the cathodic region, one cathodic peak (c1) was observed (Fig. 5, A-1 and B-1). This cathodic peak (c1) at E\u00a0=\u00a00.3\u00a0V (Ni thin film: pH\u00a0=\u00a02.5) and E\u00a0=\u00a00.25\u00a0V (Ni thin film: pH\u00a0=\u00a05.5) corresponds to the reduction of \u03b2-NiOOH (formed at a3) to \u03b2-Ni(OH)2. Based on the above-described results, it was assumed that after the 1st cycle (Fig. 5, A-1 and B-1) the surface of the Ni thin film (pH\u00a0=\u00a02.5) is still (mostly) covered by \u03b1-Ni(OH)2 (formed directly during Ni electrodeposition) and by a small amount of \u03b2-Ni(OH)2 (formed at a3). The same surface composition (i.e., \u03b1-Ni(OH)2 and small/negligible amounts of \u03b2-Ni(OH)2) were ascribed to the Ni thin film (pH\u00a0=\u00a05.5); however, the \u03b1-Ni(OH)2 was formed at a1 in the 1st cycle of the KOH-modification process.Since the crystallinity of the Ni(II) hydroxides can be controlled from highly crystalline (\u03b2-phase) to structurally disordered hydroxides by tailoring the experimental conditions, we selected the continuous CV cycling in KOH. The literature indicates that continuous potential cycling over the Ni(OH)2/NiOOH redox peaks leads to the transformation of well-crystalline \u03b2- Ni(OH)2 to a large set of disordered \u03b2-Ni(II) hydroxides with a variable excess of intersheet water, stacking-fault disorder or mechanical stresses [2], which proved themselves in enhanced electrocatalytic HCHO oxidation [16,44\u201346,76]. Due to this, the Ni thin films (pH\u00a0=\u00a02.5 and 5.5) were KOH modified in KOH up to 60 times. Fig. 5 shows the CV profiles of the A-2) Ni thin film \u2013 pH\u00a0=\u00a02.5 and B-2) Ni thin film \u2013 pH\u00a0=\u00a05.5 observed in the potential range from 0\u00a0V to +0.6\u00a0V from the 10th to 60th cycle. From the CV plots, the anodic peak (a3) was observed at E\u00a0=\u00a00.4\u00a0V (Ni thin film: pH\u00a0=\u00a02.5) and E\u00a0=\u00a00.45\u00a0V (Ni thin films: 5.5) in the forward scan. The a3 corresponds to the formation of the \u03b2-NiOOH (most probably structurally disordered) due to the diffusion of OH\u2212 to the \u03b2-Ni(OH)2 (formed in the 1st cycle) or to the NiO2\n[4,5,14] according to the reactions: \u03b2-Ni(OH)2\u00a0+\u00a0OH\u2212\n\n\u2192\n \u03b2-NiOOH\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212 or/and NiO2\u00a0+\u00a0H2O -> \u03b2-Ni(OH)2\u00a0+\u00a0OH\u2212\n\n\u2192\n \u03b2-NiOOH\u00a0+\u00a0e\u2212, respectively. With increasing numbers of cycles from 10 to 60, the current densities of the anodic (a3) and cathodic (c1) peaks increase until a steady state is reached and there is no significant current-density increase for the cycles \n\u2265\n 50 (\n\n\u0394\nj\n\n\n50\u201360 cycles\n\n\u2248\n 0). From the steady-state on, no further changes in the surface composition are expected [46]. In the reverse scan, the cathodic (c1) peak was observed for both Ni thin films and corresponds to the reduction of \u03b2-NiOOH (formed at a3 from the 10th to 50th cycle) to \u03b2-Ni(OH)2: \u03b2-NiOOH\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212\n\n\u2192\n \u03b2-Ni(OH)2\u00a0+\u00a0OH.The KOH-modified Ni thin films (after 60 cycles): pH\u00a0=\u00a02.5 and 5.5 (Fig. 5) were analysed by XRD in order to investigate their surface crystal structure after the KOH-modification process. Fig. 6\nA has the XRD patterns of the KOH-modified (green curve) and as-deposited (grey curve) Ni thin film (pH\u00a0=\u00a02.5) observed in the 2-theta ranges 10\u201325\u00b0 (left) and 32\u201336\u00b0 (right). The XRD diffractogram of the KOH-modified Ni thin film \u2013 pH\u00a0=\u00a02.5 consists of hcp \u03b1-Ni(OH)2 (ICDD 00-022-0444 [29]) with the three characteristics peaks at 2-theta of 12\u00b0, 24\u00b0, 34\u00b0 and 34.6\u00b0 and rhombohedral NiO2 (ICDD 04-012-0153) with the characteristic peak at 19\u00b0. Also, indexing this XRD pattern reveals the presence of the substrate, i.e., SiO2 (ICDD 00-033-1161), at 20\u201325\u00b0. As seen from Fig. 6A, the XRD pattern of KOH-modified Ni thin films \u2013 pH\u00a0=\u00a02.5 (green curve) and as-deposited Ni thin films \u2013 pH\u00a0=\u00a02.5 (grey curve) are similar. After the KOH-modification process, only a mild increase in the intensity of the \u03b1-Ni(OH)2 characteristic peaks is observed, indicating a slight enrichment of the amount of \u03b1-Ni(OH)2 on the surface. Fig. 6B (blue curve) shows the XRD pattern of the KOH-modified Ni thin film (pH\u00a0=\u00a05.5) observed in the 2-theta ranges 10\u201325\u00b0 (left) and 32\u201336\u00b0 (right). In order to demonstrate the changes in the composition of surface species after the KOH-modification process, the graphs also include the XRD patterns of the as-deposited Ni thin films \u2013 pH\u00a0=\u00a05.5 (black curves). From the XRD pattern of the as-deposited and KOH-modified Ni thin film \u2013 pH\u00a0=\u00a05.5 (Fig. 6, B), the following diffraction peaks were observed: characteristic peak for rhombohedral NiO2 (ICDD 04-012-0153) at 19\u00b0 and broad hump for SiO2 (i.e., substrate) at 20\u201325\u00b0 (ICDD 00-033-1161). The significant differences between the as-deposited (black curves) and KOH-modified (blue curves) Ni thin film (pH\u00a0=\u00a05.5) were observed in the 2-theta regions 23\u201325\u00b0 (left) and 33.5\u201334.5\u00b0 (right). The XRD pattern of the KOH-modified Ni thin film \u2013 pH\u00a0=\u00a05.5 (blue) indicates the presence of two different electrochemically assembled hcp \u03b1-Ni(OH)2 with the characteristic peaks at 12.5\u00b0, 24.2\u00b0, 34\u00b0 and 34.5\u00b0 that can be attributed to the 3Ni(OH)2\u00b72H2O (ICDD 00\u2013022-0444), and characteristic peaks at 12\u00b0, 23.5\u00b0, 33.9\u00b0and 34.4\u00b0 that can be ascribed to the Ni(OH)2\u00b70.75H2O (ICDD 00-038-0715).Furthermore, the indexing of the XRD patterns confirmed the absence of crystalline \u03b2-Ni(OH)2 that is formed at a3 (Fig. 5, A-2 and B-2) [3]. The absence of Ni(OH)2 (formed at a3) during potential cycling, Fig. 5, A-1 and B-1) reflection in the XRD was attributed to its disordered structure that resulted from potential cycling in KOH for up to 60 cycles [2,46] and was already confirmed in our previous research by TEM and FT-IR [46,72].In order to determine the amount of the structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH (formed at a3 during the potential cycling for up to 60 cycles) on the surface of the Ni thin films (pH\u00a0=\u00a02.5 and 5.5), the number of monolayers was calculated according to the equation (3.1):\n\n(3.1)\n\n\n\n\n\u0393\n\nNifilm\n\n\n\u00d7\n1\nm\no\nn\no\nl\na\ny\ne\nr\n\n\n\u0393\n\nstandard\n\n\n\n\n\nwhere \n\n\u0393\n\nstandard\n\n\n is the number of moles per square centimetre of structurally disordered \u03b2-NiOOH [mol cm\u22122] for 1 monolayer, i.e., 1.06\u00a0\u00d7\u00a010\u22129 mol cm\u22122 (obtained by Bode [14,37]). The \n\n\u0393\n\nNifilm\n\n\n [mol cm\u22122] was calculated using the equation (3.2):\n\n(3.2)\n\n\nQ\n=\nn\nF\n\nA\n\necsa\n\n\n\n\u0393\n\nNifilm\n\n\n\n\n\nwhere \nQ\n is the charge [As\u00a0=\u00a0C], \nn\n is the number of electrons, \nF\n is the Faraday constant (96485.33 As mol\u22121) and \n\nA\n\necsa\n\n\n is the electrochemically determined surface area of the Ni thin films (Supplementary data). The charge was calculated based on the area (\n\nidE\n\n) under the reduction peak of the \u03b2-NiOOH to \u03b2-Ni(OH)2 (c1) (3.3):\n\n(3.3)\n\n\nQ\n=\n\n\u222b\ni\n\nd\n\u03c4\n=\n\n\n1\n\u03c5\n\n\n\u222b\ni\n\nd\nE\n\n\n\n\nwhere \u03bd is the scan rate [V s\u22121] and i [A] is the peak current. The reduction peak (c1) was selected as it provides the charge required to fully reduce an oxyhydroxide and does not overlap any other electrochemical processes [14,33,71]. Thus, the Ni thin film (pH\u00a0=\u00a02.5) with an \n\nA\n\necsa\n\n\n of 1.72\u00a0cm2, and a cathodic charge of 6.7\u00a0\u00d7\u00a010\u22125C, the number of monolayers is calculated to be 0.4, depending on the overall electrochemically active surface area. In the case of the Ni thin film (pH\u00a0=\u00a05.5) with a \n\nA\n\necsa\n\n\n of 2.19\u00a0cm2, and a cathodic charge of 2.2\u00a0\u00d7\u00a010\u22124C, the number of monolayers is calculated to be 0.9, depending on the overall electrochemically active surface area. The fact that the calculated values were determined to be\u00a0<\u00a01 means that the structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH most probably covers small patches and not the entire electrochemically active surface area of both Ni thin films (pH\u00a0=\u00a02.5 and 5.5).Based on the XRD (Fig. 6) and the CV behaviour in KOH (Fig. 5), and the calculated amount of disordered-Ni(OH)2, a possible mechanism for the surface transformation during the KOH-modification process for Ni thin films (pH\u00a0=\u00a02.5 and 5.5) is as follows. The mechanism for the surface transformation of the Ni thin film \u2013 pH\u00a0=\u00a02.5 is presented in Fig. 7\n. As can be seen from Fig. 7a, the surface of the Ni thin film is covered by electrodeposited \u03b1-Ni(OH)2 before the KOH-modification process. When the electrodeposited Ni thin film (pH\u00a0=\u00a02.5) is placed in the KOH solution and exposed to prolonged cycling over the oxidation and reduction potential range (Fig. 7b), a small increase in the amount of \u03b1-Ni(OH)2 (formed at a1 in the 1st cycle, Fig. 5, A-1) is observed (proved by XRD, Fig. 6A). As the conditioning continues by potential cycling (up to 60 cycles), the amount of structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species (formed at a3, Fig. 5, A-2) does not increase significantly (Fig. 7c), as we observed a small current-density increase (\n\n\u0394\nj\n\n(a3, up to cycle 60)\u00a0<\u00a00.07\u00a0mA\u00a0cm\u22122) (Fig. 5, A-2). In addition, the calculated value of 0.4 for the monolayer confirmed that the amount of structurally disordered-Ni(OH)2 is small compared to the \u03b1-phase after the KOH-modification process.For the Ni thin film \u2013 pH\u00a0=\u00a05.5, the KOH-modification process influences the surface composition of the as-deposited Ni thin film (pH\u00a0=\u00a05.5) since the surface of the electrodeposited film is covered with Ni/NiO2 (Fig. 8\na). The changes take place in the 1st cycle (Fig. 5, A-1) at a1 (the current\u2013density increase \n\u2248\n 0.5\u00a0mA\u00a0cm\u22122) where \u03b1-Ni(OH)2 is formed by the diffusion of OH\u2212 into Ni/NiO2. Also, the potential cycling in KOH more than 60 times (Fig. 8b) increases the amount of structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species due to the larger increase in the current density at a3 (Fig. 5, A-2: \n\n\u0394\nj\n\n(a3, up to cycle 60)\u00a0<\u00a00.25\u00a0mA\u00a0cm\u22122) and the calculations, 0.9 monolayer. From these observations, it was concluded that the electrochemically active surface of the Ni thin film (pH\u00a0=\u00a05.5) is mostly composed of structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species (Fig. 8c).A formaldehyde oxidation ability of all the Ni thin films was investigated via cyclic voltammetry. Fig. 9\na shows the CV profiles of as-deposited (dot curve) and KOH modified (solid curve) Ni thin films (pH\u00a0=\u00a02.5), both observed in 1-mmol L\u22121 HCHO and 0.1-mol L\u22121 NaOH at a scan rate of 100\u00a0mV\u00a0s\u22121. From the CV profiles of the as-deposited and KOH-modified Ni thin film (pH\u00a0=\u00a02.5), a well-defined anodic peak (aHCHO) was observed at E\u00a0=\u00a00.75\u00a0V and E\u00a0=\u00a00.70\u00a0V, respectively. Also, a rapid increase of the anodic current density (aoxygen) at E\u00a0>\u00a00.80\u00a0V that correspond to the oxygen-evolution reaction. The aHCHO current increase (jas-deposited\n\u00a0=\u00a00.05\u00a0mA\u00a0cm\u22122 and jKOH-modified\n\u00a0=\u00a00.07\u00a0mA\u00a0cm\u22122) is a result of two parallel reactions [6\u201310,46] (mediated electron-transfer mechanism): oxidation of Ni(OH)2 (i.e., \u03b1-/structurally disordered-\u03b2-Ni(OH)2\u00a0+\u00a0OH\u2212\n\n\u2192\n \u03b3-/structurally disordered-\u03b2-NiOOH\u00a0+\u00a0e\u2212) and the formation of \u201cnew\u201d Ni(OH)2 (i.e., \u03b3-/structurally disordered-\u03b2-NiOOH\u00a0+\u00a0HCHO \n\u2192\n \u03b1/\u03b2-structurally disordered-Ni(OH)2\u00a0+\u00a0CO2\u00a0+\u00a0H2O). In the cathodic region, a cathodic peak (c1) was observed at E\u00a0=\u00a00.68\u00a0V for the as-deposited Ni thin film: pH\u00a0=\u00a02.5 and at E\u00a0=\u00a00.4\u00a0V for the KOH-modified Ni thin film: pH\u00a0=\u00a02.5. The cathodic current increase (c1) was attributed to the reversible transformation of the \u03b3-/structurally disordered \u03b2-NiOOH to the \u03b1/\u03b2-structurally disordered Ni(OH)2.\nFig. 9b presents the CV profiles of the as-deposited (dot curve) and KOH-modified (solid curve) Ni thin films (pH\u00a0=\u00a05.5), both observed in 1-mmol L-1 HCHO and 0.1-mol L\u22121 NaOH at a scan rate of 100\u00a0mV\u00a0s\u22121. The CV response for the as-deposited Ni thin film: pH\u00a0=\u00a05.5 (dot curve) shows a complete electrode\u2019s inactivity towards HCHO due to the absence of the HCHO oxidation peak (aHCHO). An almost negligible electrocatalytic activity for the HCHO oxidation of the as-deposited Ni thin film (pH\u00a0=\u00a05.5) can be ascribed to the presence of the native NiO2 on the surface of the film (proved by XRD, FT-IR). However, the CV profile of the KOH-modified Ni thin film: pH\u00a0=\u00a05.5 (solid curve) revealed the presence of an anodic peak (aHCHO) at E\u00a0=\u00a00.67\u00a0V, j\u00a0=\u00a00.24\u00a0mA\u00a0cm\u22122 and a cathodic peak (c1) at E\u00a0=\u00a00.15\u00a0V, j\u00a0=\u00a0\u22120.23\u00a0mA\u00a0cm\u22122. This result shows that the KOH-modified Ni thin film (pH\u00a0=\u00a05.5) is also oxidized HCHO via mediated mechanisms (as explained above).In order to be able to make a direct comparison of the produced Ni thin films for HCHO oxidation catalytic ability, the output currents were normalized to the electrochemically active surface areas (Supplementary data). From Table 1\n it is clear that the increased current densities of the aHCHO and the decreased HCHO oxidation onset potentials (or aHCHO peak potentials) were achieved in the following sequence: as-deposited Ni thin film (pH\u00a0=\u00a05.5)\u00a0=\u00a00\u00a0<\u00a0as-deposited Ni thin film (pH\u00a0=\u00a02.5)\u00a0<\u00a0KOH-modified Ni thin film (pH\u00a0=\u00a02.5)\u00a0<\u00a0KOH-modified Ni thin film (pH\u00a0=\u00a05.5).The Tafel slopes (Fig. 10\n) for the as-deposited Ni thin film (pH\u00a0=\u00a02.5), KOH-modified Ni thin films (pH\u00a0=\u00a02.5 and 5.5) were examined to describe the influence of the onset potential on the steady-state current density. The as-deposited Ni thin film (pH\u00a0=\u00a05.5) consisting of NiO2 surface species was excluded since it shows complete catalytic inactivity towards HCHO in alkaline media. The Tafel plots, which reflect the charge-transfer kinetics, were determined by fitting the CV data (Fig. 9a and b) to the equation \u03b7\u00a0=\u00a0a\u00a0+\u00a0b\n\u00d7\nlog\n\n\nj\n\n\n, where \u03b7 is the over-potential, b is the slope of the Tafel curve and \nj\n is the current density [20]. The observed slope of the KOH modified Ni thin film: pH\u00a0=\u00a05.5 (blue) is 69\u00a0mV dec\u22121 smaller than the KOH modified Ni thin film: pH\u00a0=\u00a02.5 (green), and 141\u00a0mV dec\u22121 smaller than the as-deposited Ni thin film: pH\u00a0=\u00a02.5 (wine). These results indicate that the KOH-modified Ni thin film (pH\u00a0=\u00a05.5) shows improved kinetics and thus exhibits the highest catalytic activity towards HCHO oxidation with the highest current density (0.24\u00a0mA\u00a0cm\u22122) and the lowest onset potential (0.44\u00a0V), which is lower than the reported values of 0.55\u00a0V for the nanoporous-NiPh-modified electrode [49] or 0.5\u00a0V for the Ni/IL/CPE [8], Ni(OH)2/POT (TX-100)/MCNTPE [7] and Ni/P-nanozeolite-modified electrode [10]. Assuming the Bode\u2019s diagram [34] is valid for \u03b1-Ni(OH)2/\u03b3-NiOOH transformation, our results suggest that the poorer electro-catalytic activity of KOH-modified Ni film (pH\u00a0=\u00a02.5) is due to the presence of the smaller amount of highly active disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species and higher amount of \u03b1-Ni(OH)2/\u03b3-NiOOH surface species as the \u03b1\u2013\u03b3 transformation requires a higher input energy for the transfer of 1.6\u20131.67 electrons in HCHO electrocatalytic oxidation. Upon that an enhanced electro-catalytic activity (low onset overpotential of 0.44\u00a0V) of the KOH-modified Ni thin film (pH\u00a0=\u00a05.5) is attributed to a larger amount of highly active structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH surface species which enable a transfer of only 1 electron during the charging and discharging of \u03b2-Ni(OH)2/\u03b2-NiOOH in HCHO alkaline electrolyte. Hence, this \u03b2-Ni(OH)2/\u03b2-NiOOH transformation consumes less input energy needed for electrocatalytic HCHO oxidation.This study proposes a mechanism for the formation of the highly active, structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH redox pair on the surface of two different Ni thin films, produced at pH\u00a0=\u00a02.5 and pH\u00a0=\u00a05.5. A highly active structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH species were produced from the available surface NiO2 via KOH modification process that encompasses potential cycling through the NiOOH/Ni(OH)2 redox peaks in KOH up to 60 cycles. Based on the quantitative analysis of \u03b2-Ni(OH)2/\u03b2-NiOOH (i.e., calculations based on electric charge) it was shown that the amount of \u03b2-Ni(OH)2/\u03b2-NiOOH surface species plays a crucial role in the electrocatalytic oxidation of HCHO in alkaline media. The experimental data showed the enhanced electrocatalytic activity with the lowest onset overpotential of 0.44\u00a0V vs Ag/AgCl for the KOH-modified Ni film (pH\u00a0=\u00a05.5). The improved electrocatalytic activity in later case is attributed to the higher amount of structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH species which enable a transfer of only 1 electron during the charging and discharging, which in comparison to \u03b1-Ni(OH)2/\u03b3-NiOOH thus consumes less input energy. We have shown that via finetuning of the character of the Ni-surface species, we can tune their electrochemical and catalytic performances.\n\u0160pela Trafela: Investigation, Methodology, Formal analysis, Conceptualization, Writing - original draft. Sa\u0161o \u0160turm: Funding acquisition, Writing - review & editing. Kristina \u017du\u017eek Ro\u017eman: Supervision, Conceptualization, Funding acquisition, Writing - review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This study was supported by the Slovenian Research Agency through the J2-8182 and the PR-06805 research projects and the and P2-0084 program, of which this investigation forms a part. The corresponding author gratefully appreciates the financial support from the COST action MP1407 that provided a scholarship for four training-school events in the e-MINDS project. Spela Trafela would like to thank L\u2019Or\u00e9al ADRIA and the Slovenian National Commission for UNESCO for a scholarship awarded by the National Programme for Women in Science in 2020.Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2020.147822.The following are the Supplementary data to this article:\n\nSupplementary Data 1\n\n\n\n", "descript": "\n                  The main challenge with electrocatalysis is finding low-cost electrocatalysts that can work efficiently to oxidize the HCHO. Here, we propose a mechanism for the voltammetric formation of a highly active, structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH redox pair on the surface of electrodeposited Ni thin films to achieve an extraordinary catalytic performance with respect to HCHO oxidation in alkaline media. We report electrochemical, XRD and FT-IR measurements on as-deposited and voltammetrically treated (i.e., KOH-modified) Ni thin films, and calculations based on the electrical charge to investigate the changes in the surface composition, crystal structure and related HCHO oxidation activity. We found that the KOH-modification process plays a crucial role in the formation of surface highly active, disordered \u03b2-Ni(OH)2/\u03b2-NiOOH. The KOH-modified Ni film with the largest amount of the structurally disordered \u03b2-Ni(OH)2/\u03b2-NiOOH resulted in improved catalytic performance, i.e., an onset overpotential reduced by 400\u00a0mV and a catalytic rate increased by 69\u00a0mV dec\u22121. The presented technique has a wide range of applications and provides advances with a novel design idea and a new synthesis strategy for the preparation of highly active, structurally disordered Ni(OH)2/NiOOH redox systems on the surface of Ni thin films and other Ni-based nanostructured electrocatalysts for HCHO oxidation.\n               "}
{"full_text": "The deterioration of the global environment and depletion of fossil fuel resources have driven the global community to search for alternative, sustainable, and eco-friendly energy resources [1]. Among the available energy resources, H2, which has attracted significant attention as a clean resource, is extensively utilized in various applications such as in fertilizer, food, and petrochemical industries [2,3,39]. Currently, H2 is mainly produced by the steam reforming of fossil fuels such as coal, oil, and natural gas, because they are currently the most economical resources [3,4]. However, because fossil fuels are limited energy resources that produce greenhouse gases, there has been a strong demand for developing alternatives to the current H2 production methods [2\u20134].Biomass is one of the promising alternative resources owing to its benefit as a carbon-neutral source [2,4,5]. Terrestrial biomass (e.g., sugar- and starch-based crops, lignocellulose, and agricultural residues) and marine biomass (e.g., algae) are regarded as potential energy sources. However, the terrestrial biomass has the competition issue as the usage for foods and as the agricultural land, and the use of fertilizers and water for producing terrestrial biomass causes environmental issues. In comparison, macroalgal biomass has many advantages as an energy source, including less competition as a food source and land for growth, high productivity owing to a short growth cycle, and efficient carbon dioxide fixation (Scheme 1\n) [6]. Therefore, H2 production using macroalgae can be an attractive alternative to carbon neutralization.Hydrogen can be produced from the hydrocarbon using various methods, including the primary techniques of steam reforming, partial oxidation, and autothermal reforming [2\u20134]. Among these processes, steam reforming is regarded as an economical process owing to its low-temperature operation [2\u20134]. Water-soluble bio-oils, which are thermochemically converted by fast pyrolysis or hydrothermal liquefaction of biomass, are used as a feed for the steam reforming of biomass [2,4]. As most studies have been devoted to the steam reforming of terrestrial biomass [2,7\u201310], the steam reforming of marine algal biomass is still limited. In particular, because many mineral components derived from marine biomass would be harmful to the subsequent steam reforming reaction, the post-treatment of liquefied bio-oils should be carefully considered.The steam reforming of hydrocarbons has been suggested to take place owing via two reactions, steam reforming and water gas shift reactions, as shown in Eqs. (1) and (2) [2,10].\n\n(1)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nk\n\n+\n\n(\n\nn\n\u2212\nk\n\n)\n\n\nH\n2\n\nO\n\n\u2192\nn\nC\nO\n+\n\n(\n\nn\n+\n\n\nm\n2\n\n\u2212\nk\n\n)\n\n\nH\n2\n\n\n\n\n\n\n\n(2)\n\n\nnCO\n+\nn\n\nH\n2\n\nO\n\u2192\nnC\n\nO\n2\n\n+\nn\n\nH\n2\n\n\n\n\n\nThe overall reaction can be summarized as,\n\n(3)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nk\n\n+\n\n(\n\n2\nn\n\u2212\nk\n\n)\n\n\nH\n2\n\nO\n\n\u2192\nn\nC\n\nO\n2\n\n+\n\n(\n\n2\nn\n+\n\n\nm\n2\n\n\u2212\nk\n\n)\n\n\nH\n2\n\n\n\n\n\nNi-based catalysts have been utilized in steam reforming because of their high activity for C\u2013C bond breakage (active for Eq. (1)) and low cost [11,12]. However, they are severely deactivated by coke deposition and sintering [10,13]. Many studies have been devoted to improving the stability of Ni-based catalysts using supports and additives. One of the studied methods is to use hydrotalcite-derived mixed oxide catalysts. Hydrotalcite, which belongs to a family of anionic clays, is a layered double hydroxide (LDH) of Al and Mg consisting of anions and water in the interlayers [14,15]. The structure of the hydrotalcite is closely related to that of brucite, Mg(OH)2, in which certain divalent cations in the layer structure are replaced by trivalent cations. The partial substitution of Mg2+ and/or Al3+ with other cations results in materials with isomorphous structures known as hydrotalcite-like compounds (HTLCs) [14]. The thermal decomposition of HTLC precursors leads to the formation of mixed oxides, resulting in good dispersion of metal cations, high thermal stability, and high surface area compared with those obtained from direct methods. Ni\u2013Mg\u2013Al mixed oxides derived from hydrotalcite exhibit superior activity and stability compared to Ni/Al2O3 and Ni/MgO in the reforming of hydrocarbons [16,17] and pyrolysis oil of biomass [18]. The second method is to add promoters to the Ni-based catalysts. For instance, Li et\u00a0al. reported enhanced catalytic performance of Ni/MgAl hydrotalcite-like compounds by adding Fe or Cu in the steam reforming of biomass tar derived from cedar wood (terrestrial biomass) [9,19]. Especially, Cu addition improved the metal dispersion, oxidation ability, and coke resistance of the Ni/MgAl catalyst [9]. In particular, Cu has been reported to be active in the water gas shift reaction (Eq. (2)) and thus will improve H2 selectivity [20\u201322]. Although NiCu hydrotalcite-derived catalysts are shown to be the promising catalysts for steam reforming of terrestrial biomass-derived hydrocarbons, the investigation over the steam reforming of marine algae-derived hydrocarbons (bio-oil) is limited. Considering that hydrocarbon compositions difference between model compounds and real bio-oil critically affect the catalytic performance of the steam reforming [7], bio-oil derived from macro algae would be different over the NiCu hydrotalcite-derived catalysts.In this study, we studied hydrothermally liquefied bio-oil derived from S.\u00a0japonica (macroalgae) as a clean resource for hydrogen by steam reforming reaction over NiCu hydrotalcite-derived mixed oxide catalysts (Scheme 1). A desalting process was performed to remove the minerals remaining in the raw bio-oil. Thereafter, we controlled the Ni/Cu composition of the NiCuMgAl catalysts and performed the steam reforming of the refined liquefied oil for H2 production.The hydrothermal liquefaction of brown algae (S.\u00a0japonica) was performed under 300\u00a0\u00b0C, 2\u00a0h, autogenous pressure in an autoclave reactor including macro algae (Saccharina japonica) and distilled water at a 1:10\u00a0wt ratio [23]. The chemical components of liquefied oil were analyzed by gas chromatography-mass spectrometry (GC\u2013MS, 7890\u00a0GC/5975C MSD, Agilent, USA). The elemental composition (C, H, and O) and water content were analyzed by elemental analysis (Unicube/rapid OXY cube, EA KOREA, KOR) and Karl Fischer titration.Desalted oil was prepared by adsorption process using the glass filter reactor (I.D.: 35.0\u00a0mm, O.D.: 38.0\u00a0mm, length: 890\u00a0mm, Vol.: 1.1\u00a0L) which was fed into a reactor by HPLC pump (Young Lin Instrument). We used H+ ion resin (SCR-BH, Samyang, KOR), OH\u2212 ion resin (SAR10MBOH, Samyang, KOR), and amberlyst-15 (Sigma-Aldrich, USA) ion-exchange resin. Desalting process was performed by combination H+ ion resin (500\u00a0mL) and OH\u2212 ion resin (500\u00a0mL) in two of glass reactor. The liquefied oil was fed in glass reactor (2\u00a0mL/min), after passed the resins, the oil mixed in the batch reactor for 2\u00a0h with ablerlyst-15. The mineral contents (Na, K, Ca, Mg, Si, Fe, and P) in liquefied oil were determined by the plasma atomic emission method (inductively coupled plasma atomic emission spectrophotometer, ICP-OES, Optima 7400DV, PERKIN ELMER).The chemicals used for obtaining the NixCu1.5\u2013xMg1.5Al1.0 catalyst were Ni(NO3)2\u00b76H2O (Junsei, 97.0%), Mg(NO3)2\u00b76H2O (Katayama, 99.0%), Al(NO3)2\u00b79H2O (Junsei, 97.0%), and Cu(NO3)2\u00b73H2O (Junsei, 99.0%). The gases utilized in the steam reforming study were H2 and Ar (J.B. Korea Gases Co. Ltd., >99.999%).A series of NixCu1.5\u2013xMg1.5Al1.0 catalysts were prepared by the co-precipitation of the nitrates present in the metal components [24]. An aqueous solution of Ni(NO3)2\u00b76H2O, Cu(NO3)2\u00b73H2O, Mg(NO3)2\u00b76H2O, and Al(NO3)2\u00b79H2O was slowly added into a beaker containing an aqueous solution of Na2CO3 (2\u00a0M) that was stirred at room temperature to achieve a constant pH of 10\u00a0\u00b1\u00a00.5. The pH of the solution was adjusted with an aqueous solution of NaOH (2\u00a0M). The resulting suspension was kept at 60\u00a0\u00b0C for 16\u00a0h. The precipitate was filtered, washed several times with deionized water, and dried at 110\u00a0\u00b0C for 12\u00a0h. Thereafter, the precipitate was ground to fine powders and calcined at 600\u00a0\u00b0C for 6\u00a0h in a static air atmosphere. Subsequently, the obtained material was pressed to a disk, crushed, and sieved to particles with 0.4\u20130.6\u00a0mm diameter (30\u201340 mesh size). The catalysts are denoted as NixCu1.5\u2013xMg1.5Al1.0. The molar ratio of (Ni\u00a0+\u00a0Cu\u00a0+\u00a0Mg)/Al was fixed at 3 and the molar ratio of Ni/(Ni\u00a0+\u00a0Cu) varied from 0 to 1.0. The total moles of Ni and Cu were fixed at 1.5.After calcination, the catalysts were reduced under 10% H2/Ar (v/v) for 5\u00a0h at 550\u00a0\u00b0C and cooled down to room temperature under N2 atmosphere. The catalyst surface is passivated under 1% O2/He for 24\u00a0h and the catalysts are denoted as \u201creduced\u201d catalysts.X-ray fluorescence (XRF) were obtained using X-ray fluorescence spectrometer (Shimadzu, XRF-1800, Japan) equipped with Rh-Ka radiation at 4\u00a0kW.X-ray diffraction (XRD) were obtained using an X-ray diffractometer (PHILIPS, X'Pert-MPD System, Netherlands) equipped with Cu\u2013K\u03b1 radiation (\u03bb\u00a0=\u00a00.15406\u00a0nm) at 45\u00a0kV, 40\u00a0mA.The H2-temperature programmed reduction (H2-TPR) was conducted in the Autochem\u2161 2920 (Micromeritics Instrument Corp., USA). The materials were first preheated at 100\u00a0\u00b0C for 2\u00a0h, and then reduced from 100\u00a0\u00b0C to 930\u00a0\u00b0C at a rate of 7.5\u00a0\u00b0C/min in 5% H2 in Ar flow.X-ray photoelectron spectrometer (XPS) experiment was performed in a THERMO VG SCIENTIFIC (MultiLab 2000, UK) operating Mg\u2013K\u03b1 radiation at 14\u00a0kV and 20\u00a0mA. The binding energy was calibrated using C 1s at 284.6\u00a0eV.Brunauer-Emmett-Teller (BET) surface area and pore volume were measured using a surface area & pore size analyzer (Quantachrome autosorb-iQ) via adsorption of N2 at\u00a0\u2212195\u00a0\u00b0C. The samples were degassed in vacuum at 250\u00a0\u00b0C for 3\u00a0h.Thermogravimetric\u2013differential thermal analysis (TG-DTA) was performed using a DTG-60H (Shimadzu, Japan) system below 1000\u00a0\u00b0C at a rate of 10\u00a0\u00b0C/min and 100\u00a0mL/min of air.Scanning electron microscopy (SEM) images were collected on a SU8020 (HITACHI, JPN) using 10\u201315\u00a0kV acceleration voltage and 9.8\u201310.0\u00a0mm working distance.Transmission electron microscopy (TEM) images, energy dispersive X-ray spectroscopy (EDS) analysis were obtained using TALOS F200X (Thermo Scientific\u2122, USA) with a scanning transmission electron microscopy (STEM) unit and a high-angle annular dark-field (HAADF) detector at 200\u00a0kV. The samples were dispersed in ethanol and sonicated for 2\u00a0h, prior to placing a drop of liquid on a holey carbon coated copper grid and followed by evaporation for 5\u00a0min in a vacuum oven at 25\u00a0\u00b0C for the sample preparation.H2 chemisorption was performed on a chemisorption analyzer (ASPA 2020, Micromertics) 0.5\u00a0g of sample was reduced at 400\u00a0\u00b0C for 2\u00a0h under 5% H2. After vacuum at 400\u00a0\u00b0C, the sample was cooled to 25\u00a0\u00b0C and H2 chemisorption was performed till 67\u00a0kPa. H2 desorption profiles were also obtained with decreasing the pressure. Metal dispersion was calculated based on the stoichiometry of H/Ni\u00a0=\u00a01 [22,25].N2O chemisorption was performed on a pulse chemisorption analyzer (AutoChem 2920, Micromertics) 0.1\u00a0g of sample was reduced at 400\u00a0\u00b0C for 1\u00a0h under 5% H2/He. After purging under He, the catalysts were cooled to 90\u00a0\u00b0C and N2O was introduced using repeated 250 \u03bcl pulses (95% N2O/He). Metal dispersion for Ni and Cu were calculated based on the following reaction (Eqs (4) and (5)) [22].\n\n(4)\nNi\u00a0+\u00a0N2O \u2192NiO\u00a0+\u00a0N2\n\n\n\n\n\n(5)\n2Cu\u00a0+\u00a0N2O \u2192Cu2O\u00a0+\u00a0N2\n\n\n\nThe catalytic activity test for liquefied oil steam reforming was conducted in a fixed-bed reactor made of Inconel 625 material (outside diameter (OD) of 63.5\u00a0mm and length (L) of 600\u00a0mm), which was heated by a furnace. The catalyst (0.4\u20130.6\u00a0mm) was placed in a reactor and reduced at 550\u00a0\u00b0C for 5\u00a0h with 10% H2/Ar (v/v) flow. The liquefied oil feed was then fed into a reactor using a high-performance liquid chromatography pump (Young Lin Instrument). The steam reforming was conducted at 440\u2013860\u00a0\u00b0C under atmospheric pressure with a S/C ratio of 10 and a liquid hourly space velocity (LHSV) of 0.2\u20132.3 h\u22121. The outlet gas was cooled using a trap and analyzed online using gas chromatography (HP-5890 Model). The concentrations of H2, CO, CH4, and CO2 were analyzed by gas chromatography (HP-5890, Agilent) with a thermal conductivity detector (TCD) equipped with a HayeSep DB column. After reaction, the catalysts were cooled to room temperature under Ar and passivated under 1% O2/He for 24\u00a0h. The obtained catalysts are denoted as \u201cspent\u201d catalysts.The carbon conversion of the liquefied oil, H2 yield, and the selectivity of H2, CO, CH4, and CO2 were calculated using Eqs. (6)\u2013(8).Carbon conversion of a liquefied oil:\n\n(6)\n\n\n\nX\nC\n\n\n(\n%\n)\n\n=\n\n\nC\n\nm\no\nl\ne\n\ns\n\nl\ni\nq\nu\ne\nf\ni\ne\nd\n\no\ni\nl\n,\n\nf\ne\ne\nd\n\n\n\u2212\nC\n\nm\no\nl\ne\n\ns\n\nl\ni\nq\nu\ne\nf\ni\ne\nd\n\no\ni\nl\n,\n\no\nu\nt\n\n\n\n\nC\n\nm\no\nl\ne\n\ns\n\nl\ni\nq\nu\ne\nf\ni\ne\nd\n\no\ni\nl\n,\n\nf\ne\ne\nd\n\n\n\n\n\n\n\n\nH2 yield:\n\n(7)\n\n\n\nY\n\nH\n2\n\n\n\n(\n%\n)\n\n=\n\n\np\nr\no\nd\nu\nc\nt\ni\no\nn\n\no\nf\n\ne\nx\np\ne\nr\ni\nm\ne\nn\nt\na\nl\n\n\nH\n2\n\n\n\np\nr\no\nd\nu\nc\nt\ni\no\nn\n\no\nf\n\nt\nh\ne\no\nr\ni\nt\ni\nc\na\nl\nl\ny\n\n\nH\n2\n\n\n\n\u00d7\n100\n\n\n\n\nSelectivity of product gases:\n\n(8)\n\n\n\nS\ni\n\n\n(\n%\n)\n\n=\n\n\nm\no\nl\ne\ns\n\no\nf\n\ng\na\n\ns\ni\n\n\no\nb\nt\na\ni\nn\ne\nd\n\n\n\nm\no\nl\ne\ns\n\no\nf\n\np\nr\no\nd\nu\nc\nt\n\ng\na\ns\ne\ns\n\no\nb\nt\na\ni\nn\ne\nd\n\n\n\n\u00d7\n100\n\n\n\n\nWe performed hydrothermal liquefaction of brown algae (S.\u00a0japonica) under the following conditions: 300\u00a0\u00b0C, 2\u00a0h, S.\u00a0japonica/H2O ratio of 10 (w/w), and autogenous pressure [23,26]. Table\u00a01\n and S1 show the chemical composition of the liquefied oil, as determined through elemental analysis (C, H, and O), moisture analysis, and ICP analysis. The liquefied oil was composed of 5.2\u00a0wt% C, 11.7\u00a0wt% H, 80.9\u00a0wt% O, and 82.2\u00a0wt% H2O. Furthermore, it contained a high content of mineral components (mainly \u223c2,900\u00a0ppm of Na and \u223c11,000\u00a0ppm of K, and others (Ca, Mg, Si, Fe, and P) derived from S.\u00a0japonica, as shown in Table\u00a0S1. To eliminate the minerals, we performed a desalting process using a combination of H+ and OH\u2212 ion-exchange resin and Amberlyst-15, as shown in Fig.\u00a01\na. Fig.\u00a01b and Table\u00a0S1 showed that most of the mineral components of the liquefied oil were removed to <10\u00a0ppm after the desalting process. Fig.\u00a02\n shows the main chemical components of the hydrothermally liquefied oil derived from S.\u00a0japonica (detailed chemical components are shown in Table\u00a0S2), as determined by GC-MS. As the main components of the liquefied oil were ketones (74.64%) and nitrogenous compounds (18.34%), the subsequent steam reforming of this liquefied oil should be mainly based on a ketone steam reforming mechanism.We prepared a series of NixCu1.5\u2212xMg1.5Al1.0 catalysts by co-precipitation method with different atomic ratios of Ni/(Ni\u00a0+\u00a0Cu) by varying x in the range of 0\u20131. The physicochemical properties of the NixCu1.5\u2212xMg1.5Al1.0 catalysts are summarized in Table\u00a02\n. All catalysts exhibited similar BET surface areas in the range of 100\u2013134\u00a0m2/g. Figure\u00a0S1 shows the Barret-Joyner-Halenda (BJH) pore size distribution of reduced NixCu1.5\u2212xMg1.5Al1.0 catalysts. It indicates that pore radius of reduced catalysts ranges from 2 to 16\u00a0nm. The N2 adsorption/desorption hysteresis shows H3 type related to the slit-shaped pores from aggregates of plate-like particles (Sing. et\u00a0al. [27]). SEM imaging (Figure\u00a0S2) consistently showed the agglomerated particles with platelet-like morphology without apparent difference among catalysts. The Ni and Cu metal contents determined by XRF were essentially the same as the nominal metal contents used in the synthesis.The crystalline phases were analyzed by XRD (see Figure\u00a0S3 and Fig.\u00a03\n). XRD patterns of synthesized NixCu1.5-xMg1\n.\n5Al1.0 catalysts (Figure\u00a0S3) shows diffraction peaks at 2\u03b8 values of 11.4\u00b0, 22.8\u00b0, 34.6\u00b0, and 38.9\u00b0 indicating that the as-synthesized catalysts represent hydrotalcite phases (JCPDS 22\u20130700). In addition, the Ni0.5Cu1.0Mg1.5Al1.0 and Cu1.5Mg1.5Al1.0 catalysts contained the CuO phase (JCPDS #80\u20131916) and yielded diffraction peaks at 2\u03b8 values of 35.5\u00b0 and 38.7\u00b0. After calcination at 600\u00a0\u00b0C (Fig.\u00a03a and b), the hydrotalcite layer structures collapsed, and all the catalysts transformed into mixed metal oxides consisting mainly of the MgO periclase phase (JCPDS 75\u20131525) [9,28]. No distinctive NiO peaks were observed, indicating that the Ni2+ cations were well-incorporated in the MgO phase or undetectable highly dispersed particles. Ni0.5Cu1.0Mg1.5Al1.0 exhibited additional signals of the CuO phase at 2\u03b8 values of 35.5\u00b0 and 38.7\u00b0 [21,28]. As shown in Fig.\u00a03b, the Cu1.5Mg1.5Al1.0 catalyst without Ni provided strong diffraction peaks of CuO and minor peaks of spinel phases (MgAl2O4 or CuAl2O4). The minor peaks are more likely due to CuAl2O4 because the Cu catalyst only showed a diffraction peak at 2\u03b8\u00a0=\u00a032.8\u00b0. After reduction at 550\u00a0\u00b0C (Fig.\u00a03c and d), the MgO periclase structure was still maintained. The reduced Ni1.5Mg1.5Al1.0 catalyst (without Cu) showed Ni(111) and Ni(200) diffraction peaks at 2\u03b8\u00a0=\u00a045.5\u00b0 and 51.9\u00b0. On the other hand, the reduced Cu1.5Mg1.5Al1.0 catalyst (without Ni) showed Cu diffraction peaks at 2\u03b8\u00a0=\u00a043.2\u00b0 and 49.5\u00b0. Fig.\u00a03d shows that as the Cu/(Ni\u00a0+\u00a0Cu) atomic ratio increased from 0 to 1.5, the Ni(200) diffraction peaks gradually shifted to lower angles and finally to Cu(200) peaks at 49.5\u00b0. The gradual diffraction peak shift indicates that NiCu alloys were formed in the NixCu1.5\u2212xMg1.5Al1.0 catalysts [9].The reducibility of the NixCu1.5\u2212xMg1.5Al1.0 catalysts was studied by H2-TPR (Fig.\u00a04\n). The Ni1.5Mg1.5Al1.0 catalyst exhibited a broad reduction feature at \u223c750\u00a0\u00b0C, which is related to the reduction of Ni2+ species (NiO) in the mixed oxide [18,29]. For the Cu1.5Mg1.5Al1.0 catalyst, a sharp peak was observed at 170\u00a0\u00b0C, with another small peak at 645\u00a0\u00b0C. The former is attributed to the reduction of Cu2+ species in the mixed oxide [28,30,31] and the latter might be due to the reduction of the CuAl2O4 spinel phase [9].Upon increasing the Ni content in the NixCu1.5\u2212xMg1.5Al1.0 catalyst, the reduction temperature of CuO increased from 170\u00a0\u00b0C (Cu1.5Mg1.5Al1.0) to \u223c200\u00a0\u00b0C (Ni0.5Cu1.0Mg1.5Al1.0), suggesting that Ni addition leads to a stronger interaction between the Cu2+ species and parent hydrotalcite [28,30,31]. Upon increasing the Cu content instead of Ni, the high-temperature reduction peak of Ni2+ shifted to lower temperatures as compared with that of the Ni1.5Mg1.5Al1.0 catalyst. This originates from the hydrogen spillover from the reduced Cu metal during the H2-TPR process [28,31\u201333]. In summary, H2-TPR indicated synergetic interaction between Cu and Ni in the NixCu1.5\u2212xMg1.5Al1.0 catalysts, suggesting that the reducibility of the catalyst changes with the Ni/Cu atomic ratio.The chemical states of the surface elements in the reduced catalysts were characterized by XPS. The XPS profiles for Cu 2p3/2 and Ni 2p3/2 are shown in Fig.\u00a05\n. Fig.\u00a05a shows two Cu 2p3/2 peaks at 932.6 and 934.5\u00a0eV, indicating metallic Cu0 and Cu2+ oxidation states, respectively [22,34].\nFig.\u00a05b shows three Ni 2p3/2 peaks. The peaks centered at 855.6 and 852.6\u00a0eV are assigned to Ni2+ and metallic Ni0, respectively [22,34], while the peak appearing at\u00a0\u223c\u00a06\u00a0eV at the higher binding energy is considered as the satellite peak. After the deconvolution process, the relative Ni2+/Ni and Cu2+/Cu ratios were quantified, and the results are shown in Table\u00a02. The Cu1.5Mg1.5Al1.0 catalyst without Ni contained \u223c51% of Cu0. As the Ni content was increased, the relative proportion of metallic Cu0 increased up to \u223c82% in the Ni1.0Cu0.5Mg1.5Al1.0 catalyst. On the other hand, the Ni1.5Mg1.5Al1.0 catalyst without Cu contained \u223c6.6% metallic Ni, and as the relative Cu content was increased, the relative proportion of metallic Ni0 increased to \u223c17.9% in the Ni0.5Cu1.0Mg1.5Al1.0 catalyst. These results indicate that the addition of Ni or Cu induces the reduction of Cu or Ni species, respectively, on the surface, suggesting close interaction between the Ni and Cu atoms. This result is consistent with the H2-TPR and XRD results.Further, the metal dispersion was characterized by TEM and chemisorption. Figure\u00a0S4 showed that Ni1.5Mg1.5Al1.0 catalyst had metal particles around <20\u00a0nm. Figure\u00a0S5 shows TEM, STEM images and EDS mapping for Ni0.75Cu0.75Mg1.5Al1.0 catalyst. EDS mapping showed Ni and Cu are overlapped over the catalysts and metal particles (<20\u00a0nm) are observed. Figure\u00a0S6 exhibited that Cu1.5Mg1.5Al1.0 catalysts had also 10\u201320\u00a0nm particles. However, because of local information from TEM images and particle overlap with support (mixed oxide), the accurate particle sizes are difficult to obtain. So, we additionally performed the chemisorption. The H2 volumetric chemisorption results in Table\u00a02 reveal that the addition of Cu resulted in a decrease in the amount of H2 chemisorbed on the metallic particles from 1.8% (Ni1.5Mg1.5Al1.0) to 0.8\u20130.9% (Ni1.0Cu0.5Mg1.5Al1.0 or Ni0.75Cu0.75Mg1.5Al1.0) and 1.6% (Ni0.5Cu1.0Mg1.5Al1.0). Previous studies on NiCu catalysts have indicated that alloying Ni with Cu leads to weaker H2 adsorption, which hinders accurate characterization of NiCu alloy particles [35]. The decreased chemisorbed H2 amounts prove that the NixCu1.5\u2212xMg1.5Al1.0 catalysts formed NiCu alloys. Instead, N2O probe molecules can characterize both the Ni and Cu surfaces by oxidizing the Ni and Cu species (Eqs. (4) and (5)) [22]. The metal dispersion data obtained from N2O chemisorption are summarized in Table\u00a02. The average metal crystal sizes ranged from 19\u00a0nm to 33\u00a0nm. Interestingly, the metal crystal sizes are the smallest as 19\u00a0nm at 1 to 1 Ni:Cu atomic ratio, which were smaller than those of the Ni1.5Mg1.5Al1.0 (without Cu) or Cu1.5Mg1.5Al1.0 (without Ni) catalyst, suggesting that alloying Ni with Cu at a 1:1 atomic ratio results in a higher dispersion of metallic particles.The catalytic activities of the series of NixCu1.5\u2013xMg1.5Al1.0 catalysts in the steam reforming of liquefied oil were evaluated comparatively under optimized conditions. First, the reaction conditions (LHSV and reaction temperature) were optimized to maximize the carbon conversion and H2 yield from the reaction of the liquefied oil on the Ni1.5Mg1.5Al1.0 catalyst based on a response surface methodology (RSM) [23,26,36]. Fig.\u00a06\n shows the contour plot of the carbon conversion and H2 yield with a change in the reaction temperature (440\u2013860\u00a0\u00b0C) and LHSV values (0.2\u20132.3\u00a0h\u22121). The RSM analysis indicated a maximum H2 yield of 83% and carbon conversion of 98% at 750\u00a0\u00b0C and an LHSV of 1.0\u00a0h\u22121. Therefore, all catalysts were evaluated at the same condition.The catalytic activity and product distribution after the steam reforming of the liquefied oil on the Ni1.5Mg1.5Al1.0 catalyst with time on stream are presented in Fig.\u00a07\n. This catalyst was stable during 5\u00a0h of the reaction, and the carbon conversion and H2 selectivity were 92.8 and 80%, respectively. Further, the CO2 selectivity was \u223c13%, and the CO, C2+, and CH4 selectivities were lower than 6, 2, and 2%, respectively. The catalytic activities and product selectivities obtained with the NixCu1.5\u2212xMg1.5Al1.0 catalysts are shown in Fig.\u00a08\n and Figure\u00a0S7. Fig.\u00a08a showed that the Cu1.5Mg1.5Al1.0 catalyst initially underwent fast deactivation during 1\u00a0h and then stabilized. In contrast, the other Ni- and Ni/Cu-containing catalysts exhibited stable C conversion (>90%) and H2 selectivities (76\u201378%). Fig.\u00a09\n summarizes the catalytic performances of the NixCu1.5\u2212xMg1.5Al1.0 catalysts after 5\u00a0h of the steam reforming of the liquefied oil. The catalyst containing only Cu showed the lowest C conversion of \u223c67%, while the others showed >89% C conversion. Most of the gaseous product was composed of H2, with the highest H2 selectivity of 76\u201378%, followed by CO2 selectivity of 10\u201317%. Interestingly, the Ni0.75Cu0.75Mg1.5Al1.0 catalyst showed the lowest CO selectivity (\u223c2.8%), leading to the highest H2/CO ratio of \u223c28. This result indicates that this catalyst is the most selective one for H2 production among the NixCu1.5\u2212xMg1.5Al1.0 catalysts.We further compared the H2 production rates per exposed metallic sites (determined by N2O chemisorption) of all catalysts. Fig.\u00a010\n shows the variation of the H2-production rate per exposed metallic sites and metal dispersion with the Ni/(Ni\u00a0+\u00a0Cu) atomic ratio determined by XPS. Interestingly, a volcano-type plot of the H2-production rate vs. Ni/(Ni\u00a0+\u00a0Cu) atomic ratio was obtained, which was a similar trend between the metal dispersion and Ni/(Ni\u00a0+\u00a0Cu) atomic ratio. In particular, the highest H2 production rate was achieved at a Ni:Cu atomic ratio of 1:1, at which the metal dispersion in the catalyst was the maximum. It should be noted that this catalyst also showed the highest selectivity for H2 (i.e., the highest H2/CO ratio). This result suggests that the synergetic alloying of Ni and Cu at an optimum 1:1 atomic ratio leads to smaller metallic particles and more exposed metallic sites, resulting in a higher and selective H2 production rates compared to those of the catalyst containing Ni or Cu only [9]. In general, Ni exhibits good activity for steam reforming reaction due to its high C\u2013C bond scission ability [11,12]. Furthermore, Cu prevented the sintering of Ni by forming NiCu alloy, and promoted H2 production through its high activity in the water gas shift reaction (Eq. (2)) [20\u201322]. Especially, designing smaller NiCu alloy particles based on the optimized Ni/Cu atomic ratio is critical for H2 production by the steam reforming of liquefied oil derived from S.\u00a0japonica [9].After the reaction, the spent catalysts were analyzed by XRD (Fig.\u00a011\n). These results suggested that the Ni/Cu ratio had considerable effect on the crystallite aggregation in hydrotalcite precursors catalysts [24]. Notably, the Ni0.75Cu0.75Mg1.5Al1.0 catalyst, which showed the best catalytic performance, had the smallest metal particle size compared to the other catalysts, suggesting that small metallic particles are important for H2 production. Also, TEM/STEM and EDS analysis were performed on selected spent catalysts, as shown in Figure\u00a0S8\u2013S10. Ni1.5Mg1.5Al1.0 catalyst (Figure\u00a0S8) showed the agglomerated Ni particles (>100\u00a0nm). Figure\u00a0S9 exhibited that Ni0.75Cu0.75Mg1.5Al1.0 catalyst had 40\u2013100\u00a0nm particles, smaller than those of Ni1.5Mg1.5Al1.0 catalyst although some region also showed sintered particles (>100\u00a0nm). Detected metal particles are confirmed as NiCu alloy from EDS analysis (Figure\u00a0S10). Considering both XRD and TEM results, we conclude that Ni0.75Cu0.75Mg1.5Al1.0 catalyst had smaller metal particles than Ni1.5Mg1.5Al1.0 catalyst. Cu1.5Mg1.5Al1.0 catalyst (Figure\u00a0S11) also showed very large particles (>100\u00a0nm) and also agglomerated regions with 20\u201330\u00a0nm particles.\nTable\u00a02 showed that BET surface area of NixCu1.5-xMg1.5Al1.0 (x\u00a0=\u00a00.5\u20131.5) and Cu1.5Mg1.5Al1.0 catalysts decreased to 37\u201353\u00a0m2/g and 5\u00a0m2/g after the reaction, respectively. Figure\u00a0S12 shows the BJH pore size distribution of spent NixCu1.5\u2212xMg1.5Al1.0 catalysts, indicating the pore radius of studied catalysts ranges from 6 to 16\u00a0nm, which increased compared to reduced catalysts (2\u201316\u00a0nm). Particle agglomeration and carbon species accumulation during the reaction decreased the BET surface area and also affect the pore size distribution of spent catalysts. SEM analysis (Figure\u00a0S13) showed no notable morphology change. Coke analysis of the spent catalysts (Figure\u00a0S14) revealed that only the Cu1.5Mg1.5Al1.0 catalyst showed the weight loss at 300\u2013400\u00a0\u00b0C, indicating that amorphous coke was deposited on the catalyst. This might be the cause of the initial deactivation of the Cu1.5Mg1.5Al1.0 catalyst (Fig.\u00a08) and the drastic decrease of BET surface area on Cu1.5Mg1.5Al1.0 catalyst. Other NixCu1.5\u2212xMg1.5Al1.0 catalysts (x\u00a0=\u00a00.5\u20131.5) exhibited weight increases owing to the oxidation of metallic particles [37,38]; therefore, notable coke deposition (weight loss) could not be detected. These results suggest that the NiCu hydrotalcite-derived catalysts are promising catalysts for the steam reforming of liquefied bio-oil derived from S.\u00a0japonica (macro algae).Liquefied oil from marine biomass can be a good resource for hydrogen by steam reforming it on NiCu hydrotalcite-derived mixed oxide catalysts (NixCu1.5\u2212xMg1.5Al1.0). S.\u00a0japonica is liquefied into bio-oil by hydrothermal liquefaction at 300\u00a0\u00b0C. GC-MS analysis showed that Bio-oil from Saccharina japonica mainly contain ketone and N-containing compounds. After the desalting process combined with H+/OH\u2212 ion-exchange resin and Amberlyst-15, the remaining salts are mostly removed to <10\u00a0ppm. XRD, H2-TPR, TEM, XPS, chemisorption exhibited the synergetic interaction between Ni and Cu. XRD showed the Ni diffraction peak shifts with varying Ni:Cu atomic ratio, indicating the NiCu alloy formation. With increasing Cu content, the reducibility of Ni was improved, evidenced by H2-TPR and XPS. EDS analysis also showed that Ni and Cu mapping are overlapped. N2O chemisorption showed that particle sizes also vary where 1:1 atomic ratio of Ni and Cu had the smallest particle sizes (\u223c19\u00a0nm).The steam reforming of bio-oil from S.japonica was performed over NixCu1.5-xMg1.5Al1.0 catalysts. NixCu1.5-xMg1.5Al1.0 catalysts, except Cu1.5Mg1.5Al1.0 catalyst, were stable with >89% of carbon conversion and H2 selectivity of 76\u201378% during 5\u00a0h. In particular, at an optimized Ni:Cu atomic ratio of 1:1, the synergetic interaction of Ni and Cu led to the smallest NiCu alloy particles and maximized H2 production rates along with the highest selectivity to H2. Thus, designing smaller NiCu alloy particles over NiCu hydrotalcite-derived catalysts is critical for H2 production via the steam reforming of liquefied oil from S.\u00a0japonica.\nSeong Chan Lee: Investigation, Data curation, Writing \u2013 original draft, Writing \u2013 review & editing. Jae Hyung Choi: Validation, Formal analysis, Writing \u2013 original draft, Writing \u2013 review & editing. Chul Woo Lee: Resources, Methodology, Investigation. Seung Han Woo: Resources, Methodology, Investigation. Jaekyoung Lee: Project administration, Supervision, Writing \u2013 original draft, Writing \u2013 review & editing. Hee Chul Woo: Conceptualization, Supervision, Project administration, Funding acquisition, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2017R1E1A1A01074445 and 2021R1A2C2094256).The following is/are the supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2022.03.161.", "descript": "\n                  H2 is highlighted as a sustainable energy resource, and mainly produced by steam reforming of fossil fuels, which emit greenhouse gases. Marine biomass can be an alternative because of high productivity and carbon neutrality compared to terrestrial biomass. In this work, we studied bio-oil from Saccharina japonica (macroalgae) as a renewable H2 resource by steam reforming on NiCu hydrotalcite-derived catalysts (NixCu1.5-xMg1.5Al1.0). After the hydrothermal liquefaction of S.\u00a0japonica, minerals were removed by the desalting process. GC-MS showed bio-oil mainly consists of ketone and N-containing compounds. Increasing Cu content improved the reducibility of Ni, evidenced by H2-TPR and XPS, suggesting the synergetic interaction between Ni and Cu. Chemisorption showed the catalyst had the smallest particle sizes (\u223c19\u00a0nm) at 1 to 1 Ni:Cu atomic ratio. As for steam reforming of bio-oil, NixCu1.5-xMg1.5Al1.0 catalysts, except Cu1.5Mg1.5Al1.0, were stable with >89% of carbon conversion and H2 selectivity of 76\u201378% during 5\u00a0h. Especially, at 1:1 Ni:Cu atomic ratio, the catalyst maximized H2 production rates with the highest H2/CO ratio of 28. It suggests that designing small NiCu particles is critical for H2 production. In summary, NixCu1.5-xMg1.5Al1.0 catalysts are promising for H2 production by the steam reforming of the bio-oil from macro algae.\n               "}
{"full_text": "In 2018, the global carbon dioxide emissions increased by 2.7% [1]. The challenges of meeting the growing need for electricity and transport fuel of the world must be dealt with a substantial decrease in CO2 emissions [2]. Hydrogen, which can replace fossil fuels such as coal, diesel, gasoline, and natural gas, may uniquely decarbonize the electricity and transportation sectors when produced from greenhouse gases [3]. Hydrogen has the potential to be a major source of low-carbon energy, with the possibility of reducing CO2 emissions to nearly zero. Hydrogen demand was \u223c8\u00a0EJ in 2015, and is anticipated to increase tenfold by 2050 [4]. This fact motivates the search for novel technologies for inexpensive, CO2-free H2 production on an industrial scale. Developing large-scale, economically competitive H2 production processes is essential for producing low-carbon fuels and fertilizers [5,6]. The use of methane would take advantage of the existing natural gas infrastructure, reducing the conversion costs to a hydrogen-based energy system. Dry reforming of methane (DRM), as shown in equation (1) below, is one of the viable routes to produce hydrogen [7\u20139]. Catalytic DRM reaction utilizes greenhouse gases, comprising methane and carbon dioxide, to form syngas (CO and H2) [10,11]. DRM has also been receiving extensive consideration in recent years [12\u201314] due to the formation of syngas (H2/CO) in a mole ratio close to unity, which is required for Fischer-Tropsch synthesis (FTS). Reverse water gas shift (RWGS), a major side reaction (Equation (2)) in CO2 reforming of CH4, gives rise to H2/CO mole ratio less than unity [15\u201317].\n\n(1)\nCH4 + CO2 \u2192 2CO + 2H2\n\n\n\n\n\n(2)\nCO2\u00a0+\u00a0H2 \u2192 CO\u00a0+\u00a0H2O\n\n\nNickel-based catalysts are less expensive than noble metal-based catalysts for DRM but are prone to sintering at high reaction temperatures. The importance of the catalyst support is evident because it provides a surface area for the dispersion of Ni catalyst, and its surface chemistry greatly facilitates CO2 activation [18]. Alumina (Al2O3) is cheap, abundant catalyst support for DRM due to its high surface area and thermal stability [19]. \u03b3-Al2O3 support was found to be more suitable than \u03b2-Al2O3 support for Ni-based catalyst in DRM reaction because \u03b3-Al2O3 can hold large numbers of C\u03b1 species (completely dehydrogenated carbides carbon) over its surface, which resulted in a higher CO2 conversion [20]. Kim et\u00a0al. [21] used the solvothermal method to prepare the Ni/Al2O3 nanosheet catalyst, which exhibited significantly more stable activity than Ni/Al2O3 with an arbitrary configuration. In addition, considerable efforts have been made to boost the catalytic activity and stability by tuning the textural property of the support using promoters. Fig.\u00a01\n indicates the type of promoters inducing Al2O3-supported Ni catalyst towards DRM.Among s-block elements, Alipour et\u00a0al. [22] utilized MgO-, CaO- and BaO-modified, Al2O3-supported Ni-based catalysts. They found that the moderate addition of MgO as a modifier improved the catalytic activity over 5.0\u00a0wt% Ni/Al2O3 catalyst due to the formation of MgAl2O4 phase. In a different study, the catalyst of 2.5\u00a0wt% Ni supported on Mg-Al mixed oxide showed more than 65% CH4 conversion and 75% CO2 conversion for 7\u00a0h on stream [23]. Khoja et\u00a0al. prepared 10\u00a0wt% Ni/La2O3-MgAl2O4 (1:4) catalyst and was tested in catalytic dielectric barrier discharge reactor for DRM and found more than 79% CH4 conversion and 84% CO2 conversion with H2/CO mole ratio of \u223c1.0 [24]. On the other hand, the 0.75\u00a0wt% Sr promoter on Ni/Al2O3 catalyst facilitated strong metal-support interaction and boosted the basicity, which induced the dissociation of CO2 over the catalyst and, in turn, decreased the coke deposition. It demonstrated minimum deactivation and more than 75% CH4 and CO2 conversion with H2/CO\u00a0=\u00a00. 95 [25]. Among p-block elements, the role of 5.6\u00a0wt% boron as a modifier was found to suppress 86% of carbon deposition without affecting the catalytic activity due to a more uniform distribution of Ni catalyst over the support [26]. Moreover, the incorporation of boron nitride into a nickel-based catalyst not only avoided metal particle sintering but also enhanced coke resistance [27,28]. Wei et\u00a0al. coated monolithic SiC foam with \u201cNi embedded in mesoporous Al2O3 layer\u201d, which caused a high Ni dispersion over a larger specific surface area. It showed more than 30% CH4 conversion and 40% CO2 conversion [29]. In contrast, 3.0\u00a0wt% Si modifier exhibited excellent coke resistance, and it also induced strong metal-support interaction or the formation of NiAl2O4 mixed oxide. It demonstrated more than 64% CH4 conversion, 70% CO2 conversion, and an H2/CO mole ratio of \u223c0.90 over 7\u00a0h of reaction [30]. The lack of negative peak in the TPO spectra of used 1.0\u00a0wt% Ga-promoted, loaded on Ni/Al2O3 catalyst (concerning other transition metal promotors of Cu, Gd, and Zn) indicated the retention of Ni in its metallic state. It showed more than 74% CH4 conversion and 84% CO2 conversion over 7\u00a0h on stream [31].In f-block elements, due to the development of strong metal-support interaction, 1.0\u00a0wt% of Gd promoter resulted in >83% CH4 conversion, 88% CO2 conversion, and an H2/CO mole ratio of \u223c1.0 over 7\u00a0h on stream [31]. Lanthana added basicity to the catalyst system, inducing more CO2 adsorption, increased metal-support interaction, and neutralized the acid sites of alumina support (responsible for excessive coke deposit). The formation of La2O2CO3 can prevent Ni from sintering and facilitate NiAl2O4 formation. La2O2CO3-modified, Ni/Al2O3 exhibited more than 60% CH4 conversion and CO2 conversion with H2/CO\u00a0>\u00a00.8 [32]. The presence of Ceria and praseodymium oxide presence offered additional redox properties. Mobile lattice oxygen is readily available during the redox cycle for carbon deposit oxidation. Thus, 5.0\u00a0wt% ceria addition resulted in >75% CH4 conversion and CO2 conversion with H2/CO \u223c0.8\u00a0at 800\u00a0\u00b0C [33]. Praseodymium oxide promotional addition induces the transport of electrons through the oxygen vacancies of the redox pair of Py4+/Py3+. The catalyst with 3.0\u00a0wt% Py-10\u00a0wt% Ni/delaminated clay, showed no carbon formation, 38% CH4 conversion, 43% CO2 conversion, and H2/CO\u00a0=\u00a00.7 [34]. The addition of 2.0\u00a0wt% Yb controlled the Ni particle size, resulting in the narrowest particle size distribution and the highest reducibility over Al2O3 support. It demonstrated \u223c80% CH4 conversion, 87% CO2 conversion, and H2/CO\u00a0>\u00a00.9 [35].In d-block elements, the role of Cu and Ti were examined, but each induced the formation of free NiO particles over the surface (or weak metal-support interaction). Therefore, the addition of Cu and Ti was not beneficial for DRM [30,36]. The promotion of Ni/Al2O3 catalyst with Mn resulted in coke resistance, but it suppressed the catalytic activity due to the partial blocking of the catalytic active sites \u201cmetallic Ni\u201d by manganese oxide [37]. However, adding 2\u00a0wt ratio (%) potassium further stabilized its catalytic activity. The promotion with Mo for the Ni/Al2O3 catalytic system was found to be inferior due to the formation of the MoNi4 phase and the weak interaction of Ni with support [38], while Mo promoted/modified ZSM-5 facilitated efficient coke removal for nickel-based catalysts [39]. The addition of 10\u00a0wt% ZrO2 promoter was found to enhance the dissociation of CO2 through the formation of oxygen intermediates (near the ZrO2 and nickel interface), which oxidized coke. Thus, 10\u00a0wt% ZrO2-promoted, 15\u00a0wt%Ni/Al2O3 catalyst showed more than 70% CH4 conversion and 60% CO2 conversion [40]. The 3.0\u00a0wt% Co-promotional addition into 10\u00a0wt%Ni/Al2O3 catalyst was remarkable. Co-controlled Ni particle size, causing coke depression, high CO2 conversion (>90%), high CH4 conversion (>90%), and high H2/CO ratio (\u223c0.9) at 850\u00a0\u00b0C [35].Both the 3.0\u00a0wt% W and 3.0\u00a0wt% Si as promoters into Ni/Al2O3 catalyst showed similar catalytic performance in DRM as both had attained >64% CH4 conversion, >70% CO2 conversion, and >0.90\u00a0mol H2/CO ratio over 7\u00a0h of reaction [30]. W-modified Ni/Al2O3 catalysts were examined in DRM reaction [41], where 10\u00a0wt% Ni and various tungsten loadings were tested. It was found that 11.9\u00a0wt% W reduced the carbon deposition by 76% compared to W-free catalyst. Researchers have enhanced the performance of nickel-based catalysts by adding other promoters of d-block metal oxide. Nevertheless, WO3 was found to be more reactive than other metal oxides like Fe3O4, ZnO, SnO2, and V2O5 because of its enhanced reducibility [42]. WO3 had superior properties over B2O3, TiO2, ZrO2, and MoO3 as promoters because it enhanced the interaction between NiO and \u03b3-Al2O3 support, and hence, improved the dispersion and stability of nickel particles in the catalytic system during DRM [6]. The CH4 decomposition over WO3 (forming tungsten carbide, WC), the gasification of carbon deposits by CO2, and the redox property of tungsten oxide (WO3 \u2192 WC \u2192 WO3) have sparked interest in the use of tungsten as a promoter [42]. The presence of WC during the reaction also increased the catalyst system's thermal stability, which was a must condition for high-temperature DRM reaction [43]. Al-Fatesh and his co-workers [6] employed ZrO2-supported Fe catalysts for the catalytic decomposition of methane and investigated the influence of La2O3 and WO3 dopants on the catalytic activity and stability. WO3 strongly influenced the methane conversion, hydrogen yield, and stability at 800\u00a0\u00b0C, 4000 mL/(h.gcat.) space velocity because of the higher dispersion, stabilization, and stronger interaction of iron nanoparticles on the surface of ZrO2 modified with WO3. Therefore, tungsten trioxide is worth studying owing to its high stability, coke resistance, and reactivity [44]. Overall, low-cost tungsten oxide may have redox properties, and its presence over a supported Ni system may induce Ni dispersion, enhance stronger metal-support interaction and promote coke oxidation which must favour the dry reforming of methane. In this context, this study aimed to optimize the amount of tungsten trioxide promoter for the best catalytic performance of nickel nanoparticles supported on mesoporous gamma-alumina (Ni/\u03b3-Al2O3). The effect of 1.0\u20139.0\u00a0wt% of tungsten trioxide loadings on the textural, morphological, and catalytic properties was investigated.Ammonium tungstate [(NH4)10H2(W2O7)6; 3060.45\u00a0g/mol; 99.99% trace metals basis; Aldrich], mesoporous \u03b3-alumina (meso-Al2O3, 1/8\" pellets, Alfa Aesar), and nickel nitrate hexahydrate [Ni (NO3)2.6H2O, 98%, Alfa Aesar] were purchased and were used as received. Ultrapure water was obtained via a Milli-Q water purification system (Millipore).A two-step synthesis procedure based on dry impregnation was followed to prepare all catalysts. The first step was the synthesis of mesoporous \u03b3-alumina support promoted with tungsten trioxide (x%WO3/\u03b3-Al2O3), where x\u00a0=\u00a01.0\u20139.0\u00a0wt% in 2.0\u00a0wt% increments. The second step was to load the active catalyst as nickel oxide with 5.0\u00a0wt% on the various supports. These two steps are described in detail below.The required amount of ammonium tungstate to produce the required weight percent loading of tungsten trioxide and the required amount of mesoporous \u03b3-alumina were mixed and ground together to give a solid white mixture. Ultrapure water was added dropwise to get a thick paste, which was mechanically stirred until dry. The addition of water and mechanical stirring was repeated three times to ensure homogeneous distribution of ammonium tungstate within the matrix of mesoporous \u03b3-alumina. The solid mixture was finally calcined under static air, at 600\u00a0\u00b0C, over 3\u00a0h to give white supports promoted with various loadings of tungsten trioxide (1.0, 3.0, 5.0, 7.0, or 9.0\u00a0wt%).The required amount of nickel nitrate hexahydrate was mixed and was ground with the required amount of the desired support of xWO3-doped \u03b3-Al2O3 (x\u00a0=\u00a01.0, 3.0, 5.0, 7.0, or 9.0\u00a0wt%) to produce a solid green mixture. This mixture was then converted to a paste by adding drops of ultrapure water. The paste was mechanically stirred to ensure complete drying and the formation of a green solid. The addition of water and stirring was repeated three times, and then the solid mixtures were calcined for 3\u00a0h at 600\u00a0\u00b0C to produce brown solids.Powder X-ray diffraction (XRD) patterns of the prepared catalysts were recorded on a Miniflex Rigaku diffractometer, worked at 40\u00a0kV and 40\u00a0mA, and fitted with Cu K\u03b1 X-ray radiation.The isotherms of N2 physisorption were measured by using a Micromeritics Tristar II 3020 surface area and porosity analyzer at \u2212196\u00a0\u00b0C after outgassing the samples at 200\u00a0\u00b0C for 3\u00a0h to desorb accumulated gases or vapors on the surface and into the pores. The Barrett\u2013Joyner\u2013Halenda (BJH) model was used to investigate the distributions of pore size of the samples.The H2-TPR, CO2-TPD, and NH3-TPD analyses of the synthesized and the spent catalysts were performed on a Micromeritics Auto Chem II 2920. The tests were done over a temperature range of 50\u2013800\u00a0\u00b0C and 2.40\u00a0L/h flow of 10% H2/Ar mixture for the H2-TPR analysis, 10% CO2/He mixture for CO2-TPD basicity measurement, and 10% NH3/He mixture for NH3-TPD acidity assessment. During the H2-TPR analysis of the catalyst, 0.070\u00a0g of the catalyst precursors were first heated to 150\u00a0\u00b0C and held at that temperature for 60\u00a0min in the presence of Ar at the rate of 1.8\u00a0L/h and then cooled to room temperature. Next, the sample temperature was raised to 900\u00a0\u00b0C at 10\u00a0K/min under 10% H2/Ar mixture in an automatic furnace at 1\u00a0atm. The amount of consumed H2 was determined by a thermal conductivity detector (TCD). For the NH3-TPD acidity assessment, \u223c150\u00a0mg of each tested catalyst was placed in a reaction tube. After pretreating at 600\u00a0\u00b0C for 2\u00a0h under a He flow of 50\u00a0mL/min, the sample was cooled to 170\u00a0\u00b0C and dosed for 30\u00a0min with a 10% NH3 in He (balance). After dosing, the samples were cooled to 50\u00a0\u00b0C, followed by heating to 600\u00a0\u00b0C at 5.0\u00a0\u00b0C/min, under He flow of 50\u00a0mL/min. The NH3 concentration in the output was recorded via a TCD.The mass of carbon deposits on the surface of spent catalysts was determined using a thermo-gravimetric unit (Shimadzu-TGA). The deposited carbon was removed by heating the samples under air up to 1000\u00a0\u00b0C at a 10\u00a0\u00b0C/min heating rate and recording the weight loss.Laser Raman spectra of the spent catalysts were measured, in the spectral range of 1400\u20131600\u00a0cm\u22121, on a JASCO NMR-4500 spectrometer. The wavelength of the excitation beam was fixed to 532\u00a0nm with the use of the objective lens of 100X magnification. The laser power was tuned to 1.6\u00a0mW for 10\u00a0s of exposure time at three accumulations to prevent sample damage by laser irradiation. Spectra Manager Ver.2 software (JASCO, Japan) was used to process the spectra.DRM experiments were performed at 700\u00a0\u00b0C and ambient pressure. Stainless-steel reactor (i.d.\u00a0=\u00a00.0091\u00a0m; length\u00a0=\u00a00.3\u00a0m) was used. An amount of 0.1\u00a0g of the catalyst was used for catalytic testing. The temperature was monitored using a sheathed, stainless-steel K-type thermocouple (placed at the centre of the catalyst bed). Before the catalytic examination, the catalyst was activated at 800\u00a0\u00b0C, under H2 flow, for 1\u00a0h. Methane, carbon dioxide, and nitrogen gases were mixed in a 3:3:1\u00a0vol ratio during the experiments. This gas mixture was used as a reactant feed with a space velocity of 42\u00a0L/h/gcat. The effluent gas was analysed by online GC-2014 SHIMADZU, equipped with a thermal conductivity detector, molecular sieve 5A column, and porapak Q column. Methane and CO2 conversions and deactivation factor were calculated as shown below:\n\n(3)\n\n\n\nC\n\nH\n4\n\n\n\nConversion\n\n(\n%\n)\n\n=\n\n\nmoles\n\nof\n\n\n(\n\nC\n\nH\n4\n\n\n)\n\n\ni\nn\n\n\n\u2013\nmoles\n\nof\n\n\n(\n\nC\n\nH\n4\n\n\n)\n\n\no\nu\nt\n\n\n\n\nmoles\n\nof\n\n\n\n(\n\nC\n\nH\n4\n\n\n)\n\n\ni\nn\n\n\n\n\n\u00d7\n100\n\u00d7\n100\n\n\n\n\n\n\n(4)\n\n\n\nC\n\nO\n2\n\n\n\nConversion\n\n(\n%\n)\n\n=\n\n\nmoles\n\nof\n\n\n(\n\nC\n\nO\n2\n\n\n)\n\n\ni\nn\n\n\n\u2013\nmoles\n\nof\n\n\n(\n\nC\n\nO\n2\n\n\n)\n\n\no\nu\nt\n\n\n\n\nmoles\n\nof\n\n\n\n(\n\nC\n\nO\n2\n\n\n)\n\n\ni\nn\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(5)\n\n\nDeactivation\n\nFactor\n=\n\n\n(\n\nInitial\n\nC\n\nH\n4\n\n\nConversion\n\u2212\nFinaI\n\nC\n\nH\n4\n\n\nConversion\n\n)\n\n\nInitial\n\nC\n\nH\n4\n\n\nConversion\n\n\n\n\n\n\nNitrogen physisorption was used to determine the catalysts' surface areas. Table S1 presents the BET-specific surface area (SBET), pore volume (Pv), and average pore diameter (Pd) for each promoted and un-promoted catalyst. The highest surface area was found for the un-promoted catalyst (5%Ni/\u03b3-Al2O3) with a surface area of 159.4\u00a0m2/g. Upon incorporating 1.0\u00a0wt% tungsten trioxide, SBET, Pv, and Pd of the catalyst were decreased due to the deposition of WO3 in the pore with values of 149.7\u00a0m2/g, 0.46\u00a0cm3/g and 11.4\u00a0nm, respectively. On further addition of tungsten trioxide, there were no discernible trends in surface area, pore volume and pore diameter results; however, the surface areas of the promoted systems were always lower than those of the un-promoted catalytic system [45]. Fig.\u00a02\n demonstrates the N2 adsorption-desorption isotherms, which, for all catalysts, fell within the mesoporous range of type IV isotherms, according to the IUPAC classification [46]. H1 hysteresis loop on type IV isotherm was specified by the sharp inflection in the 0.6\u20130.75 relative pressure region. It indicated the presence of uniform cylindrical mesopores.X-ray diffraction (XRD) analysis was carried out to investigate the prepared catalysts' crystallinity, as shown in Fig.\u00a03\n. The diffraction peaks with Miller indices of (311), (400), and (440) were identified at Bragg angles (2\u03b8): 37.10, 45.72, and 66.77\u00b0, respectively. These diffraction peaks were attributed to the cubic \u03b3-aluminium oxide phase (JCPDS card Nos.: 01-029-0063). No diffraction peaks were observed for nickel oxide, tungsten trioxide, and nickel aluminate (NiAl2O4), implying either their high dispersion on \u03b3-Al2O3 support or their diffraction peaks were overlapped with those of the support. The absence of tungsten trioxide diffraction peak even at 9\u00a0wt% loadings indicated that this amount was still below monolayer coverage, interacting with the surface, and thus was not detectable by XRD [46]. Variation of \u03b3-Al2O3 crystallite size upon the addition of tungsten trioxide was quite informative. The un-promoted catalyst had a crystallite size of 26.8\u00a0nm (at 2\u03b8\u00a0=\u00a066.7\u00b0) for the \u03b3-Al2O3, whereas just after the addition of 1.0\u00a0wt% WO3, the size of \u03b3-Al2O3 crystallite drastically decreased to 10.1\u00a0nm (Table S2).The reduction temperature profiles (TPR) of the fresh catalysts are shown in Fig.\u00a04\n. H2-TPR showed no reduction peaks in the WO3-\u03b3-Al2O3 catalyst system because no reducible metal oxides were present. Interestingly in the literature, WO3-loaded on silicate system had shown H2-TPR reduction peaks exclusively in the high-temperature range of 600\u20131000\u00a0\u00b0C due to reduction of W+6 state [47]. However, WO3-\u03b3-Al2O3 catalyst system had no reduction peaks, indicating that WO3-\u03b3-Al2O3 matrix was stable in H2 stream up to a wide range of temperatures. While upon loading 5.0\u00a0wt% NiO, reduction peaks appeared in various temperature regions, and all of them were detected below 1000\u00a0\u00b0C. Due to the strength of the interaction between the active NiO and mesoporous-\u03b3-Al2O3 support, different reduction temperature regions were observed. The reduction peaks in the temperature range of 500\u2013700\u00a0\u00b0C were attributed to the moderate interaction of NiO with the support. At high temperatures, nickel ions were diffused into the \u03b3-Al2O3 support, coordinated tetrahedrally and octahedrally and formed NiAl2O4 [48]. Thus, NiO was in strong interaction with \u03b3-Al2O3 support, and so the reduction peak in the temperature range of 700\u2013900\u00a0\u00b0C was due to the reduction of NiAl2O4 species [48]. At temperatures above 900\u00a0\u00b0C, some aluminum ions in NiAl2O4 may be replaced and form new NiO-WO3 species. Incorporating W with a higher oxidation state than Al in the matrix produced strong bonding over oxygen. Thus, NiO-WO3 species would be harder to reduce than NiAl2O4 species. Thus, the peak in the temperature range of 900\u20131000\u00a0\u00b0C could be attributed to the reduction peak of NiWOAl species [48]. The sudden decrease in crystallite size of Al2O3 after adding WO3 was also noticed in XRD, confirming the substitution of Al ions with W ions. The negative peaks in the temperature range of 100\u2013300\u00a0\u00b0C may be due to the hydrogen spillover in the mesopores of \u03b3-Al2O3 support [30].\nTable 1\n displays the H2 consumption quantities of the various promoted and un-promoted tungsten catalysts. The unpromoted catalyst had a reducible peak of NiAl2O4 species majorly. Upon tungsten trioxide loading, reducible species of moderately interacting NiO and strongly interacting NiWOAl were grown. 5Ni\u00a0+\u00a0xWO3/\u03b3-Al2O3 (x\u00a0=\u00a01\u20135) catalyst exhibited all types of NiO-interacting species, as discussed above, with support. With increasing tungsten trioxide loading, the total amount of reducible NiO-interacting species was increased over the catalyst surface (Table 1). Above 5\u00a0wt% WO3 loading, 5Ni+7WO3/\u03b3-Al2O3 and 5Ni+9WO3/\u03b3-Al2O3 catalysts had no reducible NiWOAl species.The CO2-temperature-programmed desorption (CO2\n-TPD) of the fresh catalysts is shown in Fig.\u00a05\nA. It depicts the CO2 desorption peaks within the temperature range of 50\u2013650\u00a0\u00b0C. The desorption peaks below 100\u00a0\u00b0C were attributed to weak basic sites/surface hydroxyl, while the peaks below 200\u00a0\u00b0C were attributed to medium-strength basic sites/carbonates, and the peaks from 200 to 400\u00a0\u00b0C were for strong basic sites/surface O2\u2212 species [49,50]. The 5.0\u00a0wt% WO3 catalyst sample exhibited a broad range of CO2 adsorption over a wide distribution of basic sites over the catalyst surface. However, it had a moderate amount of basic sites/surface anion, whereas 3.0\u00a0wt% WO3 had the maximum number of strong basic sites/surface anions. The NH3-TPD profiles (Fig.\u00a05B) showed that 5.0\u00a0wt% WO3 loading had the maximum number of acidic sites, implying that the surface of this catalyst was the most enriched with acidity. The acidic sites were claimed for carbon accumulation in DRM [51]. Overall, 5.0\u00a0wt% WO3 catalyst had a moderate number of basic sites and the maximum number of acidic sites, whereas 3.0\u00a0wt% WO3 had a moderate number of acidic sites and the maximum number of basic sites. Previously, it was reported that the surface WOx species on Al2O3 was more acidic due to the high electronegativity of the Al and the formation of more acidic bridging W\u2013O\u2013Al [52]. Therefore, upon the addition of WO3 over alumina support, a rise in acidity can be expected. However, after an optimum loading of WO3 (>5\u00a0wt%), the drastic fall in the acidity of the catalyst was noticeable in 5Ni+7%WO3/\u03b3-Al2O3 and 5Ni+9%WO3/\u03b3-Al2O3 catalysts. The drastic fall in acidity may be claimed to the coverage of the acid site of alumina by surmounting WO3 [53]. However, the same observation was found over the zirconia-supported WO3 catalyst. Zirconia support is not acidic, but upon more than 5\u00a0wt% WO3 loading, the acidity of the catalyst was fallen [54]. It indicates that above than 5\u00a0wt% WO3 loading, whether support is alumina or zirconia, the acidity of catalyst was decreased. It pointed out that bulk WO3 was formed after optimum loading, leading to a decrease in ammonia adsorption and surface acidity [47].\nFig.\u00a06\n shows the SEM images for two fresh catalysts of 5Ni\u00a0+\u00a0xWO3/\u03b3-Al2O3, where x is either 3.0\u00a0wt% (Fig.\u00a06A) or 9.0\u00a0wt% (Fig.\u00a06B). Chunky with irregular-shaped particles were observed for both samples. The morphology was not affected by varying the weight percent loading of WO3. The EDX surface elemental analysis of 5Ni+3WO3/\u03b3-Al2O3 is shown in Figure\u00a0S1. Surface oxygen could help in the gasification of carbon deposition and played a role in surface basicity [55]. Aluminium is the second element on the surface in terms of abundance (\u223c42\u00a0wt %). The loaded nickel appeared on the surface with \u223c4\u00a0wt%, which could be linked to the observed catalytic performance because nickel is the active catalyst and interacts with the support.Experimental CO2 and CH4 conversion results in Fig.\u00a07\nA and Fig.\u00a07B, respectively, showed that the catalytic performance of all catalysts was fairly stable within a reaction time of 7.5\u00a0h. The operation of the RWGS reaction during the DRM was manifested by the higher values of fractional conversion of CO2 than those of CH4 [56]. The conversions of CH4 and CO2 were 72\u201373% and 78\u201379%, respectively, over 5Ni/\u03b3-Al2O3 within 7.5\u00a0h at 700\u00a0\u00b0C. Incorporation of tungsten trioxide and increasing its loading to 5.0\u00a0wt% WO3, the CH4 conversion, and CO2 conversion increased progressively. Tungsten trioxide was also known for its ability to decompose CH4. Thus, the increase in CH4 conversion could be primarily correlated with the increase of tungsten trioxide in the catalyst [42]. The conversions of CH4 and CO2 were 79% and 83%, respectively, over 5Ni+5WO3/\u03b3-Al2O3 within 7.5\u00a0h at 700\u00a0\u00b0C. The incorporation of tungsten trioxide into the catalyst had no effect on CO2 conversion when compared to CH4 conversion. Table S3 displays that the H2/CO mole ratio was \u223c1.0 for all catalysts during the 7.5\u00a0h of time-on-stream. This molar ratio is the proper one for higher conversions, better stability, and reduction in CO2 participation in RWGS reaction [57]. Tungsten-promoted catalysts demonstrated substantial stability in syngas production, with the minor carbonaceous compound formation on the surface of catalysts likely attributable to the presence of tungsten [58]. It was reported that tungsten promoter covered particularly the deformed sites at the catalyst surface, which improved the rate of carbon gasification and thereby induced stability. Furthermore, the existence of Ni-stabilized reducible species (i.e., NiAl2O4 and NiWOAl) in the presence of WO3 may substantially affect deactivation. At 5.0\u00a0wt% WO3 loading, the deactivation factor (0.88) was the minimum. The 5.0\u00a0wt % WO3 loading was the optimum loading in terms of CH4 conversion and minimum deactivation. Further loading above 5.0\u00a0wt% loading resulted in a reduction in both CH4 conversion and deactivation factor.Thermogravimetric (TGA) analysis for the spent catalysts (Fig.\u00a08\n) was performed in the temperature range of 100\u20131000\u00a0\u00b0C after running DRM for 7.5\u00a0h at 700\u00a0\u00b0C. The catalyst with 9.0\u00a0wt% WO3 had the highest carbon deposition (88\u00a0mg/g), whereas the amount of carbon deposit formed on 5.0\u00a0wt% tungsten trioxide was the least (31\u00a0mg/g). With increasing tungsten trioxide loading from 1 to 5\u00a0wt%, the amount of carbon deposition decreased over the spent catalysts because of the reducible characteristic of tungsten [19]. In addition, the improved stability and higher resistance to carbon deposition of the tungsten-promoted catalysts could be ascribed to the cover of active sites with the tungsten promoter, particularly deformed sites, which lowered the amount of formed carbon. Sayed and his co-workers have claimed the coke resistance property of the tungsten-promoted catalyst [42]. The alternative explanation can be derived from the acidic-basic profile of the catalyst. Carbon decomposition depends on CH4 dissociation and, is then delayed in the oxidation of CHx species by CO2. Basicity enhances CO2 adsorption, whereas acidity encounters CH4 dissociation [58]. Herein, up to 5.0 wt. W%, there was fine-tuning between acidic and basic sites for proper carbon removal, but after this optimal loading, fine-tuning was lost, resulting in an increase in carbon deposition at 7.0\u00a0wt% Ni and the highest in 9.0\u00a0wt% Ni.The laser Raman spectra (Figure\u00a0S2) for all spent catalysts in the spectral region of 1400\u22121600\u00a0cm\u22121 showed two intense peaks at Raman shift of 1475\u00a0\u00b1\u00a05\u00a0cm\u22121 and 1530\u00a0\u00b1\u00a010\u00a0cm\u22121, which corresponded to the D and G bands, respectively [59,60]. The D band is related to amorphous/disordered carbon deposits, while the G band is characterized by graphitic/ordered carbon deposits [61,62]. The ratio of the intensity of disordered carbon and ordered carbon (ID/IG) was 1.15 over the unpromoted catalyst and 1.08\u20131.01 over tungsten-promoted catalysts, implying that the unpromoted catalyst had relatively more amount of disordered carbon than ordered carbon. Among tungsten-promoted catalysts, ID/IG value decreased nominally from 1.08 to 1.01 on increasing loading of tungsten trioxide promoter from 1.0\u00a0wt% to 9.0\u00a0wt%. It indicates that the degree of graphitization was nominally increased with increasing tungsten trioxide loading.Previous literature on WO3-promoted Ni/Al2O3 catalyst showed better coke-resistant but inferior catalytic activity than the unpromoted catalyst due to partial coverage of catalytic active sites by WO3 (6.3\u201326.3\u00a0wt%), less exposure of Ni species, and the formation of Ni17W3 alloy, which was less active for methane decomposition [41,42]. Our two-step synthesis methodology used less amount of WO3 (5\u00a0wt%) and showed superior catalytic activity (\u223c79% CH4 conversion and \u223c83% CO2 conversion during up to 7.5\u00a0h time-on-stream) than the earlier reported co-impregnated catalyst. The synthetic methodology may be one of the reasons for the good catalytic performance. We first synthesized mesoporous alumina support, which was then impregnated with tungsten trioxide and calcined. The second step was the dry impregnation for loading nickel precursor onto \u201cmesoporous alumina support promoted with tungsten trioxide\u201d followed by calcination. WO3-Al2O3 support in our catalyst system was not reducible under H2 steam. Consequently, WO3 interacted well with Al2O3 in this synthetic approach and was stable up to 1000\u00a0\u00b0C under H2 steam. The Ni/Al2O3 catalyst system was quite active for CH4 conversion (\u223c72%). Certainly, the potential catalytic active sites are metallic Ni, where CH4 was decomposed. On the addition of 1.0\u00a0wt% WO3 interacted strongly with \u03b3-Al2O3, and non-reducible \u201cWO3-Al2O3\u201d support was formed. Further substitution of Al ions with W ions in 5Ni+1WO3/\u03b3-Al2O3 catalyst formed NiWOAl species, which was reducible in the range of 900\u20131000\u00a0\u00b0C under the H2 stream. The strong interaction between WO3 and Al2O3 was also verified by the sudden decrease of crystallite size of Al2O3 after the 1.0\u00a0wt% addition of WO3. In total, the reducible NiO-interacting species was increased to about 38% just after the addition of 1.0\u00a0wt% WO3. Therefore, the 5Ni+1WO3/\u03b3-Al2O3 catalyst showed marginal progress in CH4 conversion (74%) and CO2 conversion (80%).When the tungsten trioxide loading was 5.0\u00a0wt%, the total reducible NiO-species concentration over the catalyst surface was increased more than double that of the nonpromoted catalyst (Table 1). The 5.0\u00a0wt% tungsten-promoted catalyst contained all types of reducible NiO-interacting species with support. The acidic-basic profile of the catalysts were quite informative. The 5.0\u00a0wt% tungsten-promoted catalyst had a moderate amount of wide range of basic sites and the maximum number of acidic sites in comparison to the other catalysts. Moderate basic sites were related to moderate CO2 adsorption. Methane decomposition increases as surface acidity increases, as reported in [63]. In earlier literature, WO3 was also recognized for its ability to decompose CH4, which was decomposed over Ni and WO3 sites to form NiC3 and WC species. WC itself was active during the methane reforming reaction [64]. WC was oxidized by carbon dioxide into WO3 and CO [42]. However, we have not found carbide phases in XRD (WC, NiC3) and reducible W+6 species in H2-TPR. Overall, it could be said that the presence of Ni metal, derived from thermally stable NiAl2O4, and NiWOAl species and decomposition of CH4 over Ni metal pivoted the path of the optimum performance in DRM reaction over the 5Ni+5WO3/\u03b3-Al2O3 catalyst, which resulted in >79% CH4 conversion and >83% CO2 conversion over 7.5\u00a0h of the time-on-stream test. Adjusting acidic and basic sites was crucial for the high catalytic performance of tungsten-promoted, Al2O3-supported Ni-based catalysts. As 5Ni+5WO3/\u03b3-Al2O3 had a moderate number of basic sites and the maximum number of acidic sites, whereas 5Ni+3WO3/\u03b3-Al2O3 had a moderate number of acid sites as well as the maximum number of basic sites. 5Ni+3WO3/\u03b3-Al2O3 catalyst performed comparably less than 5Ni+5WO3/\u03b3-Al2O3 in the mean of CH4 conversion (\u223c75%) and CO2 conversion (81%). After the optimum tungsten loading (5.0\u00a0wt%), catalytic activity declined even below the un-promoted catalyst.The systematic reaction scheme of dry reforming of methane over nonpromoted and tungsten-promoted \u03b3-Al2O3 supported Ni catalyst is shown in Fig.\u00a09\n. The dry reforming reaction can be specified by \u201crate of CH4 dissociation\u201d and \u201crate of carbon deposit oxidation by CO2\u201d. The delay in carbon deposit oxidation results in carbon deposit over the catalyst surface. Nagaoka et\u00a0al. found that CH4 was decomposed over metal active sites and acid sites of Al2O3 support [65] (Fig.\u00a09 A-9a). However, dehydrogenated methane (CHx) over acid support was not reactive towards CO2-species, causing severe coke decomposition and deactivation of catalyst (Fig.\u00a09\u03b1). Upon the addition of WO3 over alumina support, a rise in acidity is expected due to the formation of more acidic bridging W\u2013O\u2013Al [66]. In our case, 5Ni+5%WO3/\u03b3-Al2O3 catalyst system had maximum acid sites, optimum CH4 conversion and least coke decomposition. It indicated that the new support, formed by \u201cinteraction of WO3 and Al2O3\u201d as well as \u201csubstitution of Al ion by W ion\u201d, was unique and CHx species over such support remained active with CO2-species for oxidation (Fig.\u00a09B-9b-9\u03b2). Thus, 5Ni+5%WO3/\u03b3-Al2O3 catalyst had maximum CH4 conversion (\u223c79%), CO2 conversion (\u223c83%), and the least coke decomposition (31\u00a0mg/g). Above 5.0\u00a0wt% WO3 incorporation (5Ni+7%WO3/\u03b3-Al2O3 and 5Ni+9%WO3/\u03b3-Al2O3 catalyst), the interaction between WO3 and Al2O3 species was weakened, and bulk WO3 grew, resulted into a drop in acidity and less CH4 conversion (Fig.\u00a09C-9c-9\u03b3). However, catalytic activity declined below the unpromoted catalyst [67]. 5Ni+7%WO3/\u03b3-Al2O3 and 5Ni+9%WO3/\u03b3-Al2O3 catalysts showed \u223c71.4% and 70.3% CH4 conversion, respectively. It indicates that only a drop in acidity did not cause a such severe decline in catalytic activity. It might be due to the covering of potential catalytic active sites (metallic Ni) by WO3 overloading. Upon higher tungsten trioxide loading, unique WO3-Al2O3 support was defoliated and became inactive for the interaction of dehydrogenated methane (CHx) and CO2-species. It caused more coke deposition over 5Ni+7%WO3/\u03b3-Al2O3 and 5Ni+9%WO3/\u03b3-Al2O3 catalysts. The degree of crystallinity of deposited carbon over non-promoted and tungsten-promoted catalysts can be answered by Raman results. The degree of graphitization increased markedly from unpromoted to a tungsten-promoted catalyst, and it continued to increase nominally on increasing loading of tungsten trioxide.Alumina-supported Ni-based catalyst system with noble and non-noble metals-based promoters, including Mg [22], Ca, Ba, Sr [25], B [26], Si [30], Ti, Mo, Zr, W, Cu [31], Zn, Gd, La [32,34], Yb [35], Co [36], Ce [68] and Fe [69] have been reported for the dry reforming reaction. Table 2\n compares catalytic activity in terms of CH4 conversion, CO2 conversion at different reaction/catalytic conditions such as catalyst weight, catalyst surface area, internal reactor diameter, feed ratio, activation temperature, reaction temperature, and time on stream (TOS). Our catalyst 5Ni+5WO3/\u03b3-Al2O3 was superior and comparable to 5Ni+1Cu/Al [31] and Ni2Yb/\u03b3-Al2O3 [35] catalysts over a minimum catalyst amount of 100\u00a0mg and moderate reaction temperature of 700\u00a0\u00b0C.ID= Internal Diameter of reactor, SA = surface area, CA = Catalyst amount, Cg = Carrier gas, AT/RT = Activation temperature/Reaction temperature, TOS = Time on stream, C (CH4) = Conversion of CH4, C (CO2) = Conversion of CO2, Ref. = Reference.The coke resistance and high catalytic activity for dry reforming of methane increased with increasing the loading of the WO3 promoter up to 5.0\u00a0wt%. We obtained 79% CH4 conversion and 83% CO2 conversion over 7.5\u00a0h of time-on-stream when using the 5.0\u00a0wt% WO3-promoted catalyst because it possessed stable, reducible NiAl2O4 and NiWOAl species, the moderate density of basic sites with the highest density of acidic sites. Increasing the weight load percent of tungsten trioxide above 5.0\u00a0wt% decreased the catalyst stability, increased the carbon deposition on the catalyst, and decreased the CH4 and CO2 conversions due to WO3 overloading covering catalytically active Ni sites. The degree of graphitization over spent catalyst was increased markedly from unpromoted to the tungsten-promoted catalyst and nominally on increasing loading of tungsten trioxide.The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU Programmes Body (SEUPB).The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to appreciate sincerely to Researchers Supporting Project number (RSP-2021/368), King Saud University, Riyadh, Saudi Arabia. Dr Ahmed I. Osman wishes to acknowledge the support of The Bryden Centre project (Project ID VA5048). The Bryden Centre project is supported by the European Union's INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). The authors would like to thank Charlie Farrell for proofreading the manuscript. RK acknowledges the Indus University, India for supporting research.The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2022.09.313.", "descript": "\n                  Syngas production via dry reforming of methane was conducted over 5\u00a0wt%Ni\u00a0+\u00a0xWO3/\u03b3-Al2O3 (x\u00a0=\u00a01, 3, 5, 7, or 9\u00a0wt%) catalysts at 700\u00a0\u00b0C and ambient pressure for 7.5\u00a0h in a tubular fixed-bed reactor. Textural, morphological, and catalytic properties were investigated in relation to the weight percent of tungsten trioxide loading. The physicochemical properties of the catalysts were evaluated using XRD, N2-physisorption, TGA, H2-TPR, CO2-TPD, NH3-TPD, SEM, EDX, and Raman techniques. N2-physisorption analysis showed that tungsten trioxide promoter had a minor impact on the textural properties upon varying its weight percentage loading. With increasing tungsten trioxide loading, the total amount of reducible NiO-interacting species was increased over the catalyst surface. 5Ni+5WO3/\u03b3-Al2O3 catalyst showed stable 79% CH4 conversions and 83% CO2 conversion with the lowest carbon deposition due to the presence of stable metallic Ni species (derived from reducible NiAl2O4 and NiWOAl), the highly acidic sites, and moderate basic sites.\n               "}
{"full_text": "Data will be made available on request.An attractive route towards CO2-free production of hydrogen is the thermal catalytic decomposition of methane. In addition to H2 formation, carbon nanomaterials, such as fibers, single or multiwalled carbon nanotubes, or other carbon nanostructures can be grown as a potentially valuable by-product [1\u20135\n].\n\n(1)\n\n\n\n\nCH\n\n\n4\n\n\n\ng\n\n\u2192\n2\n\nH\n2\n\n\ng\n\n+\nC\n\ns\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\u2206\n\nH\n298\n0\n\n=\n74.5\n\nkJ\nmol\n\n\n\n\n\nThese carbon nanostructures have interesting properties, e.g. high electron conductivity and/or mechanical strength [6]. They can be used in a wide range of applications like in semiconductors, energy storage, catalyst supports or building blocks for strong and light-weight materials (tennis rackets or planes) [7\u201310].The most commonly used catalysts for this reaction are first row transition metals (Ni, Fe and Co) supported on different inert materials [4,11\u201315]. These metals have a high carbon solubility which is essential for carbon transport [16]. Among these transition metals, supported Ni catalysts have been most widely investigated. Ni is highly active for methane decomposition, and can usually be implemented at reaction temperatures between 500 and 700\u2009\u00b0C. However, Ni-catalysts suffer from fast deactivation due to surface encapsulation by carbon. Addition of copper to the Ni-catalysts improves the catalytic activity and lifetime [4,11,12,17\u201322]. It must be noted that the catalytic performance strongly depends on the support that is used [15,21,23]. For example, Pinilla et al. found that the carbon yield was significantly higher when using NiCu catalysts supported on Al2O3 or MgO rather than on TiO2 or SiO2\n[21].To restrict the influence of support effects, we use a graphitic carbon support that is weakly interacting (chemically inert) and thermally stable [24\u201326]. With carbon as a support, tip growth for the formation of carbon nanofibers is expected (the metal nanoparticle is lifted from the support by the growing carbon nanostructure)\u00a0[27\u201329]. Another practical advantage of using carbon as a support is that one only has to remove the metal catalyst, but not the support material from the final product, possibly avoiding harsh conditions for example to remove silica from the products. After the growth of carbon nanofibers, the graphitic support is only a small fraction of the final reaction product. For fundamental studies, it is also relevant that carbon is such a light element, that the contrast with the metal in electron microscopy makes more detailed studies possible.In this paper, we describe how the carbon yield and catalytic lifetime of carbon-supported NiCu catalysts depend on the reaction conditions. A thermal gravimetric analyzer was used to follow the carbon growth in-situ, by monitoring the sample weight. Interestingly, we found that two different operation regimes can be identified, with distinctly different reaction orders and activation energies, and different types of carbon nanostructures being formed. The highest carbon yield was obtained at the interface between these two operating regimes.Nickel(II) nitrate hexahydrate (Ni(NO3)2\u00b76\u2009H2O, \u2265\u200997%) and Copper(II) nitrate trihydrate (Cu(NO3)2\u00b73H2O, 99%) were purchased from Sigma Aldrich. Graphene nanoplatelets (xGnPC-500, later referred to as C or GNP-500) were used as support material for the catalyst synthesis. This support material consists of stacked graphitic sheets ( \u223c 500\u2009m2/g surface area, \u223c 0.9\u2009mL/g pore volume) and was obtained from XG Sciences.The catalysts were prepared via incipient wetness (co-)impregnation. The carbon support, GNP-500, was impregnated with an aqueous solution containing one or both metal precursors (Ni(NO3)2\u00b76\u2009H2O and/or Cu(NO3)2\u00b73\u2009H2O, pH \u223c1). The Ni catalyst was prepared with a 4\u2009M Ni-solution. The bimetallic catalyst was prepared using a 3\u2009M Ni, 1\u2009M Cu precursor solution. After impregnation, the samples were dried overnight under dynamic vacuum. Thereafter, the samples were treated at 330\u2009\u00b0C (mono- and bimetallic) for 2\u2009h under nitrogen flow (200\u2009mL\u2009min\u22121 gcat\n\u22121) and reduced at 280\u2009\u00b0C (Ni-Cu) or 300\u2009\u00b0C (Ni) for 2\u2009h in 5% H2/N2 (200\u2009mL\u2009min\u22121 gcat\n\u22121).Methane decomposition was performed in a thermogravimetric analyzer (TGA, PerkinElmer TGA 8000) coupled to a Mass Spectrometer (Hiden Analytical HPR-20). The sample weight was constantly monitored during the reaction. 1\u20132\u2009mg catalyst powder was loaded in a ceramic crucible sample holder (d = 6\u2009mm, h = 2\u2009mm, set-up shown in Fig. S1.1). First, the catalyst was dried at 70\u2009\u00b0C for 15\u2009min under argon. To make sure that the catalysts were in fully reduced state before catalysis, the sample was reduced in-situ at 280\u2009\u00b0C (Ni-Cu) or 330 (Ni) \u00b0C for 3\u2009h in 5% H2/Ar (total flow = 100\u2009mL\u2009min\u22121). During this step, the sample weight was closely monitored. As the sample weight stabilized within the 180\u2009min in-situ reduction, we could safely assume that reduction was completed before the catalysis started (for details see Fig. S1.2, which shows the sample weight during the in-situ reduction step). Then, the sample was heated to reaction temperature (5\u2009\u00b0C\u2009min\u22121) under Ar. As soon as the reaction temperature was reached, methane gas was introduced to the system by an external gas mixing device. The experiments were carried out with a total flow rate of 127\u2009mL\u2009min\u22121 and at atmospheric pressure. The reaction temperature was varied in the range of 450\u2013600\u2009\u00b0C, while keeping the partial pressure of CH4 and total flow constant. The partial pressure of methane was varied between 0.20 and 0.40\u2009bar and argon was used as balance gas. Blank experiments were performed to exclude reactivity of the sample holder or the support (for details, see supporting information Section S1.3.) All experiments were performed in duplo using the same catalyst batch (see section S1.4). The conversions in the experiments did not exceed 2%, which is an order of magnitude less than the equilibrium conversions at any conditions used. This comparison in shown in supporting information section S2.Nitrogen physisorption at 77\u2009K was performed on a Tristar II Plus apparatus (Micromeritics) to characterize the surface area and porosity of the support material and catalysts. The BET surface area was determined in the relative pressure range of p/p\n0 =\u20090.03\u20130.14. The total pore volume was determined from the adsorbed quantity at p/p\n0 =\u20090.995. Prior to analysis, the samples were dried at 170\u2009\u00b0C under vacuum for 24\u2009h.The presence of crystalline phases and the crystallite sizes were analyzed using powder X-ray diffraction (XRD) in a Bruker D2 Phaser 2nd Generation diffractometer with a Co radiation source (\u03bb\u2009=\u20091.7889\u2009\u00c5). A Bruker D8 Advance (Co irradiation) was used for X-ray diffraction under inert atmosphere.H2-Temperature programmed reduction (TPR, Micromeritics, AutoChem II 2920) was used to check the presence of bimetallic phases. 50\u2009mg sieved fraction (75\u2013150\u2009\u00b5m) of catalyst (after heat treatment) was loaded in a quartz U-tube in between quartz wool. Prior to reduction, the sample was dried at 120\u2009\u00b0C. Then, the sample was cooled down and heated up from 30 to 500\u00a0\u00b0C (5\u2009\u00b0C\u2009min\u22121) in 5% H2/Ar (flow = 40\u2009mL\u2009min\u22121). A thermal conductivity detector (TCD) was used to obtain the reduction profiles.Transmission Electron Microscopy (TEM) was used to determine the particle size of the fresh catalysts and to study the structure of the products formed during the decomposition experiments. A ThermoFischer Talos F200X was operated at 200\u2009kV in TEM mode to capture bright-field images of the catalyst. Scanning transmission electron microscopy Energy dispersive X-ray spectroscopy (STEM-EDX) mapping was used to map the distribution of Ni and Cu in the catalyst before catalysis.Weight loadings were determined via ICP analysis, which was carried out by an external institute (the Mikroanalytisches Laboratorium Kolbe). The elements were measured on an Arcos Model Acros Spectro ICP-OES after microwave digestion on a CEM MARS 6.For the TGA measurements, the increase in sample weight during the reaction \u2013 when exposed to methane - is directly related to the amount of carbon that is formed; the carbon yield. The carbon deposition was normalized for the amount of Ni present in the sample. This also holds for the bimetallic catalysts, as copper itself is inactive for the reaction.When the reaction temperature is reached, methane is added to the gas feed. As a consequence, the total flow rate becomes higher and the gas density changes. Both events induce a small drift in observed sample weight due to the buoyancy effect [30,31]. In addition, after triggering the gas switch, it takes a moment before the new feed composition reaches the sample. For example, at 500 and 600\u2009\u00b0C it takes about a minute before the sample weight starts to change significantly due to carbon nanostructure formation. Therefore, we have looked at when the increase in sample weight was the largest after the introduction of methane and took that as our t\u2009=\u20090 for each of the individual experiments. The results of each pair of duplo measurements were averaged. On average, the deviation between the two measurements was approximately 8% (for more details see supporting information section S1.4.).The carbon growth rate (r) was calculated by taking the derivative of the carbon yield with respect to time:\n\n(2)\n\n\nr\n\n\n\nt\n\n\n\n=\n\n\n\u2206\n\n\nC\n\n\nyield\n\n\n\n\n\u2206\nt\n\n\n\n\n\nwhere C\nyield is the carbon yield (gC/gNi), and t is the reaction time (min). The initial growth rate, r\n\n0\n, is the growth rate derived from the first two points of the reaction.In this work, the lifetime of the catalyst is defined as the time when \n\nr\n\n\n\nt\n\n\n\n=\n\n\n0.01\n\u2219\nr\n\n\n0\n\n\n\n. For the experiments with varying the partial pressure of methane, the lifetime of the catalysts was determined manually with a 10% error. For the temperature experiments, the catalysts did not deactivate to 1% of the initial growth rate within the measuring time. Therefore, the growth rate was fitted using the following equation of Borghei et al [32]:\n\n(3)\n\n\nr\n(\nt\n)\n=\n\n\n1\n\n\n\n\n\n\n1\n+\n\n\n\nd\n\u2212\n1\n\n\n\n\n\nr\n\n\nd\n\n\nt\n\n\n\n\n\n\n1\n\n\n(\nd\n\u2212\n1\n)\n\n\n\n\n\n\n\n\n\u2219\nr\n\n\n0\n\n\n\n\n\n\nin which d is the deactivation factor, r\n\nd\n the deactivation rate, r\n\n0\n is the initial/maximal growth rate. This equation was derived by a kinetic study of carbon formation over NiCu/MgO catalysts. Important to note is that r\n\nd\n might be dependent on temperature and the partial pressures of methane and hydrogen. A further explanation of the lifetime determination can be found in section S3 of the supplementary information.Powder X-ray diffraction was used to characterize the crystallite phases present in the catalysts. \nFig. 1a shows XRD patterns of the support material (black line) and pre-reduced catalysts. Diffraction peaks at 2\u03b8 =\u200930, 52 and 64\u00b0 are characteristic for graphitic materials and are attributed to the graphite support [33]. Peaks at 2\u03b8 =\u200951 and 59\u00b0 are ascribed to the presence of crystalline Ni and Cu, 2\u03b8 =\u200988\u00b0 can be ascribed to Cu. As the weight loading of Cu in the bimetallic catalyst is low, it was not possible to resolve this peak in the NiCu/C diffractogram. Peaks at 2\u03b8 =\u200934, 43 and 74\u00b0 indicate the presence of metal oxides. This indicates that the pre-reduced catalysts had partially been re-oxidized before or during the XRD measurement due to air exposure.H2-Temperature programmed reduction (TPR) was used to study the reduction behavior of the catalysts. Prior to the measurements, the catalysts only had been heat treated at 330\u2009\u00b0C under nitrogen flow and had not yet been reduced. Fig. 1b shows the reduction profiles of the carbon-supported 15% Ni (orange), 13% Cu (blue) and 11% Ni 4% Cu (green) catalysts. For the Cu/C sample, a peak in hydrogen consumption is observed at Tmax=\u2009145\u2009\u00b0C which corresponds to the reduction of copper oxides to metallic copper. The Ni/C sample shows multiple reduction peaks which can be ascribed to reduction of NiO to Ni, weakly or more strongly interacting with the support [34,35]. It was calculated that theoretically approximately 60\u2009mL\u2009H2/ gcat was needed to reduce all NiO to Ni. However, when integrating the whole peak area of the reduction profile (120\u2013500\u2009\u00b0C) a total of 118\u2009mL\u2009H2/gcat was consumed (\nTable 1). This can be explained by the fact that above 300\u2009\u00b0C Ni catalyzes the reaction of hydrogen with the graphite support to methane, resulting in a hydrogen uptake [25].In the profile of the NiCu/C catalyst multiple peaks are present. The first peak is observed at Tmax =\u2009175\u2009\u00b0C, which is shifted 30\u2009\u00b0C to a higher temperature with respect to the copper sample. The total hydrogen consumption of these peaks are listed in Table 1. The area underneath the first peak in the bimetallic sample is larger than the area of the peak in the Cu/C sample. Yet, the Cu content in the bimetallic sample is much lower (4\u2009wt% compared to 13\u2009wt%). Therefore, it can be concluded that not only the reduction of copper oxides but also the reduction of (part of) the nickel oxide contribute to the observed hydrogen uptake. From the fact that Cu and Ni influence each other\u2019s reduction behavior, it can be concluded that they are in close proximity to each other. The other peaks are observed at similar temperatures as the particles of the Ni/C catalyst. Also here, support methanation becomes significant above 300\u2009\u00b0C (see Fig. S4.1). Yet, it could also mean that there is still some isolated Ni present.Transmission electron microscopy was used to determine the average particle size and to study the metal distribution in the catalysts. We evaluated the particle sizes at the start of the reaction. In order to do this, we loaded catalyst powder in the TGA. Part of the regular method was used (as described in Section 2.3.) with a drying step, in-situ reduction and subsequent heating to 500\u2009\u00b0C in argon (reaction temperature). After that, the sample was cooled down, and transferred to the TEM (under ambient conditions). \nFig. 2 shows representative TEM images and the corresponding size distributions of the fresh Ni/C (top; a,b) and NiCu/C (bottom; c,d) catalysts. The metal particles are well visible in the bright-field TEM images due to the high contrast with the graphitic support. The metal particles (dark) are well distributed over the graphitic sheets (light gray). The average particle sizes are 8.2\u2009\u00b1\u20092.0 (Ni/C) and 10.8\u2009\u00b1\u20093.4\u2009nm (NiCu/C). Fig. S4.2 provides a comparison between the particle sizes after ex-situ reduction and the fresh catalyst before the start of the reaction. From this, it can be seen that the bimetallic particles are a little bit larger than the monometallic particles. This is possibly due to more particle growth during heating. Scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM-EDX) was performed to map the distribution Ni and Cu in the particles (Fig. S4.3 in supporting information). It was observed that both Ni and Cu are present in the same particles.\n\nTable 2 summarizes the most important properties of the support, and the catalysts. The metal weight loadings were determined via ICP.We explored how the lifetime of the carbon supported Ni-based catalysts was influenced by the presence of Cu, and operation conditions (the reaction temperature and the partial pressure of methane). In addition, we looked at the carbon products formed under different reaction conditions. First, we compare the performance of the two catalysts. \nFig. 3a shows the evolution of carbon production over time over 15% Ni/C (orange) and 11% Ni 4% Cu/C (green) catalysts, at 500\u2009\u00b0C with a partial pressure of methane (p\nCH4 = 0.34\u2009bar). First of all, it is observed that the initial growth rate over the NiCu/C catalyst is significantly higher than for the monometallic catalyst. In addition, the lifetime of the NiCu/C catalyst is much longer than for Ni/C. Together this leads to a 6\u20137 times higher final carbon yield. Thus, the bimetallic catalyst clearly outperforms the monometallic catalyst. Experiments were reproducible within an average relative error of 8% (see for details section S1.4. of the supporting information). Gas analysis during catalysis with a mass spectrometer did not show any formation of CO2 (see section S5).\nFig. 3b shows an electron micrograph of the carbon nanofibers formed using the Ni/C catalyst. Thin and thick fishbone-type fibers are observed, originating from smaller and larger particles, respectively. The Ni particle at the top (magnification in Fig. 3c) shows the presence of carbon layers around the metal particle which could have stopped further carbon growth.It was proposed in literature that Cu helps to prevent carbon layer formation on the active surface of metal Ni(Cu) particles, thereby enhancing the lifetime and performance of the catalysts [36]. Yet, work of Pinilla et al. showed that the effect of Cu depends on the type of support [21]. Here, we show that for a carbon support the presence of Cu in the catalyst leads to a significant enhancement of the catalytic performance. One possible explanation is the influence of Cu on the particle size during catalysis. It was already postulated in Section 3.1. and shown in Fig. S4.2. that NiCu particles grow more at the reaction temperature in inert atmosphere than pure Ni metal particles, possibly due to a reduced melting point of the nanoparticles by Cu addition. In literature it has been reported that the catalytic performance can be highly dependent on the size of catalyst particles\u00a0[37]. Optimal particle sizes were reported in the range of 10\u201340\u2009nm depending on the reaction conditions and the composition of the catalyst [21,38].\n\nFig. 4a and c show TEM micrographs of the catalysts after 5\u2009min reaction time at 500\u2009\u00b0C with p\nCH4 =\u20090.34\u2009bar. A lot of fibers were observed in both samples. It was clear that in the Ni/C sample, the fibers were smaller in diameter than most of the fibers in the NiCu/C sample. Also, a few very large particles were observed (50\u201360\u2009nm) in the bimetallic sample. We have evaluated the average particle size in both cases. The size distributions are shown in Fig. 4b and d. The bimetallic particles have grown from 10.8\u2009\u00b1\u20093.4\u2009nm to an average size of 23.4\u2009\u00b1\u20099.2\u2009nm. On the other hand, the Ni particles have only grown from 8.2\u2009\u00b1\u20092.0\u2009nm to 10.6\u2009\u00b1\u20092.1\u2009nm. Clearly, Cu containing particles are more prone to particle growth as they have become more than twice as large. Under our reaction conditions, the NiCu particles probably obtain a more optimal size than the monometallic catalyst particles. Hence, we will further discuss the influence of reaction conditions on the catalytic performance specifically of NiCu catalysts.We investigated the influence of the reaction temperature on the performance of NiCu/C catalysts. In \nFig. 5a the formation of carbon over time is shown for experiments at temperatures ranging from 450 to 600\u00a0\u00b0C using p\nCH4 =\u20090.34\u2009bar. Increasing the temperature resulted in higher initial growth rates (r\n0). Fig. 5b shows the relation between r\n\n0\n and the temperature in an Arrhenius plot. Surprisingly, the data cannot be fitted with a single straight line, but with two. We clearly distinguish two regimes: temperatures up to 500\u2009\u00b0C in regime 1 (trendline in red) and starting from 550\u2009\u00b0C in regime 2 (trendline in blue). This results in two apparent activation energies: 86\u2009\u00b1\u20098\u2009kJ/mol and 45.4\u2009\u00b1\u20090.4\u2009kJ/mol for regime 1 and regime 2, respectively. Assessment of the Weisz-Prater criterion showed that diffusion limitations are not present in these experiments (see supporting information Section S6 for more details) [39,40]. The activation energy is lower in the high temperature regime than in the lower temperature regime. In literature, values between the two are reported depending on the reaction conditions used. For example, Borghei and co-workers reported an Ea of 50.4\u2009kJ/mol for a NiCu/MgO catalyst (6\u201310% CH4, 550\u2013650\u2009\u00b0C) [32]. Reshetenko et al. reported values in the range of 65\u201377\u2009kJ/mol for NiCu/Al2O3 catalysts with varying Ni:Cu ratios [22]. However, for the first time we report that depending on the reaction conditions, two regimes can be distinguished, with different activation energies, indicating that different steps in the reaction sequence can be rate determining.\nFig. 5c shows the lifetime of the NiCu/C catalysts at the different reaction temperatures. It is clear that increasing the temperature resulted in a strong decrease in catalytic lifetime. The lifetime was determined by fitting Eq. 3 to the growth rates obtained (a detailed explanation can be found in supporting information\nSection 3). In addition to the lifetime, also two other valuable parameters can be obtained from this fit: the deactivation rate and the kinetic deactivation order [32]. As expected from the decreased catalytic lifetime, the deactivation rate increases rapidly with increasing temperature (Fig. S3.4). Using the Arrhenius law the deactivation energy was found to be Ed =\u200970\u2009\u00b1\u20093\u2009kJ/mol. Interestingly we could not distinguish two different regimes here (Fig. S3.4). The deactivation order was found to be 1.5\u20131.8 at 450\u2013550\u2009\u00b0C, yet becomes \u223c1 at 575 and 600\u2009\u00b0C (Fig. S3.4). This indicates a change in the deactivation mechanism at higher temperatures.The final carbon yield obtained for all experiments (after reaction time) is shown in Fig. 5d. An optimal yield was found around T\u2009=\u2009500\u2009\u00b0C. This corresponds to a maximal yield of approximately 18\u2009gC/gNi. For monometallic Ni catalysts an optimum was reported in literature at 500\u2013580\u2009\u00b0C [5,41]. We identify the optimal temperature for our Ni-Cu/C system, and find that it is exactly in between the two temperature regimes.To further explore the characteristics of the two temperature regimes, we looked at the influence of the partial pressure of methane on carbon formation. For this, we varied the partial pressure of methane from 0.2 to 0.4\u2009bar at a fixed temperature and at atmospheric pressure. \nFig. 6a shows the growth curves for the experiments with varying methane concentrations in the feed conducted at 500\u2009\u00b0C (regime 1). An increase of the initial growth rate was observed upon increasing the amount of methane in the feed (Fig. 6b). From the slope (0.91), we derived a close to first order dependence of the reaction rate on the methane partial pressure. This means that the reaction rate scales linearly with the amount of the reactant gas in the feed.The formation of carbon nanostructures from dissociated methane comprises many steps, but starts with the (dissociative) adsorption of methane, followed by recombination of hydrogen atoms yielding H2 and dissolving carbon atoms into the metal nanoparticle. A common explanation for first order reaction kinetics is that the overall reaction rate is limited by the amount of methane available at the surface of the metal catalysts. This is the case if the adsorption of methane is relatively weak (in the Henry regime of the adsorption isotherm) and hence linearly dependent on the methane concentration. In literature, the dissociative adsorption of methane is indeed several times reported to be the rate limiting step of this reaction, rather than the growth of carbon nanostructures [32,42\u201344].Increasing the CH4 partial pressure led to an enhanced carbon yield at this temperature which is illustrated in Fig. 6c. Fig. 6d shows the catalytic lifetime as function of the partial pressure. The lifetime increases when the amount of methane in the feed is increased. As the carbon supply by the metal nanoparticle seems rate limiting, we do not have an a priori expectation of the influence of the amount of methane on the lifetime, which depends on the fraction of carbon deposited as a carbon shell (leading to deactivation) rather than used for the formation of well-defined nanostructures. An explanation for the increased lifetime could come from the hydrogen produced during the reaction. If the reaction rate is higher, also the local H2 concentration increases. This H2 could react with the defective carbon covering the surface of the metal particle, rather than with the more stable carbon nanofibers that were grown. This helps to keep the surface clean and available, thereby avoiding deactivation\u00a0[5].Interestingly, in literature there has not been consensus yet about the influence of the methane concentration on the catalyst lifetime. It is reported that the lifetime is negatively influenced by increasing the amount of methane in the feed [3,45\u201347]. On the contrary, recently Hadian et al. (2022) reported a slight increase in lifetime when increasing the methane partial pressure from 0.40 to 0.90\u2009bar (total pressure 1\u2009bar) [5]. They postulated that the difference between their work and previous work arose from working on a different scale and with a higher concentrations of methane in the feed. However, our reactor scale and methane concentrations are quite similar to the conditions reported in literature. Nevertheless, the reaction temperature reported in most papers is high (650\u2013900\u2009\u00b0C) [3,45\u201347], while we are conducting experiments also at lower temperatures (500\u2009\u00b0C). We hence postulate that the lifetime of the catalysts correlates with the methane concentration depends on which of the two regimes we operate in.To clarify this point, our experiments were repeated at a higher temperature (575\u2009\u00b0C). Based on the results of Section 3.2.2, we now operate in regime 2. \nFig. 7a shows the dependence of the initial growth rate on the partial pressure at 575\u2009\u00b0C. Now, the slope of the plot (and thus the reaction order) has increased to 1.47. This suggests that the dissociative adsorption of methane is not (exclusively) the rate limiting factor. It is more likely that other elementary reaction steps, such as the dissolution and migration of carbon, and/or the formation of the final carbon nanostructures, influence the overall reaction rate. For the carbon nanostructure growth probably more than one carbon atom is involved at a time, as the carbon nanofibers form layer by layer. This is in agreement with a reaction order in methane higher than 1 [48].\nFig. 7b gives the lifetime for the catalysts as a function of the methane partial pressure. In contrast to what had been found for regime 1 (at lower temperatures), at this higher temperature the lifetime is negatively influenced by an increase in partial pressure of methane, in line with the findings in other papers [3,45\u201347]. We have also evaluated the deactivation parameters (deactivation rate and order) for these experiments (Fig. S3.5). It was found that these parameters are not or barely influenced by the partial pressure of methane, but only with the temperature. The deactivation order was found to be 1.4\u20131.8 at 500\u2009\u00b0C and 1.2\u20131.5 at 575\u2009\u00b0C. This stresses the importance of analyzing in which of the two regimes one is operating to understand the influence of reaction conditions on the lifetime of the catalyst.Carbon structures formed during the experiments at 450, 500 and 600\u2009\u00b0C using the 11% Ni- 4% Cu /C catalysts are shown in \nFig. 8. Two different carbon nanofiber structures are formed: solid (full) fishbone fibers and fibers with a hollow core, referred to as hollow fibers. Relevant details to identify the two structures are highlighted with arrows in the TEM micrographs. For example, in Fig. 8a,b (operating at 450\u2009\u00b0C) the formation of full fishbone fibers is observed. Occasionally, thin hollow fibers are also found (Fig. 8b), but at 450\u2009\u00b0C, 90% of the product consisted of full fibers and only about 10% were hollow fibers. At 500\u2009\u00b0C, different structures are formed as visualized in Fig. 8c,d. The formation of hollow fibers is now more pronounced. During this experiment, 68% of the products were full fibers and the rest were hollow fibers (Fig. 8f). During reaction at 600\u2009\u00b0C, only hollow fibers had been formed which can be seen in Fig. 8e. The average fiber diameter was determined from the TEM images, the histograms are shown in Fig. S7.1. Typically, the fiber diameter is very close to the diameter of the particles they have grown from. At 450\u2009\u00b0C the average fiber diameter is 15\u2009\u00b1\u20095\u2009nm (n\u2009=\u2009121) and at 600\u2009\u00b0C it is 17\u2009\u00b1\u20095\u2009nm (n\u2009=\u200991). Also, X-ray diffraction was used to characterize the used catalysts and products (Fig. S7.2). Peaks of metallic phases NiCu were observed in the diffractograms, indicating that the crystalline part of the catalyst nanoparticles is reduced, even after cooling down and exposure to air. The crystallite sizes range from 8.6\u2009nm (450\u2009\u00b0C) to 13.3\u2009nm (600\u2009\u00b0C). Unfortunately, the sample could not be uniformly distributed over the sample holder, due to its limited amount (\u223c5\u2009mg), and hence additional peak broadening is caused by the uneven distribution, influencing the apparent crystallite size. Next to the peaks for the turbostratic support, additional peaks indicating an interlayer spacing of 0.334\u2009nm were observed, typical for graphite. More details can be found in the supporting information (page 22\u201323). Fig. S7.3 shows the analysis at two different partial pressures. Here, no difference in average diameter was observed with different partial pressures.\nFig. 8f gives a semi-quantitative overview of the carbon structures formed at the different temperatures. From this it is clear that the dominant carbon structure changes from mainly full fibers in regime 1 to mainly hollow fibers in regime 2. To the best of our knowledge, this is the first time that the dependence of the structure of the carbon formed is related to the growth conditions.The balance between the rates for methane adsorption and dissociation (carbon supply), carbon dissolution and transport and nanostructure formation is of key importance [49]. The rate-limiting factors during the carbon formation from methane depend on the nature of the catalyst and the reaction conditions and, as we have shown in this paper, two different regimes can be distinguished. A schematic overview of the different steps in the process is visualized in \nFig. 9, based on the findings in this paper and literature.We consider three main steps: carbon supply (step 1), carbon transport (step 2), and carbon nanostructure formation (step 3). Step 1 comprises the adsorption and dissociation of methane on the catalyst surface, and the release of hydrogen gas and incorporation of carbon in the metal nanoparticle. Step 2 involves the transport of carbon atoms through the metal nanoparticle, via surface or bulk diffusion. Thereafter, carbon atoms can recombine to form solid carbon in different structures depending on, among others, the type of catalyst and the reaction conditions (step 3) [1,4,6,11,12,21,37].First, we consider the first step: methane adsorption and dissociation, yielding hydrogen and carbon atoms that are adsorbed or dissolved in the metal particle. Methane decomposition is an endothermic reaction, therefore, performing the reaction at lower temperatures will give rise to lower equilibrium conversions (see Fig. S2.1). Also kinetically, the carbon supply is slower at lower temperatures. In the low temperature regime (regime 1), a reaction order close to 1 in methane is found, with an apparent activation energy of 86\u2009kJ/mol. This is in line with step 1, the carbon supply, being the rate determining factor, with a low coverage of the metal nanoparticles surface with methane (Henry regime). Physisorption is not an activated process, but the activation energy represents the dissociation barrier of the methane molecule. In literature, values for the activation energy of methane dissociative adsorption of nickel in the range of 71\u2013127\u2009kJ/mol have been reported [50\u201353], fully in line with our experimental values in regime 1.In this low-temperature regime, a higher methane concentration increases the overall carbon yield. It also slightly increases the lifetime of the catalysts, which is determined by the balance in step 3 between desired processes (3a and 3b, forming well-defined crystalline carbon nanostructures) and step 3c (forming less ordered/amorphous carbon which covers the metal nanoparticle surface and hence causes catalyst deactivation). We postulate that the slight improvement of the lifetime of the catalyst with methane concentration is caused by the fact that a higher reaction rate also causes a somewhat higher local hydrogen concentration, which shifts the balance slightly from the undesired formation of less stable and less crystalline carbon (3c) to the formation of desired nanostructures. This also means that the deliberate addition of hydrogen to the reaction mixture probably could much further improve the carbon yield [5].Now we consider the high temperature regime 2. It is clear that the rate limiting step in this regime is different from that in regime 1, as evidenced by the different activation energy (Fig. 5b) as well as a different reaction order (Fig. 7a). If step 2 (diffusion) would be rate limiting under any conditions, this would be expected to lead to a reaction order close to 0.5 in methane concentration. This is never observed. In the high temperature regime the reaction order in methane is about 1.5, even higher than the reaction order of 1 at the lower temperatures. It is likely that at these temperatures step 3 of the process, the formation of solid carbon, becomes (partially) rate limiting.The driving force for the formation of fibers from dissolved or adsorbed carbon atoms is its higher stability. The growth of the fibers is known to occur in stages, layer by layer, which involves several carbon atoms at a time and hence would be in line with a reaction order in methane higher than 1.0. Although no direct correlation can be made, because we do not know the carbon atom concentration in the metal nanoparticle as a function of the experimental parameters.It is expected that the formation of well-defined, crystalline, carbon nanofibers (3a and 3b) is thermodynamically more favorable, but kinetically less favorable, than the formation of low-crystallinity or disordered solid carbon. The combination of this kinetic enhancement and solid carbon formation as rate determining step, induces a shift from well-controlled carbon formation to the formation of disordered or amorphous shells. This is reflected in the observed decrease in catalytic lifetime with increasing reaction temperature as the formation of carbon shells around the catalyst particle cause deactivation\u00a0[1,5,54].With increasing temperature (going from regime 1 to regime 2), also the structure of the carbon nanofibers changes from solid to hollow (Fig. 8). Different explanations are possible. A first one is that, as at these high temperatures the carbon structure formation is rate limiting, hollow nanofibers are kinetically easier to form than full solid carbon nanofibers [49]. Another possible explanation might be related to the mode of diffusion of the carbon in the metal nanoparticle. At higher temperatures the solubility of carbon in the nickel nanoparticle, which is already lowered by the addition of Cu, is further lowered. Possibly carbon surface diffusion becomes more dominant.Overall, it is clear that it is important to distinguish different operating regimes, where the transition between the regimes is dependent on the catalyst and process conditions. The sweet spot for the maximum carbon yield seems to be exactly between the two. At this transition, neither the methane decomposition nor the carbon nanostructure formation are particularly rate limiting, but the rates of these two processes are balanced.In this study, we evaluated how the operating conditions influence the carbon yield during the decomposition of methane over carbon supported Ni-based catalysts. Having Cu present next to Ni in the catalyst greatly enhanced the activity, catalyst lifetime, and, hence, carbon yield. Varying the reaction temperature showed two carbon growth regimes for the NiCu/C catalysts. Different activation energies and reaction orders were obtained in the two regimes. In addition, we found that the dominant carbon nanostructure formed changes from full, in lower temperature regime, to hollow fibers in the higher temperature regime. From this, we postulate that the rate determining step changes from methane dissociation to carbon transport and nanostructure formation. It also allows us to define the conditions at which the maximal yield is obtained, at the interface between the two regimes.\nS.E. Schoemaker: Conceptualization, Investigation, Methodology, Validation, Visualization, Writing \u2013 original draft. Tom A.J. Welling: Software, Visualization, Writing \u2013 review & editing. Dennie F.L. Wezendonk: Resources. Bennie H. Reesink: Writing \u2013 review & editing. Alexander P. van Bavel: Writing \u2013 review & editing. Petra E. de Jongh: Conceptualization, Writing \u2013 review & editing, Supervision, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is part of the Advanced Research Center for Chemical Building Blocks, ARC CBBC, which is co-founded and co-financed by the Dutch Research Council (NWO) and the Netherlands Ministry of Economic Affairs and Climate Policy (Project number: 2018.017.C). The authors acknowledge Joy Bodde for her work on the synthesis and preliminary characterization of the catalysts used in this paper.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cattod.2023.114110.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  An alternative route towards COx-free production of hydrogen is the thermal catalytic decomposition of methane. In addition to hydrogen, valuable carbon nanomaterials can be formed. Carbon nanofiber formation from methane decomposition over carbon supported NiCu catalysts was studied in-situ using a Thermogravimetric analyzer. We especially investigated how the carbon yield is influenced by reaction parameters. Based on experiments with varying temperature (450\u2013600\u00a0\u00b0C), two distinct temperature regimes were identified. Different kinetic parameters were derived for the two regimes. Activation energies of 86 and 45\u00a0kJ/mol, and reaction orders in methane of close to 1 and 1.5, were found in the low and high temperature regimes, respectively. We postulate that at lower temperature the methane dissociation is rate limiting, while at higher temperature the carbon formation plays a more critical role. At low temperatures mostly full fishbone fibers are formed, whereas at higher temperatures mainly hollow fibers are formed. The maximum carbon yield is obtained at the transition between the two regimes, when the carbon supply and carbon nanostructure formation are balanced.\n               "}
{"full_text": "Nitrophenols (NPs) are widely presented as precursors, intermediates, or by-products in various industries, such as pharmaceutical, agrochemical, fine-chemical, dye, etc. [1, 2]. Due to the electron withdrawing effect of nitro group (-NO2) and benzene ring, as well as the conjugate effect, NPs are always stable molecules, which makes them hardly to be biodegraded through traditional wastewater treatment process. Fortunately, researchers have found that if the -NO2 group of NP is reduced to amino group (-NH2), the resulted aminophenol (AP) shows improved biodegradability and reduced toxicity [2].Generally, NP conversion to AP could be achieved by constructing NaBH4-based advanced reduction system [3\u20135]. Metal nanocatalysts exhibited the characteristics of large specific surface area and high surface activity, which thus attract much attention. Since the early report using metal nanoparticles (Au, Ag, Cu, Zn, etc.) for 4-NP reduction applying NaBH4 as reductant [6\u201310], a number of nanocatalysts have been developed for 4-NP reduction, including noble metal, transition metal and metal oxide/phosphide, etc. [11\u201315]. Considering their earth abundance and relatively low cost, transition metal, especially Ni-based catalysts have drawn much attention. Metallic Ni, nickel hydroxide, nickel phosphide in sole or loaded on support are reported powerful catalysts towards NP reduction, for example 4-NP to 4-AP [16\u201325]. The presence of support can not only provide abundant adsorption and reaction sites for reactants, but also may facilitate charge transfer, especially in the case of advantageous carbon material as catalyst support. For example, as catalytic center support, reduced graphene oxide, graphene, polymeric (polycaprolactone(PCL)/chitosan) nanofiber, N-doped carbon, metal organic framework-derived porous carbon, show promotional effects for 4-NP reduction [17, 19, 24, 26, 27].Attempts of NixPy for 4-NP reduction reaction prove its capability for 4-NP conversion to 4-AP [21\u201324, 28]. However, controllable synthesis of NixPy using sustainable and environmental-benign phosphorus precursor is still a challenging work. The commonly adopted synthesis strategies, such as organic precursor decomposition, solid phase conversion, hydro/solvothermal methods, require costly or high-hazardous risk phosphorus precursors (trioctylphosphine, (NH4)2HPO4, NaH2PO2, red phosphorus, white phosphorus etc.) [21, 22, 24, 28, 29]. Recently, we have demonstrated that phosphorus-containing biomass, such as yeast cell, rice bran, can serve as sustainable, cost-effective, and environmental-benign precursor for transition metal phosphide (Co2P, FexP, Fe2P) fabrication through an anoxic pyrolysis process [25, 30\u201332]. Meanwhile, carbon and nitrogen elements in biomass can be converted to advantageous carbon material as support for metal phosphide, bringing additional benefits for catalytic reactions.Besides of phosphorus-containing biomass, typical biomolecule of nucleic acid extracted from yeast cell is also demonstrated to be an appropriate precursor for CoP/N-doped carbon composite formation [33]. Interestingly, owing to the complexing ability of nucleic acid with Co2+, the molecular level mixing of phosphorus component with Co2+ leads to a dramatically different morphology of CoP/N-doped carbon and related high efficiency for organic pollutant removal through advanced oxidation process [33]. Adenosine triphosphate (ATP) is also one important biomolecule existing in cell, which contains abundant phosphorus, carbon and nitrogen element. Similar to nucleic acid, ATP also possesses the potential to form a molecular level mixing with metal ions [34], which may bring unexpected material property after pyrolysis treatment. In this study, for the first time, we adopted ATP as phosphorus precursor for nickel phosphide/carbon composite fabrication. By simply mixing ATP with Ni2+ in aqueous solution and lyophilization, single precursor of ATP-Ni complex was obtained for the following sample synthesis. An anoxic pyrolysis process ensured the formation of nickel phosphide/carbon composite, which was proved by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterizations. The catalytic activity of as-obtained sample for 4-NP conversion to 4-AP was evaluated with NaBH4 as reductant. The present work brings new insight into metal phosphide fabrication and develops an efficient catalyst for advanced reduction process.Adenosine 5\u2032-triphosphate disodium salt (ATP-2Na, >98%) was purchased from Aladdin Co., Ltd. 4-nitrophenol (4-NP, 99%) was acquired from Adamas-beta. NiCl2\u22196H2O (98%) and other commonly used chemicals and reagents were of analytical grade and provided by Chengdu Kelong Reagent Company. All regents were used without further purification. De-ionized water was used for all experiments.A single precursor of ATP-Ni complex was firstly prepared, which was subsequently transformed to nickel phosphide/biocarbon composite by anoxic pyrolysis. Detailly, 2\u00a0g of ATP was dissolved in 40\u00a0mL de-ionized water. After adding 0.405\u00a0g of NiCl2\u22196H2O and stirring for 30\u00a0min, the clear solution was lyophilized in a freeze-drier to obtain the single precursor of ATP-Ni. Then, 1\u00a0g of ATP-Ni powder was placed in a porcelain crucible and treated by an anoxic pyrolysis process (900\u00a0\u00b0C, 2\u00a0h) under continuous Ar flow (30\u00a0mL min\u22121) in a tube furnace. The obtained black powder was thoroughly washed by de-ionized water and dried in a 60\u00a0\u00b0C oven. The finally obtained sample was denoted as Ni2P/BC. Control sample without Ni incorporation was also prepared following the same procedure and denoted as BC.X-ray diffraction (XRD) pattern of the as-synthesized sample was obtained on a Rigaku D/max-TTR III X-ray diffractometer with a Cu K\u03b1 (\u03bb\u00a0=\u00a01.54056\u00a0\u00c5). X-ray photoelectron spectra (XPS) were acquired on an X-ray photoelectron spectroscopy (Thermo Fischer, ESCALAB Xi+). The morphology of sample was observed on a field emission scanning electron microscope (FESEM, JSM-7610F, JEOL\u00a0Ltd.,\u00a0Japan) and a transmission electron microscope (TEM, JEOL JEM-2100 F). N2 adsorption/desorption measurement was conducted on a Micromeritics ASAP 2020 equipment. Sample was degassed at 150 \u00b0C for 8\u00a0h before measurement. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method. The total pore volume data was obtained at P/P0=0.986.The 4-NP reduction reaction was conducted in a 3\u00a0mL quartz cuvette adopting the as-synthesized Ni2P/BC sample as catalyst and NaBH4 as reductant. A 4-NP stock solution of 1\u00a0g L\n\u22121 was prepared. Catalyst of Ni2P/BC was dispersed in de-ionized water by ultrasonication to form a stock suspension of 1\u00a0g L\n\u22121. The NaBH4 solution of 10\u00a0g L\n\u22121 was freshly prepared before each catalytic reaction. In a typical reaction run, 50\u00a0\u03bcL of 4-NP stock solution, 1\u00a0mL of de-ionized water, 1\u00a0mL of freshly prepared NaBH4 solution were added into a 3\u00a0mL quartz cuvette in sequence. After adding 75\u00a0\u03bcL of catalyst suspension, the cuvette was put into a UV\u2013visible spectrophotometer (UV-1800\u00a0PC, MAPADA Instruments) immediately. At pre-determined time interval, a full wavelength spectrum scan (250\u2013500\u00a0nm) was conducted to monitor the concentration change of 4-NP. Correspondingly, the initial 4-NP concentration, catalyst dosage and NaBH4 dosage are 23.53\u00a0mg L\n\u22121, 35.29\u00a0mg L\n\u22121 and of 4.706\u00a0g L\n\u22121, respectively. The weight ratio and molar ratio of NaBH4:4-NP was 200 and 735, respectively.Control catalytic experiment without catalyst was conducted following the same procedure as described above.In order to test the reusability of catalyst, cycle runs were conducted by supplementing 4-NP stock solution (50\u00a0\u03bcL) and NaBH4 (50\u00a0mg) into the cuvette after every 8\u00a0min catalytic reaction.Phosphorus-containing biomass (yeast cell, nucleic acid, rice bran, etc.) has been proved to be effective and environmental-benign precursor for transition metal phosphide synthesis [25, 30\u201333, 35]. During anoxic pyrolysis treatment, high-valance phosphorus of phosphate (P5+) in biomass could be reduced to low-valance phosphorus in phosphide (P\ny\n\n\u2212) with the assistance of in-situ generated reducing gasses (CO, H2, etc.) [30, 33]. ATP is one typical biomolecule containing three phosphate groups in one molecule, which also possesses abundant carbon, oxygen and hydrogen elements. Meanwhile, the phosphate group in ATP could complex with metal ions (M\nx\n\n+) [34], leading to a single precursor of ATP-M. The molecular mixing of different precursors is believed to be beneficial for the formation of uniform sample [33, 36]. Therefore, it is considered that ATP may be one appropriate precursor for transition metal phosphide (TMP) synthesis.By simply mixing ATP and Ni2+ in aqueous solution and lyophilization, the single precursor of ATP-Ni was obtained. After pyrolyzing ATP-Ni complex at 900\u00a0\u00b0C for 2\u00a0h under Ar flow, nickel phosphide crystal with Ni2P nature was obtained as evidenced by the XRD pattern shown in Fig.\u00a01\n. Based on the Scherrer equation, the fitting results for several peaks ((111), (201), (210), (300), (211) plane) were employed for estimation of the particle size, which was calculated to be 46.3\u00a0\u00b1\u00a02.6\u00a0nm. XPS characterization results shown in Fig.\u00a02\n give another evidence of nickel phosphide formation. Besides of the typical oxidized Ni species (856.6\u00a0eV, 874.6\u00a0eV) and satellite (862.2\u00a0eV, 880.6\u00a0eV) peaks, another XPS Ni 2p peak with low binding energy of 853.4\u00a0eV is found (Fig.\u00a02a), which can be assigned to Ni\u03b4+ (0\u00a0<\u00a0\u03b4\u00a0<\u00a02) in Ni-P bonding [21, 37]. The deconvolution of XPS P 2p spectrum gives three peaks (Fig.\u00a02b), which can be assigned to P-O (133.7\u00a0eV), P-C (132.0\u00a0eV) and P-Ni (129.9\u00a0eV) bonding, respectively [21, 37]. Therefore, Ni2P is successfully synthesized by the designed experimental procedure. Considering the appearance of P-C bond in Ni2P/BC sample (Fig.\u00a02b), it is suggested that P doping into biocarbon framework exists [37]. It is also worth to mention that N doping into biocarbon is achieved as evidenced by the formation of CN bond and N-related species (Fig.\u00a02c and 2d) [38]. The abundant P and N elements in ATP molecule (16.86\u00a0wt% P, 12.71\u00a0wt% N) enables the possibility of P and N co-doping into biocarbon. Foreign element doping into carbon framework may alter the original carbon structure and provide abundant free flow of electrons, thus beneficial for catalytic reactions [24, 37, 38].\nFig.\u00a03\n shows the morphology of as-obtained Ni2P/BC sample. Thin layer sheet-like structure with wrinkles is found for biocarbon (Fig.\u00a03a). The formation of such carbon structure is believed to be related to the in-situ generated abundant gasses during pyrolysis process, which are also reported when applying biomass, guanine, hexamine as precursors for advanced carbon materials formation [32, 39, 40]. Ni2P with large particle size (hundreds of nanometers) is found on the surface of biocarbon (Fig.\u00a03b). Besides, TEM of as-synthesized sample clearly shows that Ni2P with an average particle size of 2.4\u00a0nm are inlaid on layer sheets (Fig.\u00a03c). Both large particles of hundreds of nanometers and small particles of \u223c2.4\u00a0nm exist for Ni2P. The measured lattice distance of 0.227\u00a0nm for Ni2P particle (Fig.\u00a03d) agrees well with the d spacing of Ni2P (111) plane (0.221\u00a0nm), which further implies the successful formation of Ni2P. Normally, thin layer sheet-like structure will bring high specific surface area, which is an advantageous feature for heterogeneous catalyst. Indeed, N2 adsorption-desorption measurement for Ni2P/BC sample gives a type IV isotherm with H3 hysteresis loop (Fig.\u00a04\na), implying the formation of assembled layer structure. The calculated specific surface area (S\nBET) and pore volume (V\ntotal) are as high as 261.06\u00a0m2\ng\n\u22121 and 0.3457\u00a0cm3\ng\n\u22121, respectively. The average pore diameter is measured to be 5.30\u00a0nm.Overall, we adopted the typical biomolecule of ATP as an unusual P precursor. Through the formation of ATP-Ni complex and followed anoxic pyrolysis, Ni2P nanoparticles were formed and loaded on sheet-like P/N-doped biocarbon. The advantageous features of Ni2P/BC sample, such as good crystallinity of Ni2P nanoparticle, P and N doping into biocarbon, high specific surface area, may bring unexpected catalytic performance.Supported or unsupported noble metals (Au, Pt, Pd, Ag, etc.) and transition metals (eg., nickel) are typically active catalysts for 4-NP reduction to 4-AP in the presence of NaBH4 [2, 11, 41, 42]. Since TMPs always exhibit metallic properties, it is anticipated that the developed Ni2P/BC may bring high activity towards 4-NP reduction.The capability of Ni2P/BC for 4-NP reduction to 4-AP in the presence of NaBH4 was firstly monitored by a UV-visible spectrophotometer. As illustrated in Fig.\u00a05\n, 4-NP solution presents with light yellow color, corresponding to a maximum absorbance peak at 318\u00a0nm. After adding NaBH4 into the solution, a bright yellow color is observed with the maximum absorbance peak shifting to 400\u00a0nm, indicating the generation of 4-nitrophenolate [21, 41]. Once Ni2P/BC is added into the solution, a distinctly color change from bright yellow to pale yellow and finally to colorless is observed, which is the typical characteristic of 4-NP reduction reaction [26, 41]. Correspondingly, the maximum absorbance peak shifts from 400\u00a0nm to 300\u00a0nm, which belongs to 4-AP. Control experiment was conducted to exclude the contribution of 4-NP adsorption by solid catalyst. As shown in Fig.\u00a06\na, in the absence of NaBH4, the absorbance peak belonging to 4-nitrophenolate (\u03bb\u00a0=\u00a0400\u00a0nm) was slightly increased. The possible reason is that the surface functional groups on biocarbon support and the possible existence of Na+ in Ni2P/BC from ATP precursor may induce the hydrolyzation or ion exchange of 4-NP, leading to the formation of 4-nitrophenolate (\u03bbmax=\u00a0400\u00a0nm). However, both the reaction and adsorption of 4-NP by Ni2P/BC are negligible. Around 9.6% of 4-NP is removed. Similarly, in the absence of Ni2P/BC, sole NaBH4 is also not efficient for 4-NP reduction with only 1.7% of 4-NP conversion after 30\u00a0min (Fig.\u00a06b). Therefore, both catalyst of Ni2P/BC and reductant of NaBH4 are essential for 4-NP reduction.We further monitor the concentration change of 4-NP versus time by UV-visible spectra as presented in Fig.\u00a07\na. Along with the reaction proceeding, the maximum absorbance peak belonging to 4-nitrophenolate (\u03bb\u00a0=\u00a0400\u00a0nm) gradually decreases and finally vanishes after 180\u00a0s, indicating the fully reduction of 4-NP. Meanwhile, absorbance peak belonging to 4-AP (\u03bb\u00a0=\u00a0300\u00a0nm) gradually appears and increases. Since the concentration of NaBH4 is much higher than that of catalyst dosage, it is reasonable to use pseudo-first-order reaction model to investigate the reaction kinetic. Relevant kinetic equation can be written as ln (C\nt/C\n0)\u00a0=\u00a0\u2212kt, where C\nt and C\n0 are the concentration of 4-nitrophenolate ion at time t and 0, respectively, and k represents the reaction rate constant. A good linear fitting is found between ln (C\nt/C\n0) and t as displayed in Fig.\u00a07b, which gives a k value of 0.019\u00a0s\n\u22121. Such value is comparable with other transition metal (Ni)-based catalysts for 4-NP reduction in the presence of NaBH4 (Table\u00a01\n). If comparing the normalized rate constant by introducing catalyst dosage, the developed Ni2P/BC catalyst in this study possesses a k\nnormal value of 253\u00a0s\n\u22121\ng\n\u22121, which outperforms most of reported Ni-based catalysts (Table\u00a01), and even better than a few noble metal-based catalysts [11, 43].As a composite material, both Ni2P nanoparticles and thin layer sheet-like carbon structure exist in Ni2P/BC catalyst. Carbon materials are also reported active for 4-NP reduction [44\u201346]. Therefore, it is necessary to understand the contribution of biocarbon for 4-NP reduction. As shown in Fig.\u00a07c, biocarbon sample of BC obtained from ATP pyrolysis does show catalytic activity for 4-NP reduction to 4-AP. However, it requires up to 420\u00a0s to achieve satisfactory conversion efficiency, which is much longer than that of Ni2P/BC catalyst (180\u00a0s). The calculated pseudo-first-order reaction rate constant of 0.0154\u00a0s\n\u22121 is also lower than that of Ni2P/BC catalyst (k\u00a0=\u00a00.019\u00a0s\n\u22121). Therefore, biocarbon structure not only supports Ni2P nanoparticles, but also participates in 4-NP reduction reaction. And the presence of Ni2P nanoparticles in the composite material obviously accelerates reaction.The reusability of Ni2P/BC was tested in consecutive reaction runs as illustrated in Fig.\u00a08\n. After each 8\u00a0min-reaction run, fresh 4-NP and NaBH4 were supplemented into the reaction solution for the next run. Therefore, along with the vanishing of 4-nitrophenolate peak after each run, 4-AP is accumulated in reaction cell with continuously increased absorption peak intensity (\u03bb\u00a0=\u00a0300\u00a0nm, Fig.\u00a08a). It is noticed that even after 10 consecutive runs, the conversion efficiency of 4-NP to 4-AP still remains as high as 95.8% (Fig.\u00a08b). Therefore, the good reusability of Ni2P/BC for 4-NP reduction reaction is verified.As illustrated in Fig.\u00a09\n, 4-NP reduction reaction over Ni2P/BC in the presence of NaBH4 can be described by Langmuir-Hinshelwood (L-H) heterogeneous catalytic reaction model [2, 41, 49]. Firstly, BH4\n\u2212 is adsorbed on the surface of catalyst and hydrolyzed to BO2\n\u2212 and active hydrogen species. Meanwhile, 4-nitrophenolate ions resulted from 4-NP are also adsorbed on the surface of catalyst. The contact of active hydrogen species and 4-nitrophenolate ions results in the reduction of nitro group to amino group, leading to the formation of 4-AP finally.Morphology and structure analysis indicates that the developed Ni2P/BC composite material possesses the typical features of dispersed Ni2P nanoparticles with good crystallinity, thin layer sheet-like biocarbon support and high specific surface area, N and P-doping into carbon framework, etc. Such features are beneficial for 4-NP reduction reaction. Firstly, the point of zero charge value (pHpzc) for Ni2P/BC was measured to be 7.93. The pH of 4-NP solution was determined to be 6.85. Therefore, the surface of Ni2P/BC is positively charged under investigated reaction condition, indicating that the adsorption of 4-nitrophenolate anions and BH4\n\u2212 ions on Ni2P/BC surface was favorable. Meanwhile, Ni2P could efficiently adsorb active hydrogen species owing to the electronegativity of Py\u2212 in Ni2P [37]. Secondly, N and P doping into biocarbon could change the electronic structure of carbon framework and bring strong positive polarization charges, which is favorable to attract negatively charged 4-nitrophenolate anions [24, 37, 38]. Thirdly, the high specific surface area could provide abundant adsorption sites as well as reaction sites, which is favorable for the contact between reactants. Fourthly, metal phosphide of Ni2P could exhibit metal-like property, which ensures the good electron conductivity. Meanwhile, thin layer biocarbon obtained from high temperature treatment also possesses good electron conductivity. Therefore, fast electron transfer is suggested for Ni2P/BC material, which is critical for the fast and efficient reduction of 4-NP. Overall, the high affinity of Ni2P/BC towards reactants, as well as the good electron conductivity and transfer efficiency enables its superior activity for 4-NP reduction reaction.In summary, Ni2P/biocarbon composite was successfully assembled by adopting ATP as an unusual P precursor through an anoxic pyrolysis process. Benefiting from the molecular mixing of Ni2+ and P precursor, as well as abundant N and P contents in ATP, the as-obtained biocarbon showed thin layer sheet-like structure with wrinkles and both hundreds of nanometers and \u223c2.4\u00a0nm small particles of Ni2P were dispersed on the surface of biocarbon. When applied for advanced reduction process, the developed Ni2P/BC sample exhibited the features of superior catalyst for 4-NP reduction to 4-AP, such as fast reaction kinetic, good reusability, etc. The superior catalytic activity of Ni2P/BC was suggested to be related to its high affinity for reaction substrates, high specific surface area of thin layer biocarbon and Ni2P nanoparticles with good crystallinity. This study not only enriches the synthesis method for transition metal phosphide, but also provides a highly efficient catalyst for advanced reduction process.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge funding support from the National Natural Science Foundation of China (grant number 21808147), Sichuan University \u201cChemical Star\u201d Excellent Young Talents Training Program (2021) and Institute of Engineering Technology of Petro China Southwest Oil & Gas Field Company. We would like to thank Yanping Huang from Center of Engineering Experimental Teaching, School of Chemical Engineering, Sichuan University for the instrument support (Field Emission Scanning Electron Microscope, JEOL JSM 7610F).", "descript": "\n                  Conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) through advanced reduction process is important to lower the hazardous risk of 4-NP, which requires the development of highly efficient catalyst. Herein, we proposed a novel strategy to fabricate nickel phosphide/carbon composite for 4-NP reduction in the presence of NaBH4. An unusual precursor of adenosine 5\u2032-triphosphate (ATP) was applied to provide the essential phosphorus and carbon elements for nickel phosphide and biocarbon, respectively. ATP-Ni complex was firstly prepared and then transformed to nickel phosphide/biocarbon composite by an anoxic pyrolysis treatment. Material characterizations confirmed both large particles of hundreds of nanometers and small particles of \u223c2.4\u00a0nm existing for Ni2P, which were dispersed on N/P-doped biocarbon with thin layer sheet-like structure. When applied for 4-NP reduction, Ni2P/BC exhibited fast reaction kinetic with a pseudo-first-order reaction rate constant of 0.019\u00a0s\n                     \u22121 (normalized rate constant 253\u00a0s\n                     \u22121\u00a0g\n                     \u22121) and good reusability in 10 consecutive reaction runs (95.8% conversion efficiency). It is suggested that such superior activity may originate from the high affinity of Ni2P/BC towards reactants and the good electron transfer ability of Ni2P/BC. This study not only enriches the synthesis strategy of transition metal phosphide, but also provides a superior catalyst for advanced reduction process.\n               "}
{"full_text": "The oxygen evolution reaction (OER) is a crucial half reaction to realize efficient hydrogen production via water electrolysis [1\u201320]. To date, tremendous efforts have been devoted to designing advanced OER catalysts in low price and high earth-abundance, and current studies indicate that a pre-oxidation process with surface reconstruction in phase, composition and morphology is essential for such transition metal-based catalysts, or so-called \u201cpre-catalysts\u201d [21\u201324]. Therefore, designing pre-catalysts with high reaction tendency and rich reactive surface sites is of great promise in enhancing the OER behavior.During the past few years, the Prussian blue analogues (PBAs) have attracted substantial research interests as efficient OER pre-catalysts owing to their multiple advantages including tunable chemical compositions in cationic and anionic sites, various nanostructures and high reaction tendency towards the pre-oxidation [25\u201328]. For instance, our group proposed a binary CuFe PBA pre-catalyst for highly active and ultrastable OER catalysis, which undergoes an obvious activation owing to the efficient pre-oxidation process with in-situ formed catalytically active species [29]. Zou et\u00a0al. demonstrated that the introduction of Zn ions in CoFe PBA could simultaneously enrich the catalytically active Co3+ ions and increase the surface area, thereby leading to enhanced OER performance than the binary counterparts [30]. Tour group proposed that the cationic modulation in CoNi\u2013Fe PBAs could effectively regulate the OER behavior owing to multi-metal synergy, and indicated that the catalytically active Ni and Co sites could lead to lower onset potential and easier activation kinetics, respectively [31]. Recently, our group developed a high-entropy amorphous oxycyanide pre-catalyst derived from the thermal oxidation of CoFeNiCuMn PBA, which undergoes a facile pre-oxidation process and achieves higher OER performance than the counterparts with fewer metallic elements [32]. Therefore, realizing rational elemental modulation in PBAs would be of great significance in promoting the pre-oxidation process and OER behavior.To date, the elemental modulation strategies are basically focused on the cationic sites, while systematic elemental modulation in both cationic and anionic sites of the PBA structure and the related elucidation of structure-performance relationship still remain blank [33,34]. Conducting synergistic dual elemental modulation in cationic and anionic sites of the PBA pre-catalysts is expected to be effective on further optimizing the OER behavior, and figuring out the role of components during the pre-oxidation process and OER catalysis is of high significance for designing advanced PBA-based catalysts for related applications. Aiming on the above considerations, herein we proposed a controllable dual elemental modulation in both cationic and anionic sites of the multi-metal PBA pre-catalysts, realizing highly active and ultrastable OER performance. Detailed investigations indicate that the Co ions in cationic sites is essential for the high intrinsic activity, and the multi-metal synergy could further enhance the intrinsic activity of the PBA pre-catalyst. In addition, mixed FeIIICoIII cyanide anions could benefit the OER process owing to the enriched Co3+ active sites and intermetallic synergy. As a result, the optimal NiCuCoII\u2013FeIIICoIII PBA (denoted as NiCuCoII\u2013FeIIICoIII) pre-catalyst with high intrinsic activity, enriched local Co3+ sites and optimal multi-metal synergy displays a low \u03b710 of 288\u00a0\u200bmV, and a remarkable 1.4\u201361.2 times enhancement in OER activity can be identified compared with the counterparts with variable cations. Not only that, benefitted from the facile pre-oxidation process of NiCuCoII\u2013FeIIICoIII, abundant high-valence Ni, Cu and Fe active species can be in-situ formed and accumulated, resulting in substantially enhanced OER activity with ultrastable durability. The activated NiCuCoII\u2013FeIIICoIII shows an ultralow \u03b710 of 251\u00a0\u200bmV and a 1.81 times enhancement in OER activity, outperforming most PBA-based/-derived catalysts and making the PBA pre-catalyst with dual elemental modulation a promising candidate for water electrolysis.The chemicals were purchased from SinoPharm Chemical Reagent Co., Ltd..Taking the synthesis of NiCuCo\u2161-Fe\u2162Co\u2162 as an example, 2\u00a0\u200bmmol NiCl2\u00b76H2O, 2\u00a0\u200bmmol CuCl2\u00b72H2O, 2\u00a0\u200bmmol CoCl2\u00b76H2O and 9\u00a0\u200bmmol of trisodium citrate dehydrate were dissolved in 20\u00a0\u200bmL of distilled water by vigorous stirring. Meanwhile, 2\u00a0\u200bmmol K3 [Co(CN)6] and 2\u00a0\u200bmmol K3 [Fe(CN)6] were dissolved in 20\u00a0\u200bmL of distilled water under stirring. Then, the above solution was mixed by stirring at 600\u00a0\u200brpm for 20\u00a0\u200bmin. The powdery products were collected by centrifugation, rinsed with water and ethanol, and dried under vacuum. The other PBAs were fabricated by the same operation, and the elemental modulation can be realized by simply adjusting the kinds and amounts of the salt precursors. The metal salts used in the synthesis involve metal cations (Ni2+, Cu2+, Co2+ and Fe2+) and metal anions ([Co(CN)6]3- and [Fe(CN)6]3-). The specific amounts of the salt precursors are shown in Table\u00a01\n.All the electrochemical measurements were performed in a three-electrode system linked with an electrochemical workstation (Ivium Vertex. C. EIS). All potentials were calibrated to a reversible hydrogen electrode (RHE) according to the Nernst equation and the data were presented without iR correction. Typically, 4\u00a0\u200bmg catalyst and 50\u00a0\u200b\u03bcL Nafion solution (Sigma Aldrich, 5\u00a0\u200bwt%) were dispersed in 1\u00a0\u200bmL water-isopropanol mixed solution (volume ratio of 3:1) by sonicating for at least 30\u00a0\u200bmin to form a homogeneous ink. Then 5\u00a0\u200b\u03bcL of the dispersion (containing 20\u00a0\u200b\u03bcg catalyst) was loaded onto a glassy carbon electrode with 3\u00a0\u200bmm diameter, resulting in a catalyst loading of 0.285\u00a0\u200bmg\u00a0\u200bcm\u22122. The as-prepared catalyst film was allowed to be dried at room temperature. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) with a scan rate of 2\u00a0\u200bmV\u00a0\u200bs\u22121 were conducted in O2-purged 1\u00a0\u200bM KOH solution. A Hg/HgO electrode was used as the reference electrode, a platinum gauze electrode (2\u00a0\u200bcm\u00a0\u200b\u00d7\u00a0\u200b2\u00a0\u200bcm, 60 mesh) was used as the counter electrode, and the glassy carbon electrodes loaded with various catalysts were served as the working electrodes. The electrochemical impedance spectroscopy (EIS) measurements were operated in the same configuration at 1.55\u00a0\u200bV vs. RHE from 10\u22122\u2013105 Hz.The PBA pre-catalysts with the cationic and anionic elemental modulation were fabricated via a rapid room-temperature approach by simply adjusting the metal salt precursors (Experimental section). In brief, the total amounts of metal salts were fixed, while the kinds and amounts of specific cations and anions were modified (Table\u00a01; cations: Ni2+, Cu2+, Co2+ and Fe2+; anions: [Co(CN)6]3- and [Fe(CN)6]3-). The as-obtained products were named according to the type of cations/anions, and the valence of Co and Fe was labeled. For instance, the product fabricated by Ni2+, Cu2+, Co2+, [Co(CN)6]3- and [Fe(CN)6]3- was named as NiCuCoII\u2013FeIIICoIII, in which trimetallic cations and mixed FeIIICoIII cyanide anions were involved.The X-ray diffraction (XRD) patterns in Fig.\u00a01\nA indicate that all the products are of high phase purity with high consistency to the standard pattern of PBA (JCPDS card No. 01\u20130239). Besides, the characteristic M\u00a0\u200b\u2212\u00a0\u200bC and CN bonding can be detected from the Fourier transform infrared spectra (FT-IR) and Raman spectra (Figs.\u00a0S1\u20132), further confirming the formation of PBA structure. The transmission electron microscopy (TEM) images in Fig.\u00a01B and S3-8 reveal that the PBA products are in nanocube morphology with size of 50\u00a0\u200bnm. Of note, as the number of elements increases, curved surface of nanocubes can be observed, which may originate from the lattice relax owing to the high surface energy caused by the multi-metal nature. In addition, the selected area electron diffraction (SAED) pattern of NiCuCoII\u2013FeIIICoIII in the inset of Fig.\u00a01B shows weak 4-fold symmetry, which can be indexed to the (100) and (200) facets of the PBA lattice. Of note, bright halo can be observed from the SAED pattern, demonstrating the existence of amorphous component in NiCuCoII\u2013FeIIICoIII. The high-resolution TEM (HRTEM) image further reveals the poor crystallinity, and only short-range ordering of the PBA lattice can be identified (Fig.\u00a01C). The low crystallinity can be attributed to the multi-metal feature which brings in considerable lattice mismatch due to the different ion radius of metallic elements. The low crystallinity and disordered lattice could lead to the exposure of more undercoordinated metal sites, which is beneficial to the pre-oxidation process and OER catalysis [35].The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image and element maps of NiCuCoII\u2013FeIIICoIII reveal the homogeneous distribution of Ni, Cu, Co and Fe in the nanocubes (Fig.\u00a01D\u2013E), further confirming the multi-metal feature of the product. Besides, X-ray photoelectron spectroscopy (XPS) was used to survey the compositional and valence information of the products. As shown in Fig.\u00a02 and S9-14, the mixed valence feature of metallic elements and the characteristic signals of CN can be confirmed for all the products, further demonstrating the successful elemental modulation of the multi-metal PBA pre-catalysts. Taking the NiCuCo\u2161-Fe\u2162Co\u2162 as an example, the Ni 2p spectrum can be indexed to Ni3+ (875.0 and 857.3\u00a0\u200beV) and Ni2+ (873.6 and 856.2\u00a0\u200beV), indicating the mixed valence of Ni (Fig.\u00a02\nA) [34,36]. In addition, it can be observed two dominant peaks for the Cu2+ at the binding energies 935.7 and 956\u00a0\u200beV (Fig.\u00a02B) [37,38]. Besides, Co 2p spectrum indicates the co-existence of Co2+ and Co3+ in NiCuCo\u2161-Fe\u2162Co\u2162, where the main peaks at 782.4 and 797.7\u00a0\u200beV can be labeled as Co2+ ions, and the peaks at 781.9 and 796.8\u00a0\u200beV can be assigned to Co3+ species (Fig.\u00a02C) [39]. The binding energy of the Fe 2p3/2 region can be assigned to two peaks at 708.3 and 710.0\u00a0\u200beV (Fig.\u00a02D), which match well with the oxidation states of Fe2+ and Fe3+, respectively [39]. Furthermore, the binding energy of the Fe 2p1/2 region can be deconvoluted into two peaks, which are located at 721.2 and 723.8\u00a0\u200beV corresponding to Fe2+ and Fe3+. In addition to the metallic elements, non-metallic elements such as C, N, and O were also investigated by XPS analyses. Apart from the C\u2013C reference, CN, C\u2013O, and CO bonds can be identified at the binding energies of 285.2, 286.1 and 288.6\u00a0\u200beV, respectively (Fig.\u00a02E) [29]. The presence of CN can be further verified by the N 1s spectrum, where the intensive peaks centered at 398.0\u00a0\u200beV can be detected (Fig.\u00a02F) [40]. The O 1s spectrum can be deconvoluted into two independent signals, where the peak at 533.3\u00a0\u200beV can be assigned to the C\u2013O bonds, and the peak at 532.0\u00a0\u200beV indicates the presence of crystal water in the PBA lattice (Fig.\u00a02G) [41].The OER behavior of the PBA pre-catalysts with elemental modulation on cationic and anionic sites was investigated in 1\u00a0\u200bM KOH solution using a three-electrode system. As shown in Fig.\u00a03\nA, the linear sweep voltammetry (LSV) curves of the PBA pre-catalysts with mixed FeIIICoIII cyanide anions and variable cations reveal distinct activity towards OER catalysis, and the quaternary NiCuCoII\u2013FeIIICoIII displays earlier OER onset and larger current density. For instance, NiCuCoII\u2013FeIIICoIII only requires an overpotential (\u03b7) of 288\u00a0\u200bmV to achieve a 10\u00a0\u200bmA\u00a0\u200bcm\u22122 current density, which outperforms the counterparts and previous PBA-based/-derived catalysts (Table\u00a02\n). Besides, the geometric current density (jgeo) of NiCuCoII\u2013FeIIICoIII reaches 195.8\u00a0\u200bmA\u00a0\u200bcm\u22122 at \u03b7\u00a0\u200b=\u00a0\u200b500\u00a0\u200bmV, which is 1.7\u201361.2 times larger than that of the counterparts with variable cations. Interestingly, the OER activity is not directly correlated to the number of elements in cationic sites. The PBA pre-catalysts with Co cations display higher activity, and the OER behavior could be further enhanced by introducing more metallic elements in the cationic sites. That is, the Co ions in the cationic sites of PBAs are the dominating active sites OER catalysis, and the catalytic ensemble effect guaranteed by the multi-metal synergy in cationic sites could further improve the OER behavior. Not only that, the effect of elemental modulation in the anionic sites of PBAs was also surveyed. As shown in Fig.\u00a03B, NiCuCoII\u2013FeIIICoIII with mixed FeIIICoIII cyanide anions exhibits higher OER activity than the counterparts with unary anions (NiCuCoII\u2013CoIII and NiCuCoII\u2013FeIII), where a 1.4\u20132.2 times enhancement in jgeo can be identified. Hence, the PBA pre-catalyst with Co-containing multi-metal cations and mixed FeIIICoIII cyanide anions is favorable for OER catalysis, and the improved OER activity could be ascribed to the high intrinsic activity of the local Co3+ sites as well as the optimal multi-metal synergy.Tafel plots and electrochemical impedance spectra (EIS) were studied to survey the kinetic information during OER operation. As shown in Fig.\u00a03C\u2013D, NiCuCoII\u2013FeIIICoIII displays a smaller Tafel slope of 97\u00a0\u200bmV dec\u22121 than the counterparts, suggesting the favorable electrochemical process combined by the pre-oxidation process and OER catalysis. The EIS data further prove the facile electrochemical behavior under OER operation (Fig.\u00a04\n). As listed in Fig.\u00a04C, the samples with Co-containing cations exhibit much smaller charge transfer resistance (Rct), and the introduction of other metallic elements could further reduce the Rct. For instance, NiCuCoII\u2013FeIIICoIII exhibits the smallest Rct of 73.6\u00a0\u200b\u03a9 among the tested catalysts, demonstrating its favorable reaction kinetics towards OER catalysis.The electrochemical surface area (ECSA) is another key parameter to determine the catalytic activity owing to the different contribution in active site density. The electrochemical double-layer capacitances (Cdl) were calculated to evaluate the ECSA and the influence on the intrinsic activity. As shown in Fig.\u00a05A-B and S15, NiCuCoII\u2013FeIIICoIII exhibits a large Cdl value of 1.49\u00a0\u200bmF\u00a0\u200bcm\u22122, which is 1.02\u20134.26 times larger than the counterparts. However, the larger Cdl value (equivalently, the larger ECSA) of NiCuCoII\u2013FeIIICoIII may not the only reason for the enhanced OER activity. To better evaluate the intrinsic activity of the PBAs with elemental modulation, LSV curves normalized by Cdl values were plotted [56]. As shown in Fig.\u00a05\nC\u2013D, NiCuCoII\u2013FeIIICoIII delivers the highest jCdl of 53.0\u00a0\u200bA\u00a0\u200bF\u22121 at \u03b7\u00a0\u200b=\u00a0\u200b400\u00a0\u200bmV, which is 1.5\u201316.3 times larger than the cation-modulated PBAs and 1.4\u20132.0 times larger than the anion-modulated counterparts, thereby confirming its high intrinsic OER activity. The enhanced intrinsic activity may originate from the multi-metal synergy in both cationic and anionic sites, which not only boosts the pre-oxidation process for facile generation of the active species but also promotes the OER kinetics [41]. The enhanced intrinsic activity is responsible for the high OER activity of NiCuCoII\u2013FeIIICoIII, further verifying the effectiveness of the elemental modulation in both cationic and anionic sites of the PBA pre-catalyst.Previous studies indicate that the transition metal-based pre-catalysts usually undergo obvious performance activation during the long-term OER operation owing to the in-situ formation and accumulation of the catalytically active high-valence species [57,58]. To elucidate the pre-oxidation process and evaluate the operational stability of the NiCuCoII\u2013FeIIICoIII pre-catalyst, I-t stability test and related post-catalytic characterizations were conducted. As shown in Fig.\u00a06\nA, NiCuCoII\u2013FeIIICoIII exhibits a remarkable activation with the mass activity increasing from 95.2 to 225.6\u00a0\u200bA\u00a0\u200bg\u22121 during the first 2\u00a0\u200bh at \u03b7\u00a0\u200b=\u00a0\u200b400\u00a0\u200bmV, and a dramatic 140% enhancement in OER activity can be identified. After the activation, the OER activity of NiCuCoII\u2013FeIIICoIII undergoes slight decrement, while a high mass activity of 172.4\u00a0\u200bA\u00a0\u200bg\u22121 can still be achieved even after continuous catalysis for 72\u00a0\u200bh. The final activity shows 1.81 times enhancement than that of the fresh NiCuCoII\u2013FeIIICoIII pre-catalyst, further demonstrating the crucial role of the pre-oxidation process in promoting the OER behavior. Of note, the excellent operational stability of NiCuCoII\u2013FeIIICoIII outperforms most previously reported PBA-based/-derived catalysts (Table\u00a02), making it a promising candidate for efficient and stable water electrolysis. The comparative study of the LSV curves for the fresh and activated NiCuCo\u2161-Fe\u2162Co\u2162 further verifies the key role of the pre-oxidation process in OER catalysis. As shown in Fig.\u00a06B and Table\u00a02, the \u03b710 of NiCuCo\u2161-Fe\u2162Co\u2162 reduces from 288\u00a0\u200bmV to 251\u00a0\u200bmV after the activation, and the jgeo increases from 195.8 to 246.8\u00a0\u200bmA\u00a0\u200bcm\u22122, showing a 26% enhancement in OER activity. Hence, the superior OER stability with substantially increased activity was confirmed for NiCuCo\u2161-Fe\u2162Co\u2162.The increased OER activity during the long-term catalysis could be attributed to the facile pre-oxidation process with substantial accumulation of the active species [21\u201323]. As indicated in the inset of Fig.\u00a06B, the TEM image of NiCuCo\u2161-Fe\u2162Co\u2162 after stability test (namely, NiCuCo\u2161-Fe\u2162Co\u2162-pc) reveals the obvious morphology change, where hollow nanocage built by ultrathin nanosheets can be observed, indicating the complete reconstruction of the PBA pre-catalyst. The evaluation of Cdl values before and after stability test also confirms the enlarged surface area. As shown in Fig.\u00a0S16, the Cdl value of NiCuCo\u2161-Fe\u2162Co\u2162 increases from 1.49\u00a0\u200bmF\u00a0\u200bcm\u22122 to 3.44\u00a0\u200bmF\u00a0\u200bcm\u22122 after the long-term OER operation, confirming the obviously enlarged surface area caused by the morphology changes and phase conversion owing to the pre-oxidation process. The large surface area could bring in more surface sites for OER catalysis and favor the mass transport during catalysis, which is advantageous to the improved OER performance [29]. Besides, the reaction kinetics is also improved after the activation. As shown in the EIS data in Fig.\u00a0S17, the charge-transfer resistance reduces from 73.6\u00a0\u200b\u03a9 for the fresh NiCuCoII\u2013FeIIICoIII to a remarkably low value of 7.7\u00a0\u200b\u03a9 for NiCuCoII\u2013FeIIICoIII-pc, confirming the boosted reaction kinetic of the OER catalysis along with the proceeding of the pre-oxidation process.The pre-oxidation process causes the structural reconstruction of the PBA-based pre-catalyst. As shown in Fig.\u00a06C, the post-catalytic FT-IR spectra reveal the vanished characteristic peaks of PBA at 460, 2091 and 2181\u00a0\u200bcm\u22121 and emerging signals of hydroxides at 629 and 981\u00a0\u200bcm\u22121 during the stability test, suggesting the phase conversion from PBA to metal hydroxides [40,59,60]. The emerging hydroxide phase can be further verified by means of XRD, HRTEM and SAED analyses (Fig.\u00a06D and S18). As shown in Fig.\u00a06D, (012) facets of metal hydroxide with interplanar spacing of 0.27\u00a0\u200bnm and typical six-fold symmetry can be identified for the ultrathin nanosheets in NiCuCo\u2161-Fe\u2162Co\u2162-pc. Of note, the nanosheet is of low crystallinity, suggesting the presence of amorphous component induced by the multi-metal feature that limits the crystal growth during the pre-oxidation process. The low crystallinity could bring in more undercoordinated metal ions as the active sites for OER [35]. Besides, ICP-OES and XPS analyses were conducted to survey the compositional change during the stability test. As shown in Fig.\u00a0S19, the ICP-OES data reveal that only slight change in the atomic ratio of the metallic elements can be identified, suggesting the negligible elemental leaching during long-term catalysis. Besides, the post-catalytic XPS spectra prove the presence of all metallic elements, and the signal of CN bonding declines severely with complete conversion to oxidized N and C species (Fig.\u00a0S20). As shown in Fig.\u00a06E, the N/M ratio (M\u00a0\u200b=\u00a0\u200bNi, Cu, Co and Fe) decreases from 3.64 to 0.77 after the activation, while the O/M ratio oppositely increases from 0.29 to 3.03, which are consistent with the phase conversion from PBA to metal hydroxides during the pre-oxidation [29]. Besides, as revealed from Fig.\u00a06F, the contents of high-valence Ni, Cu and Fe exhibit significant increment after activation, while the Co3+/Co2+ ratio remains nearly unchanged. That is, the in-situ formed Ni3+, Cu2+ and Fe3+ are responsible for the increased OER activity during the long-term operation, while the abundant local Co3+ sites are the dominating factor for the high catalytic activity of the fresh pre-catalyst (Fig.\u00a07\n). Hence, the facile pre-oxidation process of NiCuCo\u2161-Fe\u2162Co\u2162 with morphological, structural and compositional reconstruction was confirmed, which endows the elemental-modulated PBA pre-catalyst with substantially promoted OER behavior during the long-term operation.In summary, synergistic elemental modulation on the cationic and anionic sites of the multi-metal PBA pre-catalysts was achieved for promoting the OER performance. Detailed investigations indicate that Co-containing multi-metallic cations and mixed FeIIICoIII cyanide anions are beneficial to OER catalysis. Thanks to the high intrinsic activity of the local Co3+ sites as well as the optimal multi-metal synergy, the optimal NiCuCoII\u2013FeIIICoIII pre-catalyst displays a low \u03b710 of 288\u00a0\u200bmV, and a remarkable 1.4\u201361.2 times enhancement in OER activity can be identified compared with the counterparts with variable cations. In addition, benefitted from the facile pre-oxidation process of NiCuCoII\u2013FeIIICoIII, abundant high-valence Ni, Cu and Fe ions with high catalytic activity can be in-situ generated and accumulated, resulting in ultrastable OER performance with substantially enhanced activity. After the pre-oxidation-induced activation, an obviously reduced \u03b710 of 251\u00a0\u200bmV and a 1.81 times enhancement in OER activity can be achieved, making the elemental-modulated PBA pre-catalyst a promising candidate for water electrolysis.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (22171167 and 21927811).The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.pnsc.2022.12.001.", "descript": "\n                  Designing advanced electrocatalysts for the oxygen evolution reaction (OER) is of great significance owing to its crucial role in facilitating the production of clean hydrogen energy via water splitting. To date, it has been widely accepted that a pre-oxidation process with the in-situ generation of the catalytically active high-valence metal sites is essential for promoting the OER behavior of most transition-metal-based OER catalysts, or more felicitously speaking, pre-catalysts. Hence, exploring such pre-catalysts with high pre-oxidation reactivity is of high promise. Herein, we proposed the dual elemental modulation in the cationic and anionic sites of the multi-metal Prussian blue analogue (PBA) pre-catalysts, resulting in promoted OER behavior benefitted from the efficient pre-oxidation ability as well as the multi-metal synergy. Detailed investigations indicate that the Co-containing multi-metallic cations and mixed FeIIICoIII cyanide anions in NiCuCoII\u2013FeIIICoIII PBA (denoted as NiCuCoII\u2013FeIIICoIII) are beneficial to OER catalysis owing to the high intrinsic activity guaranteed by the local Co3+ active sites as well as the optimal multi-metal synergy. After the facile pre-oxidation process, additional high-valence Ni, Cu and Fe ions can be in-situ formed and serve as the active sites, thereby resulting in significantly improved OER behavior. For example, the OER current density of NiCuCoII\u2013FeIIICoIII exhibits 1.81 times enhancement even after 72\u00a0\u200bh continuous OER catalysis, and the required overpotential for 10\u00a0\u200bmA\u00a0\u200bcm\u22122 reduces from 288\u00a0\u200bmV for the fresh pre-catalyst to a remarkable record of 251\u00a0\u200bmV after the pre-oxidation-induced activation, making the optimal PBA-based catalyst a promising candidate for efficient and durable water electrolysis.\n               "}
{"full_text": "The discovery of carbon-based nanostructures, such as carbon fibers, carbon nanotubes (CNTs), carbon microtubes (CMTs), and carbon nanodots, appears to be beneficial for practical applications. In term of CNTs and CMTs, the structures have similarities particularly in hollow, tubular, and single- or multi- walled which only differ in the size, and since their discoveries (Ijima, 1991). Ever since the discovery, another attempt to produce similar carbon-based structures, such as CMTs is carried out. Both CNTs and CMTs are fabricated by applying certain temperature of treatments which most of them were in high temperatures with high pressure condition. Therefore, the synthesis of CMTs is influenced by three factors i.e., temperature, and to reduce the high temperature, the latter factors are catalysts and the carbon sources. The precursors for synthesizing have become a challenge, which commonly is organic compounds such as methane, methanol and acetone(Janas, 2020). Materials consisted of carbons were the most suitable precursors for synthesizing CNTs and CMTs, including hydrocarbon compounds such as methane(Kang et al., 2008) and ethanol(Kakehi et al., 2008). The drawbacks in using these two types of precursors require high temperatures of treatments even though its gaseous form reduces time reaction. Nonetheless, gaseous phases are not the only substances that could be used as precursors as a study had successfully synthesized CNTs from solid materials(Singh et al., 2002). On the other hand, the needs of carbon microtubes (CMT) for several applications are in needs such as for oil adsorption(Zhao et al., 2019), and batteries and energy storage(Huang et al., 2012);(Salahdin et al., 2022), which are synthesized from solid compounds. Hence, finding strategies via availability of precursors and selective catalyst is promising advantages for future use.In term of availability, cellulose provide advantages due to their availability of abundant material in carbon content. This organic material is commonly found in plants, for instance, angel\u2019s trumpet plant contains high variation of acidic compounds which also can be precursors for synthesizing carbon-based materials (Mokbli, 2021). Furthermore, In Indonesia, cellulose is organic waste that is annually produced by the plantations, including empty bunch of palm oil, and even more a study utilized part of palm oil tree which is the kernel shell as precursor for bifunctional catalyst (Abdullah et al., 2020). Based on its structure, cellulose has been used as biofuel (Panneerselvam et al., 2016), suggesting its high carbon content. Other reports have also successfully carbon nano- and micro-structures with mesoporous morphological features from paddy rice(Hao et al., 2019), poplar-catkin(Huang et al., 2021), and plant tissues(Zhao et al., 2019). Another potential plants that can be a precursor is kapok randu, which is a native plant to Indonesia. Cellulose fibers from kapok randu were reported to be used in the synthesis of activated carbon(Chung et al., 2013); subsequently; cellulose-fiber from this plan can be used as the precursors in synthesizing CMTs.Attempts in synthesizing carbon-based microstructures seem to be high-cost due to the use of catalysts in order to overcome the non-steady phase of carbonization. Several reports have suggested the introduction of catalysts, such as silver nanoparticles (Gea et al., 2022) which is considered as high-cost. Whilst, the use of mono-(Co) combined with bimetallic (Fe-Co) catalysts during CVD synthesis is considered as complex strategies which also takes place in high energy(Balogh et al., 2008);(Kakehi et al., 2008). Although several reports have used transitional-metals (TiC, NiCl2, SnO2) and non-metal catalysts (Sulfur) with more controllable reaction, these show limited selection in finding affordable and available catalysts for synthesizing with low-cost aspects(Huang et al., 2021);(Ariyanto et al., Feb. 2019);(Anil Kumar et al., 2022). Hence, the purpose of this present work was to evaluate the possibility of cellulose fibers isolated from kapok randu to be used as the precursor with catalysts such as Fe, Ni, and Cu via heating treatments were.The Kapok Fibers (KFs) were collected from the fruit, in which the tree was located in the sub-district Tanjung Mulia, Deli Serdang Regency, Medan, Indonesia. The chemical reagents such as HCl, NaOH, H2O2, H2SO4, H3PO4, NaOCl, Na2S2O3, NaNO3 and acetone were purchased from Sigma Aldrich Inc. Meanwhile, the metal catalysts, such as FeSO4\u00b77H2O, NiSO4\u00b7H2O, CuSO4\u00b75H2O were supplied by Bratachem. The gasses utilized to control the condition including nitrogen, methane, hydrogen and helium, were purchased from supplied by PT Aneka Gas.The amount KF that was from the fruit was separated manually via man-labor. Then, these amounts of fibers were dried directly under daylight, and from these amounts, 75\u00a0g of KFs were cut into 2\u20133\u00a0cm of sizes. The fibers were immersed in 1\u00a0L of 3.5\u00a0% HNO3 and 10\u00a0mg NaNO3 mixture for 2\u00a0h at 90\u00a0\u00b0C. Then, the mixture was filtered and the fibers were washed by using distilled water until the neutral pH was achieved. Next, the fibers were soaked into 750\u00a0ml of 2\u00a0% w/v NaOH and 2\u00a0% Na2S2O3 solution for 1\u00a0h at 50\u00a0\u00b0C, and followed by washing and filtrating processes. Afterward, these KF samples were bleached with 250\u00a0ml of 1.75\u00a0% of NaOCl solution until its temperature boiled for 30\u00a0min. After being bleached, the samples were mixed with 500\u00a0ml of 17.5\u00a0% NaOH for 30\u00a0min at 50\u00a0\u00b0C to obtain cellulosic sample. Then, this cellulosic sample was washed by using distilled water, in which hereafter, 10\u00a0% of H2O2 were used to immerse the samples for 30\u00a0min at 60\u00a0\u00b0C. The cellulosic sample then was filtered and dried in an oven at 60\u00a0\u00b0C to remove the residual water content, which was followed by storing it in a desiccator.The first step in isolating nanofiber cellulose (NFC) from KFs began by acid hydrolysis treatment, which was previously described in several studies(Gea, Panindia, et al., 2018; Gea, Zulfahmi, et al., 2018; (Zulham Efendi Sinaga et al., 2018). The cellulosic fibers were soaked in 45\u00a0% H2SO4 with ratio of 1:25 w/v% for 45\u00a0min at 45\u00a0\u00b0C. Then, into the mixture, some bi-distillation water with ratio 1:25 v/v% were added and the mixture was allowed to stand for 12\u00a0h at room temperature to form suspension. The suspension was separated and washed to reach pH 7. Next, this mixture sample was placed in an ultrasonic bath for 3\u00a0h, and then followed by homogenization step for 3\u00a0h at 8000\u00a0rpm. Finally, these nano-cellulosic kapok fibers (NCKFs) were heated at 50\u00a0\u00b0C in an oven to remove water content, and the dried samples of NCKFs were obtained which hereafter is stored in desiccator.The characterization of TGA was performed to determine as basis of later heating treatments for the growth of CMT. The TGA was carried out via TGA DTG-60 in which the thermal rate was 10\u00a0\u00b0C.min\u22121 in nitrogen condition (flow rate 30\u00a0ml.min\u22121). The initial temperature was 27\u00a0\u00b0C, and final temperature was 600\u00a0\u00b0C, whereas the starting mass of the NCKFs was 3\u00a0mg.Thermal Gravimetric Analysis (TGA) was used to obtain the decomposition temperature. The decomposition process started at 320\u00a0\u00b0C based on the TGA result, so that the NCKFs were heated inside a furnace at 400\u00a0\u00b0C for two hours to produce carbon. Afterward, the carbon samples were obtained, and the chemical activation were performed by immersing these carbon samples into 1\u00a0M H3PO4 for 90\u00a0min with ratio 1:10 w/v%. Then, this mixture was filtered, and the filtered carbon samples were dried at 150\u00a0\u00b0C for 24\u00a0h. Afterwards, 5\u00a0N HCl was added to the dried carbon samples to perform reactivation. Thus, the removal of excessive chloride ions was done by washing these dried carbon samples with distilled water to reach pH 7, and followed by additional washing with cool distilled water as well as filtering to remove the residual of phosphate anion. After being washed and filtered, the wet carbon samples were dried at 150\u00a0\u00b0C for 24, and finally this sample was named after AC.The preparation of 0.09\u00a0M Cu(NO3)2, 0.09\u00a0M Ni(NO3)2, and 0.09\u00a0M FeCl3 catalyst solutions was carried out by respectively dissolving 1.08\u00a0g of Cu(NO3)2\u00b73H2O, 1.3\u00a0g of Ni(NO3)2\u00b76H2O, and 1.21\u00a0g of FeCl3\u00b76H2O in acetone. Then, each solution was homogenized with constant stirring.The preparation of catalysts for CMTs was performed by mixing 0.09\u00a0M Cu(NO3)2, 0.09\u00a0M Ni(NO3)2, and 0.09\u00a0M FeCl3 with AC and ratio of 1:10 w/v%. Then, each of the mixture underwent ultrasonication for 2\u00a0h at 70\u00a0\u00b0C. The results of the impregnation samples were dried in oven for 12\u00a0h at 70\u00a0\u00b0C.An amount of impregnated AC samples was placed in a 25\u00a0ml porcelain dish into a gas furnace. On the surface of the dish, the end of furnace ceramic pipe was connected to gas source. During the heating process, the dish was covered to prevent small carbon particles from escaping. The first step was calcination process, which was done by streaming the impregnated AC with heat at 500\u00a0\u00b0C for two hours under inert conditions (nitrogen gas 100\u00a0ml.min\u22121). The second stage was a reduction process, where the temperature of 700\u00a0\u00b0C and hydrogen gas (with flow of 60\u00a0ml.min\u22121) were applied for two hours. In this process, metal oxides would be removed and the metals were converted into metal nanoparticles. In the third stage, the temperature in the reactor was increased to 950\u00a0\u00b0C, and followed by the increase of nitrogen gas rate to 100\u00a0ml.min\u22121. When the reactor reached the set temperature, the nitrogen gas rate was increased to 200\u00a0ml.min\u22121. The next stage was the flowing of mixture methane and nitrogen gas which respectively 1:2 ratio (rate of 100\u00a0ml.min\u22121) for two hours at 950\u00a0\u00b0C. Then, at the final process, Helium gas was flowed at 60\u00a0ml.min\u22121 into the furnace, thus; the temperature within it would drop to room temperature. Inert condition during the final process was important in order to prevent the destruction of CMTs.In this research, CMTs were analyzed by Transmission Electron Microscopy (TEM) instrument (JEM-1400) with acceleration voltage 120\u00a0kV. The photograph obtained from TEM was analyzed to observe the length and diameter of the tubes via Image-J application, as well as the structures of CMTs, which were different, depends on the catalyst used.The NCKF were analyzed by Fourier Transform Infrared (FTIR) instrument. Characterization by FTIR was done to confirm the functional groups in NCKF in Fig. 1\n.The FTIR pattern shows that the functional groups of the NCKF were the same as cellulose fibers. The \u2013OH functional group was shown at peak 3418\u00a0cm\u22121 with stretching vibration up to 2900\u00a0cm\u22121, whereas, the CH aliphatic group was at 2900\u00a0cm\u22121. It could also be clearly seen that the OH group related to carboxylate group was at 1635\u00a0cm\u22121. The bending vibrations of HCH, OCH, and CH, and rocking vibration of \u2013CH2 were respectively at 1427\u00a0cm\u22121, 1373\u00a0cm\u22121, and 1334\u00a0cm\u22121, which these three vibrations were in C6 glucose chain. The Fig. 2\n shows the morphological of NCKF.\nFig. 2 shows morphological images by SEM of NCKF obtained with 250 and 100 times of magnification. Unlike those obtained by previous studies, the NCKFs had different shapes from palm oil bunches and corncobs (Zulham Efendi Sinaga et al., 2018). Although generally, kapok randu is one of tropical tress with fruits containing cellulosic fibers, its cellulosic material has been reported to be hydrophobic-oleophilic (Wang et al., 2018). Hence, it is assumed that the thermal properties of NCKF were different due to its utilization as a precursor in the synthesis of CMTs.As the NCKFs was used as the precursor to synthesize CMTs, TGA analysis was performed to determine the temperature breakdown. The Fig. 3\n depicts the comparison of TGA analysis of NCKFs.According to Fig. 3, the initial temperature started from 27\u00a0\u00b0C to 600\u00a0\u00b0C, and the initial mass of NCKFs was 3\u00a0mg with 10\u00a0\u00b0C.min\u22121 of heating rate. The significant observation can be seen at the temperature above 300\u00a0\u00b0C as the NCKFs started to decompose and reached 60\u00a0% of mass change. Although the NCKFs differs to other biomass, this thermal analysis has confirmed similar data to what have been reported in several studies (Soykeabkaew et al., 2012). The region of initial and ending composition were above 300\u00a0\u00b0C and 600\u00a0\u00b0C respectively (Gea et al., 2020a).As it has been reported by several studies, around 32\u201347\u00a0% cellulose was isolated from various raw materials (Gea, Andita, et al., 2018; Gea et al., 2020a; (Marpongahtun et al., 2018), where its physical and chemical structures may have been different from one to another due to different alkaline treatments. The use of sodium hydroxide in cellulose isolation has been concluded to provide distinguishing impacts on the morphological structures, including the stiffness and orientation of the fibrils (Chakraborty et al., 2011).In this study, the AC was obtained from NCKFs, and in Figs. 2 and 3, the \u2013OH, CH aliphatic, and \u2013CH2 were confirmed. However, the spectrum of AC in Fig. 4\n showed different results, particularly in the presence of new groups and the occurrence of reduction. The groups, such as CC, CO, and P\u00a0=\u00a0OOH was confirmed due to the treatments with H3PO4. This could occur as the surface of biomass was carbon-derived(Oginni et al., 2019), which were indicated the presence of CC stretching band around 1600\u00a0cm\u22121, \u2013CH stretching band in interval of 2800\u20133000\u00a0cm\u22121, and -P\u00a0=\u00a0OO4 in 1100\u00a0cm-1(Xu et al., 2014).Subsequently, the confirmation of activated carbon was also performed to ensure the success synthesis. The following Fig. 5\n demonstrates the XRD result of sample activated carbon. In overall, two broad peaks were noticeable observed in 40-50\u00b0 and 60-70\u00b0 which is related to (002) (Xu et al., 2014). It also can be observed that amorphous parts of activated carbons alongside, in which it could have attributed to the random stacking of layers, that were also noted from the SEM photographic images in Fig. 5 (inset).\nFig. 6\n shows significant differences of TEM photographic images in each sample synthesized from different catalysts. The commercial AC samples were with diameter of 45\u201350\u00a0nm (Fig. 6a). The image also showed metal nanoparticles attached to the cap of the CMTs with a mean tube length of 600\u00a0nm. Meanwhile, the sample with AC synthesized from NCKF with Cu catalyst for 11\u00a0h had tube diameter and length of 50\u00a0nm and 100\u00a0nm respectively (Fig. 6c). In Fig. 6c*, with samples made of Cu catalyst, produced mostly spherical amorphous carbon particles with sizes of under 10\u00a0nm. Meanwhile, the AC from NCKF with Ni catalyst had tube diameter and length about 40\u201350\u00a0nm (Fig. 6d).The first variation was sample of AC from NCKF with Fe catalyst treated for 6\u00a0h to produce CMTs with diameters of 200\u00a0nm with the average tube length of 2\u20133 \u00b5m (Fig. 6b). Then, the second variation was commercial AC with Ni catalyst treated for 11\u00a0h, produced CMTs with diameters of 50\u00a0nm and a tube length of 150\u00a0nm (Fig. 6e). This happened due to the solubility of carbon in metal particles, which would form solid filaments alongside with the width of the diameters(Duc Vu Quyen et al., 2019). Another report has shown that the decreasing of diameter of CMTs from kapok randu for almost a half (from 20\u00a0\u00b5m to\u00a0\u223c\u00a012\u00a0\u00b5m) after carbonization at the temperature of 500\u20131000\u00a0\u00b0C(Zhao et al., 2019). Fig. 6a and 6b display tubular structure even though rough shapes were found. Therefore, high temperature used could reduce catalyst activities to bind the carbons in CMTs arrangement during the reaction, such as direct calcination at 500\u00a0\u00b0C(Wang et al., 2018). It is assumed that due to the high temperatures that leads to high pressure condition, carbon atoms begin to degrade and create uncontrollable reaction, which allows the formation of carbon clusters. At the same time, the surface of AC could react to which form graphitization too, and due to the use of the metal catalysts which implied to the uncontrollable graphitization reaction(Ariyanto et al., Feb. 2019). The reaction caused the particles to agglomerate to each other, so that the reaction results tended to lead the formation of amorphous carbon particles with a size below 10\u00a0nm(Ahmad et al., 2018) as it is shown in Fig. 6.In general, the synthesis reaction of CNTs via CVD was carried out over a time span of 30\u201360\u00a0min for the growth of CNTs by using precursors(Costa et al., 2008);(Ram\u00edrez Rodr\u00edguez et al., 2018). Several studies also mentioned that the process could take longer time(Li et al., 2009);(Fathy, 2017). In this study, the results obtained based on TEM analysis showed a small amount of pile up (especially based on Fig. 6a and b), which was assumed to occur due to thermal treatments. However, the tubes produced in this study were seen to be consistent compared to previous reports(Duc Vu Quyen et al., 2019);(Ahmad et al., 2019). The optimum growth of CMTs was 30\u201360\u00a0min, more than 60\u00a0min would not produce more CMTs as the surfaces of catalyst would have been overgrown with CMTs. In conclusion, the use of a longer time with the same temperature would reduce the yield of CMTs as previously reported (Ahmad et al., 2019).The above Fig. 7\n is the Raman shifts of pyrolysis sample of AC from NCKF with different catalysts, i.e., the impregnation of Ni, Fe, and Cu. The shifting could be clearly seen in the interval of 1500\u20131700\u00a0cm\u22121, indicating the first order of G band as well as implying the presence of CC structures. Among three of them, sample 1 and 2 had the higher intensity, and also, in Fig. 7B, the sample 2 which was treated with Cu as the catalyst, had higher intensity counts than that in sample 1 that was treated by Fe as the catalyst. The presence of this peak has confirmed the successful growth of single-wall carbon nanotube (SWCNT); as per what other studies have also reported the shifting 1500\u20131700\u00a0cm\u22121 both in red and green laser (respectively 785 and 514\u00a0nm)(Costa et al., 2008);(Li et al., 2009).Metal catalysts, mono-metals in particular, could affect the structural rearrangement structure of the carbonaceous materials(Ahmad et al., 2019). As this study only focused in the use of mono-metals, the selection of metals was based on the precursor, which is the cellulosic material(Duc Vu Quyen et al., 2019);(Ahmad et al., 2019). Both TEM and Raman results confirmed the differences in each growth samples in terms of the sample intensities as well as the length of the tubes. Copper (Cu) catalysts had amorphous structures, which were both proven by sample 3 in Raman shift results (Fig. 7a). These results were in accordance to the studies which utilized Fe and Ni catalysts, producing long-shape tube growths via CVD method(Arjmand et al., 2016). Meanwhile, another research reported Ni catalyst has successfully synthesized long-shape tubes with 10\u201340\u00a0nm diameters via microwave treatments(Burakova et al., 2018). Although this study investigated that sample Cu and Ni which are based on Fig. 6a, 6b, and 6d, the growth of the tubes via AC from NCKF may occur due the surface area. Thus, it is suggested to evaluate further about the relation of surface area with the growth of carbon microstructure, nevertheless; have been successfully obtained.The fibrous material from kapok randu that contains cellulose has the potential to be precursors for CMTs synthesis from activated carbon as the basis of growth. By using various catalysts, such as iron, nickel and copper, the growth of CMTs has been successfully obtained via thermal treatments with the introduction metal catalysts of Fe, Ni, and Cu at moderate temperature (700\u2013950\u00a0\u00b0C) even though longer graphitization takes time. Based on the results, the size diameter of the CMTs were in between 50 and 200\u00a0nm with length diameter of less than 2\u00a0\u00b5m in average, as the growth is observed in the micrograph results. The highest presence of CMTs\u2019 growth was observed in sample with nickel (Ni) catalyst, and the Raman shifts appears to show the growth with considerably peaks. Thus, further investigation related to the physical parameters of catalysts and graphitization reaction in moderate temperature is required.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank the rector of Universitas Sumatera Utara as its grants via TALENTA scheme program with given contract No. 2590/UN.5.1.R/PPM/2018.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jksus.2022.102423.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Objective\n                  As a hydrocarbon material, cellulose could be used as precursor in synthesizing carbon micro-structure (CMTs), and this study aims to investigate the potential use of cellulosic material from kapok randu as a precursor in synthesizing carbon-based structures.\n               \n                  Methods\n                  The isolation of cellulose was carried out via alkaline treatment, followed by mechanical disintegration. Meanwhile, the growth of CMTs was performed via heating treatment for 12\u00a0h with various catalysts (i.e., Fe, Ni, and Cu). Chemical characteristics were confirmed by FTIR, XRD spectra, while TEM, SEM and Raman spectra were performed to determine the growth of CMTs.\n               \n                  Results\n                  The thermal characteristic suggested that the decomposition was initiated at 300\u00a0\u00b0C. The FTIR results confirmed the presence of functional groups in accordance to cellulose fiber, such as \u2013OH (3418\u00a0cm\u22121), CH aliphatic group (2900\u00a0cm\u22121), OH (1635\u00a0cm\u22121), and CH (1334\u00a0cm\u22121). Whilst, the FTIR pattern also confirmed the presence of CC stretching at 1600\u00a0cm\u22121, \u2013CH in between 2800 and 3000\u00a0cm\u22121, indicating the activated carbon. Raman shift indicated the growth on G-band in the interval of 1500\u20131700\u00a0cm\u22121, suggesting the presence of CC structures. Based on the morphological characteristics, the growth of CMTs were successfully obtained with different diameters and lengths due to the different catalysts used. The iron (Fe) catalysts produced CMTs with a diameter about 200\u00a0nm and average length of 2\u20133\u00a0\u00b5m, whereas the Ni catalysts formed tubes with around 150\u00a0nm of length and 50\u00a0nm of diameter. Meanwhile, the Cu catalysts formed amorphous particles with diameter below 10\u00a0nm.\n               \n                  Conclusion\n                  From these results, the evaluation of cellulose isolated from kapok randu as a precursor in the growth of carbon micro-tube with distinguished characteristics was demonstrated.\n               "}
{"full_text": "Under the trend of dramatic population growth [1], rapid urbanization [2], as well as expansion of industrial and agricultural production scale [3], the issues of water pollution and scarcity are stern day after day and have become critical environmental issues facing the worldwide in the 21st century. Moreover, subject to the combined influence of multiple factors under the 2019 coronavirus disease (COVID-19) pandemic such as extreme weather [4], monetary policies [5], and supply-demand imbalance [6], the prices of fossil fuels (oil, coal, and natural gas) that underpin current world economic development and social progress have increased dramatically, which heralds global energy crisis is becoming even more intense and facing more complexity and uncertainty. Traditional water treatment (biological, physical, and chemical) technologies [7], have significant energy consumption problems in the treatment process and are unable to efficiently and cost-effectively treat wastewater containing large amounts of highly concentrated organic pollutants [8], which poses a serious burden to social and economic development. How to treat wastewater in an efficient and green way with both social benefits and economic value has become a hot topic for researchers, and on this basis has stimulated the development of a series of green and sustainable energy technologies such as microbial fuel cell (MFC).MFC is an environmentally friendly and efficient device for wastewater treatment and energy recovery [9], which can degrade organic pollutants while generating electricity, in line with the current concept of sustainable development. Specifically, MFC, as an important branch of fuel cells, has outstanding advantages in operation and function that are incomparable with other energy sources: wide sources of raw materials [10], all biodegradable organic matter can be used as MFC substrates in theory; no energy input is required [11], the air-cathode MFC is in direct contact with oxygen and does not require energy input for aeration; relatively higher energy conversion rates [12], MFC can convert the chemical energy contained in the organic matter directly into electrical energy; mild operating conditions [13], the microbial diversity in the MFC enables it to work under normal temperature and pressure; clean and environmentally friendly [14], the exhaust gas generated by MFC is mainly carbon dioxide (CO2), which has small emission and will not cause secondary pollution to the environment and does not require exhaust gas treatment.Compared with other fuel cells, MFC has broad application prospects due to the above characteristics and advantages, which contribute to the fields of environment, green renewable energy, and biomedicine. The expanded applications of MFC in emerging fields such as desalination [15], ecological restoration [16], alternative power sources [17], artificial organ power sources [18], biosensors [19], environmental monitoring [20] and coupling application [21] have emerged and achieved energy diversification. It well illustrates that the application potential of MFC as an emerging environment-friendly technology is huge. At present, the research on MFC is still mainly focused on wastewater resource treatment. Despite the dual advantage of wastewater treatment with simultaneous electrical energy production by MFC technology, the current relatively low output power has not reached the ideal state. Current stage research suggests that the wastewater treatment effectiveness and power production of MFC are influenced by various factors, mainly including MFC configurations [22], anode materials [23], cathode materials [24], inoculated microbiological [25], and proton exchange membranes (PEM) [26], etc. Among the many factors affecting MFC performance, the expensive cost [27], biofouling [28], and slow oxygen reduction reaction (ORR) kinetics [29] of the cathode have become the key factors preventing the effective operation and expected practical application of MFC. Therefore, the cathode can be considered as a pointcut to improve the overall performance of MFC.In the early stages of MFC research, the Pt/C catalyst with relatively high ORR electrocatalytic activity was generally used as a traditional cathode catalyst for MFC. The follow-up studies have shown that the shortcomings of the Pt in terms of high price [30], scarce resources [31], and easy deactivation by poisoning [32] have greatly limited the large-scale commercial application of MFC. The development of cathode catalysts with low cost, high efficiency, and good tolerance has become a crucial prerequisite for the industrialization of MFC. After extensive and in-depth research by many scholars, a series of optimized single-metal cathode catalysts with favorable ORR catalytic activity have been successfully prepared, such as transition metal macrocyclic compounds [33], transition metal oxides [34], metal sulfides [35], transition metal-nitrogen-carbon (M-N-C) catalysts [36] and metal-organic framework (MOF) catalysts [37], etc., which can be used as efficient and low-cost cathode catalysts to replace Pt/C catalysts. In recent years, bimetallic catalysts with two different metal elements or compounds as active ingredients have shown superior catalytic performance and durability than monometallic catalysts owing to the synergistic effects (geometric [38], electronic [39], and stabilizing effect [40]) between the bimetallic components. This is not limited to bimetallic catalysts, trimetallic or multimetallic catalysts also possess more outstanding catalytic properties, such as good active stability and selectivity, than monometallic catalysts, but are limited by the fact that their synthesis may be more complex, wordy, and relatively expensive than bimetallic catalysts [41], researchers have focused their attention more on bimetallic catalysts. Various types of bimetallic catalysts have been successfully applied in MFC as low-cost ORR catalysts with high activity and stability.In the last decade, there have been relatively few review articles on bimetallic ORR catalysts, especially lacking a comprehensive and systematic exposition of bimetallic ORR catalysts suitable for MFC. Based on previous studies, we reviewed from the perspectives of reaction mechanisms, advantages, and typical synthesis methods of bimetallic catalysts. The achievements of Pt-M alloys, transition metal alloys, transition composite metal oxides, transition metal macrocyclic compound-based bimetallic catalysts, MOF-based bimetallic catalysts, and bifunctional catalysts in MFC are also analyzed emphatically, to sort out a relatively clear vein for reference. Last but not least, a tentative suggestion of future research priorities for MFC cathodic bimetallic catalysts. Fig.\u00a01\n presents the thinking logic diagram of this review.Oxygen (O2) is considered to be an ideal electron acceptor superior to potassium ferricyanide [K3Fe(CN)6] and Potassium permanganate (KMnO4) due to its widespread presence in the environment and relatively high redox potential [42,43], which is mainly used in air-cathode MFC, dissolved-oxygen cathode MFC, and biocathode MFC. In air-cathode MFC (Fig.\u00a02\na), the electrons and protons generated via the decomposition of organic pollutants in the wastewater by the electro-producing microorganisms of the anode reach the cathode through the external circuit and the PEM, respectively. Then ORR occurs with the incoming O2 under the action of the cathode catalyst to form water and electric currents, which ultimately achieve the degradation and energy conversion of organic pollutants. ORR is a multi-electron reaction with a rather complex reaction process at the electrode surface which involves multi-step elementary reactions (O\u2013O breakage, transfer of protons and electrons, etc.) and numerous short-lived intermediate species (e.g. O, OH, O2\n\u2212, HO2\n\u2212 and H2O2, etc.) [44]. If specific details are not considered, ORR can be simply divided into direct 4e\u2212 and indirect 2e\u2212 pathways, the possible reaction pathways are shown in Fig.\u00a02b, where k1-k5 are the reaction rate constant. The ORR pathways are also affected by the acidity or alkalinity of the electrolyte [45], Fig.\u00a02c shows the possible reduction pathways and reduction potentials for O2 in acidic and alkaline electrolytes.From the above, it is clear that the indirect 2e\u2212 ORR pathway is more complicated than the direct 4e\u2212 ORR pathway whether in acidic or basic electrolytes. H2O2 with a strong oxidizing effect may not only re-engage O2 in the reaction through reversible reactions, reducing the reaction efficiency, but may also damage the cathode properties [46] and corrode PEM [47]. Therefore, the ideal ORR pathway should be a 4-electron reaction, but the barrier of the 4e\u2212 ORR pathway is higher than that of the 2e\u2212 ORR pathway, the thermodynamic analysis revealed that the dissociation energy of the O=O bond dissociation energy in the unstable intermediate (H2O2) (149 kJ mol\u22121) generated by the 2e\u2212 ORR pathway is much lower than that in O2 (490 kJ mol\u22121) [48]. This makes it easier for actual ORR to follow the reaction path of 2e\u2212 or 2e\u2212 combined with 4e\u2212. To address this conundrum, the ideal cathode catalyst should be designed to be highly selective for the 4e\u2212 ORR pathway to reduce O2 to water in one step at a higher potential, thus increasing the ORR efficiency to obtain high output voltage and energy conversion rates.There is a competitive relationship between the 4e\u2212 and 2e\u2212 pathways for ORR, and the ORR pathways of different catalysts strongly rely on the adsorption pattern of O2 [49]. According to Yeager et\u00a0al. [50], the adsorption modes of O2 on metal surfaces are broadly classified into three types, as shown in Fig.\u00a03\na [51]. The \u201cGriftiths\u201d mode: oxygen molecule interacts laterally with the catalytic active center, which is favorable to the O\u2013O breakage to occur direct 4e\u2212 ORR pathway, which is usually observed on the surface of the Pt catalyst; the \u201cBridge\u201d mode: oxygen molecule simultaneously interacts with two catalytically active centers, apparently favoring the 4e\u2212 ORR pathway, which may be observed on transition metal alloys (e.g. Pd\u2013Co alloys) (Fig.\u00a03b) [51]; the \u201cPauling\u201d mode: only one side of the oxygen molecule interacts with the catalytically active center, which is not conducive to the breaking of the O\u2013O bond and generally results in 2e\u2212 ORR, with most electrodes performing ORR in this mode. Recent studies have shown that Co8FeS8 microspheres containing Co, Fe, and S atoms may promote O2 dissociation and the efficient side-on (and/or bridge) adsorption pathways, N atoms with strong electronegativity may powerfully attract electrons from S and metals (Co and Fe), thereby enhancing the charge transfer rate and ORR kinetics on Co8FeS8/NSC-900 so that the main 4e\u2212 ORR pathway occurs (Fig.\u00a03c) [52]. Overall, the design of an ideal bimetallic cathode catalyst should consider the effect of O2 adsorption so that the ORR is biased towards the 4e\u2212 pathway as much as possible.Elucidating ORR mechanisms and rate-determining steps (RDS) at the atomic level remains challenging, which is mainly attributed to the extremely complex process in ORR. Three currently commonly accepted ORR mechanisms were combed by Xia et\u00a0al. [53], as shown in Fig.\u00a04\na. The \u201cDissociative\u201d mechanism: after O2 diffuses to the electrode surface to form O2\u2217 (\u2217 indicated the adsorbed state), the O\u2013O bond is broken directly to form O\u2217 intermediates which are reduced successively to OH\u2217 and to H2O\u2217; the \u201cAssociative\u201d mechanism: the O\u2013O bond is cleaved to form O\u2217 and OH\u2217 intermediates after O2\u2217 forms OOH\u2217; the \u201cPeroxo\u201d mechanism: the O2\u2217 is reduced successively to OOH\u2217 and to HOOH\u2217 before the O\u2013O bond cleavage. For many reasons, the actual ORR process may be carried out by one or more of these three mechanisms, alone or in combination.The free energy barriers gained via Density functional theory (DFT) calculations can be used to estimate the dominant mechanism in the ORR process. In general, the dissociation mechanism, which has the lowest energy barrier at low oxygen coverage, is the main ORR pathway, while the association mechanism, which presents the lowest potential barrier at high oxygen coverage, dominates the ORR process [54]. Through a new method of adsorption preference and electron affinity, Wu et\u00a0al. [55] predicted that the dominant ORR mechanism on the Pd3Cu surface in alkaline media may be one association mechanism to achieve ultra-low overpotential. Liu et\u00a0al. [56] demonstrated that the potential ORR RDS of the CoNi alloy nanoparticles (CoNi3\u2013CoN4) was the protonation reaction of O2 (O2+H++e\u2212\u2192OOH\u2217) based on DFT calculations, moreover, the energy barrier of RDS at the CoNi3\u2013CoN4 site (0.40\u00a0eV) was lower than that on the Co\u2013N4 site (0.70\u00a0eV), indicating that CoNi3\u2013CoN4 site exhibits better ORR catalytic activity (Fig.\u00a04b). Recently, Chen et\u00a0al. [57] proved by in situ Raman spectroscopy that the key intermediate species of disordered and ordered Au\u2013Cu nanocatalysts was \u2217OH during ORR, as well as the binding site of Au and Cu was the real active site of the catalyst (Fig.\u00a04c and d), laterally reflecting the superiority of the bimetallic catalyst.Furthermore, renowned scientist N\u00f8rskov [54] used DFT to draw a \u201cvolcano\u201d diagram of ORR activity trend for different pure metals to describe the relationship between ORR catalytic rate and metal-oxygen adsorption energy. As can be seen from Fig.\u00a05\na, there are significant differences in the ORR activity of different metals, with Pt and Pd at the top having the best binding strength with oxygen, being the best monometallic catalysts in terms of ORR activity, and Pt/C thus became one common commercial catalyst, proving that the binding energy between the oxygen-containing intermediate species and the catalyst surface determines the catalytic rate of ORR. Subsequently, Greeley et\u00a0al. [58] plotted another \u201cvolcano\u201d diagram (Fig.\u00a05b), which showed that Pt-based alloys not only have the same oxygen binding energy trend as pure metals but also Pt-based alloys at the top possess higher ORR activity. This conclusion is just in line with the Sabatier principle, which states that the adsorption strength of the catalyst on the reactants should be in the appropriate range to obtain the optimal catalytic activity. Structural characterization and DFT calculation [59] revealed that the interaction of Pt3Co(111) and Co\u2013N4 active sites with low overall reaction-free energy in PtCo@NGNS reduced the adsorption energy of oxygen-containing intermediates and the activation energy of the reaction, so as to synergically improved the ORR performance through an associated 4-electron mechanism (Fig.\u00a05c and d).Compared with monometallic catalysts, bimetallic catalysts exhibit synergistic effects due to metal-metal bonding interactions (Fig.\u00a06\na), significantly improving the catalytic performance [60]. DFT calculations demonstrate that in catalytic reactions, the synergistic effects between bimetals can greatly decrease the reaction activation energy [61], achieving the remarkable effect of 1\u00a0+\u00a01>2. A vivid diagram can be used to interpret the synergistic catalytic effect of bimetallic catalysts, i.e., the catalytic performance of A\u00a0+\u00a0B is greater than that of A or B (Fig.\u00a06b). The synergistic effects between bimetallic components can be classified into three types as follows:\n\n(1)\nGeometric (or strain) effect: The addition of second metal makes mismatching of lattice constants (lattice distortion), leading to changes in the atomic spacing or average metal-metal bond lengths, which alters the geometrical configuration of the catalyst while improving the catalytic activity [62,63]. Wu et\u00a0al. [64] confirmed that the Au\u2013Pt alloy exhibited higher catalytic performance than the monometallic catalyst owing to changes in grain size and lattice structure (lattice shrinkage in Au and lattice expansion in Pt) during the formation of Au\u2013Pt.\n\n\n(2)\nElectronic (or ligand) effect: The addition of second metal alters the electronic configuration of the active metal site via altering the electronic environment on the metal surface or by promoting electron transfer between metals [65,66]. The d-band center theory, which assumes that the d-orbital center (cannot be too high or too low) of the metal is linearly related to the adsorption strength of the reacting species on the metal surface, is one of the significant descriptors of ORR activity. Yin et\u00a0al. [67] discovered that the NP-Ag4Cu catalyst made the d-band center of Ag closer to the Fermi energy level due to the alloying effect of Cu, thereby optimizing the electronic structure and significantly improving the ORR catalytic activity (7 times that of NP-Ag).\n\n\n(3)\nStabilizing effect: The addition of second metal improves the stability of the catalytically active metal. The Pt6Ru1/C catalyst exhibited good SO2 resistance in ORR due to the addition of Ru that was not poisoned by SO2 [68], maintaining 60% mass activity even after SO2 poisoning, far better than commercial Pt/C catalyst (30%).\n\n\nGeometric (or strain) effect: The addition of second metal makes mismatching of lattice constants (lattice distortion), leading to changes in the atomic spacing or average metal-metal bond lengths, which alters the geometrical configuration of the catalyst while improving the catalytic activity [62,63]. Wu et\u00a0al. [64] confirmed that the Au\u2013Pt alloy exhibited higher catalytic performance than the monometallic catalyst owing to changes in grain size and lattice structure (lattice shrinkage in Au and lattice expansion in Pt) during the formation of Au\u2013Pt.Electronic (or ligand) effect: The addition of second metal alters the electronic configuration of the active metal site via altering the electronic environment on the metal surface or by promoting electron transfer between metals [65,66]. The d-band center theory, which assumes that the d-orbital center (cannot be too high or too low) of the metal is linearly related to the adsorption strength of the reacting species on the metal surface, is one of the significant descriptors of ORR activity. Yin et\u00a0al. [67] discovered that the NP-Ag4Cu catalyst made the d-band center of Ag closer to the Fermi energy level due to the alloying effect of Cu, thereby optimizing the electronic structure and significantly improving the ORR catalytic activity (7 times that of NP-Ag).Stabilizing effect: The addition of second metal improves the stability of the catalytically active metal. The Pt6Ru1/C catalyst exhibited good SO2 resistance in ORR due to the addition of Ru that was not poisoned by SO2 [68], maintaining 60% mass activity even after SO2 poisoning, far better than commercial Pt/C catalyst (30%).It is generally accepted that the catalytic activity and selectivity of bimetallic catalysts are mainly influenced by geometrical and electronic effects [69]. That is, the geometric and electronic properties are adjusted through geometric and electronic effects, breaking the activity-selectivity trade-off in the catalytic reaction and avoiding overshadowing efforts to optimize the performance of catalysts. Typically, the geometrical effect of bimetallic catalysts is always accompanied by an electronic effect. In situ X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), with strong correlation to DFT predictions, which can detect the electronic and geometric states of atoms in materials during catalytic reactions. Gibbons et\u00a0al. [38] employed in situ XANES and found that the white line peak (dashed line) of both components was almost identical in intensity and position (Fig.\u00a06c), indicating that the electronic structure of Ag was almost unchanged by the presence of Cu. Fourier transforms (FT) of EXAFS display no prominent movement of the Ag\u2013Ag distance (dashed line) for Ag and CuAg measured at 0.75\u00a0V vs RHE (Fig.\u00a06d), showing that the geometric state of Ag atoms in CuAg was not changing dramatically. DFT calculations predicted that the electronic structure of Ag changes only slightly in the presence of Cu (Fig.\u00a06e), while the density of states of the Cu atom in a Ag lattice was dramatically altered compared to pure Cu (Fig.\u00a06f). These results demonstrated that the highly active Cu-centered catalytic sites and the electronic effect rather than geometric effect was the main reason for the ORR activity of the CuAg catalyst exceeding the sum of Cu and Ag. In contrast, Qiu et\u00a0al. [39] figured out that the main reason for the CuPd/SiO2 catalyst prepared via the incipient-wetness impregnation method to exhibit higher activity than Cu/SiO2 was the high dispersion of Cu and Pd and the geometric effect between the bimetals, while the electronic effect had weaker effect towards catalytic performance. To sum up, it is just the mechanism of the synergistic effects of bimetallic catalysts that allows the two metals doped in a bimetallic catalyst to produce higher performance (catalytic activity, selectivity, and stability) than either of its components would produce if they were present alone.The ideal ORR catalysts are expected to have high ORR catalytic activity. Currently, electrochemical testing is mostly performed on an electrochemical workstation (Pine, USA) with a standard three-electrode system. Among them, the working electrode is always the study electrode, the reference electrode (e.g. SCE, Ag/AgCl, RHE, etc.) is mainly used to determine the potential of the working electrode, and the counter electrode is used to form a series circuit with the working electrode to pass current. The most main ORR electrochemical measurement techniques are cyclic voltammetry (CV) and linear scanning voltammetry (LSV). Apart from these, various common characterization and analytical techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmet-Teller (BET), and electrochemical impedance spectroscopy (EIS), have been employed to observe, understand and analyze the surface morphology, microstructure, crystal structure, elemental, specific surface area, and internal resistance variation including charge transfer resistance (Rct) of bimetallic catalysts, respectively. These analytical technologies will be briefly expounded in the applications section of this review.CV testing of ORR performance is usually performed using the standard static three-electrode system in O2- and N2-saturated 0.1M KOH electrolyte for one cycle scan of the oxidation and reduction processes, which is aimed at avoiding gas over-disturbance of the solution and thus ensuring the stability of the system. By CV analysis (Fig.\u00a07\na), Goswami et\u00a0al. [70] observed that the presence of oxidation peaks could only be observed clearly for the Pd3Cu/C in O2-saturated 0.1\u00a0M KOH solution, and the more positive reduction potential (\u20130.26\u00a0V) than Pt/C indicated a superior ORR activity. LSV testing can further investigate the ORR activity of the catalyst with the same principle as the CV method. It is just missing a back sweep while still needing to provide a high-speed stirring environment to reduce or eliminate the effect of factors such as diffusion layers and accelerate the mass transfer rate of O2 [71]. Normally, LSV testing is carried out in O2-saturated electrolyte (KOH, NaOH, or H2SO4) using a rotating disc electrode (RDE) or a rotating ring disc electrode (RRDE) as the working electrode. LSV curve measured by Kisand et\u00a0al. [72] displayed that FeCoNC catalyst had onset potential (Eonset) and half-wave potential (E1/2) similar to Pt/C catalyst (\u20130.048 and \u20130.18\u00a0V), confirming its good ORR activity (Fig.\u00a07b).Tafel slopes obtained by LSV curve fitting can further explore the kinetics of electrocatalytic reactions, it is generally accepted that the smaller the Tafel slope, the better the catalyst has the potential for ORR activity [73]. Anwar et\u00a0al. [74] revealed that the Tafel slope of the CuPt/NC catalyst (190\u00a0mV dec\u22121) was smaller than that of commercial Pt/C (213\u00a0mV dec\u22121), validating its superior ORR catalytic activity over the Pt/C (Fig.\u00a07c). Due to the limitations of the RDE test in not fully capturing the intermediate products [75], to further explore the ORR mechanism of the catalysts, RRDE testing can be used to more accurately evaluate the electron transfer number (n) and H2O2 yield of catalysts during the entire ORR process. Based on this, You et\u00a0al. [76] through RRDE testing showed that SrCO3/Fe3C (1:12) had a higher n value (3.94\u20133.96) and a lower H2O2 yield (less than 3%) compared to Pt/C, indicating a good selectivity for ORR (Fig.\u00a07d).Improving the activity and durability of ORR catalysts simultaneously is a challenging task, as catalysts with higher activity are usually less stable [77]. Numerous studies have shown that ORR catalysts will inevitably deactivate to some extent during long-term use, mainly in the form of a gradual decline in catalyst activity or selectivity over time and a shortened working life. There are many reasons for the deactivation of the catalysts, which can be divided into three categories: \u201cpoisoning\u201d deactivation [78], coking and blocking deactivation [79,80], sintering and thermal deactivation [81,82]. In particular, ORR catalysts applied to the single-chamber MFC are also susceptible to microbial contamination due to the lack of isolation by proton exchange membranes [83]. Therefore, the ideal ORR catalysts should not only have excellent ORR activity but also long-term stability and anti-inactivation stability, which means that the development of high-stability performance ORR electrocatalysts is imperative.The long-term stability of ORR catalysts is mostly assessed using the Cycle endurance test (CV and LSV), chronoamperometry (CA), thermogravimetric analysis (TGA), and others. Xiong et\u00a0al. [84] assessed the stability of the 3D Pd\u2013Cu catalyst and the commercial Pt/C using cyclic endurance experiments, as can be seen in Fig.\u00a08\na, the 3D Pd\u2013Cu catalyst maintained outstanding stability after the 1000-3000th CV cycle, Fig.\u00a08b showed that its E1/2 only decreased by 3\u00a0mV and 12\u00a0mV after the 3000th and 5000th LSV cycles, whereas the activity and stability of the commercial Pt/C decayed significantly (39\u00a0mV decrease in E1/2 after 3000 cycles). CA tests are to observe the decay of catalyst current density at a constant potential, which can be used to detect the stability of ORR catalysts under harmful environmental conditions. Lim et\u00a0al. [85] conducted methanol crossover tests using CA at a constant voltage of 0.6\u00a0V, the results showed that the addition of methanol at 200\u00a0s caused a sharp decrease in the current density of the commercial Pt/C catalyst, while the current density of NiCo2O4 did not change significantly and the initial current density decayed more slowly than Pt/C within 10,000\u00a0s (Fig.\u00a08c and d), which indicated that NiCo2O4 has better methanol crossover resistance and long-term stability performance. The TGA method, which measures the change of catalyst mass over time under programmed temperature control, is commonly used to characterize the thermal stability of synthetic materials, which will be briefly expounded in the applications section.The economic viability of ORR catalysts is another major challenge. As mentioned earlier, the noble-metal catalysts represented by Pt have long been the core of catalytic research due to their high ORR electrocatalytic performance. However, they are limited by high prices and scarcity of earth resources, in particular, Bernsmeier et\u00a0al. [86] showed that the cathode materials can account for more than 50% of the total cost of laboratory-scale MFC, making noble-metal catalysts economically inappropriate for large-scale wastewater treatment. The high maintenance costs also limit the large-scale use of MFC [87]. Since the primary role of MFC electricity production is for balancing energy consumption rather than generating additional economic benefits [88], the efficient and rational use of noble-metal materials or the search for non-expensive alternative catalysts has become a top priority in current ORR catalysis research.In the present work, scholars have primarily concentrated on the development of non-precious metal catalysts with new structures. Forming Pt-based alloys via the introduction of a second non-precious metal can improve the d-band center of the catalyst while increasing the cost-effectiveness and activity of the catalyst. Yan et\u00a0al. [89] revealed that the addition of relatively inexpensive Sn reduced the Pt loading while decreasing the unoccupied d-band of neighboring Pt in the PtSn catalyst, and the Pt50Sn50 catalyst with the highest Sn content results in the highest mass activity. The synthesis of bimetallic catalysts using earth-abundant or cheaper metals is also an effective strategy to improve ORR catalyst activity and decrease costs. A study by Qiao et\u00a0al. [90] discovered that low-cost Cu\u2013Fe alloy synthesized from two non-precious metals exhibited high activity due to electronic effects that caused a change in the d-band center energy of Cu. Currently, the economic feasibility analysis of most bimetallic ORR catalysts in MFC is based on the cost of production (COP) as the main indicator, including material cost and processing cost (e.g., power consumption). Considering that the price of Mn salts was slightly lower than the same of Fe, the Fe\u2013Mn\u2013N\u2013C catalyst using Mn as a second metal can slightly decrease the total cost, which was about 3.5 $ g\u22121 considering only consumables [91]. It was estimated that the cost of the Mn/Fe@WRC catalyst doped with Mn and Fe with watermelon rind as raw material is even less (about 0.15 $ g\u22121), which was much lower than Pt/C (33.0 $ g\u22121) [92]. In addition to the COP, In order to fabricate large-scale MFCs for practical application, an ideal ORR catalyst should also have cost-effectiveness associated with power generation, which can be interpreted as a maximum power density (MPD) normalized to the cost for comparison. The normalized MPD (mW $\u22121) is determined by the following equation [93]:\n\n\n\nN\no\nr\nm\na\nl\ni\nz\ne\nd\n\nM\nP\nD\n=\n\n\nM\nP\nD\n\n\nA\nO\nC\n\u00d7\nC\nO\nP\n\n\n\n\n\n\nWhere MPD is the maximum power density (mW m\u22122), AOC and COP are the amount of catalyst (g m\u22122) and cost of production ($ g\u22121), respectively. Such analysis provides useful information for decision-makers to understand which type of catalyst is more cost-effective or more economically feasible to use. The CoNi alloy synthesized by Hou et\u00a0al. [94] exhibited a superior normalized MPD (150\u00a0mW $\u22121) over the Pt/C catalyst. Similarly, the Cu\u2013Sn-2/AB catalyst prepared by Noori et\u00a0al. [95] was nearly 11 times more cost-effective (31.0\u00a0mW $\u22121) than the Pt/C catalyst, meeting the sustainability and economic requirements.According to current reports, various synthesis methods for bimetallic catalysts, among which the typical ones mainly include co-reduction, seed-mediated, impregnation, co-precipitation, electrodeposition, microemulsion, and microwave heating method (Fig.\u00a09\na\u2013d). In practice, the appropriate synthesis method should be selected according to the specific situation to achieve the desired synthesis effect. Furthermore, the properties of bimetallic catalysts can also be enhanced via the combination of different synthesis methods.Co-reduction aims to use reducing agents (e.g. NaBH4, N2H4) to simultaneously reduce the two metal salt precursors in the system to zero-valent metal atoms, then make them grow together to form the bimetallic catalyst. The alloys prepared by this method mostly have a relatively high alloying degree, such as Pd\u2013Pb [100], CuFe [101] and AgPd [102], etc. It is generally more difficult to synthesize core-shell structured alloys due to the limitations of the applicable conditions (e.g. inherent properties of the metal), which require the assistance of other synthetic methods. Tsuji et\u00a0al. [103] successfully gained the Ag@Ni catalyst by using the microemulsion-assisted polyol method to simultaneously reduce the mixture of AgNO3 and NiSO4\u00b76H2O, NiCl2\u00b76H2O or Ni(NO3)2\u00b76H2O for 10\u00a0min.Seed-mediated is used to first reduce one metal salt precursor into a metal seed and deposit it as a nucleus, then another metal atom is deposited as a shell by reduction or thermal decomposition to attach to the surface of the formed metal seed, forming a core-shell structure. Bimetallic catalysts prepared using this method generally have larger nanostructure sizes, such as core-shell Au\u2013Pt dendritic nanoparticles [104], Pd@Ir [105], etc. Nevertheless, the presence of strong reducing agents leads to too fast a reduction rate which breaks the conditions close to equilibrium, making it more difficult to synthesize high-quality bimetallic nanoparticles with a single structure. It was found that the independent nucleation of the second reduced metal could be avoided by surface modification [106] and surface replacement reactions(or galvanic replacement reactions) (Fig.\u00a09a) [96].Utilizing metallic salt solutions to impregnate support materials with porous structure, removing the remaining liquid after reaching equilibrium, and then via heat decomposition and activation to make the bimetallic nanoparticles dispersed in the pores on the surface of the carrier. Using an amount of the precursor solution over the pore volume of the support, producing a thin slurry, is called wet impregnation, limiting the amount of precursor solution to just filling the pore volume is termed incipient wetness impregnation [107]. There are also co-impregnation and sequential impregnation methods that are commonly used for the synthesis of loaded bimetallic catalysts. The teams of Nourozi Rad [108] and Naicker [109] respectively synthesized Ni\u2013Cu/TiO2 catalyst and Cu\u2013Ag/Al2O3 catalyst using co-impregnation and sequential impregnation methods. The impregnation method has the advantages of simple operation and low cost, but the lack of induced interaction between the precursor and the support during the catalyst drying process may lead to agglomeration of metal particles and inhomogeneous elemental composition [110], thereby causing low metal utilization.The simultaneous precipitation of cations from two metal salt solutions with the aid of the precipitating agents to acquire the desired bimetallic catalysts. The use of ammonium hydroxide as a precipitant avoids an additional washing step (e.g. removal of residual impurity ions) [111]. Suitable pH and temperature are also key factors in realizing excellent sedimentation results. Yang et\u00a0al. [112] used a low-temperature co-precipitation method to prepare FeMn catalyst with highly dispersed and selective. Compared with the impregnation method, the co-precipitation method can better control the particle size and distribution of elements in bimetallic catalysts, which is the most commonly used method to prepare composite metal oxides, such as NiCo2O4 (Fig.\u00a09b) [97], CoMoO4 [113], etc. Although this method has the characteristics of a simple process and low calcination temperatures, unnecessary co-precipitation of impurities and analytes can occur at each stage, thereby leading to the formation of aggregates.Bimetallic catalysts prepared by sequential electrodeposition of metal cations in a metal salt solution using an electrode system with an applied constant voltage have good dispersion and chemical stability. The teams of Liu [114] and Vega-Cartagena [115] both used electrodeposition to synthesize a PtPd/rGO catalyst with highly dispersed and an Ag/Pd catalyst with excellent stability in a relatively short time. The catalyst composition and performance can be further regulated by adjusting factors such as potential, temperature, and the type of metal salt precursor. Ajmal et\u00a0al. [116] synthesized a CuZn alloy with good selectivity by sequential electrodeposition of CuSO4\u20135H2O and ZnCl2 in the electrolyte at a constant voltage of \u22120.3\u00a0V. Xia et\u00a0al. [98] synthesized a Fe\u2013Cu alloy by electrodeposition in an electrolyte of FeSO4 and CuSO4 at 25\u00a0\u00b0C for the 20s (Fig.\u00a09c). Although this is a green, simple and fast method, its preparation cost and conditions are relatively high and harsh.Under the action of surface and auxiliary active agents (e.g. alcohols), immiscible liquids (e.g. oil and water) are first mixed and emulsified to form thermodynamically stable microemulsion systems, then metal salt precursors react in the microbubbles to obtain bimetallic catalysts. Although the preparation is complex and tedious, the particle size of the catalysts can be well adjusted via micelle modification and varying the size of microbubbles, the concentration of reactants, and the pH value in the aqueous phase. Sheoran et\u00a0al. [117] synthesized a series of spinel catalysts (MFe2O4; M\u00a0=\u00a0Co, Ni, Cu, and Zn) with small particle sizes, large specific surface areas, and sufficient magnetism by the microemulsion method. Currently, the water-in-oil (W/O) method is mostly used to prepare uniformly distributed bimetallic catalysts, Szumelda et\u00a0al. [99] synthesized a series of alloy catalysts such as PdAu, PdPt, PdRu, and PdIr using the W/O microemulsion method (Fig.\u00a09d), where the PdRu and PdIr systems exhibited almost uniform alloy microstructures.Microwave heating is a method that allows metal salt precursors to form bimetallic catalysts under microwave radiation. Compared to conventional heating methods, this method has milder synthesis conditions (uniform heating and fast reaction), which can not only accelerate the synthesis of particles but also obtain more dispersed catalysts. Galhardo et\u00a0al. [118] successfully synthesized one highly active PtNi catalyst uniformly distributed on a carrier by reacting for 6\u00a0min under microwave radiation at 160\u00a0\u00b0C, reducing the time by 94% compared to the traditional heating methods. In addition, it is a generally feasible method to combine it with other methods, Lingaiah et\u00a0al. [119] synthesized a series of SiO2-supported Pd\u2013Fe catalysts by combining the impregnation method respectively with the conventional heating method (calcination for 5\u00a0h) and microwave synthesis (irradiation at 100% power for 5\u00a0min). However, this method cannot effectively regulate the structure of bimetallic nanoparticles.By classifying the currently applicable bimetallic catalysts for MFC, the key contributions of six bimetallic catalysts to MFC (excellent ORR activity, high stability and economic efficiency) and their application towards energy-efficient wastewater treatment in MFC are detailed in sequential order. In general, the values of Eonset, E1/2, n, Rct, BET surface area (SBET), and open-circuit voltage (OCV) are regarded as key indicators to evaluate the ORR performance of bimetallic catalysts suitable for MFC (Table 1\n). In the process of realizing wastewater treatment and energy conversion, MFC mainly uses synthetic wastewater, sludge, high-concentration organic wastewater, and general persistent or stubborn pollutants as substrates. The effectiveness of MFC configured with bimetallic catalysts for wastewater treatment is usually characterized by chemical oxygen demand (COD) removal and pollutant degradation rates. To explore the power generation of MFC under the action of different bimetallic catalysts, the maximum stable output voltage and the MPD are mainly used as evaluation indexes, where the MPD is further divided into area power density and volume power density.Alloying of Pt with relatively inexpensive 3d group transition metals (M\u00a0=\u00a0Fe, Co, Ni, etc.) to form Pt-M alloy catalysts can reduce the amount of Pt while significantly enhancing the ORR catalytic activity. This is mainly attributed to the geometrical effect [155], the electronic effect [156], the Raney effect (increasing the effective active area of Pt) [157], and the anchoring effect (allowing Pt to be better embedded in the carrier) [158] of Pt-M alloys, enabling the modulation of Pt catalytic activity.Graphene (G) with high surface area, good electrical conductivity, thermal stability, and durability [159], can be used as excellent support for electrocatalysts. Yan et\u00a0al. [120] performed XRD analysis of the Pt\u2013Co/G (15\u00a0wt% Pt) catalyst and revealed that its reflection peaks were shifted to higher angles than Pt/C, indicating that the addition of the Co causes the lattice contraction of Pt to lower its d-band center relative to the Fermi energy level, which was favorable to the ORR. CV testing demonstrated that the higher Eonset of Pt\u2013OH generation on the surface of the Pt\u2013Co/G catalyst compared to the Pt/C catalyst may be related to the enhanced ORR activity. The OCV (0.71\u00a0V) of the MFC configured with Pt\u2013Co/G was similar to that of Pt/C (0.77\u00a0\u00b1\u00a00.01\u00a0V), indicating a comparable ORR rate, as well as obtained MPD (1378\u00a0mW\u00a0m\u22122) is quite close to that of the Pt/C catalyst (1406\u00a0mW\u00a0m\u22122). However, the long-term stability of Pt\u2013Co/G catalysts is not as good as that of commercial Pt/C catalysts, and the maximum output voltage after the cyclic operation is lower than that of Pt/C catalysts. Due to low Pt loading, The total cost of Pt\u2013Co/G was about three-quarters of that of the Pt/C catalyst, which can be used as low-cost cathode material for MFC. Studies have shown that the ORR activity sequence of the Pt-M/C catalyst was: Pt\u2013Fe/C\u00a0>\u00a0Pt\u2013Co/C\u00a0>\u00a0Pt\u2013Ni/C\u00a0>\u00a0Pt/C [160]. Zhang et\u00a0al. [121] synthesized the C/Pt\u2013Fe catalyst with only 0.5\u00a0mg\u00a0cm\u22122 loading amount of Pt, which was slightly higher than the optimum loading amount (0.1\u00a0mg\u00a0cm\u22122) [161], effectively reducing the cost. CV experiments indicated that the C/Pt\u2013Fe alloy produced the largest current density value (\u20131.53\u00a0mA\u00a0cm\u22122) compared to Pt and Fe, indicating its highest catalytic activity for ORR. It could be attributed to the presence of Fe reduced the Pt\u2013Pt bond length and increased the electron density in the 5d orbital holes. The MPD of 1148.8\u00a0mW\u00a0m\u22122 produced from the MFC equipped with C/Pt\u2013Fe was 6.5% higher than that of C/Pt (1078.2\u00a0mW\u00a0m\u22122), which makes it potentially feasible in terms of power generation and economics to be an alternative catalyst to Pt.Since Stamenkovic et\u00a0al. [162] found that the activity of Pt3Ni (111) alloy was 90 times that of Pt/C, then a boom in research on Pt\u2013Ni alloy catalysts have been set off. Yan et\u00a0al. [122] reported a Pt\u2013Ni/MWNT (atomic ratio, Pt: Ni\u00a0=\u00a01:1,15\u00a0wt% Pt) catalyst with multi-walled carbon nanotubes (MWNT) as a support, where Pt\u2013Ni particles were uniformly distributed on the MWNT as small particles (anchoring effect). The OCV of the MFC configured with Pt\u2013Ni/MWNT was 0.74\u00a0\u00b1\u00a00.01\u00a0V, which was similar to that of Pt/C (0.76\u00a0\u00b1\u00a00.01\u00a0V), indicating that the ORR rate of the Pt/Ni/MWNT catalyst was comparable to that of the Pt/C catalyst. In MFC, the Pt\u2013Ni/MWNT catalyst obtained an MPD (1.22\u00a0W\u00a0m\u22122) similar to that of Pt/C (1.40\u00a0W\u00a0m\u22122), but its price was about three-fourths of the Pt/C catalyst, effectively reducing costs. The maximum output voltage of Pt\u2013Ni/MWNT was only 10\u201320\u00a0mV lower than Pt/C during the whole operation cycle of the MFC, and the maximum voltage of each cycle was stable around 0.57\u00a0V and 0.585\u00a0V. With the contributions of excellent ORR activity, low cost, and high power, the Pt\u2013Ni/MWNT presented a highly promising air-cathode catalyst for MFC. For understanding the effect of the Pt/Ni ratio on the ORR activity of Pt\u2013Ni catalysts, Wang et\u00a0al. [123] synthesized three highly dispersed alloy catalysts with different Pt:Ni atomic ratios on carbon support. All of the Pt\u2013Ni alloy catalysts with narrow particle size distribution and well-dispersed showed a single-phase face-centered cubic structure, which was beneficial for enhancing the ORR activity. CV experiments concluded that Pt2\u2013Ni/C catalysts possessed the largest electrochemically surface area (ECSA) of 81\u00a0m2\u00a0g\u22121, thus may feature favorable ORR catalytic activity, which is known as the Raney effect. LSV measurements showed that the Pt2\u2013Ni/C catalyst exhibited the most positive Eonset (0.630\u00a0V) and E1/2 (0.51\u00a0V), it can be seen that the Pt2\u2013Ni/C catalyst exhibited the highest ORR activity. Consequently, the MFC modified with Pt2\u2013Ni/C produced 22% higher MPD (1724\u00a0mW\u00a0m\u22122) than the commercial Pt/C (1413\u00a0mW\u00a0m\u22122) and obtained the maximum OCV (0.78\u00a0V). To generate electricity directly from dairy wastewater, Cetinkaya et\u00a0al. [124] exploited CMI7000 membranes coated with Pt\u2013Ni alloy as the cathode of MFC. The results showed that the addition of Ni as an alloying element in a certain proportion can reduce the particle size and content of noble metal in Pt\u2013Ni alloy (reduce the cost) without losing the active surface area. When the Ni content was increased to Pt:Ni\u00a0=\u00a01:1, the inherent catalytic activity of the Pt:Ni(1:1) catalyst reached its maximum, reducing oxygen more easily than pure Pt. The MPD (637\u00a0mW\u00a0m\u22122) produced by the MFC with Pt:Ni(1:1) as the cathode was much higher than that of pure Pt (180\u00a0mW\u00a0m\u22122), and 85% COD removal was obtained after the MFC power output was stabilized. Overall, the Pt:Ni(1:1) catalyst can be used as an alternative catalyst for Pt in MFC applications.Transition metal alloys composed of two relatively inexpensive or nonprecious transition metals compared to Pt have the advantage of being easily available and synthesized, which solves to a certain extent the problem of still too high Pt burden in most Pt-M catalysts, thus serving as an efficient and low-cost cathode catalyst for MFC. Relatively inexpensive transition metal alloys can provide a viable method for wastewater pretreatment, W\u0142odarczyk et\u00a0al. [125] applied Ni\u2013Co (85% Ni, 15% Co) to MFC for the treatment of highly concentrated pollutant wastewater from a yeast plant, requiring only an additional oxygen supply from the air, the value of which was 27 times lower than that of aeration reactor. The MFC configured with Ni\u2013Co (85% Ni, 15% Co) alloy reduced COD by 90% within 20 days and achieved a power output of 6.1\u00a0mW. Although the electricity output was not significant, it could offer extra electrical energy to the subsequent wastewater treatment process equipment. To further investigate the electricity production performance of MFC configured with different cathodes. Thereafter, W\u0142odarczyk et\u00a0al. [126] utilized MFC modified with Ni\u2013Co alloy to treat wastewater from municipal wastewater treatment plants. The alloy containing 15% cobalt exhibited higher catalytic activity, after 8\u00a0h of oxidation and three times of anodic loading, showing the most favorable electrode voltage. The MFC with Ni\u2013Co (85% Ni, 15% Co) as a cathode catalyst obtained a slightly higher maximum power (7.19\u00a0mW) than the carbon cloth cathode (1.56\u00a0mW), and the COD reduction time to an assumed 90% was also shortened by 3 days compared to the carbon cloth electrode.By TGA testing, Papiya et\u00a0al. [127] found that the Ni\u2013Co/MGO catalyst had the lowest total mass loss (20%) when the temperature reached 600\u00a0\u00b0C (Fig.\u00a010\nb), demonstrating its good thermal stability. The MPD obtained (1003.18\u00a0mW\u00a0m\u22122)\u00a0by applying the Ni\u2013Co/MGO catalyst to the MFC was much stronger than the Pt/C catalyst (483.48\u00a0mW\u00a0m\u22122). Using conductive polymers as support carriers is a potential solution for improving chemical stability and catalytic activity [163,164], such as polyaniline (PANI), polypyrrole (Ppy), polythiophene (PTH), etc. Nguyen et\u00a0al. [128] gained the Ni\u2013Co(1:1)/SPAni catalyst using sulfonated polyaniline (SPAni) as the carrier, where catalyst particles were well-dispersed and distributed on SPAni without any agglomeration and sintering, increased the active site in favor of ORR. In single-chamber MFC equipped with Ni\u2013Co(1:1)/SPAni, MPD acquired (\u223c659.79\u00a0mW\u00a0m\u22122) was greater than Pt/C (\u223c483.48\u00a0mW\u00a0m\u22122), with the COD removal rate is up to 91.5%.Combining carbon-based materials with conducting polymers as cathode catalysts is an effective means of increasing ORR rates [165]. Papiya et\u00a0al. [129] synthesized another supported catalyst (Mn\u2013Co/SGO-PAni) by combining sulfonated graphene oxide (SGO) with PAni, XPS analysis revealed the presence of Mn, Co, C, N, and S elements, indicating that Mn\u2013Co/SGO-PAni was successfully prepared (Fig.\u00a010c), and the MFC containing Mn\u2013Co/SGO-PAni exhibited larger MPD (1392.68\u00a0mW\u00a0m\u22122) than Pt/C (481.3\u00a0mW\u00a0m\u22122). The factors affecting the ORR catalytic activity of alloys are not only related to the type and content of the transition metal, but also related to its surface stacking mode (the morphology of alloys) [166]. Yang et\u00a0al. [130] utilized RDE testing to find that the n value of core-shell Au\u2013Pd was similar to that of hollow Pt nanoparticles, the semicircle diameter in the EIS (Nyquist plots) also demonstrated that the Au\u2013Pd catalyst features a far lower Rct value than the hollow Pt nanoparticles (Fig.\u00a010d), MFC with Au\u2013Pd as cathode produced the MPD of 16.0\u00a0W\u00a0m\u22123, which synthetically confirmed its excellent ORR catalytic activity.Transition non-noble metal alloys (e.g. CuZn) have been successfully adapted to fuel cells due to their good catalytic properties [167]. Das et\u00a0al. [131] employed carbon-loaded CuZn to field-scale MFC (septic tank slurry as substrate), obtaining an MPD (0.32\u00a0mW\u00a0m\u22122) 64 times higher than without the cathode catalyst, with 68\u00a0\u00b1\u00a08% removal of COD, the 1000 cycles of CV showed that CuZn has particularly strong electrochemical stability. The positive effect of cathodic catalysts containing M-N-C structures towards ORR has been reported [168]. The Fe\u2013Mn\u2013N\u2013C catalyst prepared by Kodali et\u00a0al. [91] using the sacrificial support method with higher Eonset (0.28\u00a0V) and more positive E1/2 (0.10\u00a0V) than the Fe\u2013N\u2013C catalyst, as well as the MPD (228.4\u00a0\u03bcW\u00a0cm\u22122) obtained by Fe\u2013Mn\u2013N\u2013C as the MFC cathode was also higher than that of the Fe\u2013N\u2013C catalyst (196.4\u00a0\u03bcW\u00a0cm\u22122), indicating that the Fe\u2013Mn\u2013N\u2013C catalyst had an excellent ORR catalytic performance. The simpler preparation of Fe\u2013Mn catalysts is beneficial for practical applications, Guo et\u00a0al. [132] synthesized the FeMn2 catalyst via a simple hydrothermal method applied to MFC, obtained a higher MPD (1971\u00a0mW\u00a0m\u22122) than both FeMn (1820\u00a0mW\u00a0m\u22122) and FeMn4 (1580\u00a0mW\u00a0m\u22122), the possible ORR mechanism for the Fe\u2013Mn catalyst was shown in Fig.\u00a011\n.Transition metal oxides are characterized by high specific capacity, low cost, and commercial suitability [169], which can improve the ORR catalytic performance of catalysts by facilitating oxygen adsorption and O=O bond dissociation during the ORR process. It can be classified by their metal element composition into unitary transition metal oxides (e.g. manganese series oxides (MnOx)) and transition composite metal oxides (oxides in which two or more metals co-exist). Transition composite metal oxides exhibit better electrical conductivity and catalytic properties than monometallic oxides due to the synergistic effect between multiple metal species [170,171]. In recent years, using transition metal composite oxides to replace conventional Pt/C has become a research hotspot.Since most transition composite metal oxides are semiconductors, combining them with highly conductive carriers (e.g. carbon support) can effectively improve conductivity and catalytic activity [172]. Graphene oxide (GO) is produced by strong oxidation of G, and the functional groups generated on the surface confer its good electrical conductivity and catalytic activity [173]. The FeCoO/GO catalyst obtained more catalytic activity sites while reducing agglomeration due to the addition of GO, thereby exhibiting outstanding ORR electrochemical activity [133]. In the MFC-Fenton system, the gained MPD (461.2\u00a0mW\u00a0m\u22122) was 4.5 times higher than that of carbon felt (102.5\u00a0mW\u00a0m\u22122) and the removal rate of 80.34% for 20\u00a0mg/L oxytetracycline (OTC), indicating that the application of FeCoO/GO to MFC-Fenton was a feasible solution for the deep removal of antibiotic contaminants.Perovskite-type metal composite oxides, in general formulation of ABO3, with high conductivity, thermal stability, and ORR electrocatalytic activity. Compared with the GO/FeMnO3 catalyst, the C/FeMnO3 catalyst was homogeneously dispersed on the carbon support, with higher porosity and surface area, significantly accelerating ORR, electrochemical characterization also showed that it had a lower Rct value (69.8\u00a0\u03a9) [134]. Treat it as a single-chamber MFC cathode, a higher MPD (475\u00a0mW\u00a0m\u22122) than a Pt-based catalyst (461\u00a0mW\u00a0m\u22122) was acquired. The C/FeMnO3 catalyst maintained good temperature cycling resistance in the long-term temperature cycling mode of operation, indicating that it was a promising ORR catalyst with high-temperature resistance. Individual perovskite metal composite oxides are widely used as photocatalysts due to their photocatalytic features of semiconductors, Ahmadpour et\u00a0al. [135] confirmed that a two-chamber MFC with NiTiO3 as the cathode obtained twice as much as the MPD (76.86\u00a0mW\u00a0m\u22122) under visible illumination than under dark conditions, which was attributed to the increase of n value caused by the decrease of Rct value from 38.67\u00a0\u03a9 to 31.42\u00a0\u03a9 under light conditions. This result showed that NiTiO3 is an excellent photocatalyst in MFC-coupled photocatalytic systems.Spinel metal composite oxides with the general formula AB2O4 have the same ORR catalytic activity as ABO3, which is another promising class of ORR catalysts. Liu et\u00a0al. [136] found that the hexagonal shape of the CoGa2O4 catalyst exposed more active sites (Fig.\u00a012\na and b), which was conducive to O2 adsorption and the ORR 4e\u2212 pathway, leading to a superior catalytic effect. The high SBET (207\u00a0m2\u00a0g\u22121) and large pore size (46\u00a0nm) of the CoGa2O4 catalyst increased the active sites while promoting the diffusion of O2 and electron transfer. RDE testing demonstrated that the CoGa2O4 catalyst had an n value close to 4 (3.87), which was 48% higher than activated carbon (AC) (2.58). MFC equipped with CoGa2O4 modified AC produced 80% higher MPD (1960\u00a0mW\u00a0m\u22122) than reported Pt/C. Its total cost was also far lower than that of Pt/C. Nano spinel rod-like CoFe2O4 doped in AC (AC-CoFe2O4) was successfully used as an air cathode catalyst for MFC by Ren and co-workers, as well as suitable O2 adsorption energy and the direct 4-electron ORR pathway on CoFe2O4 was verified by DFT calculations [137]. In addition, the AC-CoFe2O4 catalyst also exhibited a much higher n value (3.85) than the NiCo2O4/AC catalyst (3.72) [138], indicating its good ORR kinetic activity, and the MPD obtained for MFC configured with 10% CoFe2O4 reached as high as 1800\u00a0mW\u00a0m\u22122, which was 1.99 times higher than that of bare AC. Huang et\u00a0al. [139] generated a CoFe2O4@N-AC catalyst in situ on nitrogen-doped activated carbon (N-AC), which showed good ORR electrocatalytic activity due to the synergistic effect between CoFe2O4 and N-AC, furthermore, the obtained MPD by MFC modified with the CoFe2O4@N-AC catalyst reached as high as 1785.8\u00a0mW\u00a0m\u22122, which was 2.39 times higher than that with a bare electrode.Reduced graphene oxide (rGO) possess large surface area and abundant surface oxygen-containing functional groups, utilizing it as one support carrier for catalysts not only can provide massive adsorption sites, but also leads to stable anchoring of the active metal, thus inhibiting its agglomeration [174,175]. The CoMn2O4/rGO-8 catalyst prepared by Hu et\u00a0al. [140] exhibited better ORR electrochemical activity than the CoMn2O4 catalyst, which was attributed to the uniformly embedding of CoMn2O4 nanoparticles on the rGO surface (Fig.\u00a012c), yielding 2.6 times larger SBET (78.4\u00a0m2\u00a0g\u22121) with pore volume of about 0.19\u00a0cm3\u00a0g\u22121, providing more abundant oxygen adsorption active sites and facilitating the mass transfer of reactants and reaction products in ORR. The CoMn2O4/rGO-8 catalyst in RDE test yielded the optimum Eonset (0.69\u00a0V) and current density (1.57\u00a0mA\u00a0cm\u22122) than the CoMn2O4 catalyst (0.64\u00a0V and 1.40\u00a0mA\u00a0cm\u22122). Loading CoMn2O4/rGO-8 onto modified graphite felt (GF) formed the GF-CoMn2O4/rGO catalyst that well bonded to the graphite fibers (Fig.\u00a012d). The MFC with modified the GF-CoMn2O4/rGO catalyst obtained significantly higher MPD (361\u00a0mW\u00a0m\u22122) than other materials. To obtain electrical energy while effectively decomposing refractory pollutants in a photocatalytic assisted MFC coupled system, Li et\u00a0al. [141] certificated that immobilization of the CoFe2O4-rGO photocatalyst on photocatalytic composite membranes (PCM) for assisted application in MFC/MBR systems can accelerate the cathodic ORR rate, producing an MPD (942\u00a0mW\u00a0m\u22123) higher than the light-free mode (871\u00a0mW\u00a0m\u22123) under natural light irradiation, with a degradation rate of up to 95% for 50\u00a0mg/L tetracycline hydrochloride (TH). These achievements indicated the potential for better and wider application of photocatalyst-assisted MFC-MBR systems in wastewater treatment.N4-transition metal macrocyclic compounds (e.g. porphyrins, phthalocyanines, etc.) are favored owing to their ORR catalytic activity comparable to that of Pt and lower cost than metal oxides. Noteworthily, the ORR catalytic activity depends mainly on the transition metal centers in the macrocyclic N4 [176], especially Fe (mainly occurs 4e\u2212 ORR) and Co (mainly occurs 2e\u2212 ORR) [177]. Thus, iron phthalocyanines (FePc) [178], cobalt phthalocyanines (CoPc) [179], iron porphyrins (FePP) [180], and cobalt porphyrins (CoPP) [181] became the most studied N4-transition metal macrocyclic compounds ORR catalysts.Carbon nanotubes (CNT), with high specific surface area, excellent mechanical strength, high thermal conductivity, and electrical conductivity [182], possess high application prospects in the aspect of acting as carbon carriers for catalysts. Using N4-transition metal macrocyclic compounds as precursors for the preparation of bimetallic catalysts is a feasible approach. Deng et\u00a0al. [142] discovered that the Co/Fe/N/CNT catalyst prepared by high-temperature pyrolysis of the CoTMPP/FePc functionalized CNT precursors mainly supported the ORR 4e\u2212 pathway. The CoTMPP and FePc attach to the CNT surface via \u03c0-stacking interactions, which facilitated the increase of catalytically active surface, while the presence of CNT also promoted electron transfer (3.75\u20133.81) and reduced the ORR overpotential. The OCV of the MFC with Co/Fe/N/CNT as cathode catalyst was 0.76\u00a0V, and the obtained MPD (751\u00a0mW\u00a0m\u22122) was higher than that of Pt/C (498\u00a0mW\u00a0m\u22122) and Co/Fe/N/graphite (618\u00a0mW\u00a0m\u22122), with no significant change in output power after 2 months. These results indicate that Co/Fe/N/CNT was a suitable material for the preparation of MFC.In 2019, Noori et\u00a0al. [143] successfully synthesized the Co-FePc/carbide-derived carbon (CDC) catalyst by introducing FePc into Co/CDC, TEM analysis revealed that FePc extensively covered the surface of Co/CDC (Fig.\u00a013\na), the imperfect molecular arrangement of Co-FePc (Fig.\u00a013b) indicated the stacking of different materials on CDC, confirming the successful doping of Co-FePc in CDC. Based on the analysis of the N2 physisorption technique, the Co-FePc/CDC had the most suitable pore volume (0.159\u00a0cm3\u00a0g\u22121) and SBET (212\u00a0m2\u00a0g\u22121) to effectively promote the electron transfer to the catalyst layer, indicating the kinetic propensity of the catalyst for ORR. Due to the synergistic effect of Co and FePc in CDC, the Co-FePc/CDC catalyst could improve the ORR catalytic activity through the 4e\u2212 ORR pathway. Electrochemical tests confirmed that Co-FePc/CDC possessed a significant oxygen reduction peak and a more positive Eonset. When it was applied to MFC (external resistance of 500\u00a0\u03a9) containing acetate-based synthetic wastewater, the maximum MPD and OCV obtained were 1.57\u00a0W\u00a0m\u22122 and 741\u00a0mV, respectively (Fig.\u00a012c), and the COD removal rate was 86%. CA tests showed that The current density response of Co-FePc/CDC at each applied voltage was almost constant for 20,000\u00a0s. The RDE test also demonstrated that the diffusion current density of Co-FePc/CDC remained almost constant until 250 cycles and varied slightly after 500 cycles, indicating the high stability of the catalyst.Metal-organic framework (MOF) is one zeolite-like material with a three-dimensional microporous network structure formed by the self-assembly of a metal source (inorganic metal ions or metal clusters) and a carbon source (organic ligands) through coordination bonds [183]. Compared with conventional porous materials, it has a greater specific surface area, a more developed and regular pore structure, and superior chemical and thermal stability [184]. Currently, MOF is widely used in catalytic reactions [185], sensing [186], biomedicine [187], and other fields, which are favored by many research scholars. MOF-based catalysts prepared with MOF as the base or sacrificial templates are prone to defective oxygen vacancies (exposing more active sites) during calcination, thus having the potential to promote ORR.N-doped MOF has been successfully applied in MFC to improve catalytic activity [188]. In 2019, Using the Cu/Co/N\u2013C#2 catalyst with hexagonal hollow structures that were synthesized via Cu and N co-doped with ZIF-67 (Co) as the cathode of MFC, which was a feasible approach to improve the catalytic performance of ORR, yielding an MPD of 1008\u00a0mW\u00a0m\u22122 and a maximum stable output voltage of 677\u00a0mV, which are 1.25 and 1.31 times higher than the 20% Pt/C catalysts [144]. The analysis revealed that outstanding performance for Cu/Co/N\u2013C#2 was mainly attributed to the high SBET (286\u00a0m2\u00a0g\u22121) and pore size (approximately 8\u00a0nm) of both larger than Co\u2013N\u2013C, which increased the effective catalytic active site and facilitated electron transfer and mass transfer. The successful doping of Cu and Co elements, especially the presence of Cu ions increases the N content through Cu\u2013N, also made an important contribution to the improvement of ORR catalytic activity. The values of the Eonset (0.25\u00a0V) and E1/2 (0.14\u00a0V) of the Cu/Co/N-C#2 catalyst are better than those of Co/N\u2013C (0.02\u00a0V and 0.24\u00a0V). CA tests revealed that with the addition of methanol, the current drops of Cu/Co/N\u2013C#2 and Pt/C were 2.6% and 6.8%, confirming that Cu/Co/N\u2013C#2 had better resistance to methanol neutrality. In addition, the current density of Cu/Co/N\u2013C#2 decreased by only about 15.4% after 8000\u00a0s, with better long-term stability than Pt/C (24.9%). In 2020, the Mn\u2013Fe@g-C3N4 catalyst was fabricated via pyrolyzing Mn-doped g-C3N4 assisted Fe-based MOFs (MIL-101), which was an outstanding air-cathode in MFC with an MPD (420\u00a0mW\u00a0m\u22122) and maximum stable output voltage (0.450\u00a0V), superior to 20\u00a0wt% Pt/C (333.9\u00a0mW\u00a0m\u22122 and 0.422\u00a0V) [145]. At high temperatures, Mn ions and N atoms can interact with the carbon layer to optimize the SBET (268.6\u00a0m2\u00a0g\u22121) and pore volume (0.119\u00a0cm3\u00a0g\u22121) of the Mn\u2013Fe@g-C3N4 catalyst with a graded porous structure, accelerating O2 transport and internal proton and external electron transfer, as well as exposing more potential catalytically active sites. The Mn\u2013Fe@g-C3N4 catalyst exhibited superior Eonset (0.393\u00a0V) and E1/2 (\u22120.042\u00a0V) over the state-of-the-art Pt/C catalysts (0.343\u00a0V and \u22120.067\u00a0V). The results indicated that its excellent ORR catalytic activity was mainly attributed to the 3D interconnected porous structure, the high conductivity framework, and the synergistic effect of the N ions with the metal ion centers. Based on CA tests, the current density of the Mn\u2013Fe@g-C3N4 catalyst decayed to 90.1% after 30,000\u00a0s with long-term durability, while the Pt/C catalyst maintained only about 59.4% of the original activity. With the addition of methanol at 1000\u00a0s, the Mn\u2013Fe@g-C3N4 catalyst performed well tolerated, no significant change in current density. In the same year, Xue et\u00a0al. [146] discovered that FeCo nanoparticles were completely dispersed in the carbon matrix without significant aggregation in FeCoNC-900 catalysts prepared with ZIF-67 (Co) as a precursor, and the metal-bound state nitrogen (M\u2212N) and pyridine state nitrogen in FeCoNC-900 promoted plasmid transport. Although the increase of the pyrolysis temperature led to the transformation of the catalyst micro-to mesoporous structure, the FeCoNC-900 with the presence of a large number of mesopores still had a relatively high SBET (864.13\u00a0m2\u00a0g\u22121) and pore volume (2.83\u00a0cm3\u00a0g\u22121), favoring the increase of ORR active sites and promoting electron transfer, exhibiting a significantly higher 4-electron transfer number than the other samples. The MPD (1769.95\u00a0mW\u00a0m\u22122) obtained by the FeCoNC-900 in MFC was superior to Pt/C (1410.31\u00a0mW\u00a0m\u22122) and exhibited superior durability and stability after 4 months of continuous operation with no significant change in MPD.Based on the synergistic effect between bimetallic components and the features of stability and functionalization of MOF, bimetallic MOF can be used as precursors for the preparation of efficient catalysts [189]. In 2020, Wang et\u00a0al. [147] obtained Cu/Co/N-HS-3 catalyst with uniform hollow structures by pyrolysis of polystyrene@Cu/CoZIFs composite precursors, which were free from aggregation and structural collapse and existed micropores on the surface. N2 adsorption isotherm characterization illustrated that the Cu/Co/N-HS-3 catalysts had a large SBET (708\u00a0m2\u00a0g\u22121) and a suitable pore structure, which is favorable for active site exposure and proton transport. The pyridine-N, graphite-N, Co\u2013N and Cu\u2013N are considered as effective ORR active sites. The Eonset (0.25\u00a0V) and E1/2 (0.13\u00a0V) values of Cu/Co/N-HS-3 were greater than 20% Pt/C (0.24 and 0.12\u00a0V). The MPD (1016\u00a0mW\u00a0m\u22122) gained also better than commercial Pt/C (908\u00a0mW\u00a0m\u22122) when applied to MFC. When methanol was added, the ORR activity of Cu/Co/N-HS-3 catalyst and Pt/C decreased by 7.5% and 18.4%, while Cu/Co/N-HS-3 maintained 84.5% electrocatalytic ORR activity after 10,000\u00a0s, confirming its superiority over Pt/C catalyst in terms of methanol poisoning resistance and long-term stability. In 2021, Li et\u00a0al. [148] used the Fe\u2013Co\u2013C/N catalyst synthesized with ZIF-L&FeTPP@ZIF-8 as precursors as an air-cathode catalyst, and exhibited a degradation efficiency of 61.64% toward 6\u00a0mg/L sulfamethoxazole (SMX) within two days and obtained an MPD of 219.45\u00a0mW\u00a0m\u22122. RDE tests revealed that Fe\u2013Co\u2013C/N had an excellent electrocatalytic activity with n values ranging from 4.01 to 4.24, indicating an ideal 4-electron pathway for its ORR process, mainly attributed to the high content of pyridine-N (62.5%) and Co (63.7%), which facilitated electron transfer and enhanced ORR catalytic activity. In the long-term stability test, although the output voltage of the MFC decayed with increasing SMX concentration, it remained stable overall (nearly 1400\u00a0h), indicating that the Fe\u2013Co\u2013C/N is a feasible cathode catalyst for antibiotic wastewater treatment of MFC.Research demonstrated that heteroatom doping and oxygen vacancy formation (at high temperatures) can lead to a reduction in active sites [190], so catalysts with higher ORR activity and stability need to be developed. In 2020, Yan et\u00a0al. [149] revealed that direct contact of FeCoS(MOF) with the substrate material (foam Ni) produced abundant oxygen defects (increased active sites) and reduced interfacial resistance (accelerated electron transfer). The layered porous FeCoS(MOF) present a multi-energy microspheres (average diameter of 1\u00a0\u03bcm) morphology, which facilitated the exposure of active sites. The microporous structure of the surface facilitated electron transfer and mass transfer, where N doping significantly improves the oxygen adsorption and conductivity, exhibiting excellent ORR activity. The FeCoS(MOF) exhibited the highest E1/2 (\u22120.208\u00a0V), which was comparable to that of Pt/C (\u22120.124\u00a0V). Poisoning tests with the addition of S2\u2212 and SCN\u2212 revealed that the FeCoS(MOF) catalyst maintained a stable current output with an insignificant current drop in both cases (15% and 12%), which was superior to that of Pt/C catalysts (more than 30%). The MPD of the FeCoS(MOF) catalyst-based MFC was enhanced to 1008\u00a0mW\u00a0m\u22122, which was 2.55 times higher than Pt/C. After 2 months of MFC operation, the output voltage of FeCoS(MOF) remained steady, while that of the Pt/C catalyst dropped to 67% of the initial voltage, revealing superior toxicity resistance and stability than that of the Pt/C cathode. The mechanical strength of the MFC with FeCoS(MOF) as the cathode decreases significantly after a long subsequent reaction, in 2022, Yan et\u00a0al. [150] also implanted B-doped graphene quantum dots (BGQDs) into FeCoMOF to obtain the BGQDs/MOF-15 catalyst with flower-like morphology (Fig.\u00a014\na), which facilitated fast electron transfer and mass transfer. The BGQDs implantation provided an efficient charge transfer rate and abundant edge active centers, which were potential active sites for ORR. Based on electrochemical tests, the value of the Eonset of BGQDs/MOF-15 (0.014\u00a0V) was significantly higher than that of other catalysts, which was even close to that of Pt catalyst (0.04\u00a0V). The MFC with BGQDs/MOF-15 catalyst as cathode produced 1.53 times more MPD (704.24\u00a0mW\u00a0m\u22122) than Pt/C (460.29\u00a0mW\u00a0m\u22122). After 50\u00a0h of operation, the current of the BGQDs/MOF-15 was maintained at 91.2%, much higher than that of Pt/C (57.5%) (Fig.\u00a014b). After 800\u00a0h, the maximum voltage output of BGQDs/MOF-15 was maintained at approximately 0.6\u00a0V compared to 0.51\u00a0V for the Pt/C electrode (Fig.\u00a014c), demonstrating the long stability of the BGQDs/MOF-15 catalyst in the MFC.During the long-term operation of MFC, microorganisms attached to the cathode and the harmful substances produced by their secretions can block the active site and lead to catalyst \u201cpoisoning\u201d deactivation, thus increasing the diffusion resistance and internal resistance of the MFC system and impeding the ORR process [191]. Combining ORR catalysts with antimicrobial components and developing bifunctional catalysts (increasing ORR activity while inhibiting microbial contamination) is an effective way to improve MFC output power and long-term stability. It has been demonstrated that Ag and Cu, two heavy metal elements, have certain antibacterial activity, reducing efficiently the adhesion and viability of microorganisms [192,193], which are the most widely used antibacterial active metal materials.Based on the sterilization performance of Ag NPs and the principle that alloying with any element does not lose its bactericidal properties [194], In 2018, Noori's team reported a C\u2013Ag3\u2013Pt catalyst with a high ORR current response (5.2\u00a0mA) and positive reduction potential (\u22120.06\u00a0V) than the C\u2013Pt catalyst, which can reduce the activation energy barrier of ORR due to the striking molecular arrangement and d-band electron sharing ability. The MFC decorated with C\u2013Ag3\u2013Pt obtained MPD up to 1030\u00a0mW\u00a0m\u22122, which was superior to C\u2013Pt (963\u00a0mW\u00a0m\u22122) and carbon catalysts (111\u00a0mW\u00a0m\u22122) [151]. After 40 days of MFC operation, no obvious signs of cracks and biological contamination were observed on the cathode (C\u2013Ag3\u2013Pt) or membrane surface, and no significant changes in output voltage. This high activity and bacterial inhibition were attributed to the synergistic effect between the Pt and Ag components and the poisoning of microorganisms via Ag NPs. The cost-benefit assessment analysis showed that the C\u2013Ag3\u2013Pt catalyst had an excellent normalized MPD (39.1\u00a0mW $\u22121), reflecting its potential economic viability for scale-up applications. Modification of Ag NP onto Co\u2013N\u2013C may be a strategy to improve ORR activity and antibacterial capacity. This is well demonstrated by Jiang and co-workers, who observed that the two-dimensional Ag/Co\u2013N\u2013C-30 catalyst with optimal Ag content presented suitable ECSA (286.8\u00a0m2\u00a0g\u22121) and graded porous structure varying from micropores to mesopores, facilitating the transfer of reactive species and electrons. The synergistic effect of the higher conductivity Ag NP and Co, N to provide antibacterial and ORR activity for the Ag/Co\u2013N\u2013C-30 catalyst, which had an Eonset of 0.88\u00a0V close to 20% Pt (0.91\u00a0V) and showed the lowest Tafel slope (51.25\u00a0mV dec\u22121). The MFC equipped with Ag\u2013Co\u2013N\u2013C-30 obtained an MPD of 560.6\u00a0mW\u00a0m\u22122 and maintained an excellent output voltage (468\u00a0\u00b1\u00a017\u00a0mV) even after 1600\u00a0h of operation [152]. Using the E. coli as the antibacterial model to quantitatively investigate the antibacterial activity of different concentrations of Ag/Co\u2013N\u2013C-30 catalyst, the results showed that the numbers of colonies and colony-forming units (CFU) decreased significantly with increasing catalyst concentration (Fig.\u00a015\na and b), among them, the best inhibition of aerobic bacteria in cathode biofilm was attributed to the selective antibacterial ability of Ag/Co\u2013N\u2013C-30.Combining Ag NPs with MOF is an effective strategy for improving the performance and bacteriostatic ability of catalysts [195]. In 2021, Zhong and co-workers produced a porous carbon catalyst (Ag/Fe\u2013N\u2013C-2:1) with relatively regular octahedral morphology via Ag/Fe co-doping UiO-66-NH2 (Zr-based MOF). The N2 adsorption isotherm and pore structure distribution indicated that Ag/Fe\u2013N\u2013C-2:1 had a hierarchical pore structure (coexistence of micro- and mesopores) that mainly developed at approximately 0.5\u20132\u00a0nm, and the maximum pore volume (0.129\u00a0cm3\u00a0g\u22121) facilitated the mass transfer of reactive species at the interface to the catalytic site. Its possession of the largest SBET (311.7\u00a0m2\u00a0g\u22121) increased the exposure of the active center and was highly conducive to facilitating the ORR process [153]. Based on electrochemical tests, the Ag/F-N-C-2:1 catalyst showed excellent Eonset (1.14\u00a0V) and Tafel slope (78\u00a0mV dec\u22121), even comparable to commercial Pt/C catalysts (1.14\u00a0V and 65\u00a0mV dec\u22121). Such excellent ORR activity was mainly attributed to the synergetic effect between Ag and Fe to optimize the d-band center (promoting O\u2013O bond breaking and charge transfer rates) (Fig.\u00a015c). Moreover, the number of E. coli colonies decreased significantly with increasing Ag/Fe\u2013N\u2013C-2:1 concentration and was significantly inactivated within 12\u00a0h, demonstrating its excellent antibacterial activity. The MPD of Ag/Fe\u2013N\u2013C-2:1 catalyst-based MFC is raised to 1285.1\u00a0mW\u00a0m\u22123, which was superior to that of Pt/C (1101.5\u00a0mW\u00a0m\u22123), and a steady maximum output voltage of 0.425\u00a0V was maintained after eight cycles of operation. Based on the fact that the application of Cu-based MOFs as cathode catalysts with antimicrobial function is not common, Wang et\u00a0al. [154] prepared CuCo@NCNTs catalyst by impregnation and pyrolysis, in which doping of Co and N promoted ORR via forming highly active sites. The N2 adsorption isotherm and pore structure distribution showed that the flowing gas could form pores to increase the SBET of the CuCo@NCNTs catalyst, with a significant increase in SBET to 96\u00a0m2\u00a0g\u22121, as well as also exhibiting a multistage pore structure with the coexistence of micropores and mesopores, contributing to mass transfer and exposure of more active centers. The CuCo@NCNTs catalyst exhibited excellent Eonset (0.91\u00a0V) and Tafel slope (44\u00a0mV dec\u22121), even better than commercial Pt/C catalysts (0.82\u00a0V and 92\u00a0mV dec\u22121). Based on antibacterial tests, this catalyst was dissolved in deionized water and dropped into the bacteria-containing culture medium, which showed a significant bacteria-inhibiting ring. Furthermore, the MPD obtained by CuCo@NCNTs as MFC cathode was up to 2757\u00a0mW\u00a0m\u22123, and the biomass of the cathode was only 0.35\u00a0\u00b1\u00a00.048\u00a0mg\u00a0cm\u22122 after one month of MFC operation, which was significantly lower than that of Pt/C cathode (0.57\u00a0\u00b1\u00a00.061\u00a0mg\u00a0cm\u22122). After 3 months of operation, a stable output voltage of 0.51\u00a0V was maintained. These results collectively indicated that CuCo@NCNTs exhibit strong stability and antibacterial ability.Throughout the history of MFC development, it is not hard to observe that the ultimate goal of MFC is to achieve energy-efficient wastewater treatment, not to add additional costs. Table 2\n summarized the key contributions and application effects of bimetallic catalysts for MFC. It can be concluded that most bimetallic catalysts are durable after long-term operation, stable in wastewater, and economically feasible, which enables MFC to treat wastewater while obtaining higher and stabler output power better than or comparable to Pt/C catalysts. Specifically, compared to simple Pt catalysts, due to electronic and geometrical effects, Pt-M (M\u00a0=\u00a0Fe, Co, Ni) alloyed with transition metals significantly improves the catalytic performance of ORR with reduced Pt utilization, obtaining appreciable MPD, but tolerance and long-term stability are still not specifically discussed. To further improve cost-effectiveness, ORR activity and stability, transition metal alloys rich in Earth-rich elements are first considered. Transition metal alloys are simple to prepare and inexpensive, maintaining good stability and high MPD even during actual wastewater treatment. Transition composite metal oxides with oxidation states and unique structures, combined with highly conductive carriers, can better improve the electrocatalytic activity, showing the incomparable advantages of metal catalysts, i.e., unique photocatalytic properties and high stability, and high output MPD makes it stand out, but it does not seem to be competitive in terms of durability and cost. Transition metal macrocyclic compound-based bimetallic catalysts possess low manufacturing cost, exhibit excellent catalytic activity, tolerance and high stability, achieving more stable MPD. Finally, MOF-based bimetallic catalysts with inherent porous structure, abundant pores and high SBET are favorable for mass transfer and improved catalytic activity, and when used as bifunctional catalysts, they achieve increased ORR activity while inhibiting microbial contamination, resulting in highly stable MPD and long-term durability, which makes it probably the most promising candidate for practical applications, but the adaptability to high-temperature pyrolysis still needs to be improved.In recent years, with the continuous deepening of the applied research on bimetallic catalysts, we can find that a large number of problems remain to be solved if the MFC decorated with bimetallic catalysts is to achieve efficient wastewater treatment and scale-up application. To facilitate future research and practical applications of bimetallic catalysts, this review offers the following insights into the existing challenges.\n\n(1)\nBimetallic synergistic catalysis is an important way to regulate and control the catalytic performance of bimetallic catalysts. The mutual accompaniment of electronic and geometric effects between bimetallic components is challenging to identify the key causes leading to enhanced catalytic performance of bimetallic catalysts. The use of more advanced in situ characterization techniques (e.g. In situ XAS) [38] combined with theoretical calculations (e.g. DFT) and experimental methods is vital to further essentially reveal the dominant effect of synergistic catalysis of different bimetallic catalysts during ORR, as well as to clarify the real active sites of bimetallic catalysts, which may provide theoretical guidance for the design of bimetallic catalysts with higher activity and stability in the future.\n\n\n(2)\nIt is well known that the synthesis methods of bimetallic catalysts commonly play a key role in their catalytic performances and behaviors. Each of the current typical synthesis methods has certain weaknesses, indicating that it is still a developing field. Among them, relatively difficult to achieve highly accurate regulation of the microstructure of bimetallic nanoparticles becomes a key limiting factor, which implies that the modification of these methods is imperative, i.e., the development of new synthesis methods should make the above problems possible and provide some reference for the future design of bimetallic ORR catalysts with accurate regulation of structure, components, and morphology on this basis.\n\n\n(3)\nAt present, the application of bimetallic catalysts in MFC coupled systems is comparatively less studied, even though applied to MFC, the substrates are also mostly from synthetic wastewater or high concentrations of organic pollutants, and there are still few studies on the targeted degradation of emerging pollutants. Therefore, in the future, the application research of bimetallic catalysts can be expanded more to MFC coupled with photocatalytic, electro-Fenton, and MBR systems, especially for the degradation of emerging contaminants (e.g. antibiotics, PAHs, personal care products, endocrine disruptors, etc.), which still need to be further strengthened.\n\n\n(4)\nIn recent years, the degradation of antibiotics in MFC decorated with bimetallic catalyst has gradually become a hotspot and gained achievements to some extent, however, the long-term stable operation research is still in the initial stage, how to achieve long-term stable operation (with high output voltage) and practical application of MFC should be the key direction of subsequent research. Given the future development trend of MFC is from laboratory scale to field scale, or even towards actual industrial power generation, the formation of strong metal-carrier interactions through the selection of suitable catalyst carriers (e.g. heteroatom-doped carbon materials, conductive polymers, etc.) may be an effective strategy to enhance the long-term stability of MFC.\n\n\n(5)\nMFC configurated with bimetallic catalysts is still facing the problem of relatively low output power when applied to on-site scale wastewater treatment and does not achieve the expected effects. Future practical applications are more likely to be as an alternative energy source: pretreating wastewater from a wastewater treatment plant while providing a small amount of electricity for subsequent low-power equipment (e.g. aerators, etc.) and for daily use in nearby rural areas (e.g. night-time toilet lighting, etc.). Alternatively, this electrical energy could also be used to build MFC self-powered biosensors for real-time monitoring of pollutants in wastewater.\n\n\n(6)\nCurrently, MOF-based bimetallic catalysts have achieved gratifying results in MFC applications owing to their remarkable specific surface area and outstanding porosity. There are still some issues such as the limited variety of MOF precursors, the relatively complicated preparation process, the adaptability to high-temperature pyrolysis, and the over-generalized cost estimation. Considering those, exploring other effective MOF precursors or appropriately broadening the choice of their organic ligands will help to expand the diversity of MOF derivatives. Developing simpler synthesis methods will be an important step toward achieving commercial production and application of MOF-based bimetallic catalysts. Besides, further cost estimation using normalized MPD, which characterizes power production costs, is significant for the overall evaluation of cost-effectiveness.\n\n\nBimetallic synergistic catalysis is an important way to regulate and control the catalytic performance of bimetallic catalysts. The mutual accompaniment of electronic and geometric effects between bimetallic components is challenging to identify the key causes leading to enhanced catalytic performance of bimetallic catalysts. The use of more advanced in situ characterization techniques (e.g. In situ XAS) [38] combined with theoretical calculations (e.g. DFT) and experimental methods is vital to further essentially reveal the dominant effect of synergistic catalysis of different bimetallic catalysts during ORR, as well as to clarify the real active sites of bimetallic catalysts, which may provide theoretical guidance for the design of bimetallic catalysts with higher activity and stability in the future.It is well known that the synthesis methods of bimetallic catalysts commonly play a key role in their catalytic performances and behaviors. Each of the current typical synthesis methods has certain weaknesses, indicating that it is still a developing field. Among them, relatively difficult to achieve highly accurate regulation of the microstructure of bimetallic nanoparticles becomes a key limiting factor, which implies that the modification of these methods is imperative, i.e., the development of new synthesis methods should make the above problems possible and provide some reference for the future design of bimetallic ORR catalysts with accurate regulation of structure, components, and morphology on this basis.At present, the application of bimetallic catalysts in MFC coupled systems is comparatively less studied, even though applied to MFC, the substrates are also mostly from synthetic wastewater or high concentrations of organic pollutants, and there are still few studies on the targeted degradation of emerging pollutants. Therefore, in the future, the application research of bimetallic catalysts can be expanded more to MFC coupled with photocatalytic, electro-Fenton, and MBR systems, especially for the degradation of emerging contaminants (e.g. antibiotics, PAHs, personal care products, endocrine disruptors, etc.), which still need to be further strengthened.In recent years, the degradation of antibiotics in MFC decorated with bimetallic catalyst has gradually become a hotspot and gained achievements to some extent, however, the long-term stable operation research is still in the initial stage, how to achieve long-term stable operation (with high output voltage) and practical application of MFC should be the key direction of subsequent research. Given the future development trend of MFC is from laboratory scale to field scale, or even towards actual industrial power generation, the formation of strong metal-carrier interactions through the selection of suitable catalyst carriers (e.g. heteroatom-doped carbon materials, conductive polymers, etc.) may be an effective strategy to enhance the long-term stability of MFC.MFC configurated with bimetallic catalysts is still facing the problem of relatively low output power when applied to on-site scale wastewater treatment and does not achieve the expected effects. Future practical applications are more likely to be as an alternative energy source: pretreating wastewater from a wastewater treatment plant while providing a small amount of electricity for subsequent low-power equipment (e.g. aerators, etc.) and for daily use in nearby rural areas (e.g. night-time toilet lighting, etc.). Alternatively, this electrical energy could also be used to build MFC self-powered biosensors for real-time monitoring of pollutants in wastewater.Currently, MOF-based bimetallic catalysts have achieved gratifying results in MFC applications owing to their remarkable specific surface area and outstanding porosity. There are still some issues such as the limited variety of MOF precursors, the relatively complicated preparation process, the adaptability to high-temperature pyrolysis, and the over-generalized cost estimation. Considering those, exploring other effective MOF precursors or appropriately broadening the choice of their organic ligands will help to expand the diversity of MOF derivatives. Developing simpler synthesis methods will be an important step toward achieving commercial production and application of MOF-based bimetallic catalysts. Besides, further cost estimation using normalized MPD, which characterizes power production costs, is significant for the overall evaluation of cost-effectiveness.In a nutshell, bimetallic catalysts can become the research hotspots for MFC cathode ORR catalysts in recent years mainly due to two points: firstly, the synergistic effects between the bimetallic components endow them with better catalytic performance than monometallic catalysts, and secondly, they are simpler to prepare than multimetallic catalysts, making them more feasible for practical applications. Although bimetallic catalysts currently suitable for MFC have been studied extensively, the lack of a collective summary of them hinders the prediction of future research directions. Therefore, this review is designed to address the aforementioned significant issues. It is noteworthy that the trend of bimetallic catalyst research from Pt-M alloys to MOF-based bimetallic catalysts is obvious, especially when using MOF-based bimetallic catalysts as ORR and bifunctional catalysts, the overall performance of MFC has been significantly improved. However, there is still much work to be done if MFC is to move from experimental scale to actual industrialization. These include the broadening of MOF precursors, simplification of synthesis methods, cost estimation using normalized MPD, and the selection of suitable catalyst carriers to improve MFC output power and long-term stability (especially in the degradation of emerging contaminants). By updating the above conclusions, MOF-based bimetallic catalysts are expected to become one of the most desirable bimetallic ORR catalysts for MFC in the future.We have consulted the Guide for Authors in preparing this manuscript and confirm that the manuscript is prepared in compliance with the Ethics in Publishing Policy as described in the Guide for Authors. No conflict of interest exists in the submission of this manuscript and is approved by all authors for publication. The work has not been published previously and is not under consideration for publication elsewhere.This study was supported by the National Key R&D Program of China (2019YFC1804102) and the National Natural Science Foundation of China (32171615).", "descript": "\n                  Microbial fuel cell (MFC) is one synchronous power generation device for wastewater treatment that takes into account environmental and energy issues, exhibiting promising potential. Sluggish oxygen reduction reaction (ORR) kinetics on the cathode remains by far the most critical bottleneck hindering the practical application of MFC. An ideal cathode catalyst should possess excellent ORR activity, stability, and cost-effectiveness, experiments have demonstrated that bimetallic catalysts are one of the most promising ORR catalysts currently. Based on this, this review mainly analyzes the reaction mechanism (ORR mechanisms, synergistic effects), advantages (combined with characterization technologies), and typical synthesis methods of bimetallic catalysts, focusing on the application effects of early Pt-M (M\u00a0=\u00a0Fe, Co, and Ni) alloys to bifunctional catalysts in MFC, pointing out that the main existing challenges remain economic analysis, long-term durability and large-scale application, and looking forward to this. At last, the research trend of bimetallic catalysts suitable for MFC is evaluated, and it is considered that the development and research of metal-organic framework (MOF)-based bimetallic catalysts are still worth focusing on in the future, intending to provide a reference for MFC to achieve energy-efficient wastewater treatment.\n               "}
{"full_text": "Hydrogen (H2) is becoming increasingly important as a future fuel compared with fossil fuels because of its advantages of clean and renewable energy generation (Dresselhaus and Thomas, 2001; Turner, 2004). Electrochemical water splitting provides an effective approach for H2 production. Water splitting consists of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), both of which require efficient catalysts\u00a0to reduce the overpotential for practical applications (Jin et al., 2015; Zhang et al., 2017a; Balogun et al., 2016). Although platinum (Pt) is regarded as a conventional HER catalyst in acidic solutions owing to its highest exchange current density and low Tafel slope, it shows an \u201cincomparable\u201d HER activity in alkaline solutions owing to the sluggish reaction kinetics (Ma et\u00a0al., 2017; Mahmood et\u00a0al., 2017; Zheng et\u00a0al., 2016). Even though non-noble metal materials have been widely explored as enhanced catalysts for HER, the greatest challenge for the use of non-noble metal materials so far is that their HER activities still underperform Pt-based catalysts, and they are susceptible to acid corrosion (Zhang et al., 2017b; Conway and Tilak, 2002). Similar obstacles are still unavoidable for non-noble metal materials for OER applications owing to their relatively high overpotentials for driving the OER process and the low energy conversion efficiencies. To date, pursuit of effective catalysts for both OER and HER in the same electrolyte, not to mention under universal pH conditions, has been extremely desirable (Zheng et al., 2014; Wang et al., 2018a, 2018b; Ellis et al., 2010). Therefore, the development of efficient and stable bifunctional catalysts for the simultaneous production of H2 and oxygen (O2) under universal pH conditions is still a significant challenge.It has been generally considered that noble metal materials, such as Ru-based catalysts, are the most promising catalysts for use as overall water-splitting catalysts owing to their promising activities for the two half-reactions in both acidic and alkaline solutions as well as their high stability under extreme conditions (Lu et\u00a0al., 2014; Jin et\u00a0al., 2016; Petrykin et\u00a0al., 2010; Seitz et\u00a0al., 2016; Kong et\u00a0al., 2016). However, the water-splitting performances of the reported Ru-based catalysts are still far from satisfactory, particularly under universal pH conditions. From the viewpoint of the structure, a two-dimensional (2D) structure can provide\u00a0great opportunities for enhancing the electrochemical performance because it largely exposes the surface area (Hang et al., 2014; Gao et al., 2012). However, undesirable drawbacks arise from the severe aggregation\u00a0or fracture that usually occurs during the electrochemical process, inevitably leading to the obvious activity decay. This renders the conventional 2D structure not an ideal candidate for efficient electrocatalysis (Zheng et\u00a0al., 2014; Chhowalla et\u00a0al., 2013; Hwang et\u00a0al., 2011; Chen et\u00a0al., 2015). Based on this, the assembly of 2D structures into unique 3D structures may provide an effective strategy to achieve efficient catalysts for water splitting under universal pH conditions because the structures can achieve a large exposure of the active sites while stabilizing the structure.To surmount this challenge, we report an efficient wet chemical approach for the synthesis of 3D hierarchical Ru-Ni nanosheet assemblies (NAs) consisting of ultrathin nanosheets as subunits and explore their high\u00a0performances for overall water splitting under universal pH conditions. The distinctive hierarchical NA structures are highly beneficial for enhancing electrochemical energy conversion. We found that the introduction of Ni into Ru largely downshifts the d-band center of the Ru-Ni NAs and effectively modulates the surface environment for HER. After air treatment at 350\u00b0C, the newly generated abundant RuO2 provides effective active sites for OER. As a result, the Ru-Ni NAs deliver high HER and OER activities as well as outstanding stability under a broad range of pH conditions. More interestingly, Ru3Ni3 NAs demonstrated potential applications for overall water splitting with a lower overpotential, smaller Tafel slope, and better stability than the reference Ir/C-Pt/C catalyst.A typical preparation of Ru-Ni NAs was introduced by adding ruthenium(III) acetylacetonate (Ru(acac)3), nickel(II) acetylacetonate (Ni(acac)2), phloroglucinol, tetramethylammonium bromide, polyvinylpyrrolidone (PVP), and benzyl alcohol into a glass vial. After capping the vial, the mixture was ultrasonicated for approximately 1 h. The resulting homogeneous mixture was then heated from room temperature to 160\u00b0C and maintained at 160\u00b0C for 5\u00a0h using an oil bath. Ru-Ni NAs with different Ru/Ni ratios (i.e., Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs) have been readily achieved by tuning the Ru/Ni precursor amount ratios (Figures S1A\u2013S1C).The detailed characterizations of Ru3Ni3 NAs were further carried out to determine the 3D assembly structure (Figures 1\n, S1D, and S1E). The high-angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) image (Figure\u00a01A) showed at first glance that all the products had a spherical outline, which indicated the high purity of product. For a close view of the Ru3Ni3 NAs, enlarged HAADF-STEM was performed, and a 3D flower-like structure assembled by hierarchical 2D nanosheet subunits was clearly observed (Figure\u00a01B). Elemental mappings and line scans showed that the flower-like Ru3Ni3 NAs had a typical core-shell structure consisting of a Ru-Ni core and Ru shell (Figures 1C and 1D). Compared with those of the pure Ru NAs, additional X-ray diffraction (XRD) peaks in the Ru-Ni alloy were observed for 3D Ru3Ni3 NAs, which further confirmed the core-shell structure of the Ru3Ni3 NAs with the Ru phase and Ru-Ni alloy phase (Figures S1D and S1E). As revealed by the high-resolution transmission electron microscopic (TEM) image of the Ru3Ni3 NAs, lattice fringes with interplanar distances of 0.204 and 0.230\u00a0nm were observed, which correlated well with the (101) plane of Ru and the (100) plane of the Ru-Ni alloy, respectively (Figures 1E\u20131G).Notably, the morphologies of Ru3Ni2 NAs and Ru3Ni1 NAs with different Ru/Ni ratios were similar (Figures\u00a0S2A, S2B, S2E, S2H, S2I, and S2L). The XRD results show that as the amount of Ni increased, the main peaks of the Ru-Ni alloy approach the standard pure Ni XRD peaks (PCPDS No. 89\u20137,129), which suggested the successful alloying of Ni into Ru. The energy dispersive spectroscopy (EDS) elemental mapping images and line scans confirm that the alloys have a core-shell structure similar to that of the Ru3Ni3 NAs (Figures S2C, S2D, S2J, and S2K). The same lattice fringes with an interplanar distance of 0.204\u00a0nm were found in the Ru3Ni2 NAs and Ru3Ni1 NAs, which correlated well to the (101) plane of Ru. Lattice fringes of the (100) Ru-Ni alloy with interplanar distances of 0.231 and 0.232\u00a0nm were also observed in the Ru3Ni2 NAs and Ru3Ni1 NAs, respectively (Figures S2F, S2G, S2M, and S2N).The direct creation of unique, 3D Ru-Ni superstructures with ultrathin building blocks is the most striking feature of the synthesis reported here, which has never been reported previously. To gain a better understanding of the growth mechanism behind the successful synthesis, characterizations of the intermediates collected at different reaction times were also carefully performed (Figures S3A\u2013S3J). At the beginning of the reaction (25\u00a0min), intermediates with messy and irregular multi-branched structures were observed (Figures S3A and S3B). Nanosheets began to form, and a portion of the assembled flower-shaped intermediates appeared at a reaction time of 40\u00a0min (Figures S3C and S3D). When the reaction reached 1.5 h, the diameter of the flower-shaped intermediates increased (Figures S3E and S3F). After the reaction progressed for 3 h, the monodispersed, hierarchical assembly became obvious (Figures S3G and S3H). A further increase in the size of the Ru3Ni3 NAs was observed after the completion of the reaction (Figures S3I and S3J). The different reaction intermediates were also further analyzed by XRD (Figure\u00a0S4), and the peaks of Ru and small peaks of the Ru-Ni alloy were detected during the initial 25\u00a0min. With the prolonged reaction time, the peak indexed to the Ru-Ni alloy became increasingly obvious and shifted to a higher angle, which indicated that more Ni was reduced and alloyed with Ru (Figure\u00a0S3K).To further understand the formation progress behind the successful synthesis, the effect of various experimental parameters on the morphology of Ru-Ni NAs was carried out. The results reveal that the combined use of PVP, phloroglucinol, and tetramethylammonium bromide was essential for the successful creation of\u00a0Ru-Ni NAs. The Ru-Ni NAs could not be obtained in the absence of any PVP or phloroglucinol (Figures S5A, S5B, S7A, and S7B). Further detailed control experiments show that high-quality Ru-Ni NAs could only be obtained in the presence of specific amount of phloroglucinol and tetramethylammonium bromide. For\u00a0example, irregular morphology was obtained when the amounts of phloroglucinol and tetramethylammonium bromide were changed (Figures S5 and S6), and a layered product with low yield was obtained when benzyl alcohol was replaced by ethylene glycol (Figures S7C and S7D). The morphology of assemblies has changed greatly without using Ni(acac)2 (Figure\u00a0S8).Considering that Ru is expected to have high activities for HER and OER, the design of Ru-based catalysts for overall water splitting is highly significant from the viewpoint of practical applications (Pu et\u00a0al., 2017; Jiang et\u00a0al., 2015), but the systematic study of Ru-based catalysts is still very limited, especially in a broad pH range. To this end, detailed HER and OER measurements were carried out in electrolytes with different pH values using Ru-Ni NAs as the candidate catalyst. All electrochemical measurements were performed in a standard three-electrode system with a saturated calomel electrode as the reference electrode and a carbon rod as the counter electrode. The reference electrodes were calibrated before the electrochemical measurements (Figure\u00a0S9). All polarization curves were recorded without iR compensation. Before the electrocatalytic measurements, all different Ru-Ni NAs were loaded on a carbon support (Vulcan XC72R carbon) by sonication. Ru loading of 20 wt % was maintained in each catalyst, and no obvious morphological changes were observed after heat treatment (Figure\u00a0S10). The resulting Ru-Ni NAs/C were then dispersed in a mixture solvent containing isopropanol and Nafion (5%) and sonicated for 30\u00a0min to form a homogeneous catalyst ink. The concentration of the Ru-Ni NAs loading on the carbon powder was controlled at 2\u00a0mg mL\u22121; 10\u00a0\u03bcL catalyst ink was uniformly dropped onto a glassy carbon electrode and dried naturally at room temperature.The HER performance of the Ru-Ni NAs/C was first explored at a slow scan rate of 5\u00a0mV s\u22121 to ensure steady-state behavior on the electrode surface. To obtain the best performance of the Ru-Ni NAs/C in HER, we first determined the effects of the annealing temperature and atmosphere on HER performance by using Ru3Ni3 NAs as the candidate material. As shown in Figures S11A and S11B, the sample annealed at 250\u00b0C for 1\u00a0h exhibited the best HER activity in both alkaline and acidic conditions (0.5\u00a0M H2SO4 and 1\u00a0M KOH solutions). Figure\u00a02\nA shows the polarization curves of the Ru-Ni NAs and Ru NAs and commercial Pt/C in 1\u00a0M KOH. In detail, at a current density of 10 mA cm\u22122, the overpotentials of Ru3Ni3 NAs, Ru3Ni2 NAs, Ru3Ni1 NAs, Ru NAs, and commercial Pt/C were 39, 42, 44, 62, and 90\u00a0mV, respectively, versus the reversible hydrogen electrode (RHE), and the Ru3Ni3 NAs showed the smallest value. The Tafel slope is an intrinsic property of the catalyst that is determined by the rate-limiting step of the HER (Cherevko et\u00a0al., 2016). Importantly, the Tafel slopes of the Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs were calculated to be 26.9, 29.9, and 30.5\u00a0mV dec\u22121, respectively (Figures 2C and S12A). In contrast, the Ru NAs and commercial Pt/C showed relatively high Tafel slopes of 58.3\u00a0mV dec\u22121 and 46.8\u00a0mV dec\u22121. The electrocatalytic stability\u00a0of the Ru3Ni3 NAs was further studied by both long-term cycling and chronopotentiometry tests,\u00a0and the polarization curves of Ru3Ni3 NAs exhibited no obvious change after 12,000 cycles (Figure\u00a02E). The Ru3Ni3 NAs showed only a slight potential increase after 10\u00a0h of chronopotentiometry at a current density of 5 mA cm\u22122 (Figure\u00a02E, inset).With the change in the electrolyte to 0.1\u00a0M KOH, the Ru-Ni NAs still showed promising HER activities. At 10\u00a0mA cm\u22122, the overpotentials of the Ru3Ni3 NAs, Ru3Ni2 NAs, Ru3Ni1 NAs, Ru NAs, and commercial Pt/C\u00a0were 119, 127, 123, 152, and 132\u00a0mV, respectively (Figure\u00a02B). In addition to the low overpotentials,\u00a0the Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs also exhibited lower Tafel slopes than Pt/C (99.7\u00a0mV dec\u22121) and Ru NAs (76.0\u00a0mV dec\u22121) (Figures 2D and S12B). The Ru3Ni3 NAs also exhibited excellent durability after 12,000 cycles and in the chronopotentiometry test in 0.1\u00a0M KOH (Figure\u00a02F), which indicated that the Ru3Ni3 NAs exhibit a superior HER activity and durability under alkaline conditions.The HER properties of the Ru-Ni NAs under acidic conditions were further investigated. Figure\u00a0S13 shows that the overpotentials of the Ru3Ni3 NAs, Ru3Ni2 NAs, Ru3Ni1 NAs, and Ru NAs were 39 and 96\u00a0mV, 39 and 115\u00a0mV, 46 and 112\u00a0mV, and 55 and 122\u00a0mV at a current density of 10 mA cm\u22122 in 0.5\u00a0M H2SO4 and 0.05\u00a0M H2SO4, respectively. The Tafel slopes of the Ru3Ni3 NAs, Ru3Ni2 NAs, Ru3Ni1 NAs, and Ru NAs were 53.9 and 67.1\u00a0mV dec\u22121, 53.5 and 64.0\u00a0mV dec\u22121, 54.2 and 66.8\u00a0mV dec\u22121, and 81.6 and 79.6\u00a0mV dec\u22121 in 0.5\u00a0M H2SO4 and 0.05\u00a0M H2SO4, respectively. The Ru-Ni NAs showed a much better HER performance than the Ru\u00a0NAs, indicating the vital role of Ni in improving the HER performance. After the working electrode was cycled for 6,000 cycles, the Ru3Ni3 NAs exhibited the best durability under acidic conditions with potential increases of only 62 and 39\u00a0mV in 0.5\u00a0M H2SO4 and 0.05\u00a0M H2SO4, respectively. In addition, after the 12-h chronopotentiometry test at 5 mA cm\u22122 in 0.5\u00a0M H2SO4 and 0.05\u00a0M H2SO4, the Ru3Ni3 NAs showed only potential increases of 36 and 49\u00a0mV, respectively (Figures S13E and S13F).The obtained Ru-Ni NAs were also successfully applied as efficient OER catalysts. Before the OER tests, the Ru-Ni NAs were also subjected to thermal annealing in air at different temperatures because Ru oxide has been discovered to be an active component for the OER (Petrykin et\u00a0al., 2010; Reier et\u00a0al., 2012). As shown in Figures S11C and S11D, the catalyst after heat treatment in air (350\u00b0C, 2 h) showed the best performance under both acidic and alkaline conditions (0.5\u00a0M H2SO4 and 1\u00a0M KOH). The TEM images show that the hierarchical structures were largely preserved (Figures S10C and S10D). We also studied the structural characterization of NAs after heat treatment by STEM image, elemental mapping, and line scan, where the core-shell structures of Ru3Ni3 NAs are largely reserved (Figure\u00a0S14). We also showed that the carbon can enhance both the electrical conductivity and the dispersion of Ru3Ni3 NAs, and thus improve the electrocatalysis (Figure\u00a0S15). To evaluate the OER performances of Ru-Ni under universal pH conditions, we tested the OER performances in both acidic (0.5 and 0.05\u00a0M H2SO4) and alkaline (1 and 0.1\u00a0M KOH) electrolytes. The commercial Ir/C catalyst was chosen as the reference because Ir is considered to be the benchmark catalyst for OER (Lettenmeier et al., 2016; Zhang et al., 2017c).Examination of the OER polarization curves in 0.5 and 0.05\u00a0M H2SO4 shows that the Ru-Ni NAs showed much better OER activities than the Ru NAs and commercial Ir/C. To drive a current density of 10\u00a0mA\u00a0cm\u22122, the Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs required overpotentials of 252\u00a0mV, 260\u00a0mV, and 268\u00a0mV in 0.5\u00a0M H2SO4, respectively (Figure\u00a03\nA). The Tafel slopes of the Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs derived from Figure\u00a03A were 45.8, 46.1, and 46.0\u00a0mV dec\u22121 in 0.5\u00a0M H2SO4, respectively. In contrast, the commercial Ir/C and Ru NAs required larger overpotentials of 328 and 277\u00a0mV in 0.5\u00a0M H2SO4, respectively. The Tafel slopes of the commercial Ir/C and Ru NAs were also larger than those of the Ru-Ni NAs (Figures 3C and S16A). Similar trends were also obtained in 0.05\u00a0M H2SO4, and the Ru3Ni3 NAs showed the lowest overpotential and Tafel slope of 312\u00a0mV and 70.8\u00a0mV dec\u22121, respectively (Figures 3B, 3D, and 6B).We further measured the OER activities in different alkaline electrolytes. The overpotentials of the Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs were 304, 309, and 301\u00a0mV in 1\u00a0M KOH, whereas the Ru NAs and commercial Ir/C showed larger overpotentials of 351 and 311\u00a0mV, respectively (Figure\u00a0S17A). The Tafel slopes of the Ru3Ni3 NAs, Ru3Ni2 NAs, Ru3Ni1 NAs, Ru NAs, and commercial Ir/C derived from Figure\u00a0S15A were 91.7, 67.9, 73.4, 111.1, and 47.1\u00a0mV dec\u22121, respectively (Figure\u00a0S17C). When the solution is replaced by a dilute alkaline solution (0.1\u00a0M KOH), in which it is more difficult for the OER to proceed (Lu and Zhao, 2015), the Ru-Ni NAs also exhibited a high activity. The overpotentials of the Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs were 394, 390, and 384\u00a0mV, respectively, which were smaller than those of the Ru NAs (439\u00a0mV) and commercial Ir/C (407\u00a0mV) (Figure\u00a0S17B). The Tafel slopes of the Ru3Ni3 NAs, Ru3Ni2 NAs, Ru3Ni1 NAs, Ru NAs, and commercial Ir/C derived from Figure\u00a0S17B were 133.8, 131.4, 130.2, 140.7, and 111.1\u00a0mV dec\u22121, respectively (Figure\u00a0S17D). All these results confirmed that the unique Ru-Ni NAs show excellent OER performances compared with the Ru NAs. In addition, in the 10-h chronopotentiometry test, the Ru3Ni3 NAs showed limited degradation after continuous electrolysis at 5 mA cm\u22122 in 0.5\u00a0M H2SO4, 0.05\u00a0M H2SO4, 1\u00a0M KOH, and 0.1\u00a0M KOH (Figures 3E, 3F,S17E, and S17F). No obvious morphological changes were observed in 0.5\u00a0M H2SO4 and 1\u00a0M KOH after the chronopotentiometry test (Figure\u00a0S18), which demonstrated that the Ru-Ni NAs are indeed \u201cacidic- and alkaline-stable\u201d OER catalysts. To further demonstrate the OER and HER stability, chronopotentiometry test at higher current density was also performed, where the Ru3Ni3 NAs still showed limited degradations after continuous OER and HER electrolysis at 10 mA cm\u22122 in 0.5\u00a0M H2SO4 and 1\u00a0M KOH (Figure\u00a0S19).As we explored the best catalysts for HER and OER under both acidic and alkaline conditions, a two-electrode\u00a0setup with anodic catalyst Ru3Ni3 NAs after air treatment at 250\u00b0C for 1\u00a0h and cathodic catalyst Ru3Ni3 NAs after air treatment at 350\u00b0C for 2\u00a0h was used to study the potential application of Ru-Ni NAs in overall water splitting under universal pH conditions. The Linear Sweep Voltammetry (LSV) plots of Ru3Ni3 NAs and Ir/C-Pt/C under different pH conditions are presented in Figure\u00a04\nA. The data clearly show that both the potentials and Tafel slopes of the Ru3Ni3 NAs are much lower than those of Ir/C-Pt/C. The Ru3Ni3 NAs show an overpotential of\u00a0280\u00a0mV in 0.5\u00a0M H2SO4, which is considerably lower than that of Ir/C-Pt/C (370\u00a0mV). The Tafel slope of the Ru3Ni3 NAs is only 96.9\u00a0mV dec\u22121, whereas that of Ir/C-Pt/C is as high as 150.1\u00a0mV dec\u22121 (Figures 4B and S20A), indicating that the reaction kinetics of the Ru3Ni3 NAs are much faster than those of Ir/C-Pt/C. Significantly, the Ru3Ni3 NAs showed excellent durability with limited degradation after a 10-h chronopotentiometry test at 5 mA cm\u22122 in 0.5\u00a0M H2SO4, 0.05\u00a0M H2SO4, 1\u00a0M KOH, and 0.1\u00a0M KOH (Figure\u00a04C). Overall, these results confirmed that the Ru-Ni NAs can serve as excellent water-splitting catalysts under universal pH conditions.It should be noted that both the HER and OER activities of the Ru-Ni NAs in different electrolytes are higher\u00a0than those of most catalysts reported to date (Tables S1\u2013S3). To explore the reasons behind the high performance, the surface structures of the different catalysts were first explored in detail. As shown in Figure\u00a0S10, no obvious morphological changes were found in the Ru-Ni NAs after heat treatment. However, the XRD peaks assigned to RuO2 appeared in the catalysts processed at 350\u00b0C in air, and the Ru3Ni3 NAs showed the highest peak for RuO2 (Figure\u00a0S21A). Considering that RuO2 plays an important role in enhancing the OER activity, the formed RuO2 greatly enhances the OER activity in the Ru-Ni NAs (Fang and Liu, 2010). XPS was also carried out to explore the surface properties of the Ru-Ni NAs. Figure\u00a0S22 shows the full scan curves of the different Ru-Ni NAs, and the positions of the Ru and Ni peaks were consistent with the literature results (Folkesson et\u00a0al., 1973). Furthermore, the XPS peaks of Ru in different catalysts after treatment at 350\u00b0C in air for 2\u00a0h were divided into Ru 3p3/2 and Ru 3p1/2 peaks, which can be further split into three peaks, corresponding to Rux+ (purple line), Ru4+ (orange line), and Ru0 (dark yellow line) (Figure\u00a05\nA) (Li et\u00a0al., 2016). It was calculated that the Ru4+ fractions in the Ru3Ni3 NAs (57.00%), Ru3Ni2 NAs (44.46%), and Ru3Ni1 NAs (44.57%) were much higher than those in the Ru NAs (29.89%) (Table S4), which confirmed the higher concentrations of RuO2 in the Ru-Ni NAs. As shown in Figure\u00a05B, the Ni 2p peaks in the Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs were composed of Ni 2p1/2 and Ni 2p3/2 peaks, which both split into two oxidized Ni peaks, namely, Ni2+ (dark yellow line) and Ni3+ (orange line) (Zhang\u00a0et\u00a0al., 2007; Gong and Dai, 2015). Ni3+ is helpful for the formation of NiOOH on the catalyst surface, resulting in a better OER performance (Lee et\u00a0al., 2012). This result indicates that both Ru and Ni in high\u00a0oxidation states are generated in the Ru-Ni NAs by treatment at 350\u00b0C for 2\u00a0h in air, and that they are beneficial for the enhanced OER performance.Compared with the peaks of Ru-Ni NAs treated at 350\u00b0C for 2\u00a0h in air, no additional peaks were generated for the Ru-Ni NAs treated at 250\u00b0C for 1\u00a0h in air (Figure\u00a0S21B). Based on XPS analysis, Ru can be successfully split into three peaks, namely, Rux+ (purple line), Ru4+ (orange line), and Ru0 (dark yellow line) (Figure\u00a0S23). It was calculated that the area ratios of the metallic Ru0 were 59.74%, 56.77%, and 58.06% in the Ru3Ni3 NAs, Ru3Ni2 NAs, and Ru3Ni1 NAs, respectively, which were higher than 56.08% in the Ru NAs. (Table S5) and indicated a large number of active sites of metallic Ru present on the surface of the Ru3Ni3 NAs. Surface valence band XPS spectra were also obtained to determine the d-band centers of the Ru-Ni NAs treated at 250\u00b0C in air (Figures 5C and 5D). The d-band center downshifted with the increasing concentration of Ni. The reported d-band centers of Pt and Ru are located at \u22122.32 and \u22121.49 eV, respectively, corresponding to hydrogen binding energies of \u22120.32 and \u22120.64 eV, respectively, and suggesting that Ru shows a stronger hydrogen adsorption than Pt (Jiao et\u00a0al., 2015). Pt is regarded as the best catalyst for HER performance owing to the suitable binding energy between the catalysts and adsorbates. Here, by alloying the catalyst with Ni, a downshift of the d-band center was observed in the Ru-Ni NAs (Figures 5C and 5D), which results in a suitable binding energy between the Ru-Ni NAs and adsorbates and boosted the HER activity of the Ru-Ni NAs (Stamenkovic et\u00a0al., 2007).We further carried out density functional theory (DFT) calculations to elucidate how the downshift effect of the Ru-Ni NAs is related to the high performance of water splitting for both the OER and HER. The Ru-Ni NA system was modeled by a hexagonal lattice (hex-Ru-Ni) based on the Ru local symmetry. It shows a good metallic behavior with uniform isotropic conductivity across the Fermi level (EF) (Figure\u00a06\nA). The d-orbital projected density of states (PDOS) were compared and showed that Ni-3d downshifted to a value lower than that obtained for the bulk face-centered cubic Ni metal (Figure\u00a06B) due to repulsion with the overlapping Ru-4d orbital, which implied a weakening in the Ni-O and Ni-H bonding. In addition, this downshifting effect appeared to be even more pronounced within the hexagonal local lattice than in the cubic lattice. Meanwhile, the Ru-4d states also downshifted compared with those in the hex-Ru metal, especially for the 4d-eg component above the EF (Figure\u00a06B), regardless of the different local symmetries. This occurs because the eg-level component is essential for the adsorption of the bond of the p-\u03c0 lone pair electrons in molecules such as H2O, O, or O2. This is because they almost remain in the non-bonding orbitals, and the adsorption stabilities are dominated by the Coulomb repulsion between 4d-eg in such p-\u03c0 orbitals. Accordingly, the Ru in hex-Ru-Ni will easily transfer electrons between the catalysis substrate and intermediate molecules and facilitate O-O bond formation. The simulated OER pathway (Figure\u00a06C) shows that the system is an energetically favorable catalyst even under U\u00a0= 0 and U\u00a0= 1.23 V, showing that\u00a0water splitting with such Ru-Ni NAs would be a substantially low-barrier process. The splitting of H2O results in an increase in energy of 1.49 eV, guaranteeing that the initiation would be very reactive\u00a0within a low overpotential. Meanwhile, there is no evident change in the energy for the evolution reaction [HO*+(H++e\u2212)]\u2192[O*+2(H++e\u2212)] (\u223c0.4 eV). An additional similarly energetic increase (1.50 eV) was found for the formation of *OOH, indicating that the O* on the Ru-Ni still stays active to oxidize OH under lower overpotential. The splitting of H for the [HOO*+3(H++e\u2212)]\u2192[O2+4(H++e\u2212)] transformation is very active. Compared with the pathway at U\u00a0= 1.23 V, we confirm the overall overpotential (i.e., \u03b7\u00a0= max{[barrier-1.23 eV]/e\u00a0= 0.306 V}) is almost the same within the range of 0.200\u20130.300 V. Further calculations of the O2 dissociation confirmed that the combined O-O on the Ru-contained surface will be easily dissociated and enter into the surrounding solution conditions (Figure\u00a0S24, Tables S6 and S7). Therefore, the OER on the Ru-Ni surface can achieve a very high performance supported by an energetic barrier-free water-splitting process. We further gain energetic insights on the alkaline HER. In the Ru-Ni surface system without partial oxidations by O-coverage, the alkaline HER performance overall is energetically downhill and the whole process gains a reaction heat of \u22120.48 eV with a small barrier of 0.16 eV. Activation barrier for the HER on this system may arise due to barrier of [H2O\u2192H\u00a0+ OH]. As found by our experimental observation, partial oxidation states were found on the surface. We further conducted the reaction energy calculation. The overall reaction heat released is found to be \u22120.97 eV, showing it to be rather more energetically favorable than the case without oxidation. The process of [H2O\u2192H\u00a0+ OH] is also energetically preferred gaining \u22120.28 eV during the bond cleavage on the partially oxidized Ru-Ni surface (Figure\u00a06D). At the same time, a comparison of the chemisorption energies sheds light on the high HER/OER performance (Figure\u00a06E). We also determined that the HER on the Ru-Ni system favors high H coverage with easy chemisorption of the 2H, and the formation of 2H\u2192H2 is energetically favorable. Meanwhile, the low O coverage will easily facilitate water splitting and further accelerate further 2O chemisorption and O2 desorption. The kinetics of possible oxygen absorption or oxygen-related intermediates (OH\u2212) is shown in absorption process in Figure\u00a06F, which will result in the formation an intermediate distorted octahedral unit. The overlapping between eg orbital of Ru2+ and O-p\u03c3 orbitals will facilitate the ion transfer. The distorted structure prompts Ru2+ (d6) to change from a low-spin state (t2g\n6eg\n0) to an intermediate-spin state (t2g\n5eg\n1), where the eg1 can point to the intermediate with high bonding possibility. We also find that the absorption energy of further absorption on vertical oxygen molecule will be lowered nearly 1\u00a0eV, which can be attributed to the Jahn-Teller effect from the extra oxygen molecule to the c-axis of the distorted octahedral unit, which decreases the whole energy. Electrons on t2g can be further excited to eg and then form a high-spin state (t2g\n4eg\n2) with energy decrease. Overall, the Ru-Ni catalytic system is found to be efficient in HER performance from acidic to the basic condition. Thus, the Ru-Ni (NAs) system exhibits a high catalytic reactivity for water splitting based on the DFT calculations. We have also made a detail comparison for the preliminary absorption behavior on the cubic Ru-Ni (111) and hexagonal close packed (hcp) Ru-Ni (001) surface to elucidate the experimental treatment and related analysis. The discussions and analysis cover the following sections: energetics, electronic structures, orbital energetic behaviors, and adsorption analysis (Figures S24\u2013S28 in Supplemental Information).In summary, for the first time, we have demonstrated a facile method for the synthesis of 3D Ru-Ni NAs, which leads to favorable 3D Ru-Ni superstructures with fully exposed active sites. The valence band spectra and DFT calculations revealed a change in the d-band center in the Ru-Ni NAs after the introduction of\u00a0Ni,\u00a0resulting in the transformation to a favorable surface environment for the OER and HER. The RuO2-decorated Ru-Ni NAs treated at 350\u00b0C in air provided additional active sites for the OER. The combined structural and electronic engineering leads to superior electrocatalytic performance for overall water\u00a0splitting under universal pH conditions, and the performance is much better than that of the commercial Pt/C and Ir/C, demonstrating an unprecedented class of nanocatalysts with exceptional activity and excellent stability for electrochemical water splitting.Our work has demonstrated a novel bifunctional catalyst for water splitting in the universal environment from experimental and theoretical perspectives. Based on the combination of XPS and DFT as an effective approach, electronic environment modulation has been interpreted as the key factor that facilitates both\u00a0HER and OER. However, an in-depth understanding of the oxidation states of the catalyst is still an open challenge because of the complex charge transfer induced by the overlap between the metal orbitals as\u00a0well as the correspondingly accurate characterization. The site-to-site sampling and analysis of surface\u00a0oxidation sites is of great significance for precise understanding of the catalyst reactivity. Therefore, we will keep working on further development and perfection on related theoretical exploration and advancement.All methods can be found in the accompanying Transparent Methods supplemental file.This work was financially supported by the Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20170003), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the start-up supports from Soochow University. B.H. thanks the support from the Natural Science Foundation of China (NSFC) for the Youth Scientist grant (Grant No.: NSFC 11504309, 21771156) and the Early Career Scheme (ECS) fund from the Research Grants Council of Hong Kong (Grant No.: PolyU 253026/16P).X.H. conceived and supervised the research. X.H., J.Y. and Q.S. designed the experiments. X.H., J.Y., and Q.S. performed most of the experiments and data analysis. X.H., J.Y., and Q.S. participated in various aspects of the experiments and discussions. B.H. and M.S. performed the DFT simulations. X.H., Q.S.,\u00a0J.Y., B.H., and M.S. wrote the paper. All authors discussed the results and commented on the manuscript.The authors declare no competing interests.Supplemental Information includes Transparent Methods, 28 figures, and 7 tables and can be found with this article online at https://doi.org/10.1016/j.isci.2019.01.004.\n\n\nDocument S1. Transparent Methods, Figures S1\u2013S28, and Tables S1\u2013S7\n\n\n\n", "descript": "\n                  Although electrochemical water splitting is an effective and green approach to produce oxygen and hydrogen, the realization of efficient bifunctional catalysts that are stable in variable electrolytes is still a significant challenge. Herein, we report a three-dimensional hierarchical assembly structure composed of an ultrathin Ru shell and a Ru-Ni alloy core as a catalyst functioning under universal pH conditions. Compared with the typical Ir/C-Pt/C system, superior catalytic performances and excellent durability of the overall water splitting under universal pH have been demonstrated. The introduction of Ni downshifts the d-band center of the Ru-Ni electrocatalysts, modulating the surface electronic environment. Density functional theory results reveal that the mutually restrictive d-band interaction lowers the binding of (Ru, Ni) and (H, O) for easier O-O and H-H formation. The structure-induced eg-dz2 misalignment leads to minimization of surface Coulomb repulsion to achieve a barrier-free water-splitting process.\n               "}
{"full_text": "There is an urgent need to develop highly efficient catalysts and technology to obtain butyl alcohol with their production based on local secondary resources of significant national economic importance.Butyl aldehyde, a limiting aliphatic aldehyde of the acyclic series, is obtained by the oxosynthesis of hydrocarbon raw materials. Its byproducts have long attracted the attention of researchers as a raw material \u2013 a starting object for the synthesis of new oxygen-containing organic compounds [1,2].In industry, aldehydes are obtained by hydroformylation (oxosynthesis) of unsaturated ethylene hydrocarbons of oil [3]. During the oxosynthesis of propylene, two isomers of butyl aldehydes are formed:\n\nUnlabelled Image\n\n\n\nOne of the most effective methods of chemical processing of carbonyl-containing compounds is heterogeneous catalytic liquid-phase hydrogenation into the corresponding alcohols, which are in huge demand in the industry [4\u20139].Skeletal nickel catalyst, characterized by high activity and low cost, has been widely used in industry as well in several studies on the liquid-phase catalytic hydrogenation of oxygen- and nitrogen-containing organic compounds [10,11].To increase the stability and activity of catalysts in the hydrogenation of saturated aldehydes, promoting additives are employed [11].Analysis of the characteristics of industrial processes for the hydrogenation of butyl aldehyde, used in various countries, showed that each of these processes, along with advantages, has several disadvantages. Butyl alcohols are produced on an aluminum\u2011nickel\u2011titanium catalyst. However, the specified catalyst has low activity and productivity. The hydrogenation of butyl aldehyde at this contact occurs under severe conditions \u2013 high temperature (up to 170\u00a0\u00b0C) and pressure (up to 25\u00a0MPa), leading to a decrease in the process selectivity and an increase in the process cost [12,13].To address this problem, the search continues for effective catalytic systems and the selection of optimal technological parameters, conditions close to the environment (T, P), and a solution of harmless environment-friendly supercritical liquids for the hydrogenation of butyl aldehyde, considering the requirements of the principles of green technology [14].The authors studied promoted-skeletal nickel catalysts with the addition of platinum group metals [15,16]. They found that the rate of hydrogenation of carbonyl-containing compounds depends on the amount of promoter and passes through a maximum rate. Ruthenium is the most active catalyst in the hydrogenation of carbonyl compounds.The development of new selective modified catalysts for the hydrogenation of aldehydes contributes to improving the technology for the production of butyl alcohols, which are used as solvents for organic substances as well as for the synthesis of pharmaceuticals.The authors studied skeletal nickel [17,18] and supported [19\u201321] catalysts in the reactions of liquid-phase hydrogenation of carbonyl-containing compounds.In this study, the kinetic regularities of butyl aldehyde hydrogenation in solution at atmospheric pressure of hydrogen on skeletal Ni-Ru and Ni-Rh catalysts were investigated.This work aims to improve industrial processes for the selective hydrogenation of normal butyl aldehyde to primary butyl alcohol by developing new and modified heterogeneous catalysts considering various technological and economic requirements.Catalyst alloys were prepared in a high-frequency furnace. Nickel and aluminum granules and ruthenium or rhodium powder were used as metals for the preparation of alloy catalysts. To examine the rate of butyl aldehyde hydrogenation, alloyed nickel catalysts with the addition of ruthenium or rhodium were prepared. The amount of ruthenium or rhodium in the catalyst varied from 0.5% to 10%. The aluminum content in the initial alloy remained constant at 50%. The activation technique for promoted skeletal nickel catalysts consists of leaching 1.0\u00a0g of the alloy in a 20% NaOH solution at 100\u00a0\u00b0C for 1\u00a0h. The resulting catalyst was washed from alkali with distilled water to a neutral medium with respect to phenolphthalein.The n-Butyl aldehyde of the grade (chemically pure) TU 6-09-3828-74 was purified before the use by simple distillation and the purity of butyl aldehyde was controlled by chromatography. Distilled water was used as a solvent. The characteristics of the substances used are consistent with the reference data [22]. The hydrogenation reaction of butyl aldehyde was carried out in a glass thermostated duck-type reactor in water at the atmospheric pressure of hydrogen (700\u00a0bar) and a temperature of 20\u00a0\u00b0C [23]. To ensure that the reaction proceeds in the external kinetic region, the stirring speed was maintained at 600\u2013700 unidirectional oscillations per min. The reactor was charged with 0.2\u00a0g of catalyst and 25\u00a0ml of solvent (H2O). Air was displaced from the reactor with a stream of hydrogen. The rocking chair was then switched on, and the catalyst was saturated with hydrogen at the temperature of the experiment for 30\u00a0min. Butyl aldehyde (0.5\u00a0ml) was added to a stream of hydrogen to the reactor. Catalytic activity was measured by the amount of hydrogen absorbed in one min. The state of the catalyst surface was monitored by the change in potential.The butyl aldehyde hydrogenation products were analyzed by gas-liquid chromatography (GLC) on a Chrom-4 chromatograph with a flame ionization detector in isothermal mode using a capillary column with a polar phase 50\u00a0m in length and 0.32\u00a0mm in inner diameter. The temperature in the column was maintained at 90\u00a0\u00b0C; temperature in the evaporation chamber was 200\u00a0\u00b0C; helium served as the carrier gas; the volume of the injected sample was 0.2\u00a0\u03bcl(microliter). Samples of the liquid reaction mixture were collected 2\u20133 times during the experiment [25]. GLC was based on the physicochemical separation of the analyzed components in the gas phase as they passed along a non-volatile liquid deposited on a solid sorbent. The GLC is caused as the components allow to separate and quantify substances in a complex mixture even if they are similar in chemical properties [26].The hydrogenation of primary aldehyde to alcohols takes place as per the following scheme:\n\nUnlabelled Image\n\n\n\nThe catalytic properties of Ni-Ru and Ni-Rh catalysts in the butyl aldehyde hydrogenation reaction were studied in water at 20\u00a0\u00b0C. The preliminary results showed that the aldehyde hydrogenation rate slightly increases with an increase in its concentration in the solution by a factor of 2, and then remains constant. The hydrogenation process is limited by the activation of hydrogen. In all cases, hydrogen is consumed for the hydrogenation reaction, which is theoretically calculated. The kinetic curves consist of two sections. In the first section, a sharp decrease in the rate of hydrogen absorption was observed and in the second section, a smoother decrease in the rate occurred.\nFig. 1\n shows kinetic and potentiometric curves of butyl aldehyde hydrogenation on skeletal Ni-Ru and Ni-Rh catalysts in water. The ordinate axis shows the rate of butyl aldehyde hydrogenation (W), expressed in hydrogen milliliters absorbed per minute, and the potential of the catalytic system (E) is shown on the down of the ordinate axis. The abscissa axis shows the number of ml of hydrogen (V) absorbed during the reaction.The saturation potentials of nickel skeletal catalysts and promoted skeletal nickel catalysts are 700\u00a0mV and 650\u00a0mV, respectively. The ratio of the saturation potentials of the catalysts shown in Fig. 1 and Table 1\n indirectly indicates the higher activity of Ni-Ru and Ni-Rh catalysts as compared to skeletal nickel catalysts (Niskel). The course of the kinetic curves shows that hydrogenation both on skeletal nickel and on promoted catalysts occurs at a decreasing rate. Varying the content in the alloy from 0.5% to 10% significantly increases the activity of the obtained catalysts.\nTable 1 shows the characteristics of Ni-Ru and Ni-Rh catalysts for the butyl aldehyde hydrogenation in water. Analysis of the data in the table shows that all the Ru and Rh additives increase the catalytic activity of the skeletal nickel catalyst during the butyl aldehyde hydrogenation by 1.7\u20131.9 times.When the hydrogenated substance is introduced into the reaction zone, the potential shift occurs, which indicates the competitive adsorption of hydrogen molecules and the hydrogenated aldehyde. The potential drop (\u25b3E) (as evident in Fig. 1 and Table 1) ranges from 105 to 120\u00a0mV. At the end of the hydrogenation reaction, the potential of the catalysts does not reach the saturation potential by 30\u201340\u00a0mV, due to irreversible adsorption by the reaction products on the surface of the catalysts.Notably, the character of the change in the potentiometric curves of the hydrogenation correlates with that in the process rate. To determine the comparative activity of alloyed nickel catalysts depending on the number of additives, the butyl aldehyde hydrogenation rate on the Ni-Ru and Ni-Rh catalysts was studied. This dependence is shown in Fig. 2\n. The butyl aldehyde hydrogenation rate (W), expressed in ml of hydrogen absorbed per minute, is plotted up the ordinate axis, and the catalyst potential shift (\u25b3E) down the ordinate axis. The abscissa axis exhibits the amount of additive (Me / Ni-Al-Me, %) on the catalyst.\nFig. 2 shows the catalysts' activity dependence after the absorption of 100\u00a0ml of hydrogen on the content of the addition of metals in the alloy during the butyl aldehyde hydrogenation in water. Fig. 2 shows that the optimal content of the additive in the alloy during the butyl aldehyde hydrogenation in the solvent (\u041d2\u041e) is different. Thus, the maximum reaction rate on the Ni-Ru catalysts is observed at the contact with the content of 1.0% Ru. The butyl aldehyde hydrogenation rate on this catalyst is 1.9 times higher in water than in skeletal nickel without additives. The maximum activity of the Ni-Rh catalysts in water corresponds to 1.0% Rh in the alloy, the butyl aldehyde hydrogenation rate on this composition is 1.7 times higher than the corresponding value for skeletal nickel without additives. A further increase in the concentration of Ru and Rh in the alloy leads to a decrease in the activity of the contacts.An increase in the reaction rate is mainly accompanied by a decrease in the value of the anodic shift of the catalyst potential (Fig. 2), which characterizes a decrease in the degree of butyl aldehyde adsorption on the catalytic surface. These Ru and Rh additives help to better activate the reactants from the surface of the mixed catalyst. This conclusion is confirmed by the fact that the surface of promoted skeletal catalysts with an increase in the number of additives in the alloy changes slightly from 28 to 40 m2/g nickel.According to the data of X-ray diffraction studies, the promoting components have a significant effect on the composition and structure of the starting alloys and catalysts. The additives create, in addition to the phases usual for the alloy \u2013 NiAl3, Ni2Al3, and eutectics (NiAl3\u00a0+\u00a0Al), new phases \u2013 F\u0445 have not yet been deciphered. The ratio NiAl3 / Ni2Al3 in the promoted alloys is 1.25 times higher than in the Ni-Al alloy without an additive. Electron microscopic research methods indicate that the additives do not affect the crystal lattice parameter of nickel; however, these additives significantly grind nickel crystals (from 5.4 to 4.7\u00a0nm); increase the specific surface of the catalyst within 110\u2013120\u00a0m2/g Investigations of the particle size distribution using optical microscopy and an electron microscope confirmed that Ru and Rh additives increase the fraction of dispersed catalyst particles, decrease diffusion inhibition in H2, leading to an increase in the reaction rate. The degree of its fatigue and deactivation depends on the ability of the catalyst to renew the initial concentration of adsorbed hydrogen and maintain it at a high level. This conclusion is supported by our data on the activation and stability of promoted catalysts.The hydrogen adsorption capacity was characterized using thermal desorption and conductometry methods.The conductometric method was used to characterize the state of hydrogen adsorbed by the catalysts. The promotion of skeletal nickel with Ru and Rh increases the fraction of weakly adsorbed hydrogen, leading to an increase in the binding energy of adsorbed hydrogen with its surface, thereby causing an increase in the total amount of adsorbed hydrogen. Large quantities of Ru and Rh in the alloy lead to a certain slowdown in the rate of hydrogen diffusion over the surface.Promotion on skeletal nickel leads to a decrease in the activation energy; the lowest value of the activation energy corresponds to the maximum activity of the catalyst, which indicates an increase in the binding energy of hydrogen with the surface. A similar conclusion was drawn from the calculation of the apparent activation energy of the butyl aldehyde hydrogenation reactions in the presence of promoted catalysts, which were 3\u20134\u00a0kcal/mol less than when the process was carried out on relatively non-promoted catalysts.To assess the amount and form of adsorbed hydrogen, the thermal desorption method was applied. The data show that the release of H2 from the surface of the catalysts under study begins at (\u221230\u00a0\u00b0C) and proceeds up to \u2265800\u00a0\u00b0C; however, it proceeds at different rates. An increase in the rate of hydrogen evolution is noted in the temperature range: the first at (\u221220\u00a0\u00b0C); the second main desorption peak is located in the range 130\u2013140\u00a0\u00b0C, and is characterized as a surface-adsorbed form of hydrogen. The third peak of desorption in the temperature range 220\u2013230\u00a0\u00b0C characterizes the structural form of hydrogen.The introduction of Ru or Rh into the skeletal nickel catalyst is accompanied by an increase in the second main desorption peak and its shift to higher temperatures up to 170\u2013180\u00a0\u00b0C, indicating a relative increase in the binding energy of adsorbed hydrogen with the catalytic surface. It confirms the conductometric measurements. The total volume of adsorbed hydrogen varies slightly from 45 to 47\u00a0ml/g.The results obtained are presented in Table 2\n and indicate that the butyl aldehyde hydrogenation process on skeletal nickel is nonselective. The addition of Ru and Rh causes an increase in the selectivity index from 0.59 to 0.94. Comparison of the obtained data on thermal desorption measurements allows to observe a tendency to an increase in the selectivity index with the strengthening of the bond of adsorbed hydrogen with the catalyst surface.Analysis of the reaction products shown in Table 2 reveals that under the hydrogenation conditions, the butyl aldehyde not only turns into butyl alcohol but also undergoes destructive endothermic decomposition [23\u201325]. It should be noted that in the first minute, both butyl alcohol and propane are formed. In the beginning, active surface-adsorbed hydrogen takes part in the formation of butyl alcohol, after a slowdown in the reaction rate, strongly adsorbed hydrogen arriving at the catalyst surface from its depths. Propane is formed during the destructive endothermic decomposition of butyl aldehyde at the time of adsorption.\nTable 3\n shows that the volume of H2 adsorbed by skeletal nickel is 17.6\u00a0ml/g, and the \u041d2 surface is 63.8\u00a0m2/g; for promoted skeletal nickel catalysts, these values increase: in the case of Ru \u2013 26.4\u00a0ml/g and 96.0\u00a0m2/g; in Rh \u2013 30.0\u00a0ml/g and 110.0\u00a0m2/g, respectively.This increase is due to an increase in the area on which energetically homogeneous hydrogen is adsorbed. This hydrogen may have a significant effect on the activity of catalysts in the hydrogenation of compounds, the rate of which is limited by the activation of hydrogen.\nFig. 3a shows the obtained chromatogram of the butyl aldehyde hydrogenation products. Fig. 3b shows the simultaneous formation of butyl alcohol and propane, and the concentration of butyl alcohol reaches 87.1% (Table 2) by the time of the absorption of 0.75\u00a0mol of hydrogen. An intense increase in the concentration of butyl alcohol occurs after butyl aldehyde disappears from the solution. IR spectroscopic studies were carried out (Fig. 4\n) in order to establish the chemical structure of n-butyl alcohol [27].IR spectra were recorded on a Shimadzu IR Prestige-21 FT-IR spectrometer with a Miracle ATR attachment (Pike Technologies) in the frequency range 600\u20134000\u00a0cm\u22121.The method has been developed for the hydrogenation of butyl aldehydes under mild conditions, which helps to reduce energy costs. Novel efficient catalytic systems based on the alloyed nickel catalyst containing Ru or Rh additives have been created, which allow increasing the hydrogenation process rate by 1.7\u20131.9 times as compared to the skeletal nickel catalyst without additives. The hydrogenation of butyl aldehydes under mild conditions proceeds with high selectivity and stability ensures the quality of the target product and is of practical interest for improving the technology for the production of butyl alcohols.None.The study was conducted at the Chair of Oil Refining and Petrochemistry, M. Auezov South Kazakhstan State University, Shymkent, Kazakhstan.", "descript": "\n                  The processes of heterogeneous catalytic hydrogenation of carbonyl-containing compounds into the corresponding alcohols mostly occur under rather harsh conditions; therefore, catalysts and optimal parameters that facilitate the process under mild conditions need to be selected. New ways of synthesizing catalytic systems and the use of harmless, environmentally friendly supercritical liquids as solvents are some of the key areas of green chemistry. At present, due to their high activity, two-component skeletal Ni catalysts are widely used in industry. This work attempts to develop highly efficient catalytic systems based on a three-component alloyed Ni catalyst containing metals (Ru, Rh) for the liquid-phase hydrogenation of aldehydes to butyl alcohols. The physicochemical and catalytic properties of modified alloyed Ni catalysts in the reaction of liquid-phase hydrogenation of butyl aldehyde to butyl alcohol have been investigated. Butyl aldehyde hydrogenation was carried out on a skeletal Ni catalyst promoted with ruthenium or rhodium in water at a temperature of 20\u201325\u00a0\u00b0C and the hydrogen pressure of 0.1\u00a0MPa. It was found that modified catalysts with high activity and selectivity reduce butyl aldehydes to butyl alcohols. The processes of catalyst preparation and butyl aldehyde hydrogenation are scalable and can be used in various processes of organic and petrochemical synthesis.\n               "}
{"full_text": "In recent years, CO2 hydrogenation is the most important process because it not only reduces global warming but also produces value-added products [1\u20135]. The hydrogen obtained from water electrolysis using renewable and sustainable energy sources such as solar, wind, geothermal, biomass etc. can be used to hydrogenate CO2 [6]. A variety of chemicals such as CO, CH4, CH3OH etc. can be obtained through catalytic hydrogenation of CO2 [7\u20139]. CO is a valuable precursor molecule that can be used as a raw material for the production of olefins, methyl alcohols and liquid hydrocarbons for the Fischer-Tropsch synthesis [10,11]. Various precious metals including Au, Pt, Pd, Rh and Ru [12\u201316] and non-precious metals Ni, Fe and Cu [17\u201319] loaded on various supports have been reported for the production of CO.Several metal catalyst have been reported to be active in CO2 methanation reaction including noble metal catalysts such as Pd, Ru and Pt [20\u201322] and non-noble metal catalysts such as Cu, Ni and Co [19,23,24] loaded on various supports such as Al2O3, SiO2, TiO2, CeO2 and ZrO2 [25\u201329]. NiO have been extensively investigated in various fields as catalysts [30,31], sensors [32\u201334] as well as batteries [35,36] due to their high catalytic activity, low cost and low toxicity. \u03b3-Alumina is widely used as a catalyst [37\u201339], adsorbent [40] and catalyst support [41] due its thermal, chemical and mechanical stabilities. Compared with individual performances of NiO and Al2O3, NiO-Al2O3 composites are more efficient and have been used as catalysts [42\u201345], batteries [46,47], sensors [48,49] and adsorbents [50,51]. The non-metal phosphorus element has been proven to serve as a structure stabilizer of alumina [52\u201354]. Meanwhile, the incorporation of phosphorus into alumina brings about the decoration of hydroxyl groups on the surface of alumina accompanied by the formation of POH groups. The dehydroxylation between POH groups results in the formation of PO groups, which will rehydrate into POH groups in the presence of water and consequently reduce the adsorption of water on the adsorption sites of the catalysts, hence enabling the improvement of the hydrothermal stability for the NiO/Al2O3 catalysts. Moreover, the reducibility of metal-alumina have been found to be increased after doping phosphorus [55].In the present work, a series of 5 wt% NiO-xP-Al2O3 (x = 0, 5, 15, 20 wt%) catalysts have been prepared by a modified sol-gel method and investigated their performances in CO2 hydrogenation reaction. The results show that addition of phosphorus can significantly increase the catalytic activity. Compared with phosphorus unloaded 5 wt% NiO-Al2O3 catalyst, phosphorus loaded 5 wt% NiO-xP-Al2O3 catalysts exhibit superior performance.Nickel (II) nitrate hexahydrate (Ni(NO3)2.6H2O, \u2265 98.5 %) and pluronic P-123 (average Mn \u223c5800) were purchased from Aldrich. Ammonium dihydrogen phosphate was purchased from Reanal. Aluminium isopropoxide (\u226598 %) and acetic acid (99.7 %) were purchased from Sigma-Aldrich. Ethanol absolute (99.96 %) was purchased from VWR chemicals. Isopropanol (99.99 %) was purchased from molar chemicals. All chemicals were used as received without any further purification.P-loaded alumina was prepared via a modified sol-gel method. 1.50 g P123 (Mav = 5800) was dissolved in the mixed solution of 30 mL absolute ethanol, 10 mL isopropanol and 0.18 mL acetic acid at ambient temperature. Then, a required amount of ammonium dihydrogen phosphate and 0.015 mol of aluminium isopropoxide were introduced into the above solution under vigorous stirring for 4 h. The product was dried overnight at 80 \u00b0C and calcined in air at 500 \u00b0C for 4 h and then at 1000 \u00b0C for 1 h.The as-obtained P-loaded Al2O3 support was loaded with 5 wt% NiO by the incipient wetness impregnation method using Ni(NO3)2.6H2O aqueous solution as metal precursors. The product was dried overnight at 80 \u00b0C and calcined in air at 500 \u00b0C for 1 h. The final sample was marked as 5 wt% NiO-xP-Al2O3 (x = 0, 5, 15, 20 wt%).The specific surface area (BET method), the pore size distribution and the total pore volume were determined by the BJH method using a Quantachrome NOVA 2200 gas sorption analyser by N2 gas adsorption/desorption at \u2212196 \u00b0C. Before the measurements, the samples were pre-treated in a vacuum at 200 \u00b0C for 2 h.XRD studies of all samples were performed on a Rigaku MiniFlex II instrument with a Ni-filtered Cu-K\u03b1 source in the range of 2\u03b8 = 20\u221280\u00b0.Imaging of the all the samples were carried out using a FEI TECNAI G2 20 X-Twin high-resolution transmission electron microscope (equipped with electron diffraction) operating at an accelerating voltage of 200 kV. The samples were drop-cast onto carbon film coated copper grids from ethanol suspension.The temperature-programmed reduction (TPR) was carried out in a BELCAT-A analyser using a reactor (quartz tube with 9 mm outer diameter) that was externally heated. Before the measurements, the 50 mg of catalyst was pre-treated in oxygen at 400 \u00b0C for 30 min and in N2 at 400 \u00b0C for 15 min. Thereafter, the sample was cooled in flowing N2 to 50 \u00b0C. The oxidized sample was flushed with N2 containing 10 % H2, the reactor was heated linearly at a rate of 5 \u00b0C/min from 50 \u00b0C to 500 \u00b0C and the H2 consumption was detected by a thermal conductivity detector (TCD).The chemical states (and the atomic ratio of the elements) were investigated by X-ray photoelectron spectroscopy (XPS). The SPECS instrument was equipped with a Phoibos150 MCD-9 analyser. The Al K\u03b1 x-ray source was operated at 14 kV and 10.8 mA (150 W) and the analyser was used in FAT mode with a pass energy of 20 eV in the case of high-resolution spectra. CasaXPS software was used for data evaluation. The binding energy was set to the adventitious carbon C1 s peak is at 284.8 eV. The peaks of the P2p were fitted with single due to closely spaced spin-orbit coupling. Specs FG 15/40 flood gun was operated during the accumulation at 0.6 V and 0.3 mA to the prevent the charging of the surface of the sample.Before the catalytic experiments in a continuous-flow reactor the as-received catalysts were oxidized in the O2 atmosphere at 300 \u00b0C for 30 min to remove the surface contaminants and thereafter were reduced in H2 at 300 \u00b0C for 60 min. Catalytic reactions were carried out at atmospheric pressure in a fixed-bed continuous-flow reactor (200 mm long with 8 mm i.d.), which was heated externally. The dead volume of the reactor was filled with quartz beads. The operating temperature was controlled by a thermocouple placed inside the oven close to the reactor wall, to assure precise temperature measurement. For catalytic studies, small fragments (about 1 mm) of slightly compressed pellets were used. Typically, the reactor filling contained 150 mg of catalyst. In the reacting gas mixture, the CO2: H2 molar ratio was 1:4, if not denoted otherwise. The CO2: H2 mixture was fed with the help of mass flow controllers (Aalborg), the total flow rate was 50 mL/min. The reacting gas mixture flow entered and left the reactor through an externally heated tube in order to avoid condensation. The analysis of the products and reactants was performed with an Agilent 6890 N gas chromatograph using HP-PLOTQ column. The gases were detected simultaneously by thermal conductivity (TC) and flame ionization (FI) detectors. The CO2 was transformed by a methanizer to methane and it was also analyzed by FID. CO2 conversion was calculated on a carbon atom basis, i.e.\n\n\n\n\nC\nO\n\n\n2\n\u2009\n\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\n%\n\n=\n\n\n\n\nC\nO\n\n\n2\n\u2009\ni\nn\n\n\n-\n\n\nC\nO\n\n\n2\n\u2009\no\nu\nt\n\n\n\n\n\n\nC\nO\n\n\n2\n\u2009\ni\nn\n\n\n\n\n\u00d7\n100\n%\n\n\n\nCH4 selectivity and CO selectivity were calculated as following\n\n\n\n\nC\nH\n\n\n4\n\u2009\n\n\ns\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\n%\n\n=\n\n\n\n\nC\nH\n\n\n4\n\u2009\no\nu\nt\n\n\n\n\n\n\nC\nO\n\n\n2\n\u2009\ni\nn\n\n\n-\n\n\nC\nO\n\n\n2\n\u2009\no\nu\nt\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\nC\nO\n\n\u2009\n\ns\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\n%\n\n=\n\n\n\n\nC\nO\n\n\n\u2009\no\nu\nt\n\n\n\n\n\n\nC\nO\n\n\n2\n\u2009\ni\nn\n\n\n-\n\n\nC\nO\n\n\n2\n\u2009\no\nu\nt\n\n\n\n\n\u00d7\n100\n%\n\n\nwhere \n\n\nC\nO\n\n\n2\n\u2009\ni\nn\n\n\n and \n\n\nC\nO\n\n\n2\n\u2009\no\nu\nt\n\n\n represent the \n\n\nC\nO\n\n2\n\n concentration in the inlet and outlet respectively, \n\n\na\nn\nd\n\u2009\nC\nH\n\n\n4\n\u2009\no\nu\nt\n\n\n\u2009\na\nn\nd\n\u2009\n\n\nC\nO\n\n\n\u2009\no\nu\nt\n\n\n\u2009\nr\ne\np\nr\ne\ns\ne\nn\nt\n\u2009\nt\nh\ne\n\u2009\na\nm\no\nu\nn\nt\n\u2009\n\n\no\nf\n\u2009\nf\no\nr\nm\ne\nd\n\u2009\nC\nH\n\n\n4\n\u2009\n\n\n and CO, respectively.\nFig. 1\n presented the XRD pattern of all the catalysts. The diffraction peaks observed at 2\u03b8 = 21.9\u00b0, 28.2\u00b0, 31.1\u00b0 and 35.8\u00b0 are assigned to diffraction planes (111), (021), (112) and (220) of AlPO4-tridymite crystal structure (JCPDS No. 11-0500) with no impurity phases [56]. The small peak at 20.6\u00b0 may be due to the presence of structural defects [57]. The peak intensity increased with phosphorous content, suggesting the growth of AlPO4-tridymite crystal phase. All the phosphorus loaded samples exhibited no characteristic diffraction peaks related with \u03b1-Al2O3 and NiO phases. 5 wt% NiO-Al2O3 catalyst exhibited main peaks at 2\u03b8 of 19.8\u00b0, 32.7\u00b0, 37.3\u00b0, 39.5\u00b0, 45.6\u00b0, 60.6\u00b0, 66.9\u00b0 are assigned to diffraction planes (111), (220), (311), (222), (400), (511) and (440) of cubic \u03b3-Al2O3 crystal phase (JCPDS No. 29-0063) [58]. There is no obvious signal of NiO phase detected in the XRD patterns of 5 wt% NiO-Al2O3 catalyst except a low intense peak at 2\u03b8 = 43.4\u00b0 as a result of overlapping with the \u03b3-Al2O3 peaks and low NiO content.The BET surface area, pore volume and pore size of the catalysts were summarized in Table 1\n. All the catalysts showed type H3 hysteresis loop indicating the presence of slit shaped pores. The pore size distribution reveals the existence of mesopores in the range 2\u20139 nm. It is clear that the introduction of phosphorus into the 5 wt% NiO-Al2O3 results in considerable decrease in the surface area and pore volume with 22.67 m2/g and 0.06 cm3/g respectively for 5 wt% NiO-20P-Al2O3 in comparison with 100.52 m2/g and 0.26 cm3/g for 5 wt% NiO-Al2O3. This can be attributed to the blocking of pores of 5 wt% NiO-Al2O3 after phosphorus loading.The morphology and particle size of some of the catalytically active catalysts were examined by TEM measurements and shown in Fig. 2\n. All the catalysts show nanoobjectives with porous structures with a building blocks of spherical shaped morphology with the size of \u223c2\u221215 nm.The reducibility of the catalysts was studied by the H2-TPR technique and the results are shown in Fig. 3\n. 5 wt% NiO-Al2O3 catalyst exhibited two reduction peaks. The low temperature peak at 406 \u00b0C was attributed to reduction of NiO not bound with Al2O3 and high temperature peak at 594 \u00b0C to the reduction of NiO that weakly interacted with Al2O3 [59\u201362].When phosphorus was added into the NiO-Al2O3 catalyst, there is a shift in reduction peaks of NiO towards lower temperature indicates that the addition of phosphorus facilitates the reduction of NiO. This observation corroborated well according to the previous work reported over Pd/xP-OMA catalyst [63]. The total H2 consumptions have been summarized in Table 2\n.The catalytic performances of all the 5 wt% NiO-xP-Al2O3 (x = 0, 5, 15, 20) catalysts were evaluated at atmospheric pressure in the temperature range from 200 \u00b0C to 600 \u00b0C. The CO2 conversion is shown in Fig. 4\na. CO2 conversion increased with temperature raising from 200\u2212600 \u00b0C. All the catalysts reach the highest conversion at 600 \u00b0C with 61.54 %, 62.89 %, 63.88 % and 66.13 % respectively for 5 wt% NiO-Al2O3, 5 wt% NiO-5P-Al2O3, 5 wt% NiO-15P-Al2O3 and 5 wt% NiO-20P-Al2O3. The CO2 conversion decreases in the order 5 wt% NiO-20P-Al2O3 > 5 wt% NiO-15P-Al2O3 > 5 wt% NiO-5P-Al2O3 > 5 wt% NiO-Al2O3.The main products identified during the catalytic reaction were CO and CH4. The CH4 and CO selectivity results are shown in Fig. 4b and c respectively. CH4 selectivity also depends on phosphorus loading, with 5 wt% NiO-xP-Al2O3 (x = 5, 15, 20) catalysts forming high CH4 than 5 wt% NiO-Al2O3 catalyst. The CH4 selectivity is high at low temperature and decreases to 64.2 %, 58.8 % and 56.7 % for 5 wt% NiO-20P-Al2O3, 5 wt% NiO-15P-Al2O3 and 5 wt% NiO-5P-Al2O3 catalysts respectively at 450 \u00b0C and then increased and decreased. The CO selectivity first increases then decreases and then again increases as the temperature increases. 5 wt% NiO-Al2O3 catalyst displayed higher CO selectivity. Gao et al. reported that, above 450 \u00b0C the formation of CO increases as a result of reverse water gas shift reaction while the CH4 formation decreases as a result of exothermic nature of CO2 methanation [64].\nFig. 5a shows the Arrhenius plot for CO2 hydrogenation over all the 5 wt% NiO-xP-Al2O3 (x = 0, 5, 15, 20) catalysts. The apparent activation energies were calculated from the slopes in the temperature range of 350\u2212450 \u00b0C resulting in values of 56.3 kJ mol\u22121 for 5 wt% NiO-Al2O3, 68.3 kJ mol\u22121 for 5 wt% NiO-5P-Al2O3, 49.1 kJ mol\u22121 for 5 wt%NiO-15P-Al2O3 and 45.5 kJ mol\u22121 for 5 wt% NiO-20P-Al2O3. The apparent activation energy decreases with increase in phosphorus loading. 5 wt% NiO-20P-Al2O3 catalyst had the lowest apparent activation energy and this is in line with the catalytic activity.\nFig. 5b shows the time on stream results of all the catalysts for CO2 hydrogenation at 600 \u00b0C. It can be seen that all the catalysts exhibit almost similar stability indicates that phosphorus addition does not improve the stability of the catalysts.The used catalysts have been characterized by XRD in order to identify possible structural changes (Fig. 6\n). Besides NiO and AlPO4 form, the XRD patterns revealed new peaks at 2\u03b8 = 44.5\u00b0, 51.8\u00b0 and 76.3\u00b0 for metallic nickel (JCPDS No. 65-2865) [65]. This metallic nickel is supposed to be formed during the pre-treatment with H2 prior to the catalytic reaction and is responsible for H2 dissociation during CO2 hydrogenation.For better understanding of the outstanding activity of phosphorous loaded catalysts, XPS studies were performed on the spent catalysts. The XPS was measured to study the chemical composition and valence states of the surface of the used 5 wt% NiO-xP-Al2O3 (x = 5, 15, 20) catalysts. The core level Ni 2p spectrum is shown in Fig. 7\na. The peak at binding energy of 856.5 eV is ascribed to Ni 2p3/2 and another peak at binding energy of 874.4 eV is ascribed to Ni 2p1/2. The two relatively weak peaks at 861.9 eV and 880.7 eV belong to shakeup satellites indicating the presence of NiO [66]. The small shoulder peak at 852.7 eV is ascribed to the presence of metallic nickel [67]. The peak at 74.4 eV is ascribed to the presence of either Al2O3 or AlPO4 [68]. However, based on the XRD results of the used catalyst, it could be expected as AlPO4. Fig. 7b. shows P 2p XPS spectra. The peak at 134.4 eV is ascribed to the presence of AlPO4 [69]. The small peak at 130.3 eV can be assigned to the Ni2P/Ni5P12 arising from the interaction of nickel with phosphorus [70]. This indicates the existence of Ni/NiO/Ni2P/Ni5P12/AlPO4 interfacial species in the catalyst during the reaction and this interfacial species increased with phosphorus loading and can be correlated with the higher catalytic activity.The formation ratio of the metal-phosphide is relatively low \u223c3\u22125 % (Table 3\n) and this atomic concentration is decreasing with the rising of the phosphate content. However, the nickel enrichment in the surface layer presumable in Ni2P/Ni5P12 form is very likely according to the P 2p spectra. The authors assume that could be responsible for the enhanced catalytic activity. In the C 1s XPS spectra, three peaks were identified. The first peak at 284.78 eV is assigned to the CC and CH\n hydrocarbon and the second peak at 286.48 eV is assigned to COH arising from surface contamination. The third peak at 288.84 eV are assigned to CO and OCO\n\n species, which can be attributed to a surface contamination component or a solvent degradation product. In the O 1s XPS spectra, two peaks were identified. The first peak at 531.9 eV is assigned to CO and the second peak at 530.63 eV is assigned to the lattice oxygen, OCO [71]. The corresponding binding energies and atomic percentages as well as the phosphate and phosphide ratio according to the deconvoluted XPS spectra are reported in Table 3.5 wt% NiO-xP-Al2O3 catalysts were prepared by a modified sol-gel method. The effect of phosphorus addition on NiO-Al2O3 catalyst in CO2 hydrogenation was investigated. The BET surface area decreased with increasing phosphorus doping. The phosphorus loaded NiO-Al2O3 exhibited enhanced performance towards CO2 hydrogenation compared with pure NiO-Al2O3. Ni/NiO/Ni2P/Ni5P12/AlPO4 interfacial species were detected as active species on the used catalysts by X-ray photoelectron spectroscopy. The formation ratio of the metal-phosphide is relatively low \u223c3\u22125 %, and this atomic concentration is decreasing with the rising of the phosphate content. However, the nickel enrichment in the surface layer presumable in Ni2P/Ni5P12 form is very likely according to the P 2p spectra and the authors assume that could be responsible for the enhanced catalytic activity. This work will not only help in understanding the role phosphorus in CO2 hydrogenation reaction but also provide insights for future design and development of high performance phosphorus loaded catalysts.The authors have nothing to declare about Credit of Statement.The authors declare no competing financial interest.This paper was supported by the Hungarian Research Development and Innovation Office through grants NKFIH OTKA PD 120877 of AS. \u00c1K, and KZ is grateful for the fund of NKFIH (OTKA)\nK112531 & NN110676 and K120115, respectively. The financial support of the Hungarian National Research, Development and Innovation Office through the GINOP-2.3.2-15-2016-00013 project \"Intelligent materials based on functional surfaces - from syntheses to applications\" and the Ministry of Human Capacities through the EFOP-3.6.1-16-2016-00014 project and the 20391-3/2018/FEKUSTRAT are acknowledged.", "descript": "\n                  A series of 5 wt% NiO-xP-Al2O3 with different phosphorus loading contents (x = 0, 5, 15 and 20 wt%) were prepared by a modified sol-gel method. A significant promotional effect of phosphorus on NiO-Al2O3 in CO2 hydrogenation is observed. All the catalysts reach the highest conversion at 600 \u00b0C with 61.54 %, 62.89 %, 63.88 % and 66.13 % respectively for 5 wt% NiO-Al2O3, 5 wt% NiO-5P-Al2O3, 5 wt% NiO-15P-Al2O3 and 5 wt% NiO-20P-Al2O3 catalysts. Ni/NiO/Ni2P/Ni5P12/AlPO4 interfacial species were detected on the surface as active species on the used catalysts by X-ray photoelectron spectroscopy. The formation ratio of the metal-phosphide is relatively low \u223c3\u22125 %, and this atomic concentration is decreasing with the rising of the phosphate content. However, the nickel enrichment in the surface layer presumable in Ni2P/Ni5P12 form is very likely according to the P 2p spectra and the authors assume that could be responsible for the enhanced catalytic activity.\n               "}
{"full_text": "With the increasingly serious global environmental degradation and energy crisis, renewable energy techniques have attracted considerable attention of scientific researchers [1\u20134]. Among numerous technologies for energy storage and conversion, over water splitting, which can achieve by green photocatalysis or electrocatalysis, has become a star because of its advantages of friendly environment, rich resources, and strong sustainability. However, the water splitting mode still faces many unavoidable barriers on the path to the large-scale practical application [5\u20137]. Some of the most difficult aspects consist in its oxygen evolution reaction (OER) as a half reaction, which is coupled with hydrogen evolution reaction (HER) for water splitting [8,9]. Since the OER process has an innate four-electron transmission mechanism different from the two-electron one in HER, the complex reaction paths slow down the electrocatalytic kinetics and the inferiority of energy barriers is more prominent for over water splitting [10]. The most straightforward approach to surmounting energy penalty is to optimize the design of catalyst. However, although noble metal catalysts (e. g., Ir- and Ru-based catalysts) have appropriate theoretical free energy, the well-known scarcity and high cost limit their large-scale application, so the research trend is biased towards the non-noble substitutes [11]. As reported, extensive research focuses on various transition metal-based oxides, hydroxides, sulfides, phosphates and selenides, etc [12\u201315]. These semiconductors can be used as excellent OER catalysts without exception since their following characteristics: outstanding inherent activity, high density of active sites, excellent electrical conductivity, and rapid transport of reaction species.Among various transition metal-based catalysts, nickel-containing catalysts undoubtedly garner focused attention because of their earth-abundant superiority and potential in the field of water oxidation [16,17]. Meanwhile, the Ni(OH)2 as a common member has been constantly optimized to improved OER catalytic performance in wide researches, including different preparation methods (hydrothermal [13], solvothermal [18], electrodeposition [19], etc. methods), multifarious modification for distinctive morphologies (hollow sphere [18], nanoplate [20], nanoparticle [18], etc.) and particular phase-dependent superiority (\u03b1 or \u03b2 phase) [21,22]. Although these studies give prominence to the advantages of Ni(OH)2 in their increasing inherent activity, there is still a need to design novel Ni(OH)2 catalysts with more excellent electronic conductivity and long-term stability through structural optimization.To our knowledge, building the one-dimensional, porous, and hollow characteristics is ideal optimized modification to serve high-performance electrocatalyst candidates. Because they can endow catalysts with high surface roughness, excellent permeability, large surface area (active site density) and plenty channels to meet the deep-seated chemical accessibility between electrolyte and electrode, as well as the rapid charge and mass transport. Moreover, the resulting low density will also save resource consumption [23\u201326]. Therefore, preparing tubular Ni(OH)2 with one-dimensional hollow structure will further improve its catalytic performance.The template-assisted method is a reliable and available method to prepare one-dimensional hollow materials, and when considering the formation of Ni(OH)2 with these unique morphologies, Cu2O is a promising choice as sacrificial template [27\u201330]. As we know, the Cu2O are perfect to be used as hard template for the preparation of hollow structures, because of its low cost, abundant reserves, reachable control of morphology and size. So various hollow or core-shell structures have been widely reported, such as copper sulfide [31], mental alloy [32], metallic oxide/hydroxide cages [33\u201336]. Among the Cu2O pre-shaped templates with various morphologies, Cu2O wires with the simple preparation are especially ideal to serve the hollow tubular construction. Here, Pearson's hard and soft acid\u2013base (HSAB) principle, namely soft Lewis acids tend to combine with soft bases while hard acids prefer hard bases, provides a new inspiration for Cu2O-templated strategy to form hollow transition metal hydroxides/oxides, which always act as superior electrocatalysts and exhibit enhanced ionic conductivity and robust stability [33\u201336]. In addition, theoretically, choosing an appropriate etchant (like Na2S2O3) to react with Cu2O can realize the synthesis of hydroxide and the etching of Cu2O in one step, which will be a simple and feasible liquid-phase preparation strategy of hydroxide with hollow structure. Therefore, it is of great significance to develop a liquid-phase synthesis strategy with Cu2O wires as templates to form Ni(OH)2.Herein, we firstly propose a synthesis strategy of Ni(OH)2 hollow tubes (Ni(OH)2 HTs) with the above-mentioned hollow, porous and one-dimensional structural features, in which the etching of as-prepared Cu2O wires sacrificial template and the formation of Ni(OH)2 occurred simultaneously. The obtained Ni(OH)2 HTs were confirmed with immensely enlarged specific surface area to create accessible active sites, numerous charge transfer channels for fast kinetics, and a stable structural foundation inherited from Cu2O wires for long-term stability. Therefore, it displayed excellent electrochemical performance when used as OER catalyst, with a low overpotential of 207\u00a0\u200bmV to reach 10\u00a0\u200bmA\u00a0\u200bcm\u22122 current density, an ideal Tafel slope of 79.8\u00a0\u200bmV dec\u22121, and undiminished sustainability of current density for 24\u00a0\u200bh. The innovative avenue for preparing hollow one-dimensional transition metal hydroxide in this work will open a new approach for the design of electrocatalysts with distinctive structure-activity relationship.All the reagents in this paper were utilized as received without further purification and had analytic purity grade. Polyvinyl pyrrolidone (PVP), pyrrole and Cu(Ac)2 were purchased from Macklin Biochemical Co., Ltd. NiCl2\u00b76H2O, Na2S2O3\u00b75H2O and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. KOH was purchased from Aladdin Reagent Co. Ltd.According to the hydrothermal method previously reported [37], the preparation process was further specifically clarified as follows. Typically, a 30\u00a0\u200bmL deionized (DI) aqueous solution of 100\u00a0\u200bmg Cu(Ac)2 was prepared. Under 40\u00a0\u200b\u00b0C water bath, 67\u00a0\u200bmg pyrrole was dissolved in 10\u00a0\u200bmL DI water (0.1\u00a0\u200bM), and then the pyrrole solution was added to the Cu(Ac)2 solution with continuous stirring for 30\u00a0\u200bmin. Afterward, the mixture was transferred into a 50-mL Teflon-lined stainless steel autoclave and maintained at 180\u00a0\u200b\u00b0C for 10\u00a0\u200bh. When cooled down to ambient temperature, the precipitate was collected by centrifugation and washed several times with DI water and ethanol. Then, the product was dried at 50\u00a0\u200b\u00b0C in vacuum for 24\u00a0\u200bh.Firstly, 333\u00a0\u200bmg PVP was dispersed in 10\u00a0\u200bmL DI water/ethanol solvent with volume ratio of 1:1. Then 5\u00a0\u200bmg as-prepared Cu2O wires and 6.66\u00a0\u200bmg NiCl2\u00b76H2O were dissolved into the PVP solution, following by ultrasonication for 20\u00a0\u200bmin and stirring for 40\u00a0\u200bmin. Afterward, 4\u00a0\u200bmL of 1\u00a0\u200bM Na2S2O3\u00b75H2O aqueous solution was prepared under ice bath and added dropwise into the above mixture solution with vigorous stirring. After dropping and a continuous stirring for 30\u00a0\u200bmin, the final product was collected by centrifugation, washed several times with DI water and ethanol, and finally dried at 50\u00a0\u200b\u00b0C in vacuum for 24\u00a0\u200bh.The compositions, structural features and chemical states of all samples were identified using X-ray diffraction of X'Pert Pro MPD with Cu K\u03b1 radiation and \u03bb as 0.154\u00a0\u200bnm (XRD, Nalytical, the Netherlands), X-ray photoelectron spectroscopy of PHI 5000 VersaProbe III (XPS, Ulvac-Phi, Japan), transmission electron microscopy of JEM-2200FS with a 200\u00a0\u200bkV accelerating voltage energy (TEM, JEOL Japan) equipped with energy dispersive X-ray spectroscopy (EDX), scanning electron microscope of SUPRA 55 (SEM, ZEISS, Germany), and nitrogen adsorption/desorption isotherm of Autosorb iQ (Quantachrome, USA).The electrochemical measurements were carried on a CorrTest electrochemical workstation and in a standard three-electrode configuration (about 25\u00a0\u200b\u00b0C room temperature). The Ag/AgCl, carbon rod and sample-modified nickel foam were used as reference, counter and working electrode, respectively. The slurry was prepared by mixing the catalyst and acetylene black in a 4:1\u00a0\u200bwt ratio in 500\u00a0\u200b\u03bcL ethanol and 20\u00a0\u200b\u03bcL Nafion. Then the slurry was pasted uniformly onto a Ni foam (1\u00a0\u200bcm\u00a0\u200b\u00d7\u00a0\u200b1\u00a0\u200bcm) and dried in vacuum to obtain a working electrode with a catalyst mass of 4\u00a0\u200bmg. The tested potentials were converted into reversible hydrogen electrode (RHE) scale via Nernst equation: ERHE\u00a0\u200b=\u00a0\u200bEAg/AgCl\u00a0\u200b+\u00a0\u200b0.0591\u00a0\u200bpH\u00a0\u200b+\u00a0\u200b0.1989. The electrochemical impedance spectra (EIS) were recorded in frequency of 0.1\u2013105\u00a0\u200bHz with an alternating current potential amplitude of 5\u00a0\u200bmV. The 1\u00a0\u200bM KOH was used as electrolyte. A constant 1.484\u00a0\u200bV (vs RHE) potential was exerted for chronoamperometric measurement. All electrochemical test results were processed without IR correction.In Fig.\u00a01\n, the scheme shows the fabrication process of Ni(OH)2 HTs, which was inspired by Pearson's HSAB principle. At the beginning, the as-prepared Cu2O wires were successfully synthesized through a previously reported hydrothermal method [37], which played the role of sacrificial templates for the further formation of Ni(OH)2 HTs. According to HSAB principle, since Cu+ possesses soft acid feature, the soft base ligand, S2O3\n2\u2212 was chosen as the coordinating etchant to facilitate a soft-soft interaction, which is more stable than a soft-hard one between Cu+ and O2\u2212 existing in Cu2O. Thus, in the step (A) in Fig.\u00a01, the following chemical reactions occurred [36]:\n\n\n\n\n\n\n\n\nCu\n2\n\nO\n\n+\n\nx\n\nS\n2\n\n\n\nO\n3\n\n\n2\n-\n\n\n+\n\n\nH\n2\n\nO\n\u2192\n\n\n[\n\n\nCu\n2\n\n\n\n(\n\n\nS\n2\n\n\nO\n3\n\n\n)\n\nx\n\n\n]\n\n\n\n2\n-\n2\n\nx\n\n\n+\n\n\n\n2\nOH\n\n-\n\n\n\n\n\n\n\n\nS\n2\n\n\n\nO\n3\n\n\n2\n-\n\n\n\n+\n\u00a0\u200b\n\nH\n2\n\nO\n\u2192\n\u00a0\u200b\n\nHS\n2\n\n\n\nO\n3\n\n-\n\n+\n\u00a0\u200b\n\nOH\n-\n\n\n\n\n\n\n\n\nNi\n\n2\n+\n\n\n\n+\n\u00a0\u200b\n\n\n2\nOH\n\n-\n\n\u00a0\u200b\n\u2192\n\u00a0\u200bNi\n\n\n(\nOH\n)\n\n2\n\n\n\n\n\n\n\n\n\nIn the above process, while the surface of Cu2O wires were etched, the OH\u2212 derived from hydrolysis and etching effect of S2O3\n2\u2212 was combined with Ni2+ to form Ni(OH)2 clinging to the surface of Cu2O. Then, due to the complete depletion of Ni2+, only the etching of Cu2O took place in the next step (B), that was, the formation of Ni(OH)2 hollow structure.The composition and morphology of Cu2O were characterized in detail. As shown in Fig.\u00a02\na, the XRD pattern of Cu2O wires templet indicates that the main diffraction peaks could be assigned to cubic Cu2O (PDF #78\u20132076), but there were also several weak peaks corresponding to Cu phase (PDF #85\u20131326), which may be caused by the excessive reduction of pyrrole as reducing agent. The SEM image (Fig.\u00a02b) suggests that the length and diameter of Cu2O wires were not identical from each other (about dozens of microns in length and 90\u2013600\u00a0\u200bnm in diameter), but these wires had identical smooth surface, solid interior and single crystal property which reflected in the TEM image and corresponding selected area electron diffraction (SAED) pattern (Fig.\u00a02c and its inset). Then, the distinct lattice fringe in HRTEM of Fig.\u00a02d exhibited an interplanar distance of about 0.21\u00a0\u200bnm in accordance with the (2 0 0) plane of cubic Cu2O. In addition, the homogeneous distribution of Cu and O elements on EDX mapping (insets in Fig.\u00a02d) and the appropriate atomic ratio of Cu/O (Figs.\u00a0S1 and 61.69/38.31) all indicated the successful preparation of Cu2O wires.Similarly, the crystallographic and morphologic structure of Ni(OH)2 HTs transformed from Cu2O wires was characterized. In the XRD spectrum (Fig.\u00a0S2a), there were several sharp diffraction peaks corresponding to Cu2O that had not been completely etched, and the characteristic peak positions of the prepared sample located at the rhombohedral \u03b1-Ni(OH)2\u00b70.75H2O (PDF #38\u20130715), indicating the successful synthesis of the final product as Ni(OH)2\u00b70.75H2O. The TEM and SEM images (Fig.\u00a03\na and Fig.\u00a0S2b) show that Ni(OH)2 tubes retained the wire-like appearance of Cu2O with nonuniform sizes and had the diameter range of 90\u2013620\u00a0\u200bnm. Furthermore, the more detailed TEM image of a broken Ni(OH)2 HT further proves the hollow structure (about 17\u00a0\u200bnm in thickness of tube wall) and significantly rougher surface of the etched Ni(OH)2 tubes when compared with the Cu2O wires template, which both are essential conditions to improve catalytic performance. In Fig.\u00a03b, the interplanar distances of approximate 0.20\u00a0\u200bnm were observed from HRTEM image corresponding to (0 1 8) plane of Ni(OH)2\u00b70.75H2O, and the polycrystalline nature could be also testified by the SAED pattern as the inset. Besides, in EDX mapping (Fig.\u00a03c), a Cu2O wire that was not completely etched (red wireframe 1) and a final as-prepared Ni(OH)2 hollow tube (red wireframe 2) were observed at the same time, which could be proved by elements distribution (Cu, Ni, O) and their atomic ratio (Fig.\u00a0S3, 5.02/32.58/62.40). Concurrently, the above results were also consistent with the XRD result in Fig.\u00a0S2a.The most obvious benefit of hollow structure and rough surface for catalysis is a substantial increase of its specific surface area. Therefore, the correlative analyses were carried out to the Cu2O wires template and the final Ni(OH)2 HTs. The N2 adsorption\u2013desorption isotherm of the Ni(OH)2 HTs (Fig.\u00a04\na) confirms a classic type \u2163 isotherm, which has an apparent hysteresis loop at 0.1-1.0\u00a0\u200bP/P0 and a part of decreasing slope at initial relative pressure range. That is, the as-prepared Ni(OH)2 HTs possessed mesoporous microstructure. And expectedly, an outstanding specific surface area of 63.1\u00a0\u200bm2\u00a0\u200bg\u22121 for Ni(OH)2 HTs was calculated by Brunauer\u2013Emmett\u2013Teller (BET) method. As for the pore size distribution (Fig.\u00a0S4a), the Ni(OH)2 HTs exhibited a mostly centralized distribution that located at 3.05\u00a0\u200bnm, meaning the major existence of mesopores in Ni(OH)2 HTs. As a comparison, the Cu2O wires template was also tested by the same ways (Figs.\u00a0S4b and c), which gave a calculated BET specific surface area of only 6.2\u00a0\u200bm2\u00a0\u200bg\u22121. Evidently, the tenfold enlargement of high specific surface area and the transformation to porous structure are sufficient to illustrate the importance of template method and the advantages of structural modification, which make more approaches for reactants to enter the active material and provide more active sites for the next electrochemical reaction.Then, the XPS was employed to further understand the surface information about elemental composition and the electronic structure of Ni(OH)2 HTs sample. As shown in survey spectrum (Fig.\u00a04b), the observation of Cu 2p, Ni 2p and O 1s peaks with the respective atomic ratio (table in Fig.\u00a04b) corroborated component analysis referred above, meaning the successful synthesis of Ni(OH)2 HTs with few residual elemental Cu. In the high-resolution XPS spectrum of Ni 2p (Fig.\u00a04c), accompanied by two shakeup satellite peaks (denoted as \u201cSat.\u201d) at 880.0 and 861.7\u00a0\u200beV, two intensive peaks that located at 873.8 and 856.1\u00a0\u200beV were conformed to Ni 2p1/2 and Ni 2p3/2, which were good consistent with Ni2+ and further proved expected synthetic Ni(OH)2. The O 1s XPS spectrum can be deconvolved into three peaks (Fig.\u00a04d). The peaks at 532.3 and 531.6\u00a0\u200beV were ascribed to the absorbed water in surface [38] and the metal-oxygen bond resulting from residual Cu2O, respectively. And the one owning a binding energy of 530.9\u00a0\u200beV was related to a hydrated phase of nickel (Ni\u2013O\u2013H) [38,39].To reflect the electrocatalytic activity of as-made catalysts, the OER measurements were performed. Firstly, the linear sweep voltammetry (LSV) curves were measured using a relatively low scan rate of 2\u00a0\u200bmV\u00a0\u200bs\u22121 in order to minimize the capacitive current. And if no additional instructions, all LSV overpotentials mentioned below were chosen uniformly at the benchmark of 10\u00a0\u200bmA\u00a0\u200bcm\u22122 for easy comparison. As comparing all samples in Fig.\u00a05\na, Cu2O was not suitable for OER catalysis because of the high overpotential (358\u00a0\u200bmV) of Cu2O wires. At the same time, introducing Ni foam as substrate did not provide more activity for various catalytic materials, which could be demonstrated by the extremely high overpotential of Ni foam (363\u00a0\u200bmV). In sharp contrast, the Ni(OH)2 HTs catalyst prepared by template method had the most negative overpotential (207\u00a0\u200bmV), even better than commercial RuO2 catalyst (226\u00a0\u200bmV). The comparison of overpotentials intuitively stressed the active advantage of Ni(OH)2 HTs as OER catalyst. In addition, the catalytic performance of Ni(OH)2 HTs outperformed that of other newly reported typical Ni-based catalysts (Fig.\u00a05b and Table\u00a0S1) in terms of overpotential at 10\u00a0\u200bmA\u00a0\u200bcm\u22122 [40\u201346]. It is worth mentioning that the designs of pure phase Ni(OH)2 through different synthetic methods and its representations in OER have been reported in previous reports. For example, Gao and coworkers successfully synthesized nanosheet-assembled \u03b1-Ni(OH)2 hollow spheres, \u03b2-Ni(OH)2 hexagonal nanoplates and \u03b2-Ni(OH)2 nanoparticles through a simple solvothermal strategy, where \u03b1-Ni(OH)2 hollow spheres showed the most effective OER performances with an overpotential of 331\u00a0\u200bmV [18]. Luan' group prepared \u03b1-Ni(OH)2 with various morphologies by lamellar reverse micelles method, including three kinds of layer-stacking Ni(OH)2 (bud-like, flower-like and petal-like) and ultra-large sheet-like Ni(OH)2. Due to the dominant structural effects, the fewer stacked layer petal-like Ni(OH)2 owning more active boundary sites exhibited the best OER activity with an overpotential of 260\u00a0\u200bmV [47]. The above studies show that the structure effects of the catalyst have a great impact on its activity. In comparison, Ni(OH)2 HT in this work has the lowest overpotential among these excellent Ni(OH)2 catalysts.Then, the kinetics advantage of Ni(OH)2 HTs could be evaluated by the Tafel slope which was calculated through LSV date at an extremely slow scan rate of 0.1\u00a0\u200bmV\u00a0\u200bs\u22121. As displayed in Fig.\u00a05c, the Tafel slope of 79.8\u00a0\u200bmV dec\u22121 for Ni(OH)2 HTs was significantly smaller than that of Cu2O wires template (189.5\u00a0\u200bmV dec\u22121) and commercial RuO2 (139.3\u00a0\u200bmV dec\u22121), indicating an excellent OER kinetics in Ni(OH)2 HTs catalyst. Besides the Tafel slop, the EIS Nyquist plot is another important characterization to investigate kinetic behavior defining the interfacial charge transfer of the catalyst [48\u201350]. Fig.\u00a05d shows the Nyquist curve of Ni(OH)2 HTs and its fitting results corresponding to total frequencies (dotted line) and high-frequency region (inset in Fig.\u00a05d) respectively. According to the obtained fitting data, Ni(OH)2 HTs possessed a expectedly low values of solution resistance (Rs, 1.71\u00a0\u200b\u03a9) and charge transfer resistance (Rct, 5.12\u00a0\u200b\u03a9), which were greatly improved when compared with Cu2O wires with the Rs and Rct fitted to 1.99 and 19.21\u00a0\u200b\u03a9 (Fig.\u00a0S5). Both Tafel slopes and EIS results suggested that a rapid charge transfer kinetics existed in Ni(OH)2 HTs to prompt more active catalytic reaction, largely due to the construction of numerous one-dimensional channels. Moreover, the estimated electrochemical double-layer capacitance (Cdl), a slope deduced from a functional relationship between capacitive current and scan rate which all extracted from the cyclic voltammetry (CV) tests (Figs.\u00a0S6a and b), was also used to evaluate the electrochemically active surface area (ECSA) which is proportional to Cdl. As shown in Fig.\u00a05e, the values of Cdl were calculated to be 8.68 and 1.42\u00a0\u200bmF\u00a0\u200bcm\u22122 for Ni(OH)2 HTs and Cu2O wires respectively, illustrating more accessible active sites in Ni(OH)2 HTs with the assistance of template method. Moreover, the ECSA results exactly corresponded to the trend originating from above calculated BET specific surface area.In order to guarantee the application of Ni(OH)2 HTs in practice, the chronoamperometry method was applied to test the long-term durability of the catalyst. With a polarization for a time period of 24\u00a0\u200bh, no significant decay appeared (Fig.\u00a05f), indicating a strong stability in Ni(OH)2 HTs. The robust structural foundation of Ni(OH)2 HTs comes from the stable Cu2O wires frame. Next, the XRD analysis (Fig.\u00a0S7a) exhibited a transformation to amorphous state in Ni(OH)2 HTs after the OER measurement while the XPS curves unveil that the binding energies of Ni 2p and O 1s (Figs.\u00a0S7b and c) had only a weak positive shift, meaning a persistent stability for the catalyst and that the real catalyst for OER might still be transition metal hydroxide with Ni2+ species as the active sites.With the structure-activity relationship revealed by all the above physical and electrochemical characterizations, the advantages of Ni(OH)2 HTs as OER catalyst can be summarized as follows. Firstly, the template-assisted method using the wire-like Cu2O as sacrificial template brought the hollow tubular interior and rough surface to Ni(OH)2 HTs, which greatly increased the specific surface area of the catalyst with more active sites. Thus, more contact opportunities between OH\u2212 in the electrolyte and the active sites could be achieved to improve the catalytic efficiency. Secondly, the tubular configuration of Ni(OH)2 HTs provided numerous channels for the rapid charge/mass transport. Coupled with the relatively good inherent conductivity of Ni(OH)2\u00b70.75H2O, Ni(OH)2 HTs obtained excellent electrochemical kinetics. Finally, the hollow structure evolved from stable Cu2O wires frame was extremely firm, which provided a structural basis for the dynamic OER long-term process.In summary, Ni(OH)2 HTs were synthesized according to HSAB principle through template-assisted method with Cu2O wires as sacrificial template. The stable hollow one-dimensional structure and rough surface from this strategy resulted in greatly increased specific surface area (raising ECSA) for abundant active sites, numerous transmission paths for fast electrochemical kinetics, and sturdy build for OER long-term stability. Benefiting from the structure-activity relationship, the Ni(OH)2 HTs as OER catalyst showed outstanding catalytic activity and kinetics with only overpotential of 207\u00a0\u200bmV to drive the current density of 10\u00a0\u200bmA\u00a0\u200bcm\u22122 and a Tafel slope of 79.8\u00a0\u200bmV dec\u22121. And there was almost no decay to maintain a current density within 24\u00a0\u200bh. The novel preparation pattern in this article provides a new strategy to design transition metal-based hydroxide materials with hollow porous tubular structure, and helps to further dig into the research on advanced catalysts for water splitting.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Key Research and Development Program of China (No. 2018YFA0703700), the National Natural Science Foundation of China (Nos. 12004031, 12034002 and 51971025), Beijing Natural Science Foundation (Grant No. 2212034), Foshan Talents Special Foundation (BKBS202003), Scientific and Technological Innovation Foundation of Foshan (No. BK22BE005) and Foshan Science and Technology Innovation Project (No. 2018/T100363).The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.pnsc.2022.09.002.", "descript": "\n                  Cu2O is an ideal template material for the preparation of transition metal hydroxide/oxyhydroxides with oxygen evolution reaction (OER) enhanced catalytic performance. Here, inspired by Pearson's principle, Cu2O wires were prepared and used as a sacrificial template to prepare Ni(OH)2\u00b70.75H2O hollow tubes (Ni(OH)2 HTs) with highly improved surface roughness. Benefiting from unique structural advantages, the Ni(OH)2 HTs showed excellent catalytic activity, rapid kinetics and a long-term stability as the OER catalyst, where an overpotential of only 207\u00a0\u200bmV was required to drive a current density of 10\u00a0\u200bmA\u00a0\u200bcm\u22122, an ideal kinetics with a Tafel slope as 79.8\u00a0\u200bmV dec\u22121 was calculated, and no obvious attenuation in chronoamperometry was discovered after operation for 24\u00a0\u200bh. This paper provides a novel template-assisted strategy to prepare high-performance transition metal-based OER catalysts possessing hollow and tubular structures.\n               "}
{"full_text": "The metal nanoparticles (MNPs) take over the growing interest in the catalytic processes. MNPs are characterized by a high surface-to-volume ratio providing a large number of active sites (surface atoms) per unit area [1]. The presence of large number of reactive surface atoms enhances the efficiency of the catalytic process as a whole [2,3]. However, the high surface energy of MNPs makes them thermodynamically unstable and more likely to aggregate during the catalytic process, suppressing the catalytic activity. Also, the use of MNPs catalysts is limited due to the difficulty of their separation and recycling.To address these obstacles in homogeneous catalysis, researchers pay their attention in developing novel, green, and efficient heterogonous catalytic systems [4,5]. Loading MNPs onto a solid matrix (metal oxide, inorganic silica, glass, polymers, etc.) has been emerged as an efficient pathway for designing eco-friendly catalytic systems that enable the recyclability and reusability of the MNPs catalysts [6]. However, the heterogeneous catalysts usually show lower activity and selectivity as compared with their homogeneous counterparts, due to the lower surface area available for the interaction between the substrates and the catalyst surface [7]. This is the critical issue that restricts the development of heterogeneous catalytic systems. In this regard, researchers continue to innovate novel heterogeneous catalysts that combines the advantages of both homogenous catalysts (high activity and selectivity, etc.) and heterogeneous catalysts (ease/safe synthetic pathway, efficient separation of MNPs, possibility of recycling the MNPs catalysts, easy catalyst separation, and thermal stability) [8].Polymeric matrices as catalyst supports have attracted a considerable interest in modern catalytic reactions due to their functionality, ease of preparation, excellent mechanical properties, and the ability of surface modifications. The preparation of polymer/MNPs nanocomposites as free-standing film enables the efficient removal and recycling of the catalyst from a catalytic reaction. This is the most required feature from economic and environmental points of view. In recent years, several approaches have been developed to fabricate polymer/MNPs nanocatalyst such as Zn/chitosan/Fe3O4 [9], Ag and Au/cellulose nanocrystals [10], TiO2/polyethersulfone [11], Ni/Hyperbranched polyaromatic polymer [12], Cu/poly(aminobenzoic acid) [13], Fe/coordination polymers [14], and (Fe/Co)/Bis(pentamethylene)pyridyl [15]. Most of these approaches faced with technical and/or economical hardships i.e., they have high capital and operational costs, unacceptable sensitivities to operational conditions, and high energy consumption as well as high sludge generation. An ideal heterogeneous catalyst is expected to overcome the above limitations as well as has a strong activity for a wide range of catalyzed reactions and enables a facile contact between the active sites and chemical substrates.The Graphene oxide (GO) shows outstanding properties such as abundant oxygenated functional groups, large surface area, strong mechanical properties, and chemical stability. These properties make GO as a strong candidate in heavy metal removal from solutions. Recently, it has been found that the dispersion of GO sheets inside polymeric chains suppress the agglomeration/aggregation of GO sheets in the adsorption of heavy metal ions such as Pb2+. The immobilization of GO sheets within a polymeric matrix facilitates the bonding between GO and metal ions leading to an increase in the Pb2+ adsorption capacity from 800 to 1730\u00a0mg\u00a0g\u22121 [16,17].Herein, it was considered worthwhile to develop an eco-friendly nanocomposite of poly(vinyl alcohol) (PVA), reduced graphene oxide (rGO), and nickel (Ni) nanoparticles, as it could be used in catalytic oxidation/reduction of water pollutants. The synthetic route satisfied the cost aspects and avoided the usage of hazardous materials. The physical shape of Ni@rGO/PVA nanocomposite (film-like structure) enabled the efficient separation and reuse of the catalyst for several cycles.An aqueous PVA solution (3% wt/V) was prepared by dissolving PVA powder (PVA, 98\u201399% hydrolyzed, molecular weight from 75000-80000, LOBA CHEMIE) in distilled water at 80\u00a0\u00b0C for 10\u00a0h until a clear solution was obtained. The polymeric solution was cooled naturally to room temperature. GO was prepared using Modified Hummers\u2019 method described elsewhere [18\u201320]. Then, 1\u00a0mL of GO (1\u00a0mg/mL) was added to 10\u00a0mL of PVA solution under magnetic stirrer for 6\u00a0h to attain a mixture of higher homogeneity. The GO/PVA mixture was casted on a polyethylene dish and allowed to dry at room temperature to get the desired film. A 10\u00a0mL of PVA was casted on a polyethylene dish and allowed to dry at room temperature to prepare pure PVA film for comparison.The free-standing film of GO/PVA was immersed into a solution of 1\u00a0M NiCl2 for 24\u00a0h. The polymeric film Ni2+/GO/PVA was removed and left to dry. The polymeric film was soaked in cold solution of NaBH4 (4\u00a0\u00b0C) to reduce Ni2+ to Ni nanoparticles. The Ni@rGO/PVA nanocomposite film was left to dry for the subsequent characterization and stored for future use. The whole preparation steps are summarized in the schematic description shown in Fig.\u00a01\n.The PVA, GO/PVA, and Ni@rGO/PVA nanocomposite films were prepared as (0.5\u00a0\u00d7\u00a00.5\u00a0cm) film and weighed accurately (W0). The samples were soaked in distilled water from 0.0 to 2\u00a0h at room temperature (\u224825\u00a0\u00b0C). The swollen films were removed and the excess water which present on the surface of the PVA films was wiped using a soft tissue and weighed again (W1). The swelling% of the polymeric films was calculated using the following equation:\n\n(1)\n\n\nSwelling\n\n%\n=\n100\n\u00d7\n\n(\n\n\nW\n1\n\n\u2212\n\n\nW\n0\n\n/\n\nW\n0\n\n\n\n)\n\n\n\n\n\nThereafter, the films were dried (at 40\u00a0\u00b0C in an oven) until constant weight was reached (W2). The gel% was calculated according the following relation:\n\n(2)\n\n\nGel\n\n%\n=\n100\n\u00d7\n\n(\n\n\nW\n2\n\n/\n\nW\n0\n\n\n)\n\n\n\n\n\nThe crystalline phase of the prepared Ni@rGO/PVA nanocomposite film was characterized using X-ray powder diffractometer (XRD) of the type Schimadzu 6000 (Japan). A scanning electron microscope (SEM) of the type PHILIPS/FEI QUANTA 250 quipped with Energy dispersive X-ray analysis (EDX) was used to study the morphology of the prepared samples. The UV/visible (UV/vis) spectra were collected using a Jasco V-770 spectrophotometer. A WITec alpha 300\u00a0R confocal Raman microscope was used to record Raman spectra. The samples were excited by 532\u00a0nm laser line. The surface chemical structure of the prepared samples was studied using X-Ray photoelectron spectrometer (XPS, Thermo Scientific K-ALPHA instrument).The catalytic reduction of Cr6+ using Ni@rGO/PVA as a catalyst and formic acid (FA) as a reducing agent was performed in a 15\u00a0mL glass beaker at ambient conditions. In short, a 10\u00a0mL aqueous solution of Cr(VI) (0.7\u00a0mM), 1\u00a0mL FA, and 0.02\u00a0g of Ni@rGO/PVA film were placed in a glass beaker under stirring. The performance of the Ni@rGO/PVA was assessed by following the decay of the 350\u00a0nm-peak of K2Cr2O7 as a function of time. The data were collected in triplicate.A 0.03\u00a0g of Ni@rGO/PVA film was soaked in a volume of 3\u00a0mL aqueous solution of 0.08\u00a0mM 4-NP and 0.5\u00a0mM of NaBH4. The catalytic reduction was followed by observing the decay of the 400\u00a0nm absorption peak using UV/vis spectrophotometer.A 0.03\u00a0g of Ni@rGO/PVA film was dipped into 3\u00a0mL solution containing 0.05\u00a0mM\u00a0MB and 0.5\u00a0M\u00a0H2O2. The catalytic oxidation cycle was observed by following the diminishing of the absorption peak at 664\u00a0nm during the reaction.\nFig.\u00a02\n displays the XRD patterns of PVA, GO/PVA, and Ni@rGO/PVA films. The diffraction peak at 2\u03b8\u00a0=\u00a019\u00b0 represented the semi-crystalline phase of the PVA matrix which is consistent with the previous report [21]. In GO/PVA nanocomposite, the main peak of PVA got weaker and broader. This was attributed to the decreasing crystallinity of PVA due to the probable interactions between GO and PVA segments [22]. The absence of the characteristic peak of GO at 2\u03b8\u00a0=\u00a011\u00b0 [23] might be due to the masking effect of PVA matrix and/or the peak is too weak to be detected. The XRD pattern of Ni@rGO/PVA film showed three new peaks at 2\u03b8\u00a0=\u00a026\u00b0, 44\u00b0, 52\u00b0, and 76\u00b0. The XRD peak at 2\u03b8\u00a0=\u00a026\u00b0 indicated the reduction of GO to rGO sheets under the action of NaBH4. The reduction of GO into rGO by NaBH4 was reported by Muda et\u00a0al. [24]. It is well known that the bulk graphite with interlayer distance of \u22480.34\u00a0nm has a distinct peak at 2\u03b8\u00a0=\u00a026.7\u00b0 [25]. Accordingly, the peak at 2\u03b8\u00a0=\u00a026\u00b0 could be attributed to the formation of restacked rGO layers i.e., confirming the removal of oxygen functional groups during the reduction. The XRD peaks at 2\u03b8\u00a0=\u00a044\u00b0,52\u00b0, and 76\u00b0 are corresponding to the crystallographic planes (111) (200), and (220) of fcc Ni nanoparticles (JCPDS #04\u20130850). The broadening and weakness of the XRD peaks of Ni nanoparticles is in accordance with their small grain size and low degree of crystallinity.SEM imaging was used to characterize the surface morphology of pure PVA, GO/PVA, and Ni@rGO/PVA films. Fig.\u00a03\n(a) shows the SEM image of the pure PVA film where a smooth surface without any distinct morphological features were observed. The GO/PVA showed densely packed distribution of GO with a flat plates shape (see Fig.\u00a03(b)). Some cracks were observed which were attributed to the sensitivity of the PVA film to the electron beam. Fig.\u00a03(c) shows the morphology of Ni@rGO/PVA film. The film is characterized with the appearance of bright dots which could be attributed to the formation of Ni nanoparticles. The SEM results clearly demonstrated the successful formation of Ni nanoparticles onto the GO/PVA film, and no free Ni nanoparticles were present in the image. The EDAX spectrum (Fig.\u00a03(d)) revealed that Ni@rGO/PVA was composed of 44.7% C, 43.89% O, and 11.41% Ni elements without any impurities from precursors.\nFig.\u00a04\n(a) depicted the Raman spectra of the pure PVA film. The Raman peak centered at \u2248 2913\u00a0cm\u22121 is due to the CH2 stretching vibrations mode. The stretching vibration of \u2013CH was appeared at \u2248 1444\u00a0cm\u22121. The fine structure between 1145 and 1100\u00a0cm\u22121 was attributed to the C\u2013O stretching modes, while the C\u2013C stretching modes are observed at 920 and 853\u00a0cm\u22121 [26,27]. The Raman spectrum of GO was shown in Fig.\u00a04(b). The two distinguished bands at 1344 and 1593\u00a0cm\u22121 were due the defective (D) and graphite (G) modes of graphitic materials, respectively. The large intensity of the D-mode was in a good indication for the formation of highly oxidized graphene sheets. The peaks appeared at 2650, 2930, and 3170\u00a0cm\u22121 could be assigned to G\u2032, and D\u00a0+\u00a0G, and 2G\u2032 modes, respectively. The appearance of these peaks is a direct evidence of oxidization of graphene sheets edges. The addition of GO to the PVA matrix did not affect the PVA structure as shown in Fig.\u00a04(c). The D and G modes of the GO at 1344 and 1593\u00a0cm\u22121, respectively, were clearly seen in addition to the main characteristic peaks of the PVA chains at 853, 920, and 2913\u00a0cm\u22121. The PVA and GO peaks did not show any significant shift indicating that GO sheets were mixed with the PVA chain through the weak van der Waals forces. In the case of Ni@GO/PVA film (Fig.\u00a04(d)), in addition to the appearance of the main characteristic peaks of PVA and GO, one can clearly observed the appearance of a low intensity Raman peak at 490\u00a0cm\u22121. It is well known that pure Ni metal is Raman in-active. Accordingly, this peak could be assigned to the formation of few Ni\u2013O bonds on the surface of Ni nanoparticles [28].\nFig.\u00a05\n(a) depicted XPS survey scan spectra of Ni2+ doped GO/PVA before and after reduction with NaBH4. The two strong peaks corresponding to the binding energies at about 284 and 532\u00a0eV are assigned as C1s and O1s orbitals, respectively. The XPS spectra showed an electronic signal at 858\u00a0eV which was attributed to the presence of Ni species within the nanocomposite matrices. The peaks in Fig.\u00a05(b) and (e) depict high-resolution spectra of Ni2p orbitals before and after reduction of Ni2+ ions, respectively. The peaks at 855.8 and 873.4\u00a0eV correspond to binding energies of Ni2p3/2 and Ni2p1/2. Four electronic signals were seen from the high-resolution spectrum of Ni2p before reduction at 856.9, 838.3, 864.4, and 867.9\u00a0eV. These peaks were assigned as the satellites peaks about Ni2p orbital [29]. After reduction of Ni2+ with NaBH4, the high resolution XPS spectrum showed two main peaks at 854.1 and 873.4\u00a0eV. These two peaks were attributed to Ni2p3/2 and Ni2p1/2 of the zero-valent Ni species [30,31].The high-resolution spectra of C1s of Ni2+-doped GO/PVA before and after reduction with NaBH4 were presented in Fig.\u00a05(c) and (f), respectively. The C1s spectrum acquired before reduction (Fig.\u00a05(c)) was resolved into three peaks centered at 284.3, 286.3, 288.2\u00a0eV, arising from C=C/C\u2013C in aromatic rings, C\u2013O of epoxy and alkoxy, and C=O groups [32]. The peak intensity of the oxygenated functionalities was reduced markedly after reduction (see Fig.\u00a05(f)).For the O1s spectrum before reduction (Fig.\u00a05(d)), the 532\u00a0eV peak was resolved into three peaks. The peak from (COOH/C=O/R\u2013O\u2013R) was observed at 532.4\u00a0eV. The peak at 533.5\u00a0eV can be related to the (O\u2013H) groups, and the signal at 536.1\u00a0eV can be assigned as an epoxy group [33]. After reduction (see Fig.\u00a05(g)), the peak area of O1s was decreased markedly as well as the epoxy peak (536\u00a0eV) totally disappeared. The atomic% of C/O from XPS analysis was increased from 1.23% before reduction to 3.14% after reduction, suggesting that some oxygen-containing groups were removed due to the reduction of GO to rGO with NaBH4.The swelling and gel% of PVA, GO/PVA, and Ni@rGO/PVA films were determined and the results were displayed in Fig.\u00a06\n. After \u224815\u201320\u00a0min, pure PVA film adsorbed large quantity of water and a gel-like swollen PVA film was formed at room temperature. It is well known that pure PVA films of moderated molecular weight are characterized by their high degree of water absorption and film completely dissolves at \u2248 37\u201340\u00a0\u00b0C [34,35]. Here, at room temperature, the swollen film lost its mechanical flexibility and teared down to small pieces i.e., no data could be collected. For GO/PVA and Ni@rGO/PVA films, the selling rate was fast within the first 40\u00a0min reaching 750% and 600%, respectively. Thereafter, it gradually slowed down reaching its equilibrium value after 90\u00a0min. The GO/PVA has the highest maximum swelling ratio (\u2248960%) as compared with that of Ni@rGO/PVA (\u2248880%). The gel% of the two samples seemed to not change markedly after soaking in distilled water after 130\u00a0min (see inset of Fig.\u00a06).GO is a two-dimensional (2D) carbon material with one-atom thickness rich with hydrophilic groups i.e. (\u2013OH) (\u2013C=O), and (\u2013COOH) groups [36,37]. These hydrophilic groups improve the miscibility of GO sheets with water-soluble PVA through the formation of hydrogen bonds with the PVA chains. Due to the 2D nature GO sheets, the formed hydrogen bonds between them will result in an increase in both tensile modulus and tensile strength of PVA matrix. On the other hand, the GO nanosheets serve as crosslinking agent that restrain the solubility of PVA film yielding highly hydrophilic and insoluble polymeric matrix [36\u201338]. These findings are consistent with previous data which reported GO as a perfect candidate for fabricating novel superabsorbent hydrogels [39].The investigated catalyst, Ni@rGO/PVA film, has been designed taking into account the environmental and industrial requirements. PVA was chosen as a supporting matrix due to its bio-compatibility, hydrophilicity, and film forming ability that shows good anti fouling properties [40]. However, PVA swells rapidly in aqueous solution until complete dissolution. To overcome this disadvantage, PVA must be crosslinked before it can be used in aqueous media. PVA can be crosslinked chemically (using glutaraldehyde, acetaldehyde, formaldehyde, etc.), and physically (electron beam, \u03b3-irradiation, freeze-thawing cycles, etc.) [41]. These methods require the use of toxic chemicals and laboratory equipment. Also, the crosslinking decreases the equilibrium degree of swelling i.e., the ability of water uptake of the PVA is reduced markedly.GO is enriched with oxygen-containing functional groups, such as (\u2013OH) (\u2013COOH), (R\u2013O\u2013R), (\u2013C=O) and epoxy groups as deduced from the results of XPS. Blending GO with PVA will introduce these polar groups to the polymeric matrix to achieve three tasks. First, the polar groups crosslink the PVA matrix through the formation of hydrogen bonds with PVA chains. This produces the three-dimensional network structures preventing the dissolution of PVA. Second, the presence of the polar groups imbibes water within the polymeric networks and increasing the equilibrium degree of swelling of the PVA matrix. Third, the oxygenated functional groups on the GO surface and \u03c0-electron system provide plentiful binding sites for the adsorption and tight fixing of the Ni2+ ions [42]. In catalytic reactions, when the Ni@rGO/PVA film was soaked within aqueous solution, the polymeric film swells (up to\u00a0\u2248\u00a0900 times of their dry weight) and the dissolved reactants enter the polymeric chains. This will allow the effective adsorption of the reactants onto the catalytic centers (Ni NPs in the present case) where the catalytic process takes place. After reduction, there is a metal\u2013support interaction, which is due to the overlap between the \u03c0-orbitals of graphene with the d orbitals of transition metal atoms. The bonding between Ni particles and graphene sheets secures them against falling off from the composite film [43].The presence of Cr6+ in the industrial wastes induces harmful issues to the humans, animals, and plants. Thus, the reduction of Cr6+ into Cr3+ (which has a low mobility and is not toxic to the most living organisms) is environmentally required to minimize chromium contamination in the environment [44]. The catalytic performance of the as-prepared Ni@rGO/PVA had been studied for the reduction of Cr6+ to Cr3+ in aqueous solution using formic acid. The UV/vis spectrum of K2Cr2O7 and formic acid solution displayed two peaks at 437 and 350\u00a0nm due to ligand-to-metal charge transfer absorption [45]. The mixture (K2Cr2O7 and formic acid) maintained its spectral profile without considerable change for several hours i.e., the addition of formic acid could not reduce Cr6+ in absence of catalyst. In contrast the dipping of Ni@rGO/PVA film in the reacting solution markedly accelerated the reduction of Cr6+ into Cr3+ in the presence of excess formic acid (Fig.\u00a07\n(a)). The progress of the reduction was followed by recording the decay of the 350\u00a0nm-peak of K2Cr2O7 as a function of time [46]. It was observed that the 350\u00a0nm-peak gradually decayed with increasing reaction time. The 350\u00a0nm-peak vanished completely with decolorization of the solution (from yellow to colorless) after 35\u00a0min indicating the reduction of Cr6+. The reduction of Cr6+ to Cr3+ was confirmed by the appearance of the green color after treating the resulting solution with excess NaOH solution [47]. It was suggested that formic acid was adsorbed onto the Ni surface to produce a Ni-formate intermediate, which was reactive for the reduction of Cr6+ [46].Due to the high concentration of formic acid (in comparison with that of Cr6+), the kinetics follows a pseudo-first-order model as follow:\n\n(3)\n\n\nln\n\n(\n\n\nC\nt\n\n/\n\nC\n0\n\n\n)\n\n=\nln\n\n(\n\n\nA\nt\n\n/\n\nA\n0\n\n\n)\n\n=\n\u2212\n\nk\n\na\np\np\n\n\nt\n\n\n\n\nwhere C0, Ct, A0, and At are the concentrations and their corresponding absorption of K2Cr2O7 at the beginning and after time t of the reaction. k\n\napp\n is the apparent rate constant of the catalytic reaction which is equal to the slop of the straight line of ln(A\nt\n/A0) vs\nt.The effect of [Cr6+] on the k\n\napp\n of the catalytic reduction was evaluated at the following conditions (0.02\u00a0g of Ni@rGO/PVA, 1\u00a0mL of formic acid, and pH\u00a0=\u00a06) as presented in Fig.\u00a07(b) and Table 1\n. The k\n\napp\n of the catalytic reduction decreased from 1.45\u00a0\u00d7\u00a010\u22123 to 0.54\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 by increasing the [Cr6+] from 0.5 to 1\u00a0mM. The reduction was achieved just in 26\u00a0min for the lowest concentration studied (0.5\u00a0mM) whereas it exhausted about 44\u00a0min for completion when [Cr6+] is increased to 1\u00a0mM. The presence of a large number of Cr6+ at higher concentrations) might block the active centers of the catalyst thereby adsorption onto and/or shielding the catalyst surface. This in turn prevents/minimizes the decomposition of the formic acid at the catalyst surface slowing down the catalytic reduction as a whole. The catalyst dosage plays a vital role in optimizing the operational parameters of the catalytic reaction and to avoid the excess use of the catalyst. The impact of catalyst dosage on the catalytic reduction of Cr6+ was investigated by varying the weight of Ni@rGO/PVA from 0.02 to 0.1\u00a0g, at the following conditions; ([Cr6+]\u00a0=\u00a00.7\u00a0mM, 1\u00a0mL of formic acid, and pH\u00a0=\u00a06). The k\n\napp\n of catalytic reduction increased from 0.95\u00a0\u00d7\u00a010\u22123 to 1.94\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 by increasing the catalyst from 0.02 to 0.1\u00a0g (Fig.\u00a07(c) and Table 1). The addition of more doses of the catalyst necessarily means adding more active centers i.e., more reactive cites are available for adsorption of Cr6+ as well as for the decomposition of formic acid. This in turn can speed up the reaction rate to attain equilibrium. It could be seen that the increase of the catalyst dosage from 0.07 to 0.1\u00a0g did not yield a significant increase in the reaction rate. Hence, the 0.07\u00a0g catalyst dosage was chosen for the subsequent study.The dosage of formic acid has a key role in the catalytic reduction of Cr6+ as it is the hydrogen donor. The study was conducted to evaluate the effect of formic acid dosage on the k\n\napp\n of the Cr6+ reduction at the following conditions (0.07\u00a0g of Ni@rGO/PVA, [Cr6+]\u00a0=\u00a00.7\u00a0mM, and pH\u00a0=\u00a06). The results were displayed in Fig.\u00a07(d) and Table 1. The results indicated that the reduction rate was accelerated with the increase of formic dosage. The k\n\napp\n was increased from 1.72\u00a0\u00d7\u00a010\u22123 to 2.67\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 as the dosage of formic acid increased from 1 to 3\u00a0mL. The use of more formic acid means the more hydrogen was produced at the catalyst surface which in turn accelerates the reduction of Cr6+ [48].The catalytic reduction of Cr6+ is strongly pH-dependent reaction and catalyzed by the dissolved and surface-bound metals [48]. The rate constant (k\n\napp\n) was calculated as function of the pH value of the reaction at the following conditions (0.07\u00a0g of Ni@rGO/PVA, [Cr6+]\u00a0=\u00a00.7\u00a0mM, and 3\u00a0mL of formic acid) and the results were given in Fig.\u00a07(e) and Table 1. The catalytic reduction of Cr6+ to Cr3+ was accelerated markedly as the acidity of the reacting solution was increased (at lower pH values). The k\napp\n was increased from 2.67\u00a0\u00d7\u00a010\u22123 to 7.29\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 as the pH value decreased from 6 to 2.The pH value controls the surface charge, substrate ionization and the hydrogen donor dissociation, when it participates in the hydrogen transfer process [49]. The lower pH values are favored for the adsorption of negatively charged formate (HCOO\u2212) and dichromate (Cr2O7)2\u2212 ions onto the catalyst surface [50]. As a result, the adsorption of the reactants onto the catalyst surface is accelerated and higher catalytic rates are promoted at lower pH values. Hence, the catalytic reduction of Cr6+ was highly pH dependent.It is nonetheless worth conducting recycling trials to evaluate the stability and reusability of Ni@rGO/PVA film. This was achieved by using thoroughly rinsed Ni@rGO/PVA film for the next catalytic cycle, followed by distilled water washing after each cycle. Fig.\u00a07(f) showed the catalytic performance of Ni@rGO/PVA film for ten catalytic cycles. It could be seen that the reduction activity decreased less than 12% of the initial activity whereas the Cr6+ conversion% lost about 7% of its initial value after the tenth catalytic cycle. The content of the segregated Ni within reacting medium was determined by atomic absorption. The collected data depicted that no Ni species were determined, suggesting that Ni was fixed in the rGO/PVA film composites. The film shape of Ni@rGO/PVA catalyst could realize instantaneous and efficient separation of the catalyst from the reacting medium. This eliminates the need for industrially inapplicable separation methods such as centrifugation, filtration, precipitation by pH shift, etc. [10]. This type of nanocomposite films could be built in fixed bed reactors under flow of reacting molecules on the way of real applications of such catalysts.The catalytic reduction of 4-NP to 4-AP by NaBH4 is considered as the foremost model catalytic reactions. This is due to it the straightforward assessment of the catalyst performance using the real-time spectroscopic analysis of an aqueous solution. The nitrophenol derivatives are essential intermediates for variety of industries such as pesticides, petrochemical, production of paper, pharmaceuticals, etc. [51]. In addition, the nitrophenol compounds are classified by U.S. Environmental Protection Agency (EPA) as non-biodegradable pollutants. The catalytic reduction of 4-NP to 4-AP is environmentally and industrially of great interest. The visual inspection of the reaction between 4-NP and NaBH4 (i.e., without catalyst) indicated the stability of the yellow color of the solution for several days without considerable change. This is attributed to the potential barrier between the standard reduction potentials (E\no\n) of the reactants; E\no\n (4-NP/4-AP)\u00a0=\u00a0\u22120.76\u00a0V; E\no\n (H3BO3/BH4\n\u2212)\u00a0=\u00a0\u22121.33\u00a0V [52]. After the dipping of the Ni@rGO/PVA film in the reacting medium, the yellowish color of the solution vanished gradually until complete decolorization.\nFig.\u00a08\n(a) showed the time-dependent UV/vis spectra of the reduction of 4-NP with NaBH4 in presence of Ni@rGO/PVA film as a catalyst. The spectra is dominated with strong peak at 400\u00a0nm which is attributed to the nitrophenolate ions [53,54]. As the reaction progressed, the 400\u00a0nm-peak faded gradually while a new peak at 298\u00a0nm appeared which is a sign of 4-AP formation [55]. The disappearance of the 400\u00a0nm-peak was taken as sign for the completion of the reaction. It could be concluded that Ni nanoparticles provided the reactive sites for the reactant adsorption and reducing the kinetic energy barrier of the reduction.As the reduction of 4-NP occurred in excess of NaBH4 ([NaBH4]/[4-NP]\u00a0=\u00a085), the catalytic reaction was described by the first-order model i.e., the rate constant of the reaction depends only on the 4-NP [56]. To evaluate the impact on k\n\napp\n, the catalytic experiments were performed by varying a single operational parameter of the 4-NP concentration, catalyst dose, NaBH4 concentration, and pH value.\nFig.\u00a08(b) displayed the plots of (ln(A\nt\n/A0) vs.\nt) at different initial 4-NP concentrations at the following conditions; 0.03\u00a0g of Ni@rGO/PVA [NaBH4]\u00a0=\u00a07\u00a0mM, and pH\u00a0=\u00a07.6. The value of k\n\napp\n decreased from 4.18\u00a0\u00d7\u00a010\u22123 to 1.36\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 as the [4-NP] increased from 0.08 to 0.16\u00a0mM concentration as listed in Table 2\n. On the other hand, the k\n\napp\n was found to increase from 2.72\u00a0\u00d7\u00a010\u22123 to 10.2\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 as the catalyst dose increased from 0.02 to 0.05\u00a0g at the following conditions [4-NP]\u00a0=\u00a00.08\u00a0mM, [NaBH4]\u00a0=\u00a07\u00a0mM, and pH\u00a0=\u00a07.6 (see Fig.\u00a08(C) and Table 2). According to Langmuir\u2013Hinshelwood (L-H) model, the co-adsorption of 4-NP and NaBH4 (to the catalyst surface) is essential for the efficient catalysis. Therefore, the increased concentration of 4-NP occupied more active sites of the catalyst and minority sites were available for active hydrogen species. This resulted in suppressing the catalytic process and decreasing k\n\napp\n value. Also, the more the catalyst dose, the more is the number of the active sites available for the adsorption of the reactants. This accelerates the catalytic reaction [57].The effect of [NaBH4] on the rate of the catalytic degradation of 4-NP was also evaluated at the following conditions: 0.05\u00a0g of Ni@rGO/PVA, [4-NP]\u00a0=\u00a00.08\u00a0mM, and pH\u00a0=\u00a07.6. According to Fig.\u00a08(d) and Table 2, one can observe that in the concentration range (5\u201310\u00a0mM) of NaBH4, the k\n\napp\n increased from 5.82\u00a0\u00d7\u00a010\u22123 to 13.3\u00a0\u00d7\u00a010\u22123\u00a0s\u22121. However, increasing the [NaBH4] to higher than 10\u00a0mM did not show significant effect on the k\n\napp\n i.e., the rate remains almost constant. The advantages of higher [NaBH4] are limited and the most would be wasted because of the limited substrate available. Beyond the optimal level, the increasing [NaBH4] would not effectively enhance the efficiency of the process.The effect of pH on the k\n\napp\n in the representative experiment of the reduction of 4-NP was performed at the following conditions: 0.05\u00a0g of Ni@rGO/PVA [4-NP]\u00a0=\u00a00.08\u00a0mM, and [NaBH4]\u00a0=\u00a010\u00a0mM. Fig.\u00a08(e) displayed the first-order kinetic curves of 4-NP over Ni@rGO/PVA at different pH values. It is quite evident that the k\n\napp\n was speeded from 5.57\u00a0\u00d7\u00a010\u22123 to 23.6\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 with raising the pH value from 6 to 9 (Table 2). With further increase in the pH value, no remarkable changes were recorded on the k\n\napp\n values. The aqueous NaBH4 solution is stable in the highly alkaline solution and the deprotonation of \u2013OH moiety of 4-NP is decreased at pH\u00a0>\u00a010 [58].The reusability of the Ni@rGO/PVA catalyst was also studied with the catalytic reduction of 4-NP. As depicted in Fig.\u00a08(f), the catalytic activity of 4-NP to 4-AP was 83% after 15 cycles. The Ni@rGO/PVA showed good catalytic performance after 15 cycles without significant loss of active sites (Ni species), due to the supporting effect of rGO/PVA matrix.The oxidative degradation of MB dye over Ni@rGO/PVA film was represented in Fig.\u00a09\n(a). The catalytic oxidation of MB was detected by following the diminishing of the absorption peak at 664\u00a0nm [59]. This experiment was performed with 0.03\u00a0g Ni@rGO/PVA [MB]\u00a0=\u00a00.05\u00a0mM, [H2O2]\u00a0=\u00a00.5\u00a0M, pH\u00a0=\u00a06 and stirring the reaction mixture at 300\u00a0rpm. It was noted that the 664\u00a0nm-peak decayed completely within 32\u00a0min. This happened in conjunction with the disappearance of the distinctive blue color of the dye. For blank reaction, separate experiments were carried out under similar conditions, once without catalyst and another without H2O2. It was noted that there was no oxidation occurred and the dye solution maintained its color for several days without considerable changes.The effect of the initial [MB] on the k\n\napp\n of the catalytic processes was evaluated with 0.03\u00a0g Ni@rGO/PVA [H2O2]\u00a0=\u00a00.5\u00a0M, pH\u00a0=\u00a06 and stirring the reaction mixture at 300\u00a0rpm. It was observed that as the initial MB concentration increased the decolorization of MB was markedly slowed down (Fig.\u00a09(b) and Table 3\n). At higher MB concentrations, the co-adsorption of the reactants (MB and H2O2) was significantly disrupted i.e., more MB molecules were adsorbed to the catalyst at the expense of H2O2. This in turn inhibited and/or reduced the production of hydroxyl radicals necessary for the efficient oxidation [21]. The effect of Ni@rGO/PVA dosage was evaluated through a series of experiments carried out at [MB]\u00a0=\u00a00.05\u00a0mM [H2O2]\u00a0=\u00a00.5\u00a0M, pH\u00a0=\u00a06 and stirring the reaction mixture at 300\u00a0rpm. The calculated data were presented in Fig.\u00a09(c) and Table 3. It could be seen that the decolorization efficiency of MB increased with increasing the catalyst dosage i.e., the k\n\napp\n increased from 0.68\u00a0\u00d7\u00a010\u22123 to 2.91\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 with increasing Ni@rGO/PVA dosage from 0.1 to 0.07\u00a0g.The effect of [H2O2] was evaluated at the following experimental conditions: 0.07\u00a0g Ni@rGO/PVA, [MB]\u00a0=\u00a00.05\u00a0mM, pH\u00a0=\u00a06 and stirring the reaction mixture at 300\u00a0rpm. Fig.\u00a09(d) and Table 3 showed the behavior of k\n\napp\n of the MB oxidation at different initial [H2O2]. It could be seen that the k\n\napp\n increased from 1.09\u00a0\u00d7\u00a010\u22123 to 4.81\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 with increasing [H2O2] from 0.25 to 1\u00a0M. The further increase of [H2O2] to 1.25\u00a0M resulted in decreasing the k\n\napp\n value to 4.42\u00a0\u00d7\u00a010\u22123\u00a0s\u22121. The excess H2O2 might serve as a scavenger of OH and forms a less reactive perhydroxyl radicals, resulting in reduction of the catalytic rate [60].The pH effect was followed by changing the pH value of the reacting solution from 5 to 2.5 with 0.07\u00a0g Ni@rGO/PVA [MB]\u00a0=\u00a00.05\u00a0mM, [H2O2]\u00a0=\u00a01\u00a0M, and stirring the reaction mixture at 300\u00a0rpm. It is worthwhile to mention that there is no remarkable degradation of the MB dye in the pH range of 6.0\u201311. In the alkaline medium, H2O2 loses its oxidation ability due to the formation of oxygen and H2O [21]. Fig.\u00a09(e) and Table 3 showed that the decrease in the pH value resulted in higher oxidation rates of MB dye. For the catalytic oxidation reaction, the k\n\napp\n\u00a0=\u00a011.5\u00a0\u00d7\u00a010\u22123\u00a0s\u22121 was achieved at a pH\u00a0=\u00a02.5. At low pH values, H2O2 yields a considerable amount of OH radicals which accelerate the oxidation process of MB dye. For pH values below 2.5, the k\n\napp\n was decreased markedly. This might be attributed to the presence of large number of H+ which in turn reacts with OH radicals to form H2O and slows down the reaction rate.In order to study the robustness and recyclability of Ni@rGO/PVA, the catalyst film was ejected manually by using a forceps and washed repeatedly with water and ethanol to get rid of any dye residuals before reuse. The reusability of Ni@rGO/PVA was carried out by using fresh film (1st run) or used ones (2nd\u20135th runs) in repeated optimum degradation cycles. Fig.\u00a09(f) depicted that Ni@rGO/PVA showed a high catalytic performance in the five reaction runs. The conversion% and activity% of MB after five successive catalytic runs reached 83% and 78%, respectively. However, the conversion% and activity% of MB decreased sharply to about 41% and 33%, respectively, in the next five runs. This might be to the accumulation of the MB molecules at the active sites of the catalyst which in turn lead to a depression in the catalytic efficiency.Ni@rGO/PVA film was successfully prepared as recoverable/separable dip catalyst for oxidative and reductive catalytic reactions. The swelling properties of Ni@rGO/PVA film showed that the GO nanosheets serve as crosslinking agent that restrain the solubility of PVA film yielding highly hydrophilic and insoluble polymeric matrix. Also, GO shows strong affinity toward Ni2+ ions leading a tight fixing of the Ni NPs onto the polymeric matrix. Using GO replaces the usage of hazardous chemical crosslinkers. The XRD, XPS, and Raman results indicated the reduction of GO and Ni2+ to rGO sheets and Ni NPs, respectively, under the action of NaBH4. The SEM result clearly demonstrated the successful formation of Ni nanoparticles onto the GO/PVA film. Ni@rGO/PVA film showed efficient catalytic activity in both reductive and oxidative reactions. After, optimization of operational parameters (pollutant concentration, catalyst dose, reducing/oxidizing agent concentration, and pH value), the calculated k\n\napp\n for the degradation of Cr6+, 4-NP, and MB are 7.29\u00a0\u00d7\u00a010\u22123, 23.6\u00a0\u00d7\u00a010\u22123, 11.5\u00a0\u00d7\u00a010\u22123\u00a0s\u22121, respectively. The recyclability of Ni@rGO/PVA film was investigated by several cycles without significant loss of activity. The prepared Ni@rGO/PVA film was shown to be simple and low-cost, eco-friendly, recyclable, and separable catalyst with potential catalytic activity for a variety of reactions.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This paper is based upon work supported by Science, Technology & Innovation Funding Authority (STDF) under grant number 45556. Also, it is acknowledged the financial support from the in\u2013home project unit, National Research Centre, project No. 12020226.", "descript": "\n                  Graphene oxide was blended with poly(vinyl alcohol) film as a solid support for nickel nanoparticles as a dip catalyst. Simple preparation steps were followed avoiding the use of hazardous materials and/or any surface treatment. Graphene oxide plays the key role in the proposed catalyst by cross-linking poly(vinyl alcohol) film, increasing the affinity of the substrate for adsorbing nickel ions, and increasing the hydrophilicity of the nanocomposite. The prepared dip-catalyst was employed in catalytic reduction and oxidation to get rid of Cr6+, 4-nitrophenol, and methylene blue as water pollutants. A reasonable control over the operational parameters of the catalytic reaction was achieved. Significantly, the free-standing film form of the catalyst enabled the facile separation of the catalyst without release of the nickel nanoparticles. Also, the recyclability of the catalyst was investigated for several runs without any considerable loss of catalyst efficiency. The proposed catalyst fits the environmental and industrial requirements because of its stability, low cost, and activity for wide range of catalytic reactions.\n               "}
{"full_text": "The rapid technological development in recent decades has led to an increase in the population in the world, which in turn has caused a noticeable increase in the consumption of fuels such as jet fuel, gas oil, and gasoline, which contain organic sulfur compounds (OSCs).) and are a major cause of air pollution (Choi\u00a0et\u00a0al., 2014). These OSCs are found in many forms like sulfides, thiol, and thiophene with their derivatives which may be described as highly harmful to health and the environment via sulfur oxides (SOx) emission during combustion. Moreover, the presence of sulfur in petroleum products also may be causing corrosion of internal combustion engines, poisoning catalytic converters, and causing air pollution, (Alwan\u00a0et\u00a0al., 2021). For all mentioned above the sulfur compounds must be removed or eliminated to allowed limits which pay attention to scientists working towards sulfur removal. There are various desulfurization technologies like hydrodesulfurization (HDS), oxidative desulfurization (ODS), extractive desulfurization, and biodesulfurization etc. for production of low-sulfur fuel. HDS is a classical technology used in large-scale processes which desulfurizes different fuels by using hydrogen at high pressure and Ni-Mo or Co-Mo catalyst under elevated temperature, but the HDS has low reactivity towards benzothiophene (BT), dibenzothiophene (DBT), and its alkylated derivatives (Alwan,\u00a02022). There are many disadvantages to using HDS as follows; it requires severe operating conditions such as high reaction temperature (between 300 and 400\u00a0\u00b0C), hydrogen at high pressure (30\u201375 bars), the huge amount of catalyst, use of large reactors, and long residence time which causes high operation cost (Choi\u00a0et\u00a0al., 2016) .The ODS technology may be described as a promising technique because it does not need to work at extremely high temperatures, as well as atmospheric pressure is enough to achieve the reaction (Alwan\u00a02021) .The ODS efficiency is a two-step process, first step the sulfur present is oxidized to sulfoxide or suldones in presence of oxidation agent such potassium ferrate, tetra\u2011butyl hydroperoxide, hydrogen peroxide ozone, molecular oxygen, etc., this oxidation reaction is a selective oxidation, it is oxidized OSCs to its sulfur forms without breaking Carbon-carbon bonds. Among these oxidant agents hydrogen peroxide H2O2 is preferred due to high oxidation reactivity and it maybe consider as green oxidant (environmentally-friendly) as well as its low cost safety, and high selectivity (Choi\u00a0et\u00a0al., 2014). The oxidation of sulfur caused increasing in polarity sulfur containing compounds. Thus, the sulfoxides and sulfones are easily removed from oil phase with polar solvent or adsorbents and this is the second step. (Zhu\u00a0et\u00a0al., 2012; Choi\u00a0et\u00a0al., 2022). The most common used solvents such as acetonitrile, methanol,dimethylsulfoxide (DMSO), acetone and, dimethylformamide (DMF). Using of solvent has many disadvantages; toxicity, disposal, reusability, cost and explosiveness, thus solvent selection may represent challenge for example recovery of DMSO challenge via similar boiling point, while acetonitrile characterized by its high polarity which extract a lots of aromatic, and methanol is a good solvent for sulfones extraction but has density closed to diesel density separation is difficult. The ease of OSCs oxidation depends on electrons densities on sulfur atom. High electron density sulfur atoms are easier to oxidize (Badoga\u00a0et\u00a0al., 2018).The ODS process require to use various transition metal oxides catalyst such as titanium, copper, cobalt, manganese, iron, tungsten, molybdenum, vanadium and so on .The metals oxides catalyst need support (carrier) like alumina .Using of synthesized CoMo/Alumina with different Co/Mo ratio for BT and DBT oxidation on fixed bed reactor was showed 30% removal for BT and 90% removal for DBT, and they reported about that using MoOx catalyst supported on alumni is very active but it has faster deactivation rate (Chica\u00a0et\u00a0al., 2006). Titanium oxide nanotubes and H2O2 exhibited good activity for DBT oxidation (Loren\u00e7on\u00a0et\u00a0al., 2014). Tian et\u00a0al. conducted ODS reaction for removing BT and DBT with H2O2 and phosphomolybic acid supported on silica and they get removal efficiency about 99% (Tian\u00a0et\u00a0al., 2016). To promote classical molybdenum based catalyst for ODS reaction of DBT at mild operating conditions M. Yaseen et\u00a0al. used 2\u00a0wt.% loading as promoter to classical molybdenum based catalyst in presence of oxidation system consists H2O2 and formic acid and (Muhammad\u00a0et\u00a0al., 2018). There are many workers interested to use carbon and its allotropes as catalyst support via its high chemical and thermal stability as well as its mechanical strength such as grapheme and carbon nanotubes (Alwan,\u00a02022).In this study the molybdenum-based catalyst was synthesized by wet impregnation for activated carbon, the molybdenum oxide represented as active phase while nickel oxide is a promoter because the molybdenum base catalyst lost its activity during oxidation desulfurization reaction so the goal for this study is the effect of adding nickel as catalyst promoter as well as to analyze the effect of some other variables on DBT oxidation to remove sulfur from model fuel. The studied variables are catalyst amount, nickel (Ni wt.%) loading, and initial sulfur concentration while the response is the sulfur removal efficiency. The experiments were designed by applied Box-Bohenken experimental design combined with Response Surface Methodology (RSM).Activated carbon AC (568\u00a0m2/g and 0.0062\u00a0cm3/ g for specific surface area and pore volume respectively) purchased from the local market was used as catalyst support, ammonium heptamolybdate (NH4)6Mo7O24\u00b76H2O (AHM) with purity 99% (HOPKIN & WILLIAMS), nickel nitrate Ni(NO3)2\u00b76H2O with purity 99% (CHD Ltd.).The catalyst was prepared by wet impregnation AC with nickel and molybdenum oxide from their precursor as follows; AHM and Ni (NO3)2\u00b76H2O are sources for molybdenum oxide and nickel oxide respectively. The molybdenum was loaded 15\u00a0wt.%, while the nickel loaded (2, 4, and 6\u00a0wt.%) to investigate the impact of nickel content as a catalyst promoter. For impregnation of 10\u00a0g from AC, two solutions were prepared as follows; first solution, 2.007\u00a0g of AHM salt (as molybdenum oxide source), second solution, 1.0297, 2.0594 and 3.0891\u00a0g of nickel nitrate hexahydrate (Ni (NO3)2\u00b76H2O) salt (as nickel oxide source) dissolved in distilled water to get loaded nickel percentage 2%, 4%, and 6% where they are symbolized as 2%NiMo/Ac, 4%NiMo/Ac and 6%NiMo/Ac respectively. These two solutions were added, followed loaded on an AC surface by co-impregnation method to precipitate cobalt and molybdenum oxides. The impregnated AC was dried at 110\u00a0\u00baC for two hours and calcination was done at 400\u00a0\u00baC for four.RSM is a practical procedure used for evaluating the relation between actual experimental results (response) with studied variables (control variables), and this is usually done by combining RSM with factorial design techniques such as central-composite design CCD and Box-Bohenken design BBD. BBD technique can reduce the required number of experiments without decreasing the accuracy of the optimization in comparison with other factorial design methods (Alwan,\u00a02021). The required experiments number to cover the studied variables system according to using BBD is:\n\n(1)\n\n\nN\n=\n2\nk\n\n(\n\nk\n\u2212\n1\n\n)\n\n+\nr\n\n\n\nWhere N is the number of experiments, k is the number of variables, and r is the replicate number of central points (3\u20136). BBD stated that the levels of the studied variables were adjusted to only three levels (-1.0, 1) with equal values for the interval between each level, thus for three variables with three levels, the number of experiments was 15\u221218 depending on the number of replicated experiments number (r in the equation). The catalyst dosage, Ni% loaded in catalyst, and initial sulfur concentration is chosen as studied (controlled) variables on DBT conversion (Table\u00a01\n), the experimental design with using of design expert version 13 as shown in Table\u00a02\n.The experiments results for the effects of catalyst dosage (x1), Ni wt.% loaded on catalyst (x2) and, sulfur initial concentration (x3) on oxidative desulfurization were fitted as second-order polynomial, and it can be used to estimate predict values and optimization the system, for three variables where the second-order polynomial represents by equation\n\n(2)\n\n\nR\n%\n=\n\n\n\u03b2\n0\n\n+\n\u2211\n\n\u03b2\ni\n\n\nx\ni\n\n+\n\u2211\n\n\u03b2\n\ni\ni\n\n\n\nx\ni\n2\n\n+\n\u2211\n\n\u03b2\n\ni\nj\n\n\n\nx\ni\n\n\nx\nj\n\n+\n\n\u03b5\n\n\n\n\nWhere R% is predicated response, \u03b20 is the intercept coefficient, \u03b2\ni\n is the linear effect (slope) of input variable xi\n, \u03b2\nij\n is interaction effect of linear by linear between two input variables xi\n, and B\nii\n is squared effect.The model fuel (DBT dissolved in n-heptane) was prepared by using three different DBT concentration (400, 600, and 800\u00a0ppm); the DBT concentration prepared according to Box-Behnken design BBD. ODS reaction for DBT was conducted under mixing of model fuel at 50\u00a0\u00b0C in presence of prepared catalyst and H2O2 \u2013CH3COOH oxidation system, where the ODS reaction was examined under the effect of three independent variables; catalyst dosage, Ni% loaded on the catalyst and, initial sulfur concentration with the range for these studied variables which shown in Table\u00a01. The total number of experiments required to cover the three-level for the three-variables system is 15 according to Eq.\u00a0(1), all experiments were arranged according to Box\u2013Behnken experimental design as shown in Table\u00a02. The oxidation reaction starts by heating 100\u00a0ml of model fuel using the magnetic stirrer heater to reach the required temperature (50\u00a0\u00b0C), 10\u00a0ml of hydrogen peroxide, and 5\u00a0ml of acetic acid with the needed dosage of catalyst added to model fuel. The reaction stopped after 60\u00a0min. Subsequently OSCs were converted into the polar compounds such as sulfoxides and /or sulfones m which separated by using acetonitrile (with 1:1 volume ration) during extraction step. The separation done in separation funnel in which the upper phase was the low sulfur fuel while the below phase was the mixture of oxidative compounds and solvent (acetonitrile).The sulfur content in the final product was measured by X-ray fluorescence (Sulfur Meter model RX-620SA/Tanka Scientific). The DBT conversion (R%) is related with initial sulfur concentration (Si) and final sulfur concentration (So) as in the following equation:\n\n(3)\n\n\nD\nB\nT\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\n(\n\nR\n%\n\n)\n\n=\n\n\nS\ni\n\u2212\nS\no\n\n\nS\ni\n\n\n\u00d7\n100\n\n\n\n\nThe XRD (Shimadzu Model XRD- 6000 \u2013Japan) patterns for prepared catalysts are shown in Fig.\u00a01\n, which contains the pattern for 2% NiMo/Ac (blue curve) and 6% NiMo/Ac (red curve). As the result show the peaks around 2\u03b8 equal to 28.9 and 28.77 are attributed to graphite (carbon) at 2% NiMo/Ac and, 6% NiMo/Ac respectively (Wang\u00a0et\u00a0al., 2015). There are many peaks for molybdenum trioxide MoO3 at 27.38\u00b0 at 2%NiMo/Ac, while peaks at 2\u03b8 equal to 32.72, and 39.26\u00b0 at 6% NiMo/Ac (JCPDS No.05\u20130508), these peaks with sharp shapes indicate that MoO3 have good crystalline and Juehan noted the result closed to this work results (Alwan\u00a02022), (Jegal\u00a0et\u00a0al., 2013) and (Dedual\u00a0et\u00a0al., 2014). NiMoO4 phase is diffracted at 2\u03b8 equal to 23.46 and 23.94 on 2% NiMo/Ac and,6% NiMo/Ac patterns respectively, according to the standard card (JCPDS No. 86\u20130361), and this good agreement with (Ghosh\u00a0et\u00a0al., 2013). Furthermore, the nickel oxide exhibited diffraction peaks at 2\u03b8 equal about 43.26 and 54.34 which agreed with Dong et\u00a0al. (Jang\u00a0and Park,\u00a02012).The presences of dispersion active metallic oxides (nickel and molybdenum) were further confirmed by EDX (BRUKER Model X Flash 6l10 Germany) elemental mapping as shown in Fig.\u00a02\n.\nTable\u00a03\n, shows the DBT conversion for all experiments done according to Box \u2013Behnken design BBD. The DBT conversion ranged between 23 and 71% whereas these results fitted with a second-order polynomial (quadratic model), this equation relate between R% as a function for a function of independent variables (catalyst dosage, Ni% loaded and sulfur initial concentration) and as with respect to actual value below:\n\n(4)\n\n\nR\n%\n=\n\n\u2212\n1.516\n+\n0.989\n\nX\n1\n\n\u2212\n0.1557\n\nX\n2\n\n+\n0.00555\n\nX\n3\n\n\u2212\n0.0121\n\nX\n1\n\n\nX\n2\n\n\u2212\n0.0011\n\nX\n1\n\n\nX\n3\n\n+\n0.00132\n\nX\n2\n\n\nX\n3\n\n\u2212\n0.043\n\nX\n1\n2\n\n+\n0.01018\n\nX\n2\n2\n\n\u2212\n0.000003\n\nX\n3\n2\n\n\n\n\n\nThe analysis variance ANOVA results for the predicated model as seen in Table\u00a04\n, ANOVA gained by Minitab software version 17. The predicted model shows good fitting for actual data due to the high value of correlation coefficient R2 (0.9719) and close value for adj. R2 (0.9213) indicates that the assumed model is reasonably well fitting with actual results. F-value for regression model is 16.77 is greater than tabulated value (F 95, 5,0,05\u00a0=\u00a04.77). Based on F-value results, the initial sulfur concentration shows the highest effect on DBT conversion (sulfur removal efficiency) followed by catalyst dosage and Ni% loaded as predicated according to their F-values 89.45, 8.07, and 0.61 for initial sulfur concentration, catalyst dosage, and Ni% loaded respectively. The optimum DBT conversion is 75.74% at 0.5\u00a0g, 6% and 700\u00a0ppm for catalyst dosage, Ni loaded and initial sulfur concentration respectively.The impact of the studied variable individually and optimization of the studied system were shown in Fig.\u00a03\n, the DBT conversion increased with an increase in initial sulfur concentration from 200\u00a0ppm until reached near 700\u00a0ppm, with further increases in initial sulfur concentration the DBT conversion decreased and this may be because of the presence of the limited number of active sites in a fixed amount of catalyst, in which these limited active sites are insufficient for conversion of BDT (Subbaramaiah\u00a0et\u00a0al., 2018) . DBT conversion was raised via the increasing of dosage (amount) of catalyst, which increased the amount of catalytic intermediate produced by reaction with oxidant agent (H2O2), in another meaning when catalyst amount increased will provide more active sites (providing more chance of surface interaction between DBT molecules and catalyst active phase) that responsible on DBT conversion (Yu\u00a0and Wang,\u00a02013), (Cheng\u00a0et\u00a0al., 2015) and (Chu\u00a0et\u00a0al., 2010). The impact of nickel weight percent loaded on DBT conversion was decreased with increasing of nickel weight percentage because increasing of amount of nickel loaded leads to less-active surface species formation which maybe caused blockage of some active cites by Ni species (Kim\u00a0et\u00a0al., 1996). Zhang\u00a0et\u00a0al.\u00a0(2008) reported that increasing nickel content led to lower nickel dispersion, Figs. 4\u20136\n\n\n show the interaction effect for each pair from studied variables.For better understanding the ODS mechanism by H2O2/CH3COOH system in presence of NiO-MoO3/Ac catalyst, by assuming is the presence of NiO as a catalyst promoter, while the MoO3 as an active phase, the reaction initiated by MoO3, involving the hydrolytic cleavage of hydrogen peroxide to produce strong oxidation agents (active hydroxyl radical (OH\u2022) (Ahmad\u00a0et\u00a0al., 2021), these active radicals were attack acetic acid to produce peracetic acid, which offers oxygen to DBT to produce DBTO (sulfoxide;contains S\u00a0=\u00a0O) and with further attack the DBTO2 (sulfones; contains O\u00a0=\u00a0S\u00a0=\u00a0O) was produced (Scheme\u00a01\n) .\n\n(5)\n\n\nD\nB\nT\n\n\u2192\n\n[\no\n]\n\n\nD\nB\nT\nO\n\n\u2192\n\n[\no\n]\n\n\nD\nB\nT\n\nO\n2\n\n\n\n\n\nIn this study, the oxidation reaction ODS for DBT dissolved in n-heptane is done using molybdenum oxide-nickel oxide supported on AC and an H2O2 \u2013 acetic acid system as an oxidant agent. The study consists of the investigation of the effect of three parameters which are arranged by combined RSM and Box-Behnken design. The studied variables were catalyst dosage, Ni% loaded, and initial sulfur concentration. Results show that DBT conversion (sulfur removal efficiency) ranged between 23 and 71%, and they were fitted with seconds\u2013order polynomial (high correlation coefficient R2\u00a0=\u00a00.9719). These results agreed with many previous studies but the most point considered is the use of nickel oxide for enhancement of the molybdenum-based catalysis activity. In contrast, the using nickel oxide caused decreasing in sulfur removal efficiency and which may mean that the deactivation of the catalyst was happen rapidly. The optimum DBT conversion is 75.74% at 0.5\u00a0g, 6% and 700\u00a0ppm for catalyst dosage, Ni loaded and initial sulfur concentration respectively.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to acknowledge to Mr. Riyadh Noaman manager of Chemical and Petrochemical Research Center / Corporation of Research and Industry Development /Ministry of Industry & Minerals and Mr. Quraish, Mr. Zuhair for their help in measuring sulfur con-tent to support in this research .", "descript": "\n                  In this study, oxidative desulfurization of dibenzothiophene (DBT) with an H2O2-acetic acid system whereas the catalyst used is molybdenum oxide supported on activated carbon (AC). The effect of loading nickel oxide as a promoter as well as the impact of catalyst dosage and the initial sulfur concentration were investigated. The ranges for these parameters are catalyst dosage (0.5\u20131.5)\u00a0g, nickel loading (2\u20136)\u00a0wt.% and initial sulfur concentration (400\u2013800)\u00a0ppm. A Response Surface Methodology (RSM) combined with Box-Behnken design (BBD) was utilized to evaluate the impacts of studied variables; the evaluation consists of the level of order significance of each factor, the interaction effects of parameters was analyzed with Analysis of variance (ANOVA) and determine the optimum conditions for oxidative desulfurization (ODS). Results showed that sulfur removal efficiency from model fuel ranged between 23 and 71%, and these results were fitted with a second-order polynomial model with a high correlation coefficient R2 (0.9719). The optimal condition for DBT oxidation is 0.5\u00a0g. Ni wt. 6% and 700\u00a0ppm for catalyst dosage, nickel loading, and initial sulfur concentration respectively.\n               "}
{"full_text": "Fuel cell technology is a potent alternative for the production of clean energy. The main fuels which are widely used to power fuel cells are hydrogen, methanol, methane, formic acid or hydrazine. Of particular interest are low temperature fuel cells that are powered by clean hydrogen giving electricity, water and heat. This is because such cell systems powered by the hydrogen offer highly efficient and environmentally friendly energy production technology (Munjewar et al., 2017; Mahapatra et al., 2014; Lenarda et al., 2007). Specifically, the polymer electrolyte membrane fuel cell (PEM), also called proton exchange membrane fuel cells (PEMFC), is one of the most popular types of fuel cell. A drawing of a PEM is shown in Fig. S1 of the supporting information. Nowadays, the polymer electrolyte membrane fuel cell (PEMFC) is the one of the most advanced fuel cells; it can be used in portable electronics, electric vehicles or stationary power plants (Lamy et al., 2009; Wang et al., 2011). PEM-FCs uses a polymer membrane as an electrolyte, which is an ion conductor and contains two electrodes: an anode and a cathode. Catalysts are very important in these systems, platinum being the most common. However platinum catalysts are very sensitive to CO poisoning and if the hydrogen is supplied from a hydrocarbon fuel, it is necessary to eliminate CO from the feed gas (Lu et al., 2016).Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.10.002.Fuel cell technology is a potent alternative for the production of clean energy. The main fuels which are widely used to power fuel cells are hydrogen, methanol, methane, formic acid or hydrazine. Of particular interest are low temperature fuel cells that are powered by clean hydrogen giving electricity, water and heat. This is because such cell systems powered by the hydrogen offer highly efficient and environmentally friendly energy production technology (Munjewar et al., 2017; Mahapatra et al., 2014; Lenarda et al., 2007). Specifically, the polymer electrolyte membrane fuel cell (PEM), also called proton exchange membrane fuel cells (PEMFC), is one of the most popular types of fuel cell. A drawing of a PEM is shown in Fig. S1 of the supporting information\n. Nowadays, the polymer electrolyte membrane fuel cell (PEMFC) is the one of the most advanced fuel cells; it can be used in portable electronics, electric vehicles or stationary power plants (Lamy et al., 2009; Wang et al., 2011). PEM-FCs uses a polymer membrane as an electrolyte, which is an ion conductor and contains two electrodes: an anode and a cathode. Catalysts are very important in these systems, platinum being the most common. However platinum catalysts are very sensitive to CO poisoning and if the hydrogen is supplied from a hydrocarbon fuel, it is necessary to eliminate CO from the feed gas (Lu et al., 2016).The next highly advanced fuel cell is the Solid Oxide Fuel Cell (SOFC). This type of fuel cell can be directly powered by a hydrocarbon stream (e.g., natural gas) without the need for carbon monoxide removing from the feed stream. In this type of fuel cell, the material of the anode or cathode does not have to contain a platinum catalyst. This provides more fuel options. The most important disadvantage of this fuel cell is the high operating temperature. However, it shows high efficiency and stability (Papurello and Lanzini, 2018; Ramadhani et al., 2017; Papurello et al., 2016). Typically, anode materials used in this type of fuel cell contain nickel oxide and others metal oxides supported usually on zeolite such us: Ni-Gd0.1Ce0.9O1.95AFL, Ni-Gd0.1Ce0.9O1.95, NiO-YSZ, NiO-Fe2O3-Ce0.8Sm0.2O2-\u03b4 (Gao et al., 2016) (see Fig. 1\n).Several sources of hydrogen are well known to power a fuel cell, which may include: alcohol, hydrocarbons, ammonia etc. Methanol is one of the most promising source of hydrogen because it is the simplest alcohol without C C bond in the molecule and provides a high H:C ratio. These properties indicate that methanol can be easily decomposed to a hydrogen rich mixture. Basically, there are four methods available for hydrogen production from CH3OH:Steam reforming of methanol (SRM)\n\n(1)\nCH3OH\u202f+\u202fH2O\u202f\u2192\u202fCO2\u202f+\u202f3H2\n\n\n\nDecomposition of methanol (DM)\n\n(2)\nCH3OH\u202f\u2192\u202fCO\u202f+\u202f2H2\n\n\n\nPartial oxidation of methanol (POM)\n\n(3)\n\n\nC\n\nH\n3\n\n\nOH +\n\n\n\n1\n2\n\n\nO\n2\n\n\u2192\nC\n\nO\n2\n\n\n\n+ 2\n\n\nH\n2\n\n\n\n\n\nOxidative Steam Reforming of Methanol (OSRM \u2013 combination of SRM and POM)\n\n(4)\n\n\nC\n\nH\n3\n\n\nOH +\n\n\n\n1\n2\n\n\nH\n2\n\n\nO +\n\n\n\n1\n4\n\n\nO\n2\n\n\u2192\nC\n\nO\n2\n\n\n\n+\n\n\n\n5\n2\n\n\nH\n2\n\n\n\n\n\nIt is worth emphasizing that a combination of steam reforming and partial oxidation of methanol is energetically favourable and the OSRM process can run in an auto \u2013 thermal manner, without the need to supply any external heat. Previously mentioned properties of methanol indicate that the OSRM reaction can be carried out in the temperature range 150\u2013330\u202f\u00b0C without the formation of carbon deposits (Mierczynski et al., 2016; Mierczynski et al., 2016). Typical catalysts used in the reforming of methanol processes are Cu, Ni, Fe, Co, Pd, supported on mono, and binary oxide systems (Mierczynski, 2016; Mierczynski et al., 2017; Mierczynski et al., 2016; Pojanavaraphan et al., 2015; Mierczynski et al., 2013; Abrokwah et al., 2016; S\u00e1 et al., 2010; Schuyten et al., 2009; Ahn et al., 2009). It is also well known that binary oxides exhibited the superior catalytic properties compared to the monometallic systems (Maniecki et al., 2009; Maniecki et al., 2009). In addition, promotion of monometallic copper or nickel catalyst by noble metals improves the catalytic activity and selectivity in methanol reforming processes (Lenarda et al., 2007; Mierczynski et al., 2016; Mierczynski, 2016; Mierczynski et al., 2017; Mierczynski et al., 2016). Modification of copper catalyst by ZrO2 cause increase of catalyst surface, stabilize crystallites size of copper, and in the same time protects crystallites against their aggregations. In addition, ZrO2 stabilizes the copper Cu+ ions on catalyst surface (Papavasiliou et al., 2007). Jeong et al. (2006) examined the influence of ZrO2 addition on yield of copper catalysts in reforming of methanol reaction and reported that system containing ZrO2 exhibited an increase of approximately 16% in methanol conversion and a CO molar fraction 7.3 times lower.Although much work has been focused on addition of noble metals or transition oxides to nickel and copper catalysts influence on their catalytic properties in OSRM reaction, there has been no study exploring the possibilities of improving the catalytic activity of nickel catalyst through an activation process carried out in a mixture of 5% H2\u201395% Ar at various temperatures or the promotion of nickel catalysts by noble metal. To fill these knowledge gaps, we prepared monometallic copper and nickel, and bimetallic Rh(Pd)-Cu(Ni) catalysts supported on various binary oxides in order to determine the most optimal catalytic composition for OSRM and to correlate their physicochemical properties with catalytic activity. In this work, we present how precious metals influence the catalytic and physicochemical properties of nickel and copper catalysts supported on selected binary oxide in OSRM process. The manuscript describes in detail the effect of partial reduction of nickel catalyst on its catalytic properties in the tested reaction. In addition, we studied how changes in the composition of binary oxide support influences on the catalytic and physicochemical properties of the copper catalysts obtained in the OSRM reaction.Monometallic copper and nickel catalysts supported on (ZrO2)x\n\n\n\u00b7\n\n(Al2O3)y binary oxide supports were prepared by a wet aqueous impregnation method. Binary oxide ZrO\n\n\u00b7\n\nAl2O3 (Zr:Al\u202f=\u202f2:1, 1:1, 1:2) systems were prepared by a co-precipitation method. In order to prepare a working range of binary oxides, the following molar ratios of Zr:Al\u202f=\u202f2:1, 1:1, 1:2 were used. Aqueous solutions of 1\u202fmol/L zirconium (IV) nitrate and 1\u202fmol/L aluminium nitrate were mixed in appreciate quantities under vigorous stirring at 80\u202f\u00b0C. A concentrated ammonia solution was then added dropwise until the pH reached values of between 10 and 11, respectively. Then the mixtures were stirred for another 30\u202fmin. The resulting fine precipitates were washed two times in deionised water and then dried at 120\u202f\u00b0C for 15\u202fh and calcined for 4\u202fh at 400\u202f\u00b0C in air atmosphere. The metal phase (i.e., Cu or Ni) was introduced on the supports using aqueous solutions of copper nitrate (V) or nickel nitrate (V). Copper or nickel loading on the catalyst surface was 20\u202fwt%. The supported catalysts were then dried for 2\u202fh at 120\u202f\u00b0C and calcined for 4\u202fh in an air atmosphere at 400\u202f\u00b0C. Bimetallic supported catalysts 1% Pd(Rh)\u201320% Cu/ZrO2\u00b7Al2O3 and 1% Pd(Rh)\u201320% Ni/ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) were prepared by an impregnation method on the surface of the previously prepared supported copper or nickel catalysts systems.The specific surface area and porosity of catalytic material were determined by the BET method based on low temperature (\u2212196\u202f\u00b0C) nitrogen adsorption in a Sorptomatic 1900 Carlo-Erba apparatus. The pore size distributions of the investigated material were defined based on the BJH method. Temperature programmed reduction TPR-H2 measurements of supported catalysts were performed in order to study their reducibility. TPR-H2 measurements were carried out in an automatic apparatus Altamira (AMI-1). The reduction behaviour of all supported copper catalyst systems was studied in the temperature range of 25\u2013900\u202f\u00b0C, with a linear heating rate of 10\u202f\u00b0C\u202fmin\u22121. In each investigation, a sample about 0.1\u202fg was placed in a micro-reactor and was reduced in a mixture of hydrogen in argon stream (5% H2\u201395% Ar) with a volumetric flow rate of 40\u202fcm3 min\u22121. The hydrogen consumption rate was monitored by a thermal conductivity detector (TCD). The TPD-NH3 system was used to study the acidity of the catalysts. The temperature programmed desorption of NH3 experiments were performed in the temperature measurements were carried out in a quartz flow micro-reactor using NH3 as a probe molecule. Before all experiments, the catalyst surface was purified in a flow of He at 600\u202f\u00b0C for 60\u202fmin. After purification, the NH3 was adsorbed on the catalyst surface at 50\u202f\u00b0C for 30\u202fmin. The temperature programmed desorption of NH3 were performed in the temperature range 100\u2013600\u202f\u00b0C using a linear temperature ramp (25\u202f\u00b0C\u202fmin\u22121). Before each TPD-NH3 experiment, physically adsorbed NH3 has been removed from the catalyst surface. All measurements were performed using IR Tracer-100 FTIR (Shimadzu) spectrometer equipped with a liquid nitrogen cooled MCT detector. Before each experiment, a catalyst was reduced at 300\u202f\u00b0C in a mixture of 5% H2\u201395% Ar mixture. A resolution of 4\u202fcm\u22121 was used in collecting all spectra. 128 scans were taken in order to achieve a satisfactory signal to noise ratio. The background spectrum was collected at 50\u202f\u00b0C after the reduction process of each catalytic material. After the reduction process, a reducing mixture was shifted to a mixture of 1\u202fvol% CH3OH in argon stream and at the same temperature spectra were collected. Powder X-ray diffraction patterns were recorded on a PANalytical X\u2019PertPro MPD diffractometer in Bragg-Brentano reflecting geometry. Cu K\u03b1 radiation (\u03bb\u202f=\u202f154.05\u202fpm) from a sealed tube was used in the 2\u0398 angle range 5\u201390\u00b0. The morphology and composition of the investigated catalyst systems were studied using S-4700 scanning electron microscope HITACHI, equipped with an energy dispersive spectrometer EDS. The XPS spectra were recorded for selected catalysts on a Specs SAGE XPS spectrometer using Mg K\u03b1 radiation source (h\u03bd\u202f=\u202f1253.6\u202feV) operating at 10\u202fkV and 20\u202fmA. The elements present on the sample surface were identified from a survey spectrum recorded over the energy range 0\u20131000\u202feV at pass energy of 100\u202feV and a spectrum acquisition step of 0.5\u202feV. The areas under selected photoelectron peaks in the spectrum were used to calculate the percentage of atomic concentrations of each species. High-resolution (spectrum acquisition step of 0.1\u202feV) spectra were collected for pertinent photoelectron peaks at a pass energy of 20\u202feV to identify the chemical state of each element. All the binding energies (BEs) were referenced to the C1 s peak (285\u202feV) coming from adventitious carbon to compensate for the effect of surface charging. The analysis area had a diameter of 0.7\u202fmm. Casa XPS software was used during analysis of the high-resolution spectra.OSRM reaction was performed out using a flow quartz micro-reactor under atmospheric pressure in the temperature range 160\u2013300\u202f\u00b0C. The following reaction mixture was used in each catalytic test: H2O/CH3OH/O2\u202f=\u202f1/1/0.4 (molar ratio) and the GHSV was 26700\u202fh\u22121 (calculated at ambient temperature and under atmospheric pressure). The total flow of the reaction mixture was 31.5\u202fcm3/min. Argon was used as a balance gas. The catalytic activity tests were done after two hours of stabilization process performed at each temperature. The mass of the catalysts used in each test was 0.2\u202fg. Before each catalytic activity test copper containing catalysts and all bimetallic systems were activated for 1\u202fh in a mixture of 5% H2\u201395% Ar at 300\u202f\u00b0C. While, the monometallic supported nickel catalyst was activated under the same conditions as well as at a higher temperature of 500\u202f\u00b0C using the same reduction conditions. The analysis of the obtained products in the investigated process was monitored using GC systems. Analysis of the organic products (methanol, methane, methyl formate, dimethylether (DME), and formaldehyde) were performed using chromatograph equipped with FID detector and 10% Carbowax 1500 on Graphpac column. While, CO and CO2 concentrations were monitored by GC system equipped with TCD detector (150\u202f\u00b0C, 60\u202fmA), and Carbosphere 60/80 (50\u202f\u00b0C) column. The hydrogen concentration was monitored also by a GC chromatograph equipped with TCD detector (120\u202f\u00b0C, 60\u202fmA) and molecular sieve 5a (120\u202f\u00b0C) column. Material balances on carbon were calculated for each run to verify the obtained results. The selectivity results towards hydrogen, carbon monoxide, carbon dioxide and DME formation in OSRM was calculated using Eqs. (5)\u2013(8). While, the methanol conversion was calculated using Eq. (9):\n\n(5)\n\n\n\nS\n\nH\n2\n\n\n\n\n\n\n%\n\n\n\n=\n\n\n(\n\n\nnH\n\n\n2\n-\no\nu\nt\n\n\n)\n\n\n\u2211\np\nr\no\nd\nu\nc\nt\ns\n\no\nf\n\nt\nh\ne\nr\ne\na\nc\nt\ni\no\nn\n\n\n\n\u2217\n\n100\n\n\n\n\n\n\n(6)\n\n\n\nS\n\nCO\n\n\n\n\n\n\n%\n\n\n\n=\n\n\n(\n\n\nnCO\n\n\nout\n\n\n)\n\n\n\u2211\np\nr\no\nd\nu\nc\nt\ns\n\no\nf\n\nt\nh\ne\n\nr\ne\na\nc\nt\ni\no\nn\n\n\n\n\u2217\n\n100\n\n\n\n\n\n\n(7)\n\n\n\nS\n\n\nCO\n\n2\n\n\n\n\n\n\n%\n\n\n\n=\n\n\n(\n\n\nnCO\n\n\n2\n-\no\nu\nt\n\n\n)\n\n\n\u2211\np\nr\no\nd\nu\nc\nt\ns\n\no\nf\n\nt\nh\ne\n\nr\ne\na\nc\nt\ni\no\nn\n\n\n\n\u2217\n\n100\n\n\n\n\n\n\n(8)\n\n\n\nS\n\nDME\n\n\n\n\n\n\n%\n\n\n\n=\n\n\n(\n\n\nnDME\n\n\nout\n\n\n)\n\n\n\u2211\np\nr\no\nd\nu\nc\nt\ns\n\no\nf\n\nt\nh\ne\n\nr\ne\na\nc\nt\ni\no\nn\n\n\n\n\u2217\n\n100\n\n\n\n\nwhere n CH3OH and n H2 is the molar flow rate of CH3OH and H2, respectively.\n\n(9)\n\n\n\n\nConv\n.\n\n\n\n\nCH\n\n3\n\nO\nH\n\n\n\n\n\n\n%\n\n\n\n=\n\n\n\nn\n\n1\n\n\nin\n\n\n\n\nCH\n\n3\n\nO\nH\n-\n\nn\n\n2\n\n\nout\n\n\n\n\nCH\n\n3\n\nO\nH\n\n\n\nn\n\n1\n\n\nin\n\n\n\n\nCH\n\n3\n\nO\nH\n\n\n\n\u2217\n\n100\n\n\n\n\nwhere\n\nnH2-out \u2013 molar flow rate of H2 feed out,\n\n\nnCO2-out \u2013 molar flow rate of CO2 feed out,\n\n\nnCOout \u2013 molar flow rate of CO feed out,\n\n\nn1\nin CH3OH, n2\nout CH3OH \u2013 molar flow rate of CH3OH feed in and feed out, respectively. Organic compounds such as: methane, formaldehyde and methyl formate formation were not detected in the obtained product. Only carbon monoxide, carbon dioxide, hydrogen and DME were formed as reaction products during the OSRM reaction.\n\n\nnH2-out \u2013 molar flow rate of H2 feed out,nCO2-out \u2013 molar flow rate of CO2 feed out,nCOout \u2013 molar flow rate of CO feed out,n1\nin CH3OH, n2\nout CH3OH \u2013 molar flow rate of CH3OH feed in and feed out, respectively. Organic compounds such as: methane, formaldehyde and methyl formate formation were not detected in the obtained product. Only carbon monoxide, carbon dioxide, hydrogen and DME were formed as reaction products during the OSRM reaction.The main goal of this paper was to optimize of the catalyst composition to suit the purpose of the OSRM process. Therefore, in the first step of our catalytic investigations we decided to carry out activity tests for copper catalysts supported on various (ZrO2)x\n\n\n\u00b7\n\n(Al2O3)y binary oxide systems in order to choose the best carrier. The results of the catalytic activity expressed as methanol conversion and selectivity towards hydrogen and other products are given in Table 1\n. The methanol conversion results showed that the most active catalyst among all studied copper systems supported on Zr and Al was the one with the lowest content of Zr. This catalyst showed the highest methanol conversion at both studied temperatures (i.e., 160 and 200\u202f\u00b0C) and also high selectivity towards hydrogen formation in the oxy-steam reforming of methanol process. Furthermore, the reactivity tests showed that increasing of the aluminium content caused an increase in methanol conversion and selectivity towards hydrogen formation. It is worth noting that carbon monoxide was not formed during the reaction which is very advantageous from an application point of view. In summary, it is clear that the activity and selectivity of copper catalysts is a function of catalyst composition according to Abrokwah et al. (2016). The authors studied various monometallic Cu, Co, Ni, Pd, Zn and Sn catalysts supported on MCM-41 in reforming of methanol. They reported that the methanol conversion values and selectivity towards main products depend mainly on the active phase of the catalyst used in the process. They also confirmed that supported copper catalysts exhibited the highest methanol conversion value \u223c82% and high selectivity to hydrogen formation. Cu/MCM-41 system also showed the lowest selectivity towards carbon monoxide formation (Abrokwah et al., 2016). Based on the obtained activity results for supported copper catalysts we decided to prepared analogous nickel catalysts supported on ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) binary oxide and test them for the same reaction. The catalytic activity results obtained for Ni/ZrO2\u00b7Al2O3 system clearly showed that this catalyst exhibited lower activity in the low temperature range 160\u2013250\u202f\u00b0C. The methanol conversion value for this catalyst at low temperature (160, 200 and 250\u202f\u00b0C) was below 15%. Increasing reaction temperature up to 300\u202f\u00b0C resulted in a significant increase in the methanol conversion value to about 94% and high selectivity towards hydrogen production. In addition, the results of the catalytic activity in oxy-steam reforming of methanol obtained at 300\u202f\u00b0C showed that the carbon monoxide was formed as one of the main products of the reaction (CO selectivity\u202f=\u202f25%). On the other hand, in the case of the monometallic supported nickel catalyst, we investigated the effect of higher temperature (500\u202f\u00b0C) of the activation process carried out in a mixture of 5% H2\u201395% Ar. The results of the catalytic activity showed that the use of a higher reduction temperature before the reactivity test did not improve the activity of the nickel catalyst. The catalysts exhibited lower methanol conversion at 300\u202f\u00b0C and selectivity towards hydrogen formation. In addition, large amounts of the dimethyl ether formed at 250 and 300\u202f\u00b0C were observed. Abrokwah et al. (2016) also claimed that Ni/MCM-41 exhibited lower activity in the SRM process compared to the monometallic supported copper catalyst. Further, we attempted to improve the catalytic activity of our systems by introducing metallic promoters. Therefore, in the next step of our reactivity studies we prepared and tested bimetallic Pd-Cu(Ni) and Rh-Cu(Ni) catalysts supported on a previously selected carrier ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2). The results of the activity tests performed in oxy-steam reforming of methanol showed that promotion of monometallic nickel catalysts by Pd or Rh significantly improves the activity. Both bimetallic Pd-Ni and Rh-Ni supported catalysts exhibited higher methanol conversion compared to the monometallic systems at 160 and 200\u202f\u00b0C. However, carbon monoxide was formed for both catalysts at high temperature. In contrast, DME formation was not observed during the OSRM process at 160 and 200\u202f\u00b0C. The catalytic activity tests performed for bimetallic Pd-Cu and Rh-Cu supported catalysts showed that there was an improvement in the catalytic activity. Notably, carbon monoxide formation was not observed in Pd-Cu and Rh-Cu catalytic systems at the reaction temperatures. The comparison of the catalytic activity obtained for bimetallic Pd-Ni and Pd-Cu supported catalysts showed that both systems exhibited practically the same values of methanol conversion and selectivity towards hydrogen formation. On the other hand, bimetallic Rh-Cu catalysts showed the highest activity and selectivity towards hydrogen formation at 200\u202f\u00b0C. The catalytic activity tests also showed that the only undesired products which were formed during the reforming process were carbon monoxide and DME. Based on the results of catalytic activity measurements we further optimized the content of rhodium in Rh-Cu bimetallic supported catalysts. Chang et al. (2012) investigated the catalytic properties of copper CuO/ZnO/Al2O3 (30/60/10) catalysts promoted by noble metals such as: Pt, Pd, Ru and Rh in oxidative steam reforming of methanol and they reported that addition of noble metals improves the methanol conversion during the reaction in all cases but also increases the formation of CO. The catalytic tests performed by authors in OSRM showed that only copper catalysts promoted by platinum prepared by co-precipitation method exhibited higher methanol conversion and low CO selectivity. We also investigated in this work the influence of the rhodium content on methanol conversion and selectivity results towards H2, CO, CO2 and DME. We prepared three bimetallic Rh-Cu catalysts supported on ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) by an impregnation method and tested for OSRM. The results obtained in oxy-steam reforming of methanol reaction are also given in the same Table 1. The reactivity results clearly indicate that the most active catalyst was the system containing the lowest content of Rh.The reactivity results clearly indicate that the most active catalyst was the system containing the lowest content of Rh. It is also worth mentioning that this catalyst also exhibited the highest selectivity to hydrogen production among of all bimetallic promoted by Rh catalysts at low temperature i.e. 160\u202f\u00b0C. While, at 200\u202f\u00b0C, the results of selectivity towards hydrogen formation showed that the 0.5% Rh\u201320% Cu/ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) catalyst exhibited also high selectivity towards hydrogen formation similar to the copper catalyst containing 1% wt. of Rh. Further increase of the Rh loading in the catalytic system resulted in decrease of the selectivity to hydrogen production.In the next step of our investigations, we determined the Specific Surface Area (SSA) and average pore size for binary oxides and all tested catalysts. The specific surface area results are presented in Table 2\n. The results of the SSA measurements clearly show that the binary oxide (ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2)) and catalyst with the highest content of the aluminium among all copper catalysts exhibited also the highest specific surface area. In addition, all investigated catalysts had average pore size below 3\u202fnm.Notably, the 20% Cu/ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) system had specific surface area value about 20% higher compared to the rest of the monometallic copper catalysts supported on the same carrier. In contrast, the nickel catalysts supported on ZrO2\u00b7Al2O3 system showed significantly lower SSA value compared to the copper catalyst supported on the same support. On the other hand, the SSA measurements obtained for bimetallic supported catalysts showed that the promotions of copper or nickel catalyst by noble metals does not cause significant changes in the specific surface area. In the case of the monolayer capacity values the results indicate that the bimetallic systems exhibited slightly higher values. Whereas, the values of the pore radius obtained for all catalytic systems were below 3\u202fnm in all cases.Next, we studied the reducibility of the monometallic and bimetallic supported catalysts. TPR-H2 measurements recorded for copper catalysts supported on various ZrO2-Al2O3 systems are presented in Fig. 2\n. The reduction measurements obtained for 20% Cu/ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) catalyst showed two unresolved reduction effects in the temperature range of 150\u2013320\u202f\u00b0C (Mierczynski et al., 2013). The maxima of hydrogen consumption peaks at about 210\u202f\u00b0C and 280\u202f\u00b0C are associated with reduction of CuO species according to the following scheme (Mierczynski, 2016):\n\nCu2+\u202f\u2192\u202fCu+\u202f\u2192\u202fCu0\n\n\n\nThe first reduction peak located at 210\u202f\u00b0C is assigned to the reduction CuO to Cu2O species. The next peak with a maximum of hydrogen consumption at 280\u202f\u00b0C is associated with the reduction of Cu2O species to metallic Cu. In the case of the rest of the supported copper catalysts the same reduction stages were visible on the TPR-H2 profiles as for 20% Cu/ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) system. Ren et al. (2015) investigated the reducibility of Cu/Al2O3 catalyst modified by ZnO, ZrO2 and MgO. They observed also two reduction stages in the case of all investigated Cu catalysts. TPR-H2 profile recorded for 20% Cu/Al2O3 catalyst modified by ZrO2 showed two unresolved reduction peaks with maxima of hydrogen consumption peaks at 220 and 270\u202f\u00b0C, respectively. These authors reported that a first reduction effect was assigned to the reduction of the highly dispersed CuO phases. The second effect is attributed to the reduction of CuO species strongly interacted with the support. They strongly suggested that the modification of 20% Cu/Al2O3 catalyst by ZrO2 improves the dispersion of CuO species on the catalyst surface. This is due to decrease in the interaction between CuO and support surface which also prevents the migration of metallic copper species onto support surface. Zhu et al. (2015) studied also the reduction behaviour of Cu/ZrO2/Al2O3 catalyst calcined in the temperature range 350\u2013650\u202f\u00b0C. The TPR-H2 profiles recorded for all investigated catalysts showed, similar as in our case, two unresolved reduction effects located in the temperature range 150\u2013300\u202f\u00b0C. These reduction stages were assigned to the two steps of the reduction process described by the following scheme Cu2+\u202f\u2192\u202fCu+\u202f\u2192\u202fCu0. It is worth emphasizing that the Cu/ZrO2/Al2O3 catalyst calcined at 350\u202f\u00b0C exhibited slightly higher reduction temperature than catalysts calcined at 450\u202f\u00b0C. The TPR-H2 profiles recorded for Cu/ZrO2/Al2O3 catalysts calcined at 750 and 850\u202f\u00b0C also showed the reduction effect with the maximum of the hydrogen consumption peak at 370\u202f\u00b0C. This reduction peak was attributed to the reduction of the CuAl2O4 spinel structure. Fig. 3\n presents a comparison of the reducibility of monometallic Cu and Ni and bimetallic Pd-Cu, Rh-Cu, Pd-Ni, Rh-Ni supported catalysts. The TPR-H2 profile recorded for 20%Ni/ZrO2\n\n\n\u00b7\n\nAl2O3 catalyst showed a two-step reduction process (see Fig. 3). The first reduction effect located in the temperature range 350\u2013450\u202f\u00b0C is associated with the reduction of unbounded NiO species. The second hydrogen consumption peak located above 450\u202f\u00b0C is assigned to the reduction of NiO species differently interacted with support. In the same Fig. 3 the reduction results of bimetallic supported catalysts are also given. The TPR-H2 profiles recorded for all bimetallic catalysts showed that addition of noble metals into monometallic supported copper and nickel catalysts facilitates their reduction. The TPR-H2 profiles of bimetallic systems showed the same reduction stages which were observed in the case of monometallic catalysts but shifted towards the lower temperature range. These shifts confirm the facilitated reduction of copper or nickel oxides after introduction of noble metal. In addition, in the case of the 1%Pd-20%Ni/ZrO2\u00b7Al2O3 catalyst a low temperature (maximum at about 90\u202f\u00b0C) consumption peak was observed in the TPR profile and was assigned to PdO reduction step. Guo et al. (2014) investigated Ni/ZrO2/Al2O3 catalysts with the different ZrO2 content. The authors reported three various reduction steps assigned to the reduction of \u03b1, \u03b2 and \u03b3 nickel oxide species differently interacted with the support present in the TPR-H2 profiles recorded for these catalytic systems. The \u03b1 species represent the unbounded NiO which are reduced at low temperature (320\u2013450\u202f\u00b0C). The reduction effect located in temperature range 450\u2013720\u202f\u00b0C was attributed to \u03b2 species correspond to NiO interacted with the support. The last high temperature effect is assigned to the reduction of NiAl2O4 spinel structure. Furthermore, they reported that increasing the ZrO2 content in Ni/ZrO2/Al2O3 catalysts leads to the growth of \u03b1 species which are reduced in low temperature. The reduction properties of supported nickel catalysts were also studied by Richardson at al. (Richardson et al., 1994). The authors observed in the TPR-H2 profile high temperature reduction effects, also assigned to the reduction of NiAl2O4 spinel structure. They have reported that the incorporation of Al3+ to the NiO structure or mutual migration of ions leads to the formation of NiAl2O4 spinel structure. These processes take place on the support surface during the heat treatment. The reduction studies performed for all catalytic material clearly indicate that all copper catalysts reduced in two steps and are connected with the reduction of CuO and Cu2O species, respectively (Mierczynski et al., 2015; Mierczynski et al., 2014; \u00c1guila et al., 2008).\nFig. 4\n presents the influence of the Rh content on the reduction behaviour of supported copper catalysts. The observed TPR profiles recorded for all bimetallic supported catalysts indicate that for all investigated bimetallic catalysts two reduction peaks are visible on the TPR curves. These two steps are connected to the reduction of CuO through Cu2O intermediates. It is worth noting that in the case of bimetallic catalysts with low Rh loading, the reduction process took place in the temperature range of 100\u2013300\u202f\u00b0C. Additionally, the first reduction stage with the maximum of hydrogen consumption located at about 140\u202f\u00b0C had the highest intensity compared to the rest of the bimetallic supported catalysts (see Table 3\n). The highest intensity of the first reduction peak recorded on the TPR-H2 curve recorded for 0.5% Rh-20% Cu/ZrO2\u00b7Al2O3 catalyst means that this system is the easiest reduced catalyst.To further explain the differences in activity we also studied the phase composition of monometallic 20% Cu/ZrO2\u00b7Al2O3 and 20% Ni/ZrO2\u00b7Al2O3 catalysts being after various treatments. We studied the phase composition of the catalysts calcined in an air atmosphere for 4\u202fh at 400\u202f\u00b0C and catalysts after reduction in a mixture of 5%H2\u201395%Ar at 300\u202f\u00b0C for 1\u202fh and reaction performed in oxy-steam reforming of methanol. The XRD results are given in Figs. 5 and 6\n\n. The X-ray diffraction studies were used to determine the changes of the phase composition after various treatments and in order to indicate the interaction between an active phase component and the support. X-ray diffraction curve recorded for 20% Cu/ZrO2\u00b7Al2O3 catalyst being after calcination confirmed the amorphous nature of the ZrO2 (see Fig. 5). The XRD diffraction pattern shows diffraction peak positioned between 30 and 35 theta angles which was attributed to amorphous ZrO2. While, the diffraction peaks positioned at 36, 38, 48 and 62\u00b0 were assigned to the CuO phase. In the same XRD curve, \u03b3-Al2O3 phase was visible at 2\u03b8 angles\u202f=\u202f46, 67 and 68\u00b0. Whereas, the XRD curve recorded for the same catalyst reduced at 300\u202f\u00b0C showed the occurrence of the diffraction peaks assigned to metallic copper and wide diffraction peak positioned between 30 and 35 2\u03b8 angle assigned to amorphous zirconia.\nFig. 6 present the phase composition studies of 20%Ni/ZrO2\n\n\n\u00b7\n\nAl2O3 catalyst. The XRD curve recorded for 20%Ni/ZrO2\n\n\n\u00b7\n\nAl2O3 catalyst calcined in an air atmosphere at 400\u202f\u00b0C shows diffraction peaks positioned at 2\u03b8 angles\u202f=\u202f36.43, 63, 75 and 79\u00b0 which are attributed to nickel (II) oxide phase. The XRD pattern recorded for this catalyst also showed a wide XRD peak attributed to amorphous zirconia. However, in the case of the same nickel catalyst after being reduced the diffraction curve showed the presence of peaks assigned to metallic nickel (2 theta angles\u202f=\u202f44.52\u00b0, 76\u00b0), NiO phase (2\u03b8 angles\u202f=\u202f36.43, 63, 75 and 79\u00b0) and amorphous zirconia. Diffraction curve recorded for the reduced nickel catalyst confirmed its partial reduction. The existence of other phases in the diffraction curve was not confirmed by the XRD technique.In order to elucidate the differences in activity measurements in OSRM process for bimetallic Rh-Cu supported catalysts the XPS high-resolution spectra of the binding energies between 920 and 970\u202feV were recorded and the results are given in Fig. 7\n and Table 4\n. The performed surface analysis of the supported copper catalysts showed that in the investigated binding energy range several peaks were visible. Photoelectron peaks visible in the XPS spectrum can be assigned to metallic copper and copper in first and second oxidation states (Kulkarni and Rao, 2003). The presented on each spectra binding energies bands located at 936, 934.5 and 932.4\u202feV were assigned according to Ertl and co-workers (Ertl et al., 1980) to Cu2+, Cu0 and Cu+, respectively.The peaks located at about 943 and 963\u202feV are satellite peaks and are characteristic only of Cu2+ species. The detailed analysis of the presented data gave evidence that increasing of the Rh content in the investigated catalyst to 1 and 2% wt. of Rh leads to higher content of metallic copper species present on the catalyst surface (see Table 4).Cu0 and Cu+ are active centres in the reaction of methanol reforming, and their number and ratio affect on the catalytic activity. Oxidation reaction of CH3OH to CH3O\u2212 occurs on metallic copper, while the oxidation of CH3O\u2212 takes place on Cu+ in order to formate species creation causing an increase in the conversion of methanol. The presence of Cu+ forms on the catalyst surface leads to greater stability of the catalyst in the process of steam reforming of methanol compared to metallic copper which sinters much easier than in the case of Cu2O species due to its higher Tamman temperature (Mierczynski et al., 2013). In our case the concentration of the Cu+ species on the catalyst surface was the highest for the 2%Rh\u201320%Cu/ZrO2\u00b7Al2O3 catalyst. The role of Cu0 and Cu+ centres in the reforming of methanol process is still unclear but based on the results of the copper species concentration on the catalysts surface it can be assumed that system with the lowest ratio between Cu0 and Cu+ exhibited the highest activity in OSRM process. In addition, these results also confirmed that the occurrence of Cu0 and Cu+ species and their ratio is a critical parameter to achieve highly active systems in OSRM (Kulkarni and Rao, 2003). These results agree well with our previous work (Mierczynski et al., 2013) performed for copper catalysts supported on Zn-Al containing systems tested in steam reforming of methanol reaction. Similar results of the catalytic activity were also confirmed by other authors (Oguchi et al., 2005). The increase of metallic copper content in the catalyst surface after reduction agree well with the temperature programmed reduction results carried out for bimetallic catalysts. This also confirmed that increase of the Rh content in the bimetallic catalysts facilitates the reduction of a copper oxide species present on the catalyst surface.In order to explain and understand the difference in activity and selectivity results in the OSRM process, we carried out acidity measurements for all catalytic material. The acidity measurements performed for supports, monometallic and bimetallic supported catalysts are given in Table 5\n. The results show that all investigated systems exhibited three kinds of the acidic centres on their surface namely weak, medium-strong and strong acid sites. The acidity measurements performed for supported copper catalysts confirmed that the highest total acidity, calculated based on the surface under the peaks, had the system with the highest Al content. This result also indicates that catalysts which showed the highest activity also exhibited the highest total acidity among of all monometallic supported catalyst. The results obtained in this work agree well with our previous investigations (Mierczynski et al., 2016; Mierczynski et al., 2017). The acidity measurements showed that nickel 20% Ni/ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) system exhibited lower total acidity compared to the monometallic copper catalyst supported on the same support.While, in the case of the bimetallic supported catalysts also their high total acidity was detected, what suggest also high activity. This tendency can be easily explained by the fact that acidic sites play crucial role during the OSRM process. These centres are indeed responsible for stabilizing of intermediates such as methoxy, monodenate and bidentate formate species and even carbonates, which are then transformed into the main products CO2 and H2 (Mierczynski et al., 2016; Hereijgers and Weckhuysen, 2009). These findings are in agreement of other published studies such as this conducted by Hereijgers and Weckhuysen (2009).To reinforce the observed activity results and hypothesis concerning the important role of the acidity centres during OSRM process, additional experiments were conducted by FTIR and shown in Fig. 8\n. As we can easily distinguish on all presented spectra that during the sorption process carried out at 50\u202f\u00b0C the formation of methoxy (peaks at 2995, 2936, 2919, 2825, 1470, 1443, 1350, 1200 and 1020\u20131100\u202fcm\u22121), formate (peaks at 2925, 2850, 1620, 1364, and 1350\u202fcm\u22121) and carbonate species (peaks at 1620, 1570\u20131440, and 1220\u202fcm\u22121) are formed on the catalyst surface. The presented results showed that in the case of the most active systems (\u00b720% Cu/ZrO2Al2O3 (Zr:Al\u202f=\u202f1:2)) the highest intensity of the IR bands assigned to methoxy, formate and carbonate species were detected. These results agree well with the hypothesis presented above that catalysts which contain the largest number of acidic centres on its surface has the highest sorption properties with respect to methanol at the studied temperature range. The sorption of methanol is one of the important stages during the oxy-steam reforming process. It is also worth mentioning that catalytic systems which showed the highest intensity of bands assigned to methoxy and formate species during the adsorption process had also the highest activity in the oxy-steam reforming of methanol process.SEM-EDS measurements were also performed for monometallic and bimetallic copper and supported nickel catalysts. This useful technique allows determining the morphology and composition of the catalyst surface. SEM images and EDS spectra collected for the investigated mono- and bimetallic catalysts supported on the selected support ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) were present on Figs. 9\u201311\n\n\n. Fig. 9 and Fig. 10 presented the images and EDS spectra collected for monometallic supported copper and nickel catalysts calcined in an air atmosphere at 400\u202f\u00b0C for 4\u202fh, respectively. The presented data clearly confirms the composition of the investigated catalysts. In both spectra the occurrence of the same elements such as Zr, Al, O were confirmed on the catalyst surface. In addition, Cu and Ni were also detected for monometallic copper and nickel catalysts, respectively. Analogical measurements were also performed for bimetallic supported catalysts and the results are given on Fig. 11. The analysis of the bimetallic catalyst confirmed the presence of the same elements which were found on the surface of monometallic systems. The only difference in the case of the bimetallic supported catalyst was the presence of rhodium and palladium for the appropriate bimetallic system.In summary, we prepared monometallic and bimetallic copper and nickel catalysts supported on binary oxides by an impregnation method and tested in oxy-steam reforming of methanol in order to determine the optimal composition of the catalyst. The physicochemical properties of the catalysts were investigated by TPR, BET, XRD, FTIR, SEM-EDS and XPS techniques and the obtained results were correlated with the reactivity results obtained in OSRM process. We found that the activity and selectivity of the tested systems are strongly dependent on their acidity and sorption properties in relation to methanol. The reactivity results confirmed that the highest active systems were copper catalysts supported on ZrO2-Al2O3 (Zr:Al\u202f=\u202f0.5) binary oxide promoted by noble metals such as Pd or Rh. These catalysts showed the highest specific surface area, the highest number of acidic centres on their surfaces. The reactivity results obtained for bimetallic copper containing systems also confirmed that system with the lowest ratio between Cu0 and Cu+ exhibited the highest activity in OSRM process. Furthermore, these results also confirmed that the occurrence of Cu0 and Cu+ species and their ratio is a critical parameter to achieve highly active systems in OSRM. In addition, copper catalysts showed higher activity and selectivity towards hydrogen formation in oxy-steam reforming of methanol compared to the monometallic supported nickel catalysts. Furthermore, the reactivity measurements carried out for monometallic supported nickel catalysts confirmed that the pre-treatment process before the activity tests has great influence on the reactivity results in oxy-steam reforming of methanol. The supported copper or nickel catalysts described in this work have an application potential in fuel cell technology, especially in Solid Oxide Fuel Cell technology owing to their high efficiency towards hydrogen generation.This work was partially funded by Polish Ministry of Science and Higher Education within the \u201cIuventus Plus\u201d Programme (2015\u20132017) (project no.0305/IP2/2015/73). Magdalena Mosinska thanks the Lodz University of Technology for a scholarship (W\u0142asny Fundusz Stypendialny P\u0141 programme, W-3D/FMN/10G 2018).I would like to thank Mr A. Kedziora for help in the research (BET measurements) carried out in the framework of the work.", "descript": "\n                  Monometallic copper, nickel and bimetallic Pd(Rh)-Cu(Ni) catalysts supported on a binary oxide containing various content of ZrO2 and Al2O3 were prepared by impregnation method. Their physicochemical and catalytic properties in oxy-steam reforming of methanol reaction (OSRM) were extensively investigated. Selecting an optimal composition of the catalyst for the OSRM process was the main goal of this work. The influence of zirconia content on the reactivity and physicochemical properties of supported copper catalysts in OSRM was also studied. The reactivity measurements showed that the supported copper catalyst was more active than the nickel catalyst. The catalytic measurements showed that the catalyst properties depend on their surface composition, acidity and adsorption properties. High selectivity of supported copper catalyst with composition 20%Cu/ZrO2\u00b7Al2O3 (Zr:Al\u202f=\u202f1:2) towards carbon dioxide and hydrogen was confirmed. In addition, the promotion effect of palladium and rhodium on the activity of monometallic supported copper and nickel catalysts in OSRM was confirmed. The most active system in the OSRM process was 0.5%Rh-20%Cu/ZrO2\u00b7Al2O3.\n               "}
{"full_text": "Due to increasing evidence of global warming in the present century, scientists at the UN Intergovernmental Panel on Climate Change have reached a consensus for reduction of greenhouse gas emissions, especially carbon dioxide, to the atmosphere [1\u20138]. This has also prompted steering committees of industrialised countries to assess their energy strategies based on mitigation of greenhouse gas emissions [9\u201314] (see Table 1\n).Extensive literature has covered on the various alternatives for cleaner energy sources [15,16,227\u2013229], smart drilling techniques [17\u201320], efficient fracturing technologies [21\u201325,230\u2013232], usage of nanoparticles [26\u201331,233], their economic aspects and advantages to mitigate CO2 emissions and reduce environmental pollution [234\u2013241]. Among the list of proposed alternative energy sources, hydrogen appears to be the most promising large-scale fuel due to its efficient storage over time and clean combustion [2,244]. Recent, there has been enormous interest in hydrogen and it's been increasing rapidly due to its potential applications in fuel cells. They also serve as an excellent replacement to batteries in the field of portable electronics, internal combustion engines as well as power plants. The demand for hydrogen in the most important sector of road transport is depicted in Fig.\u00a01\n, which illustrates that the annual hydrogen demand is projected to surge from 25 tons in 2020 to 945.5 thousand tons in 2045.Furthermore, the soaring demand for hydrogen in Japan [33] can be illustrated in Fig.\u00a02\n. This shows a gradual increase in hydrogen demand from 2015, which will increase rapidly to 21 million tons in 2035.Additionally, electric engines in any vehicle are energised by electricity from fuel cells, which is generated by conversion of clean and environmentally friendly hydrogen (and oxygen from the air) in fuel cells. A schematic diagram representing hydrogen supply from various sources, and its applications, are illustrated in Fig.\u00a03\n.It has been established that CO2 emission levels [35] to the atmosphere can be significantly decreased by substitution of traditional fuels such as diesel, gasoline and carbon with higher (H/C) ratio fuels such as natural gas or biomass, as shown in Fig.\u00a03. Therefore, production of hydrogen from hydrocarbons is regarded as the most economic and efficient way of achieving a significant degree of reduction in the emissions of greenhouse gases. Natural gas is a non-renewable energy source; essentially a blend of lighter hydrocarbons existing in the basement of gas accumulations present in porous rock which might or might not be associated with oil. It is mainly constituted of saturated hydrocarbons, mainly methane, with butane and propane in insignificant quantities, and other compounds composed of inorganic gases. Production of synthesis gas comprising of a mixture of CO along with purified H2 being obtained from natural gas by using various catalyst and is currently the most preferred choice (Table 1). With the advent of the hydrogen economy, there has been an increased focus on the transformation of petroleum gas into more ecologically friendly hydrogen fuel.The Global carbon dioxide emissions from various industrial processes and fossil fuel combustion have been estimated to be around 35.7 billion tons [36] annually, which has contributed to increased global warming [242,243]. Therefore, it is imperative to develop clean technologies for the utilisation of fossil fuels [245\u2013247] and to introduce alternative greener fuels for inhibiting the adverse effects of greenhouse gas emissions and subsequent climatic changes. Among various alternative fuels, hydrogen [37] is considered to be a sustainable energy carrier and offers near zero end-use emissions of greenhouse gases and pollutants [38].For the creation of clean fuel, like hydrogen, natural gas needs to undergo a catalytic process described as natural gas reforming. Reforming is the most common technique used in industries for production of synthesis gas via through one of three reforming processes i.e., partial oxidation of methane (POM) [39], steam reforming of methane (SRM) [40\u201348] and CO2 reforming of methane (DRM) [49\u201355]. SRM is a fully developed generation technique which utilises steam at high temperature (700\u20131000\u00a0C) for the production of H2 from natural gas. During SRM, CH4 interacts with steam with pressures ranging from 3 to 25\u00a0bar using catalyst to produce H2, CO and a moderate quantity of CO2. Eventually during the water-gas shifting reaction, steam and CO2 interact to generate CO and more H2 using an efficient catalyst. Steam reforming of methane requires rigorous energy input because of its endo-thermicity and higher H2O/CH4 ratio which results in better yields of H2, thereby making the SRM process energetically unfavourable [56\u201362] and accelerates the catalyst deactivation process [51]. On the other hand, DRM [63\u201369]can be utilized for the generation of syngas from methane and is valuable for the immediate transformation of CO2 into different compounds, Equations 1 to 8\n\n\n(1)\nSRM: CH4 + H2O \u2192 CO + 3H2 = \u0394H\u00b0\u00a0298,\u00a0\u2212\u00a0206\u00a0kJ/mol\n\n\n\n\n(2)\nPOM: CH4\u00a0+\u00a01/2O2 \u2192 CO\u00a0+\u00a02H2\u00a0= \u0394H\u00b0\u00a0298,\u00a0\u221238\u00a0kJ/mol\n\n\n\n\n(3)\nDRM: CH4\u00a0+\u00a0CO2 \u2192 2CO\u00a0+\u00a02H2\u00a0= \u0394H\u00b0\u00a0298,\u00a0\u2212\u00a0248\u00a0kJ/mol\n\n\nPOM can deliver syn gas with a H2/CO proportion of 2.0. Nonetheless, controlling this process is an arduous task due to the danger associated with explosions [70] and the presence of hot spots Also, Partial oxidation of methane needs an air separation unit (ASU), which markedly impacts the expenses associated with the reforming plant. Because of these disadvantages associated with POM, combined steam and dry-reforming of methane (CSDRM), where H2O is used in conjunction with CO2, has been considered as a worthwhile strategy for the mass production of syn gas with a H2/CO proportion of 2.0 [71,72]. The CSDRM can generate syn gas with flexible H2/CO proportions, which can be effectively controlled by modifying the feed gas (H2O, CO2 and CH4) composition. The utilized procedure is alternatively known as bi-reforming (BRM) where a 3/2/1 proportion of CH4 along with CO2 and steam produces a gas blend with basically a 2/1 proportion of H2 to CO. This formed gas is also called \u2018met gas\u2019 to underline its distinction from broadly utilized syngas blends of different H2/CO proportions. The formation of syngas with this ratio has potential applications in Fischer-Tropsch operations for the preparation of long hydrocarbon chains [73\u201375]. as well as in the production of methanol [76\u201378].Furthermore, bi-reforming [79\u201394] of CH4 has captivated massive interest from both environmental and industrial perspectives. CO2 and CH4 are the most abundant carbon-containing, ozone-depleting substances from an environmental perspective, which can be used successfully in this reaction and can undergo conversion to useful chemical products. In reality, the combination of both steam and dry reforming provides a more pragmatic route for enhancing the H2/CO ratio compared to the introduction of CH4 [95\u201397]. Additionally, this method possesses the merit of producing synthesis gas by using methane and carbon dioxide which are coined as greenhouse gases.It has been reported [98] that at lower temperatures higher conversion of methane can be achieved in the bi-reforming process. In addition to the operating conditions, catalysts also play a crucial role in bi-reforming reactions. One of the most important advantages of bi-reforming [85,98\u2013104] is that the consumption of major greenhouse gases occurs, thereby creating a significant environmental impact. These gases are water vapour, which accounts for 36\u201370% of the feed gas, CO2 at 9\u201326%, CH4 at 4\u20139% and ozone (O3) for the rest (3\u20137%) [105]. Hence, there has been a renewed interest in the application of these gases via bi-reforming of methane towards the production of value-added chemicals that are useful for both scientific and industrial communities.Additionally, bi-reforming technology can be regarded as a method for enhancing the caloric value of biogas, which is composed of CO2, H2 and CH4 through the solar reforming process [106,107]. One of the biggest stumbling blocks for the methane reforming process is related to the sudden catalyst deactivation, which might be due to sintering and coke formation on the active sites [108,109]. CH4 decomposition (Eq. (3)), CO disproportionation (Eq. (4)) and CO reduction (Eq. (6)) are the primary processes that lead to coke formation. The reaction in Equation (3) shows an endothermic reaction that is highly favourable at higher temperatures and lower pressures, whereas Equations (4) and (5) are exothermic in nature and favoured at lower temperatures [110] and higher pressures through the reverse water gas shift reaction (Eq. (6)).\n\n(4)\nCH4 \u2194 C + 2H2 = \u0394H, \u2212\u00a074\u00a0kJ/mol\n\n\n\n\n(5)\n2CO\u2194 C\u00a0+\u00a0CO2 = \u0394H,\u00a0\u2212172\u00a0kJ/mol\n\n\n\n\n(6)\nCO\u00a0+\u00a0H2 \u2194 C\u00a0+\u00a0H2O = \u0394H,\u00a0\u2212131 KJ/ mol\n\n\n\n\n(7)\nCO\u00a0+\u00a0H2O \u2194 CO2\u00a0+\u00a0H2\u00a0= \u0394H,\u00a0\u2212 41\u00a0kJ/mol\n\n\n\n\n(8)\n3CH4\u00a0+\u00a0CO2\u00a0+\u00a02H2O \u2194 4CO\u00a0+\u00a08H2\u00a0= \u0394H,\u00a0+220\u00a0kJ/mol\n\n\nSince catalysts deactivation is caused by formation of coke from the above reactions (4), (5) and 6, hence, it is desirable to establish promising catalysts that demonstrate greater selectivity, excellent stability and activity during the production of syngas. Several investigations [49,50,111], have been reported for assessing the most suitable catalyst for syngas production employing different technologies. Common catalysts that have been used in reforming reactions include catalysts such as copper, nickel supported by transition metals and other supported noble metal catalysts such as ruthenium, platinum, rhenium. Several noteworthy reviews reporting on various innovations recorded in catalyst development for DRM reactions have mainly focussed on catalysts configurations [112], the influence of process parameters [50], noble metal catalysts [49], coke deposition and management [111], development of oxygen carriers in chemical looping [113], Ni and Ni-based catalysts [51], low temperature dry reforming [114], and advances in synthesis of catalysts with mesoporous SBA-15 support [115]. Fidalgo et\u00a0al. [36], conducted a review on carbon black catalysts and activated carbon which have the unique characteristic of operating without being deactivated by carbon deposition. The catalysts role on methane decomposition and carbon dioxide reforming of CH4 was assessed, and the characteristics of carbon deposits during CO2 reforming of CH4 were listed. The influence of nanocatalysts [38] on the oxidative coupling, steam reforming and CO2 reforming of CH4 has been previously reported, which suggested that methane conversion over a nanocatalyst occurred significantly than the ordinary catalyst and there existed no interdependence between the average particle size of nanoparticles and the conversion of methane.Pakhare et\u00a0al. [49], reviewed DRM for catalysts based on metals such as palladium, platinum, rhenium and ruthenium, which involved the role of these elements on the mechanism, deactivation, kinetic behaviour of these catalysts. Abdullah et\u00a0al. [51], conducted a comprehensive review on the potential of nickel based catalysts employed during syngas production using dry reforming process. Their result suggested that strong metal support interactions were dependent on the catalyst supports and these factors were responsible for highest coke resistance, high thermal resistance and greater stability. The authors also examined the synthesis of catalyst supports from cellulosic materials and stressed the enhanced catalytic activity of the cellulose in the DRM reaction due to its superior mechanical strength and distinct structure.However, to the author's knowledge there is a lack of comprehensive literature on the synthesis, characterisation and the role of catalysts and their promoters in the generation of synthesis gas during bi - reforming of methane (BRM). Therefore, the present review encompasses in details the role of various catalysts: Ni-based, Co-based, Ru-based, mesoporous and La-based, on BRM process. Additionally, the review describes the recent progress relating to the most relevant topics on catalysts used in bi-reforming technology.Though Ni-based catalysts [116] are inexpensive, they display exhibit superior performance in comparison to precious metal catalysts. Nevertheless, sintering and formation of carbon affects the sudden deactivation of catalysts. Since bi-reforming employs low S/C ratios for adjusting the H2/CO ratio, hence catalysts involving Ni undergo deactivation by carbon deposition [76]. Two main methods have been documented to diminish the deactivation of catalyst due to formation of coke. One method described the effect of promoters such as lanthanum [117], cerium, magnesium and calcium [118,119], on the characteristics of the catalysts during the reforming process and other was aimed at controlling the particle size at the nano-level in the active metals.It has been established that during bi-reforming [120\u2013122] and DRM, deactivation of Ni supported catalysts occur due to coke formation. Hence, it is highly imperative for the development of most active and stable catalysts in bi-reforming. Several authors [120,121,123], have developed nickel catalyst of high activity and stability supported by Ce\u2013ZrO2, ZrO2 and MgO during the DRM process. Several literature studies on the effect of nickel catalysts and their promoters during bi-reforming of methane have been documented in the present review. For example, Roh et\u00a0al. [121] employed numerous supported Ni catalysts during bi-reforming reactions for production of syngas having H2/CO\u00a0=\u00a02. The supported Ni catalysts were prepared by incipient wetness method with Ni(NO3)2. The Ni catalysts were supported by small nanoparticles of ZrO2 or MgO which were highly active and stable for BRM. Fig.\u00a04\n illustrates scanning electron microscopic (SEM) images of catalysts used in the reaction and the coke formation observed when subjected to 800\u00b0C. The authors observed that the degree of carbon formation and shape varied with different catalysts. Ni catalyst with MgO\u2013Al2O3 as support generated filamentous coke but of insignificant intensity (Fig.\u00a04a). However, Ni catalyst with MgO as support generated a lot of coke from the filaments (Fig.\u00a04b) during occurrence of BRM reaction while Ni/ZrO2 exhibited a worm-like coke feature (Fig.\u00a04c). Similar shape of coke (Fig.\u00a04d) was observed for Ni/CeO2 catalyst. Nevertheless, Ni/\u03b1\u2013Al2O3 generated rod shaped-like coke (Fig.\u00a04e).The authors [121] also made a comparative study between the Ni/MgO\u2013Al2O3 catalyst and the commercial Ni catalyst supported with MgAl2O4 (Fig.\u00a05\n) at various temperatures. The methane conversion was 83% and the conversion of carbon dioxide was 71% in presence Ni catalyst supported by MgO\u2013Al2O3 at 700\u00b0C, while CO2 and CH4 conversions with Ni/MgAl2O4 were both found to be 20% lower during BRM of methane. Commercial Ni/MgAl2O4 was used as a reference catalyst. Also, coke formation was more severe with the commercial Ni/MgAl2O4 catalyst than with Ni/MgO\u2013Al2O3, which was attributed to the efficient dispersion of Ni [124] supported MgO\u2013Al2O3. The high activity and stability of Ni/MgO Al2O3 catalyst was attributed to the beneficial role of MgO which resulted from basic property, fine dispersion of nano-sized Ni and strong interaction of Ni to the support.From Fig.\u00a06\n it was clearly observed that the CH4 conversion was highest for Ni catalyst supported by MgO Al2O3 and approximately 90% methane underwent conversion which continued for 2\u00a0h. However, rapid catalyst deactivation occurred in the case of Ni \u03b1-alumina catalyst with changes in time attributed to carbon formation. Moreover, Ni/MgO catalyst exhibited around 60% CH4 conversion and Ni/ZrO2 demonstrated around 70% CH4 conversion. Both the catalysts were found to be highly stable during the reforming process. Nevertheless, the conversion of methane for Ni catalyst supported by CeO2 was initially 57% which decreased to 50% followed by its saturation. Results revealed that Ni/MgO\u2013Al2O3 possessing lowest nickel oxide crystallite size exhibited highest stability along with higher CH4 conversion with time on stream.Ryi et\u00a0al. [125] conducted tests over a catalytic nickel membrane during bi-reforming of methane for a shorter residence time of 120\u00a0ms for various CO2/H2O ratios in the range of 0\u20131.0, along with (H2O\u00a0+\u00a0CO2)/CH4 ratio of around 3.0 in the reactant feed for temperatures ranging from 923 to 1023\u00a0K.The purpose of this study was to examine the performance of bi-reforming of methane over a catalytic nickel membrane for the GTL (gas to liquid) process. GTL process possess two advantages. One is that carbon formation is reduced due to the oxidation of carbon precursor species and a desirable H2/CO can be achieved by adjusting CH4/H2O/CO2 ratio in the feed stream. Porous wall of catalytic nickel membrane was chosen for reforming studies since hydrogen passed through the catalytic nickel membrane was faster than the other gases because of viscous and Knudsen flow. Generally, the catalyst that contained relatively small size pore was more affected by internal diffusion than the one which has large sized pores.The results revealed that the change in the feed ratio of CO2/H2O strongly affected the conversion of methane and furthermore an increase in the feed ratio of CO2/H2O at a temperature of 923\u00a0K (Fig.\u00a07\n) decreased the methane conversion. The authors noted a very high conversion of methane in the range of 92.7\u201396% above 973\u00a0K, when the CO2/H2O feed ratios were in the range of 0\u20131.0 during bi-reforming of methane. The authors ascertained that with change in the CO2/H2O ratio during the reforming reaction, a change in H2/CO also occurred. Hence H2/CO molar ratio obtained at 973 k were 8.1, 5.7, 3.7 & 2 when the molar ratio of CO2/H2O were 0, 0.11, 0.33 and 1.0. However, increase in temperature to 1023\u00a0K changed the H2/CO molar ratio to 7.5, 5.3, 3.4 and 1.8 respectively for similar values of H2O/CO2. Additionally, the CO2 registered an increase with increasing temperature attributed to CO2 reforming of methane occurring at higher temperatures, which remained almost constant at \u2265 973\u00a0K attributed to the limitations of CH4 as the reacting species [125].Al-Nakoua and El-Naas [126] experimented with different molar proportions of H2O/CH4 and CO2/CH4 in a detachable reactor covered by catalyst B Nickel (33%)\u2212Chromium (5.6%)\u2212Barium (11%)/La2O3 (19%) and catalyst A which represents Ni (49%)/Al2O3 (51%). The author observed that rapid carbon deposition was observed at 700\u00b0C, 1\u00a0atm during dry reforming of methane. However, when CO2 reforming was performed in conjunction with steam reforming reaction in thinner channels deposited with a thinner layer of catalyst, a reduction in carbon deposition was noticed on the surface. The authors determined the equilibrium compositions of the CH4 and CO2 reactants, which are shown in Figs. 9 and 10 at pressures of 1, 2, 3, 4, 5, 10, and 20\u00a0bar respectively. Fig.\u00a08\n\n\n shows that CH4 conversion was highest (85\u201390%) at pressures ranging from 1 to 3\u00a0bar and at temperatures in the range of 810\u2212900\u00b0C.Similarly, CO2 conversion (Fig.\u00a09) was found to be above 80% in the pressure range of 1\u20133\u00a0bar and temperatures varying from 840 to 900\u00b0C. SEM studies (Fig.\u00a010) revealed that cracks formed in catalyst A possessed a length of 250\u00a0\u03bcm and width of about 10\u00a0\u03bcm, whereas for catalyst B the cracks were formed with a width of 20\u00a0\u03bcm and was spread up-to a certain length where the cracks were interconnected. Furthermore, the results [126] established that there was a significant improvement in the catalyst stability when the H2/CO was around 2.2 during bi-reforming of methane. These conditions were appropriate for Fischer-Tropsch applications and synthesis of methanol. Additionally, the authors observed a five-fold increase in the resistance of coke formation displayed by Ni/Al2O3 catalyst An upon addition of Cr, Ba, and La2O3 during a continuous reaction time of 140\u00a0h. The SEM results were also supported by EDX analysis.The results of catalyst activity test on Ni/Al2O3 in the ratio of 1:1 for Catalyst A is represented in Fig.\u00a011\n. The catalyst film exhibited 50% conversion of CH4 and 15\u201310% conversion of CO2 when the reaction was continuously operated for 24\u00a0h at 630\u00b0C. Furthermore, with increase in inlet pressure from 1 to 23 psig, carbon deposition was noticed. The flow rate of CO2 was 0.2\u00a0mol/h and the flow rate of CH4 was 0.8\u00a0mol/h and the steam: carbon ratio was 0.51. However, Ni\u2013Cr\u2013Ba/La2O3\u2013Al2O3 (Catalyst B) displayed CH4 conversion in the range of 50\u201375% and conversion of CO2 increased from 20% to 60%.The authors found a reduction in the conversion percentage (Fig.\u00a012\n) of methane and carbon dioxide when the reactor pressure of steam was increased up to 42 psig during continuous operation from 25 to 90\u00a0h. However, the conversion percentage of methane and carbon dioxide underwent an increase with further increase in temperature. Since catalyst deactivation has been caused by carbon deposition, hence suppression of coke formation was important. This was only achieved by optimization of the H2O/CH4 and CO2/CH4 and feed ratios.Formation of coke is usually attributed to the following reactions 9 and 10:\n\n(9)\n2CO \u2194 C\u00a0+\u00a0CO2\n\n\n\n\n\n(10)\nCH4 \u2194 C\u00a0+\u00a02H2\n\n\n\nMajor amount of coke formed in the temperature range of 850 to 900\u00b0C [127] resulted from disproportionation reaction involving carbon monoxide (reaction 9) and pyrolysis of methane (reaction 10).Son et\u00a0al. [128], observed that Ni/\u03b3-Al2O3 catalyst was rendered stable by pre-treatment with steam at a temperature operated at 850\u00b0C. Ni/Al2O3 based catalysts are relatively cheap because precious metals are not used and these catalysts can operate stably with high activity under excess steam. Ni/\u03b3-Al2O3 catalyst used in this study was prepared by incipient wetness method. Thermodynamically, the catalyst promoted very high conversion of CH4 (98.3%) and CO2 (82.4%) when subjected to bi-reforming of methane for 200\u00a0h and resulting in H2/CO ratio of 2.01. Furthermore, the results revealed that the conventional catalyst system produced 15.4% coke after 200\u00a0h while the mass of carbon deposited was around 3.6% for catalysts exposed to steam. This novel steam pre-treatment technique significantly increased the resistance towards carbon formation in the presence of catalysts, thereby improving both long-term stability and activity.Transmission electron images of fresh untreated Nickel Aluminium and fresh steam-treated NiAl (WNiAl) catalysts shown in Fig.\u00a013 (a) and (c)\n revealed that it was difficult for distinguishing nickel nanoparticles dispersed in the Nickel Aluminium catalyst, owing to their small size. However, the WNickel Aluminium catalyst showed the presence of distinct Ni nanoparticles in the range greater than 10\u00a0nm. Severe carbon deposition (Fig.\u00a013b) was noticed for Ni/Al catalysts treated with steam for 200\u00a0h. The shape resembled to wire-type resembled carbon and the size of the nanoparticles were enhanced from 4.2\u00a0nm to 23.5\u00a0nm from Hydrogen chemisorption measurements. Nevertheless, carbon coke with wire typed shape did not appear in the WNickel Aluminium catalyst (Fig.\u00a013d).Two alumina supported Ni catalysts with pore sizes of 5.4\u00a0nm and 9\u00a0nm were synthesized and tested in the bi-reforming process [129] for the production of hydrogen rich gases. Structural and functional characterisation of catalysts showed that Ni/Al2O3 with the largest pore size exhibited better characteristics i.e. higher capacity to adsorb CO2, higher surface area, higher proportion of stronger catalytic sites for hydrogen adsorption and lower Ni crystallite sizes. At all the investigated temperatures, for a CH4: CO2: H2O molar ratio of 1:0.48:1.2, a (H2+CO) mixture with H2:CO ratio around 2.5 was obtained. The optimum conditions for the production of hydrogen rich gases, were CH4: CO2: H2O\u00a0=\u00a01:0.48:6.1 and 600\u00a0\u00b0C.Dan et\u00a0al. [130] have investigated the role of Ni/Al2O3, Ni/MgO\u2013Al2O3 and Ni/La2O3\u2013Al2O3 with bimodal pore structure in the bi-reforming process. The authors observed that La2O3 and MgO promoted catalysts presented better functional and structural properties. Among all the catalysts, Ni/La2O3\u2013Al2O3 was found to be the catalyst with best stability and activity. The presence of both lanthanum and magnesium oxides contributed to excellent dispersion and stabilization of Ni nanoparticles on the catalyst surface. The catalytic activity for the bi-reforming process increased in the order Ni/Al2O3(r)\u00a0<\u00a0Ni/Al2O3\u00a0<\u00a0Ni/La2O3\u2013Al2O3\u00a0\u2248\u00a0Ni/MgO\u2013Al2O3.Lanthanide group metals (La, Ce) have been reported [131\u2013133], to be efficient promoters for Ni-based catalysts. Recently, literature reports have suggested [120,121], that during the bi-reforming of methane smaller nanoparticles of Ce\u2013ZrO2,ZrO2 and MgO supported by Ni catalysts were found to be highly stable and active. Koo et\u00a0al. [134], used a stable and extremely active magnesium oxide promoted Nickel/Al2O3 catalyst to investigate catalytic activity and coke formation during bi-reforming for potential applications in gas to liquid (GTL) processes. In their study, the incipient wetness technique was employed to synthesise Ni/Al2O3 catalysts with different concentrations of MgO. The authors used H2-chemisorption, CO2-temperature programmed desorption (TPD), BET analysis, and X-ray diffraction (XRD) to examine the characteristics of the prepared catalysts. Furthermore, the authors established that by changing the feed ratio of H2O/CO2, a H2/CO ratio of 2 was obtained during the bi-reforming reaction. Additionally, catalysts containing 20\u00a0wt % magnesium oxide (MgO) showed high coke resistance and excellent catalytic performance during the bi-reforming reaction. MgO addition to the catalyst formed a stable MgAl2O4 spinel phase at high temperatures and was quite effective in eliminating formation of coke by enhancing the adsorption of CO2 because of higher base strength on the surface of the catalyst. SEM images of reduced Ni/MgO/Al2O3 catalysts with changing concentrations of MgO content are illustrated in Fig.\u00a014\n.In particular, Cerium oxide has been widely recognized as an efficient promoter for Ni-based catalysts. This is because the redox properties of Ce4+/Ce3+ results in easier gasification of the settled coke on the surface of the catalyst and also helps in storage and delivering of active oxygen thereby enhancing the dispersion of Ni. In another study [132], Ce-promoted Ni/MgAl2O4 catalysts synthesized by co-impregnation showed higher metal dispersion than Ni/MgAl2O4 catalyst alone and demonstrated outstanding reducibility properties at lower temperatures of around 550\u00b0C, as established by XPS. The authors found that the catalytic activities of Ni\u2013Ce/MgAl2O4 catalyst were the highest and it generated enormous coke resistance during the bi-reforming reaction performed at lower temperatures with Ce/Ni ratio of 0.25. These were due to stronger metal-support interactions and powerful dynamic oxygen movement through close contact with Ni\u2013Ce. Furthermore, when no Ce was present, the NiO crystallite size in the Ni/MgAl2O4 catalyst was observed to be enormous at a value of 11.0\u00a0nm and indicated a lower metal scattering of 3.49%. The authors used Brunauer\u2013Emmett\u2013Teller, (BET) adsorption H2-chemisroption, CO2-TPD and TPR to ascertain the crystallite size of NiO, basicity and reduction temperature of the catalysts. Results revealed that the nickel oxide (NiO) crystallite size, reduction degree and dispersion of the metal were significantly affected by cerium addition to the Ni/MgAl2O4 catalyst.The authors [132] also employed Raman spectroscopy in the range of 1200\u20131800 cm-1 to investigate coke formation in the presence of Nickel Cerium/MgAl2O4 catalysts with varying Cerium/Nickel ratios. The spectra in Fig.\u00a015\n revealed two peaks in the vicinity of 1600 cm\u22121 and 1350 cm\u22121 which corresponds to G band and D band. The role of the G band is to provide useful information related to the electronic characteristics of filamentous carbon [135] while the D band arose from imperfect and polycrystalline graphite. Additionally, Ce-promoted Ni/MgAl2O4 showed a decrease in peak intensity with a Cerium/Nickel ratio of 0.25 due to minimal coke formation on the surface of the catalyst in comparison to Ni/MgAl2O4 catalyst without addition of cerium. These results were also in accordance with results obtained from TGA studies: quantification of coke deposition by TGA established a rise in graphitic and amorphous carbon with an increase in Ce/Ni ratio to 1, which was further confirmed by the increase in D band peak intensity (Fig.\u00a015) for amorphous carbon. The advantages of using a Ce-promoted Ni/MgAl2O4 catalyst in the bi-reforming reaction relate to its inherent ability to eliminate formation of amorphous coke in comparison to the Ni/MgAl2O4 catalyst.Recently, there has been renewed interest in developing Ce1-x\u2212ZrxO2 catalytic systems [136]. It has been established that addition of zirconium oxide (ZrO2) to cerium oxide results in significant improvement in the oxygen storage capacity of cerium oxide, its thermal stability, metal dispersion and its redox properties. These improvements were attributed to the preferential replacement of Ce4+ with Zr4+ ion existing in the structure of the lattice surrounding cerium oxide (CeO2). [136\u2013138], The Ce1-x\u2013ZrxO2 catalytic unit has also been regarded as an outstanding material for support in Ni-based catalyst systems [139\u2013141]. CeO2\u2013ZrO2 has been reported to be an effective promoter for the Ni/\u03b8-Al2O3 catalytic system and helps in significant suppression of coke formation with a high catalytic stability [142] Bae et\u00a0al. [143], investigated the catalytic activity of Ni/MgAl2O4 catalyst in presence of cerium oxide-zirconium oxide (CeO2\u2013ZrO2) during combined steam and CO2 reforming of methane. The synthesis of the catalysts were performed by employing an impregnation technique followed by co-precipitation process of CeO2\u2013ZrO2 components.Furthermore, the basic supports such as MgO or ex-hydrotalcite MgAl2O4 employed in this study possessed beneficial effects such as minimising coke formation due to the reduced acidic site density [144,145]. The Cerium oxide-zirconium oxide (CeO2\u2013ZrO2) component demonstrated a key role in the conversion of CO2 by increasing CO2 activation when contacted with crystallites of nickel. The catalysts synthesized by co-precipitation technique showed higher catalytic characteristics in comparison to catalysts synthesized by successive impregnation of Ni on support of MgAl2O4 with cerium zirconium oxide (CeO2\u2013ZrO2).Addition of lanthanum to Ni/Al2O3 catalysts inhibited the agglomeration of Ni particles due to the enhancement of strong metal to support interaction (SMSI). SMSI of catalysts was reported to enhance thermal stability [145\u2013147]. Park et\u00a0al. [148], synthesized 10\u00a0wt % Nickel\u2013xLanthanum/MgAl2O4 catalysts where x ranges from 0 to 5% by the co-impregnation technique during the bi-reforming of coke oven gas (COG). They conducted aging treatment with a H2: H2O: N2 ratio of 1:10:1.25 with temperature around 900\u00b0C run for 50\u00a0h. The results revealed an increase in the Ni crystallite size for all the investigated catalysts subjected to ageing. Furthermore, the lanthanum promoted catalysts exhibited greater nickel dispersion than Ni/MgAl2O4 catalyst due to their enhanced interactions between the metal and support. Results from catalytic tests performed at 900\u00b0C and at a pressure of 5 atmospheric pressure for 40\u00a0h with a CH4: H2O: CO2:H2:CO: N2 ratio of 1:1.2:0.4:2:0.3:0.3 also revealed that aged Ni\u20132.5La/MgAl2O4 catalyst showed maximum sinter stability and activity due to its enhanced nickel dispersion and surface area.Furthermore, the role of Ce/Zr ratio on the catalytic activity of Ni\u2013Cex Zr1\u2212xO2 catalyst and coke formation was demonstrated by Roh et\u00a0al. [131], during the bi-reforming reaction. The authors used co-precipitation method to synthesise Ni\u2013Ce\u2013ZrO2 catalysts having different ratios of CeO2/ZrO2 for syngas production having potential applications in gas to liquid (GTL) processes. 15% Ni\u2013Ce0.8Zr0.2O2 demonstrated excellent stability and highest activity during BRM which was attributed to the dispersion of nickel oxide having higher oxygen storage capacity and intimate contact with the support. Recently, it has been reported [131] that Cerium content along with nickel-cerium loading technique has an significant effect on the transfer of O2 occurring between nickel (Ni) and cerium (Ce). Studies have also shown an improvement in coke resistance of 12\u00a0wt % Nickel/\u03b1-Al2O3 catalyst in the bi-reforming reaction. [133,149], However, the characteristics of supports play a marked effect on coke formation [132]. For example, \u03b1-Al2O3 support caused carbon deposition due to its acidity [150]; active nickel metal and Al2O3 supports underwent interaction to form inactive NiAl2O4, resulting in deactivation of the catalyst [134]. Additionally, Baek et\u00a0al. [151], observed higher coke resistance and enhanced catalytic stability of Ni\u2013Ce/MgAl2O4 (MgO/Al2O3\u00a0=\u00a03/7) in comparison to Ni\u2013Ce/\u03b8-Al2O3 in bi-reforming process.Gao et\u00a0al. [152] develop an ideal Ni\u2013Ce/ZSM-5 catalyst by the impregnation method for the bi-reforming process. The authors noted that by adjusting the parameters properly, highest conversion i.e. 99% and 94% of CH4 and CO2 to syngas was achieved in presence of Ni\u2013Ce/ZSM-5 catalyst. Furthermore, the catalyst did not show any deactivation and maintained high activity for 40\u00a0h. SEM, XRD and H2-TPR analysis further established the structure as well as composition of the catalysts and provided better understanding of the catalytic performance.Chen et\u00a0al. [153] synthesized highly dispersed mNi/xL/Si catalysts by one-pot sol-gel process and applied to the bi-reforming process for syngas production. Results revealed that the addition of lanthanum improved the stability, catalytic activity as well as the coke resistance of these catalysts. The17.5Ni/3.0LaeSi catalyst prepared using ethylene glycol and poly (ethylene glycol) displayed the best catalytic activity, coke resistance and stability. Additionally, the H2/CO ratios in the product gas were tuned by varying the C/S ratios in the feed.Jabbour et\u00a0al. [154], employed a one-pot method followed by evaporation-induced self-assembly (EISA) to synthesise two types of catalyst namely Ni5%M5% where M represents Ca or Mg and Nix% where X corresponds to 5\u201310\u00a0wt % along with packing of mesoporous Al2O3.Low cost and widely available Mg2+ and Ca2+ containing salts were used as the additives based on their potential to yield basic properties (in their oxide form) and their positive impact on bi-reforming process [155]. Temperature programmed reduction (TPR) of calcined Ni-loaded samples displayed a strong reduction peak at higher temperatures ranging from 550 to 800\u00b0C (Fig.\u00a016\nb\u2013f), which was ascribed to the reduction of oxidised nickel (Ni) undergoing stronger interaction with the support present in the mixed spinel phase [156]. The author observed that there was no peak signifying weakly-bounded Ni species below a reduction temperature of around 500\u00b0C, which was similar to the finding for non-porous impregnated alumina samples [82,157]. The authors in their studies noted that after the reduction process, the catalysts demonstrated higher dispersion of Ni within the arranged oxide cavity, possessed elevated activities and also showed long-lasting stability in bi-reforming of CH4 performed at 800\u00b0C. They also observed that a relationship existed between carbon deposition and reactivity level in the presence of Mg free catalysts.SEM (Fig.\u00a017\nA, B) and TEM (Fig.\u00a017C, D) images for spent Ni10%Al2O3 clearly identified long carbon filaments on the exterior area of the alumina containing grains, with some grains which were found to more protected than others. These images also showed some Ni nanoparticles containing coke were situated either at the boundary between the support and the filament embedded into it. These images also resembled carbon nanotubes nucleation with a \u2018closed end\u2019 consisting a nanoparticle at either their closure or located inside the tip [158\u2013160]. For both Ni5% Mg5% Al2O3 and Ni5% Ca5% Al2O3, a peak was observed at very high temperatures of above 800\u00b0C, as shown in Fig.\u00a018\nA (e and f); this was a possible indication of free metallic Ni existing under stronger interactions or may have been related to the reduction of Mg- or Ca-derived species. Furthermore, a linear correlation was established between H2 uptake and Ni content, which confirmed that all Ni used in the synthesis was completely reclaimed in the solid after preparation. The authors also reported that, due to the endothermic nature of the bi-reforming reaction, the conversion of both CH4 and CO2 decreased at lower temperatures, and CO2 conversion was more significant below 700\u00b0C.Results also revealed that there was a beneficial effect when 5\u00a0wt% magnesium or calcium was used for the conversion of both CH4 and CO2 represented by d and e (Fig.\u00a018A, B), compared to catalyst containing 5\u00a0wt % Nickel and without additive. The enhancement in the reactivity in presence of 5\u00a0wt (wt%) magnesium or calcium was higher than with Ni7.5% Al2O3 despite the lower Ni content reported in previous literature [161\u2013163]. Jabbour et\u00a0al. [154], established that in addition to high activity levels, doping of the Ni catalysts with Mg or Ca additive resulted in excellent catalytic stability for high temperature bi-reforming operations. The authors found that mesoporous catalysts synthesized by one-pot method served as an ideal candidate for catalysing met gas production from biomass-related natural resources. The beneficial effect of nickel confinement in the pores was twofold, one in protecting the metal nanoparticles against sintering phenomenon and second against coking due to steric constraints.Kang et\u00a0al. [164], synthesized core shell structured Ni catalysts Ni/MgO\u2013Al2O3 and Ni/Al2O3 and via technique coined multi-bubble sono-luminescence and conducted tests using these catalysts for the bi-reforming process. The authors observed that Ni catalysts constituting of 10% Ni loaded on Aluminium oxide or MgO\u2013Al2O3 exhibited exemplary performance during the steam reforming of methane, achieving 97% conversion of CH4 at a temperature of 750\u00b0C. Additionally, methane conversion was 96% at 850\u00b0C during dry reforming of CH4 and demonstrated greater thermal stability for the initial duration of 50\u2013150\u00a0h. The results also established that supported Ni catalysts demonstrate excellent performance in both mixed and auto-thermal reforming of CH4, where satisfactory thermal stability was noted for the first 50\u00a0h. An interesting observation was that no significant carbon formation was obtained on surface of the investigated catalysts after the reforming reaction. Very recently, Koo et\u00a0al. [118], synthesized nickel catalysts in the nanoscale by employing a mixture of magnesium oxide\u2212aluminium oxides (MgO\u2013Al2O3) obtained from a structure resembling hydrotalcite. Their results revealed an enhancement in the coke resistance with various mixed ratios of Mg/Al for the generation of syngas during bi-reforming for applications in GTL processes.Mesoporous SBA-15 has aroused enormous interest among researchers in steam reforming [165,166], and CH4 dry reforming [68,167], process due to its high surface area, high silanol group density, uniformity of pores and enhancement in active metal dispersion with smaller crystallite size [168]. A group of Nickel/SBA-15 catalysts with Ni content ranging from 5 to 15\u00a0wt % were synthesized by Huang et\u00a0al. [169], along with 10% Ni/MgO/SBA-15 catalysts with MgO content ranging from 1 to 7\u00a0wt (wt %) during combined steam and dry reforming reaction in a continuous micro-reactor. XRD, H2-TPR and CO2-TPD techniques were used to investigate the structure of catalysts. The authors observed that selectivity of carbon monoxide (CO) for these reactions was almost 100% and they also noticed that with the change in the molar ratio of H2O/CO2, there can be effective control of the H2/CO ratio. After reaction at 850\u030a C for more than 120\u00a0h with 10\u00a0wt % of Ni/SBA-15 catalyst, the conversion of methane underwent a decrease from 98% to 85% while the conversion of CO2 reduced from 86% to 53%, respectively. Additionally, the catalyst containing 3% MgO/SBA-15 loaded with Ni demonstrated excellent catalytic activity after a reaction for 620\u00a0h and the CO2 conversion over this catalyst underwent a decrease from 92% to 77%, while no change in CH4 conversion was observed. Furthermore, certain changes in the MgO promoter enhanced the Ni0 species dispersion and resulted in an increase in the adsorption affinity of CO2, thereby inhibiting coke deposition and retarding the deactivation phenomenon.Mg\u2013Al mixed oxides derived from hydrotalcite-like materials are reported [118,154], to exhibit higher activity and stability in bi-reforming process due to its basic property, enhanced steam and CO2 adsorption, strong Ni to support interaction and fine dispersion. These catalysts were synthesized using various preparation methods such as impregnation of pre-calcined carriers or simultaneous co-precipitation of the mostly nitrate-based solution of all the constituents. Roohollahi et\u00a0al. [170], synthesized numerous Ni-based catalysts supported on mesoporous MgO\u2013Al2O3 resembling a Mg\u2013Al hydrotalcite structure with Mg/Al ratio of 1. Mg\u2013Al hydrotalcite-like components, represented by the formula [MgII\n1-xAlIII\nx(OH)2]x+(CO3\n2\u2212)x/2\u22c5mH2O, have been regarded as the best candidates for precursors employed in the synthesis of mesoporous MgO\u2013Al2O3 carriers possessing high surface area [171]. The synthesis of hydrotalcite-like components was performed at an optimized pH of 10, which were then calcined at various calcination temperatures from 500 to 800\u00b0C to obtain a homogenous texture. Results from the bi-reforming reaction conducted on the catalysts at 800\u00b0C for 36\u00a0h with feed stock constitution of CH4: CO2: H2O\u00a0=\u00a01.0:0.4:0.8\u00a0at GHSV\u00a0=\u00a0150,000\u00a0mL\u00b7gcat\u22121\u00a0h\u22121 revealed that the sample derived from the carrier calcined at 700\u00b0C exhibited the lowest nickel crystallite size (2.68\u00a0nm) and largest nickel surface area (25.01\u00a0m2/g). The results also established excellent conversion efficiencies for CH4 (93.7%) and CO2 (75.2%) and higher resistance to coke formation. The high resistance to carbon formation was due to the enhanced strength of basic sites formed in the catalyst carrier during the calcination of Mg\u2013Al hydrotalcite-like components at 700\u00b0C.He et\u00a0al. [172] investigated the role of nickel nanoparticles supported on the binary Mg\u2013Al metal oxide catalysts during bi-reforming of CH4. The successful synthesis of Ni/MgO, Ni/Mg\nx\nAl\ny\nO, and Ni/Al2O3 catalysts were also supported by XRD, TEM, and FT-IR results. The TPR profile revealed that the reduction temperature of Ni species underwent a slight decrease upon addition of Al due to the formation of the NiAl2O4 phase. Furthermore, the XPS spectra demonstrated that Ni/MgO and Ni/Al2O3 produced higher amounts of Ni0 after H2 reduction.NiO\u2013CaO catalyst has been reported [173] to exhibit high selectivity, activity and productivity in the oxidative conversion of methane to synthesis gas. Choudhary et\u00a0al. [174], have reported the role of NiO\u2013CaO catalysts during SRM, DRM and combined steam and CO2 reforming of methane to produce CO and H2 at varying temperatures ranging from 700 to 850\u00b0C and gas hour space velocities (5000 to 70,000\u00a0cm3\u00a0g\u22121\u00b7h\u22121) They characterised the catalysts using various techniques including XRD, XPS and TPR. Their results revealed that the catalysts demonstrated high activity/selectivity during all of the reforming processes tested. When CO2 reforming was performed in conjunction with steam reforming process a drastic reduction in carbon deposition from 25.96% to 1.08% was observed for a feed composition of CH4:CO2:H2O\u00a0=\u00a01.0:0.55:0.55 [174]. Furthermore, the authors noted that when the feed composition was maintained for CH4: H2O\u00a0=\u00a01:1 during the steam reforming reaction, the reaction characteristics were outside the coke formation control. Nevertheless, for the dry reforming reaction with a reactant feed composition of CH4: CO2 in the ratio of 1:1, the coke formation was obtained from a gas mixture formed at equilibrium.The authors [174] also noticed that complete conversion of methane to syngas with 100% selectivity consisting of both CO2 and H2 and during bi-reforming reaction at 800\u00b0C with GHSV ranging from 20,000 to 30,000\u00a0cm3\u00a0g\u22121\u00a0h\u22121. The authors observed that by changing the carbon dioxide/steam (CO2/H2O) ratio in the reactant feed, a significant improvement in the bi-reforming process occurred and also a desirable H2/CO ranging from 1.5 to 2.5 was seen. TPR studies were performed to measure changes in the concentration of H2 owing to reduction of nickel oxide in the catalyst. The TPR curves (Fig.\u00a019\n) revealed maximum value in the range 400 and 450\u00b0C, in accordance with the maximum peak temperature observed around 418\u00b0C attributed to reduction of bulk nickel oxide.Chen et\u00a0al. [153] synthesized mNi/xLa/Si catalysts with efficient dispersion characteristics and comprising of various weight contents of nickel and lanthanum by using sol-gel method, and tested these catalysts for bi-reforming of CH4 to generate syngas. The authors noticed an increase in the stability, catalytic characteristics and an enhancement in the resistance of carbon deposited during bi-reforming in presence of mNi/xLa/Si catalysts upon addition of lanthanum. The 17.5Ni/3.0La/Si catalyst prepared using ethylene glycol and poly (ethylene glycol) demonstrated excellent coke resistance and catalytic activity. Additionally, modification of the carbon/sulphur (C/S ratios) in the reactant caused tuning of the H2/CO ratios in the gas generated as products. Furthermore, when the bi-reforming reaction was performed in presence of 17.5Ni/3.0La/Si catalyst produced a H2/CO ratio of about 2 for the C/S ratio of 0.5.Ni-phyllosilicate (PS) intermediates were used to synthesise [175] Nickel\u2013SiO2\u2013MgO materials for its application in bi-reforming of methane., and the role of reaction temperature as well as steam on the reforming process were also investigated. The results revealed that catalytic performance was excellent and resulted in 80% conversion of CH4 and 60% CO2 conversion respectively, at 750\u00b0C for 140\u00a0h in presence of a Ni\u201330\u00a0wt % SiO2\u201355\u00a0wt % MgO catalyst. Furthermore, carbon deposition was found to be stable when the H2/CO ratio was maintained at 2. The catalytic behaviour of the investigated catalyst was ascribed to its structural stability, acidic strength and enhanced basicity for the reforming reaction conducted at high temperatures. The presence of nickel-magnesium comprising phyllosilicates in the reduced catalysts were established by TEM and XRD technique. Furthermore, a TPR profile of around 750\u00b0C substantiated the presence of strong interlinkage between nickel and Silicon dioxide\u2013Magnesium support species. A representative schematic diagram of this is illustrated in Fig.\u00a020\n.Jabbour et\u00a0al. [82], used an one pot method (Fig.\u00a021\n\n) for synthesis of mesoporous nickel\u2013alumina catalyst containing 5\u00a0wt % Nickel and possessing an ordered structure. From their observations, the ordered Ni\u2013alumina sample exhibited excellent stability in comparison to non-porous and impregnated catalyst during the bi-reforming process at 800\u00b0C over 40\u00a0h. The conversion percentage of methane was consistent with the thermodynamically expected variants. The authors also noted that nickel catalyst loaded with SBA-15 demonstrated enhanced catalytic activity than Ni/celites, however both these catalysts underwent rapid deactivation on stream which was attributed to the partial re-oxidation of the Ni active phase under the investigated conditions (see Fig.\u00a022).SBA-15 support has been employed for suppressing carbon formation in steam reforming reactions [166,167], and has aroused significant interest due to its high surface area, high silanol group density, pore uniformity. An incipient wetness method was employed by Singh et\u00a0al. [176], to synthesise SBA-15\u2212packed Ni catalyst by impregnating nickel nitrate onto the SBA-15 support. They found that the surface area decreased from 669.5\u00a0m2\u00a0g\u22121 to 538.6\u00a0m2\u00a0g\u22121 with the change in catalyst support from SBA-15 to 10\u00a0wt % Nickel/SBA-15 catalyst was confirmed by BET surface area analysis. Analysis by H2-TPR demonstrated the complete reduction of NiO nanoparticles beyond 576.85\u00b0C where the temperature of reduction from nickel oxide to metallic nickel was completely dependent on metal-support interactions which was correlated to the location, confinement effect and crystallite size of nickel oxide. CO2 and H2O had a significant role in controlling formation of carbon during bi-reforming of methane due to their unique capability in converting the partially dehydrogenated CxH1-x to a mixture of CO and H2. The authors observed that carbon dioxide conversion and methane conversion was 58.9%, and 61.6% respectively. Furthermore, the resulting H2/CO ratio was found to be 2.14 during the combined CO2 and steam reforming of CH4 under stoichiometric conditions. A steep increase in the H2 and CO yield was noticed while increasing the CO2/(CH4\u00a0+\u00a0H2O) ratio, and a considerable decrease in the ratio of both hydrogen and carbon monoxide ratio ranging from 2.14 to 1.83 was observed with a decrease in the H2O/(CH4\u00a0+\u00a0CO2) ratio. Furthermore, Ni/SBA-15 exhibited higher resistance towards both coking and sintering which was related to the efficient distribution of nickel particles and steric effects caused by SBA-15.The synthesis, catalytic activity and characterisation studies on Ni/SBA-15 catalysts during BRM has been reported [177]. The authors observed that 25\u00a0wt % Nickel/SBA-15 SGM catalyst showed the maximum conversion of CH4 (23%, 548\u030a C), which was followed Ni/SBA-15 HTM (CH4\u00a0\u2248\u00a020% at 548\u00b0C) and 10% CH4 conversion was achieved in presence of 25\u00a0wt % Ni/SBA-15 CG catalyst. CO2 and CH4 conversion were found to be 82% and 23% at 548\u00b0C, respectively. These differences in the catalytic activity were related to the degree of availability of active metal for the reaction. Due to excellent catalyst activity of these catalysts, these catalysts were employed for the formation of membrane reactor with hollow fibres and catalytic hollow fibres.The authors employed commercially available SBA-15 for comparison. SEM micrographs (Fig.\u00a023\n) revealed a needle shaped particle having a grain size of around 0.6\u00a0\u03bcm (A1 and A2). The SBA-15 particles synthesized by the sol-gel method did not display a homogeneous shape and consisted of a hard shell covering smaller particles whose grain size was approximately around 0.1\u00a0\u03bcm.Mesoporous siliceous SBA-15 material has been used as support for preparing active metal catalysts in several reforming processes [66,178]. The mesoporous SBA-15 support possessed uniform mesopores with thick framework walls, high thermal stability and wide specific surface area [179], [. Additionally, the ordered hexagonal mesostructure of SBA-15 support provided a confinement effect to anchor the nanoparticles inside its channels and also prevented deposition of carbonaceous species metal sintering [115]. Siang et\u00a0al. [180], used the incipient wetness impregnation technique to synthesise stable and active boron (B) aided catalyst for bi-reforming of methane. Results revealed that B2O3 and nickel oxide particles were scattered on the outer area of SBA-15 support possessing higher surface area. Additionally, the authors observed an enhancement in catalytic activity that underwent a linear increase with temperature due to the endothermic behaviour of the catalysed process. They obtained H2/CO molar ratio of 2.7 and 67.3% of CH4 conversion at 799.8\u00b0C which was highly significant for downstream Fischer-Tropsch (FT) applications. Furthermore, XPS measurements revealed that B facilitated the adsorption of CO2 through the electron transfer to the Ni cluster at the neighbourhood, thereby improving its catalytic activity. More importantly, analysis by XRD and Raman showed that boron doped catalyst was completely free from graphitic and amorphous carbon deposition. This was due to the incorporation of B into the octahedral sites occupied by NiO, resulting in inhibition of carbonaceous deposits.Encapsulation of Ni particles in a suitable support material has been reported [181] to enhance the sintering resistance and coke resistance of Ni catalysts. The introduction of promoters namely rare-earth metals; metal oxides, alkaline earth and alkali metals is also one of the effective strategy to prevent the sintering of active sites/supports and enhance the coke-resistant ability of catalysts [182]. Chen et\u00a0al. [153], synthesized highly dispersed mNi/xLa-Si catalysts by employing one pot sol-gel process by varying the weight percentages of nickel and lanthanum. These catalysts were subsequently applied to generate syngas during bi-reforming of CH4. The authors observed that La addition enhanced the stability, coke resistance and catalytic activity of mNi/x Lanthanum\u2013Silicon catalysts. The 17.5Ni/3.0LaSi catalyst prepared by employing poly (ethylene glycol) and ethylene glycol displayed the coke resistance, maximum selectivity and catalytic activity. One notable observation was that a H2/CO ratio of about 2 was obtained when the carbon to sulphur ratio was maintained at 0.5, for the 17.5Ni/3.0LaSi catalyst, suitable for potential applications in Fischer-Tropsch synthesis.Literature reports [183] have established that CeO2\u2013Al2O3 combinations are potential supports for reforming reactions. Furthermore, the redox properties of CeO2 resulted in a significant improvement in the oxidation of deposits thereby enhancing the lifetime of the catalysts [184,185]. Furthermore, second metal addition promoted the formation of an active phase along with modification of the support. Bimetallic systems [186] have been known to display superior catalytic activity and increased the resistance of carbon formation in comparison to their own counterparts. The bimetallic combination of Ni\u2013Sn has proved to be of considerable interest in reforming reactions. Additionally, the dispersion of nickel over the catalyst surface has been shown to be enhanced in the presence of Sn [187].Straud et\u00a0al. [188], synthesized a set of multicomponent advanced catalysts composed of Sn, CeO2 and Ni/Al2O3. A schematic diagram representing the production of syngas in the presence of the investigated catalysts are shown in Fig.\u00a023. The authors observed that addition of minute amounts of the investigated dopants improved the performance of methane reforming using CO2. From their results it was noticed that a multicomponent Sn 0.02 Nickel/Cerium-Al catalyst showed excellent catalytic characteristic and remained active over a long period of 92\u00a0h. The catalyst also demonstrated an exceptional level of stability and conversion during BRM. Comparison of dry reforming and BRM reactions over the Sn0.02Ni/Ce\u2013Al catalyst at 700\u00b0C revealed that H2/CO ratio remained above 1.6 for 24\u00a0h. This suggested that the catalyst could generate high quality synthesis gas by introduction of water into the reforming mixture.Therefore, the addition of water established the suitability of the Sn0.02Ni/Ce\u2013Al catalyst for bi-reforming of CH4. Furthermore, the results also revealed that presence of ceria created high storage capacities for oxygen and changed both the acidic and basic characteristics of support thereby enhancing the catalyst performance. The multicomponent catalyst Sn0.02Ni/Ce\u2013Al proved to be active over period of 92\u00a0h and fared well over a range of space velocities and temperatures.The remarkable level of stability and excellent conversions seen in the bi-reforming process has proved the versatility of Sn0.02Ni/Ce\u2013Al catalyst which can be upgraded to variety of CO2 containing feed stocks.Literature reports [189,190]have established the role of ZrO2 as an excellent support for reforming reactions because of its higher oxygen mobilisation, excellent thermal stability as well as its unique basic and acidic properties. The reinforced interaction between nickel and zirconium oxide makes zirconium oxide an effective support for a nickel based catalyst.Agli et\u00a0al. [191], reported that basic mineralisers affect the nucleation, rearrangement and crystallisation of gel made of zirconia during the synthesis of zirconium oxide. Hence, Zhao et\u00a0al. [190], used the hydrothermal method [192,193] with various mineralisers along with l-arginine ligand-using wetness impregnation technique [194] to synthesise Ni/ZrO2 supports for bi-reforming of methane. Results from their studies revealed that the catalysts performance depended on texture of the zirconium oxide support and its morphology was also highly affected by the mineraliser amount. In this study, the authors synthesized Zirconium oxide support with a mole ratio sodium acetate/Zr4+ as 0.5 denoted by (SAc0.5). ZrO2\u2212supported Ni catalyst was synthesized by employing sodium acetate where the mole ratio was NSAc/Zr\u00a0=\u00a00 and also showed increased catalytic activity in comparison to catalyst i.e zirconium oxide synthesized using (SC) where the ratio was Nsc/Zr\u00a0=\u00a00.5. The authors established from the results that sodium acetate would serve as a suitable mineraliser for making an excellent ZrO2 support and also in terms of its stability and activity. Furthermore, the authors [190] observed that, in general, the addition of different amounts of mineralisers to ZrO2 supports had a significant effect on textural properties, which in turn affected the behaviour of the Ni-supported catalysts on zirconium oxide and also influenced the catalytic activity of the Nickel/Zirconium oxide catalysts during bi-reforming of methane.The TEM micrographs in Fig.\u00a024\n show that all the investigated zirconium oxide supports resembled cobblestone like structure, with dimension of mesopores. Interestingly, reduction in pore volume and pore diameter along with expansion in the surface area was observed when the SAc/Zr molar ratio rose from 0.5 to 2.0. This provided the Ni/ZrO2 catalyst with a bigger crystallite size but also caused lower dispersion compared to Ni/ZrO2 (SAc0.5). From these studies, the authors noted lowering in the sintering resistance of nickel in Ni/ZrO2 (SAc2.0) catalyst than Ni/ZrO2 (SAc0.5), which was attributed to its imperfect interaction between nickel and zirconium oxide as established by H2-Temperature programmed reduction. Fig.\u00a025\n displays the catalytic characteristics of the prepared Ni/ZrO2 catalysts with varying amount of mineralizers present in the support.The figure above revealed the initial activity in the order of Ni/ZrO2 (SAc0.5)\u00a0\u2248\u00a0Ni/ZrO2 (Non)\u00a0>\u00a0Ni/ZrO2 (SC0.5)\u00a0>\u00a0Ni/ZrO2 (SAc2.0) respectively. Nevertheless, the Ni/ZrO2 catalyst without any acetate in the figure showed least stability among all the catalysts.Itkulova et\u00a0al. [195], used Group VIII metals(0.25\u20131\u00a0wt %) along with alumina as a support to synthesise 5% bimetallic Co-based catalysts. The bimetallic Co constituted catalysts were synthesized by impregnation of Al2O3 with solutions comprising of both cobalt and platinum compounds followed by a thermal treatment. The authors investigated the stability of these catalysts by varying the temperature (300\u2013800\u00b0C), composition of feed mixture and space velocity (SV) (500\u22123000\u00a0h\u22121) during bi-reforming as well as DRM. The authors observed that methane conversion was almost 100% at 750\u00b0C and 770\u00b0C for 5\u00a0wt % Co\u2013Pt (9:1)/Al2O3 catalyst during both DRM and BRM, However, the results in Fig.\u00a026\n a reveal a decrease in CO2 conversion during the reforming process (Fig.\u00a026 b) performed over the entire temperature range compared with DRM due to the suppression of CO2\u2013CH4 reaction by the competing CH4\u2013H2O interaction. The authors also observed a surge in the H2/CO ratio from 0.84 to 1.0 when 20\u00a0vol % steam was added to the feed with equal amounts of CH4 and CO2.The effect of Pt on BRM for various feed compositions was also investigated [195]. Results in Fig.\u00a027\n revealed an enhancement in the catalytic activity with increased platinum loading varying from 0.25 to 1\u00a0wt %. It was also noted that higher temperatures were necessary for the total conversion of CH4 when there was a decrease in platinum content in the catalyst. It was established that addition of 10\u201330% steam had a marked effect on the conversion of CH4, which further decreased the temperature required for conversion of methane and an increase in the ratio of H2/CO.Syngas produced during bi-reforming of methane over 5% Co\u2013Pt/Al2O3 catalyst showed a desirable H2/CO ratio >1. Pt was responsible for the formation and stabilization of highly dispersed and reduced bimetallic nanoparticles. Itkulova et\u00a0al. [196] have investigated the role of 5% Co\u2013Pt catalysts modified with 0.25\u20130.5 mass% Pt supported on alumina and modified with zirconia (ZrO2) with amounts ranging from 5 to 10 mass% of Zr in the bi reforming process in the temperature range of 300\u2013755\u00a0\u00b0C, and CO2/CH4 in 1:1 ratio. The results revealed that introduction of 20\u00a0vol% of steam into the CO2\u2013CH4 feed was highly beneficial to the performance of the bi-reforming process. The improved performance of the 5%Co\u2013Pt/Al2O3\u2013ZrO2 catalysts was attributed to the synergistic effect caused by the combination of two reactions i.e. dry and steam reforming of methane.The major issue affecting commercialisation of the reforming process is coke formation, which causes deactivation of catalysts. The most effective way for decreasing coke formation is by coupling CO2 with steam. It has been established that the support plays a significant role in suppressing formation of coke on Group VIII metals during the CO2 reforming of CH4 [197\u2013200]. Several researchers [201,202], have demonstrated that the addition of promoter such as cerium led to a marked improvement in the activity of catalyst, stability and also decreased the sintering of ZrO2 during calcination performed at high temperature. Literature reports [203\u2013207], have also established that Pt\u2013ZrO2 catalysts demonstrate high stability and activity under extreme deactivating environment.The activity for CO2 reforming of methane has been investigated by Noronha et\u00a0al. [208], on Pt\u2013ZrO2 (Fig.\u00a028\n a) and Pt\u2013Ce\u2013ZrO2 (Fig.\u00a028b) catalysts under CH4:CO2 molar proportion of 2:1. The authors noticed that the conversions of CH4 and CO2 decreased slightly in the presence of Pt\u2013ZrO2 catalysts (see Fig.\u00a029).Interestingly, after the removal of water the conversion of carbon dioxide and methane remained roughly constant, and at the same level as after interaction with water (Fig.\u00a028a). A more drastic reduction in H2/CO from 0.82 to 0.45 was also noticed after 22\u00a0h. However, the authors observed that DRM of CH4 in presence of H2O occurred differently with Pt\u2013Ce\u2013ZrO2 Pt\u2013ZrO2 catalysts. Furthermore, CH4 and CO2 conversion underwent a decrease upon addition of water during the ongoing reaction on the Pt\u2013ZrO2 (Fig.\u00a028 a) catalysts, with the decrease in conversion of CO2 being significant. This was attributed to the reaction between excess H2 and higher amount of CO2 through water-gas shift reaction [209] in the reverse mode. The reduced stability observed in the Pt\u2013ZrO2 catalyst was related to the diminishing of oxygen vacancies on the support and oxidation behaviour. Furthermore, Temperature Programed Oxidation analysis established that water addition enhanced the amount of mass of carbon deposited on the surface. Nevertheless, Pt\u2013Ce\u2013ZrO2 exhibited excellent stability in presence of H2O and its stability was due to higher number of vacancies caused by O2 on the support. Both the Pt catalysts with and without Ce were relatively stable during CO2 reforming of methane performed at 105 psig.BRM is considered to be an endothermic process which involves optimization of the temperature within the bed containing the catalyst and also a large amount of heat transfer into the reaction system occurs with the aid of external source. This suggests that catalysts used in these processes should have greater thermal conductivity, which can be attained by employing metallic supports [210]. Several authors have reported the role of numerous catalysts that operate on these supports [211,212].A promising under layer for Ni catalysts is MgO due to its high thermal stability, ability to decrease carbonisation and ability to easily form solid solutions with NiO, also aids in promoting the dispersion of reduced nickel crystallites [213]. There have been numerous studies on supported Ni catalyst with a MgO under layer: supports on metal foams [213], porous Ni plate [212] and Al2O3\u2013SiO2 [214].Danilova et\u00a0al. [215], reported the synthesis of thick porous Ni ribbon (pNirb) with a MgO under layer supported by Ni catalyst on the top. The under layer constituting magnesium oxide was synthesized by packing of the support with MgNO3 solution, then subjected to drying followed by calcination performed at 550\u030a C in presence of air designated as Support 1 and the calcination performed in flowing H2 was known as Support 2. The catalyst was reduced under the atmosphere of flowing H2 at 750\u00b0C was termed catalyst I and the catalyst reduced at 900\u00b0C was termed catalyst II. Use of these supported catalysts resulted in 49% and 56% conversion of CH4 with support 1 and support 2, respectively. The greater activity of support 2 in comparison to support 1 was attributed to efficient dispersion of Ni crystallites that was produced from solid solution reduced in presence of reaction medium. The authors further remarked that catalyst II (2.7% Ni/(p Nirb +8.6% MgO) and 4.0% Ni/(pNirb +10.4% MgO) exhibited excellent stability for CH4 conversion over a period of 18\u00a0h compared with catalyst I (4.6% Ni/(pNirb +6.0% MgO) and 4.6% Ni/(pNirb\u00a0+\u00a06.0% MgO) under the following conditions: GHSV\u00a0=\u00a062.5\u00a0L\u00a0h\u22121,CH4/CO2/H2/N2\u00a0=\u00a035/23/39/3).Recently, nanocatalysts have attracted much attention [38]. Nanocatalysts show better selectivity, outstanding stability and higher activity, due to their special crystal structure, higher amount of surface atoms in comparison to their micro-sized counterparts and larger specific surface area [216]. Many works [217] have shown that catalyst preparation with larger surface area affected significantly the physical and chemical properties, which can only be achieved by a nanocatalyst.Khani et\u00a0al. [218], synthesized novel M/ZnLaAlO4 nanocatalysts where M consists of 3%Ru, 10% Ni and 3% Pt using wet impregnation technique and characterised these by using TPR, FT-IR, TEM, XRD, FE-SEM, Thermogravimetric analysis and Differential Thermal Analysis. The authors evaluated the catalytic characteristics of the these catalysts in the SRM, DRM and BRM of methane at temperatures varying from 600 to 800\u00b0C at different gas hourly space velocities values of 10,500, 7000, 3500 h\u22121. TGA revealed that the nanocatalysts namely 3% Pt/ZnLaAlO4 and 3% Ru/ZnLaAlO4 did not exhibit any coke formation during SRM, which was also supported by FE-SEM (Fig.\u00a030\n).The authors noted that an increase in temperature during the bi-reforming (BRM) of methane increased the CH4 conversion, however decreased conversion of CO2 (Fig. 31\n). Furthermore, Fig.\u00a030 shows that 3% Ruthenium/ZincLaAlO4 demonstrated the lowest activity while 10%Ni/\u03b3-Al2O3 showed marked activity for CO2 conversion. Additionally, among the four tested catalysts (Figs. 30), 3% Ru/ZnLaAlO4 displayed the highest catalytic CH4 conversion. The authors observed a reduction in H2/CO ratio with the rise in temperature from 600 to 700\u00b0C for the investigated catalysts used in the bi-reforming process. The authors observed 3% Ru/ZnLaAlO4 a H2/CO of 2.1\u00a0at a temperature of 800\u030a C, while for 10% Ni/\u03b3-Al2O3 catalyst a lowest value of 1.6 was obtained., 3% Ru/ZnLaAlO4 was considered as the potential catalyst for potential applications based on its resistance to formation of carbon and catalytic efficiency in BRM, dry reforming and SRM. TPR profiles of the nano-catalysts showed lowest reduction temperatures at the onset for 3%Pt/ZnLaAlO4, 10% Ni/ZnLaAlO4, 3%Ru/ZnLaAlO4 at 264\u00b0C, 333\u00b0C and 230\u00b0C respectively.Potdar et\u00a0al. [219], noted that the nanocatalyst Ni\u2013Ce\u2013ZrO2 synthesized by employing co-precipitation technique exhibited excellent coke resistance and highly stable catalytic activity attributed to the greater mobility of oxygen in the carbon dioxide reforming of methane and higher surface area. Roh et\u00a0al. [121], demonstrated that higher stability and activity of Ni/MgO\u2013Al2O3 catalyst with nano dimensions was due to the beneficial effects of magnesium oxide (MgO) namely stronger interaction between nickel and support, basicity, enhanced steam adsorption and also the crystallite size of nanosized NiO. Sadykov et\u00a0al. [220], investigated the role of catalysts made of nanocomposites in the bi-reforming reaction. Nanocomposite catalysts consisting of nickel particles implanted into an oxide matrix of Yttrium or Scandinavium-stabilised Zr (YSZ, ScSZ) mixed with doped Ce\u2013Zr oxides or Lanthanum\u2013Praseodymium\u2013Manganese\u2013Chromium\u2013Oxygen (La-Pr-Mn-Cr-O) perovskite along with promoters namely Pd, Ru and Pt were synthesized via different routes [101].Soria et\u00a0al. [221], investigated the role of H2O along with Ru/ZrO2\u2013La2O3 catalyst placed in a fixed-bed Palladium reactor with membrane during bi-reforming of methane. The authors observed that addition of H2O along with CO2 during the reforming reaction significantly affected the catalyst activity. Fig.\u00a032\n shows that the presence of small concentrations of H2O (1\u20132\u00a0vol %) did not affect the conversion of CH4 appreciably, but an increase in steam to 5\u00a0vol % did result in increased CH4 conversion.Furthermore, the CO2 conversion gradually decreased with increasing concentration of H2O from 1 to 5\u00a0vol %, and the CO2 conversion exhibited lower values below 330, 375 and 450\u030a C for water content ranging from 1 to 5\u00a0vol %. Furthermore, at a designated temperature, the composition of syngas (H2/CO ratio) was altered with the change in the concentration of H2O feed.The authors [221] also investigated the stability of the Ru/ZrO2\u2013La2O3 catalyst during both bi-reforming and carbon dioxide reforming of methane at 500\u00b0C. It was observed that without steam presence in the reaction feed, the Ru/ZrO2\u2013La2O3 catalyst was very stable and 15% of deactivation was noticed. Fig.\u00a033\n shows that the addition of water had a marked effect on the stability, which increased in a significant manner with the increase in steam amount. The deactivation values were 5%, 11%, 8% for addition of 5, 1, 2\u00a0vol % H2O, respectively.Research has indicated that steam addition to the CO2 during bi-reforming of CH4 affects the reaction parameters in a temperature-dependent manner, which is noteworthy for the generation of high purity H2 using Pd-based membrane technology.Chaudhary and Mandal [101] demonstrated the CH4 conversion of synthesis gas in presence of NdCOO3 perovskite-type oxides used as a catalyst during BRM of methane. Results from their studies revealed that H2O and CH4 conversion along with the H2/CO ratio, were greatly affected by the feed ratio of CO2/H2O during the reforming process. Furthermore, the heat of reaction was strongly affected by relative concentration of oxygen in the reactant feed, space velocity and temperature. NdCOO3 perovskite-type catalyst proved to be highly efficient for carbon-free bi-reforming process.The Sr-doped Ni\u2013La2O3 catalyst has been reported [222] to generate the highest CH4 and CO2 activity along with the highest resistance to carbon deposition over the catalyst surface which was attributed to considerable involvement of a large amount of mobile lattice oxygen species as a result of C\u2013H activation in dry reforming. Yang et\u00a0al. [223], have reported the role of Sr addition to LaNiO3 perovskite catalysts during the bireforming process. Mineralogical characterisation by XRD revealed a distortion in the perovskite lattice and generation of alien phases such as La2\u2212xSrx NiO3\u00a0\u00b1\u00a0\u03b4 and Sr0.5La1.5NiO4. The authors [223] observed that the reduction behaviour was affected by the presence of these phases. The results also revealed that strontium oxide adsorbed CO2 during the bi-reforming reaction and formed strontium carbonate (SrCO3), which possessed the unique ability of inhibiting carbon sources by producing La2O2CO3. The addition of Sr particles covered the support sites thereby resulting in large-sized Ni particles by decreasing the interlinkage between the support and active metals. The authors [223] recommended using a small amount of Sr in the perovskite-based catalyst for obtaining greater resistance to carbon deposition. Furthermore, larger Ni particles were formed, with diameters of 30.8, 29.9, 27.6\u00a0nm for concentrations of 10%, 50%, 30% SrO containing La2O3\u2013 NiO3 catalysts in comparison to a La2O3\u2013NiO3 particle size of 13.5\u00a0nm.Kim et\u00a0al. [224], investigated the bi-reforming reaction of methane employing mixed oxides of La, Sr and Ni packed on \u03b2-SiC catalysts loaded with Al2O3 for assessing the conversion of carbon dioxide at a certain concentration of Al2O3 used as a modifier. The authors found that though all the investigated tested catalysts provided close activation energies values, the increase in dispersion of aluminium oxide on silicon carbide with 10\u00a0wt % Al2O3 as modifier was in agreement with the higher distribution of perovskite containing La2NiO4 crystallites. Additionally, larger amounts of adsorption of carbon dioxide on the efficiently distributed basic Sr and La oxides were also responsible for enhanced carbon dioxide conversion. CO2 and CH4 conversion also correlated well to the IA1/INi ratios obtained from XPS analyses.Park et\u00a0al. [225], have described the use of various foam catalyst embedded with metals to enhance the heat transfer of reaction during BRM process. The authors characterised heat transfers based on the Nusselt number and also used a pellet shaped catalyst to improve the heat transfer of the foam catalyst. The results revealed that the Nusselt number of foam catalyst packed with metals was larger than the pellet catalyst used conventionally. Additionally, uniform temperature distribution was noticed in the reformer throughout the catalyst bed along with the foam catalyst. Images of the various metallic foam catalysts are shown in Fig.\u00a034\n.\nFig. 35\n shows that the uncoated Al2O3 and bare Ni foam were reactive without Ni catalyst loading. Additionally, the bare Ni foam displayed greater methane conversion than the uncoated aluminium oxide. The results established that the Ni foam exhibited a significant role in improving heating characteristics of the catalyst bed and mass transfer inside the reactor. However, after wash coating of a layer of Al2O3 on the nickel foam, the conversion of carbon dioxide and methane increased to 34% and 69.1% and, respectively.The syngas flow rate along with molar ratio of hydrogen/carbon monoxide in presence of Nickel/Al2O3/Nickel foam, uncoated Nickel foam, uncoated Al2O3 bead and Al2O3/Ni foam are shown in Fig.\u00a036\n. The syngas flow rate exhibited a similar behaviour both for carbon dioxide and methane conversions. Nevertheless, the H2/CO molar ratio was different and furthermore molar ratio of H2/CO of uncoated Ni and uncoated Al2O3 bead was less than 2.0. The metallic foam catalyst is potentially useful for GTL-FPSO applications based on the enhanced mechanical properties of the catalyst and compactness of the reformer. Additionally, higher selectivity levels and activity are associated with nickel inside the coating layer, which serves as active sites for methane, water and carbon dioxide.Brush et\u00a0al. [181] reported the ability of Ni/Mo2C to catalyse the bi-reforming of methane. The authors noted that by altering the ratio of carbon dioxide: water, the resulting Hydrogen: Carbon monoxide (H2:CO) ratio could be changed from 0.91 to 3.0, which covers a wide range of H2:CO ratios common to various hydrocarbon syntheses. Most importantly, the catalytic activity changed from very high (50% conversion) to very low (10% conversion) within a time interval of 10\u00a0min. Additionally, for various inlet feed compositions similar performance was exhibited by the catalyst. However, enhanced activity was followed by greater deactivation shortly after the exposure to stream.Claridge et\u00a0al. [226], synthesized Mb and W carbide materials of larger surface area to assess the performance of carbide catalysts formed from non-metals for various CH4 reforming reactions including bi-reforming. Their study revealed that carbides activity was similar to those of iridium and ruthenium catalysts used in reforming of methane. Nevertheless, conversion values were in agreement with the values obtained during thermodynamic equilibrium. HRTEM images revealed the absence of deposited macroscopic on the catalysts during the reforming process.The present review article demonstrates a comprehensive review of various catalysts, including Ni, Co, Rh and Pt-based catalysts, used for BRM. It also describes the role of various promoters and supports on the conversion efficiencies of CH4, CO2 and H2/CO ratio. Ni catalysts supported by smaller nanoparticles of Ce\u2013ZrO2 ZrO2, MgO were observed to be extremely stable and active for BRM processes. The degree of coke formation was dependent on each investigated catalyst. Coke formation was found to be more severe in commercial Ni/MgAl2O4 catalyst than Ni/MgO\u2013Al2O3 catalyst, which was attributed to higher dispersion of Nickel over MgO\u2013Al2O3.Bi-reforming of methane over a catalytic nickel membrane for the GTL (gas to liquid) process produced a very high conversion of methane in the range of 92.7\u201396% above 973\u00a0K, when the CO2/H2O feed ratios were in the range of 0\u20131.0. GTL process possessed two advantages. One was that carbon formation was reduced due to the oxidation of carbon precursor species and a desirable H2/CO was achieved by adjusting CH4/H2O/CO2 ratio in the feed stream. Cerium containing Ni/MgAl2O4 catalysts synthesized by both co -precipitation and impregnation technique exhibited higher metal dispersion than Ni/MgAl2O4 alone and showed marked reducibility at lower temperatures around temperature 550\u00b0C as confirmed by XPS since the redox properties of Ce4+/Ce3+ resulted in easier gasification of the settled coke on the surface of the catalyst and also helped in storage and delivering of active oxygen thereby enhancing the dispersion of Ni. The catalyst Ni/\u03b3-Al2O3 promoted very higher conversion of carbon dioxide (82.4%) and methane (98.3%) when subjected to bi-reforming of methane for 200\u00a0h since the Ni/Al2O3 catalyst exhibited characteristics such as high metal dispersion, high catalytic activity large specific surface area, and stronger metal support interaction respectively. Presence of MgO in 20\u00a0wt % MgO/Ni catalyst was quite effective in preventing coke formation due to the formation stable MgAl2O4 spinel phase at higher temperatures. The presence of CeO2 in Ce1-x \u2013ZrxO2 catalytic systems caused a marked improvement in redox properties, thermal stability, and promotion of metal dispersion and also enhanced the oxygen storage ability of CeO2. A drastic reduction in carbon deposition from 25.96% to 1.08% was observed for a feed composition of CH4: CO2: H2O\u00a0=\u00a01.0:0.55:0.55 when CO2 reforming was performed in conjunction with steam reforming process in presence of NiO\u2013CaO catalyst due to its high selectivity, activity and productivity in the oxidative conversion of methane to synthesis gas. Nickel/Santa Barabara Amorphous \u221215 (SBA-15) exhibited enormous resistance both towards sintering and coking. 10% Nickel/3% MgO/Santa Barabara Amorphous 15 catalyst demonstrated higher catalytic performance after reaction for 620\u00a0h since the mesoporous SBA-15 support possessed uniform mesopores with thick framework walls, high thermal stability and wide specific surface area. Bimetallic systems such as Sn 0.02 Nickel/Cerium-Al catalyst displayed superior catalytic activity, increased the resistance of carbon formation and remained active over a long period of 92\u00a0h in comparison to their own counterparts. The remarkable level of stability and excellent conversions seen in the bi-reforming process has proved the versatility of Sn0.02Ni/Ce\u2013Al catalyst which could be upgraded to variety of CO2 containing feed stocks. Ni-based catalysts supported on mesoporous MgO\u2013Al2O3 resembling a Mg\u2013Al hydrotalcite structure with Mg/Al ratio of 1 demonstrated excellent conversion efficiencies for CH4 (93.7%) and CO2 (75.2%) and higher resistance to coke formation due to its basic property, enhanced steam as well as CO2 adsorption, strong Ni to support interaction of these catalysts. The excellent catalytic performance of Ni 30\u00a0wt % SiO2 55\u00a0wt % MgO catalyst was ascribed to its acidic strength, enhanced basicity and structural stability under high temperatures. NdCOO3 perovskite-type mixed-oxide catalysts proved to be highly efficient for carbon-free bi-reforming process. Pt\u2013Ce\u2013ZrO2 catalyst exhibited excellent stability in presence of H2O and its stability was attributed to the greater number of O2 vacancies present in the support. Lanthanum promoted catalysts exhibited greater nickel dispersion than Ni/MgAl2O4 catalyst due to their enhanced interactions between the metal and support. Ni\u20132.5La/MgAl2O4 catalyst showed maximum sinter stability and activity due to its enhanced nickel dispersion and surface area.It is highly essential to develop Ni-based catalysts containing bi-metallic and tri-metallic configuration since Ni displayed stronger stability and enhanced activity. However, the biggest constraint in this approach is the coke formation. Bi-reforming process is endothermic and would require higher activation energy to achieve the synthesis. Future studies should be undertaken to design an appropriate bi-metallic and tri-metallic catalyst that can be suitable at lower temperatures. Furthermore, the approach towards catalyst formation has a significant influence on the catalyst's capability. Therefore, selecting a suitable catalyst and its synthesis technique can provide improved SMSI, superior activity, enhanced Ni dispersion on the catalytic support and stability against coke formation.Among the evaluated inlet feed compositions, conducting bi-reforming process under a stoichiometric feed composition (CH 4: CO 2: H 2 O\u00a0=\u00a03:1:2) is considered the ideal one for selective production of syngas within the operating temperature of 750\u2013800\u00a0\u00b0C range. One of the major drawback associated with nano-based catalysts is that the up scaling of catalyst preparation from laboratory batches to continuous industrial production. Henceforth, development of reproducible and economical synthetic strategies is imperative for linking all advantages of nano-based catalysts to large-scale metgas generation facilities.\n\nXPS\n\nX-ray photo electron Spectroscopy\n\nXRD\n\nX-ray diffraction\n\nFT\n\nFischer-Tropsch Synthesis\n\npNirb\n\nThick porous Nickel carbon\n\nH2- TPR\n\nHydrogen Temperature Programmed Reduction\n\nCO2 \u2013TPD\n\nCarbon dioxide Temperature Programmed Desorption\n\nTEM\n\nTransmission electron Microscopy\n\nDRM\n\nDry reforming of methane\n\nGHG\n\nGreenhouse gas emissions\n\nSRM\n\nSteam reforming of methane\n\nPOM\n\nPartial oxidation of Methane\n\nBRM\n\nBi-reforming of methane\n\nCRM\n\nCarbon dioxide reforming of methane\n\nSEM\n\nScanning Electron Microscopy\n\nTPR\n\nTemperature Programmed Reduction\n\nGHSV\n\nGas Hourly Space Velocity\n\nSBA-15\n\nSanta Barbara Amorphous-15\n\nEISA\n\nEvaporation Induced Self-assembly\n\nWHSV\n\nWeight hourly space velocity\n\nFTIR\n\nFourier Transfer Infrared Spectroscopy\n\nTGA\n\nThermogravimetric analysis\n\nDTA\n\nDifferential Thermal Analysis\n\nCeria\n\nCerium (IV) Oxide\n\nZirconia\n\nZirconium dioxide\n\nSGM\n\nSol-Gel method\n\nCG\n\nCommercial method\n\nHTM\n\nHydrothermal Method\n\nGTL-FPSO\n\nFloating Production, Storage and Offloading\n\n\nX-ray photo electron SpectroscopyX-ray diffractionFischer-Tropsch SynthesisThick porous Nickel carbonHydrogen Temperature Programmed ReductionCarbon dioxide Temperature Programmed DesorptionTransmission electron MicroscopyDry reforming of methaneGreenhouse gas emissionsSteam reforming of methanePartial oxidation of MethaneBi-reforming of methaneCarbon dioxide reforming of methaneScanning Electron MicroscopyTemperature Programmed ReductionGas Hourly Space VelocitySanta Barbara Amorphous-15Evaporation Induced Self-assemblyWeight hourly space velocityFourier Transfer Infrared SpectroscopyThermogravimetric analysisDifferential Thermal AnalysisCerium (IV) OxideZirconium dioxideSol-Gel methodCommercial methodHydrothermal MethodFloating Production, Storage and OffloadingThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge the support from Edith Cowan University, Western Australia, Australia.", "descript": "\n                  Today, bi - reforming of methane is considered as an emerging replacement for the generation of high-grade synthesis gas (H2:CO\u00a0=\u00a02.0), and also as an encouraging renewable energy substitute for fossil fuel resources. For achieving high conversion levels of CH4, H2O, and CO2 in this process, appropriate operation variables such as pressure, temperature and molar feed constitution are prerequisites for the high yield of synthesis gas. One of the biggest stumbling blocks for the methane reforming reaction is the sudden deactivation of catalysts, which is attributed to the sintering and coke formation on active sites. Consequently, it is worthwhile to choose promising catalysts that demonstrate excellent stability, high activity and selectivity during the production of syngas. This review describes the characterisation and synthesis of various catalysts used in the bi-reforming process, such as Ni-based catalysts with MgO, MgO\u2013Al2O3, ZrO2, CeO2, SiO2 as catalytic supports. In summary, the addition of a Ni/SBA-15 catalyst showed greater catalytic reactivity than nickel celites; however, both samples deactivated strongly on stream. Ce-promoted catalysts were more found to more favourable than Ni/MgAl2O4 catalyst alone in the bi-reforming reaction due to their inherent capability of removing amorphous coke from the catalyst surface. Also, Lanthanum promoted catalysts exhibited greater nickel dispersion than Ni/MgAl2O4 catalyst due to enhanced interaction between the metal and support. Furthermore, La2O3 addition was found to improve the selectivity, activity, sintering and coking resistance of Ni implanted within SiO2. Non-noble metal-based carbide catalysts were considered to be active and stable catalysts for bi-reforming reactions. Interestingly, a five-fold increase in the coking resistance of the nickel catalyst with Al2O3 support was observed with incorporation of Cr, La2O3 and Ba for a continuous reaction time of 140\u00a0h. Bi-reforming for 200\u00a0h with Ni-\u03b3Al2O3 catalyst promoted 98.3% conversion of CH4 and CO2 conversion of around 82.4%. Addition of MgO to the Ni catalyst formed stable MgAl2O4 spinel phase at high temperatures and was quite effective in preventing coke formation due to enhancement in the basicity on the surface of catalyst. Additionally, the distribution of perovskite oxides over 20\u00a0wt % silicon carbide-modified with aluminium oxide supports promoted catalytic activity. NdCOO3 catalysts were found to be promising candidates for longer bi-reforming operations.\n               "}
{"full_text": "With the rapid development of the global economy, energy crisis and environment issues have become increasingly prominent. Carbon dioxide (CO2) is a primary greenhouse gas, while it could also be a valuable carbon source. In recent years, electrochemical CO2 reduction reaction (ECR) has received considerable attention among various CO2 conversion technologies due to numerous advantages [1,2]. For instance, ECR can be driven under ambient temperature and pressure using renewable energy source such as wind and solar power [3,4]. The external voltages as well as electrolytes solutions can be adjusted for the generation of specific products. Furthermore, ECR technology could not only mitigate CO2 emission, but also effectively convert CO2 to value-added chemicals and fuels, which has been regarded as an appealing technology path for closing the carbon circle [5\u20137]. Nevertheless, physicochemical properties of the CO2 molecule make electrochemical conversion of CO2 challenging [8,9]. In the past decades, metals or related oxides, carbon-based materials and nanocomposites have been widely investigated as electrocatalysts for ECR. Despite that great progress has been made to exploit electrocatalysts for ECR, the process is still impeded by the sluggish kinetics, poor product selectivity, catalyst stability, and high overpotential [10\u201313]. Taking copper as an example, it can electrochemically convert CO2 to CH4 with a high overpotential around 0.9\u00a0V, but the selectivity to specific products is low [14,15]. Therefore, it is highly desirable to develop electrocatalysts with high activity and selectivity for ECR.It has been well established that the catalytic activity can be improved by reducing the size of catalysts. Specifically, single atoms catalysts (SACs) with single atom as active centre have aroused huge interest due to maximum atom utilization and excellent performance in various catalytic reactions such as water splitting, CO2 reduction, N2 reduction, etc. [16\u201320]. It is worth noting that the preparation of SACs is a challenge because single atoms with high surface energy are easy to aggregate into clusters, leading to catalyst deactivation. Thus, a suitable support which could offer anchoring sites and possesses good stability will greatly improve the activity of SACs. Two-dimensional (2D) materials are appealing substrates for anchoring single transition metal atom due to their unique structures and electronic properties [21\u201323]. Moreover, it has been disclosed that the interaction between p-orbital of substrates and d-orbitals of single transition metal atom are beneficial for highly effective electrocatalysts [24,25]. For example, single transition metal atoms embedded into graphene, graphitic carbon nitride (g-C3N4), graphyne, boron nitride (BN), phthalocyanine (PC), etc. have been widely studied as promising SACs for CO2 reduction [26\u201329].Recently, a family of 2D transition metal carbides, nitrides and carbonitrides, known as MXenes, were reported and synthesized from the layered Mn+1AXn phase [30]. In Mn+1AXn, M stands for early transition metals, A denotes the group 13 or 14 elements, X denotes C or N, and n is between 1 and 3 [31]. There are a variety of MXenes that have been predicted and synthesized experimentally, which are explored for applications in many fields [32]. For instance, Li et al. reported that MXenes from the group IV to VI series are active for CO2 capture, while Cr3C2 and Mo3C2 are promising catalysts for CO2 conversion to CH4\n[33]. Both simulation study and experimental work have shown that MXenes have large surface areas, excellent electronic conductivity, tunable surface composition and great stability [32,34]. Thanks to these merits, MXenes have also been demonstrated to be promising substrates for anchoring single transition metal atoms in catalytic reactions [35]. Generally, Mn+1Nn is more difficult to synthesize compared to Mn+1Cn. Interestingly, it has been demonstrated that nitride MXenes exhibit better conductivity in comparison with carbide MXenes [36]. During synthesis, the basal plane of MXenes could be functionalized by various atoms or groups including O, OH and F, which affect their inherent properties [30]. Recent investigations have demonstrated that under high temperature treatment the F group can be eliminated and OH group can be converted to O groups [37,38]. Studies have confirmed that these different functional groups could tune the work function and electronic properties of MXenes [39].Nb-based MXenes have gained great attention in energy storage and conversion [40,41]. Pt-doped Nb-based MXene has been reported to be an excellent bifunctional OER/ORR catalyst [42], while nitride Nb2N for ECR has not been studied. In this work, for the first time, we investigated the single transition metal atoms (V, Cr, Mn, Fe, Co, Ni) embedded O group terminated Nb2N monolayer (Nb2NO2) as ECR catalysts by first-principles calculation. It is found that Nb2NO2 can be an ideal support for anchoring sing TM atoms because of excellent stability and conductivity. TM@Nb2NO2 show excellent CO2 adsorption capacity, which benefits CO2 activation and reduction. Among six SACs catalysts, V, Cr and Ni@\u00a0Nb2NO2 are identified as efficient electrocatalysts for ECR to CH4, with smaller limiting potential of \u2212\u00a00.45, \u2212\u00a00.47 and \u2212\u00a00.28\u00a0V, respectively. Meanwhile, the origin of the ECR activity was revealed by several key descriptors.All calculations were carried out by spin-polarized density functional theory (DFT) in the Vienna Ab initio Simulation Package (VASP) with projector augmented wave (PAW) [43,44]. The generalized gradient approximation (GGA) implemented Perdew-Burke-Ernzerhof (PBE) was used to calculate the exchange-correlation energy [45,46]. The empirical correction (DFT-D3) was employed to describe the van der Waals (vdW) interactions [47]. The parameter for dipole correction along z-direction are considered in our calculations. DFT+U calculations are also considered for single TM atoms. The values of U\u2212J were set to be 2.72, 2.79, 3.06, 3.29, 3.42 and 3.4\u00a0eV for V, Cr, Mn, Fe, Co, and Ni, respectively [18]. A 3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a01 TM@Nb2NO2 containing 45 atoms was constructed by anchoring one TM atom in site1 (N site) and site 2 (Nb site) (\nFig. 1a). The 18\u00a0\u00c5 thickness vacuum region in the z-direction was added to eliminate the spurious interactions from periodic boundary. The cutoff energy was set to 500\u00a0eV. The K-points for geometry optimization and electronic calculations were set to be 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a01 and 10\u00a0\u00d7\u00a010\u00a0\u00d7\u00a01, respectively. The convergence of energy and force was set to be 1.0\u00a0\u00d7\u00a010\u22125 eV and 1.0\u00a0\u00d7\u00a010\u22122 eV/\u00c5, respectively. Solvent effect was included in our calculations by using implicit solvent model based on VASPsol, and the dielectric constant of water was 78.4 [48]. Moreover, to explore structure stability, the ab initio molecular dynamics (AIMD) simulation was performed in NVT ensemble and phonon spectra was calculated based on the density functional perturbation theory (DFPT) [49,50]. The TM atom transition energy barrier on Nb2NO2 monolayer was calculated by the climbing image nudged elastic band method (CINEB) [51], transition states were confirmed by vibration frequency analysis. The Bader charge analysis was used to analyze electron transfer [52].The binding energy (Eb) of TM atoms on Nb2NO2 monolayer was calculated by Eq. 1:\n\n(1)\nEb = E(TM@Nb2NO2) \u2212 E(TM) \u2212 E(Nb2NO2)\n\n\nwhere E(TM), E(Nb2NO2) and E(TM@Nb2NO2) denote the total energies of single TM atom, Nb2NO2, and TM@Nb2NO2, respectively. With such definition, a more negative value indicates a stronger binding of TM atoms to the Nb2NO2 substrate.The gas adsorption energy (Eads) was calculated by Eq. 2:\n\n(2)\nEads = E(gas@Nb2NO2) \u2212 E(gas) \u2212 E(Nb2NO2)\n\n\nwhere E(gas), E(Nb2NO2) and E(gas@Nb2NO2) are the total energies of gas, Nb2NO2 and gas adsorbed Nb2NO2.The Gibbs free energy of ECR were calculated by the computational hydrogen electrode (CHE) method [53]. After intermediate was adsorbed on the surface of catalyst, the translation and rotation freedom could be ignored and only vibration freedom is contributed to the entropy. The free energy of H+/e- pair is equivalent to the chemical potential of H2 at standard conditions. The Gibbs free energy change (\u0394G) can be obtained by Eq. 3:\n\n(3)\n\u0394G = \u0394E + \u0394E(ZPE) \u2212 T\u0394S + \u0394G(pH) + \u0394G(U)\n\n\nin which \u0394E was the energy difference between reactants and products directly obtained from DFT calculations, \u0394E(ZPE) and \u0394S are zero-point energy correction and entropy change at temperature T of 298.15\u2009K. \u0394G(pH) is the free energy correction due to the effect of H concentration, and was calculated by the formula \u0394G(pH) =\u2009k\nBTln10\u2009\u00d7\u2009pH. In this work, the pH value was set to be zero under acidic condition. \u0394G(U) is the contribution of the applied electrode potentials. The limiting potential (UL) from potential-determining step (PDS) can be obtained by Eq. 4:\n\n(4)\nUL = \u2212\u0394Gmax/e\n\n\nwhere \u0394Gmax is the maximum free energy change in the ECR process along the most favourable pathway.After geometry optimization, the obtained lattice parameter a of clean Nb2N monolayer is 3.11\u2009\u00c5, consistent with previous study [54]. Nb2N monolayer shows a hexagonal symmetry with P63/mmc space group. O was then added on the centre of three Nb atoms, similar to the O functionalized Ti2C MXene (Fig. 1a and b) [55]. The binding energy (Eb) of O on Nb2N monolayer was calculated by the equation: Eb =\u2009(E(Nb2NO2) \u2212\u2009E(O2) \u2212\u2009E(Nb2N))/2, where E(Nb2NO2), E(O2), E(Nb2N) are the total energy of Nb2NO2, O2 and Nb2N [56]. A negative value of Eb =\u2009\u20135.34\u2009eV demonstrates that Nb2N monolayer can be easily covered by O atoms. It is possible for O group transforming to OH during ECR process. Therefore, we calculated the Gibbs free energy for H adsorption on O atoms, with \u0394G*H of \u22120.16\u2009eV. A moderate \u0394G*H indicates that the proton can easily adsorb on and desorb from the surficial O atom, which may promote protonation of the ECR intermediates. The phonon curves and AIMD simulation were performed to check its stability, as shown in Fig. 1c and Fig. S1. There are no imaginary bands in phonon spectra. The fluctuation of the total energy of Nb2NO2 is quite small and around the equilibrium. Meanwhile, the structure does not show any obvious changes, confirming that Nb2NO2 monolayer possesses excellent stability. On the other hand, the calculated density of state of Nb2NO2 exhibits metallic behaviour, indicting good capability for electron transfer (Fig. 1d). This endows Nb2NO2 monolayer excellent electrical conductivity, a prerequisite for an ideal substrate for SACs used in ECR.The stability of TM anchored Nb2NO2 will be the key for the synthesis and application of MXene based SACs. As presented in Fig. 1a and b, there are two possible anchoring sites for single TM atoms: (1) the centre site between three neighbouring N atoms and the top of Nb atom (Nb site), (2) the centre site between three neighbouring Nb atoms and the top of N atom (N site). After structure relaxation, the anchored TM atoms have slight effects on lattice parameters a. The thermodynamic stabilities of TM@Nb2N were investigated by calculating Eb (Fig. 1e and Table S1). Notably, a more negative value of Eb on N site indicates that TM atoms prefer to bind on N site. Moreover, the transition energy barriers (ET) of single TM atoms from N to Nb site were calculated to evaluate its kinetic stability. The ET were calculated by ET =\u2009ETS \u2212\u2009EIS, in which ETS is the total energy of transition state (TS) from N to Nb site, while EIS is the total energy of TM embedded in N site. As shown in Table S1, the ET of TM atoms are quite large in the range of 0.87\u20132.58\u2009eV, implying that it is difficult for TM atoms to diffuse and aggregate into clusters. These results suggest that single TM atom can firmly anchor on N site. We therefore will only consider this site as active site for further study. In addition, Bader charge analysis show that the charge transfer from TM atoms to substrate decreases with the atomic number. Consequently, V and Cr atom present higher oxidation state (+1.08 and +1.02), while Ni atom shows lower oxidation value (+0.50).CO2 adsorption on the surface of electrocatalysts is important for CO2 activation and transformation into intermediates such as *COOH and *OCHO [57]. The optimized CO2 adsorption configurations on TM@Nb2NO2 were shown in \nFig. 2. Obviously, the carbon or oxygen atom of CO2 molecule is absorbed on TM atoms. Meanwhile, it can be observed that CO2 molecule is not absorbed on TM@Nb2NO2 in linear state, but with a certain degree of bending. The corresponding adsorption energies, bond lengths of C\u2212TM and O\u2212TM, bond angels of CO2 molecule, and charge transfer between TM and CO2 molecule are summarized in \nTable 1. It is clear that the bond angle of CO2 molecule on TM@Nb2NO2 increases with the atomic number, ranging from 138.39\u00b0 to 154.34\u00b0. Specially, V@Nb2NO2 greatly deviated from the linear state, which indicates higher CO2 adsorption capacity. The bond lengths of C\u2212TM and O\u2212TM are quite close to 2.00\u2009\u00c5, demonstrating strong adsorption between substrate and CO2 molecule, consistent with previous studies [26,58]. The CO2 adsorption energies on TM@Nb2NO2 range from \u2212\u20090.77 and \u2212\u20090.30\u2009eV. The negative values indicate that CO2 adsorption on the SACs is thermodynamically favourable. V, Cr and Ni@Nb2NO2 exhibit relatively strong interaction with CO2. Bader charge analysis confirm that there is a significant net charge transfer from V, Cr and Ni atoms to CO2, with a value of \u2212\u20090.60e, \u2212\u20090.53e, \u2212\u20090.31e, respectively. Thus, CO2 molecules can be effectively activated by V, Cr and Ni@Nb2NO2, and these three SACs potentially exhibit high performance for producing specific ECR products.The ECR process starts with the hydrogenation of CO2 molecule to form *COOH (* + CO2 + (H+ + e\u2212) \u2192 *COOH) or *OCHO (* + CO2 + (H+ + e\u2212) \u2192 *OCHO) on active centres by H atom binding O or C atom. However, the side-reaction HER (* + H+ + e\u2212 \u2192 *H) may occur due to the direct interaction between proton and TM atoms, resulting in low ECR selectivity. It has been widely accepted that the Gibbs free energy change (\u0394G) for *COOH/*OCHO and *H formation can be used to evaluate the ECR selectivity versus HER selectivity [59]. Therefore, \u0394G*COOH/*OCHO were calculated and compared with \u0394G*H. As plotted in \nFig. 3, all TM@Nb2NO2 electrocatalysts prefer ECR (below the diagonal) to HER (above the diagonal). Notably, V, Cr and Fe@Nb2NO2 are ECR selective with two favourable initial protonation processes (*COOH and *OCHO), while Ni, Co and Mn@Nb2NO2 exhibit ECR selectivity only with one favourable initial protonation step (*COOH or *OCHO). Meanwhile, \u0394G*OCHO is smaller than \u0394G*COOH for V, Cr and Fe@Nb2NO2, demonstrating that the formation of *OCHO is more energetically favourable. Therefore, the *COOH reduction path and the corresponding CO product will not be considered on these three SACs in the further protonation process.The reduction products from CO2 could involve C1, C2 and C3 due to complex protonation and C\u2212C coupling. However, the formation of high carbon products (C2+) is impossible because C\u2212C coupling will not occur on SACs. Therefore, only C1 products by accepting 2e to 8e electrons, including CO, HCOOH, HCHO, CH3OH, CH4, were investigated in this work. These different products are formed by different number of protons binding C or O atoms. A possible pathway was plotted in \nFig. 4 by taking the optimized configuration of intermediates on Fe@Nb2NO2 as an example. It is obvious that only TM atom binds with the C or O atoms during the whole ECR process, demonstrating TM atom as active site.After *COOH or *OCHO formation via accepting first proton-electron pair, further hydrogenation by obtaining a second proton-electron pair will produce *OCHOH or *CO intermediates. Therefore, the binding strength between these two intermediates and active centre will decide HCOOH or CO generation. We calculated the Eads of HCOOH and CO on TM@Nb2NO2 (\nTable 2). For HCOOH formation from *OCHO, V, Cr, Mn and Fe@Nb2NO2 show a large Eads with \u2212\u20091.07, \u2212\u20091.25, \u2212\u20090.87 and \u2212\u20091.51\u2009eV, respectively. For CO formation from *COOH, the Eads of CO on Co and Ni@Nb2NO2 are \u2212\u20091.95 and \u2212\u20091.73\u2009eV, respectively. It means that both HCOOH and CO could be further protonated on these SACs instead of desorbing from the SACs as final products. In addition, the generation of *OCHOH or *CO on Fe, Co, Cr, Ni and V@Nb2NO2 are overall exothermic. The generation of *OCHOH is only slightly endothermic on Mn@Nb2NO2, benefiting the further reduction of intermediates.For further hydrogenation of *OCHOH or *CO, three possible intermediate including *CHO, *COH and *OCH could be generated. Notably, *COH from *CO (*CO + (H+ + e\u2212) \u2192*COH) on Co and Ni@Nb2NO2 underwent a larger energy uphill in comparison with the formation of *CHO (*CO + (H+ + e\u2212) \u2192*CHO). Similarly, For V, Cr, Mn and Fe@Nb2NO2, the formation of *OCH from*OCHOH (*OCHOH + (H+ + e\u2212) \u2192*OCH +\u2009H2O) are more energy consuming than the production of *CHO. Thus, it can be concluded that *CHO will be the key intermediates for the third hydrogenation process.*OCH2 and *CHOH intermediates can be produced after *CHO accepting the fourth proton-electron pair. It is evident from \nFig. 5 that the \u0394G of *CHOH on these six SACs show energy uphill, while the \u0394G for the formation of *OCH2 on these SACs show energy downhill. Therefore, these TM atoms exhibit strong oxophilicity to form TM\u2212O bonds. The four-electron product HCHO will be desorbed from the electrocatalyst if the interaction between *OCH2 and TM atom is too weak. The calculated Eads of HCHO on these SACs are in the range of \u2212\u20091.59 to \u2212\u20090.96\u2009eV, suggesting that it is difficult for HCHO to desorb and thus can be further reduced.*CHOH then accepted the fifth proton-electron pair to produce *CH and *CH2OH intermediates, while the hydrogenation products of *OCH2 are *OCH3 and *OHCH2. However, \u0394G of *CH and *OHCH2 are energetically unfavourable and will not form. In contrast, *CH2OH and *OCH3 will be the key intermediates and participate in later hydrogenation. CH3OH is the six-electron product via *OCH3 +\u2009(H+ + e\u2212) \u2192*OHCH3 \u2192 *\u2009+ CH3OH. Nevertheless, the formation of *OHCH3 only show energy downhill on Fe and Mn@Nb2NO2. The Eads on V, Cr, Mn, Fe, Co, Ni@\u2009Nb2NO2 is \u2212\u20091.15, \u2212\u20091.17, \u2212\u20090.88, \u2212\u20090.75, \u2212\u20091.07 and \u2212\u20091.03\u2009eV, respectively. Thus, CH3OH can still be stably bonded with SACs and further reduced. The eight-electron product CH4 can be generated from diverse paths such as *CH3 +\u2009(H+ + e\u2212) \u2192 *\u2009+ CH4, *OCH3 +\u2009(H+ + e\u2212) \u2192 CH4 +\u2009*O and *OHCH3 +\u2009(H+ + e\u2212) \u2192 CH4 +\u2009*OH. Remarkably, the Eads of CH4 on TM@Nb2NO2 are significantly smaller than the other C1 products, ranging from \u2212\u20090.47 to \u2212\u20090.23\u2009eV, indicating that CH4 can easily desorb from the SACs and become the final product. According to principle of minimum free energy increase at each step, the optimized paths for ECR to CH4 on TM@Nb2NO2 were concluded as below (Fig. 5):\n\n(I) V,Cr and Fe@Nb2NO2:*+CO2 \u2192 *OCHO \u2192 *OCHOH \u2192 *CHO \u2192 *CHOH \u2192 *CH2OH \u2192 *CH2 \u2192 *CH3 \u2192 * + CH4\n\n\n\n\n\n(II) Mn@Nb2NO2:*+CO2\u2192 *OCHO \u2192 *OCHOH \u2192 *CHO \u2192 *OCH2 \u2192 *OCH3 \u2192 *OHCH3 \u2192 *OH + CH4 \u2192 * + H2O\n\n\n\n\n(III) Co and Ni @Nb2NO2: * + CO2 \u2192 *COOH \u2192 *CO \u2192 *CHO \u2192 *OCH2 \u2192 *OCH3 \u2192 *OHCH3 \u2192 *OH + CH4 \u2192 * + H2O\n\n\nThus, TM@Nb2NO2 can be promising candidates in electrochemically converting CO2 to CH4.To evaluate the ECR performance of TM@Nb2NO2, the PDSs and the corresponding UL were summarized in Table 2. Generally, the lower the value of UL, the higher the activity of SACs. In path I and II, *OCHOH \u2192 *CHO was identified as PDS for V, Cr, Mn and Fe@Nb2NO2. The UL for CH4 generation on these four SACs are \u2212\u20090.45, \u2212\u20090.47, \u2212\u20090.62 and \u2212\u20090.89\u2009V. The PDS of Co and Ni@Nb2NO2 in path III is *CO \u2192 *CHO, and the corresponding UL are \u2212\u20090.57 and \u2212\u20090.28\u2009V. Intriguingly, UL for the ECR to CH4 on V, Cr, Co and Ni@Nb2NO2 are lower than that the state-of-the-art catalyst Cu (211) (\u22120.74\u2009V) [60], demonstrating potentially excellent performance of TM@Nb2NO2 for ECR to CH4. Particularly, the UL of Ni@Nb2NO2 is among the best reported in literature. Finally, we investigated the stability of Ni@Nb2NO2 by AIMD simulations with a time step of 3 fs at the temperature of 300\u2009K for 18\u2009ps (Fig. S2). It can be found that Ni atom can still stay at the vacancy, which evidenced that diffusion will not occur.The excellent activity of TM@Nb2NO2 for CH4 generations is mainly related to the interaction between active TM atom and substrate. As shown in Table S1, the Nb2NO2 monolayer is negatively charged by TM atoms. The different amount of charge transfer indicates the different interaction strength. Consequently, the single TM atom with different positive charge will contribute to different catalytic activity. After intermediate adsorption on the single TM atom, the binding strength between them will directly determine UL. According to the Sabatier principle, too strong or too weak binding strength will result in low catalytic activity [61]. As shown in \nFig. 6, the PDOS of key intermediates from PDSs of TM@Nb2NO2 exhibit deferent interaction between TM and C or O atoms. For instance, the Mn-3d orbitals and the O-2p orbitals of *OCHOH in Mn@Nb2NO2 have slight overlap, contributing to weak binding strength. Fe-3d orbitals interacts greatly with O-2p, exhibiting strong interaction. The corresponding adsorption energy of OCHOH on these two SACs is \u2212\u20090.95 and \u2212\u20091.51\u2009eV, suggesting that too strong or weak interaction could increase the free energy of PDSs.We further investigated the activity origin on TM@Nb2NO2 by using descriptors. The PDSs of TM@Nb2NO2 can be assigned to *OCHOH and *CO, therefore we distinguish them by two different areas (palegreen and slateblue in \nFig. 7). Since the d-band centre of TM atoms has often been used to correlate the catalytic properties, the locations of d band centres (\u03b5) were calculated and plotted against UL, as shown in Fig. S3 and Fig. 7a. With the increase of the TM-d electron number, \u03b5 shifts to a more negative energy level, resulting in the increase of UL. When the key intermediate is *OCHOH, there is a good linear relationship between \u03b5 and UL (UL = 0.35\u03b5 \u2013 0.40, R2 = 0.97). For *CO as key intermediate, only Co and Ni@Nb2NO2 are distinguished. Generally, the more negative the value of \u03b5, the weaker the adsorption between intermediates and catalysts. For example, it can be found that Mn@Nb2NO2 shows a lower \u03b5, while the Eads for *OCHOH is smaller, indicating weak adsorption and a large UL. However, \u03b5 is not associated with Eads for a specific TM atom in a small range, because of the neglect of the d-band shape and the effect of the TM-s and p orbitals. Thus, the linear relationship is not apparent (Fig. 7b). For *CO intermediate, the higher \u03b5 of Co atoms contributed a strong Eads of *CO and high UL.The crystal orbital Hamilton populations (COHP) were employed to analyse the bonding and antibonding states of the TM and key intermediates *OCHOH and *CO [13]. Meanwhile, the integrated COHP (ICOHP) was calculated to give a more quantitative explanation (Fig. S4). For O atom bonding with V, Cr, Mn and Fe@Nb2NO2, it shows obvious antibonding states below Fermi level, demonstrating weak adsorption. The corresponding ICOHP values are \u2212\u20091.32, \u2212\u20091.44/\u2212\u20091.57, \u2212\u20091.47/\u2212\u20091.75, \u2212\u20091.28/\u2212\u20091.61\u2009eV, respectively. V and Cr@Nb2NO2 have similar antibonding states in spin up state, resulting in similar UL. For C atom bonding with Co and Ni@Nb2NO2, there is no antibonding state below Fermi level with value of \u2212\u20092.56/\u2212\u20092.66 and \u2212\u20092.40\u2009eV, respectively, indicating strong adsorption. The more negative the ICOHP, the more stable of bonding, thus Fe@Nb2NO2 shows a large UL. A good linear relationship between ICOHP and UL was obtained for V, Cr, Mn and Fe@Nb2NO2 (UL = 1.58\u03a6 + 1.70, R2 = 0.86), disclosing the role of different metal centres in the bonding/antibonding orbital populations.Recently, charge transfers of active atoms have been reported as descriptor to explain the performance of catalysts [62]. Herein, we calculated the valence state (\u03b4) of TM atoms after adsorbing intermediates. The \u03b4 of different atoms for different binding atoms vary in a range from +\u20090.55 to +\u20091.32, indicating an increase of charge transfer from TM atoms after intermediates adsorption and different interaction strength between them. Fe atom had the largest \u0394\u03b4 increase of 0.37 after intermediates adsorption, implying a possible strong interaction between Fe and *OCHOH and a large UL. Meanwhile, an approximate linear relationship (UL = \u22123.02\u0394\u03b4 \u22120.31, R2 = 0.86) was obtained, demonstrating that binding strength between catalysts and intermediates can be represented by \u0394\u03b4. Therefore, \u03b5, \u03a6 and \u0394\u03b4 can be used as descriptors to describe the activity origin well. Meanwhile, the Eads can be a nominal descriptor for the ECR activity to CH4 due to the close connection between energy and electronic structure, while \u03b5, \u03a6 and \u0394\u03b4 can quantitively describe the intrinsic activity of ECR to CH4 on TM@\u2009Nb2NO2. Overall, the results show that Ni@Nb2NO2 is the best ECR catalyst for CH4 generation, while Fe@Nb2NO2 is not an ideal catalyst.Single TM atoms (V, Cr, Mn, Fe, Co and Ni) anchored Nb2NO2 monolayer as potential SACs for electrochemical CO2 reduction were studied by first-principles calculation. Results demonstrate that TM atoms can be stably embedded into N site and will not aggregate into clusters. CO2 molecules can be effectively activated by V, Cr and Ni@Nb2NO2 due to charge transfer and large adsorption energy. All TM@Nb2NO2 electrocatalysts exhibit high selectivity for ECR in comparison with HER. The Eads of C1 products (CO, HCOOH, HCHO, CH3OH) is too large for them to desorb from the surface of catalysts, while CH4 can easily desorb due to the small Eads. The PDS on these SACs for ECR to CH4 can be divided into two categories: *OCHOH to *CHO for V, Cr, Mn and Fe, and *COOH to *CHO for Co and Ni. The UL for CH4 generation on V, Cr and Ni@Nb2NO2 SACs are \u2212\u20090.45, \u2212\u20090.47 and \u2212\u20090.28\u2009V, exhibiting high performance for ECR to CH4 and is even better than the Cu (211) electrocatalyst. Furthermore, the adsorption energy of the key intermediates (Eads) can serve as a nominal descriptor to indicate ECR activity, while d band center (\u03b5), ICOHP (\u03a6), the change of valence state (\u0394\u03b4) can quantitatively describe the ECR activity. This work demonstrated that MXene based earth abundant metal SACs are promising for electrocatalytic CO2 reduction.\nSong Lu: Methodology, Formal analysis, Investigation, Writing \u2013 original draft, Writing \u2013 review & editing, Visualization. Yang Zhang: Investigation, Formal analysis, Writing \u2013 review & editing. Fengliu Lou: Investigation, Formal analysis, Writing \u2013 review & editing. Zhixin Yu: Conceptualization, Formal analysis, Validation, Resources, Supervision, Writing \u2013 review & editingThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is supported by the Norwegian Ministry of Education and Research. The computations were performed on resources provided by UNINETT Sigma2 - the National Infrastructure for High Performance Computing and Data Storage in Norway.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2022.102069.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  The design of highly efficient catalysts for electrochemical reduction CO2 (ECR) to value-add chemicals and fuels is important for CO2 conversion technologies. In this work, earth abundant transition metal (TM = V, Cr, Mn, Fe, Co and Ni) atoms embedded into two-dimensional (2D) Nb2NO2 (TM@Nb2NO2) as single-atom catalysts (SACs) for ECR was investigated by first-principles study. We demonstrated that Nb2NO2 can be an excellent substrate for anchoring single TM atom due to its excellent stability and electronic conductivity. Besides, V, Cr and Ni@Nb2NO2 could effectively promote CO2 adsorption and reduction. All TM@Nb2NO2 exhibit high selectivity towards CH4, and V, Cr and Ni@Nb2NO2 show low limiting potentials. The activity origin was revealed by analysing adsorption energy, d band centre, bonding/antibonding population and the change of valence state of TM atoms.\n               "}
{"full_text": "municipal solid wastewater-gas shiftvolume percentageweight percentageSyngas obtained during biomass/municipal solid waste (MSW) gasification is mainly a mixture of carbon monoxide, carbon dioxide, hydrogen, methane and nitrogen, which can be utilized for electric power generation or liquid fuel synthesis [1]. The biomass- and MSW-derived syngas, however, contains significant concentrations of impurities such as tar, HCl, alkali chlorides, particulate matter, ammonia, HCN and sulfur compounds. Tar, consisting of a mixture of aromatic hydrocarbons, causes equipment failure by its condensation and corrosion upon cooling of syngas [2\u20134]. The techniques that can efficiently remove tar compounds to the acceptable levels are still under development. One of the prospective techniques is catalytic steam reforming which converts tar into H2 and CO [5\u20137]. Different types of natural minerals (e.g., dolomite, olivine and clay minerals) and synthetic catalysts (e.g., char, activated alumina and metal-based catalysts) were proposed for tar reforming, among which Ni-based catalysts are the most common and commercially available. The utilization of Ni-based catalysts enhances syngas production due to steam reforming of hydrocarbons and other catalyzed reactions, including dry reforming, WGS and Bodouard reactions [8\u201312]. Furthermore, Ni-based catalysts facilitate simultaneous decomposition of NH3 and HCN into N2 and H2 during the reforming process, resulting in lower NOx emissions [13,14].Besides nickel, other metals as well as bimetallic and polymetallic composites have been extensively investigated as reforming catalysts [15\u201323]. For instance, monometallic Fe and bimetallic Ni-Fe catalysts have shown satisfactory reforming activity and high catalytic stability during reforming of tar compounds under certain conditions [24\u201328]. Noichi et al. [24] found that higher Fe content in Fe-Al catalysts enhanced the catalytic steam reforming activity by increasing naphthalene conversion efficiency. In NiO\u2013Fe2O3\u2013Al2O3 catalysts developed by Dong et al. [25] and Margossian et al. [26], syngas production and dry reforming activities of methane were influenced by Fe content. Furthermore, catalysts with optimized Fe content were reported to enhance thermal stability of the Ni-Fe catalysts by mitigating coke formation during tar reforming [25,26]. This superior effect was attributed to the formation of Ni-Fe alloys enriched with Fe-O species at the surface of nanoparticles that could catalyze coke oxidation [27,28].It is well known that impurities present in syngas (e.g., particles, sulfur and chlorine species) can poison the catalysts during steam reforming process [29\u201335]. H2S is notorious poison for catalysts and only a few ppmv of H2S could rapidly deactivate a Ni-based reforming catalyst [29\u201332]. Upon contact with Ni-based catalyst, sulfur species (e.g., H2S) chemisorb on metal sites forming NiS according to the reaction (1) decreasing the accessibility of active sites to hydrocarbons [29]:\n\n(1)\n\n\nNi\n+\n\nH\n2\n\nS\n\u21cc\nNi\n-\nS\n+\n\nH\n2\n\n\n\n\n\nYung et al. [30] have attempted to regenerate the spent Ni catalyst which was contaminated during catalytic tar reforming at 850\u202f\u00b0C by 43\u202fppmv H2S in syngas produced from gasification of white oak. It was found that the Ni-S species in catalyst could not be completely removed during the steam/air regeneration procedure. As a result, the catalytic activity of Ni was only partially recovered and was lower than its initial activity levels [30]. The low melting point and high surface mobility of NiS can accelerate sintering [31], which may deteriorate the activity of catalyst. Furthermore, sulfur species can increase the carbon deposition on catalyst surface, which also decreases the catalytic activity [32].The presence of HCl in syngas was reported to decrease the reforming and WGS activities of Ni catalysts [33\u201337]. Richardson et al.\n[33] found that the conversion of methane was extremely inhibited in the presence of HCl, due to the chemisorption of HCl by Ni. Coute et al. [36] demonstrated that HCl induced detrimental effect on WGS activity during steam reforming of chlorocarbons. Veksha et al. [37] investigated the mechanism for the activity loss of Ni catalysts during naphthalene reforming in the presence of 2000\u202fppmv HCl and demonstrated that naphthalene conversion is not influenced by HCl while WGS activity was poisoned due to the sintering of Ni. In the above mentioned studies, either H2S or HCl were present in gas streams during the reforming while in real syngas, these impurities are present simultaneously. To what extent the co-existence of both H2S and HCl in the gas can influence the catalytic activity during steam reforming has not yet been investigated.The purpose of this work was to investigate the influence of H2S and HCl on the poisoning of synthesized and commercial catalysts during steam reforming of tar. It has been well known that Ni is an excellent metal for steam tar reforming. In this study, the addition of Fe is attempted, because Fe is a low cost material and Fe species has high redox activity [38]. Furthermore, the addition of Fe to Ni had beneficial effect on the performance of bimetallic catalysts under certain experimental conditions [24\u201328]. Four synthesized catalysts with different loadings of Ni and Fe on alumina support and two commercial catalysts were tested in a fixed bed reactor at different temperatures with varying contents of H2S and HCl in gas. Naphthalene was used as a model tar compound as it is one of the major tar species [39] which also has high stability during tar reforming [40\u201342]. In this study, 50\u202fppmv H2S and 300\u202fppmv HCl were used as they are in the range of typical concentrations of H2S and HCl present in syngas produced from biomass/MSW [29\u201331,34\u201337]. The individual and combined effects of impurities on reforming and WGS activities of the catalysts and the reversibility of the catalyst poisoning are discussed.Four catalysts with different Ni and Fe contents were synthesized using the method described elsewhere [37]. Briefly, the catalysts were prepared by impregnation of aluminum hydroxide (H3AlO3\u00b7xH2O, Sigma-Aldrich) having particle sizes of 0.56\u20131.18\u202fmm with known concentrations of Ni(NO3)2\u00b76H2O (Sigma-Aldrich) and Fe(NO3)3\u00b79H2O (Sigma-Aldrich) in aqueous solution. After evaporation of water in a rotary evaporator (Hei-Vap Precision, Heidolph Instruments, Germany), the materials were dried overnight in an oven at 105\u202f\u00b0C and calcined at 500\u202f\u00b0C for 2\u202fh (heating rate 2\u202f\u00b0C\u202fmin\u22121) in air, followed by sieving to obtain particle sizes between 0.56 and 1.18\u202fmm. The synthesized catalysts are denoted as xNi \u2013 yFe, where x and y represent calculated molar contents of metals per 100\u202fg of the resulted catalyst.Two commercial catalysts from different manufacturers (6-holes monoliths from Xian Sunward Aeromat Co., China and 19\u202f\u00d7\u202f19\u202f\u00d7\u202f10\u202fmm rings from Pingxiang Hualian Chemical Ceramic Co., China) were crushed and sieved to obtain 0.56\u20131.18\u202fmm particles, and used as the reference materials.\nFig. 1\n shows the experimental setup for catalytic naphthalene reforming. A fixed bed reactor with a quartz frit (50\u201390\u202f\u03bcm openings) was used. In a typical run, 0.5\u202fmL of a catalyst was loaded into the reactor and heated at 15\u202f\u00b0C\u202fmin\u22121 in reducing atmosphere containing 20\u202fvol% H2 \u2013 80\u202fvol% N2 (total gas flow 50\u202fmL-STP\u202fmin\u22121) to the reforming temperature of 790, 850 or 900\u202f\u00b0C. Once the temperature was reached, the gas flow was maintained for 30\u202fmin to reduce the catalyst and then the flow was switched to 200\u202fmL-STP\u202fmin\u22121 (space velocity 24,000\u202fh\u22121) of a model syngas containing 0.14\u202fvol% naphthalene, 10\u202fvol% H2, 26\u202fvol% H2O, 0 or 300\u202fppmv HCl, 0 or 50\u202fppmv H2S and N2 (balance). In this study, 0.14\u202fvol% naphthalene was used as it is within the range of typical concentrations of naphthalene produced from biomass/MSW gasification [43,44]. Naphthalene vapors were generated by purging N2 gas through an evaporator containing heated naphthalene. H2, N2 and H2S were supplied from gas cylinders using mass flow controllers. The steam and HCl vapor were generated from aqueous solution of HCl injected by a syringe pump into an evaporator. During experiment, all the gas lines were heated and kept above 150\u202f\u00b0C to avoid vapor condensation. After reforming reactor, the model gas passed through two water traps to capture HCl and one silica gel trap to remove moisture, and then was collected in Tedlar gas bags for analysis (collection time 5\u202fmin). Concentrations of CO, CO2 and C1-C5 hydrocarbons were measured by a calibrated gas chromatograph (Agilent 7890B, USA) equipped with one flame ionization and two thermal conductivity detectors. Steam reforming of naphthalene is analogous to steam reforming of other hydrocarbons [10,45]:\n\n(2)\n\n\n\nC\n10\n\n\nH\n8\n\n+\n10\n\nH\n2\n\nO\n=\n10\nCO\n+\n14\n\nH\n2\n\n\n\n\n\nDue to WGS activity, CO is partially converted to CO2 over Ni catalysts [1,28,45]:\n\n(3)\n\n\nCO\n+\n\nH\n2\n\nO\n=\n\nCO\n2\n\n+\n\nH\n2\n\n\n\n\n\nNaphthalene conversion can be calculated by the following equation:\n\n(4)\n\n\nNaphthalene\n\nconversion\n\n\n(%)\n=\n\n\n(\n\n\nn\n\n\nC\n\n\nO\n\n\n2\n\n\n\n\n+\n\n\nn\n\n\nCO\n\n\n)\n\n\n\n\nn\n\n\nnaphthalene\n\n\n\u00d7\n10\n\n\n\n\u00d7\n100\n%\n\n\n\nwhere: \n\n\nn\n\nCO\n\n\n\n and \n\n\nn\n\nC\n\nO\n2\n\n\n\n\n are the molar concentrations of CO and CO2 generated during steam reforming of naphthalene, mol min\u22121, and \n\n\nn\n\nnaphthalene\n\n\n\n is the molar concentration of naphthalene in the feed, mol min\u22121. All experiments were triplicated and the results are presented as averages of three experimental runs.The catalysts were characterized by X-ray diffraction analysis with Cu-K\u03b1 radiation source (XRD, Bruker AXS D8 Advance), X-ray photoelectron spectroscopy with a dual anode monochromatic K\u03b1 excitation source (XPS, Kratos Axis Supra), X-ray fluorescence spectroscopy (XRF, PANalytical Axios mAx), transmission electron microscopy at 120\u202fkV (TEM, JEOL JEM-2010) and N2 adsorption at \u2212196\u202f\u00b0C (Quantachrome Autosorb-1 Analyzer). Binding energies of elements in XPS spectra were corrected against an adventitious carbon C 1s core level at 284.8\u202feV. The processing of XPS peaks was carried out in the CASA XPS software. TEM images were used to measure the size of Ni nanoparticles in spent catalysts. The diameters were calculated using ImageJ software by analysing 150\u2013200 Ni nanoparticles in each sample and assuming that nanoparticles have ideal spherical shape. Specific surface area of catalysts was calculated from N2 adsorption isotherms using BET model. Total pore volumes were calculated from N2 adsorption volume at P/P0\u202f=\u202f0.96. Temperature programmed reduction (TPR) was performed in a 5% H2/N2 gas mixture at 30\u202fmL\u202fmin\u22121 flow rate with a temperature ramp of 10\u202f\u00b0C\u202fmin\u22121 up to 900\u202f\u00b0C. Carbon content in the catalysts was measured by CHNS elemental analyser (Vario EL Cube).The properties of pristine Ni, Fe, Ni-Fe and commercial catalysts are presented in Table 1\n. Ni and Fe contents were determined from XRF analysis and used to calculate molar quantities of Fe and Ni. The molar Ni and Fe loadings per 100\u202fg of catalysts were close to the corresponding theoretical values of x and y in xNi-yFe samples. The amount of Ni in 0Fe-0.4Ni and two commercial catalysts loaded into the reactor for steam reforming of naphthalene was nearly the same (approx. 1.90\u202fmmol 0.5\u202fmL\u22121 catalyst) due to the differences in bulk density, allowing comparison between the activities of Ni in the synthesized and commercial catalysts. The synthesized catalysts had higher BET specific surface areas and total pore volumes compared to the commercial catalysts. According to the high N2 adsorption volumes at relative pressures P/P0\u202f>\u202f0.1 and hysteresis loops between adsorption and desorption branches of isotherms (Fig. S1), the synthesized materials were mesoporous. Among them, 0Fe-0.4Ni had the largest porosity (i.e. 213\u202fm2 g\u22121 and 0.31\u202fmL\u202fg\u22121, respectively) (Table 1), which is one of the reasons for its better catalytic performance stated in the following study. The specific surface areas and total pore volumes of the synthesized catalysts decreased with increasing Ni\u202f+\u202fFe contents, which can be attributed to the impregnation of the porous alumina with loaded metal species. X-ray diffraction (XRD) patterns of the synthesized catalysts in Fig. 2\n consist of broad peaks with no distinct XRD peaks and also show no sharp XRD peaks indicating that alumina, nickel and iron oxides have non-crystalline and/or nanosized structures, so that alumina provides surface area for better dispersion of catalyst. On the other hand, in commercial catalysts, the XRD peaks of NiO (commercial 1 and 2) and \u03b1-Al2O3 (commercial 1) can be clearly identified.\nFig. 3\n depicts the Ni 2p and Fe 2p core level spectra of the four pristine synthesized catalysts. Ni 2p spectra of 0Fe-0.4Ni, 0.1Fe-0.4Ni, 0.2Fe-0.4Ni and 0.5Fe-0Ni contain shake-up satellite peaks with binding energy (BE) of approx. 862\u202feV and peaks with BE of approx. 856\u202feV. In Ni-based catalysts, the binding energy of Ni2+ typically increases with the strength of NiO\u2013Al2O3 interactions from approx. 854\u202feV for unsupported and weakly bound to the support NiO to approx. 856\u202feV for strongly bound to the support NiO [46\u201349]. At high NiO\u2013Al2O3 interaction levels, the binding energy of Ni2+ in NiO of alumina supported catalysts becomes similar to that in NiAl2O4 spinel (855.8\u202feV) [48,50]. Due to the shift in binding energy, it is uncertain whether the Ni2+ state in the catalysts NiO or NiAl2O4 solely based on XPS spectra. The similar binding energies of Ni2+ in all synthesized catalysts suggest that independently on Ni\u202f+\u202fFe loading, strong interactions between NiO and alumina were maintained in the catalysts and there was no formation of new compounds with Fe species. The same can be concluded from Fe 2p core level spectra. Binding energies of Fe 2p for all catalysts were similar regardless of the presence of NiO (i.e. 711.0\u202feV for 0.1Fe-0.4Ni, 711.4\u202feV for 0.2Fe-0.4Ni and 711.0\u202feV for 0.5Fe-0Ni) and corresponded to Fe3+ in Fe2O3\n[50].TPR profiles provide useful information regarding reducibility of Ni and Fe oxides in the synthesized catalysts (Fig. 4\n). The reduction of catalysts occurred in a wide temperature range between 300 and 800\u202f\u00b0C. The catalysts 0.2Fe-0.4Ni and 0.5Fe-0Ni contained a distinct reduction peak at 475\u202f\u00b0C, which corresponds to the reduction of Fe2O3\n[51], i.e. the main Fe species in catalysts according to XPS. According to the TPR profile of 0Fe-0.4Ni, most of nickel was reduced at 500\u2013700\u202f\u00b0C with the maximum reduction temperature at 590\u202f\u00b0C, which can assigned to highly dispersed NiO having strong metal-support interactions [48,52]. Small shoulder peaks at 350, 425 and 770\u202f\u00b0C were also observed. The reduction at 300\u2013400\u202f\u00b0C is typically attributed to bulk and/or unsupported NiO, while the reduction >700\u202f\u00b0C could be attributed to nickel aluminates formed due to sintering of NiO with Al2O3\n[48,53,54], indicating that minor quantities of these species could be also present in the synthesized catalysts. According to the similar positions of H2 consumption peaks in the catalysts, the reducibility of Ni species was not influenced by the addition of Fe2O3 and vice versa.XPS and TPR data of the commercial catalysts are shown in Fig. S2. As it was reported elsewhere [37], in both catalysts, Ni2+ was in the form of NiO. However, in Commercial 2, NiO was more strongly bonded to the support compared to Commercial 1. Considering the similar Ni loading per 0.5\u202fmL catalyst bed for Commercial 1, Commercial 2 and 0Fe-0.4Ni, this allows investigation of the effects of H2S and HCl on the activity of catalysts with different strength of NiO\u2013Al2O3 interactions determined by the differences in porosity, crystalline structure, NiO dispersion etc. The addition of Fe to Ni-based catalyst provides further insight about the influence of H2S and HCl on the activity of catalysts with different metal composition.\nFig. 5\n depicts naphthalene conversion using the six catalysts in the presence and absence of H2S and HCl at 850\u202f\u00b0C. CO and CO2 were the only reaction products. No formation of C1\u2013C5 hydrocarbons was observed during the process. Naphthalene conversion over catalysts fluctuated during the first 30\u202fmin of experiment and was stabilized thereafter. In all catalysts containing Ni, the reforming activity was lower in the presence of H2S and HCl due to the poisoning effect (data in Fig. 5a against Fig. 5b). Furthermore, naphthalene conversion by 0.5Fe-0Ni was approx. 12% in the absence of H2S and HCl, and decreased to approx. 8% in the presence of H2S and HCl, suggesting the poisoning of Fe. Regardless of the presence of H2S and HCl, naphthalene conversion was stable during 5\u202fh tests. The synthesized 0Fe-0.4Ni showed comparable conversion efficiency with commercial catalysts, which was likely due to the same amount of Ni loading per 0.5\u202fmL bed in the three catalysts. These results suggest that there was similar poisoning effect on the naphthalene reforming activity for the catalysts with different strength of NiO\u2013Al2O3 interactions. Reforming activity of 0.1Fe-0.4Ni was similar to 0Fe-0.4Ni, while the higher content of Fe in 0.2Fe-0.4Ni resulted in the decreased naphthalene conversion. This could be attributed to the decreased porosity and specific BET surface area with the higher Fe content due to the occupation of surface sites (Table 1). Unlike Ni-based catalysts, 0.5Fe-0Ni merely achieved approx. 8% of naphthalene conversion, indicating that Ni is more active catalyst for naphthalene reforming compared to Fe. The lower catalytic toluene reforming activity due to Fe addition to Ni/zeolite catalyst was reported by Ahmed et al.\n[38], who found the depletion in basicity strength of this Fe-Ni/zeolite catalyst leading to suppressed steam reforming.Elemental CHN analysis (Table S1) of the pristine and spent catalysts suggests that there was no significant increase in the amount of carbon after the reforming, indicating no coking happened in the presence of H2S and HCl. This can be attributed to the relatively high content of steam in the model gas (i.e. 26\u202fvol%) that could assist in carbon gasification.\nFig. 6\n shows the TEM image of fresh 0.2Fe-0.4Ni after preheating in 20\u202fvol% H2\u201380\u202fvol% N2 and spent 0.2Fe-0.4Ni after 5\u202fh of reaction at 850\u202f\u00b0C in the presence of H2S and HCl. The comparison of the morphologies of fresh and spent catalyst indicates the absence of carbon deposition during naphthalene reforming, which is consistent with CHN analysis. After reforming, Ni was present in the form of discreet spherical nanoparticles. This is attributed to the sintering of Ni during the process [37]. On the contrary, Fe was evenly distributed over the catalyst surface (Fig. 6a). Fig. S3 shows that in other Fe-containing catalysts, Fe also remains in the dispersed state. The coverage of entire surface of the spent 0.2Fe-0.4Ni catalyst by S and Cl indicates that the chemisorption of these species occurred both on the Ni and Fe sites (Fig. 6b) [29,55,56], which explains the poisoning effect of HCl and H2S both on the reforming activity of Ni and Fe.\nFig. 7\n shows the XRD patterns of the spent catalysts after naphthalene reforming at 850\u202f\u00b0C in the presence of 50\u202fppmv H2S and 300\u202fppmv HCl. In the spent samples containing Ni element, the formation of metallic Ni phase was observed as suggested by the labelled NiO XRD peaks. As there were no XRD peaks of NiO in the fresh catalysts, these results indicate that upon reduction and reforming, Ni undergoes sintering into larger size crystalline nanoparticles, which is consistent with TEM data in Fig. 6a. Unlike NiO, the formation of crystalline FeO in 0.5Fe-0Ni was not observed as suggested by the absence of corresponding XRD peaks in this sample and even distribution of Fe in the TEM images of spent catalysts (Figs. 6a and S3). According to TPR data (Fig. 4), the reforming temperature was sufficient for the reduction of Fe2O3 to FeO. Therefore, it is likely that in the spent catalysts iron was in metallic non-crystalline state. These observations are consistent with scanning TEM data in Fig. 6, showing the differences in Fe morphology compared to Ni.There was no change in the position of NiO XRD peaks in 0Fe-0.4Ni, 0.1Fe-0.4Ni and 0.2Fe-0.4Ni with the addition of Fe (Fig. 7), which would have been observed with the formation of Ni-Fe alloys [27,57], indicating that there was no alloying between Ni and Fe in the spent catalysts. The amount of chemisorbed sulfur and chlorine species during reforming was typically low which explains the absence of XRD peaks corresponding to metal chlorides and sulphides in all catalysts.The reaction temperature is one of the most important operating variables for steam reforming. 0Fe-0.4Ni, 0.1Fe-0.4Ni and Commercial 1 were further selected to investigate the effect of temperature on catalytic activity. Fig. 8\n shows naphthalene conversion at 790, 850 and 900\u202f\u00b0C in the presence of H2S and HCl. Except for the decrease in conversion within the initial 30\u202fmin at 790\u202f\u00b0C, the activity of catalysts remained constant thereafter indicating that it is possible to maintain stable conversion efficiency of naphthalene in the presence of H2S and HCl within the studied period of time at each temperature. The catalytic activities of the three catalysts were similar in the presence of H2S and HCl at each temperature regardless of the strength of NiO\u2013Al2O3 interactions (0Fe-0.4Ni vs. Commercial 1) and the addition of Fe (0Fe-0.4Ni vs. 0.1Fe-0.4Ni). The reforming activities of all catalysts were greatly influenced by the reforming temperature, increasing from approx. 40% to approx. 100% efficiencies with the increase in reaction temperature from 790 to 900\u202f\u00b0C, respectively. These results can be attributed to the increased reaction rate of naphthalene with steam and the decreased H2S poisoning effect at higher temperature. It has been well known that H2S poisoning is caused by sulfur adsorbed on the nickel surface in the catalyst according to reaction (1). This reaction is reversible. With the increasing temperature desorption of H2S increases releasing surface active sites for the steam reforming reaction [58].To determine the respective and relative roles of H2S and HCl in the catalyst poisoning effect observed in Figs. 5 and 8, the naphthalene reforming at four different conditions was compared: (1) 50\u202fppmv H2S and 300\u202fppmv HCl, (2) 0\u202fppmv H2S and 300\u202fppmv HCl, (3) 50\u202fppmv H2S and 0\u202fppmv HCl, and (4) 0\u202fppmv H2S and 0\u202fppmv HCl. The experiments were carried out at 790\u202f\u00b0C, as the poisoning was the most prominent at this temperature. According to Fig. 9\n, in the absence of H2S, naphthalene conversion was approx. 100% both at 0 and 300\u202fppmv HCl. In the presence of 50\u202fppmv of H2S, naphthalene conversion decreased to approx. 40% both at 0 and 300\u202fppmv HCl. These results suggest that the poisoning of naphthalene reforming was caused by H2S, while HCl had negligible effect on this reaction. Furthermore, since the naphthalene conversion in the presence of H2S was similar at 0 and 300\u202fppmv HCl, it can be concluded that H2S and HCl had no synergistic effect on the poisoning of reforming activity when both impurities were present in the stream.Based on the obtained data, during the reforming of naphthalene from gas streams containing both H2S and HCl, the poisoning of catalysts is mainly caused by H2S and can be attributed to the decreased accessibility of surface active sites for hydrocarbons due to H2S chemisorption [29]. The poisoning effect on naphthalene reforming activity was similar for the catalysts with different strength of NiO\u2013Al2O3 interactions and Ni\u202f+\u202fFe contents. Increasing reaction temperature could effectively improve catalytic activity of Ni and Ni-Fe based catalysts in the presence of H2S and HCl leading to approx. 100% naphthalene conversion (Fig. 8).\nFig. 10\n shows the ratios between CO and CO2 in the gas during naphthalene reforming over the catalysts at 850\u202f\u00b0C in the presence of H2S and HCl. Steam reforming of hydrocarbons is typically presented as the combination of two reactions, namely, partial oxidation of hydrocarbon by steam into CO and H2 (reaction 2) followed by WGS reaction (3) [1,8,10,28,45]. Consequently, the lower CO/CO2 ratio is probably attributed to the higher conversion of CO into CO2 over catalysts via WGS reaction (3). Dashed black line shows the CO/CO2 ratio at thermodynamic equilibrium (CO/CO2\u202f=\u202f0.52 at 850\u202f\u00b0C). For all catalysts, the CO/CO2 ratios were higher than 0.52 indicating that thermodynamic equilibrium was not attainable. This is due to the lower space velocity and longer residence time required for the equilibration of WGS reaction over the catalysts [37]. There were significant differences in the kinetics of WGS reaction as suggested by the different CO/CO2 ratios for the catalysts. The CO/CO2 ratios of synthesized Ni and Ni-Fe catalysts increased from 0.9 to 1.0 to 1.2\u20131.5 during the 5\u202fh tests, depending on the sample. These changes were much lower compared to Commercial 1 and Commercial 2 catalysts (i.e. from 0.7 to 3.9 for Commercial 1 and from 1.0 to 2.2 for Commercial 2), suggesting higher stability of WGS activity of the synthesized catalysts. Based on the similar CO/CO2 ratios for 0Fe-0.4Ni, 0.1Fe-0.4Ni and 0.2Fe-0.4Ni, the addition of Fe to catalysts did not alter the WGS activity of catalysts. Furthermore, the lower CO/CO2 ratios over the synthesized Ni containing catalysts compared to 0.5Fe-0Ni indicate that the WGS activity over Ni was higher compared to Fe during naphthalene steam reforming.Although 0Fe-0.4Ni, Commercial 1 and Commercial 2 had similar NiO loading per catalyst bed volume, the strength of interactions between NiO and alumina support was different in the catalysts, eventually, leading to the different Ni-support interactions in the reduced catalysts. Specifically, the strength of interactions increased from Commercial 1 to Commercial 2 and, finally, to 0Fe-0.4Ni which is consistent with the increase in WGS activity in the same order (Fig. 10). One reason behind the observed phenomenon is the mechanism of WGS reaction over Ni based Al2O3 catalysts. By combining density functional theory and microkinetic modelling, it was demonstrated that Ni-support interface provides catalytically active sites for WGS reaction, serving as a storage for oxygenated Ni2+ species [59]. Therefore, the decrease in the strength of metal-support interactions in catalysts can result in the observed loss of WGS activity. In comparison, for the steam and dry reforming reactions of methane, the importance of metal-support interactions was found to be less important as the active sites for these reactions seem to be different. [59,60] Assuming that the mechanisms for reforming of hydrocarbons are similar [45], this could explain the negligible differences in naphthalene conversion over 0Fe-0.4Ni, Commercial 1 and Commercial 2 (as shown in Fig. 5).For WGS reaction, oxygenated Ni2+ sites are required [59], while higher reforming temperatures favor the reduction of NiO to metallic Ni. TPR profiles of synthesized and commercial catalysts (Figs. 4 and S2) show that the reduction temperature of Ni2+ increased from Commercial 1 to Commercial 2 followed by the synthesized catalysts indicating that the synthesized catalysts can have the higher density of oxygenated Ni2+ sites at the reforming conditions due to the stabilization of NiO by the support [48,52,61,62]. To confirm that, thermodynamic calculations using HSC Chemistry 9 software were carried out to calculate the content of oxygenated Ni2+ in catalysts in the absence and presence of NiO-support interactions. For the simplicity of calculations, it was assumed that in the absence of interactions with the support, nickel can only be oxidized into NiO. In the presence of strong metal-support interactions, Ni can form stable oxidized species at the NiO-support interface. \u03b3- and \u03b1-Al2O3 were selected as the representative support materials, which allow for the formation of NiAl2O4 spinel [62]\n.Under steam reforming conditions, H2O acts as an oxidant and the oxidation of Ni can be described by the following reactions:\n\n(4)\n\n\nNi\n+\n\nH\n2\n\nO\n\u21cc\nNiO\n+\n\nH\n2\n\n\n\n\n\n\n\n(5)\n\n\nNi\n+\n\u03b3\n-\n,\n\n\u03b1\n-\n\nAl\n2\n\n\nO\n3\n\n+\n\nH\n2\n\nO\n\u21cc\n\nNiAl\n2\n\n\nO\n4\n\n+\n\nH\n2\n\n\n\n\n\n\nFig. 11\na shows the standard Gibbs reaction energies (\u0394G\u00b0) for the oxidation of Ni as the function of reforming temperature. It can be seen that \u0394G\u00b0 increases with temperature suggesting that higher temperature causes the formation of metallic Ni. However, at the same temperature, \u0394G\u00b0 is lower when \u03b3- and \u03b1\u2010Al2O3 participate in the reaction, indicating that in the catalysts with strong NiO-support interactions, there is a higher content of oxygenated Ni2+. From the corresponding thermodynamic equilibrium constants, the content of Ni2+ can be calculated at the experimental conditions. According to Fig. 11b, in the absence of NiO-support interactions, the content of Ni2+ slightly increases with temperature and is 1.3%, 1.4% and 1.5% at 790, 850 and 900\u202f\u00b0C, respectively. In the presence of NiO-support interactions, the content of Ni2+ is much higher at each reforming temperature (Fig. 11b). Notably, the \u03b3- Al2O3 favors the stabilization of Ni2+ to a larger extent compared to \u03b1\u2010Al2O3 highlighting the importance of alumina material for the design of catalysts with tailored WGS activity. The provided thermodynamic calculations confirm that at the reforming temperatures, NiO-Al2O3 interactions can indeed stabilize nickel in the oxidized form due to the participation of support in the reaction, which could be in turn responsible for the higher WGS activity on the synthesized catalysts. Since, the XRD patterns of the spent catalysts contain only metallic Ni phase (Fig. 2), it is likely that the oxygenated Ni2+ species are mainly present at the NiO-Al2O3 interface.Previously, it was proposed that the exposure of catalysts to high concentration of HCl (2000\u202fppmv) during steam reforming of naphthalene causes the chemisorption of HCl on Ni followed by the sintering of Ni species into larger size nanoparticles. This process is irreversible and leads to a permanent loss of WGS activity [37]. The poisoning of WGS activity of catalysts by low concentrations of H2S and HCl (i.e. 50 and 300\u202fppmv, respectively) has not been investigated. Fig. 12\n presents the CO/CO2 ratios for 0Fe-0.4Ni, 0.1 Fe-0.4Ni and Commercial 1 at 790, 850 and 900\u202f\u00b0C. Among the tested catalysts, Commercial 1 showed the lowest WGS activity at each temperature. With the increase in temperature, CO/CO2 ratios for Commercial 1 catalyst decreased, indicating that WGS activity of this catalyst could be improved by increasing the reforming temperature. Since the content of oxygenated Ni2+ species is relatively high at all reforming temperatures, this could be attributed to the faster reaction rate that allows to approach closer to the thermodynamic equilibrium and/or enhanced desorption of S- and Cl-species at higher temperature. Nevertheless, the CO/CO2 ratios for Commercial 1 remained high compared to those corresponding to thermodynamic equilibrium. The CO/CO2 ratios for Ni and Ni-Fe catalysts were lower than that of Commercial 1 and closer to thermodynamic equilibrium at all temperatures, indicating higher WGS activity.Since the poisoning of Commercial 1 was more pronounced at lower temperature, the individual and combined effects of H2S and HCl on WGS activities of two representative catalysts, namely, 0Fe-0.4Ni and Commercial 1, were compared at 790\u202f\u00b0C. Fig. 13\na presents the CO/CO2 ratios at four experimental conditions: (1) 50\u202fppmv H2S and 300\u202fppmv HCl, (2) 0\u202fppmv H2S and 300\u202fppmv HCl, (3) 50\u202fppmv H2S and 0\u202fppmv HCl, and (4) 0\u202fppmv H2S and 0\u202fppmv HCl. In the context with respect to WGS reaction, the presence of H2S and HCl had negligible effect on the poisoning of 0Fe-0.4Ni indicating high stability of the WGS activity to the action of both impurities. The deterioration of WGS activity of Commercial 1 was observed even in the absence of H2S and HCl. This could be attributed to the lower strength of NiO\u2013Al2O3 interactions in this catalyst compared to 0Fe-0.4Ni. As shown in Fig. 13b and c, the sizes of Ni nanoparticles were larger in the spent Commercial 1 compared to 0Fe-0.4Ni after using condition 4, which could result in the lower WGS activity [59].For Commercial 1, CO/CO2 ratios increased both under condition 2 (HCl only) and condition 3 (H2S only), indicating that both impurities contributed to the poisoning of WGS activity (Fig. 13a). The poisoning of WGS activity in the presence of H2S was faster compared to HCl as demonstrated by the rapid increase in CO/CO2 ratio within the first 60\u202fmin of reaction (i.e. condition 2 against condition 3). The poisoning of catalyst was more pronounced in the presence of both H2S and HCl (condition 1), indicating the detrimental synergistic effect of impurities. According to Fig. 13b and c, at low concentrations of H2S and HCl, there was no change in the sizes of Ni nanoparticles of 0Fe-0.4Ni and Commercial 1. These data suggest that unlike at 2000\u202fppmv HCl in literature [37], low concentrations of H2S and HCl are unable to enhance Ni sintering, and the detrimental effect on WGS activity of Commercial 1 was most likely associated with the poisoning of catalyst surface solely via chemisorption. This could explain the increase in WGS activity of Commercial 1 with the increase in the reaction temperature from 790 to 900\u202f\u00b0C in Fig. 12 as higher temperature typically decreases chemisorption. If this hypothesis is correct and chemisorption is the main reason for the catalyst poisoning, after desorption of S and Cl species, the WGS activity of catalyst can be restored. On the other hand, if sintering causes the poisoning as observed for high concentrations of HCl (i.e. 2000\u202fppmv), the loss of WGS activity would be irreversible [37]. To test the hypothesis, the spent Commercial 1 and 0Fe-0.4Ni after 5\u202fh of naphthalene reforming at 790\u202f\u00b0C in the presence of 50\u202fppmv H2S and 300\u202fppmv HCl (denoted as Exp. 1 in Fig. 14\n) were respectively used for the subsequent 5\u202fh naphthalene reforming at 790\u202f\u00b0C in the absence of H2S and HCl (denoted as Exp. 2 in Fig. 14). According to Fig. 14a, while at the end of Exp. 1 naphthalene conversions by 0Fe-0.4Ni and Commercial 1 were both only approx. 40%, they were restored to approx. 80% by commercial 1 and approx. 85% by 0Fe-0.4Ni during Exp. 2 when H2S and HCl were removed from the gas stream. This improvement can be attributed mainly to the desorption of H2S, that has detrimental effect on the steam reforming of hydrocarbons as it was demonstrated in Section 3.2. Despite the two times increase in the catalytic activity, naphthalene conversion during Exp. 2 was still lower compared to that of the fresh catalysts utilized in the absence of H2S and HCl (i.e. approx. 100%). These data suggest that desorption of H2S was incomplete. According to Fig. 14b, for 0Fe-0.1Ni, CO/CO2 ratio during Exp.2 was similar with that during Exp.1 (i.e. 1.1) indicating that the presence of H2S and HCl had negligible effect on WGS activity of 0Fe-0.4Ni. This observation is consistent with Fig. 13a showing high stability of the WGS activity to the action of both impurities. However, after Exp. 1, CO/CO2 ratio by Commercial 1 was 4.4 and the CO/CO2 ratio drastically decreased to 2.5 during the first 30\u202fmin of Exp. 2 and remained stable for 4.5\u202fh. This value (i.e. 2.5) is comparable with the CO/CO2 value obtained for the fresh Commercial 1 utilized in the absence of H2S and HCl (i.e. 2.8), indicating that at low concentrations of impurities, the poisoning effect on the WGS catalytic activity was reversible, thus, confirming the hypothesis.The structure of catalyst played an essential role in WGS reaction but not in reforming reaction (which was strongly influenced by temperature). The stronger NiO\u2013Al2O3 interactions provided beneficial effect to catalytic activity which could be probably attributed to the formation of larger content of oxygenated Ni2+ species that serve as active sites for WGS reaction [59]. The poisoning effect of HCl and H2S on WGS was more pronounced in a catalyst with weakly bonded NiO to the Al2O3 support. At low H2S and HCl concentrations, the poisoning of WGS activity proceeds via chemisorption of S and Cl species and the loss of catalytic activity is reversible when H2S and HCl are removed from the gas stream.The effects of H2S (50\u202fppmv) and HCl (300\u202fppmv) on catalytic steam reforming of naphthalene were investigated using Ni, Ni-Fe and Fe catalysts supported on alumina at 790, 850 and 900\u202f\u00b0C. Ni had higher reforming and WGS activities compared to Fe and the activities of Ni were not significantly influenced by the addition of Fe. H2S poisoned naphthalene reforming activity of the catalysts, while the addition of 300\u202fppmv to gas stream had no effect on this reaction at 0 and 50\u202fppmv H2S. On the contrary, both HCl and H2S could poison WGS activity of the catalysts and the poisoning effect was more pronounced when both impurities were present in the gas stream. The poisoning by H2S could be only partially restored by removing H2S from the gas stream indicating the strong chemisorption of H2S on Ni. However, H2S poisoning effect could be prevented by carrying out reforming of naphthalene at higher temperatures. Specifically, the increase in temperature from 790\u202f\u00b0C to 900\u202f\u00b0C increased naphthalene conversion from approx. 40% to approx. 100%. The poisoning of WGS activity during naphthalene reforming was significantly influenced to the structure of catalyst. The stronger NiO\u2013Al2O3 interactions provided beneficial effect minimizing the loss of WGS activity. This beneficial effect could be attributed to the formation NiO-support interfaces upon reaction serving as active sites for WGS reaction. At these concentrations of H2S and HCl (i.e. 50 and 300\u202fppmv, respectively), the loss of WGS activity was reversible when H2S and HCl were removed from the gas stream.This research is supported by the National Research Foundation, Prime Minister\u2019s Office, Singapore and the National Environment Agency, Ministry of the Environment and Water Resources, Singapore, under the Waste\u2013to\u2013Energy Competitive Research Programme (WTE CRP 1501 105). The authors also acknowledge the management of Nanyang Environment and Water Research Institute and Economic Development Board, Singapore for the support.Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2018.12.119.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  H2S and HCl are common impurities in raw syngas produced during gasification of biomass and municipal solid waste. The purpose of this study was to investigate the poisoning effect of H2S and HCl on synthesized and commercial catalysts during steam reforming of naphthalene. Four synthesized catalysts with different loadings of Ni and Fe on alumina support and two commercial catalysts were selected and evaluated in a fixed bed reactor at 790, 850 and 900\u202f\u00b0C. The obtained results revealed that reforming and water-gas shift (WGS) activities of catalysts did not benefit from the Fe addition. The activities were influenced differently by H2S and HCl indicating that the reactions were catalyzed by different active sites on the nickel surface. In the presence of H2S and HCl, the poisoning of naphthalene reforming activity was caused by H2S and was not affected by HCl when both compounds were present in the gas. H2S chemisorbs on nickel surface forming NiS and decreasing the accessibility of active sites to hydrocarbons. The poisoning effect was only partially reversible. On the contrary, the poisoning of WGS activity could be caused by both H2S and HCl, and the activity could be completely restored when H2S and HCl were removed from the gas. Unlike naphthalene reforming activity, which was comparable for catalysts with similar Ni loadings, WGS activity depended on the catalyst structure and was less susceptible to poisoning by H2S and HCl in case of the catalyst with strong NiO-support interactions.\n               "}
{"full_text": "Data will be made available on request.Supplying most of the energy consumed by our society from fossil fuels is at risk of critically affecting global warming due to net CO2 emissions into the atmosphere [1]. An energy matrix transformation from the currently used net emission sources to CO2 neutrality is essential to achieve energy sustainability. An increased production of energy from renewable energy sources such as wind-, solar-, hydro-, or geopower would here be highly desired. However, several of these energy resources are highly intermittent, geographically spread, or seasonally dependent. Achieving an efficient and large-scale compatible way of storing the produced energy would be highly beneficial. In this context, ammonia emerges as a promising candidate both as a fertilizing chemical and as a potential energy vector that benefits from its high hydrogen content and easy liquefaction. Currently, ammonia is industrially produced via the Haber-Bosch process, which demands a large amount of energy and releases CO2 into the atmosphere, thus aggravating the greenhouse effect. One strategy to circumvent this is to produce ammonia in an eco-friendly process, which could be an electrochemical synthesis of ammonia from nitrogen gas using sustainably produced electricity. A severe bottleneck of electrochemical ammonia synthesis, is the low ammonia production rates of about microgram per hour per square centimeter-level, often lower than 10% Faradaic efficiencies (FEs), and stability issues. Therefore, the development of suitable materials plays an important role in mitigating such issues and achieving industrial application [2]. In this context, high-entropy alloys (HEAs) emerge as a new class of catalysts that provide unprecedented compositional diversity that hold the promise to tune reaction pathways and, thus, selectivity and rates, alongside entropic stabilization of the material.The concept of multi-component alloys with entropy stabilization came out around 2004 when two independent research groups showed that multiple-element materials containing at least five different species could be formed into a homogeneous phase [3,4]. The thermodynamically and kinetically stabilized structures of HEAs provide high fracture resistance, ductility, and physicochemical stability, thus enabling employment in harsh environments [5\u20137]. Concerning their application in catalysis, these alloys form a promising new material class and is a rapidly growing research field [5,6]. As an advantage, the multi-component form of HEAs can provide several active sites on a catalytic surface and structural stability. The complexity of the catalytic surface enables the possibility of breaking the symmetry rules imposed by the scaling relationships [8], which in principle opens the possibility of finding highly active catalysts for different reactions.It is notoriously challenging to reduce nitrogen to form ammonia electrochemically due to the scaling relationships and simultaneous ability to reduce hydrogen in a protonic system, and thus a competition between the nitrogen reduction reaction (NRR) and the hydrogen evolution reaction (HER). Although N2 is the most abundant molecule in the atmosphere, its triple bond and the lack of dipole moments make it a highly inert specie and, hence, very hard to catalyze due to the lack of nitrogen fixation on catalytic surfaces [9]. Several strategies have been reported in the literature to enhance NRR, where they all more or less are related to nitrogen fixating surfaces with either proton deficiency or electron starving (and thus low rate) [9\u201314]. Especially interesting for this work are the results reported by D. Zhang et al. [15]. They were one of the first groups reporting that HEAs could be used to reduce nitrogen, where 38.5% FE was achieved using HEA RuFeCoNiCu nanoparticles with a small size of 16\u00a0nm and signifies the promises of the approach, although the remaining challenge is to perform the reaction with NRR selectivity versus HER also at high rates.The present work focuses on two important aspects of the application of HEAs to NRR: i) a rational strategy to screen over an ample search space of quinary HEAs formed with Mo-Cr-Mn-Fe-Co-Ni-Cu-Zn working as novel catalysts for NRR, without the inclusion of Ru or other platinium group metals (PGMs), and ii) identifying relationships between HEAs intrinsic properties and their catalytic activity. The search for alternative catalysts for NRR in the wide range of compositional space found in the quinary HEAs is performed by employing the framework of the density functional theory (DFT), machine learning techniques and a probabilistic approach developed by T. A. Batchelor et al. [16]. Similar strategies were also applied, successfully, by T. A. Batchelor et al. [16], J. K. Pedersen et al. [17] and W. A. Saidi et al. [18]. They used DFT to train machines over hundreds of adsorption energies on HEAs microstates and, further, used these machines to screen for selective and active catalysts for oxygen, carbon dioxide, carbon monoxide reduction reactions and also ammonia oxidation, respectively. The estimated catalytic activities are, further correlated with intrinsic properties of the HEAs, like the average valence electron concentration and their electronegativity. This might help to understand the main properties that dictate the electrochemical transformations and it is also a simplified path to selecting promising catalysts. Finally, since a clear reaction pathway on the surface of a HEA is not possible due to their inherent randomness, a statistical analysis of the reaction pathway will be performed for the selected HEAs.The associative pathways (distal/alternating and enzymatic) are the most favorable ones when electrochemical NRR is in focus [19]. This is due to the high activation barriers to dissociating N2*\u00a0into 2\u00a0N*, that, for instance, is of the order of 1.77\u00a0eV for the case of Ru(0001) [20]. The other two likely options are the distal/alternating pathways and the enzymatic pathway [21]. Pedersen et al. [22] recently showed that species on threefold hollow sites of HEAs could partially circumvent scaling relations with the adsorption of species on top sites due to the different coordination of threefold sites compared to on top sites (also confirmed in this work). Moreover, Singh et al. [19] and Montoya et al. [23] showed that the two potential limiting steps of the NRR reaction on transition metal surfaces are the hydrogenation of N2*\u00a0forming NNH*\u00a0and the desorption of NH*\u00a0forming NH2 *\u00a0. In the distal pathway, the N2*\u00a0adsorbs on the top position while the NH*\u00a0adsorbs on the threefold hollow site. Hence, the scaling relationship between these two steps can be circumvented due to the randomness of the HEA surfaces. That allows us to seek highly active HEAs for NRR that deliver strong N2 *\u00a0bond interaction in the distal position (adsorbing on the top site), but still with lower desorption of the NH*\u00a0intermediate. Therefore, we will focus on identifying HEAs that optimize the catalytic activity and selectivity towards NRR following the distal/alternating pathway. Moreover, only for the selected catalysts, we explicitly compute, with the DFT framework, the potential limiting steps, N2(gas)+*\u2192N2*, N2*+H++e-\u2192NNH* and NH*+H++e-\u2192NH2*\u00a0for 100 microstates of the referent HEA and show how the reaction pathways can be depicted based on the statistical analysis.Tuning to the approach to characterize the reaction steps, Singh et al. [19] and Montoya et al. [23] have shown that the N*\u00a0is a descriptor of the NRR catalytic activity on transitions metals where, for the case of close-packed structures, Fe is placed on the top of the volcano curve [19]. Therefore, we can employ this descriptor to optimize HEAs elemental concentrations that maximize the local sites with similar adsorption energies as in the case of Fe, for instance. That must lead to optimal cases with reasonable potential limiting steps \u2013 at least similar to the value displayed by Fe. Moreover, it allows us to build a much more efficient screening strategy since the number of parameters is reduced. That in itself should be enough to deliver promising HEA catalysts for NRR. However, due to the break of scaling relationships, the top of the volcano curve is not completely known and, hence, including the N2*\u00a0also in the optimization process might lead to cases with even higher activity and that can also mitigate the N2 fixation issue. It is also important to highlight that the activations barrier of the NRR following the distal pathway tends to be very similar to the computed thermodynamical barriers. E. Tayyebi et al. [20] and A. B. H\u00f6skuldsson et al. [24] have shown that including activation barriers in the calculation of N2 reduction pathways leads to the same electrochemical paths predicted with thermochemistry, for Ru(0001) and W(110). Moreover, the transition states computed for small molecules like N2, are often resembling the final state of the reaction\u00a0[25]. Therefore, confirming that thermodynamical steps, in this case, can be used as an effective parameter to estimate the rates of the reaction.The approach to model the reactions and to screen over the HEA\u2019s elemental concentration pool is depicted in \nFig. 1 and based on the following steps:\n\n1.\nQuinary HEAs formed with the elements Mo-Cr-Mn-Fe-Co-Ni-Cu-Zn are randomly created. DFT calculations are performed over 1200 microstructures formed with the above-mentioned elements. For each microstructure, the adsorption energies of N2 *\u00a0and N\u00a0*\u00a0(descriptors of the reaction) are computed and stored in a database (DFT details are in the section \u201c\nDFT Calculations\u201d).\n\n\n2.\nRepresentation models of the microstructures are created and used to build neural network models that are trained on the adsorption energies from the DFT calculations. These models can compute adsorption energies almost instantaneously and, overcome the time-consuming task of performing thousands of adsorption energies with the DFT approach (details are in the section \u201c\nDeep neural networks\u201d\n).\n\n\n3.\nUsing the deep neural network models, we calculate the adsorption energy of N\u00a0*\u00a0and N2 *\u00a0(descriptors of the catalytic activity) on 2000 microstates of each of the 3000 HEAs considered here (an impossible job if DFT would be directly employed). The constraint that species concentration must be lower than 50% is used. A higher concentration of a specific species reduces the entropic effects that stabilize these catalysts. Hence, increasing the probability of structural dissociation into multiple phases, for instance. The probabilistic approach is, hence, used together with the adsorption energies to estimate catalytic activities (as described in the section \u201c\nTowards active HEAs for NRR\u201d\n) and also selectivities (as described in the section \u201c\nTowards Selectivity\u201d\n).\n\n\n4.\nInherent properties of the HEAs like the averaged valence electron concentration (VEC), averaged electronegativity (ELE) and averaged working function (WF) are correlated with the estimated activities to unravel the properties controlling the activity and selectivity.\n\n\n5.\nThe thermodynamical barriers of the potential limiting steps are calculated for 100 microstructures of a selected HEA. These are shown in the form of box plots and compared with the case of Fe (111) (details are in the section \u201c\nPotential limiting steps of the selected HEA\u201d\n).\n\n\nQuinary HEAs formed with the elements Mo-Cr-Mn-Fe-Co-Ni-Cu-Zn are randomly created. DFT calculations are performed over 1200 microstructures formed with the above-mentioned elements. For each microstructure, the adsorption energies of N2 *\u00a0and N\u00a0*\u00a0(descriptors of the reaction) are computed and stored in a database (DFT details are in the section \u201c\nDFT Calculations\u201d).Representation models of the microstructures are created and used to build neural network models that are trained on the adsorption energies from the DFT calculations. These models can compute adsorption energies almost instantaneously and, overcome the time-consuming task of performing thousands of adsorption energies with the DFT approach (details are in the section \u201c\nDeep neural networks\u201d\n).Using the deep neural network models, we calculate the adsorption energy of N\u00a0*\u00a0and N2 *\u00a0(descriptors of the catalytic activity) on 2000 microstates of each of the 3000 HEAs considered here (an impossible job if DFT would be directly employed). The constraint that species concentration must be lower than 50% is used. A higher concentration of a specific species reduces the entropic effects that stabilize these catalysts. Hence, increasing the probability of structural dissociation into multiple phases, for instance. The probabilistic approach is, hence, used together with the adsorption energies to estimate catalytic activities (as described in the section \u201c\nTowards active HEAs for NRR\u201d\n) and also selectivities (as described in the section \u201c\nTowards Selectivity\u201d\n).Inherent properties of the HEAs like the averaged valence electron concentration (VEC), averaged electronegativity (ELE) and averaged working function (WF) are correlated with the estimated activities to unravel the properties controlling the activity and selectivity.The thermodynamical barriers of the potential limiting steps are calculated for 100 microstructures of a selected HEA. These are shown in the form of box plots and compared with the case of Fe (111) (details are in the section \u201c\nPotential limiting steps of the selected HEA\u201d\n).\n\nDFT calculations:\n The projected augmented wave method was used to solve the Kohn-Sham equations implemented in the Vienna ab initio Simulation Package (VASP) [26,27]. The wave functions were expanded using plane waves with a cutoff energy of 400\u2009eV while a (4\u00d74\u00d71) k-point mesh was used to sample over the Brillouin zone. Smearing of 0.2\u2009eV was employed to obtain partial occupations using the Methfessel-Paxton scheme of second order. Spin-polarized orbitals were used in the ferromagnetic (FM) state and the Bayesian error estimation functional with van der Waals correlation (BEEF-vdW) [28] was utilized to describe the Kohn-Sham Hamiltonian\u2019s exchange and correlation term. The BEEF-vdW has been reported to be one of the most accurate functionals to describe adsorption energies on transition metal surfaces [29,30], and is the approach chosen for this study. The structural models were built into a 2\u00d72\u00d74 face-centered cubic (FCC) (111) slab with a vacuum of 20\u2009\u00c5 to avoid interaction amongst periodic images, allowing the two topmost layers to geometrically relax. In contrast, the two bottom layers were fixed to the optimized bulk structure. Atoms positions were optimized until a maximum force of 0.08\u2009eV/\u00c5 was obtained. It is common for calculation where single atoms are used forces convergence of the order of 0.03\u20130.01\u2009eV/\u00c5. However, the randomness of the HEAs adds complexity and lower convergence parameters need to be used. Other references have also employed this value [16,17]. Moreover, we tested for one case of N\u2009*\u2009with force convergence of 0.03\u2009eV/\u00c5. We got a difference of 0.02\u2009eV in the adsorption energy. Lattice parameters of the slabs were set on a weighted average basis and assuming species has FCC bulk structures, similar to the work of T. A. Batchelor et al. [16]. Moreover, Clausen et al. [31] showed that possible remaining strain effects on the adsorption energy of small molecules are alleviated by the inherent distortion of the lattice in HEAs. Bulk optimizations were performed with a k-point mesh of 15\u00d715\u00d715 in an FCC structure, and the obtained lattice parameters are summarized in Table S1.\n\nDeep neural network:\n Although the values of ALPHA could be estimated under any first-principle approach, the high amount of possible microstructures makes the calculation of the N and N2 adsorption energies non-feasible from a computational time point of view. To circumvent this issue, a representation model of the microstructures that enables establishing a deep neural network (DNN) model permitting the computation of N and N2 adsorption energies almost instantaneously was built and used together with the DFT calculations. Though the DFT calculations were performed on 1200 microstructures, after data cleaning, the DNN was trained using 784 adsorption energies for N atoms sited on the hexagonal-close-packed sites. We have shown that N adsorbs strongly on this site (see Fig. S2), which also corroborates with the results of W. A. Saidi et al. [18]. For the case of N2, 784 adsorption energies of the N2 molecule on randomly created slabs were used to train the DNN model. The representation used to feed the DNN involves the specification of four regions of the HEAs microstructures and frequency counting of species on each specific region (\nFig. 2). These regions are then concatenated into a vector defining a regression problem, \n\n\u2206\n\n\nE\n\n\nN\n,\nN\n2\n\n\n=\n\n\n\u2211\n\n\np\n\n\nR\n\n\n\n\n\n\u2211\n\n\nk\n\n\nmetals\n\n\n\n\n\nC\n\n\np\n,\nk\n\n\n\n\nN\n\n\np\n,\nk\n\n\ni\n\n\n\n\n\n, where \n\n\nN\n\n\np\n,\nk\n\n\ni\n\n\n is the number of atoms of specie k in the region p and R is the total number of regions, solved with the DNN. Each built vector represents one microstructure of a HEA of a specific concentration.The DNNs were built using the Keras library [32]. The data was trained in several networks where the best models were composed of dense sequential layers. The input layers were set with two hundred neurons and a linear activation function for the N adsorption energy and one hundred neurons together with a linear activation function for the dinitrogen adsorption energy. Six hidden layers composed of two hundred neurons each and a \u201crelu\u201d activation function were employed for the N adsorption training, while for the N2 adsorption training, one layer with 50 neurons and a \u201crelu\u201d activation function (L2 regularization function were employed in both cases). The output layers were built with a linear function. Loss function (mean squared error, MSE) between predicted adsorption energies and DFT computed adsorption energies were minimized using an Adam optimizer with a learning rate of 0.014. Our dataset utilized to build the DNN was randomly divided into a training set (80%) and test set (20%) for both N and N2 adsorption energies. The evolution of the loss function with the epoch number (training steps) is shown in Fig. S4 and confirms no overfitting phenomena.\n\nTowards active HEAs for NRR:\n A probabilistic approach based on the adsorption of N2 molecules and N atoms on the HEAs surface is employed to estimate the catalytic activity of HEAs. The basic principles of the approach were firstly proposed by T. A. Batchelor et al. [16] and are here further expounded on and extended to the use in NRR. The first assumption is that bonds formed between small molecules and catalytic surfaces have a local character, hence, are determined by the microstructure of the local site. This means that the vast composition of the HEA can be approached as an average over microstructures of the HEA, where each microstructure will contribute to the activity in a specific way, depending on the adsorption energy of N in the respective site. This is an approximation that concurs with the experimental situation with a random mixing and atomic dispersion in a real HEA. Moreover, the N2 fixation issue is accounted for by introducing the N2 adsorptions energy in the model. In a technical sense, to maximize the number of randomly created sites that deliver: i) Adsorption energies of N in between the obtained values for Fe and Ru. Ru has been proven to show activity towards NRR [33\u201335], while Fe is a known to be an efficient electrocatalyst for ammonia production [36,37] and appears at the top of the volcano plot \u2013 lower limit potential step [19]. Therefore, maximizing microstructures with similar N adsorption energy should ensure high catalytic activity for the specific HEA towards NRR. ii) To identify the number of sites that adsorbs N2 exothermically \u2013 better N2 fixation. These assumptions can be formulated into a probabilistic approach with:\n\n(1)\n\n\nP\n(\n\n\nN\n\n\n2\n\n\n)\n=\n\n\n\n\u2211\n\n\nMicrostructures\n\nwith\n\n\n\n\u2206\nE\n\n\n\n\nN\n\n\n2\n\n\n\n\n<\n\u2212\n0.5\neV\n\n\n\n\n\u2211\n\n\nAll\n\nconsidered\n\nMicrostructures\n\n\n\n\n\n\n\n\n\n(2)\n\n\nP\n(\nN\n)\n=\n\n\n\n\u2211\n\n\nMicrostructures\n\nwith\n\n\n\u2206\nE\n\n\nN\n\n\n(\nFe\n)\n<\n\n\n\u2206\nE\n\n\nN\n\n\n<\n\n\n\u2206\nE\n\n\nN\n\n\n(\nRu\n)\n\n\n\n\n\u2211\n\n\nAll\n\nconsidered\n\nMicrostructures\n\n\n\n\n\n\n\n\n\n(3)\n\n\nALPHA\n=\nP\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\u00d7\nP\n(\nN\n)\n\n\n\nwhere \n\nP\n(\n\n\nN\n\n\n2\n\n\n)\n\n is the probability of finding sites with exothermic adsorption for N2, \n\nP\n(\nN\n)\n\n is the probability of finding microstructures with energy between \u2206EN(Fe) and \u2206EN(Ru) ALPHA is the probability of the two events happening. A HEA with high ALPHA should thus deliver high activity, while each microstructure is randomly created with the constraint that its species concentration reassembles a specific HEA concentration.The Gibbs free energy variation of the reaction N2 +\u2009* \u2192 N2 *\u2009can be calculated as \n\n\u2206\nG\n=\n\u2206\nE\n+\n\u2206\nZPE\n+\n\n\n\n\u2206\n\n\nH\n\n\nvib\n\n\n+\n\u2206\n\n\nH\n\n\nrot\n\n\n+\n\u2206\n\n\nH\n\n\ntrans\n\n\n\n\n\n\u2212\nT\n\n\n\n\u2206\n\n\nS\n\n\nvib\n\n\n+\n\u2206\n\n\nS\n\n\nrot\n\n\n+\n\u2206\n\n\nS\n\n\ntrans\n\n\n\n\n\n\n where \n\n\u2206\nZPE\n\n is the variation on the zero-point energy, \n\n\u2206\nH\n\n and \n\n\u2206\nS\n\n are the variations of enthalpy and entropy, respectively, and \n\n\u2206\nE\n\n is the electronic energy change. To further validate and assess the parameters, we have suggested that a complete vibrational frequencies calculation is performed using DFT to assess the thermal effects. Performing this for conformations in the set, we find that\n\n(4)\n\n\n\u2206\nZPE\n+\n\n\n\n\u2206\n\n\nH\n\n\nvib\n\n\n+\n\u2206\n\n\nH\n\n\nrot\n\n\n+\n\u2206\n\n\nH\n\n\ntrans\n\n\n\n\n\n\u2212\nT\n\n\n\n\u2206\n\n\nS\n\n\nvib\n\n\n+\n\u2206\n\n\nS\n\n\nrot\n\n\n+\n\u2206\n\n\nS\n\n\ntrans\n\n\n\n\n\n\u2248\n0.5\n eV\n\n\n\n\nTherefore, we can estimate that \n\n\u2206\nG\n\n will be exothermic only when \n\n\u2206\nE\n\n is lower than \u2212\u20090.5\u2009eV, which is settled as a limit in Eq. 1. This condition for activity towards NRR is also found in the work of C. Ling et al. [38]. For other reactions, the analogous assessment of thermal effects is required with the corresponding change in Eq. 1.The adsorption energies of N atoms and N2 molecules on the HEAs microstructures were calculated as\n\n(5)\n\n\n\u2206\nE\n\n\n\nN\n\n\n\n=\n\n\nE\n\n\nN\n\n\n\u2212\n\n\nE\n\n\n*\n\n\n\u2212\n\n\n\n\nN\n\n\n2\n\n\ngas\n\u2212\nphase\n\n\n\n\n2\n\n\n\nand \n\u2206\nE\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n=\n\n\nE\n\n\n\n\nN\n\n\n2\n\n\n\n\n\u2212\n\n\nE\n\n\n*\n\n\n\u2212\n\n\nN\n\n\n2\n\n\ngas\n\u2212\nphase\n\n\n\n\n\n\nThe calculation of the first constraint used for the N adsorption, \n\n\u2206\nE\n\n\n\nFe\n\n\n\n\n, was performed considering 2\u00d72x5 BCC slabs on the (110) and (100) directions where results were \u2212\u20091.1\u2009eV and \u2212\u20090.88\u2009eV, respectively. \n\n\u2206\nE\n\n\n\nRu\n\n\n\n\n was calculated using a HCP structure in the (001) direction resulting in adsorption of \u2212\u20090.78\u2009eV. The recent work by Megha Anand et al. [37] highlights that the best NRR catalyst Ru, is followed by Fe in terms of effectiveness in catalytic activity. Instead of using the exact values of Fe and Ru adsorption energies as the constraints in Eq. 2, we set those to be \u2212\u20090.7\u2009eV and \u2212\u20090.9\u2009eV, therefore slightly shifted towards the Ru instead of Fe. If other reactions are targeted, with key rate-limiting steps in the adsorption energies, adjustments of the targeted training parameters are required and can also be chosen from other rate-limiting parameters without loss of generality.\n\nTowards Selectivity:\n It is well known that most of the catalysts suffer from poor selectivity towards NRR due to the competing HER \u2013 protons being more likely to be activated and reduced on the catalytic surface than dinitrogen (N2). Selectivity can, hence, be ranked by analyzing the averaged value of \n\n\u2206\nE\n(\nH\n)\n\n and \n\n\u2206\nE\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n over the microstructures of a HEA. In a first assessment, this allows the selection of the best catalysts as the ones with more positive values of \n\n\u2206\nE\n\n\n\nH\n\n\n\n\u2212\n\u2206\nE\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n\n\n[39]. We have tested, for\u00a010 microstructures, if N2 would be adsorbed exothermically once H*\u2009atoms are on the catalytic surfaces (hydrogenated surface). Unfortunately, for all cases, N2 does not adsorb exothermically. Therefore, there is a competition between N2*\u2009adsorption and H*\u2009adsorption and, this is mitigated if the term \n\n\u2206\nE\n\n\n\nH\n\n\n\n\u2212\n\u2206\nE\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n gets more positive. Moreover, the energetics of both adsorbates, H and N atoms, scale linearly (see Fig. S1 (error in the energy of hydrogen) for details, both adsorbs on a threefold site). While the relation \n\n\u2206\nE\n\n\n\nH\n\n\n\n\u2212\n\u2206\nE\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n\n\u00a0might be of interest to assess absolute values of selectivity, the main concern is to rank the HEAs faithfully based on such parameters. To approach this, and, using the scaling relation between H and N adsorption, the energetics of the hydrogen adsorption, \n\n\u2206\nE\n\n\n\nH\n\n\n\n\n can be exchanged by the energetics of N atoms adsorption, \n\n\u2206\nE\n\n\n\nN\n\n\n\n\n, to predict selectivity and, leading to: selectivity =\u2009\n\n\u2206\nE\n\n\n\nN\n\n\n\n\u2212\n\n\u2206\nE\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n.\n\n This parameter will now be named SELE.\n\nPotential limiting steps of the selected HEA:\n For the selected catalyst, a statistical approach is employed to estimate the thermodynamical barriers of the potential limiting steps.\n\n(6)\nN2 + *\u2192N2*\n\n\n\n\n(7)\nN2* + (H++e-) \u2192NNH*\n\n\n\n\n(8)\nNH* +(H++e-) \u2192NHH*\n\n\nThe computational hydrogen electrode approach, as proposed by N\u00f8rskov\u00a0[40], was applied to model the electrochemical reactions. This approach assumes a coupled electron-proton transfer simplifying the demanding calculation of solvation energies of ionic species. The free energy variation of each electrochemical/chemical reaction was calculated for 100 microstructures of the selected HEA using DFT as:\n\n(9)\n\n\n\n\nE\n\n\nad\n\n\n=\n\n\nE\n\n\nadsorbate\n\n\n*\n\n\n\u2212\n\n\nE\n\n\n*\n\n\n\u2212\n\n\u2211\n\ni\n\n\n\n\n\n\n\nn\n\n\ni\n\n\n\u03bc\n\n\ni\n\n\n\n\n\n\nwhere \n\n\nE\n\n\nadsorbate\n\n\n*\n\n\n is the self-consistent-field (SCF) energy of the adsorbed intermediate corrected by the zero-point energy (ZPE) of the adsorbate, \n\n\nE\n\n\n*\n\n\n is the SCF energy of the pure slab and \n\n\nn\n\n\ni\n\n\n is the number of species \ni\n with chemical potential \n\n\n\u03bc\n\n\ni\n\n\n. Moreover, \n\n\n\u03bc\n\n\nH\n\n\n, \n\n\n\u03bc\n\n\nN\n\n\n are the chemical potentials of hydrogen and nitrogen, respectively, that are obtained as:\n\n(10)\n\n\n\n\n\u03bc\n\n\nH\n\n\n=\n\n\n1\n\n\n2\n\n\n\n\nE\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n\n\n\n(11)\n\n\n\n\n\u03bc\n\n\n\n\nN\n\n\n2\n\n\n\n\n=\n\n\nE\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n\n\n\n\n(12)\n\n\n\n\n\u03bc\n\n\nN\n\n\n=\n\n\n\n\nE\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n2\n\n\n\n\n\n\n\n\n\n(13)\n\n\n\n\nE\n\n\n\n\nH\n\n\n2\n\n\n,\n\n\nN\n\n\n2\n\n\n\n\n=\n\n\nE\n\n\nscf\n\n\n+\nZPE\n+\n\n\n\n\n\nH\n\n\nvib\n\n\n+\n\n\nH\n\n\ntrans\n\n\n+\n\n\nH\n\n\nrot\n\n\n\n\n\n\n\n\n\n\n\n\n\n\u2212\nT\n\n\n\n\n\nS\n\n\nvib\n\n\n+\n\n\nS\n\n\ntrans\n\n\n+\n\n\nS\n\n\nrot\n\n\n\n\n\n+\nPV\n\n\n\n\nThe usual approach to depict the energy landscape of reaction pathways on transition metal surfaces needs to be adapted to fulfill the restriction imposed by the randomness of the HEAs. Indeed, every microstate of the structure (that together resemble the HEA surface) delivers one different energetics for the concerning reaction step. Therefore, what we can get is a distribution of energies for each associated transformation. Box plots are a common tool to report the overall patterns of a group. This summarizes important information about the group as the minimum, the first quartile, the median, the third quartile and the maximum. Here, the computed thermodynamical barriers are shown as a box plot.In this section, the accuracy of the developed DNN is discussed and compared to preview data reported in the literature (See the deep neural network model section). We also performed a deeper analysis of the relations between ALPHA, SELE, and intrinsic HEAs properties, with the resulting HEA(s) activities given in the subsection Computed electrocatalytic activity (ALPHA). Finally, the most promising novel HEAs for NRR are pointed out in The selected HEAs section and its potentials limiting steps investigated.\nThe deep neural network model: The DNN employed here to estimate adsorption energies of N2 molecules and N atoms on the surface of HEAs displayed reasonable accuracy with mean absolute errors (MAEs) of 0.09\u2009eV and 0.20\u2009eV, respectively (Figs. 2a and 2b). One can note that the mean absolute error of N2 adsorption is below the typical resolution of DFT while the N adsorption shows a slightly higher error. In context to this, we would like to remind the reader that we have used the hypothesis that bond formation is a local process, and hence that the adsorption energies can be obtained by specifying elements close to adsorbates and their location. However, the model assumes symmetric adsorption sites as input parameters. This might be one of the causes of the better accuracy of the model found for N2 adsorption than the model found for N adsorption since N2 sits on the top site in a very symmetric environment (only one species is accounted for in region one, Fig. 2). On the other hand, N atoms sit on threefold HPC sites. Therefore, symmetry breaking would be observed depending on the coordinating species, leading to higher errors in the built model (three species are considered in region one for this case). Still, others have reported ML predicted MAEs of about 0.2\u2009eV regarding the DFT adsorptions energies [41], which inherently also has an error of about 0.2\u2009eV within Beef-vdW functional [28,29]. Therefore, it pays off the employment of these models in pro of a considerable gain in computational time, allowing the removal of unpromising catalysts to be experimentally processed or by DFT calculations.\nComputed electrocatalytic activity (ALPHA): ALPHA and SELE were calculated for three thousand randomly created quinary HEAs of the elements Mo, Cr, Mn, Fe, Co, Ni, Cu, and Zn. The relationship between ALPHA and SELE is displayed in \nFig. 3(a) and (b). For this task, two thousand microstates of each created HEA were considered to assess the averaged quantities for SELE and the probabilities associated with ALPHA. Higher and lower values of SELE lead to lower values of ALPHA. An optimal value is obtained when SELE is between \u2212\u20090.25 and 0.0, building a volcano-shaped relationship. Interestingly, the shape obtained for this relationship is also reproduced for ALPHA vs. averaged N adsorption energies. Hence, the averaged N adsorption energies emerge as the main influencing factor for SELE. The averaged N2 adsorption appears as an almost fixed shift in SELE since they are computed as the average of cases with adsorptions higher than 0.5\u2009eV. Hence, minimal variance is revealed when comparing distinct HEAs. As expected, the cases with higher ALPHA have averaged N adsorption energy around \u2212\u20090.75\u2009eV \u2013 the \n\n\n\n\n\u2206\nE\n\n\nN\n\n\n\n\n\nRu\n\n\n\n\n\n=\u2009\u22120.78\u2009eV since ALPHA is set to maximize the probability of sites with adsorption similar to Ru.Something interesting differentiates the case studied here from the volcano-shapes reported for NRR in the literature [23,36]. HEAs with the same averaged N adsorption (SELE) can display different activities (Fig. 2a), producing a volcano relation where data is spread inside the volcano shape. Two characterizing cases were selected for further analysis to gain insights into the obtained relationship. The first case, Mo0.38Fe0.31Co0.19Ni0.06Cu0.06, has high ALPHA of 0.14 with a SELE value of \u2212\u20090.15, while the second case, Mo0.25Cr0.06Mn0.31Cu0.06Zn0.31, has ALPHA 0.00 and SELE \u2212\u20090.15\u2009eV. Though both have similar averaged N adsorption energies of \u2212\u20090.75\u2009eV and \u2212\u20090.68\u2009eV (hence, similar SELE), their distributions are completely different (Fig. 3d). Most cases end up in the required P(N) region for Mo0.38Fe0.31Co0.19Ni0.06Cu0.06. For Mo0.25Cr0.06Mn0.31Cu0.06Zn0.31, the cases are distributed on high energy and low energy values, leading to low ALPHA, yet similar SELE. The problem faced here has a multi-dimensional character, and due to the need to get averaged quantities, information is lost, thus, explaining the filled volcano-shaped relation between ALPHA vs. SELE.Adsorption energies are widely employed to characterize activities in distinct fields of electrocatalysis [23,36,37]. Here, the direct application of the similar quantity, averaged N adsorption as the descriptor of catalytic activity of HEAs towards NRR, is shown to be insufficient to uniquely characterize each HEA, as discussed above. The plot of the averaged valence electrons in the occupied d orbitals (\n\u03b3\n) vs. the averaged adsorption energies of N and N2 brings insights into how to properly explore and develop a unique descriptor for the activity of HEAs (\nFig. 4). N and N2 adsorption strength correlate with the conventional approach to analysing the d-band center of transition metals [42]. This is in our view closely associated with the number of valence d electrons in the system and, of course, the energetic position of the states. The results displayed a close linear relation for the case of averaged N adsorption energies with R2 of 0.76 (R2 = goodness of the linear relation), but a widespread data point for the case of N2 averaged adsorption energies, R2 of 0.45. Here, calculations of N2 adsorption on the microstructures are performed on the top site. So, the value of valence electrons in the occupied d orbital of the species where N2 is adsorbed must have a much stronger influence than the averaged relation inherent in \n\u03b3\n. Hence, \n\u03b3\n alone cannot fully describe the averaged N2 adsorptions. On the other hand, this issue is mitigated in the description of averaged N adsorption by the stronger influence of the three atoms coordinating the adsorbate, threefold HCP. Hence, \n\u03b3\n results in a better descriptor for the averaged N adsorption energy. Again, information is lost when performing the averages and there exists a need to introduce a second property to better correlate the averaged adsorption and \n\u03b3\n.Electronegativity measures the electron affinity of certain elements when a covalent bond is formed. Under the assumption that electronegativity would influence the redistribution of d electrons during the bond formation between adsorbate and catalytic surface, H. Xu et al. [43] showed that electronegativity could be employed together with the number of d electrons of a species as a descriptor of the O and OH adsorption energies on single metal catalysts. Using the HEA\u2019s averaged electronegativity in the plot of N2 vs. \n\u03b3\n as a color map, one sees that thought at same \n\n\u03b3\n,\nand\n\n different averaged N2 values are observed. Moreover, these values mainly vary with the weighted-averaged electronegativity of the HEAs (we will name the weighted averaged electronegative ELE from now on). Generally, higher ELE leads to more negative values of averaged N2 adsorption, and inspecting the relationship between the averaged N2 adsorption energy vs. \n\n\u03b3\n/\nELE\n\n (Fig. S5) a better R2 of 0.58 is obtained in comparison to the previews value of R2 0.45 for averaged N2 adsorption energy vs. \n\u03b3\n. This reflects in the volcano plot (Fig. 3a), where higher activities are found for the cases with higher ELE once the probability of finding N2 adsorption exothermically increases under these circumstances.As the conventional (Pauling) electronegativity scale is defined from covalent bonding, it causes concerns in metallic alloys with domination of metallic bonding. Therefore, we have also assessed how the obtained relations behave when changing the electronegativity scale (Fig. 3a-b and S6). Clearly, no change is observed when varying from the Pauling scale to the M\u00fclliken scale defined from the arithmetic mean of the ionization energy and the electron affinity (these electronegativity measures scale linearly for the species investigated here, Fig. S7). On the other hand, no correlation between ALPHA and the electronegativity employing the Allen scale (The Allen scale is the average one-electron energy of the perceived valence shell electrons in the ground state in the free atom) is observed (Fig. S6d). Moreover, as the electronegativity could be considered a non-ideal descriptor in a metallic alloy, we explored the possibility of using the averaged work functions of the HEA as a further modification of \n\u03b3\n as a descriptor of ALPHA where the work functions are computed for the pure bulk phases and, further, weight-averaged for the HEAs. This approach displayed no correlation with ALPHA (Fig. S6c), and together with the lack of correlation with the average one-electron energy in the Allen electronegativity, one can summarize that the local environment and effects beyond single atom properties are vital in constructing a descriptor for charge transfer in-between elements and catalytic activity of the HEAs.The obtained relations indicate that ALPHAs of HEAs can be conveniently described by \nELE\n and \n\u03b3\n, properties easily assessed by knowing the HEAs composition and concentrations. The relation between activity (ALPHA) as a function of \n\u03b3\n and ELE shows that higher activities are more likely to be obtained when \n\u03b3\n is between 6 and 6.5 and ELE is higher than 1.9 (Fig. S8). It is also important to emphasize that calculations were performed here in an FCC (111) surface, and, experimentally, the HEAs phases can vary. To, somehow, capture this information, another construction for the description of the catalytic activity is constructed on the VEC of HEAs (VEC and \n\u03b3\n scales linearly, hence, similar relation with ALPHA). Sheng Guo et al. [44] showed that HEAs with VEC higher than eight must likely form FCC structures. This similarity allows the removal of cases where the HEAs come with VEC smaller than 8, increasing the probability of getting an FCC phase upon synthesis procedure (emphasis here is given to an FCC lattice just because calculations were performed with this structure). \nFig. 5 graphically shows this analysis by plotting the values of ELE vs. VEC of each HEA together with their APLHA as the color map. Higher ALPHA values are more likely to be obtained when ELE is higher than 1.9 and VEC is between 7.5 and 8.5. Hence, a map towards higher ALPHA(S) is found only using intrinsic properties of the HEAs like VEC and ELE.The selected catalysts\u2019 cases presenting ALPHA higher than 0.1 are summarized in \nTable 1. We have grouped HEAs in two sets: i) the three cases with the highest ALPHA and ii) the cases with ALPHA higher than 0.1 and, hence, ranked based on SELE. The highest ALPHA is obtained for Mo0.38Fe0.31Co0.19Ni0.06Cu0.06. On the other hand, C. J. H. Jacobsen et al. [45] have shown that MoCo is a promising catalyst for electrocatalytic ammonia production. Therefore, it is not surprising that Mo0.38Mn0.06Fe0.13Co0.38Ni0.06 and Mo0.31Mn0.06Fe0.13Co0.44Cu0.06, having balanced values between Mo and Co and minor concentrations of other species, display high activity. This also confirms the robustness of the screening strategy employed in this work.The second set of compounds ranked based on selectivity displayed Mo0.44Co0.38Ni0.06Cu0.06Zn0.06 as the best option. Again, the balanced Mo-Co ratio leads to high activity while introducing Zn, Cu, and Ni in small quantities, leads to an increased value of SELE. Furthermore, species with higher VEC (VEC \n\u221d\n\n\n\u03b3\n) present lower bond strength between N atoms and the catalytic surfaces, pushing the SELE to more positive values. Interestingly, all selected cases presented ELE close to 2 since, generally, this pushes the N2 adsorption towards more negative values, thus, resulting in higher ALPHA.Though HEAs formed from quinary components of the elements Mo, Cr, Mn, Fe, Co, Ni, Cu, and Zn were randomly created in this work, the best HEAs serving as novel catalysts for NRR are mainly formed of Mo-Fe-Co and with minor or non-quantities of other species. Moreover, at least 30% of the HEAs are made of Mo for all cases. N2 molecules on transition metal surfaces correlate with the d band center position with respect to the Fermi level of the transition metal due to the \u201cpush-pull\u201d mechanism with \u03c3-donation and \u03c0\u2009*\u2009-back donation [42]. Amongst the investigated species, Mo is the one with d band center closer to the Fermi level [46], thus, resulting in a higher probability of delivering strong N2 adsorption. This in turn increases the value of ALPHA. Aiming to confirm this hypothesis, the adsorption energy of N2 is calculated (see Fig. S3 for details) and displays the stronger adsorption on Mo as compared to other species. Therefore, Mo can be considered as the main N2 molecules fixating center on the catalytic surface of the HEA during the NRR cycling. While higher amounts of Mo (yet still inside the high entropy alloy stability zone) would probably assist in the activation of N2 molecules, but would also result in higher adsorption energies for the N atoms, scaling with the H adsorption and thus increased HER. This implies that a too strong N adsorption leads to: i) very slow rates of NRR reaction, ii) catalytic surface poisoning. To retain selectivity and not to have predominant HER reactions, the concentration of Mo has to be balanced by introducing Co and Fe species. Considerable concentrations of Cr and Mn in the HEA also deliver strong adsorption of N as compared to other species. Hence, these are not optimal options to make this balance. On the other hand, Ni, Cu, and Zn can contribute to weaker adsorption energy values.The histograms of the HEAs Mo0.38Fe0.31Co0.19Ni0.06Cu0.06 and Mo0.44Co0.38Ni0.06Cu0.06Zn0.06, cases selected as the best alternatives on the two sets presented in Table 1, are displayed in \nFig. 6. The relation between the probability of N2 adsorption is directly proportional to the concentration of Mo species on the HEA, as discussed above. Here, Mo0.38Fe0.31Co0.19Ni0.06Cu0.06 presents 42% of its active sites presenting N2 adsorption in the exothermic region while Mo0.44Co0.38Ni0.06Cu0.06Zn0.06 has 50% of the cases into the exothermic region (Fig. 6). The increment in the probability of finding exothermicity in the N2 adsorption is, here, due to the increment in Mo concentration. On the other hand, the chance of finding sites with N adsorption energy close to the obtained for Ru is smaller for the case Mo0.44Co0.38Ni0.06Cu0.06Zn0.06, 20% of the sites, in comparison to Mo0.38Fe0.31Co0.19Ni0.06Cu0.06 presenting 32% of the sites in the optimal region. Mo0.44Co0.38Ni0.06Cu0.06Zn0.06 exhibits sites with N adsorption energy as positive as 1\u2009eV, and this is due to the higher concentration of species with higher VEC like Ni, Cu, and especially Zn. While this is positive to the selectivity of the HEA pushing the average N adsorption to \u2212\u20090.57\u2009eV as compared to \u2212\u20090.76\u2009eV in Mo0.38Fe0.31Co0.19Ni0.06Cu0.06, this comes at the price of lower activity.The tendency of a HEA to form a solid solution instead of dissociating into multiple phases can be determined via either combination of (Caloric and electrochemical) experimental measurements or theoretically via quantum mechanical calculations of alloy bonding, effects of lattice entropy from mixing, and temperature effects. X. Yang et al. [47] have demonstrated that estimations can be obtained via empirical data that estimates atomic sizes, formation enthalpy and configurational entropy. When the terms \n\n\u03b4\n=\n\n\n\n\u2211\n\ni\n=\n1\n\n\nN\n\n\n\n\n\nC\n\n\ni\n\n\n\n\n\n\n1\n\u2212\n\n\n\n\nr\n\n\ni\n\n\n\n\n\n\nr\n\n\nave\n\n\n\n\n\n\n\n\n2\n\n\n\n\n\n\u2264\n6.6\n%\n\n and \n\n\u03a9\n=\n\n\n\n\nT\n\n\nm\n\n\n\n\n\u0394\nS\n\n\nmix\n\n\n\n\n\n\n\n\n\n\u0394\nH\n\n\nmix\n\n\n\n\n\n\n\n\u2265\n1.1\n\n the HEA might form a solid solution. Here, \n\u03b4\n is a parameter gauging the atomic size difference that depends on, \n\n\nC\n\n\ni\n\n\n, the atomic percentage of ith component, \n\n\nr\n\n\ni\n\n\n atomic radius of ith component and \n\n\nr\n\n\nave\n\n\n the averaged atomic radius. \n\u03a9\n parameter depends on the concentration weighted averaged melting temperature, T\n\nm\n, the configurational entropy \n\n\n\n\u0394\nS\n\n\nmix\n\n\n=\n\u2212\nR\n\n\u2211\n\ni\n=\n1\n\n\nN\n\n\n\n\n\nC\n\n\ni\n\n\n\n\nln\nC\n\n\ni\n\n\n\n\n and mixing enthalpy \n\n\n\n\u0394\nH\n\n\nmix\n\n\n=\n\n\u2211\n\ni\n,\nj\n\n\nN\n\n\n\n\n\nC\n\n\ni\n\n\n\n\nC\n\n\nj\n\n\n\n\n4\nH\n\n\ni\n,\nj\n\n\n\n\n where \n\n\nH\n\n\ni\n,\nj\n\n\n is the mixing enthalpy of binary alloys computed based on Miedema macroscopic model and obtained in the work of A. Takeuchi et al. [48].Apart from predicted activity, the individual concentrations of the elements and their respective atomic radius need to be taken into account also for the predicted HEAs. As such, it is a compromise to retain an entropically stabilized structure and, at the same time, change the composition to strive for higher activity without sacrificing too much of the entropic stabilization and thus increasing the risk of precipitation and phase separation for some of the elements. For all pointed HEAs the values of \n\u03b4\n are smaller than 6.6%, and values of \n\u03a9\n are higher than 1.1 (Table 1). Hence, these HEAs would likely form a solid solution as previously described in the introduction.We selected the best case in Table 1, Mo0.38Fe0.31Co0.19Ni0.06Cu0.06, to perform a comparative analysis of the thermodynamical barriers of the potential limiting steps with the case of Fe(111). The energetics of the reactions for Eqs.(6\u20138) are displayed in box plot format for the HEA and as red lies for the case of Fe(111) (\nFig. 7).The first step, the N2*\u2009adsorption, is endothermic on the Fe(111) surface, while for at least 25% of the 100 microstates of the HEA, this reaction becomes exothermic. Since, N2 capturing is one of the main issues in NRR, the existence of local sites on the HEA surface with strong N2 bonds is considered a plus for the electrochemical NRR. The activation of the N2 *\u2009is the second investigated reaction transformation. There, in the case of Fe, the thermodynamical barriers is 1.1\u2009eV. This means, based on the computational hydrogen electrode approach, that at leads a potential of 1.1\u2009V vs. RHE is needed to activate N2*\u2009and form NNH*\u2009on iron. This picture changes for the case of the HEA. There, the lowest observed case displays a thermodynamical barrier of about 0.74\u2009eV, while at least 25% of the 100 tested microstates of the HEA display barriers lower than 1\u2009eV. Finally, for the desorption of the NH*\u2009and forming NH2 *\u2009, thermodynamical barriers go from 0\u2009eV up to 0.6\u2009eV for the HEA vs. 0.4\u2009eV for the case of Fe. Certainly, considering the distal pathway, the first activation of the N2*\u2009molecule is the PLS. As demonstrated here, the randomness of the HEA surface opens up the possibility of lower PLSs as compared with the case of Fe(111) and still keeping the desorption of the NH*\u2009in a reasonable energetic value.All the above results are expected to be directly applicable for NRR in gas-phase or in H2O/N2 vapor conditions as in a gas diffusion cell, while several additional considerations have to be taken into account in a practical application in a solid-liquid reaction cell. First, one needs to consider the low N2 solubility in water (1.3\u2009\u00d710\u22123 mol/L) [13] together with the high adsorption energies of H2O*\u2009, OH*\u2009and H*\u2009on the catalytic surface that might create a water coverage, hydroxylation or hydrogen coverage in aqueous electrochemical cells depending on the conditions regarding electrolyte pH and the employed electrochemical potential. These points can limit the N2 coverages on the catalytic surface, hence, deteriorating the delivered FE and activity (besides the dominant kinetics of HER over NRR [49]). In this context, even with an optimized catalyst, values of activity and FE could be way off from the expected due to the lack of available catalytic sites for the NRR reaction to proceed. One way to mitigate such issues is to work with a gas-diffusion electrode (GDE) that increases the N2 coverage by adjusting the back N2 pressure. In conjunction with an optimized catalyst, this strategy can facilitate the activity towards NRR due to the increased N2 coverage and the suppressed H2O presence that inhibit HER and surface coverages with water or hydroxyl groups, hence, increasing FE towards NH3. Another option is the application of an aprotic electrolyte with increased N2 solubility [50,51]. This would also promote the N2 coverage and mitigate HER. Though HEAs concentration and compositions were optimized to deliver higher catalytic activity and selectivity towards NRR, most of the discussions presented here need to be carefully evaluated when an aqueous electrochemical cell is considered. As long as the underlying electronic properties of the HEA surface is consistently scaled to lower N2 and N adsorption energies upon hydroxylation of the surface, the results can be directly transferrable. However, also potential decrease of the frequency of competing N2 fixation and effects from differently induced kinks and terraces in-between different compositions would be required to behave in a scalable way compared to the flat surfaces screened here. For differences in any of these scalings, a case-to-case investigation has to be performed for the HEAs and their corresponding hydroxylated surfaces and surface Pourbaix diagrams, to evaluate and rank the most interesting HERs.Another point that has to be carefully evaluated is the selection of parameters used in this work to define what is an efficient HEA for NRR. Within the probabilistic approach, we selected HEAs compositions that maximize the number of sites endothermically adsorbing N2 and also adsorbing N atoms (descriptor of catalytic activity) with adsorption energy close to Ru, as explained in the methods section. Even though this approach leads to the identification of active HEAs towards NRR, it also selects compositions with considerably high H*\u2009. This means that the selectivity can be deteriorated and also the activity due to the low coverage of N2. Kani et al. [13] hypothesized that the most efficient catalyst for NRR, instead, would be the one with lower hydrogen adsorption H*\u2009providing lower H coverage and also lower HER activity \u2013 higher activation barrier. This option would allow the application of lower cathodic potentials without fully covering the catalytic surface with H atoms. Moreover, hydroxylation of the catalytic surface would be partially suppressed due to the destabilization of the OH groups adsorption, hence, facilitating the existence of active sites for the N2 adsorption and lower potentials would likely lead to higher current densities. In our opinion, however, this would lead to intrinsically low NRR rates due to the increment of the thermodynamical limiting step of the NRR path. Therefore, the definition of what is a highly efficient catalyst for NRR is a gray zone that depends, amongst other things, on the experimental conditions in place and which rate one wishes to achieve. The approach presented in this work is flexible, however, and can easily be modified toward desired selectivities and rates to fit experimental conditions beyond the ones proposed in this work.By employing DFT together with Machine Learning and deep neural network techniques, a screening protocol enabling a rational selection of novel catalysts for NRR was developed to search over a large compositional space of HEAs. Activities and selectivities were computed by a probabilistic approach that incorporates the adsorption of N, H, and adsorption of N2 atoms. The computed HAE(s) activities reveal a volcano-shaped relationship with Mo0.38Fe0.31Co0.19Ni0.06Cu0.06 located on the top of the volcano. Moreover, a rank based on selectivity and activity pointed to Mo0.44Co0.38Ni0.06Cu0.06Zn0.06 as an alternative option that balances activity and selectivity. We also include a critical analysis of different aspects of electronegativity in connection to the work function of the elements showing that the local composition and charge transfer are necessary to formulate key descriptors of catalytic activity. Instead, valence electron concentration of HEAs with either different energy d-states or electronegativity, forms descriptors of the catalytic activity. The approach shows a promising pathway to conveniently screen candidates for catalytic activity and selectivity for a given catalytic reaction, here exemplified by the NRR reaction. The screening disclosed and quantified existing relationships between HEAs composition and catalytic activities towards NRR that bears the promise of accelerating the search for complex NRR catalysts.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the European Union\u2019s Horizon 2020 research and innovation programme under the call H2020-LC-SC3-2020-RES-RIA in the TELEGRAM project [grant agreement No 101006941]. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) via the project SNIC 2021/5-282, and funding by the Swedish Research Council through grant agreement no. 2019-05591.\nSupplementary information is available in the online version.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2022.108027.\n\n\n\nSupplementary material.\n\n\n\n.", "descript": "\n                  A computational approach to judiciously predict high-entropy alloys (HEAs) as an efficient and sustainable material class for the electrochemical reduction of nitrogen is here presented. The approach employs density functional theory (DFT), adsorption energies of N atoms and N2 molecules as descriptors of the catalytic activity and deep neural networks. A probabilistic approach to quantifying the activity of HEA catalysts for nitrogen reduction reaction (NRR) is described, where catalyst elements and concentration are optimized to increase the probability of specific atomic arrangements on the surfaces. The approach provides key features for the effective filtering of HEA candidates without the need for time-consuming calculations. The relationships between activity and selectivity, which correlate with the averaged valence electron concentration and averaged electronegativity of the reference HEA catalyst, are analyzed in terms of sufficient interaction for sustained reactions and, at the same time, for the release of the active site. As a result, a complete list of 3000 HEAs consisting of quinary components of the elements Mo, Cr, Mn, Fe, Co, Ni, Cu, and Zn are reported together with their metrics to rank them from the most likely to the least likely active catalysts for NRR in gas diffusion electrodes, or for the case where non-aqueous electrolytes are utilized to suppress the competing hydrogen evolution reaction. Moreover, the energetic landscape of the electrochemical NRR transformations are computed and compared to the case of Fe. The study also analyses and discusses how the results would translate to liquid-solid reactions in aqueous electrochemical cells, further affected by changes in properties upon hydroxylation, oxygen, hydrogen, and water coverages.\n               "}
{"full_text": "The efficient conversion of biogas to liquid fuels (Bio-GTL) could become a key industrial process in the future bio-economy. Biogas is an attractive renewable energy source and can be produced by anaerobic digestion of organic wastes in the presence of microorganisms. Methane and carbon dioxide are the major constituents of biogas, although traces of ammonia, hydrogen, oxygen, hydrogen sulfide, water vapor and other impurities can be present depending on the feedstock used (landfill, sewage sludge, agricultural waste, etc.) [1]. Although biogas can have different uses [2], its high concentration of CO2 and CH4 makes it ideal for syngas production by dry reforming with minimal purification. Syngas is a CO and hydrogen-rich gas mixture that can be readily converted to high-performance liquid fuels by the Fischer-Tropsch process [3,4].The catalytic reforming of biogas has been widely studied for the last two decades. Currently, the main limitation to industrialize this technology is the low catalytic stability associated with coke deposition and metal sintering under reaction conditions. Owgi et al. [5] surveyed different catalytic systems for the dry reforming of methane, analyzing the effect of active metals, support materials, promoters, and preparation methods, and concluded that the design of cost-effective and stable catalysts for biogas reforming is an unsolved challenge. Recently, significant progress has been made in understanding catalysts under working conditions using in situ and operando techniques [6\u20138]. The investigation of catalysts under realistic conditions allows correlating the dynamic structural changes of the catalyst surface in the presence of reactants with catalytic performance [9], providing unique insights that could inform the rational design of better catalysts.Nickel catalysts are favored in industrial applications for their low-cost. Different strategies are used to control the nickel particle size and mitigate coke deposition, such as the use of basic supports and doping of the catalyst with alkaline metals, which enhances the metal dispersion and favors coke gasification by the reverse Boudouard reaction [10,11]. Indeed, the addition of potassium as a promoter on nickel catalysts has been extensively investigated. However, the role of potassium in Ni-based catalysts during reforming is not fully understood. Borowiecki et al. [12] proposed that the effect of potassium as a promoter is directly related to its location and chemical state on the catalyst surface. They showed that only those potassium atoms close to nickel sites promote gasification of carbonaceous deposits produced in methane cracking. Frusteri et al. [13] suggested that potassium induces an electronic effect on Ni/MgO catalysts, inhibiting the rate of coke formation, carbon nucleation, and carbon diffusion through the nickel sites. In a previous study, we reported that introducing a potassium promoter on a Ni/MgAl2O4 catalyst changed the nature of the catalytic active sites and enhanced their coking resistance [14]. We proposed that the existence of Ni-K containing phase favors the gasification of carbonaceous deposits by the reverse Boudouard reaction and reduces the sticking probability of CO/CO2 in dissociative adsorption.At present, several questions about the effect of potassium in dry reforming remain unsolved, which limit our ability to design better catalysts. Where is the potassium located on the catalyst? What properties of the material are affected? Is potassium a carbon gasifier? In this work, we investigate how the addition of potassium affects the structural properties, reducibility, and chemical state of Ni-based catalysts, as well as its relationship with the catalytic activity and stability in the dry reforming of methane. To this end, we study a series of xK-Ni/MgAl2O4 catalysts with identical amounts of nickel but different potassium loads (x between 0 and 5\u00a0wt%). The study includes a combination of in situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with mass spectrometry (MS) to investigate the structural and redox changes occurring during the dry reforming of methane.The support, a stoichiometric aluminium magnesium spinel (MgAl2O4), was synthesized by co-precipitation and treated at 900\u00a0\u00b0C for 24\u00a0h following a previously described procedure [15]. The solid obtained was labelled as MgAl.The catalysts were prepared by wet co-impregnation of potassium (KNO3) and nickel (Ni (NO3)2\u00b76H2O) salts. Adequate amounts of these salts to obtain 1, 3 or 5\u00a0wt% potassium and 10\u00a0wt% nickel loading were diluted in water and mixed with the support. Excess water was removed by roto-evaporation at 60\u00a0\u00b0C. The samples obtained were dried overnight at 100\u00a0\u00b0C (fresh catalysts) and then calcined at 550\u00a0\u00b0C for 4\u00a0h (calcined catalysts). The catalysts were designated as xK-Ni/MgAl, where x indicates 1, 3 or 5\u00a0wt% of K2O. For comparison, an unpromoted catalyst was also synthesized by impregnating the bare support with the nickel salt solution and designated as Ni/MgAl.For every characterization study, a calcined sample of catalyst from the original synthesis batch was used, except for those techniques using spent catalysts.Nitrogen adsorption-desorption experiments of the calcined catalysts were carried out in Micromeritics Tristar II instrument to evaluate their textural properties. Prior to measurement, the samples were outgassed in vacuum at 250\u00a0\u00b0C for 4\u00a0h. The specific surface area and pore volume were estimated using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models, respectively.UV\u2013Visible spectroscopy characterization of the calcined catalysts was carried out in a Shimadzu 2101 spectrometer with a diffuse reflectance accessory. The electronic spectra were recorded in the wavelength range from 190\u00a0nm to 900\u00a0nm. The bandgap energies were estimated from the intercept of the tangents to the plots of (\u03b1\u00a0*\u00a0h\u03bd)1/2 against the photon energy [16].\nIn situ X-ray diffraction and reducibility analyses of the calcined catalysts were performed in a high-temperature chamber Anton Paar HTK 1200 coupled with an X'Pert Pro Philips diffractometer equipped with Ni-filtered Cu K\u03b1 radiation (40\u00a0mA, 45\u00a0kV) and a X'Celerator detector. The powder XRD patterns were recorded with a 0.05\u00b0 step size in the 10\u00b0 to 90\u00b0 2\u03b8 range and 30\u00a0s time steps. Measurements were carried out every 100\u00a0\u00b0C in the 25\u2013900\u00a0\u00b0C temperature range while flowing 100 NmL min\u22121 of 5% H2 diluted in argon through the chamber.XPS measurements of the calcined catalysts were carried out in a SPECS spectrometer equipped with a PHOIBOS 150 MCD analyzer working at fixed pass energy of 40\u00a0eV and 0.1\u00a0eV resolution for the studied zones. Al K\u03b1 radiation (1486.6\u00a0eV) was used at 250\u00a0W and 12.5\u00a0kV. Prior to analysis, each sample was pressed into a thin disk. All XPS spectra were recorded at room temperature with the binding energy calibrated with Mg 2p at 50\u00a0eV. The samples were reduced in situ at 800\u00a0\u00b0C during 1\u00a0h in 5%H2/Ar mixture in a high-pressure treatment cell (HTHP Cell). After reduction, the evacuation of the gases was maintained overnight before the spectrum was acquired at room temperature under vacuum. The analytical chamber operates under ultra-high vacuum (10-10 mbar).\nIn situ TPR-DRIFTS analysis of the calcined catalysts was performed using a high-temperature environmental reaction chamber with ZnSe windows supported in a Praying Mantis (Harrick) optical system and coupled to a Thermo Nicolet iS50 FTIR spectrometer with MCT detector. The spectra were recorded as an average of 64 scans with 4\u00a0cm\u22121 of spectral resolution per spectrum. 80\u00a0mg of finely ground calcined catalyst was loaded in the cell for each experiment. The temperature-programmed reduction (TPR) was performed by feeding a flow of 5% H2 in Ar (50 NmL min-1) and increasing the temperature from room temperature to 750\u00a0\u00b0C at a rate of 10\u00a0\u00b0C\u00a0min-1. The spectra were recorded in continuous series mode using the OMNIC 9.1 software and the temperature was simultaneously monitored using a software written in Labview. The effluent gases were analyzed on line by mass spectrometry (PFEIFFER Vacuum Prisma Plus).A quartz U-shaped reactor fitted in a homemade temperature programmed device equipped with a TCD detector was used to analyze the reduction (TPR) of the calcined catalysts. For the TPR quantitative analysis, the TCD signal was calibrated with a CuO pattern (Strem Chemicals 99.999%). Water and other condensable gases were trapped in a cryogenic bath of dry ice and acetone. An analogous experiment was performed by omitting the cold trap and substituting the TCD by an on line mass spectrometer (Pfeiffer Vacuum Prisma Plus) to monitor the gases evolved during the TPR experiment. The modelling of the TPR was performed using the optimization toolbox in MATLAB 2020a. The modelling involved the numerical optimization of kinetic parameters with the resolution of a system of ordinary differential equations (ODEs), which describes the TCD signal over time (see Supporting Information for details). Prior to modelling, experimental data were conditioned by removing the electric noise of the TCD using a loess filter. Background subtraction was applied using a piecewise cubic Hermite interpolating polynomial. The time window used in the fitting was clipped to that in which reduction processes were observed. The resolution of the system of ODEs was carried out with an adaptive Runge-Kutta algorithm. The optimization started from guess values for the parameters provided manually. The parameter values were then iteratively refined using a combination of Nelder-Mead and Levenberg-Marquardt optimization algorithms to minimize the sum of squared residuals between the experimental TPR signal and the model prediction. The confounding between fitted parameters was assessed by their standard errors, estimated by numerically computing the Jacobian of the objective function at the optimum.A cold-cathode Hitachi\u00ae S4800 SEM-FEG microscope was used for SEM analysis of reduced (800 \u00baC during 1\u00a0h in 5% v/v H2/N2) and spent catalysts. High resolution transmission electron microscopy (HR-TEM) micrographs of reduced catalysts were taken with a Talos\u00ae F200S FETEM microscope operated at 200\u00a0kV. At least 200 particles were measured to assess the diameter of nickel particles. The volume-surface average diameter was estimated assuming a spherical morphology with the following expression [17]:\n\n(1)\n\n\n\n\nD\n\n\np\n\n\n=\n\n\n\n\n\u2211\n\n\n\n\nn\n\n\ni\n\n\n\n\nd\n\n\ni\n\n\n3\n\n\n\n\n\n\n\u2211\n\n\n\n\nn\n\n\ni\n\n\n\n\nd\n\n\ni\n\n\n2\n\n\n\n\n\n\n\n\nwhere n\n\ni\n represents the number of particles with diameter d\n\ni\n.The nickel exposed surface area and the metallic dispersion of the catalysts were estimated by H2 chemisorption pulses based on the quantity of hydrogen chemisorbed over nickel surface atoms. The experiments were performed in a U-shape quartz reactor loaded with 800\u2009mg of sample. Firstly, the sample was reduced in situ at 800\u2009\u00b0C under 50 NmL min-1 of 5% H2/Ar for 1\u2009h, and then the temperature was decreased to 60\u2009\u00b0C in Ar flow. At this temperature, calibrated volume pulses of 250\u2009\u03bcL of pure hydrogen were successively introduced until saturation was achieved, indicating that hydrogen species covered the nickel metallic surface thoroughly. The pulses were monitored from the H2 signal (m/z= 2) by mass spectrometry (PFEIFFER Vacuum Prisma Plus).Temperature-programmed oxidation (TPO) experiments were carried out using approximately 50\u2009mg of spent catalysts to analyze the carbon deposits formed in the reforming reactions. The same equipment was used as in the TPR measurements. Samples were heated from room temperature to 900\u2009\u00b0C with a rate of 10\u2009\u00b0C\u2009min-1 under 50\u2009mL\u2009min-1 SPT of O2/He (10% v/v) flow. The signals of CO2 (m/z= 44), O2 (m/z= 32), and CO (m/z= 28) were followed on line by mass spectrometry in a PFEIFFER MS Vacuum Prisma Plus.A dispersive Horiba Jobin Yvon LabRam HR800 Confocal Raman microscope using a green laser (532.14\u2009nm) working at 5\u2009mW and with a 600 grooves mm-1 grating was used to record the Raman spectra of spent catalysts. A 50x objective (Olympus) was used in the microscope with a confocal pinhole of 1000\u2009\u00b5m.Powder X-ray diffraction (XRD) analysis of spent catalysts was performed on a Siemens D-500 diffractometer using a Ni-filtered Cu K\u03b1 radiation at 40\u2009mA and 45\u2009kV. The diffraction patterns were recorded in the 2\u03b8 range from 10\u201390\u00b0 using 0.05\u00b0 step size and 300\u2009s time steps.The catalytic activity in the dry reforming of methane (DRM) reaction was evaluated at atmospheric pressure in a Microactivity Reference (PID Eng & Tech) reactor coupled to a microGC (Varian 4900) equipped with Porapak Q and MS-5A columns and TCD detectors. A fixed-bed tubular reactor of 9\u2009mm inner diameter made of Hastelloy was used for all the experiments. Prior to each test, 200\u2009mg of non-diluted calcined catalyst (sieved to 100\u2013200\u2009\u00b5m) was reduced at 800\u2009\u00b0C for 1\u2009h in 100 NmL min-1 of 50% H2/N2. All catalytic tests were carried out at 30 NL g-1 h-1 space velocity using a molar ratio CH4/CO2 =\u20091 at 650\u2009\u00b0C for 48\u2009h. Experiments were performed at three different partial pressures of 20, 30 and 40 kPa for each reactant, using nitrogen to balance the total pressure at 100 kPa.Potassium addition leads to a change in the textural properties of the Ni-based materials prepared. Fig. S1 summarizes the textural properties of the materials after calcination. The mesoporous support presents a monomodal pore size distribution peaking at ~20\u2009nm, which remains unchanged upon nickel impregnation. The observed BET specific surface area (SBET) for the support is 83\u2009m2 g-1. After nickel incorporation, the specific surface area decreases slightly to 71\u2009m2 g-1, suggesting that nickel gets well distributed throughout the pores of the support. The surface area strongly decreases upon adding potassium to the catalyst (Table S1). This may indicate that, impregnation with potassium leads to reconstruction of the support surface, probably with formation of strong basic sites [18].\n\nFig. 1A shows the UV\u2013Vis spectra of the calcined catalysts in the 200\u2013800\u2009nm region. It contains bands of different widths, corresponding to different nickel-support interactions. The undoped Ni/MgAl material presents a broad featureless band centered at ~250\u2009nm, which is associated to O2p\u2192Al3sp electronic transitions [19]. When increasing the K content above 1\u2009wt%, this band splits in two at 252 and 212\u2009nm, respectively. The splitting strongly suggests that potassium stimulates spinel inversion, modifying the acid-base properties of the support. Spinel inversion is the partial interconversion of A and B sites in the spinel structure, giving rise to oxygen vacancies. The simultaneous addition of K and Ni precursor salts to the spinel support in the synthesis is necessary for Ni ions to participate in the defect creation process. The absorption spectrum also suggests that some nickel ions are incorporated in the spinel structure. Both doped- and undoped-Ni materials present broad bands at 380, 407, 650 and 737\u2009nm, likely associated with Ni2+ ions in tetrahedral and octahedral coordination sites [20,2122]. The low energy bands at 652 and 737\u2009nm (ascribed to \n3\n\nT\n\n1\n\n(F)\u2192\n\n3\n\nT\n\n2\n\n(P) and \n3\n\nT\n\n1\n\n(F)\u2192\n\n1\n\nT\n\n2\n\n,\n\n1\n\nE(D) transitions, respectively) are indicative of tetracoordinate nickel species. This points to the incorporation of nickel ions in the spinel structure (NiAl2O4), since only hexacoordinated Ni2+ ions can be accommodated in the rock salt-like structure of NiO. On the other hand, hexacoordinated Ni2+ ions in NiO or Ni1\u2212xMgxAl2O4 may account for the bands at 380 and 407\u2009nm. The NiO UV\u2013Vis absorption spectrum is dominated by the valence band (VB) to conduction band (CB) transition at 355\u2009nm, whereas the d-d transition in the visible region is hardly visible [23], in agreement with absorption coefficients previously reported (Table S2). The small shoulder at 380\u2009nm is associated with trapped holes associated with Ni2+ species.The deposition of Ni and K introduces defects (oxygen vacancies) in the spinel support. It was reported that the MgAl2O4 indirect bandgap is 7.8\u2009eV. However, the presence of oxygen vacancies results in interband energy states and an electron transition at 4.75\u2009eV (260\u2009nm) [24]. Solid non-stoichiometry, disorder or impurities may result in complex defects, including the generation of interstitial oxygen atoms and spinel inversion. These defects originate an optical transition at 5.3\u2009eV (230\u2009nm) [25]. Fig. 1B indicates the estimated band gap values for all the calcined catalysts. Notably, the band gap decreases from 3.37\u2009eV in Ni/MgAl sample to 3.24\u2009eV in the case of 5\u2009K-Ni/MgAl. This indicates that potassium doping favors the formation of oxygen vacancies. Wrobel et al. [26] demonstrated that potassium may be incorporated in NiO nanostructures, resulting in hole-doped materials. The presence of defects may result in coordination numbers below six for nickel ions: tetracoordinated [NiO4] polyhedral units (square-planar in K2NiO2 and tetragonal in K9Ni2O7) have been reported, the latter also containing Ni3+ ions [27]. Laporte forbidden d-d transitions of Ni2+ ions in K2NiO2, characterized by a broad band at ~625\u2009nm [28], cannot be excluded.The materials prepared may change when they are used under the high temperature and reducing conditions of a dry reforming reactor. To investigate this, in situ X-ray diffraction experiments were conducted while exposing the catalysts to increasing temperature under a reducing flow of 5% H2/Ar (\nFig. 2). The results indicate that the phases present initially in the materials are spinel phases (MgAl2O4 and likely some NiAl2O4), nickel oxide (NiO with bunsenite structure), and traces of aluminium oxide. In addition, potassium-rich samples (3\u2009wt% and 5\u2009wt%) also present some potassium nitrate (KNO3). The small amounts of potassium nitrate present in the starting materials are rapidly reduced at 400\u2013500\u2009\u00b0C (Fig. 2). Notice that these nitrate species had not decomposed during the oxidative calcination in the preparation of the materials. Virtually no K2O was detected in any measurement.NiO reduction starts in all samples at 600\u2009\u00b0C, which is accompanied by an increase in reflections characteristic of metallic nickel at 2\u03b8 values of 44.5\u00b0 and 51.8\u00b0 (Fig. 2). The potassium loading does not seem to affect the onset temperature for the NiO reduction process. However, as the temperature is increased, the Ni peaks become sharper and more intense in the 3\u2009K-Ni/MgAl and 5\u2009K-Ni/MgAl. This suggests that metallic nickel sintering may be stimulated by high potassium loading. In agreement with this, Mross reported that the incorporation of alkali ions in the NiO lattice diminishes the activation energy of recrystallization and provokes the sintering of the metal particles [29]. Also, El-Shobaky et al. [30] observed that the incorporation of monovalent ions in the NiO lattice induces the formation defects, facilitating the diffusion of ions in the outermost surface layers and the agglomeration of nickel particles.Potassium may favor the mobilization of species from the spinel at high temperature. As shown in Fig. 2, in the samples with high loads of potassium, the diffraction peak at 2\u03b8 =\u200942.7\u00b0, associated with NiO/MgO phases, persists even at 800\u2013900\u2009\u00b0C. This suggests that the incorporation of potassium in the catalyst may promote the migration of Mg2+ and Ni2+ ions from the bulk spinel lattice to the outer surface layers. We believe that, at temperatures above 600\u2009\u00b0C, potassium interacts with nickel particles and helps with the reduction and mobilization of NiAl2O4 and MgAl2O4, leading to higher final amounts of metallic Ni and NiO/MgO oxides. Also note that, in the starting materials, the relative amount of NiO also increases with the potassium loading, suggesting that potassium may exert a similar mobilizing effect under the oxidant calcination conditions.Remarkably, in potassium-rich materials (3\u2009wt% and 5\u2009wt%), a new crystalline phase is detected under high-temperature, reducing conditions. As the potassium loading is increased, a new diffraction peak appears at 32.8\u00b0 at temperatures above 700\u2009\u00b0C. The diffractograms at high temperature did not show any crystalline diffraction related to known potassium species, suggesting that K+ cations could have entered the NiO crystal lattice to form a new Ni-O-K structural phase. In more detail, \nFig. 3 shows the diffractograms recorded at 800\u2009\u00b0C during in situ reduction for all catalysts. All the samples lack the peaks expected for the (220), (311), (222), (200), and (400) reflection planes of the MgAl2O4 spinel lattice. Moreover, the (111) reflection of well-crystallized metallic nickel (2\u03b8 = 44.5\u00b0) is observed in all samples. High potassium loads (3\u2009wt% and 5\u2009wt%) lead to a new diffraction peak at an angle of 32.8\u00b0. The assignment of this peak is unclear: it might be attributed to the formation of K-Al-O phases or K-doped MgAl2O4 spinel. However, the large ionic radius of potassium would hamper the formation of this type of K-Al-O phases. Considering the UV\u2013Vis analysis discussed above, we tentatively assign these peaks to the formation of nickel potassium oxide composite layers (Ni-O-K).Praliaud et al. [31] suggested that K is mainly present in the K+ form and that a Ni-O-K surface complex is formed on the surface of nickel particles. The addition of alkali metals on the NiO surface has been extensively studied, and the formation of Ni3+ oxidation states in these phases has been proposed to occur by the following reaction:\n\n(2)\nK2O2 + 2 NiO \u2192 2 KNiO2\n\n\n\nThe formation of potassium nickelate phases leads to the stabilization of nickel in the trivalent formal oxidation state. Kim et al. [32] described the KNiO2 structure illustrated in Fig. 3. Interestingly, Ni3+ occupies pyramidal sites located between potassium layers with an adequate K-K distance to accommodate the nickel cations, minimizing the electrostatic repulsions between potassium ions. Our results suggest that a new phase, designated as Ni-O-K, is formed during the reduction treatment, which may be compatible with proposals like KNiO2.XPS measurements were conducted to investigate the electronic state of nickel both in the presence and absence of potassium and confirm the existence of a new phase designated as Ni-O-K that contains nickel in state trivalent. \nFig. 4 A includes the Ni 2p3/2 spectra recorded for both Ni/MgAl and 5\u2009K-Ni/MgAl samples without treatment and after in situ reduction at 800\u2009\u00b0C. The two samples show a main peak at 854.1\u2009eV with the corresponding shake-up satellite peak at 860.2\u2009eV. These binding energies are typical of nickel oxide (NiO) disperse on the catalyst surface [33]. Apparently, both unpromoted and K-doped samples present similar Ni 2p3/2 spectra. However, after reduction treatment, notable changes become visible in both samples. XPS analysis of the reduced Ni/MgAl sample indicates the formation of metallic nickel (850.3\u2009eV) and the presence of a weak peak at 857.1\u2009eV, which is related to Ni2+ cations in NiAl2O4 spinel. This high binding energy stems from the strong metal-support interaction [34]. By contrast, the reduced 5\u2009K-Ni/MgAl displays a broad peak at 857.1\u2009eV, while the peak assigned to Ni0 is very faint. The broad peak may be affected by the complex main line splitting due to multiplet contributions in oxides, although its high binding energy and broadening clearly indicate that the oxidation state of nickel is formally Ni3+\n[35,36]. Different authors have postulated that potassium doping inhibits the reduction of nickel and shifts the binding energy to higher values due to the presence of ionic potassium and the formation of Ni-O-K complexes [31]. Carley et al. [37] investigated the interaction between potassium and nickel single-crystal (100) surface by XPS measurements, evidencing the formation of Ni3+ unambiguously after annealing the surface at 600\u2009K. The authors suggested the formation of species between nickel and potassium where the chemical state of nickel is formally +\u20093. These observations are in good agreement with our results.The contribution from multiplet splitting, satellite peaks and plasmon loss structures often complicates the interpretation of XPS results, particularly for nickel species in different surface environments. Biesinger et al. [38] reported that additional insights can be obtained from the Ni LMM Auger peak-shape. Fig. 4B shows the Ni LMM Auger spectra for the fresh and reduced samples. The peak-shapes observed in both fresh Ni/MgAl and 5\u2009K-Ni/MgAl samples are typical of NiO species, while the reduced Ni/MgAl sample presents a LMM Auger peak-shape characteristic of metallic Ni. By contrast, the reduced 5\u2009K-Ni/MgAl sample shows a Ni LMM Auger spectrum with a significant broad peak which can be fitted to NiOOH oxyhydroxides (i.e. Ni3+), constituted by stacking faults with intercalated alkali cations [38,39]. Moreover, the confirmation of formation of a Ni-O-K oxide phase was also verified from the XPS spectra recorded in the K2p region for both fresh and reduced 5\u2009K-Ni/MgAl samples. As illustrated in Fig. S2, the deconvolution of the K2p XPS spectra show that the reduced sample has two different potassium phases. This observation can be directly related to the formation of the Ni-O-K layer.Based on these observations, we suggest that potassium interacts with nickel surface particles forming a core surrounded by a Ni-O-K phase (Ni@Ni-O-K). This phase likely presents an alkali-nickelate-type structure with nickel is stabilized in oxidation state +\u20093. In previous work, we demonstrated that CO is hardly adsorbed on K-promoted nickel catalysts [14]. Ni-O-K sites are accessible to hydrogen adsorption but not to CO adsorption. As discussed below, the importance of these sites in coke gasification is crucial for developing more stable dry reforming catalysts.To gain additional insights on the origin of the new Ni-O-K phase, we conducted in situ DRIFT spectroscopy during a reduction experiment. \nFig. 5 shows the evolution of the IR spectra recorded during the H2-TPR reaction from room temperature to 750\u2009\u00b0C for both Ni/MgAl and 5\u2009K-Ni/MgAl samples, respectively. It also displays the evolution of the main gaseous species followed by MS as a function of time-on-stream and temperature for both samples.With regards to the unpromoted Ni/MgAl sample (Fig. 5a), a complex set of bands attributable to polydentate (1510\u20131407\u2009cm-1), bidentate (1560\u20131360\u2009cm-1), and monodentate (1547\u20131373\u2009cm-1) carbonate species were initially detected in the 1600\u20131300\u2009cm-1 region [14,40]. The bands at 1637\u2009cm-1 and 3467\u2009cm-1 are characteristic of physisorbed water. Early in the TPR, the release of physisorbed water leads to a band at 3730\u2009cm-1, characteristic of isolated hydroxyl species bound to tetrahedral Mg2+ cations on the MgAl2O4 surface [41]. The thermal stability of carbonate species increases from monodentate to polydentate species, and the most thermostable carbonates are only fully removed above 500\u2009\u00b0C (Fig. 5b). All carbonate surface species were entirely removed by 750\u2009\u00b0C. In agreement with this, the products detected by MS (Fig. 5c) include the release of H2O and CO2 in two steps at approximately 200 and 400\u2009\u00b0C, suggesting the desorption of two types of labile carbonates. Above 500\u2009\u00b0C, reduction of the most thermostable carbonate-like species was clearly accompanied by the formation of CH4 and CO and the consumption of hydrogen.On the other hand, the IR evolution for the 5\u2009K-Ni/MgAl sample was significantly different to that of the unpromoted sample (Fig. 5d). Firstly, the spectra of 5\u2009K-Ni/MgAl show that potassium addition completely neutralizes the hydroxyl surface species, in agreement with our previous work [14]. Consequently, multiple overlapping absorption bands were detected in the 1800\u20131200\u2009cm-1 range at room temperature. At temperatures above 600\u2009\u00b0C, most of these bands disappear and only the most thermostable species remain on the surface (Fig. 5e). These correspond to bulk polydentate or highly ionic carbonate species (1720\u20131407\u20131444\u2009cm-1), although monodentate carbonate species formed on very strong basic Mg-O-K sites (1584\u20131323\u2009cm-1) are also thermally stable [14,41,42]. Note that the presence of carbonate species on the samples likely stems from contact with CO2 in air during preparation of the material (calcination), and this would be favored by the presence of basic potassium sites. On the other hand, not only labile carbonate species are removed during the reduction, but also nitrate species, observed in the 1550\u20131350\u2009cm-1 region, disappear at temperatures between 500 and 600\u2009\u00b0C. In terms of gases evolved during the reduction of 5 K-Ni/MgAl (Fig. 5f), an appreciable CO2 and H2O release is first observed without hydrogen consumption, indicating that labile carbonates are decomposed into CO2 and water below 400\u2009\u00b0C. By contrast, an important hydrogen consumption was detected between 350 and 550\u2009\u00b0C, peaking at 450\u2009\u00b0C. This hydrogen depletion matches the production of NO, CO, and H2O, corresponding to the reduction of nitrates and carbonates in this temperature range. Increasing the temperature further resulted in higher CO release, indicating that the reduction of carbonates by the reverse water gas shift reaction (RWGS) was accelerated above 600\u2009\u00b0C. Notably, with this material, CH4 was not detected at any temperature. Previous mechanistic studies suggested that adsorbed CO is an important reaction intermediate of CO2 methanation [43]. The presence of Ni-O-K sites may inhibit CO adsorption and the subsequent production of methane.A closer inspection of the IR spectra during reduction of 5\u2009K-Ni/MgAl at 250\u2013650\u2009\u00b0C (\nFig. 6) reveals the appearance of two bands at 2038 and 2158\u2009cm-1 accompanied by the simultaneous decrease of the bands associated with bidentate carbonate species (1540 and 1372\u2009cm-1) and potassium nitrate-like species (1565, 1418, 1510 and 1363\u2009cm-1) [44]. The bands at 2038\u2009cm-1, observed between 250 and 450\u2009\u00b0C, can be assigned to CO linearly adsorbed on small particles of metallic nickel, whereas the band at 2158\u2009cm-1 had been assigned to linear NCO adsorbed forming cyanate-nickel complexes species [45]. However, we consider it is more reasonable to assign this band to nitrosyl(carbonyl) complexes formed on reduced nickel sites, given the CO and NO released during the reduction. The formation of these nitrosyl(carbonyl) intermediates deserves attention since it allows to understand the formation of Ni-O-K sites.The interaction between NiO particles and adsorbed potassium species is accompanied by an initial reduction of small particles of nickel oxide to metallic nickel, which begins at temperatures below 250\u2009\u00b0C. The driving force of this step is the reduction of potassium carbonate species to generate highly stable alkali peroxide species and CO-linearly adsorbed on metallic nickel sites [37]. Under reducing conditions at 300\u2013500\u2009\u00b0C, potassium nitrate species are reduced, and a nitrosyl(carbonyl) complex is formed on reduced nickel sites. The thermal decomposition of this nitrosyl(carbonyl) intermediate occurs around 500\u2009\u00b0C releasing CO and NO simultaneously. Presumably, the formation of the formally Ni3+ oxidation state evidenced by XPS can be attributed to the reduction of the alkali peroxide previously formed in the low temperature reduction. This step occurs at temperatures above 550\u2009\u00b0C in which the oxygen of alkali peroxide is expended at high temperature to form trivalent alkali-nickelates (Ni-O-K sites). Fig. 6 sketches the plausible mechanism of formation of Ni-O-K sites. A similar process has also been proposed on the basis of FTIR studies of CO and adsorption experiments performed over analogous catalysts based on iron and potassium [46]. The authors hypothesized a similar mechanism to explain the formation of new phases (KFeO2 and similar) constituted by oxidized iron and potassium in close contact. In agreement with the XRD and XPS results above, it is reasonable to assume that reduction of 5\u2009K-Ni/MgAl catalyst at high temperature leads to the formation of an active phase composed of a metallic nickel core covered by Ni-O-K sites.The modelling of H2 temperature-programmed reduction (TPR) allows to evaluate the impact of potassium on the reducibility of the different materials. The H2-TPR profiles for the materials are shown in \nFig. 7. All samples display a broad reduction peak with multiple underlying components across the range 600\u2013900\u2009\u00b0C. We modelled the TPR process by considering three first-order reduction processes in an unsteady reactor model, resulting in a system of algebraic and ordinary differential equations (see Supporting Information for details). The resulting components fitted from the model are displayed in Fig. 7 and the corresponding parameters are included in Table S3. The three-component model recapitulates the TPR signal from the different materials very successfully and allows estimating the apparent activation energy for each reduction step (Fig. 7).Potassium interacts with nickel and enhances its reducibility in the Ni/MgAl2O4 catalysts, as shown in Fig. 7. The TPR results indicate that the different reduction processes are initiated at lower temperatures as potassium loading is increased. This points to an interaction between nickel and potassium species (Fig. 7). The intermediate temperature component \u03b2 (550\u2013750\u2009\u00b0C range) is associated with the reduction of NiO species with a moderate interaction with the support, whereas the component \u03b3 (> 750\u2009\u00b0C) is assigned to the reduction of NiOx complex species with a very strong metal-support interaction [47,48]. In fact, this high-temperature component can be associated with Ni2+ ions migrated into the MgAl2O4 matrix forming the non-stoichiometric nickel aluminate spinel (NiAl2O4) phase discussed above. Notably, the introduction of potassium above 3\u2009wt% significantly increases the activation energy for the reduction of the \u03b3 component, indicating that potassium interacts with the NiAl2O4 phase and makes its reduction more temperature sensitive. The results also support the existence of a weaker interaction between K and NiO, with the activation energy for the reduction of NiO increasing slightly with K loading.On the other hand, a reduction component \u03b1 is observed below 500\u2009\u00b0C, and its area under the curve notably increases with the potassium loading. Some authors have ascribed this reduction peak to bulk nickel oxide crystallites presenting very weak interactions with the support [49,50], and accordingly, it would indicate that a large amount of nickel should be reduced in the 5\u2009K-Ni/MgAl sample. However, this would be inconsistent with the characterization results above. Thus, to understand the origin of this peak, a MS spectrometer was coupled to analyze on line the gases released during the TPR process. As shown in Fig. S3, an intense peak of H2 consumption around 420\u2013430\u2009\u00b0C was observed along with the formation of CO (m/z= 28), NO (m/z= 30) and H2O (m/z= 18) in the samples with high potassium load. This indicates that the \u03b1 component in the TPR results from the reduction of potassium nitrate and carbonate species. In addition, CO2 (m/z= 44), CH4 (m/z= 15), and CO (m/z= 28) were also detected during its reduction, and they decreased with the potassium loading (Fig. S3).In summary, we conclude that Ni-O-K sites are not generated directly from the reaction of potassium oxide and nickel oxide on the K-Ni/MgAl2O4 catalysts but, instead, require an intermediate complex formed from potassium nitrate and carbonate species. These data provide additional evidence for the Ni-O-K phase. Ni-O-K sites seem available for hydrogen adsorption but not for CO adsorption.To explore the functional implications of this new Ni-O-K phase, we investigated the catalytic performance of all prepared samples in dry reforming at 650\u2009\u00b0C for 48\u2009h, using different partial pressures of methane and CO2 (20, 30 and 40 kPa) and a CH4/CO2 molar ratio equal to 1. \nFig. 8 shows the CH4 and CO2 conversion over all the prepared catalysts against time-on-stream for the three partial pressures studied. Notably, Ni/MgAl and 1\u2009K-Ni/MgAl catalysts suffered a drastic deactivation after 6\u2009h when the reaction was performed at high partial pressures (30 and 40 kPa) of reactants. This was due to the rapid accumulation of coke, which even led to reactor plugging. It is well known that coke formation is related to CH4 cracking and that dissociative adsorption of CHx* species is a rate-determining step sensitive to CH4 partial pressure. By contrast, the catalysts promoted with high potassium loads showed stable CH4 and CO2 conversions during 48\u2009h, even at high partial pressures of both reactants. This clearly indicates that potassium mitigates the carbon deposition or accelerates the gasification of carbon deposits.\nFig. S4 shows the evolution of the H2/CO molar ratio over time during the tests performed at 20 kPa for all catalysts. The Ni/MgAl catalysts present a gradual increase of H2/CO molar ratio over time, indicating that the unpromoted catalyst deactivates rapidly. Similarly, the Ni-1\u2009K/MgAl catalyst shows an initial increase in the H2/CO molar ratio, but it then stabilizes. This suggests that small amounts of K promote an equilibrium between the Boudouard reaction and carbon gasification, slowing catalyst deactivation. On the other hand, the H2/CO ratios remain stable over time for the 3\u2009K-Ni/MgAl and 5\u2009K-Ni/MgAl catalysts. Both samples show H2/CO molar ratios below the stoichiometric value of 1, pointing to stable carbon gasification and RWGS reactions. Consequently, no deactivation was observed for both potassium-rich catalysts. In agreement with these results, it has been reported that potassium promotes the RWGS reaction as it activates the CO2 molecules via carbonates and subsequent reduction into CO [51,52].To quantify the amount of nickel exposed in the different materials, H2 chemisorption experiments were carried out (\nTable 1). The results indicate that high potassium loads reduce the amount of exposed nickel sites. This may be due to i) the coverage of Ni sites by formation of a Ni-O-K layer and ii) the somewhat increased particle size (Fig. S8 and Table 1) upon K loading. The catalyst average particle sizes are slightly larger for samples with high potassium load. Some authors have reported an increased metal particle size upon addition of promoters such as Mg, K or Ce [53]. This suggests that potassium may favor the formation of larger nickel particles as a result of a weaker interaction with the MgAl2O4 support, in agreement with our TEM results (Fig. S5).We also investigated the effect of potassium on the catalyst turnover frequency (TOF), which expresses the activity of catalysts in terms of moles transformed per time unit and per mol of exposed nickel. Apparent TOF values were estimated from the run tests at 20 kPa using initial reaction rates and the dispersion of nickel estimated from H2 chemisorption (Table 1). Notably, the samples containing 3 and 5\u2009wt% K2O displayed superior TOF values for both reactants. This indicates that, although the number of nickel sites exposed is reduced upon K addition, the C-H bond cleavage and the CO2 activation in carbon gasification are greatly facilitated, consistent with the formation of a new Ni-O-K active phase.In \nFig. 9, the CH4 and CO2 consumption rates are compared with the H2 and CO production rates, respectively, in terms of Ni metal surface exposed. As can be observed in Fig. 9A, CH4 consumption and H2 production rates are directly correlated with the Ni surface area exposed and thus Ni/MgAl and 1\u2009K-Ni/MgAl catalyst, in which higher fraction of metallic nickel sites are exposed, favored these reactions. Likewise, the addition of potassium decreases the amount of exposed nickel sites due to the formation of Ni-O-K layer and both methane consumption and H2 yield are less favored. On the other hand, Fig. 9B shows that CO2 consumption and CO formation rate are also nickel structure-sensitive reactions. Remarkably, in the absence of potassium, the CO produced is significantly lower than the CO2 consumed. When potassium is added, the CO produced matches the CO2 consumed. This suggests that a sizeable proportion of CO produced on the unpromoted catalyst is dissociated to C* and O* species whereas, in the presence of Ni-O-K, CO does not adsorb and remains in the effluent. Therefore, the addition of potassium avoids the CO dissociation on nickel sites and thus decrease the accumulation of carbonaceous deposits. Moreover, this effect could open the door to enhanced low-temperature RWGS catalysts since CO dissociation favors the methanation reaction against reverse water gas shift reaction [54].Finally, the amount and nature of the carbon deposits after dry reforming of methane were studied as a function of potassium loading. The TPO profiles obtained for all samples are displayed in \nFig. 10. The m/z= 32, m/z= 44, and m/z= 28 signals were chosen to analyze the evolution of O2, CO2, and CO, respectively. The area under a TPO profile is proportional to the amount of deposited carbon, and it is clearly seen that the 5\u2009K-Ni/MgAl catalyst has the lowest amount of carbon deposited. As observed in Fig. 10, Ni/MgAl and 1\u2009K-Ni/MgAl catalysts show maximal CO2 production at 613\u2009\u00b0C, which is associated with the oxidation of highly structured carbon species such as whiskers or carbon filamentous. This type of carbon is the main one responsible for pore blocking and metal particle encapsulation [10,55]. Note that there is an abrupt decline in the CO2 signal on both Ni/MgAl and 1\u2009K-Ni/MgAl catalysts at 716\u2009\u00b0C and 750\u2009\u00b0C. The amount of carbon deposited in these samples was so high that all oxygen of stream was fully consumed, and the CO2 produced then reacted as an oxidant through the reverse Boudouard reaction (C* + CO2 \u2192 CO). This explains the concurrent formation of CO gas. The TPO profiles obtained for 3\u2009K-Ni/MgAl and 5\u2009K-Ni/MgAl catalysts show that the carbon oxidation processes occur at lower temperatures. The TPO profile of 3\u2009K-Ni/MgAl catalyst peaks at 593\u2009\u00b0C, consistent with the oxidation of carbon with a higher graphitic degree, probably small filamentous species [56]. The oxidation step occurring around 330\u2009\u00b0C is ascribed to the oxidation of amorphous carbon, and this is the only process detected on the 5\u2009K-Ni/MgAl spent catalyst.\nFigs. S6 and S7 shows the structural analysis obtained by XRD and Raman spectroscopy, respectively, for all the spent catalysts. The XRD pattern (Fig. S6) obtained for Ni/MgAl spent catalyst present an intense and well-defined peak at 2\u019f =\u200926.5\u00b0, associated with graphitic carbon species (JPDS 00-025-0284). This peak was less intense in the spent K-promoted catalysts, and it was absent in the Ni-5\u2009K/MgAl sample. On the other hand, potassium loading increased the intensity of peaks associated with Ni0, MgAl2O4 and Mg(Ni)O phases compared to the unpromoted sample.The Raman spectra of Ni/MgAl, 1\u2009K-Ni/MgAl and 3\u2009K-Ni/MgAl catalysts, Fig. S7, are characterized by two main features at 1349 and 1578\u2009cm-1, ascribed to the vibrational modes D and G of carbon species, respectively. The D-band is characteristic of structurally disordered carbon while the G-band corresponds to C-C vibration stretching of structured carbon, such as graphite [57]. The D-to-G intensity ratio (I(D/G)) is thus a valuable parameter to characterize the carbon disorder degree [58]. The I(D/G) ratios estimated for the spent catalysts follows the sequence 3\u2009K-Ni/MgAl >\u20091\u2009K-Ni/MgAl >\u2009Ni/MgAl (Fig. S7), indicating that potassium loading decreases the graphitization degree. These bands were absent in the 5\u2009K-Ni/MgAl spent sample.The morphology of spent catalysts was studied by SEM analysis (Fig. S8). As depicted in Fig. S4, the SEM micrographs show that the unpromoted catalyst becomes covered of carbon species with whisker or filamentous structure. Meanwhile, the amount of whisker carbon and the diameter of the filamentous decreases notably with the increment of potassium loading becoming wholly absent for 5\u2009K-Ni/MgAl catalyst. These observations are consistent with the XRD and Raman spectroscopy results.Taken together, the results reveal that, while the unpromoted catalyst became fully covered by carbonaceous deposits, increasing the loading of potassium led to less carbon deposition, which was also less graphitic in nature. Remarkably, deposits were fully prevented on the 5\u2009K-Ni/MgAl catalyst, and its diffractogram coincides with the pattern of the reduced catalyst prior to reaction (Fig. 2), confirming an exceptional improvement in stability accompanying the new Ni-O-K phase.The efficient conversion of biogas to syngas by dry reforming is a very promising route to produce liquid fuels, but low catalytic stability prevents its establishment as an industrial process. In this work, Ni-based catalysts promoted with potassium were tested in the dry reforming of methane and were exhaustively characterized to understand the key role of potassium in carbon deposition suppression. By means of different characterization techniques, we have established that, in the presence of 5\u2009wt% of potassium, the nickel particles form a core surrounded by a Ni-O-K inter-layer (Ni@Ni-O-K) during the reduction of the catalyst. Likely, this layer presents a structure type alkali-nickelate (KNiO2), in which nickel is stabilized in oxidation state +\u20093. The Ni-O-K phase formation induces essential changes in the electronic properties of nickel. The presence of Ni-O-K sites leads to coke-resistant catalysts with excellent activity and stability. Specifically, these new sites do not catalyze the dissociation of CO, thus avoiding the formation of methane and coke and greatly enhancing the yield of syngas and the catalytic stability. The study also provides the first insights on the formation process of the Ni-O-K phase, providing a new direction to design high-performance dry reforming catalysts for sustainable syngas production.Textural properties; XPS K2p; TPR modeling details; SEM/TEM results; H2/CO ratios results for DRM; Characterization of spent catalysts (XRD, Raman spectra and SEM); Figs. S1\u2013S8 and Tables S1\u2013S3.\nL. Azancot, V. Blay: Conceptualization, Methodology L. Azancot, L.F. Bobadilla: Data curation, Writing \u2013 original draft preparation. V. Blay, R. Blay-Roger, L. Azancot, A. Penkova, M.A. Centeno: Visualization, Investigation. L.F. Bobadilla, J.A. Odriozola\n: Supervision. L.F. Bobadilla, L. Azancot: Writing \u2013 review & editing, J.A. Odriozola, M.A. Centeno\n: Funding acquisition.The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Financial support for this work has been obtained from the Spanish Ministerio de Econom\u00eda y Competitividad \u2013 MINECO (RTI2018-096294-B-C33) co-financed by FEDER funds from the European Union and the Universidad de Sevilla-Junta de Andaluc\u00eda Program under contract US-1263288. Lola Azancot acknowledges the MINECO for her associated Ph.D. fellowship (BES-2016-0077475).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2022.121148.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Liquid fuels produced via Fischer-Tropsch synthesis from biomass-derived syngas constitute an attractive and sustainable energy vector for the transportation sector. This study focuses on the role of potassium as a promoter in Ni-based catalysts for reducing coke deposition during catalytic dry reforming. The study provides a new structural link between catalytic performance and physicochemical properties. We identify new Ni-O-K chemical states associated with high stability in the reforming process, evidenced by different characterization techniques. The nickel particles form a core surrounded by a Ni-O-K phase layer (Ni@Ni-O-K) during the reduction of the catalyst. This phase likely presents an alkali-nickelate-type structure, in which nickel is stabilized in oxidation state +\u00a03. The Ni-O-K formation induces essential changes in the electronic, physical, structural, and morphological properties of the catalysts, notably enhancing their long-term stability in dry reforming. This work thus provides new directions for designing more efficient catalysts for sustainable gas-to-liquids processes.\n               "}
{"full_text": "Lignin is an abundant biomass source that can be converted into value-added aromatic platform chemicals.\n1\n To use biomass to its fullest, the challenges involved in valorizing lignin need to be overcome.\n2\u20135\n Conventional lignocellulose delignification methods, such as Kraft and organosolv pulping, require the complete breakdown of the C\u2013O bonds in a series of steps.\n6\u20138\n However, the ability to deconstruct lignin directly from raw biomass has transformed the conventional concept of the biorefinery by capturing high-value products from lignin in the first step,\n9\n a process that is particularly effective under reductive conditions.\n3\n\n,\n\n10\u201312\n The lignin is depolymerized and selectively converted to monomers with high retention of carbohydrates in the pulp.\n10\n\n,\n\n13\n\n,\n\n14\n The monomers obtained are valuable platform chemicals with a range of applications. Further use of the remaining cellulose and hemicellulose provides additional value-added chemicals.\n15\u201317\n\nUsing noble metal catalysts (e.g., Rh, Ru, Pd, Pt), the yields of lignin monomers can reach close to the theoretical maximum.\n18\u201320\n However, these noble metal catalysts are expensive and can be susceptible to poisoning by CO or coking.\n21\n Efforts to maximize the efficiency of these noble metal catalysts with improved selectivity has led to the development of atomically dispersed heterogeneous catalysts.\n22\u201324\n All of the reaction steps take place at single-atom sites and, compared with metal nanoparticles (NPs), the reaction kinetics with single-atom catalysts are rate limited by the low concentration of H atoms available in the active atomic sites.Progress toward catalyst design to combine the activity of noble metals and low cost of earth-abundant metals is popular.\n25\n Some earth-abundant metals such as Ni have low energy barriers for both the dissociation of H2 and the diffusion of H atoms.\n26\n If H2 dissociation on Ni NPs and diffusion to active single noble metal atoms is facile, then the rate-limiting addition of H atoms to the substrate should be accelerated, improving the overall reaction kinetics and reducing the amount of the precious metal required. Such a single-atom alloy (SAA) concept has been described for a PdCu system in which facile hydrogen dissociation and spillover take place.\n27\n Because the dissociation of H2 and reaction sites on SAAs are decoupled, SAAs may not be confined to linear scaling relationships, exceeding the reactivity limit and selectivity of many catalysts.Despite numerous reports of SAAs, their application is mostly limited to catalytic process involving small substrates and in reactions such as C-C coupling, hydrogenation, and electrocatalytic processes.\n22\n\n,\n\n28\u201333\n There are only limited studies describing SAAs as catalysts for biomass transformations, with the focus on lignin model compounds as substrates.\n34\n\n,\n\n35\n Direct depolymerization of lignin using SAAs has not, to the best of our knowledge, been reported.Here, we describe a highly active catalyst for reductive lignin depolymerization based on Pt single atoms anchored onto Ni NPs supported on C (denoted as Pt1Ni/C, where Pt1 represents single Pt atoms). Using this catalyst, a yield of lignin monomers of 37% for birch sawdust was achieved under 5 MPa H2 in methanol (MeOH) at 200\u00b0C, which is significantly higher than that using single Pt atoms supported on active carbon Pt1/C or Ni NPs on active carbon Ni/C.The Pt1Ni/C catalyst was obtained by anchoring Pt atoms on Ni NPs supported on C (Ni/C) through galvanic replacement (Scheme 1\n). In the synthesis, Pt(acac)2 dissolved in toluene was added to a suspension of Ni/C in ethanol. C was chosen as the support as it is inexpensive and has a high surface area and cavities where H2 can be adsorbed.\n36\n Following washing with ethanol and hexane, the Pt1Ni/C catalyst was obtained as a black powder. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis of the Pt1Ni/C catalyst gives weight percentages of the Pt and Ni as 0.3 and 4.4 wt%, respectively. Thermogravimetric analysis (TGA) of Pt1Ni/C in air (Figure\u00a0S4) confirmed that the loading of metal NPs is \u223c5%. The specific surface area of Pt1Ni/C is 90.4 m2/g (Figure\u00a0S5). Using transmission electron microscopy (TEM), the diameter of the Ni NPs was found to be \u223c6.9\u00a0nm with a narrow size distribution (Figure\u00a0S1), a size that is similar to Ni/C (Figure\u00a0S7).The distribution of Pt on Ni/C within the Pt1Ni/C matrix was analyzed by high-angle annular dark-field scanning TEM (HAADF-STEM) (Figure\u00a01\n), which confirms that the Pt atoms are highly dispersed (the isolated Pt atoms are manifested by brightness and marked by circles; Figure\u00a01B). The Pt single atoms on the surface of the Ni/C particles were further characterized by extended X-ray absorption fine structure (EXAFS) analysis in the R space (Figures 2\nA and S8; Table S1). The oscillation manners of the Pt L3-edge in the R space for the Pt1Ni/C differ from those of the Pt foil. The Pt\u2212Pt bond at 2.76\u00a0\u00c5 was not observed in Pt1Ni/C. Compared to the Pt L3 edge of Pt foil probes by X-ray absorption near-edge spectroscopy (XANES) (Figure\u00a02B), the adsorption edge for Pt1Ni/C is \u223c11,570 eV, indicative of Pt(II) species. X-ray photoelectron spectroscopy (XPS) of Ni 2p and Pt 4f indicate that the majority of surface Ni and Pt species are in the 2+ oxidation state (Figure\u00a0S2).\n37\n\nThe X-ray diffraction (XRD) profile of the Pt1Ni/C catalyst shows a characteristic peak at 44\u00b0 corresponding to (111) reflections of face-centered cubic Ni NPs (Figure\u00a0S3), at the identical position observed in Ni/C, presumably as the low content of highly dispersed Pt does not influence the XRD pattern.The H2-temperature-programmed reaction (TPR) profiles (Figure\u00a02C) indicate that alloying takes place in the Pt1Ni/C catalyst. With the addition of single Pt atoms, the reduction temperature of Pt1Ni/C (261\u00b0C) is shifted to lower regions compared with Ni/C (275\u00b0C). The adsorption of H2 on the surface of the Pt1Ni/C catalyst was investigated through H2 temperature-programmed desorption (TPD) measurements (Figure\u00a02D). The first peak (at 425\u00b0C) in the Pt1Ni/C catalyst is at a much lower temperature than that observed for Ni/C (at 698\u00b0C), indicating that the Pt atoms provide low-barrier exit routes for H2 during the desorption process.The performance of the Pt1Ni/C catalyst was investigated in the depolymerization of lignin using birch sawdust as the substrate. For comparison, single Pt atoms and Ni NPs supported on C (Pt1/C and Ni/C) were prepared and applied in the same depolymerization reaction under identical conditions (Figure\u00a03\n). The monomer yields are used as a measure of depolymerization efficiency of the catalysts. Under 5 MPa H2 in methanol at 200\u00b0C, the yield of total monomers with the Pt1Ni/C catalyst is 37% after 18\u00a0h (Figure\u00a03, entry 1), quite close to the theoretical maximum monomer yield, which ranges from 44 to 56 wt%.\n13\n The yield of total monomers is significantly higher than that with the control catalysts (Figure\u00a03, entries 2 and 3) and reported Ni/C, with monomer yields of 24% at 200\u00b0C in methanol.\n38\n Further analysis of the product distribution shows that a combined selectivity toward 4-n-propylsyringol (S) and 4-n-propylguaiacol (G) exceeds 90% within the monomer fractions, whereas 4-n-propanolguaiacol (G-OH) and 4-n-propanolsyringol (S-OH) accounts for <5% of the monomers (Figure\u00a03, entry 1). Similar to other reported single-atom catalysts (i.e., Co1/C,\n39\n Ru1/ZnO/C,\n40\n and Pd1/CNx\n41\n), the aromatic rings are preserved with the Pt1Ni/C catalyst even under more forcing reaction conditions, unlike pure NP catalysts that lead to ring hydrogenation.\n42\n\n,\n\n43\n Saturated compounds were not observed with the Ni/C catalyst at 200\u00b0C, although traces of saturated compounds were detected at 300\u00b0C. It has been shown that Pt NPs are more active than Ni NPs in hydrogenolysis.\n44\n As the hydrogenolysis of C\u2013O bonds is highly metal dependent,\n45\n the overall yield in S and G may be attributed to the high activity of Pt atoms. Note that in the absence of the metallic sites the monomer yield is very low (Figure\u00a03, entry 4).After separating the liquid products and drying the solid residue of birch sawdust and catalysts, 0.1\u00a0g birch sawdust was added to perform the recycling test. The decrease in the activation of the catalyst may be caused by coking on the surface of the catalyst (Table S2).The mechanism of reductive fractionation involves solvolysis of the C\u2013O bonds, with the catalyst hydrogenating the reactive intermediate products generated, preventing re-polymerization.\n46\n\n,\n\n47\n Based on the similar temperatures used, both Ni NPs and single Pt atoms contribute to H2 dissociation (Figure\u00a02C). Compared with Pt1/C, which is less active with hydrogen dissociation, both Ni NPs and single Pt atoms in Pt1Ni/C served as active sites in H2 dissociation and adsorption of H atoms, so that a more abundant amount of H atoms can be produced on the surface of Pt1Ni/C. Compared with Ni/C, the better performance of Pt1Ni/C is due to the lower hydrogen binding energy of Pt atoms than Ni atoms. The H2-TPD analysis shows a large decrease in the H2 desorption temperature of the Pt1Ni/C catalyst (425\u00b0C) (Figure\u00a02D), compared with that of Ni/C (698\u00b0C), indicating that the single Pt atoms serve as active sites. Moreover, due to the single-atom nature of the Pt in the Pt1Ni/C, the aromatic structure of the phenyl rings of lignin monomers are preserved without further hydrogenation. Since ring hydrogenation requires coordination of the aromatic ring over a trimetal face, NP catalysts would lead to the hydrogenation of aromatic rings,\n42\n\n,\n\n43\n while single Pt atoms can achieve hydrogenolysis of the C\u2013O bonds without the hydrogenation of the phenyl ring.\n48\n As such, Pt single atoms played a pivotal role in Pt1Ni/C in enhancing the catalytic activity while keeping the high selectivity in the lignin depolymerization, a benefit that cannot be achieved by using Ni/C only. Other reasons for the high activity, such as support effects, cannot be excluded.The reaction conditions were optimized to obtain monophenolic compounds in higher yields (Figure\u00a03). The yield of monophenolic compounds increased from 12 wt% at 150\u00b0C to 43 wt% at 300\u00b0C at a H2 pressure of 5 MPa in methanol after 18\u00a0h (Figure\u00a04\nA), with high selectivity to S and G (>90%, Figure\u00a04B). At lower H2 pressures (and in the absence of H2), G-OH and S-OH are preferentially formed instead of G and S (Figure\u00a04C).A comparison of product distributions in water, methanol, ethanol, 1-propanol, 1-butanol, and ethylene glycol (Figure\u00a04D) shows that solvent has a remarkable impact on the monomer yield as well as the product distribution, as observed elsewhere.\n49\n\n,\n\n50\n The solubility of lignin in different solvents has been extensively studied,\n51\n with the solubility in ethylene glycol being the highest followed by methanol, ethanol, 1-propanol, 1-butanol, and H2O.\n52\n The monomer yields basically decrease with the decreasing solubility of lignin in the solvent. As in our case, a comparably lower yield of 32% in the pure ethylene glycol was obtained than that from methanol, probably due to the high viscosity and low solubility of H2 in ethylene glycol.We describe a selective hydrogenation catalyst in which monodispersed Pt atoms are anchored on the surface of Ni NPs supported on active carbon. The Pt1Ni/C catalyst affords monophenolic compounds in a 37 wt% yield at 200\u00b0C, which is close to the theoretical maximum yield. The remarkable activity and selectivity of the Pt1Ni/C catalyst may be attributed to the synergistic effects between the Ni NPs and the single Pt atoms.C powder (Vulcan XC 72R, 1 g) was mixed with 50\u00a0mL 5\u00a0M nitric acid at 80\u00b0C and stirred for 18 h. The solids were separated by centrifugation and were washed with distilled water until constant pH was achieved. The pre-treated activated carbon (200\u00a0mg) was dispersed in ethanol (20\u00a0mL) under vigorous stirring at room temperature. To the resulting suspension, a solution containing Ni(NO3)2\u00b76H2O (50\u00a0mg, 0.2\u00a0mmol) in ethanol (5\u00a0mL) was slowly added and stirring was continued for 12\u00a0h at room temperature. The reaction mixture was further heated at 40\u00b0C under stirring until all of the solvent had evaporated. The remaining solid was heated to 400\u00b0C for 1\u00a0h under H2 in a tube furnace, to afford Ni/C.The Pt1Ni/C catalyst was prepared via galvanic replacement between Ni NPs and Pt(acac)2. Ni/C was dispersed in ethanol (50\u00a0mL), and the resulting suspension was heated at 50\u00b0C for 10\u00a0min. To the reaction mixture, a solution of Pt(acac)2 (6\u00a0mg, 0.015\u00a0mmol) dissolved in toluene (5.2\u00a0mL) was added slowly. After stirring for 6\u00a0h at 50\u00b0C, the solution was cooled to room temperature. Then, the sample was collected by centrifugation and washed with ethanol (3\u00a0\u00d7 10\u00a0mL) and hexane (3\u00a0\u00d7 10\u00a0mL). After drying in vacuum at 40\u00b0C for 24 h, the Pt1Ni/C catalyst was obtained as a black powder.C powder (Vulcan XC 72R, 1 g) was mixed with 50\u00a0mL 5\u00a0M nitric acid at 80\u00b0C and stirred for 18 h. The solids were separated by centrifugation and were washed with distilled water until constant pH was achieved. This pre-treated activated carbon (100\u00a0mg) was dispersed in water (20\u00a0mL) under magnetic stirring. K2PtCl4 aqueous solution (10\u00a0mL, 0.043\u00a0mg/mL) was slowly added to the resulting reaction mixture. After stirring at room temperature for 30\u00a0min, the solid was collected by centrifugation. The isolated solid was washed with water (3\u00d7 10\u00a0mL) and dried at 60\u00b0C in vacuum for 24 h. The Pt1/C was obtained as a black powder, and the HAADF-STEM images and elements mapping of Pt1/C are in Figure\u00a0S6.Pt L3-edge XAFS data were recorded under fluorescence mode with a 32-element Ge solid-state detector at the SuperXAS beamline of Swiss Light Source (SLS) at the Paul Scherrer Institute (PSI, Villigen, Switzerland). The energy was calibrated according to the L3 absorption edge of pure Pt foil. Data analysis was performed using a standardized IFEFFIT package (including Athena and Artemis software).\n53\n\nThe Pt1Ni/C powders were suspended in acetone and the resulting suspension was ultrasonicated for 1 h. Subsequently, the acetone suspension of NPs was deposited on a C film coated with Cu grid and then analyzed by TEM (FEI Talos, operated at 200 keV). XPS analysis were performed using a monochromatic Al K\u03b1 X-ray source of 24.8\u00a0W power with a beam size of 100\u00a0\u03bcm. XRD measurements were recorded in Bragg Brentano geometry on a Bruker D8 Discover diffractometer, equipped with a Lynx Eye XE detector, using non-monochromated Cu-K\u03b1 radiation.In a typical reaction, birch sawdust (0.1 g, size 0.25\u20130.5\u00a0mm) and Pt1Ni/C (0.02 g) in MeOH (5\u00a0mL) were added into a 100-mL stainless-steel batch reactor with a glass liner. The reactor was sealed, flushed with H2 3 times, and then pressurized to 5 MPa at room temperature. The reaction mixture was heated at 100\u00b0C for 3\u00a0h under stirring at 800\u00a0rpm, and then heated to 200\u00b0C. After 18 h, the autoclave was cooled in water and then depressurized.Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Paul Dyson (paul.dyson@epfl.ch).All materials generated in this study are available from the lead contact without restriction.All data generated in this study can be found in the article and supplemental information or is available from the lead contact upon request.The authors are grateful to the Swiss National Science Foundation and EPFL for financial support. L.P. acknowledges funding from the Swiss National Science Foundation under the Early Postdoc.Mobility Grant\nP2ELP2_195109. J.L. acknowledges funding from the European Union\u2019s Horizon 2020 Research and Innovation program under the Marie Sk\u0142odowska-Curie Grant agreement no. 838686. The authors thank Dr. Maarten Nachtegaal and Mr. Urs Vogelsang for technical support at the SuperXAS beamline of SLS.All of the authors contributed to the design of the experiments and data analysis. L.C. performed the experiments, L.B. performed the HAADF-STEM tests, and J.L. performed the EXAFS measurements. L.C. and P.J.D. wrote the manuscript, and all of the authors discussed, commented on, and proofread the manuscript.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2021.100567.\n\n\nDocument S1. Figures S1\u2013S8 and Tables S1 and S2\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  Due to the highly complex polyphenolic structure of lignin, depolymerization without a prior chemical treatment is challenging, and new catalysts are required. Atomically dispersed catalysts are able to maximize the atomic efficiency of noble metals, simultaneously providing an alternative strategy to tune the activity and selectivity by alloying with other abundant metal supports. Here, we report a highly active and selective catalyst comprising monodispersed (single) Pt atoms on Ni nanoparticles supported on carbon (denoted as Pt1Ni/C, where Pt1 represents single Pt atoms), designed for the reductive depolymerization of lignin. Selectivity toward 4-n-propylsyringol and 4-n-propylguaiacol exceeds 90%. The activity and selectivity of the Pt1Ni/C catalyst in the reductive depolymerization of lignin may be attributed to synergistic effects between the Ni nanoparticles and the single Pt atoms.\n               "}
{"full_text": "Triglyceride based biomass is an appealing alternative to produce transportation fuels as they are readily available, renewable [1] and its long chain usually contains carbons between 16 and 18 [2], which is within diesel range [3] and it results in high heating value [2]. However, vegetable oils cannot be directly used as fuels due to the high amount of oxygen and consequently engine incompatibility [3,4] and therefore, in order to be used as fuels, an upgrading process is needed [3,4]. The transformation of vegetable oils into transportation fuels can be achieved by cracking, transesterification and deoxygenation [4\u20137]. A main drawback of biodiesel produced by transesterification is its inferior quality due the high amount of oxygen, resulting in high viscosity, poor chemical stability, high pour point and high cloud point, which results in poor cold flow properties [1,4,5,7]. Even tough cracking of triglycerides yields a fuel similar to oil derived diesel [6], it is not a selective process due to the formation of a wide range of hydrocarbons [4] and there is reduction of energy content due to loss of carbon [8].An appealing alternative is the deoxygenation, producing a fuel similar to conventional diesel [5,7\u20139] and with an outstanding cetane number when compared to fossil-based diesel [1,4,10]. Moreover, this approach has superior removal of oxygen when compared to thermal cracking and it is less susceptible to coke formation, due to operation at high pressures [11]. Other advantages are absence of sulfur [12], the use of established structure for storage and distribution, and green diesel can be used in diesel engines pure or blended with petrol-diesel at any ratio [13]. The removal of oxygen in deoxygenation can be achieved by three main pathways: (i) hydrodeoxygenation, (ii) decarbonylation and (iii) decarboxylation [1,2,4,7,8,10,13\u201316]. In addition, one should note that there are not many studies of deoxygenation using solvent-free operating conditions [16]. Therefore, this factor is worth investigating and unlike previous work, the solvent-free approach results in a better understanding on how to design practical reactors for the production of sustainable hydrocarbons.Currently, many studies have investigated transition-metal phosphides as hydrotreating catalysts due to some advantages such as its high performance ascribed to ensemble and ligand P effects [7], affordable price, long lifetime [14,17], globular particles, enhancing site exposure [9] and resemblance with noble metals due to P incorporation into the MeOx framework [18]. Another advantage is its low activity for side reactions such as cracking and methanation [1]. Furthermore, it exhibits metallic and acidic properties [1], ascribed to Ni\u03b4+ (small positive charge functioning as Lewis acid site) and P-OH group (Br\u00f6nsted acid sites, resulting from partial phosphate reduction) [10].Several studies have explored the use of nickel phosphides for the deoxygenation of biomass. From the reported supports, it is important to highlight the use of SiO2, Al2O3, TiO2 and CeO2\n[9,19], from which it was concluded that SiO2 showed the higher activity and CeO2 the lower [9]. This has been attributed to the surface density of Ni sites, which result from interactions with the support, and differences in acidity and reducibility also impact the catalyst activity [9]. Even though the use of Al2O3 presents some advantages such as high mechanical strength, a drawback is the formation of AlPO4 and as consequence occurs the formation of the Ni12P5 phase [20,21]. Other relevant supports employed are zeolites such as H-Y [22], H-ZSM-5 [23] and H-\u03b2 [24] and mesoporous materials such as MCM-41 [25] and SBA-15 [26]. The advantage of zeolites is their strong acidity, enhancing isomerization reactions, and as consequence the quality of the fuel produced [27]. On another hand, the use of mesostructured materials can enhance the hydrotreating activity [26]. Albeit there are many studies exploring the use of metallic phosphides, not many studies have reported the effect of the support on the deoxygenation of model molecules over nickel phosphide catalyst [7], therefore, this is worth investigating.Therefore, the aim of this work was the investigation of nickel phosphide catalysts supported on different structures (USY, H-ZSM-5 and Al-SBA-15) on solvent-free deoxygenation of oleic acid to produce diesel-like hydrocarbons. The performance of the catalysts was conducted on a batch reactor operated at 260\u2013300\u00a0\u00b0C and 50\u00a0bar. The effects of the structure, reducibility, acidity and dispersion of the catalysts on the deoxygenation of oleic acid and the temperature were investigated.The commercial zeolites used were H-ZSM-5 (HCZP 90, Clariant, Si/Al\u00a0=\u00a050) and USY (HDT 9807, Cenpes/Petrobr\u00e1s, Si/Al\u00a0=\u00a05.95). On the other hand, the Al-SBA-15 was synthetized as follows. The materials used were nickel nitrate hexahydrate (97%, Vetec), dibasic ammonium phosphate (99%, Acros Organics), oleic acid (90\u00a0wt%, Synth), nitric acid (65%, Synth), hydrochloric acid (37%, Anidrol), hydrofluoric acid (48%, Din\u00e2mica), sulfuric acid (98%, Fmaia), isopropyl alcohol (99.5%, Synth), methanol (99.8%, Synth), hexane (98.5%, Anidrol), sodium hydroxide (97%, Anidrol), ammonium chloride (99%, \u00caxodo Cient\u00edfica), sodium chloride (99%, Nuclear), ammonium hydroxide (30%, Neon), aluminum isopropoxide (98%, Acros organics), tetraethyl orthosilicate TEOS (98%, Aldrich), triblock copolymer P123 (30\u00a0wt%, Aldrich), potassium bromide (>99%, Sigma-Aldrich) and pyridine (>99%, Synth).The synthesis of the support Al-SBA-15 (Si/Al\u00a0=\u00a010 at the gel) was performed as the literature [28,29]. Firstly, a solution was prepared with 0.85\u00a0g of aluminum isopropoxide, 8.5\u00a0g of TEOS, 10\u00a0mL of HCl 2\u00a0mol L\u22121 and it was stirred for 5\u00a0h. Another solution was prepared adding 4\u00a0g of P123 to 100\u00a0mL of HCl 2\u00a0mol L\u22121 and it was stirred for 5\u00a0h. Secondly, the first solution was added dropwise to the second one and the resulting gel was mixed for 20\u00a0h at 40\u00a0\u00b0C. Then, the pH was increased to 7.5 adding NH4OH dropwise. The mixture was placed into teflon-lined stainless steel autoclaves and heated at 100\u00a0\u00b0C for 48\u00a0h. Afterwards, the product was filtered, washed thoroughly with deionized water and dried at 100\u00a0\u00b0C overnight. Finally, the material was calcined at 550\u00a0\u00b0C for 6\u00a0h. The catalysts were prepared with 10\u00a0wt% Ni2P (the active phase was calculated to be 10% of the weight of the support) and with a Ni/P mole ratio of 1.25 by incipient wetness impregnation. Initially, an aqueous solution of nickel nitrate hexahydrate was prepared. To this solution, dibasic ammonium phosphate was added, followed by dropwise addition of nitric acid until no solids remained undissolved. After drying, the calcination was performed under nitrogen (50\u00a0mL\u00a0min\u22121) with a heating rate of 2\u00a0\u00b0C\u00a0min\u22121 until 500\u00a0\u00b0C and held for 3\u00a0h. Then, the ex situ reduction was conducted under hydrogen (50\u00a0mL\u00a0min\u22121) with a rate of 5\u00a0\u00b0C\u00a0min\u22121 until 350\u00a0\u00b0C followed by a rate of 2\u00a0\u00b0C\u00a0min\u22121 until 650\u00a0\u00b0C and held for 3\u00a0h. Finally, the catalysts were passivated in 0.5% O2/N2 (50\u00a0mL\u00a0min\u22121) for 1\u00a0h at room temperature.Atomic absorption spectroscopy (AAS) to determine the amount of Si, Al and Ni was conducted on a Spectra AA 50B (Varian) spectrometer. Previously, the samples were digested using nitric acid, hydrochloric acid and hydrofluoric acid.Temperature programmed reduction (TPR) of hydrogen was obtained on a ChemBet-3000 (Quantachrome) instrument using a heating rate of 10\u00a0\u00b0C\u00a0min\u22121 up to 900\u00a0\u00b0C with a flow of 20\u00a0mL\u00a0min\u22121 of 5% H2/N2. Before the reduction, the samples were treated with N2 at 300\u00a0\u00b0C under a flowrate of 20\u00a0mL\u00a0min\u22121 for 30\u00a0min.X-ray diffraction (XRD) analysis was performed on a XRD 600 (Shimadzu) diffractometer, using Cu-K\u03b1 radiation (\u03bb\u00a0=\u00a01.54\u00a0\u00c5, V\u00a0=\u00a040\u00a0kV, i\u00a0=\u00a030\u00a0mA), a rate of 2\u00b0 min\u22121 and a range of 2\u03b8 from 5\u00b0 to 80\u00b0. The XRD analysis using small angle was conducted on a small angle X-ray scattering (SAXS) N8 Horizon (Bruker) diffractometer between the 2\u03b8 range from 0.1\u00b0 to 5.5\u00b0.Nitrogen physisorption to evaluate the specific surface area, pore volume, and pore size was obtained on a ASAP 2020 (Micromeritics) instrument at liquid nitrogen temperature (77\u00a0K). The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method, while the average pore size was obtained from the Barrette-Joyner-Halenda (BJH) method.The temperature programmed desorption (TPD) of ammonia was carried on an Autochem II 2920 (Micromeritics) equipment. First, the samples were treated at 300\u00a0\u00b0C for 2\u00a0h under He flowrate of 20\u00a0mL\u00a0min\u22121. Then, they were saturated with 10% NH3/He at 100\u00a0\u00b0C under a flow of 20\u00a0mL\u00a0min\u22121 for 1\u00a0h, followed by a 50\u00a0mL\u00a0min\u22121 He flow for 1\u00a0h. The desorption increased the temperature at a rate of 10\u00a0\u00b0C\u00a0min\u22121 up to 700\u00a0\u00b0C. The acid sites were classified into weak (\n\n\u2264\n\n 200\u00a0\u00b0C), medium (200\u00b0 \u2013 350\u00a0\u00b0C) and strong (\n\n\u2265\n\n 350\u00a0\u00b0C) [30].Transmission electron microscopy (TEM) examinations were conducted on a JEM-1400 (JEOL) instrument. Previously, the samples were dispersed in isopropyl alcohol and then supported on a carbon grid and dried at room temperature overnight.Fourier-transform infrared spectroscopy (FTIR) was performed on a Vertex 70\u00a0V (Bruker) spectrometer using 4\u00a0cm\u22121 resolution and wavelength range from 4000\u00a0cm\u22121 to 400\u00a0cm\u22121. To prepare the samples, 1\u00a0mL of pyridine was dropped on 1\u00a0g of sample followed by heating at 150\u00a0\u00b0C overnight. Then, pellets were made blending 200\u00a0mg of KBr and 1\u00a0mg of sample.The deoxygenation tests were conducted in an autoclave batch reactor of 160\u00a0mL from Parr Instruments equipped with a sampling cylinder to allow the addition of the reactant free of oxygen. First, 1.5\u00a0g of passivated catalyst was reduced in situ at 5\u201310\u00a0bar and 250\u00a0\u00b0C for 12\u00a0h under hydrogen flow (50\u00a0mL\u00a0min\u22121). After adding 50\u00a0g of oleic acid and reaching the desired temperature (260\u00a0\u00b0C, 280\u00a0\u00b0C and 300\u00a0\u00b0C), 50\u00a0bar of hydrogen was added in order to initiate the reaction and the catalytic tests were conducted for 6\u00a0h. A test with no catalyst was also performed in order to evaluate thermal effects. Afterwards, the liquid products were treated according to the literature [31]. Firstly, it was added to a test tube 100\u00a0\u03bcL of sample and 2\u00a0mL of 0.5\u00a0mol L\u22121 of NaOH in methanol and this was heated at 90\u00a0\u00b0C for 5\u00a0min. After cooling, it was added 3\u00a0mL of another solution (50\u00a0g of H2SO4 and 33.3\u00a0g of NH4Cl in 1 L of methanol) and heated at 90\u00a0\u00b0C for 5\u00a0min. Then, after cooling, it was added 3\u00a0mL of hexane and 2\u00a0mL of a saturated solution of NaCl. Finally, the material was centrifuged to separate the phases. The analyses to quantify the samples were conduct on a GC-10 Plus gas chromatograph (Shimadzu) with flame ionization detector (FID) equipped with a VA-5 (Varian) capillary column with dimensions 30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u03bcm, while the product determination was conducted using the NIST library. The conversion of oleic acid and degree of deoxygenation were determined by Eqs. (1) and (2), respectively:\n\n(1)\n\n\nX\n=\n\n\n\n\n\n\nC\n\nO\nA\ni\n\n\n-\n\nC\n\nO\nA\n\n\n\n\nC\n\nO\nA\ni\n\n\n\n\n\n\n.\n100\n\n\n\n\n\n\n(2)\n\n\nD\nO\n=\n\n\n\n\n\n\nC\n\nF\nA\ni\n\n\n-\n\nC\n\nF\nA\n\n\n\n\nC\n\nF\nA\ni\n\n\n\n\n\n\n.\n100\n\n\n\nwhere \n\nX\n\n is the oleic acid conversion (%), \n\n\nC\n\nO\nA\ni\n\n\n\n and \n\n\nC\n\nO\nA\n\n\n\n are the initial and final concentration of oleic acid (wt %), respectively, \n\nD\nO\n\n is the degree of deoxygenation (%), \n\n\nC\n\nF\nA\ni\n\n\n\n and \n\n\nC\n\nF\nA\n\n\n\n are the initial and final concentration of free fatty acids (wt %), respectively.The turnover frequency was determined by Eq. (3):\n\n(3)\n\n\nT\nO\nF\n=\n\n\nr\n\nH\nC\n\n\nL\n\n\n\n\nwhere \n\nT\nO\nF\n\n is the turnover frequency (h\u22121), \n\n\nr\n\nH\nC\n\n\n\n is the global reaction rate (mol gcat\n\u22121h\u22121) and \n\nL\n\n is the theoretical metal site concentration (mol gcat\n\u22121).An alternative to CO uptake is the determination of the theoretical metal site concentration, given by Eq. (4)\n[32]:\n\n(4)\n\n\nL\n=\n\n\n6\nn\nC\n\n\n\u03c1\nd\n\nn\nA\n\n\n\n\n\n\nwhere \n\nn\n\n is the average surface metal atom density of Ni2P (1.01.1015 cm\u22122), \n\nC\n\n is the fractional weight loading (gNi2P gcat\n\u22121), \n\n\u03c1\n\nis the density of Ni2P (7.09\u00a0g\u00a0cm\u22123), \n\nd\n\n is the crystallite size calculated from Scherrer equation (cm) and \n\n\nn\nA\n\n\n is the Avogadro\u2019s constant (mol\u22121).Assuming the behavior of an ideal batch reactor with constant volume, the reaction rate may be calculated as shown in Eq. (5)\n[33]:\n\n(5)\n\n\n\nr\n\nH\nC\n\n\n=\n\n\nm\n\nO\nA\n\n\nW\n\n\n\n\nd\nC\n\n\nH\nC\n\n\n\nd\nt\n\n\n\n1\n\nM\nM\n\n\n\n\n\nwhere \n\n\nm\n\nO\nA\n\n\n\n is the mass of oleic acid (50\u00a0g), \n\nW\n\n is the weight of catalyst (1.5 gcat), \n\n\nC\n\nH\nC\n\n\n\n is the total amount of hydrocarbons C10-C18 (wt %), \n\nt\n\n is time (h) and \n\nM\nM\n\n is the average molar weight of the products (g mol\u22121).The TPR profiles of the calcined catalysts, denoted NixPyOz/support, are illustrated in Fig. 1\n. The nickel phosphide formation occurs by reduction of Ni+2 to metallic Ni followed by phosphate species reduction [34]. The NixPyOz/H-ZSM-5 catalyst started to reduce at 600\u00a0\u00b0C and was complete at 950\u00a0\u00b0C. On the other hand, the catalysts supported on USY and Al-SBA-15 reduced at lower temperatures, both starting at 400\u00a0\u00b0C and ending at 800\u00a0\u00b0C and 870\u00a0\u00b0C, respectively. The first reduction peak at 723\u00a0\u00b0C (NixPyOz/H-ZSM-5), 509\u00a0\u00b0C (NixPyOz/USY), 640\u00a0\u00b0C (NixPyOz/Al-SBA-15) can be attributed to reduction of NiO species, while the second peak at 835\u00a0\u00b0C (NixPyOz/H-ZSM-5), 594\u00a0\u00b0C (NixPyOz/USY), 783\u00a0\u00b0C (NixPyOz/Al-SBA-15) can be ascribed to reduction of P on the P-OH bond [7,9,35].As the reduction of the nickel phosphide supported on H-ZSM-5 occurs at higher temperature, there is a stronger interaction between the support and the metals [36], which can be assigned to the strong interaction between Ni and the acid OH of the zeolite [36]. The strong interaction between the USY zeolite and PO4\n\u22123 can reduce the interaction between nickel and phosphate species, what explains its reduction at lower temperatures [9]. In addition, the results from the TPR and TPD analyses suggest that catalysts with stronger weak acid sites (desorption peaks of temperature\u00a0<\u00a0200\u00a0\u00b0C) are more difficult to reduce, thus requiring higher temperatures at the reduction. A previous study has concluded that a stronger Br\u00f6nsted acidity (e.g. weaker O\u2013H bonds) hinders the reduction of the catalysts, as it facilitates the mobility of H+ species [37]. Indeed, the TPD result (Section 3.1.4) shows that the order of strength of weak acid sites is: USY\u00a0<\u00a0Al-SBA-15\u00a0<\u00a0H-ZSM-5, which is consistent with the reducibility order given by the TPR.The wide angle XRD patterns of the supports and nickel phosphide reduced catalysts are shown in Fig. 2\n. Comparing the patterns of the zeolitic supports with patterns found in the literature, it is possible to confirm the MFI structure of the H-ZSM-5 [23,24,25] and the FAU framework of the Y zeolite [12,25,26,27]. On the other hand, the peak at around 22.5\u00b0 at the Al-SBA-15 and its reduced catalyst can be assigned to amorphous silica found on the pore wall of this ordered mesoporous material [7,26].The pattern of Ni2P/H-ZSM-5 shows peaks at 40.8\u00b0, 44.7\u00b0, 47.4\u00b0, 54.2\u00b0 and 74.8\u00b0 ascribed to the Ni2P nickel phosphide phase [35]. On the other hand, the pattern of Ni2P/USY exhibits only a minor peak 44.7\u00b0 attributed to Ni2P [35]. This difference can be explained by the high specific surface area of the USY zeolite, which enhances the crystallites dispersion [40]. The nickel phosphide supported on Al-SBA-15 exhibits peaks at 40.7\u00b0, 44.5\u00b0 and 54.2\u00b0 attributed Ni2P, while the peaks at 32.6\u00b0, 38.4\u00b0, 41.8\u00b0, 46.9\u00b0, 48.9\u00b0, 56.1\u00b0 and 60.1\u00b0 are ascribed to Ni12P5 phase [35].It is also important to highlight that the Ni2P phase is more active in hydrotreating reactions than the Ni12P5 phase [39]. Moreover, an excess of P is necessary to form the Ni2P phase [41,42]. However, even though the synthesis used excess P, this amount was not enough to form only Ni2P on the Al-SBA-15.The low angle SAXS patterns of the support Al-SBA-15 and its reduced catalyst Ni2P/Al\u2013SBA\u201315 are illustrated in Fig. 3\n. These patterns are typical of ordered mesoporous materials, exhibiting a main peak at the (100) lattice plane and two peaks of lower intensity at (110) and (200) planes, or peaks at 0.85\u00b0, 1.45\u00b0 and 1.68\u00b0, respectively [43]. Furthermore, the pattern of the reduced catalyst confirms that the structure of Al-SBA-15 is kept even at a high reduction temperature (650\u00a0\u00b0C). The TEM of this sample (Section 3.1.5), also corroborates this, showing the typical structure of the Al-SBA-15 molecular sieve.Physicochemical properties of the supports and the reduced catalysts are detailed on Table 1\n. The nominal Si/Al ratios are close to the experimental Si/Al for all catalysts. Furthermore, the Ni loading of the reduced catalysts is also close to the nominal value of 7.91\u00a0wt% Ni (that is equal to 10\u00a0wt% Ni2P). After nickel phosphide impregnation, the specific surface area and pore volume decreased. The reduction is more intense for Ni2P/Al-SBA-15, suggesting accumulation of the active phase on its channels [26,44]. The catalyst with the higher surface area is Ni2P/USY, followed by Ni2P/H\u2013ZSM-5 and Ni2P/Al-SBA-15. Although USY zeolite has a significant mesoporous contribution, the zeolite in this study did not suffer an intense dealumination process, due to the small mesoporous area compared to the total area and the low Si/Al ratio, close to 6, the value found to be a limitation of the synthesis process of the zeolite Y [39]. The Ni2P/Al-SBA-15 exhibits the higher pore volume, followed by Ni2P/USY and Ni2P/H-ZSM-5.The TPD results of the supports and reduced catalysts are given on Table 2\n and the acid profiles are shown in Fig. 4\n. The acid strength of the supports, evaluated by the temperature [45] follows the order: H-ZSM-5\u00a0>\u00a0USY\u00a0>\u00a0Al-SBA-15. The ordered mesoporous material Al-SBA-15 showed the lower acid strength when compared with the zeolites, due to the fact that the walls of its pore are amorphous [46]. Moreover, the H-ZSM-5 acidity is stronger than the USY zeolite, because the latter has a lower Si/Al, therefore, there are more Al atoms on its structure and more acid sites and its framework suffers less unbalanced during the creation of acid sites [47]. Furthermore, the amount of acid sites is related with the Si/Al ratio. The Si/Al ratio increases with the order: USY\u00a0<\u00a0Al-SBA-15\u00a0<\u00a0H-ZSM-5, while the total amount of acid sites decreases with the following order: USY\u00a0>\u00a0Al-SBA-15\u00a0>\u00a0H-ZSM-5. This occurs because a lower Si/Al means a higher amount of Al atoms and as a consequence a higher amount of acid sites [47].Regarding the acid strength of the nickel phosphide catalysts, they follow the same trend as the bare supports. Comparing the nickel phosphide catalysts and their respective supports, they show a reduction of the total amount of acid sites and this is in accordance with previous studies [22,36,48]. Comparing the USY zeolite and its catalyst, there is an increase of the amount of weak and medium sites, while there is a decrease of the quantity of strong sites, probably because the nickel phosphide creates weak acid sites and at the same time it covers strong acid sites [9,22]. In addition, the surplus acidity can be ascribed to the P-OH group on Ni2P [38], since nickel phosphide has both Br\u00f6nsted and Lewis acidity, attributed to P-OH and to electron deficiency of Ni, respectively [9,22]. Regarding the nickel phosphide supported on Al-SBA-15 and its support, a decrease of weak and strong sites takes place and we suggest that even tough occurs creation of acid sites, the destruction of them is more intense. Moreover, the Ni2P supported on H-ZSM-5 presents a reduction of the amount of weak and strong sites, which can be ascribed to the strong interaction between Ni and the acid OH on the zeolite [36].The TEM images of the supports and the reduced catalysts are illustrated in Fig. 5\n and the nickel phosphide particle size distribution of the catalysts are given in Fig. 6\n. These figures show that the average diameter of the phosphide particles are 18.84\u00a0nm, 10.25\u00a0nm and 13.10\u00a0nm for Ni2P/USY, Ni2P/H-ZSM-5 and Ni2P/Al-SBA-15, respectively. It seems that the particles have a better dispersion on USY, due to its high surface area and its mesopores, that can enhance the metallic dispersion. On another hand, the particles supported on H-ZSM-5 are not well dispersed, due to the presence of micropores contained on large crystals [48]. Moreover, the image of nickel phosphide supported on Al-SBA-15 also shows a good dispersion and it confirms that the channels and the ordered structure are maintained, even the at a high reduction temperature (650\u00a0\u00b0C) [7]. In addition, the catalysts supported on USY and Al\u2013SBA\u201315 can have nickel phosphide inside their pores because their mesopores present an average diameter of 14.29\u00a0nm and 7.23\u00a0nm, respectively, obtained by the BJH method (Section 3.1.3). On the other hand, due to its small micropores (0.53\u00a0nm\u00a0\u00d7\u00a00.56\u00a0nm), the metallic phosphide is formed on the external surface of H-ZSM-5 [38].The FTIR spectra of pyridine adsorbed on the supports and nickel phosphide catalysts are illustrated in Fig. 7\n. The bands at 1540\u00a0cm\u22121 and at 1450\u00a0cm\u22121 are ascribed to pyridine protonated by Br\u00f6nsted sites and pyridine coordinated with Lewis acid sites, respectively [49], while the band at 1490\u00a0cm\u22121 is attributed to combination of both acid sites [49,50]. Albeit some bands are not very defined on some samples, all supports and all catalysts exhibit bands assigned to Lewis sites, to Br\u00f6nsted sites and combination of both of them.Regarding zeolites, Br\u00f6nsted sites can be ascribed to OH acid, such as terminal silanol groups (Si-OH) [51] and bridged hydroxyl group (Si-OH-Al) [47], while Lewis acid sites are due to extraframework aluminum [51]. In addition, nickel phosphides catalysts present Br\u00f6nsted sites due to P-OH group, resulting of non-reduced P [9,22] and Lewis acid sites are ascribed to non-reduced and partially reduced Ni species [27]. Moreover, it is important to highlight that Br\u00f6nsted sites are required in reaction involving transformation of hydrocarbons, while Lewis sites do not present catalytic activity by themselves, however, they enhance the strength and activity of sites when associated with them [52].The product distribution correspond to diesel fuel range (C10-C25) (Fig. 8\n) [13]. The main products are C17 and C18 hydrocarbons. C17 hydrocarbons derived from decarbonylation/decarboxylation of oleic acid, while C18 hydrocarbons derived from hydrodeoxygenation of this fatty acid [3]. The gas chromatography analysis of the oleic acid showed that this reactant contained 90% of oleic acid, 1% of stearic acid and 5% of palmitic acid and the latter explains the presence of C15 and C16 hydrocarbons, derived from decarbonylation/decarboxylation and hydrodeoxygenation, respectively. Furthermore, C15 and C16 can result from scission \u03b2 and \u03b1 of oleic acid, respectively, however, this approach is hardly considered on previous studies [53].The yield of the total amount of hydrocarbons (C10-C18) at 300\u00a0\u00b0C and at 6\u00a0h follows the order: Ni2P/Al-SBA-15 (42%)\u00a0>\u00a0Ni2P/H-ZSM-5 (29%)\u00a0>\u00a0Ni2P/USY (24%) (Fig. 8). Without catalyst, the production was c.a. 0.5% of C17 and 0.6% of C18 hydrocarbons, an insignificant production when compared to the catalytic tests, therefore proving the activity of the prepared catalysts. The highest yield over the catalyst supported on the Al\u2013SBA-15 can be ascribed to its mesoporous nature, which implies less diffusional resistance. Between the zeolites, the catalyst supported on H-ZSM-5 achieved more hydrocarbons, probably due to its strong acidity and although the USY has a mesoporous contribution, it has a small amount of them. Furthermore, albeit the major phase on the Ni2P/Al-SBA-15 was the Ni12P5, which is least active than the Ni2P phase on hydrotreating reactions [39], it yielded more hydrocarbons, confirming its impressive activity (i.e. production of hydrocarbons). Comparing with previous studies, Jeon et al. (2019) obtained 54% of selectivity of C9-C17 hydrocarbons with a Ni catalyst promoted with Pt and supported on Ce0.6Zr0.4O2 on the deoxygenation of oleic acid [5]. In addition, the work of Silva et al. (2016) resulted in 43% of hydrocarbons on the deoxygenation of macauba pulp oil using Pd/C [13]. As on the present work it was obtained an amount of hydrocarbons close to the previous results, this shows that the Ni2P/Al-SBA-15 has a remarkable activity on hydrocarbons production, comparable to noble metals. As can be seen in Table 3\n, the reaction conditions of previous studies, considering the temperature, the pressure and hydrocarbon yield is similar to the ones used for this study, which permits an adequate comparison between the results. Although nickel phosphide has been tested as an active phase, no previous studies were found that compare the effects of the supports presented in this paper.Concerning the surface specific area of the catalysts, it follows the order: Ni2P/USY\u00a0>\u00a0Ni2P/H\u2013ZSM-5\u00a0>\u00a0Ni2P/Al-SBA-15, suggesting that the catalyst area is not related with the reaction pathway [54]. In addition, as the oleic acid kinetic diameter is 0.55\u00a0nm [55], which is bigger than the micropores of the zeolites USY and H-ZSM-5, thus the reaction occurs on the external crystal surface [54]. The difference between the zeolites can be attributed to the strong acidity of H-ZSM-5, resulting in lighter products (C10-C16) (Section 3.2.2), due to cracking reactions [56].Another factor that can impact the production of hydrocarbons is the crystallite size [5,36]. On the present work, the nickel phosphide crystallites sizes follows the order: Ni2P/H-ZSM-5 (10.25\u00a0nm)\u00a0<\u00a0Ni2P/Al\u2013SBA-15 (13.10\u00a0nm)\u00a0<\u00a0Ni2P/USY (18.84\u00a0nm). Indeed, as this is related with production of hydrocarbons, the catalysts with smaller crystallites produce more hydrocarbons, as shown by fact that Ni2P/Al\u2013SBA\u201315 and Ni2P/H-ZSM-5 presented a higher degree of deoxygenation (Section 3.2.2). This is in agreement with the work of Zhang et al. (2017), in which the authors investigated the effect of citric acid addition on the size of nickel phosphide nanoparticles supported on mesoporous H\u2013ZSM-5 and they concluded that the hydrodesulfurization activity of 4,6\u2013 dimethyldibenzothiophene increased with smaller Ni2P particles [36]. Moreover, it is known that Ni2P has two types of Ni structures: tetrahedral (Ni (1)) and pyramidal (Ni (2)). The fraction of the latter increases with a decrease on the particle size. As this site is responsible for the hydrodeoxygenation pathway while Ni (1) is responsible for decarbonylation/decarboxylation reactions [57]. As the main product is C17 hydrocarbons for all catalysts, we can state that the size of the Ni2P crystallites are such that the majority of sites is in the form Ni (1), thus decarbonylation/decarboxylation reactions prevail, which is an advantage due to the lower consumption of H2 when compared to hydrodeoxygenation [1].The results of the turnover frequency are summarized in Table 4\n and they are in agreement with results reported previously [7,58]. The TOF follows the order: Ni2P/Al-SBA-15\u00a0>\u00a0Ni2P/H-ZSM-5\u00a0>\u00a0Ni2P/USY, which is the same order of the production of hydrocarbons. Albeit Ni2P/USY achieved the highest metal site concentration, it had the lowest TOF, due to the lowest production of hydrocarbons.The catalytic tests were performed at 260\u00a0\u00b0C, 280\u00a0\u00b0C and 300\u00a0\u00b0C as this range of temperature is appropriate for the deoxygenation of fatty acids [59]. Fig. 9\n shows that the total amount of hydrocarbons (C10-C18) increased from 260\u00a0\u00b0C to 280\u00a0\u00b0C however, there is a decrease from 280\u00a0\u00b0C to 300\u00a0\u00b0C. Fig. 10\n exhibits the degree of deoxygenation according to the temperature. At 300\u00a0\u00b0C the highest degree of deoxygenation is ascribed to Ni2P/H-ZSM-5, followed by Ni2P/Al\u2013SBA-15 and Ni2P/USY. At 280\u00a0\u00b0C Ni2P/Al-SBA-15 achieves the highest oxygen removal and at 260\u00a0\u00b0C this catalyst has the lowest degree of deoxygenation. Fig. 11\n (c) shows that the yield of lighter hydrocarbons (C10-C16) increases with temperature, because cracking reactions are favored are higher temperatures [1]. Regarding C17 hydrocarbons (Fig. 11 (a)), there is a peak of production at 280\u00a0\u00b0C. The oleic acid conversion (Fig. 11 (d)) also exhibits a maximum hydrocarbon production at 280\u00a0\u00b0C except for the H-ZSM-5 supported catalyst. We suggest that the decrease of production at 300\u00a0\u00b0C can be ascribed to catalyst deactivation [2] and probably H\u2013ZSM-5 does not deactivate due to small pores, preventing coke deposition [47]. Regarding C18 hydrocarbons (Fig. 11 (b)), more hydrocarbons are achieved at a higher temperature, except with the catalyst supported on USY. Another interesting outcome is that Ni2P/Al-SBA-15 achieves more hydrocarbons (C10-C18) at 300\u00a0\u00b0C and 280\u00a0\u00b0C but it achieves less than the other catalysts at 260\u00a0\u00b0C. This shows that this catalysts leads to a reaction mechanism with higher activation energy, and consequently, changes in temperature have a larger impact on reaction rate.The literature shows that long reaction runs decreases the diesel yield, as higher temperatures and long reaction times lead to cracking of fatty acids and hydrocarbons into smaller molecules [1], supporting the activity decrease from 280\u00a0\u00b0C to 300\u00a0\u00b0C. For instance, Hor\u00e1\u010dcek et al. (2019) tested Mo carbide, nitride and phosphide catalysts on the deoxygenation on rapeseed oil and it was reported a decrease on the catalytic activity as temperature increased due to an increase on cracking reactions [60]. In addition to that, Yang et al. (2013) noticed a decrease on the yield of C15-C18 hydrocarbons as temperature increased from 300\u00a0\u00b0C to 325\u00a0\u00b0C and to 350\u00a0\u00b0C on the hydrotreating of oleic acid [61].According to the product distribution obtained and based on previous works, a possible reaction pathway was proposed for the deoxygenation of oleic acid over nickel phosphide catalysts, as illustrated in Fig. 12\n. Firstly, oleic acid is hydrogenated to stearic acid (1) [2,5] or it can be decarboxylated [12] and hydrogenated to heptadecane (2). Stearic acid can then form heptadecane from decarboxylation (3) [2,12,17] or it can be reduced to form an aldehyde (4) [17,33]. The aldehyde can form heptadecane from decarbonylation (5) [17] or it can reduce and result in an alcohol (6) [17,33]. Finally, hydrodeoxygenation of alcohol result in octadecane (7) [33].The catalysts present different behaviors regarding reducibility, acidity and dispersion, which influences that deoxygenation activity and the diesel yield. The total amount of hydrocarbons (C10-C18) reached at 300\u00a0\u00b0C followed the order: Ni2P/Al-SBA-15 (42%)\u00a0>\u00a0Ni2P/H-ZSM-5 (29%)\u00a0>\u00a0Ni2P/USY (24%). The catalyst Ni2P/Al-SBA-15 achieved more hydrocarbons than the other catalysts due to its mesoporous nature and its small nickel phosphide particles size. Between the zeolites, the Ni2P/H-ZSM-5 produced more hydrocarbons, on account of its strong acidity and small particle sizes. All catalysts exhibited more C17 than C18 hydrocarbons resulting from decarbonylation/decarboxylation reactions. Regarding the temperature effect, the amount of hydrocarbons increased from 260\u00a0\u00b0C to 280\u00a0\u00b0C but it decreased from 280\u00a0\u00b0C to 300\u00a0\u00b0C, probably due to catalyst deactivation.\nMariana de Oliveira Camargo: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Jo\u00e3o Louren\u00e7o Castagnari Willimann Pimenta: Writing - review & editing, Formal analysis, Investigation. Mar\u00edlia de Oliveira Camargo: Writing - review & editing, Investigation. Pedro Augusto Arroyo: Supervision, Project administration, Funding acquisition, Writing - review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We would like to acknowledge CAPES, Brazil (Coordination for the Improvement of Higher Education) for the student scholarship. We are also grateful to COMCAP at State University of Maring\u00e1 by the analyses conducted on its facilities such as SAXS, TEM and FTIR.", "descript": "\n                  An alternative to non-renewable fuels is the production of green diesel from deoxygenation of vegetable oils. In this study, the effect of the supports on nickel phosphide catalysts for deoxygenation of oleic acid to produce hydrocarbons was investigated. Nickel phosphide catalysts supported on USY, H-ZSM-5 and Al-SBA-15 were synthetized by temperature programmed reduction (TPR) of metal phosphate precursors and the prepared catalysts were characterized by atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), small angle X-ray scattering (SAXS), temperature programmed reduction of hydrogen (TPR), nitrogen physisorption, temperature programmed desorption of ammonia (TPD), transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FTIR). The catalytic tests were performed in an autoclave batch reactor at 260\u00a0\u00b0C, 280\u00a0\u00b0C and 300\u00a0\u00b0C and 50\u00a0bar of hydrogen. The production of hydrocarbons (C10-C18) at 300\u00a0\u00b0C followed the order: Ni2P/Al-SBA-15\u00a0>\u00a0Ni2P/H-ZSM-5\u00a0>\u00a0Ni2P/USY. The Ni2P/Al-SBA-15 exhibited a remarkable deoxygenation activity, compared to noble metals. All the catalysts achieved more C17 than C18 hydrocarbons, therefore, decarbonylation/decarboxylation reactions prevail. With regard the temperature effect, the amount of hydrocarbons increased from 260\u00a0\u00b0C to 280\u00a0\u00b0C but it decreased from 280\u00a0\u00b0C to 300\u00a0\u00b0C for all catalysts, which can be ascribed to catalyst deactivation.\n               "}
{"full_text": "Serious environmental and health challenges that are caused by air pollution such as global warming, weather changes, damage to plants, and spread of many diseases such as chronic asthma, pulmonary insufficiency and cardiovascular diseases, which is responsible for about 9 million deaths per year, have become a common problem in the world [1,2]. For example, the combustion of fuel is the main cause of emission of most CO pollutant, which has been called as the silent killer for the 21st century [3,4]. However, the World Health Organization has reported that, stricter guidelines for air quality standards not to exceed 4\u00a0mg/m3 for CO [5]. Therefore, many researchers have reported that the removal of pollutants from exhaust gases at ambient conditions is carried out using heterogeneous catalytic methods [6]. For example, Dey et al. study on gases of a laboratory composition containing CO and O2 and argon or nitrogen as balance, showed that silver (Ag) is the best catalyst for many catalytic oxidation reactions and is highly active for carbon monoxide (CO) oxidation at low temperature [7]. Whereas, the study concluded that the performance of silver catalysts depends on their structure, surface active sites, and different AgO reactions. So the small size silver metal (Ag0) particle is the main factor to improve the catalytic performance of the supported catalyst [7]. While Hernandez et al. reported that Ag0 is formed on the catalysts prepared by impregnation even without a reduction treatment [8]. Among the catalysts used in catalytic oxidation reactions are also transition metal oxide catalysts, for example nickel oxide catalyst supported on Al2O3 or TiO2 [9]. Mixed silver\u2011nickel oxide AgNiO2 have high activity in CO oxidation in gases of a laboratory composition containing CO and O2, and helium as balance at low temperature [10]. It was noticed that above 150\u00a0\u00b0C the removal of CO becomes complete because the structure of AgNiO2 undergoes complete and irreversible destruction with the formation of individual Ag0/AgOX and NiOX particles [10]. Among the cheap transition metal oxide catalysts, Iron oxide and their composite oxides are also used as catalyst and catalyst carriers on CO oxidation [11]. furthermore, it has been found that introduction of silver to the transition metal oxide systems strongly increased activity of the catalysts in the oxidation of CO [12,13,14], combustion of methane [15,16] or volatile organic [11,17]. The increased activity is often attributed to the donation of oxygen from surface transition metal oxide sites to silver species [18] and increased dispersion of silver [19,20]. On the other hand, Zhang et al. reported that silver catalyst supported on iron oxide, alumina and montmorillonite, as a clay binder, has a high catalytic activity for propylene oxidation with a complete removal 100% at 350\u00a0\u00b0C [21]. Clay bonding materials of zeolites or bentonite or a mixture of them are usually used, which increase the dispersion of the catalyst due to the large specific surface of these materials [22]. In general, Eqs. (1),(2) and (3) show the air oxidation of CO and HC over a catalyst [23].\n\n(1)\n\nCO\n+\n\nO\n2\n\n\u2192\n\nCO\n2\n\n\n\n\n\n\n(2)\n\nHC\n+\n\nO\n2\n\n\u2192\n\nCO\nX\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n(3)\n\nCO\n+\n\nH\n2\n\nO\n\u2192\n\nCO\n2\n\n+\n\nH\n2\n\n\n\n\nThe catalytic efficiency of the oxidation reaction of CO and HC increases with the increase in the concentration of oxygen in the exhaust gas. Therefore, a phase of air (secondary air injection) is added to the exhaust gas after remove nitrogen oxides in a reduction catalyst [24,25].\nThe main goal of the present study is to prepare a new catalyst from relatively cheap raw materials(a mixed fabrication of NiO, Ag, Fe2O3, Al2O3, and Syrian natural bentonite) as a catalyst for removing CO and CH instead of the triple catalyst that contains rare and expensive materials such as platinum, rhodium and palladium.Silver nitrate (98%) was purchased from Sigma- Aldrich, iron oxide (98%) from BDH. Nickel nitrate hexahydrate (98%) and alumina were bought from Merck. Syrian natural bentonite (63\u00a0\u03bcm), Aleppo Pylon. Commercial quartz tubes are 30\u00a0cm long and 1\u00a0cm average in diameter.X-ray fluorescence (XRF) analysis was performed on a Sequential ARL 8410 spectrometer. X-ray diffraction (XRD) patterns were recorded using a Philips (PW1830) diffractometer with Cu K\u03b1 radiation operating at 40\u00a0kV and 50\u00a0mA. the patterns were collected in a 2\u03b8 range from 3\u00b0 to 60\u00b0, with a sampling width of 0.01\u00b0 and a scanning step of 2\u00b0/min. The differential thermal analysis (DTA) was conducted with Shimadzu DTA & DTG-60H instrument. BET surface areas were determined from the nitrogen adsorption curve by the conventional multipoint technique with a Micromeritics Gemini 3. The samples for BET measurements were pretreated at 300\u00a0\u00b0C for 3\u00a0h at high vacuum. Surface morphology of catalysts was examined by scanning electron microscopy (SEM) (VEGA II xmu, TESCAN). The energy dispersive X-ray spectra (EDX) of samples were also acquired during their conventional scanning electron microscopic investigations. Gas Analyzer (Kane AUTO 5\u20131) was used for CO, CO2, O2, NO and HC determination.(NiO, Ag/ Fe2O3, Al2O3, Bentonite) catalyst was synthesized in our laboratory according to the literature [26] with some modification. Briefly, the mixed silver and nickel oxide was prepared by impregnation from a solution of AgNO3 (0.3\u00a0N) and Ni(NO3)2\u00b76H2O (3\u00a0N) on a pre-calcined base composed of iron oxide, alumina and bentonite (0.57:1:1 wt) respectively. The base was suspended on etched quartz tubes (Fig. 1a). The length of the suspended part of the base on the quartz tube was 25\u00a0cm, and the overall thickness of the base layer inside and outside was 1\u00a0\u00b1\u00a00.15\u00a0mm. The whole catalyst was dried for 24\u00a0h in air and calcined at 400\u00a0\u00b0C for 5\u00a0h.The catalytic measurements were performed by pulse method using temperature-programmed reactor (an automatic setup) equipped with two flowmeters and gas analyzer. The reactor and blank tube were placed vertically in a split-open furnace as shown in Fig. 1b. Applying the same temperature conditions to both the catalytic reactor and the blank tube allows eliminating the effect of gas expansion in the gas phase. First, the engine is running for five minutes, then the exhaust gas was collected in a vacuum air bag in order to control the exhaust gas flow velocity precisely and obtain a stable exhaust gas composition for each single experiment (one temperature and one flow velocity). The exhaust gas was mixed with air at the same flow space velocity as it entered the reactor or blank tube. No difference in pressure was observed at the reactor inlet and outlet, as well as the blank tube due to the relatively large diameter of the quartz tube that suspends the catalyst and space between each other. As a result, a rather homogeneous diffusion of the mixture gas occurs over the catalyst surface, so the mass and temperature gradients resulting from the oxidation reaction are minimal. In addition, the relatively short pulse time and the expansion of the catalyst positively affect the kinetic study, (small amount of heat released from oxidation reactions and its dissipation). The weight flow per hour of CO or HC was calculated using flow space velocity of mixture gas (F\u00a0=\u00a01000 to 1800\u00a0ml.min\u22121), CO or HC blank concentration, and ideal gas equation at laboratory temperature and pressure using Eqs. (4),(5). The weight hourly space velocity was then calculated by the Eq. (6). The catalytic oxidation of CO and HC over catalyst (NiO, Ag /Fe2O3-Al2O3-Bentonite) was carried out at atmospheric pressure in a fixed bed tubular reactor containing 3 tubes of catalyst (net weight of catalyst 5.81\u00a0g (at different temperatures (300\u00b0, 320\u00b0, 340\u00b0, 360\u00b0, 380\u00b0 and 400\u00a0\u00b0C) and different weight hourly space velocity (WHSVCO 0.193 to 0.529\u00a0h\u22121, WHSVHC 0.014 to 0.031\u00a0h\u22121). The initial concentrations of the components of the gas mixture measured in the blank tube (CO 1.71 to 2.8%, HC 394 to 525\u00a0ppm, CO2 1.1 to 2.1%, O2 13.02 to 16.46%) NO gas was not observed. The conversion of CO or HC was calculated by the Eq. (7).\n\n(4)\n\nPV\n=\nnRT\n\n\n\n\n\n(5)\n\n\nweight\nx\n\n\nflow\n\nper\n\nhour\n=\n\n\nF\n\u00d7\nP\n\u00d7\n\nMw\nx\n\n\u00d7\n\n\n\nC\nx\n\n\nblank\n\n\u00d7\n60\n\n\nR\n\u00d7\nT\n\n\n\n\n\n\n\n(6)\n\n\nWHSV\nx\n\n=\n\n\n\n\nweight\nx\n\n\nflow\n\nper\n\nhour\n\ncatalsyt weight\n\n\n\n\n\n\n(7)\n\n\nX\nx\n\n=\n\n\n\n\n\nC\nx\n\n\nblank\n\n\u2212\n\n\n\nC\nx\n\n\ncatalyst\n\n\n\n\n\n\nC\nx\n\n\nblank\n\n\n\n\n\nWhere x is either CO or HC in Eqs. (5), (6) and (7).Characterization of bentonite was carried out using XRF which aimed to analyze and determine the elemental composition. Table 1\n shows the elements present in the bentonite sample.DTA analysis of the natural bentonite in Fig. 2a demonstrates endo-thermal actions at 107.34\u00a0\u00b0C, 224.36\u00a0\u00b0C and 747.25\u00a0\u00b0C. \u00d6nal and Sarikaya reported that endo-thermal actions of the natural bentonite between 25\u00a0\u00b0C to 400\u00a0\u00b0C are due to dehydration of inter particle water, adsorbed water and inter layer water, and endo-thermal actions between 400\u00a0\u00b0C to 800\u00a0\u00b0C with the maximum rate at 668\u2013672\u00a0\u00b0C was due to the formation of dehydroxylation water [27]. Dehydroxylation is defined as the \u201closs of a water molecule from two adjacent hydroxyls molecules\u201d [28]. In addition, the third endothermic action may be due to the decay of carbonates, as XRD spectrum in Fig. 2b shows that bentonite contains calcite (CaCO3) and dolomite (MgCa(CO3)2) which is decomposed to (MgO\u00a0+\u00a0CO2\u00a0+\u00a0CaCO3) by an endothermic action at 750\u00a0\u00b0C to 800\u00a0\u00b0C [29]. XRD also showed that bentonite contains quartz, palygorskite, Kaolinite and montmorillonite. It has been found that the essential beams of an X-ray diffraction spectrum for the minerals in clay appear within 2\u00b0 to 37\u00b0 (2\u03b8) [30].Thus, the calcining temperature did not increase during the calcining of the base (Fe2O3, Al2O3, Bentonite) or the catalyst (NiO, Ag/ Fe2O3, Al2O3, Bentonite) over 400\u00a0\u00b0C. The TGA curve in Fig. 2a indicates a weight loss of about 5% in the adsorbed dehydration region (<400\u00a0\u00b0C) and a weight loss of 17% in the dehydroxylation region and carbonates decay (400\u2013800\u00a0\u00b0C).SEM images of catalyst are shown in Fig. 3\n, which shows pronounced differences in the particle shapes. The catalyst particle sizes in Fig. 3a, ranged from 2\u00a0\u03bcm to 4\u00a0\u03bcm with aggregation. Small particles of different sizes are observed at 10,000 times zoom as shown in Fig. 3b. Relatively large nanoparticles, with diameters ranging from 100 to 500\u00a0nm, have been observed at 30,000 times zoom as shown in Fig. 3c. A clearer picture was obtained by enhancing the Fig. 3c using ImageJ software which is shown in Fig. 3d. The later was used to calculate the distribution of particles according to their diameters as shown in Fig. 3e. The elemental analysis of full area in Fig. 3b were confirmed by the EDX analysis in Table 1, revealing that it consisted mainly of O, Fe, Al, Si, Ni and Ag in addition to C. This is due to the presence of dolomite and calcite in bentonite as shown in Fig. 2B.BET surface area and pore volumes of bentonite, alumina, the base (Fe2O3, Al2O3, Bentonite) and the catalyst (NiO, Ag/ Fe2O3, Al2O3, Bentonite) were determined as shown in Table 1. The decrease in the BET surface area and the pore volumes was observed when the base had been impregnated with silver and nickel to prepare the catalyst. The decrease is likely due to agglomeration of Ag on the surface of the catalyst and occlusion of small-sized pores [31], which corresponds to what is shown in Fig. 3C, d.The concentrations of CO, CO2 and HC in the gas mixture from the catalytic reactor or from the blank tube were monitored using infrared radiation in gas analyzer (Kane). Fig. 4A & B show the conversion ratios of CO and HC, respectively, during 7:30\u00a0min at all tested temperatures and WHSVs. Relative stability of the conversions was observed along the time as well.Highest conversion ratios for CO were about 99% at 400\u00a0\u00b0C with all tested WHSVCO, and at 380\u00a0\u00b0C when (WHSVCO is 0.216\u00a0h\u22121) (Fig. 4a). Then CO conversion ratio decreases with the increase of WHSVCO (96\u201372)% at 360\u00a0\u00b0C Fig. 4a, (91\u201348)% at 340\u00a0\u00b0C Fig. 4a, (74\u201322)% at 320\u00a0\u00b0C Fig. 4a and (62\u201313)% at 300\u00a0\u00b0C Fig. 4a.HC conversion ratios decrease with the increase of WHSVHC (77\u201362)% at 400\u00a0\u00b0C Fig. 4b, (63\u201339)% at 380\u00a0\u00b0C Fig. 4b, (45\u201318)% at 360\u00a0\u00b0C Fig. 4b, (26 \u2013 8)% at 340\u00a0\u00b0C Fig. 4b, (11 \u2013 4)% at 320\u00a0\u00b0C Fig. 4b and (8 \u2013 4)% at 300\u00a0\u00b0C Fig. 4b.The values of the apparent reaction rate constant are calculated when the kinematic equation is applicable to the residual concentration of CO and HC. the reaction of CO oxidation on NiO is of the first order of CO concentration at high temperature [32]. The applicability of the kinetic equation for the conversions of CO and HC was observed at all temperatures, except the temperatures in which the conversion rate of CO exceeded 98%. The shift of these points from linearity is observed in Fig. 5a & b. The catalyzed sites are likely to be occupied by CO oxidation when the conversion ratio is above 98%. Chin et al. found that \u201cO reacts much faster with CO than with CH4, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O to produce CO2 at any residence time required for detectable extents of CH4 conversion\u201d [33\u201337]. Moreover, the initial concentration of CO is much more than the initial concentration of HC, ([CO]/[HC]\u00a0=\u00a040 to 65).New catalyst was prepared by impregnation method from base of Syrian bentonite, alumina and iron oxide for use as supports of nickel oxide and silver catalyst. It was used to oxidize CO and HC from exhaust gas emissions of gasoline electricity generator. The analysis results showed that the catalyst contained varied nanoparticles size 100\u2013500\u00a0nm, and contained 10.01w% nickel and 3.22 w% silver on its surface. The surface area was 31.01\u00a0m2.g\u22121 by BET equation. The activity catalytic results showed that the catalyst has high activation for oxidation CO \u0334 99% at 400\u00a0\u00b0C with all tested WHSVCO, and it had medium activity for oxidation HC 77% at 400\u00a0\u00b0C when WHSVHC was 0.014\u00a0h\u22121.\nA. Fawaz: Conceptualization, Methodology, Investigation, Visualization, Writing \u2013 review & editing. Y.W. Bizreh: Funding acquisition, Project administration. L. Al-Hamoud: Visualization, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the Scientific Affairs in Damascus University. The authors would like to thank Prof. Wail Al-Zoubi, Dr. Maysoon Alhafez and Dr. Hanan Al-Chaghouri for their fruitful help.", "descript": "\n                  The catalytic oxidation of carbon monoxide (CO) and hydrocarbons (HC) is a very important process for maintaining health protection systems. So nickel and silver are widely used in heterogeneous catalysis. In this work, the activate of (NiO, Ag/ Fe2O3, Al2O3, Bentonite) based gasoline Oxidation Catalyst on the exhaust emissions of gasoline electricity generator was tested. Catalyst was confirmed by scanning electron microscope (SEM), Energy Dispersive X-ray (EDX) and nitrogen adsorption for determination of BET surface area. This catalyst (Ni, Ag /Fe2O3-Al2O3- Bentonite) exhibits superior catalytic performance in the oxidation of CO and HC, with air as an oxidizer. Maximum conversion in CO and HC emissions were obtained at the rate of 99% and 77%.\n               "}
{"full_text": "With the increasing harsh of the legislation on a sulfur content, as the conventional method of reducing sulfur, the hydrodesulfurization (HDS) could not meet the increasingly severe requirements of deep desulfurization [1]. Recent years, the oxidative desulfurization (ODS) has been developed because of its high activity to aromatic sulfur compounds under mild reaction conditions [2\u20136]. In the ODS process, the aromatic sulfur compounds in oils can be oxidized into the corresponding sulfones by oxidants. Therefore, it is crucial to seek suitable oxidants and efficient catalysts for the ODS process. O2 in air has been regarded as a desirable oxidant due to its advantages of low cost, and source abundant [3,7]. However, O2 is difficult to be activated effectively under mild reaction conditions. Thus, the O2 activation is vital for the ODS catalyst performance [2\u20134].Recently, noble metal-based catalysts (W, Mo, V, Ti, Co, Cu, and Pt etc.) have been developed for the aerobic oxidation [2,8\u201312]. However, the high cost of noble metals limits their applications. Theoretical and experimental results demonstrated that the reduction of metal size into nano size or even a single atom would significantly improve the metal catalytic activity and selectivity due to more low-coordination atoms available for catalysis [13\u201315]. Therefore, dispersing the metal particles into single atom is highly desirable way not only to maintain a high density of active sites but also to reduce the cost of the catalyst [16,17]. Two dimensional hexagonal boron nitride (h-BN), possesses many inherent advantages such as high thermal and chemical stability [18]. Additionally, some defects such as boron and nitrogen vacancies are often identified during the synthesis of the h-BN nanosheets [19]. Such defects make it an excellent support to anchor single metal atom for catalytic reactions [20\u201324]. Recently, Cu and Pt nanoparticles supported on the h-BN have been synthesized and applied into the ODS process [2,11]. However, the single atom catalysts (SACs) have not been realized in the ODS field. In this work, we theoretically investigated the potential of Cu and Ni atoms supported on h-BN pristine and defective surfaces as the ODS catalysts. We hope this work will shed a light for the design of the SACs for ODS reactions.In this work, all calculations were carried out by using B3LYP functional with a dispersion-corrected term (B3LYP\u00a0+\u00a0D3) implemented in Gaussian 16 program [25]. For all the atoms including the metal atoms, a triple-zeta basis set with polarization functions (6-311G(d,p)) was used to describe the electronic wave function. Such a method has been proved successful in previous studies [26]. All the coordinates were fully optimized and no imaginary frequencies were found for the reactants, intermediates, and products. The transition states (TSs) were identified by confirming the existence of an imaginary vibration mode and intrinsic reaction coordinate calculations. The temperature is 298\u00a0K used for the free energy calculations.Three cluster models are chosen to anchor the single metal atom on the h-BN monolayer, as shown in Fig. 1\n. Specifically, h-BN model containing 27 B and 27\u00a0N atoms is used to simulate the pristine h-BN with the truncation boundary closed by H atoms. Two typical monovacancies defects are considered, one is boron vacancy (VB/h-BN), which is constructed from the h-BN by removing a boron atom. Similarly, a nitrogen vacancy (VN/h-BN) is built by removing a nitrogen atom from h-BN. All the displayed geometries have been optimized. The h-BN model maintains a planar configuration with a BN distance of 1.448\u00a0\u00c5, consistent with the experiment value of 1.44\u00a0\u00c5 [27]. Both vacancies experience obvious lattice deformations. In the case of VB, the values of NN distance near the vacancy are 2.739, 2.739, and 2.878\u00a0\u00c5, consistent with the corresponding value of 2.64, 2.65 and 2.70\u00a0\u00c5 in previous report [28]. For the VN, the distances between BB near the vacancy are 2.042, 2.043, and 2.458\u00a0\u00c5, which are comparable with the values of 2.06, 2.10 and 2.10\u00a0\u00c5 in the previous report [28]. Thus, the accuracy of our models and theory is further confirmed, and will satisfy the purpose of our work. The adsorption Gibbs free energy was defined as:\n\n\n\n\u0394G\nad\n\n=\n\nG\n\n\nadsorbate\n+\nmodel\n\n\n\n\u2212\n\nG\n\nfree molecule or atom\n\n\n\u2212\n\nG\n\nfree model\n\n\n\n\nwhere G\n(adsorbate+model) denotes the total Gbbis free energies of the adsorption system, while G\n(free molecule or atom) and G\n(free model) are the total Gbbis free energies of the separate metal atom and h-BN models, separately. As the definition, positive values of \u0394Gad indicate an endothermic process, whereas negative values an exothermic process.Three types of structures for Cu and Ni atom anchored on h-BN are considered here, as shown in Fig.S1. In the case of the pristine h-BN surface, both TM atoms prefer to be adsorbed on the top of N atom of h-BN, and the adsorption height for Cu and Ni on the h-BN sheet is 2.019\u00a0\u00c5, 1.835\u00a0\u00c5, respectively, in good agreement with previous reports [21,29,30]. When the TM atom is introduced into the VB site, it is observed an outward displacement from the plane of h-BN due to the larger size of TM atoms as compared with B atoms, and three TM\u00a0\u2212\u00a0N bonds are formed. Specifically, in the case of Cu/VB, three CuN bond lengths are 1.763, 1.763, and 1.801\u00a0\u00c5, respectively, comparable with previous report of 1.83\u00a0\u00c5 [29]. In the case of Ni/VB, three NiN bonds are observed with the average bond length of 1.776\u00a0\u00c5, in line with previous report of 1.81\u00a0\u00c5 [31]. In the case of N-vacancy, the TM atom is located near the center of the defect with three TM\u00a0\u2212\u00a0B bonds formation. For example, for Cu/VN, the average Cu\u00a0\u2212\u00a0B bond length is 2.102\u00a0\u00c5, close to the value of 2.14\u00a0\u00c5 in previous report [29]. For Ni/VN, three Ni\u00a0\u2212\u00a0B bond lengths are equivalent with the distance of 1.873\u00a0\u00c5, in accordance with previous results of 1.91\u00a0\u00c5 [31]. Therefore, when TM atom is embedded in the VB or VN, the TMN3 or TMB3 moiety is formed and probably exhibit high catalytic activity for ODS reactions.\nFig. 2\n demonstrates the binding Gibbs free energy (\u0394Gb) of Cu and Ni on the pristine and defective h-BN surface. Table S1 summarizes the \u0394Gb and NPA charges for the adsorption system. Our calculations illustrate that Cu and Ni could not bind strongly on the pristine h-BN due to the slightly positive binding energy, In the previous report, although the calculated Eb was negative for Cu on h-BN, the small value of \u22125.1\u00a0~\u00a0\u22125.5\u00a0kcal/mol also indicates the weak interaction between Cu and h-BN [29,32]. The interactions become stronger when TM atom is trapped on the VB or VN. Specifically, Cu atom is adsorbed at VB and VN with the binding energy of \u221295.7 and\u00a0\u2212\u00a043.8\u00a0kcal/mol, respectively. In the case of Ni, our calculated binding energy is\u00a0\u2212\u00a0145.9 and\u00a0\u2212\u00a069.5\u00a0kcal/mol on VB and VN, respectively. Thus, both TM atoms are more stable on VB site than on VN site, this is in good agreement with previous reports [29,31]. Besides, it is vital for a single atom catalyst (SAC) that the metal atom exists as a single atom rather than as diffused or aggregated atoms, the binding energy of the TM supported on h-BN is compared with the corresponding experimental cohesive energies of metals. For example, the experimental cohesive energy per atom of the bulk metal for Cu and Ni is\u00a0\u2212\u00a080.5\u00a0kcal/mol and\u00a0\u2212\u00a0102.4\u00a0kcal/mol respectively [33]. Therefore, Cu and Ni atoms may prefer not to be clustered when they are dispersed on the VB site of h-BN. Overall, the binding energy is largely enhanced at the vacancies as compared with that on pristine h-BN, which indicates the monovacancy site can be a good anchoring site for the single metal atom.In previous study, O2 was found to be inert on the defect free h-BN surface, but activated on defected h-BN [26], and metal atom supported on h-BNNSs [20,21,29]. We performed the O2 adsorptions on the supported TM atoms, and the optimized geometries and corresponding binding energies are shown in Fig. 3\n. In the case of h-BN surface, both TM atoms have the exceptional ability for O2 activation. On Cu/h-BN and Ni/h-BN, O2 is adsorbed on top of TM atom with the OO bond length largely stretched to 1.356 and 1.418\u00a0\u00c5, respectively. And the corresponding adsorption energies are \u221229.4, \u221238.2\u00a0kcal/mol, respectively. However, the activation of O2 is quite different on TM/VB and TM/VN. Specifically, O2 is readily activated on TM/VN with the OO bond largely enlarged, and the corresponding binding energies are \u221220.0, \u221230.4\u00a0kcal/mol respectively. While in the case of TM/VB system, O2 is not easily adsorbed on both TM/VB due to positive binding energies. Therefore, the activation ability for O2 decreases in the order TM/h-BN, TM/VN and TM/VB. It is interesting to note TM/VB has the most stability but the least activity for O2 activation. With the exception on the TM/VB, O2 will be activated to the superoxide state (O2\n\u00af), which will be reflected from the NPA charges of adsorbed O2, see Table S2. As we all known, the O2 activation is a key step for the next ODS reactions. Therefore, the TM atoms supported on the pristine and the N-vacancy of h-BN will probably be an active ODS catalysts.As discussed above, O2 is not activated on the Cu/VB, the overall ODS process is further carried out on Cu/h-BN and Cu/VN surfaces.\nFig. 4a presents the optimized geometries of each elementary step on the surface of Cu/h-BN. The co-adsorptions of O2 and dibenzophene (DBT) are both nearby the Cu atom, and the OO bond is stretched to 1.348\u00a0\u00c5 (ISa). Subsequently, the activated O2 will transfer one of its O to the DBT, forming transition state (TS1a). In TS1a, the OO bond is further elongated to 1.705\u00a0\u00c5, and the dissociated O is gradually approaching the S atom of DBT with the SO bond shortened to 1.721\u00a0\u00c5. As a result, the DBTO species is formed in the intermediate (IM1a), and the remaining O atom is settled on Cu atom with a CuO bond length of 1.752\u00a0\u00c5. In the next step, Cu-O\u204e species will act as the active species for DBTO further oxidation. At last, the DBTOO is obtained in FSa by overcoming TS2a, and the single Cu atom is now free on top of N atom of h-BN surface, which will continue to play an important role for the ODS reactions.\nFig. 4b displays the geometries of each step on Cu/VN surface. In the co-adsorption configuration, O2 is bridged adsorbed between Cu and B atom of the h-BN surface. Notably, the OO bond is largely stretched into 1.431\u00a0\u00c5 (ISb). Overcoming TS1b, one of the O atoms is transferred to DBT, resulting in the formation of DBTO in IM1b. Note that in IM1b the residual O atom is now bridged bound between the B and Cu, leading to the formation of a stable five-membered ring with the BO and CuO bond distance of 1.461\u00a0\u00c5, 1.887\u00a0\u00c5, respectively. As mentioned in our previous report [26], the bridge-bound O will lose its activity on metal-free surface. Fortunately, the bridged B-O\u204e species is transformed into Cu-O\u204e species as shown in IM2b, then Cu-O\u204e species as the active species is attacking the S atom of DBTO. Finally, DBTOO is produced as shown in FSb.\nFig. 5\n displays the corresponding free energy profiles on the Cu embedded surfaces. In the case of Cu/VN, firstly, the co-adsorption of O2 and DBT is thermodynamically beneficial by an energy release of 33.9\u00a0kcal/mol. Then the first O transfer occurs, which can be written as DBT\u00a0+\u00a0O2\n\u204e\u00a0\u2192\u00a0DBTO +O\u204e. This step must overcome the energy barrier of 37.5\u00a0kcal/mol. The free energy of DBT oxidation to DBTO is slightly uphill by 5.2\u00a0kcal/mol. In the subsequent step, the residual O atom experiences the transformation from the bridged adsorption into the terminal adsorption on the Cu atom, and this step requires an energy cost of 13.3\u00a0kcal/mol. At last, the DBTO is oxidized by the Cu-O\u204e species with the Gibbs free energy decreased by 7.8\u00a0kcal/mol. In this pathway, the ODS process could take place smoothly, and the first O transfer becomes the potential-limiting step. Finally, DBTOO can be easily to desorb and the Cu/VN catalyst thus be resumed for a new round of DBT oxidation. Likewise, in the case of Cu/h-BN, the first O transfer process is still the rate-limiting step with the energy barrier of 42.4\u00a0kcal/mol, and the second O transfer process takes place easily with a slight barrier of 2.2\u00a0kcal/mol. The overall reaction is exothermic by 0.4\u00a0kcal/mol on Cu/h-BN surface. Therefore, Cu/VN and Cu/h-BN could become the efficient catalysts for ODS reactions. The single Cu atom plays an important role for O2 activation and DBT oxidation. Specifically, O2 activation on CuN or CuB3 moiety ignites the oxidation, and the second O transfer process becomes easier due to the participation of the Cu-O\u204e species.The ODS process is also investigated on Ni/h-BN and Ni/VN surfaces as shown in Fig.S2. The whole process on Ni/VN is similar to that on Cu/VN surface. At first, O2 is largely activated on NiB3 moiety with a stretched bond length of 1.326\u00a0\u00c5, which acts as the oxidant for the formation of the sulfoxide. Note that the residual O dissociated from O2 is now bridged adsorbed between the Ni and B atom. At the presence of NiB3 moiety, the bridged O could be transformed into the terminal O on Ni atom, and Ni-O\u204e active species is for the final production of sulfone. Likewise, O2 is firstly activated on Ni/h-BN and initiates the ODS reaction.The free energy profiles on Ni embedded surfaces are shown in Fig.S3. The first step of ODS is still the rate-limiting step with the intrinsic energy barrier of 46.9 and 50.2\u00a0kcal/mol on Ni/h-BN, Ni/VN surface, respectively. Although the intrinsic energy barrier is relatively high, considering the large co-adsorption energies of O2 and DBT, which are \u221250.4 and\u00a0\u2212\u00a047.7\u00a0kcal/mol on Ni/h-BN and Ni/VN, respectively, the apparent activation energy is relatively low, which is \u22123.5, 2.5\u00a0kcal/mol on Ni/h-BN and Ni/VN, respectively. Therefore, the co-adsorption energies of O2 and DBT provide significant driving force for the ODS reactions. In the second step, Ni-O\u204e will account for the sulfone formation as the active species on both surfaces. Thus, both Ni/h-BN and Ni/VN surfaces will be appropriate catalysts for the ODS reaction.In summary, we systematically calculated the possibility of the single TM atoms (Ni and Cu) embedded on the pristine and defective h-BN sheet as the ODS catalyst. Some useful conclusions can be drawn from our calculations:TM atoms anchored on the VB or VN site are more favorable than on the pristine h-BN. Except for that on TM/VB surface, O2 could binds strongly and be activated to O2\n\u2212 species on both TM/h-BN and TM/VN. The first step of DBT oxidation into DBTO by the activated O2 is the rate-limiting step, and then DBTO is finally oxidized at the presence of TM-O\u204e species. Considering the excellent stability of TM/VN, both Cu/VN and Ni/VN surfaces are expected to the good catalysts with high activity toward O2 oxidation and subsequent ODS process.\nNaixia Lv: Investigation, Formal analysis, Writing \u2013 original draft. Jinrui Zhang: Formal analysis, Data curation. Jie Yin: Resources, Methodology. Hongshun Ran: Resources, Methodology. Yuan Zhang: Formal analysis, Data curation. Tianxiao Zhu: Resources, Methodology. Hongping Li: Conceptualization, Writing \u2013 review & editing, Supervision, Funding acquisition.The authors declare no competing financial interest.This work was financially supported by the Guizhou Basic Research Project (ZK[2022]561), the Qianxinan Prefecture Science and Technology Project(2021\u22122\u221232), and the National Natural Science Foundation of China (Nos. 22078135).\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106492.", "descript": "\n                  The rational design of the single transition metal atoms (Cu and Ni) anchoring on the boron nitride (TM-BN) pristine and defective monolayer has been investigated by means of density functional theory. Our calculations revealed that both B-vacancy (VB) and N-vacancy (VN) on BN monolayer are good sites for trapping Cu and Ni atoms. TM/VN and TM/h-BN systems exhibit high catalytic activity toward O2 activation and subsequent oxidative desulfurization (ODS) reactions. Considering the stability, the TM/VN is expected to be a good catalyst for ODS process.\n               "}
{"full_text": "CO2 capture and utilization is an important option for a future carbon-neutral economy as excessive CO2 emissions affect the climate and the environment [1,2]. The catalytic conversion of CO2 to value-added chemicals is one of the most promising and researched approaches. In combination with renewable hydrogen, the conversion of CO2 into fuels can help close the carbon cycle. Various products such as alcohols, acids, aldehydes, and olefins have been synthesized through thermal, electrical and photochemical CO2 conversion [3\u20138]. Among such products, CO2 hydrogenation to CO, also known as the reverse water-gas shift reaction (RWGS, Eq. 1), has attracted attention since CO is more active than CO2 and can be further hydrogenated to variety of products [9\u201313].\n\n(1)\n\n\n\n\nC\n\nO\n2\n\n+\n\nH\n2\n\n\u2009\n\u2194\nC\nO\n+\n\nH\n2\n\nO\n\n\n\u0394\nH\n\u2009\n=\n\u2009\n41.2\n\u2009\nk\nJ\n.\n\n\nm\no\nl\n\n\n-\n1\n\n\n\n\n\n\n\n\nAfter conversion to CO, CO2 can be converted into light olefins as well as jet fuel through further hydrogenation in the Fischer-Tropsch reaction [14]. In addition, the RWGS is part of a pathway to form methanol as a CO2 hydrogenation product [15\u201319]. Many catalysts have been used for CO2 conversion to CO. In general, transition metals of the groups 8\u201310 such as Ni, Pd, Ru, and Rh have been shown to form both CO and CH4, while group 11 metals such as Cu, Ag and Au produce CO more selectively [20]. In particular, Cu-based and Mo-based catalysts were found to catalyze the almost exclusive formation of CO [9,21\u201325]. Metal-carbides, nitrides, and phosphides can also form active structures for CO2 hydrogenation reactions [26\u201328]. Among various catalysts, certain patterns have been observed. For instance, although nanoparticles of group 8\u201310 transition metals form both CO and CH4, atomic dispersion of the metals shifts their selectivity towards CO formation [29\u201331]. However, the oxidation state of the metal sites can also affect the product selectivity [32]. The addition of promoters to improve the catalyst activity and stability was also studied in detail [33\u201342]. Reina\u2019s group investigated the addition of Cu and Cs promoters to Mo2C to increase the catalyst activity for the RWGS reaction [12,13]. The addition of Cu added more active sites, whereas the addition of electropositive Cs shifted the electron density favorably. The addition of alkali metals as electropositive elements has the same positive shifting effect on the electron density in the case of the RWGS reaction [43]. Other transition metals were used to form bimetallic catalysts, which promote the CO selectivity and/or catalyst activity for CO2 hydrogenation [33,44]. Ni-based catalysts have been studied extensively for hydrogenation reactions, since they are relatively inexpensive and active materials. They tend to form both CO and CH4, but are mostly used for catalyzing the CO2 conversion to CH4 (Sabatier reaction). If Ni-based catalysts could be tuned to selectively produce CO, they would be a favorable RWGS catalysts since they are more robust at higher temperatures compared to Cu-based catalysts [45]. This is an important property since the endothermic RWGS reaction requires high temperatures, at which Cu-based catalysts suffer from low stability [9]. There have been successful attempts to shift the selectivity of CO2 hydrogenation toward CO on modified Ni-based catalysts. Le Sach\u00e9 et al. synthesized a Ru-Ni/CeO2-ZrO2 catalyst with which they achieved 91 % CO selectivity, but only at high temperatures (750 \u00b0C) [46]. Braga et al. also promoted Ni by addition of Pd. They managed to reach up to 45 % CO selectivity at 600 \u00b0C [47].In this context, Cammarota et al. noticed that the addition of Ga to Ni can stabilize formate on Ni during the CO2 hydrogenation reaction [48]. Formate has been reported to be one of the intermediates in the RWGS. Although this study was carried out with homogeneous catalysts and under high pressures (34 atm), it motivated us to consider the addition of Ga to Ni to form a Ni-based catalyst with potentially high selectivity towards CO. Following this concept, we prepared a Ni-Ga catalyst on an alumina support that produces CO almost completely selectively (98 % selectivity) at a relatively low temperature (400 \u00b0C). We also studied the reaction mechanism on both Ni/Al2O3 and Ni-Ga/Al2O3 using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), from which we concluded that the presence of hydroxyl groups on the surface strongly influences the activity of the catalyst in CO2 conversion.The supported Ni-Ga catalysts as well as the control catalyst (Ni/Al2O3) were synthesized by means of the incipient wetness impregnation method based on a previously published study [49]. The desired amounts of Ni(NO3)2\u22196H2O (Sigma-Aldrich) and Ga(NO3)3\u22199H2O (Sigma-Aldrich) were used as metal precursors and \u03b3-alumina (Merck) was used as the support. For instance, 2.48 g of nickel precursor and 3 g of gallium precursor was used for the preparation of 1Ni-1Ga/Al2O3. The amount of Ni was constant for all catalysts while the amount of Ga was varied. The procedure for the synthesis of Ga/Al2O3 was the same as for the other catalysts except that no Ni was added to this catalyst. The actual loadings are presented in Table S1 in the Supporting information. The catalysts were dried at 120 \u00b0C overnight and calcined at 700 \u00b0C for 6 h with a temperature ramp of 5 \u00b0C min\u22121.The catalytic activity tests were performed using a laboratory test bench with a fixed-bed quartz tube reactor with heating wires and a temperature controller. The outlet stream was analyzed with an online MATRIX-MG01 FTIR spectrometer (Bruker) with a 10 cm gas cell, heated at 120 \u00b0C. The spectrometer was operated with the OPUS-GA software to evaluate the spectra. For each test, 100 mg of catalyst was fixed in the reactor using quartz wool. The catalyst was reduced at the desired temperature (heating ramp 10 \u00b0C min\u22121) in-situ for 1 h under 20 % H2 flow diluted in Ar prior to each test. After reduction, the temperature was set to 400 \u00b0C and a mixture of CO2 and H2 diluted in Ar was dosed into the reactor. The CO2:H2 ratio was set to 1:4 with a weight hourly space velocity (WHSV) of 30,000 mL\u2219gcat\n\u22121 h\u22121.For the OH passivation test on Ni/Al2O3, the same procedure as above was used. However, after reduction at 700 \u00b0C, the catalyst was exposed to 10 mL min\u22121 of CO diluted in Ar. The temperature was set to 350 \u00b0C to avoid the formation of nickel tetracarbonyl (Ni(CO)4), which forms at temperatures lower than 230 \u00b0C [50]. During CO introduction, the formation of CO2 was monitored and when no more CO2 was detected in the in-line FTIR, the reactor was purged with Ar for 30 min. The temperature was then set to 700 \u00b0C with a 10 \u00b0C min\u22121 ramp to desorb potentially adsorbed CO from the surface of the catalyst. The temperature was set again to 400 \u00b0C and the same reaction mixture of CO2 and H2 was used at the above mentioned conditions. At the end of the test, the Ni content of the sample was checked with ICP-OES to ensure no Ni was lost in form of Ni(CO)4 during the passivation process (Table S1).Barrett-Joyner-Halenda (BJH) mesoporous volumes and Brunauer-Emmett-Teller (BET) surface areas were calculated from N2 physisorption isotherms collected with a Micromeritics 3Flex instrument. Before measurements, all materials were degassed overnight under vacuum (<10\u22123 mbar) at 120 \u00b0C (10 \u00b0C min\u22121 ramp rate).Hydrogen Temperature Programmed Reduction (H2-TPR) profiles was recorded on a Micromeritics Autochem 2920 II instrument. Typically, the samples (\u223c400 mg) were diluted with silicon carbide (\u223c250 mg), loaded into a quartz U-shaped quartz cell, and dried under He (50 mL min\u22121) at 150 \u00b0C. After cooling to 40 \u00b0C under He, the samples were heated to 900 \u00b0C at a ramp of 10 \u00b0C min\u22121 under a flow of 10 % H2 with balance Ar (50 mL min\u22121). The effluent gasses were passed through a cold trap with a mixture of liquid N2 and ethanol, and H2 consumption was monitored with a TCD.Additional H2-TPR experiments were carried out using a slightly modified procedure. Fresh samples (\u223c100 mg diluted in 500 mg silicon carbide) were dried and cooled under He as before. Then, the samples were heated to 700 \u00b0C (10 \u00b0C min\u22121) and held at temperature for 1 h under a flow of dilute H2 (10 % H2, bal. Ar, 50 mL min\u22121). The samples were cooled under He, and reheated again at 10 \u00b0C min\u22121 to 900 \u00b0C under dilute H2.Diffuse reflectance infrared Fourier transform (DRIFT) spectra were collected using a Bruker Vertex70 spectrometer equipped with a liquid nitrogen-cooled HgCdTe detector. The spectra were recorded from 4000 to 1000 cm\u22121 at a resolution of 4 cm\u22121 and scanner velocity of 80 kHz. The sample and background spectra resulted from averaging 10 and 100 scans, respectively. Approximately 30\u221240 mg of the catalyst sample were placed in a custom-built spectroscopic cell with a low void volume [51]. The cell was equipped with a 2-mm-thick CaF2 window (Crystran) and attached to a Praying Mantis\u2122 accessory (Harrick Scientific) in the compartment of the IR spectrometer. Prior to the in-situ experiments, the sample was activated under 80 vol% H2/Ar flow for 30 min. The subsequent steps were as follows: (1) introduction of 7 vol% CO2 at 250 \u00b0C; (2) removal of 7 vol% CO2 at 250 \u00b0C with Ar; (3) ramp-up to 350 \u00b0C under Ar; (4) introduction of 7 vol% CO2 and 30 vol% H2 at 350 \u00b0C; (5) removal of 7 vol% CO2 and 30 vol% H2 at 350 \u00b0C.The X-ray photoelectron spectroscopy (XPS) measurements were carried out on an Axis Supra (Kratos Analytical) using the monochromated K\u03b1 X-ray line of an aluminum anode. The pass energy was set to 40 eV with a step size of 0.15 eV. The samples were electrically insulated from the sample holder and charges were compensated. Spectra were referenced at 284.8 eV using the C 1s orbital of the CC bond. Before XPS measurements, the samples were reduced for 1 h at 700 \u00b0C with the same flowrates as indicated in the catalytic experiments. The samples delivered to the instrument were prepared using a glovebox to ensure no air exposure occurred.Data for X-ray diffraction (XRD) were acquired using a D8 Bruker Discover diffractometer, which was equipped with a LynxEye XE detector as well as a non-monochromated Cu-source. The XRD patterns were measured for 2\u03b8 between 10 \u00b0 and 80 \u00b0 with the step size of 0.01.High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) was conducted on a FEI Talos with 200 kV acceleration voltage in the Z contrast mode. Samples were dispersed in ethanol and placed on a carbon coated copper grid. Energy-Dispersive X-ray Spectroscopy (EDXS) analysis was performed using Bruker Esprit software.While Ni nanoparticles tend to dissociate C and O on their surface, addition of Ga to Ni has been shown to suppress this effect [49]. In order to study the effect of Ga addition to Ni on its catalytic activity and product selectivity, we synthesized three alumina-supported catalysts with various Ni to Ga molar ratios, i.e. 1Ni\u20131Ga/Al2O3 (Ni:Ga = 1:1), 2Ni\u20131Ga/Al2O3 (Ni:Ga = 2:1), and 1Ni\u20132Ga/Al2O3 (Ni:Ga = 1:2), and tested them for CO2 hydrogenation. The measured CO2 conversions and the selectivities to CO and CH4 are presented in Fig. S1 in the Supporting information. The CO selectivity of these catalysts followed the order: 1Ni\u20132Ga/Al2O3\n\n\u2245\n 1Ni\u20131Ga/Al2O3 > 2Ni\u20131Ga/Al2O3 (selectivities are reported in Table S2) and the CO2 conversion was almost the same for all three, but was slightly higher for 1Ni\u20132Ga/Al2O3 (9 %) and 2Ni\u20131Ga/Al2O3 (9.5 %) compared to 1Ni\u20131Ga/Al2O3 (7 %). This means that the overall best performing candidate in terms of both activity and selectivity was 1Ni\u20132Ga/Al2O3, which was selected for further studies. The CO2 conversion rate of these studied catalysts are presented in Table S2 for comparison. The structure of these catalysts as well as the reference Ni/Al2O3 catalysts were studied by X-ray Diffraction (XRD) and the patterns are presented in Fig. S2. No clear peaks of Ni or NiO were detected, which supports the high dispersion of Ni on all catalysts. Fig. 1\n shows the CO2 conversion and CO selectivity for 1Ni\u20132Ga/Al2O3 and Ni/Al2O3 as the reference catalyst. Ga/Al2O3 was also tested as a reference but only insignificant CO2 conversion (<2 %) was detected on this catalyst. The low conversion of CO2 on Ga/Al2O3 is caused by the lack of active nickel sites for H2 adsorption and dissociation. While the CO2 conversion was clearly higher using Ni/Al2O3 compared to 1Ni\u20132Ga/Al2O3 (\u223c30 % as opposed to 10 %), its CO selectivity was significantly lower compared to 1Ni\u20132Ga/Al2O3 (\u223c40 % as opposed to 98 %). The addition of Ga caused the shift of selectivity to CO, resulting in the observed decreased conversion level since the methanation reaction almost ceased. To compare the CO selectivity using various catalysts, they should be tested in the same range of CO2 conversion. Therefore, we have decreased the WHSV for 1Ni-2Ga/Al2O3 (Fig. S3), to bring its level of CO2 conversion close to the one reported for Ni/Al2O3 in Fig. 1. We observed that the CO selectivity on this catalyst did not change compared to Fig. S1 despite its higher CO2 conversion.When using alumina-supported transition metals as catalysts, the RWGS reaction mechanism is known to proceed through CO2 adsorption (forming carbonate or bicarbonate) followed by its reaction with dissociated H atoms to form oxygenated intermediate products (formate or carboxylate). The formed intermediate product then decomposes to form CO and H2O [52\u201355]. In order to understand the reasons behind the differences in the performance of the presented catalysts, we studied the reaction mechanisms by means of in-situ DRIFTS experiments. The IR spectra were collected when introducing and purging CO2 as well as during the CO2 hydrogenation reaction and reactant cut-off.\nFig. 2\na and b shows the IR spectra obtained during the adsorption of CO2 on 1Ni-2Ga/Al2O3 and Ni/Al2O3. The persistence of the peaks at 2344 and 2358 cm\u22121 reflects the continuous presence of gas-phase CO2. For both samples, we observed two peaks at 1649 cm\u22121 and 1442 cm\u22121, which together with the weak signal at 1220 cm\u22121, are characteristic for the formation of bicarbonates [54] formed through the reaction of CO2 with surface hydroxyl groups. However, the area of the bicarbonate peak at 1649 cm\u22121 over Ni/Al2O3 was up to 23 % larger than that over 1Ni-2Ga/Al2O3, suggesting that Ga incorporation results in less CO2 uptake and adsorption (Fig. S4). Since bicarbonates are formed due to the interaction of CO2 with surface hydroxyl groups, this observation points out that there are fewer hydroxyl species in the Ga-containing sample. Indeed, the single-beam spectrum of 1Ni-2Ga/Al2O3 showed comparatively less hydroxyl groups than on Ni/Al2O3 (Fig. S5). Therefore, it is reasonable to conclude that the surface hydroxyl groups control the adsorption CO2 and the formation of bicarbonate as the intermediate product. This observation explains the lower activity of 1Ni-2Ga/Al2O3 for CO2 hydrogenation.\nFig. 2c and d shows the spectra obtained during desorption of CO2 from 1Ni-2Ga/Al2O3 and Ni/Al2O3. The abrupt disappearance of the gas-phase CO2 peaks was accompanied by a much slower extinction of the peaks at 1649 cm\u22121 and 1442 cm\u22121, which suggests the desorption of bicarbonate species. The bicarbonates almost fully desorbed from 1Ni-2Ga/Al2O3 after 10 min of purging while a significant portion remained bound to Ni/Al2O3. According to literature, stronger adsorption of the intermediate products results in their further hydrogenation to CH4 [56]. This explains the increased CH4 formation on Ni/Al2O3, where residual bicarbonate species persisted after CO2 removal. In contrast, only weakly adsorbed CO2 participates in the formation of CO through the RWGS [24]. Hence, the weaker interaction of 1Ni-2Ga/Al2O3 with the bicarbonate species proved to be beneficial in driving the selectivity towards CO.The weak adsorption of CO2 on 1Ni-2Ga/Al2O3 can be explained by the structure of this catalyst. Fig. 3\n shows the STEM images of the 1Ni-2Ga/Al2O3 catalyst. While both small and relatively large Ni nanoparticles were formed on Ni/Al2O3 (Fig. 4\n), a uniform dispersion of Ni and Ga was observed on 1Ni-2Ga/Al2O3. The STEM images in Fig. 3 suggest that Ni and Ga have mostly covered the surface of alumina since the blue color representing Al is not prominent on the edge of the STEM images on Fig. 3. Highly dispersed transition metals can only weakly adsorb the intermediate products of CO2 hydrogenation and therefore, selectively form CO [31].The aforementioned alumina surface coverage is also supported by N2 physisorption results (Figs. S6 and S7 as well as Table 1\n). Through comparison of the BET surface area (SBET) and pore volume (Vp), we noted that impregnation of Ni on alumina did not change the initial surface area (120 m2 g\u22121for Al2O3 and 117 m2 g\u22121 for Ni/Al2O3) and pore volume (0.232 cm3 g\u22121 for Al2O3 and 0.226 cm3 g\u22121 for Ni/Al2O3) of alumina. However, for 1Ni-2Ga/Al2O3, both SBET and Vp decreased to 102 m2 g\u22121 and 0.1798 cm3 g\u22121, respectively. Although this decrease in surface area as well as pore volume is not very pronounced, it may be due to the formation of a Ni-Ga entities on the catalyst surface, which blocked some of the pores and consequently decreased the surface area. Therefore, we hypothesize that the formation of the Ni-Ga layer may have decreased the availability of the Al2O3 surface and caused the aforementioned decrease in hydroxyl group concentration. This hypothesis was also checked using Al 2p XPS spectra (Fig. S8). It can be noted that the signal intensity for Al 2p decreased for 1Ni-2Ga/Al2O3 compared to Ni/Al2O3, which supports the formation of the Ni-Ga layer.The CO2 hydrogenation reaction was studied with IR spectroscopy and the results are shown below (Fig. 5\n). After introduction of H2 on both catalysts, the peaks at 1649 cm\u22121 and 1442 cm-1 started to disappear and peaks at 1595 cm\u22121, 1393 cm\u22121 and 1375 cm\u22121 formed, which indicate the presence of formate species [57]. We concluded that bicarbonate was hydrogenated to formate upon H2 introduction, in line with previously reported CO2 hydrogenation studies on Au/Al2O3 [52]. A higher formate concentration was detected on 1Ni-2Ga/Al2O3 compared to Ni/Al2O3, as evidenced by the higher ratio of the peak areas for formate (1595 cm\u22121) and bicarbonate (1649 cm\u22121). This means that nickel alone is more capable of activating adsorbed formate species than Ni-Ga, resulting in the observed higher concentration of surface formate on the Ga-treated catalyst. This explanation is also confirmed by the spectral response upon gas cut-off (Fig. 5c and d). When the gas flows were cut off, unreacted formate on both catalyst surfaces were observed, which suggests that at least some formate species remains on the catalyst surface as spectator species, most probably those formate species far distant from Ni. Formate is also known to be a potential intermediate for the methanation reaction [56]. Therefore, we propose that the formates that remain unreacted on the 1Ni-2Ga/Al2O3 surface, poisoned some of the active sites, which resulted in the lower activity of this catalyst. The same species react further to form methane on the Ni/Al2O3 catalyst, which lowers the relative selectivity of this catalyst compared to those containing Ga.Based on the results obtained from the in-situ investigation of the catalysts, the concentration of hydroxyl groups were found to be correlated with the adsorption of CO2. To further test the importance of these surface groups for CO2 hydrogenation, we passivated the hydroxyl groups on the surface of Ni/Al2O3 using an approach described by Yang et al. [58]. Their method consists of reacting the OH groups with CO to remove them from the catalyst surface through the water-gas shift reaction (WGS) (details are described in Section 2.2). Fig. 6\n shows the activity of Ni/Al2O3 for CO2 hydrogenation with and without passivation with CO. Importantly, the CO2 conversion on the CO-passivated catalyst decreased compared to the non-passivated catalyst. This further confirms the effect of the hydroxyl groups on adsorption of CO2. Both the IR spectroscopy results and the CO passivation results indicate that the presence of hydroxyl groups facilitates the CO2 adsorption through formation of bicarbonate, which in turn results in higher CO2 conversion. Al2O3 not only provides a large surface area for the dispersion of metallic active sites for the adsorption and dissociation of H2, but also supports the presence of hydroxyl groups on the catalyst surface that can actively react with CO2 and thus participate in the reaction.To better understand the relationship of structure and activity, the surfaces of 1Ni-2Ga/Al2O3 and Ni/Al2O3 were investigated by XPS. Since the catalysts were reduced in-situ before the catalytic tests, we reduced the catalysts under the same conditions before the XPS measurements. The Ni 2p spectra for both catalysts are presented in Fig. 7\n. The presence of Ga on the surface of 1Ni-2Ga/Al2O3 is evident from the Ga 2p spectra (Fig. S9). Ni on the surface of 1Ni-2Ga/Al2O3 appears to be mostly reduced and present in metallic form. However, the Ni 2p spectrum for Ni/Al2O3, shows that Ni was not fully reduced on this catalyst, where it was predominantly present as NiOOH and Ni(OH)2. Ni 2p fitting was performed based on the approach of Biesinger et al. [59]. While CO2 adsorption on Ni/Al2O3 could be facilitated due to the presence of the hydroxyl groups, H2 adsorption and activation is less favorable in the absence of metallic Ni. Considering the similar Ni loading and the same reduction conditions, this observation demonstrates that Ga promotes the reducibility of Ni.The reducibility of these catalysts were further studied in H2-TPR experiments. In our first experiment, the calcined catalysts were reduced up to 900 \u00b0C. It is clearly visible from Fig. 8\n that the reduction of 1Ni-2Ga/Al2O3 starts at lower temperatures compared to Ni/Al2O3 (\u223c520 \u00b0C as opposed to \u223c600 \u00b0C). This confirms the promoting role of Ga for the reducibility of Ni. In the second experiment, we conducted H2-TPR on both catalysts after reducing them under the same conditions used during the catalytic tests (20 vol% H2/He flow at 700 \u00b0C for 1 h). The results showed that both catalysts were partly reduced during this treatment (Fig. S10). However, by comparison with the H2-TPR profiles for the calcined catalysts, we observed that most of the Ni on 1Ni-2Ga/Al2O3 was already reduced during the pre-treatment, while a notable portion of the Ni sites of Ni/Al2O3 catalyst remained unreduced after the pre-treatment. This is also in agreement with the aforementioned XPS results (Fig. 7).As observed in the H2-TPR profiles, the peak of H2 consumption for both catalysts occurred at 830 \u00b0C, which ensured the full reduction of Ni sites. Since, based on XPS results, the Ni sites on the surface of 1Ni-2Ga/Al2O3 were mostly reduced, we did not expect much variation in catalyst activity after reduction at 830 \u00b0C. For Ni/Al2O3, however, a higher catalyst activity would be expected compared to the treatment at 700 \u00b0C, since metallic Ni has higher H2 adsorption and dissociation activity. To study these effects, we performed the same catalytic test but with in-situ catalyst reduction at 830 \u00b0C for 1 h prior to the test (Fig. 9\n). The Ni/Al2O3 catalyst activity increased after this treatment (initial CO2 conversion was 33 % as opposed to 26 % in Fig. 1). As expected, for 1Ni-2Ga/Al2O3 catalyst, little change was observed. Our results and corresponding discussions in the literature confirm that two groups of active sites are required for the RWGS reaction. One serves for adsorption and dissociation of H2 molecules, the other for adsorption of CO2. The metal sites (e.g., on Ni/Al2O3 and 1Ni-2Ga/Al2O3) are responsible for the adsorption and dissociation of H2. A lack of these sites is the reason for the low activity of Al2O3 or Ga2O3/Al2O3. However, for high selectivity of Ni-based catalysts in CO formation, high dispersion of the metallic sites is required. Otherwise, accumulation of the metallic Ni sites may lead to the formation of CH4. On the other hand, the presence of hydroxyl groups on the metal oxide (Al2O3 in this case) is required to adsorb CO2, as shown by the DRIFTS study.We did not observe any deactivation in any of the catalytic tests we performed. However, we investigated the spent catalysts with XRD for comparison. No diffraction peak for Ni or NiO was observed for both the fresh and spent catalysts (Fig. S11). For 1Ni-2Ga/Al2O3, no sign of Ga was observed in the diffraction patterns. This suggests that no large crystalline nanoparticles (> 3 nm) of Ni, NiO or Ga2O3 were formed in the fresh or spent catalysts either during the calcination, reduction, or reaction. To confirm these observations, we also took STEM images of the 1Ni-2Ga/Al2O3 after 20 h of reaction (Fig. S12). No sign of Ni and/or Ga agglomeration and sintering could be detected, confirming that 1Ni-2Ga/Al2O3 did not sinter during the reaction.Ni-based catalysts are known for their tendency to form CH4 during the CO2 hydrogenation reaction. Although they are usually more thermally robust compared to Cu-based catalysts, they are not used for the RWGS due to their low selectivity for CO. This was also confirmed by our experiments using a Ni/Al2O3 catalyst. However, our present study showed that addition of Ga can restructure Ni particles on the surface of alumina in a way, which increased the selectivity for CO. Although this shift in selectivity came at the expense of lower catalyst activity, selective formation of CO eliminates the need for downstream separation processing in industrial applications. The lower CO2 conversion could be compensated by recycling the unreacted gases to increase the overall conversion. Using in-situ DRIFTS studies as well as other catalyst characterization methods showed that the addition of Ga led to high dispersion of Ni and high surface coverage of alumina, which limited the availability of hydroxyl groups on the surface. Our results support the view that hydroxyl groups are crucial species in the CO2 hydrogenation mechanism since they are the active sites for CO2 adsorption and formation of bicarbonates and their presence contributed to the higher activity of Ni/Al2O3 compared to 1Ni-2Ga/Al2O3. Although Ga is more expensive compared to Ni and the addition of Ga does not seem economically promising, this disadvantage is compensated by the better temperature stability of these catalysts, which might help the process in the long run. Nevertheless, further studies will be required to increase their activity.\nAli M. Bahmanpour: Conceptualization, Methodology, Formal analysis, Investigation, Validation, Writing \u2013 original draft, Data curation. Rob Jeremiah G. Nuguid: Investigation, Data curation, Writing \u2013 original draft. Louisa M. Savereide: Investigation, Data curation, Validation, Writing \u2013 original draft. Mounir D. Mensi: Investigation, Data curation, Writing \u2013 original draft. Davide Ferri: Supervision, Resources, Writing \u2013 review & editing. Jeremy S. Luterbacher: Supervision, Resources, Writing \u2013 review & editing. Oliver Kr\u00f6cher: Supervision, Resources, Project administration, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.JSL, and LMS acknowledge funding from the European Research Council (ERC) under the European Union\u2019s Horizon 2020 research and innovation program (Starting grant: CATACOAT, No. 758653). RJGN, DF, and O.K. acknowledge funding from the Swiss National Science Foundation (SNF, #172669). The authors would like to acknowledge Dr. Pascal Schouwink for XRD analysis, and Mr Sylvain Coudret for ICP-OES analysis.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2021.101881.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Ni/Al2O3 is an active catalysts for CO2 hydrogenation to both CH4 and CO. By doping with Ga, we succeeded in shifting the selectivity of these catalysts almost completely toward CO. In-situ IR spectroscopy studies showed that the catalyst activity is directly related to the concentration of surface hydroxyl groups that are responsible for the adsorption of CO2 and the formation of intermediate bicarbonates and formates on the catalyst surface. The addition of Ga improved the Ni dispersion which was concomitant with the formation of a Ni-Ga layer on the surface of alumina, thereby reducing the surface hydroxyl concentration. The reduced and weakened interaction between the intermediate products, i.e. bicarbonates and formates, and the catalyst surface, increased the CO selectivity from \u223c40 % to 98 %.\n               "}
{"full_text": "Data will be made available on request.Ethylene is mainly produced at industrial scale by steam cracking of naphtha, LPG or natural gas condensates at high temperature (between 750 and 875\u00a0\u00b0C) and short contact time, resulting in products that are difficult to separate [1,2]. This process presents a low energy efficiency and it is considered as one of the most energy-consuming process in the chemical industry [3,4], with a massive CO2 formation of ca. 1.8 kgCO2/kgEthylene\n[4].The oxidative dehydrogenation (ODH) of ethane could be a low demanding energy alternative, that can operate at temperatures up to 400\u00a0\u00b0C, with high selectivity to ethylene [1,5\u20137]. The most promising catalytic systems for the ODH of ethane are multicomponent MoVTeNbO catalysts [8\u201310] and NiO based catalysts [11\u201325].In the last 15\u00a0years, Me-doped NiO catalysts, especially Nb5+- [11\u201317], but also W6+- [18], Sn4+- [19\u201321], Zr4+- [22], Ti4+-doped catalysts [20,21], or supported NiO catalysts [22\u201325], have been proposed as highly selective in ethane ODH when working at moderate ethane conversions. In this way, it has been reported that the incorporation of high valence state cations, especially Nb5+, Ta5+, Sn4+ and W6+ into the NiO lattice, decreases the amount of electrophilic O- in NiO and improves the selectivity to ethylene during the ethane ODH [21,26\u201329]. In an opposite trend, the incorporation of low valence state cations (such as Li+\n[30] or K+\n[26]) favors the deep oxidation of ethane. Recently, it has been proposed that, by tuning the Ni3+ quantity in NiO, the reaction pathway can be modulated: low Ni3+ concentration favors the ethane ODH whereas high Ni3+ concentrations lead to the deep oxidation of hydrocarbons [30]. Furthermore, other authors suggest that a high-valence dopant in an irreducible oxide will increase the bonding of neighboring surface oxygen atoms to the oxide (which would hinder oxidation reactions through a redox mechanism), however, this behavior could change depending on the atmosphere (presence/absence of oxygen in the feed) [31].By using density functional theory, the dissociative adsorption of ethane on the surface of a Nb-doped nickel oxide catalyst has been studied [32]. Thus, it has been suggested that Nb species (or NbO2 groups) substituting Ni atoms in the surface layer of NiO could favor the strong adsorption of the O2 fed in the gas phase. In this way, the presence of Nb favors a low oxidant capacity of oxygen atoms and could act as a lower-valence dopant, activating also the surface oxygen atoms surrounding these Nb-sites [32].In addition to this, NiO is a p-type semiconductor [33\u201335] with high catalytic activity for the deep oxidation of hydrocarbons and CO [35,36]. Lemonidou et al. characterized pure and doped nickel oxide catalysts with Li, Mg, Al, Ga, Ti, and Nb determining their electrical conductivity. This study showed that all undoped and doped materials were p-type semiconductors (before and after reaction), with positive holes as the main charge carriers, where a correlation between the p-type semiconductivity and the catalytic performance was observed [34]: the lower the p-type semiconductivity the higher was the selectivity to ethylene. Additionally, these authors concluded that the reaction mechanism involves surface lattice O- species operating through a Mars-van Krevelen mechanism. On the other hand, in the case of Nb-doped NiO catalysts prepared by evaporation, the electrical conductivity decreased when the Nb-content increased. This correlates well with their intrinsic rates of ethane consumption [35].Furthermore, and as it has been proposed in the case of pure NiO [37\u201342], the catalytic performance of doped catalysts [11\u201322,43], strongly depends on the catalyst preparation method.Recently, it has been proposed that the presence of oxalic acid in the synthesis gel changes the textural and the catalytic properties of NiO [41]. This fact is explained as a consequence of changes in the catalyst precursors, the formation of Nickel (II) oxalate dihydrate being paramount to achieve high selectivity to ethylene.ODH of ethane on Nb-doped NiO catalysts proceeds through a redox mechanism. Therefore, the study of the electrochemical properties of these samples is highly interesting. However, there are just a couple of articles about NiO based catalysts that have dealt with the conductivity of p-type semiconductors, and they have related this fact with the catalytic behavior in the ODH of ethane [34,35]. Then, we have decided to undertake a deeper electrochemical study of Nb-doped NiO catalysts in order to check if a correlation between electrochemical properties and catalytic performance does exist. Thus, the technique of Electrochemical Impedance Spectroscopy (EIS) has been applied, which is a powerful non-destructive technique typically used in electrochemistry and from which the resistance values of the catalysts can be properly determined. Moreover, we have also undertaken capacitance analysis (Mott-Schottky analysis) to verify the type and extent of the semiconductor behavior. Finally, cyclic voltammetries have been carried out in order to study the electrochemical activity of the different synthesized catalysts.The obtained results show that the presence or absence of oxalic acid in the synthesis gel as well as the calcination temperature have a strong influence on both the incorporation of Nb5+ atoms in the framework of NiO and on the catalytic performance in ethane ODH. In addition, a change in the semiconducting character from p- to n-type is observed in the Nb-doped NiO catalysts prepared in specific conditions (i.e. samples synthesized in the presence of oxalic acid in the synthesis gel and calcined at 500\u00a0\u00b0C), being these catalysts the ones that presented the best catalytic performance.Nb-promoted nickel oxides catalysts (with a Ni/Nb molar ratio of 9/1) were prepared through evaporation at 90\u00a0\u00b0C of aqueous solutions of nickel nitrate (Ni(NO3)2\u00b76H2O, Sigma-Aldrich) and ammonium niobate (V) oxalate hydrate (C4H4NNbO9\u00b7xH2O, Sigma-Aldrich) with different amount of oxalic acid (C2H2O4, Sigma-Aldrich) in the synthesis gel. The solids were dried overnight in a furnace at 120\u00a0\u00b0C and, finally, they were calcined in static air for 2\u00a0h at 350 or 500\u00a0\u00b0C. Unpromoted nickel oxide catalysts have been also prepared by using the same procedure, for comparison [41]. Catalysts are named as NiNb/x-T, where\u00a0\u00d7\u00a0is the oxalic acid/nickel (OxA/Ni) ratio in the synthesis gel of 0.0, 1.0 or 3.0, and T is the calcination temperature (350 or 500\u00a0\u00b0C). The as-synthesized samples (before calcination) are named as NiNb/x-as. Nomenclature and physicochemical properties of Nb-promoted and unpromoted catalysts are shown in Table 1\n and Table S1, respectively.The specific surface areas were estimated by the Brunauer-Emmet-Teller (BET) method from N2 adsorption isotherms at 77\u00a0K measured in a Micromeritics TriStar 3000 instrument.X-ray diffraction patterns were recorded with a PANalytical CUBIX instrument equipped with a graphite monochromator, by using Cu K\u03b1 radiation (\u03bb\u00a0=\u00a00.1542\u00a0mm) and operating at 45\u00a0kV and 4\u00a0mA.High Resolution Transmission Electron Microscopy (HRTEM) was performed on a JEOL JEM300F electron microscope by working at 300\u00a0kV (point resolution of 0.17\u00a0nm). Crystal-by-crystal chemical microanalysis were performed by energy-dispersive X-ray spectroscopy (XEDS) in the same microscope equipped with an ISIS 300 X-ray microanalysis system (Oxford Instruments) with a detector model LINK \u201cPentafet\u201d (resolution 135\u00a0eV). Samples for transmission electron microscopy (TEM) were ultrasonically dispersed in n-butanol and transferred to carbon coated copper grids.Raman spectra were collected with an \u201cin via\u201d Renishaw spectrometer equipped with an Olympus microscope. The samples were excited by the 514.5\u00a0nm line of an Ar+\u00a0laser (Spectra Physics model 171) with a laser power of 2.5 mW or by the 325\u00a0nm line (UV-Raman) generated with a Renishaw HPNIR laser with a power of approximately 15 mW.Temperature-programmed reduction experiments (TPR-H2) were carried out in an Autochem 2910 (Micromeritics) equipped with a TCD detector. The reducing gas composition was 10\u00a0% H2 in Ar (total flow rate of 50\u00a0mL\u00a0min\u22121), and using a heating rate of 10\u00a0\u00b0C\u00a0min\u22121 until 800\u00a0\u00b0C.X-ray photoelectron spectroscopy (XPS) measurements were performed on a SPECS spectrometer equipped with a Phoibos 150 MCD-9 detector using a monochromatic Al K\u03b1 (1486.6\u00a0eV) X-ray source. Spectra were recorded using an analyzer pass energy of 50\u00a0eV, an X-ray power of 100\u00a0W, and an operating pressure of 10-9 mbar. Spectra treatment was performed using CASA software. Binding energies (BE) were referenced to C 1s at 284.5\u00a0eV.Electrochemical interfacial properties of the catalysts were studied in a three-electrode electrochemical cell with an Ag/AgCl 3\u00a0M KCl reference electrode and a platinum wire as counter electrode. The catalysts were connected to the potentiostat as working electrodes with 0.5\u00a0cm2 of exposed area to the electrolyte. Mott-Schottky (MS) measurements, Electrochemical Impedance Spectroscopy (EIS) tests and cyclic voltammetries were carried out to characterize the catalysts, specifically their electrochemical properties at the interface between the catalysts (electrodes) and the electrolyte. Mott-Schottky plots were performed at a frequency of 5000\u00a0Hz, scanning the potential from 1 to \u22120.5 VAg/AgCl at 0.05\u00a0V/s using an amplitude signal of 0.01\u00a0V. Before EIS measurements, catalysts were immersed in the solution for 1800\u00a0s at 0.5 VAg/AgCl. After this pre-treatment, EIS tests were carried out applying a potential of 0.5 VAg/AgCl using an amplitude of 0.01\u00a0V and scanning frequencies from 100\u00a0kHz to 0.01\u00a0Hz. Cyclic voltammetries were performed in the potential range of \u22120.1 to 0.6 VAg/AgCl at 0.01\u00a0V/s. The electrolyte used for MS and EIS tests was 0.1\u00a0M of Na2(SO)4, while for the cyclic voltammetries, a solution of 10\u00a0mM of Fe(CN)6K4\u00a0with 0.1\u00a0M Na2SO4 was employed.The oxidative dehydrogenation of ethane was carried out in an isothermal fixed-bed quartz reactor, at atmospheric pressure, in the range of 300\u2013350\u00a0\u00b0C. The feed consisted of an ethane/O2/He mixture with 3/1/26\u00a0M ratio. The total flow and the catalyst\u2019s weight were varied (25\u2013100\u00a0mL\u00a0min\u22121 and 0.1\u20131.0\u00a0g of catalyst) in order to achieve several contact times. Reactants and products were analyzed by gas chromatography using two packed columns [25]: (i) molecular sieve 5A (2.5\u00a0m); and (ii) Porapak Q (3\u00a0m).\nFig. 1\nA shows the XRD patterns of Nb-doped nickel oxide catalysts calcined at 350 or 500\u00a0\u00b0C. In all cases, the presence of diffraction maxima at 2\u03b8\u00a0=\u00a037.10\u00b0, 43.30\u00b0, 62.86\u00b0, 76.50\u00b0 and 79.22\u00b0 can be indexed to a face-centered cubic NiO phase (JCPDS-ICDD pattern number 47\u20131049, space group Fm3m). No additional maxima related to Nb-compounds were detected. XRD patterns of samples calcined at 500\u00a0\u00b0C present peaks with higher intensities, which suggest a higher degree of crystallinity and/or bigger size of NiO crystals (Table 1) [38].This is in agreement to previous results, in which the size of NiO particles in Nb-doped nickel oxide catalysts is smaller than those of the corresponding pure NiO catalysts [11,15]. This lower crystal size is attributed to the incorporation of Nb5+ into the NiO network which hinders crystallization thus leading to smaller crystallites. In addition, the presence of additional oxalic acid in the synthesis gel promotes a decrease in crystal size, in a way that increasing the concentration of oxalic acid in the synthesis gel gradually decreases the intensity of the NiO diffraction maxima. Thus, the average crystallite size estimated by the Scherrer equation shows a variation from 20\u00a0nm in the catalyst prepared without oxalic acid in the synthetic gel to approximately 10\u00a0nm in the catalysts prepared with oxalic acid (Table 1).X-ray diffraction patterns of as-synthesized catalysts (Fig. S1A, in Supporting Information) confirm the presence of Nickel (II) oxalate dihydrate (NiC2O4\u22c52H2O) and Nb-oxalate in samples prepared with oxalic acid in the synthesis gel (NiNb/1-as and NiNb/3-as), whereas Nickel(II) nitrate hexahydrate (Ni(NO3)2\u22c56 H2O and Nb-oxalate are only observed in the sample prepared in the absence of additional oxalic acid in the synthesis gel (i.e. NiNb/0-as). As a consequence of changes in the nature of reactants during the synthesis procedure, changes in the thermogravimetric analysis were also observed in the decomposition of precursors (Fig. S1, B and C): i) the decomposition of Ni-nitrate corresponds to a low intense broad peak at ca. 300\u00a0\u00b0C for sample NiNb/0-as; ii) the decomposition of Ni-oxalate shows a narrow peak (very intense) at ca. 350\u00a0\u00b0C for samples prepared with oxalic acid in the synthesis gel (i.e. NiNb/1-as and NiNb/3-as); iii) changes in the nature of exothermic peak depending on the concentration of oxalic acid in the synthesis gel. These results are similar to those reported for pure NiO [41], except that a decomposition peak at ca. 250\u00a0\u00b0C, related to the decomposition of Nb-oxalate, is observed for sample NiNb/0-as. This is in agreement with previous results observed during the preparation of unpromoted nickel oxide in the presence/absence of oxalic acid in the synthesis gel [41].\nFig. 1B shows the UV Raman spectra of Nb-doped nickel oxide catalysts calcined at 350 or 500\u00a0\u00b0C. Several bands at 571, 724, 901 and 1128\u00a0cm\u22121\n[44\u201346] have been observed (see Supporting Information for the assignment of the bands). In general, the intensities of these bands change depending on the preparation method and/or the calcination temperature of catalysts. According to the spectra, the catalyst prepared in the absence of oxalic acid in the synthesis gel and calcined at 500\u00a0\u00b0C (NiNb/0\u2013500) likely presents large NiO crystal size [46]. However, the samples prepared with oxalic acid and calcined at 350\u00a0\u00b0C (NiNb/1\u2013350 and NiNb/3\u2013350) probably presents a lower crystal size and/or a higher concentration of defects [47]. Accordingly, the different crystal size and/or the concentration of defects are mainly related to both the amount of oxalic acid in the synthesis gel and/or the calcination temperature.Raman spectra of all catalysts, using a 514\u00a0nm laser (Fig. S2) present a strong band at ca. 497\u00a0cm\u22121 (Ni-O stretching mode) [11,15,38], with a shoulder at 410\u00a0cm\u22121 (which could be related to the nonstoichiometry of catalysts [11,12]. In the case of NiNb/0\u2013350 and NiNb/0\u2013500 samples, the width of the band at 497\u00a0cm\u22121 increases with the calcination temperature. However, no significant changes are found for catalysts prepared with higher oxalic acid contents in the synthesis gel. On the other hand, bands at 790, 850 and 1071\u00a0cm\u22121 are also observed for sample NiNb/0\u2013500. The band at 1071\u00a0cm\u22121 is related to the v1 vibration mode of carbonate groups on the NiO phase [16], whereas the band at ca. 840\u00a0cm\u22121 has been related to stretching vibrations of slightly distorted NbO6 octahedra in amorphous Nb2O5\n[12]. The band at 790\u00a0cm\u22121, detected by other authors in Nb-doped NiO catalysts prepared in a similar way than that of sample NiNb/0\u2013350\n[15], has been related to vibration modes of bridging Ni-O-Nb bonds, suggesting the formation of a Ni\u2013Nb solid solution.A detailed study of these catalysts was performed by High Resolution Transmission Electron Microscopy. Fig. 2\n shows characteristic high-resolution images of the catalysts under study. The largest NiO crystals were obtained when oxalic acid was not added to the synthesis gel (Fig. 2a). For the oxalic free NiNb/0\u2013500 catalyst the size of the crystallites ranges between 20 and 50\u00a0nm. Smaller particles (size\u00a0\u2264\u00a05\u00a0nm) of amorphous material were also visible in small amount. EDS nanoanalysis showed that the large NiO crystals present some Nb incorporated (typically 6 at. % of Nb) whereas the small particles present higher concentration of niobium (55 at. % of Nb). Corresponding spectra (Fig. 2a) include both Ni and Nb atomic percentages as determined from semiquantitative analysis. Note that the limited spatial resolution of the nanoprobe prevents to determine with absolute certainty if the segregation of niobium with respect to the NiO crystals is total in the selected zones, but the percentages indicate a very clear trend.The addition of oxalic acid to the synthesis gel implied a diminishing of the NiO crystal size, as previously observed by XRD. Fig. 2b shows a group of NiO crystals of NiNb/1\u2013350 and, for that sample, crystallite size ranges between 5 and 10\u00a0nm. The amorphous particles are barely observed in this catalyst. Moreover, it must be noted that increasing the calcination temperature increases the particle size to a small extent. This fact can be seen in Fig. 2c, which shows a low magnification micrograph of several particles of NiNb/1\u2013500 catalyst. As observed, this catalyst is formed by rod like particles longer than 1\u00a0\u03bcm which in fact are constituted by nanocrystals of about 20\u00a0nm. This appreciation probably suggests that a sintering process occurs when calcined at 500\u00a0\u00b0C. On the other hand, the high magnification high resolution micrograph for NiNb/1\u2013500 sample and the enlarged detail observed in Fig. 2d show an average crystal size of\u00a0\u223c\u00a020\u00a0nm. The included EDS spectra show the atomic percentages of Ni and Nb in the two types of particles observed, revealing that the amorphous particles are mostly composed of niobium (81 at. %) whereas the large NiO crystals present a very low Nb-loading (0.6 at. %). This suggests that the presence of oxalic acid in the synthesis gel decreases the Nb-incorporation in the NiO lattice and leads to the formation of more small particles very rich in Nb on the surface of the NiO crystals.At this point it is important to mention that no significant differences were observed in the microstructure of the catalysts prepared with different concentrations of oxalic acid (Oxal/Ni ratios of 1 and 3).Nb-doped nickel oxide catalysts have been also characterized by diffuse reflectance UV\u2013vis spectroscopy (DRS). Fig. 3\nA groups together the DRS spectra of the Nb-doped catalysts calcined at 350 (black) or at 500\u00a0\u00b0C (red). According to the literature, all samples show typical NiO absorption bands [22,48] at 380, 416 and 722\u00a0nm (assigned to the octahedral Ni2+ in the NiO lattice [22]), 510\u00a0nm (attributed to charge transfer in NiO [49]) and in the 400\u2013600\u00a0nm range, which have been linked to the presence of non-stoichiometric oxygen.Catalysts treated at 350\u00a0\u00b0C exhibit higher absorbance in the 400\u2013600\u00a0nm range than catalysts treated at 500\u00a0\u00b0C. The high background absorbance in this region has been associated with an increase in non-stoichiometric oxygen concentration [37]. Therefore, samples calcined at 350\u00a0\u00b0C exhibit a higher concentration of non-stoichiometric oxygen species.\nFig. 3B presents the H2-TPR profiles of catalysts calcined at 350 or 500\u00a0\u00b0C. The maximum of the reduction peak slightly shifts to lower temperatures when increasing the OxA/Ni ratio in the synthesis gel, especially for samples calcined at 500\u00a0\u00b0C. These patterns have reduction maxima centered between 277 and 366\u00a0\u00b0C, the relative intensity of which depends on the preparation method. The presence of several reduction peaks has been related to steps proposed for the reduction of NiO [50] but also to the presence of mixed phases linked to the Nb-promoter [12]. Indeed, the high temperature signals in supported NiO catalysts have been attributed to a strong influence of the promoter. In addition, the reduction peaks at low temperature (around 200\u00a0\u00b0C) observed in samples calcined at 350\u00a0\u00b0C could be related to the presence of undoped NiO particles [51], as suggested in the microscopy characterization. It has been proposed that non-stoichiometric nickel oxide exhibits a lower initial reduction activation energy than stoichiometric NiO, thus giving reduction signals at lower temperature [51]. The enlarged part of the area around 200\u00a0\u00b0C shows that the samples calcined at 350\u00a0\u00b0C have higher intensity. The use of oxalic acid in the synthetic gel leads to a decrease in its intensity. In addition, the reducibility of Ni-O bonds in Ni-Nb-O catalysts calcined at 500\u00a0\u00b0C (NiNb/x-500 series) is slightly lower than those of Ni-Nb catalysts calcined at 350\u00a0\u00b0C (NiNb/x-350 series). The reducibility of the catalysts with oxalic acid slightly increases compared to that without oxalic acid, being more evident for catalysts treated at 500\u00a0\u00b0C. When comparing these results with those achieved for undoped NiO catalysts [41], it can be concluded that the incorporation of Nb5+ promotes a lower reducibility of Ni2+ species.XPS experiments were undertaken for Nb-doped catalysts and the quantitative results of the analysis are presented in Table 2\n. Fig. 4\n shows Ni 2p3/2\n spectra for Nb-doped NiO catalysts, whereas Fig. S3 presents the Nb 3d spectra of catalysts.In the case of the Ni 2p3/2\n core level, spectra for all the samples can be deconvoluted into two signals at 853.2 and 855.3\u00a0eV, with a satellite peak at 860.2\u00a0eV. The peak at about 853.2\u00a0eV is related to the presence of structural Ni2+ species within the lattice, while the second signal (named Satellite I) at 1.5\u20132.0\u00a0eV above the main peak has been associated with the presence of structural defects (such as Ni3+, Ni2+\u2013OH species or Ni2+ vacancies in the network [20,26,29]). In addition, a second larger satellite (named Satellite II), at about 860.2\u00a0eV, is related to metal\u2013ligand charge transfer [20,26,29]. Although these three types of signals are clearly identified, notable differences can be observed among the catalysts. Thus, for catalysts calcined at 350\u00a0\u00b0C, the intensity of the signals seems to be lower than those obtained for the catalysts treated at 500\u00a0\u00b0C regardless of the preparation method.In addition, the catalysts prepared with oxalic acid in the synthetic gel, in any quantity, show a shift towards higher binding energies, which could be associated with different electrochemical properties of the catalysts.On the other hand, spectra at the Nb 3d level are shown in Fig. S3. Typically, a niobium spectrum consists of a doublet with a spin\u2013orbit separation of 2.72\u00a0eV between the 3d5/2\n and 3d3/2\n components. A single well-defined doublet is observed for the NiNb/0\u2013350 catalyst, with peaks at 206.7 (3d5/2\n) and at 209.4\u00a0eV (3d3/2\n), respectively, which corresponds to the Nb5+ species [52].For catalysts calcined at 500\u00a0\u00b0C, a well-defined doublet corresponding to the presence of Nb5+ species is also observed, regardless of the amount of oxalic acid in the synthesis gel (Fig. S3). Certain authors [53] suggest the presence of Nb4+ species when reducing agents (such as oxalic acid) are present, especially in higher amounts, however, this is not our case. As evidenced in Fig. S3, no noticeable differences can be ascertained from Nb 3d XPS analysis regarding the different synthesis method and thermal treatment in terms of chemical state. Nevertheless, quantitative differences are suggested since dissimilar intensities were observed depending on the sample.In this sense, the results in Table 1 suggest that the use of oxalic acid in the synthesis gel and high activation temperatures maximizes the presence of niobium species on the surface of the catalyst, finding the highest value for the NiNb/1\u2013500 catalyst. Then, it is observed that the concentration of Nb on the surface of NiNb/1\u2013500 is remarkably higher (4-fold) than that of the oxalic acid-free sample (NiNb/0\u2013500). These results are in agreement with that observed by TEM. Additionally, the concentration of Nb on the surface is also determined by the calcination temperature, as the calcination at 500\u00a0\u00b0C leads to a concentration of niobium double than that catalyst activated at 350\u00a0\u00b0C (NiNb/1\u2013350).The catalytic results for the ODH of ethane using Ni-Nb-O catalysts prepared using different approaches are shown in Table 2. All catalysts resulted to be highly selective to ethylene, although some catalytic differences were observed. The catalysts were tested in a temperature range of 300\u2013350\u00a0\u00b0C in order to avoid changes in the chemical nature of the samples treated at low calcination temperature and to have a better comparison of the catalytic performance of these mixed oxides, i.e. minimizing the effect of reaction temperature on ethane conversion and selectivity to ethylene. During all catalytic tests, ethylene and CO2 were the only reaction products observed.\nFig. 5\n shows the variation of the catalytic activity per mass of catalyst (Fig. 5A), the catalytic activity per surface area of catalyst (Fig. 5B) and the selectivity to ethylene under isoconversion conditions (Fig. 5C) for Nb-doped samples, calcined at 350\u00a0\u00b0C or at 500\u00a0\u00b0C, with several OxA/Ni ratio in the synthesis gel. It is observed that the addition of oxalic acid in the synthesis gel leads to an increase in both the catalytic activity and the selectivity to ethylene regardless of the calcination temperature, with a maximum in the selectivity to ethylene for catalyst NiNb/1\u2013500. Thus, the ethylene selectivity for catalysts prepared with oxalic acid ranges between 80 and 90\u00a0%, whereas the catalysts prepared in the absence of oxalic acid exhibit an ethylene selectivity of about 75\u00a0%. In addition, samples calcined at 500\u00a0\u00b0C show higher selectivity to ethylene than those calcined at 350\u00a0\u00b0C. Note that the optimal performance was observed for an OxA/Ni molar ratio of 1.We must remark that the activity per gram of catalyst differs in an important manner compared with the activity per surface area. The presence of oxalic acid in the synthesis gel highly increases the activity per mass of catalyst. However, if considered the catalytic activity per surface area the presence of oxalic acid in the synthesis gel follows a completely different trend, which depends on the calcination temperature as well as on the amount of oxalic acid in the synthesis gel.We must inform that, in all cases, the selectivity to ethylene barely varies with the conversion of ethane under the reaction conditions employed (Fig. S4). This means that carbon oxides are mainly formed directly from ethane [54]. However, variations in the selectivity to ethylene in the ODH of ethane are observed for the different catalysts tested, the NiNb/1\u2013500 catalyst exhibiting the highest ethylene selectivity while NiNb/0\u2013350 is the least selective one.\nFig. S5 shows the variation of STY (space time yield for ethylene formation) with OxA/Ni ratio in the synthesis gel for the Nb-doped catalysts. This parameter takes into account both the catalytic activity and the selectivity to ethylene. It can be observed that the formation of ethylene is increased by the incorporation of oxalic acid in the synthesis gel, this effect being clearer in the case of the NiNb/x-350 set of catalysts. In addition, STY values for catalysts calcined at 350\u00a0\u00b0C are superior to those of the corresponding catalysts calcined at 500\u00a0\u00b0C. These results are in agreement with those of Fig. 5, which show a higher reactivity of the catalysts calcined at 350\u00a0\u00b0C.Consequently, the optimal catalyst in the Nb-doped NiO catalysts requires the presence of a certain amount of oxalic acid in the synthesis gel and a final calcination at 500\u00a0\u00b0C, leading to excellent ethylene selectivity (about 90\u00a0%). Moreover, and in agreement with the catalyst characterization results, the catalytic performance of Ni-Nb-O oxides can be explained in terms of the different physicochemical properties of the catalysts.On the other hand, the influence of the calcination temperature is not entirely clear at this time. Nevertheless, the positive influence of the calcination temperature in the Nb-containing NiO catalysts could be related to the higher presence of Nb5+ species on the surface of the NiO particles. Thus, the existence of isolated Nb5+ may be a key factor in increasing vacancies in the NiO lattice [32] which turn out to have a positive effect in ethylene selectivity in ethane ODH. Moreover, it is well known that the incorporation of Nb5+ in mixed metal oxides requires relatively high calcination temperatures [56], as observed in our case.Up to this point a conventional catalyst characterization has been conducted. However, little is known about electrochemical properties of the catalysts\u2019 surface and their possible relationship with the catalytic performance.\nFig. 6\n shows the Mott-Schottky (MS) plots for Nb-doped catalysts synthesized with OxA/Ni molar ratios of 0, 1 or 3, and calcined at 350\u00a0\u00b0C and 500\u00a0\u00b0C. For comparison, the corresponding undoped nickel oxides catalysts have been also tested (Fig. S6). It can be seen that most samples (except for NiNb/1\u2013500 and NiNb/3\u2013500 catalysts) show a linear region with negative slope in the MS plots, which is indicative of p-type semiconductivity.Indeed, NiO is a well-known p-type semiconductor which contains an excess of cationic vacancies or, in other words, an excess over the lattice O2\u2013 anions [1,11,34,35,42], resulting from a non-stoichiometric composition (Ni1-xO). Both species (cationic vacancies and lattice oxygen anions) are associated with positive holes, which are the main charge carriers in p-type semiconductor materials. Concerning cationic vacancies, their linkage with positive electron holes is given by the following reaction (using the Kr\u00f6ger-Vink notation):\n\n(1)\n\n\n\nV\n\nM\n\nX\n\n\u2192\n\nV\n\nM\n\n\u2033\n\n+\n2\n\n\nh\n\n\u00b7\n\n\n\n\nwhere VM\nX is a neuter cationic vacancy (i.e. with a null electric charge after losing electrons from its surroundings), which can take two electrons from the valence band of a neuter lattice oxygen in a regular position (OO\nX, equivalent to an O2\u2013 anion), resulting in an ionized cationic vacancy (V\u2019\u2019M) and two electron holes positively charged (h\u2022).On the other hand, electron holes are also related to lattice oxygen anions [35,57]:\n\n(2)\n\n\n\nO\n\nO\n\nX\n\n+\n\n\nh\n\n\u00b7\n\n\u2192\n\nO\n\nO\n\n\u00b7\n\n\n\n\nwhere the neuter lattice oxygen anion (OO\nX) losses an electron (reacts with an electron hole) and becomes a lattice anion with positive effective charge (OO\n\u2022), which is equivalent to the O- species [35]. Therefore, a positive hole corresponds to an electron vacancy in the valence band of a lattice OO\nX anion, which is to say that the \u201cchemical site\u201d of an electron hole corresponds to a non-stoichiometric lattice OO\n\u2022 (O-) anion [35,58]. Several studies have indicated that oxygen species on the NiO surface and within its structure can be of two types, their nature and reactivity being determinant for the catalytic properties of the oxide. Electrophilic oxygen species (O-), the non-stoichiometric oxygen (NSO), have been found to be related with the deep oxidation reactions, while nucleophilic species (O2\u2013) are usually involved in more selective oxidation reactions [1,11,42,58,59].For our catalysts, it can be seen that for the samples showing p-type semiconductivity, the negative slopes increased for the catalysts calcined at 500\u00a0\u00b0C with respect to those obtained at 350\u00a0\u00b0C (Fig. 6 and Fig. S6). Besides, as observed for undoped NiO catalysts [41], the slopes of Nb-doped NiO catalysts prepared with oxalic acid in the synthesis gel were higher than for those prepared in the absence of oxalic acid in the synthesis gel. The value of the slope is inversely related to the density of acceptor defects (cationic vacancies) in the space-charge region developed at the oxide surface (NA\n), according to the following equation [60,61]:\n\n(3)\n\n\n\nN\nA\n\n=\n-\n\n2\n\n\u220a\n\n\u220a\n0\n\ne\n\u03c3\n\n\n\n\n\nwhere \u03b5 is the dielectric constant of the oxide (a value of 12 has been assumed for NiO) [33,62], \u03b50\n is the vacuum permittivity (8.85\u00b710-14F cm\u22121), e is the electron charge (1.60\u00b710-19C) and \u03c3 is the negative slope of each straight line in MS plots for samples showing p-type semiconducting behavior.The density of cationic vacancies, NA\n, which corresponds to the density of holes (main charge carriers), for the samples with p-type semiconductivity is shown in Table 1. In general, NA\n values were higher in catalysts calcined at 350\u00a0\u00b0C than at 500\u00a0\u00b0C. Moreover, and regardless of the temperature of the thermal treatment, NA\n decreased between 1 and 2 orders of magnitude upon treating NiO with oxalic acid and after Nb doping [41]. This decrease indicates that the density of positive holes was lower in those cases than for simple NiO. Consequently, and due to the correspondence between electron holes and non-stoichiometric oxygen (NSO) species, i.e. O-, given by equation (Eq. (2)), it can be suggested that the higher the value of NA\n, the higher the predominance of NSO in the lattice and on the catalyst surface. Therefore, the selectivity of the catalyst towards specific oxidation reactions, such as the oxidative dehydrogenation of ethane to ethylene [1,11,42,59] should be lower. Our results (Table 2) are in accordance with those previously presented, in which the selectivity to ethylene was higher, in general, for Nb-doped NiO catalyst, but also for undoped NiO prepared with oxalic acid [41], than for the reference NiO catalyst.A drastic change occurred for the Nb-doped NiO catalysts prepared in the presence of oxalic acid in the synthesis gel and calcined at 500\u00a0\u00b0C (samples NiNb/1\u2013500 and NiNb/3\u2013500), in which a linear region with a positive slope could be clearly observed (Fig. 6). For these catalysts (NiNb/1\u2013500 and NiNb/3\u2013500), there was an evident modification in the semiconducting nature of the Nb-doped catalysts, evolving from p-type to n-type semiconductivity. This change can be attributed, on the one hand, to the Nb-doping, given that niobium oxide is an n-type semiconductor with oxygen vacancies and free electrons as main charge carriers [11]. Niobium, together with other high valence metals such as W (W6+) or Sn (Sn4+), is known for its ability to insert into the NiO lattice, filling Ni2+ vacancies with Nb5+ cations, whose ion sizes are compatible [1,11,34,35,55], and acting as electron donors. This substitution, as a consequence of changes in the catalyst preparation procedure, resulted in a decrease in the main charge carriers (hole) concentration, as observed for the Nb-doped NiO catalysts (Table 1), or even in a modification in the semiconducting nature of NiO from p-type to n-type semiconductivity, as observed for the NiNb/1\u2013500 and NiNb/3\u2013500 catalysts. Hence, the presence of oxalic acid when doping NiO with Nb5+ played a fundamental role, since in that case Nb-rich nanoparticles (Nb2O5, n-type) cover the surface of the NiO rich crystals. Thus, the doping procedure not only decreased the concentration of positive holes (and the concentration of electrophilic non-selective O- species), but it also changed the semiconducting character of the base NiO oxide, this way affecting its catalytic properties in terms of selectivity. Certainly, the selectivity towards ethylene formation for the NiNb/1\u2013500 and NiNb/3\u2013500 samples was the highest, reaching values of 90\u00a0% and 86\u00a0%, respectively (Table 2).Mott Schottky analyses of two representative catalysts after ODH reaction (i.e. NiNb/0-500R and NiNb/1-500R, R refers to reused) were carried out (Fig. S7A). Results show that semiconductive behavior of the samples is maintained after the ODH reaction, i.e. p-type semiconductivity remains for the catalyst formulated without oxalic acid and calcinated at 500\u00a0\u00b0C, showing a slight decrease in the acceptor density value (from 4.74\u00b71020 to 4.38\u00b71020 cm\u22123) for the sample after the ODH reaction. This could be explained considering that after the reaction, some electrophilic oxygens (reactive sites) might be consumed. In any case, the catalysts prepared with oxalic acid in the synthesis gel and calcinated at 500\u00a0\u00b0C the n-type semiconductivity is maintained after ODH reaction.\nFig. 7\n shows the Bode-module plots of the catalysts, calcined at 350\u00a0\u00b0C (Fig. 7A) and 500\u00a0\u00b0C (Fig. 7B), at an applied potential of 0.5 VAg/AgCl, where the impedance associated with the total resistance of the system corresponds to the impedance at low frequencies. The total electrical resistance of the catalysts surface has been related to their catalytic performance for the ethane oxidative dehydrogenation into ethylene [11,34,35,63]. The total resistance obtained from Fig. 7 was higher for the samples calcined at 500\u00a0\u00b0C. Additionally, the resistance increased for the samples doped with Nb5+ and prepared with oxalic acid. This fact can be explained taking into account that the total electrical resistance is inversely proportional to the density of charge carriers, NA\n, and the effect of Nb5+ to the NiO based catalysts is to partially (or totally, if oxalic acid is added to the electrolyte for catalysts calcined at 500\u00a0\u00b0C) remove the electrophilic NSO species (see Table 1). We must indicate that an undoped NiO catalyst, prepared without the addition of oxalic acid in the synthesis gel (Ni/0-500), also presented the lowest resistances [41]. Additionally, EIS tests were carried out after ODH for representative samples (Fig. S7B), showing similar impedance profiles for the catalysts after the ODH reaction even though total resistances after the reaction are somewhat higher. This is in agreement with the decrease of the acceptor densities after the ODH reaction presented for the capacitance measurements.Cyclic voltammetries were registered in order to study the electrochemical activity of the different catalysts. Fig. S6 shows, as an example, the cyclic voltammograms for the catalysts at 350 and 500\u00a0\u00b0C using the Ferro/Ferri redox couple. Note that 10 cycles were performed for each catalyst and no considerable differences were observed between the first and the tenth cycle, hence, Fig. 7C and 7D shows the different cyclic voltammograms of the catalysts for the tenth cycle. The cyclic voltammetries of Fig. 7 and Fig. S6 clearly show two peaks at\u00a0\u223c\u00a00.18 VAg/AgCl (cathodic) and at\u00a0\u223c\u00a00.30\u00a0V Ag/AgCl (anodic), typical response of the Ferro/Ferri couple. In all cases, the anodic peaks were the highest for the catalysts annealed at 350\u00a0\u00b0C, which is consistent with the increase of the catalysts active area for samples calcined at low temperatures. Besides, for a given temperature, the anodic peak values are higher for the samples doped with Nb5\n+ in the presence of oxalic acid. This behavior might be also attributed to the increase of the surface area in those catalysts, specifically to the electrochemical active area, due to the Nb5+ and oxalic acid contents.Nb-doped nickel oxide catalysts are found to be highly selective catalytic materials in the ODH of ethane, in agreement with previous results [11\u201317,21,25\u201329]. However, as presented here, the addition of oxalic acid to the synthesis gel and the selection of a suitable calcination temperature strongly influence the physicochemical characteristics and, consequently, the catalytic behavior of Nb-doped NiO catalysts. Both the addition of oxalic acid and a lower calcination temperature (350 \u00baC) resulted in an increase in the surface area of the catalysts with an influence on the catalytic activity. However, the activity normalized per surface area (Table 2) highly varies depending on the sample, so that other factors also have a strong influence on the catalytic activity. Moreover, it has been observed by Raman spectroscopy that, depending on the amount of oxalic acid in the synthesis gel and/or the calcination temperature, samples with different crystal size and/or concentration of defects have been synthesized. Electrochemical characterization has been undertaken to further refine any conclusion to be drawn.The catalytic behavior of these catalysts has been linked to several parameters, in particular: the high concentration of Ni defects and the minimum concentration of electrophilic oxygen species, as well as the highest presence of Nb5+ species on the surface of the catalyst. Although changes in physicochemical and catalytic properties have been recently proposed for undoped NiO [41], the changes presented here for Nb-doped catalysts show notable differences to those observed for undoped NiO.The influence of the presence of oxalate in the synthesis gel and the calcination temperature on the catalytic properties of bulk NiO catalysts has been previously studied [41]. Then, it would be interesting to study, in a comparative manner, the possible differences in the influence of the two synthesis parameters on the catalytic properties of the NiO and Ni-Nb-O catalysts.\nFig. S7 shows the change in selectivity to ethylene under isoconversion (10\u00a0%) conditions of NiO [41] and those achieved over Ni-Nb-O catalysts calcined at 350 or 500\u00a0\u00b0C. In the case of undoped NiO catalysts, the selectivity to ethylene initially increases with the incorporation of oxalate anions, presenting a maximum selectivity for the catalyst prepared using oxalic acid (OxA/Ni of 1) and calcined at 350\u00a0\u00b0C. In any case, catalysts calcined at 350\u00a0\u00b0C showed a greater selectivity to ethylene than those calcined at 500\u00a0\u00b0C regardless of the oxalic acid amount employed in the synthesis.In the case of Nb-doped NiO catalysts, the selectivity to ethylene increases with the incorporation of oxalic acid into the gel. However, unlike NiO catalysts, ethylene selectivity is higher for catalysts calcined at 500\u00a0\u00b0C than for those at 350\u00a0\u00b0C. Thus, the calcination step at 500\u00a0\u00b0C has a positive effect on the selectivity towards ethylene for Nb-doped NiO catalyst prepared with oxalic acid in the synthesis gel.\nFig. S8 shows the variation of the catalytic activity for the NiO and Ni-Nb-O catalysts with the OxA/Ni ratio in the synthetic gel. A similar trend is observed in the catalytic activity for the NiO and Ni-Nb-O catalysts. Thus, catalysts calcined at 350\u00a0\u00b0C are more active than those calcined at 500\u00a0\u00b0C. In addition, an influence of the presence of oxalic acid in the synthetic gel on the catalytic activity is observed, so that the catalysts prepared with an OxA/Ni ratio of 1 are the most active regardless of the presence or the absence of Nb5+ in the catalyst. The presence of Nb5+ has a significant positive effect on the selectivity to ethylene (Fig. S7) but a weak effect on the catalytic activity (Fig. S8), whereas the incorporation of oxalic acid in the synthesis gel has a positive influence on the catalytic activity for both NiO and Ni-Nb-O catalysts.Accordingly, it can be concluded that the incorporation of Nb5+ increases the selectivity to ethylene and the rate of formation of ethylene, this effect being greater if oxalic acid is incorporated into the synthesis gel. As indicated in the characterization of the catalysts, the presence of Nb5+ in Ni-Nb-O promotes a low reducibility of Ni-O bonds, as determined by TPR-H2 (Fig. 3B). This aspect is more evident in catalysts calcined at 500\u00a0\u00b0C, which explains the high selectivity to ethylene, in particular for the sample NiNb/1\u2013500 (Fig. 5).\nFig. S9 shows the reaction rates for the formation of ethylene and CO2 to the product kgcat\n-1h\u22121 (the sum corresponds to the catalytic activity for the conversion of ethane) with the oxalic acid/Ni molar ratio in the synthesis gel for catalysts calcined at 350\u00a0\u00b0C (Ni/x-350 and NiNb/x-350 series) and at 500\u00a0\u00b0C (Ni/x-500 and NiNb/x-500 series). In all cases, catalysts prepared with an oxalic acid/Ni ratio of 1 exhibit the highest reaction rate for the formation of ethylene. In addition, catalysts calcined at 350\u00a0\u00b0C exhibit the highest reaction rates, while catalysts calcined at 500\u00a0\u00b0C, especially those containing Nb, exhibit the highest ratio between the rate of ethylene formation and the rate of CO2 formation, i.e. the greatest selectivity to ethylene.Thus, the combined use of a relatively high calcination temperature (500\u00a0\u00b0C) and the inclusion of an appropriate load of oxalic acid in the synthetic gel during the preparation step resulted in excellent selectivity to ethylene (approx. 90\u00a0%). In addition, and in accordance with the characterization results, the catalytic performance of Ni-Nb-O catalysts can be explained in terms of the different physicochemical properties of the catalysts, including changes in the number of vacancies and in the size and concentration of electrophilic oxygen species.Lemonidou et al. [55] demonstrated the existence of a strong kinetic isotopic effect (KIE) on NiO and Nb-doped NiO catalysts, suggesting that: i) breaking the CH bond is the determining step in the speed of the reaction in the ODH of ethane over both catalysts; ii) these two catalysts should have similar active sites, although the abundance or surface concentration of selective and non-selective sites changes with the incorporation of Nb5+ into the NiO lattice. In this way, it has been proposed that the nature of the surface sites is strongly influenced by the valence and acid-base characteristics of the metal oxide promoters, which have a great impact on the selectivity to ethylene [21,25\u201328]. Thus, these authors have proposed a good correlation between the selectivity to ethylene and the valence of the promoter. In conclusion, the authors found that Nb5+ was the best promoter.On the other hand, the influence of the calcination temperature is not entirely clear at this time. However, the positive influence of calcination temperature in the catalytic performance in ethane ODH of Nb-doped NiO catalysts and the change in semiconductivity nature (from p- to n-type) in samples calcined at 500\u00a0\u00b0C could be related to the higher or lower incorporation into the structure of NiO particles and/or the dispersion of Nb5+ on the surface of the NiO particles, as evidenced by the XPS results (Table 1).The main promotional mechanism to explain the improvement in selectivity to ethylene in NiO catalysts doped by transition metals, and especially those doped with niobium, is related to the elimination of cationic vacancies in the NiO particles by the incorporation of Nb5+ into the nickel oxide structure [34].A second promotional mechanism for improving the selectivity to ethylene in NiO-based catalysts has been proposed for supported NiO catalysts, in which the interaction of nickel oxide with supports, determines the catalytic performance [64,65]. In this case, by using an appropriate metal oxide support, the enhanced catalytic performance has been related to the high dispersion of nickel oxide particles on the support, which leads to a lower reducibility of the nickel oxide, hindering the oxidation of ethane into carbon oxides.A third promotional mechanism could be related to the lower crystallization of NiO crystals and the interaction between NiO and promoter oxides due to the presence of highly dispersed oxides on the catalyst surface. This is the case of SnO2-promoted NiO catalysts, in which the nature of Ni species has been related to changes in the size of NiO crystallites and the presence of SnOx crystals highly dispersed on the surface of NiO [19]. Thus, the presence of oxalic acid in the synthesis gel of our optimal Ni-Nb-O catalysts seems to favor the formation of Nb-rich nanoparticles on the surface of the NiO large crystals. In fact, the characterization results suggest the presence of agglomerates of nanoparticles of an amorphous nature that concentrate niobium, while the visible isolated platelets of NiO contain a low amount of Nb.Then, in the optimal catalyst of the present article, it seems that both the first and the third promotional mechanisms occur: a little amount of Nb incorporates to the NiO lattice (first mechanism) and Nb-rich nanoparticles cover the surface of the NiO rich crystals (third mechanism). This way, the simultaneous occurrence of both promotional mechanisms leads to an enhanced ethylene formation.Moreover, a deep electrochemical study of the catalysts has been also carried out. Previous studies on promoted NiO catalysts showed that these catalysts present, similarly to unpromoted NiO, p-type semiconductivity (before and after reaction) [33\u201335]. Thus, it was observed that the addition of suitable promoters decreases the p-type semiconductivity, leading to an increase of the selectivity to ethylene. In this work, this trend has been observed but, additionally, the most selective catalysts present n-type semiconducting character. Moreover, other interesting correlations between electrochemical and catalytic properties have been also found.Then, Fig. 8\nA plots the relationship between the selectivity to ethylene and the acceptor density for these catalysts with p-type semiconductivity. These results, consequently, reveal a correlation between NSO density (directly associated with electron holes concentration) and the selectivity towards ethylene. Hence, in general, the catalysts with the highest selectivity correspond to those with the lowest NA\n values, i.e. fewer electrophilic oxygens.The electrochemical impedance (resistance) of these catalysts could be related to catalytic performance since a high resistance could hinder non-selective reactions. Accordingly, Fig. 8B shows a relationship between the selectivity to ethylene with the total electrical impedance. Since cationic vacancies and electron holes are related to the presence of electrophilic oxygen species, which in turn are associated with the total ethane oxidation to CO2, as explained before, the general trend presented in Fig. 8B is consistent. That is, higher total electrical resistances (where cationic vacancies and, therefore, O- species are partially or totally eliminated) correspond to higher selectivity values. Therefore, there is a clear inverse correlation between the total conductivity of catalysts (and the concentration of charge carriers within their structure) and the selectivity towards ethylene formation.Above, some correlations between the electrochemical properties and the selectivity to ethylene have been found. At this point, it could be interesting to find a representative electrochemical parameter linked to the catalytic activity. In this way, it makes sense that the anodic peak values could be related with the activation of the ethane both selectively and non-selectively. Fig. 9\nA shows a clear correlation between the ethane conversion at fixed conditions (conversions lower than 15\u00a0%) and the intensity of the anodic peak values, which is related to the electrochemical activity of the samples. Conversion of ethane depends on the number of active sites capable of activating ethane and also on the reactivity of these active sites. In these bulk NiO catalysts the amount of active sites highly depends on the surface area and the amount of Nb, which is supposed to be inactive in this reaction conditions. Then, we have calculated the turnover frequency (TOF), which is the parameter that considers the catalytic activity of the active sites. The determination of TOF requires the estimation of the amount of exposed surface sites. In the case of these catalysts, we have considered the surface area, the surface composition and taking into account that the amount of molecules that cover the monolayer of nickel oxide is 9.7.1014 molecules of NiO per cm2\n[66]. Interestingly, Fig. 9B shows that there is also a certain relationship between the anodic peak and the catalytic activity per surface site of the catalysts (TOF). Thus, the higher anodic peak values correspond to catalysts with an enhanced activity per surface site (molecules reacted per surface site per unit of time) but, especially, with the conversion of ethane (and also with the activity per gram of catalyst, which also takes into account the enhanced surface area, see Table 1). Therefore, there is a clear link between surface area and electrochemical active area, but also between the last one and catalytic activity towards ODH of ethane.The n-type semiconductivity observed for selected NiO based catalysts in the present article, requires simultaneously: i) the presence of Nb; ii) the use of oxalic acid in the preparation method; and iii) a calcination temperature of 500\u00a0\u00b0C. If one of these factors are absent, that n-type character is not observed.Accordingly, the presence of Nb5+ decreases the number of cationic vacancies [35] whereas high calcination temperatures favor the Nb5+ incorporation into the NiO lattice (according to our TEM data). The role of the oxalic is not straightforward to explain but its presence favors the formation of small Nb-rich particles and minimizes the Nb-insertion into the NiO lattice. Then, we can hypothesize that n-type catalysts (prepared with oxalic acid) are formed by Nb-containing NiO crystallites whose p-type character has been notoriously reduced together with many Nb2O5 nanoparticles with a clear n-type semiconductivity [67,68] located on the surface of the NiO particles, leading to an overall n-type semiconducting character. The p-type character of the catalyst without oxalic acid (NiNb-0/500) could be the result of a higher incorporation of Nb and a lower amount of small particles that, additionally, present a relatively high concentration of Ni, then maintaining the overall p-type character.Finally, we want to mention that the optimal catalyst is quite stable after 32\u00a0h on-line. Using a high contact time, W/F, of 80 gcat h (molC2)-1 and a feed rich in oxygen (the initial conversion was as high as 57\u201357.5\u00a0% with an ethylene selectivity of 70\u201371\u00a0% (yield of 40\u201341\u00a0%). After 2\u00a0h, the ethane conversion decreased until 55\u201355.5\u00a0% which remained almost stable after the rest of the experiment (Fig. S11). This slight drop in the catalytic activity could be related to the subtle fall of the surface area (32.9\u00a0m2/g for the fresh catalyst whereas 31.5\u00a0m2/g for the used sample), since no apparent variations in the electrochemical properties, in the near surface or in the crystalline phases (by XPS or XRD) have been observed in the post-mortem catalyst compared to the fresh catalyst (Fig. S12).Controlling the preparation conditions (calcination temperature of 500\u00a0\u00b0C and an appropriate amount of oxalic acid in the synthesis gel) a high and stable selectivity to ethylene of ca. 90\u00a0% can be obtained. By using these synthetic conditions two mechanisms that promote ethylene selectivity (Nb5+ incorporated into the NiO lattice and the interaction of large Nb-containing NiO crystals with tiny Nb-rich particles) take place simultaneously. Unlike the other samples, the optimal catalysts present n-type semiconductivity, in spite of the fact that the composition (Ni/Nb ratio) is the same for all the catalysts studied. In the present article, we have also shown that the behavior in the oxidative dehydrogenation of ethane of NiO catalysts can be estimated knowing their electrochemical properties. Interestingly, an inverse relationship between the density of non-stoichiometric oxygen and the selectivity towards ethylene has been clearly found in the catalysts presenting p-type semiconductivity. Thus, the catalysts with high selectivity to the olefin present low NA\n values, i.e. fewer electrophilic oxygens. Additionally, a correlation between the selectivity to ethylene and the total electrical resistance has been observed. Then, the optimal catalysts present high total electrical resistance as a result of the removal of cationic vacancies and electrophilic O- species, which are selective towards the CO2 formation. Overall, the most selective catalysts are those having low concentration of cationic vacancies and electrophilic oxygen species as well as a high amount of Nb5+ species on the surface of the catalyst.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank the the Ministerio de Ciencia e Innovaci\u00f3n of Spain, MINECO/FEDER (Projects: TED2021-129555B-I00, PID2021-126235OB-C31, PID2021-126235OB-C33, TED2021-130756B-C32 and MFA/2022/016). Y.A. thanks the Ministry of Higher Education and Scientific Research of Algeria for the National Exceptional Program for the fellowships. A.A. acknowledges Severo Ochoa Excellence Program for his fellowship (BES-2017-080329).Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2023.02.009.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  In the present article, a relationship between the catalytic performance in the oxidative dehydrogenation of ethane (ODHE) for Nb-doped NiO catalysts and their electrochemical properties has been proposed. To do so, highly stable and selective Nb-doped NiO catalysts for the ODHE to ethylene have been synthesized by optimizing synthesis parameters such as the amount of oxalic acid in the synthesis gel and the calcination temperature. These catalysts have been characterized by means of XRD, HRTEM, Raman and UV\u2013vis diffuse reflectance spectroscopies, TPR and XPS. Moreover, Electrochemical Impedance Spectroscopy (EIS), capacitance measurements (Mott-Schottky analysis) and cyclic voltammetries studies were also carried out. Electrochemical characterization indicates changes in the type of semiconductivity: p-type for samples calcined at 350\u00a0\u00b0C and the sample prepared in the absence of oxalic acid and calcined at 500\u00a0\u00b0C, to n-type for samples prepared in the presence of oxalic acid in the synthesis gel and calcined at 500\u00a0\u00b0C. According to the obtained results, the most selective catalysts present a low Nb-incorporation on the NiO lattice with a large amount of tiny Nb2O5 nanoparticles covering NiO crystallites. This work presents, for the first time, a complete electrochemical characterization of Nb-doped NiO catalysts showing a correlation between electrochemical properties and catalytic performance.\n               "}
{"full_text": "Data will be made available on request.Due to economic, social, and environmental reasons, research focusing on alternative fuel and chemical sources has gained interest. As reserves of fossil fuel are declining worldwide, combined with an increasing demand for petroleum fuels by emerging economies, the price of conventional fossil fuel shall continue to rise, increasing the risk in energy supply around the world. Furthermore, the environmental damage resulting from the combustion of fossil fuel, by releasing atmospheric pollutants and CO2, is a contributing factor to global warming [1]. As the only natural-occurring source of renewable organic carbon, biomass represents an essential feedstock to produce chemicals and liquid transportation fuels. The recalcitrant C\u2013C and C\u2013O bonds found in biomass requires a bulk depolymerisation technique to produce bio-oil, a viscous liquid that can be easily processed, stored, and safely \u2018dropped-in\u2019 into the supply chain for large scale chemical conversions in existent refineries [2]. The bio-oil produced from common thermochemical methods consists of a complex mixture of oxygenated compounds (\u223c50% oxygen), which can be upgraded to hydrocarbon fuels and specialty chemicals through proper refining methods such as hydrodeoxygenation, decarboxylation, decarbonylation, isomerisation, hydrogenation, dehydration, etc. [3\u20137].In general, bio-oil can be obtained from biomass by pyrolysis and/or hydrothermal liquefaction (HTL) methods. The bio-oil from pyrolysis is highly unstable as a result of its high oxygen content and as a result of vigorous reaction condition which can destroys the natural structure of phenolic compounds [8]. Our earlier investigation on using pyrolysis for valorization of PJ biomass has yielded only 25% bio-oil after removing the aqueous phase [9]. On the other hand, conventional HTL takes place under subcritical water conditions (250\u00a0\u2013\u00a0373\u00a0\u00b0C, 4\u201322 MPa) eliminating the need of a pre-drying step. Under these conditions, water acts both as solvent and as acid-base catalyst, improving the solvation and deoxygenation of intermediate compounds, yielding higher-quality bio-oils compared to pyrolysis [10]. However, supercritical conditions, with temperatures above 380\u00a0\u00b0C, have been shown to improve glucose conversion, reducing char production compared to subcritical HTL [11]. Besides the carbon rich bio-oil, HTL also yields solid hydrochar, gas products (mostly CO2) and an aqueous phase, which can be easily separated from the desired bio-oil product.A potential HTL catalyst must be water-tolerant, display high selectivity towards bio-oil, minimizing char and gas formation. Homogeneous catalysts consisting of base or basic salts such as NaOH, KOH, Na2CO3, K2CO3 have been intensively utilized for biomass HTL, decreasing the biochar formation and increasing bio-oil yield, however, presenting challenges in separation, extraction, and reusability of the catalyst [12,13]. Heterogeneous catalysts have the advantage of easy separation from the liquid products, improved process economics and energy efficiency. Xu et. al. utilized solid alkaline earth metal catalysts such as hydrotalcite, MgO, and colemanite for woody biomass HTL which improved bio-oil yield and quality [14]. Noble metal catalysts such as Pd/C, and transition metal catalysts based on Ni, W, Co, Mo, and Fe such as Raney nickel, Fe ore, FeS, Ni and Fe metals, CoMo/\u03b3\u2013Al2O3, etc. has been also explored for biomass HTL [15\u201317]. In particular, Ni catalysts produced bio-oil with improved yield and quality due to its hydrogenating property [18,19]. We have previously reported the deoxygenating behavior of Nb2O5 catalyst due to its oxophilic nature that can strongly bind with the oxygen groups helping to cleave the C\u2013O bond [9].As known, biomass tends to be rich in carbon but hydrogen deficient. Hence, incorporating a H-rich co\u2013reactant during the biomass HTL could has the ability to increase the bio-oil yield and quality [20,21]. As such, plastic waste composed of polyolefins (polyethylene, polypropylene, their copolymers and olefinic rubbers) could be a potential co-reactant as contains hydrogen-rich polymers [22\u201324]. Utilising plastic as a co\u2013reactant not only benefit increasing the bio-oil yield but also benefit mitigating the waste plastic landfilling environmental issue, that could result in an effective waste management strategy. Polypropylene (PP), a non\u2013oxygenated light weight polymer made of long chain molecules (C3H6)n, [25] is one of the most utilized commodity plastics and is present as the largest fraction in the waste-stream [26]. PP at the subcritical hydrothermal liquefaction conditions (350\u00a0\u00b0C, 20\u00a0min, non-catalytic) produced mainly solid (83%) [27]. However, under supercritical water liquefaction has been reported to increase the PP degradation to 91\u00a0wt.% oil (80% range naphtha hydrocarbons) at 425\u00a0\u00b0C and 2\u22124\u00a0h [28]. The same oil yield was achieved at 0.5\u00a0\u2013\u00a01\u00a0h reaction time when the temperature was increased to 450\u00a0\u00b0C [28]. This hydrocarbon oil produced from PP can then synergistically improve the biomass conversion and bio-oil quality when is co-liquefied together with biomass [29]. However, until now there has been limited investigation on catalytic liquefaction of biomass with PP and their synergetic interactions on bio-oil yields obtained.In this study, we aim to investigate the production of renewable hydrocarbons from abundantly available non-food biomass such as PJ using the hydrothermal co-liquefaction route. Prosopis juliflora (PJ) with a growth rate of 25\u00a0km2/year in India, is an abundant biomass which is resistant to drought and adaptable to different soil types and therefore, is a promising biomass source for biofuel production used in India and other tropical countries [30]. Our target also seeks to elucidate the synergetic effect occurring when polypropylene (PP) wastes are added to PJ in terms of improving the bio-oil yield. The co-liquefaction studies of PJ and PP were conducted over a broad temperature range of 340\u00a0\u00b0C to 440\u00a0\u00b0C, using different PJ/PP ratios at 60\u00a0min reaction time, where the synergy percentage effect was calculated at each condition.The first part of this study consisted of the non-catalytic co-liquefaction reactions to optimise the temperature and percentages of PP added to PJ in terms of high bio-oil yield. On the second part, a series of alumina supported metal oxide catalysts were tested for optimum conversion of biomass-plastic mixture. On this account, we firstly aimed to synthesise, characterize, and evaluate the catalytic activity of a series of transition metal oxides (Ni, Mo, W, Nb) supported on \u03b3\u2013Al2O3 for the individual and co-liquefaction of PJ and PP. The temperature, percentage of PP added to PJ, effect of catalyst, catalyst: feed ratio, were all optimized in terms of high bio-oil yield. The reaction products from the HTL process (i.e. bio\u2013oil, aqueous phase, gas, and bio\u2013 char) are all characterised and optimal process conditions are reported. The regeneration and reusability of the best performed catalyst at the best reaction condition was also studied.Ammonium molybdate tetrahydrate (81.0\u00a0\u2013\u00a083.0%), nickel (II) nitrate hexahydrate (>98.5%), ammonium metatungstate hydrate (\u226585%), niobium pentachloride (99%) were purchased from Merck, India. \u03b3\u2013Al2O3 was purchased from BASF chemicals company. Ethanol (99.9%) was purchased from Changshu Hongsheng fine chemicals. Prosopis juliflora (PJ) and single-use polypropylene (PP) (polypropylene packaging bags) were collected in and around SSN College of Engineering campus, Chennai, Tamil Nadu, India. Both the PP and PJ waste were cut into small pieces using a blade shredder and sieved to a size of 1mm.PJ and PP were characterized to understand their composition that plays a vital role in product formation. PJ is a hardwood biomass with 37.9% cellulose, 19% hemicellulose, and 37% lignin [30]. The C, H, N, S and O content of PJ was 48%, 7%, 0%, 2% and 43%, respectively, whereas PP contains 86% C and 14 % H (Table 1\n). The (H/C)eff of PJ and PP were 0.375 and 1.95, respectively. It is explicit that PP is H-rich whereas PJ is H- deficient. PP contained high amount of volatile matter (96.2%) with less amount of fixed carbon (3%) and negligible ash content. On the other hand, 79% volatile matter, 15.2% fixed carbon (non-volatile carbon) and 5.8% ash content were found in PJ. The high heating value (HHV) of a biomass is highly influenced by the composition of lignocellulosic components, extractives and detrimentally by moisture and ash contents [31]. The HHV of PJ and PP are 20 MJ/Kg and 41 MJ/Kg, respectively. The high HHV of PP is attributed to its high H content and the absence of heteroatoms.The moisture content, volatile matter, ash content and fixed carbon were analyzed according to ASTM standards E871\u201382 and E1755\u201301. Ultimate analysis was conducted using an ELEMENTAR Vario EL III elemental analyzer. Thermogravimetric analysis (TGA) was performed to determine the waste degradation rate using a Shimadzu TGA 50H thermogravimetric analyzer. TGA was performed using 10 mg of waste at a temperature of 30 to 800\u00a0\u00b0C under 20\u00a0\u00b0C/min heating rates and held at final temperature for 10 min.Alumina (\u03b3\u2013Al2O3) supported metal catalysts (Mo, Ni, Nb, W) were prepared by simple wetness impregnation method with a nominal metal content of 7\u00a0wt.%. The desired amount of aqueous solution of the metal precursor was added to the alumina support and mixed in a rotary evaporator at room temperature for 12\u00a0h (In the case of niobium pentachloride, ethanol was used as the solvent due to its decomposition in water). Water was then removed by the rotary evaporator at 50\u00a0\u00b0C, followed by drying the catalyst overnight at 100\u00a0\u00b0C, and subsequent calcination at 550\u00a0\u00b0C for 5 h in a muffle furnace. The catalysts were then labelled as Mo/alumina, Ni/alumina, Nb/alumina, and W/alumina.The catalyst structural analysis was elucidated by X \u2013ray diffraction (XRD) using Bruker D8 advance with monochromatic Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.542\u00a0\u00c5) at 30\u00a0kV and 15\u00a0mA with a step size of 0.1\u00a0\u00b0, for the range of 10\u00a0\u00b0\u00a0\u2264\u00a02\u03b8\u00a0\u2264\u00a080\u00a0\u00b0. Nitrogen adsorption-desorption data were obtained at -196\u00a0\u00b0C using a Micromeritics TriStar II 3020 surface area and porosity analyser. Prior to physisorption measurements, all samples were degassed under vacuum at 200\u00a0\u00b0C overnight. The specific surface area was determined by applying Brunauer\u2013 Emmett\u2013 Teller (BET) method and pore volume were calculated from the amount of N2 adsorbed at P/P\n\no\n of 0.99. An Inductively coupled plasma mass spectrometry (ICP\u2013MS) from Thermo Fisher iCAP RQ ICP\u2013MS was used for the bulk elemental analysis. The amount and strength of the catalyst acid sites were characterized using a Micromeritics Autochem II 2920 chemisorption analyzer (TPD-ammonia), fitted with a TCD detector for monitoring NH3 desorption profile. About 50 mg of sample was preheated for 2 h under the flow of helium gas at 400\u00a0\u00b0C. Then the sample was saturated by passing 15 vol% NH3 in He for 1\u00a0h at 100\u00a0\u00b0C. Afterward, was heated from 100\u00a0\u00b0C to 800\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C/min. In parallel, Pyridine Fourier- Transform Infrared Spectroscopy (FT-IR) was used to determine the nature of the catalyst acid sites. A known amount of pyridine was adsorbed on the 50 mg catalyst at 150\u00a0\u00b0C. The excess and physiosorbed pyridine were removed by passing N2 at 150\u00a0\u00b0C for 30\u00a0min and FT-IR was recorded using a Perkin Elmer 200 FT\u2013IR, USA spectrometer at 128 scans and 4 cm-1 resolution. A Field Emission Scanning Electron Microscope (FE\u2013SEM) \u2013 JOEL 6390LA microscope operated at 30 kV with backscattering (BSE) and Energy Dispersive X\u2013ray Spectroscope (EDAX) detectors was used for characterising the morphology of catalysts. A high-resolution transmission electron microscope (HR-TEM, JOEL/JEM 2100) operated at 200 kV, fitted with an energy dispersive X-ray (EDS) detector was used to find the particle size distribution and metal dispersion over the alumina support. X-ray photoelectron spectrometer (XPS) by Scienta O micron was used to find the oxidation states of Nb2O5. The peaks were calibrated by using C 1 s line in the carbon spectra at 284.0 eV as a reference.Hydrothermal liquefaction reactions (HTL) were carried out in a 250 ml capacity stainless steel closed high\u2013pressure batch auto\u2013reactor. The reactor was loaded with 15g of feed (biomass and/or PP), a feed: water ratio of 1:8 and pressurized to 5 MPa with nitrogen. After heating to the desired reaction temperature (320\u00a0\u00b0C to 440\u00a0\u00b0C, heating rate 10\u00a0\u00b0C/min), temperatures were maintained for 60 min under constant stirring at 740 rpm. The effect of Mo/alumina, Ni/alumina, Nb/alumina, and W/alumina catalysts on bio-oil yield was studied by varying the catalyst wt.% with respect to feed (1\u00a0wt.%, 2\u00a0wt.%, 3\u00a0wt.%, 4\u00a0wt.% and 5\u00a0wt.%). It must be noted that the catalysts were not reduced before the reaction. Before dismantling the reactor, the reaction was quenched by removing the heating jackets and immersing the autoclave in an ice water bath. The pressure was released by collecting gases using a Tedlar gas bag. Bio-oil produced from the HTL crude was separated through solvent extraction process using hexane [32]. The contents of the reactor were extracted using hexane as the solvent and transferred into a 250 ml separating funnel where the organic phase was recovered. The organic phase (bio-oil) was subjected to vacuum separation to remove excess hexane. The solid along with the catalyst was collected by filtration and washed with ethanol and dried overnight at 100 \u00b0C and analyzed for the coke deposition using an ELEMENTAR Vario EL III elemental analyzer. For the reusability tests, before conducting each test, the catalyst was regenerated by burning off the deposited coke at 400 \u00b0C in a muffle furnace [9]. The yield of bio-oil, gas, aqueous phase, and solids, higher heating value (HHV), percentages of deoxygenation in bio-oil, and carbon recovery in bio-oil are evaluated using the formulae given in the electronic supplementary information (ESI). % Synergy and % calculated yields are evaluated using the formulae:\n\n\n%\nSynergy\n=\n\n\nExperimental yield\n\u2212\nCalculated yield\n\nCalculated yield\n\n\u2217\n100\n\n\n\n\n\n\nCalculated Yield\n=\n\n\n\nx\nPJ\n\n\u2217\n\ny\nPJ\n\n+\n\nx\nPP\n\n\u2217\n\ny\nPP\n\n\n\n/\n100\n\n\nwhere x is the mass fraction, y is the % yield, PJ is Prosopis juliflora, and PP is polypropylene.Gas chromatography\u2013mass spectrometry (GC\u2013MS) was used to analyze the bio-oil, obtained from hydrothermal liquefaction process. An Agilent 7890 GC equipped with an Agilent 7683B auto\u2013injector, a HP\u20135 column and flame ionization detector (FID) was used. The injector temperature was 250 \u00b0C. The column temperature was set at 100 \u00b0C and held for 1 min, followed by ramping at 10 \u00b0C/min to 200 \u00b0C and held for 10 min. A volume of 0.5 \u03bcL liquid product was injected in a split mode ratio of 100:0. The average molecular weight of bio-oil was analyzed by gel permeation chromatography (GPC) using a Water GPC 1515 pump system provided with Styragel HT\u20136E and HT\u20133 columns linked in series. UV (Waters 2489) and RI (Waters 2414) detectors were used for finding the average molecular weight of bio-oil products. The bio-oil samples were dissolved in 1 mg/ml THF (used as an eluent with a flow rate of 1\u00a0ml/min) and filtered using a 0.45\u00a0micron filter before analysis. The system was calibrated using the narrow polystyrene standards in the range of Mw 1.3\u00a0million Da to 1350 Da.Four catalysts (Mo/alumina, Ni/alumina, W/alumina, and Nb/alumina) were studied for the co\u2013liquefaction of PJ and PP. The alumina support had a surface area of 192\u00a0m2\u00a0g\u22121 with a pore volume of 0.49\u00a0cm3\u00a0g\u22121 (Table 2\n). The metal impregnation over alumina support decreased the surface area to 127, 137, 124, and 139\u00a0m2\u00a0g\u22121, for Mo, Ni, W and Nb catalysts, respectively. Similarly, the deposition of the metal particles in the surrounding pore mouth of alumina decreased the pore volume as expected. The elemental percentage as measured by ICP\u2013 MS (Table 2) was 7.3, 6.5, 7.2, and 6.9 for Mo, Ni, W and Nb catalysts, respectively which is in good agreement with the theoretical values (standard deviation\u00a0=\u00a00.3317).\nTable 2 also reports the catalyst acidity measurement by TPD-ammonia. The acid strength is categorized as weak, moderate, and strong depending on the temperature at which ammonia was desorbed from the catalyst [33]. From the table, alumina support exhibits a total acidity of 0.76\u00a0mmol/g where about 55% are weak acid sites and 45% moderate acid sites. The metal loading to alumina support increased the total acidity where the highest acidity was found to be for Nb/alumina (1.23\u00a0mmol/g). Alumina supported Mo, Ni, and W exhibited 0.92, 0.87 and 0.98\u00a0mmol/g total acid sites, respectively. The nature of acid sites (Br\u00f8nsted/ Lewis) was examined using pyridine FT-IR spectroscopy (Fig. 1a). The characteristic absorption bands at 1425 and 1630 cm-1 represented the surface coordinated pyridine molecules with the Lewis (PyL) acid sites whereas, the absorption peak at 1540 cm-1 represents the pyridine ion adsorbed on the catalyst Br\u00f8nsted (PyB) acid sites [33]. The adsorption band at 1480\u00a0cm-1 is characteristic for a combination of Br\u00f8nsted and Lewis acid sites (PyL\u00a0+\u00a0B). Alumina support exhibits a strong Lewis acidity with a negligible Br\u00f8nsted acid site. The deposition of metals increased the Lewis and Br\u00f8nsted acid sites where the highest PyL, PyB and PyL\u00a0+\u00a0B was observed with Nb/alumina catalyst in agreement with TPD-ammonia results in Table 1.The XRD spectra of alumina support in Fig. 1b displayed three main peaks at 2\u03b8 = 37.2\u00b0, 45.5\u00b0 and 66.7\u00b0 corresponding to the d311, d400, d440 reflections of \u03b3\u2013Al2O3 (PDF 00\u2013050\u20130741) [34]. All the supported metal catalysts, apart from exhibiting the signals related to alumina, had additional peaks corresponding to the respective metal oxides (Fig. 1b). The SEM coupled with EDAX images given in Fig. S1 (a-d) in ESI indicated the absence of other elemental impurities.Firstly, non-catalytic co-liquefaction of PJ with different percentages of PP added was investigated in a temperature range of 340\u2013440\u00a0\u00b0C and the bio-oil yields are depicted in Fig. 2a. In the absence of PP, increasing the temperature from 340\u00a0\u00b0C to 420\u00a0\u00b0C increased the bio-oil yield from 13.5% to 42.5% with a concomitant decrease in solid residues (Fig. S2 in ESI). Increasing temperature stimulates the conversion of organic compounds into bio-oil, gaseous products, and other water-soluble compounds. While further increasing the temperature to 440\u00a0\u00b0C, the yield of bio-oil slightly dropped (from 42.5% to 41.2%), because of thermal cracking of bio-oil compounds following in an increase of gaseous product from 24.4% at 420\u00a0\u00b0C to 28.9% at 440 \u00b0C (Fig. S2 in ESI).On the other hand, HTL of PP alone, at the subcritical conditions (below 380\u00a0\u00b0C) yielded mainly 58.6%, 50.2% and 47.4% solid residue products, with an oil yield of 15.2, 16.5 and 17.2% at 340\u00a0\u00b0C, 360\u00a0\u00b0C, and 380\u00a0\u00b0C, respectively which is comparable to the findings by Savage. et. al and Biller. et. al. [20,27], where it was claimed that at subcritical reaction condition, PP degradation was low due to the insufficient number of reactive active sites for dehydration [27]. The appreciable oil yields started at the supercritical condition, where oil yield of 30.2\u00a0% was obtained at 400\u00a0\u00b0C, reaching a maximum yield at 420\u00a0\u00b0C (37.5%) with further decrease to 31.3% at 440\u00a0\u00b0C, similar behavior as observed with PJ. Concurrently, the solid products decreased from 58.6% (at 340\u00a0\u00b0C) to 20.2% at 420\u00a0\u00b0C, owed to the supercritical water that stabilizes the radicals minimizing coke formation [28].When adding 25% PP during liquefaction of PJ, a substantial increase in bio-oil yield at 340\u00a0\u00b0C (13.5% to 27.1%) was obtained. This yield is 97.6% higher than the calculated yield based on a weighted average of PJ and PP yields (Fig. S3a in ESI) and suggests a significant synergy occur during the co-conversion of the two materials. Similarly, the solids decreased to 30.2%, compared to 47.3% when PJ alone was used at 340 \u00b0C. Subsequently, a gradual increase in bio-oil yields up to 46.5% at 420 \u00b0C was observed, representing about a 12.8% bio-oil yield improvement for all the non-catalytic HTL reaction conducted in this study. It is known that during HTL, decomposition through free-radical formation is more prevalent and the PP is known for the rapid formation of (more stable tertiary) free radicals upon C-H cleavage during thermal decomposition [35]. These free-radicals from PP are expected to bond with the oxygen radicals from biomass, thereby promoting the cleavage of the oxygenated groups from biomass enhancing the oil fraction formation [36]. As such, the bio-oil yield dropped to 45.1% when the temperature was further increased to 440\u00a0\u00b0C.As can be seen in Fig. 2a, a further increase in PP substitution to 33%, 50%, 67% and 75% respectively, although indicated good synergy (Fig. 2b) and bio-oil yield improvement, when compared to the anticipated calculated value, the amount of solid products formation increased resulting in poor bio-oil yield when compared to the yield obtained with 25% PP added to PJ. The maximum bio-oil yield at 25% PP implies that only a small amount of PP is sufficient to be added to generate enough radicals to break down biomass. Based on the data obtained from the non\u2013catalytic HTL tests performed, 420\u00a0\u00b0C was selected as an optimum HTL reaction condition for further studies under the presence of a catalyst.The alumina supported transition metal oxide catalysts were screened for the co-liquefaction of PJ and PP at 420\u00a0\u00b0C, then compared to the non-catalytic reaction results (Fig. 3a). Initially, to distinguish the role of metals on the bio-oil composition and production, a blank experiment for the co-liquefaction reaction was carried out using only alumina support as catalyst.At 420 \u00b0C, HTL of PJ on alumina as a catalyst, resulted in 40.2% bio-oil yield, found to be 5.4% lower than that obtained from the non-catalytic reaction. Even with the addition of 25% and 33% PP to PJ, similar decrease in the bio-oil yield was observed (7.4%, 4.8% decrease in bio-oil yield at 25%, 33% PP addition, respectively). On the contrary, further increase in the PP\u00a0% to 50%, 67% and 75% improved the bio-oil yield by 9.1%, 13.1%, and 11.7%, respectively.In an interesting approach, where HTL reaction was carried out using PP only, a 32% improvement in the bio-oil yield was observed, confirming a positive effect when alumina is present compared to the non-catalytic reaction. As PP comes in contact with alumina support Lewis acid sites, the degradation of PP to lower molecular weight compounds increases sharply [37]. As is known, the catalytic degradation of PP occurs via an ionic mechanism through two steps. The first step is the abstraction of hydride ion from the hydrocarbon polymer which is promoted by the Lewis acid sites of the alumina, where the second step is the formation of a variety of hydrocarbon isomers due to isomerization reaction and \u03b2\u2013scission [35,38]. From these results, it can be inferred that alumina as a catalyst is very promising for PP conversion in terms of high oil yield as compared to PJ.With the presence of transition metal oxides, the conversion of PJ alone, contrastingly, showed an increase in the bio-oil yield for all the catalysts tested in the order of Nb\u00a0>\u00a0Ni\u00a0>\u00a0Mo\u00a0>\u00a0W (22.6%- Nb, 3.8%- Ni, 1.7\u00a0%- Mo, and %- W improvement when compared to non-catalytic conversion). Similarly, when PP alone was used, the performance of the supported metal catalysts was exceptional increasing oil yield to 73.3%, 65.3%, 57.3%, and 54.6% for Nb/alumina, Ni/alumina, Mo/alumina, and W/alumina, respectively when compared to the non-catalytic conversion.The bio-oil yields obtained for the co-liquefaction experiments with 25%, 33%, 50%, 67%, and 75% PP substitution and at 420 \u00b0C, are then compared with the calculated anticipated value based on the corresponding individual conversion from the weight fractions of PP and PJ as shown in Fig. S4 (a-e) in ESI. At 25% PP addition, an excellent synergy between PJ and PP was observed producing a high bio-oil yield of 59.4% for Nb/alumina catalyst. The bio-oil yields obtained when using the other catalysts were 47.8%, 48.7% and 49.6% for Mo/alumina, W/alumina, and Ni/alumina, respectively. Evidently, the presence of a metal oxide catalyst improves the overall conversion of solid feed, suppressing gas product formation, thereby increasing the liquid hydrocarbons yield (Fig. 3b) [21]. An increase in the aqueous phase yield was noted, indicating the extraction of organic compounds into the aqueous phase and due to increased deoxygenation reactions, such as demethoxylation in the presence of catalyst. These results suggest that adding 25% PP to PJ is an optimum value in terms of high bio-oil yield (both in the case of catalytic and non\u2013 catalytic conversion).As established Nb/alumina catalyst was found to be the best choice in terms of bio\u2013 oil yield, then it was decided to further optimize the catalyst weight percentage for the 25% PP added to PJ during HTL reaction by investigating the bio-oil yield obtained when a catalyst loading of 1, 2, 3, 4, and 5 wt.% was used. At the low catalyst loading of 1 wt.%, 24% solids, 42% bio-oil, 17.2% aqueous phase and 16.8% gases were produced. The low bio-oil liquid yield obtained can be attributed to the limited catalyst amount used to drive the conversion to oil, therefore, the oil production is rivalled by gas and solid product formation. The increase in catalyst load from 1\u00a0wt.% to 2\u00a0wt.%, increased the bio-oil yield to 59.4%, while decreasing solid residue formation by 50% (24%\u20131\u00a0wt.% to 12%\u20132\u00a0wt.%) suggesting the effective conversion of organic compounds into HTL products. Further increasing catalyst loading results in an increase in gas phase formation due to further decomposition of low molecular weight hydrocarbons from bio-oil due to strong catalyst acidity. Gradual increase in solid residue formation was also observed at increased catalyst load which would have been due to the re\u2013polymerization of oil intermediates. Hence, 2\u00a0wt.% of Nb/alumina catalyst was the optimum loading for the HTL reactions.\nTable 3\n shows the physicochemical properties of the bio-oils obtained from the non-catalytic and catalytic HTL tests conducted at 420\u00a0\u00b0C and with 25% PP addition to PJ biomass. The 75% PJ\u201325% PP blend feedstock contains 55.7%\u00a0C and 6.4% H with a net hydrogen to carbon ratio (H/C)eff of only 0.38. C and H in the bio-oil obtained from the non-catalytic HTL was 59.5% and 6.7%, respectively, with an increased (H/C)eff of 0.52. Due to the low bio-oil yield and carbon loss through solid and gas products, only 40.1% carbon was recovered into the oil phase. Evidently, the catalytic runs produced much improved bio-oil in terms of (H/C)eff and carbon recovery as can be seen in Table 3. With Nb catalyst, about 79% carbon was recovered to the oil phase with (H/C)eff as 1.13.In terms of oxygen, the non-catalytic bio-oil contained 32.3% oxygen corresponding to 16.5% bio-oil deoxygenation when compared to the feed. During the catalytic runs, up to 65.1% deoxygenation was achieved with Nb/alumina catalyst as occurrence of several reactions during the HTL, which is discussed in the following sections. It must be also noted that 1.7% sulphur was present in the feed and brought down to 0.1% when Nb/alumina catalyst was used for the HTL reaction, supporting the evidence that Nb is an efficient catalyst for desulphurization reactions as well [39]. Nb catalysts has been reported prominent for dehydration due to its Lewis and Bronsted acid sites [40]. Higher heating value (HHV) is the heat produced upon complete combustion is one of the vital properties of bio-oil which is influenced by factors such as % of heteroatom, H/C ratio, etc. The HHV of the feedstock was 23.54 MJ/Kg which was also improved while employing a catalyst and a maximum of 35.08 MJ/Kg was observed with Nb catalyst.Overall, by comparing the bio-oil properties, Nb/alumina catalyst performed exceptional in terms of improving bio-oil properties such as HHV and in terms of % deoxygenation and % carbon recovery.With respect to product distribution, the bio-oil obtained from the catalyzed HTL of PP alone at 420 \u00b0C was analyzed by GC-MS and n\u2013paraffin, i\u2013paraffin, olefin, naphthene and aromatics hydrocarbons in the range of C7 to C18 were observed. The main products identified from GC-MS were methyl cyclohexane (C7H14), methylhexane (C7H16), 2,4\u2013dimethyl\u20131\u2013heptene (C9H18), 2\u2013 decene\u20132,4\u2013dimethyl (C12H24), hexyl cyclohexane (C12H24), 3\u2013ethyl\u20135\u2013methyl\u20131\u2013propyl cyclohexane (C12H24), 1,4\u2013dicyclohexylbutane (C16H30), undecylcyclohexane (C17H34), pentadecene (C17H34), and dodecylcyclohexane (C18H36). The aromatic product includes toluene (C7H8), trimethylbenzene (C9H12) and xylene (C8H10). Escola et al. has reported the formation of C1-C5 range products with highly acid catalyst formed by the end-chain scission reaction [41]. The other two conversion pathways of PP are: (i) oligomerization of the produced olefinic gas products, and (ii) the cracking reactions occurring at random position of the polymer chain [41].The composition of the obtained bio-oils from PJ and the co-liquefaction studies with different PP concentrations were quantified also by GC-MS analysis and categorized into seven major classes. (i) guaiacolics, (ii) aromatic hydrocarbons, (iii) acids, aldehydes, and ketones (iv) alkyl phenolics, (v) catechols, (vi) naphthalene oligomers, and (vii) alkanes. The detected naphthalene compounds and undetected large naphthalene molecules were considered as naphthalene oligomers. Biomass undergoes decomposition and de\u2013polymerization during the initial HTL process temperature which further produce smaller molecules through addition, cracking, hydrogenation, oxidation and nucleophilic reactions [42].\nFig. 4\n shows the selectivity towards bio-oil components for the non-catalytic and catalytic HTL runs with 25% PP in PJ at 420 \u00b0C for 60 min and 2 wt.% catalyst. The non\u2013 catalytic HTL of 25% PP blend produced guaiacolics (42%), followed by acids, aldehydes, and ketones (26%). 12% of completely oxygen-free compounds: aromatic hydrocarbons were produced as a result of the deoxygenation reaction taking place under non-catalytic hydrothermal condition. It was noticed that 6% alkyl phenolics, 6% catechols, and 8% naphthalene oligomers were the other product classes identified (Fig. 4). During the HTL reaction, repolymerization and condensation reactions occur that produce oligomers. There were no alkanes detected from GC\u2013 MS. As it is well known, biomass constitutes cellulose, hemicellulose, and lignin components. The presence of major derivatives compounds from lignin in the bio\u2013 oil can be attributed to the more solubility of cellulose derived compounds in water.During the catalytic HTL reaction of 25% PP added to PJ, interesting results were observed. The guaiacolics selectivity decreased from 42% in case of non-catalytic to 32%, 34%, 33% and 29%, when the Mo, Ni, W and Nb catalysts are used, respectively. There was a simultaneous increase in the yield of aromatic hydrocarbons which can derive to the inference that the deoxygenation of guaiacolics takes place in the presence of metal oxide catalysts to produce aromatic hydrocarbons. Moreover, the bio-oil average molecular weight was calculated by GPC and results shown in Table 3. The non-catalytic HTL of 25% PP blend produced bio-oil with an average molecular weight of 692 g/mol. Whereas, during the catalytic HTL, the average molecular weight decreased to 526, 582, 424 and 368 g/mol for alumina supported Mo, Ni, W and Nb catalysts, respectively. This indicates the effective cleavage of C\u2013C bonds in biomass in the presence of catalyst. It has been reported that Nb2O5 possess exceptional hydrogenolysis activity by selectively cleaving Caromatic \u2013 C lignin bonds, while suppressing hydrogenation reaction, when compared to other supports such as ZrO2, Al2O3, TiO2 [43]. The exceptional dehydration capacity of Nb catalyst can be ascribed to the oxophilic nature of Nb2O5 (XPS spectra in Fig. S5 in ESI) that possess an unique dehydration potential due to the strong interaction between the Nb5+/Nb4+ and the oxygen atom of the guaiacol molecule [44]. Xia. et al. reported that the C\u2013 O bond cleavage in tetrahydrofuran ring performed by Nb\u2013 O\u2013 Nb is the result of an increase of acidity of NbOx that favors an increase in the rate of dehydration reaction [45]. The direct conversion of biomass derived carbohydrates and glucose involves the dehydration to produce hydroxymethylfurfural and Nb based catalysts has been reported to be promising for this direct conversion [46,47]. Additionally, the selectivity to acids, aldehydes, and ketones, were lesser during catalytic HTL when compared to the non\u2013 catalytic conversion (from 26%\u2013 non catalytic to 19%, 12%, 13%, and 17% with Mo, Ni, W and Nb, respectively).The decrease in selectivity to this group of compounds along with the associated increase in CO2 and CO gases infer that decarboxylation and decarbonylation reactions of the acid, aldehyde and ketone groups were taking place [48]. The decarboxylation and decarbonylation reactions are accompanied by the formation of CO2 and CO, respectively which can be observed in the gas products (Fig. S6 in ESI).In contrast, catalytic HTL increased the selectivity to alkyl phenolics when compared to non\u2013catalytic HTL (from 6%\u2013 non catalytic to 9%, 11%, 12%, and 12% with Mo, Ni, W and Nb, respectively). Alkylation of guaiacol aromatic ring is a common reaction that occurs in the presence of an acidic catalyst and an alkyl source under hydrothermal conditions [49]. Demethylation and demethoxylation of guaiacol produces CH4 and CH3OH, where this methyl group could alkylate the aromatic ring producing alkylated products as a result of the catalyst acidity (Table 1) [5]. The next class of product compound, catechols and naphthalene oligomers showed a yield decrease due to the presence of catalyst. The catechols and naphthalenes have been reported to produce coke through condensation reactions, [50] therefore, the decrease in selectivity to naphthalene oligomers and catechols implies the decomposition of these coke precursors in the presence of catalyst. The presence of acidic catalyst also promotes the formation of gases from PP by the end-chain cleavage mechanism. A complete deoxygenation of guaiacolics and alkylphenolics results in the formation of aromatic hydrocarbons and alkanes as can be seen in Fig. 4.The efficiency of the Nb2O5/alumina catalyst was further investigated for its catalytic reusability. After reaction, the catalyst was separated from the reaction mixture and regenerated by burning off the deposited coke at 400\u00a0\u00b0C in a muffle furnace. The reusability tests were then conducted using the same reaction conditions as the fresh one. There is a catalyst loss of about \u223c3.4% every time which was compensated from a fresh batch. The yield of bio-oil, aqueous phase, biochar, and gases from each reaction were quantified and the corresponding % deoxygenation and carbon recovery to bio-oil phase were calculated, and the data presented in Fig. 5\n. The catalyst performed remarkably up to 10 reaction cycles maintaining a high bio-oil yield with a marginal decrease (59.4% yield\u20131st cycle to 55.2%\u201310th cycle). The decrease in bio-oil yield was also followed by a decrease in the % carbon recovery to bio-oil from 78.9% in 1st cycle to 75.8% in 10th cycle which is not a significant loss. An increase in the biochar yield was observed from 12% to 15% after 10th cycle. These results demonstrate that the catalyst is promising for reusability.For a comparison, the catalyst retrieved after first cycle was tested for reusability without regeneration (burning coke at 400 \u00b0C), and as expected the bio-oil yield decreased from 59.4% to 55.6% whereas gas and biochar yield sharply increased. This is due to the coke deposited on the catalyst surface covering Nb2O5 active site, therefore, hindering the contact of reactant with the catalyst active acid species. Therefore, the regeneration of the catalyst was essential after every reaction cycle. The catalyst deactivation during HTL reactions have been reported to occur due to multiple factors such as leaching of active metals to the liquid medium,[51] catalyst coking that blocks the pores and masks the active sites,[51] the presence of high concentration of hetero atoms in the feed,[52] etc. On the other hand, \u0263-alumina tend to deactivate in hot water and change phase to aluminium oxide hydroxide (boehmite) that also contains Lewis acid sites [53]. However, this phase change could lead to catalyst deactivation [54].In this study, PJ was converted to bio-oil by hydrothermal liquefaction process that can be further upgraded to biofuels or platform chemicals. To improve the bio-oil yield and quality, a hydrogen rich co-reactant PP and solid acid catalysts were employed. The synergistic interaction between the PJ and PP (25% PP substitution to PJ) at HTL reaction temperature of 420 \u00b0C increased the oil yield to 46.5% from 42.5% obtained when using PJ alone. Among the catalysts, Nb based catalysts showed high selectivity and efficiency for deoxygenation of liquid biomass compounds resulting in high hydrocarbons with reduced oxygen content, making them very suitable for conversion into transportation platform fuels. Nb/Al2O3 was reasonable stable up to 10 reaction cycles. This strategy could be useful both for efficient valorisation of PJ and recycling of PP waste into high value fuels and chemicals which could potentially benefit to Indian farmers, rural industries-based bioeconomy, and municipalities strategies for plastic waste management.This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.\nSwathi Mukundan: Investigation, Writing \u2013 original draft, Supervision, Funding acquisition. Jonathan L. Wagner: Writing \u2013 review & editing, Conceptualization. Pratheep K. Annamalai: Writing \u2013 review & editing, Conceptualization. Devika Sudha Ravindran: Formal analysis. Girish Kumar Krishnapillai: Writing \u2013 review & editing, Supervision. Jorge Beltramini: Writing \u2013 review & editing, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Dr. SM thankfully acknowledge the University Grants Commission- Dr. D. S. Kothari Postdoctoral Fellowship Scheme for sponsoring the research. The first author sincerely appreciates the facilities provided by SAIF STIC, Cochin University of Science and Technology, Kochi.\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.fuproc.2022.107523.", "descript": "\n                  This study reports an efficient conversion route for prosopis juliflora (PJ) biomass into high-quality bio-oil through catalytic hydrothermal liquefaction (HTL) process with systematically substituted hydrogen-rich plastic waste \u2018polypropylene (PP)\u2019, and using alumina supported metal oxide (Mo, Ni, W, and Nb) catalysts. The HTL treatments of PJ with PP (0-75 wt.%) were investigated in both sub and supercritical water conditions. An excellent synergy between PP and PJ was observed even in subcritical conditions (97.6% synergy at 340\u00a0\u00b0C at 25% PP to PJ), while efficient liquefaction of PP alone was observed only in the supercritical conditions. The optimum temperature, and PP substitution were found to be 420\u00a0\u00b0C and 25% respectively, with 46.5% bio-oil yield, high deoxygenation (65.1%), and carbon recovery (78.9%) when using Nb/Al2O3 as the catalyst. An in-depth analysis of physicochemical properties and the bio-oil product distribution with respect to each catalyst and PP/PJ substitution ratio are discussed in detail. Among all, the Nb/Al2O3 catalyst performed well with remarkable recyclability up to 10 cycles. The produced bio-oil mixture due to its low oxygen content is very promising to be upgraded to precursors for chemicals and transportation biofuels.\n               "}
{"full_text": "\n\n\n\u2022\nThis study did not generate a new code.\n\n\n\u2022\nThe supplemental information includes all datasets generated and analyzed during this study.\n\n\n\u2022\nAny additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.\n\n\nThis study did not generate a new code.The supplemental information includes all datasets generated and analyzed during this study.Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.Syngas, a mixture of CO and H2, is widely used in modern chemical industries such as hydroformylation, Fischer-Tropsch synthesis, and the production of alkanes, olefins, and oxygenates.\n1\n\n,\n\n2\n\n,\n\n3\n Given its industrial importance, a variety of syngas production methods have been developed, mainly using natural gas, petroleum, or coal as raw materials. Among all the synthesis pathways, dry reforming of methane (DRM) attracted considerable attention recently, because this process would convert two greenhouse gases, methane and carbon dioxide, into useful syngas.\n4\n\n,\n\n5\n\n,\n\n6\n In addition, compared with an alternative steam reforming of methane,\n7\n\n,\n\n8\n DRM has the advantage of producing syngas with a low H2/CO ratio.Despite the great potential of DRM as one can imagine, there still exist some technical problems to industrialize this process. The most important one is the absence of an efficient catalyst. Many noble metal catalysts show excellent catalytic performance but are rather expensive, preventing the large-scale usage of these catalysts in the industry.\n9\n\n,\n\n10\n\n,\n\n11\n It was found that the inexpensive earth-abundant transition metal nickel gives rise to good catalytic activity for the DRM reaction as well.\n9\n\n,\n\n12\n\n,\n\n13\n\n,\n\n14\n\n,\n\n15\n Nevertheless, there exists another inevitable drawback of nickel, which is deactivation due to the deposition of coke. It was found that the deactivation mainly results from the formation of inert carbon structures on the catalyst surface during the DRM process. In the reaction network, the relevant carbon production reactions are the dissociation of methane and the disproportionation of carbon monoxide. In addition, the accumulation of coke on nickel is a macroscopic scale phenomenon contributed by different surfaces. According to our previous work,\n16\n\n,\n\n17\n (211) surface of nickel is a typical surface where coke can form easily, while (111) is not a suitable surface for carbon deposition.As carbon atoms deposit on the nickel surface, two types of Ni-carbon structures would be possible. One structure is formed through atomic carbon diffusing into the bulk phase of Ni and forming a Ni carbide structure, namely a Ni-C bulk structure.\n18\n\n,\n\n19\n\n,\n\n20\n The other is that carbon atoms adsorb on the surface of Ni and poison the active site, forming a Ni-C surface structure. While most previous studies focused on the Ni-C bulk structure, the characteristics of the Ni-C surface structure in tuning DRM activity were rarely reported. It has been reported previously that Ni(100) is a surface suitable for the deposition of carbon atoms on its surface sites.\n21\n\n,\n\n22\n\n,\n\n23\n Therefore, Ni(100) can be a typical surface for the formation of Ni-C surface structure. Besides, according to previous Wulff construction study results, Ni(100) is the secondly abundant surface; the proportion is even higher than Ni(211), over Ni nanoparticles with a size around 5\u201310\u00a0nm, the typical size of Ni particles for the DRM reaction.\n24\n\n,\n\n25\n\n,\n\n26\n\nIn order to obtain deeper understandings on the formation and dynamic behavior of this type of surface structure on Ni(100) surface, a combined density functional theory (DFT) calculation and microkinetic modeling (MKM) study was performed here. The kinetics of the DRM reaction and deposition of carbon atoms on the pristine Ni surface were studied first. Based on the results obtained, DFT calculations of the surface structures at different carbon coverages were exerted. It is worth mentioning that surface reconstruction has also been observed in the current work here, which was reported experimentally before.\n27\n\n,\n\n28\n\n,\n\n29\n\n,\n\n30\n\n,\n\n31\n Finally, we analyzed the kinetics of the DRM reaction features over carbon-covered Ni(100) and discovered a unique carbon-based Mars-van-Krevelen (MvK) mechanism.\n32\n\nThe reaction network considered here is the same as the one reported in our previous study.\n16\n\n,\n\n17\n\n,\n\n33\n All the surface adsorbates are formed from the product of methane and carbon dioxide dissociation. According to early systematic studies, among all the CHx (x\u00a0= 0\u20133) species, only C\u2217 and CH\u2217 showed noteworthy reaction activity.\n18\n\n,\n\n34\n Therefore, we only introduce here five main routes of CHx oxidation in the reaction network, namely C\u00a0+ O, C\u00a0+ OH, CH\u00a0+ O, CH\u00a0+ OH (COH), and CH\u00a0+ OH (CHO). The intermediates produced by these oxidation reactions finally transform into carbon monoxide, the product of the DRM reaction.Structures of all the adsorbates and transition states on pristine Ni(100) are optimized and presented in Figure\u00a0S1. One can find that most adsorbates, except CH3\u2217 and CHOH\u2217, prefer the 4-fold hollow site. Based on the energies calculated for these structures, the activation barriers and reaction energies of all elementary steps involved in the network can be obtained, which are shown in Figure\u00a01\n. Compared with all other elementary steps, the oxidation reactions of C\u2217/CH\u2217 possess much higher activation energies. Among these oxidation reactions, only C\u2217+OH\u2217 has a slightly lower energy barrier of 1.68\u00a0eV, while the other three oxidation reactions all have higher energy barriers of around 2\u00a0eV.Based on these energies calculated by DFT on pristine Ni(100), MKM can be further performed to obtain important kinetic information. We plot in Figure\u00a02\n the steady-state coverage of surface dominant species, the logarithm of turnover frequency (TOF), and degree of rate control (DRC) obtained from MKM studies against reaction temperatures. One can clearly find that the steady-state coverage of C\u2217 is almost 1 monolayer (ML), suggesting that C\u2217 would occupy almost all the 4-fold hollow sites. This result is consistent with the tendency of carbon accumulation on Ni(100) surface reported before.\n35\n The TOF of the overall reaction varies to a small extent as temperature increases. The calculated TOF related to the key elementary steps that determine the preferred reaction pathway is shown in Figure\u00a0S4, and the dominant route over pristine Ni(100) is found to be C\u00a0+ O/OH.One can find from Figure\u00a02C that the DRC results of reaction intermediates are quite simple since C\u2217 is the only adsorbate with high coverage (\u223c1\u00a0ML) on the surface. Therefore, C\u2217 is the rate-controlling intermediate with a constant DRC value of \u22122. The transition states of the oxidation of surface carbon, i.e. C-O and C-OH, have higher DRC values at a relatively low temperature. As temperature increases, DRC of the transition state of CO2 dissociation (CO-O) increases rapidly and is finally close to 1. More importantly, it is obvious that all the transition states with high DRC are related to carbon elimination. According to detailed MKM analyses performed by our group, when the DRC value of carbon elimination process is high, carbon atoms would be accumulated on the surface as the C\u2217 formation process is quasi-equilibrated and fast.\n8\n\n,\n\n16\n\n,\n\n17\n\n,\n\n33\n\n,\n\n36\n\n,\n\n37\n It should be mentioned that we also added the side reaction of water formation into the DRM reaction network and found that the influence of such reverse water-gas shift reaction on the kinetics is almost negligible (see Figure\u00a0S4), which is consistent with previous studies by our group.\n17\n\n,\n\n33\n\nRecent studies from our group reported that the conventional MKM approach presents some major failures.\n37\n\n,\n\n38\n\n,\n\n39\n\n,\n\n40\n\n,\n\n41\n As one can find in the above section, the main discrepancy here is that the C\u2217 coverage obtained from MKM at the steady state, i.e. 1\u00a0ML, is not consistent with the initial coverage used for DFT energy calculations, which is 0 on pristine Ni(100). Meanwhile, it was observed by DFT calculations that the adsorption of carbon atoms at the subsurface sites of Ni(100) is less stable than that on the surface.\n23\n Furthermore, according to the literature, the appropriate temperature window for DRM reaction is 650\u00b0C\u2013850\u00b0C,\n42\n also indicating the results in Section 3.1 are not consistent with the experimental results reported before. Therefore, the most useful information one can obtain from the results in Section 3.1 is that surface carbonization is very likely to occur on Ni(100). Based on this observation, we further calculated the adsorption free energy of carbon as a function of coverage over Ni(100).Since a p(4\u00a0\u00d7\u00a04) supercell of Ni(100) is used here, we only considered the surface structures covered by carbon atoms with an increment of 1/16\u00a0ML. In addition, we find that the surface will get reconstructed at high carbon coverage after structural optimization. According to a previous study, this phenomenon is named as a clock-type surface reconstruction, which is common on Rh(100), Pd(100), and Ni(100).\n27\n\n,\n\n28\n\n,\n\n29\n\n,\n\n30\n\n,\n\n31\n Therefore, all the possible structures of both original and reconstructed surfaces under different carbon coverages were considered in the DFT calculations. We find from the results that the preferred adsorption structures of multiple carbon atoms would give rise to minimized interactions between atoms.Differential adsorption free energies of carbon atom on the most stable surface structures are plotted in Figure\u00a03\n against carbon coverage from 1/16\u00a0ML to 10/16\u00a0ML. Since our slab model for Ni(100) is a four-layer p(4\u00a0\u00d7\u00a04) supercell, each addition of carbon atoms on the surface corresponds to a carbon coverage increment of 1/16\u00a0ML. The red and green curves show the differential adsorption free energy of C\u2217 on the original and reconstructed Ni(100) surface, respectively. The differential adsorption free energy of carbon atom at different coverages can be calculated with the following equation:\n\n(Equation\u00a01)\n\n\n\nG\n\na\nd\ns\n\n\n\n(\n\nN\n16\n\n)\n\n=\n\nG\n\ns\nl\na\nb\n+\nN\n\u00d7\nC\n\n\n\u2212\n\nG\n\ns\nl\na\nb\n+\n\n(\n\nN\n\u2212\n1\n\n)\n\n\u00d7\nC\n\n\n\u2212\n\n(\n\n\u03bc\n\n\nC\nH\n\n4\n\n\n\u2212\n2\n\n\u03bc\n\nH\n2\n\n\n)\n\n\n\n\nwhere \n\n\nG\n\na\nd\ns\n\n\n\n(\n\nN\n16\n\n)\n\n\n, \n\n\nG\n\ns\nl\na\nb\n+\nN\n\u00d7\nC\n\n\n\n, and \n\n\nG\n\ns\nl\na\nb\n+\n\n(\n\nN\n\u2212\n1\n\n)\n\n\u00d7\nC\n\n\n\n are differential adsorption free energy of the Nth C\u2217, total free energy of the surface structure with N C\u2217, and (N-1) C\u2217, respectively. \n\n\n\u03bc\n\n\nC\nH\n\n4\n\n\n\n and \n\n\n\u03bc\n\nH\n2\n\n\n\n are the chemical potential of methane (0.5\u00a0bar) and hydrogen (0.05\u00a0bar), respectively, at 873\u00a0K.We find from Figure\u00a03 that, when carbon coverage is lower than 6/16\u00a0ML, the reconstructed surface is unstable and will change back to the original structure after optimization, giving rise to the same adsorption energies. With surface carbon coverages between 6/16 and 8/16\u00a0ML, carbon adsorption on the reconstructed surface is more stable than that on the original one. More importantly, the differential adsorption free energies of carbon over these structures are all close to the one over the pristine surface (\u22120.74\u00a0eV), e.g. \u22120.63 eV at 6/16\u00a0ML, \u22120.67 eV at 7/16\u00a0ML, and \u22120.50\u00a0at 8/16\u00a0ML. In addition, it is worth noting that the differential adsorption free energy will not be above zero until coverage reaches 9/16\u00a0ML. From the structures shown in Figure\u00a03, the ninth carbon atom has to locate at the unstable 3-fold hollow site on the reconstructed surface. Meanwhile, upon the adsorption of the 10th carbon atom, the surface structure is significantly deformed, further suggesting that the adsorption of more carbon atoms will be strongly endergonic.It is also worth noting that, from previous experiments, the reconstruction will not occur until the coverage reaches 0.33\u00a0ML,\n30\n which is consistent with our DFT results (between 5/16 and 6/16\u00a0ML). From both DFT results and experimental evidences, it is reasonable to assume that pristine Ni(100) surface will accumulate carbon atoms and finally form a stable Ni-C surface at 8/16\u00a0ML carbon coverage.Over the reconstructed Ni(100) surface with 8/16\u00a0ML of carbon coverage, the structure of which is shown in Figure\u00a03C, there would be two possible mechanisms of the DRM reaction, namely the surface reaction mechanism and the carbon-based MvK mechanism.Regarding the surface reaction mechanism, the reaction pathways are identical to those studied over the pristine Ni(100) surface, and five routes introduced in Section 3.1 are considered here, i.e. the C\u00a0+ O, C\u00a0+ OH, CH\u00a0+ O, CH\u00a0+ OH (COH), and CH\u00a0+ OH (CHO) routes. It should be mentioned that, over the carbon-covered reconstructed Ni(100), we calculated the adsorption of surface species over all possible sites, including the 3-fold metallic sites and the 4-fold sites with one carbon atom at the center, and the structures presented in Figure\u00a0S2 are the most stable ones. From these stable structures, we find all the reactions occur at 3-fold metallic sites and therefore denote these active sites as \u2217.The energy profile of the DRM reaction following this mechanism is shown in Figure\u00a04\nA. Comparing this energy profile with the one shown in Figure\u00a01 over the pristine Ni(100), one can find that the desorption of products is much more difficult over the carbon-covered reconstructed Ni(100). In addition, the dissociation reactions of reactants have to overcome high activation energies and reaction energies, e.g. CO2 dissociation gives an activation energy of 3.01\u00a0eV and a reaction energy of 1.98\u00a0eV, suggesting that both reactants may be difficult to dissociate on this surface. Meanwhile, oxidation of C\u2217 or CH\u2217 is much easier, e.g. CH\u00a0+ OH shows an activation energy of 0.95\u00a0eV. These results indicate that carbon atoms are difficult to form and easy to be eliminated on this surface. We will show later that this is important to understand the different steady-state carbon coverage obtained from MKM simulations.The other mechanism considered is the carbon-based MvK mechanism (see Scheme 1\n). In this catalytic cycle, the DRM reaction would be initiated by the elimination of one surface carbon atom, through the reaction between CO2 and this carbon atom, to produce two CO molecules and to leave one carbon-vacancy site on the surface. This reaction is the reverse reaction of the classical Boudouard reaction, following which CO2 is dissociated in a concerted way. The transition state structure of this reaction is shown in Figure\u00a04B. This reaction is \u201cspin forbidden\u201d in gas phase and thus needs to overcome a high energy barrier.\n43\n However, according to experiments, nickel is a suitable catalyst for this reaction,\n44\n which is consistent with our computation result. Subsequently, CH4 will get dissociated at this carbon-vacancy site to close the catalytic cycle. This mechanism is quite similar to the traditional oxygen-based MvK mechanism, which is an important reaction mechanism over metal oxide catalysts,\n32\n and normally the key species participating in the catalytic cycle is lattice oxygen and oxygen vacancy.The energy profile of this carbon-based MvK mechanism is presented in Figure\u00a04B. Interestingly, we find all the reactions occur at 4-fold metallic sites and denote these active sites as #. It is obvious that dissociation of CH4 is much easier with a barrier of 1.19\u00a0eV compared with that in the surface reaction mechanism (the barrier is 1.66\u00a0eV). All the elementary steps of methane dissociation share similar energy trends compared with those on a pristine nickel surface. In addition, the barrier for carbon-assisted CO2 dissociation is 1.43\u00a0eV, which is slightly more difficult than at the 4-fold hollow sites on pristine surface but much easier than at the 3-fold sites on the reconstructed surface.We plot the steady-state coverage of surface-dominant species, logarithm of TOF and DRC obtained from MKM studies concerning the surface reaction mechanism, and the carbon-based MvK mechanism in Figure\u00a05\n against reaction temperatures. The influence of side reverse water-gas shift reaction is also found negligible over this surface (see Figure\u00a0S4).Distinct steady-state coverage of surface carbon can be found from Figures\u00a05A and 5D, when the surface reaction mechanism or the carbon-based MvK mechanism is considered. Almost no adsorbate has distinctive coverage when the surface reaction mechanism is considered, and therefore only a negligible amount of carbon (<10\u221210\u00a0ML) would be observed at the steady state. In comparison, the carbon-based MvK mechanism would suggest that the carbon atoms should occupy almost all the active sites. It should be noted that the 4-fold hollow sites are taken as the active sites in the kinetic model for the carbon-based MvK mechanism; therefore, the carbon vacancy should be readily replenished and the catalytic cycle can be closed.TOF of the carbon-based MvK pathway is much higher than that of surface reaction pathway, suggesting that these two kinetic models would give significantly different reactivity of the DRM reaction. In addition, compared with the TOF over pristine surface, TOF of carbon-based MvK mechanism is even higher. These results indicate that the mechanism driven by CO2 concerted dissociation reaction has higher reactivity than conventional DRM mechanism at high carbon coverage. This provides a detailed understanding on the observations reported in a recent study at the molecular level.\n45\n For surface reaction mechanism, the calculated TOF related to the key elementary steps that determine the preferred reaction pathway is shown in Figure\u00a0S4, and the dominant route is found to be CH\u00a0+ OH (CHO).DRC results obtained for the surface reaction mechanism are presented in Figure\u00a05C. The transition states with non-zero DRC values are CO2 dissociation and oxidation of surface CHx species, mainly CH-OH and C-OH. In contrast, the only rate-controlling transition state determined from the carbon-based MvK mechanism is C-CO2, the transition state of carbon-assisted CO2 dissociation, as shown in Figure\u00a05F. Since almost all the 4-fold hollow active sites are filled by C\u2217 at steady state, only C\u2217 has a non-zero DRC value.To analyze the relevance of these two mechanisms, we also constructed a microkinetic model combining all these reactions at all possible sites, and the migration of all adsorbates between different sites was also included. The microkinetic modeling results are presented in Figure\u00a0S5. One can find that all the active # sites are occupied by carbon atoms, while there is almost no adsorbate adsorbed in the active \u2217 sites. TOFs related to CO production obtained from the combined model are found to be identical to those of the preferred carbon-based MvK mechanism. According to the DRC results, the important transition state and intermediate are both within the carbon-based MvK mechanism. These results further suggest that the carbon-based MvK mechanism should be preferred.According to the results presented above, the C\u2217 formation process can be considered quasi-equilibrated in both mechanisms. This means that once the vacancy at the 4-fold hollow site is created, it always tends to be replenished by C\u2217. Meanwhile, the C\u2217 is difficult to form at the 3-fold hollow sites. In our simulation model, this reaction process can be described as an oscillation between 7/16\u00a0ML carbon-covered and 8/16\u00a0ML carbon-covered surfaces. Once an 8/16\u00a0ML carbon-covered surface structure is formed, the further accumulation of surface carbon will be prevented. In the meantime, a new reaction pathway, i.e. the carbon-based MvK pathway, becomes dominant. This reaction pathway firstly creates a vacancy at the 4-fold hollow sites and a 7/16\u00a0ML carbon-covered surface structure is formed, then changes to the 8/16\u00a0ML carbon-covered surface structure upon dissociation of CH4 at the vacancy sites.The above result indicates that carbon-based MvK mechanism is favored over Ni(100) surface at high carbon coverage. It is intriguing to extend this result to other typical surfaces of nickel nanoparticles. In our previous studies, the activity of Ni(111) and Ni(211) has been thoroughly studied.\n17\n\n,\n\n36\n Combining all the results reported, we found that the TOF is increasing from Ni(100) and Ni(111) to Ni(211). Therefore, one can find that Ni(211) possesses the highest activity for the DRM reaction, but the high coverage of carbon might result in deactivation and coke formation, which would be an interesting topic for future studies. It should be mentioned that a similar analysis approach introduced in the current work would be helpful for future studies on Ni(211).In addition, several key information that can be used to help design more stable Ni catalysts for the DRM reaction can be obtained from our results. Firstly, not all the surface sites would contribute to the formation of coke during the DRM, which is consistent with recent work that only part of the surface sites on nickel nanoparticles lose their activity during DRM reaction.\n45\n More importantly, different from the catalyst designing idea widely reported previously to find the surface where no carbon tends to accumulate, our research presents a new idea that the adsorbed carbon may prevent the formation of poisonous coke. Combining both strategies of preventing carbon deposition, we suggest that a low-index flat surfaces, i.e. Ni(100) and Ni(111), tends to be coke resistant, although the mechanisms might be different over these surfaces, while surfaces with defects may deactivate quickly, e.g. Ni(211). Therefore, in order to prevent coke-induced deactivation of Ni catalyst during the DRM reaction, the catalyst particles should possess as few defects as possible.In the current work, thorough understandings on the DRM reaction over Ni(100) are obtained. We find that carbon deposition and accumulation will be spontaneous over the pristine Ni(100) surface, because the steady-state coverage of carbon obtained from microkinetic modeling is as high as 1\u00a0ML. In the meantime, as the coverage of carbon on the surface increases, surface reconstruction would happen and form a stable Ni-C surface structure, which brings more interesting properties. The optimal coverage of carbon over Ni(100) surface is found to be 0.5\u00a0ML and further adsorption of carbon is endergonic. Through comparing the surface reaction and the carbon-based MvK mechanisms for the DRM reaction over carbon-covered reconstructed Ni(100) surface, we find that the latter dominates under reaction conditions and shows even higher activity and promoted coke resistance. The unique carbon-based MvK mechanism was rarely reported in previous studies on the DRM reaction, and the current work not only provides evidence of the existence of this mechanism but also quantitatively determines the detailed kinetic information such as reaction rate and the rate-controlling step over the carbon-covered reconstructed Ni(100) surface. The possible strategies for the promotion of Ni catalyst stabilities during the DRM reaction are proposed.Our work is a theoretical research of DRM reaction behavior on Ni(100) by combining DFT calculations and microkinetic modeling. The catalyst design strategy is proposed that augmenting proportion of Ni(100) and Ni(111) is beneficial to coke resistance, while the experimental investigation over this topic needs to be further conducted.\n\n\n\n\n\n\n\n\nREAGENT or RESOURCE\nSOURCE\nIDENTIFIER\n\n\n\n\n\nSoftware and algorithms\n\n\n\nVASP 5.4.1\nVASP Software GmbH\n\nhttps://www.vasp.at\n\n\n\nCATMAP\nSUNCAT, Stanford University\n\nhttps://github.com/SUNCAT-Center/catmap\n\n\n\n\n\n\nFurther information and requests should be directed to and will be fulfilled by the lead contact, Bo Yang (yangbo1@shanghaitech.edu.cn).This study did not generate new unique material.All the DFT calculations were performed with the Vienna Ab-initio Simulation Package (VASP). The electron-ion interaction was described by the projector-augmented wave (PAW) formalism, with an energy cutoff of 500\u00a0eV.\n46\n Bayesian error estimation functional with van der Waals correlations (BEEF-vdW) was used to describe electron exchange and correlation.\n47\n Spin polarization was considered for all the calculations. The Methfessel-Paxton smearing method was used here with a broadening of 0.1\u00a0eV.\n48\n All adsorption configurations were optimized by a force-based conjugate gradient algorithm and the transition states were determined by using a constrained minimization method.\n39\n\n,\n\n49\n\n,\n\n50\n All the identified transition states were further confirmed with vibrational analysis to ensure that only one imaginary frequency, corresponding to the bond breaking/formation, was obtained. The force convergence criterion was set to 0.05\u00a0eV/\u00c5 while the total energy convergence criterion was 10\u22124\u00a0eV. From previous studies, theoretical calculations with such parameter settings are sufficient to simulate the experiment results.\n39\n\n,\n\n51\n\n,\n\n52\n\n,\n\n53\n\nWe built a 4-layer p(4\u00a0\u00d7\u00a04) supercell for the Ni(100) surface and the bottom two layers were frozen during structural optimization. A vacuum layer with a height of 15\u00a0\u00c5 was used here to avoid interactions between periodic structures. A k-point mesh of 3\u00a0\u00d7\u00a03\u00d71 was used here for all the adsorption structure optimization and transition state searching.Adsorption energy was calculated from\n\n(Equation\u00a02)\n\n\n\nE\n\na\nd\n\n\n=\n\nE\n\ns\nl\na\nb\n+\na\nd\ns\n\n\n\u2212\n\nE\n\ns\nl\na\nb\n\n\n\u2212\n\nE\n\na\nd\ns\n\n\n\n\n\nwhere \n\n\nE\n\na\nd\n\n\n\n, \n\n\nE\n\ns\nl\na\nb\n+\na\nd\ns\n\n\n\n, \n\n\nE\n\ns\nl\na\nb\n\n\n\n, and \n\n\nE\n\na\nd\ns\n\n\n\n are adsorption energy, energy of the slab after adsorption, energy of the slab and energy of the adsorbate in gas phase. Although DFT-calculated gaseous energies of CO2 and CO may be inaccurate, and this error might result in flawed microkinetic modeling results,\n51\n\n,\n\n52\n\n,\n\n53\n we found that no other correction should be made for the energies of CO2 and CO while studying the DRM reaction using the above DFT parameter settings.\n17\n\nWe utilized the CatMAP package in the microkinetic simulations.\n54\n Steady state approximation and transition state theory were used for microkinetic modeling in this package. The temperature considered in our MKM study is within the range of 873\u20131073\u00a0K. To be consistent with the experimental conditions reported, the total pressure of reactants here was selected as 1\u00a0bar. The ratio between two reactants, CH4 and CO2, was fixed at a proportion of 1:1, and the conversion considered was 5%. Thermodynamic corrections were included here for both gas-phase molecules and surface adsorbates with Shomate equations and harmonic approximation, respectively. Here, we consider a model with two types of active sites, one for hydrogen atoms and the other for the remaining adsorbates. In this approach, hydrogen is adsorbed at a special \u201chydrogen reservoir\u201d site and does not compete with other adsorbates, because hydrogen has almost zero interaction with all adsorbates, including itself. This approach has been widely used in MKM studies by several groups.\n51\n\n,\n\n55\n\n,\n\n56\n More details regarding the parameter settings used in MKM studies can be found in the supplemental information.The degree of rate control (DRC) analysis method, developed by Campbell and co-workers, was applied in our study to obtain deeper understandings of the MKM results.\n57\n\n,\n\n58\n\n,\n\n59\n\n,\n\n60\n\n,\n\n61\n The values of DRC can be calculated with\n\n(Equation\u00a03)\n\n\n\nX\ni\n\n=\n\n\n(\n\n\n\u2212\n\u2202\nln\n\nr\n\n\n\u2202\n\n(\n\n\nG\ni\n0\n\n/\n\nk\nB\n\nT\n\n)\n\n\n\n)\n\n\nG\n\nj\n\u2260\ni\n\n0\n\n\n\n\n\nwhere \n\n\nX\ni\n\n\n represents the DRC of a transition state or an intermediate i, r is the rate of the overall reaction and \n\n\nG\ni\n0\n\n\n is the Gibbs free energy of i. The relative magnitude of DRC is in line with the response of reaction rates to the change in the free energy of a given intermediate or transition state. A positive DRC value means the reaction rate increases when lowering the Gibbs energy of i, while a negative value implies the opposite influence. One can find that a transition state should normally possess a positive value. In addition, the higher the DRC value is, the more important this transition state would be to the overall reaction. The maximum for a DRC value of a transition state is 1, which indicates this transition state controls the overall reaction rate completely. Meanwhile, DRC value of an intermediate is negative and equal to the coverage of the intermediate multiplied by the number of active sites required for the elementary reaction.This work is financially supported by the National Natural Science Foundation of China (22072091, 91745102, 92045301), Shanghai Rising-Star Program (20QA1406800), and ShanghaiTech University. We thank the HPC Platform of ShanghaiTech University for computing time.Z.G. performed the DFT calculations and microkinetic simulations related to carbon-covered surfaces. S.C. performed DFT calculations related to clean surface. B.Y. conceived the problem. All the authors contributed to writing the paper.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.isci.2023.106237.\n\n\nDocument S1. Figures\u00a0S1\u2013S5 and Table\u00a0S1\n\n\n\n", "descript": "\n                  Dry reforming of methane (DRM) is an efficient process to transform methane and carbon dioxide to syngas. Nickel could show good catalytic activity for DRM, whereas the deactivation of nickel surfaces by the formation of inert carbon structures is inevitable. In this study, we carry out a detailed investigation of the evolution and catalytic performance of the carbon-covered surface structure on Ni(100) with a combined density functional theory and microkinetic modeling approach. The results suggest that the pristine Ni(100) surface is prone to carbon deposition and accumulation under reaction conditions. Further studies show that over this carbon-covered reconstructed Ni(100) surface, a carbon-based Mars-van-Krevelen mechanism would be favored, and the activity and coke resistance is promoted. This surface state and reaction mechanism were rarely reported before and would provide more insights into the DRM process under real reaction conditions and would help design more stable Ni catalysts.\n               "}
{"full_text": "Methane is the primary component of natural gas, shale gas, biogas and combustible ice. Methane conversion has been a hot topic for several decades. With the fast exploration of shale gas, methane conversion has even attracted increasing attentions (Bian et al., 2017; Yi et al., 2015; Pan et al., 2010). Among various options for methane conversion, CO2 reforming of methane or dry reforming of methane (DRM) is promising, because this reaction can convert methane and carbon dioxide into valuable chemicals in a large scale with reasonable H2/CO ratio for further syntheses. Compared with the noble metal-based catalysts, the nickel-based catalyst has high activity but relatively low price. It has been therefore widely investigated for CO2 reforming (Gonzalez et al., 2016; Vasiliades et al., 2018; Pudukudy et al., 2017). During the DRM reaction, methane decomposition to carbon and hydrogen was reported to be the first and rate-determining step. The formed carbon intermediate can be then oxidized by CO2 to CO. The reactivity of such formed carbon is very important for the stability of the catalyst. If the reactivity of the formed carbon is not good, the carbon will tend to aggregate into more stable carbon species with higher graphitization degree (e.g., graphite and carbon filaments). This would cause a serious carbon deposition on the catalyst surface (Aramouni et al., 2018). The active sites of the catalyst would be covered, which further suppresses the following reaction steps and finally deactivates the catalyst (Bayat et al., 2016). Therefore, the carbon intermediate for DRM must be active and easy to be removed by CO2. Rationally controlling the methane decomposition and carbon deposition is of great importance to sustain the DRM activity.The morphology and structure of the nickel-based catalyst play key roles in the structure of the carbon species produced from methane decomposition. It was previously reported (Beltr\u00e1n et al., 2017; Goula et al., 1996) that the methane dissociation is a structure sensitive reaction on nickel surface with different activation energies on various nickel facets. For example, the activation energy of the dissociation of CH* on Ni (111), Ni (211), Ni3C (111) and Ni3C (001) facet was 1.35\u00a0eV, 0.52\u00a0eV, 1.14\u00a0eV and 0.86\u00a0eV, respectively (Liu et al., 2019; Wang et al., 2014). Ni (111) shows the highest activation energy, which means that the rate of carbon deposition on Ni (111) facet is the slowest. The Ni nanoparticle size also significantly affects the diameter of carbon nanotube (Seshan et al., 1998) and the growth rate of carbon filament (Ermakova and Ermakov, 2002; Lou et al., 2017).We previously reported that the decomposition of nickel precursor by dielectric barrier discharge (DBD) plasma, followed by the thermal treatment in the absence of the plasma, causes the Ni catalyst with enhanced coke resistance for DRM (Hu et al., 2019). In this work, the influence of reaction conditions and plasma catalyst decomposition on the structural properties (e.g., species, morphology and size) and the reactivity of the carbon intermediate formed from methane decomposition was further investigated. Ni/ZrO2 catalysts were prepared via two methods: one was decomposed by the DBD plasma, and the other one was prepared by thermal calcination. Compared with the calcined Ni/ZrO2 catalyst, the DBD plasma decomposed Ni/ZrO2 catalyst showed a significantly different Ni structure, which leads to the formation of carbon with improved reactivity towards CO2, which explains the enhanced coke resistance for DRM.The ZrO2 support was prepared by calcining Zr(NO3)4\u00b75H2O (Tianjin Kemiou Chemical Reagent) at 500\u00a0\u00b0C for 3\u00a0h. A certain amount of Ni(NO3)2\u00b76H2O (Tianjin Kemiou Chemical Reagent) was dissolved in distilled water. The ZrO2 powder was then incipiently impregnated with the prepared aqueous solution and aged at ambient temperature for 12\u00a0h. After drying at 110\u00a0\u00b0C for 12\u00a0h, one part of the dried sample was calcined at 700\u00a0\u00b0C for 2\u00a0h. The obtained catalyst was denoted as Ni/ZrO2-C. The other part was decomposed by DBD plasma, operated ca. 150\u00a0\u00b0C, under argon atmosphere for 1\u00a0h. In order to remove the undecomposed nickel nitrate, the DBD decomposed sample was washed by ionized water and alcohol, and then dried at 75\u00a0\u00b0C for 8\u00a0h. The obtained catalyst was denoted as Ni/ZrO2-P. Both of the two catalysts were reduced by hydrogen in the absence of the plasma before the activity tests for methane decomposition.The DBD plasma is a typical kind of cold plasmas with plentiful of various energetic species, like electrons, ions, radicals and excited species. The DBD plasma decomposes the nickel precursor in a rapid way. It causes a rapid nucleation but relatively slower crystal growth. This makes the DBD plasma decomposition different from the conventional thermal decomposition. The DBD plasma setup has been described in our previous works (Yan et al., 2015; Tan et al., 2018). A sinusoidal voltage with a frequency of ca. 22\u00a0kHz was generated via a high voltage generator (CTP-2000\u00a0K; Corona Laboratory, Nanjing, China). The sample powders were laid on a quartz reactor. The diameter of the quartz plate is 90\u00a0mm with a thickness of 8\u00a0nm. The DBD plasma is generated in the quartz reactor via applying an average voltage of 14\u00a0kV to a steel plate electrode.Thermal gravimetric analyses (TGA) with differential scanning calorimetry (DSC) were applied to obtain the weight loss and heat change of the catalysts. It was carried out on a Netzsch STA 449 F3 system with a heating rate of 10\u00a0\u00b0C/min (from 35 to 800\u00a0\u00b0C) under a flowing air of 100\u00a0mL/min (O2 20\u00a0mL/min, N2 80\u00a0mL/min).X-ray powder diffraction (XRD) patterns were recorded by a Rigaku D/max-2500 diffractometer with Ni-filtered Cu target and a K\u03b1 radiation source (\u03bb\u00a0=\u00a01.54056\u00a0\u00c5). The 2\u03b8 range was from 10\u00b0 to 80\u00b0 with a scanning speed of 4\u00b0/min. The acquired XRD patterns were compared using Joint Committee on Powder Diffraction Standards (JCPDSs) to identify the crystal phase of different samples. The metal particle size was calculated using Scherrer equation based on the characteristic diffraction peak.Transmission electron microscopy (TEM) was applied to study the morphology and the size of different catalysts. TEM images were obtained from a Philips Tecnai G2 F20 system equipped with an energy-dispersive X-ray spectrometer (EDX) operated at 200\u00a0kV. The catalyst powder was first suspended into ethanol and then ultrasonically dispersed for 40\u00a0min. One drop of the suspension was dripped onto a copper grid for TEM observation.Raman spectroscopy is a common method to quantitatively analyze the graphitization degree of carbon species. Raman spectra were collected on a Renishaw inVia reflex spectrometer, using a laser with an excitation wavelength of 532\u00a0nm.Temperature programmed reaction with CO2 (CO2-TPR) was performed on AutoChem II 2920 adsorption apparatus combined with a mass spectrometry. A certain amount of sample was blown with 30\u00a0mL/min He at 50\u00a0\u00b0C for 1\u00a0h to remove surface physically absorbed gas. Then, the sample was heated from 50\u00a0\u00b0C to 800\u00a0\u00b0C, at a heating rate of 10\u00a0\u00b0C/min under a gaseous mixture of CO2:He\u00a0=\u00a01:2 (30\u00a0mL/min). The gas product was analyzed by mass spectrometers.Methane decomposition was carried out in a quartz tubular fixed-bed reactor under atmospheric pressure. 80\u00a0mg catalyst was loaded in the quartz reactor and linearly heated to 700\u00a0\u00b0C under argon atmosphere. The reactor was then fed with hydrogen at 40\u00a0mL/min to reduce the catalysts for 1\u00a0h at 700\u00a0\u00b0C. After that, it was cooled down to reaction temperature under argon atmosphere. A mixed gas flow (CH4:Ar\u00a0=\u00a01:3 or CH4:Ar\u00a0=\u00a02:3) was fed into the reactor at a certain temperature. Carbon was then formed on the catalysts. The range of reaction temperature is from 450\u00a0\u00b0C to 600\u00a0\u00b0C.Generally, carbon species deposited on Ni catalysts can be divided into three types: atomic carbon (C\u03b1), amorphous carbon (C\u03b2) and graphite (C\u03b3) (Zhang et al., 2015). As a highly reactive intermediate in DRM, C\u03b1 can be easily oxidized by oxygen under 100\u00a0\u00b0C (Noh et al., 2017). C\u03b2, which is converted from C\u03b1, can be removed at 300\u00a0\u00b0C or turned into more stable form (C\u03b3). Onion-like carbon, carbon fibers and carbon nanotubes (CNTs), as the C\u03b3 species, are the most inactive carbon species and need higher temperature to be oxidized. This is the main reason for the deactivation of the catalyst during DRM (Bartholomew, 2001; Al-Fatesh et al., 2017).In this work, methane decomposition under different temperature and reaction time were conducted. After methane decomposition, the TG/DSC analyses under air atmosphere were conducted to measure the amount and reactivity of carbon deposition through the mass losses and exothermic peaks of the used catalysts. The effect of reaction temperature on carbon deposition is shown in Fig. 1\n. The mass losses of carbon on the plasma-treated and calcined catalysts are almost equal, indicating the same amount of carbon deposition on two catalysts. After methane decomposition at 450\u00a0\u00b0C, 500\u00a0\u00b0C and 550\u00a0\u00b0C for 30\u00a0min, the amount of deposited carbon is 15\u00a0wt%, 29\u00a0wt% and 38\u00a0wt%, respectively. Therefore, the carbon deposition increases with increasing temperature. Carbon oxidization is an exothermic process, as reflected by DSC curves. The peaks were fitted by a Gaussian-type function to figure out the different types of carbon (Fig S1 and Fig S2). For every sample, the peak at the lowest temperature should belong to the oxidization of metallic Ni. The exothermic peaks located at temperature higher than 400\u00a0\u00b0C for every sample should be assigned to the oxidation of onion-like carbon (<500\u00a0\u00b0C) and CNTs (>500\u00a0\u00b0C) (Guo et al., 2004, Zhang et al., 2015). Ni/ZrO2-P shows lower ratios of onion-like carbon to CNTs than Ni/ZrO2-C (Table 1\n). The onion-like carbon would encapsulate Ni particles, leading to the deactivation of active sites of the Ni catalyst soon. On the contrary, CNTs with Ni particles at the top would not completely cover the Ni active sites, as discussed below. Obviously, the degree of graphitization of the onion-like carbon should be lower than that of CNTs. Therefore, the onion-like carbon requires lower oxidation temperature. Yan et al. (2015) have reported that the plasma treated Ni catalysts with fewer Ni defect sites facilitate the formation of CNTs along with less onion-like carbon. Hence, Ni/ZrO2-P would present a higher resistance to deactivation than Ni/ZrO2-C. With the increasing reaction temperature, the peaks shift towards high temperatures with increasing CNTs. It can also be found that the exothermic peaks of Ni/ZrO2-P always center at lower temperatures than those on Ni/ZrO2-C. This means that the carbon deposited on Ni/ZrO2-P exhibits lower degree of graphitization.\nFig. 2\n presents the TG curves of the used Ni/ZrO2-P and Ni/ZrO2-C after methane decomposition for different time. The mass losses of both catalysts are less than 10\u00a0wt% after methane decomposition for 10\u00a0min. When the reaction lasts for 60\u00a0min, the mass loss (48.02\u00a0wt%) of the calcined catalyst is slightly higher than that (45.01\u00a0wt%) of the DBD catalyst. The DSC curves of the used catalysts are displayed in Fig. 3\n, and the ratios of CNTs to onion-like carbon are listed in Table 1. With the time increasing, the peaks gradually shift to higher temperatures, so the deposited carbon is more difficult to be oxidized. As compared with Ni/ZrO2-C, Ni/ZrO2-P always produces less onion-like carbon. And, CNTs on Ni/ZrO2-P need lower oxidization temperature. Therefore, the DBD plasma decomposed catalyst shows a stronger carbon resistance than the calcined one. This suggests that the carbon formed on Ni/ZrO2-P has higher reactivity, leading to an improved stability for DRM (Hu et al., 2019).XRD was used to estimate the graphitization degree of deposited carbon and nickel particle size for both catalysts. The XRD patterns of the catalysts after reactions at 450\u00a0\u00b0C, 500\u00a0\u00b0C and 550\u00a0\u00b0C for 30\u00a0min are shown in Fig. 4\n. The typical peak of graphite (002) facet at 2\u03b8\u00a0=\u00a026\u00b0 can be detected for Ni/ZrO2-C and its intensity increases with increasing temperature. Ni/ZrO2-P exhibits weaker graphite peaks than Ni/ZrO2-C, indicating lower degree of graphitization of the carbon formed. The average diameters of nickel particles were calculated using Scherrer equation, based on the typical peak of Ni (111) facet at 2\u03b8\u00a0=\u00a044.5\u00b0. The results are listed in Table 2\n. After methane decomposition at 450\u00a0\u00b0C for 30\u00a0min, the average diameters of Ni particles of Ni/ZrO2-P and Ni/ZrO2-C are 12.90\u00a0nm and 15.19\u00a0nm, which are similar to those of fresh catalysts (12.90\u00a0nm and 15.59\u00a0nm). No obvious sintering was observed, meaning that the catalysts kept stable structures. The particle size of Ni significantly increases when the reaction temperature reaches 550\u00a0\u00b0C, indicating that the Ni catalysts are likely to move and aggregate into larger particles. However, the size of Ni particles on Ni/ZrO2-P is still smaller than that on Ni/ZrO2-C. It has been reported that Ni/ZrO2-P mainly possesses Ni (111) as the principally exposed facet with fewer Ni defect sites, while the smaller Ni particles can provide more Ni (111) active sites (Jia et al., 2019). Therefore, Ni/ZrO2-P improves the formation of CNTs with lower graphitization degree, supporting the TG/DSC and XRD results.XRD patterns of the catalysts after reaction at 500\u00a0\u00b0C with different time are shown in Fig. 5\n. With the increasing reaction time, the graphite (002) peak gradually gets stronger for both catalysts. Methane decomposition for longer time would produce more graphite-like carbon with higher crystallinity. In addition, the size of Ni particles is also influenced by the decomposition time. According to Table 3\n, the Ni particles exhibit obvious aggregation with time increasing. The diameters of Ni on Ni/ZrO2-P are always smaller than those on Ni/ZrO2-C.TEM was applied to directly investigate the morphology and distribution of the CNTs on the used catalysts (Fig. 6\n). The CNTs, attached to Ni nanoparticles at the top, and the onion-like carbon, covering Ni nanoparticles, can be observed on both catalysts. The Ni particles are not completely covered by CNTs. They should be still active for methane decomposition (Li et al., 2011; Monthioux et al., 2007; Li et al., 2009). With the increasing temperature, the amount and the length of the CNTs increase rapidly. The size distribution of CNTs after methane decomposition at 550\u00a0\u00b0C for 30\u00a0min is presented in the right of Fig. 6. The average diameter of CNTs on Ni/ZrO2-P and Ni/ZrO2-C is 21\u00a0nm and 40\u00a0nm, respectively. It has been proved that the rate of carbon deposition is positive correlation with the size of Ni particles (Seshan et al., 1998). The above results support that smaller Ni particle size of Ni/ZrO2-P contributes to the lower rate of carbon deposition and more uniform distribution of CNTs with smaller diameters.The structure of the carbon deposited on the catalysts was then analyzed by Raman spectroscopy. Fig. 7\n shows the Raman spectra of used catalysts after methane decomposition at 450\u00a0\u00b0C, 500\u00a0\u00b0C and 550\u00a0\u00b0C for 30\u00a0min. The G band located at 1580\u00a0cm\u22121 is attributed to the in-plane C-C stretching vibration of graphite. The peak at 1350\u00a0cm\u22121 is named as D band, derived from amorphous carbon or imperfect graphite. The relative intensity ratio in the form of ID/IG is used to quantitatively estimate the graphitization degree of deposited carbon (Yu et al., 2017). Namely, when the degree of graphitization increases, the ratio of ID/IG decreases. The ID/IG value of the deposited carbon on Ni/ZrO2-P is 1.52 and 1.50 at 450\u00a0\u00b0C and 550\u00a0\u00b0C, respectively. The ID/IG value of the deposited carbon on Ni/ZrO2-C is 1.42 and 1.36 at 450\u00a0\u00b0C and 550\u00a0\u00b0C, respectively. Larger ID/IG values of Ni/ZrO2-P indicate lower graphitization degree of deposited carbon, consistent with the results of TEM and XRD.CO2-TPR was used to evaluate the reactivity of the deposited carbon on catalysts. The produced CO was detected by a mass spectrometer. The temperature of the CO signal reflects the reactivity of the carbon with CO2. As shown in Fig. 8\n, the peaks of Ni/ZrO2-P are found at 480\u00a0\u00b0C, 530\u00a0\u00b0C and 562\u00a0\u00b0C, while those of Ni/ZrO2-C appear at 491\u00a0\u00b0C, 554\u00a0\u00b0C and 578\u00a0\u00b0C. Therefore, the peaks of Ni/ZrO2-P shift to lower temperatures, suggesting lower graphitization degree of the carbon deposited. According to the DSC results, the peaks at 562\u00a0\u00b0C and 578\u00a0\u00b0C for Ni/ZrO2-P and Ni/ZrO2-C should belong to CNTs, and the peaks at lower temperatures should belong to onion-like carbon. The ratio of onion-like carbon to CNTs of Ni/ZrO2-P is 2.8, lower than that of Ni/ZrO2-C (3.9), indicating less onion-like carbon encapsulating Ni active sites formed on Ni/ZrO2-P. This result is consistent with the TG/DSC results, supporting that Ni/ZrO2-P has a higher resistance to deactivation due to onion-like carbon deposition.The present work demonstrates the significant effect of catalyst preparation methodology on the structure and carbon deposition of the Ni/ZrO2 catalyst for methane decomposition. The decomposition of nickel precursor by DBD plasma, followed by the thermal treatment in the absence of the plasma, causes the Ni catalyst of smaller Ni particle size with Ni (111) as the principally exposed facet. This unique catalyst structure favors the formation of the carbon with enhanced reactivity towards oxygen and carbon dioxide. This means the carbon formed on the plasma prepared Ni/ZrO2 catalyst is more easily to be removed, leading to higher reactivity and stability for DRM, as confirmed by our previous study (Hu et al., 2019). This work will be helpful for the future catalyst design beyond DRM.\nXue Hu: Methodology, Formal analysis, Investigation, Writing - original draft, Visualization. Xinyu Jia: Visualization, Investigation, Writing - review & editing. Chang-jun Liu: Resources, Investigation, Supervision, Writing - review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (21536008 and 21621004).Supplementary data to this article can be found online at https://doi.org/10.1016/j.cesx.2021.100104.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  In this work, the structural effect of the plasma decomposed Ni/ZrO2 catalyst and the thermally calcined Ni/ZrO2 catalyst on the formation and the reactivity of the carbon formed from methane decomposition was investigated. The plasma prepared Ni/ZrO2 catalyst possesses smaller Ni particle size with Ni (111) as the principally exposed facet, which favors the formation of the carbon with enhanced reactivity towards carbon dioxide. The Ni (111) facet benefits the formation of carbon nanotubes (CNTs) attached to Ni nanoparticles at the top and suppresses the formation of onion-like carbon which encapsulates Ni active sites. Besides, CNTs on the plasma-treated catalyst show smaller diameter sizes with lower graphitization degree, which are easier to be converted. Therefore, the plasma prepared catalyst presents a higher carbon resistance. The present work well explains the improved activity and stability of the plasma prepared Ni/ZrO2 catalyst for CO2 reforming of methane.\n               "}
{"full_text": "No data was used for the research described in the article.As societal understanding of the adverse implications of global warming is increasing, so is the dependence on renewable energy sources such as solar and wind [1,2]. Water electrolysis provides a means to store this intermittently produced energy in the form of hydrogen, thus facilitating the global transition away from fossil fuels [3,4]. While the hydrogen evolution reaction (HER) is a relatively fast and efficient process, its anodic counterpart, the oxygen evolution reaction (OER), suffers from slow kinetics due to the complex, four-electron transfer pathway [5,6]. Commercial OER catalysts for polymer electrolyte water electrolyzers (PEWEs) are based on non-abundant, thus expensive, Ir and Ru; consequently, research efforts target the development of efficient and stable non-noble metal catalysts for alkaline water electrolyzers (AWEs) [7,8]. Transition metal oxides containing Ni, Co and Fe are promising candidates for alkaline OER catalysis due to their relative abundance, tunable 3d electron configuration, and the versatility of available crystal structures [2,9,10]. In particular, Ni-based materials possess high activity and stability and, unlike their Co-based counterparts, have a comparatively cleaner supply chain with reduced geopolitical risk [11].The rational design of new Ni-based catalysts requires the development of structure\u2013activity relationships in order to correlate the electronic properties, local and long-range structure, and morphology of materials with their catalytic performance. However, it is well established that Ni and Co-based catalysts undergo such significant transformations of surface and sub-surface atoms under OER conditions that structure\u2013activity relationships are difficult to qualify. Indeed, the as-synthesized structure is considered merely a \u201cpre-catalyst,\u201d which undergoes dynamic reconstruction under oxidative conditions to form an amorphous, active surface known as the oxyhydroxide layer [12,13]. Simultaneously, many other reconstruction processes occur under OER conditions. Reversible processes include other potential-dependent phase transformations, the electrochemically-driven dissolution and re-deposition of surface atoms, and the adsorption and desorption of OER intermediates during the catalytic process (accompanied by the associated redox transformations of catalytic centers, and vacancy generation and refilling). Differently, irreversible transformations can include phase transformations and morphological or structural changes [14]. This review will examine the following operando transformations of Ni-based OER catalysts: phase changes involving the formation of a surface layer with a new crystalline structure; oxidation state changes of interfacial cations including Ni and Fe; the extent of lattice oxygen participation in the OER; and the uptake of Fe from an impure electrolyte into the crystal lattice.Ni oxides undergo phase transformations as a function of applied potential, with the reversible formation of an OER active surface layer under oxidative conditions being a key prerequisite for the high activity of these materials [15]. The electrochemical stabilities of Ni metal and its oxide, hydroxide and oxyhydroxide derivatives have been calculated by Huang et\u00a0al. using standard Gibbs free energies of formation (\u0394fG) obtained both experimentally and using DFT, across a range of pH values [16]. The resulting Pourbaix diagrams (Figure\u00a01\na, b), experimentally verified by electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS), illustrate the formation of new phases at the surface of NiO with applied anodic potential Ni(2+)O \u2192 Ni(2+)(OH)2 \u2192 Ni(3+)OOH \u2192 Ni(4+)O2, and the instability of these phases at low pH. The limitations of such DFT calculations are exposed by the discrepancy in experimentally determined and DFT-obtained phase stability windows and Ni(OH)2 oxidation potentials; though the latter would likely arise from an irreversible phase transformation of the disordered, hydrous \u03b1-Ni(OH)2 to the crystalline, anhydrous \u03b2-Ni(OH)2 polymorph with electrochemical cycling. In addition, the study proposed probability profiles (Figure\u00a01c) that indicate multiple phases are present at the NiO-NiOOH boundary including Ni3O4, Ni2O3, NiO2, NiO, and Ni(OH)2, and this may contribute to discrepancies in reported experimental oxidation potentials of NiO or Ni(OH)2 [16].These electrochemically-driven phase transformations are associated with significant structural changes. The Ni(2+)(OH)2 \u201cprecatalyst,\u201d with the brucite structure (P3m1), exists as two polymorps: \u03b2-Ni(OH)2, which consists of Ni2+ and OH- ions in a hexagonal close packed arrangement, and \u03b1-Ni(OH)2, which comprises planes of \u03b2-Ni(OH)2 with intercalated H2O and electrolyte ions. Under OER conditions, \u03b1-Ni(OH)2 and \u03b2-Ni(OH)2 experience reversible phase transformations to form their corresponding oxyhydroxides, \u03b3-NiOOH (with a Ni oxidation state of 3.3\u20133.67+), and \u03b2-NiOOH (with a Ni oxidation state of 2.7\u20133.0+), respectively [17\u201320]. While the high \u03b3-NiOOH oxidation state range can be attributed to the presence of Ni4+, that of \u03b2-NiOOH is less easily understood [20]. Additionally, in concentrated alkaline solutions, the \u03b3-NiOOH phase can form from the irreversible overcharging of \u03b2-NiOOH. Attempts to track the formation of these \u2013OH and \u2013OOH species with Raman spectroscopy have produced differing results, suggesting that the exact phase transformations experienced by a material depend on the precursor structure and specific heteroatom doping. For instance, D\u00fcrr et\u00a0al. used operando Raman to identify the direct and irreversible formation of \u03b3-NiOOH from their initial NiMoO4 nano-flower catalyst, without an intermediate hydroxide step [21]. Conversely, with comparable reaction conditions and spectra acquisition time, Sagu\u00ec et\u00a0al. used operando Raman to identify an initial, irreversible transformation of their Ta-doped NiO films to \u03b1-Ni(OH)2 upon immersion in the alkaline electrolyte, followed by a reversible transformation to \u03b3-NiOOH with applied anodic potential [22]. Likewise for Fe/Co-based materials (hydr)oxide catalysts undergo a similar potential-dependent surface transformation to form the OER active oxyhydroxide phase [23,24]. For example, D. Grumelli et\u00a0al. used operando X-ray diffraction to observe the surface reconstruction of Fe3O4 in OER conditions [24]. For the highly active spinel Co3O4, this process is also dependent on the Co-ion geometry: only the tetrahedral Co2+ is capable of releasing electrons under applied potential to form the surface CoOOH layer [25].Similar to the phase transformations of Ni(OH)2 polymorphs to the corresponding NiOOH structures, other Ni-based structures such as perovskite or spinel oxides experience an electrochemically-driven surface reconstruction associated with the dynamic formation of an oxyhydroxide surface layer [26\u201328]. Baeumer et\u00a0al. used operando UV-vis spectroelectrochemistry to investigate the behavior of Ni-terminated LaNiO3 (LNO) films, and discovered the formation of a 4\u00a0\u00c5-thick NiOOH surface layer at the Ni2+/Ni3+ redox peak potential during the anodic CV sweep\u00a0[28]. Post-mortem low-energy electron diffraction (LEED) revealed that this surface reconstruction is associated with the disappearance of the perovskite diffraction pattern. While the authors hypothesized an irreversible loss of long-range order in the material, it is important to note that LEED probes approximately a < 1\u00a0nm depth. Conversely, Liu et\u00a0al. used scanning transmission electron microscopy (STEM), probing the entire sample, to identify the localized amorphization of only two LNO surface layers after OER catalysis [29]. While the structural amorphization of LNO is a fully irreversible process, the formation of the \u2013OOH layer is theoretically reversible with applied potential, according to the thermodynamic stability windows outlined in Pourbaix diagrams calculated by Huang and Zhou [16,30]. However, Fabbri et\u00a0al. used operando X-ray absorption spectroscopy (XAS) to correlate the oxyhydroxide layer formation on the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-\u03b4 (BSCF) with an irreversible oxidation of Co atoms [12]. This implies that surface \u2013OOH adsorption is in fact not fully reversible, though an analogous claim has not yet been proven for Ni perovskites.Under OER conditions, interfacial cations including Ni and Fe (where present) will undergo reversible changes in oxidation state. For nickel oxide, hydroxide and perovskite catalysts, recent evidence has pointed to the reversible oxidation of Ni centers Ni(2+) \u2192 Ni(3+) \u2192 Ni(4+) during OER catalysis [12,28,31\u201333]. The formation of Ni(4+)OO under OER conditions was first identified by Diaz-Morales et\u00a0al. using in situ SERS combined with an 18O-labeled electrolyte [34]. Due to its high oxidation state, the 3d orbitals in Ni(IV) are lowered in energy to allow optimal overlap with the O 2p orbitals. This, in turn, energetically favors OH- adsorption onto the material surface [27,35]. Similar transformations occur in Co-based oxides, as evidenced by Hu et\u00a0al., who used in situ XAS to identify the formation of surface Co4+ sites on CoOOH as a function of applied potential [36]. Their findings were echoed by operando XAS studies conducted by Zhang et\u00a0al., who identified the transformation of CoOOH to Co3+/4+OOH1-x under oxidative conditions via a potential dependent deprotonation reaction [37]. Ni-based materials are often doped with Fe to enhance the OER activity, though the reversible electronic transformations of Fe under OER conditions are still widely debated [38]. For instance, XAS studies conducted by Friebel et\u00a0al. and G\u00f6rlin et\u00a0al. determined Fe3+ to be the maximum oxidation state of Fe under OER conditions, whereas Hunter et\u00a0al. used M\u00f6ssbauer and UV-vis spectroscopy to detect Fe4+ states stabilized as \u2217Fe4+=O, a finding supported by DFT calculations performed by Martirez et\u00a0al. [39\u201343]. Despite the discrepancies in reported electronic structure, it is evident that synergistic electronic interactions between Ni and Fe result in reduced overpotentials for the OER on Fe-doped materials. The bridging oxygen (\u03bc\u2013O) in Ni-O-Fe bonds can facilitate partial electron transfer as \u03c0-donation between the Ni/Fe d-orbitals, and the Ni-Fe synergy might enable a bimetallic mechanism to proceed through bridging O2 intermediates Ni-\u2217O-O\u2217-Fe [44]. Aside from electronic effects, Abbott et\u00a0al. used operando XAS to observe that the Fe-doping of nickel oxides increases the structural stability of the \u03b2-Ni(OH)2 phase [45]. The transformation of \u03b2-Ni(OH)2 to a layered \u03b2-NiOOH structure (and disordered \u03b1/\u03b3-phases) facilitates ionic diffusion to previously inaccessible metal centers, increasing the electrochemically active surface area. However, since Fe-doping enhances the intrinsic activity of the available active sites in \u03b2-Ni(OH)2, this transformation is no longer a prerequisite of high activity.Wang et\u00a0al. studied the effect of Fe doping of \u03b1-Ni(OH)2 in an anion exchange membrane electrolyzer, and reported a current of 2\u00a0A\u00a0cm\u22122 at 2.046\u00a0V and 50\u00a0\u00b0C, a performance on par with proton exchange membrane alternatives [46]. Initial rotating disk electrode (RDE) studies revealed a positive shift in the Ni2+/Ni3+ redox peak potential attributed to the Ni to Fe charge transfer, and the trigonal distortion of the octahedral symmetry that arises from Ni-O bond contraction as a consequence of Fe doping [47,48]. A similar distortion was also observed for Fe-doped LaNiO3 films by Bak et\u00a0al., who demonstrated the oxygen-octahedron distortion results in a significant increase of the DOS of both the O 2p and Ni/Fe 3d orbitals near the Fermi level, facilitating the charge transfer from transition metals to adsorbates via oxygen (Ni3+-O(OH\u2217) \u2192 Ni4+-OO\u2217) [49]. Wang et\u00a0al. used DFT\u00a0+\u00a0U calculations to model the OER mechanism on the (001) facet of \u03b3-Fe0.25Ni0.75OOH (Figure\u00a02\na), and created a free energy diagram from the reaction energies of each elementary step, as shown in Figure\u00a02b. The most favorable surface configuration was achieved with a local arrangement of one Fe3+ and two Ni4+ atoms, resulting in a low overpotential of 0.57\u00a0V (Figure\u00a02c). However, they provided no experimental evidence that such a high local concentration of strongly oxidized Ni4+ could be achieved, and it appears that they did not consider the possibility of Fe4+. Besides, it is vital to consider that irreversible, OER-driven processes such as cation dissolution or surface amorphization will likely evolve the local surface structure when evaluating DFT facet calculations.There are two main classes of OER mechanism: the conventional adsorbate evolution mechanism (AEM), in which all oxygen-containing intermediates originate from the electrolyte, and the lattice-oxygen mediated mechanism (LOM), in which the lattice oxygen participates in the reaction [50]. The latter is associated with an increased OER reactivity for Ni-based perovskites, oxides and hydroxides; thus, developing surface-sensitive spectroscopic techniques that can directly detect oxygen vacancies in situ is paramount for optimizing future catalyst design [51\u201354]. Ex situ neutron diffraction has provided indirect evidence for the oxygen vacancy content of OER catalysts, though it has crucial limitations: highly crystalline samples are required, surface changes in vacancy concentration are difficult to detect with bulk sensitivity, and a large amount of material is necessary, which precludes the possibility of operando studies [55,56]. Soft X-ray absorption spectroscopy (sXAS) at the oxygen K-edge likewise indirectly monitors oxygen vacancy dynamics; in combination with total electron yield (TEY) acquisition, a high surface sensitivity (up to 5\u00a0nm) is possible\u00a0[57,58]. Thus far, the design of an operando cell with negligible electrolyte interference in the oxygen K-edge absorption spectra limits definitive sXAS data interpretation. Recently, Mom et\u00a0al. used sXAS to study IrOx thin films of 100\u00a0nm thickness while eliminating electrolyte oxygen contribution, through a back-contacted electrolyte/IrOx interface [59]. Nevertheless, the use of TEY detection at the \u2018front\u2019 negates the concept of surface sensitivity, and illustrates the multifaceted problem of optimal cell design.As a result, conclusions about the extent of oxygen vacancy participation in Ni-based materials must be examined with caution based on the experimental methods employed. For instance, Zhang et\u00a0al. used sXAS of the oxygen K-edge to determine that oxygen vacancy participation in FeCoCrNi thin films can be promoted by the reversible formation of Ni4+ [57]. The generation of Ni4+ under OER conditions results in downshifted Ni 3d orbitals (as predicted by partial density of states (PDOS) calculations); this induces the formation of oxygen ligands with localized holes in their p-orbitals (i.e. O(2\u2212\u03b4)-) [57]. As outlined by Nong et\u00a0al., the enhanced electrophilic character of these oxygen ligands increases their reactivity towards nucleophilic acid\u2013base-type O-O bond formation (i.e. nucleophilic attack of electron-deficient O(2\u2212\u03b4)- ligands), facilitating the LOM and improving OER activity\u00a0[60]. However, obtaining these ex situ sXAS measurements involved freeze-quenching the post-mortem samples in liquid N2, before transferring them in air to the vacuum chamber for measurement; the impact of this preparation process on the surface oxygen is unknown. Moreover, systematic studies performed by Cheng et\u00a0al. concluded that varying the oxygen vacancy content in Ni and Co-based perovskites was accompanied by changes in other physiochemical properties including conductivity, degree of structural (dis)order and cation oxidation state; the predominant mechanism is determined by the combination of these and other factors\u00a0[55,61,62].Ni-based electrocatalysts uptake trace Fe impurities from unpurified KOH electrolyte, resulting in significantly enhanced OER activity and cycling stability [63,64]. Therefore, it is vital to understand the mechanism of this Fe incorporation and its role in improving OER kinetics in order to decouple and optimize the intrinsic activity of Ni catalysts. Kuai et\u00a0al. used operando XAS to determine that electrolyte Fe incorporation into 2D Ni(OH)2 nanosheets is an electrochemically driven process, occurring at the OER reactive potential during the anodic CV sweep [65]. Furthermore, by using X-ray fluorescence microscopy (XFM) to generate elemental distribution maps, they determined that Fe incorporation occurs predominantly at edge sites, which feature a higher concentration of oxygen vacancies and show higher OER reactivity [65]. The group identified a non-linear increase in OER current with Fe atomic ratio, and attributed this to the irreversible formation of a separate, insulating FeOOH phase at high overpotentials, although other disruptive electronic or structural effects may also influence OER activity. Furthermore, operando soft XAS suggested that Fe surface uptake enhances the reducibility of Ni, increasing the concentration of oxygen vacancies and improving the OER activity [65].The surface Fe sites exist in a dynamic equilibrium, undergoing reversible dissolution and re-incorporation in aqueous KOH [63]. Chung et\u00a0al. investigated the effect of electrolyte Fe concentration on the OER activity of MOxHy (M\u00a0=\u00a0Ni, Fe, Co), and determined that Fe adsorption saturates at electrolyte concentrations as low as 0.1\u00a0ppm (Figure\u00a03\na, b) and the OER activity increases linearly with Fe surface coverage (Figure\u00a03c). Importantly, maintaining the high OER activity was conditional on achieving \u201cdynamically stable\u201d surface Fe with continued re-deposition after dissolution (Figure\u00a03e) [63]. Farhat et\u00a0al. reported that Ni(Fe)OxHy subsequently cycled in purified (Fe-free) KOH will experience a loss of OER activity as the active, surface Fe atoms move into inactive bulk sites, though they provide only electrochemical evidence to support their claims [66]. The authors hypothesize that Ni atoms at the surface are now able to dissolve and redeposit in the structure preferentially with respect to Fe in so-called \u201cFe-free\u201d KOH, as the purification process involves dissolving nickel nitrate in the electrolyte [66,67]. However, Chung et\u00a0al. used ICP-MS combined with stationary probe rotating disk electrode studies (SPRDE) to prove that Ni sites have a comparatively high stability with respect to Fe in (Ni/Fe)OxHy, and Ni dissolution is negligible [63].In summary, Ni-based OER catalysts experience significant operando transformations, often leading to a highly active final state. Reversible changes include phase transformations, oxygen vacancy dynamics, intermediate adsorption/desorption, and surface atom dissolution/redeposition. Irreversible changes comprise structural and morphological transformations. This begets the obvious question for further research: is the initial \u2018precatalyst\u2019 structure a prerequisite of the high activity final state, or should the future development of Ni-based materials focus on direct synthesis of this final state? Accordingly, gaining a greater fundamental understanding of the OER mechanism is a broad but pressing concern. In particular, this should involve developing operando methods and innovative cell designs to enable direct observation of oxygen vacancy dynamics (with bulk and surface sensitivity, a high time resolution, and minimal interference from atmospheric, electrolytic, or binder oxygen). Alongside the expansion of fundamental mechanistic studies, further exploration of new classes of materials with less well-known structure-activity correlations would advance the field in new directions. In particular, Ni-based metal-organic framework (MOF) catalysts, noble metal-free high entropy alloys, and amorphous Ni-based materials provide underexplored yet promising avenues for research [68\u201371]. In addition, while it is worthwhile to decouple the intrinsic activity of Ni-based catalysts from changes induced by operando uptake of Fe from unpurified electrolyte, shifting the focus of future research to optimize Ni-based catalysts for operation in Fe-doped electrolyte may be a valuable, application-oriented approach. Finally, approaching DFT calculations with careful consideration of the reversible and irreversible changes of Ni-based catalysts during operation will enable this valuable computational method to direct and enhance experimental work, rather than acting as an addendum.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge the Swiss National Science Foundation through its SNSF PRIMA grant (grant no. PR00P2_193111).", "descript": "\n                  Nickel-based catalysts for the alkaline oxygen evolution reaction (OER) demonstrate excellent catalytic performance and stability. However, a lack of fundamental understanding of the dynamic electronic and structural changes that occur under OER conditions inhibits the rational design of new materials. Recent advances in operando spectroscopy and computational modeling techniques have helped to elucidate the electrochemically-driven transformations of Ni-based materials. For reversible transformations, this encompasses an increased understanding of the redox transformations of Ni/Fe centers, the adsorption and desorption of reaction intermediates, oxygen vacancy dynamics, phase transformations, and the mechanism of dissolution and redeposition of surface atoms. Likewise, there have been great advances in scientific understanding of irreversible transformations including phase transformations related to ageing, as well as operando surface reconstruction which involves the growth of new OER active phases.\n               "}
{"full_text": "Data will be made available on request.The global climate is facing two main challenges: first, the increased energy demand due to the growing world population, and second, growing anthropogenic CO2 emissions due to the use of non-renewable fossil fuels in major economic sectors. Ambitious targets like limiting the rise of global temperature to stay below 2\u00a0\u00b0C in the Paris Agreement and achieving net-zero CO2 emissions by 2050 in the European Green Deal will drive the current energy transition towards a low or neutral renewable carbon energy system. In line with this, the United Nations (UN) Sustainability Development Goals (SDGs) adopted by all UN members also provided a blueprint to tackle detrimental climate change and achieve a better sustainable society for all. Thus, the use of renewable energy in tackling climate change is inevitable. Renewable liquid biofuels fall under this category and provide an immediate solution for sectors like transportation.Biomass feedstock can be divided into three main categories: (1) sugar-based feedstocks such as sugar beet, sugar cane, and corn, (2) triglyceride feedstocks like animal fats, vegetable oil, and waste cooking oil, and (3) lignocellulosic feedstocks like wood and forestry residues, bagasse, grass, and leaves. The types of biofuels depend largely on the biomass source. Renewable liquid fuels, also known as advanced biofuels, produced from the non-crop and waste-based bio-feedstocks, represent an excellent option as an alternative fuel and also serve the role of bridging the transition period for existing conventional combustion engine-based fleets. For instance, hydroprocessed esters and fatty acids (HEFA) as hydrotreated vegetable oils (HVO), are the only drop-in biofuels that are commercially produced in refineries. Several examples of such commercial technologies are NEXBTL\u2122, Ecofining\u2122, Vegan\u2122, and Hydroflex which produce these advanced biofuels. Apart from these commercially available examples, the next-generation biofuels such as those derived from pyrolysis oil also possess advantages in reducing greenhouse gas (GHG) emissions and fossil fuel dependency. Pyrolysis oil can be produced using different processes, one of which is fast pyrolysis or thermal liquefaction of biomass feedstocks [1,2]. The conversion of solid biomass via a thermochemical process like fast pyrolysis results in bio-oils that can be subsequently upgraded via catalytic hydrotreating into biofuels and high-value platform chemicals. Another potential advanced feedstock like lignin can also be used to substitute fossil-based feeds. Lignin is a biopolymer consisting of phenylpropane units (coniferyl, sinapyl, and p-coumaryl alcohol) [3]. It is an important renewable carbon source and accounts for 20\u201330% of the major mass of lignocellulosic biomass. Due to the large utilization of cellulosic and hemicellulosic materials in the existing biorefineries, the remaining lignin fraction is considered a byproduct and is often burnt to produce heat and power for the mill. Thus, lignin can serve as a sustainable feed for liquid fuel production or value-added fine and platform chemicals.However, there are a few common undesired properties of the bio-feedstocks such as high oxygen content depending on the biomass (\nTable 1) and acidic nature caused by the presence of carboxylic acids. The high oxygen content contributes to detrimental properties of bio-oils like high viscosity and low heating value as compared to fossil-derived fuels [4]. Owing to the various negative characteristics of the bio-oils from these feedstocks, it is difficult to use these bio-liquids directly as engine fuel. Therefore, a refining process is required to improve the quality of the products so that the produced liquid fuel is compatible with the existing fuel grades. This process involves conventional hydrotreating technology such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), and hydrodemetallization (HDM) processes. These processes serve to remove or reduce the sulfurous, nitrogenous compounds, oxygenates, and metals from fossil feeds. Catalytic hydrodeoxygenation (HDO) has been implemented in the refineries to remove excess oxygen in the form of H2O, CO, and CO2 at various temperatures and pressure with hydrogen as a co-reactant. Moreover, the reaction is catalyzed by a selective hydrotreating catalyst. The key elements in such a process are the choice of catalyst material, reaction conditions, type of reactors, and feedstocks that are upgraded. Over the past few decades, transition metal sulfides/noble metals and non-sulfided catalysts have been studied extensively for valorizing bio-based feedstocks (Triglycerides/Fatty acids/pyrolysis oil/ lignin-derived bio-oil, etc.). In this review, we focus on the application of metal sulfides as catalysts for advanced biofuel production.Krauch and Pier discovered the transition metal sulfides (TMS) at the former Badische Anilin und Sodafabrik (BASF) in 1924 [8]. Their early findings showed that MoS2 and WS2 were effective hydrogenation catalysts, and this led to the future development of hydrotreating catalysts. The traditional industrial metal sulfides are Ni or Co-promoted Mo/W disulfides. The Ni or Co promotion in a fully sulfided catalyst gives the so-called active Ni/CoMoS phase [9]. It is postulated that the promotion weakens the Mo-S bonds via d-electron donation by Ni/Co producing a sulfur vacancy or so-called co-ordinately unsaturated sites (CUS), indicated by the green dotted oval in \nFig. 1\n[4,10]. In terms of deoxygenation, the electrophilicity of Mo thus attracts oxygen-bearing molecules [10,11]. On the other hand, the presence of H2 generates metal hydrides and sulfhydryl groups that play additional roles [12]. In other words, the catalytic activity of metal sulfides is thus governed by the type and composition of sites available to the substrate molecule which can be engineered by parameters such as promoters, support, additives, and activating conditions. The morphologies of the evolved catalyst also play a dominant role in determining the activity of the catalysts [13]. These Ni or Co-promoted MoS2 catalysts are depleted of sulfur during oxygen removal from bio-oils and require constant addition of sulfur sources like DMDS or H2S to maintain them in their active sulfide state [14]. Due to this fact, these sulfided catalyst systems are criticized in literature since they nullify the advantage of bio-oils which inherently contain no or low sulfur [15]. However, these sulfided catalysts have excellent hydrotreating selectivity and are low cost which makes them viable for commercial refinery operations. Metal sulfide catalysts will continue to have a significant role in the refining industries owing to the versatility of this type of catalytic material and also the transition toward cleaner and sustainable fuel production. This can be evident from the scientific publications regarding HDO which have increased by 10-fold since 2010 because of the escalating need for alternative sources of energy and also the increasing application of sulfided catalysts in deoxygenation applications (\nFig. 2). Existing review articles related to the application of sulfided catalysts focused on the understanding of the fundamental principles of the materials [16], and their applications in electrocatalysis [17,18], photocatalysis [18], and as supercapacitors [19].Although there are various reviews in the last decade focused on the upgrading of renewable feedstocks in the form of model compounds and/or real feedstocks over various types of catalysts (see \nTable 2); a comprehensive review solely dedicated to sulfided catalysts is still lacking. Given the significance of hydrotreating catalysts in the emerging field of advanced biofuel production, a comprehensive review of the use of sulfided catalysts for biomass conversion is needed. In this work, we have reviewed the catalytic upgrading of different important bio-feedstocks such as triglycerides, monomeric and dimeric phenolic compounds that are present in pyrolysis oil, biomass-derived pyrolysis oil, woody feedstocks, and waste lignin using metal sulfide catalysts. The reaction routes of the biomass-derived feed during the hydrotreatment have been emphasized. Furthermore, kinetics studies of upgrading the various feedstocks using sulfided catalysts and also the deactivation of catalysts are highlighted. Insights of the deoxygenation reaction and deactivation mode over metal sulfides from theoretical studies and computational approaches are also presented. Finally, the challenges and future possible research related to the valorization of different bio-feedstocks into liquid fuels employing the sulfided catalysts, and hurdles utilizing bio-feeds in the industry are also discussed in the current work.Out of all advanced biofuel technologies like \u2013 Hydrotreated Vegetable Oil (HVO), fast pyrolysis, catalytic pyrolysis, hydro pyrolysis, lignin depolymerization, and hydrothermal liquefaction (HTL), it is only HVO process that has been commercialized so far. There are already several companies like Neste, Preem, Diamond diesel, REG producing HVO biofuels at a commercial scale. The only other biofuel process which is close to commercialization is fast pyrolysis bio-oil (FPBO), where commercial fluid catalytic cracking (FCC) trials of FPBO are underway [45].HEFA/HVO (Hydroprocessed Esters and Fatty Acids) fuels are compatible with fossil-based diesel fuels due to their chemical resemblance. As a result, existing refinery facilities, and infrastructure can be used for production, transportation, and distribution for further use. \nFig. 3 shows a parametric comparison between ultra-low sulfur diesel and HVO/HEFA fuels. Thus, standalone, or blended forms of HVO/HEFA fuels contribute toward a significant reduction of GHG and particulate matter emissions. As a result, tremendous research focus has been devoted to understanding the mechanistic insights in the core upgrading process, HDO. Such renewables can be obtained from variable feedstocks like animal fat, waste cooking/vegetable oil, and tall oil from wood and forest biorefinering. It is expected that refineries will face scarcity of these feedstocks in the next few years as the demand for sustainable fuels from the heavy transport and aviation sectors accelerates. Thus, more research and technology developments are needed in pretreatment methods for low-value, ubiquitous, and high-contamination feedstocks such that they can be hydroprocessed in the refinery.However, the turnover time of a hydrotreater is much shorter when renewable oils like used cooking oil, waste animal fat, tall oil, etc. are processed compared to a hydrotreater processing fossil feeds like vacuum gas oil (VGO), etc. There are three main reasons for the shorter lifetime of catalysts in hydrotreater processing HVO feedstocks. Firstly, since the oxygen content of renewable oils is higher than sulfur in VGO, larger quantities of hydrogen are required to remove oxygen as H2O. The second reason is that hydrodeoxygenation reactions are more exothermic compared to hydrodesulfurization reactions as can be observed from the differences in the enthalpy of formation of H2O(g) and H2S(g) respectively (\u2212242\u2009kJ/mol vs \u221221\u2009kJ/mol) [47]. Last but one of the most important reason which has not received enough attention in research studies is the contaminants present in renewable feeds like iron, phosphorus, alkali metals, etc. [48\u201350]. So these three factors \u2013 depletion of hydrogen, high temperature, and contamination, combined result in accelerated catalyst deactivation and pressure build-up [51]. In this section, we will focus on the studies that have used sulfided catalysts since they are the most industrially relevant catalysts. The literature studies can be categorized in two segments based on the feedstocks studied \u2013 Model compounds which include - Fatty acids (FAs), Fatty acids alkyl esters (FAAEs), Triglycerides (TGs), and commercial feedstocks like UCO (used cooking oils), tall oil, etc. It should be noted that the HDO reaction mechanism for fatty acids, fatty acid alkyl esters, triglycerides is quite similar. Renewable oils like UCO, tall oil, etc. primarily contain free fatty acids and triglycerides so their reaction chemistry is similar as well. Typically, fatty acid alkyl esters and triglycerides undergo hydrolysis to produce fatty acids as the common intermediate in the overall reaction scheme. Deoxygenation of fatty acids over sulfided catalysts occurs in the following three ways [48,52]:\n\na)\nA so-called direct-HDO in which oxygen is removed as a water (H2O) molecule\n\n\nb)\nDecarbonylation (DCO) in which oxygen is removed as carbon monoxide (CO)\n\n\nc)\nDecarboxylation (DCO2) in which oxygen is removed as carbon dioxide (CO2)\n\n\nA so-called direct-HDO in which oxygen is removed as a water (H2O) moleculeDecarbonylation (DCO) in which oxygen is removed as carbon monoxide (CO)Decarboxylation (DCO2) in which oxygen is removed as carbon dioxide (CO2)In the direct-HDO route, there is no loss of carbon as oxygen is removed in the form of a H2O molecule while in the two latter routes, oxygen is removed in the form of CO or CO2 such that the hydrocarbon product is formed with one less carbon.The term decarbonation (DCOx) will be used to refer to decarbonylation and decarboxylation together, otherwise, they will be separately specified in the following sections of this review. The hydrodeoxygenation or \u201cHDO\u201d is a broader term to define the removal of oxygen irrespective of the three routes. However, \u201cdirect-HDO\u201d is referred to when deoxygenation occurs while producing water as the side product [48,52].The catalytic cycle for hydrodeoxygenation of fatty acid molecules (here stearic acid) over sulfided molybdenum catalysts is presented in \nFig. 4\n[53]. The following steps of this catalytic cycle, initiated with the creation of a sulfur vacancy have been also reported for phenol-like molecules, so this is also relevant for later sections discussing the phenolic models. Sulfur linked to Mo reacts with hydrogen to produce H2S and a \u201csulfur vacancy\u201d is created on the MoS2 structure. It is postulated that there is always a dynamic equilibrium of such sulfur vacancies depending on H2/H2S partial pressure. Then a heterolytic dissociation of the hydrogen molecule occurs such that one hydrogen atom binds to sulfur to form a sulfhydryl (-SH) group while the other hydrogen atom forms a metal hydride bond with molybdenum. The carbonyl moiety of the fatty acid molecule binds at the sulfur vacancy. Then a proton from the acidic SH group attacks the hydroxy group of the stearic acid as an example. A water molecule is removed, and the charge is transferred to the neighboring carbon. This cation species extracts a hydride from the next Mo atom. In the final step of this catalytic cycle, a hydrogen molecule reacts to yield metal hydride (Mo-H) and sulfhydryl (-SH) species and results in the conversion of stearic acid to octadecanal. The patent literature has reported the use of Ni or Co-promoted molybdenum sulfided catalysts for the deoxygenation of fatty acid-based feedstocks for more than three decades now [54].Several studies have been explored for HDO of fatty acid to green diesel employing supported and unsupported sulfided catalysts. The sulfidation temperature influences the formation of different Mo species (Mo4+, Mo5+, Mo6+) catalyzing deoxygenation of palmitic acid to straight-chain hydrocarbons to varying degrees over NiMo/Al2O3-TiO2 catalyst [55]. The role of unsupported MoS2 and Ni-promoted MoS2 has been studied using hexadecenoic acid by Wagenhofer et al. [56]. It is concluded that fatty acid deoxygenation over MoS2 is primarily followed via C-O hydrogenolysis to an aldehyde, hydrogenation to a primary alcohol, dehydration, and hydrogenation to corresponding alkanes (Cn pathway). On the other hand, deoxygenation over unsupported Ni promoted MoS2 mainly proceeds through a ketene intermediate, and decarbonylation via the scission of its C-C bond to yield saturated hydrocarbons (Cn-1 pathway) via alkene intermediates. However, keto-enol tautomerism of intermediate aldehyde has also been demonstrated for the HDO of fatty acid over supported sulfided NiMo/Al2O3 catalysts [52,57]. Ni or Co promotion to base MoS2 typically promotes the decarbonation pathway [53]. Surfactant-modified and magnetically reusable unpromoted and Ni(Co) promoted MoS2 over greigite (G) have been evaluated for stearic acid HDO and the deoxygenation activity was ranked in the order of NiMo/G\u2009>\u2009CoMo/G\u2009>\u2009Mo/G [58]. Interestingly, an inhibiting effect of fatty acid which preferentially occupies the active site and hinders access for the intermediate aldehydes/alcohols has also been explained via experimental and DFT studies [10,59]. Hydrogen donor solvents and acidity imparted by BEA zeolite were found to enhance the conversion/deoxygenation of stearic acid at low pressure (0.8\u2009MPa\u2009H2 and 350\u2009\u00b0C) and give a high yield of alkanes (C17 being the major one) for NiMo supported over mixed oxide (\u03b3-Al2O3-BEA-zeolite) [60].Alkyl esters like \u2013 fatty acid methyl esters (FAMEs) and fatty acid ethyl esters (FAEEs) have different decomposition pathways. As shown in \nFig. 5, \u03b2-elimination occurs only in FAEEs (or esters with alkyl groups >C1) to produce free fatty acids [61]. The FAEEs are expected to have a similar reaction mechanism like TGs via \u03b2-elimination. This reaction is not possible for FAME molecules due to the absence of the \u03b2 hydrogen.Another route through which such alkyl esters can produce fatty acids is hydrolysis. There is not a significant difference in the reaction mechanism for the hydrolysis of methyl and ethyl esters with a similar yield and product distribution [62,63]. Hydrolysis requires the presence of moisture (H+/H2O) and the Lewis acid sites (e.g., alumina as support). HDO reactions produce water so hydrolysis of alkyl esters is quite possible in the catalyst bed. Another possible route can be the direct deoxygenation of such alkyl esters.Laurent and Delmon tested sulfided CoMo and NiMo catalysts for hydrodeoxygenation of model compounds containing ester groups (Diethyl sebacate) [64]. During the reaction, a group of products has been identified demonstrating that the major pathways for deoxygenation are hydrogenation and decarboxylation. De-esterification to carboxylic acid occurs to a limited degree. The activation energy for the hydrogenation reaction was found lower for NiMo than CoMo catalysts while no appreciable differences in the decarboxylation activation energy were discerned although a higher decarboxylation degree was observed for NiMo catalysts.Krause and coworkers [65,66] explored HDO activities of methyl esters (methyl heptanoate and methyl hexanoate) in flow and batch reactors over sulfided NiMo/\u03b3-Al2O3 and CoMo/\u03b3-Al2O3. Methyl ester conversion was found higher over NiMo/\u03b3-Al2O3 and the formation of the corresponding deoxygenated products (n-heptane/heptenes and n-hexane/hexenes etc.) required more hydrogen than that with CoMo/\u03b3-Al2O3. Analysis of the reaction products revealed that primary alcohols are produced from the methyl ester via hydrogenolysis of the C-O, \u03c3-bond of the carboxyl group which upon dehydration yields alkenes, and further hydrogenation forms the n-alkanes. A second path proposed is the de-esterification to carboxylic acid and methanol which takes water from the alcohol dehydration reaction. The carboxylic acid can either further be reduced to alcohol under the reaction conditions or can be decarboxylated to alkenes. A third path can be direct decarboxylation of esters to one carbon atom with fewer product hexenes/pentenes with additional methane/carbon dioxide as shown in \nFig. 6.Coumans et al. [52,67] investigated HDO of methyl oleate using sulfided NiMo over a few supports (\u03b3-Al2O3, activated carbon, SiO2, and SiO2-Al2O3) in a fixed bed reactor under trickle flow conditions at 260\u2009\u00b0C, 30 or 60\u2009bar and WHSV of 6.5\u2009h\u22121. Hydrogenation of the double bond in methyl oleate produces predominantly methyl stearate during the early stage (\u223c10\u2009min) of the reaction. Besides, oleic acid and stearic acid are also observed in the reaction mixture. Initial hydrolysis of methyl oleate to oleic acid was found higher over NiMo/Al2O3, and NiMo/Al2O3-SiO2 catalysts than others due to the presence of surface Al3+ which acts as Lewis sites as mentioned earlier. Blockage of such sites by carbonaceous deposits deactivates Al-containing catalysts while others show stable deoxygenation activity. High hydrolysis, high stability (168\u2009h on stream), and selectivity to C18 hydrocarbons over NiMo/C were attributed to the activity of evolved metal sulfides and/or to surface acidic moieties.Triglycerides have the following three main decomposition pathways: Route 1: \u03b2-elimination; Route 2: \u03b3-hydrogen migration; and Route 3: Direct deoxygenation (DO), as shown in Fig. 5. In Route 1, the removal of hydrogen at the \u03b2 position and then hydrogenation occur alternatively in a stepwise manner from triglyceride to diglyceride to monoglyceride to glycerol, with the potential to release three free fatty acid molecules per triglyceride molecule. Also, most of the studies report the evolution of fatty acids during the deoxygenation of triglycerides [61]. However, the subsequent \u03b2-elimination of di-fatty acid ester is not possible without hydrogen and active sites. \u03b3-hydrogen migration (Route 2) is reported to occur only at a higher temperature of 450\u2009\u00b0C, so it is not likely at the typical HDO reaction conditions. Route 3 of direct deoxygenation (DO) occurs only in the presence of a highly active NiMo catalyst [61]. So it could be concluded that the triglycerides-based renewable feeds can yield fatty acids only through Route 1. Indeed, facile hydrolysis of a triglyceride molecule (triolein, glyceryl tioleate) to fatty acid intermediates has been demonstrated over sulfided NiMo/Al2O3 through route 1 [52].As mentioned earlier, oxygen from feedstocks like triglycerides, and fatty acids/alcohols are eliminated via water and carbon oxides as deoxygenation proceeds and yields hydrocarbons as mainly n-alkanes in the diesel range with high cetane values. \nTable 3 presents the state-of-the-art of sulfided catalysts for HDO of triglyceride-based feedstocks like waste vegetable/cooking oil and tall oil that have been studied exclusively via the catalytic HDO process. Kubi\u010dka and co-workers investigated the deoxygenation of rapeseed oil over sulfided Ni, Mo, and NiMo over Al2O3 in a fixed bed flow reactor [68\u201371]. Bimetallic NiMo yields a higher amount of hydrocarbon than the monometallic catalysts for a given conversion. Ni and Mo-containing catalysts promote decarboxylation and direct-HDO respectively. Fatty acids are the only intermediates over Ni/Al2O3, thus no fatty esters form, while over Mo and NiMo containing catalysts fatty alcohol and fatty ester formation proceeds due to the rapid disappearance of fatty acids. Esterification of fatty alcohols and fatty acids was observed higher over Mo/Al2O3\n[70]. The authors also detected that the hydrocarbon phase obtained (over 310\u2009\u00b0C) was mostly composed of n-alkanes, n-C18, n-C17, and i-alkanes of varying amounts (based on the reaction conditions and catalyst). Such an organic liquid product is compatible with mineral diesel, thus meeting or exceeding the required quality. However, it suffered from poor low-temperature properties necessitating further processing. Furthermore, NiMo sulfides over SiO2, TiO2, and Al2O3 have been evaluated to elucidate the interaction of the active phase and support [71]. It was found that NiMoS over SiO2 enhances hydrogenolysis of triglycerides to fatty acids at low deoxygenation degree while over TiO2 fatty ester formation increases. As the deoxygenation progresses n-C17 yield increases over NiMo/SiO2 demonstrating its preference for the decarbonation route, while the other two showed HDO preferences. Observed reactivity was thus ascribed to the differences in the support properties despite the active phase dispersion variation in the order of SiO2 >\u2009Al2O3 >\u2009TiO2.M. Toba et al. [72] studied different grades of waste oils which can be converted to paraffinic hydrocarbons over NiMo/\u03b3-Al2O3, CoMo/\u03b3-Al2O3, and NiW/\u03b3-Al2O3. Modification of the alumina support by B2O3 promoted the formation of i-paraffins over the bimetallic catalysts. NiMo/B2O3\u2013Al2O3 and NiW/Al2O3 showed high HDO and hydrogenation activity for a longer period (\u223c80\u2009h) while hydrogenation activity of CoMo/B2O3\u2013Al2O3 decreases (approximately after 10\u2009h on stream) resulting in high olefin formation. Compared to Mo-based catalysts, tungsten-based catalysts accelerated deoxygenation by decarboxylation/decarbonylation. H. Wang et al. [73] also investigated HDO of waste cooking oil over supported CoMoS, elucidating the deoxygenation activity, deactivation, and regeneration of the catalyst. Crude tall oil, distilled tall oil, and tall oil fatty acid (TOFA) were hydrotreated employing a commercial NiMo/\u03b3-Al2O3 in a trickle-bed reactor at 5\u2009MPa and 300\u2013450\u2009\u00b0C. Hydrocarbon fractions with 45\u2009wt% paraffins were obtained at the most favorable tested conditions for crude tall oil. TOFA hydrotreating yielded more than 80% n-alkanes where the decarboxylation route was dominant over the direct-HDO route at high temperatures greater than 400\u2009\u00b0C [74,75]. Soybean oil has been studied using sulfided NiMo/\u03b3-Al2O3, and CoMo/\u03b3-Al2O3\n[76]. HDO yielded higher amounts of straight-chain alkanes (66%) over the NiMo catalyst than the CoMo (43%) catalyst due to isomerization and cracking enhancement in the latter. NiMoS over Mn-modified Al2O3 was reported to enhance triglyceride conversion and subsequent deoxygenation during the HDO of waste soybean oil [77]. Refined cottonseed oil has been hydrotreated with desulphurized petroleum diesel under refinery conditions and it was found that such treatment increases the cetane number of the final product [78]. Kubi\u010dka et al. also investigated CoMo sulfides over mesoporous MCM-41 for hydrotreatment of refined rapeseed oil and the observed deoxygenation activity is lower than that for CoMo/Al2O3\n[79]. Al incorporated MCM-41 showed better deoxygenation activity towards hydrocarbon formation. Withdrawal of Al from the MCM-41 framework favored the formation of fatty esters instead of fatty acids with MCM-41. Jatropha oil has been hydrotreated using sulfided NiMo over acidic SAPO-11 and Al2O3 support. It was claimed that the higher amount of total and strong acidic sites in SAPO-11 affects the formation of different active phases of NiMo and in combination they promote decarbonation, hydrocracking and isomerization reactions [80]. Lower acid sites with alumina supported sulfided NiMo on the other hand shows high selectivity to diesel range (C15\u201318) hydrocarbon fractions.Depolymerization and deconstruction of the complex structure of biomass can be performed via various processes for example enzymatically [81], thermally using fast pyrolysis [82], and catalytically. In all these processes, the operating conditions largely affect the composition and yield of final products. For example, hydrothermal liquefaction (HTL), is a technology where bio-oils are produced using water as a medium under supercritical or subcritical conditions [43]. Depolymerization and liquefaction of biomass can also be performed under oxidative or reductive conditions producing renewable-based oils [83]. For instance, the oxidative depolymerization reaction has the benefit to produce a pool of high-value and functionalized green chemicals. On the other hand, the reductive depolymerization gives a considerable yield of deoxygenated monomers, such as BTX (benzene, toluene, xylenes) products which are of interest as platform fuel precursors. The heterogenous structure of the biomass components like lignin impacts the selectivity for linkage cleavage and consequently on the selectivity for oligomeric, dimers, and monomeric products. One of the similarities of these processes is that the depolymerized lignin fragments contain different functional groups like methoxy (CH3O-), hydroxyl (-OH), benzyl alcohol (C7H8O-), ketone (R-CO-R), and aldehyde (-CHO) groups which contribute to the high oxygen content of the bio-oils. Similar functionalities and product spectrum can be seen in bio-oils derived from the pyrolysis of biomass. The oxygen content in bio-oil contributes to negative properties like poor heating value, high viscosity, corrosiveness, thermal and chemical instability [84]. Due to such detrimental characteristics, an upgrading process like hydrotreatment which includes hydrodeoxygenation, hydrocracking, hydro-decarbonylation and decarboxylation, and hydrogenation is required before application as a biofuel. In a lab-scale experiment, the reaction atmosphere for hydrotreatment involves a temperature range of 300\u2013450\u2009\u00b0C and a hydrogen pressure of 50\u2013200\u2009bar mimicking the operating conditions of a refinery hydrotreating process. The hydrotreatment of the lignin feedstocks aims to first cleave the recalcitrant linkages such as the carbon-carbon (C-C) and ether (C-O-C) bonds present in the lignin chemical structure. Then the produced alkylphenols are further reacted to form deoxygenated aromatics and cycloalkanes. This section will present an overview of the use of sulfided catalysts in supported and unsupported form for the hydrotreatment of bio-oil model monomer compounds such as guaiacol, phenol, anisole, and cresol. More attention will be dedicated to the discussion on the reaction schemes of the hydrodeoxygenation of the oxygenates that are present in the bio-oils over sulfided catalysts. The catalytic mechanism such as the active sites of sulfided catalysts and reaction network when using different bio-oil model compounds will also be discussed.\n\nTable 4 presents the state-of-the-art of metal sulfide catalysts for hydrotreating phenolic monomers. Various catalytic systems employing mixed oxide supported and sulfided catalysts have been reported for the HDO of phenolics. Garcia-Mendoza et al. have studied the activities of NiWS supported on TiO2, ZrO2, and the mixed oxide TiO2-ZrO2 for the HDO of Guaiacol at 320\u2009\u00b0C [85]. Their results show that the NiWS supported catalysts system shows remarkable influence in shifting the distribution of the product towards deoxygenated products with NiWS supported on TiO2 showing an 80% HDO product selectivity at full guaiacol conversion [85]. The authors also speculated that the synergistic effect of NiWS and TiO2, and also the NiWS phase were responsible for the high catalytic and deoxygenation ability [85]. In a similar catalyst system, Hong et al. have shown that a 2\u2009wt% Ni loading and 12\u2009wt% W loading on such mixed oxide sulfided catalysts can give full guaiacol conversion and a 16% cyclohexane yield under different reaction conditions [86]. The study also mentions that nickel (Ni) performs better than cobalt (Co) as a promoter in catalyzing the HDO of guaiacol [86]. Another study using CoMoS supported on the mixed oxide Al2O3-TiO2 for the HDO of phenol has also shown that the mixed oxide improved the HDO activity with a better metal-support interaction than the conventional CoMoS supported on Al2O3\n[87]. The use of activated carbon as catalyst support has also been reported in the literature [88\u201390]. Mukundan et al. have prepared an amorphous highly dispersed and disordered nanosized MoS2 single-layer on activated carbon by a microemulsion technique for guaiacol HDO and found that the single-layer MoS2/C promotes deoxygenation and hydrogenation better than a multi-layered MoS2/C in the production of phenol [90]. The conclusion was made based on the ratio of phenol/catechol produced using single or multi-layered MoS2/C, where the single-layered catalyst gave a higher ratio. This result inferred the importance of the morphology of the MoS2 catalyst in affecting product selectivity. Moreover, Mukundan et al. proposed a reaction pathway for HDO of guaiacol based on the detected compounds over the course of 5\u2009h as shown in \nFig. 7\n[90].Templis et al. studied hydrotreatment of phenol over a NiMo/\u03b3-Al2O3 catalyst in reduced and sulfided form [91]. Results demonstrated that the reaction routes for phenol hydrodeoxygenation occurred via two main parallel routes, the first one is direct deoxygenation (DDO) of phenol, and the second is the hydrogenation of the phenol ring forming cyclohexanol and followed by the hydrogenolysis removing the hydroxyl group producing cyclohexene and cyclohexane. The main difference between both catalysts was that the sulfided catalyst had a high cyclohexane selectivity of more than 90% while the reduced catalyst had higher activity for phenol conversion. Their results indicated that the sulfided catalyst favored the DDO route while giving high cyclohexane selectivity. \nFig. 8a) shows the general reaction network for the HDO of phenol using a sulfided NiMo catalyst [91]. Adilina et al. also studied the classical NiMo catalysts supported on pillared clays (PILC) in reduced (NiMoPR) and sulfided (NiMoPS) forms for the HDO of guaiacol [104]. They have used techniques like quasielastic neutron scattering (QENS) and inelastic neutron scattering (INS) measurements to understand the interaction between guaiacol and the reduced and sulfided NiMo catalysts with the clay support [104]. Their results revealed that guaiacol adsorbed on these types of catalysts via two interaction modes, as illustrated in Fig. 8b): the first interaction is guaiacol adsorbed with the Ni-Mo-S site via an H-bonding interaction for sulfided catalysts and the second interaction is chemisorption of guaiacol on both the Ni-Mo site and also the clay support via phenate formation as can be observed in the reduced catalysts [104]. Their results with the sulfided catalysts also showed high activity and selectivity for guaiacol HDO.The promoters play a role in conventional hydroprocessing catalysts. Badawi et al. have demonstrated that cobalt promotes both DDO (Direct cleavage of the hydroxyl group) and HYD pathways (Hydrogenation of the aromatic ring and followed by the cleavage of the hydroxyl group) in the HDO of phenol to different extents [92]. They have performed DFT calculations and have shown that both DDO and HYD pathways occur on sulfur vacancy sites (CUS) [92]. Romero et al. have also reported the same findings [95]. Using 2-ethylphenol as a model compound [95], they have found that both Ni and Co improve the deoxygenation rate, while Ni only facilitates the HYD pathway. The reaction mechanism for DDO and HYD is illustrated in \nFig. 9, respectively [95]. The main difference between these two pathways is that HYD originated from flat adsorption by the aromatic ring while the DDO pathway adsorption occurred through the oxygen atom.In addition to Ni and Co, a study conducted by Yang et al. has demonstrated that phosphorus (P) was able to promote the phenol HDO activity over a CoMoS-supported MgO catalyst, and they proved that DDO is the major pathway in phenol deoxygenation [96]. A non-conventional hydrotreating catalyst like supported ReS2 has been reported in several studies [100,99,101,103,102]. For instance, ReS2 supported on SiO2 or \u03b3-Al2O3 supports was applied in the processing of dimethyl dibenzothiophene and guaiacol [102]. Both Re-based catalysts showed high HDS and HDO activities; ReS2 supported on SiO2 showed high HDO rates giving 40% HDO products [102]. In addition to inexpensive transition metals used as promoters, research has examined the use of rare earth and noble metals as promoters for metal sulfide catalysts in phenolics HDO [98,99]. For instance, Ir and Pt have been incorporated into RuS2/SBA-15 and used in the HDO of phenol [98]. The results have demonstrated a higher conversion rate of phenol (37\u201341%) and better cyclohexane selectivity (62\u201363%) than with the non-promoted RuS2/SBA-15 [98]. It is important to note that the use of noble metals involves high costs for catalyst production, which limits their industrial application. The sulfur content in some bio-feedstocks, such as Kraft lignin, may act as a poison to such noble catalyst systems, nevertheless, studying such a system facilitates better insight into the reaction pathways of the HDO of phenolics.Jongerius et al. studied a pool of lignin model compounds using CoMoS supported on Al2O3 under the same reaction parameters (300\u2009\u00b0C, 50\u2009bar\u2009H2, 4\u2009h, and batch system) for comparison [94]. Their main findings suggest that the mono-aromatic oxygenates underwent three distinct pathways that included HDO, demethylation, and methylation. This provided invaluable products like phenol, benzene, cresols, and toluene [94]. Less than 5% of hydrogenated products were detected in the reaction medium, indicating that hydrogenation is the least preferred reaction network for this catalyst system [94].It is commonly found in the considerable number of studies on the HDO of phenolic compounds that sulfiding agents, such as dimethyl disulfide (DMDS) or carbon disulfide (CS2), were co-fed during an experiment to create H2S to maintain the sulfidation degree of the sulfided catalyst. Results show that adding a sulfiding agent during the HDO process had a negative effect on the HDO activity of phenolics but promoted the HDO of aliphatic oxygenates such as vegetable oils and animal fats [66]. As a result, one should note that the addition of a sulfiding agent also plays a role in affecting the effectiveness of the catalyst other than the type of reactant being used. Ferrari et al. have studied the effect of H2S partial pressure and sulfidation temperature on the conversion and selectivities of phenolics [88]. It was found that the increase in H2S partial pressure reduced the formation of deoxygenated products from the HDO of guaiacol over CoMoS supported on carbon [88].Over recent decades, these traditional TMS catalysts have been tested by omitting the use of catalyst support, resulting in unsupported TMS. There are several methods to prepare unsupported TMS, that can be used in the hydrotreatment processes. One of these is a hydrothermal synthesis with synthesis parameters, such as moderate synthesis temperature (150\u2013250\u2009\u00b0C) and the absence of hydrogen pressure [112,123,122,124,105,125]. The synthesis method involved simple operation and also controllable catalyst morphology, allowing easy scale-up for industrial application. Wu et al. prepared a series of hydrophobic unsupported MoS2, NiS2-MoS2, and CoS2-MoS2 using hydrothermal synthesis with the aid of silicomolybdic acid for the HDO of 4-ethylphenol [105]. The CoS2-MoS2 catalyst achieved a 99.9% 4-ethylphenol conversion with a 99.6% ethylbenzene selectivity after 3\u2009h. The catalyst showed good recyclability after 3 runs at 225\u2009\u00b0C [105]. Cao et al. also presented results for highly efficient unsupported Co/MoS2-x (x is the molar ratio of Co/(Co + Mo)) catalysts with high dispersion, rich in defects, and curvy slabs in deoxygenation of p-cresol under mild HDO condition [121]. During the hydrothermal synthesis of catalysts, the accommodation of Co atoms in the coordinative unsaturated sites (CUS) of MoS2 and also edge sites resulted in the formation of a Co-Mo-S active phase that enhanced the HDO activity [121]. Their results further demonstrated that the Co/(Co +\u2009Mo) molar ratio of 0.3 provided the highest HDO activity and toluene selectivity, on the other hand, excessive Co introduction, causing the formation of large Co9S8 particles that hinder the HDO active sites with decreased HDO activity [121].A two-step strategy for the synthesis of Co-MoS2\u2212x catalysts involved first the hydrothermal and then subsequently the solvothermal method as explored by Wu et al. [122]. They concluded that the reducing ability of MoS2\u2212x induced by the sulfur vacancy was able to reduce the Co precursor to Co metallic while decorating the edges of MoS2\u2212x, leading to the formation of the metal-vacancy interfaces that catalyze the HDO reaction [122]. They further performed Gibbs free-energy calculations (\nFig. 10) and clarified that DDO of 4-methylphenol underwent a two-step, hydrogenation and dehydration with CH3C6H4OH+ and CH3C6H4\n- being the transition states [122]. As can be seen in Fig. 10, the Gibbs free energies for the formation of the transition state species were decreased for Co-MoS2\u2212x in comparison to MoS2\u2212x indicating that the metal-vacancy interface favored the adsorption of 4-methylphenol and lowered the reaction energy barriers, hence enhancing the HDO activity [122]. Another study by Wang et al. has proposed a reaction network for p-cresol HDO using a hydrothermally prepared CoMoS catalyst, as shown in \nFig. 11a) [112]. Two different deoxygenation routes for p-cresol have been proposed: the first is the DDO route, where the partially hydrogenated dihydrocresol is attacked and the dissociated H+ and the OH2\n+ species are cleaved in the form of H2O producing toluene [112]. The second route involves HYD where the partially hydrogenated p-cresol is fully hydrogenated to 4-methylcyclohexanol and then dehydrated to 3-methycyclohexene. The product, 3-methylcyclohexene then undergoes hydrogenation and forms methylcyclohexane [112]. The study also described a p-cresol adsorption scheme on an unsupported CoMoS catalyst [112], as shown in Fig. 11a). p-cresol could adsorb via its vertical orientation and coplanar position in relation to the DDO and HYD routes, respectively [112]. It can also be considered that there is a difference in the CoMoS properties which determine the adsorption orientation and thus the reaction route [112]. For instance, it was mentioned in their study that an increase in the number of MoS2 layers and also reduced slab length can enhance the toluene selectivity and p-cresol conversion [112].The use of a template like zeolitic imidazolate framework-67 (ZIF-67) was used to prepare a self-supported defect-rich CoMoS-x catalyst for HDO of p-cresol [120]. The main finding in this work highlighted the importance of a pre-reduction of the catalysts in decalin (300\u2009\u00b0C, 30\u2009bar\u2009H2, and 6\u2009h), and such pre-treatment promoted the formation of sulfur vacancies on the MoS2 surface and facilitated the surface restructuring of Co-Mo interfaces resulting in the in-situ generation of an abundance CoMoS active sites favoring the DDO route, as shown in \nFig. 12\n[120]. A hard template like mesoporous silica SBA-16 has also been used to synthesize an unsupported NiMoW sulfide catalyst for the HDO of guaiacol in a fixed-bed reactor [111]. The NiMoW sulfide unsupported catalyst gave a 99.6% guaiacol conversion with minimal coke formation at 400\u2009\u00b0C [111]. Adapted from the reference, shown in Fig. 12 b), guaiacol underwent HDO via demethylation (DME), demethoxylation (DMO), and transalkylation [111]. Phenol was formed by either the direct demethoxylation of guaiacol or the dehydroxylation of catechol; both reactions resulted in the production of benzene [111]. It is worth noting that phenol was first obtained from the HDO of guaiacol as a reaction intermediate caused by the higher bond dissociation energy for the hydroxy group on the aromatic ring than the methoxy group [84].Furthermore, the cleaving and HDO of dimeric phenols are of interest because the depolymerized lignin streams and bio-oils can contain not only monoaromatic compounds but also various aromatic dimers and oligomer fragments. These intermediates are present in the reaction feed, and they should be cleaved during the deoxygenation process. Hence it is necessary to study the HDO of model fragments that can mimic specific structural linkages that can be found in lignin under HDO conditions. Metal sulfides have been explored for cleaving lignin model dimers having C-O-C ether and C-C linkages. The cleavage of lignin dimers containing etheric linkages (\u03b1-O-4 or \u03b2-O-4, 4-O-5, etc) is faster due to their low bond dissociation energy while C-C linkages (5\u20135\u2032, \u03b2-1, \u03b2-\u03b2, etc.) are quite recalcitrant. The bond dissociation energy of the etheric linkages is in the order of 4-O-5 (ca. 330\u2009kJ/mol) >\u2009\u03b2-O-4 (ca. 289\u2009kJ/mol) >\u2009\u03b1-O-4 (ca. 218\u2009kJ/mol) while for the 5\u20135\u2032 linkage it is ca.\u00a0490 kJ/mol [126\u2013128]. The review for model lignin dimers (\nTable 5) is discussed in this section while Section 4 of this paper is focused on real lignin feed for hydrotreatment. These types of model dimer compound-related studies can provide a means of screening the effectiveness of catalysts and understanding reaction mechanism schemes that occur during lignin depolymerization. Various studies involving lignin dimers revealed that sulfided catalysts can effectively break down the dimers to monomers however to a different extent. Koyama et al. showed the hydrocracking of benzyl phenyl ether, diphenyl ether, benzyl phenols, diphenylmethane, and dibenzyl over sulfided NiMo/alumina, Fe2O3/alumina, and MoO3/TiO2 in the range of 340\u2013450\u2009\u00b0C [129]. Monomer yield increased with increasing temperature for model dimers having a phenolic hydroxyl group and C-O-C linkages while the C-C biphenyl bond was quite recalcitrant even up to 450\u2009\u00b0C. Additional dimers were also found to be formed due to dehydroxylation and subsequent hydrogenation reactions.As mentioned earlier, Jongerius et al. reported that with a commercial sulfided CoMo/Al2O3 the \u03b2-O-4 bonds of phenyl coumarin alkyl ether could be cleaved to monoaromatics while 5\u20135\u2032 linkages could not be broken at 300\u2009\u00b0C and 50\u2009bar of H2 pressure [94]. Shuai et al. reported a substantial yield of aromatic monomers from the selective cleavage of -CH2- linked C-C phenolic dimers over a commercial CoS2 catalyst at 250\u2009\u00b0C and 50\u2009bar of H2 following 1.5\u2009h of reaction [130]. However, \u03b2-1 and 5\u20135\u2032 C-C linked dimers could not be cleaved but rather transformed into hydroxyl dimers via demethoxylation. Surface engineering of unsupported Co-promoted MoS2 nanosulfides by Song et al. showed that diphenyl ether (4-O-5) could be cleaved efficiently to produce benzene selectively [106]. The same author also claimed that in-situ exsolution synthesized CoMoS on a sulfated zirconia support enhanced the 4\u20130\u20135 (diphenylether) C-O cleavage and the subsequent deoxygenation to benzene, toluene, and ethylbenzene [133]. Their work also compared the catalysts prepared by a conventional impregnation and physically mixed sulfated ZrO2 supported CoMo sulfide catalysts, and showed that the exsolution method represented the best method based on the activity and characterization tests. The activity enhancement was mainly attributed to the better interaction between the CoMo sulfide phase and support, Lewis acid sites of sulfated ZrO2, and highly-dispersed CoMo sulfide [133]. Ji et al. showed that nanocrystalline pyrite and marcasite (FeS2), supported over activated carbon was highly active and selective for the hydrodeoxygenation of dibenzyl ether into toluene at 250\u2009\u00b0C under an initial H2 pressure of 100\u2009bar for 2\u2009h [134]. The high activity of such a catalyst was claimed to be due to the transformation of surface FeS2 into Fe(1\u2212x)S [134]. A proposed reaction pathway for the chemical transformation of dibenzyl ether to benzaldehyde and toluene is shown in \nFig. 13a) [134]. The proposed chemical pathway involves the formation of phenylmethylium, and subsequent cleavage of the ether bond. Then hydride transfer takes place from phenylmethanolate to phenylmethylium, resulting in the formation of targeted products, benzaldehyde, and toluene. In another study, Zhang et al. proposed that the cleaving of lignin \u03b2-O-4 ether bonds can occur through a dehydroxylation-hydrogenation reaction over the acid-redox site of a NiMo sulfide catalyst which significantly lowers the bond dissociation energy and subsequently facilitates the formation of styrene, phenols, and ethers with H2 and an alcohol solvent [135]. A potential main route for the conversion of 2-phenoxy-1-phenylethanol (\u03b2-O-4-A) over the NiMo sulfide catalyst was proposed in their work which involves the adsorbed \u03b2-O-4-A firstly losing a hydroxy group and generating carbocation intermediates (PhCH\u03b4+CH2OPh) at the weak and medium acid sites of the catalyst. The carbocation intermediates were then transformed into radical intermediates (PhCH\u00b7CH2OPh) by obtaining an electron from the catalyst redox cycle (Fig. 13b). Due to the lower bond dissociation energy (BDE) of the C\u03b2-OPh ether bond in the radical intermediate (66.9\u2009kJ/mol) than the one in \u03b2-O-4-A (274.0\u2009kJ/mol), the C\u03b2-OPh bond cleaves easily, and then the generated radical species react with activated H2 or methanol to form various arene products like phenols and ethers [135].A similar strategy of peroxidation via O2/NaNO2/2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/N-Hydroxyphthalimide (NHPI) and subsequent hydrogenation over a NiMo sulfide catalyst was found beneficial for the overall cleavage of \u03b2-O-4 model compounds [136]. Recently our group studied lignin dimer hydrotreatment involving sulfided NiMo over Al2O3 and ultra-stable Y zeolite (USY). It has been found that the NiMoS-USY combination can give a high yield of mono-aromatics including deoxygenated aromatics, mono/alkyl phenols, and cycloalkanes via hydrogenolysis of etheric C-O bonds and subsequent cleavage of C-C intermediates formed by transalkylation reactions [131]. Interestingly, with the NiMoS-USY combination, recalcitrant \u03b2-1, -CH2 linked dimers and 5\u20135\u2032 linkages could be significantly cleaved due to a suitable balance between Br\u00f8nsted acidity and Ni-promoted MoS2 redox sites. This study led to a further study investigating USY with different silica/alumina ratios (SAR) as catalyst supports for NiMoS in valorizing lignin dimers [132]. Among all the studied catalysts, NiMoS on USY with an intermediate SAR of 30 was found to be most effective in cleaving \u03b2-O-4 and C-C linkages with a high degree of hydrogenolysis, hydrocracking, and HDO reactions owing to an apparent optimal balance between acidic and deoxygenation sites. Moreover, it was found that a higher surface acidity promoted the initial conversion of 2-phenethylphenylether (PPE) and 2,2-biphenol (5\u20135\u2032) via transalkylation and isomerization [132]. These results suggested that tuning the acidity and pore sizes of the USY-zeolite and further impregnating with NiMoS was an effective way to create a catalyst that is efficient in both breaking recalcitrant lignin linkages and achieving a high level of deoxygenation.Pyrolysis oil produced from the fast pyrolysis of solid biomass contains a variety of compound groups like sugars, alcohols, phenols, ketones, aldehydes, furans, and acids. These compounds contribute to various detrimental properties of pyrolysis oil which need to be addressed before the full utilization of pyrolysis oil as a fuel or other applications by hydrotreatment in petroleum refineries. Shumeiko et al. performed a series of screening tests of lab-synthesized and commercial sulfided NiMo catalysts for long-term hydrotreatment of wheat/barley (50/50\u2009wt%) straw-derived pyrolysis oil in a fixed-bed reactor aiming to produce a hydrotreated pyrolysis oil that is compatible with the petroleum refinery fraction for coprocessing [137]. The assessment of their results was based on the HDO and HDS activity. Their results showed that the catalysts synthesized by co-impregnation were better than the catalysts prepared by a two-step impregnation procedure despite having the same active NiMo phase loading and the same commercial alumina support. While the commercial catalysts performed worst among all the catalysts in terms of HDO activity, they showed the best HDS performance [137]. The difference in activity may be attributed to the different physicochemical properties of the catalysts and also preparation methods. These results suggest that the HDS activity of a sulfide catalyst is not suitable for indicating its HDO performance for pyrolysis oil. The long-term experiments (80\u2009h) in their work were useful in understanding the deactivation of the sulfided catalysts [137]. Their experimental results showed that the product quality changes, which was indicated by a gradual loss of catalyst activity with increasing time-on-stream [137]. Thus, their work demonstrated the feasibility of using biomass-derived pyrolysis oil to obtain a compatible feedstock for co-processing in a refinery, however, the stability of the sulfided catalysts needs to be fully addressed before achieving a successful deployment of the technology. Another study conducted by Zhang et al., upgraded a pyrolysis oil produced by the fast pyrolysis of forest residues with light cycle oil (LCO) as a reaction medium using a dispersed unsupported MoS2 catalyst [6]. The use of the dispersed unsupported catalysts was considered to allow better interaction between the active sites of the catalysts, hydrogen, and the heavy feedstock resulting in less solid yield. The low solid yield ranging from 0.8 to 1.8\u2009g/100\u2009g bio-oil at the end of their experiment showed that the use of dispersed unsupported materials can suppress the side reactions such as polymerization and re-polymerization of the large molecular weight compounds and reactive species that result in solid residues.Priharto et al. studied the hydrotreatment of pyrolysis oil derived from lignin-rich digested stillage over commercial sulfided NiMo and CoMo catalysts [138]. They demonstrated the feasibility of utilizing solid waste residues from bioethanol processes for the production of pyrolysis oil. The further hydrotreatment of the pyrolysis oil also resulted in an appreciable oil yield of 60\u201364\u2009wt% [138]. It should be noted that the nitrogen content in such feed should be refined employing hydrodenitrogenation (HDN), as, from their GCMS analysis, nitrogen-containing aromatic heterocyclic compounds present in the feed like indoles were converted to pyrroles. Hence, the removal of nitrogen content in pyrolysis oil by the means of hydrodenitrogenation (HDN) should be addressed in any future study with the aid of sulfided catalysts. For instance, Izhar et al. studied HDN of fast-pyrolysis oil derived from sewage sludge over a phosphorus-promoted sulfided NiMo/Al2O3 catalyst [139]. The main finding from their work showed that dissolving pyrolysis oil using a non-polar solvent like xylene improved nitrogen removal compared to using protic solvents due to the competition between denitrogenation and deoxygenation reactions [139].Lignin (an amorphous solid) is a renewable and sustainable future source of aromatic compounds for the chemical industry [140]. Lignocellulosic biomass contains up to 33\u2009wt% of lignin. Softwoods (coniferous woods, e.g. spruce), contains 27\u201333\u2009wt% lignin. Hardwoods (deciduous species, e.g. birch) have 18\u201325 w% lignin and grasses constitute 17\u201324\u2009wt% of lignin [141,142]. Chemically, lignin is a 3-dimensionally complex biopolymer composed of three basic structural units; sinapyl alcohol, coniferyl alcohol, and coumaryl alcohol. The composition of these structural units is different among the lignocellulosic biomasses. In softwoods, the coniferyl alcohol units form 90\u201395\u2009wt% of the lignin with the remaining being only the sinapyl alcohols (10\u20135\u2009wt%). In hardwood lignin, an equal amount of (50\u201350\u2009wt%) coniferyl and sinapyl alcohols are observed. These woods are devoid of the coumaryl alcohol structural unit. Grasses contain 0\u20135\u2009wt% of the coumaryl alcohol units, but the major contributions are from the coniferyl alcohol (75\u2009wt%) and the sinapyl alcohol (20\u201325\u2009wt%) units. The structural units in lignin are connected by various C-O-C ether linkages (\u03b1-O-4, \u03b2-O-4, 4-O-4) and C-C linkages (\u03b2-\u03b2, \u03b2-1, \u03b2-5, 5\u20135) (see \n\nFig. 15) to generate the three-dimensional structure of lignin. Among these two kinds of linkages, the ether linkages (65%) dominate in lignin, of which the \u03b2-O-4 alone accounts for 50% of all ether linkages. The composition of the chemical linkages in softwood, hardwood, and grass lignin are also dissimilar. For instance, softwood contains more C-C bonds than hardwood. Also, the C-O-C and C-C linkage patterns vary among the softwood/hardwood/grasses plants classes too. Lignin structure is very complex among and within the various lignocellulosic biomasses. This necessitates the exact structural determination of every isolated lignin which is very challenging to accomplish [143].The natural sources of lignins are the many agricultural residues (grain dust, sunflower stalk, bagasse, etc.) and forest residues [3]. The lignins from these sources can be utilized only after their extraction. Otherwise, they need to be co-processed along with the cellulose and hemicellulose fractions of the biomass. On the other hand, commercial lignins (also known as the technical lignins: Kraft lignin, soda lignin, hydrolysis lignin, etc.) are already extracted lignins almost devoid of the cellulose and hemicellulose fractions. Commercial lignins are usually generated as the byproducts of various commercial pulping/hydrolysis processes. For instance, Kraft lignin is generated as the byproduct of the Kraft pulping process, likewise, the soda pulping process generates soda lignin as a byproduct, and hydrolysis lignin is produced during the enzymatic hydrolysis of cellulosic biomass to ethanol [3]. The severity of the pulping processes and the chemical reagents used in the processes (NaOH, Na2S, H2SO4, etc.) [140] adversely affect the lignin structure. Bonds are broken and new ones are generated during the processes. Hence, the structures of commercial lignins are different from their natural counterparts. Currently, most of the commercial lignins produced are utilized as a combustion fuel in the pulping process to regenerate heat energy for the pulping process. This gives rise to a low value-addition of the lignin ($70\u2013150 per ton). On the other hand, the conversion of commercial lignins to chemicals (e.g.: phenols, benzenes, toluene, xylenes, etc.), and fuels significantly improves its value-addition (approximately $1300 per ton) [7].The sole way to obtain the different monomeric aromatic compounds from lignin is through its depolymerization. In general, lignin depolymerization can be achieved by techniques such as pyrolysis, gasification, hydrogenolysis (H2), chemical oxidation (O2), and hydrolysis (H2O) [144]. Other methods include microwave-assisted lignin depolymerization, biological depolymerization, and a so called lignin-first approach, as summarized in Fig. 15. There are many excellent reviews available in the literature on various lignin depolymerization techniques and their different advantages [3,140,141,143,144\u2013151]. Anyways, the produced bio-liquids needs further upgrading to produce bio-fuels. For instance, the liquid bio-oil obtained from the pyrolysis of lignin is corrosive and high in oxygen content. To produce value-added compounds from this bio-oil, an additional step involving catalytic hydrotreatment is necessary. On the contrary, chemical depolymerization especially using heterogeneous catalysts has the advantages of high product selectivity to either value-added oxygenates or deoxygenates in a single step, high efficiency of the reagents used, and moderate reaction conditions, and the ease of reaction control [140]. This part of the review focuses on the chemical reductive method (H2) of lignin valorization using sulfided catalysts.The first step in lignin depolymerization is its thermal degradation to oligomeric lignin fragments (\nScheme 1) [152]\n. The macromolecular structure of lignin is a major hurdle for employing solid catalysts during this step, since, in most of the porous solid catalysts, their pore dimensions do not match with the dimensions of the lignin macromolecules. However, homogeneous catalysts may better facilitate the first step. In the second step, the formed smaller fragments undergo further degradation to form oxygenated monomeric molecules. Solid catalysts can perform this step. In the third step, deoxygenation (catalytic HDO) of these monomers to liquid aromatic products ensues. Further catalytic hydrogenation of aromatics to cyclic aliphatic compounds follows in the final step (Scheme 1). The gaseous products of lignin depolymerization are mainly, CO2, CO, CH4, and C2-C4 alkanes. At any stage of the lignin depolymerization, the repolymerization of the depolymerized lignin fragments, aromatic oxygenates, and aromatics may occur. This repolymerization occurs mainly through C-C bond formation (on the other hand, the C-O-C bonds may undergo further breakage) leading to the formation of a solid phase composed of polycondensed aromatic structure, usually called as lignin depolymerization char [152]\n. The main challenge in lignin depolymerization is to reduce the char formation and concurrently to increase the formation of the liquid products [153].The conversion in lignin depolymerization in the literature is expressed in contrasting ways by different research groups. Some authors separate the unconverted lignin from the char through solvent extraction to calculate the actual lignin conversion to liquids and gases. While others consider only the total amount of solid products (also containing unconverted lignin) obtained after the depolymerization in their calculation. The liquid product yields are also represented in different ways in the literature. The weight of bio-oil produced after depolymerization (sometimes expressed as the wt% of a particular solvent-soluble fraction) is the common method found in the literature. In the very early reports on lignin depolymerization, both the conversion and the composition of bio-oils are rarely mentioned. Whenever the composition of the bio-oil was specified in the literature, it is briefly stated in this section of the review. However, to obtain more detailed information about the full composition of the bio-oil and conversion, it is recommended to follow the corresponding cited references. Moreover, some literature represents the char yield as \u2018char\u2019, while others refer to it as \u2018solid residue\u2019 obtained after the reaction. These terms can be synonymous, but in some cases, the \u2018solid residue\u2019 could also include char as well as unconverted lignin.Early reports on the use of sulfided catalysts in lignin conversion were mainly focused on examining to what extent lignin valorization could be achieved rather than the catalyst structure, and the activity correlations. The choice of the various sulfided catalysts for this was purely based on their reputation for sulfur tolerance and capability for desulfurization/hydrogenation/hydrogenolysis activity. One such early study was reported by Vuori et al. in 1988, where they compared the lignin liquefaction under mild thermolysis conditions (< 400 \u00b0C) and catalytic conditions [153]. A commercial sulfided CoMo/Al2O3 was employed as the hydrotreating catalyst. Kraft lignin was the feedstock and tetralin was chosen as a hydrogen donor solvent (tetralin can act both as a solvent and 4-hydrogen atom donor to the reaction medium). The liquid products of the reaction (345\u2009\u00b0C, 20\u2009bar of H2, 5\u2009h) mainly constituted phenols and acids (ether soluble fraction) amounting to 11.5\u2009wt% in yield. This yield was not very much higher than the thermolytic reaction where the liquid product yield was 8.1\u2009wt%. In the gaseous products, CH4 (from lignin) contributed around 2.7\u2009wt% yields from the catalytic reaction as compared to 1.1\u2009wt% from the thermolytic conditions, suggesting an enhancement in reaction rate under catalytic conditions. Surprisingly, the authors found more char formation with the catalytic process (33\u2009wt%) than the thermolytic process (21\u2009wt%), leading to a conclusion that the catalyst could improve the reaction rate, however, was incapable of preventing the condensation reactions leading to char. Even the presence of both the hydrogen donor solvent (tetralin) and H2 pressure (20\u2009bar) was unable to prevent condensation reactions from lignin fragments [153].In general, solvents in lignin depolymerizations can stabilize the reactive intermediates, enhance the catalyst-lignin interactions, and promote the solubility of lignin. The role of solvents in lignin depolymerization was reviewed in detail by Raikwar et al. [154]. To understand the effect of hydrogen-donor and non-donor solvents on the lignin liquefaction process, Schuchardt et al., employed a set of different solvents such as xylene, pyridine, cyclohexanol, isopropanol, and tetralin; the latter three being hydrogen-donor solvents [155]. The catalyst of choice was ferrocene in situ sulfided with CS2 or S. Among these solvents, tetralin gave the maximum heavy-oil yield of 44\u2009wt% at 65\u2009wt% of lignin conversion (400\u2009\u00b0C, 270\u2009bar at 400\u2009\u00b0C, 0.5\u2009h). This was attributed not only to the high hydrogen-donor ability of tetralin but also to its heavy-oil extraction ability.The C-C bonds in lignin are more difficult to cleave than the C-O bonds (vide supra). Therefore, catalysts for lignin depolymerization also need to be efficient for the C-C bond cleavage since a large amount of char (repolymerized lignin-containing a large amount of C-C bonds) formed during the depolymerization needs to be broken down as well for higher monomer yield. A comparative study using unsupported MoS2 and CoS2 on the model compound dimethylguaiacylmethane containing a methylene bridge and Kraft lignin showed the CoS2 catalyst to be superior in C-C bond cracking (11.7\u2009wt% yields to the aromatic monomer from Kraft lignin at 250\u2009\u00b0C, 50\u2009bar\u2009H2, 15\u2009h) than MoS2\n[130]. However, the catalyst has a limitation, not all C-C bonds could be cleaved by CoS2. It is efficient only if there are hydroxy groups on any of the benzene rings related to the ortho position of the methylene/C-C bond. In the absence of these -OH groups, no C-C bonds were cleaved. The influence of the -OH group was more effective than the -OMe group in the C-C bond cleavage. Further studies showed that the active phase of the catalyst for C-C bond breakage is not CoS2 but CoS generated by the reduction of CoS2 in the H2 atmosphere. Complementary to this, the surface composition of the catalyst after the reaction showed a larger contribution of CoS (\nFig. 16a). The recycling studies showed a decrease in catalytic activity. The Co:S atomic ratio was reduced from 1:1.7\u20131:1 in the third run and the presence of the CoO phase was noticed. Conditional experiments with CoO showed less activity, indicating that the most active phase of the reaction was its sulfided form. The decrease in activity during the recycle runs can thus be attributed to its oxidation to the oxide phase. Both the supported catalysts sulfided CoMo/Al2O3 and unsupported CoS/S2 are effective for the breakage of the C-C bonds in lignins.Lignin depolymerization behavior in the presence of a sulfided catalyst during the heating period of the batch reactor is often overlooked. This information gives an idea about the lignin fragmentations happening in the presence of the sulfided catalysts in the early hours of the reactions where there is a large temperature variation. This was investigated by Joffers et al., using Protobind 1000 lignin and a commercial sulfided NiMo/Al2O3 catalyst [157]. The time taken for the reactor to reach the desired temperature of 350\u2009\u00b0C was only 14\u2009min. Immediately following the 14\u2009min, the lignin conversion was 27\u2009wt%. This lignin conversion resulted in 24\u2009wt% yields of liquids (mainly monomeric phenols), and 3\u2009wt% of gases (CH4, CO, CO2, and C2-C5 hydrocarbons). The remaining was the lignin residue (solid). Gel permeation chromatographic (GPC) analysis of the tetrahydrofuran soluble fraction of this lignin residue (oligomers) showed a molecular weight of 3575\u2009g/mol, corresponding to 20 phenylpropane units (parent lignin had 26 units). These monomer units were further reduced to 6 after 28\u2009h of reaction [156]. The THF-soluble lignin residue still contained stronger C-C linkages between monomer units, which were difficult to break. This could be due to the reaction conditions or perhaps due to the choice of a NiMo/Al2O3 catalyst than a CoMo/Al2O3 catalyst. Characterization of the catalyst after the reaction confirmed coke formation. Moreover, the sulfur content in the catalyst was decreased to 7\u2009wt% as compared to the 9\u2009wt% in the freshly sulfided catalyst (Protobind 1000 had only 0.1\u2009wt% of sulfur in it). Fig. 16b) shows the changes in the BET surface area, pore volume, and pore diameter of the catalyst as a function of reaction time. All these textural properties decreased in the first hour of the reaction and became almost constant after 28\u2009h of the reaction. The sulfided catalyst showed good stability for longer reaction runs. The main cause of its deactivation is due to the loss of sulfur. Therefore, an additional sulfidation step is necessary to regenerate most of the initial activity.To increase the monomer yield during lignin depolymerization, i.e., effectively removing the products from the catalyst preventing their further transformation, and competition for active sites, the lignin depolymerization process was attempted in a semi-continuous mode using sulfide catalysts. In the case of both a batch reactor and a fixed-bed reactor, the semi-continuous mode involves the continuous withdrawal of the reaction products under the flow of the H2. The lignin and the solvent (if any) are already placed in the reactors; they are not in a continuous feeding mode. In one such study, a NiMo/Al2O3 catalyst was mixed with lignin (hydrolysis lignin) and packed into a tubular reactor [158]. No solvents were used in the depolymerization. At the reaction temperature (380 \u00b0C, 40\u2009bar of H2, 4\u2009h), the lignin underwent thermal degradation to smaller fragments, which were then subsequently transformed at the catalyst active site. The H2 gas which was continuously flowing through the reactor enabled the mass transfer. The liquid products were collected in a gas-liquid separator during the depolymerization, and it consisted of an aqueous phase and an organic phase. The liquid products were composed of hydrocarbons, oxygenates, and phenols. The gaseous products were CO, CO2, CH4, etc. The solid product was mainly the lignin condensation product, char, which was separated from the catalyst by sieving. The catalyst was sulfided either in situ in the presence of the lignin in the reactor using dimethylsulfide at a temperature range of 200\u2013220 \u00b0C (this temperature range is lower than the decomposition temperature of the lignin), or ex-situ (a pre-sulfided catalyst mixed with lignin and loaded into the reactor). The elemental analysis of the in situ and ex situ sulfided catalysts showed a Mo/S ratio of 1.86 and 1.49 (w/w) respectively, indicating a lower degree of sulfidation in the in situ sulfided catalyst. The pre-sulfided catalyst was more efficient (higher amount of liquid and gaseous products, and a lower amount of char) than the in situ sulfided catalysts. The sulfidation state of the catalyst is the crucial factor for its activity. Increasing the catalyst-to-lignin ratio increased the liquid and gaseous product yield, and consequently decreased the solid residue. This behavior was attributed to the proximity effect between the catalyst active sites and the reactants, which increased with an increase in catalyst amount. Similarly, an increase in H2 pressure favored a higher rate of hydrogenation and deoxygenation during the hydrocracking process. The study showed the adaptability of commercial sulfided catalysts for process modifications.A slightly modified semi-continuous batch process with a sulfided CoMo/Al2O3 catalyst was reported by Pu et al. [159]. A constant flow of H2 to maintain the reactor pressure and a reflux system to extract continuously the light aromatic products and H2O from the reactor were the main features of the semi-continuous setup. The semi-continuous approach helped to remove the gases formed during the reaction so that their contribution to catalyst properties/deactivation could be avoided. However, the contribution of H2O which was not completely removed until the reaction temperature was reached, to catalyst deactivation, could not be avoided. When the catalyst was characterized after the reaction (350\u2009\u00b0C, 80\u2009bar\u2009H2, 13\u2009h), significant changes in its composition and textural properties were observed (\nFig. 17). Coking had started at the early hours of the reaction. The carbon content in the catalyst after the first hour of the reaction was 12\u2009wt% and remained the same until the end of 13\u2009h (Fig. 17a). The sulfur content was also reduced from 7.5 to 6\u2009wt% after 13\u2009h of reaction (Fig. 17b). However, based on XPS analysis, the S/Mo atom ratio had decreased significantly when comparing the freshly sulfided catalyst to that after 13\u2009h reaction (2.2 and 1.7 respectively, Fig. 17b). Sulfur loss during the reaction appears to be inevitable for the sulfided catalysts. Other notable changes were observed in the textural properties of the catalyst. The surface area and pore diameter decreased from 193 to 185\u2009m2/g, and from 0.47 to 0.29\u2009cm3/g respectively in the first hour and remained the same (Fig. 17a), while the average pore diameter decreased from 7.9 to 6.2 during the first hour and remained almost the same until 13\u2009h. The changes in both the composition and textural properties of the catalyst occurred during the heating period where lignin started to undergo depolymerization (the 0th hour is immediately following the heating period in Fig. 17). Nevertheless, with all these changes in its properties, the catalyst was active for oligomer cracking and deoxygenation reactions during the 13\u2009h (the liquid fraction increased from 44 to 82\u2009wt%). Perhaps, the most important aspect influencing the activity of the catalyst is its sulfur content. If there was a continuous source of sulfur, it can be presumed that the catalyst could have maintained its activity for longer reaction times.Instead of the ordinary organic solvents used for lignin depolymerization, the use of slurry-oils with sulfided catalysts was also reported. Meier et al. studied the performance of a sulfided NiMo/Al2O3 catalyst in 5 different slurry-oils [160]. They were, (1) light fraction oil from bitumen and lignin coprocessing (S = 2\u2009wt%), (2) heavy fraction oil of the same bitumen and lignin coprocessing (S = 4\u2009wt%), (3) a recycled residual oil from\u00a0(2), (4) standard vacuum gas oil, and (5) lignin-derived slurry oil. The performance of the catalyst in different slurry oils is compared in \nTable 6. Since the lignin-derived slurry oil already contained phenols, the calculation of phenolic yields solely from the lignin feedstock resulted in negative values because the amount of the initial phenol in the lignin slurry oil was subtracted from the total phenols produced after the reaction. The highest yield for solid residue (coke, 10.7\u2009wt%) was obtained when the heavy fraction oil was used as the slurry oil. This amount was about 1.5 times lower than without the catalyst. The minimum yield to the solid residue (0.3\u2009wt%) was obtained with lignin oil. Although the total amount of oil obtained from lignin with different slurry oils was in the high range of 68\u201383\u2009wt%, the highest being in lignin slurry oil, the amount of phenol obtained ranged only between \u2212\u20091.4\u20134.1\u2009wt%. The phenolics yield without the catalyst was only 2.9\u2009wt%. Hence, the catalyst had only a small effect in improving the phenolic yield during the hydrocracking process. When the heavy fraction slurry oil was used in its 3rd recycle run, a higher yield to total lignin oil (79\u2009wt%), with a low yield to the solid residue (4.9\u2009wt%) was obtained. The results of using slurry-oils containing sulfur in combination with a commercial sulfided NiMo/Al2O3 catalyst appeared highly promising for maintaining the sulfidation state of the catalyst and the commercialization of the process as other noble metal catalysts would normally undergo sulfur poisoning during the reaction.Sulfided catalysts were also used for solvent-free hydrotreatment of lignin. The solvent-free attempt was aimed at alleviating the techno-economic issues resulting from solvent recovery and recycling that would arise for large-scale production. Meanwhile, solvents have the advantages that they can impart good heat and mass transfer properties which are poorer in a solvent-free reaction [161]. The earliest study on a solvent-free depolymerization process using sulfided catalysts was reported by Oasmaa et al., who mainly focused on the process and influence of lignin types rather than the catalysts [162]. Five technical lignins; 3 pine Krafts, 1 birch Kraft, and 1 organocell, over a combination of two commercial catalysts; a sulfided NiMo/aluminosilicate catalyst and 20\u2009wt%Cr2O3/Al2O3 catalyst (1:1) was used for the process. In general, the product oil yields (395\u2013400\u2009\u00b0C, 100\u2009bar of H2, 0.5\u2009h) followed the trend as organocell (71\u2009wt%) >\u2009pine Kraft (63\u2009wt%) >\u2009birch Kraft (49\u2009wt%)). Out of the produced bio-oil, the detectable aromatic yield was in the range of 19\u2009wt% for organocell, 21\u2009wt% for pine, 14\u2009wt% for birch Kraft lignins. The amount of solid residue produced after the reaction was however lower for pine Kraft lignin (4\u2009wt%) than organocell and birch Kraft lignins (7\u2009wt%). The study demonstrated that the sulfided catalyst could efficiently depolymerize the lignins to monomers under solvent free conditions at least in laboratory scale (70\u2009g of lignin).Later, a comparative study of sulfided NiMo on two different supports including activated carbon (AC), and MgO-La2O3 oxide was attempted under solvent-free conditions [161]. The sulfided NiMo/AC gave 55\u2009wt% yields (350\u2009\u00b0C, 100\u2009bar of H2, 4\u2009h) to the dichloromethane soluble fraction (average molecular weight being 700\u2009g/mol) with only 9\u2009wt% yields to the solid residue. The monomers in this fraction were largely composed of alkyl phenolics and aromatics. The NiMo/MgO-La2O3 gave 48\u2009wt% yields of the dichloromethane soluble fraction having an average molecular weight of 660\u2009g/mol, however with 12.7\u2009wt% of the solid residue. The NiMo/AC catalyst was slightly more effective in producing low molecular weight fragments from the lignin during the depolymerization. The XRD of NiMo/MgO-La2O3 after the reaction showed NiS, Ni3S4, and MoS phases on the catalyst, indicating its sulfidation state after the reaction (\nFig. 18). Only a negligible decrease in the surface area (from 29 to 23\u2009m2/g) and pore volume (from 0.16 to 0.14\u2009cm3/g) was observed between the fresh and used catalysts. However, an increase in particle size from 4.3 to 15.7\u2009nm of the supported NiMo occurred after the reaction (Fig. 18), probably due to the severity of the experimental conditions and due to the low heat dispersion effect under solvent-free conditions.The main active component of the supported sulfided Ni(Co)Mo/Al2O3 catalyst is the MoS2 phase wherein Ni and Co act mainly as promoters and Al2O3 acts as a dispersing medium. The unsupported form of sulfided catalysts has the advantage that they could be synthesized in different morphologies and compositions. Moreover, the unsupported MoS2 can offer more sulfur vacancies at the edge of its slabs. Unsupported sulfided catalysts were also studied in lignin depolymerization. Li et al., synthesized MoS2, and MoS2-based composite catalysts (MSx/MoS2, M = Ni, Co, Ag) with a flower morphology (\nFig. 19a-d) for lignin (corn stover) depolymerization [163]. A bio-oil yield of >\u200978\u2009wt% (310\u2009\u00b0C, 25\u2009bar\u2009H2, 1\u2009h) was obtained over MoS2. The performance of other sulfided catalysts (NiS2, CoS2, and Ag2S) was inferior (< 65\u2009wt% yields) to that of MoS2. A significant improvement in bio-oil yield (>85\u2009wt%) was obtained when the composite catalysts NiS2/MoS2, and CoS2/MoS2 were used (5\u2009wt% of MS2). The enhancement in the catalytic activity of MoS2 when other metal sulfide components were present was explained by the Edge Decoration (ED) model. Characterization studies showed that the parent MoS2 had the typical layer structure where the edges of the layers are wedge-shaped providing the hydrogenation sites. When NiS2 and CoS2 were present, the layer structure of MoS2 became more curved and less stacked, increasing the amount of potential surface-active sites. This was in conjunction with the enhancement in the surface area of MoS2 (5\u2009m2/g) when NiS2 and CoS2 were present (6 and 15\u2009m2/g for NiS2/MoS2 and CoS2/MoS2 catalysts, respectively). According to the ED model, the NiS2 weakened the Mo-S bond, thereby promoting the breakage of the Mo-S bond for the generation of S vacancies (active site for hydrogenation). Further comparison of MoS2 with FeS2 and CuS showed the activity trend for bio-oil production as (250\u2009\u00b0C, 1\u2009h, no H2 pressure) MoS2 (82\u2009wt%) >\u2009CuS (65\u2009wt%) >\u2009no catalyst (53\u2009wt%) >\u2009FeS2 (37\u2009wt%) [164].Another unsupported sulfided catalyst reported for lignin depolymerization is VS2 where different morphologies of catalyst (sheets and nanoflowers) were compared (Fig. 19 e-h)\n[165]. When VS2 sheets were used, the lignin conversion was at about 77\u2009wt% with 59\u2009wt% yields to the bio-oil (250\u2009\u00b0C, 20\u2009bar of H2, 1.5\u2009h). The solid residue amount accounted for nearly 22\u2009wt% yields. The use of VS2 nanoflowers decreased the conversion (65\u2009wt%) and bio-oil yield (50\u2009wt%) and increased the solid residue amount (35\u2009wt%). The flower morphology imparted steric hindrance to the reactant and eventually decreased its catalytic performance. According to the proposed mechanism, the H atoms from thermally broken H2 molecules adsorbed on the VS2, leading to the formation of -VH and -SH bonds. These bonds were unstable and could undergo breakage to form the VS2 catalyst, meanwhile transferring the hydrogen to the unsaturated reactant molecule.Narani et al. [166] found better results with sulfided a NiW/AC catalyst than NiMo/AC catalyst for the hydrotreatment of Kraft lignin in supercritical methanol (320\u2009\u00b0C, 35\u2009bar of H2, 8\u2009h). Two fractions (methanol and dichloromethane soluble fractions) of liquid products were identified. The methanol soluble oil was composed of aromatic monomers and low molecular weight (500\u2009g/mol) oligomers, and the dichloromethane soluble fraction was composed of solely high molecular weight (2725\u2009g/mol) oligomers. When sulfided NiMo/AC was used, a 57\u2009wt% yield to the methanol soluble oil was obtained. Only a trace amount of char was formed over the catalyst. The performance of sulfided CoMo/AC was inferior to that of the NiMo/catalyst (41\u2009wt% yield to methanol soluble oil with 9\u2009wt% yield to char), possibly due to the increase in acidity of the catalyst when Ni was replaced with Co. Sulfided NiW/AC increased the methanol soluble oil yield to 82\u2009wt% with no concurrent char formation. The acidity of the sulfided NiW/AC catalyst was even lower than NiMo/AC and was ascribed to the reason for the increase in the product yield (18.4 versus 44.5 \u00b5molg\u22121 of NH3 adsorption). Over the non-sulfided NiW/AC catalyst, substituted guaiacols were the predominant product. Sulfidation improved the activity towards deoxygenation (involving removal of -OCH3) leading to more phenols. By prolonging the reaction time from 8 to 24\u2009h up to 35\u2009wt% yield to monomers was obtainable. Nonetheless, this longer reaction time did not lead to over-hydrogenated compounds, highlighting the selectivity of the catalyst for phenols. The effect of different supports such as acidic ZSM-5, and basic MgO\u2013La2O3, MgO\u2013CeO2, and MgO\u2013ZrO2 were also investigated for the depolymerization. Their performance was inferior to that of AC. The MgO\u2013La2O3 (291 \u03bcmolg\u22121 of CO2 adsorption, the highest of all basic supports) gave similar results like NiW/AC. Different characterization techniques were used to analyze the structure and composition of the used catalysts. XRD of the spent NiW/AC catalyst showed peaks corresponding to WS2 and Ni3S2 phases, indicating its sulfided state. In support of this, the EDX analysis of the spent catalyst showed 2\u2009wt% of sulfur on it, which was homogeneously distributed. Morphological analysis of the spent catalyst by TEM confirmed the preservation of its spherical morphology (\nFig. 20a), however, particle agglomeration was observed. The catalyst was in its active state even after a long reaction time of 24\u2009h, indicating its high stability for longer reaction runs.Another important factor that affects the activity and stability of a sulfided catalyst during lignin hydrotreatment is the impurities in the lignin. Recently, our group has reported the role of inorganic impurities of a commercial Kraft lignin (Na, K, Ca, Fe, etc.,) on the activity of sulfided NiMo/Al2O3 catalyst [7]. These impurities in the lignin come from the Kraft pulping process which uses reagents NaOH and Na2S, and from the source wood. These inorganic impurities were deposited on the catalyst during the depolymerization, of which the major impurity element was the Na because of its higher amount in the lignin. Conditional studies using poisoned catalyst showed that at lower loadings of individual inorganic impurities, their promotor effect was prominent (the monomer yields were higher than in their absence on the catalyst, Fig. 20b). However, at their higher loadings, their poison effects were dominant. When all these elements were present together on the catalyst, their poisoning effect was much stronger. The number of moles of impurities, their strength, and their synergism were the main factors responsible for the catalyst deactivation.In summary, both supported and unsupported sulfided catalysts have been used for the hydrotreatment of various lignins. Supporting the metal sulfide active phase on various metal oxides (Al2O3, MgO\u2013La2O3, etc.) is helpful for its high dispersion. On the other hand, the unsupported catalysts have the advantages that they can be synthesized in different morphologies and compositions to tune the activity. But, the unsupported catalysts are prone to particle agglomeration (increase in their crystallite sizes) under long heat treatments. Supported sulfided catalysts were employed to study process modifications in lignin hydrotreatments. The catalysts showed good adaptability in batch, semi-continuous, and solvent-free lignin hydrotreatments. The deactivation of the sulfided catalyst occurs mainly through the removal of lattice sulfur atoms, and through the deposition of impurities from the feedstock. The latter is a common mode of deactivation in most catalytic processes and can be solved by using impurity-free feedstocks. A crucial factor governing the stability of sulfided catalysts is the sulfidation state of the catalyst. During deoxygenation reactions, there is a high risk that the removed oxygen atoms could substitute the lattice sulfur atoms, decreasing the sulfur vacancies and sulfidation state of the catalyst. The H2 gas used in the hydrotreatment could also remove some fraction of S as H2S. In these circumstances, a simple re-sulfidation of the catalyst can regenerate its sulfidation state and restore the activity. Another method to maintain the sulfidation state of the catalyst is to provide a continuous supply of sulfur sources during the hydrotreatment. The use of lignins containing a significant amount of sulfur (e.g, Kraft lignin) has the advantage of maintaining the sulfidation state of the catalyst longer than the use of sulfur-free lignins (e.g, hydrolysis lignin).The kinetics for deoxygenation of oxygenates presented in biomass-derived oils have been studied using model compounds and real feedstocks in the currently reported literature. The subject remains important as it allows a better understanding of the reaction mechanisms, and the function of catalysts and it facilitates the upscaling of the reaction process. The approach for kinetic modeling studies, for lab-scale level research, involves the construction of mathematical expressions for the mass and heat transfer phenomena, in some cases phase equilibrium, and further develops the kinetic model based on a lumped or molecular-based approach depending on the complexity of the feedstocks. The following section discusses the kinetics of HDO reactions for triglycerides, phenolics, lignin, and biomass-derived oils over metal sulfides.A kinetic study based on a sulfided CoMo/Al2O3 catalyst for the hydrotreatment of a mixture of 10\u2009wt% cottonseed oil with desulphurized diesel to produce renewable diesel has been reported by Sebos et al. [78] A plug flow approximation for the reactor was considered to study the kinetics of the HDO of the triglycerides of the feedstock. Overall kinetics were presented including the influence of internal mass transfer resistance. The first-order reaction kinetics was presented with the reaction rate constant (\n\n\n\nk\n\n\nHDO\n\n\n)\n\ncalculated as:\n\n\n\n\n\nk\n\n\nHDO\n\n\n=\n\u2212\nln\n\n\n\n1\n\u2212\nx\n\n\n\n\u2219\n\n\nm\n\n\u0307\n\n/\n\n\nm\n\n\ncat\n\n\n\n\n\nwhere \nx\n is the conversion, \n\n\nm\n\n\u0307\n\n is the mass feed (a mixture of 10\u2009wt% of refined cottonseed oil in desulfurized diesel (S < 50\u2009ppm)) and \n\n\nm\n\n\ncat\n\n\n is the mass of the catalyst. The experimental conversion values were plotted with the one proposed by the model to estimate the goodness of the model as shown in \nFig. 21A).A pseudo-first-order lump-type kinetic model was developed by Sharma et al. to study the deoxygenation of triglycerides (jatropha oil) over mesoporous titanosilicate (MTS) supported sulfided CoMo catalyst to determine the triglyceride conversion pathway at 300\u2009\u2103 and 320\u2009\u2103 [167]. The best-fitted model showed that the triglycerides were converted not only to deoxygenated (C15 \u2013 C18) and oligomerized (> C18) products but also were directly cracked to lighter (< C9) and middle (C9 \u2013 C14) range hydrocarbons (as shown in Fig. 21B). Among all, the oligomerized product formation rate is the highest at these temperatures from the triglycerides (\nTable 7).A recent study on the reaction kinetics based on the hydrodeoxygenation of stearic acid (SA) was reported by Arora et al. over a sulfided NiMo/\u03b3-Al2O3 catalyst [59]. A Langmuir-Hinshelwood (LH) type kinetic model was developed that showed good agreement with the experimental variation of selectivities with different reaction conditions. In their work, a simplified pathway for HDO of SA is shown (\nScheme 2) and the LH type rate expressions used are presented in \nTable 8. The reaction scheme includes intermediates like octadecanal (C18\u2009=O) and octadecanol (C18-OH) and explains the selectivity for the three major reaction routes (decarboxylation, decarbonylation, and direct-HDO). A single catalytic reaction site was considered here and hence an SA inhibition term is included in all the rate expressions. The presented results show that the model can predict well the increase in the conversion rate of stearic acid with the increase in temperature. The key in their work was that a phase equilibrium model was used to predict the saturation concentration of H2 in the liquid phase which depends on the reaction temperature. It was assumed in their study that the gas-liquid transport was fast compared to the rate of reaction and hence allowed the activation energies for the reactions to be predicted. The model could also predict well the conversion and selectivity with variations in residence time, pressure, and feed concentration [59].Ho\u010devar et al. explored the kinetics of HDO using model compounds involving primary/secondary alcohol (1-hexanol), aldehyde (hexanal), methyl ester (methyl hexanoate), ether (dihexyl ether), carboxylic acid (hexanoic acid) [168]. Based on the kinetics constants and activation energies obtained from mathematical modeling and DFT calculation, it was observed that the primary alcohol is more resistant to HDO which undergoes a dehydration reaction to form ethers at the studied conditions. The secondary alcohol follows the typical path like that one highlighted in Scheme 2. Interestingly, experiments with a high initial concentration of aldehyde (hexanal) led to a parallel aldol condensation reaction (C-C coupling to C12 hydrocarbon) in addition to the deoxygenation reaction (C6). However, the products of the aldol condensation reaction were not noticed for HDO of any other model compounds owing to its very low concentration and high reactivity for hydrogenation [168].The kinetic analysis of the HDO of phenolic compounds dates back to 1987 when Gevert et al. [169] studied the reaction kinetics of 4-methylphenol HDO over a sulfided CoMo/Al2O3 catalyst and concluded that the HDO reaction of methyl-substituted phenols can occur through two independent pathways, one forming aromatic products (by direct deoxygenation of phenol, DDO) and the other naphthenic products (ring hydrogenation followed by deoxygenation of saturated or partially saturated phenols to cycloalkanes, HYD). No oxygen-containing products were detected. The rate-limiting step in path 1 has been considered as the C-O bond cleavage and that in path 2 as the hydrogenation of phenol\u2019s aromatic ring [169].The reaction network is shown in \nScheme 3\u2009A) where (a) represents 4-methylphenol, (b) is toluene, and (c) represents methylcyclohexane plus methylcyclohexene. The authors also calculated that the adsorption constant for path 1 is twice as large as that of path 2. This indicated that the two reaction paths proceed on two different types of active sites. The poisoning effect of H2S on such HDO reactions was also studied which showed that H2S strongly suppresses toluene formation from path 1 whereas it hardly affects path 2, which supports their hypothesis that the two paths occur on two different catalytic active sites [169]. However, it was worth highlighting that authors tend to explain the deoxygenation pathways in different ways. For instance, in the study conducted by Wang et al. [112], they considered that the adsorption scheme of p-cresol on the sulfided catalyst surface differed between the deoxygenation paths.Later the effects of methyl substituents of methyl-substituted phenols on the HDO reaction over a sulfided CoMo/Al2O3 catalyst were studied by Simons and co-workers [97]. They also investigated the relationship between the relative reactivities of the methyl-substituted phenol species and the intrinsic properties of the reactant determined from electronic structure calculations. These properties include the electrostatic potential in the reactant molecules and also the electron-binding energies of various molecular orbitals. The reaction data were analyzed using Langmuir-Hinshelwood (LH) kinetics to determine the adsorption and rate constants leading to the two independent aromatic and cyclohexane paths.The following LH equations were considered here:\n\n(1)\n\n\n\n\n\u2202\nA\n\n\n\u2202\n\u03c4\n\n\n=\n\u2212\n\n\n\n\nk\n\n\n1\n\n\n\n\nK\n\n\nA\n\n\nA\n+\n\n\nk\n\n\n2\n\n\n\n\nK\n\n\nA\n\n\nA\n\n\n\n\n\n\n1\n+\n\n\nC\n\n\n0\n\n\n\n\nK\n\n\nA\n\n\nA\n\n\n\n\nn\n\n\n\n\n\n\n\n\n\n\n(2)\n\n\n\n\n\u2202\nB\n\n\n\u2202\n\u03c4\n\n\n=\n\n\n\nk\n1\n\n\nK\nA\n\nA\n\n\n\n\n\n1\n+\n\nC\n0\n\n\nK\nA\n\nA\n\n\n\nn\n\n\n\n\n\n\n\n\n(3)\n\n\n\n\n\u2202\nC\n\n\n\u2202\n\u03c4\n\n\n=\n\n\n\nk\n2\n\n\nK\nA\n\nA\n\n\n\n\n\n1\n+\n\nC\n0\n\n\nK\nA\n\nA\n\n\n\nn\n\n\n\n\n\nwhere the mole fractions of the methyl-substituted phenolic feed (A), the aromatic benzene (B), and the cyclohexane and cyclohexene (C) formed from two different pathways as discussed above [169].\n\n\n\nK\n\n\nA\n\n\n=\u2009equilibrium constant when A is adsorbed on the catalyst surface.\n\n\n\nk\n\n\n1\n\n\n=\u2009rate constant for the formation of the aromatic product (DDO pathway).\n\n\n\nk\n\n\n2\n\n\n=\u2009rate constant for the formation of hydrocarbon products (HYD pathway).\n\n\n\nC\n\n\n0\n\n\n=\u2009feed concentration of A.\n\n\u03c4\n=\u2009space-time variable and \nn\n =\u2009order of inhibition.The kinetic analysis of their work showed that the optimal n parameter in Eqs. (1), (2), and (3) was n\u2009=\u20092, which resulted in the best fit of the data. This result has been interpreted as the reactions for both pathways involving an adsorbed species and an active site. The assumption of having only one adsorption site, \n\n\nK\n\n\nA\n\n\n was also examined by applying two separate adsorption constants, \n\n\nK\n\n\nB\n\n\n and \n\n\nK\n\n\nC\n\n\n, for each reaction with modified Eqs. (1), (2), and (3). Their results from the regression analysis demonstrated that both \n\n\nK\n\n\nB\n\n\n and \n\n\nK\n\n\nC\n\n\n constants were identical and within the experimental errors, which is the equivalent to a single value (\n\n\nK\n\n\nA\n\n\n). Hence, a single catalytic site was considered to be present for both reaction pathways due to the same adsorption constant being calculated.\n\nFig. 22A (a) shows the variation of the adsorption constant depending on the location of the methyl groups. The \n\n\nk\n\n\n1\n\n\n rate constant leading to the aromatic products is the lowest for phenols and highest for 3,5-DMP (Fig. 22A (b)). The \n\n\nk\n\n\n2\n\n\n rate constant for the aromatic ring hydrogenation path shows a different trend from \n\n\nk\n\n\n1\n\n\n. \n\n\nk\n\n\n2\n\n\n seems to drop significantly with the methyl group in position 2 (Fig. 22A (c)). Overall, from Fig. 22 the authors showed significant variations in the adsorption and rate constants as the location and number of the substituent methyl groups varied [97]. A correlation between the derived adsorption, rate constants, and molecular parameters is also studied in this work.A kinetic study of guaiacol (GUA) conversion over a ReS2/SiO2 catalyst using an LH kinetic model was studied by Leiva et al. [170]. They observed two different kinds of active sites for the guaiacol conversion over ReS2/SiO2 and dissociative adsorption of hydrogen was considered. The two active sites are the metal ion vacancy (M) and the sulfur stable ion (X2- \u23bcM). The rate-determining step considered here was the H+ attack on the oxygen of the methoxy group. The reactions considered for the model are as follows:Adsorption of GUA:\n\n(4)\n\n\nGUA\n+\nM\n\u2194\nGUA\n\u2212\nM\nEquilibrium\n\n\nK\n\n\nGUA\n\n\n\n\n\n\nHydrogen dissociative adsorption:\n\n(5)\n\n\n\n\nH\n\n\n2\n\n\n+\nM\n+\n\n\nX\n\n\n2\n\u2212\n\n\n\u2212\nM\n\u2194\n\n\nH\n\n\n\u2212\n\n\n\u2212\nM\n+\n\n\nH\n\n\n+\n\n\n\u2212\n\n\nX\n\n\n2\n\u2212\n\n\nM\nEquilibrium\n\n\nK\n\n\nH\n2\n\n\n\n\n\n\nAddition of H\u23bc:\n\n(6)\n\n\nGUA\n\u2212\nM\n+\n\n\nH\n\n\n\u2212\n\n\n\u2212\nM\n\u2194\n\n\nGUAH\n\n\n\u2212\n\n\n\u2212\nM\n+\nM\nEquilibrium\n\n\nK\n\n\n3\n\n\n\n\n\n\nAddition of H+:\n\n(7)\n\n\n\n\nGUAH\n\n\n\u2212\n\n\n\u2212\nM\n+\n\n\nH\n\n\n+\n\n\n\u2212\n\n\nX\n\n\n2\n\u2212\n\n\n\u2212\nM\n\u2192\n\nPh\n\n+\n\nMeOH\n\n+\n2\nM\n\n\nRate\n\n\n\nconstant\n\n\n\n\nK\n\n\n4\n\n\n\n\n\n\nThe rate expression used here is:\n\n(8)\n\n\n\n\n1\n\n\n\n\nr\n\n\nGUA\n\n\n\n\n=\n\n\n1\n\n\n\n\nk\n\n\nGUA\n\n\n\n\nC\n\n\nH\n2\n\n\n\n\n+\n\n\n1\n\n\n\n\nk\n\n\nGUA\n\n\n\n\nK\n\n\nGUA\n\n\n\n\nC\n\n\nH\n2\n\n\n\n\nC\n\n\nGUA\n\n\n\n\n\n\n\n\n\nFig. 22B clearly shows the goodness of fit of the model (Eq. (8)) which follows a linear behavior. This result indicates the presence of two different active sites on the sulfided catalyst. The authors also showed that this kinetic model did not fit well the catalytic activity of ReOx/SiO2 (shown in the inset in Fig. 22B) suggesting that ReOx/SiO2 followed a different pathway for the conversion of GUA [170].The HDO of cresol isomers over sulfided Mo/Al2O3 and CoMo/Al2O3 was investigated by Gon\u00e7alves et al. [171]. They reported that over both catalysts the reactivity of cresols follows the order: m-cresol >\u2009p-cresol >\u2009o-cresol. The kinetic analysis of the HDO of m-cresol was studied in this work and the role of cobalt on the HDO of three cresols was investigated [171].\nScheme 3B shows that the HDO of cresols follows two independent pathways. The desired direct deoxygenation (DDO) pathway forms toluene (TOL) (strongly promoted by Co), whereas, the hydrogenation (HYD) route forms methylcyclohexene (MCHe) and methylcyclohexane (MCH) (not affected by Co). A pseudo-first-order reaction kinetic model was assumed to apply for the reactions. \n\n\nk\n\n\nDDO\n\n\n and \n\n\nk\n\n\nHYD\n\n\n are the kinetic rate constants for the DDO and the HYD pathways respectively, \n\n\nk\n\n\nHYD\n\n\n\u2032\n\n\n is the intrinsic rate constant for the hydrogenation of methylcyclohexene to methylcyclohexane, \n\n\nC\n\n\nTOL\n\n\n, \n\n\nC\n\n\nMCHe\n\n\n and \n\n\nC\n\n\nMCH\n\n\n represent the molar concentrations of toluene, methylcyclohexenes, and methylcyclohexane [171].\n\n(9)\n\n\n\n\nC\n\n\nCRE\n\n\n=\n\n\nC\n\n\nCRE\n,\n0\n\n\n.\n\n\ne\n\n\n\n\n\u2212\nk\n\n\nHDO\n\n\n.\n\u03c4\n\n\n\n\n\n\n\n\n(10)\n\n\n\nC\n\nTO\nL\n\n\n=\n\n\n\nC\n\nCRE\n,\n0\n\n\n.\n\nk\nDDO\n\n\n\nk\nHDO\n\n\n\n\n1\n\u2212\n\n\ne\n\n\n\u2212\n\nk\nHDO\n\n.\n\u03c4\n\n\n\n\n\n\n\n\n\n\n(11)\n\n\n\n\nC\n\n\nMCHe\n\n\n=\n\n\n\n\nC\n\n\nCRE\n,\n0\n\n\n.\n\n\nk\n\n\nHYD\n\n\n\n\n\n\nk\n\n\nHDO\n\n\n\u2212\n\n\nk\n\n\nHYD\n\n\n\u2032\n\n\n\n\n\n\n\n\n\ne\n\n\n\n\n\n\n\u2212\n\n\nk\n\n\n\n\nHYD\n\n\n\u2032\n\n\n.\n\u03c4\n\n\n\u2212\n\n\ne\n\n\n\n\n\u2212\nk\n\n\nHDO\n\n\n.\n\u03c4\n\n\n\n\n\n\n\n\n\n\n\n(12)\n\n\n\n\nC\n\n\nMCH\n\n\n=\n\n\nC\n\n\nCRE\n,\n0\n\n\n\n\n\n1\n\u2212\n\n\ne\n\n\n\n\n\u2212\nk\n\n\nHDO\n\n\n.\n\u03c4\n\n\n\n\n\n\u2212\n\n\n\n\nk\n\n\nDDO\n\n\n\n\n\n\nk\n\n\nHDO\n\n\n\n\n\n\n\n1\n\u2212\n\n\ne\n\n\n\n\n\u2212\nk\n\n\nHDO\n\n\n.\n\u03c4\n\n\n\n\n\n\u2212\n\n\n\n\nk\n\n\nHYD\n\n\n\n\n\n\nk\n\n\nHDO\n\n\n\u2212\n\n\nk\n\n\nHYD\n\n\n\u2032\n\n\n\n\n\n\n\n\n\ne\n\n\n\n\n\n\n\u2212\n\n\nk\n\n\n\n\nHYD\n\n\n\u2032\n\n\n.\n\u03c4\n\n\n\u2212\n\n\ne\n\n\n\n\n\u2212\nk\n\n\nHDO\n\n\n.\n\u03c4\n\n\n\n\n\n\n\n\n\nThe selectivity of each product i (in mol%) can be calculated from Eqs. (10)\u2013(12) as:\n\n(13)\n\n\n\n\nS\n\n\ni\n\n\n=\n\n\n\n\nC\n\n\ni\n\n\n\n\n\n\nC\n\n\nCRE\n,\n0\n\n\n\u2212\n\n\nC\n\n\nCRE\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\nFig. 23 shows that the experimental data points and those predicted by the model fit well for the selectivity of products as a function of the conversion of m-cresol validating the model. It has been shown from the values of the rate constants that the hydrogenation of methylcyclohexene to methylcyclohexane was 2.9 times higher over CoMo/Al2O3 when compared to Mo/Al2O3, showing that Co acts as a promoting agent improving the hydrogenating properties of molybdenum sulfide [171].Further, the promotional effect of isolated Co atoms decorated on monolayer MoS2 sheets (sMoS2) was studied by Liu et al. [172]. A kinetic study was developed for the conversion of 4-methylphenol to toluene and it was demonstrated that cobalt immobilization on the MoS2 monolayer (Co- sMoS2) showed a 34 times higher rate (396.4\u2009ml\u2009s\u22121molMo\n\u22121) when compared to non-promoted sMoS2 (11.7\u2009ml\u2009s\u22121molMo\n\u22121) at 30\u2009bar and 300\u2009\u00b0C (\nFig. 24a). The activity order shows: Co- sMoS2 >\u2009sMoS2 >\u2009FMoS2 >\u2009bulk MoS2. FMoS2 stands for few-layer MoS2. It was also shown that the incorporation of single Co atoms on the basal planes of sMoS2 facilitates the formation of more basal sulfur vacancy sites during hydrogen activation at 300\u2009\u00b0C that enhances the activity of the Co-doped monolayer MoS2 for the HDO of 4-methylphenol. The rate of the HDO reaction was calculated considering a pseudo-first-order reaction (Eq. (14)) [172]:\n\n(14)\n\n\nln\n\n\n\n\n1\n\u2212\nx\n\n\n\n=\n\u2212\nk\n\n\nC\n\n\ncat\n\n\n\nt\n\n\n\n\n\n\nk\n =\u2009pseudo first order reaction constant (ml s\u22121 mol\u22121).\n\nx\n =\u2009conversion of 4-methylphenol.\n\n\n\nC\n\n\ncat\n\n\n =\u2009concentration of catalyst under reaction system.\n\nt\n =\u2009reaction time (s).Recent work by Cheah et al. investigated the role of transition metals (Ni, Cu, Zn, Fe) on \u03b3-Al2O3 supported MoS2 for the HDO of propylguaiacol (PG) [173]. In this work, the authors developed a kinetic model considering the reaction network to elucidate the reaction pathway of demethoxylation and dihydroxylation of PG. The experimental results were nicely fitted to the model, thus validating the model. Initially, the authors developed a simplified pseudo-first-order kinetic model to fit the kinetic data for the HDO of PG considering the route: A =\u20094-propylguaiacol \u2192 B =\u20094-propylphenol \u2192 C =\u2009propylbenzene \u2192 D =\u2009propylcyclohexane as shown in Fig. 24b). However, this simple kinetic model could not fit the experimental results well as shown in Fig. 24b), because it did not take into consideration any of the side reactions. Thereafter, another model was proposed taking into consideration all the main side reactions, including intermediates and reactants. The fit for the reaction kinetics for all the catalysts improved with this modified model with over a 90% coefficient of determination [173]. More importantly, their work also provided a means of evaluating how the promoters (Ni, Fe, Zn, and Cu) for MoS2/Al2O3 influenced the product selectivity for different pathways and eventually the products of the reaction with the aid of the kinetic model. Their results suggested that Ni is a promoter for the Mo catalyst while doping metals such as Fe, Zn, and Cu acted as inhibitors for the formation of deoxygenated cycloalkanes. On the other hand, both Zn and Fe had a negative impact on the HDO activity for PG but changed the selectivity towards aromatics like propylbenzene at full conversion.Due to the complex nature of lignin, a typical hydrotreatment experiment produces a broad spectrum of especially liquid phase products. The kinetic modeling for the hydrotreatment of a feedstock like lignin can be important to understand the complex series of reactions involved in the transformation of lignin, how they are influenced by operating conditions and catalyst properties, and eventually aid in an effective scale-up of a lignin hydrotreatment process. Pu et al. developed a kinetic model for lignin hydrotreatment over a CoMoS-supported catalyst in a semi-batch reactor using a lumped approach [174]. There were three main lumps divided further into product groups: lignin oligomeric residues (solid): (i) THF-insolubles, THF-solubles, and solubilized oligomers; (ii) liquid product lumps: dimethoxyhenols, methoxyphenols, alkylphenols, catechols, alkanes ( < C13), alkanes ( \u2265 C13), aromatics, naphthenes, and H2O; (iii) gas products lumps: CO2, CO, CH4 and C2-C6 (light hydrocarbon) with \n\n\nv\n\n\ni\n\n\nj\n\n\n, the overall stoichiometric coefficient for component i in reaction j (\nScheme 4). The model accounted for gas hydrodynamics which was characterized by Residence Time Distributions (RTD), liquid-gas mass transfer resistance, and vapor-liquid equilibrium effects with reactions (1) to (10) from Scheme 4. This resulted in a model that fitted well with the obtained experimental data [174]. The model results were able to well describe the main overall lignin depolymerization reactions and further deoxygenation reactions of phenolic monomers in the liquid phase [174].Reaction (1): THF-insolubles \n\n\u2192\n\nk\n1\n\n\n\n\n\n\nv\n\n\nTSB\n\n\n1\n\n\n\u00b7THF-solublesB.Reaction (2): THF-solublesA +\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\n\n\n2\n\n\n\u00b7H2\n\n\n\u2192\n\nK\n2\n\n\n\n\n\n\nv\n\n\nTSB\n\n\n2\n\n\n\u00b7THF-solublesB +\u2009\n\n\nv\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n2\n\n\n\u00b7CH4 +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n2\n\n\n\u00b7H2O +\u2009\n\nv\n\n\nC\n2\n\n\nC\n6\n\n\n2\n\n\u00b7C2 \u2013 C6.Reaction (3): THF solublesB +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\n\n\n3\n\n\n\u00b7H2\n\n\n\u2192\n\nK\n3\n\n\n\n\n\nv\nSO\n3\n\n\u00b7Solubilized oligomers +\u2009\n\n\nv\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n3\n\n\n\u00b7CH4 +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n3\n\n\n\u00b7H2O +\u2009\n\nv\n\n\nC\n2\n\n\nC\n6\n\n\n3\n\n\u00b7C2 \u2013 C6 +\u2009\n\n\nv\n\n\nAP\n\n\n3\n\n\n\u00b7Alkylphenols +\u2009\n\n\nv\n\n\nAK\n1\n\n\n3\n\n\n\u00b7Alkanes (< C13) +\u2009\n\n\nv\n\n\nAK\n2\n\n\n3\n\n\n\u00b7Alkanes (\u2265 C13).Reaction (4): Solubilized oligomers \n\n\u2192\n\nK\n4\n\n\n\n\n\n\nv\n\n\nAP\n\n\n4\n\n\n\u00b7Alkylphenols.Reaction (5): Dimethoxyphenols +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\n\n\n5\n\n\n\u00b7H2\n\n\n\u2192\n\nK\n5\n\n\n Alkylphenols +\u20092\u00b7H2O +\u20092\u00b7CH4.Reaction (6): Methoxyphenols +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\n\n\n6\n\n\n\u00b7H2\n\n\n\u2192\n\nK\n6\n\n\n Alkylphenols +\u2009H2O +\u2009CH4.Reaction (7): Methoxyphenols +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\n\n\n7\n\n\n\u00b7H2\n\n\n\u2192\n\nK\n7\n\n\n Catechols +\u2009CH4.Reaction (8): Catechols +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\n\n\n8\n\n\n\u00b7H2\n\n\n\u2192\n\nK\n8\n\n\n Alkylphenols +\u2009H2O.Reaction (9): Alkylphenols +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\n\n\n9\n\n\n\u00b7H2\n\n\n\u2192\n\nK\n9\n\n\n Aromatics +\u2009H2O.Reaction (10): Alkylphenols +\u2009\n\n\nv\n\n\n\n\nH\n\n\n2\n\n\n\n\n10\n\n\n\u00b7H2\n\n\n\u2192\n\nK\n10\n\n\n Naphthenes +\u2009H2O.Grilc et al. screened a series of catalysts covering the commercial NiMo catalysts in sulfided, oxide, and reduced form and other catalysts in the hydrotreatment of a solvolyzed biomass-derived oil [175]. A complex reaction pathway was proposed and followed by constructing a lumped kinetic model based on the quantified functional groups by Fourier transform infrared spectroscopy (FTIR) [175]. Among all the tested catalysts, the commercial sulfided NiMo catalyst was found to be suitable for yielding bio-oils with high gross calorific value. The authors also discovered that the unsupported bulk MoS2 resulted in high HDO activity and selectivity which is worth further investigation [175]. The same authors then extended their work by comparing the selectivity and activity of several synthesized and commercial unsupported Mo catalysts in oxide, carbide, and sulfide form, and also unsupported WS2 nanotubes [176]. A similar lumped kinetic model was developed giving apparent kinetic constants that correspond to main reactions like hydrodeoxygenation (k1), decarboxylation (k4), decarbonylation (k3), dehydrogenation (k2), and hydrocracking of a solvolytic oil as shown in \nScheme 5a) [176]. One of the observations was that the urchin-like MoS2 possessed the highest k1 value among all other unsupported sulfided catalysts which corresponds to the removal of the hydroxyl group in the form of water. While the decarbonylation, decarboxylation, and hydrocracking reactions occurred to a lesser extent using the unsupported materials which could be explained by the absence of the use of an acidic catalyst support that can cleave the C-C linkages [176].Grilc et al. also studied the simultaneous liquefaction and hydrotreatment of biomass (Sawdust samples like beech, fir, and oak) over a sulfided catalyst (NiMo on alumina), a reduced Pd on alumina catalyst, and zeolite Y. Emphasis was placed on studying the effect of different process parameters like time, pressure, temperature, wood, and solvent type. The yield and product composition from the simultaneous reactions were identified and correlated to a lumped kinetic model accounting for liquefaction, decarboxylation, decarbonylation, HDO, and char formation reactions as shown in Scheme 5a). Their modeling results showed that reaction temperature played an important role in the liquefaction and HDO of biomass. The increase in reaction temperature from 300\u00b0 to 350\u00b0C resulted in a 2.5-fold higher yield for HDO products, while the solid residue yield decreased by 39%. However, when the reaction temperature is increased over 350\u2009\u00b0C, a lower oil yield was achieved which was mainly attributed to an increased formation of char [177]. Also, in their work, sulfided NiMo on alumina was found to achieve higher oil yield as compared to the noble metal Pd on alumina.Common pathways for catalyst deactivation include poisoning, coking, sintering, fouling/physical blockage, leaching or vapor formation, and solid-state transformations [178]. Based on the literature, it can be deduced that metal sulfide catalysts used for upgrading renewable feedstocks can lose activity due to loss of sulfur, impurities present in renewables, evolved products, coking, and sintering of the active phase [48,179\u2013182].Since bio-based renewables have high oxygen contents, often sulfides catalyst loses sulfur through a sulfur-oxygen exchange during HDO. Such an exchange more easily takes place on sulfur edges over unpromoted MoS2 than the promoted sites (in the case of CoMoS) in the presence of a high H2O partial pressure as observed via combined CO adsorption and IR studies during HDO of 2-ethyl phenol [179]. The authors also demonstrated via DFT that in the presence of a large amount of water the exchange is stronger and irreversible over MoS2 while Co promotion makes the catalyst more water tolerant and enables the poisoning to become more reversible. Hence, a continuous supply of sulfiding agents (e.g., H2S) in the feed at a sufficiently low concentration can restore the catalytic activity, while an inhibition can occur at a higher H2S concentration [183]. Resulfidation of the catalyst can also restore its initial activity as has been demonstrated for a sulfided CoMo/Al2O3 catalyst while hydroprocessing 2-hydroxydiphenylmethane (250\u2009\u00b0C, 155\u2009bar, WHSV = 0.49\u2009h\u22121) [184]. It is often criticized that the addition of such agents will however contaminate the product oils. Since TMS catalysts, e.g. Ni/Co-promoted Mo/W sulfides are also highly active in hydrodesulfurization, the final product typically contains only traces of sulfur.Bio-oil impurities depend on their source of production and prior pretreatment processes. Typical impurities include alkali, alkaline-earth metals (Na, K, Ca, Mg, etc.), phosphorus, sulfur, and nitrogen. Irreversible K deactivation of NiMoS2/ZrO2 (K impregnated as KNO3 to the catalyst at a K/(Ni+Mo) molar ratio of 1) was attributed to the preferential occupation of edge vacant sites of the promoted MoS2 during HDO of phenol and octanol [180]. Trap grease phospholipids also containing alkali metals were shown to cause severe deactivation of a commercial CoMoS/\u03b3-Al2O3 catalyst via coking and severe pore plugging while upgrading rapeseed oil [185]. Fe was found to preferentially block the Ni-promoted sites in NiMoS/\u03b3-Al2O3 while upgrading fatty acids [48]. Bio-oil phospholipids having phosphate and choline moieties have also been shown to lower the activity of NiMoS/\u03b3-Al2O3 during HDO of oleic acid [49]. Nitrogen-bearing compounds (e.g. amines, pyridines, quinolines, etc.) have been shown to cause the deactivation of TMS catalysts [186\u2013188].The presence of water in bio-oil or produced during HDO may influence the activity of TMS-based catalysts via oxidation of the sulfide phase or deterioration of the structure of the active phase [65]. Couman and Hensen et al. [52] reported that water had little influence on sulfided NiMo/ \u03b3-Al2O3 below a concentration of 5000\u2009ppm during HDO of fatty esters. Krause and co-workers reported that the inhibition effect of water can be compensated for by H2S during HDO of an aliphatic ester (methyl heptanoate) [65]. The decarboxylation route has been reported to be affected by H2O perhaps via keto-enol isomerization during HDO of fatty esters [52]. Support material used can also be affected by the water e.g. \u03b3-Al2O3 reportedly transforms to its boehmite phases or poisons the acidic sites [179]. The interaction of water with active sites may also lead to the formation of an inactive sulfate layer [179]. However, water may affect the HDO of phenolic compounds over metal sulfides to a varying extent [64,179]. The presence of a small amount of water (water/p-cresol molar ratio < 1) was found to increase the direct-HDO route of p-cresol to toluene while a high amount of water (water/p-cresol molar ratio > 1) significantly reduces the deoxygenation rate and toluene selectivity due to preferential occupancy of active sites [124]. Additionally, CO formed during HDO under reduction conditions can strongly inhibit the direct-HDO pathways owing to the lower adsorption energy of CO over metal sulfides (e.g., MoS2, CoMoS) [183,189].Coking is one of the leading challenges to deal with for hydrotreating catalysis. TMS-based HDO catalysts may deactivate via carbonaceous deposits (reactive/soft or refractory/hard) formed either by physical or chemisorbed processes [190]. Such deposits arise from the undesired side reactions (e.g. cracking, aromatization, dehydrogenation, cyclization, condensation, etc.) involving adsorbed species/precursor molecules (alkenes, aromatics, oxygenates, etc.). Condensation and rearrangement reactions typically play a major role in low-temperature coke deposition (<200\u2009\u00b0C) while dehydrogenation and hydrogen transfer reactions lead to the formation of polyaromatic hydrocarbons (>350\u2009\u00b0C) [191]. On the other hand, carbon species can be bonded to the active site via weak interaction [192] or can partially replace sulfur atoms at the edge sites of MoS2 in the form of a Mo-S-C bond in a MoS2\u2212xCx phase [193]. The latter can be synthesized via the thermal treatment of MoS2 with a mixture of dimethyldisulfide in N2/H2 which is reported to stabilize the MoS2 crystallites with a smaller size (i.e., carbon species restricts the crystal growth) with lower stacking, thus enhancing the activity instead [194]. In addition, metal carbides (Co-C and Mo-C bonds) may form which may be observed by EXAFS analysis [192].Catalyst/support combination is another critical parameter in determining the activity, selectivity, and coke formation. Unsupported TMS like MoS2 may undergo agglomeration [181,182] under reaction conditions which can be prevented in the presence of hydrogen and hydrocarbon feed [193]. It is important to note that sintering or agglomeration may occur through Ostwald ripening or coalescence to obtain higher thermodynamic stability which lowers the surface area and catalytic activity [195]. Segregation of the promoter (e.g. Ni/Co) [196,197] as sulfides (Co9S8/Ni3S2) may influence the catalyst deactivation [182]. Acidity/basicity of the active phase or support materials can also play an important role [198] in aiding coke-forming reactions. Both Lewis acid sites (LAS) have a high affinity for basic precursors and Br\u00f8nsted acid sites (BAS) via proton donation and carbonium cation formation may contribute to coke formation. BAS, in addition, promotes coupling/isomerization reactions [199]. The deactivation of MoS2 by phenolic compounds is indirect. The basicity of the phenolic compounds and their substituent nature gives different adsorption mechanisms as in \nScheme 6a). Due to their basicity, they adsorb as phenolates on the acidic alumina support. If they are adsorbed closely to the MoS2, they could block the accessibility of other reactants for deoxygenation to the MoS2 active site, resulting in the deactivation of the whole catalyst for deoxygenation reactions [200]. In addition, the oxygenates that adsorbed on the sulfide phase reduce the active phases and cause catalyst poisoning [201]. Thermal instability and repolymerization reactions of phenolic compounds in bio-oil derived from fast pyrolysis and lignin lead to premature deactivation of the catalyst [202]. Recently, Kraft lignin impurities (Na, K, etc.) have been demonstrated to deactivate a sulfided NiMoS/Al2O3 catalyst especially at high loading as discussed above in Section 4.4.2\n[7].With a representative model compound, 2-hydroxydiphenylmethane, and using a sulfided CoMo/Al2O3 catalyst, a decline in the catalytic activity was noticed after 20\u2009h of reaction (250\u2009\u00b0C, 155\u2009bar, WHSV = 0.49\u2009h\u22121). But, a simple re-sulfidation of the catalyst restored its initial activity. The mode of deactivation of the CoMo/Al2O3 catalyst could be ascribed due to the loss of sulfur. Another interesting aspect of the CoMo/Al2O3 catalyst was revealed when 4-methyl guaiacol was used as the model compound. The char formation from this experiment was around 40\u2009wt%, far less than the non-catalytic thermal reaction indicating that the CoMo/Al2O3 catalyst had generated products with less char forming tendency. This observation is somewhat contrary to the previous reports where more char was formed in the presence of the catalyst [153]. However, the role of temperature/pressure in char formation could not be neglected (vide supra). The deactivation of TMS catalysts while upgrading lignin/lignin-derived phenolic compounds has been discussed in Section 4\n.\nApart from the experimental approaches described in the previous section, computational approaches based on density functional theory (DFT) calculation, can also provide a better insight into the physiochemical properties of the sulfided-based catalytic materials and their relationship with their observed reactivity in deoxygenation. The deactivation mode of the metal sulfides can also be understood through DFT calculation. For instance, recent work by Liu et al. investigated the detailed mechanism of the in situ and ex situ substitution of sulfur atoms in the active phase (Co(Ni)MoS edge) by oxygen atoms under the presence of water via the DFT [203]. The calculation was also performed for the energy change, Gibbs free energy evolution, and reaction coordination to gain an understanding of the water deactivation of Co(Ni)MoS under HDO conditions [203]. It was outlined in their work that, for in situ oxygen substitution, the oxygen atoms from water molecule occupied the positions of the edge sulfur atoms; while for the ex situ substitution, the water molecules occupied directly the unsaturated sites after the desorption of edge sulfur as hydrogen sulfide [203]. It was found that the substitution of sulfur atoms depends on the ratio of partial pressure between hydrogen and water, and also between hydrogen and hydrogen sulfide. Moreover, it was found that the CoMoS is more prone to water deactivation than NiMoS, and when compared to MoS2, both promoted Co(Ni)MoS showed better water resistance. Another recent study by Diao et al. used DFT calculations to demonstrate and validate that the Co-doped MoS2 and Mo-doped Co9S8 enhanced the vertical adsorption of oxygenates which further undergoes C-O cleavage of diphenyl ether (DPE) and were also able to avoid benzene ring hydrogenation [204]. It was further shown that the Mo-doped Co9S8 surface promoted the adsorption and C-O bond activation in DPE. Besides, there are also excellent reviews on the summary and studies of the application of the computational approach to study other catalyst systems such as transition metal phosphides (TMP) [205], transition metal catalysts [206,207], and transition metal sulfides (TMS) [208]. With this in mind, there is still a need to engage in theoretical studies that can disclose the reaction mechanisms in the complex catalytic reaction and strive to improve the existing catalyst systems, eventually aiding the selection and implementation of these catalysts into the complex refinery.Recently, the European Commission (EC) has published its ambitious legislative package \u2018Fit for 55 targeting a 55% reduction in GHG emissions by 2030 compared to the 1990 level [209]. The presented package provides tools to tackle the climate crisis, and different solutions taking sustainability into account must be endorsed and applied. Advanced biofuels play a key role in decarbonizing the transport sector. Therefore, many research efforts have been dedicated to the development of heterogeneous catalysts for application in advanced biofuel production in the past few decades. Metal sulfides remain the core catalysts in hydroprocessing industries as they are effective in the removal of heteroatoms such as sulfur, nitrogen, oxygen, halides, and metals. This review has emphasized the use of hydrotreating catalysts, metal sulfides in the valorization of triglyceride feeds, oxygenates in monomer and dimeric form, biomass-derived pyrolysis oil, and lignin feed. Besides the type of catalysts, the catalytic performance and progression of reactions during hydroprocessing also depend largely on the reaction parameters like reactor type, reaction temperature, pressure, residence time, and solvent system. These aspects have been discussed in this work.The major challenges associated with the hydroprocessing of various renewable feedstocks and the possible future research and development in respective areas are listed in \nTable 9. Numerous research papers report the use of alumina as a catalyst support for hydroprocessing catalysts due to its good textural and mechanical properties, and low relative cost [210,211]. The acidic nature of alumina is found to be beneficial in breaking the C-O bond in anisole which is also found in the lignin structure [210]. The sulfur vacancies located at the edges of the metal sulfides act as unsaturated sites and Lewis acids sites and are found to be active in cleaving C-O linkages\u00a0[44]. Other supports like silica and activated carbon were also used as supports for NiMo hydrotreating catalysts and studied in vacuum residue hydrotreating reactions [211]. Carbon as catalyst support has also gained attention owing to its high surface area, inert nature, high thermal stability, and low cost [212]. An important conclusion made is that the effectiveness of the hydrotreating catalysts depends on the pore size diameter, pore-volume, and metal dispersion that ultimately improves the efficiency of hydroconversion. As discussed in this work, the unsupported or self-supported metal sulfides have also gained interest because of their higher activity per catalyst mass as compared to the supported sulfide catalysts [208]. In addition, the direct use of the active metal sulfides phase allows the elimination of the transport resistance interference due to the support during the reaction. With this, ExxonMobil and Albemarle Catalysts developed an unsupported catalyst by so-called NEBULA technology that claims to show superior activity as compared to the conventional hydrotreating catalysts [213,214]. Another great example is the Eni Slurry Technology (EST) process which uses highly dispersed MoS2 nanoparticles and has proven the feasibility of using unsupported catalyst materials in hydrotreating [215]. Apart from developing stable, active, and cost-effective metal sulfide catalysts, the issue related to sulfur leaching and replenishment when using sulfided catalysts remains a central research topic. Some studies have been communicated in this regard exploring the benefits of applying sulfiding agents to compensate for sulfur loss during the process [14,62,66]. There is also a need and an interest in the research community in designing metal sulfide catalysts and also understanding the mode of sulfide deactivation aided by DFT tools. A better understanding of the physicochemical properties of metal sulfides aided by the first principles approaches can eventually benefit the tailored synthesis of metal sulfides and improve desired product selectivity. A review by Raybaud in 2007 and extended by others provided insights into the understanding of the sulfide active phases, the localization and role of promoters, electronic properties, and morphological changes influenced by the operating parameters, synthesis methods, or addition of promoter [10,12,216\u2013220].In addition, when dealing with hydrotreating of complex feedstocks like lignin, the diffusion of depolymerized lignin oligomeric fragments into the catalyst pores to access active sites is likely to be limited by pore transport resistance, and therefore, the self-supported sulfide catalysts may be seen as beneficial to gain better active site accessibility. Depolymerized lignin fragments that do not undergo deoxygenation reactions due to the inaccessibility of active sites, may instead repolymerize to form solid residues like char, which is usually undesirable. Studies related to process improvement could also be another way to ensure an efficient hydroconversion of solid lignin. A recent example shows that a modification to a semi-batch reactor operating mode by injecting a lignin slurry into a reactor that has reached the desired reaction temperature can effectively avoid the repolymerization and recondensation reactions resulting in better lignin conversion [221]. Supported and unsupported versions of NiMoS catalysts was also reported effective in this regard [222,223].The hydroprocessing of various renewable feedstocks such as triglycerides, model compounds for bio-oil, and lignin-derived oils are discussed in this work. There are different challenges involved while using these different feedstocks for the scale-up of a hydrotreatment process. One of the common challenges when dealing with bio-feedstocks is the deactivation of the catalyst caused by the presence of inorganic impurities in the feedstocks. These inorganic elements can act as poisons to the catalytic sites, causing a decrease in the catalyst lifetime during the time-on-stream. Future research should focus on understanding the role of these impurities on the catalytic activity of a typical hydrotreating catalyst and also the in-depth deactivation mechanism. For instance, one recent study revealed that low concentrations of impurity elements like Na, K, Ca, and Fe promotes the deoxygenation ability of a NiMoS/Al2O3 catalyst. However, when they are present in higher concentrations, these impurities are deposited on the catalyst, ultimately leading to the poisoning of the catalyst [7]. More of these types of studies should be pursued using different bio-feedstocks as they can be seen as highly relevant for operation in refineries in terms of the stability of the catalysts and also it could possibly provide a way to regenerate, recycle and reuse the catalyst. Issues like catalyst pore plugging due to coking and inorganic impurities present in bio-feed should also be addressed and investigated in the future by studying different guard bed materials like catalysts or adsorbents to improve the catalyst lifetime. Research related to catalytic material development that is more resistant to deactivation and can be easily restored following deactivation is of high interest. In addition to these, the pretreatment of these bio-feedstocks for the removal of inorganic elements that are responsible for the catalyst deactivation is required to better improve the properties of the feedstock. The pretreatments and enhancement methods to improve the quality of the feedstocks is desirable to achieve efficient refining of bio-feedstocks. The removal of nitrogen content in bio-feedstocks also remains an area that is less explored and requires more attention. For instance, pyrolysis oil derived from sewage sludge contains high nitrogen and sulfur-bound polyaromatics compounds which reduces the quality of the product fuels and also generates toxic emissions upon combustion.To sum up, we have comprehensively reviewed the use of industrially-relevant metal sulfide catalysts for the upgrading of biomass feedstocks like triglycerides, monomeric and dimeric phenolic compounds, pyrolysis oil, and waste lignin. Various aspects such as sulfide deactivation, reaction kinetics, and mechanisms have been discussed. The challenges and future research opportunities concerning the efficient upgrading of bio-feedstocks to liquid fuel were explored. Metal sulfides will remain as a core in the processing of renewable feedstocks in existing refinery infrastructures and both the research community and industries play a significant role in realizing future biorefineries.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research is a collaboration work between Chemical Engineering, Competence Centre for Catalysis (KCK) at Chalmers, Preem AB, and RISE Research Institutes of Sweden Energy Technology Center (ETC). The authors would like to acknowledge the Swedish Energy Agency (2017-010890 and 2018-012459) and Preem AB for financial support.", "descript": "\n                  Human activities such as burning fossil fuels for energy production have contributed to the rising global atmospheric CO2 concentration. The search for alternative renewable and sustainable energy sources to replace fossil fuels is crucial to meet the global energy demand. Bio-feedstocks are abundant, carbon-rich, and renewable bioresources that can be transformed into value-added chemicals, biofuels, and biomaterials. The conversion of solid biomass into liquid fuel and their further hydroprocessing over solid catalysts has gained vast interest in industry and academic research in the last few decades. Metal sulfide catalysts, a common type of catalyst being used in the hydroprocessing of fossil feedstocks, have gained great interest due to their low cost, industrial relevance, and easy implementation into the current refining infrastructures. In this review, we aim to provide a comprehensive overview that covers the hydrotreating of various bio-feedstocks like fatty acids, phenolic compounds, pyrolysis oil, and lignin feed using sulfided catalysts. The main objectives are to highlight the reaction mechanism/networks, types of sulfided catalysts, catalyst deactivation, and reaction kinetics involved in the hydrotreating of various viable renewable feedstocks to biofuels. The computational approaches to understand the application of metal sulfides in deoxygenation are also presented. The challenges and needs for future research related to the valorization of different bio-feedstocks into liquid fuels, employing sulfided catalysts, are also discussed in the current work.\n               "}
{"full_text": "The increasingly strict environmental regulations on exhaust emissions of gasoline-powered transportation vehicles are compelling refineries around the world to produce gasoline with lower sulfur content [1\u20134]. S-Zorb gasoline adsorption desulfurization technology [5\u201310] is a typical ultra-low sulfur gasoline production technique, which has achieved a large-scale industrial application. However, S-Zorb technique still faces the problem of loss of octane number, due to the hydrogenation of the olefins present in gasoline.Aromatization and isomerization reactions are two effective approaches to recover or even enhance the octane number of gasoline because they can transform paraffins, linear olefins and naphthenes to higher octane compounds, such as isoparaffins, isomeric olefins and aromatics. Naphthenes are important components of fluid catalytic cracking (FCC) gasoline, representing 5%\u201310% (by volume fraction) of FCC gasoline fraction. Naphthenes are primarily distributed between C6\nC8, so methylcyclohexane is a representative of naphthenes in FCC gasoline fraction. Transformation of naphthenes to aromatics and isomerization products through dehydrogenation aromatization as well as isomerization of alkanes can greatly improve the gasoline RON [11\u201313]. Generally, dehydrogenation of naphthenes is complex because a series of competing reactions, such as ring contraction, ring opening, cracking, hydride transfer and isomerization, can occur simultaneously. Although dehydrogenation aromatization and isomerization of naphthenes can be accomplished over either acid [14,15] or metal catalysts [16,17], the catalytic activity of their combination is better than any single catalyst due to the synergetic effect of acid and metal [18,19].The typical catalyst for these processes is a bifunctional heterogeneous catalyst consisting of a noble metal supported on an acidic support [20\u201322]. The catalytic hydro-conversion of cyclohexane over Pt/HY catalysts were studied by Onyesty\u00e1k et\u00a0al. recently [19]. Gopal et\u00a0al. demonstrated that the maximum isomer yield for a Pt/H-zeolite catalyst could be obtained when the metal and acidic functions of the catalysts were well-balanced [23]. Choudhury et\u00a0al. noted that the hydroisomerization yield could be enhanced by the elimination of strong acid sites from the micropores [21]. Belatel et\u00a0al. investigated the MCH reaction on PtIr/sulfated zirconia catalysts and showed that no isomerization was found to take place in the absence of metals [13]. However, the scarcity and high cost of noble metal has impeded their widely application in the aromatization and isomerization of gasoline. In this work, we studied the non-noble metal Ni supported on different supports for transformation of methylcyclohexane (MCH). Fe ion-exchange ZSM-5, ZSM-5, alumina and silica were utilized to illustrate the effect of acidity and support interaction on the dehydrogenation, isomerization and cracking of MCH. To investigate whether these RON recovery catalysts can be integrated into the S-Zorb process, the reactions were carried on the S-Zorb reaction condition (1.5\u00a0MPa H2 and 673\u00a0K).The raw material ZSM-5 was supplied by Shanghai ShenTan Environmental and Advanced Materials Corporation. Inert silica and \u03b3-Al2O3 were purchased from Evonik Degussa (China) Co., Ltd and Across respectively. Fe-ZSM-5 samples were prepared through ion-exchange of ZSM-5 with iron nitrate solution (Aldrich Chemical, > 99.99% pure) at 343\u2013353\u00a0K for 2\u00a0h. Then the resulting samples were filtered, washed with distilled water, dried at 423\u00a0K for 12\u00a0h, and then calcined in air at 873\u00a0K for 3\u00a0h. In order to investigate the effect of molecular sieve structure and acid properties on catalytic performance, the inert silica and \u03b3-Al2O3 were used for reference supports. The catalysts were prepared by spraying a saturated solution of nickel nitrate on the above four supports and then dried at 423\u00a0K for 3\u00a0h and calcined at 873\u00a0K for 1\u00a0h. The obtained samples were designated as NiO/ZSM-5, NiO/ZSM-5-Fe, NiO/SiO2, and NiO/Al2O3, respectively.The crystal structure of catalysts was characterized by X-ray diffraction (X'Pert SW, SIEMENS) using Cu K\u03b1 radiation operated at 40 kV and 40\u00a0mA. Diffraction lines of 2\u03b8 from 5\u00b0 to 70\u00b0 with a scanning speed of 5\u00b0 min\u22121 were taken to determine the crystalline phase of the catalyst.The chemical compositions of the catalysts were measured using a X-Ray Fluorescence spectrometer from Rigaku Corporation.Nitrogen sorption measurements were carried out over a Quantachrome Autosorb-6B unit. The isotherms were measured at 77\u00a0K after degassing samples below 1.3\u00a0Pa\u00a0at 573\u00a0K for 8\u00a0h. The BET specific surface area was estimated using adsorption\u2013desorption data as per the ASTM 4365 standard applicable for microporous materials. Total pore volume was equal to the amount of N2 adsorbed at a relative pressure of 0.99.NH3 temperature-programmed desorption (NH3-TPD) experiments were conducted on a Micromeritics AutoChem II 2920 analyzer equipped with a thermal conductivity detector (TCD) to determine the density and strength distribution of the acid sites. Typically, a 0.1\u00a0g sample was housed in a quartz U-shaped tube and pretreated in flowing helium (50\u00a0mL min\u22121) at 823\u00a0K for 1\u00a0h. After the pretreatment, the sample was cooled down to 393\u00a0K and ammonia-saturated in a stream of 10% NH3/He flow (50\u00a0mL min\u22121) for 0.5\u00a0h. Subsequently, the physically adsorbed NH3 was removed by flowing helium (50\u00a0mL min\u22121) at 393\u00a0K for 1\u00a0h. Finally, the chemically adsorbed NH3 was desorbed, with the temperature of the sample being raised from 393 to 823\u00a0K\u00a0at a heating rate of 10\u00a0K min\u22121 and maintained at 823\u00a0K for 10\u00a0min.The acidity properties of samples were determined using the pyridine FT-IR method, which was carried out on a NICOLET 6700 Fourier-transform infrared spectrometer (Thermo Fisher Scientific Corp.). Self-supporting wafers (13\u00a0mm in diameter) were made from ca. 10\u00a0mg of catalysts. The sample was initially evacuated to 1.0\u00a0\u00d7\u00a010\u22123\u00a0Pa\u00a0at 723\u00a0K for 2\u00a0h and then cooled to 363\u00a0K to be saturated with pyridine for 5\u00a0min. Then, pyridine desorption was performed under vacuum for two consecutive periods (0.5\u00a0h each) under isothermal conditions at 473 and 623\u00a0K, followed by IR measurements. Infrared spectra were measured at a 4\u00a0cm\u22121 resolution. The sample was examined in the range of 1350\u20131800\u00a0cm\u22121. For Py-IR spectra, the bands located at 1540\u00a0cm\u22121 and 1450\u00a0cm\u22121 can be assigned to pyridine adsorbed on Br\u00f6nsted (B) and Lewis (L) acid sites, respectively [24,25]. Total B acid sites and total L acid sites, and medium and strong B acid sites and medium and strong L acid sites can be obtained from the Py-IR measurement results at 473 and 623\u00a0K, respectively. The quantitative calculation of B and L acidity by Py-IR analysis is based on the integrated Lambert\u2013Beer Law [26]:\n\n\nC\n\nSW\n\u00a0=\u00a0AS/m\u03b5\n\nwhere C\n\nSW\n (\u03bcmol g\u22121) is the concentration of B or L acid sites in reference to a unit weight of dry sample, A (cm\u22121) is the integrated absorbance, S (cm2) is cross sectional area of the sample wafer, m (g) is the weight of the dry sample, and \u03b5 is the integrated molar extinction coefficient determined by Corma et\u00a0al. (\u03b5B\u00a0=\u00a00.059\u00a0+\u00a00.004 (cm2 \u03bcmol\u22121)*A, \u03b5\u0141\u00a0=\u00a00.084\u00a0+\u00a00.003 (cm2 \u03bcmol\u22121)*A) [27].Temperature-programmed reduction (TPR) was measured with the same apparatus as that of NH3-TPD. Prior to the TPR experiments, the catalysts were dried in flowing He at 773\u00a0K for 1\u00a0h. A mixture of 10% of H2/Ar was used as the reducing gas at a flow rate of 30\u00a0mL min\u22121. The rate of temperature rise in the TPR experiment was 10\u00a0K min\u22121 up to 923\u00a0K.Adsorption properties of MCH on different samples were determined by MCH pulsed adsorption and TPD using the same apparatus as that of NH3-TPD. Approximately 20\u00a0mg of catalyst was pretreated to remove water and reduce the supported nickel oxide. Then, pulses of MCH were injected repeatedly until the TCD signal showed no further adsorption. Following the pulsed adsorption experiments, the system was purged for 1\u00a0h in a He flow (30\u00a0mL min\u22121) to remove residual MCH. Subsequently, TPD of MCH from 373 to 823\u00a0K was performed at a heating rate of 10\u00a0K min\u22121.MCH dehydrogenation reaction was performed in a bench-scale high-pressure fixed bed reactor with a 25 mm inner diameter. Activity tests were carried out with 16\u00a0g of the pelletized catalyst, particle sizes ranged from 0.10 to 0.30\u00a0mm. Prior to activity test, the catalyst was activated by in situ reduction in a flow of H2 (1.5\u00a0MPa) at 673\u00a0K for 1\u00a0h. Then, the temperature was maintained at 673\u00a0K. The experiment was carried out in a mass space velocity 5\u00a0h\u22121 for 12\u00a0h with sampling interval of 2\u00a0h. The effluent from reactor was condensed, and then the liquid samples were taken and analyzed using a GC with an OV101 capillary column (30\u00a0m) and an FID.The textural properties of the catalysts were obtained by analyzing the BET surface areas and total pore volume (Table\u00a01\n). As shown in Table 1, the BET areas follow the trend: NiO/ZSM-5-Fe\u00a0>\u00a0NiO/ZSM-5\u00a0>\u00a0NiO/SiO2\u00a0>\u00a0NiO/Al2O3. The reference sample NiO/SiO2 presents a similar pore volume as the two ZSM-5 supported samples. All of the samples have the same content of nickel oxide, which implies that the method of introducing nickel by spray impregnation is repeatable.N2 adsorption isotherms of the four samples are shown in Fig.\u00a01\n. The isotherms of ZSM-5 supported samples belong to a combination of type I and IV patterns with H4 hysteresis loops, which are indicative of micropores and mesopores according to the IUPAC classification [28]. The shape of the hysteresis loop relates to the shape of meso- and macropores, namely, the horizontal loop implies ink bottle-shaped pores, whereas a vertical hysteresis loop implies cylindrical pores [29]. With Fe modification, the hysteretic loops change slightly from horizontal to vertical, suggesting a portion of the pores shifts to cylindrical pores connected to the external surface, which is favorable for the diffusion of molecules [30]. The reference sample NiO/SiO2 and NiO/Al2O3 exhibited type IV patterns, corresponding to a mesoporous structure. In addition, the hysteretic loops of these two samples seem to be of type H3 of IUPAC classification, which is associated with porous solid having slit-shaped pores [31].XRD patterns of NiO/ZSM-5, NiO/ZSM-5-Fe, NiO/SiO2 and NiO/Al2O3 are illustrated in Fig.\u00a02\n. All of the samples display distinct diffraction peaks of NiO phase at 2\u03b8\u00a0=\u00a037.0\u00b0, 43.1\u00b0 and 62.8\u00b0, representing the existence of crystalline phase nickel oxide on the catalyst. The XRD patterns of ZSM-5 supported samples show the intact MFI zeolite structure, which has 2\u03b8 values of characteristic diffraction peaks of approximately 7.9\u00b0, 8.9\u00b0, 23.3\u00b0, 23.9\u00b0 and 24.4\u00b0. This indicates that the ferric nitrate treatment does not significantly damage the long-range order of the original zeolite framework. XRD patterns of NiO/ZSM-5-Fe show much weaker intensity of NiO diffraction peaks, suggesting that the active phase is dispersed better than other samples. In addition to the diffraction peaks of NiO phase, NiO/SiO2 exhibits a broad peak centered at 2\u03b8\u00a0=\u00a022.0\u00b0, which is the characteristic peak of amorphous SiO2, and diffraction peaks of NiO/Al2O3 at 2\u03b8\u00a0=\u00a037.4\u00b0, 46.07\u00b0 and 66.9\u00b0 are related to alumina phase.In order to investigate the acid properties of the synthesized samples, the NH3-TPD experiment was carried out. The profiles are presented in Fig.\u00a03\n. Usually, NH3 desorption is temperature-dependent and can be classified in three stages, viz.: weak (<\u00a0473\u00a0K), moderate (473\u2013723\u00a0K) and strong (>\u00a0723\u00a0K). The area of a specific peak corresponds to the amount of desorbed NH3 and can be taken as the standard to quantify the acid amount [32]. The total acid amounts are reported in Table 2\n. The acid amounts were found to follow the order: NiO/ZSM-5\u00a0>\u00a0NiO/ZSM-5-Fe\u00a0>\u00a0NiO/Al2O3\u00a0>\u00a0NiO/SiO2. The NH3-TPD profile of NiO/SiO2 displays no peak of NH3 desorption, which is in accordance with the Pyridine FTIR result (Table 2), while NiO/Al2O3 possessed moderate acid amount owing to the surface hydroxyl ions of alumina. It implies that introducing nickel by spraying onto inert silica cannot create acid sites. The profiles of the NiO/ZSM-5 sample exhibit one main peak centered at 480\u00a0K, which is attributed to NH3 bound to weak acid sites. The tailing phenomenon suggests that moderate and strong acid sites are present with much less amounts. Interestingly, two distinct NH3 desorption peaks are observed for the profiles of NiO/ZSM-5-Fe. One centered at 470\u00a0K corresponds to weak acid sites and the other at 645\u00a0K to moderate acid sites. It can be deduced that iron exchange decreased the acid amounts but enhanced the acid strength. Moreover, the decreased acid amount can be ascribed to the interaction between acid sites and Fe species [33].To further probe the strength and the nature of the acid sites (L vs. B), FTIR analyses of pyridine adsorbed catalysts were carried out at 473 and 623\u00a0K, respectively. The results are listed in Table 2. Silica shows no acidity. Alumina contains mainly L acidity. Meanwhile two ZSM-5 supports have both L and B acid sites. The acidity results at 473\u00a0K involve all acidic sites, but only acidity with moderate and strong strength is included in the results of 623\u00a0K. From Table 2 and Fig.\u00a03, it is found that ZSM-5 supported sample has the highest L and B acidic amounts in which weak acid sites are dominated. On the contrary, Fe modified ZSM-5 sample has the most L and B sites with moderate acidic strength.The TPR characterization was carried out to find out information about the interaction between the Ni active metal and supports. TPR patterns of oxide catalysts are shown in Fig.\u00a04\n. All four samples have an obvious peak with Tmax identified at 650\u2013680\u00a0K, which was attributed to the reduction of NiO particles. And the peak temperature of NiO reduction is in the order of NiO/ZSM-5-Fe\u00a0<\u00a0NiO/ZSM-5\u00a0<\u00a0NiO/SiO2\u00a0<\u00a0NiO/Al2O3. This reflects the trend of interaction between NiO and support. NiO/Al2O3 shows a wider reduction peak with the highest temperature at approximately 680\u00a0K, which corresponds to the expected reduction temperature for NiO in strong interaction with alumina supports [34]. The shoulder peak detected at around 780\u00a0K appears to be due to the reduction of NiAl2O4 spinel. The reduction of NiO/SiO2 occurred at temperature around 660\u00a0K, suggesting the decreased interaction of Ni with support. The reduction peak of the ZSM-5 supported samples shifts to lower temperatures (approximately 650\u00a0K), which implies that the metal-support interaction in NiO/ZSM-5 was weaker than those of NiO/Al2O3 and NiO/SiO2. Fe modified NiO/ZSM-5 catalysts show the lowest reduction temperature of NiO, suggesting some interaction of Fe ions with nickel oxide. Two shoulder peaks appear over the NiO/ZSM-5-Fe at approximately 736\u00a0K and 810\u00a0K respectively, which can be ascribed to the reduction of Fe2O3 to FeO and Fe gradually [35]. This suggests that ion-exchanged iron is in the form of iron oxide in NiO/ZSM-5-Fe.The heterogeneous catalytic reaction is comprised of different steps, such as surface and pore diffusion, adsorption and surface reaction [36]. Adsorption affects the reaction performance to some extent. MCH adsorptions were investigated by TPD after MCH saturation. As illustrated in Fig.\u00a05\n, MCH adsorption capacities on different Ni-based catalysts decrease in the order of NiO/ZSM-5-Fe\u00a0>\u00a0NiO/ZSM-5\u00a0>\u00a0NiO/Al2O3\u00a0>\u00a0NiO/SiO2, which is consistent with the order of the number of the moderate acid sites. This implies that the MCH adsorption capacity might be directly related with the moderate acid sites of the samples. Namely, as for acidic supports, MCH mainly adsorbs on the moderate acidic centers. It indicates that this experiment is carried under oxide form of catalyst, thus the MCH adsorption on Ni surface is excluded.The catalytic transformation of MCH over NiO/SiO2, NiO/ZSM-5, NiO/Al2O3 and NiO/ZSM-5-Fe was conducted at 673\u00a0K and hydrogen pressure of 1.5\u00a0MPa, and the conversion of MCH and yield of various products versus reaction time are presented in Fig.\u00a06\n. As Scheme 1\n depicts, the reactions of MCH over the bifunctional catalysts involve dehydrogenation aromatization, isomerization, and cracking to aromatics, linear chain isoparaffins, cyclicisoparaffins and small alkenes. Over the bifunctional catalysts, MCH can continue to lose hydrogen to give toluene mainly on the nickel sites. Due to the introduction of B acid sites, protonation of the tertiary hydrogen in MCH forms hydrogen and methylcyclohexyl carbocation. The methylcyclohexyl carbocation can isomerize, lose a proton to produce methyclohexene (MCHE) or crack by the \u03b2-scission mechanism to give 2-methyl-1-hexene, which could occur further cracking to obtain products with less than seven carbons, primarily C4s and C3s. Methylcyclohexyl carbocation can also isomerize to alkyl cyclopentyl carbocations, which can desorb to generate alkylcyclepentanes. The RON of above products are all higher than that of MCH. As shown in Fig.\u00a06a, significant differences in activity and product distribution are observed among the four samples. As for MCH conversion, NiO/ZSM-5-Fe provides the highest catalytic activity. The NiO/ZSM-5-Fe (86.7% initial conversion) is much more effective than NiO/ZSM-5 (40% initial conversion) for MCH conversion. But, it is surprised that the activity of NiO/Al2O3 is lower than that of NiO/SiO2 although NiO/Al2O3 has a certain amount of L acid sites and moderate adsorption capacity of MCH.From the reaction results over four catalysts, we deduce that the main active centers of acidic catalysis for MCH conversion should be the B acid sites with moderate acid strength. It can be seen that the conversion of MCH follows the sequence of NiO/ZSM-5-Fe\u00a0>\u00a0NiO/ZSM-5\u00a0>\u00a0NiO/SiO2\u00a0>\u00a0NiO/Al2O3. And the B acidity obtained from pyridine adsorption at 623\u00a0K is also in the order of NiO/ZSM-5-Fe\u00a0>\u00a0NiO/ZSM-5\u00a0>\u00a0NiO/SiO2\u00a0=\u00a0NiO/Al2O3\u00a0=\u00a00 (Table 2). It is indicated that among the supported catalysts, NiO supported over ZSM-5-Fe exhibits the highest MCH conversion and the least cracking activity, although the total amounts of acids over NiO/ZSM-5-Fe is lower than NiO/ZSM-5. NiO/Al2O3 catalyst with moderate adsorption capacity of MCH, which total acid amounts are lower than that of of NiO/ZSM-5-Fe, but is higher than that of NiO/SiO2, has the lowest MCH conversion activity. This reaction results illustrates that other acidic sites, such as weak acidic centers and moderate L acidic centers, may provide the adsorption of MCH, but cannot efficiently catalyze MCH conversion.A decrease of MCH conversion with the reaction time is observed which is attributed to the coke formation, blocking the active sites. Because the S-Zorb process is operated in the way of fluidized fluid beds with continuous regeneration, thus the coke can be removed in the regeneration of deactivated catalysts if this catalysis system is integrated into the S-Zorb technique.The product distributions over the four samples are illustrated in Fig.\u00a06b\u2013d. The yield of aromatic product over NiO/ZSM-5-Fe is significantly higher than those over NiO/ZSM-5, NiO/Al2O3 and NiO/SiO2 (Fig.\u00a06b). The dehydrogenation is supposed to mainly occurs on the Ni active centers. The TPR shows that the temperature of reduction peak is in the sequence of NiO/ZSM-5-Fe\u00a0<\u00a0NiO/ZSM-5\u00a0<\u00a0NiO/SiO2\u00a0<\u00a0NiO/Al2O3. The decrease of interaction of support with Ni makes the reduction of NiO more easier. Due to the interaction between Fe ions and NiO and the weakened interaction of ZSM-5 with NiO phase, the sufficient Ni reduction was achieved on the NiO/ZSM-5-Fe. Meanwhile, the enhanced dehydrogenation aromatization of MCH performances for NiO/ZSM-5-Fe and NiO/ZSM-5 catalysts could also be attributed to the synergetic effect between active Ni components and the B acid sites of ZSM-5.Hydrocracking and isomerization of MCH take place in several steps, firstly the saturated cycloalkanes are transformed to the olefins via dehydrogenation on metal sites, then the olefin intermediates protonated to carbenium ions on the B acid sites, which either isomerizes to the dimethyl substituted cyclopentene or cracks to fragments. At last, these unsaturated products can be further hydrogenated to the saturated 5-membered ring cycloparaffin or the saturated lower molecular paraffins (principally propane). The hydroisomerization activity of MCH over these four catalysts is shown in Fig.\u00a06c. It can be seen that NiO/ZSM-5-Fe catalyst exhibits the highest isomerization yield, followed by NiO/ZSM-5 catalyst, while NiO/SiO2 and NiO/Al2O3 show almost no isomerization activity. From the results in Table 2, the isomerization activity is supposed to be mainly related with the B acids at 623\u00a0K, namely the B acid with moderate strength. It is postulated that weak B acid sites cannot effectively transfer the proton onto olefins to form the carbenium ions. On the other hand, olefins adsorb on moderate B acid sites by protonation, leading to the isomerization reaction. Thus NiO/ZSM-5-Fe with more moderate strong B acid sites shows excellent isomerization activity.Although isomerization can improve the RON, another acidic catalytic cracking reaction may decrease the liquid products yield. As shown in Fig.\u00a06d, the cracking yields is in the order of NiO/ZSM-5\u00a0<\u00a0NiO/ZSM-5-Fe\u00a0<\u00a0NiO/Al2O3\u00a0<\u00a0NiO/SiO2, which is agreement with that of total acidic amounts. It is reasonable that the cracking reaction occurs both on the L and B acid sites. The acidic strength and types have less influences on the cracking activity than acid amounts.From above reaction results, NiO/ZSM-5-Fe obviously exhibits an appropriate balance between the high yield of aromatic and isomerization products and less cracking. The modification of ZSM-5 with iron can raise the amount of medium strong B acid sites and decrease the total amount of acid sites, which leads to a high yield of aromatic products by enhancing the conversion of MCH and inhibiting the ring opening and cracking of MCH simultaneously. These results show that NiO/ZSM-5-Fe can effectively catalyze the aromatization and isomerization of MCH under the S-Zorb reaction conditions. Moreover, modification of NiO/ZSM-5 with iron creats more medium strong B acid sites, which is favorable to improve aromatization and isomerization products during gasoline desulfurization, with simultaneous enhancement of the RON of fuels and desulfurization efficiency. Thus, the NiO/ZSM-5-Fe sample may be introduced as specific modifier onto S-Zorb catalysts, and further study will be investigated through the combination of NiO/ZSM-5-Fe with S-Zorb catalyst so as to recover the RON of gasoline during the S-Zorb reactive adsorption desulfurization.In summary, NiO/SiO2, NiO/Al2O3, NiO/ZSM-5 and NiO/ZSM-5-Fe catalysts were developed and evaluated for the conversion of MCH under the S-Zorb catalytic adsorption desulfurization conditions. The results indicated that the catalytic activity and the distribution of main products were significantly influenced by the interaction between NiO and supports and the acid\u2013base properties of the catalysts (including acid amount, strength and types). TPR results showed that the locations of reduction peaks gradually shifted to higher temperature in the order of NiO/ZSM-5-Fe\u00a0<\u00a0NiO/ZSM-5\u00a0<\u00a0NiO/Al2O3\u00a0<\u00a0NiO/SiO2, as well as the onset temperature of reduction. This indicated that NiO/ZSM-5-Fe and NiO/ZSM-5 was more easier to be reduced than NiO/Al2O3 and NiO/SiO2 due to the weaker interaction between NiO and ZSM-5. On the other hand, the Ni active centers over the ZSM-5 surface would be expected to catalyze the dehydrogenation of MCH into methylcyclohexene and so on. The total acid sites, especially the number of B acid sites with mediun strong acidity, also played a critical role for MCH conversion. The medium strong B acid sites were the main active sites for aromatization and isomerization reaction and it was improved significantly after modification with iron metal. So, the best result was obtained by using NiO/ZSM-5-Fe as the catalyst, with 86.7% MCH conversion and remarkable aromatization and isomerization yields of 82.1% and little cracking products (4.6%).The authors have declared that no conflict of interest exists.The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (21433001, 21406251 and 21403265), Science and Technology Development Projects of SINOPEC, China (No. 113138, 112008 and 110099), and The Young Taishan Scholars Program of Shandong Province (tsqn20161052).", "descript": "\n                  In this work, nickel metal supported on different supports (SiO2, Al2O3, ZSM-5) were prepared by spraying nickel nitrate on the supports and calcined at 873\u00a0K. Then, they were characterized by XRD, XRF, N2 adsorption\u2013desorption, NH3-TPD, MCH-TPD, H2-TPR, and pyridine-FTIR, and tested as catalysts for the dehydrogenation aromatization and isomerization of methylcyclohexane (MCH) under the conditions of S-Zorb catalytic adsorption desulfurization (T\u00a0=\u00a0673\u00a0K, P\u00a0=\u00a01.5\u00a0MPa, WHSV\u00a0=\u00a05\u00a0h\u22121). The H2-TPR results showed that the interaction of NiO with support decreased in the order of NiO/ZSM-5-Fe\u00a0<\u00a0NiO/ZSM-5\u00a0<\u00a0NiO/Al2O3\u00a0<\u00a0NiO/SiO2. The decrease of the interaction appeared to facilitate the reduction of Ni and therefore to promote the dehydrogenation aromatization of MCH.\n                  It was found that a direct correlation existed between the gasoline components yields, cracking activity and the total number of different supports acid sites measured by NH3-TPD tests. Higher total acidity of ZSM-5 resulted in gasoline loss because of higher cracking activity of MCH. The number of total acid sites of NiO/ZSM-5-Fe decreased and the medium strong Br\u00f6nsted acid sites necessary for MCH isomerization increased after the modification of ZSM-5 by iron metal. So, NiO/ZSM-5-Fe exhibited enhanced MCH conversion, aromatic and isomerization yields when compared to NiO/ZSM-5 and other Ni-based catalysts. This study shows that NiO/ZSM-5-Fe catalyst may be possible to be integrated into the S-Zorb system achieving the recovery of the octane number of gasoline.\n               "}
{"full_text": "CH4 is a major contributor to global warming. A reaction like the decomposition of methane has great environmental importance as CH4 is consumed and its concentration is depleted in the environment. Again, this reaction has great economic feature as it produces clean energy source H2 and high-quality carbon without the formation of CO/CO2 (CH4\u00a0\u2192\u00a0C\u00a0+\u00a02H2). The separation of H2 gas from solid carbon is easier than the separation of two gases in other reforming processes (stream reforming; CH4\u00a0+\u00a0H2O\u00a0\u21cc\u00a0CO\u00a0+\u00a03H2, dry reforming; CH4\u00a0+\u00a0CO2\u00a0\u21cc\u00a02CO\u00a0+\u00a02H2). However, the decomposition of methane through C\u2013H cleavage occurs at very high reaction temperature (up to 1200\u00a0\u00b0C). To decrease the reaction temperature or bond dissociate energy of C\u2013H prominently, the high temperature sustainable Fe, Co, Ni, Cu-based catalyst is required (Gajewski and Pao, 2011; Wu et al., 2009). When the electronic promotor La2O3 was added with physical mixture of Ni-Cu alloy with a mass ratio of 0.107, the rate of C\u2013H dissociation was increased (Figueiredo et al., 2010). The interaction of deposited carbon (after CH4 decomposition) with lattice oxygen at high temperatures cannot be neglected which forms CO in low quantity (Choudhary et al., 2001). The catalyst community is trying to develop a catalyst for COx-free H2 production through CH4 decomposition.Among the supported catalyst systems, Ni supported on activated carbon had drawn attention due to the in-situ generation of the active site (metallic Ni) through the reducibility of activated carbon in the carbonization process. Coal char derived from lignite coal showed minute CH4 decomposition in micropores. C\u2013H bond dissociation energy of CH4 over coal char was found four times (89\u2013105\u00a0kJ/mol) less than in uncatalyzed reaction (Bai et al., 2006). Prasad et al. (Sarada Prasad et al., 2011) prepared activated carbon from a coconut shell and impregnated Ni over it. Ni supported on activated carbon-showed an initial decrease of CH4 conversion in the first two hours and thereafter an increase in CH4 conversion. 10 wr% Ni supported on carbon (derived fromcoal liquefactionresidue) (Zhang et al., 2013) showed a continuous rise of CH4 conversion (13 to 60%) at 850\u00a0\u00b0C during 9\u00a0h time on stream. Another thermally stable support that can hold Ni during high temperature reaction was tried. Nanosized Ni (prepared by citric acid at pH controlled condition) supported over high silica ZSM (Si/Al\u00a0=\u00a0300) showed about 45% CH4 conversion for a small time (14\u00a0min) at 700\u00a0\u00b0C (Michalkiewicz and Majewska, 2014). Among Ni/HY, Ni/SiO2, Ni/H-ZSM; CO contamination was found lowest on Ni/SiO2. Ni supported on showed\u00a0\u223c\u00a010% CH4 conversion during the entire range of reaction temperatures 500\u00a0\u00b0C \u2013 800\u00a0\u00b0C for 5\u00a0h (Dong et al., 2015). 30\u00a0wt% Ni loading over SiO2 support showed 15% CH4 conversion up to 8\u00a0h time (Venugopal et al., 2007). Longevity of Ni-supported catalyst was found in the following order Ni/MgO\u00a0>\u00a0Ni/SiO2\u00a0>\u00a0Ni/LiAlO2\u00a0>\u00a0Ni/ZrO2 (Bonura et al., 2006) in which Ni supported on MgO showed>30% CH4 conversion up to 210-minutes. Ni/MgO catalyst can be regenerated in the O2 stream and utilized again for the reaction without any prior reduction step. 10-40\u00a0wt% Ni-MgO catalyst prepared by hydrothermal method showed the presence of NiO-MgO solid solution with mesoporosity (Bai et al., 2021). CH4 and N2 gas feed (1:2\u00a0vol ratio) over 30\u201340% Ni-MgO reached above 45% H2-yield within 3\u00a0h at 600\u00a0\u00b0C. Karimi et al. studied the decomposition of CH4 (in CH4: N2\u00a0=\u00a03: 17) over Ni supported on MgSiO3-(prepared by the coprecipitation method) (Karimi et al., 2021). The catalyst showed 64% CH4 conversion at 600\u00a0\u00b0C. Low La/Ni ratio in LaNiO3, La4Ni3O10, La3Ni2O7 and La2NiO4 was known for bulk carbon decomposition with high degree of graphitisation (Li et al., 2001). Ni-incorporated hydrotalcite was derived from 2: 0.7: 0.3\u00a0mol ratio of Ni: Al: La metal precursors respectively. It had strong metal support interaction and showed\u00a0\u223c\u00a030% CH4 conversion up to 24\u00a0h (Anjaneyulu et al., 2015).The Co-Al mixed oxide had Co3O4 phase and Co2AlO4 (spinel) phases (Calgaro and Perez-Lopez, 2019; Zardin and Perez-Lopez, 2017). The catalyst reduced under CH4 (than under H2) had lower particle size and showed 75% CH4 conversion at 750\u00a0\u00b0C reaction temperature. In Co-Al mixed oxide, the Co3O4 phase favoured graphene formation. 20\u00a0wt% Co-impregnated Al2O3-coated silica fabric has strong metal support interaction and showed 90% CH4 conversion up to 11.6\u00a0h at 700\u00a0\u00b0C (Italiano et al., 2010). Cu and Ni supported on Alumina-was found better than Cu supported onalumina-catalyst because of the formation of Ni-Cu alloy. After reduction, 70%Ni\u201310%Cu\u201310%Fe/Al2O3 catalysts showed the formation of Ni-Cu-Fe alloy (Chesnokov and Chichkan, 2009). Alloy formation caused a decrease in the number of contacts between metal particles and thus sintering was prevented. Upon iron addition in 70%Ni\u201310%Cu/Al2O3 catalyst, H2 concentration remained between 71 and 77% and the diffusion coefficient of the carbon atom was increased three times (carbon nanofiber yield 136\u00a0g/g). At 15\u00a0ml/min methane flow rate, 65%Ni-10%Fe-25%SiO2 catalyst showed 20% CH4 conversion at 550\u00a0\u00b0C reaction temperature (Wang et al., 2012). However, the presence of Ni-Fe redox (in Ni2-xFexAl; x\u00a0=\u00a0Fe/Al) also functions as oxygen carrier (Huang et al., 2018) which can mitigate the target of COx-free H2 production.In the mean of Ni, Co, and Cu free catalyst, a mechanochemical activation of LaFeO3 and CeO2 mixture had drawn attention. It caused an accumulation of oxygen vacancy about Fe+3 which became the sites of oxygen exchange between O2 form air to surface to bulk CeO2 (Pinaeva et al., 2013). However, in presence of oxygen; COx-free hydrogen production from CH4 was not possible over mechanochemical mixture of LaFeO3 and CeO2. If 60\u00a0wt% Fe supported on alumina catalyst was reduced under the H2 stream, iron oxide was reduced into metallic Fe (Ibrahim et al., 2015). The metallic Fe is an active site for CH4 decomposition. Fe supported on Al2O3- generated multiwalled nanotube and 77.2 % H2 yield up to 4\u00a0h at 700\u00a0\u00b0C. Decomposition of CH4, C2H4, and C2H2 over Iron-based catalysts was reported (Maroto Valiente et al., 2000; Qian et al., 2008). Jin et al. prepared activated carbon from coconut shell and impregnated the 40\u00a0wt% iron oxide and alumina (Fe/Al\u00a0=\u00a024/16) over activated carbon (Jin et al., 2013). Here, activated carbon brought in-situ reduction of Fe(NO3)3 to metallic. During N2 pre-treatment process at 870\u00a0\u00b0C, the carbon wall was burned off by Fe and created mesopores. The catalyst showed 35% CH4 conversion up to 100\u00a0h.By literature review, we come to know that the widely available and cheap Fe can be utilized for the generation of COx-free H2 through CH4 decomposition. The activated carbon as support had the additional benefits as it had in-situ generation capacity of catalytic active sites (metallic Fe) by carbon reducibility. Tungsten had appealing redox chemistry and WC had high thermal stability (Mounfield et al., 2019). In the presence of W, additional CH4 decomposition sites were previously claimed also (Patel et al., 2021). Herein, waste date pits were utilized for the preparation of activated carbon. The WO3-activated carbon support was prepared by hydrothermal method and thereafter iron was impregnated over the WO3-activated carbon support. It is expected that if tungsten oxide is used as support along with activated carbon, Ni supported on WO3-activated carbon catalyst system would be benefited by high thermal stability, in-situ reducibility, and enhanced CH4 dissociation. The prepared catalyst was investigated for CH4 decomposition reaction and characterized through X-ray diffraction, N2-physiosorption, and porosity measurement, H2-temperature programmed reduction, thermogravimetric analysis, O2-temperature programmed oxidation and X-ray photoelectron spectroscopy. The fine correlation of catalytic activity and characterization results will add a step up in the development of an industrially suited catalyst for CH4 dissociation.The following materials were used in the preparation of the newly designed catalysts; Sodium tungstate dehydrate (Na2WO4\u00b72H2O, \u2265 99% Sigma Aldrich), sodium chloride (NaCl; \u2265 99.0%, Sigma Aldrich), hydrated iron nitrate (Fe(NO3)3\u00b79H2O; 99%; Loba Chemie), hydrochloric acid (HCl; 37%, Sigma Aldrich) and waste of date pits (collected from Albaha region, Saudi Arabia).The waste of date pits was cleaned, sieved, and washed several times by deionized water. Further, it is carbonized on heating at 250\u00a0\u00b0C under an electrical oven for 24\u00a0h. The black carbonized pits were obtained, ground and sieved. Finally, black carbon powder is obtained. To activate the black powder, concentrated H2SO4 was added and the mixture was heated at 250\u00a0\u00b0C in an oven for 24\u00a0h. The obtained material was washed several times with deionized water until pH 7 is not attained. The activated carbon material was abbreviated as \u201cAc\u201d.The support WO3 nanoparticles were synthesized by hydrothermal process. 1.067\u00a0g of Na2WO4\u00b72H2O and 0.038\u00a0g of pure NaCl were dissolved in 20\u00a0ml distilled water in stainless steel autoclave and stirred the solution in the dark for 30\u00a0min. Further, 5\u00a0ml HCl solution was added dropwise in this solution. The mixture (in an autoclave) was placed in the oven at 150\u00a0\u00b0C for 10\u00a0h. The precipitate in the autoclave was washed several times with distilled water until pH 7 was not reached. Finally, sample was calcined in air at 450\u00a0\u00b0C for 5\u00a0h. The material was used for support further and abbreviated as W.The support Ac-doped WO3 nanoparticles were prepared by the following procedure. Appropriate amounts of \u201cx\u201d wt% Ac and 100-x wt% WO3 (x\u00a0=\u00a05\u201395) were added in 20\u00a0ml distilled water under the stirring conditions in the autoclave. Further, HCl solution was added to the solution, kept for 30\u00a0min at room temperature, and then placed in an autoclave under the oven at 150\u00a0\u00b0C for 12\u00a0h. The precipitate in the autoclave was washed several times with distilled water until pH 7 was not reached. Finally, the sample was calcined in air at 450\u00a0\u00b0C for 5\u00a0h. The material was used as support further and abbreviated as xW(100-x) Ac (x\u00a0=\u00a00\u2013100).30\u00a0wt% Fe loading was obtained from dissolving the specified amount of hydrated iron nitrate in 30\u00a0ml water and followed by impregnated of this solution over Ac or W or xW(100-x)Ac (x\u00a0=\u00a00\u2013100) support at 80\u00a0\u00b0C for 3\u00a0h. Further, the slurry was dried overnight at 120\u00a0\u00b0C and calcined at 600\u00a0\u00b0C for 3\u00a0h sequentially. Fe supported on activated carbon, Fe supported on tungsten oxide, Fe supported on \u201ctungsten oxide-activated carbon\u201d catalysts were abbreviated as 30Fe100AC, 30Fe100WO3,and 30FexW(100-x) Ac (x\u00a0=\u00a00\u2013100) respectively.X-ray diffraction (XRD) study of catalyst samples was carried out by Rigaku diffractometer using Cu K\u03b1 radiation source operated at 40\u00a0kV and 40\u00a0mA. 0.01 step size and 5\u2013100 scanning range were set for analysis. Phase analysis was carried out by using X\u2019pert high score plus software and JCPDS database. N2-physiosorption isotherms study of catalyst sample was carried over Micromeritics Tristar II 3020. Surface area was estimated by Brunauer-Emmet Teller (BET) method whereas pore volume and pore diameter were estimated by Barrett-Joyner-Halenda (BJH) method. The reducibility of the catalyst sample was studied by H2-temperature-programmed reduction (TPR) over Micromeritics Auto Chem II 2920, USA. 70\u00a0mg of the sample was subjected to a heat treatment at 10\u00a0\u00b0C/min up to 900\u00a0\u00b0C under 30\u00a0ml/min gas flow of 10% H2/Ar mixture gas. The thermogravimetric analysis (TGA) was carried out over 0.015\u00a0g of spent catalyst sample in the temperature range (room temperature to 1000\u00a0\u00b0C) at heating ramp 20\u00a0\u00b0C by using Shimadzu TGA-51. The TGA analysis was carried out under oxidizing gas O2. The weight loss/weight gain of catalyst sample against temperature was monitored continuously. O2-Temperature programmed oxidation (TPO) was carried out over spent catalyst system in 50\u2013800\u00a0\u00b0C temperature range by using a 10% O2/He mixture through by Micromeritics AutoChem II. Before analysis, the spent catalyst was treated under high purity Argon at 150\u00a0\u00b0C for 30\u00a0min and subsequently cooled to room temperature. The morphology of the catalyst sample was investigated by using a field emission scanning electron microscope (FE-SEM, model: JEOL JSM-7100F) and transmission electron microscope (TEM, model: 120\u00a0kV JEOL JEM-2100F). Element valance state and binding energy of electron were determined by X-ray photoelectron spectroscopy (XPS) (Themo Fisher Scientific, USA) operated through AlK\u03b1 excitation source and 20\u00a0eV pass energy.The detailed reaction set up for the CH4 decomposition reaction is shown in Fig. 1\n. Catalytic decomposition of methane was carried out over 0.15\u00a0g catalyst packed in fixed-bed stainless steel tubular micro-reactor (PID Eng & Tech micro activity reference company; L\u00a0=\u00a030\u00a0cm, I.D\u00a0=\u00a09.1\u00a0mm) at atmospheric pressure. The reactor temperature was monitored by an axially positioned K-type stainless steel sheathed thermocouple at the centre of the catalyst bed. Prior to the reaction, the catalyst was activated under 40\u00a0ml/min flow of H2 for 60\u00a0min at 600\u00a0\u00b0C. Futher reactor is purged by N2 for 15\u00a0min to remove the remnant of H2. Now, the temperature of the reactor was raised to 800\u00a0\u00b0C under flow of N2. 15\u00a0ml/min CH4 and 5\u00a0ml/min N2 (total flow rate of feed gas 20\u00a0ml/ min) was allowed to pass through the catalyst bed at 800\u00a0\u00b0C with 8000\u00a0ml/hgcat space velocity of. GC-2014 SHIMADZU (Column: Shin carbon C20380 for gases and Haysepe Q AC0209 column for water analysis; carrier gas: Argon) equipped with conductivity detector was used to analyse the feed and output gas composition. The expression for CH4 conversion, H2 yield and Carbon yield (%) are given as\n\n\n\n\n\nC\nH\n\n4\n\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n=\n\n\n\n\n\nC\nH\n\n\n4\n,\ni\nn\n\n\n-\n\n\nC\nH\n\n\n4\n,\no\nu\nt\n\n\n\n\n\nC\nH\n\n\n4\n,\ni\nn\n\n\n\n\n\u00d7\n\n100\n%\n\n\n\n\n\n\n\n\n\nH\n2\n\n\nY\ni\ne\nl\nd\n%\n\n=\n\n\n\nM\no\nl\ne\n\no\nf\n\n\nH\n2\n\n\ni\nn\n\nP\nr\no\nd\nu\nc\nt\n\n\n2\n\nx\n\nm\no\nl\n\no\nf\n\n\n\n\n\nC\nH\n\n4\n\n\n\ni\nn\n\n\n\n\n\nx\n\n100\n\n\n\n\n\n\n\n\nC\na\nr\nb\no\nn\n\ny\ni\ne\nl\nd\n\n\n\n\n%\n\n\n\n=\n\n\n\n\nW\np\n-\nW\nc\na\nt\n\n\n\u00d7\n100\n\n\nWcat\n\n\n\n\n\n\n(Where Wp\n is the weight of the product after reaction and Wcat\n is the weight of the fresh catalyst).The X-ray diffraction pattern of 30FexW(100-x) Ac (x\u00a0=\u00a00\u2013100%) catalysts are shown in Fig. 2\n and Fig. 3\n. 30\u00a0wt% Fe supported over activated carbon had only phases related to iron oxide at Bragg angle (2\u03b8) 24.07\u00b0, 33.12\u00b0, 35.60\u00b0, 39.19\u00b0, 40.80\u00b0, 49.38\u00b0, 54.10\u00b0, 57.56\u00b0, 62.33\u00b0, 63.99\u00b0, 69.55\u00b0, 71.80\u00b0 (JCPDS reference number 00\u2013024-0072) (Fig. 2\nA-C). As activated carbon amount is substituted by tungsten oxide up to 5\u201310\u00a0wt% W, the diffraction peak intensity for iron oxide is suppressed greatly in 30FexW(100-x) Ac (x\u00a0=\u00a05\u201310) catalyst. However, upon the 1:3 ratio of W and Ac (25\u00a0wt%WO3-75%Ac), the many peaks related to Fe2O3 again appeared with variation in intensity over 30Fe25W75Ac catalyst (Fig. 2\nD). The 30Fe25W75Ac catalyst also shows crystalline carbon phases at 2\u03b8 values of 27.38\u00b0, 36.10\u00b0, and 62.7\u00b0 (JCPDS reference number: 00\u2013018-0311) (Fig. 2\nB-C). When \u201c50\u00a0wt% WO3-50\u00a0wt% Ac\u201d support is prepared for 30\u00a0wt% Fe dispersion, orthorhombic tungsten oxide phases at 22.97\u00b0, 24.02\u00b0, 33.22\u00b0, 35.70\u00b0, 49.55\u00b0, 54.24\u00b0, 62.59\u00b0 (JCPDS reference number: 00\u2013020-1324), tungsten carbide phase at 35.70\u00b0, 64.11\u00b0 (JCPDS reference number 01\u2013073-0471) and Fe2(WO4)3 phases at 20.33\u00b0, 22.45\u00b0, 22.97\u00b0, 25.45\u00b0, 29.98\u00b0 (JCPDS reference number 00\u2013038-0200) are appeared additionally (\nFig. 2\nD). Upon 3:1 ratio of W and Ac respectively, the 30Fe75W25Ac catalyst shows the most intense diffraction peak patterns along with additional diffraction peaks for tungsten oxide (monoclinic phase) at 23.61\u00b0, 28.70\u00b0, 30\u00b0, 34.11\u00b0, 38.94\u00b0, 56.29\u00b0, 68.81\u00b0 (JCPDS reference number 01\u2013072-0677) (\nFig. 2\nE-F). It may be expected that on further increasing the weight ratio of W and Ac (W/Ac\u00a0=\u00a075/25, 90/10, 95/5), the peak intensity of tungsten-related phases should be increased but the opposite diffraction results are noticed (\nFig. 3\nA-C). It indicates either the addition of \u201c5-10\u00a0wt% activated carbon in tungsten oxide matrix\u201d or \u201caddition of 5-10\u00a0wt% WO3 in activated carbon\u201d brings a drop of the crystallinity of the catalyst sample. Finally, on complete substitution of activated carbon by tungsten oxide, 30Fe100W catalyst shows iron oxide, tungsten oxide and the intense peak intensity for Fe2WO6 mixed oxide (at 20.50\u00b0, 27.41\u00b0, 31.08\u00b0, 33.21\u00b0, 35.99\u00b0, 39.09\u00b0, 49.59\u00b0, 54.19\u00b0; JCPDS reference number 00\u2013015-0688) (\nFig. 3\nD-F).\nThe support \u201cactivated carbon\u201d has 0.8348\u00a0m2/g surface area, 0.002550\u00a0cm3/g pore volume, and 393\u00a0\u00c5 pore diameter. For 30FexW(100-x) Ac (x\u00a0=\u00a00\u2013100) catalyst system, adsorption isotherm, pore size distribution, surface area, pore volume, and pore diameter are shown in Fig. 4\n\nand\n\nFig. S2\n\n. The catalyst system belongs to the type IV isotherm having an H3 hysteresis loop. It indicates the presence of non-rigid aggregate-like mesopores. Upon incorporation of 5-10\u00a0wt% tungsten oxide, the surface area of 30FexW(100-x)Ac (x\u00a0=\u00a05, 10) catalyst is 2.5 times than 30Fe100Ac indicating expansion of framework (Kumar et al., 2016). Upon 25\u00a0wt% tungsten oxide incorporation, the surface area is noticed to decrease to 30% but pore volume is increased by 46%. However, upon further loading up to 50\u00a0wt% W; the surface area and pore volume of 30Fe50W50Ac are increased to 3 times and 1.8 times (with respect to 30Fe100Ac) respectively. The pore size distribution plot (dV/dlogW vs W) indicates that up to 50 % incorporation of tungsten oxide, pore size distributions remain bimodal. In the 30Fe50W50Ac catalyst, the intensity of the low pore-width range is more pronounced than the higher pore-width range. In 30FexW(100-x)Ac (x\u00a0=\u00a00\u2013100) catalyst systems, when support is made up by major WO3 than Ac (upon\u00a0>\u00a050\u00a0wt% W incorporation), the pore size distribution becomes multimodal. In 30FexW(100-x)Ac (x\u00a0=\u00a075\u2013100),the surface area decreases suddenly to 11\u201322\u00a0m2/g (against 59.15\u00a0m2/g in 30Fe50W50Ac) due to deposition of various crystallite inside the pore (Rahman et al., 2015).The H2-Temperatured programmed reduction profile of 30FexW(100-x) Ac (x\u00a0=\u00a00\u2013100) catalyst systems are shown in Fig. 5\n\nA and\n\nFig. S3\n\n. The total H2-consumption during the H2-TPR experiment is shown in \nTable S4\n. H2-TPR of Fe2O3 is constituted by two sharp peaks at 355\u00a0\u00b0C and 577\u00a0\u00b0C and a broad peak between 624\u00a0\u00b0C and 934\u00a0\u00b0C (\nFig. S3\n). The three peaks are correlated with the sequential reductions Fe2O3\u00a0\u2192\u00a0Fe3O4\u00a0\u2192\u00a0FeO\u00a0\u2192\u00a0Fe respectively (Ibrahim et al., 2015; Jozwiak et al., 2007). The activated carbon-supported Fe (30Fe100Ac) catalyst shows shifting of lower temperature reduction peak to relatively higher temperature (at 385\u00a0\u00b0C) indicating interaction of Fe2O3-species with support. The peak at 385\u00a0\u00b0C belongs to reduction of interacted-Fe2O3-species into Fe3O4. Upon incorporation of just 5\u00a0wt% WO3, a merged peak maximum at 420\u00a0\u00b0C for reduction of Fe2O3\u00a0\u2192\u00a0Fe3O4\u00a0\u2192\u00a0FeO and a broad peak at a higher temperature for the reduction of FeO\u00a0\u2192\u00a0Fe are observed. Further incorporation of 10\u00a0wt% WO3, the reduction peak of 30Fe10W90Ac catalyst is shifted towards more higher temperature (444\u00a0\u00b0C). It is noticeable that the amount of reducible iron species had decreased upon providing support as well as increasing the proportion of tungsten oxide (up to 10\u00a0wt%) in the support. It indicates that the total reducible quantity has decreased due to the interaction of Fe-species with the new support composed of xW(100-x) Ac (x\u00a0=\u00a00, 5, 10) catalyst. As well as WO3 incorporation is increased to 25\u00a0wt%, the reduction peak maxima of 30Fe25W75Ac catalyst are shifted to a higher temperature and the amount of reducible iron-species is increased to\u00a0\u223c\u00a034% with respect to 30Fe10W90Ac catalyst (\nTable S4\n). Shifting of reduction peak to a higher temperature also indicates increased metal support interaction upon tungsten oxide loading. 30Fe50W50Ac catalyst has the lower temperature reduction peak (for reduction of Fe2O3\u00a0\u2192\u00a0Fe3O4\u00a0\u2192\u00a0FeO at 470\u00a0\u00b0C and broad higher temperature reduction peak with comparable amount of reduceable species than 30Fe25W75Ac catalyst. 30FexW(100-x) Ac (x\u00a0=\u00a090\u2013100\u00a0wt%) showed the reduction peak about 530\u2013560\u00a0\u00b0C. It again shows a general trend of increasing metal-support interaction upon tungsten oxide loading. The peak pattern of 30FexW(100-x)Ac (x\u00a0=\u00a090\u2013100\u00a0wt%) has also an additional peak in the temperature region of 593 to 720\u00a0\u00b0C for reduction of WO3 crystallite or \u201cWO3 interacted species\u201d(Ramanathan et al., 2013). It is noticeable that the total concentration of reducible species at the catalyst surface is decreased sharply above 50\u00a0wt% tungsten oxide incorporation. 30Fe50W50Ac, 30Fe90W10Ac, 30Fe95W5Ac, and 30Fe100W catalysts had 174.2\u00a0cm3/g, 72.28\u00a0cm3/g, 68.24\u00a0cm3/h, and 46.32\u00a0cm3/g consumption of hydrogen. The H2-TPR pattern of the 30Fe25W75Ac catalyst is needed to address separately. It has the highest amount of reducible species over the surface (183\u00a0cm3/g H2 consumption in H2-TPR result) among other tungsten oxide incorporated catalysts. The H2-TPR peak pattern is constituted by five peaks enveloping each other at 433\u00a0\u00b0C, 506\u00a0\u00b0C, 664\u00a0\u00b0C, 816\u00a0\u00b0C and 929\u00a0\u00b0C. It indicates the 30Fe25W75Ac catalyst had the highest concentration of \u201cFe-related\u201d reducible species which interacted with the support to different extents.The thermogravimetric analysis (TGA) of spent catalysts is shown in Fig. 5\nB. The weight gain over the catalyst system in TGA analysis may be due to oxidation of \u201creduced metal species\u201d (like iron or tungsten-related metal oxide) over the catalyst surface (Ibrahim et al., 2015). In the case of spent 30Fe25W75Ac and spent 30Fe50W50Ac, weight loss due to oxidation of deposit carbon is optimum and so weight gain due to oxidation of \u201creduced metal species\u201d over these catalyst systems is not evident on TGA analysis. The carbon yield % over the different catalysts is found in the following order; 30Fe25W75Ac (140%)\u00a0>\u00a030Fe50W50Ac (107%)\u00a0>\u00a030Fe10W90Ac (120) 30Fe95W5Ac (93.3%)\u00a0>\u00a030Fe90W10Ac (66.6%)\u00a0>\u00a030Fe5W95Ac (13.3 %)\u00a0>\u00a030Fe75W25Ac (6.7%) (\nTable S5\n). Clearly, carbon yield % over 30Fe25W75Ac and 30Fe50W50Ac are higher than other catalysts. Previously, tungsten species were claimed to generate additional CH4 decomposition sites (Patel et al., 2021). It seems that the presence of 25-50\u00a0wt% of WO3 in the catalyst system cultivates the optimum amount of catalytic active sites which leads potential dissociation of CH4 into carbon and H2. It resulted in an excellent carbon yield and severe weight loss. These findings also give the sign of higher activity toward CH4 decomposition reaction over 30Fe25W75Ac and 30Fe50W50Ac catalysts. Severe weight loss over spent 30Fe25W75Ac and spent 30Fe50W50Ac catalysts also indicates that the carbon deposits over these catalysts are not inert, it is oxidizable under O2 stream. Inert carbon deposit may shade the catalytic active site permanently and causes fast deactivation. The non-inert carbon deposit over 30Fe25W75Ac and spent 30Fe50W50Ac catalysts may cause slower deactivation than other catalysts.O2-TPO of spent 30FexW(100-x) Ac (x\u00a0=\u00a00\u2013100) catalyst system is shown in Fig. 5\nC-5D. In the literature, the O2-TPO peak profile is differentiated into three regions 300\u2013500\u00a0\u00b0C for easily oxidizable \u03b1-carbon (amorphous carbon) species (Al-Fatesh et al., 2021), 500\u2013600\u00a0\u00b0C for moderately oxidizable \u03b2-carbon species (Patel et al., 2021) and\u00a0>\u00a0600\u00a0\u00b0C for higher crystallization degree of carbon species (Zhang et al., 2015). In our catalyst system, TPO peak maxima is found about\u00a0\u223c\u00a0600\u00a0\u00b0C in spent-30Fe5W95Ac, spent-30Fe50W50Ac, and spent-30Fe90W10Ac catalysts whereas, for spent-30Fe10W90Ac, spent-30Fe25W75Ac, spent-30Fe95W5Ac and spent-30Fe100W catalysts, TPO peaks are at about\u00a0\u223c\u00a0650\u00a0\u00b0C. This observation indicates that a particular amount of tungsten oxide (spent-30Fe5W95Ac, spent-30Fe50W50Ac, and spent-30Fe90W10Ac catalyst) in the support induces less crystallization degree of carbon (than30Fe10W90Ac, 30Fe25W75Ac, and 30Fe100W catalyst).The morphology of catalyst samples is shown by SEM images in \nFig. S6\n. The catalyst morphology of fresh low tungsten-containing samples (30Fe10W90Ac) or high tungsten-containing samples (30Fe90W10Ac) is not differentiable. In the case of spent 30Fe10W90Ac catalyst, carbon tubes are easily observed than in 30Fe90W10Ac catalyst. The morphology of the catalyst and carbon tube is evident in TEM images under Fig. 6\n. TEM image indicates particle size over the 30Fe25W75Ac has grown from 5.57\u00a0nm to 5.8\u00a0nm after the reaction (Fig. 6\nA-D). Fig. 6\nE shows the presence of carbon nanotubes of varying diameters. A typical multiwalled carbon tube having wall width of 3.81\u20134.74\u00a0nm and total tube width of 13.13\u00a0nm is evident in Fig. 6\nF.The X-ray photo-electron spectra of 30Fe25W74Ac catalyst is shown in Fig. 7\n\n. Fe (2p3/2) peak at 711\u00a0eV and Fe (2p1/2) peak at 725\u00a0eV and O (1\u00a0s) peak at 530.1 confirms the presence of Fe+3 oxidation state (Allen et al., 1974; Konno and Nagayama, 1980) (Fig. 7\nA- B). The presence of W(4f7/2) peak at 35.4\u00a0eV and W(4f5/2) peak at 37.6\u00a0eV confirm the presence of WO3 or W+6 oxidation state (Fig. 7\nC) (Barreca et al., 2001). The C(1\u00a0s) XPS spectra is observed at 284.7 (Barreca et al., 2001; Gr\u00fcnert et al., 1987) (\nFig. 7\n). The 30Fe25W75Ac catalyst has both carbon and WO3 but absence of carbidic carbon peak at 282.7\u00a0eV indicates that WC like species are not formed over the catalyst surface (Katrib et al., 1994). Overall, from the XPS spectra presence of Fe (III) (as Fe2O3), W (IV) (as WO3) species are confirmed. Fe2O3 and WO3 phases are already confirmed during the XRD analysis of sample.The CH4 conversion, H2-yield and ratio of H2-yield/CH4 conversion (\n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n) are shown in Fig. 8\n\nA-8C. The initial conversion of CH4 and H2-yield at 30wt.%Fe supported over activated carbon (30Fe100Ac) is just 4.43% and 1.96% respectively whereas 30wt.%Fe supported over tungsten oxide (30Fe100W) shows 20.5% initial CH4 conversion and 19.43% initial H2-yield. Markedly tungsten-oxide supported iron has a higher catalytic activity as well as ratio of \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n is\u00a0>\u00a00.94 whereas activated-carbon-supported iron has low catalytic activity and a ratio of \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\nis just half (Fig. 8\nC). During the entire time on stream (420\u00a0min), \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio remains close to 0.9 over 30Fe100W whereas \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio drops to\u00a0\u223c\u00a00.2 over 30Fe100Ac at the end of 420-minutes time on stream. A high \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio indicates a higher accumulation of CH4 or CHx over the catalyst surface followed by higher H2 release to the gas phase during the reaction (\u0141amacz and \u0141abojko, 2019). The higher activity of 30Fe100W than 30Fe100Ac catalyst toward methane decomposition reaction can be explained by X-ray diffraction and H2-TPR results. 30Fe100Ac catalyst had only reducible iron oxide as surface active species whereas 30Fe100W has iron oxide, tungsten oxide, and Fe2WO6 mixed oxide phases. Patel et. al found \u201cadditional CH4 decomposition sites\u201d over tungsten oxide-zirconia in CH4-temperature programmed surface reaction experiment (Patel et al., 2021). It was reported that Fe2WO6 may also be reduced to respective Fe and W under the hydrogen stream at reaction temperature (Pak et al., 2009). In H2-TPR results, we found the reduction peak for WO3 at 593\u00a0\u00b0C to 720\u00a0\u00b0C over 30Fe100W catalyst. That means 30Fe100W has a variety of reducible surface-active species that markedly influence the CH4-decomposition reaction. Overall, it can be said that tungsten oxide is promising support for Fe-based catalyst for CH4 decomposition reaction.Up to incorporation of 5\u201310\u00a0wt% WO3 in 30FexW(100-x) Ac (x\u00a0=\u00a05\u201310) catalyst, X-ray diffraction peaks are suppressed and surface area is increased up to 2.5 times (with respect to 30Fe100Ac catalyst). It indicates the suppression of crystallinity upon expansion of the surface (Khalid et al., 2013). H2-TPR result of 30Fe10W95Ac showed increased metal-support interaction over than 30Fe5W95Ac catalyst. Increased metal-support interaction over an expanded surface is the ideal condition for exposing more active sites for CH4 decomposition. Overall, it can be said that 30Fe5W95Ac catalyst has an expanded surface (than 30Fe100Ac) and 30Fe10W90Ac catalyst has an expanded surface (than 30Fe100Ac) as well as increased interaction of reducible Fe-species (Fe2O3, Fe3O4, FeO) with support (than 30Fe5W95Ac). 30Fe5W95Ac catalyst has 29.5% initial CH4 conversion (against 4.43% in 30Fe100Ac) and 22.5% initial H2-yield (against 1.96% in 30Fe100Ac). 30Fe10W90Ac catalyst shows 50.2% initial CH4 conversion and 48.39% initial H2-yield. The \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio over 30Fe10W90Ac also remains\u00a0\u223c\u00a00.9 up to 250\u00a0min whereas, over 30Fe5W95Ac, \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio falls to 0.5 within 70\u00a0min time on stream. Here, the role of tungsten in CH4 decomposition, higher surface area, and more \u201csurface interacted reducible Fe-species\u201d are evident over 30Fe10W90Ac catalyst which achieves higher CH4 conversion and higher \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\nratio.Up to 25\u00a0wt% W incorporation; prominent iron oxide phase is evident over 30Fe25W75Ac catalyst. The presence of Fe2O3 and WO3 phases are also confirmed by Fe(2p), W(4f) and O (1\u00a0s) XPS spectra. The decrease in surface area (up to 30%) of 30Fe25W75Ac (with respect to 3010W90Ac) is compensated by an increase in average pore diameter (up to 46%). 30Fe50W50Ac has again expanded surface area (three times than 30Fe100Ac) and other tungsten-related phases like tungsten oxide, tungsten carbide, and Fe2(WO4)3 over the surface. 30Fe25W75Ac catalyst has the highest amount of reducible species (183\u00a0cm3/g H2 consmption in H2-TPR result) over the surface among the rest tungsten oxide incorporated catalysts. These reducible iron species have interacted with the support along a wide range of temperatures as per the extent of interaction with support. 30Fe50W50Ac catalyst has also good number of reducible species (174\u00a0cm3/g H2 consumption in H2-TPR result) after 30Fe25W75Ac catalyst. Previously, tungsten species were claimed to generate additional CH4 decomposition sites (Patel et al., 2021). 30Fe25W75Ac and 30Fe50W50Ac catalysts show severe weight loss and higher carbon yield. The catalyst activity of 30Fe50W50Ac and 30Fe25W75Ac towards the CH4 decomposition reaction are also close to each other initially. It indicates that the presence of 25-50\u00a0wt% of WO3 in the catalyst induces an optimum amount of catalytic active sites leading to potential CH4 dissociation, excellent carbon yield, severe weight loss and optimum H2-yield over 30Fe25W75Ac and 30Fe50W50Ac catalysts. The initial CH4 conversion of 30Fe25W75Ac and 30Fe50W50Ac catalysts are 66.04% and 64.82 % respectively. Again, the initial H2-yield over 30Fe25W75Ac and 30Fe50W50Ac catalyst is found 63.12% and 59.81 % respectively.Fe supported over \u201cxW(100-x)Ac (x\u00a0=\u00a010\u201350)\u201d are able to show\u00a0>\u00a050% CH4 conversion, \u226550% H2-yield and\u00a0\u223c\u00a00.9 \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio initially. Severe weight loss in TGA profile is obtained over 30Fe25W75Ac and spent 30Fe50W50Ac catalysts. It indicates that carbon deposits over these catalysts are oxidizable/not inert/active. The non-inert carbon deposit deactivates 30Fe25W75Ac and spent 30Fe50W50Ac catalyst slowly than the rest catalysts. After 160\u00a0min, CH4 conversion and H2 yield of the 30Fe25W75Ac catalyst drop to 37.61% (against 66% initial CH4 conversion) and 35.2% (against 63.12% initial H2 yield) respectively. 30Fe10W90Ac catalyst is found the second best as the CH4 conversion and H2-yield don\u2019t fall below 25% after 160\u00a0min time on stream. The \n\n\nY\n\nH\n2\n\n\n\n/\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio of both 30Fe25W75Ac and 30Fe10W90Ac catalyst is also\u00a0\u2265\u00a00.9 up to 240\u00a0min. At the end of 160\u00a0min 30Fe50W50Ac catalysts showed\u00a0\u223c\u00a019% CH4 conversion, \u223c11% H2 yield and 0.56 \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio. Overall, at the end of 420\u00a0min time on stream, 30Fe25W75Ac is found best. It has\u00a0\u223c\u00a014% CH4 conversion, \u223c6% H2-yield and\u00a0>\u00a00.4 \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio at 420\u00a0min time on stream.TGA results indicate severe coke decomposition over 30FexW(100-x) Ac (x\u00a0=\u00a025, 50) catalyst. O2-TPO result indicates that coke over the 30Fe25W75Ac catalyst has a higher crystallization degree than the 30Fe50W50Ac catalyst. The carbon yield calculation of the spent 30FexW(100-x) Ac (x\u00a0=\u00a05, 10, 25, 50, 90, 95) catalyst system is shown in \nTable S5\n. Here also, the carbon yield over the spent 30Fe25W75Ac catalyst is greater than the 30Fe50W50Ac catalyst. Interestingly, the catalytic activity of the 30Fe25W75Ac catalyst is less affected by severe coke deposition but the activity of 30Fe50W50Ac drops suddenly on increasing time on stream. Initially, the activity of both catalysts is close to each other but after the end of 200\u00a0min, 30Fe50W50Ac has only\u00a0\u223c\u00a016% CH4 conversion (against 30% in 30Fe25W75Ac), 9% H2-yield (against 29% in 30Fe25W75Ac) and 0.56 \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio (against 0.97 in 30Fe25W75Ac). It indicates that coke decomposition affects the performance of 30Fe50W50Ac to a great extent but not the performance of 30Fe25W75Ac catalyst. It seems that over 30Fe25W75Ac, rate of CH4 decomposition (carbon formation) is well matched with the rate of diffusion of carbon species from metal-gas interface (where decomposition of CH4 took place) to the metal-nanofiber interface (where carbon precipitates to form carbon nanofibers). Over highly crystalline 30Fe50W50Ac catalyst (than 30Fe25W50Ac), the rate of carbon formation may not properly match the rate of carbon diffusion. So, carbon species isn\u2019t able to be transferred away in time and would cover the catalyst\u2019s active sites leading to catalyst deactivation (Chen et al., 1997).Tungsten oxide incorporation of>50\u00a0wt% in 30FexW(100-x)Ac (x\u00a0=\u00a075, 90, 95) causes a fast drop of surface area due to the deposition of various crystallite inside the pore (Rahman et al., 2015) constituted by major-tungsten oxide and minor-activated carbon. These catalysts systems have also low density of reducible reducible-species over the catalyst surface. Low surface area catalyst and few catalytic active sites on the surface conveys less initial CH4 conversion. 30Fe75W25Ac has the highest crystallinity among rest catalyst systems. 30FexW(100-x) Ac (x\u00a0=\u00a075\u201395) catalysts has low initial CH4 conversion (14\u201336%) and initial H2-yield (13\u201329%).Tungsten oxide incorporated activated carbon is found to be an excellent support for Fe based catalyst towards CH4 decomposition reaction (than activated carbon incorporated tungsten oxide catalyst) due to enhanced surface area, a higher concentration of various types of reducible surface-active species as iron oxide, tungsten oxide, and Iron tungstate. The research outcome over 30FexW(100-x) Ac (x\u00a0=\u00a00\u201350) catalyst can be pointed as follow:\n\n\u2022\n30Fe10W90Ac catalyst has a comparable surface area but higher metal support interaction than the 30Fe5W95Ac catalyst. So, the earlier one has higher activity than latter.\n\n\n\u2022\n30FexW(100-x) Ac (x\u00a0=\u00a010\u201350) catalyst shows\u00a0>\u00a050% initial CH4 conversion, \u223c50% initial H2-yield and\u00a0\u223c\u00a00.9 initial \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio.\n\n\n\u2022\n30Fe25W75Ac catalyst has the highest concentration of reducible surface-active species (compared to the rest tungsten incorporated catalysts). It shows 66.04% initial CH4 conversion and 63.12% initial H2 yield and\u00a0>\u00a00.9 initial \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n.\n\n\n\u2022\n30Fe50W5pAc catalyst has a comparable concentration of reducible surface-active to 30Fe25W75Ac catalyst. Both catalysts have severe carbon deposits, higher carbon yield, higher initial CH4 conversion and higher initial H2 yield than other catalysts due to the potential dissociation of CH4 into carbon and H2.\n\n\n\u2022\nOn longer time on stream, the activity of the 30Fe50W50Ac catalyst drops fast than 30Fe25W75Ac catalyst due to improper matching between the rate of carbon formation and the rate of diffusion over highly crystalline 30Fe50W50Ac catalyst (compared to 30Fe25W75Ac catalyst). Even after 160\u00a0min, CH4 conversion and H2 yield over 30Fe25W75Ac catalyst does not drop below 35%.\n\n\n\u2022\nInferior catalytic activity over 30FexW(100-x)Ac (x\u00a0=\u00a075, 90, 95) is due to low surface area catalyst and few catalytic active sites.\n\n\n30Fe10W90Ac catalyst has a comparable surface area but higher metal support interaction than the 30Fe5W95Ac catalyst. So, the earlier one has higher activity than latter.30FexW(100-x) Ac (x\u00a0=\u00a010\u201350) catalyst shows\u00a0>\u00a050% initial CH4 conversion, \u223c50% initial H2-yield and\u00a0\u223c\u00a00.9 initial \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n ratio.30Fe25W75Ac catalyst has the highest concentration of reducible surface-active species (compared to the rest tungsten incorporated catalysts). It shows 66.04% initial CH4 conversion and 63.12% initial H2 yield and\u00a0>\u00a00.9 initial \n\n\nY\n\nH\n2\n\n\n\n /\n\n\nC\n\nC\n\nH\n4\n\n\n\n\n.30Fe50W5pAc catalyst has a comparable concentration of reducible surface-active to 30Fe25W75Ac catalyst. Both catalysts have severe carbon deposits, higher carbon yield, higher initial CH4 conversion and higher initial H2 yield than other catalysts due to the potential dissociation of CH4 into carbon and H2.On longer time on stream, the activity of the 30Fe50W50Ac catalyst drops fast than 30Fe25W75Ac catalyst due to improper matching between the rate of carbon formation and the rate of diffusion over highly crystalline 30Fe50W50Ac catalyst (compared to 30Fe25W75Ac catalyst). Even after 160\u00a0min, CH4 conversion and H2 yield over 30Fe25W75Ac catalyst does not drop below 35%.Inferior catalytic activity over 30FexW(100-x)Ac (x\u00a0=\u00a075, 90, 95) is due to low surface area catalyst and few catalytic active sites.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors extend their appreciation to the Deanship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project no. (IFKSURG-2-055).Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2023.104781.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Production of COx-free H2 from CH4 (a major global warming contributor) over cheap catalysts is a dominant task for the scientific community to accomplish environmental-friendly clean H2 energy sources. Herein, a tungsten oxide-activated carbon-supported Fe catalyst is prepared by impregnation method, characterized by X-ray diffraction, surface area-porosity measurement, temperature programmed reduction/oxidation and thermogravimetry analysis. 30wt.%Fe supported tungsten oxide incorporated activated carbon catalyst is found superior to 30\u00a0wt% Fe supported on activated carbon incorporated tungsten oxide due to higher surface area and high concentration of reducible catalytic active sites. 30wt.%Fe impregnated over 25\u00a0wt%WO3-75\u00a0wt%activated carbon support catalyst has the highest concentration of reducible surface-active species and it had excellent performance among other tungsten oxide incorporated catalysts. The catalyst showed 66.04% CH4 conversion, 63.12% H2 yield and \n                        \n                           \n                              Y\n                              \n                                 H\n                                 2\n                              \n                           \n                        \n                      /\n                        \n                           \n                              C\n                              \n                                 C\n                                 \n                                    H\n                                    4\n                                 \n                              \n                           \n                        \n                     \u00a0>\u00a00.9 initially which didn\u2019t fall below 35 % up to 160-minutes. Improper matching between the rate of carbon formation and the rate of diffusion over a highly crystalline 30Fe50W50Ac catalyst resulted in rapid deactivation.\n               "}
{"full_text": "No data was used for the research described in the article.The reduction of organic compounds is an important chemical technology used for the fabrication of valuable components for agrochemicals, pharmaceuticals, and cosmetics [1].Different Ni catalysts have been employed for hydrogenation of hydrocarbons and functionalized organic compounds [2\u20137]. The Raney Ni is a representative example of heterogeneous catalysts used in the laboratory scale as well as in the industry [8]. In most cases, heterogeneous catalysts contain a metal active phase bonded to the inorganic or organic support. Exceptionally, Raney Ni catalyst is used in the unsupported form. Although catalytic activity of Ni compounds is in many cases lower than precious metals-based catalysts, they are studied intensively as a cheaper and more environmentally friendly alternative [9\u201311]. Not only the low price but also the abundance of Ni motivate development of catalytic processes based on this metal [12\u201314].Hydrogenation of acetophenone (APh) is a promising method to produce 1-phenylethanol (PhE), an important intermediate for fabrication of ibuprofen. This process can be carried out using gaseous hydrogen or transfer hydrogenation methods [15\u201317].Yus et al. reported on transfer hydrogenation of APh in the presence of Ni NPs and 2-propanol/NaOH as a hydrogen donor. Modest to high yields of the corresponding alcohols were obtained, however, a serious drawback of this systems was the application of stoichiometric amounts of nickel [18,19]. In the applied conditions Raney Ni formed a mixture of products, while Ni/Al2O3 was not active. In 2014, Marchi described a catalyst based on NiO/SiO2 for APh hydrogenation at 10\u00a0bar of H2. They found that the kind of solvent strongly affects the catalytic activity, however, without any impact for the selectivity to PhE [20]. The highest reaction rates were noted for C2 \u2013 C3 alcohols.Ni@C catalyst, containing Ni supported on graphene, was used for hydrogenation of APh to PhE in a flow reactor under 1\u00a0MPa of H2 at 100\u00a0\u00b0C. After 48\u00a0h of continuous reaction selectivity to PhE was 97.77% [21]. Amorphous Ni-B-P materials supported on macroporous SiO2 were used in selective hydrogenation of APh to PhE under 1.5\u00a0MPa of H2\n[22].The harmful pollutant, 4-nitrophenol (4-NP), can be transformed by hydrogenation to 4-aminophenol (4-AP) which is used for drugs synthesis [23]. Yang et al. have fabricated a new Ni catalyst supported on mesoporous carbon and employed it for reduction of 4-NP with NaBH4. Spherical Ni NPs showed the largest activity factor of 20.9\u00a0s\u22121 g\u22121\n[24]. Dong et al. prepared an Ni catalyst supported on N-doped mesoporous carbon, Ni/m-CN, by pyrolysis of Ni-MOF at 700\u00a0\u00b0C. This nanocatalyst catalyzed hydrogenation of 4-NP to 4-AP with the activity factor of 9.1\u00a0s-1g\u22121\n[25]. This is the only example of using a catalyst obtained by thermal decomposition of Ni-MOF material in hydrogenation of 4-NP.Ni(0) containing MCM-41-carbon materials, prepared by carbonization, efficiently catalyzed reduction of 4-NP with the rate constant up to 0.09\u00a0min\u22121\n[26]. Kuo et al. presented hydrogenation of 4-NP to 4-AP with application of the NiO/NiS composites containing different amounts of sulfur [27].Our interest lies in using MOFs calcination for the synthesis of catalytically active composites [28,29]. Herein, we present calcination of Ni-BDP MOF (BDP\u00a0=\u00a01,4-Bis(pyrazol-4-yl)benzene) which resulted in the formation of two new Ni composites. Remarkably, calcination under N2/O2 atmosphere (4:1) led to the Ni/NiO composite, an active catalyst of transfer hydrogenation of APh and 4-NP, used in an amount of only 8\u00a0mol%.Nickel(II) acetate tetrahydrate (Ni(OCOCH3)2\u00b74H2O, 98%), sodium borohydride (NaBH4, 98%), APh (99%) and 4-NP (99%) were purchased from Sigma-Aldrich. Water purified by an Ultrapure Water System was used in all experiments. 4,4\u2032-Benzene-1,4-diylbis(1H-pyrazole) (BDP) was prepared according to the procedure reported previously [30].Inductively coupled plasma mass spectrometry analysis (ICP-OES) was carried out in an iCAP 7400 DUO icp (Thermo Fisher Scientific). Transmission electron microscopy (TEM) images were performed using a FEI Tecnai G2 20 X-TWIN microscope. X-ray diffraction (XRD) patterns were recorded with a powder X-ray diffractometer D8-ADVANCE Bruker (Cu-K\u03b1, \u03bb\u00a0=\u00a01.54056\u00a0\u00c5). X-ray photoelectron spectra (XPS) were recorded by XPS/AES system EA10 (Leybold-Heraeus GmbH, Cologne, Germany). All acquired spectra were calibrated to adventitious carbon C1s at 285\u00a0eV. A microRaman apparatus (inVia\u2122 Renishaw) was used to register both the Raman and the emission spectra with 830 and 514\u00a0nm excitation lines, respectively. GC-FID and GC\u2013MS were performed using a Shimadzu QP 2010 SE. UV\u2013vis spectra were recorded in Varian Cary 50 UV\u2013Vis spectrophotometer. Magnetization measurements at 300\u00a0K were carried out on 0.00161\u00a0g sample of compound using a Quantum Design SQUID Magnetometer (type MPMS-XL5) with an applied field between \u221220000 Oe to 20000 Oe.\nNiBDP was prepared according to the procedure reported previously [30]. 3\u00a0mmol of ligand BDP (4,4\u2032-benzene-1,4-diylbis(1H-pyrazole)) was dissolved in 160\u00a0mL of N,N\u2032-dimethylformamide and 4\u00a0mmol of Ni(CH3COO)24H2O was dissolved in 40\u00a0mL of H2O. The two solutions were mixed and refluxed for 12\u00a0h under stirring. The obtained material was filtered off and washed with N,N\u2019-dimethylformamide, ethanol and dried in air.0.31\u00a0g of NiBDP MOF was transferred to a quartz vessel and the solid was heated at 700\u00a0\u00b0C for 10\u00a0min under a flow of N2/O2 (4:1) with heating/cooling rates of 2.5\u00a0\u00b0C/min. Yield: 0.064\u00a0g. ICP: Ni 31.7%; Elemental anal.: C 1.10%, N 0.03%, H 1.22%.0.31\u00a0g of NiBDP MOF was transferred to a quartz vessel and the solid was heated at 700\u00a0\u00b0C for 10\u00a0min. under a flow of air with heating/cooling rates of 2.5\u00a0\u00b0C/min. Yield: 0.051\u00a0g. Elemental anal. C 0.5%; N 0.04%; H 0.37%.Aqueous solutions of 4-NP (75\u00a0mL, 0.15\u00a0mmol) and NaBH4 (75\u00a0mL, 15.0\u00a0mmol) were transferred into a three-neck roundbottom flask. After several minutes of stirring, NiOBDP or Ni/NiOBDP catalyst (8\u00a0mol%), were transferred into the flask. The mixture was continuously stirred at room temperature (25\u00a0\u00b0C). The progress of the reduction reaction was monitored by taking 0.5\u00a0mL aliquots of the supernatant solution, diluting them with cold deionised water (9.5\u00a0mL, 5\u00a0\u00b0C), and examining by UV\u2013Vis spectroscopy.The transfer hydrogenation reaction was performed in a glass bottle. The catalyst (8\u00a0mol%), APh (1\u00a0mmol), NaOH (0.26\u00a0mmol), 2-propanol (3\u00a0mL), were introduced into a bottle, and then the bottle was sealed and placed in an aluminium block and heated to 60\u201380\u00a0\u00b0C for 2\u20135\u00a0h. After the reaction was finished, the catalyst was separated by centrifugation, the organic products were analyzed by means of GC-FID and GC\u2013MS.Two new Ni-based catalysts, NiOBDP and Ni/NiOBDP, were prepared by thermal decomposition of the Ni-BDP MOF in different conditions (Scheme 1\n).The composites were characterized first by XRD (Fig. 1\nA) and Raman spectroscopy (Fig. 1B). X-ray diffraction (XRD) patterns of NiOBDP\n and Ni/NiOBDP\n are shown in Fig. 1A. The major diffraction peaks of NiO at 37.2\u00b0 for (111), 43.2\u00b0 for (200), and 62.9\u00b0 for (220) planes are observed in both samples, in agreement with the standard pattern peaks of cubic NiO structure (JCPDS No. 04\u20130835). Moreover, the major peaks of Ni(0) phase (JCPDSNo. 04\u20130850) were also observed at 44.5\u00b0 for (111), and 51.8\u00b0 for (200) planes for Ni/NiOBDP\n. The Raman spectra additionally revealed the presence of NiO in both composites. The small peaks centered at 200\u00a0cm\u22121 can be assigned to wide one-phonon (1P). The strong band at 500\u00a0cm\u22121 stemmed from wide one-phonon (1P) Ni-O Raman bands. Additionally, bands at 721, 893 and 1090\u00a0cm\u22121 can be identified as the two-phonon (2P) Ni-O Raman bands [31].It is important to note that despite of very similar synthetic procedures, in one of the samples, Ni/NiOBDP\n, Ni(0) was found besides of the main fraction of NiO. Thus, thermal decomposition of Ni-BDP MOF performed under N2/O2 (4:1) atmosphere produced some amount of Ni(0), whereas only NiO was formed in the air atmosphere.The TEM images (Fig. 2\n) showed the crystallites of the NiO particles with the diameter varying from 23 to 51\u00a0nm. The average size of nanoparticles calculated using the Scherrer equation was 29.7\u00a0nm, close to the average size obtained from the TEM images.\nFig. 3\n exhibits the Ni 2p3/2 spectra of NiOBDP\n and Ni/NiOBDP\n catalysts. Both spectra are similar and the major peak at 854.6\u00a0eV with the broad satellite peak centered at 861.8\u00a0eV that can be attributed to the surface Ni2+ species [32]. The shoulder peak at 856.7\u00a0eV originated from the surface Ni3+ species [33]. The peaks of Ni2+ and Ni3+ are slightly shifted compared to the commercial NiO (853.7\u00a0eV, 855.5\u00a0eV [34]. It is noteworthy that the signal of metallic Ni was not found in the spectrum of Ni/NiOBDP\n.O1s XPS spectrum (Fig. S1A) also confirmed the presence of NiO in both samples. The deconvoluted peak located at 529.3\u00a0eV stemmed from O bonded to Ni2+. In addition, the peak located at 530.3\u00a0eV was assigned to the O adjacent to the Ni vacancy [35]. Due to the different sample preparation routes (using the air or N2/O2 mixture), various amounts of water were present on the surface of NiO. According to the XPS results NiOBDP\n contained more water than Ni/NiOBDP\n (Fig. S1A). It should also be noted that for the Ni/NiOBDP\n sample more surface defects are visible in XPS and TEM pictures.Magnetic properties of Ni/NiOBDP\n were analyzed by a vibrating sample magnetometer. The magnetization curve reached a saturation value Ms about 15.09 emu g\u22121 (Fig. 4\n). The observed hysteresis loop characterized by the remanent magnetization Mr\u00a0=\u00a00.374 emu g\u22121 and coercivity Hc\u00a0=\u00a035 Oe is an evidence of ferromagnetic behavior of the material at room temperature.It is worth to note that magnetic nature of the catalyst facilitates its separation from the reaction mixture using a simple magnet. This enables to avoid additional operations such as centrifugation, filtration or other procedures before the catalyst recycling.The evaluation of catalytic activity of the new Ni catalysts was first performed for 4-NP transfer hydrogenation with NaBH4. The progress of the transformation of 4-NP to 4-AP was monitored by UV\u2013Vis spectroscopy (Fig. 5\n).The absorption peak at 400\u00a0nm, originating from 4-nitrophenolate ion formed after addition of NaBH4, decreased in time in the presence of the Ni/NiOBDP\n or NiOBDP\n nanocatalysts confirming conversion of 4-NP. Concomitant increase in the intensity of the peak at 300\u00a0nm, corresponding to 4-AP, indicated the product formation. The reactions were carried out for 45\u00a0min for the Ni/NiOBDP\n catalyst and 55\u00a0min for NiOBDP\n. The rate constants, equal to 0.032 and 0.015\u00a0min\u22121 for Ni/NiOBDP\n and NiOBDP\n, respectively, were obtained assuming pseudo-first-order kinetics by fit the equation ln(At/A0)\u00a0=\u00a0\u00a0\u2212\u00a0kt where A0 and At represent the absorbance values of 4-NP at the start and at the time t. For comparison, the rate constant 0.043\u00a0min\u22121 was obtained for MCM-41\u2013Ni [26]. Interestingly, in the same paper the potential activity of NiO in the hydrogenation of 4-NP was mentioned but not confirmed. Higher values of the rate constants, from 0.045 to 0.854\u00a0min\u22121 were reported for NiO/NiS catalysts of different composition [27].In our case, both catalysts contain mainly NiO but the presence of Ni(0) in Ni/NiOBDP\n enhanced the catalytic activity. Our results, confirming that NiO can catalyze the reduction of 4-NP, corroborate the studies of [27]. In contrast, other authors did not obtain 4-AP from 4-NP using NiO as the catalyst [36].The XRD (Fig. S2) and XPS (Fig. S1B) data of the catalysts recovered after 4-NP hydrogenation, did not confirm unequivocally the presence of Ni(0). However, analysis of the TEM images (Fig. S2) showed some changes which could suggest the presence of Ni(0) nanoparticles dispersed on the NiO surface. Therefore, we presume that a certain fraction of NiO might be reduced to Ni(0), which might be the true catalytically active species. In fact, the catalytic tests performed with reused catalysts clearly showed increase of the rate constants to 0.048 and 0.026\u00a0min\u22121 for Ni/NiOBDP\n and NiOBDP\n, respectively, which can be attributed to the increase of Ni(0) content (Fig. S4-S5).Transfer hydrogenation of APh occurred selectively with formation of PhE as the only product. The kinetic curves for the reaction catalyzed by Ni/NiOBDP\n at different temperatures are shown in Fig. 6\n.As a result of the temperature optimization, it was found that the highest conversion of APh to PhE occurred at 80\u00a0\u00b0C with 63% conversion after 5\u00a0h reaction. The activation energy (E\na) was calculated by the temperature dependence of the hydrogenation rate. (Fig. 7\n) It was assumed that hydrogenation of APh to 1-PhE is a first-order reaction (dCAP/dt\u00a0=\u00a0-kCAP, ln(\n\n\nC\n\nAP\n\no\n\n/\n\nC\nAP\n\n)\n\n\u00a0=\u00a0kt) and based on the Arrhenius relation, k\u00a0=\u00a0A\u00b7exp(-E\na/RT), the activation energy was determined to be 88\u00a0kJ/mol (21\u00a0kcal/mol). This value of the activation energy is slightly higher than that reported for Ni\u2013B\u2013P/SiO2 catalyst (50.73\u00a0kJ/mol) [22].After optimization of the reaction conditions, NiOBDP\n was used as the catalyst. We found that in the first 3\u00a0h the reaction is slower than with Ni/NiOBDP\n and after 3\u00a0h the conversion was 39%. However, the final conversion of APh obtained after 5\u00a0h was similar for both catalysts (Fig. 8\n). After the reaction, the NiOBDP\n and Ni/NiOBDP\n nanocatalysts were separated and reused under the same conditions without any loss of their catalytic activity. The conversion of APh was 56% for NiOBDP\n-R and 65% for Ni/NiOBDP\n-R.Next, both the recovered catalysts, NiOBDP\n-R and Ni/NiOBDP\n-R, were examined using TEM, XRD and XPS methods. Figure S3 shows only minor changes in the TEM images. However, the analyses of FFT (Fast Fourier Transform) allowed to determine d-spacings values (Fig. S6). Notably, the fringes of 0.207 and 0.205\u00a0nm for Ni/NiOBDP-R and NiOBDP-R, assigned to Ni(111), evidenced the presence of Ni(0) [37].Furthermore, both samples of the used catalysts were examined by XPS with Ar+ ion etching. The spectra of the Ni 2p3/2 region (Fig. S7) exhibited a main peaks at 852.6\u00a0eV characteristic for Ni(0) [38]. Additionally, signals at 853.8\u00a0eV and a satellite at 861.03\u00a0eV were assigned to Ni2+ species. The peak at 856.72\u00a0eV is attributed to Ni3+.XRD analysis of the Ni/NiOBDP-R sample indicated some decrease in the intensity of the (111) reflection of Ni(0) (Fig. S3A).In summary, two new NiO-based catalysts were synthesized by a calcination of Ni-MOF. Calcination of MOF in a N2/O2 (4:1) gas stream led to a mixed composite Ni/NiOBDP\n while thermal decomposition in an air atmosphere provided only NiOBDP. The presence of Ni(0), formed in the absence of any reducing agent, affected the catalytic activity and Ni/NiOBDP\n provided better results in the transfer hydrogenation than NiOBDP.\nReduction of the \u2013NO2 group in 4-NP with NaBH4 proceeded for both catalysts, however, it was incomplete even after 55\u00a0min for NiOBD\n\nP indicating insufficient number of active sites for formation of 4-AP. The Ni/NiOBDP\n catalyst reacted faster due to the presence of active Ni(0) centers. During the catalytic process the structure of both catalysts was modified and their activity increased. Reduction of NiO to Ni(0) by NaBH4 was probably slow because of the presence of a strong Ni-O bond [39], however, the TEM images of the catalysts recovered after the reaction suggest the presence of ultra-small Ni(0) NPs. Consequently, the recycled catalysts exhibited higher activity in the second run than in the first one, as it was evidenced by the increase of the rate constants. Our kinetic results corroborate well with these reported for MCM-41-Ni containing mostly NiO phase [26]. The similar value of rate constant, 0.045\u00a0min\u22121 was also obtained for the NiO catalyst [27]. In general, catalysts containing higher amounts of Ni(0) reacted faster [24,25]. Similarly, Ni/NiOBDP\n was more efficient in the transfer hydrogenation of APh to PhE than NiOBDP\n. In this reaction 63% of PhE was obtained after 5\u00a0h using 8\u00a0mol % of catalyst. The TOF value for this reaction, equal 1.6\u00a0h\u22121, is comparable to the TOF values presented for transfer hydrogenation of furfural using different NiO catalysts (0.4 \u2013 2.6\u00a0h\u22121) [34].\nAdam W. Augustyniak: Conceptualization, Investigation, Writing \u2013 original draft. Andrzej Gniewek: Funding acquisition, Writing \u2013 review & editing. Rafa\u0142 Szukiewicz: Investigation. Marcin Wiejak: Investigation. Maria Korabik: Formal analysis. Anna M. Trzeciak: Conceptualization, Writing \u2013 review & editing, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Andrzej Gniewek is grateful for financial support from the National Science Centre (NCN, Poland) with grant MINIATURA 2021/05/X/ST4/00402.Supplementary data to this article can be found online at https://doi.org/10.1016/j.poly.2022.116029.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The NiOBDP and Ni/NiOBDP composites obtained by calcination of NiBDP MOF (MOF\u00a0=\u00a0metal\u2013organic framework; BDP\u00a0=\u00a01,4-Bis(pyrazol-4-yl)benzene) in air at 700\u00a0\u00b0C were characterized by XRD (X-ray diffraction), TEM (Transmission electron microscopy), XPS (X-ray photoelectron spectroscopy) and Raman spectroscopy. Both nanocatalysts presented high activity in selective transfer hydrogenation of acetophenone (APh) and 4-nitrophenol (4-NP) in mild conditions, using 2-propanol/NaOH or NaBH4/H2O as reducing agents. In both reactions, the catalytic activity of Ni/NiOBDP was higher compared to NiOBDP suggesting the participation of Ni(0) in the hydrogenation process.\n               "}
{"full_text": "CO2 and CH4 are not only typical greenhouse gases, but also important carbon-containing resources. The CH4-CO2 reforming to syngas process can combine the comprehensive utilization of CH4 and the resource utilization of CO2, which provides a technological route for the comprehensive utilization of carbon and hydrogen sources and the conversion of greenhouse gases on a large scale [18,40,49]. This process is also known as dry reforming of methane (DRM) because no water vapor is involved in the reaction. The DRM process can be used to treat industrial off-gases such as coke oven gas and choke gas containing CH4 and CO2, and is also suitable for the conversion of CO2-rich field gas and offshore natural gas resources [9,43]. Compared to water vapor reforming and partial oxidation reforming, the DRM process utilizes both CH4 and CO2, the two major greenhouse gases, and has both environmental and economic benefits [27,29,42,47]. Compared to water vapor reforming, DRM eliminates the need for an energy-intensive evaporation process and can be used in water-scarce regions. DRM is a strong heat absorption reaction, storing energy in the form of syngas, which can be used to store and transport energy. However, there are both opportunities and challenges, as CH4 and CO2 are chemically stable resource small molecules whose conversion is thermodynamically unfavorable [6,14]. The efficient activation and directional reorganization of the bonds depends to a large extent on the catalyst used [45].The synthesis of phosphides dates back to the seventeenth century, when transition metal phosphides were initially studied by hydrodesulfurization, deoxidation, and other reactions. Among them, Ni2P has the best catalytic effect on the hydrodesulfurization reaction due to the key role of the (100) crystal plane of Ni2P material as the catalytic active plane [25]. In 2005, Zhang et al. [48] used DTF to calculate the energy changes of [Ni-Fe] hydrogenase and other compound materials of Ni during hydrogen precipitation. It was found that the atomic structure between Ni and P on the active crystal plane of [Ni-Fe] hydrogenase and Ni2P is very similar, thus inferring that Ni2P also has a relatively strong catalytic activity for hydrogen precipitation reactions [4,12]. In 2013, researchers tested for the first time Ni2P catalysts synthesized by the solvothermal method and concluded that Ni2P NPs have excellent catalytic properties [34]. The catalytic activity of Ni2P NPs is substantially higher compared to other previous transition metal compounds. Since then a large number of CoP materials with different preparation methods and morphological structures have been much studied for use as catalysts [13,31,41,46]. In the same year, nanostructured CoP with different morphologies such as nanowires, nanosheets and nanoparticles by low-temperature phosphorylation reaction without organic solvent were prepared under Ar conditions [36]. However, designing and modulating metal phosphides with specific morphological structures and stability is not easy. For this purpose, MOFs were used as precursors to obtain transition metal phosphides with different morphological structures and less collapse [17,35]. By modulating the electronic structure in this way, the specific surface area of the catalyst becomes larger, the conductivity is improved, and the catalytic activity is greatly enhanced.It has been shown that different synthesis methods can affect the particle size and distribution as well as the crystalline shape of the material, resulting in differences in the properties of MOF materials [19,22,30]. The traditional synthesis methods include hydrothermal and solvothermal methods, but there are complex processes, high energy consumption, long reaction times and high requirements for instrumentation that need to be improved. Compared with traditional methods, electrochemical methods have the advantages of mild reaction conditions (room temperature and pressure), simple process, easy operation, short reaction time, and safety and environmental protection. Therefore, electrochemical synthesis techniques are widely used in the field of novel nanoparticle synthesis. In 2005, Mueller et al. [24] research workers from BASF Corporation reported for the first time the synthesis of MOF by electrochemistry. The principle is that the metal ions are first obtained by in situ anodic oxidation and then dispersed into a solution containing organic ligands and electrolyte. The organic ligands are deprotonated and combined with the metal ions to produce the MOF material.So far, there have been many research results on the synthesis of microcrystalline MOF powder or thin film by electrochemical methods. Yang et al. [39] synthesized flower-like MOF-5 with a more perfect crystal structure in a tunable ionic environment by electrochemical methods. Gascon et al.[1] synthesized a variety of typical MOF materials, such as HKUST-1 (i.e. Cu3(BTC)2), ZIF-8, MIL-100(Al), etc., by electrochemical methods, and investigated the effects of different parameters on the crystalline structure and particle size during the reaction. Similarly, Denayer et al. [38] used an electrochemical method to synthesize HKUST-1 materials with controlled particle size in a short time at room temperature by adjusting the ratio of ethanol and water in the solvent. Therefore, the synthesis of MOF materials by electrochemical methods is well regulated in terms of controlling the crystalline structure and particle size. In addition, the rapid synthesis reaction allows for continuous production, and these characteristics show important advantages in industrial settings. In this work, Cu3(BTC)2 with high yield, good crystal structure and pure components were synthesized by electrochemical method. The effects of different phosphating temperatures, phosphating ratios and phosphating methods on the performance of the synthesized cuprous phosphide were explored to explore the optimal synthesis conditions. After a series of characterizations, different catalysts were applied to the DRM reaction.1,3,5-H3BTC was purchased from Aladdin Technology Co. Tetrabutylammonium tetrafluoroborate (TBAFB) was purchased from Beijing Bailingway Technology Co. Methanol, ethanol (EtOH) and N, N-dimethylformamide (DMF) was purchased from Tianjin Comio Chemical Reagent Co. Copper electrodes (99 %) of 2\u00a0mm\u00a0\u00d7\u00a05\u00a0mm\u00a0\u00d7\u00a00.8\u00a0mm were used for both the anode and cathode of the electrochemical synthesis. NaH2PO2\u00b7H2O was purchased from Sinopharm Chemical Reagent Co.0.1\u00a0M 1,3,5-H3BTC, 0.05\u00a0M TBAFB were dissolved in 50\u00a0mL of methanol. The electrodes were inserted and electrolyzed at an applied voltage of 10\u00a0V for 2.5\u00a0h. The Cu3(BTC)2 powder was formed on the surface of the Cu electrode and fall off into the solution. The samples were then filtered and washed repeatedly with ethanol until the supernatant was clarified. The sample was dried in a drying oven at 60\u00a0\u00b0C for 12\u00a0h to obtain a sky blue sample of Cu3(BTC)2. Cu3(BTC)2 was activated at 230\u00a0\u00b0C under N2 protection for 2\u00a0h. The crystalline water adsorbed on Cu2+ was removed to obtain a catalyst with unsaturated metal coordination. The color of the Cu3(BTC)2 catalyst also changed to dark blue.Cu3(BTC)2 and NaH2PO2\u00b7H2O were placed in different proportions in two different small porcelain boats and placed in a tube furnace. The porcelain boat containing NaH2PO2\u00b7H2O was located at the upstream of the furnace. Under the protection of argon atmosphere, the boats were heated at 300\u00a0\u2103\u223c400\u00a0\u2103 for 2\u00a0h at a heating rate of 2\u00a0\u2103/min. After allowing the tube furnace to cool naturally to room temperature, argon was turned off and the black powder was collected and named Cu3P-X (X is a different heating temperature). The ratio of Cu3(BTC)2 and NaH2PO2\u00b7H2O and the phosphorylation temperature were varied to explore the optimal reaction conditions for the synthesis of Cu3P-X catalyst.The catalyst activity was evaluated using H2 reduction and CH4-CO2 reforming reactions simultaneously in a fixed-bed reactor. Weigh 0.2\u00a0g of catalyst into a quartz tube reactor with \u03d5 =\u00a08\u00a0mm and fix it with quartz wool. N2 at a flow rate of 40\u00a0mL/min was used as a shield gas. The temperature was purged at a rate of 10\u00a0\u2103/min for 70\u00a0min to reach 700\u00a0\u2103. After 700\u00a0\u00b0C, it was switched to H2 for reduction. After 2\u00a0h of reduction, the reaction was switched to reaction gas (CH4/CO2/Ar = 44.0/47.2/8.8, flow rate of 40\u00a0mL/min) for 6\u00a0h. The reaction was carried out at atmospheric pressure. Samples were taken every 30\u00a0min during the reaction. The export products were analyzed online by GC-TCD, and the conversion of the reactants CH4 and CO2. The selectivity of the products H2 and CO and H2/CO were calculated.The amount of carbon accumulation increases when the phosphating temperature reaches 400\u00a0\u00b0C. We examined the differences in the synthesis of catalysts in different solvents. The dissolution rate of copper metal flakes in the solvent was observed to be MeOH>\u00a0EtOH>\u00a0DMF during the experiment. Because the nature of the solvent affects the solubility of the organic ligand and the conductivity of the solution, this also results in differences in the specific surface area, pore volume and yield of the catalyst [15]. \nTable 1 shows the specific surface area, pore volume and yield of Cu3(BTC)2 catalysts synthesized in different solvents. It can be seen from Table 1 that the specific surface area and pore volume of the catalysts synthesized in MeOH were the largest with 800.3\u00a0m2/g and 0.411\u00a0cm3/g, respectively.During the electrochemical reaction, the electrolyte can reduce the resistance brought by the solvent and promote the dissolution of metal ions in the anode. By controlling the current density, the concentration of metal ions near the electrode can be adjusted [37]. Current density and voltage are correlated via system geometry and solution conductivity. Therefore, the electrolyte concentration and voltage regulation play a key role in the electrochemical reaction process [28]. When the voltage is fixed (10\u00a0V) and the concentration of electrolyte increases (0.01\u20130.1\u00a0M), high conductivity leads to high yield (\nFig. 1A). At a certain concentration of electrolyte (0.2\u00a0M), the increase in voltage (8\u201316\u00a0V) also brought about an increase in yield (Fig. 1B).The length of electrolysis time also has an effect on the yield. It can be seen from Fig. 1C that the yield increases with the increase of electrolysis time [32]. When the electrolysis time was less than 2\u2009h, the yield of catalyst was basically linear with the electrolysis time. The increase of reaction time brings obvious yield changes. When the electrolysis time exceeded 2\u2009h, the yield tended to be stable, indicating that the electrolysis reaction was basically completed after the electrolysis time exceeded 2\u2009h. This is also evidenced by the change in current. At the beginning of the reaction, the anode dissolution rate is high and the initial current is high. As the reaction proceeds and the reactants are consumed, the current drops sharply. When the reaction time exceeded 2\u2009h, the current was lower than 0.1\u2009A, and the reaction rate decreased significantly.\n\nFig. 2 A shows the XRD spectrum of the Cu3(BTC)2 catalyst. Among them, 6.6\u00b0, 9.5\u00b0, 11.6\u00b0, 13.4\u00b0, 14.6\u00b0, 15.0\u00b0, 16.5\u00b0, 17.4\u00b0, 19.0\u00b0, 20.2\u00b0 are the characteristic peaks of Cu3(BTC)2, which correspond to (111), (200), (220), (222), (400), (331), (422), (333), (440), (442) crystalline planes of Cu3(BTC)2\n[44], respectively. The sharp peaks in XRD indicate that the synthesized catalyst has a good crystalline structure. No peak of Cu2O (2\u03b8 = 36.4\u00b0) was observed in the XRD spectrum of the catalysts prepared by the optimized electrochemical synthesis method, indicating that the electrochemically synthesized Cu3(BTC)2 materials are pure and structurally intact [16].\nFig. 2B shows the thermogram of the Cu3(BTC)2 catalyst. As can be seen from the figure, Cu3(BTC)2 has three weight loss intervals. The volatilization of solvent molecules ethanol and water introduced during sample washing at 50\u2013130\u2009\u00b0C resulted in a mass loss of about 17.4 %. There is a continuous weight reduction process at 130\u2013310\u2009\u00b0C, about 25.3 %, mainly for the removal of water molecules adsorbed on Cu2+ coordination bonds in Cu3(BTC)2, thus obtaining free active sites. This is also the reason for the catalytic activity of the catalyst [21]. After the temperature exceeds 325\u2009\u00b0C, there is a clear weight loss peak and the sample loss decreases sharply. This indicates that the organic ligand decomposes and the Cu-MOF structure collapses [20]. The decomposition was complete by about 400\u2009\u00b0C, with 36.4 % mass remaining. The decomposition products are oxides of metallic copper, which indicates that Cu-MOF has good thermal stability below 310\u2009\u00b0C.\n\nFig. 3 A shows the SEM image of Cu3(BTC)2 catalyst. From the figure, it can be seen that the Cu3(BTC)2 particles are octahedral in shape with homogeneous crystalline phase and the particle size is in the range of 0.2\u20130.5\u2009\u00b5m.In order to further investigate the molecular structure of Cu3(BTC)2 catalyst, Raman characterization was carried out to investigate the valence bonding pattern. Fig. 3B shows the Raman diagram of the catalyst. The characteristic peaks in the low frequency region 170\u2013600\u2009cm\u22121 in the figure are attributed to the Cu2C4O8 metal cluster in the Cu3(BTC)2 catalyst structure. Among them, 177\u2009cm\u22121 is attributed to the stretching vibration of the Cu-Cu bond, and the attribution of the characteristic peak at 502\u2009cm\u22121 has not been clearly confirmed [26]. The characteristic peaks in the high frequency region 730\u20131800\u2009cm\u22121 are attributed to the organic component of the Cu3(BTC)2 catalyst structure. The characteristic peaks of 1461 and 1544\u2009cm\u22121 can be attributed to the symmetric stretching vibration and asymmetric stretching vibration peaks of -COO, indicating the complete deprotonation of 1,3,5-H3BTC carboxyl group [10]. The peak located at 1005\u2009cm\u22121 and 1610\u2009cm\u22121 corresponding for the stretching vibration peak of C =\u2009C on the benzene ring [2]. The peak located at 827\u2009cm\u22121 and 745\u2009cm\u22121 are the bending vibration peaks of the C-H bond on the benzene ring and the benzene ring vibration peaks [33]. The above results indicate that the Cu3(BTC)2 catalysts synthesized by electrochemical methods are structurally sound.We used the hydrogen precipitation properties of the catalysts to first verify their catalytic activity. \nFig. 4 shows the polarization curves of Cu3P hydrogen precipitation performance at different phosphorization temperatures. At j\u2009=\u20091\u2009mA/cm2, the starting overpotential \u03b70 is 79\u2009mV, 89\u2009mV, 97\u2009mV, 125\u2009mV and 103\u2009mV, and at j\u2009=\u200910\u2009mA/cm2, the overpotential \u03b710 is 136\u2009mV, 145\u2009mV, 155\u2009mV, 181\u2009mV and 164\u2009mV, respectively. The hydrogen precipitation performance of the prepared catalysts was reduced by both high and low phosphorylation temperatures [23].Subsequently, we performed DRM reaction tests on different catalysts and the results are shown in \nTable 2 and \nFig. 5. At first, the Cu3P catalysts prepared at elevated temperatures have more excellent activity and stability. However, the activity of the catalyst did not keep increasing with the increase of temperature. After the DRM reaction at 700\u2009\u2103 for 6\u2009h, the conversion of Cu3P-350 did not change significantly, and the stability was significantly higher than that of Cu3P-300 and Cu3P-325. In addition, the amount of carbon accumulation was significantly reduced, thanks to the dual physical and chemical domain-limiting benefits of Cu3P-350, which inhibits copper particle migration at high temperatures and keeps the catalyst highly stable. The final conversion of CH4 was 77.44 % and 66.41 % for Cu3P-350 and Cu3P-300, respectively, after 6\u2009h of reaction. The final conversion rates of CO2 were 80.15 % and 77.47 %, respectively. In addition, the inverse water gas conversion reaction results in a higher CO2 conversion than CH4 conversion, so the H2/CO in the product is less than 1. After 6\u2009h of reaction, the H2/CO of Cu3P-300 catalyst (0.94) was greater than that of Cu3P-350 catalyst (0.92). In addition to the reforming reaction, there are other side reactions such as reverse water gas conversion, CH4 cracking and Boudouard reaction [7]. The Gibbs free energy can be used to determine the possible side reactions at different reaction temperatures. The reverse water gas conversion always exists below 820\u2009\u2103, resulting in the DRM process of CO2 conversion is usually greater than CH4 conversion, and H2/CO ratio is less than 1. CH4 cracking can occur above 557\u2009\u2103. The Boudouard reaction needs to be lower than 700\u2009\u2103, so more carbon deposition will occur between 557\u2009\u2103 and 700\u2009\u2103. Previous works conducted thermodynamic simulations of the effects of temperature, CH4/CO2 ratio, reaction pressure, and other oxidants on the formation of carbon deposits [3,8]. They suggest that operating at temperatures above 850\u2009\u2103, low pressure and high CO2/CH4 ratios can achieve higher conversion rates and less carbon deposition.\n\nFig. 6 shows the carbon combustion curves of different catalysts after 6\u2009h of DRM reaction at a reaction temperature of 700\u2009\u2103. All catalysts produced a weight loss plateau from 0 to 100\u2009\u2103, which was caused by the evaporation of residual water in the catalysts that were not completely dried after the reaction. A clear weight loss plateau was produced at 400\u2013600\u2009\u2103, corresponding to the obvious exothermic peak of the DSC curve, which was attributed to the combustion of carbon. It was found that all the carbon accumulation of Cu3P was less when the phosphorylation temperature was below 375\u2009\u00b0C. The first step of DRM reaction is the adsorption of CH4. At low temperature, CH4 molecules with low kinetic energy adsorbed on the metal surface to form an intermediate state, and then further desorption or dissociation occurred. At higher temperatures, CH4 molecules tend to dissociate directly after adsorption on the metal surface. The cleavage of CH4 on the metal surface is one of the slower steps in the DRM reaction because the dissociation energy of CH3\u2014H bond is as high as 439.3\u2009kJ/mol. The total energy required for CHx\u2014H bond dissociation depends on the catalytic system, and the selection of appropriate catalyst will be helpful for CHx \u2014H bond dissociation. adford and Vannice [5] summarized the behavior of CHx species on various metals. BCHx tends to be located at the active site that causes it to form a tevalent form. CH2 is a bridge adsorption, while CH and C need to be attached to a vacancy with three or four adjacent sites. This hypothesis does not take into account the changes in metal surface structure caused by the adsorption of CHx species and the effects of adsorbed species at adjacent sites [11]. The amount of carbon accumulation increased when the phosphorylation temperature reached 400\u2009\u00b0C, which was consistent with the DRM reaction activity and stability pattern.In this work, Cu3P was synthesized by direct phosphorylation at different temperatures using Cu3(BTC)2 as the precursor, and their catalytic activity was first verified using the hydrogen precipitation performance of the catalysts. It was found that both high and low phosphorylation temperatures reduced the hydrogen precipitation performance of the prepared catalysts. The Cu3P catalysts prepared at higher temperatures showed better activity and stability. However, the activity of the catalysts did not always increase with the increase in temperature. The most effective catalyst was Cu3P-350.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the Research Foundation of Education Bureau of Hunan Province, China (Grant Nos. 20A060, 21C0869, 21C0900).", "descript": "\n                  The CH4-CO2 reforming reaction can realize the utilization of both CH4 and CO2, which is important to control the greenhouse effect and protect the environment. The development of low-cost, high-activity and high-stability reforming catalysts is the focus of research. This work used electrochemical methods to synthesize Cu3(BTC)2 in high yields. Cu3(BTC)2 was subsequently used as a precursor and Cu3P was synthesized by direct phosphorylation at different temperatures. Both high and low phosphatization temperatures can reduce the hydrogen precipitation performance of the prepared catalyst. In the CH4-CO2 reforming reaction, the Cu3P catalyst prepared at elevated temperature has more excellent activity and stability. However, the activity of the catalyst did not keep increasing with the increase of temperature. After the CH4-CO2 reforming reaction at 700\u00a0\u2103 for 6\u00a0h, the conversion of Cu3P-350 did not change significantly, and the stability was significantly higher than that of Cu3P-300 and Cu3P-325. At the same time, when the phosphating temperature is lower than 375\u00a0\u2103 Cu3P carbon accumulation are less. The amount of carbon accumulation increases when the phosphating temperature reaches 400\u00a0\u00b0C.\n               "}
{"full_text": "The so-called Guefoams (guest-containing foams) are a new family of recently developed materials that are attracting increasing attention due to their promising behaviour in several applications. Guefoams consist of multifunctional porous materials hosting granular or fibrous phases with specific functionality (guests) in open-pore foam (host) cavities [1\u20134]. There is no bonding between the host matrix phase (the phase that forms the porous skeleton) and the guest phases other than mere physical contact. For this reason, the entire surface of the guest phases is functional, and a fluid can flow through the Guefoam with a relatively low pressure drop. Guefoams were conceived to provide broader or newer functionalities to conventional foams or those incorporating new phases by full or partial embedding in the matrix phase. The Guefoams manufacturing process is simple and economically feasible for large-scale production, as it is based on the conventional replication method commonly used to produce most foams on the market today. The production consists of infiltrating a foam matrix precursor under gas pressure into preforms containing guest phases in the form of particles or fibres coated with NaCl, which is dissolved after solidification of the matrix precursor [5,6]. As an example, Guefoams with an aluminium matrix phase containing both steel and activated carbon particles as guest phases have been reported for the pre-concentration and desorption of volatile organic compounds (VOCs) by rapid magnetic induction heating [3]. Recently, magneto-inductive carbon matrix Guefoams with embedded iron nanoparticles and activated carbon as guest phase have been shown to be effective VOC preconcentrators [4].Conventional foams are used as catalyst supports, whose intricate interconnected porous structure enables the development of a higher specific surface area per unit volume than honeycomb monoliths. The interaction of fluids with active phases loaded on foams is generally greater than in honeycombs, since fluids usually adopt laminar flow in honeycomb channels, while foams favour a turbulent regime, which improves mass and heat transfer [7]. In addition, foams also undergo a moderate pressure drop, which can be tailored by controlling the size of the interconnection windows between pores.However, given the complex pore space of foams, loading of active catalytic phases is often non-trivial for these materials [8]. Some examples of loading foams with active phases have been described by electrochemical or hydrothermal processes for Ru-Ce/Ni [8], Ni-SiO2/GO-Ni [9], BiFeO3/Ni [10], Cu(NP)s/Gf-Ni [11], Co-W-B/Ni [12], Ni/SiC [13], ZSM-5/SiC [14] and MOF/SiC [15]. Recently, open-pore graphite foams derived from mesophase pitch have been developed in which TiC nanoparticles are conveniently distributed in two positions (on the pore surface and in the foam struts) [16]. Depending on their location, the TiC nanoparticles fulfil two different roles: those at the struts catalyse the pitch graphitization process and allow high thermal conductivity to be achieved, while those anchored on the pore surface can serve as metal supports for catalytic purposes. These particles at the pore surface are partially embedded in the matrix phase. A major drawback of the embedded phases is that they lose part of their surface area, which significantly reduces their surface functionality.Despite the interesting features of foams as catalyst supports, the use of Guefoams in heterogeneous catalysis has not been developed yet. These materials have great design potential, which makes them even more interesting than conventional foams as catalytic supports. Its processing, besides being simple and economically feasible for large-scale production, avoids the difficult step of active phase loading. In addition, Guefoams can be prepared with a metal matrix and their high thermal conductivity promotes heat transfer from or into the catalyst phases. This contributes to the catalytic conversion rate of a reactor having a more homogeneous radial distribution than that of a particle bed, allowing larger reactors to be built with higher catalytic efficiency. Moreover, Guefoams have the advantage of being materials that can be customized by varying design parameters such as the fraction of pores occupied by the guest phase(s) or the volume fraction that they occupy in the porous cavity, which allows modification of properties that are important for fluid dynamic applications, such as permeability or relative pressure drop [3,4]. This design versatility enables the development of catalytic reactors with higher efficiency and consequently lowers operating costs, in line with near-term expectations for heterogeneous catalysis in the context of greener and more energy-efficient chemical processes. Guefoams have such high design potential for catalytic purposes that they can successfully meet challenges that numerous scientists have proposed as research topics in the field of heterogeneous catalysis for the coming years. By using Guefoams, scientists could make scientific progress in designing reactors for multi-catalysis or simultaneous tandem catalysis by combining guest phases with different catalytic functions and differentiated localization [17,18].The preparation and applicability of Guefoams as heterogeneous catalysts were investigated in this study. Alumina particles loaded with a Ni/CeO2 active phase were used as a guest phase hosted in a Al-Si foam. The resulting material was tested for methane production by CO2 hydrogenation. This reaction is of practical relevance to reduce CO2 emissions to the atmosphere and to produce a valuable fuel that can be easily distributed through the existing natural gas network [8,9,19\u201325]. This reaction will become particularly important in a new energy scenario, where H2 will be massively obtained from renewable energy sources, as CO2 methanation will be a chemical route for energy storage. The selected active phase is one of the most efficient noble metal-free CO2 methanation catalysts of practical interest [26]. The aim of this study is therefore not to investigate the methanation reaction or the active phase behaviour, as this has already been studied by several researchers [20,27\u201329], but to use this reaction and the active phase as a proof of concept to evaluate the potential applicability of the novel Guefoam catalyst. A multidisciplinary team with metallurgical and catalytic background was necessary to optimize the manufacturing process and avoid thermal and chemical degradation of the active phase.A Ni/CeO2/Al2O3 active phase was prepared with the following nominal composition: 5\u00a0wt% Ni\u00a0+\u00a047.5\u00a0wt% CeO2\u00a0+\u00a047.5\u00a0wt% Al2O3. The composition was confirmed by ICP-AES. Commercial \u03b1-Al2O3 pellets were ground and sieved to produce Al2O3 particles of 0.75\u20131\u00a0mm, which were first impregnated with cerium (III) nitrate hexahydrate. After calcination at 500\u00a0\u00b0C for 2\u00a0h (heating at 5\u00a0\u00b0Cmin\u22121), nickel (II) nitrate hexahydrate was further impregnated, dried and calcined at 450\u00a0\u00b0C for 2\u00a0h (heating at 5\u00a0\u00b0Cmin\u22121). The above three raw materials were supplied by Alfa-Aesar (Kandel, Germany).The catalyst particles were coated with NaCl (99.5\u00a0wt%; Panreac Qu\u00edmica S.L.U., Barcelona, Spain) by spray coating with a 20\u00a0wt% NaCl-water solution, as described elsewhere [1\u20134]. The coated particles were sieved and fractions with a diameter of 1.7\u20132.2\u00a0mm were used for the Guefoam catalyst preparation.Replication method was followed to prepare the Guefoam catalyst [30,31]. The coated guest phase particles were packed with the help of vibrations into a graphite crucible with an inner diameter of 26\u00a0mm and a height of 100\u00a0mm, which had been previously sprayed with a BN coating (ZYP Coatings Inc., Oak Ridge, USA) to facilitate demoulding (see more details in [32] for the packing procedure). To prevent the particles from moving or floating during metal infiltration, a 2\u00a0mm thick graphite disk with holes of about 0.5\u00a0mm was properly fixed to the top of the compacted preform. A eutectic aluminium\u2013silicon alloy (Al-12\u00a0wt% Si), prepared with commercially pure aluminium (99.95\u00a0wt%) and silicon powder (99.9\u00a0wt%) both purchased from Alfa Aesar (GmbH & Co KG-Karlsruhe, Germany), was used as the metal matrix precursor. A solid piece of aluminium alloy was placed on top of the graphite disk and the crucible was then inserted into a gas pressure infiltration chamber [33,34]. A vacuum up to 0.2\u00a0mbar was applied, with a heating rate of 4.5\u00a0\u00b0Cmin\u22121 up to 665\u00a0\u00b0C. After 10\u00a0min at constant temperature, the vacuum was closed and the chamber was pressurized with 0.8\u00a0bar argon to infiltrate the packed preform with liquid metal.After infiltration, the chamber was rapidly cooled at 50\u00a0\u00b0Cmin\u22121 to solidify the metal. The solid was extracted by removing the surrounding excess metal. This yielded a piece 25\u00a0mm in diameter and 45\u00a0mm in length containing 350\u00a0mg of active phase particles. The sodium chloride coating was removed by dissolution with a pressurized water flow, as described in [35]. The result was an interconnected Al-Si alloy pore structure in which the guest phase particles are located inside the porous cavities without chemical or physical bonding. The Guefoam catalyst was finally calcined at 500\u00a0\u00b0C for 4\u00a0h, and catalytic tests were performed with the Guefoam catalyst before and after this heat treatment.The guest phase particles and the surface morphology of their NaCl coating were characterized using a SEM-Hitachi S3000N scanning electron microscope operating at variable voltage. The spatial distribution of the Ni active phase and the composition of the guest phase particles were analysed using the same microscope equipped with a Bruker XFlash 3001 X-ray detector for point and map analysis (EDX).Geometric parameters (circularity and aspect ratio) were determined from image analysis. Circularity is defined as 4\u22c5\u03c0\u22c5area/perimeter2, where 1.0 represents perfect circularity. The aspect ratio is the ratio between the average major and minor axes of the particles. These last two parameters were determined from measurements of over 300 particles.The density of the active phase particles was measured by densitometry using dichloromethane (density\u00a0=\u00a01.330 gcm\u22123 at 25\u00a0\u00b0C) according to the ASTMD854 standard. The use of dichloromethane avoids the dissolution of the NaCl used as coating of the active phase particles.Thermal conductivity was experimentally determined by a set-up assembled at the University of Alicante laboratories in compliance with the international standard ASTM E-1225-04, based on a relative steady-state (equal-flow) technique [35\u201337]. Each sample, with cylindrical geometry, was placed between two blocks. The bottom of the sample remained in contact with a cooled cylindrical block (refrigerated by a room temperature water flow) and the top was in contact with a brass reference block connected to a 70\u00a0\u00b0C water bath. Two sets of thermocouples were connected to the sample and three more to the brass reference so that the temperature gradients required to estimate thermal conductivity could be measured with an uncertainty of less than \u00b15%.XPS characterization was performed in a K-ALPHA Thermo Scientific device using Al-K\u03b1 radiation (1486.6\u00a0eV) and a twin crystal monochromator that yields a focused X-ray spot with a diameter of 400\u00a0\u03bcm at 3\u00a0mA\u00a0\u00d7\u00a012\u00a0kV. The binding energy scale was adjusted by setting the C1s transition to 284.6\u00a0eV.CO2 methanation experiments were carried out in a cylindrical reactor with a 64% H2\u00a0+\u00a016% CO2 gas mixture balanced with N2 (100 mlmin\u22121 total flow and atmospheric pressure). The experiments were performed with a packed bed of the active phase particles between quartz wood plugs and with the Guefoam catalyst. In both cases, the amount of catalyst particles was 350\u00a0mg. Gas composition was monitored using specific AwiteFLEX COOL gas analysers, with NDIR, electrochemical and TCD detectors for CO, CO2, CH4, O2 and H2. The catalysts were pretreated with 50% H2/N2 at 500\u00a0\u00b0C for 1\u00a0h and cooled to room temperature under inert gas. Then the reaction mixture was fed into the reactor and the gas composition was measured under steady-state conditions at selected temperatures from room temperature to 500\u00a0\u00b0C.The gas flow pressure drops generated by the novel Guefoam catalyst and a packed bed of active phase particles were determined experimentally using the setup described in Fig. 1\n.The permeability (k) can be derived from the Darcy-Forchheimer equation, which relates the fluid velocity (v) and the pressure drop (\u0394P/\u0394L). This equation contains the viscous term of Darcy\u2019s law and the inertial effects generated by the flow in the porous medium [5,37]:\n\n(1)\n\n\n\n\n\u0394\nP\n\n\n\u0394\nL\n\n\n=\n\n\u03bc\nk\n\nv\n+\n\u03c1\n\nC\ni\n\n\n\nv\n\n2\n\n\n\n\nwhere \u00b5 and \u03c1 are the dynamic viscosity and density of the fluid (taken as 1.85\u00a0\u00d7\u00a010-5 kgm-1s\u22121 and 1.184 kgm\u22123 at 25\u00a0\u00b0C, respectively). Ci refers to the inertial coefficient. The viscous loss (v\u00b7\u00b5/k) is linear with velocity and includes a viscous resistance coefficient of 1/k, which is the inverse permeability. The inertia term (\n\n\u03c1\n\nC\ni\n\n\n\nv\n\n2\n\n\n) accounts for the nonlinear pressure behaviour as a function of fluid flow by including an inertial resistance coefficient Ci.Temperature gradients within the Guefoam and the packed bed allow understanding how quickly a radial section of the reactor approaches the minimum and maximum temperatures of catalytic conversion, which mainly depends on the permeability and thermal conductivity of the material as well as fluid velocity. Radial temperature gradient calculations were performed with the ANSYS Fluent software package using a computational fluid dynamic (CFD) approach. The software was employed to simulate the above materials using simplified porous media configuration under local thermal non-equilibrium (LTNE) conditions, which assumes the difference between fluid and solid temperatures in two energy equations. Real-dimensioned reactors of 30\u00a0cm length (L) and 15\u00a0cm diameter (d) with the composition and pore volume fraction of Guefoam and packed bed were modelled following the computational domain schematic diagram and boundary conditions shown in Fig. 2\n. The system, considered to be at a constant temperature of 400\u00a0\u00b0C, was subjected to a fluid flow of 6\u00a0\u00d7\u00a0103 lmin\u22121. The fluid was deemed incompressible with analogous air physical properties and inlet temperature of 180\u00a0\u00b0C. Heat losses due to convection or radiations were assumed to be negligible. The governing energy equations are as follows [38,39]:\nFluid energy equation:\n\n\n(2)\n\n\n\n\u03c1\nf\n\n\nC\nf\n\nv\n\u00b7\n\u2207\n\nT\nf\n\n=\n\u03b5\n\nK\nf\n\n\n\n\u2207\n\n2\n\n\nT\nf\n\n+\n\nh\n\nsf\n\n\n\na\nv\n\n\n\n\n\nT\ns\n\n-\n\nT\nf\n\n\n\n\n\n\n\n\n\nSolid energy equation:\n\n\n(3)\n\n\n\n\n1\n-\n\u03b5\n\n\n\nK\ns\n\n\n\n\u2207\n\n2\n\n\nT\ns\n\n+\n\nh\n\nsf\n\n\n\na\nv\n\n\n\n\n\nT\nf\n\n-\n\nT\ns\n\n\n\n\n=\n0\n\n\n\nwhere \u03b5 is the pore volume fraction of the porous material, C is the specific heat, K is the thermal conductivity, T is the temperature, hsf is the interfacial heat transfer coefficient and av is the interfacial area density. The subscripts f and s refer to the fluid and solid phases, respectively.CO2 methanation is an exothermic reaction that typically proceeds between 200\u00a0\u00b0C and 500\u00a0\u00b0C, depending on the catalyst and experimental conditions [19]. Its enthalpy is \u2212165 KJmol\u22121 at 25\u00a0\u00b0C, but it decreases rapidly with temperature, becoming virtually nil at high temperatures close to its common operating limit [40]. In this context, it can be assumed that the heat released by the methanation reaction does not substantially alter the local temperature conditions of the catalytic monoliths, since the conversion rate at low temperatures is low and therefore the heat released can be considered negligible. At high temperatures, where the conversion rate is high, the heat released is also negligible due to the near zero enthalpy. Therefore, the effect of the heat released by the methanation reaction was not included in the modelling calculations of the temperature profiles.\nFig. 3\n shows several images of active phase particles before and after NaCl coating.\nFig. 3a and b show that the uncoated active phase particles have an angular geometry, whereas they become more spherical after coating with NaCl (Fig. 3c). This is confirmed by the circularity values listed in Table 1\n, which increase from 0.68 to 0.86 after NaCl coating, and by the aspect ratio values, which also increase from 0.68 to 0.89. Table 1 also compiles the densities determined by densitometry with dichloromethane.\nFig. 3b displays an EDX-Ni mapping on the surface of an active phase particle, which confirms the homogeneous spatial distribution of the active phase on the surface of the alumina particles. The thickness of the NaCl coating can be seen in Fig. 3d, which shows a profile after a controlled fracture. It should be noted that, since the active phase particles have an angular morphology, the thickness of the NaCl coating around each particle is not homogeneous. Using image analysis, the average diameters of both the uncoated and coated active phase particles were measured (see supplementary material for more information).For the characterization of Guefoams, the guest loading (GL) and guest occupation (GO) parameters are essential:\n\n(4)\n\n\nGL\n=\n\n\nn\ng\n\n\nN\np\n\n\n\n\n\n\n\n\n(5)\n\n\nGO\n=\n\n\nv\ng\n\n\nV\np\n\n\n\n\n\nwhere ng is the number of pores hosting a guest phase, Np is the total number of pores, vg is the average volume of guest phases and Vp is the average volume of hosting pores.GL represents the fraction of pores hosting a certain type of guest phase. This parameter is determined by the relative ratio between the amount of NaCl-coated active phase and the amount of massive NaCl spheres that do not contain active phase. In the present study, all the pores of the foam material were intended to host an active guest phase particle, i.e., the GL parameter should be as close as possible to 1 (or, as a percentage, 100%). The experimental results showed that the GL\u00a0=\u00a097% (supplementary material).The GO parameter is the ratio between the average volumes of active phase particles and the cavities containing them. For fully spherical active phase particles and cavity geometries, GO=(r/R)3, where r and R are the average radii of the uncoated active phase particles and NaCl-coated active phase particles, respectively. The calculation of the GO parameter in non-spherical geometries such as the current angular geometry of the active phase is not so straightforward. This parameter was determined in two independent ways and resulted in a percentage value of 42\u201343%, as explained in the supplementary material.Although constant GO and GL values are employed in this work, given the proof-of-concept nature of the present research, both parameters can be modified to significantly alter the fluid dynamic behaviour of the fluid passing through the material. The effect of GO and GL on critical parameters such as permeability and relative pressure drop was demonstrated in [3].The general structure of a Guefoam is depicted in Fig. 4\na. Guefoams consist of an interconnected (or open-pore) foam material hosting freely moving guest phases in their cavities, since there is no chemical or physical matrix-guest bond. The dimensions and geometry of the Guefoam herein fabricated can be seen in the photograph in Fig. 4b. Fig. 4c provides an optical micrograph of the cavities containing the guest phases (Ni/CeO2/Al2O3 active phase particles).Experiments on CO2 methanation were performed with a packed bed of active phase particles and with the Guefoam catalyst. The results of CO2 conversion and CH4 selectivity are shown in Fig. 5\n.The as-prepared Guefoam catalyst obtained after NaCl removal with water showed no activity (Fig. 5a; green triangles), and our hypothesis was that the active phase was poisoned by chlorine, consistent with what other authors have found about the inhibition of various active phases by chlorine species [43\u201345]. This hypothesis was confirmed by XPS characterization, and a detailed analysis is described in the next section. In order to reverse this chlorine poisoning, the monolith catalyst was calcined at 500\u00a0\u00b0C for 4\u00a0h, and then successful catalytic activity was achieved in CO2 methanation. The onset reaction temperature was 225\u00a0\u00b0C, and thermodynamic equilibrium was reached at 500\u00a0\u00b0C. CH4 selectivity was 100% up to 400\u00a0\u00b0C, and few CO was detected above this temperature, with selectivity dropping to 80% at 500\u00a0\u00b0C. This catalytic behaviour was compared with powder of Ni/CeO2 active phases reported in the literature, as well as with our own previous catalytic results [26], and the onset CO2 methanation temperature obtained with the novel Guefoam catalyst prepared in this study is similar to that previously measured for Ni/CeO2 powders. However, thermodynamic equilibrium was reached with Ni/CeO2 powder at 350\u00a0\u00b0C, while this temperature was shifted to 500\u00a0\u00b0C in the current study. This indicates that the catalytic activity of the Guefoam catalyst is in some way lower than that of the active phase powder, and this is the penalty to be paid for the novel supported catalyst. Nevertheless, the catalytic activity of the novel monolith is high enough to be properly used.A catalytic experiment was performed with the active phase particles subjected to the same treatments used for the preparation of the monolith catalyst (NaCl coating, washing and calcination) but without the AlSi foam support. The catalytic behaviour is similar for the particles and for the Guefoam catalyst, proving that the shape of the catalytic bed (particle or monolith) does not affect the catalytic behaviour under the experimental conditions of these tests. That is, the Guefoam catalyst is able to perform the same as the packed bed made of the same active phase particles, but with the benefits of a structured piece (easy manipulation, improved thermal conductivity due to the metal matrix, etc.).To analyse the effect of chlorine on the catalytic performance of the active phase, XPS analysis was performed on the active phase particles in the as-prepared state, after NaCl coating and washing, and after further calcination at 500\u00a0\u00b0C for 4\u00a0h. Fig. 6\n shows the spectra obtained in different energy regions corresponding to Cl2p, Ni2p, Ce3d and O1s.The presence of chlorine on the active phase was confirmed after NaCl coating and water washing (Fig. 6a), which is consistent with our hypothesis of chlorine poisoning. Calcination at 500\u00a0\u00b0C for 4\u00a0h removed part of the chlorine, and successfully activated the active phase, resulting in suitable catalytic activity.In addition to the chlorine changes, additional changes were observed on the active phase surface after NaCl coating, washing and calcination. Fig. 6b shows the Ni2p region, and differences in the position of the main peak after the different treatments are distinguished. The detailed interpretation of Ni2p spectra is still a matter of debate, but information about the electronic environment of the nickel species can be obtained from the position of the main peak [46\u201349]. It has been reported that the main peak of metallic nickel appears at around 853\u00a0eV, while cationic species of Ni2+, such as NiO or partially hydrated oxides, usually appear at 854\u00a0eV and higher energies. The values measured in our spectra confirm the presence of cationic nickel in all cases, but there is a shift in the position of the peak from 855.2\u00a0eV in the as-prepared particles to 855.9\u00a0eV after NaCl coating and washing. This 0.7\u00a0eV shift evidences a lower negative charge density on Ni2+ cations after NaCl coating and washing, consistent with a Ni2+-Cl- interaction. The Ni2+ main peak is further shifted to 856.4\u00a0eV after calcination treatment, indicating a significant change in the electronic environment of the nickel cations, which could be consistent with the partial substitution of chlorine by oxide anions.The electronic density of the cerium cations (Fig. 6c) also changes before and after the different treatments. A mixture of Ce3+ and Ce4+ cations is usually found on ceria, and the Ce3d spectra combine the contributions of both types of cations, as shown in the graph. The percentage of Ce3+ cations slightly decreases from 21 % to 18 % after NaCl coating and washing. This oxidation is consistent with the chlorine-poisoning hypothesis and proves that chlorine affects not only nickel cations but also cerium cations. Stabilization of the oxidised state of cerium cations by the presence of chlorine probably inhibits the reversible Ce3+/Ce4+ redox cycle required for proper catalytic activity. The percentage of Ce3+ cations increases significantly from 18% to 27% after calcination treatment. In accordance with the nickel behaviour, calcination treatment seems to replace chlorine by oxide also on ceria, allowing the partial reduction of Ce4+ to Ce3+, which would explain the recovery of catalytic activity.The increase in Ce3+ content upon calcination is consistent with the O1s spectra (Fig. 6d). The O1s spectra can be deconvoluted into different contributions. The peak at the lowest energy is attributed to lattice oxygen, while other peaks at higher energies are attributed to surface species such as carbonates, hydroxyl groups and chemisorbed oxygen on ceria vacancies associated with Ce3+ cations. The position of the oxygen lattice peak shifts from 428.7\u00a0eV to 429.3\u00a0eV after NaCl coating and washing, which may be a consequence of the presence of a highly electronegative element such as Cl. After thermal treatment, the position of this lattice oxygen peak shifts by 0.8\u00a0eV (from 429.3\u00a0eV to 530.1\u00a0eV), which is also consistent with the partial removal of chlorine. Special attention must be paid in this case to the surface oxygen species, whose contribution to the O1s spectra increases dramatically after calcination. This is consistent with the appearance of oxygen vacancies due to the increase in the proportion of Ce3+ cations during calcination, which are filled by chemisorbed oxygen.In summary, XPS analysis confirms the presence of chlorine on the active phase after NaCl coating and washing, which is expected to inhibit the redox processes of the nickel and cerium cations required to achieve adequate catalytic activity. The negative effect of chlorine can be partially reversed by calcination at 500\u00a0\u00b0C for 4\u00a0h, and XPS analysis provides evidence for the substitution of chlorine by oxide with the expected restoration of redox behaviour.To further analyse the potential advantages of the novel Guefoam materials in catalytic applications, the thermal conductivity measurements are discussed in this section.The measured thermal conductivities of the materials herein evaluated are summarized in Table 2\n. The thermal conductivity of the Guefoam catalyst is 43 Wm\u22121\u00b0C\u22121, which is significantly higher than that of the packed bed obtained by compacting the active phase particles, with a value of 1.7 Wm\u22121\u00b0C\u22121. The Guefoam value of 43 Wm\u22121\u00b0C\u22121 is consistent with the thermal conductivity measured for an analogous guest-free foam (i.e. GL\u00a0=\u00a00%). This confirms that the absence of chemical bonding between the matrix and the active phase is responsible for the nil contribution of the guest phases to the overall thermal conductivity of the material. Therefore, this conductivity value is consistent with the estimates that can be made with analytical models for metal foams [35\u201337], which state that.\n\n(6)\n\n\nK\n=\n\nK\ns\n\n\n\n\n\n1\n-\n\u03b5\n\n\n\nn\n\n\n\n\nwhere Ks is the thermal conductivity of the solid (matrix) and n is a parameter dependent on the pore geometry (n\u00a0=\u00a01.5 for spherical pores).Considering that \u03b5\u00a0=\u00a00.59 (value of the pore volume fraction for a guest-free foam) and that the thermal conductivity of the Al-12\u00a0wt% Si alloy (Ks) was measured to be 179 Wm\u22121\u00b0C\u22121, the value estimated by Eq. (6) is 47 Wm\u22121\u00b0C\u22121 when n is assumed to be 1.5, which is in perfect agreement with the measured value, considering the experimental measurement error.The pressure drop across the Guefoam and packed bed was determined using the setup described in Fig. 1. Fig. 7\na shows the resulting pressure drop curves.As expected, the pressure drop caused by the packed bed of active phase particles is substantially higher than the pressure drop generated by the Guefoam. Unlike other foam materials, Guefoams can experience different pressure drops depending on the orientation (Fig. 7a). This is because guest phases can block the interconnecting windows and force the fluid to take less direct paths, resulting in higher pressure drops (Fig. 7b). The pressure drop of the Guefoam was measured in the horizontal orientation and in the vertical position with gas flow from top to bottom and from bottom to top. Fig. 7a confirms, as expected in view of Fig. 7b, that the lowest pressure drop is achieved in the horizontal configuration. In this horizontal arrangement, gravity acts perpendicular to the fluid passage so that the guest phase particles lean on the lower cavity walls, away from the most direct path defined by the fluid as it passes through the material, thus offering less resistance to the fluid passage. In contrast, gravity and fluid passage are parallel in vertical arrangements and the guest phase particles lean on fluid-trajectory aligned cavity regions, sometimes even blocking the interconnecting windows that define the easiest fluid path and offering greater resistance to fluid passage.The permeability values (k) can be obtained from the curves in Fig. 7a by quadratic fits according to Eq. (1). The results are summarized in Table 2, which shows that the Guefoam catalyst is 1.7\u20132.7 times more permeable than the packed bed.The design of catalytic materials in heterogeneous catalysis is a complex process that must be considered from a holistic perspective. The success of a catalytic material in industry is not constrained to its catalytic activity, but encompasses other considerations, such as its pressure drop, which represents a large portion of the energy required to function as a catalyst, or its thermal conductivity, which allows for considerations of its scalability to useful dimensions. In this work, these considerations have been further explored in order to show a performance comparison between the studied systems, i.e. Guefoam and particle bed.In [50], conversion efficiency and pressure drop were related in a parameter defined as catalytic performance index (Ip) to compare different reactors using the following equation (slightly rewritten to replace \u0394P with \u0394P/\u0394L to normalize the pressure drop with sample length):\n\n(7)\n\n\n\nI\np\n\n=\n\n\n-\nln\n(\n1\n-\n\u03b7\n)\n\n\n\u0394\nP\n/\n\u0394\nL\n\n\n\n\n\nbeing \u03b7 the conversion efficiency.\nFig. 8a shows the Ip values obtained from the experimental characterization as a function of the conversion temperature. The graph reveals that the Guefoam (washed and calcined) has the best performance as a catalytic reactor at temperatures above 350\u00a0\u00b0C, since the conversion efficiency/pressure drop ratio results in the highest Ip index.The thermal conductivity and permeability of a catalytic material affect the heat transfer into or from the material, thus influencing the temperature gradients, which can ultimately have a significant impact on the catalytic performance of materials with large dimensions. In order to analyse the temperature gradients inside the Guefoam catalyst and the packed bed, a CFD modelling of the temperature profiles was performed. Fig. 8b and c shows radial temperature profiles on three y-z planes at 1, 15 and 29\u00a0cm inlet length of the Guefoam and the particulate bed (the dimensions and fluxes taken, which were explained in Section 2.7, correspond to those of a possible industrial application for the methanation reaction considered).Significant differences are observed in the temperature gradients of the two systems considered. As expected, the temperature for both catalyst beds is higher near the outer walls than in the middle of the reactor, since the model assumes that heating is provided by an external heat source. The lowest temperature gradients were obtained for the Guefoam catalyst due to its metallic nature (Fig. 8b), while the highest gradients were obtained for the packed bed of active phase particles (Fig. 8c), which consists of metal oxides (mainly alumina and ceria) with poor thermal conductivity.Thus, the temperature at the centre of the y-z plane, located 15\u00a0cm from the inlet, is 250\u00a0\u00b0C for the Guefoam catalyst, while it is significantly lower for the packed bed. Considering, for instance, that the onset temperature for CO2 methanation is about 250\u00a0\u00b0C (see Fig. 5), the effective heating of the Guefoam catalyst allows a larger volume of the catalyst bed to have temperatures above the threshold required for the reaction.The preparation and use of Guefoams as heterogeneous catalysts were investigated in this study. The general structure of the prepared Guefoam catalyst consists of an interconnected (or open-pore) Al-Si foam hosting freely moving guest phases in their cavities, since there is no chemical or physical matrix-guest bond. Alumina particles loaded with the Ni/CeO2 active phase were used as the guest phase. A eutectic Al-12Si alloy was chosen for the foam body to lower the melting temperature and prevent thermal sintering of the active phase during liquid metal infiltration.CO2 methanation experiments were performed using the novel Guefoam catalyst as a reaction test. The obtained activity and CH4 selectivity (close to 100%) were similar to the values obtained with a packed bed of the same active phase particles, but with the benefits of a structured reactor. Guefoam manufacture requires the coating of the active phase particles with a NaCl shell. The salt is dissolved once the Al-Si alloy is infiltrated to obtain the foam. As shown by XPS characterization, the presence of chlorine anions poisons the active phase and inhibits the catalytic activity. A critical step in the Guefoam synthesis is the final calcination (500\u00a0\u00b0C; 4\u00a0h) to replace the chlorine with oxide anions, which only then ensure the catalytic activity.The thermal conductivity of the Guefoam catalyst is significantly improved with regard to the packed bed of active phase particles. This reduces the temperature gradients in the catalytic reactor, as demonstrated by computational fluid dynamic modelling.Pressure drop measurements showed that the permeability of the Guefoam catalyst is up to 2.7 times higher than that of the packed bed, resulting in a better catalytic performance index (Ip), especially at temperatures above 350\u00a0\u00b0C.Beyond the specific conclusions drawn in the present study and to comment on the future perspectives that can be achieved with Guefoams, the authors foresee a great potential of these materials in the context of new challenges in heterogeneous catalysis, such as the design of multi-catalytic reactors or for tandem reactions by combining different guest phases as differentiated catalytic centres. The versatility in varying the GL and GO parameters in the different cavities of a material is a tool with enormous potential for the design of future catalytic reactors adapted to specific working conditions and in which the catalytic performance can be optimised to values adapted to particular needs.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Financial support from the Spanish Agencia Estatal de Investigaci\u00f3n (AEI), the Spanish Ministry of Science and Innovation and the European Union (FEDER and NextGenerationEU funds) [projects MAT2016-77742-C2-2-P, PDC2021-121617-C21 and CTQ2015-67597-C2-2-R] and the Conselleria d'Innovaci\u00f3, Universitats, Ci\u00e8ncia, i Societat Digital of theGeneralitat Valenciana[projects GVA-COVID19/2021/097 and PROMETEO/2018/076, PhD grant of C.Y. Chaparro GRISOLIAP/2017/177 and contract of E. Bail\u00f3n APOSTD/2019/030]. L.P. Maiorano also acknowledges the financial support from the University of Alicante through grant \u201c\nPrograma Propio para el fomento de la I\u00a0+\u00a0D\u00a0+\u00a0I del Vicerrectorado de Investigaci\u00f3n y Transferencia de Conocimiento\n\u201d (UAFPU2019-33).The raw data required to reproduce these findings are available to download from [https://zenodo.org/deposit/5719300]. The processed data required to reproduce these findings are available to download from [https://zenodo.org/deposit/5719300].Supplementary data to this article can be found online at https://doi.org/10.1016/j.matdes.2022.110619.The following are the Supplementary data to this article:\n\nSupplementary Data 1\n\n\n\n", "descript": "\n                  The preparation and use of Guefoams as heterogeneous catalyst is reported. The Guefoam catalyst consists of an open-pore Al-Si foam that accommodates a freely mobile guest phase (Ni/CeO2/Al2O3 particles) in its cavities, with neither a physical nor a chemical matrix-guest bond. A eutectic Al-12Si alloy was used as a low-melting matrix precursor to prevent thermal sintering of the active phase during liquid metal infiltration. CO2 methanation was chosen as the reaction test. The activity and CH4 selectivity (close to 100%) achieved with the Guefoam catalyst were similar to those obtained with a packed bed of the same active phase particles, but with the advantages of a structured reactor such as robustness and ease of handling. The thermal conductivity of the Guefoam catalyst is significantly improved with regard to the packed bed of active phase particles, which reduces the temperature gradients in the catalytic reactor, as demonstrated by computational fluid dynamic modelling. Since the permeability of the Guefoam catalyst is 2.7 times that of the packed bed, the pressure drop caused by the passage of a fluid through the novel material is reduced, resulting in a significantly higher catalytic performance index than the packed bed.\n               "}
{"full_text": "The global demand for energy fossil fuels has increased tremendously due to the increase in economic growth and pollution; notably, nonrenewable fossil fuels are continuously depleting. Several research emphases have focused on developing alternative routes for bioenergy production, such as the use of agricultural residue after harvesting as an abundant renewable natural resource [1,2]; this has led to the development of a variety of techniques by which these products can be recovered. These techniques have immense potential in regard to the production of biofuels while minimizing negative effects on the environment. Pyrolysis is an increasingly promising thermochemical technology for biomass conversion and can produce biofuels (char, bio-oil, gases) in the absence of oxygen [3\u20136]. The pyrolysis of lignocellulosic biomass is based on the thermal decomposition of its components. In particular, woody biomass is divided into hardwood and softwood, which substantially differ in the concentrations of cellulose, hemicellulose, lignin and various compounds; for example, hardwood typically has less lignin (\u223c23\u201330%) than softwood and non-wood (\u223c26\u201334%) [7,8]. In particular, lignocellulosic components show different thermal degradation results, decomposing at low temperatures. Cellulose demonstrates thermal resistance over a peak decomposition range, while lignin degradation also occurs over a broad range after reduced volatilization when compared with holocellulose. In contrast, the initial degradation of softwood begins at lower temperatures, and both the decomposition of hemicellulose and the cellulose range are broader than that of woody biomass, causing different thermal decomposition results [9\u201311].Among the bio-oils produced from pyrolyzed woody biomass, although they can be used as a burner fuels or precursor chemicals [12], their physicochemical properties should be further modified before being used due to their acidity, viscosity and high oxygen content, which make them have very low thermal stability. Several studies have suggested enhancing the quality of bio-oil, one of which is the catalytic pyrolysis of biomass [13\u201315], deoxygenation, decarboxylation, decarbonylation reactions and the secondary reaction of tar cracking take place, resulting in an increasing hydrocarbon content and decreasing oxygen content which contributes to stability, high heating values and low acidity in a bio-oil yield. HZSM-5 shows enhanced catalytic activity to remove oxygen via decarbonylation and decarboxylation and the active acid sites of the catalysts continuously perform at high temperatures; however, some disadvantages are its rapid catalytic deactivation due to the active sites being blocked by carbonaceous coke along with dehydration reactions promoting an increase in the water component of the produced bio-oil [16,17].Another catalyst candidate for deoxygenation has been alternatively emphasized for use due to its inexpensive preparation from natural elements and has been widely used as a potential upgrade for catalytic pyrolysis to produce a bio-oil. In addition, compared to the pore size of the basic catalyst, mesoporous catalysts are important to have enough space for reactions; this also reduces the diffusion limitations of large hydrocarbon molecules with minimal steric hindrance when compared to that of a microporous zeolite [18,19]. Furthermore, the occurrence of coke causing catalytic deactivation is prevented [19\u201321]. MgO has been described as a strong Lewis base affecting the effectiveness of CO2 adsorption and exhibits resistance to carbon formation on the surface of the catalyst structure during devolatilization. Further pyrolysis reactions result in the enhanced conversion of high molecular weight hydrocarbons into smaller hydrocarbons [21,22]. Several studies have indicated that the use of CaO in the pyrolysis reaction, CaO acts as CO2 absorbent and reactant in biomass pyrolysis and exhibits the ability to absorb the moisture released [23\u201326]. CaO reacts with moisture and CO2 released to generate CaCO3 at low pyrolysis temperature and further decomposed at high temperature [27], enhancing decarbonylation during the thermal degradation of lignocellulosic components causing the formation of CO [28], while dolomite exhibits a role in absorbing, reacting, and catalyzing the thermal degradation conditions of the employed lignocellulosic components. Importantly, dolomite is an abundant natural carbonate mineral that is used as an inexpensive reactant, and mesoporous heterogeneous catalysts for gasification and pyrolysis reactions enhance the mass transfer of reactants and pyrolysis products [13,18,21,22,29,30]. Natural dolomite can be converted into a highly active basic oxide that consists of CaO, MgO, SiO3, Al2O3, and others after calcination at 700\u2013900\u00a0\u00b0C affecting the thermal decomposition of MgCa(CO3)2 to an active metal oxide and modifying the chemical including both surface area and morphology [31]. Moreover, the calcined dolomite exhibits a significantly change during thermal decomposition at high temperatures, had many orderly pores on the surface, which established a pore structure. Both the surface area and pore volume values were higher than those of natural dolomite affecting the catalytic reaction, making an inexpensive and practical basic oxide catalyst for the catalytic cracking and reforming of tar in biomass pyrolysis gas [22,31]. The modification of calcined dolomite with both noble metals and several catalyst preparation techniques usually enhances its effectiveness as an active catalyst [32,33]. In particular, nonnoble metal oxides are important materials that are widely applied in various reactions to improve catalytic activity, enhancing the formation of unsaturated hydrocarbons and the effectiveness of converting unsaturated hydrocarbon compounds and aromatics into saturated hydrocarbon compounds. Therefore, the modification of small nonnoble metals, such as Ni particles, over a Lewis base catalyst results in an alternative catalyst with high resistance to Ni sintering and carbonaceous materials; moreover, it also been of interest due to its effectiveness toward C\u2013O cleavage rather than C\u2013C cleavage [29,31].Ni-modified dolomite catalysts are used to improve both the quality and quantity of bio-oil through deoxygenation, decarbonylation, and decarboxylation reactions. However, its use for the catalytic pyrolysis of softwood and non-wood biomass has not yet been reported. In this study, the effects of the catalytic pyrolysis operating parameters on lignocellulosic biomass were also determined. Rubberwood sawdust is a softwood candidate obtained from the production of wood pellets, which contain approximately 30% waste sawdust that has little or no economic value; however, this sawdust still has the potential to be used as an energy feedstock. Additionally, the cassava rhizome is a residual biomass candidate after agricultural harvesting. Farmers usually tend to burn it before planting the next crop cycle, which results in the release of small particles, smoke and air pollution to the environment. The catalytic pyrolysis of these two different lignocellulosic components using Ni-dolomite was also determined to investigate the chemical components, physiochemical properties of the crude pyrolyzed oil and production of valuable chemicals; additionally, catalytic performance was thoroughly investigated and discussed.Rubberwood sawdust (RWS) was provided from a wood pellet manufacturer in Rayong Province, Thailand, and cassava rhizome (CRZ) samples were collected after harvesting before the next crop in Uthaithani Province, Thailand. Both samples were subjected to moisture reduction, milled in a SW-2 high-speed rotary cutting mill (Hsiangtai, the People's Republic of China), sieved into a size distribution of 0.250\u20132.000\u00a0mm, dried at 105\u00a0\u00b0C overnight and stored before further pyrolysis. The biomass components were determined according to the TAPPI standard. The volatiles, ash and fixed carbon were obtained by proximate analysis using the ASTM D3172-73(84) standard. Elemental analyses were performed using a 628-CHN analyzer (LECO Corporation, USA.), while the oxygen content was calculated by the difference. Table 1\n represents the characterization of the lignocellulosic components, and elemental analyses were employed.Calcined dolomite (DM) was produced by calcining at 850\u00a0\u00b0C for 4\u00a0h in a muffle furnace. After that, the DM was allowed to slowly cool until reaching approximately 100\u00a0\u00b0C before being quickly stored in a desiccator [30]. Next, the DM was dried at 105\u00a0\u00b0C for 2\u00a0h and ground to a particle size of less than 40\u00a0\u03bcm prior to use. In this study, x% Ni loading over DM was prepared using the wet impregnation method, where x wt.% Ni(NO3)2\u00b76H2O (Sigma-Aldrich, Singapore) was placed in 10\u00a0mL of deionized water and 14\u00a0g of DM was placed in 90\u00a0mL of deionized water at a temperature of 60\u00a0\u00b0C and with constant stirring at 600\u00a0rpm for 6\u00a0h. The Ni-modified DM mixtures were filtered using vacuum-assisted filtration, rinsed with deionized water and placed into a hot air oven at 100\u00a0\u00b0C for 12\u00a0h. Then, the Ni-modified DM catalyst was crushed and filtered with a 20-mesh sieve before being calcined at 550\u00a0\u00b0C for 4\u00a0h. Ni-DM was characterized by an ASAP 2020 instrument (Micromeritics Corporate, USA) to determine the surface area, pore size, and pore volume. Prior to the test, a 0.15\u00a0g catalyst sample was placed into a sample tube and degassed under vacuum at 300\u00a0\u00b0C for 3\u00a0h. The nitrogen adsorption-desorption isotherms of the catalysts were obtained at \u2212196\u00a0\u00b0C, and the surface area was also calculated by defining the adsorption isotherm. The crystallinity of Ni-DM and DM was characterized by a D8-Advance X-ray diffractometer (Bruker Corporation, Germany) that was operated with the following parameters: 30\u00a0mA, 40\u00a0kV, a Cu K\u03b1 radiation source at 0.154439\u00a0nm, a scanning rate of 4 \u00b0min\u22121, and a 2\u03b8 scanning range of 5\u00b0\u201390\u00b0. The crystal morphology and mesopores of the catalysts were characterized by using a JSM-6400 scanning electron microscope (JEOL, Japan).Catalytic pyrolysis tests were performed in a fixed bed pyrolyzer (3.81\u00a0cm i.d. and 120\u00a0cm length; SS316), as shown in Fig. 1\n.Prior to the tests, 5\u00a0g of feedstock was placed into the hopper at the top of the pyrolyzer and dropped with the catalyst placed between the quartz wool in the middle of the pyrolyzer; the complete volatilization of the feedstock to vapor due to thermal decomposition occurred before being introduced to the catalyst layer, thereby preventing mixing between the pyrolyzed solid and catalyst. The Ni-modified DM was reduced with 5% H2 at a flow rate of 20\u00a0mL\u00a0min\u22121 at 450\u00a0\u00b0C for 1\u00a0h to reduce NiO-DM to metallic Ni-DM. Then, N2 was circulated through the pyrolyzer to the top of the hopper until reaching the desired reaction temperature (450\u2013600\u00a0\u00b0C) and a constant N2 flow rate (60\u2013240\u00a0mL\u00a0min\u22121) was kept for approximately 20\u00a0min before dropping the biomass into the pyrolyzer. The tests were performed for reaction times of 45\u201390\u00a0min. The volatile vapor due to the thermal decomposition of the biomass feedstock, which was dropped into the middle of the pyrolyzer, diffused through the glass wool and reacted at the Ni active sites and in the pores. The pyrolyzed vapors were condensed in an ice bath at the bottom of the pyrolyzer, while the noncondensable gas passed through the gas drier before being collected in a gas bag to further determine the gas composition. The obtained bio-oil from catalytic pyrolysis, which was collected in a cool trap, was diluted using acetone and separated using a funnel to determine the organic phase and aqueous phase. Then, the organic phase was evaporated at 60\u00a0\u00b0C to remove the solvent. After that, the bio-oil was obtained and weighed to determine the yield. The physicochemical properties were determined according to ASTM standards, such as the density (ASTM D1298), kinematic viscosity (ASTM D445), and higher heating value (HHV, ASTM D3286-91a); moreover, an 840-Trinoplus automated titration instrument (Metrohm AG, UK) was used to determine the modification acid number (ASTM D664). In addition, the ultimate analyses of the bio-oil from pyrolysis were performed using a CHN-628 instrument (LECO Corporation, USA.). The pyrolyzer was cooled to ambient temperature, and the solid char was collected and weighed; furthermore, the catalyst was separated from the quartz wool layer for weighing, the coke deposition was determined using the weight difference of the catalysts before and after the tests. Both solid char weight and weight of carbonaceous over the catalysts are classified as solid yield in percent by weight. The noncondensable gas yield was determined using the yield difference between the fed sample and the total yield of condensed bio-oil and solid char.1\u00a0\u03bcL of bio-oil was diluted in acetone to 1\u00a0mL before analysis using gas chromatography-mass spectrometry. A GC-7890A coupled with MS-5975C instrument (Agilent Technologies, USA.) that was equipped with an HP5MS nonpolar capillary column (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm i.d., 0.25\u00a0\u03bcm film thickness) was used, and the gas chromatography (GC) oven was heated to 40\u00a0\u00b0C for 4\u00a0min and then to 280\u00a0\u00b0C at a heating rate of 4\u00a0\u00b0C min\u22121 for 10\u00a0min. The injector and detector temperatures were set to 250 and 280\u00a0\u00b0C, respectively. Helium (99.999%) was used as the carrier gas at 1\u00a0mL\u00a0min\u22121, and a split ratio of 1:10 was used when injecting the a 1\u00a0\u03bcL sample. Electron ionization was performed at 70\u00a0eV for the molecular mass range of m/z\u00a0=\u00a050\u2013550. The National Institute of Standards and Technology (NIST) mass spectral library was used to identify the main peaks in the chromatogram to determine the chemical compounds. The noncondensable gas was collected in a sampling bag from the gas drier unit, and then 60\u00a0\u03bcL was injected into the Agilent gas chromatograph coupled with a thermal conductivity detector using the following GC program parameters: argon flow of 30\u00a0mL\u00a0min\u22121 pressure of 75 psi, oven temperature of 50\u00a0\u00b0C, injection temperature of 80\u00a0\u00b0C, and detector temperature of 90\u00a0\u00b0C.\nTable 1 lists the proximate and ultimate analyses and the lignocellulosic components in the rubberwood sawdust (RWS) compared with cassava rhizome (CRZ). The different components of both types of lignocellulosic biomass make it difficult to determine the differences in the thermal behavior [32], product yield and characteristics of the pyrolysis products.The results of the ultimate and proximate analyses (on a dry basis) of RWS and CRZ are presented in Table 1. As a result, both RWS and CRZ show no significant difference in the ultimate analyses. RWS and CRZ contain oxygen contents of 44.81\u00a0\u00b1\u00a00.23\u00a0wt% and 60.77\u00a0\u00b1\u00a00.31\u00a0wt%, respectively, which are primarily attributed to aldehydes, phenolics, carboxylic acids and oxygenated hydrocarbon compounds typically obtained in bio-oil. RWS contains 47.42\u00a0\u00b1\u00a00.35\u00a0wt%, which is higher than CRZ owing to its higher carbon content and lower oxygen content; moreover, the HHV of RWS (17.38\u00a0MJ/kg) is also higher than that of CRZ. The proximate analyses show fixed contents in RWS and CRZ, with values of 18.19\u00a0\u00b1\u00a00.42\u00a0wt% and 14.49\u00a0\u00b1\u00a00.88\u00a0wt%; this result accounts for the high char yield obtained from the catalytic pyrolysis reaction. It is notable that CRZ has an ash content of approximately 28.06\u00a0\u00b1\u00a01.41\u00a0wt%, while RWS has a lower ash content of only 2.75\u00a0\u00b1\u00a01.44\u00a0wt%. This affects the volatile content in the feedstock and favors the production of volatile vapor during catalytic pyrolysis, which results in the difference in the liquid and gas yield distribution. In particular, the high volatile matter contents indicate high volatility and reactivity, which enhance bio-oil production from pyrolysis, while a high ash content inhibits the production of bio-oil and leads to increased char and decreased gas contents [32,33].\nFig. 2\n(A) shows the X-ray diffraction (XRD) pattern of natural dolomite, which consists of CaCO3, Ca(OH)2 and Mg(OH)2. Notably, the intensity of Ca(OH)2 and the crystalline structures of natural dolomite and calcined dolomite are not very different. Moreover, the XRD pattern of DM does not show Mg(OH)2 peaks, as illustrated in Fig. 2(B). In accordance with the thermal decomposition at a high calcination temperature above 700\u00a0\u00b0C, the intensities of the diffractograms represent the crystallinity of CaO and MgO that also appear [30].Ni-modified dolomite samples with different Ni loadings was prepared to investigate their catalytic activity during the pyrolysis of lignocellulosic biomass. As shown in Fig. 2(B)\u2013(E), the calcined dolomite has the same intensity peak as the Ni-modified dolomite embedded in the calcined dolomite template. It is notable that the XRD pattern of several Ni-modified dolomite samples has a peak intensity that does not significantly change from the parent dolomite when increasing the Ni-ion concentration; additionally, the intensities of the diffraction peaks of Ni-DM are similar to the peaks of the parent calcined dolomite. The diffractograms show that the weight percentage of modified Ni in the calcined dolomite template does not change the original framework, according to the Ni concentrations, thus similar results are obtained in regard to the decreasing crystallinity of the Ni-modified DM samples when compared to the original dolomite template; this result can be attributed to the uniform metal dispersion throughout the Ni-calcined dolomite surface, which should be due to the complete crystal framework structure and lower Ni loading amount on the calcined dolomite [31]. Furthermore, the peak intensity gradually decreases with increasing Ni loading because the incorporation of Ni in the calcined dolomite template decreases the crystallinity of the calcined dolomite or causes the accumulation of NiO on the surface of the parent dolomite during calcination [30,31].\nFig. 3\n shows the surface morphologies of natural dolomite, DM and Ni-DM. DM has many orderly pores on the surface, which results in a porous structure, while Ni-DM shows the same morphology as that of calcined dolomite. This can be confirmed by the fact that calcined dolomite retains its original textural form after the metal is loaded during the wet impregnation procedure. The addition of Ni slightly decreases the SBET of the parent calcined dolomite because some Ni metal is adsorbed on the surface of the pores, leading to a reduction in the pore volume and surface area. Nevertheless, it is also quite difficult to observe the difference in the pore volume of the Ni-doped calcined dolomite to that of the calcined dolomite because Ni cations are small and can diffuse through the mesopores of calcined dolomite; thus, they may fill the pores of the calcined dolomite. Furthermore, the SEM images illustrate the relative BET surface area listed in Table 2\n, showing a total pore volume of 0.02\u00a0cm3/g and a surface area of SBET\u00a0=\u00a010.02\u00a0m2/g. The surface area and pore volume of both DM and Ni-DM are lower than those of natural dolomite.In particular, the average particle size distribution of samples is significantly affected by the product distribution of the biomass pyrolysis reaction due to the mass and heat transfer of the lignocellulosic components. Moreover, the components in the softwood and non-wood show different thermal decomposition behavior that enhances the decomposition of lignocellulosic components and volatilization to vapors coupled with deoxygenation, decarbonylation, and decarboxylation due to the catalytic activity. Thus, the influence of the average biomass size was determined to investigate the product yield and the characteristics of the produced bio-oil. As shown in Fig. 4\n(A), the average size distribution (0.250\u20132.000\u00a0mm) of both RWS and CRZ is determined to investigate the pyrolysis product distribution when other operating parameters are kept constant: reaction temperature of 450\u00a0\u00b0C, reaction time of 45\u00a0min, inert nitrogen flow rate of 180 mLmin-1, 10%Ni-doped calcined dolomite and a catalyst loading of 10\u00a0wt%. The highest bio-oil yield (32.56\u00a0wt%) is obtained from RWS when using sawdust with a size distribution of 0.355\u20130.710\u00a0mm. The bio-oil yield slightly decreases, and the generation of noncondensable gas also increases when increasing the average size distribution of the feedstock from 0.710\u00a0mm to 2.000\u00a0mm. In contrast, the bio-oil yield obtained from the catalytic pyrolysis of CRZ tends to increase and have a maximum liquid yield (30.41\u00a0wt%) when the average size distribution of CRZ is increased to 2.000\u00a0mm. It is worth noting that the yield of solid and noncondensable gases tends to decrease slightly by increasing the average size of CRZ during catalytic pyrolysis. The results indicate that an increase in the average particle size of RWS leads to a decrease in bio-oil yield and a slight change in the solid yield; in contrast the noncondensable gas yield shows an increasing trend. This explains why sufficient heat transfer to the RWS particles during pyrolysis enhances the thermal decomposition of hemicellulose and cellulose into vapors, accomplishing the production of more noncondensable gas and decreasing the biochar yield. In particular, the heat transfer resistance more strongly affects bio-oil production than an increase in the average particle size because an insufficient temperature may occur during the thermal decomposition of lignocellulosic components [30\u201334]. Therefore, the catalytic pyrolysis of RWS, which is mainly composed of hemicellulose and cellulose, is easy to thermally decompose into volatile vapor and then further decompose via deoxygenation and decarboxylation; moreover, the secondary reaction of tar into bio-oil can be induced by heat transfer with an RWS size distribution of 0.250\u20130.355\u00a0mm. The catalytic pyrolysis of CRZ shows that when the particle size is increased, the bio-oil yield tends to slightly increase and reach a maximum of 30.41\u00a0wt% at an average size distribution of 0.850\u20132.000\u00a0mm. Notably, an increase in the average particle size of CRZ does not significantly affect the bio-oil yield but causes the thermal decomposition of the lignocellulosic components in CRZ. The influence of temperature and heat transfer from the wall of the reactor to the surface of the particle also mainly affects the decomposition of cellulose components to volatile vapor and high molecular weight hydrocarbon compounds [33,34]. Next, deoxygenation, including the catalytic activities of the dolomite and Ni-modified calcined dolomite, has a sufficient influence on the bio-oil and slightly changes the gas yield. It is worth noting that the catalytic pyrolysis of CRZ with an average particle size of 0.355\u20130.710\u00a0mm results in a bio-oil yield of only 27.93\u00a0wt%. However, when increasing the average particle size to 0.850\u20132.000\u00a0mm, the yield of biooil increases to 30.41\u00a0wt%, which is lower than the bio-oil yield from RWS when using an average particle size of 0.355\u20130.710\u00a0mm. This result shows that large particles have an effect on temperature, making it insufficient to complete thermal degradation during the primary thermal degradation step and resulting in a higher solid yield and lower bio-oil yield.Typically, thermal decomposition mainly influences the temperature and greatly affects the bio-oil yield. Fig. 4(B) illustrates the influence of temperature on the product distribution when varying the temperature from 450 to 650\u00a0\u00b0C while the other parameters are kept constant: average size distribution of 0.250\u20130.355\u00a0mm (RWS) and 0.710\u20132.000\u00a0mm (CRZ), inert nitrogen flow rate of 180\u00a0mL\u00a0min\u22121, reaction time of 45\u00a0min, 10%Ni-DM, and a catalyst loading of 10\u00a0wt%. As the temperature is increased from 400 to 550\u00a0\u00b0C, it is found that the pyrolysis bio-oil yields of RWS and CRZ tend to increase from 32.36\u00a0wt% to 39.48\u00a0wt% and 30.41\u00a0wt% to 40.39\u00a0wt%, respectively. As seen from the results, the bio-oil produced from CRZ is higher than that produced from RWS because it consists of approximately 93.52\u00a0wt% hemicellulose and cellulose, which affects the thermal decomposition of volatile vapor and the subsequent deoxygenation reaction to a medium hydrocarbon vapor. Under the operating conditions, the secondary reaction and the tar carking reaction are not sufficiently accomplished; thus, it is found that the production of noncondensable gases has no significant tendencies. In addition, it is worth noting that when the average particle size of CRZ is larger, it receives sufficient heat transfer, causing thermal decomposition of the lignocellulosic component to a large amount of volatile vapor. Then, deoxygenation and decarbonylation are also enhanced to obtain medium-weight hydrocarbon compounds before being influenced by the Ni-DM catalyst, allowing the hydrocarbon chain to break into small hydrocarbon compounds at the active Ni sites on the surface of the dolomite structure to obtain the highest bio-oil yields of 39.48\u00a0wt% and 40.39\u00a0wt% when using RWS and CRZ, respectively. As the temperature is increased to 600\u00a0\u00b0C, the bio-oil yield decreases, the gas yield also significantly increases due to the secondary reaction, and the cracking of tar also occurs, causing an increase in noncondensable carbon dioxide and carbon monoxide gases [35\u201337]. In addition, the tendency of solid char is also likely to increase the process temperatures from 400 to 550\u00a0\u00b0C. The pyrolyzed solid from CRZ is found to be higher due to the influence of this temperature increase, which causes uniform heat transfer into the lignocellulosic particles in CRZ; thus, the uniform heat transfer into the particles enhances thermal degradation, devolatilization and the secondary reaction of tar cracking [30,33,35,37]. Moreover, catalytic pyrolysis of small volatile vapors over the active Ni sites results in large bio-oil and gas yields. In contrast, with an increasing process temperature up to 600\u00a0\u00b0C, the content of pyrolyzed solid decrease from both RWS and CRZ, which is due to the influence of the temperature in accomplishing the devolatilization of the lignocellulosic components. Notably, the fixed carbon remains stable. Furthermore, a high pyrolysis temperature might decrease the catalytic activity, leading to a lower yield of desirable products, such as bio-oil, which is affected by the influence of temperature on deoxygenation and is slowed with an increase in operating temperature; in contrast, coke decomposition on the surface of the Ni-DM structure causes catalytic deactivation [31,35,38]. Thus, a desirable product distribution at high temperature shows a significant decrease.\nFig. 4(C) shows the influence of the reaction time on the catalytic pyrolyzed product distribution from RWS and CRZ using the following operating conditions: average size distribution of 0.250\u20130.355\u00a0mm RWS and 0.710\u20132.000\u00a0mm CRZ, the temperature of 550\u00a0\u00b0C, inert nitrogen flow rate of 180\u00a0mL\u00a0min\u22121, 10%Ni-DM, a catalyst loading of 10\u00a0wt%, and a varying reaction time of 45\u201390\u00a0min. There is a significant decrease in the bio-oil yield of both RWS and CRZ by continuously increasing the reaction time. The results can be explained by the continuously increasing reaction time affecting the thermal decomposition of the lignocellulosic volatile vapor at high temperature. Then, deoxygenation, decarboxylation and decarbonylation reactions enhance its conversion into smaller volatile vapors and further decomposition by secondary reactions; additionally, the cracking of tar results in a large quantity of noncondensable gas. Then, the small vapor gas easily diffuses into the mesopores of the catalyst [23,32]. Therefore, an increase in reaction time with a temperature over 45\u00a0\u00b0C enhances the devolatilization of lignocellulosic components to small volatile vapors that can be further decomposed into small noncondensable hydrocarbon vapors. This reaction is still influenced by a temperature of 550\u00a0\u00b0C, causing the secondary degradation reaction and tar cracking reaction to result in a bio-oil yield that decreases to 37.64\u00a0wt% RWS and 33.56\u00a0wt% CRZ when catalytic pyrolysis is continuously performed for 90\u00a0min. These results show that longer reaction times lead to an increased noncondensable gas yield which is affected by the influence of high temperature [37,39]; this is contrary to the yield of bio-oil, while the solid char yield shows an insignificant difference due to the devolatilization of the lignocellulosic components to form stable fixed carbon [22,40,41].The influence of the inert nitrogen flow rate was investigated with the following constant catalytic pyrolysis parameters: average size distribution of RWS (0.250\u20130.355\u00a0mm), CRZ (0.710\u20132.000\u00a0mm), temperature of 550\u00a0\u00b0C, reaction time of 45\u00a0min, and 10\u00a0wt% loading of 10%Ni-DM. The inert N2 flow rate was varied from 60, 120, 180, and 240\u00a0mL\u00a0min\u22121. As shown in Fig. 4(D), the control of volatilization from pyrolysis of both RWS and CRZ at a N2 flow rate of 180\u00a0mL\u00a0min\u22121 obtains the highest bio-oil yields of 39.48\u00a0wt% and 40.39\u00a0wt% when using RWS and CRZ, respectively. Both the catalytic pyrolysis of RWS and CRZ obtain the same trend of bio-oil production when increasing the flow rate of inert N2 gas. The reason for these results is that the lower flow rate of N2 affects the longer reaction time of the devolatilization of lignocellulosic components into volatile vapor, which is influenced by the reaction temperature of 550\u00a0\u00b0C, leading to the formulation of smaller volatiles due to the deoxygenation, decarbonylation, and decarboxylation reactions. Notably, the long residence time results in a large amount of volatile vapor and further continuous cracking by the secondary reaction. The cracking of tar is affected by continuing to convert small volatile vapor into noncondensable gas that then diffuse through the surface of the Ni-DM structure and throughout the pyrolysis reactor [18,21,31,33]. In contrast, the high N2 flow rate causes a short residence time of devolatilization. All vapor gases receive insufficient heat transfer to decompose the lignocellulosic components, while the N2 carrier gas leads to the larger molecular weight volatile vapor passing throughout the pyrolysis reactor before the secondary reaction. Tar cracking and catalytic pyrolysis are also accomplished. The decreasing trend in the bio-oil yield when increasing the inert N2 flow rate is caused by a short residence time and insufficient heat transfer into holocellulose is not complete despite flowing throughout the reactor with the N2 carrier gas [30,37].The advantage of dolomite is that it is an inexpensive mesoporous strong Lewis base catalyst, providing effective mass transfer of larger hydrocarbon molecules into its pore structure with minimal steric hindrance when compared to that of a well-known commercial catalyst. DM can might absorb moisture at low temperature of pyrolysis reaction and will then be released by increasing the pyrolysis temperature, promote devolatilization and deoxygenation of holocellulose and the adsorption of both CO and CO2 on the pore structure [13,26\u201328]. The stronger basicity and more developed pore structure of Ni-DM demonstrate its strong potential in improving the deoxygenation reaction and further catalytic pyrolysis enhances C\u2013O bond cleavage rather than C\u2013C cleavage [40], which then passes through the mesoporous dolomite and undergoes a secondary reaction, namely, tar cracking to form low molecular weight hydrocarbon compounds [41\u201343]. Thus, the bio-oil yield increases with the Ni concentration on the modified DM. As seen in Table 2, the increase in impregnated Ni from 5 to 20\u00a0wt% in the DM template shows a slight change in surface area and pore volume but the concentration of Ni-modified on calcined dolomite from 5\u00a0wt% to 20\u00a0wt% has an effect on the pyrolysis yield as seen in Fig. 4(E). In particular, when using a Ni concentration of 20\u00a0wt% on DM, the highest yields of noncondensable gas for both RWS (38.27\u00a0wt%) and CRZ (33.17\u00a0wt%) are obtained which can be explained by the DM having additional catalytic performance with an increased concentration of Ni. This enhances the devolatilization of holocellulose, deoxygenation of volatile vapor due to C\u2013O cleavage, and formation of CO2 due to the Lewis basicity [13]. When the Ni concentration is enhanced to provide higher catalytic activity, the secondary reaction and further vapor cracking to small volatile vapors [31,36,42,43] is also observed. When increasing the Ni impregnated from 5\u00a0wt% to 10\u00a0wt% into calcine dolomite, a slight change in the bio-oil yield produced from RWS and CRZ occurs, with values of approximately 37.91\u00a0wt% to 39.48\u00a0wt% and 38.61\u00a0wt% to 40.39\u00a0wt%, respectively. This suggests that increasing the Ni modification on DM further promotes deoxygenation, decarbonylation, and decarboxylation; then, secondary reactions and tar cracking also occur [31,32,37], resulting in a decrease in bio-oil and leading mainly to a dramatic increase in noncondensable gas. With 20\u00a0wt% Ni-DM, a strong acid site is obtained, but the dispersion of a high Ni modification might decrease both the surface area and pore volume, which affects the deactivation of the Lewis base catalyst, including the influence of the high temperature enhancement on coke formation. The highest yield of noncondensable gas is likely the product distribution from noncatalytic pyrolysis. Furthermore, hydrogen transfer during catalytic pyrolysis is promoted by the active Ni modification, which might have inhibited carbonaceous material on the surface and in the pores of the dolomite support and slightly enhanced the solid char yield.The influence of catalyst loading (0%, 5%, 10%, 20% by weight of the feedstock) of RWS and CRZ pyrolysis on the product distribution were investigated under a constant operating condition of 550\u00a0\u00b0C, reaction time of 45\u00a0min, and inert N2 flow rate of 120\u00a0mL\u00a0min\u22121 at the appropriate 10\u00a0wt%Ni modified-DM. As shown in Fig. 4(F), the use of 10\u00a0wt% loading of 10%Ni-DM catalyst increased both the pyrolyzed -oil yield and noncondensable gas yield. This increased use of catalyst could produce a higher pyrolyzed oil yield (39.48\u00a0wt% from RWS, 40.39\u00a0wt% from CRZ) compared with the noncatalytic pyrolysis of both lignocellulosic biomasses, which were only 28.63\u00a0wt% from RWS and 27.87\u00a0wt% from CRZ because calcined dolomite can enhance the devolatilization of large-molecule compounds to small-molecule compounds, making it easier to enter the pore structure and react with 10\u00a0wt%Ni-modified at the catalyst surface due to enhanced catalytic activity, C\u2013C cleavage, and decarboxylation. Then, the secondary reaction also occurred and promoted low molecular volatile vapor to obtain pyrolyzed oil [44,45]. With an increase catalyst loading in the 10\u00a0wt%Ni-DM from 5\u00a0wt% to 20\u00a0wt% into the pyrolysis reaction, both pyrolyzed oils produced from RWS and CRZ dramatically decreased, but in contrast, it increased the amount of gaseous RWS and CRZ in both catalytic pyrolysis processes and reached the maximum yield of a noncondensable gas of 35.43\u00a0wt% from RWS and 29.12\u00a0wt% CRZ at a catalyst loading of 20\u00a0wt% and decreased the pyrolyzed oil yield. Notably, catalytic pyrolysis using 20\u00a0wt% catalyst loading enhanced the devolatilization, deoxygenation, decarboxylation, decarboxylation and secondary reaction due to the influence of high temperature, which led to the decomposition of volatile vapor to a small noncondensable gas similar to noncatalytic reactions. These results might explain why an increase in the catalyst loading enhances the large production of noncondensable gas, while the noncatalytic reaction and thermal degradation have a significant effect on the devolatilization of lignocellulosic components [30,31]. In particular, the pyrolysis of RWS, which contains hemicellulose that decomposes more easily at low temperature than the thermal decomposition and devolatilization during the pyrolysis of CRZ. The solid char yield showed a fluctuating trend when increasing the catalyst loading. Compared with the noncatalytic experiment, the production distribution of solid char (30.43\u00a0wt% from RWS, 37.59\u00a0wt% from CRZ), pyrolyzed oil yield (28.63\u00a0wt% from RWS, 27.87\u00a0wt% from CRZ) and maximized noncondensable gas (40.94\u00a0wt% from RWS, 34.54\u00a0wt% from CRZ) are obtained.The pyrolyzed oil produced from the catalytic pyrolysis of RWS and CRZ over Ni-DM in the fixed bed reactor was investigated using GC-MS. The use of the catalyst improved the activation energy of catalytic pyrolysis and promoted thermal catalytic cracking during the pyrolysis of lignocellulosic components in accordance with the enhanced devolatilization of lignocellulosic components to improve the pyrolyzed oil yield. Table 3\n illustrates the relative intensity peak and identified chemical compounds of catalytic pyrolysis oil by varying the percentage of Ni modified on calcined dolomite at operating condition of 550\u00a0\u00b0C, an inert N2 flow rate of 180\u00a0mL\u00a0min\u22121, the reaction time of 45\u00a0min using catalyst loading at 10% by weight.A large number of peaks were also observed, indicating many types of organic compounds, e.g., aliphatic, monoaromatic, polyaromatic, and oxygenated organic compounds. In particular, calcined dolomite represents a stronger Lewis basic catalyst that obtains more basic sites, and higher specific surface area and mesoporous properties affect the active catalytic performance in C\u2013O cleavage and improve the deoxygenation reaction [31,40,41,43], resulting in volatile vapor and small molecular weight oxygenated hydrocarbon compounds, e.g., phenol furan, ketone, some sugar and carbohydrate derivatives. Phenols are greatly produced from the catalytic pyrolysis of lignin components. The degradation of lignin via catalytic pyrolysis mainly obtained the main source of active free radicals, while furan was obtained by thermal decomposition, dehydration of cellulose components at low temperature and further deoxygenation, decarboxylation, and decarboxylation to produce aliphatics [46]. Thus, the catalytic pyrolysis of RWS, which contains a lignin component of approximately 29.97\u00a0wt%, might yield phenols and phenol derivatives. Then, phenolic oligomers abstract hydrogen transfer during thermal degradation at low temperature. Then, smaller alkyl phenolic and oxygenated compounds undergo secondary reactions to smaller phenols [47\u201349], while the pyrolysis of CRZ, which consists of a higher proportion of cellulose, might be converted to furan via deoxygenation and further decarbonylation and decarboxylation and is influenced by Ni-modified catalytic performance on calcined dolomite. However, the use of Ni-DM significantly reduced furan and phenol production and eliminated carboxylic compounds and cyclopentanone formation during the catalytic pyrolysis of both RWS and CRZ. Notably, long chain aliphatic hydrocarbons were formed and further converted to shorter chain hydrocarbons due to the active catalytic performance of Ni modification. Moreover, Ni-dolomite catalysts play a catalytic role with active sites where increasing Ni modification on the dolomite support reduces the activation energy of the catalytic pyrolysis and promotes thermal catalytic cracking during pyrolysis of the lignocellulosic component, which is in accordance with the enhanced devolatilization of holocellulose. In particular, the thermal decomposition of hemicellulose and cellulose into volatile vapor and further deoxygenation, decarbonylation, and decarboxylation caused by the role of Ni on the stronger Lewis basic catalyst enhanced C\u2013O cleavage rather than C\u2013C cleavage at Ni-acidic sites on the dolomite surface led to smaller amounts of oxygenated compounds. The most abundant aliphatic product of both alkanes and alkenes was observed from the use of Ni-DM in the catalytic pyrolysis of both RWS and CRZ. This result might explain why alkyl radicals readily transfer H radicals to volatile vapor during the thermal degradation of hemicellulose at low temperatures, resulting in the production of alkanes and alkenes from alkyl radicals. Moreover, the presence of a sufficient concentration of Ni-modified dolomite catalyst might convert furan to monoaromatics by enhanced deoxygenation, decarbonylation, decarboxylation, and secondary reactions, including tar cracking [21,30,31,41,42], contrary to the decrease in aldehydes and ketones and the disappearance of long chain carboxylic acids due to deoxygenation, decarbonylation, and decarboxylation at the Ni active site. Compared with the pyrolysis reaction with the use of catalyst, all the Ni-modified calcined dolomite slightly reduced the solid char yield while increasing the liquid and noncondensable gas yield at the conversion of large molecular weight volatile vapor. These phenomena are also due to various hydrocarbon conversion reactions, e.g., thermal degradation at low temperature, catalytic activity in C\u2013C cracking, deoxygenation, and oligomerization, which are catalyzed by Ni active sites on the Lewis basic support [41,42].\nTable 4\n presents a comparison of the pyrolysis oil from the pyrolysis without Ni-DM and catalytic pyrolysis by varying the percentage by weight of Ni modified on calcined dolomite at the optimal condition of 550\u00a0\u00b0C, an inert N2 flow rate of 180\u00a0mL\u00a0min\u22121, the reaction time of 45\u00a0min using catalyst loading at 10% by weight, which represented a lower oxygen content and higher H/C content than the noncatalytic pyrolysis due to the thermal decomposition of holocellulose at low temperature. It was found that RWS contained 22.71\u00a0wt% hemicellulose and 47.32\u00a0wt% cellulose, which were affected by thermal decomposition and converted to volatile vapor in the form of alkyl radicals. Furthermore, RWS easier C\u2013O cleavage and enhanced deoxygenation, decarboxylation, and decarboxylation to low oxygen volatile vapor and further reacted with the Ni-modified active site on calcined dolomite to produce bio-oil, which contained low amounts of oxygenation components. The thermal decomposition of cellulose, which mainly consisted of CRZ, was also more difficult; thus, the pyrolyzed oil from CRZ may have a higher O/C ratio affecting the HHV of approximately 27\u00a0MJ/kg. The HHV in noncatalytic pyrolysis was 26.86\u00a0MJ/kg and 24.89\u00a0MJ/kg, indicating that the presence of Ni-DM promoted thermal catalytic pyrolysis of lignocellulosic components in accordance with enhanced devolatilization into volatile vapor and further deoxygenation reaction [18,21,22] caused by the Ni active site on the stronger Lewis basic catalyst and that its pore structure led to oxygen removal during the whole pyrolysis reaction [29]. As seen from Table 4, the solid char obtained from catalyzed with Ni-modified calcined dolomite showed quite similar elemental analyses in biochar composition for both RWS and CRZ due to thermal devolatilization and further deoxygenation to pyrolyzed oil, meanwhile, the noncatalytic pyrolysis of biomass was rapidly thermally decomposed. When the pyrolysis reaction undergoes at high temperatures, some volatile vapor may be decomposed into the hydrocarbon radicals and depolymerization to form a large hydrocarbon compound as a residual component in the solid char, it was found that the elemental analyses of noncatalytic biochar exhibit a quite difference in carbon and oxygen content, has a higher O/C ratio is considered to be less candidate solid fuels due to its lower heating value. These results indicated that the practical application of catalytic pyrolysis from both softwood and non-wood could produce useful fuel-like compounds and chemicals with high heating values, and the physicochemical properties of pyrolyzed oil and solid char were higher than those of raw materials.\nTable 5\n presents the compositions of the noncondensable gases produced from the catalytic pyrolysis of RWS and CRZ using the difference percentage by weight of Ni modified on calcined dolomite at the optimal condition of 550\u00a0\u00b0C, an inert N2 flow rate of 180\u00a0mL\u00a0min\u22121, the reaction time of 45\u00a0min using catalyst loading at 10% by weight, compared with the noncatalytic reaction, are mainly composed of CO2, H2, CO and CH4, while small hydrocarbon (C2\u2013C3) gases also occurred. Both softwood and non-wood lignocellulosic biomass obtained similar components of pyrolysis of noncondensable gas due to the catalytic activity enhanced the thermal decomposition of lignocellulosic components into volatile vapor, then both C\u2013C cleavage and C\u2013O cleavage including deoxygenation, decarbonylation, decarboxylation and further the secondary reaction high temperature and longer residence time enhanced to cracking of volatile vapor into noncondensable gas [22,31,38,40,47]. As seen from Table 5, CO2 is mainly a gas component from the pyrolysis of lignocellulosic biomass due to the deoxygenation reaction of volatile vapor from the thermal degradation of holocellulose. The increased Ni-modified concentration enhanced the appearance of the CO component in the presence of Ni-DM and further enhanced the deoxygenation ability of DM. These results might explain why the deoxygenation of volatile vapors enhanced the production of phenols and monoaromatics, including some aliphatic hydrocarbon compounds. Notably, an increase in Ni-DM also leads mainly to increased production of H2, contrary to the reduction in CO2 concentration in a noncondensable gas component, which may be affected by the adsorption of CO2 the presence of Ni-DM, is able to react with CO2 then converting the CaO component in Ni-DM to CaCO3 [13,26], including the water gas-shift reaction enhancing the occurrence of the H2 component [31,34,47\u201349]. CH4 and olefins change a little, was formed when methoxy organic derivatives were formed during oxygenation and then dealkylation reactions, while the appearance of C2H4, C2H6, C3H8 hydrocarbon gases might be attributed to the degradation of alkyl groups in the oxygenated compounds during carboxylation and decarboxylation of volatile vapor. Then, secondary reactions, including the secondary reaction of alkyl groups attached to alcohol or phenol compounds, also converted lower molecular weight hydrocarbons to C2\u2013C3 gases.This study compared the catalytic pyrolysis of soft wood and non-wood biomass in a fixed bed reactor. Ni-modified calcined dolomite acted as an active catalyst in the pyrolysis reaction. The lignocellulosic components of both biomasses showed a difference in product yield due to thermal decomposition depending on their components. The highest bio-oil yield of \u223c34\u00a0wt% was obtained from the catalytic pyrolysis of rubberwood sawdust at a temperature of 550\u00a0\u00b0C, an inert nitrogen gas flow rate of 180\u00a0mL\u00a0min\u22121,a reaction time of 45\u00a0min, catalyst loading of 10\u00a0wt% with 10%Ni-modified calcined dolomite. Both pyrolyzed bio-oils obtained from softwood and non-wood biomass consisted of aliphatic (paraffin, olefin), monoaromatic, polyaromatic hydrocarbon compounds and derivative oxygenated compounds, while the physicochemical analyses indicated that oxygen compounds were removed via thermal decomposition, deoxygenation, decarbonylation and decarboxylation. Ni-DM acted as an acid site on the mesoporous dolomite parent, affecting the C\u2013C cleavage of large volatile vapors into smaller vapors, and then the secondary reaction and pore structure enhanced the production of small hydrocarbon compounds. The solid char yield from the pyrolysis of non-wood was higher than from softwood because the amount of fixed carbon and ash. The noncondensable gas product mainly consisted of CO2 and H2, with increasing Ni-DM loading reducing the CO concentration, whereas the yield of CO2 also increased due to decarboxylation rather than decarbonylation. Notably, the yield of H2 increased with an increase in the Ni-DM loading due to the dehydrogenation reaction that occurred in the secondary reaction.\nKittidech Praserttaweeporn, Methodology, Data collection, Visualization, Data curation. Tharapong Vitidsant: Conceptualization, Supervision. Witchakorn Charusiri: Conceptualization, Methodology, Visualization, Data curation, Validation, Writing \u2013 review & editing, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the Center of Excellence on Petrochemical and Materials Technology (PETROMAT), Center of Fuels and Energy from Biomass, Chulalongkorn University in the form of facilities support, and research projects through the Program Management Unit Competitiveness (PMUC), National Higher Education Science Research and Innovation Policy Council (No. C10F630094) and Srinakharinwirot University also partially supported this study.", "descript": "\n                  This study investigated the effect of process conditions during the catalytic pyrolysis of softwood and non-wood on the pyrolysis product. Ni modified on calcined dolomite (Ni-DM) was used to determine the effectiveness of the catalytic activity and the effect of doping non-noble metals on the product distribution of both pyrolyzed biomasses. The lignocellulosic biomass components showed different effects on the decomposition characteristics of the pyrolysis vapor and its devolatilization, while Ni-DM showed a catalytic effect to enhance the decarbonylation, decarboxylation, and secondary reaction of tar cracking affecting CO2 and CO removal; additionally, the catalytic activity also promoted the formation of aliphatic and olefin hydrocarbon compounds and facilitated C\u2013C cleavage and scission to smaller hydrocarbon compounds. With increased Ni loading, the yield of noncondensable gas increased. Furthermore, gas chromatography analysis indicated that the composition of gas mainly consisted of hydrogen gas, which increased significantly due to a water-gas shift during the catalytic pyrolysis of biomass at high temperature with 10% Ni-dispersed calcined dolomite acting as the catalyst. Meanwhile, the pore structure and the modified-Ni on calcined dolomite enhance decrease acids and sugars in bio-oil yield and favor the formation of alkane gases of liquid hydrocarbon fuels including the dramatically increased in alkane gases.\n               "}
{"full_text": "No data was used for the research described in the article.At present, >300 different grades of polyolefins are commercially available for different applications. Particularly, polyethylenes (PE) and polypropylenes (PP) are the most important materials produced by the petrochemical industry [1]. Together, they account for the largest segment of commercial polymeric materials [2]. Thanks to the dramatic developments in catalyzed polymerizations, the annual production of polyethylenes exceed 170 million tons, which is roughly 50\u00a0% of the global plastics production [3]. Although faced with environmental concerns, their superior mechanical performance still provides substantial advantages over their bio-based alternatives [4]. Coordination-insertion polymerizations catalyzed by transition-metal complexes are the predominant solutions for synthesizing polyethylenes on the industrial scale [5]. As already mentioned above, the major advantage of coordination-insertion chemistry is the outstanding control over polymeric microstructure, which also determines the macromolecular characteristics of polymers, such as the mechanical, thermal, and optical properties, and thus their final commercial values [6,7]. Consequently, the improvement of the catalytic performance of organometallic catalysts in ethylene (co)polymerization is a key driving force in catalyst research.Over fifty years ago, Ziegler and Natta won the Nobel Prize in Chemistry for their discovery of heterogeneous catalysts applied to olefin polymerizations [8,9]. Since then, the research field of transition-metal catalyzed olefin polymerization has seen great progress [10]. Compared to their early-transition metal rivals, the late-transition metal organometallics have been a topic of interest, because of their low oxophilicity, as well as their great potential in producing polymers containing various types of branches and polar functional groups [11\u201313]. Currently, a variety of structurally related late-transition metal organometallics have been applied in the ethylene (co)polymerization which includes \u03b1-diimine (N^N), phosphine (P^O), phenoxyimine (N^O) and pyridinylimine (N^N) types of catalysts (Fig. 1\n)[14\u201320]. Disparate catalytic performance and varying polymeric properties can be obtained via the utilization of different metal complexes. For example, phosphine-containing palladium catalysts have emerged as powerful alternatives for copolymerization of ethylene with polar monomers, leading to linear polymers [21\u201324]. However, the \u03b1-diimine Ni and Pd complexes still have a lot of advantages over the rest, including the ease of synthesis and structural modification of ligands. In addition, late-transition metal complexes have attracted more considerable attention with the report of the highly active \u03b1-diimine Ni and Pd complexes by Brookhart (Fig. 1, A), for application in ethylene polymerization [25\u201328]. The branching density and topology of polyethylenes could be predictably adjusted by modifying the ligand structure and polymerization conditions. Thus elastic polymers could be synthesized using ethylene as a single monomer [29].According to the previous studies, it was observed that small changes in the ligand structure entail significant variations in the macroscopic properties of the resulting polyethylenes [30]. N-aryl steric effects (Fig. 1. B, C, D, and E), backbone effects (Fig. 1. F and G), and remote substituent effects (Fig. 1. B and G) have been considered as main variables, influencing the catalytic behavior of \u03b1-diimine Ni(II) and Pd(II) complexes and consequently modifying polymer properties [31\u201339]. It is worth noting that the incorporation of phenyl groups on the ortho-moieties of N-aryl has helped achieve the control over the synthesis of either linear or branched polyethylenes, exhibiting ultrahigh molecular weight (up to 4\u00a0\u00d7\u00a0106 g mol\u22121). These complexes were stable in the presence of hydrogen during ethylene polymerization (Fig. 1. B) [31]. Recent research activities in \u03b1-diimine Ni and Pd catalysts are mainly devoted to the modification of complex structures to achieve more efficient and higher degree of catalytic properties in homogenous (co)polymerization. Along these lines, unique and functional polymers, such as ultra-high-molecular-weight polyethylenes, elastomeric polymers, and highly branched and high-molecular-weight polyethylenes, have been reported through the elaborated designs of \u03b1-diimine Ni and Pd catalysts in catalyzed ethylene (co)polymerization [5].Over the last few years, several high-quality reviews on Ni and Pd complexes have been published, which partially includes the \u03b1-diimine Ni and Pd complexes for ethylene or \u03b1-olefin (co)polymerizations [5,11,30,40\u201347]. However, we still see the obvious gap between the published literatures and the specifically state-of-art discussion (especially fastened in the last decade). The lack of a comprehensive overview of \u03b1-diimine Ni and Pd complexes still remains, which includes the catalytic mechanisms, catalytic behaviors, and heterogeneous catalysis applications. Hence, this review emphasizes the structural variation of \u03b1-diimine Ni and Pd complexes, their catalytic performance during ethylene (co)polymerization, catalytic mechanisms in the ethylene polymerization and copolymerization with polar monomers, and the resulting polymer properties. Although researchers have invested extensive efforts in the modifications of ligands and complexes structures, the discovery of promising catalytic systems is inevitably accompanied by trial and error. The relationship between structural variations (steric and electronic effects) of \u03b1-diimine Ni and Pd complexes and their catalytic performance is of great importance and thus need to be addressed in detail. More importantly for future application, the use of heterogeneous catalysts in the gas- and slurry-phase polymerization currently represents the predominant route in the industry [48]. Studies related to the immobilization of \u03b1-diimine Ni and Pd complexes onto solid substrates are a crucial step for their successful commercialization. This review has four main sections: i.e. i) Coordination-insertion chemistry; ii) Relationship between structural details and catalytic performance, iii) heterogeneous polymerization using supported \u03b1-diimine Ni and Pd complexes, iv) the mechanical properties and hydrophilicity of the produced polymers.Thermoplastic elastomers (TPEs) are rubber-like materials, which offer the ease of processing, recyclability, and enhanced mechanical characteristics. Their annual production is driven by industrial and consumer demands [49]. For TPEs synthesis, Dow's constrained geometry catalysts (CGC) were widely applied to produce the long-chain branches, which involve the incorporation of ethylene and higher a-olefin monomers. Short-chain branches in TPEs are achieved via copolymerization between ethylene and a-olefin monomer, catalyzed by transition-metal-based complexes. In contrast, \u03b1-diimine Ni and Pd complexes exhibit unique catalytic behaviors, producing high molecular weight polyethylenes with highly branched structures through the so-called chain-walking mechanism. This facile synthetic approach enables control of polyethylenes microstructures (either linear or branched polymers), solely using ethylene as the monomeric feed. The ethylene (co)polymerization catalyzed by the \u03b1-diimine Ni and Pd complexes provides a cost-effective alternative to more complicated and multi-step approaches to synthesize elastomeric materials, as it requires only a single step for their manufacturing [44]. A notable report describing the chain-walking mechanism was originally put forward by Fink et al. [50,51]. Soon afterward, Brookhart et al. refined this mechanistic method, which was then validated by both experimental and theoretical research [25,26,52,53]. Scheme 1\n illustrates the concept of the chain-walking mechanism for ethylene polymerization. Initiated by a cocatalyst, the chain-walking mechanism could be divided into several parts, namely chain propagation, chain transfer, and chain walking or isomerization. Cationic alkyl-metal active species provide superior activities. Hydrogen elimination and chain transfer lead to the formation of highly branched polyethylenes (Scheme 1). The catalytic metal center migrates along the polyethylene backbone via rapid \u03b2-H elimination and reinsertion as a chain-walking process [46]. The molecular weight of the obtained polyethylenes depends on the competition between the monomer insertion (chain growth) and the \u03b2-H elimination (chain transfer), where the latter process, followed by the reinsertion, leads to the formation of branched structures. More recently, Pei et al. have proposeda unique and new mechanistic model for the formation of long-chain-branches (LCBs), based on the classic chain-walking process catalyzed by \u03b1-diimine Ni complexes. The methyl branch was produced by a one-step chain walking followed by ethylene insertion. Consequently, the long-chain branching (LCBs) was directly obtained by ethylene insertion into the primary Ni-alkyl species, which is originally formed from the migration of the catalytic Ni center to the methyl terminal. Steric interactions from the ortho-aryl substituted anilines play a central role to limit the ethylene insertion to selectively generate the primary Ni-alkyl species and/or the secondary Ni-alkyl species with the \u03b1-methyl substituent. The proposed mechanism could explain the existence of methyl and LCBs without the formation of other short branches during the ethylene polymerization [54]. The bulky substituentson the ortho-N-aryl offer a steric crowding at the axial sites of the catalytic metal centers, which are perpendicular to the metal-diimine plane. This steric hindrance suppresses the associative chain transfer process. Such metal complexes bearing more bulky groups can lead to higher molecular weight polymers. Additionally, the diimine backbone influences the polymerization activity as well as the polymer molecular weight. The backbones with alkyl substituents are reported to yield higher molecular weight of polymers with narrower PDI than the planar acenaphthyl backbones [30]. Meanwhile, the steric enhancement on the diimine backbones significantly improves the thermal stability of the catalyst during ethylene polymerization. Side-arm effects (electronic effects and weak interactions) from the remote substituents of the ligands similarly influence the catalytic activity and ensuing polymer properties.Generally, the polymerization temperature and ethylene pressure have a significant impact on the catalytic performance of the \u03b1-diimine Ni and Pd complexes. The high temperature increases the rotation of the C-Naryl bond, reducing steric hindrance at axial sites. Furthermore, a high rate of chain transfer is expected at high polymerization temperature, which leads to an increased reinsertion rate and the formation of branching. As the ethylene pressure is increased, chain propagation is preferred over chain transfer, therefore more linear and less branched polymers are formed. In addition, the \u03b1-diimine Pd complexes tend to undergo the chain-walking process, as compared to the corresponding Ni complexes.Hyperbranched and amorphous polyethylenes can be produced via the \u03b1-diimine Pd catalyzed ethylene polymerization, while the \u03b1-diimine Ni complexes can produce mainly linear (few short-branches) polyethylenes with a well-defined melting point [25].Due to this unique catalytic behavior of \u03b1-diimine Ni and Pd complexes, alkyl chains, such as methyl, ethyl, propyl, butyl, and even longer branches could be generated in the polymer backbones. Various polymers with elastic, semi-crystalline, and amorphous properties could be modulated with controlled chain-walking polymerization, using different reaction conditions and /or specially-synthesized Ni and Pd catalysts [42,55].Polyethylenes possess essential characteristic like excellent chemical resistance, ease of manufacturing, and low production costs [8]. However, its nonpolar backbone also has a lack of added-value functionalities, which are important applications in many fields [56]. Functionalized polyethylenes exhibit improved surface and mechanical performance due to the incorporation of functional polar groups [19,57]. The synthesis of functionalized polymers is primarily performed by post-polymerization functionalization, free radical copolymerization, or, by use of special methods like ring-opening metathesis polymerization (ROMP). These polymerization approaches have some drawbacks, such as the use of harsh conditions or poor controls of polymer microstructure [40,58]. In contrast, flexible synthesis of functionalized polyethylene with well-characterized structures and properties is currently an important field of research in coordination-insertion polymerization [59]. Early-transition metal complexes, like the Ziegler-Nata catalysts, were certainly applied to copolymerize ethylene with polar monomers. However, researchers could not achieve any success in synthesizing copolymers, due to the poisoning effect from the polar monomers. Thanks to the low oxophilicity, the late-transition metal complexes (especially for Pd-based complexes) exhibit remarkable capacity to copolymerize ethylene with polar monomers (Scheme 2\n) [11]. Following the insertion of polar monomers, the metal center coordinated with the CC bond to form the intermediate I. There were two types of the insertion process, namely the 1,2 insertion (intermediate II) and 2,1 insertion (intermediate III). The polar group, X (Lewis-basic groups) and catalytic metal center, M (Lewis-acidic groups) in Scheme 2 potentially generated the stable metalation of X-M chelates. These chelated metalations deactivate the cationic alkyl-metal species and terminate the copolymerization process [5,60].The initially reported \u03b1-diimine Ni and Pd catalysts could surprisingly accomplish the ethylene copolymerization with methyl acrylate as a comonomer [26]. This indicated that the late transition metal catalysts could provide an effective solution for the challenging ethylene copolymerization with polar monomers, because of the low oxophilicity of the metal centers [11,40,61]. This discovery could address the deactivation problems associated with the polar groups near the metal center. However, the field of copolymerization of ethylene and polar monomers has been mainly dominated by the use of \u03b1-diimine palladium catalysts. \u03b1-Diimine nickel catalysts are generally less tolerant toward polar groups than the palladium catalysts. The nickel catalysts could only catalyze the ethylene copolymerization with a limited number of polar monomers, such as polar derivatives of norbornene, silane-based and long-chain \u03b1-olefins[40,62]. With the extensive exploration of new \u03b1-diimine Ni and Pd complexes, a variety of polar monomers have been broadly investigated in ethylene copolymerization studies. Fig. 2\n displays the collection of polar monomers applied in the ethylene copolymerization catalyzed by \u03b1-diimine Ni and Pd complexes. These polar monomers could be classified into two types of vinyl monomers: alkene-connected and long-chain polar monomers [60]. Alkene-connected polar monomers refer to the monomers, where the polar groups were directly connected to the CC bond. As reported, these types of monomers are the most challenging monomers, potentially poisoning the ethylene copolymerization [47]. Copolymer A in Fig. 2 is synthesized from the ethylene copolymerization with alkene-connected polar monomers. These copolymers led to a straightforward attachment of the polar groups to the polymer backbones. The ethylene copolymerization with long-chain polar monomers generates the copolymer B, where there is an alkyl spacer between the CC bond and polar groups. However, copolymer B can be generated from the ethylene copolymerization with alkene-connected polar monomers due to the chain-walking process. The functionalized copolymers can even be synthesized with both, in-chain and terminal polar groups, catalyzed by the \u03b1-diimine Ni and Pd catalysts. These unique structures of the long-chain polar monomers allowed the easier copolymerization with ethylene, reducing the possibility to poison the catalytic center by means of the coordination with polar sites. This strategy facilitates the polymer products with polar groups away from the polymer backbone. Both types of copolymers (A and B) provide very interesting and useful properties for future industrial application; and thus, it is currently the driving force in such research field [11,60]. Additionally, polar functionalized norbornenes are also an interesting class of polar substrates (Fig. 2). Ethylene-norbornene (E-NB) copolymers are an important class of polyolefins with the high refractive index and high transparency. They are suitable for optical applications including lenses, blister packs and medical equipment. As strong \u03c0-donors, norbornene-type monomers can efficiently coordinate with the metal centers compared to other polar vinyl monomers. The \u03b2-hydride elimination is relatively prevented by the cyclic structure. The presence of long spacers between the CC bond and the polar groups offers less likeliness for deactivation of the catalytic metal centers [63\u201366].As mentioned earlier, the \u03b1-diimine Ni and Pd complexes are unique in their capacity to control the microstructure of the resulting polymer, while allowing ethylene copolymerization with polar monomers. An additional advantage of the \u03b1-diimine Ni and Pd complexes over their early-transition-metal competitors is the ease of synthesis and higher air stability [42,45]. The synthesis largely involves two main reactions (Fig. 3\n); namely, 1) the \u03b1-diimine formation from the reaction of modified anilines and diketones in the presence of the catalytic amount of the acid; 2) the coordination reactions of \u03b1-diimine ligands and metal (Ni and Pd) salts or complexes to synthesize \u03b1-diimine Ni and Pd complexes. The resulting complexes are very stable to moisture and oxygen. This inertness of \u03b1-diimine Ni and Pd catalysts suits the industrial application, where storage stability is a well-known issue. The structural modifications of anilines and diketones initially based on ligands bring about the versatile synthesis of \u03b1-diimine Ni and Pd complexes and different coordination environments to the catalytic metal centers.Generally, the catalytic performance of a metal center is influenced by the electronic and steric effects of the ligand structures [67]. Similarly, previous researchers have reported that high pressures, low polymerization temperatures, bulky backbone substituents, and/or N-aryl groups enable polymerization of high-molecular weights, low branching densities, and thus higher melting point [30]. On the other hand, low pressures, high temperatures, and the use of less bulky ligands result in polymers with lower molecular weight, a higher degree of branching, and low melting points. The electronic effects of the ligand are also crucial. Studies on ligands with electron-withdrawing substituents induce the electrophilicity at the catalytic metal center, which indirectly promoted chain propagation, thus enabling higher molecular weight of polyethylene [43,45]. In summary, the catalytic performance can be readily tuned with variations of ligand structures and reaction conditions. In the following section \u03b1-diimine complexes are grouped in four classes based on their structural features and reaction conditions, i.e: N-aryl modifications (Table 1\n. and Figs. 4-18), backbone modifications (Table 2\n. and Figs. 19-23), binuclear complexes (Table 3\n. and Figs. 24-28), functional group modification (Table 4\n. and Figs. 29-36).In recent years, one of the most popular modifications on \u03b1-diimine Ni complexes has been the incorporation of the 2, 6-dibenzhydryl group on the ortho-N-aryl positions reported by Rhinehart et al. in 2013 (Fig. 4\n)[68]. These \u03b1-diimine Ni complexes exhibited remarkable thermal stability in ethylene polymerization. Activated by methylaluminoxane (MAO), the catalytic activity of Ni precatalysts (C1b in Fig. 4) as high as 2.81\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121 at 100 \u2103 (10\u00a0min) (Table 1) was estimated. The resulting polymer showed a well-defined and narrow molecular weight distribution (M\nw/M\nn\u00a0\u2264\u00a01.31), moderate degree of branching (63 to 75B/1000C), and high molecular weight (M\nn\u00a0>\u00a0600 000\u00a0g/mol). The robust nature and thermal stability of C2 achieved even higher activity (1.4 times in TOFs) than C1 in ethylene polymerization [69]. An increase in melting point by \u223c20\u00a0\u00b0C and fewer branching content was also observed for polyethylene. In addition, these Ni complexes exhibited the capacity for living polymerization at 75 \u2103 [70].Inspired by the C1 and C2 structures, Dai et al. have further developed the 2, 6-dibenzhydryl-substituted \u03b1-diimine Pd complexes (C3 in Fig. 5\n) containing either electron-donating (-OMe, - Me) or electron-withdrawing groups (Cl, -CF3) [71]. In this work, new synthetic strategies led to the improvement in the yield of sterically demanding ligands using the more efficient fashion (yield above 90\u00a0%). In comparison to the classical Pd complex\nA (\nFig. 1), these Pd complexes exhibited remarkable thermal stability and catalytic properties in ethylene polymerization. Higher catalytic activity up to 3.2\u00a0\u00d7\u00a0106 g of PE (mol of Pd) -1h\u22121 (60\u00a0\u00b0C, 15\u00a0min) was achieved, accompanied by a higher molecular weight (Mn\n\nup to 538\u2009000\u00a0g/mol) and lower branching density (23\u201329B/1000\u2005C) of polyethylenes (Table 1). The melting point (T\nm) of the synthesized polyethylene was as high as 99\u00a0\u00b0C. A semi-crystalline E-MA copolymer was synthesized through Pd-catalyzed copolymerization. It was notable that the slow-chain-walking behaviors of these Pd catalysts resulted in unique polymer microstructures (higher molecular weight and lower branching).Some new \u03b1-diimine Ni complexes (C4 and C4\u2032 in Fig. 5) bearing similar coordinating structures as C3 were synthesized by Guo et al. [72]. Namely, C4 and C4\u2032 displayed a very high catalytic activity of 6.18\u00a0\u00d7\u00a0106 g of PE (mol of Pd) -1h\u22121 (100\u00a0\u00b0C, 30\u00a0min) (Table 1). The synthesized polyethylene exhibited a molecular weight of more than one million with a rather narrow PDI. All Ni complexes exhibited a robust catalytic behavior combined with high activity, producing a high molecular weight of polyethylenes. Ni complexes were able to polymerize the ethylene even at 100\u00a0\u00b0C. In the ethylene polymerization, the dibromonickel-catalyzed polymerization was relatively insensitive to the electronic perturbation introduced by structure C4\u2032. In contrast, electronic effects of the Ni(acac) systems (C4) were clearly observed. Trifluoromethyl-substituted C4d catalyst exhibited exceptionally high activity and thermal stability at elevated temperatures.Simultaneous tuning of both electronic and steric effects was rarely investigated in previous studies. Muhammad et al. advanced a series of symmetrical \u03b1-diimine Ni and Pd catalysts bearing both benzhydryl N-aryl with various substituents of methoxy/fluoro groups (C5 in Fig. 5) for ethylene polymerization and copolymerization with AA and MA [73]. The six methoxy/fluoro substituents located both at para and meta-positions were able to significantly enhance and alter the electronic effects of ligand and the coordination environment around catalytic metal center. It was hypothesized that the meta-methoxy groups on C5-Pd-OMe interacted with the benzhydryl groups, increasing the steric constraints around the metal center. This palladium (C5-Pd-OMe) and nickel (C5-Ni-OMe) catalysts exhibited an increased catalytic activity [up to 5\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121] 1 (100\u00a0\u00b0C, 30\u00a0min). The resulting polymers had a high molecular weight (2.54\u00a0\u00d7\u00a0106 g/mol) and thus a high melting point (T\nm\u00a0=\u00a0112.2\u00a0\u00b0C), along with the reduced polymer branching densities (Table 1). Accordingly, improved mechanical properties were observed. For C5-Pd-OMe catalyzed copolymerization, the incorporation of monomer (MA and AA) in the polymer chains were reduced because of the ligand's bulkiness.In 2018, Guo et al. reported a series of sterically hindered and acenaphthene-based \u03b1-diimine nickel complexes with the remote R' (-OMe, -Me, -tBu, -Ph) groups in para-positions of diarylmethyl moiety (C6 in Fig. 6\n) [74]. Activated by the Et2AlCl, these nickel catalysts exhibited high activities [up to 5.1\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121] (20\u00a0\u00b0C, 30\u00a0min) and high thermal stability (stable at 100\u00a0\u00b0C) in ethylene polymerization. The synthesized polyethylenes were characterized as ultra-high molecular weight (UHMWPE) (M\nw\nup to 4.5\u00a0\u00d7\u00a0106\ng/mol) with a moderate branching in the range of 26\u201371B/1000C (Table 1). It was interesting that the presence of remote substituents (-OMe, -Me,-tBu, and -Ph) in the para-position had a strong influence on the catalytic properties of these corresponding \u03b1-diimine nickel complexes and the UHMWPE mechanical properties. These branched UHMWPE materials displayed the typical properties of thermoplastic elastomers with well-defined stress\u2013strain curves and elastic recovery.More recently in 2020, Xia et al. introduced the concerted double-layer steric strategy concept in designing a new series of \u03b1-diimine nickel catalysts. This method involved the incorporation of bulky diphenylaniline into the ligand structures (C7 in Fig. 7\n) [75]. These newly designed \u03b1-diimine Ni and Pd catalysts exhibited both significant thermal stability (stable at 150\u00a0\u00b0C) as well as very high activity [on the level of 1.03\u00a0\u00d7\u00a0109 g of PE (mol of Ni) -1h\u22121] (30\u00a0\u00b0C, 1\u00a0min) in ethylene polymerization. The resulting polyethylenes exhibited ultrahigh molecular weight (M\nw\u00a0=\u00a04.2\u00a0\u00d7\u00a0106 g/mol) with a controlled degree of branching from quasi-linear (2B/1000C) to lightly branched (32B/1000C) structures (Table 1). Ethylene copolymerization with a good incorporation of methyl 10-undecenoate was also observed. The key structural innovations introduced in C7 explained its typical catalytic performance. The first steric layer offered by the inner phenyl rings provided enough space for ethylene coordination and insertion. The second steric layer from outer phenyl rings gave rise to restraining chain transfer. This strategy of catalyst design led to simultaneously high catalytic activity and high molecular weight of polyethylene, normally better than the reported conventional modifications.\nKanai et al. initially reported the Nicomplexes with symmetric bowl-shaped \u03b1-diimine ligands, consisting of two pentiptycenyl-substituents in [(a-diimine)NiBr2] (C8a in Fig. 8\n). The Ni complexes displayed good catalytic activity in ethylene (co)polymerization [76]. The Et2AlCl /C8a system exhibited a moderate catalytic activity of 3.4\u00a0\u00d7\u00a0104g of PE (mol of Ni) -1h\u22121 (25\u00a0\u00b0C, 30\u00a0min) at 7\u00a0atm. The molecular weight of the resulting polyethylene reached up to 1.5\u00a0\u00d7\u00a0105 g/mol with low branching densities (12B/1000C) and a high melting point of 133\u00a0\u00b0C. The catalytic performance of these complexes structures was attributed to the coordination environment of nickel, which was located in a highly shielded, hemispherical, and crowded space of the two pentiptycene units. Polar monomers (UAME, UA, UCl, and UO in Fig. 2) and ethylene could be efficiently copolymerized, leading to the copolymers with 4.2\u00a0mol% incorporations of polar monomers. The copolymerization highly depended on the amount of activator and was relative to the amount of polar monomer. By lowering the molar ratio of Et2AlCl/polar monomer to 0.5, the decreased activity for ethylene copolymerization with oxygen-containing monomers was observed, while 11-chloro-1-undecene could still be efficiently copolymerized with ethylene under the same conditions.Similar to the above work, Liao et al. then proposed a novel series of sterically demanding pentiptycenyl\nN-aryl substituted \u03b1-diimine Ni and Pd catalysts for ethylene (co)polymerization (C8b to f) [77]. These newly synthesized complexes were characterized as highly bulky substituents and backbone. For ethylene polymerization (20\u201380\u00a0\u00b0C) catalyzed by Ni complexes, the catalytic activities were in the range of 0.64 to 3.74\u00a0\u00d7\u00a0106\ng of PE (mol of Ni) -1h\u22121 (60\u00a0\u00b0C, 30\u00a0min). The generated polyethylenes exhibited a moderate molecular weight of 3.77\u00a0\u00d7\u00a0105\ng/mol, tunable branching densities from 6B/1000C to 55B/1000C, and high Tm (135\u00a0\u00b0C) (Table 1). These Pd catalysts ensured respectable MA incorporation up to 4.1\u00a0mol% in the copolymerization with ethylene. Compared with the free rotation of dibenzhydryl substituents, the restricted rotation of pentiptycenyl substituents offered superior activity and a slower chain-walking process for \u03b1-diimine Ni(II) species. These special bulky groups also enhanced comonomer incorporation for \u03b1-diimine Pd(II) species. It was notable that less steric blockage of substituents at the axial positions on the catalytic metal center led to a decrease in the molecular weight of the resulting polymer.In 2016, Dai et al. reported the synthesis of novel naphthalene and benzothiophene substituted N-aryl groups on \u03b1-diimine Pd complexes (C9a, C9b in Fig. 9\n) [78]. In ethylene polymerization, these Pd complexes displayed the moderate catalytic activity around 4.1\u00a0\u00d7\u00a0105\ng of PE (mol of Pd) -1h\u22121 (60\u00a0\u00b0C, 15\u00a0min) and good thermal stability (Table 1). The produced polyethylenes achieved extremely high molecular weights (8.02\u00a0\u00d7\u00a0106\ng/mol), low branching densities (as low as 6B/1000C), and comparatively high melting points (Tm up to 127.2\u00a0\u00b0C). In ethylene-MA copolymerization, the catalytic activity was in the order of 3.03\u00a0\u00d7\u00a0104\ng of PE (mol of Pd) -1h\u22121. The E-MA copolymer possessed rather high molecular weights (M\nn up to 4.42\u00a0\u00d7\u00a0105\ng/mol). The initial \u03b1-diimine catalyst A (Fig. 1) was deactivated in the presence of a long-chain polar monomer, which was speculated to the fast chain-walking process of catalyst A. Compared to catalyst A, C9a and C9b were more efficient in the copolymerization of long-chain monomers achieving high activity. The molecular weight of the copolymers was close to 1\u00a0\u00d7\u00a0106\u2005g/mol. The surface wetting property of the resulting polymer was indeed improved via this incorporation of the polar functional groups into the polymer chains.In 2018, Na et al. demonstrated the specially designed \u03b1-diimine Pd complex containing steric thienyl-phenyl substitution (C10 in Fig. 9) [79]. The properties of hence generated polyethylene were similar to low-density polyethylene (LDPE). Tunable branching densities (16 to 37B/1000C), high melting points (Tm 101 to113\u00a0\u00b0C), and low polymer densities (0.89\u20130.92\u00a0g/cm3) were observed. Polar-functionalized low-density polyethylene (P-LDPE) was synthesized via ethylene copolymerization with polar monomers. The catalytic activities during copolymerization were up to 105\ng of PE (mol of Pd) -1h\u22121. Copolymers with high incorporation (6.8\u00a0%), high molecular weights (M\nn up to 1.24\u00a0\u00d7\u00a0106 g/mol), high melting points (118\u00a0\u00b0C), and tunable branching densities (14 to 46B/1000C) were achieved. The incorporation of polar groups significantly influenced the mechanical as well as the surface wetting properties of the resulting copolymers.\nRishina et al. investigated the catalytic effects from fluoro (C11a) and trifluoromethyl (C11b) substituted N-aryl modifications on \u03b1-diimine Ni catalyzed ethylene and propylene oligomerization (Fig. 10) [80]. Oligomerization of ethylene with C11 activated by a mixture of Et2AlCl /EASC and PPh3 at 30\u00a0\u00b0C resulted in only oligomers (i.e. oligomerization degree: 6 to 9) as mixtures of wax and liquid. A microstructure study indicated that the oligomers contained 14 to 20\u00a0mol% of methyl branches, 4 to 6\u00a0mol% of ethyl branches, and a small number of long-chain branches. In propylene oligomerization, these catalysts produced mixtures of very short oligomers (mostly dimers) at elevated temperatures from 30 to 70\u00a0\u00b0C. C11bNi generated the active species that exhibited no regioselectivity. Compared with C11b, the preference for primary insertion was observed in the oligomerization catalyzed by the C11a Ni complex.\nMundil et al. designed the series of \u03b1-diimine Ni and Pd complexes bearing fluorinated alkyl substituents at the para-N-aryl groups C12 (Fig. 10\n) [81]. These complexes were used to carry out catalyzed polymerization of ethylene, propene, and 1-hexene. Remarkably, there were no significant effects on the catalytic properties due to fluoroalkyl groups. The branching densities of the generated polyolefins were rather tunable by the ligand's backbones and ortho-substituents of N-aryl groups.In 2017, Lian and Wang et al. reported the novel synthesis of PTPE-type polyethylenes through ethylene polymerization, which was catalyzed by the Xanthene substituted N-aryl of \u03b1-diimine Ni and Pd (C13 in Fig. 11\n) [82,83]. These \u03b1-diimine Ni complexes revealed rather high activities [up to 6.94\u00a0\u00d7\u00a0106 g of PE (mol of Pd) -1h\u22121] (20\u00a0\u00b0C, 30\u00a0min) and thermal stability at 80\u00a0\u00b0C for ethylene polymerization. The generated polyethylenes exhibited high molecular weight (M\nn up to 1.53\u00a0\u00d7\u00a0106 g/mol) and notably narrow molecular weight distributions (Table 1). The remote substituents (-Ph, -CF3, \u2013NO2, and -OMe) had again a dramatic influence on the catalytic properties of ethylene polymerization. Specifically, the nickel complexes bearing the -Ph substituent (C13Ni-Ph) led to the formation of polyethylenes with exceptional elastic properties due to the branched structure of polyethylene (elastic strain recovery value of 70\u00a0% via\nC13Ni-Ph\nat 40\u00a0\u00b0C). The catalytic properties of Pd complexes were investigated in ethylene polymerization and ethylene/MA, ethylene/NB, ethylene/5-norbornene-2-yl acetate copolymerization. High molecular-weight E-MA and E-NB copolymers were produced by Pd-catalyzed copolymerization. C13Pd-Ph\nexhibited much higher activity (up to 2.5\u00a0\u00d7\u00a0104\ng of PE (mol of Pd) -1h\u22121) than other complexes, and generated polymers and copolymers with much higher molecular weight (M\nn up to 1.21\u00a0\u00d7\u00a0105 g/mol).In 2017, Li et al. investigated a series of \u03b1-diimine Ni and Pd complexes bearing nitrogen-containing cyclic groups (C14 in Fig. 12\n) [84]. In ethylene polymerization, the nitrogen atoms situated on N-aryl groups of C14a and C14b interacted with catalytic metal centers in a remarkable manner. The catalytic metal center was then highly tolerant to polar functional groups, while the polymer branching densities were significantly reduced. The generated polyethylenes were characterized as nearly perfect linear structure (branching\u00a0<\u00a01B/1000C and high Tm (>130\u00a0\u00b0C) (Table 1). For the unsymmetrical PdC14c, a moderately branched polymer (around 70B/1000C) was produced. The presence of nitrogen also improved the thermal stability of the catalysts. Continuously high activity was achieved in polymerization even after 2\u00a0h at 60\u00a0\u00b0C, while the high molecular weight of polymers was still achieved at 80\u00a0\u00b0C. Additionally, these newly designed \u03b1-diimine Ni and Pd complexes were able to copolymerize ethylene with a series of polar comonomers such as UAME and the challenging AAc. Linear E-MA copolymers with high incorporation (up to 7.5\u00a0mol%) were still achieved in copolymerization. Furthermore, the functional-group tolerance of the catalytic metal center was greatly enhanced by the presence of additional side-arm heteroatoms. The high tolerance and unique catalytic performance were attributed to a \u201csecond-coordination sphere\u201d strategy. It was hypothesized by the author that a second coordination sphere of the ligands was stronger than \u03b2-H or \u03b2-X (X being a polar group). Nevertheless, the activity was still weaker than with ethylene insertion. It was noteworthy that the experimental data and the hypothesis was backed up by a computational study.Compared to rigid steric modifications, Guo and Dai et al. worked with a series of flexible alkyl (C15) and cycloalkyl (C16) substituted N-aryl units of \u03b1-diimine Ni and Pd complexes (Fig. 13\n) [85,86]. Compared with fixed phenyl substitutions on \u03b1-diimine Ni complexes (B in Fig. 1), the flexible cyclohexyl complexes exhibited distinctive catalytic behavior. The polymers were generated with remarkably high branching densities, low Tm, and low molecular weight. PTPE-type polymers with these highly branched polyethylenes were obtained. The flexible cyclohexyl Pd catalyst exhibited a remarkably higher catalytic activity than the conventional \u03b1-diimine complexes (A in Fig. 1), providing an increased amount of molecular weight and branching density in ethylene (co)polymerization. The flexible modification of cyclohexyl \u03b1-diimine Ni and Pd complexes offered a much faster chain-walking process and higher catalytic activity. The synthesized polymers possessed higher molecular weight with appreciable comonomer incorporation. In terms of long-chain alkyl \u03b1-diimine Ni and Pd complexes, the Ni complexes presented high activities [up to 2.14\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121](20\u00a0\u00b0C, 30\u00a0min) and generated highly branched polyethylene with ultra-high molecular weight (M\nn up to 1.2\u00a0\u00d7\u00a0106 g/mol) (Table 1\n). The synthesized polyethylene also displayed exceptional capacity in mechanical elasticity like TPE-type polymers (SR value up to 88\u00a0%).\nBrookhart et al. have described two \u201csandwich-like\u201d arrangements of naphthyl substituted N-aryl groups in \u03b1-diimine Ni-based precatalysts (C17aNi in Fig. 14\n) [87]. The two 8-p-tolylnaphthylimino moieties were implanted on the \u03b1-diimine ligands to shield the Ni-axial direction and thus control the monomer insertion. The tolyl substituents are arranged perpendicular to the naphthyl rings, which were nearly coplanar with the square coordination plane. Activated by the modified methylalumoxane, these distinctive Ni complexes produced hyper-branched (up to 152B/1000C) polyethylenes with high molecular weights. It was believed that the increased axial bulk efficiently led to lower rates of chain transfer, relative to increase the chain propagation rates. Meanwhile, it resulted in high molecular weights and narrow PDIs of polyethylene. Subsequently, Vaidya et al. have developed additional similar derivatives of \u201cSandwich\u201d-type nickel complexes (C17bNi) [88]. They were applied to catalyze higher \u03b1-olefin polymerizations with precise control of the chain-walking process, which favors the \u03c9, 1-enchainment. The \u201csandwich\u201d type catalysts also provided the chance to synthesize the low-branched polyethylene with a \u201cchain-straightened\u201d semi-crystalline property (high melting point). With the activation of MAO, O'Connor et al then used the C17bNi complex to generate polyolefin-based PTPE-type block copolymers (Table 1). The 1-decene monomer was responsible for high crystallinity and hard blocks, while low crystallinity soft blocks were synthesized from ethylene monomer [89]. Various block structures were characterized as copolymers in the range of block size from diblock to heptablock. All resulting polymers behaved as elastic PTPE-type materials. More recently, Allen et al. have reported newly synthesized \u201csandwich\u201d types of \u03b1-diimine palladium catalysts (C17aPd in Fig. 14\n) for ethylene polymerization [90]. The Pd complexes were used in the ethylene polymerization with typical signs for living polymerization at 25\u00a0\u00b0C. The resulting polyethylene was hyper-branched and exhibited narrow molecular weight distribution (around 1.1). Ethylene copolymerization with MA using the Pd catalysts presented a high percentage of incorporation, which was up to 14\u00a0%.\nZhai et al. reported a new ortho-menthyl substituted N-aryl on the \u03b1-diimine Ni complexes as the syn- and anti-conformers for ethylene and 1-hexene polymerization (C18 in Fig. 15\n) [91]. Both anti- and syn-conformers of C18b could be activated by the Et2AlCl for polymerization. The catalytic activity could be achieved from 2.5 to 6.6\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121 (15 psi of ethylene pressure at room temperature and 15\u00a0min) in ethylene polymerization (Table 1). The syn- and anti-conformers of C18 exhibited a different catalytic performance. The polyethylene produced by syn-conformers tends to entail a higher molecular weight and branching density than the one obtained from the anti-conformer catalyzed polymerization. Compared to polyhexene produced from anti-conformer, the polymer produced by syn-conformer possessed a higher level of chain straightening and a higher percentage of methyl branches rather than butyl branches. This result also indicated a greater preference for the 2,1-insertion and chain-walking process for the syn-conformer C18syn.\nIn recent years, Sun et al. contributed considerable research towards the \u03b1-diimine Ni-catalyzed ethylene polymerization [45]. All finely tuned \u03b1-diimine nickel complexes in Fig. 16\n are selected examples of such modifications on the N-aryl groups [92\u2013112]. The Ni complexes exhibited outstanding catalytic activity and generated polyethylene of high molecular weight and highly branched microstructures. For instance, C19Ni were unsymmetrically synthesized with various and modified benzhydryl substitutions on one of the two N-aryl groups. Activated by MAO, MMAO, Et2AlCl, or EASC, a remarkable catalytic activity of 1.48\u00a0\u00d7\u00a0107 g of PE (mol of Ni) -1h\u22121 could be reached. Super highly branched polyethylene with branching densities as high as 337B/1000C were obtained. The polymer produced solely by ethylene monomer exhibited typical PTPE properties. The molecular weight of the resulting polyethylene was in the range of 105 to 3\u00a0\u00d7\u00a0106 g/mol, which is characteristic of UHMWPE and it exhibited a narrow molecular weight distribution (Table 1). Complexes C20 were synthesized with the incorporation of a 2,4- or 2,4,6-substitution pattern using the steric benzhydryl groups. As a remarkable feature, high activity was retained at the high thermal stability of these catalysts. Activated by MMAO cocatalyst, C20aNi (ortho-R group as -Me) exhibited high activities of up to 8.9\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121 and resulted in highly branched polyethylenes (166B/1000C) at 80\u00a0\u00b0C. In presence of relatively low amounts of EASC, C20bNi exhibited higher activities compared to C20aNi [up to 10.9\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121], retained thermal stability by maintaining high activity (3.76\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121) at 80\u00a0\u00b0C. C20c with the 2,4,6-substituted benzhydryl presented remarkable activity even at 90\u00a0\u00b0C [2.97\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121]. The synthesized polyethylene contained hyperbranched microstructures as high as 135B/1000C, which were analyzed as methyl (84.4\u00a0%), ethyl (5.6\u00a0%), and longer chain branches (10\u00a0%). C21 containing N-naphthyl ligands displayed moderate activity. Activated by either MAO or Et2AlCl, C21 yielded polyethylene with typically low branching densities and a high melting point (131\u00a0\u00b0C). The catalytic performance of unsymmetrical N-naphthyl complexes C22 was investigated to determine the influence of the bulky difluorobenzhydryl substitution. With the activation of Et2AlCl, C22 exhibited high activity, exhibiting a narrow molecular weight of polyethylene (1.22\u20131.99). Activated by MAO and Et2AlCl, C23aNi showed high activities in ethylene polymerization with the activity of up to 107\ng of PE (mol of Ni) -1h\u22121, illustrating the feature of the single-site active species and observing narrow molecular weight distributions of the resultant polyethylene. C23bNi exhibited higher activity over C23aNi, which was ascribed to the electron withdrawing nature of the para-fluorides and their influences on the active mental centers. C23cNi/EASC system generated both high activity and thermal stability, catalyzing the ethylene polymerization effectively even at 80\u00a0\u00b0C [6.01\u00a0\u00d7\u00a0106 g of PE (mol of Ni)\u22121h\u22121 (30\u00a0min)], while yielding high molecular weight (as high as 10.6\u00a0\u00d7\u00a0105 g mol\u22121) polymers for the same reaction time. Notably, the polyethylene produced by C24Ni was characterized as ultra-high molecular weight (>1\u00a0\u00d7\u00a0106\u2005g\u2005mol\u22121) with a relatively high degree of branching (115 branches per 1000 carbons). C25Ni and C26Ni displayed a moderate catalytic activities up to 106 g of PE (mol of Ni)\u22121h\u22121 (30\u00a0min) upon the activation with either MAO and EASC. Particularly, C26Ni exhibited good thermal stability in the temperatures range from 60 to 80\u00a0\u00b0C, generating the polyethylenes with high molecular weight. Activated by either MAO or Et2AlCl, C27Ni exhibited outstanding catalytic activity in ethylene polymerization [1.02\u00a0\u00d7\u00a0107 g of PE (mol of Ni)\u22121h\u22121 (30\u00a0min) ]. C27Ni bearing the equivalent difluorobenzhydryl-substituted N-aryl groups was observed to have exceptional thermal stability. C27Ni/ Et2AlCl system exhibited high activity [ 1.02\u00a0\u00d7\u00a0107 g of PE (mol of Ni)\u22121h\u22121 (30\u00a0min) ] at 100\u00a0\u00b0C, while generating the polyethylene with the high molecular weight. Significantly, the polyethylenes possessed exceptional elastomeric recovery and high elongation at break determined by DMA and stress\u2013strain testing. In summary, these research offered a promising route to alternative materials for the conventional thermoplastic elastomers (TPEs).\nZhai et al. reported a series of novel \u03b1-diimine Pd complexes containing the secondary amide (\u2212CONHMe) or tertiary amide (-CONMe2) substituents on the N-aryl groups (C28 in Figure 17\n) [113]. These Pd complexes were investigated in the catalytic performance of ethylene polymerization, ethylene/MA, and ethylene/AA copolymerization. With the replacement of two\n\ni\nPr units using -CHPh2\ngroups,\nC28 led to a significant improvement in catalytic performance. The generated (\u03b1-diimine)PdMe+\nspecies (C28) were activated by NaBArF\n4 to produce polyethylenes with a molecular weight of around 5.9\u00a0\u00d7\u00a0104 g/mol. Compared to C29, the structure C28 exhibited lower catalytic activity [3.9\u00a0\u00d7\u00a0104 g of PE (mol of Pd) -1h\u22121] (20\u00a0\u00b0C, 120\u00a0min) (Table 1). This was due to the enhanced steric effects of -CHPh2\ngroups, which counteracted the negative effect of electron-withdrawing amide units. The resulting polymers exhibited moderate branching content (77 to 81B/1000C). In addition, the Pd complex of C28 incorporates higher levels of MA and AA in the copolymerization with ethylene than\nC29.\nHu et al. recently reported the newly developed \u03b1-diimine Ni and Pd complexes with the steric enhancement of unsymmetrically pentiptycenyl-dibenzhydryl substituted N-aryl modifications (C30 in Fig. 18\n\n)\n[114,115]. It demonstrated some new catalytic features of C30 compared to previous studies [45]. Within a relatively long-lived reaction time, the increased bulk of \u03b1-diimine Ni and Pd complexes indeed raised the molecular weight of polyethylene, while this trend differed in the short polymerization time. With increased steric, the branching density initially increases followed by a decrease. In Ni-catalyzed ethylene polymerization, the activity can be as high as 6\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121 (20\u00a0\u00b0C, 5\u00a0min), producing the polyethylene with ultrahigh molecular weight (1.58\u00a0\u00d7\u00a0106 g/mol) (Table 1). The C30Ni enabled the ethylene copolymerization with UAME, generating copolymers with high molecular weight (2.13\u00a0\u00d7\u00a0105 g/mol) and branching density (138B/1000C). Compared to C30Ni, the catalytic activity of C30Pd is comparatively lower.A substantial class of structural variations relates to modifications at the reactive center backbone in the catalyst. The table below summarizes structures C31-C38 and each of these structural variations will then be described in detail below.\nSong et al. explored the synthesis and characterization of \u03b1-diimine Ni dihalides (Cl and Br), bearing the 4,5-bis (arylimino)pyrenylidene (C31 in Fig. 19\n\n)\n[116]. After activation with a very low amount of cocatalysts including MAO, EASC, and Et2AlCl, the structure C31 exhibited high activity up to 4.41\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121 (40\u00a0\u00b0C, 30\u00a0min) (Table 2). The microstructure of the synthesized polyethylene was analyzed using high temperature NMR, which revealed a high degree of branching density (up to 130B/1000) and narrow molecular weight distributions (around 2.5). This work also indicated that reaction parameters like the Al/Ni molar ratios, the reaction temperature, and polymerization time had a significant influence on the catalytic activity and the properties of the generated polyethylenes.\nLiu et al. proposed two chiral \u03b1-diimine Ni complexes containing (1R)- and (1S)- camphyl substituted backbone for ethylene and a-olefin polymerization (C32in Fig. 20\n\n)\n[117]. In this catalytic system, the chiral tunes on the ligand structure exhibited no influences on the catalytic behavior and region-selectivity for the Ni catalyzed polymerization. Activated by Et2AlCl, the catalytic activity of C32 revealed characteristics of a living polymerization for ethylene, propylene, 1-hexene, and 4-methyl-1-pentene under the optimized conditions (Table 2). Rather narrow molecular weight distributions (PDI\u00a0<\u00a01.2) were observed in the produced polypropylenes and poly(1-hexene)s with a wide range of polymerization temperatures. A high 1,3-enchainment fraction of 45\u00a0% was also observed in C32Ni-catalyzed propylene with polymerization at \u221260\u00a0\u00b0C, which was attributed to the 2,1-insertion of propylene and a chain-walking process.\nZou et al. reported a series of \u03b1-diimine Ni(II) and Pd(II) complexes with different substituents on the acenaphthyl backbones (C33a-e in Fig. 21\n\n)\n[118]. The corresponding complexes were synthesized\uff0c characterized, and applied to the ethylene polymerization and E-MA copolymerization. In terms of ethylene polymerization, NiC33 a-d complexes exhibited high activities of up to 1.6\u00a0\u00d7\u00a0107\ng of PE (mol of Ni) -1h\u22121 (20\u00a0\u00b0C, 10\u00a0min) (Table 2). The synthesized polyethylene displayed high molecular weight (M\nn) (up to 4.2\u00a0\u00d7\u00a0105 g/mol) with a molecular weight distribution of around 2.5. The structural variations C33 a-e had similar catalytic performance in ethylene polymerization. However, the polymer obtained from C33e-catalyzed polymerization was confirmed to exhibit much higher molecular weight and lower branching density than polymers from other catalysts. Substituents on the ligand backbones of PdC33 a-e had a significant influence on their catalytic performance during ethylene polymerization and E-MA copolymerization. The polyethylene and E-MA copolymers produced by the PdC33e complex exhibited higher molecular weights than polymers from PdC33 a-d.Along these lines, Zhu et al. reported the modification of acenaphthy backbones on the \u03b1-diimine Ni complexes (C33f in Fig. 21\n), which has a similar structure as C33a-e complexes [119]. Experimental and computational studies were carried out to reveal and analyze the thermal stability of the proposed \u03b1-diimine Ni complex (C33f). Compared to the initially reported \u03b1-diimine Ni complexes (A in Fig. 1), the complex C33f presented higher activity and thermal stability at elevated temperatures (Table 2). It was found that the presence of ethylene evidently affected the conformation of the C1\u2013N1\u2013Ni\u2013N2\u2013C2 five-membered ring (where the nickel center is located) of\nC33f. According to calculations, differences in the decomposition energy between C33f and A in Fig. 1\n.were observed.\nZhang et al. explored the synthesis and characterization of new nickel bromide complexes containing the rigid bidentate bis(arylimino)camphane ligands with different N-aryl substituents (C34 in Fig. 22\n)[120]. Upon activation with either MMAO or Me2AlCl, the newly synthesized NiC34 complexes exhibited high catalytic activities and thermal stabilities during ethylene polymerization (as high as up to 11.2\u00a0\u00d7\u00a0106\u2009g PE (mol Ni)\u22121\nh\u22121\n(80\u00a0\u00b0C and 30\u00a0min)), producing PEs of high molecular weights (23.7\u00a0\u00d7\u00a0105\u2009g\u2009mol\u22121) and low PDI (1.6\u20132.6) (Table 2). In this catalytic system, the introduction of di(p-fluorophenyl)methyl on the\northo-position of\nN-aryl groups resulted in increasing of both the catalytic activity and the thermal stability of the corresponding Ni complexes. The synthesized PEs were moderate to highly branched nature with the tunable branch contents governed by various ligand structures. The bulkiness of substituents in the ligand structure led to high molecular weight polymers with a low branching -degree and limited -types. The polymers produced with\northo-hydrogen\nNiC34b/Me2AlCl possessed the highest branching density with the unique terminal vinyl (\u2013CH\u2550CH2) and internal vinylene (\u2013CH\u2550CH\u2013) structures.\nLong et al. proposed the newly synthesized \u03b1-diimine Ni complex containing dibenzobarrelene-bridged backbone (C35 in Fig. 23\n\n)\n[121]. C35 exhibited an exceptional catalytic behavior in ethylene polymerization. The DBB-bridged C35 resulted in a steric hindrance around the cationic Ni center, decreasing catalytic deactivation and slowing the chain-walking process. Therefore, C35 produced linear polyethylene in ethylene homopolymerization. C35 Ni afforded a high molecular weight of polyethylenes (7.12\u00a0\u00d7\u00a0105 g/mol) with narrow distribution (1.18) and low branching densities (<1B/1000C) (Table 2). It also revealeda living polymerization behavior at room temperature, producing linear polyethylene with a high melting point (T\nm\u00a0=\u00a0135\u00a0\u00b0C) (Table 2). It facilitated the copolymerization of ethylene with the methyl 10-undecenoate to yield highly linear ester-functionalized polyethylene (T\nm\nvalues at128\u00a0\u00b0C and 1\u00a0mol\u2009% comonomer incorporation). In the presence of ester functional groups, the catalytic activity during copolymerization dropped by an order of magnitude. This was hypothetically attributed to the reversible coordination between the ester-functionalized co-monomer and the cationic Ni center.\nZhong et al. reported a series of \u03b1-diimine Ni and Pd complexes with the modifications of dibenzobarrelene-bridged backbone. (C36a in Fig. 23\n)\n[122]. These complexes also revealed high thermal stability and clear signs of living polymerization. PdC36a exhibited the ability for precision synthesis of functionalized copolymers by living ethylene copolymerization with various acrylate monomers (Table 2). The bulky enhancement of dibenzobarrelene backbone improved the insertion selectivity of methyl acrylate (MA) in a 2,1-insertion. This catalytic behavior prevented polar groups from poisoning the active PdC36a species. In this living chain-walking system, it was demonstrated that the composition, molecular weight, and branching topology of the copolymer could be controlled by the variation of the ethylene pressure. Based on backbone modifications of C36a, a series of novel dibenzobarrelene-derived \u03b1-diimine nickel complexes were also synthesized and applied in ethylene polymerization (C36b-d in Fig. 23\n). The increased steric effects on the ligand backbone and the repulsive interactions inhibited the N-aryl rotation of the \u03b1-diimine ligands and enhanced the thermal stability of the complexes. The living polymerization could be achieved at 80\u00a0\u00b0C. Bulky enhancement of dibenzobarrelene backbone also improved tolerance of Ni complexes towards the ethylene copolymerization with monomers containing polar groups. The living ethylene copolymerization with methyl 10-undecenoate was also carried out by the bulky NiC36d. PdC36e was applied in a precision synthesis of functionalized polymers by the living ethylene copolymerization with the variety of acrylate monomers [123]. The incorporation of the steric dibenzobarrelene backbone significantly improved the migratory-insertion selectivity of methyl acrylate (MA) in a 2,1-insertion manner. This bulk-enhanced strategy prevented the polar groups from poisoning palladium centers of the catalysts by a formation of the five-membered palladacycle intermediates PdC36e exhibited a living polymerization and good thermal stability (55\u00a0\u00b0C) during ethylene polymerization. Living ethylene copolymerization with MA monomer were also successfully achieved, which was in contrast to general knowledge that polar monomers poison the transition metal catalysts. PdC36 was reported to perform the (co)polymerization of petroleum-based ethylene and bio-based furfuryl acrylate by Du et al.[124].The cationic palladium catalyst exhibited higher thermal stability than the neutral chloromethyl palladium complex, while the later complex was more active at low temperature. The incorporation of tert-butyl on the dibenzobarrelene backbone improved the tolerance of the PdC36 toward polar groups such as the incorporation of furyl groups into the polymer chain. Ethylene living (co)polymerization with the furfuryl acrylate (FA) catalyzed by PdC36 was successfully carried out, which afforded copolymers with a uniform incorporation of FA. The mechanistic study indicated that FA was selectively inserted into the Pd\u2013Me bond in a 2,1-insertion mode. There was no interactions observed between the palladium center and the furyl ring. Then, the dinaphthobarrelene-based backbone of\n\u03b1-diimine Ni and Pd complexes was synthesized, providing three-dimensional confinement for ethylene (co)polymerization (C37 in Fig. 23) [125]. These increased steric effects generated a 3D-confined space around the catalytic Ni and Pd centers, which strictly shielded the back and axial direction of the \u03b1-diimine Ni and Pd complexes. This confinement was assigned to the enhanced catalytic activity, thermal stability, and living fashion for ethylene polymerization (Table 2). The synthesized polyethylene possessed narrow molecular weight distribution (1.04\u20131,45). The steric accumulation effectively favored the ethylene copolymerization with polar monomers.\nZhang et al. proposed two types of dibenzobarrenlen (backbone) and pentiptycenyl (N-aryl) substituted \u03b1-diimine Pd complexes (C38 in Fig. 23) [126]. Remarkably,the Pd complexeswere active in a very broad range of temperatures (from 0\u00a0\u00b0C up to 130\u00a0\u00b0C). The Pd complexes allowed the synthesis of polyethylene with high molecular weight (around 106 g/mol), see also Table 2. The microstructure analysis of the resulting polymers displayed highly methyl (220\u2005Me/1000\u2005C) branched features, which were close to the typical properties of commercial ethylene-propylene elastomers. In ethylene copolymerization with MA, elevating the reaction temperature from 30\u00a0\u00b0C to 90\u00a0\u00b0C gave rise to the increase in catalytic activity, molecular weight, and MA incorporation. This steric framework of C38 exhibited an abundant chain-walking process, while the formation of branches in the polymer structure was only limited to methyl branches. The region-selectivity of acrylate insertion was characterized as 1,2-insertion due to the steric constrains.A notable development of novel polymerization catalyst structures includes the integration of two active metal centers in asymmetric or symmetric binuclear complexes. It can pave the way to, for example, bimodal polymerization or synergistic activity enhancement. Table 3 summarizes these structures with the nomenclature C39-C44.\nZhu et al. reported a series of binuclear \u03b1-diimine Ni and Pd complexes containing conjugated backbones (C39 in Fig. 24\n) [127]. Activated by MMAO, the catalytic activity of such complexes could reach up to 1.05\u00a0\u00d7\u00a0106\ng of PE (mol of Ni) -1h\u22121 (RT, 30\u00a0min) (Table 3). The activity of\nNiC39b\nis almost twice as high as the remaining complexes of this family, provided identical polymerization conditions. The binuclear PdC39\nproduced polyethylene with bimodal features in GPC. This confirmed the simultaneous formation of two active species were in a binuclear catalyst system. The E/MA copolymerization was also investigated using the Pd complexes. The MA incorporation can be up to 2.36\u00a0mol% in the copolymerization catalyzed by PdC39 b.\nXing et al. reported a series of binuclear \u03b1-diimine Ni(II) complexes consisting of 4,5,9,10-tetra(arylimino)pyrenylidene-bridged ligands (C40 in Fig. 24) [128]. To mediate ethylene polymerization, different parameters like type of cocatalysts, cocatalyst ratio, polymerization time, and temperature were varied to optimize the catalytic capacity of the binuclear Ni complexes. Both NiC40 (a and b)Ni complexes exhibited high activities [1.5\u00a0\u00d7\u00a0106\ng of PE (mol of Ni) -1h\u22121] (30\u00a0\u00b0C, 30\u00a0min) in the presence of either MAO or Me2AlCl (Table 3). The Ni complexes exhibited a long time (60\u00a0min) of catalytic life when activated with MAO. The polyethylene obtained from the catalyzed polymerization revealed a minor amount of branches (7B/1000C). Compared to the analogous mononuclear complexes, these binuclear nickel complexes revealed no significant improvements in catalytic activity in ethylene polymerization.\nWang and Na et al. demonstrated the xanthene-, naphthalene- and biphenylene-bridged \u03b1-diimine binuclear Ni and Pd complexes C41 (Fig. 25\n\n)\n[129,130]. These binuclear nickel complexes exhibit good thermal stability (stay active at 80\u00a0\u00b0C) during ethylene polymerization. They exhibited the catalytic activity up to 106\ng of PE (mol of Ni) -1 (20\u00a0\u00b0C, 30\u00a0min) (Table 3). It was interesting to note that these binuclear Ni complexes exhibited higher activity and resulted in polymers with higher molecular weights than their mononuclear analogues. Polymers with a high molecular weight (M\nn), narrow PDI and low branching density were obtained from these catalysts. These results indicated that the Ni-Ni cooperativity slowed the \u03b2-hydride elimination and related chain-walking process. In terms of Pd-catalyzed ethylene (co)polymerization, the Pd-Pd cooperation had a significant impact on catalytic behavior, especially for E-MA copolymerization. No MA incorporation was observed in the case of polymerization with the binuclear Pd complexes, while the mononuclear analogues enabled MA incorporation.\nKong et al. reported a series of methylene-bridged binuclear \u03b1-diimine Ni and Pd complexes C42 (Fig. 26\n\n)\n[131]. Uponactivationwith Et2AlCl or MAO, all the nickel complexes exhibited high activity towardethylenepolymerization [catalytic activity up to 7.86\u00a0\u00d7\u00a0106\ng of PE (mol of Ni) -1h\u22121] (20\u00a0\u00b0C, 30\u00a0min). The resulting polymer displayed high melting points (Tm up to 130.9\u00a0\u00b0C) as well as high branching densities (151B/1000C) (Table 3). The binuclear Ni complexes exhibited a synergistic catalytic activity as compared to their mononuclear analogs. Furthermore, the synthesized polyethylene possessed a higher molecular weight and broader PDI (up to 4.8).\nKhoshsefat et al. reported the aryl-bridged binuclear \u03b1-diimine Ni and Pd complexes C43 (Fig. 27\n\n)\n[132]. Under the optimized conditions for ethylene polymerization ([Al]/[Ni]\u00a0=\u00a02000/1, 42\u00a0\u00b0C, 20\u00a0min), C43d reached its highest catalytic activity of 1.07\u00a0\u00d7\u00a0106\ng of PE (mol of Ni) -1h\u22121 (42\u00a0\u00b0C, 20\u00a0min) (Table 3\n). Compared to other Ni complexes, the polyethylene produced from C43d also processed the highest molecular weight with the broad molecular weight distribution (PDI\u00a0=\u00a017.8). The bulky ortho-substituents on N-aryl groups presented positive influences on the catalytic activity, molecular weights, and degree of branching, which was confirmed by the theoretical study as well.\nTakano et al. have developed a unique binuclear double-decker structure of \u03b1-diimine Pd complexes containing the macrocyclic ligands (C44 in Fig. 28\n) [133\u2013135]. In ethylene polymerization at high temperatures (at 60\u00a0\u00b0C and 100\u00a0\u00b0C), C44 exhibited more stability and higher activity than the mononuclear complexes (Table 3). The polyethylene formed by the binuclear catalyst C44 possessed less branched density (33B/1000) than the mononuclear catalysts (110B/1000C). A longer catalytic lifetime for C44 was also observed as it was still active after 18\u00a0h. In E-MA copolymerization, the use of binuclear C44 resulted in higher acrylate incorporation (5.2\u00a0mol %) in the copolymers than that formed by the mononuclear catalysts (1.1\u00a0mol%). NMR analysis confirmed branched structure for the E-MA copolymer. In E-AA (Acrylic Anhydride) copolymerization, C44 afforded copolymer containing a repeating unit of acrylic anhydride. High incorporation (up to 5.7\u00a0mol %) of cyclic and acyclic anhydride groups was evidenced in the main polymer chain, which was again much higher than the respective mononuclear complex (0.8\u00a0mol %).\nZhong et al. developed \u03b1-diimine Pd complexes with two ferrocenyl units and applied them in the ethylene (co)polymerization (C45 in Fig. 29\n) [136]. The two ferrocenyl groups were sequentially and stepwise oxidized, which increased the electron-withdrawing capacity of the \u03b1-diimine ligand. This stepwise redox control was applied to modify the catalytic properties of \u03b1-diimine Pd complexes during the ethylene homopolymerization and copolymerization with polar monomers (norbornene, methyl acrylate, and 5-norbornene-2-yl acetate). The catalytic activity decreased as the two ferrocenyl units become oxidized. It seemed that the rates of chain propagation and chain transfer were greatly affected by this stepwise redox-control strategy. The branching density of polyethylene was only slightly increased along with the oxidation, while the polymer microstructure and PDI was significantly controlled during these stepwise oxidation processes. The same ligands containing two ferrocene units was also applied to the synthesis of the \u03b1-diimine Ni complex. The oxidation process of the ferrocene groups did not alter the catalytic behavior of the corresponding Ni complex in ethylene polymerization. It was proposed that the reducing nature of the aluminum species (MAO) was too strong to carry out such stepwise redox-control strategy for Ni-catalyzed ethylene polymerization.Peng et al. reported a new \u03b1-diimine Ni complex bearing azobenzene groups with photoresponsive properties toward ethylene (co)polymerization (C46 in Fig. 30\n) [137]. The axial steric environment of the metal center was directly influenced by the light-induced trans\u2013cis isomerization. UV light can tune the properties of the ligand structure and therefore the catalytic behavior of the \u03b1-diimine Ni complexes for ethylene (co)polymerization. This light-induced control increased the polymer molecular weight and decreased the catalytic activity and the polymer branching density (Table 4). The authors suggested the incorporation of the photo-responsive functional units could induce even more dramatic changes in the electronic and steric coordination environments around the catalytic metal center.Metal-metal cooperation and synergistic effects have been extensively explored in ethylene polymerization, like the synthesis and application of the binuclear transition-metal catalysts. Contrary to this strategy, Wang et al. proposed a supramolecular chemistry strategy which was carried out to construct multinuclear catalysts for ethylene (co)polymerization. A new series of \u03b1-diimine Pd complexes was designed and synthesized with the urea-functional groups (H and N-methylated counterparts) (C47 in Fig. 31\n)[138]. The experimental results indicate that self-assembly of Pd complexes took place via the urea-based hydrogen bonding interactions, which was evidenced from the Fourier transform infrared (FTIR) spectroscopy. During ethylene polymerization and copolymerization with MA, the catalytic activity and polymer properties such as molecular weight, PDI, branching density, comonomer incorporation of the (co)polymer were modified by the various catalyst concentration, ligand structures, and reaction conditions. In order to further explore the supramolecular-induced self-assembly effects, the photo-sensitive azobenzene group was incorporated in the urea-functionalized \u03b1-diimine Pd complexes. This work indeed showed the presence of photosensitive functional group in the nickel complex influenced the microstructure of the polyethylene.The aliphatic hydrocarbon solvents (hexane and heptane) were widely applied in the industrial research for ethylene polymerization, while the academic researchers predominantly worked on properties of the catalyst in aromatic solvents (toluene). In order to bridge this gap between academic studies to practical applications, Chen et al. designed the new \u03b1-diimine Ni complex with the diaryl-methyl aniline bearing eight tert-butyl groups (C48 in Fig. 32\n) [139]. The incorporation of the multiple tert-butyl substituents in the diaryl-methyl moiety increased both the ligand's steric and electronic-donating ability. It resulted in the enhancement of the catalyst stability and polymer molecular weight in ethylene polymerization. The presence of multiple tert-butyl groups enabled solubility of the metal complexes in aliphatic hydrocarbon solvents, leading to similar polymerization properties compared to the aromatic solvents. The incorporation of tert-butyl substituent in the ligands improves the solubility of the Ni complexes in typical polymerization solvents, which improves its application potential.\nGong et al. developed a new series of acenaphthene-based sterically hindered \u03b1-diimine Pd complexes bearing bulky diarylmethyl moiety for ethylene (co)polymerization (C49 in Fig. 33\n) [140]. The \u03c0-\u03c0 interaction between the acenaphthene moiety and the phenyl of diarylmethyl moiety was suggest to occur. The \u03c0-\u03c0 interaction was considered as the capacity to freeze the N-aryl-bond rotation at room temperature, resulting the enhancement of the axial steric bulk and thus resulting in a low branching densities of the polyethylene. The highest catalytic activity after 60\u00a0min [6.73\u00a0\u00d7\u00a0104\ng of PE (mol of Pd) -1h\u22121] was achieved at 60\u00a0\u00b0C. At high temperature, the effects of the \u03c0-\u03c0 interaction was decreased while the branching densities of polyethylene was significantly increased. Consequently, the branching density and the microstructure of the polyethylene was modified by the various reaction conditions. In terms of the ethylene copolymerization with polar monomers, moderate catalytic activities (up to 6.8\u00a0\u00d7\u00a0104\ng of copolymer (mol of Pd) -1h\u22121), high molecular weight copolymers (M\nn\nup to 4.8\u00a0\u00d7\u00a0105\ng mol\u22121) and low incorporation ratios of polar monomers (up to 2.12\u00a0%) was observed. This work also indicated that the \u03c0-\u03c0 interaction effect played a critical role in copolymerization as well as ethylene homopolymerization.\nZhong et al. proposed a series of \u03b1-diimine Ni and Pd complexes with electron-donating/withdrawing groups on the dibenzobarrelene backbone for ethylene polymerization (C50 in Fig. 34\n\n)\n[39,141]. The electronic effects from the remote substitutions on the backbones influenced the catalytic performance of the corresponding Ni complexes. The electron-withdrawing halogens enhanced catalytic activity and polymer molecular weight, while electron-donating methoxy groups led to a decrease. C50 displayed the highest activity [5.6\u00a0\u00d7\u00a0105 g of PE (mol of Ni) -1h\u22121 (50\u00a0\u00b0C, 30\u00a0min)] and produced the highest molecular weight polyethylene (3.3\u00a0\u00d7\u00a0105 g/mol) (Table 4). Intra-ligand hydrogen bonding interactions (CH\u00b7\u00b7\u00b7OMe) were observed in the C50a. The weak and noncovalent interactions enhanced the catalyst thermal stability and brought about a living ethylene polymerization at high temperatures (80\u00a0\u00b0C) via inhibiting rotation of the N-aryl bonds. The dibenzobarrelene-based \u03b1-diimine Pd complexes (PdC50) exhibited the thermally robust characteristics for ethylene polymerization, due to the cooperative effect of hydrogen bonding interactions, electronic modification, and steric modification. The chloro-substituted Pd precatalyst presented the best thermal robustness for ethylene polymerization. In terms of the copolymerization with polar monomers, methoxy-substituted Pd precatalyst showed the most excellent tolerance toward both high temperature and polar groups. High MA incorporation (up to 9.5\u00a0mol%) can be achieved via the ethylene copolymerization with methyl acrylate (MA) at 80\u00a0\u00b0C. Compared to the previous strategies on weak noncovalent interactions in catalyzed ethylene polymerization, these hydrogen bonding interactions provided a fundamentally new approach in enhancing thermal stability of the \u03b1-diimine Ni and Pd complexes.\nZheng et al. investigated the \u03b1-diimine Ni complexes with bulky 8-p-tolylnaphthylamine and dibenzo-/dinaphthobarrelene backbones for ethylene polymerization (C51 in Fig. 35\n) [142]. The weak Ni\u2013phenyl interactions were considered as confining elements of the \u03b1-diimine Ni complexes. The interactions thus promoted the acceleration of the chain-growth process. 51Nia and 51Nib exhibited enhanced thermal stabilities and activities [1.39\u00a0\u00d7\u00a0106 g of PE (mol of Ni) -1h\u22121 (80\u00a0\u00b0C, 30\u00a0min)]. The synthesized PE was characterized as a linear semi-crystalline polymer. The combined experimental and theoretical study demonstrated the Ni-phenyl interactions decreased PE branching density in catalyzed ethylene polymerization. This work addressed the effects of Ni-phenyl interaction induced confinement, which provided an alternative strategy to prepare linear PE. Ni-phenyl interactions were also observed to promote E-MA copolymerization, where the MA incorporation was confirmed as 2.1\u00a0mol %.\nWang et al. reported a new family of \u03b1-diimine Ni and Pd complexes bearing axially bulky terphenyl and equatorial bulky dibenzobarrelene groups (C52 in Fig. 36\n) [143,144]. Due to the presence of bulky groups, chain transfer was limited in the ethylene polymerization. These novel nickel complexes yielded polyethylenes of ultrahigh molecular weights (M\nw as high as 1.74\u00a0\u00d7\u00a0106 g/mol). Meanwhile, the use of the unsymmetrical skeleton with both bulky terphenyl group and less bulky aniline group resulted in a high catalytic activitiy as 1.52\u00a0\u00d7\u00a0107\ng of PE (mol of Ni) -1h\u22121 (20\u00a0\u00b0C, 10\u00a0min) (Table 4). The incorporation of the electrowithdrawing trifluromethyl group into the terphenyl moiety cleary gave rise to H-F interactions between the N-terphenyl/anilinyl group and the dibenzobarrenlene backbone. This interaction efficiently suppressed the rotation of the N-terphenyl/anilinyl moiety, leading to relatively higher thermostability. The \u03b1-olefin polymerization studies revealed quite a large amount of \u03c9,1-enchainments. Due to the axial and equatorial bulkiness and the weak H-F interactions, the deactivation of the active species at high temperatures was relatively reduced. At 80\u2013140\u00a0\u00b0C, the Pd complexes efficiently catalyzed norbornene polymerizations with high catalytic activity (up to 5.65\u00a0\u00d7\u00a0107 g of PNB (mol of Ni) -1h\u22121) and yielding high molecular weights of PNB (up to 37.2\u00a0\u00d7\u00a0104 g/mol). Moreover, the Pd complexes could successfully promote the norbornene copolymerization with NB-MA, achieving moderate catalytic activities [104 g of copolymer (mol of Pd) -1h-1at 80\u00a0\u00b0C (60\u00a0min)].The immobilization of the catalytic metal center onto inorganic supports is a crucial step for gas and slurry-phase polymerization in the industry [145\u2013147]. Compared to the common supported heterogeneous catalyst, \u03b1-diimine Ni and Pd complexes provided much higher catalytic activity up to 107\ng of PE (mol of Ni) -1h\u22121 (or even higher) for ethylene polymerization. Molecular weight and molecular weight distribution of polyethylene could reasonably be controlled and modified using \u03b1-diimine Ni and Pd complexes, which gave us the chance to produce high-value polymers rather than the lower-quality polyolefins (broad dispersity and low molecular weight) from Ziegler-Natta systems [148,149]. Therefore, heterogenization of \u03b1-diimine Ni and Pd complexes provided a promising opportunity for a convenient \u201cdrop-in\u201d approach for novel catalytic solutions in industrial-scale production processes. However, previous academic studies on \u03b1-diimine Ni and Pd complexes were mainly focused on homogeneous systems [44 150]. In industry, however, the heterogeneous polymerization is still the predominant polyethylene synthesis method. Consequently, it is rare to find commercial applications of \u03b1-diimine Ni and Pd complexes for olefin polymerization [151]. This section summarizes the various recently reported immobilizing methods (either physisorption or chemisorption) for \u03b1-diimine Ni and Pd complexes. Successful immobilization of the well-defined, single-site catalysts such as \u03b1-diimine Ni and Pd complexes on inorganic supports would definitely be the best solutions for their industrial applications. Heterogeneous catalysis has the advantage over the homogenous catalysis in terms of separating the catalyst from the polymer during recycling, which is considered as the key drawback of homogenous catalysis [44]. This strategy has been the most efficient tool to establish a heterogeneous platform for olefin polymerization.The chemisorption methods for \u03b1-diimine Ni and Pd complexes could be classified based on the types of immobilization techniques: i.e i) covalent attachment or ii) surface-bound anions as shown conceptually in Fig. 37\n. The former method involved the covalent bonding between the silica supports and \u03b1-diimine Ni and Pd complexes, with the help of a linker (R in Fig. 37). The latter approach involves the treatment of the silica particles with cocatalysts (MAO, TAM, or others) to form surface-bound Al compounds. Then, the Al compounds served as the initiator to convert the complexes into active cationic metal-alkyl complexes. In this way, the complex was successfully immobilized onto the support particles through the ionic attraction to the surface-bound anions (electrostatic interactions) [150].\nWegner et al. has developed an efficient synthetic strategy for novel 2,5- and 2,6-phenyl substituted \u03b1-diimine Ni complexes (C53 in Fig. 38\n) [152]. The Ni complexes were directly supported during catalyst synthesis, without any further chemical links. These surface-bound catalysts were applied for ethylene polymerization in the gas phase. Unsupported C53a and C53b Ni complexes were benchmarked in the homogeneous solution for the comparison study, also with the gas-phase polymerization. The supported catalysts presented strong chemical stability. Even after several months, no decomposition was observed. The supported catalysts exhibited moderate activities in ethylene polymerization, which was up to 1.36\u00a0\u00d7\u00a0105\ng of PE (mol of Ni) -1h\u22121 (30\u00a0\u00b0C, 60\u00a0min) (Table 5\n). The produced polyethylene exhibited the features typical for \u03b1-diimine Ni catalyst; i.e. ranging from HDPE to LLDPE with narrow PDI and low branching densities. In general, polyethylene produced in the gas phase exhibited higher molecular weights than those obtained from solution polymerization. The growth of single polyethylene particles was investigated in gas-phase polymerization using video microscopy. All the Ni complexes exhibited strong tolerance towards hydrogen additions during ethylene polymerization.\nHuang et al. has developed an ionic immobilized C19d Ni complex (Fig. 39\n) onto the silica via different Al organic compounds (Et3Al and Et2AlCl) (C19d@SiO2\n). The immobilized Ni complexes displayed moderate activities [0.68\u00a0\u00d7\u00a0106\ng of PE (mol of Ni) -1 bar\u22121\u00a0h\u22121] (50\u00a0\u00b0C, 60\u00a0min) towards ethylene polymerization, initiated by the co-catalyst either\n\ni\nBu3Al or Et2AlCl [153]. The synthesized polyethylene was characterized as medium branching densities (30\u201350 CH3/1000C), ultrahigh molecular weights (up to 2.2\u00a0\u00d7\u00a0106\u00a0g/mol), and narrow molecular weight distributions (2.1\u20132.4) (Table 5). The supported nickel complexes produced spherical particles of polyethylene upon slurry-phase polymerization, while noteworthy, there was no reactor fouling.Based on the initial complex A, Fevero et al. recently developed a series of functionalized \u03b1-diimine Ni complexes with covalent tethers (C54-58 in Fig. 40\n) [154]. The Ni complexes were covalently attached to the mesoporous silica (MCM-41) which were applied in ethylene heterogeneous polymerization. The Ni precatalysts were directly attached as the single precursor (C56 and C57) or a binary precursor (C58) to the TMA treated silica. Under optimized conditions, the binary catalysts exhibited similar catalytic performance as the homogeneous catalysts [as high as 3.97\u00a0\u00d7\u00a0106\ng of PE (mol of Ni) -1h\u22121] (30\u00a0\u00b0C, 20\u00a0min) (Table 5). The analytical evidence from GPC and the thermal test indicated that both catalysts of the binary precursor were highly active during ethylene polymerization. C56 catalysts delivered high linear polymers while the C55 and C57 yielded branched polymers. The use of binary C58 catalyst resulted in polymers as the different functions, which was assumed to be due to the polymers' mixture. The functionalized tethers and covalent attachment both from the backbone and N-aryl groups were proven as a successful strategy to immobilize the \u03b1-diimine Ni(II) complexes on the solid support.\nTafazolian et al. reported a unique form of immobilizing the \u03b1\u2013diimine Ni(II) complex on the inorganic ZrO2\n[155]. The calcined ZrO2 and dilute sulfuric acid solution formed firstly the sulfated zirconium oxide (SZO) sites, which contained Br\u00f8nsted acids. These active sites allowed the ionic support of \u03b1\u2013diimine Ni complex on the ZrO2, which was already partially dehydroxylated at 300\u00a0\u00b0C. In the MeCN/Et2O solution, the immobilizing reaction took place between the (\u03b1-diimine)NiMe2 complex and SZO, generating the ionic support and methane (C59 in Fig. 41\n). Under 45-psi ethylene pressure, C59b polymerized ethylene monomer in toluene with the TOF of 21000\u00a0h\u22121 at 40\u00a0\u00b0C (15\u00a0min) (Table 5). The high molecular-weight polyethylenes were produced in catalyzed polymerization (M\nn\u00a0=\u00a01.53\u00a0\u00d7\u00a0105\ng/mol). The polymers exhibited moderate branching as 71B/1000C with a narrow PDI around 1.8. The elevated temperatures gave rise to a decrease in catalytic activities, polymer molecular weights, and molecular weight distribution. A steady decrease in the catalytic activity was observed with the increased reaction time in slurry-phase polymerization. Ethylene copolymerization with 10-undecenoate was also performed in this study. Compared to the polyethylenes, the synthesized copolymers presented a moderate molecular weight (M\nn\u00a0=\u00a02.97\u00a0\u00d7\u00a0104\ng/mol) with broad dispersity (PDI\u00a0=\u00a05.17).\nBahuleyan et al. have reported a leaching-free strategy to directly support the \u03b1-diimine Ni complexes on nonporous silica (C60and C61 in Fig. 42\n) [156]. This method avoids any tedious process, such as the chemical and thermal treatments of the silica substrate. The reactive amino groups on the \u03b1-diimine Ni complex provided the functionality to prepare the covalently supported catalysts. Firstly, amino-functionalized ligands reacted with the organosilane, 3-(triethoxy-silyl)propylisocyanate, which played the role of linkers on the ligands. Using the Stober method, the silica-supported ligands or complexes (after metalation with (DME)NiBr2) were prepared by the reaction between linkers with SiO2. Instead of the MAO-supported forms, the catalytic activity of the supported systems could be up to 106\ng of PE (mol of Ni) -1 bar\u22121\u00a0h\u22121 (30\u00a0\u00b0C, 60\u00a0min) with activation of a very small amount of aluminum compounds (Al/Ni 100) like EASC, MeAlCl2, and Et2AlCl (Table 5). No visible leaching from the active metal centers or pollution of the reactor was observed in the heterogeneous ethylene polymerizations. The types of cocatalysts, catalytic activities, and metal loadings were considered the main factors influencing the morphology of polyethylenes. Electron microscopic investigations indicated a fibrous architecture for the polymers.In the field of coordination-insertion polymerization catalyzed by the late-transition metal complexes, the polymeric microstructures are largely governed by catalyst structures and reaction conditions. The control over microstructure has a direct influence on mechanical and other properties of the polymers [157]. Due to the chain walking process, \u03b1-diimine Ni complexes produced mainly branched polymers using only ethylene as the monomer feedstock [158,159]. The structural modifications on the ligand backbone and N-aryl substituents of the \u03b1-diimine Ni complexes played a crucial role in controlling the chain-walking behavior, which had a significant influence on the polymeric branching densities. The branching nature of the polymers were determined by the high temperature 1H and 13C NMR, where the branching degree (per 1000C) were calculated (Fig. 43\n) [54,107,160]. In the high temperature 13C NMR spectra, the types and percentage of the various branches could be characterized clearly, like methyl, ethyl, propyl, butyl, amyl and even longer chains. These branched polymers are part of the polyolefin thermoplastic elastomers (P-TPE) [157]. Compared to the thermoplastics polymers (like HDPE), a low Young\u2019s modulus, high elongation at break, and high elastic recovery is typical for P-TPE-type materials. The stress\u2013strain curves indicated mechanical properties like tensile strength and elongation at break that are typical for P-TPE materials (Fig.\u00a044\nA). The elastic recovery of the P-TPE materials was also confirmed (Fig.\u00a044B) [82]. For example, Sui et al. synthesized a series of the elastic polyethylenes catalyzed by the unsymmetrical \u03b1-diimine Pd complexes, which displayed very nice mechanical properties [160]. High tensile strengths (18\u00a0MPa) and great elastic properties, like elongation at break close to 500\u00a0%, were observed. The molecular weight and broad PDI were considered as the main factor, influencing the mechanical performance of the polymers. In 2017, Lian et al. reported the synthesis of PE-based TPEs catalyzed by the \u03b1-diimine Ni complexes [82]. The tensile strength (3 to 28\u00a0MPa) and elongation-at-break (300\u00a0% to 1800\u00a0%) values of these polymers could be modified by different Ni complexes and polymerization conditions. An exceptional elastic recovery as high as 1605\u00a0% was noted. One year later, Fang et al. developed the newly synthesized elastomeric materials via ethylene polymerization, which exhibited good tensile strength ranging from 7.4 to 16.3\u00a0MPa and elongation-at-break values ranging from 450\u00a0% to 700\u00a0% [158]. Notably, the mechanical properties of the polymers were significantly influenced by their molecular weights and branching densities. In addition, an elastic recovery up to the strain recovery values of 83\u00a0% was observed. More recently, Liu et al. reported a series of highly branched polyethylene (161 branches /1000C) catalyzed by the trifluoromethoxy-substituted \u03b1-diimine Ni catalysts [107]. The lowest tensile strength was up to 3.3\u00a0MPa, while the highest elongation at break was observed as 984\u00a0%. The elastic recovery were determined as 71\u00a0%. The molecular weight, crystallinity, and the alkyl-branching architectures significantly influenced the tensile strength of these polymers. The distinctive properties of the produced polymers was believed to be the promising alternatives to the commercial thermoplastic elastomers.Besides the unique elastic properties, the copolymers incorporated with the polar monomers catalyzed by the \u03b1-diimine Ni and Pd complexes exhibited remarkable hydrophilicity (Fig. 45\n) [79]. This hydrophilicity could be easily determined via the water contact angles (WCA) measurements. For instance, Dai et al. synthesized the E-MA copolymers with good surface properties obtained from \u03b1-diimine Pd complexes [161]. The WCA values gradually decreased from 104\u00b0 to 54\u00b0 with increasing incorporation of MA monomer. The hydrophilicity of polar-functionalized polymers was reported to strongly related to the structures and contents of the polar groups. Normally, the presence of the hydrogen bond donors in the polar groups (such as \u2013COOH and \u2013OH units) brought about a higher hydrophilicity than the rest of polar monomers [65,78,162]. The stereochemistry of the polar monomers and the microstructures of the copolymers also affected the polymeric surface property (hydrophilicity). In contrast, with the use of \u03b1-diimine Pd complexes it was much more challenging to directly achieve the P-TPE materials due to the superior chain-walking tendency [161 163]. It led to the formation of the highly branched (amorphous) polymers with poor mechanical properties. Recently, Dai et al. reported the synthesis of polar functionalized P-TPE via the ethylene copolymerization with 10-undecenoic acid catalyzed by the \u03b1-diimine Pd complexes [163]. The polar copolymers displayed characteristics of thermoplastic elastomers with great elastic recovery (SR\u00a0=\u00a072\u00a0%-80\u00a0%) (Fig.\u00a044B), while the good surface properties enabled by polar monomers was well pronounced at the same time.This review summarizes the recent advances in the synthesis of the \u03b1-diimine Ni and Pd complexes applied in ethylene (co)polymerization. These \u03b1-diimine metal complexes achieved very high catalytic activities and thermal stability in ethylene (co)polymerization. The molecular weight, dispersity, branching densities, and melting points could be directly modulated via the finely controlled chain-walking mechanism. The synthesis of high-value polymers such as elastic polymers, LLDPEs, UHMWPEs, and functionalized copolymers can be realized by the ethylene monomer as the main feedstock. Although there are lots of advances in homogenous polymerization, the heterogeneous domains the industrial application. It is clear that the majority of current efforts in the development of \u03b1-diimine Ni and Pd complexes are more focused on homogenous (co)polymerization systems. Nevertheless, many fails and errors are inevitable along with the catalysis developments. The reports on heterogeneous polymerization catalyzed by these complexes are still rare. As the homogenous platform is well studied and established, the successful heterogenization of the well-defined \u03b1-diimine Ni and Pd complexes could become the primary research emphasis in the future. Due to the unique catalytic performance, these late-transition metal complexes can compete with or outperform present-day catalysts. This heterogeneous strategy will ensure the further control of the polymeric microstructure, morphology, and macro performance on an industrial scale.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was partially supported by Subitex (2020-2025) and China Scholarships Council (No. 201904910562).", "descript": "\n                  \n                     \u03b1-Diimine Ni and Pd complexes are one of the most examined late-transition organometallics in the application of catalyzed ethylene (co)polymerization. These organometallic catalysts provide unique advantages and particular opportunities to tailor the architectures, composition, and topology of synthesized polymers through catalyzed polymerization. Two decades after their initial discovery, they are still drawing extensive attention in both academia and industry. More recently, researchers have studied the effect of structural modifications on the \u03b1-diimine Ni and Pd complexes and their catalytic behaviors in ethylene (co)polymerization. This review highlights the recent progress in the developments of \u03b1-diimine Ni and Pd complexes achieved in the last decade. The chain-walking mechanism as a unique catalytic behavior of \u03b1-diimine Ni and Pd complexes is also addressed. The versatile synthesis of ligands and complexes enables researchers to tailor the catalytic performance and the microstructures of polyethylene. Correlations between their structural tunes and catalytic behaviors, polymer properties, and the ethylene copolymerization with polar monomers are comparatively presented and discussed. The heterogenization study of \u03b1-diimine Ni and Pd complexes on a solid support for heterogeneous catalysis is also comprehensively summarized. The review is broadly classified into four sections which includes i) the coordination-insertion chemistry in ethylene (co)polymerization, ii) the modification of ligand structure, iii) their application in the field of heterogeneous polymerization, iv) and the properties of the synthesized polymers, followed by the short summary and outlook for their potential studies and applications.\n               "}
{"full_text": "Precious metal nanoparticles supported on metal oxides are used for chemical conversions in heterogeneous and electrocatalysis. Reducing the size of the particles increases the fraction of atoms present at the nanoparticles\u2019 surface, and thus the per-atom efficiency. When the particles enter the subnano regime, quantum size effects can affect the catalytic activity. [1,2] In the ultimate limit, single metal atoms can be anchored directly onto the oxide support, and so-called single-atom catalysis (SAC) has emerged as a key strategy in heterogeneous and electrocatalysis in the last decade. [3\u20137]\nUnravelling how catalytically active metal atoms bind to the metal oxide support and interact with reactants is essential for understanding their properties. The local coordination environment has been shown to strongly influence the reactivity and stability of SACs, [8\u201311] but the structural details of the active sites are difficult to obtain from experiment. This is partly due to the structural inhomogeneity of powder supports, and partly due to the limitations of analytical techniques. In the absence of this critical information, the reaction mechanism is typically modelled computationally assuming an idealized low-index facet of the support material with the catalyst atom located at a high-symmetry site. Such models almost certainly do not represent the active catalyst, particularly for electrochemical applications, because the presence of water and/or hydroxyl groups is neglected.One approach to investigate the validity of the assumptions made in the computational modelling of SACs is to synthesize analogous systems experimentally. Such experimental modelling is achieved using single-crystalline metal-oxide supports where the atomic structure is well known. The metal of interest is evaporated directly onto the pristine surface in ultrahigh vacuum, which allows the most stable adsorption site to be determined. One can also selectively introduce molecules that might affect the stability of the system, and determine their individual impact unambiguously. For systems ultimately utilized in an aqueous solution, water is the obvious candidate. Recently, we demonstrated that Rh atoms sinter rapidly after deposition on a pristine \u03b1-Fe2O3\n\n\n(\n\n1\n\n1\n\u00af\n\n02\n\n)\n\n model support in ultrahigh vacuum (UHV) at room temperature but are stabilized as \u201csingle atoms\u201d when the same experiment is performed with 10\u22128 mbar water in the background pressure. The enhanced dispersion occurs because the adatoms are stabilized by additional coordination to two OH ligands. [12] On the other hand, adsorbates can also induce sintering, as observed for Pd/Fe3O4(001) in the presence of CO. [13]\nIn this work, we turn our attention to TiO2 as a model support. The thermodynamically stable rutile phase of TiO2, especially the (110) surface, has been widely investigated in surface science studies. [14,15] In SAC, the anatase polymorph (a-TiO2) is of particular interest because it becomes more stable in the nanoparticle form typically used as a support [16]. The reactivity of a-TiO2-supported SAC systems has been heavily investigated in recent years [17\u201323]. DeRita et\u00a0al. [24], for example, have convincingly demonstrated that Pt adatoms are active for CO oxidation. The possibility that agglomerates might be responsible for the observed activity was ruled out by using a very low Pt loading; each support particle hosted on average just one Pt atom. DFT-based calculations accompanying the experiments suggested that Pt atoms are probably not stable on the bare a-TiO2(101) surface. Moreover, a single Pt adatom on top of the bare a-TiO2(101) surface also failed to reproduce the experimentally observed binding energy and vibrational frequency of Pt-adsorbed CO molecules. These benchmark parameters were best reproduced when the Pt atoms were assumed to be coordinated to two additional oxygen atoms originating from hydroxyl groups on the surface. [24] Another study by the same group [25] reported that the coordination of Rh adatoms on a-TiO2 is sensitive to the composition of the reducing gas that was used to activate the catalyst. When Rh was pre-treated in CO at 300\u00b0C and further exposed to CO, Rh(CO)2 species formed with Rh being bound to two O2c from the lattice. (For a sketch of the a-TiO2 surface structure and the adsorption site, see Fig.\u00a01\n, below) When the system was pre-treated with H2 at 100\u00b0C, hydroxyls formed. These coordinated to the Rh(CO)2 species by adding an additional neighbouring surface OH group which substantially changed the CO binding energy. It was concluded from CO FTIR-TPD and DFT that the presence of hydroxyl groups can alter the local metal coordination and molecular desorption significantly. [25]\nHere, we present a surface science investigation study of four different metals \u2013 Pt, Rh, Ni and Ir \u2013 vapor-deposited directly onto an a-TiO2(101) single crystal support at room temperature in UHV. We find that Ir is the only metal that exhibits atomic dispersion under UHV conditions. However, the presence of water de-stabilizes the Ir adatoms, which leads to the formation of large clusters anchored at step edges. Pt, Ni, and Rh all form mostly clusters even at very low coverages, suggesting diffusion is facile on the regular terrace at room temperature. For Pt and Ni, small protrusions are observed in the STM images that we tentatively assign as isolated adatoms immobilized at defects.Room-temperature scanning tunnelling microscopy (STM) was performed in a two-vessel UHV chamber consisting of a preparation chamber (base pressure p < 10\u221210 mbar) and an analysis chamber (p < 5\u00a0\u00d7\u00a010\u221211\u00a0mbar). The analysis chamber is equipped with a nonmonochromatic Al K\u03b1 X-ray source (VG) and a SPECS Phoibos 100 analyzer for XPS, and an Omicron \u03bc-STM. The STM measurements (positive sample bias, empty states) were conducted in constant current mode with an electrochemically etched W tip. The natural a-TiO2(101) single crystal was prepared in UHV by sputtering (Ar+, 1 keV, 10 min) and annealing (610\u00a0\u00b0C, 20\u00a0min). Every fifth cycle the sample was annealed at 500\u00a0\u00b0C for 20\u00a0min in 5\u00a0\u00d7\u00a010\u22127\u00a0mbar of O2 and then in UHV at 610\u00a0\u00b0C. [26] Pt, Rh, Ni and Ir were deposited using an e-beam evaporator (FOCUS), with the flux calibrated using a water-cooled quartz microbalance (QCM). One monolayer (ML) is defined as one metal atom per surface unit cell. (The areal density of unit cells is 5.15\u00a0\u00d7\u00a01018\u00a0m\u2212\n2) The STM images were corrected for distortion and creep of the piezo scanner as described in ref [27]. The gray scale of each image is set individually to ensure that the possible adatoms and other small adsorbates are easily distinguishable. For measurements of the apparent height of adatoms or clusters, the average height of the surrounding substrate in the STM images was defined as a height of zero.\nFig.\u00a01a shows STM images of the a-TiO2(101) surface after several cleaning cycles. As is typical for this well-studied surface, a clean sample exhibits rows of bright, oval-shaped protrusions running in the [010] direction. These are attributed to the surface Ti5c and O2c atoms shown in Fig.\u00a01c, d. [28] The [10\n\n1\n\u00af\n\n] direction cannot be easily determined from these images; it was inferred from the preferred step directions. [28] Isolated dark features (highlighted with a blue arrow in Fig.\u00a01a between the rows are inhomogeneously distributed over the surface. These have previously been attributed to extrinsic Nb dopants, which are often present in natural anatase TiO2 samples. [29] Our XPS survey spectra did not show a peak that would allow us to confirm or debunk this assignment, likely because their average coverage is too low (0.02\u20130.03 ML as measured by STM). For what follows, it is important to note that surface oxygen vacancies (VOs) are not present at the surface of a-TiO2(101). Even when formed artificially, they quickly diffuse to the subsurface at room temperature [30]. This is in stark contrast to rutile TiO2(110), where VO sites are prevalent and active sites for adsorption [14].\nFig.\u00a01b shows the surface after 2 hours of exposure to the residual gas of the preparation chamber at room temperature (with the evaporator turned on but the shutter closed). Bright protrusions are observed; they appear identical to those observed after water adsorption in low-temperature studies [31]. Since water is known to desorb from regular a-TiO2(101) surface sites below 250\u00a0K [32], we conclude these water molecules are adsorbed at surface defects. The concentration agrees with that of the dark defects highlighted in Fig.\u00a01a. Interestingly, the water molecules are mobile at room temperature (Fig.\u00a02\n), but do not leave a visible defect behind when diffusing. This suggests that water and defect probably diffuse together, which makes it unlikely that the defect can be a cation substituting Ti in the anatase lattice. It could conceivably be an interstitial lattice species, or perhaps a surface site above a subsurface defect such as an oxygen vacancy. The images also exhibit a low concentration of molecular O2\n[33] species, which are in the residual gas as left-over from the oxidation step during sample preparation. This species is also most likely bound at defects, and a few examples are highlighted in orange and marked as (O2)extr in Fig.\u00a01b, consistent with the labelling in ref. [29]. These (O2)extr are also mobile at room temperature, and in a rare case we observed one of them to hop onto a dark defect, without leaving a similar defect behind (Fig.\u00a03\n). This suggests that there may be defect sites that are not visible in STM images, where adsorbates can bind more strongly than at regular surface sites. Overall, these data show that the regular anatase surface is inert at room temperature, but that defects (both visible and invisible in STM) can act as binding sites for molecular adsorbates. In what follows, we will show that these defects can also stabilize metal adatoms.A previous STM study of the Pt/a-TiO2(101) system [34] revealed that small clusters form predominantly on the terrace, with some species tentatively assigned to adatoms. Our data (Fig.\u00a04\na for a coverage of 0.05 ML) are similar to those in presented in ref. [34], and we also observe the coexistence of larger clusters and smaller features that have a uniform apparent height of 150\u2013160 pm. At a lower coverage of 0.01 ML (Fig.\u00a04b), the density and average size of the clusters is lower, and the 150\u2013160 pm species are observed again. These smallest Pt species are easily distinguished from adsorbed water by their larger apparent height at our imaging conditions (60\u201380 pm for water, see Fig.\u00a04c for a comparison), and because they are immobile in room-temperature STM movies. Given their relatively small apparent height, we tentatively assign these protrusions to Pt1 species. From the experiment alone, we cannot discount that these species could be dimers (or trimers) if such species were significantly more stable. For the higher coverage (0.05 ML), \u22487 % of the deposited Pt (according to the QCM calibration) can be attributed to possible single atoms, whereas at 0.01 ML this increases to \u224817 %.\nFig.\u00a05\na shows a high-resolution image, in which orange dots mark the approximate position of surface Ti5c atoms. Assuming that the substrate maxima imaged by STM are closer to the Ti5c than the O2c, the Pt-related protrusion is close to the position predicted by DFT calculations in ref. [34] (in between two adjacent O2c atoms, grey circles in Fig.\u00a01d). We also note that the Pt adsorption site is equivalent to the sites where the dark defects are seen in STM (Fig.\u00a05f), so it is possible that these defects stabilize the Pt atoms. Adsorbed water and O2 are also labelled in Fig.\u00a04a and 4b for ease of comparison.\nFig.\u00a04d shows an STM image of Pt deposited in a water vapor background of 2\u00a0\u00d7\u00a010\u22128 mbar. Again, a mixture of clusters and possible adatoms is observed, and the ratio of clusters and possible single atoms is comparable to that obtained in UHV. We thus conclude that water has no significant effect on the dispersion of Pt on the a-TiO2(101) terraces, at least in this low-pressure regime.\nFig.\u00a05b shows that the possible Pt adatom adsorption site is the same, independent of whether deposition was done in a water vapor background or in UHV.\nFig.\u00a06\n shows the a-TiO2(101) surface after deposition of 0.02 ML Rh (a) in UHV and (b) in a water vapor background of 2\u00a0\u00d7\u00a010\u22128\u00a0mbar. Unlike Pt, Rh forms exclusively small clusters on the surface despite the presence of the dark defects. We did not observe any features that we would attribute to single atoms, irrespective of the environment. We conclude that Rh1 species are not stable on the a-TiO2(101) surface at room temperature under our conditions. This is similar to our experience with r-TiO2(110) [37], where Rh1 species were found to sinter already at 150 K.\nFig.\u00a07\n shows the surface after deposition of 0.02 ML Ni in a) UHV and a water vapor background of 2\u00a0\u00d7\u00a010\u22128\u00a0mbar. Like Pt, Ni forms a mixture of clusters and small, uniform features that could be attributed to adsorbed single atoms. The coverage of these small species is relatively high: assuming that they are Ni1 they would account for \u224820 % of the deposited Ni, with the rest contained within larger clusters. The smallest species are easily distinguished from adsorbed water, partly by their apparent height (150\u2013170\u00a0pm), and also because they are immobile on the a-TiO2(101) surface at room temperature. Thus, in analogy to Pt, we presume that the smallest Ni species are most likely trapped at defect sites. Fig.\u00a05c and d show that these species are adsorbed at a different location on the surface than the features attributed to Pt atoms.After deposition in a water background of 2\u00a0\u00d7\u00a010\u22128 mbar, the concentration of the Ni1 species doubles from 20 % of the deposited Ni to 40 %. There is no discernible difference between the protrusions in the two experiments, so it seems that water has a significant effect on the dispersion of Ni and may play a role in stabilizing Ni at defect sites.The last metal investigated in this study was Ir. Fig.\u00a08\n shows 0.02 ML Ir deposited in UHV. Unlike Pt, Rh and Ni, Ir forms mostly uniform features with an apparent height of 130\u2013170 pm. All these features occupy the same site on the surface and are immobile at room temperature. Like Pt, Ir is located approximately between two O2c surface atoms (Fig.\u00a05). We assign these features to single Ir adatoms, which appear at a coverage of 0.011 ML on the surface. In addition to the single atoms, a small number of clusters can also be recognized. The apparent height of all features is depicted in a histogram in Fig.\u00a08b. A clear peak exists at 150 pm due to the features attributed to single atoms, with the shoulder at larger apparent heights originating from clusters. Considering that each cluster contains several Ir atoms, the coverage of the \u2248150 pm high features agrees nicely with the assignment to single atoms. If the smallest species were dimers, our QCM calibration would have to be significantly underestimating the amount of Ir deposited. Increasing the Ir coverage to 0.05 ML (Fig.\u00a08c) increases the density of clusters but does not affect the density of adatoms.\nFig.\u00a08d shows the influence of water on the Ir/a-TiO2(101) system. Deposition in water at room temperature leads to complete sintering of the single Ir atoms and the formation of large clusters at the step edges. This de-stabilizing effect of water is different to all the other metals studied here.We also performed XPS measurements on the four metals deposited in UHV and in water vapor. Fig.\u00a09\n shows an overview. For Pt, Rh and Ni, the peaks are shifted towards higher binding energy than those of the respective pure metals in the bulk. Water did not cause any drastic peak shifts, but intensity changes consistent with the propensity of dispersion/cluster formation observed in STM. The peak maxima are marked with a dotted line.Overall, this study shows that Pt, Rh and Ni readily sinter after deposition onto the a-TiO2(101) surface in UHV conditions. Rh is particularly unstable, and forms small clusters even at the lowest coverage studied with no evidence of any adatoms. Pt and Ni exhibit a mixture of small clusters and small, uniform features, which we assign as single atoms. Ir, in contrast, is highly dispersed at low metal coverages, but clusters begin to form when the coverage is increased. Our analysis of the adsorption site suggests that the adatom-assigned protrusions are between two surface O2c atoms for Pt and Ir, which is consistent with the site predicted for Pt by several DFT studies [34,35,38]. If the metal atoms bind to O, it is clear that the behaviour of the different metals can be understood in terms of the different oxygen affinities. Campbell and co-workers [39] recently studied adsorption of several late transition metals on MgO(110) and CeO2\u2212x(111), and reported the trend Ir > Ni > Pt > Rh for the oxygen affinity, which matches the relative stability observed for the UHV experiments here.One issue with the assignment of adsorption at a regular lattice site is that the diffusion barrier for Pt atoms along the [010] direction has been calculated to be 0.86\u00a0eV [38]. Such a value means that Pt atoms could diffuse at room temperature on the ideal surface, which is likely why the majority of Pt atoms form small clusters before our STM measurements are conducted. Consequently, we infer that the immobile adatoms we observe at room temperature must be trapped at defect sites.TiO2 is sometimes considered synonymous with oxygen vacancies, because the behaviour of rutile\u00a0TiO2(110) [37] in UHV is dominated by VO sites. DFT calculations suggest that Pt atoms would indeed be highly stable at surface VO sites (4.71 eV, compared to 2.20 eV on the pristine surface) [34], but it is known that VOs are preferentially accommodated in the subsurface layers. It is possible that such a large energy difference could cause a Vo to diffuse to the surface [40] in the presence of Pt atoms, but this is inconsistent with our STM results for Pt and Ni: In this case, the adatom would sit on an O2c site, not in-between as is consistently observed with Pt and Ir (Fig.\u00a05). Consequently, we conclude that the immobile Pt and Ir species are stabilized by another defect type. Ni on the other hand is slightly shifted from the Pt and Ir adatoms and could therefore possibly be stabilized by a Vo.The dark defects observed in STM are a primary candidate for the stabilization of metal adatoms because the defect is also located between two O2c atoms (compare Fig.\u00a05a and f). However, since the density of these defects is very inhomogeneous, which hinders any analysis of the number of defects covered by other species, we cannot exclude that another defect also plays a role. In any case, the nature of the dark defect is not clear. The previous assignment to substitutional Nb dopants [29] seems unlikely given the diffusion behaviour observed in the presence of water (Fig.\u00a02). Similar logic leads us to exclude that the defect is linked to substitutional Fe cations, although we do observe a small Fe2p signal in XPS survey spectra as this metal is a common contaminant in natural crystals. Nevertheless, Fe tends to be localized in patches on the surface, and its appearance differs significantly from the dark defects at standard imaging conditions. [26] Finally, we can also exclude that the dark defects are mistaken for an adsorbate, because the appearance of most candidate molecules present in the residual gas (water, O2, CO, OH) has already been established on the a-TiO2(101) surface. [29,31] We propose that the defect is most likely a dopant atom present in an interstitial site in the lattice. Hebenstreit et\u00a0al. speculated that it could possibly be a Ti interstitial.[41] While we cannot positively identify the chemical nature of the dark defect at this stage, our results suggest that extrinsic doping of the oxide could be a strategy to provide stronger binding sites capable of immobilizing expensive metals on a-TiO2(101).Turning now to the effect of water, we first note the completely different behaviour of Rh on a-TiO2(101) compared to our previous study on \u03b1\u2011Fe2O3\n\n\n(\n\n1\n\n1\n\u00af\n\n02\n\n)\n\n. In that work, depositing the metal in a background of water led to complete dispersion because Rh adatoms were stabilized by additional OH ligands [12]. One possible difference here is that water is already partially dissociated on \u03b1\u2011Fe2O3\n\n\n(\n\n1\n\n1\n\u00af\n\n02\n\n)\n\n at room temperature [42], so OH ligands are more readily available than on a-TiO2(101) where water adsorbs molecularly. Another difference is the surface geometry: On \u03b1\u2011Fe2O3\n\n\n(\n\n1\n\n1\n\u00af\n\n02\n\n)\n\n, OH groups adsorbed on nearby surface Fe cations can create a square planar environment for the Rh1 adatom [12], which is known to be energetically favourable. [43] On a-TiO2(101), this is not possible because OH groups adsorbed on surface Ti cations can at best create a threefold coordination, assuming the Rh1 remains coordinated to two surface O atoms.The only case where the dispersion seems to be aided by water is Ni, and our data suggest that \u224840\u00a0% of the deposited metal can be stabilized as isolated adatoms. The apparent height and adsorption site is the same as it was in the absence of water, so it is possible that some of the Ni1 were already partially stabilized by the water inadvertently present in the residual gas in the UHV experiments. The complete opposite effect was seen after the deposition of Ir in a water vapor background. The presence of water promotes a dramatic sintering of the dispersed species, leading to mobile clusters, which finally get trapped at the steps. Clearly, then, the effect of water is difficult to predict, and given that water and hydroxyl groups are always present on metal-oxide surfaces, its omission from computational modelling of SAC systems is likely a major oversimplification. It is also important to recognise that water can have a significant effect on the reactivity, and there is evidence that water can play a role in SAC reaction mechanisms [44,45].Finally, one of the goals of this study was to assess the suitability of the a-TiO2(101) surface as a model support for surface science studies of SAC mechanisms. While it would be possible to study adsorption at the adatoms by STM/nc-AFM, the ambiguity over the nature of the defect sites that stabilize the adatoms precludes reliable modelling of the system. In any case, the presence of clusters at low coverage will make it difficult to distinguish the reactivity of single atoms from clusters using area averaging-techniques. At present, it is difficult to recommend this system as a suitable model system for studies of single-atom catalysis.We have carried out room-temperature STM measurements of the a-TiO2(101) surface after deposition of Pt, Rh, Ni and Ir in UHV and in a water vapor background. Pt and Ni form a mixture of small clusters and, possibly, single atoms. Rh exclusively forms clusters, while Ir is highly dispersed at a low coverage. The influence of water strongly varies from metal to metal. No influence is discernible for Pt and Rh, but the dispersion of Ni is increased when deposition is performed in a water vapor background. The exact opposite effect occurs in the case of Ir, which rapidly sinters after deposition in a water vapor background. The adsorption site of the species attributed to Pt and Ir atoms is the same as calculated for Pt1 on the pristine surface; nevertheless, there is evidence that the single metal atoms are trapped at defect sites on the a-TiO2(101) surface. As such, we conclude that doping of oxide surfaces could be a viable strategy to provide strong adsorption sites for single metal atoms.\nLena Puntscher: Data curation, Investigation, Writing \u2013 original draft. Kevin Daninger: Data curation, Investigation. Michael Schmid: Writing \u2013 review & editing. Ulrike Diebold: Writing \u2013 review & editing. Gareth S. Parkinson: Funding acquisition, Supervision, Conceptualization, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.LH, KD, and GSP acknowledge funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. [864628], Consolidator Research Grant \u201cE-SAC\u201d). UD acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme grant agreement No. [883395], Advanced Research Grant \u2018WatFun\u2019). The authors acknowledge TU Wien Bibliothek for financial support through its Open Access Funding Programme.", "descript": "\n                  Understanding how metal atoms are stabilized on metal oxide supports is important for predicting the stability of \u201csingle-atom\u201d catalysts. In this study, we use scanning tunnelling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) to investigate four catalytically active metals \u2013 platinum, rhodium, nickel and iridium \u2013 on the anatase TiO2(101) surface. The metals were vapor deposited at room temperature in ultrahigh vacuum (UHV) conditions, and also with a background water pressure of 2\u00a0\u00d7\u00a010\u22128 mbar. Pt and Ni exist as a mixture of adatoms and nanoparticles in UHV at low coverage, with the adatoms immobilized at defect sites. Water has no discernible effect on the Pt dispersion, but significantly increases the amount of Ni single atoms. Ir is highly dispersed, but sinters to nanoparticles in the water vapor background leading to the formation of large clusters at step edges. Rh forms clusters on the terrace of anatase TiO2(101) irrespective of the environment. We conclude that introducing defect sites into metal oxide supports could be a strategy to aid the dispersion of single atoms on metal-oxide surfaces, and that the presence of water should be taken into account in the modelling of single-atom catalysts.\n               "}
{"full_text": "Biomass is a promising renewable source for the production of fuels, chemicals and H2\n[1\u20134] due to its availability and CO2 neutral contribution. Besides, H2 is a clean fuel whose potential as an energy carrier makes it a promising choice for its transformation into any form of energy for diverse end-use applications.In recent decades, the progress of technological strategies for H2 production from biomass has gained increasing attention in the literature [5\u20139]. Two types of routes are used for biomass conversion into H2, as are thermochemical and biological processes [10]. Thus, biomass may be converted into H2 through the following routes: i) water biophotolysis using micro-algae and cyanobacteria, ii) photofermentation, iii) dark-fermentation, and iv) hybrid reactor system [5]. The biological H2 production (biohydrogen) from microorganism metabolism is a promising technology under development, in which renewable sources can be used for the sustainable production of H2\n[11,12]. Amongst the different routes for biomass valorization, thermochemical processes have merited especial consideration in the literature [8,13,14], particularly biomass steam gasification [15\u201319], fast pyrolysis [20,21], and the steam reforming of the bio-oil obtained in the pyrolysis process [22\u201325]. More recently, the alternative route of biomass pyrolysis and in-line catalytic steam reforming has attracted remarkable attention for the production of H2 from biomass [26\u201331]. Most pyrolysis-reforming studies conducted in the literature have been performed in discontinuous mode using batch reactors. However, a great effort has been made in the recent years in order to implement a continuous feeding system, and therefore to step further in the scaling-up of this process.The choice of a suitable catalyst for these processes is of uttermost significance for the viability of their industrial implementation. Accordingly, primary catalysts, such as dolomite, olivine, \u03b3-Al2O3 or spent fluid catalytic cracking (FCC) catalyst, have been widely investigated in biomass gasification [32\u201335]. Thus, several authors have reported the activity of dolomite and olivine for reforming and cracking reactions [36,37], whereas \u03b3-Al2O3 is effective in tar decomposition and promoting H2 production [38]. The utilization of a spent FCC catalyst is of special relevance, since it increases the lifetime of a refinery waste material [39,40].Besides, commercial Ni-based catalysts have been extensively used in steam reforming processes, since these types of catalysts involve several advantages, such as their lower cost compared to noble based catalysts, as well as their high activity for breaking C\u00a0\u2212\u00a0C and O\u00a0\u2212\u00a0H bonds. However, their fast deactivation, mainly by coke deposition on the active sites, is a great challenge to face up. Accordingly, different strategies have been proposed with the aim of improving the activity and stability of the reforming catalyst, as are the use of different reactor configurations, the selection of suitable operating conditions or the optimization of the catalyst design based on supports and promoters [8,41\u201343].The strategy proposed in this study to attenuate the fast catalyst deactivation lies in the modification of the feed into the reforming step, which may be conducted in the pyrolysis reactor itself or downstream by upgrading the bio-oil produced in the biomass pyrolysis. Thus, it is well established that certain bio-oil compounds reaching the reforming catalyst bed significantly influence the mechanisms of catalyst deactivation, particularly coke formation, and therefore the performance of the catalyst in the reforming step [44]. Although huge effort has been made to assess the catalyst performance and its deactivation causes in the steam reforming process, the highly complex bio-oil composition and the problems associated with its handling boosted use of bio-oil model compounds, such as ethanol, phenol, acetic acid, toluene, or their mixture [45\u201347]. Therefore, knowledge of the reforming catalyst performance and the main species responsible for catalyst deactivation by coke deposition under real process conditions is still limited.Based on this background, this study pursues a dual objective. On the one hand, to enhance catalyst activity and stability in the reforming step in a tandem pyrolysis-reforming reactor by conditioning the biomass pyrolysis volatile stream using highly available and inexpensive catalysts. On the other hand, to provide a further understanding of the reactivity of the main bio-oil oxygenate compounds and their role in the deactivation of the reforming catalyst. The results obtained in this study will contribute to progressing towards the understanding of the main coke precursors in the reforming step and promoting the proposal of new strategies for improving catalyst stability, which are the main challenges to be faced in the industrial implementation of the pyrolysis-reforming process.Accordingly, the production of H2 from biomass (pinewood sawdust) has been carried out in a conical spouted bed reactor (CSBR) for the fast pyrolysis, and an in-line fixed bed reactor for the reforming of the volatiles produced [48]. For the conditioning of this stream, different low cost materials (inert sand, \u03b3-Al2O3, a spent FCC (fluid catalytic cracking) catalyst and olivine) have been located prior to the reforming catalyst. Continuous biomass pyrolysis has been conducted in a conical spouted bed reactor (CSBR) and the volatiles formed have been transferred into a fixed bed reactor for their conditioning and reforming. Thus, the fixed bed reactor includes two reaction sections, the first one with the guard catalyst (conditioning step) and the second one with the steam reforming one. In a previous study, this reactor configuration revealed a high efficiency for the conversion of the volatiles derived from biomass pyrolysis into a hydrogen rich syngas, with catalyst deactivation being lower than when a fluidized bed reactor is used in the reforming step [48].The biomass used in this process was pinewood waste (pinus insignis), which was crushed and sieved to a particle size in the range from 1 to 2\u00a0mm. The ultimate and proximate analyses were determined in previous studies in a LECO CHNS-932 elemental analyzer and in a TGA Q5000IR thermogravimetric analyzer, respectively [26,49]. An isoperibolic bomb calorimetry (Parr 1356) was used to determine the higher heating value. Table 1\n summarizes the main biomass features. Fig. S1 in the Supplementary Information shows the TG profile of the pinewood sawdust.Four different low-cost materials have been used in this study as guard catalysts for the conditioning of the pyrolysis volatile stream, as are: i) inert silica sand (Minerals Sibelco), ii) olivine (Minerals Sibelco), with basic character and active for reforming biomass-derived oxygenates, iii) spent FCC catalyst, supplied by Petronor Refinery in Muskiz, Spain, and iv) \u03b3-Al2O3\n(Alfa Aesar). The spent FCC catalyst and the \u03b3-Al2O3 are of acid character and active for cracking reactions [39].Prior to use, the spent FCC catalyst was regenerated by calcination with air at 575\u00a0\u00b0C for 1\u00a0h in order to burn the coke deposited during its utilization in the refinery. The FCC catalyst particles were agglomerated by wet extrusion to obtain a particle size suitable for use in the fixed bed. Bentonite (50\u00a0wt%) was used as binder to confer mechanical and thermal resistance upon this catalyst. Subsequently, the extruded sample was dried overnight and calcined with air at 575\u00a0\u00b0C for 2\u00a0h. Then, the FCC catalyst was ground and sieved to a particle size in the 0.8\u20131.6\u00a0mm range. Similarly, olivine and \u03b3-Al2O3 were also ground and sieved to attain the desired particle size (0.8\u20131.6\u00a0mm). The fraction of inert silica sand was also within this range.These conditioning catalysts were characterized by N2 adsorption\u2013desorption, X-ray Fluorescence (XRF) spectrometry and NH3-TPD. The characterization procedure has been described in the Supplementary Information.The commercial catalyst used in the reforming reactor (ReforMax\u00ae 330 or G90-LDP), denoted as G90, was supplied by S\u00fcd Chemie. The selection of this catalyst is based mainly on its availability and reliability (without reproducibility problems in its preparation), since a significant amount of catalyst is needed in all the experimental runs. Moreover, commercial G90 and other similar commercial Ni/Al2O3 catalysts have been extensively used in the literature about the steam reforming of tar compounds [50,51], biomass and sewage sludge pyrolysis volatiles [52\u201356] and pyrolysis oils produced from waste plastics [57].This commercial catalyst for CH4 reforming was provided as perforated rings (19\u00a0\u00d7\u00a016\u00a0mm), which were ground and sieved to a particle size in the 0.4\u20130.8\u00a0mm range. This particle size range showed a suitable performance in previous studies operating in fixed bed regime [48]. The chemical composition of G90 catalyst is based on NiO, whose nominal content is 14\u00a0wt%, apart from CaAl2O4 and Al2O3. The textural properties of the fresh catalyst determined by N2 adsorption\u2013desorption have been shown in previous studies [58,59]. Accordingly, the catalyst is a mesoporous material, with mean pore diameter of 12.2\u00a0nm. The results of BET surface area (19.0\u00a0m2 g\u22121) are rather low.The Ni based catalyst reduction temperature was ascertained by temperature programmed reduction (TPR), and the results are provided elsewhere [60,61]. Accordingly, the TPR profile revealed two main peaks with the prevailing one located at 550\u00a0\u00b0C, which was ascribed to the reduction of NiO which is interacting with Al2O3 support. The peak observed at higher temperature (700\u00a0\u00b0C) was associated with NiAl2O4 spinel phase. Moreover, prior to the pyrolysis-reforming experiments, in-situ catalyst reduction was carried out by feeding a stream of 10\u00a0vol% H2 with N2 at 710\u00a0\u00b0C for 4\u00a0h.The biomass pyrolysis-reforming has been carried out in a bench scale laboratory plant, whose scheme is shown in Fig. 1\n. The pyrolysis step was conducted in a conical spouted bed reactor (CSBR), whereas a fixed bed reactor was selected to perform the catalytic reforming of the volatiles from the pyrolysis step. In the latter reactor, different conditioning beds (sand, \u03b3-Al2O3, FCC and olivine) were placed prior to the reforming catalyst bed (see Fig. 1).Previous studies conducted by the research group have proven the good performance of the CSBR for the pyrolysis of different materials, such as biomass [49,62,63], waste plastics [64,65] or tires [66\u201368]. Moreover, the design of the CSBR is based on previous hydrodynamic studies [69], and its dimensions are as follows: conical section height, 73\u00a0mm; cylindrical section diameter, 60.3\u00a0mm; conical section angle, 30\u00b0; bed bottom diameter, 12.5\u00a0mm; and gas inlet diameter, 7.6\u00a0mm. Continuous removal of the char particles in this reactor is carried out by means of a lateral outlet pipe located above the bed surface (Fig. 1). The gas stream is heated to the desired temperature prior to entering the reactor by means of a preheater. Both the reactor and the gas preheater are located inside a radiant oven of 1250\u00a0W.The temperature in the fixed bed reactor, which is located inside an oven (550\u00a0W), is controlled by a thermocouple located in the catalyst bed. The pilot plant is provided with a cyclone, which retains the char and sand particles entrained from the pyrolysis bed. With the aim of avoiding steam and heavy compounds condensation, both reactors (the CSBR and the fixed bed reactor), all interconnection pipes and the cyclone are placed inside a forced convection oven, wherein the box temperature is maintained at 300\u00a0\u00b0C. Avoiding the condensation of heavy compounds before and after the fixed bed reactor is essential to carry out the analysis of the products.The solid feeding device consists of a vessel equipped with a vertical shaft connected to a piston placed below the material bed. At the same time as the piston raises, the biomass feeder is vibrated, which ensures continuous discharge of the biomass into the reactor.The water required in the conical spouted bed and in the reforming step was fed by a high precision pump (Gilson 307). It was vaporized by means of an electric heater prior to entering the pyrolysis reactor. Moreover, the plant is provided with three mass flow-meters for N2, (used as fluidizing agent in the process of heating the reaction system), air and H2 (used for the reforming catalyst reduction prior to the reforming step).The product condensation system is provided with a condenser and a coalescence filter, which ensure the collection of the non-reacted steam and bio-oil compounds prior to analysis.The pyrolysis step was carried out at 500\u00a0\u00b0C, as the condensable volatile fraction (liquid fraction) obtained in the biomass pyrolysis is maximized at this temperature [49]. Based on previous hydrodynamic studies, the CSBR contained 50\u00a0g of silica sand with a particle size in the 0.3\u20130.35\u00a0mm range. Besides, a water flow rate of 3\u00a0mL\u00a0min\u22121 was chosen for all the runs, which corresponds to a steam flow rate of 3.73 NL min\u22121. These conditions ensure a vigorous movement in the CSBR.The reforming temperature was fixed at 600\u00a0\u00b0C, which was established as the optimum one in previous biomass pyrolysis-reforming runs [26]. Thus, higher temperatures (700\u00a0\u00b0C) showed a limited effect on the reforming results [58] and may favour sintering of the metallic Ni active sites. [70].In the fixed bed reactor, the bed was divided into two sections: i) the conditioning step with the guard bed, (silica sand, \u03b3-Al2O3, spent FCC catalyst or olivine), which is located in the upper section of the reactor, and ii) the reforming catalyst bed, which is composed of a mixture of inert sand (1\u20132\u00a0mm) and a commercial Ni/Al2O3 (G90) catalyst (0.4\u20130.8\u00a0mm). A steel mesh was used to divide both fractions with the aim of easing their separation for further characterization when each experimental run was finished.The conditioning catalysts used in this study are significantly different concerning density, i.e., sand: 2600\u00a0kg\u00a0m\u22123; olivine: 3300\u00a0kg\u00a0m\u22123; FCC: 1246\u00a0kg\u00a0m\u22123, and \u03b3-Al2O3: 1666\u00a0kg\u00a0m\u22123. The bed mass of these materials was chosen in order to have the same bed volume in all the experiments (30\u00a0mL), which was that corresponding to a GHSVvolatiles of 3100\u00a0h\u22121, and the particle size of all guard catalysts was in the 0.8\u20131.6\u00a0mm range. The corresponding masses were 44.2\u00a0g of silica sand, 46.2\u00a0g of olivine, 17.3\u00a0g of spent FCC catalyst and 19.9\u00a0g of \u03b3-Al2O3. Moreover, the same bed volume of the mixture of commercial Ni/Al2O3 catalyst (G90) and inert sand was used (30\u00a0mL), which corresponds to 9.4\u00a0g and 29.0\u00a0g of reforming catalyst and inert sand, respectively.In order to compare the influence of the conditioning step on the reforming one, the same operating conditions were established. Accordingly, all the runs were conducted in continuous regime by feeding 0.75\u00a0g\u00a0min\u22121 of biomass, with the S/B ratio being 4 and the space time 15 gcat min gvolatiles\n\u22121. These conditions allow attaining conversion values close to thermodynamic equilibrium without involving high energy requirements to vaporize the water supplied. Thus, given that the steam flow rate required for attaining a suitable spouting regime in the CSBR has been set at 3.73 NL min\u22121 (3\u00a0mL\u00a0min\u22121 of water), the corresponding biomass flow rate was 0.75\u00a0g\u00a0min\u22121.Prior to the pyrolysis-reforming runs, all the elements of the plant were heated using N2 as fluidizing agent. Then, the fluidizing gas was switched to water and once temperature had been stabilized, biomass feed started.The analysis of the volatile stream was carried out at three different locations: i) after the pyrolysis step, ii) once the stream had passed the guard bed (conditioning step) and, finally, iii) at the outlet of the steam reforming reactor. Furthermore, experimental runs using different reactor configurations have been conducted under the same conditions. Thus, in the pyrolysis runs, the volatile stream is directly connected to the condensation system without passing through the fixed bed. In the conditioning runs, both reactors (pyrolyser and conditioning fixed bed) were used, with the fixed bed containing the conditioning catalyst, i.e., the reforming catalyst G90 was not introduced in the reactor. Finally, the configuration for the pyrolysis-reforming runs was made up of a CSBR and a fixed bed reactor containing both the guard and the reforming catalysts. This latter configuration corresponds to the scheme shown in Fig. 1.The mass balance in the pyrolysis runs (biomass pyrolysis and biomass pyrolysis\u00a0+\u00a0guard catalyst) was closed by weighting the char particles, i.e., the amount of char remained in the reactor plus those collected through the lateral pipe and retained in the cyclone and filter, and combining this information with that obtained by on-line chromatographic analysis. In these pyrolysis runs, cyclohexane was used as an internal standard to validate the mass balance closure. Thus, the pyrolysis product stream leaving the fixed bed reactor was analyzed in-line in a GC Agilent 6890 provided with a HP-Pona column and a flame ionization detector (FID) by means of a line thermostated at 280\u00a0\u00b0C, and the non-condensable gases were analyzed in a GC Varian 4900. Besides, the identification of the bio-oil compounds was conducted by condensation of the liquid sample and further analysis by means of a GC\u2013MS spectrometer (Shimadzu 2010-QP2010S) provided with a BPX-5 column. In the pyrolysis-reforming runs, the overall and elemental mass balances (C, H and O) were closed based on the information about the volatile stream that reached the reforming catalyst (which has been determined as mentioned above) and the information obtained in the GC and microGC analyses of the stream at the outlet of the reforming step. The mass balances closure was above 95 % in all cases, and runs were repeated at least 3 times in order to ensure reproducibility. The chromatographic analyses were conducted in-line by means of a GC Agilent 6890 provided with a HP-Pona column and a flame ionization detector (FID). In order to avoid the condensation of non-converted oxygenate compounds, the sample from the reforming reactor outlet stream has been injected into the GC by means of a line thermostated at 280\u00a0\u00b0C. The permanent gases, i.e., H2, CO2, CO, CH4 and C2-C4 hydrocarbons, were analyzed in a GC Varian 4900 once the outlet stream of the reforming reactor was condensed and filtered.The textural properties of all conditioning beds were analyzed after each experimental run by N2 adsorption\u2013desorption technique in a Micromeritics ASAP 2010 following the procedure described in the Supplementary Information.The coke formed on the spent catalysts, both guard and reforming ones, was analyzed at the end of each continuous run. The coke content was measured by Temperature Programmed Oxidation (TPO) in a Thermobalance (TGA Q5000 TA Instruments) coupled to a mass spectrometer (Thermostar Balzers Instrument). Given that the Ni active phase is oxidized at the same time as the coke combustion occurs, the CO2 formation is monitored throughout the TPO runs, according to the following procedure: i) Signal stabilization with a N2 stream at 100\u00a0\u00b0C, and, ii) oxidation with air (heating rate of 5\u00a0\u00b0C\u00a0min\u22121 to 800\u00a0\u00b0C maintaining this temperature for 30\u00a0min to ensure complete coke combustion. Similarly, the amount of coke deposited on each guard catalyst after the pyrolysis-reforming runs was determined following the same procedure. The deactivated G90 catalysts were analyzed by Scanning Electron Microscopy (SEM) in a JEOL JSM-6400 apparatus.In order to evaluate the influence the different guard catalysts have on the subsequent reforming step, volatile conversion and individual product yields have been taken as the key reaction indices. It should be noted that the definition of these reaction indices is based on the volatile products that reach the commercial Ni/Al2O3 catalyst (G90) bed (gases and bio-oil derived compounds), i.e., once the volatile stream from the pyrolysis step had passed the guard bed. Thus, the carbon contained in the char produced in the pyrolysis step was not considered, given that this product was removed from the CSBR (through the lateral pipe) prior to the conditioning step.Accordingly, the volatile conversion in the reforming step is determined as the ratio between the C moles in the product stream leaving the reforming step (Cgas) and the C moles in the volatile stream reaching the reforming catalyst (Cvolatiles):\n\n(1)\n\n\nX\n=\n\n\n\n\nC\n\n\ngas\n\n\n\n\n\n\nC\n\n\nvolatiles\n\n\n\n\n\u00a0\u00b7 100\n\n\n\n\nSimilarly, the yield of each individual product, i, has been calculated based on the pyrolysis volatile stream.\n\n(2)\n\n\n\n\nY\n\n\ni\n\n\n=\n\n\n\n\nF\n\n\ni\n\n\n\n\n\n\nF\n\n\nvolatiles\n\n\n\n\n\u00a0\u00b7 100\n\n\n\nwhere Fi and Fvolatiles are the molar flow rates of each compound i and the volatile stream at the inlet of the reforming reactor, respectively.The hydrogen yield is defined based on the maximum allowable by stoichiometry:\n\n(3)\n\n\n\n\nY\n\n\n\n\nH\n\n\n2\n\n\n\n\n=\n\n\n\n\nF\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n\nF\n\n\n\n\nH\n\n\n2\n\n\n\n\n0\n\n\n\n\n\u00a0\u00b7 100\n\n\n\nwhere FH2 is the H2 molar flow rate and F0\nH2 the maximum allowable by the following stoichiometry:\n\n(4)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nk\n\n+\n\n\n\n\n2\nn\n-\nk\n\n\n\n\n\nH\n2\n\nO\n\n\u2192\n\nn\nC\n\nO\n2\n\n+\n\n(\n2\nn\n+\nm\n/\n2\n-\nk\n)\n\n\nH\n2\n\n\n\n\n\nFinally, H2 production is defined by mass unit of the biomass in the feed:\n\n(5)\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n=\n\n\n\n\nm\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n\nm\n\n\nbiomass\n\n\n0\n\n\n\n\n\u00b7\n100\n\n\n\nwhere mH2 and m0\nbiomass are the mass flow rates of the H2 produced and biomass fed into the process, respectively.The textural properties (BET surface area, pore volume and pore diameter) of the conditioning catalysts are shown in Table 2\n. As observed, FCC and \u03b3-Al2O3 are mesoporous materials, with an average pore size of around 17.0\u00a0nm, whereas inert sand and olivine are non-porous materials with negligible BET surface area and pore volume. Apart from the characteristic features of the conditioning catalysts, their physical properties may significantly influence the pyrolysis volatile composition to be fed into the reforming catalyst bed. Thus, mesoporosity would enhance the diffusion of bulky reactants, i.e., phenolic compounds, such as guaiacol and their derivatives [71]. In the case of the spent FCC catalyst, which is based on the HY zeolite, the use of bentonite as binder provides meso and macropores to the catalyst, which minimize external blockage of the channels [72]. However, the microporous structure of this zeolite is also evident, with a microporous surface area of 57\u00a0m2 g\u22121.The chemical composition of each conditioning catalyst was determined by XRF analysis, and the results are set out in Table 2. As observed, inert sand is mainly composed of SiO2; olivine is a mineral containing MgO, SiO2 and Fe2O3; the \u03b3-Al2O3 used in this study contains a small amount of SiO2, apart from Al2O3; and the FCC catalyst agglomerated with bentonite (50\u00a0wt%) is a mixture containing Al2O3, SiO2, Fe2O3 and P2O5, among other metal oxides. Futhermore, incorporation of bentonite greatly influences the composition of the spent FCC catalyst (used in a previous gasification study [39]), as it leads to a significant increase in the amount of SiO2. Moreover, it has been widely reported that the chemical composition of the olivine plays a positive role in tar decomposition and reforming reactions [73,74] due to the presence of Fe0 on its surface [75].\nTable 2 also shows the total acidity of the conditioning catalysts determined by NH3-TPD analysis. The results obtained revealed that only the spent FCC and the \u03b3-Al2O3 catalyst contain acid sites, with a total acidity of 47 and 106 \u00b5molNH3 gcat\n\u22121, respectively, whereas in the case of olivine, a negligible acidity is observed (6 \u00b5molNH3 gcat\n\u22121). The acidity of these materials enhances cracking reactions involving bio-oil oxygenated compounds, leading to a higher amount of aromatics and paraffins.Biomass pyrolysis was conducted at 500\u00a0\u00b0C using steam as fluidizing agent. Moreover, the catalytic conditioning of fast pyrolysis volatiles was carried out at 600\u00a0\u00b0C in the fixed bed reactor (see Fig. 1). A previous study detailed the biomass pyrolysis products obtained using these low-cost conditioning catalysts, and the main mechanisms of bio-oil transformation [76].The pyrolysis products obtained were grouped into three different fractions: i) a gaseous fraction made up of CO2, CO, H2 and small amounts of C1\u2013C4 hydrocarbons, ii) a condensable volatile fraction (liquid fraction or bio-oil) composed of water and a complex mixture of oxygenated compounds, and iii) a solid residue or char, which is the non-volatilized biomass fraction. Table 3\n shows the product yields obtained once the pyrolysis stream obtained at 500\u00a0\u00b0C in the CSBR had passed the conditioning beds in the fixed bed reactor at 600\u00a0\u00b0C.As observed in Table 3, a high bio-oil yield was obtained in the biomass pyrolysis at 500\u00a0\u00b0C (75.36\u00a0wt%), which evidences the good performance of the CSBR for biomass pyrolysis due to the characteristic features of this reactor, as are high heating rates, short vapour residence times and rapid char removal from the hot reaction environment [49,77,78]. Conditioning catalysts led to a significant reduction in the bio-oil yield at the expense of gaseous product formation. In the case of inert sand, this drop is a consequence of thermal cracking reactions, whereas the decrease with the other catalytic materials is due to simultaneous thermal and catalytic cracking reactions. These results evidence that the features of the conditioning catalysts, i.e., physical properties, chemical composition and catalyst acidity (see Table 2), greatly influence the composition of the volatile stream to be fed into the reforming catalyst bed. Thus, although all conditioning catalysts tested are active for cracking (with an increase in the non-condensable gas yield in detriment of the condensable fraction (bio-oil)), the basic nature of olivine, as well as its limited porous structure (which hinders the diffusion of bulky oxygenated compounds into the bed material), led to a lower extension of cracking reactions. Besides, the mesoporous structure of FCC and Al2O3 guard catalyst, and especially the higher acidity of FCC and \u03b3-Al2O3 catalysts, Table 2, led to higher gas yields (26.11 and 32.45\u00a0wt%, respectively) due to the promotion of bio-oil cracking.The char produced is continuously extracted from the pyrolysis reactor by a lateral outlet (see Fig. 1), and its yield was not therefore affected by conditioning catalysts. Accordingly, it remained constant in all the experimental runs (17.34\u00a0wt%).In the biomass pyrolysis at 500\u00a0\u00b0C, CO and CO2, and small amounts of H2 and C1\u2013C4 hydrocarbons account mainly for the gaseous product stream. Besides, when inert sand, spent FCC and \u03b3-Al2O3 were used, CO was the main compound in the non-condensable gaseous stream, which is evidence that decarbonylation reactions prevailed rather than decarboxylation ones. The higher extent of cracking reactions when spent FCC and \u03b3-Al2O3 catalysts were used was also evidenced in the higher yield of CH4 and C2-C4 hydrocarbons in the volatile stream. However, in the case of olivine, the yield of CO2 was higher than that of CO and the other gaseous compounds due to the basic nature of this mineral, which enhanced ketonization and aldol condensation reactions leading to the formation of CO2 and water [79]. Besides, the highest H2 yield was observed in the experimental runs conducted with the olivine guard bed. Thus, the chemical composition of olivine, which contains Fe0 on its surface, promotes reforming and WGS reactions [73,75].The detailed bio-oil composition obtained in each experimental run is shown in Table 4\n. The inert nature of steam as fluidizing agent in the biomass pyrolysis at 500\u00a0\u00b0C has been demonstrated in previous studies [26,80], and has been confirmed in this one by comparing the results obtained when steam was used as fluidizing agent (see Tables 3 and 4) with those reported for N2\n[49]. Besides, this inert nature of steam is a great advantage for process viability, as the cost of the gases is reduced and the problems related to inert gas separation prior to the catalytic reforming step are avoided. However, the use of conditioning beds significantly modified the composition of the bio-oil products. As aforementioned, all conditioning catalysts were located in a fixed bed reactor at 600\u00a0\u00b0C (above that of pyrolysis), which may also have certain influence on the bio-oil composition.In comparison with the bio-oil obtained in the pyrolysis at 500\u00a0\u00b0C, the use of inert sand led to a significant drop in the amount of light alcohols, saccharides, and mainly in that of the phenolic fraction (particularly the catechol fraction), as a result of thermal cracking reactions. Moreover, the concentration of polycyclic aromatic alcohols increased from 0.27 to 2.47\u00a0wt%, respectively. The acidity of the spent FCC and \u03b3-Al2O3 conditioning catalysts (Table 2) promoted deoxygenation reactions, as well as cracking, oligomerization, alkylation, isomerization, cyclization and aromatization, leading to a considerable increase in the hydrocarbon fraction (to 6.12 and 8.49\u00a0wt%, respectively). Significant differences were observed in the distribution of the components in the phenolic fraction, with catechols being the major components with the FCC conditioning bed, whereas the phenolic fraction obtained with the Al2O3 catalyst only contained alkyl-phenols. The higher selectivity of the Al2O3 catalyst for the production of alkyl-phenols revealed the effective dealkoxylation of guaiacols and cathecols [81].In the case of the olivine conditioning bed, a decrease in the acid and furan fractions was observed, with the ketone fraction remaining almost constant. Thus, the basic nature of this material promoted the ketonization of acids over basic catalysts, as well as the aldol condensation of small ketone and aldehyde molecules to larger chain ketones by carbon\u2013carbon coupling reactions [79,82]. A reduction in the guaiacol fraction, and therefore in the overall phenolic fraction was also evidenced, whereas the yield of hydrocarbons (mainly naphthalene compounds) increased because of secondary cracking reactions.The effect of using conditioning catalysts prior to the reforming step has been analyzed, i.e., the influence the composition of the volatile stream fed into the reforming bed has on the catalyst activity and stability. Accordingly, the evolution of the volatile conversion (Fig. 2\n) and product yields (Fig. 3\n) was monitored with time on stream based on the following main reactions:Oxygenate steam reforming:\n\n(6)\nCnHmOk+(n-k)H2O\u00a0\u2192\u00a0nCO + (n\u00a0+\u00a0m/2-k)H2\n\n\n\nWater Gas Shift (WGS):\n\n(7)\nCO\u00a0+\u00a0H2O\u00a0\u2194\u00a0CO2\u00a0+\u00a0H2\n\n\n\nOxygenate cracking (secondary reaction):\n\n(8)\nCnHmOk\u00a0\u2192\u00a0oxygenates\u00a0+\u00a0hydrocarbons\u00a0+\u00a0CH4\u00a0+\u00a0CO\u00a0+\u00a0CO2\u00a0+\u00a0C\n\n\nMethane (and hydrocarbons) steam reforming:\n\n(9)\nCH4\u00a0+\u00a0H2O\u00a0\u2194\u00a0CO\u00a0+\u00a03H2\n\n\n\n\nFig. 2 shows that the volatiles derived from biomass pyrolysis are completely reformed independently of the stream composition at the inlet of the reforming catalyst bed, which is explained by the high activity of the commercial Ni/Al2O3 (G90) catalyst.It is to note that the volatile stream reaching the reforming catalyst in any experimental run is made up of a complex mixture of oxygenate compounds. Most of the research studies dealing with the mechanisms involving reforming reactions and/or catalyst deactivation have been conducted with model compounds and synthetic mixtures simulating bio-oil and tar. However, different compounds reactivity has been reported depending on whether they are reformed alone or in a mixture of different oxygenated compounds [83]. Therefore, detailed and laborious studies are required to analyse, on the one hand, the reactivity of the compounds in the biomass pyrolysis volatile stream modified by the use of conditioning catalysts (due to the high amount of species contained and their interactions) and, on the other hand, their further contribution to catalyst deactivation.As observed in Fig. 2, the experiments conducted without any conditioning catalyst showed a stable catalyst performance for the first 50\u00a0min on stream (conversion values up to 99.8 %), and it then decreased to 56.6 % after 86\u00a0min on stream, which is evidence of the deactivation of the catalyst. The runs conducted with inert sand as guard bed showed similar results of conversion, which decreased to 58.2 % for 87\u00a0min on stream. Despite the differences obtained in the volatile composition when no conditioning catalyst and inert sand were used (see Table 4), similar catalytic performance in the reforming step was observed. It seems that, in the pyrolysis-reforming experiment without any conditioning material, the volatile stream leaving the pyrolysis reactor undergoes thermal cracking reactions in parallel with catalytic ones on the reforming catalyst, with the thermal cracking leading to a volatile composition similar as the one obtained with inert sand.Regarding the experiments conducted using olivine guard bed, almost full conversion was attained for the first 30\u00a0min on stream, and it then gradually decreased to 51.6 % for 98\u00a0min on stream. However, despite the fact that the reforming catalyst begins to lose activity earlier when there is an olivine guard bed than without any conditioning catalyst or inert sand, its deactivation rate is significantly lower, which leads to greater stability over time. This fact is explained by the different nature of the volatile stream to be reformed. The presence of more refractory compounds in the volatile stream when olivine was used as conditioning catalyst, namely the hydrocarbon fraction (1.06\u00a0wt%), led to a faster initial loss of activity. Although the hydrocarbon fraction\u2019s reactivity is low for reforming reactions, its low concentration did not involve a fast catalyst deactivation with time on stream. Other authors have also reported a higher reactivity of oxygenated compounds derived from biomass pyrolysis in comparison with the hydrocarbon compounds due to the presence of C\u2550O bonds that enhance the formation of carbon oxides in the reforming step [84,85]. The lower amount of phenolic compounds when olivine was used, especially the guaiacol fraction, resulted in a greater stability over time. The lower aldehyde fraction than with inert sand may also contribute to attenuating catalyst deactivation. Thus, several authors have reported that the main coke precursors, and therefore the main responsible for catalyst deactivation are aldehydes, phenols, and saccharides [83,86]. Gayubo et al. [86] state phenolic compounds (as well as aldehydes) are the main contributors to catalyst deactivation by coke formation. Valle et al. [87] analyzed the influence the presence of phenolic compounds has on catalyst stability in the reforming of raw bio-oil. Accordingly, they approached the removal of these phenolic compounds from the raw bio-oil by accelerated aging and liquid\u2013liquid extraction methods. They observed that catalyst deactivation was lower than when raw bio-oil was used in the steam reforming, since phenols removal from the bio-oil significantly reduced coke content. Ochoa et al. [88] associated the composition of oxygenate compounds in the reaction medium with the composition of the coke formed using Fourier transform infrared (FTIR) spectroscopy. They concluded that methoxyphenols (guaicols) and levoglucosan (saccharides) have greater influence on coke formation than acids, ketones and aldehydes.The performance of the commercial Ni/Al2O3 (G90) catalyst when the spent FCC guard catalyst was used showed full conversion for the first 30\u00a0min on stream, and then sharply decreased attaining a conversion value of 38.2 % after 76\u00a0min on stream. This fast catalyst deactivation is mainly associated with the high concentration of the phenolic fraction, especially the high amount of catechol compounds (11.56\u00a0wt%). Besides, the high concentration of polycyclic aromatic alcohols (mainly composed of indenol and naphthalenol derived compounds) may also contribute to the fast catalyst deactivation in the reforming step. These results are consistent with those obtained in other literature studies. Thus, Artetxe et al. [47] investigated the steam reforming of different tar model compounds (phenol, toluene, methyl naphthalene, indene, anisole and furfural), and reported that the lowest conversion was attained when phenol was used as model compound. Trane-Restrup and Jensen [89] studied the steam reforming of cyclic model compounds in the bio-oil (2-methylfuran, furfural, and guaiacol) over Ni-based catalysts, revealing that the phenolic compound guaiacol was the most difficult to convert to synthesis gas. Likewise, Wang et al. [90] reported the lower reactivity of phenol over a Ni based catalyst compared to other oxygenated compounds, such as furfural, hydroxyacetone or acetic acid.The high concentration of hydrocarbons (6.12\u00a0wt%) in the bio-oil as a result of the acidity of the FCC catalyst also influenced the reforming reaction. This hydrocarbon fraction is mainly composed of polycyclic aromatic hydrocarbons (PAHs), namely, indene (0.12\u00a0wt%), naphthalene (2.40\u00a0wt%), fluorene (0.74\u00a0wt%), anthracene (0.65\u00a0wt%) and phenanthrene (1.89\u00a0wt%). Thus, several authors have reported the lower reactivity of large cyclic hydrocarbons with higher molecular weights than oxygenate compounds in the steam reforming reactions [47,84]. Regarding hydrocarbon reactivity in steam reforming reactions, several studies in the literature report lower reactivity of naphthalene than other bio-oil model compounds like toluene, benzene, pyrene or anthracene [41,91,92]. In fact, these aromatic hydrocarbons are well known because they undergo condensation reactions to form coke, which accelerates catalyst deactivation [93,94].The best catalytic performance was observed when \u03b3-Al2O3 was used as conditioning catalyst, with a stable volatile conversion for the first 30\u00a0min on stream and then decreasing to 39 % subsequent to 112\u00a0min on stream. Similarly to other conditioning catalysts, the initial loss of catalyst activity occurred earlier than in the runs conducted without conditioning bed or with silica sand, which is a consequence of the total acidity of the \u03b3-Al2O3, as it promotes secondary cracking reactions leading to a higher concentration of olefins and aromatic hydrocarbon compounds. However, although there is a high concentration of this hydrocarbon fraction (the highest one is obtained when Al2O3 is used as conditioning bed), these compounds do not involve a sharp decrease in volatile conversion. The reduction in the concentration of phenols in the volatile stream to be reformed, particularly that of guiacol and catechol fractions, significantly attenuated the Ni/Al2O3 (G90) catalyst deactivation. Total removal of acids and saccharides and a significant reduction in the aldehyde fraction may also contribute to attenuating the fast deactivation of the reforming catalyst. Several authors have reported that the main coke precursors, and therefore the main responsible for catalyst deactivation, are aldehydes, phenols, and saccharides [83,86].The great differences in the performance of the reforming catalysts when the spent FCC catalyst and \u03b3-Al2O3 conditioning catalysts were used are mainly due to the differences observed in the composition of phenolic compounds. Thus, whereas alkyl-phenols (15.96\u00a0wt%) are only obtained with the \u03b3-Al2O3, catechols are the major fraction with the FCC conditioning bed (11.56\u00a0wt%). A comparison of the reforming performance when olivine and Al2O3 conditioning beds are used reveals that both total concentration of the phenolic fraction and component distribution in this faction influence the performance of the reforming catalyst. Accordingly, the presence of guaicols, and especially catechols led to a fast catalyst deactivation in the reforming step.Despite the stability of G90 catalyst is greatly improved when an Al2O3 conditioning catalyst is used, the rapid deactivation of the reforming catalyst will entail working under reaction-regeneration cycles when the operation is performed at large scale.\nFig. 3 shows the evolution of the yields of gaseous product and non-converted bio-oil compounds with time on stream for the experiments conducted without conditioning (Fig. 3a), and with conditioning catalysts (Fig. 3b-e). As observed, H2 and CO2 decreased with time on stream in all cases due to the lower extension of reforming, Eqs. (6) and (9), and WGS reaction, Eq. (7), as the catalyst is being deactivated. The runs conducted without conditioning catalyst and inert sand showed a similar trend in the evolution of gaseous product yields, which is evidence of the similarities in the volatile stream reaching the reforming bed catalyst in both experiments.When olivine and spent FCC catalysts were used, the evolution with time on stream followed a similar trend as conversion, with a stable H2 and CO2 yield for the first 30\u00a0min. Regarding the FCC guard bed, a sharp decrease in H2 and CO2 yields was observed due to the attenuation of the reforming and WGS reaction (Eqs. (6) and (7), respectively) as the catalyst was being deactivated. In the case of the \u03b3-Al2O3 conditioning bed, the yields of H2 and CO2 decreased from the beginning of the reaction. Thus, H2 yield decreased from 95.9 to 28.1 % and CO2 from 93.9 % to 33.1 % for 112\u00a0min on stream.In all cases, the deactivation rate of the catalyst is faster as the concentration of the non-converted bio-oil compounds in the reaction medium is higher. In fact, an autocatalytic deactivation behaviour has been reported in the reforming of biomass pyrolysis volatiles [58,95]. These results clearly show that the oxygenated compounds, particularly the phenolic fraction (mainly catechols) produced in the pyrolysis of biomass using FCC conditioning bed, cause a much faster deactivation, which reveals that these compounds are the main coke precursors.The results obtained at zero time on stream for H2 production (based on the biomass mass unit in the feed) revealed a high performance of the overall pyrolysis-reforming strategy, with values between 9.5 and 10.2\u00a0wt%. Similar H2 production values were reported in a previous study conducted under the same experimental conditions, but using a fluidized bed reactor in the reforming step [26]. Xiao et al. [96] studied the pyrolysis-reforming of pinewood chips on a Ni/coal char catalyst, obtaining a H2 production of 10\u00a0wt% at a reforming temperature of 650\u00a0\u00b0C. Ma et al. [97] reported a H2 production value of 7.6\u00a0wt% in a three-step process (biomass pyrolysis, gas\u2013solid simultaneous gasification and catalytic reforming) using a Ni/MgO commercial catalyst. Akubo et al. [98] investigated the pyrolysis-catalytic steam reforming of six agricultural biomass waste samples obtaining a H2 production in the 3.3\u20135.1\u00a0wt% range.As concerns CO yield, a slightly decreasing trend is observed in all the experimental runs, which is a consequence of a balance involving its production by the reforming reaction (decreasing with time on stream), Eq. (6), formation by cracking reactions, Eq. (8), and deactivation of the catalyst for the WGS reaction, (Eq. (7)). As the reaction proceeded, the yields of CH4 and light hydrocarbons increased slightly, which is evidence of cracking reactions, although to a minor extent (CH4 yields lower than 2 %) due to the operating conditions used in the reforming reactor, i.e., rather low temperature and residence time.With the aim of evaluating the influence of different conditioning beds on the pyrolysis volatile composition and their relationship with catalysts performance and deactivation, the cokes deposited on the guard catalysts as well as on the commercial Ni/Al2O3 (G90) catalyst have been characterized by temperature programmed oxidation (TPO). The main causes of catalyst deactivation in the reforming processes are metal sintering and coke deposition [99]. However, previous pyrolysis-reforming studies conducted by the research group using the commercial Ni/Al2O3 (G90) catalyst evidenced that metal sintering did not occur [100]. Thus, the low reforming temperature used in this study (600\u00a0\u00b0C) is slightly higher than Ni Tamman temperature (590\u00a0\u00b0C), and metal sintering is therefore avoided. Accordingly, coke deposition is the main responsible for catalyst deactivation.TPO analyses have been conducted to the guard materials in order to ascertain the influence textural properties and their characteristic features have on the coke deposited. It should be noted that the coke is formed due to the contact of the volatile stream with these materials prior to reaching the reforming catalyst. However, due to the different duration of the runs (which depends on the stability of the G90 catalyst in the reforming step), and in order to compare the amount of coke deposited on each conditioning catalyst, the average coke deposition rate per biomass mass unit has been defined as follows:\n\n(10)\n\n\n\n\n\n\nr\n\n\n-\n\n\n\n\ncoke\n\n\n=\n\n\n\n\nW\n\n\ncoke\n\n\n/\nt\n\n\n\n\nW\n\n\ncatalyst\n\n\n\n\n\nm\n\n\nbiomass\n\n\n\n\n\n\n\nwith Wcatalyst and Wcoke being the catalyst and coke masses, respectively, mbiomass the biomass mass flow rate in the feed and t the reaction time in each run.The results of coke content and average coke deposition rate per biomass mass unit are set out in Table 5\n, and the TPO profiles are shown in Fig. 4\n.As observed in Table 5, significant differences are observed in the results for the conditioning catalysts used. Thus, the runs conducted with silica sand and olivine led to a negligible amount of coke deposition (0.13 and 0.38\u00a0wt%, respectively), which corresponds to average coke deposition rates per biomass mass unit of 0.02 and 0.05 mgcoke ggc\n-1 gbiomass\n-1. The limited porous structure of these materials (see Table 2) hindered coke deposition. Their characteristic features, i.e., the inert nature of silica sand, and the basic nature of olivine, as well as the capability of the latter to enhance reforming reactions, may contribute to attenuating coke deposition.As concerns \u03b3-Al2O3 and spent FCC conditioning catalysts, the amounts of coke deposited and the average coke deposition rates per biomass mass unit were considerable higher (0.98 and 2.04 mgcoke ggc\n-1 gbiomass\n-1, for FCC and \u03b3-Al2O3, respectively). It is well-established in the literature that the acid properties of these materials promote the formation of coke deposits due to dehydration, cracking and polymerization reactions, which take place on the acid sites [22,71,94,101,102]. Accordingly, the higher total acidity of the \u03b3-Al2O3 compared to the FCC catalyst (See Table 2) led to a higher amount of coke deposited on this conditioning bed. The selective coke deposition on the former conditioning catalyst surface attenuates the subsequent coke formation on the reforming catalyst, and therefore improves its stability. Thus, the coke precursors are deposited on the acid sites of Al2O3 conditioning catalyst, leading to a volatile stream less prone to coke formation on the G90 reforming catalyst surface. Moreover, the mesoporous structure of these materials favors coke deposition on their surface. Similarly, Li et al. [93] analyzed the main reaction pathways occurring in the catalytic cracking of different bio-oil model compounds (acetic acid, cyclopentanone and guaicol). They reported a higher coke production when guaicol was used, and ascribed it to the coke chemical structure and hydrogen to carbon effective ratios of the feedstock. Besides, these authors describe coke formation as a sequence of polymerization and polycondensation reactions involving bulky aromatic compounds formed in the catalytic cracking of guaicol, which lead to carbon deposits on the catalyst surface.The textural properties of the fresh and spent conditioning catalysts are also shown in Table 5. As observed, the coke deposited on the conditioning materials also influenced their textural properties. In the case of inert sand and olivine, no significant differences were observed prior and subsequent to use in the pyrolysis-reforming runs due to the limited porous structure of these materials. However, a sharp decrease in the SBET area was observed for the FCC catalyst (from 81.3 to 15.1\u00a0m2 g\u22121) due the full blockage of the pores, especially the micropores of the HY zeolite structure. Concerning the \u03b3-Al2O3 catalyst, the specific surface area and average pore diameter decreased from 100.6 to 83.9\u00a0m2 g\u22121 and from 16.9 to 12.3\u00a0nm, respectively, which is evidence of the partial blockage of the pores in this catalyst.\nFig. 4 shows the TPO profiles of all conditioning catalysts, wherein significant differences are revealed concerning their coke combustion temperature. As regards the \u03b3-Al2O3 catalyst, one main peak located at 475\u00a0\u00b0C is observed, whereas the main peak in the profile of the spent FCC catalyst is located at 535\u00a0\u00b0C. Thus, cokes of different nature are deposited, with the one deposited on the \u03b3-Al2O3 catalyst being more hydrogenated (higher proportion of aliphatic compounds than aromatic ones), whereas that on the FCC catalyst has a more structured aromatic composition [103,104].\nTable 6\n shows the amounts of coke deposited (CC) and the average coke deposition rates per biomass mass unit fed (rC) on the deactivated Ni/Al2O3 catalysts when different conditioning beds are used prior to the reforming. As in the previous section, this average coke deposition rate has been determined by Eq. (10). As observed, a similar average coke deposition rate was obtained in the experiments conducted without conditioning catalyst and inert sand (0.67 and 0.66 mgcoke gcat\n-1 gbiomass\n-1, respectively), which is consistent with their similar evolution of conversion and product yields with time on stream (Figs. 2 and 3, respectively).The poor performance of the reforming process observed in Fig. 2 when the FCC catalyst was used is supported by the high average coke deposition rate obtained (0.70 mgcoke gcat\n-1 gbiomass\n-1). Thus, the composition of the volatile stream reaching the reforming catalyst is responsible for this coke formation rate, and therefore for catalyst deactivation. Accordingly, the high concentration of phenolic compounds (catechols) in the volatile stream when the FCC guard bed is used increases the coke deposition rate.The lowest average coke deposition rate was obtained when \u03b3-Al2O3 was used as guard catalyst, which is consistent with the better performance observed in Fig. 2. The amount of coke deposited on the conditioning catalyst has an influence on the amount of coke deposited on the reforming catalyst. As aforementioned, the higher acidity of the \u03b3-Al2O3 favored coke promoters deposition on its surface (see Table 5), and so decreased the coke formation rate on the subsequent Ni/Al2O3 (G90) catalyst used in the reforming step. Besides, the composition of the volatile stream to be reformed, with a high fraction of hydrocarbons and the phenolic one containing only alkyl-phenols, attenuates coke deposition. Thus, several authors have reported that oxygenated compounds are more prone to form carbon deposits than aromatic hydrocarbons [47,84].Regarding the experiments conducted with the olivine conditioning catalyst, a similar average coke deposition rate as in the runs without conditioning bed and with silica sand (0.67 mgcoke gcat\n-1 gbiomass\n-1) was observed on the reforming catalyst. Thus, although the conversion of pyrolysis volatiles decreased faster during the initial minutes on stream due to the presence of refractory compounds in the volatile stream, a similar coke formation rate was observed on the Ni/Al2O3 reforming catalyst.It should be noted that use of a fixed bed regime in the reforming step may lead to severe coke formation on both the conditioning and the reforming catalysts, and therefore to operational problems, such as bed plugging [60,105].\nFig. 5\n shows the TPO profiles of the deactivated commercial Ni/Al2O3 catalyst when different conditioning beds were used prior to the pyrolysis-reforming process. In these profiles, two main peaks can be distinguished in all the catalyst samples, with the first one located at 435\u00a0\u00b0C (coke I), and the second one at 525\u00a0\u00b0C (coke II). The different combustion temperatures observed are closely related to the location and/or composition of the coke deposited. Accordingly, the lower combustion temperature (<475\u00a0\u00b0C) is ascribed to the coke deposited on Ni metallic sites (encapsulating coke with an amorphous nature). This coke fraction hinders the access of reactants to the active sites due to Ni particle encapsulation, and is therefore the main responsible for catalyst deactivation. Besides, this type of coke (coke I) is more hydrogenated, has a higher content of aliphatic compounds than the other one at higher temperature and is stemmed from the decomposition of oxygenates derived from biomass pyrolysis and the re-polymerization of phenolic compounds [58]. The peak located at the higher temperature (coke II) has been related to the coke deposited on the support aside from Ni active sites (and therefore, having less influence on catalyst deactivation), and is composed of highly ordered and condensed aromatic compounds [94,100,106]. Moreover, the SEM images shown for the deactivated G90 catalysts in Fig. S2 in the Supplementary Information, in which no specific morphology of the coke formed is observed, confirm the results obtained by TPO analysis.In the case the spent FCC catalyst is used as guard bed, the TPO profile of the Ni/Al2O3 catalyst reveals a higher proportion of coke I than coke II, which is evidence that it promotes the formation of encapsulating coke, thereby hindering the access of reactants to the metallic sites. Therefore, although coke content has a great influence on catalyst deactivation, coke structure and location also play an essential role on deactivation [94,107]. A minor peak appears at 660\u00a0\u00b0C in the TPO profile of the G90 catalyst used without any conditioning bed. As aforementioned, G90 catalyst is doped with Ca, and previous studies show that this high combustion temperature peak must be ascribed to the decomposition of CaCO3, which is formed by carbonation of the CaO contained in the commercial G90 catalyst [48]. Furthermore, all the conditioning catalysts used in the pyrolysis-reforming process led to a small peak at 610\u00a0\u00b0C, which arose from the thermal cracking of hydrocarbons or oxygenates in the reaction medium [94].Improvements have been carried out in an integrated reaction system for H2 production from biomass consisting of a conical spouted bed reactor for the fast pyrolysis and an in-line fixed bed reactor for the steam reforming of the oxygenate volatile stream. Thus, it has been proven that the provision of low cost conditioning catalysts (\u03b3-Al2O3, spent FCC catalyst and olivine) prior to the reforming reactor allows tempering the volatile stream, and therefore modifying oxygenate composition, which enables the attenuation of the fast deactivation of the reforming catalyst (G90).Coke deposition is the main cause of catalyst deactivation, which leads to the blockage of the Ni active sites. Phenolic compounds in the oxygenate stream, and especially the presence of guaiacols and catechols, have a considerable influence on coke formation due to the repolymeration of these compounds on the Ni sites. This coke has been identified by TPO analysis due to its low combustion temperature.Based on the results and features of the conditioning catalysts, it has been proven that the high total acidity of \u03b3-Al2O3 (with a high density of centers and moderate acid strength) is suitable for the selective cracking of the volatile fraction responsible for coke formation. Thus, although the initial H2 production decreases when \u03b3-Al2O3 is used as conditioning catalyst, the stability of the reforming G90 catalyst is enhanced, and therefore a longer duration of the reaction stage is feasible.Although the deactivation of the catalyst is notably attenuated with this strategy, the deactivation by coke deposition of the conditioning catalyst is also observed. Consequently, the scalability of this two-step pyrolysis-reforming process provided with a conditioning step will require regeneration strategies for both the G90 reforming catalyst and the \u03b3-Al2O3 conditioning catalyst.\nEnara Fernandez: Investigation, Visualization, Writing \u2013 review & editing. Laura Santamaria: Writing \u2013 original draft, Visualization, Writing \u2013 review & editing. Maite Artetxe: Writing \u2013 original draft, Conceptualization, Writing \u2013 review & editing, Visualization, Supervision, Project administration, Funding acquisition. Maider Amutio: Conceptualization, Writing \u2013 review & editing, Visualization, Supervision, Project administration, Funding acquisition. Aitor Arregi: Validation, Visualization, Writing \u2013 review & editing. Gartzen Lopez: Conceptualization, Validation, Writing \u2013 review & editing, Visualization, Supervision, Project administration. Javier Bilbao: Writing \u2013 review & editing, Visualization, Supervision, Project administration, Funding acquisition. Martin Olazar: Writing \u2013 review & editing, Visualization, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was carried out with the financial support of the grants RTI2018-101678-B-I00, RTI2018-098283-J-I00 and PID2019-107357RB-I00 funded by MCIN/AEI/ 10.13039/501100011033 and by \u201cERDF A way of making Europe\u201d and the grants IT1218-19 and KK-2020/00107 funded by the Basque Government. Moreover, this project has received funding from the European Union\u2019s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 823745.Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2021.122910.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The fast deactivation of the reforming catalyst greatly conditions H2 production from biomass. In order to alleviate this problem, use of conditioning catalysts in a previous conditioning step has been proposed to modify the pyrolysis volatile stream reaching the reforming catalyst. The experimental runs have been conducted in a two-step reactor system, which includes a conical spouted bed reactor for the continuous pinewood sawdust pyrolysis and an in-line fixed bed reactor made up of two sections: the conditioning and the reforming steps. Biomass fast pyrolysis was conducted at 500\u00a0\u00b0C and the reforming step at 600\u00a0\u00b0C. Different conditioning beds (inert sand, \u03b3-Al2O3, spent fluid catalytic cracking (FCC) catalyst and olivine) were used for the conditioning of biomass pyrolysis volatiles and the influence their composition has on the performance and deactivation of a commercial Ni/Al2O3 reforming catalyst has been analyzed.\n                  Considerable differences were noticed between the conditioning catalysts, with the reforming catalyst stability decreasing as follows depending on the type of material used: \u03b3-Al2O3\u00a0>\u00a0olivine\u00a0>\u00a0inert sand \u2248 no guard bed\u00a0>\u00a0spent FCC catalyst.\n                  The high acidity of \u03b3-Al2O3 (with a high density of weak acid centers) is suitable for the selective cracking of phenolic compounds (mainly guaiacol and catechol), which are the main precursors of the coke deposited on the Ni active sites. Although H2 production is initially lower, the reforming catalyst stability is enhanced. These results are of uttermost significance in order to step further in the scaling up of the in-line pyrolysis-reforming strategy for the direct production of H2 from biomass.\n               "}
{"full_text": "Global anthropogenic CO2 emissions were reduced about 8% (2.6 Gt of CO2) in 2020, as a result of the Covid-19 crisis [1]. This fact, although results from lockdown measures and economy slowdown, may turn into the starting point from which CO2 emissions progressively decline in the future if adequate actions are taken. According to IEA, governments have now the chance to accelerate the transition into a more resilient and cleaner energy system, while rebooting their economies and creating new jobs. Making the right investments, the economic growth can work together with a sustainable recovery plan, which might lead to air pollution emissions decrease of 5% by 2023 [2]. This plan, among other objectives, contemplates: (i) accelerating the installation of low carbon energy sources (such as renewable wind and solar PV) along with the expansion and modernisation of electricity grids; (ii) turn fuels production and utilization more sustainable; and (iii) boost innovation in crucial technology areas including hydrogen, batteries, CO2 utilisation and small modular nuclear reactors. In this context, Power to Gas (PtG) process is presented as an interesting alternative. This process targets the production of synthetic natural gas (SNG) through the catalytic conversion of renewably produced H2 and CO2 from flue gases according to the Sabatier reaction: CO2 + 4H2 \u21c4 CO2 + 2 H2O (\u0394H\u00b0 = - 165 kJ mol\u22121). Thus, CO2 is used as raw material instead of being emitted as a waste and renewable energy is stored in form of a low-carbon fuel such as SNG or methane. Besides, as H2 is produced via water electrolysis in low electricity demand periods, renewable power is better exploited, which promotes its development and expansion. The produced CH4 can be easily stored or widely distributed in the current gas grid and, afterwards, can be used again for power and heat generation in private homes, mobility sector or industry [3,4].The complete hydrogenation of CO2 into methane (popularly named as CO2 methanation) is a process with considerable kinetic limitations (8 electron reduction) which can only be achieved with a suitable catalyst; commonly, supported Ni or Ru highly dispersed over a basic mesoporous support. In recent years, Ni and Ru catalysts with increasingly smaller and, a priori, more active metallic particles have been designed mostly due to advances in nanomaterials synthesis techniques, which allow increasing the surface/volume ratio and the number of active sites [5,6]. The reduction of particle size not only leads to higher metallic surface areas but also to changes in particle\u2019s morphology, which according to its structure sensitivity could modify Turnover Frequency (TOF) numbers. It has been reported that low coordinated Ni nanoparticles contain more surface defects that act as surface hydrogen traps facilitating its dissociation and improving Ni specific activity [7]. On the contrary, other authors have reported that, in the case of Ru catalysts, low coordinated or monolayer sites induce lower CO2 methanation rates than larger nanoclusters, since they suffer from poisoning by the adsorption of stable carbonyls during reaction [8\u201310]. In order to obtain small particles or change their structure, diverse preparation methods have been alternatively employed, such as Incipient Wetness Impregnation (IWI) [11], one-pot Evaporation-Induced Self-Assembly (EISA) [12], Microwave-Assisted (MA) [13], Deposition-Precipitation (DP) [14], Co-Precipitation (CP) [15] and polyol method [16] or equivalent Glycerol Assisted Impregnation (GAI) [17].The main disadvantage of Ni catalysts with respect to those of Ru is their considerably lower activity at low temperature due to their inferior H2 dissociation capacity [18]. Instead, the main drawback of Ru catalysts is their exorbitant price. Nevertheless, designing appropriate Ni-Ru bimetallic systems could be a solution to balance those handicaps. Generally, these bimetallic catalysts are known to exhibit better catalytic properties compared to their monometallic counterparts such as higher conversion, fewer side reactions (selectivity) and more stability due to a synergistic effect [19\u201321]. This synergy happens as a result of specific electronic interactions and geometric positional relationships between the two metals (combination of \u201cligand\u201d and \u201censemble\u201d effects) [20]. By adding an appropriate secondary metal (Ru) to the catalytic formulation, the electronic properties of the main metal (Ni) are usually altered leading to changes in reagents adsorption and reaction intermediates formation. These changes, in turn, can modify the reaction pathway and the activation energy so that the activity of the catalyst is improved [21]. Recently, Ni-Ru bimetallic systems have proven to be very effective specifically for CO methanation [22\u201325]. Liu et al. [26] also reported enhanced catalytic activity for CO2 methanation over 10Ni-1Ru-2CaO/Al2O3 formulation due to a significant increase in H2 and CO2 chemisorption capacities, whereas Wei et al. [27] did not achieve activity improvement by adding Ru to Ni-zeolite (13X and 5A) catalysts. However, the analysis of Ni-Ru systems in terms of physicochemical and catalytic properties for CO2 methanation has not been the focus of many systematic studies so far.Regarding CO2 methanation reaction mechanism, several studies have been carried out by means of Operando FTIR or DRIFTS with the aim of determining the reaction intermediates and elementary reaction steps over supported Ni [20,28\u201330] and Ru [9,10,16,31] catalysts. Although there is still controversy regarding the identification and the role or place of some adsorbed species in the reaction pathway, two widely accepted routes have been proposed: the so-called dissociative and associative mechanisms [32]. The former assumes the dissociative adsorption of CO2 into adsorbed CO or carbonyl followed by its hydrogenation into CH4. In the latter, by contrast, CO2 is molecularly adsorbed in form of carbonates or bicarbonates, which are progressively reduced by H spillover into formate, methoxy species and, finally, methane [33]. Noteworthy, CO2 methanation mechanism over bimetallic catalysts has scarcely been studied.The main goal of this work has been to sequentially improve the low temperature activity of the conventional and industrial Ni/Al2O3 formulation by modifying the preparation method and incorporating a secondary metal, such as Ru. Additionally, this work has aimed to identify which factors are responsible for such improvement. Firstly, the influence of Glycerol Assisted Impregnation (GAI) method on the dispersion and structural characteristics of Al2O3-supported Ni and Ru particles was examined. These materials were catalytically compared with equivalent ones prepared by the conventional Incipient Wetness Impregnation method (IWI). After that, the effect of Ru incorporation on the physicochemical properties and catalytic performance of Ni/Al2O3 formulation was studied. To our knowledge, we pioneer operando FTIR analysis of CO2 methanation reaction on Ni-Ru bimetallic system, identifying the type and evolution of reaction intermediates and determining the roles of both Ni and Ru in the reaction pathway.For this work, a series of alumina-supported Ni and Ru catalysts as well as bimetallic Ni-Ru/Al2O3 samples were prepared. The two pairs of monometallic Ni and Ru catalysts were obtained by two synthesis procedures that consisted of the following steps: impregnation and calcination. According to the first procedure, the metal (Ni or Ru) nitrate solution was incorporated into Al2O3 support by Incipient Wetness Impregnation (IWI) and the resulting catalyst precursor was calcined in a muffle under air (uncontrolled atmosphere). IWI method as well as calcination procedure was the same as followed and explained in detail in our previous work [34]. However, in the second synthesis route, the metal solution is introduced by Glycerol Assisted Impregnation (GAI) method and the precursor is calcined under a controlled atmosphere. The GAI method, which was developed by Gudyka et al. [17], also consists of the typical dry impregnation but employs a glycerol/water solution as solvent instead of bare H2O. In our case, a 30 wt% C3H8O3/water solution was used. After impregnation, samples were dried overnight and calcined ex situ in a tubular reactor under 50 mL min\u22121 of 20 %H2/N2 (controlled atmosphere) at 550 \u00b0C for 2 h (with 10 \u00b0C min-1 heating rate). In both cases, the required amounts of Ni(NO3)2\u00b76H2O (Sigma Aldrich, 99.99 %) and Ru(NO)(NO3)3\n(Sigma Aldrich, Ru = 1.5 % w/v) precursors were employed in order to attain 12 wt% Ni and 3 wt% Ru nominal metal contents and the calcination temperature was chosen according to thermogravimetric results of catalysts precursors. These four catalysts were labelled according to their composition and preparation method as follows: NiAlIWI, RuAlIWI, NiAlGAI and RuAlGAI.On the other hand, once results of monometallic catalysts were analysed, three additional bimetallic catalysts were prepared by GAI method varying the Ru content from 0.5 to 1.5 wt%. In all cases, the nominal Ni content was set at 12 wt% and small amounts of Ru were incorporated by co-impregnation. After that, samples were also dried overnight at 120 \u00b0C and calcined under the same conditions described above. These samples were named Ni-xRuAl, where variable x represents the Ru content (0.5, 1.0 or 1.5 wt%).In order to determine the suitable calcination temperatures, thermogravimetric analysis was carried out in a Setaram Setsys Evolution apparatus connected in series with a Pfeiffer Prisma mass spectrometer (TGA-MS). In all cases, around 100 mg of catalyst precursor was placed in a 30 \u03bcL Al2O3 crucible and was firstly dried in situ at 125 \u00b0C. After that, the temperature was increased from 125 to 625 \u00b0C with 5 \u00b0C min\u22121 heating rate and continuously recorded along with mass loss. The catalysts precursors prepared by IWI method were calcined under 50 mL min\u22121 oxidative stream (5% O2/He), whereas the ones prepared by GAI were analysed under reductive atmosphere (5% H2/Ar). The exit gas stream composition was analysed by mass spectrometry following the 16 (CH4), 17 (NH3), 18 (H2O), 28 (CO/N2), 30 (NO), 44(CO2/N2O) and 46 (NO2) mass signals.The textural properties and crystalline phases of the supported catalysts were determined by N2 physisorption and X Ray Diffraction (XRD). The protocols for these analyses are detailed in the former work [34].The micrographs of the monometallic catalysts were obtained by a TECNAI G2 20 TWIN microscope which operates at 200 kV and is equipped with a LaB6 filament, EDAX-EDS microanalysis system and Transmission Electron Microscopy (TEM). The micrographs together with elemental maps of the bimetallic catalysts, instead, were taken by a FEI Titan Cubed G2 60\u2013300 microscope with much higher resolution. This microscope is equipped with a high-brightness X-FEG Schottky field emission electron gun, monochromator, CEOS Gmbh spherical aberration corrector and Super-X EDX system with High-Angle Annular Dark-Field (HAADF) detector for Z contrast imaging in Scanning Transmission Electron Microscopy (STEM) configuration. All powder samples were mixed with ethanol solvent and kept in and ultrasonic bath for 15 min in order to attain a good suspension. After that, a drop of suspension was spread onto a TEM copper grid (300 mesh) covered by a holey carbon film for each sample. Finally, the grids were dried under vacuum to remove the solvent. The particle size distribution of monometallic catalysts was determined by measuring the diameter (d) of at least 200 particles. After that, the mean metal dispersion (DMe\n) was estimated applying the d-FE model [35] as follows:\n\n(1)\n\n\n\nD\nMe\n\n(\n%\n)\n=\n\n\n5.01\n\nd\n\na\nt\n\n\n\n\u2211\nj\n\n\n\nn\nj\n\n\nd\nj\n2\n\n+\n2.64\n\nd\n\na\nt\n\n0.81\n\n\n\u2211\nk\n\n\n\nn\nk\n\n\nd\nk\n2.19\n\n\n\n\n\n\n\u2211\ni\n\n\n\nn\ni\n\n\nd\ni\n2\n\n\n\n\n\u00d7\n100\n\n\n\nwhere, di, dj\n and dk\n are the diameters of the \u201ci\u201d, \u201cj\u201d and \u201ck\u201d particles, ni\n is the number of particles with diameter di\n, nj\n is the number of particles with diameter dj\n (dj > 24\u00b7dat.), nk is the number of particles with diameter dk\n (dk\n \u2264 24\u00b7dat) and dat. is the atomic diameter of Ni or Ru.The resistance against oxidation of catalysts prepared by GAI method was determined by three consecutive RedOx cycles in a Micromeritics AutoChem 2920 apparatus. Previously, the samples were exposed to a 50 mL min\u22121 stream of 5%H2/Ar in order to reduce the passivated nickel layer. For each RedOx cycle, 15 oxidative pulses (5 cm3 of 5%O2/He) were injected followed by another 15 reductive pulses (5 cm3 of 5%H2/Ar). Note that between pulse injections an inert gas stream of He or Ar was continuously fed depending on the step (oxidative or reductive, respectively). The resistance to oxidation of NiAlGAI and Ni-1.0RuAl catalysts was measured at 325 \u00b0C, while that of RuAlGAI catalyst at 550 \u00b0C. The temperatures were chosen considering that Ni and Ru are oxidized at around 300 \u00b0C and 500 \u00b0C, respectively [19]. The resistance against oxidation, defined as the cycle reversibility, was calculated by the following expression:\n\n(2)\n\n\nReversibility of cycl\n\ne\ni\n\n(\n%\n)\n=\n\n\n\nn\ni\n\n(\nN\n\ni\n0\n\n)\n\n\n\nn\ni\n\n(\nN\n\ni\n2+\n\n)\n+\n\nn\n\ni\n\u2212\n1\n\n\nrem\n.\n\n\n(\nN\n\ni\n2+\n\n)\n\n\n\u00d7\n100\n\n\n\nwhere, ni\n(Ni2+) are the moles of Ni oxidized in cycle i, \n\n\nn\n\ni\n\u2212\n1\n\n\nrem\n.\n\n\n\n\nN\n\ni\n2+\n\n\n\n\n are the moles of Ni that remain oxidized from previous cycles and ni\n(Ni0) are the moles of Ni reduced or recovered in cycle i. Note that ni\n(Ni2+) as well as ni\n(Ni0) were calculated from total O2 and H2 uptakes of oxidative and reductive steps, respectively.Hydrogen Temperature Programmed Desorption (H2-TPD) experiments were also performed on a Micromeritics AutoChem 2920 apparatus. These experiments allowed us determining the hydrogen chemisorption capacity as well as chemisorption strength of monometallic and bimetallic catalysts. In a first step, the metal surface of samples was reduced and cleaned up by 5%H2/Ar gas stream at 500 \u00b0C for 30 min and then cooled down to 50 \u00b0C. After that, a 50 mL min\u22121 stream of pure hydrogen was fed long enough for complete adsorption or saturation (around 1 h). Subsequently, catalysts were flushed out with Ar for 30 min in order to remove physisorbed H2. Finally, the desorption was conducted increasing temperature up to 850 \u00b0C at 10 \u00b0C min\u22121 heating rate. From integration of TPD profiles at T < 450 \u00b0C, the metallic surface area (S\nNi) of NiAlGAI catalyst was estimated according to this equation:\n\n(3)\n\n\n\nS\nNi\n\n\n\n\nm\n2\n\n\ng\n-1\n\n\n\n=\n\n\n\nN\nA\n\n\n\n\nV\nm\n\n\n\n\u00d7\n\nV\n\ndes\n.\n\n\n\u00d7\nS\nF\n\u00d7\na\nt\n\nA\nNi\n\n\n\n\nwhere, Na is the Avogadro number, V\nm is the molar volume in cm3 mol\u22121, Vdes.\n is the volume in cm3 of desorbed H2 per gram of catalyst, SF is the stoichiometric factor and atA\nNi is the effective atomic area of Ni. In this work, a SF (Ni/H2) of 2 and atA\nNi of 6.49 \n\u00d7\n 10-20 m2 atom\u22121 were assumed.CO2 methanation reaction was performed in a downstream fixed bed reactor (ID =9 mm). In all cases, the stainless-steel reactor was loaded with 0.5 g of catalyst particles (dp\n = 300\u2212500 \u03bcm), which were diluted to 50 % (v/v) with quartz particles in order to avoid hot spots. The Ni and Ru catalysts prepared by IWI were firstly reduced at 500 and 400 \u00b0C for 1 h with 20 % H2/He, respectively. The samples prepared by GAI were also reduced but at 250 \u00b0C in order to remove the passivated oxide layer formed by having been in contact with air. After cooling down the samples to 200 \u00b0C with He (inert gas), the temperature was raised up to 400 \u00b0C in steps of 25 \u00b0C under reactant stream. This gaseous mixture was composed of 16 % CO2 and 64 % H2 (H2/CO2 = 4), balanced up to 100 % with He (total flow of 250 cm3 min\u22121). The outlet gas stream composition was analysed by GC (Agilent 7890B) once steady state was reached at each temperature. H2, He, CH4 and CO concentrations were monitored by MolSieve type columns, while that of CO2 by HayeSep type column. The produced water was retained by a Peltier cooling module upstream of the gas chromatograph to avoid molecular sieve column degradation. All reactions were carried out at atmospheric pressure and at WHSV of 30,000 mL h\u22121 gcat\n\u22121.The catalytic performance was evaluated by CO2 conversion (\n\n\nX\n\nC\n\nO\n2\n\n\n\n\n), CH4/CO products selectivity (\n\n\nS\n\nC\n\nH\n4\n\n\n\n\n or \n\n\nS\nCO\n\n\n) and yield (\n\n\nY\n\nC\n\nH\n4\n\n\n\n\n and \n\n\nY\nCO\n\n\n) which were calculated from reactor inlet and outlet molar flows according to the following equations:\n\n(4)\n\n\n\nX\n\nC\n\nO\n2\n\n\n\n\n%\n\n=\n\n\n\nF\n\nC\n\nO\n2\n\n\nin\n\n\u2212\n\nF\n\nC\n\nO\n2\n\n\nout\n\n\n\n\nF\n\nC\n\nO\n2\n\n\nin\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(5)\n\n\n\nS\n\nC\n\nH\n4\n\n\n\n\n%\n\n=\n\n\n\nF\n\nC\n\nH\n4\n\n\nout\n\n\n\n\nF\n\nC\n\nO\n2\n\n\nin\n\n\u2212\n\nF\n\nC\n\nO\n2\n\n\nout\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(6)\n\n\n\nS\nCO\n\n\n%\n\n=\n\n\n\nF\nCO\nout\n\n\n\n\nF\n\nC\n\nO\n2\n\n\nin\n\n-\n\nF\n\nC\n\nO\n2\n\n\nout\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(7)\n\n\n\nY\n\nC\n\nH\n4\n\n\n\n\n%\n\n=\n\nX\n\nC\n\nO\n2\n\n\n\n\u00d7\n\n\n\nS\n\nC\n\nH\n4\n\n\n\n\n100\n\n=\n\n\n\nF\n\nC\n\nH\n4\n\n\nout\n\n\n\n\nF\n\nC\n\nO\n2\n\n\nin\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(8)\n\n\n\nY\nCO\n\n\n%\n\n=\n\nX\n\nC\n\nO\n2\n\n\n\n\u00d7\n\n\n\nS\nCO\n\n\n100\n\n=\n\n\n\nF\nCO\nout\n\n\n\n\nF\n\nC\n\nO\n2\n\n\nin\n\n\n\n\u00d7\n100\n\n\n\nwhere Fi\n is the inlet or outlet molar flow of component \u201ci\u201d in mol s\u22121.Finally, the Turnover Frequency (TOF) numbers, which indicate the number of CO2 molecules converted per second and per active site, were calculated as follows:\n\n(9)\n\n\nT\nO\n\nF\n\nM\ne\n\n\n\n\n\ns\n-1\n\n\n\n=\n\n\n\u2212\n\nr\n\nC\n\nO\n2\n\n\n\n\n\nmol C\n\nO\n2\n\n\ng\n\ncat\n.\n\n-1\n\n\ns\n-1\n\n\n\n\n\n\nS\n\nM\ne\n\n\n\n\nmol Me\n\ng\ncat\n-1\n\n\n\n\n\n=\n\n\n\nF\n\nC\n\nO\n2\n\n\nin\n\n\u00d7\n\nX\n\nC\n\nO\n2\n\n\n\n\u00d7\nM\n\nW\n\nM\ne\n\n\n\n\nW\n\u00d7\n\nD\n\nM\ne\n\n\n\u00d7\n\nF\n\nM\ne\n\n\n\n\n\n\n\nwhere MW is the mass weight of the metal in g mol\u22121, W is the catalyst weight in g, DMe\n is the metallic dispersion and FMe\n is the mass fraction of metal in the catalyst.\nOperando FTIR spectra were collected using an IR cell from In-Situ Research Instruments, coupled to a Nicolet 6700 spectrometer equipped with a MCT detector and using a spectra resolution of 4 cm\u22121. Powdered samples were pressed at 1.5 tons into 10 mg cm-2 wafers which, prior to the experiments, were in situ activated/reduced at 500 \u00b0C for 1 h under a 5% H2/Ar flow of 20 mL min\u22121. After pretreatment, wafers were cooled down under Ar flow to 150 \u00b0C, being background spectra collected every 25 \u00b0C. CO2 adsorption tests were carried out by exposing samples to a 20 mL min\u22121 stream of 5% CO2/Ar, whereas in CO2 methanation experiments a 5% CO2: 20 % H2: 75 % Ar gas mixture was used. In both cases, experiments were carried out in two steps. Firstly, the used gas mixture was stabilised during 30 min and a series of spectra were collected at 0, 1, 3, 5, 10, 15 and 30 min. Secondly, temperature programmed adsorption (CO2/Ar flow) or temperature programmed surface reaction (TPSR, CO2/H2/Ar flow) was run from 150 to 450 \u00b0C using a heating rate of 2 \u00b0C min\u22121. Note that the depicted spectra were obtained by subtraction of those recorded under reaction/adsorption conditions every 25 \u00b0C and those corresponding to backgrounds.To determine how catalysts precursors are decomposed and the temperature required for their complete calcination, thermogravimetric analysis (TGA) was carried out (Fig. 1\n). Additionally, the gaseous products from precursors calcination were analysed by a mass spectrometer connected at the exit of the thermobalance (Figs. S1 and S2, supplementary material). Fig. 1a and b show both TG and dTG profiles of supported Ni catalysts precursors calcined under oxidative (5% O2/He, IWI catalyst) and reductive (5% H2/Ar, GAI catalyst) atmospheres, respectively. In general, the mass loss takes place in different consecutive steps that can be identified by the dTG profiles. In the case of NiAlIWI precursor calcined under O2/He (Fig. 1a), the dTG profile presents a main mass loss rate peak at 265 \u00b0C and two shoulders at 200 and 350 \u00b0C. The first shoulder can be attributed to structural water desorption from Al2O3 or water released during dehydration steps of nickel precursor (Ni(NO3)2\u00b76H2O), whereas the broad peak and the second shoulder are due to nitrate decomposition/oxidation into NOx (NO and NO2), as confirmed by MS signals (Fig. S1b). Mass loss is observed up to 475 \u00b0C approximately, suggesting that a calcination temperature of 500 \u00b0C is enough for the complete precursor decomposition into NiO/Al2O3.The TG profile of the NiAlGAI precursor (Fig. 1b) is somewhat different due to the presence of an organic compound which could be a metal alkoxide from coordination nickel cations (Ni2+) with glycerol solution [5,6,17]. In this case, the dTG profile shows 4 differentiated negative peaks among 125 and 400 \u00b0C. In agreement with MS spectra (Fig. S1d), the first one at 170 \u00b0C could be attributed to NO3\n\u2212 reduction into NO and the next two, centered at 290 and 325 \u00b0C, to the reduction of the organic template. It can be suggested that the glycerolate is decomposed into smaller molecules (such as ethylene glycol and ethanol) and surface carbon by hydrogenolysis reactions. In fact, the last mass loss rate peak centered at 360 \u00b0C matches with the appearance of methane (m/z = 15) in the product stream, suggesting that the remaining surface carbon is being reduced. In this case and according to the TG profile, a calcination temperature of 550 \u00b0C is needed to complete NiAlGAI precursor reduction.Regarding Ru/Al2O3 precursors, the TGA profiles of their respective calcinations are shown in Fig. 1c and d. By comparing those figures with the above described, it can be observed that the mass loss profile of RuAlIWI precursor (Fig. 1c) is similar to that of NiAlIWI (Fig. 1a). In fact, the same calcination steps are identified and confirmed by MS spectra (Fig. S2b): a first peak at 205 \u00b0C due to water release followed by a more intense negative peak together with a shoulder at 335 \u00b0C, which are attributed to nitrate and nitrosyl groups oxidation into NOx. Although the precursor is completely removed at 450 \u00b0C, a calcination temperature somewhat lower (400 \u00b0C) was employed in order to avoid excessive growing of RuO2 crystallites [34]. Finally, TGA profiles of RuAlGAI precursor calcined under 5%H2/Ar are shown in Fig. 1d. Note that the dTG profile presents a broad band which could be divided into 3 negative peaks at 185, 255 and 320 \u00b0C, which correspond to several calcination steps. According to MS spectra (Fig. S2d), nitrate groups and organic compounds are partially reduced and water is released as product in a first step (negative peak at 190 \u00b0C). In a second step (from 250 to 350 \u00b0C), the organic compound continues being reduced and carbon monoxide (m/z = 28) is observed in the products stream. Additionally, a small broad peak can be appreciated at around 540 \u00b0C. This peak matches with methane appearance from hydrogenation of remaining surface carbon. In this case, a temperature of 550 \u00b0C was used for precursor calcination. It must be highlighted that in all cases the observed total mass loss is similar to that expected for complete calcination of catalyst precursors: 19.9 vs. 17.2 % for NiAlIWI, 23.5 vs. 27.1 % for NiAlGAI, 10.9 vs. 9.9 % for RuAlIWI and 18.9 vs. 19.6 % for RuAlGAI.Once catalysts precursors were calcined according to TGA results, the resulting catalysts were characterized by several techniques. Some physicochemical properties are shown in Table 1\n. It should be noted that the metal content of all catalysts is close to the nominal, indicating that Ni and Ru were successfully incorporated by the two methods (IWI and GAI). In addition, the high specific surface area and pore volume of all catalysts indicate that the textural properties of starting \u03b3-Al2O3 (SBET\n = 214 m2 g\u22121 and Vpore\n = 0.563 cm3 g\u22121) were not considerably affected by the different impregnation and calcination processes. As expected, supported Ni catalysts presented lower SBET\n and Vpore\n than RuAl ones, mainly due to their higher metal content. On the other hand, the catalysts prepared by GAI method exhibited slightly lower values of such textural properties than those prepared by IWI, probably due to the higher calcination temperature.In regard to XRD analysis of reduced catalysts (not shown), both elemental Ni (XRD peaks at 2\u03b8 = 44.5, 51.8 and 76.4 \u02da) and Ru (XRD peaks at 2\u03b8 = 38.4, 42.2 and 44 \u02da) were clearly identified on NiAlGAI and RuAlIWI samples, respectively. However, broad and low-intensity peaks of Ni0 and no peaks of Ru0 were detected in NiAlIWI and RuAlGAI XRD patterns, suggesting that the crystalline phases are better dispersed than on NiAlGAI and RuAlIWI catalysts. This fact was confirmed by crystallite size calculation according to Scherrer equation (\u03c4, Table 1).The effect of the preparation method on the morphology as well as on the particle size distribution was determined by TEM. In addition, the mean metal dispersion and metal surface area (Table 1) were calculated by d-FE model [35]. The micrographs of Ni catalysts along with their corresponding particle size distribution histograms are displayed in Fig. 2\n. In both cases, quasi-spherical supported Ni particles (circled in yellow) were observed. It can be appreciated that the particle size distribution (calculated from measurement of at least 200 particles) is wider in the case of the sample prepared by GAI method. In fact, NiAlIWI catalyst presents Ni particles sizes from 2 to 10 nm, whereas the distribution of NiAlGAI sample ranges from 3 to 19 nm. In this line, the average particle sizes are 5.8 nm (D\nNi = 19.8 %) and 11.2 nm (D\nNi = 11.5 %) for NiAlIWI and NiAlGAI catalysts, respectively. Note that these values are in agreement with crystallite sizes estimated by XRD, indicating that the active phase is better dispersed on NiAlIWI catalyst. However, this catalyst presents a reduction degree of 38 % at 500 \u00b0C, i.e., less than the half of total nickel is reduced before the reaction, as determined in our previous work [34]. For that reason, the Ni reactive surface area is slightly higher for the catalyst prepared via GAI method (see Table 1).Such differences in dispersion and amount of reducible nickel are related with the calcination step. In the case of NiAlIWI catalyst, the precursor is calcined in air favouring mainly the formation of NiO highly interacting with the support or even NiAl2O4 inert phase. After reduction treatment at 500 \u00b0C, small and well distributed Ni particles are obtained but not all nickel is reduced due to the high metal-support interaction observed by H2-TPR. This high interaction between NiO and Al2O3, which was extensively studied in the literature [36,37], is also confirmed by examination and comparison of several TEM micrographs: far fewer Ni particles are visualized on NiAlIWI than on NiAlGAI catalyst, indicating a lower Ni reduction extent. On the other hand, the NiAlGAI precursor is calcined under reductive atmosphere (20 % H2/N2), avoiding the formation of Ni2+ species able to react with \u03b3-Al2O3 and assuring that all nickel will be reduced after the preparation procedure. Besides, the presence of non-volatile organic compounds apparently prevents Ni crystals from excessive growing. As the temperature increases during the calcination, it seems that incipient nickel nanocrystals are embedded in an organic matrix that acts as a barrier preventing them from sintering [17]. As a result, all Ni is reduced and quite well dispersed in form of 11 nm size particles. Noteworthy, Ding et al. [38] observed a similar Ni particle size distribution for a Ni/SiO2 prepared by the glycerol assisted impregnation and reported that glycerol resulted to be the best alkanol solvent among those studied.Analogously, Fig. 3\n shows TEM micrographs together with particle size histograms of monometallic Ru/Al2O3 samples. In both catalysts, Ru particles with different morphology were easily visualized (within yellow circles or rectangles). Ruthenium was homogeneously dispersed in form of spherical particles on RuAlGAI while a much more heterogeneous distribution was verified on RuAlIWI. The latter presents both oval and hexagonal Ru particles or even aggregates formed by several particles. In this case, the particle size distribution seems to be quite affected by the preparation method. On one side, RuAlIWI catalyst has an unimodal particle size distribution with a long tail ranging from 4 to 32 nm and a corresponding average particle size of 14.8 nm (D\nRu = 7.2 %). On the contrary, the particle size distribution of RuAlGAI sample, shown in Fig. 3b, is symmetric and much narrower. It should be noted that this catalyst presents an average particle size of 2.7 nm, which correspond to a dispersion of 34.4 %. These results clearly indicate that GAI is a more appropriate method to disperse Ru over Al2O3.In our former studies based on thermo-XRD results, we reported that RuO2 crystals tend to grow fast and agglomerate under oxidative calcination conditions due to the formation of volatile RuOx [34]. That would explain why bigger particles and so long tail are observed in the histogram of the catalyst prepared by IWI method. This fast growth is clearly avoided by GAI method, which includes a non-oxidative calcination. Furthermore, even more uniform and smaller particles are created due to the organic enclosing effect above explained. Yan et al. [10] obtained similar metallic dispersion (DRu = 32.2 %) in a 3%Ru/Al2O3 prepared by incipient wetness impregnation of Ru(III) acetylacetonate precursor and performing the calcination treatment under 10 %H2/Ar flow. As a result, the RuAlIWI catalyst contains a Ru surface area of 0.79 m2 g\u22121 while that of RuAlGAI is 3.90 m2 g\u22121.In a final step, the catalytic performance of the catalysts was evaluated in order to determine the effect of the preparation method on activity. Fig. 4\n shows the CO2 conversion (above) along with product selectivity (below) as a function of the reaction temperature for Ni/Al2O3 and Ru/Al2O3 catalysts, respectively. As previously reported [18], Ru-based catalysts were more active than Ni-based ones due to the higher ability of the former to dissociate hydrogen at lower temperature. Thus, the catalytic activity order is as follows: RuAlGAI > RuAl IWI > NiAlGAI > NiAlIWI. The activity profiles of Ni/Al2O3 samples are not so different, as shown in Fig. 4a. In both cases, the CO2 conversion (reaction rate) increases exponentially with temperature from 225 \u00b0C (onset reaction temperature) to 325 \u00b0C and then, this increase slows down as the reagents are depleted and equilibrium conversion is approached. It must be noted that the CO2 conversion is slightly higher for NiAlGAI catalyst in the studied temperature range, resulting in a T\n50 (temperature at which 50 % of CO2 conversion is obtained) only 5 \u00b0C lower. However, a more significant difference can be observed in selectivity (Fig. 4b): NiAlIWI produces more CO than NiAlGAI catalyst at mild temperatures (T \u2248 300 \u00b0C), although never more than 3.5 % of converted CO2. In fact, the CO selectivity of NiAlIWI at 300 \u00b0C is around 2.5 times higher than that of NiAlGAI catalyst (3.2 vs. 1.3 %). The small amount of carbon monoxide is produced either from reverse water gas shift (RWGS) or reforming reactions.On the other hand, the higher CO2 conversion and CH4 selectivity observed for NiAlGAI catalyst are probably related to a higher Ni surface area (8.25 vs. 5.62 m2 g\u22121). This hypothesis was supported by calculations of TOFs at 250 \u00b0C. Note that by definition, TOF assumes that reaction takes place at any point of metal surface. However, under CO2 methanation conditions, the partial H2 pressure is at least four times higher than that of CO2, which disfavors the adsorption of the latter. Consequently, metal particles will be largely covered by H2. Also, considering that the support (\u03b3-Al2O3 in this study) is able to adsorb or active CO2, it can be assumed that CO2 methanation takes place at the perimeter of metal-support interface rather than on surface, as reported in a previous work [39]. Therefore, for more realistic comparison, TOF was normalized with respect to interfacial length or perimeter (TOF/I\n0, Table 1). The total metal-support perimeter per metal surface area (I0\n) was calculated by Eq. (10), which was proposed by Kourtelesis et al. [40] and is based on developments reported by Duprez et al. [41].\n\n(10)\n\n\n\nI\n0\n\n\n\n\nm\ninterface\n\n/\n\nm\nMe\n2\n\n\n\n=\n\n\n\nS\nMe\n2\n\n\u00d7\n\u03b2\n\u00d7\n\n\u03c1\nMe\n\n\u00d7\nA\n\nW\nMe\n\n\n\n\nN\nA\n\n\u00d7\na\nt\n\nA\nMe\n\n\n\n\n\n\nwhere, S\nMe is the metallic surface area in m2 gMe\n\u22121, \u03b2 is a particle shape factor (33.3 for hemispherical particles), \u03c1\nMe is the density of the metal in g m-3, AW\nMe is the metal atomic weight of the metal in g mol\u22121, NA is the Avogadro number, and atA\nMe is the area occupied by a single metal atom (6.49\u00b710-20 m2 Ni atom-1 and 6.13\u00b710-20 m2 Ru atom-1). It can be observed that TOF/I0\n values are of the same order of magnitude, suggesting that the CO2 methanation rate per metal atom at the interface for supported catalysts with average Ni particle perimeters of 18.2 nm (NiAlIWI catalyst) and 35.2 nm (NiAlGAI catalyst) is quite similar. Recently, the structure sensitivity of CO2 methanation over supported metals has been studied by various authors. For instance, Vogt at al. [29] clearly reported structure sensitive CO2 methanation over Ni/SiO2 catalysts with small particle sizes ranging from 1 to 6 nm, concluding that the more active Ni atoms are those forming clusters of 2\u22123 nm. The high TOF of these clusters was attributed to an intermediate adsorption strength of CO on Ni, which was reported to be a reaction intermediate of CO2 methanation on Ni/SiO2 catalyst. However, Beierlein et al. [14] demonstrated that the specific activity does not depend on Ni particle size within a range from 6 to 91 nm, observing a linear correlation between the activity and Ni surface area and concluding that CO2 methanation on highly loaded Ni/A2O3 catalysts is a structure insensitive reaction. Therefore, it seems that structure sensitivity clearly depends on the range of Ni particle size studied as well as the metal-support combination used. In our case, the results are in agreement with the findings of the second authors, since the observed specific activity barely increase when decreasing particle size from 11 to 6 nm.Analogously, the light-off and selectivity curves of Ru/Al2O3 catalysts are displayed in Fig. 4c and d. In this case, the onset temperature for both samples is 200 \u00b0C and the equilibrium CO2 conversion is reached at the same temperature (\n\n\nX\n\n\n\nC\nO\n\n\n2\n\n\n\n\n\n at 400 \u00b0C = 82 %). Nevertheless, the increase in CO2 conversion with temperature for RuAlGAI is faster than for RuAlIWI catalyst, which leads to a notable 20 \u00b0C left shift of the light-off curve (i.e., superior activity at low temperature). Regarding the selectivity towards CH4, it was higher than 99.5 % in the range of studied temperatures and only trace amounts of CO in terms of ppm were detected for RuAlGAI catalyst (Fig. 4d). Considering that metal particles of RuAlGAI are five times smaller than that of RuAlIWI catalyst, one could expect a bigger difference in catalytic performance. This suggests that metal-support interface of the former is less active, as revealed by TOF/I0\n values also summarized in Table 1. Note that TOF/I0\n value is around one order of magnitude lower for RuAlGAI catalyst, suggesting that CO2 methanation is structure sensitive on RuAl catalysts. Indeed, a lower specific methanation activity on small Ru particles or clusters had already been reported by several authors [8\u201310]. According to them, CO formation via r-WGS is favoured rather than CO2 methanation on atomically dispersed or low coordinated small Ru particles. Despite that fact, a considerable T\n50 value gradient of 20 \u00b0C is observed, which evidences that a more active catalyst is obtained by GAI method.Monometallic Ni and Ru catalysts prepared by Glycerol Assisted Impregnation (GAI) achieved better methanation activity compared to those prepared by incipient wet impregnation. In a second step, bimetallic Ni-based catalysts with small Ru contents (< 2 wt%) were prepared following the GAI coimpregnation procedure. The physicochemical properties of NiAlGAI and bimetallic catalysts (Ni-0.5RuAl, Ni-1.0RuAl and Ni1.5RuAl) are shown in Table 2\n.As observed, the metal contents determined by ICP are very close to the nominal values, indicating that no relevant amount of metal was lost during the synthesis. Interestingly, the specific surface area slightly increases with Ru content: 5, 9 and 10 %, respectively. This unexpected trend can be explained by analysing the pore size distribution of the catalysts (Fig. S3, supplementary material). The monometallic catalyst (NiAlGAI) presents a narrow unimodal mesopore size distribution centered at 7.3 nm, whereas bimetallic catalysts exhibit wider and bimodal distributions with maxima between 6 and 10 nm. As already discussed, NiAlGAI catalyst presents similar particles with sizes probably above 7 nm, which partially or completely block the mesopores of the support. However, the bimodal distribution verified for bimetallic catalysts might be due to the presence of particles with well differentiated size or morphology, which might penetrate into the small pores of \u03b3-Al2O3. Ru incorporation widens the distribution but decreases its intensity, which finally results in a slight increase of SBET\n from 168 to 175 m2 g\u22121 and similar pore volume of 0.42 cm3 g\u22121. Thus, introduction of Ru makes some improvement in textural properties of Ni/Al2O3 catalyst.XRD analysis was also performed for bimetallic catalysts (not shown). However, no characteristic peaks of both metals were detected (crystallites sizes < 5 nm). This observation is in agreement with N2 physisorption results and indicates that Ni and Ru are finely dispersed.Concerning catalysts\u2019 resistance against oxidation, NiAlGAI and Ni-1.0RuAl samples were exposed to three consecutive RedOx cycles at 325 \u00b0C. Each RedOx cycle consisted of feeding 15 oxidative pulses (5 cm3 of 5%O2/He) followed by another 15 reductive pulses (5 cm3 of 5%H2/Ar). On that way, the effect of O2, fed in a concentration similar to that typically presented in flue gases, on the catalysts was estimated and their reversibility was determined. Note that the resistance to oxidation of 3RuAlGAI catalyst was measured at 550 \u00b0C in order to ensure that its oxidation was effective. The reversibility values of NiAlGAI and Ni-1.0RuAl catalysts, defined as the percentage of Ni reduced per cycle after sample being exposed to 15 oxidative pulses (Eq. 2), are shown in Fig. 5\n.It can be clearly observed that the reversibility values of the bimetallic catalyst are superior to those of monometallic one in all cycles, observing the major difference in the third cycle: 60 vs. 42 %, respectively. This indicates that incorporation of Ru provides higher resistance to oxidation and/or higher capacity to recover the reductive state than the monometallic NiAlGAI. The observed higher reversibility is due to ruthenium role as promotor of nickel reduction in the bimetallic catalyst, i.e., H2 is firstly dissociated on Ru surface and then can migrate to neighbouring NiO particles facilitating their reduction [23]. Furthermore, it can be noticed that the reversibility of monometallic catalysts decreases from 52 % (cycle 1) to 42 % (cycle 3), while that of bimetallic catalyst remain stable around 60 %. Although the decrease from second to third cycle is not so pronounced (- 2%), it seems that the reversibility value of NiAlGAI sample could keep decreasing in further consecutive cycles due to a progressive formation of NiO that is no longer able to be reduced by remaining Ni\u00b0. The fact that reversibility of bimetallic system is apparently stable, suggests that Ni particles are near to and surrounded by Ru ones, which avoids or at least slows down the formation of non-reversible NiO particles. However, as reported by Rynkowski et al. [19], the presence of Ru does not prevent the formation of spinel type oxides at long term and high temperatures. Finally, it must be highlighted that the H2 uptake was around 2 times the O2 uptake at 550 \u00b0C for RuAlGAI catalyst, suggesting, as expected, 100 % reversibility.On the other hand, the hydrogen adsorption capacity was determined by TPD. Thus, H2-TPD profiles of the samples are depicted in Fig. 6\n.It can be observed that all profiles exhibited two bands, before and after 450 \u00b0C. While the band below 450 \u00b0C can be generally attributed to H2 chemisorbed on metal particles (type I), the one at higher temperature is associated with H2 desorption from the subsurface alumina layers or with the spillover phenomenon (type II) [42]. Indeed, the H2-TPD profile of bare \u03b3-Al2O3 does not show any signal variation below 400 \u00b0C but an intense band at higher temperature, which might be related to a dehydroxylation process (Fig. 6). Likewise, the band at low temperatures can be divided into several peaks. For instance, the monometallic NiAlGAI catalyst, presents a main peak at 375 \u00b0C and additional H2 desorption below 250 \u00b0C. According to Ewald et al. [4], the main peak corresponds to hydrogen chemisorbed on Ni surface while the TCD signal at low temperatures can be ascribed to hydrogen adsorbed on the corners of large Ni particles or on better dispersed particles. Noteworthy, the main peak position shifts towards lower temperatures and its intensity increases with Ru content, suggesting that the amount of exposed Ni atoms grows accordingly. Such increase in Ni dispersion was also reported by other authors who incorporated Ru [26], Cr [12] or Fe [21]. The amounts of desorbed H2 calculated from TPD profiles integration are summarized in Table 2. Note that this parameter duplicates with co-impregnation of 1.5 % Ru on Ni/Al2O3 formulation, i.e., the fraction of exposed metal notably rises. Accordingly, the ability to supply dissociated hydrogen under methanation reaction conditions remarkably increases with the Ru content. Based on H2 desorption data, Ni dispersion on the monometallic catalyst was also estimated, resulting a value of 7.9 % (11.5 % by TEM, Table 1). In the case of bimetallic catalysts, dispersion cannot be estimated since exposed atoms of both Ni and Ru, in major and minor extent respectively, contribute in the total H2 desorption below 450 \u00b0C. Anyway, the amount of desorbed hydrogen compared to that of NiAlGAI catalyst is more than twice for Ni-1.5RuAl catalyst and hence, this suggests that its metal surface could be around double.In order to determine the morphology, size and distribution of the supported bimetallic particles, HAADF-STEM analysis was conducted. The high-angle Z-contrast annular field imaging together with EDX mapping allowed us differentiating between two or more elements, such as Al (Z = 13), Ni (Z = 28) and Ru (Z = 44). STEM micrographs together with EDX maps of bimetallic catalysts are shown in Fig. 7\n. It can be observed that Ni (red coloured) and Ru (green coloured) are homogenously dispersed as individual spherical particles, which means that no alloy is formed during the calcination at 550 \u00b0C [26]. Noteworthy, the Ni-0.5RuAl catalyst presents an average Ni particle size of 7.4 nm (calculated from around 50 particles), 4 nm lower than that obtained for monometallic NiAlGAI catalyst. This parameter is even lower for Ni-1.0RuAl and Ni-1.5RuAl, with values of 6.3 and 5.9 nm, respectively. Therefore, Ni particle size is lowered by increasing the amount of co-impregnated Ru. Regardless the metal loading (0.5, 1.0 or 1.5 %), the Ru particle size resulted to be around 4\u22125 nm for all bimetallic catalysts. Note that some of these particles are located near to Ni ones, especially for catalysts with higher Ru contents (see Fig. 7b and c). The fact that Ni and Ru particles are next to each other or in intimate contact is in agreement with the enhanced reducibility observed by H2-TPR: the neighbour Ru particle acts as H supplier via spillover mechanism favouring the reduction of Ni2+ [23].As is in the case of monometallic catalysts, Ni dispersion on bimetallic catalysts was also estimated by d-FE model and the results are summarized in Table 2. As already observed by H2-TPD, the Ni dispersion is significantly enhanced with Ru loading. In fact, Ni dispersion increases 9.4, 15.6 and 20.0 % by adding 0.5, 1.0 and 1.5 % of Ru, respectively. This behaviour might indicate that both Ru and glycerol solvent act as structural promoters during the calcination process, avoiding the excessive growing or sintering of Ni. Based on the characterization results properly discussed above, it is expected that Ni/Al2O3 catalysts performances are improved with the incorporation of small percentage of Ru in the formulation.Thus, once bimetallic catalysts were characterized and the effect of Ru on physicochemical properties of Ni/Al2O3 determined, their catalytic performance was studied. The conversion-temperature as well as the selectivity-temperature curves of bimetallic catalysts are shown in Fig. 8\n. For comparison purposes, the light-off curves obtained for NiAlGAI and RuAlGAI catalysts are also displayed. It can be clearly observed that the addition of increasing amounts of co-impregnated Ru leads to a notable increase of the sigmoid curve slope, especially at mild temperatures (from 275 to 325 \u00b0C). Accordingly, the T\n50 value is lowered 40 \u00b0C by only co-impregnating 1.5 %Ru, which indicates that the presence of Ru considerably improves the activity of Ni/Al2O3 formulation. Although different trends are observed depending on the temperature, all catalysts exhibit selectivity to CH4 higher than 98.5 %. The slightly lower \n\n\nS\n\nC\n\nH\n4\n\n\n\n\n (or higher CO production) observed for bimetallic catalysts at low temperature compared to that of NiAlGAI catalyst may be related to some desorption of CO from low coordinated and inactive Ni and Ru particles. Even so, the methane yield clearly increases with Ru content, being the productivity order at 300 \u00b0C as follows: Ni-1.5RuAl (\n\n\nY\n\nC\n\nH\n4\n\n\n\n\n = 51 %) > Ni-1.0RuAl (\n\n\nY\n\nC\n\nH\n4\n\n\n\n\n = 44 %) > Ni-0.5RuAl (\n\n\nY\n\nC\n\nH\n4\n\n\n\n\n = 32 %) > NiAlGAI (\n\n\nY\n\nC\n\nH\n4\n\n\n\n\n = 20 %). It should be noted that Ni-1.0RuAl catalyst (T\n50 = 305 \u00b0C) shows almost the same activity as 3RuAlGAI catalyst, whose noble metal content is three times higher.According to the characterization results, co-impregnation of Ru increases Ni dispersion. Besides, the presence of small Ru particles close to Ni ones considerably improves reducibility and hydrogen chemisorption capacity of nickel. Under reaction conditions, this leads to a greater amount of dissociated H2, which is an essential reaction intermediate, and hence to a superior activity. Thus, the great enhancement observed in the catalytic performance can be attributed to a synergistic effect between Ni and Ru, as also reported by Liu et al. [26].The catalytic behaviour of alumina supported Ni and Ru catalysts proved to be stable for 24-h-on stream and at stoichiometric feed ratio in the former work [34]. Then, in order to accelerate the aging of the catalyst, the stability of monometallic NiAlGAI and bimetallic Ni-1.0RuAl catalyst was evaluated for 50 h-on-stream under harsher reaction conditions: at 325 \u00b0C (far for equilibrium conversion) and under sub-stoichiometric feed ratio (H2/CO2 = 3). Noteworthy, the activity of NiAlGAI catalyst resulted to be stable during the evaluated period, observing CO2 conversion values within 35 and 37 %, as shown in Fig. 9\na. This indicates that the catalyst did not suffer from any type of deactivation such as particle sintering or poisoning [4] even though more CO was produced (Y\nCO = 1.33 %) as consequence of sub-stoichiometric feed. Besides, the CH4 selectivity also remained stable, observing values within 96.3 and 97.1 %.In the case of the bimetallic catalyst, the stability test also included three wet periods (t =2 h) in which increasing amounts of water (10, 20 and 30 mL/min) were fed interspersed by dry periods. It can be observed that, before 25 h-on-stream, the catalytic performance remained stable as observed for NiAlGAI catalyst, obtaining CO2 conversion and CH4 selectivity average values of 60 and 99 %. However, the feed of increasing amounts of water, led to a CO2 conversion drop of around 3 (\n\n\ny\n\n\nH\n2\n\nO\n\n\n\n = 0.04), 6 (\n\n\ny\n\n\nH\n2\n\nO\n\n\n\n = 0.08) and 9% (\n\n\ny\n\n\nH\n2\n\nO\n\n\n\n = 0.12) without a remarkable CH4 selectivity decrease (0.1, 0.25 and 0.5 %). This behavior indicates that water is strongly adsorbed on part of active sites, temporally rendering them unavailable for the reaction. Nevertheless, the activity was completely recovered when switching to dry conditions, indicating that water adsorption or inhibition effect is reversible at short term. Thus, based on the above activity and stability results, it can be concluded that glycerol assisted impregnation is a viable catalyst preparation method.Although it has been shown that bimetallic catalyst have enhanced catalytic properties based on characterization as well as activity results, the individual roles of both Ni and Ru on the CO2 methanation reaction mechanism are not clear yet. Such roles, as well as the identification of the reaction intermediates, will be analysed in this section by Operando FTIR study. Fig. 10\n shows the evolution of CO2 adsorption FTIR spectra with temperature for bare \u03b3-Al2O3. Immediately after 5%CO2/Ar exposure at 150 \u00b0C (see black spectrum), three clearly distinguishable bands appeared at 1653, 1437 and 1228 cm\u22121, whose intensity grows with time up to 30 min. These bands, already identified by many authors in the literature [9,43\u201346], correspond to asymmetric as well as symmetric OCO stretching (\u03bda(OCO) and \u03bds(OCO)) and OH deformation (\u03b4(OH)) vibration modes of bicarbonate species, respectively. Besides, two negative bands can be observed in the hydroxyl region (3800\u22123600 cm\u22121) at 3765 and 3665 cm\u22121 together with a narrow positive peak at 3620 cm\u22121. The negative ones are attributed to the vibration of OH\u2013 groups adsorbed along alumina surface whereas the positive one corresponds to \u03bd(OH) vibration mode of bicarbonates. The presence of negative bands clearly indicates that bicarbonates are formed from CO2 chemisorption on OH\u2013 groups of \u03b3-Al2O3, which are partially consumed after 30 min CO2 adsorption [43]. Additionally, other wide and weak bands appear at 1575 and \u2248 1330 cm-1, which might be assigned to \u03bda(OCO) and \u03bds(OCO) vibration modes of (chelating) bidentate carbonates. It is expected that carbonates are formed from CO2 chemisorption on surface O2- of \u03b3-Al2O3 acting as Lewis basic sites [44].The intensity of bicarbonate bands along with those of bidentate carbonates progressively decreases with temperature until practically disappearing at 400 \u00b0C, indicating that these species are not strongly attached to alumina. In fact, the weak-medium bond strength of bicarbonate has already been observed by CO2-TPD [34,47]. However, the increase of temperature gives rise to small bands at 1515 and 1457 cm-1, which might be related to formation of more stable organic compounds. Furthermore, additional discrete bands are observed at 1393 and 1375 cm-1, suggesting the presence of formate species. The formation of formates on alumina have already been reported and we suggest they come from reaction between bicarbonate or carbonate and residual H chemisorbed during the pre-treatment [9].After studying CO2 adsorption over the bare support, the CO2 methanation was analysed by means of Operando FTIR over monometallic Ni formulations (NiAlIWI and NiAlGAI catalysts). FTIR spectra recorded under reaction conditions from 150 to 450 \u00b0C along with their respective C-species evolution for NiAlIWI and NiAlGAI catalysts are shown in Fig. 11\n. Starting by the analysis of NiAlIWI catalyst results (Fig. 11a), note that the black spectrum, which was recorded at 150 \u00b0C after 30 min under reaction stream exposure, shows more intense bands in the carbonate region (1800-1200 cm\u22121) than bare alumina (Fig. 9). Specifically, the bands assigned to bidentate carbonates at 1574 and 1330 cm\u22121 overlap with additional new ones at 1545 and 1380 cm\u22121, which might be assigned to vibration of monodentate carbonates [3,45]. This greater number of surface carbonates could be associated with the presence of non-reducible Ni2+O2- or even NiAl2O4 able to adsorb CO2 [37]. As the temperature increases, these bands disappear giving rise to clear and intense bands at 1595, 1395 and 1375 cm\u22121, characteristic of 3 vibration modes of formates: asymmetric OCO stretching (\u03bda(OCO)), CH deformation (\u03b4(CH)) and symmetric OCO stretching (\u03bds(OCO)), respectively [9,31,47]. Complementary, the band corresponding to CH stretching (\u03bd(CH)) is observed at 2900 cm-1 (not shown), confirming the formation of formate species. After that, new increasing bands appear at 3016 cm-1 (\u03bda(CH)) and 1305 cm-1 (\u03b4(CH)), indicating the formation of methane gas [33]. Note that no bands were verified in the carbonyl region (2100\u22121800 cm\u22121) but the characteristic bands of CO gas were observed at 2175 and 2105 cm\u22121, suggesting that no detectable amount of COads could have formed on Ni\u00b0 by CO2 disproportionation.Analogously, Fig. 11b displays FTIR spectra of NiAlGAI catalyst. As expected, the same bands and/or species were identified in the carbonate region but with different concentration. In fact, the bands corresponding to carbonates are less intense at the starting temperature (150 \u00b0C) probably due to the absence of NiO or NiAl2O4 acting as basic sites in the catalyst prepared by GAI method. Notably, unlike NiAlIWI, NiAlGAI catalyst presents 3 bands in the carbonyl region (2100\u22121800 cm\u22121) located at 2020, 1920 and 1860 cm\u22121. The first is ascribed to the stretching vibration of terminally or linearly adsorbed CO on top single Ni atoms, whereas the other two can be attributed to weakly and strongly attached bridged carbonyls on neighbouring Ni atoms, respectively [20,29,48]. Interestingly, the band corresponding to linearly adsorbed CO shifts with temperature, while the others remain at the same frequency. This shift is associated with changes in CO covering on Ni surface and suggests that these CO species participate in the CO2 methanation mechanism. On the contrary, bridged carbonyls are more stable and may not react with hydrogen [29]. Furthermore, it is wide known that the \u03bd(CO) frequency (in wavenumbers) is associated with the metallic dispersion: the higher the frequency, the higher the dispersion or the lower the Ni particle size. Thus, according to the 3 \u03bd(CO) bands, NiAlGAI catalyst presents particles with different sizes indicative of highly, moderately and poorly dispersed Ni\u00b0 [48]. This observation is consistent with TEM results, according to which a particle size distribution ranging from 3 to 20 nm is observed. Noteworthy, the lack of adsorbed carbonyls on the catalyst prepared by IWI suggests that there are differences in the Ni electronic state when comparing Al2O3 supported Ni catalysts. In the case of NiAlIWI, it seems that Ni, after reduction pretreatment, is partially oxidized or positively charged (Ni\u03b4+) due to the interaction with remaining non reducible Ni2+ species or with Al3+ cations exposed on the alumina surface. As the exposed Ni has electron deficiency, NiAlIWI presents lower affinity to dissociate CO2 by H-assistance or adsorb CO and, hence, no bands are detectable within 2100\u22122000 cm\u22121. Although Ni2+ is also able to adsorb CO, no bands were observed among 2300 and 2100 cm\u22121 assignable to CO on Ni2+ sites. NiAlGAI, by contrast, has much more affinity to CO adsorption since all nickel is in reduced state (Ni\u00b0) after being calcined under reductive atmosphere (GAI method).The evolution of the main reaction intermediates and methane with temperature is clearly shown in the attached figures (Fig. 11c and d). In the case of NiAlIWI catalyst (Fig. 11c), it can be observed that the relative concentration of bicarbonates decreases as that of formates increases, following a symmetric evolution (T < 250 \u00b0C). This suggests that formates mainly arise from bicarbonates although it cannot be excluded that, in minor extent, carbonates are also reduced into formates [47]. After that, from 250 \u00b0C to 325 \u00b0C, adsorbed bicarbonates disappear and the formation rate of formates slows down up to zero, i.e., its relative concentration reaches a maximum. This slowdown or depletion matches with methane appearance, whose relative concentration increases exponentially in agreement with activity results. Finally, at higher temperatures (T > 350 \u00b0C), the relative concentration of formates decreases, while that of methane slowly increases up to 425 \u00b0C approaching to the limited thermodynamic equilibrium of an exothermal reaction. Thus, it can be assumed that formates at the metal-support interface could participate in methane formation. However, it cannot be claimed that formates are directly hydrogenated following the associative mechanism, since not bands characteristic of methoxy species (reaction intermediates) or methanol have been detected by FTIR, as reported by Solis-Garc\u00eda et al. [28]. Finally, the appearance of COgas from 300 \u00b0C together with the absence of adsorbed carbonyls indicates that this by-product could be formed from decomposition of formates as follows:\n\n(11)\n\n\nHCO\n\nO\n\n\nads\n\n\n\n\u2192\n HC\n\nO\n\n\nads\n\n\n\n +\n\nO\n\n\nads\n\n\n\n\u2192\n C\n\nO\n\n\nads\n\n\n\n + O\n\nH\n\n\nads\n\n\n\n\u2192\n C\n\nO\n\n\ngas\n\n\n\n +\n\nH\n2\n\n\nO\n\n\ngas\n\n\n\n\n\n\n\nOn the other hand, the corresponding species evolution of NiAlGAI sample is displayed in Fig. 11d. Note that, in general, the relative concentration curves for adsorbed species follow the same trend but are clearly shifted towards lower temperatures. In fact, bicarbonates are depleted or transformed into formates faster (at 275 \u00b0C) and the maximum of formates concentration curve, which is also volcano-shaped, is clearly shifted 50 \u00b0C into the left (275 vs. 325 \u00b0C). Carbonyls relative concentration, in turn, increases with temperature up to 300 \u00b0C and then starts depleting. We suggest that carbonyls, which appear from 200 \u00b0C, might arise from formates decomposition (Eq. (11)) or, less probably, from CO2 dissociative adsorption. Wang et al. [9] also studied CO2 methanation by FTIR on a 5%Ru/Al2O3 catalyst and concluded that formates are reactive towards the formation of adsorbed CO when it is close to metal particles. From 225 \u00b0C, the linearly bonded and, in minor extent, weakly attached bridged carbonyls may be hydrogenated into methane, whereas the strongly attached bridged ones remain stable. From 300 \u00b0C, some of the bridged carbonyls could be desorbed as COgas, as revealed by bands at 2175 and 2105 cm\u22121. Noteworthy, the general shift of adsorbed species evolution indicate that NiAlGAI catalyst has a greater capacity to dissociate H2 and provide H, which is essential to carry out the successive steps of reaction mechanism. This leads to a higher activity at mild temperatures, as evidenced by the higher CH4 relative concentration of NiAlGAI catalyst at 300 \u00b0C (0.59 vs. 0.48).The FTIR spectra as well as evolution with temperature of adsorbed species over RuAlGAI and Ni-1.0RuAl catalyst are shown in and Fig. 12\n. Additionally, CO2 methanation FTIR spectra of RuAlIWI catalyst are included in Fig. S4 (supplementary material). Mainly, the same species as in the case of Ni catalysts are observed in carbonate region with similar evolution. However, the position and intensity of bands appearing at carbonyl region are different, i.e., the type and distribution of carbonyl species are not the same. In fact, FTIR spectra of Ru/Al2O3 catalysts show a main band at 2015 cm\u22121 at 150 \u00b0C that can be attributed to vibration of linearly adsorbed CO over reduced Ru atoms (Ru-CO) [10,31,49]. This band is more intense to that observed for NiAlGAI catalyst, indicating that Ru has a major capacity or more affinity to adsorb CO than Ni. However, unlike RuAlIWI catalyst, RuAlGAI presents a shoulder at 1970 cm\u22121 (Fig. 12a) related to stretching vibration of terminal CO species located at metal-support interface ((Al2O3)Ru-CO) [49]. Note that the main band on both Ru catalysts red shifts with temperature from 2015 to 1990 cm\u22121 due to a decrease in Ru surface coverage by CO, whereas the position of the shoulder observed for RuAlGAI catalyst does not shift and it vanishes above 350 \u00b0C along with appearance of CO gas in the cell. Based on these observations, it can be concluded that on-top CO species participates in the reaction but the same cannot be stated for CO species adsorbed at the interface. It seems that this species may not participate in the reaction but eventually desorbed, indicating that RuAlGAI presents a higher fraction of inactive Ru atoms in agreement with the lower TOF/I\n0 value obtained.In the case of the bimetallic catalyst, it should be considered that bands appearing at 2100-1800 cm\u22121 region correspond to carbonyl species adsorbed on both Ni and Ru particles. Thus, what Fig. 12b shows is a combination of bands previously observed for NiAlGAI and RuAlGAI catalysts, characteristic of above-mentioned CO species. The difference is that a new peak is observed at 2056 cm\u22121 attributed to geminal di-carbonyls on low coordinated Ru [9,10,49], which disappear above 250 \u00b0C. According to Panagiotopoulou et al. [50], this species disappears with temperature since it is converted into linearly adsorbed CO due to H2-induced agglomeration of low coordination Ru sites into bigger Ru clusters. Noteworthy, the combination band at 2030 cm\u22121 corresponding to linearly adsorbed carbonyls is significantly more intense than on NiAlGAI catalyst, indicating that CO adsorption is promoted by the co-impregnation of 1% Ru. On the other hand, the band corresponding to weakly attached bridged carbonyls (at 1910 cm\u22121) is clearly more intense compared to that observed on NiAlGAI catalyst, which confirms that the bimetallic catalyst presents a higher Ni dispersion (26.3 vs. 11.5 % according to TEM results). As the temperature increases, bands at 2030 and 1910 cm\u22121 first blue shift up to 250 \u00b0C and then red shift to 2010 cm\u22121 and 1905 cm\u22121, respectively. The red shift matches with the appearance of CH4 band at 1305 cm\u22121, suggesting that both species could be reaction intermediates.Regarding to C-species evolutions of RuAlGAI and Ni-1.0RuAl catalysts (Fig. 12c and d), it can be seen that they are quite similar (except to that of CO), observing a shift of curves towards lower temperatures with respect to those of monometallic Ni catalysts. The shift is due to an enhanced catalytic activity, as demonstrated by H2-TPD runs. In fact, the bands corresponding to bicarbonate species vibration at 150 \u00b0C are much less intense than those observed for NiAlGAI catalyst in both cases, suggesting that bicarbonates are more easily hydrogenated into formates, which reach maximum concentration value at 175 and 200 \u00b0C, respectively. After that, formates at the interface are decomposed into carbonyls and, subsequently, part of carbonyls (most probably linear carbonyls) are hydrogenated into CH4, which relative concentration at 300 \u00b0C is 0.64 (for RuAlGAI) and 0.72 (for Ni-1.0RuAl).Finally, it should be highlighted that RuAlGAI presents a considerable higher amount of potentially reactive carbonyls (linearly bonded) but a CH4 yield similar to that of bimetallic Ni-1.0RuAl catalyst, as can be deduced by comparing its respective spectra and C-species evolution at different temperatures. This suggests that the fraction of carbonyls effectively converted into CH4 is lower in the monometallic catalyst. In fact, although Ni-1.0RuAl adsorbs less CO, it disposes of an enhanced dissociated hydrogen supply to reduce CO as a result of the Ni-Ru synergetic interaction. Based on these results, it can be concluded that an effective CH4 formation not only depends on the type and number of adsorbed carbonyls but also on the availability of adjacent H atoms to carry out the CO bond hydrogenation.To sum up, Scheme 1\n proposes and depicts the proposed reaction pathways on bimetallic Ni-1.0RuAl catalyst deduced from operando FTIR results shown in this section.Firstly, CO2 is mainly adsorbed on hydroxyl groups (OH\u2212) of \u03b3-Al2O3 to give monodentate bicarbonates (HCO3\n\u2212), whereas H2 is dissociated and adsorbed on metal surface. After that, dissociated H2 (H atoms) spillovers and reacts with bicarbonates close to metal particles yielding bidentate formates (HCOO\u2212), which are considered potential reaction intermediates in alumina supported catalysts. Specifically, formates adsorbed at the interface are decomposed into hydroxyls (OH\u2212) on \u03b3-Al2O3 support and carbonyls (CO), which, in the case of monometallic catalysts, are adsorbed either on Ni or Ru surface. However, in the bimetallic system, CO is expected to preferentially adsorb over Ru nanoparticles due to a higher affinity, whereas H2 is adsorbed on neighboring Ni particles acting as H atoms reservoir. Then, carbonyls are reduced by adjacent H atoms into formyl (COH, not observed), which are subsequently hydrogenated into CHXO species (hydroxycarbene (CH2O) or hydroxymethyl (CH2OH)). At certain hydrogenation degree (x = 1\u20133), the CO bond cleavage of CHxO species occurs (rate determining step), finally releasing CH4 and H2 molecules [16,31,51].In this work, the low temperature activity of Ni/Al2O3 formulation is systematically improved through the use of efficient synthesis (IWI vs. GAI) and the addition of Ru. Overall, catalysts prepared by GAI method presented better catalytic performance than those prepared by IWI. In the case of Ni catalysts, the formation of Ni2+ strongly interacting with the support was avoided by GAI synthesis route, resulting in a higher Ni surface area available for the reaction. Instead, GAI method led to a notable increase in the metal dispersion on RuAlGAI catalyst due to the glycerol enclosing effect but, in return, the specific activity (TOF/I\n0) of Ru nanoparticles resulted to be two order of magnitude lower since reaction is structure sensitive. On the other hand, the activity of Ni/Al2O3 was improved even more by co-impregnation of small amounts of Ru as a result of a synergistic combination. In fact, the bimetallic Ni-1.0RuAl catalyst showed remarkably higher Ni dispersion, reducibility, and CO adsorption capacity than NiAlGAI catalyst, observing a methane yield equal to that of 3RuAlGAI. Operando FTIR experiments revealed that CO2 methanation over alumina supported Ni and Ru catalysts proceeds via formation of carbonyl species mainly arising from intermediate formates decomposition, followed by its hydrogenation into CH4. In the bimetallic system, the potentially most reactive species is CO linearly adsorbed over Ru, which is more easily hydrogenated by H atoms supplied from adjacent Ni particles. We conclude that the enhanced CO2 methanation activity of bimetallic catalyst is not only due to a promoted CO adsorption but also to a higher supply of dissociated H2.Adri\u00e1n Quindimil: Methodology, Investigation, Data curation, Writing - Original Draft, Visualization.M. Carmen Bacariza: Methodology, Investigation, Data curation, Writing - Review & Editing, Visualization.Jos\u00e9 A. Gonz\u00e1lez-Marcos: Verification, Resources, Data curation, Writing - Review & Editing, Visualization.Carlos Henriques: Conceptualization, Resources, Funding acquisition, Validation, Supervision.Juan R. Gonz\u00e1lez-Velasco: Conceptualization, Resources, Funding acquisition, Validation, Supervision, Project administration.The authors report no declarations of interest.The support from the Economy and Competitiveness Spanish Ministry (PID2019-105960B-C21), the Basque Government (IT1297-19) and the SGIker (Analytical Services) at the University of the Basque Country are acknowledged. One of the authors (AQ) also acknowledges University of the Basque Country by his PhD grant (PIF-15/351).Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2021.120322.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Conventional Ni/Al2O3 catalyst, currently used for COx removal in ammonia production, admits room for improvement as catalysts for application in low temperature CO2 methanation, which is the aim of this work. The Incipient Wetness Impregnation (IWI) has been replaced by Glycerol Assisted Impregnation (GAI) method and, afterwards, a secondary metal (Ru) has been co-impregnated forming a bimetallic system. The monometallic as well as bimetallic systems have been characterized by several techniques (TGA, XRD, N2-physisorption, TEM, H2-TPR, H2-TPD, STEM-EDX and operando FTIR) and tested for CO2 methanation reaction in a downflow fixed bed reactor (conditions: P =1 bar, H2: CO2 ratio = 4 and WHSV = 30,000 mL h\u22121 g\u22121). GAI method together with a reducing calcination atmosphere (20 %H2/N2) results effective to avoid the formation of large metal particles during the synthesis, especially for Ru/Al2O3 formulation. In fact, the Ru dispersion of the catalyst prepared by GAI (RuAlGAI) is around 5 times higher than that of RuAlIWI catalyst. On the other hand, NiAlGAI presents larger population of reduced particles but bigger in size than NiAlIWI catalyst, which finally provides the former with slightly higher metal surface and superior catalytic performance. By co-impregnating small amounts of Ru (0.5, 1.0 or 1.5 wt%) the Ni surface is considerably increased which, together with Ru synergistic collaboration, results in a methane yield rise from 20 to 44 % at 300 \u00b0C. The operando FTIR results show no differences in the reaction pathway with GAI preparation method and incorporation of Ru, but different evolution of reaction intermediates concentration with temperature. The bimetallic Ni-RuAl system presents much higher capacity to adsorb CO and hydrogenate the reaction intermediates (adsorbed formates and carbonyls) by dissociated H2 than its monometallic counterparts.\n               "}
{"full_text": "The wide application of traditional fossil fuels not only produces amounts of green-house gases CO2, but also causes the emission of harmful substances such as SO2 and NOx, both of which endanger the environment [1,2]. The demand for clean energy is increasing. Solar energy, wind power, tidal and geothermal energy are important energy sources today, but they are intermittent and regionally limited. As an energy carrier, hydrogen not only has the characteristics of greenness and easiness of preparation, but also has high specific energy [3,4]. So, solar and wind energy in nature may be stored in the form of hydrogen through water splitting. The chemical energy in hydrogen can be further converted into electric energy through a hydrogen\u2013oxygen fuel cell when necessary. This strategy not only effectively stores natural resources, but also owns the facilitation of energy transport. During the whole process, hydrogen\u2013oxygen fuel cell is one of the keys [4-6].Because it is not affected by the Carnot cycle [7,8], fuel cells have high efficiency in the energy conversion. Compared with other power generation devices, fuel cells have also several advantages, for example, they do not make noise and also do not emit any harmful gases during operation [9,10]. In particular, they can provide continuous electrical energy as long as the fuel is enough. Fuel cells have become one of the most promising and clean cell systems and get wide applications in various fields [11,12]. Several important vehicle manufactures such as Ford, Toyota, BMW have realized the scale up application of hydrogen in automobiles with fuel cells.The hydrogen\u2013oxygen fuel cell converts chemical energy in hydrogen into electric energy; the cell device consists of cathode, anode and electrolyte (Fig. 1\n). During the discharge process in acidic electrolyte, the fuel H2, loses electrons, undergoes an oxidation process and then forms protons at the anode. The lost electrons arrive at the cathode via an external circuit; the oxidant O2, gets the electrons, results in a reduction reaction and then forms water at the cathode [13].Hydrogen-oxygen fuel cells involve two half reactions: the hydrogen oxidation reaction (HOR) at the anode and the oxygen reduction reaction (ORR) at the cathode. In the system of proton-exchange membrane fuel cell (PEMFC), acidic electrolyte was used. With acidic electrolyte, the overpotential of the HOR is relatively lower. In contrast, the overpotential of the ORR at the cathode is high, which highly influences the overall performance of fuel cells [14]. Precious Pt metal is needed as a catalyst for the ORR, which significantly promotes the fuel cells cost [13,15,16]. The cost of a PEMFC highly depends on the usage of Pt catalyst amount. Up to date, the specific powder density of Pt PEMFC locates at 4\u20135\u00a0kW gpt\n-1, much lower than the cost target demands (>8\u201310\u00a0kW gpt\n-1) that was proposed by the Department of Energy of the USA (total cell cost of <$30\u00a0kW\u22121).In recent years, new effective catalysts used in alkaline electrolyte for ORR have been developed [17-22]. The typical new non-precious ORR catalysts, present comparable or even better catalytic performance than Pt catalysts under alkaline conditions, are Fe-N-C catalysts [23-28]. Along with the development of anion exchange membrane, researchers then pay attentions to anion-exchange membrane fuel cells (AEMFCs) with alkaline electrolytes, with the expectation of developing non-platinum fuel cells [29-31].In alkaline electrolytes, although the non-precious Fe-N-C catalysts can be used. The overpotential of HOR on the anode end is relatively higher. Therefore, developing of an economical and efficient catalyst for HOR under alkaline conditions is highly required. Generally, the HOR was proposed to proceed with a combination of three elementary steps: Tafel, Heyrovsky, and Volmer steps. These elementary steps can be presented as follows (* is a hydrogen adsorption site on catalyst surface, Had in the Equation denotes hydrogen atoms in adsorbed state) [32-34]:\n\n(1)\n\n\n\nH\n2\n\n+\n2\n\u2217\n\u2192\n2\n\nH\nad\n\nTafel\n\nstep\n\n\n\n\n\n\n(2)\n\n\n\nH\n2\n\n+\n\u2217\n+\n\n\nOH\n\n-\n\n\u2192\n\nH\nad\n\n+\n\nH\n2\n\nO\n+\n\n\ne\n\n-\n\nHeyrovsky\n\nstep\n\n\n\n\n\n\n(3)\n\n\n\nH\nad\n\n+\n\n\nOH\n\n-\n\n\u2192\n\nH\n2\n\nO\n+\n\n\ne\n\n-\n\n+\n\u2217\nVolmerstep\n\n\n\n\nThe Tafel step is a chemical dissociative adsorption process of hydrogen molecules with the formation of two adsorbed hydrogen atoms. The dissociation energy for one H2 molecular is 4.52\u00a0eV. Due to the diatomic molecular, two adjacent empty adsorption sites are needed on catalyst surface. This was confirmed by the results that no HOR activity of Pt atoms were shown when they are atomically dispersed on a sulfur doped carbon substrate [35]. Heyrovsky step involves not only the chemical dissociative adsorption of hydrogen molecules, but also the transfer of one electron. In this process, OH\u2013 species in the vicinity of catalyst surface are needed, which was also argued to be OHad adsorbed on catalyst surface but not OH\u2013. Volmer step reveals the electron transfer from adsorbed hydrogen atoms to form one molecular water.The HOR either follows Tafel-Volmer or Heyrovsky-Volmer mechanisms [36]. In alkaline electrolyte, the catalyst should provide at least dual adjacent sites for all of the three elementary steps. In steps of Heyrovsky and Volmer, one site for hydrogen adsorption, another for OH or OH\u2013 adsorption; while in acid solution, the only required sites are those for chemisorb hydrogen. This is commonly believed to be the reason for the much lower HOR activity of catalysts in alkaline electrolyte [36,37]. All of the three steps involve Had. Thus, the hydrogen binding energy between hydrogen atoms and active sites (noted as HBE) is a critical factor for HOR, which is also used as a descriptor of HOR activity for a catalyst. Smaller HBE is more favorable for the Volmer step; while if HBE is too small, Tafel or Heyrovsky steps would be difficult to occur [32,38,39]. Especially, it should be noted that the hydrogen binding energy on catalyst surface would vary with electrolytes of varying pH values and potentials for HOR [40-43].According to the calculation of density functional theory (DFT), the metal-hydrogen bond strength on various metal surfaces were estimated with free energy of hydrogen adsorption (\u0394GH) that is correlation with the HBE. The relationship between the exchange current density of hydrogen evolution reaction and \u0394GH is present in Fig. 2\n. Pt locates at the top of the volcanic-type relationship. It indicates that Pt catalyzing the reaction results the highest exchange current density, while the binding of hydrogen on W, Fe, Co, Ni, and Pd metals is too strong, which locates on the left side of the volcanic-type relationship. Accordingly, Cu, Au and Ag metals are on the other side of the relationship. Weaker hydrogen binding will lead to weaker adsorption of H atoms on the active sites, which is not favorable for the Tafel step. In contrast, stronger hydrogen binding will not favorable for Volmer step [39,44,45]. Therefore, HBE (or \u0394GH) is usually used as a descriptor of HOR activity for a catalyst [32,38,39,46,47], an excellent catalyst requires active sites with suitable HBE value.Except the HBE, the OH\u2013 species, that is, the surface adsorbed OH/OH\u2013 species also significantly affect the HOR catalytic activity in alkaline electrolyte [49,50], since for steps of Heyrovsky and Volmer, bifunctional catalytic surface that can adsorb hydrogen atom and OH\u2013 (oxophilic surface) is needed (Fig. 3\n) [51]. Pt is an excellent component for the dissociative adsorption of molecular hydrogen, while monometallic Pt surface is not a good plat for the adsorption of OH\u2013 at the conditions relevant for HOR [52]. Ramaswamy et al. suggested that during the HOR reaction, Had is formed on Pt surface. A voltage penalty must be paid to get over repulsive force due to the transfer of negatively charged OH\u2013 anions from outer-Helmholtz plan (OHP) to negatively charged Pt surface, which causes the formation of Pt-Had\u00b7\u00b7\u00b7OHq-ad cluster and then comes to the formation of water molecule (Fig. 3A) [51]. On a bimetallic surface, quasi-specifically absorbed hydroxide species (OHq-ad) on Mp surface is able to react with Pt-Had forming water. Similar effect appears when the alloying elements come to Cu, Ni, Co, Nb, etc. It should be noted that these elements do not form Hupd duo to the oxide/hydroxide on its surface. Specially absorbed hydroxide species react with Pt-Had forming MOx(OHy) instead of M\u2212OHad (Fig. 3C). Introducing of Ru in Pt catalyst can induce more than five-fold increase of HOR catalytic activity in 0.1\u00a0M KOH [53]. It is believed that the introduction of Ru increases site amounts for the adsorption of hydroxyl species.Similar cases are Ni(OH)2-metal composite catalysts. It was found the presence of Ni(OH)2 could promote the adsorption of OH\u2013, which endows the catalyst surface with bifunctions and promotes HOR [55]. Ddekel [49] and Strmcnik et al.\n[56] also indicated that adjusting the OHad on the catalyst surface is an effective method to optimize HOR activity. Their studies suggested that the catalytic activity for HOR can be improved by hybriding or alloying of two metals sites together. In this bimetal system, the Metal-H intermediates react with the OHad on the adjacent metal sites that can bond of OH species with Metal-OHad or Metal-HUPD-OHad configurations. Thus, Davydova [57] and Koper et al.\n[58] proposed a new description of catalytic activity for HOR catalysts, i.e. binding energy of OH species on active sites (OHBE). So far, however, whether the OHBE can be used as a descriptor of HOR activity is still under debate, since there are also some studies showing that the presence of second metal sites influences the catalytic activity through modulating the HBE [59].Rotating disk electrode (RDE) with a standard three-electrode system is usually used to check the electrocatalytic activity for a HOR catalyst. Be noted that the rotating electrode is needed to weaken the influence of hydrogen diffusion in the electrolyte on the catalytic current. H2-saturated 0.1\u00a0M KOH aqueous solution is often used as the electrolyte. An appropriate amount of catalyst ink that was usually prepared by dispersing catalysts in Nafion-ethanol solution was dropped onto a glassy carbon disk electrode and was allowed to be dry, which was used as the working electrode. To avoid the contamination of Pt on the catalyst, the counter electrode cannot be Pt-based materials. Carbon rods and glassy carbon pieces are encouraged to be used. To check the catalytic activity, the linear sweep voltammetry method is used. With the increasing of potentials, hydrogen will be oxidized and then so will water. Be noted during the operation, the catalyst surface will possibly be oxidized.During the HOR process, the influence of hydrogen mass transfer on current density cannot be ignored. The influence of hydrogen mass transfer on the polarization curve should be corrected by the Koutecky-Levich formula (Equation (4)), from which the kinetic current ik\n can be calculated by Equation (4).\n\n(4)\n\n\n\n1\ni\n\n=\n\n\n1\ni\n\nk\n\n+\n\n1\n\ni\nd\n\n\n=\n\n\n1\ni\n\nk\n\n+\n\n1\n\nB\n\nc\no\n\n\n\n\u03c9\n\n\n0.5\n\n\n\n\n\n\n\n\nIn Equation (4), i is the total current actually measured, ik\n is the kinetic current density, id\n is the diffusion limited current that is related to the Nernstian diffusion potential. B, Co\n, and \u03c9 are the Levich constant, the solubility of hydrogen in 0.1\u00a0M KOH, and the speed of the RDE, respectively. Plotting 1/i vs. 1/\u03c90.5\n will help the deduce of kinetic current [60].\n\n(5)\n\n\n\ni\nk\n\n=\n\ni\n0\n\n\n\n\n\nexp\n\n\n\n\n\n\n\u03b1\nF\n\nRT\n\n\u03b7\n\n\n\n\n-\nexp\n\n\n\n\n\n\n\n\n\n\u03b1\n-\n1\n\n\n\nF\n\nRT\n\n\u03b7\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n(6)\n\n\n\n\u03b7\n\ndiffusion\n\n\n=\n-\n\n\nRT\n\n\n2\nF\n\n\nln\n\n\n\n1-\n\n\ni\nd\n\n\ni\nl\n\n\n\n\n\n\n\n\n\nWith the Butler-Volmer formula (Equation (5)) (in which \u03b1 and \u03b7 are the transfer coefficient and the overpotential; R, T, and F are the universal gas constant, 8.314\u00a0J\u00a0mol\u22121 K\u22121, the Kelvin temperature, the Faraday constant, 96,485C mol\u22121, respectively), the HOR exchange current density i0\n can be estimated by fitting of the current density with Butler-Volmer formula (Fig. 4\n). The exchange current density i0\n is the most important parameter to characterize the catalytic activity of the catalyst. Besides the exchange current density i0\n, the Nernstian diffusion overpotential (\u03b7diffusion) is another important parameter that can be obtained from equation (6), in which il\n is the hydrogen diffusion limited current density [60].Platinum group metals (PGM) including Ir, Pt, Pd and Rh show excellent catalytic performance for HOR [61-66]. Pt shows the highest catalytic activity for HOR. However, in alkaline electrolytes, the catalytic activity of PGM catalysts is greatly reduced, which is about two orders of magnitude lower than that in acidic media. For platinum group metal HOR catalysts used in alkaline environment, the current research focus is to develop various nanostructures to reduce the amount of precious metals [51,67-69]. On the other hand, researchers devote to optimizing the catalytic performance by introducing other components [41,42,70,71]. Researchers have shown the electronic structure of PGM active sites can be improved by alloying, which can promote the Volmer step and increase the HOR activity. One of the typical examples is the AuPt/C catalyst for HOR [62]. Compared with commercial PtRu/C and commercial Pt/C, the resulting AuPt/C catalyst showed superior HOR activity with a normalized current density up to 0.158\u00a0mA/cm2 in 0.1\u00a0M KOH solution. The researchers believed that the improved HOR catalytic activity is mainly due to the introduction of Au that reduces the interaction strength between Pt atoms and adsorbed hydrogen atoms. Yang et al. found Rh2P/C catalyst showed improved catalytic HOR performance than that of Rh [64]. They proposed that the presence of phosphorus atoms may help reduce the HBE of active sites on the catalyst surface and thus promote the improvement of catalytic performance. The exchange current density obtained with the Rh2P/C catalyst is 2.4 times of the Rh/C catalyst under the same conditions. Recently, P-Rh/C and P-Ru/C catalysts were also reported for HOR, further confirming the introduction of P can improve the catalytic activity [72,73]. The optimized P-Ru/C catalyst shows a normalize exchange current density of 0.72\u00a0mA\u00a0cm\u22122 which is even 2 times higher than that of commercial Pt/C [73].Based on the electrodeposition method, Liu et al. selected a variety of metals (Mg, Cr, Mn, Fe, Co, Ni, Cu, Ru, La, and Ce) to modify the Pt planar electrode, indicating that both of the electronic effect and the oxophilic effect due to the presence of introduced metals play important roles for HOR, but the former can affect the catalytic performance more significantly [74]. To distinguish the influence of the electronic effect and the oxophilic effect on the catalytic activity, core\u2013shell structure is an ideal model catalyst for the study. Considering the relatively lower price of Ir metal, Liu et al. synthesized IrNi@Ir core\u2013shell nanoparticles. The exchange current density of IrNi@Ir core\u2013shell particles reaches 1.22\u00a0mA cmIr\n-2 at overpotential of 0.05\u00a0V in 0.1\u00a0M KOH solution. In this core\u2013shell catalyst, the IrNi core changes the electronic structure of the Ir shell and optimizes the HBE on Ir sites [61]. It seems that no oxophilic effect provide contribution for the improved catalytic process, since the introduced nickel atoms locate at the core position.Although PGM present good catalytic performance for HOR, the high price of PGM still hinder their wide application. Especially in alkaline media, a large amount of PGM catalysts must be loaded to overcome the challenge of two orders of magnitude decrease of catalytic activity for HOR. It prompts researchers to develop non-precious metal catalysts [75-79]. To date, the non-precious metal-based HOR catalysts are mainly Ni-based materials (Table 1\n) [77,80-83]. Recently, Oshchepkov et al. reviewed the electrochemical behavior of Ni catalysts for the oxidation of hydrogen-containing fuels [77]. Here, we focus on the HOR catalyzed by Ni-based catalysts.Studies have shown that with partial oxidation, the catalytic performance of Ni catalysts can significantly improve [84,85]. The Ni-NiO composite structure formed by partial oxidation has a bifunctional surface, in which the metallic Ni can adsorb hydrogen and the NiO phase can adsorb OH species. The thus bifunctional surface promotes its catalytic activity. In addition, the presence of NiO may also be beneficial to optimize the adsorption of hydrogen on the surface of nickel nanocrystals. For example, by partially oxidizing of Ni nanoparticles, the exchange current density can increase from 6.2 to 56 \u03bcA/cm2 (Fig. 5\n) [86]. Based on the Arrhenius formula, Oshchepkov et al. studied the effect of temperature on the kinetics of HOR/HER with Ni electrode in the range of 298\u2013338\u00a0K [86]. It was found that partial overlaying of NiO on Ni electrode can lead to the reduction of HBE on Ni sites and promote an increase in the reaction rate of the Volmer step, thereby induce a significant enhancement of the HOR/HER kinetics [87].Oshchepkov et al. also explored how does the oxidation of Ni electrode effect its kinetics in catalyzing HOR/HER [88]. They show 10 times of enhancement of the catalytic activity for HOR and HER with Ni catalyst that was oxidized in air. The researchers also believed that the presence of NiO optimizes the adsorption of hydrogen atom on the surface of Ni.With this acknowledgement, researchers designed and prepared Ni-NiO catalysts to enhance its catalytic HOR activity. Yang et al. synthesized a variety of Ni/NiO/C catalysts at different temperature through the calcination of metal organic framework precursors. The catalytic performance is an order of magnitude higher than Ni/C. Both of the catalytic stability and CO tolerance ability of the Ni/NiO/C catalyst are better than those of Pt/C [89]. Pan et al. obtained Ni(OH)2-Ni/C catalyst by electrochemical oxidation of Ni/C catalyst and found that after electrochemical oxidation, the HOR exchange current increased by 6.8 times (Fig. 6\n) [90].Except for NiO, the introduction of CeO2 and MoO2 can also improve the performance of metallic Ni catalysts. The improving mechanism for catalytic activity is similar to that of NiO. The Ni active sites on Ni-CeO2 catalyst own a more thermo-neutral \u0394GH* than that on pristine Ni catalyst, that is, the induction of CeO2 weakened the HBE. In addition, an enhanced adsorption behavior for OH* was also found on Ni-CeO2. These two effects synergistically accelerate the Volmer step so that improves the HOR activity [91]. Similarly, Deng et al. modified Ni nanocrystals with MoO2, where Ni nanocrystals provided hydrogen adsorption sites. MoO2 as a corrosion-resistant stable oxide can not only promote the dissociation of water, but also optimize the OH adsorption due to the positively charged surface, and thus accelerating the Volmer step, eventually improving the HOR activity [92].Another strategy to optimize the performance of the Ni-based HOR catalysts is to introduce a second metal component, which will optimize the HBE of the Ni active sites through the interactions between them [71,93-95]. Usually, the introduction of the second metal component will also help the improvement of the oxidation resistance for Ni nanocrystals. Based on density functional theory, Tang et al. predicted that Ni-Ag alloys have multiple adsorption centers, and some active centers possess the optimal HBE [48]. With a low temperature physical vapor deposition method, they synthesized a series of binary Ni-Ag alloys, which can catalyze both HER and HOR with catalytic performance much high than pristine metallic Ni and with improved stability. Besides Ni-Ag bimetals, theoretical calculation also indicates that the ternary CoNiMo alloys surface shows optimized HBE (Fig. 7\n) [96]. Compared with pure Ni materials, NiMo bimetals synthesized by electroplating method exhibit significantly increased HOR activity. Significantly, the performance of ternary CoNiMo product presents 20 times higher catalytic activity than pure Ni. The presence of Co in the catalyst would adjust the structure of d-orbitals of nickel, inducing the improvement of catalytic activity. Kabir et al. synthesized NiMo catalysts by thermally reducing the transition metal precursor with carbon carrier, and suggested that the Mo component does not directly participate in HOR process, but affects the HBE of Ni active centers [97].Another metal that can significantly improve the HOR performance of Ni catalytic centers is Cu [98-101]. The Ni-Cu alloy is thermodynamically unstable. Oshchepkov et al. discussed the effect of different amounts of Cu in Ni/C catalysts on the HOR activity, and found that the addition of Cu can enhance the oxidation resistance of Ni [98]. After that, Cherstiouk et al. also prepared NiCu/C catalysts [99]. The optimized mass activity of Ni0.95Cu0.05/C catalyst was 1.5 times of that of pure Ni sample. The exchange current density reached 14 \u03bcA cm\u22122. The catalytic activity of Cu-Ni alloys varies with the preparation methods. Ni0.6Cu0.4 catalyst prepared by magnetron co-sputtering method shows significantly improved activity with the exchange current density four times of pure Ni catalyst [100].Besides, recent researchers found that W, Pd, Ru, Mo, etc. can also improve the catalytic performance of nickel. The modification of W on Ni nanocrystals can not only adjust the electronic structure of the Ni surface, but also restrain the oxidation of Ni surface [29]. Baoks et al. deposited Pd on the surface of Ni film by electrodeposition technology. Within 17% of Pd coverage, the catalytic current density increases linearly with increasing Pd coverage [102]. Recently, single atomic Ni sites loaded on Ru nanosheets were firstly studied as HOR catalysts. The single atomic Ni sites loaded on Ru nanosheets possess optimized HBE, which results in improved catalytic activity for HOR. It is believed that both of Ni and Ru centers take part in the catalytic process [103]. Metallic of Mo can also enhance the catalytic process of Ni-based catalysts [104]. Duan et al. recently reported a tetragonal nickel-molybdenum nanoalloy, MoNi4, which shows a high apparent exchange current density of 3.41\u00a0mA\u00a0cm\u22122. It is suggested that the high exchange current density is attributed to the improved filling antibonding state [104].The support highly affects the catalytic activity. There is strong interaction between metallic active species and the supports, which will cause electron transfer between them and finally turning the adsorption behavior for hydrogen, improving the catalytic performance. For nickel-based HOR catalysts, heteroatoms (N, S, P) doped carbon materials are excellent supports, which can significantly improve the performance. N atoms on the support can affect the d-orbitals of nickel and improve the HOR performance. In the Ni/N-doped carbon nanotubes catalyst (Ni/N-CNT), N-CNT was used as a support for Ni particles (Fig. 8\n) [105]. The N-CNT support makes the mass activity and exchange current density increase by 33 and 21 times; the exchange current density of Ni/N-CNT catalyst reached 28 \u03bcA cmNi\n-2 (Fig. 9\n). A series of heteroatoms-doped carbon supported Ni nanoparticles catalysts (the heteroatoms are S, B, and N) were prepared through pyrolysis of organic molecules followed by NaBH4 reduction. The results show that the catalytic activity for HOR is in the order of Ni/S-doped carbon\u00a0>\u00a0Ni/N-doped carbon\u00a0>\u00a0Ni/B-doped carbon\u2248Ni/carbon. The exchange current density of the Ni/S-doped carbon catalyst is up to 40.2 \u03bcA cm\u22122. The optimal performance can be attributed to the following two main factors. On one hand, the carbon support anchored Ni nanoparticles, which helps to prevent the aggregation of Ni nanoparticles and so to keep the Ni nanoparticles with smaller size. The smaller Ni nanoparticles make the catalyst a larger specific surface for electrochemical reaction. On the other hand, that is more important, the electronic interactions between Ni sites and the support optimized the HBE of Ni and eventually improves the catalyst activity [106]. However, systematical acknowledge about the interaction between supports and Ni centers in HOR catalysts is still lacking.The heteroatom content in carbon is obviously an important parameter for the catalyst. With SiO2 microspheres as a template and melamine as a raw material, Jiang et al. designed a N-doped carbon support with spherical shell microstructure, in which the nitrogen content can be tuned from 0 to 21.6 at%. Ni nanoparticles were loaded on the support by an impregnation-reduction method. The electrochemical measurements in alkaline media unveils the \u201cvolcano-like\u201d regularity for the N-doping content and HOR activity. The catalyst with 8.7 at% nitrogen doping shows the optimal catalytic activity with calculated exchange current density of 30 \u03bcA cmNi\n-2\n[107]. For the preparation of Ni-based catalyst with carbon support, Simonov et al. developed a simple method for Ni/C catalyst preparation by nitrate decomposition. The introduction of Ni(CH3COO)2 can prevent the gasification of the carbon support during the catalyst synthesis [108].Besides metallic Ni as HOR catalysts, some Ni-based compounds including Ni3N can also catalyze HOR. By calcining of NiO or Ni(OH)2 under a nitrogen atmosphere. Ni3N can be obtained. During the transition from Ni to Ni3N, the d band of Ni shifted downward, and the interface charge was transferred from Ni3N to the carbon carrier. It weakens the HBE on nickel sites and the resulting product shows significantly enhanced HOR activity and stability [109]. Yang et al. discussed the electron transfer effect in Ni3B/Ni heterostructure. The inter-regulated effect not only weaken H* adsorption, but also strengthen OH* adsorption, prompting the enhancement of HOR activity [110].The present studies on HOR catalysts mainly focus on the development of catalysts used in alkaline media. Although platinum group metals such as Pt, Ir, Rh, and Ru show high electrocatalytic activities in acidic media, their high cost limits the wide application. In addition, they also face a major challenge of two orders of magnitude decrease in catalytic kinetics under alkaline media. Therefore, designing of low-cost, high-efficiency, non-precious catalysts working at low pH medium highlights the research value. The common designing ideas for non-noble metal-based HOR catalysts that are mainly composed by Ni-based materials include the introduction of second component (such as oxides/hydroxides, alloying), the using of various heteroatoms doped supports, with the aim of adjusting the electronic structure of the catalytically active sites and making their electronic structure be close to the optimal state. Designing catalyst surface with bifunctions (adsorption of hydrogen and adsorption OH species at the same time) is also believed to be an effective strategy for the improvement of HOR activity. Generally, promoting the reduction of the hydrogen binding energy of Ni sites plays a vital role for the catalytic process, which would accelerate the kinetic process of the rate-determining step for HOR.On the other hand, for Ni-based HOR catalysts, some important issues should also get attention. Firstly, Can the metallic Ni keep the original crystal structure on the surface before and after the catalytic process? The possible surface re-construction on Ni surface during electrocatalytic process should be understood. Secondly, the hydrogen diffusion in the catalyst layer and the adsorption behavior of hydrogen on various Ni sites are also not fully clarified at this stage. In addition, the mechanisms of the Ni-based catalyst stability and the performance decay are also not clear. Detailed experimental studies with in-situ characterization techniques and systematically theoretical calculation would provide useful information on the above listed issues.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors are grateful for National Natural Science Foundation of China (No. 21776115). Six talent peaks project in Jiangsu Province (XCL-2018-017). Foundation from Marine Equipment and Technology Institute for Jiangsu University of Science and Technology, China (HZ20190004)", "descript": "\n                  With the rapid development of anion exchange membranes, researchers began to shift their attention from proton exchange membrane fuel cells (PEMFCs) with acid electrolytes to anion exchange membrane fuel cells (AEMFCs) with alkaline electrolytes. Non-precious metal catalysts such as Fe-N-C catalysts are available for ORR at the cathode in alkaline electrolytes. But at the anode, the catalytic reaction kinetics of Pt catalysts in alkaline media is two orders of magnitude slower than that in acid media, which prompts researchers to develop new low-cost and high-efficient non-precious metal catalysts for HOR. Up to date, the typical non-precious metal catalysts for HOR are Ni-based materials. In this minireview, we firstly introduced the elementary steps of HOR and the important activity parameters for a HOR catalyst. Secondly, we briefly describe the performance of various Ni-based HOR catalysts reported in recent years.\n               "}
{"full_text": "Over the decades, the large consumption of fossil fuels due to anthropogenic activities has released heat and greenhouse gas into the atmosphere. The concentration of CO2, one of the major greenhouse gases, in the atmosphere exceeds 410\u202fparts per million (ppm), which is much higher than 280 ppm in the pre-industrial period.\n1\n The thermal radiation from the sunlight and the surface of the earth can be trapped by CO2 molecules, augmenting the global temperature and aggravating the issue of global warming.\n2\n The over-emitted CO2 is truly one of the top concerns for human society. The CO2 reduction reactions (CO2RR) that convert CO2 molecules into value-added products such as CH3OH, CH4, CO, and HCOOH give an alternative strategy to consuming CO2 while mitigating the energy crises. In addition, realizing CO2RR by clean energies such as solar energy offers an approach to alleviate both global warming and energy crisis issues in a green concept.\n3\n To fulfill this, photocatalytic CO2 reduction reactions (PCO2RR) has gained much attention because it utilizes semiconductor materials as photocatalysts to absorb sunlight as the driving force to reduce CO2. Fig. 1\na depicts the fundamentals of PCO2RR on a semiconductor catalyst. Typically, the photocatalysis takes three main steps: i) the generation of the photo-generated carriers (e\u2212/h+ paires) within the semiconductor photocatalyst through harvesting the incident light; ii) the transfer of the photo-generated electrons and holes to the surface of the photocatalyst; iii) the catalytic reactions (CO2RR and the oxidation half-reaction) on the surface of the photocatalyst.\n4\n The photo-generated carriers (e\u2212/h+ paires) are essential to PCO2RR as they are the keys to the catalytic reactions. The photo-generated electrons in the conduction band (CB) participate in CO2RR to reduce CO2 molecules. The photo-generated holes in the valence band (VB) participate in the counter-reaction (oxidation half-reactions, such as water oxidation reaction). During the formation of the photo-generated carriers, the electrons and holes tend to recombine with each other through surface recombination and volume recombination, which prohibits catalytic efficiency. Besides, the potentials of the VB maximum and the CB minimum need to straddle the redox potentials of the oxidation half-reactions and CO2RR, respectively.\n5\n Equations (1)\u2013(8) shows some typical reaction steps involved in CO2RR, along with the reactions for H2 and O2 productions (vs. Normal Hydrogen Electrode (NHE), in aqueous solution of pH\u202f=\u202f7).\n3\n\n,\n\n6\n\nThe semiconductor catalyst needs to meet not only the kinetic barriers but also the thermodynamic requirements to achieve a successful PCO2RR. Moreover, the bandgap between CB and VB determines the wavelength range that the semiconductor can absorb from sunlight. When the value of the bandgap is larger than ca. 3.1 eV, the semiconductor catalyst can solely harvest ultra-violet illumination, which is only a small fraction of the sunlight. Increasing the light-harvesting ability of the photocatalysts requires the bandgap of the semiconductor materials to be relatively narrow. The single pure semiconductor catalyst alone usually cannot afford sufficient catalytic efficiency for PCO2RR. Great efforts are made in the modification of the semiconductor catalysts (e.g. the refinement of crystallinity, defect engineering, heterojunction construction, etc.) to increase the catalytic performance (e.g. product selectivity and activity, light harvesting capability, stability) during PCO2RR.\n5\n The goal is to design rational photocatalysts with I) the matching CB and VB potentials for CO2RR and the oxidation half-reactions; II) the narrow bandgap to absorb sufficient sunlight; III) low recombination rate of the photo-generated carries; IV) sufficient and highly efficient active sites for CO2RR and the oxidation half-reactions. This review concentrates on the most recent advanced photocatalyst designs for PCO2RR, where the superiorities of semiconductor modification and integration are highlighted (Fig. 2\n). Hybridization strategies of photocatalysts such as surface engineering and band engineering are explained with some typical examples (e.g. co-catalyst designs, photosensitizer, heterojunction construction). Then, promising results from structural engineering and single-atom active site fabrications are exposed, along with the biohybrid catalyst designs. Finally, the perspectives on the remaining challenges and future focuses are presented.Currently, mainstream works focus on modifying the semiconductor photocatalysts by hybrid engineering such as surface modification and the integration of different semiconductors to form heterojunctions. Table 1\n lists some typical hybrid photocatalysts for PCO2RR.As the catalytic reaction takes place on the surface of the catalyst, its modification with active sites or support co-catalysts is a promising way to enhance the catalytic activities of the catalyst materials. For example, An et al. modified the Fe tetraphenyl porphyrin (FeTPP) catalyst with an alkyne-functionalized supramolecular synthon to form an iron porphyrin box (PB) bearing 24 cationic groups (FePB-2(P)) that offered a synergy of porosity and charge effects. The modified FePB-2(P) exhibited a 41-times enhancement in catalytic performance, as compared to the original FeTPP, toward CO production in PCO2RR.\n8\n The modification of the metal co-catalysts on the semiconductor surface can serve as not only the active sites to capture CO2 molecules for the activation but also the electron trap to separate photo-generated carriers, demonstrating excellent PCO2RR performance (e.g, Au\u2013Cu alloy modified on TiO2 substrates for CH4 and C2H4 productions\n9\n; Ni cluster shell on NiO core for the CO generation\n10\n).Surface engineering for light harvesting enhancement is another approach to increase photocatalytic efficiency. As photocatalysis utilizes solar energy to drive the catalytic reactions, the photocatalysts\u2019 efficiency of harvesting sunlight is vital in the practical aspect. Currently, many good semiconductor catalysts such as metal oxides exhibit wide bandgap, which limits light absorption within the ultra-violet range, only a small fraction of the incident solar radiation (less than 5%).\n11\n It makes those wide-bandgap photocatalysts less attractive in industrial applications, as sunlight mostly contains visible and infrared lights. The integration of semiconductor catalysts with light-response-efficient materials allows for the improved light-harvesting ability of the photocatalysts. Modifying semiconductor catalyst surfaces with photosensitizers is one of the approaches to achieve this goal. Wang et al. constructed a number of photosensitizers of homoleptic Al (III) for PCO2RR with emission quantum yields from 10% to 40%.\n12\n\nFig. 3\na shows the molecular structures of the homoleptic Al (III) complexes. The light absorption band center can be tuned by changing the ligands attached to the Al (III) photosensitizers (Fig. 3b). Fig. 3d illustrates the PCO2RR activities of different catalysts coupled with Al (III) photosensitizers. The catalyst of [Fe(qpy)(OH2)2](ClO4)2 (FeQPY; qpy\u202f=\u202f2,2\u2032:6\u2032,2\u2033:6\u2033,2\u2034-quaterpyridine) affords the most durable and stable CO production among all the catalysts. Fig. 3c demonstrates the reaction scheme for the PCO2RR with Al (III) photosensitizers/FeQPY with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]-imidazole (BIH) as the sacrificial electron donor. This noble-metal-free system achieves a CO selectivity of 99% and a TON value of 10250 under 450 nm light illumination in PCO2RR (Fig. 3e). Das et al. developed a porous organic polymer (POP) as a light harvester for improving PCO2RR. The composite catalyst of In2.77S4 and POP was built by electrostatic interaction and exhibited a C2H4 product selectivity of 98.9% with a yield rate of 67.65 \u03bcmol\u202fg\u22121\u202fh\u22121 under irradiation of visible light.\n13\n\nTo maximize the utilization of sunlight, scientists have discovered some promising semiconductor photocatalysts with narrow bandgap to harvest visible light, such as carbon nitride,\n14\u201316\n bismuth oxyhalides,\n17\n\n,\n\n18\n CdS,\n19\n layered double hydroxides (LDHs),\n4\n\n,\n\n20\n\n,\n\n21\n etc. However, those visible light-response semiconductor materials suffer from severe recombination of the photo-generated carriers. The surface modification of the semiconductor catalyst with a metal co-catalyst such as Pd/TiO2\n\n22\n and Ag\u2013TiO2,\n23\n which can trap the photo-generated electron, emerges as a powerful strategy to separate the photo-generated carries and contributes to for an enhanced PCO2RR performance. It is well applied to visible light-response photocatalysts as well. For instance, Yue et al. reported a Bi-MOF/BiOBr photocatalyst where Bi-MOF was in-situ mounted on the BiOBr for PCO2RR with a CO yield rate of 21.96 \u03bcmol\u202fg\u22121\u202fh\u22121.\n24\n\nAnother approach is to integrate the semiconductor catalysts with metal co-catalysts. In this strategy, using plasmonic metals offer extra benefits for light harvesting. The nanocatalysts of plasmonic metal interact strongly with incident light, generating a localized surface plasmon resonance (LSPR). The LSPR effect can easily tune the light absorption wavelength from ultra-violet to near-infrared through adjusting the geometry structure of the plasmonic metal catalyst.\n25\n\n,\n\n26\n For example, Cu/TiO2 enables absorbance at 500\u2013600 nm from the LSPR of Cu particles to improve the CO production in PCO2RR.\n27\n The LSPR effect of Au particles helps achieving the PCO2RR under low-intensity irradiation at 420 nm.\n26\n\nBased on the promising results provided by the assistance of the plasmonic metal Au in light harvesting and catalytic performance, modifying Au catalyst with other metal alloys has proven to enhance the employment of photons with low energy for PCO2RR. Hu et al. fabricated plasmonic light harvesting Au rods and coupled them with a co-catalyst shell of CuPd alloy to achieve highly effective PCO2RR to CH4 production.\n28\n\nFig. 4\na demonstrates the structure of the Au/CuPd core-shell composite. Plasmonic catalysis often happens close to the surface of the catalyst (within the range of plasmon-induced local field). Pure Au nanorods are unlikely to collide with CO2 molecules, resulting in its low CO2 conversion efficiency (Fig. 4a, left). For Au/CuPd, the CuPd shell can capture CO2 molecules to increase the CO2 concentration on the surface of the catalyst, promoting the probability of further conversion and activation (Fig. 4a, right). The thickness of the shell and the Cu/Pd ratio can be easily adjusted by controlling the number of metal precursors during the synthesis. The optimized Au/CuPd catalyst can achieve a CH4 yield rate of 15.6 \u03bcmol\u202fg\u22121\u202fh\u22121, which is almost 40 times higher than the one achieved by pure Au rods (Fig. 4b). The apparent quantum efficiencies are up to 0.1% at 800 nm (Fig. 4d). The optimized thickness of the CuPd shell is believed to maximize the number of active sites on the catalyst surface and strengthen the electron-phonon scattering effect, contributing to the best CH4 production. Fig. 4c illustrates that C2H4 and C2H6 can also be detected after the long-term reaction test. This suggests the multiple proton-coupled electron transfer ability of the Au/CuPd core-shell catalyst. To avoid the scattering loss of incident light during PCO2RR, a spherical-structured gas-solid reaction system is further applied (Fig. 4e). The novel spherical-structured reaction system can realize the re-incidence of scattered photons, which benefits the catalytic reaction system with a plasmonic effect. With the optimization of the partial pressure of CO2 and the volume of H2O in the system, the production rate of CH4 can be further improved to 0.55 mmol\u202fg\u22121\u202fh\u22121 (Fig. 4f). The spherical-structured reaction system affords a catalytic performance of around 35-folds to that of the conventional reaction system which only allows one single incident photon pass. Besides, the apparent quantum efficiency in the spherical-structured reaction system reaches 0.38% at 800 nm. It offers the opportunity to utilize the Au/CuPd core-shell catalyst in PCO2RR under low-energy near-infrared irradiation.When applied in photocatalysis, single pure semiconductors often suffer from the issues of wide bandgap that limits the visible light harvesting, unmatched CB or VB potential positions to drive the catalytic reactions, etc. Integrating two semiconductors with different CB and VB potential positions to form a heterojunction becomes a useful approach to overcome these difficulties.There are three types of heterojunctions when integrating two different semiconductors. Type I is \u201cstraddling\u201d where the CB and VB of one semiconductor are entirely contained in those of another semiconductor (Fig. 5\na). In this type of heterojunction, both photo-generated electrons in CB and holes in VB of semiconductor I tend to transfer to the CB and VB of semiconductor II by the potential difference, respectively. It comes with two drawbacks. First, the CB and VB potentials of semiconductor II are less powerful to drive the catalytic redox reactions. Second, the accumulated photo-generated holes and electrons in semiconductor II exacerbate the recombination of the photo-generated carriers. Hence, the \u201cstraddling type\u201d heterojunction is not ideal for photocatalysis.Type II is \u201cstaggered\u201d where the CB and VB of one semiconductor overlap with those of another semiconductor (Fig. 5b), generating a favourable heterojunction. Once semiconductor I have more negative CB and VB potential positions than semiconductor II, the photo-generated electrons tend to transfer from the CB of semiconductor I to the CB of semiconductor II while photo-induced holes tend to transfer from the VB of semiconductor II to the VB of semiconductor I. This new possible transfer pathway created by the \u201cstaggered type\u201d heterojunction can separate the photo-generated electrons and holes in the semiconductors and significantly reduce the recombination rate of the photo-induced carriers in both semiconductor I and semiconductor II, boosting photocatalytic performance. The only drawback of this type of heterojunction is that the CB and VB that participate in the catalytic reactions are the less effective ones. Namely, the CB potential of semiconductor II is less negative than that of semiconductor I, and the VB of semiconductor I is less positive than that of semiconductor II.Type III is a \u201cbroken gap\u201d where the bandgaps of two semiconductors do not overlap (Fig. 5c). In this circumstance, the photo-generated electrons and holes between two semiconductors fail to transfer.\n29\n Therefore, the type II heterojunction is of interest in photocatalysis. Many works focus on the fabrication of type II heterojunction to boost catalytic performance. For example, a heterojunction between CsPbBr3 and graphitic carbon nitride (g-C3N4) is built based on the band position differences between g-C3N4 and CsPbBr3. It provides a stable CO yield rate of 975.57 \u03bcmol\u202fg\u22121\u202fh\u22121 in PCO2RR for 76 hours.\n30\n Zhu et al. constructed the heterojunction in a heterostructure of ZnIn2S4\u2013CdS for PCO2RR.\n31\n The ultrafast transient absorption spectroscopy proves the accelerated charge transport in the heterostructure (Fig. 6\n a and b). With the assistance of Co(bpy)3\n2+ as co-catalyst for CO2RR and TEOA as the sacrificial compound to consume photo-generated holes (Fig. 6e), it exhibits the CO yield of around 33 \u03bcmol in the first hour (Fig. 6c) under visible light irradiation and retains its initial catalytic activity after 5 cycles (Fig. 6d) of the PCO2RR measurements.As aforementioned, the type II heterojunction still shows limits concerning the CB and VB positions that participate in the catalytic reactions. Inspired by natural photosynthesis, an electron transfer pathway where the photo-induced electron in the CB of semiconductor II travels to the VB of semiconductor I to perform the recombination is proposed (Fig. 5d). It creates an electron transfer pathway with the shape of the letter Z, which is named \u201cZ-scheme\u201d in the heterojunction construction. The Z-scheme electron transfer pathway allows photo-generated carriers from the stronger CB and VB in the two semiconductors to participate in the catalytic reactions, which overcomes the difficulty raised in the type II heterojunction catalysts.In another approach, the Z-scheme is constructed without building any additional bridge between two semiconductors, this is called the direct Z-scheme system.\n6\n The construction of the internal electric field in the heterojunction between two semiconductors proves to be an effective strategy to achieve the direct Z-scheme. Wang et al. built a 2D/2D Z-scheme heterostructure of Ni\u2013CsPbBr3/Bi3O4Br for PCO2RR.\n32\n Driven by the difference in the Fermi levels of the two semiconductors, the photo-generated electrons transfer from Ni\u2013CsPbBr3 to Bi3O4Br. It leads to increased charge densities at the interface, positive on the Ni\u2013CsPbBr3 side and negative on the Bi3O4Br side, respectively. The internal electric field formed in this space charge region creates the band-bending effect. Fig. 7\na depicts the Z-scheme electron transfer pathway where the electrons are directed from the CB of Bi3O4Br to the VB of Ni\u2013CsPbBr3. Because of the bending effect, the shape of the electron pathway looks more like the letter \u201cS\u201d rather than \u201cZ\u201d. Under this circumstance, the term \u201cS-scheme\u201d is preferred over \u201cZ-scheme\u201d to highlight the band-bending effect. The Ni\u2013CsPbBr3/Bi3O4Br with the S-scheme heterostructure demonstrates an excellent 98.2% CO selectivity with a CO yield of 387.57 \u03bcmol\u202fg\u22121, which is more than 10 times higher than that of CsPbBr3 (Fig. 7b). Long et al. combined Fe2O3 and CdS to achieve the direct Z-scheme charge transfer pathway by a built-in internal electric field as well. The Z-scheme heterojunction of Fe2O3/CdS enables a CO yield of 9.3 \u03bcmol\u202fg\u22121 in the first hour during PCO2RR, which is much better than the sum of those of pure Fe2O3 and CdS.\n33\n The unique Z-scheme favors not only the separation of photo-generated electron/hole pairs but also the redox capacity of the photo-generated electrons and holes involved in catalytic reactions such as CO2RR.Alternatively, when the Z-scheme is constructed with the help of the additional bridge between two semiconductors, it is called the indirect Z-scheme system. For example, Wang et al. delicately fabricated lanthanum (La)- and rhodium (Rh)-doped SrTiO3 (SrTiO3:La, Rh) and the light absorber of Co(II) bis(terpyridine) modified molybdenum (Mo)-doped BiVO4 (BiVO4:Mo) and the RuO2 catalysts on a gold layer (SrTiO3:La, Rh|Au|RuO2\u2013BiVO4:Mo) for the PCO2RR (Fig. 7c). The Au layer in SrTiO3:La, Rh|Au|RuO2\u2013BiVO4:Mo is believed to bridge the CB of RuO2\u2013BiVO4:Mo and the VB of SrTiO3:La, Rh, which constructs an effective indirect Z-scheme for the PCO2RR to HCOO\u2212 and the H2O oxidation reaction. It affords an HCOO\u2212 selectivity of 97% with a production rate of around 20 \u03bcmol\u202fg\u22121\u202fh\u22121 (Fig. 7d).\n34\n\nThe catalyst structure plays an essential role during photocatalysis, benefiting the spatial separation of the photo-generated carriers for improved catalytic performance. Taking 2D catalysts as an example, the ultra-thin 2D nanosheets facilitate the transfer of the photo-generated electron/hole pairs to the surface of the catalyst, promoting catalytic activity. For instance, ultrathin 2D NiMgV-layered double hydroxide nanosheets afford excellent CO and CH4 productions in PCO2RR.\n35\n Liang et al. constructed a 2D dislocated bilayer MOF that allowed 100% product selectivity of CO in PCO2RR.\n36\n However, 2D structured materials often suffer from agglomeration, leading to the prolongation of the carriers\u2019 transfer and a decreased active area of the catalysts, which jeopardizes the catalytic performance. Wang et al. morphologically modified CuInZnS by introducing negatively charged Ti3C2T\nx\n to interfere with the nucleation and growth processes of CuInZnS. It creates a defect regulation in CuInZnS and results in thinner 2D nanosheets of CuInZnS with a bigger specific surface area and larger pore size than those of the pristine CuInZnS. The hybrid 2D Ti3C2T\nx\n-CuInZnS exhibits a CO production rate of 42.8 \u03bcmol\u202fg\u22121\u202fh\u22121 in PCO2RR.\n37\n With the proper synthetic routes, escalated hierarchical structures such as leaf/flower/litchi-like nanostructures can be fabricated through 2D nanostructures.\n38\n As aforementioned, Zhu et al. constructed the heterojunction in a heterostructure of ZnIn2S4\u2013CdS for PCO2RR.\n31\n The authors also took advantage of the hierarchical structure construction by anchoring 0D CdS quantum dots on the 3D ZnIn2S4 nanoflowers, which contributes to better charge transfer and separation during PCO2RR. Other than the stacked 2D nanosheets/nanofibers to construct hierarchical structures, the 3D core-shell structure is also a hot topic in this field. Li et al. reported a 3D core-shell heterostructure made of c-TiO2@aTiO2-x\n(OH)\ny\n with HO\u2013Ti-[O]-Ti surface frustrated Lewis pairs (SFLPs) on the shell in PCO2RR.\n39\n\nFig. 8\na illustrates that the SFLPs dissociate dihydrogen, forming hydrides and charge-balancing protonated hydroxyl groups at unsaturated Ti sites on the surface of the catalyst to promote the PCO2RR performance. The crystalline-amorphous heterostructure prolongs the lifetime of the electron/hole carriers (Fig. 8b). It allows a CO production rate of 5.3 mmol\u202fg\u22121\u202fh\u22121 which is 350 times of the original c-TiO2 catalyst.Wei et al. developed a hollow multi-shelled structure (HoMS) of CeO2@CeO2/TiO2 for CH4 production in PCO2RR. The high-angle annular dark field scanning TEM (HAADF-STEM) and X-ray energy dispersive spectral (EDS) mapping images in Fig. 8 e and f present the multi-shelled structure of CeO2@CeO2/TiO2. The quadruple inner shells are made of CeO2, which reduces CO2 into CO accumulated within the multi-shelled structure. The amorphous outer shell is made of TiO2, which further converts the accumulated CO into CH4 (Fig. 8g). The tandem CO2\u2013CO\u2013CH4 reaction of the HoMS CeO2@CeO2/TiO2 affords the CO and CH4 production rates of 97.6 \u03bcmol\u202fg\u22121\u202fh\u22121 and 15 \u03bcmol\u202fg\u22121\u202fh\u22121, respectively (Fig. 8 c and d). The HoMS structure can be destroyed by grinding CeO2@CeO2/TiO2 into debris, causing a dramatic catalytic performance decrease (CH4 production rate of 3.4 \u03bcmol\u202fg\u22121\u202fh\u22121). The control experiment highlights the significance of the hierarchical structure for the tandem reaction in PCO2RR.\n40\n It reveals that the rational structure construction strategy of hybrid catalysts can further promote catalytic performance by combining different functional semiconductors to achieve a complex reaction of PCO2RR for high-value-added products.The production of C2+ is difficult for CO2RR. Cu-based materials are the most known metal catalysts to facilitate the C\u2013C coupling for the formation of C2+ products.\n41\n Many Cu-based photocatalysts show catalytic activities towards the production of C2+ products, such as In\u2013Cu SA/PCN for CH3CH2OH production,\n42\n P/Cu SAs@CN for C2H6 production\n43\n and NiCo SA-TiO2 for the CH3COOH production.\n44\n With the assistance of the hierarchical structure engineering strategy, Cu-based photocatalysts demonstrate promising activity toward C2+ production in PCO2RR. Chakraborty et al. applied the operando surface reconstruction of Wurtzite phase CuGaS2 to form a 2D CuO layer-modified CuGaS2, which led to a C2H4 production of 20.6 \u03bcmol\u202fg\u22121\u202fh\u22121 with a selectivity of 75.1% in PCO2RR.\n45\n Jia et al. grew p-type Cu2O selectively on an Au bipyramid with the assistance of CTAB, fabricating a hetero-structure of dumbbell top which spatially separate Au and CuO active catalysts in an hybrid photocatalyst Au/CuO. Taking advantage of both the structural engineering and the LSPR effect, dumbbell-shaped Au/CuO affords much higher C2+ productions under near-infrared irradiation than under visible light irradiation.\n46\n\nNowadays, atomically dispersed catalysts and single-atom catalysts (SACs) have gained much attention for their maximum active site utilization and superior catalytic performance.\n47\n\n,\n\n48\n SACs have commonly supported metal catalysts that consist of isolated monometallic moieties (single metal atoms surrounded by neighboring atoms within the support), which offer well-defined active sites in the catalysts.\n49\n\n,\n\n50\n Integrating the state-of-art highly active SACs with semiconductor substrates holds great promise in the catalytic performance enhancement of PCO2RR. Anchoring the metal atoms on the substrate catalysts is currently one of the most common strategies to construct SAs such as Au, Ag, and Pt SAs on graphene.\n51\n Ou et al. fabricated Au SAs on red P support (Au1/RP) with low electronegativity to absorb CO2 for PCO2RR.\n52\n RP is a single-element constituent, which provides a uniform coordination environment for Au SAs. Fig. 9\n a-e depicts that Au SAs are evenly distributed on the RP support. Au1/RP affords a C2H6 production rate of 1.32 \u03bcmol\u202fg\u22121\u202fh\u22121 with a selectivity of 96% (Fig. 9f). The turnover frequency (TOF) also reaches 7.39 h\u22121, which is pretty high among many photocatalysts (Fig. 9g). The CO2 adsorption analysis (Fig. 9h) and the in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS, Fig. 9i) confirm that Au1/RP provides better CO2 adsorption and activation than those of pure RP. It is suggested that the P atoms around the Au SAs are electron rich and capable of serving as active sites for CO2 activation. In the meantime, the Au SAs can decrease the C\u2013C coupling energy barrier for C2H6 production in PCO2RR (Fig. 9j).Metal organic frameworks (MOFs) are popular precursors and templates in the CO2RR catalyst fabrication, as well as in the SACs synthesis.\n53\u201355\n The metal node and organic ligand structure of MOFs facilitate the formation of SAs with a post-treatment of pyrolysis.\n56\n\n,\n\n57\n Using MOFs as the precursors is a solid strategy to fabricate SAC-combined semiconductor catalysts. Other than the MOF strategy, the facile thermal polymerization strategy is also successful in SACs fabrication. Shi et al. reported a Cu\u2013In dual-metal SAC by the polymerization strategy for PCO2RR.\n58\n As demonstrated in Fig. 10\na, the metal ion salt, urea, and MIL-68 are used as precursors to fabricate the polymer. Followed by the calcination, Cu\u2013In dual site SACs are prepared. TEM results in Fig. 10 b-d show that Cu and In are evenly distributed on the CN nanosheets with isolated atom spots. The X-ray absorption spectroscopy reveals that the coordination environments of Cu and In in CuInCN are dominated by Cu\u2013N and In\u2013N rather than Cu\u2013Cu and In\u2013In, which confirms the single Cu and In atoms in the CuInCN catalyst (Fig. 10 e-p). The Cu\u2013In dual site SAC affords the superior CO yield rate of 1.2 mmol\u202fg\u22121\u202fh\u22121 that is almost ten times of the one of the original CN catalyst under visible light irradiation while maintaining the catalytic activities after 6 runs of the tests (Fig. 10 q and r).Besides, many novel synthetic strategies were discovered over recent years. For instance, Wang et al. applied a co-dissolution strategy to dissolve [PtV9O28]7\u2212 into [V10O28]6\u2212 to obtain the Pt single-atom catalyst that allowed a CH4 production rate of 247.6 \u03bcmol\u202fg\u22121\u202fh\u22121, much higher than that of Pt particles.\n59\n The numerous possibilities for the SAC synthesis of different elements hold great potential for refining the PCO2RR catalysts for better catalytic activities.For abiotic photocatalysts, the effective transfer of the photo-generated electrons into chemical bonds for CO2RR is challenging. Biohybrid catalysts offer alternative means for CO2RR for the production of biofuels and biochemicals with higher product selectivity by integrating catalyst materials with biological cells.\n60\u201366\n Many successful biohybrid semiconductor photocatalysts have been developed for PCO2RR. For example, moorella thermoacetica-based biohybrid photocatalysts are efficient for the production of acetic acid from CO2, such as the self-photosensitization of moorella thermoacetica/CdS nanoparticles (Fig. 11\n a and b)\n67\n and moorella thermoacetica/Au nanoclusters (Fig. 11 c and d).\n68\n Moreover, light-harvesting artificial cells containing cyanobacteria afford to fix CO2 into glucose.\n69\n\nOther than bacteria, protein can also be used in biohybrid semiconductor systems for PCO2RR. Saif et al. designed a \u2013NH2 group functionalized 1D protein-encapsulated CeO2 nanorods (PCNRs) for CO and CH4 productions in PCO2RR.\n70\n As depicted in Fig. 12\na, the bovine serum albumin (BSA) is applied as an efficient biotemplate to synthesize PCNRs. With TEOA as the electron donor to consume the holes and the assistance of RhB, PCNRs demonstrate excellent activity toward H2 production (Fig. 12b). When carried out in a CO2 gas environment, PCNRs show catalytic activities towards CH4 and CO productions under light irradiation of 400 nm\u202f<\u202f\u03bb\u202f<\u202f780 nm (Fig. 12c) with great suppression of H2 production. Fig. 12d and e reveal that PCNRs exhibit CO and CH4 production rates of 5.0 and 3.3 \u03bcmol\u202fh\u22121 g\u22121, which are 50 and 83 times higher than those of non-biohybrid CeO2, respectively. The authors believe that the protein hybrid PCNRs significantly enhance the material stability and facilitate the transfer of photo-generated holes to promote the separation process of photo-generated carriers.Researchers should still pay attention to some key points, concerning the future design of biohybrid photocatalysts. First, semiconductor materials that are compatible with bio cells need to be rationally constructed to protect microbial cells from deactivation while sufficiently harvesting visible light to provide enough electrons for CO2RR. Second, the interface of the biotic\u2013abiotic should be tailored for quick charge transfer, accelerating the separation rate of electron/hole pairs for better catalytic performance. Finally, the integrated bio-material should be able to efficiently utilize photo-generated electrons to produce fuel chemicals from PCO2RR.\n60\n\nAfter the design of highly efficient catalysts, many characterization techniques are required to understand the properties and uniqueness of the photocatalyst material for excellent catalytic efficiency. X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), UV\u2013Vis spectroscopy, and X-ray absorption spectroscopy (XAS) are common techniques for exploring the element valence, crystallization, bandgap value, and coordination environment information of the materials. Scanning electron microscopes (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) can directly provide visual images of morphologies and element distributions of catalyst materials. The combination of different characterization techniques is important to confirm one's assumption. For example, the analysis of XAS and XPS helps to confirm the N/O ratio in N\u2013Ti\u2013O/V[O] for the production of C2+ products on TiO2 catalysts.\n82\n For revealing the reaction mechanism of CO2RR, the density functional theory (DFT) simulations emerge as one of the most useful tools to demystify the structure-activity relationship and catalytic mechanism for complex catalytic systems such as hybrid catalysts.\n83\u201386\n In the meantime, in-situ characterizations, such as in-situ XRD, in-situ XAS, and in-situ Raman spectroscopy, are powerful experimental approaches to trace the evolution of the catalyst structures and reaction intermediates during the catalytic reaction of CO2RR.\n87\u201390\n Apart from Raman spectroscopy, in-situ Fourier transform infrared absorption spectroscopy (FTIR) can also provide evidence of reaction intermediates, uncovering the reaction pathway of CO2RR.\n91\n\n,\n\n92\n Wu et al. prepared an oxygen vacancy (Vo)-rich MoO2-x\n for PCO2RR.\n93\n The Vo-rich MoO2-x\n exhibits a CH4 production rate of 5.8 and 12.2 \u03bcmol\u202fg\u22121\u202fh\u22121 under NIR and full light irradiation in PCO2RR, which is around 10- and 7-fold to those of the Vo-poor MoO2-x\n, respectively (Fig. 13\na). Besides, the Vo-rich MoO2-x\n performs PCO2RR directly under an air atmosphere with a CO production rate of 6.5 \u03bcmol\u202fg\u22121\u202fh\u22121 under NIR irradiation (Fig. 13b). Vo-rich MoO2-x\n also demonstrates good stability in PCO2RR activity under NIR irradiation in concentrated CO2 after 4 runs (Fig. 13c). In-situ FTIR is applied to reveal the reaction mechanism of PCO2RR over the MoO2-x\n catalysts. Fig. 13d shows that carbonate species and *CO2\n\u2212 appear in the dark, suggesting the absorption and activation of CO2 on the Vo-rich MoO2-x\n surface. Additional peaks of *COOH (1593 cm\u22121), *CH3O (1170 and 1100 cm\u22121), and *CHO (1082 cm\u22121) appear and are gradually strengthened with the increase of illumination time under NIR (Fig. 13e). These intermediates are essential to the production of CH4 in CO2RR. When illuminated under full spectrum light, the IR peak intensities of the intermediates further increased (Fig. 13f). It reveals the efficient light response of Vo-rich MoO2-x\n and the possible reaction pathway (CO2\u2192 *CO2\u2192*COOH\u2192 *CO\u2192CO or *CHO\u2192*CH2O\u2192*CH3O\u2192CH4) for PCO2RR.With the advancing of characterization techniques, powerful characteristic techniques are exploited for revealing the compositional effects in hybrid catalysts for PCO2RR. Chen et al. recently applied spatiotemporally resolved surface photovoltage measurements (SPVM) on the facet and defect-engineered Cu2O catalysts (Fig. 14\na) to map the holistic charge transfer processes at the single-particle level on the femtosecond timescale.\n94\n\nFig. 14b depicts that the {001} facet has more accumulated photo-generated electrons than the {111} facet of the Cu2O octahedron, owing to the high Cu vacancies (VCu) on the {001} facet. Fig. 14c demonstrates that the anisotropic charge transfer is optimized with a truncated octahedral configuration, suggesting the contribution of the inter-facet built-in electric field to the anisotropic charge transfer. SPVM in Fig. 14d further illustrates that the moderate hydrogen-compensated VCu (H\u2013VCu) results in an efficient spatial separation of the photo-generated carriers on {111} and {001} facets. On the contrary, the extreme incorporation of H\u2013VCu leads to the quench of the photo-generated electron/hole pairs (Fig. 14e). The photoemission electron images in Fig. 14f visualize the dynamics of anisotropic electron transfer for single Cu2O particle, indicating that the ultrafast inter-facet electron transfer contributes significantly to the anisotropic electron distribution. Au being selectively deposited on the {001} facet of Cu2O can also be successfully probed by SPVM (Fig. 14 g-i). It is confirmed that the H2 evolution performance is associated with the anisotropic charge transfer of Cu2O (Fig. 14j). This powerful SPVM technique brings meaningful insights into the photo-carrier transfer dynamics, which can be transplanted to PCO2RR. With more advanced characterization techniques developed and applied to the PCO2RR catalysts, the rational design of the next-generation photocatalyst for excellent catalytic performance can be precisely and systematically guided.To sum up, semiconductor photocatalysts often suffer from unsatisfying catalytic performance (e.g. with production rate at the level of \u03bcmol g\u22121 h\u22121) in PCO2RR, owing to the poor light harvesting ability, the low separation rate of the photo-induced carriers, and stability issues. Designing novel semiconductor materials with highly efficient catalytic performance for PCO2RR that address these issues is a priority. The hybridization of semiconductor catalysts through different approaches such as surface modification and band engineering strategies can integrate the advantages of the different semiconductor catalysts and co-catalysts to prohibit the recombination of the photo-generated electron/hole pairs and promote the light response of the semiconductor catalysts for PCO2RR. The hierarchical structure construction of semiconductor catalysts also contributes to the separation of the photo-generated electron/hole pairs and sometimes can even achieve a spatially coordinated tandem reaction to produce C2+ products from PCO2RR. Active sites are essential to catalysis. Anchoring highly catalytically effective SACs on semiconductor catalysts holds great promise for the augmentation of catalytic activities in PCO2RR. To improve product selectivity, integrating biological materials that are highly selective in photosynthesis with semiconductor catalysts has been proven to be an effective solution. Moreover, by using advanced characterization techniques for in-situ probing, the underlying mechanism of the reaction pathway and catalyst structure evolution can be demystified for next-generation PCO2RR catalyst design. Other than the proposed solutions discussed above, current research on PCO2RR still needs to focus on the following perspectives:\n\n\u2022\nThe duration of the catalytic performance is always a big issue in PCO2RR. Most photocatalysts present a catalytic activity duration of dozens of hours, which is excessively low for industrial requirements (more than thousands of hours).\n95\n\n,\n\n96\n The fabrication of highly active semiconductor materials for CO2RR without accumulating residues of poisoning intermediates is forever a priority. Besides, the addition of the protective layer on the surface of photocatalysts to mitigate the decay of the catalyst structure during PCO2RR could be a possible solution to enhance the catalyst stability.\n7\n\n\n\n\n\u2022\nIn addition, the combination of photocatalysis with other catalytic methods such as electrochemical catalysis and thermal catalysis (e.g. photo-electrocatalysis) holds great potential to improve both the catalytic activities and product selectivity in CO2RR.\n97\n\n,\n\n98\n CO2 capture and storage (CCS) takes a vital part in the mitigation of over-emitted CO2 because it is energy-consuming.\n99\n Sorbent porous materials such as metal oxides,\n100\u2013102\n zeolites and amine-functionalized silicas,\n103\n covalent organic frameworks (COFs),\n104\u2013106\n MOFs,\n107\u2013109\n and porous carbons\n110\n are effective toward CO2 captures. Many of these sorbent materials also demonstrate photocatalytic activity towards CO2RR. Hence, coupling CO2-capture with PCO2RR can be a practical means in the further market for better efficiency.\n111\u2013115\n\n\n\n\n\u2022\nTo finally achieve a practical application, a rational catalysis setup needs to be designed for high catalytic efficiency and scale-up production. Many promising CO2RR systems have been developed for electrolysis, such as gas phase flow cells, solid oxide electrolysis cells (SOECs), etc.\n116\u2013118\n For the photocatalysis-related setup upgrade, there are also some interesting discoveries, e.g., the back-illuminated photoelectrochemical flow cell for the increased solar-to-fuel conversion efficiency,\n119\n the aerobic environment for the improved PCO2RR in a less restricted reaction condition.\n120\n\n\n\n\nThe duration of the catalytic performance is always a big issue in PCO2RR. Most photocatalysts present a catalytic activity duration of dozens of hours, which is excessively low for industrial requirements (more than thousands of hours).\n95\n\n,\n\n96\n The fabrication of highly active semiconductor materials for CO2RR without accumulating residues of poisoning intermediates is forever a priority. Besides, the addition of the protective layer on the surface of photocatalysts to mitigate the decay of the catalyst structure during PCO2RR could be a possible solution to enhance the catalyst stability.\n7\n\nIn addition, the combination of photocatalysis with other catalytic methods such as electrochemical catalysis and thermal catalysis (e.g. photo-electrocatalysis) holds great potential to improve both the catalytic activities and product selectivity in CO2RR.\n97\n\n,\n\n98\n CO2 capture and storage (CCS) takes a vital part in the mitigation of over-emitted CO2 because it is energy-consuming.\n99\n Sorbent porous materials such as metal oxides,\n100\u2013102\n zeolites and amine-functionalized silicas,\n103\n covalent organic frameworks (COFs),\n104\u2013106\n MOFs,\n107\u2013109\n and porous carbons\n110\n are effective toward CO2 captures. Many of these sorbent materials also demonstrate photocatalytic activity towards CO2RR. Hence, coupling CO2-capture with PCO2RR can be a practical means in the further market for better efficiency.\n111\u2013115\n\nTo finally achieve a practical application, a rational catalysis setup needs to be designed for high catalytic efficiency and scale-up production. Many promising CO2RR systems have been developed for electrolysis, such as gas phase flow cells, solid oxide electrolysis cells (SOECs), etc.\n116\u2013118\n For the photocatalysis-related setup upgrade, there are also some interesting discoveries, e.g., the back-illuminated photoelectrochemical flow cell for the increased solar-to-fuel conversion efficiency,\n119\n the aerobic environment for the improved PCO2RR in a less restricted reaction condition.\n120\n\nThere is no conflict of interest to declare.This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de Recherche du Qu\u00e9bec-Nature et Technologies (FRQNT), Centre Qu\u00e9b\u00e9cois sur les Materiaux Fonctionnels (CQMF), Institut National de la Recherche Scientifique (INRS), and \u00c9cole de Technologie Sup\u00e9rieure (\u00c9TS). Dr. G. Zhang thanks for the support from the Marcelle-Gauvreau Engineering Research Chair program.", "descript": "\n                  Using clean solar energy to reduce CO2 into value-added products not only consumes the over-emitted CO2 that causes environmental problems, but also generates fuel chemicals to alleviate energy crises. The photocatalytic CO2 reduction reaction (PCO2RR) relies on the semiconductor photocatalysts that suffer from high recombination rate of the photo-generated carriers, low light harvesting capability, and low stability. This review explores the recent discoveries on the novel semiconductors for PCO2RR, focusing on the rational catalyst design strategies (such as surface engineering, band engineering, hierarchical structure construction, single-atom catalysts, and biohybrid catalysts) that promote the catalytic performance of semiconductor catalysts on PCO2RR. The advanced characterization techniques that contribute to understanding the intrinsic properties of the photocatalysts are also discussed. Lastly, the perspectives on future challenges and possible solutions for PCO2RR are presented.\n               "}
{"full_text": "In recent decades, much attention has been paid to various methods of water remediation, including sorption (Abdel Maksoud et al., 2020; Ahsan et al., 2020; Bartczak et al., 2018; El-Sayed, 2020; Fang et al., 2020; \u017b\u00f3\u0142towska-Aksamitowska et al., 2018), ozonation (Schmitt et al., 2020), photocatalytic and catalytic approaches (Acharya and Parida, 2020; Fang et al., 2020; Kubiak et al., 2020; Kumari et al., 2020; X. Liu et al., 2020b; Lu and Astruc, 2020; Siwi\u0144ska-Ciesielczyk et al., 2020), membrane separation and sedimentation (Chen et al., 2020). However, each method has drawbacks and may lead to the production of problematic wastes requiring safe and efficient disposal. Although there is no universal solution to these problems, catalysis currently seems to have a crucial role in the development of effective processes and methods that can maximize effectiveness and minimize waste generation and energy demand (De et al., 2016).Particular interest is focused on metal-based catalysts, which have been extensively utilized in a wide range of applications (Finiels et al., 2014; Lee and Lee, 2020; H. Liu et al., 2020a; Ma et al., 2019; Yan et al., 2016; Yang et al., 2019; Zhang et al., 2007; Zheng et al., 2017; Zuo et al., 2016), including biotechnological treatment processes (El-Sayed, 2020; Jankowska et al., 2019; Zdarta et al., 2019). Such materials have been investigated in particular with regard to their use in oxidation\u2013reduction reactions of organic compounds (Ambursa et al., 2021; Parmeggiani and Cardona, 2012). For comparison, Table 1\n presents a set of studies on the catalytic oxidation of phenol and its derivatives and the reduction of 4-nitrophenol using various metal-based catalysts.As shown above, heterogeneous catalysis offers endless possibilities in the degradation of organic compounds via either oxidation or reduction reactions. Recent results have proved the usefulness of a wide range of materials in the removal of phenolic compounds from water and wastewater. It is notable that composites based on transition metals are most commonly used in the degradation of organic compounds (Verdine, 2019).Among the metals commonly applied in catalysis, nickel is one of the best known (Finiels et al., 2014; Lipshutz et al., 2003; Plumejeau et al., 2015). It is one of the most abundant elements in the Earth's crust and is approximately 5000 times cheaper than gold (De et al., 2016). Due to the long history of the use of nickel in catalysis, the literature on nickel catalysts is vast and covers an enormous number of reactions, including oxidation, reduction, hydrogenation and reforming reactions (Lipshutz, 2001; Lipshutz et al., 2003). Despite the significant success of applications of this element in industry, purely nickel-based catalysts still cannot be used for environmental applications, due to problems with selectivity, stability and activity (Kour et al., 2020; Qin et al., 2020). To overcome these obstacles, nickel\u2013carbon composites have been developed. The use of carbon as a support enables the good dispersion of the metal-containing phase on the support surface. Moreover, the properties of carbonized materials, including high chemical and thermal stability, high porosity, low density and weight, may help to fulfil the strict requirements for environmental applications (De et al., 2016).The main aim of this study was to prepare novel, 3D fibrous-like nickel-based bio\u2011carbons and test their potential use as catalysts in model reactions. As a source of carbonaceous material, spongin-based scaffolds derived from the marine demosponge Hippospongia communis were used. This biopolymer creates unique systems of channels built by interwoven fibres. Spongin chemistry is considered complex. Despite some similarities to collagen and keratin, spongin is distinguished by the presence of halogens (such as I and Br) and xylose. The physicochemical properties of spongin-based scaffolds, including thermal stability, persistence in acidic media and the presence of various heteroatoms, suggest that they can be used as an innovative precursor of bio\u2011carbons, including metallized 3D carbon materials (Petrenko et al., 2019; Szatkowski et al., 2018). In this study, the low-temperature carbonization of spongin-based scaffolds was used to generate hierarchical 3D carbonaceous structures preserving the original morphology of the spongin-based skeleton. The scaffolds underwent modification with nickel compounds via the simple and fast sorption reduction method, to obtain novel catalysts. The resulting nickel\u2013carbon composites were effective catalysts in the reduction and oxidation reactions of various phenolic compounds. The reaction kinetics and the reusability of the prepared catalysts were also investigated, and possible mechanisms of reduction and oxidation were proposed.Spongin-based skeletons of the marine sponge Hippospongia communis (Porifera: Demospongiae), purchased from INTIB GmbH (Germany), were used as a precursor material. The samples were cleaned in distilled water for 1 h, then moved to an ultrasonic bath for 40 min. The sponge skeletons were then immersed in 3 M HCl in a purification process. This process was carried out in three stages; after each stage the solution of HCl was exchanged for a fresh one with a concentration of 3 M. The first and second stages of purification were conducted for 6 h, and the third for 3 h. After the acid purification process, samples were cleaned with distilled water to neutral pH, dried, and cut into smaller pieces. The prepared material was subjected to a carbonization process. Carbonization of spongin-based samples was conducted in an R 50/250/13 tube furnace (Nabertherm, Germany) in a nitrogen atmosphere. The process was carried out in a temperature range from 400 to 600 \u00b0C, with a 1 h plateau and a heating rate of 10 \u00b0C/min, and cooling by thermal inertia to 50 \u00b0C. Before the carbonization process, samples were conditioned for 2 h in a nitrogen atmosphere at a temperature of 20 \u00b0C.The method was based on the treatment of carbon materials with a solution of nickel nitrate in a concentration of 5 mg/L. Each sample was placed in a three-neck round-bottom flask filled with 50 mL of nickel nitrate salt solution. The first stage, including sorption, was carried out for 1 h with continuous stirring (800 rpm). Next, the reduction was carried out by dropping into the former solution 50 mL of 0.5 mol/L sodium borohydride at a rate of 5 mL/min. After dosing, the reduction process was continued for an additional 30 min, again with continuous stirring (800 rpm). The sorption and reduction procedures were repeated three times. Finally, the metallized materials were dried at 60 \u00b0C.The crystalline structure of the prepared materials was evaluated by the X-ray diffraction method, using a Rigaku Miniflex 600 analyser (Rigaku, Japan) operating with Cu K\u03b1 radiation (\u03b1 = 1.5418 \u00c5). Patterns were obtained over an angular range of 10\u201380\u00b0. Parameters of the crystalline structure of the samples were calculated using PDXL: Integrated X-Ray Powder Diffraction Software (Rigaku, Japan). The analysis was based on the International Centre for Diffraction Data (ICDD) database.SEM analysis was performed using an EVO-40 scanning electron microscope (Zeiss, Germany). Transmission electron microscopy (TEM) investigations were carried out using a Hitachi HT7700 microscope (Hitachi, Tokyo, Japan) operating at an accelerating voltage of 120 kV. Materials were prepared in epoxide resin and cut into thin layers using a microtome to prepare specimens. EDS X-ray microanalysis was prepared using a Tescan apparatus (Czech Republic) with Gamma-Tec instrumentation from Princeton Inc. (USA). Energy-dispersive X-ray fluorescence spectrometry (XRF) was carried out using an Epsilon 4 spectrometer equipped with a high-resolution silicon drift detector (SDD), typically 135 eV@ Mn-K\u03b1 (Malvern Panalytical, UK).XPS analysis was performed using a Prevac spectrometer (Prevac Ltd.) with a hemispherical Scienta R4000 electron analyser with a Scienta SAX-100 X-ray source (Al K\u03b1, 1486.6 eV, 0.8 eV band) and an XM 650 X-ray monochromator (0.2 eV band). The pass energy of the analyser was set to 50 eV for the regions (high-resolution spectra) Ni 2p, O 1s and C 1s (with a 50 meV step). The base pressure in the analysis chamber was 5\u00b710\u2212\n9 mbar, and the pressure during the collection of spectra was not higher than 3\u00b710\u2212\n8 mbar. The porosity characteristics of the obtained materials were determined by the multipoint BET (Brunauer\u2013Emmett\u2013Teller) method using data for adsorption under relative pressure (p/p\n\no\n) obtained with an ASAP 2020 instrument (Micromeritics Instrument Co., USA). FTIR analysis was performed with a Vertex 70 apparatus (Bruker, Germany) using the attenuated total reflection (ATR) method. Electrophoretic mobility was measured using a Zetasizer Nano ZS instrument equipped with an autotitrator (Malvern Instruments Ltd., UK) by analysing 0.01 g of catalyst in 25 mL of 0.001 mol/L sodium chloride solution at 25 \u00b0C. Changes in the conductivity and pH values of the suspension were observed during the measurement. The pH of the suspensions was adjusted by an automatic titrator using hydrochloric acid (0.2 mol/L) or sodium hydroxide (0.2 mol/L). The zeta potential was obtained from the electrophoretic mobility by the Smoluchowski equation (Sze et al., 2003).To evaluate the possible application of the prepared metallized materials, they were used as catalysts in the reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). This reaction was carried out in a quartz cuvette containing 2.5 mL of 4-nitrophenol solution in water (concentration 10 mmol/L). After adding a water solution of sodium borohydride (0.5 mL with concentration 100 mmol/L) and 5 mg of catalyst, the reaction was started. The reduction progress was measured using a UV\u2013Vis spectrophotometer (Jasco V700, Japan) based on spectra obtained after every 60 s of the reaction. In addition, the kinetics of the reaction were calculated based on the pseudo-first-order kinetic model. The choice of this model is a consequence of the excess quantity of sodium borohydride used during the reaction, which means that its concentration can be assumed constant. The proposed model can be described with Eq. (1):\n\n(1)\n\nln\n\n\n\nC\nt\n\n/\n\nC\n0\n\n\n\n=\nln\n\n\nC\n0\n\n\n\u2013\nkt\n\n\nwhere C\n\n0\n and C\n\nt\n denote the initial concentration of 4-NP and the concentration at time t (min), and k (min\u2212\n1) denotes the rate constant.The oxidation reaction was carried out in a three-neck round-bottom flask placed in a water bath. First, the catalyst (50 mg), phenol (50 mL of water solution at concentration 0.5 mmol) and 1 mL of 31 wt% hydrogen peroxide (H2O2) were loaded into the reactor. This mixture was stirred (800 rpm) at 60 \u00b0C for 4 h under a reflux condenser and then immediately cooled in an ice bath to stop the reaction. Then methanol was added to the mixture to quench the reaction and remove excess hydrogen peroxide. The HPLC-MS system was used to analyse the efficiency of oxidation. The same starting conditions were used for the oxidation of 4-chlorophenoxyacetic acid and methylchlorophenoxypropionic acid. Due to the exothermic nature of this reaction, the concentration of reagents was kept low. Moreover, the agitation speed, reaction volume and amount of catalyst were chosen to avoid external diffusion problems.LC analysis was performed using the UltiMate 3000 RSLC chromatographic system from Dionex (Sunnyvale, CA, USA). 5 \u03bcL samples were injected into a Hypersil Gold C18 RP analytical column (100 \u00d7 2.1 mm \u00d7 1.9 \u03bcm) (Thermo Scientific, USA). In the typical procedure, the column was kept at 35 \u00b0C, and the mobile phase was 5 mM ammonium acetate in water (A) and methanol (B), at a flow rate of 0.2 mL/min. The following gradients were used: 0 min 80% B, 5 min 100% B for phenol, and 0 min 50% B, 5 min 100% B for 4-CPA and MCPP. The LC system was connected to an API 4000 QTRAP triple quadrupole mass spectrometer (AB Sciex, Foster City, CA, USA). Compounds were determined by electrospray ionization mass spectrometry (ESI-MS) in negative ion mode. The analyte was detected using the following settings for the ion source and mass spectrometer: curtain gas 10 psi, nebulizer gas 40 psi, auxiliary gas 40 psi, temperature 200 \u00b0C, ion spray voltage \u22124500 V, collision gas set to medium. For more details, see Supplementary Note 1.The evaluation of physicochemical properties is an important step in describing a new catalytically active material. The XRD patterns of the functionalized catalysts and of the carbonized spongin-based supports before functionalization are shown in Fig. 1\n.As shown in Fig. 1, the XRD patterns exhibit diffractions characteristic for different phases of nickel, carbon, and silicon dioxide. The XRD patterns of the catalysts (Fig. 1A) do not show significant differences. The nickel phase is represented by the three broad diffraction peaks at 12\u00b0, 34\u00b0 and 60\u00b0, which can be indexed respectively to Ni(OH)2 (001), Ni(OH)2 (110) and Ni(OH)2 (003) (JCPDS no. 1011134). The formation of a nickel oxide phase is confirmed by the presence of diffraction peaks derived from the NiO (220) phase at 46\u00b0 (JCPDS no. 1010095). A small but well visible peak, characteristic for the Ni (111) phase, appeared at 43\u00b0, and a peak for Ni (220) at 72\u00b0 (JCPDS no. 9008509). The diffractogram of the catalyst also contains a broad peak characteristic for C(002) (JCPDS no. 9011577). However, the intensity of this peak is significantly lower than for the corresponding phase in the XRD spectra of the pure supports (Fig. 1B). Also, features derived from various forms of silica (Fig. 1B) are not visible in the XRD pattern of the catalysts. This seems to be a result of the modification process, where the carbonized fibres are covered with a metal-containing phase. The strong effect of nickel atoms and the low content of silica compounds results in a lack of silica features in the XRD diffractogram of the catalysts.The chemical reduction of nickel ions adsorbed onto the surface of carbonaceous scaffolds is an efficient method of functionalization and is well described in the literature (Chen et al., 2017; Kuang et al., 2001; Sahiner et al., 2010). The SEM + EDS analysis provides information about the surface morphology and chemistry of the examined samples (results are shown in Fig. 2\n).From the Ni mapping presented in Fig. 3\n, it is apparent that the metal-containing phase evenly covers all of the prepared catalysts. The highest efficiency of nickel functionalization was achieved for the NiO/Ni(OH)2/Ni_600 catalyst (26.01% mass of Ni), and the lowest for the sample carbonized at 500 \u00b0C (15.19% mass of Ni). The variation in the amount of nickel on the surface of the carbonized materials may be explained by the different ability of the carbonized scaffolds to bind nickel ions during the modification process. As has been described elsewhere (Petrenko et al., 2019; \u017b\u00f3\u0142towska et al., 2021), the increase in carbonization temperature results in the formation of bio\u2011carbons with higher carbon content and decreasing content of other heteroatoms, including sulphur and nitrogen, among others. Consequently, the use as supports of bio\u2011carbons obtained at different carbonization temperatures leads to the formation of composites with different nickel loading. Owing to this fact, and referring to the results obtained in previous studies (Petrenko et al., 2019; \u017b\u00f3\u0142towska et al., 2021), a broad statement can be made that the affinity of nickel ions towards the surface of bio\u2011carbons increases with the content of carbon and the level of graphitization of the carbonized material.Moreover, XRF analysis was performed to evaluate the elemental composition. The results provide evidence of traces of bromine and iron within the structure of the prepared materials; however, the content of these elements is low. Thus, it is assumed that their presence has no significant effect on the catalytic ability; rather, these elements participate in increasing the diversity of surface functional groups. A detailed discussion is given in Supplementary Note 2, and information regarding the surface functional groups is presented in Supplementary Note 5.It should be noted that the presence of iron (Fe) and aluminium (Al) is linked to the natural origin of the spongin-based scaffolds (Szatkowski et al., 2017; Jesionowski et al., 2018), while the silicon and calcium, according to previous research (Petrenko et al., 2019), are internal elements of the spongin skeleton. As is shown, the applied purification treatment with HCl acid results in the complete removal of calcium; however, it does not lead to the total removal of silicon species. Thus, carbonized spongin-based scaffolds appear to consist of a naturally occurring composite containing carbon, oxygen, nitrogen, sulphur and silicon traces, aluminium, and iron. For catalytic purposes, the existence of small amounts of silicon dioxide or alumina (less than 1 wt%) does not exclude the use of the bio\u2011carbons as supports. Moreover, the presence of heteroatoms such as sulphur, nitrogen, and iron in the catalyst structure may enhance its catalytic properties (Moosapour Siahkalroudi et al., 2021; Wang et al., 2021).The results of higher-resolution microscopy analysis (SEM and TEM) of the obtained composites are shown in Fig. 3. SEM images of the spongin-based scaffolds before carbonization and functionalization are compared in Supplementary Note 3.The results of scanning electron microscopy analysis provide evidence that the functionalization of the carbonaceous supports through reduction of adsorbed nickel particles is an effective method. Consequently, the naturally prefabricated, three-dimensional scaffolds are tightly covered with the metal-containing phase, which forms particular structures. Interestingly, the morphology of the metal-containing phase varies with the support used. For the NiO/Ni(OH)2/Ni_400 material, the metal-containing phase creates needle-like structures with a length of around 3\u20135 \u03bcm (Fig. 3A). For NiO/Ni(OH)2/Ni_500, spherical agglomerates of the metal-containing phase are visible (Fig. 3C), while the surface of NiO/Ni(OH)2/Ni_600 is again characterized by the presence of needle-like structures (Fig. 3E), although these structures are thicker and longer than those observed on the fibres of NiO/Ni(OH)2/Ni_400. These interesting differences in the morphological structure of the catalysts cannot be related to the method of synthesis, because all supports were treated in the same way. A possible explanation of these variations may be differences in the loading of the nickel-containing phase; as was mentioned previously, the material prepared using the carbonized support obtained at the highest carbonization temperature seems to have a superior ability to bind the nickel species, and thus the agglomerates formed are larger than in the case of the other materials. Consequently, differences in surface chemistry, such as graphitization level and carbon content, may affect the efficiency of the functionalization process and indirectly the final structure of the metal-containing phase (Petrenko et al., 2019). The influence of surface properties on the loading of the metal-containing phase, as observed in this study, is a well-known phenomenon sufficiently described in other works (Alijani et al., 2021; Peng et al., 2021).The most promising aspect of the application of carbonized spongin-based scaffolds as a support for the metal-containing phase is their three-dimensional fibrous architecture. The SEM images show a unique structure of interlaced fibres, which usually form channels with diverse shapes: triangle-like, rectangular-like, pentagonal-like and hexagonal-like, with sizes ranging from 1 to 300 \u03bcm. These structures are well visible on the SEM images (for comparison, see Fig. 2 and Supplementary Note 3). Therefore, it should be emphasized that the approach proposed in this paper, where the spongin-based scaffold was used as a bio-template together with a simple method of functionalization, led to the obtaining of desirable 3D structures.The TEM images in Fig. 3B, D, F show the metal-containing phase deposited on the bio\u2011carbon fibres. It can be assumed that in nanoscale, the structure of the metal-containing phase is similar and consists of thin sheets. Moreover, it seems that the modification process results in thick aggregates with irregular structure, forming the metal-containing layer on the surface of the carbonized bio\u2011carbon (Huang et al., 2013). Additionally, in Fig. 3D, some contaminants \u2013 derived from silicon dioxide \u2013 are marked with arrows.XPS analysis was carried out to evaluate in detail the surface composition of the prepared catalysts. The spectra obtained are shown in Fig. 4\n.The Gaussian fitting method was used for a comprehensive analysis of the oxidation state of the nickel as well as the contributions of oxygen and carbon. In the case of all catalysts, the Ni 2p core-level spectra show two intense peaks at around 855.5 and 871 eV, attributed to Ni 2p3/2 and Ni 2p1/2 respectively, with corresponding satellites at around 861.3 and 879.7 eV, characteristic for Ni2+ (An et al., 2014; Cheng et al., 2017; Zhou et al., 2017). The core\u2013shell peaks are attributed to NiO bonds, in this case associated with the hydroxide, as was confirmed in XRD analysis. The XPS spectra do not show any features corresponding to the NiO phase because its characteristic peaks appear at lower binding energies. These results might be related to additional atmospheric moisture adsorption. The O 1s spectra exhibit three oxygen contributions, labelled O1, O2 and O3. The O1 peak, derived from O\u2013O\u2013C bonds, is located at 534.1 eV (peak area 27.74%) for the NiO/Ni(OH)2/Ni_400 catalyst, 534.0 eV (peak area 10.35%) for NiO/Ni(OH)2/Ni_500 and 534.2 eV (peak area 8.36%) for NiO/Ni(OH)2/Ni_600. Its existence indicates that CO2 molecules were adsorbed on the surface of each catalyst. The O2 peak is commonly ascribed to physi-/chemisorbed water within the material's interface. The O3 feature, at 531.9 eV (peak area 35.04%) for NiO/Ni(OH)2/Ni_400, 531.3 eV (peak area 44.87%) for NiO/Ni(OH)2/Ni_500 and 534.5 eV (peak area 51.48%) for NiO/Ni(OH)2/Ni_600, is characteristic for metal\u2013oxygen bonds (Payne et al., 2012; Weidler et al., 2017). A comparison of the C 1s spectra is given in Supplementary Note 4.The presented spectra do not differ significantly in terms of the surface composition, but show differences in the contents of various elements. Such results are not surprising considering the catalyst preparation method. Nevertheless, the XPS results show that NiO/Ni(OH)2/Ni_400 has the highest amount of CO2 adsorbed on the surface and produces more intense satellite peaks than the NiO/Ni(OH)2/Ni_500 and NiO/Ni(OH)2/Ni_600 catalysts.Further determination of surface functional groups was performed using FTIR spectroscopy. It was proven that the prepared materials contain various functional groups, including hydroxyl, amino and sulfoxide groups, among others. Besides, the formation of NiO groups is indicated (for detailed investigation, see Supplementary Note 5). The effect of pH on the zeta potential was also evaluated to investigate the electrokinetic behaviour of the prepared composites. The results indicate that the contents of nickel species and the electron releasing groups impact the value of the isoelectric point. Consequently, the NiO/Ni(OH)2/Ni_600 material, which has the highest content of nickel moieties, also has the highest IEP. A detailed discussion of the electrokinetic behaviour of the prepared materials is given in Supplementary Note 6.\nFig. 5\n presents the porous structure parameters, examined using the low-temperature nitrogen sorption technique.The nitrogen sorption capacity is highest for the NiO/Ni(OH)2/Ni_500 catalyst. It is slightly lower for NiO/Ni(OH)2/Ni_400 and approximately three times lower for NiO/Ni(OH)2/Ni_600. These results correspond to the decrease in the calculated BET surface area. The sorption isotherms of NiO/Ni(OH)2/Ni_400 and NiO/Ni(OH)2/Ni_500 can be classified as type IV isotherms. The well-visible hysteresis loop suggests that these materials have a mesoporous structure, with pore condensation at high pressure. The NiO/Ni(OH)2/Ni_400 catalyst has the largest pore size among the prepared materials, and NiO/Ni(OH)2/Ni_500 has the highest BET surface area. The isotherms of the NiO/Ni(OH)2/Ni_600 catalyst can be classified as type II, typical for low-porous materials containing both macropores and mesopores but no micropores. This result agrees with the finding that this sample has the lowest surface area.However, it must be recalled that the method of catalyst preparation results in coverage of the fibres of the carbonized spongin-based scaffolds with the metal-containing phase. For this reason, the surface area of the prepared materials does not exceed 10 m2/g. As a result, the three-dimensional fibrous support structure with channels of diverse shape may provide good accessibility to the catalyst surface.The catalytic properties of the prepared materials were first tested in the reduction reaction of 4-nitrophenol. This reaction is widely used as a determinant of the catalytic activity of heterogeneous materials, whether or not involving a support (Emam et al., 2017; Grzeschik et al., 2020; Hu et al., 2015; Strachan et al., 2020). Moreover, 4-nitrophenol is an important but toxic substrate used in the production of various drugs, dyes, and pesticides. Therefore, the evaluation of a fast and straightforward method of converting this compound to a functional product is potentially of great benefit from the engineering and environmental point of view (Blaser, 2006).The UV\u2013Vis spectra (Fig. 6\n) measured during the reaction in the presence of the prepared catalysts show that all of the tested materials exhibit catalytic ability in the reduction of 4-nitrophenol. The reaction time varies between 4 and 6 min, whereas without the catalyst this reaction does not occur, as explained in Supplementary Note 7. The addition of sodium borohydride led to an increase in the reaction mixture pH from 7 to 10; in such conditions, the functional groups on the surface of the catalysts are deprotonated (see zeta potential measurements in Supplementary Note 4) and are thus negatively charged. Despite the negatively charged surface of the catalyst, only for the NiO/Ni(OH)2/Ni_400 material was an induction period observed; this is apparently related to charging transformation of the surface of the catalyst before reaction (Khalavka et al., 2009; Mahmoud and El-Sayed, 2011; Sarkar et al., 2011; Wu et al., 2011). In the case of the NiO/Ni(OH)2/Ni_500 and NiO/Ni(OH)2/Ni_600 catalysts, different behaviour was observed in the reduction of 4-nitrophenol. For the first-mentioned material, a slow reduction of the peak intensity assigned to the 4-nitrophenolane anion is visible, while for NiO/Ni(OH)2/Ni_600, the peak intensity decreases rapidly after 60 s of the reaction, and the reaction is completed after 4 min. This behaviour may be related to a different path of reduction. For these materials, surface-mediated hydrogen transfer seems to play a leading role during the reduction of 4-nitrophenol.Because the reducer, sodium borohydride, was applied in significant excess, the kinetics of the reaction were calculated based on the pseudo-first-order kinetic model (Table 2\n) (Jiang et al., 2012; Sahiner et al., 2010; Yang et al., 2019; Zhu et al., 2011).The highest reaction rate constant was calculated for the NiO/Ni(OH)2/Ni_400 catalyst. However, the time of reaction is similar for each catalyst used, although the value of the rate constant varies significantly. Even though the highest reaction rate was obtained for the NiO/Ni(OH)2/Ni_400 catalyst, the correlation coefficient took the lowest value. The variations in catalytic activity seem to be related to the structure of the prepared catalyst. The NiO/Ni(OH)2/Ni_400 material has a lower content of nickel phases than NiO/Ni(OH)2/Ni_600, although the needle-like structures observed on the surface of both catalysts are thinner and shorter in the case of NiO/Ni(OH)2/Ni_400 (see Fig. 4A). This may increase the contact area between the reagents and lead to an increase in the diffusion of substrates (Pushkarev et al., 2012; Wang et al., 2014).Additionally, the NiO/Ni(OH)2/Ni_400 material is characterized by the presence of iron and aluminium, among others, which may create additional active centres of the catalyst. The fact that NiO/Ni(OH)2/Ni_500 has the lowest catalytic activity may therefore be explained by the different morphological structure of the metal-containing phase, together with the lowest content of nickel phases. Since it had the highest rate constant (k), only NiO/Ni(OH)2/Ni_400 was considered for further evaluation of reusability. This material was repeatedly used five times in the catalytic reduction reaction to assess its stability. After each cycle, the catalyst was recovered by filtration, washed several times with deionized water, and dried in a dryer at temperature 60 \u00b0C. The calculated rate constants (from the pseudo-first-order model) are compared in Table 2.Comparison of these data shows a decrease in the rate constant and an increase in the reaction time with each catalytic run, probably because of loss of activity due to the blocking of active sites of the catalyst. On the other hand, it should be noted that after the fifth cycle, the rate constant is still high \u2013 comparable to the rate constant obtained for the NiO/Ni(OH)2/Ni_500 catalyst in its first cycle. Such results prove that the formation of a metal-containing phase on the carbonaceous fibrous support provides better stability for the catalytically active phase (Dhokale et al., 2014; Gu et al., 2014; Kongarapu et al., 2017).The mechanism of 4-nitrophenol reduction is exhaustively described in the literature (Khalavka et al., 2009; Mahmoud and El-Sayed, 2011; Sahiner et al., 2010; Sarkar et al., 2011; Wu et al., 2011). In the description of new catalysts, it is essential to note which component can be assumed as the active site of the catalyst. Based on the available literature (Wunder et al., 2010, 2011) and the present results, it can be assumed that the reduction occurs by way of sodium borohydride decomposition on the crystallites of nickel moieties. In the next stage, electron transfer occurs from BH4\n\u2212 molecules towards 4-nitrophenoloane anions via Ni(OH)2/NiO/Ni grains, which play the role of electron carriers. In view of the previously described activity of carbonized spongin-based scaffolds in the reduction of 4-nitrophenol, this study provides evidence that modification with nickel hydroxide, nickel oxide and nickel leads to enhanced catalytic activity, while the carbonized spongin-based scaffolds and the metal-containing phase act synergistically during the reduction reaction. Besides, the three-dimensional, fibrous-like morphology with open channels enhances the diffusion of substrates towards the surface of the catalyst. The promising results concerning the catalytic activity of the prepared materials further encouraged us to evaluate their effectiveness as catalysts in oxidation reactions.Phenol, methylchlorophenoxypropionic acid (MCPP) and 4-chlorophenoxyacetic acid (4-CPA) were used as substrates for the catalytic oxidation reaction. These compounds are commonly used in the production of drugs (phenol) and pesticides (MCPP and 4-CPA). Their presence in water streams has been proved in several studies (Abdel Rahman and Hung, 2020; Somasundaram et al., 2018; Wang et al., 2020; Wang et al., 2016a), as have their toxic and bioaccumulation effects on the environment (Benny and Chakraborty, 2020; Piotrowska et al., 2017). The oxidation was carried out in a water solution at temperature 60 \u00b0C in the presence of hydrogen peroxide as an oxidizing agent, for a time of 4 h. The oxidation efficiency was calculated based on measurement of the concentration of the substrate after the reaction, using the calibration curve method. The obtained oxidation efficiencies are shown in Fig. 7\n.The above results show the good catalytic ability of NiO/Ni(OH)2/Ni_400. Application of this catalyst at acidic pH leads to full oxidation of MCPP and 4-CPA (oxidation yield more than 99%) and sufficient oxidation of phenol (oxidation yield 80%) (Fig. 7A). Without a catalyst, the yield of oxidation is not higher than 10%. This proves that the presence of a catalyst is essential for the oxidation of phenolic compounds. Interestingly, the NiO/Ni(OH)2/Ni_500 and NiO/Ni(OH)2/Ni_600 catalysts presented significantly weaker catalytic properties. The yield of oxidation of any substrate was not higher than 33%. In contrast to the results for the reduction of 4-nitrophenol, where the difference in catalytic activity was relatively small, the same materials used in oxidation reactions differed significantly in catalytic activity. The favourable catalytic activity of NiO/Ni(OH)2/Ni_400 may be a consequence of the morphology of the metal-containing phase forming the well-dispersed needle-like structure, the fact that it has the largest pore size among the prepared materials, as well as the chemical composition of the bio\u2011carbon. The material obtained at the lowest carbonization temperature exhibits the presence of various heteroatoms, as described in other studies (Petrenko et al., 2019; \u017b\u00f3\u0142towska et al., 2021). Owing to the chemical composition of the prepared composites, the NiO/Ni(OH)2/Ni_400 material has the most diverse elemental composition, with the highest percentage of oxygen, iron, bromine, and iodine. Thus, it can be concluded that this material, thanks to the diversification of its surface functional groups, can create various active centres, enhancing its catalytic activity. However, the nickel-containing phase seems to be the major player in the tailoring of the catalytic properties. Thus, it can be assumed that the morphology of the prepared materials and the nickel content may be the crucial factors impacting the activity of the catalyst. Consequently, as can be seen in Fig. 3, an excessive amount of nickel leads to the formation of larger nickel-containing clusters, but with lower surface area. Related to this assumption, BET data showed a higher surface area for the NiO/Ni(OH)2/Ni_500 material. For this reason, the NiO/Ni(OH)2/Ni_400 surface morphology seems to be the best suited for catalytic purposes.In the next step, the phenolic compounds were oxidized at alkaline pH (Fig. 7B). The results show that increasing the pH negatively affects the efficiency of oxidation of phenol, MCPP, and 4-CPA. (NiO/Ni(OH)2/Ni_500 was excluded from testing because it had the lowest catalytic activity.) These results may be explained by the fact that in a water solution with pH in the range 2\u20133, decomposition of hydrogen peroxide produces hydrogen radicals (OH\n\n), which play the leading role in the oxidation of organic compounds (Xing et al., 2018; Zhang et al., 2019). At higher pH, the main product of hydrogen peroxide decomposition is hydroxyl ions. Therefore the concentration of radicals is significantly lower, which results in lower reaction efficiency. The significantly higher catalytic ability of NiO/Ni(OH)2/Ni_400 catalysts may be a result of their having the largest pore size among the materials, and the highest content of surface oxygen and the Ni2+ phase together with the presence of other heteroatoms (Fe, Al, S, among others). In consequence, the synergistic action of several factors impacts the final catalytic activity in oxidation reactions.The results of zeta potential measurements can provide further important information on the influence of pH. The pKa value was 9.94 for phenol and 3.56 and 3.75 for 4-CPA and MCPP respectively. It can be concluded that when the pH of the reaction is 3 the compounds are in a protonated state, with the surface groups of each catalyst positively charged. For reactions carried out at pH 8 the 4-CPA and MCPP molecules are deprotonated, while phenol is still in the protonated form, and the catalysts have negatively charged surface functional groups. It appears, therefore, that at acidic pH, the repulsive electrostatic interaction between the catalyst surface and the substrate molecules does not hamper the oxidation process significantly. Moreover, the prepared catalysts are involved in the formation of hydroxyl radicals (at pH 3) or hydroxyl ions (at pH 8), which attract molecules of the phenolic compounds. An increase in the pH of the reaction mixture, therefore, has a significant negative effect on the oxidation efficiency.As regards possible interactions between the substrates and catalysts, \u03c0-\u03c0 interactions may be considered the most important. However, in the oxidation reactions, the catalyst acts via electron transfer. A more detailed examination of interactions between the substrates and catalysts will not be made here.To obtain a useful catalyst, not only high catalytic activity is essential. Such a material should also be stable over catalytic cycles. Thus, reusability experiments were carried out to evaluate the possibility of multiple application of the NiO/Ni(OH)2/Ni_400 material in repeated oxidation of phenol, MCPP, and 4-CPA.As the results (Fig. 8\n) show, an apparent decrease in oxidation efficiency is visible in the case of each compound. In the oxidation of MCPP and 4-CPA, the reduction of catalytic ability after the fifth run reaches 80%. The lowest decrease in catalytic activity is observed for the oxidation of phenol, where the catalyst retains 40% of its activity after the fifth catalytic run. Interestingly, the reaction with this compound gives the lowest yield in the first catalytic cycle. However, after the second, third, fourth, and fifth reaction runs, the calculated activity of NiO/Ni(OH)2/Ni_400 in the oxidation of phenol is higher than for both 4-CPA and MCPP after the same number of runs. The observed reduction in catalytic activity may be related to loss of catalyst mass during the recovery and washing process. However, deactivation of active sites by poisoning could not be excluded. This is the case especially for 4-CPA and MCPP, as the intermediate products of their oxidation contain chlorine, which can act as a poison on the catalyst surface. This assumption may explain the significant decrease in oxidation efficiency on repeated reuse (Mork and Norgard, 1976; Paquin et al., 2015).Because the mechanism of phenol oxidation is extensively discussed in the literature, it will not be evaluated in detail here. However, to investigate the mechanisms of 4-CPA and MCPP oxidation, MS spectra before and after oxidation were recorded (see Supplementary Note 8).Evolution in materials science and catalysis has led to work that merges two different topics: metals and biopolymers. There is an increasing number of reports on carbon materials derived from biomass. Most biopolymers (lignin, collagen, silk, cellulose, starch, chitin, chitosan, dextran, pectin, alginate, carrageenan) can be successfully used as precursors of carbon materials (Boury and Plumejeau, 2014; Lee et al., 2011, 2012; Qu et al., 2016; Q. Wang et al., 2016b; Zhao et al., 2016). Biopolymers of biological origin are of particular importance for obtaining carbon materials (Zhang et al., 2010). This is related to their chemical structure, which is rich in heteroatoms such as nitrogen or sulphur; the presence of these increases the diversity of surface functional groups. Therefore, these carbon materials are well suited as scaffolds for metal\u2013carbon composites (Lee et al., 2011, 2012; Zhang et al., 2010).Among the many biopolymers commonly applied in the preparation of bio\u2011carbons, spongin-based scaffolds seem to be the most promising choice. Spongin belongs to the \u201ccollagen suprafamily\u201d (Petrenko et al., 2019), and it is the main skeletal protein building the skeletons of sponges in the class Demospongiae, which can grow up to 70 cm. These spongin-based skeletons have been known since ancient times as natural (bath) or commercial sponges, and have been used for cosmetic or medical purposes. Their use for the preparation of bio\u2011carbons is economically feasible, because they are cultivated under marine farming conditions on a large scale worldwide, creating a sponge market worth more than US$20 million (\u017b\u00f3\u0142towska et al., 2021). Moreover, in contrast to other biomass materials used to prepare bio\u2011carbons, which are fragile and can be applied only in the form of powder, these ready-to-use scaffolds preserve the unique structure of Demospongiae sponge skeletons even after carbonization at temperatures as high as 1200 \u00b0C. Carbonized spongin-based scaffolds are mechanically robust and can be used to prepare bio\u2011carbons with a strictly designed shape.Nevertheless, this study and other recently published work (Petrenko et al., 2019; \u017b\u00f3\u0142towska et al., 2021) are just the beginning of efforts to discover the full potential of bio\u2011carbons derived from spongin-based scaffolds. Further research may focus on the atomistic simulation of bio\u2011carbons to provide an additional overview regarding the optimization of properties. Therefore, significant effort should be put into understanding the mechanism of transformation from organic precursor to inorganic carbon and the effect of additives on the structural and chemical properties of the resulting bio\u2011carbon. Moreover, knowledge of the mechanism of functionalization and effect of the bio\u2011carbon structure on the properties of the resulting composite may provide additional insight and represent a milestone in the development of efficient bioinspired materials. Besides, the detailed characterization of spent catalysts may be considered as a subject of future evaluation, encompassing changes in composite morphology, elemental content, BET surface area, leaching of the metal-containing phase, and deeper catalytic study, including the mechanistic aspects. Likewise, the heat conduction of modified bio\u2011carbon-based composites in catalytic applications is another potential area of research.Spongin-based scaffolds are indisputably promising materials for use for catalytic purposes. Their carbonization has been shown to lead to bio\u2011carbons with exceptional stability, robustness and physicochemical properties. The relatively simple functionalization method demonstrated in this study, which does not require the use of extreme synthesis conditions or toxic and expensive reagents, opens up new possibilities for the preparation of bio-inspired materials, which is in line with the philosophy of sustainable development.A spongin-based fibrous scaffold isolated from the marine demosponge Hippospongia communis was utilized as a precursor of a carbon support and subjected to carbonization (at various temperatures) followed by modification with nickel and nickel oxide. For the first time, a relatively simple method of functionalization of carbonized scaffolds was applied to obtain nickel-based bio\u2011carbon composites. Morphological and physicochemical analysis revealed moderate differences in the chemical nature of the prepared materials. It was shown that the temperature of carbonization influences the effectiveness of the modification process. However, this study mainly focused on the characterization of the prepared materials and their evaluation as potential catalysts for oxidation or reduction reactions of various phenolic compounds. The results confirm the promising activity of all of the materials in the reduction of 4-nitrophenol; the slight differences in the calculated rate constants were ascribed to the different nickel oxide contents in the tested catalysts. The mechanism of reduction was also predicted. In oxidation tests, the NiO/Ni(OH)2/Ni_400 catalyst showed excellent catalytic ability in the oxidation of MCPP and 4-CPA (99% oxidation efficiency at pH 3) and good activity in the oxidation of phenol (80% yield at pH 3). The use of hydrogen peroxide as an oxidizing agent eliminates the formation of additional toxic products of oxidant decomposition.This remarkable catalytic activity may be ascribed to (i) the diverse chemical composition of the nickel phase, consisting of nickel hydroxide Ni(OH)2, nickel oxide NiO and nickel(0), which creates various active centres of the catalysts; (ii) the even dispersion of the metal-containing phase on the fibres of the carbonized supports; and (iii) the unique needle-like morphology, which acts synergistically with the 3D structure of the supports to provide good diffusion and high contact area between the catalysts and reagents. This study provides evidence that spongin-based scaffolds can be utilized to produce a structured carbonaceous material that can be successfully functionalized with nickel moieties using a simple sorption\u2013reduction approach. As a result, it becomes possible to produce unique composites based on bio\u2011carbon functionalized with nickel species, with impressive catalytic performance in the removal of emerging contaminants.\nSonia \u017b\u00f3\u0142towska: Conceptualization, Investigation, Writing \u2013 original draft, Writing \u2013 review & editing, Visualization. Zuzanna Bielan: Investigation. Joanna Zembrzuska: Investigation, Writing \u2013 review & editing. Katarzyna Siwi\u0144ska-Ciesielczyk: Investigation. Adam Piasecki: Investigation, Resources. Anna Zieli\u0144ska-Jurek: Investigation. Teofil Jesionowski: Supervision, Writing \u2013 review & editing, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The work was supported by the National Science Centre, Poland, project Etiuda no. 2019/32/T/ST8/00414 (S.\u017b.), and by the Ministry of Education and Science, Poland (K.S.-C., T.J.). Sonia \u017b\u00f3\u0142towska and Teofil Jesionowski would like to thank Professor Monika Mazik of TU Bergakademie Freiberg for assistance during the catalytic tests.\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2021.148692.", "descript": "\n                  Three different 3D fibrous-like NiO/Ni(OH)2/Ni\u2011carbonized spongin-based materials were prepared via a simple sorption\u2013reduction method. Depending on the support used, the catalysts were composed of carbon, nickel oxide, nickel hydroxide and zero-valent nickel, with the surface content of the nickel-containing phase in the range 15.2\u201326.0 wt%. Catalytic studies showed promising activity in the oxidation of phenolic compounds in water and in the reduction of 4-nitrophenol. The oxidation efficiency depends on the substrate used and ranges from 80% for phenol at pH 2 to 99% for 4-chlorophenoxyacetic acid (4-CPA) and methylchlorophenoxypropionic acid (MCPP). In the reduction reaction, all catalysts exhibited superior activity, with rate constants in the range 0.648\u20131.022 min-\n                     1. The work also includes a detailed investigation of reusability and kinetic studies.\n               "}
{"full_text": "The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.PE-based polymers represent one of the most important materials class in our daily life due to their superb mechanical properties, chemical stability and low production costs [1]. In this field, the coordination-insertion polymerization via transition-metal complexes plays a crucial role in synthesis of polyethylene materials. Compared to the early-transition metal complexes, the late-transition metal-based analogues exhibit superior performance in such catalyzed polymerization [2\u20134]. These complexes are tolerant to moisture and air, enabling a long-term storage and delivery without inert atmosphere protection. Furthermore, due to the low oxophilicity, they are even able to perform the ethylene copolymerization with polar monomers, yielding PE copolymers containing polar groups [5\u20138]. Compared to the conventional multi-site heterogeneous catalysts, the single-site catalytic metal allows for the production of PE with narrow molecular weight distributions (PDI). The polymeric microstructures can be modulated via a modification of the complexes structures, leading to a variation in the molecular weight (Mw), PDI, melting transition, density, crystallinity, and branching density. The macroscopic characteristics such as mechanics, surface wettability, chemical and thermal stability, optics, and viscosity of PE can indirectly be altered by suitable modifications of the complex structures [9,10]. Thus, late-transition metal-catalyzed polymerization can be considered as one of the most promising approaches for future industrial olefin polymerization [2,10\u201313].The a-diimine Ni/Pd complexes are one of the best-studied and long-examined late-transition metal-based catalysts for ethylene polymerization. The initial work (A in Fig. 1\n) of Brookhart et al. demonstrated the high activity (107\u00a0g of PE (mol of Ni)\u22121\u00a0h\u22121) and great potential of such catalysts [14,15]. One of the major advantages of these complexes is the ability to produce thermoplastic-elastomers (TPEs) like branched PE via chain-walking mechanism, by use of ethylene as the only reactive monomer [16\u201318]. The chain walking process involves a competition between the chain growth and chain walking during the monomer insertion process. Cationic alkyl-metal active species migrate along the polyethylene backbone via rapid \u03b2-H elimination and reinsertion, leading to the formation of branched structures [19,20]. As reported previously, the remote substitutions, N-aryl groups and backbones are considered as the main factors that control the catalytic behavior of the a-diimine metal complexes. For example, the steric accumulation on the N-aryl group (e.g.\nB in Fig. 1) significantly changes the catalytic performance of the Ni center [21\u201323]. These bulky substituents (especially in ortho position of the N-aryl) act as a shield and offer confined space for monomer insertion, which slows down both the chain-transfer and the chain-walking process. A limited coordination space around the metal center favors a chain-growth mechanism, leading to a more linear and high molecular weight PE [21,24\u201331]. Additionally, electron-withdrawing groups could considerably enhance the catalytic performance of the cationic alkyl-metal center \u2013 the fluorine effect is of great importance in the application of the olefin polymerization, as the fluorine atom has an electron-withdrawing character due to its inductive effects [32\u201335]. In contrast to saturated electron-withdrawing groups, the fluorine exhibits \u03c0 interactions with the aromatic rings. The interaction between the catalytic metal centers with fluorine atoms has been described in previous work, where it was shown to have great influences on the catalytic behavior in polymerization process [26,36\u201339]. The side-arm (i.e. steric and electronic) effects of the ligand moiety create distinctive coordination environments around the catalytic metal center, leading to important variety of polymer properties [2,7,40\u201344].Currently, numerous studies related to the synthesis of the new a-diimine Ni (II) complexes have been reported, where the symmetric arrangement of bulky N-aryls has been the focus [20,45]. However, the catalytic activity is decreased by the limited monomer access into the confined coordination space of the active metal center. The ligands containing bulky groups on both N-aryls tend to exhibit lower (1 or 2 order of magnitudes) catalytic activity than A in Fig. 1\n[21,46]. As reported previously, it is difficult to simultaneously achieve both the high catalytic activity and high molecular weights. The unsymmetrical a-diimine Ni complexes bearing different anilines have been reported since the initial study of benzhydryl-substituted ligands [47,48]. The dibenzhydryl substitutions as the steric groups were incorporated on the first N-aryl, while the second N-aryl moiety was maintained as the less bulky (methyl group) substituent. The catalytic properties of such unsymmetrical a-diimine Ni complexes can be modified by the structural tunes of the solo N-aryl substitution with typical electronic and steric features [20,30,48,49].In this work, a simplified synthetic methodology was followed to synthesize a series of new unsymmetrical a-diimine Ni complexes (Ni-OH, Ni-FOH, Ni-PhOH and Ni-PhFOH in Fig. 1). The aim of our catalyst design was to realize the typical modulation of ethylene monomer insertion and chain-walking process while still maintaining high activity during the ethylene polymerization. It can be considered as a balancing combination of high activity and suppressed \u03b2-H elimination. Different steric enhancements (dibenzhydryl substitutions) on the para/ortho-N-aryls offer various environments to the coordination-insertion process. In order to systematically check the fluorine effect on the catalytic performance during the ethylene polymerization, the fluorine moiety on the N-aryls was synthesized and incorporated on these Ni complexes. The Cl atoms are selected as the affiliation of metal center rather than Br, leading to the formation of the a-diimine Ni dichlorides. The various polymerization conditions were optimized in detail, including co-catalyst, Al/Ni ratio, polymerization temperature, lifetime, and ethylene pressure. This facile strategy is expected to realize the further modifications of the catalytic behaviors of the proposed a-diimine Ni complexes and polymeric microstructure, such as the branches, Mw, PDI and melting transitions. Additionally, the design of ligands involved the incorporation of hydroxyl group, which could then be used to tether the Ni complexes to solid surfaces for gas- and slurry-phase polymerization. Although the hydroxyl group is very electron donating and active to the activated metal center, no reduction in the catalytic performance of the a-diimine Ni complexes was observed.The synthesis of the a-diimine ligands and Ni (II) complexes was carried out under air. Contrary to this, ethylene polymerizations were performed under Schlenk techniques and inert argon atmosphere protection. All the solvents and starting materials were purchased from Sigma-Aldrich and Chemie-Brunschwig. The 1H, 13C and 19F NMR spectrum of a-diimine ligands and Ni complexes were recorded on the Bruker Avance III 400 NMR spectrometer. Elemental analysis was carried out using Elementar-UNICUBE analyzer. Mw and PDI, branching density were determined by a 1260 infinity ii HT-GPC at 160\u00a0\u00b0C with 1,2,4-trichlorobenzene as the solvent against PS standards. The ESI-HRMS results were analyzed by an UHR-TOF BRUCKER Doltonik (Bremen, Germany) maXis with an ESI-quadrupole time-of flight (qToF) mass spectrometer. The melting points were determined by the differential scanning calorimeter (DSC, 214-Polyma) with second-heating-scan curves. 1H and 13C NMR spectra of the polyethylene were measured by using an ARX-300 spectrometer at 140\u00a0\u00b0C in bromobenzene- d\n6.\nL-OH\n\n\n\n\n\nA solution of 3-((2,6-dibenzhydryl-4-methylphenyl)imino)butan-2-one (1.01\u00a0g, 2.0\u00a0mmol) and 4-amino-3,5-dimethylphenol (0.33\u00a0g, 2.4\u00a0mmol) in toluene (50\u00a0mL) with a catalytic amount of para-toluenesulfonic acid (0.035\u00a0g, 0.2\u00a0mmol) was stirred under refluxed over 36\u00a0h. Afterwards, the mixture was cooled down to the room temperature and concentrated under reduced pressure by rotary evaporator. The rest solid was purified by silica column chromatography (Heptane 3/1 Ethyl acetate) to afford L-OH as yellow crystalline solid (0.38\u00a0g, 30.0\u00a0%). 1H NMR (400.2\u00a0MHz, DMSO\u2011d\n6): \u03b4(ppm) 0.67 (s, 3H), 1.78 (s, 3H), 1.86 (s, 6H), 2.11 (s, 3H), 5.18(s, 2H), 6.49 (s, 2H), 6.61 (s, 2H), 7.00 (d, 4H, J\u00a0=\u00a08\u00a0Hz), 7.07 (d, 4H, J\u00a0=\u00a08\u00a0Hz), 7.17\u20137.30 (m, 12H), 8.86 (s, 1H). 13C NMR (100.6\u00a0MHz, DMSO\u2011d\n6): \u03b4(ppm) 13.9, 15.4, 15.5, 17.6, 21.0, 22.1, 28.3, 31.2, 51.7, 114.4, 125.0, 126.2, 126.2, 128.0, 128.2, 128.5, 129.0, 129.3, 130.9, 131.1, 140.2, 142.1, 143.1, 145.2, 152.8, 167.6, 169.6. Anal. Calcd for C45H42N2O (626.84): C, 86.22; H, 6.75; N, 4.47. Found: C, 86.22; H, 7.01\u00a0N, 4.17.\nL-FOH\n\n\n\n\n\nBased on the similar procedure described for L-OH, L-FOH was synthesized via the reaction of 3-((2,6-bis(bis(4-fluorophenyl)methyl)-4-methylphenyl)imino)butan-2-one (3.48\u00a0g, 6\u00a0mmol) and 4-amino-3,5-dimethylphenol (1.28\u00a0g, 6\u00a0mmol) with a catalytic amount of para-toluenesulfonic acid (0.105\u00a0g, 0.6\u00a0mmol) in toluene. L-FOH was isolated as yellow crystalline solid (1.85\u00a0g, 39.8\u00a0%). 1H NMR (400.2\u00a0MHz, DMSO\u2011d\n6): 1H NMR (400.2\u00a0MHz, DMSO\u2011d\n6): \u03b4(ppm) 0.84 (s, 3H), 1.76 (s, 3H), 1.85 (s, 6H), 2.13 (s, 3H), 5.21(s, 2H), 6.49 (s, 2H), 6.58 (s, 2H), 6.97\u20137.13 (m, 16H), 8.86 (s, 1H).13C NMR (100.6\u00a0MHz, DMSO- d\n6): \u03b4(ppm) 15.5, 15.7, 17.7, 21.0, 50.0, 54.9, 114.4, 114.9, 115.1, 115.2, 115.4, 125.1, 128.0, 130.7, 130.8, 131.0, 131.1, 131.4, 138.2, 138.3, 139.0, 139.1, 140.1, 152.9, 159.4, 159.5, 161.8, 161.9, 167.5, 169.4. 19F NMR (376.5\u00a0MHz, DMSO- d\n6): \u03b4(ppm) \u2212116.7, \u2212116.4. Anal. Calcd for C45H38F4N2O (698.81): C, 77.35; H, 5.48; N, 4.01. Found: C, 77.23; H, 5.56; N, 3.63.\nL-PhOH\n\n\n\n\n\nBased on the similar procedure described for L-OH, L-PhOH was synthesized via the reaction of 3-((2,4,6-tribenzhydrylphenyl)imino)butan-2-one (3.33\u00a0g, 5\u00a0mmol) and 4-amino-3,5-dimethylphenol (0.69\u00a0g, 5\u00a0mmol) with a catalytic amount of para-toluenesulfonic acid (0.087\u00a0g, 0.5\u00a0mmol) in toluene. L-PhOH was isolated as yellow crystalline solid (1.21\u00a0g, 31.0\u00a0%). 1H NMR (400.2\u00a0MHz, DMSO- d\n6): \u03b4(ppm) 0.72 (s, 3H), 1.77 (s, 3H), 1.85 (s, 6H), 5.16 (s, 2H), 5.39 (s, 1H), 6.49 (s, 2H), 6.60 (s, 2H), 6.87\u20136.97 (m, 12H), 7.11\u20137.23 (m, 18H) 8.85 (s, 1H). 13C NMR (100.6\u00a0MHz, DMSO- d\n6): \u03b4(ppm) 16.0, 16.1, 18.1, 52.2, 55.6, 114.8, 125.4, 126.4, 126.5, 126.7, 128.5, 128.6, 128.9, 129.0, 129.2, 129.2, 129.6, 131.4, 137.7, 140.7, 142.5, 143.4, 144.6, 146.1, 153.4, 168.1, 170.1. Anal. Calcd for C57H50N2O (779.04): C, 87.88; H, 6.47; N, 3.60. Found: C, 88.04; H, 6.48; N, 3.68.\nL-PhFOH\n\n\n\n\n\nBased on the similar procedure described for L-OH, L-PhFOH was synthesized via the reaction of 3-((2,4,6-tris(bis(4-fluorophenyl)methyl)phenyl)imino)butan-2-one (1.54\u00a0g, 2\u00a0mmol) and 4-amino-3,5-dimethylphenol (0.28\u00a0g, 2\u00a0mmol) with a catalytic amount of para-toluenesulfonic acid (0.035\u00a0g, 0.2\u00a0mmol) in toluene. L-PhFOH was isolated as yellow crystalline solid (0.79\u00a0g, 44.7\u00a0%). 1H NMR (400.2\u00a0MHz, DMSO- d\n6): \u03b4(ppm) 0.90 (s, 3H), 1.71 (s, 3H), 1.84 (s, 6H), 5.19 (s, 2H), 5.49 (s, 1H), 6.37 (s, 2H), 6.49 (s, 2H), 6.87\u20136.90 (m, 4H), 6.95\u20137.08 (m, 18H) 8.86 (s, 1H). 13C NMR (100.6\u00a0MHz, DMSO- d\n6): \u03b4(ppm) 14.4, 15.7, 16.4, 18.1, 22.5, 28.8, 31.7, 50.5, 53.5, 114.8, 115.2, 115.3, 115.5, 115.5, 115.6, 115.8, 125.5, 128.9, 130.8, 130.9, 130.9, 131.0, 131.2, 131.3, 131.4, 137.9, 138.5, 138.5, 139.2, 139.2, 140.4, 140.4, 140.5, 146.0, 153.4, 159.9, 159.9, 162.3, 162.4, 167.9, 169.7. 19F NMR (376.5\u00a0MHz, DMSO- d\n6): \u03b4(ppm) \u2212116.8, \u2212116.3. Anal. Calcd for C57H44F6N2O (886.98): C, 77.19; H, 5.00; N, 3.16. Found: C, 77.39; H, 5.14; N, 3.09.\nNi-OH\n\n\n\n\n\nNiCl2\u00b76H2O (0.209\u00a0g 0.88\u00a0mmol) and L-OH (0.55\u00a0g, 0.88\u00a0mmol) were dissolved in DCM (10\u00a0mL) and EtOH (2\u00a0mL) and stirred at room temperature overnight. The mixture was concentrated under vacuum pump and wash with diethyl ether (20\u00a0mL) and heptane (10\u00a0mL). The precipitated compound was filtered and washed by the excess diethyl ether and heptane, affording Ni-OH as orange powder (0.44\u00a0g, 66.7\u00a0%). ESI-MS (positive-ion mode): m/z 719.2334 ([M\u00a0\u2013\u00a0Cl]+). Calcd: m/z 719.2316. Anal. Calcd for C45H42N2ONiCl2 (756.44): C, 71.45; H, 5.60; N, 3.70. Found: C, 71.33; H, 5.27; N, 3.71.\nNi-FOH\n\n\n\n\n\nBased on the similar procedure and molar ratios described for Ni-OH, Ni-FOH was isolated as orange powder (0.51\u00a0g, 62.2\u00a0%). ESI-MS (positive-ion mode): m/z 791.1930 ([M\u00a0\u2013\u00a0Cl]+). Calcd: m/z 791.1957. Anal. Calcd for C45H38F4N2ONiCl2 (828.40): C, 65.25; H, 4.62; N, 3.38. Found: C, 65.36; H, 4.66; N, 3.21.\nNi-PhOH\n\n\n\n\n\nBased on the similar procedure and molar ratios described for Ni-OH, Ni-PhOH was synthesized as orange powder (0.25\u00a0g, 92.5\u00a0%). ESI-MS (positive-ion mode): m/z 871.3008 ([M \u2013 Cl]+). Calcd: m/z 871.2960. Anal. Calcd for C57H50N2ONiCl2 (908.63): C, 75.35; H, 5.55; N, 3.08. Found: C, 75.00; H, 5.78; N, 2.81.\nNi-PhFOH\n\n\n\n\n\nBased on the similar procedure and molar ratios described for Ni-OH, Ni-PhFOH was synthesized as yellow powder (0.66\u00a0g, 81.5\u00a0%). ESI-MS (positive-ion mode): m/z 472.1342 ([M \u2013 2Cl]2+). Calcd: m/z 472.1350. Anal. Calcd for C57H44F6N2ONiCl2 (1016.58): C, 67.35; H, 4.36; N, 2.76. Found: C, 67.06; H, 4.39; N, 2.52.\nNi-OH: Single crystals of C45H42Cl2N2NiO [Ni-OH] were crystallized. A suitable crystal was selected and mounted on a STOE IPDS 2 diffractometer. The crystal was kept at 173(2) K during data collection. Using Olex2, the structure was solved with the SIR2008 structure solution program using Direct Methods and refined with the SHELXL refinement package using Least Squares minimization [50\u201352]. Crystal Data for C45H42Cl2N2NiO (M\u00a0=\u00a0756.41\u00a0g/mol): monoclinic, space group P21/c (no. 14), a\u00a0=\u00a018.926(3) \u00c5, b\u00a0=\u00a09.4716(10) \u00c5, c\u00a0=\u00a021.408(4) \u00c5, \u03b2\u00a0=\u00a0100.789(13)\u00b0, V\u00a0=\u00a03769.8(10) \u00c53, Z\u00a0=\u00a04, T\u00a0=\u00a0173(2) K, \u03bc(MoK\u03b1)\u00a0=\u00a00.694\u00a0mm\u22121, Dcalc\u00a0=\u00a01.333\u00a0g/cm3, 22,350 reflections measured (3.874\u00b0\u00a0\u2264\u00a02\u0398\u00a0\u2264\u00a050.416\u00b0), 6702 unique (R\nint\u00a0=\u00a00.0791, Rsigma\u00a0=\u00a00.0541) which were used in all calculations. The final R\n1 was 0.0438 (I\u00a0>\u00a02\u03c3(I)) and wR\n2 was 0.1274 (all data shown in Table S1.). Supplementary crystallographic data (CCDC 2202319) is available free of charge from The Cambridge Crystallographic Data Centre CCDC.\nNi-FOH: Ni-FOH was characterized by the similar procedure for Ni-OH. Crystal Data for C45.5H39Cl3F4N2NiO (M\u00a0=\u00a0870.84\u00a0g/mol): orthorhombic, non-centrosymmetric space group P21212 (no. 18) with Flack \u22120.01(1), a\u00a0=\u00a022.900(2) \u00c5, b\u00a0=\u00a019.6803(17) \u00c5, c\u00a0=\u00a010.5741(7) \u00c5, V\u00a0=\u00a04765.5(7) \u00c53, Z\u00a0=\u00a04, T\u00a0=\u00a0173(2) K, \u03bc(MoK\u03b1)\u00a0=\u00a00.624\u00a0mm\u22121, Dcalc\u00a0=\u00a01.214\u00a0g/cm3, 30,791 reflections measured (2.728\u00b0\u00a0\u2264\u00a02\u0398\u00a0\u2264\u00a050.368\u00b0), 8293 unique (R\nint\u00a0=\u00a00.0549, Rsigma\u00a0=\u00a00.0400) which were used in all calculations. The final R\n1 was 0.0468 (I\u00a0>\u00a02\u03c3(I)) and wR\n2 was 0.1448 (all data shown in Table S1.). Supplementary crystallographic data (CCDC 2202320) is available free of charge from The Cambridge Crystallographic Data Centre CCDC.\nNi-PhFOH: Ni-PhFOH was characterized by the similar procedure for Ni-OH. Crystal Data for C58H46Cl4F6N2NiO (M\u00a0=\u00a01101.48\u00a0g/mol): monoclinic, non-centrosymmetric space group Cm (no. 8) with Flack 0.01(1), a\u00a0=\u00a010.6554(12) \u00c5, b\u00a0=\u00a018.6022(16) \u00c5, c\u00a0=\u00a013.4204(14) \u00c5, \u03b2\u00a0=\u00a095.166(9)\u00b0, V\u00a0=\u00a02649.3(5) \u00c53, Z\u00a0=\u00a02, T\u00a0=\u00a0173\u00a0K, \u03bc(Mo K\u03b1)\u00a0=\u00a00.631\u00a0mm\u22121, Dcalc\u00a0=\u00a01.381\u00a0g/cm3, 18,123 reflections measured (3.048\u00b0\u00a0\u2264\u00a02\u0398\u00a0\u2264\u00a051.508\u00b0), 4991 unique (Rint\u00a0=\u00a00.0805, Rsigma\u00a0=\u00a00.0488) which were used in all calculations. The final R1 was 0.0466 (I\u00a0>\u00a04u(I)) and wR2 was 0.1218 (all data). A solvent mask was calculated and 74 electrons were found in a volume of 216A3 in 2 voids per unit cell. This is consistent with the presence of one heptane molecule per formula unit which accounts for 58 electrons per unit cell. (All data shown in Table S1.). Supplementary crystallographic data (CCDC 2202318) is available free of charge from The Cambridge Crystallographic Data Centre CCDC.The polymerization reactions were carried out in a 300\u00a0mL stainless B\u00fcchi steel autoclave. The polymerization setup was equipped and connected with multifunctional systems including vacuum, argon pipeline, monomer feeding tank, catalyst injection, temperature control, and magnetic stirrer with controlling units. The temperature inside the reactor was controlled by the connected thermostat. The reactor was initially vacuumed and filled up with argon for three time. The desired amount of mixture of dry solvent (toluene) and co-catalysts was injecting inside to wash and clean the impurities in the reactor. The washing procedure was maintained for 15\u00a0min under 90 \u2103. Then, the solution mixture was released by the argon pressure, which kept the inert atmosphere in reactor. At the selected polymerization conditions, the distill toluene (30\u00a0mL) was injected into the autoclave, followed by the injection of co-catalyst dissolved in dry toluene (50\u00a0mL). Ni complexes were purified using the Schlenk manipulations under argon. The required amount of Ni complexes was introduced with dissolution of the rest 20\u00a0mL toluene. The reactor was immediately pressurized into certain ethylene pressure while the magnetic stirrer was also initiated at the same time. After the required time for ethylene polymerization, the monomer pressure was released out of the autoclave. The mixed solution containing polymers was removed out and quenched by the mixture of HCl and EtOH (ratio 1:10). The polymer was collected, washed with EtOH, and then dried under vacuum oven at 60\u2103 for further catalytic calculation and characterization.The synthesis of the here proposed a-diimine Ni complexes could be divided into several parts, namely the synthesis of bulky anilines, a-imino-ketones, a-diimine ligands and Ni complexes (Fig. 2\n). The synthetic procedure of the bulky anilines has previously been reported, which involved the reaction between the normal anilines and diphenylmethanol catalyzed by zinc chlorides [53\u201355]. Subsequently, one-equivalent 2, 3-butanedione reacted with these bulky anilines with a catalytic amount of para-toluene sulfonic acid in dichloromethane under reflux, producing the a-imino-ketone precursors. The unsymmetrical a-diimine ligands were synthesized via the imine formation reaction of a-imino ketones with 4-amino-3,5-xylenol. The most conventional starting material for the synthesis of a-diimine Ni(II) complexes is the nickel(II) bromide 2-methoxyethyl ether complex ((DME)NiBr2) [11]. As (DME)NiBr2 is very sensitive to moisture, the synthesis of a-diimine Ni complexes has to be carried out under inert atmosphere. The aid of air-stable nickel(II) chloride hexahydrate (NiCl2\u00b76H2O) as the coordination affiliation is rarely reported [17]. Additionally, the price of (DME)NiBr2 is even higher than NiCl2\u00b76H2O. The inert-protection procedure during the synthesis and the high price of chemicals definitely increases the catalyst cost, which hampers their commercialization. Therefore, the more efficient and cheaper NiCl2\u00b76H2O was applied to produce the a-diimine Ni chlorides in ethanol/DCM mixture, which was completely carried out at ambient. The components of the a-diimine ligands and Ni complexes synthesized in this work are not commercially available, nor was their synthesis reported previously.The a-diimine ligands (L-OH, l-FOH, L-PhOH, and L-PhFOH) were characterized by elemental analysis, 1H, 13C, and 19F NMR spectroscopy (Figs. S1\u2013S10). The NMR analysis was also employed to verify the a-diimine Ni (II) complexes (Ni-OH, Ni-FOH, Ni-PhOH, and Ni-PhFOH). However, owing to the paramagnetic behavior of the Ni based complexes, the chemical shifts of the 1H NMR signals were very broad and difficult to determine their complex structures comparing to the ligands (Figs. S11\u2013S14) [56]. As a consequence, the a-diimine Ni (II) complexes were further characterized by Elemental Analysis, ESI-HRMS, and a single-crystal X-ray diffraction study. Particularly, the single crystals of Ni-OH, Ni-FOH, and Ni-PhFOH were isolated from slow-diffusion of heptane (nonpolar solvent) into a dichloromethane solution (polar solvent) and further verified via X-ray crystallographic analysis. It was also found that storage in the solvents was critical to protect the single-crystals from decomposition. The molecular structures of the complexes are shown in Figs. 3\u20135\n\n\n with selected bond lengths and angles. In the Ni complexes, the Ni atom is situated at the center of a distorted-tetrahedron structure. Two nitrogen donors belonging to the unsymmetrical a-diimine ligands coordinate with Ni chlorides. The two N-aryls are perpendicular to the planar coordination sphere of Ni and aliphatic backbone. The phenyl rings of the N-aryls surround the coordinated Ni center, which partially shields it and controls the monomer insertion rates. Thus via the various steric and electronic effects, the coordination environment is finely tuned, as evidenced by the variation of bond lengths and bond angles among these different Ni complexes. The characteristic tetrahedral geometry of a-diimine Ni (II) complexes favors the improved catalytic activity in catalyzed ethylene polymerization [57].As a first step, the initiating effects of different activators (co-catalysts) were screened for the in situ polymerization using the synthesized Ni complexes. Numerous alkyl-aluminum compounds have been previously reported as activators for late-transition metal precatalysts [17]. In this work, initiation effect of various activators such as modified methylaluminoxane (MMAO), ethylaluminum sesquichloride (EASC), diethylaluminumchloride (Et2AlCl), and trimethylaluminium (TMA) were studied. Although the activation mechanism of these co-catalysts might be slightly different, the monomer insertion of ethylene into the active cationic alkyl-metal species remains the same. The screening of co-catalyst initiation were performed using Ni-FOH in toluene with a constant ethylene pressure of 10\u00a0bar and a temperature of 30\u00a0\u00b0C (see Table 1\n).As shown in Table 1, all co-catalysts except TMA exhibited remarkable capacity to initiate Ni-FOH. MMAO is the most efficient activator for Ni-FOH in ethylene polymerization resulting in a catalytic activity of 13.0\u00a0\u00d7\u00a0106\u00a0g of PE (mol of Ni)\u22121\u00a0h\u22121. Furthermore, the GPC analysis revealed the high molecular weight (1.36\u00a0\u00d7\u00a0106\u00a0g\u00a0mol\u22121) for the isolated PE in the MMAO-Ni-FOH system (see entry 1, Table 1). High activities were also observed for the Et2AlCl-Ni-FOH and EASC-Ni-FOH system (12.7 and 8.8\u00a0\u00d7\u00a0106\u00a0g of PE (mol of Ni)\u22121\u00a0h\u22121) (entries 2 and 3, Table 1). However, the molecular weight of the PE decreased for Et2AlCl and EASC compared to the MMAO-Ni-FOH system. This particularly demonstrates the competition between the chain-growth and chain-transfer process [58]. The latter led to a decrease in the molecular weight of PE. Somehow, the Et2AlCl-Ni-FOH and EASC-Ni-FOH system seemed to provide the active species, which typically favored the chain-transfer process in ethylene polymerization. TMA is not a suitable activator for the Ni-FOH complex (entry 4, Table 1). Nevertheless, the main cause of this poor activation performance of TMA-Ni-FOH still remains unclear. As previously reported, TMA was also applied as the significant linker to covalently tether the OH/NH2-containing late-transition complexes to solid substrate for ethylene heterogeneous polymerization [59,60]. It is also likely that TMA reacts with the Ni complex (Ni-FOH) and generates the a-diimine Ni dimethyl or heterobinuclear Ni(I) species, rather than initiating the catalytic metal center for polymerization [61].Furthermore, the influence of various polymerization conditions, such as Al: Ni ratio, polymerization temperature, catalyst lifetime and ethylene pressure, for the MMAO-Ni-FOH catalytic system was investigated to improve the catalyst\u2019s performance (entries 1\u201316, Table 2\n and Fig. 6\n). Initially, the influence of the Al:\u2006Ni molar ratio was optimized with an ethylene pressure of 10\u00a0bar, a polymerization time of 10\u00a0min and a polymerization temperature of 30\u00a0\u00b0C (entries 1\u20135, Table 2). The catalytic activities of the systems displayed an upward trend as the ratio was increased from 500:\u20061 to 1500:\u20061. The highest activity (21.84\u00a0\u00d7\u00a0106 g (PE) mol\u22121 (Ni) h\u22121) was observed at a molar ratio of 1500:\u20061. Above this molar ratio, the catalytic activity steadily dropped to 7.44\u00a0\u00d7\u00a0106 g (PE) mol\u22121 (Ni) h\u22121 (Fig. 6). This decreased activity was plausibly attributed to the presence of the OH moiety in the para-N-aryl, as higher Al:\u2006Ni molar ratios generates O\u2212 ions from OH. These O\u2212 ions are strong electron-donating group, which weaken the catalytic capacity of the Ni centers [62]. All PE samples exhibited the high molecular weight, typically around 1.2\u20131.3\u00a0\u00d7\u00a0106 g\u00a0mol\u22121. However, no obvious correlation between the PE molecular weights and the catalyst activity was observed.The effect of temperature on the performance of the MMAO-Ni-FOH system was tested by conducting polymerization under selected temperature gradients with a constant Al: Ni ratio of 1500:1 (entries 3, 6\u20138, Table 2). It was observed that the reaction temperature had a significant influence on the catalytic activity and the molecular weight (Fig. 6) of the PE; namely, the molecular weight diminished (from 1.81 to 0.45\u00a0\u00d7\u00a0106 g\u00a0mol\u22121) with increased temperatures. The MMAO-Ni-FOH system achieved the highest activity of 21.84\u00a0\u00d7\u00a0106 g (PE) mol\u22121 (Ni) h\u22121 at 30\u00a0\u00b0C. A high molecular weight for the formed PE at 0\u00a0\u00b0C was attributed to the low rate rotation of the C-N-aryl bond and chain transfer, which decreased the rate of the monomer insertion and activity [63]. Along with a higher activity and molecular weight, a better balance between the chain propagation and chain transfer mechanism could be achieved at 30\u00a0\u00b0C. As expected, both catalytic activity and the molecular weight decreased significantly at higher polymerization temperatures (60 and 90\u00a0\u00b0C). A persuasive explanation can be made that the increased temperature gave rise to the increased rotation of C-N-aryl bond. High rate rotation of the C-N-aryl bond led to the fast chain transfer (low Mw) and thermal damages to the metal center (chain termination). A polymerization temperature of 90\u00a0\u00b0C yielded an activity of 2.8\u00a0\u00d7\u00a0106 g (PE) mol\u22121 (Ni) h\u22121 and a molecular weight of 0.4\u00a0\u00d7\u00a0106 g\u00a0mol\u22121 (entry 8, Table 2). Meanwhile, higher temperature also reduced the concentration of the ethylene monomer in toluene solution, therefore decreasing the monomer access. However, the catalytic performance of Ni-FOH/MMAO also suggests great thermal stability compared to systems reported in previous works [17].Additionally, variation of the polymerization time was used to check the lifetime of the activated Ni species (entries 3, 9 \u2013 13, Table 2). As shown in Fig. 6, the highest activity (27.7\u00a0\u00d7\u00a0106 g (PE) mol\u22121 (Ni) h\u22121) was achieved around 5\u00a0min after the beginning of the polymerization; then the activity gradually decreased from 5\u00a0min to 120\u00a0min, leading to 7.7\u00a0\u00d7\u00a0106 g (PE) mol\u22121 (Ni) h\u22121. The reason for this decrease might be due to a monomer diffusion limit. As the synthesized PE was not fully dissolved in toluene, a polymeric diffusion barrier around the metal center was created during the ethylene polymerization. This reduced the possibility of the coordination-insertion process between the Ni center and ethylene monomer. There is the possibility that the Ni complexes were partially deactivated during the long-term polymerization. However, the catalytic activity of the Ni complexes was still maintained at high level even after 2-hour reactions, demonstrating the robust nature of these synthesized catalysts. To further investigate the influence of ethylene pressure, polymerizations were performed between 5 and 20\u00a0bar (entries 3, 14 \u2013 16, Table 2), as a higher monomer pressure leads to a higher ethylene concentration in toluene solution. Consequently, higher activity (up to 29.1\u00a0\u00d7\u00a0106 g (PE) mol\u22121 (Ni) h\u22121) and molecular weight (Mw\u00a0=\u00a01.53\u00a0\u00d7\u00a0106 g\u00a0mol\u22121) were observed at 20\u00a0bar (Fig. 6). This result demonstrates a superior performance of the catalyst under increased polymerization pressure.The structural modifications of the a-diimine Ni complexes were of significant importance to control the catalytic behavior and the properties of the synthesized PE. In order to determine the catalytic performance, Ni-OH, Ni-FOH, Ni-PhOH, and Ni-PhFOH were applied to the ethylene polymerization under the previously optimized conditions (entries 3, 17 \u2013 22, Table 2). All the Ni complexes exhibited a high catalytic (8.7 \u2013 23.7\u00a0\u00d7\u00a0106 g (PE) mol\u22121 (Ni) h\u22121) activity for ethylene polymerization at 30\u00a0\u00b0C. It was apparently observed that the F effect and the incorporation of the sterically demanding groups generated positive influences on the catalytic activity and molecular weight of the resulting PE. Both, Ni-FOH and Ni-PhOH exhibited higher catalytic activity compared to Ni-OH, whereas the catalytic activity of Ni-FOH was also higher than Ni-PhOH. These findings together indicated that the F effect plays a crucial role in improving the catalytic activity at 30\u00a0\u00b0C. The highest activity of these Ni complexes was achieved using Ni-PhFOH as the pre-catalysts, probably due to the combination of the F effects and the steric enhancement (Fig. 7\n). There was a trend concerning the molecular weight of the produced PE for different ligand structures: Ni-PhFOH (1.71\u00a0\u00d7\u00a0106 g\u00a0mol\u22121)\u00a0>\u00a0Ni-PhOH (1.52\u00a0\u00d7\u00a0106 g\u00a0mol\u22121)\u00a0>\u00a0Ni-FOH (1.38\u00a0\u00d7\u00a0106 g\u00a0mol\u22121)\u00a0>\u00a0Ni-OH (1.15\u00a0\u00d7\u00a0106 g\u00a0mol\u22121) observed, which could be understood as follows. The restricted access from axial directions of the metal center suppressed the chain-transfer process more efficiently, which leads to a further increase of molecular weight via chain propagation. However, the F and steric effects did not have a great impact on the catalytic activity of the Ni complexes at 90\u00a0\u00b0C. Although the catalytic performance was maintained at high activity [> 106 g (PE) mol\u22121 (Ni) h\u22121], a minor decrease was nevertheless observed for all Ni complexes (Fig. 7) \u2013 probably due to a boost of the rotation rate of the C-N-aryl bond at the high reaction temperature. The coordination between the protons from the alkyl-N-aryl groups and the metal center terminated the catalytic active species. The presence of the -CHPh2 groups at the para-N-aryl led to an increase in the molecular weight of PE, thus Ni-PhFOH and Ni-PhOH systems yielded a higher molecular weight PEs compared to Ni-FOH and Ni-OH systems. Compared to the symmetrical modifications on \u03b1-diimine Ni complexes with the incorporation of the 2, 6-dibenzhydryl groups (B in Fig. 1), one of the biggest advantages of the proposed Ni complexes (Ni-PhFOH, Ni-PhOH, Ni-FOH and Ni-OH) is the remarkably high catalytic activity in ethylene polymerization [21,22]. Even after 1\u00a0h, the catalytic activity was still maintained at the level of 107 g (PE) mol\u22121 (Ni) h\u22121 (entry 11, Table 2). Derivative Ni-Sym. from a previous research [46]. similar to B\n[21,22]. was selected as a benchmark reference for this work (as shown in Fig. 7). The steric bulkiness of both sides of the N-aryls limited the space for monomer insertion, thus reducing the catalytic activity. Furthermore, the rigid structure of the symmetrical Ni complexes also created fewer opportunities to modify the molecular weights and other properties of the resulting PE. Contrary to this, the unsymmetrical designs of the a-diimine Ni complexes (Ni-PhFOH, Ni-PhOH, Ni-FOH, and Ni-OH) offered both a tailored catalytic activity and a high molecular weight PE (Fig. 7). Meanwhile, this work combines the excellent catalytic features of previously described unsymmetrical a-diimine Ni complexes [64\u201369]. The incorporation of the aliphatic backbone is more likely to generate the PE samples with higher Mw (up to 1.81\u00a0\u00d7\u00a0106 g\u00a0mol\u22121) in comparison to the aromatic backbone. A high catalytic activity and thermal stability of such unsymmetrical a-diimine Ni complexes were also observed in this work. Compared to the previous derivatives with aliphatic backbone, the steric enhancements on both the para- and ortho-position of the N-aryls allows for generation of a stable single-site catalytic Ni center during the ethylene polymerization[69]. In the current work a high catalytic activity, high PE Mw and narrow PDI were observed. In addition, the presence of the terminal hydroxyl group offers reactive site for covalent immobilization of these outstanding a-diimine Ni complexes on inorganic substrates for applications in heterogeneous polymerization.Compared to the currently applied Ziegler-Natta heterogeneous catalysis in ethylene polymerization, one of the main advantages of single-site catalysts is the formation of PE with narrow molecular weight distribution. As shown in Fig. 8\n, the PE samples catalyzed by the a-diimine Ni complexes typically exhibit a narrow PDI, enabling better mechanical properties of the resultant polymer [70]. However, broader PDIs (higher than 3) were observed for the PE samples generated at a polymerization temperature of 90\u00a0\u00b0C for all the Ni complexes. It illustrates that deactivation and chain termination occurs at higher temperatures during ethylene polymerization.In addition, the steric hindrance from the para-N-aryl potentially squeezed the free volume of the phenyl rings at the ortho-N-aryl, which increased the possibility for the coordination between the CH and the active Ni centers [71]. Besides a narrow PDI, branched structure is another potential advantage of the a-diimine Ni complexes, enhancing the polymer mechanical flexibility, such as the tensile strength and elongation at break [72]. The chain-walking mechanism allowed the \u03b1-diimine Ni and Pd complexes to produce high molecular weight PE with highly branched structures [73]. The branched microstructures of the resulting PE samples was determined by the high-temperature 1H and 13C NMR (Figs. 9\u201311\n\n\n).The calculation of the branching density and peak assignments of the PE simples were based on the methods, which were previously reported [19,25,74]. As shown in Fig. 9, the 1H NMR spectra of the PE-3\u2032 sample (entry 3 in Table 1) shows a significant quantity of terminal methyl groups in the polymer chains, which is interpreted as evidence for the PE branching (52.5B/1000C). 13C NMR spectra also helps to characterize various types of branching architectures. For instance, the 13C NMR spectra of the PE-3\u2032 sample (Table 1) revealed multiple branching types in Fig. 10, including methyl (50.5\u00a0%), propyl (15.2\u00a0%), butyl (10.6\u00a0%), and long chain moieties (23.7\u00a0%). The polymer microstructure of sample PE-3 (entry 3 in Table 2.) exhibited less variation in the branches (26.2B/1000C) than the PE-3\u2032 sample (Table 1).Obviously, only methyl branches were detected in the 13C NMR spectra for the PE-3 sample (Table 2.), shown in Fig. 11. Interestingly, such branched elastomer like PE was obtained via ethylene polymerization in this work by using ethylene only as the monomer feedstock; remember, the difference in the synthesis of PE-3\u2032 (Table 1) and PE-3 (Table 2) was the use of a different co-catalysts (Et2AlCl or MMAO), applied for ethylene polymerization. This surprising finding suggests that the variation of the co-catalysts plays a role in altering the chain-walking behavior at the hence initiated Ni center\u2014the exact details remaining unclear. A possible explanation could be that the co-catalysts initiation can induce the selectivity of the activated Ni center, where either the chain propagation (MMAO and PE-3 (Table 2.) or chain transfer (Et2AlCl and PE-3\u2032 (Table 1) is favored upon ethylene insertion. It was observed that the molecular weight of PE-3 (Table 2) was almost twice as high as PE-3\u2032 (Table 1), also indicating a higher rate of chain transfer if activated with Et2AlCl as compared to MMAO. Therefore, a high rate of chain transfer increased the chances of chain-walking process thus leading to the formation of branched structures. A similar observation was made in catalyzed propylene polymerization, where the MMAO activation formed a much higher syndiotacticity in comparison to the in-situ activated hafnocenes. This stereoselectivity was assumed to be attributed to the effects of counter anion [75]. Moreover, higher temperatures led to an increasing rate of the chain-walking process as well as branching, which was due to the C-N-aryl rotations of the Ni complexes. As shown in Fig. S15, for the PE-8 samples (Table 2), the 13C NMR spectra shows the presence of both the methyl (66\u00a0%) and propyl groups (34\u00a0%) as the branches (69.9B/1000C), if synthesized at 90\u00a0\u00b0C. The branching density of the PE sample also suggests a higher rate of the chain-transfer process as compared to PE-3 (Table 2), which was synthesized at 30\u00a0\u00b0C [57]. The variation of the melting points was mainly due to the different branching densities and structures, which led to the formation of various crystallinity and microstructures of the PE samples. The branching properties of PE samples were directly modified by the so-called chain-walking process in ethylene polymerization, catalyzed by such Ni complexes. The competition between the chain-growth and chain-walking process can be modulated by the applied polymerization conditions or complexes structures. The higher degree of branching densities, the lower melting points are characterized (Fig. 12\n). For example, hyperbranched and amorphous PE can be prepared via the \u03b1-diimine Pd complexes, which exhibits higher selectivity to chain-walking process than \u03b1-diimine Ni complexes in ethylene polymerization [14,20]. The \u03b1-diimine Ni complexes can produce mainly linear (few short-branches) PE with a well-defined melting point. DSC (second-heating-scan) curves revealed diverse microstructures of the selected PE samples, which were initially controlled various ligands structures and polymerization conditions. The melting points of the produced PE range from 67.9 up to 127.5\u00a0\u00b0C.In conclusion, this work presents a series of novel unsymmetrical a-diimine Ni complexes for ethylene polymerization, combining the benefits of high activity and high molecular weight PE. The two unsymmetrical N-aryl groups created the unique coordination surroundings toward the active metal center. The dibenzhydryl groups (-CHPh2/-CHPhF\npara\n\n2) on N-aryl suppressed the axial direction of the Ni coplanar and retarded the rate of chain transfer, which gave rise to high molecular weight of polyethylene (Mw\u00a0=\u00a01.8\u00a0\u00d7\u00a0106\u00a0g\u00a0mol\u22121). Meanwhile, the less bulky N-aryl provided enough space for monomer insertion, leading to the high catalytic activity (29.1\u00a0\u00d7\u00a0106\u00a0g of PE (mol of Ni)\u22121\u00a0h\u22121). The incorporation of the fluorine atoms on the bulky substitutions brought about the significant increase on the catalytic activities of these Ni complexes and PE molecular weight (Mw\u00a0=\u00a01.7\u00a0\u00d7\u00a0106\u00a0g\u00a0mol\u22121). It also indicated that the polymerization conditions played a crucial role in controlling the catalytic behaviors and PE microstructures, including the co-catalysts, polymerization temperature and ethylene pressure. High melting transitions (up to 127.5\u00a0\u00b0C) were observed among the selected PE samples. The presence of the strong electro-donating hydroxyl group at the para-N-aryl did not negatively influence the catalytic activity, while it provided the opportunities to further functionalized the newly synthesized a-diimine Ni complexes. These unique unsymmetrical structures must generate a gradient polarizing effects at the metal center, i.e. more electron withdrawing on the F-rich and highly steric side. Such F-effects brought about positive influences on the both the catalytic performance of Ni complexes and chain-growth process in ethylene polymerization. It is very interesting to explore the role of such \u201cmetal center polarization\u201d in future catalyst designs, which is the \u201ckey\u201d to further optimize the catalytic behaviors of such related catalysts. This N-aryl moiety with \u2013OH group also enables the capacity to covalently tether theses Ni catalysts on inorganic nanoparticles (SiO2, Al2O3, MgCl2, etc.) for ethylene heterogeneous polymerization.\nRuikai Wu: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing \u2013 original draft. Lucas Stieglitz: Investigation, Writing \u2013 review & editing. Sandro Lehner: Investigation, Writing \u2013 review & editing. Milijana Jovic: Investigation, Writing \u2013 review & editing. Daniel Rentsch: Investigation, Writing \u2013 review & editing. Antonia Neels: Investigation, Writing \u2013 review & editing. Sabyasachi Gaan: Project administration, Writing \u2013 review & editing, Supervision. Bernhard Rieger: Writing \u2013 review & editing, Supervision. Manfred Heuberger: Project administration, Writing \u2013 review & editing, Supervision, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by Subitex grant, Switzerland (2020\u20132025) and China Scholarships Council (No. 201904910562). The NMR hardware was partially granted by the Swiss National Science Foundation (SNSF, Grant 206021_150638/1). The authors are grateful to Feng-Sen Sun (LMU) for his kind help with ESI-HRMS measurements and interpretations.Supplementary data to this article can be found online at https://doi.org/10.1016/j.eurpolymj.2023.111830.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n\n\n\nSupplementary data 2\n\n\n\n\n\n\nSupplementary data 3\n\n\n\n\n\n\nSupplementary data 4\n\n\n\n", "descript": "\n                  A series of new tailored a-diimine Ni (II) complexes (Ni-OH, Ni-FOH, Ni-PhOH, and Ni-PhFOH) containing bulky ortho-N-aryl groups with various dibenzhydryl substitutes was successfully synthesized, characterized and applied in ethylene polymerization. The a-diimine ligands and Ni (II) complexes were characterized by 1H, 19F, and 13C NMR, elemental analysis, and high resolution electrospray ionization mass spectrometry (ESI-HRMS). The X-ray crystallographic study of metal complexes Ni-OH, Ni-FOH, and Ni-PhFOH revealed their distorted tetrahedral geometry. An unsymmetrical steric-enhancement design approach was employed to modulate the competition between the monomer insertion and the chain-walking process in ethylene polymerization. This facile design resulted in a high catalytic activity and yielded high molecular weight PE. The catalytic activity of these complexes was optimized by varying the polymerization conditions (temperature, time, and ethylene pressure), use of co-catalysts and variation of the Al/Ni ratio. When activated with modified methylaluminoxane (MMAO), these Ni complexes exhibited the activity as high as 29.1\u00a0\u00d7\u00a0106\u00a0g of PE (mol of Ni)\u22121\u00a0h\u22121), with a molecular weight of 1.81\u00a0\u00d7\u00a0106\u00a0g\u00a0mol\u22121. Their thermal stability was well pronounced at elevated temperatures; high activity of 2.88\u00a0\u00d7\u00a0106\u00a0g of PE (mol of Ni)\u22121\u00a0h\u22121 and high molecular weight PE (0.86\u00a0\u00d7\u00a0106\u00a0g\u00a0mol\u22121obtained at 90\u00a0\u00b0C). PEs with tunable branches and high melting points (127.5\u00a0\u00b0C) were obtained, which is a typical feature of LLDPE. The incorporation of fluorine atoms on N-aryl groups had a strong positive influence on the catalytic activity of the Ni complexes and favored the chain-growth process in the ethylene polymerization. Surprisingly, the simultaneous presence of terminal hydroxyl group in these complexes did not adversely affect their catalytic activity, while offering the possibility for covalent attachment to the solid supports for future heterogeneous polymerization.\n               "}
{"full_text": "Data will be made available on request.Hydrothermal liquefaction (HTL) is a promising thermochemical process for the conversion of wet biomass into energy-dense bio-oil which can be further refined into platform chemicals and liquid transportation fuels to replace fossil-derived petrochemicals (Bampaou et al., 2022). HTL is typically conducted in the presence of subcritical water (280 \u2013 374\u00a0\u00b0C, 10 \u2013 22\u00a0MPa) which exhibits a low dielectric constant, resulting in non-polar organic solvent like properties to assist with biomass solubilization, and high dissociation to promote fast acid-base catalyzed reactions (Gollakota et al., 2018). Therefore, the hot pressurised water environment aids the solubilisation and depolymerisation of biomass into smaller reaction intermediates which can be further deoxygenated and condensed into the desired bio-oil product. Despite this partial deoxygenation through dehydration and decarboxylation reactions, the bio-oil retains a high heteroatom content requiring significant upgrading through secondary processes such as hydrotreatment, hydrogenolysis and cracking reactions, which increase the cost of processing (Mukundan et al., 2020; Wagner et al., 2018). Besides small amounts of solid char and reaction gas, consisting predominantly of CO2, HTL also produces large volumes of aqueous phase (AP) product, which contains a mixture of short organic acids, phenolics and other polar hydrocarbons, resulting in a high total organic carbon (TOC) and chemical oxygen demand (COD). While AP carbon may be recovered via secondary processes such as aqueous phase reforming, gasification, anaerobic digestion or algae cultivation, these processes are expensive and tend to produce lower value products (Silva Thomsen et al., 2022; Watson et al., 2020).Therefore, there is a considerable need for employing catalysts that can improve solid biomass depolymerisation, convert the undesired AP products into bio-oil, and promote deoxygenation and other heteroatom removal reactions to reduce the cost for subsequent upgrading processes (Mukundan et al., 2022). The most commonly used catalysts are homogenous bases such as NaOH, KOH, K2CO3, Na2CO3, etc., which help to improve carbohydrate conversion and reduce solid formation (Biller et al., 2016) (Wang et al., 2013). However, as these homogenous base catalysts are difficult to recover and recycle post reaction, they may be more suitable for HTL of dry biomass, where the majority of the HTL aqueous phase, and hence the catalyst, can be recirculated (Biller et al., 2016).Unlike homogenous bases, heterogenous catalysts can be more easily recovered from the product mixture through simple physical processes, such as filtration or gravity. Multiple studies have explored the use of noble metals such as Pt, Pd or Ru, which have shown high activity for biomass conversion and oxygen removal (Duan & Savage, 2011; Yang et al., 2016), but due to their high costs are unlikely to be viable at commercial scale. Moreover, despite the easy separation from the HTL AP, heterogenous catalysts become contaminated with solid HTL products, particularly the inorganic ash (e.g., SiO2, CaO, MgO, K2O, Al2O3, TiO2) which account for up to 20\u00a0% of total biomass weight (Caillat & Vakkilainen, 2013), impeding their long-term cyclability. A potential solution to this problem is the use of inexpensive ferromagnetic catalysts such as Fe, Co, or Ni and their oxides, which can be magnetically separated from the solid product post reaction, as already demonstrated for other applications such as biodiesel production (Gardy et al., 2018; Shylesh et al., 2010; Zhang et al., 2021). HTL studies with inexpensive NiMo and CoMo catalysts (Prestigiacomo et al., 2019) and carbon nanotube supported Fe, Ni, and Co catalysts (Liu et al., 2021) resulted in significant increases in bio-oil O/C ratios compared to non-catalytic reactions from 0.11 to 0.16, and 0.29 to 0.69, respectively. Despite this, studies on the magnetic catalyst recovery and subsequent recycling are rare in this field, and do not fully address the separation of catalyst and solid HTL products, particularly the inorganic ash. Fe powder catalysts used during HTL of empty oil palm fruit bunch was regenerated by heating the biochar/catalyst mixture at 1000\u00a0\u00b0C under N2, with the biochar acting as reducing agent to regenerate metallic Fe, while the removal of ash (4.6\u00a0% for selected biomass feed) was not discussed (Miyata et al., 2017). Another study employed Fe powder for the HTL conversion of the macroalga Cladophora socialis, pre-treated with HCOOH to reduce its ash content from 21.2\u00a0% to 5.7\u00a0% (Nguyen et al., 2021). After reaction, the catalyst was magnetically separated, heated to 550\u00a0\u00b0C under air to burn off the biochar and finally reduced at 700\u00a0\u00b0C in the flow of H2/Ar gas to regenerate metallic Fe. However, it was found that ash build-up in the catalyst during repeated catalyst recycling led to catalyst deactivation and the use of high temperatures and hydrogen reduction may not be economically viable.The current study focuses on developing a low-cost ferromagnetic catalytic system for biomass HTL, that can be effectively separated from the solid reaction products and reused multiple times with minimal deactivation. Unlike the existing literature, the study specifically addresses the purification and reusability of catalysts post reaction, supported by detailed material analysis, without the need for high-temperature catalyst regeneration. Initially, two different ferromagnetic metal oxide catalysts, FeOx/C and NiOx/C, were tested for HTL of draff (brewer\u2019s spent grains), an example lignocellulosic residue, and baselined against the performance of a homogeneous base catalyst (Na2CO3). The two metals were chosen for their significantly lower cost compared to Co, as well as their high cracking and hydrogenating activities. Based on the improved recovery of FeOx/C over NiOx/C after reaction, the former was selected for subsequent catalytic reusability studies and characterised to identify its active oxide phase and stability. The novelty of this study lies in (i) synthesising the active magnetite phase (Fe3O4) on activated carbon support by simple wetness impregnation method, (ii) the multi-functionality of the catalyst in both depolymerising and deoxygenating the biomass polymer resulting in remarkable bio-oil yields and composition, (iii) separation of the catalyst from the reaction solid phase by simple magnetic retrieval, and (iv) retained catalyst activity for up to 5 reaction cycles without high-temperature catalyst regeneration or hydrogen reduction. This work extends the current literature and knowledge on the metal active phase, catalyst separation and reusability for HTL reactions.Draff was provided by Chivas Brothers from the Strathclyde distillery unit. Iron (III) nitrate nonahydrate (99+ %) and nickel (II) nitrate hexahydrate (99\u00a0%), were purchased from Fischer Scientific UK, dichloromethane (99 \u2013 99.4\u00a0%) from Honeywell, anhydrous sodium carbonate from Fischer Chemicals, and activated charcoal (DARCO, 100mesh particle size) from Sigma Aldrich. Deionised water was used for all catalyst synthesis and HTL experiments.The activated carbon supported Ni and Fe oxide catalysts, abbreviated as NiOx/C and FeOx/C throughout the article, were prepared by simple wetness impregnation method with an active metal loading of 7.5\u00a0wt%. Prior to use, the activated carbon support was dried overnight at 70\u00a0\u00b0C in the oven. The appropriate amount of aqueous solution of the Ni or Fe precursor was added to the activated carbon support and stirred for 8\u00a0h at room temperature. Water was then removed using a rotary evaporator at 50\u00a0\u00b0C, followed by overnight drying at 100\u00a0\u00b0C, and subsequent heat treatment at 550\u00a0\u00b0C for 5\u00a0h under the flow of 30\u00a0mL\u00a0min\u22121 N2 in a tube furnace.The moisture content in draff was calculated from the weight-loss during oven-drying of as-received draff at 110\u00a0\u00b0C for 12\u00a0h. The weight of dried draff is the total solids. The dried draff was heated to 550\u00a0\u00b0C for 4\u00a0h in a muffle furnace in air. The weight loss is the volatile solid while the remaining solid is the ash content (Sluiter et al., 2008).The elemental analysis (C, H, N, S, and O) was performed using a Thermo Fisher Scientific Flash SMART elemental analyser coupled with TCD at Grant Institute, University of Edinburgh. The furnace temperature was set to 950\u00a0\u00b0C and 1060\u00a0\u00b0C for CHNS and O, respectively. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to quantify the elemental percentage in the materials. The catalyst was digested by reacting 5\u00a0mL of aqua regia with 0.25\u00a0g in a microwave accelerated reaction system at 220\u00a0\u00b0C for 40\u00a0min. After cooling, the sample was filtered through a 0.45\u00a0\u00b5m pore PTFE syringe filter. X-ray powder diffraction (XRD) patterns of the materials were recorded using a Bruker D8 advance with monochromatic Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.542\u00a0\u00c5) at 30\u00a0kV and 15\u00a0mA with a step size of 0.1\u00b0, for the range of 10\u00b0 \u2264 2\u03b8\u00a0\u2264\u00a080\u00b0. Nitrogen adsorption\u2013desorption data were obtained at \u2212196\u00a0\u00b0C using a Micromeritics TriStar II 3020 surface area and porosity analyser. Prior to physisorption measurements, all samples were degassed under vacuum at 200\u00a0\u00b0C overnight. The specific surface area was determined by applying Brunauer\u2013 Emmett\u2013 Teller (BET) method and pore volume were calculated from the amount of N2 adsorbed at P/Po\n of 0.99. X-ray photoelectron spectrometer (XPS) by Thermo NEXSA with a monochromated Al k\u03b1 X-ray source (1486.7\u00a0eV), was used to find the oxidation states of catalysts. The peaks were calibrated by using C 1\u00a0s line in the carbon spectra at 284.0\u00a0eV as a reference. Raman spectroscopy was performed using a Horiba Jobin Yvon Raman microscope under ambient conditions. Pump radiation was supplied by a red argon laser operating at a wavelength of 633\u00a0nm.Hydrothermal liquefaction reactions (HTL) were carried out in a 300\u00a0mL capacity stainless steel closed high\u2013pressure Parr reactor (Model 4560) at temperatures of 300\u00a0\u00b0C, 320\u00a0\u00b0C, and 340\u00a0\u00b0C, with a reaction of 1\u00a0h after reaching the set-point temperature. The reactor was loaded with as received draff (corresponding to 10\u00a0g dry weight), and deionized water (dry draff: H2O\u00a0=\u00a01:10). Catalysed experiments (FeOx/C, NiOx/C, Na2CO3) were conducted with a 1:20 catalyst:draff mass ratio. The catalysts were used as synthesised and did not go through the traditional reduction step. Once complete, the reactor was cooled, and the gas was measured using an inverted water-filled measuring cylinder. Initial screening experiments indicated that the gas consisted predominantly of CO2 (over 98\u00a0%), in line with the existing literature. The contents of the reactor were passed through Whatman filter paper (grade 1), and the aqueous phase was recovered. The reactor assembly was flushed with dichloromethane (DCM) to extract any remaining residues and combined with the filter retentate to solubilize and recover the bio-oil into the solvent, followed by rotary evaporation. The remaining solid phase was dried at 70\u00a0\u00b0C overnight to quantify the overall solid recovery, consisting of biochar, ash, and catalyst.The catalyst recovery from the solid phase post reaction is given in Fig. 1\n. To recover the catalysts, the solids were first suspended in DCM, thoroughly mixed using vortex (Grant instrument PV-1) and centrifuged at 3000 RPM. After decanting the supernatant, the solids were oven-dried at 70\u00a0\u00b0C for an hour, re-suspended in deionized water and catalyst was recovered using a magnetic bar. The recovered catalyst was dried overnight at 70\u00a0\u00b0C before used for the reusability studies. To compensate for the unavoidable losses associated with catalyst filtration and recovery (2\u00a0% to 18\u00a0% of catalyst loading), fresh catalyst was added for repeatability studies. The reusability tests were conducted with the same procedure as mentioned above. All the experiments were performed in duplication, and the errors are represented as the standard deviation.Bio-oil was analysed by GC\u2013MS, CHNS(O) for elemental quantification, bomb calorimetry to determine the calorific value, and muffle furnace for measuring the boiling point distribution. The bio-oil was analysed using a Shimadzu GC-2010 Plus with Restek Rtx- 5 Sil MS column (30*0.25*0.25). The injection temperature was 220\u00a0\u00b0C with a split ratio of 50.0. The column temperature was set to 50\u00a0\u00b0C and held for 5\u00a0min, then ramping to 250\u00a0\u00b0C at the rate of 15\u00a0\u00b0C\u00a0min\u22121 and held there for 1\u00a0min and finally increased to 300\u00a0\u00b0C at 5\u00a0\u00b0C\u00a0min\u22121 and held for 1\u00a0min. Cal2K oxygen bomb calorimeter (Digital data systems) was used to find the calorific value of bio-oil using 0.2\u00a0\u00b1\u00a00.02\u00a0g of each sample. The tests were performed in triplication. CHNS(O) analysis is discussed in section 2.3. The boiling point distribution of the different bio-oils were analysed using a muffle furnace (in the presence of air). Alumina crucible was used, and the empty weight of the crucible was noted. Then, bio-oil was sampled into the crucible and the final weight was noted. The weight of bio-oil was maintained around 0.7\u00a0\u00b1\u00a00.1\u00a0g for better comparison. The crucibles were placed in the furnace and the temperature was set to 100\u00a0\u00b0C at the heating rate of 5\u00a0\u00b0C\u00a0min\u22121. Once set temperature was reached, the oven was turned off and the samples were let to cool, and the weight loss was noted. For the consecutive temperatures (200\u00a0\u00b0C, 300\u00a0\u00b0C, 400\u00a0\u00b0C, 500\u00a0\u00b0C, and 600\u00a0\u00b0C), the furnace was initially set to the previously analysed temperature and then ramped to desired temperature and the weight loss was noted. The total organic carbon in aqueous sample was analysed using Analytik Jena Multi N/C 2100 S HTC TOC/ TN CLD system with HT1300 solids combustion module and AS 60 Autosampler. A thermogravimetric analyser (Mettler Toledo TGA/DSC 1 STAR system) was used to find the decomposition profile. The sample was heated from 25\u00a0\u00b0C to 800\u00a0\u00b0C in the flow of air (20\u00a0mL\u00a0min\u22121).The yields (%) of each phase, % deoxygenation in bio-oil, and % carbon distribution into each phase, % carbon recovery, % energy recovery, H to C effective ratio, and % catalyst recovery were calculated using the following formulae:\n\n(1)\n\n\nYield\n\nof\n\n\nbio - oil\n\n\n\n(\\%)\n\n=\n\n\nMass\n\nof\n\n\n\nbio\n\n-\noil\n\n\n\ng\n\n\n\n\nMass\n\n\nof\n\n\n\ndry\n,\n\nash\n-\nfree\n\ndraff\n\n\n\n\ng\n\n\n\n\n\u2217\n100\n\n\n\n\n\n\n(2)\n\n\nYield\n\nof\n\nsolids\n\n(\n%\n)\n=\n\n\nMass\n\nof\n\nsolid\n\nafter\n\nreaction\n\n\n\ng\n\n\n-\nweight\n\nof\n\ncatalyst\n\n\n\ng\n\n\n\n\nMass\n\nof\n\ndry\n,\n\nash\n-\nfree\n\ndraff\n\n\n\ng\n\n\n\n\n\u2217\n100\n\n\n\n\n\n\n(3)\n\n\nY\ni\ne\nl\nd\n\no\nf\n\n\na\nq\nu\ne\no\nu\ns\n\n\np\nh\na\ns\ne\n\n\n(\n%\n)\n=\n100\n-\ny\ni\ne\nl\nd\n\n\no\nf\n\n\n(\nb\ni\no\n-\no\ni\nl\n+\ns\no\nl\ni\nd\ns\n+\ng\na\ns\n)\n\n\n\n\n\n\n(4)\n\n\nYield\n\nof\n\ngas\n\n\n\n%\n\n\n=\n\n\n\n\nn\n\ngas\n\n\n\nM\n\ngas\n\n\n\n\n\n\nMass\n\n\n\nof\n\ndry\n,\n\nash\n-\nfree\n\ndraff\n\n\n\ng\n\n\n\n\n\u2217\n100\n=\n\n\n\nPV\n\nRT\n\u2217\n\nM\n\ngas\n\n\n\n\n\n\nMass\n\nof\n\ndry\n,\n\nash\n-\nfree\n\ndraff\n\n\n\ng\n\n\n\n\n\u2217\n100\n\n\n\nwhere ngas\n\u00a0=\u00a0number of mols of gas, Mgas\n\u00a0=\u00a0molecular weight of gas, assumed to consist exclusively of CO2 (44\u00a0g\u00a0mol\u22121), P\u00a0=\u00a01\u00a0atm (101,325\u00a0Pa), R\u00a0=\u00a08.314\u00a0m3 Pa/Kmol\u22121, T\u00a0=\u00a0temperature (K) at which the gas was collected, V\u00a0=\u00a0recorded volume of gas in m3.\n\n(5)\n\n\nDeoxgenation\n\n\n\n%\n\n\n=\n\n\nMolar\n\nratio\n\nof\n\n\n\n\n\nO\n\nC\n\n\n\n\nin\n\ndraff\n-\n\n\nMolar\n\n\n\nratio\n\nof\n\n\n\n\nO\n\nC\n\n\n\n\n\nin\n\nbio\n-\noil\n\n\nMolar\n\nratio\n\nof\n\n\n\nO\nC\n\n\n\nin\n\ndraff\n\n\n\u2217\n100\n\n\n\n\n\n\n(6)\n\n\nC\na\nr\nb\no\nn\n\n\nd\ni\ns\nt\ni\nb\nu\nt\ni\no\nn\n\n\n\n\n%\n\n\n=\n\n\nM\na\ns\ns\n\n\no\nf\n\n\nC\n\n\ni\nn\n\n\np\nr\no\nd\nu\nc\nt\n\n\nM\na\ns\ns\n\n\no\nf\n\n\nC\n\n\ni\nn\n\n\nd\nr\na\nf\nf\n\n\n\n\u2217\n\n100\n\n\n\n\n\n\n(7)\n\n\nC\na\nr\nb\no\nn\n\n\nr\ne\nc\no\nv\ne\nr\ny\n\n\n\n\n%\n\n\n=\n\n\nM\na\ns\ns\n\n\no\nf\n\n\nc\na\nr\nb\no\nn\n\n\ni\nn\n\n\nb\ni\no\n-\no\ni\nl\n\n\u2217\n\ny\ni\ne\nl\nd\n\n\no\nf\n\n\nb\ni\no\n-\no\ni\nl\n\n\nM\na\ns\ns\n\no\nf\n\nc\na\nr\nb\no\nn\n\ni\nn\n\nd\nr\na\nf\nf\n\n\n\n\n\n\n\n\n(8)\n\n\nE\nn\ne\nr\ng\ny\n\n\nr\ne\nc\no\nv\ne\nr\ny\n\n\n\n\n%\n\n\n=\n\n\nH\nH\nV\n\n\no\nf\n\n\nb\ni\no\n-\no\ni\nl\n\n\u2217\n\ny\ni\ne\nl\nd\n\n\no\nf\n\n\nb\ni\no\n-\no\ni\nl\n\n\nH\nH\nV\n\n\no\nf\n\n\nd\nr\na\nf\nf\n\n\n\n\n\n\n\n\n(9)\n\n\n%\n\n\nC\na\nt\na\nl\ny\ns\nt\n\n\nr\ne\nc\no\nv\ne\nr\ny\n=\n\n\nc\na\nt\na\nl\ny\ns\nt\n\n\nu\ns\ne\nd\n\n\nf\no\nr\n\n\nr\ne\na\nc\nt\ni\no\nn\n\n\n\n\ng\n\n\n-\n\nc\na\nt\na\nl\ny\ns\nt\n\n\nr\ne\nc\no\nv\ne\nr\ne\nd\n\n\n\n(\ng\n)\n\n\n\nc\na\nt\na\nl\ny\ns\nt\n\n\nu\ns\ne\nd\n\n\nf\no\nr\n\n\nr\ne\na\nc\nt\ni\no\nn\n\n\n\n\ng\n\n\n\n\n\n\u2217\n\n100\n\n\n\n\n\n\n(10)\n\n\nH\n/\nC\n\ne\nf\nf\ne\nc\nt\ni\nv\ne\n=\n(\nH\n-\n2\nO\n-\n2\nN\n)\n/\nC\n\n\n\n\nwhere H, C, O, and N are the moles of hydrogen, carbon, oxygen, and nitrogen, respectively in the sample (Huang et al., 2016).Draff is a key by-product from the malting process of barley in whiskey distilleries, where barley is mixed with water to convert the starch into soluble sugars. The insoluble solid fraction is recovered as draff and usually used as wet animal feed. Scottish based draff typically contains around 17\u201325\u00a0% cellulose, 22\u201328\u00a0% hemicellulose, 12\u201328\u00a0% lignin, as well as proteins (15\u201324\u00a0%) and lipids (\u223c10\u00a0%) (Foltanyi et al., 2020). The draff used in this study had a residual water content of 69.8\u00a0%, below the typical HTL water to biomass ratio of 10:1, and hence no additional dewatering or drying is required (see supplementary material). Instead, some make-up water is required, which may be added by recycling some of the HTL aqueous phase product, or by co-feeding pot ale or spent lees, which are aqueous-phase by-products from the malting processes and contain dilute concentrations of organic acids and alcohols (Akunna & Walker, 2017).The dried draff contained 48.9\u00a0% carbon, 7.2\u00a0% hydrogen, 3.8\u00a0% nitrogen, 0.2\u00a0% sulphur and 34.5\u00a0% oxygen, and the remaining 4.4\u00a0% can be attributed to minerals in the ash phase (5.1\u00a0%). Using a typical protein nitrogen content of 16\u00a0%, the maximum draff protein content can be estimated as 23.8\u00a0%, within the typical range stated above. Oxygen is present as different functional groups in draff such as fatty acids, phenolics, aldehydes, ketones, amino acids, etc., as well as oxides in the ash phase, explaining the slight difference between ash content (5.1\u00a0%) and non-organic minerals (4.4\u00a0%).To determine the optimal temperature for catalytic studies, initial screening experiments were conducted at three typical HTL temperatures (300\u00a0\u00b0C, 320\u00a0\u00b0C, and 340\u00a0\u00b0C) and the % yield and % carbon distribution into bio-oil, aqueous, gas and solid phases are given in Fig. 2\n. In Fig. 2a, the maximum solid residue yield of 16\u00a0% was observed at 300\u00a0\u00b0C, reducing to 13.5\u00a0% at 320\u00a0\u00b0C and 340\u00a0\u00b0C, indicating improved biomass depolymerisation at higher temperature. While 300\u00a0\u00b0C is sufficient to break the glycosidic bonds in carbohydrates, previous HTL studies with protein-rich biomass have shown that higher temperatures are required for full protein and lipid depolymerisation (Seshasayee & Savage, 2021; Shakya et al., 2015). Bio-oil yields increased from 27.4\u00a0\u00b1\u00a01.7\u00a0% at 300\u00a0\u00b0C to 41.2\u00a0\u00b1\u00a02.2\u00a0% at 320\u00a0\u00b0C, before reducing slightly to 38.5\u00a0\u00b1\u00a03.0\u00a0% at 340\u00a0\u00b0C. At the same time, aqueous phase yields reduced from 52.6\u00a0\u00b1\u00a02.7\u00a0% at 300\u00a0\u00b0C to 41.4\u00a0\u00b1\u00a00.3\u00a0% at 320\u00a0\u00b0C, indicating increased recombination of polar organic molecules, such as glycoaldehydes, organic acids, furfurals and phenols, into less-polar components which preferentially fractionate into the oil phase as the reaction temperature is increased (Gollakota et al., 2018). The small reduction in oil yields at 340\u00a0\u00b0C may be caused by increased cracking at high temperatures (Jindal & Jha, 2016), although gas yields remained constant at around 3.6\u00a0% at all three reaction temperatures. The data clearly suggest that HTL temperatures of 320\u00a0\u00b0C are sufficient to depolymerize draff and maximise carbon extraction into the oil phase, avoiding excessive cracking and other undesirable side reactions.Oil yields from this study are significantly higher than those reported previously for non-catalytic HTL of draff at 300\u00a0\u00b0C for 1\u00a0h (\u223c14\u00a0%) (D\u00e9niel et al., 2017), but lower than from pyrolysis in a twin coaxial screw reactor at 450\u00a0\u00b0C (51\u00a0%), containing both organic and aqueous phases (Mahmood et al., 2013). These differences may be explained by a variety of factors, including differences in biochemical composition, water loading, heating rates etc. The carbon distributions given in Fig. 2b follow similar trends to the mass yields, but with a significant shift to the oil phase, ranging from 36.4\u00a0\u00b1\u00a03.5\u00a0% at 300\u00a0\u00b0C to 58.3\u00a0\u00b1\u00a03.5\u00a0% at 320\u00a0\u00b0C, indicating the preferential fractionation of carbon to this product. While an excellent carbon balance of >99\u00a0% was obtained at 320\u00a0\u00b0C and 340\u00a0\u00b0C, only 88\u00a0% of carbon was recovered at 300\u00a0\u00b0C, due to the high viscosity of the bio-oil/solid product mixture, making it difficult to extract all the products completely.Two ferromagnetic metal oxide materials, FeOx/C and and NiOx/C, were tested as potential magnetically recoverable low-cost catalysts for the conversion of draff, both in the absence and presence of homogenous Na2CO3 base catalyst (Fig. 3\n). While all catalytic runs resulted in a significant reduction in solid yields, the effect was more pronounced for the metal-oxide catalysts on their own than the metal oxide \u2013 base combinations (Fig. 3a). These trends are slightly different to the catalytic conversion of corn straw over Zn/Ni/Co-Fe2O4 (Chen et al., 2021) and food wastes over mixed metal oxides based on Al, Si, Fe, and Ca (Cheng et al., 2020) where the presence of base (NaOH and red mud/red clay base catalysts, respectively) reduced the solid residue yield compared to the single catalytic system. In both cases, it was reported that the base to acid site density was crucial for improving bio-oil yield.A potential explanation for the discrepancy with the current study is the difference in biomass composition. While corn straw and food wastes are rich in cellulose and carbohydrates, respectively, draff contains higher fractions of lignin, protein, and lipids with recalcitrant CC and CO bonds which require strong acid/metal sites to cleave. HTL of lignin under basic conditions produces phenolic monomers which are prone to repolymerise into oligomers increasing solid formation (Ciuffi et al., 2021). Base catalysts have also been found to be detrimental for the conversion of protein and lipid-rich feedstocks due to the inefficiency of alkali catalysts to break the peptide and ether linkage bonds (Shah et al., 2022).Consistent with the reduced solid yields, all catalytic reactions resulted in an increase in the HTL aqueous phase (AP), indicating improved biomass depolymerisation. Bio-oil yields were significantly enhanced for the NiOx/C (47.9\u00a0%) and FeOx/C (49.3\u00a0%) catalysed reactions, but addition of base resulted in a reduction of oil yields for all three reaction systems. It is known that HTL follows a multi-step reaction sequence, starting with the hydrolysis of biomass into reactive intermediates in the aqueous phase, followed by the deoxygenation and condensation into organic bio-oil compounds. Therefore, although the presence of base appears to increase the initial biomass depolymerisation during the non-catalytic experiments, it inhibits the subsequent conversion of AP products, resulting in reduced bio-oil production.The trends are even more pronounced for the product carbon distribution, where the NiOx/C and FeOx/C catalysts increase carbon recovery to the bio-oil to 75.1\u00a0\u00b1\u00a02.9\u00a0% and 78.6\u00a0\u00b1\u00a02.1\u00a0%, respectively, while reducing AP carbon by 18.9\u00a0\u00b1\u00a00.4\u00a0% and 14.8\u00a0\u00b1\u00a00.9\u00a0% (Fig. 3b). Besides improved bio-oil yields, the lower AP carbon reduces the demand for downstream waste-water treatment, improving the viability of the process. In contrast, addition of base increases the carbon concentration in the AP between 21.9\u00a0% for FeOx/C to 69.4\u00a0% for the non-catalytic runs, while reducing bio-oil carbon by 5\u00a0%, further demonstrating the inhibitive effect of base on the conversion of AP intermediates into bio-oil.Non-catalytic HTL of draff increased the carbon content from 48.9\u00a0% in the biomass to 68.7\u00a0% in the bio-oil, while reducing oxygen content from 34.5\u00a0% to 19.8\u00a0%, hydrogen from 7.2\u00a0% to 6.8\u00a0% and nitrogen from 3.8\u00a0% to 2.9\u00a0% (Table 1\n). Despite the reduction in hydrogen content, the effective H/C ratio, (H/C)eff, which indicates the required amount of hydrogen and energy during bio-oil refining (Karatzos et al., 2014), increased from 0.64 to 0.70, due to the large reduction in oxygen content. Similarly, the higher heating value (HHV) increased from 22.1\u00a0MJ\u00a0kg\u22121 in the biomass to 30.3\u00a0MJ\u00a0kg\u22121 in the bio-oil, corresponding to an energy recovery of 54.3\u00a0% into the bio-oil. Oil properties can be improved both by hydrogenation reactions to eliminate O and N as water and ammonia, or through non-hydrogen reactions such as cracking, releasing oxygen in the form of carbon oxides to reduce the heteroatom content, and hence improving the (H/C)eff value. The presence of base catalyst increased the bio-oil carbon content to 73.6\u00a0%, while reducing the hydrogen and oxygen contents to 6.7\u00a0% and 15.5\u00a0%, respectively, raising the (H/C)eff to 0.74, with a HHV of 32.4\u00a0MJ\u00a0kg\u22121. However, the reduced bio-oil yields and bio-oil carbon recovery lowered the overall bio-oil energy recovery to 49.1\u00a0%.The addition of the as-synthesised metal catalysts resulted in a significant increase in both bio-oil carbon (81.3\u00a0% and 80.0\u00a0%) and hydrogen contents (8.3\u00a0% and 9.5\u00a0% for FeOx/C and NiOx/C, respectively), leading to high effective H/C ratios (1.06 and 1.24), and HHVs (37.8\u00a0MJ\u00a0kg\u22121. and 38.6\u00a0MJ\u00a0kg\u22121), comparable to the HHV of heavy fuel oil (40\u00a0MJ\u00a0kg\u22121) (Karatzos et al., 2014). Together with the high oil yields, the addition of FeOx/C and NiOx/C result in excellent energy recoveries of 84.4\u00a0\u00b1\u00a00.8\u00a0% and 83.7\u00a0\u00b1\u00a00.7\u00a0%, respectively, indicating that the catalysts simultaneously improve bio-oil yield and bio-oil composition. These values are comparable to the highest HTL energy recoveries of 87\u00a0% reported to date (for halophytic microalga,\nTetraselmis\nsp.) (Eboibi et al., 2014), and significantly higher than those obtained from non-algae biomass (up to 75.6\u00a0% for sewage sludge derived bio-oil) (Anastasakis et al., 2018). Particularly, the high hydrogen content of 9.5\u00a0% in the bio-oil produced over NiOx/C is remarkable and may be ascribed to the excellent hydrogenation and effective water dissociation activity of nickel-based catalyst (Subbaraman et al., 2011; Wang et al., 2020). Nickel oxides have been previously found to yield superior bio-oil hydrogen content during HTL of microalgae compared to other metal oxides such as Fe, Co, Mg, and Mo (Wang et al., 2018). Zhu et al. reported the exceptional catalytic activity of Ru/Ni/Ni(OH)2/C for room temperature hydrogenation of naphthalene, where it was reported that the role of Ru was hydrogen activation, while Ni acts as a bridge in transferring the hydrogen species to the naphthalene over the Ni(OH)2 sites (Zhu et al., 2017). Meanwhile, using FeOx/C catalyst, the H % still increased to 8.3\u00a0%, indicating good hydrogenation properties. A potential mechanism is the reaction of partially oxidised Fe3O4, with water to generate in-situ hydrogen. For example, Fe3O4 catalyst has been reported to effectively dehydrate cellobiose to 5-hydroxymethylfurfural in water, while retaining its activity and oxidation state even after several uses (Bhalkikar et al., 2015). Addition of base to the metal-oxide catalysed reactions resulted in reduced bio-oil carbon and hydrogen contents, while increasing the fractions of oxygen and nitrogen. Together with the lower bio-oil yields overall, the energy recovery is significantly reduced to 67.9\u00a0% for FeOx/C\u00a0+\u00a0Na2CO3 and 60.6\u00a0% for NiOx/C\u00a0+\u00a0Na2CO3, and hence the presence of base appears detrimental to the metal-oxide catalysed HTL of draff.A commonly used qualitative analysis of bio-oil composition is its simulated boiling point distribution, which determines the fraction of compounds within pre-defined boiling point ranges, and hence the downstream refining requirements. Here, boiling point distributions were obtained by heating bio-oil samples in a muffle furnace under controlled heating rates and recording the resulting weight losses at 100\u00a0\u00b0C intervals (Fig. 4\n). As expected, bio-oil weight losses up to 100\u00a0\u00b0C were low for all reactions (\u223c1 \u2013 2\u00a0%), as most volatiles would be lost during solvent evaporation of the oil recovery process. Given the high overall carbon recoveries of\u00a0>\u00a099\u00a0% (See Fig. 3b) and low gas yields (3.4\u00a0%), volatile formation during HTL of draff appears to be low for both the standard and catalytic reactions, and the low boiling point compounds are most likely attributed to residual water or DCM solvent in the bio-oil.Non-catalytic HTL produced relatively heavy bio-oils, with the majority of compounds in the 400 \u2013 600\u00a0\u00b0C boiling point range. The results are consistent with the low effective H/C ratio of these oils, suggesting limited cracking and hydrogenation during bio-oil formation. Therefore, significant upgrading will be required to convert these bio-oils into useful transportation fuels or chemicals. It is also notable, that the majority of compounds fall outside the suitable range for standard GC analysis (<240\u00a0\u00b0C), demonstrating that this method is not suitable for bulk bio-oil characterisation. Therefore, commonly reported compositional analysis by GC provides only limited insight into the true molecular make-up of bio-oils. Addition of base catalyst as limited effect on the formation of low-boiling point compounds but increases the production of compounds in the 300 \u2013 400\u00a0\u00b0C range from 5.8 to 7.5\u00a0%. This may be the result of improved biomass depolymerisation into smaller aqueous phase intermediates and in-situ hydrogen production via formate (HCOO-Na+) formation, aiding the reduction of heavier hydrocarbons (S\u0131na\u01e7 et al., 2004). At the same time, the increase in lower boiling point compounds is outweighed by the notably reduction in heavier compounds in the 400 \u2013 600\u00a0\u00b0C region, resulting in an overall decrease of bio-oil production, and hence reduced overall energy recovery.Both metal oxide catalysts resulted in a significant increase in the formation of low-boiling compounds in the gasoline (100 \u2013 200\u00a0\u00b0C) and particularly jet fuel (200 \u2013 300\u00a0\u00b0C) range, consistent with the high effective H/C ratios and energy densities. Acidic metal oxides have been proven effective catalysts for C\u2013heteroatom bond, aiding the deoxygenation of shorter chain organics, reducing their solubility in the aqueous to oil phase. Ni and Fe oxide catalysts are also well-known for cracking reactions to produce lighter hydrocarbon compounds (Qiu et al., 2022). The metal oxides also enhanced the production of compounds in the 300 \u2013 400\u00a0\u00b0C and 400 \u2013 500\u00a0\u00b0C range, while significantly reducing the formation of heavies (boiling points\u00a0>\u00a0500\u00a0\u00b0C) from 25\u00a0% for the non-catalytic run to 1.5\u00a0% and 3.5\u00a0% for FeOx/C and NiOx/C, respectively. Crucially, around two thirds of compounds produced with FeOx/C (66.5\u00a0%) and NiOx/C (63.5\u00a0%) have boiling points below 400\u00a0\u00b0C, indicating their potential for upgrading to vital chemicals and fuel additives.While the addition of base to the metal oxide catalyst reactions resulted in only small reduction of overall bio-oil yields, particularly for FeOx/C, it shifted the boiling point distribution towards heavier compounds, consistent with the lower hydrogen and energy content in these products. Cracking reactions are commonly catalysed by acid-sites on the metal oxide catalysts, which are likely blocked by the base resulting in reduced bio-oil conversion.Both metal oxide catalysts investigated for HTL of draff showed excellent activity in not only increasing bio-oil production, but also improving bio-oil composition through reduced heteroatom content, increased effective H/C ratio and heating content, and improved boiling point distribution. Despite this, commercial application of these catalysts for the HTL of biomass is only feasible if the catalysts can be effectively recovered and recycled. Particularly the separation of catalyst from solid ash poses a major challenge for biomass HTL, as unlike char, the ash cannot be readily volatilised or incinerated. Draff used in the current study had an ash content of 5.1\u00a0%, comparable to the amount of catalyst loaded to the system, and hence most of the ash must be removed prior to recycling to avoid system accumulation. By employing the two ferromagnetic catalysts FeOx/C and NiOx/C, it was hoped that their magnetic properties could be exploited for post reaction recovery. Both fresh materials exhibited magnetic properties and were attracted to a magnet placed outside the sample vial. However, the magnetic affinity of FeOx/C was visibly stronger than that of NiOx/C, increasing the amount of material that could be dragged up at a time (see supplementary material).After reaction, initial attempts to magnetically separate the catalysts from the solid reaction products were unsuccessful, as residual oil and char trapped in the solids acted as a binder between the catalyst and ash, inhibiting the separation. However, after adapting the procedure to improve bio-oil extraction through thorough mixing with DCM and suspending the resulting catalyst-ash mixture in water, the majority of FeOx/C catalyst (96\u00a0%) could be recovered. In contrast, none of the NiOx/C could be recovered. NiOx/C displayed two sharp peaks with a 2\u03b8 values of 52.15\u00b0 and 61.06\u00b0 corresponding to the JCPDS 00-001-1260 card for Ni metal (see supplementary material). The formation of metallic Ni during heat treatment in N2 is ascribed to the low reduction temperature required for Ni oxide reduction to metal. For example, it has been previously reported that Ni nitrate precursor, supported on carbon, first decomposed into Ni oxides, before reducing into metallic Ni due to oxygen transfer to the carbon support, during high temperature treatment (723\u00a0K) under N2 flow (Gandia & Montes, 1994). However, post reaction, the catalyst no longer exhibited any peaks corresponding to Ni oxides or Ni metals, explaining the loss of magnetic properties, and indicating poor catalyst stability under hydrothermal conditions. Hence this catalyst appears unsuitable for this process and could not be used for further catalyst reusability studies.To test the stability of FeOx/C catalyst during HTL of draff, its reusability was tested over 5 reaction cycles (Fig. 5\n). The catalyst recovery ranged between 80\u00a0% and 98\u00a0% and the difference in catalyst weight was compensated using fresh catalyst. Reaction yields were corrected to discount the contribution of fresh catalyst using the following equation:\n\n(11)\n\n\n\nY\n\ncorrected\n\n\n=\n\nY\n\nactual\n\n\n-\n\n\nm\n\nFC\n\n\n\nm\n\nTC\n\n\n\n\u00d7\n\n\n\nY\n\nFC\n\n\n-\n\nY\n\nNC\n\n\n\n\n\n\n\nwhere Y is yield of different product fractions, mFC and mTC are the weights of fresh and total catalyst, respectively, and YFC and YNC are the yields obtained from the initial catalytic and non-catalytic reactions, respectively.The results show a gradual decrease in the carbon recovery to the bio-oil from 80\u00a0% initially to 71.7\u00a0% after 5 reaction cycles, while the solid formation doubled from 1.8\u00a0% to 3.6\u00a0%. There were no significance differences in the aqueous and gas yields during the reusability studies. The increased solid formation resulted in increased material losses during product extraction, resulting in a slight decrease in overall carbon balance closure. The increase in solid formation also impeded the catalyst separation post reaction, reducing catalyst recovery from 96\u00a0% after the first run, to only 86\u00a0% retrieved by 5th reaction cycle. Nonetheless, even after 5 cycles, the catalyst showed a significant improvement in oil yields and reduced solid formation compared to the non-catalytic experiments.The catalysts were supported on commercial activated charcoal, with a surface area of 870\u00a0m2 g\u22121. The N2 adsorption\u2013desorption isotherm profiles (see supplementary material) indicate that the carbon support consists mostly of mesopores (type IV with H3 hysteresis loop, IUPAC classification isotherm), with a pore volume of 0.6\u00a0cm3 g\u22121 and pore size of 5.7\u00a0nm. As expected, following FeOx and NiOx deposition the material surface area reduced slightly to 706 and 637\u00a0m2/g, pore volume decreased to 0.5\u00a0cm3 g\u22121 and pore size to 3.7 and 3.8\u00a0nm, respectively with FeOx/C and NiOx/C catalysts, due to metal oxide deposition inside the pores, however the overall mesopore structure was retained. The bulk metal loading in the catalyst (Fe in FeOx/C and Ni in NiOx/C) was confirmed as 7.4\u00a0% and 7.2\u00a0% using ICP-AES analysis (see supplementary material).Iron oxide exist in various phases, including \u03b1-Fe2O3 (hematite), \u03b3-Fe2O3 (maghemite), and Fe3O4 (magnetite). While both maghemite and magnetite are magnetic, fully oxidised hematite is generally nonmagnetic. Therefore, given the strong magnetic affinity of the material, the presence of significant amounts of \u03b1-Fe2O3 in the catalyst is unlikely. Analysis by XRD (see supplementary material) revealed the presence of a broad reflection around 25\u00b0, corresponding to amorphous carbon, and several reflections at 35.07 \u00b0 (220), 41.38 \u00b0 (311), 50.44 \u00b0 (400), 62.91 \u00b0 (422), 67.22 \u00b0 (333), and 74.11 \u00b0 (440), which could be matched to the JCPDS-ICCD diffraction pattern 082\u20131533 of Fe3O4 (Silva et al., 2013). To further confirm the formation of Fe3O4, Raman analysis (see supplementary material) revealed a broad band around 670\u00a0cm\u22121 which may correspond to Fe3O4, however Fe3O4 is a poor Raman scatterer and tends to oxidise under laser irradiation inducing phase changes (Li et al., 2012; Schwertmann & Cornell, 2008). This may explain the presence of band around 243\u00a0cm\u22121 and 290\u00a0cm\u22121, which have been previously attributed to the Eg Fe- O symmetric bending of \u03b1-Fe2O3 (M\u00e4kie et al., 2011), and the band around 228\u00a0cm\u22121, which can be attributed to the A1g Fe- O symmetric stretching of \u03b1-Fe2O3. The oxidation state of the metal oxides was further characterized by high resolution XPS and the deconvoluted XPS spectra of the Fe 2p region (see supplementary material). Fe 2p exhibits two oxidation states, +2 and\u00a0+\u00a03. The spectral bands at 712.1\u00a0eV and 725.8\u00a0eV corresponds to the 2p3/2 and 2p1/2 of Fe\u00a0+\u00a03 of Fe3O4, while the bands at 710.4\u00a0eV and 723.8\u00a0eV can be attributed to the 2p3/2 and 2p1/2 of Fe\u00a0+\u00a02. Stoichiometrically Fe3O4 can be expressed as FeO.Fe2O3, with a ratio of Fe2+ to Fe3+ of 0.33 to 0.67. The results from the deconvoluted peaks of Fe 2p region provided a ratio of 0.3:0.7 which is consistent with the values published earlier (Yamashita & Hayes, 2008). A potential explanation for the formation of Fe3O4, rather than fully oxidised Fe2O3, is the use of N2 during heat treatment, which may limit the availability of oxygen to form the more common Fe2O3 phases. With NiOx/C, the characteristic spectral bands at 853.4\u00a0eV and 870.7\u00a0eV corresponds to the Ni 2p3/2 and Ni 2p1/2 of Ni-O with a satellite peak at 861.5\u00a0eV. The bands at 856.3\u00a0eV and 873.9\u00a0eV corresponds to the Ni 2p3/2 and Ni 2p1/2 of Ni(OH)2 with a satellite peak at 878.6\u00a0eV (Nesbitt et al., 2000). As can be observed, Ni is mainly present as Ni(OH)2 which could be due to the immediate conversion of Ni(NO3)2\nprecursor to Ni(OH)2 which is strongly bound to the carbon surface (Sturgeon et al., 2014). The formation of Ni metal was not observed in the XPS which could be due to the surface oxidation of the Ni atoms.To gain further insight into the behaviour and performance of the promising FeOx/C catalyst, the as synthesised and spent materials (recovered after 5 reaction cycles) were thoroughly analysed to identify the active sites and surface changes during the HTL reaction. The FeOx/C catalysts retrieved after 5 reaction cycles were re-characterised to identify any changes in physicochemical properties. From ICP analysis, the amount of Fe reduced from 7.4\u00a0% in the fresh to 5.9\u00a0% in the spent catalyst, indicating some degree of metal leaching over the course of the 5 reactions (see Supplementary Information). The decrease in iron content could be due to the leaching of iron from the catalyst into the solid collected after reaction. As it has been reported, metal leaching from activated carbon support is possible due to the low surface oxygen groups in the carbon that makes a poor support-metal interaction. To check the possible leaching of iron into the solid phase, the solid residue obtained after catalyst recovery was characterised with an iron content of 3.6\u00a0%, compared to 2.4\u00a0% in the ash obtained from draff (obtained by heating at 550\u00a0\u00b0C for 4\u00a0h). The other minor elements found by SEM-EDS in ash were Ca, P, Mg, Na, and K. Besides leaching, there is also the possibility that some of the biomass ash was retained in the recovered catalyst, reducing its apparent Fe content, and vice versa.However, as described above, the ash retained some of the solid catalyst which may partially account for this increase in Fe content. The XRD patterns of the catalyst retrieved after 5 reaction cycles (before and after ash removal) are less crystalline (see supplementary material), but still display the diffraction patterns related to Fe3O4, as well as small peak corresponding to the \u03b1-Fe2O3 phase. These results suggest that Fe3O4 may participate in redox reactions under hydrothermal conditions, which may explain its excellent activity for bio-oil deoxygenation and hydrogenation. High-resolution XPS analysis of the spent materials did not detect any signals related to Fe, which may indicate carbon deposition on the catalyst surface XPS is sensitive only up to 10 atomic layers. Interestingly, a significant amount of N was observed during the survey scan which could indicate surface adsorption of biomass nitrogen. For comparison, an XPS survey scan of biochar produced by non-catalytic HTL of draff was performed and similar signals related to N were observed, supporting this observation (see supplementary material).Overall, despite the small metal losses and phase transformation, the catalyst maintained good stability after 5 reaction cycles as demonstrated by the high activity compared to the non-catalytic experiments. It was demonstrated that the magnetic properties of the catalysts could be effectively exploited to separate the catalyst from the solid HTL reaction products, particularly ash, although the process needs to be further developed to reduce the number of processing steps and increase the lifetime of the catalyst, such as; (i) pre-treatment of biomass to remove ash; (ii) synthesis of a core\u2013shell catalyst where the active species is encapsulated in the core and covered by metal oxide such as Al2O3 or SiO2; (iii) increase of catalyst particle size or iron oxide deposition into monoliths. It has to be noted that the metal oxide should be stable in HTL conditions since \u03b3- Al2O3 tend to phase transfer to boehmite under pressurised water.In summary, a cheap, safe, stable, and magnetically separable Fe3O4 catalyst supported on activated carbon was developed for HTL of lignocellulosic biomass. Catalyst addition not only increased bio-oil mass yields by 49.3\u00a0%, to reach non-optimised carbon and energy recoveries of 82\u00a0% and 84.3\u00a0%, respectively, but also produced an oil with an exceptionally high calorific value of 37.8\u00a0MJ\u00a0kg\u22121. The catalyst was separated and reused up to 5 times, however, further optimisation of reaction and recovery conditions are required to achieve commercial relevant system.\nSwathi Mukundan: Data curation, Formal analysis, Investigation, Writing \u2013 original draft. Jin Xuan: Funding acquisition, Investigation, Writing \u2013 review & editing. Sandra E. Dann: Methodology, Formal analysis, Writing \u2013 review & editing. Jonathan L. Wagner: Conceptualization, Methodology, Project administration, Supervision, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by UKRI-EPSRC through grant EP/V011863/1. The authors sincerely acknowledge the facilities and assistance provided by Loughborough Materials Characterisation Centre (LMCC) at Loughborough University. Sincere thanks to MrsBethany Taylor and Dr Tanya Radu from the Department of Water Engineering, Loughborough University for the ICP and TOC analysis. Further, we would like to acknowledge Chivas Brothers for providing the brewer\u2019s spent grains used in this study.Supplementary data to this article can be found online at https://doi.org/10.1016/j.biortech.2022.128479.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  This article reports a safe, low-cost, and industrially applicable magnetite supported on activated carbon catalyst that can be magnetically retrieved from the solid and reused multiple times without the need of a regeneration step. The FeOx/C catalyst improved the bio-oil yield by 19.7\u00a0\u00b1\u00a00.96\u00a0% when compared to the uncatalysed reaction at 320\u00a0\u00b0C for the HTL of draff (brewer\u2019s spent grains). The use of homogeneous Na2CO3 base as a catalyst and co-catalyst, improved carbon extraction into the aqueous phase. The exceptional catalytic activity can be attributed to the Fe3O4 phase which can produce in-situ H2 that improves the biomass decomposition and oil property with an energy recovery of \u223c84\u00a0%. The FeOx/C catalyst was separated using magnetic retrieval and maintained its catalytic activity even up to 5 reaction cycles showing potential as a cheap catalyst for HTL reactions and can be scaled-up for industrial applications.\n               "}
{"full_text": "Excellent mechanical and chemical properties enable polyolefin widely used in automation, electronics and other industrial fields [1\u20133]. Current development trend indicates that the polyolefin industry will continue to maintain a rapid growth rate of 4%\u20135% in the next few years [4]. Furthermore, catalysts for olefin polymerization have always been the focus of researchers [5]. Since the Ziegler-Natta catalysts and Metallocene catalysts are hard to realize the synthesis of polyethylene with high branching density [5], Brookhart et\u00a0al. reported the \u03b1-diimine(\u0399\u0399) nickel complexes as the catalysts for the ethylene polymerization [6,7]. Based on the unique \u201cchain walking\u201d mechanism [8], \u03b1-diimine nickel catalysts with varied ligand structures could greatly enrich the microstructure of polymer chains and then affect the properties of the resultant polymers [9,10], which provides a new path for the preparation of polyolefin. In the next two decades, researchers have conducted a series of in-depth researches on the catalytic performance by adjusting external reaction conditions [11\u201313] and ligand structures [14\u201316]. As one of the important routes to obtain high-end polyethylene products, the introduction of branched chains would further enrich the microstructure of polyolefin. So the presence of 1-hexene in the ethylene polymerization would contribute to enrich the microstructure and manipulate the product properties [17]. However, systematic investigation of ligand effect on ethylene/1-hexene copolymerization and microstructure manipulation of products have not been reported in detail before.In this contribution, we investigated the ethylene/1-hexene copolymerization performance of three catalysts with different steric effect, naming (Ar1N=C(Me)\u2013C(Me)=NAr1) NiBr2 (Cat. A), (Ar1N=C(An)\u2013C(An)=NAr1) NiBr2 (Cat. B), and (Ar2N=C(An)\u2013C(An)=NAr2) NiBr2 (Cat. C) (where Me\u00a0=\u00a0methyl, An\u00a0=\u00a0acenaphthene, Ar1\u00a0=\u00a02,6-(i-Pr)2C6H3, Ar2\u00a0=\u00a04-MeO-2,6-(Ph2CH)2C6H2) respectively, under different reaction conditions. The ligand steric effect of these three \u03b1-diimine nickel complexes on branching distribution was detected by nuclear magnetic resonance (NMR) in detail. In addition, electron paramagnetic resonance (EPR) experiments were used to investigate the chemical valence of Ni species and the proposed polymerization mechanism in the presence of co-catalyst.Toluene (99.5%), ethanol (99.7%), chlorhydric acid (36.0%\u201338.0%) were purchased from Sinopharm Chemical Regent Co. Ltd. Ethylene (99.99%) and argon (99.99%) were purchased from Jin Gong Gas Co. Ltd. Catalysts (Cat. A, Cat. B, and Cat. C, Fig.\u00a01\n) were provided by the University of Science and Technology of China [6,16]. C6D4Cl2 (o-DCB) were purchased from Qingdao Tenglong Weibo Technology Co. Ltd. Polymerization-grade ethylene and argon were further processed by the purification system. Toluene was refluxed in the presence of sodium and indicator benzophenone before it was used for ethylene polymerization.All polymerization process involving air and/or moisture-sensitive compounds were performed under argon atmosphere under strict anhydrous and anaerobic conditions through the Schlenk techniques [18,19]. Olefin polymerization was carried out in a 100\u00a0mL three-necked flask. Co-catalyst methylaluminoxane (MAO) and catalyst toluene solution were injected to initiate polymerization. It was quenched by the addition of acidified ethanol (5\u00a0wt % HCl) after the polymerization was completed. The product was washed by anhydrous ethanol for more than three times and dried at 40\u00a0\u00b0C to constant weight.The detailed polymerization conditions were summarized in Table 1.\n1H NMR spectra were recorded at 120\u00a0\u00b0C on a Varian Mercury-Plus 300\u00a0MHz spectrometers. Chemical shift (\u03b4) were expressed as parts per million and 1H NMR spectra were referenced using the solvent o-DCB. Sample was prepared as 40\u00a0mg/mL and the cumulative scan was 100 times. It was used to investigate the branching density (B) of the products.\n13C NMR spectra were obtained on an Agilent DD2 600\u00a0MHz spectrometer and CDCl3 was used as the solvent. The concentration of prepared sample was 50\u00a0mg/mL and the cumulative scan was 3000 times. The peak of the main chain methylene was set to \u03b4 30 as an internal standard. It was used to detect the branch distribution of the products.Gel permeation chromatography (GPC) measurements were carried out using a PL-GPC220 high temperature gel permeation chromatography at 160\u00a0\u00b0C. And 1,2,4-trichlorobenzene was used as the solvent. It was used to investigate the molecular weight (M\nw) and polydispersity index (PDI) of the product.Differential Scanning Calorimetry (DSC) measurements were conducted under nitrogen in the temperature range from \u221280\u00a0\u00b0C to 160\u00a0\u00b0C with heating or cooling rates of 10\u00a0\u00b0C/min on a TA Q200 instrument. It was used to detect the glass transition temperature (T\ng) and melting temperature (T\nm) of the products.EPR measurements were carried out using a Bruker A300 electron spin resonance spectrometer at room temperature. The concentration of sample was 2.5\u00a0mmol/L of toluene solution.The experiments focused on exploring the tuning in catalytic performance of Cat. A/B/C (Fig.\u00a01) under different reaction conditions, such as reaction temperature (T), co-catalyst/catalyst ratio (Al/Ni ratio), co-monomer concentration ([1-hexene]) and reaction time (t) etc. Polymerization conditions and results (activity, M\nw, PDI, T\ng, T\nm, B) are summarized in Table 1.Catalyst activity was the most intuitive reflection of reaction conditions on the catalytic performance. Fig.\u00a02\n illustrated the reactivity of these three catalysts under different reaction conditions in detail and completely different catalytic performance was observed. Among them, the performance of Cat. A and Cat. B were more greatly affected by temperature and catalytic activity changed obviously from 25\u00a0\u00b0C to 75\u00a0\u00b0C [20] as shown in Fig.\u00a02(a). The activity of Cat. A was increased from 1.8\u00a0\u00d7\u00a0105\u00a0g polymer/(mol Ni\u00b7h) to 4.04\u00a0\u00d7\u00a0105\u00a0g polymer/(mol Ni\u00b7h). As for the Cat. B, the activity dropped from 8.94\u00a0\u00d7\u00a0105\u00a0g polymer/(mol Ni\u00b7h) at 25\u00a0\u00b0C to 4.04\u00a0\u00d7\u00a0105\u00a0g polymer/(mol Ni\u00b7h) at 75\u00a0\u00b0C.Comparatively, the activity of Cat. C maintained at a low value (Fig.\u00a02(a)), which might be due to the steric hindrance generated by the ortho-position substituents [21], and the blocked coordination process between metal active site and ethylene monomer. On the other hand, previous studies have already shown that a bulky catalyst with dibenzhydryl group as ortho-position substituent could only exhibit good thermal stability and reactivity under high pressure(\u22654\u00a0atm) [22].Compared to the change in reactivity, effect of Al/Ni ratio on the activity was all in an increasing trend (Fig.\u00a02(b)), which was similar to the results reported before [23], which might be due to the adequate reactant for the polymerization provided by co-catalyst promoter. When the Al/Ni ratio approaching to 1800, its effect on catalytic activity of Cat. B tended to be weakened due to enough cocatalyst existing. When the comonomer 1-hexene increased from 0\u00a0mol/L to 0.30\u00a0mol/L, the overall activity of the Cat. A increased (Fig.\u00a02(c)). This increase in the catalytic activity of the catalyst could be attributed to the \u201cco-monomer effect\u201d [24]. However, Cat. B and Cat. C may have slightly reduced activity due to steric hindrance. Besides, Cat. C even maintained good thermal stability in the 2\u00a0h polymerization process (Fig.\u00a02(d)), due to the hindrance of a bulky ligand.As the temperature increased continuously from 25\u00a0\u00b0C to 75\u00a0\u00b0C, T\ng value of poly(ethylene-co-1-hexene) in the presence of Cat. A decreased from \u221246.20\u00a0\u00b0C to \u221264.54\u00a0\u00b0C (Fig.\u00a03\n(a)). Cat. B showed a moderately rise till 75\u00a0\u00b0C, T\ng value of the product decreased from \u221257.09\u00a0\u00b0C to \u221267.40\u00a0\u00b0C. The absence of absorption peak in the DSC curves under this condition (T\u00a0=\u00a075\u00a0\u00b0C) indicated that the product was completely amorphous [20,25]. In general, the effect of catalyst Cat. C on the crystallinity of poly(ethylene-co-1-hexene) was most obvious with the increase of temperature (Fig.\u00a03(a)), During this process, T\ng values of the product catalyzed by Cat. C also showed a slight downward trend, which was indirectly resulted by the unique \"chain walking\" ability of catalyst. In addition, T\nm decreased with the increase of branching density. Under the microscope, with the increase of e branching density of polyethylene, the number of active segments in the poly(ethylene-co-1-hexene) increases, so a lower temperature is needed to cease the movement of the segments.The effect of Al/Ni ratio on T\ng was not obvious. As shown in Fig.\u00a03(b), T\ng values decreased while the Al/Ni ratio changed from 600 to 1800. The T\nm and T\ng values of the copolymer prepared by these three catalysts all decreased to a certain extent, and the trend gradually slowed down as the steric hindrance of the catalyst increased. Due to the conformation of Cat. C, poly(ethylene-co-1-hexene) with the highest crystallinity and T\nm values was tuning over a wide temperature range (58.40\u201375.12\u00a0\u00b0C). Similarly, T\ng and T\nm values of all the samples decreased slightly with the increase of the concentration of co-monomer 1-hexene (Fig.\u00a03(c)), which may be closely related to the modification of branch topologies. It was shown from Fig.\u00a03(d) that the effect of reaction time on the preparation of poly(ethylene-co-1-hexene) catalyzed by Cat. C was negligible. T\nm values of the obtained copolymers range from 69.38\u00a0\u00b0C to 73.44\u00a0\u00b0C. Besides, the reaction time didn't have a great effect on the phase transition temperature of products catalyzed by Cat. A and Cat. B.Obviously, M\nw values of the obtained poly(ethylene-co-1-hexene) decreased rapidly with the increase of reaction temperature, (Fig.\u00a04\n(a)). When catalyzed by Cat. B, the M\nw values of poly(ethylene-co-1-hexene) was as high as 2.08\u00a0\u00d7\u00a0105\u00a0g/mol at 25\u00a0\u00b0C, but only 8.78\u00a0\u00d7\u00a0104\u00a0g/mol at 75\u00a0\u00b0C. M\nw values of poly(ethylene-co-1-hexene) obtained with Cat. A and Cat. C also showed similar but smoother trends. This might be attributed to the fact that the chain transfer rate was greatly accelerated compared to the chain growth rate as the temperature increased [20].Among the catalysts involved, Cat. A and Cat. B was greatly affected by the tuning of Al/Ni ratio (Fig.\u00a04(b)). In contrast, due to its large sterically hindered ligand structure and low reaction pressure, the M\nw values in the case of Cat. C remained a low value. In addition, as the concentration of 1-hexene increased from 0 to 0.3\u00a0mol/L (Fig.\u00a04(c)), the product obtained in the presence of Cat. A had a significant increase in M\nw after further improvement of the co-monomer concentration (from 1.22\u00a0\u00d7\u00a0105\u00a0g/mol to 2.15\u00a0\u00d7\u00a0105\u00a0g/mol). Due to the influence of the catalyst N-aryl ligand \"hugging\" on the metal site and the steric effect of 1-hexene, the insertion of the monomer was restricted. The products obtained by Cat. B and Cat. C seemed to be slightly affected by the concentration of 1-hexene. The effect of reaction time is a good reflection of thermal stability, as shown in Fig.\u00a04(d). In comparison, Cat. C got a clearer distinction in chain growth rate and chain transfer rate, which leading to an almost linear increase in the M\nw values of polyolefin obtained once got sufficient reaction time. To the best of our knowledge, the PDI of polyethylene obtained by alkyl backbone ligand catalyst was smaller than that of the corresponding acenaphthene backbone catalysts [26,27], which was basically same as that mentioned in Table 1: such as the PDI of the obtained polymerobtained in the presence of Cat. B (Table 1, Run 9. Cat. B) is higher than Cat. A (Table 1, Run 2. Cat. A).Reaction temperature was believed to affect the microstructure greatly. From 75\u00a0\u00b0C to 25\u00a0\u00b0C, the branch density of poly(ethylene-co-1-hexene) prepared by Cat. A, Cat. B, and Cat. C vary from 106 branches/1000C (Run 3 in Table 1\n) to 79 branches/1000C (Run 1 in Table 1), from 116 branches/1000C (Run 12 in Table 1) to 87 branches/1000C (Run 10 in Table 1), and 54 branches/1000C (Run 21 in Table 1) to 46 branches/1000C (Run 19 in Table 1), respectively.In order to detect the poly(ethylene-co-1-hexene) microstructure in detail, 13C NMR investigations were further conducted (Fig.\u00a05\n). According to the resonance peak assignments in literatures [28\u201330], the branch distribution was quantitatively analyzed. The integrated area of 13C NMR spectra was consistent with the theory mentioned above. The order of the integrated l area of butyl and propyl groups (Cat. C\u00a0<\u00a0Cat. A\u00a0<\u00a0Cat. B) indicated Cat. B has the best catalytic activity under the same condition.In addition, the branch distribution of Cat. A/B/C under different temperatures (25/50/75\u00a0\u00b0C) has been described in Fig.\u00a06\n. It can be found that the increasing temperature had a great impact on the enrichment of branched polyolefin microstructures. As temperature increased, the branches distribution of samples obtained with the three catalysts changed greatly. First, proportion of methyl group on the polyolefin chain decreased gradually, and that of long-chain branches (more than five carbon atoms) increased steadily. At the same time, ethyl or propyl groups gradually present from absence, and the insertion of the co-monomer (1-hexene) promoted to the butyl group with higher proportion. Due to the bulky steric hindrance and the reaction conditions (low pressure), the proportion of intermediate groups of poly(ethylene-co-1-hexene) obtained by Cat. C was substantially zero proportion under the applied temperature (Fig.\u00a06(c)).The microstructure of poly(ethylene-co-1-hexene) was vividly illustrated in Fig.\u00a06(d) that the insertion of 1-hexene enabled us to obtain a more abundant branched structure [8,26,31]. For poly(ethylene-co-1-hexene) prepared with Cat. A and Cat. B, it was especially noticeable that the proportion of butyl increases significantly. However, when Cat. C with significant steric hindrance enhancement was used, poly(ethylene-co-1-hexene) with abundant short-chain microstructure has never been obtained (Fig.\u00a06(c)). Furthermore, this phenomenon confirmed the existence of the \u201cprimary insertion\u201d and \u201csecondary insertion\u201d stage (Fig.\u00a07\n). It was suggested that the insertion of the monomer demonstrated a preference for the \u201cprimary insertion\u201d stage [32]. Comparatively, the activity of the catalyst would be largely affected by the steric hindrance in the \u201csecondary insertion\u201d stage [33]. Thereby, the monomer might prefer to insert into the primary alkyl species.During the polymerization process, the metal active site at the end of chain may undergo one-step \u03b2-H elimination and subsequent monomer reinsertion. Once the chain is grown at this active site, methyl group in the main chain would be formed. Similarly, as the polymerization process continues, after two-step \u03b2-H elimination and the following monomer reinsertion, a nickel-active species with \u03b1-ethyl groups can be formed, repeatedly forming a branched polyethylene structure. But Pei et\u00a0al. believed that it was difficult to form a branched group with larger steric hindrance, such as ethyl group [34]. At this point, it coincided with the previous view that the metal active site becomes inactive once it inserts into the polymer chain [35] (i.e. in the \u201csecondary insertion\u201d stage). In addition\uff0cPei and co-workers described the mechanism by which long chain branches form in detail [34]. Branch distribution shown in Fig.\u00a06(d) reached a good consensus with Pei's research.In order to better understand the internal changes of catalytic system and explore the role of MAO in Cat. B/MAO catalytic system, EPR analysis under different Al/Ni ratios was carried out (Fig.\u00a08\n(a)). Only Cat. B or MAO in toluene had no signal. However, with the addition of the co-catalyst (MAO) in the catalyst solution, an unpaired electron signal (g\u00a0=\u00a02.218) belonging to Ni(\u0399) species was obviously detected [36], accompanied by a radical signal (g\u00a0=\u00a02.002) [36]. Due to the interaction of the excessive Lewis acid MAO with the electrophilic acenaphthene-based ligand, unpaired electrons were attracted or captured from metal active center to form a ligand-based and carbon-centered radical, thus, the coordination process between metal active centers and olefin monomers was hindered or weakened to some extent (Fig.\u00a09\n) [37,38], and the effect of this process becomes more obvious with the increase of Al/Ni ratio (the increasing trend of catalyst activity decreased). Most interestingly, as the Al/Ni ratio increased, the intensity of signal (g\u00a0=\u00a02.218) was significantly enhanced. This means that the concentration of Ni(\u0399) species would increase with the growth of Al/Ni ratio. It demonstrated that the existence of inactivated Ni(\u0399\u0399) species was motivated by MAO and promoted the olefin polymerization. Because this effect on the performance of the catalyst is more significant than that of being reduced to Ni(I), the catalytic activity of the system increases with the increase of the Al/Ni ratio.When Al/Ni ratio was adjusted from 600 to 1800, the signal intensity of Ni(\u0399) species decreased (Fig.\u00a08(b)). Considering the fact that the activity of the catalyst increases slowly, it was believed that MAO played the role of reductant more because of the large excess. In other word, more Ni(\u0399\u0399) species were further reduced to zero oxidation state after conversion to Ni(\u0399) species. This view was also confirmed by the appearance of black particle precipitation in the solution [35].In this paper, the catalytic performance of three \u03b1-diimine nickel catalysts with different ligands for ethylene/1-hexene copolymerization under different reaction conditions. The results showed that the external reaction conditions (temperature, Al/Ni ratio, [1-Hexene], reaction time) had great influence on the polymerization process. Different ligand structures led to the effect of external conditions differ from one to another, but in short, changes of reaction temperature showed most significant influence on the catalytic performance. Specially, the proportion of the \"branch-on-branch\" structure (sec-butyl group) of the polymer chain also increased rapidly, well indicating the tuning of the \u201cchain-walking\u201d ability. The main reason for the changes of catalytic performance was the relative rate of chain growth and chain transfer. On the other hand, the change in Ni species was also noticeable. The introduction of co-catalyst also played a role of reductant to a certain extent, and promoting the olefin polymerization process.The authors declare that they have no competing interests.Financial support from the National Key Research and Development Program (2016YFB0302403) is gratefully acknowledged. Besides, we would like to thank Dr. Shengyu Dai for providing the catalysts in this research.", "descript": "\n                  The structure of polyolefin has an important influence on its performance and application. Ethylene/1-hexene copolymerization is one of the important ways to control the structure of the polyolefin. However, research on the ethylene/1-hexene copolymerization catalyzed by nickel complexes with different steric ligands remains to be refined. Here, three \u03b1-diimine nickel catalysts are used to study the ligand effect on catalytic performance in the ethylene/1-hexene copolymerization. Reaction activity, molecular weight, phase-transition temperature and branching density of the resultant copolymer are measured to evaluate the catalytic performance. The results indicate that the steric ligands could exert great effect on the copolymerization. As for the chemical valence of Ni species, detailed EPR demonstrate that the presence of excess co-catalyst can reduce Ni(II) to the lower valence and affect the catalytic performance.\n               "}
{"full_text": "In the industry, the catalytic hydrogenation of edible oils is typically carried out in a slurry reactor through a semi-batch process, with H2 gas injected at an elevated temperature and pressure [1]. One of the most imperative aspects of the process is the usage of a catalyst to catalyse the reaction. Although Ni-based catalysts are vastly applied in industrial processes due to their superior performance and cost effectiveness, constant improvement is vital to further enhance the activity and selectivity of the catalysts. Various preparation parameters have been studied e.g. support type, active metal content, reduction temperature, presence of doping agents, synthesis methods, etc. [2\u20135]. To illustrate, some of these properties such as the pore volume, pore length, particle size, total surface area and Ni crystallite size play a crucial role in determining the activity and selectivity of the catalysts [6,7]. In particular, a high pore volume, shorter pore length, smaller particle size, larger total surface area and smaller Ni crystallite size would potentially benefit the catalytic activity of the catalyst [8,9].The co-precipitation of a Ni salt and a silica source using an alkali source such as sodium carbonate is one of the common and conventional techniques to synthesise Ni catalyst precursors i.e. silica-supported Ni carbonate. The aforementioned substance yields supported Ni catalysts upon reduction [10]. Nitta and colleagues have concluded that compounds of Ni, particularly hydroxide or carbonate compounds of Ni, tend to form strong interactions with the silica carrier or support. Consequently, this results in the generation of Ni hydrosilicate phases, particularly nickel phyllosilicates, on the external layers of the support phase, which function as anchoring sites for Ni particles, leading to their stabilisation and dispersion [11]. This phenomenon is also supported by various other work concerning the synthesis of silica-supported Ni catalysts [2,12\u201314]. The presence of these metal-support interactions (MSI) could potentially influence the properties and performance of heterogeneous supported catalysts in terms of their active metallic area and dispersion, reducibility and stability or resistance to thermal sintering, and subsequently the activity and selectivity [3,15,16]. As described in some studies, many factors could influence these properties, including the catalyst synthesis technique. For instance, it was reported that the ageing step is of substantial significance in dictating fundamental catalyst properties, which controls the degree of formation of less reducible silicates, therefore affecting final metallic dispersion and surface area properties [17,18]. In particular, ageing time and ageing temperature were reported to play an important role in tuning nickel silicate formation, ultimately influencing catalyst activity and applicability [18].Among numerous catalyst synthesis techniques, ultrasonic technology has emerged over the years as an innovative method to effectively modify the properties and performance of heterogeneous catalysts [19\u201321]. The incorporation of ultrasonic irradiation into catalyst synthesis could result in various changes in particle morphology, surface composition, metal dispersion, structural or geometric properties, electronic configurations and catalyst reactivity [19,22,23]. In some cases, the use of ultrasound is able to activate less reactive, but also less costly, catalytic metals [24]. To illustrate, in liquids irradiated by ultrasound, the phenomenon known as acoustic cavitation induces the formation and subsequent implosion of numerous short-lived micro-bubbles with extremely high temperatures and pressures [25]. These transient, localised hot-spots facilitate various physical and chemical reactions during catalyst synthesis, which brings about the enhancement and promotion of nucleation rates and dispersion of active metals on the support surface [26]. Nevertheless, it is also imperative to understand the underlying mechanisms involved in driving the positive effects demonstrated by ultrasonic irradiation in catalyst synthesis, which is an aspect that many literature sources lack. For instance, in various catalysts synthesised for photocatalytic, catalytic cracking and gas reforming applications, authors have reported the increase in reducibility, metal dispersion, particle uniformity, BET surface area etc. [19,27\u201334] with the use of ultrasound during synthesis but detailed explanations for the reason behind the improvement were generally lacking. It is also important to note that performing routine calibration experiments is imperative to accurately reflect the actual acoustic power dissipated by the ultrasonic source, which could allow effective replications as well as comparisons between different bodies of work. This in fact is a critical aspect that a large majority of literature sources lack, whereby only the electric powers specified by the ultrasonic generators (as provided by manufacturers) are detailed [19,27\u201329,32,34,35].Prompted by this knowledge gap, this present work aims to study the phenomenon and mechanisms that take place during the synthesis of sequentially precipitated catalysts with ultrasound employed during the ageing step. Specifically, nickel-silica hydrogenation catalysts were synthesised via sequential precipitation, with ultrasonic irradiation applied at varying ultrasonic intensities during the ageing step, whereby the actual acoustic powers supplied were determined. The catalysts were evaluated based on several characterisation tests, in which the variation in catalyst phase compositions due to sonication was analysed and the ultrasonic irradiation synthesis mechanism was outlined. To further appraise the performance of the synthesised catalysts, the partial hydrogenation of sunflower oil was carried out to ascertain the activity and selectivity of the catalysts.Chemicals used in this work were of analytical grade, obtained from R&M Chemicals Malaysia, utilised without supplemental purification. The ultrasonic system employed in this study was fitted with a 20\u00a0kHz probe. The diameter of the probe tip was approximately 1\u00a0cm (Sonics and Materials, VCX 750, 750\u00a0W). O\u2019life Sunflower oil (Sime Darby Food and Marketing Sdn. Bhd., Malaysia) was employed as the feed for the hydrogenation reaction. The iodine value (IV) of the feed was 124, with its composition listed in Table 1\n.To obtain the calorimetry data for the ultrasonic system used, a beaker of 200\u00a0ml deionised water was subjected to irradiation with ultrasonic amplitudes of 20, 30 and 40%, total duration being 30\u00a0min. The temperature of the system was not regulated. The power output of the ultrasonic system, Q, was obtained using the following equation:\n\n(1)\n\n\nQ\n=\n\n\nm\n\n\nC\n\n\np\n\n\n\u0394\nT\n\n\n\u0394\nt\n\n\n\n\n\nwhere m is the total mass of water, Cp\n specific heat capacity of water (4.18\u00a0kJ\u00a0kg\u22121\u00a0\u00b0C\u22121), \u0394T/\u0394t (\u00b0C s\u22121) is the temperature gradient. Temperature readings were repeated at 30\u00a0s intervals and taken at three varied positions in the beaker, noting the mean value.To ascertain the heat dissipated by the system, water at an equal volume to the ultrasonic set was heated using a portable electric heater of 1000\u00a0W, while being stirred. Similarly, the process took 30\u00a0min and readings were repeated at 30\u00a0s intervals and taken at three varied positions in the beaker, noting the mean value. The power output due to heating, QH\n, can be acquired using Eq. (2). Hence, the heat dissipated, QHL\n, can be obtained using the following equation:\n\n(2)\n\n\n\n\nQ\n\n\nH\nL\n\n\n=\n\n\nQ\n\n\nH\n\n\n-\nQ\n\n\n\n\nSubsequently, the acoustic energy intensity provided by the probe, IUS\n, is calculated via the following equation:\n\n(3)\n\n\n\n\nI\n\n\nU\nS\n\n\n=\n\n\nQ\n+\n\n\nQ\n\n\nH\nL\n\n\n\n\nA\n\n\n\n\n\nwhere\nQ\u00a0+\u00a0QHL\n is the output power, Pout\n, and A is the cross-sectional area of the surface producing ultrasonic waves, determined as 0.8\u00a0cm2 for the ultrasonic probe.In addition, the ultrasonic density provided by the probe to the liquid body, \u03c1US\n, is calculated via the following equation:\n\n(4)\n\n\n\n\n\u03c1\n\n\nU\nS\n\n\n=\n\n\nQ\n+\n\n\nQ\n\n\nH\nL\n\n\n\n\nV\n\n\n\n\n\nwhereV is the volume of water (cm3) in the beaker used for the calorimetry test.Solutions of nickel sulphate hexahydrate (Ni(SO4)2\u00b76H2O) and magnesium sulphate heptahydrate (Mg(SO4)2\u00b77H2O) were mixed with a molar ratio of 3:1 and subsequently heated to 50\u00a0\u00b0C. Then, 10\u00a0wt% sodium carbonate (Na2CO3) was dosed until the pH of the precipitated suspension was 8.8, in a duration of 10\u00a0min. The suspension was then heated to 90\u00a0\u00b0C, followed by the addition of 2\u00a0wt% sodium metasilicate pentahydrate (Na2SiO3\u00b75H2O) solution, in a duration of 10\u00a0min. The suspension was then allowed to age for 30\u00a0min at 90\u00a0\u00b0C. Throughout the whole synthesis process, the suspension was stirred constantly. After terminating the synthesis procedure, the resulting mixture was filtered and thoroughly rinsed using deionised water three times. Next, the filtered precipitate was dried at 100\u00a0\u00b0C for 5\u00a0h in an oven. Lastly, the dried precipitate was calcined in a chamber furnace at 400\u00a0\u00b0C for 4\u00a0h. The unsonicated sample is labelled as A.For catalysts subjected to ultrasonic irradiation, the dosing of 2\u00a0wt% Na2SiO3\u00b75H2O solution was delivered under the influence of a sonicator, in the same duration of 10\u00a0min as the unsonicated sample for consistency. The probe was immersed at a depth of 2\u00a0cm, pulsed at 2\u00a0s on/off, under temperature-controlled conditions. Three sonicated catalysts were synthesised with ultrasonic amplitudes of 20, 30 and 40%, corresponding to ultrasonic intensities of 7.07, 20.78 and 27.72\u00a0W\u00a0cm\u22122, labelled as B, C and D, respectively. The aforementioned ultrasonic amplitudes were chosen considering their interesting impacts on catalyst synthesis in a previous work [36].The morphology of the samples was characterised with a field emission scanning electron microscopy (FE-SEM, Fei Quanta 400F). SEM scans were obtained with a Fei Quanta 400F microscope. The beam current was 1 \u00b5A and the accelerating voltage was 20\u00a0kV.A field emission scanning electron microscope (FE-SEM) fitted with an Oxford Instruments X-MAX energy dispersive X-ray (EDX) analyser was used to obtain the elemental information of the catalysts. The accelerating voltage was 20.0\u00a0kV with an acquisition live-time of 45\u00a0s.The PANalytical X\u2019Pert-PRO diffractometer was used to obtain diffractograms for the catalysts. Cu-K\u03b1 X-ray radiation used was of the wavelength 1.54060\u00a0\u00c5. The beam current was 40\u00a0mA. While the voltage was 45\u00a0kV. Crystallite sizes of the samples was ascertained via the Debye-Scherrer\u2019s formula.The specific surface areas (SBET) and the porosities of the catalysts were acquired with the Micromeritics 3Flex Surface and Catalyst Characterisation Analyser by using the Brunauer-Emmett-Teller method. Adsorption and desorption runs were executed at \u2212195.681\u00a0\u00b0C (77\u00a0K) using N2 gas. The catalysts were degassed at 150\u00a0\u00b0C for 4\u00a0h before the adsorption experiments.Temperature programmed reduction was carried out with the Micromeritics AutoChem II 2920 chemisorption analyser fitted with a thermal conductivity detector. Approximately 30\u00a0mg of the sample were carefully inserted into the quartz U-tube reactor and then placed in the tubular furnace. Catalysts were subjected to pre-treatment under Ar flow at 20\u00a0cm3/min, with the temperature raised from ambient to 100\u00a0\u00b0C and held for 60\u00a0min to remove physisorbed and/or weakly bound species. Subsequently, the catalysts were analysed in 9.47% of H2/Ar at a flow rate of 25\u00a0cm3/min from ambient to 900\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C/min. The vapour produced is removed via a cold trap filled with chilled coolant. H2 adsorbed by the catalysts was detected by the thermal conductivity detector and ascertained via the peak area of the H2-TPR profile. The TPR curves were further processed and deconvoluted using the OriginPro software via Gaussian multi-curve fitting.Pulse chemisorption analysis was conducted using the Micromeritics AutoChem II 2920 chemisorption analyser. Samples were first degassed in an inert argon gas flow for 60\u00a0min at 100\u00a0\u00b0C. Then, the catalysts were reduced at 500\u00a0\u00b0C in H2 for 2\u00a0h and degassed once more for 30\u00a0min prior to the chemisorption analysis. For the analysis, hydrogen gas was pulsed every 6\u00a0min until no further uptake was detected. The Ni metal dispersion (%) is calculated with the following equation:\n\n(5)\n\n\n%\nD\ni\ns\np\ne\nr\ns\ni\no\nn\n=\n\n(\n\n\n\n\n\n\nV\n\n\nm\n\n\n\n\n\n\nV\n\n\nm\no\nl\n\n\n\n\n\n\n\n\n\n\nM\n\n\n%\n\n\n\n\n\n\nW\n\n\na\n\n\n\n\n\n\n)\n\n\n(\n\n\nF\n\n\ns\n\n\n)\n\n\n\n\nwhere\nVm\n is the volume of hydrogen gas chemisorbed (cm3/g STP); Vmol\n is the molar volume of the adsorptive (cm3/mole STP); M%\n is the percentage of Ni metal by weight as grams of Ni per gram of sample; Wa\n is the atomic weight of Ni (g/mole); Fs\n is the stoichiometry factor, taken as 2 for hydrogen on Ni.The active metal surface area (m2/g) is calculated with the following equation:\n\n(6)\n\n\nM\ne\nt\na\nl\n\ns\nu\nr\nf\na\nc\ne\n\na\nr\ne\na\n=\n\n\nF\n\n\ns\n\n\n\n\nn\n\n\na\n\n\n\n\nN\n\n\nA\n\n\n\n\nA\n\n\ng\n\n\n\n\n\nwhere\nFs\n is the stoichiometry factor, taken as 2 for hydrogen on Ni; na\n is the number of moles of gas adsorbed (cm3/g STP); NA\n is Avogadro\u2019s constant; Ag\n is the cross-sectional area of the active adsorptive atom (nm2) (with the assumption that a single Ni atom occupies 0.0649\u00a0nm2) [37].The average metal particle size (nm) is calculated with the following equation [38]:\n\n(7)\n\n\nA\nv\ne\nr\na\ng\ne\n\np\na\nr\nt\ni\nc\nl\ne\n\ns\ni\nz\ne\n=\n\n\n6\n\n\n\n(\n\n\nA\n\n\nS\nm\n\n\n)\n\n\n(\n%\nD\ni\ns\np\ne\nr\ns\ni\no\nn\n)\n\n\n(\n\u03c1\n)\n\n\n\n\nX\n\n100\n\n\n\nwhere\nASm\n is the active metal surface area (m2/g); \u03c1 is the density of the metal (g/cm3).X-ray photoelectron spectroscopy (XPS) was conducted with the JEOL JPS-9030 photoelectron spectrometer. Al K\u03b1 (1486.6\u00a0eV) was used as the excitation source to probe the sample surface information at a depth of 1 \u2013 12\u00a0nm. The pressure in the analysis chamber during experiments was less than 5\u00a0\u00d7\u00a010\u221210 Torr. A hemispherical electron-energy analyser working at a pass energy of 30\u00a0eV was used to collect core-level spectra. The samples were dispersed in ethanol and placed on silicon wafers, which were mounted on a sample holder and directly transferred into the analysis chamber. Step size was adjusted to 0.1\u00a0eV, dwell time was set at 100\u00a0ms and the high-resolution spectra were recorded with 10 scans. Charge effects were corrected by using the C 1\u00a0s peak at 285\u00a0eV. A Shirley background was applied to subtract the inelastic background of core-level peaks. The model peak to describe XPS core-level lines for curve fitting was a product of Gaussian functions. The XPS spectra were processed and deconvoluted using the OriginPro software.Atomic absorption spectroscopy (AAS) was conducted with a Perkin Elmer AAnalyst400 AA spectrometer to obtain the concentration of leached nickel from the hydrogenated oil samples. Sample readings were taken three times to obtain an average value.Prior to carrying out the hydrogenation reaction tests, the catalyst samples involved were reduced in pure hydrogen gas for activation. The samples were reduced in a tubular furnace. Before reduction, the samples were heated to 500\u00a0\u00b0C with a heating rate of 10\u00a0\u00b0C/min in N2. Upon reaching 500\u00a0\u00b0C, the samples were subjected to a flow of hydrogen gas and held for 2\u00a0h. Lastly, fully hydrogenated palm stearin (iodine value <0.5) was used to coat the catalysts to prevent oxidation, thus forming fat-coated catalyst granules of 22\u00a0wt% Ni.Partial hydrogenation was carried out in a 1.5 L pressurised batch reactor (Buchiglasuster Eco-Clave). The temperature of the reactor was regulated by a high precision temperature regulatory system (Huber Unistat Tango Nuevo), with the use of a silicone oil jacket around the circular reactor and a temperature probe inside the reactor. Briefly, 750\u00a0ml of sunflower oil was added into the reactor with the aid of a vacuum pump and heated to a set-point temperature of 180\u00a0\u00b0C. Prior to dosing the catalyst, the reactor was vacuumed to remove any air or moisture that will poison and deactivate the catalyst. Subsequently, a catalyst dosage of 2\u00a0g/L was used and hydrogen gas of 5 barg was charged into the reactor to initiate the reaction, while the slurry was vigorously stirrer at 1500\u00a0rpm. Oil samples (4\u20135\u00a0ml) were taken from the bottom of the reactor for each time interval. The reaction was carried out for 90\u00a0min. Obtained samples were labelled and filtered using filter papers to remove the catalyst particles.The iodine value measures the degree of unsaturation of fats and oils, which represents the mass (g) of iodine consumed per 100\u00a0g of oil. Products collected from the reactor were subjected to iodine value tests, in accordance with the American Oil Chemist\u2019s Society (AOCS) Official Method Tg 1a-64, performed 3 times to obtain average values. The resulting iodine value was calculated with the following formula:\n\n(8)\n\n\nI\no\nd\ni\nn\ne\n\nv\na\nl\nu\ne\n=\n\n\n\n\nB\n-\nS\n\n\n\u00d7\nN\n\u00d7\n12\n.\n691\n\n\nm\na\ns\ns\n\no\nf\n\ns\na\nm\np\nl\ne\n,\n\ng\n\n\n\n\n\nwhere\nB is the volume of titrant of blank set, mL; S is the volume of titrant of sample set, mL; N is the normality of the sodium thiosulphate solution.Gas chromatography was carried out in order to ascertain their respective fatty acid compositions of the reaction products. A PerkinElmer Clarus 500 gas chromatograph fitted with a PerkinElmer COL-ELITE-2560 capillary column (100\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm ID\u00a0\u00d7\u00a00.20\u00a0\u03bcm df). Helium was utilised as the carrier gas at 1.3\u00a0ml/min. Prior to the tests, esterification of the hydrogenation products was carried out to convert them into fatty acid methyl esters (FAME). The oven, injector and flame ionisation detector (FID) temperatures were 175, 210 and 250\u00a0\u00b0C, respectively. The injection volume was 1\u00a0\u03bcL and the split ratio was 100:1. The relative areas of the fatty acid peaks obtained from the chromatogram were used to determine the product distribution of the oil samples.The ultrasonic system was assessed for its calorimetry and results are presented in Table 2\n. The heat loss, QHL\n, calculated was 0.8\u00a0W.The morphology of the catalysts was interpreted via the SEM scans in Fig. 1\n. While the non-sonicated catalyst exhibited rough surfaces with the presence of conglomerates, the morphology of the sonicated catalysts all appeared to be smooth and uniform, which confirmed the capability of ultrasound to reduce the extent of particle aggregation [39]. As demonstrated later, the combination of ultrasound irradiation and co-precipitation generated a synergistic effect whereby a uniform environment was provided for the nucleation and growth of metal particles, while concurrently averting small particles from agglomerating [40], thus preventing the formation of aggregates such as those in the non-sonicated catalyst. In particular, the increased ultrasonic intensity instigated more violent micro-bubble implosions, hence inducing stronger shock waves and micro-jets with velocities of approximately 400\u00a0km/h. Consequently, vigorous inter-particle collisions and surface pitting could occur, simultaneously suppressing the aggregation of catalyst particles, thus producing particles with increased dispersion [41]. This observed phenomenon resonated with the work of other researchers, in which a lower degree of particle agglomeration was noticed with the use of ultrasound during synthesis [40,42].From the EDX and XPS analyses of the synthesised catalysts in Table 3\n, it is apparent that they were constituted of the required elements to form the catalysts, which indicates the efficacy of the synthesis procedure. As the typical approximated penetration depth of an EDX electron beam is 0.4\u00a0\u03bcm [43], the measured atomic percentage was considered to represent the bulk atomic percentage of the catalysts. On the other hand, characterisation by XPS detects the elemental composition of the samples at the surface layer, which corresponds to a penetration depth of approximately 1 \u2013 12\u00a0nm, hence demonstrative of the superficial composition of the catalysts [43]. As the catalysts investigated were all synthesised according to the same starting materials, it will be useful to draw comparisons based on their bulk and surface atomic distributions to ascertain the changes due to the difference in synthesis conditions. As shown in Table 3, there were decreasing trends for the Mg/Ni and Si/Ni atomic ratios with the increase in ultrasound intensity during the ageing process from data obtained from both EDX and XPS analyses. It is worth noting that while the difference was small for the Mg/Ni ratios between the EDX and XPS methods, there was a substantial gap in the Si/Ni ratios between the two aforementioned methods. We believe the observed change in Mg/Ni and Si/Ni atomic ratios was mainly due to the effect of ultrasonic intensity (or lack thereof) on the sequentially co-precipitated Ni-Mg hydroxycarbonates and siliceous materials during the ageing process.A conceptualisation demonstrating the action of ultrasound during the ageing process is presented in Fig. 2\n. It is herein noted that, for the sake of accuracy, prefixes such as hydro- or hydroxy- are used to represent the actual phases of the precursors present during synthesis. After the synthesis process, the aforementioned phases were subjected to dehydration and calcination and subsequently transformed into their respective oxides or silicates, which were then used for characterisations mentioned in this present work. In conventional synthesis of sequentially precipitated catalysts without the presence of ultrasound, the formation of Ni silicate proceeded as the silica precursor was added during the ageing process, which occurs under an alkaline condition at elevated temperatures. Prior to the ageing process, co-precipitates of Ni-Mg hydroxycarbonates were present in the suspension. As the ageing process was initiated, silicate ions (in the form of Na2SiO3) were dosed into the ageing solution, which then: (i) attached epitaxially to the co-precipitates [44], forming nickel hydrosilicates, in which Ni-Mg hydroxycarbonates would then be anchored; (ii) attached to existing hydrosilicates and underwent polymerisation to form silica clusters [45]. This attachment of silicate ions is illustrated by Step 1 in Fig. 2. In the presence of ultrasonic irradiation, the Ni-Mg hydroxycarbonate co-precipitates can experience erosion, causing the shrinkage of the size of the co-precipitates, which will eventually lead to higher Ni dispersion as we will see in Table 4\n. The increase in ultrasound intensity also led to lower Mg/Ni ratios, beginning with sample A at 0.329 (theoretical ratio\u00a0=\u00a00.333 with Ni:Mg\u00a0=\u00a03:1) and down to 0.268 for sample D. During the formation of Ni-Mg hydroxycarbonate nanoparticles, Mg has the tendency to migrate to surficial layers due to its lower surface energy compared to Ni. It is noted that Ni has a surface energy of 2.080\u00a0J/m2, while Mg has a surface energy of 0.688\u00a0J/m2\n[46]. As a result, it is quite likely that a significant amount of Mg was eroded away from the surface layers of Ni-Mg hydroxycarbonate co-precipitates when the ultrasonic intensity was increased. As shown in Table 3, the difference in the Mg/Ni ratio for the surface and bulk (XPS \u2013 EDX) decreased as the ultrasonic intensity increased from samples A \u2013 C, indicating the increased extent of Mg erosion from the catalyst surface. However, sample D registered an increase in the Mg/Ni ratio difference between the surface and bulk, likely due to the re-attachment of the Mg ions at higher ultrasonic intensities, as similar investigations reported the inefficiency of ultrasonic irradiation at high powers causing the re-agglomeration of particles [47]. In a similar manner, higher ultrasound intensity has also disrupted the formation of hydrosilicates and silica clusters and led to the drop in the bulk Si/Ni ratio, from 0.333 to 0.244.Following the attachment of silicate ions to form metal hydrosilicate and silica clusters, the individual silicate-attached Ni-Mg hydroxycarbonates would clump together through the -O-Si-O- bonding provided by silicate ions in the ageing solution [45,48]. Small aggregates begin to form from the clumping of nano-sized silicate-attached Ni-Mg hydroxycarbonates (Step 2 of Fig. 2), while further clumping of small aggregates lead to bigger aggregates, eventually forming the catalyst particle network (Step 3 of Fig. 2). The significantly higher Si/Ni ratios as observed by XPS (from 0.468 to 0.333) compared to EDX (from 0.333 to 0.244) was mainly a result of silicate ions attaching to and growing on the outer surface of the aggregated catalyst particles as silica clusters. Attachment of silicate ions within the pores of the catalyst particles is thought to be insignificant compared to the outer surface due to slower mass transfer in the narrow pores. In the present work, ultrasound irradiation was only active during the first 10\u00a0min of the ageing process for the sonicated samples, while the remaining ageing period was carried out without ultrasonic irradiation. As higher surface Si/Ni density was also observed for catalyst samples sonicated with high intensity, it can be inferred that the growth of silicate ions on the surface of the catalyst particles took place predominantly following the termination of irradiation, since a high ultrasonic intensity can disrupt the formation of hydrosilicate and silica clusters. It should be noted that the discrepancy between the bulk and surface layer Si/Ni ratios in this present work was in stark contrast with the typical synthesis method of simultaneously co-precipitating the active and support phases. Compared to the sequential precipitation method used in this work, whereby the support phase was added after the co-precipitation of active phases, the simultaneous co-precipitation of both active and support phases could result in a more uniform and homogeneous distribution of the elements [49,50], thus the clustering of the elements would be less drastic in that case.The XRD diffractograms for the catalysts are shown in Fig. 3\n. Peaks representing the phases of NiO (ICDD: 01-073-1523), MgO (ICDD: 01-089-7746) and nickel silicate (Ni3Si2O5(OH)4) (ICDD: 01-083-1648) were detected. Diffraction patterns of all samples exhibit reflections at the 2\u03b8 value of 36\u00b0, 43\u00b0, 63\u00b0, 75\u00b0 and 79\u00b0, which correspond to NiO(111), NiO(200), NiO(220), NiO(222) and Ni(311) planes, respectively [51,52]. These results agreed well with those of several other investigations [53,54]. The SiO2 support was amorphous, which did not contribute to any peaks on the diffractogram [55,56]. The NiO crystallite size was ascertained using the Scherrer equation, referencing the characteristic peak at 43\u00b0, with the results presented in Fig. 3. In general, the diffractograms registered reflections that appeared broad, which signified the poorly crystallised state of the catalysts. This also indicated the presence of small and well-dispersed phases and highly developed surfaces [57,58]. In addition, the lack of sharp peaks also indicated the absence of large crystalline domains or metal oxide clusters, which could be of great advantage for metal dispersion during catalyst reduction [59]. On the other hand, the presence of nickel silicate, formed by a strong interaction between Ni and Si, is also detected at the 37\u00b0 peaks. Poorly crystallised nickel silicate compounds with imperfect nickel antigorite structures are commonly formed when the co-precipitating nickel salt and silicate solutions are at temperatures lower than 100\u00a0\u00b0C, with structures such as Ni3Si2O5(OH)4\n[60]. Due to the incorporation of ultrasound during synthesis, the samples registered an increase in amorphicity, evidenced by the decrease in crystallite size of 2.56\u00a0nm for the unsonicated catalyst, to a range of 2.28 \u2013 2.41\u00a0nm, for the sonicated catalysts. Pertaining to the sonicated catalysts, the crystallite size decreased with the increase in ultrasonic intensity. This may be owed to the highly turbulent mixing induced by sonicating the suspension during synthesis, in which unique conditions due to acoustic cavitational bubble collapse were produced. This highly turbulent mixing is facilitated by the presence of acoustic streaming and high mass transfer [61] and subsequently resulted in a higher packing disorder in the samples, forming samples that were more amorphous [25]. Furthermore, as the cavitation bubbles collapsed violently, localised regions of extremely high cooling rates, in the range of 1011 K/s, were generated, which could inhibit the growth of crystals [25].H2-TPR was used to probe the redox properties of the unsonicated and sonicated catalysts, with results presented in Fig. 4\n. The reduction profiles of the catalysts are characterised by multiple reduction peaks fused together into a much broader reduction band from 100 to 800\u00a0\u00b0C, with a peak ca. 400 \u2013 500\u00a0\u00b0C. This observation denotes the homogeneous distribution of small particles over the support, with the presence of complex and intimate interactions between the active phase and support, which supports the XRD diffractograms and is also in accordance with other researchers [6,45,57,62], as well as Ni catalysts prepared via other techniques such as sol\u2013gel [63]. From the figure, it is observed that the unsonicated catalyst had the highest maxima of 448.9\u00a0\u00b0C, while the sonicated catalysts had maximum reduction peaks in the range of 431 \u2013 435\u00a0\u00b0C. Moreover, sample D, irradiated with the highest ultrasonic intensity presented the lowest reduction peak at 431.4\u00a0\u00b0C. The presence of ultrasound during synthesis has marginally increased the reducibility of the catalysts, which is also a finding supported by other relevant investigations concerning ultrasound-assisted catalysts synthesis [29,64]. The increase in reducibility also signified the decrease in extent of metal-support interactions, in which phases with weaker interaction with the support (i.e. NiO) would be more readily reduced to Ni0.To gain more insight into the TPR results, the broad peaks were deconvoluted into symmetrical peaks via Gaussian multipeak curve-fitting to evaluate the relative percentage of the different reducible species, as well as their peak positions, presented in Fig. S1 (Supplementary information). In total, three main peaks could be identified, which were labelled as \u03b1, \u03b2 and \u03b3. The reduction temperatures reported can be classified into different types of nickel species, which are directly associated with the degree of interaction with the silica support. This also indicates that Ni species of different extents of interaction coexist within a catalyst sample. Henceforth, the three peaks are classified based on their reducibility and explained based on their relative proportion and individual peak position. It is noted that no reduction peaks corresponded to the reduction of Mg species due to their extreme difficulty in reduction under the present conditions applied [59]. The first low temperature peak (\u03b1) at ca. 181 \u2013 198\u00a0\u00b0C represented a minor portion approximately 6% or less of the total reduction profile. These are ascribed unambiguously to the reduction of higher Ni(III) oxides in trace quantities (Ni2O3 to NiO), which is a common occurrence for precipitates calcined at temperatures 400\u00a0\u00b0C or lower, as per this present study [13,63]. However, since the XRD results exhibited no obvious Ni2O3 signals, it is suggested that the Ni2O3 phase present was too sparse in amount and also highly dispersed on the support [52]. On the other hand, the \u03b2 peaks at ca. 411 \u2013 422\u00a0\u00b0C were due to the NiO species possessing weak interactions with the silica support [65,66]. Lastly, the \u03b3 peaks at ca. 495 \u2013 514\u00a0\u00b0C were due to the Ni species having strong interactions with the silica support, hence forming Ni silicate, which presented difficulty in reduction [65,66].Based on Table 4, one can see that the composition of \u03b1 for all samples remained in the range of approximately 3\u20136%, representing their minor role in the catalysts. However, the percentages of \u03b2 and \u03b3 varied significantly with the presence of ultrasound and its intensity. In general, the weaker NiO phase, \u03b2, observed an increase in proportion with the increase in ultrasound intensity, from 32.1% in the unsonicated catalyst and up to 42.5% in the sonicated counterparts. Accordingly, the Ni phase with stronger interactions, \u03b3, also observed a declining trend with the increase in ultrasound intensity, registering 61.7% in the unsonicated catalyst and down to 53.4% for sample D, which suggests the possibility of Ni silicate erosion or inhibition by ultrasonic irradiation. Interestingly, the increase in ultrasonic intensity from 20.78 to 27.72\u00a0W\u00a0cm\u22122 did not lead to significant variations in the relative percentages of \u03b2 and \u03b3, which indicates that there is a limit to the extent of Ni silicate erosion caused by ultrasonic irradiation. Considering the bond linkages of -Si-O-Ni- present in Ni hydrosilicate species [45] and the bond dissociation energy of Ni-O as 391.6\u00a0kJ\u00a0mol\u22121\n[67], calculations have shown that it is in fact plausible for acoustic micro-bubble implosions to generate sufficient energy for the bond breakage of Ni-O [68], which erodes away the Ni hydrosilicate structure\n\n.\n\nOn the other hand, with a significantly higher bond dissociation energy at 440\u00a0kJ\u00a0mol\u22121 for the Si-O bonds present in the bond linkages of -Si-O-Si-, it became more difficult to instigate bond breakage once silicate ions were attached to the hydrosilicates to form silica. Consequently, this allowed the silica clusters to act as a protective layer, enclosing the Ni silicate phases to impede its erosion caused by ultrasonication. Instead of Ni silicate erosion, it appeared that the increasing ultrasonic intensity has caused considerable agitation that suppressed the build-up of silica clusters by deterring the polymerisation of silicic acid [69], leading to the decreasing trend of Si/Ni ratios as shown previously in Table 3.Apart from the compositional change in Ni phases in the catalyst, the increase in ultrasonic intensity has also resulted in the shrinkage of Ni-Mg hydroxycarbonate nanoparticles, leading to higher Ni surface area and dispersion. In particular, Ni dispersion was increased from 8.79% to 17.81%, while the Ni surface area was increased from 58.55\u00a0m2/g Ni to 118.53\u00a0m2/g Ni, as seen in Table 4. This suggested that Ni was more dispersed across the support due to the action of ultrasonic irradiation during synthesis, whereby the increase in intensity amplified this phenomenon. As discussed earlier, the improvement in Ni dispersion and surface area could be due to the erosion of Ni-Mg hydroxycarbonates caused by ultrasound irradiation, with higher intensity lead to more severe erosion. Given that the pH remains unchanged at 8.8 during the application of ultrasound irradiation, it is thought that the eroded Ni and Mg ions would re-precipitate and form smaller hydroxycarbonate nanoparticles, which in turn lead to higher Ni dispersion and Ni surface area. Generally, the increase in total metal surface area and dispersion are good indicators of enhanced catalytic activity, as more active sites equate to more area available for reaction to occur.The chemical and electronic properties of the calcined catalysts were studied using XPS. As per Fig. 5\n, the spectra show features associated with Ni 2p3/2. For all catalysts, the Ni 2p3/2 is accompanied by the presence of a satellite peak at 861 \u2013 863\u00a0eV, congruous to the existence of Ni2+ instead of metallic Ni0\n[70]. It is noted that these satellite peaks manifest due to the paramagnetic state of the Ni2+ species and electron shake-up. Notably, these peaks are typically ca. 6\u00a0eV higher than the main Ni 2p3/2 peak [71], as noted by the \u0394Esat values in Table 5\n.The Ni 2p3/2 binding energy of the unsonicated catalyst resonated well with reported values of 856.3 \u2013 856.7\u00a0eV [72,73]. However, it is discovered that the Ni 2p3/2 binding energy of the sonicated catalysts experienced a shift to lower values at 855.7 \u2013 855.9\u00a0eV. Since XPS analysis is surface sensitive, the discrepancies in binding energies indicated that the Ni species on the surface have been altered electronically [2], in this case due to the incorporation of ultrasound into the synthesis procedure of the catalysts. Furthermore, a decrease in binding energy also indicated a lowered extent of metal-support interaction between the Ni phase and Si phase [2,74]. In this case, a shift to lower binding energies due to ultrasound has also been reported by other researchers [75], which led to a change in electron density that may affect the bonding of chemical intermediates to the active sites, thus ultimately affecting the reaction pathways during hydrogenation [76\u201378]. Furthermore, this indication of weakening in metal-support interactions is also corroborated with the TPR studies demonstrated earlier, noting the shift in main reduction peaks to lower temperatures in sonicated samples.To gain more insight into the XPS results, the Ni 2p3/2 XPS core level region was deconvoluted via Gaussian multi-peak curve-fitting and fitted with three doublets assigned to NiO, Ni silicate and associated satellite features, as shown in Fig. 5. As the NiO binding energies have been reported to be significantly lower at a range of 854 \u2013 855\u00a0eV [72,73,79], it is suggested that another Ni phase exists at a higher binding energy, which is assigned to the Ni silicate phase. Hence, this agreed well with the two Ni phases detected in the XRD and TPR analyses. In addition, it is known that a relationship can be derived using the difference between Ni 2p3/2 and Si 2p binding energies, \u0394ENi-Si\n[80]. The existence of Ni silicates in the catalyst sample would give \u0394ENi-Si values of 753.2 \u2013 753.8\u00a0eV [73,81,82]. From Table 5, the \u0394ENi-Si values of the catalysts synthesised agreed excellently with this, thus substantiating the presence of the aforementioned phases. Literature has also reported similar binding energy values of nickel silicates [73].The total area of respective signals, when averaged over the whole system, allows one to approximate the relative amount of species considered [83]. In this case, the area under the curve of each deconvoluted peak in Fig. 5 were used to estimate the relative surface proportion of each nickel species, which are presented in Table 5. Echoing the results from the TPR analysis, sonication has also imparted considerable discrepancies on the NiO and Ni silicate surface compositions. The proportion of NiO on the surface was increased from 18.6% in the non-sonicated catalyst to 29.5 \u2013 40.7% in the sonicated catalysts, in which this proportion has also increased with higher ultrasonic intensities. Conversely, the Ni silicate phase has also seen a decrease in relative proportion when ultrasound was used during the synthesis, as it dropped from 81.4% in the unsonicated catalyst to a range of 59.3 \u2013 70.5% in the sonicated counterparts. This aforementioned trend observed in the XPS analysis was similar to those obtained from the TPR analysis, albeit with a greater magnitude of change. This was due to the fact that XPS presents details on surface composition, which could vary slightly from those of the bulk provided by TPR measurements [84].It was reported that the precipitation of silicates without the use of surface modifiers tend to clump to each other, which results in the formation of secondary aggregates [48], hence in the case of no sonication (samples A) and low power sonication (sample B), silicate ions would adhere to existing silicate networks in an unperturbed manner during suspension ageing, resulting in a higher degree of aggregation within a shorter period of time. With such a phenomenon, the relatively unhindered growth of the silicate network is more prominent, which brought about longer and more severe ageing conditions on the outer surface of the catalyst particles for samples A, as validated by the XPS analysis showing the high concentration of Ni silicates on the surface (81.4%). As opposed to the above case, the inclusion of ultrasonic irradiation during support loading and the early stage of ageing could effectively disperse the aggregates and suppress the growth of the catalyst particles, mainly by delaying the growth of the silicate network. The erosion caused by high intensity ultrasonic irradiation coupled with the delayed growth of silica on the outer surficial layers eventually limited the extent of ageing, as exhibited by the lower Ni silicate concentration in samples C (61.2%) and D (59.3%). Such a delayed growth in particles could have also played a role in enhancing the Ni dispersion in sonicated catalysts, as smaller particles result in an increase in total exposure of Ni active sites. Herein, the conceptualisation of the two different states of ageing is shown in Fig. 6\n.The textural and structural properties of the calcined nickel catalysts were evaluated using N2 physisorption, in which the adsorption\u2013desorption isotherms for the catalysts can be seen in Fig. S2 (Supplementary information). According to the IUPAC classification, the sonicated samples (B \u2013 D) exhibited Type IV(a) isotherms, characteristic of mesoporous substances with capillary condensation accompanied by hysteresis. Contrariwise, the non-sonicated sample A exhibited a hybrid of the Type II and Type IV(a) isotherms, denoting the presence of mesopores and macropores, which was also accompanied by a hysteresis loop. Regarding the hysteresis loops exhibited by the catalysts, they can be ascribed to the Type H2(b) hysteresis loop, which signifies the presence of ink-bottle pores with a wider size distribution of neck widths, typical of mesoporous ordered silicas obtained after hydrothermal treatment [85].The incorporation of ultrasound is said to have altered the textural properties of the catalyst, resulting in more uniform mesoporous structures with higher BET surface areas, as shown in Table 6\n. The increase in surface areas was also noted in other studies, in which ultrasound acted as a dispersant tool for catalyst phases, which might increase the total surface area available for reaction [36,64]. The imploding micro-bubbles in the irradiated suspension as a result of acoustic cavitation facilitated the nucleation and fine dispersion of particles, creating phases of more uniform and well-defined morphology that resulted in increased surface areas from 192.5\u00a0m2/g to a range of 228.9 \u2013 289.7\u00a0m2/g [86]. According to Coenen [87] and Ghuge et al. [45], it is the nickel antigorite (hydrosilicate) phase instead of the silica support that gives rise to a high BET surface area, with the former typically exhibiting >300\u00a0m2/g while the latter presenting surface areas in the range of 20\u201350\u00a0m2/g. If one is to refer to the results from the TPR studies (Table 4), it can be assumed that sample A with the highest Ni silicate percentage (61.7%) would exhibit the highest BET surface area. However, Table 6 shows otherwise with sample A exhibiting a significantly lower BET surface area compared to the sonicated samples. As discussed earlier, the amount of silica clusters attached to the hydrosilicate declined with the usage of ultrasound, which in turn led to higher hydrosilicate to silica ratio in the samples synthesised with ultrasonic irradiation, thus giving rise to a higher BET surface area. In fact, this observation is also correlated with the increase in ultrasonic intensity from sample B \u2013 D, whereby the formation of more silica clusters were suppressed, causing a further increase in hydrosilicate to silica ratio, thereby progressively increasing the BET surface area as per Table 6. As a result, sonicated catalysts exhibited higher range in the overall BET surface area than the unsonicated catalyst. Nevertheless, the pore volume and average pore width of the catalysts were similar in range, noting that the non-sonicated sample possessed a larger average pore width due to the presence of macropores, which can also be observed in the pore size distribution in Fig. 7\n. One can see that all samples had a uniform pore size distribution in the mesoporous range, while the non-sonicated sample also contained pores in the macroporous range. The presence of larger macropores is likely due to the unperturbed and prolonged ageing [88,89] relative to that experienced by the sonicated catalyst, which contributed to the increase in average pore width for catalyst A. Overall, the presence of pores above 3.5\u00a0nm was a good indication that the catalysts were suitable for the hydrogenation of edible sunflower oil, which are approximately twice the size of the triglyceride molecules of 1.5 \u2013 2\u00a0nm [8,44]. However, the presence of narrower pores with average width slightly below 3.5\u00a0nm in catalyst B might have posed significant diffusional and mass transport problems for the movement of triglycerides. Table 6 collates the relative pore flow rate for each catalyst, with catalyst B as the reference. Employing the flow continuity equation, it is inferred that the unsonicated catalyst A had a 96.4% higher flow rate than sample B. The smaller flow area due to smaller pores denote the increased hindrance in mass transfer, which results in the slower diffusion rate of reactants and products in and out of the pores, leading to decreased activity relative to samples possessing higher pore flow rates.All synthesised catalysts were subjected to catalytic activity tests via the partial catalytic hydrogenation of sunflower oil. The decline in IV throughout the stipulated reaction time of 90\u00a0min for each catalyst sample was noted and presented in Fig. 8\n. The hydrogenation performance was tracked by observing the decrease in the IV. The sunflower oil utilised as the reactant has an initial IV of 124, which was tested with the above-mentioned iodine value test. As anticipated, the IV for all sample sets decreased as the reactant was progressively saturated during hydrogenation. Furthermore, the gradient of the graph signified the rate of decline in the IV, whereby a sharper gradient represented an increase in hydrogenation activity. Catalyst C, synthesised with an ultrasonic intensity of 20.78\u00a0W\u00a0cm\u22122 registered the highest activity among the catalysts synthesised, attaining a final IV of 37.9 after a reaction time of 90\u00a0min. On the other hand, the unsonicated catalyst produced an IV drop to 53.8 in 90\u00a0min. The least active catalyst was catalyst B, synthesised with an ultrasonic intensity of 7.07\u00a0W\u00a0cm\u22122. Although possessing higher Ni surface area and dispersion than its unsonicated counterpart, catalyst B presented a smaller pore size below 3.5\u00a0nm. Coenen [7] postulated that while it is beneficial to possess a high Ni dispersion and surface area, oil/fat hydrogenation is also structure-sensitive, whereby pores narrower than 3.5\u00a0nm would considerably impair the mass transport and pore diffusivity of bulky triglyceride molecules, hence causing pore congestion and affecting overall catalytic activity. The initial activity of catalyst B was higher than that of catalyst A due to its advantages in Ni dispersion and surface area. However, at IV\u00a0=\u00a090, the activity of catalysts A and B began to diverge as shown in Fig. 8, the percentage of bulky triglyceride molecules (C18:2 and cis-C18:1) for sample B remained relatively high compared to that of sample A, as shown in Fig. 9\n. As the narrower pores in sample B effectively impede the diffusion of these molecules, the reaction and conversion of such molecules would be affected negatively, thus resulting in a slower drop in IV. In addition, as presented in Table 6, the unsonicated catalyst A has a 96% higher flow in the pores than that of catalyst B, indicating that despite a higher intrinsic catalytic activity possessed by catalyst B, its mass transfer at the pores was severely impacted thus affecting the overall catalytic activity.Based on the catalyst characterisation results, the geometric effects imparted by ultrasonic irradiation have led to the sonicated catalysts possessing superior catalytic activity compared to their unsonicated counterpart, with the exception of the sonicated catalyst B due to its disadvantages in pore characteristics. The action of ultrasound during synthesis has induced the formation of more reducible Ni species available on the catalyst superficial layers, which on the other hand are composed of particles of lower agglomeration, thus leading to increased Ni surface area and dispersion after reduction. The presence of more well dispersed active sites ultimately allowed more reactants and intermediates to adsorb and react. Nevertheless, it was also discovered that catalyst D, albeit sonicated with the highest ultrasonic intensity, performed marginally poorer than catalyst C, whereby the hydrogenation activity of the latter overtook the former during mid-hydrogenation. This could be due to the large Ni surface area exhibited by catalyst D, allowing more surface area contact with the reacting medium, hence increasing the propensity of Ni lixiviation. Indeed, AAS analyses performed on the hydrogenation products of catalysts C and D confirmed that the concentration of Ni present in the hydrogenation product of catalyst D was ca. 30% higher than that of catalyst C at 90\u00a0min (0.00294\u00a0mg Ni/g oil vs. 0.00226\u00a0mg Ni/g oil), which caused the gradual deactivation of catalyst D over time.The hydrogenation activity of the synthesised catalysts was demonstrated via the IV tests, which is a direct representation of the number of carbon\u2013carbon double bonds present in a sample, without any discrimination towards the product composition. Product analyses are also very crucial in distinguishing the behaviour of catalysts during edible oil hydrogenation. The synthesised catalysts were appraised by monitoring the evolution of saturated fat (C18:0), trans-C18:1 and cis-C18:1 at particular IV levels which represent a reaction time up to 90\u00a0min, as depicted in Fig. 9. The respective yield and selectivity of the catalysts are shown in Table 7\n.From Fig. 9 it can be seen that the composition profile of the hydrogenated fats depicted similar trends for all catalysts used. The elimination of carbon\u2013carbon double bonds, as well as cis\u2013trans isomerisation during hydrogenation can be tracked throughout the plots. Overall, a depletion of C18:2 and increase in C18:0 is observed, accompanied by a variation of cis-C18:1 and trans-C18:1 composition throughout the reaction. Another observation was the overall increase in trans-C18:1 coupled with the overall decrease in cis-C18:1 for all hydrogenation runs. It has been documented that trans-C18:1 are preferentially formed during the initial phase of hydrogenation [90]. On the other hand, cis-C18:1 was either isomerised to trans-C18:1 or fully hydrogenated to C18:0. It can be seen that the formation of trans-C18:1 was generally less favoured in the hydrogenation runs using sonicated catalysts, whereby the selectivity of trans-C18:1/cis-C18:1 at IV 90 and 70 were at the lower range of 10 \u2013 12.5% and 18.42 \u2013 24.39%, respectively. Meanwhile, the unsonicated catalyst presented higher trans-C18:1/cis-C18:1 selectivities of 20.46% and 30.43% at IV 90 and 70, respectively.While Fig. 9 gives a useful outline of the catalytic performance of each sample over the course of the reaction, it is also highly crucial to analyse the product distribution correlated to a specific target IV. In particular, the concentration of trans-C18:1 and C18:0 are of paramount significance in deciding the final oil quality. Fig. 10\n demonstrates the percentage of C18:0 and trans-C18:1 fatty acids at an IV of 70. In this case, this particular level was chosen for comparison as it is a common target IV for oleomargarine products and it also corresponds to the point where the formation of C18:0 starts to be significant [6]. This extent of hydrogenation resembles to that in the manufacture of fatty acid components required for shortenings and margarine in the industry [91]. Employing the sonicated catalysts, the hydrogenation products obtained at an IV of 70 presented trans-C18:1 at equal to or less than 10%, while the hydrogenation products of the unsonicated counterpart registered trans-C18:1 levels of 13.7%, with the lowest trans-C18:1 level observed using catalyst D. On the other hand, the sonicated catalysts had similar percentages at approximately 29% in C18:0 production, while the unsonicated catalyst was slightly higher at 34%.Generally, as the overall hydrogenation proceeded, the isomerisation of fatty acid products increased with conversion. However, it is noted that the mechanisms related to hydrogenation and cis/trans isomerisation are heavily interlinked. With regards to the formation of trans fatty acids (TFAs) during hydrogenation, the addition\u2013elimination mechanism proposed by Horiuti and Polanyi [92] is commonly adopted to illustrate the process. The mechanism states that both the hydrogenation and isomerisation processes are governed by a half-hydrogenation state mechanism. Firstly, carbon\u2013carbon double bonds of C18:2 or C18:1 molecules are adsorbed on active sites. It is noted that C18:2 bonds tend to adsorb to the surface to a stronger degree and are thus first hydrogenated. Therefore, this also explains the consecutive drop in linoleic content throughout the reaction as per Fig. 9. After adsorption, one of the double bonds are half-hydrogenated by the addition of one hydrogen atom from the active sites. Further reaction resulting in the saturation of C\u2013C bonds necessitates the addition of another hydrogen atom. Nevertheless, typically without the presence of a second hydrogen atom, the first hydrogen atom detaches from the half-hydrogenated intermediate, thus re-establishing the double bond with either a cis-C18:1 or trans-C18:1 configuration [56]. Particularly, the latter configuration is more thermodynamically favoured than the former [93]. As hydrogenation is supplemented by isomerisation reactions, it is suggested that the change in the electronic characteristics of the sonicated Ni catalysts could also influence the isomerisation reactions. In this case, the enhanced removal of the chemisorbed half-hydrogenated intermediate from the active sites could prevent the further isomerisation to TFAs. This alteration in electronic properties may be used to explain the decrease in TFA formation for the sonicated catalysts, relative to the unsonicated counterpart [2,94]. Hence, one of the functions of ultrasound in this case of catalyst synthesis might be as a selectivity modifier for the Ni catalyst. Through sonication, the bonding strength of the adsorbates on the active sites were altered, which resulted in the increase of the energy barrier required for fatty acid isomerisation or the decrease of the energy barrier for hydrogenation that hastens cis-C18:1 to C18:0 conversion [94]. In particular, the shift from a higher to lower binding energy in the sonicated catalysts as observed in the XPS results (Table 5) complemented this finding. The changes in the electronic properties and electron densities of catalysts as observed by the shift in binding energies is known to alter the adsorption of reacting species and hence the selectivity towards the final products [95\u201397]. In this case, the decrease in binding energy denotes the increase in electron density of Ni species on the catalyst surface, thus leading to weaker interactions between the adsorbed or hydrogenated intermediates and the active sites, allowing easier desorption thus diminishing the transformation of unsaturated fat molecules or cis isomers to trans isomers [98], which was also a phenomenon reported by other researchers such as Iida et al. [99]. Similarly, Li et al. [75] has noticed a negative shift in binding energy for Ru species in Ru-B catalysts after sonication, which facilitated the adsorption of the oxygen atom of carbonyl groups in the cinnamaldehyde molecule, ultimately enhancing the hydrogenation of the molecule.To further assess the hydrogenation performance of the synthesised catalysts in this work, comparisons to novel catalysts obtained from recent literature were made, with a compilation presented in Table 8\n. Compared to most studies, it can be seen that catalyst C in this present work was able to achieve an IV drop to 70 in a shorter reaction time of 40\u00a0min, while containing a low amount of saturated fats and trans-fats, particularly the latter. Although noble metal (e.g. Pt and Pd) catalysts required milder reaction conditions (approximately 100\u00a0\u00b0C) and produced lesser saturated fats, the present catalyst was able to achieve a relatively low trans-fats percentage of 10%. It should also be noted that the saturated fats content for noble metal catalysts were comparatively lower due to their lower extent of reaction. Compared to the other Ni-based hydrogenation catalysts such as Ni/ZnO/Al2O3, Ni/SiO2, Ni-Mg-Ag/D and Ni-Ce/Al2O3, the novel sonicated catalyst in this work has produced at least half the amount of trans-fats, while maintaining comparable if not better hydrogenation activity. Nevertheless, it should be noted that these studies obtained from the literature were executed with disparities in reaction conditions and oil variation, hence the overall activity and trans-fats and saturated fats contents would have also been affected. For instance, the Ni-Ce/Al2O3 catalyst although requiring only 33\u00a0min of reaction time for hydrogenation, was employed to hydrogenate a feed oil with the lowest initial IV of 115. Therefore, this present work has exemplified the potential of incorporating ultrasonic technology into the synthesis of such Ni hydrogenation catalysts, with positive and applicable end results i.e. the enhancement of hydrogenation activity and the lowering of detrimental trans-fats for edible oil applications. For the latter application, edible oil manufacturers and catalyst developers who seek an alternative technique to lower their trans-fats production substantially could take the ultrasound-assisted catalyst synthesis procedure into consideration for their processes.This present work was performed to investigate the synthesis of silica-supported nickel catalysts using the ultrasonic technique and its application in the partial hydrogenation of edible oil. It was successfully shown that the incorporation of ultrasonic irradiation into the ageing process of sequentially precipitated catalysts have led to benefits in terms of the catalyst characterisation properties and their performance in the catalytic hydrogenation of sunflower oil. Further understanding into the proposed mechanisms indicated that the usage of ultrasonic irradiation, as well as variations in its intensity, had profound impacts on the overall characteristics and compositions (hydroxycarbonates, silicates and silica clusters) comprising the catalysts. It is noted that ultrasonic irradiation had a significant influence on the ageing properties of the catalysts, which directly affected the formation of silica clusters, followed by hydroxycarbonates and Ni silicates. For instance, ultrasonic irradiation led to the erosion of hydroxycarbonate phases and the suppression of silica clusters, which once formed, serve as protective enclosing to avert Ni silicate erosion. As the ultrasonic intensity was increased, greater Ni dispersion was achieved due to increasing extents in hydroxycarbonate erosion, while increase in BET surface areas was also noted due to the decrease in silica to hydrosilicate ratio as a result of the suppression of silica clusters. On the other hand, it was also observed that upon the termination of ultrasonic irradiation, the growth of the silica network on the surface of the catalysts during ageing also led to a higher Si composition on the external layer of the catalyst particles. In general, these effects in turn imparted alterations to the geometrical and electronic properties of the catalysts, improving catalyst reducibility, surface area and dispersion of the material.Hydrogenation using the catalyst synthesised at an ultrasonic intensity of 20.78\u00a0W\u00a0cm\u22122 achieved the fastest decrease to IV\u00a0=\u00a070 in 40\u00a0min, compared to 58\u00a0min achieved by its non-sonicated counterpart. Furthermore, the usage of ultrasonic irradiation prompted the modification of product selectivity by altering the electron density and subsequently the adsorption capability of the nickel species with hydrogenation intermediates. This led to a lower production of trans-fats in the sonicated catalysts compared to the non-sonicated catalyst. To illustrate, at IV\u00a0=\u00a070, the sonicated and non-sonicated catalysts produced 7 \u2013 10% and 13.7% of trans fats, respectively. Generally, the effects of sonication on nickel catalysts used for sunflower oil hydrogenation were positive and promising, which could potentially be extended to other oil types or catalyst systems. Overall, the present work has shown that the incorporation of ultrasound during catalyst synthesis offers attractive benefits, in which the activity and selectivity of the synthesised catalysts could be significantly modified by a brief exposure to ultrasonic waves.\nMitchell S.W. Lim: Investigation, Data curation, Visualization, Writing - original draft. Thomas Chung-Kuang Yang: Supervision, Resources, Funding acquisition. T. Joyce Tiong: Conceptualization, Methodology, Resources. Guan-Ting Pan: Data curation, Investigation. Siewhui Chong: Data curation, Visualization, Validation. Yeow Hong Yap: Writing - review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to acknowledge the technical guidance provided by Van Wu at the Precision Analysis and Material Research Center (National Taipei University of Technology). This research was financially supported by The Ministry of Science and Technology, Taiwan under the grant number 108-2221-E-027-072.Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2021.105490.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Sequentially precipitated Mg-promoted nickel-silica catalysts with ageing performed under various ultrasonic intensities were employed to study the catalyst performance in the partial hydrogenation of sunflower oil. Results from various characterisation studies showed that increasing ultrasonic intensity caused a higher degree of hydroxycarbonate erosion and suppressed the formation of Ni silicates and silica support, which improved Ni dispersion, BET surface area and catalyst reducibility. Growth of silica clusters on the catalyst aggregates were observed in the absence of ultrasonication, which explained the higher silica and nickel silicate content on the outer surface of the catalyst particle. Application of ultrasound also altered the electron density of the Ni species, which led to higher activity and enhanced product selectivity for sonicated catalysts. The catalyst synthesised with ultrasonic intensity of 20.78 Wcm\u22122 achieved 22.6% increase in hydrogenation activity, along with 28.5% decrease in trans-C18:1 yield at IV\u00a0=\u00a070, thus supporting the feasibility of such technique.\n               "}
{"full_text": "Hydrogen is called to play a key role in future energy demand as a substitute for fossil fuels [1]. In fact, in the post-pandemic crisis scenario, the alternatives for the production and use of renewable H2 are taking on great relevance, as for example in the economic recovery package of EU (NextGenerationEU) with \u20ac750 billion focused around the European Green Deal, where renewable energy projects highlight, especially wind, solar and kick-starting a clean hydrogen economy, with full economy decarbonization as the target for 2050 [2].Currently, the most common and economic process for H2 production is the steam reforming (SR) of natural gas, accounting for 76% of the global production, and whose global warming potential per kg of hydrogen produced is 11.956\u00a0kg CO2-eq [3]. Although the optimal technology for \u201cgreen H2\u2033 production is the water electrolysis, in the energy transition period until this solution becomes a reality H2 could be obtained with limited CO2 emissions from lignocellulosic biomass, by thermochemical process, particularly gasification, partial oxidation and SR of biomass derivatives (as bioethanol and bio-oil). Some of these processes are currently in a pilot-scale demonstration or at a commercial stage but they require improvements to produce larger competitive volumes [4]. The SR of bio-oil, obtained by fast pyrolysis of lignocellulosic biomass, has gained increased attention [5], due to the good prospects of a strategy to combine the delocalized bio-oil production (with well-developed technologies and with low infrastructure costs) [6], with centralized bio-oil SR in a bio-refinery with units designed ad hoc for selective H2 production. The liquid state and higher volumetric energy density of bio-oil facilitates its transportation, storage and treatment compared to biomass [7]. In addition, the SR of bio-oil avoids the costly dehydration steps required for the use of bio-oil as fuel or for its valorization in other catalytic processes [8].Bio-oil is composed of an oxygenate mixture with the presence of different functional groups (carboxyl, ester, carbonyl, ether, phenolic and hydroxyl groups) and variable water content (depending on the origin of the biomass). The SR reaction of oxygenated hydrocarbons (CnHmOk) to produce syngas (H2\u00a0+\u00a0CO) can be described by the following equation:\n\n(1)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nk\n\n\n\n+\n\n\n(\nn\n-\nk\n)\n\n\nH\n2\n\nO\n\n\u2192\n\nn\nC\nO\n\n\n+\n\n\n(\nn\n+\n\nm\n2\n\n-\nk\n)\n\n\nH\n2\n\n\n\n\n\nIn addition to the main reaction (Eq. (1)), the water gas shift (WGS) reaction (Eq. (2)) takes place, and thus the overall SR equation for the oxygenates is defined by Eq. (3).\n\n(2)\n\n\nCO\n\n+\n\n\nH\n2\n\nO\n\n\u2194\n\nC\n\nO\n2\n\n\n+\n\n\nH\n2\n\n\n\n\n\n\n\n\n(3)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nk\n\n\n\n+\n\n\n(\n2\nn\n-\nk\n)\n\n\nH\n2\n\nO\n\u2192\n\nn\nC\n\nO\n2\n\n\n\n+\n\n\n(\n2\nn\n+\n\nm\n2\n\n-\nk\n)\n\n\nH\n2\n\n\n\n\n\n\nH2 yield is also affected by reactions occurring in parallel to oxygenates SR and WGS reactions, such as decomposition/cracking (Eq. (4)), which affects the catalyst stability due to coke deposition, SR of decomposition products (CH4 and hydrocarbons, (Eqs. (5) and (6)), and interconversion of oxygenates (Eq. (7)).\n\n(4)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nk\n\n\n\u2192\n\n\nC\nx\n\n\nH\ny\n\n\nO\nz\n\n\n+\n\ng\na\ns\n\n\n(\nC\nO\n,\n\nC\n\nH\n4\n\n,\n\nC\n\nO\n2\n\n,\n\n\nC\na\n\n\nH\nb\n\n,\n\n\nH\n2\n\n,\n\n\u2026\n)\n\n+\nc\no\nk\ne\n\n\n\n\n\n\n\n(5)\n\n\nC\n\nH\n4\n\n+\n\n\nH\n2\n\nO\n\n\u2194\n\nC\nO\n\n+\n\n3\n\nH\n2\n\n\n\n\n\n\n\n\n(6)\n\n\n\nC\na\n\n\nH\nb\n\n\n+\n\na\n\nH\n2\n\nO\n\n\u2192\n\na\nC\nO\n\n+\n\n\n\na\n\n+\nb\n/\n2\n\n\n\n\nH\n2\n\n\n\n\n\n\n\n(7)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nk\n\n\n\u2192\n\n\nC\nx\n\n\nH\ny\n\n\nO\nz\n\n\n\n\n\nMoreover, the reactions for coke formation from gaseous products (Eqs. (8) and (9)) and its gasification reaction (Eq. (10)) should be considered, as they may affect catalyst stability and also the products yields when they are highly promoted.\n\n(8)\n\nHydrocarbon decomposition: CaHb\u00a0\u2192\u00a0(b/2)H2\u00a0+\u00a0aC\n\n\n\n\n(9)\n\nBoudouard reaction: 2CO\u00a0\u2194\u00a0C\u00a0+\u00a0CO2\n\n\n\n\n\n(10)\n\nCoke gasification: Coke\u00a0+\u00a0H2O\u00a0\u2192\u00a0CO\u00a0+\u00a0H2\n\n\nOne of the main problems or bottlenecks of bio-oil SR is the rapid deactivation of the catalyst, which justifies that it receives a great attention, in order to prepare stable catalysts. Deactivation studies have generally been carried out with model oxygenates [9\u201320], with mixtures of oxygenates [21\u201323]\n, and studies with raw bio-oil are limited. [24\u201329]. The results show the importance of the nature and location of the coke in the deactivation of the catalyst. Thus, the formation of carbon filaments has a reduced incidence in the deactivation, whose responsibility falls mainly on the formation of amorphous coke encapsulating the Ni sites. There is also a general tendency to relate the formation of deactivating amorphous coke to the SR of some families of oxygenates (phenols, carboxylic acids, furfural and saccharides, mainly).This paper delves into the clarification of deactivation by coke of a catalyst derived from NiAl2O4 spinel, which has been previously proven to have high activity and selectivity to H2 in the reforming of raw bio-oil and, more interestingly, it can be fully regenerated by coke combustion at 850\u00a0\u00b0C (with spinel reconstruction) [30]. For that purpose, we have studied the influence on the deactivation behavior and coke deposition of individual oxygenate compounds with varied functional groups present in bio-oil (acetic acid, acetaldehyde, acetol, ethanol, acetone, catechol, guaiacol and levoglucosan). The evolution along time on stream of the conversion and products yields in the SR of each individual compound have been analysed, as well as the amount, nature, morphology and location of the coke deposited on the catalyst used by means of several techniques: temperature programed oxidation (TPO), X-ray diffraction (XRD), N2 adsorption\u2013desorption, scanning and transmission electron microscopy (SEM, TEM) and Raman spectroscopy. The experimental conditions used are similar to those previously used in the SR of raw bio-oil with the same catalyst [31], which has allowed a direct comparison of the catalyst performance in the SR of each individual oxygenate with that obtained in the SR of raw bio-oil. The results have allowed stablishing the main responsible of catalyst deactivation during SR of bio-oil, as well as the coke characteristics that mainly affect the deactivation of the catalyst. Consequently, interesting information is obtained to adjust the composition of the raw bio-oil in order to attenuate catalyst deactivation by coke. In addition, the oxygenates of greatest interest as model compounds for the comparison tests of catalyst deactivation for the SR of raw bio-oil are identified.The pure oxygenate compounds selected as representative of the major families of oxygenates in bio-oil are the following: acetic acid (AA) (Romil LTD, purity\u00a0>\u00a099.9 %), acetaldehyde (AD) (Merck KGaA, purity\u00a0\u2265\u00a099 %), acetone (A) (AppliChem GmbH, purity\u00a0\u2265\u00a099.9 %), acetol (AT) (hydroxyacetone, Alfa Aesar GmbH, purity\u00a0=\u00a095 %), ethanol (E) (Merck KGaA, purity\u00a0\u2265\u00a099.9 %), 1,2-benzenediol or catechol (C) (Sigma-Aldrich, purity\u00a0\u2265\u00a099 %), levoglucosan (L) (1,6-Anhydro-\u03b2-D-glucopyranose, Acros Organics, purity\u00a0>\u00a099%), and 2-methoxyphenol or guaiacol (Alfa Aesar GmbH & Co, purity\u00a0>\u00a098 %) dissolved in 50\u00a0wt% of ethanol (G\u00a0+\u00a0E) due to its low solubility in water. Acetone, acetaldehyde, 1,2-benzenediol (catechol) and 2-methoxyphenol (guaiacol) are representative of relevant families of compounds in bio-oils such as ketones, aldehydes and phenols (among these, mainly guaiacols and catechols) [32,33]. Acetic acid, levoglucosan and acetol are present in remarkable concentrations in the bio-oil obtained from pyrolysis of pine sawdust [26,34]. The study of the catalyst behavior in the SR of ethanol is interesting because it may be cofed with bio-oil for its stabilization and because the SR of bio-oil/bio-ethanol mixture (BO\u00a0+\u00a0E) is an interesting route for sustainable H2 production from two biomass derived feeds [35].The catalyst precursor (Ni-Al spinel, NiAl2O4) was prepared by co-precipitation method with a nominal Ni content of 33\u00a0wt% from Ni(NO3)2\u00b76H2O and Al(NO3)3\u00b79H2O with a NH4OH 0.6\u00a0M solution as a precipitating agent. The precipitation was carried out at 25\u00a0\u00b0C until the pH was fixed at 8. After aging for 30\u00a0min, the precipitate was filtered, washed with distilled water to remove the ammonium ions and dried at 110\u00a0\u00b0C for 24\u00a0h. Lastly, the catalyst was calcined at 850\u00a0\u00b0C for 4\u00a0h [30].The physical properties of the fresh catalyst and deactivated samples (BET surface area, pore volume and mean pore diameter), were characterized by adsorption\u2013desorption of N2 in a Micromeritics ASAP 2010. Temperature Programed Reduction (TPR) was carried out in a Micromeritics AutoChem 2920 for determining the reducibility of the metal species. The amount and nature of coke deposited on spent catalyst samples has been determined by Temperature Programed Oxidation (TPO) in a TA-Instruments TGA-Q5000IR thermobalance, coupled in line with a mass spectrometer (Thermostar Balzers instrument) for monitoring the signal of CO2. The coke content has been quantified from the CO2 spectroscopic signal, due to Ni oxidation during combustion process masks the thermogravimetric signal in samples with low coke content [30]. The X-Ray Diffraction (XRD) analysis of the reduced fresh and spent catalysts was carried out in a Bruker D8 Advance diffractometer with a CuK\u03b11 radiation, from 10\u00b0 to 80\u00b0 with step of 0.04\u00b0 in 2\u03b8 and measurement time of 103\u00a0min. The scanning electron microscopy images of the fresh or spent catalysts were taken with a Hitachi S-4800\u00a0N field emission gun scanning electron microscope (FEG-SEM), with an accelerating voltage of 5\u00a0kV and secondary electron detector (SE-SEM) and a Hitachi S-3400\u00a0N microscope with an accelerating voltage of 15\u00a0kV, using a backscatter electron detector (BSD-SEM). The transmission electron microscopy (TEM) images were obtained in a Phillips CM-200 microscope using an accelerating voltage of 200\u00a0kV. The Raman spectra were carried out in a Renishaw InVia confocal microscope using an excitation wavelength of 514\u00a0nm, taking a spectrum in several areas of the sample for assuring reproducibility.The N2 adsorption\u2013desorption isotherm and the BJH pore distribution of the fresh-reduced catalyst are shown in Figs. S1a and S1b, respectively, in the Supporting Information. An isotherm of type IV according to the IUPAC classification is observed in Fig S1a, which is associated with capillary condensation taking place in mesopores, with a hysteresis of the type H2, attributed to a difference in mechanism between condensation and evaporation processes occurring in pores with narrow necks and wide bodies (often referred to as 'ink bottle' pores). The BET surface are, pore volume and mean pore diameter for the fresh-reduced catalyst (Table 1\n) are 65.1\u00a0m2/g, 0.24\u00a0cm3/g and 13.1\u00a0nm, respectively. The TPR profile of the fresh catalyst (Figure S1c) has a maximum H2 uptake at 760\u00a0\u00b0C, corresponding to the reduction of Ni species incorporated in the NiAl2O4 spinel structure [30,36]. The XRD pattern of the fresh catalyst prior reduction (black curve in Figure S1d) shows intense peaks at 2\u03b8\u00a0=\u00a037.2, 45.2 and 65.7\u00b0 corresponding to the cubic structure of NiAl2O4 spinel, whereas the XRD of the fresh-reduced catalyst (blue curve in Figure S1d) shows peaks corresponding to Ni0 (diffraction angle at 44.5\u00b0 in (111) plane, 51.8\u00b0 in (200) plane and 75.5\u00b0 in (110) plane, JCPDS n\u00b0 00\u2013004-0850) and Al2O3 (37.3\u00b0, 45.6\u00b0 and 66.8\u00b0, JCPDS n\u00b0 01\u2013077-0396). This result indicates that the reduction treatment (at 850\u00a0\u00b0C for 4\u00a0h) completely converted NiAl2O4 spinel into reduced Ni crystals supported on Al2O3 (Ni/Al2O3), as previously reported [36]. The mean Ni crystal size in the reduced catalyst (calculated with the Debye-Scherrer equation using the diffraction peak at 2\u03b8\u00a0=\u00a052\u00b0) is 9\u00a0nm (Table 2\n).Runs have been carried out in an automatized reaction system (MicroActivity-Reference, PID Eng & Tech,) that has been described in detail elsewhere [37], with a fluidized bed reactor. The catalyst (with particle size of 150\u2013250\u00a0\u00b5m to avoid internal diffusional limitations) is mixed with inert solid (SiC, 37\u00a0\u00b5m particle size) in order to ensure good fluid dynamic behaviour of the catalytic bed (inert/catalyst mass ratio\u00a0>\u00a08/1).Prior to each steam reforming reaction, the catalytic bed is reduced in-situ by using H2-N2 flow (10\u00a0vol% of H2) at 850\u00a0\u00b0C for 4\u00a0h, thus forming the active Ni0/Al2O3 catalyst. The operating condition for the kinetic runs have been: atmospheric pressure; 600 and 700\u00a0\u00b0C, that are suitable for attaining high conversion in the SR of bio-oil; space time of 0.034 gcatalyst\nh/goxygenate in order to favor catalyst deactivation by coke formation during not excessively long runs (of 5\u00a0h duration); steam-to-carbon (S/C) molar ratio of 3 (except for levoglucosan, with S/C\u00a0=\u00a06 due to its low water solubility), which is suitable for promoting WGS reaction (necessary to enhance H2 yield) but without excessive penalty of energy requirements. This S/C ratio has been set by co-feeding water (307 Gilson pump) with the feed (injection pump Harvard Apparatus 22). The reaction products were analysed in a Micro GC Varian CP-490 connected in-line to the reactor through an insulated line (130\u00a0\u00b0C) to avoid condensation of the products. The gas chromatograph is equipped with three analytic channels: molecular sieve MS5 for quantifying H2, O2, N2, CH4 and CO; PPQ column for light hydrocarbons (C2-C4), CO2 and water; and Stabilwax for oxygenated compounds (C2+) and water.In order to quantify the results, the following reaction indices were used:\n\n\n(11)\n\nCarbon\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\nt\no\n\ng\na\ns\ne\ns\n:\nX\n\n=\n\n\n\n\nF\n\n\nout, gas\n\n\n\n\nF\n\n\nin\n\n\n\n\n\nwhere Fout,\ngas is the molar flow rate of the total carbon in gaseous product (CO2, CO, CH4 and light hydrocarbons, in C units contained) at the reactor outlet, and Fin is the molar flow rate of the oxygenate at the reactor inlet in C units contained.\n\n\n(12)\n\n\n\nH\n\n\n2\n\n\n\ny\ni\ne\nl\nd\n:\n\n\nY\n\n\n\n\nH\n\n\n2\n\n\n\n\n=\n\n\n\nF\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\nF\n\n\n\n\nH\n\n\n2\n\n\n\n\no\n\n\n\n\n\nwhere FH2 is the H2 molar flow rate in the product stream and \n\nF\n\n\nH\n2\n\n\no\n\n, is the stoichiometric molar flow rate, which is calculated as (2n\u00a0+\u00a0m/2 \u2013 k)/n Fin, according to the global stoichiometry for the SR of each oxygenate (CnHmOk) (including the WGS reaction) (Eq. (3)).In order to assess the catalyst activity, selectivity and stability for the different feeds studied, the evolution with time on stream (TOS) of carbon conversion to gas and yield of H2 at 600 and 700\u00a0\u00b0C is shown in Fig. 1\n (acetic acid (a), acetaldehyde (b), ethanol (c), acetol (d) and acetone (e)) and Fig. 2\n (catechol (a), mixture guaiacol\u00a0+\u00a0ethanol (b) and levoglucosan (c)).At 600\u00a0\u00b0C, the initial H2 yield (fresh catalyst) varies between 42 % (for acetone, Fig. 1e) and 61 % (for acetaldehyde, Fig. 1b), with similar values (near 50 %) for the rest of oxygenates, thus evidencing similar reactivity at this temperature. The increase in temperature up to 700\u00a0\u00b0C enhances the carbon conversion to gas and H2 yield at zero time on stream in the SR of all oxygenates, with this increase being more significant for acetaldehyde, ethanol and acetol (almost 100 % conversion), and also for acetic acid and catechol (around 93 % conversion). However, it is less noticeable for the guaiacol\u00a0+\u00a0ethanol mixture, acetone and levoglucosan, thus evidencing the lower reactivity towards SR reactions at high temperature of the latter oxygenates. The low increase with temperature of the carbon conversion for the guaiacol\u00a0+\u00a0ethanol mixture (Fig. 2b), compared to that obtained with ethanol (Fig. 1c), gives evidence of a low effect of temperature for guaiacol, whose reactivity for SR reactions is noticeably lower than that of ethanol, acetic acid, acetaldehyde, acetol and catechol. Moreover, the initial H2 yield in the SR of ethanol at 700\u00a0\u00b0C (70 %) is lower than that obtained with the other oxygenates (around 80 %), in spite of its high carbon conversion (100 %). This result reveals the higher selectivity of the catalyst for H2 forming reactions (steam reforming and WGS) in the SR of acetic acid, acetol and acetaldehyde compared to ethanol, whose reforming produces significant CH4 formation (not shown). The high H2 yield (80%) obtained in the SR of levoglucosan at 700\u00a0\u00b0C, in spite of its incomplete carbon conversion, should be attributed to the high S/C ratio used in the SR of this oxygenate (6), that significantly promotes WGS reaction.Regarding the stability of the catalyst, overall, the conversion and H2 yield remain constant or even increase (for acetic acid (Fig. 1a), acetol (Fig. 1d) and catechol (Fig. 2a)) at 600\u00a0\u00b0C and slightly decrease with TOS at 700\u00a0\u00b0C in the SR of all the studied oxygenates, except for guaiacol\u00a0+\u00a0ethanol mixture. The increase in conversion and H2 yield with TOS can presumably be explained by the formation of a remarkable amount of filamentous coke (as shown later), which leads to an improved Ni dispersion and better accessibility of reactants due to the tip-growth mechanism of carbon filaments [38]. Conversely, in the SR of the guaiacol\u00a0+\u00a0ethanol mixture (Fig. 2b) there is a fast decrease in conversion and H2 yield at both 600 and 700\u00a0\u00b0C, until the values corresponding to the thermal reaction routes (in the absence of catalyst) are reached. Therefore, this result evidences a much faster deactivation rate of the catalyst in the SR of this mixture than for rest of the oxygenates studied.In order to identify the causes responsible for the deactivation of NiAl2O4 catalyst, and for a better understanding of its deactivation behavior in the SR of the different oxygenates, a thorough characterization of the spent catalyst samples has been performed, by using complementary techniques, that include TPO, SEM and TEM images, XRD, TPR, Raman spectroscopy and N2 adsorption\u2013desorption. These techniques have allowed determining the amount, nature, morphology and structure of the coke deposited in the catalyst, as well as the changes in the metal sites and porous structure of the catalyst. It should be noted that Ni oxidation was ruled out as deactivation cause, as no significant reduction peaks were observed in the H2-TPR profiles of selected deactivated catalyst samples (results not shown here), thus indicating the absence of oxidized species. This is an expected result, which was previously observed in the oxidative steam reforming (OSR) of raw bio-oil with this type of catalyst [39], and it is due to the highly reducing environment in the SR reaction, with a high H2 content. The results of the rest of characterization techniques are presented in the following sections. The spent catalyst samples have been denoted as X-N, where\u00a0X\u00a0identifies the oxygenate feed (AA\u00a0=\u00a0acetic acid; AD\u00a0=\u00a0acetaldehyde; E\u00a0=\u00a0ethanol; AT\u00a0=\u00a0acetol; A\u00a0=\u00a0acetone; C\u00a0=\u00a0catechol; (G\u00a0+\u00a0E)\u00a0=\u00a0guiaiacol\u00a0+\u00a0ethanol; L\u00a0=\u00a0levoglucosan) and N is the SR temperature (600 or 700\u00a0\u00b0C).\nFigs. 3 and 4\n\n show the TPO profiles of the deactivated catalysts used in the SR of the oxygenates at different temperatures, obtained from the spectroscopic signal of CO2 released during coke combustion (as explained in section 2.3). These results provide qualitative information on the nature and/or location of the coke in the structure of the catalyst [40]. Several authors differentiate the amorphous and filamentous coke contents of coke deposited on Ni catalysts by deconvolution of the TPO profiles. Thus, Hu et al. [41], relate each type of coke to one of the two peaks of the TPO, so that the amorphous/paraffinic coke burns at lower temperature than the graphitic/filamentous coke. This identification of the two types of coke allowed verifying that with the addition of Fe there is a higher attenuation of the deposition of the graphitic/filamentous coke in the Fe-Ni/Al2O3 catalysts used in the steam reforming of toluene. The same authors characterize the coke over a Ni/\u03b1-Al2O3 catalyst in the steam reforming of two hydrocarbons (toluene and methylnaphthalene) and two oxygenates (phenol and ethanol) distinguishing amorphous coke from carbon nanotubes (CNTs), whose combustion is identified with the peak at higher combustion temperature [42]. Subsequently, these authors have verified the relevant effect of the steam reforming temperature (in the 500\u2013800\u00a0\u00b0C range) on the content of the two types of coke and on the quality of the CNTs, proving the existence of a maximum of both at 650\u00a0\u00b0C [43]. The identification of these two types of coke of different nature by deconvolution of coke combustion TPO profiles was also used to quantify the formation of CNTs on Ni catalysts from the volatiles from polyolefins pyrolysis [44].The size and location of combustion peaks in Figs. 3 and 4 evidence differences in the amount and nature of coke deposited with the different oxygenates and at different temperature. Noticeably, in the TPO profiles of Figs. 3 and 4 there is apparently a unique combustion peak, with maximum in the 500\u2013550\u00a0\u00b0C range for the SR at 600\u00a0\u00b0C. This maximum shifts towards higher combustion temperature for the catalyst used in the SR at 700\u00a0\u00b0C, which suggests that the coke evolves into a more condensed and graphitic-like structure, with lower H/C ratio, and therefore, a higher combustion temperature is required [40].A shoulder burning at low temperature is also observed in the TPO profiles for the samples deactivated at 600\u00a0\u00b0C with catechol and guaiacol\u00a0+\u00a0ethanol (Fig. 4), which is more noticeable for the latter, but is not observed in the SR of the non-phenolic oxygenates (Fig. 3). This result confirms the previously reported relevant role of phenolic compounds as precursors of the coke burning at low temperature (deposited near metal sites, causing its partial or total encapsulation), deposited in the SR of bio-oil over this catalyst [8]. Taking into account the similar amount of this coke type deposited for catechol and for the mixture (guaiacol\u00a0+\u00a0ethanol), with only 50\u00a0wt% of guaiacol, it can be concluded that the latter is more prone to its formation.The results of TPO profiles also provide information on the content of coke deposited, estimated from the total area under the TPO profiles, because the calculation of coke content from TGA results (Figure S2) is masked by the mass increase due to Ni oxidation. The results of coke content are gathered in Table 1, which also includes the average coke deposition rate, calculated assuming linear coke deposition over the reaction. The coke content notably decreases with reforming temperature in the SR of non-phenolic compounds, but, conversely, it increases in the SR of phenolic compounds (catechol and guaiacol), which suggests a different mechanism of coke formation and evolution for the two groups of compounds. At 600\u00a0\u00b0C, the amount of coke follows the order: ethanol\u00a0>\u00a0acetone\u00a0>\u00a0acetic acid \u2248 catechol\u00a0>\u00a0guaiacol\u00a0+\u00a0ethanol\u00a0>\u00a0acetaldehyde \u2248 acetol\u00a0>\u00a0levoglucosan, whereas at 700\u00a0\u00b0C the order is guaiacol\u00a0+\u00a0ethanol\u00a0>\u00a0catechol \u226b acetone\u00a0>\u00a0ethanol\u00a0>\u00a0acetic acid\u00a0>\u00a0acetaldehyde\u00a0>\u00a0acetol\u00a0>\u00a0levoglucosan. It should be noted that S/C ratio used in the SR runs with levoglucosan (S/C\u00a0=\u00a06), is significantly higher than that used with the rest of oxygenates (S/C\u00a0=\u00a03), which contributes to the lower coke content obtained with levoglucosan at any temperature.Comparing the results of Table 1 with the deactivation results (Figs. 1 and 2), it is noteworthy that there is no direct relationship between the amount of coke and the deactivation rate. This result has been also reported in previous works on oxygenates reforming [38,45\u201349]\n, and is explained by the fact that other characteristics of the coke (morphology, structure and location) have a greater impact on deactivation than its content. In addition, it is observed that with similar TPO profiles in terms of peak position (as is the case of coke for the SR of guaiacol\u00a0+\u00a0ethanol and catechol at 600\u00a0\u00b0C) the deactivation rate is different (much faster in the SR of guaiacol\u00a0+\u00a0ethanol). Consequently, although the TPO profile of the coke provides valuable qualitative information on the level of condensation and heterogeneity of the coke, to understand the deactivation of the catalyst it is necessary to complete the information on the coke with other characterization techniques of the deactivated catalyst, which will be shown in subsequent sections.The textural properties of fresh and deactivated samples (BET surface area, average pore diameter and pore volume) have been determined by means of N2 adsorption\u2013desorption and are displayed in Table 1. The N2 adsorption\u2013desorption isotherms of spent catalyst samples are shown in Fig. 5\n (ethanol and (guaiacol\u00a0+\u00a0ethanol) feeds) and Figure S3 of Supplementary Information (rest of feeds).All the samples have isotherm of type IV, but differently to the fresh-reduced catalyst, a H3-type hysteresis cycle is observed for most of the isotherms of catalyst samples used in SR of pure oxygenate compounds, which does not exhibit any limiting adsorption at high P/P0, and is associated to aggregated of plate-like particles giving rise to slit-shape pores [50]. Overall, the shape of the isotherms in Figs. 5 and S3 for spent catalysts does not change with SR temperature, but there are significant differences in the values of textural properties (BET surface area, mean pore diameter and pore volume, gathered in Table 1). In the region of low partial pressures (P/P0 \u2248 0), the volume adsorbed in samples of catalyst used in the SR of aliphatic oxygenates and catechol at 600\u00a0\u00b0C increases noticeably due to the high BET surface area of these samples (which is more than double that of fresh catalyst). This result can be explained by the deposition of porous carbon structures, such as carbon filaments (in agreement with the SEM images shown later, in Figs. 9 and 10), and is consistent with the high stability observed in the SR at 600\u00a0\u00b0C of these oxygenates. In the samples of catalyst used at 700\u00a0\u00b0C and with the lowest values of coke deposition (SR of acetaldehyde, acetol and levoglucosan), the physical properties resemble those of the fresh catalyst. On the other hand, the significantly lower total volume adsorbed at high pressures (P/P0 \u2248 1) in the sample of the catalyst used in the SR of (guaiacol\u00a0+\u00a0ethanol) mixture at 700\u00a0\u00b0C (Fig. 5a) evidences the partial blockage of the mesopores, thus causing a decrease in BET surface area and mean pore volume (in spite of a high presence of carbon filaments). This partial blockage of the porous structure would contribute to a rapid deactivation, as observed in the SR at 700\u00a0\u00b0C of this feed (Fig. 2b).The XRD was carried out to analyze the crystalline structure of the catalyst and of the coke, and also to determine the average size after reaction of Ni metal crystals, by means of Debye-Scherrer equation, at 2\u03b8\u00a0=\u00a051.8\u00b0 (Ni0 (200) plane). Fig. 6\n shows the XRD diffractograms of spent catalyst samples used in the SR of oxygenates at 600\u00a0\u00b0C (graph a) and at 700\u00a0\u00b0C (graph b). The XRD diffractogram of the fresh catalyst is also shown in Fig. 6 for comparison. The same diffraction peaks as in the fresh-reduced catalyst are observed in the spent catalyst samples. Therefore, the presence of NiO is not detected, in agreement with H2-TPR results, which corroborates the high reducing capacity of the reaction medium to keep the active metal in a reduced state.Moreover, the XRD pattern of most of the spent catalysts shows the presence of a broad peak at a diffraction angle 2\u03b8\u00a0=\u00a026 \u00b0, which corresponds to high crystallinity cokes (graphite carbon), a characteristic peak usually identified in catalysts used in the steam reforming of pure oxygenate compounds (or some mixtures), such as ethanol [51], acetone [52] or acetic acid [53]. The intensity of this peak is in a reasonable agreement with carbon amounts (Fig. 5). Thus, its intensity is high for all the samples deactivated at 600\u00a0\u00b0C, except for levoglucosan, and for the samples deactivated at 700\u00a0\u00b0C with phenolic compounds or acetone in the feed, but it is not observed for levoglucosan, acetaldehyde and acetol, due to their low coke content (<3%). Nevertheless, there is not a linear relationship between the intensity of this peak and the amount of coke, which is consequence of differences in the crystallinity level of the different coke deposits.The calculated average values of Ni0 particles for fresh-reduced and spent catalysts are gathered in Table 2. It should be noted that the calculation is possible only with low-moderate coke content (below 120\u00a0wt%), because a high coke content hinders the measurement of metal crystal size from XRD diffractograms (as it masks the Ni0 diffraction peaks). The values of average size of Ni0 crystallites of all used catalysts in Table 2 are around that of fresh catalyst (9\u00a0nm), except for SR of levoglucosan, that slightly increases. This moderate sintering of Ni0 crystals could be the responsible of the moderate deactivation rate observed in the SR of levoglucosan (Fig. 2c), in spite of the low coke content deposited in these experiments (Fig. 5).\nFigs. 7 and 8\n\n\n\n show the BSD-SEM images for the catalyst used in the SR of oxygenates at 600 and 700\u00a0\u00b0C, respectively. The BDS-SEM images allow determining the presence of some type of elements on the external surface of the particles based on the brightness intensity [47]. Thus, the high brightness intensity of the fresh catalyst (Figure S4) indicates the presence of heavy elements (Ni and Al) constituting the catalyst phases (Ni crystals and Al2O3). In contrast, the particles of the spent catalysts are generally homogeneous and exhibit a low brightness intensity (dark appearance), which indicates the majority presence of coke on the particle external surface. However, the catalyst used in the SR of levoglucosan at 700\u00a0\u00b0C (Fig. 8h) shows an homogeneous high brightness intensity, similar to that of the fresh catalyst, which confirms the very low coke deposition observed in the TPO results. On the other hand, the catalyst used in the SR of acetaldehyde at 700\u00a0\u00b0C (Fig. 8b) shows heterogeneous particles, some with high brightness intensity and others with a dark appearance, which is indicative of the heterogeneous coke deposition.Additionally, these images also show differences in the particle shapes and textures that can be correlated with the coke content (Table 1). When the coke content is low (below 20\u00a0wt%), the particle shape of the spent catalysts (SR of levoglucosan at 600\u00a0\u00b0C (Fig. 7h) and acetaldehyde, acetol and levoglucosan at 700\u00a0\u00b0C (Fig. 8b, 8d and 8\u00a0h, respectively)) is similar to that of the fresh catalyst, being irregular with a smooth surface and sizes in between 150 and 250\u00a0\u03bcm (original catalyst particle size). When the coke content is moderately high (between 20 and 120\u00a0wt%), the particle texture of the spent catalysts (SR of acetic acid, ethanol and acetone at 700\u00a0\u00b0C (Fig. 8a, 8c and 8e, respectively)) changes to a rough surface keeping the original catalyst particle size. In particular, the catalyst used in the SR of acetic acid at 700\u00a0\u00b0C (Fig. 8a) shows bare catalyst particles with fragments of coke shells, evidencing the low mechanical strength of the superficial coke shells. When the coke content is high (above 200\u00a0wt%), the particles of the spent catalysts (rest of the experiments) have a rough surface and are remarkably smaller than the original catalyst particle size, which may indicate a collapse of the catalyst particles due to the excessive coke growth.\nFigs. 9 and 10 show the SE-SEM images of the spent catalyst surfaces. In general, at 600\u00a0\u00b0C (Fig. 9), the images show the formation of carbon filaments from all the model compounds with different characteristics (heterogeneous in size and texture). In particular, the carbon filaments from SR of ethanol (Fig. 9c) show a rough surface, indicating the growth/deposition of carbon along the filaments. Additionally, the formation of an amorphous carbon phase is observed in the catalyst used in the SR of guaiacol\u00a0+\u00a0ethanol (Fig. 9g), and in comparison with the catalyst used in the SR of ethanol (Fig. 9c), this carbon phase is inferred to be formed from guaiacol. At 700\u00a0\u00b0C (Fig. 10), the SE-SEM images clearly show the predominant formation of carbon filaments from acetic acid, acetaldehyde, ethanol, acetone and catechol. The aforementioned peculiar feature of the carbon filaments from ethanol is highly noticeable at this temperature (Fig. 10c), indicating the growth/deposition of carbon along the filaments is favored. A second carbon phase in between the filaments is observed in the SR of guaiacol\u00a0+\u00a0ethanol (Fig. 10g), probably due to the formation of pyrolytic carbon from guaiacol (as explained in discussion section) which is more predominant on some regions of the catalyst surface (Figure S5). On the other hand, the surface of the spent catalysts with low coke content (SR of acetol and levoglucosan, Fig. 10d and 10\u00a0h, respectively) resembles that of the fresh catalyst, which confirms no significant coke deposition.To study the location of coke on the catalyst surface, Figure S6 shows contrasts of BSD-SEM and SE-SEM images for selected spent catalyst samples. In the spent catalyst with carbon filaments, Ni crystals are often visualized on the tip of the filaments, but not for all the cases. Interestingly, for the catalyst used in the SR of acetic acid at 600\u00a0\u00b0C (Figure S6c), a large filament was captured showing various Ni crystals along it, which indicates that various Ni crystals may be involved in the growth of large filaments. To complement these observations, selected spent catalyst samples were also analyzed using TEM, and the images (Figures S7 and S8) evidence the formation of hollow carbon filaments (carbon nanotubes) with thick walls (probably multiwall carbon nanotubes, MWCNT) and the presence of Ni crystals on the tip of or along the filaments with no evidence of sintering. Particularly, the catalysts used in the SR of guaiacol\u00a0+\u00a0ethanol at 600 and 700\u00a0\u00b0C (Figure S7) showed two carbon phases (amorphous and filaments). The presence of Ni crystals on the tip of the filaments is an expected observation based on the tip growth mechanism commonly described for the formation of carbon filaments on different Ni catalysts used in the SR of oxygenates [38,42,54,55]. It also explains the catalyst stability observed in the experiments for the SR of acetic acid, acetaldehyde, ethanol, acetol, acetone, catechol and levoglucosan (Figs. 1 and 2) in spite of the high content of filaments, because Ni crystals are exposed and accessible for the reactants. However, the rapid catalyst deactivation observed for the SR of guaiacol\u00a0+\u00a0ethanol in Fig. 2b is associated to the formation of a second carbon phase at 600 and 700\u00a0\u00b0C, which is also observed in the SR of raw bio-oil [31,34]. This carbon phase is formed from guaiacol and has an amorphous nature at 600\u00a0\u00b0C and a pyrolytic nature at 700\u00a0\u00b0C based on the coke combustion characteristics (Fig. 4a and 4b, respectively).\nFig. 11\n shows the Raman spectra of selected spent catalyst samples to further study the structural properties of coke. All the samples show the typical D (corresponding to disordered aromatic structures, at\u00a0\u223c\u00a01343\u00a0cm\u22121) and G (corresponding to condensed, ordered or graphitic aromatic structures, \u223c1589\u00a0cm\u22121) bands as commonly found for various carbon structures, and the corresponding second-order bands in the 2500\u20133500\u00a0cm\u22121 region (Figure S9) [31,56,57]\n. At 600\u00a0\u00b0C, the G and D bands have similar features for all the spent catalyst samples (SR of ethanol, acetol, guaiacol\u00a0+\u00a0ethanol and levoglucosan) with noticeable different intensities for the D band. The intensity ratio between the D and G band (ID/IG) determined from deconvolution (procedure described in the SI document and results summarized in Table 3\n) is notoriously higher for the coke formed from the SR of ethanol, and consecutively decreases for the coke corresponding to acetol, levoglucosan and guaiacol\u00a0+\u00a0ethanol. At 700\u00a0\u00b0C, the D and G band features are significantly different. Thus, the coke formed from ethanol has narrow D and G bands, the G band has a shoulder at 1605\u00a0cm\u22121 and noticeable higher ID/IG ratio. The spectra of the coke formed from levoglucosan present a high noise level, which is coherent with the low coke content determined from the TPO analysis.Moreover, the Raman spectra mostly correspond to carbon nanotubes (CNT) with different structural qualities, in coherency with the results of SEM and TEM analyses, which revealed the presence of CNT in the catalysts used in the SR of ethanol, acetol, guaiacol\u00a0+\u00a0ethanol and levoglucosan at 600\u00a0\u00b0C and ethanol and guaiacol\u00a0+\u00a0ethanol at 700\u00a0\u00b0C. The spectrum for ethanol at 700\u00a0\u00b0C is very close to that of MWCNT, exhibiting dominant narrow D and G bands [58\u201360]. The intensity ratio between the D3 (assigned to amorphous carbon) and G bands (ID3/IG) (listed in Table 3) provides an indicator for measuring the quality of carbon nanotubes [59], indicating that those formed from ethanol at 700\u00a0\u00b0C would have the highest purity having the lowest ID3/IG ratio (0.06).It is also observed that the Raman spectra for the catalyst used in the SR of guaiacol\u00a0+\u00a0ethanol at 600 and 700\u00a0\u00b0C is typical of carbon structures with different degree of order [61], which is in agreement with the formation and deposition of a second carbon phase between the filaments. This result is in agreement with the BSD-SEM (Fig. 8g) and SE-SEM (Fig. 10g) images discussed above. Based on the ID/IG ratio, being higher at 700\u00a0\u00b0C than at 600\u00a0\u00b0C, the second carbon phase is predominantly amorphous with ordered domains below 2\u00a0nm, but it is more structured at 700\u00a0\u00b0C, which is consistent with the higher combustion temperature observed in the TPO analysis (Fig. 4b). This relationship between TPO and Raman spectroscopic analyses has been also observed for this catalyst used in the SR of raw bio-oil [31], evidencing the formation of carbon filaments and amorphous carbon with different degree of order, which indicates an analogy between coke deposition in the SR of guaiacol and raw bio-oil.The deactivation of the catalyst in the oxygenates SR can be explained by the steps in Fig. 12\n, where the nature of the coke is key.The characterization of deactivated catalyst samples (amount and morphology of coke deposits, sections 3.2.1, 3.2.4 and 3.2.5, as well as physical, metallic and textural properties, section 3.2.2 and 3.2.3) has allowed establishing the deactivation causes of the NiAl2O4 spinel derived catalyst in the SR of the different oxygenates at 600 and 700\u00a0\u00b0C. Firstly, Ni oxidation has been ruled out as a deactivation cause, due to the absence of reduction peaks (TPR) or NiOx diffraction peaks (XRD measurements) in all the spent catalysts, which is coherent with the highly reducing atmosphere along the SR reactions, and is in agreement with the results reported for the SR of raw bio-oil [31]. Secondly, Ni sintering does not appear to be a relevant cause of deactivation of this catalyst, since it is not observed a significant increase in the average Ni0 crystal size, except for SR of levoglucosan, where a slight deactivation is observed (Fig. 2c). Nevertheless, a similar moderate Ni sintering has been reported in the SR of raw bio-oil at 700\u00a0\u00b0C with this catalyst [31], which does not explain the rapid deactivation for this reaction. Consequently, the main cause of the rapid deactivation of the NiAl2O4 derived catalyst in the SR of bio-oil and of the guaiacol\u00a0+\u00a0ethanol mixture must be attributed to coke deposition.By relating the results of TPO analysis and SEM images of the deactivated samples (sections 3.2.1 and 3.2.4) to the deactivation rate of the catalysts, it has become clear that deactivation is directly related to the nature of the coke, in agreement with previous results in literature for different catalysts [38,45\u201349]\n. Thus, a large amount of filamentous coke is deposited in the SR of most of the pure oxygenates studied (especially at 600\u00a0\u00b0C), but it does not cause a significant impact on the activity of the catalyst. The increase in SBET (Table 1) and the BSD-SEM images (Figs. 7 and 8) for the catalyst used in the SR of oxygenates (such as acetic acid, acetaldehyde, ethanol, acetol and acetone) evidences the deposition of a porous and filamentous coke with contents in the catalyst above 20\u00a0wt%, but that does not hinder the access of reactants to metal sites in the reaction time studied. However, for a high time on stream, it can create a slight plug on pores or it may grow as clumps of entangled filaments that encapsulate metal particles [47]\n, which can originate a decrease in activity as that observed at high reaction time in the SR of ethanol (Fig. 1c).The low values of SBET in the catalyst used in SR of guaiacol\u00a0+\u00a0ethanol (only slightly above that of fresh catalyst) is explained by the formation of both i) filamentous coke that is probably stacked on the surface of the catalyst and causes an increase in BET surface area (with high contribution of ethanol to the formation of this type of coke), and ii) an amorphous carbon phase in between the carbon filaments, probably due to the formation of pyrolytic carbon from guaiacol, which is promoted at high temperature, and that clogs the porous structure and contributes to the rapid deactivation observed for the mixture (guaiacol\u00a0+\u00a0ethanol) (Fig. 2b). This formation of pyrolytic coke by repolymerization of phenols in bio-oil is well established in the literature [62].According to the literature, the importance of the properties of the catalyst in the nature of the coke should be pointed out. Thus, Zhang et al. [48], observed the prevalent formation of amorphous coke from guaiacol on Ni/Al2O3 catalyst, whereas carbon nanotubes are preferentially formed on Ni/SBA-15 catalyst.Due to the filamentous nature of coke, the catalyst stability is high in the SR of non-phenolic oxygenates, (Fig. 1), as well as in the SR of catechol (Fig. 2a) and levoglucosan (Fig. 2c). Conversely, the catalyst undergoes complete deactivation after 300\u00a0min reaction in the SR of the guaiacol\u00a0+\u00a0ethanol mixture at both temperatures studied. Taking into account the high stability observed in the SR of ethanol (Fig. 1c), it can be concluded that guaiacol is the responsible of the rapid catalyst deactivation observed in Fig. 2b.The origin of long and heterogeneous carbon filaments in the SR of aliphatic oxygenates (Figs. 9 and 10) can be attributed to the reaction of CO (Boudouard reaction, Eq. 9) and CH4 decomposition (Eq. 8) [51,63,64]. As CH4 decomposition is favoured above 750\u00a0\u00b0C, in the conditions of this study the main origin of this coke is probably the exothermic Boudouard reaction, whose extent is favoured at lower temperature. Moreover, in the SR of ethanol at 600\u00a0\u00b0C with the same catalyst, the contribution to the formation of filamentous coke by the route of dehydration to ethylene over the acid sites of the Al2O3 support followed by the ethylene decomposition on the Ni-Al2O3 interface has been proved [36]. Also, acetone is an important precursor of filamentous coke [14,65], which can explain the higher amount of coke deposited in the SR of acetone than in the SR of acetic acid.The formation of filamentous coke is also significant in the SR of the phenolic compounds, as revealed by SEM images (Fig. 9f, 9g, 10f and 10g), and the high combustion peak located at high temperature in the TPO profiles (Fig. 4). But differently to the SR of aliphatic oxygenates, the presence of a small coke fraction burning at low temperature (amorphous and encapsulating coke) is observed in the SR at 600\u00a0\u00b0C of catechol and more notoriously in the SR of the guaiacol\u00a0+\u00a0ethanol mixture (Fig. 4a). For this latter feed, the formation of this amorphous carbon phase could explain the lower amount of filamentous coke deposited at this temperature (Fig. 9g) compared to the SR of ethanol (Fig. 9c). Thus, the formation of encapsulating coke on metal sites hinders the mechanisms of filamentous coke formation, which requires diffusion of C species through Ni metal particles, their precipitation on the base of the Ni crystallite and the formation of a carbon filament growing in size [51,66]. This synergy in the mechanism of formation of each type of coke from each oxygenate makes it difficult to understand the mechanism of coke formation from a complex mixture such as raw bio-oil.The difference in the results of coke amount at 600 and 700\u00a0\u00b0C can be explained by the effect of temperature on the reactions involved in their formation (mainly Boudouard reaction (Eq. 9) and polymerization reactions) and their elimination (gasification reaction, Eq. (10)). Thus, the polymerization and gasification reactions are favored with the increase in temperature, whereas the Boudouard reaction is disfavored. Consequently, the increase in the reaction rate of gasification and the lower extent of Boudouard reaction explain the sharp reduction of coke amount on the catalyst observed in the SR of aliphatic oxygenates at 700\u00a0\u00b0C [67]. Nevertheless, in the SR of phenolic oxygenates the coke amount is higher at 700\u00a0\u00b0C, especially for the guaiacol\u00a0+\u00a0ethanol mixture, because guaiacol polymerization (with pyrolytic carbon formation, Fig. 10g) is favored to a greater extent than gasification. A similar result was previously reported for other heavy oxygenates like glucose and m-xylene [14,20].This effect of temperature on coke formation is very important in the reforming of levoglucosan, where the coke amount is 16.7\u00a0wt% at 600\u00a0\u00b0C and 0.5\u00a0wt% at 700\u00a0\u00b0C. Considering the ease of cracking of this oxygenate [68] it can be understood that the increase in the cracking rate favors the SR of the intermediates to a greater extent than their polymerization, which explains the low coke deposition. The extent of thermal cracking is different for each oxygenate in the bio-oil depending on its functionality, which affects the results of the raw bio-oil SR, and in particular deactivation. Moreover, increasing the temperature above 700\u00a0\u00b0C also favors the gasification of the coke retained in the catalyst, attenuating its development towards filamentous structures. However, as aforementioned, this strategy has the unfavorable effect of Ni sintering. It should be noted that this problem is minimized with the NiAl2O4 spinel derived catalyst, which recovers its spinel structure by controlled calcination, recovering the dispersion and size of the Ni0 crystals in successive reaction-reduction-regeneration cycles [30].In a previous study of the deactivation of the same NiAl2O4 spinel derived catalyst in the SR of raw bio-oil [31] was found that the coke is mainly constituted of short and heterogeneous filaments, representing much lower amounts than those formed in this work from aliphatic oxygenates, being remarkable the presence of amorphous and encapsulating coke. Based on the results of the present work, the formation of this coke may be attributed to the high content of guaiacols and catechols, and heavier phenolic compounds in raw bio-oil, whose polymerization significantly inhibits the mechanisms of formation of abundant and long carbon filaments on the catalyst surface from aliphatic oxygenates present in bio-oil.Consequently, the phenolic compounds have a relevant role in the deactivation of the NiAl2O4 derived catalyst during the SR of raw bio-oil. Nevertheless, the decrease in carbon conversion [31] is faster than that observed in the SR of the guaiacol\u00a0+\u00a0ethanol mixture (Fig. 2b), which evidences the significant contribution of other compounds in bio-oil, most probably heavier phenolic compounds, to the deactivation of the catalyst. Moreover, a synergistic effect of the presence of different compounds in bio-oil (with different functionalities) could also contribute to a more rapid deactivation in the SR of bio-oil than in the SR of each pure oxygenated compound. Consequently, in order to establish a mechanism that faithfully represents the reality of coke formation and catalyst deactivation in the SR of raw bio-oil, the study of pure oxygenated model compounds is not sufficient, but studies of co-feeding of binary mixtures and progressively more complex mixtures are required. However, based on the results of this work, it is advisable to separate the phenolic components from the raw-bio-oil to mitigate the deactivation by coke, although this implies a decrease in the H2 yield and the formation of a byproduct stream.The deactivation of the NiAl2O4 spinel derived catalyst in the SR of oxygenates at 600\u2013700\u00a0\u00b0C is a consequence of coke deposition, whose effect on the deactivation rate highly depends on the oxygenates nature, which determines the coke nature and its deactivation ability. Thus, the formation of filamentous coke from the aliphatic oxygenates by the Boudouard reaction has a reduced deactivation effect, because it does not blocks the porous structure of Al2O3. However, the formation of amorphous and Ni-encapsulating coke in the SR of guaiacol leads to a rapid deactivation of the catalyst. The increase in temperature from 600 to 700\u00a0\u00b0C has low impact on deactivation because it favors the extent of encapsulating coke formation reactions by polymerization but attenuates the formation of filamentous coke by promoting its gasification.Presumably, in the SR of raw bio-oil a synergy between the mechanisms of coke formation from the different oxygenates present is to be expected. But according to the results of this work, the formation of encapsulating coke from phenolic oxygenates is preferential and inhibits the formation of filamentous coke from aliphatic oxygenates. Consequently, the results of this work can be applied to: i) use guaiacol as oxygenate model to test the stability of new catalysts and adapt the reaction conditions in order to minimizing deactivating coke, and ii) design of pretreatment methods of bio-oil in order to eliminate the guaiacol and phenolic components in order to minimizing the formation of this coke.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work has been carried out with the financial support of the Ministry of Science and Innovation of the Spanish Government (grant RTI2018-100771-B-I00 funded by MCIN/AEI/10.13039/501100011033 and by \u201cERDF A way of making Europe\u201d), the European Commission (HORIZON H2020-MSCA RISE 2018. Contract No. 823745) and the Department of Education, Universities and Investigation of the Basque Government (Project IT1645-22, IT1218-19 and PhD grant PRE_2021_2_0147 for L. Landa). The authors thank for technical and human support provided by SGIker (UPV/EHU/ERDF, EU).Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2022.124009.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The catalyst stability, mainly affected by coke deposition, remains being a challenge for the development of a sustainable process for hydrogen production by steam reforming (SR) of bio-oil. In this work, the influence of oxygenates composition in bio-oil on the deactivation by coke of a NiAl2O4 spinel derived catalyst has been approached by studying the SR of a wide range of model oxygenates with different functionalities, including acetic acid, acetone, ethanol, acetaldehyde, acetol, catechol, guaiacol and levoglucosan. A fluidized bed reactor was used in the following conditions: 600 and 700\u00a0\u00b0C; steam/carbon ratio, 3 (6 for levoglucosan); space\u2013time, 0.034 gcatalyst\n                     h/gbio-oil (low enough to favor the rapid catalyst deactivation), and; time on stream, 5\u00a0h. The spent catalysts were analyzed with several techniques, including Temperature Programed Oxidation (TPO), X-ray Diffraction (XRD), N2 adsorption\u2013desorption, Scanning and Transmission Electron Microscopy (SEM, TEM) and Raman Spectroscopy. The main factors affecting the catalyst stability are the morphology, structure and location of coke, rather than its content, that depend on the nature of the oxygenate feed. The deposition of pyrolytic and amorphous coke that blocks the Ni sites inhibiting the formation of filamentous carbon causes a rapid deactivation in the guaiacol SR. Conversely, the large amount of carbon nanotubes (CNTs) giving rise to a filamentous coke deposited in the SR of aliphatic oxygenates only causes a slight deactivation. The increase in the temperature significantly reduces coke deposition, but has low impact on deactivation.\n               "}
{"full_text": "Hydrogen is potential as a fuel source has touted for decades, but the technology has never gotten off the ground on a sizeable scale. This is due to complexities with its safety, efficient storage, and very crucial concerns regarding the cost of infrastructure for H2 delivery. This novel finding may have found the solutions to both these encounters. Ammonia as a clean and invulnerable energy carrier comprising H2, offers a potential solution to the complications of storage and expense for usage to produce on-demand in situ hydrogen (David et al., 2014, Yao et al., 2020). As a global commodity, million tons of ammonia produce per annum majorly from the Haber\u2013Bosch process. Moreover NH3 is stable and non-explosive, thus secure to transport, handle, and store (Grinberg et al., 2015). Ammonia can be easily liquefied under moderate circumstances (\u221233.4\u00a0\u00b0C at atmospheric pressure or 8.46 bars at 20\u00a0\u00b0C) (Grinberg et al., 2016).Last several years, several metals like as Cobalt (Podila et al., 2015, Podila et al., 2016, Zhang et al., 2013, Zhang et al., 2014a, Podila et al., 2017), Iron (Ohtsuka et al., 2004), Nickel (Hu et al., 2018, Zhang et al., 2014b, Kurto\u011flu et al., 2018), Platinum (Wu et al., 2019, Vajo et al., 1985), Rhodium (Leewis et al., 2006) and Ruthenium (Yin et al., 2004a,b,c, Duan et al., 2010, Huang et al., 2019, Ju et al., 2019) have been examined for ammonia decomposition using various supports and Ru is detected as the best metal catalyst. However, overpriced and unavailability of ruthenium discouraged for its practical application in fuel cells. Thus, there is an inspiration to hunt for economical substitutes such as transition metal oxides. Nickel and Cobalt based catalysts showed better activity for ammonia decomposition.Various materials as support have been examined for H2 production from NH3 such as MgO, SiO2, Al2O3, TiO2, ZrO2, Carbon (activated), porous (meso and micro) materials, multi walled (MWCNTs), etc. (Podila et al., 2015, Zhang et al., 2014b, Yin et al., 2004a,b,c, Ju et al., 2017, Wang et al., 2021, Bell et al., 2020, Gu et al., 2021, Lorenzut et al., 2010). For all tested supports, the multi-walled carbon nano tubes supported metal system has noticed to be the most active, and this is due to high surface area and electronic conductivity. High cost, methanation are the difficulties for the usage of carbon nanotubes as a support material in this reaction. On other hand, studies published that support basicity is essential to a productive catalyst for the NH3 decomposition reaction. The oxides of lanthanum and cerium are favourable for ammonia decomposition reaction and improve catalyst activity because of their basicity (Huang et al., 2019, Lucentini et al., 2019, Lucentini et al., 2021, Huang et al., 2020).The perovskite-type oxides (ABO3) are fascinating materials because they display high ion conductivity, high electron conductivity, and fabulous chemical stability over a wide range of temperature, which has used as catalyst for many catalytic reactions (Amini et al., 2019, Xiaolong et al., 2010, Kajita et al., 2002). Perovskite-like oxides can be edited to create a broad range of catalysts by changing either the A-site and/or the B-site cations with other metal cations, creating solid solutions that favors modification of the physicochemical properties (Sarshar et al., 2011). The La2O3 and CeO2 as support or as promoters has been studied for Ni and Co catalyst for ammonia decomposition. It was found that these oxides enhance catalyst intrinsic activity, and they are active for ammonia decomposition even at high metal loading in these catalysts (Lucentini et al., 2021, Y. Yu et al., 2020, Zhang et al., 2005a).On this basis, it can see those compounds with perovskite structure ABO3 (A\u00a0=\u00a0La, B\u00a0=\u00a0Ni/Co) shows interesting behavior towards ammonia decomposition reaction. Accordingly, to gain improved activity, we have sought to extend our studies by introducing the Ce along with La in-catalyst preparation. To the extent of our knowledge in literature, only one report is available for ammonia decomposition reaction using perovskite type material (Pinz\u00f3n et al., 2021). Moreover the authors used much diluted ammonia as reactant (5\u00a0%NH3 in Ar).Therefore, our ambition in this study is to synthesize ABO3 (A\u00a0=\u00a0La, Ce B\u00a0=\u00a0Ni/Co) type perovskite materials for ammonia decomposition reaction by combustion synthesis method. The Citric Acid (CitA) used as organic molecule as fuel. The NH3 decomposition activity will study over these perovskite type materials for environmentally friendly hydrogen production.The ABO3 type perovskite material synthesized using combustion synthesis method. The citric acid (CitA) used as organic fuel and the molar ratio between metal nitrate and CitA maintained as 1:1. The chemicals Lanthanum Nitrate (Merk 99%), Nickel Nitrate (Fluka), Cobalt Nitrate (Aldrich), Cerium Nitrate (Aldrich) and Citric Acid (Aldrich) were utilized in catalyst preparation. To prepare LaNiO3 the required molar amount of lanthanum nitrate and nickle nitrate dissolved in 50\u00a0mL of deionized water. To obtain a molar ratio of metal nitrate and CitA as 1, dissolved a required amount of CitA in 50\u00a0mL of de-ionized water and mixed to the metal nitrate solution. The primary solution (metal nitrate\u00a0+\u00a0CitA) was heated at 80\u00a0\u00b0C temperature to evaporate water until the solution become viscous gel. Next, the viscous gel heated at 90\u00a0\u00b0C for 24\u00a0h in a water bath. Then the gel dried at 150\u00a0\u00b0C for 24\u00a0h in an oven. The resultant swelled solid ground to powder and executed heat treatment at 650\u00a0\u00b0C for 6\u00a0h in a muffle furnace. The prepared perovskite material denoted as LaNi. The same procedure followed for LaCoO3, La0.5Ce0.5NiO3 and La0.5Ce0.5CoO3 perovskite type materials. These samples specified as LaCo, LaCeNi and LaCeCo respectively.The surface area results and pore-size distribution analysis carried out using Nova Station Quanta chrome device with N2 as the sorbate at \u2212196\u00a0\u00b0C. The samples out-gassed under vacuum for two hours at 200\u00a0\u00b0C before analyzing sample. The multi point BET method and DFT method was used for surface area and pore size distribution measurements.X-ray diffraction studies performed using devise from Inel. The fine powder of calcined catalysts loaded in holder and patterns recorded using Co K\u03b1 (\u03bb\u00a0=\u00a01.78\u00a0\u00c5) radiation. The phases were identified using PDF database.The micromeritics Auto Chem HP 2950 instrument employed to perform H2 temperature -programmed reduction (H2-TPR) tests. For this a 150\u00a0mg of samples was placed into the instrument reactor cell and the catalyst was reduced using 10% H2 in argon with 50\u00a0mL/min flow. The catalyst temperature increased at 10\u00a0\u00b0C\u00b7min\u22121 from 50 to 800\u00a0\u00b0C. The outlet flow examined with a detector of thermal conductivity (TCD).Sample basicity was studied by CO2 temperature-programmed desorption (CO2-TPD). The analysis performed on micromeritics Auto Chem HP 2950 instrument with TCD detector. Prior to analysis, each sample treated with hydrogen flow at 550 \u00b0C for two hours. Then He gas used to flush the sample for 1\u00a0h and cool down to 40\u00a0\u00b0C in a He gas flow. After cooling the sample was exposed to CO2 gas for one hour at 40\u00a0\u00b0C. Next, the sample replaced with helium gas for one hour. Finally the sample was heated progressively up to 700\u00a0\u00b0C with a rate 10\u00a0\u00b0C\u00b7min\u22121.The morphology of prepared samples was studied using of Field Emission Scanning Electron Microscope (FEI Quanta FEG450) unit with an Everhart Thornley detector (ETD) and a back scattering electron detector (VCD). The elemental mapping and compositions of the catalysts measured by EDS.The catalysts performance tests conducted in a fixed-bed reactor at normal pressure. Prior to the test, a 0.1\u00a0g sample reduced in N2 and H2 mixture (1:1) gas flow for five hours at 550\u00a0\u00b0C. After the reduction step, the catalysts exposed to N2 gas for 1\u00a0h at 550\u00a0\u00b0C and cool to 300\u00a0\u00b0C. 100% NH3 gas fed into the reactor with 6000\u00a0h\u22121 GHSV. The tests operated at 300\u2013600\u00a0\u00b0C temperature range. The temperature increased via a sequential increments with 50\u00a0\u00b0C intervals. At each temperature, the reaction was executed till to reach steady state. Reaction products were analyzed online by using gas chromatography unit with TCD and a Poropak Q column. The stability of catalyst tested by conducting the time on study test at 550\u00a0\u00b0C for 50\u00a0h and the products were analyzed continuously.The physical properties of the all-calcined catalysts obtained from the corresponding nitrogen adsorption\u2013desorption isotherms (Fig. 1\nA). It is clear from the Fig. 1A that all catalysts showed isotherms of type IV with H3 type hysteresis. The results specify existing aggregates of plate like particles with slit shaped pores. The specific surface area, pore volume and pore width of all prepared perovskite type catalysts are displayed in Table 1\n. It can see that the LaNiO3 catalyst showed higher surface area than the LaCoO3 oxide. LaNi and LaCo catalyst showed similar average pore width, but the pore volume is high in case LaNi catalyst. After the substitution of 50% of cerium in place of La the surface areas of both Ni and Co catalyst increased. However, the LaCeNi catalyst showed two times higher surface area than the surface area of LaCeCo catalyst. C.A. Franchini et al. (2014) studied Ce substitution in LaNiO3 mixed oxide with different Ce loadings for glycerol steam reforming. According to this report the replacement of La3+ (1.17\u00a0\u00c5) with Ce4+ (0.97\u00a0\u00c5) the cerium oxide lattice remarkably affects and increases mean particle diameter. On other hand, many studies reported that segregated CeO2 will form along with perovskite with the substitution of 50% of Ce in place of La (Soongprasit et al., 2012, Su et al., 2014, Pecchi et al., 2008, Kirchnerova et al., 2002). This segregated CeO2 may help in increase of surface area of catalyst. Fig. 1B presents the pore-size distribution of calcined catalysts. From Fig. 1B it clear that the pore-width of Ce substituted catalyst increased also increase pore volume. The catalysts prepared in the present work showed highest surface areas compared to the surface areas of catalysts reported in literature especially in case of Ni catalysts with and without Ce substitution (Amini et al., 2019, Franchini et al., 2014, Soongprasit et al., 2012, Pecchi et al., 2008, Pereniguez et al., 2012).The powder Xrd patterns of all calcined samples are presented Fig. 2\n. The diffraction patterns of LaCo and LaNi catalysts in Fig. 2 related to characteristics peaks for perovskite type oxide with rhombohedral phase. The Xrd results showed LaCo and LaNi catalysts contain single phases LaCoO3 [PDF: 01-086-1665] and LaNiO3 [PDF: 00-012-0751] respectively. The LaCo catalyst showed more crystallinity than the LaNi catalyst (from the signal intensities in Fig. 2). Consequently, decreased in surface area will detect in LaCo catalyst than the LaNi catalyst. These results are in good resemblance with surface area results in Table 1 where we detected lower surface area for LaCo catalyst than the LaNi catalyst. After substituting cerium new phases are detected in Co and Ni catalysts. The LaCeCo catalyst showed formation of CeO1.66 [PDF: 01-089-8430], Co3O4 [PDF: 01-074-2120] along with LaCoO3 perovskite oxide phase. Similarly, the LaCeNi catalyst showed formation of CeO1.66 [PDF: 01-089-8430], NiO [PDF: 00-044-1159] along with LaNiO3 perovskite oxide phase.The TPR profiles of LaCo, LaNi, LaCeCo and LaCeNi samples are shown in Fig. 3\n. The LaCo catalyst displayed three major peaks. The two overlapped peaks between 200 and 500\u00a0\u00b0C related to the reduction of surface adsorbed oxygen on the catalyst and reduction of Co3+ to Co2+. Next the peak between 500 and 700\u00a0\u00b0C could be conferred to the reduction of Co2+ to Co0 (Pere\u00f1\u00edguez and Ferri, 2018, Zhao et al., 2017). The hydrogen consumption profile of LaNiO3 showed three peaks. The first two unresolved reduction peaks in 300\u2013400\u00a0\u00b0C corresponds to the partial reduction of perovskite network leading to the formation of the intermediate oxygen deficient La2NiO4 perovskite phase. The third peak registered at 477\u00a0\u00b0C matches to reduction of La2NiO4 to Ni0 (Franchini et al., 2014, Batiot-Dupeyrat et al., 2003).The 50% substitution of La by Ce in perovskites brings some modification in redox behavior. In general after addition of Ce, reducibility of active site is favored due to the lattice oxygen mobility toward oxygen deficient areas (Soongprasit et al., 2012). The H2-TPR profiles of the LaCeCo showed three major signals. The combined low temperature peaks having peak maxima at 313 and 367\u00a0\u00b0C connected to reduction of Co3+ to Co2+ and reduction of segregated CeO2. The high temperature peak with maxima at 510\u00a0\u00b0C is corresponds to the reduction of Co2+ to Co0 species (Sarshar et al., 2011). The TPR peaks of the LaCeNi catalyst are shifted to lower temperatures (Fig. 3d) indicating easier reduction of Ni species upon substitution of Ce. The half substitution of cerium in place of lanthanum in LaNiO3 leads to an increase of the first peaks with respect to the third one. This result can be ascribed to the existence of isolated NiO and CeO2. The total H2 consumption results of all catalysts for TPR are displayed in Table 1. The hydrogen consumption increased for LaCeNi catalyst compared to that of LaNi catalyst. On other hand, the hydrogen consumption decreased for LaCeCo catalyst in comparison to that of LaCo catalyst. However the reducibility shifted towards low temperature peak in LaCeCo catalyst than in LaCo catalyst (Fig. 3). The results emphasize that the addition of Ce encourages metal reducibility at lower temperature in both Co and Ni systems. In addition substitution of Ce enhances metal reduction in case of Ni system (Table 1) (Amini et al., 2019, Lima et al., 2006, Gallego et al., 2009).The basicity of catalyst is crucial for ammonia decomposition reaction (Yin et al., 2004a,b,c). Basicity strength and basic site distribution of catalysts were studied using carbon dioxide temperature programmed desorption study. The CO2-TPD patterns are displayed in Fig. 4. All catalysts showed desorption peaks at two temperature regions. The first desorption peak observed below 100\u00a0\u00b0C and the second desorption peak observed in 200\u2013500\u00a0\u00b0C region. As per the literature reports, the low temperature desorption peak below 100\u00a0\u00b0C corresponds to weakly adsorbed CO2 on catalyst surface and the desorption peak registered at 200\u2013550\u00a0\u00b0C related to moderate basic sites. The quantification results of CO2 desorption in 200\u2013550\u00a0\u00b0C range presented in Table 1. The results clearly refer to that the LaNi catalyst is more basic than the LaCo catalyst. After Ce substitution in La based perovskite the moderate basic sites increased. The LaCeCo catalyst displayed highest basicity among all prepared catalysts.The morphologies of all prepared samples studied by scanning electron microscopy which are presented in Fig. 5\n\n. The morphology of LaNi catalyst in Fig. 5a exhibited flake-like structure. At higher magnification the image Fig. 5e clearly showed that the flake-like structure is with interconnected porous in nature. The LaCo catalyst showed an open structure which contain nano-sized interconnected spherical particles (Fig. 5b & f). After substitution of Ce the morphology of catalyst changed. The LaCeNi and LaCeCo catalysts showed morphology of irregular agglomerated particles with porous nature. It is observed from Fig. 5c and Fig. 5d that the agglomeration seems more in LaCeCo catalyst than in LaCeNi catalyst.For further investigation of metal homogeneity in perovskite-type catalyst elemental mapping performed in several areas. The representative mapped regions are presented in supplementary information. The elemental mapping image of LaNi and LaCo catalysts in Fig.S1 and Fig.S3 clearly showed the distribution of metal is homogeneous. The EDS analysis of LaNi and LaCo catalysts are presented in Fig.S2 and Fig.S4. The quantification results showed 47 and 53 atomic percentage of La and Ni for LaNi catalyst and 48 and 52% of La and Co for LaCo catalyst respectively. The elemental mapping image of LaCeNi and LaCeCo catalysts are presented in Fig. 6\n. The LaCeNi catalyst showed homogeneous metal distribution from metal mapping image displayed Fig. 6A. The selected area EDS spectra of LaCeNi catalyst presented in Fig.S5. The quantitative results showed presence of 28, 28 and 44% of La, Ce and Ni in LaCeNi catalyst. The elemental mapping image of LaCeCo catalyst showed in Fig. 6B. An agglomeration of cobalt clearly observed from the Fig. 6B. On other hand, the La and Ce metals distributed homogenously. The EDS results displayed in Fig.S6 and observed 29, 28 and 43% of La, Ce and Co in selected region.The results of catalytic performance for ammonia decomposition reaction over perovskite type catalysts are presented in Fig. 7\n. The activity experiments performed from 300 to 600\u00a0\u00b0C. As the NH3 decomposition reaction endothermic, all prepared samples showed increased activity with increased temperature (Podila et al., 2016, Bell et al., 2020). The results in Fig. 7 clearly showed that the LaNi and LaCo catalyst are active for hydrogen production from NH3 decomposition. As reported by literature the nitrogen desorpton is the rate limiting step for ammonia decomposition. Thus metal nitrogen binding energy is a crucial parameter in the design of good NH3 decomposition catalyst (Bell and Torrente-Murciano, 2016, Yin et al., 2004a,b,c). From the literature reports after Ru the nitrogen binding-energy of Co-based catalysts is nearest to the ideal value (Huang et al., 2020, Torrente-Murciano et al., 2017). However, the ammonia decomposition activity also depends on catalytic support and metal support interaction. In this present study, the La based Ni perovskite type catalyst showed higher activity than La based Co perovskite catalyst. This is due to metal support interaction. Zhang etal reported that La and Ce promoter influence the physico chemical properties of Ni catalyst and causes for increased activity for ammonia decomposition (Zhang et al., 2005b, Zhang et al., 2015, Deng et al., 2012). After the substitution of Ce in La based Ni/Co perovskite derived catalyst the activity rasied substantially. Exclusively LaCeNi catalyst showed 99% ammonia conversion at 550\u00a0\u00b0C.The Fig. 8\n shows the activation energy (Ea) values calculated from Arrhenius graphs (ln(k) vs. 1/T) using the synthesized perovskite derived catalysts for NH3 decomposition from 400 to 550\u00a0\u00b0C at 6000\u00a0h\u22121 space velocity. The catalysts LaCo, LaNi, LaCeCo and LaCeNi displayed a clear activation \u00e9nergies 55.7,43.0, 48.4 and 35.9\u00a0kJ\u00a0mol\u22121 respectively. A comparison of Hydrogen formation rate (H2 mmol/gcat/min) and apperent activation energy (Ea) values of perovskite-derived catalysts from this work and other Ni/Co catalysts reported in literature displayed in Table 2\n. From the literature, it is clear that many reported Ni/Co systems showed less hydrogen production rate compared to that of perovskite-derived catalysts in this work. There are few reports showed good hydrogen production rate but the catalyst preparation of these systems are difficult for large-scale application (Hu et al., 2019). Recently Pinzon et al reported lanthanum based perovskites for ammonia decomposition but with 5\u00a0%NH3 in Ar reactant (Pinz\u00f3n et al., 2021). It is clear from Table 2 that the catalysts LaCo and LaNi showed good hydrogen production rate and after Ce substitution i.e., LaCeCo and LaCeNi catalysts the hydrogen production rate increased.The activation energy values of corresponding perovskite-derived catalysts are far lower than the Ea values of Ni/Co-based catalysts for decomposition of NH3 such as 40\u00a0%Ni/MgLa [54 KJ mol\u22121] (Y. Yu et al., 2020), 20\u00a0%Ni/LaMg [181 KJ mol\u22121](Hu et al., 2019), 10% Ni/Al2O3 [53.9 KJ mol\u22121](Y. Yu et al., 2020), 4\u00a0%Ni/Al2O3 [99.5 KJ mol\u22121](Deng et al., 2012), 4\u00a0%Ni/CeO2 [71.0 KJ mol\u22121](Deng et al., 2012), 5\u00a0%Co/MgLa [67 KJ mol\u22121](Podila et al., 2016), 20\u00a0%Co/LaMgO [167 KJ mol\u22121](Hu et al., 2019), 5\u00a0%Co/CNTs [92 KJ mol\u22121](Pei Yu et al., 2020), 90\u00a0%Co/Al [123\u00a0kJ\u00a0mol\u22121] (Gu et al., 2015) and LiNiO3 [108\u00a0kJ\u00a0mol\u22121] (Pinz\u00f3n et al., 2021) etc. The La based Ni catalyst showed lower activation energy in comparison to that of Co catalyst. The replacement of 50% of La with Ce induced significant rise in activity which results decrease in activation energy. These results are obvious that the addition of cerium will enhance the catalytic ability for NH3 decomposition reaction. Among all synthesized catalysts, the LaCeNi catalyst exhibited lowest activation energy. Liu et al. (2016) reported that the La2O3 increases weak and medium strength basicity also it improves the reducibility of metal especially in case of Ni species. The tpr results in Fig. 3 clearly showed the reducibility more favorable in La-based Ni than Co system. Thus, the La based Ni perovskite catalyst showed higher activity than the La based Co perovskite catalyst. After the 50% substitution of La by Ce the medium strength basicity and metal reducibility boosted (Figs. 3 and 4). Thus, the activity increased in comparison to that of without cerium. On other hand the surface area of catalyst increased significantly after the substitution of cerium. Hence the La-Ce based Ni/Co perovskite catalysts showed increased activity. The agglomeration of cobalt in LaCeCo catalyst (from Fig. 6B) is the reason for lower activity than LaCeNi catalyst. The time on study test performed using LaCeNi catalyst at 550\u00a0\u00b0C for 50\u00a0h and the results displayed in Fig. 9\n. It is noteworthy that the LaCeNi catalyst showed extremely stable performance in terms of NH3 conversion for 50\u00a0h at 550\u00a0\u00b0C.The La based Ni/Co perovskite catalysts were prepared and used as catalyst precursor for NH3 decomposition. The LaNi sample showed very attractive activity at lower temperature compared to that of LaCo catalyst. The 50% substitution of La by Ce has a substantial effect on the catalytic activity. The NH3 conversion raised extensively both in Ni and Co catalysts. The combination of La, Ce and Ni i.e. LaCeNi catalyst displayed the best performance out of all the synthesized catalysts. The higher catalytic activity of the LaCeNi sample is due to raised surface area, easily reducible and appropriate basicity. The uniform inter-distribution of metal-oxide components confirms good dispersion of nickel species as a result is the achieving extremely stable catalyst.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The Project was funded by Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, under the grant No. G: 779-135-1441. The authors, therefore, acknowledge with thanks DSR for technical and financial support.Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2021.103547.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The La based perovskite type LaMO3 (M\u00a0=\u00a0Ni, Co) oxides were prepared by combustion synthesis method using citric acid as organic fuel. These catalyst precursors tested for ammonia decomposition. The LaNiO3 and LaCoO3 catalysts showed good activity for NH3 decomposition. The LaNiO3 catalyst displayed greater activity than LaCoO3. This due to high surface area and easily reducibility of Ni species. A 50% of La was substituted by Ce in both LaNiO3 and LaCoO3 catalysts. A remarkable effect on catalytic performance was observed with the partial substitution of La by Ce in perovskite catalyst especially at lower temperatures. The La0.5Ce0.5NiO3 catalyst exhibited highest activity among all prepared samples. The achieved superior activity is due to boost in surface area, reducibility and suitable basicity. The SEM elemental mapping of La0.5Ce0.5NiO3 catalyst concluded that metal oxide constituents dispersed homogeneously. The La0.5Ce0.5NiO3 catalyst showed excellent stable catalytic performance during 50\u00a0h time on study at 550\u00a0\u00b0C.\n               "}
{"full_text": "Data will be made available on request.Due to the dependence on the non-renewable energy sources, there is a serious increase in environmental problems. With increasing environmental problems and decreasing fossil fuels, it is essential to seek alternative energy sources. In this context, hydrogen, which is a sustainable, low-cost, environmentally friendly, and highly efficient energy source, stands out as a green energy carrier [1\u20137].In order to use hydrogen actively today, it must be produced, stored, and transported efficiently. At this point, promising improvements have been experienced with the use of chemical hydrides such as NaBH4, LiBH4, NH3BH3, LiH, NaH, CaH2, MgH2, and AlH3. Among these hydrides, NaBH4 comes forward as a hydrogen storage material due to its advantages such as high hydrogen capacity (10.8%), recyclability of byproducts, non-flammability, chemical stability, and sustainability [2,8,9]. The release of stored NaBH4 occurs by hydrolysis and methanolysis reactions [10\u201313].\n\n(1)\n\nNa\n\nBH\n4\n\n+\n2\n\nH\n2\n\nO\n\n\n\u2192\ncatalyst\n\n\nNa\n\nBO\n2\n\n+\n4\n\nH\n2\n\n\u2191\n+\nheat\n\n\n\n\u2212\n300\n\nkj\n/\nmol\n\n\n\n\n\n\n\n\n(2)\n\nNa\n\nBH\n4\n\n+\n4\n\nCH\n3\n\nOH\n\u2192\nNaB\n\n\n\nOCH\n3\n\n\n4\n\n+\n4\n\nH\n2\n\n\n\n\n\nHydrolysis of NaBH4 for hydrogen production is quite cheap and simple, but the reaction temperature can limit this application. For this reason, methanol is frequently used as an alternative method for hydrogen release by a methanolysis reaction due to its suitable reaction rate and high activity at low temperatures (\u2264 0\u00a0\u00b0C) [14\u201316].In NaBH4 hydrolysis and methanolysis reactions, the lack of a catalyst slows down the reaction to a great extent. Thus, researchers focused on new types of catalysts to accelerate the reaction. These catalysts were homogeneous catalysts containing acid or metal complexes and heterogeneous catalysts containing metal or non-metal catalysts [17,26,27]. Although both catalyst types accelerate the reaction as reported in these studies, heterogeneous catalysts have become more advantageous due to the fact that homogeneous catalysts are not deactivated; they are irreversible, and their industrial usage are limited [18\u201325]. In addition to all these, catalysts such as carbon nanotubes, carbon spheres and particles, etc. which do not contain metal and are generally carbon-based, are frequently preferred today due to their environmentally friendly, low cost and stable catalytic activity nature [28\u201332].Carbon spheres are attracting a lot of attention due to their smooth surfaces, unique structures, and potential applications (such as in drug release, hydrogen storage, catalyst supports and lithium batteries). Researchers have developed different methods for the preparation of carbon spheres, such as hydrothermal synthesis, chemical vapor deposition, and carbonization pathways [33\u201335].However, alternative sources are needed for the preparation of carbon spheres. Among them, starch is one of the most abundant natural biopolymers in nature. It contains two different polysaccharides, namely linear (1, 4)-linked \u03b1-D-glucan amylose and highly (1, 6)-branched \u03b1-D-glucan amylopectin. Additionally, it has advantages such as being available from plant sources, being biodegradable and being a cheap product. Starch is generally produced from raw materials such as wheat, corn, peas, and potatoes. Among them, potato starch is a very suitable material due to its spherical morphology and the preparation of micron-sized carbon spheres [36\u201341]. Starch is used in a wide range of areas such as food, paper, textile, and pharmaceutical industries. It has also been used as a template for synthesizing alloy and metal nanoparticles in hydrogen production with NaBH4 [42,43].There are many carbon based catalysts in the literature such as CNTs, graphene, C60, carbon quantum dots, carbon fibers, activated carbon, carbon black, carbon nitride, boron carbon nitride, covalent organic frameworks, metal-organic frameworks etc. In this study, starch being one of the natural, inexpensive, accessible polysaccharide biopolymer was used as a carbon source for the production of carbon spheres via hydrothermal synthesis. Hydrothermal carbonization has received much attention as a promising large-scale application. The method is simple and inexpensive. Also modifications were made with simple mechanisms and increased the performance of the catalyst. Modified forms of carbon spheres used as the catalysts in hydrogen production by NaBH4 methanolysis was reported for the first time with this study. The synthesized catalysts were characterized by using Fourier-transform infrared spectrometer (FT-IR), thermogravimetric analyzer (TGA), zeta potentiometry (ZP), and scanning electron microscopy (SEM). The parameters affecting the methanolysis such as ambient temperature, amount and type of catalyst and NaBH4 concentration were investigated. Activation parameters such as activation energy (Ea), activation enthalpy (\u0394H#) and activation entropy (\u0394S#) and also hydrogen (H2) production rate were determined in the temperature range of 273\u2013303\u00a0K. In addition, catalysts reuses, and regeneration efficiencies were investigated.In the synthesis of carbon spheres (CSs), potato starch (Sigma-Aldrich) as a carbon source and potassium hydroxide (KOH,\u00a0\u2265\u00a085%, Sigma-Aldrich) for the preparation of alkaline conditions were used. Iron(II)chloride tetrahydrate (FeCl2.4H2O, Sigma-Aldrich) and ammonia (25%, Merck) were used for magnetic property. Ethylenediamine (EDA, 99.0%, Sigma-Aldrich), taurine (TA, 99.0%, Sigma-Aldrich), and poly(ethyleneimine) (PEI, 50% solution in water, Sigma-Aldrich) and epichlorohydrin (ECH, 99%, Sigma-Aldrich) were used as modifying agents, and dimethylformamide (DMF,\u00a0\u2265\u00a099.0%, Isolab) was used as reaction medium. Sodium hydroxide (NaOH, 98%, Sigma-Aldrich) was used for the deprotonation before modification. Hydrochloric acid (HCl, 36.5\u201338%, Sigma-Aldrich) was used for protonation of modified CSs. Sodium borohydride (NaBH4, 98%, Merck) was used as hydrogen carrier/source and methanol (\u2265 99.8%, Tekkim) was used as reaction medium in hydrogen gas production. Ethanol (\u2265 99.9%, Isolab) and distilled water (DW) were used for cleaning/purification throughout the experiments.Natural resources like starch were used as a carbon source when creating carbon spheres. Aqueous solutions of starch were prepared freshly before hydrothermal synthesis. Optimum condition for the spherical uniform CS synthesis were studied via altering all effective parameters. CS synthesis carried out in alkali condition. Starch (0.3\u00a0M) was put into KOH solution (10\u00a0mM, 70\u00a0mL) and then the mixture was immersed in oil bath pre-heating at 80\u00a0\u00b0C. After 30\u00a0min, the obtained solutions were put into Teflon autoclaves and placed in an oven at chosen temperature. To ensure the dehydration process, these prepared solutions/mixtures were maintained at the working temperature for the enough time. The autoclave was taken out of the oven after this time and quickly left to cool. The cooled autoclave was opened, and the solid portion was removed using centrifugation or decantation. The obtained product (CS) was cleaned by soaking in DW for 2\u00a0days. The washing water was renewed every 3\u20134\u00a0h to remove unreacted species. After the cleaning process, the CSs were dried in an oven at 50\u00a0\u00b0C for one day and taken in a closed tube.To decide mass production condition, effect of the same parameters as starch concentration (0.1\u20130.4\u00a0M), base (KOH) concentration (1.0\u2013100.0\u00a0mM), reaction temperature (120\u2013200\u00a0\u00b0C), and reaction time (6\u201324\u00a0h) was evaluated. After CS synthesis, second hydrothermal process was applied for the production of mag-CSs. For this, a method similar to the method described in the literature was followed [44]. For the preparation of mag-CS, 0.3125\u00a0g FeCl2.4H2O was dissolved in 10.0\u00a0mL water and stirred for 10\u201315\u00a0min; then 25% ammonia (1.6\u00a0mL) was added and stirring continued for another 10\u201315\u00a0min. Then, 0.625\u00a0g of CS was added to the mixture and mixed for another 5\u00a0min. The mixture was then transferred to the Teflon autoclave. The autoclave was placed in the oven at 180\u00a0\u00b0C for 3\u00a0h. At the end of the period, mag-CS was cleaned with a water-ethanol (1:1\u00a0v/v) mixture. Obtained mag-CS was dried in an oven or lyophilizer and then put the magnetic particles in the sealed vial for later use.CSs were modified using different sources having different functional groups used as catalysts in the methanolysis of NaBH4, as previously reported in the modification of Hal nanotube catalysts [9]. 4.0\u00a0g of CS was mixed with 0.3\u00a0M in 200\u00a0mL of NaOH (aq.) solution for 1\u00a0h. Then, the CS-containing solution was centrifuged three times at 4500\u00a0rpm for 10\u00a0min, followed by redispersion in distilled water to remove the residue NaOH after filtration. After base treatment, Na cation containing CS (CS-Na) was dried in a 50\u00a0\u00b0C oven for 1\u00a0day. The resulting solid (CS-Na) was used in other step to modification with EDA, TA, and PEI by using linking agent as ECH.Chemical modifications of CS with both EDA and PEI were carried out similarly to each other, with some changes in the literature [9]. The synthesis was briefly started by dispersing 4.0\u00a0g of EDA-Na in a 100\u00a0mL single-necked flask containing 55.0\u00a0mL of DMF. In order to get a good dispersion, the reaction flask was closed with a rubber septum and stirred at 600\u00a0rpm for about 15\u00a0min at RT. Then, the round bottom flask was dipped in an oil bath, and the temperature, which was set at 90\u00a0\u00b0C, was allowed to reach equilibrium for 5\u00a0min, and 3.0\u00a0mL of ECH added dropwise to the CS-Na dispersion with a syringe and mixed at 700\u00a0rpm for 1\u00a0h. Then, 3.0\u00a0mL of ethylene diamine mixed with 5.0\u00a0mL of DMF and dropped into the mixture through a syringe, with the reaction continuing for 20 more min after the final drop was injected. To get rid of contaminants like unreacted EDA, ECH, and NaCl, the EDA-CS was centrifuged at 4500\u00a0rpm, washed once with DMF, and then several times with an ethanol/water mix solution (1/1, v/v). The resulting EDA-CS was dried in an oven at 50\u00a0\u00b0C for 24\u00a0h.The modification of CS with PEI was also performed under identical circumstances as the EDA-CS modification procedure. The same quantity of CS-Na and ECH as used in the EDA-CS modification reaction was added to the DMF solution. Instead of EDA, 3.0\u00a0mL of PEI was added dropwise via syringe, and the reaction was continued for 1\u00a0h to establish linkages among CS and PEI with ECH to form PEI-CS. The washing and drying procedures were also performed using the same method as used in the EDA-CS approach described above. After dispersing 3.0\u00a0g of EDA-CS in 50.0\u00a0mL of acid solution (1.0\u00a0M HCl) for 1\u00a0h at room temperature with steady stirring at 400\u00a0rpm, the cleaned and dried modified CS was protonated with HCl. After that process, the protonated EDA-CSs were cleaned using excess water and continued with centrifugation several times at 4500\u00a0rpm before drying in an oven at 50\u00a0\u00b0C. The prefix \u201cH\u201d was then used to define samples as H-EDA-CS and H-PEI-CS. Both products were stored in closed tubes for the characterization and the catalysis reactions after drying.CSs were also modified with TA by using same technique given in Section 2.3.1. Briefly, 50.0\u00a0mL of DMF and 100\u00a0mL of reaction flask containing 1.0\u00a0g of CSs and 3.0\u00a0mL of ECH were added. The mixture was stirred at 90\u00a0\u00b0C for 1\u00a0h at 750\u00a0rpm. 3.0\u00a0mL of DW was used to dissolve 0.2\u00a0g of TA in a separate reaction vial. The mixture was stirred for an additional hour after the addition of the TA aqueous solution drop by drop. The elimination of unreacted ECH and TA was completed by washing TA-CS with an ethanol-water solution (1:1\u00a0v/v). The produced TA-CSs were put into closed vials for characterization and catalysis reaction after drying in an oven at 50\u00a0\u00b0C.Modified CS as called H-EDA-CS was used in the hydrogen production reaction as a catalyst via NaBH4 methanolysis. Catalyst performance was determined by measuring the amount of H2 gas released for the period of reaction using the water replacement method. The work was carried out in a 50.0\u00a0mL single neck reaction flask immersed in a temperature controlled 25\u00a0\u00b0C water bath. The reaction initiated by dispersing 50.0\u00a0mg of catalyst (H-EDA-CS) in 20.0\u00a0mL of methanol. Then 125\u00a0mM NaBH4 was added and the displacement of the released H2 gas with water at a stirring speed of 1000\u00a0rpm was continuously noted. Additionally, Hydrogen Generation Rate (HGR) were calculated changing some parameters such as NaBH4 concentration, type and amount of catalyst. To calculate activation parameters, the reaction was realized at different temperatures. Obtained results were given by comparing with each other and with the literature.Reuse of the H-EDA-CS catalyst was tested by using conversion% and catalytic activity%. The process started with dispersing 50.0\u00a0mg of H-EDA-CS catalyst in 20.0\u00a0mL of methanol and followed by the addition of 0.0965\u00a0g of NaBH4, and the reaction was continued until the hydrogen production finished. After the first hydrogen production finished, the same amount of NaBH4 was added to the reaction medium without adding a new amount of catalyst. This process was repeated 10 times in total. After 10 repeated uses, the catalyst washed tree times with an ethanol-water mixture (1:1\u00a0v/v), centrifuged at 4500\u00a0rpm for 10\u00a0min, and dried in an oven at 50\u00a0\u00b0C. Then the catalyst was regenerated according to the literature [9] by 30.0\u00a0mL HCl (1.0\u00a0M) for 0.5\u00a0h. After regeneration, the catalyst was reused in hydrogen production for three repetitive uses.To determine surface morphology of CS and its modified forms, a scanning electron microscope (SEM, Hitachi Regulus 8230) working at 1.0\u201310\u00a0kV was used. The sample was placed on carbon band-attached aluminum SEM stubs suspended with a drop of ethanol, after drying process the samples coated with several nanometers gold under vacuum, and the SEM image of the freeze-dried sample was determined. To characterize the magnetic properties of the mag-CS, Vibrating Sample Magnetometer (VSM, Lake Shore, 7407 model) was used. The FT-IR analyses of the CS-based catalyst and modified-CS were performed by using a Bruker Tensor FT-IR spectrophotometer in the 4000\u2013400\u00a0cm\u22121 spectral range. A thermogravimetric analyzer was used to complete the thermal analyses of CS-based catalysts (TGA, SII EXSTAR 6000, Japan). The same amount of starch, bare CS, and CS-based catalysts (5\u00a0mg) were put in a TGA pan, and the weight loss against temperature was recorded by increasing the temperature from 30 to 1000\u00a0\u00b0C with a heating rate of 15\u00a0\u00b0C\u00a0min\u22121 and a gas flow rate of 200\u00a0mL.min\u22121. Zeta potential measurements on CS-based catalyst before and after chemical modification were performed by dispersing the CS-based catalyst in 1\u00a0\u00d7\u00a010\u22123 M KCl aqueous solution at approximately 0.1\u00a0mg\u00a0mL\u22121 concentration at 25\u00a0\u00b0C using a zeta potential analyzer (Malvern Inst.).Starch was chosen because it is an environmentally benign, low-cost, and being biocompatible substance that may be employed as the catalyst after modification of the CS via different functional group containing agents. Hydrothermal synthesis was used to obtain CSs. The reaction conditions and SEM images of obtained CSs were given in the Table 1\n. To determine optimum conditions, the effect of the starch concentration, base concentration, reaction temperature, and reaction time was systematically studied. In reactions carried out at temperatures below 160\u00a0\u00b0C, the CS yield was either very low or no product could be obtained. Polydisperse spheres were obtained when the starch concentration was >0.3\u00a0M and the base concentration was <10\u00a0mM. The results were evaluated based on the yield and the homogeneity. Optimum conditions in CS preparation by hydrothermal synthesis were to keep the mixture which was prepared with 0.3\u00a0M starch and 10\u00a0mM KOH in an oven at 200\u00a0\u00b0C for 18\u00a0h. Thus, the CSs used in all of the studies were synthesized by the method given above.Magnetic CSs were prepared in two steps and optimized conditions were used for the first step. Subsequently, the obtained CSs were mixed with the production medium of Fe3O4 particles to obtain mag-CSs. Fe3O4 magnetic particles were freshly formed in the basic medium of CSs applying second time hydrothermal treatment. Obtained mag-CSs digital photo in aqueous media by applying magnetic field and VSM result was given in supplementary file as Fig. S1. The synthesized magnetic CS particles were dispersed in water. The magnet was located close to this vial, and it was observed that the mag-CS were rapidly oriented towards the magnet (Fig. S1a). This sensitivity will help to ensure that the application potential of the particles is high, and when used as a catalyst, it will help to slow down the reaction rate or to stop it. VSM result is given in the Fig. S1b. The results for mag-CS, Hci: 68.929\u00a0Oe, Ms.: 0.22891\u00a0emu, Mr.: 25.841E-3\u00a0emu, slope at Hc: 393.83E-6\u00a0emu/Oe were obtained.The SEM images of obtained CS were shown in Fig. 1a with different magnifications. It is apparent that the synthesized CS is in spherical shape, smooth and has a diameter smaller than 8\u00a0\u03bcm. New functional groups were also introduced on the CS by using different modifying agent. Epoxy groups are known to be reactive in alkaline conditions [9,45]. Therefore, the activation of CSs was achieved by using alkaline solution (0.3\u00a0M NaOH, RT, 1\u00a0h), thus increasing the density of \u2013O\u2212Na+ (deprotonation of \u2013OH) on the CS surface. Fig. 1b illustrates CS formation and its modifications. Accordingly, amine modification of CS in DMF occurred in two steps: The first step was the rapid reaction of hydroxyl groups forming CS-Na dispersion with ECH, and the second step was the addition of amine sources to the medium to form modified CSs.Additionally, some modified CSs were protonated by HCl treatment, and the samples were coded as using the prefix H-. FT-IR spectroscopy, TGA and ZP measurements were used to characterize the CS and its modified forms. FT-IR spectra of both CS and modified CS were shown in Fig. S2 (see Supplementary Information). In CS spectra, the broad band between 3600 and 3000\u00a0cm\u22121 correspond to hydroxyl group vibration. A peak at 2926\u00a0cm\u22121 can be attributed \u2212CH vibrations. The peak at 1208\u00a0cm\u22121 shows the C\u00a0\u2212\u00a0O stretching. In modified forms, the peaks/band at 3400\u20133200\u00a0cm\u22121 for NH stretching and 2930\u00a0cm\u22121 for -CH stretching from modified amine sources.TGA was used to determine the thermal stability of starch, bare CS, and modified CS. Fig. S3 shown the TGA thermogram. The thermogram showed that bare CS had high thermal stability and a primary decomposition temperature of between 350 and 550\u00a0\u00b0C. The residual mass at 900\u00a0\u00b0C was determined as 50% for bare CS. The degradation temperature of the modified CS and starch (precursor) was found to be 38 and 20% at 900\u00a0\u00b0C for H-EDA-CS and starch, respectively. Because of the presence of amine groups, main degradation occurred earlier (200\u2013450\u00a0\u00b0C) in modified CS.Zeta potential measurements were performed following CS changes in order to understand the catalytic performance of the synthesized catalyst and as a control modification. Table 2\n shows the recorded zeta potential values for all of the catalysts. The zeta potential values of the samples were determined by mixing the CS with 1.0\u00a0mM KCl at 25\u00a0\u00b0C.Due to the -OH group on the surface, CS had a negative ZP value (\u221263.4\u00a0mV), but after amine derivatization, the ZP values for PEI and EDA modification agents resulted with a change from negative to positive. Because of the presence of sulfonic acid, the ZP of TA modified CS is slightly higher when compared to bare CS. When the bare CS had \u221263.4\u00a0mV ZP value, after second step modification, protonation of EDA and PEI modified CS, ZP values increased too much as +45.5\u00a0mV and\u00a0+\u00a048.3\u00a0mV for H-PEI-CS and H-EDA-CS, respectively.In the current study, CSs obtained systematic modification using a variety of amine group-containing resources before being protonated with HCI. In the methanolysis of NaBH4, these amine modified CSs acted as a catalyst in the production of H2. From the H2 evolution curves, the HGR, measured in mL.min\u22121 or mL.min\u22121.g\u22121, was calculated. Based on the graphs of the H2 generation vs time, the rate of reaction was determined at the half point of the NaBH4 conversion slopes (r50).The effects of CSs modified with different amine groups on H2 production were determined by using them as catalysts in the NaBH4 dehydrogenation reaction in methanol. To clarify activity of the catalyst, in a parallel reaction settings control experiment was performed in the absence of the catalyst. Self-dehydrogenation of the NaBH4 and modified CS-based catalysts generated the equal quantity of hydrogen gas from NaBH4 dehydrogenization in methanol and achieved 100% conversions at varied rates of reaction, as seen in Fig. 2\n.As shown in Fig. 2, the hydrogen production rate was calculated to be 19 mLmin\u22121 for self-methanolysis and 26\u00a0mL.min\u22121 for CS (HGR: 529\u00a0mL.min\u22121.g\u22121). Hydrogen production rates for mag-CS, TA-CS, PEI-CS, H-PEI-CS, EDA-CS and H-EDA-CS were calculated to be 33, 46, 24, 36, 30 and 85\u00a0mL.min\u22121, respectively, while HGR values were calculated as 668, 930, 481, 713, 608, and 1705\u00a0mL.min\u22121.g\u22121. When modified CS was used as a catalyst, the reaction rate increased, and the reaction was completed faster than self-one by 100% conversion. The dehydrogenation reaction of NaBH4 in methanol is considered an acid-catalyzed reaction [16]. When the catalytic effects or reaction rates of the modified CS catalysts in the NaBH4 dehydrogenation in methanol were evaluated, it was realized that the reaction rate increased with the use of the catalyst with a high acid character. When the measured zeta potential values of the catalysts are examined, it is seen that the highest value belongs to H-EDA-CS with +48.3\u00a0mV (Table 2). According to the literature, the existence of NaCl in the reaction medium increases the rate of a reaction by increasing the concentration of CH3OH2\n+. Therefore, catalysts protonated with HCl (H-PEI-CS and H-EDA-CS) performed better than PEI and EDA modified CS (non-protonated form) and bare CS. Among CS and its modified forms, H-EDA-CS spheres were found to be the most efficient, therefore, the study continued with the use of H-EDA-CS spheres as catalysts and the impact of different factors on the dehydrogenation of NaBH4 in methanol was examined. Activation parameters of the reaction were calculated in the range of 273\u2013303\u00a0K by using H-EDA-CS catalyst.To evaluate catalyst effect on the dehydrogenation of NaBH4, the reaction was carried out using 12.5, 25, 50, and 75\u00a0mg of H-EDA-CS catalyst while holding all other parameters constant. Fig. 3a shows that changing the catalyst amount had no effect on the amount of H2 produced. The reaction rate increased from 43\u00a0mL.min\u22121 to 96\u00a0mL.min\u22121 with the addition of 75\u00a0mg of catalyst. In Fig. 3b, however, as catalyst amount increases from 12.5\u00a0mg to 75\u00a0mg, contrary to the reaction rate, the HGR decreases from 3460\u00a0mL.min\u22121.g\u22121 to 1282 mLmin\u22121\u00a0g\u22121, respectively. Slope of ln(H2 production rate) vs ln(NaBH4 conc.) graph was calculated to be 0.4741. Based on the amount of H-EDA-CS catalyst, the reaction kinetic can be assumed to be between pseudo-zero-order and pseudo-first-order. For the catalysis of such metholysis reaction, various catalys have been reported as the catalysts providing both pseudo-zero order and first order kinetic results. A condition that could lead to zero order rates; when two or more reactants are involved, One is used in a small amount and the others in higher amount. This situation commonly occurs when a reaction is catalyzed on a solid surface (heterogeneous catalysis).Five different NaBH4 concentrations were studied using 50\u00a0mg H-EDA-CS catalyst with the given condition in order to investigate the initial NaBH4 concentration effect on the rate of hydrogen production in NaBH4 dehydrogenation.The plot of H2 produced volume as a consequence of time shows that the amounts of H2 produced and the rate of H2 production increase proportionally when the concentration of NaBH4 increases, as shown in Fig. 4a. As seen in Fig. 4b, when the NaBH4 concentration is increased from 50\u00a0mM to 250\u00a0mM, the HGR increases from 673\u00a0mL.min\u22121.g\u22121 to 2933\u00a0mL.min\u22121.g\u22121. With the increase in the amount of NaBH4, the volume of H2 produced per unit time increases. The slope of the graph of ln(H2 production rate) vs ln(NaBH4 concentration) was found to be 0.9201 (Fig. 4c). This value confirms that the reaction kinetic is first order with respect to NaBH4 concentration.Temperature effect on NaBH4 dehydrogenation reaction catalyzed by CS and modified CS in methanol was examined by carrying out the reaction at four different temperatures, 273, 283, 293 and 303\u00a0K, using 125\u00a0mM NaBH4 in 20\u00a0mL methanol solution. The rate constants at different temperatures (273\u00a0K\u2013303\u00a0K) were calculated by using R50 value. Then the calculated \u201ck\u201d values for the dehydrogenation of NaBH4 catalyzed by CS and modified CS catalysts were used to construct graphs as lnk - 1/T from Arrhenius Eq. (3) and ln(k/T) - 1/T from Eyring Eq. (4). Activation parameters (Ea, \u0394H#, and \u0394S#) for the dehydrogenation reaction of NaBH4 with the use of H-EDA-CS as a catalyst were calculated.\n\n(3)\n\nln\nk\n=\nln\nA\n\u2212\n\n\nEa\nRT\n\n\n\n\n\n\n\n(4)\n\nln\n\n\nk\nT\n\n\n=\nln\n\n\n\nk\nB\n\nh\n\n\n+\n\n\n\u2206\n\nS\n#\n\n\nR\n\n\u2212\n\n\n\u2206\n\nH\n#\n\n\nR\n\n\n\n1\nT\n\n\n\n\n\nwhere, k is the rate constant, Ea is the activation energy, R is the gas constant (8.314\u00a0J.K\u22121.mol\u22121), T is temperature, kB is Boltzmann constant (1.381\u00a0\u00d7\u00a010\u221223 J.K\u22121), h is Planck constant (6.626\u00a0\u00d7\u00a010\u221234 J.s), \u0394H# is activation enthalpy, \u0394S# is the activation entropy.\nFig. 5a shows a graph of reaction temperature vs produced volume of hydrogen for NaBH4 dehydrogenation in methanol. When the effect of temperature on the dehydrogenation of NaBH4 with methanol is examined, the reaction time shortens with the increase in temperature, and all of the reactions at various temperatures produced approximately 250\u00a0mL H2. When the reaction temperature was increased from 273\u00a0K to 303\u00a0K, the reaction rate increased from 27\u00a0mL.min\u22121 to 91\u00a0mL.min\u22121. Again with the same temperature increase, the HGR value increased from 534\u00a0mL.min\u22121.g\u22121 to 1824\u00a0mL.min\u22121.g\u22121 (Fig. 5b). At other temperatures, the rates of generated H2 were between these two limits.The lnk - 1/T graph using Eq. (3) and the ln(k/T) - 1/T graph using Eq. (4) were plotted for the H-EDA-CS catalyst at temperatures up from 273\u00a0K to 303\u00a0K. The related graphs are shown in Fig. 6a and b, respectively. A relatively low activation energy of 26.14\u00a0kJ.mol\u22121 was calculated between 273 and 303\u00a0K temperature values and the comparison of the result obtained with the literature is given in Table 3\n.As seen in Table 3, the activation energy of the H-EDA-CS (26.14\u00a0kJ.mol\u22121) catalyst described in this study is comparable to the activation energies of metal-containing and metal-free catalysts published in the literature. For example, the Ea value for H-EDA-CS is quite close to the Ea of 21.7\u00a0kJ.mol\u22121 for the MGCell-PEI+ catalyst [31], 18.94\u00a0kJ.mol\u22121 for C-KOH-S-P catalyst [48], 20.84\u00a0kJ.mol\u22121 for p(2-VP)++C6 catalyst [49], 26.20\u00a0kJ.mol\u22121 for Co\u2013B-50GO catalyst [57], 24.01\u00a0kJ.mol\u22121 for p(CO) catalyst [53], 24.29\u00a0kJ.mol\u22121 for AC@Pt\u2013Ru\u2013Ni catalyst [58], 24.03\u00a0kJ.mol\u22121 for SiO2@PAA catalyst [62] and 24.10\u00a0kJ.mol\u22121 for p(MTMA) catalyst [50]. Furthermore, the Ea for NaBH4 dehydrogenation in methanol catalyzed by H-EDA-CS is considerably lower than the other previously reported activation energies, for example 34.80\u00a0kJ.mol\u22121 for the CAP catalyst [54], 38.41\u00a0kJ.mol\u22121 for the Co-Cr-B/NG catalyst [59], 38.80\u00a0kJ.mol\u22121 for the Co1B/GNS catalyst [60].Because of their high reactivities, primary alcohols can be used instead of water in the production of hydrogen with NaBH4. Methanol is known to have the highest reactivity of all of the primary alcohols. Therefore, methanol has become a feasible alternative for H2 generation [63,64]. Also, among other alcohols, methanol has a higher acidity than water. The carbon atom linked to the oxygen in the methoxy ion is less electropositive than the hydrogen, resulting in a more stable structure [65].\n\n(5)\n\n\nNaBH\n4\n\n\u2194\n\nNa\n+\n\n+\n\nBH\n4\n\u2212\n\n\n\n\n\n\n\n(6)\n\n\nBH\n4\n\u2212\n\n+\n\nH\n+\n\n\u2194\n\nBH\n3\n\n+\n\nH\n2\n\n\n\n\n\n\n\n(7)\n\n\nBH\n3\n\n+\n3\n\nCH\n3\n\nOH\n\u2192\nB\n\n\n\n\nCH\n3\n\nO\n\n\n3\n\n+\n3\n\nH\n2\n\n\n\n\n\n\n\n(8)\n\nB\n\n\n\n\nCH\n3\n\nO\n\n\n3\n\n+\n\nCH\n3\n\nOH\n\u2194\nB\n\n\n\n\nCH\n3\n\nO\n\n\n4\n\u2212\n\n+\n\nH\n2\n\n\n\n\n\n\n\n(9)\n\n4\nB\n\n\n\n\nCH\n3\n\nO\n\n\n4\n\u2212\n\n+\n2\n\nH\n+\n\n+\n7\n\nH\n2\n\nO\n\u2194\n\nB\n4\n\n\nO\n7\n\n2\n\u2212\n\n\n+\n16\n\nCH\n3\n\nOH\n\n\n\n\nEq. (2) makes assumptions the reaction between NaBH4 and methanol. Eqs. (5), (6), (7), and (8) explain the mechanism of methanol and NaBH4 in details. The recoverability of methanol by hydrolysis of byproducts is defined by Eq. (9).The reaction mechanism of the H-EDA-CS catalyst with NaBH4 is shown in Fig. 7\n. Hereunder the reaction first starts with the dehydrogenation of NaBH4, and an active intermediate is formed. As a result of the reaction of the intermediate product formed afterwards with methanol, as shown in Eq. (7), 4\u00a0mol of H2 and a tetramethoxyborate anion are released as by-products. As mentioned before, these released by-products make the reaction environment more basic and cause methanolysis to slow down.The repeated use of catalyst in the methanolysis of NaBH4 is a very important parameter to determine its catalytic stability and feasibility for industrial applications. As a consequence, ten consecutive uses of H-EDA-CS as a catalyst in the production of H2 from the dehydrogenation of NaBH4 in methanol were performed, and the obtained results were given in Fig. 8a. As shown in Fig. 8a, the activity of the H-EDA-CS catalyst decreased to 82% after the 1st use, and it continued to decrease after the 2nd and subsequent uses, reaching 43% after the 10th usage. It is important to emphasize that 100% conversion is obtained at every usage, and the H-EDA-CS catalyst demonstrated little decrease in activity even after the 10th cycle. The activity decrease can be solved by regenerating the used catalyst by treating it with HCI. Fig. 8b represents the three reuse cycles after regeneration of the H-EDA-CS catalyst with HCl.The catalyst was treated with 30.0\u00a0mL of 1.0 M HCl and used three times in a row after three repeated uses. In the first/initial use, the activity of the catalyst was higher. With the repeated use of the catalyst, the activity decreased when the conversion was 100% at each use, as seen in Fig. 8a. Fig. 8b, the experiments were repeated and the catalyst was regenerated after the first (3 repetitions) use, and it was used again 3 times after each generation. In Fig. 8b obtained with the new test after correction, all values were given in comparison with the catalyst pre-regeneration (first use) activity. Activity % was calculated as 100, 91, 80 and 79% for initial use, 1st regeneration, 2nd regeneration and 3rd regeneration, respectively. According to the literature, the decrease in the activity percentage of the H-EDA-CS catalyst can be related to the facile reaction of BH4\n\u2212 anions with positively charged amine groups and the tetramethoxyboron generated in the environment as the reaction progressed [3,45]. As a result of this study, it is evaluated that the CS-based catalyst will be a useful catalyst in industrial-scale hydrogen gas production since it can be regenerated and has high reusability performance, which is critical for practical applications.CS was synthesized systematically via hydrothermal method and optimum conditions were determined. Then CS was modified and used as a catalyst for dehydrogenation of NaBH4 in methanol to produce H2 gas, a clean fuel source. Zeta potential, thermal analysis, and FT-IR spectroscopy measurements were used to confirm the chemical modification of CS. When used directly as a catalyst in the dehydrogenation of NaBH4, the CS HGR value was 529\u00a0mL.min\u22121.g\u22121. Surprisingly, the protonation of EDA modified CS (H-EDA-CS) had significantly better catalytic performance than bare CS and maximum HGR value was determined to be 3460\u00a0mL.min\u22121.g\u22121.The calculated HGR value of H-EDA-CS showed an increase with an increase on temperature and NaBH4 concentration but a decrease with increasing amount of catalyst. Moreover, the calculated activation energy value was 26.14\u00a0kJ.mol\u22121 for H-EDA-CS. When compared to conventional catalysts, the calculated Ea values of the CS catalyst are relatively low in the methanolysis of NaBH4. These values outperform the majority of noble metal-based catalysts reported in the literature. Moreover, reusability studies of the H-EDA-CS catalyst indicated that the activity of the catalyst was more than 50% after 7th usage with reaching 100% conversion in each usage. Additionally, after various usage the loss of activity can be easily recovered by regeneration. As a result, starch, one of the world renewable, natural, and plentiful biopolymers, can be easily synthesized and changed using the hydrothermal process, and used as an efficient catalyst in the synthesis of a greener energy source, H2, from the dehydrogenation of NaBH4 in methanol.\nSultan Butun Sengel: Conceptualization, Methodology, Project administration, Writing \u2013 review & editing, Visualization, Supervision, Funding acquisition. Hatice Deveci: Resources, Data curation, Formal analysis, Writing \u2013 original draft, Visualization. Harun Bas: Resources, Visualization, Data curation. Vural Butun: Supervision, Funding acquisition, Writing \u2013 review & editing.The authors report no declarations of interest.This work was supported by the Commission of Scientific Research Projects of Eskisehir Osmangazi University (ESOGU BAP, FBA-2021-1620).\n\n\n\nSupplementary material\n\nImage 5\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106650.", "descript": "\n                  Carbon sphere (CS) was successfully synthesized by using starch via hydrothermal process with optimization of all conditions and parameters, systematically. The optimized CS was modified and used as a catalyst in the dehydrogenation of sodium borohydride in methanol for hydrogen production. Factors affecting the H2 production rate such as reaction temperature, catalyst type and amount, NaBH4 amount were investigated. Activation parameters for the dehydrogenation reaction of NaBH4 with the use of amine modified and protonated CS (H-EDA-CS) catalyst were calculated to be 26.14\u00a0kJ.mol\u22121, 23.75\u00a0kJ.mol\u22121, \u2212192.19\u00a0J.mol\u22121.K\u22121, for Ea, \u0394H# and \u0394S#, respectively. Maximum HGR value was calculated as 3460\u00a0mL.min\u22121.g\u22121 at 25\u00a0\u00b0C. Moreover, reusability studies of the H-EDA-CS catalyst were made and the activity of the catalyst was found to be above 50% even after the 7th use. The catalyst was also regenerated 3 times, and the % activity results for initial use, 1st, 2nd and 3rd regeneration were calculated as 100%, 91%, 80% and 79%, respectively.\n               "}
{"full_text": "In order to improve the petroleum refining efficiency, various catalysts are widely used in refinery industry. The fluid catalytic cracking (FCC) catalyst is widely used in the petroleum refineries worldwide (Muddanna and Baral, 2019). FCC catalyst tends to lose the catalytic activity with the metal deposition from crude oil. After repeated use, FCC catalyst will be deactivated and become spent FCC catalyst (SFCCC). SFCCC contribute large amounts of solid wastes in petrochemical industry, and about 200,000\u2013400,000 tons of SFCCC are produced worldwide every year (Alonso-Fari\u00f1as et al., 2020).Currently, SFCCC is mostly treated by landfills, although it contains a variety of metals (Ni, V, Sb, La, etc.). However, SFCCC has been considered to be toxic to environment (Alonso-Fari\u00f1as et al., 2020). SFCCC has been identified as hazardous waste in China since 2016 (Xue et al., 2020), and was subject of directive regulations in Europe (EC\u00a0Commission Decision, 2014). However, the risk of SFCCC is still controversial. SFCCC is not included in the \u201cHazardous Waste Listings\u201d by United States Environmental Protection Agency (US EPA) (USEPA, 2016). Studies have evaluated the risk and pollution characteristics of SFCCC in China. Zhang et al. (2019) found that SFCCCs have no ignitability, reactivity, corrosivity and toxicity to mice and rabbit. Study of Liu et al. (2016) showed that SFCCC did not belong to the hazardous solid waste. Unfortunately, few studies have been conducted on the evaluation of ecological and environmental hazards of SFCCC, and the main pollution components of SFCCC are still unclear. In the process of stacking and landfill of SFCCC and under the action of rainfall, the metal elements in SFCCC (Ni, Sb, V, La, etc.) will be leached with the rain and flow into groundwater, rivers, lakes and other water bodies, thus polluting the aquatic environment. However, to the best of our knowledge, there are no reports on the toxicity evaluation of SFCCC on aquatic organisms.The freshwater green microalgal species Raphidocelis subcapitata (formerly known as Pseudokirchneriella subcapitata or Selenastrum capricornutum) has been frequently employed to evaluate the toxicity of chemicals and wastes as the test organism (Fu et al., 2017; Sousa et al., 2018), and was recommended as assay microalga by Organization for Economic Co-operation and development (OECD), US EPA, or International Organization for Standardization (ISO) (OECD, 2011; ISO, 2012; USEPA, 2012).In this study, the ecotoxicity assays using R. subcapitata were conducted to evaluate the effect caused by metals in SFCCC leachates. To achieve this experiment, we collected 17 SFCCC samples from different petroleum refineries and prepared SFCCC leachates. Pearson's correlation analysis was conducted to discover the relationships between the metal concentrations of leachates and the values of toxicity. Furthermore, we designed the toxicity prediction models of the SFCCCs by multiple linear and non-linear regression models. Through our study, the ecotoxicity and key toxigenic factors of SFCCC can be first clarified, which provides reference for the management of SFCCC and formulation of measures to reduce toxicity.17 Spent FCC catalyst samples were collected from spent catalyst warehouses in different petroleum refineries. All SFCCCs were FCC catalysts deposed with metals from crude oil. FCC catalysts are Y molecular sieve, which is supported by SiO2-Al2O3 and loaded with rare earth metals. The label of those SFCCCs were Ha, Ji, Sha, Yan, Sh, Go, Ba, Qi, QD, Ur, Zho, ZD, Zhe, Fu, So, Ya-1, and Ya-2. The collected SFCCC samples were oven-dried at 75\u202f\u00b0C for 24\u202fh to remove moisture and stored in desiccators for 2\u202fh.In this study, the mixed solution of nitric acid and sulfuric acid was used as the leaching agent based on the Chinese solid waste extraction procedure for leaching toxicity \u2013 sulfuric acid and nitric acid method (HJ/T299-2007) (Wang et al., 2019; Li et al., 2021). The method simulated the leaching process of metal components from SFCCC into the environment under the influence of acidic precipitation during the nonstandard landfill disposal or stockpiling of SFCCC.Aqueous extraction treatment was carried out on the dried SFCCCs samples. The mixture of concentrated sulfuric acid and concentrated nitric acid with a mass ratio of 2:1 was added to the MilliQ water (about 2 drops in 1\u202fL water) to prepare the extract with the pH of 3.20\u202f\u00b1\u202f0.05 (Wang et al., 2019). The 150\u202fg SFCCC was mixed with 1500\u202fmL extract (1:10, m/v) in 2000\u202fmL PTFE bottle. The bottles were fixed on the flip oscillator (Polyfutai, ZKF-WF, Beijing, China) and oscillated for 18\u202f\u00b1\u202f2\u202fh at 30\u202f\u00b1\u202f2\u202frpm and 23\u202f\u00b1\u202f2\u202f\u00b0C. The leachates were filtered by 0.45\u202f\u00b5m cellulose acetate membrane washed with dilute nitric acid and the filtered leachates were stored at 4\u202f\u00b0C. The concentration of metals in leachates was measured using an inductively coupled plasma-optical emission spectrometer (ICP-OES) (PE, OPTIMA 8000, USA). Metal concentration data were averaged for at least three measurements.\nRaphidocelis subcapitata FACHB-271 was obtained from the Freshwater Algae Culture Collection at the Institute of Hydrobiology (FACHB) in China. Microalga was acclimatized in ISO 8692\u00a0standard algal medium (ISO, 2012) under continuously illumination of white fluorescence light at 100\u202f\u00b1\u202f4\u202f\u03bcmol m-2 s-1 at a temperature of 24\u202f+\u202f1\u202f\u00b0C in the illumination incubator (Jiangnan, GXM-508F-4, Ningbo, China) for two weeks.The fresh algal medium was prepared by adding the stock solution of medium to MilliQ water supplemented with different SFCCC leachate concentrations (\nTable 1).Generally, these dosing levels were designed to be a geometric progression after finishing a range-finding preliminary test. The tests were carried out in 250\u202fmL flasks containing 100\u202fmL of medium. An initial inoculum (1\u202f\u00d7\u202f104 cells mL\u22121) in the exponential growth phase of R. subcapitata was added to medium. Microalgae incubated under the same condition, but without added SFCCC leachates, were used as a control. All assays were carried out in triplicate for 96\u202fh. Microalgal cultures were maintained under the same conditions as were used for microalgal acclimatization in incubator. The microalgal cell density measurements were performed every 24\u202fh. The cell density in the inoculum was determined by counting under microscope (Olympus, CKX53, Japan) in the haemocytometer. The pH of all test solutions was measured at test initiation and termination using a pH meter (Sartorius, PB-10, Germany).The growth curves of R. subcapitata under different initial concentrations of SFCCC leachates were plotted versus time, and the area (A) under the growth curve of microalgae based on cell density for each treatment was estimated using the following equation (USEPA, 2012):\n\n(1)\n\n\nA\n=\n\n\u2211\n\ni\n=\n1\n\nn\n\n\n\n(\n\n\n\n\n\nN\n\n\ni\n\u2212\n1\n\n\n+\n\n\nN\n\n\ni\n\n\n\u2212\n2\n\n\nN\n\n\n0\n\n\n\n2\n\n\n)\n\n\n(\n\n\n\nt\n\n\ni\n\n\n\u2212\n\n\nt\n\n\ni\n\u2212\n1\n\n\n\n)\n\n\n\n\n\nwhere N\n0 and N\n\ni\n are the cell densities at test initiation (t\n0), time of the i\nth counting (t\ni) after test initiation.The percent of inhibition (% I) of each concentration of SFCCC leachate relative to control was estimated using the following equation (USEPA, 2012):\n\n(2)\n\n\n%\n\nI\n=\n\n\n\n\nA\n\n\nC\n\n\n\u2212\n\n\nA\n\n\nT\n\n\n\n\n\n\nA\n\n\nC\n\n\n\n\n\u00d7\n100\n\n\n\nwhere A\n\nC\n is the area under the microalgal growth curve of the control, and A\n\nT\n is the area of SFCCC leachate treatment group.The values of percentage of inhibition were used to calculate the median effective concentration values for 96\u2009h (96\u2009h EC50) using the means of a probit analysis (Finney, 1971).For each treatment and control, means and standard deviations were calculated from three replicated determinations. The relationships between the toxicity and the metal factors were analyzed by Pearson correlation using SPSS 19.0 software at significance levels of p\u2009<\u20090.05 and p\u2009<\u20090.01. Some measured metal concentrations in the leachates were below the detection limits, so the statistical analysis of those data was performed using the half value of the detection limit (Farnham et al., 2002). Before the correlation analysis, a logarithmic transformation was conducted for the factors that were not normally distributed.In this study, SPSS 19.0 was used to perform multiple linear regression (MLR) and non-linear regression (MNLR), which are used to derive a mathematical relationship between the 96\u2009h EC50 values and the concentrations of metals which are significantly correlated with EC50.The linear equation containing all those variables can be constructed in:\n\n(3)\n\n\ny\n=\n\n\u2211\n\ni\n=\n1\n\nn\n\n\n\n\n\u03b2\n\n\ni\n\n\n\n\nx\n\n\ni\n\n\n\n+\nC\n\n\n\nwhere, y, \u03b2\ni, x\ni and C represent ln(EC50), parameters of the model, concentrations of metals and constant of the model, respectively.The best-fit MNLR model was built using the stepwise selection method. The exponential model was introduced based on the fitting result of MLR:\n\n(4)\n\n\ny\n=\n\n\n\u03b1\n\n\n1\n\n\n+\n\n\n\u03b1\n\n\n2\n\n\n\u00d7\nexp\n\n(\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\n\n\n\u03b2\n\n\ni\n\n\n\n\nx\n\n\ni\n\n\n+\nC\n\n\n)\n\n\n\n\nwhere, \u03b1\n1 and \u03b1\n2 are constants of the model.A total of 27 elements of 17 SFCCC leachates were determined, of which Fe, Cu, Zn, P, As, Be, Ti, Cr, Cd, Ag, Pb, Ba, Zr, and Se were not detected in all samples. The concentrations of the other 13 metals are shown in Table S1. The results shown that, V, Sb, La, Ce, Na, and Ca are the metal elements with high concentration in the leachates. The concentrations of Ni, Al, Cu, Zn, Mo, Mn, and Co are relatively low, and the concentrations in most samples are less than 1\u2009mg\u2009L\u22121. The concentrations of metals vary greatly among different SFCCC leachates.The effects of different concentrations of 17 SFCCC leachates on the growth curves of R. subcapitata are illustrated in Fig. S1. The growth lag period of R. subcapitata was about 24\u2009h, and tiny changes between the control and the treatments were observed in the early stage of test (Fig. S1). The pH in each tests experienced a trend of change from weakly acidic (5.3\u20136.1) to weakly alkaline (7.7\u20138.2). The calculated 96\u2009h EC50 values based on the area under the growth curve of microalgae are listed in \nTable 2.The 96\u2009h EC50 values of the 17 SFCCC leachates varies greatly due to the large differences in metal concentrations. Qi SFCCC was the most toxic to R. subcapitata with a 96\u2009h EC50 value of 1.38%. Yan, ZD, Ba, and Ji SFCCC also exhibit high toxicity to R. subcapitata, with 96\u2009h EC50 value of 2.35%, 2.79%, 3.11% and 3.81%, respectively. Ha SFCCC was the least toxic, with 96\u2009h EC50 values exceeding 100% to R. subcapitata.In order to identify the toxicogenic factors of SFCCC leachate, bivariate correlation analysis was conducted between SFCCC toxicity (96\u2009h EC50 value) and concentrations of metals in SFCCC leachates, and the results are shown in \nTable 3. It is clear that the concentration of Ni (C\nNi) (r\u2009=\u2009\u22120.751, p\u2009=\u20090.001) and La (C\nLa) (r\u2009=\u2009\u22120.726, p\u2009=\u20090.001) showed inversely significant correlation with EC50 value at the 0.01 level, which suggests that an increase in C\nNi and C\nLa generates a more toxic leachate. In addition, EC50 value was inversely correlated with the concentration of Mn (C\nMn) (r\u2009=\u2009\u22120.581, p\u2009=\u20090.014), Co (C\nCo) (r\u2009=\u2009\u22120.565, p\u2009=\u20090.018), Ce (C\nCe) (r\u2009=\u2009\u22120.568, p\u2009=\u20090.017), and Ca (C\nCa) (r\u2009=\u2009\u22120.523, p\u2009=\u20090.031) at 0.05 level. The concentration of other metals, V, Sb, Al, Co, Zn, Mo, and Na, did not correlate with the toxicity of SFCCC leachates.In order to establish the prediction model of the toxicity of SFCCC, MLR model was used to analyze the contribution of each metal to the toxicity. In this model, the linearity relationship between metals and EC50 values is assumed. The concentration of metals with significant correlation with EC50 values were selected for MLR analysis. The collinearity diagnostic test confirmed that C\nCe and C\nCa are highly intercorrelated with C\nLa and C\nMn, respectively. Therefore, C\nNi, C\nLa, C\nMn, and C\nCo were selected for MLR analysis, and none had a variance inflation factor (VIF) more than 1.9. The Result of MLR analysis was shown in \nTable 4. When P of the independent variable is greater than 0.05, the variable should be excluded from the model. As shown in Table 4, only the significance test results of C\nNi and C\nLa meet the criteria, therefore the regression model is defined as:\n\n(5)\n\n\nln\n(\nE\n\n\nC\n\n\n50\n\n\n)\n=\n3.518\n\u2212\n1.112\n\u00d7\n\n\nC\n\n\nN\ni\n\n\n\u2212\n0.107\n\u00d7\n\n\nC\n\n\nL\na\n\n\n\n\n\n\n\n\nFig. 1 shows the relation between the estimated values of ln(EC50) using MLR model (Eq. (5)) and the experimental ln(EC50) values. Although the MLR model can describe the trend of EC50 values, however the prediction accuracy is not high (R2 =\u20090.783). In addition, it can be clearly seen from Fig. 1 that the predicted value and experimental value are more in line with the logarithmic model. Therefore, this study considered to introduce a logarithmic model based on MLR to simulate the regression relationship between metal concentrations and EC50 values of SFCCC leachates.In order to construct a more fitting model to better simulate the response relationship of metal concentrations and 96\u2009h EC50 values of SFCCC leachates, the multiple non-linear regression (MNLR) model was tested. The logarithmic equation (Eq. (4)) was introduced and fitted with the results of toxicity and metal concentrations. The MNLR model was constructed as follows:\n\n(6)\n\n\nl\nn\n\n(\n\nE\n\n\nC\n\n\n50\n\n\n\n)\n\n=\n0.817\n+\ne\nx\np\n\n(\n1.356\n-\n1.736\n\u00d7\n\n\nC\n\n\nN\ni\n\n\n-\n0.262\n\u00d7\n\n\nC\n\n\nL\na\n\n\n)\n\n\n\n\nThe analysis of variance (ANOVA) for MNLR model was provided in \nTable 5. The sum of squares metric from Eq. (6) is 92.703 and the residual is 1.667. Low mean squares for residuals indicate the robustness of the proposed MNLR model for predicting the toxicity of SFCCC.The ln(EC50) values calculated by MNLR model was fitted with the experimental ln(EC50) values. The fitting result was shown in \nFig. 2. As can be seen in Fig. 2, the predicting result of MNLR model (R2 =\u20090.926) are better than that of MLR model (R2 =\u20090.783). The MLNR model established in this study can be used as a toxicity prediction model for SFCCC.Ni, V, Sb, Mo, Mn, Co, La, and Ce are the characteristic metals in SFCCC leachates, which is similar to the previous studies (Zhou et al., 2020; Aung and Ting, 2005). Among the 27 metal elements, C\nNi, C\nMn, C\nCo, C\nLa, C\nCe, and C\nCa have significant correlation with the 96\u2009h EC50 values of R. subcapitata. The Ni, Co, Ca, and Mn in SFCCC are from the deposition of metals in crude oil. La and Ce are the main catalytic active components of fresh FCC catalyst. In this study, the main toxic ingredients of SFCCC to microalgae were identified for the first time. Researches should pay more attention to these elements in the development of SFCCC treatment and detoxification technology.At present, many studies have adopted various methods to remove or recover La (Mouna and Baral, 2019; Zhao et al., 2017; Lu et al., 2020; Innocenzi et al., 2015), Ni (Aung and Ting, 2005; Muddanna and Baral, 2019; Bayraktar, 2005), and Ce (Zhao et al., 2017; Lu et al., 2020; Innocenzi et al., 2015) in SFCCC. However, metals such as Al, V, and Sb concerned in SFCCC metal removal study did not show toxic effects on R. subcapitata. Satoh et al. (2005) assayed the toxicity of Cu, As, Sb, Pb, and Cd to eight microalgae. The results showed that the 72\u2009h IC50 value of Sb to microalgae was 7.9\u201345.9\u2009mg\u2009L-1, which was significantly higher than that of other metals (Cu: 4.2\u201311.7\u2009mg\u2009L\u22121; As: 1.6\u201312.0\u2009mg\u2009L\u22121; Pb: 2.5\u201321.4\u2009mg\u2009L\u22121; Cd: 2.9\u201313.8\u2009mg\u2009L\u22121). The results of low toxicity of Sb to microalgae in the research of Satoh et al. (2005) are consistent with the results of this study.\nMeisch and Bielig (1975) studied the effects of pentavalent vanadium on the growth of Scenedesmus obliquus and Chlorella pyrenoidosa, and found that V had no toxic effects on microalgae. Furthermore, vanadium was able to overcome completely a limited iron-deficiency in the algae and the chlorophyll formation was stimulated in Scenedesmus obliquus in presence of vanadium (Meisch and Bielig, 1975).In this study, the concentrations of Ni and La were significantly correlated with the 96\u2009h toxicity of SFCCC to R. subcapitata (P\u2009=\u20090.001). The high toxicity of Ni and La to microalgae has been reported. Deleebeeck et al. (2009) researched the toxicity of Ni (\u2161) to R. subcapitata. Under different conditions, the 72\u2009h EC50 value of Ni to R. subcapitata was 0.082\u20131.120\u2009mg\u2009L-1. For the toxicity of La (\u2162) on microalgae, the 96\u2009h EC50 value of La (\u2162) to Chlorella vulgaris and Phaeodactylum tricornutum is 10.077\u2009mg\u2009L\u22121 and 5.665\u2009mg\u2009L\u22121 respectively, in the research of Sun et al. (2019). The toxicity category of Ni and La according to \u201cHazardous Substances (Classification) Notice\u201d by USEPA (2017) is highly toxic (0.1\u201310) mg L-1.According to the results of this study, C\nNi and C\nLa can be used as indicators of toxicity of SFCCC leachate. The prediction model for toxicity of SFCCC was established according to C\nNi and C\nLa in leachate (Eq. (6)) (R2 =\u20090.926). In the future, when evaluating the ecotoxicity of SFCCC, the 96\u2009h EC50 value of SFCCC to R. subcapitata can be roughly obtained by preparing the leachate and measuring the C\nLa and C\nNi. The work in this study can provide a quick method for the determination of SFCCC ecotoxicity and provide assist for the hierarchical management and control of SFCCC.However, it should be noted that this study only explored the toxic effects and toxic factors of SFCCC on microalgae. However, metals are not equally toxic to different organisms. For example, some studies have shown that Sb has a high toxicity to large plants (Feng et al., 2020; Park et al., 2021). More studies are needed to investigate the toxic effects of SFCCC on a variety of organisms.The ecotoxicity of 17 SFCCC leachates to R. subcapitata was assayed in this study. The toxicity of the 17 SFCCCs varied enormously with the range of 96\u2009h EC50 values are 1.38%- >\u2009100%. The concentration of Ni (p\u2009=\u20090.001), La (p\u2009=\u20090.001), Mn (p\u2009=\u20090.014), Ce (p\u2009=\u20090.017), Co (p\u2009=\u20090.018), and Ca (p\u2009=\u20090.031) in SFCCC leachates showed significant correlation with 96\u2009h EC50 values. The predictive models for the toxicity of SFCCCs were established with the concentrations of Ni and La in leachates by multiple linear and non-linear regression models. The main toxic ingredients of SFCCC to microalgae were identified for the first time in this work. This study is significance for toxicity determination and management of SFCCC.This work was supported by the SINOPEC Ministry of Science and Technology Basic Prospective Research Project (CN) through a research scheme (A-531).\nYue-jie Wang: Conceptualization, Methodology, Software, Writing - original draft. Chen Wang: Data curation, Writing - review & editing, Supervision. Ling-ling Li: Formal analysis, Investigation, Resources. Yan Chen: Visualization, Investigation. Chun-hong He: Investigation. Lu Zheng: Investigation.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We are grateful to Engineer Mingzhe Li and Engineer Huaji Wang, our colleague of Key Laboratory, who contributed a lot to the measurement of metal concentration.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ecoenv.2021.112466.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  The 17 spent fluid catalytic cracking refinery catalysts (SFCCCs) from different petroleum refineries were collected and the leachates of SFCCCs were prepared. The ecotoxicity of SFCCC leachates to Raphidocelis subcapitata was assayed. The results showed that the toxicity of the 17 SFCCCs differ greatly. Ji SFCCC was the most toxic to R. subcapitata with a 96\u202fh EC50 value of 1.38%, while Ha SFCCC was the least toxic, with the EC50 value was >100%. The relationships between the toxicity of SFCCCs and the metal concentrations in leachates were analyzed. The concentration of Ni (p\u202f=\u202f0.001), La (p\u202f=\u202f0.001), Mn (p\u202f=\u202f0.014), Ce (p\u202f=\u202f0.017), Co (p\u202f=\u202f0.018), and Ca (p\u202f=\u202f0.031) in leachates showed significant correlation with EC50 values. The predictive model for the ecotoxicity of SFCCCs were established with the concentrations of Ni and La in leachates as:\n                  ln(EC50)\u202f=\u202f0.817\u202f+\u202fexp(1.356\u00a0\u2013\u00a01.736\u202f\u00d7\u202fC\n                     Ni -\u00a00.262\u202f\u00d7\u202fC\n                     La) (R2 =\u202f0.926).\n                  The main toxic ingredients of SFCCC to microalgae were identified for the first time in this work. The results and predictive model of this study are significance for toxicity determination and management of SFCCCs.\n               "}
{"full_text": "Biomass-derived energy cycle is one of the most sustainable alternatives to existing fossil fuel-derived energy platform [1]. A key intermediate for biomass conversion, 5-hydroxymethylfurfural (HMF) is of great interest as it can be obtained from the most earth-abundant organic materials, cellulosic matter, and can be converted to various kinds of important chemicals and fuels [2]. Among them, its oxidation product, 2,5-furandicarboxylic acid (FDCA) is receiving a great attention as a precursor for producing biomass-derived polymer, polyethylene 2,5-furandicarboxylate (PEF). PEF is a renewable candidate for replacing petroleum-derived polymer, polyethylene terephthalate (PET) [3].Typically, the FDCA has been produced by thermochemical oxidation of HMF under high pressure O2 or air (3\u201320\u00a0bar) at 30\u2013130\u00a0\u00b0C usually using precious metal catalysts such as Pd [4], Ru [5], Pt [6], and Au [7]. As a promising alternative route, electrochemical oxidation of HMF in aqueous solution has several advantages over the conventional oxidation approaches. (i) It doesn't require harmful oxidant as the water serves as an oxygen donor. (ii) It can be performed at ambient temperature and pressure. (iii) Tunable applied potential can control the selectivity thereby providing mechanistic insights. (iv) Various important reduction reactions like CO2 reduction reaction and hydrogen evolution reaction could be integrated, significantly increasing the worth of electrochemical biomass upgrading. (v) There remains a room for increment in economics when the electricity is replaced with renewable energy sources. In the same vain, electrocatalytic HMF oxidation reaction (HMFOR) represents a desirable strategy to replace kinetically unfavored oxygen evolution reaction (OER) in water electrolysis, increasing the overall energy efficiency.Electrochemical HMFOR also adopted noble metal-based nanoparticles like Au, Pd, PdAu alloys as catalytic anodes. For example, Chadderdon and co-workers exploited carbon-supported Au and Pd nanoparticles as the electrocatalyst [8]. They showed high conversion yield (\u223c100% at 0.9\u00a0V vs reversible hydrogen electrode, RHE), but the selectivity was only 83%. In a recent work, Choi and co-workers achieved a high Faradaic efficienty (\u223c100% at 1.54\u00a0V vs RHE) over gold anode by adopting an mediator (2,2,6,6,-tetramethylpiperidine-1-oxyl, TEMPO) [9]. However, use of the organic mediator would increase the separation/recovery cost. To replace the precious metals, diverse non-noble metal-based electrodes including nickel oxide/hydroxide [10], Co-P [11], Ni2P/Ni foam [12], Ni3S2/Ni foam [13], Porous Ni [14], NiB\nx\n [15], NiCo2O4 [16], Ni3N [17], and NiFe layered double hydroxide (LDH) [18] have been studied. Interestingly, most active electrocatalysts include Ni species, which are both active for HMFOR and OER. In particular, the introduction of Fe into layered nickel hydroxide has been effective strategy to enhance performance of OER as well as HMFOR [19]. However, reducing the activity toward the OER is essential to result in high selectivity for the HMFOR. Considering that HMFOR and OER are competitive with each other, it is hardly understandable how Fe insertion into nickel hydroxide catalyst simultaneously enhances both reactions. Previosuly, the Fe-related species have been addressed only in individual reactions. Boettcher group revealed that incorporation of Fe-impurities existed in KOH electrolyte increased OER activity for NiOOH [19]. Choi group reported that the FeOOH itself showed the inferior HMFOR activity compared to the NiOOH [20]. The role of Fe in Ni-based composite under conditions considering both oxidation reactions remains an unresolved issue.To address this issue, herein we comparatively studied the role of Fe in the layered Ni(OH)2 catalysts in both oxidation conditions; HMFOR and OER. For this purpose, layered Ni(OH)2 catalysts were prepared with controlled Fe content (0 to 5 at.%). Their structural properties were scrutinized with X-ray spectroscopic analyses. Their general morphologies, crystalline phases and chemical states were well retained after insertion of Fe, but with increasing Fe content, the crystallite size and layer number decrease due to the peeling of layers promoted by Fe intercalation. Electrochemical analysis performed in Fe-free KOH solution revealed that the OER activity increases up to 0.4 at.% with increasing Fe content, and then decrease showing volcano relation with the Fe content. Combined analysis based on the spectroscopic and electrochemical results suggested that traces of Fe penetrate the interlayer of Ni(OH)2 and enlarge the interlayer distances, resulting in a formation of NiFe LDH by replacing the Ni site with Fe during the electrochemical reaction. Although this Fe-promoted peeling of the Ni(OH)2 layers and facilitated OER activity up to optimal Fe content, the performance of the HMF conversion to FDCA was gradually decreased with Fe content due to charge consumption to OER. The HMFOR activity was the highest in the Fe-free Ni(OH)2, showing HMF conversion (99.9%), FDCA yield (94.2%), and Faraday efficiency (FE) for FDCA (98.0%). This comparative study offers a way of controlling selectivity and provides a guideline for rational design of Ni-based HMFOR-selective electrocatalysts.Fe(X)-Ni(OH)2 (X\u00a0=\u00a00.4 to 5 at.%) was synthesized by a simple microwave-assisted method [21], with some modifications. Ni(NO3)2\u00b76H2O (99.999%, Sigma-Aldrich) and Fe(NO3)3\u00b79H2O (\u226599.95%, Sigma-Aldrich) were dissolved in 10\u00a0mL of deionized (DI) water. NaOH pellet (98%, Samchun) was dissolved in 40\u00a0mL of DI water. Above tow solutions were mixed and stirred for 10\u00a0min. The resulting slurry was centrifugated at 8000\u00a0rpm for 15\u00a0min, and washed with DI water several times. They are dispersed in DI water again and undergone microwave heating at 90\u00a0\u00b0C for 1\u00a0h. The product was collected with centrifugation at 12000\u00a0rpm for 20\u00a0min and dried. The synthesis of Ni(OH)2 was carried out in the same manner as Fe(X)-Ni(OH)2, but without Fe precursor.The Fe content was analyzed using an inductively coupled plasma optical emission spectrometry (ICP-OES) analyzer (iCAP 6000 Series, Thermo, US). Transmission electron microscope (TEM) images were taken with a transmission electron microscope (Tecnai F20 G2, FEI, USA) operated at 200\u00a0kV. X-ray diffraction (XRD) was measured on a diffractometer (D8 Advance, Bruker AXS, Germany) with a Cu K\n\u03b1\n radiation using a LynxEye line detector. X-ray photoelectron spectroscopy (XPS) analysis was performed to analyze the chemical states of the samples with a spectrometer (Nexsa, Thermo Fisher Scientific, USA) equipped with a monochromatic Al K\n\u03b1\n X-ray source (1486.6\u00a0eV).Polypropylene (PP) centrifuge tubes were cleaned with a H2SO4 solution (0.5\u00a0M). 2\u00a0g of Ni(NO3)2\u00b76H2O was dissolved in 4\u00a0mL of DI water. 20\u00a0mL of 1\u00a0M KOH was added to precipitate Ni(OH)2, which serves as adsorbent for Fe impurities. The mixture was shaken and cetrifugated, and the supernatant was decanted. The Ni(OH)2 was washed with 20\u00a0mL of DI water and 2\u00a0mL of 1\u00a0M KOH. The tube was filled with 100\u00a0mL of 0.1\u00a0M KOH solution for purification. The solid was redispersed and mechanically agitated at least 10\u00a0min, followed by at least 3\u00a0h of resting for adsorption of Fe in KOH solution. The mixture was centrifugated, and the purified KOH supernatant was kept in a H2SO4-cleaned PP bottle for storage before use.Electrochemical experiments were performed using a Biologic VSP multichannel potentiostat electrochemical analyzer at room temperature (RT) and atmospheric pressure in a three-electrode configuration in a H-cell divided by Fumasep FBM-PK membrane. Hg/HgO and Pt mesh were used as the reference electrode and counter electrode, respectively. Catalyst inks were prepared by mixing 5\u00a0mg of catalyst with 40\u00a0\u03bcL of Nafion (5\u00a0wt% in a mixture of isopropanol and water, Sigma-Aldrich) in a solution of 800\u00a0\u03bcL DI water, 200\u00a0\u03bcL of isopropanol (99.9%), and the mixture was sonicated for 30\u00a0min to produce homogeneous slurry. Afterwards, 42\u00a0\u03bcL of the catalyst ink was drop-cast onto carbon paper (CP, 1\u00a0cm2), and dried at RT. The resulting catalyst loading on CP was 200\u00a0\u03bcg\u00b7cm\n\u22122. A Fe-free 0.1\u00a0M KOH (pH\u00a013) was used as the electrolyte, and HMF (5 mM) was added only for HMFOR test. Linear sweep voltammetry (LSV) for the HMFOR was conducted from 0.9\u00a0V to 1.9\u00a0V (vs RHE) at a scan rate of 10\u00a0mV\u00a0s\n\u22121 in Fe-free 0.1\u00a0M KOH. For OER test, LSVs were conducted without HMF following the same procedure. The electrochemical impedance spectra (EIS) were recorded at 1.1\u00a0V (vs RHE) and an AC potential amplitude of 10\u00a0mV from 200\u00a0kHz to 100 mHz. Series resistance (Rs) arises from a combination of resistance in solution, resistance in the catalysts themselves, and resistance in the glassy carbon substrate [22]. The LSVs in this paper were plotted after compensating the ohmic drops with Rs values. The electrochemical double-layer capacitance (C\ndl) was determined from cyclic voltammetry (CV) measured in the non-Faradaic window with a series of scan rates (12.5, 25, 50, 100, and 200\u00a0mV\u00b7s\n\u22121). Theoretically, the charging current (i\nc) is equal to the product of C\ndl and scan rate (v), as the following equation: i\nc\u00a0=\u00a0v\u00b7C\ndl. Hence, the C\ndl is the slope obtained from a linear fitting of i\nc as a function of v.HMF and its oxidation products were quantified by high-performance liquid chromatography (HPLC). After constant potential electrolysis performed at different applied potentials of 1.39\u00a0V, 1.44\u00a0V, and 1.54\u00a0V (vs RHE), 100\u00a0\u03bcL solution aliquots were taken from the anode compartment before and after the HMFOR and diluted with 1500\u00a0\u03bcL of 5\u00a0mM H2SO4 for HPLC analysis (YL9100 HPLC system containing a PDA detector). As the intermediates and FDCA exhibit a different light absorption profile, different detection wavelengths were chosen. 266\u00a0nm, 285\u00a0nm, 258\u00a0nm, 289\u00a0nm and 284\u00a0nm were set as detection wavelengths for FDCA, HMF, HMFCA, DFF and FFCA, respectively. 5\u00a0mM H2SO4 (99.999%, Sigma-Aldrich) was used as the mobile phase in isocratic mode with a flow rate of 0.5\u00a0mL\u00a0min\u22121 at 40\u00a0\u00b0C. Aliquots of the diluted samples (20\u00a0\u03bcL) were injected into a Coregel 87H3 column (Concise Separations). Product identities and concentrations were determined from calibration curves obtained using standard solutions of known concentrations.The following equations were used to calculate HMF conversion (%), product yields (%), and FE for FDCA (%). Here, F represents Faraday constant, 96485C mol\u22121.\n\n\nHMF\n\nconversion\n\n\n%\n\n=\n\n\n\nHMF\n\nconsumed\u00a0(mol)\n\n\nInitial\n\nHMF\n\n(\nmol\n)\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\nProduct yield\n\n\n%\n\n=\n\nProduct formed\u00a0(mol)\n\nInitial\n\nHMF\n\n(\nmol\n)\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\nFE\n\nfor FDCA\n\n\n%\n\n=\n\nCharge consumed for producing FDCA\nTotal charge passed\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n=\n\n\nFDCA formed\u00a0(mol)\n\u00d7\n\n\n6\n\u00d7\nF\n\n\n\n\nTotal charge passed\n\n\u00d7\n100\n%\n\n\n\nLayered Ni(OH)2 catalysts containing controlled Fe contents were prepared by a simple microwave-assisted synthesis at low temperature (90\u00a0\u00b0C for 1\u00a0h). The detailed synthetic procedure is described in experimental section. Since excess amounts of Fe can produce FeO\nx\n and FeOOH, which are catalytically less active for the HMOER and OER [20,23], the Fe content was controlled from 0.4 to 5 at.%. Successful incorporation of desired amount of Fe were confirmed with ICP\u2013OES (Table S1). Hereafter, these samples are denoted as Fe(X)-Ni(OH)2 (X\u00a0=\u00a0Fe content). TEM images show that Ni(OH)2 and Fe(X)-Ni(OH)2 have a morphology of quasi-hexagonal nanoplate with lateral sizes about 30\u201350\u00a0nm (Fig. 1\n).The Fe(X)-Ni(OH)2 nanostructures were further scrutinized with XRD and XPS. Their XRD patterns are in good agreement with the Ni(OH)2 standard (JCPDS card no: 14\u20130117) (Fig. 2a). The edge plane direction of (001) diffraction peak at 2\u03b8\u00a0=\u00a019.3\u201322.1\u00b0 became broader with increased Fe content. Scherrer analysis for the diffraction demonstrates that the crystallite size and layer number in Ni(OH)2 nanosheets decrease with increasing Fe content (Fig. 2b). Introduction of Fe species between the Ni(OH)2 layers promoted the peeling of nanosheets [19,24]. Their chemical structures were accessed with XPS analysis (Fig. 2c,d and Fig. S1). Ni 2p XPS spectra of the Fe(X)-Ni(OH)2 samples almost overlapped with that for Ni(OH)2 (0% Fe) (Fig. 2c). The predominant peaks centered around 855.3\u2013855.6\u00a0eV (Ni 2p3/2) and 873.0\u2013873.2\u00a0eV (Ni 2p1/2) well matched with the Ni2+ [25]. Marginal shift to lower binding energy with increased Fe content indicates slight reduction possibly due to formation of Fe3+ species, as confirmed with Fe 2p XPS spectra for Fe(4)-Ni(OH)2 and Fe(5)-Ni(OH)2 (Fig. 2d) [18,26]. These peaks at 713.1\u00a0eV (Fe 2p3/2) and 725.4\u00a0eV (Fe 2p1/2) correspond to the Fe3+ species in FeOOH, which are catalytically less active [20,23]. It is rarely detectable when the Fe content is too small (< 3 at.%). Overall, XRD and XPS suggested the decreased crystallite size, decreased layer number, reduced oxidation states of Ni species, and increased amount of less active FeOOH with increased Fe content in Ni(OH)2.Electrocatalytic HMFOR and OER activities were measured on three-electrode setup using the catalyst-loaded carbon paper as working electrode. Fe-free 0.1\u00a0M KOH solution was used to exclude the effect of Fe existed in the base electrolyte only concerning the Fe species in catalytic material. The OER activity was measured without HMF dispersion in the electrolyte. The OER activities were compared with the OER currents at 1.7\u00a0V (vs RHE), which are not overlapped with the oxidation currents of Ni(II) species appeared in the potential rage of 1.4\u20131.5\u00a0V (vs RHE). The OER activity has a volcano relation with Fe content in the catalysts (Fig. 3a,b). The OER current is the highest in Fe(0.4)-Ni(OH)2; the current densities derived at a potential of 1.7\u00a0V (vs RHE) were 2.99, 7.01, 6.57, 5.39, and 3.95\u00a0mA\u00a0cm\u22122 for the Ni(OH)2, Fe(0.4)-Ni(OH)2, Fe(2)-Ni(OH)2, Fe(4)-Ni(OH)2, and Fe(5)-Ni(OH)2, respectively (Fig. 3a). The Tafel analysis showed the same activity trend (Fig. 3b); the Tafel slopes are 192, 143, 155, 159, and 186\u00a0mV dec\u22121 for the Ni(OH)2, Fe(0.4)-Ni(OH)2, Fe(2)-Ni(OH)2, Fe(4)-Ni(OH)2, and Fe(5)-Ni(OH)2, respectively. The OER activity parameters are summarized in Table S2. Interestingly, the onset potentials for the oxidation of Ni(II) to Ni(III) shifted to higher potential with increasing Fe content, indicating unfavorable transition of Ni(II) to Ni(III), resulting in decrease of active species for the OER. However, the OER current (at 1.7\u00a0V vs RHE) rather increased with Fe content up to 0.4 at.%. The combined analysis from structural and electrochemical characterizations suggest that even traces of Fe impurity can penetrate the interlayer of Ni(OH)2 and form NiFe LDH by replacing the Ni site with Fe during the electrochemical reaction [19], facilitating the OER up to optimal Fe content (0.4 at.%).The HMFOR activity was evaluated with HMF dispersion in the electrolyte. In the presence of HMF, the following reactions can occur together and compete.\nOER:\n\n\n\n4OH\n\u2212\n\n\u00a0\n\u2794\n\n\n2H\n2\n\nO\n+\n\nO\n2\n\n+\n\n4e\n\u2212\n\n\n\n\n\nHMFOR:\n\n\nHMF\n\n\n\n\nC\n6\n\n\nH\n6\n\n\nO\n3\n\n\n\n+\n\n6OH\n\u2212\n\n\n\n\n\n\n\n\u2794\nFDCA\n\n\n\n\nC\n6\n\n\nH\n4\n\n\nO\n5\n\n\n\n+\n\n4H\n2\n\nO\n+\n\n6e\n\u2212\n\n\n\n\nUnlike the activity trend in OER, the kinetics of HMFOR enhances with increasing pH, thereby resulting in high rates and yields for FDCA. However, further increase of pH over 14 could induce polymerization of HMF and produce insoluble humins, thereby decreasing the effective HMF concentration [27]. Therefore, designing non-noble metal-based catalysts, which can be active at pH\u00a0<\u00a013, is of prime importance. For this reason, we adopted Fe-free 0.1\u00a0M KOH (pH\u00a0\u223c\u00a013) as an electrolyte solution for both HMFOR and OER. The HMFOR activities were compared with the HMFOR currents at 1.5\u00a0V (vs RHE), which can neglect the effect of OER and observe only the effect of Fe content on the HMFOR. In the series of Fe(X)-Ni(OH)2, the HMFOR currents decreased with increasing Fe content (Fig. 3c); the current densities derived at a potential of 1.5\u00a0V (vs RHE) were 2.53, 2.36, 1.63, 1.26, and 1.04\u00a0mA\u00a0cm\u22122 for the Ni(OH)2, Fe(0.4)-Ni(OH)2, Fe(2)-Ni(OH)2, Fe(4)-Ni(OH)2, and Fe(5)-Ni(OH)2, respectively. The higher HMFOR current density in Ni(OH)2 were likely attributed to the abundance of Ni(II) species in surface, which are the main source for the Ni(III) species, initiating the indirect HMFOR process [20]. In electrooxidation of organic compound, Ni(II) is directly oxidized to Ni(III) in electrode surface and then the Ni(III) oxidize organic reactant by reducing themselves [28]. They showed similar Tafel slopes, might indicating the reaction rates of the Fe(X)-Ni(OH)2 were limited by the same step, which occured on Ni(OH)2 (Fig. 3d). To show the involvement of Ni(III) species in the indirect HMFOR, the surface electrochemistry was investigated with CV measurements in absence and in presence of HMF (Fig. S2). The oxidation current for Ni2+ to Ni3+ increased with addition of HMF due to continuous regeneration of Ni2+ by the indirect HMFOR. In this regard, the reduction peak area for Ni3+ to Ni2+ decreased with addition of HMF as the Ni3+ species already reduced to Ni2+ by the indirect HMFOR. Additionally, double-layer capacitance (C\ndl) of Ni(OH)2 was determined by measuring a series of CVs at various scan rates (12.5\u2013200\u00a0mV\u00b7s\u22121) to evaluate electrochemically active surface area of electrodes (Fig. S3). The C\ndl obtained from the linear regression of current densities plotted against the scan rates was determined to be 561 uF\u00b7cm\u22122, which was comparable to previous C\ndl values of Ni(OH)2-based electrodes (100 and 576 uF\u00b7cm\u22122) [29,30]. The HMFOR activity parameters are summarized in Table S3.Constant potential electrolysis for HMF conversion to FDCA was performed to analyze the production yield and Faradaic efficiency (FE) for FDCA. For full conversion of HMF, stoichiometric amount of charge was passed (20.84C). As the applied potential strongly influences on the selectivity toward HMFOR, the constant potential electrolysis was performed at a series of potentials (1.39\u00a0V, 1.44\u00a0V, and 1.54\u00a0V vs RHE), where the OER currents are very minor (Fig. 3c and Fig. S4) [18,20]. The reaction time required to consume the stoichiometric amount of charge for the full conversion of HMF decreased with increased applied potentials as the higher potential results in the higher current (Fig. S4). At the potentials of 1.39\u00a0V and 1.44\u00a0V (vs RHE), HMF was almost completely converted (> 99.9%) with FDCA yields of 94.2% and 90.4%, respectively (Fig. 4a). At an applied potential of 1.54\u00a0V (vs RHE), even the stoichiometric amount of charge for full conversion of HMF was passed, the HMF conversion was 95.5%, lower than those of 1.39\u00a0V and 1.44\u00a0V. Also, the FDCA yield decreased to 71.3% and unreacted HMF (4.5%) was found in the product solution, indicating that the passed charge was not used only for the HMF conversion (Fig. 4a). With increasing potentials, the FE decreases mainly due to charge consumption to the competing OER (Fig. 4b). For the HMF conversion at 1.54\u00a0V, 87.1% of the passed charge was used for the HMF conversion, while 12.9% of the charge was consumed for the competing OER. Additionally, quantitative analysis based on the HPLC measurement revealed that the yields of the other intermediate compounds are 15.7%, 1.0%, and 6.9% for FFCA, HMFCA, and DFF, respectively. The HMF conversion, production yield, and FE were summarized in Tables S4\u2013S6. The gap between HMF conversion (95.5%) and products yield (94.9%) could originate from the crossover of organic compounds through the membrane to the counter electrode compartment [31], and the uncontrollable degradation of HMF and intermediates due to the base-induced polymerization of aldehydes [32], which can make it difficult to assess the true production yield.Herein, layered Ni(OH)2 catalysts with controlled Fe content (0 to 5 at.%) were prepared for comparative investigation on the role of Fe in competing oxidation reactions; HMFOR and OER. While the OER activity increases up to 0.4 at.% and then decrease showing a volcanic relation, the HMFOR activity decrease with the introduced Fe content. It is explained with structural characterization that even traces of Fe largely affected to enlarge the interlayer spacing of Ni(OH)2 and form NiFe LDH by replacing the Ni site with Fe during the electrochemical reaction, boosting the OER performance and lowering the selectivity toward HMFOR. Among the samples, the Fe-free Ni(OH)2 showed the highest HMF conversion (99.9%), FDCA yield (94.2%), and FE for FDCA (98.0%). This finding offers a promising way of controlling selectivity and provides guideline for rational design of Ni-based HMFOR electrocatalysts.\nBora Seo: Conceptualization, Methodology, Data curation, Investigation, Writing \u2013 original draft. Jongin Woo: Investigation, Data curation. Eunji Kim: Investigation. Seok-Hyeon Cheong: Investigation. Dong Ki Lee: Writing \u2013 review & editing, Funding acquisition. Hyunjoo Lee: Writing \u2013 review & editing, Supervision, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by C1 Gas Refinery Program through the National Research Foundation (NRF) of Korea (NRF-2015M3D3A1A01065435) and KIST Institutional Program (Atmospheric Environment Research Program, Project No. 2E31690). This work was also supported by NRF grant funded by the Ministry of Science and ICT (No. 2022M3H4A1A02091720). The XAS experiments performed at Beamline 1D of the Pohang Accelerator Laboratory (PAL) were supported in part by the Ministry of Education and Pohang University of Science of Technology (POSTECH).\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106501.", "descript": "\n                  5-hydroxymethylfurfural oxidation reaction (HMFOR) has been considered as promising anodic reaction alternating oxygen evolution reaction (OER). The introduction of Fe into layered nickel hydroxide has been effective strategy to enhance performance of OER as well as HMFOR. However, considering that HMFOR and OER are competitive with each other, it is hardly understandable how Fe simultaneously enhances both reactions. Herein, we provide an insight toward the role of Fe in the layered Ni(OH)2 in both oxidation reactions. While the OER activity of the layered Ni(OH)2 electrocatalyst has volcanic relation with Fe content, the HMFOR activity was highest in the Fe-free Ni(OH)2.\n               "}
{"full_text": "Data will be made available on request.Plastic has become an indispensable product in modern society. Likewise, plastic waste is ubiquitous and accumulates in huge quantities. The production of plastic in the EU has been reported to reach 55\u00a0Mt per year in 2020, accompanied by over 29.5\u00a0Mt of plastic waste (Soup, 2022). Unfortunately, the Covid-19 pandemic has further intensified this issue (Behera, 2021), and it is estimated that the global daily plastic waste generated since the outbreak began is 1.6 million tons (Benson et al., 2021).Despite this, only a small fraction of plastic waste is recycled, with the vast majority being either discarded, landfilled, or incinerated (Soup, 2022). In addition to causing severe environmental damage, discarding and landfilling plastic waste is a huge waste of resources. The energy potential in 1 ton of plastic waste is substantial; it equals that of more than 1 ton of coal or more than 4 barrels of oil (Themelis et al., 2011). Although incineration can convert the chemical energy in plastic waste into power (Al-Salem, 2019), it is crucial to note that plastic is a carbon-based material and releases over 2.8\u00a0kg of CO2 per kilogram of plastic burned, contributing significantly to carbon emissions (No-burn, 2021).In order to reach \u2018net zero emissions\u2019, low-carbon processes that convert plastic waste into high-value products are urgently needed. A number of thermochemical processes, including pyrolysis (Qureshi et al., 2020), hydrothermal liquefaction (Veksha et al., 2020), gasification (Nanda and Berruti, 2021), and chemolysis (R. X. Yang et al., 2022a, 2022b), have been extensively studied to convert waste plastics into more valuable products, such as syngas and plastic oil. Syngas obtained from the gasification of plastics has a LHV of 16\u201325\u00a0MJ/m3 (Lopez et al., 2018). Additionally, plastic oil obtained from pyrolysis or hydrothermal liquefaction has a HHV of 40\u201345\u00a0MJ/kg (Sharuddin et al., 2016), which is comparable to that of gasoline (HHV: 42.5\u00a0MJ/kg). However, these energy products still contribute to greenhouse gas emissions when they are combusted (Mani et al., 2009).Based on the elemental composition of plastic waste, the complete conversion of plastic waste into H2 and value-added carbon products via pyrolysis and catalytic cracking is recognized as the most promising and attractive solution for plastic recovery. Here, H2 has been recognized as one of the most promising energy carriers for the future (Ishaq et al., 2022). The generation of carbon products instead of COx, results in no CO2 emissions. More importantly, as high-value products, carbon products have critical applications in many industries, such as metallurgy (Zhang et al., 2016), batteries (Zhang et al., 2021), catalysis (Yoon et al., 2005), adsorbents (Wong et al., 2018), refractories (Thethwayo and Steenkamp, 2020), and so on.Numerous studies have also been conducted to co-produce carbon and hydrogen from plastic wastes. The main strategy is to deploy a metallic catalyst for reforming/cracking plastic pyrolysis volatiles. For instance, Wu and Williams reported a hydrogen yield of 133\u00a0mmol/g using Ni\u2013Mg\u2013Al catalyst for the pyrolytic reforming of PP while obtaining filamentous carbon on the catalyst surface (Wu and Williams, 2010). Yao et al. used Ni/ZSM5-30 catalysts for the pyrolytic reforming of HDPE and obtained a hydrogen yield of 30.11\u00a0mmol/g (without steam) (H. Yang et al., 2022a, 2022b). Aboul-Enein et al. obtained 27.8% CNT yield by pyrolysis of LDPE by using a 10% Ni\u2013Mo/Al2O3 catalyst (Aboul-Enein et al., 2017).The construction of a highly efficient and stable metallic catalysts remains a major challenge in the process. The metallic catalyst could deactivate quickly as carbon (coke) is deposited on the catalyst surface (Wong et al., 2022). Furthermore, Barbarias et al. reported that in a continuous HDPE pyrolytic reforming experiments, the catalytic effect of Ni-based catalysts decreased significantly, and a slight irreversible deactivation persisted even after regeneration (Barbarias et al., 2019). The separation of carbon and metallic catalysts is another major challenge. Carbon applications generally require a high carbon purity, especially in the battery industry (Wissler, 2006). To obtain carbon with a high purity, it is necessary to purify the carbon to remove the residual metals in carbon products by using strong acids (Ma et al., 2022). However, the use of strong acids would inevitably create new environmental impacts (Shin et al., 2014).Biocarbon catalysts derived from a woody biomass pyrolysis serve as ideal catalysts for the thermal cracking of plastic waste pyrolysis volatiles to co-produce carbon and hydrogen. On one side, a metal-free carbon framework is thermally stable and has a high tolerance with respect to a coke deposition (Konwar et al., 2014; Wang et al., 2020). In this way, the catalyst's lifetime could be significantly extended. More importantly, spent biocarbon catalysts (with plastic waste derived carbon coating on the surface) could be directly used as functional carbon products with no need for an additional separation process (He et al., 2021). On the other side, woody biocarbon catalyst has a hierarchical porous structure originating from wood (Chen and Pilla, 2021). The tortuous and complex pore channels have been reported to increase the tortuosity of gas flow, which increases the residence time of the gas in the high-temperature region and significantly enhances thermal cracking reactions (Wang et al., 2017).In this study, biocarbon catalysts are fabricated by pyrolysis and subsequent carbonization of woody sawdust waste at a specific temperature. Thereafter, the catalysts are applied to catalytic cracking of plastic waste pyrolysis volatiles to produce metal-free carbon and hydrogen-rich gases. Specifically, the influence of the catalyst to plastic waste ratio (mass), and the influence of the plastic types have been investigated.Three types of sphere plastic samples (from Goodfellow), i.e. PP, LDPE, and HDPE, with the average particle size around 2\u20134\u00a0mm were used in this study. The elemental compositions of the plastic samples are listed in Table S1.The biocarbon catalyst used in this study was prepared by pyrolysis followed by a subsequent carbonization of woody sawdust, which consists of a mixture of pine and spruce bought from Svenska Cellulosa AB (SCA). Before each experiment, the sawdust was sieved, and only particles with a size larger than 1.25\u00a0mm were used. The aim was to retain the porous structure of biocarbon and to lower the pressure drop of the biocarbon catalyst bed. The pyrolysis and carbonization of sawdust were conducted at a temperature of 900\u00a0\u00b0C for a duration of 2\u00a0h in a nitrogen atmosphere and using a vertical electrical heating furnace.In this study, the experiments were carried out by using a two-stage reactor, as shown in Fig. 1\n. And the detailed information of the models in the experiments was provided in Table S2. The whole system consists of a carrier gas supply section, a two-stage fixed bed, a cooling system, Micro GC, and a gas collection system. For the reactors, the furnace on the left was the pyrolysis reactor, and the one on the right was the catalytic cracking reactor. Also, ball valves were installed at the top of the pyrolysis reactor to drop the raw materials.Each test used 5\u00a0g of plastics and different amounts of a biocarbon catalyst, based on the cases described in Table 1\n. Before the test, biocarbon was placed in the middle of the cracking reactor. Nitrogen at a flow rate of 200\u00a0ml/min was injected into the reactor as the carrier gas, and the cooling bath was set at \u221215\u00a0\u00b0C. Then two reactors were heated up to the set temperature shown in Table 1.Once the temperature is reached, plastic samples were quickly dropped into the pyrolysis reactor from the top valve. Hot gaseous compounds from the cracking reactor first passed through a series of condensed bottles in the cooling bath. Condensable liquids were condensed and collected in the bottles. Non-condensable gases passed through a Micro-GC instrument for fast online determinations of the chemical compositions. The total volume of the gases was measured by using a gas clock. Finally, the gases were collected in a gas bag for further chemical composition determinations. The experiment was terminated when only nitrogen is detected by the Micro-GC. After the reactor was cooled down, the spent biocarbon catalysts together with plastic-derived carbon were collected from the cracking reactor.For the continuous feeding experiment, 5\u00a0g of plastic was used for each testing cycle with 10\u00a0g of biocarbon catalyst placed in the cracking reactor. The Micro-GC was used to monitor the gas composition changes in the continuous feeding experiment. When the hydrogen peak was lower than the nitrogen peak, this test ends, and the gas collection was stopped. After changing the gas bag, the plastic sample is re-fed and a new test is started.The gas yield is calculated by multiplying the density of a single component by its volume, and then summing the results for all gas components. Futhermore, the liquid yield is calculated by measuring the weight increase of the cooling bottles before and after analysis.The ultimate composition analysis of biomass was conducted by using a \u201cVario EL cube\u201d elemental combustion analyzer (Elementar Analysensysteme GmbH, German) located at KTH. The oxygen content in all cases was determined by difference (100 %-C %-H %-S %) on a dry basis. Also, a Raman test was conducted by using Tyrode I located at KTH. TGA of plastics and carbon products were performed by using a NETZSCH STA 449 F3 Jupiter thermogravimetric analyzer located at KTH. SEM test was conducted by Ultra 55 in KTH. The composition of the syngas was determined by using a micro gas chromatograph with thermal conductivity detector (Micro GC-TCD, Agilent). The surface area and pore size distribution were tested by BET Surface Area Instrument ASAP 2060 in Uppsala University.In this study, the performance of the biocarbon catalyst is first investigated, which is quantified in terms of the mass ratio of the catalyst to the plastic. Specifically, PP, the common material to make personal protective equipment (PPE) (Bratovcic, 2021), is used as feedstock, and three different C/P ratios i.e. 0, 1, 2 (see Table 1, cases 0,1,2) are used. The results are shown in Fig. 2\n.Carbon, liquid, and gases are three major types of products which are obtained in all cases. For the non-catalytic case, i.e. case 0, the yields of the carbon, liquids, and gases are 51.58\u00a0wt%, 7.93\u00a0wt%, and 40.49\u00a0wt%, respectively. The carbon yield (weight percent) is calculated by difference, i.e., carbon wt.%\u00a0=\u00a0100%-liquid wt.%-gas wt.%. Because a certain percentage of carbon is flushed away by nitrogen and thereby hard to collect during the test. Liquids are mainly believed to be wax, adhering to the glass bottle walls. Due to the relatively low yield, the composition of liquid is not analyzed.H2 and CH4 are two major components of the gas, which contain some C2, C3 compounds. The yield of CH4 (15.32\u00a0mmol/g) is slightly higher than that of H2 (14.09\u00a0mmol/g). The case using a C/P ratio of 1, i.e. case 1, shows an 11.03\u00a0wt% higher carbon yield, a 17.14\u00a0mmol/g higher H2 yield, and a 5.26\u00a0mmol/g lower CH4 yield compared to case 0. The case using a C/P ratio of 2, i.e. case 2, shows that the hydrogen yield further increases to a value as high as 34.31\u00a0mmol/g (68.6 mg/gplastic). Meanwhile, the C2 yield decreased from 3.17\u00a0mmol/g (case 1) to 2.17\u00a0mmol/g. The result indicates that the increase of the C/P ratio from 1 to 2 further enhances the formation of H2 by facilitating the cracking of C2 compounds. Notably, the slight increase of the CH4 yield indicates that a further increase of the catalyst loading seems not to be favorable for the CH4 cracking. Compared to the non-catalytic case, all catalytic cases show higher carbon yield, but lower liquid and gas yields. The result indicates that the deployment of the biocarbon catalyst promotes a cracking of hydrocarbons derived from plastic pyrolysis into carbon and gas compounds. The transfer of carbon from gases to solids results in a reduced mass yield, but an increased molar yield of gases. The carbon and hydrogen distribution results are shown in Fig. S1 in the supplement. Compared to the non-catalyst case, catalyst case 1, with a C/P ratio of 1, the amount of hydrogen in the PP converted to H2 increased from 22.7% to 54.3%. Furthermore, the amount of carbon in the PP converted to carbon increased from 57.1% to 76.3%. Case 2, with a C/P ratio of 2, showed slightly lower conversion rates of the hydrogen and carbon in the PP to H2 (53.5%) and carbon (68.7%) compared to Case 1. However, Case 2 showed the lowest distribution ratios of hydrogen and carbon for C2\u2013C4 compounds, indicating that the addition of biocarbon catalyst promoted the cracking of the C2\u2013C4 compounds. In general, the use of the biocarbon catalyst significantly promotes the conversion of carbon in plastic into carbon products and hydrogen in plastic into hydrogen. A carbon yield as high as 580.6 mg/gplastic and a H2 yield as high as 68.6 mg/gplastic are obtained when using a C/P ratio of 2, which further confirms the advantages of using a biocarbon catalyst. Compared to similar studies, showed in Table S3, using biocarbon catalyst have a higher carbon yield and a desirable hydrogen yield.Another two types of commonly used plastics i.e. LDPE and HDPE (see Table 1, cases 3,4) have also been tested to show the biocarbon catalytic performance for different types of plastic waste. Based on the above results, the C/P ratio was fixed at a value of 2. The corresponding results are shown in Fig. 3\n.Carbon, liquid, and gases are still the main products of the process, but the product yields vary for different plastic types. Compared to the results of Case 2, Case 3, using LDPE as the raw material, shows a higher carbon yield of 64.12\u00a0wt%, but a lower gas yield of 31.93\u00a0wt%. The H2 and CH4 yields decrease to 28.4\u00a0mmol/g and 8.78\u00a0mmol/g, respectively, while the C2 yield increase to 5.48\u00a0mmol/g. Case 4, which uses HDPE as the raw material, shows a higher gas yield (46.4\u00a0wt%) with an increase in CH4 yield to 14.31\u00a0mmol/g. This demonstrates that, for different types of plastics, the catalytic effect of biocarbon is slightly different. The best catalytic cracking effect is found in the test using PP where the highest H2 yield (34.3\u00a0mmol/g) is obtained. Moreover, the carbon and hydrogen distribution results, shown in the Supplement Fig. S2, indicate that for PP, the percentage of hydrogen in plastic convert into H2 is the highest (53.5%). The overall cracking effect of biocarbon catalysts for LDPE and HDPE is slightly weaker compared to that for PP. In terms of carbon and hydrogen distributions, the catalyst effect for LDPE appears to be better, as a higher percentage of carbon in the plastic is converted into carbon, and a higher percentage of hydrogen in the plastic is transformed into hydrogen, compared to the results obtained for HDPE. TGA curves of different plastic waste are provided in the Supplement Fig. S3. It can be seen that the decomposition temperature of the plastic waste is in following the order: PP, LDPE, HDPE. It could be seen that the catalytic effect of the biocarbon catalyst decreases with the higher decomposition temperature of the plastic, resulting in less hydrogen in plastic being converted into hydrogen and less carbon in plastic being converted into carbon products.Tests with continuous feeding of the plastic sample (PP, case 5) are conducted to study the stability of the biocarbon catalyst. As mentioned in the experimental section, 5\u00a0g of plastic is fed for each testing cycle and 10\u00a0g of biocarbon catalyst is placed in the cracking reactor. A complete collection of all produced gases for each cycle is challenging, as some gases can be retained in the reactor. However, based on the previous experiment, the peak area of gases detected by GC-MS have the same trend during the experiment, which means the gas fraction remained stable during the experiment. Therefore, the yield of CH4 has been verified to be closely related to the yield of other gaseous products. And a stable gas component to CH4 volumetric ratio for each cycle is calculated as the indicator of the catalyst performance. The results are shown in Fig. 4\n. As seen, during the nine-cycle experiment, the H2 to CH4 ratios are maintained at values between 1.65 and 2 and the C2 compounds to CH 4 ratios are maintained at values between 0.3 and 0.4. The result indicates that the catalyst remained relatively stable during this test period. In total 45\u00a0g of plastic is used which indicates that more than 20\u00a0g of carbon is produced and mixed with 10\u00a0g of biocarbon catalysts. The relatively stable performance also indicates that the carbon products from plastics are prone not to cause a severe blockage of the pores in the biocarbon catalyst. If a more detailed analysis is performed, it can be seen that from the sixth test to the seventh test, the H2 to CH4 ratio plummets from a value of approximately 2.0 to about 1.7 and stabilizes at a value of approximately 1.65 in the subsequent tests. This means the accumulation of carbon products (from plastic) still has a certain effect on the biocarbon performance. Specifically, the production of CH4 seems to be promoted which indicates a certain decrease in the deep cracking ability of the catalyst. A more detailed characterization of the spent biocarbon catalyst will be conducted to understand the potential reason.The fresh biocarbon catalyst appearance is characterized by using SEM determinations. The corresponding SEM images are shown in Fig. 5\n. Fig. 5a and b shows the appearance of biocarbon particles: Fig. 5a shows the aggregation state of the biocarbon particles with irregular shapes, and Fig. 5b focuses on the appearance of the individual biocarbon particles. In general, biocarbon pieces have a longitudinal length of about 1\u00a0mm or more, which is the result of the use of sawdust with particle size larger than 1.25\u00a0mm. Moreover, from Fig. 5a and b, the biocarbon pieces retain the original appearance of the woody sawdust, which is consistent with literature findings (Bridgwater et al., 1999). Correspondingly, elongated channels in the length direction could be observed on the surface of the biocarbon pieces, as shown more clearly in Fig. 5b. Also, Fig. 5c and d shows enlarged views of a specific biocarbon particle in the x-axis and y-axis directions. It can be seen that the biocarbon catalyst has a hierarchical and regular macroporous structure (Fig. 5c) with open channels through the length direction (Fig. 5d), which have been verified to be originated from the vessels in natural wood (Wang et al., 2017). Specifically, the open channels have inner diameters ranging between 10 and 30\u00a0\u03bcm and outer diameter between 13 and 35\u00a0\u03bcm. Instead of straight channels, channels with a certain tortuosity have been observed (Fig. 5d). Moreover, additional pores have been detected along the channel walls.Porous properties of the fresh biocarbon catalyst are characterized by applying a Nitrogen adsorption-desorption analysis. As shown in Table S4, the fresh biocarbon catalyst has a surface area of approximately 40\u00a0m2/g and a micro-pore surface approximately 37\u00a0m2/g indicating a micropore-rich structure of the biocarbon catalyst. N2 adsorption-desorption isotherm, shown in Fig. S4a, shows a IV-type isotherm, which confirms the coexistence of micropores and mesopores. Notably, owing to a micropore-rich structure, the adsorption and desorption isotherms does not overlap. The pore size distribution figure, i.e. Fig. S4b shows that the pore size of the pores inside the biocarbon catalyst. Specifically, pore sizes are concentrated in the range between 8 and 20\u00a0\u00c5, which belongs to the micropore size range. This indicates that, in addition to the regular elongated macropores that formed channels, micropores that cannot be detected by the SEM also existed inside the biocarbon catalyst. The channel walls seem to be the only place where the micropores exist.From the catalyst perspective, a hierarchical macro- and micro-porous structure with elongated channels could provide a large surface area for reactions (Yang et al., 2019). A regular macroporous structure (Fig. 5c) with open channels could significantly promote a diffusion of the intermediates (diffusion into and outside of the catalyst) while maintaining a relatively low pressure drop (Custodis et al., 2016; Trogadas et al., 2016). Moreover, the channels with certain tortuosity have been reported to be able to extend the residence time of intermediates along the channel walls (Wang et al., 2017). Macropores and micropores on the channel walls could further promote the diffusion of intermediates and increase the tortuosity of the intermediates pathway (Bai et al., 2016). On a macroscopic level, the catalyst bed assembly using a biocarbon catalyst with a certain particle size can also form channels with a certain degree of tortuosity. Combining all of these, the residence time of intermediates i.e., the components that forms the plastic pyrolysis volatiles in the biocarbon catalyst bed would be significantly increased. Also an increased residence time at high-temperature catalyst bed would inevitably promote the cracking reactions of the intermediates (Hu et al., 2016). Correspondingly, the excellent performance of biocarbon catalysts for plastic volatiles cracking could be attributed to this. Additionally, due to the steric hindrance effect caused by the pore size (Jae et al., 2011), large organic molecules may encounter obstacles when accessing the catalytic site in the pore (Zhang et al., 2019), resulting in the micropores on the channel walls exhibiting selective catalytic properties for intermediates of different sizes. This could potentially explain the differences in product yield distributions among different cases, due to the different pyrolysis intermediates of PP, LDPE, and HDPE (Honus et al., 2018).The surface morphology and textural property of spent biocarbon catalyst are further characterized to show the catalyst change during the reactions. Specifically, the spent catalyst from the continuous feeding tests is used.\nFig. 6\n shows the SEM images of the spent biocarbon catalyst. As shown in Fig. 6a and b, compared with the fresh biocarbon catalyst, there is no significant change in the particle size for the spent catalyst. It implies that the rapid flushing and reaction of plastic pyrolysis volatiles does not result in a significant breakage of biocarbon particles during the experiment. However, the catalyst surface, especially the exposed open channels is partially covered, as seen in Fig. 6b. To observe the surface of the spent biocarbon more clearly, a higher magnification is chosen, and the enlarged views are displayed in Fig. 6c and d. Extra carbon products covering the surface of the elongated channels are observed clearly. Undoubtedly, these extra carbon products are generated from a plastic pyrolysis and a subsequent cracking of the pyrolysis volatiles. With a further magnification, the carbon products do not have regular morphologies, as shown in Fig. 6d. The results indicate that carbons derived from plastic could remain on the surface and inner pores of the biocarbon catalyst, which may further lead to a pore blockage of the catalyst. The XRD analysis results of both fresh and spent catalyst are provided in Fig. S5. It can be observed that the peak representing the graphite 002 crystal plane has shifted towards the right and has become sharper in the XRD figure. This shift indicates that the catalyst tends towards graphite gradually after use due to prolonged heating and the deposition of carbon products in the pores of the biocarbon catalyst. However, the peak corresponding to the graphite 100 crystal plane does not show significant changes. Table S4 also provides the textural property characterization results for the spent catalyst sample. It can be seen that the spent catalysts have a surface area value lower than 1. This, in turn, indicates a non-porous structure. Correspondingly, it makes no sense to show adsorption-desorption isotherm. The result indicates that the micropores on the channel walls were almost completely blocked. This further confirmed that the carbon derived from the plastic could cause blockage of the catalyst, and thereby cause a deactivation of the catalyst.After the continuous feeding test, plenty of individual carbon products that do not adhere to the biocarbon catalysts are also obtained. This indicates that only a certain percentage of carbon could remain on the surface or inside the inner pores of the catalyst. This is also the reason why the catalyst performance is kept relatively stable after 9 testing cycles. SEM images of these individual collected carbon products are given in Fig. 7\n. As shown in Fig. 7a and b, the overall size of the carbon products are relatively small (<50\u00a0\u03bcm) and the vast majority have a spherical shape. A clear comparison view between biocarbon particles and carbon spheres are provided in Fig. 7b, where several fine biocarbon particles are mixed with carbon products. More apparent carbon spheres could be observed from enlarged views shown in Fig. 7c and d. It can be seen that some of these carbon spheres are independent of each other, and that some of them are stacked to form larger carbon spheres or even carbon strips. More specifically, the diameters of the carbon spheres varies from 10\u00a0\u03bcm to 50\u00a0\u03bcm or larger. This is because the catalysts used in the present study are metal-free, it is believed the cracking of the pyrolytic volatiles results in the agglomerated carbon product in the absence of the metal catalyst (Veksha et al., 2022). This mechanism is similar to the nucleation process observed in the formation of carbon black, where hydrocarbon molecules crack at high temperatures and form spherical carbon on the surface (Jun et al., 2022; Smith, 1982).Raman determinations of the individual carbon products have also been conducted to show the degree of order of the carbon atoms. The corresponding Raman spectra result is shown in Fig. S6. A D-peak represents carbon atoms in a disordering state, and a G-peak represents carbon atoms in the ordering state. These are the two major peaks for Raman spectra (Muzyka et al., 2018). Also, the D-peak is defined as the band between 1320\u00a0cm\u22121 and 1365\u00a0cm\u22121. Furthermore, the G-peak is defined as the band between 1520\u00a0cm\u22121 and 1600\u00a0cm\u22121 (Ferrari, 2007). The ID/IG ratio represents the degree of material defects and disordered structures (Cheng et al., 2021). The ID/IG value of the carbon products is around 1. This demonstrates that the carbon atoms in the individual carbon products have a certain degree of order. Moreover, a clear 2D peak that represents the graphite layer structures could also be observed. This also indicates a certain degree of order of carbon atoms. According to the literature, carbon products from PP plastic belong to soft carbon, which could be used for a series of applications such as batteries (Yaqoob et al., 2022), adsorbents (Shen et al., 2022), and industrial additives (Asl et al., 2018). The regular spherical shape observed by SEM, and the certain degree of order of carbon atoms detected by Raman spectra both indicate the formation of promising carbon products that may be utilized in a variety of applications. Notably, biocarbon derived from biomass is hard carbon, which is not graphitizable (Alvira et al., 2022). To improve the electrochemical performance of hard carbon, many studies have been reported attempts to coat additional soft carbon in the surface and internal pores of hard carbon (He et al., 2021; Lee et al., 2007). This is practically the same as done in the current study. Therefore, the spent biocarbon catalyst also has promising application potential. A further exploration of the applications of the individual carbon products as well the spent biocarbon catalysts mixed with plastic derived carbon will be the focus of our future research.A hierarchical porous biocarbon catalyst has been prepared via the combination of using a biomass pyrolysis and a subsequent carbonization at a certain temperature. The catalyst performance for a waste plastic pyrolysis and an in-line catalytic cracking has been studied to coproduce hydrogen-rich gases and carbon products. The main conclusions are as follows:Compared to the non-catalytic case, all catalytic cases show higher carbon and H2 yields. The deployment of the biocarbon catalyst promotes a cracking of hydrocarbons derived from the plastic pyrolysis into carbon and gas compounds. An increae of the catalyst-to-plastic (C/P) ratio from 0 to 2, leads to an increase of the conversion rate of H in plastics to H2 from 22.7% to 53.5%. Also, a maximum H2 yield of 34.31\u00a0mmol/g plastic (68.6 mg/gplastic) could be obtained by pyrolysis (550\u00a0\u00b0C) and catalytic cracking (C/P\u00a0=\u00a02, 900\u00a0\u00b0C) of PP. The biocarbon catalyst is suitable for different plastics and all applications result in carbon yields of more than 50\u00a0wt% (500 mg/gplastic) as well as high hydrogen yields above 28\u00a0mmol/g plastic (56 mg/gplastic). Moreover, in the continuous experiments, the catalytic activity of the biocarbon catalyst does not decrease significantly, which indicates that the catalyst has a longer lifetime.Biocarbon catalysts have a hierarchical macro- and micro-porous structure consisting of elongated and tortuous channels. The tortuous channels and the existence of the macropores and micropores on the channel walls could significantly increase the tortuosity of the gas flow. Combined with tortuous channels formed by the accumulation of biocarbon of a certain size, the residence time of gases could be significantly promoted. This allows the biocarbon catalyst to effectively facilitate the cracking reactions resulting in an increase of the hydrogen and carbon yields.A spent biocarbon catalyst contains a certain amount of plastic derived carbons coating on the surface of the catalyst. The individual carbon products have regular spherical shapes and the carbon atoms have a highly ordering state. Both the individual carbon products and the spent catalyst have the potential to be used in a variety of applications.Yanghao Jin: Investigation, Validation, Formal analysis, Visualization, Writing \u2013 original draft. Hanmin Yang: Investigation, Validation, Formal analysis, Visualization, Supervision. Yanghao and Hanmin contribute equally to this article, Guo Shuo: Investigation, Conceptualization., Ziyi Shi: Validation, Formal analysis, Visualization. Tong Han: Conceptualization, Methodology, Data curation, Supervision, Writing-Reviewing and Editing, Formal analysis. Ritambhara Gond: Investigation, Weihong Yang: Resources, Writing-Reviewing and Editing, Supervision, Project administration. P\u00e4r G. J\u00f6nsson: Supervision, Writing-Reviewing and Editing.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Yanghao Jin reports financial support was provided by Sweden's Innovation Agency. Hanmin reports financial support was provided by Chinese scholarship Concil. Tong Han reports a relationship with Chinese Scholarship Council that includes: funding grants.The supply of raw materials for the experiments by Envigas is greatly appreciated. The financial support by VINNOVA-Swedish Innovation Agency with the project number 2021\u201303735 is higher appreciated. Tong Han and Hanmin Yang, would also like to acknowledge funding from Chinese Scholarship Council (CSC).The following is the Supplementary data to this article.\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2023.136926.", "descript": "\n                  Carbon and H2 recoveries from plastic waste enable high value-added utilizations of plastic waste while minimizing its GHG emissions. The objective of this study is to explore the use of a metal-free biocarbon catalyst for waste plastic pyrolysis and in-line catalytic cracking to produce H2-rich gases and carbon. The results show that the biocarbon catalyst exhibits a good catalytic effect and stability for various plastic wastes. Increasing the C/P ratio from 0 to 2, induce an increase in the conversion rate of C and H in plastics to carbon and H2 from 57.1% to 68.7%, and from 22.7% to 53.5%, respectively. Furthermore, a carbon yield as high as 580.6 mg/gplastic and an H2 yield as high as 68.6 mg/gplastic can be obtained. The hierarchical porous structure with tortuous channels of biocarbon extends the residence time of pyrolysis volatiles in the high-temperature catalytic region and thereby significantly promotes cracking reactions.\n               "}
{"full_text": "With the reduction of industrial hydrogen production costs and the development of hydrogen fuel cell technology, hydrogen has gradually become one of the most promising clean energy in the 21st century [1]. Noticeably, hydrogen storage is a crucial link in rolling out infrastructure construction to build a \u201chydrogen economy,\u201d especially in terms of the extensive applications in hydrogen compressor, fuel cell vehicle (FCV), as well as grid-scale hydrogen energy storage [2\u20135]. More specifically, it is still urging to develop hydrogen storage technologies with the characteristics of high gravimetric capacity, low cost, high safety, and reliability. Compared with the high-pressure gaseous and low-temperature liquid storage technologies, the solid-state hydrogen storage technology is highly promising, due to its relatively higher gravimetric or volumetric density, safety, and economy [6,7]. Owing to the high theoretical gravimetric capacity (7.6\u00a0wt.% for pure MgH2) and relatively abundant resources, Mg-based materials have been receiving widespread coverage from researchers as one of the most promising carriers for on-board hydrogen storage devices [8\u201310]. But the relatively high desorption temperature (>300\u00a0\u00b0C) and slow sorption kinetics make them difficult to meet the requirements of the fuel cell module (<85 \u00b0C) and on-board hydrogen storage devices. Besides, Mg-based materials still have the problem in terms of the capacity fade during the ab/desorption cycles.Doping additives or catalysts, such as transition metal-based catalysts (such as Nb, Ti, V, and Ni, etc.), has been considered as one of the most effective strategies to improve the hydrogen storage performance of Mg/MgH2. It can effectively accelerate the dissociation and combination of hydrogen atoms and decrease the activation energy for hydrogen desorption [11\u201314], thus improving the dehydrogenation kinetics and lowering the dehydrogenation temperatures [15\u201321]. For example, Wang et\u00a0al. [15] demonstrated that the NNb2O5 (10\u00a0wt.%)-doped MgH2 composite has an excellent desorption performance, releasing 5.0\u00a0wt.% H2 at 250\u00a0\u00b0C within 3\u00a0min. By exfoliating the Ti3AlC2 powders to synthesize 2D Ti3C2 (MXene), Liu et\u00a0al. [20] obtained MgH2 containing 5\u00a0wt.% Ti3C2 that can release 6.2\u00a0wt.% H2 at 300 \u00b0C within 1\u00a0min, exhibiting superior dehydrogenation kinetics to counterparts doped with other Ti-based catalysts. In particular, metallic Ni is also an efficient and low-cost catalyst to significantly improve the hydrogen storage performance of Mg/MgH2. To name a few, Liu et\u00a0al. [21] added the porous Ni@rGO to MgH2 by ball milling, and the formed MgH2+5\u00a0wt.% Ni@rGO nanocomposite can still release 6\u00a0wt.% H2 at 300 \u00b0C within 10\u00a0min after the 9th cycle. Although the excellent catalytic effects, the amount of doped catalyst should be limited largely, especially for some heavy elements since it would lower the practical hydrogen storage capacity of the Mg/MgH2 system. Therefore, the catalytic activity of the doped catalyst should be effectively improved to promote the dehydrogenation kinetics of MgH2 as much as possible, so as to maintain the high hydrogen storage capacity of Mg/MgH2.Recently, it is interesting to find that the reduction in size and improvement of dispersity would increase the catalytic activity of the catalyst [22\u201325]. For instance, Zhang et\u00a0al. [23] employed a wet-chemical method to prepare the NbH\nx\n (\u223c10\u201350\u00a0nm-sized nanoparticles) and doped it into MgH2 to improve its hydrogen storage properties, which can release 7.0\u00a0wt.% H2 within 9\u00a0min at 300\u00a0\u00b0C. In addition, their experimental results also concluded that the smaller the particle size of the NbH\nx\n was, the better catalytic effect on hydrogen storage performance of MgH2 would be. In this regard, Chen et\u00a0al. [24] reported that the well-distributed Ni nanoparticles (NPs) can provide more active catalytic sites for the absorption and desorption cycles. Specifically, the homogeneous distribution of super Ni NPs (uniform size of \u223c10\u201320\u00a0nm) on the surface of MgH2 was achieved by breaking the 1D fibrous Ni via ball milling. Accordingly, the MgH2 doping with 4 mol% Ni NPs composites can dehydrogenate 7.02\u00a0wt.% H2 within 11\u00a0min at 325\u00a0\u00b0C. More impressively, the ultrafine catalyst with homogeneous dispersity can be tailored from some precursors, such as some transition metal MXene and metal organic frameworks (MOF) [26\u201330]. For example, Jia et.al [27] obtained ultrafine Ni NPs (2\u20133\u00a0nm) from Ni-MOF-74 in the MgH2 matrix by a mechanochemical-force-driven procedure, which improved the hydrogen absorption/desorption processes of Mg/MgH2 and was proven by theoretical calculations and experiments. Huang et\u00a0al. [30] used MOF as a precursor to homogeneously disperse metallic Ni on Ti3C2. The synthesized MgH2+10\u00a0wt.% Ni@C-MXene composite can release about 5.6\u00a0wt.% H2 within 2\u00a0min at 300 \u00b0C and absorb approximately 5\u00a0wt.% H2 within 2\u00a0min under 3.2\u00a0MPa at 150 \u00b0C, possessing an excellent cycling stability (e.g., without obvious decay for both capacity and kinetics after 10 cycles). Particularly, a common Ni-based metal-organic complex named nickel acetylacetonate (Ni(acac)2) has been often used as a precursor for high-efficiency catalysts [31]. In comparison with other Ni-based compounds (e.g., NiCl2 and NiF2), Ni(acac)2 as catalyst precursor has a lower melting point (238 \u00b0C) [32,33], which is beneficial for the smaller particle size and better dispersion [34]. Nonetheless, many ultrafine catalysts-doped Mg/MgH2 systems still show the obvious degradation of cycle stability. [35].It has been found that the addition of carbon can be used as a grinding aid to inhibit the grain aggregation and growth of Mg/MgH2 during cycle-life (kinetics). Various carbon-based materials, such as activated carbon, carbon nanotubes, graphite, graphene, and its derivatives, are considered as additives [36\u201340], among which carbon nanotubes and graphite are typical representatives for the ideal candidates. For example, Liu et\u00a0al. [41] supported the Co/Pd catalysts on bamboo-shape carbon nanotubes to obtain MgH2\nCo/Pd@B-CNTs composite, which can absorb 6.68\u00a0wt.% H2 at 250\u00a0\u00b0C within 10\u00a0s. Wang et.al [42] developed a graphene-guided and growth process to prepare N-doped Nb2O5@C nanorods and the MgH2 with 10\u00a0wt.% N-doped Nb2O5@C can release 6.2\u00a0wt.% H2 from 170 \u00b0C to 270 \u00b0C, which has a capacity retention of 98% after 50 cycles. It should be noted that expanded graphite (EG) is one of the cheapest and most efficient carbon materials [37].The above descriptions indicate that a suitable transition metal catalyst precursor and carbon materials (especially the EG) could be introduced into the Mg/MgH2 matrix to in situ form the ultrafine and well-dispersed catalyst with high catalytic activity, thereby improving the dehydrogenation kinetics and cycle stability of Mg/MgH2 system while maintaining the high hydrogen capacity (e.g., over 7\u00a0wt.%) for the target of the on-board application. Herein, a facile one-step high-energy ball milling technique has been developed to in situ form ultrafine Ni nanoparticles catalyst in the MgH2 matrix, combining the nickel acetylacetonate as a precursor and EG. On one hand, the in situ formed ultrafine Ni nanoparticles catalyst from the Ni(acac)2 can significantly improve the desorption kinetics of MgH2. On the other hand, the cycle performance of Mg/MgH2 is improved by the low-cost and effective EG. Consequently, the formed MgH2\nNi-EG nanocomposite with the optimized doping amounts of Ni and EG can release 7.03\u00a0wt.% H2 within 8.5\u00a0min at 300 \u00b0C after 10 cycles. The exceptional hydrogen storage performance was credited to a 26.9% decrease in the dehydrogenation activation energy in comparison with pure MgH2. In addition, the evaluation process of Ni(acac)2 and was revealed on the basis of the microstructural characterization analysis.The high-purity Ni(acac)2 (99%, Aladdin), MgH2 (98%, Aladdin), Ni powder (99%, Maclin), and expandable graphite (XingRuiDa Graphite manufacturing Co., Ltd.) were used as raw materials. The received expandable graphite was annealed in an Ar atmosphere at 1300\u00a0\u00b0C for 2\u00a0h, and then sintered in a H2 atmosphere at 400\u00a0\u00b0C for 4\u00a0h to obtain EG. Ni(acac)2 was doped into the commercial MgH2 at mass percentages of x = 1\u00a0wt.%, 3\u00a0wt.%, 5\u00a0wt.%, 7\u00a0wt.%, and 10\u00a0wt.%, respectively, via a vibration-type ball mill (QM-3C, Nanjing, China) at 1200\u00a0rpm for 5\u00a0h under H2 pressure of 1.5\u00a0MPa. Further, the EG was introduced into MgH2 with Ni(acac)2 together by ball milling. The mass ratio of MgH2, Ni(acac)2, and EG is 97:1.5:1.5 (denoted as MgH2\nNi-EG). For the comparison, pure Ni powder was also doped into MgH2 (donated as MgH2\nNip-EG) by ball milling for 5 h The ball-to-sample radio was around 50:1 during the milling process, which was conducted for 30\u00a0min after every 30\u00a0min pause.The X-ray diffraction (XRD) equipped with Cu K\u03b1 radiation (\u03bb\u00a0=\u00a00.15,418\u00a0nm) operated at 45\u00a0kV and 40\u00a0mA was used to identify the phase of the samples. The XRD data was captured in a 2\u03b8 range of 15\u00b0\u223c85\u00b0 with a step of 0.026\u00b0. A scanning electron microscope (Zeiss Supra-40) and a transmission electron microscope (JEM-2100, Japan) were used for observing morphologies and microstructures of the samples. X-ray photoelectron spectroscopy (XPS) spectra (Thermo Fisher Scientific K-Alpha) were performed with a monochromatic Al K\u03b1 X-ray source at a base pressure of 5\u00a0\u00d7 10\u22129 mbar to obtain the relevant valence information about the sample. The XPS data were fitted using Avantage software.The hydrogen sorption properties of materials were measured by using PCT Pro2000, in which the sample with a mass of 180\u00a0\u00b1 5\u00a0mg was loaded into a stainless-steel sample holder. For non-isothermal dehydrogenation tests, the sample was heated at a heating rate of 2\u00a0K/min under a vacuum environment for desorption. For isothermal measurements, the sample was heated to the preset temperature at a heated rate of 5\u00a0K/min and then start the next dehydrogenation experiments. In addition, the sample with the mass of 9.5\u00a0\u00b1 0.5\u00a0mg was loaded into an alumina crucible for thermal analysis (DSC, Setaram SENSYS Evolution) and it was heated from room temperature to 450 \u00b0C at different rates (2, 7, 10 and 15\u00a0K/min, respectively) under an argon atmosphere. Before the measurement for DSC, the MgH2\nNi-EG and MgH2\nNip-EG composites were activated via dehydrogenation and hydrogenation procedures at 300 \u00b0C.Ni(acac)2 precursors with different mass fractions were added to MgH2 via vibratory-type high-energy ball milling to investigate the effect of Ni(acac)2 addition on the hydrogen storage performance of MgH2. Fig.\u00a01\n shows the XRD results of MgH2+x wt. Ni(acac)2 (x = 1, 3, 5, 7, and 10) samples after high-energy ball milling which can provide sufficient energy for the reaction between them. Obviously, the diffraction peaks associated with the \u03b2-MgH2, \u03b3-MgH2, and a little amount of MgO phases can be well indexed in the ball-milled MgH2+x wt. Ni(acac)2 samples. Characteristic peaks related to Ni(acac)2 cannot be found, indicating the chemical reaction between MgH2 and Ni(acac)2 or the decomposition of Ni(acac)2 during the ball milling process. However, there are no diffraction peaks of Ni and/or Ni-based compounds in the XRD patterns. It may be due to the small content of the Ni phase or the small size in situ formed Ni particles from Ni(acac)2 (as evidenced by the HRTEM observations in the following section), which might result in the corresponding diffraction peaks being too weak to detect. In addition, it should be noted that the peak intensity of the MgO phase become stronger with the increase of the mass percent of Ni(acac)2 (Fig. S1), which suggests that the O element might come from the C\u00a0=\u00a0O group of Ni(acac)2 to facilitate the formation of the MgO phase.Non-isothermal dehydrogenation curves of MgH2+x wt.% Ni(acac)2 samples (x = 1, 3, 5, 7, and 10) were firstly tested to explore the influence of Ni(acac)2 precursor on the hydrogen storage performance of MgH2, which was also compared with the pure MgH2 treated under the same conditions of ball milling. Specifically, the temperature at which the sample releases 0.1\u00a0wt.% H2 was used as the initial dehydrogenation temperature during the non-isothermal dehydrogenation test. As shown in Fig.\u00a02\n(a), it can be clearly seen that the initial dehydrogenation temperature of the MgH2+x wt.% Ni(acac)2 samples decrease with the increase of the addition of Ni(acac)2, i.e., around 260 \u00b0C, 254 \u00b0C, 245 \u00b0C, 243 \u00b0C and 234 \u00b0C for x = 1, 3, 5, 7, and 10, respectively. These initial hydrogen release temperatures of all MgH2+x wt.% Ni(acac)2 samples are lower than 265 \u00b0C of as-milled pure MgH2. In addition, the kinetics of MgH2+x wt.% Ni(acac)2 samples in the subsequent dehydrogenation are better than that of the as-milled MgH2 sample. It should also be noted that the hydrogen storage capacity of the MgH2+x wt.% Ni(acac)2 samples is decreased with the increase of the addition of Ni(acac)2.Furthermore, the first and second-cycle isothermal dehydrogenation curves of the MgH2+x wt.% Ni(acac)2 samples and as-milled MgH2 at 300 \u00b0C under initial H2 pressure of 0.05\u00a0bar were given and compared in Fig.\u00a02(b) and (c), respectively. Obviously, all the MgH2+x wt.% Ni(acac)2 samples demonstrate the much better dehydrogenation kinetics than the as-milled MgH2 sample. Fig.\u00a02(b) shows that the MgH2+x wt.% Ni(acac)2 samples can release H2 ranging from 7.26\u00a0wt.% to 6.66\u00a0wt.% at 300 \u00b0C in the first cycle, with increasing the doping amount of Ni(acac)2 from 1\u00a0wt.% to 10\u00a0wt.%. And the increment in the doping amount of Ni(acac)2 could also speed up the dehydrogenation kinetics of MgH2 in the first dehydrogenation process. Noticeably, the dehydrogenation kinetics of the MgH2+x wt.% Ni(acac)2 are further accelerated in the second cycle (Fig.\u00a02(c)), especially for the MgH2+x wt.% Ni(acac)2 samples with relatively small Ni(acac)2 amount. For example, the MgH2+1\u00a0wt.% Ni(acac)2 sample spends \u223c 1580s to release 6.9\u00a0wt.% H2 in the first dehydrogenation process, as shown in Fig.\u00a02(b-c), which is significantly reduced to be \u223c 758\u00a0s in the second dehydrogenation process. When the addition of Ni(acac)2 exceeds a certain value (i.e., higher than 3\u00a0wt.%) at 300 \u00b0C, the dehydrogenation kinetics of MgH2+x wt.% Ni(acac)2 samples would keep unimproved in the second dehydrogenation cycle, while the hydrogen capacity is decreased correspondingly, as shown in Fig.\u00a02(c). In other words, the MgH2+3\u00a0wt.% Ni(acac)2 sample may exhibit the best combination of the dehydrogenation kinetics and hydrogen storage capacity (\u223c 6.9\u00a0wt.% H2 for the second dehydrogenation test) at 300 \u00b0C, showing the sufficient catalytic effect without scarifying the hydrogen capacity. This optimized doping amount is very important for designing the subsequent experiments, which will be discussed later.In addition, the isothermal dehydrogenation kinetics curves of the MgH2+x wt.% Ni(acac)2 samples were measured at a lower temperature of 275 \u00b0C under initial H2 pressure of 0.02\u00a0bar to further understand the effect of Ni(acac)2 addition on the dehydrogenation kinetics of MgH2, as shown in Fig.\u00a02(d), exhibiting the significantly enhanced dehydrogenation kinetics performance in comparison with the as-milled MgH2 sample. Noticeably, when the added amount of Ni(acac)2 is increased to 5\u00a0wt.%, the improvement in the dehydrogenation kinetics of MgH2 will be not obvious anymore. For instance, the MgH2+5\u00a0wt.% Ni(acac)2 sample releases 6.27\u00a0wt.% H2 within 10\u00a0min and 6.5\u00a0wt.% H2 within 13\u00a0min, respectively. Similarly, the 10\u00a0wt.% Ni(acac)2-doped sample can release the slightly less hydrogen capacity of 6.15\u00a0wt.% within 10\u00a0min, which might be caused by the more addition of Ni(acac)2. Different from that at 275\u00a0\u00b0C, it is noted that the dehydrogenation kinetics of the MgH2+10\u00a0wt.% Ni(acac)2 sample can be significantly faster than MgH2+5\u00a0wt.% Ni(acac)2 sample at two lower temperatures of 250 \u00b0C under initial H2 pressure of 0.002\u00a0bar and 225 \u00b0C under initial H2 pressure of 0\u00a0bar, respectively, as shown in Fig.\u00a03\n. It is reasonable to see in Fig.\u00a03(a) that MgH2+10\u00a0wt.% Ni(acac)2 sample with more addition results in a lower reversible hydrogen storage capacity than the MgH2+5\u00a0wt.% Ni(acac)2 sample at 250 \u00b0C. However, Fig.\u00a03(b) shows that both MgH2+5\u00a0wt.% Ni(acac)2 and MgH2+10\u00a0wt.% Ni(acac)2 sample can desorb equivalent H2 capacity of 5.6\u00a0wt.% within 120\u00a0min at the lower temperature of 225 \u00b0C, but the dehydrogenation kinetics of the former is slower than the latter. For example, the MgH2+5\u00a0wt.% Ni(acac)2 sample releases 3.6\u00a0wt.% H2 with 1\u00a0h, significantly lower than 4.6\u00a0wt.% H2 for the MgH2+10\u00a0wt.% Ni(acac)2 sample. The above dehydrogenation curves of MgH2+x wt.% Ni(acac)2 tested at various temperatures indicate that appropriately controlling the amount of the additive could achieve the optimal combination of the fast desorption kinetics and high hydrogen storage capacity at a certain temperature.The results of dehydrogenation tests suggest that MgH2+3\u00a0wt.% Ni(acac)2 sample might have the optimal combination of reversible hydrogen storage capacity and desorption kinetics at 300 \u00b0C. Subsequently, the cycle stability of the MgH2+3\u00a0wt.% Ni(acac)2 sample was further measured at 300 \u00b0C under initial H2 pressure of 0.05\u00a0bar using isothermal dehydrogenation mode, as shown in Fig.\u00a04\n(a). It is found that the capacity of MgH2+3\u00a0wt.% Ni(acac)2 sample is decayed rapidly from 7.15\u00a0wt.% in the first cycle to 6.73\u00a0wt.% in the fifth cycle during the dehydrogenation cycle-life (kinetics), showing the relatively poor cycle stability. This phenomenon may be related to the agglomeration of Mg/MgH2 and catalysts during high-temperature cycles [25,40,43], leading to the incomplete hydrogenation of Mg (also evidenced by our XRD results in Fig.\u00a08(b) in the later section). Therefore, the prepared EG with the same weight fraction was introduced to improve the cycle stability of the MgH2, denoting as MgH2+3\u00a0wt.% EG. And the XRD pattern and SEM image of EG were shown in Fig. S2 and Fig. S3, respectively. Owing to the huge improvement in the cycle stability from EG, as shown in Fig.\u00a04(b) and (c), the MgH2+3\u00a0wt.% EG sample can maintain the hydrogen capacity of 7.05\u00a0wt.% after 5 cycles. To balance the dehydrogenation kinetics and cycle stability, the Ni(acac)2 and EG were added to MgH2 together with a mass ratio of 1:1, namely MgH2:Ni(acac)2:EG=97:1.5:1.5 (donated as MgH2\nNi-EG)).Accordingly, the hydrogen storage performance, including the isothermal hydrogenation and dehydrogenation measurements, was characterized for the MgH2\nNi-EG nanocomposite. Fig.\u00a05\n(a) shows the isothermal dehydrogenation curves of MgH2\nNi-EG nanocomposite at different temperatures of 275 \u00b0C, 300 \u00b0C, and 320 \u00b0C, respectively. Excitingly, the MgH2\nNi-EG nanocomposite can release 7.0\u00a0wt.% H2 within 9.3\u00a0min at 300 \u00b0C and within 4.2\u00a0min at 320 \u00b0C, respectively. To the best of our knowledge, this should be the best performance in the literature in terms of the dehydrogenation hydrogen capacity of 7.0\u00a0wt.% for the Ni catalyst. The final dehydrogenation capacity of the MgH2\nNi-EG nanocomposite is reduced to 6.8\u00a0wt.% at 275 \u00b0C, which is still significantly better than pure MgH2 that only can desorb 0.55\u00a0wt.% H2 at 300 \u00b0C within the same period of dehydrogenation time. Fig.\u00a05(b) shows the hydrogen absorption performance of the MgH2\nNi-EG nanocomposite at several different temperatures ranging from 50 \u00b0C to 125 \u00b0C under initial H2 pressure of 50 bar It turns out that MgH2\nNi-EG nanocomposite can absorb 5.6\u00a0wt.% H2 within 60\u00a0min at 100 \u00b0C, and the finally reach 6.3\u00a0wt.% within 2 h When the temperature increases to 125\u00a0\u00b0C, the hydrogen absorption kinetics of the sample is greatly accelerated, absorbing 6.0\u00a0wt.% H2 within 40\u00a0min and 6.51\u00a0wt.% H2 within 75\u00a0min. Even at a low temperature of 50 \u00b0C, 2.42\u00a0wt.% and 3.44\u00a0wt.% H2 can still be absorbed within 1\u00a0h and 2\u00a0h, respectively, which is much better than that of the pure as-milled MgH2. In addition, the hydrogen absorption kinetics of MgH2\nNi-EG and MgH2+3\u00a0wt.% Ni(acac)2 samples are given and compared in Fig. S4, which shows that the hydrogen absorption kinetics of MgH2\nNi-EG sample are slower than MgH2+3\u00a0wt.% Ni(acac)2 sample at relatively low temperatures of 50 \u00b0C and 100 \u00b0C. When the temperature increasing to 125 \u00b0C, the MgH2\nNi-EG sample exhibits a better hydrogen absorption kinetics and higher hydrogen absorption capacity than that of the MgH2+3\u00a0wt.% Ni(acac)2 sample, consistent with the dehydrogenation kinetics performance.\nFig.\u00a05(c) gives the dehydrogenation kinetics curves of MgH2\nNip-EG composite at the same temperatures of 275 \u00b0C, 300 \u00b0C, and 320 \u00b0C, respectively, to compare with the MgH2\nNi-EG nanocomposite using Ni(acac)2 as a precursor. Since the content of Ni in Ni(acac)2 is \u223c22.6\u00a0wt.%, the actual Ni content in the MgH2\nNi-EG nanocomposite is 0.33\u00a0wt.%. Therefore, the metallic Ni powder with the amount of 0.33\u00a0wt.% and EG with 1.5\u00a0wt.% were selected to prepare the MgH2\nNip-EG sample for comparison. Fig.\u00a05(c) shows that the MgH2\nNip-EG composite needs 25.5\u00a0min to desorb 7.0\u00a0wt.% H2 at 300 \u00b0C, obviously longer than that 9.3\u00a0min for the MgH2\nNi-EG nanocomposite. In addition, the MgH2\nNi-EG nanocomposite also shows faster dehydrogenation kinetics at both 275 \u00b0C and 320 \u00b0C. As a concluding point, Fig.\u00a05(d) compares the dehydrogenation kinetics curves of different samples at 300 \u00b0C. Although the MgH2+3\u00a0wt.% Ni(acac)2 sample shows the best dehydrogenation kinetics, the poor cycle stability and the loss of theoretical hydrogen storage capacity due to the reaction of Ni(acac)2 with MgH2 lead to the necessity to further optimize its performance. Thanks to the excellent catalytic performance of the catalyst, the MgH2\nNi-EG nanocomposite can not only speed up the dehydrogenation kinetics rate but also increase the dehydrogenation capacity, compared to the MgH2+3\u00a0wt.% EG, MgH2\nNip-EG, and as-milled MgH2 samples. Thus, replacing part of Ni(acac)2 by EG to improve the cycle stability of the composite is necessary, as indicated by our experimental results.Further, the effects of Ni(acac)2 and EG on the dehydrogenation process of MgH2\nNi-EG nanocomposite was analyzed by DSC in Fig.\u00a06\n(a), which indicates that the dehydrogenation peak temperatures of MgH2\nNi-EG nanocomposite are 293.20 \u00b0C, 321.35 \u00b0C, 329.20 \u00b0C, and 340.04 \u00b0C at the heating rates of 2, 7, 10, and 15\u00a0K/min, respectively. And the pure MgH2 and MgH2\nNip-EG were also studied by the DSC method, as shown in Fig. S5 and Fig. S6. Based on DSC curves at different heating rates, the dehydrogenation activation energy (Ea\n) can be calculated using Kissinger's method as follows [44]:\n\n(1)\n\n\n\n\nd\n\n(\n\nl\nn\n\n\u03b2\n\nT\n\nm\na\nx\n\n2\n\n\n\n)\n\n\n\nd\n\n(\n\n1\n\nT\n\nm\na\nx\n\n\n\n)\n\n\n\n=\n\n\u2212\n\n\nE\na\n\nR\n\n\n\n\nwhere Ea\n is the apparent activation energy (kJ\u00b7mol\u22121), \u03b2 is the heating rate(K/min), Tmax\n is the absolute temperature for the maximum reaction rate(K), and R is the gas constant(J/(K\u00b7mol)), respectively. Based on Eq.\u00a0(1), the Ea\n can be obtained by linearly fitting the slope of the plot with ln(\u03b2/T2\n\nmax\n) versus 1/T (Fig.\u00a06(b)). Accordingly, the dehydrogenation activation energy (Ea\n) of MgH2\nNi-EG nanocomposite is linearly fitted to be 114.7 kJ\u00b7mol\u22121, which is lower than 133.0 kJ\u00b7mol\u22121 for MgH2\nNip-EG sample and 157.1 kJ\u00b7mol\u22121 for pure MgH2, respectively. On the other hand, Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation can be expressed as [42]:\n\n(2)\n\n\nln\n\n[\n\n\u2212\nln\n\n(\n\n1\n\u2212\n\u03b1\n\n)\n\n\n]\n\n=\n\n\n\u03b7\n\nln\n\n\nk\n+\n\n\n\u03b7\n\nln\n\nt\n\n\n\n\nwhere \u03b1 is the dehydrogenation reaction fraction at time t, \u03b7 is Avrami index, k is the dehydrogenation rate constant and t is the reaction time, respectively. Combining the Arrhenius equation, the dehydrogenation activation energy of the MgH2\nNi-EG sample can also be obtained by a linearly fitting plot with lnk versus 1000/T.\nFig.\u00a06(c) shows the fitting results of the dehydrogenation kinetics curves for the MgH2\nNi-EG sample at 275\u00a0\u00b0C, 300\u00a0\u00b0C, and 320\u00a0\u00b0C, respectively, contributing to a dehydrogenation activation energy of 118.1 kJ\u00b7mol\u22121 in Fig.\u00a06(d). It should be noticed that the dehydrogenation activation energy of the MgH2\nNi-EG sample calculated by the above two methods is very close. Both the high reversible hydrogen storage capacity of 7\u00a0wt.% and the faster dehydrogenation kinetics for the MgH2\nNi-EG sample can be understood by the decreased dehydrogenation activation energy.The excellent reversible hydrogen storage capacity of the MgH2\nNi-EG sample over 7.0\u00a0wt.% was also confirmed by the dehydrogenation PCI curves (Fig.\u00a07\n) at 300 \u00b0C, 320 \u00b0C, and 340 \u00b0C, respectively. Consistently, the dehydrogenation PCI curves in Fig.\u00a07(a) show that MgH2\nNi-EG nanocomposite has a hydrogen storage capacity of \u223c7.1\u00a0wt.% at all temperatures. In addition, based on the dehydrogenation plateau pressure corresponding to the different temperatures (Table S1), the linearly fitted slope of the van't Hoff curve (Fig.\u00a07(b)) denotes the dehydrogenation reaction enthalpy (\u0394H) of 76.5\u00a0kJ mol\u22121 for the MgH2\nNi-EG sample, which is almost equivalent to that of pure MgH2, indicating that the addition of Ni(acac)2 and EG hardly influences the thermodynamic property of MgH2.The XRD patterns at different states of MgH2\nNi-EG nanocomposite during ball milling and the cycle-life(kinetics) process were obtained in Fig.\u00a08\n to clarify the evolution process of Ni(acac)2. Fig.\u00a08(a) shows that the diffraction peaks of \u03b2-MgH2, \u03b3-MgH2, and trace amount MgO phases can be found in the as-milled MgH2\nNi-EG sample. As mentioned earlier, the formation of the MgO phase may be due to the reaction between MgH2 and Ni(acac)2 during ball milling (Fig.\u00a08(c)). However, it should be noted that neither metallic Ni nor Ni-based compounds were detected from the XRD patterns in the as-milled, dehydrogenated, and rehydrogenated samples. It might be due to the low concentration of Ni being ultrafine particles. In this regard, the XRD pattern of as-milled, dehydrogenated, and rehydrogenated MgH2+10\u00a0wt.% Ni(acac)2 sample with a higher amount of Ni were obtained in Fig.\u00a08(b), in which Mg2Ni and Mg2NiH4 phases can be well-indexed. Admittedly, the in situ formed Mg2Ni/Mg2NiH4 can actively affect the re/dehydrogenation process of MgH2 as a \u201chydrogen pump\u201d [45].XPS was also used to analyze the chemical states of Ni in the sample after ball milling. Similarly, an effective XPS signal related to the Ni might still not be detected in the MgH2\nNi-EG nanocomposite because of the too low concentration of Ni. Therefore, the high-resolution spectrum of the Ni 2p was obtained from the as-milled MgH2+10\u00a0wt.% Ni(acac)2 sample, which is shown in Fig.\u00a08(d). The Ni 2p spectrum exhibits two 2p1/2 and 2p3/2 contributions located at 852.38 and 869.98\u00a0eV, respectively, which can be assigned to Ni0. And the characteristic peak at 873.78\u00a0eV is the satellite peak of Ni0 2p1/2, also implying that the valence state of metal Ni is 0. In addition, Fig.\u00a09\n gives the HRTEM images of the microstructure and distribution of the in situ formed ultrafine Ni particles in the as-milled MgH2\nNi-EG nanocomposite. As marked by the yellow lines, two typical diffraction rings in SAED patterns can be indexed to (211) planes of MgH2 (PDF#01\u2013074\u20130934) and (220) planes of metallic Ni (PDF#01\u2013070\u20130989) phases, respectively, in the as-milled MgH2\nNi-EG sample. In addition, the HRTEM image of the as-milled MgH2\nNi-EG nanocomposite in Fig.\u00a09(c) also confirms the existence of these two phases. More specifically, the interplanar spacing of d\n(111)\u00a0=\u00a00.287\u00a0nm for \u03b3-MgH2, d\n(101)\u00a0=\u00a00.255\u00a0nm for \u25a1-MgH2, and d\n(111)\u00a0=\u00a00.204\u00a0nm for Ni are measured, respectively. In addition, a Fast Fourier Transform (FFT) pattern of the area circled by the red circle in Fig.\u00a09(c) is also given in its insert, showing the (111) plane of the metallic Ni phase. It should be noted that the ultrafine metallic Ni (4\u20135\u00a0nm) is in situ formed and dispersed uniformly in the MgH2 matrix, as shown in Fig.\u00a09(c), which is consistent with the reference work [27]. Therefore, the evolution process of Ni(acac)2 can be described as follows:\n\n(3)\n\n\nMg\n\nH\n2\n\n\n+\n\n\nNi\n\n\n\n(\nacac\n)\n\n2\n\n\n\u2192\n\n\nMgO\n\n\n+\n\n\nNi\n\n\n\n\n\n\n\n(4)\n\n\nMg\n\nH\n2\n\n\n+\n\n\nNi\n\n\n\u2192\n\n\nM\n\n\ng\n2\n\n\nNi\n\n\n+\n\n\nH\n2\n\n\n\n\n\n\n\n(5)\n\n\nM\n\ng\n2\n\n\nNi\n\n\n+\n\n\nH\n\n2\n\n\n\n\u2192\n\n\nM\n\n\ng\n2\n\nNi\n\nH\n4\n\n\n\n\n\nThe cycling stability of MgH2\nNi-EG nanocomposite has been evaluated in Fig.\u00a010\n for the purpose of the potential practical application. During the cycling testing, the MgH2\nNi-EG nanocomposite was operated to absorb H2 at 300 \u00b0C under initial H2 pressure of 50\u00a0bar for 25\u00a0min and then started the dehydrogenation kinetics test at the same temperature under initial H2 pressure of 0.05 bar Fig.\u00a010 shows that the MgH2\nNi-EG nanocomposite is able to maintain a high hydrogen storage capacity of 7.03\u00a0wt.% even after 10 hydrogen ab/desorption cycles. It achieves a hydrogen capacity retention of up to 97.2% (Fig. S7), which is referred to the second-cycle capacity. Surprisingly, the MgH2\nNi-EG nanocomposite exhibits faster dehydrogenation kinetics with the increase of the cycle number. There is no doubt that the in situ formed ultrafine and uniformly dispersed metallic Ni from Ni(acac)2 precursor and the EG combined in the MgH2\nNi-EG nanocomposite can significantly improve the hydrogen storage performance of MgH2, including the dehydrogenation kinetics, high hydrogen capacity, and cycle stability.\nFig.\u00a011\n shows the TEM results of the MgH2\nNi-EG sample after 10th dehydrogenation. It can be proved from the SAED patterns (Fig.\u00a011(a)) that the main phases of the dehydrogenated MgH2\nNi-EG sample are Mg and Mg2Ni phases, which is consistent with the XRD results (Fig.\u00a08(b)). The interplanar spacing d\n(101)\u00a0=\u00a00.245 and d\n(200)\u00a0=\u00a00.245 can be also well-indexed for Mg (PDF#01\u2013089\u20137195) and Mg2Ni (PDF#01\u2013075\u20131249) phases, respectively, in the HRTEM image (Fig.\u00a011(b)). Note that the in situ formed Mg2Ni distributed around the Mg particles can act as catalytic active sites. In addition, the EDX analysis of the dehydrogenated MgH2\nNi-EG sample (Fig.\u00a011(c-f)) shows that the Ni and C elements are still homogeneously distributed on the MgH2 matrix, in lieu of aggregation after 10 dehydrogenation/hydrogenation cycles. Therefore, the TEM observations reveal that the in situ formed Mg2Ni/Mg2NiH4 phase has been well-maintained with the cycles, in which the high-dispersibility of Ni and C elements might contribute to such superior cycling stability of the MgH2\nNi-EG system.\nFig.\u00a012\n schematically summarizes the roles of the in situ formed Mg2Ni/Mg2NiH4 and EG in improving the hydrogen storage performance of MgH2 and the catalytic mechanism. More specifically, the Mg2Ni/Mg2NiH4 converted from the ultrafine metallic Ni and EG are homogeneously dispersed on the interface of the MgH2 particles. First, the highly dispersed Mg2Ni/Mg2NiH4 provides a large number of active sites for the hydrogen absorption/desorption reactions of MgH2, which can reduce the dehydrogenation activation energy and accelerate its hydrogen de/absorption kinetics. Secondly, the presence of EG can inhibit the grain agglomeration and growth of Mg/MgH2 at high temperatures and thus improve the cycle stability of the MgH2\nNi-EG nanocomposite. With the combined action of the in situ formed Mg2Ni/Mg2NiH4 and EG, the MgH2\nNi-EG nanocomposite not only can ensure suitable dehydrogenation kinetics but also show good cycle stability.In this work, a facile one-step high-energy ball milling process is developed to in situ form ultrafine Ni nanoparticles with uniform dispersity from the nickel acetylacetonate precursor in the MgH2 matrix. On one hand, the in situ formed ultrafine Ni nanoparticles catalyst from the Ni(acac)2 can significantly improve the desorption kinetics of MgH2. On the other hand, the cycle performance of Mg/MgH2 is improved by the low-cost and effective EG. After tailoring the amounts of the catalyst, the MgH2\nNi-EG nanocomposite can combine the individual functions from the ultrafine metallic Ni catalyst and EG to release 7.03\u00a0wt.% H2 within 8.5\u00a0min after 10 cycles, showing an exceptional hydrogen storage performance. The activation energy of dehydrogenation is reduced to 115\u00a0kJ/mol from 157.1\u00a0kJ/mol for pure MgH2. Additionally, the MgH2\nNi-EG sample can absorb 2.42\u00a0wt.% H2 within 1\u00a0h at a temperature close to room temperature (50 \u00b0C). As a result, the ultrafine metallic Ni (<5\u00a0nm) in situ formed and the Mg2Ni/Mg2NiH4 generated in the subsequent hydrogen absorption and desorption process play a critical role in the improvement of the hydrogen ab/desorption kinetics of MgH2. Our work provides a methodology to significantly improve the hydrogen storage performance of MgH2 by combining the in situ formed and uniformly dispersed ultrafine metallic catalyst from precursor and EG.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to acknowledge financial support from the National Basic Research Program of China (2018YFB1502100). XSY acknowledges the support from the PolyU grant (No. G-YW5N).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jma.2021.12.003.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  It has been well known that doping nano-scale catalysts can significantly improve both the kinetics and reversible hydrogen storage capacity of MgH2. However, so far it is still a challenge to directly synthesize ultrafine catalysts (e.g., < 5\u00a0nm), mainly because of the complicated chemical reaction processes. Here, a facile one-step high-energy ball milling process is developed to in situ form ultrafine Ni nanoparticles from the nickel acetylacetonate precursor in the MgH2 matrix. With the combined action of ultrafine metallic Ni and expanded graphite (EG), the formed MgH2\n                     Ni-EG nanocomposite with the optimized doping amounts of Ni and EG can still release 7.03\u00a0wt.% H2 within 8.5\u00a0min at 300 \u00b0C after 10 cycles. At a temperature close to room temperature (50 \u00b0C), it can also absorb 2.42\u00a0wt.% H2 within 1 h It can be confirmed from the microstructural characterization analysis that the in situ formed ultrafine metallic Ni is transformed into Mg2Ni/Mg2NiH4 in the subsequent hydrogen absorption and desorption cycles. It is calculated that the dehydrogenation activation energy of the MgH2\n                     Ni-EG nanocomposite is also reduced obviously in comparison with the pure MgH2. Our work provides a methodology to significantly improve the hydrogen storage performance of MgH2 by combining the in situ formed and uniformly dispersed ultrafine metallic catalyst from the precursor and EG.\n               "}
{"full_text": "Data will be made available on request.The search for sustainable and clean energy sources has become one of the most important topics as the global crisis on energy and environmental issues is accompanied with unknown uncertainties. Hydrogen is endowed with clean features and possibility of large-scale productions via sustainable routes. Among several approaches for obtaining hydrogen, Fujishima and Honda devised a promising strategy through water splitting on a photosensitive semiconductor device [1], which has aroused great interest in the production of hydrogen by semiconductor photocatalysis [2].Many semiconductor photocatalysts have been explored so far. They are represented by not limited to metal oxides (TiO2, ZnO, WO3 and Bi2O3) [1,3\u20136], metal sulfides (CdS, ZnS and MoS2) [7\u201312], Bi-based compounds [13\u201315], Ag-based photocatalysts and g-C3N4 and etc [16\u201318]. Among these composites and their variants, ternary metal sulfides have been featured by their photocatalytic properties, tunable band gap, and good visible light absorption ability. As a typical member, ZnIn2S4 has received extensive attentions from scientists due to its suitable visible light absorption band gap (2.34\u20132.48\u00a0eV) and its reaction-friendly electrical properties [19\u201323]. It is associated with three different crystal polymorphs including trigonal, cubic, and hexagonal, with latter two active and suitable for photocatalytic hydrogen production under light irradiation [24]. With the existence of a surfactant, Bai et al. created a series of ZnIn2S4 that resembled flowers. The pH of the reactant was crucial in achieving the highest production of H2, which was 1545\u00a0\u03bcmol/g/h [25]. However, the weak ability to separate and transfer photo-induced charges and unavoidable photocorrosion are associated with the sulfide yet hindering practical application in photocatalysis. As a result, attempts have been made to limit carrier recombination and design an effective strategy to mitigate photocorrosion.To suppress high recombination rates of photogenerated charge carriers, numerous efforts have been taken place through modifications of photocatalysts, including morphology control [26,27], elemental doping [28], defect engineering [29], and heterojunction construction [30\u201332]. Among them, heterojunction construction is considered as a highly effective method to enhance photocatalytic performance. So far, various heterojunctions have been engineered and grouped to type-I, type-II, p-n type, Schottky junction, Z-type heterojunction, and S-type heterojunction etc. Despite of endeavors to separate photoinduced charges and facilitate charge transfers to heterosites, the mitigations to severe photocorrosion happening on photosensitive semiconductors have been overlooked. Very recently, intensive attentions were emphasized to this field [33\u201336]. For instance, with the \u201ctwo birds with one stone\u201d composite photocatalyst, Chun successfully solved the two pressing issues of S-metal bonding of sulfide readily oxidized by photogenerated holes and Ag+ of Ag-based photocatalyst [17]. According to Hao\u2019s team, the photocorrosion of CdS can be effectively suppressed through the photocorrosion-recrystallization process [37].To reach low-cost and potential industrial applications, water splitting photocatalysts are further developed by substituting noble metal with transition metal (TM) complexes [38\u201340]. The Co(III)-dimethylglyoxime (dmgH) complex is considered as an essential unit for hydrogen production in cobalt oxime, according to earlier researches [41,42]. However, the complex\u2019s low stability makes it unsuitable for sustained usage. A similar compound of nickel butanedione oxime (Ni(dmgH)2) has been noticed as a catalyst for the electrooxidation of methanol. This is ascribed to the cheap raw materials and simple synthesis method, and moreover the increased electrochemical reactivity of nickel ions [43]. By combining Ni(dmgH)2 with graphitic carbon nitride (g-C3N4) submicron lines and using the triethanolamine as a sacrificial agent, the complex catalyst designed by Cao\u2019s team exhibited efficient hydrogen production (1.18\u00a0\u03bcmol/h) under visible light irradiation [44]. This indicates that it has considerable potential in the field of photocatalysis. Despite these prior arts, the Ni(dmgH)2 has been less commonly used for photocatalysis compared with other TM complexes, and its potential in the field is yet to be explored.Aiming to improvement of charge transfer in heterojunction, inhibition of sulfide photocorrosion, and facile preparation of photocatalysts, we have designed and constructed a novel ZnIn2S4/Ni(dmgH)2 (ZIS/NID) composite system for vigorous hydrogen evolution under UV\u2013visible illumination. A high hydrogen evolution rate of 36.3\u00a0mmol/g/h was reached and associated with a large amount bubbles visually observable, in competence to hydrogen gas evolution seen in electrolysis [45]. It is further found that the sulfur ions generated by the photocorrosion of sulfide got involved in an in situ formation of the Ni-S intermediate which behaves as a co-catalyst and an electron trap to influence the electron transfer path. Thus, the photocatalytic hydrogen evolution capacity of the catalyst was greatly enhanced.Zinc chloride (ZnCl2), nickel nitrate hexahydrate (Ni(NO3)2\u00b76H2O), ammonium hydroxide solution (NH3\u00b7H2O, 25.0\u00a0\u223c\u00a028.0%), absolute alcohol (CH3CH2OH), hydrochloric acid (HCl, 36.0\u00a0\u223c\u00a038.0%) and barium chloride (BaCl2) were purchased from Sinopharm Chemical Reagent Co., Ltd. Indium chloride (InCl3) and Thioacetamide (TAA) were purchased from Shanghai Maclean Biochemical Technology Co., Ltd. Dimethylglyoxime (dmgH) was purchased from Shanghai Aladdin Chemistry Technology Co., Ltd. All chemicals were analytical grade and used without further purification.The synthesis of ZIS followed protocol as documented in the literature [46]. An excess amount of thioacetamide (TAA, 6\u00a0mmol) was dissolved in 100\u00a0mL deionized water by adding 1\u00a0mmol ZnCl2 and 2\u00a0mmol InCl3 at a stoichiometric ratio. The solution was adjusted to pH 1\u00a0\u223c\u00a03 with hydrochloric acid, and the flask was put in a 353\u00a0K water bath without stirring for 6\u00a0h. The flask naturally cooled down to room temperature when the reaction was completed. Centrifugation was used to collect the product, which was then washed several times with deionized water and anhydrous ethanol. For characterizations, the final sample was dried under vacuum for 6\u00a0h at 333\u00a0K to obtain ZIS.A simple chemical precipitation method was used to synthesize Ni(dmgH)2 and ZnIn2S4/Ni(dmgH)2 composites. First, Ni(NO3)2\u00b76H2O (2.6\u00a0mmol) and ZIS (2.6\u00a0mmol) were dispersed in 60\u00a0mL of 95% ethanol and sonicated for 30\u00a0min to obtain solution A. Dimethylglyoxime (5.2\u00a0mmol) was dissolved in 60\u00a0mL of 95% ethanol to obtain solution B. Liquid A was added to liquid B alternately drop by drop under stirring and heating at 70\u00a0\u00b0C. Then the pH of the mixed solution was adjusted to 8\u201310 with 25%-28% NH3\n\u00b7H2O and the solution was mixed continuously under a water bath at 70\u00a0\u00b0C for 2\u00a0h to form a red suspension. The product was collected after centrifugation, washed repeatedly with water and ethanol, and then dried in a vacuum oven. Here, the amount of Ni(dmgH)2 in the composite was calculated based on the amount of Ni(NO3)2\u00b76H2O.Pure Ni(dmgH)2 (NID) was produced by using a similar strategy only without the addition of ZIS. The final catalyst was named as ZnIn2S4/Ni(dmgH)2 with a molar ratio of 1:1 (abbreviated as ZIS/NID-1). Similarly, different molar ratios of ZnIn2S4/Ni(dmgH)2 (i.e., 0.5 and 2) were defined as ZIS/NID-0.5 and ZIS/NID-2 by changing the amount of ZnIn2S4, respectively. The synthesis process of the ZnIn2S4/Ni(dmgH)2 was shown in Scheme 1\n.X-ray diffraction (XRD) measurements were carried out with a 0.02\u00b0 step on a Bruker D8 ADVANCED diffractometer (Cu-K\u03b1, \u03bb\u00a0=\u00a00.15406\u00a0nm; 40\u00a0kV; 40\u00a0mA). Fourier transform infrared (FT-IR) spectra were collected on a Nicolet 6700 spectrophotometer. A JSM-6610LV scanning electron microscope (SEM), and a 200\u00a0kV JEM-2100F transmission electron microscope (TEM) were employed to study surface morphology evolution. The x-ray photoelectron spectra (XPS) measurements were recorded using a Perkin-Elmer PHI 5000C spectrometer with a monochromatic Al K\u03b1 incident source. A UV\u2013vis spectrophotometer (UV-2550, Japan) was used to obtain the diffuse reflectance spectra. Steady-state photoluminescence (PL) spectra were recorded on a fluorescence spectrometer (F-4600 FL Spectrophotometer) with an excitation wavelength of 320\u00a0nm. Electrochemical measurements were carried out on an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Corporation, China). During these experiments, samples were dry-pressed into thin films.A photocatalytic online analysis system (Lab Solar-III AG, Beijing Perfect Light) was used in conjunction with a gas chromatograph to perform the usual PHP test (GC-6890A). Firstly, 20\u00a0mg of photocatalyst was added into 100\u00a0mL aqueous solution, which was comprised of 10\u00a0vol% triethanolamine (TEOA) as the sacrificial reagent, then kept stirring until the reaction was completed. The above reactor was bubbled with nitrogen for 30\u00a0min before the photocatalytic experiment to guarantee that the reaction system was anaerobic. A 300\u00a0W Xe lamp (Beijing Perfect Light, PLS-SXE300) was used to irradiate the reactor. Meanwhile, a circulating condensate system was used to keep that the reaction was at 6\u00a0\u00b0C. Finally, the amount of hydrogen generated was measured by using a gas chromatograph with a thermal conductive detector (TCD). Visible light was simulated by equipping the xenon lamp with a 420\u00a0nm cut-off filter. By using different band-pass filters on the Xenon lamp (DT420 and DT475), under the identical circumstances, the apparent quantum efficiencies (AQEs) were obtained. The following equation was used to calculate the quantum efficiency.\n\n(1)\n\n\n\nIn a three-electrode electrode cell, the photoelectrochemical analysis was conducted in a 0.5\u00a0M Na2SO4 aqueous solution. The reference electrode was Ag/AgCl chloride, while the counter electrode was Pt., and electrolyte was Na2SO4 (0.5\u00a0mol L-1) aqueous solution. Drop coating method was used to make the work electrode. The specific steps were as follows. 10\u00a0mg of the photocatalyst was dispersed in 20\u00a0\u03bcL of nafion, 0.5\u00a0mL of ethanol, and 0.5\u00a0mL of distilled water. The suspension was then sonicated for 0.5\u00a0h to form a homogeneous dispersion. 50\u00a0\u03bcL of the suspension was dropwise injected onto ITO and then dried at room temperature. For the photocurrent measurements, the light source was a 300\u00a0W Xe lamp. Electrochemical impedance spectra (EIS) were obtained in the frequency range from 105 to 0.1\u00a0Hz (applied potential of \u22120.2\u00a0V). The steady-state photocurrent density of the photoanode was further checked by using transient photocurrent measurements with chopper illumination. The Mott-Schottky plots were performed at 1.0, and 0.5\u00a0kHz with Na2SO4 aqueous solution as the electrolyte (0.5\u00a0M). The linear sweep voltammetry (LSV) was carried out at a scanning speed of 0.05\u00a0V/s.The crystal structure and phase compositions of the ZIS/NID, NID, and ZIS composites were evaluated by X-ray diffraction (XRD) as depicted in Fig. 1\n(a-b). For the pure ZnIn2S4, the diffraction peaks at 21.1\u00b0, 27.7\u00b0, 47.3\u00b0, 52.4\u00b0, and 56.4\u00b0 are respectively indexed to (006), (102), (110), (116), and (022) crystallographic planes in the hexagonal ZnIn2S4 (JCPDS No.65\u20132023) [47,48]. (110), (200), (130), (002), (112), and (240) crystal facets are found situating at 9.98\u00b0, 10.6\u00b0, 26.3\u00b0, 27.5\u00b0, 29.3\u00b0, and 36.0\u00b0, respectively (Fig. 1(a)), illustrating that the Ni(dmgH)2 produced has the same diffraction peaks as the published results (JCPDS No.55\u20131252) [44,49\u201352]. All characteristic signals can be observed in the sign of ZnIn2S4 and Ni(dmgH)2 for the ZIS/NID sample, showing that the composite has been properly constructed. Furthermore, as the amount of ZnIn2S4 integrated with the composite grows, the peak intensity of ZnIn2S4 increases, showing the presence of the combination.Similar findings were also confirmed by FTIR spectra where the surface functional groups are revealed in Fig. 1(c). For pure Ni(dmgH)2, IR spectra showed characteristic Ni-N vibrations at 428 and 520\u00a0cm\u22121\n[53,54], while the peaks at 989,1101, 1238 and 1367\u00a0cm\u22121 may be attributed to NO stretching vibrations [55,56]. The characteristic peak at 1571\u00a0cm\u22121 can be attributed to CN stretching vibrations, the weak band at 1650\u20131823\u00a0cm\u22121 to OH bending vibrations [56], and the CH symmetric stretching vibrations were located at 2920\u00a0cm\u22121\n[55]. The peaks in the spectra of pure ZnIn2S4 at 1616\u00a0cm\u22121 and 3430\u00a0cm\u22121 could be connected to the stretching vibrations of the adsorbed hydroxyl group and the HO-H group of the adsorbed water molecule [48]. The above XRD and FTIR results denote successful synthesis of the ZnIn2S4/Ni(dmgH)2 photocatalyst.SEM and TEM were used to characterize the morphology and microstructure of the synthesized photocatalysts. In Fig. 2\n(a-b), the NID owns long but thin microrods morphology with the length ranging from 1\u00a0\u03bcm to 20\u00a0\u03bcm (both just grown and fully grown), and pure ZIS (Fig. 2(c-d)) exhibits a microsphere morphology with a rough surface. Fig. 2(e-f) show the SEM images of ZIS/NID, and it can be observed that ZIS/NID exhibits micron-level \u201crod\u201d and \u201csphere\u201d interaction structures associated with ZIS and NID. Fig. 2(g-i) depict the TEM images of ZIS/NID. The ZIS/NID exhibits a three-dimensional structure of ZIS microspheres loaded on the smooth surface of NID microrods. In addition, according to the composite\u2019s HR-TEM image (Fig. 2(l)), for ZIS/NID, distinct lattice stripes at a distance of 0.32\u00a0nm and 0.29\u00a0nm can be seen, which agrees well with the value of the (102) and (104) plane in hexagonal ZnIn2S4\n[24,57,58]. The result is also confirmed by the selected area electron diffraction (SAED) pattern in Fig. 2(j). Interestingly, the contact between ZIS and NID can be seen clearly. This feature is critical for charge transfer between the two components to be effective.The composites contained C, O, Ni, Zn, S, In, and N components, as determined by energy dispersive spectroscopy (EDS) (Fig. S1). Spatial distributions of elements in the ZnIn2S4/ Ni(dmgH)2 sample were investigated using elemental mapping analysis, as shown in Fig. S2(a-h). C, N, O, and Ni elements were consistently distributed throughout the composite, while S, In, and Zn components spread in the microsphere.The surface compositions and chemical states of the ZnIn2S4/Ni(dmgH)2 composites were investigated by X-ray photoelectron spectroscopy (XPS) measurements. The XPS survey spectrum of the ZIS/NID-1 sample (Fig. 3\n(a)) shows that it consists mainly of C, N, O, Ni, Zn, In, and S elements, which agrees well with the EDS results. As shown in Fig. 3(b), the C 1s binding energy peak at 285.78\u00a0eV can be attributed to sp2 CC bonds in Ni(dmgH)2, and the peak of 284.8\u00a0eV is assigned to adventitious carbon. The remaining weak peaks at 288.27\u00a0eV can be assigned to the CO bonds, possibly from the absorbed CO2 and defective sample surface [59\u201361]. The peak at 531.17\u00a0eV is related to oxygen in water adsorbed on the catalyst surface, and the O 1s peak at 532.32\u00a0eV is related to the OH group in Ni(dmgH)2 (Fig. 3(c)). The peaks of 400.45\u00a0eV and 403.16\u00a0eV in the N 1s spectrum (Fig. 3(d)) are resulted from the CN (pyrrolic-type N) bond and its satellite peaks, respectively [62]. The Ni 2p spectrum shows four peaks at 854.8\u00a0eV, 872.1\u00a0eV, 858.78\u00a0eV, and 876.08\u00a0eV (Fig. 3(e)), attributing to Ni 2p3/2, Ni 2p1/2 and their satellite peaks, which indicate the presence of Ni2+ in the complex [18,51,63]. According to Fig. 3(f), S 2p can be split into two separate signals at 161.3\u00a0eV and 162.5\u00a0eV, which correspond to S 2p3/2 and S 2p1/2, respectively [64]. Fig. 3(g) shows two peaks at 444.7\u00a0eV (In 3d5/2) and 452.23\u00a0eV (In 3d3/2) in In 3d high-resolution XPS spectra, denoting existence of In3+ cation. Two main peaks can be assigned to Zn 2p3/2 (1021.61\u00a0eV) and Zn 2p1/2 (1044.64\u00a0eV), respectively in Zn 2p XPS figure (Fig. 3(h)), demonstrating the existence of Zn2+ ions [65,66].In addition to ascertaining the chemical valence states of surface elements, X-ray photoelectron spectroscopy (XPS) characterization facilitates the investigation of charge transfers among elements following their bonding schemes. Generally, a positive shift in binding energy implies a reduction in electron density, while a negative shift signifies an increase in electron density [67]. Thus, the migration pathways of electrons in heterojunction photocatalysts can be assessed by analyzing the shifts in binding energy observed in XPS spectra for heterogenous systems [68,69]. A noticeable shift towards lower binding energy of the peaks of S 2p, In 3d, and Zn 2p complex were found in the XPS of ZnIn2S4/Ni(dmgH)2\u20131 compared to those of ZnIn2S4 (Fig. 3(f-h)). Contrarily, the Ni 2p peak in the ZnIn2S4/Ni(dmgH)2\u20131 complex exhibits a significant shift towards higher binding energy, as compared to pure Ni(dmgH)2 (Fig. 3(e)). These changes in binding energy suggest that electron charge transfers happen from Ni(dmgH)2 to ZnIn2S4 in the ZnIn2S4/Ni(dmgH)2 composite. Overall, the XPS results indicate a robust interfacial coupling effect between Ni(dmgH)2 and ZnIn2S4, which may facilitate separation and migration of photogenerated carriers, ultimately leading to an improved photocatalytic performance of the ZnIn2S4/Ni(dmgH)2 complex [48,57].As shown in Fig. 3(i), the UV\u2013vis diffuse reflectance spectroscopy (DRS) was used to assess the optical absorption parameters of pure Ni(dmgH)2, ZnIn2S4, and ZnIn2S4/Ni(dmgH)2 composites. Ni(dmgH)2 microrods exhibit a broad visible spectral absorption range, with an absorption edge of roughly 580\u00a0nm, whereas pure ZnIn2S4 has a shorter absorption range. When comparing pure ZnIn2S4 with ZnIn2S4/ Ni(dmgH)2, the light absorption edge of the ZnIn2S4/Ni(dmgH)2 composite displays a clear red shift, showing that the coupling of Ni(dmgH)2 and ZnIn2S4 leads to the formation of a photocatalyst complex that exhibits a broader light absorption range compared to pure ZnIn2S4, as supported by relevant literature [70,71]. The Kubelka-Munk formula was employed to calculate the band gap values (Eg) of Ni(dmgH)2 and ZnIn2S4 in general [64].\n\n(2)\n\n\n\u03b1\nh\nv\n\n=\n\nA\n\n\n\n(\nh\nv\n-\n\nE\ng\n\n)\n\n\nn\n/\n2\n\n\n\n\n\n\nThe absorption coefficient, Planck constant, optical frequency, constant value, and band gap energy of the photocatalyst are denoted by \u03b1, h, v, A, and Eg, respectively. ZnIn2S4, ZIS/NID-0.5, ZIS/NID-1, ZIS/NID-2, and Ni(dmgH)2 had Eg values of 2.04, 1.63, 1.48, 1.63, and 1.52\u00a0eV, respectively, as shown in Fig. 3(j).Under light irradiation, the photocatalytic hydrogen evolution (PHE) performance of Ni(dmgH)2, ZnIn2S4, and ZnIn2S4/Ni(dmgH)2 composites with various ratios were examined using 10% triethanolamine (TEOA) as sacrificial agents. According to the experimental findings (Fig. 4\n(a-b)), a single fraction of Ni(dmgH)2 displayed a minimal PHE activity. With a hydrogen evolution rate of 7.4\u00a0mmol/g/h, similar results were seen when utilizing pure ZnIn2S4. The fast recombination of photogenerated electron and hole pairs is primarily responsible for the pure photocatalyst\u2019s inactive PHE reaction. The application of ZIS/NID composites, however, offers the possibility of significantly boosting PHE activity, and the component ratio of a single catalyst has a significant impact on its PHE performance. The Ni(dmgH)2-ZnIn2S4 composites (ZIS/NID-0.5, ZIS/NID-1, and ZIS/NID-2) own different PHE rates of 13.6, 36.3, and 25.4\u00a0mmol/g/h, respectively. A comparison with other recent works on photocatalytic hydrogen evolution was summarized in Table S1.To study possible reaction sites in PHE, 100\u00a0mL of 10% vol methanol solution and 50\u00a0mL of 0.35\u00a0M Na2S/0.25\u00a0M Na2SO3 solution were used as sacrificial agents for comparison experiments. From Fig. 4(c), the amount of hydrogen evolution was greatly reduced, respectively. This is not surprising since the methanol was volatile, causing the solution\u2019s methanol content to decrease. It was easily oxidized by h+ to produce formic acid, which would corrode the photocatalyst. In the Na2S/Na2SO3 case, too many sulfur ions would interrupt the reaction equilibrium and be hazardous to the formation of NiS intermediate. Furthermore, different sacrificial agents also had varying redox potentials, which would affect their consumption of cavities and thus lead to different amounts of hydrogen evolution. Moreover, it was discovered that the H2 production rate of pure ZnIn2S4 grew slowly after 3\u00a0h of photoreaction while the H2 production rate of ZIS/NID-1 increased after 6\u00a0h (Fig. 4(d)). As can be seen in Fig. 4(e), the AQE of the ZIS/NID-1 was also tested under single wavelength irradiations of 420\u00a0nm and 475\u00a0nm. The AQE reached a surprising value of 20.45%at 420\u00a0nm, and although it significantly decreased as input light wavelength increased, it still reached 5.02%at 475\u00a0nm. Four cycles of PHE experiment, each lasting three hours, were carried out to examine the stability of the ZIS/NID-1 composite in the photocatalytic hydrogen evolution reaction. As shown in Fig. 4(f), no discernible deactivation of PHE performance was observed after four additional runswithout anyaddition of hole scavengers, demonstrating the high stability of the ZIS/NID-1 composite photocatalyst. After the photocatalytic process, the stability of this photocatalyst was further supported by XRD, SEM and XPS, which revealed no changes to the crystal structure and elements (Fig. S3).It is important to note that the color of the photocatalyst, water, and triethanolamine suspension changed significantly before and after light irradiation. As shown in Fig. 5\n(a-c), the suspension system color changes from red to black and back to red imply a photochromic phenomenon, whereas the black color during the light irradiation denotes the enhancement of the light absorption and maximization of conversion of photon energy to other forms. When the photocatalytic reaction was completed and left overnight, its color surprisingly returned to red again. The color transition under light irradiation was reversible to some extent, which confirmed its considerable hydrogen evolution activity after four cycles of experiments. A comparison of the color of pure ZnIn2S4 before and after the experiment was also performed. In Fig. 5(d-f), the original pale yellow color became partly cloudy after the reaction and remained almost unchanged after standing all night, which may be caused by the widely reported photocorrosion of the sulfide catalyst [17,36]. By coupling ZnIn2S4 and Ni(dmgH)2, the photocorrosion problem associated with the sulfide was effectively mitigated, thus greatly enhancing the photocatalytic hydrogen evolution performance.The experiments were also repeated to compare the performance of photocatalytic hydrogen evolution under visible and UV\u2013visible light irradiation by equipping a 420\u00a0nm cut-off filter (Fig. S4). From Fig. S4(a-b), the ZIS/NID-1 composite achieved a hydrogen evolution rate of 4.4\u00a0mmol/g/h, which was about 3.4 times higher than that of pure ZnIn2S4. Also after four cycles, the rate did not decrease significantly, revealing its good stability (Fig. S4(c)). The excellent hydrogen evolution performance under both visible and UV\u2013vis irradiation confirmed the applicability of this photocatalyst under a wide range of conditions. In addition, visible hydrogen bubbles produced by simulated sunlight during and after irradiation and real sunlight irradiation recorded by the picture and movie (Fig. S5 and movie1, 2 and 3) verified this statement.To verify the photoelectrochemical properties of the synthesized ZIS/NID-1 complexes, relevant characterizations were performed. The steady-state PL spectra of ZnIn2S4, Ni(dmgH)2, and ZIS/NID-1 were measured at the excitation wavelength of 320\u00a0nm. It can be seen that there are strong emission peaks around 470\u00a0nm and the ZIS/NID-1 catalyst exhibited the lowest fluorescence intensity compared to pure ZnIn2S4 and Ni(dmgH)2, indicating that it was more favorable for the suppression of recombination of photogenerated carriers via radiative decay (Fig. 6\n(a)). The photocurrent of the ZIS/NID-1 composite was much larger than those of ZnIn2S4 and Ni(dmgH)2. The current increased with time, in line with the increase of hydrogen evolution rate of this photocatalyst with time up to the threshold. The increase of photocurrent and PHE rate indicate substantial enhancement of the charge transfer at the interface and suppressions of the recombination rate of photoinduced electron and hole pairs (Fig. 6(b)). Additionally, it can be seen from Fig. 6(c) of the electrochemical impedance spectrum (EIS) that the ZIS/NID-1 photocatalyst has a smaller semicircle than these for pure ZnIn2S4 and Ni(dmgH)2. The ZIS/NID-1 electrolyte interface\u2019s charge transfer resistance was lower, resulting in a faster interfacial charge transfer and better carrier separation. Fig. 6(d) shows the linear scanning voltammetry (LSV) curves of the three prepared samples. It is clear that ZIS/NID-1 displays the lowest hydrogen evolution overpotential, which means that ZIS/NID-1 is the most favorable candidate where hydrogen precipitation reactions can occur.The band energies of Ni(dmgH)2 and ZnIn2S4 were also studied to understand possible transport paths for photo-generated carriers during photocatalysis. At AC frequencies of 1.0 and 0.5\u00a0kHz, the Mott-Schottky (M\u2212S) curves of Ni(dmgH)2 and ZnIn2S4 were recorded. Positive slopes were seen in both ZnIn2S4 microspheres and Ni(dmgH)2 microrods, as illustrated in Fig. 6(e-f), which correlated to n-type semiconductor characteristics. In general, n-type semiconductors have a conduction band potential (ECB) that is approximately 0.1\u20130.3\u00a0V lower than their flat band potential (Efb) [72\u201374]. From the tangent intercept of the curve, the Efb of ZnIn2S4 microspheres and Ni(dmgH)2 microrods were determined to be \u22120.57 and \u22120.89\u00a0V, respectively (vs. Ag/AgCl.PH\u00a0=\u00a07). For ZnIn2S4 microspheres and Ni(dmgH)2 microrods, respectively. The observed flat-band potentials were \u22120.27\u00a0V and \u22120.59\u00a0V (vs. NHE), according to the Nernst equation. According to the literature [22,27], n-type semiconductors\u2019 Fermi levels (Ef) are close to their CB. There, the VB potentials (EVB) of ZnIn2S4 microspheres and Ni(dmgH)2 microrods were calculated using the equation (EVB\u00a0=\u00a0ECB\u00a0+\u00a0Eg) and were determined to be\u00a0+\u00a01.77 and\u00a0+\u00a00.93\u00a0V (vs. NHE), respectively, based on the band gap energy obtained from UV\u2013vis DRS. Fig. 6(g) showed the DMPO spin-trapping ESR spectra of \u22c5O2\n\u2212 over ZIS/NID-1 photocatalyst under dark and after 10\u00a0min light irradiation. The \u22c5O2\n\u2212 was not detected under dark or light conditions. In addition, the VB of Ni(dmgH)2 was calculated to be 0.94\u00a0V based on E\nNHE\n/V\u00a0=\u00a0\u03a6\u00a0+\u00a00.78\u00a0eV \u2212 4.44 (E\nNHE\n: potential of normal hydrogen electrode and \u03a6 of 4.6\u00a0eV: electron work function of the analyzer), as shown in Fig. 6(h). It was approximately the same as the valence band calculated by the Mott-Schottky (M\u2212S) curves.Based on the above experimental results and characterizations, we systematically investigated the possible photocatalytic mechanisms leading to high HER performances of the present ZnIn2S4/Ni(dmgH)2 system. A possible direct Z-scheme for the current system was first presumed according to the band alignment as determined by the above PL and M\u2212S method. Charge transfer mechanism was shown in Fig. 7\n(a). Under light irradiation, the photogenerated e- in ZnIn2S4 CB will combine with the photogenerated h+ in Ni(dmgH)2 VB, leaving h+ in ZnIn2S4 VB and the e- in Ni(dmgH)2 CB active for hydrogen evolution. Then the h+ in ZnIn2S4 VB was consumed by TEOA and the e- in Ni(dmgH)2 CB react with the H+ to form H\u00b7, and sequentially the H2\n[75]. However, since the CB potential of Ni(dmgH)2 was more negative (-0.59\u00a0eV, vs. NHE), the aggregated electrons on the CB of Ni(dmgH)2 can convert O2 to \u22c5O2\n\u2013 (-0.33\u00a0eV, vs. NHE). This, however, was not consistent with the DMPO spin-trapping ESR spectra results, vetoing the presumption of the Z-scheme in the current system at the beginning of the photocatalysis.Alternatively, the charge migration of the type-II scheme was then envisaged and considered more reasonable than in the Z-scheme at the interface between Ni(dmgH)2 and ZnIn2S4 (Fig. 7(b)). Due to their respective narrow band gaps, Ni(dmgH)2 microrods and ZnIn2S4 microspheres were both stimulated to produce electron-hole pairs when exposed to light irradiation. The photogenerated electrons in the Ni(dmgH)2 CB will be spontaneously injected into the CB of ZnIn2S4 through the type II heterojunction formed when the ZnIn2S4/Ni(dmgH)2 heterojunction was constructed. The majority of photogenerated holes in the ZnIn2S4 VB will be transported to the Ni(dmgH)2 VB. Therefore, the recombination of photoexcited electrons and holes can be greatly suppressed by the combination of Ni(dmgH)2 microrods and ZnIn2S4 microspheres, enhancing the photocatalytic activity. As a result, the holes on the VB of ZnIn2S4 were moved to Ni(dmgH)2 VB, where they could be consumed. The electrons were focused on the CB of ZnIn2S4, which could react with H ions to generate H2 and greatly suppress photocorrosion.Despite the successful inferring of type-II heterojunction for the current photocatalytic system, origins photochromic-like phenomenon that occurred during the photocatalysis still remain elusive. It has been noticed the suspension system became dark in color, but both the photocurrent and PHE ability enhanced with light irradiation time. These three evidences denote a new chemical composition was created in the heterojunctions, with the properties of i) facilitating charge migrations between hetero-sites, ii) color changes, and iii) reversable photochemical reaction process to return original semiconductors. In fact, the color appearance in the heterojunction is in line with the color changes for the pure ZnIn2S4 with both systems are less favorable for light transmission (Fig. 5(b) v.s. Fig. 5 (e)). This infers photocorrosion happens on the sulfide, regardless its combination with heterojunctional sites. Therein, the sulfur tends to be oxidized, also meaning more electronegativity for the sulfur element itself during the photocorrosion process. Meanwhile, the coordination complex Ni(dmgH)2 also suffers from photostability under irradiation, as can be deduced from the Co(dmgH)2 with similar structures [76]. The hydrogen bond joining the organic ligands behave low photostability and lead to changes of Ni coordination stages. A possible Ni-S composition was then formed between a chemically oxidative S and weakly coordinated Ni from both sides. The cross-interfacial formation has indeed been observed between sulfide and bare nickel even with a noble metal buffer [77]. The resulted Ni-S component has the pigmentary color of black as the NiS and acts as the interface to bridge charge transfer between Ni(dmgH)2 and ZnInS4, as denoted in Fig. 7(c). After light irradiation, the complex returns to the original coordination and the color of the heterojunction returns to the apparent color of red as the Ni(dmgH)2, while the sulfur may be leached or transformed to other chemical groups in the aqueous system. The suspension system is thus owning a combined feature of type-II and all-solid-state Z-scheme during the photocatalysis thanks to the appearance of the Ni-S interfacial component. Due to the facilitation of electron-hole annihilation at the interface, the electrons in the CB at the Ni(dmH)2 site and holes at the VB of ZnIn2S4 became active during the photocatalysis and moreover with larger redox activities compared to these in the previous type-II structures due to redox potentials (Fig. 7). A single ZnIn2S4/Ni(dmgH)2 heterojunction, thus, has the possibility to transfer from the type-II to Z-scheme photocatalyst where additional but more active photocatalytic sites are created, resulting in the boosting of hydrogen evolution (Fig. 4).The above proposition was supported by the analysis of ions in the suspensions during the photocatalysis. The experimental details can be found in Supporting experimental section of the SI. The Ni2+ and SO4\n2- in the black suspension (NiS was not completely oxidized and decomposed) were qualitatively detected and in the suspension that had faded to red to restore the intrinsic color of the Ni(dmgH)2. As shown in Fig. S6(a), A, B and C were blank without a detector, adding dmgH to the black suspension after centrifugation, adding dmgH to the red suspension after centrifugation, respectively. The color became darker from A, B to C, and no obvious red precipitate can be seen in solution B, indicating that the Ni2+ present in the solution at this time was a trace, and the majority of Ni2+ was located in NiS and Ni(dmgH)2, the red precipitate (Ni(dmgH)2) in the C solution was visible. And as seen in Fig. S6(b), the solution became increasingly turbid from D, E to F, probably because of the increasing number of fine white precipitates (BaSO4) in the solution.The possible hydrogen-producing reactions of ZIS/NID heterojunctions under light irradiation with TEOA as a sacrificial agent are shown in Eqs. (3)\u2013(14) below:\n\n(3)\n\n\nZnI\n\nn\n2\n\n\nS\n4\n\n+\nh\nv\n\u2192\nZ\nn\nI\n\nn\n2\n\n\nS\n4\n\n\n(\n\ne\n-\n\n-\n\nh\n+\n\n)\n\n\n\n\n\n\n\n(4)\n\n\nN\ni\n\n\n(\nd\nm\ng\nH\n)\n\n2\n\n+\nh\nv\n\u2192\nN\ni\n\n\n(\nd\nm\ng\nH\n)\n\n2\n\n\n(\n\ne\n-\n\n-\n\nh\n+\n\n)\n\n\n\n\n\n\n\n(5)\n\n\nTEOA\n+\n\nh\n+\n\n\u2192\nT\nE\nO\n\nA\n+\n\n\n\n\n\n\n\n(6)\n\n\nZnI\n\nn\n2\n\n\nS\n4\n\n+\n8\n\n\nh\n\n+\n\n\u2192\nZ\n\n\nn\n\n\n2\n+\n\n\n+\n2\nI\n\n\nn\n\n\n3\n+\n\n\n+\n4\n\nS\n0\n\n\n\n\n\n\n\n(7)\n\n\n2\n\nH\n2\n\nO\n+\n4\n\nh\n+\n\n\u2192\n\nO\n2\n\n+\n4\n\nH\n+\n\n\n\n\n\n\n\n(8)\n\n\n\n\n\n(9)\n\n\n\nS\n0\n\n+\n2\n\nH\n2\n\nO\n+\n\nO\n2\n\n+\n2\n\nh\n+\n\n\u2192\nS\n\nO\n4\n\n2\n-\n\n\n+\n4\n\nH\n+\n\n\n\n\n\n\n\n(10)\n\n\nN\n\ni\n\n2\n+\n\n\n+\n\nS\n0\n\n+\n2\n\ne\n-\n\n\n\u2192\nhv\n\nN\ni\nS\n\n\n\n\n\n\n(11)\n\n\nZ\n\nn\n\n2\n+\n\n\n+\n2\nI\n\n\nn\n\n\n3\n+\n\n\n+\n4\n\nS\n0\n\n+\n8\n\ne\n-\n\n\u2192\nZ\nn\nI\n\nn\n2\n\n\nS\n4\n\n\n\n\n\n\n\n(12)\n\n\n2\n\nH\n+\n\n+\n2\n\n\ne\n\n-\n\n\u2192\n\nH\n2\n\n\n\n\n\n\n\n(13)\n\n\nN\ni\nS\n+\n4\n\nh\n+\n\n+\n2\n\nH\n2\n\nO\n+\n\nO\n2\n\n\u2192\nN\n\n\ni\n\n\n2\n+\n\n\n+\nS\n\nO\n4\n\n2\n-\n\n\n+\n4\n\nH\n+\n\n\n\n\n\n\n\n(14)\n\n\n\nThe above heterojunction type and ion detections during the photocatalysis allows us to detail the chemical reaction paths as follows. When the reaction solution is black in color, NiS has not yet been completely oxidized and decomposed into Ni2+ and SO4\n2- ions. As to why its weak ion quantity can also be detected. This is also due to the reaction course described by Eqs. (3)\u2013(14) has been in dynamic equilibrium, or it may be because when the black suspension is separated into solid and liquid. The whole system is inevitably contacting oxygen, and then a very small portion of NiS is thus oxidized. In contrast to the black faded red suspension, relatively high amounts of Ni2+ and SO4\n2- ions are present in the solution that can be detected due to the complete oxidation of the in situ generated Ni-S active intermediate during the reaction, and this result verifies the rationality of the proposed mechanism from the side. The amount of Ni2+ free in the solution is still very small compared with the amount of Ni(dmgH)2 generated again after adding the dmgH. It worth noting that the loss of catalyst is not much, which is known from the fact that the amount of hydrogen evolution does not drop significantly after several cycles. It is important to note that the solubility product constants of NiS and Ni(dmgH)2 are about 3.2\u00a0\u00d7\u00a010-19 and 4\u00a0\u00d7\u00a010-24, respectively. So theoretically Ni(dmgH)2 is more stable than Ni-S weakly bonded in a squeezed space at the interface [78]. Being exposed to air at room temperature and pressure, Ni-S composition will tend to be broken and the coordinating group next to Ni convert it back to Ni(dmgH)2 and returns its color.In summary, we have successfully synthesized pure ZnIn2S4 microspheres and ZnIn2S4/Ni(dmgH)2 heterojunction photocatalysts using a simple low-temperature and low-pressure co-precipitation technique. The optimized ZnIn2S4/Ni(dmgH)2 heterojunction photocatalyst exhibited a remarkable photocatalytic hydrogen evolution activity of 36.3\u00a0mmol/g/h under simulated solar illumination, which was approximately 4.9 times higher than that of pure ZnIn2S4. The enhanced photocatalytic performance was attributed to several synergistic effects: (i) the addition of ZnIn2S4 microspheres onto the surface of Ni(dmgH)2 microrods improved light absorption; (ii) a well-designed type-II heterojunction structure facilitated the separation of photogenerated charge carriers and successfully mitigated the issue of photocorrosion commonly found in sulfide photocatalysts; and (iii) the in-situ formation of NiS active intermediate under simulated solar irradiation effectively enhanced the light absorption and utilization of the catalyst. Our work presents a novel approach for improving the photocatalytic performance of sulfides and provides a reference for the design and construction of sulfide composite systems for various photocatalytic applications in the future.\nShangshu Liu: Investigation, Formal analysis, Validation. Feng Li: Conceptualization, Project administration, Funding acquisition, Data curation, Writing \u2013 review & editing. Taohai Li: Methodology, Supervision, Data curation, Resources. Wei Cao: Supervision, Data curation, Resources, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Financial supports from the National Natural Science Foundation of China (21601149), China Scholarship Council, University of Oulu, Finland and the European Research Council (ERC) under the European Union\u2019s Horizon 2020 research and innovation programme (grant agreement No. 101002219) are acknowledged.\nData availability.\nAll data generated or analyzed during this study are included in this published article (and its supplementary information files) and available from the corresponding author upon reasonable request.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2023.03.123.The following are the Supplementary data to this article:\n\nSupplementary video 1\n\n\n\n\n\n\n\n\n\nSupplementary video 2\n\n\n\n\n\n\n\n\n\nSupplementary video 3\n\n\n\n\n\n\n\n\n\nSupplementary data 4\n\n\n\n", "descript": "\n                  In this work, a novel photocatalyst of ZnIn2S4/Ni(dmgH)2 was designed by a simple chemical precipitation method and used to enhance hydrogen evolution under visible light irradiation. Along with vigorous discharges of hydrogen bubbles, an optimal rate of 36.3\u00a0mmol/g/h was reached under UV\u2013Vis light for hydrogen evolution, nearly 4.9 times of the one from pure ZnIn2S4. The heterojunction exhibits steady hydrogen evolution capability and owns a high apparent quantum efficiency (AQE) of 20.45% under the monochromatic light at 420\u00a0nm. By coupling ZnIn2S4 with Ni(dmgH)2, an extraordinary photochromic phenomenon was detected and attributed to the active Ni-S component in situ formed between the nickel and sulfur composites under light irradiation. The emerging sulfide benefits light absorption of the system and separation of photogenerated electron and hole pairs. Besides providing a promising photocatalyst for visible light hydrogen production, the present work is hoped to inspire new trends of catalytic medium designs and investigations.\n               "}
{"full_text": "Within the last few decades, rapid technological development has led to major climate changes, which are the most urgent issues that currently need to be faced. Burning carbon-derived fuels releases enormous amounts of carbon dioxide into the atmosphere, which constitutes more than 60% of global warming [1,2]. Currently, remarkable effort is being made to decrease CO2 emissions as well as to effectively capture and store CO2. Carbon dioxide can be used as an environmentally friendly carbon source to produce synthetic fuels and other chemicals (methane, methanol, ethanol, etc.), tackling both CO2 capture issues and resource depletion.Solid Oxide Electrolysis Cells (SOECs) are electrochemical devices that are able to convert water into hydrogen and oxygen by an electrolysis process occurring at the triple-phase boundary of the fuel electrode [3]. When CO2 is fed to the electrode as an additional fuel, the co-electrolysis of CO2/H2O may occur. During this process, both steam and carbon dioxide undergo parallel splitting reactions resulting in the formation of a mixture of CO and H2 (syngas) on one side and the formation of oxygen on the opposite side, which can be further utilized to produce useful chemicals [4,5]. However, due to the presence of both CO2 and H2 from H2O electrolysis, the reverse Water-Gas Shift reaction (rWGS, Equation (1)) occurs simultaneously.\n\n(1)\nCO2\u00a0+\u00a0H2 \u2194 CO\u00a0+\u00a0H2O \u0394H298K\u00a0=\u00a041\u00a0kJ\u00a0mol\u22121\n\n\n\nSo far, there is no agreement on the influence of rWGS on CO production: some studies show that carbon monoxide production is mainly dependent on the electrolysis, while others state that rWGS produces a significant amount of CO [6] [\u2013] [12]. Furthermore, the total amount of CO produced depends on the temperature, voltage applied to the cell, and the composition of the inlet gases [5]. The current studies involves the modified construction of the conventional SOECs for more efficient performance when in co-electrolysis mode as well as fabrication of novel materials that will replace e.g. Ni-YSZ cermet [13\u201316].The methanation reaction, in which carbon oxides are converted into CH4, is a promising method of CO2 utilization and also provides a solution for the transportation of low-grade energy [17]. Methanation, also known as the Sabatier reaction, can be expressed by Equation (2). For SOECs working in co-electrolysis mode, reactions of CO methanation can occur simultaneously (Equations (3) and (4)) [17] [\u2013] [19]. As one can see from Equations (1)\u2013(4) the methanation process is exothermic, while rWGS is endothermic, and the selectivity may be adjusted by changing the working temperature [20].\n\n(2)\nCO2 + 4H2 \u2192 CH4 + 2H2O \u0394H298K\u00a0=\u00a0\u2212165\u00a0kJ\u00a0mol\u22121\n\n\n\n\n\n(3)\nCO\u00a0+\u00a03H2 \u2192 CH4\u00a0+\u00a0H2O \u0394H298K\u00a0=\u00a0\u2212206\u00a0kJ\u00a0mol\u22121\n\n\n\n\n\n(4)\n2CO\u00a0+\u00a02H2 \u2192 CH4\u00a0+\u00a0CO2 \u0394H298K\u00a0=\u00a0\u2212247\u00a0kJ\u00a0mol\u22121\n\n\n\nAlthough the methanation process is thermodynamically favorable, it is an eight-electron process, which results in the presence of a kinetic barrier [17,18,21]. To transcend this limit, the use of proper catalysts is required, among which Ni is the most commonly used, due to its high catalytic activity, selectivity, and low cost [20,22,23]. Several studies proved that the methanation is most efficient at temperatures not exceeding 400\u00a0\u00b0C, which stands in contrast with the SOEC working regime [24] [\u2013] [27]. Moreover, at temperatures above 450\u00a0\u00b0C, the co-electrolysis reaction is favored, decreasing the selectivity of CH4 [26,28]. In the low-temperature regime of methanation, the degradation of Ni catalysts is not significant. However, at higher temperatures of the working SOEC, Ni catalysts tend to agglomerate and coke, leading to a decrease in their performance [29] [\u2013] [32]. J. Gao et\u00a0al. showed that the addition of steam to the feed gas slightly, but not significantly, decreases the CO2 conversion and CH4 selectivity [24].Other catalysts with promising properties for CO2 electrochemical conversion, such as coking resistance, are noble metals such as Pt, Rh, and Ru [20,33]. However, their high cost considerably restricts their use for methanation [23,34,35].Besides using monometallic catalysts, many studies are focused on incorporating a second metal, to e.g. Ni-based catalysts, to form a bimetallic system that couples the advantages of both metals. Bimetallic catalysts exhibit different catalytic properties due to modification of the electronic structure and geometry [35]. Among the most popular doping approaches, one can find the incorporation of other transition metals such as Fe or Co. Cobalt and iron can be easily dissolved in Ni metal, what can result in the formation of Ni\u2013Co and Ni\u2013Fe alloys and intermetallics [21]. What is more, the addition of a secondary metal may lead to increased stability and resistance to deactivation at higher temperatures [32]. Monometallic iron catalysts exhibit a high reaction rate, but low methanation selectivity, while NiFe alloys have an improved CO2 conversion rate [21,36]. The unarguable advantage of Fe doping is its low price and high abundance. The promotion of Ni catalytic properties under the influence of Fe strongly depends on the Ni/Fe weight ratio. The work of C. Mebrahtu et\u00a0al. has shown that the optimal Ni/Fe weight ratio on an (Mg, Al)Ox substrate is 0.1, while the studies by D. Pandey et\u00a0al. revealed that the greatest enhancement of CH4 selectivity and CO2 conversion was observed for 75\u00a0wt% of Ni and 25\u00a0wt% of Fe on an alumina substrate [37,38]. The enhancement of the catalytic properties can be associated with iron ions acting as a protective element to the nickel, as suggested by M. A. Serrer et\u00a0al. [39].Another element studied as a secondary metal introduced into the Ni lattice is Co. The catalytic properties of cobalt are similar to those of nickel and its addition was shown to improve the dispersion of Ni and resistance to deactivation. Although the price of Co is higher than Ni, it is still much lower than the noble metals [22]. M. Guo et\u00a0al. have shown that a small amount of Co (0.2\u00a0M Co/Ni ratio) added by impregnation to Ni/SiO2 leads to a decrease in catalytic properties, while molar ratios higher than 0.4 enhanced the low-temperature methanation catalysis [40]. Studies by C. Jia et\u00a0al. revealed that NiCo@TiO2/SiO2 nearly doubled the turnover frequency (TOF) compared to monometallic catalysts [41]. The enhancement in TOF values indicates that cobalt promoted the intrinsic catalytic activity as a result of a synergistic effect. Furthermore, the CH4 selectivity was above 95% and the CO2 conversion exceeded 50% for the aforementioned bimetallic catalysts. L. Xu et\u00a0al. have synthesized NiCo on Al2O3 with different Co/(Ni\u00a0+\u00a0Co) ratios [42]. They proved that the addition of 20% Co to the total metal amount exhibits the best catalytic properties. Moreover, the synergistic effect between nickel and cobalt resulted in a decrease in the activation energy for CO2 methanation, which further increased the CO2 conversion rate. The addition of Co may also have a beneficial effect on the reducibility of the Ni and metal dispersion, as suggested by B. Arlafei et\u00a0al. [22] However, they observed a positive influence of Co only in the case of low Ni loading (not exceeding 10% weight). It is noteworthy that long-term tests were performed on novel NiCo catalysts, proving their high stability.So far, there has been a lack of work on the bimetallic catalysts for high-temperature methanation on Solid Oxide Electrolysis Cells. The study most similar to those presented in this article is the work of H.Y. Jeong et\u00a0al., in which Fe ions were incorporated into the Ni/YSZ electrode by the wet infiltration method. They observed an increase in CO selectivity and an increase in the rWGS reaction while the SOEC was working in co-electrolysis mode [43].Herein, a series of Solid Oxide Electrolysis Cells modified by impregnation with Co was prepared, resulting in the samples containing 1.8, 3.6, and 5.4\u00a0wt% Co metal after reduction. The changes in the phase composition and electronic structure were studied. It was found that a small amount of Co impregnated into the conventional Ni-8YSZ fuel electrode greatly enhances the performance of the SOEC for co-electrolysis of H2O/CO2 mixtures with direct methanation. The addition of 3.6\u00a0wt% Co resulted in a nearly 3-times higher methane peak concentration at the outlet compared to the unmodified cell. The wide variety of the characterization techniques made it possible to determine the reasons behind the enhancement being three-fold: the Co-YSZ interface increases the basicity of the cell, the formation of NiCo2O4 delivers active sites for the reactions, and the formed structures highly develop the active surface area of the electrocatalyst.All electrolysers were modified by the wet impregnation method. For this purpose, a 1\u00a0m solution of Co(NO3)2 6H2O (Merck, 99.9%) in 10 v/v% EtOH in DI was prepared by dissolving the exact amount of nitrate salt in the solvent (5.82\u00a0g in 20\u00a0cm3 of the solvent). According to our previous experience in catalyst preparation, \u03b2-cyclodextrin (\u03b2CD, Sigma Aldrich, \u226597%) was added into the precursor solution at an amount equal to 0.05\u00a0mol \u03b2CD per every mole of Co2+ cations (1.135\u00a0g). The native cyclodextrin acted as an ion capping agent, altering the size and dispersion of the forming nanoparticles [44].The SOECs used in the study were delivered by the S.-F. Wang group from National Taipei University of Technology, Taiwan. The 1-inch diameter half cells were composed of a 400\u00a0\u03bcm NiO/YSZ cermet support with two different porosity levels and a 15\u00a0\u03bcm thick YSZ electrolyte. Prior to each modification step, the half cells were reduced at 850\u00a0\u00b0C under an H2 atmosphere to further increase the porosity of the cermet layer by NiO reduction. The cells were then impregnated using 100\u00a0\u03bcL of the precursor solution, which was found to be the maximum sorption volume. The half cells were transferred to a vacuum chamber for 15\u00a0min to ensure good penetration of the precursor solution throughout the fuel electrode. The cells were dried at 120\u00a0\u00b0C and sintered under an air atmosphere at 400\u00a0\u00b0C for 4\u00a0h to decompose all nitrates and organics. A series of three samples were prepared by repeating the impregnation steps 1, 2, and 3 times, which corresponds to 1.8, 3.6, and 5.4\u00a0wt% of Co0 in the metallic phase. A reference sample was prepared according to the same heat treatment routine but omitting the impregnation steps.X-ray diffraction patterns of the impregnated electrodes were collected using a Bruker D2 PHASER XE-T with a Cu-K\u03b1 radiation source before and after the testing procedure. The cross-sectional morphology of the pristine and spent fuel electrodes was verified using a Scanning Electron Microscope (SEM, FEI Quanta FEG 250) with an Energy-Dispersive X-ray spectroscope (EDX, EDAX Genesis APEX 2i) and Apollo X SDD detector. TEM imaging was performed using a JEOL 2100\u00a0F (Tokyo, Japan) microscope operating at 200\u00a0kV coupled with energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, UK). The X-ray Photoelectron Spectroscopy (XPS) spectra were collected using an Omnicron NanoTechnology X-ray photoelectron spectrometer with a 128-channel collector. XPS measurements were undertaken in ultra-high vacuum conditions, below 1.1\u00a0\u00d7\u00a010\u22128\u00a0mbar. Photoelectrons were excited by an Mg-K\u03b1 X-ray source with the anode operating at 15\u00a0keV and 300\u00a0W. The obtained spectra were deconvoluted using the XPSPEAK41 software. X-ray Absorption Spectroscopy (XAFS) and novel Scanning Transmission X-ray Microscopy (STXM) measurements were performed at the PIRX (former PEEM/XAS) and DEMETER beamline, respectfully, at the SOLARIS National Synchrotron Radiation Centre, Krak\u00f3w, Poland [45]. The used synchrotron energy range ensured the collection of the Ni-/Co-L2,3 and O\u2013K edges of the as-prepared and spent cells. Powdered samples were dispersed onto carbon tape and placed on measurement plates for spectra collection. The measurements were performed using total electron yield (TEY) and/or partial fluorescence yield (PFY) using an SDD window C2 detector from Amptek depending on the self-absorption of the samples under high vacuum conditions. The energy resolution was 200\u00a0meV and better, and the beam size (h x v) was 250\u00a0\u03bcm\u00a0\u00d7\u00a040\u00a0\u03bcm. The STXM imaging was performed on powdered samples dispersed onto an Si3N4 membrane. The series of image stacks of the samples were collected under a He atmosphere. Detection of the transmitted radiation was performed using a photomultiplier tube. The collected images were analyzed using the aXis2000 software. Elemental maps were formed as the difference between the signals collected at the absorption peak and pre-peak energies presenting the global distribution of the selected element.The H2-TPR (temperature-programmed reduction) and O2-TPO (temperature-programmed oxidation) measurements were performed using an in-house-built apparatus equipped with a TCD detector (Buck Scientific, USA), coldtrap, and a heated gas transfer line. Each time, the same amount of the sample was placed in a quartz reactor with an internal measurement of the bed temperature. The powders were degassed at 200\u00a0\u00b0C for 20\u00a0min in a stream of 5\u00a0N He prior to the measurements. The samples were reduced under a flow of 40\u00a0ml\u00a0min\u22121 5\u00a0vol%H2 in an Ar gas mixture. The tests were performed up to 900\u00a0\u00b0C with a heating rate of 10\u00a0\u00b0C min\u22121. Afterwards, the samples were cooled down to RT and the gas stream was switched to a mixture of 5\u00a0vol% O2 in He. The powders were equilibrated for 1\u00a0h in an O2\u2013He stream and followed by oxygen uptake tests carried out using the same regime as during the TPR measurements. The profiles were collected using the PeakSimple software with a frequency of 1\u00a0Hz.Prior to each SOEC test, the LSCF (La0.6Sr0.4Co0.2Fe0.8O3-\u03b4\n, Electro-Science Laboratories 4421A, USA) and the LSC (La0.6Sr0.4CoO3, Fiaxell SOFC Technologies, Switzerland) layers were screen-printed onto the air-facing side of the cell with an intermediate drying step at 120\u00a0\u00b0C between each layer. The deposited pastes gave as a result of a \u223c30\u00a0\u03bcm thick porous base layer of LSCF underneath a \u223c10\u00a0\u03bcm thick LSC layer used for better electron transfer to the Au current collector. The air electrode was sintered in-situ during SOEC start-up to protect the prereduced Ni-8YSZ side from reoxidation and possible breakdown. The active surface area of the air electrode was equal to 0.78\u00a0cm2. The SOECs were mounted onto the in-house-built measuring rig. The exact scheme of the unit and prior-measurement cell preparation steps were described in detail in our previous paper [46]. In brief, the modified SOEC was mounted onto an alumina tube and sealed using Ag-based conductive paste and dielectric ceramic adhesive (552\u20131105, Aremco). Au and Pt wires were used to maintain the electrical connection. The air electrode was contacted by an Au mesh and spring-loaded alumina interconnector. The set-up was placed inside a high-temperature furnace. The SOEC was each time heated up under flowing N2 to 850\u00a0\u00b0C and then the feeding gas was switched to 47 mLSTP min\u22121 H2 to perform the reduction of the modified fuel electrode material. The cell was held for 45\u00a0min at this temperature and then cooled down to 700\u00a0\u00b0C. The SOEC was equilibrated under flowing H2 for 12\u00a0h prior to electrocatalytic tests.After 12\u00a0h of SOEC reduction, the gas stream was switched to a CO2/H2O/H2 mixture. All of the gas components were dosed using electronic flowmeters. Hydrogen was supplied from the H2 generator while the other gases were provided from pressure tanks (Air Liquide) with purity over 99.9%. The water vapor was introduced by a controlled H2\u2013O2 mixture burning in an external reactor heated up to 700\u00a0\u00b0C loaded with Pt catalyst. The H2O vapor concentration in the inlet mixture was set to 20\u00a0vol% for all runs. The rest of the inlet mixture was composed of CO2/H2 at a ratio of 1:2 by vol. for comparison tests. The mixture composition was chosen to be far from the equilibrium ratio (H2:CO2\u00a0=\u00a04) to obtain a stronger response in CH4 concentration change for each of the fabricated SOECs. The gas mixture was supplied at the total flow rate of 28 mLSTP min\u22121. After switching to the testing mixture, the SOEC underwent conditioning at OCV until a stable voltage was achieved and later with a 1.3\u00a0V applied potential unless a stable current was observed. The resulting exhaust gases were analyzed using the FTIR-based measuring unit described in our previous work [46]. The spectrophotometer (PerkinElmer Spectrum 100) was equipped with a 10\u00a0cm length heated gas cell (60\u00a0\u00b0C) and ZnSe optical windows. The inlet/outlet flow rates were controlled using electronic flow meters. The exhaust stream was passed through a coldtrap (3\u00a0\u00b0C) followed by Nafion dryer tubing for complete water vapor removal prior to entering the FTIR. The measurements of the concentrations of gases in the outlet mixture were carried out from 700\u00a0\u00b0C down to 500\u00a0\u00b0C with a 20\u00a0\u00b0C step and 60\u00a0min delay for thermal stabilization and reaction equilibration. The spectra were collected every 5\u00a0min in the range of 4000\u2013500\u00a0cm\u22121 with a spectral resolution of 4\u00a0cm\u22121. Each scan was composed of five accumulations. The concentration of CO, CO2, and CH4 was retrieved via spectra integration performed at 3760\u20133520\u00a0cm\u22121 for CO2, 2226\u20132143\u00a0cm\u22121 for CO, and 3250\u20132650\u00a0cm\u22121 for CH4 using the SpectraGryph software and calibration curves specially designated for our set-up and presented previously elsewhere [46,47]. The amount of H2 was determined as the difference between 100% and the sum of the CO2, CO, and CH4 concentrations. Simultaneously, continuous electrical measurement of the current flowing through the cell was carried out using a Gamry potentiostat/galvanostat under 1.3\u00a0V. A set of additional tests under various potentials and gas mixture ratios (maintaining 20\u00a0vol% of H2O) was also performed. The applied potential was changed in the range of 1.1\u20131.6\u00a0V and the H2/CO2 volume ratio at 1.3\u00a0V was switched within 0.25 and 7.The quality of the prepared cells for efficient co-electrolysis and methane production was described by means of the CO2 conversion (\n\n\nX\n\nC\n\nO\n2\n\n\n\n\n), CH4/CO yields (\n\n\nY\ni\n\n\n) and CH4 selectivity (\n\n\nS\n\nC\n\nH\n4\n\n\n\n\n) catalytic coefficients calculated from the measured molar flow values using Equations (5)\u2013(8). The yields were calculated considering the CO2 input stream.\n\n(5)\n\n\n\nX\n\nC\n\nO\n2\n\n\n\n\n(\n%\n)\n\n=\n\n\n\n\nn\n\u02d9\n\n\nC\n\nO\n2\n\n\n\ni\nn\n\n\n\u2212\n\n\n\nn\n\u02d9\n\n\nC\n\nO\n2\n\n\n\no\nu\nt\n\n\n\n\n\n\n\nn\n\u02d9\n\n\nC\n\nO\n2\n\n\n\ni\nn\n\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(6)\n\n\n\nS\n\nC\n\nH\n4\n\n\n\n\n(\n%\n)\n\n=\n\n\n\n\nn\n\u02d9\n\n\nC\n\nH\n4\n\n\n\no\nu\nt\n\n\n\n\n\nn\n\u02d9\n\n\nC\n\nO\n2\n\n\n\ni\nn\n\n\n\u00d7\n\n\n\nX\n\nC\n\nO\n2\n\n\n\n100\n\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(7)\n\n\n\nY\n\nC\n\nH\n4\n\n\n\n\n(\n%\n)\n\n=\n\n\n\n\nn\n\u02d9\n\n\nC\n\nH\n4\n\n\n\no\nu\nt\n\n\n\n\nn\n\u02d9\n\n\nC\n\nO\n2\n\n\n\ni\nn\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(8)\n\n\n\nY\n\nC\nO\n\n\n\n(\n%\n)\n\n=\n\n\n\n\nn\n\u02d9\n\n\nC\nO\n\n\no\nu\nt\n\n\n\n\nn\n\u02d9\n\n\nC\n\nO\n2\n\n\n\ni\nn\n\n\n\n\u00d7\n100\n\n\n\nwhere: \n\n\n\nn\n\u02d9\n\ni\n\no\nu\nt\n\n\n\n and \n\n\n\nn\n\u02d9\n\ni\n\ni\nn\n\n\n\n are the molar flow rates of a specified gas (\n\ni\n\n) at the outlet and inlet of the reactor, respectively. The reference equilibrium compositions were calculated with the usage of the Gem module from the HSC Chemistry\u2122 software. The calculations were performed under free-flow reactor conditions and atmospheric pressure for an idealized system using the Gibbs energy minimalization method disregarding the electrocatalytic reactions (OCV catalytic measurements).The XRD patterns of the prereduced and modified SOEC fuel electrodes are presented in Fig.\u00a0S1. As the NiO/YSZ composite underwent the reduction step prior to the modification steps, the phase composition consisted of ionic conductor 8YSZ (8\u00a0mol.% Yttria Stabilized Zirconia) and metallic Ni. The impregnation steps from the novel precursor solution required cyclic firing of the cermet electrode at 400\u00a0\u00b0C to decompose the cobalt nitrate and organics existing in the precursor solution. This resulted in the formation of a small amount of surficial NiO/NiOOH layer via low-temperature reoxidation of the Ni metal. Despite the introduction of the Co ions into the electrode, no additional phases were clearly distinguished from XRD measurements. It is caused by the fact that the amount of Co species present in the electrode is below the detection limit of the XRD equipment used for pattern collection. Additionally, the created species were most likely forming highly nanocrystalline objects, which in fact would produce even broader and lower in intensity diffraction peaks that could be lost within the noise. There was also no clear evidence of shifting in the peak position of the NiO/Ni lattices, even though the formation of the mixed NiO\u2013Co3O4 oxide was very possible. A slight increase in the lattice parameters was observed according to the Rietveld refining method of the peaks assigned to NiO, but due to the low crystallinity of the resultant phase and broad peaks, the outcomes were designated as unreliable. Co is characterized by an infinite solubility in the Ni lattice when the metallic form is considered. The results obtained during the studies on Ni\u2013Co alloy nanoparticles and their oxidation in the work of L. Han et\u00a0al. followed by the presentation of the Ellingham and NiOx-CoOx phase diagram clearly indicated that within the used amounts of Co, it is highly probable to obtain mixed (Ni,Co)O monoxide with traces of metastable NiCo2O4 spinel structure [48]. Even though the XRD studies gave little information regarding the internal changes in the phase composition, they revealed the partially nanometric nature of the modified electrode due to the highly broadened peaks coming from the NiO phase.The cross-section SEM analysis of the unmodified cell presented in Fig.\u00a0S2 revealed that the microstructure of the pristine prereduced electrode consisted of micrometric grains of well-sintered 8YSZ and Ni. The NiO-YSZ composite was primarily designed to reach around 35\u201340% of total open porosity after the reduction. There were also two clearly distinguishable levels of porosity in the outer and functional layer (FL). The reduction step allowed for easier penetration of the infiltrant solution. The SEM images of the fractures of the reference and modified SOECs after 1, 2, and 3 impregnation procedures are presented in Fig.\u00a01\n.The structure of the electrodes differs greatly depending on the amount of introduced Co precursor solution. There are two new species clearly distinguishable in the images. The first is composed of nanoparticles of the Co oxides-hydroxides formed on the surface of the 8YSZ grains formed via non-reactive deposition. The new Co species are rather well-dispersed and clearly distinguishable on the 8YSZ grains forming uniform nanoparticles (NPs). Secondly, the paper ball-like structures that were determined to be a surficial oxide formed during the sintering steps under ambient air atmosphere. The latter one was described as Co species delivered during reactive deposition. Similar structures were also present in the reference electrode material, but of slightly less developed microstructure. Based on the simple EDX point analysis, the formed oxide scale consisted of Ni, O, and a small amount of Co. This indicated that a new structure of mixed composition formed on the surface of the Ni metal grains. The oxide scale seemed to exhibit a highly developed surface area of offbeat microstructure. The look of the scale may also stands for the formation of the spinel-like structures, as those tend to form powders of complicated morphology when in nanometric form [49,50]. Spinels are a group of materials that have been widely studied for electrochemical application by the members of our team and other international groups with groundbreaking properties followed by promising potential always being uncovered [51] [\u2013] [53]. The arrangement of the nanoparticles on the 8YSZ grains was different for all three samples. With the increasing amount of introduced Co, the separated NPs started to form a rather continuous layer with bigger distinguishable objects. The coalescence of the repeatedly deposited Co oxides is best seen for the 5.4\u00a0wt% Co sample (Fig.\u00a01D). In the case of reactive deposition, each additional cycle of impregnation tends to further modify the microstructure of the paper ball-like structures creating bigger, clumped-up agglomerates of Co\u2013Ni oxides. The previously mentioned NiOx-CoOx phase diagram diminishes the fact of spinel formation in the homogeneous mixture. Even so, the surficial character of the Co deposition can lead to a reaction at the Ni\u2013Co interface and form a spinel-like contact mixed layer as those two transition metals exhibit rather high reactivity [54]. To better understand the internal structure of the modified samples, a series of TEM images (Fig.\u00a01E and F) were captured using lamellae cut from the cells. An exemplary image of the 3.6\u00a0wt% Co sample accurately represents the general structure after Co incorporation. The imaging proved the bimodal behavior of the Co ions, i.e. reactive and non-reactive deposition. Co partially dissolves into reoxidized Ni and simultaneously forms homogeneous nanoparticles on the surface of 8YSZ. Additionally, a highly developed structure consisting of spherical polycrystalline nanoparticles was formed. The HRTEM was further utilized to analyze this porous structure deeply. From the image, we can see that the outer surface of the structure is nearly amorphous. This was caused by the low sintering temperature coupled with the addition of the cyclodextrin and most likely implies a high external active surface area [48,55]. Deeper inside, three exemplary interplanar distances were marked. The interplanar distance of well-defined lattice fringes equal to 0.46\u00a0nm was extremely close to the values observed by L. Huang et\u00a0al. and A. Cetin et\u00a0al., and assigned to (111) planes of NiCo2O4 [56,57]. The interplanar distances of 0.26\u00a0nm and 0.16\u00a0nm corresponded to the (220) planes of NiCo2O4 and (311) planes of Co3O4, respectively [58]. Slight deviation of the interplanar distances in the ideal NiCo2O4 phase arose from the nonideal stoichiometry of the compound and very probable coexistence of intermixed NiOOH\u2013CoOOH layer double hydroxide (denoted as NiCo2(OH)6) [59]. Parallel to the mixed oxides, free CoxOy nanoparticles were also formed closer to the surface as the Ni ions were bound by previously deposited Co, and diffusion in the surface proximity was slowed down [60,61]. Based on those results it was stated that Ni ions are being consumed by reacting with Co ions and the surficial layer consists of finely dispersed spinel-like particles and free CoxOy nanoparticles. The NiO unreacted layers can be found most likely closer to the surface of the pristine Ni grains.To quantitively study the surface composition and identify the valence state of the elements, a series of XPS measurements was performed on the as-prepared and spent electrodes (after SOEC tests). The exemplary results of the 3.6\u00a0wt% Co as-prepared sample presented in Fig.\u00a02\n shows that for both the core level spectra of Ni2p and Co2p, the peaks were fitted to the two spin-orbit doublets corresponding to 2p3/2 and 2p1/2. For each of the elements, two bands were deconvoluted after Lorentzian-Gaussian fitting and ascribed to the valence state of either +2 or +3. For Ni (Fig.\u00a02A), the peaks located at 853.7\u00a0eV and 871.8\u00a0eV were assigned to Ni2+, while the doublet around 855.4\u00a0eV and 873.2\u00a0eV to Ni3+. These values are in agreement with the variety of the research dedicated to the NiCo2O4-based structures in electrocatalysis [60,62] [\u2013] [64]. There are also two clearly visible shake-up satellite peaks within the Ni2p spectra. The slight positive shift (\u223c0.4\u00a0eV) of the Ni2+ peaks in the modified samples compared to the reference also indicates the formation of a new mixed phase [60,65,66]. Moving forward to the Co2p spectra (Fig.\u00a02B), the Co3+ oxidation state was indexed to the doublet located at 779.6\u00a0eV and 795.2\u00a0eV, while Co2+ to the one located at 781.4\u00a0eV and 797.0\u00a0eV. The position of the peaks is similar to the studies of X. Tong et\u00a0al. on urchin-like NiCo2O4 and the wide range of data retrieved by other groups [60,63,67,68]. In the case of the Co2p core-level spectra, the 2p3/2 peaks are shifting towards higher binding energies of \u223c0.3\u20130.4\u00a0eV per modification step. This is also an indication of the mixed oxide phase formation and substantial decrease of Co oxidation state, i.e., taking up more electrons [69]. It is due to the higher amount of available Co ions and prolonged sintering time allowing for better interdiffusion. Well-fitted doublets of both elements determined the ratio of Ni3+ to Ni2+ and Co3+ to Co2+. The changes in those ratios, depending on the amount of Co introduced, are depicted in Fig.\u00a02E. With the increasing concentration of surficial Co ions, the ratio of Ni3+ to Ni2+ increased drastically from 1.22 up to 2.22. This directly indicates the formation of the mixed Ni\u2013Co oxide layer, as in the case of the introduction of a minor amount of Co into NiOx where the structure can become self-doped with Ni3+. It was previously described that the Co doping of NiO results in reaching higher Ni valence states as it facilitates the formation of an Ni2O3 component, playing the role of p-type dopant [70] [\u2013] [72]. A further increase of the Co amount over 3.6\u00a0wt% resulted in a slight lowering of the concentration of Ni3+ within the structure. A similar issue was described elsewhere and stated that the increase in the amount of free CoxOy clusters formed on the surface acts as a carrier trap and limits the charge transfer from Ni to Co [71]. In the case of Co, the ratio of Co3+/Co2+ increased only slightly over the following cycles of impregnation (1.29\u20131.48). The same behavior was further observed and described thanks to the XAS measurements. The increase in the amount of Ni3+ has a substantial influence on the electrochemical performance of catalysts in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) though similar explanations may also apply in this research [73] [\u2013] [76]. In the case of post-mortem measurements (Fig.\u00a02C and D) it is clearly visible that an Ni metallic form came up in the XPS spectra and overpowering peaks of Ni3+ and Co3+ appeared. This is due to the presence of water vapor content in the stream during the cooling routine. It also indicates the high activity of the metals towards reacting with H2O and the formation of NiOOH\u2013CoOOH phases, which can have a crucial impact on the water molecules bonding during electrolysis [77].The electrocatalytic activity of the modified cells towards co-electrolysis with simultaneous methanation was tested under the conditions of a working SOEC. The measurements were performed under 20\u00a0vol% water vapor CO2 stream with H2 to prevent the oxidation of the metallic phase and ensure a higher FTIR response due to the changes in the CH4 concentration. The testing runs were performed both under OCV and a thermoneutral bias of 1.3\u00a0V. The CH4 concentration changes for the reference and the modified Co-impregnated cells versus SOEC temperature are presented in Fig.\u00a03\nA and B. The obtained current densities shown in Fig.\u00a03C indicated that, surprisingly, only a slight difference in the SOEC electrical efficiency was visible between the samples.Despite their similar electrical efficiency, the CH4 outlet concentration for the presented cells changes drastically after the incorporation of those minimal amounts of Co into the structure. The outcomes indicate superbly promising properties of the modified cells. The peaking methane concentration at OCV for the sample containing 3.6\u00a0wt% Co in the reduced state reached an outstanding 2.1% being over 2.5 times the value of the unmodified cell (\u223c0.8%). In the case of the 1.8 and 3.6\u00a0wt% Co-impregnated cells, the maximum concentration peak shifted towards a slightly higher temperature compared to the reference. It was caused by higher efficiency in the electrocatalytic CO2/H2O mixture splitting overcoming the existing thermodynamic limitations of the methanation reaction at high temperatures. Generally speaking, those samples exhibited elevated catalytic activity at the temperature range suitable for the proper work of an SOEC. To better represent the increase in catalytic activity, a series of describing parameters (CH4 yield, CO2 conversion, CH4 selectivity, and CO yield) were calculated, and the results are depicted in Fig.\u00a04\n. All of the calculations were done for the samples subjected to the tests at 1.3 V applied bias. The dashed lines were added as a visual guide to better represent the general trends. The efficiency of methane generation was determined by means of the CH4 yield parameter. At 1.3\u00a0V and a temperature equal to around 640\u00a0\u00b0C, all of the new cells easily reached the thermodynamic equilibrium compositions simulated for the idealized systems, in contrast to the reference cell. The reason was most probably two-fold. Firstly, the modified cells in fact reached a higher level of electrochemical efficiency, which can be best seen in Fig.\u00a04. Secondly, the methane-formation kinetics improved as the CO2 conversion coefficient greatly increased. As the CO2 conversion went significantly over the level of the simulated thermodynamic equilibrium, the direct CO2 hydrogenation reaction in particular had to be facilitated by the novel composition of the working electrode. At the same time, relatively comparable currents indicated the tendency of the Co-impregnated samples to alter the ratio of electrolyzed H2O and CO2 towards the second component. In the case of the modified samples, there was an outstanding, over twofold increase in methane yield. The efficiency of the methanation was clearly enhanced with subtle shifts in the maximum point. The cells with 1.8, and 3.6\u00a0wt% Co revealed altered temperature-dependent catalytic response. Further addition of the Co into the structure did not increase the methane yield greatly but definitely changed the general activity look of the profile to be more like the non-modified sample. The aim of this study was to increase the high-temperature co-electrolysis and methanation efficiency (>600\u00a0\u00b0C), so the 3.6\u00a0wt% Co-impregnated sample was chosen as the most promising one. Moving on to the CH4 selectivity, the samples were following similar trends as in the case of the CH4 yield. There is clear and visible evidence of the increased efficiency of methane formation at elevated temperatures. Even more pronounced is the activity of the 5.4\u00a0wt% Co impregnated cell, but the shifting to the lower temperature range is also noticeable. This could be further explored regarding the methanation catalysts working under conventional conditions. For both the reference and modified samples, the evidence of electrolysis is indisputable. In all cases, the CO2 conversion and CO yield parameters are placed high above the thermodynamic equilibrium point, pointing out the role of electrochemical H2O/CO2 splitting with the further formation of CH4 being independent of electrochemical reactions on the electrode. There are still several disputes over the impact of direct CO2 electrolysis and rWGS on the final outcome of the SOEC [78]. In the case of these studies, it is believed that the modified electrode material is responsible for the increased CO2 direct electrolysis in parallel to the rWGS reaction. The minor increase in the currents flowing through the electrolyte followed by the comparable CO yields, despite the higher CH4 yields, is basic evidence for the aforementioned statement. The increased rate of CO2 electrolysis on the surface of the electrode may be mostly due to the higher basicity and higher tendency to react with water to form oxyhydroxides of the Co species compared to the NiOx. This was predicted based on the high tendency of Co to undergo oxidation in a wet atmosphere and to further react the Co(O)OH species with CO2 forming a corresponding metal carbonate. A series of reaction enthalpies and Gibbs energy changes (Rea module) were calculated and followed by the thermodynamic equilibrium composition (Gem module) simulation using HSC Chemistry for Ni and Co under the working SOEC idealized inlet gas mixture composition. The results of the simulations are presented in the Supplementary Materials (Figs. S3 and S4). The higher ability of Co to form oxides-hydroxides as well as adsorb and bond the CO2 should increase the overall basicity of the electrode and increase the retention time of CO2 on the surface [79]. This would result in two things: A) increase of the CO2 electrolysis rate, and B) increase of the possibility of CO2 hydrogenation to form CH4 [80]. The obtained results prove the proposed mechanism of the increase in catalytic activity after Co addition. To develop the description of the routes leading to the enhanced efficiency of the Co-impregnated Ni-YSZ SOECs, a series of more sophisticated measurements were performed, and are discussed further in the text.To further examine the changes in the material properties after the introduction of the guest Co ions, a series of XAS and STXM measurements followed by a throughout spectra analysis with the assistance of the beamline specialists were performed at the SOLARIS synchrotron facility (Cracow, Poland). During the energy scans, the spectra for Ni-, Co-L2,3 and O K absorption edges were collected for the as-prepared and spent samples. Due to the high amount of Ni in the base structure, the spectra of the Ni-L2,3 edges were very similar for all of the samples with only slight evidence of Ni3+ forming on the surface after the tests (Figs. S5 and S6) [81]. The fluorescence signal was recognized as unreliable due to the strong self-absorption of Ni. A minor shift of the Ni edge and post-edge distortions may correspond to the formation of Ni\u2013Co species. Much more reliable results with clear evidence of the electronic structure changes are presented in Fig.\u00a05\n in the form of L2,3-edges of Co both in the total electron yield (TEY) and fluorescence (PFY) signal. To assist in the analysis, a series of certified reference materials were also subjected to the spectra collection. The XAS spectra are mostly dominated by the Co2p core-hole spin-orbit coupling, splitting the spectra into two regions \u2013 L3, L2 white lines parts. The changes in the shape of the absorption edge are clearly noticeable. The total TEY signal increased with the increase in the amount of Co in the structure. When taking the PFY signal as a reference point, it was observed that the ratio of the Co ions dissolved deeper into the oxide scale is higher for lower Co amounts. After repeated the impregnation steps, the intensity of both signals reached a ratio of \u223c1:1, showing that Co ions are much more likely to form surficial CoxOy oxides than to further dissolve into the NiOx. This finding is in line with XPS results and analysis of the TEM images described previously. Disregarding the absorption intensity, the shape of the edges also slightly changed depending on the Co concentration. Looking at the reference spectra of CoO and Co3O4, there is a clear difference between the pre- and post-edge shape. The higher relative intensity and slight distortions of the pre-edge part, coming mostly from the CoO edge, can be easily observed in the case of the 1.8\u00a0wt% Co-impregnated sample. An even more pronounced difference in this region of the spectra can be seen in the PFY signal. As the Co concentration increased, its oxidation state slowly increased as the shape of the absorption edge started to very closely resemble that of Co3O4. According to research by Zhang B. et\u00a0al., where the edge of the Co strongly resembled our result, it was stated that the Co2+/Co3+ admixture varied around the 1:1 ratio [82]. Based on experimental and simulational research by Chang C.-F. et\u00a0al., it was established that the created substructure of Co\u2013Ni oxides contains mostly low spin Co3+ and high spin Co2+ ions [83]. The absorption edges of the samples after the SOEC testing revealed that the cobalt was reduced on the start-up procedure of the cell (resembling the Co0 edge). Due to the presence of residual water vapor and noninert storage of the samples after the tests, a nanometric layer of the CoxOy oxide built up on the surface of the metal, with a higher share of Co3O4 in the case of the 5.4\u00a0wt% Co-impregnated sample. This indicated a lower stability and higher tendency of this sample to get oxidized, mostly due to the higher amount of initial free Co oxide species.The O K absorption edges were measured and the results for the as-prepared 3.6\u00a0wt% Co-impregnated sample are presented in Fig.\u00a06\n. The additional absorption edges of the reference materials were also included to resolve the composition of the surficial layer as the NiO, Co3O4, and NiCo2O4 compounds are characterized by slight shifts in the edge energy. The precise description of the pure NiCo2O4 preparation is detailed in the Supplementary Materials. Fortunately, the 8YSZ O K edge and the features coming from the carbon tape are located at an energy range that is not interfering with the shapes of the edges of Ni/Co-based compounds. Based on the obtained shape of the edge, three regions in superposition were highlighted and corresponded well with the placement of the absorption edges of the NiCo2O4, Co3O4, and NiO, respectively. The existence of the mixed Ni\u2013Co spinel-like structure was identified based on the small pre-edge feature peaking at 529\u00a0eV, which indicated the existence of an increased share of Ni3+ ions in the surficial layer of the electrode [84]. The position of the Co edge pre-peak feature is also clearly visible and surely represents the Co3O4 nanoparticles formed within the spinel structure and on the surface of the 8YSZ ionic conductor.The formation of NiCo2O4 increases the abundance of the Ni2+/Ni3+ and Co2+/Co3+ redox couples and through this can deliver a high number of active sites for performing chemical reactions. The recent studies focus on the use of NiCo2O4 normal and inverse spinel structure as the electrocatalyst. The position of the Ni and Co cations in the tetrahedral (Td) or octahedral (Oh) sites determines the final performance of the catalyst [85]. The degree of the inversion in the Ni\u2013Co spinel greatly changes the DOS and alters the electronic properties of the complex oxide. This further affects the SOEC performance as the purely reduced state is not most likely to happen. A high amount of water vapor increases the pO\n\n2\n and establishes an equilibrium between the formation and disintegration of the Ni\u2013Co oxide mixed compound. It is generally agreed that the existence of a high share of Ni3+ ions plays a crucial role in the electrochemical enhancement of the catalyst. The feature located at \u223c529\u00a0eV (Fig.\u00a06) can be taken as newly appearing with the unoccupied e\n\ng\n state of Ni3+ (3\u00a0d7) hybridized with O2p [86,87]. Studies conducted by M. Cui et\u00a0al. revealed that Co3+ located at Td sites is thermodynamically unstable and tends to get reduced into the more stable Co2+ valence [88]. As a result of this change, the Ni2+ gets oxidized into Ni3+ to reach the charge neutrality of the lattice. It leads to the generation of a new hole state of Ni3d-character and shifting of the E\n\nF\n closer to the valence band. The synergistic interaction between the Co and Ni may substantially increase the electronic conductivity and, in extreme cases, induce a metallic character of the NiCo2O4. The measurements at the O K edge can give a clear description of the interactions between the elements for the spinel-like structures that evolved in future samples.To better represent the distribution of the elements regarding the properties of their electronic structure, a series of STXM images was collected. A representative set of the images of the as-prepared sample containing 3.6\u00a0wt% Co is presented in Fig.\u00a07\n. This novel method of examination of the space distribution of the elements of the given absorption edge energy sheds new light on the results obtained during SEM imaging. Clear evidence was obtained of the above-mentioned statements regarding the formed structures of the Co-8YSZ facet and the Ni\u2013Co intermixed oxide species. A reference scan performed much below the absorption edges of Ni and Co resembled the TEM image presented in Fig.\u00a01. Elemental maps were obtained as the difference between the energy scan below and at the absorption edge. After reaching the energy of the Co-L3 edge, the Co nanoparticles and NiCo2O4-like structure come up within the imaging region. Moving up to the higher scanning energy located at the Ni-L3 edge, clear evidence of the bimodal state of the Ni was observed. The dark area is ascribed to the 8YSZ particle being impermeable to the radiation. The Co was homogeneously distributed over the sponge-like surficial structure of the reoxidized Ni particle, which indicates the presence of the NiCo2O4-like structure of reacted oxides. Additionally, not clearly visible in the TEM and SEM pictures, nanoparticles of CoxOy were identified both on the surface of the YSZ creating no obvious interlayer and as the inclusions of spherical nanoparticles embedded within the homogeneous structure of the mixed Ni\u2013Co oxide. Minor bigger agglomerates were also noticed in the images, which indicated that the impregnation with the addition of \u03b2CD is a promising method for homogeneous modification of SOC cells. According to the Ni maps, the ions are found in the homogeneous surficial oxide scale. Interestingly, the formed structure resembles a core-shell particle where the internal part is mostly composed of metallic Ni (slight shift of the absorption edge). The gradient-like distribution of Ni ions beautifully revealed the layer-by-layer buildup of the structure where the metallic core is surrounded by a NiCo2O4-like structure and peripheral Co-enriched phase with CoxOy inclusions. The complex structure greatly represents the diffusion limitations of the Co ions, but at the same time brings more difficulties for the formulation of final conclusions regarding the primary reasons for the enhanced operation of the SOEC in co-electrolysis accompanied by methanation.To better understand the interaction of Co and Ni species in the samples on start-up of the cell and reduction under flowing H2, a series of H2-TPR measurements were performed using our in-house-built TPx system. The corresponding reduction profiles are shown in Fig.\u00a08\n. The reduction kinetics of all of the samples is beyond the obvious, as the process ends much before 400\u00a0\u00b0C indicating the formation of the highly nanometric and porous structures which greatly reduce the reduction temperature. In general, for all of the samples, the shape of the reduction profile is composed of two distinctive regions, denoted as the lower temperature region (\u03b1) and higher temperature region (\u03b2). The first H2 consumption peak (\u03b1) located at around 250\u00a0\u00b0C was related to the existence of the mesoporous surficial structure of paper ball-like oxide scale with an amorphous structure (see Fig.\u00a01) as it is more likely to undergo fast oxygen uptake and release than the layers underneath. According to the literature and previous findings, the \u03b1 peak can be attributed to the reduction of surface-active oxygen species adsorbed onto the surface of the mixed (Co,Ni) oxyhydroxides and/or smaller nanoparticles of (Ni,Co) oxides as the low temperature is sufficient to overcome the energy barrier of their release [89,90]. As a similar \u03b1 peak appeared in the reference sample, it indicates a significant amount of Ni3+ ions, presumably both in the form of pure and Co-doped NiOOH. The low temperature during the sintering step of the prereduced samples under an ambient air atmosphere resulted in the formation of copious amounts of surficial oxyhydroxides of low crystallinity. This is in agreement with previously analyzed TEM images (Fig.\u00a01).The splitting of the \u03b2 reduction peak, visible mostly for the 1.8\u00a0wt% Co-impregnated sample, is related to the coexistence of a NiCo2O4 spinel-like phase and embedded CoxOy nanoparticles. To better represent the deconvolution of the overlapping reduction processes, a series of H2-TPR measurements was also performed for the 8YSZ particles impregnated with NiO, Co3O4, and NiCo2O4. The detailed procedure is included in Supplementary Materials and simulated the preparation steps covered in this research. The results are presented in Fig.\u00a09\n. By comparison, it is clearly visible that the resultant H2-TPR profile of the Co-containing cells is a superposition of the NiO-YSZ and Co3O4-YSZ reduction profiles and highly resembles the one of NiCo2O4-YSZ. The high-temperature \u03b2 region was further deconvoluted into two subregions, namely \u03b21 and \u03b22. The first was attributed to the reduction of the NiO subphase being in strong interaction within Ni\u2013Co spinel-like structure. Due to this interaction between the Ni and Co, the reduction temperature was significantly lower for the Co-impregnated samples. Based on research by Y. Yi et\u00a0al., the reason for this shift is bidirectional [91]. Firstly, the synergistic effect of the coexistence of Ni\u2013Co spinel-like compound facilitates the reaction between Ni3+ and Co2+ through the charge transfer reaction \n\nN\n\ni\n\n3\n+\n\n\n+\nC\n\no\n\n2\n+\n\n\n\u2192\nN\n\ni\n\n2\n+\n\n\n+\nC\n\no\n\n3\n+\n\n\n\n, which facilitates the easier reduction of Ni ions. The second reason is the higher dispersion of the NiO subphase throughout the whole structure and increased mesoporosity of the oxide scale after the introduction of Co. As in this research, the well-defined peak around 300\u00a0\u00b0C was attributed to the reduction of the bulk Ni2+ to Ni0 based on the general data on the reduction behavior of nanometric NiO. The secondary process rising within the \u03b21 peak is the simultaneous reduction of Co3+ to Co2+ [92]. The \u03b21 was shown to undergo a slight shift towards higher temperatures, but still lower than for the unmodified sample. This indicates that the additional heating steps increase the crystallinity and average particle size of the scale.The \u03b22 peak at around 340\u00a0\u00b0C was likened to the reduction of Co species. The position of the assigned process is also in agreement with the profile of the Co3O4-YSZ reference material shown in Fig.\u00a09. According to the findings of Niu J. et\u00a0al. on NiCo2O4-based catalysts for toluene conversion at a temperature similar to that observed during this study, the reduction of Co2+ to metallic Co occurs [93]. Considering the reference material of Co3O4-YSZ, the main reduction peak of the aforementioned process was also shifted towards lower temperatures. The modified structure of the cells is hereby experiencing the synergistic effects of the mixing of the transition metals. On the one hand, the Co ions help to reduce the Ni ions at the lower temperature, while the metallic Ni speeds up the reduction of Co ions into the metallic form, most likely by the hydrogen spillover mechanism [94,95].With the increasing number of impregnation steps, the \u03b21 and \u03b22 start to fully overlap each other, creating a profile that highly resembles that of the reference material NiCo2O4-YSZ (Fig.\u00a09). This indicates that the abundance of the Co ions and longer diffusion time facilitated the formation of the proper Ni\u2013Co mixed compound. For the 5.4\u00a0wt% Co-impregnated sample, the bimodal and enhanced reductivity are extinguished. It seems that the existence of the layer of lower crystallinity is of major importance.O2-TPO experiments were also performed to resolve the changes in the redox chemistry of the electrode due to the composition of the altered catalyst. The results of the measurements are presented in Fig.\u00a08 and additional profiles of the reference materials are shown in Fig.\u00a09. Agreeing with the Gibbs free energy calculations for the oxidation of Ni and Co metals and the TPO profiles presented in Fig.\u00a09, the higher the amount of Co added into the electrode, the lower its oxidation temperature. This is direct evidence of the higher susceptibility of the modified material towards oxygen uptake. Taking into account that SOECs work under high water vapor pressures and the previous conclusions over the role of oxyhydroxides and Me3+/Me2+ pairs, the higher ability to bind oxygen and form the surficial layer of the Ni\u2013Co mixed oxide is of great importance for the final performance of the cell.A novel behavior, previously undescribed in the literature, of the NiCo2O4-YSZ material was discovered and recorded. The behavior on reduction of the NiCo2O4-YSZ reference catalyst material was in line with other reports. Even so, the further O2-TPO measurements revealed that the material, despite being subjected to the oxidizing atmosphere, was able to evolve additional oxygen from the lattice at elevated temperature (Fig.\u00a09, right). A cyclic heating-cooling experiment was performed to ensure a reliable result. During the heating, the NiCo alloy supported on the YSZ substrate underwent full oxidation until \u223c450\u00a0\u00b0C. A further increase of the temperature over 600\u00a0\u00b0C caused the evolution of the oxygen from the catalyst material into the oxidant stream, even though it contained 5\u00a0vol% O2 of He. This was recorded as a negative TCD detector signal. The cycle was completed with the cooling of the catalyst bed in the same stream of the gas mixture. On cooling, the process was fully reversed at around 600\u00a0\u00b0C again showing a positive TCD peak of a similar area. This interesting feature of the NiCo2O4 compound will be further studied to understand the chemical and physical causes of the discovery. The primary hypotheses are threefold. The first states that the compound disintegrates into two major compounds \u2013 NiO and CoO \u2013 with the simultaneous release of the redundant oxygen. The second covers the issues concerning the change of the crystallographic structure of the spinel. And finally, the most promising one: the reduction on oxidation may directly correspond to the oxygen activity coefficient and oxygen bond strength. When subjected to a slightly oxygen-depleted atmosphere, the lower pO\n\n2\n causes the partial reduction of the surface of the spinel causing the kinetics of the lattice oxygen release to overedge the kinetics of oxygen uptake. The results of those analyses will be of significant scientific value to the field of materials science and catalysis. Furthermore, according to this research, the behavior of the spinel structure may explain the increased catalytic activity of the electrode after the introduction of Co. The increase in the oxygen exchange rate should highly influence especially the electrocatalytic activity in the context of water splitting in the SOEC.To better illustrate the dependencies between the actual inlet mixture composition and the final output of the SOEC, a series of tests under various H2/CO2 ratios and operating voltages were performed. The results are presented in Fig.\u00a010\nA\u2013C. The representative temperature of 640\u00a0\u00b0C and the 3.6\u00a0wt% Co-impregnated sample were chosen for all tests. When the influence of voltage changes was examined, the inlet mixture was set to 2:1 H2 to CO2, and for the mixture change-related test, a thermoneutral bias of 1.3\u00a0V was selected. While increasing the bias applied to the SOEC, the CH4 concentration increased linearly, followed by a similar trend observed for the CO concentration (Fig.\u00a010A). Due to the increased conversion, the corresponding concentration of CO2 dropped in the same manner. Despite this, the cell maintained the same linear dependence of the current increase with the increase in voltage (Fig.\u00a0S7). This indicates that the cell performed well, even though a relatively low temperature was maintained. The increase in the CH4 concentration was a result of the shifting of the thermodynamic equilibrium by increasing the amount of the reactants \u2013 mainly CO and H2 \u2013 which enabled the high rates of the methanation. When changing the H2/CO2 vol. ratio at a fixed bias (1.3 V), an increase in the CH4 concentration was observed until the 4:1\u00a0vol ratio was reached. After that point, the CH4 yield remained almost unchanged due to the highly depleted carbon source for the methanation reaction to happen. An interesting view of the electrochemical aspects of the work of the SOEC was seen when the change of the CH4 in the outlet stream (Fig.\u00a010B) and the current change (Fig.\u00a010C) were compared. After the increase of the share of H2 in the inlet mixture, the CH4 easily reached the thermodynamic equilibrium concentration while the current flowing through the SOEC kept decreasing. The fixed amount of H2O vapor followed by the decreasing share of CO2 limited the electrolysis efficiency due to the lack of reactants to be reduced. This is proof that the CO2 can also be directly electroreduced on the electrode material, in parallel to the fast, ongoing rWGS reaction.To better represent the stability of the SOEC with additional modifications, a prolonged test for 12\u00a0h was carried out. The cell was subjected to the same start-up procedure and after the gas switch to the H2/CO2 mixture, the concentration changes were monitored every 15\u00a0min. The Time-On-Stream measurements are shown in Fig.\u00a010D. The results gave us clear evidence that the Co impregnation does not change the stability of the pristine cell, while increasing the catalytic activity and maintaining its performance for over 12\u00a0h. During the first hours of electrolysis, a slight decrease was observed mostly due to Ni particle growth and sintering. This is a normal behavior for SOCs, and after a few hours the CH4 concentration remained nearly unchanged. A similar trend was noticed for the CO and CO2 concentration changes, where after 6\u00a0h of testing, the value oscillated around 18% and 23.5%, respectively. This shows that the introduction of Co into the structure enormously enhances the catalytic performance with no negative impact on the cell stability issues.Post-mortem imaging was done to observe the structural changes after reduction and 12\u00a0h of operation in SOEC mode. The SEM and TEM images of the spent electrodes are depicted in Fig.\u00a011\n. An additional collection of elemental maps using \u03bcEDS was performed for the 3.6\u00a0wt% Co-impregnated sample. After the reduction step, the Ni and Co got fully reduced and the bimetallic alloy formed on the surface of the Ni particles. The Ni grains looked homogeneous and no clear evidence of the formation of secondary phases was found. This is due to an interesting aspect of the binary Ni\u2013Co phase diagram, which states that both elements can form solid solutions throughout the whole range of concentrations, due to their close proximity on the periodic table and similar atomic structures [96,97]. In contrast, the Co in its metallic form created a distinctive structure of spherical nanoparticles on the 8YSZ grains. It demonstrates nearly no solubility of the Co in 8YSZ with the tendency of the metal to undergo dewetting and form a network of homogeneous nanoparticles. This is true in the case of the 1.8, and 3.6\u00a0wt% Co-impregnated samples, while for the 5.4\u00a0wt% Co-impregnated sample the amount of Co was so significant that a strong agglomeration process occurred. Also, a higher tendency towards oxidation led to the possible formation of the secondary flake-like phase visible in Fig.\u00a011. Based on the previous XAS measurements, it may be composed of cobalt oxides that were prereduced and not fully dissolved into the Ni matrix, further oxidized during the cell cooldown scheme. The highest homogeneity was assigned to the sample containing 3.6\u00a0wt% Co in the structure. The TEM images and elemental maps are in line with the aforementioned analysis showing the Ni\u2013Co homogeneous mixture and Co nanoparticles anchored to the surface of the 8YSZ. As to the cell's performance, the existence of both the Co-YSZ system resulting in higher basicity, and NiCo2O4 spinel-like oxide for providing active sites and the increase in specific surface area can be ascribed as the major causes of its enhancement.The purpose of this work was to successfully modify conventional Solid Oxide Electrolysis Cells via Co-impregnation and characterize the changes it caused to the internal microstructure, phase composition, and electronic structure in detail. The modifications consisted of the introduction of small amounts of Co into the Ni-YSZ cermet material of the cells via the wet impregnation method. The addition of \u03b2CD resulted in the homogeneous dispersion of the Co ions throughout the material bulk. It was observed that the Co ions formed three types of substructures, namely nanoparticles of CoxOy supported on the surface of the 8YSZ, Ni\u2013Co mixed spinel-like compound on the interface between the Ni core and outer layer, and CoxOy nanoparticles embedded into the spinel scale. The XPS results indicated that Co induced the formation of a high amount of Ni3+ ions. This directly increased the number of the available catalytic sites through the active Ni3+/Ni2+ and Co3+/Co2+ couples for reactions to happen. The performed modifications increased the CH4 concentration in the outlet stream over 2.5 times and ensured better efficiency of the H2O/CO2 co-electrolysis and conversion. The addition of Co increased the CO2 conversion from 47% up to 57% at 700\u00a0\u00b0C. The search for the possible causes of the enhancement through XAS and STXM measurements resulted in direct proof of the existence of a significant amount of the intermixed Ni\u2013Co compound which induced changes in the shift of the E\n\nF\n band energy, generated the inverse spinel structure, and introduced a significant amount of active surface species. The STXM measurements clearly evidenced a Co-graduated structure of core-shell-like Ni grains. The H2-TPR and O2-TPO studies revealed that highly developed and nanometric structures were formed after the Co introduction. The modified cells were characterized by a lower reduction temperature due to the synergistic effects of the coexistence of Ni and Co. A highly novel discovery concerning the interesting behavior of the NiCo2O4 supported on the 8YSZ was made. Even though the TPO experiment was carried out in an oxidizing atmosphere, the powder evolved additional oxygen at high temperatures, only to reverse this process when cooling down. Tests under various operating voltages and H2/CO2 inlet ratios led to the general proof of simultaneous direct and indirect (rWGS) routes of CO2 electroreduction. The 3.6\u00a0wt% Co-impregnated sample was characterized by the most homogeneous distribution of Co species across the cermet material and small, spherical nanoparticles developed within the structure. The addition of the secondary metal into the Ni-YSZ conventional cermet material revealed highly promising results to be further applied in the field of H2O/CO2 co-electrolysis with the simultaneous single-process of methanation for the buildup of advanced conversion systems. Further studies on the bimetallic synergy and strange spinel behavior under different pO\n\n2\n and elevated temperatures are planned to better understand the phenomena.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the 5th Polish-Taiwanese/Taiwanese-Polish Joint Research Project PL-TW/V/4/2018 granted by the National Centre for Research and Development of Poland and the Ministry of Science and Technology of Taiwan. This publication was developed under the provision of the Polish Ministry of Education and Science project: \u201cSupport for research and development with the use of research infrastructure of the National Synchrotron Radiation Centre SOLARIS\u201d under contract nr 1/SOL/2021/2. We acknowledge SOLARIS Centre for the access to the Beamline PIRX and Beamline DEMETER, where the measurements were performed.The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2022.08.057.", "descript": "\n                  To study the synergy between the transition metals for enhancing the electrochemical and chemical activity, a series of SOECs were modified with a small amount of Co ions, namely 1.8, 3.6, and 5.4\u00a0wt% in the reduced state. The addition of \u03b2CD into the precursor solution allowed for extremely fine dispersion of Co species across the Ni-YSZ cermet structure. The sample containing 3.6\u00a0wt% Co reached an outstanding over 2.5-times-higher concentration of CH4 in the outlet stream. At the same time, the Co greatly enhanced the electrochemical efficiency of water and CO2 co-electrolysis. Full characterization involving STXM imaging allowed for better understanding of the synergy between the Ni and Co host metal and made it possible to find the causes of the increased activity. It revealed the complexity of the substructures formed within the electrode. A novel discovery was described regarding the NiCo2O4 spinel structure subjected to the O2-TPO measurements. Despite the applied oxidizing atmosphere, the catalyst evolved oxygen at elevated temperatures in a reversible manner. The performance tests indicated the roles of both rWGS and direct electrolysis of CO2 in the electroreduction process. The addition of Co did not influence the prolonged degradation of the cell.\n               "}
{"full_text": "The market for proton exchange membrane fuel cells (PEMFCs) is growing as the EU has highlighted hydrogen as a key factor in the energy market in their Green Deal plan [1]. One of the ways in which hydrogen shall be used is to power heavy-duty vehicles, where large storage capacity and fast refueling are crucial. To increase the competitivity of such fuel cell electric vehicles, the production cost of fuel cells needs to be reduced. Platinum-based electrocatalysts are one of the main cost-determining components, leading to an enormous research interest in the reduction of Pt content. Among the various strategies towards this goal, the alloying of Pt is one of the most promising and advanced approaches [2].Along with the strive for cost reduction, the durability of the electrocatalysts is a major issue to be tackled. The widely used, commercially available Pt/C catalysts show a number of disadvantages with respect to oxygen reduction reaction (ORR) performance at the cathode of the PEMFC. Apart from their poor efficiency with respect to the Pt loading, their limited durability is a major drawback for reliable long-term application and sustainability [3]. Pt itself, however, is crucial for ORR especially in acidic environments such as the PEMFC. The most active transition metals at ORR and therefore worth considering for alloying with Pt are Cu, Ni, and Co [4]. The most investigated and most promising Pt-based nanoparticles (NPs) tackling durability issues are Pt\u2013Co and Pt\u2013Ni NPs, often in the form of core-shell NPs with a Pt shell [5,6]. Further effort to simultaneously improve both ORR activity and durability are sought by investigation of ternary and quaternary systems [7,8]. Apart from the composition of the electrocatalyst, a high number of active sites is targeted to achieve high catalytic efficiency [9]. For this reason, the incorporation of NPs and even single-metal atom catalysts in carbonaceous matrices is currently a subject of intense investigation [10,11].One of the challenges, and an important factor influencing the fuel cell performance, is the synthesis route of the catalyst, which also adds up to the production cost. In most approaches, the catalyst NPs are synthesized on a carbon support, mixed with a binder and ionomer, and sprayed onto the membrane, which leads to NPs being deposited in locations where they are not accessible for the catalytic reactions [12]. Using direct electrodeposition of Pt alloy NPs onto the gas diffusion layer (GDL) of the PEMFC, the NPs are deposited on the most active sites of the carbon support\u00a0because the metal ions should occupy the locations where the local electric field is highest during the electrodeposition\u2014locations on the substrate where the path is short and the local charge density is high. Furthermore, this approach combines both synthesis and fixation (or distribution) of NPs into a single step, leading to a cost reduction of the synthesis route [13].Fundamental research on the mechanisms of NP formation by electrodeposition has been carried out by Ustarroz et\u00a0al., including the electrodeposition of Pt NPs for potential application at ORR [14\u201316]. The electrodeposition of Pt alloy NPs has also been shown promising for alkaline ORR and other catalytic reactions [17,18].A common synthesis route for metallic NPs is by pulse electrodeposition. An extensive study on the pulse electrodeposition of Pt NPs for ORR was carried out by Huang et\u00a0al. [19]. A pulse reverse process was employed by Sriwannaboot et\u00a0al. for the codeposition of Pt\u2013Co alloy onto carbon cloth substrates [20]. Egetenmeyer et\u00a0al. thoroughly investigated the effects of pulse electrodeposition parameters for Pt, Pt\u2013Ni, and Pt\u2013Co NPs deposited onto GDLs, and determined optimum electrodeposition parameters for each alloy [21]. Santiago et\u00a0al. were able to reduce the rather high Pt NP size commonly obtained by electrodeposition down to below 10\u00a0nm by deposition onto a rotating disk electrode (RDE) [22]. Wang et\u00a0al. achieved Pt particle sizes of 3\u201310\u00a0nm via the use of a complexing agent [23]. Remarkably, pulse electrodeposition offers several advantages compared with other NP fabrication schemes. First, synthesis and anchoring to a substrate such as GDL is achieved in one step. Second, NPs nucleate onto the most active sites of the GDL, as aforementioned, as opposed to other post-synthesis approaches for which the catalyst is distributed on the GDL irrespective of local electrical properties and depth, so that many NPs may be located at inaccessible or unfavorable locations. The main disadvantage of pulse electrodeposition compared with conventional liquid-phase synthesis and high-temperature calcination [24] is that an increase in the catalyst loading is achieved at the expense of increasing particle size, which is often undesirable for catalytic purposes.Interestingly, Liu et\u00a0al. synthesized a Pt\u2013Ni alloy with low Pt content via a carbothermal shock method, which was shown to be effective at ORR in acidic media and PEMFC testing [25]. This is in contrast to most ORR electrocatalysts investigated for PEMFC, which usually rely on high-Pt content alloy NPs.Although the synthesis of Pt\u2013Ni NPs for ORR [26,27], as well as the electrodeposition of Pt\u2013Ni alloys in general [28,29] is well advanced, Mo-containing Pt alloys have been scarcely investigated. Huang et\u00a0al. showed that the doping of Pt3Ni NPs with Mo led to extraordinary ORR performance, making the ternary Pt\u2013Ni\u2013Mo system an interesting candidate for ORR studies [30]. These NPs were obtained by decomposition of acetonate and hexacarbonyl precursors. The electrochemical co-deposition of Mo, which is usually achieved using sodium molybdate [31], may lead to the formation of intermediate Mo oxide species due to partial reduction of Mo(VI) [32]. However, even NPs containing Mo oxide show improved electrocatalytic properties [8,33]. The coordination of Mo with electronegative elements like oxygen or nitrogen can move the d-band center of Mo so that its binding capacity with reaction intermediates (O\u2217, OH\u2217, and OOH\u2217) increases, thus making Mo atoms moderately active towards ORR [34]. Therefore, the introduction of oxidized Mo atoms to Pt\u2013Ni should not be, in principle, deleterious for the ORR performance.Considering the above results, it seems that (i) Pt\u2013Ni is both an adequate candidate for ORR and (ii) the incorporation of a third alloying element can improve electrocatalytic performance. Yet, the expected improvement is not always attained. For example, Sorsa et\u00a0al. electrodeposited Pt\u2013Ni from liquid crystalline solution onto GDL substrates, but they did not observe any significant improvement of the electrodeposited Pt\u2013Ni over commercial Pt/C [35]. Hence, further investigation on this topic is still to be performed.In this work, pulse electrodeposition is used to deposit Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) particles directly onto the microporous layer of a commercial GDL. The GDL is composed of a woven carbon cloth with a microporous, PTFE-coated carbon layer, where the catalysts shall be applied. For such a complex three-dimensional substrate, electrodeposition is an especially suitable method [36]. In the deposition process, all particles are deposited on the most active sites of the carbon support's surface where they guarantee excellent contact with both the support and the proton exchange membrane (PEM) in the fuel cell. Although the use of Pt\u2013Ni NPs for ORR, and the electrodeposition of Pt\u2013Ni alloys are rather developed, the combination of both, in addition to the direct deposition onto the GDL, has been reported very scarcely. The Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) NPs with different compositions are chemically and structurally characterized, and ORR in acidic media is investigated. Finally, fuel cell performance and durability tests in a PEMFC prototype are carried out after hot-pressing the obtained cathodes with the PEM and a commercial Pt/C electrode as anode.The synthesis of Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) NPs was performed by pulse electrodeposition from aqueous solution in a three-electrode electrochemical cell. An Autolab PGSTAT204 potentiostat/galvanostat was used with a Pt wire as counter electrode (CE), an Ag|AgCl (3\u00a0M KCl) reference electrode (RE) and a working electrode (WE). The WE consisted of a 2\u00a0cm by 2\u00a0cm GDL, supplied by Freudenberg, mounted on a Cu support to ensure electrical connection and a homogeneous charge distribution over the entire area of the GDL during electrodeposition. The excessive area of the Cu support was isolated with polyimide tape (Fig. 1\n).The aqueous electrolytes were loaded with nickel chloride, sodium hexachloroplatinate, boric acid, and ammonium chloride, based on an earlier study [28]. For the deposition of Pt\u2013Ni\u2013Mo(O), sodium molybdate (as Mo precursor) as well as citric acid as complexing agent were added (Table 1\n). All chemicals for electrolyte preparation were of analytical grade and had been supplied by Merck. The pH was adjusted to 2.7 by addition of sulfuric acid solution.The electrodeposition was carried out in stagnant conditions at 30\u00a0\u00b0C with pulses of varying current density to obtain different compositions. The choice of pulse deposition parameters was initially orientated on the study on the electrodeposition of Pt\u2013Ni NPs carried out by Egetenmeyer et al. [21] and then altered by empirically optimizing pulse deposition parameters with respect to particle size, catalyst loading, and composition. It was observed that lower particle sizes were obtained at the expense of the catalyst loading, so that finally the process parameters were chosen as a compromise between particle size and catalyst loading. Pulse on-time and off-time were kept constant at 5\u00a0ms and 70\u00a0ms, respectively, whereas the number of cycles was changed with the current density to obtain identical deposited charges and, assuming similar Faradaic current efficiencies for each electrolyte, similar catalyst loading (Table 2\n). The total charge was increased for the Pt\u2013Ni\u2013Mo(O) deposition due to the addition of citric acid, which was observed to compromise the current efficiency. This is in accordance with previous studies, where the addition of citrate was found to increase Mo content in the deposits while lowering the current efficiency [37]. It was also observed during initial studies that higher current densities were needed for the ternary NPs to obtain the desired particle size and composition.The same three-electrode set-up used for NP electrodeposition was used for cyclic voltammetry (CV) studies of the NP/C assemblies in 0.5\u00a0M\u00a0H2SO4 to activate the catalysts and to determine their electrochemically active surface area (ECSA). To this end, 30 cycles at 200\u00a0mV/s and 5 cycles at 50\u00a0mV/s were recorded in a potential window between 0 and 1.3\u00a0V versus reversible hydrogen electrode (RHE). ORR was studied by CV in O2-saturated 0.1\u00a0M HClO4 after recording a background CV in N2-saturated electrolyte, both at 10\u00a0mV/s between 0 and 1.1\u00a0V versus RHE and in static conditions. Potentials applied against Ag|AgCl were converted to RHE scale. The curves were corrected for Ohmic drop (iR-correction) after determination of the instrumentation resistance by electrochemical impedance spectroscopy (EIS) [38]. A Pt/C GDE with a Pt loading of 0.3\u00a0mg/cm2 was tested as a reference. It should be noted that the procedure for ORR measurements employed here differs from the usual approach using RDE since the electrocatalysts are directly deposited onto the GDL substrates and cannot be deposited directly onto an RDE. As a result, a quantitative determination of ORR parameters, such as the half-wave potential, is not feasible. Nevertheless, the measurements give qualitative information to show trends between catalysts of different compositions, in addition to the comparison with a commercial GDE.All deposited NPs, as well as the unloaded GDL substrate, were analyzed by scanning electron microscopy (SEM) on a Zeiss Merlin electron microscope to evaluate particle size, distribution, and loading of the substrates, using an acceleration voltage of 1\u00a0kV. For energy-dispersive X-ray spectroscopy (EDX), an acceleration voltage of 20\u00a0kV was used. However, quantification of the electrocatalyst NPs was not feasible by EDX due to the very low loading of NPs. For this reason, a chemical analysis method was employed (see below). Particle sizes were determined by image analysis of the SEM micrographs using ImageJ, taking the average of 100\u2013200 particles per sample. SEM was also used for post analysis after PEMFC testing on a TESCAN LYRA3, after the cathode was delaminated from the membrane electrode assembly (MEA). For cross-sectional observation, one MEA was cut, embedded in epoxy resin, mechanically ground and polished and subsequently ion milled by Ar ions on a Gatan PECS II.Transmission electron microscopy (TEM) was conducted on a JEOL JEM-2011 in high resolution and diffraction mode to study the crystalline structure of the NPs. Samples for TEM were prepared by scratching the catalyst NPs, together with part of the carbon support, off the electrodes, dispersing the samples in ethanol, and dropping them onto the carbon film of a TEM copper grid. Electron energy loss spectroscopy (EELS) was performed on a FEI Tecnai G2 F20 STEM. Both TEM were working at an acceleration voltage of 200\u00a0kV.For determination of both composition and loading of the substrates, NPs were dissolved in aqua regia, consisting of hydrochloric acid and nitric acid in a ratio of 3:1 in volume, and the concentration of Pt, Ni, and Mo in the solution was determined by inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7500ce spectrometer for each variation of electrodeposition parameters. In addition, the Faradaic current efficiency was determined by relating the total charge converted during electrodeposition to the absolute masses of Pt, Ni, and Mo and assuming that all metals had been fully reduced from their initial oxidation state.X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5500 spectrometer to study the surface state of the NPs, recording the core spectra for C1s, Ni2p, Pt4f, and Mo2p, before and after Ar ion sputtering for 30\u00a0s, as well as on the disassembled MEAs after PEMFC testing. Peak fitting of the spectra was done by the software XPSPEAK. Energy calibration was done by positioning the main C1s peak at 284.5\u00a0eV.For the PEMFC testing, the cathodes containing the electrodeposited Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) NPs were hot-pressed together with a Nafion 212 membrane (Chemours) and a commercial GDE with a Pt loading of 0.3\u00a0mg/cm2 as anode using a pressure of 0.5\u00a0MPa at 110\u00a0\u00b0C for 3\u00a0min to form the MEA.All MEAs were tested in a single-cell fuel cell tester using coated stainless steel flow plates. The active area of the fuel cell was 2.9\u00a0cm2. A Greenlight G20 test station was used for PEMFC tests, with a temperature of 80\u00a0\u00b0C for both gases and a dewpoint temperature of 65\u00a0\u00b0C. The flow of H2 and air were 0.042 and 0.10 l\nn\n/min (normal liter per minute), respectively.The cells were activated by cycling between 0.9\u00a0V and 0.6\u00a0V for 2\u2009000 cycles before a polarization curve was obtained. For durability testing, an accelerated stress test (AST) consisting of 20\u2009000 additional cycles were performed, with polarization curves obtained after 10\u2009000 cycles and at end of test (EoT).For all NPs, the amplitude of the deposition potential increases with the amplitude of the current density (Fig. 2\n). During pulse deposition of Pt\u2013Ni NPs, the amplitude ranges initially between\u00a0\u22121.4 and\u00a0\u22122.0\u00a0V versus Ag|AgCl depending on the applied current density. This potential drops towards the end of the deposition to within\u00a0\u22121.0 to\u00a0\u22121.3\u00a0V. The potential established during off-time changes from just below 0\u00a0V to slightly positive potential. In turn, the potential amplitudes for the deposition of ternary Pt\u2013Ni\u2013Mo(O) NPs ranges from\u00a0\u22122.0\u00a0V to\u00a0\u22124.0\u00a0V due to the higher current densities applied. Those latter potentials do seem uncommonly high, however they are based on an empirical determination of the process parameters. They may thus be the result of a significant resistivity of the GDL substrate. The potential does not change significantly until the end of deposition, whereas the potential during off-time changes to\u00a0\u22120.6\u00a0V towards the end, showing a trend which is inverse to the one observed for binary NPs.The results of the ICP-MS shows that all four depositions of Pt\u2013Ni resulted in a Pt content of 67\u201380\u00a0at% (Table 3\n) and a Pt loading of 3.7\u20134.1\u00a0\u03bcg/cm2. Assuming that the metal is completely reduced, which is fairly true for this case, the current efficiency is ca. 25% for all Pt\u2013Ni depositions. However, Pt\u2013Ni\u2013Mo(O) appeared to pass a threshold as the current density increased from 77 to 100\u00a0mA/cm2, where the two lower current densities resulted in similar composition to the Pt\u2013Ni with 78 and 66\u00a0at% Pt and low amounts of 1 and 2\u00a0at% Mo, respectively. The higher current densities on the other hand resulted in very low Pt content of only 5\u00a0at% while Ni became the main constituent. The Mo content increased to 24 and 21\u00a0at%. Note that the composition of the ternary NPs is given in atomic percentage disregarding the oxygen content. The occurrence of oxygen in the Mo-containing NPs originating from incomplete reduction of Mo(VI) precursor was deduced by XPS, as shown later on. The total catalyst loading was less uniform for the Pt\u2013Ni\u2013Mo(O), ranging from 2.5 to 8.1\u00a0\u03bcg/cm2. The current efficiency was also significantly reduced when comparing the ternary Pt\u2013Ni\u2013Mo system to the binary Pt\u2013Ni, with current efficiencies as low as 2\u201312%.The GDL substrates show the typical homogeneous microporous layer, consisting of aggregates of globular carbon particles (Fig. 3\na). Fluorine emissions in EDX confirmed the presence of PTFE (not shown).After the pulse electrodeposition, Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) NPs are homogeneously distributed on the surface and near-surface region of the microporous layer, which appears darker in the SEM images compared with the deposited NPs (Fig. 3). All Pt\u2013Ni NPs show a uniform, spherical shape. The particle size is spread between 20\u00a0nm and 80\u00a0nm, with an average particle size between 40\u00a0nm and 50\u00a0nm independent of composition or process parameters (Table 3), and are agglomerated in very few cases (Fig. 3b\u2013e). The Pt\u2013Ni\u2013Mo(O) NPs have the tendency to a spherical structure, although less defined and with higher roughness than the Pt\u2013Ni NPs. While the particle size of Pt78Ni21Mo1 is around 80\u00a0nm (Fig. 3f), the other ternary NPs lie between 40\u00a0nm and 50\u00a0nm. For Pt5Ni74Mo21, a mixture of large (>100\u00a0nm) and small (<50\u00a0nm) NPs is appreciated (Fig. 3i). This observation is in agreement with the large distribution in particle size determined for this composition, while all other compositions show relatively narrow size distributions (Fig. 4\na\u2013h). While commercial Pt/C has significantly smaller particle size, Fouda-Onana et al. electrodeposited Pt particles with a size of 50\u00a0nm and found that despite the larger particle size, the utilization rate was high due to all particles having triple phase boundaries (TPBs) [39].In TEM observations, the Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) NPs are clearly distinguished from the C support by phase contrast (in bright field TEM, the NPs appear darker than the carbon-based support; Fig. 4i,m). The binary NPs are of well-defined, globular form, whereas the ternary NPs are more irregular (Fig. 4j,n). The high-resolution mode reveals the nanocrystalline structure of Pt\u2013Ni NPs, indicated by diffraction planes with different orientations (Fig. 4k). This nanocrystallinity is also clearly observed in the selected area electron diffraction (SAED) pattern of a single NP with a diameter (\u00f8) <50\u00a0nm, where almost any diffraction direction is present, confirmed by the quasi-continuous rings observed in the pattern (Fig. 4l). The corresponding d-spacings match the face-centered cubic (fcc) phase for a Pt\u2013Ni alloy. For Pt67Ni33, the average cell parameter determined from SAED was 3.88\u00a0\u00c5, which corresponds well with the d(111) spacing of 2.2\u00a0\u00c5 measured in the high-resolution image.In contrast to the Pt\u2013Ni NPs, the Pt\u2013Ni\u2013Mo(O) NPs show more diffuse diffraction rings, which are more characteristic of an amorphous material, even when the Mo content is very low (Fig. 4p). Some additional diffraction spots indicate the presence of Mo oxide species such as MoO3 or other non-stoichiometric oxides [40]. Diffraction planes in high-resolution TEM are only appreciated in extremely few occasions at the NPs\u2019 surfaces, showing crystals smaller than 5\u00a0nm (Fig. 4o). Other than that, diffraction planes are not observed, leading to the conclusion that most Pt\u2013Ni crystals are too small to diffract and that they are completely mixed in with the molybdenum oxide species in a sort of composite.From SAED data, the existence of a homogeneous alloy is clear for the Pt\u2013Ni NPs, the ternary Pt\u2013Ni\u2013Mo(O) NPs were analyzed by EELS to confirm the distribution of Pt, Ni, and Mo. However, even for the NPs with the highest Mo content (Pt5Ni74Mo21), Mo was not detectable by EELS due to the high delay of the Mo signal, and the high amount of carbon causing a large background signal (and resulting in a very low Mo content with respect to carbon). In contrast, Pt and Ni are clearly distributed evenly over the NPs (Fig. 5\n).The Pt4f spectra of the surfaces of the deposited Pt\u2013Ni NPs, exemplary shown for Pt73Ni27, show emissions of metallic Pt at 71.4\u00a0eV and of Pt(I) at 72.3\u00a0eV, which can be assigned to platinum hydroxide [41]. In the Ni2p spectra, no contribution of metallic Ni is observed and most superficial Ni is bound in Ni(OH)2 (Fig. 4q,r).After Ar ion sputtering, the XPS spectra show a contribution of metallic Ni, as well as a higher fraction of metallic Pt with respect to the hydroxide species. It must be noted that, in contrast to bulk material, the contribution of the NPs' surface cannot be eliminated by sputtering. This means that after sputtering, there is still a significant contribution of the surface state, and the actual bulk state at the NPs\u2019 cores is assumed to contain even higher fractions of metallic and less oxidized species than represented in the XPS spectra after Ar ion sputtering.For the ternary Pt\u2013Ni\u2013Mo(O) NPs, very similar observations are made with respect to the Pt4f and Ni2p emissions (Fig. 4s and t). The Mo3d detail spectra do not show any evidence of metallic Mo at 228.0\u00a0eV [41]. The initial surface state shows the occurrence of Mo(VI) exclusively, which points to the presence of MoO3. After sputtering, the lower oxidation state Mo(IV) is found, which may correspond to MoO2 (Fig. 4u). Therefore, Mo(VI) was not fully reduced to Mo(0) in spite of the presence of Ni(II) and citrate in the electrolyte. It can be argued that sufficient complexing of molybdate species with citrate, and therefore the full discharge of Mo(VI) to Mo(0), does not occur at the rather low pH of the electrolyte of 2.7. At this pH, the citrate mostly exists in its protonated form, thus hindering complexation of metal ions.For both binary and ternary NPs, a drastic reduction of O1s emissions (not shown) were observed after sputtering, indicating that the NPs exhibit an oxidized surface and a metallic core. Due to the fact that contributions of the NPs' surfaces are present even after sputtering, O1s emissions are not completely suppressed, nor can it be completely ruled out that there remain low amounts of residual oxygen in the NPs\u2019 cores.Combining all knowledge obtained on the ternary Pt\u2013Ni\u2013Mo system, the following conclusions can be drawn. Taking into account the existence of Pt(0), Ni(0), Mo(IV), and Mo(VI) shown by XPS, the absence of concrete diffraction rings for the Pt\u2013Ni fcc phase in SAED, the very small (<5\u00a0nm) crystalline regions in high resolution TEM, and the homogeneous distribution of Pt and Ni throughout the NPs as evidenced by EELS, it is concluded that Pt\u2013Ni crystals and molybdenum oxide species are coexistent and randomly distributed among the NPs. Thus, the Pt\u2013Ni\u2013Mo(O) NPs consist of a heterogeneous compound of Pt\u2013Ni and molybdenum oxide, which due to their nanosized nature and random distribution, appear as a homogeneous compound even on the nanoscale.The GDL substrates exhibit a rather low electric conductivity, leading to a relatively high double-layer capacitance observed in CV. Nevertheless, the hydrogen adsorption (H\nads\n) and desorption peaks (H\ndes\n) used for the determination of ECSA are well defined, as shown exemplarily for Pt73Ni27 (Fig. 6\n).The ECSA of both the Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) is comparable with what is found in literature (Table 3) [2], showing that despite the electrodeposited catalysts having a large particle size, the Pt utilization is high.The electrodeposited Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) NPs all show lower reduction potentials at ORR compared with the Pt/C (Fig. 7\na). Among them, the Mo-containing NPs are inferior to the binary alloy NPs with comparable Pt/Ni ratio. Interestingly, the best performance among the electrodeposited NPs is observed for Pt67Ni33. A diffusion-controlled region is not observed for any of the samples; this is related to the fact that kinetics is limited because the measurements were carried out in stagnant conditions, and also due to the GDL substrates which are not perfect conductors.The advantage of electrodeposited Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) NPs becomes apparent when the Pt mass activity at ORR is considered (Fig. 7b). Pt67Ni33 shows the highest Pt mass activity among the binary NPs. In addition, the Ni-rich ternary NPs, which also have significantly higher amounts of Mo, show the highest mass activity with respect to Pt content. This shows that the addition of Mo does not compromise the catalytic activity and may indeed lead to improvement, however, the stability at ORR of these low-Pt content NPs in acidic conditions may be compromised. Liu et al. recently found that a Ni-rich Ni\u2013Pt alloy electrocatalyst can be employed successfully in PEMFC with only little performance loss over 30\u00a0h of constant operation [25]. Among the Pt-rich ternary NPs, Pt66Ni32Mo2 shows the highest Pt mass activity, corresponding to the Ni/Pt ratio of the binary Pt67Ni33. Zalitis et al. showed that ultrathin catalyst layers of 200\u00a0nm thickness can be employed to overcome limitations by internal resistances such as experienced here with the use of the GDL. In this way, the mass activity can be further increased by one order of magnitude at comparable Pt loading [42].An increase in total load of electrodeposited NPs on the GDL is expected to increase the half-wave potential at ORR. However, the electrodeposition of higher amounts of catalyst could easily lead to NP growth rather than nucleation of more particles.Another way of improving the ORR of the electrodeposited NPs is by addition of an ionomer such as Nafion, which can lead to a significant increase in ECSA and ORR performance by improving the wettability of the GDL [43].In terms of kinetics, the determined Tafel slopes (b) show a significant improvement with respect to the commercial Pt/C electrode, for which a Tafel slope of 103\u00a0mV is obtained. Interestingly, all binary Pt\u2013Ni binary alloy NPs exhibit an almost identical Tafel slope of 65\u201366\u00a0mV (cf. Table 3). The ternary Pt\u2013Ni\u2013Mo(O) NPs show a higher spread in b, where Pt66Ni32Mo2 has the lowest Tafel slope of 62\u00a0mV (Fig. 7c). The measured Tafel slopes correspond well with the one determined by Fortunato et al. on pure Pt NPs synthesized by electrodeposition [44].The cross-sectional image of a hot-pressed MEA prior to testing in the fuel cell shows that the catalyst layer (CL) formed by the electrodeposited NPs is homogeneous and well adhered to the membrane, ensuring proton conductivity (Fig. 7d).The polarization curves show that the low catalyst loading results in low current density (not shown). However, the catalysts activity is similar independent of composition, both initially and after AST. Interestingly, the Pt67Ni33 and Pt66Ni32Mo2 again show the highest activity. The major drop in PEMFC performance takes place during the first 10\u2009000 cycles, whereas there is not as significant drop from 10\u2009000 to 20\u2009000 AST cycles (Fig. 8\n). This is in accordance with literature [2] and can be explained by the dissolution of unstable catalytic sites, such as surface Ni and Mo in the initial half of the AST, leaving the particles more stable in the second half of the AST. No decrease in activity could be seen for the commercial Pt/C, which after 20\u2009000 AST cycles show a peak power density of 0.377\u00a0W/cm2.In general, the SEM micrographs of the NPs after AST in the PEMFC show that the particle size is reduced (cf. Table 3), related to a loss of material by dissolution in the acidic environment (Fig. 8). The exception is Pt80Ni20, where the particle size is slightly increased (Table 3). This increase in particle size can be explained by Pt dissolution from smaller, less stable particles, and re-deposition onto larger particles. It should be noted that a non-negligible amount of particles remained attached to the membrane rather than the GDL after disassembling the MEA for post-analysis. The Pt\u2013Ni particles have kept their spherical morphology, while the Pt\u2013Ni\u2013Mo(O) NPs have increased roughness after testing. It is also clear that the two Ni-rich samples with very low Pt content experience the most severe dissolution of catalyst material. However, the PEMFC performance at EoT is similar to the other samples. This may be due to the high surface area endowed by the rough morphology of these NPs.The XPS spectra obtained from the catalyst layers after PEMFC testing are compromised by the aforementioned loss of material due to particles remaining on the membrane; however, it was generally observed that in comparison with the initial surface state, both Pt(0) and Ni(0) were present in higher fractions with respect to their oxidized forms (Fig. 9\n). For the Mo-containing catalysts, only Mo(VI) was detected after the PEMFC tests. Contrarily to what might be expected, Ni2p emission levels are close to their initial values (cf. Fig. 4q\u2013u) while Pt emissions are significantly lower.Although electrodeposition is a very common and widely used synthesis method, its employment in the synthesis of ORR electrocatalysts for PEMFC is rather unexplored. Few studies have investigated this topic in recent years, and the processes need to be further optimized in view of the electrocatalysts\u2019 properties and performance, and especially in terms of actual testing in PEMFC (Table 4\n).Binary Pt\u2013Ni and ternary Pt\u2013Ni\u2013Mo(O) NPs were successfully synthesized by pulse electrodeposition from aqueous media directly onto the GDL of a PEMFC. The so-prepared carbon-supported catalysts show high specific activities at ORR and the applicability in PEMFC was demonstrated in a single-cell, with Pt67Ni33 and Pt66Ni32Mo2 showing the highest activity both in ORR measurements and in the PEMFC.NP particle sizes range around 50\u00a0nm, and Tafel slopes at ORR of around 65\u00a0mV are achieved at low catalyst loadings of 4\u00a0\u03bcg/cm2. Most importantly, very high ORR mass activities up to 10 mA/\u03bcg\nPt\n are reached at half-cell electrochemical tests in 0.1\u00a0M HClO4, owing to the favorable distribution of electrocatalyst NPs along the catalyst layer as a result of the electrodeposition process. In the MEAs produced for PEMFC testing, almost all NPs are expected to contribute to the ORR reaction due to their direct contact with both carbon support and PEM, resulting in extremely high efficiency with respect to Pt utilization.For industrial application in PEMFC, the total power output of the fuel cell would need to be increased. To this respect, a higher total amount of catalyst may be needed, achieved by either increasing the Pt load in electrodeposition or by increasing the fuel cell's active area. An increase of loading by electrodeposition is easily achievable. However, by simply increasing the deposition time, additional deposition will occur on already deposited material and lead to particle growth. This would most likely improve half-wave potential and obtainable currents, but adversely affect mass activity. Further optimization is possible by tuning the NP particle size, or by optimizing the electrical properties by the use of thin carbon layers adapted to electrodeposition, to reduce intrinsic resistances.With respect to the ternary system NPs, an obvious advantage over binary Pt\u2013Ni NPs is not observed. Since Mo was mostly found in an oxidized state and the crystallinity of the ternary NPs as observed by SAED was lower, the question remains open as to whether a true, metallic ternary alloy would yield superior ORR activity.Overall, the demonstrated electrodeposition process provides a promising alternative to the conventional methods of ORR electrocatalyst synthesis. In addition to the facile synthesis which applies the catalyst NPs directly onto the GDL, the utilization of the metal electrocatalyst can be seen as close to 100%.\nKonrad Eiler: conceptualization, validation, formal analysis, investigation, data curation, writing\u2014original draft, writing\u2014review & editing, visualization. Live M\u00f8lmen: conceptualization, software, validation, formal analysis, investigation, data curation, writing\u2014original draft, writing\u2014review & editing, visualization. Lars Fast: conceptualization, methodology, validation, resources, supervision, project administration. Peter Leisner: conceptualization, resources, supervision, project administration, funding acquisition. Jordi Sort: conceptualization, resources, writing\u2014review & editing, supervision, funding acquisition. Eva Pellicer: conceptualization, validation, resources, writing\u2014review & editing, supervision, project administration, funding acquisition.The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. Specific data can be obtained from the corresponding authors on request.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sk\u0142odowska-Curie grant agreement No 764977, the Generalitat de Catalunya under project 2017-SGR-292, and the Spanish government under project PID2020-116844RB-C21. The authors want to express their thanks to Freudenberg, Germany, who gladly supported the GDL material for this study.", "descript": "\n                  Proton exchange membrane fuel cells (PEMFCs) are an important alternative to fossil fuels and a complement to batteries for the electrification of vehicles. However, their high cost obstructs commercialization, and the catalyst material, including its synthesis, constitutes one of the major cost components. In this work, Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) nanoparticles (NPs) of varying composition have been synthesized in a single step by pulse electrodeposition onto a PEMFC's gas diffusion layer. The proposed synthesis route combines NP synthesis and their fixation onto the microporous carbon layer in a single step. Both Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) catalysts exhibit extremely high mass activities at oxygen reduction reaction (ORR) with very low Pt loadings of around 4\u00a0\u03bcg/cm2 due to the favorable distribution of NPs in contact with the proton exchange membrane. Particle sizes of 40\u201350\u00a0nm and 40\u201380\u00a0nm were obtained for Pt\u2013Ni and Pt\u2013Ni\u2013Mo(O) systems, respectively. The highest ORR mass activities were found for Pt67Ni33 and Pt66Ni32\u2013MoO\n                        x\n                      NPs. The feasibility of a single-step electrodeposition of Pt\u2013Ni\u2013Mo(O) NPs was successfully demonstrated; however, the ternary NPs are of more amorphous nature in contrast to the crystalline, binary Pt\u2013Ni particles, due to the oxidized state of Mo. Nevertheless, despite their heterogeneous nature, the ternary NPs show homogeneous behavior even on a microscopic scale.\n               "}
{"full_text": "Currently, fossil fuel resources, such as petroleum, coal, and natural gas, play a crucial role in meeting the growing demand for energy and chemicals [1]. However, these resources are unsustainable, and the environmental problems caused in their utilization are also very serious, especially the greenhouse effect and fog-haze weather [2,3]. Biomass, which consists mainly of cellulose, hemicellulose, and lignocellulose, has the potential to replace fossil fuels owing to its wide distribution, abundance, low cost, and renewable nature [4\u20137]. 5-hydroxymethylfurfural (HMF) is described as an important biomass-based platform compound and a versatile intermediate, derived from cellulose and C6 sugars, for connecting biomass and the chemical industry. The C\u2013O, C=O, and furan ring of HMF make it very flexible and enable it to be transformed into fuel molecules, such as 2,5-diformylfuran (DFF), 2,5-furandicarboxylic acid (FDCA), ethyl levulinate (EL), 2,5-dimethylfuran (DMF), and 2,5-dihydroxymethylfuran (DHMF) by various methodologies [8\u201313]. In particular, DMF, formed by selective hydrogenolysis of HMF, is proposed as a promising and sustainable liquid fuel for transportation. Compared to bio-ethanol and bio-butanol, DMF possesses higher energy density (31.5 MJ/L), a higher octane number (RON\u00a0=\u00a0119), a higher boiling point (92\u201394\u00a0\u00b0C), lower volatility, and water immiscibility [14]. In addition, DMF can react with ethylene to form p-xylene in a Diels-Alder reaction, which is a potent pathway towards biomass transformation [15].The selective hydrogenolysis of HMF to DMF is a key process in the efficient utilization of biomass resources and has garnered considerable attention [16]. However, the heterogeneous metal-catalyzed conversion of HMF produces various products, including 5-methylfurfural (MF), 5-methylfurfurylalcohol (MFA), DMF, and DHMF. Thus, there is an urgent need to achieve high DMF selectivity. To date, several catalysts, mainly including noble metal or supported noble metal catalysts, have been employed for the selective hydrogenolysis of HMF [17\u201319]. In 2007, Roman-Leshkov et\u00a0al. initiated research on HMF selective hydrogenolysis and showed that the DMF yield from fructose could reach 71% over a modified Cu-Ru/C catalyst with the assistance of hydrochloric acid (HCl) and sodium chloride (NaCl) [20]. Subsequently, Chidambaram and Bell first introduced the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) as the solvent in the HMF-to-DMF reaction and achieved 32% selectivity in an H2 atmosphere with Pd/C as the catalyst, but the poor solubility of H2 in the ionic liquid led to an unsatisfactory result [21]. Chatterjee et\u00a0al. developed the hydrogenation of HMF in supercritical carbon dioxide using a Pd/C catalyst under milder conditions (80\u00a0\u00b0C, 1 MPa H2, 2\u00a0h) with high conversion (99%) and selectivity (99%) [22]. Zu et\u00a0al. reported that a Ru/Co3O4 catalyst exhibited outstanding catalytic performance for the production of DMF and showed that Ru and CoO\nx\n were responsible for hydrogenation and breaking the C\u2013O bond, respectively [23]. More recently, the CoFe layered double oxide (CoFe-LDO) has been widely applied on account of its controllable acid sites and higher surface area, and the catalyst Ru/CoFe-LDO has been shown to allow selective hydrogenation of HMF to DMF in the presence of H2\n[24]. With the continuous development of catalytic science, a series of novel catalysts such as Pd-Co9S8/S-CNT, Pt/rGO, Ru/NaY and Pd-GVL/C have also been used in the conversion of HMF into DMF [25\u201328]. Nonetheless, the high price and low availability of precious metals limit their application in industrial production.The non-noble metal catalysts Ni, Co, Fe, and Cu have gradually become the focus of intense research for the conversion of HMF into DMF. However, some results have implied that the selectivity of monometallic catalysts for DMF is low [29]. Kong et\u00a0al. used a commercial Raney Ni catalyst to achieve the complete conversion of HMF at 180\u00a0\u00b0C and 1.5\u00a0MPa H2 pressure in 1,4-dioxane. The poor DMF selectivity was due to the high hydrogenation ability of Ni, which would lead to the generation of some by-products [30]. On the other hand, Gorte's group pointed out that HMF selective hydrogenolysis is a sequential reaction, and DMF may continue to react as an intermediate to form either ring-hydrogenated (e.g. 2,5-dimethyltetrahydrofuran, DMTHF) or ring-opened (e.g. 2-hexanone and 2,5-hexanedione) as by-products in the presence of monometallic catalysts [31,32]. Based on these conclusions, the yield of DMF depends on the relative rates of DMF formation and consumption. It is important to develop a catalyst with excellent C=O/C\u2013O hydrogenation ability but to inhibit C=C/C\u2013C reaction ability. Metal alloying or designing bimetallic catalysts for the hydrogenolysis of HMF to DMF is a promising option. For example, a Ni\u2013Fe alloy formed on the surface of carbon nanotubes (Ni\u2013Fe/CNTs) favors C\u2013O bond breaking and was employed to catalyze the selective hydrogenolysis of HMF to DMF by Yuan et\u00a0al. [33]. Resasco et\u00a0al. showed that a Ni\u2013Fe bimetallic catalyst showed better performance than monometallic catalysts because of the oxyphilic Fe atoms [34]. Fang et\u00a0al. prepared a NiZn catalyst by a coprecipitation method using hydrotalcite-derived mixed oxides as raw materials for selective conversion of HMF to DMF. The excellent effect of the catalyst was attributed to the formation of the alloy and the electronic modification of Ni [35]. A NiCu3/C nanocrystal designed with a core-shell structure exhibited efficient hydrogenation ability, achieving 98.7% DMF yield at 180\u00b0C and 3.3 MPa H2 pressure [36]. Moreover, given that FeCoNi/hexagonal\u2013BN and Ni\u2013MoS2/mAl2O3 catalysts had achieved idealized results under certain conditions, the synergetic catalysis of polymetals and the structure-activity relationship between carriers and metals were well recognized [37,38]. Although these Ni-based catalysts have obtained good results for the catalytic hydrogenolysis of HMF, the preparation of the catalysts is complex and hydrogen is needed as a reducing agent, and improvement will be needed for industrial demand in the future. Therefore, it is necessary to use cheap raw materials and to prepare efficient catalysts in a simple way.With the aim of constructing a highly reactive non-noble catalytic system for the selective hydrogenolysis of HMF to DMF, mesoporous TS-1-supported Ni\u2013Cu bimetallic catalysts were prepared through a solid-phase grinding synthesis method. In this method, the reduction was effected by the gas generated during the roasting of the precursor [54]. To thoroughly evaluate the performance of the catalyst, the effect of reaction conditions, such as reaction temperature, H2 pressure, and catalyst dosage, was also investigated. It is shown that the catalysis will not proceed in the direction of side reactions owing to a strong interaction between Ni and Cu, ensuring high selectivity for the target product. Importantly, a possible reaction route and mechanism over the 40%Ni\u20135%Cu/TS-1 catalyst were obtained.5-Hydroxymethylfurfural (\u226598.0%) was purchased from Shanghai D&B Biological Science and Technology Co., Ltd. (Shanghai, China). 2,5-Dimethylfuran (99.0%) was purchased from Nine-Dinn Chemistry Co., Ltd. (Shanghai, China). 5-Methylfurfural (98.0%) was purchased from Shanghai Yien Chemical Technology Co., Ltd. (Shanghai, China). 2, 5-Dihydroxymethylfuran (98.0%) was purchased from Shanghai Macklin Biochemical Co. Ltd. (Shanghai, China). 5-Methylfurfuryl alcohol (95.0%) was purchased from Shanghai Bidepharm Technology Co., Ltd. (Shanghai, China). Nickel nitrate hexahydrate (A.R. grade), cupric nitrate trihydrate (A.R. grade), citric acid monohydrate (A.R. grade), tetrahydrofuran (A.R. grade), and octane (C.P. grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).The hollow-structured TS-1 was synthesized by the classical hydrothermal synthesis method [39]. Ni\u2013Cu/TS-1 catalysts were prepared by a simple solid-phase grinding synthesis method. Typically, nickel nitrate hexahydrate (10\u00a0mmol, 2.91\u00a0g), cupric nitrate trihydrate (1.15\u00a0mmol, 0.279\u00a0g) were taken in a mortar and ground into a powder. Then, citric acid monohydrate (12\u00a0mmol, 2.52\u00a0g) was mixed with the above powder by continuous grinding until homogeneous. Afterwards, TS-1 was added to the mixture and ground for 30\u00a0min to a viscous paste. The resulting Ni\u2013Cu/TS-1 precursor was dried at 120\u00a0\u00b0C for 10\u00a0h. Next, the dried precursor was calcined in a tubular furnace under nitrogen atmosphere at 370\u00b0C for 3 h. The heating rate from 50\u00a0\u00b0C to 370\u00a0\u00b0C was 3\u00a0\u00b0C/min. The as-prepared catalyst was defined as 40%Ni\u20135%Cu/TS-1. Ni\u2013Cu/TS-1 catalysts with different metal contents, single-metal catalysts, Ni\u2013Cu/MCM-22, Ni\u2013Cu/Al2O3, Ni\u2013Cu/H-Beta and Ni-Cu/SiO2 catalysts were prepared through the same procedure.The X-ray diffraction (XRD) patterns of the catalysts were recorded by a Bruker diffractometer with Cu K\u03b1 radiation and diffraction angle (2\u03b8) ranging from 10\u00b0 to 80\u00b0. Fourier-transform infrared spectra (FT-IR) of the catalysts were collected by the KBr pellet technique on a Nicolet 370 infrared spectrophotometer in the range 400\u20134000\u00a0cm\u20131. Thermogravimetric and derivative thermogravimetric (TG-DTG) measurements were made on a Netzsch Model STA 409 PC instrument. The heating rate from room temperature to 800\u00a0\u00b0C was 20\u00a0\u00b0C/min using \u03b1-Al2O3 as the standard material. X-ray photoelectron spectroscopy (XPS) was performed using monochromatic Al K\u03b1 (1486.6\u00a0eV) as the radiation source (Thermo Scientific K-Alpha+, USA). All binding energies (\u00b10.2\u00a0eV) of samples were recalibrated based on the sp2 hybridized C1s line of graphitic carbon at 284.8\u00a0eV. The XPS Peak 4.1 program was used for curve fitting after a Shirley-type background subtraction. The nitrogen adsorption measurements of the catalysts were performed on a Micromeritics ASAP 2460 sorption analyzer. The catalysts were out-gassed at 200\u00a0\u00b0C for 4\u00a0h before measurement. The surface acidity of the catalysts was measured by temperature-programmed desorption of ammonia (NH3-TPD) using AutoChem II 2920 equipment. NH3 was used as probe molecule to estimate acidity and the TPD data were collected from 50\u00a0\u00b0C to 700\u00a0\u00b0C. The surface morphology of the catalyst was characterized by scanning electron microscopy (SEM) using a Quanta600F instrument and X-ray energy spectrometer (EDX, IE350). The electron beam accelerating voltage was 20 kV and the surfaces of the materials were sprayed with gold.In a typical procedure, HMF (1\u00a0mmol, 0.126\u00a0g), the catalyst (0.050\u00a0g), and THF (10.0\u00a0mL) were added to a 100-mL autoclave with Teflon liner, which was sealed and purged with H2 several times. The hydrogenolysis of HMF was conducted under the chosen temperature and pressure conditions with stirring. After the reaction, the reactor was quenched in ice-water, and the liquid products were analyzed by GC (Nexis GC-2030) with an FID detector and a capillary column (Shimadzu SH-Rtx-1701). The structural characteristics of the products were further identified by GC-MS and comparison with retention times of pure chemicals. The reused catalyst was washed with THF and then vacuum dried at 50\u00a0\u00b0C for the next run.The necessary parameters for GC were as follows: injection volume 1.0\u00a0\u03bcL, split ratio 1:40, temperatures of the injection port and detector both 250\u00a0\u00b0C. The temperature program was set as 40\u00a0\u00b0C for 3\u00a0min, 40\u2013100\u00a0\u00b0C (10\u00a0\u00b0C/min), 100\u00a0\u00b0C for 1\u00a0min, 100\u2013250\u00b0C (40\u00a0\u00b0C/min), 250\u00a0\u00b0C for 2\u00a0min. All compounds were quantified based on the internal standard method using octane as the internal standard. The equations were as follows:\n\n\n\nHMF\n\nconversion\n\n(\n%\n)\n\n=\n\n(\n\n1\n\u2212\n\n\nM\no\nl\ne\ns\n\no\nf\n\nH\nM\nF\n\n\nI\nn\ni\nt\ni\na\nl\n\nm\no\nl\ne\ns\n\no\nf\n\nH\nM\nF\n\n\n\n)\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\nDMF\n\nselectivity\n\n(\n%\n)\n\n=\n\n\nM\no\nl\ne\ns\n\no\nf\n\nD\nM\nF\n\n\nI\nn\ni\nt\na\nl\n\nm\no\nl\ne\ns\n\no\nf\n\nH\nM\nF\n\u2212\nm\no\nl\ne\ns\n\no\nf\n\nH\nM\nF\n\n\n\u00d7\n100\n%\n\n\n\n\nThe XRD patterns of TS-1 and as-synthesized supported catalysts with different metal content are displayed in Fig.\u00a01\n. The characteristic peaks at 2\u03b8\u00a0=\u00a07.9\u00b0, 8.9\u00b0, 23.2\u00b0, and 24.4\u00b0, which are observed for TS-1, are due to diffraction by crystalline MFI zeolite. The intensity of the MFI contribution is weakened due to the metal doped in samples b to g while the original crystal remained, indicating that the structure of MFI prepared by the solid-phase grinding synthesis method is undamaged. Based on the XRD patterns of the monometallic 40%Ni/TS-1 catalyst, the peaks at 2\u03b8\u00a0=\u00a037.2\u00b0 (1 1 1) and 43.7\u00b0 (2 0 0) can be indexed as NiO (JCPDS card No. 47-1049) and the reflection peaks at 44.5\u00b0 (1 1 1) and 51.8\u00b0 (2 0 0) can be assigned to Ni (JCPDS card No. 04-0850), which indicates that a small amount of NiO was formed along with Ni0 during the decomposition of the catalyst precursor. This is consistent with the conclusions reached by Abu-Zied and Asiri [40]. In the XRD analysis of the 40%Cu/TS-1 catalyst, the evident peaks at 43.3\u00b0 and 50.4\u00b0 are reflections of Cu0, corresponding to the (1 1 1) and (2 0 0) planes, respectively. Interestingly, a diffraction peak is found at 36.4\u00b0, which corresponds to a Cu2O phase. The diffraction peaks at 35.2\u00b0 and 38.5\u00b0 are attributed to CuO. These phenomena can be explained by the equation: Cu+O2\u2192Cu2O, CuO. In the case of the as-prepared Ni\u2013Cu/TS-1 catalysts, new peaks appear at 2\u03b8\u00a0=\u00a044.3\u00b0, and 51.8\u00b0 and their width increases gradually with increase in the Ni:Cu ratio. The latter two characteristic peaks are located between Cu0 and Ni0, indicating formation of a Ni\u2013Cu alloy structure [41]. On the other hand, the diffraction peaks of CuO\nx\n are not obvious for some reason, possibly because the reducing gas generated by the decomposition of citric acid during calcination of the precursor occupies a dominant position, so the reduction reaction takes precedence. In addition, the content of copper in the bimetallic catalysts is less, and the formation of a Cu-Ni alloy inhibits the oxidation process.The FT-IR spectra of the catalysts are shown in Fig.\u00a02\n. In these as-synthesized samples, the peaks at 3450\u00a0cm\u20131 and 1630\u00a0cm\u20131 are assigned to the stretching and bending vibrations of H2O. The peak observed at 970\u00a0cm\u20131 arises from the stretching vibration of Si\u2013O\u2013Ti bonds in the framework of sample a, not observed for b-g samples, which can be assigned to the presence of metal species side-on bound to the silicon of Si\u2013O\u2013Ti unit, thus weakening the [Ti\u2013O\u2013Si] bond [42]. The strong bands at 1100\u00a0cm\u20131 and 1230\u00a0cm\u20131 are ascribed to the Si-O-Si asymmetric stretching region. The band at 806\u00a0cm\u20131, which originates from the Si-O-Si symmetric stretching vibration of TS-1, is wider than the others, indicating that the loaded nickel and copper species have replaced Si in the framework. The shoulder bands at 545\u00a0cm\u20131 and 455\u00a0cm\u22121 are respectively attributed to the typical structure of MFI TS-1 zeolite and the Si-O bending vibration. Based on the above phenomena and similar characteristic peaks of each sample, it can be concluded that the metal has been successfully loaded onto the carrier.Citric acid, nickel nitrate, and copper nitrate produce complexes during the preparation of catalysts by the solid-phase grinding synthesis method. Subsequently, when they are calcined under nitrogen, the reducing gas generated by the decomposition of the precursor converts Ni2+ and Cu2+ into Ni and Cu, and the Ni\u2013Cu alloy structure is formed due to complexation. The decomposition process and the thermal stability of the samples are proved by the thermo-gravimetric (TG) analysis experiments. The results are shown in Fig.\u00a03\n. It can be seen that the curves of TS-1 have only a small mass loss with the increase in temperature, indicating that TS-1 has high thermal stability. In the case of the precursor, major weight loss, amounting to about 32%, is observed in the TG-DTG plots from 350\u00a0\u00b0C to 400\u00a0\u00b0C. This shows that rapid decomposition of the complex occurs to form NiO and CuO\nx\n, with release of CO or CO2. For the 40%Ni\u20135%Cu/TS-1 catalyst, the weight increase from 200\u00a0\u00b0C to 400\u00a0\u00b0C is owing to the oxidation of the metals according to the previous report [37], whereas the weight loss at approximately 400\u2013500\u00a0\u00b0C is reasonably could be attributed to the destruction of the Ni\u2013Cu alloy structure.To obtain further understanding of the surface composition and chemical state of the 40%Ni\u20135%Cu/TS-1 catalyst, X-ray photoelectron spectroscopy (XPS) was conducted. All the measurements were calibrated by the C1s binding energy at 284.8\u00a0eV. The XPS spectra are shown in Fig.\u00a04\n. It is worth noting that the catalyst may cause the oxidation of some metals when transferred to the XPS chamber [43]. It can be observed in the survey spectra (Fig.\u00a04 (a)) that the main peaks appeared at the positions of 532.8, 853.8, 103.6, 459.9, 931.6, and 283.7\u00a0eV, corresponding to O, Ni, Si, Ti, Cu, and C, respectively. This is consistent with the energy dispersive spectroscopy (EDS) mapping. In the Ni 2p XPS spectrum, the peaks at 852.3\u00a0eV and 870.1\u00a0eV are assigned to metallic Ni 2p3/2 and Ni 2p1/2, respectively [44]. Two other satellite peaks can be discerned with binding energies at 861.4\u00a0eV and 879.5\u00a0eV. Simultaneously, the two main peaks located at 855.0\u00a0eV (Ni2+ 2p3/2) and 856.9\u00a0eV (Ni2+ 2p1/2) demonstrate the presence of NiO on the surface of the catalyst [45].The peaks at binding energy values of 934.8\u00a0eV and 953.7\u00a0eV with strong satellite peaks, which correspond to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively, signify the existence of CuO (Fig.\u00a04 b) [41]. Peaks due to Cu metal or CuO species can be seen centered at around 932.4\u00a0eV (Cu 2p3/2) and 952.8\u00a0eV (Cu 2p1/2). In the Cu 2p XPS analysis, Cu0 and Cu+ cannot be clearly identified because of their overlapping signals, so the Auger spectra of Cu are used for recognition. The peak with kinetic energy 918.0\u00a0eV is attributed to Cu0, indicating that this is part of the catalyst [55]. Based on the above introduction and previous reports, the phenomenon of partial migration of the characteristic peaks of Ni 2p and Cu 2p in the 40%Ni\u20135%Cu/TS-1 catalyst indicates the formation of a Ni\u2013Cu alloy [46].The O 1s spectrum of 40%Ni\u20135%Cu/TS-1 can be deconvoluted into three underlying peaks. The resolved peaks with the binding energies around 529.6\u00a0eV and 531.5\u00a0eV may belong to surface lattice oxygen and adsorbed oxygen, respectively, which can transfer electrons to Ni2+ and Cu2+. The Ni\u2013Cu interaction can be enhanced by this electronic effect, which is favorable to the formation of an alloy [47]. Based on previous report [48], the geometry of the bimetallic structure is different from that of the origin metals, metal Ni has more d band holes than metal Cu, d electrons of Cu0 can flow to the unoccupied d orbit of Ni0 which leads to the d electron density of metal Ni active site increase. The high electron density of metal Ni active site favors the electron transfer from metal site to the lower unoccupied molecular orbital of C-O and C=O, which can effectively promote the activation of C-O and C=O bonds, and improve the HMF conversion. Furthermore, the peak at 533.6\u00a0eV is attributed to lattice defect species of the catalyst, which the surface-adsorbed oxygen species on oxygen vacancies belonging to defect-oxide and surface hydroxyl-like groups adsorbed on metal ions [49].N2 adsorption-desorption isotherms and pore diameter distribution curves of as-prepared catalysts are depicted in Fig.\u00a05\n. All samples showed type IV Langmuir adsorption-desorption isotherms with a type H1 hysteresis loop, which is characteristic of an ordered mesoporous structure according to the IUPAC classification [50]. The TS-1 isotherm closured at a lower relative pressure (P/P0=0.42), and the Ni\u2013Cu/TS-1 catalysts moved to a higher value, indicating the production of larger pores during the catalyst preparation process. This is consistent with the result of the pore size distribution. The main physicochemical properties of the different materials are summarized in Table\u00a01\n. The TS-1 support possessed a high specific surface area, 371.21\u00a0m2/g, as determined from the Brunauer-Emmett-Teller (BET) equation. With increasing metal loading, the surface areas of catalysts b, c, d, and e were reduced to 239.00, 233.72, 233.22 and 221.62\u00a0m2/g, respectively. The total pore volumes were calculated to be 0.37, 0.35, 0.34 and 0.24 cm\u00b3/g for b, c, d and e, respectively. These values indicate that the metal phase not only covered the surface of the carrier, but was also inserted into the hollow structure of TS-1, resulting in the blockage of available pores. Compared with TS-1, the pore volumes of these samples also followed a similar decreasing trend, indicating filling of the pores by alloy particles.The surface acidities of TS-1 and 40%Ni\u20135%Cu/TS-1 were determined using NH3\u2010TPD experiments. The profiles are shown in Fig.\u00a06\n. It is well known that the surface acidity can be divided into weak (<250\u00a0\u00b0C), medium (250\u2013400\u00a0\u00b0C), and strong (>400\u00a0\u00b0C) acidic sites according to the desorption temperature of ammonia. For TS-1 and 40%Ni\u20135%Cu/TS-1, one peak around 100\u00a0\u00b0C was observed, which could be ascribed to weak acidity. On the other hand, the maximum desorption strength increased with the introduction of Ni and Cu in the 40%Ni\u20135%Cu/TS-1 catalyst, resulting in the formation of a strongly acidic site (407\u00a0\u00b0C, 0.483\u00a0mmol/g). The actual situation is that nickel atoms enter the framework of titanium silicalite-1 and replace silicon and titanium atoms in the process of catalyst preparation, and because nickel presents a positive bivalent state, it can provide lone electron pairs to form Lewis acid sites. In addition, NiO increases the Lewis acidity of the catalysts. The proper acid strength is helpful for the hydrogenolysis of the C\u2013O bond and ensures the specificity of its target product DMF; this is discussed later.The surface morphology and structure of the catalysts were characterized by the SEM technique, with the images of the TS-1 and 40%Ni\u20135%Cu/TS-1 samples shown in Fig.\u00a07\n. It can be noticed that the materials exhibited a similar spherulite nanostructure. Comparison of images (a) and (d) shows that the introduction of Ni and Cu had caused a significant change in the morphology of the catalyst. Some grooves and cracks appeared in the 40%Ni\u20135%Cu/TS-1 catalyst, which indicates that the acting force of Ni\u2013Cu alloy affected the hollow structure of TS-1 (Fig.\u00a07. b and e). Elemental mapping along with the EDS spectra were employed to investigate the distribution of atoms on TS-1. As shown in Fig.\u00a08\n, the results showed that the 40%Ni\u20135%Cu/TS-1 catalyst contained elements of C, O, Si, Ti, Ni, and Cu. Considering the corresponding EDS analysis, it is evident that Ni and Cu were uniformly incorporated into the TS-1 framework without partial agglomeration.The catalytic performance of various metal catalysts and the support for the selective hydrogenolysis of HMF was investigated, carrying out the reaction in THF at 180\u00a0\u00b0C and 0.5\u00a0MPa H2 pressure for 7\u00a0h in a 50\u00a0mL Teflon-lined stainless steel autoclave. The results are summarized in Table\u00a01. For control purposes, a blank experiment without any catalyst was first conducted, and it showed 3% HMF conversion (Table\u00a02\n, entry 1). This phenomenon may be caused by the hydrogen atmosphere in the enclosed environment. The carrier material was also submitted to the same evaluation for comparison, the result showing that HMF conversion of 24.4% with negligible DMF selectivity can be obtained under the given conditions (Table\u00a02, entry 2). Moreover, the main by-product of this reaction was MF. It is well known that the Lewis acid sites of TS-1 are conducive to the activation of C\u2013O bonds, thus facilitating C\u2013O bond rupture. When the 40%Ni\u20135%Cu catalyst was used, the selectivity for the target product DMF reached 41.1% (Table\u00a02, entry 3). On account of the poor hydrogenolysis capacity of this catalyst, the product distribution was not specific. To verify the synergistic relationship between metal sites and acid sites, physically mixed 40%Ni\u20135%Cu and TS-1 catalysts were introduced into the selective conversion of HMF to DMF (Table\u00a02, entry 4). It can be seen that the conversion of HMF increased to 77.2% and the selectivity for DMF increased to 68.8%. This result suggests that co-operation between the metal and carrier plays a pivotal role in determining catalytic performance. However, because of the simple mixing, excellent activity had not yet been achieved. The activity of monometallic catalysts was also researched. The 40%Ni/TS-1 catalyst achieved high conversion (100%) but with low selectivity (67.4%) (Table\u00a02, entry 5). This could be ascribed to the excessive hydrogenation ability of Ni metal, with the DMF being used as an intermediate to generate some by-products. Thus, the selectivity of this catalyst for DMTHF, 2-methylfuran (MeF) and 2,5-hexandione (HD) accounted for 32%. In contrast, the 40%Cu/TS-1 catalyst showed lower catalytic activity and its product distribution remained at the intermediate stage of DHMF, MF and MFA, suggesting that the individual Cu species has insufficient ability for selective hydrogenation of HMF to DMF (Table\u00a02, entry 6). Therefore, further investigation of the catalytic performance of Ni\u2013Cu/TS-1 focused on different Ni/Cu ratios. Surprisingly, the incorporation of Cu was beneficial to improve the selectivity of the target product, which may be because tilted furan ring formed on the Cu surface inhibited the side reactions of DMF [51]. In the case of the 40%Ni\u20131%Cu/TS-1 catalyst, 100% HMF conversion was obtained but the selectivity for DMF was 85.3% (Table\u00a02, entry 7). Over-hydrogenated products were the main by-products with a selectivity of 14.7%, which may have been due to the low Cu content. In fact, by changing the Ni/Cu ratio, the product distribution was changed. The best results were obtained when the Cu load was increased from 3% to 5%, which increased the DMF selectivity from 90.2% to 97.3% (Table\u00a02, entry 8\u20139). In light of this, it would appear that an appropriate Cu content may be beneficial by inhibiting the excessive hydrogenation capacity of the catalyst. The rate of C=C/C\u2013C bond breaking was reduced, which may be caused by the interaction force formed between Ni and Cu. This was consistent with the results of XRD and other characterization methods. Hence, the HMF to DMF conversion process could be synergistically accelerated by the combination of Ni, Cu, and TS-1. This was also confirmed by the result for the 40%Ni\u20137%Cu/TS-1 catalyst, indicating that an excess of the Cu source can worsen the activity of the catalyst (Table\u00a02, entry 10). Therefore, the 40%Ni\u20135%Cu/TS-1 catalyst was selected as the appropriate catalyst in all subsequent experiments.The reaction temperature affects the product distribution to some extent; therefore, the influence of different temperatures on the selective hydrogenolysis of HMF to DMF was investigated using the 40%Ni\u20135%Cu/TS-1 catalyst. The results are shown in Fig.\u00a09\n. Clearly, the conversion of HMF and the selectivity for DMF are positively correlated with the reaction temperature. When the reaction occurred at the lower temperature of 120\u00a0\u00b0C, conversion of HMF was considerably lower (27.8%) with very little DMF selectivity (3.4%), and DHMF and MF selectivity accounted for a large proportion. DHMF and MF can further generate DMF through the cleavage of C\u2013O/C=O. The hydrogenolysis of HMF was evidently not complete. When the reaction temperature increased to 140\u00a0\u00b0C, 51.8% HMF conversion and 20.7% DMF selectivity were obtained. At this point, the highest activity of the catalyst was not realized. With a further increase in the reaction temperature to 160\u00a0\u00b0C, the conversion of HMF and the selectivity for DMF increased to 90.6% and 78.1%, respectively. As usual, the selectivity for the main intermediates DHMF, MF, and MFA continued to decrease. It is noteworthy that higher temperature favors activation of the C\u2013O/C=O bonds, thereby promoting the cleavage of carbonyl and aldehyde groups [53]. To achieve the highest reactivity, the reaction was conducted at 180\u00a0\u00b0C. The HMF was fully converted and the selectivity for DMF reached 97.3%. Surprisingly, the percentage decrease in the selectivity of the target product DMF was negligible at 200\u00a0\u00b0C. This may be because the addition of Cu species and the strong interaction in the Ni\u2013Cu alloy increase the adsorption of the furan ring, thus preventing the occurrence of some side reactions. Overall, the most suitable reaction temperature over the 40%Ni\u20135%Cu/TS-1 catalyst was chosen to be 180\u00a0\u00b0C.The performance of the 40%Ni\u20135%Cu/TS-1 catalyst for the selective hydrogenolysis of HMF to DMF was also examined at various H2 pressures at 180\u00a0\u00b0C for 7\u00a0h. As shown in Fig.\u00a010\n, catalytic activity is closely related to H2 pressure. Initially, the reaction rate of HMF and the selectivity for DMF gradually increased with increase in H2 pressure. When the reactor was in hydrogen atmosphere, but the initial pressure was 0\u00a0MPa, the conversion of HMF was 40.6% and the selectivity for DMF was 22.5%. As the H2 pressure reached 0.25\u00a0MPa, the HMF conversion and DMF selectivity were, respectively, 82.5% and 70.6%. In terms of dynamics, the increase in the concentration of dissolved hydrogen promotes the transformation of intermediates DHMF, MF and MFA to DMF. With an increase in H2 pressure to 0.5\u00a0MPa, the conversion of HMF increased to 100% and the selectivity for DMF was nearly equivalent (97.3%). When the H2 pressure was further increased to 0.75 or 1.0\u00a0MPa, no noticeable change in the conversion of HMF and the selectivity for DMF was obtained. In general, higher reaction pressure may cause opening or excessive hydrogenation of the furan ring [52]. In this study, even though hydrogen pressure increases to a certain extent, the Ni\u2013Cu alloy structure in the catalyst still shows lower reactivity to DMF, meaning that the surface oxygen can effectively prevent deep hydrogenolysis of the furan ring. Based on the survey of H2 pressures, 0.5\u00a0MPa was deemed appropriate for this catalytic system.As demonstrated, the dosage of catalyst is a significant parameter for selective conversion of HMF to DMF, and should accelerate the mass transfer of the reaction mixture. The dosage effect was investigated by varying the catalyst weight between 20 and 60\u00a0mg. As shown in Fig.\u00a011\n, when the dosage of the catalyst was increased from 20\u00a0mg to 50\u00a0mg, the conversion of HMF rapidly increased from 46.1% to 100%, and the selectivity for DMF increased from 62.1% to 97.3%. The reason is mainly that the generation of more active sites accelerates the hydrogenolysis process, so that the \u2013CHO and \u2013CH2OH of HMF are continuously activated. The almost complete conversion of DHMF (4.3%), MF (18.7%) and MFA (14.9%) further implied the necessity of an appropriate amount of catalyst. However, on further increasing the catalyst amount to 60\u00a0mg, the DMF selectivity fell to 92.5%. It is possible that the excess of catalyst may have caused some side reactions, which generated by-products like DMTHF, HD and MeF that affected the selectivity for DMF. Based on the above discussion, the amount of 40%Ni\u20135%Cu/TS-1 catalyst was precisely controlled at 50\u00a0mg for the further studies.The selective hydrogenolysis of HMF to DMF is a sequential reaction in which DMF may continue to produce secondary by-products. Under normal conditions, HMF first generates some O-containing intermediate products (IP) through the hydrogenolysis process, including DHMF, MF and MFA. The IP then form DMF. Finally, DMF produces over-hydrogenated products (OP), such as DMTHF, HD, MeF and so on. The product distribution in the selective hydrogenolysis of HMF to DMF at 180\u00a0\u00b0C over 40%Ni/TS-1, 40%Cu/TS-1, and 40%Ni\u20135%Cu/TS-1 catalysts is shown in Fig.\u00a012\n. It is obvious that the bimetallic catalyst is more advantageous than the monometallic catalyst. From the trend of product distribution (Fig.\u00a012 a), the reactivity of the 40%Cu/TS-1 catalyst was relatively low. The maximum conversion of HMF reached 88.7% by extending the reaction time, but the selectivity for DMF was only 70.0%. Overall, the products mainly stayed in the stage of O-containing intermediates. This phenomenon is consistent with the results of some Cu-based catalysts used in hydrogenolysis reactions [56]. Results for the performance of the 40%Ni/TS-1 catalyst are presented in Fig.\u00a012 (b). DMF selectivity initially increased with the reaction time. When the reaction time increased to 5\u00a0h, HMF was almost completely converted, with an 80.4% maximum selectivity for DMF. Notably, intermediate products were formed in the minimal reaction time and then quickly diminished. Subsequently, over-hydrogenation or ring opening of the furan ring may have occurred during the reaction. In summary, a challenging problem that arises in this domain is that the single-metal catalysts (40%Ni/TS-1, 40%Cu/TS-1) cannot ensure the specificity of the target product DMF. Broadly speaking, as shown in Fig.\u00a012 (c) and Table S1, the 40%Ni\u20135%Cu/TS-1 catalyst prepared by the solid-phase grinding synthesis method exhibited excellent properties. When the reaction time was 1\u00a0h, the intermediates DHMF, MF and MFA accounted for a large proportion of products. After 1\u00a0h, it was found that the selectivity for DMF increased almost linearly with time, and a maximum DMF selectivity of 97.3% was obtained after 7\u00a0h. At this point, the HMF conversion reached 100%. In addition to HMF and DMF, the proportion of intermediates also decreased gradually in the reaction mixture. Within the limits of the experiment, the 40%Ni\u20135%Cu/TS-1 catalyst exhibited little activity for DMF; however, when the reaction time was 9h, the selectivity for DMF still remained at 97.0%. This behavior of inhibiting side reactions is the main reason for maintaining the high selectivity for DMF, the alloying metals preventing the furan ring from lying down on the surface of the 40%Ni\u20135%Cu/TS-1 catalyst.From the discussion regarding Table\u00a02, it is known that the acidic sites of TS-1 are conducive to the hydrogenolysis of the hydroxyl group, while the metal sites of Ni and Cu are favorable for the hydrogenation of the aldehyde group. To further explore the role of the support in bifunctional catalyst, the selective hydrogenolysis of HMF was evaluated under the conditions of different catalyst supports. The comparison is summarized in Fig.\u00a013\n. For the Ni\u2013Cu/MCM-22, Ni\u2013Cu/Al2O3, and Ni\u2013Cu/H-Beta catalysts, the conversion of HMF and the selectivity for DMF were lower. In the case of Ni\u2013Cu/SiO2, the selectivity for DMF decreased because of side reactions. This indicates poor synergism between the hydrogenolysis reaction and active sites of the bifunctional catalyst. Additionally, the overall acidity of various catalysts can be calculated from the NH3-TPD experiments, as shown in Table S2. Appropriate Lewis acid sites played an important role in the selective hydrogenolysis of HMF into DMF. The catalysts with acid amount of about 1.50\u00a0mmol/g were beneficial to the production of DMF.Nowadays, the preparation of catalysts with good stability is of great significance for commercial applications. Recycling of the 40%Ni\u20135%Cu/TS-1 catalyst was performed to investigate the reusability of the catalyst under the optimum reaction conditions. The catalyst was recovered by centrifugation after the reaction, washed at least five times with THF, and used directly for the next run without further reactivation. The results are shown in Fig.\u00a014\n. It was surprising that the 40%Ni\u20135%Cu/TS-1 catalyst still maintained good catalytic performance after six consecutive experiments. There was a slight decrease in HMF conversion (85.6%) and DMF selectivity (80.9%) in the fourth cycle. At the same time, the selectivity for the intermediates DHMF, MF and MFA was 4.2%, 4.3%, and 10.6%, respectively. To verify the leaching of metal ions in the catalyst, a thermal filtration test was conducted. The 40%Ni\u20135%Cu/TS-1 catalyst was removed from the reaction solution by filtration after 3 h, and the filtrate without catalyst was then allowed to react for 4 h. As shown in Fig.\u00a015\n, there was no obvious change in the yield of DMF after the removal of the catalyst, which indicates that the active sites were not leached. Next, the catalyst that had been used four times was recalcined under nitrogen atmosphere and used for a fifth and sixth reaction. HMF conversion and DMF selectivity reached 100% and 96.5%, respectively. A possible explanation is that organic matter in the reaction liquid covers the active sites of the catalyst. To confirm the stability of the catalyst after the selective hydrogenolysis of HMF, the XRD patterns and FT-IR spectra of the used catalyst were obtained, and are shown in Fig.\u00a016\n. There were almost no differences in the XRD patterns and FT-IR spectra between the spent and fresh catalysts, suggesting that the structure of the catalyst was still intact.Based on the abovementioned experimental results and previous publications, the following detailed reaction pathways for selective hydrogenolysis of HMF to DMF over Ni\u2013Cu/TS-1 catalysts are proposed. As illustrated in Scheme\u00a01\n and scheme S1, the \u2013CHO of HMF is first hydrogenated to form the intermediate DHMF on the metal sites and the \u2013CH2OH of HMF is deoxygenated on acid sites to transform it to MF. Then, DHMF and MF generate intermediate MFA through hydrogenolysis of the C\u2013O bond and hydrogenation of the C=O bond, respectively. Finally, the \u2013CH2OH of MFA is further hydrogenated to DMF.Thus, a plausible reaction mechanism was proposed. First, H2 and substrate HMF are adsorbed onto the surface of the Ni\u2013Cu/TS-1 catalyst. In a closed environment, hydrogen is dissociated into hydrogen atoms by interaction with the (1 1 1) surface of the metal. In the reaction of HMF to give DHMF, the carbonyl carbon atom is attacked by a hydrogen atom, such that the aldehyde group of HMF becomes a hydroxyl group. On the other hand, the oxygen atom of the HMF hydroxyl group is stimulated by the Lewis acid site of the catalyst, and is automatically dehydrated after being attacked by a hydrogen atom, leading to the formation of the intermediate MF. In addition, the Cu in the Ni\u2013Cu/TS-1 catalysts is more favorable for the reaction of C\u2013O and C=O and inhibits the excessive hydrogenolysis of C\u2013C. The possible explanation is that, owing to the overlap of the 3d band of the surface Cu atoms and the aromatic furan ring, a repulsion occurs between the Cu (1 1 1) plane of Ni\u2013Cu/TS-1 catalysts and the furan ring [50]. Similarly, the intermediates DHMF and MF are further transformed into MFA through the above process. Ultimately, MFA is easily deoxygenated on the acidic sites to produce DMF.In summary, the selective hydrogenolysis of HMF to DMF takes place over Ni\u2013Cu/TS-1 catalysts with an appropriate Ni/Cu ratio in the presence of THF solvent under H2 atmosphere. Detailed characterization of the catalysts was performed to unravel the Ni\u2013Cu alloy species formed on the surface of the carrier material. Control experiments revealed that there is synergy between the hydrogenolysis of the hydroxyl group in HMF over the Lewis acid TS-1 and the hydrogenation of the aldehyde group over metal particles during the reaction. Reusability studies further showed that the Ni\u2013Cu/TS-1 catalysts are relatively stable. Based on the concept of economy and sustainability, this study provides an approach for the efficient and targeted hydrogenation of biomass-based furan derivatives into fine chemicals and fuel additives.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (21606082), China Postdoctoral Science Foundation (2019M662787), Scientific Research Fund of Hunan Provincial Education Department (20B364), Hunan Provincial Innovation Foundation for Postgraduate (CX20200522) and the Opening Fund of Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science (CHCL21003).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2020.100081.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Production of biofuels from biomass resources has received wide attention because of the current energy crisis. Recently, significant interest has been directed towards the selective hydrogenolysis of 5-hydroxymethylfurfural (HMF) to produce biofuel 2, 5-dimethylfuran (DMF). In this study, non-noble Ni\u2013Cu catalysts with various Ni/Cu ratios, supported on titanium silicalite-1 (TS-1), were prepared through a solid-phase grinding method and used to catalyze the hydrogenolysis of HMF to DMF under a hydrogen atmosphere. The structure and surface morphology of the catalysts were characterized by X-ray diffraction, Fourier-transform infrared, thermogravimetry, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia, scanning electron microscopy, and N2 adsorption-desorption techniques. Under the optimized reaction conditions, the conversion of HMF and the selectivity for DMF over a 40%Ni\u20135%Cu/TS-1 catalyst could reach 100.0% and 97.3%, respectively. Importantly, because the strong interaction of the Ni\u2013Cu alloy structure prevents further reaction of the furan ring, almost no by-products are produced. The metal sites of Ni and Cu and the acid sites of TS-1 combine to provide a synergistic effect, which is beneficial to the hydrogenolysis of HMF. In addition, reusability experiments showed that the catalyst maintained good activity and stability.\n               "}
{"full_text": "All data related to this study included in the article and supplemental information will be provided by the lead contact upon request.Oxygen evolution reaction (OER), a four-electron-involved anodic process, normally suffers from sluggish kinetics, which thus is the bottleneck in various electrochemical applications.\n1\u20137\n It has always been a formidable challenge to design outstanding OER catalysts with low costs and high activity ahead of the proposal of single-atom catalysts (SACs).\n8\u201312\n Thanks to maximal atomic-utilization efficiency, abundant active species, and inspiring catalytic activity, SACs have been regarded as one of the most promising solutions.\n13\u201319\n M\u2212N4, M\u2212N2O2, and M\u2212N3C1 (M\u00a0= transition metals) represent common coordination modes in SACs, which are validated to be capable of tuning the electronic structures of the central metal atom and thus the catalytic performance.\n20\u201325\n For instance, Jiang et\u00a0al. demonstrated that a low N coordination number could favor the formation of COOH\u2217 intermediates of the Ni SA\u2013N2\u2013C, leading to superior CO2RR activity.\n26\n Zhou and co-workers employed density functional theory (DFT) computation and machine learning and found that both metals and coordination modes have a great impact on the OER descriptor (\u0394G\u2217O\u2212\u0394G\u2217OH).\n27\n Compared with other commonly used transition metals such as cobalt (Co) or nickel (Ni), iron (Fe) is more abundant and environmentally friendly. It has attracted particular interest not only because of its high abundance but also because it shows intriguing interaction capability with coordinating atoms/centers, which may lead to unusual coordination modes.\n28\n Therefore, it is encouraged to explore more Fe-SACs with novel coordination configurations by prudent structural design and to unveil their OER catalytic behavior.Covalent organic frameworks (COFs) with well-defined and tailorable structures could provide a platform for the variable coordination of isolated metal atoms by the confinement effect and coordination interaction with specified atoms of the organic skeleton.\n29\u201335\n For example, a series of dioxin-linked metallophthalocyanine COFs could construct a typical M\u2212N4 coordination mode.\n36\u201338\n The Salen-COFs derived from the ortho-hydroxybenzaldehyde and ethylenediamine through Schiff-base condensation reactions could lead to M\u2212N2O2 moieties.\n27\n An M\u2212N2 site can be anchored on a 2, 2\u2032-bipyridine-based COF.\n39\n\n,\n\n40\n Thus, by tuning the backbones and functional groups as well as the coordinating geometry, unusually coordinated Fe-SACs could be constructed, which remains a great challenge in practice. Meanwhile, those SACs with specific coordinating structures are a necessity to perform reliable theoretical computations, which is conducive to understanding the catalytic mechanism at the molecular level and to developing highly efficient OER catalysts.Herein, we confine Fe single-atom species at a low operation temperature (\u221260\u00b0C) in a two-dimensional (2D) COF, which is achieved based on the Schiff-base condensation reaction between the 1,3,5-triformylphloroglucinol (Tp) and 4,4\u2032,4''-(1,3,5-triazine-2,4,6-triyl)trianiline (Tta) ligands. Extended X-ray absorption fine structure spectroscopy (EXAFS) and aberration-corrected high-angle annular dark-field scanning TEM (AC HAADF-STEM) reveal that the metal species are dispersed uniformly and isolated on the COF carriers with an unusual Fe\u2013NO atomic arrangement in the skeleton. As a result, the as-prepared Fe single atom embedded in the COF catalyst (Fe-SAC@COF) demonstrates a fairly low overpotential of 290\u00a0mV at 10 mA cm\u22122 and a Tafel slope of 40\u00a0mV dec\u22121, which, to the best of our knowledge, surpasses all reported atomically dispersed Fe-based OER electrocatalysts. Moreover, DFT calculations reveal that the Fe\u2013NO-coordinated Fe-SAC@COF processes a much lower energy difference of potential-determining step than that of Ni species and thus behaves superiorly toward OER. This work may inspire more novel coordinated SACs in judicious COFs in achieving highly active electrocatalysis.The SAFe-COF is synthesized by a two-step process, as schematically demonstrated in Figure\u00a01A. First, Tp-Tta COF was prepared through the Schiff-base condensation reaction between the two ligands under a solvothermal condition\n41\n (see experimental details in the experimental procedures). Then, the as-prepared Tp-Tta COF powder was used as the Fe loading carrier, where the process was performed at a low temperature of \u221260\u00b0C to prevent the species from aggregating.\n42\n Subsequent treatment in N2H4\u00b7H2O-containing alkaline reducing agent was conducted to produce the targeted Fe-SAC@COF.Powder X-ray diffraction (PXRD) patterns of the Tp-Tta COF sample fit with the simulation results, which implies successful synthesis (Figure\u00a01B). The intense peak at 5.8\u00b0 (2\u03b8) corresponds to the reflection of the (100) plane, while the broad peak at \u223c26\u00b0 is assigned to the (001) plane, which is attributed to \u03c0-\u03c0 stacking between successive layers of the 2D Tp-Tta COF. The Fourier transform infrared spectroscopy (FT-IR) peak at 3,000\u20133,500\u00a0cm\u22121 of the Tta ligand, corresponding to \u2013NH2 bonds, disappears, while two strong peaks at 1,623 and 1,286\u00a0cm\u22121 can be assigned to the\u00a0\u2013C=O and \u2013C\u2013N bonds, respectively (Figure\u00a01C). This indicates the formation of the \u03b2-ketoenamine-linked framework. In the solid-state 13C nuclear magnetic resonance (NMR) spectrum, the characteristic peaks at 182.0 and 105.1 ppm can be assigned to \u2013C=O and \u2013C=C, which confirms the success of the condensation reaction between the two ligands (Figure\u00a0S1). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show a fibrous morphology for the Tp-Tta COF (Figures S2 and S3). Moreover, the morphology, PXRD, and FT-IR patterns are retained after soaking in 1\u00a0M KOH electrolyte solution for the Tp-Tta COF, which demonstrates decent stability and meets the basic prerequisites for alkaline OER electrocatalysis (Figures S4 and S5).No impurities were detected in the PXRD patterns and FT-IR spectra after the coordination of Fe, which indicates that the Fe-SAC@COF sample retains crystallinity and has sturdy chemical bonds (Figures 1B and 1C). Meanwhile, the fibrous-like morphology is maintained in the SEM image, implying the stable structure of\u00a0Fe-SAC@COF (Figure\u00a01D). The N2 adsorption-desorption isotherm of Fe-SAC@COF\u00a0exhibits a smaller Brunauer-Emmett-Teller (BET) surface area of 470 m2 g\u22121 compared with pristine Tp-Tta COF (716 m2 g\u22121), which could be attributed to the increment of mass of the Fe atoms (Figure\u00a0S6). Meanwhile, the pore size distribution of Fe-SAC@COF (M=Fe, Ni) by the non-local DFT (NLDFT) indicates that the size is sub-1\u00a0nm, which is similar to that of pristine COF (Figures S7, S8, and S11). Additionally, the mass ratio of Fe in Fe-SAC@COF is 1.0 wt\u00a0%, detected by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) results (Table S1). To visualize the relationship between coordination configuration and catalytic activity, Ni-SAC@COF was prepared through a similar synthetic approach (Figures S9\u2013S11).The composition analysis by X-ray photoelectron spectroscopy (XPS) reveals that metals in Fe-SAC@COF and Ni-SAC@COF are in approximate\u00a0+3 and\u00a0+2 valence states, respectively (Figures S12 and S13). No clusters or nanoparticles are displayed in the TEM images of the as-obtained Fe-SAC@COF and Ni-SAC@COF samples, while the metal elements show a homogeneous distribution (Figures 2A, S14, and S15). This implies that the metal atoms are probably dispersed in the single-atom form. AC HAADF-STEM images were further performed to identify the distribution state of the coordinated atoms. As shown in Figures 2B and 2C, uniform and high-density bright dots are observed, which verifies the atomic dispersion of the metal atoms in the COF skeleton.X-ray absorption fine structure (XAFS) spectra were performed to reveal the interface structure at the atomic level. The rising-edge position of the X-ray absorption near-edge structure spectra (XANES) of both Fe-SAC@COF and Ni-SAC@COF samples are between the corresponding metallic foils and metal oxides (Figures\u00a02D and 2G). This suggests that the metal atoms in COFs are in oxidized states, which are consistent with the XPS results. Furthermore, the FT-EXAFS spectrum was applied to confirm the metal coordination environment (Figures 2E and 2H). Fe-SAC@COF presents only one main peak at about 1.47\u00a0\u00c5, which is attributed to the Fe\u2013N/O scattering path, and no Fe\u2013Fe bond (2.17\u00a0\u00c5) can be detected, confirming the single-atomic state of Fe in Fe-SAC@COF. Similarly, Ni-SAC@COF also shows one peak in the R space (at a shorter length than the typical M\u2013M distance), confirming no metal-metal bonds. Besides, the wavelet transforms (WTs) were carried out to analyze metal-atom, K-edge EXAFS oscillations (Figures 2F, 2I, S16, and S17). The WT maximum is observed only at \u223c4\u00a0\u00c5\u22121 for M-SAC@COF (M=Fe, Ni), which could be assigned to M-N/O bonds. No WT maximum, corresponding to the M\u2013M bond, is detected as in the metallic foil and metal oxide, suggesting the presence of mononuclear metals. EXAFS fitting was performed to extract the structure parameters and the quantitative chemical configuration of metal atoms (Figures S18 and S19; Tables S2 and S3), which gives a coordination number of 4 for both Fe and Ni centers. The coordination of Fe with O atoms is confirmed by the O1s spectrum, where Fe-SAC@COF shows an emerging deconvoluted Fe\u2013O peak (Figure\u00a0S20). Moreover, the interlayer spacing of Fe-SAC@COF is calculated to be \u223c3.4\u00a0\u00c5 based on the (001) reflection peak at \u223c26\u00b0 in the XRD pattern. As there is one O in each layer and the measured Fe\u2013O/N bond distance is 1.97\u00a0\u00c5, the three O atoms are all bound within the framework can be ruled out. Besides, if Fe binds with 2\u00a0N and 2 O atoms from the adjacent layers, the valence state would be 2+, which is inconsistent with the XPS and EXAFS results. Thus, Fe is most likely coordinated with one O and one N of the COF, while the other 2 O are coordinated with the O of the acetate. The Fe\u2013NO3 coordination mode agrees with the EXAFS fitting results (Figure\u00a0S18) as well as the geometry optimization (see details in the supplemental information).The electrocatalytic performance of pristine Tp-Tta COF, Fe-SAC@COF, and Ni-SAC@COF were evaluated in 1\u00a0M KOH electrolyte in a typical three-electrode testing configuration. Fe oxide nanoparticles with an average size of 5\u00a0nm were prepared on the Tp-Tta COF (denoted as Fe-NP/COF) for comparison (Figures S21\u2013S23). As revealed in the representative linear sweep voltammetry (LSV) curves, the Fe-SAC@COF electrode exhibits superior catalytic activity with a low overpotential of 290\u00a0mV at a current density of 10 mA cm\u22122 (Figure\u00a03A), which outperforms the Ni-SAC@COF (337\u00a0mV), Fe-NP/COF (359\u00a0mV), and pristine Tp-Tta COF electrodes (430\u00a0mV). Though Fe-SAC@COF and Ni-SAC@COF have the same coordination mode, the former, as visualized in Figure\u00a03B, exhibits much higher activity toward OER. Accordingly, the Fe-SAC@COF electrode has the lowest Tafel slope of 40\u00a0mV dec\u22121 among all samples (Ni-SAC@COF, 45\u00a0mV dec\u22121; Fe-NP/COF, 51\u00a0mV dec\u22121; pristine COF, 129\u00a0mV dec\u22121), suggesting its fast kinetics for OER (Figure\u00a0S24). Moreover, Co-SAC@COF was also synthesized and tested, which is inferior to Fe-SAC@COF (onset potential, 359\u00a0mV; Tafel slope, 48\u00a0mV dec\u22121), further verifying the influence of metal centers on catalytic activity (Figures S25 and S26). Correspondingly, Fe-SAC@COF exhibits a larger turnover frequency (TOF; 1.27 s\u22121 at 1.63 V) than the other two samples (Ni-SAC@COF, 0.68 s\u22121 and Fe-NP/COF, 0.39\u00a0s\u22121), implying its high intrinsic catalytic activity. The mass activity of Fe-SAC@COF\u00a0is 9.20 A mg\u22121 at 1.63 V, which is about 1.95 and 5.05 times higher than Ni-SAC@COF and Fe-NP/COF, respectively. Figure\u00a03C shows the comparison of the Tafel slopes, overpotentials, and TOFs with other Fe-based SACs for OER catalysis. Apparently, these values of the Fe\u2013NO-coordinated Fe-SAC@COF are state of the art among single-atom Fe-based OER electrocatalysts (see specific values in Table S4). The superiority of Fe-SAC@COF might be attributed to its highly effective atomic utilization and unique metal-coordination species. Besides, the overpotential is significantly increased when introducing potassium thiocyanate in the electrolyte, which confirms that the single Fe atoms are the electrocatalytic active center (Figure\u00a0S27).Furthermore, electrochemical double-layer capacitances (Cdls) were carried out to evaluate the electrochemical surface areas (ECSA) using cyclic voltammetry (CV) measurements (Figures S28\u2013S30). The Fe-SAC@COF electrode shows a larger Cdl value of 17.9 mF cm\u22122 than those of Ni-SAC@COF (6.5 mF cm\u22122) and Fe-NP/COF (4.3 mF cm\u22122), demonstrating more exposed active atoms for Fe-SAC@COF (Figure\u00a03D). On the other hand, electrochemical impedance spectroscopy (EIS) plots were conducted to reveal the charge-transfer kinetics. The Fe-SAC@COF electrode shows the smallest semicircle, indicating the fastest charge-transfer rate on the Fe-SAC@COF catalyst (Figures 3E and S31). In addition, both Fe-SAC@COF and Ni-SAC@COF samples show a negligible change in LSV curves after 2,000 cycles (Figures S32 and S33). Meanwhile, the Fe-SAC@COF electrode maintains a stable current density at around 10 mA cm\u22122 for 24\u00a0h consecutive operation, indicating outstanding stability during long-term OER operations (Figure\u00a0S34). The XPS spectrum of Fe-SAC@COF after OER catalysis clearly shows that the composition and electronic structure are maintained (Figure\u00a0S35). The TEM images of Fe-SAC@COF and Ni-SAC@COF show original fibrous-like morphologies and uniform distribution of metal elements (Figures S36 and S37). These characters demonstrate the excellent structural stability of the two samples.To reveal the impact of metal species and their coordination modes on the catalytic performance, the OER mechanism models were studied using DFT calculations implemented on the Gaussian 16 program. As the bonding mode between metal ions and the COF is the chelation with N and O in keto-enamine form, the cluster model is used as depicted in Figures 4A and S38A (calculation details are described in the supplemental information). The associated four-electron-involved reaction pathways for the Fe-SAC@COF and Ni-SAC@COF cluster models (that are Fe\u2013NO3 and Ni\u2013NO3, respectively) under alkaline conditions are illustrated in Figures 4A and S38B, respectively. Specifically, step A shows the adsorption of OH\u2212 on the Fe metal center; step B indicates the formation of Fe\u2013O from Fe\u2013OH; step C represents the subsequent formation of Fe\u2013OOH; and step D is the process of decomposition of adsorbed \u2013OOH to\u00a0O2. Accordingly, the free energy diagram and the Gibbs free energy change (\u0394G) of all steps are summarized in Figure\u00a0S39. The potential determining step (PDS), i.e., the step that has the largest \u0394G, is the one to form M\u2212O intermediates (step B) for both Fe-SAC@COF and Ni-SAC@COF catalysts. The \u0394G of the PDS for the Fe-SAC@COF\u00a0is 0.79 eV, which is much lower than that of Ni species (1.35 eV). This explains why the former catalyst shows faster OER kinetics. Furthermore, Fe-SAC@COF with a different coordination model (Fe-NO4) is calculated, whose PDS (the step to form M\u2212OH) is distinct from that of Fe-SAC@COF (Fe-NO3). The \u0394G of the PDS is 1.10 eV, which is 0.31 eV higher than Fe-SAC@COF (Fe-NO3). The overpotential versus difference between the \u0394GO\u2217 and \u0394GOH\u2217 for these\u00a0three models shows a volcano-like shape (Figure\u00a04B). The Fe\u2013NO3 model of Fe-SAC@COF is at the summit (the lowest overpotential), which means that it has the most moderate interaction with the oxygenated intermediates during OER. The Fe\u2013NO4 and Ni\u2013NO3 models, instead, exhibit too strong and too weak interactions, respectively. Therefore, it can be concluded that both coordination modes and the metal species of the same coordination mode could have a great impact on the PDS and thus the catalytic activity of OER.We have developed stable and atomically dispersed Fe-SACs coordinated on Tp-Tta COFs with novel and precise coordination modes alleviating pyrolysis treatment. AC HAADF-STEM and EXAFS confirm the atomic dispersion and specific coordination environment. Fe-SAC@COF, with the uncommon coordination configuration, shows outstanding OER activity with the overpotential and Tafel slope higher than the other atomically dispersed Fe-based OER electrocatalysts. DFT calculations reveal that Fe-SAC@COF processes lower \u0394G of the PDS for OER than that of the Ni-SAC@COF catalyst, which explains its faster OER kinetics. This work demonstrates that COFs may serve as variable coordination supports for the design and synthesis of novel-coordinated and stable SACs, which might lead to the exploration of highly active electrocatalysts.Further information and requests for resources and materials should be directed to and will be fulfilled by the lead contact, Dr. Wei-Qiao Deng (dengwq@sdu.edu.cn).This study did not generate new unique reagents.All solvents and materials in this study were purchased from commercial sources without further purification. Fe(II) acetate, Ni(II) acetate tetrahydrate, potassium hydroxide, methanol, ethanol, 1,4-dioxane, mesitylene, acetone, tetrahydrofuran, and hydrazine monohydrate were obtained from Aladdin (Shanghai, China). Tp and Tta were purchased from Jilin Chinese Academy of Sciences - Yanshen Technology.To prepare the Tp-Tta COF, Tp (0.3\u00a0mmol) and Tta (0.3\u00a0mmol) were dispersed in 3\u00a0mL mixture solution with 1.5\u00a0mL 1,4-dioxane and 1.5\u00a0mL mesitylene in a Pyrex tube. After being sonicated for 5\u00a0min, 0.3\u00a0mL aqueous acetic acid (6 M) was added, and the mixture was sonicated to afford a homogeneous dispersion. Subsequently, the mixture was subjected to three freeze-pump-freeze cycles, then the tube was sealed off and heated at 120\u00b0C for 72 h. The precipitate was collected by centrifugation and washed with tetrahydrofuran (THF; 3 \u00d7 40 mL) and acetone (3 \u00d7 40\u00a0mL). The collected powder was dried at 60\u00b0C under vacuum for 12\u00a0h to afford the Tp-Tta COF.Taking Fe-SAC@COF as an example, the Fe precursor was prepared by the dispersion of Fe(OAc)2 in 20\u00a0mL mixed solution of water and ethanol (v/v\u00a0= 1:9) under stirring being held for 1\u00a0h at \u221260\u00b0C. Subsequently, 50\u00a0mg COF material was added into the mixture and continuously stirred for 12 h. Then, 5\u00a0M N2H4 H2O+0.05\u00a0M KOH solution (20\u00a0mL) was injected quickly. This mixture was allowed to react for another 12 h, and the resulting powder was washed at \u221260\u00b0C and naturally dried at 25\u00b0C. The metal content is confirmed by ICP-AES. Ni-SAC@COF was prepared using a similar procedure except using Ni(OAC)2\u00b74H2O.In a 50\u00a0mL round flask, 50\u00a0mg Tp-Tta COF and 2\u00a0mg Fe(OAc)2 were dispersed in 20\u00a0mL methanol, and the mixture was stirred for 72\u00a0h at 50\u00b0C under N2 atmosphere. The resulting solid was fully washed with THF and methanol. Then, the obtained material was dried at 60\u00b0C for 12 h.FT-IR spectra were obtained from a VERTEX 70v spectrometer (Bruker) in a range of 4,000\u2013500\u00a0cm\u22121. PXRD patterns were taken using a D-MAX 2500 diffractometer (Rigaku) with Cu-K\u03b1 radiation. Solid-state 13C CP/MAS NMR spectra were taken in a Bruker 300 MHz NMR. TEM images were carried out with Thermo Fisher Scientific Talos F200X G2, and the samples were prepared by drop-casting the powder from ethanol on copper grides. SEM images were obtained with a SEU8010 scanning electron microscope. Gas adsorption and desorption were recorded on a Quantachrome instrument. XPS measurements were carried out on a Thermo ESCALAB 250XI spectrometer. ICP (Optima 2100DV) analyses were used to determine the mass concentration of metal. AC HAADF-STEM was performed using a FEI Themis Z microscope.XAS spectra of all catalysts were recorded under ambient conditions in a transition mode at beamline 1W1B of Beijing Synchrotron Radiation Facility (BSRF), using a Si (111) double-crystal monochromator. The acquired EXAFS data were processed according to standard procedures using the ATHENA module implemented in the IFEFFIT software packages.\n43\n For all XAS data, the experimental absorption coefficients as a function of energies \u03bc(E) were processed by background subtraction and normalization procedures. For EXAFS modeling, EXAFS of the metal foil is fitted, and the obtained amplitude reduction factor S0\n2 value was set in the EXAFS analysis to determine the coordination numbers (CNs) in M-SAC@COF (M=Fe or Ni) catalysts.Electrochemical measurements were carried out in a three-electrode system connected on an electrochemical workstation (Bipotentiostat model CS2350) in 1\u00a0M KOH at room temperature. The working electrode was prepared by coating the catalyst ink onto a Ni foam (NF; 1 \u2217 1.5\u00a0cm). An Hg/HgO electrode and a platinum foil were used as the reference and counter electrodes, respectively. In a typical process to prepare the catalyst ink, 10\u00a0mg catalyst (0.5\u00a0mg carbon nanotube) was suspended in 2\u00a0mL mixed solvent containing ethanol and 5% Nafion (v/v\u00a0= 9/1) to form a homogeneous ink. After sonication for 10\u00a0min, the catalyst ink was dripped on the two sides of the working electrode. The optimal mass loading is measured to be about 1\u00a0mg cm\u22122. Moreover, the optimal metal content for all SAC samples is measured to be \u223c1.0 wt\u00a0%. All potentials were converted versus reversible hydrogen electrodes (RHEs) based on the Nernst equation: E (versus RHE)\u00a0= E (versus Hg/HgO)\u00a0+ 0.098\u00a0V\u00a0+ 0.059\u2217pH. The electrolyte was pre-saturated with Ar gas for 30\u00a0min before OER tests. During OER testing, the catalyst was first subjected to CV activation for 20 cycles with a scan rate of 50\u00a0mV s\u22121, and polarization curves were performed with LSV mode at a scan rate of 5\u00a0mV s\u22121. The electrochemical impedance measurement (EIS) was measured in frequency ranges from 100 kHz to 0.1\u00a0Hz at a potential of 300\u00a0mV (versus RHE). ESCA was carried out by testing C\ndl\n in non-Faradaic potential regions with various scan rates from 20 to 100\u00a0mV s\u22121. No iR compensation was employed in all tests.The TOF was calculated by the following equation: TOF= (J \u00d7 A)/(4\u00a0\u00d7 F \u00d7 n), in which J (A cm\u22122) is the current density at a given overpotential, A (cm\u22122) is the surface area of the electrode, F stands for the Faraday constant (96,485 C mol\u22121), and n represents the number of active sites (mol). In this study, the metal Fe atom is regarded as the active site. Mass activity (mA mg\u22121) values were calculated from the electrocatalyst metal loading m and the measured current density j (mA cm\u22122) at \u03b7\u00a0= 400\u00a0mV: mass activity\u00a0= J/m.Geometry optimizations and frequency calculations were carried out at the B3LYP/6-31G (d,p) level. For each geometric stationary point, the single-point energy calculations were performed using an extended 6-31++G(d,p) basis set, as well as the SMD solvation model, considering the solvent (water) effect.\n44\n Only reactant and product states in the OER process, as well as intermediate states, are proposed and evaluated. The overall reaction scheme of OER reaction in an alkaline environment is:\n\n(Equation\u00a01)\n\n\n4\n\nOH\n\u2212\n\n\u2192\n\nO\n2\n\n\n(\ng\n)\n\n+\n2\n\nH\n2\n\nO\n\n(\nl\n)\n\n+\n4\n\ne\n\u2212\n\n\n\n\n\nWe consider four elementary steps for OER, with each consisting of a single-electron transfer step reaction written as the following equations.\n\n(Equation\u00a02)\n\n\nM\n+\n\nOH\n\u2212\n\n\u2192\nM\n-\nOH\n+\n\ne\n\u2212\n\n\n\n\n\n\n\n(Equation\u00a03)\n\n\nM\n-\nOH\n+\n\nOH\n\u2212\n\n\u2192\nM\n-\nO\n+\n\nH\n2\n\nO\n\n(\nl\n)\n\n+\n\ne\n\u2212\n\n\n\n\n\n\n\n(Equation\u00a04)\n\n\nM\n-\nO\n+\n\nOH\n\u2212\n\n\u2192\nM\n-\nOOH\n+\n\ne\n\u2212\n\n\n\n\n\n\n\n(Equation\u00a05)\n\n\nM\n-\nOOH\n+\n\nOH\n\u2212\n\n\u2192\nM\n+\n\nO\n2\n\n\n(\ng\n)\n\n+\n\nH\n2\n\nO\n\n(\nl\n)\n\n+\n\ne\n\u2212\n\n\n\n\n\nHere, the asterisk refers to the active site of metal-decorated calculation models. The free energy change of each elementary step is described by the following expressions:\n\n(Equation\u00a06)\n\n\n\u0394\n\nG\nA\n\n=\n\nG\n\nM\n-\nOH\n\n\n\u2212\n\nG\n\nOH\n\u2212\n\n\n\u2212\n\nG\nM\n\n\u2212\ne\nU\n,\n\n\n\n\n\n\n(Equation\u00a07)\n\n\n\u0394\n\nG\nB\n\n=\n\nG\n\nM\n-\nO\n\n\n+\n\nG\n\n\nH\n2\n\nO\n\n(\nl\n)\n\n\n\n\u2212\n\nG\n\nM\n-\nOH\n\n\n\u2212\n\nG\n\nOH\n\u2212\n\n\n\u2212\ne\nU\n,\n\n\n\n\n\n\n(Equation\u00a08)\n\n\n\u0394\n\nG\nC\n\n=\n\nG\n\nM\n-\nOOH\n\n\n\u2212\n\nG\n\nM\n-\nO\n\n\n\u2212\n\nG\n\nOH\n\u2212\n\n\n\u2212\ne\nU\n,\n\nand\n\n\n\n\n\n\n(Equation\u00a09)\n\n\n\u0394\n\nG\nD\n\n=\n\nG\nM\n\n+\n\nG\n\n\nH\n2\n\nO\n\n(\nl\n)\n\n\n\n+\n\nG\n\n\nO\n2\n\n\n(\ng\n)\n\n\n\n\u2212\n\nG\n\nM\n-\nOOH\n\n\n\u2212\n\nG\n\nOH\n\u2212\n\n\n\u2212\ne\nU\n,\n\n\n\nwhere U denotes the applied electrode potential and G represents the free energy for each species.The free energy of H2O(l) can be derived from the gas state of water as the following equation:\n\n(Equation\u00a010)\n\n\n\nG\n\n\nH\n2\n\nO\n\n(\nl\n)\n\n\n\n=\n\nG\n\n\nH\n2\n\nO\n\n(\ng\n)\n\n\n\n+\nR\nT\nln\n\n(\n\np\n/\n\np\n0\n\n\n)\n\n,\n\n\n\nwhere GH2O(g) is the free energy of H2O(g), which can be directly obtained by DFT calculations. R is the ideal gas constant. Because it is hard to calculate the electronic energy of an oxygen molecule in a high-spin ground state exactly, the free energy of an oxygen molecule in gas (GO2(g)) was derived as the following equation:\n45\n\n\n\n(Equation\u00a011)\n\n\n\nG\n\n\nO\n2\n\n\n(\ng\n)\n\n\n\n=\n2\n\nG\n\n\nH\n2\n\nO\n\n(\ng\n)\n\n\n\n\u2212\n2\n\nG\n\n\nH\n2\n\n\n(\ng\n)\n\n\n\n+\n4.92\n\neV\n\n\n\n\nThe free energy of OH\u2013 was derived as\n\n(Equation\u00a012)\n\n\n\nG\nOH\n\n=\n\nG\n\n\nH\n2\n\nO\n\n(\nl\n)\n\n\n\n\u2212\n\nG\n\nH\n+\n\n\n,\n\n\n\n\n\n\n(Equation\u00a013)\n\n\n\nG\n\nH\n+\n\n\n=\n\n1\n2\n\n\nG\n\n\nH\n2\n\n\n(\ng\n)\n\n\n\n\u2212\nR\nT\nln\n10\n\u00d7\npH.\n\n\n\n\nThe thermal corrections to the free energy of each reactant, product, and intermediate state were calculated by frequency calculations at room temperature (298.15 K) based on the optimized geometry. The other parameters were set: T\u00a0=\u00a0298.15 K, p\u00a0= 0.035 bar, and P0\u00a0= 1 bar.This work was supported by the National Key Research and Development Program of China (no. 2017YFA0204800) and the Natural Science Foundation of Shandong Province (no. YDZX2021001). H.W. thanks the financial support from the Program of Qilu Young Scholars of Shandong University (no. 62460082163005), the Natural Science Foundation of Shandong Province (no. ZR2021QB201), and the Science Foundation for Outstanding Young Scholars of Shandong Province.Conceptualization, H.W. and W.-O.D.; methodology, X.W., L.Y., and L.S.; investigation, X.W., W.Z., and G.R.; writing \u2013 original draft, X.W. and L.S.; writing \u2013 review & editing, H.W. and W.-O.D.; funding acquisition, H.W. and W.-O.D.; resources, X.W. and L.S.; supervision, H.W. and W.-O.D.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2022.100804.\n\n\nDocument S1. Figures S1\u2013S39 and Tables S1\u2013S4\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  The exploration of environmentally friendly and highly efficient oxygen evolution reaction (OER) catalysts is vital to large-scale, electrochemical renewable-fuels generation. Here, we report an iron single-atom catalyst (SAC) confined in a covalent organic framework (Fe-SAC@COF), which constitutes an unusual Fe\u2013NO coordination in the skeleton. The as-prepared Fe-SAC@COF exhibits a high mass activity of 9.20 A mg\u22121, which is 1.95 times higher than Ni species of the same coordination and 5.05 times higher than nanoparticulate Fe counterpart. Moreover, it shows, to the best of our knowledge, a record-low overpotential (290\u00a0mV) and Tafel slope (40\u00a0mV dec\u22121) among the reported atomically dispersed Fe-based catalysts and surpasses the benchmark Ir/C catalyst. The density functional theory calculation shows that the Fe\u2013NO coordination exhibits low binding energy of oxygenated intermediates, which leads to an outstanding electrocatalytic OER performance. This work provides design strategies toward unusually coordinated SACs by prudent COF confinement for advanced electrocatalysis.\n               "}
{"full_text": "The increasing number of automobile vehicles on roads therefore the concentration of pollutants gasses emitted is also increases. The unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and sulfur oxides (SO2) are main pollutants produced from the internal combustion engines of an automobile vehicle. These gases are mainly produced due to the incomplete combustion of regions in oxygen deficiency of the engine. The application of catalytic converter in automobile vehicles for reduces the toxic gasses emissions from the internal combustion engines [1,2]. The catalysts present in a catalytic converter are reduction catalyst and an oxidation catalyst. The catalytic reaction is the reaction between catalyst surfaces and remaining gases present in the automobile vehicle exhaust. The activity of catalytic converter is highly dependent upon the types of catalyst was used. Carbon monoxide (CO) is one of the very poisonous gases present in the atmosphere and also represents as the silent killer of 21st century. The CO is produced into the atmosphere by partial combustion of carbon-containing compounds [3,4]. The CO decreases the amount of oxygen gas that reaches the blood and might because sleepiness, slow reflexes, impaired vision and decision and even it may cause personal death as CO gas replace oxygen by binding to the iron atom presented in blood hemoglobin(Hb) and so Hb becomes failure to carry out oxygen to the brain. The more exposure to CO can diminish the amount of oxygen absorbed by the brain so much that the victim may become unconscious and may suffer from the brain damage or person death from hypoxia [5,6]. The low-temperature catalytic oxidation of CO is very important process for all life support in enclosed atmospheres such as submarines and spacecraft. Due to increasing the cost of noble metals the catalytic conversion of CO over transition metal oxide catalysts (TMOs) has been much interest [7,8].The transitional metal oxides (TMOs) constructions, buildings, designing and structures have been represented as one of the most important useful elements in the area of catalysis, fuel cells, energy storage and air pollution control and so on. These materials structured units frequently contains certain physical and chemical properties that are very helpful for their efficient applications. The TMOs catalysts are novel types of mixed oxide that surface area chemistry can be considered in the similar way as the effective oxide catalysts [9,10]. It's a cheaper cost, environmentally friendly and simply available catalyst for ambient conditions CO oxidation. TMOs represents a huge structural differences due to their capacity to produced phases of different metal to oxygen ratios showing several steady oxidation states of the metal ions. The structural defects in TMO are highly influence by their catalytic performances.\u00a0The location of surface ions will change from the massive structure [11,12]. The surface metal oxides are highly influenced by the coordination of metal\u00a0cation\u00a0and oxygen\u00a0anion, which modify the catalytic activity of these compounds. The nanosized metal oxides are represented by special and crucial applications particularly in the catalysis field. The superior activity of TMOs nanoparticles in CO oxidation was associated with the smaller particle size, higher surface area and closely covered the surface coordination unsaturated sites [13,14]. In the TMO catalysts the nickel oxides (NiOx) are novel types of mixed oxide catalyst that surface area chemistry can be considered in the similar way as the effective oxide catalysts. There are different preparation conditions have been developed for the production of nickel oxide catalyst with unique structure and higher activity. The better catalytic activity of nickel oxides has been suggested for particles with the smaller sizes [15,16].Nickel oxide (NiO) is an earth-abundant transition metal oxide with superior redox property, electrochemical performance and gas sensing property. The Ni-based catalysts are commonly studied for their potential capacity to catalyze the dry-reforming reaction on an industrial scale. In addition, the several researchers have studied NiO catalysts with various morphologies for CO oxidation and found that ring-like and flower-like NiO demonstrated high activity. The Ni is the best catalyst for reaction of virtue of its life, high activity and selectivity towards CO oxidation at a lower cost. The Ni nano-particle catalysts have been represented as an excellent catalyst from lower cost, thermally, activity and selectivity point of view [17,18]. The influence of NiO particle size on catalytic activity has been subjected of continuous interest due to its importance from both primary and practical viewpoints. The high performances of NiO are associated to the presence of both intra and inter-particle porosity of catalysts and highly Ni metal particles dispersion on the catalyst surfaces. The NiO has a bimodal pore structure, those represents that huge performances for CO oxidation. It will provide more favorable pore size for the chemisorptions of reactants. The massive amorphous NiO have formed and highly dispersed on the catalyst surface, resulting in the formation of abundant surface Ni2+ ions [19,20].The potential chemisorptions of Ni have transitional binding energies; therefore, it has an intermediate value of the heat adsorption. The performance of NiO catalysts also depends upon the calcinations conditions of precursors and subsequent pretreatment of the catalysts. The synergetic effect between Ni and other metal oxides over bimetallic catalysts can reduce nickel size to improve the metal particle dispersion and accelerate the activation of adsorbed CO, thereby improving the catalytic activity. The reaction of NiO and O2 is influenced by the oxygen concentration and temperature. The Ni\u2013Co composite oxides could be potential catalysts for CO oxidation at low temperatures. The synergetic interaction between Ni and Co affecting the catalytic physicochemical properties and activity taking into account the diverse morphologies of bimetallic oxide catalysts and catalytic mechanism is worth in the further explanation [21,22]. The unique Ni0, Ni2+ and Ni3+ species for the nickel-containing samples are make them better catalytic activity. The valence of nickel for all catalysts was maintained as Ni2+ after the reaction. The Ni deposited on CeO2 has highly low even any activity for CO oxidation reaction between 20 and 120\u00a0\u00b0C. The addition of nickel, which is in a good agreement with the argument of metallic gold, is active species for CO oxidation. The addition of nickel component, benefit the catalytic performance of ceria supported catalysts for CO oxidation. The different oxidation state of Au by the addition of Ni and compared the activity of Ce-supported Au catalysts with after the normalization by Au amount [23,24].The nickel-based catalysts such as Ni/Al2O3, Ni-CeO2/Al2O3, Ni/CeO2 and Ni/C have been studied. Under ultra-high vacuum conditions the NiO-catalysts showed high reactivity for CO oxidation below room temperature. The Ni nanoparticles are initially more reactive for CO oxidation at room temperature; however the CO oxidation reactivity decreases with reaction time due to the structural instability of Ni nanoparticles causing agglomeration of nanoparticles with time under reaction conditions. The lithium doping NiO catalysts also improved their performances for CO oxidation. The increase in catalytic activity with Ni3+ concentration improves the performances of catalysts for CO oxidation. The CO adsorption is a slow processes and O2 adsorption is a fast processes. The NiFe2O4 catalysts also showed improved performances for CO oxidation at 120\u2013180\u00a0\u00b0C [25,26]. The TiO2 is a reducible metal oxide with several crystal structures and titanium-supported metal catalysts have also been used for converting CO into CO2. The titanium supported nickel oxide catalysts are more active in CO oxidation than silica or alumina supported catalysts. The surface oxygen present on the catalyst was responsible for formation of both CO and CO2 for there is no oxygen in the feed gas and catalyst was not pre-reduced. The different nickel loadings and reaction conditions are highly impact on the performance of catalysts for CO oxidation. The NiO and TiO2 may react in the solid phase at the presence of oxygen gas to form NiTiO3 (NiO\u00a0+\u00a0TiO2\u2192NiTiO3) [27,28].The reduction of NiO to Ni0 on the surface, since there was no oxygen in the feed, the formation of CO and CO2 represents that the catalyst itself supplies the oxygen. The NiO lattice oxygen O2\u2212and/or oxygen from the TiO2 support participated in the oxidation of CO. The correlation between Ni phase structures and surface acidity of Al2O3 supports calcinations at different temperatures are very important for CO oxidation reaction. The interaction between Ni nanoparticles and Al2O3 supports due to the rapid decrease of specific surface area and acidity of Al2O3 supports [29,30]. A lot of efforts have been made to improve the stability of Ni/Al2O3 catalysts by increasing the Ni loadings. The variation of Al2O3 properties will affect the dispersion of active Ni particles and metal-support interaction, thus significantly impacting their catalytic activity and stability. All the reduced catalysts mainly consisted of Ni2+, possibly in oxidic, hydrated or carbonated form, with smaller fraction of Ni0. The small and highly dispersed Ni particles (about 3\u00a0\u2212\u00a08\u00a0nm) were formed on it, which can strongly interact with the support. The increase of calcinations temperature, both surface area and acidic sites of Al2O3 decreased, forming weaker interaction between metal nanoparticles and support. As a result, the Ni oxide particles can be easily reduced to highly active Ni particles (20\u221240\u00a0nm). The particle size and interaction of Ni and Al2O3 support highly influences on the performance of catalysts [31,32].The Physico-chemical property of the support should simultaneously influence the catalytic performance of catalysts reduction in the surface area and acid sites of Al2O3 supports, Ni oxide particle size increases with the weakening of metal-support interaction and becomes more and more reducible conditions. The activation energy for CO oxidation reaction on NiO is not affected at lower temperature by the addition of foreign ions to the nickel oxide lattice. The reason of catalytic oxidation of CO on NiO and Cu2O surfaces electron transfer processes occur between the gas and solid phase, as evidenced by the solid-phase reaction. The excess oxygen contents of NiO would probably influenced the initial rate of oxidation [33,34]. The monovalent cation increases the valence state therefore the activation energy of the processes was decreases. The activation energy of reaction is the interaction of CO with oxide surfaces and controlling step in the high-temperature interval of reaction. The addition of foreign ions into the NiO lattice might modify the concentration and distribution of holes and electrons by suitable changes of the Fermi level in semi-conductor. The catalytic activity of amorphous NiCuO2 alloys depends on some surface changes. The Ni-based alloys with HCL acid treatment also observed very high activity for CO oxidation. After HCL treatment the alloy surfaces were covered with a thick oxide film, which could not be reduced during the CO oxidation [35,36].The oxide layer on the as-received alloy was thicker than that on the HCL treated alloy. The acid treatment and oxidation the surface becomes porous and surface areas are increased by 10 times. The increase of surface area leads to an increase in the number of active sites per unit area, and their activity. The formation of additional active sites is possibly associated to the presence of an oxide phase therefore; the catalytic activity of activated alloys was higher than that of the untreated alloys. The additional formation of active sites is associated to the presence of an oxide phase [37,38]. In catalytic activity, the major role is played by the structure and orbital conformity of the active centers with reagent molecules. At high temperatures, the oxidation of CO occurs by the reaction of adsorbed CO molecule with a lattice oxygen atom and oxide surface is initially reduced then oxidized by the atmospheric oxygen. The influence of particle size on catalytic activity has been subjected of continuous interest due to its importance in both primary and practical viewpoints. Future studies will focus on the development and utilization of Ni oxide catalysts for high catalyst selectivity. The achievement of NiO catalysts has encouraged huge amount of basic work dedicated to use the role played by each element and nature of active sites. The objective of present study is to review the factors that influence the CO oxidation reaction on special attention under the catalyst compositions, crystal size, pre-treatment and preparation conditions [39,40]. Table\u00a01\n\nNickel (II) oxide\u00a0is the\u00a0chemical compound\u00a0with the formula\u00a0NiO and its principal oxide of nickel. NiO\u00a0is a basic metal oxide. The mineralogical form of\u00a0NiO,\u00a0bunsenite is very rare. NiO\u00a0adopts the\u00a0NaCl\u00a0structure with\u00a0octahedral\u00a0Ni2+\u00a0and O2\u2212\u00a0sites. The simple structure is commonly known as the rock salt structure. Like many other binary metal oxides,\u00a0NiO\u00a0is often non-stoichiometric, meaning that the Ni:O ratio deviates from 1:1.\u00a0The NiO\u00a0was also as component in the\u00a0nickel-iron battery, also known as the Edison Battery, and is a component in\u00a0the fuel cells. Ni (III) oxide\u00a0is the\u00a0inorganic compound\u00a0with the formula Ni2O3 and also referred to as\u00a0black nickel oxide. Nickel oxide hydroxide\u00a0is the\u00a0inorganic compound\u00a0with chemical formula NiO(OH). It's a black solid that is insoluble in all the solvents but attacked by base and acid [41,42].\u00a0The crystalline structures of various NiO and NiCo2O4 unit cells are shown in the Fig.\u00a01\n.Nickel oxide\u00a0is highly insoluble and thermally stable so that it suitable for glass, optic and ceramic applications. It is a green crystalline solid and primary oxide of\u00a0nickel. Although it is rare in nature, several million kilograms are produced\u00a0annually.\u00a0The certain perovskite structured\u00a0oxides\u00a0are electronically conductive application in the cathode of solid oxide\u00a0fuel cells\u00a0and oxygen generation systems. They are compounds containing at least one oxygen anion and one\u00a0metallic\u00a0cation.\u00a0Ni oxide compounds are basic anhydrides and react with acids and strong reducing agents in redox reactions. Nickel oxide can be reacted with acids to form salts and other compounds e.g. nickel sulfamate for electroplating and nickel molybdate for hydrodesulfurization catalysts. The gold doped nickel oxide films can be used as transparent electrodes in an optoelectronic devices. The NiO is an attractive conversion reaction-based\u00a0on anode\u00a0material for its lower cost, nontoxicity and high theoretical capacity [43\u201345].Nickel oxide usually taken the relatively simple rock salt lattice. The natural\u00a0cleavage\u00a0plane of NiO is (100), and studies have shown that the resulting surfaces are high quality, relaxing the slightly ideal bulk terminated (100) surface. Structural determinations of adsorbents have been performed on both this surface and the polar (111) surface. To circumvent surface charging problems almost all of these studies have been performed on highly oriented NiO thin films. The NiO(100) thin film on the surface is the single crystals structures. The film preparation involved oxidation of Ni(100) surface at increasing temperature. The NiO(100) thin films are known to contain a high density of\u00a0surface defects, which could drastically affect the adsorption properties [46,47].Nickel is more familiar because of its use in pure\u00a0metal\u00a0or in the form of alloys for its many domestic and industrial applications. The nickel\u00a0constitutes\u00a0about 0.007 percent of Earth's crust and fairly common\u00a0constituent\u00a0of igneous rocks. The most important sources of nickel are\u00a0pentlandite found with nickel-bearing\u00a0pyrrhotite of which certain varieties 3 to 5% nickel. In Ni2O3(with nickel in the +3 oxidation state) which is roasted in air to give nickel oxide, NiO (+2 state), which is then reduced with\u00a0carbon\u00a0to obtain the metal. Nickel (atomic number 28) has high electrical and thermal conductivity and it's shown in the Fig.\u00a01. More than half of the nickel produced is used in\u00a0alloys\u00a0with iron (particularly in\u00a0stainless steels). Nickel is also used in electrically resistive, magnetic and many other kinds of alloys, such as\u00a0nickel silver. Finely divided nickel is employed to catalyze the\u00a0hydrogenation\u00a0of unsaturated organic\u00a0compounds\u00a0(e.g. fats and oils). Natural nickel consists of five stable isotopes: nickel-58 (68%), nickel-60 (26%), nickel-61 (1.20%), nickel-62 (3.60%) and nickel-64 (0.91%). It has a face-centered cubic\u00a0crystal structure. Nickel is ferromagnetic up to 358\u00a0\u00b0C or 676\u00b0F (Curie point). The metal is uniquely resistant to the action of alkalies and frequently used for containers to concentrated solutions of\u00a0sodium\u00a0hydroxide [48\u201350].Nickel reacts slowly with strong acids under ordinary conditions to liberate\u00a0hydrogen\u00a0and form Ni2+\u00a0ions. In its compounds nickel exhibits oxidation states of \u22121, 0, +1, +2, +3 and +4 though the +2 state is by far the most common. The Ni2+\u00a0produced large number of coordination numbers 4, 5, and 6 and all main structures are octahedral, trigonal bipyramidal, tetrahedral and square shapes. Nickel in the +2 state have a variety of applications like NiCl2, Ni(NO3)2\u00b76H2O, Ni(SO3NH2)2\u00b74H2O in electroplating baths. The NiSO4 or Ni2O3 highly used in nickel plating, fuel cells, storage batteries and electroplating baths and preparation of catalysts. In the sulfide nickel is in the +2\u00a0oxidation state, but in other compounds cited in the +3 state. The\u00a0nickel carbonyl compound, in which nickel exhibits a zero oxidation state, is used primarily as a carrier of\u00a0CO\u00a0in the synthesis of acrylates (plastics). It is colorless volatile liquid is formed by the action of CO on finely divided nickel and is characterized by an\u00a0electronic configuration\u00a0in which the nickel atom is surrounded by 36 electrons. This type of configuration is quite comparable to that of the noble-gas atoms [51,52].The size and morphologies of primary particles are often crucial factors in determining the catalytic performance of nickel oxides in structure-sensitive reactions. The small particles sizes are resultant more exposed surfaces are desirable in the catalytic oxidation reactions because sufficient activity sites can be accessed by the reactants. The oxygen movement of catalysts as shown in the Fig.\u00a02\n, which can be reflected by the reducibility, is decisive for initiating the oxidation reactions. The NiO catalyzed oxidation of CO and suggested that high concentration of Ni2+ could result in the weak Ni\u2013O bonds, which might be ensure that the good catalytic activity. The nano-sized Ni oxide catalysts with controlled surface properties such as size, shape, morphology, coordination, atomic arrangement and orientations are an important key to understand the catalytic reactions [53,54].Nickel is sometimes found free in nature but is more commonly found in the ores. The nickel atom has a radius of 124pm and a Vander Waals radius of 184pm.\u00a0The Ni0/NiO core-shell nanostructures are synthesized through a facile combustible redox reaction. The hetero-phase boundary with different crystalline orientations offered dual properties, which helped in bifunctional catalysis. Hexagonal Ni/NiO nanostructures as showed in the Fig.\u00a03\n manifested ferromagnetic behavior and catalyst could be collected easily at the end of catalytic reduction. The Ni/NiO core-shell catalysts at nanoscale had outstanding catalytic performance (reduction of 4-nitrophenol to 4-aminophenol) compared with pure NiO catalysts beyond a reaction time of ~9\u00a0min [55,56].The Ni nanocubes 15\u201340\u00a0nm diameter and high surface area contributes to the high turnover rate. From the activity order of various Ni nanocatalyst observed that the nanorods shape with highest catalytic activity and nanocubes shape Ni catalyst has shown lower catalytic activity. The activity order of CO oxidation over various Ni nanoparticle catalysts was as follows: Nanorods> Nanobelt> Nanowires> Nanoflowers> Nanospheres> Nanocubes. The high catalytic activity of nano NiO or Ni2O3 catalyst indicates that the oxidation state of Ni species may not be only cause for catalytic performances in CO oxidation [57,58].The Ni2O3 has high surface defects such as situation cluster, pits and more number of surface irregularity represents better activity for CO oxidation. The various Ni/O coordination ratios affect the number of deficiency sites, produced in various nano shapes. The lattice development can be represented by the occurrence of Ni2+ ions in this ionic lattice. Finally, observed that the Ni nanoparticles with small index surface planes and simple deficit production are attractive for the catalytic applications. The average diameter of Ni nanowires was varying from 80 to 90\u00a0nm, while the average length of Ni nanowires is equal to 5\u00a0\u03bcm. Nickel nanorods\u00a0are complete particles ranging from 20 to 110\u00a0nm with specific surface area (SSA) in the 30\u201360m2/g range. The nanoflower look constituted of small grains of approximately 50\u201365\u00a0nm which are assembled in a flower-shaped structure. The nanoflower size determined from the TEM investigations as discussed in the Table\u00a02\n follows normal distribution with a mean diameter of 42\u00a0nm [59,60].The nickel nanobelt prepared by the hydrothermal method and their morphology was confirmed by the TEM images (Fig.\u00a04\ne), also indicating size of 15\u201330\u00a0nm in thickness, 30\u2013160\u00a0nm in width and variable lengths up to microns. The Ni2O3\u00a0nanocubes\u00a0(25\u00a0nm size) inhomogeneous dispersions over the Ni2O3\u00a0nanocubes at the graphene nanosheets. The nickel spheres\u00a0are broad size particles ranging from 24 to 30\u00a0nm with specific surface area in the 40\u201350m2/g range. The transformation of nanospheres to nanorods seems to be caused by the irreversible binding of surfactant on the central region of growing nanoparticles. In summary nickel nanocubes, nanorods and nanowires were successfully synthesized by one-step reduction approach in a solvothermal environment. The reactions occur on the surfaces of NiO nanoparticles. Increasing the surface area of nanoparticles usually increases the rate of chemical reactions. TEM image of NiO nanoparticles represents that the non-spherical particle shape with smooth and uniform particle morphology, Fig.\u00a03F with average diameter (taken as average particle diameter) is nearly equal to 32\u00a0nm. The physicochemical properties of nickel nanoparticles are different as compared to the bulk counterparts owing to the fact that surface area to volume ratio increases and quantum effects as the size is decreases [61,63].The TEM analysis is used for the identification of internal composition of nanoparticles including their shape, size, distribution and defects. The NiO nanoparticles are studied broadly because of their electro-catalysis, high chemical stability, super conductance characteristics and electron transfer capability. The NiO is a p-type semiconductor metal oxide having a bandgap ranging from 3.6 to 4.0\u00a0eV depending upon the nature of defects and their density. It is an antiferromagnetic material having lower temperature TN of ~523\u00a0K and besides a high isoelectric point of ~10.7, it also shows high ionization. The development of a facile preparation process that allows convenient production of NiO nanoparticles is necessary for practical application. The oxidation of Ni2+ to NiO2 in initial conditions then transforming of NiO by treating with ethanol in the presence of a surfactant at room temperature.\u00a0Nickel oxide nanoparticle is prepared in rod and hexagonal shape by hydrolysis precipitation method from the solution of nickel chloride in the aqueous solution as a dissolving agent. The NiO nanoparticles, nanodots or\u00a0nanopowder\u00a0are white spherical high surface area metal particles. Nanoscale nickel oxide particles are typically 10\u201330 nanometers with specific surface area in the 130\u2013150m2/g range. The NiO films composing of nanoparticles maintained the porous microstructure and represents best electro chromic performance. The NiO associated with the injection or extraction of ions and electrons corresponding with the transformation between Ni2+ and Ni3+. The NiO films composing of nanoparticles maintained the porous microstructure and represents excellent catalytic performance [64,65].The catalytic oxidation of CO has long been studied on various NiO catalysts due to their high CO oxidation capacity and low cost in contrast to the expensive noble metal catalysts. The various Ni oxide catalysts have been observed for CO oxidation since has importance in the environmental protection. The catalytic activity for CO oxidation strongly depends upon the metal ion concentration on the surface and surface crystalline. Carbon dioxide is produced by the reaction of CO with oxygen adsorbed on the metal ions of catalysts surface. A surface concentration of oxygen was monitored by the subsequent reaction and partial pressures of the reactants. The oxygen species associated with Ni in the Ni2O3 catalyst are very active and may be dominated by the low-temperature catalytic oxidation of CO. The higher activity of Ni2O3 nanoparticles in CO oxidation was attributed to the small particle size, high surface area, high concentration of hydroxyl groups and more densely populated surface coordination to the unsaturated sites [66\u201368].In the study of CO oxidation over various bulks NiOx at low temperature represented that the ranking for CO oxidation in mixture of unit ratio of O2/CO by decreasing activity followed the sequence: Ni2O3 > NiO. The CO oxidation over NiO and Ni2O3 occurred via Langmuir\u2013Hinshelwood mechanism while over NiO in MvK mechanism represents. The more energy was needed for activating the CO\u2013Ni2+ bond in the reaction which accounted for low activity of NiO. By opposition, the high reactivity of Ni2O3 was applied not only for the moderate strength of CO\u2013Ni3+ bond but also abundant defects/oxygen vacancies on its surface phase [69,70]. The best catalytic activity of Ni2O3 nanorods might be associated with the high oxygen ad-species concentration and low-temperature reducibility. The surface morphology effect of Ni2O3 having hollow and solid sphere morphologies for CO abatement. The better catalytic activity by using hollow spheres of Ni2O3 was a result from beneficial effects such as morphology leading to the high surface area, higher Oads/Olatt molar ratio, believed to be proportional to the active oxygen and higher Ni average oxidation state. The NiO represents strong oxygen storage/release capacity due to the fact that they can easily undergo a rapid reduction-oxidation cycle through the interaction with reducing or oxidant agents accompany by the formation of nickel ions in the various oxidation states [71,72].The addition of lattice oxygen of catalysts incomplete oxidation of CO and strong correlation between labile lattice oxygen and catalytic activity suggest that the reaction could proceed via the MvK model. To improve the efficiency of NiO catalysts many strategies can be pursued, such as external morphology control, doping, optimization of the active Ni phase with the nature of support. Interestingly, the effect of surface morphology of Ni2O3 which was easily tune has to be deeply investigated may have a significant impact on the density of vacancies, of defects, as well as on the Ni average oxidation state and textural properties of the materials. The reaction of surface oxygen species with gas-phase CO is considered to be the rate-determining step in CO oxidation on the Ni2O3 catalyst. In Table\u00a03\n the light-off characteristic was representing that the activity of catalysts with the increase of temperature. The characteristic temperature T10, T50 and T100 represents that the initial oxidation of CO, half conversion and full conversion of CO respectively. The nickel oxide catalyst is also able for oxidation of CO at a low temperature due to the presence of lattice oxygen on their surfaces. The nickel forms very complex species with oxygen in the presence of cations and water make a characterization of supported nickel oxide catalysts. The surface area of Ni2O3 is more active than that of NiO in CO oxidation, whereas it failed insufficient surface oxygen species due to its lower specific surface area. Therefore, the redox properties and total activities of nickel oxides were influenced by both the crystal phases and textural properties [73\u201375].The NiO have produced highly dispersed on the catalyst surface, resulting in the formation of abundant surface Ni2+ ions. The Ni2+ ions partially substitute Co3+ ions to form Ni\u2013Co spinel catalysts, generating an abundance of surface oxygen vacancies, which are vital for CO oxidation. The Ni0.8Co0.2 catalyst represents highest catalytic activity and good stability for CO oxidation at 120\u00a0\u00b0C. The more cobalt content results in decline its activity, suggesting that the amorphous NiO phase is dominant active phase in place of Co3O4 for CO oxidation. The introduction of cobalt in Ni0.8Co0.2 catalyst can alter the morphology of catalyst from plate-like to flower-like structure and then to dense granules [20]. The Ce-supported Ni-Au catalyst has taken much interest due to its high reactivity on CO oxidation. The easily transformation between Ce3+ and Ce4+ nanosized cerium oxide applied to reducible oxide support to deposit the different metals or oxide. The introduction of Au and Ni ions are uniformly converted to deposited species on the surface of CeO2 nanorods during synthesis. The valence of Ni in AuNiCe catalysts remained same as Ni2+ after CO oxidation reaction. The reduction temperatures for Ni-containing species were much lowers in AuNiCe than the gold-free NiCe catalyst, representing that the interaction between Au and Ni oxide to form Au-O-Ni structure. The Ni deposited on CeO2 has completely lower level in CO oxidation at 20 and 120\u00a0\u00b0C and addition of metallic Au0 species favor for CO oxidation reaction over Ni-Au-Ce-O catalyst at 100\u00a0\u00b0C. The order of reaction for CO oxidation over Ni-Au-Ce-O catalyst is AuNiCe> AuCe> NiCe. The initial small-size Au\u03b4+ species interacting with Ni oxide and ceria support effectively kept the active Au atoms/clusters survived after the catalytic measurement. The main contribution of Ni oxide is to tune the electronic structure of active Au species in AuNiCe catalyst [23].All the nickel oxide catalysts, such as Ni/Al2O3, Ni-CeO2/Al2O3 and Ni/CeO2 are highly active for CO oxidation. The CO oxidation increased with the increase in temperature. The NiAl2O4 spinel structure is composite metal oxide produced by bonding between alumina as the support and NiO as the active material at high temperatures. It has been reported that the Ni metal sites on the Ni-oxide catalysts have high catalytic activity for CO oxidation. The CeO2 which has redox properties was added to the Ni-oxide catalysts to act as a promoter. As the content of CeO2 was increased, the surface morphology of catalyst has been changed; thus the CeO2 does cover the nickel surface [19]. The NiO catalyst showed high reactivity for CO oxidation below room temperature. The CO molecules reacted with oxygen at NiO to form reaction intermediates such as carbonate species can diffuse to the uncovered Al2O3 sites and these can be either reside on the uncovered Al2O3 or catalytically active NiO for gaseous CO2 conversion. With a lower NiO density, the reaction intermediates reside at the uncovered Al2O3 sites increase. The average NiO particle size of 1\u00a0nm or less than 1\u00a0nm and particle size was suggested to increase with increasing pre-annealing temperature. The chemisorptions of Ni2O3 catalysts are shown in the Fig.\u00a04. The initial catalytic activity and time-dependent change in activity of catalysts are highly dependent on the pre-annealing temperature at a reaction temperature of 30\u00a0\u00b0C [22].The rate of CO oxidation increases with lithium doping in Ni-oxide catalyst as compared to iridium doping. The preparation conditions and composition of catalyst is not positive in all the cases. The improved in catalytic activity with higher Ni3+concentration and relate to the activity with electronic property of the solids. The addition of lithium increases in the hole concentration of nickel oxide. The small amount of CO2 could be produced due to pre-adsorbed of oxygen on the catalysts surfaces. Therefore the precipitation lattice oxygen was moved out. To reduce the availability of holes at nickel sites that participates in the rate-determining step of reactions [24]. In NiFe2O4 catalyst the electron transfer between Fe2+ and Fe3+ ions and between Ni3+ and Ni2+ ions in the material. The smaller particle size in composite phase increases surface area with wide range of pore size distribution in the composite materials. The bi-modal size distribution in the composite phase with respect to mono-modal distribution over pure materials. This is probably due to the asymmetry in particle size with two different crystal structures [17].The TiO2 is a reducible metal oxide with several crystal structures. Titanium possesses a variety of oxidation states and titanium-supported nickel oxide catalysts have also used for converting CO into CO2. The Ni\u2013TiOx interaction can promote the catalytic activity and stability. Titanium supported nickel catalysts are more active in oxidation than silica or alumina supported catalyst. The surface oxygen present on the (NiO\u00a0+\u00a0TiO\u00a0\u2192\u00a0NiTiO3) catalyst was responsible for the formation of both CO and CO2. The NiO lattice oxygen (O2\u2212) and/or oxygen from the TiO2 support participated in the oxidation of CO to CO2. Pure TiO2 cannot oxidized CO but additional of reduced nickel oxide improved the CO oxidation process. The 8\u00a0wt.% Ni/TiO2 catalyst is complete oxidation of CO done at 50\u00a0\u00b0C. The side reaction producing CO2 from CO and oxygen was fast reacts [16]. The correlation between phase structures and surface acidity of Al2O3 supports over NiO catalyst is very important for their catalytic reactions. The high calcinations temperature not only affects the growth in Ni particle size, but also weakened the interaction between Ni nanoparticles and Al2O3 supports due to rapid decrease of the specific surface area and acidity of Al2O3 supports. The Ni/Al2O3 catalysts suffer from a series of drawbacks, such as sintering of the active Ni nanoparticles and supports due to the exothermic nature of CO oxidation reaction.A lot of efforts have been made to improve the stability of Ni/Al2O3, including increasing Ni loadings. As compared to \u03b1, \u03b2 the \u03b3-Al2O3 could act as effective supports for nickel oxide catalysts. Additionally, the variation of Al2O3 properties will further affect the dispersion of active particles of Ni and their metal-support interaction. The pore size distribution was suggesting that the growth of particle size and formation of large pores due to the inter-particle voids. All reduced catalysts mainly consisted of Ni2+with smaller fraction of Ni. The Ni particles were smaller in size are better dispersed on \u03b3-Al2O3 support than the larger size of Ni particles. The intensity of Ni 2p3/2 increased as the calcinations temperature of Al2O3 support was increased. The Ni particles are dispersed into the porous channels on low-temperature calcinations supports and also the oxidation of surface atoms screened some of Ni atoms. The CO oxidation is a structure sensitive reaction; the metal crystallite size together with the Physico-chemical property of support and simultaneously influences the catalytic performance of catalysts. The best catalytic performance due to the moderate particle size (20\u201340\u00a0nm) and metal-support interactions. Future study should be focused on the developing higher surface area \u03b1-Al2O3 with higher Ni loadings and promoters to further improve the catalytic performance [25]. The self-propagating high-temperature synthesis method has been produced highly active Ni-Cu\u2013Cr\u2013O mixed spinel catalyst for CO oxidation. The CuO reduction may be moderated by the presence of Cr ions on the surface. The various range of compositions reaction temperatures and catalytic activity of each material were measured for each particular process including the oxidation of CO. The catalytic activity may be improved if their surface area was increased. The high heating and cooling rates can produce defect structures with large lattice strains, often relieved by the formation of defects in the bulk or on the surface. The strong influence of point defects on catalytic behavior and oxide surfaces as showed in the Fig.\u00a05\n, point defects have been shown to act as active centers. The relative ease of preparation, high thermal and chemical stability and good catalytic activities of new oxide catalysts offer promise for environmental applications [18]. The catalytic activity of amorphous NiCuO2 catalysts also depends on the some surface changes and morphology of untreated alloy also changes the following activation. The catalytic reactions occur on thin oxide film, which is not destroyed after the catalytic reaction. After the acid treatment and oxidation over surface becomes porous and surface areas were increased. The increase of surface area leads to an increase in the number of active sites per unit area and affects the activity. The formation of additional active sites is probably associated to the presence of an oxide phase and therefore, the catalytic activity of the activated alloys is higher than that of the untreated alloys. The additional formation of active sites is probably associated to the presence of an oxide phase [15].The correlation between phase structures and surface acidity of Al2O3\u00a0supports calcined at different temperatures are very important in the catalytic performance of Ni/Al2O3\u00a0catalysts for\u00a0CO oxidation. The phase structures and surface acidity of Al2O3\u00a0supports are adjusted by their calcinations conditions. The high\u00a0calcinations\u00a0temperature not only led to the growth in Ni particle size, but also weakened the interaction between Ni\u00a0nanoparticles\u00a0and Al2O3\u00a0supports due to the rapid decrease of the specific surface area and acidity of Al2O3\u00a0supports. The Ni-O catalysts supported on Al2O3\u00a0calcinations at 1200\u00a0\u00b0C represents that the best\u00a0catalytic activity\u00a0for CO oxidation under the different reaction conditions [21]. The more oxygen content of nickel oxide and probably influences the initial rate of oxidation. The impurity affects directly the oxidation reactions by changing the overall activation energy. The monovalent cation increases, while cations with a higher valence than decreases the activation energy of processes. The addition of nickel over CuMnOx catalyst highly effects on their structural properties and catalytic performance for ambient conditions CO oxidation. The promoted NiO has an efficient catalyst for CO oxidation. The addition of small amount of Ni in CuMnOx catalyst exhibited improvements in the carbon tolerance properties and activity also. Addition of Ni can reduce the size of CuMnOx catalysts; therefore, the huge amorphous NiO phases have been produced and mostly dispersed over the catalyst surfaces, resultant in the production of rich surface. The higher deviation in surface areas and total mesopores volume is affected by the catalytic performances. The complete conversion of CO was achieved at 75\u00a0\u00b0C over CuMnNiOx catalyst, which was lower by 10\u00a0\u00b0C over CuMnOx catalyst. The performance of catalysts synthesized in RC conditions for CO oxidation was as follows: CuMnNiOx > CuMnOx. The CuMnNiOx catalyst has represented that the best catalytic activity towards CO oxidation at ambient conditions. The occurrence of uniform pore size distribution on CuMnNiOx catalyst surface was the main reason of increasing catalytic activity [26].The efficiency of nickel oxide catalysts for reactions with CO molecules is strongly dependent on the chemisorptions process. The chemisorptions of CO gasses is very important step, which increases the concentration of reactant on the catalyst surfaces which chemisorbed on CO molecules applying on the more energy to be easily obtain the chemical reactions. The distinct reaction mechanisms are stable with the observed kinetics. The initial reaction mechanism represents that the highly accepted CO oxidation reaction on catalyst surface that participants the O2 adsorption to produced O2* precursors, which divide on a vicinal vacancy. In the second mechanism, the O2 activation occur via the kinetically applicable with CO*-assisted O2 dissociation step lacking of the definite conditions of stable O2* precursors. In the CO oxidation process, the oxygen was first adsorbed on the catalyst surfaces with the energy of activation. When the temperature is increases in certain amount so that the adsorption of oxygen reaches on certain proportions, therefore any CO passing over the catalyst surfaces either reacts directly with the adsorbed oxygen or initially adsorbed then reacts, after which the CO2 being produced was desorbed. The performance of Ni2O3 catalysts for CO oxidation reaction is measured on the activation energy of procedure [50\u201352]. The activation energy data are very helpful for modeling and planning of catalytic converter. The CO molecules and O atoms initiate to disperse on the catalysts surface and once a CO molecule and O atom combines each other, they recombine and produced CO2. A catalytic reaction on the surface of catalyst and gas surface interface includes the observation of gas adsorption, dissociation, diffusion and desorption. The mechanisms of CO oxidation on the surfaces of nickel oxide catalysts are top tactic in nature as shown in the Fig.\u00a06\n, thus responsible for loss and uptake of bulk oxygen for production and disappearance of vacancies on these systems as attractive oxidation catalysts [53,54].The amount of CO2 molecules chemisorbed corresponded to the amount of oxygen atoms pre-adsorbed on the catalyst surfaces. The equal concentration of oxygen atoms in the gas phase over surface, therefore, the heterogeneous exchange reaction was taking places. In stable oxides the rate-determining step will be the reaction between COads and Oads, while for nickel oxide that has an inferior M-O bond energy, the rate-determining step will be the adsorption or dissociation of molecular oxygen on the metal surface [55,56].\n\n(1)\nO2 \u00a0 + \u00a02 * \u00a0 \u2192 \u00a02Oads\n\n\n\n\n\n(2)\nCO + * \u00a0 \u2192 \u00a0COads\n\n\n\n\n\n(3)\nCOads \u00a0 + \u00a0Oads \u00a0 \u2192 \u00a0CO2 \u00a0 + \u00a02 *\n\n\nThe molecular chemisorptions of CO discussed in the Eqn (1-3) can be done at the higher temperatures, which represents that the appearance of reactive oxygen forms. Where * represent a free site on the metal surface. The CO2 produced is simply adsorbed and does not influence the rate significantly, since it's rapidly desorbed into the gas phase. The reaction rate will be proportional to the surface exposure of Oads and COads. The CO is reacting with chemisorbed oxygen either by adding on it from the gas phase or produced an adsorbed state subsequently to the adsorbed oxygen. In the reaction conditions, the rate was proportional to the O2 pressure and independent of CO pressure. The rate of CO oxidation was determined by following the rate of production of CO2 when the CO was adsorbed and rate of disappearance of CO. The COad was oxidized by the adsorbed oxygen (Oad), which was the rate-determining process. Therefore, the mobility of active oxygen (Oad and/or OL) is crucial for CO oxidation. The more surface area and huge mesoporosity of nickel oxide catalyst can be easily access of reactants to the active sites and diffusion of products thus from CO2 on the catalyst surface. The CO molecule is an electron-donor probe during the adsorption, so that the active interface oxygen species may adsorbs the CO molecule [57,58].The intensity of Ni2+/3+\u2013CO at ambient conditions is slightly weaker than that of only CO adsorption as represents in the Fig.\u00a07\n, which could be the preferential adsorption of O2 molecules on the surface of catalyst to form O2\u2212 species in the oxygen vacancies, restraining in the adsorption of CO. This process of electron transfer activates the lattice oxygen availability on the Ni species. The surface capping oxygen and lattice oxygen vacancy are main oxygen sources for this reaction. An oxygen vacancy was created when the adsorbed CO picks of oxygen from the Ni2O3 surface, probably capping oxygen. In the kinetic measurements the mass of carbon was also balanced i.e. decrease in CO corresponds to the formation of CO2. In the Eley\u2013Rideal mechanisms the dissociative adsorption of O2 on the active Ni sites, followed by the reaction of surface oxygen with gaseous CO and producing CO2. Furthermore, a mean-field model was constructed for several modeling and simulation of CO oxidation, as well as calculation of the Ni2O3 surface coverage. The most important reactions Eqn (4-5) for CO oxidation over the catalysts as followed. Another redox reaction is highly\u00a0exothermic reaction with\u00a0the aluminum oxide support. The function of substrate is not constant the small supported particles, but smooth to the advance adsorption and activation of oxygen. The exact reaction mechanism for two-stage reduction behavior of Ni2O3 is still ambiguous. The Ni2O3 has tetragonal distorted spinel structure, which contained different types of Ni-O bonds. The two-stage reduction of Ni2O3 could be attributed to the different reducibility of Ni-O bonds [58\u201365].\n\n(4)\nCOads \u00a0 + \u00a0Oads \u00a0 \u2192 \u00a0CO2 (g)\n\n\n\n\n(5)\nNi2O3 \u00a0 + \u00a03 CO \u00a0 \u2192 \u00a02 Ni \u00a0 + \u00a03 CO2\n\n\n\nFor non-reducible supports, such as SiO2 and Al2O3, the catalytic performances were much strongly dependent on the Ni2O3 diffusion as represents in the Eqn\u00a0(6); in fact, the oxygen adsorption has done over the metal sites. Thermodynamic results represent that the total oxidation of CO into CO2 is always done in the present conditions, provide the proofs of kinetic restriction.\n\n(6)\n2 Al\u00a0+\u00a0Ni2O3\u00a0\u2192\u00a02 Ni\u00a0+\u00a0Al2O3\n\n\n\nOn the basis of above explanation, the reduction of most reducible Ni-O bonds, the crystal structure of neighboring atoms would immediately undergo some transformation and postulate that the transformed structures could be more stable, therefore requiring a higher temperature for further reduction. After the surface layer or sub-layers of the Ni2O3 particles are initially reduced then formed relatively stable NiOx species would cover the inner core as shown in the Fig.\u00a08\n. Despite of the Eley\u2013Rideal mechanism in particularly CO oxidation, the Langmuir\u2013Hinshelwood mechanism was considered kinetically favored from the fundamental standpoint [65\u201368].The high activity could be associated with the huge specific surface area, abundant surface oxygen species and excellent low-temperature reducibility. The activity of Ni2O3 catalysts with similar crystal structures decrease significantly with increase in the calcinations temperature. The Mars-van-Krevelen mechanism is operational in the Ni2O3 catalysts for oxidation reactions, which involves the process of releasing and replenishing lattice oxygen. Therefore, the reducibility of catalyst will be closely related to the catalytic activity. The surface defect sites represent that high oxidation activity because of catalyst closely related to the catalytic activity. The dissolution was assigned Ni3+ to Ni2+ surface reduction and reconstruction was same for all nickel oxides because all oxides and metal ions in electrolyte follow the same thermodynamic standard potentials. A pure nickel oxide metal may not be suitable for practical applications because of its defects. However, the property of metal oxide can be improved by introducing foreign metal cations into its lattice. There will be also an interaction between different metal oxides [69\u201372].Nickel mainly acts as adsorption center for CO. The Ni2+\u00a0receives an electron from CO or CO2\u00a0and will be reduced into Ni0. Then the reduced Ni0 would be restored into Ni2+\u00a0by extra oxygen then the next cycle starts. To obtain the independent kinetic parameters for adsorption and desorption of O2 and CO2 were modeled numerically. In Ni oxides the most important challenges is to obtain a single-phase because in all the procedures to obtained result is significant for core-shell structures. To improve the activity of Ni-oxides the oxygen defects have been introduced by thermal reduction which reduces Ni3+\u00a0to more active Ni2+\u00a0and improves the conductivity as shown in the Fig.\u00a09\n. The model of Eley\u2013Rideal mechanism represented that the CO oxidation by a Ni2O3-SiO2/Al2O3 catalyst in the absence of O2. Furthermore, the quantum mechanical calculations from the (100) surface, the CO oxidation on nanosized Ni2O3 occurs through an Eley\u2013Rideal mechanism, whereas on the (011) plane, a Langmuir\u2013Hinshelwood type mechanism contributes. Despite of the aforementioned arguments for Eley\u2013Rideal type mechanism in particularly CO oxidation, the Langmuir\u2013Hinshelwood mechanism was considered kinetically favored from a fundamental standpoint [73\u201375]. The surface complexes are produced by interaction of CO2 with surface oxygen in close proximity to Ni sites. The present kinetic model is based on the different types of active nickel sites are supposed to be equal. The adsorbed CO was oxidized by the high valence state Nin+\u00a0cations, on the catalysts' surface to form adsorbed monodentate nitrate (Ni(\n\nn\n\u00a0\u2212\u00a01)+\u2013O\u2013CO) and metal cations are reduced as Ni(\n\nn\n\u00a0\u2212\u00a01)+. The lower valence state of Ni oxides is also active for CO oxidation; the Ni3O4 is rich in electrochemical properties due to the mixed-valence of Ni. The dependence of OO bond length on the oxidation state of Ni given the limited number of structurally characterized of Ni2O3 compounds. The Ni compounds in such higher oxidation states are usually unstable and decompose to release oxygen atoms. The mechanism of NiO catalysts for CO oxidation is shown in the Fig.\u00a010\n. To obtain the independent kinetic parameters for adsorption and desorption of oxygen and carbon dioxide were modeled numerically [76\u201378].In order to further investigate the nature of surface reaction mechanism of NiO catalysts, the in situ of CO and O2 co-adsorption are obtained under the CO\u00a0+\u00a0O2 reaction conditions. As far as the catalyst development was concerned, it is critical to explore the structure-activity correlation of catalysts. When the surface\u2013adsorbed CO reacts with activated O over the Ni-oxide catalysts, it follows the Langmuir\u2013Hinshelwood (L\u2013H) mechanism. The O2 molecules preferentially adsorb on the nickel oxide catalyst surface in an oxygen-enriched atmosphere, forming surface active oxygen species (such as O2 or O), which occupy the surface vacancies. The CO molecules are adsorbed on the surface of NiO to form Ni2+\u2013CO species and adsorbed CO reacts with the active oxygen species on surface oxygen vacancies and transformed into gaseous CO2. Finally, the surface oxygen vacancies are regenerated by gaseous O2 and completing the catalytic cycle direct surface oxidation mechanism. The reaction over reduced Ni/TiO2 catalyst takes place via the direct surface oxidation reaction mechanism, in which the adsorbed CO and oxygen species are involved. The CO can be oxidized by oxygen in NiO or by active oxygen within the TiO2 support via the non-selective oxidation mechanism over oxidized Ni/TiO2 which may contain NiO and NiTiO3\n[79,80]. The CO is oxidized via a direct oxidation mechanism only when the nickel is reduced to Ni0 or when its surface oxygen species concentration is very low. The CO is oxidized by lattice oxygen in NiO or by active oxygen in the TiO2 support via the non-selective mechanism over oxidized Ni/TiO2, while it's efficiently converted into CO2 via a direct oxidation mechanism when Ni/TiO2 is reduced or partially reduced. The highly dispersed Ni particles were formed on it, which can strongly interact with the support. The CO oxidation process is highly exothermic and thermodynamically favorable reaction. The \u03b1-Al2O3 is a very promising support candidate for CO oxidation catalysts even at low Ni loadings [81,82].The NiAl2O4 spinel structure is a composite metal oxide produced by bonding between alumina as the support and NiO as the active material at high temperatures. It has been reported that the Ni metal sites on the Ni-oxide catalysts have a high activity for reforming hydrocarbons. The kinetic equations which were found to describe the reaction on pure nickel oxide are also operative in the case of nickel oxide catalysts, containing foreign atoms. In the low-temperature regions all catalysts operates practically with the same value of activation energy. This indicates that in these temperature intervals the added ions do not affect directly the catalytic processes. The modification of excess oxygen contents of nickel oxides and influence the initial rate of oxidation. The higher temperatures impurity presence directly influences the initial rate of oxidation reaction by changing the activation energy [83,84]. The value of activation energy depends upon the functions of various ions added. The introduction of monovalent cations increases, while cation with a valency higher than two decreases the activation energy of the processes. The reverse effects are obtained by introducing foreign anions into the nickel oxide lattice. The CO behaves like as an electron donor and possible to conclude from the effect of impurities on the activation energy of oxidation reaction that the interaction of CO with oxide surface is the controlling step in the high-temperature interval of the reaction. The addition of foreign ions into the nickel oxide lattice might be modifications the concentration and distribution of electrons by suitable changes of the Fermi level of the catalysts [85,86].This can be associated to the experimentally observations by the additions of activation energy in the heterogeneous processes involving formation and/or destruction of acceptor and donor levels in the catalysts. Although the effect has been observed on the overall activation energy of heterogeneous reactions process occurred on the catalytic materials. In the nickel oxide catalysts treated with HNO3 acid shows very high activity in CO oxidations. During acid treatment the nickel alloy surfaces were covered with a thick oxide film, which could not be reduced during the CO oxidation. The increase of surface area leads to an increase in the number of active sites per unit area and also affects the activity. The different treatment leads to a more surface area and higher number of active sites per unit area. The additional formation of active sites is probably associated to the presence of an oxide phase. The creation of more active sites is probably associated to the presence of an oxide phase and catalytic activity of activated alloys is higher than that of the untreated alloys. The amorphous nickel alloys represent unique and original compositions and surface structures to the reacting molecules unlike conventional crystalline metals [87,88].They possess several properties: a high reactivity due to their metastable structure, a high density of low co-ordination sites and defects, chemical homogeneity and easy reproducibility, which make them interesting materials in the heterogeneous catalysis. Most of the amorphous alloys show exceptional activity and selectivity in the CO oxidation reactions. The activation energy not only depends on the surface composition and structure, but it mainly depends on the number of active sites, whose nature is similar. The main reasons for high CO oxidation activities measured on both the actual composition of products as well as the highly defective structure of new materials [89,90]. The high heating and cooling rates can produce the defect structures with large lattice strains, often relieved by the formation of defects in the bulk or on the surface. The strong influence of point defect improved on the catalytic behavior. The relative ease of preparation, high thermal and chemical stability and good catalytic activities of the nickel oxide catalysts present assure for the environmental applications. The angular orientation of CO on NiO(l11)(1\u00a0\u00a0\u00d7\u00a0\u00a01) has also been dependent of the \u03c0*/\u03c3* resonances in the CO was adsorbed on the (100) nanofaceted octopolar reconstruction, with its orientation found on NiO(100). This geometry gives a tilt angle for CO away from the NiO(111) surface normal, which is within the uncertainty of the experimental value. The assessment of synthesis conditions and properties of small Ni-O clusters is at the limits of capabilities of contemporary experimental technique. To understanding and interpretation of the available and emerging experimental results must be based on\u00a0the theoretical studies to represents the physical and chemical mechanisms behind observed properties of such systems [90\u201392].The performances and selectivity of nickel oxide catalysts in catalytic converter are crucial for CO oxidation reaction. The catalyst deactivation and failure over time in catalytic performances is creating problem in the practice of catalytic process. The major reason for catalyst deactivation is divided into three parts: Chemically, mechanically and thermally. The lead, sulfur poisoning, carbon formation and sintering is the major cause of catalyst deactivation. The dispersion of active phase rapidly decreases, which is one of the major reason for catalyst deactivation. The initial decrease in catalytic activity can be recognized to the formation of carbonate species on the catalyst surface and occupation of active sites on the catalyst by CO2 and moisture. The deactivation\u00a0is main reason for failure of catalytic surface area due to the crystalline development of catalytic phase [93,94].The main reason of poisoning is due to the highly adsorption of feed impurities; therefore, the poisoned catalysts are very tough to regenerate. The sulfur poisoning and thermal degradation of catalyst is one of the main reasons for Ni-oxide catalysts deactivation. The poisoning is highly effects of reactants, impurities present on the sites or existing for the catalysis. The deposition of chemical poisoning on nickel oxide catalysts surfaces cause of deactivation is shown in the Fig.\u00a011\n. The nickel oxide catalysts become \u201cpoisoned\u201d when their surfaces are covered by carbon species formed during the reactions with carbon-containing molecules, such as when the CO dissociates into carbon and oxygen. Carbon deposited on the catalyst surface blocks the active sites and prevents further reactions from taking place, thus \u201cpoisoning\u201d and ultimately deactivating the catalyst. Due to consumption of SO2 caused by the formation of sulfate is one of the main reason of Ni2O3 catalyst deactivation [95,96].The other important sources of nickel oxide catalysts deactivation are ammonia in high temperature. In high-temperature oxidation of ammonia represented that the catalyst undergoes phase and chemical transformations mainly to the formation of low-selective nickel oxide catalysts. The deactivation processes are showed in the Fig.\u00a012\n and it was accompanied by structural changes: recrystallization and decrease in the specific surface area of system. Under the critical conditions of reaction (catalyst decay), recrystallization processes and decrease in the specific surface area of catalyst diminish its limiting load. The deactivation of catalyst can be used in developing theoretical and practical foundations for design of high-performance catalysts for CO oxidation [97,98].The deactivation caused by water vapor can be contributed to the competitive adsorption. The installing of reactor downstream of the desulfurized and precipitator is an excellent way to avoid the deactivation. When the excess O2\u00a0present in the flue gas, the trace residual SO2\u00a0can be oxidized into SO3, a reaction catalyzed by the metal active sites. The transformation of NiO into NiSO4\u00a0on the NiOx/Al2O3\u00a0catalyst significantly deactivated the catalyst's activity. The deactivation process of SO2\u00a0would be improved in the case of moisture vapor. The deactivation of Ni-oxide catalysts may be explained by the chemical interaction of Ni-oxides and support materials resulting in the formation of inactive phases of Ni cations [99,100]. The Ni/Al2O3 catalysts suffer from a series of drawbacks, such as sintering of the active Ni nanoparticles and supports due to the exothermic nature of CO oxidation reaction and severe choking. Therefore, a lot of efforts have been made to improve the stability of Ni/Al2O3, including increasing Ni loadings. The variation of Al2O3 properties will further affect the dispersion of active particles and metal-support interaction. The activity of reused Ni catalysts highly depends on the catalyst's chemical composition and recycling conditions. Ideally, a Ni catalyst should be reused in various times as possible without any treatment. The Ni particles are severely sintered, as the crystal size was increased from about 3\u00a0nm to 16\u00a0nm. The small Ni nanoparticles are easily deactivation of supported metal catalysts is due to severe coke formation in catalytic CO oxidation processes, which is highly influenced by the nature of active metal and support, the dispersion and particle size of active component, metal-support interaction, as well as the reaction conditions [101,102]. The carbon deposition occurs faster on small particles and more readily on Ni step planes. For Ni/\u03b1-Al2O3, due to the reduction in surface area and acid sites of Al2O3 supports, Ni oxide particle size increases with the weakening of metal-support interaction and becomes more and more reducible. As a result, Ni/\u03b1-Al2O3 is the most active, stable and coke-resistant catalyst among all the Ni/\u03b1-Al2O3catalysts, which can be attributed to its stable, nonporous and non-acidic support with low surface area, as well as easily reducible Ni species. The \u03b1-Al2O3 is a very promising support candidate for CO oxidation catalysts even at low Ni loadings [103,104].The nickel oxide catalysts regenerations processes are shown in the Fig.\u00a013\n as applied for total removal of blinding layers, SO2 to SO3 conversion rate, mechanical constancy and deactivation rate in all the regenerated catalyst are better than the other catalysts. The off-site regeneration processes are more sophisticated and demanding than on-site rejuvenation processes; it offer more efficient cleaning and reconstitution of catalyst with complete improvement of activity\u2014sometimes better than the fresh catalyst performances. The fresh catalyst activities by removing the carbon deposit and returning the sintered Ni2O3 catalyst particles close to the optimum size. Sintering is highly removed by reducing and controlling the temperature of reaction, although the new developments have pay attention on the encapsulating metal crystallites to remove the mobility, while remains allowing for the entrance of reactants and products [105\u2013107]. To reduce the deactivation of nickel oxide catalyst, added a certain amount of support materials like SiO2, TiO2 and Al2O3 into the nickel oxide catalyst and also increases the lifetime of catalyst. The regeneration treatment processes by which the carbon was burning off the Ni2O3 catalyst with the aid of oxidized gasses. However, the oxidative regeneration of Ni2O3 catalyst if not carefully controlled with respect to the oxidation conditions may frequently placed an undesirable over-oxidation of the Ni component of catalyst. To regenerate the spent Ni-oxide catalyst at the higher temperatures in the presence of reducing gas in a fluid solids regenerator. The self-regenerating effect of catalyst with steams as dilutions agents for the reactants. The nitrate NO3\u2013, sulfate SO4\n2\u2013 and ammonium NH4\n+ are the three main components on the poisoned of Ni-oxide catalysts. The water washing, thermal regeneration and reductive regeneration were used to regenerate the catalytic activity of Ni/\u03b1-Al2O3 catalyst. The regeneration of deactivated Ni2O3 catalysts is highly dependent on the chemical, economical and environmental factors [108\u2013110].The nickel oxide is one of the best transition metal oxide catalysts for low-temperature CO oxidation. The synthesis and calcinations strategies are highly influences the performance of Ni2O3 catalysts for CO oxidation. The relative ease of preparation, lower cost, high thermal and chemical stability and more activity of Ni2O3 catalysts offer better performances for automobile vehicle pollution control applications. The addition of suitable promoters and would results to improvement in the performances of nickel oxide catalysts towards CO oxidation. The CO oxidation is highly influenced by the crystal size of nickel oxide catalysts and increases with reducing the crystal size of catalyst till certain limit and further CO oxidation conversion decreases. After the review of all papers observed that the Ni/TiO2 catalyst is highly active for total oxidation of CO at 50\u00a0\u00b0C temperature. The results analysis show that the catalytic activity of pure Ni2O3 is not very high in the CO oxidation, but the addition of Co3O4/Fe2O3/CeO2/TiO2 significantly increases the catalytic activity at low temperatures. On the basis of studying the isotope exchange of oxygen and position of the Fermi level in the system MiO\u2212Ni1\u2212xO, an explanation was proposed for the compensation effect in the reaction of CO oxidation on nickel oxide\u00a0catalyst. In deactivation analysis observed that the Ni-oxide catalyst easily regenerated without loss of catalytic activity and gave equal turnover rate as the fresh catalyst. There are lots of papers available for CO oxidation over Ni-oxide catalysts, but this review paper provides important information about the pure and substituted Ni-oxide catalysts for CO emissions control.None.", "descript": "\n                  The low-temperature catalytic oxidation of carbon monoxide (CO) is very important process for all human health protection systems. The major sources of CO produced into the environment are automobile exhaust, so that the various types of catalysts are used in the catalytic converter for oxidation of CO. As compared to noble metal oxide catalysts the transitional metal oxide catalysts are very active, lower cost, easily available and fast regenerated. Among the various transitional metal oxide catalysts the nickel oxide (NiO) is one of the best catalysts for CO oxidation at a lower temperature. A small amount of NiO was deposited on mesoporous Al2O3 using atomic layer deposition and subsequently oxidation at different temperatures. Furthermore, as the pre-annealing temperature increased and improved resistance towards poisoning due to the CO oxidation was observed. The Ni/TiO2 catalyst show that the best efficiency in selectivity, performances and stability in the heterogeneous catalysis. The performances of NiO nanoparticles are highly dependent on the crystallite size, surface area and pore volume of the catalysts. The certain attention has been paid on recovery of Ni nanoparticle catalysts from reaction systems and their reuse for several times without losses of catalytic activity in CO oxidation. This investigation will shown scientific basis for potential design of Ni nano-particle catalysts for CO oxidation.\n               "}
{"full_text": "\n1. Introduction\nWaste plastic is a kind of organic polymer solid waste rich in carbon and hydrogen, and its typical types are polyethylene and polypropylene [1]. Compared with the traditional processing technology of waste plastics, pyrolysis technology has prominent advantages. Pyrolysis technology is the process of depolymerization and reforming of waste plastics under thermochemical action to produce hydrogen-rich gas [2]. This technology can provide the harmless and resource-based treatment of waste plastic, with outstanding advantages of safety, environmental protection, high efficiency and energy savings [3]. In recent years, the catalytic pyrolysis of plastics to produce hydrogen-rich gases has attracted extensive attention [4,5]. Catalysts play an important role in the reforming process of plastics to produce hydrogen, which can help the long chain break into short chains and break chemical bonds to promote the formation of small molecular gas products [6,7]. Therefore, an efficient and stable catalyst is of great significance, but a much larger problem exists that is closely related to the catalyst itself [8]. In the reaction, numerous olefins generated by pyrolysis easily form coke and deposit on the surface of the catalyst; this results in catalyst inactivation, also known as catalyst poisoning [9]. Therefore, to effectively avoid catalyst poisoning, slow the catalyst inactivation rate and find a simple method of catalyst regeneration are important research directions for hydrogen production from waste plastics [10]. Moreover, achieving effective bond breaking of macromolecular organic compounds such as olefins and generating more small gaseous molecules are the key steps to increasing hydrogen production [11].In terms of catalysts, precious metals such as platinum and rhodium have good catalytic performance, but their high price limits their wide application [12]. Inexpensive transition metals such as iron and nickel also have excellent catalytic activity [13], and their catalytic activity increases with increasing reduction degree [14,15]. The application of transition metals in the cracking of plastics has also attracted attention [16,17]. Yao et al. [18] compared the catalytic activity of iron and nickel in the cracking of plastics, and they found that iron catalysts have better gas and carbon production performance for single metal catalysts, with hydrogen and carbon nanotube production rates of 22.9 mmol H2/gplastic and 195 mg/gplastic, respectively. Cai et al. [19] specifically studied the conversion of different types of plastics by iron catalysts; their study showed that various plastics can be converted into high-value hydrogen, liquid fuels and carbon nanotubes with iron catalysts. In another paper [20], the catalytic cracking of polypropylene with an iron catalyst supported by alumina was described in detail, and the effect of iron active substances on the yield of gas, liquid oil and solid carbon was explored. Ann et al. [21] also confirmed that the presence of Fe in the catalyst could promote the selectivity of deoxidation products, promote demethoxidation, demethylation and deoxidation reactions in the process of volatile catalytic reforming, and significantly improve the selectivity of phenol and H2 formation. Many studies have shown that the active material iron has excellent catalytic performance in the catalytic reaction of hydrogen production from plastic cracking [22\u201324].The catalytic performance depends not only on the active components but also on the performance of the catalyst support, especially the physical and chemical properties and surface structure of the catalyst support [25,26]. Molecular sieves are usually selected as cracking catalyst carriers due to their conventional pore structure and high specific surface area, but they have poor hydrothermal stability and high cost [27]. Most pyrolysis of plastics for the production of hydrogen occurs under the condition of water vapour. Therefore, the hydrothermal stability of the catalyst support is an important parameter of the catalyst [28]. Activated carbon has a large specific surface area and developed pore structure. Additionally, due to the stability of the activated carbon structure, it shows strong chemical stability and hydrothermal stability in the reaction; therefore, it is widely used in the study of hydrogen production from methane [29,30]. The surface chemistry of activated carbon determines the initial catalytic rate, and the pore structure determines the stability of catalysis [31]. Therefore, mesoporous carbon with a larger specific surface area and pore capacity shows better catalytic activity and stability in the catalytic hydrogen production reaction [32].Based on the good performance of transition metal iron and carrier activated carbon in catalytic hydrogen production, this study develops a new activated carbon-based iron catalyst for the catalytic cracking of polymer waste plastic to produce hydrogen. The supported activated carbon-based iron catalyst was prepared by using the wet impregnation method. Amorphous carbon was chosen as the carrier because it contains two-dimensional graphite layers or three-dimensional graphite microcrystals with very small diameters and a large number of irregular bonds on the edges of the microcrystals [33]. This characteristic is conducive to its combination with active substances. Compared with the regular form of carbon, the prepared catalyst has better catalytic performance and stability. The experimental instrument used is a self-developed two-stage tubular furnace. The effects of the amount of iron, the ratio of raw material to catalyst and the amount of water vapour on the production of hydrogen from plastic catalytic cracking were studied. Surface microscopic analysis, material structure analysis and thermogravimetric analysis of the new activated carbon-based iron catalyst were carried out. Additionally, the related characterization of the spent catalyst was carried out to analyse its carbon deposition and deactivation.\n2. Experimental\nRaw polypropylene (PP) with a particle size less than 5 mm was purchased from Shanghai Myrell Chemical Technology Co., Ltd. (China). The catalyst support amorphous activated carbon was in powder form and purchased from Guangzhou Xiting Experimental Equipment Co., Ltd.Iron nitrate hexahydrate (Fe(NO3)3\u00b79H2O, AR, 98%), the experimental cylinder gas nitrogen (N2, purity>99.99%) and the chemical reagent isopropyl alcohol (AR, 99.5%) were purchased from Shanghai MacLean Biochemical Co., Ltd., Guangzhou Shengying Gas Co., Ltd., and Guangzhou Chemical Reagent Co., Ltd., respectively.The powdered activated carbon (AC) was initially sieved to obtain a powder with a diameter of 0.15-0.18 mm. Its physical and chemical properties are shown in Table 1\n. The contents of ash, volatile and fixed carbon are 25.26 wt.%, 14.07 wt.% and 60.66 wt.%, respectively. AC is mainly carbon and contains a small amount of hydrogen and oxygen. The preparation method of the activated carbon-based catalyst was the wet impregnation method. The detailed experimental method was as follows: a certain proportion of AC and Fe(NO3)3\u00b79H2O were dissolved in 200 ml of deionized water, and the two were completely dissolved after stirring thoroughly. The solution was placed on a magnetic stirrer and stirred at a speed of 600 rpm for 5 hours. The stirred solution was then placed in a drying oven for evaporation of water to obtain the catalyst precursor. After grinding, the precursor was calcined in a tube furnace under an inert atmosphere. The catalyst was obtained by calcination at 900 \u00b0C for 2 hours. The catalysts are named 5Fe/AC, 10 Fe/AC, 15 Fe/AC and 20 Fe/AC according to the proportion of active material iron.Thermogravimetric mass spectrometry (TGMS) (NETZSCH STA 449 F5, NETZSCH QMS 403) was used to detect the weight loss characteristics of the catalyst. The surface morphology of the catalyst was characterized using a Hitachi S-4800 scanning electron microscope (SEM). The inside structure of the catalyst was examined with a high-resolution transmission electron microscopy (HR-TEM) using a JEOL JEM-2100HR (200 kV). Powder X-ray diffraction (XRD) analysis of the catalysts was performed by using an X\u2019Pert Pro MPD operated at 40 kV and 40 mA with Cu K\u03b1 radiation. The XRD patterns were recorded at a diffraction angle of 2\u03b8 between 10\u00b0 and 80\u00b0, and Jade 6.5 software was used for data analysis. The graphitization and purity degree of the obtained solid carbon materials were examined by using Raman spectroscopy (labRAM HR800, HORIBA JY, France) with an excitation wavelength of 532 nm.A two-stage pyrolytic-catalytic vertical tube furnace is shown in Fig. 1\n. Before the beginning of the experiment, the PP and catalyst were placed in a stainless steel reactor in a certain proportion from bottom to top, and then the reactor was sealed. Before the reaction was heated, nitrogen was pumped into the reactor to expel air. After the nitrogen atmosphere was stabilized in the reactor, the temperature of the catalyst section was initially raised to 900 \u2103. After the temperature of the catalytic section was stabilized, the temperature in the PP pyrolysis section was heated. The temperature in the pyrolysis stage rose to 500 \u2103 after 15 minutes, and water vapour was added at the same time. Gas collection began when the temperature of the pyrolysis section reached 450 \u2103, and the continuous collection time was 30 minutes. The reaction gas was passed through a wash cylinder containing isopropyl alcohol and then condensed and dried for collection. After the reaction was complete, nitrogen flow was continued until the reactor temperature dropped to room temperature.Polypropylene is mainly decomposed into tar, carbon and hydrogen during pyrolysis (Eq. (1), (2), (3)). In the catalyst stage, under the action of catalyst and water vapour, tar underwent a catalytic cracking reaction and further decomposed into carbon monoxide, carbon dioxide, hydrogen and other small molecular gases (Eq. (4)), but tar and carbon dioxide and carbon and water vapour also underwent other side reactions (Eq. (5), (6)). Finally, after pyrolysis and catalytic reforming reactions, PP produced gas products dominated by hydrogen and solid products dominated by carbon. In our previous study, it was found that C3H6 was the main gas produced by PP pyrolysis. In a two-stage reaction unit, CH4 is the main gas produced by PP after pyrolysis at a high-temperature stage without catalyst. This result shows that even if no catalyst is added in the two-stage reaction device, the secondary high temperature will cause the decomposition of C3H6 into CH4, and the catalyst mainly acts on the further decomposition of CH4.\n\n(1)\n\n\n\n\n(\n\nC\n3\n\n\nH\n6\n\n)\n\nn\n\n\u2192\nC\n+\n\nH\n2\n\n\n\n\n\n\n\n(2)\n\n\n\n\n(\n\nC\n3\n\n\nH\n6\n\n)\n\nn\n\n\u2192\n\nC\nX\n\n\nH\ny\n\n\n(\nT\na\nr\n)\n\n+\n\nH\n2\n\n\n\n\n\n\n\n(3)\n\n\n\nC\nX\n\n\nH\ny\n\n\n(\nT\na\nr\n)\n\n\u2192\nC\n+\n\nH\n2\n\n\n\n\n\n\n\n(4)\n\n\n\nC\nX\n\n\nH\ny\n\n\n\nT\na\nr\n\n\n+\n\nH\n2\n\nO\n\u2194\nC\nO\n+\n\nH\n2\n\n+\n\n\nCO\n\n2\n\n\n\n\n\n\n\n(5)\n\n\n\nC\nX\n\n\nH\ny\n\n\n\nT\na\nr\n\n\n+\n\n\nCO\n\n2\n\n\u2194\nC\nO\n+\n\nH\n2\n\n\n\n\n\n\n\n(6)\n\n\nC\n+\n\nH\n2\n\nO\n\u2194\nC\nO\n+\n\nH\n2\n\n\n\n\n\nThe calculation method of each value is as follows:The content of the produced gases (CO, H2, CO2, CH4 and C2H4) was detected with a gas chromatograph (GC, Agilent 7890A).The mol% of CO, H2, CO2, CH4 and C2H4 were calculated using Eq. (7):\n\n(7)\n\n\nmol\n%\ni\n=\n\n\n\n%\n\n\ni\n\n\n\n\n\n%\n\nco\n\n\n+\n\n%\n\nH\n2\n\n\n+\n\n%\n\nC\n\nO\n2\n\n\n\n+\n\n%\n\nC\n\nH\n4\n\n\n\n+\n\n%\n\n\nC\n2\n\n\nH\n4\n\n\n\n\n\n\n100\n%\n\n\n\n\nwhere i represents CO, H2, CO2, CH4, and C2H4, and \n\n%\n\nc\no\n\n\n, \n\n%\n\nH\n2\n\n\n, \n\n%\n\n\nC\nO\n\n2\n\n\n, \n\n%\n\n\nC\nH\n\n4\n\n\n, and \n\n%\n\n\n\nC\n2\n\nH\n\n4\n\n\n are the percentage of the specified gas obtained from the GC results.The H2 yield was calculated using Eq. (8):\n\n(8)\n\n\n\nH\n2\n\n\ny\ni\ne\nl\nd\n\n\n\n\nm\nm\no\nl\n\n\ng\n\nP\nP\n\n\n\n\n\n=\n\n(\n\n%\n\nH\n2\n\n\n\u00d7\n\nC\n\nN\n2\n\n\n\u00d7\nt\n)\n\n/\n\n(\n22.4\n\u00d7\n\n%\n\nN\n2\n\n\n)\n\n\n\n\n\nwhere \n\n%\n\nH\n2\n\n\n and \n\n%\n\nN\n2\n\n\n represent the percentage of gas obtained from the GC results. \nt\n represents the gas collection time, and \n\nC\n\nN\n2\n\n\n represents the volume flow of nitrogen.The theoretical H2 yield was calculated using Eq. (9):\n\n\n\n\nt\nh\ne\no\nr\ne\nt\ni\nc\na\nl\n\n\nH\n2\n\n\ny\ni\ne\nl\nd\n\n\n\n\nm\nm\no\nl\n\n\ng\n\nP\nP\n\n\n\n\n\n=\n\n\n\nM\n\nH\n,\nP\nP\n\n\n+\n\nM\n\nH\n,\n\nH\n2\n\nO\n\n\n\u00d7\nt\n\n\n\u00d7\n1000\n/\n2\n\n\n\n\nwhere \n\nM\n\nH\n,\nP\nP\n\n\n and \n\nM\n\nH\n,\n\nH\n2\n\nO\n\n\n are the moles of hydrogen in PP and water, respectively.The H conversion (%) was calculated using Eq. (10):\n\n(10)\n\n\nH\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\n\n(\n%\n)\n\n=\n\nH\n2\n\n\ny\ni\ne\nl\nd\n/\n\nt\nh\ne\no\nr\ne\nt\ni\nc\na\nl\n\n\nH\n2\n\n\ny\ni\ne\nl\nd\n\u00d7\n100\n%\n\n\n\n\n1.1 Pyrolysis-catalytic reforming of PP for hydrogen productionIn the hydrogen production from PP pyrolysis and steam reforming experiment, the loading of iron in the catalyst, the addition ratio of catalyst to PP and the addition amount of water vapour are all important factors. Therefore, the influence of each parameter on gas production and hydrogen production was analysed.\nFig. 2\n shows the influence of iron loading on the experimental results. The range of iron loading is 0%-25%, and 0% loading is the blank comparison experiment. Pure activated carbon also has a certain ability to catalyse the cracking of plastics and depends on its developed pore structure and surface oxygen-containing functional groups, especially the content of ash. The addition of active material iron significantly improved the performance of the catalyst. With increasing iron loading, the gas production and hydrogen production of the plastic cracking continued to increase. When the iron content was 15%, the gas production and hydrogen production reached peak values of 172.09 mmol/gPP and 112.71 mmol/gPP, respectively. At this time, the catalytic capacity of the composite catalyst was approximately 3 times that of activated carbon. Furthermore, the hydrogen production performance was not improved by continued addition of iron loading (more than 15%). This was potentially due to the excessive iron bearing on the catalyst surface, which led to a decrease in the pore patency and a reduction in the catalytic contact area.\nFig. 2 also shows the composition of the produced gas. Due to the addition of water vapour, hydrogen is the most abundant gas, followed by carbon monoxide. Carbon dioxide and small molecule hydrocarbons are rarely produced. Carbon deposition is inevitable in the cracking reaction of polymer compounds. The formation of carbon oxides and hydrocarbons indicates that the addition of water can release the deposited carbon in the form of gas, thus reducing the carbon deposition on the surface of the catalyst and ensuring the catalytic stability over a long time.The key to ensuring hydrogen production and reducing cost is to explore the appropriate ratio of PP and catalyst. Fig. 3\n shows the effect of the PP-to-catalyst ratio on gas production, hydrogen production and gas composition. With increasing catalyst proportion, gas production and hydrogen production gradually increase. When the ratio of PP to catalyst is 1:0.75, gas production and hydrogen production basically reach the equilibrium state. When the ratio increases to 1:1, the gas production increases slightly, but the hydrogen production does not increase further. Considering the expense, the ratio of 1:0.75 is determined to be the appropriate addition ratio of PP and catalyst. Within a certain range, increasing the amount of catalyst can improve the reflection efficiency. When the contact point between the catalyst and PP is saturated, the increase in excessive catalyst affects the performance of catalysis. Additionally, the change in the ratio of PP to catalyst has no significant effect on the gas composition. Under the most appropriate conditions, the proportion of hydrogen is 65.5% and that of carbon monoxide is 22.8%.In the steam reforming reaction, the amount of water vapour added is an important factor in determining the hydrogen production capacity. Fig. 4\n shows the effect of the amount of water vapour added on the reaction result. In the reaction without water, the gas and hydrogen production of polypropylene cracking were 57.66 mmol/gPP and 38.73 mmol/gPP, respectively. Although the anhydrous cracking reaction of polypropylene produced little gas, the content of hydrogen in the gas was considerable. The addition of water greatly increased gas production and hydrogen production. The maximum gas yield (172.09 mmol/gPP) was achieved at a water content of 6 ml/h. When the amount of water reached 6 ml/h, the hydrogen yields reached a maximum peak (112.71 mmol/gPP). However, an additional increase in water content did not lead to a higher hydrogen yield. First, too much water vapour surrounded the catalyst and prevented the contact of the hydrocarbon molecules with the active site. In addition, a large amount of water vapour formed an excessive scour effect on the catalyst, which caused a decreased the catalytic performance. The addition of water had a certain saturation point, and supersaturation could have a negative effect. The determination of the equilibrium point among water, PP and catalyst in the reaction was the key to ensuring a low cost and high yield of hydrogen.The produced hydrogen can be traced back to the hydrogen in polypropylene and water. To understand the transformation degree of hydrogen in the reaction, the conversion rate of hydrogen was calculated, as shown in Table 2\n. Table 2 calculates the hydrogen conversion rate based on the actual and theoretical hydrogen production data. In the absence of water, 54.24% of the hydrogen in polypropylene can be successfully decomposed and reformed into hydrogen molecules. Other hydrogens are converted into large or small hydrocarbon molecules. With increasing water addition, the hydrogen conversion rate gradually decreases. This indicates that most of the water is not involved in the reaction, and only a small portion of the water successfully participates in the cleavage reaction and produces hydrogen.To analyse the contribution of water in the reaction of hydrogen production by steam reforming of polypropylene more directly, the transformation of hydrogen in water is analysed separately. The premise of this analysis is to assume that all hydrogen elements in polypropylene are converted to hydrogen. The difference in water content corresponds to different hydrogen contribution amounts. When the water content is 6 ml/h, its contribution degree to hydrogen is the largest, showing that the water is at its maximum strength in the reaction. This result also matches the water content choice for the optimal experimental conditions. Thus, for water to function optimally, it is necessary to add the proper amount of water. Too much or too little affects its optimal response capacity.Based on the above analysis results, the optimal experimental conditions are an iron loading of 15%, a PP to catalyst ratio of 1:0.75 and a water content of 6 ml/h. To better evaluate the catalytic stability of the catalyst under long-term operation, 10 cycle experiments were carried out on the reaction under optimal conditions, and the results are shown in Fig. 5\n. With increasing catalyst frequency, the hydrogen production performance of the catalyst decreased gradually, and the hydrogen content decreased from 65% to 47%. The selectivity of hydrocarbons and carbon dioxide increased, while the selectivity of carbon monoxide did not change significantly. As the catalyst was reacted for a longer time, the more inclined it was to produce hydrocarbons, and its catalytic capacity for small molecular hydrogen clearly decreased. The degradation of the catalyst performance in the process of recycling was due to the accumulation of carbon on the catalyst surface. With increasing reaction time, carbon deposition further increased, the initial active site slowly lost its activity, and a new active site was generated on the discontinuous graphite layer, whose activity was not as high as that of the original active site; thus, the overall catalytic activity was reduced.1.2 Fresh and used catalyst microscopic and textural characterizationThe physical and chemical properties of catalysts directly affect the production performance of the catalytic reforming reaction. To understand the catalyst change from catalysis, fresh and used catalysts were characterized and analysed with a focus on the surface microstructure and material composition changes of the catalysts; also, the inevitable carbon deposition phenomenon was detected and analysed.\nTable 3\n shows the specific surface area data of the activated carbon and catalyst. The specific surface area, pore volume and pore size of the activated carbon were 839.38 m2/g, 0.35 cm3/g and 4.82 nm, respectively. Compared with activated carbon, the specific surface area of the synthesized catalyst decreased from 562.27 m2/g to 619.24 m2/g, which was caused by the addition of iron and the covering of surface holes. However, the catalyst still had good pore structure data, and the excellent specific surface area and pore volume could ensure the catalytic performance.Fig. 6. shows the SEM images of fresh and used catalysts of 15Fe/AC. The used 15Fe/AC catalyst was recycled 10 times. Fig. 6\n (a) and (b) show the microscopic SEM images of the fresh 15Fe/AC catalyst. The surface of the fresh catalyst shows nanosized spherical particles, which are densely distributed on the surface of the activated carbon support. In addition, numerous tiny pore structures are observed on the surface of the activated carbon. This shows that a large number of iron active sites are widely distributed on the surface of the catalyst, ensuring the development of catalytic performance. Fig. 6 (c), (d), (e) and (f) show the microscopic images of the surface of the used 15Fe/AC catalyst. The surface of the used catalyst is covered with a layer of graphite carbon, some of which are carbon nanospheres and some are carbon nanotubes. The carbon nanospheres are approximately 50-100 nm in diameter and are from the incomplete graphitization of carbon nanospheres; carbon nanotubes are approximately 20-100 nm in diameter. Under the coupling effect of many factors, two kinds of carbon nanostructures were produced. Morphological differences in carbon nanomaterials are closely related to the size of the metal particles on the surface of the catalyst and the difference in the force between the metal and the support. Relevant studies have shown that larger metal particles on the surface of the catalyst correlate to a larger inner diameter of the generated carbon nanotubes, and stronger interaction between the metal active components and the support correlated to a smaller the diameter of the generated carbon nanotubes [34].TEM was used to further observe the morphology and size of carbon deposition, as shown in Fig. 7\n. The resulting carbon nanotubes are approximately 50 nm in diameter, with the smallest being 20 nm in diameter. Additionally, the nanotube is a hollow structure and has a relatively thick wall. The wall of the tube is relatively smooth and basically free of other impurities. The darker black spots in the image are catalyst particles, and very few particles are observed embedded in the tube wall. The active metal is distributed in the middle and end of the carbon tube, and the growth pattern of carbon nanotubes is a typical apex growth pattern. The active component of the metal appears quasiliquid during the growth of the carbon nanotubes and migrates continuously with the growth of the carbon nanotubes.\nFig. 8\n shows the XRD test results of fresh and used catalyst. The activated carbon support is composed of amorphous carbon, and the diffraction peaks of new substances such as Fe2O3, Fe, Fe5C2, FeC2, and Fe3O4 appear in the catalyst after metal iron is loaded; the iron active material in the catalyst is present in these four forms. The diffraction peak intensity of iron is the highest, while those of iron oxides and carbides are relatively small. As the iron loading increases, the diffraction peak of iron-related active substances gradually increases. The used catalyst structure is shown in Fig. 8 (b). Compared with the fresh catalyst, the diffraction peaks of Fe3C and C appear in the used catalyst without water, and the intensity of the diffraction peak of iron decreases. This indicates that carbon deposition appears on the surface of the catalyst after the reaction, and carbon deposition covers most of the active sites. For the used catalyst with water in the reaction, the diffraction peak of iron is completely covered, only the diffraction peak of Fe2O3 remains, and the diffraction peak of carbon deposition is also increased. This shows that the catalytic performance of the catalyst is mainly derived from the role of Fe2O3 during long-term use, and Fe2O3 has better stability in the catalytic process.The microscopic image and material structure analysis showed that the surface of the catalyst was a graphite carbon deposit. To further determine the amount of carbon deposition and the degree of graphitization, TG, TDA and Raman spectroscopy were carried out on the used catalyst 15Fe/AC.The TG and DTA curves are shown in Fig. 9\n. Fig. 9 (a) shows the thermogravimetric curve. The fresh catalyst began to lose weight at approximately 500 \u00b0C and continued to lose weight until 660 \u00b0C, with a weight loss ratio of approximately 50%. The weight loss ratio of the used catalyst (65%-70%) was significantly higher than that of the fresh catalyst. From Fig. 9 (b), compared with fresh catalyst, the used catalyst showed a wider weight loss temperature range, and a larger interval between the start and end temperatures of oxidation indicated a greater diversity of carbon structures. The used catalyst contained more ordered graphite structure carbon materials, so its weight loss began at a higher temperature and a wider weight loss interval. Moreover, the weight loss rates of the used catalysts with water content 0 and water content 6 were approximately 70% and 65%, respectively, which further indicated that the addition of water could inhibit the formation of carbon deposition.To understand the degree of graphitization of carbon deposition, Raman spectroscopic analysis was performed on the catalyst containing the carbon deposition, and the results are shown in Fig. 10\n. The Raman spectra near 1350 cm-1 and 1580 cm-1 correspond to D and G peaks, respectively. The Raman-active D peaks are observed due to defects (such as branches, openings, bends), amorphous carbon or the edge of the lamellae's crystal surface activating the vibration absorption mode of the six-member ring. The G peak is the Raman characteristic peak corresponding to the C-C bond stretching vibration (E2g vibration mode) of sp2 hybrid carbon atoms between graphite lamellae. In the literature, the ratio of the D peak to G peak intensity, ID/IG, is used to calculate the graphitization degree and crystallization index [35]. Theoretically, the ratio is zero for pure carbon nanotubes, and a smaller ratio correlated to a higher graphitization degree [36]. Analysis of the ratios obtained from peaks in the figure shows that the degree of graphitization of carbon deposition without water is higher, and the addition of water increases the number of defects in graphite carbon and the amount of amorphous carbon, reducing the degree of graphitization and crystallinity of carbon deposition. In addition, the Raman G' peak at 2700 cm-1 is related to the elastic scattering of the two phonons, and the ratio of IG'/IG can also indicate the purity of graphite carbon to a certain extent. A higher IG'/IG ratio correlates to a higher purity of the carbon nanomaterial. The purity of the carbon nanomaterials prepared by the reaction without water is higher.1.3 Comparison with literature-reported studiesTo better present the experimental results of this study, a comparison is performed with the results from similar studies, as shown in Table 4\n. The H2 yield (38.73 mmol/g) was relatively significant in the reaction without water participation. Similarly, the H2 yield of the Fe-\u03b3Al2O3 catalyst is 22.9 mmol/g with PP as the raw material. This result further indicates that, to some extent, the catalytic support performance of the disordered carbon structure is better than that of the ordered molecular sieve structure. In the catalytic conversion with added water, the H2 yield was 112.71 mmol/g, which was outstanding in comparison other reported studies. This result also shows the feasibility and superiority of the experimental method and technology in this study.2 ConclusionsIn this study, an iron catalyst supported by activated carbon was prepared and applied to research on hydrogen production by catalytic reforming of plastic polypropylene. The experimental apparatus was a two-stage pyrolysis catalytic furnace. The characterization test showed that there were some structures on the surface of the prepared catalyst, such as Fe2O3, Fe, Fe5C2, and Fe3O4, which acted as active sites in the catalytic reaction. The experimental results showed that the 15Fe/AC catalyst had good hydrogen production performance, and the H2 yield could reach 38.73 mmol/gPP in the catalytic reforming reaction without water and 112.71 mmol/gPP in the reaction with water. After the analysis of the water contribution degree, the optimal water content was determined to be 6 ml/h. In addition, the carbon deposition of the used catalyst was analysed in depth. The results showed that the carbon deposited by the reaction without water had a higher graphitization degree and purity. The addition of water reduced the degree of graphitization of carbon deposition but effectively increased the yield of H2. This study provides a novel method for hydrogen production from plastic thermal conversion.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This paper is financially supported by National Key R&D Program of China (2019YFC1906803), and CAS Project for Young Scientists in Basic Research (YSBR-044).", "descript": "\n                  The purpose of this study is to explore a method for the high-yield production of hydrogen by pyrolysis and steam reforming of polymer plastics. The developed Fe-based catalyst supported on activated carbon was applied to reactions with polypropylene for hydrogen production. The effects of iron loading (%) in the catalyst, the total catalyst amount, and the water content in the reaction atmosphere on the performance of hydrogen and gas production were investigated. Under the optimal conditions, the hydrogen yield without water added reached 38.73 mmol/gPP, and this yield was significantly improved by adding water into the reaction atmosphere. By optimizing the amount of water added, the hydrogen yield reached 112.71 mmol/gPP. The surface morphology and structural components of the fresh and used catalysts were characterized, and the morphology and quantity of carbon deposition on the catalyst were analysed. The catalytic stability of the 15Fe/AC catalyst was determined by repeating the test 10 times under the optimal reaction conditions. As the reaction time increased, the selectivity of the catalyst for hydrogen decreased and that for hydrocarbons increased. Moreover, the experimental method used in this study had excellent hydrogen production capacity. Thus, this study provided a novel method for the high-efficiency production of hydrogen by pyrolysis and steam reforming of polymer plastics.\n               "}
{"full_text": "Data will be made available on request.Dry Reforming of Methane (DRM) has great potential to contribute to current efforts towards a sustainable energy future. Beyond that, this catalytic reaction turns two of Earth\u2019s most abundant greenhouse gases, CO2 and CH4, into valuable synthesis gas [1], thus mitigating global warming. Both products of DRM, H2 and CO, are building blocks in the synthesis of various fuels and chemicals via heterogeneous catalysis, e.g. generation of hydrocarbons via Fischer-Tropsch synthesis or methanol production [2,3]. Especially the Fischer-Tropsch reaction benefits from the low H2/CO ratio obtained by DRM [4]. DRM is represented by following Eq. (1):\n\n(1)\n\n\n\n\nCH\n\n\n4\n(\ng\n)\n\n\n+\n\n\nCO\n\n\n2\n(\ng\n)\n\n\n\u21cc\n2\n\nH\n\n2\n(\ng\n)\n\n\n+\n2\n\nCO\n\ng\n\n\n\n(\n\u0394\n\nH\nr\n298\n\n=\n+\n247\nkJ\n\n\nmol\n\n\n\u2212\n1\n\n\n)\n\n\n\n\nThe endothermic nature of DRM necessitates high operating temperatures in order to achieve high conversions, usually between 650\u2009\u00b0C and 1000\u2009\u00b0C [5]. Unfortunately, DRM is still not a mature industrial process, despite its great environmental potential [4]. High operating temperatures and associated deactivation phenomena of sintering and coke formation are the biggest obstacles. Steam reforming, partial oxidation, or autothermal reforming of methane are the dominant technologies for syngas production from methane on an industrial level; however, they mainly yield hydrogen-rich products [6]. In contrast, due to the introduction of an additional carbon source, DRM leads to CO-rich syngas, beneficial for certain downstream processes (e.g. production of acetic acid [6]). Consequently, the interest in DRM is still extremely high, and further developments are necessary to obtain effective catalyst materials that are stable at the required operation temperatures.Various catalyst materials have been extensively studied for their capability of promoting DRM, with Ni-supported catalysts mostly utilised for industrial processes due to their high activity and low cost [7,8]. Unfortunately, they suffer from deactivation by carbon deposition and/or Carbon Nano Tube (CNT) formation [9]. Noble metal-based materials (e.g. Pt, Pd, Rh, Ru\u2026) are less prone to such adverse side reactions, although their expensive nature makes them unsuitable for large-scale applications [1,5]. By adding small amounts of noble metals to e.g. Ni-based catalysts with subsequent alloy formation, both drawbacks, carbon formation and high cost, can be alleviated [10]. Furthermore, use of bimetallic catalysts enables tuning of particle size and dispersion, thus increasing overall performance [11]. Wang et al. calculated the thermodynamically limiting temperatures in DRM for carbon formation (carbon is not stable above this limit) as a function of the educt ratio and found that in a CO2 excess this temperature decreases, meaning that this excess can prevent carbon formation at temperatures below 700\u2009\u00b0C [12]. During the search for highly active materials, Co- and Fe-based materials were tested as well [9,13] with Co reaching almost the activity level of Ni [4]. Moreover, perovskite-type materials have lately drawn much attention as promising substitutes to conventional catalysts, as they allow a design approach, making cost effective and highly active materials available. For example, Dama et al. have demonstrated high performance for CaZr0.8Ni0.2O3-\u03b4\n[14]. Additionally, perovskites are often used as precursors for DRM catalysts that release the active metal upon reductive treatment, which is accompanied by partial decomposition of the perovskite into the respective oxides [15,16]. Unfortunately, the rich redox and defect chemistry of the perovskite surface \u2013 which is highly beneficial for DRM \u2013 is lost during this process.Although numerous studies in the past intensively investigated DRM on supported metal catalysts, the reaction mechanism is still debated without general agreement [17]. One reason for this may be related to the fact that the mechanism depends significantly on the utilised materials and the combination of active metal and nature of the oxide support [6,18]. In principle, the reaction mechanism can be divided in the following major steps: The first step is the dissociative adsorption of methane, which occurs on the metal and is established as the rate limiting step for e.g. Ni [4]. Dissociative adsorption of CO2 occurs in parallel on the surface of the support, which is considered a fast reaction step. It has to be emphasised that CO2 activation is strongly dependent on the used materials, with basic or redox-active supports or surface defects strongly enhancing this process (e.g. oxygen vacancies on perovskite surfaces are extremely active for CO2 activation already at low temperatures around 400\u2009\u00b0C) [14,19]. In a subsequent reaction step, the formed H from CH4 dissociation can either desorb as H2 or move to the support to form OH-groups that can be observed below 800\u2009\u00b0C [4]. In the latter case, a competing reverse Water Gas Shift (rWGS) type reaction is dominating \u2013 especially as it is thermodynamically favoured compared to the DRM-pathway at lower reaction temperatures. Both pathways result in the formation of NiO and NiC which can recombine and lead to the desorption of CO. These two possible pathways are depicted in \nFig. 1. OH-groups can react with activated CO2 (forming e.g. formate) or with CHx groups to form CHxOH. The most complex part of the reaction mechanism is the oxidation of the intermediates and the subsequent CO and H2 desorption with many different possible pathways reported in literature. For instance, Iglesia and Wei outlined different routes of CHx oxidation via surface oxygen [20]. Most models were established for Ni-based materials, but they are claimed to be transferable to noble metal systems in literature (Wittich et al. [6].)The H2/CO ratio obtained by DRM (which ideally should be 1, according to Eq. 1) depends strongly on the reaction temperature: the rWGS-type pathway occurs under similar reaction conditions and reduces the ratio as can be seen in the rWGS equation:\n\n(2)\n\n\n\n\nH\n\n\n2\n(\ng\n)\n\n\n+\n\n\nCO\n\n\n2\n(\ng\n)\n\n\n\u21cc\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n(\ng\n)\n\n\n+\n\n\nCO\n\n\n\n\ng\n\n\n\n\n\n(\n\u0394\n\n\nH\n\n\nr\n\n\n298\n\n\n\n=\n\n+\n42.1\nkJ\n\n\nmol\n\n\n\u2212\n1\n\n\n)\n\n\n\n\nAside from rWGS, a large number of additional possible side reactions influence the DRM process, a brief summary of which is given by Aramouni et al. [4]. Higher operating temperatures are generally favourable for DRM, as CO and CO2 hydrogenation or the Boudouard reaction, for example, occur at lower reaction temperatures. The latter, together with methane dehydrogenation, which is more pronounced at high reaction temperatures (>730\u2009\u00b0C), is the main source for carbon deposition on DRM catalysts. The degree of catalyst deactivation by coking depends on the utilised materials. The choice of support has a strong influence on coking resistance, with highly redox-active oxides promoting the removal of formed carbon deposits during DRM [5,14]. Basic oxides and perovskites have been reported to exhibit increased ability for carbon gasification [21]. Perovskites in particular are promising alternatives due to their rich surface redox chemistry and thermal stability [19]. In addition, perovskites provide the opportunity to incorporate dopant materials on both A- and B-site, enabling the synthesis of materials that contain additional promoters and catalytically active elements. For instance, addition of Ca to DRM catalysts has been reported to improve carbon removal from the surface [14].Doping Ni or Co on the B-site of perovskites leads to the formation of metallic nanoparticles upon reduction or in reducing reaction environments via exsolution as was reported previously [22]. Unlike the precursor method mentioned above, exsolution preserves the perovskite structure \u2013 meaning its beneficial properties are not lost. Due to their high dispersion and strong anchoring to the surface, the metallic nanoparticles provide an ideal system for DRM. Additionally, Neagu et al. showed that small nanoparticles (around 20\u2009nm) that were produced via exsolution are still stable in high temperature reducing conditions, even though the surface area of the perovskite was around 1\u2009m2 g\u22121\n[23]. High metal dispersion in combination with available surface oxygen enhances coking resistance of these type of system as reported by Dama et al. [14].This was the motivation to use Co- and Ni-doped perovskites for the present study, which was focussed on comparing the effect of the formation of the metal nanoparticles during DRM (in-situ exsolution) to the effect of pre-reduction with H2/H2O (with the formation of the nanoparticles prior to the catalytic reactions). Both phenomena were studied by catalytic testing as well as with in-situ surface chemical analysis by Near Ambient Pressure X-Ray Photoelectron Spectroscopy (NAP-XPS) and operando X-Ray Diffraction (XRD). For this purpose, perovskites with the nominal compositions Nd0.6Ca0.4FeO3-\u03b4 (B-site undoped), Nd0.6Ca0.4Fe0.9Co0.1O3-\u03b4 (B-site Co-doped), and Nd0.6Ca0.4Fe0.97Ni0.03O3-\u03b4 (B-site Ni-doped) were investigated. The obtained results were supplemented by Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-Ray analysis (EDX).The catalysts (Nd0.6Ca0.4FeO3-\u03b4, Nd0.6Ca0.4Fe0.9Co0.1O3-\u03b4, and Nd0.6Ca0.4Fe0.97Ni0.03O3-\u03b4) were prepared via a modified Pechini synthesis method, as outlined in detail in previous works [22,24]. To that end, the respective starting materials Nd2O3 (99.9 %, Strategic Elements), CaCO3 (99.95 %, Sigma-Aldrich), Fe (99.5 %, Sigma-Aldrich), Ni(NO3)30.6\u2009H2O (98 %, Alfa Aesar), and Co(NO3)30.6\u2009H2O (99.999 %, Sigma-Aldrich) were mixed in stochiometric amounts and dissolved in either HNO3 (65 % Merck) or H2O, both doubly distilled. The salts were complexed using citric acid (99.9998 % trace metal pure, Fluka) in excess of 20\u2009mol% with regard to the cations. After evaporation of the liquid on a heater, the resulting gels were ignited by further heating, leading to the formation of powders. These powders were calcined for 3\u2009h at 800\u2009\u00b0C. The catalyst Nd0.6Ca0.4Fe0.97Ni0.03O3-\u03b4 had a reduced Ni content and underwent an additional annealing step at 1200\u2009\u00b0C, as phase impurities were observed even after the calcination step of the original synthesis. The products were ground to ensure homogeneity. Respective surface areas and morphological characterisation of the pristine perovskites can be found in reference [22]. The weights used for the synthesis are listed in the Supporting Information (SI) in Tables S1 to S3.The powder XRD measurements were carried out on a PANalytical X\u2032Pert Pro diffractometer (Malvern Panalytical, Malvern, UK) in Bragg-Brentano geometry using a mirror for separating the Cu K\u03b11,2 radiation and an X\u2032Celerator linear detector (Malvern Panalytical, Malvern, UK). For the operando experiments, an Anton Paar XRK 900 chamber (Anton Paar, Graz, Austria) was used. After sample preparation, the catalysts were pre-treated with oxygen (600\u2009\u00b0C, 40\u2009min, 0.5\u2009L\u2009min\u22121 O2) before switching to the reaction atmosphere. The DRM reaction was carried out in CH4 excess (48\u2009mL\u2009min\u22121 CH4, 16\u2009mL\u2009min\u22121 CO2, and 50\u2009mL\u2009min\u22121 Ar at ambient pressure) at increasing temperatures (going from 300\u2009\u00b0C to 700\u2009\u00b0C with 50\u2009\u00b0C steps). The methane excess was chosen for two reasons: Firstly, to simulate real biogas conditions, as \u2013 dependent on the feedstock \u2013 biogas generally consists of around 66 % CH4 and 32 % CO2\n[25,26]. Secondly, CH4 excess leads to a reducing atmosphere and, therefore, may promote in-situ exsolution. At each step, reaction conditions were held for 10\u2009min (to achieve equilibrium) before an operando XRD measurement (about 30\u2009min) was carried out. For interpretation of the data and assignment of the diffraction peaks, the PDF-4\u2009+\u20092019 database (International Centre for Diffraction Data) [27] in combination with the HighScore Plus [28] software (PANalytic) was used. Assignment was performed by comparison with database structures and measurements and validated by performing Rietveld refinements.A Quanta 250 FEGSEM (FEI Company) microscope was used to record SEM images for morphology examination. Additionally, EDX was performed with an Octane Elite X-Ray detector (EDAX Inc). To obtain a satisfactory surface sensitivity, an acceleration voltage of 5\u2009kV was used for imaging and of 10\u2009kV for EDX measurements.\nIn-situ NAP-XPS experiments were performed on 200\u2009nm thick catalyst films, which were grown on Yttria Stabilised Zirconia (YSZ) single crystals in (100) orientation with a size of 5\u00d75x 0.5\u2009mm (CrysTec) via Pulsed Laser Deposition (PLD). The detailed manufacturing process is described in the SI and in previous work [24]. The exact setup of the samples as well as the NAP-XPS sample stage is described in previous work as well [29]. Monochromatic Al K\u03b1 radiation with a spot size of 350\u2009\u00b5m was used for excitation achieving an energy resolution of around 0.2\u2009eV. Heating was done using a near-IR laser with a wavelength of 970\u2009nm and a maximum output of 100\u2009W, the temperature was monitored with a S-type thermocouple mounted on the sample stage as well as a pyrometer (LumaSense Technology). Both temperature measurement methods were previously calibrated utilising electrochemical impedance spectroscopy and the known temperature dependent resistivity of the YSZ substrate. Therefore, the high frequency offset could be attributed to the ohmic resistance of YSZ, which then was used to calculate the temperature [30]. Similar to the other experiments, all catalysts were initially oxidised in 1 mbar of O2 at 600\u2009\u00b0C. NAP-XPS spectra of all relevant core levels including carbon, sulphur, and the fermi edge were recorded simultaneously. After ensuring equal initial states for all samples, the gas phase was changed to reaction conditions (CH4:CO2 = 2:1) at a total pressure of 1 mbar. The reaction temperature was increased in steps of 50\u2009\u00b0C from 400\u2009\u00b0C to 700\u2009\u00b0C. At each step, a full set of spectra was collected (survey, Nd4d, Ca2p, Fe2p, Co2p, Ni2p, O1s, C1s, S2p, fermi edge). To evaluate the recorded XPS spectra, the software Casa XPS was employed. The background was approximated with a Shirley background and binding energies were calibrated using a combination of the Fermi edge and the Ca2p3/2 peak (364.1\u2009eV). The peaks were fitted with a \u201cGaussian/Lorentzian product form 30\u2033 function as implemented in Casa XPS (\u201cGL(30)\u201d) without asymmetry restriction. Two constraints were applied to the fits of the Ca2p spectra: the difference in binding energy between 2p1/2 and 2p3/2 was fixed at 3.55\u2009eV, and the area ratio of the 2p1/2 and the 2p3/2 was set to 1:2. Fe2p spectra were fitted with four components, namely Fe(II) and Fe(III) for the 2p1/2 and 2p3/2 transition each. Spin orbit splitting for the 2p1/2 and 2p3/2 fits were set to 13.6\u2009eV and the difference in binding energy was fixed at 1.5\u2009eV between Fe(II) and Fe(III). For the area ratio of the Fe 2p1/2 and 2p3/2 transition, a fixed value of 0.4:1 was used. This deviates from the theoretical value of 0.5:1, but results in a better fit most likely due to uncertainties in the background determination.To test the activity of the catalysts, a fixed bed reactor system operating at ambient pressure was used as described in previous works [22,24]. In short, it consists of a home-built gas mixing system made from steel tubes (Burde & Co, Vienna, Austria) and fittings (Swagelok, Solon, USA) and an optional saturator filled with water. The catalyst is fixed in a quartz glass tube with 6\u2009mm diameter (4\u2009mm inner diameter) in an oven, and a Micro-Gas Chromatography system (Micro-GC, Fusion 3000\u2009A, Inficon) is used to analyse the gas composition of the reaction gas mixture after passing through the reactor every 2\u20133\u2009min to monitor the catalytic activity. The amounts of the catalysts were chosen in such a way to yield conversions between 10 % and 20 % at 700\u2009\u00b0C, ensuring that the thermodynamic equilibrium is not reached. The K-type thermocouple was placed inside the catalyst bed to ensure that measured temperature matches the actual temperature of the catalyst. To achieve comparable starting conditions for all catalysts, an oxidative pre-treatment in pure oxygen was performed before each test (10\u2009mL\u2009min\u22121 O2, 600\u2009\u00b0C, 30\u2009min). For experiments with metallic nanoparticle exsolution prior to the actual DRM experiment, an additional reducing pre-treatment in humidified H2 was performed for one hour: pure H2 was bubbled through a water-filled saturator at room temperature at a flow rate of 10\u2009mL\u2009min\u22121 (leading to a H2/H2O ratio of ~32:1). The ideal temperature for the reducing pre-treatment was determined in previous experiments and was chosen such that exsolution of the B-site dopant occurs without decomposition of the remaining perovskite [22]. The chosen temperatures were 625\u2009\u00b0C for the Ni-doped catalyst, 575\u2009\u00b0C for the Co-doped material, and 700\u2009\u00b0C for the undoped sample. The order of the \u201cexsolution willingness\u201d of the B-site metals \u2013 with Co at lowest temperatures followed by Ni and Fe only at the highest temperatures \u2013 is confirmed by Temperature Programmed Reduction (TPR) experiments in the SI (Fig. S6).Afterwards, the catalyst was cooled to 400\u2009\u00b0C in Ar (total flow of 12\u2009mL\u2009min\u22121). The gas phase was then changed to the reaction mixture, with flows of 3.0\u2009mL\u2009min\u22121 CH4, 1.5\u2009mL\u2009min\u22121 CO2, and 6.0\u2009mL\u2009min\u22121 Ar (CH4:CO2 = 2:1). With this gas mixture, a temperature ramp from 400\u2009\u00b0C to 700\u2009\u00b0C with a rate of 1\u2009\u00b0C\u2009min\u22121 was performed. In case of the purely oxidatively pre-treated samples, the temperature ramps were performed twice to check for irreversible changes during the first ramp (e.g. exsolution, deactivation). Comparing the catalytic performance directly to known materials is difficult, as quantities that could be used for such comparisons strongly depend on the experimental setup: In literature, the catalytic activity is often quantified by the conversion (a comprehensive summary of different catalysts used for DRM for a variety of different conditions and setups is given in Ref. [4]); however values one gets for conversion vary heavily with experimental parameters (like weight of the catalyst, active surface, space time velocities\u2026). Another quantity commonly used is the Turn Over Frequency (TOF). For this method to be applicable, however, the surface structure and number of active centres have to be known very well and should not change during the reaction [31]. As our perovskite-type oxide catalysts are dynamic systems, the number of active centres is changing under reaction conditions. Especially when exsolution occurs, it is not straightforward to determine TOF values. To account for this fact, the catalysts were compared with respect to their specific activity as outlined in previous work [19,32]. This means that the produced amount of CO was normalised to the gas flow and the surface area of the catalyst. The respective BET areas and a detailed explanation of how the calculation of the specific activity was performed can be found in the SI. For a series of high temperature measurements, a different setup was employed. A Pfeiffer PrismaPro QMG 250 mass spectrometer was used to monitor the composition. The catalysts were again pre-treated by oxidation prior to the catalytic reaction (20\u2009mL\u2009min\u22121 O2, 600\u2009\u00b0C, 30\u2009min), which was followed by a reducing pre-treatment at the respective temperature specified above (20\u2009mL\u2009min\u22121\n, H2/H2O of ~32:1, 60\u2009min). Afterwards, the catalyst was cooled to 400\u2009\u00b0C in Ar and the gas atmosphere was switched to the reaction mixture. In contrast to the other catalytic experiments, the gas flow was four times higher to account for the increased activities of the catalysts at higher temperatures. For these high temperature experiments, the temperature was raised from 400\u2009\u00b0C to 950\u2009\u00b0C with a rate of 2\u2009\u00b0C\u2009min\u22121.Two sets of experiments were performed as catalytic tests for each catalyst: During the first set, measurements were performed immediately after an oxidising pre-treatment as described in the experimental section \u2013 samples treated that way will be designated \u201coxidised\u201d. For the second set, an additional reducing pre-treatment step before measurements was conducted (the corresponding samples are labelled \u201creduced\u201d). These different catalysts pre-treatments led to two different starting conditions: either a fully oxidised perovskite surface or a reduced material with oxygen vacancies and exsolved nanoparticles embedded in the parent oxide. In the first case (only oxidising pre-treatment), the possibility of in-situ exsolution at higher DRM reaction temperatures still exists, which may lead to an increase in catalytic performance. In case of the latter experiment, exsolution already occurred during pre-reduction in H2/H2O, and the formed nanoparticles are present throughout the whole catalytic measurement.For the B-site undoped sample (Nd0.6Ca0.4FeO3-\u03b4), the catalytic results for the two experiments are shown in Figs. S1\u2013S3 (see SI). Reduction prior to the test clearly led to an earlier onset of CO formation, with the start of CO formation being already observable at 400\u2009\u00b0C. In contrast, the oxidised sample exhibited CO production only above 500\u2009\u00b0C. Moreover, for the reduced sample, the CO concentration reached a first local maximum at 450\u2009\u00b0C, before an intermediate drop. It started to rise again around 520\u2009\u00b0C with the CO concentration constantly increasing. Interestingly, the H2 concentration was not rising during the first CO production peak. H2 was only produced at the highest reaction temperatures. This means, that two different processes for CO formation occur:\n\n(i)\nThe first CO formation peak at low temperatures took place as a consequence of the reductive pre-treatment, during which oxygen vacancies were formed in the perovskite lattice. CO2 can react with those vacancies \u2013 refilling them while releasing CO in the process.\n\n\n(ii)\nThe second increase of the CO concentration (at higher temperatures), which also was observed in the oxidised sample, was accompanied by H2 production. This means that at higher temperatures DRM took place.\n\n\nThe first CO formation peak at low temperatures took place as a consequence of the reductive pre-treatment, during which oxygen vacancies were formed in the perovskite lattice. CO2 can react with those vacancies \u2013 refilling them while releasing CO in the process.The second increase of the CO concentration (at higher temperatures), which also was observed in the oxidised sample, was accompanied by H2 production. This means that at higher temperatures DRM took place.However, as can be seen in both Fig. S1a and d in the SI, the H2/CO ratio was not reaching the theoretical value of 1, indicating side reactions (as discussed in the introduction). The main reason for a value below 1 is that the competing rWGS reaction dominates at lower to medium temperatures. As shown in previous works, perovskite-type oxides can also be efficient for rWGS reaction [19]. The SEM images (SI, Fig. S3) taken after the catalytic reactions did not display any metallic nanoparticles for both experiments, but in case of the pre-reduced sample, particles with a diameter of more than 100\u2009nm are visible on the surface, which were identified as CaCO3. As highlighted in previous works [19], the investigated materials can form CaCO3 particles at higher temperatures under CO2-rich reaction conditions. This CaCO3 phase could also be confirmed by operando XRD experiments, as discussed in Section 3.3 below. In Fig. S4 a SEM image with visible CaCO3 crystallites is shown, and additional EDX analysis confirms their chemical nature. This formed CaCO3 phase blocks catalytically active sites and, therefore, leads to deactivation of the catalyst. The difference in activity at 700\u2009\u00b0C between the two pre-treatments was also significant as the activity increased roughly five-fold (Fig. S2a).For the Co-doped catalyst Nd0.6Ca0.4Fe0.9Co0.1O3-\u03b4, a clear activation effect of the reductive pre-treatment could be confirmed as well. CO formation started already at 400\u2009\u00b0C with the reduced Co-doped catalyst, while in case of the oxidised perovskite the CO formation onset was at 560\u2009\u00b0C (SI, Fig. S1b and e). The two consecutive runs for the oxidised Co-doped perovskite are nearly identical (SI, Fig. S2b), with only a slight decrease (83 % of the first run) in activity at 700\u2009\u00b0C. A slight deactivation effect may be connected to the formation of CaCO3 and CaO as seen in the operando XRD measurements (see Section 3.3) and in the SEM image with EDX analysis (SI, Fig. S4). The activity of the reduced sample was about one order of magnitude higher. The SEM images taken after the catalytic reaction (SI, Figs. S3a and S3d) also show a clear difference between the samples. Nanoparticles with a diameter of around 33\u2009nm could be found on the surface of the reduced sample, while on the oxidised sample no nanoparticles could be observed.In \nFig. 2, the catalytic results for Nd0.6Ca0.4Fe0.97Ni0.03O3-\u03b4 are displayed. The onset of CO production occurred at approximately 500\u2009\u00b0C for the reduced perovskite, while the oxidised sample showed CO formation only above 550\u2009\u00b0C. In case of the oxidised sample, a small initial increase of the CO production could be observed around 550\u2009\u00b0C, followed by a drop between 570\u2009\u00b0C and 590\u2009\u00b0C. This resembles the behaviour of the undoped sample; however, in this case no pre-formed oxygen vacancies were present. In this case it is a side effect of the onset of in-situ nanoparticle exsolution as explained in detail in the SI. Regarding the selectivity of the Ni-doped samples (Fig. S1c and f, SI), predominantly CO formation by rWGS was observed at low temperatures. Onset of significant H2 formation via DRM was observable only at higher reaction temperatures (see also the explanation presented in Section 3.2).The in-situ formation of Ni-nanoparticles could also be confirmed via SEM images taken after the catalytic experiments. In Fig. 2b, nanoparticles are shown after the runs with an oxidic pre-treatment. In contrast, the nanoparticles for the pre-reduced catalyst are substantially bigger (Fig. 2a). Their mean diameter (26\u2009nm) was around twice the size of the particles on the oxidised catalyst (14\u2009nm). The exact distribution of the nanoparticles is displayed in Fig. S5 (SI). The sample with bigger nanoparticles exhibited an increase in catalytic activity. These bigger nanoparticles appear to be beneficial for the activation of methane.When comparing the three catalysts after application of reducing pre-treatments, it becomes apparent that the Ni-doped perovskite performed best with a specific activity for CO of 1.5\u221910\u22126 mol\u2009s\u22121 m\u22122, while the Co-doped catalyst only showed an activity of 0.75\u221910\u22126 mol\u2009s\u22121 m\u22122. The undoped sample exhibited an activity of only 0.37\u221910\u22126 mol\u2009s\u22121 m\u22122. The measurements are compared in Fig. S7 (SI).The tested perovskite catalysts exhibit significant activity with respect to the rWGS reaction at intermediate temperatures (~500\u2013700\u2009\u00b0C), a known competitive side reaction of DRM [19]. This causes a shift of the product ratio within this temperature range from an equal distribution to a CO excess of about 10:1 in the present study. It is, however, known that rWGS becomes less dominant for DRM applications above 800\u2009\u00b0C [1]. As the main intention of this study was to examine the exsolution behaviour and its impact on the DRM activity, the initial focus was put on the intermediate temperature region. To get more insights into the DRM capabilities of the investigated perovskite catalysts, a second set of catalytic reactions up to 950\u2009\u00b0C was conducted to check if the H2/CO ratio of the product gas stream increases at higher temperatures. An overview of the results of these high temperature measurements is given in \nFig. 3.During the high temperature measurements, onset of DRM occurred later than in the above-mentioned catalytic tests. This delayed onset was caused by the lower sensitivity of the MS towards low concentrations compared to the micro-GC. Also, the higher flow rates used for these experiments led to lower overall conversion. After onset of DRM, the H2/CO ratio increased steadily up to a value of around 0.5 in case of the undoped and Ni-doped samples, and it even exceeded 0.6 for the Co-doped catalyst at 950\u2009\u00b0C. The potential to tune the desired H2/CO ratio by varying the composition of the perovskite lattice becomes evident. Powder XRD measurements after the reactions were performed, which confirmed that the perovskite structure was intact even after the high temperature reactions (Fig. S9, SI). Additionally, a SEM image of the used Ni-doped catalyst (Fig. 3b) shows particles with an average diameter of 60.9\u2009nm, which is larger than in the sample that was exclusively used at lower temperatures (26\u2009nm, cf. Fig. 2a). An EDX mapping (Fig. 3c, possible due to the larger particle size) revealed that Ni is accumulated within the particles, supporting their Ni-rich composition. It should be noted that the achieved H2/CO ratio is still not as high as in recent literature (e.g. from Ignacio de Garcia et al. [33]). The reason for this lies in the pronounced oxygen vacancy formation and stabilisation of our perovskite oxide samples which results in a high rWGS activity and therefore decreases the H2/CO ratio [19].To investigate structural changes that occur during DRM and upon in-situ exsolution, operando XRD measurements were performed. For each catalyst, experiments with both pre-formed nanoparticles and in-situ exsolution were conducted. For the undoped catalyst (Nd0.6Ca0.4FeO3-\u03b4), the perovskite lattice structure (\nFigs. 4a as well as S11, SI) was preserved even at high temperatures and even if a reducing pre-treatment in wet H2 was applied before reaction. This highlights the stability of the perovskite host lattice, which does not decompose under reaction conditions and holds true for the doped samples as well. However, it should be noted that exsolution as well as Ca segregation were observed meaning that the perovskite structure is a dynamic one and does undergo changes, but it remains the predominant phase. This dynamic structure, including its rich oxygen chemistry of the perovskite surface is beneficial for DRM (e.g. by preventing coking). For the undoped sample, metallic Fe formed upon pre-reduction, which was oxidised to Fe3O4 (peak at 35.4\u00b0) during DRM at lower temperatures. Between 650\u2009\u00b0C and 700\u2009\u00b0C, Fe3O4 was transformed back into metallic Fe (peak at 44.2\u00b0). For the oxidised sample, the formation of Fe3O4 could be observed above 550\u2009\u00b0C, which transitioned into Fe above 650\u2009\u00b0C as well. A comparison of the intensities of pre-reduced and oxidised samples indicates that pre-reduction led to more metallic Fe being formed during DRM (noticeable by a stronger Fe metal signal). However, in case of the pre-reduced sample, also a stronger CaCO3 peak (29.4\u00b0) was observed which transformed into CaO (36.8\u00b0) above 650\u2009\u00b0C. This may be a consequence of the more pronounced Fe exsolution; however, due to the substantially higher activity observed on the pre-reduced catalysts, a significant loss of active perovskite surface due to Ca surface segregation can be compensated. Additionally, a small diffraction peak corresponding to graphite formation was detected at 26.3\u00b0 with no significant difference between the two experiments.\nFig. S12 summarises results for the Co-doped perovskite (Nd0.6Ca0.4Fe0.9Co0.1O3-\u03b4) with only oxidative treatment. Exsolution of B-site elements are represented by a diffraction peak forming at 44.5\u00b0. This peak can be attributed either to a Co hcp phase or a Fe bcc phase. Alternatively, a mixed bcc phase consisting of both B-site metals is possible [34]. However, alloy formation cannot be assessed with XRD alone, as the diffraction peaks are too close to each other. The second peak of the bcc phase, located at around 64.5\u00b0, begins to form at 650\u2009\u00b0C, which might indicate additional Fe exsolution taking place. Furthermore, the formation of CaCO3, with a diffraction peak at 29.3\u00b0, could be observed starting at 550\u2009\u00b0C. The intensity of this signal increased up to 650\u2009\u00b0C, but it vanished at 700\u2009\u00b0C, were CaCO3 transformed into CaO, shown by the new peak at 31.2\u00b0. Even though the second peak of CaO (at 53.5\u00b0) overlaps with a perovskite peak, a change of the intensity ratios of the perovskite peaks in this region further indicated formation of CaO. Additionally, a signal corresponding to graphite started forming at 550\u2009\u00b0C, thus indicating that at least some carbon deposition occurred. However, as the peak remained small, it can be assumed that the carbon deposition was not severe. This assumption is further supported by the fact that no CNTs could be seen in the SEM images. Alternatively, the formation of a graphite layer around the exsolved nanoparticles would be possible, but since the diffraction peak is not growing, ongoing coking was not observed. The pre-reduced sample showed similar behaviour upon heating (Fig. 4b). Exsolution was visible already at 500\u2009\u00b0C and the corresponding diffraction peak continued to grow with rising temperature. Similar to the oxidised catalyst, Co and Fe could not be separated and appear in a combined peak at 44.5\u00b0. CaCO3, CaO, and graphite were also observed.In case of the oxidised Ni-doped catalyst (Nd0.6Ca0.4Fe0.97Ni0.03O3-\u03b4, Fig. S13), sharper XRD reflexes were observed, as this sample had to be sintered at higher temperatures to achieve phase purity. In the measurement of the reduced Ni-doped catalyst, the formation of an additional phase similar to a perovskite phase was observed. At 700\u2009\u00b0C the perovskite diffraction peaks had shoulders on their left edges (Fig. 4c). This is most likely a Ruddlesden-Popper phase (marked with \u201cRP\u201d). In case of the oxidised sample, peaks corresponding to Fe and Ni appear at 44.8\u00b0 and 44.2\u00b0, respectively, at 650\u2009\u00b0C. This indicates in-situ exsolution of the dopant and the B-site cation. The Ni particles (43.5\u00b0) on the surface of the pre-reduced sample (exsolved during the reductive pre-treatment) were observed to switch back to an oxidic state upon exposure to the DRM reaction environment at lower temperatures (i.e. the nanoparticles on the surface are oxidised). Only at higher temperatures (700\u2009\u00b0C), the metallic state re-emerges due to the reducing reaction environment. Furthermore, a diffraction peak at 44.1\u00b0 possibly corresponding to Fe is present above 500\u2009\u00b0 in the oxidised sample. In contrast to the measurements with Nd0.6Ca0.4FeO3-\u03b4, neither CaCO3 nor CaO could be observed. In both Ni-doped samples, graphite was observed in small amounts.Using refinement techniques, the peak widths of the undoped and the Co-doped samples were analysed. This is related to crystallite sizes, however only relative trends are reported here, as reasonable absolute quantification would require further knowledge about broadening caused by the XRD instrument. The Ni-doped perovskite (which was additionally sintered during synthesis) exhibited very narrow diffraction peaks in the XRD data (visible broadening is mainly caused by the XRD setup itself), indicating larger perovskite crystallites and high order. The broadening of the perovskite phase itself was similar (and much more pronounced than in the case of Ni-doping) for the undoped and Co-doped samples. In both cases, the catalytic reaction led to a narrowing of the perovskite phase peaks. This indicates that the pristine samples consist of relatively small perovskite crystallites, but the morphology slowly changes towards bigger and less imperfect crystallites at high reaction temperatures. Furthermore, the peak width of the metallic phase in the Co-doped sample confirms that pre-reduction led to larger crystallites (indicated by a narrower peak) \u2013 this agrees with the SEM results.To further investigate the changes of the catalyst surfaces during DRM reactions, in-situ NAP-XPS experiments were performed. Special attention was paid to the chemical state of the elements on the B-site of the catalyst as the formation of a metallic B-site species in XPS spectra is an indicator for exsolution.The spectroscopic results for the B-site elements for the oxidised samples are summarised in \nFig. 5 (the full series is shown in the SI, Figs. S14\u2013S16). In case of the undoped Nd0.6Ca0.4FeO3-\u03b4, no formation of a metallic phase occurred even at 700\u2009\u00b0C. The same experiment with the corresponding pre-reduced catalyst revealed that even at 700\u2009\u00b0C under DRM condition or in H2/H2O atmosphere no Fe exsolution could be triggered (Fig. S14d).For the oxidised Co-doped catalyst Nd0.6Ca0.4Fe0.9Co0.1O3-\u03b4, the Co2p3/2 peak exhibited a gradual shift into a metallic state (from 780.2\u2009eV to 777.5\u2009eV [29], Fig. S15a). First indications of a metallic Co2p contribution, corresponding to in-situ exsolution, were found at 500\u2009\u00b0C. The amount of metallic species was increasing with the temperature up to 700\u2009\u00b0C in DRM gas atmosphere. The in-situ pre-reduction of the catalyst revealed that metallic Co formed on the sample surface during the reduction step. When switching to DRM conditions at 400\u2009\u00b0C, however, the Co turned oxidic again. This indicates that the formed nanoparticles are present in an oxidic form at lower temperatures. They are transformed back into a metallic state between 550\u2009\u00b0C and 600\u2009\u00b0C (Fig. S15b). Interestingly, the ratio of metallic to oxidic Co is significantly larger than in the oxidised sample. This could suggest that the reductive pre-treatment led to the formation of bigger or more nanoparticles on the surface as already shown above (Fig. 2).For the oxidised Ni-doped catalyst, a shift of the Ni2p signal was observed from the oxidic state at 854.7\u2009eV into a metallic state at 852.2\u2009eV between 600\u2009\u00b0C and 650\u2009\u00b0C (Fig. S16a) [35]. The in-situ measurement on the related pre-reduced catalyst demonstrated that the reduction step led to the formation of metallic Ni (Fig. S16b). Switching to DMR conditions at 400\u2009\u00b0C caused the re-oxidation of the exsolved nanoparticles. Above 600\u2009\u00b0C, the nanoparticles became metallic again. The Fe2p signal does not show the formation of metallic Fe during the course of any experiment (Fig. S14). This is a discrepancy to the results of the XRD measurements. Possible reasons for this are discussed in the summary.The different behaviour between the Ni- and Co-doped catalysts, respectively, (in the Ni2p and Co2p spectra a fast switch to the metallic state within one temperature step was observed for the Ni-doped sample, while the formation of Co metal occurred more gradually in the Co-doped material) could be explained by the amount of B-site doping. Whereas the amount of Co-doping was 10 %, only 3 % Ni could successfully be incorporated into the perovskite. The Ni reservoir in the sub-surface region is therefore much smaller. Thus, Ni depletes much faster upon exsolution. Moreover, remaining trace amounts of Ni within the host lattice may be below the detection limit of XPS. In case of Co-doping, the larger reservoir allows for a more gradual growth of the nanoparticles with increasing reaction temperature. The importance of the concentration of the exsolving element for exsolution properties was already proven by Gao et al. [36]. As no metallic Fe could be detected even at 700\u2009\u00b0C, the measurements indicate that the formed nanoparticles are indeed pure Ni and Co, respectively. TPR results (shown in the SI) support that there is a temperature window, where solely the more easily reducible dopant element is reduced, while reduction of Fe starts only at even higher temperatures (the exact temperatures depend on the conditions).The key finding of an analysis of the C1s spectra for all materials was the clear absence of dominant coking, as no strong carbon signals occurred throughout the reaction (Fig. S17). The observed carbon species were mostly present at lower temperatures and, in fact, vanished at higher reaction temperatures. According to Dama et al., the NiCx peak, indicating the coking of the nanoparticles, forms at binding energies around 280\u2009eV [14]. This peak was entirely absent during all measurements, and no carbonaceous structures were observed in any post-reaction SEM images, proving that all tested catalysts exhibited improved coking resistance. This is probably a consequence of the rich oxygen surface chemistry of perovskites, which hinders the formation of carbon deposits on the surface. Furthermore, it was reported that alkaline earth metals are promoting the re-oxidation of surface carbon on Ni-based catalysts [37] and Ca is known to be an exceptionally good promoter [14]. Additionally, with respect to NiCx, Dama et al. distinguished between C-C compounds with binding energies around 285\u2009eV and COx compounds with higher binding energies around 290\u2009eV. In both these energy regions, peaks could be observed, however, no intense signal occurred, indicating that only traces of carbon were present on the surface. Concerning the C1s spectra and the carbon surface chemistry, the investigated perovskites show a quite different behaviour:The experiment with the pre-reduced undoped catalyst (Nd0.6Ca0.4FeO3-\u03b4) showed no C1s signals during the pre-treatments. When switching to DRM conditions, four species appeared (\nFig. 6a). At ~292.5\u2009eV, the gas phase peak becomes visible. This signal grew weaker as the temperature increased. This indicates that the CO2 near the surface was reacting. The CO3\n2- species (289.2\u2009eV) is present above 400\u2009\u00b0C up to 700\u2009\u00b0C. Its amount decreased with rising temperature as well. Between 285.5\u2009eV and 286.0\u2009eV, adventitious carbon is present; however, the signal decreased upon further heating. The species with the lowest binding energy (283.9\u2009eV) is only visible at the 550\u2009\u00b0C and 600\u2009\u00b0C steps and corresponds to graphitic carbon.The pre-reduced Co-doped catalysts displayed only two contributions to the C1s signal at lower temperatures (Fig. 6b). The carbonate species at 289.6\u2009eV was present between 400\u2009\u00b0C and 550\u2009\u00b0C and vanished at higher temperatures. Adventitious carbon could be detected below 550\u2009\u00b0C between 284.8\u2009eV and 285.6\u2009eV. Above 600\u2009\u00b0C, no carbon signals could be detected.When a reductive pre-treatment was applied to the Ni-doped catalyst, three different carbon species were visible (Fig. 6c). The species at the highest binding energy (around 292.5\u2009eV) could be assigned to the gas phase signal. It was only visible at 400\u2009\u00b0C and 500\u2009\u00b0C, indicating increased CO2 conversion at higher reaction temperatures. Between 289.3\u2009eV and 289.7\u2009eV, a carbonate species could be observed at the two lowest temperatures. Below 550\u2009\u00b0C, adventitious carbon could be detected between 285.1\u2009eV and 285.6\u2009eV.The O1s peak fitting revealed two relevant species (\nFig. 7). The main peak, in case of all three materials between 528.3 and 528.9\u2009eV, can be attributed to lattice oxygen of the perovskites host lattice, as reported in literature [38,39]. The smaller peak at higher binding energies consists of a carbonate component and a hydroxide component. Both of these species are important for the mechanism of DRM [40]. Even though the resolution of the lab-based XPS system is not sufficient to separate them clearly [38], the analysis of the C1s region confirms the presence of carbonate as discussed above. The signal from the C1s spectra can be used to calculate and fit the corresponding O1s contribution. The remaining peak intensity can be assigned to surface hydroxyl groups. The details for this calculation can be found in the SI (Tables S6\u2013S12). It was observed that the carbonate amount is largest below 600\u2009\u00b0C in each measurement. This means that surface carbonate at lower temperature most likely derives from adsorbed CO2, which is reacting faster at higher temperatures. Alternatively, the formed CaCO3 is transforming into CaO as observed in the XRD experiments (Section 3.3).In the experiment with the undoped pre-reduced catalyst, the lattice oxygen did not shift under DRM reaction conditions; its position stayed constant at 528.9\u2009eV (Fig. S18d). The hydroxyl component did not shift as well and stayed at 530.4\u2009eV [41]. Interestingly, the signal contribution assigned to the carbonate shifted to higher binding energies during reaction. As the temperature increased, the carbonate species shifted from 531.4\u2009eV to 532.0\u2009eV [42]. A closer look into the carbonate amount contributing to the shoulder of the main oxygen peak reveals that the carbonate amount peaks at 600\u2009\u00b0C at 39 % (similar to the maximum amounts found for the oxidised sample as seen in Fig. S18a). In contrast to the latter, there was, however, carbonate present at higher temperatures as well. The carbonate species at lower temperatures, stemming from adsorbed CO2, is most likely reacting similarly to the measurement with the oxidised catalyst. The carbonate remaining at higher temperatures most likely corresponds to CaCO3 which was already observed in the operando XRD measurements.In case of the oxidised Co-doped catalyst, the hydroxyl contribution was constant at 531.0\u2009eV and a carbonate peak was present at 531.5\u2009eV. Interestingly, in contrast to the undoped oxidised catalyst, carbonate is present in all measurements. The amount of carbonate in this sample was the highest for all measured catalysts. At 500\u2009\u00b0C, the amount peaked at 64 % before declining again to around 40 % at 700\u2009\u00b0C (Table S9). The general trend of the carbonate amount with temperature was similar to the other experiments (i.e. rising at first but declining at higher temperatures). The decrease of the amount of carbonate coincided with the temperature at which metallic Co could be observed in the measurements of the oxidised catalyst (Fig. 4). However, the Co-doped catalyst appears to exhibit a tendency to form more CaCO3, as the carbonate amount on the surface was also significant at higher temperatures. During the measurement with the pre-reduced catalyst, the hydroxyl component shifted from 530.5\u2009eV to 531.1\u2009eV. The carbonate species was only present up to 550\u2009\u00b0C (at 531.5\u2009eV). As seen in Table S10\n(SI), the carbonate amount remained constant at around 47 % between 400\u2009\u00b0C and 550\u2009\u00b0C. At higher temperatures, the carbonate amount dropped to 0, as no carbonate could be observed in the C1s region. One has to keep in mind that the areas fitted for the C1s of carbonate were already very low, the decrease of surface carbonate (through the start of DRM reaction) led to the signal dropping below the detection limit and made fitting not feasible anymore.In case of the corresponding reduced catalyst with Ni-doping, the reductive pre-treatment caused formation of surface carbonate visible at 400\u2009\u00b0C and 500\u2009\u00b0C. At these temperatures, the amount of carbonate in the region with high binding energy was nearly 50 % (Fig. S18f). However, at higher temperatures (>550\u2009\u00b0C), it completely vanishes due to the increased catalytic activity. The binding energy of the OH- species remain unchanged at 531.0\u2009eV during the whole measurement.The catalytic results confirm that reductive pre-treatment increased the catalytic activity significantly for all three perovskite catalysts. Compared to samples with only an oxidation step, SEM images revealed that in case of B-site doped pre-reduced perovskites the exsolved nanoparticles were significantly bigger on average, e.g. 14.1\u2009nm compared to 25.5\u2009nm in case of the Ni-doped perovskite, thus providing a possible explanation for the increased activity. This clearly shows that to achieve enhanced catalytic activity, pre-reduction is necessary. The Ni-doped perovskite exhibited the highest catalytic activity of all tested materials after nanoparticle exsolution. A pronounced rWGS activity could be observed at intermediate reaction temperatures. The selectivity shifts towards the desired DRM reaction only at high reaction temperatures above 800\u2009\u00b0C. For the Ni- and Co-doped perovskites, a H2/CO ratio of 0.5 and 0.6, respectively, could be observed at 950\u2009\u00b0C. Kapokova et al. observed similar behaviour in their studies [43]. They reported an increase in the H2/CO ratio with rising temperature. However, the increase in H2 formation in that work was not as high as in our case, most likely due to the fact that perovskites used in their studies are extremely active for the rWGS reaction [19].XRD and operando XRD measurements confirmed host lattice stability even at the highest reaction temperatures (950\u2009\u00b0C) for all materials and experiments. Deactivating phases such as CaCO3 and graphite were observed, but no severe coking and CNT formation as SEM images revealed. The B-site doped catalysts exhibited in-situ exsolution under DRM conditions \u2013 the Co-doped sample showed an exsolution onset of 550\u2009\u00b0C, exsolution on the Ni-doped material started above 600\u2009\u00b0C.Comparing operando XRD and in-situ NAP-XPS results, some discrepancies stand out at first glance. In the XRD results, the undoped catalyst exhibited formation of metallic Fe which could not be observed in the NAP-XPS studies. Moreover, the XRD measurements show a graphitic phase at high reaction temperatures which could not be observed in any XPS spectra. An explanation for this deviation could be the difference in the respective operating pressures: While operando XRD was performed at ambient pressure (1\u2009bar), only a pressure of 1 mbar was accessible in case of NAP-XPS. This, of course, leads to different reaction rates on the surface. The pressure dependence of the chemical potential of the reaction environment plays a role as well. Additionally, more educt is reacting on the surface during operando XRD measurements due to larger partial pressures. This increasing reaction rate in turn leads to different rates at which oxygen vacancies are formed and side products (such as carbonaceous species) are deposited on the surface. The increased rate of the oxygen vacancy formation can, of course, cause preferred exsolution of B-site elements as a way to increase stability of the material [45]. Another difference between operando XRD and in-situ NAP-XPS is the sensitivity of the respective method. While XRD is a bulk method, XPS only probes surfaces. As it is also possible for nanoparticles to form at grain boundaries within the bulk, more exsolved metal can potentially be detected with XRD. Furthermore, two different types of samples were used for the different methods. While powders were used directly for the XRD measurements, thin film samples had to be prepared with PLD for the XPS measurements.The graphical scheme in \nFig. 8 summarises the generally accepted parts of the DRM mechanism. The scheme depicts the steps of H2 and CO2 activation as well as the hydrogen spill-over. The most complex and not yet fully understood step, the oxidation of the intermediates, is not shown. The formed COx species constitute the catalytically active carbonate species observed in the XPS measurements at lower temperatures. As mentioned above, the most complex part of the reaction mechanism is the reaction of the COx groups with either OH or adsorbed hydrogen to form CHxOH, which is subsequently oxidised and desorbs as CO and H2. For this step, a lot of different pathways can be found in the literature [20]; however, no fully agreed upon pathway has been established. Based on our findings, we propose a switch of the reaction mechanism depending on the temperature \u2013 the main property affected by this switching is the behaviour of the adsorbed hydrogen: At temperatures below 600\u2009\u00b0C, we observed a low H2/CO ratio, which indicates that hydrogen is not recombining to H2, but is instead spilling over to the oxide where it forms OH groups with lattice oxygen atoms. These OH groups on the surface can form water and subsequently desorb from the surface, leaving behind an oxygen vacancy that can be refilled by an adsorbed CO2. This pathway is also shown in Fig. 1 as the \u201crWGS-type pathway\u201d due to its similarity to the rWGS reaction \u2013 albeit with a different source for the adsorbed hydrogen. At higher temperatures, we observed an increase of the H2/CO ratio which suggests a change in the system. The NAP-XPS analysis also revealed that the OH contribution in the O1s region is decreasing with rising temperature. This supports the assumption that the adsorbed hydrogen is recombining directly on the nanoparticles instead of spilling over to the support. This pathway is depicted in Fig. 1 as \u201cDRM-type pathway\u201d.The correlation between in-situ nanoparticle exsolution during DRM and exsolution by pre-reduction in wet H2, respectively, and the subsequent catalytic performance was studied for three different perovskites: Nd0.6Ca0.4FeO3-\u03b4 (B-site undoped), Nd0.6Ca0.4Fe0.9Co0.1O3-\u03b4 (B-site Co-doped) and Nd0.6Ca0.4Fe0.97Ni0.03O3-\u03b4 (B-site Ni-doped). Although both exsolution scenarios led to the formation of Co or Ni nanoparticles on the surface of the B-site doped materials, the catalytic results show that nanoparticle exsolution by pre-reduction is enhancing the surface activity significantly more compared to in-situ exsolution. SEM investigations and determinations of particle size distributions reveal a possible reason for this performance difference: Nanoparticles formed during the reducing pre-treatment were bigger on average than their counterparts that were exsolved in-situ. At intermediate reaction temperatures, rWGS is a significant side reaction leading to reduced H2 production. At high temperatures, the selectivity changes and DRM is the dominant pathway leading to an obtained H2/CO ratio of 0.5 and 0.6 for the Ni- and Co-doped perovskites, respectively. These results correlate with the observed amount of hydroxyl groups on the perovskite surface (formed by H-spill over) by NAP-XPS. Of all tested perovskites, the Ni-doped catalysts showed the highest total activity.In addition to catalytic tests, operando XRD and in-situ NAP-XPS measurements were performed during DRM. The host perovskite lattice was stable up to the highest reaction temperatures for all tested materials. Metallic phases, corresponding to the exsolved nanoparticles, could be detected by both methods in case of the B-site doped catalysts. At high reaction temperatures, the formation of trace amounts of CaCO3 and graphite was observed. Both processes are undesired, as they can lead to surface deactivation. Interestingly, no formation of carbon nanotubes or big amounts of carbon deposits could be observed in case of the Ni-doped catalyst. The rich oxygen chemistry of the perovskite is a likely reason, as it facilitates effective removal of undesired carbon species, as observed by NAP-XPS.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n\u00b0 755744 / ERC - Starting Grant TUCAS).\nFlorian Schrenk: Conceptualization, Investigation, Formal analysis, Validation, Writing \u2013 original draft, Writing \u2013 review & editing. Lorenz Lindenthal: Investigation, Formal analysis, Visualization, Writing \u2013 review & editing. Hedda Drexler: Investigation, Formal analysis, Software. Gabriele Urban: Investigation, Formal analysis, Raffael Rameshan: Data curation, Investigation, Formal analysis. Harald Summerer: Investigation, Formal analysis, Resources. Tobias Berger: Investigation, Formal analysis, Software. Thomas Ruh: Data curation, Validation, Writing \u2013 review & editing. Alexander K. Opitz: Supervision, Validation, Writing \u2013 review & editing. Christoph Rameshan: Conceptualization, Funding acquisition, Project administration, Supervision, Writing \u2013 original draft, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The X-ray measurements were carried out within the X-Ray Center of TU Wien; SEM images were recorded at the USTEM, TU Wien. The authors acknowledge TU Wien Bibliothek for financial support through its Open Access Funding Programme.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2022.121886.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Nanoparticle exsolution is regarded as a promising alternative to classical catalyst synthesis routes. In this work, we compare the catalytic performance of nanoparticles formed by in-situ exsolution during dry reforming of methane with particles pre-formed by reductive pre-treatment. The experiments were conducted on three perovskite-type oxides. Using a combination of in-situ and operando spectroscopic investigations (x-ray diffraction, near ambient pressure x-ray photoelectron spectroscopy) and the correlation to the obtained catalytic results, we could highlight that pre-formed nanoparticles strongly enhance the activity compared to in-situ exsolution. Scanning electron microscope images recorded after catalytic tests revealed that nanoparticles formed during reductive pre-treatment are bigger on average than particles formed in-situ. Furthermore, B-site doping with Co or Ni significantly enhanced the catalytic activity. Importantly, the perovskite host lattice was stable in all experiments, thus providing the necessary enhanced oxygen surface chemistry which is the key to the coking resistance of the investigated materials. Additionally, we observe a temperature dependent change of mechanism leading to different product ratios.\n               "}
{"full_text": "", "descript": "\n                  An attempt has been made to investigate and optimize the recovery of Ni and Al through sulphuric acid (3.0\u22125.5 mol/L) leaching under different operating conditions. From the leaching experiments, it was possible to extract 98.5% of NiO and 40.7% of Al2O3 under the conditions of 5.5 mol/L H2SO4, reaction time of 4 h, solid-to-liquid ratio 0.2 g/mL, temperature of 358 K, particle size <100 \u03bcm, 200\u2212250 r/min with 5.0 g catalyst dosage. The leached liquor Al was separated by selective crystallization using 1.4 mol/L KOH and Ni was separated by selective precipitation using 0.3 mol/L H2C2O4. From the studies, it is possible to recover around 97.9% of NiO having 98.3% purity, around 25% of Al2O3 was also recovered as alum-(K) having 99% purity and 14.7% of Al2O3 as a salt of Al\u2212K\u2212C2O4\u2212SO4.Sulphuric acid was found to be a suitable leaching agent for selective leaching and it was also observed that alum-(K) can be selectively crystallized from sulphate solutions. The study also indicated the effective extraction and recovery of nickel and aluminium which were well supported by characterization studies using TG-DTA/DTG and XRD techniques.\n               "}
{"full_text": "In recent decades, stringent environmental regulations have strictly limited the sulfur contents in engine fuels to below 10\u00a0\u00b5g\u2022g\u22121 with increasing global concerns about the contamination of the eco-environment. Organosulfur compounds in engine fuels have been confirmed to be one of the main sources of eco-environmental contaminants since sulfur dioxide, which damages both the eco-environment and human health. These compounds are emitted after combustion in engines [27,46,16,3,71]. Thus, it becomes a significant challenge for oil refineries to produce road engine fuels with ultralow or nearly zero sulfur content. Various efforts have been made to develop desulfurization techniques, such as oxidation desulfurization, adsorption desulfurization, biodesulfurization and hydrodesulfurization (HDS). Among all these techniques, HDS has been concretely confirmed to be the most efficient, economical and practical desulfurization technique [6,26,56,49,25]. After decades of development, the efficient design of HDS catalysts with high catalytic performances has been confirmed as the core of the HDS technique. The most successful commercial HDS catalyst is bimetallic Ni(Co)Mo(W)S2 supported on \u03b3-Al2O3 due to its relatively low cost and comparable activities to noble metal supported catalysts [47,32,52,19,51]. However, with the increasing contents of highly refractory sulfides derived from dibenzothiophene in the harmonic blends, the activities of these kinds of catalysts cannot easily meet the requirements for the production of road engine fuels with ultralow sulfur contents [8,50].Massive literatures have demonstrated that the higher homologues of dibenzenthiophene, especially sulfides similar with 4,6-dimethyldibenzothiophene(4,6-DMDBT), in road engine fuels are the most difficult to remove [21,17]. Thus, the effective removal of sulfur atoms from this kind of sulfide is the bottleneck for the improving the performances of HDS catalysts [34,70]. 4,6-DMDBT can undergo HDS via either direct desulfurization (DDS) pathway or the hydrogenation desulfurization (HYDS) pathway. Because the methyl groups severely restricts the DDS pathway due to the sterically hindering of the C-S-C bond in the thiophene ring adsorbing on the active sites. and the latter one has been acknowledged as the main pathway. the methyl groups located at the 4- and 6-positions sterically hindering the C-S-C bond, which severely [7,38,11]. Thus, efforts should be made to improve the hydrogenation activity of HDS catalysts to further enhance their catalytic performance.Our previous experimental work and the density functional theory (DFT) calculation results have already proven that the selectivity for both the DDS pathway and the HYDS pathway are highly correlated to the morphology of the active sites [12,45,69,68]. We found that the activity and selectivity of the DDS pathway correlates to the coordination unsaturated Mo sites (CUS) from the corner of the NiMoS crystals, and the activity and selectivity of the HYDS pathway correlates to the CUS from the edge sites of the NiMoS crystals. The formation of both corner CUS and edge CUS is affected by the composition and structure of support materials because the morphology of the NiMoS2 slabs can be modulated by the MSI [2,5]. For this purpose, the modification of \u03b3-Al2O3 surfaces by other oxides, metal oxides and nonmetal oxides has been widely investigated. Although the incorporated modifier could greatly disperse on the \u03b3-Al2O3 surfaces at the initial stage, it agglomerates after a long period of usage, which would lead to a quick deactivation of the corresponding HDS catalysts. In situ synthesized binary oxides, such as Ga2O3-Al2O3 [10,24], SiO2-Al2O3 [9,55,57,54], ZrO2-Al2O3\n[10], MgO-Al2O3\n[53], B2O3-Al2O3 [43,33] and TiO2-Al2O3 [37,40], have been explored as HDS catalyst supports to overcome this problem. Among these investigated binary oxides, TiO2-Al2O3 attracts the most scientific attention due to its improved hydrogen transfer capacity caused by the existence of Ti3+ species, which results in the improved hydrogenation activity of HDS catalysts. V. Santes [20] prepared TiO2-Al2O3 binary oxides via three different preparation methods and found that the sol-gel method provides the highest surface area and excellent pore structures. The mesopores of the prepared TiO2-Al2O3 binary oxides is disordered, and its pore size distribution is relatively wide. Duan and coauthors [13] also successfully synthesized TiO2-Al2O3 composites which were used as HDS catalyst support. They proposed that the incorporated TiO2 weakened the MSIs of a NiW supported catalyst since less strong Al-O-W linkages were formed. Morris and coauthors [36] proposed a solvent evaporation-induced self-assembly strategy to synthesize TiO2-Al2O3 composites with highly ordered mesopores. Recently, we also synthesized TiO2-Al2O3 composites with highly ordered mesostructures and used them as HDS catalyst support to uncover the effect of the incorporation of Ti species on the NiMo/TiO2-Al2O3 catalyst [72]. All these studies have established that the type of active phase and the dispersion, morphology, MSIs and sulfidation degrees are closely related to the incorporated TiO2 species. However, few literatures have shed light on the effect of solvent evaporation temperature on the surface states of highly ordered mesoporous TiO2-Al2O3 composites, and the effect of solvent evaporation temperature on the catalytic performance of highly refractory sulfides with the corresponding HDS catalysts has not been published.Here, we report the strategy of synthesizing highly ordered TiO2-Al2O3 composites at different solvent evaporation temperatures to test the hypothesis that the synthesis temperature influences the surface states of TiO2-Al2O3 composites and the corresponding NiMo supported catalysts as well as their HDS performance. Several advanced characterization techniques were performed on both the synthesized TiO2-Al2O3 binary oxides and the corresponding catalysts. Finally, the catalytic performance of the catalysts were assessed.Highly ordered TiO2-Al2O3 with TiO2 content uniformed at 20\u00a0wt.% were synthesized at different solvent evaporation temperatures according to a reported method [1,61,65]. Here is an example: 1.76\u00a0g of aluminum isopropoxide (99.8%, Rhawn) was precisely weighed and dissolved in 10.0\u00a0mL of anhydrous ethanol, dropwise addition of 1.6\u00a0mL of fuming HNO3 (68.0%, Xi'an Sanpu Fine Chemical Co. Ltd.) was followed. Simultaneously, 2.00\u00a0g of P123 (98.0%, Sigma\u2013Aldrich) was dissolved in 10.0\u00a0mL of anhydrous ethanol, this solution was added dropwise into the flask within 5\u00a0min. After suspended for 4\u00a0h, 0.47\u00a0g of titanium tetraisopropanolate (98.0%, Aladdin) was added within 5\u00a0min. The mixture was transferred to a porcelain vessel after being stewed for 4\u00a0h, which was then placed into an oven to evaporate the solvent at the required solvent evaporating temperature for 2 days. Finally, the obtained powder was dehydrated in a 120 \u00b0C oven and calcined at 550 \u00b0C in a muffle furnace. Such obtained products were denoted TA-x, where x is the solvent evaporation temperature in Celsius with values of 50, 60, 70 or 80.The Ni and Mo precursors were loaded via the proposed incipient wetness coimpregnation method [36,52,71,72]. The pelleted and crushed grains between 20 and 40 mesh were collected. Then, the coimpregnation solution containing both the Ni precursors and Mo precursors was impregnated onto the aforementioned grains. The prepared samples were dehydrated in a dry-air flow overnight at room temperature and dried at 120\u00a0\u00b0C for 6\u00a0h before it was calcined at 550\u00a0\u00b0C in a muffle furnace. The prepared catalysts with uniform NiO loadings of 4\u00a0wt.% and MoO3 loadings of 12\u00a0wt.% were denoted NiMo/TA-x.Bruker D8 Advance powder diffractometer was used for performing both the wide-angle and small-angle XRD characterization of the synthesized TA-x serial composites. For all the tested samples, small angle XRD patterns with 2\u03b8 values from 0.5\u00b0 to 6\u00b0 were recorded at scanning rate of 0.02\u00b0s\u22121, and wide angle XRD patterns with 2\u03b8 values from 20\u00b0 to 80\u00b0 were also recorded at scanning rate of 0.5\u00b0s\u22121. The N2 physical adsorption-desorption characterization of the synthesized TA-x serial composites was performed on a Micromeritics ASAP 2020 volumetric analyzer. Prior to the test, the samples were completely degassed at 400\u00a0\u00b0C for 12\u00a0h at vacuum. The BET method was employed to calculate the specific surface areas and the BJH method was employed to determine the pore sizes distribution. A JEOL JEM-2100 instrument, whose acceleration voltage is 200\u00a0kV, was used to observe the mesopore arrangement of the synthesized TA-x serial samples, and images were taken. An Magna 560 FT-IR instrument was used to perform the pyridine absorbed FTIR characterizations of the synthesized TA-x serial composites. The tested TA-x sample (0.1\u00a0g) was completely degassed for no less than 4\u00a0h under vacuum conditions at 350\u00a0\u00b0C, after cooled to 50\u00a0\u00b0C, the saturated pyridine vapor was pulsed for 30\u00a0min. The FTIR spectra were recorded after the pyridine desorbed at 200\u00a0\u00b0C for 0.5\u00a0h. Then, the pyridine was further desorbed at 350\u00a0\u00b0C for another 0.5\u00a0h, and the FTIR spectra were recorded again.The prepared NiMo/TA-x serial oxide catalysts were temperature programmed and reduced by H2 on a self-built device. The pretreated catalyst (0.3\u00a0g) was loaded onto a quartz tube and the reduction gas (5 v% H2 loaded by 95 v% N2) was pulsed into the quartz tube with temperature programming to 800\u00a0\u00b0C at heating rate of 10\u00a0\u00b0C/min; simultaneously, the H2-TPR profile was recorded on a mass spectrograph. The Fourier transformed Raman (FT-Raman) spectra for the NiMo/TA-x serial catalysts were recorded on a Renishaw inVia Reflex apparatus in the wavenumber range of 300\u20131400\u00a0cm\u22121 to disclose the effect of the solvent evaporation temperature on the coordination states of the NiMo precursors on the TA-x composite surface.After the catalysts were fully sulfided by CS2 cyclohexane solution with concentration of 2.2\u00a0wt.% at 320\u00a0\u00b0C for no less than 6\u00a0h, an FEI Tecnai G2 F20 instrument was used to observe the morphologies of the Ni-promoted MoS2 crystals. Statistical works referred to reported methods were performed based on the HRTEM images of the Ni-promoted MoS2 slabs [18,15,70\u201372]:\n\n(1)\n\n\n\n\nL\n\u00af\n\n\n\n=\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\nn\ni\n\n\nl\ni\n\n\n\n/\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\nn\ni\n\n\n\n\n\n\n\n\n\n(2)\n\n\n\nN\n\u00af\n\n=\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\nn\ni\n\n\nN\ni\n\n\n\n/\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\nn\ni\n\n\n\n\n\n\n\n\n\n(3)\n\n\n\nD\n\nM\no\n\n\n=\n\n(\n\nMoe\n+\nMoc\n\n)\n\n/\nMot\n=\n\n\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n6\n\n(\n\nm\ni\n\n\u2212\n1\n)\n\n\n\n/\n\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n\n(\n\n3\n\nm\n\ni\n\n2\n\n\u2212\n3\n\nm\ni\n\n+\n1\n\n)\n\n\n\n\n\n\n\n\n\n(4)\n\n\n\nf\n\nM\no\ne\n\n\n=\nMoe\n/\nMot\n=\n\n\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n6\n\n(\n\nm\ni\n\n\u2212\n2\n)\n\n\n\n/\n\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n\n(\n\n3\n\nm\n\ni\n\n2\n\n\u2212\n3\n\nm\ni\n\n+\n1\n\n)\n\n\n\n\n\n\n\n\n\n(5)\n\n\n\nf\n\nM\no\nc\n\n\n=\n\nD\n\nM\no\n\n\n\u2212\n\nf\n\nM\no\ne\n\n\n\n\n\n\nHere, \n\nL\n\u00af\n\n is the statistical average slab length of MoS2 crystals, li\u2005is the length of slab i, ni is the number of MoS2 slabs whose slab length is li; \n\nN\n\u00af\n\n is the statistical average stacking number of the MoS2 crystals, and Ni is the stacking number of slab i. Mot, Moe and Moc are the numbers of the total Mo atoms, the edge Mo atoms and the corner Mo atoms, respectively. mi is calculated from the slab length (L\u00a0=\u00a03.2(2mi - 1) \u00c5). XPS characterizations of the sulfide NiMo/TA-x serial catalysts were performed on a PHI-5000 Versaprobe III spectrometer whose radio source was Al K\u03b1 radiation, and the C 1s peak with a binding energy of 284.6\u00a0eV was used to calibrate the binding energy (BE) scales. Both the recorded Mo 3d XPS spectra and the recorded Ni 2p XPS peaks were differentiated to check the effects of the solvent evaporation temperature on the covalent states of both Mo and Ni species as well as on the formation of NiMoS phases over the investigated sulfide catalysts.A model oil containing 4,6-DMDBT was used to assess the effect of the solvent evaporation temperature on the catalytic performance of the prepared NiMo/TA-x serial catalysts. Prior to the test, the catalysts were completely dehydrated at a temperature of 200\u00a0\u00b0C in an oven for no less than 6\u00a0h. Then, precisely weighed 1.0\u00a0g of the dehydrated catalyst was loaded into the reactor with both ends sealed by quartz sand. The temperature of the reactor was increased to 320\u00a0\u00b0C, and the sulfidation solution composed of 97.8\u00a0wt.% cyclohexane and 2.2\u00a0wt.% CS2 was pumped into the reactor at a flow rate of 10\u00a0mL\u2022h\u22121 by an Eldex-Optos 1LM micropump. H2 with a pressure of 4\u00a0MPa was simultaneously fed to the reactor at a flow rate of 1000\u00a0mL\u2022h\u22121. After complete sulfidation for 5\u00a0h, the sulfidation solution was switched with a reaction solution composed of 99.5\u00a0wt.% cyclohexane and 0.5\u00a0wt.% 4,6-DMDBT, and the reactor temperature was adjusted. The weight hourly space velocity (WHSV) of the reaction solution was adjusted, and the volumetric ratio of H2/oil was fixed at 150. The liquid products were carefully collected after being fully stabilized for no less than 6\u00a0h and were off-line analyzed by means of GC\u2013MS. The activation energies and the reaction rate constants for the overall 4,6-DMDBT HDS reaction, DDS pathway and HYDS pathway over the investigated catalysts were calculated by the following equations reported elsewhere [41,18,72]:\n\n(6)\n\n\n\nE\na\n\n=\n\n\nR\nT\nl\nn\n\n(\n\n\nx\n1\n\n/\n\nx\n2\n\n\n)\n\n\n\n/\n\n\n(\n\n\nT\n1\n\n\u2212\n\nT\n2\n\n\n)\n\n\n\n\n\n\n\n\n(7)\n\n\n\nk\nHDS\n\n\n=\n\n\nW\n\u00b7\n\u03c9\n\u00b7\nln\n\n(\n\n1\n\n/\n\n\n1\n\u2212\nx\n\n\n)\n\n\n\n/\n\nM\n\n\n\n\n\n\n\n(8)\n\n\n\nk\nHDS\n\n\n=\n\nk\n\nH\nD\nS\n\n\n\u00b7\n\nS\n\nD\nD\nS\n\n\n\n\n\n\n\n\n(9)\n\n\n\nk\nHYDS\n\n\n=\n\n\nk\nHDS\n\n\u2212\n\nk\nDDS\n\n\n\n\n\nHere, W is the WHSV counted in h\u22121. \u03c9, x (x\n1, x\n2) and M are the concentration, conversion and molar mass of 4,6-DMDBT, respectively. T(T1, T2) is the reaction temperature. SDDS is the product selectivity of the DDS pathway, namely, the selectivity of 3,3\u2032-DMDBT in the products. The turn over frequencies (TOFs) with 4,6-DMDBT conversion lower than 15% were also calculated [39]:\n\n(10)\n\n\nC\n=\n\nB\n\n\n\u00d7\n\n\nS\n\n/\n\n(\n\n\nf\n\n\n\u00d7\n\n\nm\n\n\n)\n\n\n\n\n\nHere, F is the molar flow rate of 4,6-DMDBT, nMo\n is the molar quantity of the Mo species loaded on the supported catalyst and SMo\n\u2005is the sulfidation degree of Mo species.The solvent evaporation temperature is normally considered to be a crucial factor for the successful synthesis of TiO2-Al2O3 composites. XRD characterizations of the synthesized TA-x serial composites were performed, and both the wide-angle and the small-angle XRD patterns are displayed in Fig.\u00a01\n. Fig.\u00a01A show that there is a broad diffraction peak with a 2\u03b8 degree centered at approximately 30\u00b0, which is attributed to the (101) facet of \u03b3-Al2O3 for sample TA-50 [72]. This result suggests that the main crystal phase for the synthesized TA-50 sample is \u03b3-Al2O3. There were barely any obvious diffraction peaks observed for the synthesized TA-60, TA-70 and TA-80 samples. These results indicate that the prepared TA-60, TA-70 and TA-80 samples are amorphous TiO2-Al2O3 with TiO2 species highly dispersed in the aluminum oxide phases.The small-angle XRD patterns displayed in Fig.\u00a01B clearly show that there are two different diffraction peaks for the investigated TA-50, TA-60 and TA-70 samples. The diffraction peak with a 2\u03b8 degree of approximately 0.75\u00b0 is assigned to the (100) facet, and the diffraction peak with a 2\u03b8 degree of approximately 1.5\u00b0 is assigned to the (110) facet of the two-dimensional hexagonal P6mm symmetric group [65,36,28,61]. This indicates the existence of ordered two-dimensional hexagonal mesopores in samples TA-50, TA-60 and TA-70. The highest intensity in the diffraction peak centered at 0.75\u00b0 was observed for sample TA-60, indicating that 60\u00a0\u00b0C is the most suitable solvent evaporating temperature for the synthesis of highly ordered mesoporous TiO2-Al2O3 binary composites among the four investigated temperatures. The absence of the diffraction peak at 0.75\u00b0 for sample TA-80 suggests failure in the synthesis of highly ordered TiO2-Al2O3 at a solvent evaporation temperature of 80\u00a0\u00b0C.The N2 physical adsorption-desorption characterization results of the investigated TA-x serial composites are displayed in Fig.\u00a02\n, and the surface areas, the pore volumes and the average pore diameters of the investigated samples are summarized in Table\u00a01\n.\nFig.\u00a02A clearly shows that the N2 physical adsorption-desorption isotherms exhibit a large H1-type hysteresis loop for samples TA-50 and TA-60. This indicates the existence of abundant, highly ordered cylindrical mesopores with narrow pore distributions for these two samples [59,66,58]. The hysteresis loop for sample TA-70 becomes flatter than those observed for TA-50 and TA-60, indicating a decrease in the amount of mesopores for sample TA-70 compared to samples TA-50 and TA-60. Although the shape of the hysteresis loop for sample TA-70 remained H1 type, the parallelism of the adsorption isotherm and the desorption isotherm declined, suggesting the existence of ordered mesopores and the lower orderliness in the mesopores [67]. The shape of the hysteresis loop for sample TA-80 is observed as H1 type in the low pressure region and H4 type in the high pressure region, suggesting the existence of both ordered cylindrical mesopores and disordered slit mesopores between different particles. Moreover, the relative pressure range of sample TA-80 is much wider than that of the other three samples, suggesting a wider pore size distribution for sample TA-80 than for the other three samples. Fig.\u00a02B clearly shows that the pore size distribution profoundly changed with the evaporation temperature of the solvent. Table\u00a01 shows that the specific surface areas declined in the order of TA-60 (241\u00a0m2\u2022g\u22121) > TA-70 (215\u00a0m2\u2022g\u22121) >TA-50 (214\u00a0m2\u2022g\u22121) > TA-80 (165\u00a0m2\u2022g\u22121), and the pore volumes of the synthesized samples declined in the order of TA-50 (0.41\u00a0cm3\u2022g\u22121) \u2248 TA-60 (0.40\u00a0cm3\u2022g\u22121) > TA-80 (0.36\u00a0cm3\u2022g\u22121) > TA-70 (0.30\u00a0cm3\u2022g\u22121). The pore size distribution became narrower, and the calculated average pore diameter decreased with increasing solvent evaporation temperature in the solvent evaporation temperature range of 50 to 70\u00a0\u00b0C. Although the most likely pore diameters for samples TA-70 and TA-80 are almost the same, the pore size distribution for sample TA-80 became wider, and the calculated average pore diameter increased to approximately 7.6\u00a0nm from that of approximately 4.6\u00a0nm for sample TA-70. All the observed reversed changing trends between samples TA-70 and TA-80 could be attributed to the formation of disordered slit mesopores between different particles in sample TA-80.The mesopore arrangements of the synthesized TA-x serial samples were investigated via the TEM method, and representative TEM images are displayed in Fig.\u00a03\n. It clearly shows that the shape of the mesopores for the synthesized TA-50 sample is hexagonal, and the arrangement of the highly ordered mesopores is honeycomb-like. The observed pore walls became thicker, and the orderliness of the mesopores increased when the solvent evaporation temperature increased from 50\u00a0\u00b0C to 60\u00a0\u00b0C. The orderliness of the mesopores for sample TA-70 became much poorer, and the orderliness of the mesopores declined and even became disordered for sample TA-80. Which in line with those observed from XRD and N2 physical adsorption-desorption. All these changes in the morphology of the mesopores can be explained by the following facts. The self-assembly rates for both the Ti4+ species and the Al3+ species by the P123 micellar are relatively slow at 50\u00a0\u00b0C, which results in a relatively thin pore wall and weak linkage between the TiO2 species and the Al2O3 species. This leads to a wider pore diameter and lower stability of the synthesized TA-50 sample. The acceleration in the self-assembly rate of Ti and Al species with P123 favors the formation of highly ordered TiO2-Al2O3 composites, and thus, the specific surface area increases. On the other hand, the acceleration in the evaporation rate of ethanol favors the deposition of Ti hydroxides and Al hydroxides on the P123 micellar; thus, the thickness of the pore wall increased, and the pore diameter slightly decreased. When the solvent evaporation temperature further increased to 70\u00a0\u00b0C or even 80\u00a0\u00b0C, the evaporation rate was too high to efficiently form highly ordered mesopores with relatively high specific surface areas; although the thickness of the mesopores further increased. Moreover, the generated ethanol steam blast partially destroyed a proportion of the formed ordered mesostructures and resulted in a relatively wide pore size distribution; thus, a relatively large calculated average mesopore diameter for the TA-80 sample was observed [65,61,72].It is well accepted that the acidity of the solid is always closely related to the bond linkages between different components, which are often affected by the imposed conditions, such as the solvent evaporation temperature. Thus, Py-FTIR characterization of the TA-x serial composites was performed, and the details of the acidity property changes are displayed in Fig.\u00a04\n. The three observed IR bands from Fig.\u00a04 are designated Lewis acid sites (LAS, with a wavenumber of 1453\u00a0cm\u22121), both LAS and Br\u00f8nsted acid sites (BAS, with a wavenumber of 1490\u00a0cm\u22121) and BAS (with a wavenumber of 1540\u00a0cm\u22121). The amounts of LAS and BAS were calculated according to the reported equation [23,64], and the results are summarized in Table\u00a02\n.\n\n(11)\n\n\nC\n=\nB\n\n\u00d7\n\n\nS\n\n/\n\n(\n\nf\n\n\u00d7\n\n\nm\n\n\n)\n\n\n\n\n\nHere, C is the concentration of the acid sites, S is the integrated absorbance, B is the surface area of the tested sample, f is the extinction coefficient with values of 3.03\u00a0cm\u2022mmol\u22121 for the calculation of the BAS amount and 3.80\u00a0cm\u2022mmol\u22121 for the calculation of the LAS amount, and m is the mass of the tested sample.The results from Table\u00a02 show that the amount of weak LAS detected for sample TA-60 increased with the solvent evaporating temperature to approximately 82.1\u00a0\u00b5mol\u2022g\u22121 from that of approximately 52.0\u00a0\u00b5mol\u2022g\u22121 for sample TA-50. It then sharply decreased to approximately 56.3\u00a0\u00b5mol\u2022g\u22121 for sample TA-70 with a further increase in the solvent evaporating temperature. A similar variation trend was observed for the amount of strong LAS. The variation trends in the LAS are caused by the fact that the deposition rate of both the Al hydroxide precursors and the Ti hydroxide precursors increased as the solvent evaporating temperature increased from 50\u00a0\u00b0C to 60\u00a0\u00b0C, resulting in more naked Al species, which are considered the sources of LAS. With the solvent evaporating temperature further increased to 70\u00a0\u00b0C, the thickness of the pore wall profoundly increased (proved by the relatively small mesopore diameter for sample TA-70), resulting in fewer Al species located on the surface of the synthesized composite; thus, the detected amounts for both weak and strong LAS declined. For sample TA-80, the detected amount of weak LAS profoundly increased to approximately 74.2\u00a0\u00b5mol\u2022g\u22121, and the amount of strong LAS increased to 35.0\u00a0\u00b5mol\u2022g\u22121 since the collapse of the mesopore wall formed amorphous TiO2-Al2O3 composites whose acidity property profoundly changed and was totally different from the highly ordered mesoporous TiO2-Al2O3 composites. This can be attributed to the faster deposition of titanium hydroxide and aluminum hydroxide on the P123 micelles, the more Ti-OH-Al groups existing on the surface, and the generation of H+ from the surface Ti-OH-Al groups being the main source of B acid. Thus, the higher solvent evaporating temperature favors the formation of BAS.The effect of the solvent evaporating temperature on the changes in the MSI over the NiMo/TA-x serial catalysts was checked by H2-TPR characterization, and the results are displayed in Fig.\u00a05\n.From the recorded H2-TPR profile for sample NiMo/TA-50, four different H2 reduction peaks appeared at different reduction temperatures. The first broad H2 reduction peak with a reduction temperature from 300\u00a0\u00b0C to 400\u00a0\u00b0C was assigned to the reduction of NiO species, and the existence of this H2 consumption peak suggests that NiO precursors and MoO3 precursors do not form the so-called active NiMoO precursors very well. The peak at approximately 430\u00a0\u00b0C represents the H2 consumed by the reduction of Mo(VI) to the Mo(IV) species. The H2 reduction peak at approximately 650\u00a0\u00b0C was assigned to the reduction of the higher coordinated Mo(VI) species to the lower coordinated Mo(IV) species from the NiMoO precursors that strongly interacted with the Al2O3 and TiO2 species from the support materials. The peak at a reduction temperature higher than 700\u00a0\u00b0C was assigned to the reduction of the tetra-coordinated Mo(IV) species [14,71,29]. The H2 consumption peak with a reduction temperature lower than 400\u00a0\u00b0C disappeared with the solvent evaporation temperature higher than 60\u00a0\u00b0C, suggesting that the higher solvent evaporation temperature favors the high dispersion of NiO precursors into MoO3 precursors. The largest area for the H2 consumption peak with a reduction temperature of approximately 430\u00a0\u00b0C was observed for catalyst NiMo/TA-60, indicating the highest efficiency for the formation of NiMoO precursors with a solvent evaporating temperature of 60\u00a0\u00b0C. Both the reduction temperature and the integrated area for the H2 consumption peak at approximately 650\u00a0\u00b0C did not change much for catalysts NiMo/TA-50 and NiMo/TA-60. This result suggests that the MSIs for these two samples are similar. The integrated area for the H2 consumption peaks of catalysts NiMo/TA-70 and NiMo/TA-80 increased compared to those of catalysts NiMo/TA-50 and NiMo/TA-60, indicating intensified MSIs over catalysts NiMo/TA-70 and NiMo/TA-80. Moreover, the reduction temperature of the H2 consumption peak at approximately 650\u00a0\u00b0C for catalyst NiMo/TA-80 is relatively lower, which can be attributed to the poorer uniformity between the TiO2 and Al2O3 components caused by the relatively fast deposition rates for the Ti hydroxide precursors and the Al hydroxide precursors.To further confirm the existence states of the Mo species over the oxide NiMo/TA-x serial catalysts, FT-Raman characterization on the prepared catalysts were performed, the recorded spectra are displayed in Fig.\u00a06\n. After decomposition, three different vibration peaks were observed, as shown in Fig.\u00a06. The relatively weak vibration peak at a Raman shift of approximately 325\u00a0cm\u22121 and the broad vibration peak at a Raman shift of 850\u00a0cm\u22121 were attributed to MoO4\n2\u2212 precursors, which is believed to be difficult in transferring into NiMoS active phases [22,63,62]. The peak at approximately 945\u00a0cm\u22121 is assigned to the Mo7O24\n6\u2212 precursors, which is believed to be easy in transferring into NiMoS active phases [21]. The proportions of the MoO4\n2\u2212 precursors and the Mo7O24\n6\u2212 precursors were summarized in Table\u00a03\n. The proportion of the Mo7O24\n6\u2212 precursors increases in the line of NiMo/TA-80 (53%) < NiMo/TA-50 (55%) < NiMo/TA-70 (58%) < NiMo/TA-60 (62%), suggesting that the superior solvent evaporating temperature (with a value of 60\u00a0\u00b0C) is favorable for the formation of the active precursors, which is highly consistent with the H2-TPR characterization results.The physicochemical properties of the support materials and the oxide precursors affect the morphologies of the resulting NiMoS2 nanoclusters and the covalent states of the active metals of the corresponding sulfide-supported catalysts [35,44]. HRTEM images of the investigated NiMo/TA-x serial catalysts were taken to observe the changes in the morphology of the MoS2 crystals, and the statistical results based on the HRTEM images are listed in Table\u00a04\n. Fig. S1 reveals that most of the MoS2 slabs from the investigated NiMo/TA-x serial catalysts were monolayer and bilayer MoS2 slabs with relatively short MoS2 slab lengths, resulting in a relatively high dispersion of Mo species. There is an obvious decline in the MoS2 slab length when the evaporation temperature of the solvent increased from 50\u00a0\u00b0C to 60\u00a0\u00b0C to 70\u00a0\u00b0C, while the stacking of the MoS2 crystals slightly declined. The slab length of MoS2 crystals increased to approximately 3.1\u00a0nm over the other two catalysts. Thus, the dispersion degrees of Mo species over catalysts NiMo/TA-60 and NiMo/TA-70 are higher. Moreover, the distributions of MoS2 slab length over these two catalysts seems much narrower than the other two catalysts. Suggesting relatively higher uniformity in the dispersion of active metals over catalysts NiMo/TA-60 and NiMo/TA-70 than over catalysts NiMo/TA-50 and NiMo/TA-80. This can be attribute to the moderate MSI between the active metals and the support materials revealed by H2-TPR characterization results and the higher proportions of Mo7O24\n6\u2212 precursors revealed by Raman characterization results.The calculated fraction of corner Mo atoms (fMoc) and the fraction of edges Mo atoms (fMoe) are summarized in Table\u00a04.XPS characterization of the investigated sulfide NiMo/TA-x serial catalysts was performed to check the effect of solvent evaporation temperature on the covalent states of the active metals. Both the XPS peaks for the Mo 3d orbital and the XPS peaks for the Ni 2p orbital were recorded and decomposited. The corresponding decomposition details for the Mo 3d orbitals are displayed in Fig.\u00a07\n, and the Ni 2p orbitals are displayed in Fig.\u00a08\n. The Mo sulfidation degrees and the NiMoS active phases proportions calculated from the decomposition results are listed in Table\u00a05\n.The Mo 3d orbital are composed of Mo 3d5/2 (228.9\u00a0eV) and Mo 3d3/2 (231.7\u00a0eV) orbitals for Mo4+(MoS2) species, Mo 3d5/2 (230.5\u00a0eV) and Mo 3d3/2 (233.6\u00a0eV) orbitals for Mo5+(MoOxSy), Mo 3d5/2 (232.7\u00a0eV) and Mo 3d3/2 (236.0\u00a0eV) orbitals for Mo6+(MoO3). The decomposition results for the Ni 2p orbital are composed of decomposited peaks for NiSx species (853.5\u00a0eV), NiMoS phases (855.5\u00a0eV) and NiO species (857.3\u00a0eV) [42,62,15,71]. The sulfidation degrees of both the Mo species (Mosul) and the Ni species (Nisul) reached their summits over catalyst NiMo/TA-60 (with values of 58.4% and 87.8%, respectively), given that Mosul and Nisul had values of approximately 51.7% and 81.2%, respectively, over catalyst NiMo/TA-50. Then, Mosul and Nisul decreased to 53.6% and 84.2%, respectively, over catalyst NiMo/TA-70 and were even lower over catalyst NiMo/TA-80 with a further increase in the solvent evaporation temperature.The above observed changes can be explained by that the MSIs of catalysts NiMo/TA-50 and NiMo/TA-60 are weaker than those of the other two catalysts, and it is more difficult for the active metals strongly interact with the support to be sulfided. Thus, the lower sulfidation degrees were observed over catalysts NiMo/TA-70 and NiMo/TA-80. For catalyst NiMo/TA-50, it has been proven by the H2-TPR results that NiO precursors and MoO3 precursors cannot form the so-called active NiMoO precursors very well; thus, lower sulfidation degrees of active metals were observed. Because the NiMoS phase is acknowledged as the real active phase for the sulfide NiMoS supported catalyst, the proportions of the NiMoS phase were also listed in Table\u00a05. The details show that it increases in the order of NiMo/TA-80 (45.9%) < NiMo/TA-50 (51.8%) < NiMo/TA-70 (54.2%) < NiMo/TA-60 (57.5%), which is in line with both the MSIs between the active metals and the support materials and the proportion of Mo7O24\n6\u2212 precursors over the oxide NiMo/TA-x serial catalysts evidenced by both the H2-TPR and Raman characterization results. These results suggest that the solvent evaporation temperature not only affects the sulfidation process but it also affects the formation efficiency of the NiMoS phases.The above results and discussions clearly demonstrate that the solvent evaporation temperature greatly influenced the physicochemical properties of the synthesized TA-x serial materials, which further played important roles on the corresponding NiMo/TA-x serial catalysts. However, which solvent evaporating temperature is the most moderate needs to be further confirmed. Thus, the HDS performances were evaluated at different reaction temperature. The results displayed in Fig.\u00a09\n show that the 4,6-DMDBT conversions increase with the reaction temperature over all four investigated NiMo/TA-x serial catalysts. At the defined reaction temperature, The 4,6-DMDBT conversions increase in line of NiMo/TA-80 < NiMo/TA-50 < NiMo/TA-70 < NiMo/TA-60 at each defined temperature, suggesting the same order of HDS activity of these investigated catalysts.The calculated activation energies over the investigated NiMo/TA-x serial catalysts summarized in Table\u00a06\n reveal an increasing order in the activation energies: NiMo/TA-60 (108\u00a0kJ\u2022mol\u22121) < NiMo/TA-70 (118\u00a0kJ\u2022mol\u22121) < NiMo/TA-50 (135\u00a0kJ\u2022mol\u22121) < NiMo/TA-80 (137\u00a0kJ\u2022mol\u22121), suggesting the superior catalytic activity over catalyst NiMo/TA-60 and the poorest catalytic activity over catalyst NiMo/TA-80, which on the same level with those reported ones [60,72]. This result is highly consistent with the NiMoS phase proportions determined by the aforementioned XPS characterizations.The HDS performances at different WHSVs were also determined, and the overall HDS reaction rate constants (kHDS) were calculated. The details from Fig.\u00a010\n clearly show that the conversions of 4,6-DMDBT decrease with increasing of WHSV. At each investigated WHSV, the 4,6-DMDBT conversions increased in the same order observed at different reaction temperatures. The calculated kHDS increases in the order of NiMo/TA-80 (303\u00a0\u00b5mol\u2022h\u22121) < NiMo/TA-50 (405\u00a0\u00b5mol\u2022h\u22121) < NiMo/TA-70 (510\u00a0\u00b5mol\u2022h\u22121) < NiMo/TA-60 (566\u00a0\u00b5mol\u2022h\u22121), with the same standard reported elsewhere [65,61]. This suggests that catalysts NiMo/TA-80 and NiMo/TA-50 are far less active than catalyst NiMo/TA-70, which is less active than catalyst NiMo/TA-60. TOF values vary in the line of NiMo/TA-80 (1.8\u00a0h\u22121) << NiMo/TA-50 (2.4\u00a0h\u22121) < NiMo/TA-70 (2.7\u00a0h\u22121) < NiMo/TA-60 (2.9\u00a0h\u22121). Moreover, the TOF values for catalyst NiMo/TA-80 are only approximately 3/4 of those for catalyst NiMo/TA-50, suggesting that the existence of highly ordered mesopores favors the hydroconversion of refractory sulfides. The product distributions with a 4,6-DMDBT conversion of 50\u00b11% over each investigated catalyst were calculated to further confirm the influence of the solvent evaporation temperature on the selectivity of the individual HDS pathways over the NiMo/TA-x serial catalysts, and the details are summarized in Table\u00a07\n.In the typical 4,6-DMDBT HDS theory, 3,3\u2032-DMBP is recognized as the only product of the DDS pathway, and other products are believed to be either the desulfurization products or the intermediates for the HYDS pathway [60,34,72]. The selectivity of 3,3\u2032-DMBP decreases in the order of NiMo/TA-60 (18%) > NiMo/TA-70 (15%) \u2248 NiMo/TA-50 (15%) > NiMo/TA-80 (12%), indicating the same trend in DDS selectivity of the investigated catalysts. This variation trend is highly related to the acidity properties of the corresponding TA-x samples except for catalyst NiMo/TA-80 and its corresponding support. This result suggests that the DDS selectivity for the 4,6-DMDBT HDS reaction is not only related to the morphology of the MoS2 nanoclusters but also highly related to the cracking activity provided by the acid sites from the support. The calculated specific reaction rate constants for the DDS pathway (kDDS) over the NiMo/TA-x serial catalysts varied more profoundly than the selectivity of 3,3\u2032-DMBP. It profoundly increased to 85\u00a0\u00b5mol\u2022h\u22121 over catalyst NiMo/TA-60 from approximately 56\u00a0\u00b5mol\u2022h\u22121 over catalyst NiMo/TA-50 and then decreased to approximately 71\u00a0\u00b5mol\u2022h\u22121 over catalyst NiMo/TA-70. The relatively low kDDS with a value of only approximately 28\u00a0\u00b5mol\u2022h\u22121 over catalyst NiMo/TA-80 can be attributed to the destruction of highly ordered mesopores of the synthesized TA-80 sample. In the present work, the kDDS values for catalysts NiMo/TA-50, NiMo/TA-60 and NiMo/TA-70 were correlated to the product of fMoc and Mosul, and the result is displayed in Fig.\u00a011\n. It reveals that the rate constants of the DDS pathway can be correlated to the product of the fMoc values and the Mo sulfidation degrees linearly with R2 of 0.98893 and the slope for the fitted line of 1302. Since the loadings of Mo species on the investigated NiMo/TA-x serial catalysts were uniform at 12\u00a0wt.% counted by MoO3, the product of fMoc and Mosul is the actual amount of corner Mo atoms of the Ni-promoted MoS2 nanoclusters. Thus, the results demonstrate that the DDS reaction of 4,6-DMDBT can only take place over the corner active sites of the MoS2 nanoclusters. Finally, the kHYDS values for catalysts NiMo/TA-50, NiMo/TA-60 and NiMo/TA-70, fMoc, fMoe, Mosul and the proportion of the NiMoS phase (PNiMoS) were correlated. The correlation result is displayed in Fig.\u00a012\n.It is well acknowledged by the reported works that all the corner Mo atoms and only a portion of the Mo atoms located at the edge sites of the NiMoS phase are active sites [31,48,30,4]. Based on this theory, fMoc was taken as an independent variable, while the product of fMoe and PNiMoS was taken as another variable in the correlation works in the present work. The results show that kHYDS is linearly related to the product of 85fMoc+fMoe\u2022PNiMoS and Mosul with an R2 of 0.99966 and a slope for the fitted line of 78. The coefficient for fMoc is 85 times that for the product of fMoe and PNiMoS in the correlating equation, suggesting that the activity of a single corner active site for the HYDS pathway of 4,6-DMDBT is approximately 85 times that of a single edge active site. Moreover, the slope for the fitted line of kDDS and factor fMoc\u2022Mosul is approximately 17 times that of kHYDS and the factor (85fMoc+fMoe\u2022PNiMoS)\u2022Mosul, suggesting that the activity of the DDS of 4,6-DMDBT is approximately 17 times that of a single edge active site for the HYDS of 4,6-DMDBT over a single corner site. Thus, the overall HDS activity for 4,6-DMDBT of a single corner active site is approximately 100 times that of a single edge active site. Moreover, the activity for the HYDS of 4,6-DMDBT is approximately 5 times that for the DDS of 4,6-DMDBT over the corner active sites. These correlation results well explained the HDS product distribution for 4,6-DMDBT over the NiMo/TA-x serial catalysts summarized in Table\u00a07.Hexagonal TA-x serial composites with a TiO2 content of 20\u00a0wt.% were successfully synthesized at different solvent evaporating temperatures. The corresponding NiMo/TA-x serial catalysts were prepared. A suitable solvent evaporating temperature (60\u00a0\u00b0C) favors the formation of narrow dispersed highly ordered mesopores with a relatively high specific surface area and excellent acidity properties due to the high match in the hydrolyzation rate of the precursor salts and the self-assembly efficiency of the mesopore structure directing agent and the hydroxides. Moderate MSIs and a high proportion of active Mo7O24\n6\u2212 species were obtained with a moderate solvent evaporating temperature (60\u00a0\u00b0C), which led to a higher dispersion degree, higher fMoc value, higher sulfidation degree and higher proportion of the NiMoS phase, resulted in the highest activity of catalyst NiMo/TA-60. The kinetic studies show that catalyst NiMo/TA-60 exhibits superior catalytic performance due to the relatively high proportion of corner Mo sites. DDS reaction can take place over the corner sites and HYD reaction can take place over both the corner sites and the edge sites. The activity for the HYDS reaction of a single corner active site is approximately 85 times that of a single edge active site, and the activity for the overall HDS of a single corner active site is approximately 100 times that of a single edge active site. These promising findings demonstrate that the direction for the design and development of highly active HDS catalysts for the removal of highly refractory organosulfides such as 4,6-DMDBT is to increase the proportion and amount of corner active sites.This work is funded by the National Natural Science Foundation of China (No. 22,178,283 and No. 21,908,174), the National Postdoctoral Program for Innovative Talents (BX20190280), the Postdoctoral Research Foundation of China (2019M663778), the State Key Laboratory of Heavy Oil Processing (SKLOP201902002, SKLOP202102004), the Natural Science Foundation of Shaanxi Province (2022GY-136, 2019JLP-10, 2020JM-517).\nGuangheng Wang: Investigation, Data curation, Methodology, Writing \u2013 original draft. Zegao Zhao: Investigation, Data curation. Wenwu Zhou: Conceptualization, Supervision, Methodology, Project administration, Funding acquisition, Writing \u2013 review & editing. Zhiping Chen: Methodology, Funding acquisition, Formal analysis, Supervision. Anning Zhou: Funding acquisition, Supervision, Writing \u2013 review & editing, Formal analysis, Supervision. Yating Zhang: Writing \u2013 review & editing, Supervision. Xingyu Yang: Funding acquisition, Investigation. Fei Yao: Investigation.The authors declare that they have no known competing interests on this work. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors greatly acknowledge the financial support from the National Natural Science Foundation of China (Nos. 22178283 and 21908174), the Key Research and Development Program of Shaanxi(Program No. 2022GY-136), the National Postdoctoral Program for Innovative Talents (BX20190280), the Postdoctoral Research Foundation of China (2019M663778), the State Key Laboratory of Heavy Oil Processing (SKLOP201902002, SKLOP202102004), the Natural Science Foundation of Shaanxi Province (2019JLP-10, 2020JM-517).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2022.100319.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Binary TiO2-Al2O3 serial composites(TA-x) and the NiMo supported catalysts(NiMo/TA-x) were successfully synthesized at different solvent evaporation temperatures,. The materials were characterized by several advanced characterization techniques, and the catalytic hydrodesulfurization (HDS) performances were evaluated. The results revealed that the synthesized TA-x serial composites are amorphous mixtures of TiO2 and Al2O3 with highly ordered two-dimensional hexagonal mesopores. Both the mesostructures and the acidities of the synthesized TA-x composites changed at different evaporation temperature of the solvent. The metal-support interaction(MSI), the existence states and the coordination states of the Mo precursors can be modulated by the changes in the physicochemical property of the TA-x composites caused by the solvent evaporating temperature. A moderate solvent evaporation temperature is favorable for the doping of Ni species into the MoS2 slab and Mo sulfidation degree, thus enhancing the proportion of the NiMoS active phases. Catalyst NiMo/TA-60 exhibits superior activity and the highest direct desulfurization(DDS) pathway selectivity due to the excellent acidity and pore structure property, the highest dispersion and the highest Mo sulfidation degree, a moderate MSI, the highest efficiency in the formation of NiMoS phase and the highest proportion of corner Mo atoms. Moreover, the rate constants of the specific HDS pathways can be correlated with the specific types of active sites: the DDS pathway (kDDS) is closely related to the proportion of corner active sites and the hydrogenation desulfurization pathway (kHYDS) is closely related to the proportion of brim sites. The activity of a single corner active site is found to be approximately 100 times that of a single edge active site. These findings are promising in the design of HDS catalysts for inferior distillates.\n               "}
{"full_text": "Extensive use of fossil fuels has led to excessive emission of CO2 to the atmosphere, causing global warming and rising sea levels [1]. The electrocatalytic conversion of CO2 into valuable chemicals and fuels would be an effective approach to reduce the atmospheric CO2 concentration, reduce harm to ecological environments, and mitigate the energy crisis [2,3]. Among the products of the CO2 electroreduction reaction (CO2RR), the two-electron transfer product CO is one of the most important, as it can be utilized in well-established gas-to-liquid conversion technologies such as the Fischer\u2013Tropsch process [3\u20135]. To date, metal-based materials [6\u201313], molecular complexes [14\u201320], and carbon-based catalysts [21\u201323] have been designed and applied in the CO2RR to produce CO. Noble metals such as Au, Ag, and Pd are favorable for the conversion of CO2 to CO [6,12], but their high cost and scarcity limit their commercial application. Homogeneous molecular catalysts such as metalloporphyrin and metalophthalocyanine complexes have well-defined active sites and structures. However, they usually work in organic electrolytes, and their low current density and poor stability impede their application [24\u201326]. Although carbon materials doped with non-metal atoms show some activity for the CO2RR, their performance still needs to be improved. It is therefore urgent to fabricate highly efficient CO2RR electrocatalysts that have high selectivity and appreciable current density to meet commercial application standards.Recently, single-atom catalysts (SACs), wherein single metal atoms are dispersed on carbon supports, have shown high activities for electrocatalysis and organic catalysis due to their high atom efficiency and unique coordination environment. In particular, single-atom Ni catalysts with Ni-Nx centers anchored on carbon supports have been developed for the CO2RR towards CO production [27\u201335]. Nevertheless, these Ni SACs supported on carbon materials such as graphene [36\u201339], N-doped nanosheets [40\u201343], and N-doped carbon nanotubes [44,45] have been unable to achieve high current density while maintaining high Faradaic efficiency (FE) for the CO2RR. This is probably because the microporous supports cannot completely expose the single-atom active sites. Thus, excellent SACs with high exposure of the single-atom active sites are needed to achieve current densities suitable for commercial applications.Carbon aerogels with 3D crosslinked networks are known to have a hierarchically porous structure, large specific surface areas, and high electron conductivity [46\u201351]. These unique features endow them with great electrocatalytic potential for improving current density by exposing accessible active sites, facilitating mass transport, and accelerating electron transfer [52\u201355]. Although a multitude of carbon-based aerogel materials have been applied for the hydrogen evolution reaction (HER) [56,57], oxygen evolution reaction [58], and oxygen reduction reaction [59], rarely have carbon aerogels supporting single-atom sites for the CO2RR been reported.In the past several years, using metal-organic frameworks (MOFs) as precursors to prepare carbon-supported SACs has attracted much attention because MOFs can inherit the unique properties of the intrinsic materials and achieve high surface areas as well as uniform distribution of metal centers [60\u201365]. However, the pyrolysis of MOFs usually results in microporous carbonaceous materials, which limit mass transportation and hinder the accessibility of active sites during catalysis. In contrast, SACs supported on hierarchical carbon materials containing micro-, meso-, and macropores, such as carbon aerogels, may circumvent this problem. So far, however, no report has been published on using a MOF as a precursor to prepare carbon aerogel-supported SACs.Herein, Ni SACs supported on carbon aerogels were successfully prepared for the first time by pyrolyzing aerogels composed of a Ni/Zn bimetallic zeolitic imidazolate framework-8 (Ni/Zn-ZIF-8) and carboxymethylcellulose (CMC) under N2 flux at 1000\u00a0\u200b\u00b0C for 4\u00a0\u200bh. The obtained Ni SACs are denoted as Ni-NCA-X (X\u00a0\u200b=\u00a0\u200b10, 20), where X is the weight ratio of CMC in the Ni/Zn-ZIF-8/CMC. During the annealing process, the ZIF frameworks and CMC were transformed into porous carbon aerogels with numerous N atoms, which served as anchoring sites to bind single Ni atoms. The Zn species were removed by evaporation at high temperatures (> 970\u00a0\u200b\u00b0C) [30,66]. For comparison, single-atom Ni supported on microporous N-doped carbon (Ni-NC) was also prepared by pyrolyzing Ni1/Zn2-ZIF-8 under the same conditions. Compared with Ni-NC, the optimal Ni-NCA-10 exhibited excellent CO2RR activity, with a more positive onset potential of \u22120.466\u00a0\u200bV vs. the reversible hydrogen electrode (RHE) and a CO Faradaic efficiency (FECO) above 95% over a wide potential range from \u22120.5\u00a0\u200bV to \u22121.0\u00a0\u200bV. It should be noted that all the potentials mentioned in this work were with reference to the RHE. Importantly, an industrial current density of 226\u00a0\u200bmA\u00a0\u200bcm\u22122 with a high FE of 95.6% was achieved for Ni-NCA-10 in a flow-cell reactor at \u22121.0\u00a0\u200bV. Moreover, after 20\u00a0\u200bh of continuous electrocatalytic measurement, the Ni-NCA-10 showed little decline in FECO, suggesting its relative electrochemical stability.All reagents and chemicals were obtained commercially and used without further purification: 2-Methylimidazole (Aladdin), carboxymethylcellulose (Adamas), Ni(NO3)2\u00b76H2O (Adamas), Zn(NO3)2\u00b76H2O (Adamas), KHCO3 (99%, SCR), carbon paper (Toray).The Ni/Zn bimetallic zeolitic imidazolate framework (Ni/Zn-ZIF-8) was prepared using a modified method of preparing ZIF-8. First, 0.3921\u00a0\u200bg of Zn(NO3)2\u00b76H2O and 0.1916 g of Ni(NO3)2\u00b76H2O (the molar ratio of Zn metal ions to Ni metal ions remained 2:1) were dissolved in 15\u00a0\u200bmL methanol. Then, 1.297\u00a0\u200bg of 2-methylimidazole (2-MeIM) dissolved in 15\u00a0\u200bmL methanol was added to the above solution under stirring. The molar ratio of Zn and Ni metal ions to 2-MeIM remained 1:8. The mixed solution was transferred to a 100\u00a0\u200bmL Teflon-lined autoclave. After an ultrasonic bath for 15\u00a0\u200bmin, the autoclave was kept at 100\u00a0\u200b\u00b0C for 12\u00a0\u200bh and then cooled down to room temperature. The precipitate was collected by centrifugation, and the obtained violet solid was washed thoroughly with methanol (3\u00a0\u200b\u00d7\u00a0\u200b20\u00a0\u200bmL) and then dried at 70\u00a0\u200b\u00b0C in a vacuum for 24\u00a0\u200bh. The dried violet solid is denoted as Ni1/Zn2-ZIF-8. Materials with different ratios of Zn and Ni were prepared by the same method and are denoted as Ni1/Zn1-ZIF-8 and Ni4/Zn1-ZIF-8. Ni1/Zn2-ZIF-8 was selected as the optimal material for the subsequent synthesis of aerogel materials.The violet Ni1/Zn2-ZIF-8 powder (0.9\u00a0\u200bg) was dispersed in water/acetone (10 mL/0.5\u00a0\u200bmL, V/V\u00a0\u200b=\u00a0\u200b20:1) and sonicated for 2\u00a0\u200bh to form a homogeneous solution. Carboxymethylcellulose (CMC, 0.1\u00a0\u200bg) was dispersed in water (10\u00a0\u200bmL) under stirring at 80\u00a0\u200b\u00b0C, then was stirred for 3\u00a0\u200bh to form a homogeneous jelly-like solution at room temperature. The as-prepared Ni1/Zn2-ZIF-8 solution was mixed with the CMC solution and stirred for 3\u00a0\u200bh, then sonicated for 3\u00a0\u200bh. The Ni1/Zn2-ZIF-8/CMC gel was frozen at\u201319\u00a0\u200b\u00b0C for 12\u00a0\u200bh and freeze-dried under vacuum for 48\u00a0\u200bh to obtain Ni1/Zn2-ZIF-8/CMC aerogel, denoted as Ni1/Zn2-ZIF-8/CMC-10 (where 10 is the weight ratio of CMC in the Ni1/Zn2-ZIF-8/CMC). Ni1/Zn2-ZIF-8/CMC-20 and pure CMC aerogel were synthesized using the same method.The as-prepared Ni1/Zn2-ZIF-8/CMC-10 was placed in an alumina crucible and transferred to a vacuum tube furnace. Before carbonization, the air in the furnace was expelled by blowing nitrogen for 1\u00a0\u200bh. Then the temperature was heated to 1000\u00a0\u200b\u00b0C and kept there for 4\u00a0\u200bh under a N2 atmosphere. After naturally cooling to room temperature, Ni-NCA-10 was obtained. C-CMC, Ni-NC, and Ni-NCA-20 were obtained by using the pure CMC aerogel, Ni1/Zn2-ZIF-8, or Ni1/Zn2-ZIF-8/CMC-20 as the respective precursor under the same pyrolysis conditions. The residual Zn species in all the samples were etched with dilute HCl; this was followed by washing with distilled water and drying to yield purified samples.As shown in Scheme 1\n, bimetallic zeolitic imidazolate frameworks (Ni/Zn-ZIF-8) with different ratios of Ni and Zn were synthesized by solvothermal reactions of Zn(NO3)2\u00b76H2O, Ni(NO3)2\u00b76H2O, and 2-methylimidazole. Then the Ni1/Zn2-ZIF-8 was chosen as the optimal material for the subsequent synthesis of aerogels. CMC was utilized as an agglomerant and mixed with Ni1/Zn2-ZIF-8 to form Ni1/Zn2-ZIF-8/CMC aerogel after freeze-drying. The aerogels with different contents of CMC are denoted as Ni1/Zn2-ZIF-8/CMC-X (X\u00a0\u200b=\u00a0\u200b10 and 20, where X is the weight ratio of CMC in the Ni1/Zn2-ZIF-8/CMC). Finally, Ni SACs supported on carbon aerogels (denoted as Ni-NCA-X, X\u00a0\u200b=\u00a0\u200b10, 20) were successfully synthesized by pyrolyzing the aerogels composed of Ni1/Zn2-ZIF-8/CMC-X under N2 flux at 1000\u00a0\u200b\u00b0C for 4\u00a0\u200bh. For comparison, pure carbon aerogel (C-CMC) and single-atom Ni supported on microporous N-doped carbon (Ni-NC) were also prepared by pyrolyzing the pure CMC and Ni1/Zn2-ZIF-8 under the same conditions.As shown in Fig.\u00a0S1, the powder X-ray diffraction (PXRD) patterns of the Ni1/Zn2-ZIF-8 and Ni1/Zn2-ZIF-8/CMC-X aerogels were consistent with those of the simulated ZIF-8, suggesting that both had the same crystalline phase as ZIF-8. Additionally, with the introduction of Ni2+, the color of the materials changed from white to violet, indicating that a portion of the zinc nodes were replaced by nickel atoms (Fig.\u00a0S2) [64]. These results indicated that Ni had replaced a portion of the Zn with the Ni becoming trapped by N atoms and subsequently forming Ni-Nx sites during the annealing process. The Ni content in Ni-NCA-10 and Ni-NCA-20 was 0.209\u00a0\u200bwt% and 0.153\u00a0\u200bwt%, respectively, according to inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements (Table\u00a0S1). Fig.\u00a0S3a shows that the four samples C-CMC, Ni-NC, Ni-NCA-10, and Ni-NCA-20 exhibited similar PXRD patterns, and only two broad peaks were observed, centered at 2\u03b8\u00a0\u200b=\u00a0\u200b25.1\u00b0 and 43.8\u00b0. The two peaks were assigned to the (002) and (101) planes of graphitic carbon [67]. No diffraction peaks for Zn-based species or Ni nanoparticles appeared, indicating that Ni SACs were formed and elemental Zn had been vaporized during pyrolysis above 1000\u00a0\u200b\u00b0C. The Raman spectra in Fig.\u00a0S3b demonstrate that all the carbon-based samples had high ratios of D-band and G-band intensity (ID/IG) with values of 0.98 for Ni-NCA-10, 0.97 for Ni-NCA-20, 1.0 for Ni-NC, and 0.99 for C-CMC. The high ID/IG pointed to the presence of numerous defects due to the insertion of Ni metal and elemental N [68]. These results indicated these materials would be beneficial for the CO2RR.X-ray photoelectron spectroscopy (XPS) measurement was conducted to investigate the surface compositions and chemical states of Ni-NCA-10, Ni-NCA-20, and Ni-NC. It can be seen from the XPS survey spectra (Fig.\u00a01\na) that C, N, O, and Ni elements were present in the corresponding carbon aerogels. The high-resolution XPS spectra of elemental Ni (Fig.\u00a01b) showed that the binding energy of the Ni 2p3/2 peak was observed at 855.04\u00a0\u200beV in the Ni-NC, Ni-NCA-10, and Ni-NCA-20 samples, located between the peaks for Ni(II) metal (856.0\u00a0\u200beV) and Ni (0) metal (854.3\u00a0\u200beV). This phenomenon demonstrated that the valence state of single-atom Ni in Ni-NCA-10 and Ni-NCA-20 was situated between Ni (0) and Ni(II) [44]. The XPS N 1s spectra of Ni-NC, Ni-NCA-10, and Ni-NCA-20 permitted the differentiation of pyridinic N (398.2\u00a0\u200beV), Ni\u2013N (398.9\u00a0\u200beV), pyrrolic N (399.7\u00a0\u200beV), graphitic N (401.1\u00a0\u200beV), and oxidized N (402.7\u00a0\u200beV) (Fig.\u00a01c and Figs.\u00a0S4a\u2013b) [69]. Further XPS analysis demonstrated that Ni-NCA-10 contained a high ratio of pyrrolic N to pyridinic N (Table\u00a0S2), which would facilitate the adsorption of CO2 and enhance the CO2RR activity.The coordination environment of the nickel atoms in the samples was further analyzed by synchrotron-based X-ray absorption spectroscopy (XAS). As shown in Fig.\u00a01d, the Ni K-edge X-ray absorption near edge structure (XANES) curves of Ni-NCA-10 demonstrated that the Ni absorption edge was located between Ni foil and NiO, implying the positively charged single Ni atoms were between Ni(0) and Ni(II). This result was consistent with the XPS (Fig.\u00a01b). A weak pre-edge peak positioned at 8333\u00a0\u200beV corresponded to the dipole-forbidden but quadrupole-allowed transition from 1s to 3d [70]. There was also a strong absorption peak at about 8339\u00a0\u200beV, associated with the 1s to 4p electronic transition [70].To further investigate the Ni\u2013N structure of Ni-NCA-10, the Ni K-edge X-ray absorption fine structure (EXAFS) was also analyzed. As presented in Fig.\u00a01e, a main peak centered at ca. 1.35\u00a0\u200b\u00c5, ascribed to the characteristic Ni\u2013N coordination path, was observed for Ni-NCA-10 and the Ni-phthalocyanines (NiPc) molecule [33,71]. The peak at 2.17\u00a0\u200b\u00c5 for Ni\u2013Ni coordination was absent in the Ni K-edge EXAFS curve of Ni-NCA-10, revealing there were no Ni particles. According to the fitting result based on the EXAFS curve in Fig.\u00a01f and Table\u00a0S3, the Ni species in Ni-NCA-10 were coordinated with four nitrogen atoms and one oxygen atom from the coordinated water molecule. All these results strongly confirmed the atomic dispersion of single Ni sites in the Ni-NCA-10.The morphologies of the samples were characterized by scanning electron microscopy (SEM). As shown in Fig.\u00a02\na and Fig.\u00a0S5a, the Ni-NCA-10 and Ni-NCA-20 clearly displayed a 3D network structure with interconnected hierarchical micro-, meso-, and macropores, while the Ni-NC inherited the dodecahedral morphology of the Ni1/Zn2-ZIF-8 (Fig.\u00a0S5b). Only micropores could be observed. Compared with the Ni-NCA-10, the pure C-CMC showed an irregular, lumpy morphology with an uneven distribution of pores (Fig.\u00a0S5c). The fine structure of Ni-NCA-10 was further investigated using transmission electron microscopy (TEM); Fig.\u00a02b shows a large number of pores with an average size of 156\u00a0\u200bnm (Fig.\u00a0S5d). Notably, no metal nanoparticles (NPs) were visible in the TEM or high-angle annular dark-field scanning TEM (HAADF-STEM) images of Ni-NCA-10 (Figs.\u00a02b and c). This result was consistent with the PXRD (Fig.\u00a0S3a). A number of bright atomic-size dots related to the heavier Ni elements were clearly visible in the aberration-corrected HAADF-STEM image (Figs.\u00a02c and d). The 3D crosslinked porous network structure was particularly obvious in the HAADF-STEM image in Fig.\u00a02e. The corresponding EDS mapping revealed the homogeneous distribution of Ni, C, and N elements. All these results indicated that interconnected, hierarchically porous N-doped carbon aerogels supporting Ni single atomic sites had been successfully fabricated.To further investigate the porous structure of these carbon-based materials, N2 adsorption\u2013desorption isotherms were tested at 77\u00a0\u200bK. As shown in Fig.\u00a02f, typical IV isotherms with obvious hysteresis loops in the P/P0 range of 0.45\u20131.0 were observed for Ni-NCA-10 and Ni-NCA-20. This phenomenon indicated the presence of meso- and macropores. In addition, the pore size distribution of these materials (Fig.\u00a0S6) showed that the Ni-NCA-10 and Ni-NCA-20 had large amounts of meso- and macropores, with pore sizes ranging from 2 to 65\u00a0\u200bnm. In contrast, the Ni-NC had only micropores of ca. 0.6\u00a0\u200bnm (Fig.\u00a0S6). Moreover, Ni-NCA-10 had a Brunauer\u2013Emmett\u2013Teller (BET) area of 1200\u00a0\u200bm2\u00a0\u200bg\u22121, which was much larger than that of Ni-NC (786\u00a0\u200bm2\u00a0\u200bg\u22121) and Ni-NCA-20 (555\u00a0\u200bm2\u00a0\u200bg\u22121) (Table\u00a0S4). Thus, it is reasonable to infer that the highly porous Ni-NCA-10 had a hierarchical micro-, meso-, and macroporous structure, which would be favorable for expediting mass and electron transport and further promoting the catalytic activity of the CO2RR. The interaction between the carbon aerogels and the CO2 molecules was demonstrated by the CO2 adsorption measurements. As displayed in Fig.\u00a02g, the Ni-NCA-10 and Ni-NCA-20 exhibited high CO2 adsorption capacities of 44\u201356\u00a0\u200bcm3\u00a0\u200bg\u22121 at room temperature, implying that the Ni sites supported on highly porous carbon aerogel materials interacted strongly with CO2 molecules and would thus promote catalytic conversion.We next investigated the CO2RR properties of the Ni SACs supported on hierarchically porous carbon aerogel. The CO2RR measurements were conducted in a two-compartment electrochemical H-cell separated with a Nafion-117 proton exchange membrane. Linear sweep voltammetry (LSV) demonstrated that Ni-NCA-10 displayed a larger current density in CO2-saturated (1\u00a0\u200batm) 0.5\u00a0\u200bM KHCO3 solution than in an Ar-saturated electrolyte (Fig.\u00a0S7), suggesting the activity may have originated from CO2 reduction. Notably, no liquid products were detected by off-line 1H NMR spectroscopy (Fig.\u00a0S8) and only CO and H2 were found via gas chromatography (GC).Ni-NCA-10 also showed an excellent FECO of > 95% in a wide potential range from \u22120.5\u00a0\u200bV to \u22121.0\u00a0\u200bV, reaching a maximum of 99.7% at \u22120.8\u00a0\u200bV (Fig.\u00a03\na). The highest current density for Ni-NCA-10 was 47165.0\u00a0\u200bmA\u00a0\u200bmg\u22121(Ni), achieved at \u22121.2\u00a0\u200bV; this was about 1.2 and 3.4 times larger than for Ni-NCA-20 (39673.2\u00a0\u200bmA\u00a0\u200bmg\u22121(Ni) and Ni-NC (13762.3\u00a0\u200bmA\u00a0\u200bmg\u22121(Ni)), respectively (Fig.\u00a03b). The corresponding CO partial current density (j\nCO) of the Ni-NCA-10 was 34688.9\u00a0\u200bmA\u00a0\u200bmg\u22121(Ni), which was about 1.38 and 5.2 times larger than for Ni-NCA-20 (25163.3\u00a0\u200bmA\u00a0\u200bmg\u22121(Ni)) and Ni-NC (6650.2\u00a0\u200bmA\u00a0\u200bmg\u22121(Ni)), as shown in Fig.\u00a03c. The Ni-NCA-10 outperformed most of the reported Ni SACs (Table\u00a0S5). Its excellent current density can reasonably be attributed to its high porosity, which exposed more active sites to substrates. Moreover, the hierarchical micro-, meso-, and macropores promoted the diffusion of the electrolyte and CO2 to the active sites.To further confirm whether the activity originated from CO2 reduction, we performed 13C-labeled CO2 isotope experiments, and the products were determined by gas chromatography\u2013mass spectrometry (GC-MS). When using 13C-labeled CO2 to replace 12CO2 in 0.5\u00a0\u200bM KH12CO3 during the CO2RR process, the signals for 13CO (m/z\u00a0\u200b=\u00a0\u200b29) and 12CO (m/z\u00a0\u200b=\u00a0\u200b28) were observed. However, when conducting the CO2RR with 13C-labeled CO2 in 0.5\u00a0\u200bM KCl electrolyte, only 13CO was detected, and no signal for an m/z of 28 was observed. These results indicated that the CO originated from the CO2 in equilibrium with bicarbonate anions in a CO2-saturated KHCO3 aqueous solution (Fig.\u00a0S9) [70,72].To confirm whether the atomically dispersed Ni sites were the active centers and exclude the influence of elemental Zn on the performance, N-doped carbon (NC) was prepared by pyrolyzing the ZIF-8/CMC aerogel without Ni sites. The Ni and Zn element contents in the NC are provided in Table\u00a0S1. Unlike with the Ni-NC, Ni-NCA-10, and Ni-NCA-20, H2 was the major product for NC, implying that the atomically dispersed Ni sites were the active centers for the CO2RR, and the presence of Zn did not affect the CO2 reduction performance (Fig.\u00a0S10).To investigate the stability of the aerogel materials, electrochemical stability testing was conducted. As shown in Fig.\u00a03d, there was a slight decay in current density and FECO efficiency at an applied potential of \u22120.9\u00a0\u200bV for 20\u00a0\u200bh in CO2-saturated 0.5\u00a0\u200bM KHCO3 solution, indicating Ni-NCA-10 had electrochemical stability. Moreover, no Ni NPs were observed in the TEM image of Ni-NCA-10 after electrocatalysis (Fig.\u00a0S11a), nor had the Ni-NCA-10 morphology changed after testing (Fig.\u00a0S11b).To achieve an industrial current density for the CO2RR, a flow-cell reactor was assembled using a gas diffusion electrode (Fig.\u00a04\na and Fig.\u00a0S12). In this electrochemical test, the optimal catalyst, Ni-NCA-10, was selected as a typical sample to evaluate its CO2RR performance. Due to the faster diffusion of CO2 to the catalyst, the current densities for Ni-NCA-10 in the flow-cell significantly exceeded those in the H-cell. As shown in Fig.\u00a04b and Fig.\u00a0S13, it was clearly demonstrated that the j\nCO in the flow-cell achieved an industrial-level value of 226\u00a0\u200bmA\u00a0\u200bcm\u22122 at \u22121.0 V. Surprisingly, the current density was greatly enhanced in the flow-cell, while the FECO remained > 90% at the applied potentials. The calculated turnover frequency (TOF) of Ni-NCA-10 reached 271810\u00a0\u200bh\u22121\u00a0\u200bat \u22121.0\u00a0\u200bV, which was much larger than that of Ni-NC (Fig.\u00a04c). This result proved that the carbon aerogel structure containing hierarchical micro-, meso-, and macropores played a significant role in improving the activity for the CO2RR. Moreover, the excellent TOF surpassed the values for most reported catalysts toward CO2-to-CO conversion (Fig.\u00a0S14 and Table\u00a0S6). The long-term durability test also indicated only slight changes in the applied potential on the Ni-NCA-10 aerogel at a high current density of 100\u00a0\u200bmA\u00a0\u200bcm\u22122, indicating its good CO2RR stability (Fig.\u00a04d and Fig.\u00a0S15).We took electrochemical impedance spectroscopy (EIS), electrochemical active surface area (ECSA), and Tafel slope measurements to understand the intrinsic catalytic activity of the carbon aerogels with Ni single atoms. As shown from the EIS Nyquist plots in Fig.\u00a0S16a, Ni-NCA-10 and Ni-NCA-20 exhibited smaller semicircles in comparison with that of Ni-NC, which may partially explain the high electron conductivity and excellent CO2RR activity of these aerogel materials. Subsequently, the double-layer capacitance was calculated as a measure of the ECSA. The results showed that the Ni-NCA-10 had a much higher Cdl value of 31.0\u00a0\u200bmF\u00a0\u200bcm\u22122 than Ni-NC, demonstrating that the Ni-NCA-10 aerogel had a far greater number of active sites (Figs.\u00a0S16b and S17). In addition, the Ni-NCA-10 showed a lower Tafel slope of 149.3\u00a0\u200bmV dec\u22121 than the Ni-NC (189.1\u00a0\u200bmV dec\u22121, Fig.\u00a0S16c), which indicated that faster kinetics occurred in the Ni-NCA-10. Based on the analysis, the 3D crosslinked aerogel structure tended to expose more active sites and accelerate the reaction kinetics, thereby achieving high TOF.To identify the reaction intermediates and mechanism of the CO2RR, in situ electrochemical Fourier transform infrared spectroscopy (FTIR) measurements of Ni-NCA-10 were conducted. The experiment was performed at a potential of \u22120.8\u00a0\u200bV in 0.5\u00a0\u200bM KHCO3. Figs.\u00a05\na and b show bands located at 1395\u00a0\u200bcm\u22121 and 1386\u00a0\u200bcm\u22121 in the spectra, assigned to a bidentate \u2217COO\u2212 intermediate and a carboxyl intermediate of \u2217COOH, respectively, which implied the coexistence of these two intermediates [72,73]. The \u2217COO\u2212 intermediate was active in the system and easily hydrogenated to form \u2217COOH, which is the key intermediate for the formation of CO [74].Density functional theory (DFT) was used to calculate the free energy to further understand the catalytic mechanism. Using the XANES and EXAFS results, we first calculated the water desorption energy from the Ni centers. This value (0.08\u00a0\u200beV) was too small to affect subsequent calculations. We therefore built a stable model with Ni\u2013N4 coordination (Fig.\u00a05c) to calculate the free energies for the CO2RR and HER processes. In the typical pathway for the CO2RR on Ni sites (Fig.\u00a0S18), the CO2 molecule was first absorbed on the Ni\u2013N4 site and formed the \u2217COOH intermediate by a proton-coupled electron transfer process. Subsequently, the \u2217COOH was converted to \u2217CO via another proton-coupled electron transfer process, and finally, the \u2217CO was desorbed from the Ni\u2013N4 to obtain CO. It is worth noting from the DFT that the formation of the absorbed intermediate, \u2217COOH or \u2217H, was the rate-determining step for the CO2RR and HER [67], respectively. Additionally, the free energy of \u2217COOH formation on Ni\u2013N4 was 1.48\u00a0\u200beV, which was smaller than for the formation of \u2217H (1.55\u00a0\u200beV) in the HER process (Figs.\u00a05d and S19). This result indicated that the CO2RR occurred preferentially at the Ni\u2013N4 site rather than the HER, which concurred with the experiments.We have successfully designed and prepared Ni single atomic sites supported on N-doped carbon aerogels as catalysts, via the pyrolysis of Ni/Zn-ZIF-8/CMC aerogel to expose a greater number of active sites for CO2 molecules and electrolytes. The electrocatalytic performance for the electrochemical reduction of CO2 was thereby enhanced. Compared with the single-atom Ni supported on microporous N-doped carbon, the Ni-NCA-10 aerogel exhibited a high CO Faradaic efficiency of over 95% in a wide potential range of \u22120.5 to \u22121.0\u00a0\u200bV vs. RHE in 0.5\u00a0\u200bM KHCO3 electrolyte. Notably, the as-prepared Ni-NCA-10 achieved an industrial current density of 226\u00a0\u200bmA\u00a0\u200bcm\u22122 with excellent CO Faradaic efficiency in a flow-cell reactor. The exceptional electrocatalytic performance of Ni-NCA-10 can reasonably be attributed to its hierarchical porous structure, high surface area, and uniformly accessible dispersed active Ni sites. The control experiments and theoretical calculations demonstrated that the formation of \u2217COOH over the Ni\u2013N4 sites was the rate-determining step for the CO2RR. This work might shed light on promising perspectives for designing excellent electrocatalysts containing hierarchical micro-, meso-, and macropores to achieve the electrochemical reduction of CO2 with industrial-level current densities.R. Cao and Y.-B. Huang proposed the concept. H. Guo performed the experiments. D.-H. Si conducted the DFT calculation. H. Guo and Y. B. Huang co-wrote the manuscript. All authors participated in data analysis and manuscript discussion.The authors declare no competing financial interests.The work was supported by the National Key Research and Development Program of China (2018YFA0208600, 2018YFA0704502), NSFC (21871263, 22071245, and 22033008), and Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZZ103). The authors thank the beamline BL14W1 station for XAS measurements at the Shanghai Synchrotron Radiation Facility, China.The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.esci.2022.03.007.", "descript": "\n                  Finding highly efficient electrocatalysts for the CO2 electroreduction reactions (CO2RR) that have high selectivity and appreciable current density to meet commercial application standards remains a challenge. Because their reduction potentials are similar to that of the associated competitive hydrogen evolution reaction and the CO2 activation kinetics are sluggish. Although single-atom catalysts (SACs) with high atom efficiency are one class of promising candidates for the CO2RR to produce CO, single-atom active sites supported on microporous carbons are not fully exposed to substrates and thus lead to low current density. Carbon aerogels with interconnected channels and macropores can facilitate mass transport. But few reports describe utilizing them as supports to anchor SACs for efficient electrocatalysis. Herein, N-doped carbon aerogels supporting Ni single atomic catalyst sites (denoted as Ni-NCA-X, X\u00a0\u200b=\u00a0\u200b10, 20) were fabricated by pyrolyzing Ni/Zn bimetallic zeolitic imidazolate framework (Ni/Zn-ZIF-8)/carboxymethylcellulose composite gels. Owing to abundant hierarchical micro-, meso-, and macropores and high CO2 adsorption, the Ni single active sites in the optimal Ni-NCA-10 were readily accessible for the electrolyte and CO2 molecules and thus achieved an industrial-level CO partial current density of 226\u00a0\u200bmA\u00a0\u200bcm\u22122, a high CO Faradaic efficiency of 95.6% at \u22121.0\u00a0\u200bV vs. the reversible hydrogen electrode, and a large turnover frequency of 271810 h\u22121 in a flow-cell reactor at \u22121.0\u00a0\u200bV. Such excellent CO2RR performance makes Ni-NCA-10 a rare state-of-the-art electrocatalyst for CO2-to-CO conversion. This work provides an effective strategy for designing highly efficient electrocatalysts toward the CO2RR to achieve industrial current density via anchoring single-atom sites on carbon aerogels.\n               "}
{"full_text": "The recent discovery of shale gas reserves has caused a decrease in the price of natural gas, encouraging its use as feedstock for the production of valuable chemicals [1]. Ethylene is, by far, the most important chemical feedstock for Petrochemistry, being directly used in the production of a wide number of commodity chemicals [2]. Currently, the main industrial route to obtain ethylene is steam cracking, which is an energy intensive and non-catalytic process [2,3]. In fact, steam cracking is considered as the most energy consuming process of the chemical industry, due to its endothermic character and the need for high reaction temperatures. Moreover, the absence of catalysts leads to the formation of many reaction products so that the separation costs are also high. Among all the possibilities, the oxidative dehydrogenation (ODH) of ethane is established as one of the most interesting alternatives to steam cracking, being an exothermic process with lower energy requirements and no thermodynamic limitations [3\u20135]. The energy consumption of ODH is expected to be substantially lower than any of the current alkene production technologies due to its exothermic nature. Furthermore, the deposition of coke is prevented provided that the presence of oxygen can oxidize coke to form carbon oxides. Despite the large amount of research efforts, industrial scale application of the ODH of ethane has not been implemented up to date due to the relatively low ethylene selectivity shown by the catalysts currently available. The main problem with most of the catalysts studied for the ODH of ethane is the excessive formation of carbon oxides (COx) which limits the selectivity to ethylene [3\u20136]. In this sense, among all catalytic systems based on reducible metal oxide catalysts, the most promising ones are multicomponent MoV(Te,Sb)NbO mixed oxides [7,8] and modified NiO materials [9\u201321]. In the latter case, it has been reported that pure NiO exhibits an important formation of CO2 and low ethylene selectivity [9\u201320]. However, the role of promoters [9\u201315] and/or supports/diluents [16\u201325] in NiO based catalysts is still under discussion. In this way, several investigations have shown the effect of different promoters on the catalytic behavior of NiO-based materials [9\u201325], with the presence of many promoters reducing the formation of carbon dioxide. Nevertheless, at this moment, Nb-promoted catalysts are the most effective ones [9\u201315], although the presence of other elements, such as Sn4+, W6+, Zr4+, Ti4+ [10\u201315], with dopant contents lower than 10 at.%, favor small changes in the characteristics of NiO particles, thus leading to the best catalytic performance (selectivity to ethylene up to 80\u201390 %).Alternatively, supported/diluted NiO catalysts also show a high ethylene selectivity, especially by using Al2O3 [16\u201319], porous clays [20] or other supports based on transition metal oxides [21\u201325]. In this case, after the incorporation of NiO contents of ca. 10\u221230 wt%, changes in both physico-chemical characteristics and catalytic performance are observed. These changes have been related to a decrease in crystal size of NiO particles but, in addition, some interaction between NiO and the support (decreasing the reducibility of Ni-O bonds) could be also necessary. Thus, it is known that the decrease of the NiO crystallite size [9,21], the elimination of non-stoichiometric oxygen species, a decrease in the reducibility of Nin+ sites, an increased Lewis acidity [15,26,27] or a lower electron conductivity [28] can give rise to high selectivity to ethylene during the ODH of ethane [9,11,12,17]. Interestingly, a similar increasing effect in the selectivity to the olefin in the ODH of ethane has been observed when oxalic acid is incorporated during the preparation procedure [15].Generally, nickel oxide catalysts with small NiO particle size (below 20 nm, although less than 10 nm is preferred) present optimum catalytic performance in the ODH of ethane [15]. However, the drastic change observed in the redox properties of NiO must rely on the modification of the chemical nature of NiO (coordination, surface environment, oxidation state) [9,15]. This could be achieved by decreasing NiO particle size. However, it is possible to increase the selectivity to ethylene by optimizing the active phase-support interaction, without substantially decreasing NiO particle size, as observed in TiO2-supported nickel oxide catalysts [28]. Thus, and according to these observations, it seems that a small particle size might not be a sufficient requirement to improve the catalytic performance of NiO-based catalysts in ODH.In order to shed some light into the chemical nature of selective NiO catalysts for the oxidative dehydrogenation of ethane, we have synthesized Al2O3-supported nickel oxide catalysts, but with varying degrees of nickel oxide-support interaction by using modifying the catalyst preparation procedure. In this way, it will be possible to determine the influence of the interaction between nickel oxide and the support, minimizing the possible interference of both the NiO crystal size effect and the support employed.Accordingly, we have followed two different synthetic approaches for a series of NiO/Al2O3 catalysts: i) addition of oxalic acid as an organic additive to NiO/Al2O3 system; and ii) incorporation of Nb5+ as a dopant during the preparation of the Al2O3-supported materials. In the latter case, the synthesis of Nb5+-promoted NiO/Al2O3 catalysts has been carried out by incorporating Nb5+ in one or two steps: a) using a Ni2+/Nb5+-containing solution to be directly impregnated on Al2O3; and b) Nb5+ is firstly impregnated on Al2O3, and a Ni2+-containing solution is subsequently added on the NbOx-Al2O3 support. The results are discussed in terms of the specific chemical and structural features found in selective and unselective materials.Al2O3-supported NiO catalysts were prepared by wet impregnation of \u03b3-Al2O3 (SBET =210 m\u00b2/g, ABCR) with aqueous solutions of nickel nitrate Ni(NO3)2\u00b76H2O (Sigma-Aldrich, 99 %). The catalysts are named as xNi/AL, where x is the NiO wt%.Alternatively, oxalic acid was added to the nickel nitrate solution, with Ni/oxalic acid molar ratios of 1/1 and 1/3 (i.e. 15Ni/AL-o1 and 15Ni/AL-o3 catalysts, respectively). For comparison, a 15 NiO wt% Al2O3 catalyst was prepared by a mechano-chemical procedure, by mixing and grinding the corresponding amounts of nickel oxide and alumina in an agate mortar (i.e. Ni+AL(PM) sample).Nb-containing alumina-supported nickel oxide catalysts were prepared by following two strategies, using an aqueous solution of C4H4NNbO9\u00b7xH2O (Sigma\u2013Aldrich): i) direct impregnation of \u03b3-Al2O3 by an aqueous solution of promoting compounds and subsequent impregnation with an aqueous solution of nickel nitrate (15 wt% NiO) named as Ni/Nb/AL; ii) \u03b3-Al2O3-supported Ni-Nb-O mixed oxides were prepared by wet impregnation method using aqueous solutions of nickel nitrate and the niobium compound, with a Nb/(Ni + Nb) atomic ratio of 0.1; which has been named as (Ni+Nb)/AL. All catalysts were dried overnight at 100 \u00b0C and finally calcined at 500 \u00b0C for 2 h (5 \u00b0C/min). The characteristics of these catalysts are shown in Table 1\n.Catalytic tests were carried out under steady state conditions in a fixed bed quartz reactor (i.d. 20 mm, length 400 mm) at temperatures in the 300\u2212500 \u00b0C range. Feed consisted of an ethane/O2/He mixture with 3/1/26molar ratio. The total flow and the catalyst weight were varied (25\u2212100 mL min\u22121, 0.1\u20131.0 g of catalyst and 0.3\u20130.5 mm particle size) in order to achieve several contact times (W/F). For some selected experiments an ethane/O2/He mixture with 3/3/24molar ratio was employed.Reactants and products were analyzed by gas chromatography using two packed columns [20]: (i) molecular sieve 5A (2.5 m); and (ii) Porapak Q (3 m).N2-adsorption isotherms were recorded in a Micromeritics ASAP 2000 device. The materials were degassed in vacuum at 300 \u00b0C prior to N2 adsorption. Surface areas were estimated by the Brunauer-Emmet-Teller (BET) method.X-ray diffraction patterns were collected in an Enraf Nonius FR590 diffractometer with a monochromatic CuK\u03b11 source operated at 40 keV and 30 mA.Raman spectra were obtained in an inVia Renishaw spectrometer, equipped with an Olympus microscope, using a wavelength of 325 nm (UV-Raman), generated with a Renishaw HPNIR laser with a power of approximately 15 mW.UV\u2013vis diffuse reflectance spectroscopy measurements of the solids were carried out within the 200\u2212800 nm range using a Varian spectrometer model Cary 5000. The value of band gap Eg is calculated by extrapolating the linear fitted region at [F(R(\u221e))h\u03c5]2 = 0 in the plot of [F(R(\u221e)) h\u03c5]2 versus h\u03c5. Additional information in supporting information.Temperature-programmed reduction experiments (H2-TPR) were performed in an Autochem 2910 (Micromeritics) equipped with a TCD detector. The reducing gas composition was 10 % H2 in Ar, with a total flow rate of 50 mL min\u22121. The materials were heated until 800 \u00b0C, with a heating rate of 10 \u00b0C min\u22121.X-ray photoelectron spectroscopy (XPS) measurements were performed on a SPECS spectrometer equipped with a Phoibos 150 MCD-9 detector using a monochromatic Al K\u03b1 (1486.6 eV) X-ray source. Spectra were recorded using an analyzer pass energy of 50 eV, an X-ray power of 100 W, and an operating pressure of 10\u22129 mbar. Spectra treatment was performed using CASA software. Binding energies (BE) were referenced to C 1s at 284.5 eV.Selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and Scanning-TEM (STEM)-Energy-Dispersive Spectroscopy (EDS) maps were performed on a JEOL JEM300 F electron microscope by working at 300 kV (point resolution of 0.17 nm). Crystal-by-crystal chemical microanalysis was performed by energy-dispersive X-ray spectroscopy (XEDS) in the same microscope equipped with an ISIS 300 X-ray microanalysis system (Oxford Instruments) with a detector model LINK \u201cPentafet\u201d (resolution 135 eV). Samples for transmission electron microscopy (TEM) were ultrasonically dispersed in n-butanol and transferred to carbon coated copper grids.The catalytic performance of supported nickel oxide catalysts during the oxidative dehydrogenation (ODH) of ethane at 400 \u00b0C is summarized in Table 1. As mentioned in the experimental section, the catalysts tested consist of a set of nickel oxide materials supported on \u03b3-alumina, with Ni-loadings from 5 to 30 wt% NiO. In addition, catalysts with a 15 wt% NiO prepared with different amounts of oxalic acid and promoted with Nb5+ were also evaluated.The ethane conversion and the selectivity to ethylene strongly depend on NiO-loading and the catalyst preparation procedure. For comparison, it has been also included the catalytic results of a mechano-chemical NiO-Al2O3 mixture, named as Ni+AL(PM). In all cases, ethylene was the main reaction product. In addition, CO2 was the only product detected from oxidation reactions.The variation of the selectivity to ethylene with the ethane conversion during the ethane ODH on representative Al2O3-supported nickel oxide catalysts (xNi/AL series) is shown in Fig. 1\na. On the other hand, Fig. 1b allows the comparison of catalytic results over catalysts with 15 wt% NiO with or without additional synthetic modifications: i) Nb-promoted catalysts, i.e. Ni/Nb/AL and (Ni+Nb)/AL samples; ii) unpromoted catalysts prepared from synthesis gels containing oxalic acid (i.e. 15Ni/AL-o1 and 15Ni/AL-o3 samples).As it can be observed in Fig. 1a and Table 1, the selectivity to ethylene has a clear dependence on the NiO-loading. Thus, the selectivity to ethylene gradually increases with NiO-loading (in the range of 88\u201395 %), reaching a maximum of ca. 95 % over sample 15Ni/AL (i.e. 15 wt% NiO). On the contrary, further increasing NiO-loading on Al2O3 (up to 30 wt% NiO) leads to a decrease in the selectivity to ethylene during the ODH of ethane (down to ca. 72 %).In the case of Nb-promoted catalysts, the (Ni+Nb)/AL sample (synthesized in one step) presents a slight decrease in the selectivity to ethylene (ca. 86 %) compared to that achieved using the reference sample, 15Ni/AL. Moreover, Ni/Nb/AL catalyst (prepared in two steps) presents a remarkably lower selectivity (ca. 71 %).The method used to prepare the catalysts, and especially the presence of oxalic acid in the synthesis gel, may also have an important influence in the catalytic performance. Fig. 1b shows the variation of selectivity with ethane conversion of samples with 15 wt% of NiO. Lower selectivity to ethylene is observed over samples prepared with oxalic acid in the synthesis gel (see 15Ni/AL-o1 and 15Ni/AL-o3), especially notorious when high oxalic acid contents are used. Thus, both Nb-promoted NiO/\u03b3-Al2O3 catalysts and Nb-free NiO/\u03b3-Al2O3 catalysts (where the oxalic acid has been directly added) show a poorer selectivity to ethylene than the corresponding catalyst of xNi/Al series (i.e. 15Ni/AL sample). The selectivity to ethylene for catalysts prepared with a 15 wt% NiO decreases according to the following trend: 15Ni/AL > (Ni+Nb)/AL > 15Ni/AL-o1 > Ni/Nb/AL > 15Ni/AL-o3 > Ni+AL(PM).\nFig. 2\n shows the variation of the space-time yield for ethylene formation of studied catalysts. For xNi/AL series, the formation of ethylene per nickel site increases with the NiO-loading, achieving its maximum value for 20Ni/AL catalyst (Fig. 2). However, the highest space-time yield for ethylene formation was observed for samples 15Ni/AL-o1 and (Ni+Nb)/AL (Fig. 2). This is so because of the higher C2H6 reaction rates for the latter cases (Figure S1, supporting information).The present study has been undertaken using a low concentration of oxygen (C2H6/O2/He = 3/1/26 M ratio) so achieving high conversions without running out of oxygen is complicated. Then, we have carried out a few experiments with the optimal catalyst 15Ni/AL using more oxygen in the feed (C2H6O2/He = 3/3/24) and higher contact times to reach higher conversions. Thus, an ethane conversion of 51.2 % and a selectivity to ethylene of 71.7 %, so that the ethylene yield obtained was 36.7 %, was obtained when working at 450 \u00b0C and a contact time, W/F, of 205 gcat h (molC2H6)\u22121 (Table S1, supporting information). This yield could be enhanced by optimizing the reaction conditions.In order to understand their catalytic performance, the catalysts were characterized by several physicochemical techniques. Table 1 summarizes their main physicochemical characteristics. Fig. 3\n displays the XRD patterns of Al2O3-supported NiO catalysts synthesized in the absence or the presence of increasing amounts of oxalic acid or Nb-oxalate in the synthesis gel. For comparison, XRD pattern of the mechano-chemically mixed Ni+AL(PM) sample is presented in Figure S2-A (supporting information).All the materials show diffraction lines corresponding to face-centered cubic NiO phase, space group Fm-3m (JCPDS: 78\u22120643), and \u03b3-alumina (JCPDS: 10\u22120425). However, depending on the catalyst preparation procedure (the NiO-loading and/or the presence/absence of Nb5+ as promoter), differences in the relative intensity of diffraction maxima of the two crystal phases were observed. X-ray diffraction patterns of the catalysts from xNi/AL series show a progressive increase in the relative intensity of NiO diffraction maxima as the nickel oxide loading increases (see Fig. 3, patterns a to e). For the same nickel oxide content (we have considered the optimal Ni/Al ratio), the incorporation of increasing amounts of oxalic acid leads to an increase in the relative intensity of the NiO peaks (Fig. 3, diagrams c, h and i).Likewise, both the use of niobium as a promoter and the method followed to incorporate it, modify the relative intensity and peak width of NiO maxima, for a fixed Ni/Al content (again, we have considered the optimal NiO content of 15 wt%). Thus, NiO diffraction maxima appear to be narrower in Ni/Nb/AL sample than in (Ni+Nb)/AL (\nFig. 3, patterns f and g, respectively), which could be related to differences in NiO crystal size and/or degree of crystallinity, depending on the extent of interaction with \u03b3\u2013Al2O3 support.In order to get further insight into the crystalline nature of NiO-based catalysts, these materials were analyzed by high-resolution transmission electron microscopy. Catalysts of the xNi/AL series are made up of \u03b3\u2013Al2O3 and NiO crystallites of 5\u221210 nm in size up to the composition 15Ni/AL, NiO showing a good dispersion over the support (Figs. 4\na and 4b). In the 20Ni/AL catalyst, platelet-like NiO crystals of 30\u221250 nm begin to appear (Fig. 4c), which are very abundant, and in the form of agglomerates in 30Ni/AL sample (Fig. 4d).The addition of oxalic acid tends to modify the microstructure of the catalysts. After adding oxalic acid in 1:1 Ni/oxalic acid molar ratio (catalyst 15Ni/AL-o1), crystallinity, the degree of dispersion on the support and the average size of the NiO crystallites, remain almost unchanged with respect to that observed in catalyst 15Ni/AL. However, for 1: 3 Ni/oxalic acid molar ratio (i.e. 15Ni/AL-o3 sample), NiO particles display higher crystallinity, even though the average crystallite size hardly changes (Fig. 4e). Note that despite the similarity between X-ray patterns of 15Ni/AL-o3 and 30Ni/AL catalysts (Fig. 3, patterns i and e, respectively), their microstructure is quite different.In the case of niobium promoted catalysts, microstructural differences are found depending on the preparation method. The Nb-promoted NiO/\u03b3-Al2O3 catalyst prepared in two steps (Ni/Nb/AL sample) is formed by NiO crystallites of 10\u221215 nm distributed on the support, as well as agglomerates of large crystals of \u223c 50 nm. EDS maps (Figure S3) show a homogeneous distribution of niobium on the \u03b3-Al2O3 support and the absence of niobium in areas where NiO crystals are observed. Interestingly, well-dispersed crystallites of approximately 5\u221210 nm size with a characteristic d spacing of approximately 7 \u00c5 can be also observed in this catalyst. Provided the chemical composition of the sample and the elemental distribution, this periodicity is compatible with the (002) d spacing of Ni2O3 (d\n002 = 7.29 \u00c5). These crystallites are clearly visible in Fig. 4f. It is important to mention that this is the only catalyst in which this type of crystals have been observed, and the details of the preparation method in this particular case must be the origin of the formation of this species.For Nb-promoted NiO/\u03b3-Al2O3 prepared in one step, (Ni+Nb)/AL, NiO crystallites of 10\u221215 nm distributed on the support with agglomerates of larger NiO crystals of \u223c 50 nm were observed. These large crystals are less abundant than in the catalyst prepared in two steps. EDS maps performed (Figure S4) show that, although in low concentration, niobium is effectively distributed and associated with nickel on the support.Al2O3-supported NiO catalysts were also characterized by UV Raman spectroscopy (using an excitation wavelength of 325 nm) in order to elucidate the spin-phonon interaction in the materials [29,30]. Fig. 5\n shows the UV Raman spectra of alumina-supported nickel oxide catalysts. For comparison, the UV Raman spectrum of Ni+AL(PM) sample is presented in Figure S2 (pattern B).As reported in the literature, the UV Raman spectra of bulk NiO are characterized by the presence of five Raman bands [29,30]: i) two bands at ca. 510 and 580 cm\u22121, assigned to one-phonon (1P) transverse optical (TO) and longitudinal optical (LO) modes; ii) two weak bands at \u223c740 cm-1 and \u223c900 cm\u22121 and an intense band at ca. 1100 cm\u22121 related to two-phonon modes, i.e. the second-order transverse optical mode (2TO), the combination of TO + LO modes and the second-order longitudinal optical (2LO) modes, respectively. Among all these bands, the most intense ones are those located at 580 and ca. 1100 cm\u22121.We must notice that, when NiO is antiferromagnetically ordered or defect-rich, the intensity of one-phonon scattering (1P, LO and TO modes) increases significantly [29,31]. In addition, a very low intensity of the band at ca. 1124 cm\u22121 has been observed in silica-supported nickel oxide (with 3 wt% NiO), which has been attributed to the presence of very small NiO crystals [32]. In addition, the presence of only one band at ca. 570 cm-1 has been recently reported for NiO supported on Nb5+-containing siliceous porous clay heterostructure catalysts [33]. This observation was attributed to a high NiO dispersion as a consequence of an effective active phase-support interaction, what would lead to a decrease in NiO particle size and/or the generation of defects.As expected, the intensity of the most characteristic bands (1P LO band at \u223c580 cm\u22121 and 2P 2LO band at \u223c1100 cm\u22121) increases when the nickel oxide loading increases (Fig. 5). However, the relative growth of both bands differs depending on the specific structural and chemical features of the catalyst. Thus, an increase in the intensity of 2P 2LO band (I1100) higher than that of 1P LO band (I580) is observed at high NiO-loading. Therefore, the relative intensity of 1P LO band simultaneously increases with the decrease of NiO particle size and/or the presence of oxygen defects [31]. This fact would mean that the increase in the nickel oxide loading favors an increase of the NiO crystal size and/or a decrease in the concentration of oxygen defects.UV Raman spectra of catalysts with the same NiO-loading (with or without Nb5+ or oxalic acid in the synthesis gel) are comparatively shown in Fig. 5 (spectra c, f to i). Differences are observed in the relative intensity of the LO band (I580) and 2LO band (I1100) depending on the catalyst preparation procedure. Catalysts 15Ni/AL and (Ni+Nb)/AL present an I580/I1100 ratio higher than 1; whereas Ni/Nb/AL, 15Ni/AL-o1 and 15Ni/AL-o3 catalysts present I580/I1100 ratios lower than 1. These results could be partly explained in terms of the presence of NiO particles with different crystal sizes. However, the distribution and the mean crystal size of NiO in the reference catalyst (15Ni/AL) and sample 15Ni/AL-o1 are very similar, despite being the relative I580/I1100 ratio much higher in the reference catalyst.In the same way, Nb-promoted NiO/Al2O3 catalysts prepared in either one or two synthesis steps ((Ni+Nb)/AL and Ni/Nb/AL, respectively) also present significant differences in their UV Raman profiles (Fig. 5, spectra f and g, respectively). Despite showing similar NiO crystal size, both catalysts display different relative I580/I1100 ratio, being higher in the material prepared in one-step (i.e. (Ni+Nb)/AL sample). In addition, this would also underline the low capability of niobium oxide in the dispersion of NiO [34].The DR-UV\u2013vis spectra of prepared catalysts are shown in Fig. 6\n, in which spectra a to e correspond to Al2O3-supported NiO catalysts with different Ni-loadings, while spectra f to i are those corresponding to catalysts prepared in the presence of oxalate anions, Nb-promoted and unpromoted catalysts. For comparison, DR-UV\u2013vis spectrum of NiO-Al2O3 mixture, Ni+AL(PM) sample, has been also recorded (Figure S2-C).Bulk NiO shows bands at 715 nm and 377 nm, which can be assigned to octahedrally coordinated Ni2+ species in the NiO lattice [17,19,23,35,36]. Additionally, a band at 510 nm can be also assigned to charge transfer in NiO crystals [36,37]. On the other hand, it has been reported that supported nickel oxide catalysts, such as NiO-Al2O3 [38,39], NiO/Silica-Alumina [39] or NiO/Al2O3 [17,19], can also present a doublet (at 600\u2212645 nm) and a band at 416\u2013430 nm, which were attributed to Ni2+ species with tetrahedral (Td) and octahedral (Oh) coordination, respectively. Nevertheless, a band at 630 nm has been also observed in NiO supported on TiO2/\u03b3-Al2O3 [40], which has been attributed to the absorption of surface-dispersed nickel oxide species in tetrahedral coordination. We must note that the accommodation of Ni2+ species in both tetrahedral and octahedral coordination could lead to nickel aluminate as surface spinel phase in samples calcined at higher temperatures [17,24,36,38\u201340].According to our results, the DR-UV\u2013vis spectra of samples of xNi/AL series with NiO-loading up to 10 wt% NiO suggest the presence of Ni2+ species with tetrahedral (doublet at 600 and 640 nm) and octahedral (band at 410 nm) coordination, without the appearance of the band at 715 nm, typical of bulk NiO (Fig. 6 a\u2013b). These results could indicate the existence of highly dispersed nickel species with high interaction between support and part of Ni-containing crystallites (Ni2+ tetrahedral diffused into the \u03b3-Al2O3 lattice) [17,24,36,38,39]. At this point it is important to mention that NiAl2O4 spinel was not detected neither by X-ray diffraction nor by electron microscopy. When nickel loading increases, the interaction with the support reaches a maximum (15 wt% NiO or below), and from there, NiO crystals begin to grow, reaching larger size. Then, the bands corresponding to Ni2+ in octahedral coordination in NiO begin to be observed in the spectrum of the 15Ni/AL catalyst, thus indicating the simultaneous presence of nickel sites linked to the support as well as bulk NiO (Fig. 6, spectra c\u2013e). The above analysis is in strong agreement with what was observed by electron microscopy, where NiO crystals of 5\u221210 nm size are observed in 10Ni/AL and 15Ni/AL, in the last one co-existing with agglomerates of larger polygonal NiO crystals.In Nb-promoted catalysts, the nature of Ni2+-species depends on the catalyst preparation procedure. Spectrum of Ni/Nb/AL sample shows an intense band at 715 nm, Ni2+(Oh), in addition to a small band at 630 nm, Ni2+(Td) (Fig. 6, spectrum g). Thus, the presence of niobium species on the surface of support, as in the Ni/Nb/AL catalyst, limits the NiO-support interaction, thus favoring NiO crystallization. However, the distribution of niobium associated to nickel in (Ni+Nb)/AL sample (one-step synthesis), limits the growth of NiO crystals as the band at 715 nm shows lower intensity (Fig. 6, spectrum f). Accordingly, agglomerates of large NiO crystals are more abundant in Ni/Nb/AL sample, as observed by electron microscopy.Bands corresponding to NiO at 377 and 715 nm are particularly intense in catalysts 15Ni/AL-o1 and 15Ni/AL-o3, indicating that crystallized NiO has a weak interaction with the support. This fact is in agreement with electron microscopy data, where an increase in the crystallinity of NiO is observed although the size of the crystallites does not increase significantly.\nTable 1 summarizes the energy band gaps (Eg, in eV) of supported nickel oxide catalysts calculated from Kubelka-Munk function (Figure S5). In general, Eg values decrease when increasing the Ni-loading. In the same way, for a fixed NiO loading (15 wt%), the lowest band gap values are observed in Nb-containing catalysts (3.50 and 3.58 eV) and catalysts prepared in the presence of oxalic acid in the synthesis gel (3.25 and 3.55 eV). Samples from xNi/AL series with NiO contents in the range 5\u221215 wt% display the highest Eg among the samples analyzed (4.05\u22123.81 eV) (Table 1), which are also the most selective catalysts in the ODH of ethane. In our case, small differences are observed for all the catalysts, however, these band gap values alone cannot explain their catalytic properties as it may be influenced by various factors such as crystallite size, structural parameter, carrier concentrations, presence of impurities and lattice strain [41\u201344].In order to study the reducibility of catalysts and NiO-support interaction, TPR-H2 experiments were performed. Fig. 7\n shows the TPR-H2 profiles of xNiO/AL catalysts with different NiO-loading (Fig. 7, patterns a to e) and catalysts with 15 wt% NiO, with or without promoters, and synthesized by different preparation procedures (Fig. 7, patterns f to i). For comparative purposes, TPR-H2 profile of a mechano-chemical mixture, Ni+AL(PM) sample (Fig. S2), is also included.The shape of reduction profiles depends on the strength of NiO-support interaction. In the prepared catalysts, profiles show three main features corresponding to (in order of decreasing reduction temperature) (Fig. 7): i) the reduction of small and highly dispersed NiO particles for which the above interaction is strong [17\u201319,36], which gives a peak at ca. 520\u2212550 \u00b0C (observed in all catalysts); ii) the reduction of NiO particles that tend to form small agglomerates with medium strength interaction with the support, that originates a reduction peak slightly above 450 \u00b0C (observed in sample with Ni-loading of 15 wt%); and iii) the reduction of large NiO crystals with weak interaction with the support, which gives a peak at ca. 340 \u00b0C, similar to that observed in pure NiO [17\u201325] (observed in sample 30Ni/AL).Additionally, differences in reducibility can be clearly observed as a function of the catalyst preparation procedure when comparing catalysts with 15 wt% NiO (Fig. 7, patterns c, f-i). In this sense, Nb-promoted catalysts show a peak of high reducibility at 330 \u00b0C, which is more intense and constitutes the main feature of the reduction profile in Ni/Nb/AL. On the other hand, (Ni+Nb)/AL catalyst shows lower reducibility, with the main peak appearing at 530 \u00b0C. These results are consistent with the structural characterization. Thus, higher reducibility of Ni-species in Ni/Nb/AL catalyst can be interpreted on the basis of a low interaction between NiO and support (NbOx/Al2O3), which facilitates the growth of crystals and the formation of agglomerates that are more easily reduced. In this way, it has been proposed that Nb2O5 has not shown good properties as a NiO diluter/support, being unable in these conditions to eliminate a large proportion of non-selective sites [34].In catalysts prepared with oxalic acid in the synthesis gel, the reduction profiles show a unique peak around 400\u2212450 \u00b0C, in contrast with the xNi/AL series with Ni-loading below 15 wt% NiO, that presents a reduction peak at ca. 520 \u00b0C. According to TEM data, catalysts with 15 wt% of NiO and different amounts of oxalic acid (i.e. 15Ni/AL-o1 and 15Ni/AL-o3) present a similar size distribution of the NiO crystals, although crystallinity clearly improves compared to 15Ni/AL when increasing amounts of oxalic acid are used. A poor interaction with the support is the reason of this change as well as the decrease in reducibility. This is also in agreement with results from DR-UV\u2013vis spectra.It is worth mentioning that nickel aluminate-like species present a reduction peak at \u223c 800 \u00b0C, as reported in the literature [17,38]. This peak is not observed in the reduction profiles of the catalysts under study. This is in agreement with the structural and microstructural characterization of the samples, where the formation of NiAl2O4 has not been observed in any case. However, the possible presence of this phase cannot be completely ruled out, although, if present, it should be in low concentration in samples with low NiO-loading.According to the TPR-H2 experiments (Fig. 7), the catalyst 15Ni/AL presents the highest NiO-support interaction among the samples with a Ni-loading of 15 wt% NiO, also showing the maximum relative intensity of LO (1 P) band in UV Raman spectra (Fig. 5).\nFig. 8\n shows Ni 2p3/2\n core-level XPS spectra for selected Al2O3-supported NiO catalysts, whereas Figures S4 and S5 displays the XPS results of additional NiO-based materials. Ni 2p3/2\n core level spectra present the characteristic features of NiO, i.e. a main peak (ca. 856 eV) together with two satellites at 1.5\u20132.0 and 7.0 eV over the main peak (Sat I and Sat II, respectively) [13,15,33]. Sat I can be attributed to the presence of a wide variety of defects, such as Ni2+ vacancies, Ni3+ species or surface Ni2+-OH species; while Sat II is usually assigned to ligand-metal charge transfer. Changes in the relative intensity of Sat I signal have been related to the variations in the concentration of defects or in the particle size, which can be favored when an effective NiO-support interaction takes place [9,15,17\u201321,33]. Unfortunately, not a clear relationship between the relative intensity of Sat I / Main peak and the selectivity to ethylene has been observed.O 1s core level spectra are shown in Fig. 9\n and Figure S6. In general, the O 1s signal of alumina appeared at higher binding energy (532.2 eV) than the signal for nickel oxide (530.5 eV) in all cases, as seen elsewhere [45]. The contribution of the alumina O 1s signal is bigger for the catalysts with Ni-loading lower than 20 wt% (Fig. 9), with a symmetric display of the peak. However, a shift of the band to lower binding energy is observed for catalysts with 30 wt% of NiO (Fig. S6), samples prepared with Nb (especially for Ni/Nb/AL sample (Fig. 9)) or a mechano-chemically treated sample (Fig. S6), in agreement with a worst dispersion of the NiO. A strong interaction of the oxygen anions with the Ni2+ cations for these catalysts was also suggested as a shift to lower binding energy occurred.Al 2p and Ni 3p spectra for the catalysts with different Ni amount (Fig. 10\n), show a single peak at 74.8 eV for Al 2p, corresponding to the Al3+ species in octahedral coordination, together with a signal at 68.5 eV, attributed to Ni 3p core level [45]. This latter Ni 3p core level peak increases in intensity as the NiO-loading increases. On the other hand, a small shift to lower binding energies when increasing the Ni loading is observed, likely associated with a progressively higher interaction of Al3+-bonded oxygen sites with Ni2+ cations, resulting in a distortion of the Al2O3 octahedral network [45].In Nb 3d XPS spectra (Fig. S7), Nb-containing catalysts presented a classical doublet with a split spin-orbit of the components of 2.78 eV which is related to the unique presence of dispersed Nb5+ [33].The TPR experiments undertaken confirm the close relationship between reducibility of nickel oxide species and the interaction of NiO particles with the support, which seems to determine the selectivity to ethylene during ethane ODH. Fig. 11\n plots the relative hydrogen consumption of the band at 330 \u00b0C (reduction degree), related to NiO with low interaction with the alumina support, and the selectivity to ethylene at isoconversion conditions. It can be observed that the presence of NiO with low interaction with the alumina support must be avoided in order to achieve high ethylene formation, since an inverse relationship between the hydrogen consumption of the NiO reduction peak and the selectivity to the olefin has been observed. Accordingly, a higher interaction between NiO particles and support decreases the reducibility of Nin+ species, and the concentration of electrophilic oxygen species, thus favoring a more controlled oxygen supply during catalytic cycles, and a higher selectivity to the olefin.This NiO-support interaction determines not only NiO crystal size, but also the definition/crystallinity of the crystals (and the concentration of defects in the active phase). As discussed in the UV Raman spectra, an increase in the relative intensity of the 1 P LO band (I580) with respect to that for 2 P 2LO band (I1100) means that the NiO crystallite size decreases and/or the amount of oxygen defects increases. As the crystal size in all the catalysts is not constant, an accurate estimation of the number of defects cannot be undertaken through this technique. Interestingly, as observed by TEM, the reference catalyst 15Ni/AL and those with oxalic acid (samples 15Ni/AL-o1 and 15Ni/AL-o3) present very similar NiO crystal size (Fig. 4). However, the I580/I1100 ratio in the corresponding UV Raman spectrum of 15Ni/AL (Fig. 5, spectrum c) is remarkably higher than that observed for 15Ni/AL-o1 and 15Ni/AL-o3 catalyst (Fig. 5, spectra h and i). Accordingly, a different concentration of oxygen species can be proposed for the reference catalyst (15Ni/AL).Similarly, both Nb-containing catalysts present similar NiO crystal size (Fig. 4), but the catalyst prepared in one step (Ni+Nb)/AL displays a higher I580/I1100 ratio in the corresponding UV Raman spectra than the one prepared in two steps (Ni/Nb/AL) (Fig. 5, spectra f and g), which also results in a higher ethylene selectivity.Al2O3-supported nickel oxide catalysts prepared by a conventional wet impregnation method (without oxalic acid in the synthesis gel) showed high selectivity to ethylene during the ethane ODH. In this way, catalysts with NiO-loadings of 15\u201320 wt% NiO display interesting catalytic properties (with a selectivity to ethylene higher than 90 %), in agreement with previous results from other authors [16\u201319]. The slightly lower selectivity to ethylene observed in the catalysts with 5\u201310 wt% NiO could be related to the presence of highly dispersed Ni2+(Td) species but also to available unselective alumina sites, which present poor catalytic activity under our reaction conditions. The lower selectivity to ethylene observed over catalysts with Ni-loading higher than 20 wt% NiO can be related to the presence of big crystals of NiO, as deduced from TEM (Fig. 4), UV Raman (Fig. 5) and DR-UV\u2013vis (Fig. 6) spectra, with weak NiO-support interaction, presenting high reducibility (Fig. 7).It is especially noteworthy that the positive effect of the incorporation of Nb5+ in unsupported Nb-promoted NiO catalysts, which has been widely reported in the scientific literature [9\u201315], is not observed in supported Nb-promoted NiO/Al2O3 catalysts, regardless of the preparation method. In fact, both (Ni+Nb)/AL and Ni/Nb/AL catalysts present lower selectivity to ethylene than that observed for Al2O3-supported NiO catalysts displaying a NiO-loading from 10 to 20 wt%. The characterization results of these catalysts, as well as those prepared in the presence of oxalic acid in the synthesis gel, clearly indicate the presence of NiO particles with low interaction with the support, whose reducibility and crystallinity increases when increasing the concentration of oxalic acid in the synthesis gel.For Al2O3-supported nickel oxide catalysts, the presence of oxalic acid during the synthesis has a deleterious effect on the catalytic performance, i.e. the higher the amount of oxalic acid in the synthesis gel, the lower is the selectivity to ethylene. This observation can be interpreted in terms of an increase in the reducibility of Nin+ sites due to a lower NiO-support interaction, which is not only related to crystallite size, but also to the crystallinity.The addition of an organic additive, such as oxalic acid, in the synthesis gel, during the preparation of unsupported materials, could help to decrease NiO particle size, thus leading to a material with lower amount of electrophilic oxygen, as observed in bulk SnO2-NiO catalysts [15]. However, the presence of a support can hinder the interaction of the organic additive with the active phase, due to a favored coupling between the additive and the support. This fact can lead, as in the case of NiO/Al2O3 system, to NiO particles presenting a bigger particle size and a higher crystallinity, even when high amounts of oxalic acid are used, thus leading to a lower selectivity to the olefin in the ODH of ethane. Moreover, this is in agreement with the characterization results by XRD, diffuse reflectance UV\u2013vis and UV Raman.On the other hand, from XPS results (Figs. 8\u201310), it can be concluded that the samples with low reducibility (i.e. samples of xNi/AL series with NiO-loading lower than 20 wt. %) present: i) the characteristic features of NiO (with band at ca. 856 eV) and two satellites, Sat I and Sat II (at 1.5\u20132.0 and 7.0 eV, respectively, over the main peak) [9,15,17\u201321,33]; ii) a bigger contribution of the alumina O1 s signal (according to a higher dispersion of NiO); and iii) a shift to lower binding energy in the Al 2p + Ni 3p signal, confirming also the interaction between NiO particles and Al2O3 support [45]. All of these characteristics suggest the correlation between the selectivity to ethylene and a higher and more effective NiO-Al2O3 interaction.It must be noted that the amount of carbon detected on the catalysts after use is negligible. This positive aspect is due to the oxidative conditions employed since the possible coke formed is oxidized into carbon dioxide. Moreover, TPR experiments of representative used catalysts were undertaken. As it can be seen in Figure S8 the differences between the fresh and the used catalysts are hardly perceptible. Then, the most characteristic features that define a TPR assay: the hydrogen consumption, the onset temperature and the temperature for the maximum hydrogen consumption, seem to keep unaltered after the reaction.A tight correlation has been found between the NiO-Al2O3 interaction (as concluded from H2-TPR) and the selectivity to ethylene during the oxidative dehydrogenation of ethane on alumina-supported nickel oxide catalysts. This interaction depends not only on the NiO crystal size and Ni-loading, but also on the synthesis method employed. Interestingly, NiO crystals with similar size have shown remarkably different interactions with the support and, consequently, different levels of ethylene formation.The preparation method has been shown to be of capital importance to synthesize selective catalysts. Then, the use of oxalic acid in the preparation of NiO/\u03b3-Al2O3 catalysts leads to lower NiO-alumina interaction, higher crystallinity of NiO particles and, subsequently, to a lower selectivity to ethylene during the ethane ODH.The presence of Nb5+ in alumina-supported Ni-Nb-O catalysts (i.e. (Ni+Nb)/AL) has not improved the selectivity to ethylene with respect to the corresponding Nb-free catalyst (i.e. 15Ni/AL), in contrast with the positive effect reported in bulk NiO catalysts. Nevertheless, the alumina-supported Ni-Nb-O catalysts, also prepared in presence of oxalate anions, are more selective to ethylene than the corresponding Nb-free samples prepared with oxalic acid in the synthesis gel (i.e. 15Ni/AL-o1) or than the nickel oxide supported on NbOx/\u03b3-Al2O3 (i.e. Ni/Nb/AL). Therefore, the presence of oxalate anions in the synthesis gel hinders the interaction between NiO and \u03b3-Al2O3, favoring the formation of supported NiO particles with high crystallinity.\nYousra Abdelbaki, Investigation, Discussing, Writing - review & editing. Agust\u00edn de Arriba, Investigation, Discussing, Writing - review & editing. Benjam\u00edn Solsona, Supervision, Conceptualization, Validation, Writing - review & editing, Funding. Daniel Delgado, Investigation, Discussing, Writing - review & editing. Ester Garc\u00eda-Gonz\u00e1lez, Investigation, Discussing, Writing - review & editing. Rachid Issaadi, Methodology, Investigation, Jos\u00e9 M. L\u00f3pez Nieto, Supervision, Conceptualization, Writing - review & editing, Funding.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Authors would like to acknowledge the Ministerio de Ciencia, Innovaci\u00f3n y Universidades in Spain through projects CRTl2018-099668-B-C21, MAT2017-84118-C2-1-R and PID2019-106662RB-C44 and Ministry of Higher Education and Scientific Research of Algeria for the National Exceptional Program for the fellowships. A.A. acknowledges Severo Ochoa Excellence Program for his fellowship (BES-2017-080329). EGG acknowledges Dr. E. Urones for the valuable assistance in the use of electron microscopy facilities as well as to the CNME (Spain).Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2021.118242.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Nickel oxides supported on \u03b3-alumina (Ni-loading from 5 to 30 wt% NiO) have been synthesized and tested in the oxidative dehydrogenation (ODH) of ethane in order to determine the importance of the NiO-support interaction. The best performance was achieved by the catalyst with 15 wt% NiO; higher NiO-loadings lead to the formation of unselective bulk-like NiO and lower Ni-loadings present high proportion of free alumina surface sites. The presence of oxalic acid and/or niobium in the synthesis gel resulted in the formation of NiO particles with similar size, but higher crystallinity and reducibility than the standard 15 wt% NiO catalyst. The obtained results have revealed that, in addition to NiO crystal size, the nickel oxide-support interaction determines the catalytic performance of these catalysts.\n               "}
{"full_text": "Fast depletion of conventional crude oil reserves and increasing demand for clean transportation fuels greatly stimulate the research and development of heavy oil upgrading technologies (Bellussi et al., 2013; Browning et al., 2021; Saab et al., 2020). Efficient conversion of vacuum residue (VR), the heaviest fraction of crude oil, into lighter fractions such as gasoline and diesel is considered as one of the greatest challenges in the modern petroleum processing industry (Omajali et al., 2017; Prajapati et al., 2021). VR is mainly comprised of high boiling point polycyclic aromatic hydrocarbons with large amount of sulfur (S), nitrogen (N), and metals (usually vanadium (V) and nickel (Ni)) (Pham et al., 2022; Prajapati et al., 2022), resulting in coke formation on both catalyst and equipment in the refining process (Tsubaki et al., 2002; Fortain et al., 2010; Kim et al., 2017). Among the various VR conversion technologies developed up to date, slurry-phase hydrocracking technology is considered as the most efficient and economic one because of its great feedstock flexibility, high conversion efficiency, and high light distillates yield. It is recognized that catalyst is the key in this process, because it determines the feedstock conversion and the liquid product yield (Saab et al., 2020; Morawski and Mosiewski, 2006; Looi et al., 2012).Slurry-phase hydrocracking catalysts include homogeneously dispersed catalysts of oil-soluble dispersed catalysts and water-soluble dispersed catalysts and heterogeneous solid powder catalysts (Pur\u00f3n et al., 2013; Nguyen et al., 2015). Oil-soluble dispersed catalysts exist as organometallic compounds can effectively depress the gas and coke formation due to its homogeneously dispersed heavy oil to adequately contact with reactant molecule (Chen et al., 2022; Kang et al., 2019). Water-soluble dispersed catalysts are prepared with multiple steps such as dispersion, emulsification and dehydration, which greatly increase the operation complexity and cost (Liu et al., 2009; Luo et al., 2011). Natural mineral catalysts as solid powder catalysts, which have the advantages of low cost and broad sources, were extensively used in the early stage of slurry-phase hydrocracking technology. Nevertheless, they were replaced gradually by other types of catalysts due to their inferior catalytic activities and property instability (Yue et al., 2016, 2018; Manek and Haydary, 2017). Supported metal catalysts are composed of active metal and support, in which the active metals are usually Mo, Co, Ni or their binary/trinary combinations, and the supports are commonly acidic materials such as alumina, silica-alumina, zeolites and even natural minerals (Looi et al., 2012; Leyva et al., 2007, 2009). As compared with natural mineral catalyst, supported metal catalyst has the advantages on the enhancement of hydrocracking reactivity and adjustable performance, thus it has attracted increasing attention in slurry-phase hydrocracking.In supported metal catalysts, the metal species are considered as the active centers to hydrogenate the polycyclic aromatic hydrocarbons and olefins, and to quench the free radicals in the feedstock and intermediate products to avoid the over-cracking reactions and condensation reactions to form gas and coke. In addition, the support also plays a crucial role in determining the catalytic performance by affecting the metal dispersion on the catalyst surface and thus influencing the feedstock conversion. There were many reports on supported catalysts employed in the slurry-phase hydrocracking process. The MoS2/SiO2\u2013ZrO2 bifunctional catalyst was applied in the slurry-phase hydrocracking of decalin-phenanthrene mixture to study the effect of Si/Zr molar ratio on performance of catalyst, the analysis results revealed that Br\u00f6nsted acid on SiO2\u2013ZrO2 support was mainly contributed to the catalytic performance (Ma et al., 2021). Looi et al. (2012) prepared a series of catalysts using alumina supports with different pore sizes and investigated their catalytic performance in residual oil hydrocracking, the result showed that the residue oil conversion was about 50\u202fwt% and the highest yield of liquid products was 97\u202fwt% at 400\u202f\u00b0C, and more acid sites benefited to the residue oil conversion. Yue et al. (2018) prepared the slurry-phase hydrocracking catalyst by using a hydrothermally treated natural bauxite mineral as the support and assessed their performance by using a high temperature coal tar as the feedstock, and they found that the high feedstock conversion and high liquid yield were attributed to the suitable support acidity and the weaker interaction between the active metal and the support. S\u00e1nchez et al. (2018) investigated the catalytic performance of a bifunctional MoS2/ASA (amorphous silica-alumina) catalyst in the slurry-phase hydroconversion, and found that the presence of moderate Br\u00f6nsted acid sites promoted the cracking, isomerization and ring-opening reactions. Despite numerous reports available on supported slurry-phase hydrocracking catalysts, there is a lack of systematic and deep understanding on the influences of support properties such as composition, pore structure and acidity on catalytic performance.Herein, we present a thorough study on the effects of support properties on the catalytic performance of supported metal catalysts in the VR slurry-phase hydrocracking process. A series of Mo catalysts supported on SiO2, \u03b3-Al2O3, amorphous silica-alumina (ASA) and ultra-stable Y (USY) zeolite, respectively, were prepared by the conventional impregnation method. The pore structure and acidity of the different supports were examined by N2-adsorption-desorption and pyridine adsorbed Fourier transform infrared (Py-FTIR) spectroscopy, the morphology of metal sulfide species on the corresponding catalysts were investigated by high resolution transmission electron microscopy (HRTEM), and the catalytic performances of the different catalysts were compared in the VR slurry-phase hydrocracking.In the present study, SiO2, \u03b3-Al2O3, ASA and USY zeolite with Si/Al ratio of 2.7 were used as the supports to prepare slurry-phase hydrocracking catalysts. SiO2, \u03b3-Al2O3 and USY zeolite were obtained from Shanghai Aladdin Bio-Chem Technology Co. Ltd., Fujian Yucheng Environmental Protection Technology Co. Ltd. and Nankai University, respectively. ASA was prepared as follows: 25\u202fmL of an aluminum nitrate solution with a concentration of 2\u202fmol/L and 30\u202fmL ammonia water (25\u202fwt% of ammonia) were simultaneously and slowly added into 50\u202fmL water to maintain pH value of 8\u20139\u202fat 60\u202f\u00b0C under agitation. Then, 2.7\u202fg of sodium silicate (27.68% SiO2 and 8.95% Na2O) was added into the above solution to obtain a mixture with a SiO2/Al2O3 molar ratio of 1:1. Finally, the resulting mixture was aged for 1\u202fh, filtered with distilled water, dried at 120\u202f\u00b0C for 10\u202fh, and calcined at 500\u202f\u00b0C for 3\u202fh to obtain ASA sample.A series of catalysts supported on the different materials were prepared by the conventional incipient wetness impregnation method with an aqueous solution of ammonium molybdate tetrahydrate ((NH4)6Mo7O24\u00b74H2O, 98%, Adamas). The resultant samples were aged at 30\u202f\u00b0C for 12\u202fh, dried at 120\u202f\u00b0C for 10\u202fh, and calcined at 600\u202f\u00b0C for 4\u202fh to obtain the corresponding supported Mo oxide catalysts, which are designated as Mo/SiO2, Mo/USY, Mo/\u03b3-Al2O3 and Mo/ASA, respectively. The MoO3 content in each catalyst is 5\u202fwt% according to the literature (Kim et al., 2018; Ancheyta et al., 2003).N2 adsorption-desorption measurement was taken on a Micromeritics ASAP 2460 apparatus at \u2212196\u202f\u00b0C. The surface area (S\nBET) of sample was determined by using Brunauer-Emmett-Teller (BET) equation, and the pore volumes (V\ntotal) and average pore diameters (D\np) were obtained by Barrett-Joyner-Halenda (BJH) method. Adsorbed pyridine Fourier transform infrared spectroscopy (Py-FTIR) measurement was carried out on a MAGNAIR 560 FTIR instrument, and the spectra were recorded at 250 and 350\u202f\u00b0C, respectively. H2 temperature programmed reduction (H2-TPR) was conducted on an ASAP-2920 equipment using a thermal conductivity detector (TCD). Around 20\u202fmg of sample was firstly pretreated in Ar steam at 300\u202f\u00b0C for 30\u202fmin, and then cooled to 50\u202f\u00b0C. The H2-TPR profiles were acquired from 50 to 950\u202f\u00b0C at a heating rate of 10\u202f\u00b0C/min in a 10\u202fvol% H2/Ar stream.High resolution transmission electron microscopy (HRTEM) images of MoS2 slabs were collected on a Tecnai G2 F20 instrument at 200\u202fkV, and at least 200 slabs were measured to calculated for each sample. The average lengths (\n\n\nL\n\u00af\n\n\n) and stacking numbers (\n\n\nM\n\u00af\n\n\n) according to the Eqs. (1) and (2) (Liu et al., 2017; Escobar et al., 2018):\n\n(1)\n\n\nAverage\n\nslab\n\nlength\n:\n\nL\n\u00af\n\n=\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\n\nx\ni\n\n\nL\ni\n\n\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\nx\ni\n\n\n\n\n\n\n\n\n\n\n(2)\n\n\nAverage\n\nstack\n\nnumber\n:\n\nM\n\u00af\n\n=\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\n\nx\ni\n\n\nm\ni\n\n\n\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\nx\ni\n\n\n\n\n\n\n\nwhere L\n\ni\n, x\n\ni\n and m\n\ni\n denote the length, the number and the layer number in a stack of MoS2 slabs.The MoS2 dispersion (D\nMo) was acquired by Eq. (5), f\nMoe is the ratio of Mo atoms located at edge sites of MoS2 slabs, and f\nMoc is the fraction of Mo atoms at corner sites, which were estimated by Eqs. (3) and (4) (Hensen et al., 2001; Kasztelan et al., 1984):\n\n(3)\n\n\n\nf\nMoe\n\n=\n\n\nMo\nedge\n\n\nMo\ntotal\n\n\n=\n\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n\n6\n\n(\n\n\nx\ni\n\n\u2212\n2\n\n)\n\n\n\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n\n(\n\n3\n\n\nx\ni\n\n2\n\n\u2212\n3\n\nx\ni\n\n+\n1\n\n)\n\n\n\n\n\n\n\n\n\n\n(4)\n\n\n\nf\nMoc\n\n=\n\n\nMo\ncorner\n\n\nMo\ntotal\n\n\n=\n\n6\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n\n(\n\n3\n\n\nx\ni\n\n2\n\n\u2212\n3\n\nx\ni\n\n+\n1\n\n)\n\n\n\n\n\n\n\n\n\n\n(5)\n\n\n\nD\nMo\n\n=\n\nf\nMoe\n\n+\n\nf\nMoc\n\n=\n\n\n\nMo\nedge\n\n\n\n+\nMo\n\ncorner\n\n\n\nMo\ntotal\n\n\n=\n\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n\n6\n\n(\n\n\nx\ni\n\n\u2212\n1\n\n)\n\n\n\n\n\n\u2211\n\ni\n=\n1\n\nt\n\n\n(\n\n3\n\n\nx\ni\n\n2\n\n\u2212\n3\n\nx\ni\n\n+\n1\n\n)\n\n\n\n\n\n\n\nwhere Moedge, Mocorner and Mototal are the numbers of Mo atoms along on edge sites, corner sites and total Mo atoms on MoS2 slabs, t represents the total number of slabs, and x\n\ni\n denotes the number of Mo atoms on edge sites of each MoS2 slab, acquired by L\n\ni\n\u202f=\u202f3.2\u202f\u00d7\u202f(2x\n\ni\n\u20131).X-ray photoelectron spectroscopy (XPS) was taken on a Thermo ESCALAB 250 spectrometer with a monochromatic Al K\u03b1 source. C 1s peak at 284.6\u202feV was used as reference to calibrate the binding energy. The XPSPEAK41 software was employed to analyze the experimental results. The relative contents of each species of MoS2, MoS\nx\nO\ny\n and Mo6+ oxide for an individual sulfide catalyst were determined through their peak areas. For instance, the relative MoS2 content [MoS2] (%) was calculated by Eq. (6):\n\n(6)\n\n\n\n[\n\nMoS\n2\n\n]\n\n\n(\n%\n)\n\n=\n\n\nA\n\nMoS\n2\n\n\n\n\nA\n\nMoS\n2\n\n\n+\n\nA\n\nMoS\nx\nO\ny\n\n\n+\n\nA\n\nMo\n\n6\n+\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\nwhere A\n\nX\n represents the peak area of species X in Mo 3d XPS envelope.VR provided by SINOCHEM Quanzhou PetroChemical Co., Ltd. was employed as the feedstock to assess the slurry-phase hydrocracking performance of catalysts, and its properties are shown in Table 1\n.The catalyst assessment for VR hydrocracking was carried out in a 300\u202fmL stainless-steel autoclave equipped with a stirrer. 40\u202fg of VR, 1.2\u202fg of catalyst and 0.88\u202fg of sulfur powder were added into the reactor. Prior to the hydrocracking reaction, the sulfuration of catalyst in situ was conducted at 350\u202f\u00b0C under an initial H2 pressure of 11\u202fMPa for 5\u202fh, subsequently, the VR hydrocracking reaction was performed at 430\u202f\u00b0C with a volumetric H2 to oil ratio of 850 (v/v) for 3\u202fh under a stirring rate of 600\u202frpm. After reaction, the mixture of the reaction product and catalyst was collected after the autoclave rapidly cooled to room temperature and was separated by centrifugalization and filtration. The liquid product was divided into four fractions in a SYD-9168 vacuum distillation apparatus according to the boiling point (BP) range, the four fractions of naphtha, middle distillate, vacuum gas oil (VGO) and VR are in the range of BP\u202f<\u202f180\u202f\u00b0C, 180\u2013350\u202f\u00b0C, 350\u2013500\u202f\u00b0C and BP\u202f>\u202f500\u202f\u00b0C. Additionally, the solid residue including coke and the used catalyst was washed with toluene. The VR conversion and the yields of gas, naphtha, middle distillate, VGO and coke were acquired by the following Eqs. (7)\u2013(12):\n\n(7)\n\n\nVR\n\nconversion\n\n(\n\nwt\n\n%\n\n)\n\n=\n\n\n\nM\nf\n\n\u2212\n\nM\np\n\n\n\nM\nf\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n(8)\n\n\nGas\n\nyield\n\n(\n\nwt\n\n%\n\n)\n\n=\n\n\nM\ng\n\n\nM\nt\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n(9)\n\n\nNaphtha\n\nyield\n\n(\n\nwt\n\n%\n\n)\n\n=\n\n\nM\nn\n\n\nM\nt\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n(10)\n\n\nMiddle\n\ndistillate\n\nyield\n\n(\n\nwt\n\n%\n\n)\n\n=\n\n\nM\nm\n\n\nM\nt\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n(11)\n\n\nVGO\n\nyield\n\n(\n\nwt\n\n%\n\n)\n\n=\n\n\nM\nv\n\n\nM\nt\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n(12)\n\n\nCoke\n\nyield\n\n(\n\nwt\n\n%\n\n)\n\n=\n\n\nM\nc\n\n\nM\nt\n\n\n\u00d7\n100\n%\n\n\n\n\nwhere M\nf, M\np are mass of >500\u202f\u00b0C fraction in the feed and product, M\ng, M\nn, M\nm, M\nv and M\nc denote mass of gas, <180\u202f\u00b0C fraction, 180\u2013350\u202f\u00b0C fraction, 350\u2013500\u202f\u00b0C fraction, and coke in product, meanwhile, M\nt means the total mass of feed.VR slurry-phase hydrocracking performance of Mo catalysts supported on the different supports was assessed, and the resulting product was distilled to obtain the different distillate fractions. The VR conversions and the yields of naphtha and middle distillate obtained over different catalysts are shown in Fig. 1\n. It can be seen that the VR conversion over Mo/ASA is 75.2%, only slightly higher than those (73.9% and 73.5%) over Mo/\u03b3-Al2O3 and Mo/USY, but significantly higher than that (67.4%) over Mo/SiO2. The results demonstrate that, as compared with the other catalysts, Mo/ASA can effectively convert the heavy fraction with large molecules in VR into lighter fractions with smaller molecules. The yields of naphtha and middle distillate over the different catalysts are in the order of Mo/ASA\u202f>\u202fMo/\u03b3-Al2O3\u202f\u2248\u202fMo/USY\u202f>\u202fMo/SiO2, indicating that Mo/ASA favors the production of naphtha and middle distillate.\nFig. 2\n displays the yields of naphtha, middle distillate, VGO, gas and coke obtained over the different catalysts. The naphtha yield over Mo/ASA is 19.5\u202fwt%, slightly lower than that (21.6\u202fwt%) over Mo/USY, but obviously higher than those (14.3\u202fwt% and 15.9\u202fwt%) over Mo/SiO2 and Mo/\u03b3-Al2O3, while the middle distillate yield over Mo/ASA is 35.2\u202fwt%, much higher than those over the others. The VGO yields obtained over the different catalysts follow the order of Mo/\u03b3-Al2O3 (22.8\u202fwt%)\u202f\u2248\u202fMo/SiO2 (22.7\u202fwt%)\u202f>\u202fMo/ASA (18.3\u202fwt%)\u202f>\u202fMo/USY (16.4\u202fwt%). The yields of unconverted residue decrease in the order of Mo/SiO2 (21.5\u202fwt%)\u202f>\u202fMo/USY (17.5\u202fwt%)\u202f\u2248\u202fMo/\u03b3-Al2O3 (17.2\u202fwt%)\u202f>\u202fMo/ASA (16.4\u202fwt%). The yields of gas are in the order of Mo/USY (15.6\u202fwt%)\u202f>\u202fMo/SiO2 (11.7\u202fwt%)\u202f>\u202fMo/\u03b3-Al2O3 (10.2\u202fwt%)\u202f\u2248\u202fMo/ASA (10.1\u202fwt%), and the yields of coke increase in the order of Mo/ASA (0.5\u202fwt%)\u202f\u2248\u202fMo/\u03b3-Al2O3 (0.6\u202fwt%)\u202f<\u202fMo/SiO2 (0.8\u202fwt%)\u202f<\u202fMo/USY (1.7\u202fwt%). By comparing the above results, it is concluded that Mo/ASA exhibits the best overall performance among all the catalysts due to its highest VR conversion, highest yield of naphtha and middle distillate, and relatively lower yields of gas and coke.The hydrogen consumption can be considered as an index of catalyst hydrogenation activity, because VR slurry-phase hydrocracking reaction is accompanied with hydrogen consumption (Bianco et al., 1994; Kang et al., 2020). Fig. 3\n shows the H2 pressure profiles during the VR slurry-phase hydrocracking process involving the different catalysts. Before the reaction, the initial H2 pressure in the reactor was 11.0\u202fMPa at room temperature, when the reaction temperature was increased to 430\u202f\u00b0C, the pressure in reactor increased up to 21.5\u202fMPa, then the pressure in reactor gradually decreased with the prolonging reaction time, and the decreasing tendencies were different for the reaction systems involving the different catalysts. After the hydrocracking reaction was terminated by cooling the reactor to about 200\u202f\u00b0C, the pressures in the reactors loaded with different catalysts were dramatically dropped, the residual pressures were in the order of Mo/ASA\u202f<\u202fMo/USY\u202f<\u202fMo/\u03b3-Al2O3\u202f<\u202fMo/SiO2. This indicated that Mo/ASA has the highest hydrogenation activity among the four catalysts. The Mo catalysts on the different support present different slurry-phase hydrocracking performances, thus it is necessary to deeply analyze properties and pore structures of these supports and their derived catalysts to understand the underlying reasons.The textural properties of catalysts can significantly impact their VR hydrocracking performance. It is widely accepted that macro-/meso-porous structure in supported catalysts benefits the diffusion of bulkier molecules in VR and thereby provides higher accessibility of active sites to reactants molecules, promoting feedstock conversion and improving the selectivity to target products (Leyva et al., 2014; Zheng et al., 2019). To understand the effects of pore structures of the supports on the catalytic performance of their derived catalysts, N2 adsorption-desorption measurements were conducted to compare the textural properties of the different supports, and the results are shown in Fig. 4\n. It can be seen that the N2 adsorption-desorption isotherms of USY zeolite and \u03b3-Al2O3 belong to type II ones with a H4 hysteresis loop, but those of SiO2 and ASA belong to type IV ones with a H1 hysteresis loop and a H3 hysteresis loop, respectively, indicating that the different supports have different pore structures. The surface areas, pore volumes and average pore diameters of the different supports calculated from the N2 adsorption-desorption data are summarized in Table 2\n. It is seen that the surface area (585\u202fm2/g) of USY zeolite is much larger than those (386\u202fm2/g, 198\u202fm2/g, and 226\u202fm2/g) of ASA, \u03b3-Al2O3 and SiO2, whereas ASA has the largest external surface area (357\u202fm2/g) among all the supports, with USY zeolite having the smallest external surface area (52\u202fm2/g). The average pore volumes of different supports are in the order of SiO2 (0.89\u202fcm3/g)\u202f>\u202fASA (0.76\u202fcm3/g)\u202f>\u202fUSY zeolite (0.34\u202fcm3/g)\u202f>\u202f\u03b3-Al2O3 (0.25\u202fcm3/g), but their mesoporous volumes are in the order of SiO2 (0.88\u202fcm3/g)\u202f>\u202fASA (0.75\u202fcm3/g)\u202f>\u202f\u03b3-Al2O3 (0.17\u202fcm3/g)\u202f>\u202fUSY zeolite (0.07\u202fcm3/g). These results suggest that, among different supports, ASA that simultaneously has the largest external surface area, larger average pore volume and mesoporous volume should be the most suitable for the preparing slurry-phase hydrocracking catalyst, because its larger average pore volume and mesoporous volume are beneficial for the diffusion of bulkier molecules in VR and their generated intermediate molecules onto the active sites of catalyst, and its largest external surface area favors metal dispersion and thereby generates more active metal sites to restrain gas and coke formation, this can provide the highest VR conversion, highest total yield of naphtha and middle distillate, and lowest yields of coke and gas obtained over Mo/ASA, as shown in Figs. 1 and 2.The acid properties of the different supports were characterized by Py-FTIR, the results measured at 250 and 350\u202f\u00b0C are shown in Fig. 5\n. The bands at 1540 and 1450\u202fcm\u22121 are attributed to Br\u00f6nsted and Lewis acid sites, respectively (Schweitzer et al., 2022). The Py-FTIR spectrum of USY zeolite has a stronger adsorption peak at 1540\u202fcm\u22121 and a weaker adsorption peak at 1450\u202fcm\u22121, indicating that USY zeolite has a large amount of B acid sites, ascribed to the bridging Si\u2013OH\u2013Al groups because of the replacement of Si4+ in the crystallite framework by Al3+ (Tang et al., 2019). However, it has only a very small amount of L acid sites. The spectrum of \u03b3-Al2O3 has an adsorption peak at 1450\u202fcm\u22121 but no apparent peak at 1540\u202fcm\u22121, and that of ASA has two weaker peaks at 1450 and 1540\u202fcm\u22121 for both 250 and 350\u202f\u00b0C, illustrating the existence of small amounts of B and L acid sites in ASA. No obvious pyridine adsorption peak is observed for SiO2, indicating its negligible acid sites. The amounts of acid sites calculated according to Py-FTIR spectra measured at 250 and 350\u202f\u00b0C for the different supports are summarized in Table 3\n. Notably, the acid sites determined at 250\u202f\u00b0C are considered as weak ones, while the acid sites determined at 350\u202f\u00b0C can be taken as moderate and strong ones (Gafurov et al., 2015; Phung and Busca, 2015). No acid site exists in SiO2, as shown in Table 3, and the USY zeolite has the largest amount of acid sites including weak, moderate and strong ones, especially B acid sites. Thus, \u03b3-Al2O3 and ASA present the total acid amounts standing between those of SiO2 and the USY zeolite, with the former having only L acid sites and the latter having only a smaller amount of L acid sites but a larger amount of B acid sites, which is ascribed to the bridging hydroxyls in connection with tetrahedrally coordinated Al species on silica (Valla et al., 2015). It indicates the amounts of acid sites in the different supports are in the order of USY zeolite > \u03b3-Al2O3\u202f>\u202fASA\u202f>\u202fSiO2.By comparing the acidity characterization results and the hydrocracking reaction results in Figs. 1 and 2, it is found that Mo/SiO2 prepared from SiO2 with the largest average pore volume and mesoporous volume but without acid sites gives the lowest VR conversion among the different catalysts. Mo/USY prepared from USY zeolite with the smallest external surface area and mesoporous volume but with the largest amount of acid sites displays a higher VR conversion than Mo/SiO2. Mo/\u03b3-Al2O3 prepared from \u03b3-Al2O3 with a relatively larger external surface area and a slightly larger mesoporous volume but with a larger amount of L acid sites, gives a VR conversion comparable to that of Mo/USY but much lower yields of gas and coke than Mo/USY. Mo/ASA prepared from ASA with the largest external surface area, larger mesoporous volume and more B and L acid sites presents the highest VR conversion, the highest yields of naphtha and middle distillate, and the yields of gas and coke comparable to that of Mo/Al2O3 but much lower than those of Mo/SiO2 and Mo/USY. Therefore, it can be concluded that both support acidity and pore structure can significantly impact VR conversion and the distribution of various distillates of hydrocracking product. By further comparing the acidity properties of ASA and \u03b3-Al2O3, and the catalytic performance of their corresponding supported catalysts, it is interesting to note that, despite of the lower amount of acid sites of ASA than \u03b3-Al2O3, Mo/ASA shows a slightly higher VR conversion, because ASA has abundant B acid sites and a larger mesoporous volume that can promote the hydrocracking reactions of VR following the carbenium ion mechanism (Weitkamp, 2012).The reducibility of Mo species on the four supported catalysts were investigated by H2-TPR and the results are given in Fig. 6\n. In the H2-TPR profiles, two H2 reduction peaks are observed: the low-temperature peak can be attributed to the reduction of octahedrally coordinated Mo6+ species to tetrahedrally coordinated Mo4+ species, and the high-temperature peak can be assigned to the further reduction of tetrahedrally coordinated Mo4+ species to Mo (Wang et al., 2002; Liu et al., 2012; Lv et al., 2018; Zhang et al., 2019). The low-temperature peaks of the four catalysts are in the range of 400\u2013500\u202f\u00b0C and shift to high temperatures in the order of Mo/ASA\u202f<\u202fMo/\u03b3-Al2O3\u202f<\u202fMo/USY\u202f<\u202fMo/SiO2. It is also noted that the high temperature reduction peaks of Mo/ASA, Mo/SiO2 and Mo/\u03b3-Al2O3 are centered at 816, 820 and 857\u202f\u00b0C, respectively, whereas no obvious high temperature reduction is observed for Mo/USY. This suggests that the reducibilities of Mo species in the four catalysts are different, possibly due to the different metal-support interaction (Fan et al., 2007). Generally, it is considered that the ability of a metal oxide to adsorb and activate hydrogen is related to the reduction temperature, and a metal oxide that can be easily reduced by hydrogen usually has a higher hydrogenation activity (Cheng et al., 2020). Therefore, the higher yields of naphtha and middle distillate and the lower yields of gas and coke obtained over Mo/ASA can be attributed to the easier reduction of Mo species in Mo/ASA.HRTEM characterization was performed to observe the morphology of MoS2 slabs as the active phase in the hydrocracking reaction, the representative images obtained are shown in Fig. 7\n. The statistical results of the lengths and stacking numbers of MoS2 slabs are presented in Fig. 8\n and Table 4\n. The two-dimensional thread-like fringes with layer stacking spacing of about 0.65\u202fnm can be assigned as MoS2 slabs (Zheng et al., 2019). The lengths of MoS2 slabs are mainly from 3 to 7\u202fnm for all catalyst, as shown in Fig. 8a. In addition, the average length of MoS2 slabs on different catalysts reduces following of Mo/SiO2 (6.2\u202fnm)\u202f>\u202fMo/USY zeolite (5.2\u202fnm)\u202f>\u202fMo/\u03b3-Al2O3 (4.8\u202fnm)\u202f>\u202fMo/ASA (4.3\u202fnm). It is found that the MoS2 slabs on Mo/SiO2, Mo/\u03b3-Al2O3 and Mo/ASA are mainly stacked in 1\u20132 layers, but those on Mo/USY are mainly stacked in 2\u20135 layers, as shown in Fig. 8b. The average stacking number of MoS2 slabs on Mo/USY catalyst is 3.0, obviously higher than those on the others (1.7 for Mo/SiO2, 1.5 for Mo/\u03b3-Al2O3 and 1.6 for Mo/ASA). The results reveal that Mo/ASA exhibits small MoS2 particles with the lowest stacking number and the shortest slab length among all catalysts, implying the more exposure active sites and the weaker space resistance that benefit to improve the hydrocracking activity (Liu et al., 2019). The dispersion degrees of Mo species (D\nMo) and the proportion of Mo species located along edge sites of MoS2 slabs (f\nMoe) with high hydrogenation activity were also estimated based on the HRTEM results. The values of D\nMo and f\nMoe for the different catalysts increase as Mo/SiO2 (0.17 and 0.19)\u202f<\u202fMo/USY (0.20 and 0.23)\u202f<\u202fMo/\u03b3-Al2O3 (0.21 and 0.25)\u202f<\u202fMo/ASA (0.23 and 0.27). The result indicates that the highest dispersion and the largest proportion of Mo species located along edge sites of MoS2 slabs on Mo/ASA, possibly due to the appropriate interaction of ASA with Mo species. Generally, the D\nMo and f\nMoe values of MoS2 slabs can determine the hydrogenation activity of catalyst, because MoS2 slabs with larger D\nMo and f\nMoe values can expose more hydrogenation active sites, favoring the hydrogenation reaction to avoid over-cracking reaction and the condensation reaction of polycyclic aromatic hydrocarbons to yield gas and coke (Jiang et al., 2017). Therefore, the different yields of naphtha and middle distillate, gas and coke over the four catalysts can be attributed to their different D\nMo and f\nMoe values.The obtained Mo 3d XPS spectra and their deconvolution results are shown in Fig. 9\n and Table 5\n. The Mo 3d XPS envelope includes three Mo 3d doublets, the doublet with binding energies at 229.0\u202f\u00b1\u202f0.2 and 232.1\u202f\u00b1\u202f0.2\u202feV for Mo 3d5/2 and Mo 3d3/2 are assigned to MoS2 species (Mo4+), the binding energies at 230.9\u202f\u00b1\u202f0.2 and 234\u202f\u00b1\u202f0.2\u202feV for Mo 3d5/2 and Mo 3d3/2 are related to MoS\nx\nO\ny\n oxysulfide compounds (Mo5+), and the binding energies at 232.6\u202f\u00b1\u202f0.2 and 235.8\u202f\u00b1\u202f0.2\u202feV for Mo 3d5/2 and Mo 3d3/2 are attributed to MoO3 species (Mo6+) (Ninh et al., 2011). Table 5 lists the relative content of MoS2 species that represents the sulfuration degree of Mo/SiO2 (63%)\u202f<\u202fMo/USY (66%)\u202f<\u202fMo/\u03b3-Al2O3 (70%)\u202f<\u202fMo/ASA (74%). In view of the fact that the active Mo sites are generally located at edge and corner sites, and the hydrogenation reaction mainly occurs along the edge sites of MoS2 crystals (Alsalme et al., 2016; Zheng et al., 2021), the contents of Mo atoms located at edge sites (Moe) in the different catalysts were calculated by Moe\u202f=\u202fMoS2\u202f\u00d7\u202ff\nMoe, the results are summarized in Table 5. It can be seen that the contents of Mo atoms located at edge sites are in the order of Mo/SiO2 (10.7)\u202f<\u202fMo/USY (13.2)\u202f<\u202fMo/\u03b3-Al2O3 (14.7)\u202f<\u202fMo/ASA (17.0), indicating the hydrogenation activity increases following as the above tendency, it is consistent with HRTEM result. The XPS result demonstrates that the hydrocracking product distributions over the different catalysts are related with the sulfuration degrees and the contents of Mo atoms located at edges sites of Mo species. Mo/ASA catalyst with the highest sulfuration degree and content of Mo atoms located at edge sites of Mo species presents the highest hydrogenation activity to restrain the over-cracking reaction producing gas and the condensation reaction forming coke.In this study, four Mo catalysts supported on four supports (ASA, \u03b3-Al2O3, USY zeolite and SiO2) were compared by investigating the effects of their pore structure, acidity and Mo species properties on their surface on the VR slurry-phase hydrocracking performance. The hydrocracking reaction results show that the VR conversions obtained over the four catalysts are in the order of Mo/ASA\u202f>\u202fMo/\u03b3-Al2O3\u202f\u2248\u202fMo/USY\u202f>\u202fMo/SiO2, which is closely related to the acid site amounts and pore structures of supports. Moreover, the highest VR conversion over Mo/ASA is attributed to the larger mesoporous volume of ASA that is beneficial for the diffusion of the large molecules in VR to contact with the active sites on catalyst. Additionally, the appropriate amount of acid sites, especially B acid sites, of ASA enhances the catalytic cracking of VR. Therefore, both the appropriate amount of acid sites and larger mesoporous volume of supports are essential to catalytic activity. Importantly, the catalyst Mo/ASA exhibits the highest yield (54.7\u202fwt%) of naphtha and middle distillate and the lowest yields (10.1\u202fwt% and 0.5\u202fwt%) of gas and coke among all catalysts, because the Mo species in Mo/ASA possess simultaneously the highest sulfuration degree, highest dispersion degree of MoS2 slabs, and largest proportion of Mo atoms located at the edges sites. These features endow Mo/ASA with the highest hydrogenation activity, and thereby it can effectively restrain the over-cracking reaction of intermediate products and the condensation of polycyclic aromatic hydrocarbon to reduce the yields of gas and coke.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge National Key Research and Development Program of China (2018YFA0209403), National Natural Science Foundation of China (21908027) and Qingyuan Innovation Laboratory Program (00121002) for financing this research.", "descript": "\n                  To deeply understand the effects of support properties on the performance of Mo-based slurry-phase hydrocracking catalysts, four Mo-based catalysts supported on amorphous silica alumina (ASA), \u03b3-Al2O3, ultra-stable Y (USY) zeolite and SiO2 were prepared by the incipient wetness impregnation method, respectively, and their catalytic performances were compared in the vacuum residue (VR) hydrocracking process. It is found that the Mo/ASA catalyst exhibits the highest VR conversion among the different catalysts, indicating that both the appropriate amount of acid sites, especially B acid sites and larger mesoporous volume of ASA can enhance the VR hydrocracking into light distillates. Furthermore, Mo catalysts supported on the different supports show quite different product distributions in VR hydrocracking. The Mo/ASA catalyst provides higher yields of naphtha and middle distillates and lower yields of gas and coke compared with other catalysts, it is attributed to the highest MoS2 slab dispersion, the highest sulfuration degree of Mo species, and the most Mo atoms located at the edge sites for the Mo/ASA catalyst, as observed by HRTEM and XPS analyses. These features of Mo/ASA are beneficial for the hydrogenation of intermediate products and polycyclic aromatic hydrocarbons to restrict the gas and coke formation.\n               "}
{"full_text": "Data will be made available on request.Considering the current status of the global energy policy, the coming years will be marked by important challenges, associated with the desired energy transition to a circular bioeconomy. In this scenario, gasification of biomass is of particular interest since it can convert a variety of low-value feedstocks into a valuable gas product, providing a flexible renewable source for the production of baseload electricity, transportation fuels and various chemicals [1\u20133]. During biomass thermal conversion, additional contaminants are also produced which require cleaning and conditioning of the raw gas.Despite the recognized potential of biomass gasification, its transition to an industrial context still faces technical shortcomings, mostly resulting from the formation of undesired tar which causes several operating problems [4,5]. In addition, the H2:CO molar ratio of the biomass-derived gas shows typical values\u00a0<\u00a02.0, which implies further adjustments to fulfil the quality demands of different end-use applications [6]. Accordingly, the integration of heterogeneous catalysts as a part of the gasification concept is crucial to connect the divergence between the conversion of biomass-derived gas with profitable products.Catalytic hot gas upgrading has been widely investigated, as reported in previous reviews provided by e.g. Guan et al. [7], Sutton et al. [8], Zhang et al. [9] and Shahbaz et al. [10]. Ni-based materials revealed greater activity for the conversion of tar to CO and H2 but are relatively expensive and require continuous regeneration. In addition, deactivation by sulfur chemisorption on Ni active sites and microstructural ageing are other recognized limitations [11,12], making these type of materials unattractive in an industrial context that present severe process conditions.On the other hand, Fe-based materials has shown a growing interest for gasification applications because of their low-cost, lower environmental impact and effective performance toward tar cracking and H2 production [13,14]. However, complications resulting from carbon deposition is expected for in-situ applications due to unconverted char in the gasifier bed and the catalytic mechanisms involving tar side reactions [15]. There is also evidence that redox changes of iron active sites, induced by the thermochemical conditions of the biomass-derived raw gas, might result in a progressive decline of catalytic activity [16]. Gas-solid interactions with S-containing compounds may also have implications on the selectivity of chemical reactions [17]. The design of suitable operating conditions is therefore crucial for extending catalyst lifetime, which is a crucial issue in the context of process economics.Thermodynamic modelling has been extensively applied to support the optimization of biomass gasification technologies [18\u201322], giving qualitative and quantitative information about the operational limits. Though the relevance of such approach, theoretical analysis specifically addressing catalyst performance are limited in literature. Studies are mostly focused on the evaluation of specific reactor parameters, such as the gasifier agent, temperature and composition of biomass feedstocks, with the objective of improving producer gas quality and overall gasification efficiency. Subsequently, there is a particular need of understanding the interactions of iron-based materials with biomass-derived gas within the gasifier, as well as simplified modelling approaches to evaluate the operating conditions required for efficient operation.Therefore, the present study aimed to develop a graphical approach to support the operation of iron-based materials during biomass gasification. A combination of experimental data and thermodynamic modelling was applied as guidelines to elucidate the dependence of catalyst performance on the process conditions. This is expected to provide comprehensive guidelines for the selection of proper operating conditions for catalysts, which is still dictated mainly by empiricism, and to minimize the impact of some underestimated deactivation mechanisms. In addition, a mathematical model for gasification of biomass is provided by applying mass balances, energy balances and thermodynamic equilibrium predictions, aiming to establish appropriate conditions for the betterment of carbon conversion. Though the fundamentals discussed here are of general applicability, the work is focused on autothermal gasification of biomass, where primary measures are preferable to enhance process efficiency.A biomass gasification model was employed to support the design of temperature diagrams. It was based on thermodynamic equilibrium approach that consists in evaluating the composition of the biomass-derived products using minimization Gibbs energy. The proposed model is based on steady-state calculations of energy associated with all the intervenient species and includes the following general assumptions: i) perfect mixing and uniform temperature and pressure; ii) heat losses through the gasifier are neglected; iii) the biomass feedstock is represented by an equivalent molecule comprising carbon (C), hydrogen (H) and oxygen (O). The presence of nitrogen (N) and sulfur (S) is neglected, and the fraction of ashes in the biomass feedstock is only considered for its impact on the overall mass balance; iv) the model assumes that gasification reaction rates are fast and residence time is long enough to reach chemical equilibrium; v) reaction products mainly consist of H2, CO, CO2, CH4, H2O, N2 and unconverted carbon (char). Details concerning the thermodynamic equilibrium approach was provided in previous investigations [18,19].Based on the aforementioned considerations, mass and energy balances, per unit of biomass on a dry and ash-free basis (daf), were implemented. The principle of mass conservation applied to the gasification process can be described by a single global equation and expressed as follows:\n\n(1)\n\n\n\n(\n\n1\n\n1\n\u2212\n\nW\nw\n\n\n\n)\n\n\u2219\n\n(\n\n1\n+\n\n\nW\nA\n\n\n1\n\u2212\n\nW\nA\n\n\n\n\n)\n\n+\n\nW\n\nG\nA\n\n\n\u2212\n\nY\n\nC\nh\na\nr\n\n\n\u2212\n\nY\n\nG\na\ns\n\n\n\u2212\n\n(\n\n\nW\nA\n\n\n1\n\u2212\n\nW\nA\n\n\n\n)\n\n=\n0\n\n\n\nwhere \n\n\nW\nw\n\n\n is the fraction of moisture in biomass (\n\nk\n\ng\n\n\nH\n2\n\nO\n\n\n\u2219\nk\n\ng\n\nf\nu\ne\nl\n\n\n\u2212\n1\n\n\n\n), \n\n\nW\nA\n\n\n is the fraction of ash in dry biomass (\n\nk\n\ng\n\nA\ns\nh\n\n\n\u2219\nk\n\ng\n\nd\nr\ny\n,\nf\nu\ne\nl\n\n\n\u2212\n1\n\n\n\n), \n\n\nW\n\nG\nA\n\n\n\n is the ratio between the gasifier agent and biomass on a dry and ash-free basis (\n\nk\n\ng\n\nG\nA\n\n\n\u2219\nk\n\ng\n\nd\na\nf\n,\nf\nu\ne\nl\n\n\n\u2212\n1\n\n\n\n), \n\n\nY\n\nC\nh\na\nr\n\n\n\n and \n\n\nY\n\nG\na\ns\n\n\n\n are the mass yields of char and producer gas, respectively. The \n\n\nW\n\nG\nA\n\n\n\n may comprise different components, being formulated according to the following equation:\n\n(2)\n\n\n\nW\n\nG\nA\n\n\n=\nE\nR\n\u00b7\n\nW\ns\n\n\u00b7\n\n[\n\n1\n+\n\n\nM\n\nN\n2\n\n\n\nM\n\nO\n2\n\n\n\n\u2219\n\n(\n\n\n1\n\nX\n\n\nO\n2\n\n,\nG\nA\n\n\n\n\u2212\n1\n\n)\n\n\n]\n\n+\n\nW\n\n\nH\n2\n\nO\n\n\n\n\n\nwhere ER is the equivalence ratio, \n\n\nW\ns\n\n\n and \n\n\nW\n\ns\nt\ne\na\nm\n\n\n\n are the stoichiometric amount of gas mixture required for a complete combustion of biomass (\n\nk\n\ng\n\nA\ni\nr\n/\n\nO\n2\n\n\n\n\u2219\nk\n\ng\n\nd\na\nf\n,\nf\nu\ne\nl\n\n\n\u2212\n1\n\n\n\n) and the steam to biomass mass ratio (\n\nk\n\ng\n\n\nH\n2\n\nO\n\n\n\u2219\nk\n\ng\n\nd\na\nf\n,\nf\nu\ne\nl\n\n\n\u2212\n1\n\n\n\n), respectively, \n\n\nX\n\n\nO\n2\n\n,\nG\nA\n\n\n\n is the molar fraction of O2 in \n\n\nW\ns\n\n\n, \n\n\nM\n\nN\n2\n\n\n\n and \n\n\nM\n\nO\n2\n\n\n\n are the molar mass (\n\nk\ng\n\u2219\nm\no\n\nl\n\n\u2212\n1\n\n\n\n) of N2 and O2, respectively. The \n\n\nY\n\nG\na\ns\n\n\n\n and \n\n\nY\n\nC\nh\na\nr\n\n\n\n relates to the abundances of the biomass-derived products and can be defined as follows:\n\n(3)\n\n\n\nY\n\nC\nh\na\nr\n\n\n=\n1\n\u2212\n\nY\n\nG\na\ns\n\n\n\n\n\n\n\n\n(4)\n\n\n\nY\n\nG\na\ns\n\n\n=\n\n\n\u2211\ni\nn\n\n\nY\n\ni\n,\nG\na\ns\n\n\n\n\ni\n=\n\nH\n2\n\n,\nC\nO\n,\n\n\nC\nO\n\n2\n\n,\n\n\nC\nH\n\n4\n\n,\n\nN\n2\n\n\na\nn\nd\n\n\nH\n2\n\nO\n\n\n\n\nEnergy calculations take into account reference conditions, at normal room temperature (Tref\u00a0=\u00a0298\u00a0K) and pressure (Pref\u00a0=\u00a01\u00a0atm). Considering the biomass on a dry and ash-free basis, one easily obtains the following relation for the enthalpy balance:\n\n(5)\n\n\n\n\n\u0394\nH\n\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n\u2212\n\u0394\n\nH\n\nr\ne\na\nc\nt\na\nn\nt\ns\n\n\n\u2212\nQ\n=\n0\n\n\n\nwhere \n\n\n\n\u0394\nH\n\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n\n and \n\n\u0394\n\nH\n\nr\ne\na\nc\nt\na\nn\nt\ns\n\n\n\n denote the enthalpy changes of biomass-derived products and reactants (\n\nk\nJ\n\u2219\nk\n\ng\n\nd\na\nf\n,\nf\nu\ne\nl\n\n\n\u2212\n1\n\n\n\n) at reference conditions, according to:\n\n(6)\n\n\n\u0394\n\nH\n\nr\ne\na\nc\nt\na\nn\nt\ns\n\n\n=\n\n(\n\n\n\n\nc\np\n\n\u203e\n\n\nf\nu\ne\nl\n\n\n+\nE\nR\n\u2219\n\nW\ns\n\n\u2219\n\n\n\nc\np\n\n\u203e\n\ns\n\n+\n\nW\n\n\nH\n2\n\nO\n\n\n\u2219\n\n\n\nc\np\n\n\u203e\n\n\n\nH\n2\n\nO\n\n\n\n)\n\n\u2219\n\n(\n\n\nT\n\na\nd\nb\n\n\n\u2212\n\nT\n\nr\ne\nf\n\n\n\n)\n\n+\nL\nH\n\nV\n\nf\nu\ne\nl\n\n\n\n\n\n\n\n\n(7)\n\n\n\u0394\n\nH\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n=\n\u0394\n\nH\n\np\n,\nG\na\ns\n\n\n+\n\u0394\n\nH\n\np\n,\nC\nh\na\nr\n\n\n\n\n\n\n\n\n(8)\n\n\n\u0394\n\nH\n\np\n,\nG\na\ns\n\n\n=\n\nY\n\nG\na\ns\n\n\n\u2219\n\n[\n\n\n\n\nc\np\n\n\u203e\n\n\nG\na\ns\n\n\n\u2219\n\n(\n\n\nT\n\na\nd\nb\n\n\n\u2212\n\nT\n\nr\ne\nf\n\n\n\n)\n\n+\nL\nH\n\nV\n\nG\na\ns\n\n\n\n]\n\n\n\n\n\n\n\n(9)\n\n\n\u0394\n\nH\n\np\n,\nC\nh\na\nr\n\n\n=\n\nY\n\nC\nh\na\nr\n\n\n\u2219\n\n[\n\n\n\n\nc\np\n\n\u203e\n\n\nC\nh\na\nr\n\n\n\u2219\n\n(\n\n\nT\n\na\nd\nb\n\n\n\u2212\n\nT\n\nr\ne\nf\n\n\n\n)\n\n+\nL\nH\n\nV\n\nC\nh\na\nr\n\n\n\n]\n\n\n\n\n\nThe parameter Q denotes the heat exchanged between the reactor and the environment, being negative, positive or zero in cases where the gasification process is exothermic, endothermic or adiabatic, respectively. The thermodynamic data, such as the average specific heat (\n\n\n\nc\np\n\n\u203e\n\n\n) and the lower heating value (LHV) of reactants and products, was obtained through empirical formulas [23]. In the case of the \n\n\nW\ns\n\n\n and \n\n\nY\n\nG\na\ns\n\n\n\n parameters, the \n\n\n\nc\np\n\n\u203e\n\n\n values were determined as follows:\n\n(10)\n\n\n\n\n\nc\np\n\n\u203e\n\n\nW\ns\n\n\n=\n\n1\n\nM\n\nW\ns\n\n\n\n\u2219\n\n(\n\n\n\n\nc\np\n\n\u203e\n\n\nO\n2\n\n\n\u2219\n\nX\n\n\nO\n2\n\n,\nG\nA\n\n\n\u2219\n\nM\n\nO\n2\n\n\n+\n\n\n\nc\np\n\n\u203e\n\n\nN\n2\n\n\n\u2219\n\n(\n\n1\n\u2212\n\nX\n\n\nO\n2\n\n,\nG\nA\n\n\n\n)\n\n\u2219\n\nM\n\nN\n2\n\n\n\n)\n\n\n\n\n\n\n\n(11)\n\n\n\n\n\nc\np\n\n\u203e\n\n\nG\na\ns\n\n\n=\n\n\n\u2211\ni\n\n\n\n\n\nc\np\n\n\u203e\n\ni\n\n\u2219\n\nX\n\ni\n,\nG\na\ns\n\n\n\n\n\n\n\n\nDepending on the reaction to be promoted, oxidation or reduction of the active sites can significantly affect the performance of metal catalysts; this can be described by reaction (12), for a generic metal active site, which depends on partial pressure of oxygen (\n\np\n\nO\n2\n\n\n). Based on this principle, the behaviour of the metal active sites under gasification conditions will depend on the redox conditions imposed by the biomass-derived gas.\n\n(12)\n\n\n2\n\n(\n\nx\ny\n\n)\n\nM\ne\n+\n\nO\n2\n\n\u21cc\n\n(\n\n2\ny\n\n)\n\n\nM\nx\n\n\nO\ny\n\n\n\n\n\nThe partial conversion of biomass through gasification involves a set of parallel reactions, including onset of fully oxidized species (reactions 13 and 14), whose extension is dictated by the working conditions of the gasifier [24]. The Water-Gas-Shift (WGS) reaction (15) may also play a role in the pO2 associated with biomass-derived. WGS reaction can be expressed as the sum of reaction (13) and (14) and, although it suggests the absence of oxygen, kinetic restriction may affect this redox-type mechanisms [18].\n\n(13)\n\n\n2\nC\nO\n+\n\nO\n2\n\n\u21cc\n2\nC\n\nO\n2\n\n\n\n\n\n\n\n(14)\n\n\n\n\n2\nH\n\n2\n\n+\n\nO\n2\n\n\u21cc\n\n\n2\nH\n\n2\n\nO\n\n\n\n\n\n\n(15)\n\n\nC\nO\n+\n\nH\n2\n\nO\n\u21cc\n\n\nC\nO\n\n2\n\n+\n\nH\n2\n\n\n\n\n\nStill, one may estimate ideal redox conditions and dependence of \n\np\n\nO\n2\n\n\n on temperature or producer gas composition on assuming gas phase equilibrium. An ideal condition may be defined in the gas phase, as follows:\n\n(16)\n\n\n\n\np\n\nC\nO\n\n\n\np\n\nC\n\nO\n2\n\n\n\n\n=\n\n1\n\n\n\nK\n13\n\n\u2219\n\np\n\nO\n2\n\n\n\n\n\n\n\n\n\n\n\n(17)\n\n\n\n\np\n\nH\n2\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\n=\n\n1\n\n\n\nK\n14\n\n\u2219\n\np\n\nO\n2\n\n\n\n\n\n\n\n\nwhere \n\n\np\ni\n\n\n (i\u00a0=\u00a0CO, CO2, H2 and H2O) represents the partial pressure of gas species, K13 and K14 denote the equilibrium constants of reaction (13) and (14), respectively, calculated from thermodynamic data (\n\n\nK\ni\n\n=\nexp\n\n[\n\n\u2212\n\u0394\n\nG\ni\n\n/\nR\nT\n\n]\n\n\n). On the other hand, to analyse risks of carbon precipitation on catalyst surface, or onset of metal carbides at sufficiently high temperatures, one should consider the carbon activity (\n\n\na\nC\n\n\n) in the gas phase which is mainly imposed by the Boudouard reaction and described as follows:\n\n(18)\n\n\n2\nC\nO\n\u21cc\nC\n+\nC\n\nO\n2\n\n\n\n\n\n\n\n(19)\n\n\n\na\nC\n\n=\n\nK\n18\n\n\u2219\n\n\np\n\nC\nO\n\n2\n\n\np\n\nC\n\nO\n2\n\n\n\n\n\n\n\n\nRisks of carbon deposition correspond to conditions when \n\n\na\nC\n\n\u2265\n1\n\n or \n\n\nK\n18\n\n\u2219\n\np\n\nC\nO\n\n2\n\n\u2265\n\np\n\nC\n\nO\n2\n\n\n\n\n, i.e. for CO-rich conditions, and/or low temperatures, as \n\n\nK\n18\n\n\n rises with decreasing temperature.Onset of the carbide phase may also occur at the onset of carbon (reaction 20) or on reaching sufficiently high activity of carbon (reaction 21), mainly at relatively high temperatures. Though this may be interpreted as a negative impact on catalytic performance, carbide catalysts have also been proposed for relevant gas phase processes, namely Mo carbides proposed as catalysts for reverse water gas shift [25] or utilization of CO2 [26]. Note also that this concept has been extended to the so-called MXenes, after previous functionalization of carbides of different transition metal elements (Ti, V, Cr, \u2026) [27]. Thus, one must also examine the thermochemical conditions when the carbide phase may be present in biomass gasification.\n\n(20)\n\n\n3\nF\ne\n+\nC\n\u27fa\n\n\nF\ne\n\n3\n\nC\n\n\n\n\n\n\n(21)\n\n\n3\nF\ne\nO\n+\nC\n\u27fa\n\n\nF\ne\n\n3\n\nC\n+\n1.5\n\nO\n2\n\n\n\n\n\nThe experimental data used to determine the \n\n\na\nC\n\n\n and \n\np\n\nO\n2\n\n\n values associated with biomass derived gas was compiled by collecting and organizing experimental results from the literature [14,28\u201353], regarding gasification experiments with distinct biomass feedstocks and different operation conditions.Thermodynamic calculations were applied as guidelines to investigate the stability range of iron-based catalysts and their compatibility with the thermochemical conditions of biomass gasification. The analysis was performed on assuming simplified model systems and computing diagrams in a form of planar representations [54]; this is based on derivation of representative reactions for 2-phase equilibria, and then extracting the relevant values of oxygen partial pressure (\n\np\n\nO\n2\n\n\n), carbon dioxide partial pressure (\n\np\n\n\nC\nO\n\n2\n\n\n), water partial pressure (\n\np\n\nH\n2\n\nO\n\n), hydrogen sulphide partial pressure (\n\np\n\nH\n2\n\nS\n\n) and/or activity values (\n\n\na\ni\n\n\n) to establish stability ranges for expected phases. For example, interactions of iron oxides with biomass-derived contaminants such as H2S corresponds to a quaternary system Fe\u2013O\u2013S\u2013H and phase equilibrium at constant temperature and total pressure will still depend simultaneously on H2S, \n\np\n\nO\n2\n\n\n and \n\np\n\nH\n2\n\n\n (or \n\np\n\nH\n2\n\nO\n\n). Early studies of the mechanism of reaction of iron with H2S at high temperatures [55] reported linear dependence on time and suggested that kinetics relies on mixed transport of cation vacancies and holes in a dense non-stoichiometric \n\n\n\nF\ne\n\n\n1\n\u2212\n\u03b4\n\n\nS\n\n scale, combined on migration of H2 in a top porous layer. One may then assume ready re-equilibration with H2O, under the thermochemical conditions of gasification, depending on oxygen partial pressure (14). Direct formation of H2O is expected for reaction of H2O2 with iron oxides, as depicted for FeO. Thus, one analysed the relevant 2-phase equilibrium reactions vs \n\np\n\nO\n2\n\n\n, to account for changes in redox conditions and vs the \n\np\n\nH\n2\n\nS\n:\np\n\nH\n2\n\nO\n\n ratio, to account for the combined effects of other gases. The corresponding reactions are shown in Table 1\n.In the case of solid carbon interactions (graphite is the stable carbon form) involving metallic iron, its carbide \n\n\n\nF\ne\n\n3\n\nC\n\n and oxides (FeO, \n\n\n\nF\ne\n\n3\n\n\nO\n4\n\n\n, \n\n\n\nF\ne\n\n2\n\n\nO\n3\n\n\n), one may describe the corresponding 2-phase equilibrium as a function of carbon activity (Table 2\n).The dependence of \n\np\n\nH\n2\n\nS\n:\np\n\nH\n2\n\nO\n\n ratio and \n\np\n\nO\n2\n\n\n, or on \n\n\na\nC\n\n\n and \n\np\n\nO\n2\n\n\n are determined numerically for given values of \n\np\n\nO\n2\n\n\n, being the outcomes used to plot the corresponding stability diagrams. Log scales are applied for their closer relation with corresponding chemical potential differences from the reference state \n\n\u0394\n\n\n\u03bc\nO\n\n2\n\n=\nR\nT\nl\nn\n\n(\n\np\n\nO\n2\n\n\n)\n\n\n and \n\n\u0394\n\n\u03bc\nC\n\n=\nR\nT\nl\nn\n\n(\n\na\nC\n\n)\n\n\n. The same approach was applied to obtain the equilibrium conditions for other iron-containing catalytic systems. The FactSage software package (version 7.3) has been used to support the development of the stability diagrams. The thermodynamic properties required for the analysis, such as standard enthalpies of formation (\n\n\u0394\n\nH\ni\n\u00b0\n\n\n), standard entropies (\n\n\nS\ni\n\u00b0\n\n\n), and specific heat (\n\nc\n\np\ni\n\n\n), were taken from the FactPS, SGTE 2017 and FToxide databases. The thermodynamic analysis was conducted at a temperature range of 600\u20131000\u00a0\u00b0C that is typical for biomass gasification processes.Under direct gasification conditions, where air is used to partially convert the biomass feedstock, \n\n\np\n\nO\n2\n\n\n\n will be mainly dictated by the CO:CO2 molar ratio (Eq. (16)) in the producer gas. A higher contribution of the H2:H2O molar ratio to the \n\np\n\nO\n2\n\n\n (Eq. (17)) is expected for biomass steam gasification. In this case, steam will behave as a mild oxidant and promotes both the conversion of CO and higher yield of H2. Thus, a commonly used parameter to characterize the redox environment of a catalytic process is the reduction factor [56], which is defined as the quotient between the contents of both reductive gases and the contents of fully oxidized gases, as follows:\n\n(22)\n\n\nR\n=\n\n\n\np\n\nC\nO\n\n\n+\n\np\n\nH\n2\n\n\n\n\n\np\n\nC\n\nO\n2\n\n\n\n+\n\np\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\n\nIron-based materials tend to lose activity over operation time due to carbon deposition which leads to blocking of active sites and, eventually formation of iron carbide (Fe3C), with consequent deterioration of catalyst performance by metal dusting [57]. In the case of primary catalysts, these phenomena can be controlled to some extent by the betterment of carbon conversion, which is strongly affected by the biomass properties and the gasifier operating parameters such as residence time, bed temperature, gasification agent and equivalence ratio [58]. Although residence time cannot be directly controlled, the bed temperature and amount of oxidant are monitoring during operation and can be adjusted to guarantee thermodynamic conditions for complete carbon conversion. Note that the discrepancies between theoretical and experimental measurements decreases in the case of solid carbon [59], suggesting that the accumulation of carbon in the gasifier bed can be predicted with a reasonable accuracy.\nFig. 1\n shows the evolution of the gasification adiabatic temperature (\n\n\nT\n\na\nd\nb\n\n\n\n) and minimum temperature for complete conversion of carbon (\n\n\nT\n\n0\n,\nC\nh\na\nr\n\n\n\n), as a function of the equivalence ratio (ER). The analysis was performed for atmospheric air, using model biomass compounds (cellulose and lignin) and real biomass feedstocks (eucalyptus and rape seed). The crossing of both temperatures in the diagrams allows the definition of different operating windows for the gasifier. The conversion of biomass under carbon-free conditions is achievable for zones II and III, avoiding excessive carbon precipitation on the surface of catalysts. Actually, gasification should be driven across Zone III, where autothermal conditions are guaranteed. It is also clear from the results that the conversion of carbon is affected by the O:C molar ratio associated with the biomass feedstock. The minimum ER required to attain complete conversion of cellulose (O:C\u00a0=\u00a00.83) inside the gasifier is ER\u00a0\u2248\u00a00.085 (Fig. 1-a); this value increases to ER\u00a0\u2248\u00a00.33 in the case of lignin (O:C\u00a0=\u00a00.22), because the low O:C molar ratio requires additional supply of oxygen, to reach the stoichiometric ratio, and shifts the carbon-free operating windows to higher equivalence ratio. A similar behaviour is observed when eucalyptus (O:C\u00a0=\u00a00.73) and rape seed (O:C\u00a0=\u00a00.30) are considered as biomass feedstocks. Note that direct gasification of biomass is typically carried out at temperatures between 700 and 900\u00a0\u00b0C with ER ranging from 0.15 to 0.30 (shaded area on diagrams) [3]; this means that the use of lignin-rich feedstocks will result in higher accumulation of unwanted carbon in the gasifier bed, with subsequent negative impact on gasification efficiency. Higher tar content can also be expected in biomass-derived gas, consisting of more stable compounds such as polyaromatic hydrocarbons, since lignin is more difficult to decompose compared to other biomass components (cellulose and hemicellulose) [60,61]. Accordingly, the selection of proper gasification conditions, as well as suitable biomass feedstocks, are critical to minimize the formation of unwanted products. In this regard, it should also be taken into consideration that performance indexes are in trade-off relationship. For example, increasing the ER to improve carbon conversion will reduce the cold gas efficiency, due to increasing fractions of fully oxidized gases (Eqs. (13) and (14)). A reasonable compromise can be obtained by driving the process through Zone 3 (Fig. 1), near the intersection between \n\n\nT\n\na\nd\nb\n\n\n\n and \n\n\nT\n\n0\n,\nC\nh\na\nr\n\n\n\n.On the other hand, highly basic alkaline oxides (K2O, Na2O \u2026) and alkaline earth oxides (BaO, SrO, CaO, MgO) interact with acidic gases (CO2, HCl \u2026), as indicated by free energy of carbonation reactions (Table 3\n) and are likely to promote the catalytic activity of water-gas-shift and reforming reactions and oppose methanation [62]. There is also convincing evidence that the fractions of alkaline and alkaline earth metals (AAEMs) in the solid fraction (ashes) of gasification may assist this catalytic activity [63,64]. However, the volatility and low melting temperatures of alkaline oxides (e.g.\u00a0\u2248\u00a0740\u00a0\u00b0C for K2O), or low eutectic temperatures in relevant systems (e.g. K2O\u00a0\u2212\u00a0SiO2 and Na2O\u00a0\u2212\u00a0SiO2 [65]) raise major difficulties, unless one considers alternative concepts based on alkaline-containing compounds, as reported for sodium titanates [66], which were proposed to upgrade the H2:CO ratio and to remove tars.Iron-based materials can exhibit distinct catalytic behaviour during biomass gasification due to the variable oxidation state of their active sites. Previous studies have reported metallic iron (Fe) as the main active phase for tar decomposition [28,67] because of its higher ability to break C\u2013C and C\u2013H bonds in aromatic hydrocarbon compounds compared to the corresponding iron oxides. Still, when the purpose of the catalyst is to increase the production of H2 through the WGS reaction, the spinel magnetite (Fe3O4) shows enhanced catalytic activity, which relies on the reducibility of the Fe3+ \u2194 Fe2+ redox couple in the octahedral sites of Fe3O4 [56,68]. In the case of chemical looping gasification, where transition metal oxides are applied as oxygen carriers to promote oxidation reactions, performance of Fe-based catalysts relies on cycling between oxidation to hematite (Fe2O3), which provides higher oxygen storage, and reduction to lower valence states in the gasifier [69,70].\nFig. 2\n shows thermodynamic predictions for the Fe\u2013O\u2013C system, which is presented vs \n\n\np\n\nO\n2\n\n\n\n and \n\n\na\nC\n\n\n. The corresponding thermochemical conditions of biomass gasification were calculated from experimental data, and were superimposed in this diagram (symbols); this comprises gasification experiments with different gasification agents (air, steam and O2-steam mixtures). Note that relatively small variations of \n\np\n\nO\n2\n\n\n in the gasifier atmosphere can have practical consequences on the prevailing phase of Fe-based catalysts, ranging from a prevailing relevance of wustite (FeO) for gasification at the highest temperatures, and gradual shift to a distribution from magnetite (Fe3O4), wustite and metallic Fe at lower temperatures. Risks of carbon deposition at relatively low temperatures (600\u00a0\u00b0C) may be minimized by maintaining the redox conditions in the Fe3O4 range, which also lowers the risk of collapse by excessive volume changes on reducing magnetite to wustite (\u221216%) or wustite to metallic Fe (\u221242%). Fe3O4 shows the widest redox window, whereas the redox window of wustite (FeO) narrows with decreasing temperature [71]. The reduction factor of the producer gas (R) is shown in the secondary vertical axis and is also a useful guideline to prevent deposition of carbon, by keeping R\u00a0>\u00a01, as shown in Fig. 2 for 600\u00a0\u00b0C. Otherwise, one may design structural changes in the active sites of magnetite-based catalysts, as pointed out for WGS catalysts [56]. One may also consider the incorporation of promoters (e.g. La, Sr, Ce, \u2026) into iron oxides catalysts for H2-enriched gas production, mainly when it is unfeasible to adjust R, and to seek enhanced oxygen storage [72].Risks of carbon deposition decrease with increasing temperatures, as shown in Fig. 2, at the highest temperatures. Note that the chemical potential differences \n\n\u0394\n\n\u03bc\nC\n\n=\nR\nT\nl\nn\n\n(\n\na\nC\n\n)\n\n\n and \n\n\u0394\n\n\u03bc\n\nO\n2\n\n\n=\nR\nT\nl\nn\n\n(\n\n\np\nO\n\n2\n\n)\n\n\n which separate the average experimental conditions from onset of carbon (at \n\n\na\nC\n\n\n\u00a0=\u00a01) increase with temperature. In fact, carbon deposition is not expected at temperatures \u2265900\u00a0\u00b0C, even when the reducing factor is high. Onset of carbide (Fe3C) also seems unlikely under typical conditions of gasification because the activity of carbon is shifted to sufficiently low values below the Fe/Fe3C equilibrium. Still, the experimental conditions may reach the Fe/Fe3C boundary at intermediate temperatures, as shown for 700\u00a0\u00b0C in Fig. 2. Thus, unconverted char in the gasifier bed may still shift the \n\n\na\nC\n\n\n to higher values, raising concerns about their impact on Fe3C formation. The tendency to carbon precipitation with decreasing operating temperature may contribute to the formation of iron carbide (Fe3C) resulting from interactions of carbon with metallic iron [73]. The risk of metal dusting and their negative effects on catalytic activity during long-term operation might be minimized by alloying Fe with other elements, promoting the formation of a protective oxide layer [74]. Formation of iron carbonate (FeCO3) is unlikely under biomass gasification conditions, since FeCO3 is unstable at temperatures above\u00a0\u2248\u00a0600\u00a0K, even in pCO2-rich atmospheres [57].It is well-know that sulfur impurities in biomass-derived gas is one of the major concerns associated with the use of metal-based catalysts. Though some mechanistic investigations showed that adsorption of H2S onto iron surface can induce oxide-metal bond scission with negative impact on WGS performance [75], the influence of gas-phase sulfur on the catalytic behaviour of iron species during exposure to biomass-derived gas is still poorly understood. The poisoning effect of sulfur on the catalytic activity of iron active sites was generally explained by a simple site-blocking mechanism, leading to formation of iron sulphide (FeS) which causes a sharp drop in the rate of catalytic conversion, as well as poorer product selectivity [17].\nFig. 3\n shows gas-solid thermodynamic predictions for the sulfur tolerance of iron species, and superimposed calculations for \n\n\nH\n2\n\nS\n:\n\nH\n2\n\nO\n\n vs \n\np\n\nO\n2\n\n\n values associated with producer gas compositions. The phase stability diagrams are analysed for the combined effects of hydrogen sulphide and water vapour (\n\n\n\np\nH\n\n2\n\nS\n:\n\n\np\nH\n\n2\n\nO\n\n)\n, at fixed temperatures, for complete description of the quaternary system Fe\u2013O\u2013S\u2013H. In this case, the analysis is not restricted to specific values of pH2O, as proposed earlier to assess the sulfur tolerance of Fe-based oxygen storage materials for chemical looping combustion [76]. Dependence on (\n\n\n\np\nH\n\n2\n\nS\n:\n\n\np\nH\n\n2\n\nO\n\n)\n also allows one to emphasize that operation in steam-rich conditions offer prospects to upgrade sulfur tolerance in biomass gasification, as found on comparing the average results from air gasification (black triangles in Fig. 3) and results from steam gasification (green squares) or oxy-steam gasification (red diamonds). Thus, one may expect substantial gains in tolerance to H2S when the fraction of \n\np\n\nH\n2\n\nO\n\n in the producer gas is high. For example, sulfur tolerance up to \n\np\n\nH\n2\n\nS\n\u2248\n\n 30\u00a0ppm is expected at 800\u00a0\u00b0C and \n\nlog\n\u2061\n\n(\n\np\n\nO\n2\n\n\n)\n\n=\n\n \u221217.8\u00a0atm if one assumes a gas composition with \n\np\n\nH\n2\n\nO\n=\n\n 0.10\u00a0atm, whereas this value increases to \n\np\n\nH\n2\n\nS\n\u2248\n\n 75\u00a0ppm when \n\np\n\nH\n2\n\nO\n=\n\n 0.25\u00a0atm.\nFig. 3 also suggests that sulfur tolerance is poorest for wustite (FeO) and may the optimized for Fe3O4-based catalysts, mainly in the intermediate range of \n\np\n\nO\n2\n\n\n. However, this does not translate in real advantages if one considers biomass gasification catalysts, since most operation conditions fall in a narrow range of redox conditions, usually near the FeO/Fe3O4 borderline; this conclusion may also be extended for potential applications in chemical looping gasification, since the oxidising step requires complete conversion to Fe2O3, under conditions when sulphates become highly stable, even for sulfur contents below the ppm range. Note that a typical standard for atmospheric air quality is in the order of 0.2\u00a0ppm of SO2, at room temperature, and this corresponds to similar contents of H2S at higher temperatures [77]. Thus, one should not expect regeneration on cycling between H2S-contaminated reducing producer gas and the oxidising step in fairly cleaner air.The so-called Fe/CaO catalysts in the Fe\u2013Ca\u2013O\u2013C system have received special attention because of their high activity towards tar conversion and H2 promotion during steam gasification. It is often based on the activity of Fe3O4 for the enhancement of WGS reaction [56], combined with the ability of calcium oxide (CaO) to favour in-situ CO2 absorption, shifting the reaction equilibrium to higher H2 yields. The CaO is also active in reforming reactions but is easily deactivated by biomass tar, resulting in the decline of catalytic performance [78]. Thus, significant cumulative formation of calcium carbonate (CaCO3) results in the suppression of the CO2-sorption ability, requiring regeneration cycles at high temperatures, which causes particle coarsening or agglomeration and severe pore blockage [79]. To overcome these constraints, the promotion of brownmillerite phase (Ca2Fe2O5) is a proposed option, which is expected to retain the catalyst performance by enhancing its thermal stability and redox tolerance over multiple operation cycles. Catalytic performance of Ca2Fe2O5 has been ascribed to co-existence of octahedral and tetrahedral sites of the brownmillerite structure, and the lower coordination was interpreted as O-vacancies facilitating the mobility of oxygen [80]. However, this interpretation is somewhat arguable taking into account that direct measurements of oxygen permeability are lower than for ferrite perovskites and also because significant changes in oxygen stoichiometry are related mainly to reductive decomposition rather than changes in occupation of the structural tetrahedral positions of Ca2Fe2O5 [81]. Therefore, one revised the extended phase stability of the Ca\u2013Fe\u2013O\u2013C system Fig. 4\n as a guideline for coexistence of Ca2Fe2O5 with other phases, and oxygen storage or CO2 storage ability related to onset of secondary phases, including formation of carbonate.Ready onset of CaCO3 at 600\u00a0\u00b0C implies greater risks of CaO deactivation for log (pCO2)\u00a0>\u00a0\u22120.86\u00a0atm, due to the limited thermodynamic stability of the brownmillerite structure (A2B2O5). Higher pO2 values results in improved CO2 tolerance relative to carbonation of CaO, but these gains are insufficient to guarantee Ca2Fe2O5 stability under typical gasification gas compositions, as indicated by the experimental data points (symbols). Increasing the gasification temperature (T\u00a0\u2265\u00a0700\u00a0\u00b0C) provides CO2 tolerance of Ca2Fe2O5, even in CO2-rich atmospheres, and minimize the risks of massive decomposition of the brownmillerite structure when exposed to biomass-derived gas, in close agreement with evidence in relevant literature [82]. The wide redox range for Ca2Fe2O5 extends from the actual range of biomass-derived gas up to oxidising conditions. In fact, the compiled information from a wide variety of experimental data on biomass gasification falls almost entirely within the thermochemical phase boundary of the brownmillerite phase, even at 700\u00a0\u00b0C. Thus, the Ca2Fe2O5 phase allows prospective operation under much wider redox ranges, compared to pure metallic Fe or its oxides (FeO or Fe3O4). Note that the Fe/FeO and FeO/Fe3O4 boundaries are clearly located inside the stability range of Ca2Fe2O5. Thus, this phase delays onset of metallic Fe, and minimizes its catalytic promotion of carbon deposition, raising prospects for higher H2 production at moderate reaction temperatures (\u2248700\u00a0\u00b0C), enhanced gasification efficiency [83] and catalyst stability during long-term operation.Chemical looping gasification by means of transition metal oxides provides an alternative option for biomass thermal conversion. Biomass is partially converted by the lattice oxygen of metal oxide and steam, aiming to obtain N2-free producer gas with a low tar content [84]. Tar conversion through oxidation reactions is also expected, namely earlier precipitation of metallic particles and their impact on C\u2013C bonds, increasing the carbon conversion efficiency. Particular attention has been given to the application of ferrite materials such as NiFe2O4, CuFe2O4, MnFe2O4 and CoFe2O4, as oxygen carriers, [85,86]. Ni- and Co-based compounds are known for their higher catalytic activity but also raise the highest environmental concerns during operation [86], including carcinogenic effects at least in the case of Ni. Ni- and Co-based compounds are also less affordable than corresponding Mn-based compounds. Manganese ferrite is less expensive and also raises lower concerns about safety. Reduced (Mn,Fe)xOy nanoparticles were successfully tested in biomass gasification with impact on tar conversion [14].The reducibility of spinels and their reversibility in reduction/reoxidation cycles can be related to the high flexibility of the AB2O4 spinel structure which allows incorporation of diverse combinations of divalent and trivalent transition metal ions in both tetrahedral A-sites and octahedral B-sites.Thermodynamic modelling of the Cu\u2013Fe\u2013O, Ni\u2013Fe\u2013O, Co\u2013Fe\u2013O and Mn\u2013Fe\u2013O systems (Fig. 5\n) show that the stability windows of spinels, in terms of temperature-pO2 ranges differ significantly but does not reach the conditions of biomass gasification (circles); these results combine a wide range of experimental data of producer gas compositions, obtained by gasification at different temperatures, using air, steam and O2-steam mixtures as gasifying agent. Thus, one observes a significant gap between the redox conditions of producer gas and the phase boundary of ferrites in the reducing side, and the widest gap is observed for CuFe2O4.Decomposition of ferrites under conditions of biomass gasification is a gradual multistep process, as detailed for \n\nC\no\n\n\nF\ne\n\n2\n\n\nO\n4\n\n\n:\n\n\n\nC\no\n\n\nF\ne\n\n2\n\n\nO\n4\n\n\u2192\nC\no\nO\n+\n2\n/\n3\n\n\nF\ne\n\n3\n\n\nO\n4\n\n+\n\nx\n1\n\n\nO\n2\n\n\u2191\n\n\n\n\n\n\n\n\n\u2192\n\n(\n\n1\n+\n\u03b4\n\n)\n\n\n(\n\nC\no\n,\nF\ne\n\n)\n\n+\n\n(\n\n2\n/\n3\n\u2212\n\u03b4\n\n)\n\n\n\nF\ne\n\n3\n\n\nO\n4\n\n+\n\nx\n2\n\n\nO\n2\n\n\u2191\n\n\n\n\n\n\n\n\n\u2192\n\n(\n\n1\n+\n\u03b4\n\n)\n\n\n(\n\nC\no\n,\nF\ne\n\n)\n\n+\n\n(\n\n2\n\u2212\n\u03b4\n\n)\n\nF\ne\nO\n+\n\nx\n2\n\n\nO\n2\n\n\u2191\n\n\n\n\n\n\n(23)\n\n\n\u2192\n3\n\n(\n\nC\no\n,\nF\ne\n\n)\n\n+\n\nx\n3\n\n\nO\n2\n\n\u2191\n\n\n\n\nVolume changes induced by these reduction steps are relatively high (Table 4\n), with corresponding risks of mechanical disintegration upon redox cycling, mainly if one considers complete reduction of both oxide components. These risks are somewhat minimized if one considers only reduction of cobalt, while retaining magnetite as the main oxide phase; this minimizes the volume changes and also maintains structural similarity between magnetite and \n\nC\no\n\n\nF\ne\n\n2\n\n\nO\n4\n\n\n. However, this step of reduction only covers a relatively small fraction of the experimental results reported for biomass gasification, as shown in Fig. 5. The effective oxygen supply in this early reduction step is also relatively small.Decomposition of other ferrites follow a similar sequence of decomposition steps, except for the first step which only occurs at sufficiently high temperatures in the case of \n\nN\ni\n\n\nF\ne\n\n2\n\n\nO\n4\n\n\n (>800\u00a0\u00b0C) and is not observed in the case of \n\nC\nu\n\n\nF\ne\n\n2\n\n\nO\n4\n\n\n. The final stage yields complete reduction to a bimetallic alloy and is slightly displaced from the corresponding conditions for reduction of pure wustite (FeO) to metallic Fe, as shown by a dotted blue line for the Ni\u2013Fe\u2013O and Co\u2013Fe\u2013O systems. In these cases, one observes a significant fraction of gasification experiments within the redox range of complete reduction; this indicates higher oxygen supply ability for chemical looping within the redox range of gasification. Thus, one cannot find a clear advantage of CuFe2O4 relative to other ferrites, in what concerns the oxygen storage ability and redox conditions for charge/discharge. Effective application of Cu-based materials as oxygen carriers should also take into consideration greater risks of microstructural ageing derived from the low melting point of metallic Cu (\u22481085\u00a0\u00b0C) and readier sintering. Greater risks of microstructural degradation may also be caused by contaminants such as alkaline, which may induce low melting eutectics [87]. Thus, gasification temperatures should be limited to minimized these risks [88]. Ni- and Co-based compounds also offer greater catalytic potential for a wide variety of processes, such as the production of H2 through the promotion of WGS reaction and degradation of tar compounds [89], except possibly for their simultaneous promotion of carbon deposition.MnFe2O4 follows a somewhat different multistep reduction, mainly because MnO is hardly reduced by fuels:\n\n\n\nM\nn\n\n\nF\ne\n\n2\n\n\nO\n4\n\n\u2192\nM\nn\nO\n+\n\n(\n\n\n2\n3\n\n\u2212\n\u03b4\n\n)\n\n\n\nF\ne\n\n3\n\n\nO\n4\n\n+\n\nx\n1\n\n\nO\n2\n\n\u2191\n\n\n\n\n\n\n\n\n\u2192\nM\nn\nO\n+\n2\nF\ne\nO\n+\n\nx\n2\n\n\nO\n2\n\n\u2191\n\n\n\n\n\n\n\n\n\u2192\n2\nF\ne\n+\nM\nn\nO\n+\n\nx\n2\n\n\nO\n2\n\n\u2191\n\n\n\n\n\n\n(24)\n\n\n\u2192\n\n(\n\n2\n+\n\u03b4\n\n)\n\n\n(\n\nM\nn\n,\nF\ne\n\n)\n\n+\n\n(\n\n1\n\u2212\n\u03b4\n\n)\n\nM\nn\nO\n+\n\nx\n3\n\n\nO\n2\n\n\u2191\n\n\n\n\nIn addition, MnFe2O4 shows limited stability under oxidising conditions, undergoing complete oxidation to trivalent state of both oxide components, as follows:\n\n(25)\n\n\nM\nn\n\n\nF\ne\n\n2\n\n\nO\n4\n\n\n\u2192\n\nO\n2\n\n\n1\n/\n3\n\n\nM\nn\n\n3\n\n\nO\n4\n\n+\n\n\nF\ne\n\n2\n\n\nO\n3\n\n\n\u2192\n\nO\n2\n\n\n0.5\n\n\nM\nn\n\n2\n\n\nO\n3\n\n+\n\n\nF\ne\n\n2\n\n\nO\n3\n\n\n\n\n\nThus, the spinel phase is not retained in both limiting conditions of chemical looping cycles, except possibly for less common processes when a specific redox pair (e.g. CO2/CO) may still allow an oxidation step within the intermediate redox range. For example, \n\nM\nn\n\n\nF\ne\n\n2\n\n\nO\n4\n\n\n was proposed for a chemical looping reaction between methane and CO2 [90].In the present study, one re-examined the thermodynamics of iron-based catalysts under the experimental conditions of biomass gasification. A combination of experimental data and thermodynamic modelling allows one to assess the dependence of catalyst performance on the thermochemical conditions of biomass gasification, by superimposing these results on phase stability diagrams of Fe-based catalysts. Thermodynamic modelling of biomass conversion showed that conversion of carbon inside the reactor is strongly dependent on the O:C molar ratio associated with the biomass feedstock and gasifier temperature. Lignin-rich feedstocks lead to higher accumulation of unconverted carbon in the gasifier bed, with expected negative impact on catalyst stability and process efficiency. This risk can be minimized with sufficient equivalence ratio to ensure operation of the gasifier at a temperature slightly above the theoretical value required for complete carbon conversion, and by selecting appropriate biomass feedstocks.Thermodynamic predictions for the Fe\u2013O\u2013C system indicated that changes in the redox atmosphere of the gasifier can have significant impact on the catalytic behaviour of Fe active sites. Greater redox tolerance of Fe3O4 phase is expected at 600\u00a0\u00b0C. At higher gasification temperatures, the catalytic promotion of H2 through the WGS reaction requires precise control of the reduction factor (R\u00a0<\u00a01), and modification of Fe-based catalysts to retain the redox tolerance of active sites. Conversion of tars over metallic Fe is challenging because the required oxygen partial pressure may cause reoxidation.Coke deposition and sulfur contamination of iron active sites can be assessed by suitable stability diagrams, with planar representations in \n\np\n\nO\n2\n\n\n vs activity of carbon or vs the partial pressure ratio \n\np\n\nH\n2\n\nS\n:\n\n\n\n\np\n\nH\n2\n\nO\n\n in the gas atmosphere. Experimental conditions of biomass gasification were superimposed in the diagrams, and confirm that carbon precipitation on Fe surface is expected under gasification conditions at relatively low temperatures. Accumulation of unwanted carbon in the reactor bed may raise concerns about the impact of Fe3C formation at higher temperatures. Thermodynamic modelling of the Fe\u2013O\u2013S system revealed that poisoning by H2S can cause degradation of Fe-based catalysts, with tolerance limits differing according to process conditions, including significant differences between gasification with air and with steam.The Ca\u2013Fe\u2013O\u2013C system was examined as guideline for Ca2Fe2O5; this shows ready carbonation at 600\u00a0\u00b0C, while enhancing the stability at higher temperatures. The corresponding results suggest that thermodynamic stability of brownmillerite phase at 600\u00a0\u00b0C requires higher redox potential in the biomass-derived gas to avoid decomposition of Ca2Fe2O5 structure, with subsequent formation of carbonate phases. Accordingly, the in-situ application of those materials may involve higher gasification temperatures, as suggested by the wide gap between the upper and lower limits of resistance to CO2 for the Ca2Fe2O5 phase at temperatures above 700\u00a0\u00b0C.Stability phase diagrams of typical ferrites (AB2O4, with A\u00a0=\u00a0Cu, Ni, Co and Mn) were also computed to evaluate their reactivity at gasification conditions, and prospects for chemical looping. These systems provide conditions for onset of bimetallic (Fe,Cu), (Fe.Ni) or (Co,Fe) particles. Similar conditions were also observed in terms of reduction steps and corresponding oxygen supply, except for slight differences in the conditions for complete reduction of both oxide components, and greater risks of microstructural ageing of oxygen storage materials in the Cu\u2013Fe\u2013O system. The Mn\u2013Fe\u2013O system shows a more complex sequence of reduction/oxidation steps in chemical looping.\nLu\u00eds Ruivo: Conceptualization, Methodology, Investigation, Formal analysis, Visualization, Validation, Writing \u2013 original draft, Writing \u2013 review & editing, Tiago Silva: Conceptualization, Investigation, Formal analysis, Writing \u2013 review & editing, Daniel Neves: Conceptualization, Investigation, Formal analysis, Writing \u2013 review & editing, Lu\u00eds Tarelho: Conceptualization, Visualization, Validation, Writing \u2013 review & editing, Supervision, Jorge Frade: Conceptualization, Investigation, Formal analysis, Visualization, Validation, Writing \u2013 review & editing, Supervision, Project administration.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the financial support through projects NOTARGAS (ref. POCI-01-0145-FEDER-030661) and CHARCLEAN (PCIF/GVB/0179/2017). Thanks to the Portuguese Foundation for Science and Technology (FCT)/Ministry of Science, Technology and Higher Education (MCTES) for the financial support to CESAM (UIDP/50017/2020, UIDB/50017/2020, LA/P/0094/2020), and CICECO \u2013 Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020), through national funds. The authors also acknowledge the Portuguese Foundation for Science and Technology for providing financial support to the PhD scholarship granted to Lu\u00eds Ruivo (ref. SFRH/BD/129901/2017).The following are the supplementary data related to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\n\n\nMultimedia component 2\nMultimedia component 2\n\n\n\n\n\nMultimedia component 3\nMultimedia component 3\n\n\n\n\n\nMultimedia component 4\nMultimedia component 4\n\n\n\n\n\nMultimedia component\n\n5\n\nMultimedia component\n\n\n\n\n\nMultimedia component\n\n6\n\nMultimedia component\n\n\n\n\n\nMultimedia component\n\n7\n\nMultimedia component\n\n\n\n\n\nMultimedia component\n\n8\n\nMultimedia component\n\n\n\nSupplementary data related to this article can be found at https://doi.org/10.1016/j.energy.2023.126641.", "descript": "\n                  The present work intended the development of a graphical approach to support the operation of conventional iron-based catalysts under gasification conditions. A combination of experimental data and thermodynamic modelling was used as guidelines to elucidate the dependence of catalyst performance on the thermochemical conditions of producer gas. The outcomes are represented by stability diagrams in a form of planar representations for easier identification of appropriate operating windows. Attention was focused not only on potential deactivation mechanisms resulting from gas-solid interactions, but also on the stability of relevant catalytic phases when exposed to biomass-derived gas atmospheres at temperatures in the range 600\u2013900\u00a0\u00b0C. The results suggest that controlled process parameters contributes to enhance the tolerance of iron-based materials to deactivation by carbon deposition, H2S poisoning and/or carbonation. Selected examples also show that the redox potential imposed by producer gas can have a significant impact on the stability of relevant active phases, with subsequent impact on catalyst performance. To overcome these constrains, one should considerer suitable composition changes to enhance their redox properties, possibly combined with microstructural or nanostructural development during materials processing.\n               "}
{"full_text": "The oxygen evolution reaction (OER) is one of the most relevant anodic reactions within electrochemical cells, where it is coupled to the hydrogen evolution reaction (HER)\n1\u20136\n or the CO2 reduction reaction (CO2RR) to energy-dense carbon compounds at the cathode.\n7\u20139\n It is, therefore, of high relevance for electrochemical energy conversion and storage technologies. Oxygen generation from water oxidation at the anode is typically carried out in acidic or alkaline conditions. Operation in alkaline conditions allows the use of cheap, efficient, and stable non-precious-metal catalysts, in contrast to acidic conditions, in which only expensive and scarce noble-metal-based catalysts such as IrO2 and RuO2 exhibit significant stability.\n10\n In alkaline conditions, the best performance and highest stabilities were observed for Ni-based multimetallic catalysts,\n11\u201315\n which led to their widespread use as OER catalysts.\n10\n\n,\n\n16\u201319\n However, sluggish kinetics of the four-electron OER requires a significant anodic overpotential to achieve relevant geometric current densities, reducing the efficiency of the conversion of electrical to chemical energy. Hence, identifying efficient, cheap, and stable OER catalysts comprising earth-abundant elements is of fundamental importance and has been a prominent field of research during the last 20 years.\n20\u201323\n Among non-noble multimetallic metal-based OER catalysts reported so far, mixed nickel/iron/cobalt oxides in particular have shown stable low overpotentials at relevant geometric current densities.Benchmarking these novel OER electrocatalysts is of utmost importance but remains highly challenging, given that methods for evaluating performance (activity and stability) are non-standardized, making fair and reliable comparison extremely difficult. Notably, different catalyst supports are used, to which OER is very sensitive,\n24\n\n,\n\n25\n and variation in characterization methods and experimental setups further adds to disparities between reported performances. As a matter of fact, very few benchmarking studies have been carried out so far. The last significant efforts in providing meaningful benchmarking studies were carried out in 2012,\n26\n 2013,\n27\n and 2015.\n28\n These studies, conducted on OER electrocatalysts deposited on Au/Ti or glassy carbon electrodes, performed water oxidation at a current density of 10 mA cm\u2212\n2 with observed overpotentials higher than 300\u00a0mV. A relevant figure of merit from these studies is the overpotential (denoted as \u03b7\n10) required to achieve 10 mA cm\u2212\n2 current density per geometric surface area at ambient temperature and 1 atm O2. The \u03b7\n10 value is indeed the benchmarking criterion generally used in literature.\n26\u201328\n Some OER electrocatalysts reported during the last five years exhibit \u03b7\n10 overpotentials much closer to 200\u00a0mV, representing a significant advancement in the field. We, thus, found it timely to provide a fair comparison of the performances of the most active catalysts of this new generation.Here, we establish a protocol to benchmark a range of anodes, consisting of various catalysts synthesized on the same nickel foam (NF) support. Because of its conductivity, mechanical strength, relative inertness at alkaline pH, and low cost, nickel is an efficient current collector and a good support for active material deposition. Furthermore, nickel foam shows extended geometric surface areas and fine three-dimensional structures, which make it attractive as a support for heterogeneous catalysts.\n25\n\n,\n\n29\n Although various porous metallic foams have been used in the past for water oxidation,\n30\u201332\n a recent resurgence in the usage of NF as a support material has occurred. This was partly driven by the development of energy-storage electrochemical systems in alkaline conditions, such as solar-driven water splitting and electrocatalytic CO2RR using gas-fed flow cells. We have followed this trend and used NF as the catalyst support exclusively.\n17\u201319\n\nFor this study, we selected nine promising multimetallic, non-precious-metal catalysts reported in the literature (Table 1\n). Several selection criteria were adopted such as the \u03b7\n10 overpotential in alkaline conditions, the Tafel slope, the maximum current density, and the long-term stability, as these data were available in previous publications.\n33\u201341\n We also made sure to select materials composed of a variety of transition metals with different morphologies and synthetic procedures in order to broaden the scope of our comparison. These catalysts were preferentially chosen on the basis that they display low \u03b7\n10 overpotentials (below 300\u00a0mV). However, we also included the Co-based catalyst developed by Nocera et\u00a0al. (CoPi),\n34\n despite it exhibiting a \u03b7\n10 value greater than 300\u00a0mV in 1\u00a0M KOH, given that it is widely used in literature. The NiMoFe-O catalyst was included as it was reported to be the highest performing catalyst at the time of writing.\n27\n\n,\n\n42\n CoV-O and CoV-OOH were chosen to provide a comparison between two catalysts based on cobalt and vanadium that were synthesized by using different methods. In the publications, these catalysts were characterized under very different conditions. In particular, various electrode supports were used (glassy carbon, Cu plate, or Ni foam), illustrating the difficulty of comparing them just on the basis of the literature data. In our study, we exclusively used the same Ni foam as the conductive support. The syntheses followed the reported procedures as strictly as possible with slight adaptations to enable deposition on a 1\u00a0cm2 NF support.The materials were characterized and their kinetic performances for the OER in alkaline conditions analyzed. Geometric current densities of OER catalysts must attain several hundreds of milliamperes per square centimeter to facilitate CO2 reduction and solar-driven water splitting under lab-scale conditions.\n1\n\n,\n\n2\n\n,\n\n43\n Therefore, we not only benchmarked the catalysts at the commonly reported value of 10 mA cm2 but also at a more relevant current density of 100 mA cm\u2212\n2,\n17\n\n,\n\n44\u201347\n according to a standardized protocol. Catalysts must be compared in terms of intrinsic activities, which requires normalization of the current densities by the effective electrochemical surface areas (ECSAs). This is in general a very challenging issue, quite often incompletely addressed. We have, thus, been careful in providing an array of experimental procedures for the determination of concentrations of active sites and specific activities, and we propose a best-practice protocol for researchers in the field. Finally, we report a novel NF-based support with different morphology and increased structuration, leading to significant improvements in the performance of almost all studied catalysts. By doing so, we report two OER catalysts with overpotential values of 195 and 198\u00a0mV required for a current density of 10 mA cm\u2212\n2, and overpotentials of 247\u00a0mV required for 100 mA cm\u2212\n2, among the lowest values reported so far. We demonstrate the outstanding electrochemical and compositional stability of these two catalysts, evaluated by long-term electrolysis in a flow cell (Figure S1). Furthermore, we stress the importance and necessity of performing galvanostatic tests as well as surface leaching quantification in order to assess the stability of the catalyst.Nine catalysts were selected among the most active materials reported in the literature (Table 1). Cu-O and NiMoFe-O were cathodically electrodeposited on Ni foam (Figure\u00a01\nA) from aqueous solutions of the metallic precursors. This simple method resulted in the formation of dendritic structures as shown by SEM analysis (Figures 1B, 1C, S2, and S4). NiMoFe-O showed a particularly strong adhesion to the substrate because the presence of nickel in the as-deposited catalyst ensures a continuous interface with NF in terms of composition and, thus, a small lattice mismatch. EDX was used to confirm the elemental composition (Figures S3 and S5). CoPi was anodically electrodeposited, which resulted in the slow formation of large dendrites with a poor adhesion to the NF support (Figures 1D, S6, and S7) . FeCoW (Figures 1E, S8A, S8B, and S9) and CoV-OOH (Figures 1F, S10, and S11) were synthesized from nanomaterial dispersions mixed with a Nafion ink and drop-cast and dried onto the surface of the support to form thick layers, which remained well attached to the support despite large cracks (Figures S8B and S10C). The major limitation of this method is the hydrophobicity of dry Nafion, which promotes the formation of an air film trapped at the catalyst/electrolyte interface and limits their contact area. In order to reach a stable OER activity, these catalysts must be kept under oxidative conditions (10\u00a0min at 5 mA cm\u2212\n2) in an aqueous electrolyte in order for the Nafion\u2019s hydrophilic domains to swell and become predominant in the bulk.\n48\n A modification in the nanostructure of FeCoW was observed after this process, revealing the formation of a thin layer structure (Figures S8C and S8D). We conclude that once the Nafion network is fully hydrated the metallic sites are exposed to the electrolyte. This allows dissolution/precipitation equilibria at the interface, resulting in the formation of highly nanostructured surfaces. NiFe-OOH (Figures 1G, S12, and S13) was made through galvanic exchange of the Ni-based substrate with a Fe3+ precursor, followed by the deposition of a bimetallic oxy-hydroxide. As the support is the only source of nickel, it is etched during the reaction. This not only ensures a very high adhesion of the catalyst on the support but also slightly modifies the material by etching its Ni backbone. Nevertheless, this method is particularly interesting as it is very simple. NiFeSe- and CoFeSe-derived oxides were synthesized by a more complex three-step procedure involving the hydrothermal formation of layered double hydroxides (LDHs), followed by selenization and subsequent reoxidation. The SEM images and EDX elemental analysis of the resulting NiFeSe-dO (Figures 1H, S14, and S15) and CoFeSe-dO (Figures 1I, S16, and S17) catalysts revealed the formation of very dense, thick, and mechanically stable deposits at the surface of NF with fine nanostructures in the range of 30\u2013100\u00a0nm. However, the formation of a selenide involves hazardous synthesis steps and its reoxidation leads to toxic selenite waste products. CoV-O was synthesized by a simple one-step hydrothermal procedure involving the coprecipitation of Co and V in a mixed-phase composed of a fine LDH nanostructure (Figures 1J, S18, and S19)\u2014this method is simple but results in a very low loading on NF.It should be noted that the atomic compositions of the catalysts differed slightly from those reported in some cases. For example, in the case of NiMoFe-O (Figure\u00a0S5), the use of NF as the support led to small modifications in the chemical composition of the film. The tungsten content in FeCoW is also lower than expected (Figure\u00a0S9). Additionally, as the procedure for CoFeSe-dO synthesis could not be reproduced, it was modified in order to reach a Co:Fe ratio comparable with the Ni:Fe ratio in NiFeSe-dO (Figures S15 and S17). In spite of the modifications we had to make in the procedures, the morphologies of the catalysts were comparable with those in the literature, as illustrated by the SEM images in Figures S14 and S16.The catalytic activity of each material was measured under alkaline conditions, in an aqueous 1\u00a0M KOH electrolyte solution. Chronopotentiometric steps (CP steps) were performed at different fixed current densities (j\u00a0= 0, 5, 10, 25, 50, and 100 mA cm\n\u22122) for 5\u00a0min each under stirring. This method is better than linear sweep voltammetry as it ensures that exclusively the OER response is measured and other contributions to the current are eliminated, such as the oxidation of Ni(OH)2 to NiOOH in Ni-containing materials.\n49\n Furthermore, these CP steps give some information regarding the stability of the potential measured at different current densities on a short timescale. A possible drawback resides in some additional ohmic drop associated with the accumulation of oxygen bubbles at the surface of the electrode.\n16\n\n,\n\n50\n\nThe j-\u03b7 profiles and overpotentials at j\u00a0= 10 and 100 mA cm\n\u22122 of the tested catalysts are displayed in Figures 2A and 2B, respectively (see Figure\u00a0S20 for error bars), with the exact values of the overpotentials at j\u00a0= 10 mA cm\n\u22122 given in Table S1. Additionally, the electrodes were held at a fixed current density (j\u00a0= 50 mA cm\n\u22122) for 30\u00a0min in static conditions to verify that measured potentials were stable over a longer reaction time. In all cases, this confirmed no clear degradative reactions (see Figure\u00a0S21 for stability data).The overpotential \u03b7\n10 value is considered a figure of merit for OER catalysts. All the catalysts characterized in this study have \u03b7\n10 values between 211 and 347\u00a0mV, a significant 136\u00a0mV range (Figure\u00a02; Table 2\n). The lowest overpotentials were obtained for NiFeSe-dO (\u03b7\n10\u00a0= 211\u00a0mV) and CoFeSe-dO, (\u03b7\n10\u00a0= 212\u00a0mV). The highest overpotentials were obtained for CoV-O (\u03b7\n10\u00a0= 331\u00a0mV) and Cu-O (\u03b7\n10\u00a0= 347\u00a0mV). Comparison with literature data is shown in Table S1.Oxygen evolution must be carried out at higher current densities in order to meet the requirements for the electrochemical conversion and storage of renewable energy, such as solar-driven water splitting and CO2 reduction technologies.\n17\n\n,\n\n43\u201347\n\n,\n\n51\n We, therefore, focus on the catalytic activities of the nine catalysts at 100 mA cm\u20132 (Figure\u00a02; Table 2). Although NF requires an overpotential as high as 563\u00a0mV, the nine catalysts enable an important drop in \u03b7\n100 as compared with that of their support. Despite differences in the slopes, the trend in OER activity is the same at 10 and 100 mA cm\u20132. NiFeSe-dO remains at the head of the group with a low overpotential of \u03b7\n100\u00a0= 264\u00a0mV, followed by CoFeSe-dO and NiFe-OOH with the same overpotential value of 289\u00a0mV, and by FeCoW with \u03b7\n100\u00a0= 293\u00a0mV. CoV-O (\u03b7\n100\u00a0=397\u00a0mV) and Cu-O (\u03b7\n100\u00a0=432\u00a0mV) show the lowest performances at all current densities.The dependence of the OER kinetics on the applied potential for the catalysts is well illustrated through Tafel analysis (Figure\u00a0S22; Table S1). The fastest increase in current density upon potential increase occurs at the surface of NiFe-OOH, with a slope as low as 36\u00a0mV dec\n\u22121. NiFeSe-dO and FeCoW also showed low Tafel slope values of 55 and 56\u00a0mV dec\u2212\n1. The obtained values for the other catalysts ranged from 63 to 83\u00a0mV dec\n\u22121 (Figure\u00a0S22). We systematically observed larger Tafel slopes with respect to reported ones, except for NiFe-OOH (Table S1). This is likely because chronopotentiometric steps were used to measure Tafel slopes in place of linear sweep voltammetry (LSV) scans, which is the general methodology employed in the recent literature. Also, as a consequence of the CP steps, the accumulation of O2 bubbles at the surface of the electrodes might contribute some extra resistance, especially at higher current densities, resulting in increased Tafel slope values.\n16\n\n,\n\n52\n\n,\n\n53\n\nComparing catalysts with different morphologies in terms of their intrinsic activities requires the determination of the density of electrochemically active sites, a very important yet challenging analysis.\n54\n\n,\n\n55\n Depending on parameters such as their nanostructure, porosity, and lattice structure, catalysts can show very different interactions with the surrounding electrolyte.\n56\u201359\n The density of electrochemically active and accessible sites can vary a lot from one catalyst to another. A range of techniques can be used to relate the total OER activity of a catalyst to the intrinsic activity of each active site.\n54\n\n,\n\n55\n\nTwo main methodologies are considered here in order to estimate the density of accessible active sites. The first is through the determination of the ECSA whereas the second involves analysis of the pre-OER redox peaks. Both methods have serious limitations; however, they are complementary techniques, therefore combined analysis enables general trends to be established.OER catalysts behave as capacitors: upon application of a potential, a charge build-up is observed at the catalyst-electrolyte interface. The capacitance of a catalyst in the absence of any Faradaic process is the double-layer capacitance C\nDL. ECSAs can theoretically be calculated from C\nDL values; however, they are difficult to obtain accurately.\n27\n\n,\n\n60\u201366\n Indeed, ECSA\u00a0= C\nDL /C\nS where C\nS is the specific capacitance of the material, which corresponds to the capacitance of an atomically smooth planar surface of the same material per unit area under identical electrolyte conditions. Although C\nDL can be experimentally determined by measuring the non-Faradaic capacitive current associated with double-layer charging from the scan-rate dependence of the cyclic voltammograms (CVs), it is almost impossible to determine reliable values of C\nS for each sample.\n60\n\n,\n\n66\n Note that in previous studies for determination of ECSAs of various OER catalysts, the same value of C\nS\u00a0= 0.040 mF cm\u2212\n2 in 1\u00a0M NaOH, based on typical reported values for metallic surfaces, was applied.\n27\n\n,\n\n28\n However, one should be aware that this only gives an estimation of the ECSAs given that the C\nS value varies significantly from one material to another.\n27\n In our case, C\nDL values were measured by cyclic voltammetry in the range\u00a0+0.95 \u2013\u00a0+1.05\u00a0V versus RHE, as it is a non-Faradaic region for all our catalysts (except Cu-O, which was characterized between\u00a0+0.66 and\u00a0+0.76\u00a0V versus RHE). However, OER catalysts can show potential-dependent conductivity variations, therefore inaccuracies can arise from lower conductivities in potential regions prior to water oxidation.\n54\n\n,\n\n55\n\n,\n\n67\n\nHere, we report the C\nDL values for the nine materials and for the NF support, measured in 1\u00a0M KOH by using electrodes with 1\u00a0cm2 geometric areas (Figures 3A and S23). We indeed observed large differences: some of the studied catalysts have C\nDL values in the range of 1 mF, slightly larger than that of the Ni foam (0.9\u00a0mF), whereas CoFeSe-dO, NiMoFe-O, and NiFeSe-dO have much larger C\nDL values of 3.65, 2.40, and 2.35 mF, respectively. This result is in line with the observation of extremely fine nanostructures for these three catalysts (Figures 1I, 1H, and 1C).Most OER catalysts show redox features at potentials below the OER onset. These processes are generally attributed to the oxidation of the metal sites. For instance, NiII hydroxide oxidizes to NiIII oxyhydroxide prior to OER catalysis.\n54\n\n,\n\n65\n\n,\n\n66\n The amount of charge (Q) necessary to oxidize all the electrochemically active metal sites in a catalyst can be estimated through the integration of the oxidation wave. Q is directly proportional to the number of electrochemically accessible active sites (N) in the material according to the relation N\u00a0= Q/(n\n\ne\n\u00b7Q\ne\n), where n\n\ne\n is the number of electrons of the oxidation process observed and Q\n\ne\n is the charge of an electron. It is generally assumed that this oxidation is a one-electron process, although this is a very strong assumption because the initial state of the catalyst is likely to be a mixed-valence state, with delocalized energy bands.\n68\n Therefore, electron transfers are more complex than isolated one-electron transfers\u2014here, we chose to compare Q values without assuming an arbitrary value for n\n\ne\n. However, one should be aware that Q can account for bulk sites, which might not participate in the OER reaction, especially given that recent studies have shown the prominent role of surface sites in OER catalysis.\n69\n\n,\n\n70\n Another limitation of this method comes from the superposition of the oxidation wave and the OER onset in some cases\u2014the deconvolution of these two contributions can be challenging. The simultaneous OER wave can also block a fraction of the active sites because of the formation of O2 bubbles and interference from mass transport limitations at high current density.The estimated oxidation charges Q for each of our nine catalysts with 1\u00a0cm2 geometric areas are displayed in Figure\u00a03\nA. Most catalysts have Q values comprised between 200 and 480 mC, except NiMoFe-O, NiFeSe-dO, and CoFeSe-dO, which stand out once again with values as high as 600, 800, and 980 mC, respectively. NiFe-OOH has a surprisingly low value of 60 mC. Despite some discrepancies (e.g., Cu-O and NiFe-OOH), it is interesting to observe that the C\n\nDL\n values correspond quite well with the obtained Q values. Among the nine catalysts, CoFeSe-dO shows the highest density of active sites, followed by NiFeSe-dO and NiMoFe-O.The current densities measured for each catalyst at a fixed overpotential of 250\u00a0mV are displayed in Figure\u00a03B. We observe that NiFeSe-dO has the highest activity, followed by CoFeSe-dO, FeCoW, and NiFeOOH. The correlation of these data with those in Figure\u00a03A provides the following conclusions. First, NiFeSe-dO has a much higher intrinsic activity than CoFeSe-dO, given that the current density is significantly higher than that of CoFeSe-dO, despite exhibiting a lower density of active sites. Therefore, NiFeSe-dO is the most active catalyst thanks to a combination of a high density of active sites and high intrinsic activity of these sites, whereas the high activity of CoFeSe-dO is mainly due to the large density of active sites. Second, FeCoW and NiFe-OOH display quite high activities but have a low density of active sites, as shown by the very low C\nDL and Q values. This suggests that these two catalysts have relatively high intrinsic activities.The evaluation of the density of active sites of metal-based (oxy)hydroxide materials is a highly challenging task. The two methods used here both suffer from limitations. Estimating ECSA by using C\nDL measurements requires the use of specific capacitances and strongly depends on the conductivity of the material, which can vary upon application of a potential. The integration of pre-OER oxidation peaks relies on the assumption that single electron transfer steps operate, and calculation is often complicated by the deconvolution of OER catalytic wave. We show a correlation between the results of these two methods and stress the need to cross-check data by using two complementary techniques.\n54\n\n,\n\n66\n This should be considered as a best-practice protocol for researchers aiming to evaluate intrinsic catalytic activities.The evaluation of the total number of moles of metal atoms (n\nM) in a catalyst on a 1\u00a0cm2 geometric area electrode can provide access to other useful information, namely the mass activity or molar activity. A high molar activity effectively translates into a lower cost, as a lower number of metal atoms are required to perform OER catalysis at a given overpotential.A n\nM value can be obtained through the dissolution of the catalyst layer and analysis by using ICP-MS. The metal content of the nine catalysts are displayed in Figure S24A. We observe that NiFeSe-dO and CoFeSe-dO have the lowest metal content whereas FeCoW has the highest. The metals molar activity was calculated by dividing the current density at \u03b7\u00a0= 250\u00a0mV by the metal content of each catalyst (Figure S24B). The data clearly show that not only do NiFeSe-dO and CoFeSe-dO display the largest current densities but they do so with the lowest amount of metals; therefore, they show extremely high metal molar activities (520 and 634 mA cm\u22122 mmol\u22121, respectively). In contrast, all other catalysts have much lower molar activities, in the 20\u201350 mA cm\u22122 mmol\u22121 range. In the case of FeCoW, although the number of metals is high, only a small fraction is involved in the OER. Consequently, an improved exposition of the metal sites to the electrolyte might be key in the enhancement of the OER activity of this catalyst.The NF support used in this study benefits from a relatively high ECSA of approximately 15\u00a0cm2 cmgeo\n\u22122 (per geometric square centimeter), estimated by using the C\nDL measurement for NF and a C\nS measurement for a Ni plate electrode (Figure\u00a0S25). In order to further increase the surface area of this support, we used a straightforward method for the electrodeposition of nickel dendrites on NF.\n71\n The deposition of the metallic branched structures in the presence of protons at very high current density generates H2 bubbles at the surface of the electrode, creating a porous dendritic morphology (Figures 4\nA and S26). As a result of this increased structuration, the double-layer capacitance of NF was greatly increased from 0.9 to 4.9 mF (Figure\u00a0S27), leading to an estimated surface area of approximately 82\u00a0cm2 cmgeo\n\u22122.In line with our prior conclusion regarding the benefit of a higher surface area and a larger density of accessible active sites for OER activity, a significantly higher activity was observed for this dendritic nickel foam (NiNF) than for NF. A 80\u00a0mV decrease of its \u03b7\n10 value (\u03b7\n10|NiNF\u00a0= 331\u00a0mV), and a 129\u00a0mV decrease of its \u03b7\n100 value (\u03b7\n100|NiNF\u00a0= 434\u00a0mV) were measured. This marked increase in activity makes NiNF a highly interesting anodic support material for electrolytic cells. NiNF was used as a new support for the deposition of the OER catalysts described above. We first detail the results obtained with the most active catalysts, namely NiFeSe-dO and CoFeSe-dO, deposited on NiNF (Figures 4B and 4C). From the SEM images, it appears that the nanostructure of NiFeSe-dO and CoFeSe-dO was maintained on this support, resulting in a hierarchical porous structure composed of three levels of porosity: the large pores of the NF (\u2248 500\u00a0\u03bcm), the pores formed by the nickel dendrites (\u2248 1\u201310\u00a0\u03bcm), and the meso- and/or macropores resulting from the layered structure of the catalysts (\u2248 30\u2013100\u00a0nm) (Figures 4B and 4C). The C\nDL value of NiFeSe-dO on NiNF was a factor of four greater than on the NF, reaching a high C\nDL value of 9.6 mF (Figures S27\u2013S27A). The C\nDL value for CoFeSe-dO was roughly three times higher, giving an extremely large C\nDL value of 10.3 mF (Figures S27\u2013S27B). Therefore, the use of this highly porous support enables a significant increase in the density of accessible active sites. To determine how the increase in C\nDL impacted the catalytic activity, the NiNF-deposited catalysts were evaluated by using our standard electrochemical characterization procedure detailed in the previous sections (Figure\u00a05\n). For both catalysts, the \u03b7\n10 and \u03b7\n100 values decreased upon substitution of NF with NiNF. For NiFeSe-dO, replacing NF with NiNF decreased \u03b7\n10 from 211 to 198\u00a0mV and \u03b7\n100 from 264 to 247\u00a0mV (Figure\u00a05A). With CoFeSe-dO, the improvement was even more significant with \u03b7\n10 decreasing from 212 to 195\u00a0mV and \u03b7\n100 decreasing from 289 to 247\u00a0mV (Figure\u00a05B). This represents a substantial improvement in the performance of these OER catalysts (Table 2). The \u03b7\n10 obtained for CoFeSe-dO and NiFeSe-dO are among the lowest values reported in the literature so far. The Tafel slope decreased from 55 to 54\u00a0mV dec\u22121 in the case of NiFeSe-dO and from 72 to 63\u00a0mV dec\u22121 in the case of CoFeSe-dO (see Figures S28 and S29; Table 3\n).For the other catalysts, we also observed a large increase in the C\nDL by shifting from NF to NiNF as the support (Figures S30 and S31). In all cases, apart from FeCoW, the C\nDL values increased by a factor between 2.4 and 5, reflecting the improvement in the specific surface area provided by NiNF. FeCoW shows a unique 9.7-fold increase in its C\nDL, larger than that of the support itself (Figure\u00a0S31). This is likely due to an altered morphology of this catalyst when moving from NF to NiNF, leading to an increased density of accessible active sites. Additionally, a decrease of the overpotentials \u03b7\n10 (by 5 to 80\u00a0mV) and \u03b7\n100 (by 14 to 129\u00a0mV) (Figures S32\u2013S38; Table 2) and a decrease of the Tafel slopes (by 1 to 30\u00a0mV dec\u2212\n1, except for Cu-O) (Figures S32\u2013S38; Table 3) was observed. The gain of 20 to 30\u00a0mV dec\u2212\n1 in most cases is significant as it induces a large decrease in the overpotential at high current densities. In particular, NiFe-OOH displays a remarkably low Tafel slope of 20\u00a0mV dec\u2212\n1 when deposited on NiNF.Stable operation under continuous flow conditions at high current density is an important property of OER systems. We designed a water-splitting experiment under flow conditions (Figure\u00a0S1), in order to test our best catalysts (NiFeSedO-NiNF and CoFeSedO-NiNF) under conditions closer to industrial applications. The catalyst was loaded in a two-compartment cell separated by a Nafion membrane with a platinum mesh-based cathode. The anolyte and catholyte were 1\u00a0M KOH aqueous solutions. These solutions were recirculated in each compartment from separate containers. A current density of 100 mA cm\u20132 was applied for 8\u00a0h and the potential response as well as FEO2\n were measured over time. Online monitoring of the elements present in solution during electrolysis also allowed quantitative measurement of metal leaching from the catalysts.The flow experiment described above was performed by using NF, NiFeSedO-NiNF (Figure\u00a06\nA), and CoFeSedO-NiNF (Figure\u00a06B) as the anodes. As illustrated by Figure\u00a06, these two catalysts show extremely stable potentials over 8\u00a0h of electrolysis at a high current density of 100 mA cm\u20132. NF performs oxygen evolution at 1.96\u00a0\u00b1 0.06\u00a0V versus RHE. The initial increase in potential is attributed to the oxidation of nickel, which occurs at the surface of the foam but also reaches its subsurface during the first hour of electrolysis under such a high current density. Both NiFeSedO-NiNF and CoFeSedO-NiNF perform oxygen evolution at potentials as low as 1.58\u00a0\u00b1 0.02\u00a0V versus RHE, with outstanding stability, which represents a major improvement as compared with that of NF. The Faradaic efficiency for O2 production (FEO2\n) was evaluated by measuring the amount of oxygen produced at the anode (Figure\u00a06). During the first hour of the experiment, the headspace of the anolyte container was saturated in gas. After this equilibration period, FEO2\n was very stable, with mean values of 98.0%\u00a0\u00b1 1.5%, 97.8%\u00a0\u00b1 3.0%, and 98.5%\u00a0\u00b1 1.9% for NF, NiFeSedO-NiNF, and CoFeSedO-NiNF, respectively. This confirms that oxygen evolution was the only process occurring at the surface of these catalysts.The concentrations of Ni, Co, and Fe in the anolyte were measured every hour by ICP-MS (Figure\u00a06). In the case of NF, the Ni concentration is in the range 40\u2013120 ppb. This concentration does not increase over time, which proves the very high stability of this support in 1\u00a0M KOH under a high current density. The concentration of Fe was comprised between 250 and 450 ppb and remained stable over time. This Fe content in a KOH electrolyte is common.\n72\n\n,\n\n73\n In the case of NiFeSedO-NiNF, we observe a small increase in the Ni concentration at the beginning of the experiment, as it reaches 300 ppb. After this small increase, the concentration slowly stabilized at around 100 ppb, which corresponds to the background concentration measured in the case of NF. No additional Ni was dissolved over the course of the reaction (Figure\u00a06A). As a result, NiFeSedO-NiNF is an extremely stable catalyst in these conditions. As expected, no Co was detected for NF or NiFeSedO-NiNF. In the case of CoFeSedO-NiNF, Ni, and Co concentrations in the range 200\u2013700 ppb were measured (Figure\u00a06B). This means that some Co and some Ni are dissolved from the surface of the catalyst at the beginning of the experiment, but do not accumulate in the solution, thus revealing the absence of continuous dissolution over the course of the electrolysis. The Fe concentrations remained constant for both catalysts and correspond to the background concentration measured with bare NF. In conclusion, these two catalysts have shown a high chemical and mechanical stability over 8\u00a0h under continuous flow conditions, at a high current density of 100 mA cm\u20132 and an electrolyte flow of 9\u00a0mL min\u20131. For practical development of these catalysts, evaluation of their stability during electrolysis over months would be required.For the first time, some of the most active OER catalysts reported during the last 4 years have been compared under identical reaction conditions after deposition on the same Ni foam support. For the reliability of the comparison, we used a standardized protocol to characterize the catalysts in terms of activity, the density of active sites, and stability. Specifically, j\u2013E curves were not obtained from LSVs, but instead from chronopotentiometric steps in 1\u00a0M KOH at different fixed current densities, up to 100 mA cm\u20132, to determine overpotentials. A range of complementary techniques was used in order to evaluate the density of accessible active sites of each anode. These procedures allowed for a reliable comparison of the catalysts.The highest activities were observed for Se-doped bimetallic oxides, NiFeSe-dO and CoFeSe-dO, with remarkably low overpotentials of 211 and 212\u00a0mV at 10 mA cm\u2212\n2, and 264 and 289\u00a0mV at 100 mA cm\u2212\n2 on a NF support, respectively. Their excellent overall OER activity is due to a combination of a high intrinsic activity and a high density of accessible active sites (Figure\u00a03). The importance of selenium in precursor materials for OER has been previously discussed.\n37\n\n,\n\n38\n\n,\n\n74\n\n,\n\n75\n During OER, Se is removed in solution, in the form of selenate and selenite ions, whereas oxygen atoms are incorporated. The metal selenide, thus, serves as a templating precursor to oxides and/or hydroxides that are the actual active species. Such Se-derived oxides display greater activity than oxides/hydroxides prepared by other methods. Furthermore, it is likely that the substitution of Se by O allows more active sites to be exposed, in line with the significantly higher densities of available active sites for the two Se-doped materials as compared with the other materials (Figure\u00a03A). Moreover, in all cases, including here, significant amounts of Se atoms (0.5\u20132\u00a0mol %) are retained in the material after a prolonged reaction. These atoms, which can be referred to as Se-doping, seem to improve the OER activity of the catalytic sites. A recent theoretical study indeed showed that Se-doping resulted in a substantial decrease of the energy barrier of the rate-determining step of the OER.\n74\n NiFeSe-dO and CoFeSe-dO are unique, thanks to very high metal molar activities (Figure\u00a0S24): these catalysts can perform OER at high current density while consuming few metal resources.The combination of Ni and Fe sites provides the highest activity as has been observed in previous studies.\n11\n\n,\n\n75\u201378\n In keeping with the very high performances measured for CoFeSe-dO, a similar beneficial association between Co and Fe is illustrated by the high catalytic activity of FeCoW, just below that of NiFeSe-dO and CoFeSe-dO, with a \u03b7\n10 value of 235\u00a0mV and a \u03b7\n100 value of 293\u00a0mV on the NF support. Analysis using NF indicates that FeCoW displays the highest intrinsic activity of the catalytic sites (Figure\u00a03), but a very poor metal molar activity (Figure\u00a0S24). Therefore, further structuration leading to higher active surface areas of the deposit would be needed to improve the OER activity of FeCoW. Similar reasoning applies to NiFeOOH.Catalysts based on the association of Co and V are less active than the materials discussed above. CoV-OOH shows a strikingly higher OER activity than CoV-O, which we attribute to the 50-fold difference in mass loadings. Indeed, the hydrothermal synthesis procedure used for CoV-O led to the deposition of only 0.5\u00a0mg cm\u20132, whereas 25\u00a0mg cm\u20132 were drop-cast in the case of CoV-OOH. We do not discuss further the other catalysts, CoPi, Cu-O, and NiMoFe-O, as they show considerably lower activities than the most active catalysts discussed above.We improved the structuration and the porosity of the electrodes through modification of the NF support by depositing dense dendritic and porous Ni structures. This was achieved by using a fast and very simple electrodeposition procedure, where H2 bubbles act as templates for the formation of pores. A large increase in the double-layer capacitance and greater OER performance than the untreated NF was observed. We tested this novel high-surface-area support, NiNF, with 9 different catalysts, and in all cases, we obtained increased double-layer capacitances, reflecting higher accessibility of active sites. We measured enhanced performances, as shown from decreased overpotentials and Tafel slopes. Specifically, the overpotentials of NiFeSe-dO and CoFeSe-dO further decreased to 198 and 195\u00a0mV at 10 mA cm\n\u22122 respectively, and to 247\u00a0mV at 100 mA cm\u2212\n2. These are among the lowest overpotentials ever reported in the literature.NiFeSedO-NiNF and CoFeSedO-NiNF are the most active catalysts studied in this work. We decided to test their long-term stability. Most reports assess the stability of OER catalysts by applying a constant current and measuring the potential response. This method is relevant for preliminary tests because it enables fast analysis of extremely unstable catalysts. However, a constant potential response is not sufficient to claim that a catalyst is stable. Indeed, Kibsgarrd et\u00a0al.\n79\n underline that catalyst corrosion can not only impair its activity but also improve it in certain cases because of factors, including corrosion, that can lead to increased roughness. Moysiadou et\u00a0al.\n80\n draw attention to the observation that a catalyst can maintain stable macroscopic features during electrolysis, such as a stable potential, while experiencing a loss of mass because of a partial decomposition of its surface. As a result, activity monitoring does not provide a reliable measure of the overall stability of a catalyst. Transition metals at the surface of an oxygen evolution catalyst experience dissolution/redeposition equilibria during catalysis.\n56\n\n,\n\n81\n Depending on the solubility of the metals considered, irreversible dissolution might occur. Moreover, the strong flow of electrolyte hitting the surface and the large amount of O2 bubbles generated can add to mechanical instability. For these reasons, leaching of metals from the surface of the catalyst is considered as a major route of decomposition, and a catalyst can only be claimed stable if leaching is negligible. This phenomenon can be detected by analyzing the composition of the electrolyte after electrolysis by inductively coupled plasma-mass spectrometry (ICP-MS).\n82\n\n,\n\n83\n Both electrochemical and compositional stabilities must be evaluated if one wants to prove the stability of a catalyst. This work proposes a methodology to perform stability evaluation by measuring galvanostatic features, O2 Faradaic efficiency, and surface leaching all at once. This rigorous stability check, using a water-splitting system, showed that NiFeSedO-NiNF and CoFeSedO-NiNF perform oxygen evolution in flow conditions, at a high current density of 100 mA cm\u20132, with outstanding stability, be it on the macroscopic or on the microscopic scale. Constant potentials as low as 1.58\u00a0V versus RHE were measured at the anode. Extremely small amounts of transition metals were leached from the catalysts\u2019 surfaces, resulting in Ni, Co, and Fe concentrations generally comprised between 100-700 ppb in the electrolyte. These results thus prove the excellent chemical and catalytic stability of the two catalysts.The benchmarking study presented here provides a reliable comparison between OER anodes comprised of catalysts deposited on porous supports. Nine of the most active and relevant precious-metal-free multimetallic OER catalysts were synthesized on NF and compared in alkaline conditions (1\u00a0M KOH) using a standardized protocol. The overpotentials at 10 and 100 mA cm\u20132 showed similar trends in OER activities. We identified NiFeSe-dO and CoFeSe-dO as the two best catalysts on NF, both showing \u03b7\n10 values of \u2248 210\u00a0mV and \u03b7\n100 values of 264 and 289\u00a0mV, respectively. We propose a protocol to assess the density of available active sites by using complementary techniques (combining double-layer capacitance measurements and pre-OER oxidation wave integration) and provide a qualitative comparison between intrinsic activities. NiFeSe-dO, in particular, stands out as a catalyst with high intrinsic activity, exhibiting higher currents than CoFeSe-dO despite displaying a comparatively lower density of active sites. With insights from the relationship between available active sites and overall activity, we further enhanced the surface area of NF by using an electrodeposition technique to texture the surface and form NiNF. OER catalysts displayed better performances (decreased overpotentials and Tafel slopes) when deposited on this new substrate, in the best case giving \u03b7\n100 values of 247\u00a0mV for CoFeSe-dO and NiFeSe-dO. Electrochemical and compositional stabilities of these catalysts were both evaluated. They perform OER catalysis in flow conditions at 100 mA cm\u20132 maintaining a constant potential without undergoing significant leaching. The information gained from this study not only enables fair and reliable comparison of the best reported OER catalysts on NF but also highlights the important role of the support in oxygen evolution seeing as the enhanced performance was observed after structuration of the NF to form NiNF. The integration of these improved NiNF-based catalysts in electrolytic cells can increase overall energy efficiency and enhance the viability of electrically driven energy conversion and storage.Further information and requests should be directed to and will be fulfilled by the lead contact, Marc Fontecave (marc.fontecave@college-de-france.fr).There are restrictions to the availability of catalysts described in this work due to an ongoing patent submission.All data in this manuscript are available upon request to the lead contact.Chemical reagents were purchased in reagent grade from Alfa Aesar and Merck. Nickel foam (1.6\u00a0mm in thickness, purity 99.5%, density 0.45\u00a0g cm\u20133, 95% porosity, and 20 pores cm\u20132) was purchased from Goodfellow. Oxygen 5.0 was purchased from Linde. All electrochemical experiments were performed with a VSP300 BioLogic potentiostat and the Biologic EC-Lab software was used for data analysis. Hydrothermal syntheses were performed in a Carbolite Gero CWF1213 furnace. SEM images were collected on a SU-70 Hitachi FEGSEM equipped with a X-max 50\u00a0mm2 Oxford spectrometer for energy dispersive X-ray spectroscopy (EDX) measurements. Inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements were performed on a Thermo Scientific iCAP 6300 duo device. Inductively coupled plasma-mass spectroscopy (ICP-MS) measurements were performed on a ICP-QMS 7900 Agilent apparatus.NF was used as the support for all the catalysts. 1\u00a0cm2 square foams were cut, and an additional section was left for electrical contact. This area was partially covered with epoxy glue in order to limit the 1\u00a0cm2 area as precisely as possible. These foams were pre-treated through soaking in a 3\u00a0M HCl solution for 10\u00a0min, in order to remove the nickel oxide layer formed at the surface when in contact with air. Then the substrates were sonicated for 5\u00a0min in ethanol, 5\u00a0min in water, and dried with compressed air before use. The synthesis procedures of the different catalysts and of the dendritic Ni support are described in the supplemental information. Mass loadings between 0.5 and 40\u00a0mg cm\u20132 were obtained.The catalysts surface morphologies were examined using SEM, and their elemental compositions were measured using SEM-EDX. In some cases, this analysis was complemented with elemental analysis using ICP-OES. This study was performed by dissolving the powder sample in an aqueous nitric acid solution.A two-compartment cell separated by a glass frit was used for electrochemical measurements. The electrolyte was an aqueous solution of 1\u00a0M KOH. A three-electrode arrangement was used with a platinum mesh counter electrode (Goodfellow, 2.25\u00a0cm2) and a Ag/AgCl/KClsat reference electrode (BioLogic), which was very regularly calibrated against potassium ferrocyanide in order to ensure the absence of any shift in its potential. The potentials were reported versus RHE according to the following equation: ERHE\u00a0= EAg/AgCl\u00a0+ 0.197\u00a0+ 0.059\u00b7pH. The working electrode was positioned in the cell in order to minimize the distance to the reference electrode (\u2248 1\u00a0mm), thus avoiding a large contribution from the cell in the ohmic drop (the resistance was always between 0.1 and 0.25\u00a0\u03a9). Before each set\u00a0of experiments, O2 was flowed through the working electrode\u2019s compartment for 20\u201330\u00a0min. This is an important step as it prevents any contribution from the O2 partial pressure pO2\n in the thermodynamic potential calculation: ENernst\u00a0= EH2O\n/O2\n \u2013 0.059\u00b7log pH\u00a0+ 0.015\u00b7log pO2\n. In an O2-saturated solution pO2\n\u00a0= 1, and the thermodynamic potential is given by the following equation: ENernst\u00a0= EH2O\n/O2\n \u2013 0.059\u00b7log pH.Each material was characterized in 10\u00a0mL of 1\u00a0M KOH aqueous solution following a precise protocol divided into 3 steps. Step 1: consecutive LSV scans were performed at a scan rate of 10\u00a0mV s\n\u22121 until the response was stable. Step 2: in order to study the OER kinetics, it is important to avoid any transient oxidation process such as the oxidation of Ni(OH)2 to NiOOH. For this purpose, chronopotentiometric steps (CP steps) were performed at different fixed current densities (j\u00a0= 0, 5, 10, 25, 50, and 100 mA cm\u20132) for 5\u00a0min each with stirring. In some cases, a stable potential was not obtained after 5\u00a0min, so the CP steps were extended by 5 additional min. The (j, E\nj\n) data points were collected. The overpotential at a given current density j (\u03b7\n\nj\n) was calculated according to the following equation \u03b7\n\nj\n\u00a0= E\nj\n \u20131.23 with E\nj\n the potential measured at the current density j, in V versus RHE. The (j,\u03b7\n\nj\n) points were plotted in a j-\u03b7 graph. Tafel slopes were obtained by plotting \u03b7 against log j. The linear fit of these plots: \u03b7\u00a0= a\u00a0+ b\u00b7log j gives the Tafel slope b. Step 3: the short-term stability of the different samples in each electrolyte was tested by running electrolysis at a fixed current density j of 50 mA cm\n\u22122 for 30\u00a0min under stirring. The pH of the electrolyte in the anode compartment did not change during electrolysis.The double-layer capacitance C\nDL values were determined electrochemically in an aqueous solution of 1\u00a0M KOH. All measurements were conducted in the voltage range\u00a0+0.95 \u2013\u00a0+1.05\u00a0V versus RHE as it is a non-Faradaic region for most of the studied samples as well as for the NF support. An exception was made for Cu\u2013O, which shows a Faradaic process in this region and therefore the double-layer capacitance was measured in the range\u00a0+0.66 to +0.76\u00a0V versus RHE for this sample. The difference between the anodic and cathodic charging currents \u0394j was obtained from CV scans at different scan rates (from 20 to 600\u00a0mV s\u22121). The double-layer capacitance is given by \u0394j/2\u00a0= v\u00b7CDL where v is the scan rate. ECSAs could theoretically be obtained by using the relation ECSA\u00a0= C\nDL/C\nS where C\nS is the specific capacitance of the sample, which corresponds to the capacitance of an atomically smooth planar surface of the same material per unit area under identical electrolyte conditions. However, it is impossible to determine reliable values of C\nS for each sample. Note that in previous studies, for determination of ECSAs of various OER catalysts, the same value of C\nS\u00a0= 0.040 mF cm\n\u22122 in 1\u00a0M NaOH, based on typical reported values for metallic surfaces, was applied.\n27\n\n,\n\n28\n However, one should be aware that this only gives an estimation of the ECSAs given that the C\nS value varies significantly from one material to another.\n27\n\nThe charge passed during the pre-OER oxidative process was evaluated. Each sample was maintained at a reducing potential (+0.4\u00a0V versus RHE) for 20\u00a0min in order to start from a fully reduced catalyst. LSV scans were recorded from\u00a0+0.40 to\u00a0+1.62\u00a0V versus RHE, with scan rates ranging from 1 to 100\u00a0mV/s. Between each scan rate, the sample was kept at\u00a0+0.4\u00a0V for 5\u00a0min. An exponential background was subtracted from the i\u00a0= f(E) curve in order to deconvolute the contributions from the pre-OER redox process and the OER catalysis, which often occur in similar ranges of potential. The charge Q passed during the pre-OER process was calculated from the integration of the oxidative wave: Q\u00a0= (1/v)\u00b7\u222bi(E)dE with\u00a0v the scan rate in V/s. The charge value was taken in a range where Q is independent from the scan rate.Each catalyst deposited on NF with a 1\u00a0cm2 geometric area was carefully removed from the support. Note that this operation was not possible in the case of NiFe-OOH and CoV-O. The collected powders were dissolved in 65% HNO3 at room temperature for 10\u00a0days in Teflon tubes. The solutions were diluted with 2% HNO3 and analyzed by using inductively coupled plasma-mass spectroscopy (ICP-MS). The quantity of each metal in each catalyst was calculated. The total number of moles of metals n\nM is the sum of the number of moles of each metal contained in the catalyst. For instance, in the case of NiMoFe-O, n\nM\u00a0= n\nNi\u00a0+ n\nMo\u00a0+ n\nFe. This assumes that all the metals are potentially OER active, which is only a rough estimation as the exact nature/composition of the active sites is unknown.Stability measurements were performed in a 2-electrode electrochemical flow cell FLC-Standard purchased from Sphere Energy (Figure\u00a0S1). The potential was measured by using a leak free Ag/AgCl/KCl3.4 M micro reference (Innovative instruments). The gas produced in the anodic compartment was analyzed by gas chromatography every 30\u00a0min by using a SRI 8610C gas chromatograph equipped with a packed Molecular Sieve 5\u00a0\u00c5 column for permanent separation. Argon (Linde 5.0) was used as carrier gas, the flow rate was regulated by using a mass flow controller (Bronkhorst). A thermal conductivity detector (TCD) was used to quantify O2. The FEO2\n was calculated by dividing the measured amount of oxygen by the theoretical amount of O2 expected: FEO2\n\u00a0= nO2,measured/nO2,expected\u00a0= nO2,measured\u00b74F/Q where nO2,measured and nO2,expected are the measured and expected amounts of O2, Q the charge passed, and F the Faraday constant. An aliquot of anolyte was collected every hour. The aliquots collected were analyzed via inductively coupled plasma-mass spectrometry (ICP-MS).This work was supported financially by funding from TOTAL S.A. Parts of this work were supported by Institut de Physique du Globe de Paris (IPGP) multidisciplinary program PARI and by Paris\u2013IdF region SESAME grant number 12015908.Conceptualization A.P. and M.F.; methodology A.P., C.E.C., D.K., H.N.T., and M.F.; investigation A.P., C.E.C., and D.K.; writing \u2013 original draft A.P. and M.F.; writing \u2013 review & editing A.P., C.E.C., D.K., H.N.T., M.S., and M.F.; funding acquisition, M.S. and M.F.; supervision C.E.C. and M.F.A European patent has been submitted in relation to this work (EP21315007.1).Supplemental information can be found online at https://doi.org/10.1016/j.joule.2021.03.022.\n\n\nDocument S1. Supplemental experimental procedures, Figures S1\u2013S38, Table S1, and Supplemental references\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  Active and inexpensive oxygen evolution reaction (OER) electrocatalysts are needed for energy-efficient electrolysis applications. Objective comparison between OER catalysts has been blurred by the use of different supports and methods to evaluate performance. Here, we selected nine highly active transition-metal-based catalysts and described their synthesis, using a porous nickel foam and a new Ni-based dendritic material as the supports. We designed a standardized protocol to characterize and compare the catalysts in terms of structure, activity, density of active sites, and stability. NiFeSe- and CoFeSe-derived oxides showed the highest activities on our dendritic support, with low overpotentials of \u03b7\n                     100 \u2248 247\u00a0mV at 100 mA cm\u20132 in 1\u00a0M KOH. Stability evaluation showed no surface leaching for 8\u00a0h of electrolysis. This work highlights the most active anode materials and provides an easy way to increase the geometric current density of a catalyst by tuning the porosity of its support.\n               "}
{"full_text": "The constantly rising worldwide energy requirement with environmental alarms has become reason to search the clean and renewable energy to alternate outdated non-renewable fossil fuels [1,2]. The solar and wind energy sources may be transformed into electrical energy and are regarded as alternating and unpredictable founded over natural weather. All-weather exploitation requires dominant significance for efficient energy storage as chemical energies [3]. Because of null carbon footprint release, great gravimetric energy density, and recyclable advantages, green hydrogen is often considered a talented energy vector for viable energy systems for future research. Due to the availability of ample water resources, electrocatalysis of water-splitting may alter the produced electricity as hydrogen storage with great purity in insignificant situations is an attractive and accessible energy conversion approach. It is sustainable and elevated for overall energy productivity, thereby is still more excessive than outdated thermocatalytic reactions originated from the techno-economic analysis [4\u20139].The effectual photocatalytic materials exhibit extended solar response, ample catalytic sites, and proper band alignment positions. The assortment of a suitable photocatalytic candidate is imperative in developing an innovative photocatalytic system [10,11]. In the past, the researchers employed two measures to investigate a suitable band structure of the photocatalysts. The first measure comprises the fabrication of micro-heterostructures via elemental doping and heterojunction construction [12,13]. The second measure involves developing macroscopic structures with numerous dimensions like hierarchical, 0D, 1D, 2D, and 3D structures [14,15]. Heterojunction, which comprises an interface between two photocatalytic materials, can hinder the unification of charge carriers by supporting their transportation to various constituents [16]. 2D heterostructures with abundant unique features possess more benefits than bulk materials, accelerating photocatalytic efficiency. 2D architectures demonstrate atomic geometry for thickness and the highest surface-to-volume ratio compared to alternative dimensional nanomaterials [17\u201319]. Commonly, a monolayer of 2D materials composes of atomically thick, covalent bonds of lattice, which in turn contain dangling-bond of free nanosheets, may show strange electronic and optical merits. Moreover, van der Waals (vdW) forces are often identified between nearby layers in 2D layered-structures [20]. Due to the lack of directly chemical bond proficiency, the gathering for 2D derivatives of vdW architectures may be regarded as out of restrictions from lattice matching [19].Heterostructures, commonly composed of various components associated with significant interfaces [21\u201324], are widely analyzed to avoid the difficulty endowed with the hybrid containing sole functioning with exotic properties [25] in the form of tunneling and confinement effects [26]. A strong approach to accelerate the HER activity is suggested to produce suitable heterogeneous interfacial contacts in tailoring the adsorption/desorption energy for critical reactions to promote the kinetics of chemical reactions [27]. Certainly, as a good platform, 2D-materials are suggested to be illustrated by graphene, as they belong to larger surface area and have higher electron transfer capability with amusing active sites, thereby leveraging towards electrochemical water splitting process [28]. However, in utmost 2D layers, the non-defective basal planes become inactive during catalytic processes [29\u201332]. Thus, the rational architecture of 2D heterostructures, as well as deuterogenic heterogeneous contacts, may create newly generations to optimize the electrochemical reactions with the adjustment of electron density having higher density of active sites, and in turn furnish an interfacial-created electric field, creation of a strong synergism to promote the kinetics attributed to surface catalysis [33].Very recently, effective progress has been made in build-up 2D heterostructures for electrocatalytic water-splitting [34\u201336]. Numerous reviews focusing the combinatorial approaches for 2D heterostructures systems as the building blocks of hybridized nanoparticles, nanorods, and nanocubes, respectively, which may be classified as 0D\u20132D [37], 1D\u20132D [38], and 3D\u20132D [39] depending on several dimensions [40\u201342]. Among 2D-based heterostructures, 2D/2D structure is the most effectual due to exhibiting high surface area and low transmission resistance [16,43\u201347]. However, essential progress significantly acknowledged in recent literature attributing to 2D vertical- and lateral-heterostructures are still extensive to be realized for electrocatalytic processes. Hence, an extensive study emphasizes 2D manifold heterostructures like stacking, lateral, vertical, and core-shell architectures. The electrocatalytic water-splitting implementations are suggested as important facilitation for researchers to be better comprehension in recent studies for this field.The current paper lies in its comprehensive analysis of the latest advancements in the field of catalyst design; offers insights into the rational design of 2D heterostructured materials for efficient and sustainable water splitting. It also provides a critical review of the various techniques employed in synthesizing and characterizing these materials, highlighting the advantages and limitations of each approach. The paper further explores the mechanisms underlying the improved performance of 2D heterostructured photo- and electro-catalysts for HER, providing a valuable resource for researchers in this field. Overall, the paper offers a holistic approach to designing and optimizing 2D heterostructured photo- and electro-catalysts for HER, potentially advancing the development of clean and renewable energy sources. The focus on rational design provides a novel approach to HER electrocatalysis that has the potential to accelerate the development of efficient and sustainable hydrogen production technologies.Photocatalysis and electrocatalysis are two different processes that involve using a catalyst to speed up a chemical reaction.Photocatalysis is a process in which a catalyst absorbs light energy and uses it to initiate a chemical reaction. The process typically involves a semiconductor material such as titanium dioxide, which absorbs light energy and generates excited electrons and holes. These electrons and holes can then react with water and other molecules to form reactive species, such as hydroxyl radicals, which can be used to degrade pollutants or produce hydrogen gas [48]. The overall reaction can be summarized as:\n\nLight energy\u00a0+\u00a0Catalyst\u00a0\u2192\u00a0Excited electrons and holes\n\n\nExcited electrons and holes\u00a0+\u00a0Reactants\u00a0\u2192\u00a0Products\u00a0+\u00a0Reactive species\n\n\nLight energy\u00a0+\u00a0Catalyst\u00a0\u2192\u00a0Excited electrons and holesExcited electrons and holes\u00a0+\u00a0Reactants\u00a0\u2192\u00a0Products\u00a0+\u00a0Reactive speciesElectrocatalysis, on the other hand, involves using a catalyst to facilitate an electrochemical reaction. The catalyst typically speeds up the reaction by providing an alternate pathway for the reaction to occur with lower energy barriers. For example, in water electrolysis, to produce hydrogen gas, a catalyst such as platinum can be used to speed up the reaction at the electrodes [49]. The overall reaction can be summarized as follows:\n\nWater\u00a0+\u00a0Electrical energy\u00a0+\u00a0Catalyst\u00a0\u2192\u00a0Hydrogen gas\u00a0+\u00a0Oxygen gas\n\n\nWater\u00a0+\u00a0Electrical energy\u00a0+\u00a0Catalyst\u00a0\u2192\u00a0Hydrogen gas\u00a0+\u00a0Oxygen gasIn both photocatalysis and electrocatalysis, the catalyst plays a crucial role in facilitating the reaction and increasing the efficiency of the process.The chemical vapor deposition approach (CVD) is gaining considerable interest as an effectual technology that may be employed to prepare high-quality, low-defect transition-metal dichalcogenides (TMDCs) nanostructures or growth of thin films proceeding for numerous substrates [50\u201352]. Developments in vapor-phase deposition comprise the adjustment of precursors ratio. Hence final product components may be easily regulated, and well-contacted nanostructure-substrate interfaces are favorable for charge migration. The exploration of the HER mechanism may also be executed by high-quality catalysts produced by the CVD method. The metal starting materials, comprising metal films [53,54], metal oxides [55,56], metal halides [57,58], or metalorganics [59], are deposited in the furnace in the middle of the heating condition. In contrast, Te, Se, or S powder is sited upstream for directional flow of gas carrier. This is a standard CVD synthesis technique (generally Ar or N2 combined with a particular concentration of H2). The vapor of sulfur, selenium, or tetraethyl will move downstream by increasing T, causing the metal precursor to being converted into the TMDCs corresponding to the high T zone (Fig.\u00a01a\n\u2013c) [50]. Current research reported that 2D monolayer heterostructures could be synthesized by reversing the carrier gas flow direction [60]. Generally, the lateral heterostructures comprising two distinctive TMDCs monolayers are delicate, and it may be difficult to survive the multistep development. Zhang et\u00a0al. suggested an advanced step-by-step synthesis method for preparing different 2D TMDCs (in-plane) micro heterostructures. This technique encompasses switching the direction in which the gas is flowing.The monolayer TMDCs nanosheet needed to be produced on the substrate using the CVD technique before implementing this strategy. For the sequential procedure, previously formed monolayer was positioned downstream of Ar movement that moved from the opposite orientation throughout T fluctuation, cooling the current single layer TMDCs substances and averting thermal deterioration. During this period, the flow of Ar in the opposite direction assisted in inhibiting an uncontrolled nucleation before sequential development stage (Fig.\u00a01d). This powerful CVD strategy may be employed to prepare a range of in-phase 2D adjacent heterostructures (i.e. WS2\u2013 WSe2, WSe2\u2212MoS2, and so on, see Fig.\u00a01d, e), in addition to numerous-heterojunctions (i.e., WS2\u2013WSe2\u2212MoS2, WS2\u2212MoSe2\u2013WSe2). As a result, the synthesis of superiority TMDCs, or heterostructures, by the CVD process compels the thoughtful regulation of various fundamental factors in which artificial T is the foremost one. Generally, a higher T will enhance crystallinity; however, the nanostructure may become unstable under extreme conditions.In conclusion, the T is the most significant characteristic that has to be carefully selected. Another contributing factor is the distance between the source of the chalcogen and the metal. Furthermore, if CVD apparatus contained two independent heat sources, it would not be possible to modify the distance. Ar or N gas carrier flow should be attuned to a reasonable rate to place TMDCs nanosheets on the substrates. It is mandatory to calibrate the T gradients inside the tube beforehand if there is just one heating source to categorize where the reactants should be placed. In this specific situation, carrier gas flow and distance should be regulated at once. Research on graphic layout of CVD tools has a long history and is currently resurgent with prospects of precise distance among substrates and reactants. This makes it challenging to duplicate experimental findings because of minor changes in CVD set up.CVD has several advantages over other synthesis methods for producing 2D heterostructures. One significant advantage is the ability to precisely control the growth process, including the thickness and composition of the films, as well as the orientation and alignment of the layers. This control allows for producing high-quality films with well-defined interfaces, which is critical for the performance of heterostructures. Additionally, CVD can be easily scaled up for large-scale production, making it an attractive option for commercial applications. CVD can also synthesize a wide range of materials, including those that are difficult to synthesize using other methods. Moreover, CVD enables the synthesis of complex heterostructures with unique properties, such as tunable bandgaps, magnetic properties, and electrical conductivity. Overall, the advantages of CVD make it a valuable method for synthesizing 2D heterostructures with tailored properties and potential applications in various fields.The un-exfoliated TMDCs, like MoS2\n[61], MoSe2\n[62,63], and WS2\n[64], are semiconducting materials comprising of 2H phase. Most of the unpredicted catalytic active sites were explored in these substances, which significantly confines their catalytic activity. By exfoliating the bulk TMDCs, layered TMDCs nanosheets can be prepared with a greater surface area and active sites [65,66]. Mechanical exfoliation is a direct technique that may be employed to synthesize single-layer TMDC nanosheets. The synthetic procedure of mechanical exfoliation is correlated to the fabrication of 2D graphene [67]. This method produces single- or multiple-layer TMDC nanosheets at yield rates that are too low for electrocatalysis, though appropriate for device fabrication or mechanism research. Li insertion has been accessed as an efficacious technique for attaining layered TMDCs materials substantially. As a result, it can accomplish the necessities of practical catalytic applications. However, Li insertion decreases the layer number of bulk TMDCs and tunes their crystal structure; thus, it causes to improve catalytic progress headed for HER. Further, three different lithiation techniques can often be utilized to exfoliate large TMDCs substance. Among them, the preliminary technique is chemical exfoliation procedure, which uses organolithium compounds like butylLi (BuLi) [68\u201370], MeLi [71], or LiBH4\n[72] (Fig.\u00a02a\n-c). This strategy includes the saturation of TMDCs powder in the solution that enclosed the Li bases and the associated organic solvent. At the same time, continuous ultra-sonication would be applied to the mixture for an extended period (normally more than two days), which would enhance the efficiency of the exfoliation procedure. However, investigations are required to obtain a large quality yield of single-layer nanosheets and better control over the Li insertion process. A large-quality yield electrochemical Li inclusion technique was explored to prepare 2D one-layer nanomaterials using a regulated lithiation process to address the preceding restrictions [73\u201375].The noteworthy discrepancy lies in the fact that the Li inclusion was executed in a Li battery cell (Fig.\u00a02d). During the discharge procedure, large TMDCs at the cathode of a cell were progressively exfoliated to layer nanosheets when Li was introduced into the cathode from the anode. Several nanosheets containing monolayers or a few layers were recovered after washing, sonicating, and centrifuging the samples (Fig.\u00a02e-g). The yield attained via electrochemical Li insertion is considerably greater than that obtained by chemical exfoliation (normally 10\u201320%) and can reach over 90% for MoS2, TaS2, and TiS2\n[76]. Though, this technology entails certain shortcomings, like a challenging technique that entails assembling battery cells. These drawbacks make this method less desirable than others. Furthermore, additional additives that are often employed throughout the fabrication process of electrodes have the potential to introduce impurities into the goods that are eventually synthesized [18]. An innovative liquid ammonia-assisted lithiation (LAAL) technique has been designed for TMDCs exfoliation. This technique is an effective source of ultrathin 2D nanosheets [61,77]. Before beginning the LAAL technique (depicted in Fig.\u00a02h), Li metal must be encircled in a quartz tube and shielded from the atmosphere with Ar. Afterward, the tube is emptied of its contents and immersed in a bath of liquid N. Simultaneously, exceptionally pristine gaseous ammonia is injected, and it slowly transforms into a liquid state during the process. Once the powder is submerged into the liquid ammonia, its color progressively varies from blue tint to colorless due to lithiation process, hence reaction's progression is monitored. Evaporation is utilized to eliminate ammonia gas from the tube once the \u201cblue hue\u201d has been totally eradicated from the tube.Furthermore, the ultrathin 2D nanosheets may be fabricated via introducing water in Li intercalated system (Fig.\u00a02i\u2013k). The LAAL method depicted three clear advantages relative to above mentioned techniques: 1) Time required to accomplish this process will be less, normally falling within an hour. Furthermore, a notable shift made it possible to intuitively estimate the response process without any additional signal; 2) 1T phase TMDCs nanosheets exhibiting a single layer or a few layers gained with a large quality yield (\u223c 82%); 3) The intense lithiation technique will result in plenty of Sulphur vacancies (S-vacancies) and large amount of edges, both of which will enhance the electrochemical efficacy of exfoliated TMDCs and nanosheets. Due to strong reaction that takes place when water and metal Li encounter one another, as well as the usage of liquid ammonia, it is significant that each stage of the process be executed with care to assure the user's wellbeing. It is possible to create a thinner sheet of TMDCs material by exfoliating the bulk substances. As a result, exfoliation of the bulk materials may be employed. Due to complex reaction procedure in liquid environment, it is difficult to prepare TMDCs nanosheets with preferred features, such as definite structure and layer quantity, making electrocatalysis challenging to explore.Exfoliation is another popular method for synthesizing 2D heterostructures with several advantages. One of the main advantages is that exfoliation is a simple and cost-effective method that can be performed using standard laboratory equipment. This method involves the mechanical or chemical exfoliation of bulk materials to obtain thin 2D sheets with unique properties that can be stacked to create heterostructures. Additionally, exfoliation can produce a wide range of 2D materials, including those that are difficult to synthesize using other methods, and the resulting heterostructures have sharp and well-defined interfaces. Exfoliated 2D heterostructures have shown promising properties for applications in catalysis. Furthermore, the ability to combine different 2D materials through exfoliation opens new possibilities for the synthesis of novel heterostructures with unique properties. Therefore, exfoliation is a valuable method for synthesizing 2D heterostructures with tailored properties and potential applications in various fields.Hydro/solvothermal is an efficient and applicable technique to prepare TMDCs-based nanomaterials at a large scale. Materials with various structures and phases may be prepared by tuning T, reaction duration, metal precursors, surfactants, and other factors, making hydro/solvothermal an appropriate technique to prepare nanostructured materials [78\u201380]. Currently, MoSe2 nanosheets prepared via hydrothermal method are an effective HER catalytic material. By varying the reaction T, combined with the quotient of NaMoO4\u20222H2O and Se starting materials to the reductant (NaBH4), the products revealed dissimilar crystal structure and disordered degree (Fig.\u00a03a\n) [62]. It showed that an increased concentration of NaBH4 would raise 1T MoSe2 ratio, which displayed enhanced HER efficiency (Fig.\u00a03b). Conversely, slower reaction T carried the high active sites, though went beside the 1T surface creation. The MoSe2 nanosheets with excellent HER catalytic performance was generated by cautiously controlling the disordered degree and 1T surface ratio. Though hydrothermal technique was commonly employed to prepare nanostructured materials that might easily be oxidized during synthesis, the process could affect the product's purity. Manufacturing TMDCs containing nanomaterials also utilizes the solvothermal technique to avert potential oxidation [81,82]. The selectivity of the final products is a prominent advantage of solvothermal reactions. For instance, 2H WS2 would be generated by carefully adding hexamethyl disilazane, and 1T WS2 would finally be fabricated (Fig.\u00a03d) [83]. The resultant products will exhibit lots of active sites. The numerous uniting forms of reactants will give advantage to the creation of heterostructured nanomaterials, like MoS2/CuS [84], MoS2/CdS [85], MoS2-graphene [86], and MoS2/CoSe2\n[87], with extra interfaces and acceptable level structures (Fig.\u00a03e-g).Hydrothermal or solvothermal synthesis is another method that has gained attention for synthesizing 2D heterostructures due to several advantages. One advantage of this method is that it can be performed at relatively low temperatures and pressures, making it a more environmentally friendly and energy-efficient method compared to other synthesis methods. Additionally, hydrothermal or solvothermal synthesis can produce high-quality 2D heterostructures with excellent crystallinity and uniformity, essential for the optimal performance of heterostructures. This method can also be used to synthesize a wide range of materials, including complex compositions and heterostructures with well-defined interfaces. Furthermore, the hydrothermal or solvothermal method allows for controlling the size, shape, and morphology of the resulting 2D heterostructures, which can impact their physical and chemical properties. Overall, the hydrothermal or solvothermal method is a promising approach for synthesizing 2D heterostructures with controlled properties and potential applications in various fields.Template transformation is one of the most effective methods to attain desired superior topological materials. The 2D heterostructures prepared using template-assisted synthetic approaches can hold a targeted size, morphology, and composition [88]. Depending on the category of templates, the template-assisted approaches can be categorized into two methods: hard-template method and soft-template method [89].The hard-template method is considered one of the most common approaches for preparing hybrids. The hard template is a rigid material that can directly evaluate the final sample's size, structure, morphology, and components [90]. For 2D heterostructure fabrication, the material/compound precursor with 2D geometry is generally chosen as the hard template. Depending on considerably various solubilities (Ksp) between metal sulfide and hydroxide, Zhang et\u00a0al. prepared the CoNi hydroxide sheets in advance as a hard template to partially reconstruct into an ultrathin CoNi hydroxysulfide shell using ethanol-modified surface sulfurization route, preparing a 2D Co1.8Ni(OH)5.6@Co1.8NiS0.4 (OH)4.8 core\u2013shell heterostructure (Fig.\u00a04a\n) [91]. As the Co1.8Ni(OH)5.6 sheets were immersed in Na2S ethanol solution under room temperature, Co1.8Ni(OH)5.6 endured an interfacial reaction with hydrolyzed HS\u00af and transformed into Co1.8Ni(OH)0.4(OH)5.6 shell. The resultant Co1.8Ni(OH)5.6@Co1.8NiS0.4 (OH)4.8 sample displayed a well-maintained hexagonal plate-like structure of the Co1.8Ni(OH)5.6 precursors (Fig.\u00a04b). Hou et\u00a0al. synthesized a confined carburization-engineered technique to prepare the unique 2D NiOx/Ni ultrathin heterostructure nanosheets (Fig.\u00a04c), which can be demonstrated as a complete hard template transformation [92]. First, ultrathin Ni(OH)2 nanosheets were used as the starting materials (Fig.\u00a04d). Later, an oxidative self-polymerization reaction of the dopamine precursor to polydopamine (PDA) was induced on the surface of Ni(OH)2, producing a core\u2013shell 2D Ni(OH)2@PDA heterostructure (Fig.\u00a04e). Ultimately, the 2D Ni(OH)2@PDA as the hard template was subject to annealing treatment in nitrogen environment to entirely convert into 2D ultrathin NiOx/Ni heterostructure nanosheets (Fig.\u00a04f,g).Wang et\u00a0al. prepared the holey 2D transition metal carbide/nitride heterostructure nanosheets (h-TMCN) using controlled thermal annealing of the Mo/Zn bimetallic imidazolate frameworks (Mo/Zn BIFs) (Fig.\u00a04h) [93]. In their work, Mo/Zn BIF precursors with 2D thin flake-like structures were fabricated using coordination and complexation of Zn2+, [MoO4]2\u00af, and 2-methylimidazole (Fig.\u00a04i). Subsequently, they were annealed in nitrogen environment at 800\u2013900\u00a0\u00b0C to assist the total pyrolysis of Mo/Zn BIF and the development of Mo2C and Mo2N nanocrystals (Fig.\u00a04j). Li et\u00a0al. prepared a 2D hierarchical Mo2C/C nanosheet hybrid by water-soluble sodium chloride cube crystal-template approach as depicted in Fig.\u00a04k\n[94]. Obtained results displayed sheet-like Mo2C that were evenly attached on the surface of carbon nanosheets (Fig.\u00a04l\u2013n). The resultant Mo2C/C hybrids revealed noteworthy HER performance in both alkaline and acid medium, associated with the strong synergistic catalytic effect and charge transfer ability.Xu et\u00a0al. employed the electrostatic assembly of positively charged \u03b2-Ni(OH)2 nanosheets and negatively charged functionalized single-layer graphene (FGR), resulting in face-to-face hybridization between Ni(OH)2 and FGR [95]. Later, the Ni(OH)2-FGR heterotemplate was conversed into Ni2P-FGR by a low-temperature phosphorization reaction (Fig.\u00a04o). TEM micrograph of Ni(OH)2-FGR showed a wrinkled sheet structure of FGR combined with ultrathin Ni(OH)2 nanosheets (Fig.\u00a04p). The TEM micrograph of Ni2P-FGR illustrated that ultrathin Ni2P nanosheets covered the FGR with wrinkles with 3.2\u00a0nm in thickness (Fig.\u00a04q). Depending on controllable experiments, the stress transfer-induced mechanism was proposed to rationalize the FGR assisted growth of ultrathin Ni2P nanosheets from Ni(OH)2 nanosheets during phosphorization (Fig.\u00a04r) [95].The soft templates comprise micelles or vesicles, macro- and microemulsions, some polymers, and biological molecular assemblies [96]. The advantages of the soft-template technique usually involve comparatively moderate experimental conditions and simple execution [97]. Combining various template approaches has also been applied to prepare 2D heterostructures. For instance, Zhuang et\u00a0al. coupled soft template with hard-template approaches to prepare targeted 2DPC-RuMo heterostructure with RuMo nanoalloy-embedded 2D porous carbon (PC) nanosheets for high-performance alkaline HER activity [98].In situ/operando characterization techniques have been employed to understand the working mechanism of these catalysts. This section will discuss these techniques and their application in studying the working mechanism of 2D heterostructured photo- and electro-catalysts for HER. In situ/operando characterization techniques allow us to monitor the catalyst's structure and activity under reaction conditions. These techniques provide information about the catalyst's surface chemistry, electronic properties, and reaction intermediates [99,100]. Some commonly used in situ/operando characterization techniques for 2D heterostructured photo- and electro-catalysts for HER:\n\n(i)\nX-ray photoelectron spectroscopy (XPS): XPS is a powerful technique for investigating catalysts' chemical composition and electronic structure. In situ, XPS can be used to study the changes in the surface chemistry of the catalyst during the reaction. This technique can also provide information about the oxidation states of the catalyst and the adsorbed species [101,102].\n\n\n(ii)\nTransmission electron microscopy (TEM): TEM can be used to study the structural changes of the catalyst during the reaction. In situ TEM can provide information about the catalyst's morphology, size, and crystal structure under reaction conditions. This technique can also be used to study the formation and evolution of reaction intermediates [101].\n\n\n(iii)\nFourier-transform infrared spectroscopy (FTIR): FTIR can study the adsorption and desorption of gasses on the catalyst surface. In situ, FTIR can provide information about the surface species and reaction intermediates formed during the reaction [103].\n\n\n(iv)\nRaman spectroscopy: Raman spectroscopy can be used to study the chemical and structural changes of the catalyst during the reaction. In situ Raman spectroscopy can provide information about the formation and evolution of reaction intermediates [104].\n\n\nX-ray photoelectron spectroscopy (XPS): XPS is a powerful technique for investigating catalysts' chemical composition and electronic structure. In situ, XPS can be used to study the changes in the surface chemistry of the catalyst during the reaction. This technique can also provide information about the oxidation states of the catalyst and the adsorbed species [101,102].Transmission electron microscopy (TEM): TEM can be used to study the structural changes of the catalyst during the reaction. In situ TEM can provide information about the catalyst's morphology, size, and crystal structure under reaction conditions. This technique can also be used to study the formation and evolution of reaction intermediates [101].Fourier-transform infrared spectroscopy (FTIR): FTIR can study the adsorption and desorption of gasses on the catalyst surface. In situ, FTIR can provide information about the surface species and reaction intermediates formed during the reaction [103].Raman spectroscopy: Raman spectroscopy can be used to study the chemical and structural changes of the catalyst during the reaction. In situ Raman spectroscopy can provide information about the formation and evolution of reaction intermediates [104].The working mechanism of 2D heterostructured photo- and electro-catalysts for HER can be understood by studying the changes in the surface chemistry, electronic properties, and reaction intermediates during the reaction. In situ/operando characterization techniques can provide valuable insights into the working mechanism of these catalysts. For example, in situ XPS can be used to study the changes in the oxidation states of the catalyst and the adsorbed species during the reaction. In situ TEM can be used to study the formation and evolution of reaction intermediates and the structural changes of the catalyst. In situ, FTIR and Raman spectroscopy can study the surface species and reaction intermediates formed during the reaction.The electronic structure is a single aspect that regulates heterogeneous catalysts' adsorption capabilities in all intermediates categories [105\u2013108]. Therefore, modifying the electrical catalyst band is an ideal strategy to influence its catalytic progress. Such as, disrupting electronic arrangement of a catalyst commonly intends to enhance its hydrogen binding energy (HBE) for alkaline HER, comparable to the approaches employed in acid conditions.Hydrogen adsorptive hydrogen bonds are reinforced when the d-band center of metallic catalytic substance shifts closer to Fermi energy, and vice versa when the center of D-band transfers away from Fermi level [108\u2013110]. The most effectual technique for creating the D-band vacancy of a metallic catalyst is to alloy one metal with another. The HBE of most metallic catalysts can be changed by alloying with another metal, as depicted in Fig.\u00a05a\n, generally due to mass electron shifting among two separate metal sites [111,112]. In certain belongings, this kind of substantial electron movement can offer an electronic structure of non-metallic sites that work with metal alloys to improve whole catalytic activity. A succession of RuCo alloys enclosed in an N-doped graphene sheet is one such example [113]. The alkaline HER is catalyzed by very active meal Ru, while stability and efficiency of RuCoC alloy are considerably higher (Fig.\u00a05b). Additionally, DFT studies demonstrate higher activity is strongly associated with the unusual electron arrangement of carbon shell. Because of alloying of RuCo, the variation of electron transport from atom to carbon shell happened (Fig.\u00a05c). Consequently, the CH bond of carbon heightened, causing to decrease HBE of catalyst; therefore, enhancing entire efficiency. Alike findings were attained aimed at numerous alloy catalysts. Relative to Pt/C, a PtNi/NiS nanowire prepared by Huang et\u00a0al. showed 5.58 times advanced current density values at \u22120.07\u00a0V vs RHE [114]. Zheng et\u00a0al. developed PtNi alloy in the form of hexagonally packed nano-multipods which displayed considerably greater HER activity than Pt/C [115]. Remarkably, alloying causes to trigger of non-noble metal substances, which ordinarily have reduced HER activity by modifying the electronic structure [116\u2013118]. Furthermore, exceptional HER progress was revealed by Chen et\u00a0al. for Cu-Ti bimetallic electrocatalysts [117]. The electronic arrangement of CuTi was attuned by modifying Ti's content in the catalyst. Meanwhile, HBE of catalytic material remained modified to a suitable level by placing them at uppermost mark of volcano plot. The Cu-Ti catalyst showed incredible activity, even though neither Cu nor Ti is a favorable HER candidate [117]. A comparable development was presented for MoNi4 alloy [119]. MoNi4 alloy shows an enhanced electronic structure that stemmed into surprising water dissociation aptitude because of rebuilt electronic arrangement of alloy, compared to Mo, Ni, and MoO2, which show relatively slow alkaline HER kinetics. Such characteristics of MoNi4 assured their significant HER efficiency using alkaline conditions.2D nanomaterials have attained considerable attention because of their extraordinary properties relative to bulk counterparts. Numerous 2D nanomaterials have been prepared to date, but specifically, TMDCs and graphene have attracted significant interest owing to their exceptional properties [120\u2013122].\u00a0Earth-rich and reasonably priced TMDCs, phosphides chalcogenides, and nitrides exhibiting numerous structures have been synthesized in large quantities during the past few decades due to fast-evolving fabrication methods [65,123,124]. Various TM-based materials were perceived to reveal exceptionally high interaction with H by precise electronic morphological engineering [123,125].Carbon-doped MoS2 (C-MoS2), which may be developed by sulfurization of Mo2C has substantial alkaline HER efficiency that is relatively comparable to Pt/C (Fig.\u00a06a\n) [126]. The materials' electrical structure is tuned by including MoS2 with carbon, which results in exceptional activity. The carbon in the catalyst did not function as a dual active site; instead, it induced vacant 2p orbitals perpendicular to the MoS2 basal plane. This generated an environment that was beneficial for the adsorption of water (Fig.\u00a06b). Therefore, the catalyst shows an improved inclusive performance in addition to an enhanced rate of water dissociation while operating in alkaline settings. Defect engineering and incorporation of heteroatom are considered effective methods for carbon-possessing catalysts [106,127\u2013129]. Further, defect engineering and the incorporation of heteroatom can be directly utilized to increase valence orbital energy for carbon sites, which eventually results in an enhancement of interaction among H* and carbon intermediates. It has been proven especially for alkaline HER. Though, the efficacy of carbon-based materials is often enhanced when they are combined with extra catalysts rather than functioning as an only catalytic phase in HER using alkaline conditions [130,131]. Because of this, a comprehensive examination of these resources will not be delivered during this assessment. However, modifying the charge density of the catalytic surface is another route commonly utilized to increase the natural capacity of transition metals to absorb electrons. Conjoining catalysts based on transition metals with other substances is a significant method normally used to attain this objective. Zou et\u00a0al. conducted a study on regulating the charge shifting of Mo2C-based HER catalytic materials [132]. By coating Mo2C with N-rich carbon, superior progress for catalytic HER was acquired for entire pH scales (Fig.\u00a06c). Aforementioned effect is recognized as lonely effect for enhanced H adsorption capability of catalyst was accomplished only by covering Mo2C with N-rich carbon.Later, studies revealed that substantial electron-extracting capacity of N sites created an electron shifting that drew electrons from Mo2C to N via C sites. This transfer happened because Mo2C sites could attract electrons. This mechanism activated the C atoms and N sites, which stemmed into the production of HER-active C sites responsible for working in conjunction with Mo2C (Fig.\u00a06d). Hence, the total activity of HER might be improved in all situations irrespective of pH. Similar strategies were also designated for other basic HER catalytic substances. These approaches are effective for altering H adsorption proficiency of the catalysts and improving interaction between the catalyst and water (OH) [133\u2013135]. The MoP@C catalyst is an excellent depiction of a distinctive case since it showed exceptional HER efficiency in a basic environment (Fig.\u00a06e) [135]. In catalyst, development of Mo-C bond was caused by the presence of carbon on MoP surface, which in turn had a substantial influence on electronic arrangement of molybdenum. In conclusion, the recently upgraded Mo site is now appropriate for the dissociation of water. In contrast, the adjacent P sites were accountable for the recombination of hydrogen (Fig.\u00a06f). It was revealed that such a structure delivered an appropriate platform for the processing of alkaline HER. Its forthright construction, which may speed up two processes simultaneously, makes it the most operative approach for generating high-quality HER activity.Though the part that OH\u2013 plays in alkaline HER has not been computed so far, it is apparent that the interaction between catalyst and OH\u2013 is one of the most momentous features in evaluating the catalyst's activity. On the other hand, it is highly thought-provoking to attain stability among desorption and adsorption abilities headed for H* and OH* on a single site because of faintly understood scaling connection among H* and OH* intermediates binding energy. Fabricating dual active sites responsible for individually presenting exceptional functionalities is one of the most broadly acknowledged routes for gaining control over OH* and H* in a distinctive way. A sequence of catalysts was revealed by Markovic et\u00a0al. via decorating bare Pt with metal/metal hydroxide sites [136\u2013138]. This was accomplished according to perception described above. Fabrication of a hybrid material is predominant aim of having standard HBE values for HER via espousing a material over top of an acidic HER volcano plot. It is just like Pt with an oxophilic metal that offers adequate OH interaction sites in place of water dissociation. Fig.\u00a07a\n shows a comparison of oxophilicity of several regularly used materials; a larger oxophilicity anticipates a stronger OH binding energy, and vice versa [139]. For HBE, the OH binding energy of a catalyst must be moderate to generate an appropriate interaction between the catalyst and the water. Consequently, a few choices are notable for their sufficient oxophilicity, such as Ni, Ru, and Co. These materials reveal extraordinary activity; hence, they are employed widely in developing catalysts with dual active sites [130,140,141].Based on the achievements of the alloys with dual sites, in preference to Pt the non-noble materials have been used as H* interaction sites to design worthwhile catalysts. In HER technique, it has been verified that MoS2 and g-C3N4 can absorb hydrogen completely, like Pt. Yang et\u00a0al. investigated that MoS2 can be used as the layered double hydroxide (LDH) and H-active material as OH-active sites for excellent HER efficiency (Fig.\u00a07b) [142]. DFT calculations reported that MoS2/LDH heterostructures exhibit excellent progress that may be associated with decreased activation energy (Fig.\u00a07c) which is the consequence of water dissociation procedure. MoS2/LDH system suggests separate sites for dissociation of water (on LDH) and hydrogen adsorption (on MoS2), similar to the Pt/Ni(OH)2 system. These sites correspond to the overall alkaline HER procedure by acting as synergists (Fig.\u00a07d) [142].Thickness and the lateral dimension of 2D materials exhibit an abundant impact on electrocatalytic aptitudes of these resources [143]. (1) When the lateral dimension of 2D materials is lowered, supplementary defects and edge locations appear; (2) the width of 2D materials may be tuned to modify their electrical structures, which results in amendable catalytic activity; (3) catalytic progress augmentation might rise from deficiencies (in-plane) produced through structural disorder when the thickness is abridged to the atomic level; (4) monolayered 2D materials have the highest hypothetical surface area, which provides maximum prospective for catalytic activity. The control of morphological aspects of MoS2 was offered by Kibsgaard et\u00a0al., nanoscale that reveals these effects may alter the surface structure at nanoscale, enhancing HER efficiency (Fig.\u00a08\n) [144]. The lattice modification in many 2D materials provides enormous vacancies for catalytic progress when thicknesses are abridged to sub-nanometer level. Thickness management also enables using 2D morphologies to reveal a catalyst's maximum area and generate large electrochemical surface area (ECSA). Moreover, the development of freestanding Pd nanosheets was offered by Huang et\u00a0al. for catalytic and plasmonic activities [145]. The electrocatalytic oxidation process of formic acid involves superior ECSA for Pd nanosheets, which stemmed into higher density (than commercial Pd black). Consequently, a number of approaches have been designed to increase the ECSA of metal and metal alloy nanosheets (for instance, Pt\u2212Cu, Rh, and Pd\u2212Cu) [146,147].Developing a Z-scheme heterojunction may increase the photogenerated carriers' split-up and retain a photocatalytic reaction's high redox capacity. These aspects have considerable benefits for Z-scheme heterojunction. The development of 2D/2D stacked heterojunction causes prolonged contact area among two semiconductors, increases the number of charge migration channels, and decreases the charge transfer distance, stimulating the migration and split-up of photogenerated exciton pairs. In the meantime, the higher specific surface area of these heterojunctions can give additional reactive sites, which is beneficial to reactant adsorption. Prominently, developing a 2D/2D heterojunction (Z-scheme) may completely integrate and exploit previously described benefits of Z-scheme and 2D/2D heterojunction. Furthermore, the 2D/2D heterojunction (Z-scheme) photocatalysts can be used in various photocatalytic processes. This section discusses photocatalytic H2 generation by 2D/2D heterojunction (Z-scheme) [148].Hydrogen has been proposed as an excellent energy vector to replace existing fossil fuels due to its high energy density (237.2 kJmol\u22121), pure combustion product (H2O), and long-term sustainability [149,150]. Among the different strategies for producing H2, sunlight-driven water splitting is a promising option since it can turn unlimited solar energy into chemical energy [151,152]. The photocatalyst must satisfy the necessary thermodynamic conditions for overall water splitting, which are that the CB (conduction band) position be more negative than the reduction potential of H\n+/H2 (0\u00a0V vs NHE at pH\u00a0=\u00a00) and the VB (valance band) position be more positive than the oxidation potential of O2/H2O (1.23\u00a0V vs NHE at pH\u00a0=\u00a00). However, most photocatalytic H2 production reactions were carried out with sacrificial agents (lactic acid, methanol, triethanolamine, Na2S, Na2SO3, and so on) to install the problematic water oxidation reaction with the hole-sacrificial agent reaction, which raises the price of H2 production. Many semiconductor photocatalysts have been created and used in photocatalytic H2 generation up to this point. However, their activities need to be enhanced since they fall far short of the desired value of solar-to-hydrogen efficiency (STH) of 20% [153\u2013155]. A promising H2 generation photocatalyst should have enough redox ability for water splitting, broad and strong light absorption, and effective separation of photogenerated carriers, according to the preceding description of the photocatalysis mechanism (Fig.\u00a09\n). The 2D/2D Z-scheme heterojunction can theoretically fulfill all of these requirements. Zhu et\u00a0al. created a 2D/2D black phosphorus (BP)/BiVO4 Z-scheme heterojunction and achieved total water splitting for H2 generation [156]. BP nanosheets were created by ultrasonic exfoliation of bulk BP in an N-methyl-2-pyrrolidone solution, whereas BiVO4 nanosheets were created using a hydrothermal technique with sodium dodecylbenzene sulfonate as a template. Electrostatic interactions easily hybridized the thin BP and BiVO4 nanosheets to generate a 2D/2D BP/BiVO4 heterojunction. Dark-colored BiVO4 nanosheets were constructed on the surface of BP nanosheets, as illustrated in Fig.\u00a09a, b. The closely connected BP ((040), d-spacing of 0.26\u00a0nm) and BiVO4 ((\u2212121), D-spacing of 0.31\u00a0nm) heterojunction is visible in HR-TEM images (Fig.\u00a09c, d). In all photocatalytic H2 and O2 generation processes, the 2D/2D BP/BiVO4 heterojunction outperformed BP and BiVO4 alone, with or without sacrificial agents (Fig.\u00a09e, F).The increased photocatalytic H2 and O2 generation activity showed that the charge transfer mechanism was Z-scheme; the authors confirmed the Z-scheme charge transfer process using femtosecond Time-domain reflectometry spectroscopic analysis. Photogenerated electrons in the CB of BiVO4 were swiftly transported to the VB of BP and merged with its photogenerated holes via the intimately connected 2D/2D interface when exposed to visible light (\u03bb > 420\u00a0nm). Consequently, electrons in the CB of BP and holes in the VB of BiVO4 with sufficient redox capacity could achieve total water splitting with H2 and O2 production rates of 53.34 and 34 \u03bcmol g\u22121 h\u22121, respectively, with an apparent quantum efficiency of 0.89% at 420\u00a0nm (Fig.\u00a09g). With the assistance of a co-catalyst Co3O4, the H2 and O2 production rates were increased to 260 and 153.34 \u03bcmol g\u22121 h\u22121, respectively (Fig.\u00a09h). Neither BP nor BiVO4 can accomplish total water splitting on their own. These findings highlight the benefits and efficiency of the 2D/2D Z-scheme heterojunction in photocatalytic water splitting for H2 generation.Li et\u00a0al. created a sulfur-vacancy-confined in ZnIn2S4 (Vs-ZnIn2S4)/WO3 2D structure in the Janus bilayer Z-scheme heterojunction with higher visible light photocatalytic H2 generation [157]. Vs-ZnIn2S4 nanosheets were created using a low-temperature refluxing approach followed by a desulfurization process between lithium and ZnIn2S4. Heat treatment of liquid exfoliated WO32H2O nanosheets from bulk WO3\n\u00b72H2O produced WO3 nanosheets. The Janus bilayer Vs-ZnIn2S4/WO3 heterojunction was created by electrostatic self-assembly by including WO3 nanosheets into the Vs-ZnIn2S4 precursor, and the remaining steps were the same as for Vs-ZnIn2S4 fabrication. The authors opted to experiment with Vs-ZnIn2S4 nanosheets because introducing sulfur vacancies might considerably increase the photocatalytic H2 generation activity of ZnIn2S4 nanosheets by enhancing light usage and charge separation. Fig.\u00a010a\n, b indicates that WO3 nanosheets were closely adhered to the surface of ZnIn2S4, showing that the 2D/2D Vs-ZnIn2S4/WO3 heterojunction was successfully synthesized. Additionally, a new peak associated with WS bonds developed in the Vs-ZnIn2S4/WO3 heterojunction's Raman spectra indicated strong interfacial adhesion between the two components. Fig.\u00a010c depicts the as-prepared samples having visible light (\u03bb > 400\u00a0nm) photocatalytic H2 generation activities. Compared to Vs-ZnIn2S4, all Vs-ZnIn2S4/WO3 exhibited increased H2 production activity; however, WO3 showed no H2 production activity. The best H2 production activity of 7.81\u00a0mmol g\u22121 h\u22121 was achieved by the WO3 (10 wt%)/Vs-ZnIn2S4 composite, which was about 2.9 and 1.57 times greater than the plain Vs-ZnIn2S4 (2.68\u00a0mmol g\u22121 h\u22121) and the WO3 (10\u00a0wt.%)/ZnIn2S4 composite (4.97\u00a0mmol g\u22121 h\u22121). Furthermore, increased loading of NiS quantum dots as a co-catalyst on the WO3 (10 wt%)/Vs-ZnIn2S4 composite may boost its photocatalytic H2 generation activity, which reached 11.09\u00a0mmol g\u22121 h\u22121 at the optimal NiS (1.0 wt%) loading level. Because of the low CB position of WO3, the energy band positions of WO3 and Vs-ZnIn2S4 (Fig.\u00a010d) indicate that photogenerated carrier transfer in the WO3/Vs-ZnIn2S4 composite accompanied the Z-scheme mechanism rather than the type II mechanism; instead that, the photocatalytic H2 production activity could not be enhanced (0.06\u00a0eV). To further evaluate the Z-scheme mechanism, radical intermediate detection tests were performed. Because of their restricted redox capacity, WO3 could only produce \u2022OH (Fig.\u00a010e) and Vs-ZnIn2S4 could only produce \u2022O2 (Fig.\u00a010f) under visible light irradiation; however, both the \u2022O2 and \u2022OH could be identified with relatively strong signals in the 2D/2D Vs-ZnIn2S4/WO3 heterojunction, which strongly validated the Z-scheme charge transfer mechanism.\nAdvantages\n\nEfficient charge separation: The 2D/2D Z-Scheme heterojunction structure can facilitate efficient charge separation, which benefits HER. Electrons and holes generated in one layer can be quickly transferred to the other layer, reducing recombination and enhancing photocatalytic activity [158].\nBroad range of visible light absorption: 2D materials such as graphene, MoS2, WS2, etc., have a wide range of visible light absorption capabilities. This enables the heterojunctions to absorb more light and increase the energy available for HER.\nHigh stability: 2D materials are generally more stable than their 3D counterparts, making them ideal for photocatalytic applications [159].\nLimitations\n\nLimited active sites: The 2D/2D Z-Scheme heterojunctions can have a limited number of active sites for HER. The active sites are the locations where hydrogen evolution occurs, and if there are not enough active sites, the photocatalytic activity will be limited [159].\nDifficult to fabricate: Fabricating 2D/2D Z-Scheme heterojunctions can be challenging and require specific techniques; it can limit their widespread use in HER applications.\nLow HER efficiency (compared to some traditional Z-schemes): Despite their advantages, the HER efficiency of 2D/2D Z-Scheme heterojunctions can still be low due to the limited active sites and other factors. This means more research is needed to optimize their performance for practical applications [148].The theory of heterostructure originates from semiconductor physics. The heterostructures consist of several heterojunctions; interfaces among dissimilar components, and significantly the heterostructures are semiconductor materials in which chemical configuration changes with position. The theory of heterostructure has led the system over semiconductor physics owing to integration and intercrossing of knowledge network. Generally, heterostructures can also be defined as composite structures consisting of interfaces aimed at dissimilar solid-state materials, covering semiconductors, insulators, and conductors [160]. Heterostructures are regarded as a crucial aspect of developing catalytic advancement of 2D materials since they overcome every material's intrinsic limits and generate innovative features. In this regard, assemblage and creation of heterostructures, usually based on 0D, 1D, and 2D materials, is an effective approach to attain higher electrocatalytic improvement. During past years, advancement. Previous fundamental studies have focused on synthesizing 2D material-based heterostructures to produce advanced electrocatalysts. Hence, this section highlights the fabrication and development of 2D-heterostructures.2D materials can show significant catalytic advancement due to their remarkable features. However, these catalytic yields cannot strive with catalysts dependent on noble metals owing to higher restacking problems. Currently, vdWs heterostructures could propose novel methods to achieve the entire response of 2D materials. Two different 2D materials can be merged to produce 2D/2D heterostructures to compensate for particular weaknesses and abridged interfacial contact resistances, generating enhanced catalytic performances [161\u2013163]. Yang et\u00a0al. prepared 2D rGO (reduced graphene oxide) and WS2 heterostructure by hydrothermal method that resulted in developed HER activity due to improved charge transfer kinetics [164]. Hereafter, Tang et\u00a0al. [165] developed a vdW heterostructure comprising nitrogen and graphene-doped MoS2 by mesoporous magnesia as a template. The fabrication of porous graphene skeleton was employed by CVD and integration of Mo/S/N sources for the growth of nitrogen-doped MoS2 nanosheets over graphene skeleton (G@N-MoS2) as shown in Fig.\u00a011a\n, b. The fabrication technique of material endorsed for active regulation towards electronic and physical structures of each component as well as the hybrid material to hold stronger interfacial interactions. Furthermore, the presence of N-MoS2 was confirmed by HR-TEM having layer to layer distance of 0.62\u00a0nm as displayed in Fig.\u00a011c. Additionally, the micrographs reveal the dispersion of N-MoS2 nanosheets over graphene to prepare face-to-face vdW heterostructures as depicted in Fig.\u00a011d. The resultant heterostructure produce effectual multifunctional electrocatalytic developments due to their excellent electronic and structural features.The HER was also explored for N-MoS2 relative to pristine MoS2 in which G@N-MoS2 catalyst provides superb HER activity in acidic media having a current density of 10\u00a0mA cm\u22122 and low overpotential (243\u00a0mV) as illustrated in Fig.\u00a011e. Moreover, an onset potential (100\u00a0mV) which is higher relative to resultant counterparts in alkaline media as depicted in Fig.\u00a011f. The exceptional ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) activity (Fig.\u00a011g and h respectively) was also presented by G@N-MoS2 catalyst by alkaline media. The partial current density was explored for G@N-MoS2 catalysts that reveal current density findings were quite close to Pt/C for ORR and half-wave potential was relatively reduced relative to other catalysts that hold overpotential (20\u00a0mV) of fabricated catalyst by current density (10\u00a0mA cm\u22122) that is lower relative to Ir/C catalysts. Developments in electrocatalytic activities in this study are attributed to various factors; primarily the electronic structures of MoS2 can be proficiently controlled by nitrogen doping to provide shortened bandgap energies [166], larger spin densities [167] that stemmed from supporting interfacial charge transfer. Then, interfacial interaction between MoS2 and graphene can increase adsorption energy, and finally, the resultant 3D mesoporous structures may improve proton transport in addition to active site exposure. Regardless of valuable tri-functional progress of G@N-MoS2 catalyst, the vital routes remain undefined, and further research is mandatory. 2D/2D heterostructures can also be used as self-sustaining electrodes like 1D/2D heterostructures towards direct energy conversion systems. Duan et\u00a0al. worked on [168] advancement of flexible film by integrating porous C3N4 nanosheets (PCN) using nitrogen-doped graphene (PCN@-N-graphene) by adopting simple vacuum filtration approach.Currently, catalytic experiments endorse flexible film with porous structure (hierarchical) for higher mass transport. The layered structure of graphene and C3N4 exhibits superior interfaces to improve charge transfer. Afterward, more advancement was carried out for self-supporting PCN@-N-graphene electrode with slight onset potential (\u2212 0.008\u00a0V) closer to commercial Pt, an unusual exchange current density (0.43\u00a0mA cm\u22122), along with exceptional durability after 5000 cycles. These essential features allow this 2D/2D heterostructure to have superior flexibility, conductivity, and catalytic development as promising material for advantageous electrocatalysis applications. Voiry et\u00a0al. [169] presented the electronic coupling among gold (Au) substrate and MoS2 that causes reduced contact resistance of systems and increases the electron injection to catalyst active sites from substrate. The basal plane is usually less active for 2H MoS2 relative to 1T MoS2 towards HER due to its inferior charge transfer kinetics and poor conductivities. Hence, charge transfer facilitation is acknowledged as a practical approach to improving basal planes of 2H MoS2\n[170]. Furthermore, Au substrates have plenty of d electrons which can be caused to enhance charge transfer in 2H MoS2 by coupling between them. Hereafter, the electrons introduced from the Au substrate (towards 2H MoS2 basal plane) resulted in increasing charge transfer accompanied by adsorption of hydrogen reactants onto 2H MoS2 basal planes, thus, promoting the electrocatalytic development of 2H MoS2. Normally, this research provides innovative findings on the part of charge transport and contact resistance on catalytic advancement of 2D materials; the results of this section expose that catalytic development of 2D materials can be considerably improved by preparing 2D/2D heterostructures that provide pathways for the progress of 2D materials in electrocatalytic applications.This type of heterostructure offers several advantages, such as enhanced charge transfer, tunable bandgap, and high surface area. The enhanced charge transfer between the two materials can lead to faster reaction rates and improved performance for HER. The tunable bandgap allows for the optimization of the heterostructure's performance for HER. Moreover, the high surface area of 2D materials provides a large number of active sites for HER, further increasing their efficiency.However, some disadvantages to using 2D/2D heterostructures for HER exist. Firstly, synthesizing these heterostructures can be challenging and may require specialized equipment. Additionally, the stability of the heterostructures can be limited, and they may be prone to degradation over time. Furthermore, their performance can be sensitive to environmental conditions, such as temperature and humidity, affecting their efficiency. Finally, the toxicity of some materials fabricating 2D/2D heterostructures can pose environmental risks if not handled properly.In summary, 2D/2D heterostructures have several advantages that make them attractive for improving the efficiency of HER in electrochemical water splitting. However, their synthesis can be challenging, and their stability and performance can be sensitive to environmental conditions. Overall, further research is needed to address these challenges and fully realize the potential of 2D/2D heterostructures for HER.The preparation of 1D/2D heterostructures involves the fabrication of exceptional development of electrocatalysts that allow optimally sized pores for gas diffusion or mass transfer. Additionally, a large diversity of 1D/2D heterostructures have been designed to enhance their features and microstructures [171,172]. For instance, Li et\u00a0al. [173] prepared (carbon nanotubes) CNT/graphene (CNT/G) heterostructures by oxidation of few-walled CNTs that was employed as catalyst in ORR. The exfoliation approach was exposed to external walls of CNTs to prepare nano-sized graphene. Graphene exhibiting massive amounts of defects assist in advancement of ORR catalytic sites after annealing in ammonia. Moreover, internal walls of CNTs remained together that aids as conductors for charge transfer. Thus, CNT/G catalyst yields exceptional production for ORR experiments having half-wave potential (\u223c 0.76\u00a0V) and onset overpotential (\u223c 0.89\u00a0V). Chen et\u00a0al. [174] prepared metal-free NG (N-doped graphene) with N-doped CNT (NG-NCNT) heterostructured that comprises four-electron mechanisms for ORR experiments, which shows that entire component of the heterostructured catalyst delivers active sites to intensify electrochemical experiment. While the utilization of CNTs assists in isolating graphene layers that produce pores in its structure and result in increasing gas diffusion. Consequently, these characteristics reveal that synergistic effects can boost catalytic development of 1D/2D heterostructures. Furthermore, outstanding findings were obtained for as-prepared hydrogel relative to some transition-metal complex and noble metal oxides (i.e., IrO2) catalysts [175\u2013177]. Hence, 1D/2D heterostructures that behave as self-supporting electrodes displays exceptional properties and superb development so that they can be directly employed in energy conversion devices.These heterostructures offer several advantages, such as high efficiency, enhanced stability, faster reaction rates, flexibility in design, and low cost. Due to their unique structure and properties, 1D/2D heterostructures can facilitate the transfer of charge carriers, promote faster reaction rates, and reduce corrosion and degradation. Furthermore, their flexible design enables the optimization of their performance for HER. In some cases, 1D/2D heterostructures can be synthesized using low-cost and abundant materials, making them more attractive for large-scale applications.However, there are also some disadvantages to using 1D/2D heterostructures for HER. Firstly, the fabrication and synthesis of these structures can be complex and challenging, which limits their widespread use. Additionally, while they can enhance the stability of some materials, 1D/2D heterostructures can be unstable and prone to degradation over time. Moreover, their performance can be sensitive to environmental conditions, such as temperature and humidity, affecting their performance. Scaling up the production of 1D/2D heterostructures for large-scale industrial applications can also be difficult. Finally, some materials fabricating 1D/2D heterostructures, such as heavy metals, can be toxic and pose environmental risks if not handled properly.In summary, 1D/2D heterostructures have several advantages that make them promising candidates for enhancing the efficiency of HER in electrochemical water splitting. However, their complexity, limited stability, sensitivity to environmental conditions, lack of scalability, and potential toxicity are essential considerations that must be addressed to realize their potential fully.For electrocatalysis, the dispersion of single-metal atoms can be made over 2D materials to prepare 0D/2D heterostructures. Fei et\u00a0al. [178] prepared minor numbers of discrete cobalt atoms dispersed on NG (Co-NG) for HER electrocatalyst that display excellent electrocatalytic development for HER in conjunction with little overpotential (\u223c 170\u00a0mV) at 10\u00a0mA cm\u22122. Similarly, Cheng et\u00a0al. [179] synthesized 0D/2D heterostructures, which consist of distinct Pt atom clusters on N-doped graphene nanosheets (Pt/NGNs). The dispersion and size of Pt atom clusters were specifically measured by atomic layer deposition (ALD), as shown in Fig.\u00a012a\n. To explore the adsorption abilities of Pt atoms, both theoretical and experimental techniques were implemented that show the absorption of Pt atoms over nitrogen sites in NG. Findings showed extraordinarily higher catalytic progress accompanied by excellent HER stability obtained for Pt/NGNs catalysts relative to Co-NG electrocatalysts and considerably commercial Pt/C. TMDCs can also be hosted as supports to obtain excellent catalytic potential of 0D/2D heterostructures. For instance, Cheng et\u00a0al. [180] made a study on the preparation of Rh/MoS2 heterostructures via MoS2 nanosheets (as rapid H2-desorbing element) morphology as depicted in Fig.\u00a012b-F, and Rh nanoparticles (as strong H-adsorbing element). Resultant heterostructures displayed a slight Tafel slope (24\u00a0mV dec\u22121) and low overpotential (47\u00a0mV) at 10\u00a0mA cm\u22122, along with good stability (Fig.\u00a012g-j). The extraordinary development of catalysts was attributed to the presence of Rh atoms that depicts prompt capturing of hydronium ion since these atoms behave as strong H-adsorbing element. Moreover, the strength of MoS2/rGO catalyst was monitored for 1000 cycles which resulted in minor cathodic current losses. Large current density is another noteworthy factor in the performance of electrocatalysts which is normally unnoticed. In this regard, a research group fabricated an electrocatalyst based on 0D/2D heterostructure composed of MoS2 nanosheets and Mo2C nanoparticles (MoS2/Mo2C) via carbonization of MoS2 (in situ) as depicted in Fig.\u00a012k-m [181]. The resultant heterostructure exhibit rough surfaces and exceptionally exposed active sites at micro- and nanoscale. Further research is mandatory regarding industrial/practical applications. Normally, 0D/2D heterostructures are advantageous for electrocatalysis; but the poor long-term strength impedes practical/industrial application and further signs of progress are essential (Fig.\u00a012n-o).Due to their unique structure and properties, 0D/2D heterostructures can improve the stability of materials during HER, promote faster reaction rates, and reduce corrosion and degradation. These heterostructures offer several advantages, such as high efficiency, enhanced stability, facilitated charge transfer, versatility in design, and low cost. Moreover, their flexible design enables optimization of their performance for HER. In some cases, 0D/2D heterostructures can be synthesized using low-cost and abundant materials, making them more attractive for large-scale applications.However, there are also some disadvantages to using 0D/2D heterostructures for HER. Firstly, the fabrication and synthesis of these structures can be complex and challenging, which limits their widespread use. Additionally, while they can enhance the stability of some materials, 0D/2D heterostructures themselves can be unstable and prone to degradation over time. Moreover, their performance can be sensitive to environmental conditions, such as temperature and humidity, affecting their performance. Scaling up the production of 0D/2D heterostructures for large-scale industrial applications can also be difficult. Finally, some materials fabricating 0D/2D heterostructures, such as heavy metals, can be toxic and pose environmental risks if not handled properly.In conclusion, 0D/2D heterostructures have several advantages that make them promising candidates for enhancing HER efficiency in electrochemical water splitting. However, their complexity, limited stability, sensitivity to environmental conditions, lack of scalability, and potential toxicity are important considerations that must be addressed to realize their potential. Tables\u00a01\n and 2\n contain various 2D Heterostructures for HER performances.This brief overview discusses a variety of 2D materials and their heterostructures, including improved photo and electrocatalysis for H2 evolution reactions and the design of catalysts. Different heterostructure types, such as 2D/2D, 1D/2D, and 0D/2D, are explained regarding the water-splitting process. The author's suggestions in conformity with future problems and prospects will clear the path for readers by thoroughly addressing the fundamental features, recent advancements, and related challenges. Some perspectives are given below:While photocatalysis and electrocatalysis have shown great potential in various applications, there is still a need for further optimization and improvement of the catalysts. This includes improving the catalysts' efficiency, selectivity, stability, and cost-effectiveness.There is a need for more fundamental research to understand the mechanisms behind photocatalysis and electrocatalysis better. This includes studying the surface chemistry, reaction kinetics, and charge transfer processes involved in these reactions.The development of photocatalysis and electrocatalysis has led to new opportunities in energy conversion, environmental remediation, and chemical synthesis. However, there is still a need for more interdisciplinary collaborations between researchers from different fields to realize the potential of these catalysts fully.There is a need to explore new materials and synthesis methods for photocatalysts and electrocatalysts. This includes developing new types of materials such as metal-organic frameworks, perovskites, and 2D materials and exploring novel synthesis methods such as atomic layer deposition and plasma-enhanced CVD.The wrinkle/buckle broadly occurs in self-supporting 2D materials. Yet, relevant research on the interface's uniformity in 2D architecture has become unique and is suggested to be critical in interface architecture to modulate the catalytic efficiency. Moreover, by increment in complexity for catalyst components, thereby necessary reaction process for 2D heterostructure in the electrocatalytic mechanism requires detailed studies, particularly for the OER. Depending on in\u2009situ or operando characterization techniques like Raman, FTIR, TEM, and XANES spectroscopy are required to control the momentary engineering of reconstruction and trace out the exact active sites for the desired reactions. In this way, comprehensive reaction kinetics and thermodynamic mechanisms offer valuable support to design efficiently 2D heterostructures towards electrocatalytic reactions rationally. Further, despite DFT provision for the reaction mechanism of specific active sites for single 2D nanomaterials founded over abridged models, still is challenging to curate a decisive route towards the 2D heterostructure containing multifunctional active sites.Secondly, solar-based water-splitting strategy for effective implementation, an STH (10%) of broad commercialization standard, becomes desirable. The greatest STH transformation performance is evaluated as 30%, achieved by the InGaP/GaAs/GaInNAsSb triple-connection solar cell associated with two series-junction polymeric electrolyte-membrane-electrolyzers. Recently, numerous standard PV-EC (integrated photovoltaic electrolyzer) device systems commonly face great Ohmic resistance, massive connections, and less integrated designs, reducing competence. Additionally, depending on standard formula, the magnitude of STH becomes parallel to close relationship between chemical-energy output and solar-energy input. In addition, reducing solution resistance, accelerating the movement and stability of catalysts, and optimizing mass transfer for the electrolysis device become achievable core strategies for gaining substantial STH performance when the performance of solar cells accelerates towards theoretical value.Thirdly, 2D heterostructures show unlimited potential for electrocatalytic water-splitting systems, while application-based research is still beginning. It is believed that the knowledge of the catalytic nature of nanomaterials is associated with the aid of in\u2009situ characterizations and DFT outcomes and producing rationally prepared technologies to improve the massive efficiency and stability. Consequently, large-scale execution for electrocatalytic implementations may be realized.2D heterostructures have revealed great perspectives in water-splitting but are resurgent in application-oriented research. We assume that by considering the catalytic nature of these materials with the aid of in situ characterizations and DFT calculations and designing more rational synthesis technologies for enhancing the overall efficiency and stability, the large-scale implementation of electrocatalytic applications will be acquired.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors thank all individuals and organizations who permitted them to republish their figures and other relevant information.", "descript": "\n                  Two-dimensional (2D) structures show atomic geometry of ultrahigh thickness and have the highest surface-to-volume ratio. A monolayer of 2D materials commonly comprises atomically thick covalent lattice bonds, which contain free nanosheets with dangling bonds that might exhibit odd electrical and optical properties. The gathering for 2D derivatives of vdW architectures may be viewed as being outside the bounds of lattice matching due to the lack of directly chemical bond proficiency. Heterostructures are frequently composed of multiple parts associated with significant interfaces and are extensively studied to prevent problems caused by hybrids with unique functions, such as tunneling and confinement effects. To optimize the adsorption/desorption energy for important reactions and to advance the kinetics of chemical reactions, heterostructures formation is a strong strategy to accelerate Hydrogen evolution reaction (HER) activity is proposed. This mini-review deals with various 2D material and their heterostructures, from catalysts design to enhanced photo and electrocatalysis for H2 evolution reactions. Various forms, including 2D/2D, 1D/2D, and 0D/2D of heterostructures, are explained in water splitting reaction. By thoroughly addressing the fundamental aspects, recent developments, and associated challenges \u2060\u2014 the author's recommendation in compliance with future contests and prospects will pave the way for readers.\n               "}
{"full_text": "Lithium\u2013oxygen (Li\u2013O2) batteries have received wide attention because they have much higher energy density (3500 Wh kg\u20131) than current Li-ion batteries [1-7]. Based on their electrolytes, Li\u2013O2 batteries can be categorized into four types: aqueous, aprotic, hybrid, and all-solid state. Among these, aprotic Li\u2013O2 batteries are regarded as promising candidates for energy storage systems owing to their simple construction and excellent reversibility. A typical Li\u2013O2 battery includes a lithium foil anode, a separator, and a cathode electrocatalyst (Fig. 1\n), with the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurring during the discharge and charge processes, respectively [8, 9].Though Li\u2013O2 batteries are still in their infancy, research on them has already attracted wide attention around the world (Fig. 2\n). The first Li\u2013O2 battery was demonstrated by Abraham and Jiang, laying a solid foundation for investigating the first Li\u2013air batteries [10]. However, the development of Li\u2013air batteries was at a standstill for the next ten years because the electrochemistry process remained unclear. In 2006, mass spectrometry confirmed the evolution of O2 during the oxidation process of Li2O2 [11]. After that, primary research to understand the electrochemistry processes during Li2O2 formation and decomposition attracted worldwide attention to the reversibility of Li\u2013O2 batteries.During the discharge process (ORR), O2 accept an electron to generate the intermediate (LiO2). This is followed by electrochemical reduction to form Li2O2 films (the surface growth model) or, in a disproportionation reaction, to produce Li2O2 discs (the solution growth model). To be specific, under a solvent with a high donor number, LiO2 can dissolve in the solvent and undergo a disproportionation reaction to form toroidal Li2O2 in the solution (Fig. 3\n). In a solvent with a low donor number, LiO2 can be stabilized on the catalyst surface. A continuous surface disproportionation reaction or a second reduction reaction then occurs to form Li2O2 films (Fig. 3). Usually, Li2O2 formed by the solution growth model better promotes the discharge capacity and rate performance than that which is formed by the surface growth model.\n\n(1)\nO2 + Li+ + e- = Li\n\n\n\n\n(2)\nLiO2* + e- + Li+ = Li2O2\n\n\n\n\n\n(3)\nLiO2* + LiO2* = Li2O2 + O2\n\n\n\n\n\n(4)\nO2 + Li + e- = LiO2(sol)\n\n\n\n\n\n(5)\n2LiO2(sol) = Li2O2 + O2\n\n\n\nFor the charge process (OER), Li2O2 is electrochemically oxidized to release O2 and Li+. Two different charge mechanisms \u2014 one-step oxidation and two-step oxidation \u2014 have been proposed to explain the OER process. During one-step oxidation, Li2O2 can be decomposed by two electrons [11]. During two-step oxidation, a one-electron oxidation process can occur at a low charge voltage, followed by the one-step oxidation process at a high charge potential [21]. However, a conclusion has not been reached on whether LiO2 exists during the OER process, since an intermediate (LiO2) cannot be identified by in situ surface enhanced Raman spectroscopy. Further study suggests that the formation of LiO2 depends on the donor number of the solvent during the charge process (Fig. 4\n) [22]. Soluble LiO2 can be formed in a high donor-number solvent, where crystalline Li2O2 is generated through a disproportionation reaction. In contrast, a solvent with a low donor number is beneficial for forming Li2-xO2, which is further oxidized to release O2.Despite their high energy density, the development of Li\u2013O2 batteries is currently prevented by several disadvantages, such as serious charge polarization and solvent degradation [23-26]. The formation of insulating Li2O2 can inhibit the transportation of O2 and passivate the cathode electrocatalyst, leading to high resistance, low rate performance, and restricted cyclic stability. Moreover, the decomposition of solvents and carbon-based cathodes results in the formation of by-products, which can cause cell polarization and unsatisfactory performance [27-30]. Furthermore, the high activities of intermediates (e.g., LiO2) can lead to parasitic reactions with the cathode and electrolyte, triggering serious corrosion.To decrease the ORR and OER polarizations and enhance the catalytic performance, research has focused on constructing efficient cathode electrocatalysts to improve the discharge and charge kinetics. According to previous reports, the rational construction of electrocatalysts such as carbon materials [31], noble metals [32, 33], and transition metal oxides [34-36] are confirmed to be efficient strategies for promoting catalytic activity. Within the scope of this review, we examine recent progress on cathode electrocatalysts for fast ORR and OER kinetics. Therefore, we first review recent progress in oxygen catalysts for Li\u2013O2 batteries using advanced carbon materials, transition metal oxides, noble metal-based catalysts, and conductive metal\u2013organic frameworks (MOFs). Then we discuss the discharge and charge mechanisms of new photo-assisted and single-atom catalysts for improving catalytic performance. These two aspects have the ability to facilitate the high-speed development of Li\u2013O2 batteries. Finally, we identify future challenges in developing cathode electrodes for Li\u2013O2 batteries.Carbon materials are potential candidates due to their abundant porosity as well as excellent electric conductivity. Numerous carbon-based catalysts (carbon nanotubes [37-40], carbon nanocubes [41], carbon nanofibers [42, 43], carbon spheres [44, 45], etc.) have been widely developed as electrocatalysts, promoting the discharge process to enhance the discharge capacity via their high surface areas. Unfortunately, the pore structure of carbon materials becomes blocked by the insulating discharge products of Li2O2, causing solvent decomposition and inferior cycling stability. To alleviate this issue, well-designed architectures [46-49] have been constructed to obtain high-energy Li\u2013O2 batteries. Recently, porous carbon nanospheres were developed as efficient electrocatalysts, demonstrating a high capacity of 20300 mAh g\u20131 [31]. Discharge products were observed on the surface of the nanospheres due to strong adsorption between LiO2 and functional groups. At the same time, the open pores of the nanospheres facilitated the transport of Li+ ions and O2 and did not become blocked with extensive Li2O2 deposition (Fig. 5\na). Since a low amount of surface functional groups can obviously inhibit the carbon oxidation side reaction, the carbon nanospheres\u2019 abundant porosity and the absence of oxygenic groups led to good cycling stability for 330 cycles (Figs. 5b and c).Though a porous carbon-based catalyst can promote performance, the sluggish OER kinetics of Li2O2 with porous carbon still hinder Li\u2013O2 battery development [50]. Density functional theory (DFT) calculations show that the catalytic activities of porous carbon materials can be enhanced by heteroatom doping, due to strong adsorption between the lone-pair electrons of heteroatoms and carbon materials [51]. In particular, the introduction of N atoms with high electronegativity can capture electrons from C atoms, indicating the high oxidation charge state of the adjacent C atoms compared to pure carbon materials [50]. O2 tends to adsorb on the positively charged carbon atoms, which promotes breakage of the O\u2013O bond and the reduction of O2 to Li2O2 for enhanced catalytic performance. In addition, pyridinic N doping can effectively promote the deposition of Li2O2 better than graphitic or pyrrolic N due to fast electron transfer between Li2O2 and pyridinic N, which differs from active oxygen sites such as C\u2013O and C=O in carbon materials [52, 53]. In a typical demonstration of N-doped carbon nanotube fabrication on stainless-steel mesh (N-CNTs@SS) for use as an electrode [54], the electrode deliver an OER overpotential below 1.0 V and outstanding cycling performance for 232 cycles due to the high electronic conductivity and superior mechanical strength conferred by N doping. Under bending and stretching stress, the light-emitting diode (LED) display screen of N-CNTs@SS-based Li-O2 batteries remained unchanged (Fig. 6\na), with negligible variation in the discharge\u2013charge curves and open-circuit voltage (Figs. 6b\u2013d). Furthermore, the N-CNTs@SS yielded little LiOH compared to pure hydrophilic catalysts, suggesting its excellent resistance to H2O (Figs. 6e and f).Compared to N-doped carbon catalysts, whereby O2 tends to interact with C atoms near the N atoms, B atoms near C atoms are positively charged due to their low electronegativity, which can facilitate the adsorption of O2 on B sites [52]. Furthermore, theoretical calculations reveal that the B\u2013O group as an active site via B implantation can activate the \u03c0 electrons of carbon, enhancing the charge transfer and decreasing the OER energy barrier [55]. A novel B-doped reduced graphene oxide (B-rGO) has been proven to substantially improve the catalytic performance of Li\u2013O2 batteries, delivering a high discharge capacity of 18000 mAh g\u20131 and a low charge overpotential of 0.85 V at 0.1 A g\u20131 [56]. DFT calculations confirmed that the doping of B atoms into the rGO activated its electrons, resulting in good rate performance. Additionally, the B-rGO displayed a strong affinity towards Li5O6 clusters, enhancing the decomposition of Li2O2. This explained the good performance of B-rGO cathodes.As mentioned above, though carbon material-based Li\u2013O2 batteries can exhibit high discharge capacity, carbon materials are easily corroded by oxygen intermediates and singlet oxygen, which can promote the degradation of carbon catalysts. In addition, carbon materials can also be oxidized when the charge voltage is higher than 3.5 V [57]. As a result, parasitic products can passivate the surface of carbon catalysts, leading to high interface resistance as well as low charge polarization, causing limited cycle life.Noble metals are critical for Li\u2013O2 batteries due to their tunable d orbital states, which allow manipulation of the interactions between intermediates and noble metals, thereby affecting OER catalytic activity [18, 33, 49, 58-66]. Usually, pure noble metals show high charge overpotentials, since the high d band center leads to strong adsorption interactions with LiO2 [67]. A strategy for enhancing the OER kinetics of noble metals is to form high-index planes. Anisotropic Pt with abundant high-index planes was developed as an efficient electrocatalyst, delivering outstanding electrochemical performance relative to commercial Pt catalyst [68]. The high-index [411] facets and atomic steps of anisotropic Pt were confirmed by high-resolution transmission electron microscopy (HR-TEM) images and fast Fourier transformed (FFT) patterns (Figs. 7\na\u2013d). In addition, theoretical calculations suggested that high-index Pt showed higher binding energy towards intermediates than commercial Pt, thereby promoting the OER process with a low energy barrier. Anisotropic Pt electrocatalyst thus displayed a larger discharge capacity of 12,985 mAh g\u20131 and a lower overall overpotential of 0.51 V than pure Pt (capacity: 6272 mAh g\u20131; overall potential: 1.36 V, Fig. 7e).Another tactic to enhance OER performance is alloying with different noble metals. Pt-based alloys have shown great potential for fast OER kinetics due to their tunable electron states. PtIr alloy was confirmed as an efficient OER electrocatalyst. The low Lewis acidity of Pt atoms on the PtIr surface indicated a downshifting of the d-band center, weakening the adsorption interaction with LiO2 and leading to low overpotentials for the ORR (0.11 V) and OER (0.33 V). Later, a PtAu alloy was applied as an electrocatalyst to improve OER electrochemical performance [69]. Due to its high electronegativity, the Au atom tends to receive electrons from Pt atoms, leading to low eg electrons of Pt in PtAu (Figs. 8\na and b). The lower eg electrons in PtAu can result in an upward shift of the Pt d-band center, leading to a stronger affinity for LiO2 than a PtRu alloy (Fig. 8c). Benefiting from the strong adsorption strength between LiO2 and PtAu electrode, Li2O2 nanosheets can be formed by the surface growth model. As a result, a good Li2O2/catalyst interface can possess rapid charge kinetics and a low OER energy barrier (0.84 eV in Fig. 8d). In contrast, due to the weak adsorption energy towards LiO2, Li2O2 discs can be generated by the solution growth model on the PtRu electrode (Fig. 8f). The inferior Li2O2/PtAu interface results in a high OER energy barrier (1.01 eV in Fig. 8e) and high charge overpotentials, delivering poor cycling stability. Therefore, the electron occupancy of Pt can tune the d-band position and thus change the adsorption strength towards LiO2 as well as the OER overpotential (Fig. 8g).Though utilizing noble metal-based catalysts can obviously facilitate the OER kinetics, the commercial application of noble metals is limited by their high cost and possible side reactions arising from solvent degradation [70-72]. Therefore, designing low-cost, high-performance electrocatalysts with high OER catalytic activities is crucial for Li\u2013O2 batteries.Currently, various non-noble metal electrocatalysts containing metal nitrides and transition metal oxides (Co3O4 [3, 73-75], MnO2 [76-78], NiCo2O4 [79, 80], perovskites [81-86], etc.) are used as advanced electrocatalysts in Li\u2013O2 batteries. Compared to transition metal-based catalyst such as Fe2O3 and MnO2, Co-based electrocatalysts exhibit higher OER and ORR kinetics, which entail high discharge capacity and cycling stability [87, 88]. DFT calculations further confirm that the charge overpotentials of transition metal oxides show volcano relationships with surface acidity [89]. Among all the transition metal oxides, Co-based catalysts with medium surface acidity at the vertex position of the volcano plot can decrease the activation energy of the potential-determining step, thereby achieving a high OER activity [89]. In addition, due to mild fabrication procedures, the catalytic activities of Co-based electrocatalysts are easily manipulated by various strategies, such as morphological [90, 91], facet [92, 93], and doping engineering [94]. Open-structured Co9S8 has been confirmed as an excellent catalyst, simultaneously achieving an ideal storage matrix for discharge products as well as superior discharge and charge performance [95]. During the discharge process, hydrangea-like Li2O2 were observed on the surface of a CoO-PCF electrode, leading to a high charge overpotential and poor cycle life caused by insufficient Li2O2/electrode interfaces (Figs. 9\na\u2013c). In comparison, hydrangea-shaped Li2O2 was homogeneously deposited on the edge of Co9S8 nanorods, suggesting enough interface contacts were present between Li2O2 and the Co9S8 electrode (Figs. 9d\u2013f). DFT calculations suggested that the (440), (311), and (111) facets of Co9S8 nanorods showed stronger oxygen adsorption than the (111), (220), and (200) facets of CoO-PCF cathodes (Figs. 9g\u2013l), which was beneficial for heterogeneous nucleation that formed Li2O2 by the surface growth model, thereby enhancing the Co9S8 cathode\u2019s cycling stability.Though Co-based catalysts have been confirmed to accelerate the ORR and OER kinetics, vacancy engineering on Co-based electrocatalysts can further improve their catalytic performance. Theoretical calculations show that the vacancy defects in Co-based electrocatalysts optimize the adsorption interaction with LiO2 and delocalize the surrounding electrons, which is critical for improving ORR/OER performance. Recently, cobalt oxides with Co vacancies (Co3\u2212xO4) have been demonstrated as cathode electrocatalysts; tuning the electronic state of Co3O4 and thus manipulating the potential-determining step in the charge process enhanced the OER catalytic performance [96]. The differential charge density distributions confirmed charge redistribution on the (111) and (220) slabs of Co24O32 and Co23O32 (Figs. 10\na\u2013d). Additionally, for the (111) slabs of Co24O32 and Co23O32, the obvious overlap of electron density promoted rapid charge transportation, facilitating the ORR and OER processes (Figs. 10a and b). The Gibbs free energy curves of Co23O32 further confirmed the low ORR (0.31 V) and OER (0.45 V) overpotential for the (220) slab and (111) slab, respectively (Figs. 10e and f). Co3\u2212xO4 thus displayed a high discharge capacity of 13,331 mAh g\u20131 and a high overall discharge/charge overpotential (1.38 V). This research provides a way to increase the energy efficiency by engineering vacancies in transitional metal oxides.As discussed above, though the utilization of Co-based catalysts has resulted in remarkable performance improvement, there are gaps in our understanding of their OER and ORR mechanisms, impeding the development and design of highly efficient catalyst. In addition, the structure\u2013activity relationship between the electrocatalysts\u2019 reaction processes and function should be further established by analyzing and detecting intermediates.Besides transition metal oxides, flexible metal sites in conductive metal\u2013organic frameworks can modulate the adsorption strength towards oxygen intermediates, making conductive MOFs efficient electrocatalysts. Copper tetrahydroxyquinone (Cu-MOF) was employed as a cathode to remarkably enhance the OER and ORR kinetics of Li\u2013O2 batteries [97]. TEM images and electron energy loss spectroscopy (EELS) indicated that the discharge product was nanocrystalline Li2O2 with amorphous regions (Figs. 11\na and b). According to DFT calculations, heteromorphic Li2O2 had higher electronic conductivities and was thermodynamically favorable for the formation of Li2O2, which promoted the formation of amorphous Li2O2. Therefore, Cu-MOF displayed a low charge overpotential (below 3.7 V) and an excellent cycle life of up to 300 cycles at 1000 mAh g\u20131. Elsewhere, a high-valence conductive nickel catecholate framework (NiIII-NCF) was constructed as a cathode to further improve the redox kinetics via spin manipulation (Figs. 11c and d) [98]. Compared to a low-valence nickel catecholate framework (NiII-NCF), NiIII-NCF showed a smaller energy difference between the d-band center of Ni and the p-band center of O, which greatly promoted electron exchange between Ni and oxygen intermediates. Therefore, during the discharge process, NiIII-NCF displayed strong adsorption to LiO2, leading to the formation of Li2O2 nanosheets (Figs. 11e\u2013i) with high discharge voltage. NiIII-NCF electrocatalysts with abundant Li2O2/NiIII-NCF interfaces delivered low ORR/OER polarization (0.81 V at 200 mA g\u20131) and high energy efficiency \u2014 specifically, a high discharge capacity of 16,800 mAh g\u20131 and good reversibility for 200 cycles at 500 mAh g\u20131.Compared to conventional solid catalysts, single-atom catalysts have unique electronic states, unsaturated coordination environments [99-101], and controllable single atom\u2013support interactions, enabling tuning of the discharge\u2013charge mechanism [102-104]. Additionally, the atomic dispersion of metals indicates high atom utilization and accelerated redox kinetics [105-107]. Single-atom catalysts incorporated into Li\u2013O2 batteries presently include Se single atoms supported on Ti3C2 [108], Co single atoms supported on nitrogen-doped carbon [109, 110], Ru single atoms on nitrogen-doped carbon [111], and Pt single atoms on g-C3N4 nanosheets [112]. Novel Co single-atom catalysts on hollow N-doped carbon spheres (N-HP-Co SACs) were found to obviously enhance the redox kinetics for Li\u2013O2 batteries [109]. In the OER process, UV\u2013vis spectra showed an absorption peak centered at 260 nm, suggesting that charging was dominated by a single-electron oxidation process (Figs. 12\na and b). DFT calculations further confirm that the N-HP-Co SACs demonstrated weaker adsorption towards LiO2 than a Pt/C electrode did, indicating LiO2 tended to dissolve in the solvent rather than in the cathode (Figs. 12c and h). The one-electron process of Li2O2 oxidation delivered rapid kinetics and excellent catalytic performance. Therefore, the N-HP-Co SACs yielded a low OER overpotential (0.29 V).Aside from Co SACs, Se SACs also have shown excellent performance, accelerating the redox kinetics of Li2O2 formation and decomposition. Recently, Se single atoms supported on Ti3C2 MXene (SASe-Ti3C2) have been reported as an electrocatalyst [108]. According to DFT calculations, the SASe-Ti3C2 delivered a stronger LiO2 affinity (0.98 eV) than Ti3C2 (0.43 eV). The charge difference density further confirmed more abundant electron transfer by SASe-Ti3C2 than by Ti3C2; the Se\u2013C bond can be regarded as a highway for transferring electrons (Figure 13\na\u2013f). The strong adsorption strength between LiO2 and SASe-Ti3C2 can lead to the formation of Li2O2 nanoarrays by the surface growth model, thereby reducing the OER overpotential. In comparison, the weak interaction between LiO2 and Ti3C2 caused the formation of Li2O2 nanodisks, triggering high charge overpotentials. The Gibbs free energy further proved that the SASe-Ti3C2 exhibited a lower OER overpotential (0.88 V, Fig. 13h) than the Ti3C2 (1.6 V, Fig. 13g). The SASe\u2013Ti3C2 electrode thus displayed a high discharge capacity of 17260 mAh g\u22121 and excellent cycling stability (170 cycles) with a low overall discharge\u2013charge overpotential (1.10 V).Though single-atom catalysts show great potential, they have major stability issues that need to be addressed. Since single atoms have higher surface energies than the corresponding metal particles, large particles can form due to accumulation. Therefore, the loading mass of single atoms should be enough low to prevent agglomeration. In this regard, efficient preparation strategies for various single atoms are necessary for the development of single-atom catalysis.Though a solid catalyst can decrease the charge overpotential, it is still high in terms of applications. In response, photocatalysis has been introduced to further decrease the charge overpotential. Under light illumination, the electron can be excited from the valence band (VB) to the conductive band (CB), enhancing ORR and OER performance. A suitable photocatalyst should yield a redox potential of O2/Li2O2 between the CB and VB. During the ORR process under light illumination, the electron will transfer from the VB to the CB, resulting in the reduction of O2 to Li2O2 (Fig. 14\na). Furthermore, the hole in the VB can be reduced by the electron from the external circuit. The discharge potential is the voltage between the VB and the potential of Li+/Li. In comparison, the oxidation of Li2O2 is conducted via the holes in the VB to form O2 and Li+, and the electron will transfer from the VB to the external circuit (Fig. 14a). The charge overpotential is the voltage between the CB and the potential of Li+/Li.While photocatalysts can enhance energy efficiency, only the ultraviolet region, which occupies 4% of the solar radiation spectrum, can be absorbed by the semiconductors currently used [113]. In addition, high electron-hole recombination rates can cause incompatibility between the carrier lifetime and charge\u2013discharge rate [4]. Therefore, manipulating the band gap of semiconductor catalysts to allow absorption of a large cross-section of visible light and achieve long carrier lifetime is the principle behind photo-assisted Li\u2013O2 batteries; this involves defect engineering, nanostructure design, and heterojunction interface design. To date, a variety of photocatalysts have been developed, including TiO2 [114], siloxene nanosheets [115], Au/C3N4 [116], ZnS [117], polyterthiophene [118], and Fe2O3 [119]. In particular, the siloxene nanosheets were developed as an efficient photocatalyst with extremely high energy efficiency [115]. Under light illumination, siloxene nanosheets with abundant sites aided in the formation and oxidation of Li2O2. As a consequence, a siloxene nanosheets oxygen electrocatalyst delivered a high discharge voltage of 3.51 V and a high energy efficiency of 185% (Figs. 14b and d). Furthermore, the siloxene nanosheets showed excellent rate performance (129% energy efficiency at 1 mA cm\u20132, Fig. 14c), offering a possible strategy to design efficient catalysts for photoconversion and storage systems.Though photocatalysts can decrease the OER overpotential, the high recombination of electrons and holes can compete with the OER and ORR processes, leading to decreased energy efficiency. To suppress the recombination of electrons/holes and increase absorption in the visible light range, Au-supported C3N4 with N vacancies (Au/Nv-C3N4) was used as a photocatalyst [116]. O2 tended to be adsorbed on the Nv of Nv-C3N4 rather than on C3N4, which benefited O2 activation (Figs. 15\na and b). The projected density of state (PDOS) further confirmed strong orbital hybridization between O2 and Nv (Fig. 15c). When Au was loaded on the Nv-C3N4, obvious hybridization between the Au cluster and Nv occurred (Fig. 15d), promoting the ORR catalytic performance. Therefore, during the discharge process, plasmonic Au NPs adsorbed visible light to form hot electrons and holes. The electrons and holes were separated by the interface between Au and Nv-C3N4, which prolonged the lifetime. The electron migrated from Au to C3N4, then to the NV-induced defect band to achieve the reduction of O2 to Li2O2. In a typical charge process, the hole in Au NPs oxidized the Li2O2 to release O2 and lithium ions (Fig. 15e). The achieved discharge and charge voltages were 3.16 and 3.26 V, respectively, indicating a high energy efficiency of 97%. In addition, the batteries cycled stably for 50 cycles. In the future, tuning the loading mass of metal particles should further enhance the utilization efficiency of solar light and improve the redox kinetics.With the increasing requirement for high energy density during the last decade, Li\u2013O2 batteries have received wide attention. Yet despite their ultrahigh energy density, Li\u2013O2 batteries also face challenges before they can be used in practical applications, including sluggish OER and ORR kinetics, and solvent degradation. In this review, we have discussed recent progress in developing highly efficient cathode electrocatalysts. We elucidated the research process and explored the main challenges with respect to carbon materials, noble metals, non-noble metals, SACs, and semiconductor photocatalysts. Systematic strategies for developing cathode structures to solve the corresponding issues were explored in depth, and we highlighted the intrinsic relationship between catalytic performance and catalyst electronic structure. Though great progress has been reported in the design and fabrication of advanced catalysts, Li\u2013O2 battery development remains in the early stages, and numerous challenges must be overcome before commercialization. In what follows, we summarize several obstacles that should be tackled in future research.Given the significant effect of cathode catalysts on the performance, optimizing catalysts\u2019 electronic structure at the molecular level should be further investigated to tune the adsorption energy towards LiO2, thereby manipulating the overpotential and cycling stability. Li2O2 films can be formed by the surface growth model due to the strong adsorption strength towards LiO2 in solvents with a low donor number, delivering a low discharge capacity and inferior rate stability. In contrast, weak adsorption strength towards LiO2 is obtained by using an electrolyte with a high donor number, entailing the formation of Li2O2 discs with high discharge capacity. Adsorption between LiO2 and cathode electrocatalysts can alter the reaction energy barrier and tune the ORR/OER kinetics of the rate-determining step (RDS), thereby affecting the overpotential, energy efficiency, and long-term cycling stability. In addition to the influence on the discharge growth model and OER/ORR reaction kinetics, accumulation of side products and high charge transfer resistance can also be triggered due to the degradation of cathode catalysts when oxygen-containing intermediates are produced. The superoxide radicals formed during the cycling processes can attack the defect sites of cathode electrocatalysts, while singlet oxygen formed in the charge process can further cause severe degradation of cathode catalysts. The parasitic products Li2CO3 and LiOH originating from singlet oxygen can passivate the catalyst electrode and result in high charge voltage and low energy efficiency. Therefore, suitable catalysts should be developed to convert singlet oxygen to 3O2, which inhibits the side reactions and remarkably enhances the cycling stability.Further, although great improvements have been achieved by using advanced cathode electrocatalysts, the lack of direct evidence proving the OER and ORR mechanisms restricts the development of highly active catalysts. To reveal the catalytic mechanisms, in situ characterization techniques such in situ Fourier transform infrared (FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS), X-ray absorption fine-structure (XAFS), Raman spectroscopy, electrochemical quartz crystal microbalance (EQCM), ultraviolet-visible (UV-vis) absorption spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy should be established to monitor intermediates and confirm the reversibility under working conditions.Apart from issues with cathode electrocatalysts, the stability of the Li metal anode has been a serious issue for Li\u2013O2 batteries. Due to the complicated solvent components of Li\u2013O2 batteries, the Li anode can react with O2 and solvents, leading to Li anode corrosion and the formation of lithium dendrites, and thereby causing inferior anode stability and battery short-circuiting. Therefore, having a good understanding of Li stripping and plating mechanisms as well as protecting the Li anode via strategies such as forming Li-based alloys, constructing SEI layers, and introducing a lithium host should be developed for Li metal batteries. The solvent is also an important component determining the stability and cycle life, since it can be attacked by oxygen-containing intermediates. We propose exploiting advanced liquid electrolytes to achieve high ionic conductivities and inhibit attacks by oxygen intermediates. Solid-state electrolytes may be potential candidates because of their high stability towards intermediates at a high charge voltage. Unfortunately, solid-state electrolytes usually exhibit poor cycle life and high polarization compared to liquid electrolytes due to the former\u2019s low ionic conductivity and interface issues between the electrolytes and electrodes. By fully utilizing the advantages of liquid and solid-state electrolytes, mixtures of liquid and solid-state electrolytes could be explored to guarantee high ionic conductivity as well as high interface and solvent stability for realizing high-performance Li\u2013O2 batteries.The authors declare no competing interests.This study was financially supported by National Science Fund for Distinguished Young Scholars (No. 52025133), Tencent Foundation through the XPLORER PRIZE, and the Fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP202004), China Postdoctoral Science Foundation (No. 2021M700211).", "descript": "\n                  Lithium\u2013oxygen (Li\u2013O2) batteries have great potential for applications in electric devices and vehicles due to their high theoretical energy density of 3500 Wh kg\u20131. Unfortunately, their practical use is seriously limited by the sluggish decomposition of insulating Li2O2, leading to high OER overpotentials and the decomposition of cathodes and electrolytes. Cathode electrocatalysts with high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities are critical to alleviate high charge overpotentials and promote cycling stability in Li\u2013O2 batteries. However, constructing catalysts for high OER performance and energy efficiency is always challenging. In this mini-review, we first outline the employment of advanced electrocatalysts such as carbon materials, noble and non-noble metals, and metal\u2013organic frameworks to improve battery performance. We then detail the ORR and OER mechanisms of photo-assisted electrocatalysts and single-atom catalysts for superior Li\u2013O2 battery performance. Finally, we offer perspectives on future development directions for cathode electrocatalysts that will boost the OER kinetics.\n               "}
{"full_text": "Data will be made available on request.In recent years, with the tremendous dependency on oil as a valuable feedstock, there has been an urge and interest for researchers in biomass utilization for the production of fuels and fine chemicals [1]. This urge has led to increase in the research for an appropriate substitute to petroleum with lignocellulose being the most abundant, carbon-sustainable and bio-renewable biomass. In variety of these lignocellulose one of the examples being furfural which can undergo a great variety of reactions to yield a wide range of chemicals [1,2]. Taking these points into consideration, industrially these reactions can be carried out either in liquid phase or vapour phase. Liquid phase reactions are carried out in high pressure reactors wherein high conversion, selectivity and yield can be achieved [2,3]. However, these liquid phase reactions give best activity, the use of solvent and high pressure may limits its industrial application and alternatively vapour phase reactions play a major role for some of these reactions [3]. The industrial applications of the vapour phase can be employed as the reaction can be carried out in vapour phase reactor in fixed bed down flow reactor at atmospheric pressure via heterogeneous catalyst [4]. However, among various catalytic vapour phase reactions, hydrogenation of furfural and dehydrogenation of cyclohexanol are two industrially important reactions which mainly produce furfural alcohol and cyclohexanone respectively [1\u20134].The catalytic vapour phase hydrogenation of furfural can yield furfuryl alcohol, 2-methyl furan, tetrahydrofurfural, tetrahydrofurfuryl alcohol, 2-pentanol and few more by-products among which furfural alcohol is widely used in manufacturing various synthetic fibers, rubbers and resins [4]. It is also an important chemical intermediate for the production of lysine, plasticizers and lubricant [5]. The industrial application of hydrogenation of furfural, copper chromite catalyst have been employed widely as commercial catalysts [6]. However, the toxic nature of chromium based catalysts greatly hinders its use in industries. Therefore, in order to develop Cr free catalytic system focus has been shifted to metal-based monometallic and bimetallic catalysts such as Au, Pd, Ru, Cu, Ni, Pd\u2013Cu, Cu\u2013Co, Pt\u2013Sn catalysts [5,7] Additionally, amorphous alloys such as Ni\u2013P, Ni\u2013B, Ni\u2013P\u2013B, Co\u2013B, Cu\u2013Al layered double hydroxide (LDH) [4,8] and Cu on different supports Al2O3, SiO2 and SBA-15 as catalyst for hydrogenation of furfural have been studied. For instance, hydrogenation of the furan ring to THFA, hydrogenolysis of the CO bond to methylfuran, decarbonylation to furan, and further hydrogenation to THF are observed with furfural alcohol [2]. Hence, more advance, selective, stable and economically feasible catalytic systems are required for this reaction.Meanwhile, the catalytic dehydrogenation of cyclohexanol to cyclohexanone is another industrially important reaction since cyclohexanone can be further transformed into caprolactam and adipic acid which are two major raw materials in synthesizing polyamide fibre [9]. The heterogeneous catalytic dehydrogenation of cyclohexanol may lead to various products namely cyclohexanone, phenol, cyclohexene, cyclohexenyl and cyclohexanone. The commercial catalysts reported for the dehydrogenation of cyclohexanol are either Cu\u2013Zn\u2013Al or Cu\u2013Mg catalysts, for which the method of preparation is very crucial and tedious to obtain equilibrium conversion of cyclohexanol, which is in the range of 60\u201368% and temperature varying from 493 to 533\u00a0K [4,5,10\u201312]. Hence, various other catalytic systems need to be designed for improved cyclohexanol conversion and selectivity towards cyclohexanone. Precisely, cyclohexanol dehydrogenation on precious metal catalysts [2\u20134] has received much attention however, are highly expensive. Taking into consideration all the downsides of already reported catalysts, we developed Cu\u2013MgO co-precipitated catalyst in our previous study which proved to be very efficient in catalyzing both furfural hydrogenation and cyclohexanol dehydrogenation independently and simultaneously as well [13\u201315]. The activity of copper catalysts depends mainly on the nature of support, method of preparation, dispersion and crystallite size of the active component.In the present work we have focused on the co-precipitation method with different precipitating agents as the process involves simple reaction set up, scalability and homogeneity with controlled morphology at atomic scale [16,17]. Additionally, the diversity in the morphology, particle size, type of reductant and ability for interfacial tension allows the developed material for the target reaction in efficient way [16,17]. The present work is a continuation of our previous study in understanding the influence of different precipitating agent's on the characteristics of Cu\u2013MgO catalysts and their activity towards hydrogenation of furfural and dehydrogenation of cyclohexanol, respectively [12\u201314]. Co-precipitated Cu\u2013MgO catalysts were prepared using a series of five different precipitating agents. The as-synthesized catalysts were characterized by various analytical and spectroscopic techniques to investigate the effect of precipitating agents on morphology and properties of Cu\u2013MgO catalyst. The study revealed that the catalyst prepared by using potassium carbonate as precipitating agent displayed very good activity towards hydrogenation of furfural to lead to a selectivity of about 99% to furfural alcohol and dehydrogenation of cyclohexanol to lead to a selectivity of 100% to cyclohexanone.Furfural (99% purity, ACS grade), Cyclohexanol (99% purity, ACS grade), Furfuryl alcohol (99% purity, ACS grade) and Cyclohexanone (99% purity, ACS grade) were purchased from Sigma Aldrich. Cu(NO3)2.3H2O (99% purity), Mg(NO3)2.6H2O (99.5% purity) are also purchased from Sigma Aldrich. K2CO3.3H2O, KOH, Na2CO3, (COOH)2 and NH3 were purchased from M/s. LOBA Chemie, India. All chemicals in this work are of reagent grade and used as received without further purification.The Cu\u2013MgO catalysts with 16\u00a0wt % Cu were synthesized by co-precipitation method using different precipitating agents. For instance, aqueous solution containing requisite amounts of 1\u00a0M each of Cu (NO3)23H2O and Mg (NO3)2.6H2O were prepared and mixed together. The mixed solution was precipitated using an aqueous solution containing 1\u00a0M K2CO3.3H2O till pH of the solution reaches approximately 9. The co-precipitated mass was thoroughly washed, filtered and dried at 393\u00a0K for 12\u00a0h. The dried sample was then calcined in air at 723\u00a0K for 4\u00a0h to obtain CuO\u2013MgO and reduced in H2 flow at 523\u00a0K for 4\u00a0h to obtain Cu\u2013MgO prior to catalytic reaction. This Cu\u2013MgO co-precipitated catalyst is designated as CM-A. In a similar manner, other Cu\u2013MgO catalysts were prepared using different precipitating agents- Na2CO3, KOH, NH3 as well as (COOH)2 and are designated as CM-B, CM-C, CM-D and CM-E respectively.The XRD pattern of both, calcined and reduced catalysts were recorded on an M/S. Rigaku's Miniflex diffractometer with Ni filtered Cu K\u03b1 as a radiation source at a 2\u03b8 scan speed of 2omin\u22121. The crystallite size of Cu was calculated by XLB method on the same instrument. The catalysts were characterized for specific surface area by N2 adsorption at 77\u00a0K by BET method using a Micrometrics Pulse Chemisorb 2700 instrument. Before measurements, the samples were dried in oven at 393\u00a0K for 12\u00a0h and flushed in-situ with Helium gas for 2\u00a0h. Surface morphologies of as-synthesized and calcined catalysts were examined by field emission scanning electron microscope (FE-SEM, Carl Zeiss Sigma VP FE-SEM). FT-IR spectra were recorded using a Varian 2000 IR spectrometer (Scimitar series) for the functional group analysis of calcined materials. Temperature programmed reduction (TPR) studies of the catalyst were performed on an indigenous pulse reactor with 6% H2\u2013Ar as reducing and carrier gas respectively. The temperature was increased linearly at a ramp of 5\u00a0K\u00a0min\u22121 from room temperature to 973\u00a0K where the isothermal conditions were maintained for 30\u00a0min. The change in the H2 concentration was monitored by micro TCD and recorded on GC work station. The elaborated experimental details of TPR are discussed elsewhere [13]. M/S. Kratos Axis 165 XPS spectrometer, with Mg-K\u03b1 radiation (1253.6\u00a0eV) was used for obtaining XPS data. XPS analysis was used to study the chemical composition and oxidation state of the catalyst surfaces. In the XPS study, C 1s line binding energy value of 285\u00a0eV (accuracy with in \u00b10.2\u00a0eV) as a reference level and the relative atomic sensitivity factors of 4.871 and 0.168 for Cu2p3/2 and Mg 2p, respectively for determining Cu/Mg surface composition were chosen. Prior to the ESCA studies, all the catalysts has been reduced in 6% H2 balance He flow at 523\u00a0K for 4\u00a0h. During the data acquisition the background pressure was kept slow at 10\u00a0bar. DTA/TGA profiles of Cu\u2013MgO samples (in dried form) were recorded on M/S. Metler Toledo (Switzerland) instrument at a heating rate of 10\u00a0Kmin-1.Vapour phase hydrogenation of furfural to furfural alcohol and dehydrogenation of cyclohexanol to cyclohexanone were carried out separately in a fixed bed quartz reactor (200\u00a0mm long and 8\u00a0mm i. d.). About 1\u00a0g of catalyst packed at the center of reactor between two plugs of quartz wool was reduced in a flow of 6% H2 in He mixture at 523\u00a0K for 4\u00a0h followed by lowering the temperature of the reactor to 453\u00a0K and replacing the H2/He mixture with ultra-pure H2 (99.9% H2 which was further purified by passing it through de-oxo and molecular sieve traps in a series to remove oxygen and moisture, etc.). Hydrogenation reaction of furfural was carried out by injecting furfural at a flow rate of 1.2\u00a0mL/h with the help of a syringe pump (Secura FT, M/S. B. Braun, Germany) with H2/furfural molar ratio\u00a0=\u00a02.5\u00a0at a reaction temperature of 453\u00a0K. For dehydrogenation of cyclohexanol the reaction temperature of 523\u00a0K was maintained. For both the reactions the gas hourly space velocity (GHSV) was maintained at 0.05\u00a0mol h\u22121gcat\u22121. The product mixture was collected every hour in an ice-cold trap. The reaction products were analyzed by gas chromatography (GC-7820 A\u00a0(M/s. Agilent, USA) equipped with a flame ionization detector and a capillary column HP-5, 19091J-413 (30\u00a0m length, 0.32\u00a0mm inner diameter and 0.25\u00a0\u03bcm film thickness) and GC-MS (QP-5050, M/S. Shimadzu instruments, Japan) using a DB-5MS capillary column of 0.32\u00a0mm dia. and 25\u00a0m long (M/S. J & W Scientific Instruments, USA).The conversion of furfural to furfural alcohol and cyclohexanol to cyclohexanone is currently been studied over various catalytic system majorly metal based, metal oxide based and bimetallic catalyst to obtain high conversion and selectivity due to availability of active sites [4,5,7,8]. In our previous reports we have studied the activity of Cu\u2013MgO catalyst prepared by co-precipitation method using K2CO3 as a precipitating agent which gave high conversion for furfural to furfural alcohol in vapour phase [18]. In present study, we have selected five different precipitating agents namely K2CO3, KOH, Na2CO3, (COOH)2 and NH3 and synthesized the catalysts following the same method. To study and correlate the influence of these precipitating agents on structural and chemical properties of Cu\u2013MgO catalyst, the as-synthesized catalyst were characterised by various modern analytical and spectroscopic techniques.The results of XRD analysis of both the calcined and reduced samples of copper catalysts are presented in Table 1\n.The predominant phases observed in the calcined catalysts (Fig. 1\na) were CuO (ICDD file No.5-661) and MgO (ICDD file No. 4-829). The Cu2O peaks have matched with the ICDD file No. 5-667 was observed in all the catalysts. The XRD results of Cu/MgO reduced catalysts, CM-A, CM-B, CM-C, CM-D and CM-E exhibited the formation of CuO phase matches with the ICDD file No. 4-836.The XRD pattern of CM-A catalyst revealed the presence of Cu metallic particles to be either in amorphous or in microcrystalline form as shown from Fig. 1b. Thus, the higher surface area of CM-A catalyst may be due to the interacted species formed between Cu and MgO. It was interesting to observe that the catalysts prepared with alkali metal containing precipitating agents consisted of smaller crystallites of Cu. The surface areas of these catalysts were relatively higher as a result of the smaller Cu-crystallites. The other catalysts namely CM-D and CM-E, showed bigger crystallites of CuO. In these catalysts, the intensity of d lines of Cuo phase reduces at the expense of CuO phase. This may be due to incomplete reduction of the bulk CuO or due to re-oxidation of the Cuo species to Cu+ and Cu2+ formed over other catalysts. Only in Cu\u2013MgO co-precipitated catalyst using K2CO3 as a precipitating agent, MgO particles were in microcrystalline form indicating a possible interaction of MgO with Cuo/Cu+ species. It is reported that MgO has the ability to stabilize the metal particles, prevent sintering and volatilization [19,20].The crystallite size of Cuo as represented in Table 1 was calculated by X-ray line broadening technique using Debye-Scherrer equation mentioned in Equation (1). The crystallite size of Cu from XRD in CM-A catalyst was lower. The XRD pattern of CM-A showed that Cu and MgO phases were in micro crystalline form, as a consequence of which the Cu dispersion and metal surface area was higher [18]. The catalysts CM-D and CM-E showed bigger crystallites of Cu and the copper dispersed in bulk on the surface of the support as shown in Table 1. These catalysts has smaller surface areas due to bigger crystallites on the surface of the support. This explains that copper crystallite size is dependent on the type of precipitating agent employed in the synthesis process.\n\n1\n\n\nD\n=\nk\n\u03bb\n/\n\u03b2\n\nCos\n\n\u03b8\n\n\n\nwhere,D\u00a0=\u00a0Crystalline Sizek\u00a0=\u00a0Scherer's constant (0.94).\u03bb\u00a0=\u00a0X-ray Wavelength (1.54178 A).\u03b2\u00a0=\u00a0Full width at half maximum (FWHM).\u03b8\u00a0=\u00a0Bragg angle corresponding to (hkl) reflection.\nTable 1 represents the BET-surface area and the crystalline phases of various Cu\u2013MgO co-precipitated catalysts using different precipitating agents studied for the hydrogenation of furfural and dehydrogenation of cyclohexanol. The surface area of MgO was found to be 38 m2g-1 and the surface areas of the Cu\u2013MgO co-precipitated catalysts were more or less same when the precipitating agent containing alkali metal was used. Higher surface area of Cu/MgO co-precipitated catalyst is reported in our previous communication [13]. However, when the precipitating agent was either oxalic acid or ammonia the resulting catalyst exhibited very low surface area. This indicated that addition of Cu to MgO reduced the surface area of the catalyst probably due to the coverage of the MgO surface by bigger crystals of Cu.The TPR profiles of Cu\u2013MgO catalysts prepared by different precipitating agents are shown in Fig. 2\n. Their corresponding H2 uptakes calculated from the TPR peaks has been presented in Table 1\n. Two stages of reduction of CuO to Cuo were observed in CM-A and CM-C catalyst. The standard CuO was found to reduce at a Tmax of 606\u00a0K (profile is not shown in Fig. 2). Most of the acidic Cu in the Cu\u2013MgO co-precipitated catalysts seems to reduce at Tmax in the range of 523\u2013563\u00a0K. But a complete reduction seemed to occur at \u223c673\u2013713\u00a0K. Thus, it appears that the reduction of Cu\u2013MgO samples took place in two stages with the first peak corresponding to the reduction of CuO and the second H2 consumption peak at high temperature may be due to the reduction of Cu2O to Cuo. XRD results of calcined catalysts indicated the presence of both CuO and Cu2O.Many literature reports suggest that the reduction peak of bulk CuO to metallic Cu directly takes place in a single step [21\u201323]. The reduction patterns of CM-A and CM-C co-precipitated catalysts using K2CO3 and Na2CO3 as precipitating agents showed peaks at 610\u00a0K and 722\u00a0K in CM-A and peaks at 654\u00a0K and 713\u00a0K in CM-C. The low temperature reduction peak indicated the presence of easily reducible Cu+2 species. A similar observation is reported with Cu/Al2O3 [11]. The defect sites known to be present in MgO probably contribute to the formation of surface interacted species with copper in the CM-A and CM-C catalysts. These interacted species got reduced at lower temperatures compared to the reduction of bulk CuO (i.e., larger crystallites) which reduce at higher reduction temperatures [21]. The XRD patterns also confirmed that the CM-A co-precipitated catalyst showed poorly crystalline CuO phase and Table 1 shows the CM-A catalyst has higher surface area. Wang et al. reported the presence of three peaks in different temperature regions in the impregnated Cu/SiO2 catalysts [23]. The major peak corresponds to the reduction of larger CuO clusters and the minor peaks reduced to of small CuO clusters and highly dispersed Cu (II) species, respectively.The Fig. 3\n represents the thermal changes and behaviour of the various Cu\u2013MgO dried samples in the temperature range of 298\u20131273\u00a0K with the help of their corresponding DTA patterns. The endotherms in the low temperature region of 343\u2013473\u00a0K correspond to the removal of physically adsorbed water and dehydroxylation of some hydroxyls associated with Cu and Mg cations showing a weight loss of 2\u201310% in almost all the catalysts except CM-D which showed only one major endotherm at \u223c608\u00a0K that may be attributed to the dissociation of oxalates of Cu and Mg forming their corresponding oxides.Jongen et al. found that copper oxalate shows a very small endotherm between 303 and 523\u00a0K corresponding to dehydration process (with a 3% wt. loss) and a major endotherm at 523\u2013573\u00a0K (with \u223c40% wt. loss) attributed to the decomposition of copper oxalate to CuO and CO/CO2 [24]. The samples CM-A, B and C showed a minor endotherm \u223c303\u2013543\u00a0K corresponding to partial dehydroxylation of the hydroxyl groups of Cu and Mg. The CM-B catalyst prepared using KOH showed a major endotherm at \u223c633\u00a0K corresponding to the transformation of the hydroxides of Cu and Mg to their corresponding oxides by complete dehydroxylation. However, the samples obtained from carbonate precipitating agents; CM-A and CM-C showed the major peak at \u223c703\u00a0K and 623\u00a0K respectively. The major peaks along with shoulder peaks of these samples were found to exist at 653\u00a0K and 728\u00a0K in CM-A and 703\u00a0K in CM-C. The DTA patterns of dried samples of single oxides namely, CuO and MgO showed endotherms at 493\u2013533\u00a0K and \u223c633\u00a0K respectively. Thus, it clearly indicated that the major endotherms of CM-A and CM-C correspond to the dehydroxylation process and the shoulder peaks associated with them may be ascribed to subsequent decarboxylation (CO2 elimination). The transitions, which occur in the range of 673\u2013773\u00a0K were attributed to the removal of CO2 associated with Cu containing samples [25,26]. The high temperature endotherm observed at 1073\u20131123\u00a0K in the DTA patterns of CM-B and CM-C was probably due a partial transformation of CuO to Cu2O [25,26]. CM-E sample showed two endotherms at 573\u00a0K and 673\u00a0K, which were attributed to the dehydroxylation of hydroxyls associated with Cu and Mg with a corresponding percentage weight loss of 28% and 5%, respectively.The XPS patterns of reduced catalysts for Cu 2p and Mg 2p are shown in Fig. 4\n. The binding energy values of Cu 2p3/2, Mg 2p and O 1s along with the Cu/Mg atomic ratios, Cus/Cup (intensity ratio of the Cu 2p satellite peak to Cu 2p parent peak) of catalysts are given in Table 2\n. XPS analysis of Cu\u2013MgO catalysts prepared by co-precipitation, impregnation and solid-solid wetting method has been discussed in our earlier work [14]. The binding energy values of Cu 2p3/2 in metallic copper and in CuO are reported to be 932.7 and 933.6, respectively [27]. The presence of another minor band of C 1s at 289\u00a0eV in some of the samples indicates carbon contamination due to CO3\n2\u2212species with the exposure of samples to air. The presence of significant amount of CuO species on the surface in the reduced samples may be due to the re-oxidation of Cuo on exposure to air.The higher Cus/Cup observed in case of CM-B, CM-C, CM-D and CM-E samples probably indicate the presence of more amounts of Cu2+ species. The satellite peaks observed in case of Cu+2 compounds are due to the shake-up transitions by ligand to metal 3\u00a0d charge transfer [28]. These satellite peaks are not seen in Cu+ compounds or in metallic Cu because of completely filled 3\u00a0d orbital. In fact, transition metal ions with unfilled 3\u00a0d orbital are well reported to show satellite peaks in the core level XPS spectra due to electron shake-up [29]. The higher intensity of CuO phase (Fig. 2) observed in the case of CM-B, CM-C, CM-D and CM-E (reduced) catalysts support the above finding. The Cu/Mg ratio is although higher with CM-A catalyst, the lower Cus/Cup in this sample suggests the surface enrichment of other species viz., Cuo or Cu2O apart from Cu2+. Our earlier observations in the hydrogenation of furfural to furfural alcohol over Cu\u2013MgO co-precipitated catalyst prepared with K2CO3 as a precipitating agent indicated that the presence of more Cuo species in this catalyst may be responsible for higher activity and selectivity towards the formation of furfural alcohol [18]. Secondly, O 1s spectra show the binding energy values in the range of 530.8\u2013532.2\u00a0eV for the Cu\u2013MgO catalysts which is a characteristic feature of metal oxides. The Mg 2p binding energy values are almost the same in all the catalysts.\nFig. 5\n shows the FE-SEM images of uncalcined and calcined Cu\u2013MgO catalysts prepared by five different precipitating agents. FE-SEM analysis revealed that changing the precipitating agent could affect the morphology of the final catalyst [30]. The sample morphology differs from each other to some degree in terms of shape, size and distribution of copper over the catalyst surface. In CM-A catalyst, employing K2CO3 as the precipitating agent led to the formation of a very peculiar rod like structure, which is retained in calcined sample as well. It can be seen that Cu particles are very well dispersed on the rod like MgO (CM-A; a-d). Hence there is less aggregation of copper particles on MgO support. The rods seem to be approximately of 4\u20135\u00a0\u03bcm in length. Since the morphology of synthesized material varies according to the functional group of the precipitants (carbonates or hydroxides) used in synthesis method, catalyst prepared by using Na2CO3 as precipitating agent (CM-B) also displayed rod like morphology similar to that of CM-A and the same morphology was observed even after calcinations (CM-B; a-d). This perhaps may be due to presence of same functional group (CO3\u2212) in precipitating used for synthesizing both the catalyst. These observations are in good agreement with those reported by Jung et al. [31]. Even in this case the active species was dispersed on rod shaped surface having length may be between 5 and 6\u00a0\u03bcm.CM-C catalyst prepared by KOH as precipitating agent showed better dispersion of Cu and Mg species which was observed in this case and is well supported by BET results which showed better surface area as compared to rest of the samples. While for CM-D catalyst prepared by NH3 as precipitating agent led to aggregation of both copper and magnesium on catalyst surface (CM-D; a-d). The low surface area reported for this catalyst can be attributed to poor dispersion of active Cu species on catalyst surface which is clearly seen in SEM image. Later on when oxalic acid was used as a precipitating agent, catalyst (CM-E; a-d) showed spherical morphology. Additionally, BET analysis of this catalyst showed lowest surface area. The blockage of pores by Cu and the compact structure can be one of the reasons for this observation.Moreover, the information regarding the formation of the Cu\u2013MgO was obtained by FT-IR analysis. Fig. 6\n presents the FT-IR spectra of the uncalcined (Fig. 6.1) and calcined (Fig. 6.2) catalysts. The broad band observed in range 3300\u20133650\u00a0cm\u22121is assigned to O\u2013H stretching due to the presence of hydroxyl as well as both adsorbed and interlayer water in uncalcined Cu\u2013MgO. These broad bands are observed for all the uncalcined catalyst prepared by different precipitating agents with a slight shift in band in each case. Two distinct peaks in finger print region is attributed to metal-OH stretching vibrations [32]. The presence of \n\n\n\nC\nO\n\n3\n\n2\n\u2212\n\n\n\n from precipitating agents K2CO3 and Na2CO3 contributes to the carbonyl stretching at around 1380\u00a0cm\u22121 in uncalcined FT-IR spectra of CM-A and CM-B respectively [32]. Supporting, the FT-IR spectrum recorded for all the calcined catalysts showed absence of O\u2013H stretching peaks. The disappearance of M-O-H stretch and appearance of metal-O stretching in finger print region of calcined spectra suggests successful synthesis of Cu\u2013MgO catalysts [33]. The finger print region shown separately gives clear insight picture of the respective characteristic peaks of M\u00a0\u2212\u00a0OH and M\u00a0\u2212\u00a0O for uncalcined and calcined samples.After acquiring in-depth knowledge of prepared catalyst by characterisation, we have tested all prepared catalysts for their catalytic activity in hydrogenation and dehydrogenation reaction respectively. Fig. 7\n shows the hydrogenation activity data for furfural to furfuryl alcohol. The graph displayed shows that the CM-A catalyst showed higher conversion of furfural and selectivity of furfural alcohol, 98% and 99% respectively and with 97% yield of furfural alcohol.The presence of more Cuo species has been attributed to the presence of defect sites of MgO which are reported to be more reactive and that the adsorption properties for metal species can be qualitatively different from those of regular surface sites [34,35]. It is also reported that the defect sites at the metal support interfacial region in case of Pt\u2013TiO2 system is helpful to coordinate the oxygen atom of the CO group via lone pair of electrons and thus activate the hydrogenation of CO group [36]. The higher conversion of CM-A catalyst compared to other catalysts was due to the presence of small crystallites of Cu as found from XRD, correspondingly higher surface area as shown in Table 1. However, the BET surface area was comparable in case of CM-A and CM-C, the uptake of H2 gas was impactful in case of the CM-A catalyst and hence affected positively in the catalytic activity. The higher conversion of CM-A catalyst was also attributed due to the presence of small crystallite size of Cu as seen in the XRD data in Table 1.\nFig. 8\n a, b and c represents the conversion, selectivity and yield with time on stream over various Cu\u2013MgO co-precipitated catalysts studied. The CM-A catalyst showed a steady and higher conversion of 99% and selectivity of 99% and therefore higher yields of furfuryl alcohol throughout the 300\u00a0min operation. CM-B catalyst showed an initial conversion of 45.5%.Additionally, from the second hour onwards, the conversion goes on decreasing and in the fifth hour of the activity run, it drastically decreased to 11.6%. Over CM-C catalyst, the initial conversion of 63.9% decreased to 43.1%, whereas over CM-D catalyst, initially the conversion was59.6% which decreased to 47.1% in 300\u00a0min run. CM-E catalyst showed a conversion of 57.4% and ended to 53% in 5\u00a0h. The selectivity towards furfuryl alcohol over all the catalysts is almost 99%.\nFig. 9\n shows the activity data for cyclohexanol conversion towards cyclohexanone over the Cu\u2013MgO catalysts prepared by different precipitating agents. The CM-A and CM-C catalysts shows a 64.3% cyclohexanol conversion with 100% selectivity towards cyclohexanone. The selectivity towards cyclohexanone is reported to be governed mainly by metal-support interaction, electronic and steric influence of the support, morphology of the metal particles, selective poisoning, influence and nature of second metal, effect and pressure and steric effects of substituents at the conjugated double bond [37]. Dehydrogenation of cyclohexanol is known to be an equilibrium-controlled reaction showing a maximum conversion of 68.89% at 523\u00a0K [38]. In the present investigation, Cu\u2013MgO catalyst prepared using K2CO3 as a precipitating agent resulted in relatively smaller Cu particles compared to CM-B, CM-C, CM-D and CM-E catalysts. The defective sites at Cu and MgO interfacial region and suitable particle size of Cu appears to be the key factor in governing the selectivity towards cyclohexanone.\nFig. 10\n a, b and c show the activity data relating to the conversion, selectivity and yield against time on stream over the catalyst prepared by different precipitating agents in the dehydrogenation of cyclohexanol.The CM-A and CM-C catalyst show a consistent activity of 64% conversion of cyclohexanol up to 5\u00a0h but the conversion over CM-B, CM-D and CM-E dropped down to 10%, 4.8% and 2.5% from 42.7%, 45% and 60.2% respectively. The large number of Cuo species on the surface of CM-A and CM-C catalysts which were responsible for higher interaction with the defect sites of MgO may have helped in yielding superior activity for cyclohexanol conversion.It is reported that various kinds of defects like steps, kinks, edges etc., impurities and vacancies at the surface of supports can interact with metal species [13,39]. There are reports with experimental evidence that the growth of metal clusters and films is initiated at the defect sites, in particular on surface vacancies. The adsorption of Cu and Pd moieties on MgO support with exposed (001) planes has been reported [39,40]. The high activity exhibited by the catalyst CM-A is due to the large number of smaller Cu crystallites at the surface evidenced by XRD data as shown in Table 1\n. Thus, the CM-A catalyst showed higher activity for dehydrogenation of cyclohexanol to cyclohexanone compared to other catalysts. Hence, K2CO3 was found to be a preferable precipitating agent in the preparation of highly active Cu\u2013MgO catalyst for both hydrogenation of furfural and dehydrogenation of cyclohexanol. Presence of smaller crystallites of Cu and surface enrichment by Cuo/Cu+ species in the Cu\u2013MgO catalyst prepared using K2CO3 appeared to be the reason for good hydrogenation and dehydrogenation activities.The literature available reveals that in Cu\u2013MgO catalysts, Cuo species acts as a catalytically active site [15,38\u201340]. Based on XPS analysis, the present of surface basic sites (oxygen vacancies) in present in the catalysts can be close contact with metallic Cuo sites and play a projecting role in facilitating the hydrogenation of aldehyde and dehydrogenation of alcohols significantly [8,18,41\u201343]. Considering above point, we propose a plausible mechanism for hydrogenation of furfural and dehydrogenation of cyclohexanol over Cu\u2013MgO co-precipitated catalyst. It is well represented in Scheme 1\n.The hydrogenation pathway starts with adsorption of hydrogen molecule over Cu\u2013MgO surface where dissociation of molecular hydrogen occurs on Cuo species followed by stepwise interaction of furfural with metallic copper and basic site of catalyst to yield furfuryl alcohol. Initially, carbonyl carbon of aldehydic group in furfural interacts with oxygen vacancies (basic sites) on the Cu\u2013MgO catalyst surface which highly activates the CO bond. This CO bond with enhanced nucleophilic character can be easily attacked by the dissociated hydrogen atom present on the surface of Cuo species. Therefore, the carbonyl oxygen (CO) gets one H atom while the second hydrogen is abstracted by carbonyl carbon (CO). Finally the desired product, furfuryl alcohol is formed which gets desorbed from the catalyst surface. This proposed mechanism well agrees with mechanism suggested in various published literature [41,44].In dehydrogenation pathway, basic sites on the Cu\u2013MgO catalyst surface play a crucial role in conversion of cyclohexanol to cyclohexanone. The oxygen vacancies (basic sites) act as a nucleophile and abstracts a proton from O\u2013H of cyclohexanol to form a negatively charged alkoxide intermediate. This intermediate undergoes \u03b2-H elimination to produce cyclohexanone which finally gets desorbed from the catalyst surface along with H2. This proposed mechanism is well supported by reported literature work [42,45]. Hence, in our case, reason for high conversion and selectivity for both the reactions over Cu\u2013MgO catalyst can be attributed to uniform distribution and high dispersion of active small crystallites of Cuo species over MgO support, as well as high synergistic interaction between active Cuo and MgO support.Highly efficient Cu\u2013MgO catalysts were prepared by co-precipitation method using five different precipitating agents such as K2CO3, KOH, Na2CO3, (COOH)2. 2H2O and NH3. The as-synthesized catalysts were investigated for their hydrogenation activity towards furfural to furfuryl alcohol and dehydrogenation activity of cyclohexanol to cyclohexanone individually. K2CO3 was found to be superior precipitating agent in yielding a Cu\u2013MgO catalyst that is highly active and stable towards both the mentioned reactions. The catalytic activity of all Cu\u2013MgO catalyst were tested for time on stream analysis and it was observed that the CM-A catalyst was active for 300\u00a0min yielding 96.71% furfuryl alcohol in furfural hydrogenation at 453\u00a0K and 64.3% cyclohexanone in cyclohexanol dehydrogenation at 523\u00a0K. The studied different analytical techniques revealed that the catalysts developed by different precipitating agents affected the reaction in different effective ways and solely depended on the physiochemical properties of as-synthesized catalyst. The physiochemical properties depicted by the XRD showed almost same crystallite size and BET surface area for CM-A and CM-C with the enrichment of Cu0/Cu\u00a0+\u00a0species as the results of higher catalytic activity. However, the results obtained from the TPR analysis depicted that the H2 uptake is better in case of CM-A which might have enriched the catalytic activity giving better results when compared with other catalysts.I hereby declare that I am submitting this manuscript on behalf of my co-author. My co-author is aware of this submission. This manuscript has not been previously published, is not currently submitted for review to any other journal, and will not be submitted elsewhere before a decision is made by this journal.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by Centre for Nano and Material Sciences (CNMS), JAIN (Deemed-to-be University), Bangalore and funding support was through the basic research grant of JAIN JU/MRP/CNMS/11/2022 and (11(39)/17/005/2017SG). Authors would like to thank Nano Mission, DST, Government of India, for partial financial support SR/NM/NS-20/2014. Meanwhile, authors thank to Director, Indian Institute of Chemical Technology, Hyderabad for his keen interest.", "descript": "\n                  The major industrial application and study of hydrogenation of furfural-to-furfural alcohol and dehydrogenation of cyclohexanol to cyclohexanone, over different precipitating agents namely K2CO3, Na2CO3, KOH, NH3 and (COOH)2.2H2O was carried out in vapour phase reactor. Efforts were made to study the effect of these different precipitating agents on Cu\u2013MgO catalyst and its activity towards the hydrogenation and dehydrogenation reactions due to its industrial applications. The prepared co-precipitated Cu\u2013MgO catalysts were characterized by using various modern analytical and spectroscopic techniques which include FE-SEM, BET, XRD, XPS, TPR and DTA. The characterization data revealed that different precipitating agents strongly influenced the physiochemical properties of the developed heterogeneous catalysts. Additionally, FE-SEM images revealed that employing different precipitating agents resulted in various morphologies for the final catalysts. The hydrogenation and dehydrogenation reactions over the Cu\u2013MgO catalyst revealed that the catalyst prepared by K2CO3 as precipitating agent exhibited high catalytic activity. Meanwhile, The presence of more Cuo/Cu+ species on this catalyst with smaller Cu crystallite size as evidenced by XPS and XRD results seems to be accountable for its high activity towards the formation of furfural alcohol and cyclohexanone compared to the other catalysts with different precipitating agents. Additionally, the time on stream (T.O.S) studies performed and it revealed that the catalyst was fairly stable for 300\u00a0min showing consistency in its activity towards both reactions. The yield obtained for furfural alcohol and cyclohexanone was 97% and 64%, respectively.\n               "}
{"full_text": "Deliberate consideration of materials for photoelectrochemical (PEC) water splitting is crucial, given that photoelectrodes must absorb the wide range of the solar spectrum to maximize photogenerated charge carriers, must have long carrier diffusion length and lifetime to minimize charge recombination, and must be cost-effective for practical application.\n1\n\n,\n\n2\n Silicon (Si), which has been extensively used in the photovoltaic (PV) industry, is a promising candidate for the photoanode because of its long carrier diffusion length, abundance, and well-established technologies to obtain highly crystalline and large-area wafers.\n3\n\n,\n\n4\n However, the utilization of silicon as a photoelectrode material raises the question in the view of relatively negative valence band position compared to the water oxidation potential, poor catalytic activity, and instability in aqueous solution. Silicon suffers from two main deleterious mechanisms, which contribute to the low stability.\n5\n The first is the self-oxidation of Si into SiOx, which suppresses the charge transfer at the Si/electrolyte interface because of electrical insulating property. The second mechanism is that at high pH electrolyte, silicon is chemically etched naturally.\n6\n\nThe research to date has been devoted to solving the challenges of silicon by introducing chemically stable and catalytic active materials as protection layers.\n7\n\n,\n\n8\n Since Kenney et\u00a0al.\n9\n first reported the Ni/n-Si photoanode by e-beam evaporator in 2013, various transition metals and their oxides and transparent conductive oxides have been used as both catalytic and passivation layers by high-vacuum processes such as e-beam evaporator and atomic layer deposition (ALD).\n10\n\n,\n\n11\n The introduction of a single passivation layer of silicon such as CoOx,\n12\n NiOx,\n13\u201315\n and MnO\n16\n thin film successfully protected the Si photoanode against surface corrosion and facilitated oxygen evolution reaction (OER) kinetics. Recent studies have shown that introducing heterogeneous catalysts on silicon via high vacuum processes (e.g., ALD, sputtering) exerts synergistic effects on PEC performance. Yang et\u00a0al.\n17\n introduced biphasic Co3O4/Co(OH)2 by the plasma-enhanced ALD method and showed the effects of multifunctional catalysts on OER activity. Oh\u2019s group\n18\n adjusted a similar strategy, in which double-layered CoO/Co3O4 films were formed using ALD to derive the collective characteristics in both high photovoltage and catalytic properties. However, those methods require the consumption of a large amount of energy and high cost, which conflict with the ultimate goal of sustainable energy production. The electrodeposition method, which is a facile and cost-effective method, has been applied to synthesizing various nanostructured photoactive materials and electrocatalysts by substituting the high vacuum processes.\n19\n In designing Si-based photoanodes, Switzer\u2019s group\n20\n first designed inhomogeneous Co nanoparticles (NPs) on a silicon surface by electrodeposition in 2015, which showed high water oxidation performance. This strategy was followed by the work of several groups to fabricate pinched-off photoanodes; NPs with core-shell structures exhibited improved photovoltage\n21\n\n,\n\n22\n and alloy NPs\n23\n\n,\n\n24\n exhibited excellent PEC properties compared to single metal NPs. Despite that the electrodeposition method has the merit of synthesizing materials with various nanostructures, there are limited reports using spherical NPs as catalysts for silicon photoanodes. Controlling the electrodeposition reaction to obtain various morphologies is quite difficult for the silicon substrate compared to other conductive substrates (e.g., Ni foam, fluorine-doped tin oxide glass) because of its fast corrosive behavior in the electrolyte.Another challenge of Si photoanodes is that additional bias is required for driving OER spontaneously. The photovoltage generated in\u00a0silicon is usually <0.7 V, which is far below the voltage for spontaneous water splitting. It is estimated that at least 1.6\u00a0V is needed to meet the thermodynamic voltage of 1.23\u00a0V for water splitting and to compensate over 0.35\u00a0V for the energy losses and kinetics overpotentials at the semiconductor/liquid interfaces.\n2\n\n,\n\n25\n Because of insufficient photovoltage output, efforts have been made to design integrated devices such as PEC tandem photoanodes,\n26\n\n,\n\n27\n integrated PV/PEC devices,\n28\n and PV/electrolyzer devices.\n29\n Although high solar-to-hydrogen (STH) efficiency >25% is estimated for tandem-based light absorbers when Si is integrated with 1.6\u20131.8 eV band gap materials,\n30\n\n,\n\n31\n an additional complicated buried junction is necessary to minimize charge recombination and facilitate charge transfer.\n32\n In terms of tandem device fabrication, the utilization of PEC cells and PV devices is attractive because of device fabrication costs, solar light use, and the possibility of achieving relatively high STH efficiency.In this article, we report the incorporation of heterogeneous Ni-based catalysts synthesized by the electrodeposition method on Si substrates and investigate their synergetic effects on PEC water splitting. We show that the morphology and water oxidation property of Ni-based catalysts can be easily tuned by changing the precursors in the electrolyte, resulting in Ni NPs and Ni(OH)2 thin films. A systematic study reveals that heterogeneous Ni NPs/Ni(OH)2/n-Si shows 2.5 times higher stability and higher enhanced charge transfer kinetics than Ni(OH)2/n-Si and 130 mV lower onset potential than Ni NPs/n-Si. One step further, by combining heterogeneous Ni NPs/Ni(OH)2/n-Si photoanodes and perovskite/Si tandem solar cells, the wired tandem cell device shows a photocurrent of 9.8 mA cm\u22122 without external bias corresponding to the STH conversion efficiency of 12%.The electrodeposition of Ni-based catalysts used in this study was conducted in the three-electrode system as shown in Figure\u00a01\nA. Recently, the electrodeposition of distinctive morphologies of Co-based catalysts on the n-Si surface has been conducted by controlling the kinds of additives that affect the pH of the electrolyte.\n33\n We used different electrolytes to control the reaction accurately in real time to synthesize different morphologies of the Ni-based catalysts. The electrodeposition of Ni(OH)2 was conducted under nickel nitrate electrolytes by applying a constant current of \u22120.15 mA cm\u22122 versus a saturated calomel electrode (SCE) (Figure\u00a01B). It has been reported that in the presence of water, nitrate ions are reduced to nitrite ions, generating hydroxide ions. The hydroxide ions near the working electrode surface combine with the nickel cations by an electrochemical (EC) reaction to form Ni(OH)2 films.\n34\n The inset indicates the schematic of the Ni(OH)2 film on the silicon surface. Similarly, Ni NPs were pulse electrodeposited under nickel sulfate electrolyte containing boric acid as an additive; Figure\u00a01C shows the duration and amplitude of the deposition potential versus SCE. The fabrication of heterogeneous Ni NPs/Ni(OH)2 on top of the silicon surface was conducted by a two-step electrodeposition, as described above. Figure\u00a01D presents the photographs of electrodeposited Ni(OH)2 (orange), Ni NPs (green), and Ni NPs/Ni(OH)2 (blue), respectively, on the silicon surface. It is well known that the local pH near the electrode surface can increase during the electrodeposition of iron-group metals, generating hydroxides.\n35\n In this respect, the addition of boric acid in the electrolyte can suppress the formation of hydroxides as a buffer reagent. The surface morphologies of Ni-based catalysts were analyzed by field emission scanning electron microscopy (FESEM). Figures 1E\u20131G show the scanning electron microscopy (SEM) images of Ni(OH)2, Ni NPs, and Ni NPs/Ni(OH)2, respectively. The SEM images reveal that various surface morphologies of Ni-based catalysts can be obtained through electrodeposition by controlling the reaction parameters. The shape of nickel particles was tuned by electrodeposition in a nickel chloride electrolyte containing glycine as an additive. As shown in Figure\u00a0S1, the morphology of the Ni NPs became branch shaped, which reveals the merits of electrodeposition as a simple and facile synthetic approach to control morphology. To investigate the effects of conditions of applying deposition currents in detail, we performed 3 types of electrodeposition of Ni NPs. We reported the pulse electrodeposition of Ni particles, in which the number of deposition cycles affects the PEC performance.\n21\n Compared to the continuous electrodeposition, it is possible to promote more nucleation and obtain uniform films because ions can be replenished during pulse-off time, decreasing the concentration variation. Figure\u00a0S2A shows the different electrodeposition modes of Ni NPs. By applying a direct current (DC) of \u22120.8 mA cm\u22122, Ni NPs showed size variations (Figure\u00a0S2B). However, the more uniform size distribution of Ni NPs was observed when pulse electrodeposition (denoted as pulse-1) was conducted with the duration (ton/ton+toff) of 0.5 in the same electrolytes and applied current density (Figure\u00a0S2C). This tendency is observed when applying a high current density of \u22128 mA cm\u22122 for a few seconds and conducting the same pulse electrodeposition (denoted as pulse-2). As shown in Figure\u00a0S2D, the increased number of Ni NPs with decreased particle size was formed uniformly on silicon substrates. We conducted successive electrodepositions of Ni NPs on Ni(OH)2/n-Si using 3 types of deposition mode. Since the electrodeposition of catalysts occurs uniformly on the conductive substrates, where the resistance of the surface is constant, the size and coverage of the Ni NPs on Ni(OH)2/n-Si were quite different from those of Ni NPs/n-Si (Figures S2E\u2013S2G). It was observed that there were no significant morphological differences between DC and pulse-1 electrodeposition. However, pulse-2 electrodeposition showed an increased number of nickel particles whose size was smaller than that of DC and pulse-1 electrodeposition. By applying high current density in the initial step during pulse electrodeposition, Ni(OH)2 films were partially etched, and more nucleation of Ni NPs occurred.Transmission electron microscopy (TEM) was conducted to clarify the element distributions and morphologies of electrodeposited Ni-based catalysts. To prevent the surface from the damage induced by a focused ion beam (FIB), the application of carbon and Pt coating was introduced. Energy-dispersive spectroscopy (EDS) was carried out to investigate the elements of the fabricated Ni-based catalysts on the silicon surface. As shown in Figures 2A and 2B, Ni(OH)2 films were uniformly formed, showing a thickness of 50\u00a0nm, and the size of Ni NPs corresponded to 50\u00a0nm. EDS analysis revealed the difference in the presence of oxygen between Ni(OH)2 and Ni NPs. As shown in Figure\u00a0S3, high-resolution TEM images and the electron patterns indicate the amorphous Ni(OH)2 thin films and crystalline Ni NPs. With continuous electrodeposition, the thickness of the Ni(OH)2 films significantly decreased to 3\u00a0nm because of electrical and chemical etching during the electrodeposition of Ni NPs. Uniform oxygen distribution on the silicon surface shows the existence of Ni(OH)2 thin layers, as shown in Figure\u00a02C. X-ray photoelectron spectroscopy (XPS) was carried out to identify the chemical state of the electrodeposited Ni-based catalysts. Figure\u00a02D shows the Ni 2p spectrums of Ni(OH)2//n-Si, Ni NPs/n-Si, and Ni NPs/Ni(OH)2/n-Si, which result from the spin-orbit splitting of the p orbital (Ni 2p3/2 and Ni 2p1/2). The Ni(OH)2/n-Si photoanode showed the main peak of Ni 2p3/2 at 855.2 eV and Ni 2p1/2 at 872.8 eV and the binding energy difference (Ni 2p3/2, Ni 2p1/2) of 17.6 eV, which is assigned to nickel hydroxide.\n19\n\n,\n\n36\n The Ni 2p peaks of Ni NPs/n-Si indicated Ni0 (852.4 eV, 869.7 eV), where the binding energy difference between the Ni 2p3/2 and Ni 2p1/2 of 17.3 eV is indicative of metallic nickel and Ni2+ or nickel hydroxide (855.4 eV, 873.4 eV).\n37\n\n,\n\n38\n Heterogeneous Ni NPs/Ni(OH)2/n-Si photoanodes showed the peaks of 853.5 and 854.8 eV, which correspond to the metallic Ni and Ni2+, respectively.\n39\u201341\n\nFigure\u00a02E shows the O 1\u00a0s spectrums of Ni NPs/n-Si, Ni(OH)2/n-Si, and Ni NPs/Ni(OH)2/n-Si, in which different chemical states of oxygen were identified. The O 1\u00a0s peaks of Ni NPs/n-Si indicated typical metal-oxygen bonds (529.3 eV) and surface hydroxide groups (531.8 eV), and that of Ni(OH)2/n-Si showed metal hydroxides (530.5 eV) and surface \u2013OH groups (531.6 eV). Combined Ni NPs/Ni(OH)2/n-Si revealed the peaks of 531 and 532.1 eV, which correspond to metal hydroxides and surface \u2013OH groups, respectively.\n19\n\n,\n\n37\n\n,\n\n42\n\n,\n\n43\n\nThe PEC water oxidation behavior of n-Si coated with Ni-based catalysts was investigated under 100 mW cm\u22122 AM 1.5\u00a0G irradiation in 1\u00a0M NaOH electrolyte. Figure\u00a03\nA shows the current density versus potential (J-V) curves of Ni(OH)2/n-Si, Ni NPs/n-Si, and Ni NPs/Ni(OH)2/n-Si photoanodes. The light-limited photocurrent density of Ni(OH)2/n-Si at 1.23\u00a0V versus reversible hydrogen electrode (RHE) was 31.4\u00a0mA\u00a0cm\u22122, Ni NPs/n-Si of 11.44 mA cm\u22122, and Ni NPs/Ni(OH)2/n-Si of 29.6\u00a0mA cm\u22122. As shown in Figure\u00a0S4A, the Ni NPs (branched)/n-Si photoanode also showed a high photocurrent density of 25.8 mA cm\u22122 at water oxidation potential. The onset potential of the photoanode in the J-V curves was defined as the potential at which the photocurrent density recorded 1 mA cm\u22122. Compared to the onset potential for Ni NPs/n-Si of 1.15\u00a0V versus RHE, both the Ni(OH)2/n-Si and the Ni NPs/Ni(OH)2/n-Si photoanode showed the onset potential of 1.02\u00a0V versus RHE. The morphology-controlled Ni NPs (branched)/n-Si photoanode recorded the onset potential of 1.03\u00a0V versus RHE. For comparison with the previously reported Si-based photoanodes, the current densities at water oxidation potential, onset potentials, and stability values are summarized in Table S1. Applied bias photon-to-current efficiency (ABPE) of the Si-based photoanodes, which ascertains the external bias dependence of actual PEC devices, was calculated based on the linear sweep voltammetry (LSV) curves, as shown in Figure\u00a03A. Compared to the Ni NPs/n-Si photoanodes, which showed the maximum value of 0.18% at 1.2\u00a0V versus RHE, both Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si photoanodes recorded the maximum value of 1.76% at 1.14\u00a0V versus RHE and 1.75% at 1.13\u00a0V versus RHE, respectively, which is 9.7 times higher than that of Ni NPs/n-Si (Figure\u00a0S6A). The Ni NPs (branched)/n-Si photoanode recorded the maximum ABPE value of 1.23% at 1.14\u00a0V versus RHE (Figure\u00a0S4B). XPS analysis was performed to scrutinize the oxidation states of the Ni-based catalysts after LSV measurements. As shown in Figure\u00a0S5, after PEC measurements, Ni 2p peaks of all of the Ni-based catalysts exhibited the presence of Ni2+ (855.2 and 872.9 eV).\n44\n The J-V behavior of Ni-based catalysts/p++-Si electrode, in which p++-Si is not photoactive material, was measured in a 1 M NaOH electrolyte under dark conditions to evaluate the oxygen-evolving activity of the catalysts. Compared to the PEC performance of n-Si photoanodes, the EC performance of Ni-based catalysts showed contradictory behavior (Figure\u00a03B). The overpotential to produce the current density of 10 mA cm\u22122 of Ni(OH)2 was 300\u00a0mV, which is 60\u00a0mV higher than that of Ni NPs. With the integration of both catalysts, Ni NPs/Ni(OH)2 requires an overpotential of 250\u00a0mV for producing 10 mA cm\u22122, which is quite similar to that of Ni NPs. Using the J-V curve of Figure\u00a03B, the Ni NPs/Ni(OH)2 showed the Tafel slope of 52.45\u00a0mV dec\u22121. We calculated the photovoltage generated at n-Si photoanodes, where photovoltage can be derived by the difference in onset potential between n-Si under light illumination and metallic p++-Si under dark conditions with the same catalysts. The calculated photovoltage generated at Ni(OH)2/n-Si in 1\u00a0M NaOH was 520\u00a0mV, and Ni NPs/n-Si showed the photovoltage as 150\u00a0mV lower than Ni(OH)2/n-Si. The integrated Ni NPs/Ni(OH)2/n-Si photoanodes showed a photovoltage of 500\u00a0mV (Figure\u00a03C). By combining the EC and PEC characteristics of different Ni-based catalysts on silicon, their contradictory behaviors came together to exert synergistic effects on both catalytic activity and junction behavior at the interfaces. The electrochemical impedance spectroscopy (EIS) was performed to elucidate the effects of Ni-based catalysts on the kinetics of charge transfer during the water oxidation reaction (Figure\u00a03D). The EIS analysis was conducted at an external bias near the onset potential\u00a0to exclude any possible intricate factor. The equivalent circuits used to fit the measured EIS data consisted of charge transfer resistance (Rct) and capacitance (C) elements, where Rct,1 indicates the contact resistance of silicon, Rct,2 the resistance between silicon and Ni-based catalysts, and Rct,3 the resistance between Ni-based catalysts and electrolytes. In the Nyquist plots, small semicircular arcs mirror the low charge transfer resistance at the interfaces. As summarized in Table 1\n, the Ni(OH)2/n-Si photoanode showed the high charge transfer resistance of 16.1\u00a0\u03a9 cm2 at the n-Si/catalyst interface and low charge transfer resistance of 2.7\u00a0\u03a9 cm2 between the catalyst/electrolyte interface. Ni NPs/n-Si showed the opposite results, in which the resistance between the catalyst/electrolyte interface was significantly higher than that of the n-Si/catalyst. By introducing both catalysts, the charge transfer resistances of the Ni NPs/Ni(OH)2/n-Si photoanode were buffered, in which the Rct,2 value is similar to that of Ni NPs/n-Si and the Rct,3 value decreased compared to the Ni(OH)2/n-Si photoanode.To scrutinize the PEC performance of fabricated Si-based photoanodes, we calculated and evaluated the efficiencies of photoanodes. In the case of photoanodes that oxidize water to generate oxygen, the amount of hole injected to the catalyst/electrolyte interface represents the charge injection efficiency. Na2SO3 was used as a hole scavenger; the oxidation of sulfite is thermodynamically and kinetically more favorable than the oxidation of water, resulting in eliminating the injection barrier. Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si photoanodes showed similar charge injection efficiencies up to 1.3\u00a0V versus RHE; however, at potentials >1.3\u00a0V versus RHE, the Ni NPs/Ni(OH)2/n-Si photoanode exhibited almost 100% compared to the Ni(OH)2/n-Si of 90% (Figure\u00a0S6B). The chronoamperometric measurements of Ni(OH)2/n-Si, Ni NPs/n-Si, and Ni NPs/Ni(OH)2/n-Si photoanodes were carried out under chopped light (on/off) at 1.5\u00a0V versus RHE to investigate the charge recombination at the transient state (Figure\u00a0S6C). All of the photoanodes instantly reacted to the light irradiation to the same extent, which indicates negligible leakage current in the light-off condition.\n8\n The Ni(OH)2/n-Si photoanode revealed the weak current spike and a slight decrease in photocurrent spectra when the light was switched on, which indicates that accumulated carriers at the transient state would lead to the electron-hole recombination.\n45\n By introducing Ni NPs, both Ni NPs/n-Si and Ni NPs/Ni(OH)2/n-Si photoanodes showed the constant photocurrent density, which would indicate the reduced charge recombination. Incident photon-to-current efficiency (IPCE), which reveals the overall efficiency of water splitting the photoanodes, was measured at a bias of 1.23\u00a0V versus RHE in a 1 M NaOH electrolyte (Figure\u00a03E). Compared with the Ni NPs/n-Si, which exhibited lower than 10% in the wavelength range of 400\u2013800\u00a0nm, both the Ni(OH)2/n-Si and the Ni NPs/Ni(OH)2/n-Si photoanode reached the efficiency of \u223c80% at nearly 800\u00a0nm. When the sufficient bias of 1.5\u00a0V versus RHE was applied for reaching the saturated current density of all photoanodes, Ni NPs/n-Si photoanodes showed the significantly improved value in the range of 400\u2013800\u00a0nm, which is similar to the Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si photoanode (Figure\u00a0S6D). All photoanodes can absorb visible light under the sufficient applied voltage and are highly reactive to the visible spectrum. The chronoamperometric measurements were conducted to determine whether the Ni-based catalysts could behave as a passivation layer. As shown in Figure\u00a0S7A, although the Ni(OH)2/n-Si showed the cathodic onset potential shift of 130\u00a0mV compared to Ni NPs/n-Si, Ni(OH)2/n-Si was able to withstand 5,000\u00a0s and degradation of the photocurrent density was observed; at the same time, Ni NPs/n-Si maintained its performance for 25,000\u00a0s without degradation. Ni NPs (branched)/n-Si operated up to 2,000\u00a0s and drastically lost performance, as shown in Figure\u00a0S4C. After the stability test, the surface of Ni(OH)2/n-Si and Ni NPs/n-Si was partially peeled off, resulting in exposure of the silicon surface to the alkaline electrolyte, which accelerates the degradation of photoanodes (Figures S7C\u2013S7H). By introducing relatively stable Ni NPs on highly active Ni(OH)2/n-Si, the stability of Ni NPs/Ni(OH)2/n-Si was improved 2.5 times higher than that of Ni(OH)2/n-Si. Based on these results, integrating both Ni NPs and Ni(OH)2 displays their synergetic effects in catalytic activity and stability. The long-term stability test of heterogeneous Ni NPs/Ni(OH)2/n-Si was then conducted in a mild alkaline K-borate buffer (K-Bi) solution (pH 9.5). As shown in Figure\u00a03F, a remarkable 6-day operation duration of Ni NPs/Ni(OH)2/n-Si was observed without any decay. By comparing the stability of the photoanode in high alkaline 1\u00a0M NaOH and mild 1\u00a0M K-Bi electrolyte, fascinating long-term stability can be attributed to the suppression of Ni NPs/Ni(OH)2 to be cracked and chemical etching of silicon. After a 6-day operation in 1\u00a0M K-Bi, the Ni2+/3+ peak disappeared and showed activated J-V characteristics (Figure\u00a0S7B). The morphology changes in Ni NPs/Ni(OH)2, such as cracks and pinholes, were observed as shown in Figures S7I\u2013S7K.The PEC performance of photoelectrodes is significantly suppressed when the rate constant of the surface charge transfer (ktrans) is lower than that of the charge recombination (krec).\n46\n\n,\n\n47\n We conducted intensity-modulated photocurrent sinusoidal spectroscopy (IMPS) to further investigate charge transfer and recombination kinetics, unveiling photocurrent in regard to sinusoidal modulation of DC illumination. IMPS is a useful method for deriving the ktrans and krec of PEC devices, where the apex of the semicircle indicates ktrans\u00a0+ krec and the ratio of the normalized real photocurrent intercepts at low frequency and high frequency reveals a charge transfer efficiency defined as ktrans/(ktrans\u00a0+ krec). Through the apex of the semicircle and the normalized real photocurrent intercepts, it is possible to calculate both ktrans and krec. Figures 4A and 4B show Nyquist plots consisting of the complex photocurrent of Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si photoanodes at various applied potentials. In both photoanodes, two semicircles were observed due to the presence of surface states.\n48\n The calculated ktrans and krec of Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si at water oxidation potential are shown in Figures 4C and 4D. The values of ktrans and krec of Ni(OH)2/n-Si at 1.25\u00a0V versus RHE are 0.51 and 12.26 s\u22121, corresponding to 196.2 and 8.15\u00a0ms, and those of Ni NPs/Ni(OH)2/n-Si are 0.8 and 17.71 s\u22121, corresponding to 124.9 and 5.64\u00a0ms. The increase in ktrans and similar krec were observed, which demonstrates that introducing Ni NPs on Ni(OH)2/n-Si expedites charge transfer at the surface. When the n-type semiconductor is illuminated, photoexcited charges are separated in the presence of the space charge layer. The migration of photoexcited charges induces the inverse potential called photopotential in the electrode, reducing the potential across the space charge layer. The band bending of n-type semiconductors is significantly affected by the charge recombination, photoexcited hole accumulation, and intrinsic potential of a semiconductor-liquid junction. Since the extent of band bending for n-type semiconductors serves as a driving force for charge separation and transport, the increase of band bending is beneficial in terms of PEC operation.\n22\n The open-circuit potential (OCP) difference between dark (OCPdark) and sunlight irradiation (OCPlight) of heterogeneous Ni-based catalysts on Si photoanodes was obtained in 1\u00a0M NaOH electrolytes to compare the extent of band bending. As shown in Figure\u00a0S8, the value of |OCPlight \u2212 OCPdark| was 0.21\u00a0V for Ni(OH)2/n-Si and 0.18\u00a0V for Ni NPs/n-Si. The electrodeposition of Ni(OH)2 and Ni NPs on silicon induce band bending at the semiconductor/catalyst junction. Ni(OH)2 has an electrolyte-permeable structure, in which electronic charges can be balanced by solution ion movement, resulting in a decrease in the electrostatic potential drop at the catalyst/electrolyte interface.\n49\n Depositing Ni(OH)2 onto Si leads to an adaptive semiconductor/electrocatalyst junction, which can enlarge the effective junction barrier height.\n50\n In the case of Ni NPs, when metal NPs with a high work function are introduced to the semiconducting silicon surface, inhomogeneous barrier heights are formed by the pinch-off effect. This inhomogeneous contact induces band bending at the interface, where photogenerated charges are moved to a low barrier region to oxidize water.\n5\n Heterogeneous Ni NPs/Ni(OH)2/n-Si photoanodes showed a value of 0.25 V, which implies that introducing both Ni NPs and Ni(OH)2 enlarges the band bending of the Si photoanodes. Considering that the krec of both Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si was similar, thin Ni(OH)2 between partially formed Ni NPs and n-Si affects the intrinsic potential of the Si/electrolyte junction. There was a discrepancy between the value of OCP and photovoltage, which is defined as the onset potential difference between photoactive n-Si and metallic p++-Si. The extent of band bending is determined by the difference between the Fermi level of the photoanode and the redox level of the electrolyte, called the built-in potential (Vbi). Although the ideal OCP should be the same as Vbi, the photovoltage is defined by the quasi-Fermi-level difference between electrons and holes. The OCP value is influenced by the PV losses from charge separation, recombination, and so on.\n51\n Whether catalysts are ion permeable or ion impermeable can affect the barrier height change and OCP. With ion-permeable catalysts, an adaptive junction is formed, in which the barrier height changes depending on the oxidation level of the catalysts. Dense catalysts form a buried junction with constant barrier height.\n50\n Considering the IMPS results, fast charge recombination kinetics probably affected the smaller OCP value. The oxygen evolution of the Ni NPs/Ni(OH)2/n-Si photoanode was measured by gas chromatography (GC) at 1.5\u00a0V versus RHE. As shown in Figure\u00a0S9A, the blue dots indicate the faradic efficiency value >90%, demonstrating that most of the photogenerated charges are consumed for the OER. The generated amounts of O2 were close to the theoretical value, which is calculated based on the passed charge (Figure\u00a0S9B). To assess the catalytic ability of Ni NPs/Ni(OH)2 as oxygen-evolving catalysts, turnover frequency (TOF) was calculated by integrating the cyclic voltammetry (CV) curves of Ni2+/3+ to derive the number of Ni active sites (Figure\u00a0S9C). The calculated TOF of Ni NPs/Ni(OH)2/n-Si was 6.648\u00a0\u00d7 103 h\u22121. In summary, heterogeneous Ni NPs/Ni(OH)2 catalysts facilitate charge transfer kinetics and enlarge the band bending at the interfaces, leading to enhanced water oxidation property.The fabricated heterogeneous Ni NPs/Ni(OH)2/n-Si photoanode generated the photovoltage of 500\u00a0mV; however, it is not ample enough for driving the spontaneous water splitting reaction. We fabricated a wired tandem cell device consisting of Ni NPs/Ni(OH)2/n-Si photoanodes as a light absorber and a perovskite/Si tandem solar cell as a voltage supplier. Although the ultimate goal of designing a tandem device is a wireless tandem configuration for commercial applications due to the ease of assembly, wireless tandem devices to date still have difficulty in achieving high STH efficiency >10% for commercial applications.\n26\n\n,\n\n52\n Therefore, we wired high-performance Si photoanodes and perovskite/Si tandem solar cells under parallel illumination to obtain high STH efficiency. The two-electrode system measurement was conducted to evaluate the performance of the front Ni NPs/Ni(OH)2/n-Si photoanodes and the rear perovskite/Si tandem solar cell in 1\u00a0M NaOH electrolyte under 1-sun irradiation. The structure of perovskite/Si tandem cells is shown in Figure\u00a0S10A. Under the two-electrode system, the operating current density (Jop) and operating voltage (Voc) of the cell can be defined. As shown in Figure\u00a0S10B, the fabricated perovskite/Si tandem solar cell device showed a short-circuit current density (Jsc) of 19.31 mA cm\u22122 and open-circuit voltage of 1.802 V. The corresponding fill factor was recorded at 77.9% and a power conversion efficiency (PCE) of 27.1% was achieved, as recently reported.\n53\n The schematic of the wired tandem cell device is shown in Figure\u00a05\nA. The active area ratio of the front photoanode to the rear perovskite cell was 1:1. Based on the intersection of the J-V curves of heterogeneous Ni NPs/Ni(OH)2/n-Si photoanodes and a perovskite/Si tandem solar cell, it can be assumed that the photocurrent of 8.8 mA cm\u22122 can be achieved without external bias (Figure\u00a05B). The actual wired tandem cell device performed for 2\u00a0h in chopped light, with an interval of 5\u00a0min without a sign of degradation. As shown in Figure\u00a05C, the average photocurrent density of the combined tandem cell was 9.8 mA cm\u22122, which corresponds to 12% STH efficiency.We successfully demonstrated the synthesis of heterogeneous Ni-based catalysts via electrodeposition on Si substrates and investigated their PEC performances. The morphology of Ni-based catalysts was controlled by changing the precursors in the electrolyte; Ni NPs with spherical and branched shapes and Ni(OH)2 films were obtained. We showed that a combination of Ni NPs and Ni(OH)2 showed synergistic effects on PEC water oxidation by taking advantage of the high catalytic activity of Ni(OH)2 and the high stability of Ni NPs. The high photocurrent density of 29.6 mA cm\u22122 at 1.23\u00a0V versus RHE and stability >140\u00a0h was achieved. These results showed the possibility of synthesizing multifunctional heterogeneous catalysts via the electrodeposition method, which permits efficient interfacial charge transport kinetics at the catalyst-electrolyte interface, replacing the high vacuum processes in existence. Finally, we fabricated wired tandem cell devices and evaluated their water oxidation properties. Without external bias, the wired tandem cell generated a photocurrent of 9.8 mA cm\u22122, which corresponds to an STH efficiency of 12%. Our systematic study reveals that introducing heterogeneous catalysts on Si photoanodes by the low-cost electrodeposition method is an underlying strategy to enhance the PEC characteristics of Si photoanodes. Although silicon photoanodes covered with electrodeposited catalysts may not exert sufficient stability compared to the photoanodes prepared using high vacuum processes, the large photovoltage and high catalytic activity can be achieved without the buried junction. We believe that this study can be a breakthrough in view of the synthesizing catalysts for silicon, opening the doors for the fabrication of high-performance PEC water splitting devices without additional bias.The lead contact for this paper is Ho Won Jang (hwjang@snu.ac.kr).This study did not generate new unique reagents.The authors declare that the data supporting the findings of this study are available within the article and Supplemental Information. All other data are available from the Lead Contact upon reasonable request.Nickel sulfate hexahydrate (NiSO4\u22c56H2O), nickel chloride hexahydrate (NiCl2\u22c56H2O), nickel nitrate hexahydrate (Ni(NO3)2\u22c56H2O), hydrogen peroxide solution (H2O2, 35%), hydrochloric acid (HCl, 35%), sulfuric acid (H2SO4, 95%), potassium nitrate (KNO3), glycine (C2H5NO2), 1\u00a0N sodium hydroxide standard solution (1\u00a0N NaOH), and 1\u00a0N standard potassium hydroxide solution (1\u00a0N KOH) were procured from Daejung Chemical. The buffer oxide etchant (6:1) was purchased from J.T. Baker. Boric acid (H3BO3) and dipotassium phosphate (K2HPO4) were sourced from Junsei. Poly(triaryl amine) (PTAA), dimethylformamide (DMF), N-methyl-2-pyrrolidone\u00a0(NMP), polyethylenimine (PEIE), and toluene were purchased from Sigma-Aldrich. Formamidinium iodide (FAI), methylammonium bromide (MABr), phenethylammonium iodide (PEAI), and phenethylammonium thiocyanate (PEASCN) were procured from GreatCell Solar. Lead iodide (PbI2) and lead bromide (PbBr2) were sourced from TCI Chemicals.On top of the silicon bottom cell with a 20-nm-thick ITO recombination layer, a PTAA/perovskite/C60 layer was sequentially deposited. PTAA solution (5\u00a0mg/mL in toluene) was spin coated at 6,000\u00a0rpm for 25\u00a0s and annealed at 100\u00b0C for 10\u00a0min. Perovskite solutions were prepared by dissolving FAI, MABr, CsI, PbI2, and PbBr2. Their molar ratios were adjusted to form stoichiometric (FA0.65MA0.20Cs0.15)Pb(I0.8Br0.2)3 in a DMF and NMP (4:1 [v/v]) mixed system. An additive 2D perovskite solution was prepared by adding 2 mol% Pb(SCN)2 and 2 mol% PEAX (X\u00a0= I, SCN) to the 3D perovskite solution. The solution was spin coated at 4,000\u00a0rpm for 20\u00a0s on PTAA film. Subsequent immersing of spin-coated film in diethyl ether (DE) for 30\u00a0s was conducted. The color of the films turned dark brown, which indicates that perovskite films were crystallized. Then, the films were annealed at 100\u00b0C for 10\u00a0min. C60 layers (C60, bathocuproine [BCP], Ag electrode) were deposited by the thermal evaporator. A 0.2 wt% of PEIE (80% ethoxylated) solution in methylalcohol was\u00a0spin coated at 6,000\u00a0rpm for 30 s. ITO films were deposited on the C60/PEIE layer using radiofrequency sputtering at room temperature (working pressure: 2\u00a0\u00d7\u00a010\u22123 mTorr). A 150-nm-thick Ag metal grid was deposited using a thermal evaporator on the ITO film.An n-Si (100) wafer (1\u201310\u00a0\u03a9 cm) was cut into 1.5\u00a0\u00d7 1.5\u00a0cm2 pieces. The wafers were cleaned with acetone, isopropanol alcohol, and ultrapure water by ultrasonication. To remove residual contaminants, Si pieces were cleaned using a piranha etching process; soaked in 3/1 v/v concentrated H2SO4/H2O2 solution for 10\u00a0min, and immersed in a buffered HF etchant for 30 s. Then, wafers were soaked in 5/1/1 (by volume) concentrated H2O, HCl, and H2O2 at 80\u00b0C for 30\u00a0min. The Si pieces were rinsed with ultrapure water and dried under a N2 flow.Before electrodeposition, ohmic contact was formed by scratching the backside of silicon and applying an InGa alloy (Sigma-Aldrich). Then, copper wire was attached by depositing silver paste on the InGa alloy. After the silver paste dried, the Si surface was covered with adhesive tape, except for the active area (1\u00a0\u00d7 1\u00a0cm2), to prevent contact with the electrolyte. n-Si was soaked for 30\u00a0s in the buffer oxide etchant (6:1, J.T. Baker) to remove the residual SiO2 layer and organic solvent from the surface. Then, Ni-based catalysts were deposited on the silicon surface by electrodeposition. The electrodeposition of Ni-based catalysts was conducted in a standard three-electrode\u00a0system: an encapsulated Si electrode as the working electrode, a Pt mesh as the counter electrode, and a SCE as the reference electrode. In the case of Ni NPs (sphere), Ni aqueous plating solution was prepared by dissolving 0.1\u00a0M nickel sulfate hydrate (NiSO4\u22c56H2O, Daejung) and 0.1\u00a0M boric acid (H3BO3, Junsei). Pulsed electrodeposition was conducted by applying \u22128 mA cm\u22122 for 2\u00a0s and \u22120.8\u00a0mA cm\u22122 for 20 s. For the deposition of Ni NPs (branched), 0.1\u00a0M nickel chloride hydrate, 0.3\u00a0M glycine, and 0.6\u00a0M K2HPO4 were dissolved in deionized (DI) water, and a plating solution was kept at 65\u00b0C. The electrodeposition of Ni NPs (branched) was conducted by applying \u22120.5 mA cm\u22122 for 40 s. Ni(OH)2 films were synthesized by dissolving 0.004\u00a0M nickel nitrate hydrate (NiNO3\u22c56H2O, Daejung) and 0.01\u00a0M KNO3 (Daejung). The electrodeposition of Ni(OH)2 was conducted by applying \u22120.15 mA cm\u22122 for 100 s. After electrodeposition, the Si pieces were rinsed with DI water, dried under nitrogen gases, and adhesive tapes were detached.After electrodeposition of catalysts on the Si surfaces, Si pieces were processed to fabricate the electrodes. The backside of the Si surface was scratched, and InGa eutectic was applied to establish ohmic contact. The silver paste was deposited on the contact and Cu wire. After the silver paste layer dried, the whole surface of the silicon except the active area was covered with an epoxy-based resin. The samples were dried for 12\u00a0h in air to cure the resin.PEC measurements (Ivium Technologies, Nstat) were performed with a three-electrode system using Ag/AgCl (saturated) reference electrode and a Pt plate as a counter electrode in 1\u00a0M NaOH electrolyte (pH 14). A Xe arc lamp (Abet Technologies, LS150) was used as a light source, and the light intensity was calculated to 1 sun (100 mW cm\u22122, AM 1.5 G) using a reference photodiode. For measuring PEC performance, the potential was swept toward the anodic direction and the quartz vessel was used to avoid UV absorption. The IPCE was carried out using a monochromator and light source. The applied potential was 1.23 and 1.5\u00a0V versus RHE to compare the effect of applied voltage on samples. EIS was conducted near the onset potential (1.02 V versus RHE for Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si, 1.15 V versus RHE for Ni NPs/n-Si). The EIS data were fit to the equivalent circuits, which were discussed in the text, using the Z plot 2.x software. The sweeping frequency ranged from 250 kHz to 0.1\u00a0Hz using alternating current with an amplitude of 10\u00a0mV. The measured potential versus Ag/AgCl was converted to the RHE scale according to the Nernst equation:\n\n(Equation\u00a01)\n\n\n\nE\n\nR\nH\nE\n\n\n=\n\nE\n\n\nA\ng\n\n/\n\nA\ng\nC\nl\n\n\n\n+\n0.059\npH\n\u00d7\n\nE\n\n\nA\ng\n\n/\n\nA\ng\nC\nl\n\n\nO\n\n\n\n\nwhere E\n\nRHE\n is the converted potential versus RHE, E\n\nO\n\n\nAg/AgCl\n\u00a0= 0.198\u00a0V at 25\u00b0C and E\n\nAg/AgCl\n is the experimentally measured potential versus the Ag/AgCl reference. The GC system (Agilent GC 7890B) was used to calculate the faradic efficiency and amount of generated oxygen. Two-compartment electrochemical cells consisting of glass body and quartz glass windows was used for GC measurements. The amounts of H2 and O2 were measured. The LSV curves in 1.0\u00a0M NaOH with or without 0.5\u00a0M Na2SO3, which was used as the hole scavenger, were used to calculate the charge injection efficiency (\u03a6inj) at the electrode-electrolyte interface:\n\n(Equation\u00a02)\n\n\n\nJ\n\nP\nE\nC\n\n\n=\n\nJ\n\na\nb\ns\n\n\n\u00d7\n\n\u03a6\n\ns\ne\np\n\n\n\u00d7\n\n\u03a6\n\ni\nn\nj\n\n\n\n\n\n\n\n\n(Equation\u00a03)\n\n\n\nJ\n\nN\n\na\n2\n\nS\n\nO\n3\n\n\n\n=\n\nJ\n\na\nb\ns\n\n\n\u00d7\n\n\n\u03a6\n\ns\ne\np\n\n\n\n\n\nwhere JPEC is the observed photocurrent density and Jabs is the photocurrent density, assuming that absorbed photons are converted to current completely. The TOF was calculated by the produced moles of oxygen and the number of moles of the active sites; produced moles of oxygen were determined by GC, and the number of moles of the active sites was calculated by integrating the CV curve of Ni2+/3+ of 0.78\u20130.9\u00a0V versus RHE. A scan rate of 10\u00a0mV s\u22121 was used. The turnover number (TON) and TOF are defined as follow:\n\n(Equation\u00a04)\n\n\nT\nO\nN\n=\n\n\nn\n\no\nx\ny\ng\ne\nn\n\n\n\nn\n\nN\ni\n\n\n\n\n\n\n\n\n\n(Equation\u00a05)\n\n\nT\nO\nF\n=\n\n\nT\nO\nN\n\nt\n\n\n\n\nIMPS measurements were conducted under white light illumination, with 10% modulation intensity using a potentiostat (PP211, Zahner) and EC workstation (Zennium, Zahner) in the three-electrode configuration. The frequency of the modulation was swept from 100 kHz to 0.1\u00a0Hz. The ktrans and krec can be calculated as follows:\n\n(Equation\u00a06)\n\n\n\nk\n\nt\nr\na\nn\ns\n\n\n+\n\nk\n\nr\ne\nc\n\n\n=\n2\n\u03c0\nf\n\n\n\n\n\n\n(Equation\u00a07)\n\n\n\nk\n\nt\nr\na\nn\ns\n\n\n=\nt\nr\na\nn\ns\nf\ne\nr\n\ne\nf\nf\ni\nc\ni\ne\nn\nc\ny\n\u00d7\n\n(\n\n\nk\n\nt\nr\na\nn\ns\n\n\n+\n\nk\n\nr\ne\nc\n\n\n\n)\n\n\n\n\nSTH efficiency (\u03b7\n\nSTH\n) was calculated under the two-electrode measurement as follows:\n\n(Equation\u00a08)\n\n\n\n\u03b7\n\nS\nT\nH\n\n\n=\n\n\n\nJ\n\no\np\n\n\n\u00d7\n\n1.23\n\nV\n\u00d7\n\n\u03b7\nF\n\n\n\nP\n\ni\nn\n\n\n\n\n\n\nwhere J\n\nop\n is the current density (mA cm\u22122) at zero bias and \u03b7\n\nF\n is the faradic efficiency. The STH efficiency was calculated using the faradic efficiency of the Si photoanodes.The morphology of the Ni-based catalysts was examined by FESEM (MERLIN Compact, JEISS) and TEM (JEM-2100F, JEOL). XPS (AXIS-His, Kratos) analysis was carried out to confirm the surface bonding of Ni(OH)2, Ni NPs, and Ni NPs/Ni(OH)2 on a Si substrate. To clarify the exact bonding form of Ni and O, the narrow Ni and O spectrum was analyzed using CASA XPS.This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Ministry of Science and ICT (MSIT) (2019M3E6A1103818), the Korean Ministry of Science, ICT and Future Planning (MSIP) (2012R1A3A2026417), the Basic\u00a0Science Research Program through an NRF grant funded by MSIP (2017R1A2B3009135), the Creative Material Discovery Program through an NRF grant funded by the MSIT (2018M3D1A1058793), the Basic Research Laboratory of the NRF funded by the MSIT (2018R1A4A1022647), and Korea Hydro & Nuclear Power Co., Ltd. (no. 2018-Tech-21). S.A.L. acknowledges the Global Ph.D. Fellowship Program through the National Research Foundation of Korea funded by the Ministry of Education (2017H1A2A1044293).S.A.L. fabricated all of the Si photoanodes and wrote the manuscript. I.J.P. fabricated the perovskite/Si tandem solar cells. J.W.Y. assisted in the PEC measurements. J.P. measured the IMPS of the photoanodes. T.H.L. carried out the TEM analysis. C.K. performed the GC measurements. J.M. and J.Y.K. participated in the discussion of the data and wrote the manuscript. H.W.J. led the overall project.The authors declare no competing interests.Supplemental Information can be found online at https://doi.org/10.1016/j.xcrp.2020.100219.\n\n\nDocument S1. Figures S1\u2013S10 and Table S1\n\n\n\n\n\nDocument S2. Article plus Supplemental Information\n\n\n\n", "descript": "\n                  Selecting moderate semiconducting materials for photoelectrochemical (PEC) cells is essential to achieve high solar-energy conversion. Despite the advantageous features of silicon, such as earth abundance and narrow band gap, silicon suffers from severe photocorrosion. Here, heterogeneous nickel-based catalysts produced via electrodeposition are investigated to expedite water oxidation and protect the silicon from corrosion. The morphology of the catalysts and their PEC performances are demonstrated in detail. Synergistic Ni nanoparticles (NPs)/Ni(OH)2/n-Si photoanode shows a high photocurrent density of 29.6 mA cm\u22122 at 1.23\u00a0V versus RHE and operates over 140 h. Owing to the insufficient photovoltage generated by a single photoanode, we introduce a perovskite/Si tandem solar cell as a voltage supplier. The fabricated wired tandem device shows a photocurrent density of 9.8 mA cm\u22122, corresponding to a solar-to-hydrogen (STH) conversion efficiency of 12% without external bias. Our work may present a promising pathway toward a design of spontaneous energy conversion devices.\n               "}
{"full_text": "Data will be made available on request.Nowadays, chemical production is heavily dependent on fossil resources. Alternatives are required for sustainable development and the use of renewable carbon sources such as biomass is an attractive option. In particular, lignin is interesting for this purpose, as nowadays it is mainly used for energy generation and thus highly underutilized. Due to its oxygenated propylphenolic backbone structure, it could serve as an attractive source for phenol and alkylphenols, which are currently produced from fossil resources on a large scale [1]. Examples of strategies that have been proposed and explored to obtain low molecular weight phenols from lignin are acid/base catalyzed depolymerizations [2\u20135], fast pyrolysis [6], and reductive catalytic fractionation [7,8]. However, the obtained monomeric products are usually highly functionalized, for example by methoxy groups, hindering their direct utilization [9]. As a result, demethoxylation is needed to funnel the product mixtures into products with reduced complexity and increased value [10,11].A well-known demethoxylation strategy involves catalytic hydrotreatment with metal-based catalysts in combination with hydrogen. Exploratory catalyst studies for demethoxylation have been reported using heterogeneous catalysts in metallic, oxide, sulfide, phosphide, nitride or carbide form. Precious metal catalysts, such as Au/TiO2 and carbon-supported Pt and Pd catalysts, show excellent performance for the conversion of (propyl)guaiacol to demethoxylated phenols and 80\u00a0+\u00a0% selectivities have been reported at conversions >\u00a097% [12\u201314]. However, based on green chemistry and engineering principles, there is a need for the use of cheaper and more abundantly available metal catalysts [15]. A number of studies have been reported for the demethoxylation of guaiacols using Ni, Fe and Mo bases catalysts and an overview is given in \nTable 1. Both batch and continuous set-ups have been used and typical temperatures are between 285 and 400\u00a0\u00b0C, with hydrogen pressures between 1 and 90\u00a0bar. Ni catalysts, either supported on TiO2 or SiO2, particularly gave good results for the demethoxylation of 4-n-propylguaiacol (89% selectivity at 95% conversion, 285\u00a0\u00b0C, 1\u00a0bar) whereas worse results were obtained using guaiacol [16,17]. Various types of Mo based catalysts have also been used for catalytic demethoxylation, ranging from bulk MoO3 to Mo-oxides, sulphides, nitrides and phosphides on various supports (AC, SiO2, and SBA-15). When considering bulk MoO3, good performance was obtained when using guaiacol in batch set-ups though performance dropped considerably in continuous fixed-bed reactors. In addition, selectivity to phenols was limited and large amounts of fully deoxygenated products (e.g., benzene and toluene) were obtained [18,19]. When considering supported Mo- catalysts, excellent performance was found for MoP/SiO2 when using 4-n-propylguaiacol as the feed (350\u00a0\u00b0C, 90\u00a0bar) [20\u201322]. Good results were also achieved with the Fe based catalysts such as FeOx/CeO2 with conversion and selectivity levels similar to those for Ni catalysts, though catalyst stability was fair with up to 41% reduction in activity over a time on stream (TOS) of 10\u00a0h (400\u00a0\u00b0C, 1\u00a0bar) [23].The use of supported Cu catalysts for the guaiacol demethoxylation with high selectivity is limited (Table 1). The best results were reported in batch using Cu/AC as the catalyst [33]. Performance was reasonable, with 79% selectivity to demethoxylated products though at a low conversion of 24% (350\u00a0\u00b0C, 50\u00a0bar). Only one study reports the use of a continuous reactor set-up in combination with Cu/C, though only 3.6% guaiacol conversion was reported (350\u00a0\u00b0C, 40\u00a0bar, Table 1). Furthermore, support studies, stability testing, and demethoxylation of crude bioliquids (enriched in guaiacols) in continuous set-ups has not been explored systematically for supported Cu catalysts. To overcome this knowledge gap, a series of supported Cu catalysts were investigated for the demethoxylation of guaiacol. The effect of inorganic supports (SiO2, ZrO2, TiO2 (various forms), MoO3-ZrO2, and MoO3-TiO2), solvent (toluene, n-octane), WHSV, and temperature on catalyst performance (activity, selectivity, and stability) was studied. In addition to common inorganic supports (viz. SiO2, ZrO2, and TiO2), MoO3-ZrO2 and MoO3-TiO2 were also investigated because the introduction of MoO3 on ZrO2 or TiO2 is known to have a positive effect on catalyst performance for the hydrodeoxygenation of lignin-derived monomers (e.g., m-cresol, anisole, and guaiacol) [36\u201338]. The Mo species are supposed to form coordinatively unsaturated sites during reactions, which promote the HDO pathway, possibly via an oxygen vacancy-driven mechanism [36]. Besides guaiacol, also other relevant feeds such as 4-n-propylguaiacol, and a guaiacol mixture isolated from pyrolysis oil were tested using the best Cu catalyst in the series.SiO2 (99.5% trace metals basis, 10\u201320\u00a0nm particle size), ZrO2 (< 100\u00a0nm particle size), TiO2 (P25, \u2265 99.5% trace metals basis, 21\u00a0nm primary particle size), anatase TiO2 with a low surface area (denoted as LSA, \u2265 99%, average diameter of 156\u00a0nm), anatase TiO2 with a high surface area (denoted as HSA, 99.7% trace metals basis, < 25\u00a0nm particle size), rutile TiO2 (99.5% trace metals basis, < 100\u00a0nm particle size), silicon carbide (about 200 mesh particle size), copper nitrate trihydrate (99\u2013104%), ammonium molybdate tetrahydrate (99.98% trace metals basis), guaiacol (\u2265 99%), 4-n-propylguaiacol (\u2265 99%), 4-n-propylphenol (99%), 2-sec-butylphenol (98%), p-propyl anisole (\u2265 99%), benzene (\u2265 99.7%), and cyclohexene (99%) were supplied by Sigma-Aldrich. n-Octane (\u2265 98%) was obtained from Alfa Aesar. Toluene (\u2265 99%) and tetrahydrofuran (THF) stabilized with BHT (for analysis) were obtained from Avantor and Boom B.V., respectively. Phenol (> 99.5%), 4-methylphenol (> 99.0%), 2,3-dimethylphenol (> 98.0%), 2,3,6-trimethylphenol (> 98.0%), 1,2-dimethoxybenzene (> 99.0%), 4-methylguaiacol (> 98.0%), catechol (> 99.0%), n-propylbenzene (> 99.0%), n-decane (> 99.0%), and n-dodecane (> 99.0%) were purchased from TCI. p-Xylene (99%), m-xylene (99%), and o-xylene (99%) were obtained from abcr GmbH. High purity (> 99.99\u00a0mol%) N2, He, and H2 were purchased from Linde. Certified gas mixtures used in this study, including 1\u00a0vol%\u00a0O2/N2 5\u00a0vol%\u00a0H2 in Ar, and 10\u00a0vol% NH3 in He were also supplied by Linde. A guaiacols enriched feed from pyrolysis oil fractionation was provided by the Biomass Technology Group B.V. [39].Supported Cu catalysts (5\u00a0wt%) were prepared using an incipient wetness impregnation method by the following procedure. Copper nitrate trihydrate (0.1053\u00a0g) was dissolved in an amount of Milli-Q water equal to the measured pore volume for water of the support. Then, the solution was added dropwise to the support (2.000\u00a0g) with mixing at room temperature. After 6\u00a0h, the mixture was dried at 100\u00a0\u00b0C for 12\u00a0h, followed by calcination at 400\u00a0\u00b0C (2\u00a0\u00b0C\u00a0min\u22121) for 2\u00a0h in static air. The catalysts were pelletized, crushed, sieved, and the 100\u2013200\u00a0\u00b5m fraction was used in the experiments. For catalyst characterization purposes, the Cu catalysts were reduced ex situ under a H2 flow (200\u00a0mL/g Cat, 350\u00a0\u00b0C, 2\u00a0h) and then passivated under 1% O2/N2 (room temperature, 2\u00a0h). The MoO3-ZrO2 and MoO3-TiO2 (P25) mixed oxides supports with \u2018sub-monolayer\u2019 coverage of MoO3 (about 4 Mo/nm2) were also synthesized by an incipient wetness impregnation method [38]. This involved dissolving the appropriate amount of ammonium molybdate tetrahydrate in Milli-Q water, support impregnation, drying, and calcination at 550\u00a0\u00b0C (2\u00a0\u00b0C\u00a0min\u22121) for 4\u00a0h in static air. The nominal MoO3 content in the MoO3-ZrO2 and MoO3-TiO2 (P25) are 3 and 5\u00a0wt%, respectively.The catalytic hydrotreatment experiments with lignin-derived model compounds and a guaiacols enriched feed were carried out in a continuous down-flow fixed-bed reactor (stainless steel) with an outer diameter of 6.35\u00a0mm and an inner diameter of 4.55\u00a0mm. Typically, the supported Cu catalyst (100\u00a0mg) was mixed with SiC (200\u00a0mg). The mixture was loaded into the reactor and reduced in situ at 350\u00a0\u00b0C for 2\u00a0h under a H2 flow (20\u00a0mL\u00a0min\u22121). An experiment was started by heating the reactor to the desired temperature, followed by increasing the pressure to 10\u00a0bar using a back pressure valve using a H2 flow (20\u00a0mL\u00a0min\u22121). Subsequently, the H2 flow rate was set to the desired value (typically 10\u00a0mL\u00a0min\u22121) and feeding of the reactant (5\u00a0wt% guaiacol in n-octane or toluene, 5\u00a0wt% 4-n-propylguaiacol in toluene, or 5\u00a0wt% crude feed in toluene) at the desired Weighted Hourly Space Velocity (WHSV, h-1) was started using an HPLC pump. The WHSV was calculated based on the feed flow rate (reactant and solvent) and the catalyst intake as shown in Eq. (1).\n\n(1)\n\n\nWHSV\n=\n\n\nF\n\n\ng\n\n\n/\nw\n\n\n\nWhere Fg is the flow rate of the feed (g/h) and w is the weight of catalyst (set at 0.100\u2009g). Aspen plus simulations indicate that all reactions were carried out in the vapor phase, see supporting information for details.The reaction products were separated in a gas-liquid separator at room temperature, and condensable products were collected at 1\u2009h intervals. The feed and liquid product composition were analyzed offline using GC-MS (Agilent 6890 series GC system equipped with an HP973 mass detector) and GC-FID [Agilent 8860 gas chromatograph equipped with a flame ionization detector and an HP-5 capillary column (30\u2009m x 320\u2009\u00b5m x 0.25\u2009\u00b5m)]. n-Dodecane was added to the feed and used as an internal standard (IS). The liquid products were diluted about 20 times using THF containing 500\u2009ppm n-decane as an IS before analyses.Conversion of feed component g and selectivity for a product i were calculated on a molar basis using Eqs. (2) and (3).\n\n(2)\n\n\n\n\nX\n\n\ng\n\n\n\n\n\n%\n\n\n\n=\n\n\n(\nC\n\n\ng\n,\n0\n\n\n\u2212\n\n\nC\n\n\ng\n\n\n)\n/\n\n\nC\n\n\ng\n,\n0\n\n\n\u00d7\n100\n\n\n\n\n\n\n(3)\n\n\n\n\nS\n\n\ni\n\n\n\n\n\n%\n\n\n\n=\n\n\nC\n\n\ni\n\n\n/\n(\n\n\nC\n\n\ng\n,\n0\n\n\n\u2212\n\n\nC\n\n\ng\n\n\n)\n\u00d7\n100\n\n\n\n\nHere Cg,0, is the molar fractions of guaiacol or 4-n-propylguaiacol in the feed, Cg is those molar fractions in the product mixture, and Ci is the molar fraction of a product i.The molecular weight distributions of the feed and liquid product after hydrotreatment were determined by gel permeation chromatography (GPC) analyses using an Agilent HPLC 1100 system equipped with three MIXED-E columns (length 300\u2009mm, i.d. 7.5\u2009mm) in series and a GBC LC 1240 refractive index detector (RID). All samples were diluted by THF to a concentration of about 10\u2009mg\u2009mL-1, and toluene was used as internal reference.The reproducibility of a selected set of experimental conditions was determined and shown to be good (Fig. 5).Inductively coupled plasma-optical emission spectrometry (ICP-OES) was used to quantify the Cu content in the supported Cu catalysts. Before analyses, the samples were dissolved in aqua regia (HF was used for Cu/SiO2), and then the Cu content was determined using an PerkinElmer Optima 7000 DV equipped with a solid-state CCD array detector.H2-TPR measurements were conducted using a Micromeritics AutoChem 2920 system, equipped with a thermal conductivity detector (TCD). Samples (about 100\u2009mg) were pretreated under Ar at 400\u2009\u00b0C for 1\u2009h and then cooled to 50\u2009\u00b0C. After that, the H2-TPR profiles were collected under 5\u2009vol%\u2009H2 in Ar (30\u2009mL\u2009min-1) from 50\u2009\u00b0C to 800\u2009\u00b0C using a temperature ramp of 10\u2009\u00b0C\u2009min-1.Surface acidity was quantified by NH3-TPD measurements, which were carried out on a Micromeritics AutoChem 2920 system. The passivated Cu catalysts (about 100\u2009mg) were reduced under 5\u2009vol%\u2009H2 in Ar at 350\u2009\u00b0C for 1\u2009h. Then a gas mixture containing 10\u2009vol% NH3 in He (50\u2009mL\u2009min-1) was used to saturate the acid sites at 100\u2009\u00b0C for 1\u2009h. After that, the samples were purged under a He flow until the baseline was stable. TPD measurements were carried out from 100\u00b0 to 600\u00b0C at a rate of 10\u2009\u00b0C\u2009min\u22121 under a He flow (50\u2009mL\u2009min-1).X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance diffractometer, operating at 40\u2009kV and 40\u2009mA using Cu-K\u03b1 radiation (\u03bb\u2009=\u20091.5544\u2009\u00c5). Data were collected using a coupled 2\u03b8 / \u03b8 configuration, between 2\u03b8 values of 5\u201380\u00b0 with a step size of 0.02 and a scan time of 1.000\u2009s.The specific surface area and pore properties of the supported Cu catalysts were determined by N2 physisorption, conducted at 77\u2009K using a Micromeritics ASAP 2420 system. Before analysis, the samples were degassed at 250\u2009\u00b0C under vacuum for 6\u2009h. The specific surface area was determined using the Brunauer-Emmett-Teller (BET) method at relative pressures (P/P0) ranging from 0.05 to 0.25. The total pore volume was obtained from the single-point desorption point at a relative pressure of 0.98. The pore diameter was calculated using the adsorption branch from the N2 isotherm according to the Barrett-Joyner-Halenda (BJH) method. This approach eliminates tensile strength effects, which lead to an artificial peak at 4\u2009nm in the pore size distribution [40].The microstructure of the Cu/TiO2 catalysts was examined with a probe and image aberration corrected Themis Z microscope (Thermo Fisher Scientific) operating at 300\u2009kV equipped with a Ceta camera. The samples were first ultrasonically dispersed in ethanol and then deposited on a carbon-coated gold grid. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the Cu/TiO2 catalysts were obtained using the same microscope in STEM mode with a convergence semi-angle of 26\u00a0mrad and a probe current of 100 pA. Energy dispersive X-ray spectroscopy (EDS mapping) results were achieved with a Dual X EDS system (Bruker) with a probe current of 250 pA. Data acquisition and analysis were done using Velox software (version 2.8.0).The coke amount on the spent catalyst were determined using thermogravimetric analysis (TGA) by a TGA 4000 from PerkinElmer. The samples were heated in an air flow (30.0\u2009mL\u2009min-1) from 50\u00b0 to 900\u00b0C with a heating rate of 10\u2009\u00b0C\u2009min-1, and the coke amount was obtained from the TG curves.The oxygen vacancy concentration of the support was quantified by oxygen storage capacity measurements on a Micromeritics AutoChem 2920 system. Firstly, the sample (0.06\u20130.34\u2009g) was pretreated in a 5\u2009vol%\u2009H2 in Ar stream (25\u2009mL\u2009min-1) at 550\u2009\u00b0C for 2\u2009h. Afterward, the sample was purged with He until a constant baseline was obtained. Then, successive pulses of 10\u2009vol%\u2009O2 in He were injected into the sample at 550\u2009\u00b0C until O2 consumption ceased. The O2 uptake was used to calculate the concentration of oxygen vacancies on the sample by assuming that one O2 molecule saturates two oxygen vacancies [25,36].A series of supported Cu catalysts on non-reducible (SiO2 and ZrO2) and reducible supports (TiO2, MoO3-ZrO2, and MoO3-TiO2) was prepared using an incipient wetness impregnation method, all with about 5\u2009wt% Cu (determined by ICP-OES, see Table S1). The focus of this study is the use of TiO2 supported Cu catalysts (Cu/TiO2-P25, Cu/TiO2-A HSA, Cu/TiO2-R, and Cu/MoO3-TiO2), which were characterized in detail using H2-TPR, NH3-TPD, XRD, N2 physisorption, HAADF-STEM, and TGA (for spent catalyst) to rationalize catalyst performance (\nTable 2).The N2 physisorption data reveals that the TiO2 supported Cu catalysts have a specific surface area of 27\u201361\u2009m2/g, pore volume of 0.25\u20130.37\u2009cm3/g, and pore diameter 19\u201321\u2009nm. The Cu/TiO2-A HSA shows the highest specific surface area as well as lowest pore volume and pore diameter among these four catalysts [41].H2-TPR was used to determine the reducibility of catalysts and the results are given in \nFig. 1. Two or three peaks are present in all samples in the 100\u2013300\u2009\u00b0C region, which are associated with the reduction of Cu-oxides. The lower temperature peak between 136 and 198\u2009\u00b0C is assigned to the reduction of monomeric Cu species directly interacting with the support, whereas the second peak at higher temperatures (195\u2013296\u2009\u00b0C) is believed to be due to the reduction of bulk CuO on the support [42,43]. For Cu/TiO2 on P25, two peaks are observed in this region due to reduction of bulk CuO with different particle sizes [42,44]. For Cu/TiO2-A HSA, the reduction peak for bulk CuO shifted to lower temperature and merged with that of monomeric Cu species, indicating the presence of low amounts of bulk CuO and a high dispersion of Cu species. Based on these data, it was deemed sufficient to reduce the catalysts before reaction at 350\u2009\u00b0C for 2\u2009h (pure H2) to ensure quantitative reduction of Cu species to metallic Cu.A distinct reduction peak centered at 383\u2009\u00b0C is observed for Cu/TiO2-A HSA, which is associated to reduction of support (TiO2-A HSA) [45]. This reduction step is promoted by the presence of Cu, as the support alone shows only a high temperature reduction peak at about 674\u2009\u00b0C (Fig. S1). The Cu/MoO3-TiO2 also showed a clear peak at 737\u2009\u00b0C, which is due to the reduction of Mo species [46].XRD was used to examine the various phases and crystallinity of the Cu catalysts (reduced at 350\u2009\u00b0C for 2\u2009h under pure H2), and the XRD patterns are shown in \nFig. 2. Clear peaks of the various TiO2 crystal phases are present at 2\u03b8 =\u200925.3\u00b0 (anatase (101)) and 2\u03b8 =\u200927.4\u00b0 (rutile (110)). Integration shows that the TiO2-P25 contains about 16\u2009wt% of the rutile phase, in line with literature data [47\u201349].In three of the samples (Cu/TiO2-P25, Cu/TiO2-R and Cu/MoO3-TiO2-P25), clear peaks from Cu species were present at 43.3\u00b0 and 50.4\u00b0, corresponding to the (111) and (200) planes of Cu, respectively (JCPDS PDF No. 04\u20130836). For Cu/TiO2-A HSA Cu peaks are absent, indicating that the Cu species are highly dispersed, which is in line with the TPR results.The average crystallite size of the Cu species in the samples showing clearly visible Cu peaks were determined using the Scherrer equation and the results are given in Table 2\n[50]. The average Cu size varies between 27 and 81\u2009nm, the smallest for Cu/TiO2-P25, and the largest for Cu/MoO3-TiO2. The differences in Cu crystal sizes may be due differences in the metal-support interactions as well as the specific surface area of the materials (Table 2).Surface acidity was quantified by NH3-TPD, and the profiles are shown in \nFig. 3. The surface acidity decreases in the order: Cu/TiO2-A HSA >\u2009Cu/TiO2-P25\u2009>\u2009Cu/MoO3-TiO2 >\u2009Cu/TiO2-R. The Cu/TiO2-P25, Cu/TiO2-A HSA, and Cu/TiO2-R mainly show weak acidic sites, while the Cu/MoO3-TiO2 mainly contains strong acidic sites.High-angle annular dark-field STEM (HAADF-STEM) was conducted to investigate the morphologies and particle size of Cu species for the fresh Cu/TiO2-P25 and Cu/TiO2-A HSA, and distinct differences were found between these two samples. The Cu nanoparticles (small bright ones) are highly dispersed on the TiO2-P25 (\nFigs. 4a and 4b), with an average diameter of 2.6\u2009nm. Beside these small Cu particles, larger ones (> 10\u2009nm) are also present (confirmed by the EDS mapping) on the fresh Cu/TiO2-P25 (Fig. S2). The presence of these larger Cu particles is consistent with the XRD results, which showed an average particle size of 26\u2009nm. Based on these measurements we conclude that the Cu species on the support have a bimodal distribution.In contrast to Cu/TiO2-P25, the HAADF-STEM images of Cu/TiO2-A HSA reveal the presence of hollow TiO2 spheres and small Cu nanoparticles (Fig. S3). The average diameter of the Cu nanoparticles is about 1.6\u2009nm, which is somewhat smaller than that in Cu/TiO2-P25. In addition, the XRD data for Cu/TiO2-A HSA show that larger Cu particles are absent, and imply that the Cu size distribution is not bimodal and that only smaller Cu particles are present.A series of supported Cu catalysts on non-reducible (SiO2 and ZrO2) and reducible supports (TiO2, MoO3-ZrO2, and MoO3-TiO2) was prepared. Four different types of TiO2 were investigated: anatase (both high and low surface area, denoted as TiO2-HSA and TiO2-LSA, respectively), rutile (TiO2-R) and P25. Initial catalytic screening experiments to assess support effects were performed using guaiacol in octane (5\u2009wt%) as the feed in a continuous fixed bed reactor at 300\u2009\u00b0C, a WHSV of 16\u2009h-1 and 10\u2009bar pressure. Experiments were run for a TOS of at least 3\u2009h to ensure steady state operation and the conversions at 3\u2009h were used for evaluation. The conversion of guaiacol and the selectivity to individual products are shown in \nFig. 5 (see also Fig. S4).The main products are the desired (alkylated)phenols such as the parent phenol, methyl- and dimethylphenols formed by demethoxylation and methyl transfer, with minor amounts of methoxybenzene and dimethoxybenzene (\nScheme 1). The balance closure on molar basis is not quantitative, and this is likely due to the formation of some gas-phase components, and GC-undetectable oligomers. The presence of the latter was confirmed by GPC measurements showing a small peak at higher molecular weight values (Fig. S5).Strong support effects were observed on catalyst performance (300\u2009\u00b0C, 10\u2009bar, and WHSV of 16\u2009h-1). The conversion for Cu/SiO2, Cu/TiO2-A LSA (low surface area anatase TiO2), Cu/TiO2-R (rutile), and Cu/MoO3-ZrO2 are all below 15%. Better results were obtained using Cu/ZrO2, Cu/TiO2-P25, and Cu/TiO2-A HSA (high surface area anatase TiO2) with conversion levels between 20% and 30%. The by far most active catalyst is Cu/MoO3-TiO2 with a guaiacol conversion of 70%.Blank experiments with support only (TiO2-P25 or MoO3-TiO2) were performed and the results are shown in Fig. S6. It was found the TiO2-P25 has limited activity (conversion of 8%), while the MoO3-TiO2 (P25) is much more active (conversion of 31%). However, performance of the bare supports is considerably less than for the supported Cu catalysts, indicating that Cu plays a major role in the catalytic cycle (vide infra).It is of interest to compare the activity data for the TiO2 supported catalysts in the series. Among them, guaiacol conversion increases in the order anatase (LSA) <\u2009rutile <\u2009anatase (HSA) <\u2009P25. The latter consists of a mixture of anatase and rutile phases and this appears to be favored when considering activity. Mo doping has a very positive effect on conversion, which is particularly evident when comparing Cu/TiO2-P25 (conversion of 29%) and Cu/MoO3-TiO2 (conversion of 70%). The promoting effect of Mo doping on titania supports is in line with recent results from our group [38] and was speculated to be due to i) the formation of additional oxygen vacancies which promote the deoxygenation reaction and/or ii) the formation of active Mo oxycarbide or Mo oxycarbohydride phase.However, activity is not the only catalyst performance criterium and selectivity should be considered as well. Regarding product distribution, demethoxylated phenols like phenol, methylphenols and dimethylphenols are the major products. In addition, small amounts of methoxybenzene and dimethoxybenzene were detected. This product distribution indicates that demethoxylation in combination with methyl transfer are the dominant reactions (Scheme 1). Selectivity to non-aromatics is low, indicating limited overhydrogenation of the aromatic rings [51,52]. Methyl transfer to form methylated phenols is assumed to be catalyzed by acid sites on the catalyst via Friedel\u2212Crafts-type reactions with a carbonium ion (CH3\n+) as an intermediate [23,53,54].For a proper comparison of the best catalysts in the series in terms of selectivity at equal conversion, the WHSV was tuned to obtain a conversion of about 40% for three of the most active catalysts (300\u2009\u00b0C, 10\u2009bar). The results are given in Fig. 5 (right). The highest selectivity to alkylated phenols (76%) was obtained using Cu/TiO2-P25, and only minor amounts of dimethoxybenzene was formed. The parent phenol selectivity in this case is up to 55%. The amounts of dimethoxybenzene are considerably higher for the Cu/MoO3-TiO2 catalyst. As such, we can conclude that i) the TiO2 based catalysts (e.g., TiO2-P25, TiO2-LSA, and MoO3-TiO2 supported Cu catalysts) perform best and ii) the best catalyst in terms of selectivity towards alkylated phenols is Cu/TiO2-P25.When considering the catalytic performance data for the TiO2 supported catalysts, guaiacol conversion increases in the order anatase (LSA) <\u2009rutile <\u2009anatase (HSA) <\u2009P25. Thus, a mixture of anatase and rutile phases as in P25 TiO2 appears to be favored when considering catalyst activity. Friedrich et al. have proposed that the interface and particularly the disorder of the TiO2 lattice between the anatase and rutile phase has a positive effect on catalyst performance and plays a major role [55]. In addition, mixed-phase TiO2 was shown to have superior photocatalytic activity [56,57], which was ascribed a larger charge separation of the anatase-rutile phase junction [57,58]. It could be one of the possible explanation for the good performance of the P25 based catalyst, as there is close contact between the anatase and rutile phases in P25 TiO2\n[49].However, catalyst characterization studies showed that other properties like surface area, Cu particle size, and acidity are also different for the set of catalysts and may affect performance. Attempts have been undertaken to find correlations between these catalyst features and performance like conversion and selectivity. Unfortunately, no clear relations between these relevant properties and performance was obtained. This implies that other factors play a role as well and may interfere. Oxygen vacancies (Ov) have been proposed to play a role in the molecular mechanism for the demethoxylation of guaiacol (or anisole) when using supported catalysts with reducible supports like MoO3 and TiO2\n[18,45,59,60]. For instance, Liu et al. performed studies on Ag/TiO2 as a demethoxylation catalyst and proposed that Ov are formed by the reduction of the TiO2 surface by spillover hydrogen [45].Unfortunately, the amounts of oxygen vacancies on the supports of the Cu based catalysts could not be quantified by oxygen storage capacity measurement due to the presence of Cu. To get some information on the possible involvement of oxygen vacancies, the oxygen storage capacity of the support (TiO2-P25, TiO2-A HSA, TiO2-R, and MoO3-TiO2) without Cu was quantified. A clear trend was found between the oxygen vacancies of the support and the conversion of guaiacol at a WHSV of 16\u2009h-1 (\nFig. 6), indicating these oxygen vacancies may play an important role in the catalytic cycle.In addition, the presence and levels of impurities in the different types of titania supports may also affect performance. P25 TiO2 shows the presence of about 180\u2009ppm of trace metals, among others K, Ag, Pd, Pt and Sb (Table S2). The total amount of trace elements in TiO2-A HSA is much higher (1018\u2009ppm), the majority being Na. The presence of Na in anatase TiO2 was shown to be detrimental for the photocatalytic degradation of certain substrtaes (malachite green oxalate and 4-hydroxy benzoic acid) [61]. A such, differences in type and levels of impurities in the titania supports may also affect catalyst performance for the demethoxylation reactions reported in this study.Based on literature precedents and the experimental correlation between oxygen vacancies and conversion [45], a mechanism is proposed for the guaiacol demethoxylation on Cu based catalysts (\nFig. 7). It involves two active sites, viz., Cu and oxygen vacancies close to the Cu sites. It assumes that hydrogen is activated and chemosorbed on reduced Cu sites, and after spill over create oxygen vacancies on the TiO2 surface. Interaction of the substrate with these oxygen vacancies leads to demethoxylation [23,45]. The mechanism for the formation of alkylated phenols is still under debate. Either the initially formed phenol reacts with guaiacol to form methylated phenols and catechol [45,51] or Friedel-Crafts-type reactions occur with the involvement of CH3\n+ species that are known to be active in intra- or inter-molecular methylation reactions [23,53]. The formation of oligomers, minor byproducts, may be related to the presence of acidic sites on the surface.The stability of the Cu/TiO2-P25 catalyst was determined in the continuous set-up for an experiment at a TOS of 100\u2009h (300\u2009\u00b0C, 10\u2009bar, and WHSV of 16\u2009h-1). Excellent stability was demonstrated with a near constant conversion level of about 27% (\nFig. 8, detailed selectivity data are given in Fig. S7). The high stability of the Cu catalyst is remarkable as catalyst deactivation is significant for the catalytic hydrodeoxygenation of lignin-derived guaiacols to phenols or BTX [14,23,29,53,62,63]. As such, these Cu-based catalysts combine good activity and selectivity with stability which is highly beneficial when considering potential industrial application.The desired demethoxylated phenols like phenol and methylated phenols are the major products with a selectivity of about 79% throughout the run. The only major side product was dimethoxybenzene, in line with the exploratory runs at much shorter TOS\u2019s (vide supra). The selectivity towards the latter increases slightly over time, indicating some changes in the catalyst structure.As such, the spent Cu/TiO2-P25 after 100\u2009h-on-stream was characterized by several techniques (HADDF-STEM, ICP-OES, and TGA) to identify changes in composition and texture. HADDF-STEM (Fig. 4) reveal that the original small Cu nanoparticles (< 3\u2009nm) are still present, though qualitatively the amount is lower than in the original samples. The latter is indicative for some aggregation to larger particles (Fig. 4f). Aggregation is typical for Cu based catalysts due to the high mobility of Cu species at elevated temperatures [64]. Besides aggregation, also some leaching of Cu also occurred as revealed by ICP-OES, showing that the Cu contents decreased from 5.0 wt% for the fresh to 3.7\u2009wt% for the spent catalyst. Carbon deposition on the catalyst was quantified by TGA (Fig. S8). Based on these data, it can be concluded that carbon deposition on the catalyst is minimal.The effect of the solvent on catalyst performance and particularly product selectivity was determined for the Cu/TiO2-P25 catalyst in the continuous set-up (300\u2009\u00b0C, 10\u2009bar) using toluene and n-octane at different conversion levels (18\u201340%). The latter was varied by operating the continuous set-up at different WHSV values. The selectivity versus the conversion in both solvents is given in \nFig. 9a. Distinct differences in selectivity trends are observed for both solvents. n-Octane gives the highest selectivity to phenol and demethoxylated phenols at low conversion, whereas the trend is opposite at higher conversions. Thus, toluene is favored for the reaction and best results were a 58% selectivity of phenol, and 81% of demethoxylated phenols at a conversion level of 35%. Possible reactivity of toluene was checked by performing a blank experiment with toluene alone and the catalyst at 300\u2009\u00b0C. Clear peaks from toluene derived products were not detected in the product mixtures, indicating that toluene is inert at the prevailing reaction conditions.The effect of reaction temperature was studied in the continuous set-up for Cu/TiO2-P25 with toluene as the solvent at a fixed WHSV of 21\u2009h\u22121 and a pressure of 10\u2009bar. The results are shown in Fig. 9b. The guaiacol conversion increases with temperature and values up to 98% were obtained at 360\u2009\u00b0C. The selectivity of demethoxylated phenols also increased to 87%, though the selectivity to the parent phenol dropped from 54% to 48%. These values correspond to a yield of demethoxylated products of about 85%, which is by far better than reported for Cu/AC in batch (79% selectivity at 24% conversion giving a yield of 19%, Table 1) and other Cu based catalysts (yields between 1.2% and 11%).The gas-phase product was collected for the experiment at 360\u2009\u00b0C (10\u2009bar, WHSV = 21\u2009h\u22121), with 5\u2009wt% guaiacol in toluene as reactant. CH4 was the only product identified, though present in only very minor amounts (0.28%). In addition, the carbon balance closure considering gas and liquid phase products, as identified and quantified with GC, was calculated for this experiment and was found to be reasonably good (86.6%).Catalytic demethoxylation using the Cu bases catalysts was extended to another model compound (4-n-propylguaiacol) and a crude bioliquid (a pyrolysis oil fraction enriched in guaiacols). The former was selected as it is a major component of lignin oils obtained by the reductive catalytic fractionation (RCF) of lignocellulosic biomass [20]. The demethoxylation of 4-n-propylguaiacol using Cu/TiO2-P25 was carried out in the continuous set-up at 360\u2009\u00b0C, 10\u2009bar, with 5\u2009wt% 4-n-propylguaiacol dissolved in toluene as a reactant and a WHSV of 21\u2009h\u22121. The results are given in \nTable 3. The conversion level of 4-n-propylguaiacol was 94%, which is slightly lower when compared to guaiacol (98%) at similar conditions. The main products were higher alkylated phenols (i.e., propylphenols and methylated propylphenols) with a total selectivity to demethoxylated phenols of 69%. The latter is somewhat lower than found for guaiacol (87%), though comparison is cumbersome as the substrate conversion is different for both experiments.A bioliquid enriched in guaiacols, obtained by pyrolysis oil fractionation was used as the feed to extent the synthetic strategy to more complex feeds. The feed is enriched in guaiacols, and like guaiacol (16\u2009wt%), methylguaiacols (35\u2009wt%), 4-ethylguaiacol (9\u2009wt%), eugenol (9\u2009wt%), and 4-n-propylguaiacol (1\u2009wt%), as shown in Table S3. The reaction was carried out in the continuous set-up at 360\u2009\u00b0C, 10\u2009bar, with 5\u2009wt% crude feed dissolved in toluene as a reactant and a WHSV of 23\u2009h\u22121. The catalytic reaction was conducted without major operational issues for a TOS of 6\u2009h and a total of 14.1\u2009g of the product was obtained. The product was analyzed in detail using GC-FID and was shown to contain mainly (methylated)-phenols like 2-methylphenol, 4-methylphenol, and 2,4-dimethylphenol (\nFig. 10). The presence of mainly methylated phenols is not surprising as the feed contains high amounts of methyl substituted guaiacols. Based on the analyses data, the total guaiacol conversion was estimated at 87% and the selectivity to demethoxylated phenols was about 81%. Overall, it can be concluded that demethoxylation of a complex bioliquid is well possible using the Cu/TiO2-P25 catalyst.A series of novel non-precious metal catalysts based on Cu and supported on inorganic supports (TiO2, SiO2 and ZrO2) was tested for the demethoxylaton of guaiacols. Cu/TiO2-P25, was shown to be the best catalyst in the series when considering selectivity at equal conversion levels. A clear correlation was found between the oxygen storage capacity of the support (TiO2-P25, TiO2-A HSA, TiO2-R, and MoO3-TiO2) and guaiacol conversion, indicating that this property plays an important role in the catalytic cycle. Further optimization studies showed that the selectivity to demethoxylated phenols can be up to 87% at 98% conversion. In addition, the catalyst shows excellent stability for a 100\u2009h TOS experiment. The synthetic methodology was successfully extended for the demethoxylation of 4-n-propylguaiacol and a guaiacols enriched pyrolysis oil fraction, showing high substrate flexibility. Further studies are in progress to obtain biobased phenol, a valuable bulk chemical, now primarily obtained from fossil resources, by selective dealkylation of the alkylated phenols formed in this study. These studies will be reported in due course.\nHuiazhou Yang: Investigation, Writing \u2013 original draft. Xiatian Zhu: Investigation, Writing \u2013 original draft. Helda Wika Amini: Investigation, Writing \u2013 original draft. Majid Ahmad: Investigation, Writing \u2013 original draft. Gert H. ten Brink: Investigation, Writing \u2013 original draft. Boy Fachri: Supervision, Writing \u2013 review & editing. P. Deuss: Supervision, Validation, Writing \u2013 review & editing. H.J. Heeres: Conceptualization, Funding acquisition, Supervision, Validation, Writing \u2013 review & editing.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Huaizhou Yang and Xiaotian Zhu reports financial support was provided by China Scholarship Council.H.Y. and X.Z. acknowledge the China Scholarship Council for funding their PhD studies (grant number 201706160156 and 201707040079, respectively). The authors thank Hans Heeres (Biomass Technology Group, BTG) for providing the crude feed isolated from pyrolysis oil. Leon Rohrbach, Erwin Wilbers, Marcel de Vries, and Hans van der Velde are acknowledged for technical and analytical support. We also thank Jos Winkelman for performing the Aspen plus simulations.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcata.2023.119062.\n\n\n\nSupplementary material.\n\n\n\n.", "descript": "\n                  Lignin is an attractive feedstock for low molecular weight biobased phenols using depolymerization strategies such as reductive catalytic fractionation or fast pyrolysis. Such strategies often yield a product mixture enriched in lignin-derived monomers with methoxy substituents. Selective catalytic hydrodeoxygenation (HDO) is an effective methodology to demethoxylate these monomers into valuable alkylated phenols. Here, we report the use of non-precious Cu based catalysts supported on SiO2, ZrO2, TiO2 (various forms), MoO3-ZrO2, and MoO3-TiO2 in a continuous fixed-bed reactor at elevated temperature and pressure (300\u2013360\u00a0\u00b0C, 10\u00a0bar) for the selective demethoxylation of guaiacol. Among the various catalysts, Cu/TiO2-P25 was found to be an effective and highly stable catalyst (100\u00a0h on stream) with a selectivity of 87% to demethoxylated compounds like phenol and cresols at a guaiacol conversion of 98%. A correlation was found between the oxygen storage capacity of the support (TiO2-P25, TiO2-A HSA, TiO2-R, and MoO3-TiO2) and guaiacol conversion, indicating that this property plays a role in the catalytic cycle. Besides, the demethoxylation of 4-n-propylguaiacol and a realistic guaiacol-rich feed isolated from a representative pyrolysis oil was successfully demonstrated. 87% of the guaiacols present in the feed were converted to demethoxylated phenols with a selectivity of 81%.\n               "}
{"full_text": "Data will be made available on request.The replacement of fossil fuels is of paramount importance. For renewable energy to be sustainable, it must be limitless and provide net-zero CO2 emissions. This demand still depends on several factors including biomass source, regional location and available technologies. Lignin, as a key component of the plant cell wall, has been identified as a major potential source of aromatic renewable resources. From an energy point of view, it has a high C/O ratio and accounts for 40\u00a0% of the carbon-based energy content in biomass [1]. However, a vast amount of generated lignin, from paper and bio-ethanol production, is predominantly incinerated and converted into heat or thermal energy and less than 2\u00a0% was sold for the production of value-added chemicals [1].Lignin consists of polymerized three-dimensional monomers, including syringyl (S), p-hydroxyphenyl (H), and guaiacyl (G) units, which have been investigated as model compounds for catalytic hydroconversion [2]. The most common linkages of carbon\u2013oxygen (\u03b2-O-4) and carbon\u2013carbon (\u03b2-5, 5\u20135, \u03b2-1 and \u03b2-\u03b2) are formed from these 3D monomers, particularly from S and G monomer units. The \u03b2-O-4 bonds are the most abundant interunit linkages, making up almost 50\u00a0% of all intermonomer linkages in softwoods and 60\u00a0% in hardwoods [3]. The cleavage of two \u03b2-O-4 bonds, which are located one on each side of the aromatic unit, is required to produce one monomer unit [4]. Moreover, it has been observed that a higher S/G ratio gives a higher yield of monomers during catalytic depolymerization. It has been reported that during catalytic depolymerization at 250\u00a0\u00b0C of birch lignin (S/G\u00a0=\u00a03) a monomer yield of 50\u00a0mol % is found, whereas a poplar lignin (S/G\u00a0=\u00a01.5) produced yields of 44\u00a0mol % [5]. Interestingly, Shuai et al. observed a monomer yield of 78\u00a0mol % using high-syringyl transgenic poplar lignin (S/G\u00a0=\u00a038) [6].With lignin being an integral part of the plant cell wall, its extraction is one of the challenges to achieve a suitable S/G ratio with a high quality. The extraction, depending on its process conditions, may add further structural complexity to the native physicochemical properties of the lignin. The lignin product is often contaminated or not fully extracted with a significant amount of residual carbohydrates or process chemicals [7]. These challenges have created ambiguities to understand the structural composition of lignin, and hence their chemical reaction network during the depolymerization of processed lignins. Consequently, identifying practical extraction and pretreatment conditions yielding a high quality lignin suitable for facile depolymerization requires further attention and development [2,7,8].Numerous approaches and methodologies have been investigated for Kraft lignin depolymerization. The catalytic reductive depolymerization with hydrogen has been studied extensively as a means to liquify Kraft lignin and selectively produce monomers. Typically, conventional hydrotreating catalysts based on supported molybdenum sulfides (MoS2), promoted by cobalt (Co), nickel (Ni) or iron (Fe) have been studied [9,10]. Recent studies of these transition metal based catalysts showed that they exhibited high activity and selectivity for the CO bond cleavage [1,11,12]. However, the sulfate pulping process contributes to high ash and sulfur contents in Kraft lignin [13]. The major problem, along with the coke deposition via bimolecular condensation reactions, is that inorganic contaminants can lead to catalyst deactivation during the catalytic depolymerization of Kraft lignin [14].Compared to Kraft lignin, hydrolysis lignin, containing less ash and sulfur, can be produced from conversion of lignocellulosic biomass during enzymatic hydrolysis, which results in solid lignin (\u226560\u00a0wt%), and unreacted cellulose [15]. However, limited studies have investigated catalytic reductive depolymerization of enzymatic hydrolysis lignin. Tymchyshyn et al. used a MoRu/AC catalyst to depolymerize hydrolysis lignin in acetone solvent and obtained low molecular weight bio-oils (380\u00a0g/mol) with high yields of around 85\u00a0wt% at 340\u00a0\u00b0C [15]. It was also reported that an aromatic monomer yield of 12.1\u00a0wt% was obtained for hydrolysis lignin over a 5\u00a0wt% Ni/AC catalyst at 240\u00a0\u00b0C for 4\u00a0h with 30\u00a0bar H2 in methanol [16]. Bai et al. used 15\u00a0wt% Ni/Al2O3 to depolymerize hydrolysis lignin and obtained a yield of 10.3\u00a0wt% of aromatic monomers at 320\u00a0\u00b0C after 7.5\u00a0h under 28\u00a0bar H2 in ethanol [17]. The effect of reaction conditions on the depolymerization of hydrolysis lignin has been investigated in a semi-continuous process over a sulfided NiMo/\u03b3-Al2O3 catalyst [18]. A full conversion was achieved and the liquid products, mainly aromatics, naphthenes, and phenols increased under the severe reaction conditions of 380\u00a0\u00b0C, and 70\u00a0bar H2. Recently, Sang et al. examined the depolymerization of hydrolysis lignin over an unsupported Ni catalyst in supercritical ethanol and achieved complete liquefication, with the highest monomer yield of 28.9\u00a0% at 280\u00a0\u00b0C for 6\u00a0h with 20\u00a0bar H2\n[19]. Importantly, these unsupported catalysts decrease the mass transfer limitations inherent to supported catalysts, to achieve a more complete liquefication and prevent char formation during the depolymerization [19]. More recently, the direct conversion of hydrolysis lignin into cycloalkanes over a NiMo/\u03b3-Al2O3 catalyst was carried out in a single step at 320\u00a0\u00b0C for 7.5\u00a0h [20]. The highest obtained overall yield of cycloalkanes was 104\u00a0mg/g enzymatic hydrolysis lignin, with a ethyl-cyclohexane selectivity of 44\u00a0wt% [20].However, there are to the best of our knowledge, no published studies where the efficiency of catalytic valorization in reducing conditions have been compared using Kraft and hydrolysis lignin, which is the objective of the current work. In this work, we introduce a facile preparation method for an unsupported NiMoS catalyst that is highly active with a high surface area. This catalyst can be a key factor in enhancing the hydrodeoxygenation and hydrogenation capabilities, and simultaneously decreasing the unwanted repolymerization reactions producing char. In addition, the reaction pathways for both lignins are proposed and discussed to reveal the key steps in their depolymerization and how they differ.Two different lignins were investigated. Kraft lignin is a three-dimensional polymer that has undergone a hydrolytic degradation process [21,22]. It was supplied by Sigma-Aldrich as a brown dry powder. The enzymatic hydrolysis lignin was kindly provided by Sekab (Sweden). Prior to all experiments, the lignin samples were dried at 80\u00a0\u00b0C in an oven. The chemicals used were of analytical grade and were not further purified. The reagents used can be found in Supplementary Information (SI).Unsupported NiMoO4 catalysts were synthesized by a nanocasting method using mesostructured silica as a hard template. SBA-16 and MCM-41, consisting of only silica, were used as templates. Hard templating is an important strategy to synthesis crystalline mesoporous materials. The unique structure of the hard template restricts the crystallization or aggregation of the precursors, and a mesoscopic phase having a structure opposite to that of the template can be obtained with the removal of template material by the appropriate method [23].A mixture of ammonium heptamolybdate tetrahydrate and nickel nitrate hexahydrate, in a molar ratio of 1:1, was dissolved in ethanol. The aqueous mixture was added to the mesoporous silica and stirred for 2\u00a0h at room temperature. Subsequently, ethanol was evaporated gradually using a water bath at 65\u00a0\u00b0C. The obtained paste was then calcined at 200\u00a0\u00b0C for 6\u00a0h. The resulting solid was re-impregnated again, followed by calcination at 450\u00a0\u00b0C for 6\u00a0h at a heating rate of 6\u00a0\u00b0C/min. Lastly, the silica template was removed from the mesoporous composite by 0.5\u00a0M NaOH using a vacuum filtration process. The solid products were washed with deionized water several times and then dried at 110\u00a0\u00b0C. Elemental analysis using ICP confirmed the absence of templates and showed the successful removal of the template. The absence of silica was also confirmed with XPS, XRD and TEM-EDS. The oxide forms of the unsupported catalysts will be denoted NiMo-SBA and NiMo-MCM, respectively, according to the mesostructured SBA-16 and MCM-41 templates used in their synthesis.Elemental composition of the catalysts was determined by using an inductively coupled plasma (ICP) and was performed by ALS Scandinavia AB after digestion of the solids in an acid solution.Powder X-ray diffraction (XRD) was used to examine the crystallinity of the catalysts. This was done using a Bruker D8 Advance (40\u00a0kV; 40\u00a0mA; Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01,542\u00a0\u00c5); 2\u03b8 range of 20-80\u00b0; 1\u00b0/min scan speed). The textural properties of the samples, such as pore volume, surface area and pore size were determined by nitrogen physisorption using a TriStar 3000 analyzer. Prior to N2-physisorption measurements, approximately 300\u00a0mg sample was degassed overnight at 300\u00a0\u00b0C under a continuous flow of nitrogen gas. After drying, the N2-physisorption isotherms were collected at\u00a0\u2212195\u00a0\u00b0C under vacuum. The specific surface area and pore size were determined using the Brunauer-Emmett-Teller equation (BET) and the Barret\u00a0\u2212\u00a0Joyner\u00a0\u2212\u00a0Halenda equation (BJH), respectively.The lignins and catalysts were thermally characterized by TGA with a TGA/DSC 3+\u00a0Star system (Mettler Toledo, Switzerland) featuring automated temperature and weight control as well as data acquisition. The samples were analyzed, without any pre-treatment, from 25 to 800\u00a0\u00b0C (to 900\u00a0\u00b0C for catalysts) with a 5\u00a0\u00b0C/min heating rate. Dry air and nitrogen were used as carrier gases for comparison.Scanning electron microscopy (SEM) was used to study the surface structure and morphologies of the sulfided catalyst samples with a JEOL JSM6400, operating at 25\u00a0kV. The shape and size of the metal species in the catalysts were examined using transmission electron microscopy (TEM), with a FEI Titan 80\u2013300 microscope (field emission gun; a probe Cs corrector; Gatan image filter Tridium; 300\u00a0kV). X-ray photoelectron spectra of the fresh sulfided catalyst was recorded using a PerkinElmer PHI 5000 Versa Probe III scanning XPS Microprobe apparatus equipped with a monochromatic Al K\u03b1 source with a binding energy of 1486.6\u00a0eV and the beam size diameter of 100\u00a0\u00b5m. The reference used is so-called adventitious carbon (AdC) using the C 1\u00a0s peak from the surface contamination layer and its binding energy (BE) is set to 284.6\u00a0eV.The NH3 temperature-programmed desorption (NH3-TPD) experiments were conducted in a Differential Scanning Calorimeter (DSC, Setaram Sensys), where the gas flow was regulated with mass flow controllers (MFC, Bronkhorst), and the outlet gases detected with a mass spectrometer (MS, Hiden Analytical HPR 20). 50\u00a0mg of NiMoO4 and NiMoS unsupported catalysts were pre-treated at 400\u00a0\u00b0C for 2\u00a0h in an argon flow of 20\u00a0mL/min. This was followed by exposing the catalyst to 4\u00a0vol% NH3 in Ar (10\u00a0mL/min) for 2\u00a0h at 120\u00a0\u00b0C. Thereafter the catalyst was flushed with Ar for 6\u00a0h and then the NH3 desorption was studied while increasing the temperature from 120 to 700\u00a0\u00b0C at a ramp rate of 5\u00a0\u00b0C/min.Elemental analyses (EA) were performed to determine the C, H, N, and S content in the feed lignins, and lignin oils using a vario MICRO cube analyzer. The MICRO cube analyzes the CHNS content of organic compounds in one single run. The amount of oxygen was determined by difference from the CHNS content. All analyses were performed twice and the average value is given.The water content in organic samples was determined by Karl Fischer (KF) titration using a Metrohm Titrino 807 titration equipment. The sample was added to a glass container with Hydranal\u00ae (Riedel de Haen) and the titrations were performed with Karl Fischer titrant Composite 5\u00a0K (Riedel de Haen). All analyses were performed twice, and the average value is given.\n31P NMR technique was used for the characterization and quantification of hydroxyl and carboxylic acid groups in lignin oils using an earlier method involving a prior derivative phosphitylation step [24]. The amount of different hydroxyl groups (mmol OH/g) in lignin oil samples was calculated according to [24]. The 13C solid-state NMR was conducted on a Bruker Avance III 500\u00a0MHz spectrometer equipped with a 4\u00a0mm MAS BB/1H probe. The rotor was spun at 10\u00a0kHz and a cross-polarization time of 1.5\u00a0ms was used.The GC-TCD technique was used to characterize gases formed during lignin hydroconversion. The gas samples were analyzed by a calibrated GC (450-GC, Varian) that was equipped with a TCD detector. A GS-GASPRO column (30\u00a0m, 0.32\u00a0mm) was used to separate and quantify the concentration of H2, CO, CO2, CH4, and C\n2\u00a0+\u00a0light hydrocarbons. The quantification was performed from the calibration of each gas using reference mixtures. All measurements were carried out in triplicate and the average value is provided.GCxGC-MS analysis was performed on organic liquid samples using an Agilent 7890B apparatus equipped with a closed cycle cryogenic jet modulation (ZX2 Model) from Zoex Corporation, two parallel detectors, an FID and a quadrupole MSD, and two columns. The first column was a moderately polar VF1701ms column (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u03bcm) and the second column was a nonpolar DB-5 MS UI column (1.2\u00a0m\u00a0\u00d7\u00a00.15\u00a0mm\u00a0\u00d7\u00a00.15\u00a0\u03bcm). The temperature of the injector port and MS interface were set at 250\u00a0\u00b0C. The column flow was set at 0.8\u00a0mL/min. The oven temperature program was set initially at 40\u00a0\u00b0C and held for 1\u00a0min, then heated up to 280\u00a0\u00b0C at 3\u00a0\u00b0C/min. The modulation time was 6\u00a0s. The data was processed via GC image 2.5 (Zoex Corporation) using GC Project and Image Investigator functions. The 2014 NIST library (Match Factor\u00a0>700), together with high resolution mass spectral data, were used for compound identification. To assess the relative standard deviation of each analysis, an internal standard was loaded before analysis by GCxGC. From GC Image Investigator, the relative standard deviations (RSD) in the first-dimension retention time, the second-dimension retention time and peak volume were less than 0.1\u00a0%.The hydroconversion experiments were carried out in a 450\u00a0mL stainless-steel Parr reactor. The influence of catalyst loadings (ranging from 5 to 20\u00a0wt% based on a constant 5\u00a0g of dry lignin always loaded into reactor), pressures (50\u201380\u00a0bar H2) and temperatures (330\u2013400\u00a0\u00b0C) have been investigated (Table 1\n). In a typical experiment, the reactor was loaded with catalyst and lignin (5\u00a0g) in 70\u00a0mL of hexadecane as a co-processing solvent to reduce the exothermic heat effects. To create a baseline for the study, reaction experiments using only hexadecane with lignin were included in the experimental plan. The sulfidation of the NiMoO4 catalyst was performed in-situ in the liquid phase, with the lignin feedstock and with the addition of (0.25\u20130.75\u00a0mL) dimethyl disulfide (DMDS). In-situ sulfidation of the unsupported NiMoO4\u2013SBA was carried out simultaneously with hydroconversion of Kraft and hydrolysis lignins. The advantage of in-situ activation is the simplification of the startup procedure. The amount of DMDS added depends on the catalyst amount used. During the sulfiding step for all experiments, a mixture of lignin, catalyst, and solvent were blended. Meanwhile, a small amount of DMDS (250, 500 or 750\u00a0\u03bcL) was added to the corresponding reaction mixture to maintain the sulfidity of the catalyst (5, 10 or 20\u00a0wt%, respectively). Under identical operating conditions, some experiments were performed to compare products yields from in-situ sulfided versus pre-sulfided NiMoO4..After the reactor was closed, it was flushed for 3\u20135 times with nitrogen to remove the air and thereafter purged three times with hydrogen. After leak testing, the reactor was initially pressurized with 16\u201325\u00a0bar H2 (depending on the target pressure for reaction conditions) and heated up to the designated temperature at an approximate heating rate of 5\u00a0\u00b0C/min while stirring at 1200\u00a0rpm. The time zero was set once the desired reaction temperate and pressure were reached. In the case of our study, duplicate reaction experiments were repeated for many of our experiments and the relative standard deviation (%, RSD) was determined to be within 9.0\u00a0%.After each experiment, different fractions of lignin products were recovered and analyzed. The typical products are gas, liquid bio-oil, unconverted lignin and solid char, as depicted in the product recovery protocol (Fig. S1, Supplementary Information (SI)). The pressure was recorded after the reactor was cooled to room temperature. The hydrogen consumption was determined from the difference in the initial and final pressures at room temperature [25,26]. Hydrogen was considered as an ideal gas and other gases produced during the reaction, were accounted for in the calculation of the hydrogen consumption. The pressure was released to atmospheric pressure and the gas products were collected in a 1 L Tedlar gas bag to determine its composition by using GC-TCD. The blended organic product was recovered, filtered and labelled as lignin oil. Subsequently, the reactor was rinsed with acetone, filtered, and this product fraction was labelled as the acetone soluble phase. Moreover, the solid phase (char, unconverted lignin, and catalyst) was washed with acetone to remove any adsorbed organics.The unconverted lignin was determined by suspending 0.5\u00a0g of solid residue in 15\u00a0mL of DMSO and thereafter stirring for 24\u00a0h at room temperature. After filtration of the solution, the solids were washed with acetone and dried. The weight difference between the solids before and after the washings was assumed to be the weight of unconverted lignin. The conversion, char, and monomer yields were determined using Eqs. (1) to (3).\n\n(1)\n\n\nConversion\n\n\n\n\nwt.\\%\n\n\n\n=\n\n\n\ninitial lignin feed\n\n\n\n\ng\n\n\n\n- unconverted lignin\n\n\n\n\ng\n\n\n\n\n\n\ninitial lignin feed (g)\n\n\n\nx 100\n\n\n\n\n\n\n(2)\n\n\nChar yield\n\n\n\n\nwt.\\%\n\n\n\n=\n\n\n\n\n\nsolid fraction\n\n\n\n\ng\n\n\n\n\n\n-(unconverted lignin (g) + catalyst\n\n\n\n\ng\n\n\n\n)\n\n\n\n\n\n\ninitial lignin feed (g)\n\n\n\nx 100\n\n\n\n\n\n\n(3)\n\n\nMonomer yield\n\n\n\n\nwt. \\%\n\n\n\n=\n\n\n\n\nmonomer\n\n\n\n\ng\n\n\n\n\n\n\ninitial lignin feed (g)\n\n\n\nx 100\n\n\n\n\nThe elemental composition was measured using ICP and the unsupported NiMoO4-SBA catalyst had a Mo/Ni ratio of 1.08). The pore structural parameters are summarized in Table 2, which displays high surface areas in the range of 155\u2013207\u00a0m2/g for none sulfided catalysts. A sulfidation treatment of the unsupported NiMoO4-SBA leads to an inevitable decrease of the catalyst surface area from 155 to 53\u00a0m2/g and a pore volume decreased from 0.19 to 0.08\u00a0cm3 g\u22121, which may be due to the inclusion of sulfur to form NiS2 and MoS2 new phases. These catalysts exhibit the type-IV isotherm, confirming their ordered mesostructured morphology. Moreover, the pore size distributions correlate with those of the templates used and exhibited a narrow pore size distribution at around 5\u00a0nm for both as prepared and pre-sulfided catalysts. While using MCM-41 as a template, relatively larger cages and mesoporous channels, corresponding to the MCM-41 template, were observed. The pore diameter of the unsupported NiMoO4-MCM catalyst enlarged to 11.8\u00a0nm with a significantly larger pore volume of 0.61\u00a0cm3 g\u22121.The crystal structures of the unsupported NiMoO4 were characterized by XRD (Fig. 1\n). The broad peaks indicate a small crystallite size of the samples. Comparison of the NiMoO4 fabricated via the two different hard templates, SBA and MCM, reveals that there are distinct differences in the XRD patterns (Fig. 1a). \u03b1-NiMoO4 and \u03b2-NiMoO4 phases have a monoclinic crystal structure (group space C12/m1). The characteristic peak of the \u03b2 phase is 2\u03b8\u00a0=\u00a026.4\u02daand for the \u03b1 phase it is 2\u03b8\u00a0=\u00a028.7\u02da. From the structural point of view, the relevant important differences between both phases are the molybdenum ion coordination in the crystal structure, being octahedral clusters, [MoO6], for the \u03b1-NiMoO4 and tetrahedral [MoO4], for the \u03b2-NiMoO4 powder [27]. It has been found that for the preparation of highly active unsupported NiMo catalysts, the \u03b2-NiMoO4 phase is most favorable for hydrodesulfurization (HDS) reactions [28\u201331]. In this work, the unsupported NiMoO4 fabricated from the SBA hard template which exhibited the \u03b2-NiMoO4 phase was therefore selected for the catalytic hydroconversion.The XRD patterns of the pre-sulfided NiMoO4-SBA are depicted in Fig. 1b. The sulfided NiMoS showed strong diffraction peaks consistent with the relatively good crystalline structure of MoS2 (JCPDS-ICDD 371492), NiS2 (JCPDS-ICDD 89\u20133058), and \u03b2-NiS (JCPDS-ICDD 12\u20130041). The presence of peaks at 2\u03b8 value of 14.5, 33, 39 and 59 corresponded to the (002), (100), (103) and (110) planes of MoS2\n[32]. As revealed by the XRD patterns, peaks due to NiS2 and \u03b2-NiS species were also detected. Some peaks at 2\u03b8 value of 27, 31, 35, 38, 45, and 53 were observed, matching well with the NiS2\n[33]. Moreover, the presence of peaks can be clearly indexed to \u03b2-NiS at 2\u03b8 value of 33, 37, 41, 49, 51, and 57 corresponding to (300), (220), (221), (131), (410) and (330) planes [34]. Obvious diffraction peaks from other compounds, such as Ni3S2 and Ni3S4, were also observed at 2\u03b8 value of 43.5, and 48 respectively [35,36]. No obvious ternary Ni-Mo-S and NiMoO4 oxide patterns were observed. It is noteworthy to mention that before XRD analysis, the pre-sulfided catalyst was passivated under a flow of 25\u00a0mL/min of 2\u00a0% O2 in Ar for 2\u00a0h and thereafter transferred into another N2 atmospheric bottle to avoid air contact.The SEM image of the sulfided NiMoS-SBA catalyst showed two major structural features (Fig. S2a, SI). Small pore sizes can be seen in the material with an average pore size of 5\u201310\u00a0nm (white insert). These smaller pores are the fine intra-aggregate pores of the material. The larger sized pores (>50\u00a0nm) may be attributed to the secondary pores formed by the combination of primary particles. The TEM result of the freshly sulfided NiMoS-SBA catalyst is given in Fig. S2b (SI). Areas with black thread-shape fringes that have spacings of about 0.5\u00a0nm indicate a high purity of the active components and are characteristic of the (002) basal planes of crystalline MoS2 (XRD pattern Fig. 1b). This has been confirmed by Yoosuk et al. [37]. As well-known from the intercalation model, slab growth occurs in parallel and perpendicular directions [2]. But for NiMoS in Fig. S2b, the slabs seem curved. These results are in good agreement with the recent study of Yoosuk et al. [37], who also suggested a reduction in the slab length and form when Ni was incorporated into the Mo sulfide. In these directions, the promoting Ni atoms are bonded (in intercalation positions) by Van der Walls forces [36]. It is generally accepted that the best hydrotreating MoS2 catalysts are promoted with Ni or Co atoms located at the edges of MoS2 slabs [2], which is discussed later in connection to the lignin depolymerization. In good agreement with the literature data, it can be assumed that the formation of NixSy active sites occurred at the rims of MoS2 sheet crystals.\nFig. 2\n displays the TGA and DTG profiles of the pre-sulfided and as prepared unsupported NiMoO4-SBA catalysts. For the unsulfided NiMoO4-SBA catalyst, it shows two weight loss areas, totaling about 6.4\u00a0wt% up to the reaction temperature of 400\u00a0\u00b0C, together with corresponding endothermic peaks (Fig. 2b). The first weight loss was about 3.4\u00a0wt% from 75 to 170\u00a0\u00b0C, based on a calculation of the first derivative of the weight loss curve at 70.4\u00a0\u00b0C and might be attributed to the loss of physically absorbed and chemically bonded water. The following weight loss was about 3.0\u00a0wt% from 210 to 415\u00a0\u00b0C, which can be due to the total phase transformation of \u03b1-NiMoO4 (octahedral MoO6) to pure \u03b2-NiMoO4 (tetrahedral MoO4) above 286\u00a0\u00b0C, which was reported by Pillay et al. by measuring the XRD pattern at high temperature to characterize the \u03b1-NiMoO4 and \u03b2-NiMoO4 phase transitions [38].After the sulfidation of the unsupported NiMoO4-SBA catalyst, the TGA thermogram showed a 6.5\u00a0wt% weight loss up to 400\u00a0\u00b0C, which was similar as for the unsulfided NiMoO4-SBA. Besides, it also shows two endothermic weak peaks (pre-sulfided NiMoS, Fig. 2b) that may refer to a decomposition of the \u03b1-NiMoO4 and \u03b2-NiMoO4 formed due to oxygen contact. At higher temperature, a weight loss was observed from 400 to 650\u00a0\u00b0C and thereafter another weight loss a higher rate in the 650\u2013900\u00a0\u00b0C range. It is likely that these weight losses are originating from decomposition of different sulfur species on the catalyst since they were not found on the non-sulfided catalyst (Fig. 2b, blue line).In order to investigate the nature of the surface species, XPS analysis of the sulfided catalyst was conducted and shown in Fig. 3\n. Based on XPS analysis, the atomic percentage of various elements present at the surface of the catalyst is given in Table 3\n. Note that the sulfided sample could be partially oxidized due to oxygen contact when storing and transferring the sample to the XPS instrument, which could possibly explain the high oxygen content (Table 3). XPS was used to investigate the chemical states of Mo, Ni and S in the pre-sulfided NiMoS, shown in Fig. 3. The XPS spectra showed that the binding energies of Mo 3d5/2 and Mo 3d3/2 were located at 228.7\u00a0eV and 232.1\u00a0eV respectively, owing to the Mo4+ in MoS2\n[39,40] and there is also a weak peak located at 235.2\u00a0eV assigned to Mo6+ 3d5/2 of MoO3 formed due to oxidation of Mo (Fig. 3b). Another weak peak at the binding energy at 225.9\u00a0eV is ascribed to S2s of S2 in MoS2\n[39,40]. Besides, there are two strong peaks at 161.5 and 162.7\u00a0eV in the S2p spectrum (Fig. 3c), which are assigned to the S2p3/2 and S2p1/2 binding energies for S2\u2212 of MoS2 and NiS2. Moreover, there are three peaks present in the Ni 2p spectrum (Fig. 3d) at 853.3, 856.4, and 861.7\u00a0eV, which could be assigned to NiS2, nickel oxide and nickel hydroxide, respectively [36,40].NH3-TPD profiles of NiMoO4-SBA and NiMoS-SBA catalysts are presented in Fig. S3 (SI) to evaluate their surface acidity. Both catalysts exhibited moderate (200\u2013400\u00a0\u00b0C) and strong (>400\u00a0\u00b0C) acid sites. The NH3-TPD profile of NiMoO4-SBA displayed a relatively wide peak at 300\u00a0\u00b0C and a minor peak at 550\u00a0\u00b0C, which were attributed to moderate and strong acid sites, respectively. Whereas after sulfidation, it can be observed that there is an important decrease in the number of moderate acid sites, while an increase in the acid strength for sulfided NiMoS-SBA.Chemical structures and thermal characterization of lignins were carried out through thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), elemental analysis (CHONS), and solid state 13C NMR. TGA curves, under oxidizing atmosphere (Fig. 4\n), showed that the water content corresponds to the weight loss of about 2.3\u00a0wt% for Kraft lignin and 1.5\u00a0wt% for hydrolysis lignin at 100\u00a0\u00b0C, whereas the remaining weight at the end corresponds to the ash content of 3.5\u00a0wt% for Kraft lignin and only 0.5\u00a0wt% for hydrolysis lignin. Elemental CHONS analyses, ash, water content as well as the H/C and O/C atomic ratios are reported in Table 4\n.The chemical structures of Kraft and hydrolysis lignin were analyzed by means of solid-state 13C NMR (Fig. 5\n). Both lignins showed small signals in the area 200\u2013160\u00a0ppm, indicating a low amount of CO structure, attributed to CO bonds in Ar-CHO or R-O-CO-CH3 at 177.5\u00a0ppm and 181\u00a0ppm, respectively [41]. In the Carom region (110\u2013150\u00a0ppm), the intense peak at 150\u00a0ppm corresponds to Carom-O, whereas three weak signals were observed at 115, 123 and 127\u00a0ppm corresponding to Carom-C and Carom-H [42,43]. The 13C NMR spectrum of hydrolysis lignin showed characteristics of typical lignocellulosic biomass, which is primarily composed of cellulose (62\u2013110\u00a0ppm) and lignin [44]. Fu et al. showed that hydrolysis lignin is covalently attached onto cellulosic moieties, indicating that the cellulose from lignocellulosic biomass is not fully isolated [15,45\u201347]. Note that the high O elemental content result for hydrolysis lignin (38.1\u00a0wt%) may be partly due to the presence of cellulosic oxygen content. Moreover, methoxy groups were detected at 58.0\u00a0ppm for both lignin types. The CH, CH2, and CH3 saturated aliphatic signals (<50\u00a0ppm) could not be clearly observed and distinguished. Some identical peaks were observed in both lignins, but the peak intensities were higher for Kraft lignin than the hydrolysis lignin. By comparing these different analytical results, we can observe that both lignins are connected randomly through \u03b2-\u03b2 or \u03b2-aryl ester, G-type \u03b2-O-4 and \u03b2-5 linkages, and cellulosic units for hydrolysis lignin.TGA and DSC curves for hydrolysis and Kraft lignin under inert atmosphere are given in Fig. 6\n. TGA analysis showed that Kraft and hydrolysis lignins start to decompose in the range of 175\u2013460\u00a0\u00b0C and 250\u2013405\u00a0\u00b0C, respectively. Thus, reactions involving lignins are already expected to take place to a certain extent when heating up the reactor to the final chosen temperature, in our case 400\u00a0\u00b0C [48]. Moreover, under inert atmosphere at 550\u00a0\u00b0C, the char yields of Kraft (43\u00a0wt%) and hydrolysis lignin (31\u00a0wt%) were higher than in an oxidizing environment (see Fig. 4 and Fig. 6). These results show that depolymerization of lignin without a catalyst to facilitate hydrogenation and deoxygenation reactions, is likely to yield a high char formation, particularly for Kraft lignin. It can also be seen that the thermal properties of the lignin depend on their source due to their structural variations. However, under catalytic hydroconversion, the conversions of the Kraft and hydrolysis lignins were found to be higher than 90\u00a0%, which was confirmed by performing DMSO extraction of the unconverted lignin. In the subsequent sections, we examine and discuss in detail the individual product yields of the hydrotreated lignins.For each lignin, the results of non-catalyzed and catalyzed hydrotreatment reactions are presented in Table 5\n. The conversions of both lignins were in the range of 91.0\u201399.5\u00a0wt%. The lowest lignin-oil yields were observed for the non-catalyzed experiments (KES0, and HES6), which correlate well with the least water content formed upon the reaction (0.3 and 0.7\u00a0wt%) and the highest char formation (52.9 and 38.6\u00a0wt%), showing a low degree of deoxygenation without the catalyst. These results also provide clear evidence that thermal depolymerization reactions play a role. Interestingly, the hydrolysis lignin results in significantly less char for uncatalyzed reactor experiments than Kraft lignin, which is consistent with the TGA results in Fig. 6. When the unsupported NiMoS-SBA catalyst (in situ sulfided) was used under the same conditions, the lignin-oil yield increased to 65.1 and 83.7\u00a0wt%, whereas the char yields were repressed to 20.6 and 8.3\u00a0wt% for Kraft (KES5: 400\u00a0\u00b0C, 80\u00a0bar H2, 10\u00a0% catalyst, 5\u00a0h) and hydrolysis lignin (HES8: 400\u00a0\u00b0C, 80\u00a0bar H2, 10\u00a0% catalyst, 5\u00a0h), respectively. The characterization results demonstrated that the sulfided catalyst had two separated sulfide phases rather than a trinary Ni-Mo-S phase. Thus, the presence of both phases of MoS2 and NixSy likely improved the lignin conversion. This is mainly due to a synergism between NixSy and MoS2, evidently reflected by the major XRD peaks of pre-sulfided NiMoO4-SBA (Fig. 1b). Moreover, the XPS analysis confirms the presence of both phases. It was also reported by Wang et al. that a synthesis of NiS2/MoS2 has higher surface area, resulting in the exposure of more active sites [49]. The hydrodeoxygenation activity is considered enhanced in the presence of both NiS2 and MoS2 which could be described by a Remote Control (RC) model via hydrogen spillover [49]. According to the RC model the two separated sulfide phases of the catalyst are described as a donor phase (promoter, NiS2) and an acceptor phase (active component, MoS2), and thus spillover hydrogen was created on NiS2 which then migrated to MoS2\n[49]. It was also reported that a Ni-Mo binary sulfide phase is more active than either of the single Mo or Ni sulfide phases and the maximum synergy depends on Ni/(Mo\u00a0+\u00a0Ni) ratio to achieve a well dispersed active phase [37].A comparison is made between pre-sulfidation and in-situ sulfidation method (see Table S1) and it is found that both methods give similar results. Based on these results, we consider that the DMDS added was adequate to activate both in-situ or pre-sulfided NiMoO4-SBA catalysts.The influence of temperature, pressure, residence time and catalyst loadings were investigated. The effect of temperature (330 and 400\u00a0\u00b0C) on the product yield was investigated in experiments KES1 (330\u00a0\u00b0C, 50\u00a0bar H2, 5\u00a0% catalyst, 5\u00a0h) and KES2 (400\u00a0\u00b0C, 50\u00a0bar H2, 5\u00a0% catalyst, 5\u00a0h) using Kraft lignin. Higher temperature led to a slight increase in lignin liquefaction yields from 48.7 to 49.7\u00a0wt% and a clear decrease in char residue from 40.5 to 30\u00a0wt%. We also observed that both the water and gas contents increased to\u00a0>2.0\u00a0wt%, indicating that the removal of oxygen from the biomass starts above 330\u00a0\u00b0C. Moreover, a longer residence time of the reaction (KES3 (400\u00a0\u00b0C, 50\u00a0bar H2, 5\u00a0% catalyst, 12\u00a0h) led to further suppression of the char yield to 27\u00a0wt%, and large increases in lignin oil, water and gas content were observed. Hydrogen consumption was measured for the 5 and 12\u00a0h residence times and it increased from 1.5 to 3.9\u00a0mmol per g of Kraft lignin, simultaneously as the water yield increased from 2.3\u00a0% to 7.9\u00a0%, due to a higher degree of deoxygenation being achieved.At a constant temperature of 400\u00a0\u00b0C, the pressure was increased from 50 to 80\u00a0bar (KES2: 400\u00a0\u00b0C, 50\u00a0bar H2, 5\u00a0% catalyst, 5\u00a0h) versus (KES4: 400\u00a0\u00b0C, 80\u00a0bar H2, 5\u00a0% catalyst, 5\u00a0h). This higher pressure ensured a higher solubility of hydrogen in the oil, which resulted in an increase from 49.7 to 61.5\u00a0wt% of the oil yield and thereby a higher availability of hydrogen in the vicinity of the catalyst. As seen in Table 5 (KES2 vs KES4), this increases the reaction rate and further decreases the unreacted lignin from 9.0 to 4.7\u00a0wt%. Furthermore, higher degrees of deoxygenation are favored by increasing the catalyst loadings from 5 to 10\u00a0wt%. Consequently, higher lignin-oil yields of 65.1 and lower char yields of 20.6\u00a0wt% were achieved for Kraft lignin (KES4 vs KES5) with increased catalyst loading. Similarly, for hydrolysis lignin (compare HSE7, HSE8 and HSE10), higher loadings of catalyst resulted in increased oil yield and decreased char formation. This could be explained by that when increasing the loading more active sites are available for the adsorption of lignin and in addition, an enhanced hydrogen spillover from NixSy could occur, resulting in a promotion of the hydrogenation-dehydration reactions.A comparison of lignin-oil and char yields for Kraft and hydrolysis lignins is presented in Fig. 7\n. Under identical operating conditions, hydrolysis lignin results display higher lignin oil formation and lower char yields in comparison to Kraft lignin. One of the reasons is that hydrolysis lignin was obtained from enzyme catalytic conditions, making it more active and free from ash and sulfur contents (Table 4), than the lignin materials obtained from chemical processes [20,50]. Generally, additional central factors may be the different chemical structures and compositions of the lignin-types, and in particular their different S/G ratios [51,52]. Moreover, the hydrolysis lignin consisted of both lignin and cellulose (see Fig. 5) and thereby the hydrolysis lignin contains less lignin per mass, and this could also be a factor for producing less char since it is known that lignin often gives large amount of char.To investigate the stability of the sulfided NiMoO4-SBA catalyst, three consecutive conversions of hydrolysis lignin were conducted using the recycled catalyst (Table S4). Upon completion of the reaction (HES10), the catalyst used was regenerated by calcining the spent catalyst in air at 500\u00a0\u00b0C for 5\u00a0h to burn off any remaining unseparated char or solids. The recycled catalyst was then used for the next reaction cycles as described in the Reaction experiments section 2.5. Table S4 shows that for the experiments with recycled catalyst the conversion and oil yields are similar during repeated cycles. A catalyst weight loss of 10.6\u00a0wt% was observed for the first cycle, which is similar to the 8.5\u00a0wt% from the TGA thermogram results (Fig. 2a). The second and third cycles were further performed and resulted in negligible weight losses up to 2.0\u00a0wt%. Thus, the spent NiMoS-SBA catalyst was found to have good stability and performance comparable to a duplicated HES10 experiment (Table 5).The gaseous phase composition was quantified, and the results are given in Table 6\n. The results show a maximum of 5.6\u00a0wt% gas product yield measured at room temperature after the reaction. The dominant gas products were CH4 (0.1\u20133\u00a0wt% on lignin), and CO2 (0.1\u20132.4\u00a0wt%), with small quantities of olefins C\n2-C4 (<0.7\u00a0wt%). The gas-phase formation may be explained by reactions occurring during lignin hydroconversion, along with gas phase reactions. The olefins are derived from the CC cleavage of alkyl chains or via dehydration of intermediate small alcohols derived from the cleavage of the \u03b2-O-4 ether linkage. The formation of CH4 can be explained by demethylation, which is favored under our conditions. Another possible pathway for methane formation is the reaction of the released CO2 and CO with H2. This was also demonstrated for model compounds such as formic and acetic acid over a Ru/TiO2 catalyst [53,54]. Moreover, the formation of CO2 and traces of CO can result from decarboxylation and water gas shift reactions. The decarboxylation of \u2013COOH to form larger amounts of CO2 product was observed for the catalytic hydroconversion at 400\u00a0\u00b0C (KES2), but not at 330\u00a0\u00b0C (KES1). These results are consistent with those reported previously, suggesting that decarboxylation of carboxylic acids occur at a temperature higher than 350\u00a0\u00b0C [52,55].When increasing the temperature (KES1 versus KES2) and residence time (KES2 versus KES3) for Kraft lignin over the NiMoS-SBA catalyst, the yields of CH4 and CO2 increased, whereas it was similar when increasing the pressure (KES2 versus KES4). With increasing catalyst content from 5\u00a0wt% (KES4) to 10\u00a0wt% (KES5), the yields of CH4 and CO2 were similar. However, a significant decrease in gas yield was observed for hydrolysis lignin while increasing the catalyst loadings from 5, 10 and up to 20\u00a0wt% (HES7, HES8, and HES10). In contrast to our results, Chowdari et al.\n[56] found that the total yield of gases slightly increased when increasing the catalyst loading for a 20NiMoP/AC from 5\u00a0wt% to 10\u00a0wt%, respectively from 8.6 to 9.4\u00a0% at 400\u00a0\u00b0C [56]. This can be due to several differences, such as much higher selectivity for the decarboxylation and demethylation with their supported 20NiMoP/AC catalyst and the conditions under which it was used.The oxygen and hydrogen contents in the lignin-oils are displayed in the form of Van Krevelen diagrams (Fig. 8\n). The hydrolysis lignin used in this study has a relatively high O/C ratio of 0.51, while that of Kraft lignin is considerably lower, 0.39. This is likely due to differences in the extraction methods, feedstock origins and their different compositions (e.g. cellulose in hydrolysis lignin). Based on the van Krevelen diagrams, the lignin oils show significantly lower O/C ratios (0.02\u20130.11) and an increase in H/C ratios (1.97\u20132.26), suggesting that the hydroconversion reactions have occurred to a large extent. It is interesting to note that the estimated Higher Heating Values (HHV) (Tables S1 and S2) are high and similar to those of traditional petroleum-based fuels [57]. Despite the higher starting O/C ratio, hydrolysis lignin showed overall lower oxygen contents in the oils, which possibly can allow easier conversion into high-quality fuels compared to the oil from Kraft lignin. Also, the presence of hydrogen and catalyst resulted in a reduction in sulfur content in the lignin-oils (<0.02\u00a0wt%), showing the efficiency of the catalyst for HDS reactions (see Tables S1 and S2). In the absence of catalyst (KES0 and HES6), the values of oxygen were higher (8.3\u201311.6\u00a0wt%) than for all experiments with unsupported NiMoS-SBA catalyst (1.9\u20136.3\u00a0wt%), which clearly demonstrates the beneficial effect of the catalyst. The above results suggest that NiMoS-SBA is an effective catalyst for hydrogenation, deoxygenation and desulfurization of lignin under the selected conditions. At high temperature and pressure (KES4), the H/C ratio increases, and it can further increase with extended reaction time as was evident for KES3 versus KES2. For reactions performed at 400\u00a0\u00b0C and 80\u00a0bar, an increase in catalyst loading (KES5 versus KES4) leads to a decrease in the O/C ratio, which results in that >87\u00a0% of the oxygen was removed, compared with the initial feedstocks used.\n31P NMR analysis on the lignin-oils was performed to help elucidate the changes that occurred during the reaction. This offers the unique ability to distinguish hydroxyl groups attached to p-hydroxyphenyl, guaiacyl, and syringyl units. Fig. 9\n shows 31P NMR spectrums of lignin oils obtained under the selected conditions. A compressed compilation of hydroxyl groups in lignin oils and their typical chemical integration ranges are summarized in Fig. 9 along with the quantitative data (Table 7\n) clearly shows that Kraft and hydrolysis lignin oils were rich in p-hydroxyphenyl and guaiacyl OH groups, while syringyl and condensed OH groups (C5-substituted OH) were observed in small amounts for the highly catalyzed Kraft lignin (10\u00a0wt% loaded, KES5) and less catalyzed (5\u00a0wt% loaded, HES7) and uncatalyzed hydrolysis lignin (HES6 and HES7). Small amounts of carboxylic acid (133.6\u2013136\u00a0ppm) and aliphatic OH (145\u2013150\u00a0ppm) can be distinguished for Kraft and hydrolysis lignin oils as well.The total quantity of monomer in the lignin-oils is of high interest to indicate the effect of the unsupported catalyst and various operating conditions on the target product classes in this study. Therefore, all lignin-oils were subjected to GCxGC analysis, with a correction for the hexadecane solvent contribution (Fig. 10\n). The lignin-oil phase product comprises a complex mixture of monomeric compounds. The monomer composition detected by GCxGC is quite similar for both lignins when other variables are kept constant (listed in Fig. 10). For noncatalyzed reactions shown in the 13P NMR spectrum, the total OH content (Fig. 9, uncatalyzed) in the lignin-oils was lower and dominated by p-hydroxyphenyl (phenolic OH groups). This implies that hydrogenolytic cleavage of aryl-O-aryl and aryl-O-aliphatic linkages in the lignin only partially occurred. A series of operating conditions were examined with the presence of the unsupported NiMoS-SBA catalyst and exhibited an important impact on lignin depolymerization, namely enhanced cleavage of COC linkages between lignin units. 13P NMR spectrums showed the presence of a larger amount of p-hydroxyphenyl and guaiacyl in the area 138\u2013140\u00a0ppm (Fig. 9, catalyzed). By increasing the temperature to 400\u00a0\u00b0C for Kraft lignin, the methoxyphenol were converted to alkylphenolics by O-demethylation reactions, as confirmed by the GCxGC analysis (KES1 to KES2, Fig. 10). This suggests that most of the O-demethylation (\u2013OCH3) and hence higher CH4, CO2 and water yields (Table 5 and Table 6) resulted from cleavage of guaiacol, syringyl and C5-substituted OH groups occurring at the higher temperature.By increasing the residence time (KES2 to KES3), both alkylated phenolic and aliphatic OH were increased for Kraft lignin (Table 7 and Fig. 10), whereas the carboxylic acid (COOH) was significantly suppressed, leading to the enhancement of CO2 formation (Table 6). This is probably due to the reduction of the carboxylic acids accompanied by an increase in solubility of the Kraft lignin in the fluid phase [58]. A similar effect on content of carboxylic acid was observed for hydrolysis lignin while increasing the residence time from 5 to 12\u00a0h at 400\u00a0\u00b0C (HES8 to HES9), however; this resulted in slightly decreased phenolic and aliphatic OH contents according to NMR results (Table 7). It is noteworthy to mention that part of the phenolic OH detected by 31P NMR includes those in oligomeric compounds which are not detected by GCxGC analysis. Thus, the enhancement of alkylphenolic and aromatic compounds when increasing the residence time from 5\u00a0h to 12\u00a0h, as shown in the GCxGC results (HES8 to HES9, Fig. 10), can be explained by a deep cleavage of the oligomeric compounds (Table 7). As can be seen in Fig. 9, the C5-substituted OH units for uncatalyzed (0\u00a0wt%, KES0) and less catalyzed Kraft lignin (5\u00a0wt%, KES4) were not observed, whereas it was observed for the highly catalyzed Kraft lignin (10\u00a0wt%, KES5). For hydrolysis lignin, the uncatalyzed (0\u00a0wt%, HES6) and less catalyzed (5\u00a0wt%, HES7) lignin showed small amounts of C5-substituted OH units, however; none were observed at the higher catalyst loading (10\u00a0wt% HES8). Although the reaction conditions were nearly identical, the depolymerization and cleavage of CO bonds differed to a certain extent between the lignins.Upon increasing the catalyst loading from 10\u00a0wt% (HES8) to 20\u00a0wt% (HES10) for hydrolysis lignin, all hydroxylic groups and particularly the phenolic OH content decreased by eightfold (Table 7), which was accompanied by a significant reduction in the char yield and an enhancement in lignin-oil yields (HES10, Table 5). These changes also resulted in an increase in aromatic, and a decrease in alkylated phenolics and aliphatic OH/Ketones yields (HES10, Fig. 10). As shown by comparison between the Kraft and hydrolysis lignins in Fig. 9, the high correlation between alkyl phenolics and aromatics is strongly dependent on the composition and structural complexity of the lignins which influences their hydroconversion reactivity.The total monomer yield ranged from 25.1 to 47.0\u00a0wt% for Kraft lignin (KES0 to KES5, Fig. 10) and from 32.7 to 76.0\u00a0wt% for hydrolysis lignin (HES6 to HES10) under the various reaction conditions. Alkylphenolics are the dominant chemical group from Kraft (7.0\u201322.7\u00a0wt%) and hydrolysis (11.9\u201324.8\u00a0wt%) lignin oil, except at higher than 10\u00a0wt% of catalyst loading for both the Kraft and hydrolysis lignins in which case the aromatics were the major compound from Kraft (18.3\u00a0wt%) and hydrolysis lignin oil (39.4\u00a0wt%). However, the proportion of the oil products composed of aromatics/naphthalenes varies between the lignin oils, where higher aromatic yields of 14.7\u00a0wt% were obtained for hydrolysis lignin (HES7) compared to 7.3\u00a0wt% for Kraft lignin oil (KES4) at the same operating conditions. When comparing the 10\u00a0wt% loading of catalyst at the same conditions (HES8 vs KES5), it is apparent that hydrolysis lignin produces higher lignin-oil yields of 83.7\u00a0wt% compared to Kraft lignin-oil of 65.1\u00a0wt% (Table 5), corresponding to 64.3 and 47.0\u00a0wt% of monomer yields respectively (Fig. 10). This has been previously confirmed using model compounds over NiS2/MoS2\n[50]. It was reported that only an appropriate proportion of donor phase (NiS2) to acceptor phase (MoS2) could produce the maximum HDO activity. More specifically, the hydrogenation activity was enhanced in the presence of NiS2, leading to an increase in cyclohexane derivative selectivity and deoxygenation degree. The authors claimed that an optimal Ni/(Ni\u00a0+\u00a0Mo) molar ratio of 0.3 was important to achieve the highest activity with 99.8\u00a0% deoxygenation degree [50]. In our study, this ratio, determined from XPS analysis, was about 0.33, which is in good agreement with the reported optimum.In a recent study, Chowdari et al.\n[56] reported a total monomer yield of 45.7\u00a0wt% over 10\u00a0wt% bimetallic 20NiMoP/AC for Kraft lignin at 400\u00a0\u00b0C and 100\u00a0bar with no added solvents. These findings are in line with our results for Kraft lignin, with monomer yields in the range of 42.3\u201347.0\u00a0wt%. However, the aromatics were reported to be lower (8.7\u00a0wt%) for the 20NiMoP/AC catalyst in comparison to the unsupported NiMoS-SBA (18.3\u00a0wt%), whereas opposite levels in the yields of alkylphenolics resulted (25.0\u00a0wt% for 20NiMoP/AC and l6.1\u00a0wt% for NiMoS-SBA). A higher solid yield was obtained for our operating conditions, but notably at lower pressures of 50\u201380\u00a0bar. The adjustment of pressure, residence time (from 5 to 12\u00a0h) and catalyst loadings can likely further reduce the char formation and significantly increase lignin-oil and monomeric yields. Interestingly by increasing the catalyst loading from 5\u00a0% to 10\u00a0% (KES4 to KES5), the degree of deoxygenation is strongly increased from about 46.0 to 60.5\u00a0% and therefore resulted in an increment of aromatics from 7.3 to 18.3\u00a0wt% respectively for Kraft lignin oil. This was also evident by a reduction of the calculated oxygen content of the oils (Tables S2 and S3) and indicating that the unsupported catalyst exhibits better selectivity for deoxygenated products under nearly identical conditions.In the case of hydrolysis lignin, the effect of catalyst loading (from 5 to 20\u00a0wt%) on lignin oil yields and composition was investigated at 400\u00a0\u00b0C using the same unsupported NiMoS-SBA catalyst. By increasing the catalyst loading from 5 to 10\u00a0wt%, the monomer yield was significantly enhanced from 46.6 to 64.3\u00a0wt% with a suppression of char to 8.3\u00a0wt% (HES8). While increasing the residence time (from 5 to 12\u00a0h), the monomer yield was further enhanced from 64.3 to 70.6\u00a0wt% with a suppression of char from 8.3 down to 3.9\u00a0wt% for HES8 and HES9 respectively. Upon increasing the catalyst loading to 20\u00a0wt%., the total monomer yield was still further increased to 76.0\u00a0wt%. The char formation was suppressed considerably at the highest catalyst loading from 8.3 to 4.6\u00a0wt% (HES10) due to the depolymerization reactions involving the bimetallic NiMoS-SBA catalyst. These results are also in agreement with data reported by Chowdari et al.\n[56], suggesting that the repolymerization reactions leading to char are likely thermal and not catalytic, while the depolymerization reactions are catalytic. However, Chowdari et al. found that higher temperature (>400\u00a0\u00b0C) leads to the formation of more char and less oil yield [56]. In our study, the main objective was to explore reaction conditions that are favorable using an unsupported NiMoS-SBA catalyst. The results obtained at 20\u00a0wt% loading of catalyst for hydrolysis lignin showed that over 87\u00a0wt% of lignin can be depolymerized, which consists of 39\u00a0wt% of aromatics/naphthalenes with the lowest alkylphenolic yield of 10.1\u00a0wt%.Like a previously published work [59], TGA analysis was used to evaluate the volatility of lignin-oils resulting from biomass conversion. Comparison of the volatilities of Kraft (KES5) and hydrolysis (HES8) lignin-oils were performed with oils produced under the same operating conditions (400\u00a0\u00b0C, 80\u00a0bar, 5\u00a0h). Fig. 11\n shows that a complete weight loss for both lignin-oils was observed when increasing the temperature to 500\u00a0\u00b0C under N2 flow. The hydrolysis lignin oil contains less thermally stable compounds, as indicated by its higher volatilization rate at 200\u00a0\u00b0C. In addition, it requires more energy (higher temperature) to completely volatilize the produced Kraft lignin-oil (>300\u00a0\u00b0C). This difference may be due to the influence of the origin of the lignins and their extraction processes. More importantly, both lignin-oils followed the TGA curve of diesel oil, particularly for hydrolysis lignin-oil [59]. This implies that the hydrolysis lignin-oil contains a higher share of low boiling point compounds than the Kraft lignin-oil.Based on the above analysis and discussion, lignin hydroconversion to produce lignin-oils, gas and residual solids involves various reaction pathways depending on the lignin composition, and operating conditions. Numerous studies have proposed and summarized schemes for the conversion of Kraft lignin by depolymerization [56,60\u201363]. In contrast to Kraft lignin, limited studies have investigated reaction pathways of the depolymerization of enzymatic hydrolysis lignin. Chudakov et al. and Pikovskoi et al.\n[64,65] proposed a unique macrostructural composition of hydrolysis lignin consisting of about 7000 peaks of deprotonated molecules [M\u2014H]\u2212. The largest detected molecules were decamers with molecular weights up to 1600\u00a0Da, containing up to 10 aromatic units with an average molecular weight of 150\u00a0Da per structural unit [64,65]. Pikovskoi et al. also claimed that depolymerization of hydrolysis lignin released coniferous lignin corresponding to the abundant guaiacyl structural unit of about 196\u00a0Da. In addition to Chudakov and Pikovskoi suggestions, the hydrolysis lignin in this work showed the presence of cellulosic units by means of solid-state 13C NMR (Fig. 5), as depicted in Scheme 1\n.According to the reported data in Table 5, Table 6 and Fig. 10 the non-catalytic experiments favored the formation of solid-char. We suggest that the highly reactive intermediates (oxygenated compounds) formed during the thermal decomposition of lignin results in condensation (Scheme 1A) and large char formation. Generally, the alkylation reactions predominantly have a significant role to restrain the condensation reaction by multiple substitutions of the abundantly formed positively charged species (e.g., R\u2014+O\u2014CH3) with electron-rich active species (negatively charged Aryl, Ar\u2013\u2014O\u2014R) that jointly affect the electron distribution of both lignins [67,68]. Without the presence of catalyst (Scheme 1A), a negatively charged aromatic, ring rich in electrons, is mostly formed (Ar\u2013\u2014O\u2014R) and subjected to substitution of phenol-ether and/or alkylphenolic (Ar\u2014R\u2014+OH and Ar\u2014+O\u2014R). In addition to the resonance effect, the inductive effect of the positively charged moieties (R\u2014+O\u2014R) leads to higher electron densities, which are involved in lignin condensation by forming benzylic carbocations and thereby enhance char formation [67,68]. This suggestion is supported by the fact that the formed alcohols/ketones, olefins and CH4 were present in the lowest amounts and only account respectively for less than 0.2 and 0.3\u00a0wt% of products for both lignins during uncatalyzed reactions (KES0 and HES6).In the presence of the unsupported NiMoS-SBA catalyst (Scheme 1B\u2013E), the hydrotreatment of both lignins becomes relevant and contributes to heterogeneous catalyzed processes at higher temperatures. Due to the difference in structural and chemical composition of hydrolysis lignin, a higher yield of small ketone and aliphatic alcohols in hydrolysis lignin-oils (13.2\u00a0wt%) were obtained due to the depolymerization and ring opening of cellulose and furan units present in hydrolysis lignin, as was evident from the 13C NMR spectrum and GCxGC results (Fig. 5 and Fig. 10). According to Shuai and Saha [69] alcohols, ketones and aldehydes can block the electron-rich sites on the aromatic ring and the benzylic cation on the side chain, which would reduce the lignin condensation. The larger amount of alcohols and ketones formed in hydrolysis lignin (Fig. 9) could be one reason for the lower char amount and enhanced monomeric yields in the liquid phase (Scheme 1B). Huang et al. [68] examined lignin depolymerization in the presence of ethanol and suggested that the C-alkylation and O-alkylation reactions are important reactions for decreasing the condensation and thereby the char formation. We therefore suggest that during the depolymerization of hydrolysis lignin, higher stability of the aromatics (negatively charged moieties) via C- and O-alkylation reactions mainly come from cleaved and dehydration of the aliphatic OH and ketones. Additionally, these small ketones/alcohols derived from lignin can be hydroconverted to small alkenes via cracking dehydration (Scheme 1C). The primary stable monomers obtained from lignin (Scheme 1D) may undergo secondary hydrogenation reactions of oxygenates to form more stable monomer products (Scheme 1E).Generality across different mechanisms can be accepted considering the complexity of the types of lignin depolymerized and may play a role in the identification of effective catalysts. In this study, evidence suggests that a combination of mechanisms hinders the formation of oligomeric CC linkages and thus blocks the re-condensation reactions. From our set of experiments and the literature [6,66\u201368], Scheme 2\n summarizes the most important aspects hindering the repolymerization over the unsupported NiMoS-SBA catalyst. We suggest that the condensation reaction rate was slowed down by blockage of the electron-rich sites (aromatic ring) via hydrogen spillover generated on NixSy, which prevented the formation of benzylic cations. Thereafter, the negatively charged aryl molecules either undergo hydrogenolysis interactions in series or in parallel reactions with the supplied hydrogen (H+) and/or with alkylated species present in the lignin-oils. These alkylated species are present in greater quantities from hydrolysis lignin (13.2\u00a0wt%) and thus serve as promoters for depolymerization compared to the lower quantities of alkylated species from Kraft lignin (6.8\u00a0wt%).We also suggest that the stabilization of aromatic rings is enhanced by a combination of interunit C(aryl)\u2014C(alkyl) linkages and the supplied hydrogen species as shown for hydrolysis lignin (Scheme 2B), whereas these factors are less dominant in the case of Kraft lignin (Scheme 2A). This accounts for the higher resulting monomeric yields and lower char formation from hydrolysis lignin compared to Kraft lignin. In this study, our hypothesis is built on the importance of the inductive effect that provides higher electron densities on methoxy groups (R\u2014+O\u2014CH3) due to the formation of interunit C(aryl)\u2014C(alkyl) linkages, which create higher electron densities on the aromatic ring to further stabilize it and block the reactive benzylic positions.Based on literature, our proposal agrees with that of by Shuai et al.\n[6,69] that found that formaldehyde acted as a lignin stabilizer by blocking the reactive benzylic positions of intermediates [6]. Also in agreement with our results, Huang et al.\n[68] suggested that ethanol is a capping agent acting as a scavenger for formaldehyde formed by removal of methoxy groups from the lignin, that thereby suppresses repolymerization reactions involving formaldehyde [68].To summarize, lignin valorization is more efficient with hydrolysis lignin compared to Kraft lignin, which could be seen by a significantly higher amount of monomer bio-oil produced and lower char formation. This is be explained by (i) differences in the structure of hydrolysis lignin, which facilitates thermal decomposition, (ii) less lignin per mass unit in hydrolysis lignin, due to the presence of cellulose (iii) less inorganic ash in the hydrolysis lignin which could negatively affect the catalytic reactions and (iv) suppressing the repolymerization by reactions with components formed from the cellulose.In this study, we have for the first time according to our knowledge compared the reductive catalytic lignin depolymerization using Kraft and hydrolysis lignin and we found large differences. We have synthesized a highly active unsupported NiMoS catalyst that was used in this work. The use of the unsupported NiMoS-SBA demonstrated a potentially promising approach to obtain bio-oils with a high proportion of aromatic and alkylphenolic compounds. The influence of the operating conditions (temperatures, pressure and time) with various catalyst loadings of 5\u201320\u00a0wt%, were evaluated in terms of product yields and composition.Catalytic hydrotreatment experiments with hydrolysis lignin exhibited deeper deoxygenation performance in comparison to Kraft lignin. The increase in the reaction temperature, between 330 and 400\u00a0\u00b0C, dramatically enhanced the cleavage of COC bonds, especially increasing alkylated phenolics from 7.0 to 20.1\u00a0wt% yield respectively. On the other hand, it was observed that as the pressure increased from 50 to 80\u00a0bar and residence time increased, the yields in oil and monomeric compounds also increased, and char formation could be suppressed to a certain extent. Interestingly, the monomers yield was the highest and the char was the lowest for hydrolysis lignin at comparable reaction conditions. In addition, the NMR and GCxGC analysis demonstrated that with an increase in catalyst loading, the phenolic OH groups were decreased in the product oil which resulted in an increment in the aromatics yield. Comparing the volatilities, lignin-oils from Kraft and hydrolysis lignin showed a complete volatilization which indicates a high content of low boiling compounds, particularly for hydrolysis lignin-oil.The unsupported NiMoS catalyst displayed notable deoxygenation activity, with 87\u00a0wt% lignin-oil yield from hydrolysis lignin, with less than 5\u00a0wt% char yield. Remarkably the significant reduction of char from hydrolysis lignin compared to Kraft lignin led to an increase in water and lignin-oil yields, resulting in higher selectivity and yield of aromatics. These results highlighted the importance of the chemical stability and the nature of processed lignins that arise directly from the oligomer composition of the lignin, and their correlation between depolymerization yields and the ratio of COC and CC linkages in the lignins. From the experimental results, the higher monomeric yield from hydrolysis lignin can be explained by that the hydrolysis lignin more easily depolymerizes. Moreover, the hydrolysis lignin also contains less lignin (since it contains both lignin and cellulose) and less ash, which also are important reasons for the lower char production and higher bio-oil yield when using hydrolysis lignin. In addition, the molecular binding mechanism of hydrolysis lignin is a key to generate higher-value small molecules through depolymerization. Results suggested that the high electron densities of the formed small molecules from cellulose decomposition could interact with the aromatic ring and influence the reactivity of the benzylic carbocations formation. Apart from effective hydrotreating cleavage, the catalyst demonstrates a good activity and stability over multiple regeneration cycles; however, long testing will be required in a pilot scale reactor to assess its total lifetime in the future.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We would like to acknowledge the collaboration with Preem, Borealis and RISE in this project. We would like to thank Sekab for kindly providing us the hydrolysis lignin. We would like to acknowledge Vinnova (Vinnv\u00e4xt) and Swedish Energy Agency (P47511-1) for the funding. We would like to thank the Swedish NMR Centre for the access to NMR facilitates and Chalmers Material Characterization Laboratory (CMAL) for CHONS, XRD, XPS, SEM and TEM access/measurements.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2022.139829.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Catalytic hydroconversion of Kraft and hydrolysis lignins was for the first time compared in a batch reactor over an unsupported NiMoS-SBA catalyst. We also report the effect of key reaction parameters on the yields and properties of the products. The results obtained at 20\u00a0wt% catalyst loading for hydrolysis lignin showed the highest monomer yield of 76.0\u00a0wt%, which consisted of 39\u00a0wt% aromatics with the lowest alkylphenolics yield of 10.1\u00a0wt%. Identical operating conditions, 400\u00a0\u00b0C, 80\u00a0bar, 5\u00a0h at 10\u00a0wt% catalyst loading, were used to compare both lignins and the highest monomer yield (64.3\u00a0wt%) was found for the hydrolysis lignin, consisting of 16.0\u00a0wt% alkylphenolics and 20.1\u00a0wt% aromatic compounds. These values are considerably higher than those for Kraft lignin with its 47.0\u00a0wt% monomer yield. We suggest that the reason for high yields of monomeric units from hydrolysis lignin is that it is more reactive due to its lower ash and sulfur contents and the chemical structural differences compared to the Kraft lignin. More precisely, the bio-oil from hydrolysis lignin contained higher yields of small molecules, sourced from ring-opening of cellulose in the hydrolysis lignin, which could stabilize the reactive oligomeric groups. These yields were two to seven times higher from kraft and hydrolysis lignin, respectively, compared to those obtained without catalyst. The results showed that the NiMoS-SBA catalyst is a promising catalyst for reductive depolymerization of lignin and in addition that the regenerated catalyst had good stability for multiple reaction cycles.\n               "}
{"full_text": "The increasing growth and modernisation of the economy intensify the demand for transportation fuel and power consumption rapidly in the last few decades (Seifi and Sadrameli, 2016). Dependence on fossil energy as the ultimate energy source has resulted in the exhaustion of the world\u2019s petroleum reserves and has led to energy price crisis (Charusiri and Vitidsant, 2017; Fazril et al., 2020). The shortage and price hike of fossil fuels accompanied by vast CO2 emissions have triggered a worldwide search for alternative and sustainable resources (Sousa et al., 2018). A wide consumption of fossil fuel in combustion largely contributes to the increase in greenhouse gas emissions in the atmosphere, thereby resulting in global climate change, acid rain and ozone layer depletion (Wang et al., 2017). Thus, the reduction of anthropogenic CO2 emissions is important to mitigate global warming (Arstad et al., 2014). These environmental issues have inspired researchers to find an alternative green fuel from renewable resources (Asikin-Mijan et al., 2018). Green fuel consists of free-oxygenated hydrocarbon compound, which is also known as clean-burning fuel with significantly low sulphur and aromatics contents and high cetane number, lubricity and renewability (Alsultan et al., 2017). Green fuel is carbon neutral because the CO2 released by its combustions is neutralised by the CO2 utilisation for plant growth, which can be used back as a raw material for biofuel generation; as a result, the net CO2 level in the atmosphere is unaffected (Pattanaik and Misra, 2017).Extensive research on biofuel generation from many renewable feedstocks, such as edible biomass and triglyceride-based biomass, has gained momentum in recent years. Triglyceride-based sources have a molecular network similar to hydrocarbon, which plays a substantial role in biofuel production (Seifi and Sadrameli, 2016). WCO can be a promising feedstock for biofuel production because the main composition in waste cooking oil (WCO) is triglyceride, and the fatty acid fractions are derived from cooking oil such as sunflower, palm, soybean, and coconut (Wang et al., 2017). These sources are considered economically viable because of their low cost and high availability, and their valorisation can also solve the issues associated with their disposal. The disposal of WCO is important in terms of the economy, environment protection and personnel safety. The abundance of WCO produced annually throughout the world has increased the demand for its rational disposal and reutilisation (Chen et al., 2014). The lack of a proper disposal collection system can result in significant disposal, odour and pollution problems (Chang et al., 2017). In the United States, 10 million tons of WCO are produced each year, whilst China generates 5 million tons of WCO annually (Lam et al., 2016). The WCO collected from restaurants, hotels, university cafeterias, hospitals, and refectories are disposed to landfills, evacuated to sewers, and utilised in the soap industry (Trabelsi et al., 2018). The use of WCO as the second generation biofuel feedstock not only improves the quality of the environment but also solves the waste disposal problem (Romero et al., 2016; Hafriz et al., 2018). Furthermore, the cost of raw material is a dominant part of the overall cost of green fuel production. Thus, the use of non-expensive, sustainable and non-value-added WCO could improve the overall financial viability (Wako et al., 2018) and promote a circular economy. However, the direct use of WCO as fuel in a diesel engine can cause problems and air pollution because of particulate matter deposits (Trabelsi et al., 2018). Therefore, several pre-treatment processes, including pre-heating, mixing with gas oil with low viscosities, emulsification, pyrolysis and transesterification, have been used to improve the properties of these edible oils (Trabelsi et al., 2018). Amongst the pre-treatment methods, pyrolysis deoxygenation (DO) is considered an inexpensive and effective way to convert WCO into lighter fractions of gasoline boiling range (Wako et al., 2018). Pyrolysis DO is a thermochemical method that breaks the chemical bonds of materials and converts them into a potential fuel product in three phases, which are liquid product, carbon-rich solid residues, and gaseous product, under oxygen-absence circumstances at a rapid heating rate (Chen et al., 2014; Lam et al., 2016; Dong and Zhao, 2018). Pyrolysis product is predominantly composed by alkanes, alkenes, dienes, aromatic compound, carboxylic acids with carbon ranging from 4 to 20 and other unsaturated compounds (Trabelsi et al., 2018). In addition to pyrolysis DO, hydrodeoxygenation (HDO) technologies have been used extensively to produce hydrocarbon-based fuel (Asikin-Mijan et al., 2016). HDO is a cracking process under high pressure in a H2 gas environment, which produces water as a by-product (Asikin-Mijan et al., 2018). However, it requires a high amount of H2 gases and high reaction pressure. Hence, due to the considerable consumption of H2, complicated equipment and high reaction operating conditions in HDO process, DO reaction is preferred because it is more economical, effective, more flexible in the choice of raw materials and suitable for industrial practices (Wang et al., 2017; Alsultan et al., 2017).The pyrolysis DO of edible oils using a selected heterogeneous catalyst is a potential candidate for renewable process-based industrialisation instead of a homogeneous catalyst. The heterogeneous catalyst is easily recycled and regenerated and is environmentally friendly, thereby increasing the product yield and decreasing the cost of liquid fuels (Wako et al., 2018). In recent years, as an environmentally friendly and low-cost value-added industrial mineral, dolomite is mainly used in construction and agricultural fields as a fertilizer. Recently, dolomites have attracted much attention as a promising basic solid catalyst for biofuel production (Mao et al., 2017; Shahruzzaman et al., 2018). Dolomites, CaMg(CO3)2, which consist of calcium carbonate (CaCO3), magnesium carbonate (MgCO3) and a very small percentage of other compounds, is widely found in sedimentary rocks deposited in marine and continental lacustrine settings (Shajaratun Nur et al., 2014). Based on recent studies by Mohammed et al. (2013) and Zhou et al. (2017) with focus on the calcination behaviour of dolomite, dolomite with high calcite content requires temperature above 900\u00a0\u00b0C for calcination, which contributes to the excellent performance in catalytic reactions.Low-cost transition metal oxides (TMOs) (e.g., Ni, Co, W, Mo, Cu, Fe and Zn) that has been extensively investigated as alternative to expensive noble metals (e.g., Pt and Pd) proven to successfully enhance the catalytic activity in DO process thus increase the yield of hydrocarbon fractions (Alsultan et al., 2017). Asikin-Mijan et al. (2018) reported that Ni-based catalyst was proven to be a good potential catalyst in improving the yield of pyrolysis oil via DO reaction. However, the presence of high acidity sites resulted in extensive deactivation due to catalyst coking and tar formation. Meanwhile, Co-based catalyst respond differently because its acidity was lower than that of Ni catalyst. Nevertheless, this paper (Asikin-Mijan et al., 2018) also reported the synergy effect of the acidity and basicity of Ca/CaO catalyst and improved the quality of green fuel and reduction in coke formation during DO reaction. The conjugation of basic and acid metals is needed to become highly selective towards the high formation of light hydrocarbon fractions (Alsultan et al., 2017). To the best knowledge of the author, no other report has covered in-depth studies of using a low-cost Malaysian dolomite via catalytic pyrolysis of WCO, to date. The present study aims to synthesise Malaysian dolomite-based catalyst, which has a basic characteristic and has been doped using selected transition metals, such as nickel (Ni), zinc (Zn), copper (Cu), cobalt (Co), and iron (Fe), which has an acidic characteristic. The physicochemical properties of the synthesised catalyst were examined. The present work also investigates the activity of the synthesised catalysts in the DO reaction of WCO. The yield and the characteristic of the liquid products were also evaluated in this study.Analytical grade nitrate salts of various metals, including iron(III) nitrate nanohydrate, (Fe(NO3)3\u00b79H2O) (99.95%), Cobalt(II) nitrate hexahydrate (Co(NO3)2\u00b76H2O) (99.95%), nickel(II) nitrate hexahydrate, (Ni(NO3)26H2O) (99.95%), Copper(II) nitrate trihydrate (Cu(NO3)2\u00b73H2O) (99.95%) and zinc nitrate hexahydrate (Zn(NO3)2\u00b76H2O) (99.95%) were purchased from Systerm Chemicals Sdn. Bhd. Grinded Malaysian dolomite (CaMg(CO3)2) was obtained from Northern Dolomite Sdn. Bhd. (Perlis, Malaysia). The grinded Malaysian dolomite was calcined at 900\u00a0\u00b0C for 4\u00a0h and was denoted as CMD900. Malaysian dolomite contained 31.0% of CaO and 20.0% of MgO. The WCO was collected from residential area in Selangor, Malaysia and treated by centrifugation at 6000\u00a0rpm for 30\u00a0min to remove the sediments from the WCO. Subsequently, the treated WCO was filtered using a wire sieve strainer (250 \u00b5 mesh size) to decant the food residue. The composition of the WCO used was analysed using GC\u2013MS analysis with 1.97% hydrocarbon and 98.02% oxygenated compound. The composition of carboxylic acid in WCO is presented in Table 1\n. Industrial-grade nitrogen gas, N2, (99.95%) was supplied by Smart Biogas Sdn. Bhd. GC analytical-grade n-Hexane (>98%) was acquired from Merck.The 5\u00a0wt% of x/CMD900 catalyst was prepared by using simple liquid\u2013liquid blending (precipitation technique) with 5\u00a0wt% loading of a transition metal x\u00a0=\u00a0Fe, Co, Ni, Cu and Zn onto CMD900. A known amount of metal nitrate was dissolved in deionised water. A total of 14\u00a0g of CMD900 was dispersed into 100\u00a0ml of deionised water under vigorous stirring at 60\u00a0\u00b0C. Subsequently, the aqueous solution of a metal nitrate was added dropwise into the slurry of CMD900 with agitation for 4\u00a0h. The mixture was stirred vigorously (with 400\u00a0rpm) and heated at 60\u00a0\u00b0C for 4\u00a0h. The sample was recovered by filtration and rinsed several times with deionised water. The sample was dried at 120\u00a0\u00b0C overnight. Subsequently, the dried sample was grinded and sieved using a mesh with a size of 250\u00a0\u00b5m. Finally, the sample was calcined at 900\u00a0\u00b0C for 4\u00a0h under a continuous flow of N2 gas. The aforementioned procedure was repeated using Fe(NO3)3.9H2O, Co(NO3)2\u00b76H2O, Ni(NO3)2\n\u00b76H2O, Cu(NO3)2\u00b73H2O and Zn(NO3)2\u00b76H2O and will be denoted as Fe/CDM900, Co/CDM900, Ni/CDM900, Cu/CDM900 and Zn/CDM900, respectively.The XRD analysis was performed to identify the phase and chemical composition of the synthesised catalysts using Shimadzu XRD-600 with scan ranges from 2\u03b8\u00a0=\u00a020\u201380\u00b0 and scanning rate at 2\u00b0/min. Cu K\u03b1 radiation source was generated at 40\u00a0kV and 40\u00a0mA with a broad focus of 2\u00a0kW and 7\u00a0kW, respectively. The XRD patterns were compared in accordance with the Joint Committee on Powder Diffraction Standard (JCPDS) file. The surface area and porosity of the synthesised catalysts were determined by the Brunauer\u2013Emmett\u2013Teller (BET) method using a quantachrome instrument (model Autosorb-1). The sample was degassed at 150\u00a0\u00b0C overnight and was flown with N2 gas in a vacuum chamber at \u2212196\u00a0\u00b0C for the desorption and adsorption processes. The basicity of the synthesised catalysts was investigated using temperature-programmed desorption (TPD) with CO2 as probe molecules. The TPD-CO2 was performed using a Thermo Finnigan TPD/R/O 1100 instrument equipped with a thermal conductivity detector (TCD). The samples were pretreated with N2 gas flow at 30\u00a0min at 250\u00a0\u00b0C. Subsequently, the samples were exposed to CO2 for 1\u00a0h to allow the adsorption process. The desorption of CO2 was detected by TCD under helium gas flow from 50\u00a0\u00b0C to 900\u00a0\u00b0C for 30\u00a0min. The surface morphology of the synthesised catalysts was examined by scanning emission microscopy (SEM) using JEOL SEM (JSM-6400). The sample was dispersed on the stub coated with a thin layer of gold by using BIO-RAS sputter.The DO reactions were conducted in a fractionated cracking system with two condensers as depicted in Fig. 1\n. In a typical experiment, 150.0\u00a0g of WCO and 7.5\u00a0g of catalyst amount were added to the reactor. The reaction was performed under inert N2 flow at a flow rate of 150\u00a0cm3/min. The DO reaction was performed at a reaction temperature of 390\u00a0\u00b0C\u00a0\u00b1\u00a05\u00a0\u00b0C for 30\u00a0min. The vapour generated during the DO reaction flowed through a Graham condenser (250\u2013270\u00a0\u00b0C) and condensed into a liquid product in a water-cooling condenser (25\u00a0\u00b0C). The liquid product and residual oil\u2013coke were collected from the collecting and reaction flasks, respectively. The gas product was released through the gas outlet without conducting further analysis. Two liquid fractions and a small amount of soap were observed in aqueous (top layer) and organic phases (bottom layer) of the collecting flask. These phases were then separated by decantation. The soap was removed by filtration using a filter paper while the pyrolysis oil was analysed with GC\u2013MS. The yield of the pyrolysis oil and the conversion of WCO was calculated by using mass balance (Eq. (1) and (2)) (Choi et al., 2018).\n\n(1)\n\n\nYield\n\no\nf\n\no\ni\nl\n\np\nr\no\nd\nu\nc\nt\n\n\n(\n%\n)\n\n=\n\n\nmass\n\no\nf\n\no\ni\nl\n\np\nr\no\nd\nu\nc\nt\n\n(\ng\n)\n\n\nmass\n\no\nf\n\nW\nC\nO\n\n(\ng\n)\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(2)\n\n\nConversion\n\no\nf\n\nW\nC\nO\n\n\n\n\n%\n\n\n\n=\n\n\nmass\n\no\nf\n\nW\nC\nO\n\n\n\n\ng\n\n\n\n-\nm\na\ns\ns\n\no\nf\n\nc\no\nk\ne\n\n\n(\ng\n)\n\n\n\nmass\n\no\nf\n\no\ni\nl\n\nW\nC\nO\n\n(\ng\n)\n\n\n\u00d7\n100\n%\n\n\n\n\nThe composition of pyrolysis oil products was analysed by gas chromatography\u2013mass spectrometry (GC\u2013MS, Shimadzu QP2010) equipped with non-polar ZB-5MS column (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u00b5m) in a split mode. The identification of unsaturated hydrocarbons was performed by interpreting GC\u2013MS data and by comparing with the National Institute of Standards and Testing (NIST) library. The hydrocarbon fraction (%) was determined by the total area of the chromatogram of saturated (n-alkane) and unsaturated (n-alkene) straight-chain hydrocarbons (C8-C20), as shown in Eq. (3). Meanwhile, product selectivity was determined by Eq. (4) (Dong and Zhao, 2018), and the percentage removal of oxygenated compound was calculated by Eq. (5):\n\n(3)\n\n\nHydrocarbon\n\n\n\n\n%\n\n\n\n=\n\n\n\u2211\nA\nr\ne\na\n\no\nf\n\na\nl\nk\ne\nn\ne\n\n(\nC\n8\n-\nC\n20\n)\n+\n\u2211\nA\nr\ne\na\n\no\nf\n\na\nl\nk\na\nn\ne\n\n(\nC\n8\n-\nC\n20\n)\n\n\n\u2211\nA\nr\ne\na\n\no\nf\n\nt\no\nt\na\nl\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\n\n\n\n\n(4)\n\n\nProduct\n\ns\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\n\n\n\n%\n\n\n\n=\n\n\nArea\n\no\nf\n\nd\ne\ns\ni\nr\ne\nd\n\np\nr\no\nd\nu\nc\nt\n\n\nTotal\n\na\nr\ne\na\n\no\nf\n\nt\nh\ne\n\np\nr\no\nd\nu\nc\nt\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(5)\n\n\nPercentage\n\nr\ne\nm\no\nv\na\nl\n\no\nf\n\no\nx\ny\ng\ne\nn\na\nt\ne\nd\n\nc\no\nm\np\no\nu\nn\nd\n\n\n\n\n%\n\n\n\n=\n\n\n\u03a3\nA\nr\ne\na\n\no\nf\n\no\nx\ny\ng\ne\nn\na\nt\ne\nd\n\nc\no\nm\np\no\nu\nn\nd\n\no\nf\n\nW\nC\nO\n-\n\u03a3\nA\nr\ne\na\n\no\nf\n\no\nx\ny\ng\ne\nn\na\nt\ne\nd\n\nc\no\nm\np\no\nu\nn\nd\n\no\nf\n\np\ny\nr\no\nl\ny\ns\ni\ns\n\no\ni\nl\n\n\n\u03a3\nA\nr\ne\na\n\no\nf\n\no\nx\ny\ng\ne\nn\na\nt\ne\nd\n\nc\no\nm\np\no\nu\nn\nd\n\no\nf\n\nW\nC\nO\n\n\n\u00d7\n100\n%\n\n\n\n\nPyrolysis oil generated using Ni/CMD900 was selected, and its properties were analysed. The analysis behaviour of the biofuel properties has been compared with the other properties of fuel, such as pyrolysis oil from palm oil, diesel fuel, and hydrocarbon biofuel, based on literatures. The property analysis was carried out in accordance to the pre-established American Society for Testing and Materials (ASTM) standard. The pyrolysis oil density was measured with pycnometer using ASTM D 4052-09 standard, and dynamic viscosity was measured using the Fenske routine viscometer type of tube model 150 L938 (ASTM D 445-09). The acid value was tested by simple titration using ASTM 974-08e1 standard. Cloud and pour points were tested by a Petrotest machine using standard ASTM D 2500 \u2013 66 and ASTM D97-87, respectively.\nFig. 2\n shows the XRD pattern of undoped dolomite, CMD900, and that with dopant catalysts (Fe/CMD900, Co/CMD900, Ni/CMD900, Cu/CMD900, and Zn/CMD900) after calcination at 900\u00a0\u00b0C for 4\u00a0h. The XRD pattern of calcined CMD900 was mainly composed of CaO and MgO. The intensified peaks that corresponded to CaO at 2\u03b8 were 32.4\u00b0, 37.6\u00b0, 54.1\u00b0, 64.3\u00b0, and 67.6\u00b0 (JCPDS File: 37-1497) and MgO peaks at 2\u03b8 were 43.1\u00b0, 62.5\u00b0, 74.9\u00b0, and 78.8\u00b0 (JCPDS File: 71-1176), which were in agreement with those in (Zhou et al., 2017). The high intensities of the diffractograms indicated the high crystallinity of CaO\u2013MgO. The characteristic diffraction peaks of Fe/CMD900 exhibited at 2\u03b8 were 44.6\u00b0, 58.2\u00b0, and 77.1\u00b0 (JCPDS File: 01-089-6466). For Co/CMD900, the major phases of CoO at 2\u03b8 were 34.5\u00b0 and 77.2\u00b0 (JCPDS File: 01-042-1300). For Ni/CMD900, the diffraction pattern clearly observed at 2\u03b8 were 43.2\u00b0 and 48.5\u00b0 (JCPDS File: 01-089-5881). The characteristic CuO peaks observed 2\u03b8 were 37.6\u00b0, 43.8\u00b0, and 51.2\u00b0 (JCPDS File: 01-077-1898). A peak in the Zn/CMD900 pattern corresponded to ZnO, which was slightly observed at 2\u03b8\u00a0=\u00a037.8\u00b0 and 48.6\u00b0 (JCPDS File: 01-036-1451). The result shows that the reflection peaks of CaO and MgO were slightly shifted along with the reduction in intensity with the presence of metal oxide that was well dispersed onto the CMD900 (Shajaratun Nur et al., 2014). This finding was consistent with the crystallinity calculation, in which the crystallite size of the modified dolomite catalyst was slightly altered after loading with transition metal (Table 2).The average crystallite sizes of the CMD900 and all doped CMD900 samples were estimated by Debye\u2013Scherer\u2019s equation based on the significant peaks of CaO and MgO (Table 2\n). The crystallite sizes of doped CDM900 were in the order of Fe/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Ni/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0CMD900. The average crystallite size increase upon the addition of metal oxide resulted in the expansion of the crystalline structure.The specific surface area, pore volume, and average pore size for CMD900 and doped CMD900 catalysts with various transition metals are presented in Table 2. The surface area of CMD900 was 12.02\u00a0m2/g. Meanwhile, all the doped CMD900 catalysts show the least improvement in terms of textural properties because the surface area has a low porous structure. The surface area was in the order of Fe/CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Ni/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0CMD900. The low surface area of all synthesised catalysts might be caused by the sintering effect of the catalyst after calcination, which can create severe particle agglomeration (Waheed et al., 2016). Table 2 reveals that the pore sizes in CMD900 are macroporous (e.g., 63.07\u00a0nm). Nevertheless, the pore diameter of all doped CMD900 catalysts exhibited slight reduction, and they mainly consisted of mesoporous structure with a pore diameter that was within the range of 2\u201350\u00a0nm, except for Co/CMD900, which was still in the macroporous range size (Shajaratun Nur et al., 2014). The pore size of all the synthesised catalysts increase in the order of CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Ni/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0Fe/CMD900. However, this increase can still provide a wide channel for the diffusion of reactant for the catalytic activity (Asikin-Mijan et al., 2018). In addition, the reduction of pore volume after dispersion with metal dopant onto CMD900, except Fe/CMD900, were shown in Table 2. The trend of the pore volume was presented as follows: Fe/CMD900\u00a0>\u00a0CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Ni/CMD900\u00a0>\u00a0Cu/CMD900. The decrement in pore size might be due to the pore blockage caused by the metal dopant particles that filled inside the CMD900 pore (Waheed et al., 2016).The basicity characteristic of all synthesised catalysts was summarised in Table 2 and Fig. 3\n. The basicity trend was increased in the order of Ni/CMD900\u00a0>\u00a0CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Fe/CMD900. The transition metal doped CMD900 catalyst (expect Ni/CMD900) showed low intensity on CO2 desorption peaks at low temperature (624\u00a0\u00b0C to 702\u00a0\u00b0C) when compared with the CDM900 catalyst (Table 2 and Fig. 3). This finding suggested that the presence of a considerable amount of weak acid sites on the CDM900 catalyst surface was due to the successful dispersion of 5\u00a0wt% of transition metals on the CMD900 catalyst surface. According to Asikin-Mijan et al. (2018) the presence of acid sites was attributed to the Bronsted acid sites associated with the bridging of OH groups and/or the Lewis acid sites associated with the presence of transition metal ions. The TPD profiles of CMD900 and all doped CMD900 catalysts shown in Fig. 3 exhibited considerably high basic strength with CO2 desorption peak at a temperature of more than 500\u00a0\u00b0C. The basic strength distribution from the catalyst active sites is expressed in terms of CO2 desorption temperature. The basic strength (Tmax\u00a0=\u00a0822\u00a0\u00b0C) and basic density (7306.37\u00a0\u03bcmol/g) of Ni/CMD900 catalyst significantly increased because of the synergy effect promoted by the interaction between NiO-CaO/MgO and the increment in active sites that resulted from the dispersion of NiO on the CMD900 surface. The characteristic of basicity is important to inhibit coke formation and enhance the cracking reaction (Kay Lup et al., 2017).The morphological features of CMD900 and doped CMD900 catalysts at 60,000\u00d7 magnification were shown in Fig. 4\n. The SEM images of CMD900 showed an agglomeration structure of particles and appeared in finer and smaller uniform sizes because of the sintering effect of metal oxides during calcination. The individual grain\u2019s segregation can be clearly observed and identified. However, the SEM morphology of the active metal-doped catalyst rendered significant changes in morphology for surface structure into larger irregular aggregate and rough surface. The active metal-doped catalysts also displayed a more agglomerated structure, which indicates well-dispersed metal oxide on the dolomite catalyst surface. The segregated grains of dolomite become increasingly invisible. As evidence, the crystallite size of active metal-doped catalysts increased with the dispersion of transition metals, thereby implying the growth of the catalyst crystal. A similar observation was reported by Azri et al. (2020) when metal was added to dolomite as an effective catalyst for glycerol hydrogenolysis. In addition, the active metal-doped catalysts comprised small cluster, which resulted in high surface area of the catalyst (Table 2).The conversion of WCO and WCO deoxygenated product distribution for all synthesised catalyst are shown in Fig. 5\n. The conversion of WCO followed the sequence: Ni/CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0CMD900. The DO of WCO will lead to the production of the liquid product via decarboxylation and decarbonylation with the release of gas and acid phase as the by-products, respectively. The liquid products obtained from all DO reactions were divided into two separate fractions, namely, pyrolysis oil and acid phase with insignificant soap formation. The pyrolysis oil produced were in the order of Ni/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0CMD900. This finding shows that TMO is a good promoter and has played an important role in tuning product selectivity towards monofunctional hydrocarbon intermediates, which further converted into the desired liquid product (pyrolysis oil).The acid phase composed of water and a high amount of carboxylic compound was detected at the bottom phase of the liquid product. This finding suggested that the decarboxylation reaction was more preferred than the decarbonylation reaction. Ni/CMD900 catalyst produced a lesser amount of acid phase (0.1%) in the liquid product as compared with other catalysts. Nevertheless, white precipitate, (or soap) was observed in all liquid products catalysed by doped CMD900 catalysts. Ni/CMD900 shows the lowest amount of soap formed (6.7%) but still slightly higher than the un-doped CMD900 catalyst (4.8%). Supposedly, the formation of undesirable by-product (coke) must be avoided during the DO reaction for an ideal reaction. However, all the reactions with all catalysts tested yielded substantial coke formation in the following order: CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0Ni/CMD900. Asikin-Mijan et al. (2018) reported that the coke generated via polymerisation reaction in the DO of triolein was expected to occur because of the poor acidic properties of active-doped metal catalyst in CaO surface. The mass balance profile suggested that Ni/CMD900 catalyst can perform actively for the DO reaction of WCO because of the high amount of basic site on the catalyst surface (Table 1). Based on a previous study (Romero et al., 2016), the composition of gas produced via DO of WCO was analysed using GC-TCD. The gas was collected for certain reaction time at a certain temperature, and the gaseous product consisted of hydrocarbon gas, CO2, CO, CH4, H2 and O2 (except calcined dolomite), using various catalysts, such as calcined Malaysian dolomite and commercial acid catalyst (e.g. FCC, Zeolite NaY, HZSM-5).\nFig. 6\n shows the main composition determined by GC\u2013MS analysis for the produced pyrolysis oil catalysed by all synthesised catalysts with different catalyst dopants. This study showed that WCO was thermally cracked to a liquid hydrocarbon product and a small quantity of oxygenated compounds. As shown in Fig. 6, the trend of liquid hydrocarbon yield was in the order of Ni/CMD900\u00a0>\u00a0CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Cu/CMD900. The oxygenated compound yielded in the following order: Cu/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0CMD900\u00a0>\u00a0Ni/CMD900. Based on these findings, the DO performance of Ni/CMD900 resulted in the highest yield of liquid hydrocarbon and the lowest yield of the oxygenated compound, suggesting that the synergistic energy effect from acid\u2013base interaction between NiO and CaO-MgO performed actively in the DO of WCO as mixed metal oxides. NiO improved the yield production of liquid products comprising mainly hydrocarbons, thereby presenting a potentially high-value chemical feedstock or fuel source. Based on Fig. 6, CMD900 catalyst contains the unique basic properties of CaO-MgO, which helps in oxygen removal by absorbing more CO2 in the gas phase. This finding is line with the result shown by Lin et al. (2010) in the pyrolysis of biomass using dolomite (MgO-CaO) catalyst as related to the reduction of tar formation with the increase in H/C ratio. The observation showed that the presence of MgO and CaO will produce a better route for oxygen removal through catalytic DO. The reduction in oxygenated compounds in pyrolysis oil product by synthesised Ni/CMD900 catalysts were also due to the existence of a large density of basic site Ni-promoted catalyst (4.40\u00a0\u00d7\u00a01021 atom/g) as compared with other metal-promoted catalysts and hence improved catalyst stability during DO reaction. The efficiency of DO reaction has been calculated on the basis of the reduction of oxygenated compound present in pyrolysis oil generated using synthesised transition metal-doped dolomite catalyst through area of GC\u2013MS analysis data (Eq. (5)). The trend of oxygenated compound removal was in the order of Ni/CMD900 (79.8%)\u00a0>\u00a0CMD900 (76%)\u00a0>\u00a0Co/CMD900 (71.9%)\u00a0>\u00a0Fe/CMD900 (67.9%)\u00a0>\u00a0Zn/CMD900 (65.6%)\u00a0>\u00a0Cu/CMD900 (62.4%). This finding showed that the catalysts can reduce oxygenated compound via desired catalytic DO pathway whilst improving the quality of the final fuel product. Based on Zhang et al. (2016), strong basic site generated from oxygen on the metal oxides created stronger forces via the abstraction of alpha hydrogen in carbonyl compound and followed by C-O scission to form hydrocarbon compound.\nFig. 7\n shows a formation of a high number of oxygenated by-products, such as alcohol and carboxylic acid. Fe/CMD900, Cu/CMD900, Co/CMD900 and Zn/CMD900 are amongst the catalysts with high alcohol value (e.g. 15.1%, 13.8%, 13.8% and 7.1%, respectively). For carboxylic acid, the alcohol value of the aforementioned catalysts is 4.6%, 11.0%, 5.4% and 11.7%, respectively. Ni/CMD900 catalyst contains less carboxylic acid (3.6%) with a minor quantity of ketone (2.6%), and the absence of aldehydes indicates a slight occurrence of such oxidation reaction in the DO reaction as compared with other transition metal-promoted catalysts. High acid value was obtained with presence of high amount of carboxylic acid group in the oxygenated compound. The produced pyrolysis oil contains a lower acid value with the presence of these catalysts, starting with CMD900 (33\u00a0mg KOH/ g) followed by Fe/CMD900 (40\u00a0mg KOH/ g)\u00a0<\u00a0Ni/CMD900 (47\u00a0mg KOH/ g)\u00a0<\u00a0Co/CMD900 (64\u00a0mg KOH/ g)\u00a0<\u00a0Cu/CMD900 (75\u00a0mg KOH/ g)\u00a0<\u00a0Zn/CMD900 (78\u00a0mg KOH/ g) as compared with the 186\u00a0mg KOH/g acid value of pyrolysis oil generated from non-catalytic WCO pyrolysis (Hafriz et al., 2018). The presence of transition metals doped on dolomite catalyst increased the acid value of pyrolysis oil produced as compared with the CMD900 catalyst. This phenomenon is in accordance with the low reduction of carboxylic acid upon the incorporation of transition metals on CMD900. Other researchers have suggested that carboxylic acid groups are more difficult to hydrogenate with a present transition metal relative to other functional types in bio-oil, such as ketone, aldehyde carbonyls and alkenes (Laurent et al., 1999). As reported by Li et al. (2013), lower acid value had relationship with good cold flow properties, such as the cold filter plugging point and freezing point.\nFig. 8\n presents the further breakdown of the main compounds into individual chemical groups, as determined by GC\u2013MS analysis pyrolysis oil produced for CMD900 and transition metal-doped CMD900 catalyst. The main chemical groups can be classified into seven components according to their structure, namely, alkanes, cycloalkane, alkene, cycloalkene, diene, alkyne, and aromatic. The figure shows that the hydrocarbon product is dominated by aliphatic hydrocarbon, namely, alkene and alkane, for all synthesised catalysts. Evidently, aliphatic alkene was dominating the composition as compared with alkane in this liquid product with the order of Ni/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Cu/CMD900. Meanwhile, the aliphatic alkane yielded in the following order: Ni/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0CMD900\u00a0>\u00a0Cu/CMD900. In the case of Ni/CMD900 catalyst, the highest concentration in alkane was detected as tetradecane (C14H30) and tridecane (C13H28).Meanwhile, 8-heptadecene (C17H38) and 1-tridecene (C13H26) were the most abundant in alkene. The aliphatic alkane for Fe/CMD900, Zn/CMD900 and Cu/CMD900 were mostly tetradecane and tridecane, except in Mo/CMD900, in which tetradecane and dodecane (C12H26) were detected in the produced pyrolysis oil. The alkenes present for all tested metals promoted catalysts and were mostly from 8-heptadecene and 1-tridecene. The detailed characterisation of hydrocarbon fractions distribution for the deoxygenated liquid product conversion (Fig. 9\n) showed that the hydrocarbon fractions for Ni/CMD900 catalyst was composed of a mixture of n-C14 and n-C17. Meanwhile, Co/CMD900 catalyst contained n-C14 and n-C10 fractions. Zn/CMD900 and Fe/CMD900 were mostly composed by n-C13 and n-C12, whereas Cu/CMD900 contained the shortest C range, which was n-C13 and n-C10.\nFig. 10\n shows the composition of biofuel as determined by the carbon number of the gasoline, kerosene and diesel fraction as the petroleum product. Interestingly, CMD900 catalyst exhibits a high degree of selectivity (48.9%) of gasoline range (C4-C12). Doped CMD900 catalyst for gasoline fraction were presented in the following order: CMD900\u00a0>\u00a0Co/CMD900, Fe/CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0Ni/CMD900\u00a0>\u00a0Zn/CMD900. However, catalytic DO over Ni/CMD900 catalyst were found predominantly selective towards diesel range (55.2%) when compared with other doped metal in the order of Ni/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0Cu/CMD900\u00a0>\u00a0CMD900. In addition, Ni/CMD900 also shows higher fraction towards kerosene (38.8%). The DO reactivity towards kerosene was in the order of Ni/CMD900\u00a0>\u00a0CMD900\u00a0>\u00a0Co/CMD900\u00a0>\u00a0Fe/CMD900\u00a0>\u00a0Zn/CMD900\u00a0>\u00a0Cu/CMD900.The properties of the pyrolysis oil derived from Ni/CMD900 were investigated, and the results are shown in Table 3\n. For comparison purposes, Table 3 also displays the specified values for the pyrolysis oil (palm oil), diesel and hydrocarbon biofuel. The results show that the fuels derived from WCO using Ni/CMD900 as catalysts possessed acceptable values for the given properties when compared with other fuels, excluding the comparison of acid value with diesel. Acid value is referred to as the oil quality indicator to monitor the oil degradation during the storage period. According to the ASTM standard for fuel application, the maximum value of acid number is 0.5 mgKOH/g (Yasin et al., 2013). The number of the acid value of pyrolysis oil is high, and pyrolysis oil can be degraded at extensive storage period when compared with mineral diesel. Therefore, studies on reducing the acid value of pyrolysis oil can be conducted by modifying the synthesised catalyst and the ratio of blending with diesel in the future. However, Ni/CMD900 is an excellent low-cost catalyst for the DO of WCO; its typical bifunctional (NiO-CaO/MgO) properties could focus on the distribution of the product and improved the fuel properties.The distribution and composition of the product were obtained through mass balance and GC\u2013MS analysis, respectively. The general catalytic pyrolysis of WCO pathway under atmosphere free oxygen over modified Malaysian dolomite catalysts can be established in Fig. 11\n, as adapted from Lestari et al. (2009). The reaction pathways consist of liquid hydrocarbon, gas and solid and soap phase reactions. The liquid phase reactions of the WCO DO process dominantly involve decarboxylation and decarbonylation reactions to produce alkanes and alkenes by releasing carbon dioxide, carbon monoxide and water, as illustrated in Reactions (1) and (2) (Hafriz et al., 2018; Kamil et al., 2020; Snare et al., 2008). Based on Reaction (3), the triglyceride of WCO also undergone hydrogen abstraction at higher temperatures to produce diene formation. Meanwhile, alkenes and diene will also undergo Diels\u2013Alder reaction to produce cycloalkenes, as shown in Reaction (4). Idem et al. (1996) mentioned that cycloalkenes are the main hydrocarbons used for synthesising the cycles and aromatic hydrocarbons. In a hydrogenation reaction (Reaction (5)), two hydrogen atoms are added across the double bond of cycloalkenes, thereby resulting in a stable form product called saturated cycloalkanes. A side reaction, such as dehydrogenation, could occur during the removal of hydrogen from of cycloalkenes and alkenes to produce a small amount of aromatic and alkynes in a liquid hydrocarbon product, as mentioned in Reaction 6.Gas is a secondary product that is generated via the DO of WCO. According to Idem et al. (1996), the hydrocarbon radicals Ru or Rs are formed by eliminating of ethylene molecules during secondary cracking. The straight and branched-chain hydrocarbons of which in the C1\u2013C5 range in the gas phase product were yielded through the successive elimination of ethylene molecules from the saturated and unsaturated hydrocarbon radicals followed by disproportionation, isomerisation, and subsequent hydrogen transfer reactions, as illustrated in Reaction 7. Dandik and Aksoy (1998) reported that the gaseous product generated from the catalytic pyrolysis of canola oil consisted mostly of hydrocarbon gases in the C1\u2013C5 range. In addition, the methanation reaction (Reaction 8) could occur as a pathway because of the conversion of carbon monoxide and carbon dioxide to methane (CH4) through hydrogenation. In a review of the DO of fatty acid Hermida et al. (2015) also mentioned the water gas shift in Reaction 9 due to the presence of carbon monoxide and water via decarbonylation reaction in producing hydrogen and carbon dioxide.Solid and soap phase reactions are the side reactions involved. In the solid-phase reaction, the polymerisation of aromatic hydrocarbon and the condensation of WCO generate the coke formation, as mentioned in Reactions 10 and 11, respectively. In the soap phase reaction, calcium carbonate is produced during the absorption of CO2 by CaO because of the high formation of CaO composition in calcined Malaysian dolomite. The oxygenates, such as carboxylic acid (RCOOH), are expected to react with calcium carbonate (CaCO3) in producing fatty acid soap (Ca(RCOO)2), as illustrated in Reaction 12.This study demonstrated that the calcined Malaysian dolomite (CMD900) was successfully dispersed with various transition metals, such as Fe, Co, Ni, Cu and Zn. The result indicated that amongst all of the doped catalyst, Ni/CMD900 catalyst demonstrated the best performance in terms of the DO reaction of WCO with high conversion (68.0%), high yield pyrolysis oil (36.4%) and low coke formation (32.0%). The high availability of active basic sites on the Ni/CMD900 catalyst surface plays an important role in affecting the catalytic behaviour in the DO reaction of WCO. CMD900 can also remove high oxygen content with only 19.8% of oxygenated compound detected and produced the maximum yield of hydrocarbon product (80.2%) of C13 and C17 fractions. In addition, Ni/CMD900 catalysed reaction rendered higher selectivity towards diesel when compared with other doped CMD900 catalysts.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the financial support from the Ministry of Higher Education of Malaysia for Fundamental Research Grant Scheme (FRGS/11/TK/UPM/02) and AAIBE Chair of Renewable Energy Grant No. 201801 KETTHA for funding this research publication.", "descript": "\n                  In the present work, nickel (Ni), zinc (Zn), copper (Cu), cobalt (Co) and iron (Fe) are tested as catalyst dopants on Malaysian dolomite calcined at T\u00a0=\u00a0900\u00a0\u00b0C (CMD900). The physicochemical properties of all synthesised catalyst are investigated by X-ray diffraction, Brunauer\u2013Emmett\u2013Teller surface area, temperature-programmed desorption of carbon dioxide and scanning emission microscopy. The synthesised catalysts are tested on the basis of the deoxygenation (DO) reaction of waste cooking oil to produce liquid fuels under N2 atmosphere. The chemical composition of the liquid product is identified by gas chromatography\u2013mass spectroscopy. The overall study suggests that Ni/CMD900 catalyst exhibits the highest performance with over 67.0% conversion and high selectivity (80.2%) with a high proportion of saturated linear hydrocarbons that corresponds to green diesel. Result indicates that Ni/CMD900 is a highly potential DO catalyst with 19.8% oxygenated compound, which is favourable for decarboxylation and/or decarboxylation predominates.\n               "}
{"full_text": "Proper control of atmospheric CO2 content to fight climate change is one of the central challenges of mankind. Anthropogenic CO2 is mainly produced via combustion of fossil fuels and is currently producing a notable environmental impact, such as in global warming [1]. Fortunately, CO2 can be captured and transformed into other valuable chemicals (e.g., formaldehyde, methane, methanol or bicarbonate, among others [2\u20136]) with the help of transition metal (TM)-based catalysts [7], mainly through Au, Pd, Cu, Ru, Ni, Pt, Pd, Rh or Fe [8\u201314]. However, since some of those TMs are expensive and scarce, it is still imperative to develop better catalysts to increase the efficiency and reduce the cost of the CO2 conversion.In the last few years, single-atom catalysts (SACs) finely dispersed into different supports have emerged as new promising materials for catalysis [15,16]. SACs aim to combine the large activity and selectivity of homogeneous catalysts but with the separation and reutilization capabilities of a heterogeneous catalyst [17,18]. Supported SACs have a higher activity/mass relationship due to a better metal utilization than nanoparticles, which makes them also more cost-efficient for catalysis. Some of the early successful reactions were carried out in oxide and graphene supported SACs including CO oxidation [19], water-gas shift reaction [20,21], hydroformylation of olefins [22] and methanol and benzene oxidations [23,24]. The great activity of SACs is correlated to their low coordination numbers, which means they can be partially deactivated if they diffuse through the support and aggregate forming nanoparticles. For this reason, it is not only important to find a good SAC for a given application, but it is also critical to disperse it into a support that can stabilize it (i.e., prevent the metal atoms from clustering). In that sense, many efforts are devoted to preventing SAC surface migration by depositing the TM in surface vacancies [25\u201328], or spatially confining them in microporous materials (e.g., zeolites or metal-organic frameworks) [29\u201332].From all those promising supports, we have turned our attention to zeolites [33], where important successes were achieved in the last years by encapsulating different TM atoms in structures with different Si/Al ratio to carry out CO oxidation [34], methane conversion to higher hydrocarbons [35], to methanol and to acetic acid [36] or n-hexane isomerization [37], among others [16]. Pd SACs were also anchored to mesoporous silica SBA-15 [38] and used to hydrogenate alkynes. Finally, Ru and Rh SACs were recently encapsulated in the fully silicated MFI structure (i.e., TM1@Silicalite-1 or simply TM1@S-1) [39,40] and the resulting catalysts were promising for H2 production from ammonia borane hydrolysis and ammonia synthesis, respectively. The encapsulation of metal atoms in zeolites requires a strict control of experimental conditions, since high pH and/or temperature might lead to particle aggregation [41]. Available synthesis techniques include direct synthesis from inorganic or ligand-assisted metal precursors, multistep post-synthesis encapsulation (e.g., two-step dry-gel-conversion) or ion-exchange followed by reduction, as described by Chai et al., [42]. From the very large number of potential SAC \n+\n support combinations, only less than 10 TMs have actually been synthesized as SACs. Such a small number evidences the need of a systematic screening for catalysts with large activity whilst still being stable at operative conditions. In a previous study [43], we computationally assessed the structure and stability of all period IV-VI TM1@S-1 (except for Tc due to its radioactive nature), showing that TMs can be encapsulated via dispersion + electrostatic interactions in the MFI framework, which can be quite strong since the adsorption energies range from -0.48 eV (for Cu and Zn) to -1.67 eV (for Pt).Herein, we evaluate the potential activity of this set of SACs for CO2 conversion. Due to the large number of systems included in this screening study and the large amount of possible reaction products, it was not feasible to build the full reaction profiles for each SAC. Instead, we focus on the first steps of CO2 activation, which involve the adsorption of reactants (CO2 and H2), direct CO2 dissociation through the redox mechanism (CO2\n\n\u2192\n CO + O), H2 dissociation (H2\n\n\u2192\n H + H) and hydrogen-assisted CO2 dissociation through the associative mechanism, either via formate (CO2 + H \n\u2192\n HCOO) or carboxylate (CO2 + H \n\u2192\n COOH) intermediates.Note that the experimental viability of TM1@S-1 synthesis was already proven for Rh1@S-1 and Ru1@S-1 [39,40], so the results from this study will serve to assess how adequate are those catalysts in comparison to other non-synthesized TM1@S-1 and for proposing novel catalysts capable of adsorbing and converting CO2. The results obtained here will provide a solid theoretical background from which potential catalytic activity can be predicted, paving the road for further experimental and computational studies on this topic.The MFI Silicalite-1 has a microporous Si96O192 unit cell composed by SiO4 tetrahedra (T) units positioned at 12 non-equivalent T sites (i.e., T1-T12). The framework O atoms are located at 26 distinct O sites (i.e., O1 \n-\n O26). This arrangement leads to a 3D pore system with straight ten-membered-ring (10-MR) channels in the [010] direction intersected by sinusoidal 10-MR channels in the [100] direction, as shown in Fig. 1\n. The most stable structures of each one of the 29 TM1@S-1 SACs were taken from our previous study [43]. In summary, we employed Density Functional Theory (DFT) calculations to find the most stable site to adsorb each TM in the pristine S-1 structure by optimizing the geometry of each TM placed in all possible non-equivalent positions of all pores. With this procedure, we located only 6 preferred sites in S-1 by the entire set of TM adatoms. Most TM atoms adsorb via van der Waals (vdW) interactions into the sinusoidal 10-MR channels at distances larger than 2.7 \u00c5. Among all possible positions in these channels, only 3 specific adsorption sites are preferred by the TM atoms, which are denoted as Channel X (XA, B or C) sites (see Fig. 1). The only exceptions are group 3 T Ms (i.e., Sc, Y and La), which prefer to adsorb in the middle of the quadrilaterals formed by two O16 and two O25 atoms (i.e., O16(\n\u00d7\n 2) \n-\n O25(\n\u00d7\n 2) site); Group 10 T Ms (i.e., Ni, Pd and Pt) and Ru atoms, which are found closely coordinated with O18 and O23 atoms (i.e., O18 \n-\n O23 site) and weakly coordinated to O16(\n\u00d7\n 2); and finally, Rh atoms, which are located in the middle of the quadrilaterals formed by O8, O11, O18 and O26 atoms (i.e., O8 \n-\n O11 \n-\n O18 \n-\n O26 site). In all those systems the porous support provides protection against SAC sintering via 3D confinement, except in group 3 TMs where the S-1 also accepts part of their electron density changing their electronic structure. Notice that, Ru and Rh are the only TMs experimentally encapsulated in Silicalite-1, and their coordination according to EXAFS fittings agree with our DFT-based predictions, both suggesting TM coordination with four O atoms. However, DFT results slightly overestimates the average Ru \n-\n O distance (r (DFT) = 2.15 \u00c5) by a 7% [40] and the Rh \n-\n O distance (r (DFT) = 2.40 \u00c5) by a 18% [39] with differences in TMO bonds lower than 0.01 \u00c5 between DFT and DFT-D3 geometries. For a more complete description of those sites, the reader is referred to the original work [43]. Additionally, images of each TM location are compiled in Fig. 1.To be consistent with our previous study [43], the Vienna Ab Initio Simulation Package (VASP) [44] was used to perform all periodic DFT calculations by employing the Perdew-Burke-Ernzerhof [45] exchange-correlation functional, plus the Grimme D3 dispersion correction (PBE-D3) [46]. The valence electron density was expanded in a 600 eV kinetic energy plane-wave basis set, which gave total energy variations below 0.01 eV. The effect of core electrons on the valence electron density was accounted through the Projected Augmented Wave (PAW) method [47], as implemented in VASP by Kresse and Joubert [48]. Spin-polarization was taken into consideration to reflect the TM1@S-1 magnetic properties. Due to the large size of the simulation cells (i.e., around 290 atoms with a = 20.09 \u00c5, b = 19.74 \u00c5 and c = 13.37 \u00c5), only the \n\u0393\n -point was used to sample the Brillouin zone.The most stable site for the adsorbed species was obtained by screening several initial geometries with different positions and orientations. The tolerance for the conjugate gradient algorithm to minimize energy and forces on atoms was set to 10\u22125 eV and 0.01 eV/\u00c5, respectively. Adsorption energies \n(\n\u0394\n\nE\n\na\nd\ns\n,\ni\n\n\n)\n were calculated as:\n\n(1)\n\n\n\n\u0394\nE\n\n\na\nd\ns\n,\ni\n\n\n=\n\nE\n\ni\n-\nS\nA\nC\n\n\n-\n\nE\n\nS\nA\nC\n\n\n-\n\nE\n\ni\n(\ng\n)\n\n\n\n\nwhere \n\nE\n\ni\n-\nS\nA\nC\n\n\n is the total energy of adsorbed \ni\n species in the TM1@S-1 SAC, \n\nE\n\nS\nA\nC\n\n\n is the energy of the clean SAC (i.e., the relaxed pristine TM1@S-1 structure) and \n\nE\n\ni\n(\ng\n)\n\n\n is the energy of species \ni\n in gas-phase and in its ground electronic state. With this definition, negative values of \n\n\n\u0394\nE\n\n\na\nd\ns\n,\ni\n\n\n indicate favorable adsorption. The latter term was calculated in a simulation cell with the same parameters than the TM1@S-1 using only the \n\u0393\n -point. The energy barriers (\n\u0394\n\nE\n\u2260\n\n) and reaction energies (\n\u0394\n\nE\nr\n\n) were calculated as:\n\n(2)\n\n\u0394\n\nE\n\u2260\n\n=\n\nE\n\nT\nS\n-\nS\nA\nC\n\n\n-\n\nE\n\nR\n-\nS\nA\nC\n\n\n\n\n\n\n\n(3)\n\n\u0394\n\nE\nr\n\n=\n\nE\n\nP\n-\nS\nA\nC\n\n\n-\n\nE\n\nR\n-\nS\nA\nC\n\n\n\n\nwhere \n\nE\n\nT\nS\n-\nS\nA\nC\n\n\n is the energy of the transition state (TS), \n\nE\n\nR\n-\nS\nA\nC\n\n\n is the energy of the initial configuration (i.e., adsorbed reactants), and \n\nE\n\nP\n-\nS\nA\nC\n\n\n the energy of final configuration (i.e., adsorbed products). All TSs were located by using the Climbing-Image Nudged Elastic Band (CI-NEB) method [49]. The initial guesses for the employed intermediate images were created through the Image Dependent Pair Potential (IDPP) interpolation procedure [50] as implemented in the Atomic Simulation Environment (ASE) [51]. All adsorption minima and TSs were characterized through frequency calculations by computing the elements of the Hessian matrix as finite differences of 0.03 \u00c5 length and considering only displacements of the adsorbate. Note that the Zero Point Energy (ZPE) term is included in all reported energy values unless otherwise indicated. Additionally, the Gibbs free energy of adsorption, reaction and free energy barriers were calculated by correcting the respective \nE\n values in Eq. 1\u20133 with the corresponding temperature/pressure correction to the free energy. For gas-phase species, the correction was obtained using the ideal gas approximation, whereas for adsorbed species the harmonic oscillator model was used for all degrees of freedom [52].Finally, to further understand the interaction between metal atoms, reactant molecules and the support, we computed the atomic charges on the supported transition metal atoms (\n\nQ\n\nT\nM\n\n\n), the total net charges on the zeolite (\n\nQ\n\nZ\ne\no\n\n\n) and on the adsorbed CO2 and H2 species (\n\nQ\n\n\n\nC\nO\n\n2\n\n\n\n and \n\nQ\n\n\nH\n2\n\n\n\n) through a Bader analysis of the electron density [53].Prior to CO2 conversion by either the redox or associative mechanisms, CO2 and H2 species must adsorb on the SAC and, for H2, it must also dissociate to give rise to adsorbed H species that are required in the associative pathway. In this section, we study the molecular adsorption of CO2 and the adsorption and subsequent dissociation of H2 species, evaluate their geometry and their electronic structure.CO2 physisorbs on the pristine S-1 structure mainly by vdW interactions, leading to a weak \n\n\n\u0394\nE\n\n\na\nd\ns\n,\nC\n\nO\n2\n\n\n\n (see Fig. 2\na\u2013d and Table S1 in the SI). However, the presence of single TM atoms in general leads to a stronger chemisorption of CO2, which typically interacts with the metal atom through the C \n-\n O bond (i.e., \u03b7(CO) configuration in Fig. 3\n) with an average C \n-\n TM distance of around 2 \u00c5. During this adsorption step, the C \n-\n O bond is elongated from the gas-phase value of 1.18 \u00c5 to 1.22 \n-\n 1.46 \u00c5 and the O \n-\n C \n-\n O angle is bent from 180\u00b0 to 120 \n-\n 150\u00b0. This activated configuration leads to a significantly strong adsorption energy (i.e.,\n\n\u2009\n\n\n\u0394\nE\n\n\na\nd\ns\n,\nC\n\nO\n2\n\n\n\n\u2272\n-\n 1 eV). In general, the binding strength of CO2 on the supported TMs decreases along a period and moving up along a group (Fig. 2a), with a limit on group 11 and 12 TMs, where CO2 weakly physisorbs with no noticeable perturbation from its gas-phase geometry. Other exceptions are those TMs with semi-occupancy of d orbitals (i.e., Mn and Re with s2d5\n) and group 4 TMs, where CO2 does not have a stable configuration (i.e., Hf) or where it binds in a \u03b7(O) configuration instead of \u03b7(CO) (i.e., Ti and Zr). Notice that, for Hf, \n\n\n\u0394\nE\n\n\na\nd\ns\n,\n\n\nC\nO\n\n2\n\n\n\n is calculated by considering CO + O as the adsorbed state.During the adsorption process, there is a significant TM-to-CO2 charge transfer, which is evidenced by the final oxidation state of the metal atom (i.e.,\n\n\nQ\n\nT\nM\n\n\n>\n0\n) and the negative net charge on CO2, as shown in Fig. 2b. The atomic Bader charges on the TM atoms decrease along a period, which in part can be rationalized due to the higher electronegativity of the metal atom. On group 11 and 12 T Ms, the charge transfer is negligible, due to the weak CO2 \u2013 TM physisorbed interaction, and therefore the net charges on the TM and CO2 are almost zero. Despite the fact that in most cases there is only charge transfer between the TM atom and CO2 (i.e.,\n\n\nQ\n\nT\nM\n\n\n+\n\nQ\n\n\n\nC\nO\n\n2\n\n\n\n\u2248\n0\n), in the case of group 3 TMs, there is also a significant TM-to-zeolite charge transfer evidenced by the negative net Bader charge on the zeolite. This charge transfer occurs due to the very strong interaction between group 3 TM atoms and the zeolite support, as discussed in our previous work [43]. When this occurs, part of the charge density still remains in the zeolite after CO2 adsorption. Interestingly, stronger adsorption energies are correlated with longer C \n-\n O bond lengths, while more negative \n\nQ\n\n\n\nC\nO\n\n2\n\n\n\n are correlated with longer C \n-\n O bond lengths and lower O \n-\n C \n-\n O angles (see Fig. S1 in the SI).H2 also physisorbs into the pristine S-1 via vdW interactions leading to a weak \n\u2009\n\n\n\u0394\nE\n\n\na\nd\ns\n,\n\nH\n2\n\n\n\n (see Fig. 2g\u2013i and Table S2 in the SI). In the presence of TMs from groups 3, 4, 5, 9 and 10, as well as for Os, H2 chemisorbs with \n\n\n\u0394\nE\n\n\na\nd\ns\n,\n\nH\n2\n\n\n\n values up to \n-\n 0.99 eV and in a \u03b7(HH) configuration, as illustrated in Fig. 3. In those TMs, the equilibrium H \n-\n TM distance lays between 1.5 \u00c5 and 2.5 \u00c5 and the H \n-\n H bond is elongated from 0.75 \u00c5 (gas-phase) up to 1.00 \u00c5. Within this sub-set, the H2 binding strength increases when moving right along a period and down in a group. Additionally, H2 breaks spontaneously (with no energy barrier) in Ru, Rh and Pt. In these cases, \n\n\n\u0394\nE\n\n\na\nd\ns\n,\n\nH\n2\n\n\n\n values are calculated by considering H + H as the adsorbed state. In the remaining TMs (i.e., groups 5, 6, 7, 8, 11 and 12, except Os and Ru), H2 weakly physisorbs and no relevant change with respect the gas-phase geometry or charge transfer is observed. Noticeably, \n\n\n\u0394\nE\n\n\na\nd\ns\n,\n\nH\n2\n\n\n\n and \n\nQ\n\n\nH\n2\n\n\n\n are correlated with the H \n-\n H distance in the adsorbed state, as shown in Fig. S2 in the SI.The reaction energies and energy barriers for H2 dissociation are compiled in Fig. 2j,k and in Table S2 in the SI. The pristine S-1 framework does not dissociate H2, since the weak binding with the framework atoms results in a gas-phase-like highly endoergic reaction with a prohibitive energy barrier larger than 5 eV. However, H2 dissociates in almost all considered SACs with low energy barriers, being barrierless for Ru, Rh and Pt, as mentioned above. Interestingly, H2 dissociation is exoergic in almost all group 3\u20139 TM SACs, with \n\u0394\n\nE\nr\n\n up to \n-\n 2.20 eV and \n\u0394\n\nE\n\u2260\n\n\n\n\u2264\n 1.02 eV. Those barriers are comparable to other good H2 dissociation catalysts such as CeO2 [54], TM1/CeO2 [55], (Ni2, Cu2)/MgO [56] or TM carbides [57], which show \n\u0394\n\nE\n\u2260\n\n values from almost zero to 0.8 eV. On the other hand, in TMs from groups 11 and 12 the reaction becomes endoergic and \n\u0394\n\nE\n\u2260\n\n increases significantly, with prohibitive values for group 12 SACs. The most common dissociation pathway for TMs where H2 adsorbs in \u03b7(HH) configuration involves further H \n-\n H bond elongation, yielding to the coadsorbed H + H on the TM atom. Otherwise, when H2 only physisorbs around 3 \u00c5 from the TM, it first needs to get closer to the metal, relocate as \u03b7(HH) and then, break.It is worth noting that for TM SACs adsorbed very close to the S-1 zeolite wall (i.e., group 3, group 10, Ru and Rh SACs) the CO2 adsorption typically promotes a small surface reconstruction, where the TM \n+\n CO2 pair separates slightly from the zeolite wall to reduce the repulsive CO2\n\n-\n zeolite interactions. Similarly, the H2 adsorption only promotes a small surface reconstruction on top of Ni and Pd TM SACs due to the H2 smaller size and weaker H2\n\n-\n zeolite repulsion. However, the cleavage to H + H leads to an equivalent reconstruction of group 3, Rh and Pt TMs. For a more precise picture of the surface reconstruction the reader is referred to Section S4 of the SI, where the original TM1@S-1 structure and the deformation caused by CO2/H2 is compiled for the full set of TMs. Also, all geometry files for each stationary point characterized in this work have been uploaded to a public repository (see Appendix A).The redox mechanism involves the C \n-\n O bond breaking by direct CO2 dissociation to CO \n+\n O. The calculated reaction energies and energy barriers for this step are compiled in Fig. 2e-f and Table S1 in the SI. For the un-aided CO2 dissociation in the pristine S-1 with no TM, the weak interactions among the support, the reactants and the products lead to a gas-phase-like highly endoergic reaction with a prohibitive energy barrier (\n\u0394\n\nE\n\u2260\n\n > 6 eV). However, the presence of single TM atoms stabilizes the reaction products, yielding to affordable reaction barriers and even barrierless dissociations in a few cases.\n\n\u0394\n\nE\nr\n\n and \n\u0394\n\nE\n\u2260\n\n follow a similar trend than the \n\n\n\u0394\nE\n\n\na\nd\ns\n,\nC\n\nO\n2\n\n\n\n along the periodic table, meaning that the reaction is more favourable in TM SACs from the bottom left of the periodic table and less favourable when moving right along a period and up along a group. As shown in Fig. 2e, the reaction is highly exoergic (i.e., high negative values up to \n-\n 2.8 eV) for early TMs, and proceeds with very low reaction barriers (i.e., 11 TM1@S-1 catalysts belonging to groups 3 \n-\n 8 break CO2 with \n\u0394\n\nE\n\u2260\n\n<\n 0.30 eV, 5 of them with \n\u0394\n\nE\n\u2260\n\n<\n 0.10 eV). These values contrast with the typical \n\u0394\n\nE\n\u2260\n\n ranging from 0.38 eV to 0.90 eV reported for other catalysts [58], such as flat metal surfaces [59\u201361] or supported metal clusters [62\u201364], placing many TM1@S-1 from groups 3 \n-\n 8 as extremely active towards direct C \n-\n O bond cleavage. In those TMs, CO2 is expected to be transformed via redox pathway, because it follows a single unimolecular step with a very low reaction energy barrier. In contrast, TM1@S-1 of groups 10 \n-\n 12 are poor catalysts for the direct CO2 dissociation, with prohibitive energy barriers due to their inability to stabilize the reaction products and their weak interaction with CO2. This implies that hydrogen-assisted associative pathways could dominate in these cases.As suggested by Pallasana and Neurock [65] and popularized by N\u00f8rskov et al. [66], the correlation between reaction energies and energy barriers in heterogeneous catalysed reactions is expected to follow the Br\u00f8nsted-Evans-Polanyi (BEP) [67,68] relationship. Specifically, the transition state energy \n(\n\nE\n\nT\nS\n\n\n) and the dissociative reaction energy (\n\nE\nr\n\n) defined by Wang et al. [69] as Eqs. 4 and 5 (without ZPE) are used to compare with the universal BEP relation for extended TM surfaces.\n\n(4)\n\n\nE\n\nT\nS\n\n\n=\n\nE\n\nT\nS\n-\nS\nA\nC\n\n\n-\n\nE\n\nS\nA\nC\n\n\n-\n\nE\n\ni\n(\ng\n)\n\n\n\n\n\n\n\n\n(5)\n\n\nE\nr\n\n=\n\nE\n\nP\n-\nS\nA\nC\n\n\n-\n\nE\n\nS\nA\nC\n\n\n-\n\nE\n\ni\n(\ng\n)\n\n\n\n\n\nIndeed, a quantitative BEP relationship for CO2 dissociation on TM1@S-1 emerges when plotting \n\nE\n\nT\nS\n\n\n in front of \n\nE\nr\n\n as shown in Fig. 4\n. The obtained scaling line has a similar slope than the universal BEP relation [69], but with \n\nE\n\nT\nS\n\n\n values about 1 eV lower. This fact evidences that S-1 encapsulated TM SACs are much more active for C \n-\n O bond scission than their extended TM counterparts. Note that several TM1@S-1 feature negative \n\nE\n\nT\nS\n\n\n and \n\nE\nr\n\n values, unachievable by extended metal surfaces, which only get as low as \n\nE\n\nT\nS\n\n\n = 0.88 eV and \n\nE\nr\n\n = \n-\n 2.00 eV. Unfortunately, the activation of H2 does not seem to follow a clear BEP relationship as shown in Fig. S3 in the SI.The most common dissociation pathway involves further C \n-\n O bond elongation of the adsorbed CO2 in \u03b7(CO) configuration, yielding to the reaction products CO and O, coadsorbed on the TM atom. There are, however, a few alternative dissociation pathways followed by some TM1@S-1, which we describe below. In the case of La1@S-1, the CO2\n\n+\n La pair needs to slightly separate from the S-1 wall in order to have enough space to accommodate the products. During this translation, the CO2 molecule dissociates in an apparently barrierless process, with the imaginary frequency of the located TS corresponding to the translation of the CO2\n\n+\n La pair moving away from the S-1 wall. For Ti and Zr, where CO2 is adsorbed in \u03b7(O) configuration, it breaks with no energy barrier before reaching the \u03b7(CO) configuration, and the imaginary frequency of the TS corresponds to a rotation from the \u03b7(O) to the \u03b7(CO) configuration, which also includes an elongation of the C \n-\n O bond. On the other hand, Hf spontaneously breaks the CO2 into CO + O. Next, for TMs of group 11, the reaction is highly endoergic, and the system must overcome a very large endothermicity to bring the CO2 closer from its initial physisorbed state to a \u03b7(CO) configuration. Then, the C \n-\n O bond breaks and the O atom is first adsorbed onto the TM. Since the coadsorbed CO \n+\n O products are not stable, the adsorbed O species diffuses to the opposite side of the TM and then CO stays adsorbed through the C atom. Finally, for the case of group 12 TMs, the reaction is even more endoergic due to the low stability of the reaction products. The CO2 dissociation proceeds through a direct gas-phase-like elongation of the C \n-\n O bond from its \u03b7(C) physisorbed state leading to adsorbed O and a weakly physisorbed CO in a \u03b7(C) configuration. Further information along with snapshots of the reaction pathway are found in Section S4 of the SI.As mentioned above, many TMs exhibit extremely low reaction energy barriers to convert CO2 through the redox pathway. Since the dissociation is a unimolecular step, it is not expected that bimolecular steps (such as hydrogenations in the associative pathway) will be kinetically relevant, even in the case of low energy barriers. For this reason, we have decided to consider this alternative pathway only in a subset of TM1@S-1 in which the redox mechanism has a significant energy barrier. This includes the late TMs in group 10 (i.e., Ni, Pd and Pt), where CO2 adsorbs strongly but it is hard to break, and also Ru and Rh, which have been already synthesized experimentally, proven to be highly active catalysts, and can dissociate H2 spontaneously.In those five TMs, CO2 adsorbed as \u03b7(CO) can react with a coadsorbed H species by forming an O \n-\n H bond leading to the carboxyl intermediate (i.e., COOH), which is bonded to the TM via the C atom (see Fig. 3). Alternatively, the H atom can react with the C atom forming a C \n-\n H bond leading to the formate intermediate (i.e., HCOO), which is first produced in its monodentate configuration (m\u2212HCOO), bonded to the metal through one of its O atoms. Then, m\u2212HCOO rearranges itself leading to the lower energy bidentate configuration (b\u2212HCOO), in which the formate is bonded to the metal through both its O atoms. The reaction energies and energy barriers for both pathways (plus the redox for comparison) are compiled in Fig. S4 and Table S3 in the SI. To compare those pathways in actual operative conditions, the Gibbs free energy diagrams at 600 K and 1 atm were built and shown in Fig. 5\n. To our knowledge, CO2 conversion in TM1@S-1 is currently not assessed experimentally, so those conditions were selected as they are generally used in similar systems [40,70] or other CO2 conversion catalysts [71]. Finally, the geometry for each minimum and TS are shown in Section S4 of the SI.The present results suggest that CO2 conversion steps on group 10 TMs (i.e., Ni, Pd and Pt) should follow the associative mechanism via COOH or HCOO intermediates, as the free energy barrier for their formation are much lower than for the direct CO2 dissociation (Fig. 5). In the case of Ni, the HCOO pathway is much more favoured than the COOH pathway. However, after H2 dissociation, the H + H recombination is barrierless, implying that the activity of the associative mechanism will be limited by the small amount of H species present in the catalyst. For Pd and Pt, the dissociation of H2 is very favoured and there will be a strong competition between the HCOO and COOH pathways, as both exhibit similar free energy barriers. For this reason, kinetic modelling techniques such as kinetic Monte Carlo [61] or microkinetic modelling should be employed to truly identify which pathway is more dominant. However, developing an accurate model for the TM1@S-1 catalysts requires further study due to their uncommon 3D morphology and is out of the scope for this work. Finally, we show that Rh and Ru would follow the redox mechanism, since, apart from being unimolecular, have lower free energy barriers compared to COOH or HCOO associative pathways.CO2 adsorption and subsequent activation have been investigated by means of periodic DFT calculations for the full set of 3d, 4d, and 5d TM SACs supported on zeolite S-1, namely TM1@S-1. The steps considered include adsorption and dissociation of CO2 and H2 species, as well as CO2 reaction with adsorbed H to produce COOH or HCOO intermediates. The catalytic properties are mainly controlled by the encapsulated TM atoms, since CO2 and H2 interact very weakly with the S-1 framework structure. In general, CO2 binds strongly to TM atoms and receives electron density from the TM, ending up being negatively charged. These effects are particularly strong for those TMs in the bottom left of the periodic table and become less pronounced when moving right along a period or up in a group. On the other hand, H2 chemisorbs on TMs from groups 3, 4, 5, 9, 10 and Os with negligible charge transfer from the TM. Noticeably, TMs from groups 11 \n-\n 12 hardly interact with CO2 and H2, where both species are physisorbed. TMs in groups 3 \n-\n 9 exhibit very low energy barriers for direct CO2 dissociation, suggesting that CO2 conversion reactions on these SACs would follow the redox mechanism. However, the COOH or HCOOmediated associative mechanism is more favoured in group 10 TMs, according to the free energy profiles. Finally, we predict group 11 \n-\n 12 TMs SACs to have a very poor catalytic activity for CO2 conversion, due to their weak interaction with CO2 and H2 as well as high energy barriers.Combining the present results with the stability assessment of supported TM SACs in our previous contribution [43], we conclude that groups 3 and 10 TM SACs, as well as Rh1@S-1 and Ru1@S-1 are very promising candidates for CO2 conversion reactions. All these catalysts exhibit low energy barriers and are predicted to have a strong resistance to aggregation/sintering due to their strong interactions with the zeolite wall. These results are in agreement with experimental observations stating that Ru and Rh were encapsulated in S-1 and used in different catalytic processes [39,40] with good stability and superior catalytic activity. With this contribution, we hope to narrow down the materials space for novel CO2 conversion SACs and provide a solid theoretical background from which observed experimental features can be interpreted, understood, and discussed.\nGerard Alonso: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Estefan\u00eda L\u00f3pez: Methodology, Investigation, Writing - review & editing. Ferm\u00edn Huarte-Larra\u00f1aga: Methodology, Investigation, Writing - review & editing. Ram\u00f3n Say\u00f3s: Project administration, Funding acquisition, Writing - review & editing. Hector Prats: Conceptualization, Methodology, Formal analysis, Writing - review & editing. Pablo Gamallo: Supervision, Methodology, Investigation, Funding acquisition, Writing - review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Support to this research is granted by the Spanish Ministry of Science, Innovation and Universities (Grants RTI2018-094757-B-I00, MCIU/AEI/FEDER, UE and MDM-2017-0767) and by the Generalitat de Catalunya (Grant 2017SGR0013 and P.G. Serra Hunter Associate Professorship). Authors thank to the Red Espa\u00f1ola de Supercomputaci\u00f3n (RES) for the supercomputing time granted (QS-2021-1-0035 and QS-2020-3-0023).Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2021.101777.The following is Supplementary data to this article:\n\n\n\n\n\n\n\n\n\n", "descript": "\n                  Zeolite-supported single-atom catalysts (SACs) have emerged as a novel class of cheap and tuneable catalysts that can exhibit high activity, selectivity and stability. In this work, we conduct an extensive screening by means of density functional theory calculations to determine the usefulness of 3d, 4d and 5d transition metal (TM) SACs-supported in MFI-type Silicalite-1 zeolite for CO2 conversion. Two reaction mechanisms are considered, namely the redox \n                        -\n                      direct CO2 dissociation \n                        -\n                      and associative \n                        -\n                      hydrogen-assisted CO2 dissociation \n                        -\n                      mechanisms. Early TM SACs exhibit the lowest energy barriers, which follow the redox mechanism. These energy barriers raise when going right in the periodic table up to group 10, where they become prohibitive and the associative mechanism should dominate. By also considering their resistance to aggregation, we support the use of Sc, Y, La, Ru, Rh, Ni, Pd and Pt as potentially active and stable catalysts for CO2 conversion, given their low energy barriers and strong interaction with the zeolite framework.\n               "}
{"full_text": "During the past decades, the selective hydrogenation of alkynes/alkynols has drawn significant attention. The partial hydrogenation of carbon-carbon triple bonds (C\u2261C) in the presence of carbon-carbon double bonds (C=C) is primarily applied in the purification of pyrolysis gas, which is of paramount significance in industrial sectors.\n1\n Likewise, it is an efficient tactic for the preparation of fine chemicals and pharmaceuticals like linalool, vitamins, and natural products through the selective hydrogenation of unsaturated alkynes or alkynols.\n2\u20135\n\nIn this review, we discuss the origin and evolution of efficient catalysts for the hydrogenation of alkynes/alkynols, for example, modified Pd-based catalysts,\n2\n\n,\n\n3\n\n,\n\n6\u201315\n alloy/intermetallic catalysts,\n16\u201321\n and single-atom catalysts (SACs) with corresponding representatives.\n22\u201339\n Analogously, some existing issues including \u201crun-away\u201d\n40\n\n,\n\n41\n and \u201cgreen oil\u201d\n21\n\n,\n\n42\n problems in the gas phase, as well as hydrogen solubility\n43\n and metal leaching\n44\n in the liquid phase are analyzed. In a similar fashion, several kinds of typical hydrogenation mechanisms like the Horiuti-Polanyi mechanism (so-called dissociative mechanism),\n40\n\n,\n\n45\n the associative mechanism,\n45\n and the Eley-Rideal mechanism\n46\n are discussed, and their applicability in hydrogenation reactions are compared. Multiple techniques, including in situ Fourier transform infrared spectroscopy (FTIR),\n29\n\n,\n\n32\n\n,\n\n33\n\n,\n\n47\n\n,\n\n48\n temperature-programmed desorption (TPD),\n32\u201335\n\n,\n\n40\n\n,\n\n48\n\n,\n\n49\n H2-D2 exchange,\n50\u201352\n solid-state nuclear magnetic resonance (NMR)\n48\n\n,\n\n53\n\n,\n\n54\n spectroscopy, as well as density functional theory (DFT) calculations,\n29\n\n,\n\n31\u201333\n\n,\n\n36\n are proposed for insight into the mechanism and structure-performance relationship in selective hydrogenations.Exploring an efficient catalyst to obtain both high activity and good selectivity toward alkenes remains a key challenge in the semi-hydrogenation process.\n55\n Two types of catalysts, namely homogeneous and heterogeneous catalysts, are the major contributors. Despite the high selectivity and explicit mechanism of homogeneous catalysts, the difficulties in separation and recycling restrict their industrial applications to some extent.\n56\n Compared with homogeneous catalysts, heterogeneous catalysts not only share the advantages of recyclability and regeneration but also the explicit and uniform location of active metal sites in some heterogeneous catalysts provide visualized models favoring the investigation of the structure-performance relationships.\n57\n\n,\n\n58\n On this basis, various heterogeneous catalysts, including traditional Pd-based catalysts, intermetallic compounds or alloys, and SACs, gradually spring up as promising candidates for selective hydrogenations (Figure\u00a01\n and Table\u00a01\n).Pd is one of the most efficient components in hydrogenation processes, inspired by its superior ability in dihydrogen dissociation.\n106\n\n,\n\n107\n However, the selectivity toward C=C bonds exhibits a huge fluctuation on account of the desorption energy of alkenes on the Pd sites.\n108\n Specifically, ethylene has the following three adsorption patterns: ethylidyne adsorbed on Pd-trimers, di-\u03c3-bonded C2H4 adsorbed on Pd-dimers, and \u03c0-bonded C2H4 adsorbed on Pd single atoms, respectively\n108\n (Figure\u00a02A). Noteworthily, the desorption energy of ethylene is lower than that for the over-hydrogenation only when C2H4 is \u03c0-bonded on Pd single atoms, thus leading to high selectivity toward ethylene. In contrast, when the ethylene is bonded on Pd species in ethylidyne or di-\u03c3-bonded C2H4 modes, the excessive hydrogenation is more favorable than the desorption of ethylene, which is detrimental to the selectivity and causes the production of the over-hydrogenation product, ethane. In this regard, despite the excellent activity of Pd-based catalysts, the selectivity toward alkenes is severely deteriorated under the ensemble effect of Pd species. Correspondingly, several components, like Pb,\n6\n S,\n2\n\n,\n\n7\u201310\n C (subsurface carbon),\n11\n CO,\n12\n\n,\n\n13\n and other p-block elements\n14\u201316\n have been adapted to promote the selectivity toward alkenes by covering the corner of edge sites of Pd counterparts.The Lindlar catalyst, typically Pd/CaCO3 modified by both lead and quinoline, has been regarded as the benchmark in the selective hydrogenation reaction.\n6\n The selectivity toward alkenes is promoted since the Pd species are partially poisoned by lead and quinoline. Similarly, sulfur-containing substances like thiols can be exploited as a \u201ctoxicant\u201d in Pd-complex systems.\n1\n\n,\n\n7\u201311\n\n,\n\n50\n Anderson and co-workers discovered a particular palladium sulfide phase (Pd4S) that was eligible in the semi-hydrogenation reactions.\n7\u20139\n The Pd4S active sites not only exhibited high conversion and selectivity in the semi-hydrogenation of dienes and alkynes but also showed high endurance even at high pressure (up to 18 bar), which is a challenging topic but is overlooked in the hydrogenation industry.\n8\n Inspired by the promising performance of the Pd4S phase in the high-pressure hydrogenation reactions, Javier and co-workers designed a nanostructured Pd3S phase with controlled crystallographic orientation, denoted as Pd3S@C3N4,\n2\n which showed unparalleled performance in the liquid-phase hydrogenation reactions. In fact, the above-mentioned sulfur-modified Pd catalysts\n7\u20139\n mimic enzyme catalysts by imitating the tactics of ensemble and electronic density control. The stable phase of Pd-S compounds provides plenty of space for tailoring the active site with the most selective ensembles at the molecule level. Recently, Zheng and co-workers demonstrated a distinct Pd-sulfide/thiolate interface, denoted as Pd@SPhF2, showing good performance in the selective hydrogenation process.\n10\n On the premise of the Eley-Rideal mechanism, the Pd@SPhF2 catalyst showed negative adsorption capacity of alkynes or alkenes and therefore exhibited high selectivity in the hydrogenation of 1-phenyl-1-propyne (>97% selectivity toward 1-phenyl-1-propene at full conversion).The above strategies provide us some enlightenment about how the surface coordination environment regulates the reaction performance. In addition to organic sulfides mentioned above, various metal sulfides show a promotion effect on alkene selectivity.\n53\n\n,\n\n94\n Li and co-workers disclosed that CuS nanoplates contributed to the dispersion of Pd elements by forming stable anchored active sites, accordingly attaining high selectivity, activity, and stability simultaneously.\n94\n Similarly, Zheng et\u00a0al. demonstrated that the modification of rhodium sulfide over Pd nanosheets facilitated the formation of surface PdS\nx\n ensembles, which not only promoted the hydrogenation activity but also accounted for the isomerization of cis alkenes to trans alkenes.\n53\n Likewise, the subsurface carbonous compounds can promote the selectivity in a similar manner. Schl\u00f6gl et\u00a0al. demonstrated that the near-surface region of Pd played an important role in the selective hydrogenation process.\n11\n The carbonaceous substance from feeding organic molecules could occupy the interstitial lattice sites and separated the adjacent Pd ensembles, accounting for the enhancement in selectivity (Figure\u00a02B). The importance of subsurface chemistry is evident, and is beneficial for the in-depth understanding of surface and subsurface dynamics as well as the rational design of catalysts.Other p-block elements, like boron (B), can also boost alkene selectivity by covering the redundant Pd active sites.\n14\n\n,\n\n15\n Edman Tsang and co-workers employed BH3THF as a reagent and designed a novel catalyst, Pdint-B, Pd nanoparticles modified with interstitial boron atoms.\n14\n The catalyst exhibited good chemical and thermal stability owing to the strong electronic interaction within the host-guest sites between Pd and BH3THF. Meanwhile, Philip and co-workers compared the selective hydrogenation activity over boron-modified Pd catalysts using DFT calculations.\n15\n It is clear that the Pd(111)-B, Pd(111) surface modified with boron atoms, shows higher activity than clean Pd(111) in the hydrogenation of acetylene and 1,3-butadiene, validating the feasibility of boron modification for the purpose of increasing the hydrogenation performance. Apart from the above inorganic elements, CO has been selected to be a good promoter in the hydrogenation process, especially in industrial applications. Javier et\u00a0al. investigated the activity of Pd-based catalyst in the absence and presence of CO with the assistance of DFT calculations.\n12\n It was demonstrated that the CO tends to reduce the Pd ensembles by forming densely packed overlayers. Thus, the over-hydrogenation and the polymerization can be prevented by suppressing the adsorption of unsaturated reactants, dihydrogen, and the hydrocarbon intermediates. However, the ratio of CO/H2 should be controlled within a certain limit (usually below 0.1) to prevent the formation of oligomers.\n13\n\nThe above poison strategy aims at promoting the selectivity by covering partial Pd active sites. However, due to sacrificing a mass of active phases and the utilization of toxic compounds, the strategy is deemed to be environmental unfriendly, with low atomic efficiency. Therefore, an alternative method employing a second or third metal, called alloys or intermetallics (Figure\u00a03A), was proposed.\n16\u201321\n\n,\n\n60\n\n,\n\n61\n\n,\n\n71\n\n,\n\n93\n\n,\n\n105\n On this basis, various Pd-based alloys/intermetallics like PdAg,\n16\n PdCu,\n17\n PdAu,\n18\n Pd-Zn,\n19\n Pd-Ga,\n20\n and Pd-In\n21\n have been investigated. Some non-precious metal alloys have also been developed on the premise of the site separation strategy. Under the construction of alloys/intermetallics, the continuous bulk metal ensembles can be separated or isolated by the second metal, promoting the selectivity toward alkenes.Zhang and co-workers reported a series of IB-metal-alloyed Pd-based catalysts, namely PdAu/SiO2,\n18\n PdAg/SiO2,\n16\n and PdCu/SiO2,\n17\n and compared their activities in the selective hydrogenation of acetylene. The similar activation energies among the three catalysts indicated an analogous mechanism, but shared different selectivity toward ethylene, which might imply different electronic effects between Pd and the second metals (Figures\u00a03B and 3C). Specifically, the PdCu/SiO2 catalyst possessed the highest electron density of Pd species, and thus suppressed the adsorption of C=C bonds and resulted in the higher selectivity toward ethylene (Figure\u00a03D). In addition to the elements of group IB, Zhang and co-workers constructed PdZn intermetallic nanostructure through alloying Pd with Zn.\n19\n The Pd species in the PdZn catalyst weakened the \u03c0-bonding adsorption of ethylene and further prevented its over-hydrogenation, resulting in high selectivity toward ethylene. The ingenious arrangement of Pd species provided two adjacent but isolated sites, which was feasible for the adsorption and activation of acetylene through moderate \u03c3-bonding patterns, and responsible for the superior activity thereof (Figures\u00a02A and 3E). Recently, Su and co-workers found that adding a Ga phase could destroy successive Pd atom ensembles, and designed supported Pd2Ga intermetallic catalysts\n20\n showing high thermal stability under employed reaction conditions by taking the advantage of covalent interactions between nanocrystals. However, the selectivity toward ethylene over the PdGa catalyst remained to be improved, probably due to the nonuniformity of nanoparticles prepared by a wet impregnation method. Recently, Fan and co-workers developed a calcite-supported PdBi intermetallic compound, PdBi/calcite. Owing to the isolated and electron-rich Pd sites, the PdBi/calcite catalyst showed weak adsorption of ethylene and superior stability (over 99% ethylene selectivity at full conversion) over a wide range of reaction temperatures (423\u2013573 K).\n105\n\nIn addition to Pd-based alloy catalysts, some non-precious intermetallics have been investigated to replace the use of noble metals. N\u00f8rskov and co-workers screened about 70 bimetallic compounds, including Fe, Ni, Co, Cu, Pd, Pt, Ag, Au, Zn, Cd, Hg, Ga, Tl, Ge, Sn, and Pb, for the hydrogenation process (Figures\u00a03F and 3G).\n60\n Among these catalyst models, the low-cost Ni-Zn catalyst exhibited comparable activity (turnover rate) and selectivity. Theoretical predictions provided a novel and efficient strategy for the rational design of alloy hydrogenation catalysts free of noble metals. In addition, Javier et\u00a0al. developed a ternary Cu-Ni-Fe catalyst that exhibited good performance in the semi-hydrogenation of propyne (selectivity of 80% at full conversion).\n71\n The three metal phases performed their own function, where Cu was used for a basement, Fe severed as the structural promoter to enhance propylene selectivity, and Ni facilitated the spillover of hydrogen and further prevented the oligomerization. The ternary catalyst was a great achievement, promoting the selectivity without the use of noble metal species and the potential poisoning step. Under the guidance of the site-isolation concept, a low-cost replacement for Pd-based catalyst, Al13Fe4, was constructed by Armbr\u00fcster et\u00a0al.\n61\n The electron structure of both Fe and Al was changed by the tight chemical bonding, and thus exhibited remarkable performance in the semi-hydrogenation of acetylene. Inspired by the above results, Wang et\u00a0al. synthesized intermetallic NixGay and NixSny nanocrystals via a solution-based co-reduction strategy,\n93\n favoring the formation of alloys with uniform sizes. The isolated sites between Ni and Sn/Ga as well as the electronic effect within the active sites accounted for the good catalytic performance, making NixMy good candidates for the Pd-based catalysts.Compared with the \u201cselective poison\u201d strategy, the construction of alloys/intermetallics promotes the atomic utilization efficiency to a great extent. However, multi-element alloys are usually prepared through wet impregnation, leading to inhomogeneous active sites and showing negative impacts on catalytic performance. Thereupon, the single-atom strategy with the maximal economic efficiency emerged.\n22\n By means of advanced techniques like Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray adsorption spectroscopy (XAS), and FTIR spectroscopy with CO adsorption, the explicit structure of SACs can be revealed. On this basis, the single-atom catalyst systems can be modeled by DFT calculations and the structure-performance relationship can be further interpreted.Graphene, nitrogen-rich carbon (C3N4), aluminum oxide (Al2O3), and silicon oxide (SiO2) are widely utilized as relatively inert supports for metal species.\n23\u201329\n\n,\n\n109\n\n,\n\n110\n These inert supports can modify the configuration of metal species\u2019 morphologic and electronic aspects, forming metal/support interfaces, which are important to selective hydrogenation. Lu and co-workers prepared atomically dispersed Pd on graphitic carbon nitride (g-C3N4), Pd1/C3N4, through an atomic layer deposition (ALD) technique, which showed 95% ethylene selectivity in the semi-hydrogenation of acetylene.\n109\n The Pd1/C3N4 also exhibited a higher stability (more than 100 h) in either reducing or oxidizing conditions than g-C3N4-supported Pd NP catalysts, and thus appearing to be a promising candidate for promoting selectivity as well as coking resistance in hydrogenation systems. A similar structure-performance relationship was also disclosed over Pd1/graphene where the alkyne adsorbed through mono-\u03c0-adsorption rather than a di-\u03c0-adsorption manner and thus showed excellent selectivity.\n23\n\n,\n\n24\n In addition, Javier and co-workers established a stable single-site Pd catalyst, namely [Pd]mpg-C3N4, which was confirmed to be efficient for the three-phase hydrogenation of alkynes and thus provide more potential in industry applications.\n28\n Recently, Ma et\u00a0al. constructed the single Pd atoms supported on nanodiamond-graphene, Pd1/ND@G.\n26\n The atomically dispersed Pd atoms over Pd1/ND@G were strongly anchored on the support through Pd-C bonds, which not only avoided the formation of \u03b2-H species but also favored the desorption of ethylene, and thus showed remarkable selectivity. Similarly, Li and co-workers demonstrated a mesoporous N-doped carbon nanosphere, Pd/MPNC, which had high activity, excellent ethylene selectivity, and good long-term stability in the semi-hydrogenation of acetylene.\n111\n Considering the high cost of noble metals, Lu and co-workers designed a novel metal trimer catalyst (Ni1Cu2/g-C3N4) exhibiting high efficiency in the semi-hydrogenation of acetylene.\n29\n The Cu atomic grippers could boost the loading of Ni species through dynamic and synergetic metal-support interactions, providing an atom-by-atom fabrication approach for the rational design of catalysts. Similar performance was also disclosed on Cu0.5/Al2O3\n\n25\n and Cu1/ND@G\n27\n when applied in the semi-hydrogenation of acetylene.Porous materials such as zeolites, metal-organic frameworks (MOFs),\n30\n and zeolite imidazolate frameworks (ZIFs)\n91\n with ordered channel structure as well as unique confinement effects have become indispensable supports in heterogeneous catalysis.\n57\n These porous supports can provide accessible space to stabilize metal species and construct single-site catalysts. For example, Corma et\u00a0al. designed FeIII-(OH) single sites embedded within MOFs, which exhibited good acetylene semi-hydrogenation performance under simulated front-end industrial conditions.\n30\n Inspired by the channel confinement effect of the zeolite, Gong and co-workers demonstrated that Pd clusters encapsulated within sodalite (SOD) zeolite, namely Pd@SOD, could catalyze the semi-hydrogenation of acetylene.\n31\n The narrow channels of SOD cages (0.28\u00a0\u00d7\u00a00.28\u00a0nm) restricted the free diffusion of acetylene and ethylene, but allowed dihydrogen to enter the pore channels smoothly. Under such circumstances, the dihydrogen molecules underwent cleavage on encapsulated Pd clusters, accompanied by OH species transferring to the surface of SOD cages through spillover and reacting with acetylene. Recently, Li and co-workers constructed Ni(II) species confined within different zeolites, Ni@FAU\n32\n and Ni@CHA,\n33\n respectively. These two zeolite catalysts exhibited comparable hydrogenation performance but followed different mechanism patterns, which might originate from the faint difference of the confining environments between faujasite and chabazite zeolites (Figure\u00a04A). Owing to the stronger local electric field of chabazite than that of faujasite, the Ni(II) sites were more tightly confined in Ni@CHA and triggered the direct dissociation of dihydrogen. By contrast, Ni@FAU with a lower coordination number possessed higher affinity for acetylene and further induced the acetylene-promoted hydrogenation mechanism.Intermetallics or alloys provide an alternative strategy toward selective hydrogenation and thus inspired the construction of the single-atom-alloy catalysts.\n16\u201321\n\n,\n\n60\n\n,\n\n61\n\n,\n\n71\n\n,\n\n93\n\n,\n\n105\n In essence, the single-atom-alloy catalysts can be rationally designed by regulating the formation and aggregation energies within host/guest metals in equilibrium,\n34\u201339\n thus reducing the dosage of noble metals to a great extent. Equipped with simple and specific configurations, the single-atom alloys accommodate the ideal catalyst models for surface science investigations and DFT calculations, beneficial to in-depth investigations of the structure-performance relationship.\n112\n As for the selective hydrogenation of alkynes, the facile dissociation of dihydrogen and the easy desorption of hydrocarbon intermediates are crucial factors controlling the selectivity, which are still difficult to realize simultaneously. To solve the problem, Sykes and co-workers constructed the Pd/Cu(111) single-atom-alloy catalyst with isolated Pd atoms over a Cu(111) surface.\n34\n Through low-temperature scanning tunneling microscopy (STM), it was disclosed that molecular hydrogen first dissociated on the single-dispersed Pd sites among 0.01 ML Pd/Cu(111) catalyst. Then the individual H adatoms underwent transfer from Pd sites to bare Cu(111) terraces (that is, hydrogen spillover) (Figure\u00a04B), providing the first direct observation of hydrogen spillover. The lower energy barrier for the dissociation of dihydrogen and the easy desorption of alkenes over Pd/Cu(111) than over bare Cu(111) surfaces were also confirmed via TPD and DFT calculations. Similarly, the Pt/Cu(111) single-atom-alloy catalyst with low concentration of Pt doped on Cu(111) surface was also found to be active for the facile dissociation of dihydrogen and the subsequent hydrogenation of 1,3-butadiene.\n35\n Inspired by the unique characteristics of trace amount metal supported on Cu(111), Zheng et\u00a0al. designed Pd atoms supported on different crystal faces, Pd1/Cu(100) and Pd1/Cu(111), and their hydrogenation behaviors were compared.\n36\n It was disclosed that extremely diluted Pd1/Cu(111) was inert for the hydrogenation of phenylacetylene unless Cu(100) was introduced (Figure\u00a04C). That is, the spillover of dihydrogen could only occur on the Cu(100) face despite the clear evidence of H adatoms spillover from Pd single sites to the bare Cu(111) face.\n36\n The essence of dihydrogen spillover over Pd/Cu single-atom alloy remains a controversy. Nevertheless, under the guidance of the single-atom-alloy strategy, diverse metal species in the form of single-dispersed atoms stabilized on peculiar crystal facets, for example Rh/Cu(111),\n37\n Pd/Au(111),\n38\n and Pt/Ag(111),\n39\n were constructed for various reactions not limited to selective hydrogenations.Apart from inert supports as mentioned above, many reducible metal oxides, like titanium oxides (TiO2),\n113\n zinc oxide (ZnO),\n102\n ferric oxide (FexOy),\n50\n\n,\n\n114\n cerium oxide (CeO2),\n99\n\n,\n\n114\n and Ga2O3,\n115\n can be employed as support materials in the selective hydrogenation reactions. The metal oxide support can interact strongly with the active metal sites under certain conditions, called strong metal-support interaction (SMSI).\u00a0For example, Francisco and co-workers found that moderate thermal treating of Pt/TiO2 favored the deep diffusion of Pt phase into the bulk TiO2 structure.\n113\n Correspondingly, the strong intimate relationship between Pt and TiO2 could be induced, facilitating the following hydrogenation process. Similarly, Wang et\u00a0al. reported that Pd/In2O3 synthesized through facile wet impregnation was a promising alternative for the hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to the corresponding alkenol 2-methyl-3-buten-2-ol (MBE), a key component for the fabrication of vitamin E.\n102\n The In2O3 support or substrate could form a loose layer to cover the exposed Pd active site under employed conditions on the premise of SMSI, ensuring the high selectivity toward alkenols. On the basis of SMSI, Choi et\u00a0al. disclosed a novel mechanism, dynamic metal-polymer interactions (DMPI), which can be regarded as the organic version of SMSI.\n50\n The organic support polyphenylene sulfide was flexible and could cover Pd active sites under employed conditions via the strong metal-polymer interactions. This controllable interface could regulate the passages of reactants and prevent the over-hydrogenation. Apart from the concept of SMSI and DMPI, Zhu and co-workers designed a porous yolk-shell structure via reverse strong metal-support interaction.\n104\n The fully encapsulated core-shell configuration Pd-FeOx nanoparticles transformed into a porous yolk-shell structure Pd-Fe3O4-H under H2 treatment. Being exposed to the reactants, the Pd active sites exhibited excellent catalytic performance in the selective hydrogenation of acetylene with a turnover frequency of (TOF) of 6.46 s\u22121 under the rule of SMSI in a reverse route. In addition to the common reducible support mentioned above, Ga2O3 exhibited strong electron transfer as well as the decaying adsorption of CO and H2, which should be a promising candidate for the SMSI.\n115\n On this basis, Xu and co-workers constructed Pd/Ga2O3 and disclosed that the formation of SMSI was triggered by the coexistence of PdGa alloy as well as the Ga2O3, thus providing both high propylene selectivity and propyne conversion under mild reaction conditions (303 K, atmospheric pressure). Noteworthily, the unique SMSI feature was also disclosed over layered double hydroxide (LDH).\n116\n Feng et\u00a0al. reported that the Pd/MgAl-LDH-Al2O3 exhibited high activity and selectivity in semi-hydrogenation of acetylene owing to the reduced acidity as well as the SMSI.Generally, the hydrogenation of alkynes is a two-phase system, where the gaseous acetylene/propyne/vinylacetylene and dihydrogen react over the solid catalyst.\n1\n\n,\n\n42\n\n,\n\n60\n The target products ethylene/propylene/1,3-butadiene are key components for the manufacture of polyolefins.\n117\n Alkenes are obtained in large scale through the cracking of naphtha, which contains a considerable number of impurities; i.e., 0.5%\u20138% alkynes.\n1\n These highly unsaturated compounds may cause serious damage in olefin polymerization; for example, the break of polymerase chains and the deactivation of the Ziegler Natta catalyst.\n118\n Thus, the content of triple-bond chemicals in an ethylene/propene stream must be reduced to an acceptable level (<5 ppm). Typically, the front-end and tail-end semi-hydrogenation is supposed to be the most efficient route to the elimination of trace alkynes.\n17\n\n,\n\n119\n\nThe front-end hydrogenation, which requires high concentration of dihydrogen in the feed gas, easily causes the over-hydrogenation of acetylene and the \u201crun-away\u201d of reaction temperature, and thus makes it more challenging to obtain high selectivity in the front-end semi-hydrogenation.\n40\n In contrast, the ethylene feed gas in the tail-end process has been purified before the hydrogenation of trace amounts of alkynes. Therefore, the tail-end hydrogenation generally requires near-stoichiometric dihydrogen (dihydrogen/alkynes\u00a0= 1\u20132), which is easier to realize and more popular in practice.\n120\n For the selective alkyne hydrogenation in the gas phase, the selectivity toward the target\u00a0alkene products is affected by many factors; for example, the drastic reaction temperature,\n65\n the excess amount of hydrogen, and the inner construction of catalysts.\n91\n On this basis, multiple hydrogenation catalysts have been investigated to avoid the over-hydrogenation process, which is not explained in detail here. Besides, some side reactions, such as couple crossing or polymerization of alkynes, are nonnegligible. Under certain circumstances, especially on the catalysts that have strong affinity toward C\u2261C bonds, alkynes tend to form coke and C4\u2013C6 hydrocarbons (green oils) in the presence of alkenes.\n121\n The above adverse reactions may cause a huge waste of alkene cuts and will lead to catalyst deterioration as well as breaking off the polymer chains. Thus, it is urgent to design suitable catalysts that can suppress the side reactions and ensure high selectivity toward alkenes.The selective hydrogenation of C\u2261C bonds can also occur in the liquid phase to produce fine chemicals. Nowadays, the hydrogenation of alkynols is considered a fundamental process for the synthesis of fine chemicals and intermediate chemicals.\n48\n One important example is the synthesis of intermediate substance like linalool and MBE, which are prevalently utilized in the production of vitamin E, vitamin K, and provitamin \u03b2-carotene.\n4\n\n,\n\n5\n\n,\n\n14\n The demand of efficient hydrogenation catalysts in the liquid phase is now at a high pitch. Generally, the durability of hydrogenation catalysts in the liquid phase does not draw much attention owing to the relatively mild conditions compared with the hydrogenation in the gas phase.\n55\n However, there are some severe problems to solve in liquid-phase hydrogenations. For example, the dihydrogen solubility in liquid phase is limited, thus hampering the conversion of alkynes.\n43\n On the other hand, the selectivity toward alkenes/alkanols remains to be promoted since the active sites exhibit stronger adsorption affinity of liquid medium than that of dihydrogen in the gas phase. Therefore, the alkenes/alkanols tend to be over-hydrogenated to saturated alkanes/alcohols instead of being desorbed as products, corresponding to low selectivity. Besides, it is important to identify whether the active sites are heterogeneous or not, which is crucial for industry applications, and this may require hot filtration test in batch reaction or long-term fixed-bed reaction. However, many liquid-phase hydrogenation catalysts equipped with organic modifiers have a huge risk of leaching, which further leads to declines in catalytic activity and/or selectivity when applied in practical operation. That is to say, the exploitation of hydrogenation catalysts with anchored active sites appropriate for hydrogenation in the liquid phase is still a challenging task.The adsorption and activation of dihydrogen is considered the crucial step in the hydrogenation process, which can be classified as homolytic dissociation and heterolytic cleavage, respectively.\n11\n The homolytic dissociation of dihydrogen generally happens on the surface of VIII group metals such as Pt,\n122\n Pd,\n123\n and Rh.\n37\n These metals have segmental occupied d orbitals, where the \u03c3 electrons from dihydrogen can be accommodated. On the other hand, metals can donate d electrons to the antibonding orbits of dihydrogen (Figure\u00a05A). Accordingly, two hydrides are formed by homolytic cleavage of the weak H\u2013H bond. It is acknowledged that metal ensembles, where more than two metal atoms are present in the vicinity, are more beneficial for the homolytic cleavage of dihydrogen than single-atom metal species.\n96\n However, this might lead to over-hydrogenation, since the hydride formed after homolytic dissociation will migrate to the near-surface region of metal counterparts, forming the subsurface hydride, which is detrimental to the selectivity.\n11\n\nAnalogously, hydrogen species can be heterolytically cleaved into H+/H\u2212 through the concerted effect of metal species and basic materials such as supports. The formed H+ and H\u2212 then bond with metal atoms and a proton acceptor, for example N,\n125\n O,\n33\n and C\n23\n atoms, respectively. It is well-known that heterolytic dissociation of dihydrogen usually happen in homogeneous catalytic systems like enzymes,\n125\n the frustrated Lewis pairs (FLPs), and the classical Lewis pairs (CLPs).\n99\n\n,\n\n126\n\n,\n\n127\n However, some heterogeneous catalysts, like supported metal oxides,\n98\n\n,\n\n128\n\n,\n\n129\n SACs,\n28\n\n,\n\n96\n\n,\n\n98\n\n,\n\n130\n and metal encapsulated zeolites,\n33\n\n,\n\n131\n are also prone to dissociate dihydrogen heterolytically. For example, Li et\u00a0al. demonstrated that dihydrogen underwent heterolytic dissociation by Ni (II) sites and the adjacent zeolite framework oxygen atoms, forming hydride (Ni-H) and proton (O-H), respectively.\n33\n The heterolytic cleavage of dihydrogen was confirmed by in situ FTIR spectroscopy of H2/D2 activation (\u03bdO\u2212H\u00a0= 3,610\u00a0cm\u22121, \u03bdO\u2212D\u00a0= 2,600\u00a0cm\u22121, \u03bdO\u2212H/\u03bdO\u2212D\u00a0\u2248 1.4) as well as the Bader charges of Ni\u2013H (\u22120.31 eV) and O\u2013H (1.0 eV) by DFT calculations. Lu and co-workers compared the Bader charge of H in Pd-H (\u22120.30 eV) and O-H (0.62 eV), which provided strong evidence for the heterolytic cleavage of hydrogen.\n24\n Unlike the homolytic dissociation of dihydrogen, which needs adjacent metal atoms, heterolytic dissociation of hydrogen requires the joint efforts of both metal and support, which serve as the hydride acceptor and proton acceptor, respectively. Lacking metal ensembles, the analogous FLPs/CLPs patterns of hydrogen dissociation enable the higher selectivity toward alkenes in alkyne hydrogenations, and therefore become a hot spot in heterogeneous hydrogenation systems.\n99\n\n,\n\n126\n\n,\n\n131\n On the premise of CLPs, Guo and co-workers demonstrated that the Ni-doped ceria, namely Ni@CeO2(111), exhibited high activity in acetylene hydrogenation.\n99\n DFT calculations revealed that the oxygen vacancies facilitated the heterolytic dissociation of dihydrogen, producing the Ce-H and O-H, respectively. The CLPs pattern of dihydrogen activation avoided the overstabilization of C2H3\n\u2217 intermediate and corresponded to the low barrier for the formation of ethylene. Similar structure-performance relationship was disclosed over Pt@Y zeolite, confirming the wide applicability of CLPs and the corresponding selective specificity in hydrogenation reactions.\n131\n\nFor insight into the mechanism of alkyne hydrogenation, some unique catalyst systems and their corresponding reaction pathways are discussed in the following sections. The Langmuir-Hinshelwood mechanism\n124\n\n,\n\n132\n with the co-adsorption of reactants on surface plays a dominating role in heterogeneous hydrogenation catalysis. In contrast, the Eley-Rideal mechanism,\n133\n featuring the direct reaction between a molecular reactant with an adsorbed one, is rarely reported for alkyne hydrogenation.The Langmuir-Hinshelwood mechanism, which remains the most common one among heterogeneous catalytic systems, can be divided into two routes (Figure\u00a05B). Proposed in 1934, the Horiuti-Polanyi mechanism, or the so-called dissociative mechanism, plays a primary role in hydrogenation processes.\n134\n It entails dihydrogen homolytic dissociation on the metal surface, followed by the successive addition of H atoms to the adsorbed alkynes. This routine is firstly disclosed on the surface of Ni\n86\n and Pd\n135\n with intrinsic ability to split dihydrogen, leaving the hydrogen atoms bonded with metal and establishing the stable metal hydrides (M-H). Subsequently, the so-called dissociative mechanism was discovered over Au nanoparticles by Javier and co-workers.\n42\n Despite the filled d orbitals of Au-based catalyst and the restricted activity toward dihydrogen dissociation, the catalyst exhibited decent performance in the selective hydrogenation of propyne with \u223c90% conversion and selectivity to propylene. Additionally, the Horiuti-Polanyi mechanism was found to be applicable over alloys and metal phosphides such as Pd3Ga7\n\n69\n and Ni2P,\n92\n involving molecular hydrogen dissociation followed by the successive addition of hydrogen atoms to the tri-bonded compounds.However, for SACs without adjacent metal-metal pairs available for the homolytic dissociation of dihydrogen, the dissociation of dihydrogen must take place via an alternative pathway, namely heterolytic dissociation.\n96\n Typically, the dihydrogen molecules undergo heterolytic cleavage, leaving one of the hydrogen atoms bound to the metal atom and the second one to the heteroatom of the support like N, C, or O. Inspired by this kind of dissociation routine, various SACs, e.g., Pd1-O/graphene,\n23\n Pd4S,\n2\n Ni@CHA,\n33\n and Pd/mpg-C3N4,\n28\n were rationally designed. For example, in a typical system of Ni(II)-encapsulated zeolite, namely Ni@CHA, the dihydrogen firstly undergoes heterolytic cleavage under the effect of coordinately unsaturated Ni(II) site and the surrounding oxygen atoms in the six-membered ring of chabazite zeolite.\n33\n This type of hydrogen heterolytic dissociation was verified not only via DFT calculations but also through isotope-labeled FTIR spectra, providing strong experimental evidence for the CLPs. As shown in Figure\u00a05C, the stretching pattern of bridging hydroxy ions (\u03bdO-H\u00a0= 3,610\u00a0cm\u22121) shifted to the red region (\u03bdO-D\u00a0= 2,600\u00a0cm\u22121) in the deuterium labeling experiment, according with the theoretical ratio of the frequencies for harmonic oscillation of H2 and D2 molecules against a rigid wall (1.41).\n136\n Similarly, Lu and co-workers demonstrated that the hydrogenation of 1,3-butadiene over Pd1-O/Graphene followed the typical Horiuti-Polanyi mechanism where the heterolytic dissociation of dihydrogen was found to be the rate-determining step.\n23\n Considering the different dissociation patterns of dihydrogen with the Horiuti-Polanyi mechanism, the hydrogenation with heterolytic cleavage of dihydrogen was denoted as pseudo-Horiuti-Polanyi mechanism by Lu et\u00a0al.Noteworthily, the Horiuti-Polanyi mechanism occurs over metal surfaces in most cases, which is feasible for the cleavage of dihydrogen. However, this kind of hydrogenation mechanism generally leads to mostly cis alkene products in the hydrogenation of internal alkynes, even though the trans alkenes are thermodynamically more stable than the cis ones.\n137\n\n,\n\n138\n On this basis, Zheng et\u00a0al. reported that the defective Rh2S3-x exhibited high selectivity toward trans alkenes in the hydrogenation of internal alkynes.\n53\n The dihydrogen underwent dissociation at the defects of the solid surface and formed the frustrated hydrogen pair, which could modulate the cis-to-trans isomerization without over-hydrogenation. That is to say, the isomerization of cis/trans alkenes can be modulated by altering the hydrogenation mechanism, providing a new thought for the rational design of novel catalysts.Despite the most crucial step of dihydrogen dissociation in the hydrogenation process, there are some relatively inert metal sites that are invalid for the direct splitting of dihydrogen. In this sort of catalyst, the hydrogen cannot be dissociated by active center independently but with the assistance of alkynes, denoted as associative mechanism (Figure\u00a05B). The alkyne-assisted pathway was first disclosed on Ag nanoparticles by Javier et\u00a0al. in 2013.\n45\n The adsorption sites of alkynes and the energy barriers over Ag(211) under a different mechanism were compared using DFT calculations. As shown in Figure\u00a05D, the associative mechanism required relatively lower activation barriers than the classical Horiuti-Polanyi mechanism. Later, this acetylene-promoted associative mechanism was reported with over Pd@SOD\n37\n and Cu1/ND@G\n27\n systems. On the premise of the associative mechanism, it was found that the above SACs exhibited higher selectivity than the corresponding metal ensembles.However, the associative mechanism is merely confirmed by DFT calculations, lacking direct experimental evidence.\n45\n This can be ascribed to the fact that the intermediate phases of Horiuti-Polanyi mechanism and associative mechanism are identical or similar; for example, (C=CH\u2217, CH\u2013CH2\u2217), making it difficult to distinguish by means of spectroscopic protocols.\n32\n\n,\n\n33\n Recently, Li et\u00a0al. conducted the H2-D2-C2H2 pulse-response experiments, providing the first in-depth evidence on the alkyne-associative mechanism.\n32\n The signals of HD (m/z\u00a0= 3) in the absence and presence of acetylene were compared over Ni@CHA and Ni@FAU catalysts, respectively. As shown in Figure\u00a05E, no HD signal was detected over Ni@FAU under the mixture flow of H2-D2 at rational temperature, indicating that the dihydrogen could not be efficiently activated under employed condition. However, it appeared immediately upon the introduction of acetylene, confirming that the dihydrogen was dissociated after the injection of acetylene molecules. That is, the hydrogen and deuterium underwent dissociation and formed HD only after the combination of Ni(II) sites and alkynes. The so-called alkyne-promoted/assisted mechanism is different from the conventional Horiuti-Polanyi mechanism,\n134\n which is rarely seen in heterogeneous hydrogenation but widely observed in homogeneous systems.\n89\n\n,\n\n139\n\n,\n\n140\n For the Ni@CHA catalyst, the HD signals were clearly detected in the H2-D2 stream at the very beginning and gradually decreased upon the introduction of acetylene due to the hydrogenation of acetylene by HD, deriving a typical Horiuti-Polanyi mechanism. In short, the H2-D2-C2H2 pulse-response experiments provide direct evidence to distinguish the associative mechanism and Horiuti-Polanyi mechanism. Under the guidance of the associative mechanism, considerable amounts of trans alkenes could be obtained from the hydrogenation of internal alkynes like di-phenylacetylene and 1-phenylpropyne, in accordance with the views of Zheng et\u00a0al.\n53\n That is, the associative mechanism may lead to the formation of trans alkenes in the selective hydrogenation of internal alkynes.The associative mechanism, where the dihydrogen dissociated heterolytically, is widely observed in organometallic chemistry and homogeneous catalysis. For example, the associative mechanism has been found to boost the trans-alkene selectivity in various metal complexes like (IMes)Ag\u2217Rp,\n89\n Ni\u2217Ln\n139\n and Cp\u2217Ru.\n140\n However, this mechanism is rarely seen in a heterogeneous hydrogenation system. The alkyne-promoted mechanism provides a specific hydrogenation process that compensates for the conventional Horiuti-Polanyi mechanism. The associative pattern was also disclosed to be suitable in heterogeneous ammonia synthesis by Li et\u00a0al.\n141\u2013143\n The associative mechanism was also found to be efficient in ammonia synthesis over Ru/H-ZSM-5\n142\n or Fe3/\u03b8-Al2O3\n\n143\n catalyst, further proving its wide application in hydrogenation reactions.Hu and co-workers summarized the Horiuti-Polanyi mechanism and non-Horiuti-Polanyi mechanisms in hydrogenation catalysis from DFT calculations.\n144\n\n,\n\n145\n The universality of the Horiuti-Polanyi routine, proposed 100 years ago, has been confirmed over various types of catalysts. For the catalysts showing weak adsorption or dissociation ability of dihydrogen, for example Ag(211) and Ni(111), the prevalence of associative mechanism may occupy the dominating role.Proposed by Eley and Rideal in 1938, the Eley-Rideal mechanism illustrates the route that only one of the reactant molecules adsorbs on catalyst surface and the other one participates in the reaction without adsorption (usually from the gas phase).\n46\n\n,\n\n146\n The reaction scheme of C\u2261C bond hydrogenation is briefly described in Figure\u00a05B. Typically, the Eley-Rideal mechanism can be divided into two categories: (1) adsorbed alkynes molecules, \u2217C\u2261H reacting with dihydrogen in the gas phase, H2(g); (2) adsorbed hydrogen molecules, \u2217H2 reacting with alkynes in the gas phase, C\u2261H(g).\n\n(1)\n\u2217C\u2261H reacting with H2(g)\n\n\n\n\u2217C\u2261H reacting with H2(g)\nIt is acknowledged that the Eley-Rideal mechanism is extensively applied in the hydrochlorination of acetylene, in which the \u2217C2H2 reacting with HCl(g).\n147\n However, as for the selective hydrogenation process, the activation and dissociation of H2 molecules remains very significant in the hydrogenation process.\n54\n Thus, the reaction pattern of \u2217C2H2 reacting with H2(g) is rarely seen in alkyne hydrogenation. In a very early study,\n148\n Butt and co-workers investigated the performance of benzene hydrogenation over supported Ni catalysts. The kinetic results of benzene hydrogenation matched well with a typical Eley-Rideal mechanism, proceeding via the molecular addition of dihydrogen to adsorbed benzene.However, there remain some controversies about the feasibility of Eley-Rideal mechanism (\u2217C\u2261H reacting with H2(g)), especially in terms of alkyne hydrogenation. The pathway of adsorbed acetylene reacting with molecular dihydrogen from the gas phase was found to be inapplicable by Hafner et\u00a0al. using DFT calculations.\n149\n\n,\n\n150\n The high activation energy of 200\u00a0kJ/mol as well as the strong Pauli repulsion between dihydrogen and acetylene molecules suppressed the impending hydrogenation process theoretically. Therefore, more straightforward approaches to distinguish the peculiar Eley-Rideal mechanism and to investigate the origin thereof are urgent required.\n\n(2)\nC\u2261H (g) reacting with \u2217H2\n\n\n\nC\u2261H (g) reacting with \u2217H2\nAs mentioned above, the dissociation of dihydrogen represents an essential step in the hydrogenation of alkynes, whether in homolytic or heterolytic patterns.\n11\n Therefore, the hydrogenation pattern of adsorbed dihydrogen molecules reacting with alkynes in gas phase is easier to accept in the Eley-Rideal mechanism. For example, Zheng et\u00a0al. demonstrated that the poisoned Pd-based catalyst, Pd4S@SPhF2, might follow the typical reaction scheme.\n10\n The direct participation of internal alkynes from the gaseous phase might be speculated since neither PhC\u2261CCH3 nor PhCH\u00a0= CHCH3 could adsorb on the catalyst surface.There remain some controversies about the feasibility of the Eley-Rideal mechanism. Hafner et\u00a0al. assumed the pathway of adsorbed acetylene reacting with molecular dihydrogen from the gas phase using DFT calculations.\n149\n\n,\n\n150\n This route was claimed to be inapplicable in alkyne hydrogenation process limited by the high activation energy of 200\u00a0kJ/mol as well as the strong Pauli repulsion between dihydrogen and acetylene molecules. Therefore, more straightforward means are required to distinguish the peculiar Eley-Rideal mechanism and to investigate the origin thereof.For a better understanding of the selective hydrogenation reaction, comprehensive characterization of the catalytic configuration and reaction process is highly desired, which is a hot and challenging topic. For the structure and configuration of catalysts, several advanced characterization strategies, such as Cs-corrected HAADF-STEM;\n26\n\n,\n\n27\n FTIR spectroscopy with CO adsorption;\n22\n and XAS, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES),\n17\n\n,\n\n18\n\n,\n\n22\u201333\n\n,\n\n59\n\n,\n\n91\n have been extensively discussed, especially for SACs.\n151\n Herein, some representative characterization techniques relevant to the hydrogenation process and structure-performance relationship investigation are summarized, including in situ FTIR,\n32\n\n,\n\n33\n TPD,\n30\n pulse-response experiments,\n32\n\n,\n\n50\n H2-D2 exchange,\n24\n\n,\n\n30\n and solid-state NMR.\n53\n\n,\n\n54\n\n\nIn situ FTIR is supposed to be a fast and sensitive characterization technique, since it provides accessible information on intermediate species through the vibrations of organic functional groups.\n31\n\n,\n\n47\n Li et\u00a0al. reported the in situ temperature-dependent FTIR spectroscopy of acetylene hydrogenation over Ni@CHA catalyst.\n33\n The competitive adsorption of reactants, i.e., dihydrogen and acetylene, could be analyzed, providing useful information on the detail reaction pathway.\n32\n\n,\n\n33\n As shown in Figure\u00a06A, the chemisorbed acetylene on Ni@CHA (3,010 and 2,925\u00a0cm\u22121) could be weakly captured only after the pretreatment with helium but completely disappeared after the pretreatment of dihydrogen. That is, the Ni (II) sites showed stronger affinity of dihydrogen that acetylene and subsequently hindered the adsorption of latter. On the contrary, the stretching bands of C\u2013H (acetylene) were strongly bonded to Ni(II) sites in Ni@FAU whether pretreatment was in helium or dihydrogen. The distinct adsorption behaviors toward reactants over Ni@CHA and Ni@FAU may imply the diverse hydrogenation mechanisms.As mentioned above, the Ni(II) species confined within faujasite and chabazite displayed different adsorption affinity of acetylene and dihydrogen shown by FTIR spectra (Figure\u00a06A).\n32\n\n,\n\n33\n The intrinsic competitive adsorption behaviors on Ni@FAU and Ni@CHA can be interpreted from TPD experiments. As shown in Figure\u00a06B, the desorption temperature of dihydrogen was higher than that of acetylene over Ni@CHA catalyst, indicating the stronger affinity of dihydrogen than acetylene. It revealed that the dihydrogen could be easily dissociated over Ni@CHA with trace hydrogen spillover around the zeolite (small peak at \u223c500 K). In contrast, the Ni@FAU catalyst only exhibited a moderate desorption peak of acetylene but no desorption signals of dihydrogen, suggesting the very weak adsorption of dihydrogen. On the premise of the different desorption behaviors between Ni@FAU and Ni@CHA, two different hydrogenation mechanisms were disclosed.\n32\n\n,\n\n33\n\n,\n\n57\n Similarly, on the premise of the TPD investigation, Zheng et\u00a0al. disclosed that the Pd@SPhF2 exhibited sore adsorption of dihydrogen but no desorption peak of alkynes (PhC\u2261CCH3), suggesting a typical Eley-Rideal mechanism.\n10\n\nThe dissociation of dihydrogen is acknowledged to be prerequisite in most hydrogenation reactions. The kinetic isotopic effect can provide strong evidence that the dissociation of dihydrogen is the rate-determining step, which also complies with the strong Pauli repulsion between dihydrogen and acetylene.\n149\n\n,\n\n150\n Additionally, the dissociation sites can be investigated by comparing the HD formation rate. Lu and co-workers compared the HD formation rate over Pd1-O/graphene and Pd-NPs/graphene via H-D exchange.\n23\n As shown in Figure\u00a06C, the HD formation rate over Pd-NPs/graphene was about 12 times higher than that over Pd1-O/graphene, indicating that the hydrogen dissociation over Pd1 single atoms was extremely hindered and the rate-determining step could be identified. The HD formation rates decreased on both catalysts after the introduction of butadiene, which could be attributed to the stronger adsorption of butadiene on Pd1 single atoms and Pd-NPs than dihydrogen.Choi and co-workers performed the H-D isotope exchange over Pd/PPS and Pd/SiO2 catalysts.\n50\n As shown in Figure\u00a06D, the absence of hydrogen activation ability over Pd/PPS was verified since no HD formation could be detected when feeding an H2-D2 mixture to the catalyst. The signal of HD appeared immediately upon the feeding of acetylene, revealing the activation of dihydrogen in the presence of co-adsorbed acetylene. The H-D isotope exchange results provided strong evidence for the alkyne-promoted hydrogenation process over Pd/PPS catalyst (i.e., the associative mechanism). Similarly, Li et\u00a0al. disclosed the strong affinity of acetylene over Ni@FAU catalyst and confirmed the alkyne-promoted mechanism though H2-D2-C2H2 pulse-response experiments as discussed previously (Figure\u00a05E).\n32\n\nThe heterolytic dissociation of dihydrogen favors the high selectivity toward ethylene owing to the lack of \u03b2-H species on metal surface. However, it is difficult to capture the M-H\u2212 species limited by the characterization methods. Theoretically, the solid-state 1H magic-angle spinning (MAS) NMR spectroscopy can provide clear evidence on the heterolytic cleavage of dihydrogen. Geoffrey and co-workers demonstrated that the dihydrogen underwent heterolytic cleavage on the In2O3-x(OH)y catalyst, forming In-OH2\n+ and In-H\u2212, respectively.\n54\n The stretching bands of In-OH2\n+ and In-H\u2212 attributed to 1,220 and 1,300\u00a0cm\u22121 could be clearly captured through FTIR spectroscopy. In the 1H MAS NMR measurements (Figure\u00a06E), the chemical shifts at 4.05 and 1.14 ppm, attributed to the In-OH2\n+ and In-H\u2212, respectively, could be captured at room temperature, in good consistency with FTIR results. On these grounds, the NMR measurement makes a quantitative compensation for the characterization of dihydrogen hydrolysis. Recently, Zheng and co-workers demonstrated that the dihydrogen could be heterolytically dissociated into the frustrated hydrogen pair over defective Rh2S3-x catalyst.\n53\n The frustrated hydrogen pair could stereo-selectively mediate the cis-to-trans isomerization of alkene via 1H MAS NMR. As shown in Figure\u00a06F, the chemical shift signals at 6.25 and 6.13 ppm corresponding to the \u03b1-C-H (Hc) and \u03b2-C-H (Hd) confirmed the formation of trans-1-phenyl-1-propene, with weak signals at 6.32 and 5.65 ppm attributed to the \u03b1-C-H (Ha) and \u03b2-C-H (Hb). The deuterium labeling experiments (Figure\u00a06F) demonstrated that the intensity of Hd decreased drastically after the D2 was charged while the intensities of others specie kept nearly unchanged. In such a way, the isomerization process was illustrated where the alkene inserted into the metal-D bond with the elimination of the original \u03b2-C-H (Hd) after the rotation of C\u2013C single bond.In addition to the above-mentioned experimental protocols, DFT calculations play a significant role in studying the selective hydrogenation process, providing comprehensive information from precise structure of catalyst to the reaction mechanism as well as structure-performance relationship. First, the structure and configuration of heterogeneous catalysts such as zeolite,\n33\n oxides,\n86\n\n,\n\n128\n carbon-nitride,\n28\n and alloys\n36\n can be optimized and modeled. Li et\u00a0al. optimized the structure of Ni@CHA by calculated energies, and Ni2+ sites were found to sit stably in the six-membered rings with Al atoms in the para or meta position (Figure\u00a07A). Second, various modes of reactant adsorption as well as the dissociation of dihydrogen among multiple active sites can be interpreted. For instance, Zheng et\u00a0al. measured different energy barriers of dihydrogen spillover among different Cu sites in Pd/Cu\n86\n catalyst\n36\n (Figure\u00a07B). The small energy barriers (0.11\u20130.25 eV) verified the facile spillover of H atoms. Third, the spectroscopy signature of adsorption and reaction intermediates can be validated. As shown in Figure\u00a07C, the stretching bands of CH2\u2217 and CH3\u2217 were well predicted via DFT calculations, in accordance with the experimental observations and assignments.\n33\n Noteworthily, with the optimized structures, the hydrogenation routine can be predicted accurately. For example, Li et\u00a0al. compared the energy barrier of Horiuti-Polanyi mechanism and associative mechanism in acetylene hydrogenation over Ni@FAU, revealing the priority of associative mechanism (Figure\u00a07D). This is in line with the H2-D2-C2H2 pulse-response experiments where dihydrogen is activated with the assistance of adsorbed acetylene molecules.Finally, DFT calculations can provide multiple insights for the rational design of hydrogenation catalysts. N\u00f8rskov et\u00a0al. performed DFT calculations to identify relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces.\n60\n As a result, cheap Ni-Zn alloys were predicted to be efficient catalysts for acetylene semi-hydrogenation (Figures\u00a03F and 3G), which were successfully verified by experimental studies. Hopefully this strategy might change the current trial-and-error mode of catalyst development.In this review article, we have summarized recent progress in the selective hydrogenation of C\u2261C bonds to the corresponding C=C bonds. This is not only an industrially relevant process known as semi-hydrogenation but also a popular model reaction. Efficient heterogeneous catalysts are being pursued and the detailed mechanism is still hotly discussed. Various strategies, including covering/poisoning the corner/edge active sites by organic compounds, segregation of active sites by adding the second metal, and the site-isolation approach by forming SACs, were discussed with concrete examples. Then, some key issues in the semi-hydrogenation process, for example the thermal run-away in front-end hydrogenation, the formation of green-oil and coke in tail-end hydrogenation, the solubility of dihydrogen, and the potential leaching of active sites in liquid-phase hydrogenation, are listed, which remain key challenges in industrial semi-hydrogenations. Finally, the detailed reaction mechanism of selective hydrogenation was discussed. Typical mechanisms, including the Horiuti-Polanyi mechanism, associative mechanism, and Eley-Rideal mechanism, are compared and the origin thereof discussed. Understanding reaction mechanisms and the further structure-performance relationships undoubtedly gives huge benefits to the rational design of robust catalysts for semi-hydrogenations. To achieve this goal, some important research methodologies, including spectroscopic investigations and theoretical calculations, were briefly detailed.The semi-hydrogenation of acetylene to ethylene in the gas phase and the semi-hydrogenation of MBY to MBE in the liquid phase have been industrialized on a large scale for decades, both using Pd-based catalysts. The innovation of catalytic materials offers great opportunities to construct a new generation of semi-hydrogenation catalysts with improved performance and economy. SACs appear to be ideal solutions to the new semi-hydrogenation processes. However, there is still a long way from the laboratory to industry. Objectively speaking, new semi-hydrogenation catalysts with comprehensive performance (substrate conversion, product selectivity, catalytic stability, and catalyst cost) surpassing the commercial ones are yet to be reported and confirmed. Apart from the thermo-catalytic semi-hydrogenation systems, electrocatalytic and photocatalytic or photothermal catalytic semi-hydrogenations are attracting growing attention in recent studies. For example, layered double hydroxide (LDH)-derived copper catalysts\n152\n and Cu dendrites deposited through an electrochemical method\n117\n show remarkable performance in the electrocatalytic semi-hydrogenation of acetylene to ethylene. Pd1/TiO2,\n153\n Au-Pd/C-TiO2,\n154\n and Pd1/N-graphene\n155\n catalysts show good performance in the semi-hydrogenation of acetylene under photothermal irradiation. Interestingly, Pt/TiO2 photocatalyst shows high substrate conversion and styrene selectivity in the hydrogenation of phenylacetylene under 385-nm monochromatic light irradiation, in significant contrast to the thermo-catalytic process.\n156\n These achievements might pave the way to new semi-hydrogenation processes, especially in the liquid phase.This work was supported by the National Natural Science Foundation of China (21872072, 22025203), the Frontiers Science Center for New Organic Matter, Nankai University (63181206), and Haihe Laboratory of Sustainable Chemical Transformations, Tianjin.X.D. and L.L. proposed the outline and completed the writing. J.W. arranged the table and collected the copyright. N.G. and L.L. revised the manuscript.The authors declare no competing interests.", "descript": "\n                  The selective hydrogenation of carbon-carbon triple bonds to the corresponding double bonds, the semi-hydrogenation process, plays a very important role in polymer and fine chemical industry. Various heterogeneous catalysts have been exploited for the selective hydrogenation of alkynes and alkynols in the gas and liquid phase. Herein, the recent progress in developing semi-hydrogenation catalysts, from traditional Pd-based monometallic catalysts to intermetallic compounds and single-atom catalysts, is summarized. The activation of dihydrogen during hydrogenation and the full hydrogenation mechanism, along with relevant research methodologies, are discussed. This review provides a comprehensive overview on the catalysts and mechanisms of industrially relevant semi-hydrogenation processes, addresses some existing debates, and sheds light on future catalyst design for hydrogenation.\n               "}
{"full_text": "3D metal-embedded microporous carbocatalystsPre-exponential factorsPristine biocharBrunauer-Emmett-TellerBarret\u2013Joyner\u2013HalendaCatalytic fast pyrolysisCatalytic fast co-pyrolysisCarbon NanotubesCorn strawDeionizedDerivative thermogravimetryActivation energyEnergy dispersive X-ray spectroscopyFe-embedded microporous carbon catalystFourier transform-infraredHydrogen to carbon effective ratioHigher heating valueLow-density polyethyleneLower heating valueMonocyclic aromatic hydrocarbonsNi-embedded microporous carbon catalystPolycyclic aromatic hydrocarbonsPellet biocharPolyethylene terephthalatePolypropylenePolystyrenePolyvinyl chloridePlastic wasteThermogravimetryThermogravimetric analysisTemperature programmed oxidationScanning electron microscopyTransmission electron microscopeWheat strawWS and PE blendWS, PE, and catalysts blendsWS and PW blendWS, PW, and catalysts blendsX-ray diffractometerX-ray photoelectron spectroscopyZn-embedded microporous carbon catalystBiomass is regarded as one of the most renewable and sustainable energy sources with huge reserves [1]. Exploration of biofuels and well-defined chemicals from biomass through fast pyrolysis has received extensive attentions [2]. At present, most efforts were devoted to further upgrading bio-oil due to the high oxygen content, low heating value, low pH value, etc.[3]. Alternatively, catalytic fast pyrolysis (CFP) is one of the most prevailing and promising techniques for the conversion of biomass directed toward valuable biofuels and chemicals [4].However, the CFP technology is commonly plagued by the low yield of target products and rapid deactivation of catalysts [5,6]. These huge challenges are mainly related to the low hydrogen to carbon effective (H/Ceff) ratio of biomass [7\u20139]. To mitigate these issues, it is reasonable to incorporate hydrogen-rich co-reactant with biomass in CFP process to modify the reactions of oxygen elimination by substituting decarbonylation and decarboxylation with dehydration [7,10]. Tremendous quantities of plastic waste produced each year can represent a cheap and abundant hydrogen source to be co-fed with biomass during the CFP process [11]. It is discerned that co-feeding of biomass with plastic waste in CFP process is remarkably beneficial for the environment and energy recapture [5,6,12].Zeolites and metal oxides are the most extensively used and the highest-efficiency catalysts to manufacture considerable petrochemicals (aromatics and olefins) [13]. However, these catalysts are expensive and still suffer from relatively severe deactivation due to the low anti-deactivation abilities [5,14]. A lot of efforts have been devoted to developing alternative carbonaceous catalysts due to the adequate environmentally benign nature, biocompatibility, and great tolerance to coke deposition [15,16]. Diverse carbonaceous materials, including activated carbon, carbon nanotubes, carbon dots, and graphene, and graphene oxide, are recognized as the very promising carbocatalysts [17,18]. Yet, the commonly used carbonaceous precursors for the synthesis of carbocatalysts present the huge limitations considering the economic feasibility, sustainability, and scalable production [16].Alternatively, biochar as the emerging carbonaceous material from biomass pyrolysis or gasification has attracted considerable attention in catalysis due to its low cost, tunable pore structure, and high stability [19]. For instance, biochar has been used as a promising catalyst for CFP of biomass [20], which could contribute to the generation of aromatics by catalyzing the deoxygenation reaction to eliminate the oxygen content in the resulting liquid product [21]. Besides, biochar has been reported as an inexpensive catalyst with fair performance in tar removal [22]. The deactivated biochar catalyst could be readily disposed by gasification or combustion for the recoveries of energy and loaded metals [22]. However, biochar usually possesses a low specific surface area, pore volume, and thermal stability, hindering the applications of biochar for effective catalysis [23].Recently, many efforts have been also devoted to developing three-dimensional (3D) porous structures and improving the catalytic abilities of biochar by incorporating metals species on the surface of biochar [22,24]. Metal chlorides (e.g., ZnCl2) with pore-forming ability are scalable and green porogens to fabricate biochar with 3D porous structures [24]. The metal dopant on biochar was able to create efficient active sties (Lewis acid sites), and the 3D porous structure allowed more active sites to be exposed, both of which were commendably desired for catalysis [25]. Thus, 3D porous biochar doped with the catalytically active sites (metal or metal oxides nanoparticles) could endow biochar with a significant increase in catalytic abilities [26].It is discerned that biochar is a hard carbon material mainly containing amorphous carbon atoms [23]. In contrast to amorphous carbon, graphitic carbon is usually composed of a hexatomic ring lattice, giving rise to a high specific surface area [27] and thereby increasing the number of active sites for catalysis [28]. If biochar can be modulated with a graphitic structure, the properties of biochar will be significantly improved [29]. Recently, some studies have also introduced graphitic carbon into amorphous carbon matrix, which exhibited excellent catalytic activities for electrocatalysis [30]. More importantly, it was reported that the 3D porous graphitic biochar exhibits an outstanding advantage of combining high stability and porous structure [31]. The high graphitization degree and abundant porosity of 3D porous graphitic biochar indicated an outstanding photocatalytic activity [32]. Biochar with highly-perfect graphitization and porous structure could also serve as a good support for the introduction of metal nanoparticles, which was beneficial for the effective photocatalysis [32].It should be also noted that the catalyst pore structure play a very crucial role in the CFP of biomass, micropores with the pore size of c.a. 0.52\u20130.59\u00a0nm was conducive to the higher aromatic yield, while the mesopores and macropores resulted in the higher coke and lower aromatic production [33]. Our previous study has also evidenced that the microporous structure of the catalyst contributed to the generation of monocyclic aromatic hydrocarbons [6]. Interestingly, it has been found that biomass pelletization is a promising pretreatment way for biochar production with compressed structures [34]. Biomass pelletization could rearrange the same particles and the original cell structure could be compressed [35], resulting in pellet biochar (PBC) developed with more compressed microporous structures.Hence, developing 3D metal-embedded microporous carbocatalysts (denoted as 3DMeMCs) with excellent catalytic activities has motivated our curiosity. Prior to the carbonization of biomass pellets, corn stover was selected as the carbonaceous precursor for pelletization due to its extensive abundance in agricultural wastes. Then we develop a single-step energy-efficient strategy, using three widely used metal chlorides (ZnCl2, FeCl3, and NiCl2), to fulfil the synchronous pore-forming, metal-doping, and graphitization for synthesizing 3DMeMCs. The simplified and facile synthesis route can remarkably reduce the consumptions of energy and chemicals. The intrinsic (surface chemistry, degree of graphitization, etc.) and extrinsic (specific surface area, morphology, etc.) characteristics of the as-synthesized catalysts were instigated by using a series of characterization techniques including SEM, EDX, TEM, TPO, TGA, FTIR, XRD, XPS, and Raman.To the best of our knowledge, the development of 3DMeMCs for catalytic fast co-pyrolysis of biomass and plastic waste has not been researched yet. It is discerned that H2 emerging as a clean and eco-friendly energy carrier; and syngas as a mixture of H2 and CO is receiving extensive attention due to its high calorific value and board applications as a precursor for the Fischer-Tropsch synthesis [36]. Carbon nanotubes derived from plastic waste possesses a variety of applications due to unique physicochemical properties [37]. Therefore, the catalytic abilities of the as-synthesized catalysts were subsequently examined in the on-line and ex-situ catalytic fast co-pyrolysis of biomass and plastic waste for aromatics, syngas, as well as valuable carbons under different scenarios. To further explore the catalytic performances and synergistic effects during co-pyrolysis, the thermal decomposition behaviors of individual reactant and co-reactants were investigated by thermogravimetric analysis (TGA), followed by the calculations of kinetic parameters. Eventually, a plausible reaction mechanism regarding practical ex-situ catalytic fast co-pyrolysis of biomass and plastic waste over 3DMeMCs were proposed in terms of the experimental observations.Corn stover (CS) and wheat straw (WS) were supplied by the Shangzhuang experimental station of the China Agricultural University. The proximate and ultimate analysis of CS and WS were given in Table S1. Both CS and WS were initially dried at 105\u00a0\u00b0C to remove the physically bound moisture until the weights were constant. Afterwards, they were pulverized using a high-speed miller (RT-34, Taiwan RongCong Precison Technology Co., China) into the particle size of 20 \u2013 40 mesh. Low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) in the form of 100 mesh powder were all purchased from Huachuang Plastic Ltd., China. 40\u00a0wt% of LDPE, 35\u00a0wt% of PP, 18\u00a0wt% of PS, 4\u00a0wt% of PET and 3\u00a0wt% of PVC were thoroughly mixed to simulate real-word plastic waste. All chemicals and reagents, including HCl (12\u00a0mol/L), ethanol (>99.7%), ZnCl2, FeCl3\u00b76H2O, and NiCl2\u00b76H2O, were supplied by Beijing Lanyi Chemical Co., Ltd. (Beijing, China).Biomass pellets (approximately 4\u00a0mm in diameter and 12 \u2013 14\u00a0mm in length) were manufactured with the raw ground CS by a 9KLP-125 pelleter (Yongfeng Machinery Equipment Co., Ltd, Henan, China). The biomass pellets were carbonized at 500\u00a0\u00b0C under N2 atmosphere with a heating rate of 10\u00a0\u00b0C/min and maintained for 1\u00a0h. After cooling to room temperature, the pellet biochar was crushed and sieved to 20 \u2013 30 mesh for the future use. The pulverized pellet biochar was named as PBC. The raw ground CS was also carbonized at the same condition to achieve pristine biochar (denoted as BC) for comparison.The synthesis procedures of 3DMeMCs are depicted in Fig. 1\n. PBC was leveraged as the precursor of 3DMeMCs. Zn-embedded microporous carbon catalyst (termed as Zn@C) was synthesized starting with ZnCl2 dissolved in deionized (DI) water, and PBC was then immersed into the ZnCl2 solution with the ZnCl2 to PBC weight ratio of 1:1. Inspired by Zhu et al., both ZnCl2 and FeCl3 were used as the activating agent and functional material [38]. Similarly, the Fe-embedded microporous carbon catalyst (Fe@C) and Ni-embedded microporous carbon catalyst (Ni@C) were prepared in light of the procedure but using FeCl3 and NiCl2 as substrates, respectively. The three slurries were stirred for 4\u00a0h at 1000\u00a0rpm and set for overnight. The slurries were subsequently air-dried at 105\u00a0\u00b0C for 12\u00a0h; and then these solid mixtures were annealed at 700\u00a0\u00b0C for 4\u00a0h under N2 flow of 100\u00a0mL/min at a heating rate of 10\u00a0\u00b0C/min. The obtained solid mixtures were successively with 0.1\u00a0mol/L HCl, ethanol (washing the constituents that undissolved in water), and DI water to remove the metal chlorides left and organic matters on the surface, prior to ultimately being dried at 105\u00a0\u00b0C. Finally, these composites defined as 3DMeMCs were cooled to room temperature under N2 flow and the 3DMeMCs were stored for the subsequent catalytic tests.The hyphenated technique of Pyrolyzer (Py-3030D, Frontier Laboratories Ltd., Japan) and gas chromatography/mass spectrometry (7890A/5975C inert, Agilent technology) were applied to conduct the catalytic fast pyrolysis and analyze the products synchronously. WS was well mixed with PW or PE in a mass ratio of 1:1 for co-pyrolysis. As such the reactants and catalysts were also blended conformably at the mass ratio of 1:1 for catalytic fast co-pyrolysis (CFcP). 2\u00a0mg of samples were subjected to the on-line catalytic fast co-pyrolysis system each time. The reaction temperature for all runs was implemented at 500\u00a0\u00b0C and reaction time was kept at 20\u00a0s. High-purity He (99.99%) was utilized as the inert gas can carried gas at a constant flow of 1\u00a0mL/min, and the split ratio was set at 1:150. The temperature of gas transmission pipeline between the micro-pyrolyzer and GC injector was maintained at 250\u00a0\u00b0C, while the GC/MS injector temperature was set at 270\u00a0\u00b0C. An HP-5MS capillary column (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25 um) was used to identify the hot pyrolysates. For MS analysis, the mass-to-charge ratio was kept in the range of 35 \u2013 550\u00a0m/z, electron ionization was set at 70\u00a0eV. Compounds were identified by comparing the spectral data with that in the NIST Mass Spectral library.The ex-situ CFcP of WS and PW was performed to evaluate the catalytic capacities of as-synthesized PBC and 3DMeMCs for desired products by using a two-stage tube furnace reactor, as shown in Fig. S1. 1.5\u00a0g of WS and 1.5\u00a0g of PW in a mass ratio of 1:1 by sharking to obtain the mixture as constant feedstock for fast co-pyrolysis; approximately 1.5\u00a0g of catalyst was first placed in the catalytic fixed-bed into the quartz tube (inner diameter 20\u00a0mm; length 550\u00a0mm). The quartz wool was utilized to separate and hold the catalyst and the blend in place. Prior to all tests, the ex-situ CFcP system was purged with Ar at a flow rate of 30\u00a0mL/min for 20\u00a0min to remove the air present in the system. The region of fast pyrolysis was set at 500\u00a0\u00b0C, while the catalytic region was maintained at either 500 or 800\u00a0\u00b0C. To achieve relatively consistent temperatures, the pyrolysis and catalysis beds were pre-heated and equilibrated at the set temperature for 10\u00a0min. Thereafter, the pyrolysis bed was pushed into the furnace to initiate the experiments and kept for 10\u00a0min. After the experiment, the condensers and adaptors were then washed with ethyl acetate (10\u00a0mL) in small aliquots. The ethyl acetate-soluble organic phase was named as bio-oil, while the water-soluble fraction was the aqueous phase product. In the meantime, bio-oils were recovered by flowing the air over the ethyl acetate-soluble phase at room temperature for 4\u00a0h to ensure the complete evaporation of ethyl acetate. In this study, the aqueous phase products were not considered for analysis due to the small amounts. The non-condensable pyrolytic vapors escaped as gas at the end of the condensers were collected for analysis. All ex-situ CFcP experiments were operated in triplicate; the product yields were calculated as the mean values of three tests, and standard deviations were all less than 5%.Thermogravimetric analysis (TGA) of WS, plastics (PE, PP, PS, PET, PVC, and PW) as well as their blends was carried out by using a thermogravimetric analyzer (SDT Q600, TA Instruments, USA) to investigate their thermal degradation behaviors. For each experiment, \u223c5\u00a0mg of the sample was placed in an alumina crucible and heated from room temperature to 600\u00a0\u00b0C at a heating rate of 20\u00a0\u00b0C/min with a N2 flow rate of 50\u00a0mL/min. TGA of WS or plastics alone were first conducted. As for the measurements concerning thermal degradation of WS and PW or PE blends, the WS to PW/PE mass ratio was set as 1:1. In addition, the same thermogravimetric analyzer was employed to conduct TGA for catalytic co-pyrolysis of co-reactants with the catalysts. The co-reactants to catalyst mass ratio was also set at 1:1.The elemental compositions (C, H, N, and S) of the reactants, carbonaceous materials, and bio-oils were tested by using an elemental analyzer (Elementar Vario ELIII, Germany). The amount of moisture, volatile matter, and ash were analyzed by using a fully automatic measuring industrial analyzer (YX-GYFX 7705B, U-Therm, China). The fixed carbon was determined by the subtraction method.The surface morphology of these carbonaceous materials was characterized by a scanning electron microscopy (SEM, SU3500, Hitachi Ltd., Japan). The elemental compositions and distributions in these carbonaceous materials were analyzed through the energy dispersive X-ray spectroscopy (EDX, SDD3310, IXRF Systems, USA) connected with the SEM. A transmission electron microscope (TEM, FEITecnaiG2F30) operated at 200\u00a0kV was used to further characterize the surface morphology of the carbonaceous materials. The textural properties of the carbonaceous catalysts were measured with N2 adsorption\u2013desorption isotherms at a liquid nitrogen temperature of 77\u00a0K using TristarII3020 (Micromeritics, USA). All the catalysts were degassedin vacuum at200\u00a0\u00b0Cfor6\u00a0h. The Brunauer-Emmett-Teller (BET) equation was used to calculate the surface area, the desorption branch of the isotherm was utilized to calculate the pore size distribution, and the pore volume was measured according to theBarret\u2013Joyner\u2013Halenda(BJH) method.Temperature programmed oxidation (TPO) of these carbonaceous materials were operated by using the thermogravimetric analyzer (SDT Q600, TA Instruments, USA) to investigate the thermal stabilities and char-inorganics quantifications of the carbonaceous materials. All TPO experiments were conducted in the synthetic air (N2:O2\u00a0=\u00a080/20 v/v%) flowing at 50\u00a0mL/min with a temperature ramping rate of 20\u00a0\u00b0C/min up to 600\u00a0\u00b0C. The TGA of these carbonaceous materials was also carried out by using the thermogravimetric analyzer under N2 atmosphere to further evaluate the thermal stabilities of the carbonaceous materials that would be used as catalysts during the CFcP process. The chemical structures and metal species on the carbonaceous materials were determined by X-ray diffractometer (XRD, XD-3, Persee Ltd., Beijing, China) operated at 36\u00a0kV and 20\u00a0mA, 2\u03b8\nscale of 10\u201380\u00b0 at a step size of 0.02\u00b0, and a scanning speed of 2\u00b0/min. The graphitization degree of the carbonaceous catalysts was analyzed by using a Raman spectrometer (SENTERRA \u2161, BRUKER, Germany) at ambient temperature. The spectra (Raman shift from 200 to 3500\u00a0cm\u22121) were recorded with spectrograms at a wavelength of 532\u00a0nm.The functional groups of the carbonaceous materials were characterized by Fourier transform-infrared (FTIR) spectroscopy (Spectrum 400, PerkinElmer, USA). The spectra were recorded at a range of 400 \u2013 4000\u00a0cm\u22121. Surface chemical compositions were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi, ThermoFisher Scientific) equipped with a dual anode monochromatic K\u03b1 excitation source. The binding energies of all elements were corrected against the adventitious carbon C 1s core level at 284.8\u00a0eV. The deconvolution of the XPS spectra was performed by using the XPS peak4.1 software [39].The chemical compositions of bio-oils were analyzed by a GC\u2013MS (QP2010 SE, Shimadzu, Japan) with RTX-5MS (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u00b5m) capillary column. The GC was initially heated to 60\u00a0\u00b0C for 5\u00a0min and then programmed to heated to 270\u00a0\u00b0C at a rate of 5\u00a0\u00b0C/min and maintained at 250\u00a0\u00b0C for 5\u00a0min. The injection volume was 1\u00a0\u00b5L and injection temperature was held at 300\u00a0\u00b0C. The interface temperature was adjusted to 280\u00a0\u00b0C and the ion source temperature was set at 200\u00a0\u00b0C for the mass selective detector. High-purity helium (99.999%) was used as the carrier gas at a stable flow of 3\u00a0mL/min, and the split radio was 1:10. The identification of bio-oils was determined according to NIST database of MS spectra library. The peak area percentages of chemical compounds based on GC/MS results were applied to predict the product selectivity. A mixture of C6 \u2013 C12 aromatic hydrocarbons ((\u226599.5%, Macklin, China) was used calibrated standards to quantify the main aromatic hydrocarbons in the bio-oils. All the GC/MS measurements were conducted in triplicated to assure reproducibility.All gaseous products were collected by a 2 L gas bag and then offline characterized by a Shimadzu GC-2014C gas chromatography (GC, Shimadzu Corp., Kyoto, Japan) with a thermal conductivity detector (TCD). A standard gas mixture consisting of H2, CO, CH4, CO2, C2H4, C2H6, C3H6, and C3H8 was employed to calibrate the proportion (vol.%) of gaseous compositions. The gas fractions (\u2265C3) were not observed or negligible in this study. All GC measurements were also implemented in triplicate to ensure reproducibility.The weight of bio-oils was determined on the basis the weight difference of the container before and after the test. The mass of biochar was calculated by the left residue in the pyrolysis bed, while the mass of gaseous products was measured by combining the total gas volume and the gas density of each gaseous fraction [39]. The mass of carbon (coke) deposited on the catalyst and waxy loss (CW) was defined as the following equation:\n\n(1)\n\n\nMass\n\no\nf\n\nC\nW\n=\nf\ne\ne\nd\ns\nt\no\nc\nk\n\nm\na\ns\ns\n-\nb\ni\no\no\ni\nl\n\nm\na\ns\ns\n-\nb\ni\no\nc\nh\na\nr\n\nm\na\ns\ns\n-\ng\na\ns\n\nm\na\ns\ns\n\n\n\n\nYields (wt%) of the bio-oil, biochar, and gas were determined according to the following formula:\n\n(2)\n\n\nYieldwt\n%\n=\n\n\nmass\n\no\nf\n\na\n\np\nr\no\nd\nu\nc\nt\ng\n\n\nmass\n\no\nf\n\nd\nr\ni\ne\nd\n\nf\ne\ne\nd\ns\nt\no\nc\nk\ng\n\n\n\u00d7\n100\n%\n\n\n\n\nThe carbon yield of bio-oils was defined as the following equations.\n\n(3)\n\n\nCarbon\n\ny\ni\ne\nl\nd\n\n\n\n\nC\n%\n\n\n\n\n=\n\n\nmoles\n\no\nf\n\nc\na\nr\nb\no\nn\n\ni\nn\n\nb\ni\no\n-\no\ni\nl\n\n\nmoles\n\no\nf\n\nc\na\nr\nb\no\nn\n\nf\ne\nd\n\ni\nn\n\n\n\u00d7\n100\n%\n\n\n\n\nH2 and syngas (H2 and CO) yields were expressed as the volumes of H2 and syngas generated divided by the total mass of mixed feedstock (eqs. (4) and (5)).\n\n(4)\n\n\n\nY\ni\ne\nl\n\nd\n\nH\n2\n\n\n\n\n\n\n\nNmL\n\ng\n\n\n\n\n=\n\n\nvolume\n\no\nf\n\n\nH\n2\n\n\nm\nL\n\n\nmass\n\no\nf\n\nd\nr\ni\ne\nd\n\nf\ne\ne\nd\ns\nt\no\nc\nk\ng\n\n\n\n\n\n\n\n\n(5)\n\n\nYiel\n\nd\n\nsyngas\n\n\n\n\n\n\n\nNmL\n\ng\n\n\n\n\n=\n\n\nvolumes\n\no\nf\n\n(\n\nH\n2\n\n+\nC\nO\n)\n\nm\nL\n\n\nmass\n\no\nf\n\nd\nr\ni\ne\nd\n\nf\ne\ne\nd\ns\nt\no\nc\nk\ng\n\n\n\n\n\n\nThe higher heating value (HHV) of bio-oils were determined by the modified Dulong formula as follow [40]:\n\n(6)\n\n\nH\nH\nV\n\n\n\n\nMJ\n/\nk\ng\n\n\n\n\n=\n0.3578\n\u00d7\nC\n+\n1.1357\n\u00d7\nH\n-\n0.0845\n\u00d7\nO\n+\n0.0594\n\u00d7\nN\n+\n0.119\n\u00d7\nS\n\n\n\n\nThe lower heating value (LHV) of the gaseous products was evaluated using the following equation [39].\n\n(7)\n\n\nL\nH\nV\n\n\n\nM\nJ\n/\nN\n\n\nm\n\n3\n\n\n\n\n=\n0.126\n\u00d7\nC\nO\n+\n0.108\n\u00d7\n\nH\n2\n\n+\n0.358\n\u00d7\n\n\nCH\n\n4\n\n+\n0.665\n\n\n\n\n\u00d7\nC\n\nn\n\nH\n\nm\n\n\n\n\nwhere CO, H2, CH4 and CnHm (\u2265C2) are the volume fractions of those species.The morphologies and microstructures of the as-synthesized materials were initially examined by using SEM images, as shown in Fig. 2\na\u2013e. BC reveals an irregular morphology and smooth surface with a very limited porous structure (Fig. 1a). With the aid of pelletization of biomass prior to carbonization, more exfoliation and pores started to emerge in resulting PBC (Fig. 1b), suggesting that pelletization could act as a pore-forming step in the carbonization process. Fig. 2c-e shows the representative SEM images of Zn@C, Fe@C, and Ni@C, respectively. The three as-synthesized catalysts were found to be comprised of a great number of interconnected sheets crosslinking each other, generating the apparent 3D porous structures. Compared to PBC, more abundant pores were generated within the 3DMeMCs. Interestingly, Fe@C and Ni@C had microspheres half-embedded or completely enclosed in the porous carbon matrix (Fig. 2d and e). Obviously, the well-developed porous structures of 3DMeMCs are beneficial to accelerate mass transfer, adsorption, and exposure of more active sites for catalysis.Further insight into the morphology and structures of as-synthesized materials was observed by the typical TEM images (Fig. 2f-i). TEM analysis substantiates the crosslinking of the three-dimensional structures of as-synthesized materials, giving rise to thin nanosheets of diverse thickness. Obviously, 3DMeMCs showed a typical 3D network with open-hollow and interconnected micropores (Fig. 2g-i), which were expected to provide more active sites for catalytic conversions. More specifically, Zn@C was endowed with a randomly stacked graphite-like feature with a high level of disorder, related to a high porosity and specific surface area. TEM images of Fe@C (Fig. 2h) and Ni@C (Fig. 2i) reveal a light-colored carbon matrix embedded with highly-dense and dark-colored fine particles distributed all over the carbon matrix. For the sake of Fe@C, the light-colored carbon matrix was thin, especially on its edges; the dark-colored particles were homogeneously distributed with the nanoscales. TEM image of Ni@C indicates that the Ni-impregnated biochar matrix consisted of film-like porous materials and nanoparticles. Based on the high-resolution TEM (HRTEM) images as shown in Fig. S2, the surface of PBC (Fig. S2a) was not decorated with nanoparticles, but only revealed microporous networks; Zn@C (Fig. S2b) presented a sp3\n-dominated structure with highly disorder. A relatively apparent lattice fringes with a distance of 0.24\u00a0nm appeared along the edges of Fe@C (Fig. S2c) and a much more obviously lattice fringes with a distance of 0.20\u00a0nm were observed throughout Ni@C (Fig. S2d), implying the high degree of crystalline of the synthesized Fe@C and Ni@C.The SEM-EDX (Fig. 2j-n) and elemental mappings (Fig. 2o-r) demonstrate the main coexistence of C and O in the as-synthesized materials. Fig. S3 show the extra elemental mappings, suggesting the presence of small amounts of trace minerals, i.e., Mg, P, K, Ca, etc. (for details, please see Table S2). As the metallic nanoparticles were prone to agglomerating to decrease the surface energy, thereby rendering a significant decrease in catalytic performances [41]. In this study, the EDX and elemental mappings have evidenced that the Zn, Fe, and Ni nanoparticles were homogeneously dispersed over the microporous carbon network of 3DMeMCs, contributing to the improvement of catalytic efficiencies. Interestingly, more non-agglomerated metal nanoparticles homogeneously penetrated insides the micropores or channels of the highly microporous carbon skeleton were obviously discerned for Ni@C than Zn@C and Fe@C. The subtle differences among the morphologies of 3DMeMCs were caused by different metallic precursors, resulting in different metallic nanoparticles.Further information on the porous structures of these materials were probed by N2 adsorption\u2013desorption isotherms at 77\u00a0K. The N2 adsorption\u2013desorption isotherms and pore size distribution of these carbonaceous materials are presented in Fig. 3\na and b, respectively. BC possess an IV-type isothermal curve with a small hysteretic loop of desorption branch in the relative pressure (P/P0\n) range of 0.45 \u2013 0.98 (Fig. 3a). The hysteresis loop of BC, extending from the medium relative pressure region, was attributed to the capillary condensation in the mesopores [42]. The observations strongly showed the coexistence of micropores and mesopores in BC. In contrast, PBC and 3DMeMCs show a type-I adsorption\u2013desorption isotherm with sharp adsorption inflections at low relative pressures and well-developed plateaus (Fig. 3a), suggesting the predominance of micropores within the carbonaceous materials. The rich microporous structures of 3DMeMCs might be derived from the defects or pores and slits among the graphite-like sheets [23]. It is also noted that the isotherms of Fe@C showed a sharper increase of N2 uptake at the low relative pressure region (P/P0\n\u00a0<\u00a00.1) than others, affirming the formation of a larger number of micropores with the activation of FeCl3\n[31]. As shown in Fig. 3b, BC exhibited a broad pore size distribution, with a maximum at approximately 4\u00a0nm, indicating the co-existence of micropores and mesopores. However, PBC and 3DMeMCs showed narrow pore size distributions in the range of 0.7 \u2013 1.3\u00a0nm with a sharp maximum at nearly 1.0\u00a0nm, substantiating the presence of well-developed microporous structures of PBC and 3DMeMCs. These results were in agreement with the adsorption\u2013desorption isotherms.The textural characteristics of these materials are summarized in Table 1\n. The BET surface area and porosity of these materials were entirely different. Due to the rugged surface feature, BC displays a low BET surface area (about 105\u00a0m2/g) and total pore volume (0.13\u00a0cm3/g), while the BET surface area and total pore volume of PBC were dramatically augmented to 539\u00a0m2/g and 0.27\u00a0cm3/g, respectively. Apparently, PBC formed undergoing biomass pelletization favored the generation of larger surface area and pore volume as compared to BC. Notably, the apparent BET surface area of 3DMeMCs was significantly dependent upon the species of metal ions. Ni@C with the modulation by NiCl2 integration possesses a lower BET surface area (645\u00a0m2/g) as compared to Zn@C (714\u00a0m2/g) and Fe@C (964\u00a0m2/g), which was ascribed to the dispersion and penetration of more active Ni species into the micropores of the carbonaceous skeleton, thereby resulting in the obstruction of the generation of micropores [41].Besides, the microporosity of 3DMeMCs could be manipulated by varying the types of metal ions. The micropore volumes of Zn@C, Fe@C, and Ni@C were 0.30, 0.42, and 0.28\u00a0cm3/g, respectively; the total pore volumes of these materials were 0.36, 0.48, 0.33\u00a0cm3/g, indicating that the proportions of micropores to the total porosity were determined to be 83, 88, 85%, respectively. There results suggested that 3DMeMCs exhibited well-developed microporosities, which were essential to accelerate the pyrolytic volatiles access to the micropores for rapid adsorption or desorption. Noted that Fe@C showed the highest porosity than Zn@C and Fe@C. The higher microporosity of Fe@C was attributed to more volatile matters and gaseous fraction under the high carbonization temperature (700\u00a0\u00b0C). Nevertheless, Ni@C exhibited the lowest porosity. This could be explained that the micropores induced by the NiCl2 and pore channels were blocked by the formed metallic Ni and nickel oxides. The collapse of the well-defined microporous structure caused by the catalytic graphitization of amorphous carbon also resulted in the decreases of surface area and pore volume. As demonstrated in Table 1, the average pore diameter (2.04\u00a0nm) of Ni@C was larger than that of Zn@C (2.01\u00a0nm) and Fe@C (1.99\u00a0nm), which validated the obstruction of micropores in Ni@C. Overall, the BET specific surface area, total pore volume, and micropore volume obey the following order: BC\u00a0<\u00a0PBC\u00a0<\u00a0Ni@C\u00a0<\u00a0Zn@C\u00a0<\u00a0Fe@C. Table 1 also lists the average pore diameter of these materials. The average pore diameter of BC (5.12\u00a0nm) was larger than that of PBC and 3DMeMCs. The average pore diameter (\u223c2.0\u00a0nm) of 3DMeMCs, most of which was derived from micropores, were beneficial for the diffusion and transport of pyrolytic volatiles.Thermal stability of these materials was tested by TGA, the plots of weight and derivate weight with regard to oxidation temperature are presented in Fig. 3c and d, respectively. As shown in Fig. 3c, the weight losses between 300 and 600\u00a0\u00b0C were caused by the oxidation of carbonaceous materials; however, the remaining residues were originated from minerals and incorporated metal oxides. PBC exhibits the largest weight loss (94.4%) during oxidation, indicating the least ash (including metal oxides) in the materials. The curve of Zn@C at the end experienced a weight loss of 91.7%, indicating that Zn@C was virtually free of impurities. However, the curves of Fe@C and Ni@C showed that 76.5 and 56.1% of weight were combusted, respectively. It was inferred that the large amounts of Ni and Fe components were retained after the catalyst synthesis. These trends were in line with the observations by XPS, EDX, and elemental analysis.The oxidation peaks in derivate weight plots represent the type of carbons and thermal stabilities, as shown in Fig. 3d. It has been reported that the oxidation peak at lower temperature is related to amorphous carbon, whereas the peak at higher temperatures is ascribed to graphitic carbon that is more stable and less reactive [43]. BC and PBC contained two distinct oxidation peaks. Compared to BC, PBC showed an increased decomposition temperature at around 400 and 430\u00a0\u00b0C, indicating the decomposition of oxygen-containing functional groups and more carbon content [44]. All 3DMeMCs revealed only one decomposition step with inflection temperatures; and the maximum temperatures of weight loss in the derivate peaks of 3DMeMCs shifted to higher oxidation temperatures as compared to PBC, indicating the high crystallinity of carbon nanomaterials in 3DMeMCs [45]. Zn@C and Ni@C, the onset and end temperatures were shifted to higher temperature (\u223c575\u00a0\u00b0C) than Fe@C (470\u00a0\u00b0C), implying that the amorphous carbon in Ni@C and Zn@C was transformed into semi-crystalline carbon with higher thermal stabilities [46]. This was well aligned with the observation by XRD and TEM that higher degree of graphitization was gained for Ni@C. Overall, the degree of graphitization according to TPO results follows the order of Ni@C \u2248 Zn@C\u00a0>\u00a0Fe@C\u00a0>\u00a0PBC\u00a0>\u00a0BC. Note that the carbon content in Zn@C was comparatively higher than that of Ni@C according to DTG intensity at around 575\u00a0\u00b0C. There was no weigh loss found in these materials after 600\u00a0\u00b0C, confirming that there was no secondary decomposition or weight loss of active products.\nFig. 3e further illustrates a TG and DTG curves of as-synthesized materials in N2 atmosphere and the curves (particularly Zn@C) was almost unchanged (less than 5% of weight loss) up to 600\u00a0\u00b0C, suggesting that 3DMeMCs presented superior thermal stabilities which enable them for utilization in the wide range of temperatures. However, the TG curve of PBC involved in Fig. 3e showed a higher weight loss (\u223c10%) due to the further pyrolysis of PBC in the range of 500 \u2013 600\u00a0\u00b0C. Thus, 3DMeMCs are proper for catalytic pyrolysis under the oxygen-free atmosphere.During the synthesis with relatively high temperature for 3DMeMCs, the initial amorphous carbon configuration (sp3\n-dominated) of PBC could be rearranged for graphitization in terms of its low-energy and stable properties [26], giving rise to the transformation of carbon configuration. More intuitive evidences were shown by XRD and Raman analysis (Fig. 3f and g). As demonstrated in Fig. 3f, there is no distinct peak found in the XRD pattern of BC, which is probably due to the destruction of its intrinsic structure. The baseline deflection (2\u03b8\u00a0<\u00a030\u00b0) was obviously found in the XRD patterns of PBC and Zn@C, evidencing the amorphous feature. Note that there was no Fe phase that was discerned in the XRD patterns of Fe@C, which probably resulted from the limited detection depth of XRD over encapsulated Fe species with thick carbon nanoshells or the amorphous Fe species on the surface [41].Upon modulation with NiCl2 followed by carbonization/thermal treatment, the diffraction peaks around 44.5, 51.3, and 76.4\u00b0 were associated with crystallin planes of metallic Ni [41], indicating the successful reduction of nickel ions to metallic state during the synthesis, which was in accordance to other studies reported elsewhere [47,48]. The finding was also in line with the HRTEM observation (Fig. S2d) that the lattice spacings of \u223c0.20\u00a0nm [48]. In addition, the distinct diffraction peak at approximately 44.1\u00b0 are observed, indicating the presence of crystalline planes of graphite carbon [49]. The graphitic carbon was probably produced by the catalytic graphitization of amorphous carbon by the anchored Ni nanoparticles. That\u2019s because the catalytically active metals could effectively lower the energy barrier of the solid-state transformation from amorphous carbon to graphitic carbon. In this study, the peak attributed to graphitic carbon (especially in Ni@C) was strong at 700\u00a0\u00b0C, which might be due to the fact that the transformation of the energetically less favorable amorphous carbon to a more favorable phase of graphitic carbon by the catalytically active metal could be significantly enhanced above 600\u00a0\u00b0C [50]. However, a weak amorphous peak of Ni@C was still found, suggesting that most of amorphous carbon were transformed into graphitic carbon, but not completely. It should be noted that the transition of pure sp3\n-carbon in tetrahedral amorphous (ta-C) into thoroughly graphitic (sp2\n) carbon (g-C) can be categorized into three basic steps: 1) from tetrahedral amorphous (ta-C) to amorphous carbon; 2) from a-C to nanocrystalline graphite (ng-C) with apparent ordered aromatic clusters; 3) from ng-C to complete graphite (g-C) [26].Generally speaking, the enhanced porous structure is usually accompanied with the improvement of the defected carbon structures [25]. In this study, the defected carbon structures of as-synthesized materials were further characterized by Raman spectra. As depicted in Fig. 3g, three distinct peaks of the carbonaceous materials were detected. Typically, the D band at \u223c1350\u00a0cm\u22121 is associated with the defect sites or disordered carbon, and G band at \u223c1580\u00a0cm\u22121 is assigned with E2g mode stretching vibration of the sp2\n-hybridized carbon network of graphite [32]. The 2D band at \u223c2700\u00a0cm\u22121 is ascribed to the two phonon lattice vibration, which is typical symbol of graphitic carbon [31]. The intensity ratio of D band to G band (ID/IG\n) indicates the degree of crystallization or defect density of carbon materials [31]. Overall, the ID/IG\n value of PBC and 3DMeMCs were in the order of PBC\u00a0<\u00a0Ni@C\u00a0<\u00a0Fe@C\u00a0<\u00a0Zn@C. The ID/IG\n ratio of PBC was calculated to be 0.82 and an intense 2D band related to the two-phonon double resonance was clearly found, confirming that the pretreatment of pelletization could endow PBC with a considerable degree of graphitization. However, the increased ID/IG\n ratio of Zn@C was estimated to be 0.99, resulting from the formation of micropores and enhanced structure disorder during the carbonization [51] It is noted that the Raman spectra of Fe@C and Ni@C presented narrow D and G bands as well as the gradually decreased ID/IG\n value, affirming the enhanced transformation of amorphous carbon into graphitic carbon. The above-mentioned findings indicated that 3DMeMCs (especially Fe@C and Ni@C) presented highly microporous structures with a high degree of graphitization. In addition, the ID/IG\n value of standard graphite was much lower than that of all 3DMeMCs [52], implying that there were still large amounts of sp3\n carbon present in 3DMeMCs.Moreover, the surface functional groups in PBC and 3DMeMCs were characterized by FTIR analysis (Fig. 3h). The typical characteristic peak of porous graphitic carbon were manifested at 1637\u00a0cm\u22121 corresponding to the skeletal vibration of CC bonds in aromatic ring [32]. The peak range of 1470 \u2013 1580\u00a0cm\u22121 could be assigned to ketonic CO stretching vibration, and the peak at 1072\u00a0cm\u22121 ascribed to C\u2013O/C\u2013O\u2013C stretching vibration [32]. Upon fabrication by metal chlorides, the new stretch appeared at the range of 2280 \u2013 2380\u00a0cm\u22121 was derived from the production of metal oxides. There results were in line with the observations from the high-resolution XPS spectra of C 1s and O 1s (Fig. 4\n). As compared to the FTIR bands of PBC, some bands decreased in intensities or even disappeared for all 3DMeMCs. Such variations explained the decompositions of the surface functionalities during the one-step thermal process [53]. Obviously, the decrease of disappearance of the bands between 950 and 1710\u00a0cm\u22121 implied that most oxygenated functionalities on the surface of 3DMeMCs were removed at 700\u00a0\u00b0C.To further elucidate the surface chemical composition and the valence state of elements, XPS measurements were conducted as shown in Fig. 4. The XPS full survey spectra (Fig. 4a) of PBC and 3DMeMCs exhibited that all as-synthesized materials primarily consisted of C and O. The strong peaks of C 1s, O 1s of PBC suggested that PBC was in high purity and the characteristic peaks of Zn 2p, Fe 2p, and Ni 2p peaks were observed in Zn@C, Fe@C, and Ni@C, respectively. The summary of XPS atomic distribution is listed in Table S3. According to the XPS data (Table S3), PBC contained 84.27 at% C and 15.73 at% O, and no other impurity elements were detected. The C atom ratio of Zn@C was found to considerably increase through the surface fabrication of ZnCl2, while the C content was reduced in Fe@C and Ni@C because of the incorporation of metal components. The atomic percentages of Zn (0.90%), Fe (14.35%), and Ni (6.81%) were achieved for Zn@C, Fe@C, and Ni@C, respectively. This result indicated that successful incorporation of Zn, Fe, and Ni elements into carbon skeleton, which was consistent with the EDX (Table S2) and CHNS elemental analysis (Table S4). Besides, the proportion of O was found to increase from 15.73 at% to 34.68 at% and 26.03 at% in Fe@C and Ni@C, suggesting that more metal oxides were present on the surfaces of Fe@C and Ni@C.The high-resolution XPS scans of the C 1s, O 1s, Zn 2p, Fe 2p, and Ni 2p regions, including the curve-fitting spectra for the representative catalysts, namely PBC and 3DMeMCs, are demonstrated in Figs. 4 and S3. The high-resolution C 1s XPS spectra of PBC and 3DMeMCs can be deconvoluted into four dominant peaks. As depicted in Fig. 4b-e, the main peak of C1s around 284.7\u00a0eV was attributed to C\u2013C, CC, and graphite in the carbon matrix [26]. The peak around 285.4\u00a0eV was ascribed to C\u2013OH, C\u2013O\u2013C, and COOR [26]; while the adjacent peak at around 287.7\u00a0eV belonged to CO [54]. Noteworthily, a peak (290.5\u00a0eV) originating from the \u03c0\u2013\u03c0* shake up was found [54], indicating the presence of graphitization. According to the deconvoluted peak area calculation (as shown in the inserted tables), PBC and Zn@C had higher CC/CC and aromatic C atomic ratios than Fe@C and Ni@C. The high-resolution O 1s spectra of these materials are shown in Fig. 4f-i, and they all contained three dominant peaks. Isolated OH or carbonyl group (CO) was the dominant oxygen-containing functional groups, followed by CO groups such as phenolic hydroxyl group (C\u2013OH) in PBC. 3DMeMCs with the modulation by metal chlorides (e.g., ZnCl2) could enhance the abundance of COOH, acting as the main oxygen-containing acidic functional group on the surface of the catalysts [55]. CO groups like C\u2013OH could offer binding sites to the carbon substrate for metals (like Zn) [56]. The Fe@C retained more oxygen atoms on the carbon surface, forming abundant oxygen-containing functional groups.The high-resolution spectra of Zn 2p, Fe 2p and Ni 2p are displayed in Fig. S4. The given Zn 2p XPS survey profile (Fig. S4a) suggested the two spin orbitals Zn 2p3/2 and Zn 2p1/2 at around 1022.2\u00a0eV and 1045.4\u00a0eV, respectively [57]. The trace amount of Zn composition was left in Zn@C even though a thorough washing step was performed. The melting ZnCl2 was intercalated into carbon layer with the formation of ZnO or Zn complexes in the high-temperature region, triggering the difficult removal [58]. The XPS spectrum of Fe 2p spectra regions was delineated in Fig. S4b, The peak at around 712.0\u00a0eV (Fe 2p3/2) was the characteristic of Fe(III) and Fe(II) [59], another main peak at 725.9\u00a0eV (Fe 2p1/2) was assigned to Fe(III) [59]. That was because the surface metallic Fe was readily oxidized to generate Fe(III) [60]. The presence of metallic Fe was also obtained with the distinctive satellite peak at approximately 720.2\u00a0eV, which was probably accompanied with the formation of Fe3C in the carbon matrix [61]. In the Ni 2p spectrum (Fig. S4c), the binding energy of Ni 2p3/2 peak at 855.1\u00a0eV and the satellite peak at 861.2\u00a0eV indicated the presence of Ni (II) bound to oxygen [62,63], affirming the observation by O 1\u00a0s analysis (Fig. 3i). The broad and intense photoelectron Ni 2p1/2 at the binding energy of 872.7\u00a0eV and the satellite peak (878.9\u00a0eV) could be assigned to metallic Ni, which was in agreement with XRD observations. Typically, the binding energy value of metallic Ni located at 872.7\u00a0eV was higher than other studies reported elsewhere, indicating an extremely strong Ni-support interaction [41]. Thus, most Ni present on the surface of Ni@C was in the metallic form instead of oxidized sates.To first get insight into the catalytic abilities of 3DMeMCs in the CFcP of WS and PW blends, the on-line micro-pyrolyzer integrated with GC/MS analysis was applied to determine the relative contents of each chemical group in the condensable products obtained from fast co-pyrolysis of WS and PW blends with/without catalysts (Fig. 5\n). The detailed chemical compounds could be categorized into the following groups: aliphatic olefins, cyclic olefins, aliphatic alkanes, cyclic alkanes, aromatic hydrocarbons, phenolics, and other oxygenates. The relative contents of these groups are presented in Fig. 5a, 5d and Table S5. The chemical compounds in the condensable product from fast pyrolysis of WS alone were abroad, containing acids, furans, phenols, phenolics, aldehydes, ketones, alcohols, esters, and aromatic hydrocarbons. As shown in Fig. 5a, the primary categories were phenolics and other oxygenates with the high relative contents of 19.0% and 70.6%, respectively.Importantly, the oxygenated compounds significantly decreased with a concomitant increase in hydrocarbon contents when WS was co-pyrolyzed with PW. A very low relative content (7.4%) of oxygenates was obtained and the relative content of phenolics was even decreased to zero during the fast co-pyrolysis of WS and PW. These phenomena indicated that the introduction of hydrogen-sufficient feedstock could be beneficial for suppressing the formation of oxygenates, especially phenolic compounds [64]. A comparison between the experimental and theoretical (the average of the experimental data gained from the fast pyrolysis of WS or PW alone) results are also presented in Fig. 5a. The relative content of oxygenates were much lower in the experimental case, which suggested that the addition of PW to fast pyrolysis of WS could result in an additional deoxygenation reaction. Conversely, the relative content of aromatic hydrocarbons was augmented from 18.2% to 36.9%, suggesting that an apparent synergistic effect took place for the generation of aromatic hydrocarbons by co-pyrolyzing WS with PW.Based on the chromatograms (Fig. 5c) of the condensable products originating from the fast co-pyrolysis of WS and PW blends with/without catalysts, the chemical components in condensable products gained from CFcP of WS and PW blend over PBC and 3DMeMCs were concluded in Fig. 5d and Table S5. In general, aromatic hydrocarbons were the most abundant in each case. More specifically, the CFcP of WS with PW over Fe@C and Ni@C resulted in a noticeable selectivity (over 35%) toward aromatic hydrocarbons. A more profound content (46.9%) of aromatic hydrocarbons was achieved by introducing Zn@C in the CFcP process, whereas the content (24.5%) was approximately halved in the case of CFcP by using PBC. Hence, 3DMeMCs was in favor of the formation of aromatic hydrocarbons as compared to PBC, indicating their higher aromatization activities.Yet a higher content of aliphatic and cyclic hydrocarbons was achieved using PBC as the catalyst. These observations implied that PBC solely containing carbon matrix cannot present sufficient efficiency for aromatic hydrocarbons formation, even though it had offered measurable catalytic activity. The higher efficiency of 3DMeMCs can be explained by its larger BET surface area, larger pore volume, and more metal active sites than PBC. The microporous structures of 3DMeMCs could accelerate the formation of aromatic hydrocarbons since the reactants in the micropores had a higher chance to collide with the active sites than in the large-pore catalysts, resulting in the improved conversion efficiencies [65]. It is also observed that the higher catalytic activities related to deoxygenation were differentiated by the species of metal active sites. Note that there were high relative contents (26.2%) of oxygenates (phenolics and others) in the condensable product resulting from CFcP of WS and PW by using Fe@C as the catalyst. This suggested that Fe@C showed a weaker deoxygenated ability than Zn@C and Ni@C.In terms of aromatic hydrocarbons that were mostly monocyclic aromatic hydrocarbons (MAHs), their yields were varied rather marginally. Apparently, styrene was the dominant aromatic hydrocarbon in all cases. As demonstrated in Fig. 5b, co-pyrolyzing WS with PW showed a higher yield of MAHs than fast pyrolysis of WS or PW alone. Similar to the chemical compositions in the condensable products, the experimental value was more than 2 times than theoretical value when WS and PW were co-fed in the fast pyrolysis process. These observations further confirmed the synergistic effect between WS and PW on the formation of MAHs occurring in the process.According to the pyrograms (GC/MS area: abundance versus time) as shown in Fig. 5c, MAHs were predominant in all cases by using carbonaceous catalysts for the fast co-pyrolysis process. This result was strongly related to the shape selectivity imposed by the microporous structures of the as-synthesized catalysts, which was in favor of the formation of molecules with six-membered carbon rings [66]. As given in Fig. 5e and Table S6, the strong abundances of MAHs in condensable products from CFcP of WS and PW over PBC, Zn@C, Fe@C, and Ni@C were 6.08\u00a0\u00d7\u00a0107, 15.8\u00a0\u00d7\u00a0107, 7.30\u00a0\u00d7\u00a0107, 19.4\u00a0\u00d7\u00a0107, respectively. Compared with PBC, 3DMeMCs gave rise to an enhanced formation of MAHs because of the increase amounts of metal active sites available for cyclization, oligomerization, aromatization, etc. Indeed, MAHs were enhanced during the CFcP process via Diels-Alder reaction between WS-derived furans and PW-derived olefins over the active sites of 3DMeMCs [5,10]. Among the 3DMeMCs, the yields of MAHs presented strong dependence upon the metal species. Ni@C showed the largest quantity of MAHs from CFcP of WS and PW than the other catalyst; and Fe@C was less effective for the production of MAHs, obeying the order of Ni@C\u00a0>\u00a0Zn@C\u00a0>\u00a0Fe@C. That\u2019s because the metal active sites in the CFcP might function differently. The higher yield of MAHs for Ni@C and Zn@C should be also ascribed to more metal actives and/or higher degree of graphitization, as evidenced by the EDX, XPS, and TPO results described in \nSection 3.1\n. Accordingly, 3DMeMCs can be selected as appropriate catalysts for the production of aromatic hydrocarbons especially MAHs through CFcP of WS and PW.3DMeMCs was thereafter examined during practical ex-situ CFcP process to determine their catalytic abilities. The product yields from practical ex-situ CFcP of WS and PW over PBC and 3DMeMCs at 500\u00a0\u00b0C are listed in Table 2\n. The product yield from fast co-pyrolysis of WS and PW mixture in the absence of catalysts were not provided in Table 2 because waxy products were hard to collect and calculate their yields. With the use of these carbonaceous catalysts, both the yields of liquid and gaseous products remarkably went up due to the active acid sites of the carbonaceous catalysts. Compared with PBC, 3DMeMCs were superior to the bio-oil production, peaking at 64.16\u00a0wt% when Zn@C was used as the catalyst. In general, these as-synthesized showed comparable gaseous and biochar yields at approximately 14\u00a0wt% and 17\u00a0wt%, respectively.Inspired by Williams et al. [37,43], the higher catalytic temperature (namely 800\u00a0\u00b0C) was in favor of the gaseous production and valuable carbon. In this study, we also augment the catalytic temperature to 800\u00a0\u00b0C for H2 or syngas production (Table S7), together with valuable carbons. Indeed, the gas yield was dramatically enhanced to around 48\u00a0wt% when 3DMeMCs was employed as the catalysts. Interestingly, PBC exhibited a higher gas yield at 52.78\u00a0wt%, and PBC also revealed a higher bio-oil yield (23.85\u00a0wt%) than 3DMeMCs.The carbon yield and qualities of the bio-oils produced from ex-situ CFcP process at 500\u00a0\u00b0C were investigated by CHNS elemental analysis, HHV and GC/MS. Representative results of CHNS elemental analysis, carbon yield, HHV, and characterization of chemical compounds by GC/MS are listed in Table 2 and Fig. 6\n. The carbon content (73.18\u00a0wt%) of the bio-oil derived from the ex-situ CFcP process in the presence of Ni@C was higher than that by using the other carbonaceous catalysts. In contrast, the oxygen content in the bio-oil from the process over Ni@C showed the lowest value. These findings suggested that the presence of metals embedded on the carbon matrix might be beneficial for the deoxygenations during the process [67], which in turn elevated both the carbon and hydrogen contents. Table 2 also demonstrated the carbon yield and HHV of bio-oils produce from the ex-situ CFcP process at 500\u00a0\u00b0C. Given the high mass yield and carbon content of bio-oils in the presence of 3DMeMCs, the carbon yield of the bio-oils produced from the process over 3DMeMCs was drastically promoted when comparing with the carbon yield (37.54\u00a0C%) by using PBC as the catalyst. Of these 3DMeMCs, Zn@C revealed the highest carbon yield (60.38\u00a0C%) than Fe@C and Ni@C, which was due to the highest mass yield (64.16\u00a0wt%) of bio-oil. However, the HHV of bio-oil from the process using Ni@C as the catalysts was 38.52\u00a0MJ/kg, which was higher than that (greater than31\u00a0MJ/kg) using PBC, Zn@C, and Fe@C. This finding suggested that the bio-oils generated from ex-situ CFcP of WS and PW at 500\u00a0\u00b0C in the presence of as-synthesized catalysts are a promising replacement of petroleum-derived fuels.The chemical compounds in the bio-oils are characterized and categorized, as shown in Fig. 6a and b. The bio-oil from CFP of WS alone was applied as a control to examine the synergistic effect between biomass and plastic waste during the ex-situ CFcP process at 500\u00a0\u00b0C. The oxygenated compounds such as furans, phenols, alcohols, ketones, aldehydes, aromatic hydrocarbons, etc. were the dominate species in the control. With the introduction of PBC and 3DMeMCs as the catalysts during the ex-situ CFcP process at 500\u00a0\u00b0C, the O-species content was reduced to less than 7%; particularly, Fe@C presented a lowest oxygenated content (less than 3%). Moreover, there was no oxygenates found in the bio-oils when the catalytic temperature was elevated to 800\u00a0\u00b0C over 3DMeMCs (Fig. 7\nc).On the contrary, the hydrocarbon content reached over 93% by using 3DMeMCs at 500\u00a0\u00b0C; and Fe@C showed the highest content (97.52%) of hydrocarbons in the bio-oil. More importantly, it was found that more than 50% of aromatic hydrocarbons were present in the bio-oils when the PBC and 3DMeMCs were used as catalysts at 500\u00a0\u00b0C; while over 90% of aromatic hydrocarbons in the bio-oils were achieved over the carbonaceous catalysts at 800\u00a0\u00b0C (Fig. 7c). The abovementioned results were comparable with the observations reported elsewhere, which were found that approximately 70% of aromatics were enriched in the bio-oils [68,69]. The rest hydrocarbons in the bio-oils obtained at 500\u00a0\u00b0C were olefins, occupying around 32%. Regarding the hydrocarbons especially aromatic hydrocarbons, their carbon numbers were primarily ranged from C8 to C16, which are lumped in the jet fuel range [70]. As shown in Fig. 6b, Fe@C was superior to other carbonaceous catalysts for the production of C8 \u2013 C16 MAHs, with the selectivity reaching 59.50%. The concentrations of the dominant MAHs (i.e., ethylbenzene and styrene) are summarized in Table 2. It was noticed that styrene was the most abundant compound in all bio-oils, which mainly originated from the decomposition of PS; the concentration of styrene was all over 18\u00a0mg/mL, maximizing at 35.99\u00a0mg/mL when PBC was applied as the catalyst. These outcomes implied that the metals embedded on the carbon matrix contributed to the Diels-alder and aromatization reactions. Moreover, these results are similar with the findings observed from on-line CFcP tests, reaffirming the stable capacities of 3DMeMCs. Yet, most (over 50%) of the aromatic hydrocarbons in bio-oils gained at 800\u00a0\u00b0C were polycyclic aromatic hydrocarbons (PAHs), and PBC was in favor of the polycyclic aromatic hydrocarbons with up to 65% of selectivity (Fig. 7d). According to the concentration of the aromatic hydrocarbons (Table S7), styrene and naphthalene were the two main aromatic compounds found in the bio-oils obtained at 800\u00a0\u00b0C.It is well known that the gaseous product was more of potential value comparing to liquid product during CFP process [45]. In this study, the gaseous product was quantified by relatively pure gas fractions with highly thermodynamic stabilities.[71] The releasing properties of the gaseous products from the CFcP process catalyzed at 500 and 800\u00a0\u00b0C are correspondingly explained in Fig. 6c, d and Fig. 7a, b. As for the fast pyrolysis of WS, CO and CO2 were the predominant gaseous fraction through the cleavage of COC, CO, and OCO groups, which are enriched in the biomass [72]. Even though the fast co-pyrolysis of WS and PW could benefit the yields and qualities of bio-oils, gas as the byproduct was slightly influenced in terms of evolved gaseous compositions (Fig. 6c) as well as H2 and syngas yields (Fig. 6d). That was due to the incomplete decomposition of PW, which was confirmed by the high amounts of waxy products. Notably, the percentages of CO and CO2 were sharply declined by using the PBC and 3DMeMCs at 500\u00a0\u00b0C. Due to the co-feeding of biomass and plastic waste, O-species were expelled by reacting with intermediate olefins through Diels-Alder reaction followed by dehydration, instead of decarboxylation and decarbonylation reactions [6].Importantly, Fe@C catalyzed at 800\u00a0\u00b0C exhibited the lowest CO (17.56\u00a0vol%) and CO2 (8.90\u00a0vol%) proportions, as shown in Fig. 7a. The use of catalysts both at 500 and 800\u00a0\u00b0C improved the H2 proportion. There observations were in agreement with the results reported by Gupta\u2019s group that the introduction of catalysts during catalytic fast co-pyrolysis/gasification of biomass and plastic wastes could efficiently enhance H2 yield and syngas production [73]. In particular, H2 proportion was found to ascend in the most pronounced way from 1.58\u00a0vol% in the absence of catalysts to over 24\u00a0vol% when using Fe@C and Ni@C as the catalysts at 800\u00a0\u00b0C (Fig. 7a). It was proven that Fe and Ni were the effective elements to enhance dehydrogenation reaction to produce H2 and valuable carbon [43,45]. In addition, the proportion of light hydrocarbons especially CH4 and C2H4 were observed to significantly increase over the carbonaceous catalysts up to 28.27\u00a0vol% and 20.15\u00a0vol% when Zn@C served as the catalyst at 800\u00a0\u00b0C, implying that the catalysts could be conducive to the catalytic waxy intermediates into light hydrocarbons.The LHV of gaseous products was calculated and shown in Fig. 6c and 7a. According to Eq. (7), the more share of CO and H2, but the less proportion of CO2 would contribute to the LHV of gas [74]. The gaseous products from CFcP process over the carbonaceous catalysts possessed the higher LHV than gas obtained from the process without the use of catalyst. 3DMeMCs were the more advantageous catalysts to gain the higher LHV both at 500 and 800\u00a0\u00b0C due to the higher proportions of CO, H2 but the lower CO2 proportion. More importantly, Zn@C and Fe@C at 800\u00a0\u00b0C showed the higher LHV (around 30\u00a0MJ/Nm3), presenting comparable value with natural gas.To further detect the catalytic capacities for the production of H2 and/or syngas (H2 plus CO), the H2 and syngas yields as a function of the catalyst species at 500 and 800\u00a0\u00b0C are illustrated in Fig. 6d and 7b, respectively. It was apparently found that the H2 and syngas yields remarkably went up to the high levels especially from the tests conducted at 800\u00a0\u00b0C (Fig. 7b). Among these 3DMeMCs, Ni@C was the optimal catalyst to yield the highest H2 and syngas yields at 156.81 and 272.54 NmL/gfeedstock, respectively.Since the slight and negligible changes of carbonaceous catalysts at 500\u00a0\u00b0C after experiments, and there was no obvious carbon deposition found on the catalysts. Accordingly, these spent catalysts at 500\u00a0\u00b0C were not considered for detailed analysis. In this regard, the characteristics of carbon deposited on PBC and 3DMeMCs at 800\u00a0\u00b0C were demonstrated in Fig. 8\n. Apparently, there were obvious carbon deposition on the surface of the spent carbonaceous catalysts (Fig. 8a \u2013 d). Particularly, it was observed that the presence of a dense entangled growth of filamentous carbons covering the surface of Ni@C (Fig. 8d and e).According to the high-resolution SEM image as shown in Fig. 8f, a lattice fringes with a distance of 0.34\u00a0nm was found throughout spent Ni@C, suggesting the formation of multi-walled carbon nanotubes (CNTs). These observations evidenced that Ni@C favored the decomposition of hydrocarbons into CNTs and H2, which also confirmed by the higher H2 yield from Ni@C at 800\u00a0\u00b0C, as described in \nSection 3.3.3\n. Based on the SEM-EDX analysis (Fig. 8i-k), there were some Ni nanoparticles found on the surface of Ni@C as well, which was probably explained by the reason that the nickel oxides were reduced by C and/or active pyrolytic vapors into metallic Ni.The spent carbonaceous catalysts (800\u00a0\u00b0C) were also analyzed by TPO to determine the properties of carbon deposition. As shown in Fig. 8g, the TPO profiles of spent catalysts were comparable with fresh catalysts. However, it was found that all spent catalysts presented single oxidation peaks at over 500\u00a0\u00b0C in the derivate TPO profiles (Fig. 8h), indicating the presence of high crystallinity of CNTs in spent carbonaceous catalysts [45]. The onset and end temperatures of spent Ni@C were shifted to higher temperature (\u223c550\u00a0\u00b0C) than other catalysts, suggesting the large amounts of CNTs formed on the Ni@C. In this regard, the fresh, spent Ni@C at 500 and 800\u00a0\u00b0C were compared in terms of derivate TPO profiles, as shown in Fig. 8l. It was confirmed that spent Ni@C at 800\u00a0\u00b0C revealed higher oxidation temperature peak and higher carbon oxidation rate than the comparable fresh Ni@C and spent Ni@C at 500\u00a0\u00b0C.Additionally, biochar generated from co-pyrolysis of biomass and plastics can be used as inexpensive absorbent, solid fuel, carbon sequestration, and soil amendment [10]. In this study, the thermal stabilities of biochar were tested by TPO, the profiles of weight and derivate weight on the basis of oxidation temperature are depicted in Fig. S5a and b. The weight losses occurred in the oxidation temperature range of 300\u2013600\u00a0\u00b0C, the remaining residues were recognized by the minerals and/or incorporated metal oxides. Compared with WS-derived biochar, biochar from co-pyrolysis was shifted to higher oxidation temperatures, suggesting its higher thermal stability. There were no weight losses found in both biochar samples after 600\u00a0\u00b0C, verifying that there was no secondary decomposition or weight losses taking place.TGA was finally conducted to investigate the relationships between weight variation and temperature, which could further determine the thermal degradation behaviors and reaction mechanisms. Fig. 9\n presents the TG and DTG curves of WS, PW, WS and PW blend (WS\u00a0+\u00a0PW), and WS, PW, and catalysts blends (WS\u00a0+\u00a0PW\u00a0+\u00a0CA). As plotted in Fig. 9a, the thermal degradation of WS mainly happened at between 150 and 400\u00a0\u00b0C and gradually continued up to 600\u00a0\u00b0C, which was due to devolatilization and most lignocellulosic fractions (cellulose, hemicellulose, and lignin) were degraded in the temperature range [75]. The decomposition of PW displayed a sequence of stages in the temperature from 300 to 500\u00a0\u00b0C. Considering the complex mixtures of PE, PP, PS, PET, and PVC, the TGA of these five model components of PW were also studied, the thermal degradation behaviors of these plastics are illustrated in Fig. S6a. The thermal degradation behavior of PVC was different from that of other plastics. Apparently, the thermal degradation of WS\u00a0+\u00a0PW blend was much more complicated than that of WS or PW alone.The DTG curve (Fig. 9c) of WS was peaked at approximately 320\u00a0\u00b0C, which was assigned to the time when the maximum degradation rates of hemicellulose and cellulose [76]. There was no an clear peak assigned to lignin decomposition because lignin was a small fraction in WS and the degradation of lignin took place in a wide temperature range of 200 \u2013 500\u2103 [76]. The DTG curve of PW showed that PW was primarily degraded from 400 to 500\u00a0\u00b0C, which was evidenced by the DTG curves of individual plastic components as shown in Fig. S6b. As expected, the DTG curves of WS\u00a0+\u00a0PW blend revealed distinct peaks, which was due to the different characteristics of individual plastic components.The thermal degradation of WS\u00a0+\u00a0PW\u00a0+\u00a0CA were significantly different from that of WS\u00a0+\u00a0PW blend (Fig. 9b), suggesting that the introduction of catalysts had an essential influence in the thermal degradation behaviors. In particular, the TG curve of WS\u00a0+\u00a0PW\u00a0+\u00a0BC was almost linear and the amount of residue after thermal degradation of WS\u00a0+\u00a0PW blend over BC was nearly 40%, suggesting that BC was not stable during the thermal degradation stage and it could be further decomposed at the high temperature range. In contrast, WS\u00a0+\u00a0PW blend with PBC or 3DMeMCs exhibited the stable and comparable thermal degradation behaviors. It was reaffirmed that PBC with high degree of graphitization can be considered as a more promising carbonaceous material for direct use or the synthesis of catalysts than BC. To gain more insight into the effect of the types of catalysts on the synergistic effect between biomass and PW, TGA of WS and PE (typical model component of PW) blend were performed as shown in Fig. S7. Likewise, the TG curves of WS, PE, and catalysts blends (WS\u00a0+\u00a0PE\u00a0+\u00a0CA) showed stable and comparable thermal degradation behaviors.The peaks of DTG curves (Fig. 9d) of WS\u00a0+\u00a0PW\u00a0+\u00a0CA was shifted slightly to the lower temperature regions as compared to the peaks of the DTG curves of WS\u00a0+\u00a0PW blend without the introduction of a catalyst. The observation indicated that the catalysts except BC could decrease the degradation temperatures and be good for the decomposition of WS\u00a0+\u00a0PW blend, which was in line with the CFcP results [76]. It is noted that the DTG curves of WS\u00a0+\u00a0PE\u00a0+\u00a0CA experienced two distinct peaks (Fig. S7d). Compared to the DTG curve of WS and PE blend (WS\u00a0+\u00a0PE), the DTG curve of WS\u00a0+\u00a0PE\u00a0+\u00a0CA (especially WS\u00a0+\u00a0PE\u00a0+\u00a0Fe@C) was shifted to the lower temperatures. In addition, the results regarding kinetic studies were specifically described in Supplementary S2. The kinetic parameters including activation energy (E), pre-exponential factor (A), and correlation coefficients (R2\n) were calculated and provided in Figs. S8\u2013S10 and Tables S8\u201310. It was consolidated that there was a synergistic effect during co-pyrolysis and the introduction of PBC and 3DMeMCs in the thermal degradation of biomass and plastics could significantly reduce the activation energy (E).\nNotably, PBC became a highly porous carbonaceous material due to the treatment of pelletization of corn stover. That\u2019s possibly due to the fact that large amounts of gaseous species (e.g., H2O, CO2, and CO) generated during the carbonization that was not prone to escape due to the compacted structure of corn stover, the gradually accumulated gas retained might affect the carbon structure, which would finally evaporate and leave more pores (especially micropores) in PBC [25]. Additionally, in the pelletization process, the fraction of lignin was molten during the grinding process with moderate temperature, which severed as the binder for pelletization. Accordingly, the grinding together with pelletization for manufacturing biomass pellets could etch biomass structure, which possibly endowed the carbon structure with more micropores after carbonization. These findings were evidenced by N2 sorption analysis in Fig. 3a, 3b and Table 1. Accordingly, PBC exhibited suitable properties to be applied as the carbon skeleton.In the synthesis of 3DMeMCs, metal chlorides (i.e., ZnCl2, FeCl3, and NiCl2) were utilized as both the activating agent and the catalysts to fulfil the synchronous carbonization and graphitization of biomass carbon, according to the following equations, but they did not necessarily happen in a sequential order. The produced metal oxides and CO2 could react with carbon over 700\u00a0\u00b0C; thus, the carbon lattices expanded irreversibly and led to high microporosity. As for Zn@C, PBC with decent porosity offered more channels for the storage of ZnCl2 in the impregnation step [77]. ZnO derived from the decomposition of ZnCl2 was formed into the pores of Zn@C (eqs. (8)\u2013(9)), which might be further reduced into metallic Zn by carbon, H2, and CO (Eq. (10)).\n\n(8)\n\n\n\n\nZnCl\n\n2\n\n+\n\nH\n2\n\nO\n\u2192\nH\n\n\n\nZ\nn\n\n\n\n\nC\nl\n\n\n\n2\n\n\n\n\nO\nH\n\n\n\n\n\n\n\u2192\nZ\nn\n\n\n\n\nO\nH\n\n\n\n2\n\n+\nH\nC\nl\nT\n\u2264\n\n105\n\u00b0\n\nC\n\n\n\n\n\n\n(90\n\n\nZn\n\n\n\n\nO\nH\n\n\n\n2\n\n\u2192\nZ\nn\nO\n+\n\nH\n2\n\nO\nT\n\u2265\n\n105\n\u00b0\n\nC\n\n\n\n\n\n\n(10)\n\n\nZnO\n+\n\n\n\nC\n,\n\nH\n2\n\n,\nC\nO\n\n\n\n\u2192\nM\ne\nt\na\nl\nl\ni\nc\nZ\nn\nT\n\u2265\n\n700\n\u00b0\n\nC\n\n\n\n\nWith regard to Fe@C, the changes of the chemical states for Fe@C are illustrated in the following equations. On the basis of eqs. (11)\u2013(14), amorphous Fe species such as Fe(OH)3, FeOOH were initially converted into Fe2O3 at 400\u00a0\u00b0C [22], and then reduced into Fe3O4 and/or FeO by a carbon matrix, and reduced gases (H2 and CO) generated from biomass pyrolysis at 500 \u2013 700\u00a0\u00b0C [78,79]. The iron oxides might be further reduced into metal atom by carbon, H2, and CO (Eq. (14)). The carbon consumed for reduction would contribute to the generation of porous carbonaceous structure.\n\n(11)\n\n\nFe\n\n\nCl\n\n3\n\n+\n\nH\n2\n\nO\n\u2192\nF\ne\n\n\n\n\nO\nH\n\n\n\n3\n\n+\nH\nC\nl\nT\n\u2264\n\n105\n\u00b0\n\nC\n\n\n\n\n\n\n(12)\n\n\nFe\n\n\n\n\nO\nH\n\n\n\n3\n\n\u2192\nF\ne\nO\nO\nH\n\u2192\n\n\nFe\n\n2\n\n\nO\n3\n\nT\n\u2265\n\n400\n\u00b0\n\nC\n\n\n\n\n\n\n(13)\n\n\n\n\nFe\n\n2\n\n\nO\n3\n\n+\n\n(\nC\n,\n\nH\n2\n\n,\nC\nO\n)\n\n\u2192\n\n\nFe\n\n3\n\n\nO\n4\n\n/\nF\ne\nO\nT\n\u2265\n\n500\n\u00b0\n\nC\n\n\n\n\n\n\n(14)\n\n\n\n\nFe\n\n2\n\n\nO\n3\n\n/\n\n\nFe\n\n3\n\n\nO\n4\n\n/\nF\ne\nO\n+\n\n(\nC\n,\n\nH\n2\n\n,\nC\nO\n)\n\n\u2192\nM\ne\nt\na\nl\nl\ni\nc\nF\ne\nT\n\u2265\n\n700\n\u00b0\n\nC\n\n\n\n\nLikewise, amorphous Ni species such as Ni(OH)2 was first formed at the low temperature (Eq. (15)), which were thereafter transformed into NiO and/or Ni2O3 (Eq. (16)) [22]. The nickel oxides were further reduced into metallic Ni (Eq. (17)) by carbon, H2 and CO at the elevated temperature (700\u00a0\u00b0C), which could eventually act as the catalyst for the conversion of amorphous carbon into graphitic carbon [22,80].\n\n(15)\n\n\nNi\n\n\nCl\n\n2\n\n+\n\nH\n2\n\nO\n\u2192\nN\ni\n\n\n\n\nO\nH\n\n\n\n2\n\n+\nH\nC\nl\nT\n\u2264\n\n105\n\u00b0\n\nC\n\n\n\n\n\n\n(16)\n\n\nNi\n\n\n(\nO\nH\n)\n\n2\n\n\u2192\nN\ni\nO\n/\n\n\nNi\n\n2\n\n\nO\n3\n\nT\n\u2264\n\n650\n\u00b0\n\nC\n\n\n\n\n\n\n(17)\n\n\n\n\nNiO\n/\nN\ni\n\n2\n\n\nO\n3\n\n+\n\n(\nC\n,\n\nH\n2\n\n,\nC\nO\n)\n\n\u2192\nM\ne\nt\na\nl\nl\ni\nc\nN\ni\nT\n\u2265\n\n700\n\u00b0\n\nC\n\n\n\n\nThese observations revealed that the formation of micropores and charring reactions took place during the synthesis of 3DMeMCs. The Lewis acid sites were produced in the one-step thermal treatment by introducing metal chlorides with the synergistic effect of metal cations (Zn2+, Fe3+, and Ni2+) and Cl-, which could enhance the charring reaction and develop intricate microporous structure [81]. Additionally, the generation of metal oxides was conductive to the formation of microporous structures. Overall, PBC could be fabricated and applied as a promising carbonaceous material for the synthesis of 3DMeMCs, and metal chlorides could serve as the activating agents and catalysts for micropore development.According to these observations, the plausible reaction mechanisms regarding ex-situ catalytic fast co-pyrolysis of biomass and plastics over 3DMeMCs are outlined in Fig. 10\n. Broadly speaking, large amounts of biomass-derived oxygenates or plastics-derived large molecular hydrocarbons from individual catalytic fast pyrolysis of biomass or plastics were transformed into light hydrocarbons, which could form the hydrocarbon pool inside the micropores and/or on the surface of 3DMeMCs and were ultimately converted into aromatic hydrocarbons through oligomerization and aromatization over the acid sites [65,75,82]. Although the formation of aromatic hydrocarbon could take place in several routes, it was mainly ascribed to the interactions of biomass-derived furans and plastics-derived light olefins [5,6]. The Diels-Alder reaction between furans and light olefins was the most dominant reaction followed by dehydration to produce MAHs.More specifically, metal chlorides (e.g., ZnCl2) used to modulate biochar after the one-step thermal process could generate Lewis acid sites [83,84]. Importantly, the Lewis acid sites associated with metal species (e.g., Zn species) contributed to facilitating the Diels-Alder reaction [85], which was the dominant effect on the formation of aromatic hydrocarbons. These Lewis acid sites could also improve the dehydrogenation and dehydrocyclization of alkane and olefins meanwhile promoting the rate of aromatization reaction [83,85]. Meanwhile, it was clearly found that the aromatic hydrocarbons obtained at 500\u00a0\u00b0C were present in the form of monocyclic alkyl-aromatic hydrocarbons. That could be explained that the alkylation of aromatic hydrocarbons was accelerated by the Lewis acid sites, producing alkyl-aromatic hydrocarbons at the expense of olefins and aromatic hydrocarbons [86].Among the 3DMeMCs, Ni@C and Fe@C showed the highest share of metal species, as evidenced by EDX, XPS, and elemental analysis. The higher share of Ni and Fe species would form more Lewis acids sites than those of Zn species, which therefore promoted aromatization reactions to produce the higher content of aromatic hydrocarbons when using Ni@C and Fe@C as the catalyst during the ex-situ CFcP process at both 500 and 800\u00a0\u00b0C. In this study, Ni@C at 500\u00a0\u00b0C exhibited the best catalytic performance for deoxygenation to produce higher HHV bio-oils. Furthermore, Ni@C at 800\u00a0\u00b0C was also in favor of gaseous products (e.g., H2 and/or syngas) along with carbon nanotubes at the cost of intermediate hydrocarbons through dehydrogenation and cracking reactions.Additionally, it has been reported that the mineral components in biochar matrix played an vital role in acid sites during the catalysis [87]. It was reported that Al, S, and P species (like AlPO4) were found to show high surface acidity with a mixture of both Lewis and Br\u00d8nsted acid sites [87,88]. In this study, some strong acid sites associated with Al, P and Si species (inherent elements in carbon matrix) should be formed on the surface of 3DMeMCs, which could enhance the hydrogen transfer reaction to achieve more aromatic hydrocarbons and alkanes in the consumption of olefins [86]. As a result, the incorporation of metal chlorides coupled with the one-step thermal process to modulate biochar not only introduced many external Lewis active sites inside the micropores and on the surface of 3DMeMCs, but also fabricated physicochemical properties of 3DMeMCs. The presence of these catalysts was conducive to altering the product distribution and enhancing the production of aromatic hydrocarbons, together with dehydrogenation of intermediate hydrocarbons (CnHm) to generate H2 and valuable carbons especially at the high catalytic temperature (800\u00a0\u00b0C) [37]. Therefore, 3DMeMCs with microporous structures possessed enough active acid sites, enhancing the catalytic conversion of biomass and plastic waste into valuable aromatic hydrocarbons, syngas, and carbons.To summarize, the 3D metal-embedded microporous carbocatalysts were successfully synthesized from pellet biochar incorporated with metal chlorides in a sing-step energy-efficient thermal process, fulfilling the synchronous pore-forming, metal-doping, and graphitization. The physicochemical characteristics of the as-synthesized catalysts were comprehensively investigated by a sequence of techniques. These as-synthesized catalysts were further tested in the online and ex-situ catalytic fast co-pyrolysis of wheat straw and plastic waste to evaluate their catalytic abilities. As expected, an apparent synergistic effect happened for the production of aromatic hydrocarbons during ex-situ fast co-pyrolysis of biomass and plastic waste; and these catalysts (especially Fe@C) at 500\u00a0\u00b0C favored the formation of MAHs with the high relative content up to 60%. Besides, Ni@C at 800\u00a0\u00b0C exhibited the better catalytic performances than the other catalysts for the production of H2 (157 NmL/gfeedstock), syngas (273 NmL/gfeedstock), and carbon nanotubes. In addition, the thermal degradation behaviors and kinetic analysis regarding co-pyrolysis of wheat straw and plastic waste over the as-synthesized catalysts were determined in detail. A plausible reaction mechanism was elucidated for ex-situ catalytic fast co-pyrolysis over these catalysts. Overall, this study developed a simple, rapid, and facile method to synthesize the green, promising, and highly efficient 3D metal-embedded microporous carbocatalysts for catalytic fast co-pyrolysis/gasification of biomass and plastic waste for the production of aromatic hydrocarbons, syngas, and valuable carbons.\nXuesong Zhang: Writing \u2013 review & editing, Conceptualization, Methodology, Project administration, Funding acquisition. Ruolan Xu: Investigation, Conceptualization, Methodology, Software, Formal analysis. Quan Liu: Data curation. Ge Kong: Formal analysis. Hanwu Lei: Writing \u2013 review & editing. Roger Ruan: Writing \u2013 review & editing. Lujia Han: Writing \u2013 review & editing, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by Start-up Funding for High-end Talents of China Agricultural University, Chinese Universities Scientific Fund (10092001), and China Agriculture Research System of MOF and MARA.Supplementary data to this article can be found online at https://doi.org/10.1016/j.ecmx.2021.100176.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  This study explored an energy-efficient and cost-effective method to synthesize three-dimensional metal-embedded microporous carbocatalysts. Pellet biochar manufactured with compressed and porous structure was used as the carbonaceous precursor, which was modulated by diverse metal chlorides in the single-step thermal process, fulfilling the synchronous pore-forming, metal-doping, and graphitization. The as-synthesized carbocatalysts were characterized in detail by using N2 physisorption, SEM, TEM, EDX, XRD, TPO, TGA, FTIR, XPS, Raman, CHNS elemental analysis, etc. It was found that the metal-embedded carbocatalysts possessed well-developed 3D microporous structures with the highest specific surface area of 964\u00a0m2/g. The catalytic activities of these catalysts were investigated during on-line and ex-situ catalytic fast co-pyrolysis of wheat straw and plastic waste. It was observed that the carbon yield of bio-oils could reach over 60\u00a0C% by using Zn@C as the catalyst at 500\u00a0\u00b0C, and the HHV of bio-oils peaked at 38.52\u00a0MJ/Kg in the presence of Ni@C at 500\u00a0\u00b0C. Moreover, these carbcatalysts at 500\u00a0\u00b0C favored production of hydrocarbons with a relative content up to 98%; in particular, monocyclic aromatics presented the highest selectivity (nearly 60%). Among metal-embedded carbcatalysts, Ni@C at 800\u00a0\u00b0C was in favor of H2 (157 NmL/gfeedstock) and syngas (273 NmL/gfeedstock) production; importantly, Ni@C also promoted the generation of carbon nanotubes. Additionally, the thermal degradation behaviors and kinetics of non-catalytic and catalytic co-pyrolysis of biomass and plastic waste over the as-synthesized catalysts were also tested by thermogravimetric analysis. Finally, a rational reaction mechanism regarding ex-situ catalyst fast co-pyrolysis of biomass and plastic waste over catalytically active sites on the as-synthesized catalysts was elucidated. Accordingly, this work provides a great potential of using the promising carbocatalysts to co-valorize biomass and plastic waste into the integrated harvests of monocyclic aromatics, syngas, and valuable carbons.\n               "}
{"full_text": "\n\n\nThe published article includes all datasets generated or analyzed during this study.\n\n\nAll data reported in this paper will be shared by the lead contact upon request.\n\n\nThis paper does not report original code.\n\n\nThe published article includes all datasets generated or analyzed during this study.All data reported in this paper will be shared by the lead contact upon request.This paper does not report original code.Electrocatalytic overall water splitting, including hydrogen-evolving reaction (HER) and oxygen-evolving reaction (OER), is a promising strategy that converts electrical energy into chemical energy. Pt- and Ir-based noble metals are generally known as the state-of-the-art catalysts for HER and OER, respectively. However, the low abundance and high prices of the catalytic materials limit the large-scale commercial application of water-splitting technologies. Despite many efficient noble-metal-free catalysts for OER and HER, such as metal nitrides,\n1\n metal chalcogenides,\n2\n\n,\n\n3\n metal phosphides,\n4\n and metal carbides,\n5\n there is still a big gap in performance between these catalysts and noble metal-based materials. It is highly desirable to explore efficient and cost-effective bifunctional electrocatalysts for water splitting.Alloying Pt or Ir with 3d transition metals (e.g. Ni, Co, or Cr) has been regarded as a promising strategy to concurrently reduce the noble metal loading and promote catalytic activity.\n6\n\n,\n\n7\n\n,\n\n8\n However, these traditional alloys tend to degrade and dissolve into electrolytes during electrochemical cycling due to the segregation of the transition metal atoms toward the catalyst surface.\n9\n A new class of high-entropy alloys (HEAs), defined as materials containing five or more near-equimolar principle metal components,\n10\n has been substantiated as efficient and stable electrocatalysts recently.\n11\n\n,\n\n12\n In most cases, the performance of HEAs is simply attributed to the synergetic effect between multiple elements.\n13\n The mechanism of such multi-component synergy in HEAs remains elusive. In-depth understanding of the structure-performance relationship should be elucidated.Herein, we synthesize PtIrCuNiCr HEA electrocatalysts by a simple laser scanning ablation (LSA) strategy developed by our group.\n12\n Ni are electron-rich atoms with both paired and unpaired d electrons, while Cu and Cr are atoms with all the d orbitals of full and half-empty, respectively. On the basis of the Brewer-Engel valence bond theory,\n14\n the electrode surface with both pairs of d electrons and half-empty d orbitals will help the electron transfer as well as the adsorption and desorption of intermediates during the electrocatalytic reaction process. Consequently, the noble-metal mass activity of PrIrCuNiCr for HER and OER is 13.0 and 9.3-fold higher than those of Pt/C and Ir/C, respectively. As bifunctional electrocatalysts for overall water splitting, PtIrCuNiCr achieves an overpotential of ca. 190\u00a0mV at 10\u00a0mA\u00a0cm\u22122, far surpassing the reported catalysts. Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and X-ray diffractometer (XRD) manifest that the incorporation of Cu, Ni, and Cr atoms in HEAs induces remarkable shrunk surface strain. Theoretical calculations reveal that the effective strain can push the HEA toward the top of volcano plots in electrocatalytic HER and OER due to the improved adsorption kinetics of reaction intermediates (i.e., \u2217H, \u2217OH, and \u2217O). The lattice strain effect of HEA can be reasonably engineered by tailoring the particle radius and configurational entropy, thereby achieving optimal interaction with the intermediates. This work enables us to better understand the relationship between radius, entropy, strain, and electrocatalytic activities of HEAs. It also provides a systematic strain regulation strategy for designing high-performance HEA catalysts for overall water splitting.The HEA nanoparticles (NPs) were fabricated on graphene substrates by the LSA strategy (Figure\u00a01\nA).\n12\n Typically, various salt precursors with the same atom ratios were firstly loaded onto graphene and then transferred in hexane and exposed under laser with a pulse duration of 5\u00a0ns (the laser parameters are described in Table\u00a0S1 and Figure\u00a0S1). Due to the coupling of laser photons and salt electrons, the electron temperature immediately rises.\n15\n The surface microns of the salts layer are transformed into a melting pool when the fluence of laser light exceeds that of the melting threshold (Figure\u00a01B). As the laser pulse duration is much longer than the electron cooling rate (\u223c in the order of 1 ps), the excited electrons can keep transmitting their energy to the salt lattices, leading to the sublimation, ionization, erosion, and/or explosion of the molten salts.\n16\n As a result, highly compressed plasma containing a mass of electrons, ions, atoms, vapors, and clusters forms. The rapid heating of the salt layer and the medium in the vicinity of the plasma plume lead to the generation of cavitation bubbles which play essential roles in the subsequent NPs formation due to their confinement effect.\n17\n During plasma or bubble expansion and collapse, significant shock waves are created behind the ablation surface and expanded inside the salt layers with mechanical solid energy, resulting in the ejection of the salt layer into NPs. Although a laser pulse lasts only 5\u00a0ns, the ablation steps take place over several orders of magnitude in time, from electronic absorption of laser beam energy (10\u22129\u00a0s), through nanomaterial nucleation and growth (10\u22126\u201310\u22124\u00a0s), to particle condensation (\u223cms).\nFigure\u00a02\nA shows a transmission electron microscope image of the PtIrCuNiCr NPs. These HEA NPs are uniformly distributed on the graphene surface with an average radius of 2\u00a0nm (Figure\u00a02B). The inductively coupled plasma mass spectroscopy demonstrates that the actual HEA NPs loading on graphene is approximately 5\u00a0wt\u00a0% (Figure\u00a0S2). Energy-dispersive X-ray spectroscopy maps display Pt, Ir, Cu, Ni, and Cr elements throughout these NPs (Figure\u00a02C). Homogeneous distribution of individual elements with no obvious segregation has been found even at the atomic scale (Figure\u00a02D), manifesting the high-entropy character of the PtIrCuNiCr sample. Although the five compositions are thermodynamically insoluble,\n18\n the clear atomic lattices and positions of every atom signify that the atomic arrangement order is stabilized by the high-entropy microstructures. The corresponding line profiles (Figure\u00a02E) show that the atomic ratio of Pt, Ir, Cu, Ni, and Cr in each projected atomic column randomly fluctuates with significantly small variation. Based on the atomic ratios, the equation calculates the configurational entropy (\u0394Smix) of the PtIrCuNiCr HEA NPs to be 13.3\u00a0J/mol\u00b7K by the equation of \u0394S\n\nmix\n\u00a0= -R\n\n\n\n\u2211\n\ni\n=\n1\n\n5\n\n\n\nC\ni\n\n\nln\n\n\nC\ni\n\n\n\n, where R is the molar gas constant and C\n\ni\n is the atomic ratio of the i element. The \u0394S\n\nmix\n value is high enough to bestow the PtIrCuNiCr high-entropy feature. The valence states of the HEA components are monitored by X-ray photoelectron spectroscopy (XPS, Figure\u00a0S3 and Table\u00a0S2). The metallic bonding states of all five elements have been determined, implying the metallic character of HEA NPs. Notably, characteristic XPS peaks of Cu, Ni, and Cr show metallic and oxidized bonding states. This is because the NPs are exposed to air before characterization, and these 3d transition metals inevitably undergo surface oxidation. The XPS peaks of Cl element were not detected, indicating Cl was excluded in the form of chlorine during the LSA process. The XRD pattern (Figure\u00a02F) confirms the face-centered cubic phase of the HEA NPs, distinguishing the high-entropy solid-solution structure from amorphous materials.Despite the well-maintained atomic arrangement stabilized by the high-entropy characteristic, severe lattice strain of HEAs inevitably forms due to atoms with different radii across the lattice sites. X-ray absorption fine structure analysis manifests that the metal-metal bonds in HEAs are either longer or shorter than the Pt-Pt or Ir-Ir bonds in pure foils (Figure\u00a0S4 and Table\u00a0S3), demonstrating the presence of severe lattice distortion. The lattice distortion of HEA can induce lattice strain. Compared with XRD characteristic peaks of Pt (Figure\u00a0S5), the broadening and shifting peaks of the HEA sample clearly demonstrate the increased strain from its distorted lattices.\n19\n We simply calculated the strains of (111), (220), and (200) peaks using the Wilson method (Table\u00a0S4).\n20\n The strain in HEA NPs is four to five times that in pure Pt. The severe lattice strain can induce HEAs with a thermodynamical nonequilibrium state.\n13\n Our previous work found that such a state contributes to the higher potential energy of catalysts, thereby contributing to lowering the energy barrier in catalytic reactions and improving performance.\n21\n In addition, the stacking fault (SF) number in HEAs is much higher than that in Pt. Aberration-corrected HAADF-STEM images demonstrate the presence of the SFs on the HEA surface (Figure\u00a03\nA). These SFs can lower the coordination number of surficial atoms,\n22\n enhancing the HEA adsorption capability. As is shown in Table\u00a0S3, the coordination numbers of Pt and Ir atoms in HEAs are significantly lower than that in the pure metal foil. The unsaturated Pt and Ir sites serve as the active centers during the electrocatalysis to promote the interactions with intermediates (e.g. H\u2217, O\u2217, and OH\u2217) and improve OER and HER.\n23\n\nTo confirm the strain types in the PtIrCuNiCr NPs, the XRD patterns of Pt and HEAs are refined and compared. In comparison to the (111), (200), and (220) planes in Pt (Figure\u00a03B), those peaks in HEAs become broadening, weak, and shift toward a higher Bragg angle. The interplanar spacing change and the Bragg peak shift mainly result from Type I and II strains which act over a large and short distance, respectively (Figure\u00a0S6b).\n19\n As is shown in Figure\u00a03A, the interplanar spacings of (111) and (200) planes in HEAs are 0.215 and 0.187\u00a0nm, respectively, both of which are smaller than those in pure Pt (0.227 and 0.196\u00a0nm, respectively). The variation trends of the spacings are consistent with the XRD results (Table\u00a0S4), although the values are slightly discrepant due to different detection errors of HAADF-STEM and XRD.In Figure\u00a03A, many crystal defects such as atom dislocations and SFs have been found, contributing to the formation of Type III strain (Figure\u00a0S6). Type IV strain can cause lattice expansion or compression.\n19\n In our\u00a0case, the incorporation of 3d transition metal atoms in HEAs leads to lattice compression with small interplanar spacing (Figure\u00a03A). To further obtain the strain statistical and spatial distributions, we further investigated the strain at the atomic scale through HAADF-STEM imaging and performed geometric phase analysis to calculate the atomic strains. In the HEA NPs shown in Figure\u00a03C, the strain \u03b5xx is perpendicular to the (200) plane, \u03b5yy is in the (200), and \u03b5xy is the sheer strain. We take the average strain of the entire plane as a baseline (0%) for measuring the strain distribution across the plane. Broad distributions of normal strain and shear strain have been found across the HEA sample. These results are consistent with the XRD analysis (Figure\u00a03B).The strain fields modify the electronic properties of PtIrCuNiCr by distorting the local bonding character (Figure\u00a0S4), which can regulate the adsorption properties of the HEAs during electrocatalytic water splitting. The electrocatalytic HER and OER activity of HEAs in 1\u00a0M KOH was evaluated in a typical three-electrode setup. Commercial Pt/C and Ir/C, which are considered as typical HER and OER catalysts, respectively, are examined for comparison. For unit mass of Pt, the ECSA for PtIrCuNiCr/C (2.5\u00a0wt% Pt) is 178.8\u00a0m2/gPt, subsequently higher than that of Pt/C (with 20\u00a0wt\u00a0% Pt, 65.75\u00a0m2/gPt) (Supplementary Inforamtion\nFigure\u00a0S7). Figure\u00a04\nA shows the linear sweep voltammetry curves for HER. The PtIrCuNiCr catalyst exhibits remarkable catalytic activity with an onset overpotential of \u223c0\u00a0mV and low overpotential of only 200\u00a0mV to drive 100\u00a0mA\u00a0cm\u22122 (vs 274\u00a0mV for Pt/C). For OER (Figure\u00a04B), the HEA sample requires an overpotential of only 176\u00a0mV to achieve 10\u00a0mA\u00a0cm\u22122 which shows much higher activity than that of Ir/C (238\u00a0mV) as well as the recently reported noble metal-based catalysts (Table\u00a0S5). After being normalized with the noble metal mass (Figure\u00a0S8), the mass activity of PtIrCuNiCr reaches 1.57 A/mg at \u22120.3\u00a0V versus RHE for HER and 0.81\u00a0A/mg at 1.5\u00a0V versus RHE for OER, which are 13.0- and 9.3-fold higher than those of Pt/C (0.12 A/mg for HER) and Ir/C (0.087 A/mg for OER), respectively. In terms of such remarkable activities toward HER and OER, the PtIrCuNiCr HEA NPs were further used as both anode and cathode for overall water splitting (Figure\u00a04C). Strikingly, it delivers a current density of 10\u00a0mA\u00a0cm\u22122\u00a0at an overpotential of 190\u00a0mV, outperforming the Ir- or Pt-based electrocatalysts reported so far (Table\u00a0S6). The HEA electrode presents excellent durability (Figure\u00a04D), manifesting the superior stability of the HEA catalyst for overall water splitting.The appropriate adsorption energy of intermediates on the catalyst surface is crucial in improving catalytic performance. Volcano plots are often used to describe the relationship between reaction overpotentials and adsorption energy.\n24\n The calculated difference in binding energies of \u2217O and \u2217OH (\u0394GO\u2217-\u0394GOH\u2217) and the covalent metal-hydrogen bond absorption energy (EM-H) are verified to describe well the trend\u00a0of OER and HER activities on catalyst surfaces, respectively.\n25\n\n,\n\n26\n As shown in Figures\u00a0S9 and S10, \u0394GO\u2217-\u0394GOH\u2217 and EM-H are closely related to the radius (r)\u00a0of catalysts.\n27\n With the catalyst radius of 2\u00a0nm, we correlated the EM-H with the exchange current density (j0) derived from the Tafel plots to obtain a volcano plot of the HER case (Figure\u00a05\nA). The \u0394GO\u2217-OH\u2217 was associated with the overpotential for the current density of 1\u00a0mA\u00a0cm\u22122 to achieve a volcano plot of OER (Figure\u00a05B). Both the pure metals of Pt and Ir are located in the left legs of the OER volcano plots or the right legs of the HER volcano plots, manifesting strong adsorption of intermediates during the reactions. In contrast, the PtIrCuNiCr approaches to the top of the volcano, indicating that the HEA catalyst has moderate adsorption energy of intermediates than Pt and Ir. The theoretical calculation reveals that the catalyst strain plays significant role in the optimization of the adsorption energy (See details in supplemental information theoretical calculations). Unlike pure metals whose strain only depends on the radius (Figure\u00a0S11), HEAs can regulate strain through entropy and radius (Figures\u00a0S12 and S13). Catalyst radius can induce strain by modifying the chemical bonds of atoms at corners and edges.\n27\n High-entropy character of HEAs induces the inherent distortion of the lattice structure, thus releasing the local strain in materials.\n28\n Therefore, the catalytic behavior of HEAs differs from that of the pure metals at the same size due to the strain caused by the high-entropy character. In our case, the entropy-driven strain weakens the adsorption energy of catalytic intermediates on HEAs, facilitating the desorption of products on the HEA surface and further improving the electrocatalytic activities. Figures\u00a05C and 5D show the dependence between the effective strain, catalyst radius, and the HER/OER performance mapping of HEAs as well as pure Pt and Ir catalysts. HEAs show larger j0 than Pt and Ir at the radius of 2\u00a0nm due to the higher strain caused by the high-entropy effect, indicating a fast reaction rate. Thus, the theoretical findings are consistent with the experimental results. To find out the optimal strain for electrocatalytic activity, the relationship between strain of HEAs and the average distance \n\n\nD\n\u00af\n\n\n (kcal/mol) to the volcano apex is investigated (Figure\u00a0S14). It is found that when the catalyst strain is around 1.08% and 1.25%, respectively, the activity of HER and OER reaches the maximum. Thus, regulating the strain of HEAs by radius and entropy is an effective avenue to improve their electrocatalytic activity.In this work, we synthesize highly active and durable HEA electrocatalysts by a simple laser scanning ablation strategy. Applied as both anode and cathode, the HEA catalyst of PtIrCuNiCr exhibits the lowest overpotential (ca. 190\u00a0mV) to achieve a current density of 10\u00a0mA/cm2 in electrocatalytic overall water splitting. Theoretical calculation demonstrates that strain in HEAs can regulate the binding energy of intermediates during electrocatalysis by changing metal-metal bonding energy. Unlike pure metals, HEA can adjust the strain through radius and entropy to enhance electrocatalytic activity. This work offers a new trial of strain engineering to develop efficient and durable HEA electrocatalysts for overall water splitting. It will inspire a new understanding of catalytic mechanisms of HEAs and guide the search for efficient HEA catalysts in a vast materials database.Due to the short synthesis time, it is challenging to synthesize HEA catalysts with very uniform particle size by the laser scanning ablation method. Therefore, the effect of the particle size and atomic ratio of the elements on the catalysis activity was studied by the theoretical calculation instead of experiment in this work.\n\n\n\n\n\n\n\n\nREAGENT or RESOURCE\nSOURCE\nIDENTIFIER\n\n\n\n\n\nChemicals, Peptides, and Recombinant Proteins\n\n\n\nH2PtCl6\u00b76H2O\nAladdin Co., Ltd\n18497-13-7\n\n\nH2IrCl6\u00b76H2O\nAladdin Co., Ltd\n16941-92-7\n\n\nCuCl2\u00b72H2O\nAladdin Co., Ltd\n10125-13-010125-13-010125-13-0\n\n\nNiCl2\u00b76H2O\nAladdin Co., Ltd\n7791-20-0\n\n\nCrCl3\u00b76H2O\nAladdin Co., Ltd\n10060-12-5\n\n\nGlucose\nXilong Scientific Co., Ltd\n14431-43-7\n\n\nNH4Cl\nMacklin\n12125-02-9\n\n\nKCl\nMacklin\n7447-40-7\n\n\nNaCl\nMacklin\n7647-14-5\n\n\nHexane\nXilong Scientific Co., Ltd\n110-54-3\n\n\nNafion solution\nDuPont\n31175-20-9\n\n\nKOH\nMacklin\n1310-58-3\n\n\nCommercial 20%Pt/C\nJohnson Matthey\nCat#S128513\n\n\n\n\n\nFurther information and requests for resources should be directed to and will be fulfilled by the lead contact, Bing Wang (bingwang@nju.edu.cn).This study did not generate new unique reagents. All chemicals were obtained from commercial resources and used as received.A typical process is as follows:\n29\n 4\u00a0mmol of glucose, 6\u00a0mmol of NH4Cl, and 80\u00a0g of KCl and NaCl with the weight ratio of 51:49 were first treated by ball-milling, and dried at 150\u00b0C for 8\u00a0h to form brown mixture. Then the mixture was pyrolyzed at 1050\u00a0\u00b0C under N2 for 1\u00a0h with the heating rate of 35\u00b0C/min, followed by natural cooling to room temperature. Graphene forms after being ultrasonically rinsed with distilled water and ethanol several times.A pulsed fiber laser (SLT-PTM-100, Jiangsu Yanchang Sunlaite new energy Co. Ltd, China) was used during the laser scanning ablation (LSA) process. The parameters of the laser are displayed in Table\u00a0S1. A Gaussian laser beam with a peak power of 20\u00a0kW was focused (Figure\u00a0S1) and supplied at normal incidence to the graphene surface.Various chloride salts were mixed in ethanol with 0.01\u00a0M for each metallic element. Taking the synthesis of PtIrCuNiCr as an example, the salt precursors of H2PtCl6\u00b76H2O, H2IrCl6\u00b76H2O, CuCl2\u00b72H2O, NiCl2\u00b76H2O, CrCl3\u00b76H2O were first mixed in ethanol. The mixed solution was then directly dropped onto graphene with a loading of \u223c0.1\u00a0ml/mg, followed by ultrasonic treatment for ensuring the uniform salt load on graphene. The loaded substrates were transferred to a vacuum oven for drying at room temperature.For the synthesis of HEA nanoparticles (NPs) by the LSA method, the precursors-loaded graphene was\u00a0firstly dispersed in hexane with 0.5\u00a0mg/ml by magnetic stirring. Hexane is used because the salt precursors are insoluble in it, ensuring the salts remain on substrates. Moreover, the oxygen-free structure of hexane is conducive in preventing the oxidation of the HEA NPs during LSA process. The solution was irradiated under agitation with the pulse laser for 30\u00a0min, ensuring all the substrates were irradiated by the laser beam.H2 evolution reaction (HER) and O2 evolution reaction (OER) electrocatalytic experiments were performed on a CHI 660D electrochemical workstation with a three-electrode cell system. In this system, Ag/AgCl (sat.\u00a0KCl) and carbon rod electrodes were used as the reference electrode and counter electrode, respectively. To prepare the working electrode, 10\u00a0mg of the HEA NPs on graphene was dispersed in the mixture of\u00a0400\u00a0\u03bcL ethanol and 50\u00a0\u03bcL 5% Nafion solution for 20\u00a0min by ultrasonication to form homogeneous inks.\u00a050\u00a0\u03bcL of the ink was carefully dropped onto a nickel foam (NF, 0.5\u00d70.5\u00a0cm2), resulting in a HEA NPs/graphene loading of 4.4\u00a0mg\u00a0cm\u22122. The electrocatalytic electrode was dried at room temperature naturally. Based on the results of inductively coupled plasma mass spectroscopy (ICP-MS), the HEA NPs loading on graphene is approximately 5\u00a0wt.% with 2.5\u00a0wt.% of Pt and 1.5\u00a0wt.% of Ir. As such, the actual HEAs NPs loading on NFs was about 0.22\u00a0mg\u00a0cm\u22122 with Pt and Ir loading of 0.11 and 0.07\u00a0mg\u00a0cm\u22122, respectively. Similar to the HEA catalyst, 10\u00a0mg of the commercial Pt/C (20 wt%XC-72) or Ir/C (5 wt%XC-72) was dispersed in the mixture of 400\u00a0\u03bcL ethanol and 50\u00a0\u03bcL 5% Nafion solution by ultrasonic treatment for 20\u00a0min. Then 50\u00a0\u03bcL of the ink was dropped onto a NF with the same area and dried at room temperature. The loading amount of the commercial Pt/C and Ir/C electrocatalyst on NFs was both 4.4\u00a0mg\u00a0cm\u22122. As the mass ratio of Pt and Ir in samples is 20\u00a0wt.% and 5\u00a0wt.%, respectively. The actual Pt and Ir NPs loading on NFs was 0.88\u00a0mg\u00a0cm-2 and 0.22\u00a0mg\u00a0cm-2, respectively. The HER and OER electrochemical experiments were conducted in a 1.0\u00a0M KOH aqueous solution at room temperature. All potentials for OER and HER reported herein were referenced to the reversible hydrogen electrode (RHE) using the equation ERHE\u00a0= EAg/AgCl\u00a0+ 0.197\u00a0+ 0.059\u00a0\u00d7\u00a0PH. The measurement was conducted under rotation to remove the produced bubbles with 90% IR correction.The overall water splitting was investigated in a two-electrode system with 1.0\u00a0M KOH electrolyte, in which PtIrCuNiCr-NF served as both anode and cathode with a loading of 4.4\u00a0mg\u00a0cm\u22122. The durability was assessed at a constant potential of 1.5\u00a0V for 100\u00a0h.The morphology of as-prepared samples was examined by transmission electron microscopy (TEM, Tacnai G2 F20, FEI). Energy-dispersive X-ray spectroscopy (EDS, Elite T EDS System) equipped on the TEM was employed to record the element distribution of HEAs on graphene. Aberration-corrected high-angle-annular-dark-filed scanning transmission electron microscope (HAADF-STEM) analysis is characterized using Thermofisher Themis Z (FEI) with 200\u00a0kV. An EDS instrument with the SuperX detector equipping the HAADF-STEM was used to obtain the element distribution of HEAs at the atomic scale. Geometric phase analysis (GPA) was conducted with Digital Micrograph software to obtain the strain information on the surface of HEA NPs. The crystal structures of the samples were measured by a powder X-ray diffractometer (XRD, Ultima III, Rigaku Corp., Japan) using Cu-K\u03b1 radiation (\u03bb\u00a0= 1.54178\u00a0\u00c5, 40\u00a0kV, 40\u00a0mA). The\u00a0atomic ratios of HEM NPs were analyzed by PerkinElmer AVIO500 ICP-MS. The solutions were prepared by digesting the samples in aqua regia followed by dilution with 2% hydrochloric acid. The surface composition of the samples was performed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with the non-monochromatic Al K\u03b1 X-ray as the X-ray source. The binding energy of C1s (284.6\u00a0eV) was used to calibrate the other binding energies.The absorption energy of intermediates on the catalyst surface has been widely accepted as a descriptor for the activity of electrocatalysts. The absorption energy varies with the size of catalyst particles due to the changed coordination environment caused by the chemical bonds of the surface atoms.\n30\n\n,\n\n31\n Since metal atoms are coordinated to several neighbor atoms on the surface, we first consider the metallic bond strength. For the metallic bond strength of NPs composed of n layers of atoms, we made a hypothesis that the bond strength and radius of NPs conform to a Gaussian function:\n32\n\n\n\n(Equation\u00a01)\n\n\n\n1\n\nE\n\nM\n\u2212\nM\n\nr\n\n\n=\n\n1\n\nE\n\nM\n\u2212\nM\n\n\u221e\n\n\n+\n\n(\n\n\n1\n\nE\n\nM\n\u2212\nM\n\n\nD\ni\na\nt\no\nm\ni\nc\n\n\n\n\u2212\n\n1\n\nE\n\nM\n\u2212\nM\n\n\u221e\n\n\n\n)\n\n\ne\n\n\u2212\n\n\n\n(\n\nn\n\nE\n\nM\n\u2212\nM\n\nr\n\n\n)\n\n2\n\n\n3\n\n\u03c3\n2\n\n\n\n\n\n\n\n\nwhere \n\n\nE\n\nM\n\u2212\nM\n\nr\n\n\n is the metallic bond strength for an NP with radius of r (nm), \n\n\nE\n\nM\n\u2212\nM\n\n\u221e\n\n\n is the bond strength for bulk materials, \n\n\nE\n\nM\n\u2212\nM\n\n\nD\ni\na\nt\no\nm\ni\nc\n\n\n\n is the covalent bond strength for a diatomic molecule, \n\n\u03c3\n\n is a controlling constant for dimension consistency (ca. 1000\u00a0kJ\u00a0mol-1). As the size of nanoparticles (NPs) decreases, metallic bonds become weaker (Figure\u00a0S8).\n33\n Driven by the stability of the system, interaction between hydrogen/oxygen adsorbates and metal goes stronger (Figure\u00a0S9). According to the covalent metal-hydrogen/oxygen bond dissociation energy and volcano plot of bulk materials, for NPs composed by n layers of atoms, we can assume:\n\n(Equation\u00a02)\n\n\n\n1\n\nE\n\nM\n\u2212\nH\n\nr\n\n\n=\n\n1\n\nE\n\nM\n\u2212\n\nH\nO\n\n\n\u221e\n\n\n+\n\n(\n\n\n1\n\nE\n\nM\n\u2212\nH\n\n\nD\ni\na\nt\no\nm\ni\nc\n\n\n\n\u2212\n\n1\n\nE\n\nM\n\u2212\nH\n\n\u221e\n\n\n\n)\n\n\ne\n\n\n\n(\n\nn\n\nE\n\nM\n\u2212\nH\n\nr\n\n\n)\n\n2\n\n\n3\n\n\u03c3\n2\n\n\n\n\n\n\n\n\n\n\n(Equation\u00a03)\n\n\n\n1\n\n\u0394\n\nG\n\n\nO\n\u2217\n\n\u2212\nO\n\n\nH\n\u2217\n\n\n\nr\n\n\n\n=\n\n1\n\n\u0394\n\nG\n\n\nO\n\u2217\n\n\u2212\nO\n\n\nH\n\u2217\n\n\n\n\u221e\n\n\n\n+\n\n(\n\n\n1\n\n\u0394\n\nG\n\n\nO\n\u2217\n\n\u2212\nO\n\n\nH\n\u2217\n\n\n\n\nD\ni\na\nt\no\nm\ni\nc\n\n\n\n\n\u2212\n\n1\n\n\u0394\n\nG\n\n\nO\n\u2217\n\n\u2212\nO\n\n\nH\n\u2217\n\n\n\n\u221e\n\n\n\n\n)\n\n\ne\n\n\n\n(\n\nn\n\u0394\n\nG\n\n\nO\n\u2217\n\n\u2212\nO\n\n\nH\n\u2217\n\n\n\nr\n\n\n)\n\n2\n\n\n3\n\n\u03c3\n2\n\n\n\n\n\n\n\n\nwhere \n\n\nE\n\nM\n\u2212\nH\n\nr\n\n\n is the binding energy between an NP with a radius of r (nm) and hydrogen adsorbates, \n\n\nE\n\nM\n\u2212\nH\n\n\u221e\n\n\n is the hydrogen absorption energy for bulk materials, \n\n\nE\n\nM\n\u2212\nH\n\n\nD\ni\na\nt\no\nm\ni\nc\n\n\n\n is the covalent metal-hydrogen bond dissociation energy, \n\n\n\n\u0394\nG\n\n\n\nO\n\u2217\n\n\u2212\n\n\nO\nH\n\n\u2217\n\n\n\n\n is the free energy change of the intermediate step in OER. \u03c3 is 350\u00a0kcal\u00a0mol\n-1 and 10\u00a0eV for metal-hydrogen and metal-oxygen, respectively. We also benchmarked some data within ab initio Density Functional Theory (DFT) methods. The M-H bond dissociation energy, M-H absorption energy, and free energy change of proton desorption of Pt, Ir, Cu, Ni, Cr at atomic level and for bulk materials are displayed in Table\u00a0S7. \u0394GO\u2217-OH\u2217 at atomic level was carried within the GGA-PBE functional in DMol3.\n34\n Each atomic structure is fully relaxed until forces acting on atoms are less than 0.05\u00a0eV/\u00c5.The DFT calculation is carried out within the GGA-PBE functional in Quantum ESPRESSO code.\n35\n The calculations are carried out using separable norm-conserving pseudopotentials and a plane-wave basis set with the kinetic energy cutoff of 40 Ry and Gamma k-points. Each atomic structure is fully relaxed until forces acting on atoms are less than 0.01\u00a0eV/\u00c5.\n\n(Equation\u00a04)\n\n\n\n\u03b5\n\u00af\n\n=\n\n\n\n\u2211\ni\n\n\n\u03b5\ni\n\n\n/\n\nN\ni\n\n\n\n\n\nwhere N is the concentration of each principle element in the HEA.Entropy is essential for the study of HEAs. The entropy of configuration is given by:\n\n(Equation\u00a05)\n\n\n\nS\n\nc\no\nn\nf\n\n\n=\n\u2212\nR\n\n\n\u2211\n\ni\n=\n1\n\nn\n\n\n\nX\ni\n\n\nln\n\n\nX\ni\n\n\n\n\n\n\nwhere \n\n\nX\ni\n\n=\n\n\nN\ni\n\n/\n\nN\nA\n\n\n\n, where N\n\ni\n is the number of \n\ni\n\n atoms, N\n\nA\n is Avogadro\u2019s constant, R is a universal gas constant. The S\n\nconf\n dependent strain is unique in HEAs materials. For NPs with the same radius r, the overall metal distribution in HEAs is analogous to the coloring problem.\n36\n As shown in Figure\u00a0S11, different entropy of HEAs exhibit distinct strain distribution.Thus, unlike pure metals, HEAs can adjust strain through entropy and particle radius, to optimize the electrocatalytic performance. The relationship between strain, entropy, and radius of HEA catalysts is revealed in Figure\u00a0S12.The electrocatalytic activity of the catalyst is described by the average distance \n\n\nD\n\u00af\n\n\n (kcal/mol) to the apex in the volcano plot of HER and OER. \n\n\nD\n\u00af\n\n\n can be calculated by:\n\n(Equation\u00a06)\n\n\n\nD\n\u00af\n\n=\n\n\n\u2211\ni\n\n\n(\n\nE\ni\nr\n\n\n\n\u2212\n\nE\n\na\np\ne\nx\n\n\n)\n/\n\nN\ni\n\n\n\n\nwhere \n\n\nE\ni\nr\n\n\n is the binding energy of principle elements, \n\n\nE\n\na\np\ne\nx\n\n\n\n is the apex binding energy in the volcano plot. When the absolute value of \n\n\nD\n\u00af\n\n\n is 0, the electrocatalytic activity achieves optimum. According to the Equations 2, 3, 4, and 6, the effect of strains in HEAs on the electrocatalytic activity is disclosed in Figure\u00a0S13.This project is supported by the National Key Research and Development Program of China (2021YFF0500501), Major Research Plan of the National Natural Science Foundation of China (91963206), National Natural Science Foundation of China (22279053, 52072169, 51627810, 51972164), Program for Guangdong Introducing Innovative and Enterpreneurial Teams (2019ZT08L101), Fundamental Research Funds for the Central Universities (14380180), Civil Aerospace Technology Research Project (B0108), and Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory.B.W., X.Z., Y.Y., and Z.Z. conceived the idea and designed the present work. B.W., W.L., L.H., and C.W.\u00a0carried out the experiments. X.Z. and Y.L. carried out the theoretical calculations. C.W. and X.Y. performed detailed microscopic characterizations. B.W. and X.Z. drafted the paper.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.isci.2023.106326.\n\n\nDocument S1. Figures\u00a0S1\u2013S14 and Tables\u00a0S1\u2013S7\n\n\n\n", "descript": "\n                  Developing active and cost-effective bifunctional electrocatalysts for overall water splitting is challenging but mandatory for renewable energy technologies. We report a high-entropy alloy (HEA) of PtIrCuNiCr as a bifunctional electrocatalyst for overall water splitting, which shows a low overpotential of ca. 190\u00a0mV at the current density of 10\u00a0mA\u00a0cm\u22122. Compared with pure metals, HEAs exhibit remarkable surface strain due to severe lattice distortion in their crystal structures. Theoretical calculations reveal that the strain can regulate the binding energy of intermediates on catalysts by adjusting the metal-metal bonding energy. It pushes the HEA toward the top of volcano plots to achieve superior electrocatalytic activity for both hydrogen and oxygen evolution reactions. The strain effect of HEAs on electrocatalysis can be well engineered by tuning the catalyst radius or configurational entropy. This work renders a systematic strain regulation strategy for designing a high-performance HEA catalyst for overall water splitting.\n               "}
{"full_text": "No data was used for the research described in the article.Ammonia (NH3) plays a crucial role in agriculture, pharmaceuticals, textile industry, and plastic production [1]. In addition, It is also considered an important energy storage medium and a promising carbon-free energy carrier because of its high content of hydrogen (17.6\u00a0wt%) [1]. The main industrial process for the synthesis of NH3 is carried out through the reaction of nitrogen (N2) and hydrogen (H2) at elevated temperatures (400\u2013500\u00a0\u00b0C) and pressures (150\u2013300\u00a0atm) using the so-called Haber-Bosch process [2]. The latter method relies on the H2 produced from fossil fuels and it requires intensive energy use leading to expensive operational costs [3]. In the past decade, the electrochemical synthesis of NH3 with renewable energy (wind or solar) input has attracted enormous interest as an alternative route [4]. Especially, the use of N2 molecule in the electrochemical reduction process became a hot-topic [4]. While interesting progress in the development of N2 reduction catalyst has been made, the low reaction rate of NH3 evolution limits its widespread application [5,6]. The reduction of N2 to NH3 also suffers from low activity and selectivity due to the high stability of the triple non-polar bond (NN) [5]. Alternatively, the moderate dissociation energy of the N\u00a0=\u00a0O bond (204 kJmol\u22121) in nitrate ion (NO3\n\u2013) makes it an attractive choice over N2 since it promises better kinetics for NH3 production [7,8]. Nitrates are common in nature in the form of metal nitrate deposits (e.g sodium nitrate (NaNO3)) and they are considered one of the major surface and groundwater pollutants that are strictly regulated by environmental agencies due to their harmful effects [9]. NO3\n\u2013 source mainly comes from fertilizers, nuclear wastes, industrial wastewater, and livestock excrements with a wide range of concentrations up to ca. 2\u00a0M [4]. High levels of nitrate consumption may lead to serious immediate health problems (e.g. cancer) [10]. Because of this risk, the USEPA (U.S. Environmental Protection Agency) established that the maximum concentration of NO3\n\u2013 in water should not exceed 10\u00a0mg\u00a0N/L [11]. Many common technologies were used to treat water contaminated with NO3\n\u2013 such as ion exchange, biological denitrification, and reverse osmosis [10]. Although, the ion exchange and the reverse osmosis are inapplicable for commercial applications because of the high cost of their additional pretreatment and posttreatment [12]. On the other hand, biological denitrification suffers from the slow potabilization system, and the risk of biological contamination since the process requires phosphorus resources and a certain amount of organic matrix, which can result in the generation of organic pollutants [12]. Using electrochemical methods to remove NO3\n\u2013 contaminants from wastewater has been an important and attractive topic in the environmental research field [4]. The efficient procedure of electrogenerated NH3 from NO3\n\u2013 would serve dual purposes: First, for water purification, and second, for waste NO3\n\u2013 reuse to produce a highly useful chemical product (NH3) [4]. Although the electrocatalytic nitrate reduction reaction (NO3RR) is attractive, worth to mention that sometimes during NO3RR may occur side reactions like the formation of products with low oxidation states such as nitrite (NO2\n\u2013), nitrogen dioxide (NO2), nitrous oxide (N2O), nitric oxide (NO), hydrazine (NH2NH2), hydroxylamine (NH2OH), etc [4,13]. For example, NH2NH2 and NH2OH evolve preferably in acidic media, whereas NO2\n\u2013 and NH3 are considered the main products in the neutral or basic environment [13]. Prior studies indicate that the transformation of NO3\n\u2013 into NO2\n\u2013 is the rate-determining step of NO3RR [14]. Recently, the scientific focuses were mainly on obtaining highly active and selective catalysts for NO3RR.The activity of an electrocatalyst depends on various parameters such as medium pH, applied potential, surface area, crystal planes, composition, etc [15]. Mono metallic (Cu, Ru, etc.) and bimetallic (Pd\u2212Cu, Pt\u2212Cu, Pd\u2212Sn, etc.) catalysts are among the most explored materials in electrocatalytic reduction of NO3\n\u2212\n[16\u201320]. For example, Pt and Pd have been shown to exhibit a high activity toward NO3\n\u2212 reduction to NH3\n[20,21]. Another outstanding monometallic catalyst is the metal Ru. By using Ru clusters with internal strains, Yu et al. achieve 100 % Faradaic efficiency (FE) for NO3\n\u2212 reduction. They claimed that strains in Ru clusters help the formation of hydrogen radicals, which accelerates the hydrogenation of NO3\n\u2212 during the reduction process [22]. In addition to monometallic nanoparticles (NPs), a bimetallic catalyst like CuPd has been proven to exhibit excellent performance for NO3RR as well [14]. Although noble metals are efficient as catalysts, their high cost limits their widespread application. In addition to cost, an ideal catalyst for NO3RR should offer good activity and selectivity [7]. Only recently, the development of low-cost catalysts which are selective for NO3\n\u2212 reduction gained interest. Zhang et al. demonstrated that oxygen vacancies in semiconductor TiO2 electrodes improve the overall FE for NO3\n\u2212 reduction by weakening the N-O bond [3,23]. Using metallic Cu NPs, Shih et al. studied the impact of facets on the electrocatalytic NO3\n\u2212 reduction. The authors reported that different crystal facets contribute in different ways to the kinetics and the mode of electrocatalytic NO3RR [24]. On the other hand, Sargent et al. have developed a Cu-Ni alloy catalyst with a unique electronic structure that has yielded a 6-fold increase in NO3\n\u2212 reduction activity compared to regular Cu electrodes [25]. Transition metal phosphides (TMPs) have emerged as promising low-cost candidates for electrocatalytic reduction reactions [26\u201330]. One of the most attractive features of TMPs is the charge transfer effect, M(\u03b4\n+) \u2192 P(\u03b4\n\u2212), which allows reversibly to produce adsorbed hydrogen atoms (H) on the catalyst surface [26,30,31]. Moreover, TMPs have been developed because of their good conductivity due to d\u2010electron configuration [32]. Recently, amorphous nickel phosphide (Ni2P) deposited on carbon cloth was demonstrated as an efficient catalyst for NO3RR [33]. Yao et al. have developed a procedure to grow Ni2P with (111) facet on Ni foam which yielded 0.056\u00a0mmol\u00a0h\u22121 mg\u22121 NH3 with FE equal to 99.2 % [34]. Further, Yang et al. also introduced Ni2P catalyst for electrochemical NH3 synthesis using NO2\n\u2212 as a source. Their catalyst showed a low onset potential of \u223c0.2\u00a0V vs. RHE with FE exceeding 90 % [26,35].Herein, we report for the first time the NO3RR to NH3 using different phases of iron phosphide. We synthesized colloidal Fe2P and FeP NPs and deposited them on a titanium (Ti) substrate by using a spin-coating approach. To activate the iron phosphide electrodes, a short heat treatment at 450 \u00baC was carried out. The Fe2P catalyst showed the highest yield (0.25\u00a0mmol\u00a0h\u22121 cm-\u22122) and FE (96 %) at \u2212\u00a00.55\u00a0V vs. RHE for NH3. Using NO3\n\u2212 as a starting species, we revealed the complex reaction pathways occurring during NH3 generation. The recycling test confirmed that FeP catalysts exhibited excellent stability during the NO3RR. To get relevant information about the fundamental origins behind the better performance of Fe2P compared to FeP and to determine the free energies of intermediates, density functional theory (DFT) calculations were performed.The preparation of Fe2P and FeP was performed using a modified solvothermal synthesis procedure reported by chouki et al. [36]. Details of the synthesis procedure and the experimental measurements can be found in Supporting Information (SI).Following the activation of catalysts at 450\u00a0\u00b0C, we recorded the XRD pattern of iron phosphide samples (\nFig. 1). The diffraction patterns are assigned to different phases: hexagonal for Fe2P (P\n\n\n\n6\n\n\u0305\n\n2m, PDF\u266f 1008826) and orthorhombic for FeP (Pbnm, PDF\u266f 9008932).The Rietveld refined crystal parameters for these phases gave: a =\u2009b =\u20095.910\u2009\u00b1\u20090.0070\u2009\u00c5, and c =\u20093.543\u2009\u00b1\u20090.0040\u2009\u00c5 for Fe2P, and a =\u20093.105\u2009\u00b1\u20090.0006\u2009\u00c5, b =\u2009197\u2009\u00b1\u20090.0004\u2009\u00c5, and c =\u20095.783\u2009\u00b1\u20090.0007\u2009\u00c5 for FeP. These values are in good agreement with the literature [37]. The reliability of the refinement was assessed by the low values of the weighted profile (R\nwp) factor, which is equal to R\nwp =\u20096.34 % for Fe2P and R\nwp =\u20094.20 % for FeP. The processed data meet the established criteria (R\nwp < 20 %) for good refinement [38].Transmission electron microscopy (TEM) studies revealed that the FeP catalyst is composed of microsphere-like objects with typical sizes ranging from 400 to 1300\u2009nm (\nFig. 2a). Fig. 2c,d shows the uniform distribution of Fe and P elements (at% ratio1:1) in the given agglomerate. The Fe2P sample contained three types of particles: nanospheres (NSs), nanocubes (NCs), and nanorods (NRs) (Fig. 2f). While the Fe2P NSs and Fe2P NCs show uniform average diameters (8\u2009\u00b1\u20093.2\u2009nm and 10\u2009\u00b1\u20092.3\u2009nm), the Fe2P NRs average sizes (7\u2009\u00b1\u20091.0\u2009nm in diameter and 15\u2009\u00b1\u20092.7\u2009nm in length) varies. High-resolution TEM (HR-TEM) studies indicated that the lattice spacing of 0.28\u2009nm of the orthorhombic FeP (Fig. 2e) and 0.15\u2009nm of the hexagonal Fe2P (Fig. 2g) correspond respectively to the (200) and (102) crystal planes.The morphology of Fe2P and FeP thin films was studied using scanning electron microscopy (SEM). As can be seen from Fig. 2h, in the FeP film that is composed of microspheres, larger voids are formed. As determined from the cross-section SEM image, the thickness of the film is around \u223c26\u2009\u00b5m (Fig. 2i). The Fe2P thin film contains large aggregates (Fig. 2j) and densely packed NPs (Fig. 2k). The thickness of the Fe2P film is about \u223c10\u2009\u00b5m (Fig. 2l).Electrocatalytic NO3\n\u2212 reduction experiments using iron phosphide films were performed in a mixture of 0.2\u2009M NaNO3 and 0.5\u2009M sodium hydroxide (NaOH) at pH 13. The catalytic activities of Fe2P and FeP catalysts for NO3\n\u2212 reduction were evaluated using the linear sweep voltammetry (LSV) method. LSV tests displayed that both catalysts have negligible electrochemical activity toward hydrogen evolution reaction (HER) in 0.5\u2009M NaOH in the potential window from 0.0 to \u2013 0.55\u2009V vs RHE (\nFig. 3a). To our expectation, the current density of Fe2P and FeP cathodes were markedly increased upon adding NaNO3 into the solution, which showed an onset potential at about \u2212\u20090.3\u2009V vs. RHE for Fe2P and \u2212\u20090.34\u2009V vs. RHE for FeP. To explain the observed improved activities for NO3RR we also conducted experiments using metallic iron (Fe) thin film. Fe is widely used for catalysis [39]. Recently, it was introduced as a good catalyst for NO3\n\u2212 reduction to NH3\n[4]. For comparison, we also provided LSV characteristics of Fe, iron oxide, Ti, Pt, and iron phosphate thin films. The LSV test showed that the Fe catalyst is not active for HER (Fig. S2). However, it showed a good activity toward NO3\n\u2212 reduction. It exhibited an onset potential at about \u2212\u20090.34\u2009V vs. RHE which is comparable to FeP and Fe2P catalysts (Fig. 3b). However, after repeated LSV cycles using Fe thin film the current greatly reduced which indicates an ongoing corrosion process (Fig. S2). Pt exhibited an onset potential at about \u2212\u20090.22\u2009V vs. RHE which is lower than FeP and Fe2P catalysts (Fig. S3). However, the high cost and scarcity of Pt limit its widespread use. On the other hand, Ti plate alone and pristine iron oxide deposited on Ti showed low current densities and high onset potentials >\u2009\u22120.4\u2009V vs. RHE proving that they have negligible contributions to NO3\n\u2212 reduction (Fig. S3). It is noteworthy that the pristine iron phosphate thin film produced a lower current for NO3\n\u2212reduction than the initial FeP and Fe2P thin films. In addition, during the repetitive cyclic test, the activity of iron phosphate film decreased significantly which confirms once again its unsuitability for NO3RR when is used as a stand alone film (Fig. S4). It seems that metal oxide and metal phosphate surface termination causes a synergistic effect which improves the activity of the iron phosphide films.Tafel analysis was performed as a useful metric for interpreting the polarization curves [36,40,41]. Fig. 3c shows the Tafel relationships, potential versus log|j| (logarithm of current), for the HER and the NO3RR recorded in 0.5\u2009M NaOH and a mixture of 0.5\u2009M NaOH / 0.2\u2009M NaNO3 (pH 13), respectively. The HER process usually goes through three reaction pathways: the Heyrovsky reaction (e.g. desorption step: H (ads) +\u2009H3O+ + e\u2212\u2009\u2192 H2 +\u2009H2O), where H (ads) represents a hydrogen atom adsorbed at the active site of the catalyst), the Volmer reaction (e.g. discharge step: H3O+ + e\u2212 \u2192 H (ads) +\u2009H2O, or the Tafel reaction (e.g. discharge or recombination step: H (ads) +\u2009H (ads) \u2192 H2). The Volmer reaction is considered slow since the adsorption of hydrogen on the active sites will result in a slope, higher than 116\u2009mV dec\u22121\n[41]. In the case of the NO3RR, the reduction of nitrates proceeds according to the proposed mechanism: NO3\n\u2212 (sol) \u21cc NO3\n\u2212 (ads) and NO\u2212 (ads) +\u2009H (ads) +\u2009e\u2212\u2009\u2192 NO2\n\u2212 (ads) +\u2009OH\u2212. The produced NO2\n\u2212 (ads) either desorbs from the surface of the electrode or reduces to give NH3 according to the reaction: NO2\n\u2212 (ads) +\u20095\u2009H (ads) +\u2009e\u2212\u2009\u2192 NH3 +\u20092OH\u2212\n[42]. The obtained Tafel slopes using Fe2P and FeP in 0.5\u2009M NaOH were 184\u2009mV dec\u22121 and 205\u2009mV dec\u22121 respectively. The high Tafel slope values in 0.5\u2009M NaOH suggest that the proton discharge is the rate-determining step on the surface of the catalysts [41]. However, the Tafel slopes decreased when we introduced NO3\n\u2212 into the solution. The obtained values of 155\u2009mV dec\u22121 for Fe2P and 158\u2009mV dec\u22121 for FeP reveal that the NO3RR process follows a similar mechanism to that of Volmer-Heyrovsky where the Volmer reaction is the rate-limiting process. As expected, Ti showed the highest Tafel slope of 212\u2009mV dec\u25001. Since lower Tafel slope correlates to higher catalytic activity [36], the catalysts are ranked in the following order: Fe2P\u2009>\u2009FeP >\u2009Ti.Few catalysts are available for efficient NH3 generation at low potentials. Recently, Wu et al. demonstrated the role of Fe single-atom catalyst (SAC) for NO3RR. The authors showed that at \u2212\u20090.55\u2009V vs RHE the Fe catalyst yielded a current density of 4.30\u2009mA\u2009cm\u22122 which corresponds to 331\u2009\u03bcg\u2009h\u22121 mgcat.\u22121 NH3 yield at 39 % FE [4]. Using CuO nanowire arrays, Zhang et al. demonstrated a more efficient NH3 evolution process. However, they applied a very high potential (\u22120.85\u2009V vs. RHE) to get 95.8 % FE [1]. In our study, the applied potentials are under \u2264\u2009\u22120.55\u2009V vs. RHE intending to suppress the competing HER process. Potentiostatic tests for NO3RR at \u2212\u20090.50\u2009V vs. RHE for three cycles revealed the stability of the electrodes (Fig. 3d). While the FeP shows decent current stability, the Fe2P suffers from the decline of current which could be a sign of an ongoing corrosion process. During operation at high current densities, factors like internal resistance losses, accessibility of catalytic surfaces to reactants (liquid-solid-gas interfaces), electron transfer rate, and bonding strength may influence the catalyst performance [43].LSVs were recorded to follow the activity of the iron phosphide films before and after the runs (Fig. S5). For the FeP catalyst, there is a slight drop in current. However, the onset potentials remained steady even after the third run. In the case of Fe2P, both current and onset potential varied. To avoid corrosion of the film, the stability tests were also performed under reduced applied potentials (\u22120.37\u2009V vs. RHE). The results show that both films are stable in this case (Fig. S6).Electrochemical impedance spectroscopy (EIS) was employed to measure the charge transfer resistance (R\nct) at the surface of electrocatalysts in a mixture of 0.5\u2009M NaOH / 0.2\u2009M NaNO3\n[44,45]. Nyquist plots of Fe2P and FeP films were recorded under applied potentials in the range from \u2212\u20090.1 to \u2212\u20090.6\u2009V vs. RHE (\nFig. 4a,b). The R\nct values were obtained after fitting the EIS data with the relevant circuits (Fig. S7a). The low R\nct values in the presence of NO3\n\u2013 are consistent with fast charge-transfer kinetics. The Ti was found to be active only in the potential range above \u2212\u20090.57\u2009V vs. RHE. This result indicated that the Fe2P and FeP catalysts have fast electron transfer and promising catalytic performance for NO3\n\u2212 reduction.The detection of NH3 has been identified electrochemically using FeP or Fe2P as the working electrodes (2\u2009cm2 geometric area) following the long-term NO3RR experiment under chronoamperometric conditions in deoxygenated 0.5\u2009M NaOH / 0.2\u2009M NaNO3 solution. During the NO3RR experiments, we used a bipotentiostate where one of them was attached to the iron phosphide working electrode which generated the NH3 while the other was used for detection. The detection followed independent NH3 oxidation using voltammetric diagnostic experiments in a manner analogous to that described previously for the detection of the CO2 reduction products [46]. The analytical concept has been based on the previous observations postulating proportionality of the Pt-induced NH3 oxidation currents on NH3 concentration in the mM range [47]. Historically, electrooxidation of NH3 attracted broad interest with respect to wastewater treatment [48], and in NH3 fuel cells [49,50]. In this context, the concept of electrooxidation of NH3 to N2 has been widely explored despite the complexity of the mechanism for the oxidation of NH3.During NH3 detection, the second working electrode modified with Pt catalytic NPs (deposited on glassy carbon) was placed in the vicinity of the FeP, or Fe2P working electrode [51]. Fig. 4d illustrates the results of a series of blank NH3 oxidation cyclic voltammetric (CV) experiments (curves a - d) performed in 0.5\u2009M NaOH which contain intentionally added NH3 in the concentration range from 1 to 10\u2009mM.Single voltammetric peaks of NH3 oxidation have emerged at potentials ranging from 0.55 to 0.70\u2009V vs. RHE. Although the peak potentials are somewhat concentration-dependent, the voltammetric peak responses are well-defined. The black line stands for the typical response of Pt NPs in NH3-free alkaline (0.5\u2009M NaOH) medium [52]. Here, in the potential range from 0.0 to 0.4\u2009V vs. RHE, hydrogen adsorption peaks are developed and at potentials higher than 0.7\u2009V vs. RHE, the reversible oxidation of platinum to platinum oxides has been observed. In between the hydrogen peaks and formation of Pt oxides, platinum exists mostly in the metallic form. It is apparent from Fig. 4d (curves a \u2013 d) that the oxidation of NH3 to N2 is catalyzed by metallic platinum, rather than Pt oxides (evident from the decrease of the oxidation currents at/above 0.8\u2009V vs. RHE). The proportionality of the peak-current densities on NH3 concentration is evident from the Fig. S7b. Since the Fe2P film gave the highest NO3RR current densities (relative to FeP) our discussion will be mainly on Fe2P (Fig. S8). The long-term (2\u2009h) chronoamperometric reduction of NO3\n\u2212 has been performed (at the Fe2P working electrode) in a two-chamber electrolytic cell (subjected to continuous saturation with argon) upon application of \u2212\u20090.55\u2009V vs. RHE (Fig. S8).By comparing the net voltammetric peak-current density (Fig. S9a) with the analogous current density values originating from blank experiments at different concentrations (working curve in the Fig. S7b), the NH3 concentration generated following the NO3RR at the Fe2P electrode upon application of \u22120.55\u2009V vs. RHE has been estimated to be equal to 4.7\u2009mM. Judging from the amount of charge (188\u2009C\u2009cm\u22122) transferred during the electrolysis for 7200\u2009s at the 2\u2009cm2 Fe2P-electrode (Fig. S8a), and by assuming 100 % efficiency of the 8-electron reduction of NO3\n\u2212 to NH3, the NH3 concentration equal to 4.9\u2009mM has been obtained. By comparing the above concentration values 4.7 and 4.9, the FE toward the production of NH3 can be postulated to be on the level of 96 %.The appearance of a single peak in the voltammogram supports our view that NH3 is the main N2 reaction product (Fig. S9a). In particular, no NH2NH2 is expected to be formed here because its oxidation peak on platinum would appear at less positive potentials, namely starting from 0.2\u2009V vs. RHE [53], which is not the case in this study. The fact that some current increase is observed at potentials higher than 0.8\u2009V vs. RHE should be attributed to the system\u2019s further oxidation, namely to the oxidation of N2-product to nitrogen oxo species [54]. This \u201ctailing effect\u201d observed at positive potentials does not seem to interfere with the analytical diagnosis based on the determination of the peak current density (Fig. S9a). Any formation of sizeable amounts of NO2\n\u2212 and nitrogen oxides (N2O, NO, N2O3, etc.) [54] would result in the sizeable reduction peak currents at about 0.2\u2009V vs. RHE in the reduction voltammetric scans. As demonstrated during voltammetric diagnostic experiments in solutions containing the intentionally introduced NO2\n\u2212 at various concentrations (Fig. S9b). No such responses have been obtained in the analyzed solutions after electrolysis at \u2212\u20090.55\u2009V vs. RHE. Because our present results are consistent with the view that NH3, together with hydrogen, are generated at Fe2P during NO3RR, estimation of the selectivity efficiency has also been based on this assumption. Remembering that formation of H2 is the two-electron reaction, and conversion of NO3\n\u2212 to NH3 is the eight-electron process, it can be rationalized from the 96 % FE (calculated for the NH3 generation) that the selectivity (molar) efficiency is equal to ca. 84 %. To validate this result, some attention has been paid to the dynamics of hydrogen evolution in the NH3-containing NaOH. Thus, we have performed an additional chronoamperometric experiment in 0.5\u2009M NaOH containing 4.6\u2009mM NH3, namely, to simulate hydrogen evolution in the presence of NH3 generated during NO3\n\u2212 reduction in an alkaline medium. Inset b of Fig. S8 shows that the addition of NH3 tends to decrease hydrogen evolution (dashed line), relative to the performance at NH3 free conditions (solid line). Based on the comparison of the H2-evolution current (after 1000\u2009s from the dashed line in Fig. S8b) and the current recorded after 1000\u2009s during NO3RR electrolysis, as well as remembering that different numbers of electrons are involved in both processes, the selectivity (molar) efficiency can be estimated to be on the level 80 %. The obtained values, 80 % and 84 % are comparable, and the difference between them may reflect the uncertainty in the assumption about the hydrogen evolution efficiency in the presence of NH3 formed during the NO3RR electrolysis. At \u2212\u20090.55\u2009V vs. RHE, the Fe2P catalyst has exhibited 0.25\u2009mmol\u2009h\u22121 cm\u22122 or 2.10\u2009mg\u2009h\u22121 cm\u22122 reaction rates toward NH3 generation. Upon application of less negative potentials, \u2212\u20090.50 and \u2212\u20090.37\u2009V vs. RHE, the yields have been lower, 1.50\u2009mg\u2009h\u22121 cm\u22122 and 0.42\u2009mg\u2009h\u22121 cm\u22122, respectively. While the FeP catalyst has also been characterized by the comparably high FE of 94 % for NH3 generation at \u2212\u20090.55\u2009V vs. RHE, the reaction rate has been found under such conditions to be lower (0.12\u2009mmol\u2009h\u22121 cm\u22122 or 1.0\u2009mg\u2009h\u22121 cm\u22122), when compared to the performance of Fe2P. Upon application of less negative potentials to FeP, \u2212\u20090.50 and \u2212\u20090.37\u2009V vs. RHE, the reaction rates have been rather low, 0.71 and 0.19\u2009mg\u2009h\u22121 cm\u22122, respectively. The obtained yields at \u2212\u20090.55\u2009V vs. RHE using the active Fe2P phase were found to be higher or comparable to what is reported in the literature (Table S1).To validate the formation of NH3 during the electrochemical NO3RR mass spectrometry (MS) analysis was used as another proof (Fig. S10). The sudden rise of ionic current at 28\u2009s indicates the presence of fragment ions with a mass-to-charge ratio equal to m/z\u2009=\u200917 for NH3. In addition, the absence of H2 (m/z\u2009=\u20092) during NO3RR suggests that HER is suppressed during NH3 production.Fourier transform infrared (FTIR) was used to identify the characteristic vibration components present in the FeP and Fe2P samples (\nFig. 5a,b). The C-H bending vibration at 3000\u2009cm\u22121 which comes from surface passivated organics is present in FeP and Fe2P samples [55]. Similarly, the intensive band at 1740\u2009cm\u22121 in FeP and 1730\u2009cm\u22121 in Fe2P samples are attributed to the bending vibrations of CO bonds [56,57]. Stretching vibrations of -CH3 are assigned to the bands at 1352 and 1455\u2009cm\u22121 in the Fe2P sample [58]. In the case of the FeP, the CH3 band is located at around 1371\u2009cm\u22121\n[59,60]. The bands located at 1086\u2009cm\u22121 (FeP) and 850\u2009cm\u22121 (Fe2P) confirm the presence of phosphate (P-O) species [61,62]. Comparison of FTIR spectra recorded from the FeP film before and after the stability tests shows insignificant changes. However, in the case of Fe2P, the intensity of the P-O peaks dramatically decreased after the test, which confirms the masking or leaching of phosphorus during the NO3RR. The composition and the chemical state of FeP and Fe2P thin films were characterized by XPS. The XPS survey spectrum of the FeP and Fe2P electrodes before and after NO3\n\u2212 reduction is shown in Fig. 5c,d. XPS narrow scans of Fe 2p, O 1\u2009s, and P 2p regions recorded from the FeP sample (before the NO3\n\u2212 reduction) are shown in \nFig. 6a,c and e. The spectrum of Fe 2p displays characteristic peaks at 711.4 and 725.0\u2009eV, corresponding to 2p3/2 and 2p1/2 levels of Fe2+\n[63]. The positions of the peaks suggest that Fe appears in the form of iron oxide and iron phosphate (major phase) [64]. The peaks at 715.0 and 729.8\u2009eV are assigned to Fe3+, indicating the coexistence of Fe3+ and Fe2+ in the FeP sample. The peaks at 719.2 and 734.3\u2009eV are satellite peaks, which belong to Fe 2p3/2 and Fe 2p1/2, respectively [63]. The characteristic O 1\u2009s line shows an intensive broad signal which contains several individual peaks. The peak at 530.4\u2009eV confirms the divalent valence state of O in the FeP. The other two peaks at 531.6 and 533.4\u2009eV are attributed to the C\u2013O and CO bands arising from functional groups absorbed on the sample surface [65]. The three signals within the P 2p envelope of FeP are attributed to surface oxidized P species and P from the iron phosphide [64]. The two peaks at 134.0 and 133.3\u2009eV could be from the PO4\n3\u2013 or P2O5 due to the unavoidable oxidation of P species either during the synthesis or heat-treatment process. The peak at 130.8\u2009eV reflects the binding energy of P 2p1/2 which can be assigned to P bonding to Fe. Selected regions of the XPS spectra of Fe2P recorded before NO3\n\u2212 reduction are shown in Fig. 6g,i and k. The photoelectron Fe 2p signal exhibits a doublet at 711.4\u2009eV (Fe 2p3/2) and 724.8\u2009eV (Fe 2p1/2) due to spin-orbit splitting. These peaks can be attributed to the oxidized state of Fe formed on the Fe2P surface, demonstrating the oxidation state of Fe2+\n[64,66]. A small peak at 714.8 is assigned to Fe3+, indicating the coexistence of Fe3+ and Fe2+ in the Fe2P sample [63]. The O 1\u2009s shows a peak at 530.3\u2009eV, confirming that the valence state of O is divalent (Fe-O, metal-containing oxygen bond). The two shoulder peaks at 531.8 and 533.6\u2009eV are attributed to the C\u2013O and CO bands arising from molecules absorbed on the sample surface [65]. The P 2p signal in the Fe2P sample is almost missing which can be explained by the fact that oxide-rich iron phosphate forms on the Fe2P surface and masks the signal of P 2p [67,68].To gain a better understanding of Fe2P and FeP catalyst surface chemistry, we also recorded XPS spectra after the NO3\n\u2212 reduction test (Fig. 6). The results of the quantitative analysis of the XPS are shown in detail in Table S2 and Table S3. The XPS peak intensities can be converted to atomic concentrations (at%) using the sensitivity factors determined experimentally or simply calculated [69]. For the Fe2P sample, the atomic and the mass concentration (mass %) of Fe 2p, O 1\u2009s, and Ti 2p before and after the test did not show any drastic changes. However, the analysis shows that P 2p decreased more than 3 times after the test. The at% of P 2p decreased from 4.71 % to 1.19 %. In the case of FeP, the at% of Fe 2p increased from 13.99 to 27.35. On the other side, the at% of P 2p decreased from 15.27 to 2.892. The extensive decrease of P 2p is explained by the fact that phosphorus transformed to phosphate (hidden by surface oxygen species) or consumed due to corrosion during the reduction of NO3\n\u2212.The reaction mechanism for NO3RR catalyzed by Fe2P and FeP electrocatalysts has been studied using DFT at the RPBE+D3 level of theory in an aqueous solution. Our models have been constructed based on the intensities of the XRD patterns (Fig. 1) and XPS data analysis, leading to the building of the oxidized Fe2P (111) and phosphate-coated FeP (101) surfaces (see Fig. S12). These are also characterized by the presence of a vacancy that will act as an active site where the substrates will interact with the catalytic surface along the NO3RR. The reduction of NO3\n\u2212 into NH3 entails the transfer of nine protons and eight electrons, however, with the assumption that both NO3\n\u2013 and its protonated form of nitric acid (HNO3) are in equilibrium, the modeling focuses on the study of the HNO3 adsorption and NH3 desorption phenomena as the initial and final steps to the global process defined by the electrochemical equation HNO3 (ac) +\u20098\u2009H+ + 8 e\n\u2013 \u21cc NH3 (ac) +\u20093\u2009H2O (l). In this context, \nFig. 7 gathers the structures of each elementary reaction step and the free energies associated with each one at room temperature and pressure conditions when there is no applied potential (U = 0) and pH =\u200914 (basic). Interestingly, both the oxidized Fe2P (111) and phosphate-coated FeP (101) surfaces present spontaneous binding free energies for HNO3 adsorption, \u20130.34 and \u22120.51\u2009eV, respectively. This fact is due to the presence of low-coordinated surface Fe atoms, specifically three-fold (3c), making them electrophilic and therefore expecting greater interactions with the substrates than other surfaces constituted by higher coordinated surface Fe atoms (see Fig. S13). Both, the oxidized Fe2P (111) and phosphate-coated FeP (101) surfaces present similar catalytic profiles for the first four reduction steps, that is, the formation of *NO2, *HNO2, *NO, and *NOH intermediate species (see Fig. 7, more details at Fig. S15).Both the production of NO2 and NO are spontaneous processes with \u20131.62 and \u22121.42\u2009eV for Fe2P (111) and \u20131.49 and \u22121.14\u2009eV for FeP (101), expected values given the great potential of these species to coordinate with metal centers. With a contrary trend, the formation of nitrous acid and nitroxyl is non-spontaneous with values of 1.17 and 1.19\u2009eV for Fe2P (111) and 0.89 and 1.26\u2009eV for FeP (101). This last, that is, the nitroxyl formation as a consequence of the fourth hydrogenation, *NO +\u2009H+/e\n\u2013 \u21cc *NOH, represents the step with the highest thermodynamics impediment, expecting a maximum overpotential of \u20131.19 and \u22121.26\u2009V vs. CHE (computational hydrogen electrode) equivalent to the SHE, or \u20130.36 and \u22120.43\u2009V vs. RHE (pH = 14) for Fe2P (111) and FeP (101), respectively. Interestingly, our calculations estimate values of anodic potentials very close to those observed experimentally, validating the construction of our models. Up to this point, all substrates interact with the Fe2P (111) and FeP (101) surfaces through, at least, two binding points involving the two three-coordinated surface Fe atoms. A greater distance of them in the FeP (101) makes the metal nitride species and its subsequent hydrogenated amino *NH and *NH2 intermediates less stabilized than the ones on the Fe2P (111) surface. Finally, the NH3 desorption is calculated as just 0.49\u2009eV for the oxidized Fe2P (111) surface while for the phosphate-coated FeP (101) one a heavy value of 1.15\u2009eV is observed, indicating a possible catalyst poisoning for this second case.In this work, we report the use of iron phosphides as highly efficient noble metal-free catalysts in NO3RR studies. A modified solvothermal synthesis procedure using triphenylphosphine precursor was used to prepare the Fe2P and FeP catalysts. Impressively, the Fe2P catalyst shows the highest FE (96 %) and yield (2.10\u2009mg\u2009h\u22121 cm\u22122) at \u2212\u20090.55\u2009V vs. RHE for NH3 generation. For the FeP catalyst, at\u2212\u20090.50 and \u2212\u20090.37\u2009V vs. RHE, the yields were found to be 0.71\u2009mg\u2009h\u22121 cm\u22122 and 0.19\u2009mg\u2009h\u22121 cm\u22122, respectively. The recycling test confirmed that both FeP and Fe2P catalysts exhibited excellent stability during the NO3RR at \u2212\u20090.37\u2009V vs. RHE. Herein, the reported catalytic activities also consider the presence of metal oxide and metal phosphate surface terminations, which contributes in a synergistic way to the observed enhanced activities of iron phosphide films. DFT calculations supported the experimental observations and explained the mechanism and the fundamental origins behind the better performance of Fe2P as compared to FeP. This study demonstrates the tremendous potential of iron phosphide catalysts as efficient cathodes toward NO3RR to NH3. Hence, this work could be also extended to other TMPs for selectively converting different nitrogen oxides into valuable green NH3 under benign conditions.\nT. Chouki: designed and conducted the experiments, analyzed the data and wrote the manuscript. M. Machreki: contributed to EIS measurements. I. A. Rutkowska and B. Rytelewska: contributed to electrochemical tests. P. J. Kulesza: contributed to electrochemical tests, data analysis, fund-raising, and Project administration. G. Tyuliev: carried out the XPS measurements. M. Harb and L. Miguel Azofra: carried out the DFT calculations. S. Emin: supervised the work, and contributed to data analysis, manuscript writing and editing, fund-raising, and Project administration. Pawel J. Kulesza: Project administration. Saim Emin: Project administration.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the Slovenian Research Agency under the trilateral project for scientific cooperation between the Republic of Slovenia, the Republic of Austria, and the Republic of Poland (N2-0221). T. Chouki and M. Machreki acknowledge the scholarships provided by the Public Scholarship, Development, Disability, and Maintenance Fund of the Republic of Slovenia (Ad futura program: 11011-25/2018) for Ph.D. studies at the University of Nova Gorica. S. Emin acknowledges the financial support from the Slovenian Research Agency (research core funding: P2-0412). L. M. Azofra acknowledges the KAUST Supercomputing Laboratory using the supercomputer Shaheen II for providing the computational resources. I.A. Rutkowska, B. Rytelewska, and P.J. Kulesza were supported by the National Science Center (NCN, Poland) under Opus Lap Project 2020/39/I/ST5/03385.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2023.109275.\n\n\n\nSupplementary material.\n\n\n\n.", "descript": "\n                  The electrochemical reduction reaction of the nitrate ion (NO3\n                     \u2212), a widespread water pollutant, to valuable ammonia (NH3) is a promising approach for environmental remediation and green energy conservation. The development of high-performance electrocatalysts to selectively reduce NO3\n                     \u2212 wastes into value-added NH3 will open up a different route of NO3\n                     \u2212 treatment, and impose both environmental and economic impacts on sustainable NH3 synthesis. Transition metal phosphides represent one of the most promising earth-abundant catalysts with impressive electrocatalytic activities. Herein, we report for the first time the electrocatalytic reduction of NO3\n                     \u2212 using different phases of iron phosphide. Particularly, FeP and Fe2P phases were successfully demonstrated as efficient catalysts for NH3 generation. Detection of the in-situ formed product was achieved using electrooxidation of NH3 to nitrogen (N2) on a Pt electrode. The Fe2P catalyst exhibits the highest Faradaic efficiency (96 %) for NH3 generation with a yield (0.25\u00a0mmol\u00a0h\u22121 cm-\u22122 or 2.10\u00a0mg\u00a0h\u22121 cm\u22122) at \u2212\u00a00.55\u00a0V vs. reversible hydrogen electrode (RHE). The recycling tests confirmed that Fe2P and FeP catalysts exhibit excellent stability during the NO3\n                     \u2212 reduction at \u2212\u00a00.37\u00a0V vs. RHE. To get relevant information about the reaction mechanisms and the fundamental origins behind the better performance of Fe2P, density functional theory (DFT) calculations were performed. These results indicate that the Fe2P phase exhibits excellent performance to be deployed as an efficient noble metal-free catalyst for NH3 generation.\n               "}
{"full_text": "The search for environmentally friendly alternative energy sources has risen to the top of the global priority list. Climate change, caused by greenhouse gas emissions associated with fossil fuels, poses a serious threat [1]. Natural gas, oil, and coal combustion are the primary sources of greenhouse gas emissions, with these three fossil fuels accounting for 20, 39, and 41% of all hydrocarbon-related CO2 emissions, respectively [2]. Furthermore, the world's supply of fossil fuels is finite and will eventually run out, especially as the demand for energy sources rises due to population growth and industrialization. As a result, the world requires clean alternative energy sources that do not pollute the environment. As an environmentally friendly energy source, hydrogen is considered one of the best solutions to the energy-environment problem [3]. Hydrogen is regarded as one of the best solutions to the energy-environment problem as an environmentally friendly energy source, particularly if produced from waste materials to promote circular economy [4]. Because hydrogen has three times the energy storage capacity per weight of the average liquid hydrocarbon, it is one of the best alternatives to fossil fuels. Hydrogen can be burned directly as a fuel, and the byproduct of its oxidation is the only pollutant-free water on the planet [5\u20137]. It has the highest combustion energy per unit mass of any commonly used fuel substance, and the volume of energy produced is 2.4, 2.8, and 4 times that of methane, gasoline, and coal, respectively [8]. Furthermore, hydrogen can be mixed with natural gas and used for combustion and heating in commercial and multi-family buildings, as well as increasing the power system flexibility in gas turbines [5].Furthermore, hydrogen is vital due to its role in fuel cell technology, which converts chemical energy into electricity and has numerous applications in power generation and automotive power [5,9]. However, most hydrogen applications today, whether pure or mixed, are concentrated in the industrial field, and demand for hydrogen in the industry is rapidly increasing due to the continued development of the global economy. The most common industrial uses of hydrogen are oil refining (33%), ammonia production (27%), methanol production (11%), and direct reduction of iron ore for iron and steel production (3%). Hydrogen can be produced locally from various sources, including water, oil, gas, biofuels, and so on, allowing different countries' energy needs to be met without relying on external energy suppliers [7]. In general, hydrogen production methods can be divided into three categories [10\u201312]. The first category includes green hydrogen production methods, in which hydrogen is produced through water electrolysis using electricity generated from renewable energy sources. Renewable energy sources include solar, wind, geothermal, hydro, and ocean thermal energy conversion. Purple hydrogen production methods based on nuclear energy belong to the second category. Nuclear fission reactors produce heat, which is then converted into steam, which is then used to power turbines, generate electricity, and electrolyze water to produce hydrogen. Furthermore, nuclear reactors' high temperatures can be used to produce hydrogen via thermochemical water splitting and steam reforming methods. The blue category of hydrogen production methods includes traditional methods based on fossil fuel-based processes.Nowadays, conventional methods account for approximately 96% of global hydrogen production, with steam reforming of methane (SRM) accounting for approximately 50%. Because methane is the primary component of natural gas, the world's methane reserves are plentiful [13]. Methane also has the highest H/C ratio of any hydrocarbon, making it the best hydrogen resource [14]. However, methane steam reforming emits approximately 830 million tonnes of CO2annually, making it one of the most significant contributors to global warming [5]. As a result, an environmentally friendly method of utilizing methane in hydrogen production is required. Catalytic decomposition of methane (CDM) is the best option for utilizing methane in hydrogen production with zero CO2 emissions [5,15,16].The only products in the CDM are gaseous hydrogen and solid carbon. The produced hydrogen does not need to be separated [5]. According to Weger et\u00a0al. [17,18], using CDM instead of SMR in hydrogen production can reduce COx emissions by about 27%, potentially reducing climate change. Furthermore, CDM is a moderately endothermic reaction requiring a lower operating temperature than SRM, so it is more cost-effective [5]. Equations 1 and 2 represent SMR and CDM reactions [19].\n\n(1)\n\n\n\n\nC\nH\n\n4\n\n+\n2\n\nH\n2\n\nO\n\n\u2192\n\n4\n\nH\n2\n\n+\n\n\nC\nO\n\n2\n\n\n\n(\n\u0394\nH\n=\n41.3\n\nk\nJ\n/\nm\no\nl\n\n\nH\n2\n\n)\n\n\n\n\n\n\n\n(2)\n\n\n\n\nC\nH\n\n4\n\n\n\u2192\n\nC\n+\n2\n\nH\n2\n\n\n\n(\n\u0394\nH\n=\n37.8\n\nk\nJ\n/\nm\no\nl\n\n\nH\n2\n\n)\n\n\n\n\n\nAside from producing COx-free hydrogen, CDM has another valuable advantage in producing high-value-added multifunctional carbon nanomaterials, which are a valuable byproduct with a wide range of beneficial applications [20]. The primary goal of the methane decomposition process in early applications was the preparation of nano-carbon, not hydrogen [21]. In the CDM process, single and multi-walled carbon nanotubes, as well as nanofibers, are produced with distinct physical and chemical properties, which are primarily determined by the catalyst used and the experimental parameters used [14]. Carbon nanotubes produced by CDM have excellent electronic properties, high axial strength, high thermal stability, and high stiffness. Furthermore, the carbon nanofibers produced have macroporous and mesoporous structures, a high surface area, metal and semiconductor properties, high conductivity, tenacity, and mechanical strength [14]. As a result of the numerous applications of these nanocarbons (nanotubes and nanofibers), the CDM is a cost-effective process for hydrogen production from methane. Among the applications are the hydrogen storage medium for fuel cells, polymer nanocomposites, supercapacitors, water treatment, electrode material in batteries, and catalysis [6,14,22,23].A methane molecule is a highly stable inert molecule with four extremely strong C\u2013H bonds formed by sp3 hybridization, each with an energy of about 435\u00a0kJ\u00a0mol\u22121 [5,24]. As a result, methane decomposition is an endothermic process that requires a high reaction temperature (>1200\u00a0\u00b0C) in the absence of a catalyst to be completed with a significant product yield [5,24]. As a result, in this reaction, a suitable catalyst is always used to reduce the reaction temperature (>400\u00a0\u00b0C) by providing a pathway with lower activation energy [5,24]. Metals or carbon-based materials are the most commonly used catalysts in CDM reactions. Activated carbon, carbon black, coal chars, glassy carbon, carbon nanotubes, acetylene black, soot, graphite, diamond powder, and fullerenes are examples of carbon-based catalysts [25].However, studies have revealed that methane conversion over carbon-based catalysts is lower than that over metal-based catalysts [6]. Metal-based catalysts are classified as supported or non-supported. The most common active metals used in the CDM reaction are nickel, cobalt, and iron [5]. The catalytic activity of Ni, Co, and Fe metals in the CDM reaction is due to their non-filled 3d-orbitals, which promote methane molecule dissociation. The transfer of electrons from the catalyst containing one or more of these transition metals (Ni, Co, and Fe) to the unoccupied anti-bonding orbitals of methane molecules facilitates their dissociation [5].Herein, we present a significant contribution to research using methane in hydrogen production via COx-free methods. In the current study, non-supported pure and mixed cobalt and iron oxide catalysts with varying Co/Fe ratios were synthesized from nitrate precursors using a very simple preparation method in which water was the only solvent used. The prepared catalysts were then used as decomposers of methane into hydrogen and carbon. To the best of our knowledge, this will be the first study using non-supported Co\u2013Fe mixed oxides to catalyze this reaction.Iron Nitrate hexahydrate (Fe(NO3)3\u00b76H2O 433.01\u00a0g/moL; 99.99%; Aldrich), and cobalt nitrate hexahydrate (Co(NO3)2\u00b76H2O, 98%, Alfa Aesar) were purchased and were used as received. Ultrapure water was obtained via a Milli-Q water purification system (Millipore).The weight of an empty crucible was measured and recorded. Pure oxides were prepared by the calcination of iron nitrate hexahydrate or cobalt nitrate hexahydrate at 600\u00a0\u00b0C for 3\u00a0h at a heating rate of 10\u00a0\u00b0C/min. In the case of mixed oxides and depending on the desired composition and mass of the catalyst, calculated masses of iron nitrate hexahydrate (equivalent to an atomic ratio of 25, 50 and 75%) and that of cobalt nitrate hexahydrate (equivalent to an atomic ratio of 75, 50 and 25%) were poured inside the crucible. The mixture was ground thoroughly in the crucible to obtain a fine powder mixture. Ultrapure water was added dropwise to the ground powder mixture in the crucible to form a paste. It was well stirred, and the water was allowed to evaporate under ambient conditions overnight as a drying process. The weight of the crucible plus the sample was measured, and subsequently, that of the sample was determined after drying overnight. Thereafter, the dried sample was calcined at 600\u00a0\u00b0C for 3\u00a0h at a heating rate of 10\u00a0\u00b0C/min. The mixed oxides containing 25, 50 and 75% of Fe metal were abbreviated as 25Fe\u00a0+\u00a075Co, 50Fe\u00a0+\u00a075Co and 75Fe\u00a0+\u00a025Co, respectively.Methane decomposition experiments were carried out at a reaction temperature of 800\u00a0\u00b0C under atmospheric pressure. The reactions were performed in a packed bed reactor of stainless steel (internal diameter, 0.0091\u00a0m; height, 0.3\u00a0m). A catalyst mass of 0.30\u00a0g was carefully positioned in the reactor over a ball of glass wool. Stainless steel, sheathed K-type thermocouple positioned axially close to the catalyst bed, was used to measure the temperature during the reaction. Prior to the start of the reaction, activation of the catalysts was performed at 700\u00a0\u00b0C in an atmosphere of H2 at a flow rate of 40\u00a0ml/min. This lasted for 60\u00a0min, and the remnant H2 was purged with N2. The feed volume ratio was maintained at 3:2 for methane and nitrogen gases during the experiments, respectively. In addition, the space velocity was kept at 5.0\u00a0l/h/gcat. The reactor outlet was connected to an online gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) to analyze its composition. The methane conversion, carbon yield and hydrogen yield were thus computed according to Equations (3)\u2013(5), respectively.\n\n(3)\n\n\n\n\nC\nH\n\n4\n\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\n\n(\n%\n)\n\n=\n\n\nC\n\nH\n\n4\n,\ni\nn\n\n\n\u2212\nC\n\nH\n\n4\n,\no\nu\nt\n\n\n\n\nC\n\nH\n\n4\n,\ni\nn\n\n\n\n\n\n\u2217\n\n100\n\n\n\n\n\n\n(4)\n\nC\na\nr\nb\no\nn\n\nY\ni\ne\nl\nd\n\n\n%\n\n=\n\n\n\nW\np\n\n\u2212\n\nW\ncat\n\n\n\nW\n\nc\na\nt\n\n\n\n\n\u2217\n\n100\n\n\n\n\n\n(5)\n\n\nH\n2\n\n\nY\ni\ne\nl\nd\n\n\n%\n\n=\n\n\nm\no\nl\ne\ns\n\no\nf\n\n\nH\n2\n\n\np\nr\no\nd\nu\nc\ne\nd\n\n\nm\no\nl\ne\ns\n\no\nf\n\nC\n\nH\n4\n\n\ni\nn\n\nt\nh\ne\n\nf\ne\ne\nd\n\nX\n\n2\n\n\n\n\u2217\n\n100\n\n\nWhere, CH4,in is methane in the feed, CH4,out is methane in the product, WP is the product's weight after reaction, and Wcat is the weight of the fresh catalyst.N2 adsorption-desorption isotherms of the catalysts were measured by N2 adsorption-desorption at 196\u00a0\u00b0C using a MicromeriticsTristar II 3020 surface area and porosity analyzer.The quantity of carbon deposits on the spent catalysts was measured using TGA analysis. A platinum pan was filled with 10\u201315\u00a0mg of the used catalysts and carefully positioned inside the device. Heating was done from room temperature up to 1000oC at a 20\u00a0\u00b0C/min\u22121 temperature ramp under an air atmosphere. The change in mass was continuously monitored as the heating progressed.Powder X-ray diffraction (XRD) patterns of the prepared catalysts were recorded on a Miniflex Rigakudi_ractometer that was equipped with Cu K, X-ray radiation. The device was run at 40\u00a0kV and 40\u00a0mA.In order to study the morphology of the catalyst and to elucidate carbon deposition on the used catalysts, Scanning Electron Microscopy - Energy-Dispersive X-ray spectroscopy (SEM/EDX) interpretations of the fresh and used catalyst samples were performed using a JSM-7500F (JEOL Ltd., Japan) scanning electron microscope.Transmission electron microscopy (JEOL JEM-2100F) with high resolution to give larger magnification was used to conduct the TEM measurement of both the fresh and used catalyst. The electron microscope operated at 200\u00a0kV produces the active metal nickel particle sizes and depicts the morphology of carbon deposit on the used catalyst. Before the TEM measurement, the catalysts were first dispersed ultrasonically in ethanol at room temperature. After that, the drop from the suspension was placed in a lacey carbon-coated Copper grid to produce the images.Laser Raman (NMR-4500) Spectrometer (JASCO, Japan) was used to record Raman spectra of the fresh and used catalyst samples. The wavelength of the excitation beam was set to 532\u00a0nm, and objective lens of 100\u00a0\u00d7\u00a0magnification was used for the measurement. The laser intensity was adjusted to 1.6\u00a0mW. Each spectrum was received by averaging 3 exposures in 10\u00a0s. Spectra were recorded in the range 1200\u20133000\u00a0cm\u22121 (Raman shift) and processed using Spectra Manager Ver.2 software (JASCO, Japan).To confirm the crystal structure and phase purity of the fresh and used catalysts, powder X-ray diffraction (XRD) has been used. The XRD diffraction patterns of the fresh and used mixed oxide catalysts are shown in Fig.\u00a01\n. The XRD diffraction pattern of all Fe\u2013Co mixed oxide fresh catalysts matched the reference patterns of Co3O4 (PDF- #:76\u20131802) and Fe2O3 (COD 1546383). The patterns of Co3O4 were detected at 2\u03b8 of 31.43, 36.8, 44.9, 55.7, 56.5, 59.6, and 65.4o, while the patterns of hematite phase (Fe2O3) were found at 2\u03b8 of 24.3, 33.7, 35.6, 41.2, 49.7, 54.4, 57.1, 62.6, and 64.2o. Furthermore, the characteristic patterns of cobalt ferrite (CoFe2O4) spinel were also found individually or overlapped with Fe and Co oxides in all catalysts. The patterns corresponding to the CoFe2O4 were located at 2\u03b8 of 18.3,30.0, 35.4, 43.1, 57.0, and 65.1o [26,27].The intensity of all diffraction peaks decreases remarkably with the increase of the Fe2O3 ratio in the fresh 50Fe\u00a0+\u00a050Co and 75Fe\u00a0+\u00a025Co catalysts, which could be attributed to a synergistic effect of Fe and Co oxides to form the CoFe2O4 spinel structure. No other phases or peak position shifts were observed in these three catalysts' XRD patterns. This suggests that the preparation method used in this study produces mixtures of cobalt and iron oxides as well as cobalt ferrite without forming an extensive solid solution. According to Abdelkader et\u00a0al. [28], no solid solution is expected between Co and Fe at calcination temperature lower than 1000\u00a0\u00b0C.Considering the used catalysts, the XRD diffractogram of all used Fe\u2013Co catalysts matched the reference patterns of a graphitic carbon diffraction peak at 2\u03b8 of 26.4\u00b0 (JCPDS No. 41\u20131487), irrespective of the Fe/Co ratio. The intensity of the peaks corresponding to carbon nanotubes/graphene is relatively stable. The diffraction peaks corresponding to the metallic forms of Co and Fe were detected at 2\u03b8 of 44 and 51.2o in all used Fe\u2013Co catalysts. Moreover, the intensity of the main diffraction peak at 2\u03b8 of 44o increased by increasing the ratio of Fe2O3 in the used 50Fe\u00a0+\u00a050Co and 75Fe\u00a0+\u00a025Co catalysts. This behaviour could be explained by forming Fe\u2013Co alloy due to CoFe2O4 formation in fresh catalysts. The Fe\u2013Co alloy is reported to have diffraction patterns at 44.80 and 65.28o [29].The presence of the Co, Fe, and Fe\u2013Co alloy in the used catalysts is due to the reduction step prior to the reaction in addition to the expected in-situ further reduction of these catalysts by the hydrogen produced from the methane decomposition reaction.The fresh and used mixed oxide catalysts were characterized by Raman spectroscopy measurements. Fig.\u00a02\n shows the Raman spectra of the fresh and used catalysts. The Raman spectra of the three fresh catalysts show bands around 275 and 560\u00a0cm\u22121 attributed to hematite Fe2O3 and a band at 1092\u00a0cm\u22121, which is attributed to CoO [30,31]. In the case of the used catalysts, two distinct bands were observed at around 1565\u00a0cm\u22121 and 2660\u00a0cm\u22121, respectively. The band at 1565\u00a0cm\u22121 is called G-band and is related to the vibration of sp2 bonded carbon atoms in a 2D hexagonal lattice and represents crystalline carbon [32,33]. The band at 2660\u00a0cm\u22121 is called the 2D band and is common to all sp2 carbon materials (appears in the range 2500\u20132800\u00a0cm\u22121) [34]. In the used catalysts, no bands appear in the 1000\u20131500\u00a0cm\u22121 range corresponding to the D band (disorder-induced band) [35]. The D band usually indicates the amount of disorder or defects in the sample. The appearance of the G and 2D bands without the D band indicates that the carbon deposited on the used catalysts is well-ordered carbon nanotubes and/or graphene [33,35]. However, the sharp non-split appearance of the G band and the 2D band is characteristic of graphene and not graphite or carbon nanotubes [36]. In graphene, the number of graphene layers affects the shape, position and relative intensity of G and 2D Raman bands [36]. According to many studies, as the number of layers increases, the intensity of the G band increases significantly, while that of the 2D band decreases [37]. From Fig.\u00a02, the relative intensity of G and 2D bands is quite different in the three used catalysts, which indicates the formation of different types of graphene depending on the number of layers formed. The band at 275\u00a0cm\u22121 in the case of the samples 25Fe\u00a0+\u00a075Co and 75Fe\u00a0+\u00a025Co and the small band at 570\u00a0cm\u22121 in the case of sample 50Fe\u00a0+\u00a050Co correspond to hematite Fe2O3 [31]. The bands at 2230\u00a0cm\u22121 in the case of the used 75Fe\u00a0+\u00a025Co sample and at 2150\u00a0cm\u22121 in the case of the 25Fe\u00a0+\u00a075Co sample are likely attributed to the laser scribed graphene [38].The SEM and EDX profiles of fresh and used mixed oxide catalysts are shown in Fig.\u00a03\n. All of the elements claimed in the fresh or used catalysts are seen in the EDX patterns. All the fresh catalysts contain Co and Fe with ratios relative to their expected ratios in the catalysts. At the same time, the used catalysts contain C, Co and Fe with a carbon ratio of over 70%. However, the ratios of Co and Fe are still relative to their ratios in the catalysts and the order of the catalysts according to the carbon ratio in each catalyst is 50Fe\u00a0+\u00a050Co\u00a0>\u00a075Fe\u00a0+\u00a025Co\u00a0>\u00a025Fe\u00a0+\u00a075Co. The SEM images of fresh and used mixed oxide catalysts were used to study their surface morphology and to evaluate the carbon deposition behaviour over the used catalysts. The SEM images of fresh catalysts show mixtures of two different kinds of grains with two different morphologies corresponding to the two different oxides consisting of the catalysts. The first kind of grains is the highly dense, variously sized grains corresponding to iron oxide, and the second is the irregular-sized grains with a sponge-like structure corresponding to cobalt oxide (Fig.\u00a03A\u2013C). The SEM images of used 50Fe\u00a0+\u00a050Co and 75Fe\u00a0+\u00a025Co catalysts show that the catalyst particles are covered with bamboo-like fibrous carbon or carbon nanotubes with a relatively higher density over the 50Fe\u00a0+\u00a050Co catalyst (Fig.\u00a03E and F). The SEM image of the 25Fe\u00a0+\u00a075Co catalyst shows a different morphology of the produced carbon as it seems mainly like aggregates of sticked thin platelets with a few carbon nanotubes (Fig.\u00a03D). This indicates that the carbon formed over this catalyst is mainly graphene. The presence of CNT on the surface of both 50Fe\u00a0+\u00a050Co and 75Fe\u00a0+\u00a025Co catalysts may be attributed to the dispersion and stabilization of Fe and Co oxides in the CoFe 2O4 structure. In this context, the aggregated Fe or Co oxide particles can be considered the active sites in graphene growth. The combination of Raman and SEM analysis indicates that the carbon deposited over the mixed oxide catalysts is most likely a mixture of different types of highly crystalline sp2 carbon, i.e., graphene and carbon nanotubes.El-Ahwany et\u00a0al. [39] studied the methane decomposition over Fe/MgO catalysts with different iron loadings. They reported the growth of different types of carbon nanomaterials, including graphene nanoplatelets and carbon nanotubes. They found that the graphene nanoplates grow on high Fe-loaded surfaces while carbon nanotubes grow on the low iron-loaded catalyst surfaces.\nFig.\u00a04\n shows the TEM image of fresh and used 50Fe\u201350Co catalyst in order to understand in-depth morphology. Similar to the SEM image, the TEM image of the fresh 50Fe\u201350Co catalyst shows the presence of two different kinds of grains with two different morphologies corresponding to the two different oxides. The first morphology is represented by large black lumps, which are likely to correspond to cobalt oxide and/or CoFe2O4 species, and the other is represented by less dark, variously sized grains with irregular shapes that correspond to iron oxide. Unlike the SEM image, no development of carbon tubes is observed in the TEM image of the used 50Fe\u201350Co catalyst; instead, transparent graphene nanoplatelets seem to be grown. This observation supports our suggestion that the carbon deposited over the catalysts is a mixture of graphene and carbon nanotubes.The textural properties of the fresh and used mixed oxide catalysts were studied by the N2 adsorption/desorption measurements, as shown in Fig.\u00a05\n. According to IUPAC calcification, all the isotherms in Fig.\u00a05 belong to type IV with H3 hysteresis loops characteristic of the mesoporous materials and are usually found on solids consisting of aggregates of particles forming slit-shaped pores with non-uniform size and shape [40]. However, it can be seen from Fig.\u00a05 that the nitrogen uptake of the fresh catalysts started at a relative pressure range of 0.7\u20130.85, whereas the nitrogen uptake of the spent catalysts started at the relative pressure range of 0.4\u20130.5. This indicates that the used catalysts exhibit a highly porous structure compared to the fresh catalysts, which can be attributed to the enhanced mesoporosity by the deposited carbon. The same result has been observed by Deyab et\u00a0al. [41], who studied the preparation of nanocomposite material of Ni\u2013Fe alloy and graphene by the chemical vapour deposition method using methane as a carbon source over Ni\u2013Fe alloy as a substrate. They found enhancement in the mesoporosity of the composite compared to the alloy due to the improvement in the dispersion of NiFe alloy nanoparticles on the surface of graphene.The catalytic activity of pure and mixed cobalt and iron oxides towards methane decomposition is shown in Fig.\u00a06\n as methane conversion (a) and hydrogen yield (b). It is clear from Fig.\u00a06 that the mixed oxides catalysts have higher catalytic activity than the pure oxides catalysts, and the change in Fe/Co atomic ratio plays an essential role in the performance of the mixed oxides. The significant difference in the activity between pure and mixed oxides shows the importance of mixing the two oxides and clarifies the impact of this mixing on the catalytic activity. Except for pure oxides, all catalysts were found to be highly stable until the end of the reaction time (425\u00a0min), with no drop in hydrogen yield, and it appears that they will retain their activity for long periods before deactivation. As shown in Fig.\u00a06, the 50Fe\u00a0+\u00a050Co mixed oxide catalyst exhibits higher catalytic activity in terms of methane conversion and hydrogen yield than the other mixed oxide catalysts within all reaction periods. The hydrogen yield of 18 and 22% was obtained at the initial reaction period of 10\u00a0min using 25Fe\u00a0+\u00a075Co and 75Fe\u00a0+\u00a025Co catalysts, respectively. Following that, the hydrogen yield gradually increases with reaction time, reaching 24% at 200\u00a0min for the 25Fe\u00a0+\u00a075Co catalyst and 33% at 325\u00a0min for the 75Fe\u00a0+\u00a025Co catalyst, and these values remain unchanged until the reaction completion at 420\u00a0min (Fig.\u00a06).On the other hand, the 50Fe\u00a0+\u00a050Co catalyst demonstrated noticeable activity during the initial stage of the reaction, yielding approximately 50% of the hydrogen at 10\u00a0min. After that, the catalytic activity decreased slightly to 44% at 20\u00a0min, then increased again to reach a steady-state value of 48% at 120\u00a0min and remained constant until 420\u00a0min of reaction time. These results indicate the presence of a synergetic effect between cobalt and iron oxides, especially in the case of 50Fe\u00a0+\u00a050Co and 75Fe\u00a0+\u00a025Co catalysts, which gives their preference over 25Fe\u201375Co catalyst. The low hydrogen yield at the beginning of the reaction can be attributed to the in situ consumption of H2 released from methane decomposition, which continued the reduction of unreduced Co and Fe active sites left from the pre-reduction step [32]. This in situ reduction leads to an overall increase in the number of active sites available for the reaction, which consequently increases the methane decomposition and carbon formation until reaching equilibrium [32]. However, it can be noted from Fig.\u00a06 that the equilibrium is reached faster in the case of 50Fe\u00a0+\u00a050Co catalyst, which means less number of unreduced active sites left from the pre-reduction step and faster reduction of the remaining unreduced sites. This can be attributed to the fact that the uniformly mixing of iron and cobalt oxides improve the reducibility of both oxides [28]. Increasing the ratio of iron or cobalt oxides decreases the degree of the mutual improvement in the reducibility of the two oxides, which likely depends on an atom-to-atom mechanism, which explains the importance of the Co/Fe atomic ratio and the superiority of mixed oxides over pure oxides.Again, the higher catalytic activity of 50Fe\u00a0+\u00a050Co among the others can be explained by the presence of CoFe2O3 species, which allows more active sites for methane decomposition reaction. This suggests that the Fe/Co ratio in this catalyst is sufficient for generating a significant percentage of CoFe2O3 species, which improves metal oxide dispersion and stabilization during the reaction. In contrast, the presence of either non-interacted Fe or Co oxides may be responsible for the low activity of 75Fe\u00a0+\u00a025Co and 25Fe\u00a0+\u00a075Co mixed oxide catalysts.Furthermore, it is known that the Fe\u2013Co mixture has higher carbon capacity compared to Co or Fe, which means better durability of catalyst towards carbon deposition [42]. This high carbon capacity that prevents the quick deactivation of the catalyst due to the formation of encapsulating carbon is another reason for the higher activity of the mixed oxides compared to pure oxides and explains the gradual deactivation of pure iron oxide. In addition, the high carbon capacity of the Co\u2013Fe mixture results from a mutual effect between the two oxides, so that it depends on the Fe/Co molar ratio, and it is optimum in the case of a 50Fe\u00a0+\u00a050Co catalyst. Awadallah et\u00a0al. [43] studied the methane decomposition over Fe\u2013Co/MgO catalyst with a total metal content of 50\u00a0wt%, and Fe/Co atomic ratio equals one. They reported achieving more than 80% hydrogen yield and 76% carbon yield using this catalyst. They explained this high catalytic activity as it is due to the existence of large numbers of non-interacting Fe2O3 and Co3O4 oxide phases on the surface of MgO support. As a result, higher adsorption and faster solubility of the reacting methane molecules are achieved, keeping the active Fe or Co metals exposed to the reactant gas for a longer time.The used mixed oxide catalysts were analyzed by the TGA technique under an air atmosphere in order to study the thermal stability of the produced carbons. The TGA curves of the used catalysts are shown in Fig.\u00a07\n. It was observed that the TGA curves are all stable to a temperature up to about 500\u00a0\u00b0C, which confirms that the carbon deposited on these catalysts is not amorphous carbon which could decompose at a lower temperature (200\u2013350\u00a0\u00b0C) [32]. This observation comes in accordance with the Raman analysis results, which confirm the crystalline nature of the carbon produced over the used catalysts. In addition, this relatively high degradation temperature reflects the high thermal stability of the produced carbon. The onset-end temperature ranges of the TGA curves of 25Fe\u00a0+\u00a075Co, 50Fe\u00a0+\u00a050Co and 75Fe\u00a0+\u00a025Co catalysts are 530\u2013930\u00a0\u00b0C (wt. loss\u00a0=\u00a048%), 560\u2013945\u00a0\u00b0C (wt. loss\u00a0=\u00a041%), and575- >1000\u00a0\u00b0C (wt. loss = >52.5%), respectively. These values of onset-end temperature ranges indicate that the carbon deposited on the 50Fe\u00a0+\u00a050Co catalyst exhibits a higher degree of graphitization and lesser defects in the structure than the carbon deposited over the other two mixed oxide catalysts [32,44]. In addition, the carbon yield over the 50Fe\u00a0+\u00a050Co and 75Fe\u00a0+\u00a025Co catalysts is much higher than that over the 25Fe\u00a0+\u00a075Co catalyst, reflecting the effectiveness of mixing the two oxides in the decomposition reaction of methane. This behaviour could be attributed to the presence of more CoFe2O4 species in these catalysts than in the third 25Fe\u00a0+\u00a075Co catalyst, as evidenced by the XRD results (Fig.\u00a01). Furthermore, at temperatures above 500\u00a0\u00b0C and immediately before beginning the thermal degradation of the used catalysts, a slight weight gain can be observed, which increases by increasing the iron ratio in the catalyst composition. This is likely due to the oxidation of Fe in the catalysts, which explains the direct proportion between the weight gain and the percentage of iron in the sample [32]. The small weight loss peak observed in the 50Fe\u00a0+\u00a050Co sample at around 900\u00a0\u00b0C is likely due to the weight gain resulting from the oxidation of residual Fe and/or Co, which takes place after the oxidation of all carbon deposits.Carbon yield as a function of the catalyst composition of the mixed oxide catalysts is shown in Fig.\u00a08\n. The carbon yield over the catalysts 25Fe\u00a0+\u00a075Co, 50Fe\u00a0+\u00a050Co and 75Fe\u00a0+\u00a025Co is 30.4, 57.7 and 40%, respectively. These results come in accordance with the catalytic activity results shown in Fig.\u00a06 and confirm the superiority of 50Fe\u00a0+\u00a050Co catalyst over the other two catalysts.Iron\u2013Cobalt mixed oxide catalysts with different Co/Fe atomic ratios were prepared from nitrate precursors using a simple preparation method and water as the only solvent. According to the results of the XRD analysis, the employed method of preparation produces mixtures of cobalt and iron oxides as well as cobalt ferrite species without forming an extensive solid solution between cobalt and iron. Due to carbon deposition on their surfaces, the spent catalysts have a much more porous structure than the fresh catalysts. Moreover, the combination of Raman, SEM, and TEM analysis suggested that the carbon deposited on these catalysts is a mixture of graphene and carbon nanotubes. The Co/Fe atomic ratio significantly impacts the catalytic activity of Co\u2013Fe mixed oxide catalysts for the conversion of methane to hydrogen and carbon. The catalyst 50Co+50Fe (Co/Fe\u00a0=\u00a01) exhibits greater activity and, as a result, produces more hydrogen and carbon than the other two catalysts. Equal mixing of the two oxides appears to optimize the mutual improvement of the reducibility of both cobalt and iron oxides, which has a positive effect on the catalytic activity of the Co\u2013Fe catalyst mixture. The Co\u2013Fe unsupported catalyst system is a cheap and promising candidate for the decomposition of methane into hydrogen and nanocarbons.The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU Programmes Body (SEUPB).The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project no. (IFKSURG-2-055). Dr Ahmed I. Osman wishes to acknowledge the support of The Bryden Centre project (Project ID VA5048). The Bryden Centre project is supported by the European Union's INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB).", "descript": "\n                  Herein, non-supported pure and mixed cobalt and iron oxide catalysts were synthesized from nitrate precursors using a simple, environmentally friendly preparation method in which water was the sole solvent. The prepared catalysts were then used to decompose methane into hydrogen and carbon (graphene nanosheets and carbon nanotubes). The fresh and spent catalysts were characterized by employing X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM) and N2 adsorption-desorption techniques. In addition, the spent catalysts were subjected to thermo-gravimetric analysis (TGA) in order to measure the quantity of carbon deposits on the spent catalysts. The results indicated that the carbon deposited over these catalysts is a mixture of graphene nanosheets and carbon nanotubes (CNT). The results indicated that the mixed oxide catalysts exhibit higher catalytic activity than the pure oxides and that Fe: Co atomic ratio represents the key factor in the catalytic activity of these mixed oxides. After 420\u00a0min under the reaction feed, the 50Fe\u00a0+\u00a050Co catalyst shows the highest catalytic activity towards methane conversion of about 52.6% compared to 41.6% and 31.8% for 75Fe\u00a0+\u00a025Co and 25Fe\u00a0+\u00a075Co catalysts, respectively.\n               "}
{"full_text": "Data will be made available on request.Recently, there has been a widespread development of materials and techniques to buffer the intermittent renewable energy supply via diverse applications such as supercapacitors [1\u20133] and lithium-ion batteries [4,5]. In addition, hydrogen production via proton exchange membrane water electrolysis cells (PEMWEs) stands in the focus of current academic and industrial research. Aside from energy storage for the grid, heat or mobility, PEMWEs can also be a key element in industrial markets that demand hydrogen, such as the ammonia chemical industry, chemical stock synthesis or fuel synthesis (power-to-x) [6]. According to the International Energy Agency (IEA), global demand for hydrogen was estimated at 87 million metric tonnes (MMT) year in 2020 and is forecasted to increase by 13\u00a0MM\u00a0T/year until reaching 528 MMT in 2050 [7]. To reach a net-zero emission scenario in 2050 without further investment in fossil-fuel-based carbon capture, utilization and storage (CCUS) an extra 190\u00a0MT of hydrogen power produced by water electrolysis would be necessary to provide 2000\u00a0GW of net capacity. To put this development into perspective, by the year 2030 a target for 40\u00a0GW in electrolysis capacity should be reached according to the European Green Deal, with an associated cost of EUR 20 to 40 billion without accounting for the electricity costs [8]. While commercial alkaline electrolyzer systems are currently more economical at a lower price per kilowatt [9], PEMWEs have experienced a greater cost reduction due to R&D efforts. The interest in PEMWE development stems from their ability to provide higher current densities and to work at higher temperatures and pressure as compared to alkaline electrolyzers, which makes them a more interesting option for industrial scaling [10]. However, their most challenging limitation is the requirement of Platinum Group Metals (PGMs) such as Ir and Pt to improve the slow electrode reaction kinetics under harsh acidic conditions and high potentials. In particular, the oxygen evolution reaction (OER) occurs in a multi-step reaction that is favoured on the active sites of Ir-based oxide catalysts [11]. While Ru-based oxides have shown higher activity than IrO2 for the OER, the latter sustains the most balanced equilibrium between high activity and durability in the acidic environment [12]. Since PEMWEs should withstand periods of >50 k operating hours under high current densities and transients without showing significant degradation, the choices are further narrowed. Unfortunately, the supply of Ir and Pt is very limited and costly. Just in the first quarter of 2021, the price per ton of Iridium increased sharply by four times, which is the highest price increase registered in the last 20 years [13]. Hence, to upscale the PEMWE production the capital expenditure (CAPEX) cost including noble metal cost has to decrease. To establish Ir catalysts as a commercially viable and scalable option in PEMWEs, the catalyst loading needs to be reduced while maximizing the catalytic activity. For maintaining a high catalytic activity with low loadings, the electrochemically active surface area (ECSA) should be as high as possible. A common strategy to reduce the catalyst loading is to develop alloys with synergistic effects that increase the intrinsic activity [14] as is observed when combining Ir with Cu, Co, or Ni [15,16] or forming core-shell structures [17] that benefit from an Ir-rich surface [18\u201320]. Another approach is to develop completely PGM-free catalysts based on alloys of more abundant metals, i.e., Co, Ni, Fe or Mo derived from metal-organic frameworks (MOF) [21\u201324]. However, the most known commercial catalysts are still unsupported, Ir-based nanoparticles. Several synthesis approaches have been tested to produce nanoparticles with different characteristics [25]. It is not possible to support the nanoparticles on carbon to increase the surface area, as the carbon degrades. This has the disadvantage that the ECSA of Iridium black is small compared to Pt/C catalysts used in fuel cell systems [11,26\u201328]. Recently developed self-supported catalyst nanostructured catalysts present a possible solution to this problem since the catalyst is applied on the substrate (e.g., a gas diffusion electrode) directly without using binder materials. With this approach, it already has been shown that microstructure tuning and modulation of the catalysts\u2019 electronic structure with heteroatoms can produce highly active and stable catalysts [29\u201332]. However, multi-step processes are often used at the lab scale to synthesize catalyst particles and coatings, which can impose limitations in the industrial scaling. On the other hand, physical vapour deposition (PVD) is a well-known technique in the industry. The plasma process yields homogenous composition in the layers and allows flexible operative conditions such as the direct deposition of oxides or control of the morphology modifying, e.g., the sputtering angle or chamber pressure [33]. In recent studies, PVD has been used to produce highly active self-supported catalysts with tunable morphologies using a co-sputtered templating metal [11,34,35]. High ECSAs are achieved by selective dissolution of the templating metal in an acid-leaching processing step, which creates an interconnected network of the active metal. Several publications concerning this method report large ECSAs and activities [11,34,35]. On the other hand, the high catalyst performance observed with traditional academic testing techniques such as the thin-film rotating disk electrode (TF-RDE) hardly ever translates to real operation conditions seen on full membrane electrode assembly (MEA) systems [36]. The step from lab testing to an industrial application is thus very wide. Testing in a liquid acidic environment under mass transport limited conditions does not describe accurately those of a Membrane-Electrode-Assembly. Furthermore, it has been discussed that the degradation trials on OER could have been systematically misinterpreted [37\u201340]. Due to the method limitations, the oxygen evolved during the reaction is trapped close to the surface of the catalyst causing early failure during the test, while the catalyst features remain unchanged [39]. Gas diffusion electrode (GDE) setups have been introduced as a bridging tool as they include realistic constraints (real catalyst loadings, membrane layer, gas diffusion/porous transport layers, three-phase boundary) while keeping the fast screening capabilities of the TF-RDE and retaining the ability to measure the potential drop of the anode in a three-electrode setup. While initially designed for oxygen reduction reaction (ORR) studies [41\u201343], an increasing number of publications with GDE setups in different configurations also prove its flexibility to explore different reactions such as the OER [25]. It is expected that this technique becomes a standard in the electrochemical community and is used more systematically to develop catalyst layers in a fast and cost-effective manner before applying MEA tests [44]. In the present study, we use a GDE setup modified to accommodate electrolysis conditions to perform an electrochemical characterization of the OER in three series of IrCo catalysts produced by PVD with different sputtering Co:Ir ratios. In particular, we aim to study the influence of the deposition parameters on the reaction performance. To that end, we use morphological and chemical characterization techniques (SEM-EDS, XAS, XRD, XPS) to follow the development of the catalyst during different steps in the material preparation (magnetron sputtering followed by acid leaching). The features observed (mesoporosity, chemical distribution, crystallinity) are further discussed alongside the electrochemical characterization of the ECSA of the catalyst by cyclic voltammetry (CV) and OER mass activity. Our findings indicate a direct relationship between the deposition parameters and the electrochemical results. Furthermore, this study underlines the interesting synergy of the PVD with the GDE method to fast-track catalyst film optimization for industrial applications.De-ionized ultrapure water (resistivity >18.2\u00a0M\u03a9\u00a0cm, total organic carbon (TOC)\u00a0<\u00a05\u00a0ppb) from an Aquinity P \u2212102 system (Membrapure, Germany) was used for electrolyte preparation and the cleaning of the GDE half-cell. Carbon gas diffusion layers (GDL) with a microporous layer (MPL) (Sigracet 29BCE, 325\u00a0\u03bcm thick, Fuel Cell Store) served as a substrate for the sputtering of the catalyst film. A polytetrafluoroethylene (PTFE) disk (Bola, 0.12\u00a0mm thickness), a GDL without an MPL (Freudenberg H23, 210\u00a0\u03bcm thick, Fuel Cell Store), a porous transport layer (PTL) (ANKURO Int. GmbH, 0.3\u00a0mm thickness, 50% open porosity), and a Nafion membrane (Nafion 117, 183\u00a0\u03bcm thick, Chemours, Wilmington, DE, USA) were used for the cell assembly (see Fig. 1\n). As a counter electrode (CE) a platinum wire of 0.5\u00a0mm diameter (99.99%, Junker Edelmetalle GmbH) was used, which was folded several times at one side to increase the active surface area. Another Pt wire was used to manufacture a hydrogen reference electrode (RE) using a borosilicate glass capillary of 40\u00a0mm in length and 6\u00a0mm in diameter. Additionally, self-manufactured borosilicate glass frits (6\u00a0mm internal diameter, 20\u00a0mm length) were used to hold the RE during the electrochemical measurements. Perchloric acid (70% HClO4, Suprapur, Merck) was used for electrolyte preparation. O2 (99.999%, Air Liquide) and Ar (99.999%, Air Liquide) were used for magnetron sputtering, acid leaching, and electrochemical measurements.To prepare the self-supported nanoporous catalyst film, a linear sputtering magnetron reactor (Univex 400, Leybold GmbH, Germany) was used. The process chamber was evacuated to a pressure of 1.7 \u00b7 10\u22125\u00a0Pa. The film substrate (GDL) was placed on a holder in a load lock at atmospheric pressure and then evacuated to a base pressure of at least 10\u22124\u00a0Pa. From there, a swivelling arm allowed the holder to enter the process chamber with minimal interruption. During the deposition, an Ar plasma was ignited at the magnetron electrode at a working pressure of 5\u00a0Pa and flushed through the individual magnetron sources at a flow rate of 100 sccm. For the IrxCo1-x film deposition, two magnetrons were equipped with planar targets of Co (99.95%, Evotec GmbH, Germany) and Ir (99.95%, MaTecK, Germany) of 177 x 25\u00a0\u00d7\u00a01.5\u00a0mm located at the upper part of the chamber. The RF generators (Cito 136, COMET) operated at a driving frequency of 13.56\u00a0MHz. Further information about the sample preparation process and the reactor configuration can be found in the SI. A mask of 5\u00a0cm\u00a0\u00d7\u00a05\u00a0cm on the substrate holder limited the sputtered area during the deposition. The sample was allowed to oscillate in a linear trajectory between the two respective magnetrons. The sputtering was initiated when the sample reached the position below each magnetron. At that point, the sample holder was programmed to oscillate with an amplitude of 1\u00a0mm to increase the homogeneity of the deposition. The holder reached an acceleration of 100\u00a0mm\u00a0s\u22122 and a maximum linear velocity of 50\u00a0mm\u00a0s\u22121. The RF power was chosen as 225\u00a0W for Co and 50\u00a0W for Ir. The alternating sputtering process was performed for 500 cycles in all series, modifying the deposition time to achieve three different element ratios as seen in Table 1\n. The average deposition time was 4\u20135\u00a0h. Before the measurements, a calibration of the sputtering process was performed where Ir was sputtered continuously for 20\u00a0min on a substrate. The final Ir loading was measured by mass gravimetry, and the thickness homogeneity was verified using a profilometer (Alpha Step D-600, KLA). Assuming a linear dependency of loading with the sputtering time, three series were produced with a nominal Ir loading of 0.250\u00a0mg\u00a0cm2 and different Co:Ir deposition time ratios. The resulting Co:Ir ratios were determined experimentally by EDX on the as-prepared samples (Table 1).As part of the Ir\u2013Co catalyst film preparation, the samples were leached after the deposition in 1\u00a0M HClO4 to create a nanoporous self-supported Ir structure by selectively dissolving the Co under potential-controlled conditions according to the method developed by Sievers et al. [11]. The individual steps of the acid leaching procedure are summarized in Table 2\n and described more in detail within the Result and Discussion section. Once the samples were leached, they were cleaned in distilled water and left to dry in air before further manipulation.The GDE was prepared using a Nafion membrane (Nafion 117, 183\u00a0\u03bcm thick, Fuel Cell Store) hot pressed to the sputtered gas diffusion layer (GDL). In this study, the Nafion membrane was activated as described by Schr\u00f6der et al. [25]. A concentric circular steel punch (BOEHM, Germany) was used to cut small disks from the GDL and the assembly material. First, a disk of \u00d8 3\u00a0mm was cut from the sputtered GDL. Using an in-house built hot press (Fig. S1) with a modified soldering iron and 6\u00a0kg steel weights, a \u00d8 10\u00a0mm Nafion membrane was hot pressed on top of the catalyst layer at 120\u00a0\u00b0C using 84\u00a0kg\u00a0cm\u22122 for 30\u00a0s.As indicated in Fig. 1, a \u00d8 20 mm\u00a0Gas Diffusion Layer (GDL) without a microporous layer (MPL) was placed directly over the flow field of the stainless-steel bottom cell. On top, a \u00d8 20\u00a0mm Teflon disk with a \u00d8 3\u00a0mm center hole was used as a sealant for the liquid and electrical insulator. Embedded inside, a \u00d8 3\u00a0mm PTL disk was positioned to allow the gas flow to contact the GDE on top and to serve as a current collector. Last, a Teflon upper cell was pressed against the assembly and secured tightly with a metal clamp. Both the Teflon upper cells and the CE and RE were cleaned before every use according to the following protocol. First, they were placed overnight in a tank with concentrated HNO3 and concentrated H2SO4 solution 1:1 in volume. Afterwards, they were rinsed and boiled in distilled water for 1\u00a0h in at least 5 cycles. The unused materials were kept in a glass vial and boiled always one last time before use. Furthermore, the Pt wire was flame annealed every time it was used to remove any organic contaminations. After every trial, all the assembly components were discarded and replaced with new ones to decrease the influence of contaminations.All the experiments were conducted with a Potentiostat (ECi-211, Nordic Electrochemistry ApS, Denmark). The Potentiostat also controlled the gas switching between humidified Ar and O2 during the experiments. An overview of the experimental protocol is presented below in Table 2.The GDE half-cell (Fig. 1) and a glass bubbler were placed inside an insulating glass chamber during the measurements (See Fig. S1 in SI). Precise temperature control (\u00b10.1\u00a0\u00b0C) was achieved through a constant flow of distilled water recirculated in between the double glass walls with a water heating system (Lauda RC6 SC). The GDE half-cell was placed in the middle of the chamber, supported on an aluminium laboratory jack (Laborboy, Sigma Aldrich) and insulated with a PTFE plate in between. Before the start of the measurements, the system was allowed to equilibrate at a constant temperature for at least 30\u00a0min. All the temperature references correspond to the set point defined in the water heating system. To prevent any shifts in reference potential due to contaminations on the RHE electrode, the RE was protected in a glass frit manufactured by an in-house technical glassblower. In addition, the RHE electrode was calibrated before each measurement in a separate GDE cell against a Pt GDE with the same molarity and electrolyte as the testing GDE cell, i.e., 1\u00a0M HClO4 electrolyte. The H2 gas was supplied through an in-house electrolyzer, connected to the gas flow through lines of the GDE cell. The RHE offset was measured by cyclic voltammetry in a potential interval between \u22120.005 and 0.005\u00a0V at 100\u00a0mV\u00a0s\u22121 for 200 cycles. The acceptable range for initial RHE values was defined as \u00b1 0.003 VRHE. In case of a larger deviation, the RHE was remade, and the calibration procedure was repeated to avoid large iR-correction errors. Before the measurements, Ar was purged through the flow field as a conditioning step and cyclic voltammograms were recorded at a scan rate of 100\u00a0mV\u00a0s\u22121 in a potential range between 0.025 and 1.2 VRHE until a stable cyclic voltammogram could be observed (ca. 30 cycles). The ECSA of the catalyst (Table 3\n) was determined by integrating the Hupd area in the potential window of 0.025\u20130.25 VRHE of the last CV acquired using a fixed conversion coefficient of 176\u00a0\u03bcC\u00a0cm\u22122 [11] according to the following formula:\n\n(1)\n\n\nE\nC\nS\nA\n\n\n[\n\n\nm\n2\n\n\ng\n\n\u2212\n1\n\n\n\n]\n\n=\n\n\nQ\n\nH\n\nu\np\nd\n\n\n\n\n\nL\n\nI\nr\n\n\n\u00d7\n176\n\n\u03bc\nC\n\nc\n\nm\n\n\u2212\n2\n\n\n\n\n\n\n\n\nThe OER activity was determined through a galvanostatic step protocol with increasing currents based on Schr\u00f6der et al. [25] and scaled accordingly to account for the loading difference. An AC signal (5\u00a0kHz, 5\u00a0mV) was applied during the current steps to obtain an online resistance measurement between the working and reference electrode (\u223c10\u00a0\u03a9) which was used for an iR-correction of the measured potential values.The morphology of the unleached catalyst layers, i.e., after the deposition process, was characterized using secondary electron imaging (SEM), see Fig. 2\n. As seen in Fig. 2a, b and c, all catalyst series featured a similarly packed globular structure. Similar morphologies have been previously observed in studies of catalyst films prepared on carbon paper substrates using comparable process conditions [33]. The size of the globular features was not substantially different between the respective series, ranging from 0.1 to 0.9\u00a0\u03bcm in diameter. However, the SEM micrographs indicate further development of nanoporous structures. That is, the surface of the globules exhibits a certain degree of roughness, which is especially distinct for the Co-rich series (Ir28Co72; in the following the notation refers to the elemental composition obtained by EDX point analysis before the acid leaching), see Fig. 2a as well Fig. S2b for a closer look. Finally, yet importantly, EDX top-down mapping of different representative areas on the catalyst films, see Fig. S3, revealed that in all cases Ir and Co were homogeneously distributed across the film. As mentioned before in the Method section, Co was removed from the sputtered films in a process that is referred to as acid leaching. As the Pourbaix diagrams show for the respective catalyst film constituents, metallic Ir is stable under the leaching conditions while Co is oxidized to soluble Co2+ ions and does not form a passive film [45,46]. Hence, Co dissolution starts spontaneously when a sputtered sample is submerged in a de-aerated 1\u00a0M HClO4 aqueous solution [11], giving the solution had a characteristic pink tone. The color of this solution has been described extensively as a result of the complexation of Co2+ complex in water to form [Co(H2O)6]2+. To confirm this, Cl\u2212 ions were added to the solution from concentrated HCl and the temperature was raised. Both effects shift the equilibrium to [CoCl4]2- as a direct consequence of Le Chatelier principle [47], which shows a distinct blue color, see Fig. S4. To attain better control of the acid leaching process and to minimize Ir oxidation before ECSA determination of the metallic surface, the samples were submitted to an electrochemical cycling protocol (Table 2) between 0.05 VRHE and 0.5 VRHE with a scan rate of 100\u00a0mV\u00a0s\u22121 starting directly after the electrolyte was added to the upper cell compartment. The cycling continued until a stable CV was achieved. This was typically the case after 30 potential cycles. Along this process, the initial Co to Ir ratios were changed significantly. In every case, the relative amount of Co decreased to under 10% in weight according to the EDX. Using XPS for a more surface-sensitive analysis of the pre-leached and leached samples, see Fig. S5, we observed a trend in the decrease in the Co:Ir ratios after leaching following the series, albeit not proportional to the initial ratios (see Fig. S6). This discrepancy could perhaps be attributed to the drastic change in morphological differences and chemical gradients to form a more stable Ir shell with a Co core after the acid leaching [11,47,48]. The process of acid leaching has been well described for Pt-based alloys for the oxygen reduction reaction. It has been shown by low energy ion scattering (LEIS) that the exposition of PtM (M\u00a0=\u00a0Fe, Co, Ni, etc.) surfaces automatically leads to a full depletion of all non-noble atoms from the surface and the formation of \u201cskeleton\u201d or core shell surfaces [48]. In the here reported work, sparse colonies of Ir-rich dendritical structures were formed of the GDL carbon substrate, which was also left exposed over large areas. The development of this porous structure differs substantially from the preparations on glassy carbon in a former study [11], see Fig. S2. The reason for this difference might be the three-dimensional structure of the gas diffusion electrode or the hydrophobicity. The initially Co-rich sample, Ir28Co72 presents the biggest size of the dendrites and area of the exposed substrate. Both features appeared to decrease together with the Co:Ir ratio when comparing Ir28Co72 with the Ir45Co55 and Ir75Co25 series (Fig. 2d, e and f respectively).\n\n\n\n\n\n\n\n\n\nSeriesBy element wt.%\nwt.% norm.\n\n\nUnleached\nLeached\n\n\nIr\nCo\nIr\nCo\n\n\n\n\nIr28Co72\n\n27.7\u00a0\u00b1\u00a01.9\n72.3\u00a0\u00b1\u00a01.9\n95.0\u00a0\u00b1\u00a00.8\n5.0\u00a0\u00b1\u00a00.8\n\n\nIr45Co55\n\n44.9\u00a0\u00b1\u00a00.7\n55.1\u00a0\u00b1\u00a00.7\n96.2\u00a0\u00b1\u00a01.5\n3.8\u00a0\u00b1\u00a01.5\n\n\nIr75Co25\n\n75.3\u00a0\u00b1\u00a05.1\n24.7\u00a0\u00b1\u00a05.1\n91.9\u00a0\u00b1\u00a00.8\n8.1\u00a0\u00b1\u00a00.8\n\n\nSeriesBy element at.%\nat.% norm.\n\n\nUnleached\nLeached\n\n\nIr\nCo\nIr\nCo\n\n\nIr28Co72\n\n10.5\u00a0\u00b1\u00a00.9\n89.5\u00a0\u00b1\u00a00.9\n85.4\u00a0\u00b1\u00a02.1\n14.6\u00a0\u00b1\u00a02.1\n\n\nIr45Co55\n\n19.8\u00a0\u00b1\u00a00.7\n80.2\u00a0\u00b1\u00a00.7\n88.6\u00a0\u00b1\u00a04.1\n11.4\u00a0\u00b1\u00a04.1\n\n\nIr75Co25\n\n48.8\u00a0\u00b1\u00a06.7\n51.3\u00a0\u00b1\u00a06.7\n77.8\u00a0\u00b1\u00a01.9\n22.2\u00a0\u00b1\u00a01.9\n\n\n\n\n\nThe element distribution of representative leached areas can be found in the EDX mapping of Fig. S3 of the SI. An as-sputtered XRD analysis indicated that the elements are found in a heterogeneous film with a low degree of crystallinity, as it is normal for sputtered catalysts that do not experience a heat treatment [26,49,50]. While the overall structure remains amorphous, the shift to lower theta values and narrowing of the Ir (111) Bragg peak after leaching, see Fig. S7, suggests that it might experience a slight increase in crystallinity, which has also been reported in similar studies [11,34,51]. Since the first studies on AuAg nanoporous structure formations via selective leaching, several studies have emerged to explain the behaviour of homogeneous bimetallic alloys [52\u201357] as well as the change in electronic properties due to the formation of core-shell nanoparticles. However, a former study using a similar magnetron-sputtering and acid-leaching process to create a self-supported Pt\u2013CoO network revealed that no alloy was formed in the bimetallic deposition or leaching process [35]. A further look into the oxidation state and the small range structures of the Ir\u2013Co series was conducted by ex-situ X-ray Absorption Spectroscopy (XAS) of the leached samples, see Fig. 3\n and Figs. S8\u201310 of the SI. Data were collected at both, the Co and Ir edge, however, due to the low Co content the data quality is significantly lower for the Co edge than for the Ir edge. Therefore, we draw our conclusions mainly from the data obtained from the Ir LIII K-edge. The X-ray absorption fine structure (EXAFS) results reveal mixed metallic and oxide structures, see Fig. 3. The presence of Co\u2013Co1 and Ir\u2013Ir1 coordination indicates that a proportion of Co and Ir remains metallic after acid leaching and exposure to air. Furthermore, the presence of Ir\u2013Co1 coordination shows a partial alloy character with a similar trend as observed in the Co content of the leached samples by EDX: Ir75Co25 > Ir28Co72 > Ir45Co55. In addition, Ir\u2013O1 and Co\u2013O1 coordination is seen indicating partial oxidation of the samples. Interestingly, the data from all series indicate a similar Ir\u2013O1 bond length, indicating that the Co content has no measurable effect on lattice strain.Considering the mixed chemical nature of the material, we describe it as IrxCo1-x nanoclusters rather than an IrCo alloy. In this context, the self-supported structure is achieved by the dissolution of a sacrificial templating metal in a selective acid leaching process under potential conditions, coupled with surface restructuring processes in the material due to diffusive forces. In an earlier study from the same authors concerning the leaching behaviour of co-sputtered noble and non-noble metals in a Pt\u2013Cu system, a mechanism of acid leaching process leading to self-supported nanostructured catalysts was already discussed [34]. As the non-noble metal dissolves in acid, hydrogen gas evolution starts spontaneously. Some of the gas can be trapped in interior cavities and mechanically push the material around to nucleate pores. At the same time, the catalyst-rich areas undergo a surface diffusion process due to the electrochemical and mechanical forces, which promote the redeposition of catalysts in neighbouring regions. The structures created in such a process depend on the irregularities of the morphology and porosity at the surface. Surface diffusion of catalyst particles is evidenced by an Ir enrichment and depletion of Co over the surface observed in the EDX maps (Fig. S3) and reinforced by the XPS results (Fig. S6). This process would be in agreement with the different morphologies observed in the series between the as-deposited and leached state for the different EDX Co:Ir ratios and the different initial distributions of Ir and Co-rich areas. A previous study of a very similar Ir\u2013Co catalyst already demonstrated that Co dissolves from all areas in contact with the acid solution leaving a percolated Ir network with the same domain size as the initial deposition [11], which corresponds well with the results presented here.After sputtering and acid leaching, each series of the catalyst layers was assembled into the GDE setup for electrochemical testing. The aim of the electrochemical testing was twofold: first, the electrochemically active surface area (ECSA) of the leached Ir was determined. This was achieved by determining the Hupd area in cyclic voltammetry [58]. The leaching conditions were designed to dissolve the Co while preserving Ir in metallic state, as IrO2 does not display any Hupd area. We assume that after leaching any oxidized Ir surface would be reduced and that there is a direct relationship between metallic Ir surface before activation and ECSA after activation. The second aim was to activate the catalyst layer and determine its activity for the OER.The electrochemical characterization is exemplified in Fig. 4\na which depicts the CV and OER activity of a leached Ir-rich (Ir28Co72) nanostructured IrCo film. It is seen that after leaching, the CV displays a pronounced Hupd area indicative of metallic Ir, allowing a straight-forward ECSA determination of 52.6\u00a0\u00b1\u00a04.8 m2g-1, Fig. 4a. After recording the CV, the gas was switched, and oxygen was flushed through the cell at 1 sccm for 20\u00a0min to guarantee a saturated oxygen atmosphere. The OER activity was determined before and after activation and benchmarked to published data from a commercial IrO2 black powder (Alfa Aesar) [25]. It is worth mentioning that the commercial sample was prepared with a different loading (1\u00a0mg\u00a0cm\u22122) than the samples in this study. However, it is still considered to be a useful reference since the OER activities were measured using the same protocol and setup configuration. The first set of OER activities revealed that the catalyst surface was not yet completely activated into IrOx. Yet, the OER overpotential in this state was around 40\u00a0mV lower as compared to the benchmark. Recording another set of CVs in Ar atmosphere after the first OER measurements confirmed that remainders of metallic Ir were present from a decreased but still discernible Hupd area. In addition to the reduced Hupd area, the double-layer capacity was increased (Fig. S11b). To complete the oxidation of the metallic Ir, a potentiostatic activation step was applied at 1.70 VRHE for 20\u00a0min in O2 atmosphere (Fig. S12), after which a second set of OER activities was recorded. The fact that the overpotential was reduced by an additional 10\u00a0mV as compared to before activation indicates the further formation of the active IrO2 phase. Nevertheless, recording another set of CVs in Ar atmosphere in step 2.3 of the protocol (Table 2) shows that a complete, irreversible oxidation of the surface has not yet been achieved and still some Hupd area is visible (Fig. S11c). Despite the incomplete activation, the OER mass activity at 1.50 VRHE was 101.5 Ag\u20131\nIr, roughly eight times higher than that of the commercial benchmark catalyst Table 3). In the third and last step of the protocol the temperature was increased from 30 to 60\u00a0\u00b0C and OER activity was determined for one last time (Fig. 4c). The raise in temperature leads to a clear decrease in overpotential of ca. 40\u00a0mV even at the lowest (20 Ag\u20131\nIr) current densities (Fig. S13). At this temperature, an OER activity of 117.8 Ag\u20131\nIr was determined at 1.46 VRHE. Interestingly, in addition to a temperature-induced kinetic activation, the raise in temperature leads to an additional activation via further oxidation. This can be seen by the fact that the relative OER mass activity increases to a threefold value. In comparison, the benchmark catalyst is mostly oxidized in its initial state. A further description of the contributions to the decreased overpotential due to temperature and activation contributions can be found in the SI. In addition to the activation, it is seen that with the temperature rise the Tafel slope decreased slightly from 62 to 49\u00a0mV dec\u22121. This is a small, but still, significant change, which can be explained by the temperature dependency of each rate constant for every step of the reaction according to Arrhenius\u2019 equation [59]. The electrochemical response of the Ir45Co55 series presented in Fig. 4d\u2013f shows a similar development as the former discussed Co-rich series. From the initial CVs after acid leaching the measured Hupd region after acid leaching was determined to be 33.7\u00a0\u00b1\u00a01.9 m2g-1. A possible explanation for the decreased surface area could be a smaller size of the features formed after the Co leaching (Fig. 2e) and less internal porosity. Since the measured surface area at the initial step (Fig. S11a) was only around half of the Co-rich series (Fig. 2a), the oxide formation after the activation in Step 2.3 (Table 2) also rendered a smaller oxide capacitive layer (Fig. S11d). However, at this point the Ir45Co55 samples still presented a comparable Hupd area to the Ir28Co72 series, indicating that the sample was not completely oxidized. Nevertheless, the overpotential still decreased by about 10\u00a0mV after activation as in the case of the Co-rich samples due to the oxidation of the metallic surface (Fig. 4e). As a result, the activity measured after activation at 30\u00a0\u00b0C and 1.50 VRHE increased to 57.6 Ag\u20131\nIr, four times greater than the value of the commercial sample (Table 3). Still, it was approximately two times lower than the Ir-rich (Ir28Co72) samples under the same conditions. OER activity measured at 60\u00a0\u00b0C was nearly doubled from the previous step, i.e., 91.8 Ag\u20131\nIr at 1.46 VRHE. The sharp activity increase reinforces the hypothesis that the samples only experience full activation during the protocol at high temperatures. Even at the start of this step the overpotential already decreased by 45\u00a0mV compared to the activity recorded after activation (Fig. S13). Since the reversible reduction in the overpotential due to temperature increase is approximately 25\u00a0mV, the further decrease supports the argument of a dynamic activation process. Additionally, the Tafel slope also decreased from 64 to 52\u00a0mV dec\u22121 between the activation and high-temperature OER respectively. The Ir-rich Ir75Co25 series exhibited the lowest values for the surface area and activity throughout the OER measurements. After acid leaching, the ECSA was determined to be 21.4\u00a0\u00b1\u00a02.0 m2g-1, see Table 3. This is in good agreement with the observed top-down morphology from the leached sample at the SEM (Fig. 2f), which featured the smallest clusters in all three series. The reduction in overpotential after activation was also minimal, i.e., ca. 3\u00a0mV (Fig. S13). The OER activity after activation at 30\u00a0\u00b0C was 44.3 Ag\u20131\nIr at 1.50 VRHE, which is approximately 30\u00a0mV lower than that of the commercial benchmark under the same conditions. As also observed for the other IrCo series, the mass activity improved at 60\u00a0\u00b0C, reaching 62.7 Ag\u20131\nIr at 1.46 VRHE as compared to the 10.14 Ag\u20131\nIr of the commercial benchmark. Interestingly, even though the mass activity results were at the lowest of the series in absolute numbers, a similar reduction in overpotential at high temperatures was observed as compared to Ir45Co55. (Fig. S13). The Tafel slope was the highest of the series and only decreased from 68 to 58\u00a0mV dec\u22121 between activation and high-temperature OER respectively, which was the smallest change in all series (see Table 3). Along the series, the ECSA, the Tafel and the mass activity followed the trend defined by the initial as-deposited Co content Ir28Co72\u00a0>\u00a0Ir45Co55\u00a0>\u00a0Ir75Co25. When combined with the catalyst morphology, this trend strongly suggests that a high initial Co content increases the catalyst utilization by increasing the ECSA in a dynamic process as the catalyst is activated. On the other hand, the specific activity was found to correlate with the XAS results and the Co content after leaching from the EDX results, which hints at a positive influence from the remaining Co in the structure. A summary of the main electrochemical results can be found in Table 3 below.Some additional factors need to be considered together with the electrochemical results. As mentioned in the methods section, the deposition time for the magnetron targets was defined between 1\u00a0s and 6\u00a0s for Co and kept constant at 3\u00a0s for Ir in each cycle. The Ir loading calibration was performed in a continuous deposition of 1200\u00a0s. In a preliminary test, it was confirmed for Ir that the loading for the continuous deposition matched the loading for the cycled deposition by mass gravimetry. However, when measuring the expected ratios by EDX they were found to be different from the nominal. While EDX is a versatile tool to determine the spatial resolution of the thin catalyst layer and the element distribution on the substrate, it is known that absolute quantification using automatic standardless EDX profiles is generally poor [60]. We found that using 15\u00a0kV for the analysis was a compromise between good surface sensitivity and exciting the higher energy lines for better elemental analysis (Co K\u03b1\u00a0=\u00a06.924\u00a0keV, Ir L\u03b1\u00a0=\u00a09.147\u00a0keV) to maximize the number of counts. However, in the acid-leaching process, the catalyst loading is further reduced which leads to larger errors in the elemental quantification. Hence, we assumed the initial loading was unchanged for the electrochemical mass activity results, while it is likely that both the surface area and the mass activity might be larger than what was measured. The quantification of the changes in the Ir loading during electrochemical measurements is not trivial. Unlike other PGM catalysts (Pt, Pd), iridium is known to fully dissolve only in extremely aggressive conditions requiring high temperatures, pressures, and strong acids [61,62]. Therefore, the preparation of the samples for conventional ex-situ techniques such as Inductively Coupled Plasma Mass Spectrometry (ICP-MS) that relies on the analysis of the dissolved species is non-standard and complex. However, in recent years, some approaches have been taken to quantify the Ir loading or loss during the electrochemical measurements. One of the most relevant methods is the Scanning Flow Cell (SFC) coupled with an ICP-MS system, which allows to perform time-resolved measurements of the material loss during an electrochemical protocol. Unfortunately, it also does not provide information about the remaining catalyst in the deposited layer [63]. Furthermore, there is not yet a compatible design to combine the high-current capabilities of the GDE method with the access to analytics of the SFC ICP-MS. Additionally, most techniques have been optimized so far for the study of supported catalysts with Ir nanoparticles which are known to present higher degradation rates compared to self-supported catalysts [11,34,35]. Since the purpose of this study was to assess the performance of different Ir-based catalysts under the same conditions and using a comparative approach, a quantitative study of the Ir loading loss or the formation of transient species was not performed. In addition, speculations about specific activity changes in correlation to XAS data were made with data measured at 30\u00a0\u00b0C in combination with the ECSA measurement in metallic state by Hupd. However, the increase in the double layer capacity of the CVs due to the oxidation to IrOx after the OER at higher temperatures (see Fig. 2f) would have resulted in different surface areas and thus different specific activities. Therefore the specific activity reported at 60\u00a0\u00b0C has to be taken with caution. Other in-situ methods such as the mercury underpotential deposition could have also been considered [64]. However, this was not possible, as the membrane would need to be removed to avoid poisoning, impeding further electrochemistry. For the same reason, most material characterization methods in this study have been limited to the after-leaching state. Further insight into the dynamic catalyst activation at high temperatures and its link to the morphology may be achieved with in-operando XAS methods as soon as they are developed. Nevertheless, these limitations were considered as boundary conditions to help the discussion and understanding of our results.In this study, we applied the GDE method to perform activity measurements of PVD-produced catalysts for the OER. First, three series of Ir\u2013Co catalysts with equal 250\u00a0\u03bcg/cm2 Ir loading were sputtered on carbon substrate using different Co:Ir weight ratios (Ir28Co72, Ir45Co55 Ir75Co25). To create a self-supported nanoporous structure with increased ECSA, Co was removed in an acid-leaching step. This is rendering a distinct dendritical surface morphology with Ir-rich clusters and slight changes in crystallinity. During the process, a mixed metallic and oxide structure with local Ir\u2013Co coordination is formed. A higher initial Co content leads to larger surface areas after leaching, outperforming the OER activity of a commercial IrOx catalyst benchmarked at 30\u00a0\u00b0C and 60\u00a0\u00b0C. Overall, the performance followed the Co:Ir series Ir28Co72\u00a0>\u00a0Ir45Co55\u00a0>\u00a0Ir75Co25\u00a0>\u00a0IrOx, where the best-performing catalyst at 60\u00a0\u00b0C reached more than a tenth-fold increase in mass activity over the commercial sample. The performance increase as compared to the benchmark catalyst, accounting for loading and preparation differences, can be due to higher dispersion in addition to a ligand effect. The latter is supported by the specific activity trend correlation with the remaining Co after acid leaching and XAS coordination data. A strain effect, by comparison, was not supported by the XAS data. The temperature increase and dynamic surface activation due to oxidation of metallic Ir, both observed by CV and the OER activity, had a positive influence on the catalyst activity. The authors acknowledge that the complex mechanisms behind the influence of the Co content and the electrochemical performance may not be fully explained from the measurement results, but also remain beyond the scope of this study. On the other hand, it was demonstrated that the flexible and reproducible characteristics achievable from the nanostructured PVD-produced catalysts in combination with the three-electrode GDE setup can reveal further insights into the electrode evolution under more realistic conditions than traditional methods such as RDE, helping to fast-track OER catalyst experimental research.\nPablo Collantes Jim\u00e9nez: Methodology, Investigation, Writing \u2013 original draft. Gustav Sievers: Writing \u2013 review & editing, Supervision, Conceptualization. Antje Quade: Investigation, Methodology. Volker Br\u00fcser: Supervision, Methodology. Rebecca Katharina Pittkowski: Investigation, Methodology. Matthias Arenz: Writing \u2013 review & editing, Supervision, Conceptualization, All authors checked and approved the final version of the manuscript.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Gustav Sievers has patent #DE102016013185B4.The authors gratefully acknowledge the financial support by the German Federal Ministry of Education and Research (BMBF) in the framework of the VIP\u00a0+\u00a0Projekt. 03VP06451 (3DNanoMe). The authors thank Adam Clark from the SuperXAS beamline X10DA at the Paul Scherrer Institute (PSI) for measuring the XAS data via mail-in service. MA and RKP acknowledge funding from the Swiss National Science Foundation (SNSF) via project No. 200021 184742 and the Danish National Research Foundation\nCenter for High Entropy Alloys Catalysis (CHEAC)\nDNRF-149.The following are the Supplementary data to this article.\n\nFig. S1\nFig. S1\n\n\n\n\n\nFig. S2\nFig. S2\n\n\n\n\n\nFig. S3\nFig. S3\n\n\n\n\n\nFig. S4\nFig. S4\n\n\n\n\n\nFig. S5\nFig. S5\n\n\n\n\n\nFig. S6\nFig. S6\n\n\n\n\n\nFig. S7\nFig. S7\n\n\n\n\n\nFig. S8\nFig. S8\n\n\n\n\n\nFig.S9\nFig.S9\n\n\n\n\n\nFig. S10\nFig. S10\n\n\n\n\n\nFig. S11\nFig. S11\n\n\n\n\n\nFig. S12\nFig. S12\n\n\n\n\n\nFig. S13\nFig. S13\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.jpowsour.2023.232990.", "descript": "\n                  The scarce supply of Ir used to catalyze the sluggish oxygen evolution reaction in acidic water electrolysis calls for unconventional approaches to design more active catalysts with minimal resource usage for their commercial scaling. Industrial-ready production methods and laboratory scale tests that can reflect the catalyst behaviour realistically need to be included in this process. In this work, we benchmarked three series of self-supported Ir\u2013Co catalysts with low Ir loading produced by physical vapour deposition under relevant current densities in a gas diffusion electrode setup. It was seen that after selective acid leaching of the Co, a nanoporous structure with a high electrochemically active surface area and a mixed oxide and metallic character was formed. Depending on the initial Co:Ir deposition ratio over ten times higher oxygen evolution mass activities could be reached as compared to a commercial, unsupported IrOx nanoparticle catalyst used as a benchmark in the same setup configuration. The presented integrative catalyst design and testing strategy will help to facilitate bridging the gap between research and application for the early introduction of next-generation catalysts for water splitting.\n               "}
{"full_text": "In many cases, the aqueous streams caused by pharmaceutically contain organic pollutants such as caffeine (CAF), which is the most commonly used legal drug throughout the world in the form (beverages or combined) [1, 2], and it is toxic and poorly biodegradable. These polluting agents are also in very high concentrations, so that releasing these molecules into the water resources, affects aquatic life and ecosystems beings, adversely [3]. In these cases, it is necessary to use less conventional techniques to remove the pollutants and convert persistent chemicals into environmentally benign compounds, such as advanced oxidation processes (AOPs) like Fenton process, electro catalytic oxidation, photocatalytic oxidation and catalytic wet peroxide oxidation [4, 5, 6, 7, 8]. The oxidation process with H2O2 using a heterogeneous catalyst is commonly known as catalytic wet peroxide oxidation (CWPO). CWPO is one of the promising methods for the rate of pollutant degradation at mild temperature and pressure conditions [9, 10, 11, 12], providing that, a suitable catalytic system is used, such as catalysts based on natural and pillared clays (PILCs). These materials are porous, developed by molecular design methods, prepared by exchanging the cations located in the interlayer space of clays with large inorganic polyoxo/hydroxo cations [13, 14, 15].Clay minerals, a large family of aluminosilicates (Si4+ and Al3+) structures with a variety of chemical composition, structure and surface properties, very reactive materials due to their small particle size, high surface area and adsorption properties [16, 17]. Bentonite, with a layer structure containing a larger amount of mesoporous, has been widely used in the catalysis field. It constitutes an abundant mineral resource and an effective catalyst support because its strong metal support interaction [18, 19, 20]. Their quality depends on several parameters such as color and swelling behavior which are influenced by the crystal chemistry and mineralogical composition [21]. Bentonites consist mainly of montmorillonite, which is a dioctahedral clay of the smectite group with the 2:1 layer linkage [22, 23]. As the previous research demonstrated, most studies focus on using cheaper transition active metals, such as Ni or Cu. Both metals are widely used as catalysts for a variety of processes, their wide usage can be attributed to their several characteristics such as being very strong catalytic, optical, electrical, mechanical and antifungal/antibacterial. Moreover, Copper (Cu) has been used in catalyst based on the activated carbon (AC), nano-zerovalent copper (nZVCu) functionalized hydroxyapatite (HA), alginate [24, 25, 26] and on the perovskite (LaNiO3, NaNi0.9Cu0.1O3 and LaNi0.5Cu0.5O3) [27,28]. Thus, the objective of the present study is to evaluate the potential use of Moroccan yellow clay as a superb natural support of Copper/Nickel catalysts, in order to enhance its catalytic activity using simple impregnation method for degradation of organic pollutants in aqueous solution [29, 30, 31].The Response Surface Methodology (RSM) was widely used in many researches for the optimization of different liquid effluent treatment processes. In fact, RSM is a statistical technique applied to reduce the number of experiments, to optimize and analyze the experimental independent parameters that affect the process' efficiency, and to generate a mathematical model, which describes the processes' behavior. Central composite, Doehlert, and Box\u2013Behnken are three classes of response surface designs. Yet, Box\u2013Behnken design is more advantageous because it creates an experimental design with a few test runs, which makes the experiments economically feasible and beneficial [32, 33]. As far as we know, and according to the extensive literature review, CWPO of CAF onto CuNi-YC was not fully investigated. Therefore, the objective of this work is to evaluate the yellow clay extracted from the North of Morocco, exhibit its characterization as a natural eco-friendly and low-cost material and highlight its availability and usefulness when it will be modified using Nickel and Copper by impregnation method. Thus, work will allow the determination of its physicochemical properties and then the identification of its field of use as a catalyst for CWPO, which has never been studied before. In order to examine its effectiveness in oxidation after its modification, the caffeine (CAF) molecule was used in this study as a persistent and hardly removable pollutant to be removed from aqueous solution in a batch reactor. Furthermore, the optimization of degradation's efficiency is an interesting study; hence, a response surface methodology based on Box\u2013Behnken Design (BBD) was used with a three-level factorial design, to optimize the effects of three significant factors: impregnated copper (%), temperature (20\u201360 \u00b0C) and H2O2 dosage (8.2\u221710\u22122 \u2013 24.6\u221710\u22122 mol.L\u22121) which influence the CWPO process.The following chemicals were used in the catalysts\u2019 preparation (CuNi-YC samples): Copper (II) Nitrate Hexahydrate (Cu (NO3)2.6H2O, Sigma-Aldrich, 99.99% purity), Nickel (II) Nitrate Hexahydrate (Ni (NO3)2.6H2O, Sigma-Aldrich, 99.99% purity), Hydrochloric Acid (37%, w/w), Sodium Hydroxide 97% (NaOH) and Hydrogen Peroxide (30%, w/w) Sigma-Aldrich. The Caffeine (C8H10N4O2, Sigma-Aldrich) were used as molecule models for the degradation by catalytic wet peroxide oxidation (CWPO) tests. All chemical materials were used without further purification. The deionized water has been used throughout the experiments.The catalysts support is referred to as yellow Clay (YC). The one used in this work has been taken from a natural basin of the Tidiennit massif in the North of Morocco. Fraction up to 63 \u03bcm. Cu\u2013Ni samples were synthesized by the wet impregnation method where Cu(NO3)2\u20226H2O, and Ni(NO3)2\u20226H2O were mixed to obtain the following Cu:Ni weight ratios: 1:0, 1:1 and 0:1 and the obtained catalysts were denoted as CuNi10-YC, CuNi11-YC and CuNi01-YC, respectively. Every solution should contain as much as metal nitrate to get 10 wt% metal in the final catalyst powder. In this process, 10 wt% metal was dissolved in 50 mL of deionized water. Then, the YC was dropped into this aqueous solution with stirring speed of 200 rpm at 75 \u00b0C for 4 h, to obtain a slurry. After impregnation, the slurry was dried at 100 \u00b0C overnight, and then calcined at 500 \u00b0C for 4 h.In order to evaluate CAF mineralization using the catalysts (CuNi10-YC, CuNi11-YC and CuNi01-YC), an amount of 1 g.L\u22121 of each catalyst was added to 100 mL CAF solutions with a concentration of 40 mg.L\u22121; then, stirred to maintain a uniform suspension. Before starting the CWPO reaction, the adsorption of CAF by the catalysts was performed until reaching the equilibrium, which happen to be at 15 min. This step is a controlling experiment to compare between adsorption of CAF and the CWPO conversion; on the other hand, to insure that the decrease of CAF concentration is attributed to CWPO conversion; then, CWPO reactions were started for each catalysts by adding H2O2 to the solutions respecting Table\u00a01\n. After 120 min of reaction, the solution was centrifuged to remove particles; then, analyzed using UV\u2013vis spectrophotometer VWR UV-6300PC at \u03bbmax of 272 nm. The degradation efficiency of CAF was evaluated in Eq. (1); Where C0 and Ct are CAF concentrations (mg.L\u22121) at the time of withdrawal [34, 35, 36, 37, 38].\n\n(1)\n\n\nCAF\n\nConversion\u00a0\n\n(\n%\n)\n\n=\n\n[\n\n\n\nC\n0\n\n\u2212\n\nC\nt\n\n\n\nC\n0\n\n\n]\n\n\u00d7\n100\n\n\n\n\nThe reaction of H2O2 with the CAF was also carried out for comparison. The total organic carbon (TOC) analyses were determined using an analyzer (TOC-VCSN, Shimadzu) at the end of each reaction to investigate the total mineralization of the CAF in the solutions. The TOC values after 2 h of CWPO reaction were calculated using Eq. (2) [4]. Both, TOC measurements and analytical determinations of CAF concentrations were performed at least twice in order to ensure reproducibility of the measurements.\n\n(2)\n\n\nTOC\n\n(\n%\n)\n\n=\n\n[\n\n\n\nTOC\ni\n\n\u2212\n\nTOC\nf\n\n\n\nTOC\ni\n\n\n]\n\n\u00d7\n100\n\n\n\n\nThe adsorption of Methylene blue dye on raw clay, CuNi10-YC, CuNi11-YC and CuNi01-YC was performed and found to be following Langmuir adsorption isotherm with a monolayers capacity of 30.40 mg.g\u22121, 69.12 mg.g\u22121, 63.54 mg.g\u22121, 58.06 mg.g\u22121 respectively. As methylene blue was reported to have flatwise adsorption from water with effective area per molecule on the surface of 130 \u00c52 [39,40]. Therefore, Langmuir specific surface area has been calculated instead of BET using N2, because it reflects a better interpretation of the effective surface area when the adsorption of CAF is carried out from water solution. The following equation was used (3), Where X is the monolayers capacity mentioned above for each catalyst in moles per gram; N is Avagadro number (6.019\u22171023 mol\u22121) and A is the area of methylene blue molecule.\n\n(3)\n\n\nSpecific\u00a0surface\u00a0area\u00a0\n\n(\n\n\nSSA\u00a0m\n2\n\n.\n\ng\n\n\u2212\n1\n\n\n\n)\n\n=\n\nX\nm\n\n.\nN\n.\nA\n\n\n\n\nBox-Behnken Design (BBD) was used for the experimental design of the CAF degradation using CuNi-YC, in order to investigate the effect of the main parameters: [H2O2], catalyst, and temperature with X1, X2 and X3 are the studied coded variables which are calculated by Eq. (4). The independent factors were studied at three different levels, low (\u22121), medium (0), and high (+1). The predicted response (Y) fitted by second-order polynomial equation is the most commonly used (5), where Y is the measured response, \u03b20 is the intercept parameter; \u03b2i, \u03b2ii, and \u03b2ij represent the linear effects, the quadratic effects, and the interaction effects, respectively. Xi and Xj are the studied factors. K is the number of the optimized factors and \n\n\u03b5\n\n is the random error. Hence, NemrodW software was used to process and design the experiments data Table\u00a01 [32, 33].\n\n(4)\n\n\n\nx\ni\n\n=\n\n\n\nX\ni\n\n\u2212\n\nX\n\ni\n,\n0\n\n\n\n\n\u0394\n\nX\ni\n\n\n\n\n\n(\ni\n=\n1,2,3\n)\n\n\n\n\n\n\n\n(5)\n\n\nY\n=\n\n\u03b2\n0\n\n+\n\n\u2211\n\ni\n=\n1\n\nk\n\n\n\n\u03b2\ni\n\n\nX\ni\n\n\n+\n\n\u2211\n\ni\n=\n1\n\n\nk\n\u2212\n1\n\n\n\n\n\u2211\n\nj\n=\n2\n\nk\n\n\n\n\u03b2\n\ni\nj\n\n\n\nX\ni\n\n\nX\nj\n\n\n\n+\n\n\u2211\n\ni\n=\n1\n\nk\n\n\n\n\u03b2\n\ni\ni\n\n\n\nX\ni\n2\n\n\n+\n\u03b5\n\n\n\n\nThe analysis test of variance (ANOVA) was applied to evaluate the results, where the determination coefficient (R2 and adjusted R2) and p-value (probability) (p < 0.05), are the main parameters used to evaluate the effectiveness, the statistical significance and the prediction capability of the model [41, 42].Plasma-Atomic Emission Spectrometry (ICP-AES) was inductively used to test the Cu and Ni contents in the prepared catalysts using a FR-T-RR-01, CURI. The phase and crystallinity of the catalysts and YC were identified by X-ray diffraction (XRD) using X'Pert Pro PANalytical diffractometer equipped with a detector operating Cu K\u03b1 radiation (\u03bb = 1.540598 \u00c5; 40 kV and 30 mA). The X-ray fluorescence (XRF) was used to explore the chemical composition of raw YC. The catalysts' morphology was illustrated by a scanning electron microscopy (SEM) using QUANTA 200 FEI instrument at 30 kV. BET surface area was carried out by N2-adsorption at 77 K using a Micromeritics ASAP 2010 instrument.The powder X-ray diffraction patterns of the support and synthetized catalysts are shown in Figure\u00a01\n. In the diffractogram of raw YC Figure\u00a01a, the peaks at 2\u03b8 = 19.9\u00b0, 35.0\u00b0, 61.84\u00b0 which corresponded to the d101, d107, d060 represents the characteristic reflection of montmorillonite (JCPDS card NO. 29\u20131499). The peaks at 2\u03b8 = 20.9\u00b0, and 26.6\u00b0 of quartz (JCPDS card NO. 46\u20131045) and dolomite are observed at 23.43\u00b0 (JCPDS card NO. 36\u20130426). These findings indicate that the support was a typical bentonite [43]. Chemical composition of raw (YC) which is an important mineral resource of Moroccan was given in Table\u00a02\n, indicates that the predominant oxide is silica followed by alumina Al2O3 associated with the material phases. In addition, it may be concluded referring to the highly intensive d101 features of the sample that Al is highly available in the octahedral centers of YC [44]. The high Mg and Ca contents of the raw Yellow clay (Table\u00a02) illustrate the significant amount of Mg2+ and Ca2+ contribution from dolomite to the framework Mg and interlayer Ca cations [45]. The diffraction peaks of CuO were detected in Figure\u00a01b around 35.43\u00b0, 38.66\u00b0, 48.7\u00b0 and 61.58\u00b0 corresponded to the d002, d111, d-202 and d113 respectively (JCPDS card NO. 01-089-2530) [29,31], and the characteristic peaks of NiO were detected in Figure\u00a01c and d at 2\u03b8 = 37.2\u00b0, 43.3\u00b0 and 62.87, which ascribed to the plane d111, d200, d220 of the cubic phase NiO, respectively (JCPDS card NO. 47\u20131049) [46] for a mixture of copper and nickel oxide are seen at 62.8\u00b0 with the d220 to the crystalline structure of (Cu\u2013Ni)O Figure\u00a01c. The crystallite size of CuO and NiO calculated using the Scherrer formula [8] was 11.47 and 12.57 nm respectively.Some physicochemical properties of the raw YC and the other three catalysts are reported in Table\u00a03\n. As it can be seen, the samples showed an increase in the nickel loading results and a decrease in specific area using both BET and Langmuir surface area methods, relative low surface area probably associated to the micro-pore filled intrinsic impregnation procedure used, due to the deposition of the Ni and Cu hydroxyl nitrate [47, 48, 49]. Which is in a good agreement with the experimental study given CWPO reaction, because the removal efficiency of CAF with adding H2O2 to the solutions was used by the oxidation not by adsorption. The final catalysts were also analyzed by ICP in order to determine the Cu and Ni contents in the prepared catalysts. The results show that the catalysts formed are CuNi10-YC, CuNi11-YC and CuNi01-YC with Cu\u2013Ni 87.41\u20130.01; 45.05\u201343.4 and 0.01\u201385.38 mg.L\u22121 respectively. These results are in agreement with the expected stoichiometric ratio of Cu to Ni used Table\u00a03.\nFigure\u00a02\n provides a SEM micrograph, which illustrates the morphology of YC, CuNi10-YC, CuNi11-YC and CuNi01-YC catalysts. Figure\u00a02. a shows YC structure while it was being organized into aggregated patches of various agglomeration with different sizes. The morphological appearance of the following Cu:Ni weight ratios: 1:0, 1:1 and 0:1 catalysts is illustrated in Figure\u00a02b, c and d respectively. The samples appear to have smaller agglomerates comparing to YC alone. The most probable cause of the observed changes in the morphological appearance of the catalysts may be due to the strong immobilized CuO and NiO nanoparticles on natural yellow clay support. These obtained results are already confirmed by XRD analysis and the same results was reported by Alakhras et al [50] when the Titania (TiO2) loaded zeolite material. However, the sizes of individual particles in the agglomerate cannot be clearly seen in these micrographs.We have adopted the response surface methodology through Box\u2013Behnken design for investigating the statistic analyze and optimizing the impact of the three factors (H2O2 dosage, impregnated copper (%) and temperature) screened concerning the degradation of CAF using the local clay as a support. The number of experiments used in this study are 17, calculated using the following formula (6), where k = 3 is the number of the studied factors and C0 = 5 is the number of central points (numbers 13\u201317), the (%) degradation of CAF in this study was observed in the range of 40\u201386 % (Table\u00a04\n) [51]. In addition, the regression model was observed in terms of the three factors which are expressed through the following second-order polynomial Eq. (7).\n\n(6)\n\n\nN\n=\n2\n\u00d7\nk\n\u00d7\n\n(\n\nk\n\u2212\n1\n\n)\n\n+\n\nC\n0\n\n\n\n\n\n\n\n(7)\n\n\nY\u00a0\n\n(\n%\n)\n\n=\n69.8\n\u2212\n2.375\n\nX\n1\n\n+\n16.125\n\nX\n2\n\n+\n3.75\n\nX\n3\n\n\u2212\n11.025\n\nX\n1\n2\n\n\u2212\n2.525\n\nX\n2\n2\n\n\u2212\n1.775\n\nX\n3\n2\n\n\u2212\n1.25\n\nX\n1\n\n\nX\n2\n\n+\n0.0\n\nX\n1\n\n\nX\n3\n\n+\n0.5\n\nX\n2\n\n\nX\n3\n\n\n\n\n\nThe results of ANOVA used for checking the validation of this work's model are presented in Table\u00a05\n. Certainly, the p-value corresponding to caffeine conversion is an extremely low probability (p-value less than 0.05) indicating that the model is highly significant, [33]. In addition, the determinant regression coefficient (R = 0.990 and the adjusted regression coefficient (adjusted R = 0.976) for both responses are closer to 1. Furthermore, the larger the ratio and the lower the p-value are, the more significant the corresponding parameter will be in the regression model. Therefore, these results show that the models fit well and the experimental data could be well modeled for both responses [33, 41].Besides, the obtained p-value implies the importance of each factor in obtaining an efficient removal of CAF. Therefore, it can be seen in Table\u00a06\n that all the model's terms such as linear (x1, x2 and x3) quadratic (x1\n2, x2\n2 and x3\n2) and interactive effects (x1 x2, x2 x3 and x1 x3) are statistically significant. Hence, the results show that the catalyst has the most significant effect on the caffeine conversion.\nFigure\u00a03\n displays the residuals plots versus the normal plot probability of the responses residuals, which show a random distribution. This confirms the adequacy of the models. Most points relatively follow the straight-line x = y. This indication is in accordance with the previous results obtained in Table\u00a05.\nFigure\u00a04\n shows the 3D response surface plots and there matching 2D contour plots corresponding to the effect of the three parameters on the removal efficiency of CAF using 1 g.L\u22121 of catalyst.The results show that the 3D and 2D plots related to impregnated copper and temperature effects, at 0.082 mol.L\u22121 of H2O2. It is shown in Figure\u00a04A and B that the removal efficiency of CAF increased along with the increase of reaction temperature at higher amount of impregnated copper. When the temperature's reaction reached 60 \u00b0C, the conversion of CAF efficiency was the highest (>80%), which is in a good correlation with the Arrhenius theory of the temperature's positive influence on the rate constant, by enhancing the feasibility of the degradation process. However, the removal efficiency decreases if the temperature continued to rise, because the temperature was too high making the faster decomposition of H2O2, and the utilization rate of H2O2 was greatly reduced according to Eq. (8) [51, 52].\n\n(8)\n2H2O2 \u2192 2H2O + O2\n\n\n\nThe 3D and 2D plots related to H2O2 and catalyst copper content, at 20 \u00b0C temperature. It is shown in Figure\u00a04C and D that the maximal conversion of CAF (>80%) was achieved at higher amount of impregnated copper and medium volume of H2O2. However, at the lowest impregnated copper and an excess of H2O2, the CAF conversion reached its minimum. In addition, it was also obvious that CuNi10-YC was the most efficient catalyst and has the active phase for hydrogen peroxide as well as CAF molecules: the catalyst has the activation sites for H2O2 and caffeine. However, when nickel was incorporated in clay matrix, it led to a decrease in the CAF conversion [29]. The results could explain that the presence of nickel reduces the Langmuir surface and leads to a distortion of the clay matrix. It was more important that although the TOC value decreased until 27% in the presence of nickel.CuNi01-YC catalyst was used in the following experiments. The effect of hydrogen peroxide's reaction and temperature treatment performance of CAF wastewater were investigated. The removal efficiency of CAF with different amount of peroxide dosage was illustrated in Figure\u00a04E and F. Hydrogen peroxide has the same behaviour as temperature on the removal efficiency of CAF. In fact, the degradation rate reached the maximum value, when peroxide dosage was \u22640.164 mol.L\u22121. However, once the initial concentration of H2O2 is higher than 0.164 mol.L\u22121, the removal efficiency drops down. This might be ascribed to the scavenging effect of excess H2O2 on the active hydroxyl radical HO\u2022 to produce HO2\n\u2022 less active as illustrated by (equation: 9, 10, 11) [51,52].\n\n(9)\nH2O2\n\u2192 2HO\u2022\n\n\n\n\n\n(10)\nHO\u2022 + H2O2\n\u2192 HO2\n\u2022 + H2O\n\n\n\n\n(11)\nHO2\n\u2022 + HO\u2022 \u2192 O2 + H2O\n\n\nThe CAF conversion reached its maximal 86.16 %, when H2O2 dosage was equal to 0.15 mol.L\u22121, copper impregnated (10 %) and temperature value attained 60 \u00b0C Table\u00a07\n.\nFigure\u00a05\n shows the CAF conversion under optimum conditions after using Box-Behnken design. It was seen that CAF could be mostly (87.03%) degraded with 68.85 % as the final value of TOC mineralization after 120 min in the coexisting system of CuNi10-YC and H2O2, which is in a good agreement with the predicted value given by the Box-Behnken model. On the contrary, in the presence of CuNi10-YC or H2O2 alone, the CAF conversion after 120 min was only 14.39 % and 5.4 %, respectively. The low percentage of caffeine decomposition in the presence of the peroxide alone, can be attributed to the deficient homogenous decomposition H2O2 in hydroxyl radical [53]. In addition, the effect of both metals copper and nickel impregnated on yellow bentonite clay ameliorate the oxidation efficiency of the raw clay and as we can see in Table\u00a03 the amount of copper (CuNi10YC) leached after 120 min of reaction, was lower than nickel (CuNi01YC). Accordingly, CuNi10YC catalyst is expected to exhibit high CAF oxidation and stability and not losing the Cu metal by leaching in the medium. When both metals (CuNi11YC) were impregnated, the CAF conversion decreases which can be explained by the decrease in the catalyst's surface area (CuNi11YC), the high amount of leached nickel leads to a distortion of the clay matrix.Additionally, to the high efficiency of the eco-friendly synthesized catalyst CuNi10-YC in degrading CAF molecule from wastewater. The stability plays an important role in checking out the best photocatalyst from others. Figure\u00a06\n shows the degradation percentages of the CAF solution (40 ppm) after 120 min of oxidation, it also shows that an important conversion is obtained (~87%) in each reuse. A slight decrease in the mineralization percentages was obtained from the TOC values during the four recycling tests. This indicates that the catalyst remains stable in successive reuses. The insignificant reduction in oxidation performance observed could be caused by the inevitable loss of the catalyst mass during washing and centrifugation processes.For germination test, populations of corn kernels were exposed to a pollutant (CAF molecule) dissolved in distilled water, its toxicity was estimated to evaluate the toxic effects, such as an inhibition of the germination rate (Figure\u00a07\n). It was observed that in only 6 days the germination of corn kernels in distilled water (control or Blank) was totally (100 %). On the other hand, the germination rate was lower for untreated aqueous solutions containing CAF, which did not exceed 50 % (Figure\u00a07A). However, the germination rate in the presence of catalyst (CuNi10-YC) CAF aqueous solutions reached 100 % in 6 days (Figure\u00a07A and B). According to the caffeine conversion and TOC results, the CuNi10-YC catalyst has a significant ability to degrade CAF molecules from wastewater.The objectives of this work were successfully achieved, through incorporating copper/nickel into yellow clay prepared using wet impregnation method, and characterizing it by XRD, XRF, ICP, SEM, BET and Langmuir surface area analysis. The CWPO test for degradation of caffeine has successfully confirmed the usefulness of the modified clay as a catalyst. Thanks to Box-Behnken's design, the optimal conditions for degradation were selected. It has revealed that the highest CAF conversion was achieved was 86.16 % using CuNi10-YC catalyst. A feasible CAF conversion reaction system could be applicated as follows: 1 g.L\u22121 of catalyst, temperature of 60 \u00b0C, 40 mg.L\u22121 of CAF and time of 120 min, where CAF could be completely oxidized, and the final value of TOC mineralization more than 68.85%. This study has reported relevant and original results demonstrating that CWPO using CuNi-YC processes could be a cost-effective, stable and efficient alternative treatment for the removal of caffeine, since the germination test has given a positive index of good degradation. Another advantage of using this modified clay is its possibility of removing different type of organic compounds. Therefore, it would be interesting to continue testing on other persistent organic pollutants, and why not testing a real wastewater not only batch processes, but also column processes in a pilot scale.Ouissal Assila: Conceived and designed the experiments; Performed the experiments; Wrote the paper.Morad Zouheir, Karim Tanji: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.Redouane Haounati: Contributed reagents, materials, analysis tools or data; Wrote the paper.Farid Zerrouq: Analyzed and interpreted the data; Wrote the paper.Abdelhak Kherbeche: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.No data was used for the research described in the article.The authors declare no conflict of interest.No additional information is available for this paper.The Authors thank the general services (DRX, MEB, ICP, etc.) of the innovation centers University of Fez/Morocco (Sidi Mohammed Ben Abdellah), as well as the general research services of the University of Las Palmas de Gran Canaria (Spain).", "descript": "\n                  Copper and nickel were incorporated into the prepared yellow clay (YC) using one of the most widely used methods, for the preparation of heterogeneous catalysts, which is the wet impregnation method (IPM) and its application as a heterogeneous catalyst for Caffeine (CAF). Several catalysts Cooper Nickel's Catalysts (Cu\u2013Ni) were applied to the yellow clay with different weight ratio of Cu and Ni, in order to explore the role of both metals during the catalytic oxidation process CWPO. Furthermore, the CuNi-YC catalysts, were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), Langmuir's surface area, Brunauer Emmett Teller (BET), scanning electron microscope (SEM) and inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), so as to get a better understanding concerning the catalytic activity's behavior of CuNi-YC catalysts. The optimization of the catalytic activity's effects on the different weight ratios of Cu and Ni, temperature and H2O2 were also examined, using Box-Behnken Response Surface Methodology RSM to enhance the CAF conversion. The analysis of variances (ANOVA) demonstrates that Box-Behnken model was valid and the CAF conversion reached 86.16%, when H2O2 dosage was equal to 0.15 mol.L\u22121, copper impregnated (10%) and temperature value attained 60 \u00b0C. In addition, the regeneration of catalyst's cycles under the optimum conditions, indicated the higher stability up to four cycles without a considerable reduction in its conversion performance.\n               "}
{"full_text": "Hydrogen energy is one of the most promising candidates to replace traditional fossil energy due to its cleanliness, abundant resources, and environmentally friendly features [1\u20136]. Among the various solid hydrogen storage materials, Mg-based materials have attracted extensive attention worldwide because of their high hydrogen storage capacity, low price, and rich natural resources [7]. However, poor hydrogenation/dehydrogenation kinetic performances and high dehydrogenation temperature are the two main drawbacks of Mg-based materials [8].We show here that the addition of catalysts such as doping with transition metals (TM) [9\u201316], metal oxides [17\u201322], or metal sulfides [23\u201327] is a highly effective and simple way to improve the thermodynamic and kinetic properties of Mg-based materials. Cui et\u00a0al. [10] introduced the TM (Ti, Nb, V, Co, Mo, Ni) into MgH2 by coating and suggested that TM with lower electro-negativity had a better catalytic action on the hydrogenation performance of MgH2. Khatabi et\u00a0al. [13] and Yu et\u00a0al. [14]studied the catalytic effect of the TM on the hydrogen storage properties of MgH2 and found that the addition of TM could weaken the Mg-H bond and decrease the energy barrier for dehydrogenation from MgH2. Lu et\u00a0al. [15] prepared a core-shell Mg@Pt nanocomposite and revealed that Pt transformed H-stabilized Mg3Pt, which acted as a \u201chydrogen pump\u201d for the dehydrogenation of Mg and enhanced the hydrogen storage properties of Mg. Valentoniet al. [17] introduced a VNbO5 catalyst into MgH2 by ball milling and revealed that hydrogen (>5.0\u202fwt.%) resulted in negligible degradation of a 15\u202fwt.% VNbO5-doped sample after 70 cycles of hydrogen absorption\u2013desorption. Zhang et\u00a0al. [18] reported that the Mg-H bonds of MgH2 could be elongated and weakened under the catalytic reaction of Mn3O4; a MgH2+\u00a010\u202fwt.% Mn3O4 composite could release hydrogen at 200\u00a0\u00b0C. Xie et\u00a0al. [23] introduced NiS into Mg by ball milling and found that NiS reacted with Mg to form Ni, MgS, and Mg2Ni after the first absorption\u2013desorption cycle. These multi-phase catalysts formed in situ greatly improved the hydrogenation kinetics of Mg, and the apparent activation energies of hydrogenation and dehydrogenation for NiS-doped Mg decreased to 45.45\u00a0kJ\u00a0mol\u22121 and 64.71\u00a0kJ\u00a0mol\u22121, respectively. However, the gradual aggregation of Mg particles after hydrogenation and dehydrogenation cycles is inevitable, leading to a decrease in hydrogen storage performance. To solve such problems, carbon materials are often compounded with TM to improve the thermodynamic and kinetic properties of Mg-based materials [28\u201340]. Here, 1D porous Ni@C nanostructures were adopted to enhance the dehydrogenation and hydrogenation performance of MgH2 based on An et\u00a0al. [31], whose study showed that a Ni@C-doped MgH2 composite showed outstanding hydrogen storage performance. At 300 \u00b0C, the 5\u202fwt.% Ni@C-doped MgH2 sample could release 6.4\u202fwt.% H2 in 10\u00a0min whereas bare MgH2 could not release any hydrogen under the same condition. Liu et\u00a0al. [38] synthesized a nano-V2O3@C composite and introduced it into Mg through ball milling. The dehydrogenated MgH2-V2O3@C composite could absorb hydrogen at ambient temperature and completely rehydrogenate at 150\u202f\u00b0C after only 700\u202fs. Theoretical studies suggested that the presence of V was responsible for the improvement in the hydrogen absorption and desorption performances of MgH2.In our previous work, carbon materials with TM or their oxides such as Y2O3@rGO [41], V2O3@rGO [42], Ni@rGO [43], and Ni-TiO2@rGO [44] were successfully synthesized using graphene oxide (GO) and TM compounds. These materials significantly improved the hydrogenation and dehydrogenation performances of Mg-based materials. This showed that catalysts based on nickel compounded with carbon materials can greatly improve the thermodynamic and kinetic properties of Mg-based materials [31,35,37]. Here, carbon-supported nickel sulfide (Ni3S2@C) composites were prepared using cheap cation exchange resins and Ni(CH3COO)2. Moreover, the impacts of Ni3S2@C on the hydrogenation and dehydrogenation kinetics and thermodynamics of MgH2 were discussed.Four different cation exchange resins (Table\u00a01\n), AmberliteIR-120 (H) (resin-1; Alfa Aesar), Amberlite\u00ae IRN77 (H) (resin-2; Alfa Aesar), Dowex Marathon MSC (H) (resin-3; Sigma\u2013Aldrich), and Amberlyst\u00ae 15 (H) (resin-4; Alfa Aesar), were used to synthesize Ni3S2@C composites. The preparation of Ni3S2@C is shown schematically in Fig.\u00a01\n. First, five grams of resin was dispersed in 100\u202fml of hydrochloric acid solution at a concentration of 15% (by weight) and magnetically stirred for five hours at ambient temperature. The resin was then washed with deionized water to remove excess acid and impurities. A nickel ion (Ni2+) solution was obtained by dissolving 2.5\u202fg of Ni(CH3COO)2\u00a04H2O (Analytical reagent; XILONG SCIENTIFIC) in 100\u202fml of deionized water followed by magnetic stirring for one hour. The as-treated resin was then added to the solution containing nickel ions. The exchange process between Ni+ ions and the resin was completed through magnetic stirring for 24\u202fh. The exchanged resin was then cleaned and dried for 24\u202fh in a frozen drying oven. Subsequently, the as-dried resin was heated to 500 \u00b0C for five hours under a nitrogen atmosphere. The obtained samples were milled for two hours with a ball-to-powder weight ratio of 40:1 at 500 revolutions per minute (rpm). The four milled samples corresponding to resin-1, resin-2, resin-3, and resin-4 were named Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4, respectively. In addition, 5\u202fg of the treated resin-4 was dried, carbonized, and milled under the same conditions, yielding carbon (C).Commercial MgH2 powder (98%; Langfang Beide Trading) was mixed with 10\u202fwt.% of C, Ni3S2 (99.9%; Jiuding Chemical), Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4. The resulting six mixtures were milled for five hours, and the ball-to-powder weight ratio was maintained at 40:1 at a speed of 500\u202frpm. The six milled composites were denoted as MgH2-C, MgH2-Ni3S2, MgH2-Ni3S2@C-1, MgH2-Ni3S2@C-2, MgH2-Ni3S2@C-3, and MgH2-Ni3S2@C-4, respectively.The phase structure of the samples was determined by X-ray diffraction (XRD; Minflex 600; Cu-K\u03b1 radiation, 40\u202fkV, and 200\u202fmA) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, USA; Al-K\u03b1 X-ray source). All XRD tests were performed by scanning the sample from 2\u03b8\u202f=\u202f10\u00b0 to 2\u03b8\u202f=\u202f90\u00b0 with a scanning speed of 5\u00b0\u00a0min\u22121. The microstructure of the samples was determined by field-emission scanning electron microscopy (FE-SEM; SU8020, HITACHI) and transmission electron microscopy (TEM; FEI Tecnai G2, f20 s-twin, 200\u202fkV). The distributions of Ni, S, and C in the samples were determined using an energy-dispersive X-ray spectrometer (EDS) attached to an FE\u2013SEM. The Bruner\u2013Emmett\u2013Teller (BET) surface areas were measured with a Micromeritics Tristar II instrument at 77.3\u202fK. The pressure\u2013composition\u2013temperature (PCT) was measured on an automatic Sievert-type device with a hydrogen pressure of 35\u00a0atm for the hydrogenation process and the lowest pressure of 0.06 atm for the dehydrogenation process. The hydrogen absorption and desorption properties of the samples were determined using a Sievert-type device. The non-isothermal hydrogenation tests were performed by heating the dehydrogenated samples from ambient temperature to 390\u00a0\u00b0C at 1\u00a0\u00b0C\u00a0min\u22121 under 60\u00a0atm of H2. The non-isothermal dehydrogenation tests were performed by heating the rehydrogenated samples from ambient temperature to 390\u00a0\u00b0C at 0.5\u00a0\u00b0C\u00a0min\u22121 under 0.001\u00a0atm of H2. The isothermal hydrogenation/dehydrogenation measurements were performed at various temperatures under 60\u00a0atm of H2 for hydrogen absorption and 0.001\u00a0atm of H2 for hydrogen desorption. The dehydrogenation performances of the composites were evaluated by differential scanning calorimetry (DSC, Mettler Toledo). The samples were heated from ambient temperature to 500\u00a0\u00b0C under an Ar atmosphere (flow rate: 75\u00a0ml\u00a0min\u22121) at a heating rate of 5\u00a0\u00b0C\u00a0min\u22121.The microstructures of the Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4 composites examined by FE\u2013SEM are displayed in Fig.\u00a02\n. All four Ni3S2@C samples prepared by different precursors possessed a spherical structure before ball milling as shown in Fig.\u00a02(A\u2013D). These spherical samples were broken into fine particles after milling for two hours (Fig.\u00a02 (A1, B1, C1, and D1)). EDS mapping revealed that all four Ni3S2@C composites contained C, S, and Ni. The XRD patterns of the resultant Ni3S2@C samples are presented in Fig.\u00a03\nA. There were two wide peaks at about 22\u00b0 and 44\u00b0 from the respective (002) and (100) planes of carbon [32]. The diffraction peaks of Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4 appeared at 21.76\u00b0, 31.10\u00b0, 37.80\u00b0, 44.35\u00b0, 49.73\u00b0, 50.11\u00b0, and 55.16\u00b0 from the respective (010), (\u2212110), (111), (020), (120), (\u2212120), and (\u2212121) planes of the Ni3S2 phase (JCPDS card no. 85-1802). The diffraction peak of C could not be detected in the XRD profiles of the Ni3S2@C composites because the diffraction peak intensity of C was weaker than that of Ni3S2. However, the EDS analysis results in Fig.\u00a02 confirm that Ni3S2@C was successfully synthesized using cation exchange resin and nickel acetate as raw materials. Furthermore, the specific surface areas and pore size distributions of the Ni3S2@C composites were investigated by nitrogen adsorption and desorption isotherms at 77.3\u202fK (Figs.\u00a03B and C). The BET surface areas of Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4 were calculated as 273.9\u00a0m2\u00a0g\u22121, 319.5\u00a0m2\u00a0g\u22121, 331.7\u00a0m2\u00a0g\u22121, and 330.3\u00a0m2\u00a0g\u22121, respectively. All four synthesized catalysts had a large BET surface area. The BET surface areas of the composites prepared by macroreticular resins were noticeably larger than those prepared by gel resins. Based on the desorption isotherm curves, the Barret\u2013Joyner\u2013Halenda (BJH) desorption average pore diameters of Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4 were calculated as 37.3\u202f\u00c5, 36.0\u202f\u00c5, 39.5\u202f\u00c5, and 40.1\u202f\u00c5, respectively (Fig.\u00a03C), indicating that all four catalysts were mesoporous. To further study Ni3S2@C, the Ni3S2@C-4 composite was selected as a typical representative and was investigated by the XPS method. Fig.\u00a03(D\u2013F) displays the high-resolution XPS spectra of Ni3S2@C-4. The peaks observed at 855.9\u202feV and 873.6\u202feV were assigned to the respective Ni 2p3/2 and Ni 2p1/2 orbitals of Ni3S2\n[45,46], and the other two peaks at 861.0\u202feV and 879.4\u202feV corresponded to the accompanying satellite peaks of Ni 2p3/2 and Ni 2p1/2 (Fig.\u00a03D). In addition, weak peaks at 853.1\u202feV and 870.8\u202feV appeared in the Ni 2p3/2 and Ni 2p1/2 orbitals of NiO [47] due to the long exposure of the sample to air during fabrication. The peaks at 163.6\u202feV and 164.8\u202feV corresponded to the respective S 2p3/2 and S 2p1/2 orbitals of Ni3S2\n[48] (Fig.\u00a03E). In the C 1\u202fs spectrum, the peak located at 284.8\u202feV was assigned to the C-C bond [29,31,32]. The results indicated that the Ni3S2@C-4 composite was successfully prepared using ion exchange resin and nickel acetate as raw materials.The microtopographies of the as-synthesized Ni3S2@C and MgH2-Ni3S2@C-4 composites were further investigated by TEM, and the corresponding results are displayed in Fig.\u00a04\n. The particle size of Ni3S2 ranged between 5\u202fnm and 20\u202fnm (Fig.\u00a04A and B), and the d-spacing of 0.410\u202fnm corresponded to the (010) plane of Ni3S2 (Fig.\u00a04C). The SAED image in the inset of Fig.\u00a04D displays MgH2 with crystal indices of (111) and (002) planes. The MgH2-Ni3S2@C-4 composite particles were distributed dispersively as shown in Fig.\u00a04(D, E). For the MgH2-Ni3S2@C-4 composite, the interplanar spacings of 0.207\u202fnm and 0.219\u202fnm were well matched with the (020) plane of Ni3S2 and the (111) plane of MgH2, respectively (Fig.\u00a04F).To study the effect of Ni3S2@C on the hydrogen absorption and desorption kinetic performances of MgH2, hydrogenation and dehydrogenation analyses of different MgH2-Ni3S2@C composites were performed. Fig.\u00a05\nA shows the hydrogenation curves of MgH2, MgH2-C, MgH2-Ni3S2, MgH2-Ni3S2@C-1, MgH2-Ni3S2@C-2, MgH2-Ni3S2@C-3, and MgH2-Ni3S2@C-4. For the unmodified MgH2 sample, the onset hydrogenation temperature was about 120\u00a0\u00b0C, and 7.36\u202fwt.% hydrogen was absorbed when the temperature reached \u223c225\u00a0\u00b0C. The onset hydrogenation temperature of MgH2 was greatly reduced after the addition of Ni3S2@C. After the addition of Ni3S2, Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4, the resulting MgH2 composites absorbed 4.39\u202fwt.%, 3.74\u202fwt.%, 2.94\u202fwt.%, 5.15\u202fwt.%, and 5.68\u202fwt.% of H2 at 100 \u00b0C, respectively. The onset hydrogenation temperatures of the MgH2-Ni3S2@C-1 and MgH2-Ni3S2@C-2 composites decreased to 50 \u00b0C, and MgH2-Ni3S2, MgH2-Ni3S2@C-3, and MgH2-Ni3S2@C-4 absorbed hydrogen at ambient temperature. The addition of Ni3S2 and Ni3S2@C dramatically reduced the initial hydrogenation temperature of MgH2. The MgH2-Ni3S2@C-4 composite exhibited excellent hydrogenation performance. Fig.\u00a05B shows that the hydrogen desorption kinetic property of MgH2 was also significantly improved when it was modified by the addition of the Ni3S2@C composites. When MgH2-Ni3S2@C-1, MgH2-Ni3S2@C-2, MgH2-Ni3S2@C-3, and MgH2-Ni3S2@C-4 were heated to 275\u00a0\u00b0C with a heating rate of 0.5\u00a0\u00b0C/min, 6.16\u202fwt.%, 5.61\u202fwt.%, 5.50\u202fwt.%, and 6.35\u202fwt.% of H2 were released, respectively. However, MgH2 could not release any hydrogen at this temperature until the temperature exceeded 300\u00a0\u00b0C. It is obvious that the catalyst containing Ni3S2 promoted the hydrogen desorption of the samples. The Ni3S2@C composite exerted an especially excellent catalytic action on the hydrogenation and dehydrogenation performances of MgH2. This may be due to the presence of C, which can make the Ni3S2 with catalytic activity disperse more evenly and inhibit the agglomeration of particles during the milling process [34]. The MgH2-Ni3S2@C-4 sample could absorb more hydrogen at 100\u202f\u00b0C and release more hydrogen at 275\u202f\u00b0C than any other sample, showing good hydrogen absorption and desorption performance.The Ni3S2@C composites showed excellent catalytic activity for the hydrogen storage of MgH2, and the Ni3S2@C-4 composite was selected as a representative to further study its effect on the hydrogen storage performance of MgH2. Fig.\u00a06\n presents the isothermal hydrogenation and dehydrogenation curves of the MgH2-Ni3S2@C-4 composite. For comparison, the isothermal hydrogenation/dehydrogenation curves of MgH2 are also plotted in the figure. The MgH2-Ni3S2@C-4 composite could absorb 6.08\u202fwt.% H2 in 10\u00a0min at 150\u00a0\u00b0C and could absorb 6.0\u202fwt.% H2 in 157\u202fmin even at 50\u00a0\u00b0C (Fig.\u00a06A), whereas MgH2 could only absorb 0.70\u202fwt.% H2 at 150\u00a0\u00b0C (Fig.\u00a06C). Hence, the hydrogenation property of MgH2 was significantly improved under the catalytic effect of Ni3S2@C-4. The catalytic effect of Ni3S2@C-4 on the hydrogenation performance of MgH2 was better than that of Ni-V [12], NiS [23], Ni@rGO [43], Fe3S4\n[25], and Co@C [29]. For example, MgH2-Ni@rGO and MgH2-20\u202fwt.% Fe3S4 took 100\u202fmin and 20\u202fmin to reabsorb 5.94\u202fwt.%[43] and 3.41\u202fwt.% of H2\n[25] at 150 \u00b0C, respectively. Furthermore, under the same hydrogenation time (10\u202fmin), Mg-Ni-V [12], Mg-5\u202fwt.%NiS [23], and MgH2-Co@C [29] absorbed only 1.0\u202fwt.%, 3.5\u202fwt.%, and 2.71\u202fwt.% of H2, respectively. The dehydrogenation kinetics of MgH2-Ni3S2@C-4 were determined at different temperatures (225 \u00b0C, 250 \u00b0C, 275 \u00b0C, 300 \u00b0C, and 325 \u00b0C) (Fig.\u00a06B). The sample desorbed 5.61\u202fwt.% H2 in 160\u202fmin at 250 \u00b0C. When the temperature reached 300 \u00b0C, the dehydrogenation capacity increased to 6.15\u202fwt.% H2 in eight minutes. However, unmodified MgH2 hardly released any hydrogen at temperatures below 275 \u00b0C and took 160\u202fmin to release 4.38\u202fwt.% H2 at 300 \u00b0C (Fig.\u00a06D). Mg-Ni-V [12], Mg-5\u202fwt.%NiS [23], and MgH2-Co@C [29] only released 3.0\u202fwt.%, 3.1\u202fwt.%, and 5.74\u202fwt.% of H2, respectively, in 30\u202fmin at 300 \u00b0C. Hence, versus the abovementioned materials, MgH2-Ni3S2@C-4 could release more hydrogen at a faster rate, showing excellent hydrogen desorption performance.To further explore the effect of Ni3S2@C-4 on the hydriding reaction of MgH2, the Johnson\u2013Mehl\u2013Avrami\u2013Kolmogorov (JMAK) and Arrhenius equations were employed to calculate the apparent activation energy (E\na). The JMAK equation can be written as follows [12,18]:\n\n(1)\n\n\nln\n\n[\n\n\u2212\nln\n\n(\n\n1\n\u2212\nf\n(\nt\n)\n\n)\n\n\n]\n\n=\nn\nln\nk\n+\nn\nln\nt\n,\n\n\n\nwhere f(t) is the time-dependent function, n is the Avrami exponent, and k is an effective kinetic parameter. Based on the isothermal hydrogenation/dehydrogenation curves at different temperatures, the values of n and nlnk were obtained by fitting the JMAK plots between ln[\u2013ln(1\u2013f(t))] and lnt (Fig.\u00a07\n(A\u2013D). Based on these JMAK plots, the corresponding apparent activation energies were calculated by the Arrhenius equation:\n\n(2)\n\n\nln\nk\n=\n\u2212\n\n\nE\na\n\n\nR\nT\n\n\n+\nln\n\nk\n0\n\n,\n\n\n\nwhere Ea, R, T, and k\n0 represent the apparent activation energy, the gas constant (8.314\u00a0J\u00a0K\u22121\u00a0mol\u22121), the absolute temperature, and the Arrhenius pre-exponential factor, respectively. The Arrhenius plots of MgH2-Ni3S2@C-4 and MgH2 are displayed in Fig.\u00a07(E, F). The hydrogenation apparent activation energy of the dehydrogenated MgH2-Ni3S2@C-4 composite was calculated to be 39.6\u00a0kJ\u00a0mol\u22121, which is lower than that of MgH2 (93.6\u00a0kJ\u00a0mol\u22121). Table\u00a02\n presents the hydrogenation apparent activation energies of some common Mg-based composites. It is obvious that in comparison with the catalysts listed in Table\u00a02, Ni3S2@C-4 can more effectively reduce the hydrogenation reaction potential barrier and improve the hydrogenation kinetics performance. Moreover, the dehydrogenation apparent activation energy of the rehydrogenated MgH2-Ni3S2@C-4 composite was calculated as 115.2 kJ\u00a0mol\u22121 (lower than that of MgH2) according to the slope of the straight line shown in Fig.\u00a07F. The hydriding reaction potential barrier for the hydrogen absorption and desorption processes of MgH2 was significantly reduced by the addition of Ni3S2@C-4. The decrease in the hydriding reaction potential barrier promoted the diffusion of hydrogen atoms in the Mg matrix and improved the hydrogenation and dehydrogenation kinetics of MgH2.DSC analysis was employed to further study the dehydrogenation performance of the MgH2-Ni3S2@C-4 composite. Fig.\u00a08\n displays the DSC curves of the MgH2-Ni3S2@C-4 composite and MgH2. The endothermic peak of MgH2 appeared at 434.3\u00a0\u00b0C; when Ni3S2@C-4 was added, the endothermic peak temperature was reduced to 296.3\u00a0\u00b0C (Fig.\u00a08), which was 138.0\u00a0\u00b0C lower than that of MgH2. The endothermic peak temperature significantly decreased with the addition of Ni3S2@C-4, indicating an improvement in dehydrogenation performance.\nFig.\u00a09\n(A, B) displays the pressure\u2013composition\u2013temperature (PCT) curves of the MgH2-Ni3S2@C-4 composite and MgH2. For MgH2-Ni3S2@C-4, a complete reversible hydrogenation and dehydrogenation cycle occurred at 275 \u00b0C. In contrast, only the hydrogenation process occurred for MgH2, and the dehydrogenation process did not occur at 325 \u00b0C. Based on the PCT curves in Fig.\u00a09, the hydriding reaction thermodynamic behavior was described by the Van't Hoff equation, which can be expressed as follows [23]:\n\n(3)\n\n\nln\n\nP\n\nH\n\n\n2\n\n\n\n=\n\n\n\n\u0394\n\nH\n\n\nR\nT\n\n\n\u2212\n\n\n\n\u0394\n\nS\n\nR\n\n,\n\n\n\nwhere \n\nP\n\nH\n\n\n2\n\n\n\n, \u2206H, and \u2206S represent the plateau pressure of hydrogenation/dehydrogenation, the reaction enthalpy change, and the reaction entropy change, respectively. Generally, enthalpy change is regarded as an important parameter of the thermodynamic behavior of a reaction process. The hydrogenation/dehydrogenation enthalpy changes for the MgH2-Ni3S2@C-4 composite and MgH2 were obtained from the Van't Hoff plots shown in Fig.\u00a09(C, D). The hydrogenation and dehydrogenation reaction enthalpy changes of MgH2 were calculated as \u221282.0\u00a0kJ\u00a0mol\u22121 H2 and 89.5\u00a0kJ\u00a0mol\u22121 H2, respectively (Fig.\u00a09D). When MgH2 was composited with Ni3S2@C-4, the hydrogenation and dehydrogenation enthalpy changes of MgH2-Ni3S2@C-4 decreased to \u221274.7\u00a0kJ\u00a0mol\u22121 and 78.5\u00a0kJ\u00a0mol\u22121, respectively (Fig.\u00a09C). These results further demonstrate that the addition of Ni3S2@C-4 decreased both the hydrogenation potential barrier of Mg and the stability of MgH2, thereby improving the reversible hydrogen storage property of MgH2.To study the reversible hydrogenation and dehydrogenation cycle stability of MgH2-Ni3S2@C-4, 30 cycles of hydrogen absorption\u2013desorption were performed for MgH2-Ni3S2@C-4 at 300 \u00b0C and for pure MgH2 at 375 \u00b0C (Fig.\u00a010\n). The maximum hydrogen storage capacities of MgH2-Ni3S2@C-4 and pure MgH2 were 6.52\u202fwt.% and 6.92\u202fwt.%, respectively. After 30 cycles of hydrogen absorption\u2013desorption, the hydrogen storage capacity of the MgH2-Ni3S2@C-4 composite had no decay. The hydrogen storage capacity and the capacity retention ratio of pure MgH2 decreased to 6.45\u202fwt.% and 93.2%, respectively, implying that the addition of the Ni3S2@C-4 composite enhanced the stability of the hydrogen absorption\u2013desorption cycle.Therefore, the comprehensive hydrogen storage performance of MgH2 was significantly enhanced by the addition of Ni3S2@C-4. To explore the catalytic mechanism of Ni3S2@C-4, the XRD patterns of hydrogenation and dehydrogenation for both pure MgH2 and MgH2-Ni3S2@C-4 were investigated (Fig.\u00a011\n). Fig.\u00a011(A, B) shows that the as-prepared MgH2 was completely dehydrogenated into Mg and H2 at 380\u202f\u00b0C, and the rehydrogenated product was MgH2. For the MgH2-Ni3S2@C-4 sample, both Mg2Ni and MgS phases appeared in the first dehydrogenation cycle. In addition, the MgH2-Ni3S2@C-4 composite was fully activated at 380\u202f\u00b0C and then subjected to hydrogen absorption and desorption at different temperatures to obtain the diffraction spectra (Fig.\u00a011(C, D)). Fig.\u00a011(C) shows that the rehydrogenated MgH2-Ni3S2@C-4 sample was mainly composed of Mg2Ni, MgS, \u03b2-MgH2, and \u03b3-MgH2 when it hydrogenated at 100\u202f\u00b0C, 200\u202f\u00b0C, and 300\u202f\u00b0C, respectively. Additionally, some Mg2Ni reacted with hydrogen to form Mg2NiH4 at 300\u202f\u00b0C (Fig.\u00a011C). As the hydrogenation temperature increased to 380\u202f\u00b0C, Mg2Ni and \u03b3-MgH2 were completely converted to Mg2NiH4 and \u03b2-MgH2, respectively. In the dehydrogenation process (Fig.\u00a011D), Mg2NiH4 was completely dehydrogenated at 250\u202f\u00b0C. However, some \u03b2-MgH2 could not release hydrogen until the temperature reached 300\u202f\u00b0C. The MgS phase remained unchanged during the hydrogenation and dehydrogenation process. Apparently, in the first dehydrogenation cycle, Ni3S2 reacted with Mg to generate Mg2Ni and MgS, thus forming multi-phase in situ catalysts (Mg/Mg2Ni, Mg/MgS, and Mg/C). This multi-phase interface provided more active sites and diffusion paths for hydrogen atoms to enhance the hydrogenation/dehydrogenation properties of Mg/MgH2\n[50]. During the dehydrogenation process, Mg2NiH4 was dissociated into Mg2Ni and H2 at the interface of MgH2/Mg2NiH4 and caused MgH2 to break down [11]. Hence, Mg2NiH4 acted as a \u201chydrogen pump\u201d to drive MgH2 to dissociate, thus reducing dehydrogenation temperatures and improving the hydrogen desorption performance of the MgH2-Ni3S2@C-4 composite. Furthermore, carbon prevented Mg, Mg2Ni, and MgS grains from agglomeration and also provided more active sites for hydrogen atoms [31]. The synthesis and catalytic mechanism of Ni3S2@C-4 during hydrogenation/dehydrogenation processes are schematically presented in Fig.\u00a012\n. MgH2 particle surfaces were covered by high-activity Ni3S2@C-4 after ball milling. Mg2Ni and MgS were formed by the reaction of Ni3S2 and Mg during the first dehydrogenation process. MgS remained unchanged during the hydrogenation and dehydrogenation reactions. Hence, the onset temperature and the apparent activation energy of Mg decreased with the formation of multi-phase catalysts. Consequently, the integrated hydrogen storage performance of MgH2 was significantly improved under the catalytic action of Ni3S2@C.Here, Ni3S2@C catalysts were successfully prepared using four different cation exchange resins: AmberliteIR-120 (H)(resin-1), Amberlite\u00ae IRN77 (H) (resin-2), Dowex Marathon MSC (H) (resin-3), and Amberlyst\u00ae 15 (H) (resin-4). The nitrogen adsorption and desorption isotherm measurement results indicated that all four types of Ni3S2@C catalysts were mesoporous materials with a large BET surface area. The comprehensive hydrogen storage performance of MgH2 was significantly improved by the addition of Ni3S2@C. The catalytic effects of the Ni3S2@C composites prepared using macroreticular resins (resin-3 and resin-4) on the hydrogen storage performance of MgH2were better than those prepared using gel resins (resin-1 and resin-2). The MgH2-Ni3S2@C-4 composite exhibited excellent comprehensive hydrogen storage performance. It absorbed hydrogen at ambient temperature, took only 10\u00a0min to absorb 6.08\u202fwt.% H2 at 150 \u00b0C, and took eight minutes to release 6\u202fwt.% H2 at 300 \u00b0C. The hydrogenation and dehydrogenation apparent activation energies of MgH2-Ni3S2@C-4 were 39.6\u00a0kJ\u00a0mol\u22121 and 115.2\u00a0kJ\u00a0mol\u22121, respectively, which were much lower than those of MgH2 (93.6\u00a0kJ\u00a0mol\u22121 and 141.5\u00a0kJ\u00a0mol\u22121, respectively). Ni3S2 of Ni3S2@C reacted with Mg to form Mg2Ni and MgS during the first desorption process. The multi-phase (Mg/Mg2Ni, Mg/MgS, and Mg/C) interface provided more active sites to improve the hydrogen storage performance of MgH2.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We thank LetPub (www.letpub.com) for linguistic assistance during the preparation of this manuscript. This work was supported by the National Natural Science Foundation of China (grant number 51571065), the Natural Science Foundation of Guangxi Province (grant numbers, 2018GXNSFAA294125, 2018GXNSFAA281308, 2019GXNSFAA245050), the Innovation-Driven Development Foundation of Guangxi Province (grant number AA17204063), and the Innovation Project of Guangxi Graduate Education (grant number YCSW2020046).", "descript": "\n                  Carbon materials have excellent catalytic effects on the hydrogen storage performance of MgH2. Here, carbon-supported Ni3S2 (denoted as Ni3S2@C) was synthesized by a facile chemical route using ion exchange resin and nickel acetate tetrahydrate as raw materials and then introduced to improve the hydrogen storage properties of MgH2. The results indicated the addition of 10\u202fwt.% Ni3S2@C prepared by macroporous ion exchange resin can effectively improve the hydrogenation/dehydrogenation kinetic properties of MgH2. At 100\u202f\u00b0C, the dehydrogenated MgH2-Ni3S2@C-4 composite could absorb 5.68\u202fwt.% H2. Additionally, the rehydrogenated MgH2-Ni3S2@C-4 sample could release 6.35\u202fwt.% H2 at 275\u202f\u00b0C. The dehydrogenation/hydrogenation enthalpy changes of MgH2-Ni3S2@C-4 were calculated to be 78.5\u00a0kJ\u00a0mol\u22121/\u221274.7\u00a0kJ\u00a0mol\u22121, i.e., 11.0\u00a0kJ\u00a0mol\u22121/7.3\u00a0kJ\u00a0mol\u22121 lower than those of MgH2. The improvement in the kinetic properties of MgH2 was ascribed to the multi-phase catalytic action of C, Mg2Ni, and MgS, which were formed by the reaction between Ni3S2 contained in the Ni3S2@C catalyst and Mg during the first hydrogen absorption\u2013desorption process.\n               "}
{"full_text": "Electrochemical water splitting represents a promising strategy to provide sustainable clean energy source from the conversion of water into chemicals and fuels [1\u20135]. It is composed of two half reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), both of which are potential candidates for future clean energy sources [1,3,6\u20139]. The main limiting factor of these reactions is represented by their sluggish kinetics [1,10,11].The hydrogen evolution reaction (HER) is one of the most often studied electrocatalytic processes because of its industrial and technological interest for producing hydrogen gas through a limited number of reaction steps [12\u201314].The reaction occurs at cathode via a two-electrons reaction [15]:\n\n(1)\n\n\n2\n\nH\n+\n\n+\n2\n\ne\n\u2212\n\n\u2192\n\nH\n2\n\n\n\n\n\nFrom an industrial perspective on water electrolysis, HER is often conducted in alkaline media to achieve higher stability of the catalyst materials [14]. In alkaline medium, HER proceeds through two steps [11,12,16,17]:\n\n1.\nFirst, the catalyst splits a H2O molecule (Volmer step) into a hydroxyl ion (OH\u2212) and an adsorbed hydrogen atom (Hads);\n\n\n2.\nthen, a hydrogen molecule is formed via either the interaction of the Hads atom and water molecule (Heyrovsky step) or the combination of two Hads atoms (Tafel step).\n\n\nFirst, the catalyst splits a H2O molecule (Volmer step) into a hydroxyl ion (OH\u2212) and an adsorbed hydrogen atom (Hads);then, a hydrogen molecule is formed via either the interaction of the Hads atom and water molecule (Heyrovsky step) or the combination of two Hads atoms (Tafel step).However, the kinetics of HER in alkaline medium are slow if compared with those in acid environment because of the low concentration of available protons. As a consequence, this process will require additional effort to obtain protons by water dissociation near or on electrode surface.The state-of-art HER catalyst is platinum (Pt) and its alloys, but the scarcity and cost of Pt, universally considered a critical raw material, limit its large-scale application for electrolysis [15]. In past decades, extensive research has been focused on the development of practical alternatives to Pt, as efficient and renewable energy sources [16,18]. These have resulted in the identification of a variety of promising HER catalysts free of precious metals such as sulfides, phosphides, carbides, nitrides, selenides, and borides [3,19,20].At the same time, enhancing the efficiency of noble metals utilization may also provide a realistic approach to the development of high-performance and cost-effective catalysts. While Pt is well-known to be effective for the adsorption of Hads atoms, the overall sluggish HER kinetics in alkaline solutions stems from the insufficient catalyzing capability of Pt toward the cleavage of the H\u2212OH bond. A possible solution consists in the creation of catalysts with a combination of metal oxides and Pt, where the oxides promote the dissociation of H2O and the nearby Pt facilitates the adsorption and recombination of H\n\nads\n into molecular H2 [15,18,21].The transition metal oxide NiO is considered a valuable candidate as active material for electrochemical water splitting thanks to its Earth abundance and low cost [22]. Furthermore, NiO nanostructures (interconnected networks, nanosheets, microflowers) increase electrolyte permeability through the active material, making more favorable the mass transport at the electrode-solution interface [23]. Thanks to unique catalytic properties, nanostructured NiO is often used as a high-performance OER catalyst [24\u201327]. Recent literature reports also evidenced that NiO is particularly interesting due to its high stability for HER in alkaline electrolytes [4].Heterostructured materials on the nanoscale have exhibited great potential in this area. These classes of catalysts, with double or multiple types of active sites on the surface, exhibit remarkable advantages for the HER in alkaline solutions. A synergistic electronic interaction between the metal and the oxide has been proposed as the reason for the enhanced HER performance [4,14,21]. In particular, Pt\u2013NiO catalysts can be the key for designing efficient and cheap catalyst at which Pt favors H+ adsorption and NiO promotes the adsorption of OH\u2212 species [28,29].Unfortunately, there are no reports on overall water splitting using only NiO-based materials (decorated or not) as bifunctional electrocatalysts, except for two ones [1,28]. Mondal et\u00a0al. tested the performance of porous hollow nanostructured NiO electrodes for overall water splitting taking advantage of their high surface areas, porous microstructures, inner hollow architectures [1]. Similarly, Bian et\u00a0al. synthesized a hierarchically structured Pt/NiO/Ni/CNTs with a low loading of Pt NPs for efficient OER and HER, taking advantage of the presence of the NiO/Ni heterojunction to boost the overall water splitting performance [28].Here, we report a new strategy for overall water splitting electrodes, exploiting NiO nanostructures (microflowers, \u03bcFs) on graphene paper (GP), decorated with ultralow content of Pt nanoparticles (NPs). NiO \u03bcFs are synthesized by a chemical-based method and decorated with a colloidal solution of Pt NPs. Our hybrid metal-oxide catalyst unfolds outstanding activity toward HER, with an overpotential of 66\u00a0mV at a constant current density of 10\u00a0mA\u00a0cm\u22122. The present electrocatalyst shows a high rate of hydrogen generation as evidenced by the remarkable turnover frequency (TOF) values despite the low amount of loaded Pt. An alkaline electrolyzer is tested using Pt\u2013NiO \u03bcFs electrode and undecorated NiO \u03bcFs as cathode and anode, respectively. We demonstrate that this all NiO-based electrolyzer can sustain a current density of 10\u00a0mA\u00a0cm\u22122 with a potential of 1.57\u00a0V. The present work represents a valid strategy for the development of cost-effective electrocatalysts with a very small content of noble metal for widespread water electrocatalysis application.NiO microflowers (\u03bcFs) were synthesized from a chemical solution method trough a bain-marie configuration [24]. The obtained \u03bcFs powder were dispersed in an aqueous solution of deionized water and ethanol and sonicated for 15\u00a0min at room temperature to achieve a higher dispersion of the nano-structures.Pt nanoparticles (NPs) dispersion was produced through a green chemical reduction method at room temperature with ascorbic acid (AA) as reducing agent [20]. 30\u00a0\u03bcL of 33\u00a0mM AA were dispersed in 30\u00a0mL of 0.2\u00a0mM H2PtCl6 (Sigma-Aldrich, St. Louis, MO, USA, \u226599.9%) aqueous solution. The dispersion was then stirred for 5\u00a0min and used without further purification.Graphene paper (GP) substrates (1\u00a0\u00d7\u00a01.5\u00a0cm2, 240\u00a0nm thick, Sigma Aldrich, St. Louis, MO, USA) were rinsed with deionized water and ethanol and dried in N2 to clean the surface from any impurity. NiO \u03bcFs were deposited by drop casting by using 20\u00a0\u03bcL of NiO dispersion. The samples were then dried on a hot plate at 80\u00a0\u00b0C for 10\u00a0min. Pt NPs were dispersed onto the electrode by subsequent addition of drops with NP dispersion in order to vary the catalyst loading. The mass of NiO (0.30\u00a0mg) on GP was measured by a Mettler Toledo MX5 Microbalance (sensitivity: 0.01\u00a0mg). Decorated samples are labelled according to the number of Pt dispersion drops (e.g. 5Pt\u2013NiO indicates NiO catalyst decorated with 5 drops of Pt NPs dispersion).Surface morphology was analyzed by using a Scanning Electron Microscope (SEM, Gemini field emission SEM Carl Zeiss SUPRA 25, Carl Zeiss Microscopy GmbH, Jena, Germany). SEM images were analyzed by using ImageJ software [30].Transmission electron microscopy (TEM) analyses of Pt decorated NiO \u03bcFs dispersed on a TEM grid were performed with a Cs-probe-corrected JEOL JEM ARM200F microscope at a primary beam energy of 200\u00a0keV operated in scanning TEM (STEM) mode and equipped with a 100\u00a0mm2 silicon drift detector for energy dispersive X-ray (EDX) spectroscopy. For EDX elemental mapping, the Pt X-rays signal was collected by scanning the same region multiple times with a dwell time of 0.5\u00a0ms. TEM images and EDX spectra were analyzed by using DigitalMicrograph\u00ae software [31].The evaluation of Pt amount on NiO \u03bcFs was analyzed by Rutherford backscattering spectrometry (RBS, 2.0\u00a0MeV He+ beam at normal incidence) with a 165\u00b0 backscattering angle by using a 3.5\u00a0MV HVEE Singletron accelerator system (High Voltage Engineering Europa, Netherlands). RBS spectra were analyzed by using XRump software [32].Electrochemical measurements were carried out at room temperature by using a VersaSTAT 4 potentiostat (Princeton Applied Research, USA) and a three-electrode setup with a graphite rod as counter electrode, a saturated calomel electrode (SCE) as reference electrode, and the prepared electrodes as working electrodes. 1 M KOH (pH 14, Sigma Aldrich, St. Louis, MO, USA) was used as supporting electrolyte. Cyclic voltammetry (CV) curves were recorded at a scan rate of 10\u00a0mV\u00a0s\u22121 in the potential range \u22120.7\u00a0\u00f7\u00a0\u22121.5\u00a0V vs SCE in order to stabilize the electrodes. The HER activities of decorated catalysts were investigated using linear sweep voltammetry (LSV) at scan rate of 5\u00a0mV\u00a0s\u22121 in the same potential windows of CVs. Electrochemical impedance spectroscopy (EIS) was performed with a superimposed 10\u00a0mV sinusoidal voltage in the frequency range 104\u00a0\u00f7\u00a010\u22121\u00a0Hz at a potential just after the onset potential (E\n\nonset\n, the minimum potential at which a reaction product is formed at an electrode). Tafel plots were extrapolated from polarization curves by plotting the overpotential (\u03b7) as a function of the log of the current density. Mott\u2013Schottky (M\u2013S) analyses were conducted on bare and decorated samples in the potential range 0\u20131\u00a0V vs. SCE, at 1000\u00a0Hz frequency. Chronopotentiometry (CP) analysis was employed to study the stability of the samples in a 1\u00a0M KOH solution for 15\u00a0h at a constant current density of 10\u00a0mA\u00a0cm\u22122.Current density was normalized to the geometrical surface area and measured potentials vs SCE were converted to the reversible hydrogen electrode (RHE) according to the equation [33]:\n\n(2)\n\n\n\nE\nRHE\n\n=\n\n\u00a0E\nSCE\n\n+\n0.059\n\u22c5\npH\u00a0\n+\n0.244\n\n\n\n\nAll measured potentials (\u03b7') were manually corrected by iR\n\nu\n-compensation as follows:\n\n(3)\n\n\n\u03b7\n=\n\n\u03b7\n\u2032\n\n\u2212\ni\n\nR\nu\n\n\n\n\nwhere i is the electrode current and R\n\nu\n [\u03a9] is the uncompensated resistance (extracted from EIS).The turnover frequency (TOF) is defined as the rate of production of oxygen molecules per active site:\n\n(4)\n\n\nT\nO\nF\n=\n\nI\n\n2\nn\nF\n\n\n\n\n\nwhere I is the measured current at a fixed overpotential, the term 2 represents the number of electrons involved in the HER, F is the Faraday constant and n is the number of moles of the active sites [34]. Once the Pt amount is known, the number of active Pt moles can be calculated as follows:\n\n(5)\n\n\n\nn\nPt\n\n\n[\n\ng\u00a0cm\n\n\u2212\n2\n\n\n]\n\n=\n\n\n\nDose\nPt\n\n\n[\n\nat\u00a0cm\n\n\u2212\n2\n\n\n]\n\n\n\n\nN\nA\n\n\n[\n\nat\u00a0mol\n\n\u2212\n1\n\n\n]\n\n\n\n\n\n\nwhere DosePt is the RBS Dose, representing the amount of Pt atoms per cm2, and N\n\nA\n is the Avogadro's number.Finally, the mass activity is defined by the ratio between a fixed current density and the catalyst loading (obtained by multiplying n\nPt for the atomic weight of the catalyst):\n\n(6)\n\n\nMass\u00a0activity\n=\n\n\nj\n\n[\n\nA\u00a0cm\n\n\u2212\n2\n\n\n]\n\n\n\ncatalist\u00a0loading\u00a0\n\n[\n\nmg\u00a0cm\n\n\u2212\n2\n\n\n]\n\n\n\n\n\n\n\n\nFig.\u00a01\n(a) shows SEM images of NiO \u03bcFs on GP. Our catalyst totally recovers the surface of graphene electrode with an irregular thickness due to the agglomeration of \u03bcFs. Fig.\u00a01(b) of the same figure shows a tilted view of the electrode. After decoration, Pt NPs (mean size of 2\u00a0nm) spread onto NiO \u03bcFs (bright particles on STEM Z-contrast image in Fig.\u00a01(c)). STEM EDX elemental map in Fig.\u00a01(d) allowed us to confirm the effective presence of Pt decorating NiO catalyst. RBS analyses (Fig.\u00a01(e)) confirmed Pt presence and allowed us to quantify the Pt loading, by using a flat substrate covered with the same drops containing Pt NP dispersion used for the electrode fabrication. We assume that after drop casting, the measured Pt loading on a flat substrate is the same of that on NiO \u03bcFs. Moreover, this NP density is not dependent from the type of surface since it is an intrinsic quantity. The Pt loading is related to the area of Pt peak in the RBS spectrum (at around 1.8\u00a0MeV) [35]. As expected, Pt amount increases with the number of drops: 1.2\u00a0\u00d7\u00a01016\u00a0at\u00a0cm\u22122, 1.8\u00a0\u00d7\u00a01016\u00a0at\u00a0cm\u22122, 3.4\u00a0\u00d7\u00a01016\u00a0at\u00a0cm\u22122, for 5, 10, and 20 drops, respectively (Fig.\u00a01(f)). Pt NP density cannot be verified by SEM analysis because of the rough surface and shadowing effect caused by NiO nanostructures. Thus, the Pt amount (D\n\nRBS\n, from RBS [34]) was joined with Pt NP diameter (Fig.\u00a0S1) to evaluate the density N of NPs decorating NiO \u03bcFs, through the following relation [33,37]:\n\n(7)\n\n\n\nD\n\nR\nB\nS\n\n\n\n=\n\nN\n\n\n\u03c1\n\na\nt\n\n\n\n\nV\n\nN\nP\n\n\n\n\n\nwhere \u03c1\n\nat\n is the Pt atomic bulk density (6.62\u00a0\u00d7\u00a01022\u00a0at\u00a0cm\u22123) and V\n\nNP\n is the volume of a single NP (in cm3) based on the size of NPs (measured from SEM images). Following these considerations, the NP density was found to vary from 2\u00a0\u00d7\u00a01010\u00a0NPs\u00a0cm\u22122 to 6.2\u00a0\u00d7\u00a01010\u00a0NPs\u00a0cm\u22122 (Fig.\u00a01(f)). Finally, from eq. (5) the Pt loading can be easily calculated, confirming the extremely low content of Pt in our decorated electrodes. The obtained values for RBS Pt dose, NP density, Pt loading are reported in Table 1\n.To evaluate the electrochemical performance of bare and decorated NiO \u03bcFs on HER in alkaline conditions, electrochemical analyses were performed in 1\u00a0M KOH (Fig.\u00a02\n). Polarization curves (Fig.\u00a02(a)) clearly show how the presence of Pt drastically reduces the activation barrier for the H2 production, confirmed by a variation in the onset potential and overpotential at a constant current density of 10\u00a0mA\u00a0cm\u22122 from 247 to 66\u00a0mV (Table 1). Two types of behavior can be distinguished as a function of the quantity of Pt:\n\n(i)\nfor no (or ultralow) Pt loading, both overpotential and onset potential appear at relatively high voltages, indicating that high energies are required to overcome the adsorption of H+ and subsequent production of H2 steps;\n\n\n(ii)\nby increasing the density of NPs (10Pt and 20Pt), overpotential and onset potential drastically reduce pointing out an enhanced catalytic action of Pt against HER.\n\n\nfor no (or ultralow) Pt loading, both overpotential and onset potential appear at relatively high voltages, indicating that high energies are required to overcome the adsorption of H+ and subsequent production of H2 steps;by increasing the density of NPs (10Pt and 20Pt), overpotential and onset potential drastically reduce pointing out an enhanced catalytic action of Pt against HER.The morphology of samples after HER was compared to the pristine ones, as shown in Fig.\u00a0S1, without any significant morphology variation.Tafel slopes of Pt decorated NiO \u03bcFs are reported in Fig.\u00a02(c). Pt decoration leads to a decrease of Tafel slope to a value of 82\u00a0mV\u00a0dec\u22121 for 20Pt\u2013NiO sample. Tafel slope values allow a deep understanding of HER catalytic mechanism. Three possible pathways (illustrated in Fig.\u00a02(d)) for the HER reaction in alkaline medium can be distinguished [11,12,38]:\n\n(i)\nelectrochemical hydrogen adsorption (Volmer step, H2O\u00a0+\u00a0e\u2212\u00a0\u2192\u00a0Hads\u00a0+\u00a0OH\u2212) at the active site of the catalyst;\n\n\n(ii)\nH2 formation through an electrochemical desorption step (Heyrovsky step, Hads\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212\u00a0\u2192\u00a0H2\u00a0+\u00a0OH\u2212);\n\n\n(iii)\nH2 formation through a recombination step between two adsorbed hydrogen atoms (Tafel step: Hads\u00a0+\u00a0Hads\u00a0\u2192\u00a0H2).\n\n\nelectrochemical hydrogen adsorption (Volmer step, H2O\u00a0+\u00a0e\u2212\u00a0\u2192\u00a0Hads\u00a0+\u00a0OH\u2212) at the active site of the catalyst;H2 formation through an electrochemical desorption step (Heyrovsky step, Hads\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212\u00a0\u2192\u00a0H2\u00a0+\u00a0OH\u2212);H2 formation through a recombination step between two adsorbed hydrogen atoms (Tafel step: Hads\u00a0+\u00a0Hads\u00a0\u2192\u00a0H2).Usually, the rate-determining step (RDS) can be evaluated from the value of the Tafel slope [35\u201337,39\u201341]. Our values (by considering the kinetic analysis and mechanism for HER that are based on Butler-Volmer equation [39]) suggest that the RDS for our catalysts is represented by Volmer step and by the initial hydrogen adsorption. NiO-based materials have been widely proved to be optimal catalysts for OH\u2212 adsorption [21,28,29]. The high Tafel slope value for bare semiconductor electrode clearly demonstrates that the HER proceeds much slower in NiO sample Conversely, Pt loading causes a reduction of Tafel slopes (Fig.\u00a02(e)). This evidence suggests that HER is limited by the hydrogen adsorption, with a poor efficiency in the NiO case. Conversely, the presence of Pt in decorated catalysts reveals an enhanced adsorption of hydrogen atoms on the surface. Pt decoration not only reduces the activation barrier for the activation of the HER (evidenced by the lowest overpotential for the most decorated sample), but also favors the hydrogen adsorption at the catalyst surface.Nyquist plots from EIS analysis in Fig.\u00a03\n(a) remark the role of Pt NPs in the HER. They were acquired in the so-called turnover region, just after the onset potential of each sample in order to appreciate a good HER activity [24,42]. The experimental EIS spectra were fitted (continuous lines) by the Armstrong-Henderson equivalent circuit [43] (Fig.\u00a03(b)) and the extracted fitting parameters are reported in Fig.\u00a03(c and d). In the Armstrong-Henderson circuit different elements can be recognized [13,16,17,24,42,44\u201348]:\n\n1.\nRu is the uncompensated resistance;\n\n\n2.\nRct is the charge transfer resistance at the electrode-electrolyte interface;\n\n\n3.\nCdl is the double layer capacitance (here we used constant phase elements to take into account the non-ideal behavior of the nanostructured electrodes);\n\n\n4.\nRp is strictly related with the mass transfer resistance of adsorbed intermediate Hads (in particular it well describes the adsorption/desorption of intermediates at the electrode surfaces);\n\n\n5.\nCp is the Hads related capacitance (usually called pseudocapacitance) in the HER mechanism.\n\n\nRu is the uncompensated resistance;Rct is the charge transfer resistance at the electrode-electrolyte interface;Cdl is the double layer capacitance (here we used constant phase elements to take into account the non-ideal behavior of the nanostructured electrodes);Rp is strictly related with the mass transfer resistance of adsorbed intermediate Hads (in particular it well describes the adsorption/desorption of intermediates at the electrode surfaces);Cp is the Hads related capacitance (usually called pseudocapacitance) in the HER mechanism.The adequately fitted experimental data reveal how these 5 parameters vary with Pt decoration (Fig.\u00a03(c and d)). Ru and Cdl do not appreciably change, as expected since the Pt NPs coverage is quite limited and most of the interface among NiO \u03bcFs and electrolyte is unchanged. A clear reduction in both Rct and Rp with Pt loading indicates that Pt accelerates the electron transfer kinetics, probably enhancing the availability of electrons at surface. Cp, related to Hads adsorption, decreases as the amount of Pt increases. Pt NPs act as effective active sites for hydrogen adsorption. Consequently, the higher the number of active sites, the lower their occupancy and therefore the value of Cp.To quantitatively evaluate the effect of Pt decoration of NiO \u03bcFs we performed Mott\u2013Schottky (M\u2013S) analysis (SI for details). The M\u2013S plot typically reports the inverse of squared capacitance (C\n\u22122) measured as a function of potential applied to the sample, as reported in Fig.\u00a04\n(a) [24,49\u201354]. By increasing E, C\n\u22122 goes to zero as the applied potential increases, indicating the presence of a capacitance at the electrode-electrolyte interface. Such behavior is typical of a p-type semiconductor, as NiO is [55,56]. The intercept with x-axis (E\n\nFB\n) represents the so-called flat band potential [49\u201354,57\u201359]. For a planar semiconductor electrode, the quantity \u0394E\n\nM\u2212S\n\u00a0=\u00a0E\n\nFB\n\u00a0\u2212\u00a0E\n\nOC\n (where E\n\nOC\n is the open circuit potential) represents the bending of the semiconductor energy bands [24] resulting from the alignment of the Fermi level of the electrode and the redox potential of the electrolyte (violet points in Fig.\u00a04(b)). After the loading of Pt NPs, we observed a clear shift of EFB towards more positive potential, up to 0.454\u00a0V in 20Pt\u2013NiO case (Table 2\n). Even if our electrodes are nanostructured semiconductors, such evidence reveals a considerable difference in energy band bending due to Pt decoration. Moreover, it is possible to correlate the effect of decoration on energy band position of NiO with catalytic properties of the electrodes by considering the value of onset potential (E\n\nonset\n, green points in Fig.\u00a04(b)). Onset potential is usually considered an important indicator for the catalytic activity along with the exchange current density in electrocatalysis [34]. For a cathodic reaction, onset potential is the highest potential at which a reaction product (H2 in our case) is formed at an electrode [60]. A commonly used method for the determination of this value is the intersection point between the tangent lines of the Faradaic and non-Faradaic [60\u201362]. Fig.\u00a04(b) clarifies the effect of Pt loading on \u0394E\n\nM\u2212S\n and E\n\nonset\n. As the amount of Pt NPs increases, the energy band bending grows, index of the creation of a nano Schottky junction at the metal-semiconductor interface [24]. Pt decoration increases the energy band bending, because of electron spillover effects, leading to space charge regions and localized electric field [37]. At the same time, a drastic reduction of E\n\nonset\n is observed in presence of Pt NPs. These two evidences confirm that surface decoration of NiO \u03bcFs is highly effective in tuning the catalytic properties of our nanostructured electrode. The increase in bending of semiconductor energy levels leads to accumulation of electrons below the semiconductor surface, considerably reducing the activation barrier for H2 production (as described in eq. (1) and demonstrated by the decreased values of E\n\nonset\n for 10Pt and 20Pt\u2013NiO electrodes) and making the HER mechanism more favorable at lower overpotentials.\nFig.\u00a04(c) schematizes the effect of Pt loading on NiO band position at the electrode-electrolyte interface.TOF is a crucial parameter for evaluating the HER performance of a catalyst because it reflects the intrinsic electrocatalytic activity of the electrode [2,20,34,63]. As presented in Fig.\u00a05\n(a), Pt-decorated samples show markedly high TOF values. It is worth to note that the TOF value of 2.07 s\u22121\u00a0(at an overpotential of 50\u00a0mV) found for 20Pt\u2013NiO is comparable (and even superior) to those reported in literature and confirms that the present Pt NP decorated catalyst owns extraordinary efficiency of hydrogen generation (see Table S1).The obtained results are now compared with the state-of-art. Fig.\u00a05(b) shows the comparison of mass activity measured at 10\u00a0mA\u00a0cm\u20132 and the overpotential for 10\u00a0mA\u00a0cm\u20132 (based on geometric area) for our decorated electrodes with other Pt-based catalysts under alkaline conditions [2]. In our samples, an increase of the mass loading leads to a reduction of the overpotential for the HER, without a significant decrease of intrinsic activity. This comparison, together with the TOF values, makes our Pt decorated NiO \u03bcFs valuable candidates as cathode electrodes for the HER.Motivated by the excellent HER performance of catalysts and the high OER activity of our previously reported NiO \u03bcFs on GP [24], we investigated the overall water-splitting performance under alkaline condition by employing 20Pt\u2013NiO \u03bcFs as the cathode and NiO \u03bcFs as the anode (a scheme of the Pt\u2013NiO||NiO is reported in Fig.\u00a06\n(a)). Our as-constructed alkaline electrolytic cell requires a low potential of 1.57\u00a0V to afford a current density of 10\u00a0mA\u00a0cm\u22122 (Fig.\u00a06(b)) and Supplementary Video 1, which is comparable or smaller to other electrocatalysts reported in Table 3\n.The following is the supplementary data related to this article:\n\nMultimedia component 2\nMultimedia component 2\n\n\n\nSupplementary video related to this article can be found at https://doi.org/10.1016/j.ijhydene.2022.08.005.In addition, the overall water-splitting durability of the two electrodes was tested using chronopotentiometry for 24\u00a0h (Fig.\u00a06(c)) showing a good stability over prolonged times (with an increase of overpotential of 50\u00a0mV after 24\u00a0h).Our results reveal that the present Pt\u2013NO||NiO electrodes represent highly efficient electrocatalysts.In conclusion, we developed a high-efficiency HER catalyst by synthesizing low-content Pt-NP decorated low-cost NiO microflowers (\u03bcFs) onto a graphene paper substrate. By varying the loading of Pt NPs, the role of decoration on catalytic performance of the materials was elucidated in terms of energy band bending of NiO and density of active sites. The Pt\u2013NiO catalyst with optimized NP loading shows an overpotential of only 66\u00a0mV at current density of 10\u00a0mA\u00a0cm\u22122 and a promisingly low Tafel slope of 82 mV\u00a0dec\u22121. The performance of NiO \u03bcFs were also supported by high intrinsic activity, in terms of TOF of 2.07\u00a0s\u22121 at an overpotential of 50\u00a0mV. An alkaline all NiO-based electrolyzer was developed by using Pt\u2013NiO as cathode and bare NiO as anode, requiring a low potential of 1.57\u00a0V to afford a current density of 10\u00a0mA\u00a0cm\u22122 and a good long-term stability. The high activity, and low-cost of the present Pt\u2013NiO \u03bcFs pave the way for large-scale and long-term applications of NiO-based catalysts for overall water splitting.L.B. fabricated the nanostructured electrode, acquired and analyzed data, generated the figures, and drafted the manuscript; S.B. analyzed the electrochemical data; M.S. performed STEM and EDX analysis; A.T., F.P., and S.M.: conceived the idea, contributed to data analysis and interpretation; S.M. supervised the project. All authors have given approval to the final version of the manuscript.This research was funded by the project AIM1804097\u2500Programma Operativo Nazionale FSE \u2212FESR \u201cRicerca e Innovazione 2014\u20132020\u201d and was supported by the project \u201cProgramma di ricerca di ateneo UNICT 2020-22\u201d linea 2.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors wish to thank G. Pant\u00e8 and S. Tat\u00ec (CNR-IMM Catania, Italy) for technical support.The following are the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2022.08.005.", "descript": "\n                  Electrochemical water splitting represents a promising alternative to conventional carbon-based energy sources. The hydrogen evolution reaction (HER) is a key process, still if conducted in alkaline media, its kinetics is slow thus requiring high amount of Pt based catalysts. Extensive research has been focused on reducing Pt utilization by pursuing careful electrode investigation. Here, a low-cost chemical methodology is reported to obtain large amount of microflowers made of interconnected NiO nanowalls (20\u00a0nm thick) wisely decorated with ultralow amounts of Pt nanoparticles. These decorated microflowers, dispersed onto graphene paper by drop casting, build a high performance HER electrode exhibiting an overpotential of only 66\u00a0mV at current density of 10\u00a0mA\u00a0cm\u22122 under alkaline conditions. Intrinsic activity of catalyst was evaluated by measuring the Tafel plot (as low as 82\u00a0mV/dec) and turnover frequencies (2.07\u00a0s\u22121 for a Pt loading of 11.2\u00a0\u03bcg\u00a0cm\u22122). The effect of Pt decoration has been modelled through energy band bending supported by electrochemical analyses. A full cell for alkaline electrochemical water splitting has been built, composed of Pt decorated NiO microflowers as cathode and bare NiO microflowers as anode, showing a low potential of 1.57\u00a0V to afford a current density of 10\u00a0mA\u00a0cm\u22122 and a good long-term stability. The reported results pave the way towards an extensive utilization of Ni based nanostructures with ultralow Pt content for efficient electrochemical water splitting.\n               "}
{"full_text": "Converting atmospheric carbon dioxide (CO2) to reduced, value-added and energy-dense molecules is of great interest for economic and environmental reasons [1,2]. Among various approaches [3\u20136], the direct electrochemical reduction of CO2 (CO2RR) into high-value chemical feedstock and fuels has received high attention because it can provide a carbon-neutral energy network by connecting with electricity production from intermittent renewable sources (solar and wind, etc.) [7\u20139] at ambient temperatures and pressures. Efforts have been devoted to the development of metal or metal-derived catalysts for the reduction of CO2 to gaseous products [10\u201320], such as carbon monoxide and methane. Among the catalysts, Cu-based ones [21\u201329] exhibit notable catalytic properties for the conversion of CO2 into multi-carbon hydrocarbons and oxygenates by optimizing the physicochemical properties, including the morphologies [30], chemical states [31\u201333], alloying [34\u201338], and so forth. However, selective formation of valuable liquid fuel products, such as ethanol, is still found with low efficiency because of their complex multi-electrons processes.Recently, multi-component tandem catalysts have been proved to enhance the catalytic activity, selectivity and understanding of the structure-property relationships for CO2 reduction accompanied by a multi-step conversion reaction [39, 40]. Using Au nanoparticles deposited on a Cu substrate as an example, a two-step electrochemical reduction mechanism was proposed for the enhanced electrochemical reduction of CO2 to ethanol with the existence of the high concentration CO intermediate [41]. This can be seen as a model to study the tandem catalyst, however, a catalyst in reality is usually in powder and the design of such a catalyst faces several challenges. First, very few metals have been used to prepare the tandem catalysts for producing ethanol and thus the pairing of metals is tricky. Second, the combination of metals may result in the formation of alloys in experiments, suppressing the special functions provided by the individual metals. Third, the fast transfer of the CO intermediates generated on one metal to the second metal before diffusing to the electrolyte/leaking into air is critical for the selective conversion of CO2 to ethanol. A well-designed structure that can resolve the above challenges is thus expected to enhance the catalytic efficiency of the conversion of CO2 and its selectivity towards ethanol.In this work, we use the non-noble metal, carbon-supported Ni nanoparticles (namely, Ni/C), as the catalytic center for converting CO2 to CO, and carbon-supported Cu nanoparticles (namely, Cu/C) as the selective catalyst to accept CO and turn it into ethanol, which have been demonstrated separately but not have been combined for this purpose [42, 43]. The key question is how to combine them in a tandem structure without alloying them and also realize the orientated transfer of CO from Ni/C to Cu/C. We designed a one-dimensional (1D) core-shell structure as shown in Fig.\u00a01a, wherein Ni/C is the core and Cu/C the shell. The metals are in the form of small clusters distributed in mesoporous carbon rather than pure metal wires. In this way, first, the clusters of Ni and Cu are separated by the carbon without forming alloys; second, as the core, Ni/C clusters catalyze the transformation of CO2-to-CO, after which the CO intermediates diffuse to the Cu/C clusters-formed-shell and experience the further reduction to ethanol rather than directly flowing into the electrolyte. The mesopores in the carbon skeleton provide channels for such an orientated mass transfer route, while the 1D composite fibers form a freestanding network that helps the transport of electrons and the stable electrochemical performance as a robust electrode. The material is named Ni(CNFs)@Cu(CNFs), where CNFs are carbon nanofibers. The catalytic testing shows that the ethanol formation was maximized at -1 V vs. RHE (VRHE), with a remarkable Faradaic efficiency (FE) and total current density of 18.2 % and 16 mA cm-2, respectively, and also catalytically stable for at least 100 h. By involving control samples that have the similar structure but contain only Ni/C, only Cu/C, or carbon-supported Ni-Cu alloy (namely, Ni-Cu/C) clusters, it is demonstrated that the trends in ethanol production originate from the tandem catalysis mechanism, that is, Ni/C reduces CO2 to CO near the Ni/C-Cu/C interfaces, driving a high CO coverage and facilitating the following C-C coupling reactions for the selective formation of ethanol. Furthermore, density functional theory (DFT) calculations revealed that Ni(CNFs) in core can play an important role in stabilizing COOH*, which efficiently supplies CO for the dimerization of the *CO intermediates and thus ethanol production.Nafion perfluorinated resin solution (5 wt% in lower aliphatic alcohols and water) and KHCO3 (ACS Reagent 99.7%) were purchased from Sigma Aldrich. Carbon dioxide (CO2, 99.999%), argon (Ar, 99.999%) and carbon monoxide (CO, 99.999%) were supplied by Beijing Beiwen Gases Company. Deionized water (Milli-Q Millipore 18.2 M\u03a9 cm-1) was used throughout the experiments. All chemicals and solvents were commercially available and used as obtained without further purification.The inner precursor solution for NiAc/PAN was prepared by mixing 0.3 g polyacrylonitrile (PAN, Sigma-Aldrich, Mw\u00a0=\u00a0150,000 g mol-1), 0.1 g nickel acetate (NiAc, Sigma-Aldrich, Mw\u00a0=\u00a0248.84 g mol-1) and 4 mL dimethylformamide (DMF, AR, Beijing Chemical Works). After 4 h of stirring at room temperature, the NiAc/PAN inner precursor was obtained. The outer precursor solution for preparing CuAc/PAN was made by mixing 0.3 g PAN, 0.1 g copper acetate (CuAc, Sigma-Aldrich, Mw\u00a0=\u00a0199.65 g mol-1) and 4 mL DMF solution. After 4 h of stirring at room temperature, the CuAc/PAN outer precursor was obtained. Both of inner and outer precursors were transferred into syringes which were equipped with a coaxial nozzle. The coaxial electrospinning setup is illustrated in Fig. S1. In addition, the feeding rates were 0.5 mL h-1 (outer precursor) and 0.4 mL h-1 (inner precursor). Subsequently, a flat Al foil covered with non-dust cloth was used as a collector and put about 15 cm away from the nozzle tip. A voltage of 18 kV was applied to the solution to start the spinning process with a high voltage source (SL50P60, Spellman High Voltage Electronics Corporation).The pristine films of NiAc/PAN@CuAc/PAN were placed in a ceramic boat, heated to 800 \u00b0C at a ramp rate of 3 \u00b0C min-1 and kept for 2 h under Ar flow. After that, the furnace was cooled down to room temperature naturally.The preparation process was same to that of Ni(CNFs)@Cu(CNFs), except that the solution of NiAc-CuAc/PAN was prepared by mixing 0.3 g of PAN, 0.1 g of NiAc, 0.1 g of CuAc and 4 mL DMF.The preparation process was same to that of Ni(CNFs)@Cu(CNFs), except that the solution of NiAc/PAN or CuAc/PAN was prepared by mixing 0.3 g of PAN, 0.1 g of NiAc or CuAc and 4 mL DMF.The preparation process was same to that of Ni(CNFs)@Cu(CNFs), except that the solution of PAN was prepared by mixing 0.3 g of PAN and 4 mL DMF.The morphology of the samples was characterized by the scanning electron microscope (Hitachi S4800), field emission transmission electron microscope and energy-dispersive X-ray spectroscopy (FEI Tecnai G2 F20 U-TWIN). A Rigaku D/MAX-TTRIII (CBO) X-ray power diffractometer was used to get powder X-ray diffraction (PXRD) patterns by using Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.5418 \u00c5). Raman spectra were collected using a Renishaw inVia Raman microscope with a laser wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB250Xi apparatus with an Al K\u03b1 X-ray source. The N2 adsorption/desorption curve was carried out by Brunaue-Emmett-Teller (BET) measurements using a Micrometritics TriStar II 3020 analyzer.5 mg of catalyst was mixed with 50 \u00b5L of Nafion solution, 475 \u00b5L of ethanol and 475 \u00b5L of DI water and then placed in an ultrasonic bath for at least 30 min to achieve a homogeneous ink. Then, 50 \u00b5L of the catalyst ink was pipetted onto a carbon paper electrode (0.5 cm2). The loading mass of the catalyst was 0.5 mg cm-2. In addition, since these membranes are flexible and self-supporting, Ni(CNFs)@Cu(CNFs) membrane was also cut into the certain size or shape (for example, \u223c0.5 mg) and directly used as working electrodes for bulk electrolysis of CO2RR.All the electrochemical measurements were carried out in a home-made gas-tight two compartment electrochemical cell with a proton exchange membrane (Nafion 117, Dupont) as the separator, equipped with Ag/AgCl reference electrode and platinum counter electrode. Each compartment contained 30 mL of electrolyte. Before the electrolysis, the electrolyte was pre-saturated with CO2 (99.999%, the pH value of the saturated solution was measured to be 6.8) by bubbling the gas for 30 min to remove the air in the cell. During the measurement, CO2 was continuously bubbled into the electrolyte at a flow rate of 20 mL min-1. The electrochemical tests were performed in a CHI660D electrochemical workstation (Shanghai Chenhua Instrument Co., China) at room temperature with 0.1 M KHCO3 aqueous solution as the electrolyte. All of the applied potentials were recorded against an Ag/AgCl (saturated KCl) reference electrode and then converted to those versus reversible hydrogen electrode (RHE) using E (vs. RHE)\u00a0=\u00a0E (vs. Ag/AgCl)\u00a0+\u00a00.197 V\u00a0+\u00a00.0591 \u00d7 pH. All potentials were recorded with iR compensation. In addition, the electrocatalytic CORR in 0.1 M KHCO3 solution was performed with similar procedures.Gas products of electrocatalysis were analyzed by an online gas chromatograph (Shimadzu GC 2014) with molecular sieves C13X, Al2O3 column (50 m, 0.53 mm, 10 \u03bcm), Rt-Q-BOND PLOT column (30 m, 0.32 mm ID, 10 \u03bcm) and equipped with two flame ionization detectors (FIDs) and a thermal conductivity detector (TCD). A GC run was initiated every 15 min. High purity Ar (99.999%) was used as the carrier gas.Liquid product was quantified using 1H NMR (Bruker Advance 400 spectrometer, 400 MHz) via water suppression using a pre-saturation method. Electrolyte (700 \u00b5L) was mixed with 35 \u00b5L of 10 mM dimethyl sulfoxide and 50 mM phenol as internal standards in D2O for the 1H NMR analysis. The gaseous products were sampled and analyzed online every 15 min during the reaction, and the averaged result was used for discussion. The liquid products were collected and analyzed after the operation for 1 h.In this work, the Faradaic efficiency (FE) of gas products was calculated from the concentration determined by GC using the following equitation:\n\n(1)\n\n\n\n\nFE\n\n%\n\n=\n\nppm\n\n\n\u00d7\n\nflow\n\nrate\n\n\n\u00d7\n\n(\n\n\nnF\n\nP\no\n\n\nRT\n\n)\n\n\u00d7\n\n\n(\n\nj\nTot\n\n)\n\n\n\u2212\n1\n\n\n\n\u00d7\n100\n\n\n\nwhere ppm is the concentration of gas (CO, CH4 or H2 etc.) determined by GC, n is the electron transfer number, F is the Faradaic constant, Po is the pressure, T\u00a0=\u00a0273.15 K and jTot is the total current density.The FE of liquid products was calculated as follows:\n\n(2)\n\n\n\n\nFE\n\n%\n\n=\n\n\nn\n\u00d7\nC\n\u00d7\nV\n\u00d7\n\ne\n\n\n\u00d7\n\nN\nA\n\n\u00d7\n\n\n\n10\n\n\n\u2212\n3\n\n\n\nQ\n\n\n\u00d7\n100\n\n\n\nwhere n is the electron transfer number, C (mol L-1) is the concentration of liquid products in the electrolyte, V (mL) is the volume of the electrolyte, F is the Faradaic constant, e (C mol-1)\u00a0=\u00a01.6 \u00d7 10-19, NA (Avogadro Number)\u00a0=\u00a06.02 \u00d7 1023 and Q (C) is the total amount of charge passed through the system.We carried out the first-principle DFT calculations on the CO2RR activity of Ni(CNFs)@Cu(CNFs) by the projector augmented wave (PAW) method-based Vienna Ab Initio Simulation Package (VASP) [44, 45].The functional of Perdew-Burke-Ernzerhof (PBE) with generalized gradient approximation (GGA) was considered for the electron exchange-correlation [46]. The cutoff energy was set as 450 eV while the force tolerance and energy tolerance were set as less than 0.03 eV \u00c5-1 and 10-4 eV, respectively. A Monkhorst-Pack k-point mesh of 3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a01 grid was used to sample the Brillouin zone. A 20 \u00c5 vacuum was added along the z direction in order to avoid the interaction between periodic structures. The DFT-D3 method was employed to consider the van der Waals interaction [47].The free energies of the CO2 reduction steps (CO2RR) were calculated with the following equation [48]:\n\n(3)\n\n\n\n\n\u0394\n\nG\n\n\n=\n\n\n\n\u0394\n\n\n\nE\nDFT\n\n\n+\n\n\n\n\u0394\n\n\n\nE\nZPE\n\n\n\u2212\n\n\nT\n\n\u0394\n\nS\n\n\n\n\nwhere \u0394EDFT is the DFT electronic energy difference of each step, \u0394EZPE and \u0394S are the correction of zero-point energy and the variation of entropy, respectively, which are obtained by vibration analysis, T is the temperature (T=300 K).The Ni(CNFs)@Cu(CNFs) was prepared by using the coaxial electrospinning approach (Fig. S1 ). This technique has the capability to afford large reactive interfaces and efficient mass transfer pathways for the carbon materials [49\u201355]. Briefly, the polyacrylonitrile (PAN), metal precursors (NiAc or CuAc), and solvent (dimethylformamide, DMF) were sequentially added into a glass vial. After the mixture was stirred for 4 h, the pristine NiAc/PAN@CuAc/PAN nanofibers were obtained by electrospinning, which were then transformed into the porous Ni(CNFs)@Cu(CNFs) by heating in Ar (Fig. S2). The preparation of other samples is detailed in the Material and methods. The morphology of the as-obtained catalysts was characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Taking Ni(CNFs)@Cu(CNFs) as a representative (Fig.\u00a01\nb, c), the nanofibers have an average diameter of \u223c300 nm, similar to those of CNFs, Ni(CNFs), Cu(CNFs), and Ni-Cu(CNFs) (Fig. S3). Fig.\u00a01b shows the nanoclusters distributing in the CNFs and further characterizations using HRTEM show that the clusters are around 5 nm which have typical lattices of Ni and Cu. It is noted that the lattice of Ni is not seen as clear as that of Cu because it is in the core of the fiber (Fig.\u00a01c). Moreover, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image shows a clear contrast between a core and a shell, indicating the core-shell structure, which was further confirmed by the linear energy dispersive X-ray spectroscopy (EDS) scan across a single fiber (inset in Fig.\u00a01d). The corresponding element mapping images also show the Ni/C-core and Cu/C-shell configuration and reflect the uniform distribution of Cu, Ni, N and C elements (Fig.\u00a01d, e). Altogether, electron microscopy provides evidence for the synthesis of the porous, core-shell, and metal cluster/carbon fiber structures. The as-prepared material could probably enable the orientated mass transfer route (CO2-to-CO via Ni/C and then CO-to-ethanol via Cu/C), which is expected to be favorable for catalyzing the reduction of CO2 to ethanol selectively.The as-prepared catalysts were characterized by X-ray diffraction (XRD), nitrogen (N2) adsorption/desorption, Raman and X-ray photoelectron spectroscopy (XPS) measurements. XRD measurement was carried out to investigate the bulk and near-surface crystal structures. Fig.\u00a02\na shows XRD patterns of the as-obtained carbon materials. A broad peak was observed for all the materials at 2\u03b8 around 23\u00b0 originating from the (002) reflection of the graphitic carbon. Typical characteristic patterns of metals including Ni and/or Cu were observed in the samples of Ni(CNFs)@Cu(CNFs), Ni(CNFs) and Cu(CNFs), suggesting the crystalline metal clusters as distributed in the carbon fibers. Compared with the typical diffraction for Cu (43.5\u00b0 (111) and 50.6\u00b0 (200), PDF No. 004-0836) and Ni (44.5\u00b0 (111) and 51.8\u00b0 (200), PDF No.04-0850) [56], the Ni-Cu(CNFs) sample shows the shift of the Cu and Ni diffraction positions, indicating the presence of CuNi alloy phase [57]. It reminds us that separating the metals in core and shell is significant not only to the orientated mass transfer but also to the prevention of metal alloying.The N2 adsorption/desorption and porosity measurements were carried out to investigate the surface and bulk properties of the materials (Fig.\u00a02c, d). All the materials present the typical type-IV N2 adsorption-desorption isotherms with a hysteresis loop, suggesting the presence of mesoporous structures. Based on the N2 sorption isotherms, the specific surface area (SBET), pore volume (VP), and pore size (DP) were calculated using BET and Barrett-Joyner-Halenda (BJH) models. The SBET of the materials are in the range of 45 to 1082 m2 g-1 as shown in Table S1. The Ni(CNFs)@Cu(CNFs) catalyst shows high surface area and pore volume, which may be due to the coexistence of two metals and the formation of the core-shell interfaces. On the one hand, metal causes the combustion of carbon during calcinations [58]. Along with the decomposition of the polymers, mesopores appeared in the carbon fibers. Compared to the alloying of Ni and Cu during the preparation of Ni-Cu(CNFs) which reduced the amount of metal species, all the Ni and Cu clusters participated in the perforation process and formed more pores in the case of Ni(CNFs)@Cu(CNFs). On the other hand, there is an intrinsic interface between Ni/C-core and Cu/C-shell during the electrospinning, such a phase separation may result in more voids/holes on the interface after heat treatment as compared to the dense fiber materials like Ni(CNFs) and Cu(CNFs). The corresponding pore size distribution curves in Fig.\u00a02d show the hierarchical pore distributions and markedly improved pore volumes (from 0.03 cm3 g-1 for CNFs to 0.15 cm3 g-1 for Ni(CNFs)@Cu(CNFs)). The high surface area and unique porous structure provide the catalyst with abundant active sites to adsorb the reactant molecules dissolved in the electrolyte and channels to deliver the reaction species between the active sites, making the electro-catalytic process more efficient. Raman spectroscopy measurement was carried out to characterize the carbon structure (Fig. S4, ), where the intensity ratios of D band (1345 cm-1) to G band (1590 cm-1) are subject to small oscillation from 0.94 to 1, indicating the similar graphitization degree, which is in line with the broad carbon peaks as observed in the XRD patterns (Fig.\u00a02a).The composition and oxidation states of the elements at the surfaces of all the catalysts have been investigated by XPS measurements as shown in Figs.\u00a02e, f and S5, S6, S7. In the high-resolution Cu 2p spectrum (Figs.\u00a02e and S5), all the samples exhibit only two main peaks associated with Cu0 and a series of satellite peaks [59, 60] from CuO, suggesting that Cu(CNFs), Ni(CNFs)@Cu(CNFs) and Ni-Cu(CNFs) also contain oxides (at least on the surface). Cu0 is generally used to catalyze the formation of multi-carbon products during CO2RR, while the oxides (i.e. Cu oxides) can assist the CORR to generate multi-carbon oxygenates and hydrocarbons due to its metastable surface features that bind CO strongly [33, 61]. In the high-resolution Ni 2p spectra (Figs.\u00a02f and S6), the Ni 2p3/2 binding energy signals of Ni(CNFs), Ni-Cu(CNFs) and Ni(CNFs)@Cu(CNFs) were split into metallic Ni0 and low-valent Ni\u03b4+ (855.7 eV), which was reported to be due to the Ni-N bonding, as a typical active site for electroreduction of CO2 to CO [62\u201364]. Additionally, the Ni clusters are surrounded by N-doped carbon. Ni was reported to be efficient in stabilizing COOH* intermediate but the desorption of CO is difficult. On the contrary, carbon facilitates the escape of CO and thus the combination of Ni and carbon optimizes the stabilization of COOH*and timely desorption of CO. Carbon also helps to suppress the hydrogen evolution reaction (HER), thereby resulting in improved activity and selectivity towards CO formation [42, 65]. The N 1s XPS spectra reveal the existence of four types of nitrogen species [66], including pyridinic N (398.7 eV, Ni), pyrrolic N (400.2 eV, Nii), graphitic N (401.1 eV, Niii) and oxidized N (402.6 eV, Niv) species (Fig. S7). Quantitatively, the total N content in Ni(CNFs)@Cu(CNFs) was determined to be 7.64 at%, similar to the other catalysts. The pyridinic N generally presents higher chemical activity and tends to capture transition-metal atoms as individual atoms [62]. Therefore, the N 1s XPS spectra of the other samples are displayed (Fig. S7f) for comparison, the relative content of pyridinic N in core-shell catalysts is slightly higher than the control samples. These results further indicated that core-shell samples possess higher N contents, forming metal-N motifs, and then regulating the surface electronic structures of metal species. Especially, affording effective N-coordinated Ni (i.e. Ni-N motifs) that could facilitate the CO2 adsorption and CO desorption process boosted the conversion of CO2 into CO [62].To quantitatively analyze and compare catalytic activity, the core-shell catalyst was loaded in a gas-tight H-type cell configuration coupled with an online gas chromatography (GC). It was compared with CNFs, Ni(CNFs), Cu(CNFs) and Ni-Cu(CNFs) catalysts using the same set-up and conditions. The liquid products were analyzed after the reaction by quantitative nuclear magnetic resonance spectroscopy (NMR). The electrolysis was performed in a 0.1 M KHCO3 (pH=6.8) electrolyte saturated with CO2. The average current densities of the electrodes made of the five catalysts at potentials between -0.7 and -1.1 VRHE are summarized in Fig.\u00a03\na. In comparison to CNFs, Ni(CNFs), Cu(CNFs) and Ni(CNFs)@Cu(CNFs), the Ni-Cu(CNFs) catalyst shows the highest current densities. This result is similar to the report [57] using Cu/Ni alloy nanoparticles embedded in a nitrogen-carbon network that demonstrated a higher intrinsic activity for the production of CO as well as current density at a low applied potential toward CO2 reduction than the individual Cu or Ni particles due to the higher CO2 adsorption capacity. The FEs of the products were calculated and the results of which are presented in Fig.\u00a03b-f and S8a . For Ni(CNFs)@Cu(CNFs), CO with a FE of 29.3% was detected as the major gaseous C1 product accompanied by H2 as a byproduct at the positive potential. C2H5OH began to evolve at a potential of -0.8 VRHE, but CO species were still the main products. When more negative potentials were applied, the FE of C2H5OH dramatically increased to an optimal value of 18.2% at -1 VRHE and then decrease at more negative potential, which may be due to the competition from HER. It is noted that other C2 products such as C2H4 were also produced although the content was low (FE \u223c1%-2%). In addition, it is observed that the FE for CO (Fig.\u00a03e) and H2 (Fig. S8b) production decrease with increasing the FE associated with C2H5OH product, suggesting the consumption of CO during the reaction [40, 67].As the control sample, CNFs showed negligible activity for electrochemical CO2 reduction. As for Ni(CNFs) and Ni-Cu(CNFs), CO was preferentially produced throughout a broad potential range (from -0.7 to -1.1 VRHE, Fig.\u00a03c and f). The catalytic performances of Ni(CNFs) and Ni-Cu(CNFs) are typical as the reported Ni-based electrodes [68, 69]. In the case of Cu(CNFs), CO and formate were observed as the major products at low overpotentials. It is noteworthy that CO2 reduction rates increased and CH4, C2H4 and C2H5OH were detected from the Cu(CNFs) electrode at more negative potentials, agreeing well with previous measurements on Cu-based catalysts [70]. Interestingly, although Cu(CNFs) produced formate with a maximum FE of 18%, the FE of formate on the Ni(CNFs)@Cu(CNFs) and Ni-Cu(CNFs) were less than 2%. As a key intermediate to formate, *COOH could also be reduced to other products such as CO, indicating that the binding energy of *COOH to the catalyst has been changed when there was Ni/C, which prefers the CO2-to-CO route. The above observation manifests that the formate formation pathway is less favored over the Ni(CNFs)@Cu(CNFs) catalyst.A long-term electrolysis was performed at a stationary -1 VRHE for 100 h to test the stability of Ni(CNFs)@Cu(CNFs) catalyst. Compared to the carbon paper electrode, the self-supporting character of the sample maintained a steady current density of \u223c16 mA cm-2 with negligible drop throughout the stability test (Fig.\u00a03g and S9). Note that the periodic fluctuation of the current-time curve is due to the changing of electrolyte every 12 h. Furthermore, the morphology and chemical structure of Ni(CNFs)@Cu(CNFs) after 100 hours of reaction was investigated and no noticeable changes could be observed relative to the fresh one (Fig. S10). Impressively, according to Fig.\u00a03h, the FE of core-shell samples reached 18.2% at -1 VRHE, which outperformed many of the reported CO2 to C2H5OH reduction catalysts under similar electrolysis conditions (Table S2), that is, an H-type cell consisted of a carbon paper (the carrier electrode of as-prepared catalyst) as the working electrode, Pt as the counter electrode, and Ag/AgCl as the reference electrode in CO2-saturated 0.1 M KHCO3 solution [30, 40, 41, 70-78]. The unique structure of Ni(CNFs)@Cu(CNFs) avoids the typical powdering and binding procedures, inhibits metal particles from agglomerating and enables the orientated mass transfer between the active sites, resulting in the high current density and selectivity in reducing CO2 towards ethanol.The electrochemical testing indicates that Ni(CNFs) core and Cu(CNFs) shell act synergistically to promote the formation of ethanol. We postulate a tandem catalysis mechanism, that is, Ni/C reduces CO2 to CO near the Ni/C-Cu/C interfaces and the CO species transfer to the Cu/C sites to produce ethanol. To prove the hypothesis of the two-sites mechanism, we carried out direct CO electroreduction tests on Ni(CNFs) and Cu(CNFs) at -1 VRHE for 1h in 0.1 M KHCO3. Under pure CO feeding, the FE of C2H5OH of Cu(CNFs) increased and reached almost the same value compared to Ni(CNFs)@Cu(CNFs) catalyst, while Ni(CNFs) showed a negligible production of ethanol under the same conditions, suggesting that it could not further promote the formation of *CO intermediates, which is the necessary reaction intermediate for C2+ products generation (Fig.\u00a04\na). Comparing the performances of Cu(CNFs), Ni(CNFs)@Cu(CNFs) and Ni-Cu(CNFs) (Fig.\u00a03, 4a), it is clear that the core-shell structure results in the suppression of H2 evolution, increase of sufficient local CO concentration and the subsequent CO-insertion caused ethanol production. Additionally, the absolute FE for CO reduction was lower than those of CO2 reduction due to the CO is sparingly soluble in water (Henry's law [79, 80]). The insights gained from the Ni(CNFs)@Cu(CNFs) catalyst provide a new possibility for developing highly active tandem catalysts.Regarding the detailed CO2RR mechanisms of using Ni(CNFs)@Cu(CNFs) as the catalyst, DFT calculations on the free energies of the intermediates along the reaction pathway (CO2 to ethanol) were carried out. Firstly, for electrochemically reducing CO2 to CO, we considered two different graphene structures (bridge-top and top-fcc) in a carbon coated model on Ni (Fig.\u00a04b and S11-13) [81]. For comparison, Ni(111), pristine graphene and Ni-N4 embedded graphene (Ni-N4/Gr) were used as the models, wherein Ni-N4 has been reported as an efficient Ni catalyst for CO2RR to CO [82, 83]. The generation of CO by CO2RR involves the exchanges of two electrons and two protons: (i) CO2\u00a0+\u00a0*\u00a0+\u00a0H+\u00a0+\u00a0e-\u00a0\u2192\u00a0COOH*; (ii) COOH*\u00a0+\u00a0H+\u00a0+\u00a0e-\u00a0\u2192\u00a0CO*\u00a0+\u00a0H2O; and (iii) CO*\u00a0\u2192\u00a0CO\u00a0+\u00a0* (* represents the active site of catalyst). As shown in Fig.\u00a04b, Gr/Ni (111) presents a lower free energy of COOH* (0.62-0.75 eV) than Ni-N4/Gr (1.56 eV) and pristine graphene (2.17 eV), indicating that COOH* formation is more favorable, which is calculated to be the rate-determining step (RDS) for all the catalysts. Although Ni (111) shows a lowest free energy (-0.01 eV) change for COOH*, it requires a large energy penalty for CO desorption, manifesting an overall difficult CO production. Thus, Ni(CNFs) stabilizes COOH* without affecting the easy CO* desorption and improves the catalytic activity for electrochemically reducing CO2 reduction to CO. Subsequently, the preferential formation of ethanol on Cu(CNFs) was proposed as follows (Fig.\u00a04c and S13): CO\u00a0\u2192\u00a0*CO\u2192 *CO\u00a0+\u00a0*CO\u00a0\u2192\u00a0*CO\u00a0+\u00a0*CHO\u00a0\u2192\u00a0*COCHO\u00a0\u2192\u00a0*COHCHO\u00a0\u2192\u00a0*COHCHOH\u00a0\u2192\u00a0*CCHOH\u00a0\u2192\u00a0*CHCHOH\u00a0\u2192\u00a0*CHCH2OH\u00a0\u2192\u00a0*CH2CH2OH\u00a0\u2192\u00a0*CH3CH2OH. It can be seen that the formation of *CO is exergonic, indicating that the CO is prone to adsorb on the Cu(CNFs) surface. The potential-determining step (PDS) of ethanol formation is *CO\u00a0+\u00a0H+\u00a0+\u00a0e-\u00a0\u2192\u00a0*CHO and the energy barrier of which is 0.88 eV (\u0394G). On the contrary, the hydrogenation is difficult with \u0394G\u00a0=\u00a01.23eV, illustrating that the formation of double carbon (C2) intermediate (*CO - *COH, orange lines) is not preferable. Further, the C-C coupling by the dimerization of *CO intermediates is promoted to form the *COCHO intermediates (\u0394G\u00a0=\u00a00.50 eV) and the rest hydrogenation steps to the final product ethanol are facile due to exothermic process or endothermic process consuming a small energy (blue lines), indicating that the ethanol pathway is favorable on Cu(CNFs) surface, which is in agreement with the experimental results.In summary, to realize a highly selective production of ethanol through CO2RR, an orientated mass transfer route is proposed and achieved by making a Ni(CNFs)@Cu(CNFs) membrane catalyst. It is found that the unique Ni/C-core@Cu/C-shell design synergistically resulted in the suppression of H2 production, the local CO concentration increase and a tailored transfer of the CO species from the inner Ni/C to outer Cu/C for the selective formation of ethanol. Compared with the control catalysts with individual metals or alloys, an impressive FE for ethanol of 18.2% at -1 VRHE in 0.1 M KHCO3 for at least 100 h was achieved using the Ni(CNFs)@Cu(CNFs) as the catalyst, which outperforms many of the reported CO2 to ethanol reduction catalysts under similar electrolysis conditions. The concept of tandem catalysis for selective formation of ethanol was further demonstrated by performing CO reduction on Ni(CNFs) and Cu(CNFs) catalysts. DFT calculations suggest that the combination of the Ni(CNFs) and Cu(CNFs) can enhance the stabilization of oxygenic C2 intermediate. The design and realization of the orientated mass transfer in a CO2RR could facilitate the preparation of unique hierarchical structures for tailored reactions used in catalysis or batteries.The authors declare that they have no conflicts of interest in this work.We acknowledge the financial support from the National Natural Science Foundation of China (Grants No. U20A20131 and 51425302).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.fmre.2021.08.021.\n\n\nImage, application 1\n\n\n\n\n\nImage, application 2\n\n\n\n", "descript": "\n                  Electrochemically reducing CO2 to ethanol is attractive but suffers from poor selectivity. Tandem catalysis that integrates the activation of CO2 to an intermediate using one active site and the subsequent formation of hydrocarbons on the other site offers a promising approach, where the control of the intermediate transfer between different catalytic sites is challenging. We propose an internally self-feeding mechanism that relies on the orientation of the mass transfer in a hierarchical structure and demonstrate it using a one-dimensional (1D) tandem core-shell catalyst. Specifically, the carbon-coated Ni-core (Ni/C) catalyzes the transformation of CO2-to-CO, after which the CO intermediates are guided to diffuse to the carbon-coated Cu-shell (Cu/C) and experience the selective reduction to ethanol, realizing the orientated key intermediate transfer. Results show that the Faradaic efficiency for ethanol was 18.2% at -1 V vs. RHE (VRHE) for up to 100 h. The following mechanism study supports the hypothesis that the CO2 reduction on Ni/C generates CO, which is further reduced to ethanol on Cu/C sites. Density functional theory calculations suggest a combined effect of the availability of CO intermediate in Ni/C core and the dimerization of key *CO intermediates, as well as the subsequent proton-electron transfer process on the Cu/C shell.\n               "}
{"full_text": "The increasingly high energy demands and the environmental issues related to energy consumption have caused great global concern over the past few decades (Li et al., 2017; R\u00f6nsch et al., 2016). Increasing global population is the main factor behind the increase in energy consumption. Natural gas has recently become a crucial resource due to its high energy density, ease of transportation and limited polluting effect. Synthetic natural gas has been produced from coal or biomass via syngas (CO\u00a0\uff0b\u00a0H\n\n\n\n2\n\n\n) in different ways. CO methanation is seen as an important means for transforming coal into natural gas. In fact, the production of methane from coal via syngas not only plays a highly important role in the efficient and comprehensive utilisation of coal but also provides a practical way of supplementing the shortage of natural gas reserves (Li et al., 2016; Zhang et al., 2018b).Due to its excellent activity and relatively low cost, a Ni-based catalyst has become one of the most popular catalysts for methanation. In addition to altering the surface area of the carrier, engineering the support\u2019s morphology is considered to be a powerful way of modifying the metal\u2013support interactions in oxide-supported catalysts, which can influence both the size and distribution of the metal nanoparticles. Silica (SiO2) has been widely applied in many methanation catalysts due to its rich pore volume and surface area (Li et al., 2019). Among the different catalyst structures, hollow-structured SiO2, which comprises a void space inside a distinct shell, has received a great deal of attention due to its intriguing physicochemical properties and huge potential (Liang et al., 2017; Yu et al., 2018). The special features of the hollow structures, such as their high surface area and high loading capacity, means that they serve as excellent platforms for catalysts in terms of improving the diffusion of the active components and offering adequate reaction sites (Yao et al., 2019). However, the common methods for preparing SiO2 hollow microspheres, namely, sol\u2013gel, hydrothermal and microemulsion methods, involve a synthesis process that is cumbersome and time consuming. Hence, identifying a simple and quick synthesis method for producing hollow-structured SiO2 microspheres has become a major focus in recent years.In the present study, highly uniform hollow nanoflowers, SiO2 nanospheres within a range of 400 to 500\u00a0nm, were synthesised through a method known as flash nanoprecipitation (FNP). This method presents new technology that allows us to quickly prepare nanoparticles through rapidly colliding different reaction solutions in a mixed mould. The advantages of FNP technology are obvious, mainly include the following points. (1) Fast processing. (2) Simple equipment. (3) Narrow size distribution. (4) Good reproducibility. (5) The experiment can not only be carried out at laboratory scale with small amounts of solutions but also can be easily scaled up to pilot scale. Thus, it has received an increasing amount of attention in recent years (Bteich et al., 2017; Grundy et al., 2018; Morozova et al., 2019). Flash nanoprecipitation (FNP) has previously been demonstrated to produce core\u2013shell and Janus colloids from homopolymer blends. Wang et al. (2015) obtained fluorescent nanoparticles which possessing a narrow size distribution (50\u00a0nm) with desirable fluorescence properties through Flash nanoprecipitation (FNP) method. Lorena et al. (2018) prepare colloids with internally structured geometries from blends of block copolymers and homopolymers by using FNP method.Mo\u2013polydopamine (PDA) is a typical organic\u2013inorganic coordination complex which can be reconstructed into hierarchical spheres (Huang et al., 2016) or hierarchical nanoflowers (Sun et al., 2017) to provide a stable template for the hydrolysis of SiO2. A hollow spherical structure with high strength and stability and a self-assembly characteristic for the generated Mo\u2013PDA complex monomer in a two-dimensional laminar structure provides a stable framework for tetraethyl orthosilicate (TEOS) hydrolysis when applied to the process of adjusting the PH value (Cui et al., 2014; Ma et al., 2015). Chitosan (CTS), a natural cationic polysaccharide, can be introduced during the process of synthesis (Wang et al., 2018b) and, as a polymer chain, can be bound to the MoDo spherical framework to form a large nanoflower-like SiO2.In our study, a series of SiO2 methanation catalysts with different morphologies are synthesised using the FNP method. Here, highly uniformed nanospheres and nanoflowers act as carriers for the active component, Ni, during the methanation. The effect of morphology on catalytic performance is subsequently investigated. Compared to solid smooth nanospheres and hollow nanospheres, hollow nanoflowers have larger specific surface areas. This was expected to result in a higher dispersion of Ni during the impregnation process, which would result in the nanoflower catalyst demonstrating better catalytic activity than the other candidates.A new simple and generic method known as flash-nanoprecipitation (FNP) was developed to produce nanoparticles with desired particle sizes. Syringe pumps, glass syringes and multi-inlet vortex mixer are the main components of FNP technology (Scheme\u00a01). FNP is a rapid process to prepare nanoparticles within only 1 s using a multi-inlet vortex mixer system. FNP technology has a wide range of applications in the field of drug nanoparticle preparation, but the preparation process of inorganic nanoparticles is rarely reported. In this work, different morphologies SiO2 nanospheres are synthesised within a few minutes using FNP technology which provides a fast and time-saving synthesis method for the preparation of hollow flower-like SiO2.\n\nFor the synthesis of solid SiO2, 50\u00a0mL of deionised water was dripped with 1\u00a0mL of ammonia (25%) to form solution A. Then, a 2\u00a0mL solution of TEOS was added to 50\u00a0mL of ethanol and stirred well to form solution B. Two 50\u00a0mL syringes were used to extract solutions A and B before both solutions were rapidly mixed with an injection speed of 40\u00a0mL/min in a two-channel mould. The mixed solution was then collected and washed through centrifugation to obtain solid SiO2 nanospheres. Just as shown in Scheme\u00a01(a).For the synthesis of hierarchical flower ridge-like SiO2 micro/nanostructure, 0.6 g of dopamine and 0.5 g of ammonium molybdate were each dissolved in 100\u00a0mL of deionised water, mixed with a magnetic stirrer for 5\u00a0min and labelled as solutions A and B, respectively. 6\u00a0mL of TEOS was dissolved in 400\u00a0mL of absolute ethanol and magnetically stirred for 5\u00a0min; this solution was labelled solution C. Two 50\u00a0mL syringes were used to extract 50\u00a0mL of solutions A and B, and another two 50\u00a0mL syringes were used to extract solution C. After being dispersed into a four-channel mould, the injection speed of solutions A and B was set to 40\u00a0mL/min while that for solution C was set to 80\u00a0mL/min. The precise injection speed of the syringe pump effectively controlled the alcohol\u2013water level with a stoichiometric ratio of 1:2. The synthetic route is shown in schematic diagram 1 (b)The reaction solution was collected and 1.2\u00a0mL of 25% wt ammonia was added to adjust the pH to 9.2. The brown-coloured solution was centrifuged at 4000 rpm and washed with ethanol and deionised water before being left to dry at 120\u00a0\u00b0C. Hierarchical flower ridge-like SiO2 micro/nanostructures were obtained following calcination at 400\u00a0\u00b0C in air atmosphere and were subsequently labelled MoDoHSiO2.Large-sized hierarchical flower ridge-like SiO2 micro/nanostructure was prepared as follows. 2 g of CTS was added to 50\u00a0mL of deionised water. Then, 2\u00a0mL of glacial acetic acid was added. The solution was thoroughly stirred to dissolve CTS. When CTS completely dissolved, a further 50\u00a0mL of deionised water was added, and the solution was continuously stirred. After adding 0.5 g of ammonium molybdate, a milky solution was obtained and labelled solution A. Thereafter, 0.6 g of dopamine was dissolved in 100\u00a0mL of deionised water and labelled solution B, while 6\u00a0mL of TEOS was thoroughly dissolved in 400\u00a0mL of absolute ethanol through magnetic stirring and labelled solution C. The remaining steps were the same as those used in preparing the hierarchical flower ridge-like SiO2 micro/nanostructure. The solution was labelled CTSMoDoHSiO2 following calcination in air at 400\u00a0\u00b0C. Just as shown in Scheme\u00a01(c)A Ni-based SiO2 catalyst was synthesised using the traditional impregnation method, with the Ni load on the catalyst set at 30\u00a0wt%. First, 0.5 g of the as-obtained SiO2 micro/nanostructure carrier was weighed and dissolved in 30\u00a0mL of deionised water. 0.75 g of Ni(NO3)2\n\n\u22c5\n6H2O was added and the mixture was stirred for 30\u00a0min. The mixed solution was evaporated to a dry state in a water bath at 80\u00a0\u00b0C, thoroughly ground and calcined in air at 400\u00a0\u00b0C (heating rate 3\u00a0\u00b0C/min) for 2 h, resulting in NiSSiO2, NiMoDoHSiO2 and NiCTSMoDoHSiO2, respectively.The crystal structure of the different materials was determined by X-ray diffractometer (XRD) with CuK\n\u03b1\n (40\u00a0kV, 40\u00a0mA, k \n=\n 1.5406\u00a0\u00c5 and 2 \n\u03b8\n range from 10\n\u00b0\n\u201390\u00b0) radiation. The samples were subjected to X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250 Xi) by using Al K\n\u03b1\n radiation (1486.6\u00a0eV). All binding energies (BE) were calibrated using the C1s peak (BE \n=\n 284.8 eV) as a standard. Morphology and microstructure of the different samples were determined by transmission electron microscopy (TEM, Tecnai G2 F30 S-TWIN (300 KV), high resolution TEM (HRTEM) and selected area electron diffraction (Talos-F200X). The Brunauer\u2013Emmett\u2013Teller\u200b (BET) specific surface area and the Barrett\u2013Joyner\u2013Halenda (BJH) pore size distribution characteristics of the catalyst were measured by a nitrogen adsorption desorption analyse. The H2 reduction behaviour of the different samples was tested by a hydrogen temperature programmed reduction (H2-TPR) apparatus with a blend gas of 10% H2/Ar (30\u00a0mL.min\u22121) as a reducing gas and the temperature was increased to 800\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C\n\u22c5\nmin\u22121. A stainless steel tube with a length of 75\u00a0cm and an inner diameter of 10\u00a0mm is used in the evaluation device. First, take an appropriate amount of 10 mesh quartz sand into the reaction tube to a position detectable by the thermocouple (K-type, WRNK-191), then put in an appropriate amount of quartz wool, and finally pour the required amount of catalysis. The catalyst evaluation device is a four-channel methanation fixed bed reaction platform. The reaction temperature is mainly adjusted by a temperature controller (AI-518P) and syngas is measured and fed in through a mass flow controller (S49 33/MT). The water in the product is condensed by the condenser, and further dried to remove water through a drying column. The experimental data is monitored online by gas chromatography (SHIMADZU, GC 2014), detector is a thermal conductivity detector and the column model is TDX-01. Sampling interval is 17 min. The external standard method is used to calibrate the chromatography. The CO conversion and CH4 selectivity calculation formula as fallow \n\n\n\n\n\n\nX\n\n\nCO\n\n\n\n(\n%\n)\n\n=\n\n(\n\n\nV\n\n\nCO,in\n\n\n\u2212\n\n\nV\n\n\nCO,out\n\n\n)\n\n\u2215\n\n\nV\n\n\nCO,in\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\nS\n\n\nCH4\n\n\n\n(\n%\n)\n\n=\n\n\nV\n\n\nCH4,out\n\n\n\u2215\n\n(\n\n\nV\n\n\nCO,in\n\n\n\u2212\n\n\nV\n\n\nCO,out\n\n\n)\n\n\u00d7\n100\n%\n\n\n\n\n\nCatalytic activity of the catalysts was measured on a fixed bed microreactor. During the test, 0.15 g of catalyst sample was introduced in a stainless steel microreactor. Firstly, the catalyst was heated to 500\u00a0\u00b0C under nitrogen with a flow of 60\u00a0mL\n\u22c5\nmin\u22121 and then reduced with a 60\u00a0mL\n\u22c5\nmin\u22121 H\n\n\n\n2\n\n\n flow for 2 h. Reaction was then performed with a weight hourly space velocity (WHSV) of 26,000\u00a0mL\n\u22c5\ng\u22121\n\n\u22c5\nh\u22121 (H2/CO with molar ratio of 3:1 and total flow rate (STP) of 65\u00a0mL\n\u22c5\nmin\u22121). Gas composition was evaluated by chromatography. The stability tests of different samples was carried out at 300\u00a0\u00b0C.\n\nFig.\u00a01a shows the XRD patterns of SiO2 carriers with different morphologies following calcination at 400\u00a0\u00b0C for 2 h. The XRD patterns of the three samples exhibited diffraction peaks only for SiO2. While (NH4)6Mo7O24 was used in the synthesis process for the MoDoHSiO2 and CTSMoDoHSiO2 samples, no diffraction peaks for MoO2 appeared following calcination. A possible reason for this is that (NH4)6Mo7O24 was coated with TEOS during the hydrolysis process and was covered with SiO2 following calcination, which cannot be detected.Field emission scanning electron microscopy (FESEM) was used to examine the morphology and microstructure of SiO2 with different morphologies. As shown in Fig.\u00a01b, the SSiO2 exhibited uniform and unmixed slick three-dimensional monodispersed SiO2 slippery microspheres with an average diameter of approximately 500\u00a0nm. The smoother surface of the SSiO2 may result in a poor distribution of the active components during impregnation. Fig.\u00a01c shows the MoDoSiO2 sample, which exhibited a uniform hierarchical flower-like microsphere geometry structure with an average diameter of approximately 400\u00a0nm. The nanosheets overlapped to form a flower ridge-like structure that likely has a high surface area, which will improve the impregnation efficiency. Fig.\u00a01d shows CTSMoDoSiO2 nanospheres formed in the presence of CTS. It is clear that, in the presence of CTS, the morphology had undergone little change compared to the MoDoSiO2 sample; however, the size significantly increases to around 1 \n\u03bc\nm (Sun et al., 2018b).The pore size distribution and specific surface area of the different SiO2 carriers following calcination were obtained from the nitrogen adsorption/desorption measurement (Zhang et al., 2018a). Fig.\u00a01e shows that all the samples exhibited type IV isotherms except for the SSiO2, suggesting the presence of mesoporous structures (Wang et al., 2018a). The type-H\n\n\n\n3\n\n\n hysteresis loop appearing at a relative pressure of p/p0 > 0.5 indicates that slit-like pores were formed (Lu et al., 2019), which may be a result of the accumulating of flower ridge-like particles. The SSiO2 has the smallest specific surface area, only 14.9 m2/g. The analysis results of N\n\n\n\n2\n\n\n adsorption/desorption curve and pore volume and pore size distribution diagram show that the adsorption amount of SSiO2 is small. Moreover, the FESEM characterisation shows that the surface of SSiO2 is relatively smooth. Therefore, SSiO2 has a small specific surface area. The MoDoHSiO2 and CTSMoDoHSiO2 exhibited large surface areas of 244 and 194 m2/g, respectively. This can be explained by the fact that the introduction of the flower ridge-like structure enhanced the surface area. The pore size distribution curves (Fig.\u00a01f) were acquired according to the Barret\u2013JoynerHalenda model. In comparison, the MoDoHSiO2 exhibited small pores of around 6\u00a0nm, while CTSMoDoHSiO2 pores were over 20\u00a0nm. This may be related to the particle size of the sample itself. In general, the flower ridge-like micro/nanostructure with a large specific surface area and rich mesopores enhanced the contact area between the active component and the reaction gas, which promoted the activity of the catalyst (Das et al., 2019).\n\n\nAll the synthesis in our work proceeded according to the self-assembly of the MoPDA complex (Dandan\u00a0Wang et al., 2016), as shown in Fig.\u00a02. Due to the slight solubility of Mo\u2013dopamine chelates in ethanol, with the mixture of ethanol and H\n\n\n\n2\n\n\nO in the mould, an interface was formed where the hydrophobic groups of the Mo\u2013dopamine complexes pointed towards the water and the hydrophilic groups were orientated outward (Dandan\u00a0Wang et al., 2016). A rapid polymerisation of molybdate anion and dopamine occurred when the different solutions were flash mixed in the mould to produce an orange\u2013red colour (Dandan\u00a0Wang et al., 2016). With ammonia added dropwise into the solution, the self-polymerisation of the dopamine was initiated along the interface (Dandan\u00a0Wang et al., 2016). At the same time, the TEOS uniformly hydrolysed on the surface of the microspheres and SiO2 with different morphologies were formed. Table\u00a01 lists the different synthesis methods for the preparation of different morphologies and sizes of SiO2. As can be seen from the table, the traditional method of preparing hollow SiO2 was cumbersome and time-consuming. For the novel FNP synthesis technology, a variety of reaction solutions can be quickly mixed within 1 min. The entire synthesis process can be completed within 10\u00a0min. FNP technology provides a fast and time-saving synthesis method for the preparation of hollow flower-like SiO2.\n\nFig.\u00a03a shows the XRD patterns of the catalyst precursors formed via immersion and calcination for the SiO2 microspheres with different morphologies. The dominant diffraction peaks are well matched with the NiO, where the peaks at 2\n\n\u03b8\n=\n37\n\n.3\u00b0, 43.3\u00b0, 62.8\u00b0 and 74.4\u00b0 belong to the (111), (200), (220) and (311) crystalline planes of NiO, respectively (Song et al., 2019). In particular, the sharp peak of NiO in the NiSSiO2 sample indicates the formation of large-sized NiO particles (Ren et al., 2018). Meanwhile, the broader and weaker diffraction peaks of NiO in the NiCTSMoDoHSiO2 and NiMoDoHSiO2 samples can be assigned to the smaller NiO particle size (Song et al., 2019). From the XRD patterns in Fig.\u00a03a, we can roughly infer that during the calcination process, the NiMoDoHSiO2 and NiCTSMoDoSiO2 samples had smaller NiO particle sizes and higher metal dispersion compared to the NiSSiO2 sample. To verify our inference, hydrogen pulse chemisorption (H\n\n\n\n2\n\n\n-PULSE), high-resolution transmission electron microscopy (HRTEM) and inductively coupled plasma (ICP) spectrometry were performed, with the results presented in Table\u00a02 and Fig.\u00a03b\u2013f.\nFig.\u00a03b\u2013f shows the methanation catalyst precursors formed following calcination and H\n\n\n\n2\n\n\n reduction. It is clear that Ni particles were present. There were massive Ni particles with different sizes on the smooth surface of the NiSSiO2 samples (Fig.\u00a03b), which were extremely inhomogeneous. In terms of the NiMoDoHSiO2, as Fig.\u00a03c and 3d show, Ni particles with a particle size of around 3\u00a0nm were uniformly supported on the flower-like surface. This was consistent with the results subsequently obtained using hydrogen temperature programmed reduction (H\n\n\n\n2\n\n\n-TPR) and H\n\n\n\n2\n\n\n-PULSE. For the NiCTSMoDoHSiO2 sample (Fig.\u00a03e, f), Ni particles with an approximate size of 7\u00a0nm were formed on the surface, which were slightly larger than the 3\u00a0nm particles of the NiMoDoHSiO2 sample. The size of the CTSMoDoSiO2 support was around 1 \n\u03bc\nm, which may lead to the formation of large Ni particles during impregnation.\nThe redox properties of all the calcined samples and the interaction between the metal particles and the support were determined via H\n\n\n\n2\n\n\n-TPR (Song et al., 2019), as shown in Fig.\u00a04. The NiSSiO2 catalyst exhibited a sharp reduction peak at 325\u00a0\u00b0C, which could have been caused by the reduction of the \u2018free state\u2019 NiO resulting in a weak interaction with the support (Song et al., 2019). This form of NiO, always characterised by a large particle size, had similar properties to the bulk NiO and could be easily reduced. The reduction peaks of the NiMoDoHSiO2 and NiCTSMoDoSiO2 samples appeared at 362\u00a0\u00b0C and 359\u00a0\u00b0C, respectively. The higher reduction temperatures can be attributed to NiO with smaller particles or to the stronger interaction and support (Sach\u00e9 et al., 2018). The NiMoDoHSiO2 sample had the highest reduction peak area compared to the other two catalyst samples, indicating that more NiO was reduced during the reaction. This can also be attributed to the weaker interaction between the NiO and the support (Zou et al., 2010). Fig.\u00a04 shows that smaller NiO was present on the NiMoDoHSiO2 and NiCTSMoDoSiO2 samples following the impregnation method and that the interaction between the NiO and the support was weak.From the H\n\n\n\n2\n\n\n-PULSE and ICP results, as presented in Table\u00a02, it is clear that NiCTSMoDoSiO2 possessed the highest metal dispersion (1.52%) and metal surface area (2.82 m2/g). The poorest metal dispersion was demonstrated by the NiSSiO2 sample, which was due to the smooth surface inhibiting the effective distribution of the active components. This result was in line with the conclusions drawn from the results shown in Fig.\u00a03.\nThe surface oxidation state of Ni and other different elements were studied using X-ray photoelectron spectroscopy (XPS) (Xue et al., 2019). The typical survey shown in Fig.\u00a05a involves four distinct peaks of Ni 2p, Si 2p, C 1s, Ni 2p and O 1s. Mo 3d was not detected even though (NH4)6Mo7O24 was used in the synthesis process for the MoDoHSiO2 and CTSMoDoHSiO2 samples. This indicates that Mo was not present on the surface of the catalyst; more precisely, the Mo was covered by SiO2. The XPS results for the Ni 2p 3/2 peak are shown in Fig.\u00a05b. In terms of the NiMoDoHSiO2 and NiSSiO2 samples, the peaks that occurred at 852.3 and 851.7 eV can be attributed to Ni0 (ref. Ni\n\n\n\n\n0\n\n\n=\n852\n\u00b1\n\n 0.4 eV) (Shan et al., 2014). Meanwhile, the peak that appeared at 857.1 eV was attributed to Ni\n\n\n\n2\n+\n\n\n for the NiCTSMoDoHSiO2 sample. Compared to the peak position of Ni\n\n\n\n2\n+\n\n\n in the other two samples, the peak position shifted towards a higher energy band (857.1 eV) for the NiCTSMoDoHSiO2 sample. These results indicate that a stronger Ni and SiO2 interaction was obtained with the NiCTSMoDoHSiO2 sample than with the other samples, which is in line with the TPR results (Fig.\u00a04). For the NiMoDoHSiO2 sample, a metallic Ni peak clearly appeared at 850.2 eV (ref Ni\n\n\n\n\n0\n\n\n=\n852\n.\n6\n\neV\n\n), signalling a shift to lower binding energies. A possible reason for this is that the small NiO particles reduced the interaction between NiO and the carrier. On comparing the satellite peaks of the three samples, the satellite peaks of the NiMoDoHSiO2 and NiCTSMoDoHSiO2 samples almost disappeared. A satellite peak originated at the long-range scattering of the structure of NiONiONiOin the lattice of a NiO nanoparticle, the surface region of which had interdigitated Ni and O atoms (Grosvenor et al., 2006; Tang et al., 2019). By checking this satellite peak, we could roughly judge whether a NiONi structure had formed on the surface region of the catalyst. The lack of a satellite peak meant that small NiO nanoclusters had formed (Akri et al., 2019) on NiMoDoHSiO2, which is consistent with the H\n\n\n\n2\n\n\n-PULSE results.The O1s XPS spectra, presented in Fig.\u00a05c, displayed three main peaks, which were labelled O\n\n\n\nI\n\n\n (lattice oxygen), O\n\n\n\nII\n\n\n (deficient oxygen) and O\n\n\n\nIII\n\n\n (surface oxygen) (Bao et al., 2015). From the peak area, more surface adsorbed oxygen was formed on all three catalyst samples, which may have been caused by the hydroxyl species of water molecules adsorbed on the surface. As shown in Fig.\u00a05c, the highest percentage content of lattice oxygen (bonding of oxygen atoms and metals) was obtained for NiSSiO2, meaning this catalyst demonstrated good stability (Guo et al., 2014). The largest difference between the three catalyst samples was related to the defective oxygen. In fact, defective sites always display abundant low oxygen coordination (Zhao et al., 2018). The NiSSiO2 sample was almost without defective oxygen, whereas the NiMoDoHSiO2 and NiCTSMoDoHSiO2 samples had very clear defective oxygen peaks at 532.7 and 533.2 eV, respectively. These oxygen defects were most likely to be introduced by the flower ridge-like structures. This indicates a good relationship with the sample morphology. The peak that appeared at 102.8 eV in Fig.\u00a05d was attributed to the bonding of SiO.\nThrough the various characterisations, we can conclude that the NiMoDoHSiO2 catalyst demonstrated a uniform morphology with small Ni particles and high metal dispersion. Therefore, it can be concluded that the performance of the NiMoDoHSiO2 sample was the best. To verify this conclusion, we tested the CO methanation performance of the catalyst in a fixed bed reactor. The test temperature is 200\u00a0\u00b0C\u2013500\u00a0\u00b0C, and the CO synthesis gas flow rate is 65\u00a0mL/min. We evaluate the catalyst based on the CO conversion and CH4 selectivity. It is clear from the performance test results shown in Fig.\u00a06a and 6b that the NiMoDoHSiO2 sample demonstrated the best catalytic performance, with 100% CO conversion and 90% CH\n\n\n\n4\n\n\n selectivity at 250\u00a0\u00b0C. The by-product of the reaction is CO2 and the selectivity is 9.8%. In addition, we tested the stability of the three catalysts for 50 h at 300\u00b0C. From Fig.\u00a06c and 6d, we can see that the activity of the NiCTSMoDoSiO2 and NiSSiO2 did not decrease following 50 h reaction. However, the previously best performing sample, the NiMoDoHSiO2 sample, exhibited a \u2018cliff\u2019 decline after 25 h of reaction with a rapid decrease from the original 100% CO conversion to a 10% conversion. While in the previous characterisation we discovered that the NiMoDoHSiO2 sample had small Ni particles and high metal dispersion, the interaction between the active component and the support was the weakest according to the H\n\n\n\n2\n\n\n-PLUSE and XPS results for the NiMoDoHSiO2 sample. The first thing that caught our attention was the agglomeration of the active component, which caused a rapid decline in performance. Therefore, we performed an HRTEM characterisation of the catalyst before and after testing to observe whether a clear agglomeration occurred after the stability test.\n\nFig.\u00a07a shows the XRD patterns of the catalyst sample following the 50 h stability test. Here, it was concluded that the sharper Ni peaks that appeared at 2\n\n\u03b8\n=\n44\n\n.5\u00b0, 51.8\u00b0 and 76.3\u00b0 belonged to the (111), (200) and (220) crystalline planes of Ni, respectively. The sharp Ni diffraction peaks mean that larger Ni particles were formed following the stability test.\nFig.\u00a07b\u2013f shows the HRTEM images of the different samples following the stability test. After stability test, the Ni particles became larger with a clear agglomeration. For the NiMoDoHSiO2 sample (Fig.\u00a07d), Ni particles increased from the initial 3\u00a0nm (Fig.\u00a03d) to around 20\u00a0nm. Therefore, the rapid decline of activity during the stability test can be attributed to the aggregation of the active components (Benavidez et al., 2012; Ouyang et al., 2013). The metal\u2013support interaction continuously existed on the surface of the SiO2 support, but the interaction force here was the weakest among all the supports. The smaller particles result in a weak interaction between the carrier and the active components, meaning agglomeration can easily occur and activity will be rapidly lost during a strong exothermic methanation reaction (Margossian et al., 2017). For the NiCTSMoDoHSiO2 sample, Ni particles with a size of around 7\u00a0nm (Fig.\u00a03f) were formed on the surface, which is slightly larger than the 3\u00a0nm particles formed on the NiMoDoHSiO2 sample. The size of the CTSMoDoSiO2 sample was around 1 \n\u03bc\nm, which may have caused the formation of large Ni particles during the impregnation. In fact, small-sized particles can provide more active sites during the reaction. However, small particle size does not always mean better performance. Indeed, the smaller the particle size, the greater the instability during the reaction and easier the migration and agglomeration on the surface of the carrier (Gao et al., 2015; Munnik et al., 2014). Peter Munnik (2014) reported that the most suitable particle size is 6\u20138\u00a0nm. The active components in this particle size range can maintain an appropriate interaction between the carriers and the active components, which results in better stability (Farmer and Campbell, 2010; Li et al., 2008). Compared to the results shown in Fig.\u00a03f, Ni exhibited slight agglomeration in the NiCTSMoDoSiO2 sample (Fig.\u00a07f). Therefore, the better stability of the NiCTSMoDoHSiO2 sample was attributed to the appropriate Ni particle size and the suitable interaction between the carriers and the active components (Dias and Assaf, 2003).\nCarbon deposition on the catalyst surface is another major factor that causes a rapid decrease in catalytic activity. To further confirm the deposited coke of the spent catalysts, the used catalysts were examined using thermogravimetric analysis (TGA), with the results presented in Fig.\u00a08. The weight loss at 134\u00a0\u00b0C may have been caused by the evaporation of the water that had adsorbed on the catalyst surface during heating. A significant weight increase occurred at 274\u00a0\u00b0C, which was caused by Ni being calcined into NiO under air-atmosphere conditions (Liu et al., 2017). Comparing the TGA curves of the three samples, the NiMoDoHSiO2 sample clearly had the most serious carbon deposits. Therefore, it can be argued that active components with small particle sizes perform poorly in terms of resistance to carbon deposition. In terms of the NiCTSMoDoHSiO2 sample, the Ni particles with a particle size of around 7\u00a0nm not only provided suitable interaction but also performed well in terms of resistance to carbon deposition. Meanwhile, to ascertain whether graphitic and amorphous carbons had accumulated in the spent catalysts (Kopyscinski et al., 2011), XRD analysis was performed (Liu et al., 2014), with the results shown in Fig.\u00a07a. No fresh diffraction peak at 26.55\u00b0 (attributed to graphitic carbon) was observed, which indicated that the deposited carbon was amorphous (Ohtomo and Hwang, 2004) or that the amount of graphitic carbon was below the XRD detection limit.Hollow-structured SiO2 with its intriguing physicochemical properties and huge potential, has been widely applied in many methanation catalysts. In this work, highly uniform hollownanoflowers, silica (SiO2) nanospheres with different sizes, were synthesised through a rapid, time-saving method known as flash nanoprecipitation. High flexibility, reduced technological cost, and high process efficiency make FNP as an attractive method of industrial applications. Furthermore, the size and morphology of the nanoparticles can be independently controlled by adding CTS during synthesis process. Our results demonstrate that this process is highly promising for the production of structured SiO2 nanoparticles in a continuous and scalable way with independent and precise control over particle size, morphology and composition. CTSMoDoHSiO2 nanoparticles as an excellent catalyst carrier are highly sought after in methanation industrial applications due to its suitable particle size and interaction between active components.Highly dispersed and uniform flower-like SiO2 nanoparticles with different sizes were formed using the FNP method. At the same time, this method will developed a commercial solution for preparing methanation supports at lower costs and with higher flexibility than conventional processes. The size of the SiO2 was effectively enlarged in the presence of CTS. Following impregnation and calcination, the MoDoHSiO2 samples exhibited the largest specific surface area and highest metal dispersion. Ni particles with a particle size of 3\u00a0nm were successfully attached to the surface of the MoDoHSiO2 sample. The weaker interaction between the support and the active component meant that Ni significantly agglomerated during the stability test for the NiMoDoHSiO2 sample and its stability performance was poor. For the NiCTSMoDoHSiO2 sample with a particle size of 7\u00a0nm, there was no obvious agglomeration following the stability test. Overall, the catalysts with smaller particles deactivated faster and to a larger extent than those with medium sized particles.Even though FNP stands out as an one-step continuous process that operates at room temperature, consumes little energy, and has potential to scale up, the underlying microscopic mechanisms responsible for the self-assembly are still elusive, which makes the experimental study of the FNP process difficult. Furthermore, it is hard task to systematically search and screen in the experiments all relevant process parameters, such as feed ratio, feed concentration, flow rate, molecular properties. Therefore, computer simulations can provide considerably more microscopic level information than experiments, and are therefore useful tools in the study of complex mechanisms and morphologies.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by International Science and Technology Cooperation Project of Shihezi University, PR China (No. GJHZ201804), International Science and Technology Cooperation Project of Bingtuan, PR China (No. 2018BC002), Science and Technology Innovation Talents Program of Bingtuan, PR China\n (No. 2019CB025).", "descript": "\n                  Hollow-structured SiO2, which comprises a void space inside a distinct shell with its intriguing physicochemical properties and huge potential, has been widely applied in many methanation catalysts. However, the common methods for preparing SiO2 hollow microspheres are cumbersome and time consuming. Highly uniform hollow nanoflowers, silica (SiO2) nanospheres with different sizes, were synthesised through a rapid, time-saving method known as flash nanoprecipitation. An assembling particle mechanism of the hollow structure of Mo\u2013polydopamine complex was established and tetraethyl orthosilicate underwent uniform hydrolysis on the surface of the hierarchical structure. Spherical SiO2 samples with different morphologies were prepared as catalyst carriers, and Ni-based methanation catalysts were prepared using an impregnation method. Ni particles with size of 3\u00a0nm were successfully attached to the surface of MoDo\u2013H\u2013SiO2, while the particle sizes of Ni on CTS\u2013MoDo\u2013H\u2013SiO2 was 7\u00a0nm. The small particles (3\u00a0nm) were found to significantly increase in size (20\u201350\u00a0nm), decrease by 90% in stability test with a weight hourly space velocity (WHSV) of 26,000 mL.g\u22121.h\u22121, which is detrimental to catalyst stability. However, the medium sized particles (7\u00a0nm) remained confined via a suitable interaction involving the support, thus displaying enhanced stability, with 100% CO conversion at 250 \u00b0C and no obviously decrease in stability test Although more active sites can be provided with smaller active metals, catalysts with small sized particles deactivate faster and to a larger extent than catalysts with medium sized particles. Thus, the smaller the particle size of the active component, the worse the stability.\n               "}
{"full_text": "Data will be made available on request.area/ \n\n\n\nm\n\n\n2\n\n\n\n\ndiameter/ \n\nm\n\n\ndiffusion coefficient/ \n\n\n\nm\n\n\n2\n\n\n\n\n\ns\n\n\n-\n1\n\n\n\n\nflow/ \n\n\n\nm\n\n\n3\n\n\n\n\n\ns\n\n\n-\n1\n\n\n\n\nequilibrium constant/ -length/ \n\nm\n\n\nmolar mass/ \n\nkg\n\n\n\nmol\n\n\n-\n1\n\n\n\n\nflux/ \n\nmol\n\n\n\nm\n\n\n-\n2\n\n\n\n\n\ns\n\n\n-\n1\n\n\n\n\npressure/ Pareaction rate/ \n\nmol\n\n\n\nm\n\n\n-\n3\n\n\n\n\n\ns\n\n\n-\n1\n\n\n\n\nideal gas constant/ \n\nJ\n\n\n\nmol\n\n\n-\n1\n\n\n\n\n\nK\n\n\n-\n1\n\n\n\n\nselectivity/ -temperature/ \n\nK\n\n\nvolume/ \n\n\n\nm\n\n\n3\n\n\n\n\nmole fraction/ -conversion/ -Shell thickness/ \n\nm\n\n\nporosity/ -effectivness factor/ -stoichiometric coefficient/ -extent of reaction/Thiele modulus/ -Power-to-X technologies are an opportunity to store electrical energy in the form of chemical compounds [1]. For this purpose, excess renewable energy is used for hydrogen generation via electrochemical water-splitting. Subsequently, hydrogen is converted to chemicals with existing infrastructure, such as methane (Substitute Natural Gas), ammonia, or methanol. In particular, the synthesis of carbon-based products also offers the possibility for reducing carbon dioxide emissions, by consuming it as a reactant. However, the volatile supply of surplus renewable energy makes these processes technologically challenging, due to unsteady process conditions [2,3].For example, the synthesis of methane, methanol, and ammonia is often conducted heterogeneously catalyzed in wall-cooled multi-tubular fixed-bed reactors. These reactors are designed for intensive reaction heat removal, to keep the reactor temperature within desired bounds. Nevertheless, changes in process conditions can lead to uncontrollable reactor behavior, known as thermal runaway. In this case, the reaction heat release leads to an increase of reactor temperature and in consequence, to a further increase of heat release, as the reaction becomes faster. This causes a feedback loop, as reaction rate rises approximately according to the Arrhenius-law (even in presence of internal mass transport limitation), while the heat removal rate increases linearly with coolant temperature. The resulting reactor temperature rise may cause selectivity decrease, catalyst deactivation or even material damage [4,5]. Evidently, effective heat management is crucial for safe reactor operation and has been researched for decades with focus on stationary reactor operation [6]. Nevertheless, if load-flexible reactor operation is expected, controllable conditions have to be maintained at all possible steady states, and also during all dynamic transitions in between. Hence, recent works also consider safe reactor design under dynamic conditions [7\u20139]. Kreitz et al. [10], for example, studied the dynamic operation of micro-structured reactors. Even though such reactors exhibit a large heat transfer areas, temperature peaks of about 150\u00a0K were observed for low-frequency changes of the inlet conditions. Such studies help to identify infeasible operation conditions in advance. However, the number of possible dynamic scenarios, which can be considered is limited (e.g., due to computational power or experimental effort), and in practical applications unforeseen situations might arise, e.g., due to aging of the catalyst or fouling in the coolant system. Thus, it is indispensable to design reactors, where runaway conditions can be generally avoided.An effective opportunity is heat release control by external mass transport limitation of the reactants to the catalyst pellets, as shown in computer-based studies by Zimmermann et al. [11,12]. This can be done, e.g., by applying an inert shell onto the active catalyst pellets, resulting in so-called core\u2013shell catalyst pellets. The inert shell is tailored, such that the mass transport through the inert shell becomes rate-determining particulary at critical reactor temperatures. In this case, the effective reaction rate and thus the heat release rate is approximately independent of temperature, as shown in Fig. 1\n. Hence, a further increase of reactor temperature is prevented by the linearly increasing heat removal rate, minimizing the risk of uncontrollable conditions. Besides influencing the effective reaction rate of the catalyst pellets, the inert shell may also affect selectivity of the catalyst pellets. This can occur in principle due to different mass transport rates of reactants and products through the shell, influencing the chemical equilibrium.These potential benefits have to be distinguished from the properties of core\u2013shell pellets prepared with zeolitic materials (a.k.a. \u2019membrane-encapsulated catalysts\u2019), which arise from the component-specific permeability of zeolites, such as protection against catalyst poisons, increased sintering resistance and shape-selectivity [13]. However, if these component-specific permeabilities are not required less expensive materials can be used. In this case, the selectivity of the catalyst pellets does not depend on the material properties at all, if the diffusion rate through the inert shell is rate-determining, but rather on the properties of the reacting components. Furthermore, zeolitic core\u2013shell materials are often manufactured at sub-millimeter scale, which is too small for application in industrial fixed-bed reactors, due to the high pressure loss involved. However, so-called \u2019egg-shell\u2019 catalyst pellets are frequently used in industrial application. These consist of an inert, sometimes even non-porous core surrounded by a catalytically active shell. A typical application is the oxidation of o-xylene to phthalic anhydride [14]. By employing the \u2019egg-shell\u2019 concept, internal mass transport resistances in the catalyst pellets are reduced and thus the often cost-intensive active material is used more effectively in the reactor.Capece & Dave [15] presented an approach to prepare coated catalyst pellets at lab-scale by fluidized-bed coating. In this process, pellets are fluidized in a gas stream, while a suspension is sprayed onto them in a fluidization chamber. While the liquid suspension evaporates, a solid layer forms around the substrate pellets. The procedure is subject to a complex interplay of material properties of the substrate pellets, the suspension, as well as different process conditions (e.g., spray rate, gas temperature, and velocity) and requires extensive experimental know-how. Werner et al. [16] summarize several fundamental phenomena. After coating, a calcination step is required for removing binding agents, which adds an additional challenge. The applied coating might crumble off, delaminate, or even tear the pellets apart [17]. Nevertheless, a successful procedure allows for coating pellets of various shapes and sizes with controllable coating thickness according to the demands of the catalytic process. In addition, fluidized bed-coating is established at industrial scale and has proven to be suitable to prepare the aforementioned \u2019egg-shell\u2019 catalyst pellets [18].The aim of this work is twofold. First, the interplay of mass transport through the inert shell and chemical equilibrium of a multi-component multi-reaction system is investigated based on first physical principles. Based on this analysis, the prediction of the influence of an inert shell on the activity and selectivity of the catalyst pellets is possible. Due to its high exothermicity and use as a \n\n\n\nCO\n\n\n2\n\n\n\n neutral (or even negative) fuel source, \n\n\n\nCO\n\n\n2\n\n\n\n methanation is employed as case study. Second, industrial \n\nNi\n/\n\n\nAl\n\n\n2\n\n\n\n\nO\n\n\n3\n\n\n\n methanation catalyst pellets are coated with an inert shell at kilogram-scale in a fluidized-bed coating apparatus. The obtained catalyst pellets are characterized via Dynamic Image Analysis, Scanning Electron Microscopy (SEM) and X-ray Computed Tomography (XCT), in order to determine the structure and integrity of the shell. In the case of hard X-ray tomography, such analysis is non-invasive and can cover large fields of view, therefore providing a representative interpretation of catalyst structure. Finally, the catalytic activity and selectivity of the catalyst pellets are investigated with respect to their dependence on temperature and compared to pellets without inert shell and to crushed catalyst pellets.To validate the model based results, spherical \n\nNi\n/\n\n\nAl\n\n\n2\n\n\n\n\nO\n\n\n3\n\n\n\n catalyst pellets (SPP2080-IMRC, [19]) are coated with an inert shell, as schematically shown in Fig. 2\n. For this purpose an aluminum oxide suspension is prepared. At first, polyvinyl alcohol (PVA, Mowiol(R) 8\u201388, Kuraray Europe GmbH) is stirred into distilled water at 343\u00a0K. After complete dissolution of PVA, pseudo-boehmite powder (Disperal P2W (R), Sasol Germany GmbH) is added and vigorously stirred for 30\u00a0min. Subsequently \n\n\u03b1\n\n-alumina powder (1.65 \n\n\u03bc\nm\n\n, BA-2, xtra GmbH) is added and stirred for another 90\u00a0min. In total, the mass fractions in the suspension are 5 % pseudo-boehmite powder, 10 % \n\n\u03b1\n\n-alumina powder, 1.5 % polyvinyl alcohol and 83.5 % distilled water.As the catalysts pellets\u2019 availability is limited, 0.15\u00a0kg thereof is diluted with 1.35\u00a0kg inert \n\n\u03b3\n\n-alumina pellets (2.5\u00a0mm, Sasol Germany GmbH) and put into the fluidization chamber of a fluidized bed coating pilot plant. The suspension is dosed into the fluidization chamber of a pilot plant by a bottom-spray two-fluid nozzle (Mod. 940, D\u00fcsen-Schlick GmbH, Germany). A peristaltic pump conveys the solution from a tank to the nozzle. The cylindrical fluidization chamber (inner diameter 200\u00a0mm) is made of temperature-resistant borosilicate glass. Additionally, ambient air is sucked in by a pressure blower and heated up, before entering the fluidized bed chamber through a perforated disk. After passing the fluidized bed, the air enters a calming zone and overspray particles are separated via a cyclone and a filter. Every 10\u00a0min samples are taken and the coated catalyst pellets are separated from the inert pellets into ceramic dishes for calcination in a furnace. The furnace is heated from ambient conditions to 823\u00a0K with a heating rate of 1\u00a0K/min, to remove the organic binder and to calcine the applied pseudo-boehmite. The temperature was held for 3.5\u00a0h and subsequently the pellets were cooled down to ambient temperature in the closed furnace. Samples taken after 0\u00a0min (calcined catalyst pellets without coating), 10\u00a0min, 30\u00a0min, and 50\u00a0min process time were then investigated in detail.A pellet imaging system CAMSIZER\u00ae(Retsch Technology) was used to quantify the size of the coated pellets. With this equipment several parameters can be measured for an arbitrary pellet collective of \n\n\n\nd\n\n\nP\n\n\n\nfrom\n\n20\n\n\u03bcm\n\n to 30\u00a0mm at the same time. The principle of dynamic image analysis according to ISO-13322\u20131 and \u22122 is applied. The sample is placed on a vibrating chute via the feed hopper. In the chute they are separated and subsequently fall through a camera field to be measured (two cameras are available, basic and zoom camera). The cameras binarize the captured shadow and calculate several parameters. Depending on the pellet shape, a different pellet diameter can be used as the basis for displaying the pellet size distribution. In this work, the \n\n\n\nd\n\n\narea\n\n\n\n-mode was chosen (50 measurements per second with both cameras), according to which the respective apparent catalyst pellet volume is calculated.Samples for cross-sections are embedded in transparent epoxy resin, ground manually under watercooling (grit 180 to grit 2500), polished semiautomatically using polycrystalline diamond suspension (3 \n\n\u03bc\nm\n\n) and water-based lubricant for 8\u00a0min at 15\u00a0N and finished semiautomatically using alumina suspension (0.06 \n\n\u03bc\nm\n\n) for 3\u00a0min at 15\u00a0N. During preparation, the height of the samples is measured using a Nikon Digimicro MS-11C to ensure a centrical surface for SEM investigations. After preparation, samples are sputter-coated with gold to prevent charge build-up.SEM analyses are performed using a FEI Scios DualBeam (ThermoFisher Scientific, Waltham, MA, USA) microscope equipped with a TEAM Trident system (EDAX, AMETEK GmbH, Weiterstadt, Germany). Secondary electron (SE) and backscattered electron (BSE) contrast are used to image topography and microstructure. EDS is performed for integral and local analyses of the chemical composition.X-ray computed tomography (XCT) measurements were carried out using a Zeiss Xradia Versa 520 X-ray microscope (Pleasanton, United States). Selected whole catalyst pellets were scanned with a 4X objective lens in binning 2 mode using a tungsten X-ray source. Measurements were performed at 40\u00a0kV and 76 \n\n\u03bc\nA\n\n using a low energy filter to optimize transmission and signal to noise ratio. The chosen setting provided an optical magnification of 3.95 and voxel size of 2.85 \n\n\u03bc\nm\n\n. 2041 projections were acquired over an angular range of 0 to 360 \u00b0 with an exposure time of each 1000\u00a0ms. The total measurement time per sample was about 2\u00a0h. Tomographic reconstructions were performed with the commercial software package Zeiss XMReconstructor, using a filtered back-projection type algorithm. The tomograms were corrected for beam hardening. Image analysis of the tomography data was performed with Avizo v.9.7.0 (Thermo Fisher Scientific) as discussed in detail in SI D.2.To perform catalytic activity measurements, three catalyst pellet spheres are placed into a quartz glass tube (\n\n\n\nd\n\n\ntube\n\n\n=\n8\n\nmm\n\n), with each sphere separated by a quartz glass bead (2.5\u00a0mm diameter). The spheres are fixed with quartz glass wool from each side and 0.5\u00a0g silicon carbide is placed upstream of the catalyst spheres, to ensure isothermal and uniformly distributed gas flow. The silicon carbide is also kept in place by quartz glass wool. Type K thermocouples are placed before and behind the packing. The latter is considered as reaction temperature. The setup is also used with powder of the calcined SPP2080-IMRC catalyst (415 \u2013 500 \n\n\u03bc\nm\n\n sieve fraction) of crushed catalyst spheres, which is diluted with a 1:9 ratio in silicon carbide powder.After the glass tube is placed into a furnace and sealed, gases (\n\n\n\nCO\n\n\n2\n\n\n\n 3.0, \n\n\n\nH\n\n\n2\n\n\n\n 5.0, \n\nHe\n\n 5.0, Westfalen AG) are supplied via mass flow controllers (El-Flow\u00aeSelect, Bronkhorst Deutschland Nord GmbH). The product gas is cooled down to 276\u00a0K to condense water and a constant flow of 15 Nml/min nitrogen (\n\n\n\nN\n\n\n2\n\n\n\n 5.0, Westfalen AG) is added as internal standard. Potentially remaining water is separated with a membrane, before analyzing the product gases using gas chromatography (490 Micro GC System, Agilent Technologies, Inc.).Before catalytic activity measurements, the catalyst is dried at a furnace temperature of 393\u00a0K with 120 Nml/min of a 1:1 mixture of \n\n\n\nH\n\n\n2\n\n\n\n and He. Afterward, the furnace temperature is increased to 673\u00a0K and the catalyst is reduced for eight hours at the same gas composition. Subsequently, the catalyst is aged at reaction conditions (\n\n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n = 20 Nml/min, \n\n\n\nF\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n = 80 Nml/min, \n\n\n\nF\n\n\nHe\n\n\n\n = 100 Nml/min, p \u00a0=\u00a01.2 bara) at 773\u00a0K for eight hours. Five product gas samples are taken at each furnace temperature, following a step change profile from 773 to 523\u00a0K in steps of 25\u00a0K. The temperature difference before and behind the packing was below 7\u00a0K and the furnace temperature is up to 15\u00a0K higher than the reaction temperature. The carbon balance was closed to more than 99 % in all cases.In the first section of the results, the reaction rates of catalyst pellets are derived in the presence of mass transport limitation through an inert shell. Based on this, the effect of an inert shell on the selectivity of core\u2013shell pellets is discussed using the \n\n\n\nCO\n\n\n2\n\n\n\n methanation system as example. In the second section, the fluidized-bed coating results of industrial \n\nNi\n/\n\n\nAl\n\n\n2\n\n\n\n\nO\n\n\n3\n\n\n\n methanation catalyst pellets with an alumina shell are demonstrated. A detailed characterization of the obtained alumina shell based on XCT analyses is presented in the third section. The fourth section deals with the analysis of catalytic activity measurements, which are related to the model-based predictions from the first section and the texture data from section two and three.The activity and the selectivity of a catalyst pellet is determined by calculating the fluxes across the outer pellet surface. In the following, this is done for a core\u2013shell catalyst pellet at the limit of very fast reaction rates in the pellet cores in a simplified manner with negligible temperature gradients at steady-state. The presence of a very fast reaction rate in the context of this work is discussed in SI A. The procedure can be extended to more complex cases (e.g., non-negligible temperature gradients in the catalyst pellet, complex pellet geometries, non-negligible mass transport through the gas boundary layer) if necessary.If the shell is very thin, its curvature can be neglected and slab geometry may be assumed. Following Fick\u2019s first law for an ideal gas, the flux of a component \n\ni\n\u2208\n\n\n\n1\n,\n.\n.\n,\nC\n\n\n\n\n through a shell of thickness \n\n\u03b4\n\n is\n\n(1)\n\n\n\n\nN\n\n\ni\n\n\n=\n-\n\n\n\n\nD\n\n\ni\n\n\n\n\nRT\n\n\n\n\n\n\ndp\n\n\ni\n\n\n\n\ndr\n\n\n\u2248\n\n\n\n\nD\n\n\ni\n\n\n\n\nRT\n\n\n\n\n\n\np\n\n\ni\n,\ncore\n\n\n-\n\n\np\n\n\ni\n,\nbulk\n\n\n\n\n\u03b4\n\n\n.\n\n\n\nHence, to calculate the fluxes of all components through the inert shell, the partial pressures at the interface between catalyst pellet core and shell have to be determined. In the presence of fast reaction rates, the chemical composition at the core\u2013shell interface approaches the equilibrium composition (\n\n\n\np\n\n\ni\n,\ncore\n\n\n\u2248\n\n\np\n\n\ni\n,\neq\n\n\n\n). Consequently, the equilibrium condition holds for each linearly independent reaction \n\nj\n\u2208\n\n\n\n1\n,\n.\n.\n,\nR\n\n\n\n\n.\n\n(2)\n\n\n\n\nK\n\n\nj\n\n\n=\n\n\n\n\u220f\n\n\ni\n=\n1\n\n\nC\n\n\n\n\n\np\n\n\ni\n,\neq\n\n\n\n\n\u03bd\n\n\ni\n,\nj\n\n\n\n\n\n\n\nThe number of components is typically larger than the number of linearly independent reactions, and thus the equation system has to be supplemented by \n\nC\n-\nR\n\n equations. These are the stoichiometric relations, which express the mass conservation of chemical reactions [20\u201322]. Accordingly, the C fluxes given by Eq. 1 are related to R potentials \n\n\n\n\u03be\n\n\nj\n\n\n\n, called the extent of reaction.\n\n(3)\n\n\n\n\nN\n\n\ni\n\n\n=\n\n\n\n\u2211\n\n\nj\n=\n1\n\n\nR\n\n\n\n\n\n\u03bd\n\n\ni\n,\nj\n\n\n\n\nd\n\n\n\u03be\n\n\nj\n\n\n\n\ndr\n\n\n\n\n\nInserting Eq. 1 and assuming the diffusion coefficient independent of the composition (e.g., in the Knudsen diffusion regime), these can be integrated from bulk conditions to equilibrium conditions. With \n\n\n\n\u03be\n\n\nj\n,\nbulk\n\n\n=\n0\n\n, the result reads\n\n(4)\n\n\n\n\nD\n\n\ni\n\n\n\n\n(\n\n\np\n\n\ni\n,\neq\n\n\n-\n\n\np\n\n\ni\n,\nbulk\n\n\n)\n\n\nRT\n\n\n=\n\n\n\n\u2211\n\n\nj\n=\n1\n\n\nR\n\n\n\n\n\n\u03bd\n\n\ni\n,\nj\n\n\n\n\n\u03be\n\n\nj\n,\neq\n\n\n.\n\n\n\nEvidently, the extent of reaction \n\n\n\n\u03be\n\n\nj\n\n\n\n is defined in a similar manner as the product yield at reactor scale, but modified by the components\u2019 diffusion coefficient. Furthermore, the extent of reaction offers a convenient opportunity to calculate the pellet reaction rates with Gau\u00df\u2019 theorem\n\n(5)\n\n\n\n\nr\n\n\neff\n,\nj\n\n\n=\n\n\n\n\u222b\n\n\n\nV\n\n\npellet\n\n\n\n\n\n\nr\n\n\nj\n\n\ndV\n\n\n\n\nV\n\n\npellet\n\n\n\n\n=\n\n\n\n\u222b\n\n\n\nA\n\n\npellet\n\n\n\n\n\n\nd\n\n\n\u03be\n\n\nj\n\n\n\n\ndr\n\n\ndA\n\n\n\n\nV\n\n\npellet\n\n\n\n\n\u2248\n\n\n\n\nA\n\n\npellet\n\n\n\n\n\n\nV\n\n\npellet\n\n\n\n\n\n\n\n\n\u03be\n\n\nj\n\n\n\n\n\u03b4\n\n\n,\n\n\n\nfrom which all other catalyst performance measures, such as activity and selectivity, can be derived.Solving the equation system analytically is only possible for simple systems, as demonstrated in SI B. In general, numerical solution techniques are required, due to the non-linear nature of the equilibrium conditions (Eq. 2). For this reason, it is illustrative to consider a specific example such as carbon dioxide methanation (\n\n\n\nCO\n\n\n2\n\n\nM\n\n) with the reverse water gas shift reaction (RWGS) as side reaction, as shown in Fig. 3\n. Carbon monoxide methanation (COM) is a linear combination of \n\n\n\nCO\n\n\n2\n\n\nM\n\n and RWGS. Thus, two equilibrium conditions determine the system.\n\n(6)\n\n\n\n\nK\n\n\n\n\nCO\n\n\n2\n\n\nM\n\n\n(\nT\n)\n=\n\n\n\n\np\n\n\n\n\nCH\n\n\n4\n\n\n,\neq\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n,\neq\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n,\neq\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n,\neq\n\n\n4\n\n\n\n\n\n\n\n\n\n\n(7)\n\n\n\n\nK\n\n\nRWGS\n\n\n(\nT\n)\n=\n\n\n\n\np\n\n\nCO\n,\neq\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n,\neq\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n,\neq\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n,\neq\n\n\n\n\n\n\n\n\n\n\n\n\nK\n\n\n\n\nCO\n\n\n2\n\n\nM\n\n\n\n and \n\n\n\nK\n\n\nRWGS\n\n\n\n are calculated from the equilibrium constants of the steam methane reforming reaction and the water gas shift reaction taken from literature, as given in SI C.The results for a 4:1 mixture of hydrogen and carbon dioxide at 1\u00a0bar are shown in Fig. 4\n (a) at the limit of Knudsen diffusion, where diffusion coefficients differ simply by the square root of their molar masses. Furthermore, they are compared to the hypothetical case where all diffusion coefficients are the same (Fig. 4 (b)), as in this case, the mass transport through the inert shell does not shift the equilibrium partial pressures and the equilibrium state corresponds to that of the surrounding gas bulk.From this comparison it is evident, that the mass transport through the inert shell influences the equilibrium state in the active core significantly. Two limiting cases, whether \n\n\n\nCO\n\n\n2\n\n\nM\n\n or RWGS is preferred can be distinguished. At high temperatures, RWGS is preferred and a significant drop in carbon dioxide partial pressure is observed, whereas the hydrogen partial pressure is almost the same as in the gas bulk. The drop of hydrogen partial pressure is only about a fifth of what is expected according to the RWGS reactions\u2019 stoichiometry, due the faster diffusion of hydrogen compared to carbon dioxide. In turn carbon monoxide and water partial pressures build up, also not according to the stoichiometry of the reaction, but according to Eq. 4 with a ratio of 1.25. Hence, the total pressure in the catalyst pellet core does not correspond to the bulk pressure. In fact, a slight overpressure is present in the catalyst pellets, which is the opposite of what might be expected for an equimolar reaction.At low temperatures \n\n\n\nCO\n\n\n2\n\n\nM\n\n dominates. In this case hydrogen and carbon dioxide are present in a stoichiometric ratio with respect to \n\n\n\nCO\n\n\n2\n\n\nM\n\n in the gas bulk, with almost complete carbon dioxide consumption in the pellet core. However, hydrogen is again present in significant amounts in the catalyst pellet core, as it diffuses much quicker through the inert shell. The present hydrogen surplus is beneficial with regard to possible coke formation, which is not expected to happen in the presence of low \n\n\n\nCO\n\n\n2\n\n\n/\n\n\nH\n\n\n2\n\n\n\n ratios, as discussed by Gao et al. [23]. Furthermore, methane and water partial pressures build up with a ratio of 0.47. In case of \n\n\n\nCO\n\n\n2\n\n\nM\n\n, the ratio is closer to the reactions stoichiometry, as water and methane have similar molar masses. As in the case of RWGS, a slight overpressure is present in the pellet core, which is also in opposite of what is expected from a highly mole number reducing reaction.In between these limits, mass transport shifts the chemical equilibrium in favor of \n\n\n\nCO\n\n\n2\n\n\nM\n\n. The reason for this is based on three synergistic effects:\n\n1.\nThe overstoichiometric hydrogen partial pressure in the pellet core shifts the \n\nCOM\n\n and \n\n\n\nCO\n\n\n2\n\n\nM\n\n in favor of methane.\n\n\n2.\nMethane is removed quicker from the pellet core than carbon monoxide, due to its higher Knudsen diffusion coefficient in the inert shell.\n\n\n3.\nThe total pressure in the catalyst pellets is elevated compared to the surrounding bulk pressure, which favors mole number reducing reactions.\n\nTherefore, as predicted by Le Chatelier\u2019s principle, methane becomes the preferred product and carbon monoxide formation is shifted towards higher temperatures (approx. +80\u00a0K) than in the hypothetical case of equal molar masses. This is beneficial for \n\n\n\nCO\n\n\n2\n\n\nM\n\n, where 750\u00a0K is often the upper feasible reactor temperature. The procedure presented in this section can be readily applied to other reaction systems to determine the influence of the inert shell on the catalyst pellet behavior.The overstoichiometric hydrogen partial pressure in the pellet core shifts the \n\nCOM\n\n and \n\n\n\nCO\n\n\n2\n\n\nM\n\n in favor of methane.Methane is removed quicker from the pellet core than carbon monoxide, due to its higher Knudsen diffusion coefficient in the inert shell.The total pressure in the catalyst pellets is elevated compared to the surrounding bulk pressure, which favors mole number reducing reactions.The applied fluidized-bed coating procedure results in very little overspray and uniform pellet growth, as shown in Fig. 5\n. Starting from the pellets without coating, the Sauter mean pellet diameter \n\n\n\nd\n\n\n32\n\n\n\n increased by 0.28\u00a0mm after 50\u00a0min. The subsequent calcination step led to no determinable shrinkage of the coated catalyst pellets and they remained mechanically stable.As shown in Fig. 6\n, a clear distinction between the core of the catalyst pellets and the coating is noticeable. As analyzed in ptychographic X-ray computed tomography measurements in SI D and by Weber et al. [19], the catalyst pellet core exhibits a distinct sponge-like structure, with approximately spherical macropores embedded in a mesoporous matrix. NiO is distributed as nanoparticles, and is therefore not visible at the given magnification. In the shell of the catalyst pellets, more dense particles could be observed in a less dense matrix, which might be explained by presence of \n\n\u03b1\n\n-alumina embed in pseudo-boehmite. However, clear assignment of the phases via SEM is hardly possible. Furthermore, noticeable voids are present in the shell, which are not an issue, as long as they do not directly connect the gas bulk with the pellet surface. If the latter would be the case, bypassing of the bulk gas through the shell would become possible, and the above described effects might not be present. For this reason, a more detailed analysis of the catalyst pellets is done via XCT.The XCT volume renderings of the four scanned catalyst pellets (after 0, 10, 30, and 50\u00a0min coating duration) are shown in Fig. 7\n(a,d,g,j), respectively, with isotropic voxel sizes of 2.85 \n\n\u03bc\nm\n\n. The obtained XCT were segmented into different labels for further quantitative image analysis. The retrieved labels in Fig. 7 (a,d,g,j) represent the core of the catalyst pellet (gray) with the core-pores (green) as well as the shell (blue) with the shell-pores (orange).The resolution of the XCT is not sufficient for a full analysis of the porosity as shown previously in [19]. However, in the present case it can provide a qualitative measure on the differences between the core and shell and their respective contribution to the overall resolved porosity. As the voxel size and measurement parameters are identical for each tomogram, this comparison is possible and observed differences can be carefully discussed. Furthermore, larger voids in the shell as indicated by the SEM images (Fig. 6) can be identified. The porosity distributions of the XCT depending on the \n\n\n\nd\n\n\neq\n\n\n\n (equivalent spherical pore diameter) of the detected pores are shown in Fig. 7 (b,e,h,k) for each coating time and the obtained porosity and mean \n\n\n\nd\n\n\neq\n\n\n\n values for the XCT are summarized in Table 1\n.In particular, the contribution of the shell porosity to the overall resolved porosity \n\n\n\n\u03b5\n\n\ntot\n\n\n\n is increasing with longer coating time, while the observed \n\n\n\nd\n\n\neq\n,\nshell\n\n\n\n of the pores in the shell are not changing significantly. In the distribution of the \n\n\u03b5\n\n weighted \n\n\n\nd\n\n\neq\n\n\n\n depending on the distance to the catalyst pellet center (\n\n\n\nd\n\n\ncenter\n\n\n\n) the two different pore labels (core-pores and shell-pores) can be readily identified. The measured \n\n\n\nd\n\n\neq\n\n\n\n for the shell-pores are showing generally larger pores in the shell compared to the core. The resolvable \n\n\n\nd\n\n\neq\n\n\n\n distribution of the core is quite homogeneous and similar for all four samples. However, the resolution of the chosen XCT method is not sufficient for a complete analysis of the catalyst pore structure, which ranges from few nm up to several \n\n\u03bc\nm\n\n and thus requires a combination of different imaging techniques as shown in [19] (see also SI D). It is rather sufficient to identify larger outliers of the macrcoporosity being present in the catalysts.In addition to a qualitative comparison of measurable porosity and pore diameters, XCT was used to analyze the thickness of the coated shell, and particularly to assess the presence of a closed shell. As shown in Fig. 6, it is also possible to determine the thickness of the shell from SEM images, however this only provides very local information limited to 2D. XCT allows for a 3D analysis of the shell as shown in Fig. 8\n for three different coating times (10, 30, and 50\u00a0min).The cuts through the XCT volumes in Fig. 8 (a-c) illustrate the increasing shell thickness with increasing coating duration. To investigate the thickness of the shell in more detail, two surfaces where computed, one for the filled catalyst core and one for exterior of the filled shell. Cuts through the rendering of the surfaces are shown in Fig. 8 (d-f). It can be observed that after 10\u00a0min coating time a closed shell was not obtained, while for 30\u00a0min coating time the thickness increased and only few voids in the shell still remained. After 50\u00a0min coating duration, a completely closed shell with increased shell thickness could be observed. Furthermore, the shortest distance of the exterior surface of the shell to the surface of the filled core was computed for each surface point. The resulting distributions of the shell thickness (\n\n\n\n\u03b4\n\n\nshell\n\n\n\n) are presented in Fig. 8 (g-i). The distributions clearly show the increased shell thickness with increasing coating time and allow to retrieve a mean \n\n\n\n\u03b4\n\n\nshell\n\n\n\n value together with its standard deviation as summarized in Table 1. The mean \n\n\n\n\u03b4\n\n\nshell\n\n\n\n increased from about 15 \n\n\u03bc\nm\n\n after 10\u00a0min, over 40 \n\n\u03bc\nm\n\n after 30\u00a0min and 105 \n\n\u03bc\nm\n\n after 50\u00a0min coating time.In summary, the XCT results allow for a quick qualitative inspection, whether a closed shell is obtained after a certain coating duration. Furthermore, precise quantitative information about the shell thickness can be retrieved, which in combination is hardly possible with any other method. The resolution of the here applied XCT method is not sufficient for a full porosity analysis, only voids larger than about 3 \n\n\u03bc\nm\n\n can be detected. A detailed study of the shell porosity is possible in future studies employing hard X-ray nanotomgraphy, which allows sub 100\u00a0nm resolution on samples that can cover the full thickness of the shell and direct retrieval of advanced pore network models for the macropores and quantitative information about the mesoporosity are available.[24].To validate the computational predictions of the influence of an inert shell on the pellet reaction rate and apparent selectivity, catalytic activity experiments have been performed in a lab-scale reactor, with results shown in Fig. 9\n. At the given conditions, detectable carbon dioxide conversion are present starting at about 523\u00a0K. The calcined catalyst powder and the calcined pellets without coating (\n\n\n\n=\n\n\n\u0302\n\n\n\n0\n\nmin\n\n coating time) show slightly decreased conversion and methane selectivity, compared to their counterparts, which have not been calcined. The results of the latter are given by Weber et al. [19].The carbon dioxide conversion of the catalyst pellets increases much slower with temperature, than the carbon dioxide conversion of the catalyst powder, which indicates the presence of mass transport limitations. The limiting component is likely carbon dioxide, due to its much larger molar mass, compared to hydrogen, even though both reactants are present in a stoichiometric ratio with regard to \n\n\n\nCO\n\n\n2\n\n\nM\n\n. Apart from that, also the methane selectivity is shifted. As hydrogen diffuses much quicker into the catalyst pellets than carbon dioxide, its partial pressure is close to that of the surrounding gas bulk. Consequently, the \n\n\n\nCO\n\n\n2\n\n\nM\n\n and the \n\nCOM\n\n equilibria (Fig. 3), which are present in the center of the catalyst pellets in the presence of mass transport limitations, are shifted towards the side of methane. Furthermore, as methane has a lower molar mass than carbon dioxide, it diffuses much quicker out of the catalyst pellet. Both effects combine to an increase of about 8 % in methane selectivity from 500\u2013550\u00a0K. The coated catalyst pellets continue these trends. The carbon dioxide conversion drops and the methane selectivity rises with increasing shell thickness, which indicates that an additional mass transport limitation is induced by the inert shell. Comparing catalyst pellets without shell to the pellets with the thickest shell at 773\u00a0K, reveals an increase in methane selectivity by 30 %.Apart from that, the mass transport limitation of the inert shell does also shift the temperature dependence of the carbon dioxide consumption rate. As shown in the Arrhenius plot (Fig. 9 (c)) the logarithm of the carbon dioxide consumption rate over the inverse temperature is a straight line for the catalyst powder, as expected in absence of transport limitations. In case of the catalyst pellets without coating, the slope is equal to that of the powder at low temperatures, but starts to decrease with rising temperature, which marks the onset of mass transport limitation. At a temperature of about 700\u00a0K, the slope is about half the value of that of the powder, which indicates the presence of internal mass transport limitation.The slope in the Arrhenius plot is further decreased for the pellet sample after 10\u00a0min coating time. However, it does not approach zero, as expected from Fig. 1. This indicates, that the pellets are operating between mass transport limitation in the active core and the inert shell. The reason for this is a very thin shell, which is not completely closed, as indicated by the XCT results in Fig. 8. In contrast, the shells of the samples after 30 and 50\u00a0min coating time are (almost) completely closed. This is also reflected in the Arrhenius plot, since for these samples the slope approaches zero at high temperatures, which indicates mass transport limitation exclusively by the inert shell.Understanding and controlling mass transport phenomena at catalyst pellet scale yields favorable properties for industrial reactor operation. For instance, in the context of carbon dioxide methanation in fixed-bed reactors, it was shown by computer-based studies, that an inert shell on the catalyst pellets yields a well-controllable heat release rate, if the diffusion of the reactants through the inert shell is rate-determining [11,12]. This, e.g., minimizes the risk of reactor runaway and allows for reliable reactor heat control even at unsteady conditions.To validate these results and to demonstrate the feasibility of large-scale production, commercial \n\nNi\n/\n\n\nAl\n\n\n2\n\n\n\n\nO\n\n\n3\n\n\n\n methanation catalyst pellets were coated with an inert alumina shell via fluidized-bed coating. After calcination, the obtained catalyst pellets were analyzed via Dynamic Images Analysis, REM and XCT. In particular, XCT revealed that a certain coating duration is required to obtain a closed shell, while the shells are generally quite homogenous.Catalytic activity experiments confirmed the computer-based predictions. The apparent activation energy of the pellets with fully closed shell is significantly decreased at high temperatures, which indicates the expected presence of mass transport limitation through the inert shell. For catalyst pellets without (fully closed) shell, these effects were not observed, which underlines the necessity of a defect-free shell for the mass transport through the inert shell to become rate-determining at high temperatures.Furthermore, the coated catalyst pellets with the thickest shell exhibit a methane selectivity, which is up to 30 % higher than that of the uncoated catalyst pellets at the presence of external mass transport limitation. As concluded by a model based analysis, this effect is rooted on in the interplay of the differing component mass transport rates through the inert shell, which in turn shift the equilibrium composition inside the catalyst pellets. Compared to carbon dioxide, hydrogen with the lower molar mass diffuses quicker through the inert shell into the active catalyst pellet core. Thus, hydrogen is excessively present in the pellet cores, which shifts the carbon dioxide methanation equilibrium and the carbon monoxide methanation equilibrium towards the side of methane. Additionally, the pressure is slightly elevated compared to bulk conditions and methane is removed quicker from the pellet core than the heavier side product carbon monoxide. In summary, according Le Chatelier\u2019s principle, methane becomes the favored product.The mathematical and experimental methods used in this work can be directly applied to other reaction systems to check whether an inert shell also has a positive effect on the selectivity towards the desired product. As a rule of thumb, the formation of the product with the higher diffusion coefficient in the inert shell becomes more preferred. It is expected, that the presented core\u2013shell catalyst pellet concept enhances reactor performance also for other challenging applications.Ronny Zimmermann, Jens Bremer, and Kai Sundmcher have patent #WO2020/234337Al pending as Inventors.This research work was conducted within the DFG Priority Program SPP2080 \u201cCatalysts and reactors under dynamic conditions for energy storage and conversion\u201d and was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) \u2212406914011. (Gef\u00f6rdert durch die Deutsche Forschungs-gemeinschaft(DFG)-406914011.) This research work was also supported by the Center of Dynamic Systems (CDS), funded by the EU-program ERDF (European Regional Development Fund).Furthermore, this project has received funding from the European Union\u2019s Horizon 2020 research and innovation programme under grant agreement No 731019 (EUSMI).The authors acknowledge the Paul Scherrer Institut, Villigen, Switzerland for provision of synchrotron radiation beamtime at beamline X12SA\u2013cSAXS of the Swiss Light Source and Ana Diaz and Mirko Holler for support during beamtime. This work was partly carried out with the support of the Karlsruhe Nano Micro Facility (KNMF), a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT), which provided access to FIB instruments via proposal 2020\u2013023-028494. The authors thank Sabine Schlabach for support during FIB sample preparation.Ronny Zimmermann is also affiliated with the International Max Planck Research School (IMPRS) for Advanced Methods in Process and Systems Engineering, Magdeburg, Germany. Generous product samples by Sasol Germany GmbH are gratefully acknowledged.Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cej.2022.140921.The following are the Supplementary data to this article:\n\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Catalyst research is concerned with synthesizing increasingly active materials, leading to safety issues at reactor scale, unless the reaction heat release is controllable. Computational studies predict that core\u2013shell pellets with catalytically active core and inert shell are beneficial for this purpose, compared to established concepts such as catalyst pellet dilution. At high temperatures, reactant diffusion through the shell becomes rate-determining, resulting in a well-controllable heat release rate, which prevents further temperature increase. Here, industrial catalyst pellets were coated in a fluidized-bed pilot plant, demonstrating large-scale production feasibility. The obtained pellets were characterized via Dynamic Image Analysis, Scanning Electron Microscopy and X-ray Computed Tomography. Conducted CO2 methanation experiments confirm the predicted trends, if the applied shell is fully closed. Furthermore, mathematical and experimental studies demonstrate, that the inert shell shifts selectivity. Based on this work, safer and yet economical reactor operation is anticipated also for other reaction systems.\n               "}
{"full_text": "\n\n\nNo data was used for the research described in the article.\n\n\nNo data was used for the research described in the article.Recently, with the development of industrial industries such as petrochemical industry, coking plant, pharmaceutical and papermaking, a large number of industrial wastewater containing high salt organic pollutants was produced [1,2]. Saline organic wastewater was characterized by the biological toxicity, high color and large concentration of organic pollutants. If this kind of wastewater was discharged directly without treatment, it would cause great pollution to the water environment and endanger the health of animals, plants and human beings [3,4]. At present, the traditional methods used for the treatment of saline organic wastewater including biological methods, physical methods and chemical methods. Biological methods were widely used for wastewater treatment, however, most microorganisms were affected by salinity in the high-salt environment limiting the treatment performance. Physical methods achieved the transformation of pollutants, but failed to degrade organic pollutants completely [5,6]. Therefore, it was urgent to find an economical and efficient technology to treat saline organic wastewater. Chemical methods could completely degrade organic pollutants, among which, advanced oxidation processes (AOPs) had the characteristics of strong oxidation ability, non-selectivity, and rapid reaction [7]. Ozone oxidation was an efficient technology used in the treatment of industrial wastewater due to its strong oxidation capacity and easy operation [8]. However, selectivity of ozone oxidation and low solubility of ozone in aqueous solution led to a low utilization rate of ozone, so the ozone oxidation alone could not completely degrade refractory organic pollutants [9]. Catalytic ozonation promoted the decomposition of O3 and produced more reactive oxygen species (ROS) such as hydroxyl radical, superoxide radical, and singlet oxygen [10\u201312].Heterogeneous catalytic ozonation could effectively avoid or reduce the loss of active components, and improve the reusability and stability of catalysts [13,14]. A variety of heterogeneous catalytic oxidation catalysts used for the treatment of refractory pollutants in wastewater, such as activated carbon (AC) [15], metal oxides (Al2O3, TiO2 and MnO2) [16,17] and metal composite materials [18]. Among these catalysts, Al2O3 was used as the catalyst support due to its large specific surface area, low cost and stable mechanical properties [19]. Zhao et\u00a0al. prepared the Mn-Cu-Ce/Al2O3 catalysts with an impregnation calcination method and the trimetal oxide catalysts were used for catalytic ozonation treatment of coal chemical wastewater (CCW). For Mn-Cu-Ce/Al2O3 catalysts, Al2O3 with the porous structure and large specific surface area were conducive to the adsorption of organic matter, and the metals were highly distributed on the surface of the Al2O3 support. In addition, the synergistic interaction of trimetallic oxides greatly enriched the catalytic active center and improved the catalytic performance. Compared with ozonation alone, the removal rate of CCW was increased by 31.6% after the addition of Mn-Cu-Ce /Al2O3 catalysts [20].Wei et\u00a0al. designed and synthesized a highly efficient catalysts (CuCo/NiCAF) with the core-multi-shell structure by loading Cu-Co bimetal on Al2O3. Compared with the Al2O3 support, the treatment performance of the synthesized catalysts was significantly improved after metal loading, that was, the removal rates of total TOC were 86.7% for CuCo/NiCAF and 48.0% for \u03b3-Al2O3\n[21]. ZSM5 zeolites loaded with metallic oxides (Ce, Fe, or Mn) were used to remove the nitrobenzene from wastewater, and the load of Ce, Fe, or Mn oxides increased the catalytic performance comparing with ZSM5 zeolites alone [22]. Transition metals (such as Fe, Cu, Mo and Co) were used to modify the catalysts, however, it was inevitable that a small amount of metal ions leaching into the wastewater during the process of catalytic ozonation when the metals were directly loaded on the pure support [23,24]. Therefore, it was very important to enhance the interaction between metal and support, reduce the leaching of metal ions, and improve the stability of catalysts. Xu et\u00a0al. prepared an amino functionalization catalysts (MnO2\nNH2-GO) enhanced the bridging covalent bond between the oxygen groups of MnO2 and GO, preventing the uncoupling of GO and MnO2. MnO2\nNH2-GO showed a higher catalytic performance and more stable performance than MnO2-GO in the process of catalytic ozonation [25]. Yang et\u00a0al. prepared the CuO/SiO2 catalysts through the atomic layer deposition, which significantly improved the stability of CuO/SiO2 catalysts during the catalytic ozonation treatment of organic matter in wastewater [26].It was important to find a nontoxic and efficient metal for catalytic oxidation process. Calcium-based catalysts showed prominent advantages in the green environmental protection. Calcium was almost pollution-free to the environment and avoided the problem of secondary pollution to the water environment after ozone oxidation. Secondly, calcium as a strong base site was conducive to the decomposition of ozone into reactive oxygen species and showed a good catalytic activity to degrade organic pollutants. Liao et\u00a0al. used CaO as a heterogeneous catalyst to ozonize nitrotoluene wastewater, and the results showed that CaO could promote the decomposition of ozone to produce more \u2022OH [27]. Besides, Hsu et\u00a0al. used CaO as the ozone catalysts for phenol removal and found that CaO could promote the ozonation process effectively removing phenol [28]. However, there is little reports about the combination of CaO active substance with catalysts support, and the stability of calcium-based catalysts should been attached more attention.Pectin is mainly composed of D-galacturonic acid (GalA) and extracted from plant cell walls, showing the properties of nontoxic and biocompatible. Besides, it has abundant electron-rich functional groups such as carboxyl and hydroxyl groups, and shows a strong affinity for metal ions (Affinity of metal ions to pectin is in the following order: Ca2+\u2248 Cu2+\u2248 Zn2+\u2248 Cr3+> Ni2+ > Pb2+ > Cd2+) [29,30]. The strong interaction between GalA in pectin and Ca2+ formed a three-dimensional network of cross-linked pectin molecules [31]. Shao et\u00a0al. prepared a green adsorbent of chitosan-pectin gel pellets in alkaline solution, and used the adsorbent to remove heavy metals in water [32]. The addition of pectin not only formed a stable porous structure, but also introduced a large number of carboxyl functional groups. So it was conducive to increasing the specific surface area and active adsorption sites. Cu(II), Cd(II), Hg(II) and Pb(II) were effectively adsorbed by chitosan/pectin gel beads, and the maximum adsorption capacities were 169.4, 177.6, 208.5 and 266.5\u00a0mg/g, respectively. Wang et\u00a0al. used Ca2+ as the cross-linking agent and prepared pectin-Ca microspheres through the interaction between Ca2+ and pectin [33]. By the modification of microspheres surface, a new adsorbent of pectin/poly (M-phenylenediamine) microspheres was prepared, and the adsorption performance of prepared adsorbent for Pb2+ was significantly improved. Pectin was mainly used for adsorbent preparation, but rarely used for the preparation of catalysts.In this work, the surface of Al2O3 carrier was coated with pectin, and calcium was loaded as the metal active substance. After calcination, a new green and efficient catalyst (Al2O3-PEC-CaxOy) was prepared. The introduction of pectin made Ca2+ more firmly loaded on the support, besides, a porous carbon layer on the surface of the Al2O3 carrier was formed after pectin calcination. The influence of preparation conditions such as pectin content, Ca2+ concentration, calcination temperature, and calcination time on the catalytic performance were investigated and optimized. Besides, the catalysts prepared under the optimized conditions were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and specific surface area pore analyzer (BET). The influence of operation conditions on removal rate of COD during the heterogeneous catalytic ozonation process were optimized. The generation of reactive oxygen species during the catalytic oxidation process was analyzed by the electron paramagnetic resonance technique and quenching test. Moreover, the reusability and stability of the catalysts were evaluated. Finally, the catalytic mechanism of treating saline organic wastewater by heterogeneous catalytic ozonation with Al2O3-PEC-CaxOy as catalysts was revealed.Al2O3 with a particle size of 3\u20135\u00a0mm and calcium chloride (analytical reagent grade (AR)) were purchased from Sinopharm Chemical Reagent Co., LTD. (Beijing, China). Pectin (CAS: 9000\u201365\u20135) was purchased from Beijing J&K Scientific Co., LTD. Quencher L-histidine (\u226599%) was purchased from Shanghai Aladin Biochemical Technology Co., LTD. P-benquinone (p-BQ, 99%) and sodium bicarbonate (NaHCO3, \u226598%) were purchased from Shanghai Maclin Biochemical Technology Co., LTD. Phosphate buffer (0.2\u00a0mol/L) was purchased from Shanghai Yuanye Biotechnology Co., LTD. All the chemicals were used without any further purification and the aqueous solutions were prepared with ultrapure water producing by UK ultrapure water machine (18.2 M\u03a9). The detailed information about industrial wastewater before treatment in this work was shown in Table S1.Al2O3 carrier was impregnated in pectin solution and oscillated at 25\u00a0\u00b0C for 6\u00a0h to obtain the modified alumina carrier (Al2O3-PEC). The modified Al2O3-PEC was impregnated in calcium chloride solution, oscillated at 25\u00a0\u00b0C for 6\u00a0h and then kept for 12\u00a0h. Subsequently, the solid was placed in an oven and dried at 60\u00a0\u00b0C for 6\u00a0h to obtain the modified Al2O3-PEC-Ca2+. Al2O3-PEC-CaxOy catalysts were prepared by calcination of the solids at different temperatures (500\u20131000\u00a0\u00b0C) for 2\u00a0h under argon atmosphere with a heating rate of 5\u00a0\u00b0C/min. In conclusion, the surface of Al2O3 carrier was coated with pectin, and then the coated carrier was modified with calcium metal. The preparation process of the Al2O3-PEC-CaxOy catalysts was shown in Fig.\u00a01\n.SEM (ZEISS GeminiSEM 300, Germany) and TEM (JEOL JEM-F200, Japan) were used to analyze the surface morphology of the catalysts. The surface element composition and valence of the catalysts were determined by X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, USA). The pore size, surface area and pore volume of the catalysts were determined by an automatic specific surface and porosity analyzer (Micromeritics ASAP 2460, USA) using N2 adsorption-desorption isotherm at 77\u00a0K. The crystal structure of the catalysts was analyzed by Japanese Rigaku Ultima IV X-ray diffractometer (XRD) with a scanning angle range of 10\u00b0\u201380\u00b0 and a scanning speed of 5\u00b0/min. The surface carbon profiles of the catalysts were characterized by Raman spectrometer (Horiba LabRAM HR Evolution, Japan). The reactive oxygen species generated during the catalytic process were analyzed by electron spin resonance spectroscopy (EPR) using Bruker EMXnano EPR spectrometer. The zero charge pH point (pHpzc) of the catalysts was detected using a Zeta potential detector (Malvern Zetasizer Nano ZS ZEN3600, UK).The catalytic ozonation system was composed of oxygen cylinder, ozone concentration detector, ozone generator, ozone catalytic reactor and absorption device of tail gas, as shown in Fig.\u00a02\n. Ozone was produced by ozone generator (Beijing Tonglin ozone) with pure oxygen. The ozone concentration at the outlet was controlled by adjusting the power of ozone generator. The catalytic ozone system was carried out in a self-made cylindrical tubular reactor, in which 50\u00a0mL wastewater was injected through the top of the reactor and filled with spherical catalysts. Ozone was injected into the solution through a microporous titanium aerator (0.22\u00a0\u03bcm) at the bottom of a self-designed cylindrical tubular reactor. The flow rate of ozone was 30\u00a0mL/min and the concentration of ozone was 4\u201320\u00a0mg/L. Absorption device of tail gas was carried out using KI solution (20%). The samples were filtered through a filter (0.45\u00a0\u03bcm) for quantitative analysis of COD. As comparison, the adsorption experiments of Al2O3-PEC-CaxOy catalysts were carried out with the same experimental setup in the absence of ozone. H2SO4 and NaOH solutions were used to adjust the initial pH value of wastewater from 2 to 12. Measurement of chemical oxygen demand (COD) was carried out by a UV/VIS spectrophotometer (DR5000, USA).Effects of pectin content, concentration of calcium ion, calcination temperature and calcination time on the preparation of Al2O3-PEC-CaxOy catalysts were investigated, and the optimization of preparation conditions for the Al2O3-PEC-CaxOy were shown in Fig.\u00a03\n.Effect of pectin content on the catalytic performance of the Al2O3-PEC-CaxOy was shown in Fig.\u00a03(a). The removal rate of COD was first increased and then decreased with the increase of pectin content. The removal rate of COD was reached to 59% when the amount of pectin content was increased to 2.5% w/v, but the removal rate of COD was decreased when the pectin content was higher than 2.5% w/v. The introduction of pectin made more Ca2+ firmly loaded on the support, and a porous carbon layer on the surface of the Al2O3 carrier was formed after the calcination of pectin. So the increase of pectin content was benefit for the treatment of saline organic wastewater. However, a thick carbon layer was formed when the pectin was coated on the surface of Al2O3 carrier, so the specific surface area of the Al2O3-PEC-CaxOy catalysts was decreased with the further increase of pectin content [34]. In addition, the viscosity of solution was increased with the increase of pectin, which blocked the channel of Al2O3 carrier and eventually led to the removal rate of COD decreasing. Therefore, the pectin content of 2.5% w/v was selected as the optimal condition for Al2O3-PEC-CaxOy catalysts preparation.During the catalytic ozonation process, metal oxides were used as active sites to catalyze ozone and increase the generation of reactive oxygen species. Insufficient modification of metal ions led to the decrease of active site, while the excessive load caused the accumulation of metal oxides on the surface of Al2O3 carrier and ultimately reduced the catalytic activity [35,36]. The effect of Ca2+ concentration on the catalytic performance of the Al2O3-PEC-CaxOy catalysts was shown in Fig.\u00a03(b). The removal rate of COD showed a trend of increase when the Ca2+ concentration was increased to 0.3\u00a0mol/L. However, the further increase of Ca2+ concentration led to the agglomeration of metal oxides on the catalysts surface and reduced the specific surface area of the catalysts, which inhibited the improvement of the catalytic performance of Al2O3-PEC-CaxOy\n[37]. Therefore, Ca2+ concentration of 0.3\u00a0mol/L was selected as the optimal condition for Al2O3-PEC-CaxOy preparation.Calcination temperature is an important parameter to determine the catalytic performance of catalysts. Effect of calcination temperature (500\u20131000\u00a0\u00b0C) on the catalysts performance was investigated (Fig.\u00a03(c)). The catalytic performance for the prepared catalysts was lower when the calcination temperature was 500\u00a0\u00b0C. Pectin coated on the catalysts surface could not be carbonized to form the carbon layer when the calcination temperature was 500 \u00b0C, and the color of the catalysts was yellow-brown (Fig. S1). In addition, the pectin coating entered the wastewater making the increase of organic pollutants in the wastewater during the process of catalytic oxidation. The removal rate of COD was increased obviously with the increase of calcination temperature. Meanwhile, the Al2O3-PECCaxOy catalysts had the highest catalytic performance when the calcination temperature was 800\u00a0\u00b0C. The formation of carbon layer was benefit for improving the treatment performance. The removal rate of COD was decreased when the calcination temperature was further increase. The specific surface area and pores of the catalysts prepared at different calcination temperatures of 500\u20131000\u00a0\u00b0C were shown in the Table S1. The further increase of calcination temperature led to the decrease of the specific surface area of the catalysts and the aggregation of the active site [38,39]. In conclusion, the calcination temperature of 800 \u00b0C was selected as the optimal condition for Al2O3-PECCaxOy preparation.Calcination time is also an important factor affecting the performance of the prepared catalysts. Effect of calcination time (1\u20135\u00a0h) on catalytic performance of Al2O3-PECCaxOy catalysts for saline organic wastewater treatment was shown in Fig.\u00a03(d), the generation of the carbon layer on the surface of Al2O3-PEC-CaxOy catalysts was incomplete at the calcination time of 1\u00a0h, and the calcium metal oxide was not fully formed. The removal rate of COD was increased obviously when the calcination time was extended from 1\u00a0h to 2\u00a0h, but the removal rate of COD was decreased slightly with the continuous increase of calcination time. The prolongation of calcination time increased the possibility of component agglomeration, decreasing the dispersion of metal oxides [40]. Therefore, the calcination time of 2\u00a0h was selected as the optimal condition for Al2O3-PEC-CaxOy preparation.In summary, the optimal preparation conditions were obtained and the optimal conditions for Al2O3-PEC-CaxOy preparation were pectin content of 2.5% w/v, Ca2+ concentration of 0.3\u00a0mol/L, calcination temperature of 800 \u00b0C and calcination time of 2\u00a0h.Structure and morphology of the Al2O3-PEC-CaxOy catalysts prepared under the optimal conditions were analyzed by various characterization methods. The XRD patterns of Al2O3 and Al2O3-PEC-CaxOy catalysts were observed in Fig.\u00a04\n. Peaks at 2\u03b8\u00a0=\u00a019.5\u00b0, 37.4\u00b0, 45.7\u00b0 and 66.9\u00b0 were good resolution and sharp diffraction for the Al2O3-PEC-CaxOy catalysts, which were attributed to the crystal phase of Al2O3 (JCPDS 50\u20130741). The results showed that PEC coating and Ca2+ introduction did not damage the crystallization and structural integrity of the Al2O3 carrier. Besides, peaks at 2\u03b8\u00a0=\u00a032.36\u00b0, 37.4\u00b0, 45.7\u00b0, 60.74\u00b0 and 66.9\u00b0 for the Al2O3-PEC-CaxOy catalysts were significantly enhanced, which was attributed to the characteristic peaks of CaO (JCPDS99\u20130070) and CaO2 (JCPDS85\u20130514). According to the XRD results, the Ca element was successfully deposited on the surface of Al2O3-PEC-CaxOy catalysts.The morphology and structure of Al2O3 and Al2O3-PEC-CaxOy catalysts were analyzed by the SEM. As shown in Fig.\u00a05\n, comparing with the pure Al2O3 support, the modified Al2O3-PEC-CaxOy catalysts still maintained the good surface morphology and pore structure, and there was no obvious agglomeration. The results indicated that the calcium loading on the Al2O3-PEC-CaxOy catalysts was uniform. In addition, Al2O3-PEC-CaxOy catalysts with rough and porous structure were due to the pectin coating after calcination. The rough porous structure provided active sites for the catalytic oxidation process and was conducive to the adsorption of organic pollutants on the catalyst surface, which promoted the interaction between the catalysts and organic pollutants.As shown in Fig.\u00a06\n, the morphology and structure of Al2O3 and Al2O3-PEC-CaxOy catalysts were further observed by TEM. Compared with pure Al2O3, the Al2O3-PEC-CaxOy catalysts showed rough surface after pectin modification and calcium introduction, and the results showed that PEC was successfully coated on the carrier surface. In addition, the shape of Al2O3-PEC-CaxOy catalysts was regular and there was no obvious agglomeration phenomenon, indicating that the active components were uniformly dispersed. The elements distribution of C, O, and Ca for the Al2O3-PEC-CaxOy catalysts was analyzed by elemental mapping. As shown in Fig.\u00a07\n, the elements of C, O, and Ca were evenly dispersed, indicating that Ca2+ was successfully loaded on the surface of Al2O3 and dispersed evenly. Moreover, the porous carbon layer was formed after the calcination of coated pectin, which could improve the adsorption performance and catalytic performance. The characterization of catalysts before and after calcination were carried out by Raman spectroscopy (Fig. S2), it confirmed the existence of carbon layer on the surface of the Al2O3-PEC-CaxOy.The element composition and oxidation state of the Al2O3-PEC-CaxOy catalysts were further analyzed by the XPS. As shown in Fig.\u00a08\n(a), the elements peaks of C, O and Ca were obviously observed, and the atomic concentrations of these elements were determined by the XPS and listed in Table S3. The Ca 2p spectra of Al2O3-PECCaxOy catalyst were divided into two peaks at 347.5\u00a0eV and 350.8\u00a0eV, corresponding to Ca 2p 3/2 and Ca 2p 1/2 respectively. The result confirmed the formation of Ca oxides (Fig.\u00a08(b)). In addition, the presence of Ca-O bond at 530.8\u00a0eV was also found [41]. Therefore, the XPS analysis results confirmed that the Ca element was successfully loaded onto the Al2O3-PEC-CaxOy, which was consistent with the results of XRD characterization.As shown in Fig.\u00a08(c), C 1\u00a0s spectra of Al2O3-PEC-CaxOy were divided into four peaks. The divided peaks of 284.6, 286.1, 288.2 and 289.8\u00a0eV were corresponding to CC/C=C, CO-C/COH, C=O and HOC=O. The formation of COH or C=O structures at the surface of Al2O3-PECCaxOy catalyst provided more catalytic sites (especial carbonyl groups) to rapidly convert O3 into free hydroxyl radical [42]. Thus, the generated carbon layer provided active sites for O3 decomposition. As shown in Fig.\u00a08(d), O 1\u00a0s spectra of Al2O3-PECCaxOy catalyst were divided into three peaks at 530.9\u00a0eV, 532.2\u00a0eV and 533.2\u00a0eV, which were corresponding to lattice oxygen (O lat), chemisorption oxygen (O ads) and physical adsorption oxygen (O surf), respectively [43]. O lat was mainly derived from metal-oxygen bonds (Ca2+-O), which played an important role in the catalytic oxidation process.Specific surface area, pore volume and pore size of the carrier and Al2O3-PECCaxOy catalysts were shown in the Table\u00a01\n. After modification, the specific surface area of the catalysts was significantly reduced. The decrease of specific surface area of catalyst was due to the blocking of porous structure by carbon layer and metal oxide. In addition, the agglomeration of internal pores of the Al2O3-PEC-CaxOy catalyst also reduced the specific surface area of the catalysts during the high-temperature calcination. The N2 adsorption-desorption isotherm was shown in the Fig.\u00a09\n(a), isotherm of the Al2O3-PECCaxOy catalysts was the type IV and the hysteresis ring was type H3, indicating that the prepared catalysts with abundant mesoporous pores. The curve of pore size distribution was shown in the Fig.\u00a09(b). The pore size of the Al2O3-PECCaxOy catalyst was mainly located in the mesoporous region (2\u00a0nm < pore size < 50\u00a0nm), which further confirmed the existence of mesoporous structures. The generation of mesoporous structures for the prepared catalysts was benefit for the improvement of adsorption performance.In order to verify the catalytic ozonation performance of the prepared catalysts, the systems for COD removal with ozonation, adsorption and catalytic ozonation were analyzed under the same operating conditions. As shown in Fig.\u00a010\n(a), the removal rate of COD in the ozonation system was only 33%. The removal rate of COD for saline organic wastewater was slightly increased to 37.6% in the Al2O3+O3 system. The removal rate of COD was increased to 47% in the Al2O3\nCaxOy+O3 system, indicating that the calcium load played an important role in the catalytic ozonation process. The removal rate of COD was significantly increased to 49% in the Al2O3-PEC+O3 system, which indicated that the porous carbon layer formed after the calcination of coated pectin was helpful to improve the treatment performance. Meanwhile, the removal rate of COD for Al2O3-PECCaxOy catalysts was increased to 62% after further loading of Ca metal. The results indicated that the loading of Ca metal could increase the active site of the catalysts and improve the catalytic performance. The removal rate of COD was increased by 29% in the Al2O3-PEC-CaxOy+O3 system comparing with ozonation alone, indicating that the prepared catalysts had a significant performance on removing the COD from the saline organic wastewater.In order to confirm the COD removal was mainly depended on catalytic ozonation process rather than adsorption process, adsorption experiments were carried out using the prepared Al2O3-PEC-CaxOy catalysts. As shown in the Fig.\u00a010(b), the removal rate of COD reached to 10% after five times of adsorption process, indicated that the carbon layer generated on the surface of the catalysts had a good adsorption performance. The contribution of adsorption process to the removal of COD was low when the saturation adsorption was reached, so the COD removal was mainly depended on the catalytic ozonation after five times of adsorption process.The comparison of catalyst performance between the prepared Al2O3-PEC-CaxOy catalysts in this work and those reported in the literature was shown in the Table\u00a02\n. The prepared Al2O3-PEC-CaxOy catalysts in this work showed a good catalytic performance for the treatment of saline organic wastewater. In addition, most of the literature reported that the catalysts were preparation using transition metal ion as active sites. Besides, Ca2+ was almost pollution-free to the environment and avoided the problem of secondary pollution to the water environment after ozone oxidation. So the prepared Al2O3-PEC-CaxOy catalysts showed environment friendly and high efficient performances.Influence of catalysts dosage on the treatment of saline organic wastewater was shown in the Fig.\u00a011\n(a). The removal rate of COD was increased from 41% to 59% when the dosage of Al2O3-PEC-CaxOy catalysts was increased from 100 to 400\u00a0g/L. The results showed that the increase of catalysts dosage could provide more active sites and enhance the reaction between solution phase and catalyst surface, leading to the decomposition of ozone and the generation of more reactive oxygen species [52]. However, the removal rate of COD did not further increase when the catalysts dosage was further increased to 600\u00a0g/L, indicating that the excess catalysts could not further improve the removal rate of COD when the ozone content in the system was limited. The further increase of Al2O3-PEC-CaxOy catalysts caused the reduce of the available surface area of the catalysts during the treatment of saline organic wastewater. Therefore, 400\u00a0mg/L of catalysts dosage was selected as the optimal condition for the treatment of saline organic wastewater.Ozone concentration is an important factor affecting the catalytic ozonation process. As shown in the Fig.\u00a011(b), the removal rate of COD was increased from 46.3% to 59.0% when the ozone concentration was increased from 4 to 12\u00a0mg/L. The result indicated that the higher concentration of ozone was beneficial to the degradation of organic pollutants. The higher concentration of ozone not only increased the possibility of contact between ozone and organic pollutants, but also promoted the contact between ozone and the active site of catalysts. So the higher concentration of ozone accelerated the generation of reactive oxygen species, and improved the treatment performance of COD [53]. However, the removal rate of COD did not increase significantly when the ozone concentration was gradually increased to 20\u00a0mg/L. Due to the number of active sites on the catalysts surface was limited, so the utilization rate of ozone reached saturation and excessive ozone could not produce lots of reactive oxygen species. In addition, the production of high concentration of ozone needed more oxygen consumption and energy consumption, resulting in the increase of costs. Therefore, 12\u00a0mg/L of ozone concentration was selected as the optimal condition for the treatment of saline organic wastewater.Initial pH of solution directly affects the decomposition efficiency of ozone, degradation of organic pollutants and surface properties of catalysts, so the pH value is an important factor affecting the catalytic performance of catalysts [54]. The initial pH value of saline organic wastewater was adjusted from 2.0 to 10.0 by H2SO4 or NaOH solution. As shown in Fig.\u00a011(c), the removal rate of COD was increased continuously when the pH value was increased from 2.0 to 7.0, and the removal rate of COD was decreased slightly under alkaline conditions. The surface charge of the prepared catalysts plays an important role in the decomposition of O3 and the formation of \u2022OH during the process of catalytic ozonation. Most of the -OH groups on the catalysts surface were in a neutral state when the pH value of the saline organic wastewater was close to pH\npzc\n, which was beneficial to accelerate the decomposition of O3 and generate ROS [55]. As shown in Fig.\u00a011(d), the point of zero electric charge was 7.73. When the pH value of wastewater was close to the pH\npzc\n =7.73 of Al2O3-PEC-CaxOy catalysts, the removal rate of COD reached to the highest value (62%). Metal oxides were easily leached from the catalysts and the active sites were reduced at the lower pH (pH\u00a0=\u00a02.0), resulting in the lower removal rate of COD. The concentration of \u2022OH was increased and the quenching reaction between \u2022OH was enhanced with the increase of pH value of solution, which hindered the further increase of \u2022OH concentration. Therefore, the neutral condition of the saline organic wastewater was selected as the optimal condition for the treatment of saline organic wastewater.After the investigation of the operating parameters, 400\u00a0mg/L of catalysts dosage, 12\u00a0mg/L of ozone concentration, and neutral condition of the saline organic wastewater were selected as the optimal conditions for the treatment of saline organic wastewater.EPR technology was used to reveal the generated reactive oxygen species during the catalytic ozonation process [56]. The spin capture reagent of DMPO (5, 5-dimethylpyrrolidine N-oxide) was used in the EPR experiments to detect the \u2022OH and \u2022O2\n\u2212, and 2,2,6,6-tetramethylpiperidine (TEMP) was used in the EPR experiments to detect 1O2. A four-line characteristic spectral height ratio of 1:2:2:1 was observed for the DMPO-\u2022OH, indicating the formation of \u2022OH (Fig.\u00a012\n(a)). In addition, as shown in Fig.\u00a012(b), six characteristic peaks of the DMPO-\u2022O2\n\u2212 was observed. A three-line characteristic spectral height ratio of 1:1:1 was observed for the DMPO-\u2022O2\n\u2212, indicating the formation of 1O2 (Fig.\u00a012(c)). The generation of DMPO-\u2022OH, DMPO-\u2022O2\n\u2212and TEMP-1O2 had no significant signal in the ozonation system without adding catalysts. In conclusion, the generation of \u2022OH, \u2022O2\n\u2212and 1O2 promoted the degradation of organic matter in the saline organic wastewater.Contribution of generated reactive oxygen species of \u2022OH, \u2022O2\n\u2212 and 1O2 was further determined by the quenching experiments in the Al2O3-PECCaxOy+O3 system. Methyl orange (MO) was used as the simulated wastewater, bicarbonate anion (HCO3\n\u2212) was used as the scavenger for quenching \u2022OH (9.7\u00a0\u00d7\u00a0108\nM\n\u22121s\u22121), p-benzoquinone (p-BQ) was used as the scavenger for quenching \u2022O2\n\u2212 (3.5\u20137.8\u00a0\u00d7\u00a0108\nM\n\u22121s\u22121) [57], and L-histidine was used as the scavenger for quenching 1O2 (3.2\u00a0\u00d7\u00a0107\nM\n\u22121s\u22121) [30]. Radical quenching experiments were performed using the same Al2O3-PEC-CaxOy catalysts and the results were shown in Fig.\u00a012(d). The degradation of MO was inhibited in the presence of HCO3\n\u2212 (10\u00a0mM), while, the degradation of MO was relatively rapid in the presence of L-histidine in the catalytic ozonation system. Moreover, an obvious inhibition of COD removal was observed in the presence of p-BQ (10\u00a0mM).In addition, quenching experiments with different concentrations of scavengers (10\u00a0mM, 20\u00a0mM, and 50\u00a0mM) were carried out. As shown in Fig. S3, the inhibition effect on MO was significantly increased with the increasing amount of quencher. NaHCO3 had a strong affinity to Lewis acid sites on the catalysts surface, and inhibited the catalytic decomposition of O3 to \u2022OH radical. The inhibition effect on MO removal was obvious when the concentration of p-BQ scavenger was increased to 50\u00a0mM. The reason for this phenomenon was that p-BQ not only consumed \u2022O2\n\u2212, but also reacted with O3 in the catalytic oxidation system. Excessive p-BQ directly consumed O3 (2.5\u00a0\u00d7\u00a0103\nM\n\u22121s\u22121) during the catalytic oxidation process [58,59], so the decomposition of ozone in the reaction system was inhibited. These results indicated that the generation of \u2022OH, \u2022O2\n\u2212, and 1O2 were the main reactive oxygen species attributed to the organic matter removal.Some researches reported that hydroxyl on the surface of metal-based catalysts were widely regarded as the catalytic active sites. Phosphate was a strong Lewis base that could replace the hydroxyl on the surface of catalysts and reduce the number of hydroxyl groups, which hindered the interaction of O3 with the Lewis acid site of the catalyst and ultimately hindered the generation of reactive oxygen species [60]. In order to verify the contribution of hydroxyl on the surface of the catalysts, phosphate (10\u00a0mM) was used for hydroxyl quenching. As shown in Fig.\u00a012(d), the removal rate of MO was inhibited in the presence of phosphate during the catalytic ozonation process. The results indicated that the hydroxyl on the surface of the catalysts were acted as the catalytic active sites for O3 decomposition to generate \u2022OH, \u2022O2\n\u2212 and 1O2.XPS analysis of the Al2O3-PECCaxOy catalysts before and after the catalytic ozonation process was shown in Fig.\u00a013\n(a), the content of C\u2212C/C=C on the surface of the carbon layer decreased significantly (from 48.85% to 17.09%) after the catalytic ozonation process, while, the content of C=O also decreased. The C\u2212OH or C=O structure on the surface of the carbon layer provided catalytic sites for O3 decomposition in the process of catalytic oxidation [61]. There was no change for O1s in the Al2O3-PEC-CaxOy catalysts before and after catalytic oxidation (Fig.\u00a013(b)). O lat was the key to hydroxylation of metal oxides, and O lat as a strong Lewis base absorbed protons from water generating Ca-OH and promoting catalytic oxidation process.Oxygen vacancies (Ovs) of Al2O3 and Al2O3-PEC-CaxOy catalysts were detected by the EPR. As shown in Fig.\u00a013(c), a signal peak was found at g\u00a0=\u00a02.0025 for Al2O3-PEC-CaxOy catalysts, but no obvious signal was found for Al2O3 catalysts. The signal peak of g\u00a0\u2248\u00a02.00 was attributed to the electrons captured by Ovs, which indicated that the carbon layer of the synthesized catalysts containing Ovs and the result was consistent with the XPS spectrum of O 1\u00a0s. O lat generated under the reaction of Ovs and continuously supplied in the catalytic oxidation system, which maintained the stability of the catalysts and further provided the active sites for the catalysts. More Ovs could improve the efficiency of electron transfer and promote the catalytic oxidation process when O lat content was remained balanced. Therefore, Ovs accelerated the electron transfer among oxygen in the process of catalytic ozone oxidation, maintaining the catalytic performance and promoting the decomposition of O3 to generate more reactive oxygen species.Mechanism diagram of treating saline organic wastewater with Al2O3-PECCaxOy catalysts was shown in Fig.\u00a014\n. During the process of non-free radical oxidation, the adsorption of O3 and organic pollutants on the surface of the catalysts was crucial. Non-radical oxidation pathways for treatment of saline organic wastewater were divided into two categories: (1) Organic matter was adsorbed on the surface of the catalysts and oxidized by O3; (2) Indirect degradation of organic pollutants with 1O2 generated by catalytic decomposition of O3\n[62]. Meanwhile, radicals of \u2022OH and \u2022O2\n\u2212were confirmed according to the electron paramagnetic resonance and radical quenching experiments, which had a higher oxidation potential to achieve the organic matter removal.Stability and reusability of catalysts are the important factors that should be considered in the industrial applications. The used catalysts were collected from the solution and washed several times with ultrapure water for future use. As shown in Fig.\u00a015\n(a), the removal rate of COD was gradually decreased from 62% to 50.5% after five cycles. The adsorption performance of the catalysts was reduced after consecutive adsorption during the process of ozonation, which reduced the contact between the ozone and the pollutants on the catalysts surface. In addition, intermediates generated by organic pollutants oxidizing and inorganic salts in wastewater occupied the catalytic active sites, which reduced the catalytic performance after five cycles. The removal rate of COD was basically stable at about 50% after further use. In addition, Al2O3-PEC-CaxOy catalyst still maintained a good crystallinity after twenty cycles (Fig.\u00a015(b)). The XPS characterization of Al2O3-PEC-CaxOy catalysts before and after twenty cycles was shown in Fig.\u00a015(c, d), the peak value of Ca element was only changed slightly after twenty cycles. In this study, the losing of metal active sites was controlled through the cross-linking between pectin and Ca2+during the catalytic oxidation process. The surface of prepared Al2O3-PEC-CaxOy catalysts maintained a good morphology, and no obvious damages were observed (Fig.\u00a015(e, f)). The results indicated that the catalytic performance of the catalysts did not decrease significantly, and the Al2O3-PEC-CaxOy catalysts had a good catalytic stability and reusability. According to the above results, the Al2O3-PEC-CaxOy catalysts showed a good stability in the long-term application.In this work, a green and efficient catalyst (Al2O3-PEC-CaxOy) was successfully synthesized. The optimal conditions for Al2O3-PEC-CaxOy preparation were investigated and summarized as pectin content of 2.5% w/v, Ca2+ concentration of 0.3\u00a0mol/L, calcination temperature of 800 \u00b0C and calcination time of 2\u00a0h. Compared with the ozone oxidation alone (removal rate of COD was 33%), the removal rate of COD was 62% in the Al2O3-PECCaxOy+O3 system under the conditions of 400\u00a0mg/L catalyst dosage, 12\u00a0mg/L ozone concentration and neutral saline organic wastewater, which was 1.9 times of that of ozonation alone. The carbon layer formed after calcination could increase the adsorption performance of Al2O3-PEC-CaxOy catalysts and contribute to the removal of organic pollutants. In addition, the structure of C\u2212OH and C=O on the surface of the carbon layer provided catalytic sites to rapidly convert O3 to \u2022OH, \u2022O2\n\u2212 and 1O2. At the same time, the surface -OH and calcium metal oxides also provided more catalytic active sites for Al2O3-PEC-CaxOy catalysts during the process of catalytic ozonation, which promoted the decomposition of O3 and accelerated the generation of reactive oxygen species. The Al2O3-PEC-CaxOy catalysts had a good catalytic stability and reusability after twenty cycles of continuous operation, and the interaction between pectin and calcium ions could improve the stability and catalytic performance. Therefore, the result showed the Al2O3-PEC-CaxOy catalysts with a great significance for long-term practical application with the economical and effective performances.Treatment of saline organic wastewater by heterogeneous catalytic ozonation with Al2O3-PEC-CaxOy as catalystsThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (22125802 and 22108012), Natural Science Foundation of Beijing Municipality (2222017), and Fundamental Research Funds for the Central Universities (BUCTRC-202109). The authors gratefully acknowledge these grants.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2023.100447.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Al2O3-Pectin-CaxOy (Al2O3-PEC-CaxOy) was prepared as catalysts to improve the treatment of saline organic wastewater with heterogeneous catalytic ozonation. Compared with ozonation alone (33%), the removal rate of COD (62%) was significantly increased for Al2O3-PEC-CaxOy catalysts prepared under the optimized conditions. The introduction of pectin made Ca2+ firmly loaded on the support, and avoided the loss of active sites in the process of catalytic oxidation. In addition, the formation of porous carbon layer on the surface of the Al2O3 support after pectin calcination was conducive to improving the catalytic activity of the catalyst. Electron paramagnetic resonance (EPR) and radical quenching experiments showed that hydroxyl radical (\u2022OH), superoxide radical (\u2022O2\n                     \u2212) and singlet oxygen (1O2) were the reactive oxygen species (ROS) that attributed to the organic matter removal. Both free radical and non-free radical pathways were involved in the degradation of organic pollutants during the catalytic ozonation process. The removal rate of COD was only decreased slightly after twenty times of continuous operation for Al2O3-PEC-CaxOy catalysts, indicating that Al2O3-PEC-CaxOy catalysts with the good catalytic stability and reusability. It showed a great significance for long-term practical application with the economical and effective performances.\n               "}
{"full_text": "No data was used for the research described in the article.Activated CarbonAberration Corrected Scanning Transmission Electron MicroscopyAcetylacetonateAtomic Layer DepositionBorane Tert\u2013Butylamine Complex (BTB)Black Pearls\u00ae 2000Carbon\u2013carbon bondOrdered Mesoporous CarbonCO2 Reduction ReactionsCarbon NanoparticlesCross Polarized-Magic-Angle Spinning-Nuclear Magnetic ResonanceCovalent Triazine FrameworksChemical Vapor DepositionDensity Functional TheoryDry Reforming of MethaneDefective GrapheneDinuclear Heterogeneous CatalystDiffuse Reflectance Infrared Fourier Transform SpectroscopyActivation EnergyExfoliated Graphitic Carbon NitrideEnergy Dispersive X-ray SpectroscopyElectron Energy Loss SpectroscopyEthylene GlycolateEuropean Patent OfficeEley-RidealElectromagnetic Spin ResonanceExtended X-ray Absorption Fine StructureIron PhthalocyanineGas Chromatography-Mass SpectrometryGraphene NanosheetHigh Angle Annular Dark FieldHydroxyapatiteHydrogen Evolution ReactionsHollow N-doped Carbon SphereHigh-Resolution Transmission Electron MicroscopyInfraredIsolated Single Atomic SitesIncipient Wetness ImpregnationSolid-State Magic-Angle Spinning-Nuclear Magnetic ResonanceMolybdenum CarbideMetal\u2013Organic FrameworkMulti-Walled NanotubesNear Ambient Pressure X-ray Photoelectron SpectroscopyNitrogen-doped CarbonNoble MetalNuclear Magnetic ResonanceNanoparticleNanotube Arrays Supported by a Ni FoamOxygen Evolution ReactionsOxygen Reduction ReactionsOperando Raman SpectroscopyPorous Nitrogen-doped CarbonPhosphomolybdic AcidPolypyrrolePreferential Oxidation ReactionTetra(4-tert-butyl-phenyl)porphyrinato PlatinumPolyvinylpyrrolidoneSingle atomSingle Atom AlloySACs Single Atom Catalyst Single Atom CatalystsStrong Metal\u2013Support InteractionScanning Transmission MicroscopyScanning Tunneling MicroscopyTermolecular Eley-RidelTurnover FrequencyTemperature Programmed ReductionTemperature Programmed DesorptionUltra-High VacuumUnderpotential DepositionUltra VioletUltra Violet OzoneWater Gas ShiftWeight Hourly Space VelocityWorld Intellectual Property OrganizationX-ray Absorption Near Edge StructureX-ray Absorption SpectroscopyX-ray Photoelectron Spectroscopy1,3-Propanediols2-DimensionalThe first appearance of the term \u201ccatalysis\u201d can be backdated to the early 19th Century. J\u00f6ns Jacob Berzelius, who was one of the founders of modern chemistry, had successfully discovered the existence of catalytic energy [1], whereas the first-ever documented catalyst application can be traced even earlier to the alchemical era (4th century BC) [2]. Initially, industrial catalysts were mainly used in the Deacon process (the transformation of HCl gas in oxygen to H2O and Cl), sulphuric acid production, Ostwald process (production of nitric acid from ammonia), and the Haber-Bosch process (production of ammonia via nitrogen fixation). To-date, its usage has been extended further into the majority of current industrial chemical productions [3]. Evidence has shown that the development of catalysts was one of the key factors that influence the sustainability and performance of catalytic processes, in terms of economic viability, technology feasibility, and environmental impacts [4,5]. Ever since the commercial usage of catalysts has been widespread around the globe. The subsequent innovation and revolution of catalyst development have ceaselessly progressed (Fig. 1\n). This review focuses on the emerging catalyst technologies from the last decade to state-of-the-art, atom efficiency and sustainable single atom catalysts (SACs) as well as highlighting the frontiers for future research into clean energy and growth.The term SAC refers to a catalyst where individual metal atoms are isolated and dispersed across/throughout a supporting material [13]. To retain such high atomic dispersion efficiency, strong interactions between the isolated metal atoms and the coordination sites are exerted, creating a unique tunable electronic structure. The single atom-level dispersion of metal atoms on a support is not only able to maximize the atomic efficiency by offering a greater number of active sites but at the same time generate uniform and well-engineered active sites [14]. These sites can be altered to finely tune the reactivity and selectivity of chemical reactions by manipulating coordination sites (e.g., ligand tuning) and defect designs [15]. In addition, undesirable deleterious side reactions can be mitigated if requiring more than one atomic active site, due to the high dispersion and isolation [16]. These compounded features lead to a remarkable catalytic performance which has been found to outperform conventional mono/bimetallic catalysts with bulk nanoparticles. Therefore, SACs (plural of a single atom catalyst) are currently noted as \u201crising stars\u201d for the future of sustainably producing fine chemicals [17] and are often envisioned as the natural bridge between heterogeneous and homogeneous catalysis [18]. Fig. 2\n presents some of the major research contributions related to the use of single atoms as catalysts in chronological order.Although single atom sites have a long history in heterogeneous catalysis spanning over five decades, the term SAC was first introduced in 2011 by Qiao et al. [19]. This work serves as an attempt to examine whether a single atom (denoted as M1 where M is the active metal used) can provide better catalytic activity, selectivity, and stability as compared to other nanometer-sized and sub-nanometer-sized particles (i.e. clusters or atom ensembles). In general, the synthesized Pt1/FeOx SAC catalysts show greater reactivity for both CO oxidation and the Preferential Oxidation Reaction (PROX) reaction (by 2\u20133 times) than that of bulk nanoparticle-based Au/Fe2O3 catalysts, capable of providing high reactivity for CO oxidation [20,21]. In addition, the Pt1/FeOx SAC was found to be fully regenerated even after 2 cycles of thermal treatments. These findings were no doubt a bombshell to the research community, given that the commercial Pt/Al2O3 catalysts are often less favorable than Au/Fe2O3 catalysts for both CO oxidation and PROX reaction due to their higher cost. This research proved that a high atomic efficiency level catalyst is one of the feasible directions for the future of sustainable catalysis research, especially chemical reactions that require high noble metal loadings. A year later, a study conducted by Kyriakou et al. [22] found that lower activation energy (Ea) for hydrogen dissociation can be achieved by using isolated Pd atoms across a Cu(111) surface under ultra-high vacuum conditions for hydrogenation reactions. The authors used the term Single Atom Alloy (SAA) to describe this class of material. In the same year, Knurr and Weber [23] explored the theoretical design of SACs for the solvent-driven CO2 reduction process via a first-principle calculation. Lin et al. [24], on the other hand, extended the utility of SACs to the water gas shift (WGS) reaction in 2013. Both Pt1/FeOx and Ir1/FeOx catalysts showed extraordinary catalytic performance for WGS, presenting a specific rate of around 2\u20133 times higher than previous Pt/MoC and Au/CeOx catalysts. Another interesting finding was reported in the work conducted by Wang et al. [25], which investigated the selective catalytic reduction of NO in the presence of H2 with an Rh1/Co3O4 catalyst. The surface of Rh1/Co3O4 had reconstructed into RhCon/Co3O4, an SAA at 220\u00a0\u00b0C. More interestingly, this re-constructed catalyst offered superior catalytic performance, when compared with its original form. In addition to the Rh\u2013Co alloy, the catalytic activity enhancement on CO oxidation attributed to the single transition metal-atom substitution in V4O10 has also been studied based on Density Functional Theory (DFT) calculations [26]. Aside from this, an advanced fabrication technique \u201catomic layer deposition (ALD)\u201d has been discovered to design and synthesize a SAC with atomic-level control on its composition and thickness [27].Thereafter, numerous works have discovered the potential of noble metal-based SACs for various reactions, which include, but are not limited to glucose oxidation using Au1/Pd nano clusters [28], the reduction of I3\n- to I\u2212 using Pt1/FeOx [29], HCHO oxidation with Ag1/hollandite manganese oxide [30], syngas-to-C2 oxygenates conversion with Rh-based SACs [31], N-alkylation and \u03b1-alkylation using Ir1-doped polypyrrole [32] and Zn1/N-co-doped porous carbon [33], CH4 oxidation using Pt1/La2O3 (NM indicates noble metal) [34]. A major use of SACs and SAAs has been found for hydrogenation reactions, specifically, nitroarenes using Pt1/FeOx [35], 1,3-butadiene using both Pd1/graphene [36] and Pt1/m-Al2O3 [37]. Other reactions have been NO reduction with Pt1/FeOx [38], hydroformylation of olefins using Rh1/ZnO [39], oxygen evolution reaction (OER) using Rh1, Ru1, Ir1 and Pd1 supported on Co3O4 [40], and extensively for CO oxidation using various atomic catalysts such as Au1, Rh1, Pd1, Cu1, Ru1, Ti1/FeOx [41]. In addition, the study on the use of a SAA has also been extended for other reactions such as the Ullmann reaction of aryl halides using Au\u2013Pd SAA [42], the dehydrogenation of propane using Cu\u2013Pd SAA [43], the hydrogenation of C2H2 using Ag\u2013Pd SAA [44], the oxidation of formic acid using Ag\u2013Pt SAA [45], the selective hydrogenation of furfural using Pd atoms alloyed into a Cu nanoparticle supported on alumina [46] and the dehydrogenation of 4-hydroxypentanoic acid [47]. In addition to the reactions shown above, the use of single atoms has been effective for electrochemical applications, work can be found starting from 2015 and has been extensively explored from 2016 onwards. These applications encompass Oxygen Reduction Reactions (ORR) using Pt1/TiN [48], Pt1/zeolite-templated carbon [49] and have recently extended to a SAA which contains Pt1 and Co nanoparticles that co-encapsulated on N-doped graphitized carbon nanotubes [50]. Hydrogen Evolution Reactions (HER) have been studied using Pt1 over 2D g-C3N4 [51], CoP-based nanotubes [52], curved carbon supports [53], and TiO2 nanosheets modified with graphene [54]. Additionally, the electrochemical NO reduction was explored using Pt-CTF/CP [55], N2 reduction using Mo1/N-doped carbon [56] and Ru1/N-doped porous carbon [57]. The electrochemical CO2 reduction reaction (CO2RR) was investigated using various atom systems utilizing metals such as Pt1, Pd1, Cu1 and Ag1/graphene [58].Despite the astounding performance of noble metal-based SACs, the high unit price and the low abundance of noble metals are undoubtedly the key compromising factors that hamper the wide employment of SACs [59,60]. With the motivation for enhancing the sustainability of SACs deployment, a notable number of research works have been conducted to examine the potential of noble metal-free SACs. In 2013, Wang et al. [26] performed a systematic DFT calculation to examine the potential of four single-atom transition metals for CO oxidation, where three of them were non-noble metals (i.e. Sc1, Ti1, and Co1). To the best of the authors\u2019 knowledge, this is the earliest reported application of non-noble metal-based SACs. The attention on non-noble metal-based SACs has grown rapidly since 2018. Their applications have been extended to non-oxidative CH4 conversion using Fe1/SiO2 [61]; HER with Co1 on N-doped graphene [59], graphitic carbon nitride [62], N-doped graphyne [63], and Ni1 on \u03b1-SiX (X\u00a0=\u00a0N, P, As, Sb, Bi) [64]; ethylene benzene oxidation using Co1/CN [65]; electrocatalytic ethanol oxidation using a hybrid material of Pd nanoparticles and Ni1 single atom [66]; oxidative desulphurization with Cr1/multiwalled carbon nanotubes [67]; CO oxidation using Ni1 over FeOx [68], phosphorene nanosheet [69], Sc1 and Fe1 on honeycomb borophene/Al(111) heterostructure [70], and Ti1 on MXene [71]; acetylene hydration using Zn1 with S/N co-doped defective graphene supports [72]; the electrochemical CO2RR with Ni1 on N-doped porous carbon [73], graphene nanosheets [74], carbon black [75] and other ZnN4-based SACs [76]; CO2 hydrogenation with various non-noble metal-based SAC (Mn1, Fe1, Co1, Mo1) supported on graphitic carbon nitride [77]; ORR has been carried out with Co1 on defective N-doped carbon graphene [78], Fe1 supported on N-doped porous carbon [79], hierarchically structured porous carbon [80], cellulose-derived nanocarbon [81] and phosphomolybdic acid cluster [82]; and the Oxygen Evolution Reaction (OER) has been carried out on Fe, Co and Ni-based SACs on N-doped graphene [83], N-doped biomass-derived porous carbon [84] as well as \u03b3-graphyne monolayer [85]. Lastly, the production of H2O2 was found to be effective via hydrogenation routes Ni-based SACs [86]. In fact, based on the comparative studies conducted in some of these works, the non-noble metal-based SACs were found to be more attractive and preferable. For instance, Liang et al. [68] found that the molecular interaction between a Ni atom and an adsorbed CO gas molecule is weaker than CO binding irreversibly with Pt1 and Ir1 atoms. This further leads to a lower barrier in CO2 formation (i.e., enhances the CO oxidation process) under the use of Ni. A recent computational study also suggested that the Fe-, Co- and Ni-based SACs were capable of providing a comparable catalytic activity for CO oxidation as compared with the noble metal-based SACs (e.g., Pd-, Pt-, Ru- and Rh-based SACs) [87]. However, on the flip side, some studies focused on exploring strategies to combat the economic drawbacks of noble metal-based catalysts (e.g., Wang et al. [88] found that the use of monolayer WO3 has great potential to reduce the Pt noble metal usage on the catalyst fabrication).In addition to the aforementioned non-noble metals, in the 2020s, the use of rare earth elements (e.g. La, Y, Ce, and Sc) as SACs were proposed. It is anticipated that the multi-shell electrons of the rare earth metals will lead to strong adhesive bonding between the rare earth element and a support material [89]. Strong metal-support interactions are favorable as they have been found to reduce the chance of atoms aggregation, a common problem during the synthesis of SACs. In other words, it can promote scalable mass production of SACs. To-date, yttrium, scandium, and praseodymium-based SACs (Y1, Pr1 and Sc1) have been tested for N2 and CO2 reduction reactions [89,90]; while erbium SACs (Er1) have been tested for the effective photocatalytic CO2 reduction [91]. On the other hand, the impact of integrating a second metal species into a SAC has also started to get more attention from researchers. It is believed that such bimetallic catalysts can offer better catalytic performance than that of monometallic catalysts given the synergetic effect between the two metal species [90,92]. Evidently, the bimetallic Ir1Mo1/TiO2 SAC synthesized in a recent work [92], is capable of offering a greater selectivity (>96%) for a hydrogenation process, as opposed to the low selectivity of their monometallic forms (i.e., less than 40% for Ir1/TiO2 catalysts and negligible activity for Mo1/TiO2 catalysts). A similar strength of bimetallic SAC has also been discovered by Kaiser and co-workers [93]. The addition of Pt single atoms into the Au SAC can inhibit the sintering effect up to 800\u00a0K, which eventually leads to a 2-fold increment in catalysts' shelf life. The potential of such bimetallic SAC has been investigated for hydrogenation (e.g. Ir1Mo1/TiO2 catalysts [92]; Pt1Sn1/N-doped carbon [94]), HER (e.g. Ru1Pt1/N-doped carbon catalysts [95]; Ni1Co1/N-doped carbon catalysts [96]), Fenton reaction (e.g. hyaluronic acid-coated Fe1Co1/N-doped carbon catalysts [97]), acetylene hydrochlorination (e.g. Pt\u2013Au-based bimetallic SAC [93]) and dichlorination (e.g. Fe1Cu1/N-doped porous carbon catalysts [98]). Meanwhile, some studies have focused on the versatile and scalable techniques for the mass production of SACs, e.g. dry ball milling [99,100], pyrolysis [60], one-pot pyrolysis [101], incipient wetness impregnation [49], anti-Ostwald method [30], photochemical strategy [102], thermal emitting strategy [103], cascade anchoring strategy [104], surface organometallic chemistry [105], and electrochemical deposition [106]. Some of those even focus on exploring ways to realize ultrahigh metal loading for SAC (e.g. 10\u201320\u00a0wt% Ru catalysts [107], >20\u00a0wt% Cu1-based SACs [108,109], >20\u00a0wt% Co1-based SACs [110]), where most synthesis methods are through thermal processing (e.g., pyrolysis [65,111] and annealing [108]).\nFig. 3\n(a) and (b) present the accumulative journal publications between 2011 and 2020, which directly relate to SAC research. Interestingly, China and the United States, the top publication hubs for SAC research, collectively account for 67.1% of the total scholarly outputs. It is then followed by Australia, Singapore, South Korea, Japan, Canada, the United Kingdom, Germany, and Spain, with a relatively gentle annual growth in publication numbers. In terms of patent applications, China has the largest patent filing numbers (>90% of the total patent filed) thus far, based on the European Patent Office (EPO), and World Intellectual Property Organization (WIPO) databases, as shown in Fig. 3(c) and (d). All the afore-presented trends have evidently shown the growing interest and potential in SAC research and development. This review paper, therefore, attempts to provide useful insights related to the development of SACs, to pave the way for potential future SAC research, especially for applications in the field of clean energy. The review is organized as follows: Section 2 outlines the synthesis and fabrication methods, specially designed for SACs. Whereas the characterization methods used in SAC research are discussed in Section 3. This is followed by current SAC state-of-the-art applications as well as the respective up-scaling challenges highlighted in Sections 4 and 5, respectively. It is then rounded up by a concluding remark in Section 6.A single atom catalyst comprises highly dispersed single atoms of a metallic species onto a support material. However, innovative synthesis methods are needed given the thermodynamically instable nature of single atoms, so that the agglomeration phenomena during the synthesis and reaction processes (as surface free energy is lower in metal cluster form as compared to single atoms form [117]) can be avoided, if not, be reduced [118]. The description of these methods is presented in the following sub-sections, where some of the respective key remarks are summarized in Table 1\n.Co-precipitation is a classical method used for the synthesis of nanoparticle (NP) based catalysts by precipitating the metal in the form of hydroxide from a salt precursor with the aid of a base[119] (see Fig. 4\n(a)). In 2011, Qiao et al. successfully produced the Pt SAC by dispersing the Pt atoms onto defects in FeOx [19]. This was carried out via the co-precipitation of aqueous ferric nitrate (1\u00a0mol/L) and hexachloroplatinic acid (0.076\u00a0mol/L) with sodium carbonate (1\u00a0mol/L). The co-precipitation was conducted at 50\u00a0\u00b0C and at pH 8. To mitigate the tendency of forming metal clusters, the Pt loadings were kept low (around 0.17\u00a0wt%). This, in fact, is the major limitation of this method, some practical applications physically need higher metal loadings [118]. The obtained slurry was dried and calcined at 333\u00a0\u00b0C and 473\u00a0\u00b0C respectively for 5\u00a0h each. Finally, the catalysts were reduced using H2/He at 473\u00a0\u00b0C for 0.5\u00a0h. The resultant SACs were proven to demonstrate remarkable stability and reactivity for CO oxidation. In addition to CO oxidation, the co-precipitation strategy has been applied to synthesize effective SACs for the WGS reaction [24], NO reduction [38], electrooxidation catalysis [148], CO2 activation [149] and carbon\u2013carbon (C\u2013C) coupling reactions [150]. However, due to the requirement of using low metal loadings (<1%) the capabilities of supported SACs are limited as metal oxide supports are not suitable for electrochemical testing [151]. There are other common pitfalls associated with the co-precipitation method, such as it being (a) time consuming, (b) not suitable for reactants that have a significant difference in precipitation rates, (c) trace impurities may be precipitated together with the desired product [152], and (d) some metal atoms are anchored in the carrier interface which makes it unable to serve as an active site for the catalytic reaction [118,122].Impregnation is a widely used and standard method of producing supported nanoparticles due to its simple execution, low waste generation, and cost effectiveness [155]. In general, this method starts with the dispersion of an aqueous solution, containing metal precursors, onto a support material. The mixture is then dried and calcined to anchor the metal atoms onto the support, be that in defect sites, onto the surface, or into the porous network of a mesoporous material, this process is depicted in Fig. 4(d). There are two iterations of the impregnation method, including wet impregnation (excess amount of solution is used; recycling is needed to mitigate waste [121]) and dry impregnation (also name as incipient wetness impregnation (IWI); which requires a lower solution volume but it is difficult to ensure uniform dispersion of metal atoms on the support [122]). For instance, Choi et al. [49], the pioneered work which synthesized Pt1/zeolite-templated carbon via a wetness impregnation protocol. In their work, the mixture of carbon and H2PtCl6\u00b75.5H2O was dried at 353\u00a0\u00b0C under reduced pressure (0.3\u00a0bar); the products were further dried and reduced using H2 flow at 523\u00a0\u00b0C for 3\u00a0h. As a result, SACs with a relatively high Pt loading (5\u00a0wt%) were synthesized. In another attempt from Yang and co-workers [48], Pt atoms have been found to atomically disperse on TiN under a low metal loading of 0.35\u00a0wt% using the dry impregnation approach. Note that higher metal loadings could lead to the formation of NPs or clusters due to atom agglomeration. It is postulated that this could be due to a weak precursor\u2013support interaction [49]. To-date, SACs derived from impregnation methods have been applied to oxygen reduction [48,49], formic acid and methanol oxidation [48], formaldehyde oxidation [156], and CO oxidation [157].Strong electrostatic adsorption (SEA) is a special type of wet impregnation in which the pH of the solution is adjusted to maximize the electrostatic interaction between metal precursors and the oxide surface [123] (Fig. 4(b)). It is anticipated that the oxide surface will be protonated (i.e., positively charged) under a pH condition lower than the point of zero charge (PZC; pH condition that led to zero interactions between metal precursor and support); while it is deprotonated (i.e., negatively charged) under a pH condition higher than the point of zero charge [121,158]. Taking advantage of this feature, monolayers of O\u2212, OH, and OH2+ can be formed by controlling the pH values of the solution, which further results in the capability of anchoring varied metal precursors onto the support [159]. Note that the ligand that connects the metal precursor with the surface will be removed during calcination [160]. Using the study performed by DeRita and co-workers [153] for example, SEA is used to enhance the adsorption of Pt atoms onto the TiO2 surface. By controlling the pH at 12.2 (greater than PZC), the surface will be deprotonated and thus, attract the metal ion complexes of the precursor (i.e., [(NH3)4Pt]2\n+). The ligand can then be removed through calcination to form a SAC [159]. In 2019, a US research group has attempted to synthesize a Pt1/SiO2@Al2O3\nvia SEA with the use of H2PtCl6 as the Pt-precursor [158]. Interestingly, given that the metal ion complexes (PtCl6\n\u2212) are carrying negative charges, the Pt atom are adsorbed at a pH lower than PZC instead. Some other successful SEA applications for SAC synthesis include Pt1/CeO2 [160,161] and Rh1/phosphotungstic acid [162] catalysts for CO oxidation; Pd1 (and Pt1)/hydroxyapatite [163]; and Ni1/hydroxyapatite for dry methane reforming [164]. Nevertheless, the metal loadings of the SAC synthesized from SEA method are usually kept low (<5wt%) [125]. Besides, the adsorption behavior is highly sensitive to the types of functional groups and the presence of defects on the support surface [124]. This is problematic since this method is heavily dependent on the electrostatic interaction to achieve atomic dispersion on the support surface.The ball-milling method, or the so-called mechanochemistry strategy, has gained interest in the synthesis of heterogeneous catalysts due to its (a) efficiency to homogeneously mix multiple precursors together [165]; and (b) is simple to scale-up [126]. In the early discovery of SACs, the ball-milling method, Fig. 4(c) was been proposed by Deng et al. [99] to synthesize FeN4/graphene nanosheets. During the ball-milling process, kinetic energy is used to break the chemical bonds of the substrates as well as the macrocyclic structure of the added metal\u2013ligand complex, slowly at 450\u00a0rpm for 20\u00a0h [99,166], while the shearing action of the milling process is known to generate a large amount of heat which can thermally decompose organic molecules [167]. The resultant compounds such as FeN4 or CoN4 then interact with the defect sites of graphene that were formed during the ball-milling process, to form the desired SACs. Similar to other methods mentioned, the metal loadings must be kept low (\u223c2.7\u00a0wt%) to avoid agglomeration of the high-surface-energy metal atoms [168]. Nevertheless, such an approach possesses limited scalability that is required, this is due to (a) unique precursors which are usually expensive and not commercially available (e.g., metallophthalocyanine); and (b) specific operational conditions (e.g., under argon atmosphere) [127]. Therefore, the ball milling-assisted approach which is coupled with calcination (or other thermal treatments) approach has recently attracted more attention from research groups [100,169]. Recently, kg-scale fabrication of noble atom-based SACs has been successfully demonstrated in the work developed by He et al. [169]. In this work, the noble metal precursor Pd(acac)2 and support precursors Zn(acac)2 were mixed at a weight ratio of 1:400 and subsequently distributed into four agate grinding jars. Each jar contains 50 grinding balls of 6\u00a0mm diameter and 20 with a 10\u00a0mm diameter. The mixtures were ground for 10\u00a0h at a speed of 400\u00a0rpm. To note, the use of the acac ligand generally aids the anchoring process of the metal precursors into the bulk support during the ball-milling process [169]. The resulting milled mixtures were calcined at 400\u00a0\u00b0C for 2\u00a0h. He and co-workers [169] have successfully produced Pd1/ZnO, Ru1/ZnO, Rh1/ZnO, and Pd1/CuO via the afore-proposed procedure. Interestingly, no significant scaling-up effect was observed as the kg-scale, Pd1/ZnO exhibits almost the same catalytic performance (about 92% styrene yield via hydrogenation of phenylacetylene) as compared to the small-scale fabrication (10\u00a0g-scale) under the same conditions (10\u00a0mg of catalyst and 0.5\u00a0mmol of phenylacetylene were used, the reaction was conducted for 20\u00a0min at a temperature of 50\u00a0\u00b0C) The ball-milling method has been successfully applied to the fabrication of SACs for oxidation of benzene [99], hydrogenation of acetylene [170], phenylacetylene [169], 2-methyl-3-butyn-2-ol [171], glycerol hydrogenolysis [172], organic pollutant degradation [173], Fenton-like reaction [174], HER [175], ORR [176,177], CO oxidation [169], and photoreaction [128]. Nevertheless, (a) restricted scalability especially for co-catalyst synthesis which inevitably involves liquid-phase processes [128]; and (b) the occurrence of metallic impurities from the machinery on the catalysts are the other common weaknesses of this approach [178].Atomic Layer Deposition (ALD) is another prominent synthetic technique for the fabrication of SACs given its capability to precisely control the atomic deposition, dispersion of metal species and coating thickness [27,129]. This strength makes it a prominent approach particularly for studying the insights on SAC synthesis [122] and the influences of various parameters involved in the synthetic processes [14,122]. Generally, a complete cycle of ALD encompasses two main steps, shown in Fig. 4(e). In the first step, metal precursors such as methylcyclopentadienyl-trimethylplatinum (MeCpPtMe3) will react with oxygen atoms which were adsorbed onto the substrate's surface. The subsequent O2 exposure was oxidized into the metal precursors and thus, form a new adsorbed O2 layer on top of the metal surface (metal\u2013oxygen (M\u2212O) species). In a complete cycle, the surface of the support is exposed to a 1\u00a0s pulse of metal precursor, followed by 20\u00a0s pulse of N2 to purge the system and then a 5\u00a0s pulse of O2, continually. To note, the metal loading can be finely tuned by altering the number of ALD cycles, e.g., Pt loading of 2.1\u00a0wt% or 7.6\u00a0wt% on N-doped graphene nanosheets can be achieved via 50 and 100 ALD cycles, respectively [179]; 150 ALD cycles can increase the Pt loading on graphene to 10.5\u00a0wt% [27]. Thus far, ALD has been applied to generate Pt-graphene SACs for the methanol oxidation reaction [27], an N-doped graphene support for HER [179], and a CeO2 support for CO oxidation [180]. This synthetic procedure has now been extended to synthesize Pd and Fe-based SACs [36,181]. In a recent work conducted by Zhang et al. [182], a Pt\u2013Ru bimetallic catalyst fabricated using the ALD method has proven to provide 50 times higher reactivity for HER as compared to a commercial Pt catalyst (Pt mass loadings for bimetallic catalysts and commercial catalysts are 61.2\u00a0\u03bcg\u00a0cm\u22122 and 1.24\u00a0\u03bcg\u00a0cm\u22122, respectively). On the other hand, the Pd1/graphene catalysts produced via the ALD method are capable of achieving 100% butene selectivity, up to 95% conversion in the selective hydrogenation of 1,3-butadiene conversion, where the attained selectivity to 1-butene (the desired product which can be used as co-monomer in polyethylene production) was more than 70% [36]. This is superior as compared with conventional Pd/C catalysts which routinely achieve \u223c60% selectivity, at the same conversion. Other to-date applications of ALD include, but are not limited to Co1-modified Pt nanoparticles catalysts for ORR [183], Ru1/PtNi catalysts for methanol oxidation [184]. However, the ALD process is slow and demands greater energy consumption, as well as requiring an ultraclean surface [185]. All these compounded issues lead to a higher fabrication cost [127]. Together with the low stability of the synthesized SAC [122,131], both serve as the key factors that need to be considered under large-scale production [186].Chemical etching is a convenient and straightforward method to re-disperse nanoparticles on substrates as single atoms. Contrary to other synthesis approaches where monoatomic particles are embedded onto the substrate as a \u201cbottom-up\u201d approach, the chemical etching strategy is a \u201ctop-down\u201d approach where larger clusters are first embedded on the substrate and then redistributed as single atoms [142]. As an example, Feng et al. [187] prepared nanoparticles of Ru, Rh, Pd, Ag, Ir, and Pt on an activated carbon support, respectively. Chemical etching was performed using a mixture of CO and CH3I at 240\u00a0\u00b0C for 6\u00a0h. The dispersion of Rh nanoparticles to single atom was studied, and the work found that I\u2022 radicals and CO promote the breakage of Rh\u2013Rh bonds. Chemical etching using H2O2 on MoS2 catalyst [188] has also successfully synthesized single-atom vacancy catalysts to improve the HER performance, showing that S-vacancies provided an effective surface electronic structure for boosted electrical transport properties. Few-layer Ti3\u2013xC2Ty nanosheets [189] are first prepared by etching Ti3AlC2 in a solution of lithium fluoride and hydrochloric acid. From this process, some Ti atoms will be ripped off, and when [PtCl6]2- complex ions are stirred with the nanosheets, Pt4+ ions will be adsorbed onto the surface, forming a SAC. Collective researchers from both Chen and Tang's laboratory [143] also demonstrated that nanoparticle graphene with doped Ni can be chemically etched by HCl solution, causing the structure of Ni nanoparticles to be inherited by the graphene. With an etching time of less than 6\u00a0h, Ni SACs have been identified via Selected Area Electron Diffraction (SAED) and High-Resolution Transmission Electron Microscopy (HRTEM).Using the deposition\u2013precipitation strategy, a metal solution is mixed and reacted with support molecules to form a uniform suspension. The reaction temperature and pH are precisely controlled to deposit the metal species on the support [144], forming SACs. This technique was popular for oxide support, for example, Fu et al. [190] synthesized Au-ceria SAC\nvia the dropwise addition of HAuCl4 into the suspension of ceria particles in an aqueous solution of (NH4)2CO3 at a pH of 8. The deposition\u2013precipitation strategy was gradually extended for different SACs, such as Wang et al. [145], who studied Au/Sn\u2013TiO2, concluding that the creation of oxygen vacancies on the TiO2 surface by single-site Sn had led to better selective activation. For this, Zhang et al. [145] also mentioned that when oxygen defects are generated via the deposition\u2013precipitation method, the metal species precursor can be reduced to form single metal atoms. Mochizuki et al. [191] demonstrated the preparation of Au1/NiO SACs by the deposition\u2013precipitation strategy by mixing and calcination of HAuCl4 and NiO support. Atomic-resolution HAADF-STEM was used to directly observe Au single atoms on NiO where the loading was 0.93\u00a0wt%. Yang et al. [192] reported a \u223c1\u00a0wt% loading of Au atoms on titania, showing applications for SACs in WGS reactions. The work uses an Au deposition\u2013precipitation strategy with UV irradiation of the titania support in ethanol. Excess Au loadings were also removed using sodium cyanide, leaving atomically dispersed Au on titania. This shows that the deposition\u2013precipitation strategy is a versatile method for even when an excess of metal loading was introduced, removal is possible.The interaction between single metal atoms and the surface atoms of support materials decides the nature of the catalyst stability. The coordination site strategy assigns support sites with chemical linkers to adsorb or bind metal atoms, preventing migration and agglomeration [132]. One of the most common synthesis methods for a SAC is to use specialized N-based linkers, exploiting coordinate bonds or N-binding to implant single atoms into a larger matrix such as in a metal\u2013organic framework. Recently, Gong et al. [193] used polypyrrole (PPy) molecules as N2-based linkers, which can fill into the 1D channel of a (non-nitrogenous) MOF (constructed by divalent Mg2+/Ni2+ ions and 2,5-dioxido-1,4-benzenedicarboxylate ligand) to form a PPy@MgNi-MOF-74 composite, illustrated in Fig. 5\n(a). The composite is then annealed at 900\u00a0\u00b0C to anchor the N atoms from PPy onto a porous carbon composite, followed by the thermal decomposition of PPy (removing the carbon from MOF linkage). This ultimately forms a high-performance NiSA-N2-C electrocatalyst that has been used for the reduction of CO2 to CO [193]. For the application of an H2 evolution photocatalyst, Li et al. [51] used a liquid-phase reaction with g-C3N4 and H2PtCl6, followed by low-temperature annealing. This procedure synthesized a Pt/g-C3N4 SAC. Here, FT-EXAFS was used to deduce a coordination number of 5, which suggests that Pt single atoms were bonded on the top of the five-membered rings of the C3N4 network. The photocatalyst was able to give a high H2 generation rate of 162.8\u2013318\u00a0\u03bcmol\u00a0h\u22121 with a Pt loading content from 0.075 to 0.16\u00a0wt%. In general, most works [132,194] implement this strategy by ushering the single atom onto coordinated sites with a chemical linker, followed by a thermal method and other post-treatment to remove and carbonize unrequired synthetic reagents. The most critical variable for controlling the synthesis of SACs using this strategy is the concentration of the metal producing the single atoms and the annealing temperature [51]. A well-established coordination strategy known as surface organometallic coordination (SOMCs) from coordination compounds has been developed to produce single-site atom catalysts [105]. During the grafting of SOMCs, several fragments of reaction intermediates are linked to a surface-supported metal (e.g., Metal-M; M\u2212H, M\u2013R, M\u00a0=\u00a0CR2, M\u00a0=\u00a0O, M\u00a0=\u00a0NR, M\u2013O\u2013OR), enabling direct access of the metal atom to the surface. Cop\u00e9ret [195] has reported that the selective formation of isolated sites of metal via SOMF on a heterogeneous catalyst is more favorable than via wetness impregnation methods, which favor the formation of larger nanoparticles or clusters. He also demonstrated that a single-site heterogeneous catalyst can be prepared using SOMC combined with thermolytic molecular precursors (TMP). These TMP can be removed readily upon thermal treatment, giving a high flexibility SOMC/TMP procedure to engineered heterogeneous catalysts. For instance, few well-defined isolated site based SACs (e.g. Cr(III) supported on Al2O3) have been developed by the combination of SOMC and thermolytic molecular precursors (TMPs), yielding a greater catalytic activity, selectivity and stability of Al2O3 than the Cr(III)/Al2O3 bulk catalyst [196]. In 2017, a combination of SOMC and ALD approach was being developed by Liu to produce a SAC with uniform nuclearity Pt/Al2O3 [197]. By pairing the ALD strategy for mitigating agglomeration, the Pt organometallic complexes can remain monatomic even upon high-temperature thermal treatments and post ligand removal (Fig. 5(b). Notably, the Pt/Al2O3 SAC does not exhibit any thermal sintering, even at 400\u00a0\u00b0C, whereas samples without ALD overcoats exhibited significant particle agglomeration under similar conditions. Alternatively, there were also efforts to synthesize SACs for hydrogenation via less energy intensive SOMC (without subsequent thermal treatment). An ultra-low Pt single loading (\u223c1\u00a0wt%) was coordinated on phosphomolybdic acid (PMA)-modified active carbon [198]. The Pt SAC was synthesized by anchoring Pt on the four-fold planar geometry on PMA. The hydrogenation of N\u2013O, CO, CC, and CC groups were studied, and the Pt-PMA/AC SAC exhibited comparable or better turnover frequencies (TOF) compared to a conventional Pt/AC catalyst. This demonstrates that single atoms have the potential to be anchored into organometallic compounds, which stabilizes the SAC structure while providing high catalytic activity.Poorly defined vacancy defects on ordered support may lead to difficulties in identifying and controlling the precise structure of the attained SAC. Therefore, a defect design strategy aims to engineer the defects on a support to stabilize the SAC surface, allowing a higher possible metal loading, providing a large number of individual active sites, as well as improve the selectivity of the desired process [133]. One of the directions followed has led to a number of works being carried out to study the effect of O2 vacancies in mixed metal oxides to stabilize active sites [200,201]. Additionally, vacancy defects on graphene or layered materials have been probed in a similar way [202,203], as well as, heteroatom defects in crystals, cation and anion vacancies [204,205]. Shen and co-authors [206] demonstrated that defects in graphene can alter the charge distribution on a carbon plane, giving evidence that graphite with more exposed edges can provide superior electrocatalytic performance. Apart from the carbon-based materials, other defect-rich oxide materials also were used as supports in the synthesis of SACs (e.g., CoO [207], TiO2 [199], and CeO2 [208]). Wang et al. [207] have employed an adsorption method to prepare an Rh-based SAC (Rh/CoO), which was stabilized in the defect sites on the surface of CoO nanosheets through the electrostatic interaction. The developed Rh/CoO SAC displayed higher selectivity toward propene hydroformylation to butyraldehyde compared to Rh/bulk-CoO. TiO2 is also another suitable support for SAC preparation that has abundant defective sites. From Fig. 5 (c), a wrap-bake-peel synthetic strategy to fabricate an enzyme-like Cu-based SAC supported on the TiO2 was proposed by Lee et al. [199]. The amorphous TiO2 was first coated on SiO2 core before the Cu atoms were adsorbed on the amorphous surface. Then, a second layer SiO2 coating was introduced to form the SiO2\u2013TiO2\u2013Cu1\u2013SiO2 core\u2212shell materials with a sandwich-like structure. Subsequent steps such as calcination, baking, and NaOH leaching were performed to ensure the single Cu atoms were located at the Ti sites of anatase TiO2. This synthetic strategy gas facilitated a valence control of co-catalyst Cu atoms and generated a reversible valence change in photoactivation that enhance photocatalytic hydrogen generation activity.It was widely known that higher thermal stability of SACs can be granted by fabricating the catalysts under higher temperatures since the precursor\u2013support interaction will be enhanced under high temperatures [134]. Some of the state-of-art techniques reported are strong metal-support interaction (SMSI) promoted production [125], the non-defect stabilized approach [133], and the high thermal redispersion approach [209].For instance, based on a work published in 2015, the single atom active sites on a support can be formed SMSI approach during pyrolysis [60]. It is attractive due to its capability in yielding SACs with good metal dispersion although in higher metal loadings (5\u201320\u00a0wt%) [101,210]. Cheng et al. [210] reported the performance of the pyrolysis-assisted method in fabricating a series of atomically dispersed transition metals (M refers Ni, Co, NiCo, CoFe, and NiPt) on N-doped carbon nanotubes with metal loading of 20\u00b14\u00a0wt%. Findings showed that the M single-atom/N-doped carbon nanotube fabricated via a pyrolysis-assisted method exhibited outstanding catalytic performance not only on ORR and HER but also in electrochemical CO2 reduction [101,211]. In general, the pyrolysis-assisted method consists of three general steps, (a) mixing of metal and support precursors, (b) pyrolysis, and (c) post-treatment, e.g., washing, drying, acid treatment (for removal of residual unencapsulated metal or unwanted ligands). Taking the deposition of Co1 on the N-doped carbon as an example (Fig. 6\n(a)), the first step was to prepare the catalysts precursors by mixing urea, glucose, CoCl2\u00b76H2O, NaSCN, and ethanol at room temperature. Then, the mixture was treated with ultrasonic treatment to form a homogeneous mixture. It was subsequently, dried at 60\u00a0\u00b0C for 8\u00a0h to remove trace ethanol residues. The resultant solids were then ball-milled to form the desired catalyst precursor. Next, the obtained materials were pyrolyzed (>600\u00a0\u00b0C), and subsequently cooled to room temperature under an inert (N2) atmosphere. This resulted in the formation of Co1/N-doped carbon SAC with a uniform distribution of Co atoms. Before the SACs can be used, it was cooled and treated in acid (0.5\u00a0M, H2SO4 at 80\u00a0\u00b0C for 8\u00a0h) to remove any non-coordinated Co atoms and precursor impurities. To note, the yielded Co1/N-doped carbon catalyst possesses the greatest catalytic activities for ORR and HER as compared to the other two conventional catalysts (Pt/C catalysts and Co/N-doped carbon catalysts), as well as excellent reusability where there was no degradation after 11,000 continuous catalytic cycles [211]. Recently, pyrolysis was also used as an efficient tool to downsize metal from the range of 100\u20131\u00a0nm. Based on the findings discovered by Wei et al. [134], the sintering and atomization occurred during the pyrolysis of metal-NP (i.e., Pd, Pt, Au), where atomization (i.e., the transformation of metal-NP to SA) took place at high temperature (900\u20131000\u00a0\u00b0C); whereas sintering was observed between 300 and 1000\u00a0\u00b0C. This represents an attractive top-down synthesis path to derive SACs from a metal NP. In fact, the pyrolysis-assisted approach appeared to be promising for the synthesis of SACs with ultra-high metal loading (>20\u00a0wt%) [65,111].Aside from this, a recent method to synthesize single-atom metals from bulk metals on the catalyst surface using high temperatures were also being developed [214]. This thermal emitting strategy provided an economically attractive advantage for producing single atom nano-catalyst from their bulk materials. Qu and co-authors [214] have used pyrolysis at 900\u00a0\u00b0C to create Zn nodes and defect sites on a pyrolyzed zeolitic imidazolate framework-8 (ZIF-8). Then, a Cu foam was reacted with NH3 gas to form Cu(NH3)x. These Cu(NH3)x species were then trapped by the defects in the pyrolyzed N-rich carbon support, which forms the Cu-single atom N-rich carbon support (Cu\u2013SAs/N\u2013C) catalyst. The work also showed that Co or Ni could be used to replace the Cu bulk metal, confirming its application towards a wide range of SAC syntheses. Later, Qu and co-authors [103] used this thermal emitting strategy to synthesize a high-performance Pt-SAC on defective graphene (Pt\u2013SAs/DG) (Fig. 6(b)). For this synthesis dicyandiamide (DCD) and Pt dispersed on graphene oxide (GO) were placed under Ar flow and heated to 1100\u00a0\u00b0C. At this temperature the DCD underwent pyrolysis, generating NH3 gas. Due to the strong coordination interaction between NH3 and Pt atoms, the volatile Pt(NH3)x species was formed. The Pt entities were oxidized by O atoms based in the GO, forming Pt\u03b4+ (0\u00a0<\u00a0\u03b4\u00a0<\u00a04) species. After that, O content from the GO was removed through thermal processing to produce graphene with defects (DG). Lastly, the isolated Pt SAs/DG catalysts were formed by trapping the Pt\u03b4+ (0\u00a0<\u00a0\u03b4\u00a0<\u00a04) species on the DG. Notably, the prepared Pt\u2013SAs/DG catalyst demonstrated high activity for HER and selective oxidation of various organosilanes. This demonstrates that even the surface of the catalyst can be freely changed by using the thermal emitting strategy, giving a high potential for future SAC synthesis on a large-scale production.Zhao and co-authors [104] recently proposed a cascade anchoring strategy to synthesize a SAC with a high-loading with the possibility for large-scale production. The work demonstrates that the cascade anchoring strategy could synthesize metal-Nx moieties, anchored on carbon support (M\u2212NC) with flexibility for the metal atom. Moreover, the metal loading for the synthesized catalyst was reported up to 12.1\u00a0wt%. First, metal ions (e.g. Mn, Fe, Co, Ni, Cu, Mo, and Pt) were fixated using a chelating agent (such as glucose). The isolated metal ions are then anchored onto the O2-rich porous carbon support where the glucose can bind to the carbon support due to interaction with O-containing groups (Fig. 6(c)). Next, the compound was mixed with a nitrogen source melamine, and pyrolyzed over 600\u00a0\u00b0C to react the carbon-N species with melamine to form a SAC with M-Nx moieties. A similar anchoring strategy using immobilization and pyrolysis was also performed for the synthesis of Pt-single atoms on a commercial carbon black, Black Pearls\u00ae 2000 (PtSA@BP) [215] for the HER reaction. This work used a tetra(4-tert-butyl-phenyl)porphyrinato platinum (PtTBPP) complex as the N and Pt precursor while simultaneously providing a monodisperse purpose. The molecules achieved porphyrin adsorption equilibrium after being stirred for 12\u00a0h. After washing (by ethanol and deionized water) and drying under reduced pressure, the PtTBPP/BP hybrid underwent two-stage pyrolysis, giving a final Pt loading of 2.5\u00a0wt%.Nevertheless, SACs synthesis at temperatures >1000\u00a0\u00b0C is still challenging to be achieved via conventional strategies. To overcome this technical barrier, two ultra-high temperature-assisted strategies, namely the high-temperature shockwave strategy [212] and the high-temperature arc-discharge strategy [213] were developed. The former method utilized periodic on-off shock heating (on-state temperature can achieve up to 2727\u00a0\u00b0C; while the off-state period operates for 10x longer, leading to a lower average temperature of 127\u00a0\u00b0C) to stabilize the single metal atom on the support. This makes the strategy compatible with various materials. Yao and co-workers [212], for the first time, synthesize Pt single atoms that are deposited on the CO2-activated carbon nanofiber via a high-temperature shockwave strategy. The metal precursor, H2PtCl6 was dispersed on the carbon nanofiber and subsequently subjected to the heating shockwave. Their studies revealed that the dispersion of Pt clusters can be eliminated by increasing the heating cycles (Fig. 6(d)). Interestingly, all the weak Pt\u2013Pt bonds and types 1\u20133\u00a0Pt\u2013C bonds were transformed into stronger types 4\u201310\u00a0Pt\u2013C bonds after multiple cycles of shock heating, thus, resulting in higher thermal stability.The latter strategy utilized high-temperature arc discharges to in-situ form and stabilize Pt single atoms on molybdenum carbide (MoC) support at a temperature up to 4000\u00a0\u00b0C (higher than the limit of the former strategy). To synthesize Pt1/MoC via this method, a mixture of Pt and Mo powders was first prepared and filled into the anode-side graphite tube (Fig. 6(e)). Whereas the cathode is made of a pure graphite rod. The synthesis process begins when the arc starts to be generated from the arc chamber. The entire process took tens of minutes (other methods take hours to complete the synthesis) which makes it an efficient and attractive fabrication tool. The resultant SACs exhibit excellent performance and thermal stability for the hydrogenation of quinoline. However, both the aforementioned methods are still in their infancy, and follow-up works are needed to upscale the overall synthesis process to cope with the demands for mass production of SACs.A freeze-drying method has also gained interest from the catalysis community as it enables a large specific surface area of the support, while more importantly, the mobility of the metal precursors can be mitigated simultaneously [135]. A few researchers have therefore incorporated the freeze-drying process into SACs fabrication to better design and control metal precursor deposition. Taking a Co-based SAC as an example (Fig. 7\n(a)), Fei et al. [59] mixed the metal precursor (CoCl2\u00b76H2O) and support precursor (graphene oxide) at a ratio of 1:135. The mixture was then sonicated in deionized water, and subsequently freeze-dried for 24\u00a0h to produce brownish powders. Next, the powder was annealed at 750\u00a0\u00b0C for several hours to activate the Co1/N-doped graphene. The work has proven that the obtained catalysts can work as effective HER catalysts under both acidic and alkaline environments. Thus far, the freeze-drying method has been extended to create Fe-based SACs for ORR [216] and Fenton-like reaction [217], Pd-based SACs for selective hydrogenation [218] and Zn-air batteries application [219], Ni-based SACs for CO2 methanation [220], Pt-based SACs for HER [221], Co-based SAC for lithium-sulfur batteries application [222], and bimetallic SACs (e.g., Co1\u2013Fe1-based SACs for HER [223]). Despite the potential of having greater reactivity and stability of catalysts as compared to the conventional oven-drying approach [224], the high operational cost still appears as one of the main conundrums for the freeze-drying assisted strategy [136].Photochemical methods rely on a powerful light source (e.g. UV lights) to deposit single atoms on support via ion reduction. In years gone by the photochemical method was used for solar O2 production in metal deposition onto a semiconductor photoanode [139]. For instance, the deposition of Co on a ZnO electrode in a K3PO4\u2013CoCl2 precursor solution under UV light emission to oxide the Co2+ ions, adhering it to the ZnO surface142. This method was recently adopted into Pt SAC preparation by Liu et al. [102]. They have successfully developed an \u201cultra-low\u201d Pt loading and highly stable atomically dispersed Pd1/TiO2 SACs for catalytic hydrogenation of aldehydes (Fig. 7(b)). The single atom Pt-loaded (up to 1.5\u00a0wt% loadings) TiO2 nanosheets on ethylene glycolate (EG) were synthesized by stepwise UV-induced radical formation which causes Cl\u2212 removal by EG radicals [102]. The adsorption of photons and electronically excited states were reported to be the two key steps in the photochemical reduction process. Later, this photochemical method was used to synthesize Pd-loaded (001)-exposed anatase nanocrystals and Pd-loaded TiO2 SACs for the purpose of styrene hydrogenation and CO oxidation [226]. For solar water oxidation, an Ir dinuclear heterogeneous catalyst (DHC) on a Fe2O3 surface was prepared by ultraviolet ozone (UVO) cleaner system, providing a paradigm for SACs with multiple active sites [227]. A photochemical solid-phase synthesis method was used to adsorb PtCl6\n2\u2212 onto an N-doped porous carbon and directly reduced by UV light to Pt atoms achieving 3.6\u00a0wt% loadings [228]. Another novel method to prevent atom nucleation is by using iced photochemical reduction [225]. This strategy (Fig. 7(c)) coupled with the freeze-drying technique, freezes the atom-carrying aqueous solution before UV treatment and successfully synthesizes a stable SAC with Pt atoms on various support (e.g., amorphous carbons, mesoporous carbon, graphene, multi-walled carbon nanotubes, TiO2\nNP, and ZnO nanowires) for HER.The electrochemical Method is a widely accepted electrocatalyst preparation method by most researchers; this is because the metal ions can be easily deposited onto cathodes in an electrolyte solution without any complicated procedure [229]. Therefore, it is one of the attractive, scalable, facile, and cost-effective methods which is capable of synthesizing and activating single atom electrocatalysts with the aid of electrochemical potential. To-note, the size, and the respective metal loadings can be fine-tuned by altering the deposition parameters such as metal ion concentration and deposition time [140]. More importantly, the SACs derived from this method are binder-free that can be used directly in electrocatalysis [140]. Some works have proved its capability in SAC fabrication. Notably, in the work conducted by Fan et al. [230], the formulated Ni\u2013C-based catalysts were successfully activated via the series of treatment processes, i.e., HCl leaching treatment (for removal of redundant Ni metal) and electrochemical cyclic-potential treatment (for activation). Unexpectedly, throughout the entire activation process, Ni atoms were atomically dispersed and anchored onto the carbon support, which therefore formed Ni-based SACs (Fig. 8\n(a)). On the other hand, some works have attempted to in-situ grow the Pt single atoms onto Ni foam [52], single-walled carbon nanotubes [231], and a bismuth ultramicroelectrode [140]. Aside from that, Zhang et al. [232], for the first time, proposed the use of the electrochemical method to synthesize an atomic Pt and Co co-trapped carbon catalyst. Thus far, the SACs fabricated using this method have shown superior HER activity and electrocatalytic stability as compared to the conventional Pt/C (20\u00a0wt% Pt) catalysts (e.g., the catalytic activity of the PtSA-NT-NF catalysts synthesized by Zhang et al. [52] were 4 times greater than that of Pt/C catalysts). More recently, a self-terminating electrodeposition technique for SACs fabrication was proposed [141]. Generally, by controlling the electrical potential at the underpotential deposition (UPD) state, the metal-support bonding can become dominant as compared to the metal\u2013metal bonding, which favors the formation of SACs. The continuous atom growth will be terminated once the surface-limited reaction has reached saturation. This further ensures the formation of SACs instead of metal clusters (Fig. 8(b)). More interestingly, it can be operated under ambient temperature which makes it an energy-efficient SAC fabrication method. Moreover, the two ultra-high temperature-assisted strategies (i.e. high temperature shockwave strategy and the high temperature arc-discharge strategy) presented in Section 2.2.3 can also be, arguably, categorized as electrochemical methods as the use of electric fields was involved.The mass-selected soft-landing approach is an emerging technique for SAC synthesis. It starts with the vaporization of metal atoms using a high-frequency laser, where the vaporized metal atoms are selected by a mass filter to control the deposition of the metal on the support surface [118,146] (Fig. 9\n). To-date, since this method is capable of providing exact tuning and control of the size of metal deposited on the support surface, it has been employed in a couple of SAC works for generating fundamental insights [13,233]. For example, to study the impact of Pt atom number on electrochemical catalysis, Weber and co-workers [234] have synthesized Pt\nn\n/Indium Tin Oxide (ITO) catalysts (1\u00a0=\u00a0n\u00a0\u2264\u00a014) using the mass-selected soft-landing method. In their work, the Ptn\n+\u00a0clusters (and atoms) produced from the laser vaporization were channeled through a quadrupole mass filter to generate a beam containing the desired cluster (or single atom) size which was then deposited on the ITO surface. Earlier in 2000, one of the pioneering works [235] had also attempted to verify the catalytic kinetics of Pdn/MgO(100) catalysts (1\u00a0= n\u00a0\u2264\u00a030) for the trimerization reaction using a mass-selected soft-landing method where they found the SAC (Pd1/MgO(100)) can significantly reduce the activation energy requirement. Aside from that, this approach has been applied to synthesize bimetallic SACs for OER and ORR [236]; Pt1/glassy carbon substrate catalysts for ORR [237]; Pd1/TiO2 catalysts for CO oxidation [238]; and Au1/TiO2 catalysts for CO oxidation [239]. Nonetheless, the need for ultrahigh vacuum conditions and low production yields has inevitably lowered the scalability of this method [13,118].The existence and spatial distribution of isolated single atoms are crucial for understanding the relationships between structures and the properties in SACs. However, it is quite challenging to identify the single atoms at an atomic-scale due to the requirement of high spatial resolution tools. In recent decades, techniques have been developed towards an atom-scale resolution that can be employed to the SACs at different ensemble level signals, of all responsive species, which can minimize the deceptive information and accurately reflect the real active atomic species. Hereby, in this section, we provide an overview of various advanced characterization techniques for SACs, which can be classified into several categories: (a) High-resolution electron microscopy, (b) X-ray irradiation spectroscopy, (c) in\nsitu spectroscopy, and (d) magnetic resonance spectroscopy.With the development of advanced microscopes, the direct observation of NPs or atoms in/on catalyst supports has been realized. With the aid of different electron microcopies such as Aberration-Corrected High Angle Annular Dark Field/Scanning Transmission Electron Microscopy (AC-HAADF/STEM), the fine distributions and precise location of single atoms in the catalyst can be evaluated, differentiated using bright and dark field imaging.Transmission electron microscopy is one of the most widely used characterization methods to determine the structure and particle size of active sites in heterogeneous catalysts. It provides a clear observation of the catalyst morphology and structural change before and after interactions between metal atoms and supports [240,241]. Lately, the improvement of spatial resolution in electron microscopes has granted the possibility of identifying the active sites of SACs at a single atom level. Specifically, HAADF-STEM is one of the high-end techniques in which the high angle annular dark field imagery is coupled with a standard scanning transmission electron microscope to provide the bright/dark contrast of different elements at a sub-angstrom level resolution [242]. For HAADF-STEM, the intensity of images obtained follows the thickness of samples and the atomic number up to an exponential value of 1.4\u20132, but still, it is not powerful enough to identify the isolated single atoms in a SAC [243]. To improve the precision, the aberration-corrected HAADF-STEM is often used to distinguish the atoms from the support materials based on their different values and, further provide direct evidence of the existence of SACs [244,245]. For example, Qiao et al. [19] reported about Pt/FeOx catalysts and demonstrated how individual Pt atoms uniformly dispersed on a FeOx surface using HAADF-STEM (Fig. 10\n(a)). Both individual Fe (Fig. 10(b)) and Ir (Fig. 10(c)) atoms can be clearly distinguished from other lighter metal atoms (highlighted by red circles) such as Si, Al, Na, C and O [246,247]. However, there is also a shortcoming where the isolated atoms in the images are not clearly shown due to the phase contrast caused by the multiple metal loadings, impurities, and the inhomogeneity of substrates (e.g. lighter elements are usually invisible when imaged together with heavier, differences in contrast). Thus, Electron Energy Loss Spectroscopy (EELS) or Energy Dispersive X-ray Spectroscopy (EDX) are often combined with HAADF-STEM to provide an in-depth insight into the catalyst [246]. Based on this method, metal atoms (e.g. Fe, Co, Ni) can be clearly distinguished from the carbon-supported SACs via\nSTEM/EDX, where metal atoms are reported to be stabilized by the carbon surface [59,246,248] (Fig. 10(d)). However, when the atomic number of single metal atoms and supports are close, it will be challenging to obtain obvious contrast in HAADF-STEM [249]. In recent work, Guo et al. [250] prepared the Cu/Al2O3 SAC with a high loading of 8.7\u00a0wt%, but there were only a few Cu single atoms observed by HAADF-STEM owing to the weak image contrast of Cu and Al2O3. Hence, coupling of HAADF-STEM with other in-situ spatial resolution synchrotron characterization techniques is value-added to probe the structural information of the SAC (i.e, HAADF-STEM-Syn Infrared and HAADF-STEM-Syn X-ray Diffraction).Another persuasive technique for directly observing the SAC structure is by using Scanning Tunneling Microscopy (STM), in which the surface images of the conducting or semiconducting materials at the atomic level can be attained [251,252]. The typical advantage of STM is to in-situ track the reaction process on a well-defined surface and provides the opportunity to explore catalytic mechanisms in real time [253,254]. STM operates under ultra-high vacuum at the broadband temperature range from near \u2212273\u00a0\u00b0C to around 1027\u00a0\u00b0C [255,256]. Over the decades, STM has been widely used in industries to track heterogeneous catalyst performance, as such, STM is applied to study the soft-landing deposition and atomic-layer deposition synthesized methods of SACs for the application in 1,3-butadiene hydrogenation [257], CO oxidation [258], and NO reduction [259]. For SAAs, STM can directly capture the images of metallic single atom entities (e.g., Pt [257], Ni [260], Au [261], etc.) deposited on singe crystal surfaces together with reactive hydrogen [22,257]. Fig. 11\n(a and b) shows Pt atoms existing as isolated protrusions substituted onto the Cu(111) surface and H atoms spillover onto the Cu surface [257]. More recently, ultrahigh vacuum high-speed STM has been adopted to observe the real-time growth process of single Ni atoms on graphene. The catalytic effect of individual Ni atoms at the edges of a growing graphene flake was captured at the millisecond time scale by STM (Fig. 11(c and d)), providing an overall picture of the diffusion of mobile nickel atoms that catalyzes the graphene growth on the edges of SAC islands [260].X-ray Absorption Spectroscopy (XAS) is a state-of-the-art synchrotron technique to characterize the local environment of atoms in materials by measuring the variation in the absorption coefficient under the scanning of X-ray radiation in an energy range around the absorption edge [198,262]. XAS can be divided into three regions, pre-edge, near edge, and extended range (Fig. 12\n(a)). When the measurement range is near the absorption edge, it stands for X-ray absorption near edge structure (XANES); while the extended X-ray absorption fine structure (EXAFS) is usually measured beyond the absorption edge in the range of 50 to >1000\u00a0eV. From both forms of characterization (which are often obtained in the same scan), detailed structural information can be provided by these techniques, including the bond lengths, the angle between chemical bonds, oxidation states, and the number of coordinating species. EXAFS is usually used to show the coordination and absence of metal\u2013metal interactions, indicating that the supported metal atoms are individual and that particles or clusters are absent. However, if the elements of the materials are sensitive to electronic and oxidation environments, XANES will be a more suitable characterization technique. For instance, Qu et al. [214] reported the preparation of Cu single atoms on N-doped carbon (Cu SAs/N\u2013C). EXAFS results revealed that Cu\u2013SAs/N\u2013C exhibits a dominant Cu\u2013N coordination at 1.48\u00a0\u00c5, and there was no Cu\u2013Cu coordination since no Cu\u2013Cu characteristic peaks were observed. As for comparison, the main peak of Cu\u2013Cu coordination was observed at 2.24\u00a0\u00c5 on a Cu reference foil. XANES was then performed for further analysis, and it showed the intensity of the line for Cu\u2013SAs/N\u2013C located between those for the Cu foil and CuO, revealing its typical electronic structure (Cu\u03b4+, 0\u00a0<\u00a0\u03b4\u00a0<\u00a02) (Fig. 12 (b)).X-ray Photoelectron Spectroscopy (XPS) is another widely used technique to characterize the chemical states and electronic structures of surfaces [39,75,265]. It has been used in many SACs to determine the chemical state and analyze their electronic environment, such as Pt/Fe\u2013N\u2013C [266], Pt/CeO2 [267], ZnNx/C [268]. Taking Chen and co-workers' work as an example [269], two peaks in the Pt 4f spectrum at binding energies of 75.8 and 72.4\u00a0eV, ascribing to 4f5/2 and 4f7/2 level were attained. From the XPS analysis, we can clearly distinguish that those peaks are between the Pt2+ and Pt0 states. The peak positions were between those of Pt(II) and Pt(0), suggesting that Pt atoms carry partially positive charge through electron transfer between metal and supports owing to enhanced metal\u2212support interactions. More recently, with the rapid development of the in-situ technique, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was discovered to track the surface of a catalyst particle at a relatively high temperature in the gas phase (mbar pressure range). With the aid of this technique, the study of dynamic modifications at single atom-support surfaces in the vapor phase environment can be investigated, providing a sophisticated defect design of next-generation SAC catalysts [270]. Most importantly, not only the surface but also bulk-dissolved elements can be detected. The element species that may influence the chemisorption or charge delocalization of SAC atom can be analyzed, which provides a precise reaction mechanism of the catalyst in the gaseous phase [162].By comparing with X-ray spectroscopy, the transmission infrared spectroscopy or Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) uses a lower wavelength of infrared light reflection and transmission to evaluate the spatial resolution of the SACs (e.g. Acidic sites-Pyridine/NH3 as probe and Basic sites-CO2/CO as probe) or the change of organic phases of catalysts at time-resolved mode [271,272]. In addition, DRIFTS is commonly used in conjunction with EXAFS, where EXAFS provides information about the electronic and geometric structure of the SACs, while DRIFTS follows the evolutionary formation of the surface bonding between single atoms and specific acidic and basic sites, aiding the understanding of the catalyst-absorbance mechanism in operando environment [273,274]. For instance, the infrared frequency of CO adsorbed on isolated metal atoms is different from that on clusters/NPs, this is because single metal atoms generally have different chemical states due to their different coordination structures with the support [275]. Referring to Fig. 12(c), on increasing the loading of Rh on ZrO2, the surface structure and mechanism for CO molecule adsorbed can be studied via absorbance peaks (e.g., atop, bridging, and a terrace site). In short, Infrared spectroscopy can be used to provide insights into the surface adsorption mechanism with respect to the single atom loading in SACs. Taking Au\u2013Pd SAA supported on ion exchange resins (Au\u2013Pd/resin) for the Ullman reaction as an example [42]. DRIFTS recorded the bonding profile of CO (probe molecule) on the Au\u2013Pd/resin SAC with different Au/Pd ratios, in which two absorption bands were observed at 1895 and 2020\u00a0cm\u22121 which signifies the CO bridged and on-top adsorption onto the Au\u2013Pd (Fig. 12 (d)).Operando Raman Spectroscopy (ORS) is an in-situ technique to analyze the structures of materials at various scales from bulk to nanoscale layers through different photon energies. The application of ORS can provide unique means for a deeper fundamental understanding of layered nanomaterials and atomic-scale catalysts [276,277]. In 2019, the first attempt of investigating the atomically dispersed Rh metal on phosphotungstic acid (Rh/PMA SAC) during CO oxidation via ORS was proposed by Yan's group [278]. Based on the spectra shown in Fig. 13\n(a), it shows that even after CO oxidation reaction at temperatures 573\u00a0K, the heteropoly acid structure of the Rh/PMA remains intact, suggesting that no formation of metal oxides after CO adsorption of Rh single atom. Apart from investigating the dispersion of single atoms, ORS has been used to investigate the molecular fingerprints of the MoSx species present during the electrochemical HER in an HClO4 electrolyte [279]. Based on the Raman profile (Fig. 13(b and c)), the structural evolution of MoSx films during HER can be attained. As such, the peak at 2530\u00a0cm\u22121 was captured at potentials relevant to H2 evolution, which corresponded to the S\u2013H stretching vibration of MoSx\u2013H moieties. MoSx-AE showed additional two peaks at 520 and 550\u00a0cm\u22121 which are not seen for MoSx-CE. These were ascribed to the \u03bd(S\u2013S) terminal, and \u03bd(S\u2013S) bridging vibrations, respectively. Hereby, ORS can be acknowledged as one of the spectroscopic techniques that are typically used to determine vibrational and rotational modes of active species bonds on the atomic metal\u2013metal coordination.Nuclear Magnetic Resonance (NMR) for single atom detection is a solid-state spectroscopic technique used to identify the electron structures or chemical bonding of the core structure of a catalyst. The application of solid-state magic-angle spinning-nuclear magnetic resonance (MAS-NMR) has been widely reported to investigate the co-relationship of the precursor ligand bonding during the synthesis of SACs [280,281]. In 2017, Zhang and co-authors [37] have synthesized a highly active Pt-based SAC (Pt/m-Al2O3) by impregnating the Pt atom on the mesoporous Al2O3 support for selective hydrogenations and CO oxidation reaction. Based on the MAS-NMR analysis shown in Fig. 14\n (a), they found that most of the Al3+ species were in tetrahedral, pentahedral, and octahedral shapes, differing from nanoparticles Pt/Al2O3 where the Al3+ pentahedral peak was not seen. This further confirmed that the significant pentahedral-coordinated Al3+ species have resulted from the calcination and reduction process, in which the Pt-atom was bound onto the Al surface via bridging O atoms. In addition, Zhang and co-authors [198] also synthesized a SAC with Pt on a phosphomolybdic acid-modified active carbon (Pt-PMA/AC) for hydrogenation reactions. A 31P MAS-NMR was used to study the electron structure of the support with and without the Pt atom loading. Through the MAS-NMR study, it revealed changes in the environment for P bonding in the Pt-loaded PMA/AC. Lately, Shao et al. [245] adopted the 13C cross-polarized-MAS NMR (CP-MAS NMR) technique to analyze carbon bonding on an Ir-based porous organic polymer with aminopyridine functionality (Ir/AP-POP). Various carbon bonding (such as CO, Py-C, Ar\u2013C, and Py-C) were identified in the Ir/AP-POP catalyst which assisted in understanding the catalytic mechanism for converting CO2 to formate by quasi-homogeneous hydrogenation (Fig. 14(b)).Electromagnetic Spin Resonance (ESR) is another characterization technique to investigate materials with unpaired electrons. This can be used to analyze the atomic state and coordination environment of SACs. For instance, the partial reduction of Cu(II) to Cu(I) due to graphene-induced charge transfer in a mixed-valence Cu/functionalized graphene SAC (G(CN)\u2013Cu) can be confirmed through ESR measurement [282] (Fig. 15\n(a)). Based on the fresh catalyst ESR spectra, only the unpaired electron signal from isolated paramagnetic Cu(II) cations (d9) was observed, whereas the Cu(I) cations (d10) were not detected. The authors make a comparison of the ESR spectra of the G(CN)\u2013Cu catalyst dispersed in hexane before and after adding H2O2. Notably, there was an increase in the intensity of Cu(II)-induced signal after adding the peroxide, this suggests that the Cu(I) was oxidized to Cu(II). Adding to that, ESR also can analyze the Curie behavior of the catalyst (inset in Fig. 16\n(a)), such as the non-presence of magnetically interacting Cu(II) centers in the catalyst. This observation indicated that there are no antiferromagnetic interactions or any formation of bulk CuO clusters. Recently, Jin et al. [283] also reported the superior performance of the partially oxidized Ni single\u2010atom sites in polymeric carbon nitride for elevating photocatalytic H2 evolution. Based on these findings, the change of oxidation state in Ni has modulated the catalyst's electronic structure, leading to an optimized photocatalytic activity, where fewer unpaired electrons were observed in deeply oxidized Ni single atoms (Fig. 15(b)). ESR can be concluded as a powerful technique to identify the electronic configurations of metal atoms in SACs, specifically for unpair electron detection.Along with the state-of-art characterization techniques, there are also some supplementary techniques that also provide information on the thermal stability of SACs (Thermogravimetric analysis, TGA), Brunauer\u2013Emmett\u2013Teller specific surface area of SACs (N2 adsorption), single atom metal loading on the SACs (Inductively Coupled Plasma Mass Spectrometry, ICP-MS), and dispersion of the single atoms on the SACs (H2/N2O chemisorption [94,285,286]). With the aid of the supplementary characterization (Table 2\n), a more in-depth understanding of the intrinsic physicochemical properties of the catalyst can be elucidated. For instance, Zhang et al. reported that the fabricated PtCu SAA can yield a high turnover frequency that reaches up to 2.6\u00a0\u00d7\u00a0103 molglycerol\u00b7molPtCu\u2013SAA\n\u22121\u00a0h\u22121 in glycerol hydrogenation, which is to our knowledge the largest value among reported heterogeneous metal catalysts [172]. Under this context, the dispersion of the Pt atoms on the SAA is highly important, and such an important piece of information only can be extracted from ICP-MS and N2O chemisorption. Another example can be derived from a study reported by Li et al., with the aid of N2 physisorption, they managed to observe that the Zn SACs are having multiple types of pores (i.e, micro, meso, and macro), which induces a large surface area of 1002\u00a0m2\u00a0g\u22121, that is \u223c10 folds higher than the commercial zinc catalyst [287,288].As shown in Table 3\n, SACs have been applied to various heterogeneous reactions (e.g. C\u2013C coupling, oxidation, reforming reactions, and hydrogenation) for chemical and fuel production. Herein, we aim to provide a comprehensive summary of SAC usage by discussing: (a) How does the heterogeneous structure of SACs affect the catalytic activity; (b) How does the structure evolution process response to a wide variety of intrinsic and extrinsic factors and (c) What are the underlying catalytic mechanisms in different possible reactions. This section summarizes all the recent experimental activities of SACs including the operating conditions, type of reactors as well as catalyst loading that affect the yield and selectivity of the desired products. Besides that, the Computational DFT calculation which includes the binding energy, the electronic structure, and possible catalytic reaction routes of the SACs is also discussed. As known, the first-principles DFT calculations allow the investigations of the energetics of processes at the atomic-level with high precision and provide quantum mechanical-based insights into the related electronic structure of these processes and their influence on catalyst reactivity [289,290]. It is worth mentioning that not only well-understood catalyst mechanisms but also controversial hypotheses are cautiously discussed.The mechanistic fundamental questions of C\u2013C coupling reactions on heterogeneous catalysts are still not fully understood and the metal active phases are debated [432,433]. This is because the atoms on the surface of NPs usually have different coordination numbers along with variable chemical environments (electronic effects) than those of their neighboring atoms which influences the catalytic activities [434,435]. Another relevant issue related to the use of NPs is the infeasibility of using high loadings of expensive noble metals, this reduces their attractiveness for bulk production. Thus, the synthesis of highly dispersed noble metal SACs as a catalyst is highly desirable as it could overcome this issue by maximizing the metal atomic efficiency, as well as reducing the catalyst cost [13,19]. However, single atoms are known for aggregation, leading to clusters and finally nanoparticles. In order to avoid their aggregation and induce stabilization of the single atoms, several methods have been proposed: a) the cascade anchoring strategy of the atom onto a metal oxide support [24,35]; b) a reductionist approach by alloying with other alloys to form an SAA [436,437]; or c) depositing single atoms into different oxide supports or organic frameworks for superior metal-support interactions [27,36,438]. As a whole, SACs are an attempt to bridge homogeneous and heterogeneous catalysis closer to understanding and revolutionizing the C\u2013C coupling field.The catalytic activity of a SAC depends closely on the nature of the active isolated metal atom and the presence of functional groups, if attached by linkers, the support used can also induce a significant electronic perturbation to the atomic active site for electron charge transfer. As a result, the single dispersed atoms of metal anchored on to a support surface are expected to have a reduced number of coordination sites for reactants or intermediates in comparison with metal NPs supported on a surface due to the absence of different active sites. More importantly, once the metal is anchored on a solid support, it can be easily regenerated for the next cycle of reaction without any complex treatment [17]. Table 4\n outlines the applications of SACs in the C\u2013C coupling process.As proclaimed by Tao's group [284], the developed Pd-based SAC anchored on TiO2, Pd1/TiO2\nvia a deposition\u2212precipitation method was highly selective and active for more than 10\u00a0C\u2013C reaction cycles of phenylacetylene and iodobenzene (Fig. 16(a)). Based on XANES and EXAFS, the coordination of Pd atoms attached to TiO2 was confirmed where each Pd atom is bound to four oxygen atoms of the TiO2 surface support, forming PdO4 units, and the exposed surface of the lattice fringe of the TiO2 is (101) with an average TEM particle size of 20\u201325\u00a0nm (Fig. 16 (b)). To further explore the deactivation and detachment of Pd atoms from Pd1/TiO2 catalyst after the reaction (Fig. 16(c)), the authors have also performed the durability test of the spent Pd single atoms on a TiO2 support by measuring the concentrations of Ti and Pd in a solution (after centrifugation), followed by characterization using XPS (Fig. 16(d)). Surprisingly, the peak positions of Pd 3d5/2 were the same, indicating that the Pd atoms on TiO2 supports have the same chemical oxidation state before and after the reaction. Regarding the deactivation study, the authors have performed a hot filtration-leaching analysis; notably, no metals were detected in the filtered solution, indicating the strong bonding between the Pd atoms (active sites) and TiO2 (support). Moreover, computational investigations based on DFT calculation have also been carried out to identify the most thermodynamically favorable structure, which corresponds to Pd single atoms anchored to four oxygen atoms of TiO2 through Pd\u2013O\u2013Ti bonds.Lately, Chem and co-workers [441] reported a heterogeneous catalyst consisting of Pd single atoms anchored on exfoliated graphitic carbon nitride (Pd-ECN) for the C\u2013C reaction of bromobenzene with phenylboronic acid pinacol ester while benchmarking with homogeneous and other bulk heterogeneous catalysts (Fig. 17\n (a)). Microwave-assisted deposition was used in this study to deposit palladium on ECN, a pristine high-surface area form of graphitic carbon nitride. STEM coupled with EXAFS was used to investigate the presence of Pd single atoms, while XPS was adopted to study the electronic properties of the Pd atoms incorporated in the ECN. In addition, the DFT calculations were also carried out to understand the promising C\u2013C coupling performance of Pd-ECN. Molecular dynamics simulations performed at different temperatures show that Pd atoms were confined within a given cavity, even though they still have some degree of freedom. This simulation evidence agreed with the experimental XPS observations, suggesting that the Pd atoms occupy two preferred positions: in the first one, Pd was located close to the surface plane, while in the second one, the metal was in between the two-top planes.In the reaction mechanism, the first step corresponds to the molecular adsorption of bromobenzene to the metal center and the consequent change of the Pd coordination number. The authors reported that the ability of Pd to change its coordination is crucial to the observed catalytic performance. In the second step, bi-hydrated potassium phenylboronic acid pinacol ester was adsorbed, and the cation from the salt occupied the nearest neighbor empty cavity. Thanks to the displacement of Br\u2013, phenylboronic acid pinacol ester coordinates, the subsequent trans-metalation was found to be the rate-determining step of the overall reaction (Fig. 17(b and c)). After the elimination of the boronic pinacol ester, the new C\u2013C bond was formed. The elimination of the product restores the initial coordination of the Pd atom. Even though the overall reaction mechanism catalyzed by Pd-ECN reflects that reported for Pd(PPh3)4 molecular catalyst, in the latter case the role of the ligands is crucial: the elimination of two ligands occurs prior to the reaction, and it opens the coordination sphere of Pd, allowing the coordination of the substrate. A third ligand was then released during the trans-metalation step.As mentioned above, heterogeneous alloyed SACs or known as SAA can be prepared using two metals, via isolation of the single metal atom by another metal atom. The synergistic effect between the two metals will alter the geometric and electronic structures of alloyed SACs, potentially inducing exceptional catalytic performance for various reactions [22,443]. Zhang and co-workers [42] have reported a durable and efficient Au -Pd SAA for the Ullmann reaction of aryl halides in water. The investigated Pd-based SAC exhibited an excellent Ullman coupling activity, not only of aryl bromides and iodides but also of the less reactive aryl chlorides. As known, aryl chlorides are less expensive, readily available, and more sustainable than their analogous aryl bromides and iodides. For this reason, their utilization as substrates is highly desirable. However, in the literature, only a few examples of Pd NP catalysts were reported to be worked well with aryl, but the bi-metallic SAA presented by Zhang and collaborators furnishes a valid alternative. The Au\u2013Pd SAA was prepared with an ion exchange-NaBH4 reduction method, and the presents of Pd single atoms were isolated by the Au atoms as confirmed by the EXAFS and DRIFTS analysis. The DRIFTS result was in good agreement with the EXAFS result, indicating that the Au alloyed Pd single atom configuration was formed at Au/Pd\u00a0\u2265\u00a04. Remarkably, the Au\u2013Pd SAA managed to convert \u223c90% of aryl without deactivation over the course of eight cycles.Another class of C\u2013C coupling reaction includes the hydroformylation of olefins to produce aldehydes, which are important intermediates to produce other chemicals. Wang et al. [207], have recently developed CoO-supported Rh single-atom catalysts (Rh/CoO) with remarkable selectivity towards propene hydroformylation. The authors investigated the yield and selectivity of butyraldehyde by increasing the Rh weight loading (e.g., 0.2, 1.0, and 4.8\u00a0wt.%). The highest turnover frequency (TOF) number of 2065 h\u22121 and selectivity of 94.4% for butyraldehyde were obtained when the catalyst with the lowest Rh weight loading. Furthermore, the stability of 0.2% Rh/CoO was also studied by recycling the catalyst five times. Over the cycles, the catalyst remained highly active, similar to the initial reaction, the selectivity slightly decreased to 94.0% by the final cycle. To further understand the facilitation and adsorption of propene on Rh single sites in an atmosphere containing both H2 and CO (syngas), a DFT investigation was performed. The DFT calculations showed indeed that after the adsorption of H2 and CO, Rh atoms moved from the original lattice position, leading to a reconstruction of Rh active atoms that facilitate the adsorption of propene. Furthermore, DFT calculations also showed dissociative adsorption of the H2 molecule, leading to the formation of an OH group on the CoO surface, while CO preferentially binds to the Rh active site, due to the strong interaction with Rh single atoms. DFT was also used to investigate the reaction mechanism that proceeds through three consecutive steps: a) one of the adsorbed H atoms will attack the CC bond in the adsorbed propene molecule; b) CO will insert into the opened CC bond; and c) the second adsorbed hydrogen atom will then combine with the C atom in the reactively formed terminal CO to form the final product. The remarkable activity and selectivity, and high stability of 0.2% Rh/CoO are of high importance for potential applications in industrial processes by reducing the cost and pollution efficiently. Another remarkable example from Zhang's group, thermally stable Rh-based SACs which favor the hydroformylation of olefins has been synthesized [39]. Notably, the Rh1/ZnO SAC has demonstrated a very high TON of 40,000 with 99% selectivity towards aldehyde products, which is to our knowledge the largest value among reported heterogeneous metal catalysts. This level of selectivity has not been previously reported for Rh, without the use of specific support materials such as polymers (ligand steric effects), or zeolites (confinement effects), adding great impact and benefit to the SACs. Also, the fabricated SAC has shown a high stability profile in terms of recycling, where no obvious leaching or aggregation of Rh active metals were observed after a 4th run of experiments.As shown in Table 2, all the examples mentioned imply that SACs can be a valid alternative to bridge both homogenous and heterogeneous catalysts for C\u2013C coupling reactions. However, the development of SACs with notably improved performances relies on the contribution of both theoretical and experimental investigations. DFT methods can be applied to investigate crucial aspects that are not directly accessible by experiments, such as the nature of the active sites of different SACs and the ways substrate molecules interact with single atoms [444]. Overall, the electronic metal-support strong interactions are a critical concern to increase the catalytic performance of coupling, and thus, modulation of the charge density of anchored single metal atoms should be emphasized in future research.Selective hydrogenation represents essential processes in the organometallic chemistry process, particularly in the petrochemical and fine chemical industries. For petrochemicals, selective hydrogenation is the most common route to eliminate the impurities such as alkynes and dienes in the ethylene industry for downstream polymerization [445]. Meanwhile, in the pharmaceutical industry, alkenyl, carbonyl, and carboxyl functional groups of the feedstock are required to be selectively reduced through H2 to their corresponding alkenes, alcohols, and amine products which are key intermediates for fine chemicals production [446]. Notably, a quarter of the chemical industrial processes include at least one hydrogenation step, and therefore it is not surprising that the selective hydrogenation reaction is one of the hot topics investigated in the catalysis field [447]. However, it is a challenging task when two or multiple functional groups coexist in the substrate and also, the hydrogenation of CC bonds is much easier than that of the CO bonds, thermodynamically favored by 35\u00a0kJ/mol [448].A new generation of catalysts for selective hydrogenation reactions has been developed to tackle the challenges by introducing the \u201cactive site isolation\u201d strategy. Atom assemblies and isolation techniques can exhibit different physicochemical properties in altering different hydrogenation mechanisms and show a better catalytic hydrogenation activity compared to NP counterparts [449,450]. Lately, SACs have also been widely applied in the selective hydrogenation of styrene, acetylene, glycerol, crotonaldehyde, and nitriles; mainly attributed to lowering the activation barriers, governing catalytic reactivity, enhancing the adsorption model, and also possessing uniform single active sites [213,451]. For example, isolated Pd atoms on a Cu surface lower the reaction barriers of both hydrogen uptake and subsequent desorption from the Cu metal surface, enhancing the selectivity of styrene hydrogenation [22]. Meanwhile, anchoring single Rh (Rh1) atoms to Mo edge vacancy sites of 2-dimensional MoS2 could also facilitate the H2 dissociation in hydrogenation [401]. There are also studies reporting that encapsulating Ni atoms with transition metals can improve covalent chemical bonding, owing to the inherent vulnerability of nickel-based SACs under acidic hydrogenation conditions [452].A comprehensive summary of the catalytic performances of different SACs for selective hydrogenation reactions is listed in Table 5\n. Although the operational conditions, reactor configuration, catalyst loading, temperature, and pressure may vary greatly, it has been shown that a low Pt loading can be incorporated into graphite shells, generating \u2018carbon onions\u2019 (Pt/C) via an arc-discharge method. This was found to provide remarkable conversion and reaction selectivity, comparable with other hydrogenation SACs [453]. Notably, the Pt@C exhibits a high catalytic reactivity and stability towards the chemo-selective hydrogenation of functionalized nitroarenes under mild reaction conditions. High selectivity of p-chloroaniline at >99% was obtained using EtOH under optimized reaction conditions, 40\u00a0\u00b0C, 1.0\u00a0bar of H2 pressure, 40\u00a0mg of catalyst loading, and 1\u00a0h reaction time. On top of that, the synthesized Pt/C catalyst displays a superior reusability performance over at least 10 cycles and without any loss in hydrogenation activity and selectivity, suggesting that the graphitic shells of carbon \u2018onions\u2019 prohibited a chemical coarsening of the Pt single atoms, which alters the effective penetration channels for the transport of ions and electrons during the catalytic reaction [453]. The proposed encapsulated graphite shells of the SAC were also processed using HRTEM images (Fig. 18\n(a\u2013c)), with a well-distributed Pt(111) interplanar size of 0.255\u00a0nm. This clearly shows the effect of arc medium concentration, where Fig. 18 (a) utilizes a 0.975\u00a0mM salt which generates Pt nanoparticles, whereas an arc medium concentration of 0.0195\u00a0mM created a dispersed catalyst that was atomically resolved (Fig. 18 (b)). The x-ray diffractograms in Fig. 18 (d), clearly show that the Pt SAC does not have a (111) feature, contrary to the Pt NP catalyst.Lately, Zhang and colleagues [198] have studied the anchoring effect of mesoporous \u03b3-Al2O3 on Pt atoms, likely on the catalyst's stability through unsaturated pentahedral Al3+ coordination for the hydrogenation of a ketone. The superior catalytic activity highlights the applicability of the catalyst for hydrogenation reactions in a small amount of Pt species loadings on Al2O3. As reported in a previous study, aromatic rings normally coordinate with multiple metal atoms before undergoing hydrogenation and remain to interact with the metal surface during stepwise hydrogenation [457]. However, the proposed mechanism is not possible for SACs as found in this study [456] where the hydrogenation ring on Pt/Al2O3 is almost fully suppressed. Alternatively, the Pt species favors CO bond adsorption and activation forming an \u03b71(O) configuration [458,459]. In the \u03b71(O) configuration, the acetophenone will be adsorbed on to a Pt single-atom site for reacting with H2 to form 1-phenylethanol. Then, the intermediate product transfers to the Al2O3 support where it is strongly bound before desorbing into the solution phase, suppressing the deactivation of a Pt site, and enhancing the reusability of the catalyst for the next cycle. A similar result has been obtained by Lucci et al. [257] in which the synthesized Pt/Cu(111) SAA can be reused more than six times TPD cycles with constant selectivity of butadiene to butene (\u223c25%) as shown in Fig. 19\n(a) [257]. Further quantification of Pt atoms through CO titration after each run has highlighted the durability of Pt/Cu(111) SAC where the concentration of Pt atoms on the surface layer of the catalyst remained consistent with the number of Pt atoms present prior to each hydrogenation reaction, suggesting that the single isolated Pt atoms in Cu are capable of H2 spillover without breaking C\u2013C bonds as well as reduce the possibility of Pt poisoning. As seen in Fig. 19(b), the addition of Pt onto Cu NPs enhanced the rate of hydrogenation, where a higher hydrogenation activity was observed in Pt0.2Cu14/Al2O3 than Pt0.1Cu14/Al2O3. Additionally, the addition of Pt single atoms to the catalyst could lower the hydrogenation reaction temperature (onset at 40\u00a0\u00b0C), which is 35\u00a0\u00b0C, i.e., lower than that of the monometallic Cu15/Al2O3 catalyst under the same conditions. In order to demonstrate the hydrogenation capability of the Pt\u2013Cu SAC in different stressful conditions (with impurities), the authors tested the SAC in the presence of excess propylene and found that the propylene has no significant effect on the activity and selectivity of hydrogenation to butadiene as displayed in Fig. 19(c). At below 120\u00a0\u00b0C, \u223c100% of butadiene was converted accompanied by a minor propylene concentration (<1%) converting to propane, implying that the Pt\u2013Cu SAC maintains active and is stable. The use of Pt in Cu NPs (Pt1Cu20/Al2O3) has also been shown recently for the liquid phase hydrogenation of furfural, here a promoted Cu nanoparticle was found to be far more active and selective than bulk bimetallic alloys and monometallic equivalents [249]. Additionally, for the same reaction, an array PdCu SAA was created by Islam and co-workers, with decreasing Pd content to determine the atomic limit required for efficient hydrogenation [46], critically finding that 0.0067\u00a0wt% of Pd could be used to radically improve the reactivity of a Cu host nanoparticle on \u03b3-Al2O3. Another work using a Pd\u2013Cu SAA is by Jiang and co-workers [454]. This work exploited the change in the structure coordination during the impregnation of ultra-low Pd loading (50\u00a0ppm) in a host Cu nanoparticle. They found that the Pd1/Cu SAC is highly effective for both hydrogen spillover and selective hydrogenation. Notably, The Pd1/Cu catalysts displayed excellent catalytic performances in the semi-hydrogenation of phenylacetylene to styrene at 303\u00a0K under 0.1\u00a0MPa H2. The selectivity of \u223c96% towards styrene was achieved at a conversion of 100%. This work was supported by DFT calculations which highlights the benefit of atom arrangement on the activity of the catalyst itself, globally finding the rate of reaction for Pd\u2013Cu (111) was substantially lower than a Pd\u2013Cu(100).There is another interesting well-dispersed single/pseudo Pt SAC catalyst impregnated on mesoporous WOx developed by Zhang's group [460]. Under the reaction conditions of 160\u00a0\u00b0C, 1\u00a0MPa H2 pressure, and a high glycerol concentration (50%), the Pt/WOx SAC processed a very high space-time yield (3.78 gPt\n\u22121 h\u22121) towards 1,3-propanediol (1,3 PD) as shown in Fig. 20\n(a). Based on the transition of the catalyst in preparation (Fig. 20(b\u2013d)), a well homogenously dispersed Pt atom of 2.59\u00a0wt % was impregnated on WO3 support without any formation of nanoclusters, suggesting that the isolation of Pt over WOx was successfully achieved. To further understand the physiochemical property of SACs a mechanism for how the Pt/WOx SAC behaves for the selective glycerol hydrogenation was proposed in Fig. 20(e), [460]. Firstly, the unshared\u2010pair of electrons in the glycerol's O atoms was trapped by the unoccupied W6+ d orbital, forming an ether\u2010like bond with a W atom (Step 1). The strong interaction with the W atom will weaken the bond between the O and H atoms and facilitate the oxidation of an H atom (Step 2). Then, extraction of protons from the terminal O atom of WO occurred and formed W\u2013OH, while the W6+ species was partially reduced (Step 3). This finding was proven by Raman spectroscopy, where the terminal WO band intensity decreased significantly after contacting with glycerol. In step 4, the WOx Br\u00f8nsted acid sites were consumed, catalyzing the dehydration pathway and stabilizing the formation of the secondary carbocation from glycerol. Owing to the oxophilic characteristic of W species, the H2 is assumed to be heterolytically dissociated on the interface between Pt and WOx, exhibiting both acid sites (H\u03b4+) and hydrogenation sites (H\u03b4\u2212). Thus, rapid hydrogenation of 3-hydroxypicolinic acid can occur and gives rise to a high yield of 1,3-PD (Step 5). This study provided an in-depth synergistic mechanism between Pt and WOx species for the production of 1,3-PD. The design of the WOx supported pseudo\u2010single atom Pt catalyst yielded a high selectivity of 1,3-PD (45.7%) under a low H2 pressure environment.Despite many experimental works reported on the robust nature of the SACs for selective hydrogenation reactions, there is still a lack of molecular calculations and microkinetic simulations to back up the hydrogenation activity and mechanisms [463\u2013465]. In 2018, Thirumalai and co-authors [461] reported the reactivity of a series SAAs consisting of Au, Ag, and Cu nanoparticles doped with single atoms of Pt, Pd, Ir, Rh, and Ni in the hydrogenation of acetylene to ethylene via DFT calculations. They reported that from the d-band model generated by Hammer and N\u00f8rskov, AuPd and AgPd were chosen as the potential candidate for hydrogenation. The findings indicate that the acetylene most likely adsorbs at the FCC sites of AuPd and AgPd. By comparing the binding energies of acetylene at the FCC site and atop sites, it reveals that acetylene was less stable on the atop sites by 0.111\u00a0eV for AuPd and 0.008\u00a0eV for AgPd, meanwhile the ethylene adsorption at the FCC sites and atop sites differ by 0.23\u00a0eV for AuPd and 0.019\u00a0eV for AgPd. Notably, the acetylene and ethylene prefer bonding as \u03b1-bonded complexes on pure metal surfaces. However, in the presence of a reactive Pd atom in a relatively inert host, they bind strongly with the surface through \u03b1-complexes in an atop conformation. Furthermore, the energetics of hydrogenation is more favorable for single atom alloys rather than their respective pure host metals (Fig. 20(f)), deducing that the single atom alloys are favorable for the formation of the vinyl intermediate, attributing to the strong adsorption on the Pd atom which resulted in excellent selectivity towards the formation of ethylene. The co-adsorption energy of intermediate on AuPd and AgPd are very close, suggesting that the Pd atom in the alloys is mainly responsible for driving the reaction forward. As noted, the energy barrier for ethylene desorption is almost negligible on single atom alloys, resulting in immediate desorption which prevents further hydrogenation of ethane.Moreover, a DFT study in conjunction with Scaling Relations Kinetic Monte Carlo (SRMC) simulations reported by J\u00f8rgensen and Gronbeck [462], found that the main reaction mechanisms for hydrogenation of acetylene\u2212ethylene will be C2H2 adsorption. However, the C2H2 adsorption is exothermically stronger on Pd(111) as compared to Cu(111) and Pd/Cu(111). Meanwhile, the H2 dissociative adsorption is barrier less on the Pd-containing surfaces, but it is high for the Cu(111). The observation for the absence of an H2 dissociation barrier on Pd and a considerable barrier on Cu is consistent with the previous studies [22,466]. Based on these simulations, it can be concluded that the Pd/Cu(111) SAA has a selectivity higher than that of Pd(111), mainly due to the weak binding of ethylene on Cu as compared to Pd while a strong binding of C2H4 at the edges and corners sites hinders the ethylene desorption before further hydrogenation. In short, acetylene\u2212ethylene hydrogenation should contain minority sites that readily dissociate hydrogen and the majority sites where ethylene is weakly adsorbed. Nonetheless, all the above findings have opened a promising avenue to the rational design of SACs for selective hydrogenation. Despite much-perceived subjectivity on the stability of SACs under high-temperature conditions for selective hydrogenation reactions, many experimental works have demonstrated that SACs can provide stable activity under many reaction cycles or reaction times for this application.SACs have emerged as a new frontier in catalysis for hydrogen production from methane reforming and water-gas shift reactions. Dry Reforming of Methane (DRM) has received much attention in the hydrogen production sector, as this sustainable process exploits two major greenhouse gases (GHG), carbon dioxide and methane to produce industrially important syngas. Table 4 shows the application of SACs for hydrogen production, specifically in DRM and the WGSR over the last five years. Numerous supported precious metal SACs (e.g., Pd, Pt, Ru, and Rh) and a few transition metals SACs (Ni and Co) have been used for DRM. Despite the high coking resistance of precious metal-based catalysts, the use of precious metals in the synthesis of catalysts is often regarded as a non-sustainable approach due to the high cost and low availability of noble metals, which are typically limited for large-scale applications [467]. On the other hand, Ni-based catalysts are generally more economical and are more abundant. Despite the low cost and high availability, Ni exhibits a high sintering tendency, poor deactivation resistance, and high affinity toward coke deposition on the active sites under a reaction temperature over 800\u00a0\u00b0C [468,469]. Ni-based catalysts are well-known for their high sintering tendency via particle migration and Ostwald ripening under a high reaction temperature and steam environment. The sintering and agglomeration of the active phase increase the particle size and reduces the dispersion of the metal atoms [469,470]. The number of atomically dispersed active species also decreases significantly and eventually leads to poor activity performance. Furthermore, severe carbon deposition on the Ni active site was also reported extensively in the literature. With such bulky NPs formed from the sintering phenomenon, a side reaction of methane pyrolysis readily takes place on the metallic NP sites due to the very high adsorption energy. The methane pyrolysis reaction could produce a layer of carbonaceous material on the metallic surface which in turn leads to catalyst deactivation after a long period of reaction time [471].As previously mentioned, the coking resistance and stabilization of an atomically dispersed active phase on a support material during a long time on stream are still regarded as one of the few major challenges encountered in the DRM and are yet to be resolved by academic and industrial practitioners. In line with the efforts in addressing the coking resistance and stabilization issue, numerous previous works have reported the considerable effect of CeO2 on the size, dispersion, stability, and deactivation resistance of active sites [203]. For the synthesis of SACs, it is summarized that the support material should possess a high affinity with the active phase leading to a high dispersion and strong metal-support interaction (SMSI) [164]. A list of SACs used for methane reforming, steam reforming, and water gas shift reactions is tabulated in Table 6\n. In a recent work by Tang et al. [368], a novel bimetallic Ni/Ru SAC supported on CeO2 was synthesized and proposed for the DRM application. As shown in AP-XPS spectra (Fig. 21\n(a)), the catalytic surface of Ce0.95Ni0.025Ru0.025O2 SACs consists of two sets of atomically dispersed Ni and Ru species. Also, the fraction of Ce3+ during the reaction at 550\u00a0\u00b0C was much higher than before the reaction, as evidenced by the formation of a plateau in the region of 885.2\u00b11.5\u00a0eV, implying that no overlapping of photoemission features of Ce3+ and Ce4+ of CeO species in the catalyst. Based on the experimental findings, both active species were found to be highly active for methane reforming with a high turnover rate of 73.6H2 per site per second at 833\u00a0\u00b0C (Fig. 21(b)). The conversion of CH4 on Ce0.95Ni0.025Ru0.025O2, 91% was much higher than that of Ce0.95Ru0.05O2 and Ce0.95Ni0.05O2 at 700\u00a0\u00b0C, implying that there is a positive synergistic effect between Ni and Ru cations which enhance the DRM activity. Furthermore, computation studies also uncovered the synergetic effects and complement functions of the atomically dispersed Ni and Ru under a low concentration of 2.5 metal atomic %. Ni atoms are highly active in the adsorption of CH4, and Ru atoms have a high affinity toward CO2. From the operando studies of chemical and coordination environments, both Ni and Ru single atoms anchored on the CeO2 surface remained in a cationic form instead of a metallic state. This was found to promote the catalytic performance of the SAC significantly up to 600\u00a0\u00b0C, outperforming NP counterparts, as depicted in Fig. 21 (c).Akri et al. [164] synthesized a highly active and carbon-resistant Ni SAC supported on hydroxyapatite (HAP) using a Strong Electrostatic Adsorption (SEA) method. The 0.5\u00a0wt.% Ni SAC exhibited the highest CO2 and CH4 reaction rates of 816.5\u00a0mol/(gcat h) and 1186\u00a0mol/(gcat h), respectively, which was four and five times higher than the reaction rate of nickel NP catalysts. The 0.5\u00a0wt.% Ni SACs displayed excellent carbon deposition resistance as evidenced by the negligible weight loss of spent Ni SACs when characterized under thermal gravimetric analysis (TGA). Despite its high activity performance and carbon deposition resistance, the Ni SACs suffered from poor stabilization of the atomically dispersed Ni phase. Severe sintering and aggregation of Ni atoms were observed in the 0.5\u00a0wt.% Ni SACs, which led to its high deactivation rate after a few hours of reaction. The same research team also attempted to reinforce the stability of the previous Ni SACs by using a polyvinylpyrrolidone (PVP) assisted preparation method. A small amount of PVP was added during the co-precipitation process of the Ni SAC. It was found that the catalytic stability of Ni SACs improved significantly with very little carbon deposition on the catalyst surface. Such improvement in the catalytic behavior could be attributed to the highly dispersed Ni single atoms on the surface, hindering the inner active site from sintering phenomena during the reaction.In another study by Akri et al. [369], the stabilization effect of ceria-doped hydroxyapatite (Ce-HAP) for atomically dispersed Ni species was investigated. From the in-situ XPS and Temperature Programmed Reduction (TPR), both characterization techniques unambiguously revealed that the ceria-doped HAP stabilized the atomically dispersed Ni from sintering and aggregation under a high temperature-reducing H2 environment. Despite the high reduction temperature, the Ni(OH)x and NiO species on the Ce doped support remained unreduced and displayed high resistance behavior as compared to the undoped counterparts. In the end, the Ce species was reported to act as a stabilizing anchor for the atomically dispersed Ni rather than to suppress carbon deposition. As compared to the SAC work by Tang et al. [368], the 2\u00a0wt.% Ni SAC supported on Ce-doped HAP in Akri et al. [369] delivered similar catalytic performance and superior stability under identical reaction conditions, without using a precious metal. A low 0.5% Ni/HAP SAC (20 times less Ni loading) exhibited a comparable CH4 reforming activity to that of the 10% commercial Ni/HAP under similar reaction conditions.Despite several experimental studies reporting that Pt SAC is highly active for low temperature (120\u2013400\u00a0\u00b0C) selective reforming and WGS, the arguments on: a) The characteristic behavior of Pt atoms in the WGS reaction at low temperature, b) The stability of Pt atoms under a reducing atmosphere and an elevated temperature, and c) The isolated Pt atoms behave only as spectators in the process [475,476]. The fundamental questions have finally been resolved by Ammal and Heyden [477] through a DFT calculation, in which positively charged single Pt atoms stabilized on a TiO2 (110) surface can be as active as Pt clusters for the WGS reaction at low and high temperatures. The calculation revealed that the interface edge Pt and single Pt2+ sites exhibited a high WGS activity at low temperatures whereas the corner Pt interface sites become active at higher temperatures. As such, the single Pt2+ sites acted as a stabilizer on an active reducible surface such as TiO2 while the oxygen vacancies in the support play a significant role in enhancing the WGS activity. A possible reaction pathway for the WGS was also proposed, containing the redox, carboxyl, and formate pathways, shown in Fig. 22\n(a). As reported, the redox reaction was the dominant pathway between the temperature range of 200\u2013400\u00a0\u00b0C, while high TOFs are possible for this active site. Meanwhile, the carboxyl pathway with redox regeneration was less favorable than the formate pathway with redox regeneration. Its rate was very close to that of the classical redox pathway at temperatures below 300\u00a0\u00b0C. In addition, the single Pt2+ sites tend to stabilize on the CeO2(110) sites with H as ligands, owing to the similar characteristics and advantages of both homogeneous and heterogeneous catalysts [478,479]. Based on the free energy profiles as displayed in Fig. 22(b), we can also clearly see that the presence of additional surface H atoms could reduce the energy barrier for the interfacial H-transfer process (TS23) by about 0.2\u00a0eV, compared to the CO-assisted redox pathway (TS18), suggesting that the associative carboxyl with redox regeneration pathway is likely the most favorable pathway.The excellent catalytic activity and stability of Pt nanoclusters reported by Ammal and Heyden [477] are in good agreement with Guo et al. [468]. In 2014, Li's group [473] synthesized stable and highly active Pt-based SACs for methanol steam reforming using the desorption-absorption method, by embedding the isolated precious metal atoms of Pt and Au onto a ZnO surface. A spin-polarized DFT calculation coupled with STEM characterization was performed to investigate the intrinsic nature of the active sites of the catalyst. The DFT calculation revealed that the corresponding formation energies of single Pt and Au atoms were 0.22 and 0.86\u00a0eV, respectively, which were much lower than the reservoirs in equilibrium with large metal counterparts. This observation indicates that the embedded Pt and Au are thermodynamically stable and resistant to segregation during the catalytic reactions and thus, providing a stronger binding toward the intermediates, as well as lowering reaction barriers. The enhancement of the catalytic activity can be seen where the TOF found in the single Pt sites embedded onto ZnO(1010) surfaces are over 1000 times higher than that of the pristine ZnO. The hypothesis was further confirmed by electron beam irradiation, where the isolated Pt single atoms were found to be relatively stable after anchoring onto ZnO(1010). All the HAADF images show no Pt or Au clusters/particles in the synthesized Pt1/Au1/ZnO SAC.In short, to cater to higher reforming rates, multi-functional SACs should be developed with a significant number of interfacial sites, resulting from the presence of individually dispersed metal atoms on the support. This could avoid the coking resistance and stabilize the atomically dispersed active phase on a support material under a long time-on-stream. Moreover, due to the involvement of multiple species and commonly complex reaction mechanisms in the catalytic reforming process, the usage of DFT for the study of reaction-free energy to unravel potential reaction pathways provides many useful insights for designing SACs from first-principles. Furthermore, although methane-based reactions are the most applicable for catalytic reforming in the industry, more effort should be carried into branching out toward other chemical species to understand the possibilities of SAC in additional applications.Extending to the energy matrices, novel SACs have been widely applied in the selective oxidation field. As such, the unique catalytic performance of SACs has demonstrated a huge prospect in various oxidation reactions such as CO oxidation or PROX, aerobic oxidation of alcohols, formaldehyde oxidation, and methane oxidation [13,480]. The sub-nanometer clusters of single metals were reported to have a better enhancement in catalytic activity or selectivity compared to larger bulk nanoparticles [481,482]. Apart from that, the utilization efficiency of the metal catalyst and selectivity, either for the adsorption or desorption activities of the active species can be modified via metal atom isolation, which directly influences the reactions kinetics [480,483]. Due to the interesting behavior found in metal SACs, these have attracted numerous researchers to have an in-depth understanding of their behavior and mechanism [102].However, a common problem faced for selective oxidation reactions is the decrease in size from a nanoparticle to a single atom, in which the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters of the catalyst [13]. This can be prevented by implementing a high surface area support material that could interrelate well with the metal atoms, and postulating an isolated metal that can accommodate the metal surfaces, metal oxides, and carbon materials in the system [484,485].According to Duprez and Cavani [486], selective oxidation is achievable using the famous Mars and van Krevelen mechanism that involves different transition metal ion oxides that display redox properties such as Cu, V, Mo, Cr, Te, Sb, Bi, and Fe [487]. Among all the metal ion oxides, the atomically dispersed Co and Cu catalysts have been reported to exhibit the highest catalytic activity in the selective oxidation of benzyl alcohol and 5-hydroxymethylfurfural (M\u00a0=\u00a0Fe, Cr, Co, Ni, Cu) [488]. In 2017, The first pioneering work of non-noble Co-based SAC for selective oxidation was reported by Guan's group. An atomically dispersed Co on N2-doped graphene (denoted as Co-NG) in an ammonia medium via pyrolysis technique was synthesized [489]. Notably, a high benzyl alcohol conversion (94.8%) and benzaldehyde selectivity (97.5%) were achieved over 6\u00a0h and 120\u00a0\u00b0C using a small amount of Co-NG (5\u00a0mg). However, under the absence of N2 doping, a much lower conversion of 42.5%) than Co-NG in selective oxidation of benzyl alcohol was attained. As reported previously, the single metal atom on a carbon matrix can be stabilized by introducing N atoms as an \u201canchor\u201d [99]. The N2 doping does not solely strengthen the interaction between the metal atom and the support but also promotes electron transfer, which resulted in a firmly anchored, atomically dispersed metal atom on supports [490,491]. A possible catalytic reaction mechanism for the aerobic oxidation of benzyl alcohol over Co-NG was also postulated as follows: Firstly, the oxygen molecules were weakly adsorbed on the Co center, followed by electron transfer activation to form a superoxide species (Co 3d orbitals to O2 2p antibonding orbitals). Lastly, the superoxide species will react with the hydrogen bonding of benzyl alcohol to produce benzaldehyde [492,493].Furthermore, Harrath et al. [364] also studied the catalytic mechanism of M/ZrO2 SAC (M\u00a0=\u00a0single atom of Rh, Pd, Ir, Pt, Fe) for a one-step conversion of CH4 to CH3OH. Their work also found that Rh/ZrO2 SAC induced the dissociative adsorption of H2O2 on its surface with great binding energy (\u22122.87\u00a0eV), favoring the selective oxidation of CH4 pathway. Subsequent steps in the reaction pathway of Rh/ZrO2 SAC include the adsorption of methane, and formation of C\u2013H bond (which forms a methyl radical and HOO\u2013Rh site) to further produce CH3OH or by-product CH3OOH (Fig. 23\n(a)). Apart from that, the non-noble Fe/ZrO2 SAC was also expected to give high selectivity of methanol due to the lower energy barrier for methyl radical formation (0.49\u00a0eV lower) and methanol formation (0.13\u00a0eV lower) on O\u2013Fe/ZrO2 compared to O\u2013Rh/ZrO2 (Fig. 23(b)). Notably, the pathway to produce the CH3COOH by-product via a Fe/ZrO2 SAC was suppressed due to a kinetically thermodynamic unfavorable energy barrier of 2.77\u00a0eV, suggesting a high selectivity for the main product CH3OH can be obtained.Inspired by the excellent results of the application of SACs in various selective oxidative reactions as shown in Table 7\n, more studies further challenged the selective benzylic C\u2013H oxidation of hydrocarbon derivatives under mild conditions. This is because most studies have reported that selective oxidation of saturated C\u2013H bonds is difficult and aggressive conditions (>120\u00a0\u00b0C and 10\u00a0bar oxygen pressure) are usually required to obtain a high selectivity of desired products [494]. In 2019, Bakandritsos together with his co-workers disclosed a mixed-valence Cu-based SAC for oxidative homocoupling of benzylamines [282]. Fig. 24\n(a) shows that the G(CN)\u2013Cu SAC was synthesized using the coordination of Cu(II) ions to CN-functionalized graphene (cyano-graphene, G(CN)) where the graphene-induced charge-transfer reduced the Cu(II) ions anchored to G-CN into the Cu(I). Surprisingly, the G(CN)\u2013Cu SAC was able to yield an excellent conversion (up to 98%) and selectivity (up to 99%) under mild conditions (85\u00a0\u00b0C, 1bar). In addition, the G(CN)\u2013Cu SAC remained at a very high conversion rate (94%), even after 5 recycling steps with no change in product selectivity (98%) (Fig. 24(g)). This observation was supported by the TEM images, in which a clear detection of Cu atoms can be observed before and after the catalytic reaction (Fig. 24(b\u2013f)), suggesting the active sites of the Cu are not prone to sintering even during a high reaction temperature. In addition to that, the XPS analysis also further confirmed the high catalytic activity of the G(CN)\u2013Cu SAC, in which there was no change of the Cu mixed valance state from 1st to 5th cycles (Fig. 24(h)). The G(CN)Cu also displayed a high turnover frequency (TOF\u00a0=\u00a013\u00a0h\u22121) at low temperatures (<100\u00a0\u00b0C), proving that it has a strong electron-withdrawing character (withdraw electron from CF3-substituted benzylamine) than that of the current best performing NP catalysts in the literature (e.g., CuO nanoflakes and Cs/MnOx) [495,496]. Through the DFT analysis and EPR measurement, a possible oxidative amine coupling mechanism for the study was proposed as shown in Fig. 24(i). Firstly, the oxidative dehydrogenation of the benzylamines started with O2 reduction in the active copper enzymes through its preferential coordination with Cu(I) centers (step 1), which leads to the formation of copper-oxyl intermediate between the Cu ions (step 2). In order to yield the formation of an imine, a two-hydrogen abstraction from the neighboring amine by the reactive oxyl species was essential (step 3). In step 4, the hydroxyl radicals produced were trapped and finally, the NH3 and N-benzylidene-benzylamine were produced through the amine\u2013imine coupling. Lastly, the catalyst was regenerated and can be reused for further oxidative coupling of benzylamines.With respect to theoretical screening works, there have been few studies investigating the fundamental mechanism of oxidation via SACs [498,499]. For instance, the fundamental insights of CO oxidation catalyzed by using single Au atoms supported on Thoria (Au/ThO2) through DFT with Hubbard-type On-site Coulomb interaction simulation (DFT\u00a0+\u00a0U) were reported lately [498]. From the computational study, three main steps mechanism of facilitation of the Au-doped ThO2 (111) surface for CO oxidation was analyzed: 1) Reaction between the gaseous phase CO between the lattice O2\n\u2212 through Mars-van Krevelen (MvK) mechanism, 2) the adsorption process of gaseous phase of O2\n\u2212 at the vacancy site to form the activated O2\n\u2212, 3) CO molecule reacts with O2\n\u2212 to form the intermediate of OCCCO* which breaks down into CO2, and O* adatom. Based on the findings, the developed Au-doped ThO2 (111) showed a positive catalytic activity for CO oxidation with a lower adsorption rate and the rate-limiting step was determined to be the adsorption of O2 which takes place at the ThO2 site on the surface.Han et al. [500] investigated the Pd stripe and Pd single atom-doped Cu(111) surfaces for COOCH3 selective oxidation. Specifically, three structures of Pd monolayer, Pd4Cu8 and Pd single atom (Pd1) on Cu(111) were studied as shown in Fig. 25\n(a)). For the conversion of COOCH3 to dimethyl oxalate (DMO), the strain effect decreases the activation barrier on Pd1\u2013Cu(111), while the ligand effects caused a non-dominant increase in the activation energy barrier (Fig. 25(b)). This effect was similar for Pd monolayer but was the contrary for Pd4Cu8/Cu(111). From Fig. 25(c), it can be clearly seen that the Pd1\u2013Cu(111) was showing an exothermic reaction energy associated with low activation energy, indicating that the oxidation reaction occurs much easier on the Pd1\u2013Cu(111) as compared to its counterparts. In addition, two possible reaction pathways related to COOCH3 oxidation were studied through microkinetic analysis. In both pathways (Fig. 25(d and e)), the results were in good agreement that the DMC (108.8\u00a0kJ/mol) was more favorable to be produced compared to the DMO (192.6; 107.5\u00a0kJ/mol) due to a lower energy barrier of the rate-determining (Fig. 25(d)). A similar observation was also depicted in Fig. 25(e) in which the activation barrier of the Pd4Cu8/Cu(111) and Pd1\u2013Cu(111) surfaces was 107.5\u00a0and 91.6\u00a0kJ/mol, respectively [500].Lately, another remarkable investigation of the synergistic effect of tri-metals in a Crown Jewel-Structured (IrPd)/Au SAC for selective oxidation has been revealed by Zhang and co-authors [493]. The presence of negatively charged Au and Ir atoms has elucidated two kinds of charge transfer modes, creating a synergistic effect that enhanced the catalytic activity of the (IrPd)/Au SACs to a maximum level. In order to further understand the synergistic effect, a DFT analysis was performed. The study revealed that electron transfer between O2 and anionic Au and Ir atoms possessed a hydroperoxo\u2013like species via donating an excess electronic charge to the antibonding orbital. The adsorbed O2 molecule on the (111) face of Au of (IrPd)Au model clusters possessed the highest negative charge numbers, suggesting that this is the key factor that leads to an enhancement of synergistic catalytic activity for selective aerobic oxidation.Over the decades, photocatalysts and electrocatalysts have attracted huge attention as a method of addressing the global environmental issue and energy crisis [501,502]. Along this line, SACs have been engaged as promising candidates in the fields of photocatalysis and electrocatalysis due to their high catalytic activity, stability, and pathway selectivity [19,503]. In recent years, SACs have been utilized for a wide range of applications, such as hydrogen evolution, oxygen evolution, CO2 reduction, pollutant removal and degradation, and chemical synthesis [504\u2013506].Photocatalyst is a unique class of materials that can accelerate chemical reactions on exposure to a specific type of light (UV, UV\u2013Vis, or Visible). Photocatalysts offer sustainable and environment-friendly catalytic solutions by utilizing green and inexhaustible solar light to facilitate chemistry reactions compared to traditional thermal activation processes. In general, photocatalysts work using the same principle as semiconductors, when the photocatalyst is exposed to light, an electron in the valence band can absorb the energy of photons and is excited to the conduction band, leaving a hole (positive charge) in the valence band. Therefore, the electron\u2013hole pair is produced in this process, which can provide both oxidation and reduction environments to accelerate chemical reactions [507]. Various bulk materials have shown photocatalytic capabilities, including metal oxides (TiO2, V2O5, ZnO, Al2O3, Fe2O3) [508], carbon dots [509], metal\u2013organic-frameworks (MOFs) [510], 2D materials [511], and plasmonic metals [512]. However, current photocatalysts are facing great challenges because of fast photogenerated electron\u2013hole recombination, limited visible-light response, and slow electron transport [513]. Along this line, SACs have emerged as capable photocatalysts that could be the answer to overcoming the typical problems that hinder conventional photocatalysts.On the other hand, electrocatalyst is a type of catalyst used to increase the rate of electrochemical reactions by facilitating the conversion between electrical and chemical energy [514]. The reaction processing in electrolysis is dominated by circuit-induced carriers, which can drive reactions far from their equilibrium potential, enabling access to difficult reaction pathways. Solid metals or oxide electrodes are usually used as heterogeneous electrocatalysts and the electrochemical processes occur at or near the liquid\u2013solid interface. These reactions usually include multistep ion/electron coupled electron transfer with high reaction kinetics, requiring efficient catalysts to accelerate the processes [515]. Homogeneous electrocatalysts are soluble or dispersed in solutions, activating the reactions in the solutions. The processes are indirect electron transfers instead of the direct electron transfer between electrode and an electrolyte. A vast array of materials has been used as electrocatalysts, including noble metals, noble metal oxides, transition-metal-based materials, MOFs, and metal-free-carbons [516\u2013520]. Electrocatalysts have been widely used in energy storage and conversion, metallurgy, and chemical synthesis applications. However, a bottleneck of wide spread electrocatalyst usage is the high cost of noble metals and low natural abundance, limiting the large-scale development of electrocatalysts. Noble metals are usually present in the forms of nanoparticles in conventional catalysts, however, the previous concept of efficient nanoparticles is flawed by the fact that these are in fact to be considered bulk materials due to extended terrace sites. Reducing the size of catalysts is an efficient method to expose more high-energy active sites. SACs offer promising access to address this issue due to natively possessing a maximum atom utilization. In this section, we highlight and introduce recent advances in electrocatalysis and photocatalysis using SACs, focusing on applications in CO2 conversion and hydrogen fuel cells (Table 8\n).The CO2 released in the atmosphere from both large, industrial point sources, and small, mobile sources are considered the main culprit for global warming. Its capture and utilization have received growing attention since it is a promising strategy for reducing its concentration in the atmosphere, and simultaneously obtaining valuable chemicals and fuels [528]. However, due to the very stable structure of the CO2 molecule, its conversion requires the utilization of catalysts. In this regard, electrocatalytic CO2 reduction reaction (CO2RR) holds great promise among various chemical approaches [529,530], since it can be carried out under ambient conditions with promising activity [530]. For this process to be environmentally friendly, the energy input should be obtained from a renewable and non-CO2 emitting electricity source, and combined with the utilization of \u2018green\u2019 electrocatalysts. Noble metal atoms are known for their superior activity, selectivity, and long-term stability in CO2RR, but their high cost and scarcity hinder their extensive use [531,532]. However, this issue is reduced in SACs, where the metal loading is notably decreased.The first example of electrochemically driven CO2RR over SACs was proposed in 1974 by Meshitsuka et al. [533] who showed that cobalt and nickel phthalocyanines attached to graphite electrodes are active catalysts for the electrochemical reduction of carbon dioxide. Since then, SACs have been extensively explored and several other promising SACs for CO2RR have been proposed [50,534,535]. Unlike gas-phase reactions, electrochemical reactions would have the additional requirement of high-conductivity support materials, such as carbon or doped metal oxide. The introduction of heteroatoms in the support matrix was found to be a useful strategy for modulating the electronic structures and stabilizing the metal atoms, resulting in an overall enhanced catalytic activity [536,537]. Furthermore, the process of \u201canchoring\u201d the metal atom to the support involves a charge transfer among the central metal sites and the substrate [523]. This type of metal-support interaction has been extensively investigated in SACs to regulate the electronic structure of catalysts, which consequently affects the intrinsic activity of active sites toward various electrocatalytic reactions [538]. Moreover, the local environments of the metal active sites in atomically dispersed metals determined the significantly different behavior between SACs and their bulk and nanoparticle counterparts [539,540]. For instance, while the bulk and nanoparticle electrodes of Mn, Fe, Co, and Ni mainly produce H2 [540], the M\u2212N\u2013C (M\u00a0=\u00a0Mn, Fe, Co, Ni) SACs show great activity for the electrochemical CO2 conversion to CO, and the suppression of the competing HER [541].The introduction of SACs into CO2RR has yielded a high efficiency towards desired fine C1\u2013C5 chemicals [542,543]; notably, SACs have shown promising results in boosting the catalytic conversion of CO2 up to nearly 100% in some cases [544]. Very recently, Li and collaborators [521] reported a single-Fe-atom catalyst tuned with phosphorus (Fe\u2013N/P\u2013C) on commercial carbon black as a robust electrocatalyst for CO2 reduction (inset in Fig. 26\n (a)). The single-Fe-catalyst was synthesized by pyrolyzing a mixture of activated carbon black (ACB) with Fe3+ (Fe3+\u2212ACB), urea, and triphenylphosphine in an argon atmosphere. Fourier transform-EXAFS sheds light on the coordination configuration of the single-Fe-atom catalysts. These findings, together with elemental composition and oxidation state analysis from in-situ XPS and AC-STEM-EDX (Fig. 26 (a) and (b)) confirm the presence of atomically dispersed Fe atoms without Fe aggregation (no Fe\u2013Fe bonding from XAS measurements). A high mass-normalized turnover frequency of 508.8\u00a0h\u22121\u00a0at a low overpotential of 0.34\u00a0V, and a high Faradaic efficiency of 98% have been determined for the Fe\u2013N/P\u2013C SAC, that support the outstanding catalytic activity for the CO2 conversion to CO DFT calculations have been performed to further investigate the catalytic mechanism. The theoretical results have shown that the HER is largely restricted on the P-tuned Fe\u2013N\u2013C catalyst. Moreover, Bader charge analysis underlines a lower oxidation state of Fe which contributes to the CO2 activation and CO desorption. (Fig. 26 (c) and (d)).Even though the conversion of CO2 to CO is appealing since it is a key step in the preparation of Fischer\u2212Tropsch synthetic fuels, extensive efforts need to be devoted to fine-tuning SAC coordination environments, to accurately change catalytic selectivity with multiple electron-reducing products which still remains a great challenge and are rarely investigated at present. The electroreduction of carbon dioxide into methanol, which involves a six electron transfer process, has been studied by Yang and collaborators [522] who prepared isolated Cu atoms decorated \u2018through-hole\u2019 carbon nanofibers (CuSAs/TCNFs), with abundant and homogeneously distributed Cu single atoms (CuSAs) for efficient electrochemical CO2RR, with high stability. CuSAs/TCNFs exhibit 44% Faradaic efficiency and \u221293\u00a0mA\u00a0cm\u22122 partial current density of methanol. Moreover, the preparation of the CuSAs/TCNFs membrane fulfills the industrial production requirements. DFT calculations underline that the desorption of the adsorbed *CO on CuSAs/TCNFs model is slightly endergonic, suggesting that the CO desorption does not occur, and the *CO intermediate is further hydrogenated to methanol. In recent work, Cai and co-workers [523] proposed a carbon dot (CDs)-supported SAC, which consists of a single-Cu-atom coordinated to two N and two O sites bound to the edge of graphitic carbons (Cu-CD), as an efficient electrocatalyst for the conversion of CO2 to CH4 (eight electron transfer process). The unique structure of the synthesized Cu-CD catalyst allows for high Faradaic efficiency of 78%, superior CH4 catalytic selectivity at high negative bias, and suppression of the HER. Moreover, the limiting step for CH4 production was found to be a lower energy value than that reported in other works.Proton exchange membrane fuel cells (PEMFCs) are among the most promising devices to convert chemical energy to electrical energy [545\u2013547]. In these devices, the cathode catalyzes the Oxygen Reduction Reaction (ORR), and the anode catalyzes the oxidation of fuels, such as hydrogen (HOR). The desired ORR is a four-electron process leading to the production of water (O2\u00a0+\u00a04H+\u00a0+\u00a04e\u2212 \u2192 4H2O), that involves the cleavage of the exceptionally strong OO bond, whose bond energy is 498\u00a0kJ\u00a0mol\u22121. Therefore, in order to overcome the slow kinetics of the ORR, efficient electrocatalysts are required. On the other hand, the anode reaction is the hydrogen oxidation reaction (H2 \u2192 2H+\u00a0+\u00a02e\u2212), which is a relatively simpler reaction than ORR [514]. Platinum-based materials are the most widely used electrocatalysts for both the ORR and HOR in PEMFCs. However, its extension to large-scale and industrial applications is hindered by high costs and low reserves. Based on that, the development of non-Pt catalysts is of paramount importance. In this regard, the work of Jasinski [548] on the ORR activity of cobalt phthalocyanine paved the way for the development of atomically dispersed M\u2013N\u2013C materials, which exhibit strong ORR performance and great potential for substituting noble metal Pt-based catalysts. DFT was used to investigate the adsorption energy of oxygen intermediates involved in the ORR process on M\u2013N\u2013C, consisting of carbon nanostructures functionalized with pyridinic nitrogen atoms and transition metals [549]. The study revealed differences of up to 0.7\u00a0eV among the adsorption energies of the oxygen intermediates on different moieties, which underlined the importance of precisely determining the local site structures in M\u2013N\u2013C materials for understanding their reactivity.Recently, a Cu SAC was proposed by Cui and collaborators [550]. The catalyst was prepared via a pyrolysis method using Cu phthalocyanine (CuPc) as the precursor and carbon nanotubes as carriers. Aberration-corrected STEM and operando XAS techniques have been used to determine the morphology and electronic properties of the catalyst. The Cu SAC showed higher ORR stability and comparable ORR activity with respect to Pt/C in an alkaline medium, which makes this catalyst a capable non-noble ORR catalyst for fuel cell applications. Moreover, DFT calculations have been performed and suggest that the transformation process from OOH* to O* is the rate-determining step of the ORR on the Cu SAC.Characterization techniques such as XAS, along with computational methods have been used to gain insights into the dynamic evolution of active sites in operando processes, whose information is crucial for an in-depth understanding of the catalytic behavior of SACs. For instance, Han and co-workers [524] demonstrated the substrate-induced activity improvement of CuN2C2 SACs embedded into sp2-hybridized carbon graphite frameworks. Specifically, the authors state that the increase of the geometry distortion of single-atom CuN2C2 active sites, formed when going from a graphene-like material to a small-diameter carbon nanotube (CNT), leads to an improved ORR activity. Indeed, the higher strain in CNT substrates implies a more significant distortion of the CuN2C2 moieties during the ORR, which results in strengthening the Cu\u2013O bonds of Cu and the oxygen atoms of the adsorbed species, while weakening the original Cu\u2013N/Cu\u2013C bonds. As a consequence, a higher electron transfer to the adsorbed O2 molecules is achieved, thus enhancing the ORR activity up to six-fold.Photocatalytic reduction of CO2 to value-added carbon-based fuels and chemicals is one of the most active research fields. In recent years, SACs have been developed and applied extensively as a new class of high-efficient catalysts for photocatalytic CO2 reduction reaction (CO2RR), using natural sunlight as an energy source. Apart from their high atom utilization, large specific surface area, and uniformity of active sites, the heterogeneous single atom photocatalysts have many other favorable features including improved energy efficiency with good excitation under visible light, and reducing recombination of photo-generated charges as well as high selectivity towards CO2 adsorption. Despite efforts being made to develop an efficient photocatalyst with high H2 generation performance, the poor visible light utilization rate, low quantum yield, severe aggregation of electron pairs caused by photogenerated electrons, and its low stability are some of the barriers and hindrances in bringing these advanced photocatalysts towards practical applications and commercialization.An example of applying SACs for photocatalytic CO2RR was by Gao et al. [551], two types of single atoms, palladium, and platinum, were supported on graphitic carbon nitride and investigated as photocatalysts for photocatalytic CO2RR. From the DFT calculations, the graphitic carbon nitride support itself offers a source of hydrogen atom (H*) from the hydrogen evolution reaction (HER). The deposition of Pd and Pt atoms onto carbon support evidently improves the visible light absorption performance, which renders them an ideal candidate for photocatalytic CO2RR. For Pd and Pt-based catalysts, the former produces more HCOOH as a product of CO2 reduction and the latter prefers to form CH4 from CO2 with a much higher rate-determining barrier of 1.16\u00a0eV, as compared to that of Pd catalysts (0.66\u00a0eV). The CO2 reduction pathways to HCOOH and CH3OH on Pd/g-C3N4 and CO2 to CH4 on Pt/g-C3N4 catalysts were illustrated in detail, as shown in Fig. 27\n(a) and (b), respectively.Noble metals including Pd, Au, and Pd are commonly used as co-catalysts for the photocatalytic reduction of CO2 due to their inherent low activation energy and effective charge separation. However, such rare elements are still considerably expensive which impedes its large-scale commercial application. Recently, the incorporation of earth\u2013abundant transition metals (e.g. Cu, Ni, Co, and Fe) as an economic alternative for photocatalytic CO2RR applications has sparked great interest among the scientific community. Chen et al. [419] designed and developed a Cu single-atom catalyst with three-dimensional ordered mesoporous TiO2 (Cu0.01/3DOM-TiO2) from a template-assisted in-situ pyrolysis method. The proposed synthesis strategy not only caters to a wider light absorption range but offers some specific active sites for the absorption and transformation of CO2 molecules via different pathways (Fig. 27(c)). From the results, the novel single atom photocatalyst exhibits a high methane selectivity of 83.3% with a formation rate of 43.5\u00a0\u03bcmol\u00a0g\u22121\u00a0h\u22121 under a gas\u2013solid system (Fig. 27(d i-ii)). Whereas under a liquid\u2013solid system, the same single atom catalysts favored the formation of ethylene with a selectivity of 58.4% and formation rate of 6.99\u00a0\u03bcmol\u00a0g\u22121\u00a0h\u22121 (Fig. 27(d iii-iv)) [551]. From the reaction mechanism study it was revealed that methane is generated from the *CHO intermediates in the gas\u2013solid system while ethylene is produced as the main product from dimerization of *CO and *CHO in the liquid\u2013solid system [551].Zhang et al. [526] investigated the photocatalytic performance of cobalt-based photocatalysts with Co single atoms isolated and anchored on a commercial \u2018super conductive\u2019 carbon black (Co-SA@SP-800). The as-prepared photocatalyst demonstrates a significant improvement in photoactivity, CO selectivity, and cycling stability, which is mainly due to the highly active isolated Co\u2013N4 single atomic sites with conductive carbon support. Considering the unique electronic structure of Co SACs toward photocatalytic CO2 reduction, Di et al. [525] introduced ultra-thin Bi3O4Br nanosheets to isolated single atoms Co as an active site and form Co\u2013Bi3O4Br catalysts for CO2 photoreduction reaction. From the results, the designed catalysts exhibited an improved selective CO formation rate of 107.1\u00a0\u03bcmol\u00a0g\u22121\u00a0h\u22121, which is \u223c4 and 32 times higher than that of the atomic layered Bi3O4Br and bulk Bi3O4Br nanoparticles, respectively [525].Apart from cobalt-based SACs, both nickel and iron are two low-cost earth\u2013abundant transition metals that can be decorated as SACs for photocatalytic reactions. Zhang et al. [527] designed a highly efficient photocatalytic system by dispersing single-site iron atoms and anchoring on a porous crimped graphitic carbon nitride (g-C3N4) polymer. Surprisingly, the synergistic effect of Fe and g-C3N4 support promoted the solar-photon-driven activities [527] and led to a higher photocatalytic hydrogen generation rate of 3390\u00a0\u03bcmol\u00a0h\u22121\u00a0g\u22121. Similarly, Jin et al. [283] decorated the same support with partially oxidized Ni single atoms with abundant unpaired d-electrons, which improved the absorption performance of visible light and mobility of charge carriers. As a result, the photocatalytic H2 production rate was improved by 30-fold as compared to that of the bulk g-C3N4 and other kinds of polymeric semiconductors.The unique and promising features of SACs have created a huge application potential in many areas. However, the scalability of this novel material remains a challenging barrier to mass production [14]. Several SAC production challenges need to be addressed to achieve maximum commercial value as shown in Fig. 28\n.Various synthesis methods have been considered including physical and chemical methods. Despite many SACs synthesis methods having been developed, the upscaling of laboratory settings into commercial production has not been effective [552]. Physical methods require complex and expensive equipment, while chemical methods cannot be adapted to synthesize SACs containing other transition metals [553]. Besides, a larger-scale synthesis of SACs containing almost any transition metal with high metal loading, with a single synthetic strategy has proven to be elusive. He et al. [169] emphasized that the fabrication cost for SACs remains feasible for commercial production. Additionally, some experiments such as mass-selected soft-landing [554] and atomic layer deposition [555] are hindered due to expensive experimental requirements and low production efficiency. To date, the largest quantity of SACs reported in the literature was \u223c1\u00a0kg under controlled laboratory environmental conditions [553]. To overcome the challenges of large-scale applications, the adoption of advanced manufacturing such as robotics and automation, not to forget nanotechnologies as well as the use of AI and machine learning is recommended. As such, 3D printing provides a convenient and precise ability to design and print geometrically complex functional SACs that integrate isolated atom, photoactive and catalytic functionalities.The performance of SACs can vary with the support material. Su et al. [556] highlighted that the limited diversity of support material for SACs has led to a narrow range of active site structures. Many works on the investigation of carbon-based support materials for SACs have been performed. The support material does not only create a strong bonding between metal atoms and the support surface, but it also affects the support for atom anchoring sites to stabilize the metal atoms [557]. Besides, the support material can improve the catalytic reaction as well. Cheng et al. [552] emphasized that a support material with a high surface area and a large number of anchoring sites can synthesize high-loading SACs. In addition, the surface material with these properties can improve the stability, selectivity, and activity of the SACs. Jing Liu et al. [479] stated that many researchers have experimentally demonstrated the application of support materials such as noble metal oxides and 2D materials to synthesize SACs. Despite the size of these metallic atoms, not all atoms are located on the surface of the catalyst and hence, are not fully accessible to the reactant. Complete accessibility can only be achieved if every single atom is well dispersed and stabilized on the support. Also, the functional groups and defects on the support surface can develop different SAC structures which can affect the stability of SAC. The selection of support material might be challenging for the mass production of SACs.To produce SACs on a commercial scale, the stability performance of SACs needs to be addressed. Cheng et al. [552] emphasized the stabilization of supported highly dispersed single atoms during catalysis can be challenging due to high surface-free energy and low coordination numbers of single atoms. With the consideration of the dynamic operation conditions, the stability of SACs can be influenced by many factors such as temperature, pressure, support material, surface condition, and reactant [558]. Mostly, in harsh reaction conditions, SACs will undergo loss of active sites under particle migration and coalescence and atomic (or Ostwald) ripening deactivation [559]. In order to prolong the lifespan and stability of the SACs, utilizing colloidal nanocrystals to independently control particle size and particle loading on the SAC is desired. However, it is very challenging to isolate a clear and precise mechanism of different species of SAC in specific reactions, due to a lack of studies in literatures. In short, further research and understanding of the stabilization of SACs under dynamic working environments is required prior to large-scale application.In the synthesis of SACs, the characterization methods are essential in determining the quality of the SAC. With the advancement in SACs, advanced characterization equipment is necessary such as STM, EXAFS, AC-STEM, and often DRIFTS [552]. These forms of characterization are in-situ methods of determining the structure of a SAC, while it undergoes its chemical reaction. Li et al. [560] highlighted that the in-situ methods can capture the reaction intermediates, identify active sites, and monitor dynamic behaviors of both geometric structure and electronic environment of catalytic sites. However, Li et al. [560] added that most of the in-situ methods are performed only for the characterization of SACs without simulating catalytic activity, simultaneously. Moreover, each characterization method has its advantage and limitation. Therefore, a detailed SAC characterization under dynamic operating conditions required the integration of in-situ characterization techniques to discover the characterization of SACs under dynamic conditions. Additionally, the cost of the characterization of SACs can be expensive before the characteristics of SACs can be fully validated for commercial use.As previously mentioned, the utilization of SACs in energy and chemical applications is still in its infancy (works are mainly at a lab-scale where pilot testing and prototyping are still pending), in which the Technology Readiness Level (TRL) is currently between 1 and 3 [561]. To date, there is no literature reporting on economic or environmental analyses of SACs in any applications. Thus, in order to provide an overview of the commercialization feasibility of SACs, this review article also provides preliminary economic and environmental analyses based on the available resources and information. The assessment portfolio was made based on the catalyst synthesis cost, production cost, CO2 emission factor, and lifespan of the catalysts (they are ordered clockwise based on increasing CO2 emissions). This representation highlights that the economic metric does not correlate with the environmental impact. Nevertheless, these findings, it is sufficient to identify the most attractive and potential catalyst, as reported in many research articles [562,563].The first step of this investigation was the selection of possible reaction which offers suitable characteristics for integration. Based on the data available, the selected reaction was dry reforming of methane (DRM, Figure S1) and six individual case studies were chosen based on the catalyst types, noted as (A) Ni-based SAC [369]; (B) Ni/Al2O3 derived from Metal\u2013Organic Framework (MIL-53) [564]; (C) Ni/Al2O3\u2013CeO2 [565]; (D) Ni/TNT [566]; (E) Ni/Al2O3 [565] and (F) Pure Ni [567] (note that H2 is considered as the targeted product). The calculations step and assumptions used are included in the Supplementary information (Tables S1-S8). As presented in Fig. 29\n, the Ni-based SAC (Ni/HAP-Ce) was the most preferred catalyst for DRM, in terms of both being environmentally friendly and economical. According to the greater details stated in the supplementary information, the cost of preparation for Ni/HAP-Ce via co-precipitation was found to be the lowest among other counterparts, also, noteworthy to mention that, the cost was about 3.5 times lower than that of the Ni-MOF. In addition, the total production cost (USD/kg H2) of Ni-SACs was also calculated to be the lowest compared to other catalysts, which is 26.4% and 1.23% lower than pure Ni (Worst) and Ni-MOF (2nd Best), respectively. Whereas the environmental matrix is measured in terms of CO2 emissions (kg CO2/kg H2 produced). Notably, the CO2 emissions in catalytic DRM using a Ni-SAC are far lower than other catalysts, i.e., in the range of 2.3\u20137.5 times lower as compared to that of other catalysts. Overall, given its promising performance in the studied aspects (economic, environmental, and lifespan), SACs emerge as an attractive frontier catalyst to be exploited further too aid the commercialization purpose and strengthen other chemicals and fuel production routes. Future work on conducting rigorous \u201cIntegrated Economic, Environmental, and Energy\u201d assessments (3Es) is required to further provide a bigger picture of the frontier of SACs, these assessments could provide decision makers in the commercialization process with feasibility data in determining the most favourable synthesis method of SACs and also suggesting the future research direction, challenges, and debottlenecking of the application of SACs in different field.The performance of SACs has gained attention in many sectors as the new frontier in catalysis science, especially in clean energy. This paper highlights the development of SAC synthesis methods from conventional (i.e., co-precipitation or sequential incipient wetness impregnation) to new synthesis methods (i.e., coordination site, defect design, photochemical and electrochemical). As the development of SAC synthesis is improved, advanced characterization methods such as high-resolution electron microscopes, x-ray irradiation spectroscopies, magnetic resonance spectroscopies, and other wavelength in-situ spectroscopies are used to provide an in-depth understanding of the characteristics of the SAC for clean energy application. On top of that, recent SAC experimental outcomes are reviewed with the consideration of operating conditions, catalyst loading, and types of reactor configurations. Based on the experimental and DFT method output, SACs appear to be an alternative to bridge both homogeneous and heterogenous catalysts for clean energy applications including coupling, oxidation, hydrogenation, and reforming reactions. Through the nano-engineering strategy, bulk nanoparticle catalysts can be downsized by using large surface area support materials, both the surface area and density of defect sites are favorable for the incorporation of single atoms of the active metal, providing some unique catalytic performance such as higher stability and flexible physiochemical properties. Besides that, studies demonstrated that SAC contributes to lowering the reaction activation barrier, enhancing adsorption pathways, processing uniform single active sites, and governing catalytic reactivity for selective routes. The commercialization and large-scale production of SACs require addressing various key challenges including the utilization of affordable support materials, stability of SACs under a mass production environment, and introducing of simple and effective in-situ characterization methods. From this review, it can be observed that the application of SACs is at the forefront of clean energy and chemicals production research, offering strategic alternatives to classical heterogeneous catalytic systems with a major impact on the economy and the environment.The review was conceptualized by ACML, SYT, MJT, and GK. Data was acquired by ACML and SYT, the overall investigation was carried out by ACML, SYT, BSH, XZ, KWC, VB, WDL, BLFC, and CLY. The review was written by all authors and was reviewed and edited by ACML, MJT, and GK.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Dr Martin Taylor reports financial support was provided by UK Research and Innovation. Dr Kin Wai Cheah reports financial support was provided by UK Research and Innovation.MJT and KWC acknowledge funding through the THYME project (UKRI, Research England). A.C.M. Loy would also like to acknowledge the Australian Government Research Training Program for supporting this project. The research contribution from S.Y. Teng is supported by the\u00a0European Union's Horizon Europe Research and Innovation Program, under Marie Sk\u0142odowska-Curie Actions grant agreement no.\u00a0101064585\u00a0(MoCEGS).The following is the Supplementary data to this article:\n\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.pecs.2023.101074.", "descript": "\n                  The emergence of single atom sites as a frontier research area in catalysis has sparked extensive academic and industrial interest, especially for energy, environmental and chemicals production processes. Single atom catalysts (SACs) have shown remarkable performance in a variety of catalytic reactions, demonstrating high selectivity to the products of interest, long lifespan, high stability and more importantly high atomic metal utilization efficiency. In this review, we unveil in depth insights on development and achievements of SACs, including (a) Chronological progress on SACs development, (b) Recent advances in SACs synthesis, (c) Spatial and temporal SACs characterization techniques, (d) Application of SACs in different energy and chemical production, (e) Environmental and economic aspects of SACs, and (f) Current challenges, promising ideas and future prospects for SACs. On a whole, this review serves to enlighten scientists and engineers in developing fundamental catalytic understanding that can be applied into the future, both for academia or valorizing chemical processes.\n               "}
{"full_text": "Converting anthropogenic CO2 into valuable fuels (e.g. CH4) using green hydrogen generated, for instance, from water electrolysis driven by renewable electricity is key to enable the energy transition of the chemical industry [1,2]. In the conversion of carbon dioxide to methane, large quantities of heat are released due to the exothermicity of the reaction (CO2 + 4 H2 \u2192 CH4 + 2 H2O, \u0394H298K = \u2013165 kJ/mol) and, in the absence of heat removal, the adiabatic temperature rise would be rather significant (773 K) [3]. When employing this technology in large-scale Power-to-Gas (P2G) processes, the conversion of CO2 can vary significantly due to the fluctuations in the production of renewable electricity that is used to generate the hydrogen required for the process (for every mole of CO2 4 mol of H2 are needed). This results in large temperature swings as a function of time on stream (typical fluctuations are in the order of minutes) [4]. Hence, the catalyst subjected to these fluctuations undergoes accelerated aging that leads to lower metal surface area (sintering) and porosity (pore-collapsing) [4]. In order to compensate for the accelerated deactivation, one could either use an excess of catalyst, or use a fluidized reactor in which the catalyst is continuously replenished. These strategies, however, could make the process economically unattractive at high catalyst consumption rates. This challenge was highlighted before by Prof. J.D. Grunwaldt and co-workers [4]. The authors argue that coupling of thermo-/electro- catalytic processes with dynamic energy and feed supply will render additional complexities to the chemical industry as reactors are often operated within a narrow operational window for optimal performance. Clearly, new catalysts and reactor concepts are needed to facilitate the commercial take-up of renewables in the chemical industry in the near future.Supported Ru, Ni, Rh, and/or Co metals on different metal oxide supports (TiO2, Al2O3, SiO2, ZrO2, CeO2\u2026) have been extensively studied for CO2 methanation [5,6,15\u201318,7\u201314]. Among them, Ni-based catalysts are the most researched materials, since doped or promoted Ni catalysts have shown good CO2 conversion, high selectivity to methane, and low cost compared to noble-based catalysts. In this catalyst, it has been demonstrated that the support plays a key role, not only modifiying the dispersion of the active phase and textural properties, but also its activity for CO2 activation. High-energy lattice metal oxides such as, cerium oxide and titanium, possess excellent redox properties due to their par M3+/M4+ and it exhibits high oxygen storage capacity [19\u201321]. As a result, ceria provides a large amount of oxygen vacancies with medium basicity, facilitating CO2 activation-dissociation and metal-support interaction [9,11,29,30,17,22\u201328]. Nanoshaped ceria (e.g. nanorods or nanocubes) has been synthesized to support Ni, Co or Ru to enhance its catalytic activity [13,15,31\u201333]. These nanoshaped ceria supports expose well-defined crystal planes that can facilitate stabilization of metal clusters for catalytic applications at elevated temperatures, which makes them suitable for CO2 methanation. In order to compare their activities, Sakpal et al. studied the influence of Ni loading, Ni cluster size, and distribution on three types of nanoshaped ceria. In this report, the authors concluded that the Ni cluster size and distribution, determined by the shape of the ceria support, was the decisive factor in the observed catalytic performance [34]. In general, nanorods-shaped ceria exhibited the highest activity compared to typical polyhedral ceria and nanocubes, mainly due to stronger metal-ceria interaction, large fraction of oxygen vacancies, and high oxygen mobility [32,33,35].In this context, ceria has been reported to help the metal dispersion and prevent deactivation due to metal sintering, which is one of the main drawbacks in CO2 methanation [12,22,24,36\u201338]. Despite the good stability reported in CO2 methanation on promoted Ni-ceria based catalysts, its long-term stability under fluctuating conditions remains elusive. Some stability tests have been reported, but often these studies were conducted close to the maximum equilibrium conversion where excess of catalyst can mask the catalyst deactivation. For instance, Ocampo et al. [37] have shown that it is possible to mitigate the catalyst deactivation of Ni/CexZr1-xO2 catalysts for CO2 methanation depending on the ratio of ceria and zirconia. In this study, however, the rate of deactivation was measured from the beginning at thermodynamic equilibrium regime. While significant improvements have been achieved in the past by supporting Ni catalysts on ceria-containing supports, the utilization of conversion levels close to the thermodynamic equilibrium to study the stability of these catalysts generates uncertainty on the validity of the results. [25,26,30,39].Since the appearance of hotspots and the consequent metal sintering are one the main causes for catalyst deactivation in CO2 methanation, different approaches for structuring the catalyst have been proposed in the last few years, aiming at improving heat and mass transport. Ricca et al. [38] studied the temperature profile inside the catalytic bed for 10 wt.% Ni/CeO2-ZrO2 supported on Al-foam and SiC monolith compared to the powdered catalyst. They observed that the temperature increase inside the reactor bed was reduced in the order powder > Al-foam > SiC monolith. Similarly, Frey and co-workers [40] studied the hotspots appearance and the temperature profiles on Ni/CeO2 based catalysts supported on open foams of Al, Al2O3, and SiC, showing that the highest conversion was obtained on SiC support. In this material, the higher rates per reactor volume led to the formation of hotspots according to IR thermography, which negatively affected the selectivity to methane and the catalyst stability. To mitigate these issues, the authors grew carbon nanofibers on the SiC to improve the hydrodynamic, thermal, and catalytic properties of the structured catalyst. This configuration drastically increased the heat removal, improving the catalyst performance [41]. In the same line, Fukuhara and co-workers [42,43] studied different Al-honeycomb configurations (plain, stacked, segmented, multi-stacked), combining shifted positions of the honeycomb stacks and free spaces or non-catalytic honeycomb stacks. These results showed that structuring of the Ni-Ceria catalyst improved the heat and mass transfer inside the reactor, leading to enhanced activity and stability. The authors, however, measured the stability of these materials near the equilibrium conversion, thus complicating interpretation of the results obtained.The selection of the material of the support is also important, since not only the heat transfer is a determining parameter. In addition, catalyst loading and adherence, cell density or hydrodynamic design are also important for its feasibility [44]. For instance, Schollenberger et al. [45] proposed a mixed Al-steel honeycomb to optimize the CO2 conversion level and the heat transfer. Among other metallic supports, FeCrAlloy\u00ae steel has been extensively proposed due to its good heat transfer, flexibility to create different shapes, very high cell density and ease to segregate an Al2O3 \u03bc-layer to improve the catalyst loading showing excellent catalyst adherences [46\u201352]. For instance, Hernandez Lalinde et al. [46] tested a Ni/Al2O3 catalyst on FeCrAlloy plates obtaining good catalyst impregnation and homogeneous temperature profile during methanation reaction.In the present study, we show that by supporting Ni catalyst on CeO2 nanorods it is possible to prevent catalyst deactivation observed during methanation reaction when using conventional Ni supported on commercial CeO2. Our catalyst showed high selectivity to methane of c.a. 95\u201399 % even under fluctuating reaction conditions, where more severe deactivation is anticipated due to the large temperature swings. We demonstrate that this excellent performance is not caused by excess of catalyst as the performance of the materials was assessed far from the maximum conversion (c.a. 20 % of the equilibrium conversion). Furthermore, we show that structuring this catalyst on metallic FeCrAlloy \u03bc-monoliths can enhance its activity and stability.Synthesis of nanorods shaped CeO2 was performed by hydrothermal process previously reported in our group [13]. In a typical synthesis, 24 g of NaOH (Sigma Aldrich) and 2.17 g of Ce(NO3)3\u00b7H2O (Sigma Aldrich) were separately dissolved in 35 mL and 5 mL of deionized H2O, respectively. Then, both solutions were slowly mixed and stirred for 30 min. The resulting slurry was transferred into a Teflon bottle (125 mL) and filled 80 % with water. The Teflon bottle was introduced in a sealed autoclave. The hydrothermal treatment was performed for 24 h at 100 \u00b0C to obtain nanorods CeO2. The resulting precipitate was separated by centrifugation (9000 rpm for 10 min) and washed with deionized water until pH 7 was reached. The sample was dried at 100 \u00b0C for 4 h, followed by calcination at 500 \u00b0C (heating rate: 5 \u00b0C/min) for 5 h in air (flow rate: 100 mL/min). On the other hand, octahedral CeO2 with an average particle size below 50 nm was obtained from commercial Sigma-Aldrich and the same calcination step at 500 \u00b0C (heating rate: 5 \u00b0C/min) for 5 h in air (flow rate: 100 mL/ min).Deposition of the desired amount of nickel on the prepared nanorods or octahedral ceria was performed by wet impregnation. Typically, 3 g of ceria was added to 60 mL of water under continuous stirring. In another flask, 0.744 g of commercial Ni(NO3)2\u00b76H2O (Alfa Aesar) was dissolved in 20 mL H2O and slowly added to the ceria slurry under stirring. Then, the pH was adjusted to 8 by adding dropwise 0.1 M NaOH aqueous solution. The mixture was stirred at room temperature for \u223c165 and \u223c315 min for octahedral and nanorods shapes, respectively, in order to obtain similar Ni particle sizes [34]. Finally, the catalysts were centrifuged and dried at 100 \u00b0C for 3 h, followed by calcination at 500 \u00b0C for 5 h in air (100 mL/min) with a heating rate of 5 \u00b0C/min.On the other hand, FeCrAlloy\u00ae sheets (Fe72.8/Cr22/Al5/Y0.1/Zr0.1, GoodFellow) with 0.05 mm in thickness were used to manufacture cylindrical multichannel monoliths. As described elsewhere [53], flat and corrugated foils were co-rolled in pairs resulting in cylindrical metallic monoliths with 15.8 mm in diameter and 20 mm in height with calculated cell density of 2004 cpsi and an exposed surface of 152 cm2 (Fig. 1\n). Then, the manufactured monolith were calcined in air at 900 \u00b0C for 22 h (heating ramp of 10 \u00b0C/min) in order to form an external porous Al2O3 \u03bc-layer by segregation from the FeCrAlloy material that facilitates the catalyst impregnation [52,53]. The calcined monolithic structure was immersed 1 min in an aqueous colloidal suspension of the desired catalyst (Ni/CeO2 oct or Ni/CeO2 rods). The channels of the monolith were gently cleaned with an airbrush to avoid obstructions. Then, the impregnated monolith was dried at 100 \u00b0C for 1 h and weighed. The impregnation process was repeated until the desired amount of catalyst was loaded on the monolithic structure. Finally, the structured catalyst was calcined at 500 \u00b0C for 5 h in air, with a slower heating rate of 2 \u00b0C/min in order to avoid fissures or fractures in the catalytic layer [54]. The typical thickness of this layer was c.a. 2 \u03bcm.To obtain homogeneous thin layers of catalyst overcoating, a stable colloidal suspension of the catalyst with optimal rheological properties was mandatory for the impregnation process. The optimization of the slurry was aimed at avoiding particles agglomeration to obtain well-controlled homogeneous thin layers over the monolith walls. This was done to prevent diffusional problems, catalyst loss, fractures, and/or peeling. The main variables to control were the particle size, viscosity, and pH of the suspension [48,55]. Particularly, the colloidal suspensions were prepared by slowly adding 20 wt.% catalyst, previously sieved below 38 \u03bcm, in deionized water. The colloidal suspensions were aged for 24 h before starting the impregnation, always under continuous stirring at room conditions.The structural analysis of the two synthesized catalysts (named Ni/CeO2 rods and Ni/CeO2 oct) and their prepared nano-shaped ceria supports (named CeO2 rods and CeO2 oct) was conducted by X-Ray Diffraction (XRD) on a Bruker D2 Phaser diffractometer with Cu K\u03b1 radiation. Ni phase of the synthesized and reduced catalysts were compared by XRD on an X\u2019Pert Pro PANalytical instrument with Cu K\u03b1 radiation. N2-physisorption at 77 K (Micromeritics Tristar) was performed to determine textural properties of the catalysts and ceria supports. Ni loading was determined by XRF (Philips PW 1480). The surface morphology and Ni particle size and dispersion were analyzed by Scanning Electron Microscopy (SEM) in a JEOL JSM-6490 instrument, and by Transmission Electron Microscopy (TEM) micrographs recorded on a Philips CM-200 instrument equipped with energy dispersive X-ray detector (EDX).In order to analyze the reducibility of the synthesized catalyst, reductive thermogravimetric analysis (H2-TGA) was conducted using a Mettler Toledo TGA/DSC3 +. The gas flow consists of 20 mL/min Argon protective gas and 50 mL/min 90:10 H2:Argon as reactive gas. The sample was weighed in a 70 \u03bcL aluminium oxide crucible. Then, the sample was placed inside the analysis chamber and left stabilizing under the H2 environment for 30 min at 25 \u00b0C. Afterwards, the temperature was increased with 10 \u00b0C/min rate to 900 \u00b0C. The sample was kept at 900 \u00b0C under H2 environment for 10 min. Then, the reactive gas was changed from 90:10 H2:Argon to pure argon. The sample was then actively cooled to room temperature under Argon atmosphere to safely resume the measurement (i.e. avoiding explosive H2/O2 mixtures).Finally, XRD, N2-physisorption and TEM analysis were performed in the same instruments and conditions already described on the used catalyst (named \u201cpost\u201d).CO2 methanation was carried out in a tailored-made setup at atmospheric pressure, using a cylindrical stainless-steel reactor (Hastelloy C276) of 40 cm in length, 15.8 mm of inner diameter and 2.8 mm of wall thickness, placed in the center of a cylindrical oven of 30 cm in length. Two thermocouples were used. One of them, which controlled the temperature of the oven, was placed in the center of the internal wall of the oven, in contact with the reactor. The other one, placed in the center of the reactor, was used to measure the real temperature achieved in the center of the catalyst (monolith or powder), providing the increment of temperature in the radial section. The feed consisting of a CO2:H2 mixture at the stoichiometric ratio of the reaction (10 % and 40 %, respectively) was balanced with N2 (50 %) using calibrated mass flow controllers (Brooks). Conversion curves vs temperature from 200 \u00b0C to 400 or 500 \u00b0C were performed in two different total flow rates (10 and 50 mL/min), keeping constant the feed composition. Outgoing gases were analyzed by an on-line GC (Varian CP-3800) equipped with an Agilent CP-Molsieve 5A, PoraPlot Q column and TCD detector. The catalysts (powders and monoliths) were placed in the center of the reactor using quartz wool, always loading c.a. 0.1 g of Ni/CeO2 catalyst. The powdered catalysts were sieved in the 125\u2013250 \u03bcm range for the catalytic tests, according to the previous work carried out in our group [13]. Before catalytic tests, the catalysts were activated in situ with a heating rate of 5 \u00b0C/min in 100 mL/min of H2/N2 flow (25:75 volumetric ratio) at 400 \u00b0C for 2 h and then cooled down in N2.The stability of the catalysts in CO2 methanation in fluctuating conditions (changing the total flow rate between 10 and 50 mL/min to provide high and low conversion levels) was evaluated at 300 \u00b0C, which was found to be the temperature where the CO2 conversion rate is maximal in these operation conditions, according to the previous conversion vs temperature analysis. All the stability tests were carried out during 100 h, varying the two conditions several times.To elucidate the structural properties of the prepared materials, XRD measurements were carried out. Fig. 2\n shows the diffractograms of the prepared samples once calcined. All the samples maintained the cubic fluorite type structure characteristic of CeO2 (Fm \n\n3\n\u00af\n\n m, JCPDS 34-0394). A close inspection of the diffraction line corresponding to the (111) crystallographic plane of CeO2 (Fig. 2b) indicates a small contraction of the Full Width at Half Maximum (FWHM) when the Ni was present. Such feature can be attributed to partial migration of the Ni2+ in the ceria structure [41]. It should be noted that this decrease of the lattice parameter (Table 1\n) when Ni is loaded on both ceria shapes (i.e. octahedral and nanorods), can be attributed to the smaller ionic radii of Ni2+ and Ce4+ (0.69 and 0.97 \u00c5, respectively). Notably, the NiO phase is also recognizable (Fm \n\n3\n\u00af\n\n m, JCPDS 47-1049), particularly in the Ni/CeO2_oct sample. As plotted in Fig. 2c, the analysis of the diffractograms of calcined and activated catalysts (i.e. before and after reduction in H2/N2 flow 25:75 volumetric ratio at 400 \u00b0C for 2 h) evinces the reduction of NiO phase to Ni (Fm \n\n3\n\u00af\n\n m, JCPDS 04-0850). Table 1 also includes the CeO2 and Ni crystallite sizes estimated by the Scherrer\u2019s Equation on the (111) crystallographic plane. Remarkably, the crystallite size of Ni and ceria on the reduced Ni/CeO2 rods reached values of 9.6 and 14.1 nm, respectively, which are significantly lower than those obtained on the Ni/CeO2 Oct (26.8 and 27.3 nm for Ni and CeO2 phases, respectively). However, it has to be pointed out that the peaks associated to Ni phase are small, decreasing the accuracy of Ni crystallite size calculation on the reduced catalysts. The textural properties obtained by N2-physisorption (Table 1), indicate that ceria nanorods exhibits a higher surface area than the octahedral samples with values of 53 and 32 m2/g, respectively. The increase in surface area was accompanied by a drop in the average pore size of the ceria support from 5 nm in the octahedral CeO2 to 2.6 nm in the nanorods, which are in line with previous reports [13,56]. Notably, the deposition of nickel catalyst on these supports did not affect the surface area as evidenced by the negligible change in BET surface area, pore sizes, and volumes.Similarly, XRD and N2-physisorption analysis of the structured samples on the monoliths were conducted in order to check the stability of the catalysts after impregnation process. As expected by the simple impregnation method used, the catalysts perfectly preserve their structural and textural properties (see supporting information, Figure S.1 and Table S.1).From the SEM images of the supports (Fig. 3\na and b) one can immediately recognize the different shapes of the commercial ceria with an octahedral-like shape and the synthesized ceria nanorods. The latter exhibited a size of c.a. 1 \u03bcm in length and only few nanometers in diameter. The SEM analysis of the Ni catalysts are identical to their respective supports, since Ni particles are undistinguishable (Fig. 3c and d).In the micrographs obtained by TEM (Fig. 4\n), the octahedral and nanorods ceria shapes are also distinguishable. Despite low contrast between Ni and Ce in TEM micrographs, identification and measurement of Ni particles have been attempted to estimate the Ni particle sizes distribution. Considering the notable dissimilarity of the Ni/CeO2 nanorods shape, the Ni particle size distribution in this sample is more reliable with 700 measurements, while only 132 measurements are available for the octahedral sample (Fig. 5\n). This analysis indicates Ni had an average particle size of c.a. 7 \u00b1 4 nm in Ni/CeO2 nanorods. In contrast, the Ni/CeO2 octahedral catalyst had a wider particle size distribution, as evidenced by the large Ni particles of about 70 nm present in the sample, and where only over 55 % of the measured particles are in the 4\u201312 nm range. In this case, majority of Ni particles are averaged to c.a. 16 \u00b1 13 nm. Detailed elemental mapping via energy dispersive X-ray spectroscopy (EDX) supported the previous observations regarding metal dispersion (Fig. 6\n). Here, it can be noted the highly heterogeneous distribution of Ni nanoparticles on the Ni/CeO2-Octahedral (Fig. 6a, Ni) as compared to the narrower distribution of Ni particles with smaller cluster size (Fig. 6b, Ni). Moreover, in the case of Ni/CeO2 nanorods, the averaged particle size (Fig. 5b and Table 2\n) is similar to that estimated by Scherrer calculation of the XRD Ni peak (see Table 1 above). However, in the case of the octahedral shaped catalyst, the Ni crystallite size detected and estimated by XRD (Table 1) is higher than that determined by TEM micrographs (Fig. 5a and Table 2). This is caused by the relatively broad particle size distribution on Ni/CeO2-Octahedral, as observed with TEM, combined with the fact that XRD is much more sensitive for larger particles. The relatively large particles therefore dominate the averaged particle size determined by line-broadening.Considering the average Ni particle size by TEM of 16 \u00b1 13 nm and 7 \u00b1 4 nm for Ni/CeO2 oct and Ni/CeO2 nanorods, respectively, Ni dispersion has been calculated according to the relationship between particle sizes and apparent dispersion described by Larsson [57]. Table 2 reports the estimated apparent Ni dispersion. As expected, higher dispersion was obtained for Ni on nanorods ceria shape. The Ni loadings according to XRF analysis reached values of 3.3 and 2.7 wt.% for Ni/CeO2 octahedral and nanorods, respectively. While these results indicate that both catalysts had similar metal loading, the resulting metal surface areas were different possible due to the differences in surface area and metal-support interaction [34,58\u201361].Conversion curves for CO2 methanation on activated Ni/CeO2 catalysts, octahedral and nanorods shapes in powders and monoliths structures, at 10 and 50 mL/min total flow rate (6 and 30 L h\u22121\u00b7gcat\n\u22121, respectively) are shown in Fig. 7\n. The set temperature was controlled with a thermocouple inside the oven on the external wall of the reactor, while the real temperature inside the catalyst bed was \u223c 20 \u00b0C lower. This internal temperature was measured with a second thermocouple in the center of the catalytic bed or \u03bc-monolith. Thus, the results shown in Fig. 7 indicate the temperature value inside the reactor. Here, one can note that the monolith samples showed temperatures several degrees higher than the powders and closer to the set point, even at similar conversion levels, at high values (T > 300 \u00b0C). The smaller temperature difference between the external reactor wall and the center of the catalyst bed can be associated with the enhanced heat transfer in the \u03bc-monoliths. In addition, testing of the calcined \u03bc-monolith without any catalyst confirmed that the metallic monolith has not catalytic activity for CO2 methanation at the reaction conditions herein employed.The conversion achieved as a function of temperature and space velocity, shown in Fig. 7, indicate that the inflection point of the conversion curve, where the variation of CO2 conversion (rate) with temperature is maximal, is around 300 \u00b0C at 6 L h\u22121 gcat\n\u22121, with c.a. 50\u201360 % of CO2 conversion (Fig. 7a). As expected, increasing the gas hourly space velocity to 30 L h\u22121\u00b7gcat\n\u22121 led to lower conversions (c.a. 20\u201330 %) (Fig. 7b).Based on these results, the stability tests were carried out at 300 \u00b0C for 100 h in order to study the catalyst behavior in the kinetic regime. Here, it is important to mention that the selectivity to methane was found in all cases to be around 90\u201399 %. Moreover, carbon balance was closed above 95 % in all cases during all the reaction time. Indeed, only in the tested points at 450\u2013500 \u00b0C at 6 L h\u22121 gcat\n\u22121 on both samples (Fig. 7a), a small amount of CO was produced (maximal selectivity about 10 %, only found at 6 L h\u22121 gcat\n\u22121 in the 60\u201380 % range of CO2 conversion level). In addition, elemental analysis of the powdered samples carried out after stability tests indicated negligible carbon deposited even after c.a. 100 h of operation. The high selectivity of group VIII-X metals (e.g. Ni) towards methane in comparison to metals in group XI (e.g. Cu, Ag) can be rationalized in terms of the electronic structure of the metal center. Broadly speaking the as the center of the d-band of the metal is closer to the Fermi level the stronger the interaction of the adsorbates involved in the hydrogenation of carbon dioxide and carbon monoxides with the metal surface [62,63]. This results in the filling of the anti-bonding states (2p*) of the CO molecule via backdonation that weakens the internal bond of the molecule, facilitating CO bond dissociation [64,65]. In this context, metals in the group XI with fully occupied d-band weekly interact with the adsorbates as anti-bonding states between the metal atoms and the adsorbate are filled. This weak interaction in the case of Cu and Ag metals leads to the formation of \u03b71(O)-CO bonding to the metal surface, while in the case of the Ni, Ru and Pt the \u03b72(CO) surface species are favored [66,67]. This results in the formation of CHxO products in the case of Cu and Ag catalysts while in the case of Ni, Ru, Pt the dissociation into C* and O* species leads to methane formation in the presence of hydrogen. In the case of Ni supported on CeO2 it is believed that COx* species can be stabilized on the oxygen vacancies on the support, which favors the activation of carbon dioxide in the presence of Ni [56,68]. In this sense, it is not surprising that on both catalysts (i.e. Ni-CeO2 nanorods and octahedral) the selectivity observed was 95\u201399 %.In order to analyze the activity of the prepared catalysts and their stability under fluctuating conditions (i.e. varying the conversion level by only changing the total flow rate) we conducted long-term stability studies for periods of at least one week per catalyst. The complete stability tests (100 h) are reported in the supporting information (Figure S.2). Fig. 8\n presents the performances with several cycles (high and low conversion) during 50 h as CO2 converted per total amount of Ni, discarding thereby the effect of slight variation on the amount of catalyst loading on each monolith. As it is shown in Fig. 7 on these catalysts the conversion of CO2 at 300 \u00b0C and 6 and 30 L h\u22121 gcat\n\u22121 varied in the ranges of 50\u201360 % and 20\u201330 %, respectively. Since these catalysts are operating at relatively similar levels of conversion and far from equilibrium limitations it is possible to compare their initial activity at low and high space velocities. Fig. 9\n plots the activities at both space velocities to facilitate the analysis of metal oxide support (nanorods vs. octahedral ceria) and structuration (powered vs. \u03bc-monoliths) at the beginning of the reaction, where catalyst deactivation effects are minimal.Notably, nanorods shaped catalysts showed higher activity than the octahedral catalysts on both powdered and \u03bc-monolithic forms. This is in good agreement with the higher BET surface area obtained on nanorods and the well-dispersed Ni clusters indicated by TEM analysis. This is also supported by the disappearance of the Ni peak in the reduced XRD on Ni/CeO2 nanorods. Moreover, as it was discussed previously, some migration of Ni2+ into ceria lattice cannot be discarded, which can stabilize Ni as NiCeO3 spinel. In previous studies similar observations have been reported. Konsolakis et al. [15] showed that metal cations can be stabilized as spinel species in CeO2. For instance, Du et al. [33] studied the morphology dependence of the catalytic activity of Ni/CeO2 for CO\n2\n methanation. The authors observed higher activity with nanorods shaped catalysts than with nanopolyhedral structures. This higher activity was ascribed to stronger anchoring of Ni nanoparticles providing better metal dispersions. One can anticipate that higher affinity between the Ni clusters and metal oxide support should also improve the stability of the catalyst to metal sintering. In this line, our studies on the long-term stability of the catalysts indicate that the powdered octahedral shaped catalyst (blue line) suffered fast deactivation from the beginning. In sharp contrast, nanorods shaped sample (green line) showed stable activity over periods of \u223c50 h of operation under fluctuating operation (Fig. 8). As mentioned earlier, CeO2 nanorods expose a large fraction of (111) facets [56,69], which are richer in defects providing a large number of oxygen vacancies with high ion mobility. This in turn can increase the metal \u201cwettability\u201d of the surface leading to more robust catalysts while enhancing the activity by pre-activating the CO2 molecule. In addition, the higher reducibility on nanorods-shaped ceria according to reported H2-TPR analysis [56,69] and the performed H2-TGA (see Fig. S4), supports the observed higher catalytic activity of this Ni/CeO2 nanorods sample. Here, it can be observed that CeO2 nanorods undergo a significant weight loss (c.a. 7.5 wt. %) when compared to the CeO2 octahedral (c.a. 2.5 wt. %) after reducing the catalyst at 900 \u00b0C in H2. Addition of nickel facilitated the reduction of octahedral the CeO2 leading to a weight loss of c.a. 4 wt. %, while in the case of the nanorods the extent of reduction remained constant, reaching a value of c.a. 7.4 wt. %. Similar results were observed by Gong et al. [31] during CO2 methanation. In that case, the authors assigned the higher CO2 uptake and activity of Ni supported nanorods ceria to the larger fraction and mobility of oxygen vacancies as analyzed by in situ IR and DRIFTS.Notably, structuring the octahedral Ni/CeO2 sample clearly improved stability too (orange line vs blue line). Deactivation of the powdered sample from the beginning is in good agreement with observations by Ocampo et al. [37], Zhou et al. [39] and Iglesias et al. [28]. In contrast to previous work on CO2 methanation using Ni/CeO2 catalysts, where high Ni loadings ranging from 10 to 26 wt.% yielded good stability at conversion levels close to thermodynamic equilibrium [25,26,30], our work demonstrates that Ni/CeO2 on octahedral ceria powder easily deactivates under harsh reaction environments, such those exerted during dynamic reactor operation. These results would suggest that it is possible to mitigate catalyst deactivation by supporting the catalyst on a metallic \u03bc-monolith, thanks to the highly efficient heat diffusion inside the reactor.Ni/CeO2 nanorods not only provides higher activity due to the nano-shaped ceria, as discussed above, it also inhibits Ni sintering and deactivation [33], showing good stability under stressful and fluctuating conditions. This is supported by post-reaction TEM analysis of the powder samples (Fig. 10\n and Table 3\n). Nanorods-shaped catalyst hinders the sintering compared to the octahedral sample, since the averaged Ni particle sizes increases during the stability tests from 16 nm to 23 nm in the case of Ni/CeO2 oct, but only from 7 nm to 9 nm for the nanorods-shaped catalyst. Moreover, as is shown in Fig. 10, the particle size distribution becomes flatter, increasing the relative frequency of particle sizes in 15\u201330 nm range.In addition, XRD analysis shown the deactivation by sintering, where the peak associated to Ni phase increased (see supporting information, Fig. S.3). The calculation of crystallite size by Scherrer equation (summarized in Table 4\n) supports the higher sintering of Ni particles in the octahedral catalyst. As it was discusses above, the XRD primarily detects large clusters. However, the increase of the Ni particles size follows the same trend. Thus, in Ni/CeO2 nanorods, Ni particles increased from 7 to 9 nm by TEM and from 9.6\u201313.2 by XRD (factor of 1.3\u20131.4), while in Ni/CeO2 octahedral, averaged Ni particles increased from 16 to 23 nm by TEM and from 26.8\u201350.5 by XRD (factor of 1.4\u20131.9). On the other hand, N2-physisorption analysis demonstrates that the catalyst keeps its textural properties during the catalytic tests (summarized in Table 4).Hence, in the case of Ni/CeO2 nanorods, structuring by deposition on the monolith does not further improve stability, as it is observed in Fig. 8, since the nanorods support already significantly hinders the catalyst deactivation by Ni sintering. However, and remarkably at higher space velocity, the activity of nanorods supported on monolith is increased compared to the powder sample, indicating that monolithic structure improves the contact between catalytic surface and reactant flow, as demonstrated by Fukuhara and co-workers [42,43]. Our estimations of the coating layer indicate that for both catalysts, Ni/CeO2 nanorods and Ni/CeO2 octahedral, the thickness of the catalyst layer is around 2 \u03bcm, which can explain the fast rates of heat and mass transport in the monoliths.Ni/CeO2 catalyst for CO2 methanation exhibits good activity and high selectivity to methane (above 95 %). However, in stressful and fluctuating conditions, it undergoes fast deactivation. Two approaches were developed in order to improve its stability, including: (1) synthesis of nanorods-shaped ceria to support the Ni and (2) catalyst structuring on metallic multichannels \u03bc-monolith. It was observed that nanorods shaped catalysts provided higher activity, attributed to the enhancement of formation and mobility of oxygen vacancies and the increase of Ni-support interaction and dispersion. Moreover, this nanoshaped catalyst already exhibited high stability in the powdered form, indicating that nanorods can delay Ni sintering. On the other hand, supporting Ni/CeO2 octahedral powder catalyst on the monolith provided enhanced stability during fluctuating conditions, compared to the same catalyst in fixed bed operation. Moreover, catalyst structuring on the \u03bc-monolith resulted in slightly higher catalytic activity than the powder form, indicating the relevance of efficient heat and mass transfer in the methanation reaction.\nNuria Garc\u00eda-Moncada: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing - original draft, Writing - review & editing. Juan Carlos Navarro: Investigation, Formal analysis, Writing - review & editing. Jos\u00e9 Antonio Odriozola: Conceptualization, Writing - review & editing. Leon Lefferts: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing - review & editing. Jimmy A. Faria: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing.The authors report no declarations of interestThe authors acknowledge the financial support from ADEM, a green deal in energy materials program of the Ministry of Economic Affairs of The Netherlands (www.adem-innovationlab.nl). We acknowledge Ir. Ties Lubbers from University of Twente for the support in the characterization of the catalysts and relevant discussions on the physico-chemical properties of these nano-structured materials.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2021.02.014.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Coupling inherently fluctuating renewable feedstocks to highly exothermic catalytic processes, such as CO2 methanation, is a major challenge as large thermal swings occurring during ON- and OFF- cycles can irreversible deactivate the catalyst via metal sintering and pore collapsing. Here, we report a highly stable and active Ni catalyst supported on CeO2 nanorods that can outperform the commercial CeO2 (octahedral) counterpart during CO2 methanation at variable reaction conditions in both powdered and \u03bc-monolith configurations. The long-term stability tests were carried out in the kinetic regime, at the temperature of maximal rate (300 \u00b0C) using fluctuating gas hourly space velocities that varied between 6 and 30 L h\u22121\u00b7gcat\n                     \u22121. Detailed catalyst characterization by \u03bc-XRF revealed that similar Ni loadings were achieved on nanorods and octahedral CeO2 (c.a. 2.7 and 3.3 wt. %, respectively). Notably, XRD, SEM, and HR-TEM-EDX analysis indicated that on CeO2 nanorods smaller Ni-Clusters with a narrow particle size distribution were obtained (\u223c 7 \u00b1 4 nm) when compared to octahedral CeO2 (\u223c 16 \u00b1 13 nm). The fast deactivation observed on Ni loaded on commercial CeO2 (octahedral) was prevented by structuring the reactor bed on \u03bc-monoliths and supporting the Ni catalyst on CeO2 nanorods. FeCrAlloy\u00ae sheets were used to manufacture a multichannel \u03bc-monolith of 2 cm in length and 1.58 cm in diameter, with a cell density of 2004 cpsi. Detailed catalyst testing revealed that powdered and structured Ni/CeO2 nanorods achieved the highest reaction rates, c.a. 5.5 and 6.2 mmol CO2 min\u22121\u00b7gNi\n                     \u22121 at 30 L h\u22121\u00b7gcat\n                     \u22121 and 300 \u00b0C, respectively, with negligible deactivation even after 90 h of fluctuating operation.\n               "}
{"full_text": "Electrochemical water splitting has been considered as a promising technology for sustainable production of clean and efficient energy by converting the electrical energy from intermittent-renewable resources such as solar and wind energies. Electrochemical water splitting provides simultaneous hydrogen and oxygen production according to two couple reactions. The hydrogen evolution reaction, HER (acidic medium: 2H+\u00a0+\u00a02e\u2212 \u2192 H2 and alkaline medium: 2H2O\u00a0+\u00a02e\u2212 \u2192 H2\u00a0+\u00a02OH\u2212) is carried out with the involvement of two electrons at the cathode, while the OER (acidic medium: 2H2O \u2192 O2\u00a0+\u00a04H+\u00a0+\u00a04e\u2212 and alkaline medium: 4OH\u2212 \u2192 O2\u00a0+\u00a02H2O\u00a0+\u00a04e\u2212) occurs with the involvement of four electrons at the anode [1,2]. The greater overpotential in OER makes the reaction kinetically and energetically disadvantageous without an adequate catalyst. Typically, the catalysts used in the reaction contain expensive noble metals such as Ir or Ru for OER and Pt for HER, which makes the process economically not-feasible for commercial applications [3]. Recently, in the field of OER in an alkaline environment, remarkable results for the replacement of the noble metals in the catalysts by abundant, cheap, stable, and operating at low overpotential (\u03b7) transition metals have been achieved [4] and among them, Fe, Ni and Ce are promising choice [5].Generally, the mechanism of the OER reaction in an alkaline environment is considered as a step-wise process as follows:\n\n(1.1)\nOH\u2212\u00a0+\u00a0Cat\u2734 \u2192 HO\u2734\u00a0+\u00a0e\u2212\n\n\n\n\n\n(1.2)\nHO\u2734\u00a0+\u00a0OH\u2212 \u2192 O\u2734\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212\n\n\n\n\n\n(1.3)\nO\u2734\u00a0+\u00a0OH\u2212 \u2192 \u2734OOH\u00a0+\u00a0e\u2212\n\n\n\n\n\n(1.4)\nHOO\u2734\u00a0+\u00a0OH\u2212 \u2192 Cat\u2734\u00a0+\u00a0O2\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212\n\n\nWhere (\u2734) represents an active surface site of the catalyst (Cat) [6\u20138]. In the reactions shown above, the steps that involve a charge transfer are of crucial importance, because their mechanism is directly dependent on the applied potential. The diffusion of any species or the surface reactions are weakly dependent on the applied potential. It is interesting to note that the direct recombination between oxygen atoms to form O2 in reaction step (1.3) is not taken into consideration due to the large activation barrier that is expected from this process, leading us to think of a more probable associative mechanism to anode using the HOO\u2734 surface intermediate [8]. As previously anticipated, hydrogen production by electrolysis of water is, however, associated with substantial energy losses. Most of the overpotential giving rise to those losses is related to the electrochemical processes at the anode, where O2 evolution takes place [9]. So the total overpotential to conduct an OER reaction could be assimilated as the sum of three determining factors, respectively: changes in Gibbs free energy, electronic conductivity and electro-catalytic active surface area, as expressed in Equation (2);\n\n(2)\n\u03b7tot\u00a0=\u00a0Eapp \u2013 E\u00b0\u00a0=\u00a0\u03b7\u0394G\u00a0+\u00a0\u03b7Con\u00a0+\u00a0\u03b7Surf\n\n\nWhere \u03b7tot is the total overpotential for the catalyst to reach the given current density. Eapp is the potential actually applied, E\u00b0 represents the thermodynamic equilibrium potential of water oxidation (2H2O (l) \u2192 O2 (g)\u00a0+\u00a04H+ (aq)\u00a0+\u00a04e\u2212 E\u00b0\u00a0=\u00a01.23\u00a0V), \u03b7Con is a value strictly related to the resistance of the system and can be idealized as the potential drop between the active sites in which the reaction takes place and the external circuit. Knowing therefore the four pivotal coupled reactions of the OER and the intrinsic factors that mark the total overpotential of the catalyst, it is easy to imagine how fundamental is the optimization of the \u0394G of the catalyst (and hence its intrinsic electronic structure), as the value of the overpotential \u03b7\u0394G is the only one factor strictly correlated to the kinetics of the four reaction steps. The main purpose in the \"evolution of a catalyst\" will therefore be to regulate the \u0394G of the reaction by modifying the electron transport and the active surface area to intrinsically increase the catalytic activity and decrease the overpotential.In recent years, research has been focused on the use of Fe, Ni, and Ce based catalysts for water splitting because of their lower cost and high availability. Metals like, Fe and Ni easily form NiFe alloys [10]. Besides, the Fe3+ and Ni2+ cations could be organized in NiFe2O4, spinel structure, where Ni2+ ions occupy one-eighth of the tetrahedral positions and Fe3+ ones occupy half of the octahedral ones. It was reported [11] that this NiFe oxide compound outperforms the individual Fe3O4 spinel and NiO in OER electro-catalytic activity [12]. In addition, Fe3+ and Ni2+ cations could be included in layered double hydroxides (NiFe-LDHs). They are ionic crystals composed of stratified positively charged structures and anionic counterpart [13]. This unequal structure provides an opportunity by simple regulation of the size and morphology of the crystallites to increase the intrinsic electrical conductivity and reduce the reaction barrier, which results in a decrease of the OER overpotential [14,15]. Xiang et al. observed that the surface of NiFe-LDH having an intermediate hydroxyl layer, possessed higher activity in OER than a normal NiFe-LDH. He proposed a synergistic effect between the NiFe-LDH and the hydroxyl layer, which increases the absorption of the OH intermediates [16]. Hollow-structured NiFe-LDH microsphere was prepared by Zhong et al. using a template-free synthesis [17].Ceria has noteworthy properties due to the dynamic in the cerium ions valence states and generation of oxygen defects as a result of temperature variation, size effect, doping with metal ions or application of electric potential [18,19]. The formation of oxygen defects is accompanied with the localization of the electrons on Ce 4f orbital leading to the formation of two Ce3+ ions. Although it is easy to imagine that the Ce cation closest to the location of the oxygen defect is actually the one that will reduce, in reality a computational study has shown the Ce3+ resulting from the oxygen vacancy is actually located in a location far from the neo-generated defect [20]. This process is possible thanks to the ability of the cerium atom to adjust its electronic configuration in a flexible way to environmental variations. Thus, the oxygen storage capacity of cerium oxide is closely related to the quantum effect of electrons located on the Ce 4f state, which provides excellent electronic transfer capabilities due to the reversible Ce3+-Ce4+ redox cycle [21,22].In our recent study [7] we demonstrated increase in the specific surface area and facile electron transfer by doping of CeO2 with Fe3+ ions. This effect was most pronounced for the samples with Fe:Ce molar ratio of 1:9 and 3:7. The physicochemical analyses demonstrated formation of Ce\u2013O\u2013Fe interface layer and stabilization of finely dispersed hematite- and ceria-like entities in its vicinity. The modification of the binary ceria-iron oxide materials with nickel [23] provided the formation of finely dispersed NiO nanoparticles nearby the Fe\u2013O\u2013Ce defects and the existing in their vicinity hematite-like structures. It was also reported that the presence of Ni in CeO2\u2013Fe2O3 catalysts facilitated the formation of \u03c7-Fe2C5 carbides during the methanol decomposition with the formation of carbon nanofibers (CNFs) which incorporate the catalyst nanoparticles.CNFs are classified among carbonaceous materials with high conductivity due to their predominantly graphitic character [6]. It has already been proven that the structural characteristics of these materials can increase the performance in many applications, in which electrical conductivity plays a role in providing them, thus crediting their use as potential elements in electrocatalytic processes [6].In the light of the above, it has been estimated that the dynamism of the class of catalysts in Fe\u2013Ce\u2013Ni, and species derived from them, could be very interesting for the OER reaction. In fact, this study has the double purpose of:\n\n(I)\nStudy the evolution of the catalyst in OER in an alkaline environment (1\u00a0M KOH) and the sudden lowering of the overpotential by making structural and electro-conductive changes, starting from two different molar ratios of mesoporous catalysts of mixed oxides in Fe\u2013Ce\u2013Ni, passing through the same catalysts in reduced form (hence the presence of the NiFe alloy), and ending with a carbon-coating process obtained by vapor phase decomposition of methanol and the formation of metal nanoparticles encapsulated within carbon nanofibers.\n\n\n(II)\nPropose a facile and innovative possibility of carbon-coating with low-temperature methanol treatment, which with a targeted study, could allow a reuse of the mesoporous mixed metal oxide catalysts, mainly metal-carbon containing phases, into promising catalysts from electro-catalytic point of view. Such reuse would lead to greater sustainability of the hydrogen economy, as the same catalyst could be used for the production of hydrogen by two different techniques, using low-cost metals and decreasing the costs for recycling metals and manufacturing additional catalysts.\n\n\nStudy the evolution of the catalyst in OER in an alkaline environment (1\u00a0M KOH) and the sudden lowering of the overpotential by making structural and electro-conductive changes, starting from two different molar ratios of mesoporous catalysts of mixed oxides in Fe\u2013Ce\u2013Ni, passing through the same catalysts in reduced form (hence the presence of the NiFe alloy), and ending with a carbon-coating process obtained by vapor phase decomposition of methanol and the formation of metal nanoparticles encapsulated within carbon nanofibers.Propose a facile and innovative possibility of carbon-coating with low-temperature methanol treatment, which with a targeted study, could allow a reuse of the mesoporous mixed metal oxide catalysts, mainly metal-carbon containing phases, into promising catalysts from electro-catalytic point of view. Such reuse would lead to greater sustainability of the hydrogen economy, as the same catalyst could be used for the production of hydrogen by two different techniques, using low-cost metals and decreasing the costs for recycling metals and manufacturing additional catalysts.Two mesoporous Fe\u2013Ce oxides with Fe/Ce molar ratio of 5:5 and 9:1 were prepared by template-assisted hydrothermal technique as described in Ref. [7]. For this purpose, CeCl3\n\n.\n 7H2O and FeCl3\n\n.\n 6H2O were used as precursors of Ce and Fe, respectively, and cetyltrimethylammonium bromide (CTAB) was used as a template. An aqueous solution of the metal salts mentioned was added drop by drop to the aqueous solution of the template under magnetic stirring at room temperature, subsequently the temperature was increased up to 323\u00a0K and left to react for 30\u00a0min. Then, 40\u00a0mL of an aqueous solution of NH4OH at 12.5% (up to pH of about 10) were added drop by drop to cause the precipitation reaction of the metal hydroxides in the solution. The emulsion was kept under magnetic stirring at 323\u00a0K for the whole night. The hydrothermal treatment was carried out at 373\u00a0K for 24\u00a0h. The solid obtained was calcinated for 10\u00a0h at 573\u00a0K.2\u00a0g of the obtained mixed oxides were loaded with nickel (Fe/Ni molar ratio 2:1) by incipient wetness impregnation with 1\u00a0ml aqueous solution of Ni(NO3)2\n\n.\n 6H2O. After drying for 24\u00a0h at room temperature, the samples were calcinated for 3\u00a0h at 773\u00a0K in air. The obtained composites were denoted as 5Fe5Ce_Ni and 9Fe1Ce_Ni, respectively.250\u00a0mg of the catalysts were subjected to reduction in a hydrogen flow (flow rate of 35\u00a0mL/min) at 773\u00a0K for 2\u00a0h. Thus obtained materials were denoted as xFeyCe_Ni_Alloy, where x/y was the Fe/Ce molar ratio.The encapsulation of the reduced mixed metal oxide nanoparticles inside the carbon nanofibers was obtained by decomposing the methanol on the surface of the catalysts, inside a flow type reactor using a mixture of argon as carrier gas (with a flow rate of 30\u00a0mL/min) and methanol vapor extracted by bubbling the carrier gas inside a saturator at 273\u00a0K. Typically, 200\u00a0mg of the xFeyCe_Ni_Alloy catalyst were pre-treated insitu in argon at 373\u00a0K for 20\u00a0min and the temperature was increased up to 773\u00a0K (rate of 20\u00a0K/min). Then, the methanol mixture (methanol partial pressure of 1.57\u00a0KPa) was introduced into the reactor for 2\u00a0h. The input and output flow composition was periodically analyzed on-line by a SCION 456-GC gas chromatograph, equipped with flame ionization and thermo-conductivity detectors and PORAPAC-Q column. Absolute calibration method and carbon based material balance were used for the elucidation of the conversion and products distribution during the methanol decomposition. The products selectivity was calculated as Si = Yi/X*100, where Si and Yi were the selectivity and the yield of the \u201ci\u201d product and X was the conversion. As for the carbon mass balance, the correction factor from the solid fraction of carbon acquired by the catalysts during the cooking process was also introduced, and determined by elemental analysis (EA). Thus obtained modifications were denoted as xFeyCe_Ni_@C. The presented above synthetic procedures are illustrated in Scheme 1\n.Powder X-ray diffraction patterns were collected on a Bruker D8 Advance diffractometer with Cu K\u03b1 radiation and a LynxEye detector with constant step of 0.02\u00b0 2\u03b8 and counting time of 17.5\u00a0s per step. Mean crystallite sizes were determined by Topas-4.2 software.The M\u00f6ssbauer spectra were recorded at room temperature by Wissel (Wissenschaftliche Elektronik GmbH, Germany) electromechanical spectrometer working in a constant acceleration mode. A57Co/Rh (activity\u00a0\u2248\u00a010\u00a0mCi) source and \u03b1 Fe standard were used. The spectra were fitted using WinNormos software.X-ray photoelectron measurements have been carried out on the ESCALAB MkII (VG Scientific, now Thermo Scientific) electron spectrometer with a base pressure in the analysis chamber of 5\u00a0\u00d7\u00a010\u221210\u00a0mbar (9\u00a0\u00d7\u00a010\u22128\u00a0mbar during the measurements), equipped with twin anode MgK\u03b1/AlK\u03b1 non-monochromated X-ray source that used excitation energies of 1253.6 and 1486.6\u00a0eV, respectively. The measurements are provided only with AlK\u03b1 non-monochromated X-ray source (1486.6\u00a0eV). The pass energy of the hemispherical analyzer was 20\u00a0eV, because of their nature and lower signal for Ce3d, Ni2p and Fe2p, 50\u00a0eV pass energy was used. The instrumental resolution measured as the full width at a half maximum (FWHM) of the Ag3d5/2, photoelectron peak is about 1\u00a0eV. The energy scale has been calibrated by normalizing the C1s line of adventitious hydrocarbons to 285.0\u00a0eV for electrostatic sample charging. The data was analyzed by SpecsLab2 CasaXPS software (Casa Software Ltd). The processing of the measured spectra includes a subtraction of X-ray satellites and Shirley-type background. The peak positions and areas are evaluated by a symmetrical Gaussian-Lorentzian curve fitting. The relative concentrations of the different chemical species are determined based on normalization of the peak areas to their photoionization cross-sections, calculated by Scofield.Raman spectra were recorded by using Raman Microscope Senterra II (Bruker). Samples were placed onto glass (approximately 10\u00a0mg) and analyzed using the vertical 20\u00d7 objective in an 180\u00b0 backscattering arrangement. The Raman spectrometer parameters used to analyze the samples include: 532\u00a0nm laser wavelength and an exposure time of 100\u00a0s, resolution was 4\u00a0cm\u22121 for all samples, laser power was 6.5\u00a0mW.FTIR spectra were recorded on a Bruker Vector 22 spectrometer with a resolution of 1\u20132\u00a0cm\u22121, in the region 4000-400\u00a0cm\u22121, accumulating 128\u2013220 scans using pellets produced from approximately 2\u00a0mg of sample diluted in 100\u00a0mg KBr.Low-temperature nitrogen physisorption was studied by a Quantachrome Instruments NOVA 1200e (USA) apparatus. The specific surface area was determined from Brunauer Emmett Teller (BET) equation, the total pore volume was obtained at a relative pressure of about 0.99, and the pore size distribution was obtained by using Non-Local Density Functional Theory (NLDFT) method and the equilibrium model for cylindrical pores.TEM analysis was performed by means of JEOL JEM 2100 high resolution transmission electron microscope at accelerating voltage 200\u00a0kV. Selected area electron diffraction (SAED) mode was applied for diffraction patterns accumulation and HRTEM imaging was used for lattice fringes registration.JEOL 2100 XEDS: Oxford Instruments, X-MAXN 80\u00a0T CCD Camera ORIUS 1000, 11 Mp, GATAN at accelerating voltage of 200\u00a0kV was applied for the elemental composition determination and mapping. Samples for TEM investigations are prepared as the suspension of the corresponding sample was dropped on standard Cu TEM grids and then dried in pure ambience.The elemental analysis (EA) was performed on Vario Macro Cube (Elementar Analyzensysteme GmbH) equipment for determination of C, H, N, S. The combustion temperature for the system was 1150\u00a0\u00b0C in an oxidizing atmosphere to form gaseous reaction products: CO2, H2O, N2, NOX, SO2 and SO3. The individual gases are transported by a carrier gas to be measured by a thermal conductivity detector (TCD).The temperature programmed oxidation (TPO) was performed in a Netzsch STA 449-F5 \u201cJupiter\u201d thermos-microbalance. Around 10\u201315\u00a0mg of sample was loaded in the thermogravimetry (TG) instrument and heated from 30\u00a0\u00b0C to 900\u00a0\u00b0C, with an isothermal step at 400\u00a0\u00b0C for 20\u00a0min, while during the all process the temperature increase was performed with a rate of 10\u00a0\u00b0C/min under a flow of synthetic air (80\u00a0mL/min, purity 4.7 AGA).All electrochemical measurements were performed in a three-electrode system controlled by a potentiostat/galvanostat AUTOLAB PGSTAT302\u00a0N at room temperature.A rotating ring disk electrode (RRDE), with a Pt ring and a glassy carbon disk of 5\u00a0mm of diameter (0.196\u00a0cm2) was used as working electrode (WE), while reversible hydrogen electrode (RHE) installed in a Luggin capillary as a reference electrode and a glassy carbon rod for the counter electrode (CE) were used. Before the electrochemical test, the working electrode (RRDE) was polished with alumina slurry. The ink was grafted onto the RRDE maintaining the following conditions: ink concentration of 10\u00a0mg/mL, Nafion\u00ae 15\u00a0wt% in catalytic layer (CL) and total catalyst loading on WE of 1018\u00a0\u03bcg/cm2. For this purpose, the ink was prepared by mixing a quantity between 6 and 9\u00a0mg of catalyst with the respective quantity in microliters of a 5% Nafion\u00ae solution (Sigma Aldrich) and about 0.6\u00a0mL of a water/isopropanol solution (3:1). The mixed solution was sonicated for 30\u00a0min and then used as catalyst ink. Then, 20\u00a0\u03bcL catalyst ink was drip-evenly applied to the disk electrode surface of the RRDE (in 4 aliquots of 5\u00a0\u03bcL each).N2 (99.99% Air Liquide) was employed to deaerate all solutions. Before electrocatalytic studies, all composites were submitted to activation process based on 50 cyclic voltammograms (CVs) between 0.05 and 1.1\u00a0V vs. RHE, at a scan rate of 0.1\u00a0V/s in the deaerated supporting electrolyte (1\u00a0M KOH). After that, a blank voltammetry was recorded in the same conditions with a scan rate of 0.02\u00a0V/s. In order to determine the activity of the catalysts in OER, a linear sweep voltammetric curve between 0.7 and 1.8\u00a0V vs. RHE (positive going scan) was recorded at 0.005\u00a0V/s and a rotation speed of 1800\u00a0rpm. All current density values in this work were referred to the geometric area of the working electrode.XRD patterns of the initial Ni\u2013Fe\u2013Ce oxides and their modifications are shown in Fig. 1\nA and B.The reflections at 2\u03b8\u00a0=\u00a028.5\u00b0, 33.1\u00b0, 47.5\u00b0, 56.3\u00b0, 59.0\u00b0, 69.7\u00b0, 76.6\u00b0, 76.7\u00b0 and 79.1\u00b0 in the patterns of the mixed 5Fe5Ce_Ni and 9Fe1Ce_Ni oxides are associated with the lattice planes (111), (200), (220), (311), (222), (400), (331), and (420) of the fluorite-like cubic ceria, respectively (JCPDS 65\u20135923). The presence of reflections at 2\u03b8\u00a0=\u00a024.1\u00b0, 33.1\u00b0, 35.6\u00b0, 40.9\u00b0, 49.4\u00b0, 54\u00b0, 57.6\u00b0, 64.2\u00b0, 72.2\u00b0, 75.3\u00b0 are attributed to the crystallographic planes (012), (104), (110), (113), (024), (116), (018), (214), (1 0 10) and (220), respectively, of rhombohedral hematite (JCPDS 33\u20130664). The reflections at 2\u03b8\u00a0=\u00a037.2\u00b0, 43.2\u00b0 and 62.7\u00b0 are assigned to the crystallographic planes (111), (200) and (220) of NiO (JCPDS 47\u20131049), respectively. The cerianite reflections are broader and less pronounced for 9Fe1Ce_Ni probably due to the lower content of highly dispersed ceria [7]. Besides, the increase of the intensity and the decrease of the width of the hematite reflections for this sample indicate the segregation of a significant portion of larger \u03b1-Fe2O3 nanoparticles (Table 1\n, Fig, 1B). After the treatment in hydrogen atmosphere (Fig. 1A\u2013B, samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy), the reflections of cerianite are still distinguished. The observed slight increase in the ceria unit cell parameters after the reduction (Table 1) could be due to the release of the incorporated in the ceria lattice iron ions and generation of additional oxygen vacancies in it. The reduction transformations of the parent mixed oxides are also confirmed with the appearance of additional reflections at 2\u03b8\u00a0=\u00a043.7\u00b0, 50.8\u00b0, 74.6\u00b0 and 2\u03b8\u00a0=\u00a044.5\u00b0, 51.6\u00b0, 76.3\u00b0 typical of Ni\u2013Fe alloys (JCPDS 38\u20130419) and metallic Ni (JCPDS 01-071-4655), respectively.Generally, the crystallite size of the metal phases is below 35\u00a0nm (Table 1) indicating that ceria hinders their agglomeration [23].The carbon coating process, carried out on the reduced catalysts (paragraph 2.2, Fig. 1A\u2013B in black), leads to the appearance of reflections at 2\u03b8\u00a0=\u00a037.8\u00b0, 39.8\u00b0, 40.7\u00b0, 42.8\u00b0, 45\u00b0 and 46\u00b0 (in the short angle magnification in Fig. 1A) of FeC3 (JCPDS 35\u20130772) and reflections at 2\u03b8\u00a0=\u00a040.9\u00b0, 43.3\u00b0, 44.2\u00b0, 45\u00b0, 46.5\u00b0, 50\u00b0, 50.3\u00b0, attributable to Fe5C2 (JCPDS 51\u20130997). The carbides are in coexistence with NiFe alloy and Ni0 nanoparticles, while the reflection at 2\u03b8\u00a0=\u00a026.5\u00b0 in both samples (5Fe5Ce_Ni_@C and 9Fe1Ce_Ni_@C) are associated with the (002) plane of graphite-2H (JCPDS 41\u20131487).In order to assess the textural changes in the specimens under investigation, N2-physissorption measurements were carried out. The absorption-desorption curves, shown in Figure S1-A and B, can be classified as type IV-a isotherms, typical of mesoporous solids (according to the IUPAC [24]), for both catalyst series (5Fe5Ce_Ni-based and 9Fe1Ce_Ni-based). The hysteresis loop, on the other hand, can be classified as type H3. The absence of sharp step-downs in the desorption curve suggests a cylinder-like pores with shrinkage-free pore-necks. As can be seen from the data obtained from the XRD analysis (Table 1), the reduction process of the metal oxides of nickel and iron leads to the formation of metal alloys and CeO2 with an overall reduction in the particle size. This process inevitably affects the BET surface area of the samples in oxidized and reduced state. The sample 9Fe1Ce_Ni_Alloy possesses higher surface area compared to its analogue before the reduction due to the larger exposed surface area given by particles of reduced nanometer size. Such effect is not observed for the 5Fe5Ce_Ni-based materials. It could be assigned to the higher amount of ceria in them, which does not take part in the reduction reaction and hinders the sintering of the metal nanoparticles. The growth of the carbon nanofibers around the nanoparticles after the treatment in methanol causes a predictable increase in the specific surface area of the samples. The significantly higher SBET for 5Fe5Ce_Ni_@C as compared to 9Fe1Ce_Ni_@C it may be due to a greater growth of the carbon component, combined with the development of a significant portion of micropores (Table 2\n and Figure S2-E and F).Moessbauer spectroscopy was used to analyze in detail the energy levels of the Fe nuclei contained in the samples and to obtain precise information on the ferrous phases. In Fig. 2\n, the Moessbauer spectra are shown and the parameters of the samples under study as: isomer shift (\u03b4), quadrupole splitting (\u0394), quadrupole shift (2\u03b5), hyperfine field (Bhf), full width at half-maxima (\u0393exp) and the relative weight of each component (G) obtained from the least squares fitting, are summarized in Table 3\n.The Moessbauer spectra of the parent mixed oxides are well-fitted with sextets and doublets. The parameters of the sextets are attributable to \u03b1- Fe2O3 with average crystallite size above 10\u201312\u00a0nm. The doublets possess \u03b4\u00a0=\u00a00.35\u00a0mm/s and relatively high value of quadrupole splitting (around 0.9\u00a0mm/s), which could be assigned to Fe3+ in different environment. In accordance with the XRD data (Fig. 1, Table 1) they probably belong to ultra-finely dispersed hematite-like entities and Fe3+ ions, inserted in the ceria lattice. The sextet part in the spectra is larger for 9Fe1Ce_Ni, which corresponds to the assumption done on the base of the XRD analyses (Fig. 1, Table 1) for the segregation of bigger portion of larger hematite particles.The spectra of the reduced samples represent superimposition of two sextets (Sx1 and Sx2) and one singlet (Sn) with isomer shift about 0\u00a0mm/s. The Sx1 has a higher magnetic field and narrower lines and can be assumed as (Ni, Fe)-alloy with a body centered cubic cell of kamacite (\u03b1-(Ni, Fe) alloy, bcc) [25]. The Sx2 with a lower magnetic field and wider lines can be attributed to (Ni, Fe) alloy with a face centered cubic cell of highly distorted taenite (\u03b3-(Ni, Fe)-alloy, fcc). The Sn could be attributed to \u03b3-(Ni, Fe) alloy with low nickel content [26,27].The proportion of each component varies with the composition of the parent materials. Predominantly \u03b1-(Ni, Fe) alloy and negligible amount of \u03b3-(Ni, Fe) alloy with less Ni content are registered after the reduction of 9Fe1Ce_Ni, while \u03b3-(Ni, Fe) alloy is mainly detected when ceria is doped with lower content of Fe and Ni (5Fe5Ce_Ni). Five additional sextets appear after the treatment of the samples under the methanol decomposition medium. The two sextets (Sx3, Sx4) with isomer shift of around 0.18\u00a0mm/s and Bhf\u00a0\u2248\u00a020\u00a0T correspond to Fe3C, while the Sx5, Sx6 and Sx7 with \u03b4\u00a0\u2248\u00a00.20\u00a0mm/s are typical of Fe5C2 carbide [28,29]. The relative part of Sx1 and Sx2, belonging to \u03b1-(Ni, Fe) and \u03b3-(Ni, Fe) alloys, respectively, is significantly bigger for 5Fe5Ce_Ni_@C, indicating lower degree of carbides formation, as was also assumed on the base of the XRD analyses (Table 1, Fig. 1A). Note, that here small doublet peaks are also distinguished, probably due to the partial preservation of Fe3+ included into the Ce\u2013O\u2013Fe interface. Just the opposite, the higher amount of Fe and Ni in the sample (9Fe1Ce_Ni_@C) promotes the transformation of all iron phases predominantly to Fe5C2 carbide under the reaction medium.In order to visualize the morphology and microstructure of the samples, Transmission Electron Microscopy (TEM) was used. In Figures S3-S4-S5-S6-S7, the images at different magnifications of the samples in the mixed metal oxides are presented. For 5Fe5Ce_Ni (Figures S3-A and S8-A), aggregates, consisting of almost spherical particles with average size of 20\u201333\u00a0nm, which are surrounded by a multitude of smaller particles (7.5\u201312.4\u00a0nm) are well-distinguished. At higher magnification (Figs. S3\u2013B) the porous texture of the clusters is detectable. The HR-TEM images (Figs. S3\u2013C) shows presence of particles with interplanar distances of 2.7\u00a0\u00c5, which could be assigned to the plane (200) of cubic CeO2 (PDF 96-434-3162). In addition, particles with d-spacing of 2.22\u00a0\u00c5 and 2.41\u00a0\u00c5, attributable to the (\u22123 1 3) plane of Fe2O3 (PDF 96-210-8029) and (222) plane of cubic NiO (PDF 96-901-3981) are visible. The presence of CeO2 and NiO is also confirmed by the SAED analysis (Figs. S3\u2013D) (Fig. 1).Larger nanoparticles with average crystallite size of 44\u00a0nm are well seen in the TEM images of 9Fe1Ce_Ni (Figs. S4\u2013A and Figs. S8\u2013B). The high resolution image (Figs. S4\u2013C) exhibits interplanar distances of 2.08\u00a0\u00c5 and 2.68\u00a0\u00c5, which correspond to planes (400) and (104) of NiO (PDF 96-901-3981) and hematite (PDF 96-901-5504), respectively. The presence of cubic NiO is also confirmed by SEAD pattern (Figs. S4\u2013D).TEM images of the samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy are shown in Figs. S5 and S6. Polygonal particles with average crystallite size of 16.4 and 25.6\u00a0nm, respectively (Figure S8-C and D), are well distinguished. The SAED images of both samples are rather similar and demonstrate co-existence of ceria and metallic Ni (PDF 96-210-2273) and Fe (PDF 96-901-5259) particles. The interplanar distances (Figs. S5\u2013C) of 2.68\u20132.7\u00a0\u00c5 in the HRTEM pattern of 5Fe5Ce_Ni_Alloy is attributed to the plane (200) of distorted cubic ceria lattice. In the pattern of 9Fe1Ce_Ni_Alloy, besides the interplanar distances of 3.1\u00a0\u00c5 and 1.76\u00a0\u00c5, typical of (200) plane of CeO2 and of (200) plane of metallic Ni and Fe, respectively, the additional ones of 2.75\u00a0\u00c5 and 2.60\u00a0\u00c5 are detected. Despite that these values differ significantly from the values reported in the literature, they could be carefully associated with the planes (204) and (124) of the orthorhombic metallic Fe (PDF 96-411-3934).The high resolution images in Fig. 3-C and S7-C clarify the layering and presence of the distorted crystallographic planes of graphite-2H (3.1\u00a0\u00c5 - 3.34\u00a0\u00c5), values confirmed for both samples by the SAED patterns shown in Fig. 3-E and S7-D. Fig. 3\n-D shows d-spacing of about 2.7\u00a0\u00c5, coming from particles adhered to the outer wall of the nanofiber, and attributable to the (200) plane of CeO2, which evidently does not seem to take an active part in the growth process of the fibers.The carbon nanofibers (CNFs) in Fig. 3-A and S7-A, can be classified, with the nomenclature \"bamboo-like'-CNFs or \"stacked-cups\"-CNFs, due to the characteristic bamboo structure of the trunk of the fiber. The mechanism proposed by Terrones et al. [30] for the growth of CNFs from the pyrolysis of acetonitrile on Co particles, provides a mechanism that can be factored into three main phases: (I) the decomposition of the organic molecule on the active metal surface, (II) diffusion of carbon inside the metal particle, and formation of thermodynamically more stable carbides, (III) layering of graphite and encapsulation of the nanoparticle with the formation of a \"carbonaceous restriction\" which will act as a nucleation point for the subsequent growth of the nanotube until the formation of a new nucleation point and subsequent formation of a second segment. This \"impulse growth\" explains both the characteristic bamboo-shape of the nanofiber and the particular pear-like shape of the promoter particles, which always possesses a particular truncated-conical deformation in the opposite direction to the direction of growth of the CNF. The presence of thermodynamically stable carbides appears to be crucial for the growth of CNFs, in fact extensive studies conducted by Matter et al. [31] have clarified how the formation of the more stable Fe carbides increases the possibility of the formation of nanofibers compared to catalysts having Ni as the active metal, as the Ni carbides are thermodynamically less stable [32]. This last assumption in fact, would agree with the Moessbauer data (Fig. 2, Table 3) and specifically, would rationalize the total disappearance of the Sn- \u03b3 -(Ni, Fe) Alloy, with a low nickel content, due to an easier formation of iron carbides in this alloy compared to other alloys with a higher nickel content. Although the formation of carbides favors the formation of CNFs, Ni nanoparticles can also favor the growth of CNFs as exposed by Wang et al. [33] by the decomposition process of methane upon Ni-NPs. Therefore, a combined contribution of these two centers in the growth of the fibers is not excluded.The relative elemental dispersion was visualized by EDS analysis, with the TEM working in STEM mode. The analysis of a single sample 5Fe5Ce_Ni_@C is shown in Fig. S9. From the Fe and Ni samples it is shown that the metal particles are mainly located in the nanofiber tips, confirming a tip-growth mechanism. Cerium and oxygen are more dispersed in the image and almost always in marginal portions outside the larger grains. This suggests a mechanism of extrusion of Fe and Ni particles, with a progressive removal from the CeO2 support, during the formation of the larger nanofibers, while the smaller particles undergo a carbon coating process that \"engulfs\" the ceria in a thin carbon layer. The mapping of the oxygen in the central part of the image is almost superimposable with that of the cerium, given the presence of CeO2 in the sample. But away from the central fiber in the more marginal areas of the image it is possible to see how even the carbon, finely dispersed in the contour, probably has distinguishable oxygen functionalization.In order to clarify the amount of carbon deposits on the samples with different composition, temperature programmed oxidation-thermogravimetric analyses were performed (Fig. 4\n). The thermogravimetric profiles consist of three main features. The first one (I), starting at about 300\u00a0\u00b0C, characterizes with mass increase of 2.6% and 9% for 5Fe5Ce_Ni_@C and 9Fe1Ce_Ni_@C, respectively. It can be predominantly attributed to the oxidation of the iron and nickel species present in the system (iron carbides, Ni\u2013Fe alloys and metallic Ni).The second feature (II), manly occurs during the isothermal step at 400\u00a0\u00b0C, as superimposition of two effects: 1) weight loss due to a principle of carbon decomposition; 2) weight increase due to the further continuation of the oxidative process. The slope of the curves in this segment of the process is different between the two specimens. The sample 9Fe1Ce_Ni_@C, being composed predominantly of Fe and Ni, during the isothermal process acquires much more mass converting into metal oxides, and at the same time decomposing part of the low crystallinity carbon, losing a total of 4% of its weight during the (II) feature. On the other hand, the sample 5Fe5Ce_Ni_@C, having a lower amount of carbides/alloys, ends the oxidative evolution during the initial phases of the isothermal process, and shows a decrease in weight of 12.6% almost exclusively due to the decomposition of carbon. The third observed feature (III), can be rationalized as the decomposition of the CNFs and it starts for both samples within 420\u2013460\u00a0\u00b0C range. Although the catalysts underwent the same decomposition time of methanol (2\u00a0h), it can be seen that sample 5Fe5Ce_Ni_@C acquired more carbon than 9Fe1Ce_Ni_@C, with an attributed to carbon weight loss of 31\u00a0wt% and 19\u00a0wt%, respectively. However, since the total percentage of carbon loss could not to be calculated accurately, due to the feature (II) effect (oxidation/decomposition process), elemental analysis (EA) was adopted to obtain more accurate results in this respect. Table 4\n shows the percentage of C, N, H, and S obtained from the samples with CNFs, confirming with good approximation the results obtained from TPO-TG analysis.Raman spectroscopy was used in order to elucidate the nature of the defects present in the samples both from the point of view of the cubic lattice of the ceria shown in Fig. 5\n, and from the point of view of the defects present in the carbon nanofibers, as shown in Fig. 6\n.The Raman spectra of samples 5Fe5Ce_Ni and 9Fe1Ce_Ni before and after the reduction process, shown in Fig. 5, introduces a superimposition of peaks belonging to ceria, hematite and probably bunsenite, which are difficult to resolve. Within the range of Raman shifts between 150\u00a0cm\u22121 and 1600\u00a0cm\u22121, it is possible to discern a predominant broad peak centered at ca. 435\u00a0cm\u22121 (especially in the sample with the higher cerium content), which can be rationalized as the resultant from the union of the vibrational contributions of the Eg vibrational modes of the hematite and the vibrational modes of the crystallographic plane (111) of the cubic fluorite-like structure of CeO2. Specifically, this is a first-order vibrational band (F2g band) associated with Ce\u2013O bond stretching, with Ce and O respectively 8-fold and 4-fold coordinated. This broad band is generally preceded by the band of a triple degenerate vibrational mode (with F1u symmetry) positioned at\u00a0\u223c\u00a0285\u00a0cm\u22121 [34,35].The vibrational mode D1, located at ca. 540\u00a0cm\u22121 is associated with the presence of defects within the fluorite CeO2 due to the introduction of low valence doping cations. The relative intensity of this band is directly proportional to the doping rate, due to oxygen vacancies generated extrinsically in the ceria to ensure the electroneutrality of the support. Taken into consideration the intensity and width of this band, the contribution made by one of the A1g modes of the hematite, usually positioned at 498\u00a0cm\u22121 [34], particularly distinguishable in the 9Fe1Ce_Ni sample, cannot be excluded. Another band, partially visible in the spectrum of 5Fe5Ce_Ni, is the one commonly called \"Intrinsic Defect Band-D2\", located at ca. 600\u00a0cm\u22121 and belonging in this case to defects due to changes in the oxidation state of cerium, with the formation of oxygen vacancies of the type \"Ce3+-VO\u2734\u2734\", this time due to a compensation of charge neutrality for reasons intrinsic to the nature of the redox couple Ce3+/Ce4+ [35].The vibrational band at 661\u00a0cm\u22121 (with higher relative intensity for sample 9Fe1Ce_Ni), is a longitudinal optical mode (LO), and is prohibited in Raman scattering. However, this mode can be activated by disturbances in the hematite lattice [36] and contributions from NiO cannot be excluded, given the presence of major vibrational modes in the region under consideration [37]. It is easy to see from Fig. 5 how the vibrational band, belonging to the hematite as well as part of the band belonging to the sorted ceria, considerably decrease their relative intensity after the reduction process leaving place, for the F2g and D1 peaks of the ceria and the triply degenerate vibrational bands with a broadening of the latter peaks probably due to the small particles size. Such a Raman response is in agreement with the variations in the unit cell parameters of the ceria (Table 1), and the migration-reduction process of Fe3+ and Ni2+ ions described above.The Raman spectra of samples 5Fe5Ce_Ni_@C and 9Fe1Ce_Ni_@C are shown in Fig. 6. In both spectra the G band at 1523\u00a0cm\u22121 and a G\u2032 band at 2683\u00a0cm\u22121 are visible, both related to carbon with sp2 hybridization of graphene [38]. The disordered graphite-like edges of the graphene layers give rise to a D-band at 1333\u00a0cm\u22121, the ratio of the relative intensities between the D-band and the G-band is indicative of the degree of disorder of CNFs. The ID/IG ratio was 1.23 for 5Fe5Ce_Ni_@C and 1.44 for 9Fe1Ce_Ni_@C, defining a higher degree of disorder for carbon nanofibers grown on nanoparticles with a high Fe and Ni content. The latter sample also presents several peaks in the Raman shift region between about 200\u00a0cm\u22121 and 700\u00a0cm\u22121. Some of these peaks, such as the one clearly visible at 660\u00a0cm\u22121 and those around ca. 185, 285, 435 and 540\u00a0cm\u22121, can be identified as the already mentioned vibrational modes of hematite and CeO2 respectively. The broad peak at about 1000\u00a0cm\u22121 with reduced relative intensity can be attributed to the 2P vibrational modes of finely dispersed NiO at reduced relative intensity due to its fine dispersion [39]. The presence of metal oxides of iron and nickel in this sample can be attributed to a passivation process of the metallic Ni or Ni\u2013Fe alloys not encapsulated within the nanofibres and therefore adhered to the surface of the nanofibers or the carbon edges of the defective portion as shown in the TEM images (Fig. 3-D and S7-B).The surface functionalization of all samples was studied by Fourier Transform Infrared Spectroscopy (FTIR), (Fig. 7\n). Although of modest intensity, the broad bands located between 3300 and 3500\u00a0cm\u22121, were associated with the vibrational stretching modes of the H\u2013O\u2013H bonds belonging to the interstitial water molecules, followed by the vibrational bending mode of the same bonds, with signal located at 1621\u00a0cm\u22121 [40].The FTIR spectrum of 9Fe1Ce_Ni shows two intense bands at 560\u00a0cm\u22121 and 475\u00a0cm\u22121, which have been attributed to the stretching Fe\u2013O and bending O\u2013Fe\u2013O vibrations in \u03b1-Fe2O3 [41]. The same characteristic peaks were found at lower intensity for the sample with lower iron content. After the reduction process, the two peaks located at positions of 526\u00a0cm\u22121 and 1423\u00a0cm\u22121, in the sample 5Fe5Ce\u2013Ni_Alloy, were associated with symmetric and asymmetric vibrations, respectively, of the Ce\u2013O bond in a distorted cubic lattice of CeO2 doped with lower valence cations [42]. While the peaks at 661\u00a0cm\u22121 and 1542\u00a0cm\u22121 were attributed to stretching vibrations of the Ce\u2013O bond in the undoped CeO2, crediting the previously introduced hypothesis that during the reduction process a part of the Fe3+ and Ni2+ ions migrates towards the surface of the support, with the formation of metal nanoparticles and a subsequent rearrangement of the cubic ceria lattice [43,44].The FTIR spectra of 9Fe1Ce_Ni_@C and 5Fe5Ce_Ni_@C show a broad band in the region between 3300\u00a0cm\u22121 and 3500\u00a0cm\u22121, which can be associated with the stretching vibrations of the O\u2013H bond, this peak may be generated by the formation of hydroxyl groups on the surface of the nanofibers (C\u2013OH and OC\u2013H), or it may be due to the absorption of atmospheric water molecules [45]. The band at 1580\u00a0cm\u22121 and the broad band located at about 1300\u00a0cm\u22121 were attributed, respectively, to the stretching vibrations of the CC and C\u2013C bonds forming part of the backbone of the fibers and the carbonaceous material around them [46].The presence of a band at about 1700\u00a0cm\u22121 confirms the presence of CO groups. The higher intensity of this band for the 9Fe1Ce_Ni_@C sample suggests greater presence of oxidized carbon compared to its carbonaceous counterpart [47]. In both samples a low intensity peak is visible at ca. 524\u00a0cm\u22121, probably due to CeO2 nanoparticles dispersed on the surface of the nanofibers (as shown in TEM images in Fig. 3-D and S7-B).X-ray Photoelectron spectroscopy (XPS) was performed to characterize the chemical state and relative abundance of elements on the surface of the catalysts under study (shown in Table 5\n). The Ce 3d region for the samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy, introduces the first surface difference between the 2\u00a0M ratios of the catalysts. Fig. 8\n-B shows three 3d3/2-3d5/2 spin-orbit-split doublets, belonging to the different photoemission configurations of the final 4f state and attributable to the majority presence of Ce4+ on the surface of the catalyst [48]. On the other hand, the 3d Ce region of sample 9Fe1Ce_Ni_Alloy (Figs. S10\u2013C), which is the region of the binding energies of the 3d electrons of Ce, shows a solvable XPS spectrum with ten Voigt-like contributions of Ce4+ and Ce3+ [48]. There the peak centered at 883\u00a0eV and the relative contributions in green (including the peak with binding energy of 918\u00a0eV), belong to the electrons of the 3d3/2 and 3d5/2 orbitals of the final state of Ce4+, while the peak centered at 880.7\u00a0eV and the related satellite peaks in blue, belong to the final state of Ce3+ [48].The Ni 2p XPS spectra, from the mixed metal oxides samples (Figs. S10\u2013B), exhibits a main peak, with BE of 855.2\u00a0eV with a shake-up peak in the Ni 2p3/2 region, typical of the presence of finely dispersed NiO nanoparticles [49].The samples composed of reduced metal oxides (Fig. 8-C), has a XPS Ni 2p3/2 spectrum, similar to its precursor of mixed metal oxides, with the addition of a peak with a binding energy of 852.8\u00a0eV, attributable to the presence of Ni0, deriving from the H2-reduction process [50]. The Fe 2p region of the 9Fe1Ce_Ni sample (Figs. S10\u2013E) shows only a weak peak at about 724\u00a0eV resulting from the interference due to the Auger electrons (LMM) of the nickel, while in the corresponding reduced catalyst, the main peak of Fe 2p at 711\u00a0eV is clearly visible together with the shake-up peak in the range of 716\u2013722\u00a0eV for both molar ratios of catalysts, confirming the presence of Fe3+ [51] ions (Fig. 8-A and S10-A). The presence of metal oxides on the surface of the reduced catalysts can be rationalized with an atmospheric passivation process induced by the nano-alloys of Ni\u2013Fe or by the nanoparticles of Ni0 (Table 1, Table 3).The encapsulation of the reduced metal oxide nanoparticles inside the carbon nanofibers further complicates the XPS analysis, due to the shielding effect that carbon exerts on the photoelectrons of the nanoparticles inside. The XPS signals reported for these catalysts can be considered as a result from photoelectrons coming from the atoms located proximal to the inner walls of the nanofibers, or proceeds from small entities, not encapsulated but adhered to the walls of the nanofibers (as shown in TEM Fig. 3-C and D).The Ni 2p3/2 spectra (Figs. S10\u2013D) show approximately the same characteristics of the corresponding reduced metal oxides, in agreement with XRD and Moessbauer (Tables 1 and 3), while the C 1s spectrum (Fig. 8D) shows the contribution of two different peaks subdivided into four Voigt-type contributions centered at 285\u00a0eV, 285.8\u00a0eV, 287\u00a0eV, and 290.6\u00a0eV that are attributable to the binding energies of the C\u2013C, C\u2013O, CO bonds and to the \u043f-\u043f* electrons resonance of the CNFs walls respectively [52] (Table 5). The presence of binding energies belonging to oxygenated functionalization is consistent with the FTIR characterization (Fig. 7) and EDX images (Fig. S9) previously discussed.The linear sweep voltammograms of the 5Fe5Ce_Ni-based and 9Fe1Ce\u2013Ni-based samples conducted at room temperature in a deaerated 1\u00a0M KOH aqueous solution are shown in Fig. 9\n-A and 9-B, respectively. Based on the curves shown, it can be easily noticed that the reduction treatment and the subsequent encapsulation inside the carbon nanofibers on both Fe:Ce molar ratios of the mixed metal oxides, considerably increased the current density (j), produced during the OER reducing the overpotential (\u03b7) required to perform it.Specifically, the linear sweep voltammetric curves showed first of all the poor activity of the catalysts composed of mixed metal oxides with 5Fe5Ce_Ni presenting a current density less than 10\u00a0mA/cm2 at 1.8\u00a0V vs. RHE (overpotential of 570\u00a0mV), while 9Fe1Ce_Ni, with the same catalyst amount placed on the surface of the RRDE (equal to 1018\u00a0\u03bcg/cm2), presented an overpotential of 450\u00a0mV at a current density of 10\u00a0mA/cm2. The reduction treatment and consequent formation of metallic species inside the catalysts visibly improved the activity towards the OER reaction, leading the samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy to present an overpotential at 10\u00a0mA/cm2 corresponding to 340 and 390\u00a0mV, respectively. The performance of both series of catalysts took a qualitative leap forward after the partial transformation of the metal species into carbides and their subsequent encapsulation within the carbon nanofibers. In fact, the 5Fe5Ce_Ni_@C sample shows (at the same quantity of catalyst grafted onto the RRDE as the previous ones) an overpotential of 280\u00a0mV at 10\u00a0mA/cm2. The same overpotential was also reached by the catalyst 9Fe1Ce_Ni_@C. Both CNFs-containing catalysts, under the same or similar operating conditions, show lower or at least competitive values of overpotential, compared to IrO2 or Ir-based catalysts commercially in use for OER in alkaline environment. (e.g. IrO2 with \u03b7\u00a0=\u00a0281\u00a0mV at 10\u00a0mA/cm2 [53,54]; black iridium with \u03b7\u00a0=\u00a0295\u00a0mV at 10\u00a0mA/cm2 [55].Regarding the registered current at the ring (Fig. 9 A-B, lower panel) as an indication of the evolved oxygen (which is reduced at the ring), it is very similar for all the pairs of materials synthesized with the same method. This evolution of the ring current can only confirm that, albeit with different molar ratios of Fe/Ce and Fe/Ni, the pairs of catalysts (mixed metal oxides, reduced metal oxides, CNFs) undergo very similar surface anodic processes, considering that all the experiments were conducted with the same catalyst load on the RRDE (1018\u00a0\u03bcg/cm2). The reduction reaction on the surface of the Pt ring of the produced and solubilized oxygen was used to try to calculate the faradaic efficiency of O2 for the two CNFs [56]. In this regard, Fig. S11 shows the efficiency trend of the two samples, in an interval between 1.48\u00a0V and 1.52\u00a0V. It can be seen that sample 9Fe1Ce_Ni_@C presents an initial trend of increasing efficiency with a maximum equal to 17% and a subsequent lowering as the applied potential increases. The values of the sample 5Fe5Ce_Ni_@C, are between 20 and 26.5% at low potentials, while when the potential increases, also in this case, a decrease in efficiency is noted. The decrease of the theoretical efficiency in the two samples as the applied potential increases, may be ascribed to the formation of oxygen bubbles, which, not dissolving in the electrolyte solution, are not reduced by the Pt ring, inexorably lacking the calculation [56].In order to conceptualize the increase in activity upon reduction and nanofiber growth of the two catalysts with different molar ratios, the considerations must include both a morphological/structural aspect due to the changes made during the synthesis process, and an aspect related to the reactions that occur on the electrode-electrolyte interface during the positive scanning of the potential in the OER reaction. A first consideration can be made regarding the variation of activity between catalysts composed of mixed metal oxides and their reduced analogues. For the latter, the catalytic activity is greatly increased for both molar rations of Fe/Ce, while it is worth noting that the sample 5Fe5Ce_Ni_Alloy, has a higher activity, than the corresponding 9Fe1Ce_Ni_Alloy, despite that its precursor 5Fe5Ce_Ni is with a very low activity. Such behavior may be due to (i) the bigger surface area of 5Fe5Ce_Ni_Alloy compared to 9Fe1Ce_Ni_Alloy, and (ii) presence of less defective CeO2 support with a balanced presence of oxygen vacancies and a dispersion of metallic particles of Ni and Fe, passivated by their corresponding metal oxides. This assumption is in good consistency with the previously discussed XRD (Table 1), N2-physisorption (Table 2), Raman (Fig. 5), FTIR (Fig. 7), and XPS (Figs. 8 and S10) characterizations.The higher performance of the samples encapsulated inside the carbon nanofibers can be rationalized, bearing in mind both the superior surface area and the higher electro-conductive properties provided by the nanofibers which exhibit a free movement of \u043f-electrons coplanar to the sp2-hybridized carbon atoms [6,57,58]. However, although the overpotential values, reached at the current density of 10\u00a0mA/cm2, are the same for both CNF-based samples (280\u00a0mV), the 9Fe1Ce_Ni_@C catalyst has a slightly higher activity (Fig. 9A\u2013B), and this may be due to a greater presence of defects on the surface of the nanofibers (as shown by Raman spectrometry in Fig. 6). The imperfections of the fibers and functional groups (as shown in Fig. 7), allow a greater electrochemically active surface even though this sample has a much lower surface area than 5Fe5Ce_Ni_@C (Table 2).These hypotheses can be further accredited by electrochemical characterization derived from the Electrochemical Impedance Spectroscopy (EIS), carried out at 1.6\u00a0V vs RHE with 10\u00a0mV of amplitude (r.m.s.) of the catalysts. The overlapped Nyquist plots are shown in Fig. 10\n A-B, while the fitting derived from the relative equivalent electrical circuit model for each catalyst under study is reported in Fig. S12 (Supplementary Informations).A full semi-circular loop was observed for each study case, the samples 5Fe5Ce_Ni and 9Fe1Ce_Ni, have a total polarization resistance (Rp), of 338\u00a0\u03a9 and 76\u00a0\u03a9, respectively. Such high values are usually considered proportional to an equally high charge transfer resistance by the catalysts [59]. The reduced homologous catalysts show a huge decrease in Rp equal to 14.2\u00a0\u03a9 and 18.4\u00a0\u03a9, this may be due to a fine dispersion of Ni metal nanoparticles and Ni\u2013Fe alloys within the CeO2 which is reflected in a more efficient synergistic action in the charge transfer process. This charge transfer is slightly faster in the case of 5Fe5Ce_Ni_Alloy, which may be one of the reasons for the observed higher activity in the OER compared to 9Fe1Ce_Ni_Alloy (Fig. 9A\u2013B). This behavior could be associated with the majority presence of Ce4+ in the sample with lower iron content, compared to the 9Fe1Ce_Ni_Alloy system, which presents ceria nanoparticles with a higher content of oxygen vacancies (as shown in Table 1 and in Fig. 8-B and S10-C). The equivalent cell series resistance (Rs), located at the point of intersection with the real axis at high frequency of the Nyquist plot, has a value included for all samples between 4.5 and 5\u00a0\u03a9, with the exception of the sample 5Fe5Ce_Ni (8.5\u00a0\u03a9). This is mainly associated to ionic conduction in the electrolyte, which is similar for all the experiments since it relays on the characterization system. In the CNF-based catalysts, the 9Fe1Ce_Ni_@C sample shows an Rp equal to 4.4\u00a0\u03a9, compared to the 5Fe5Ce_Ni_@C (7.8\u00a0\u03a9). The shown lower increase in the imaginary component of the impedance (-Z'') together with a semi-circle of smaller diameter in the low frequency zone, are an evidence of the improvement of charge- and electron-transfer properties and of the capacitive characteristics of CNFs compared to their respective precursors [60,61].Cyclic voltammetry (CV) studies have been carried out for the as-synthetized composites, at first in the potential range between 0.05 and 1.15\u00a0V vs. RHE at the scan rate of 0.02\u00a0V/s. Fig. 11\n A-B shows the voltammograms of the catalysts under examination at room temperature in a de-aerated solution of 1\u00a0M KOH. The non \"rectangular\" shape of the voltammograms indicates an important contribution of the pseudo-capacitive nature of the process within the aforementioned potential range [62], from which, however, it can be appreciated the difference in absolute area under the CV curves (AUC), that follows positive growth following the order: Mixed Metal oxides\u00a0<\u00a0Reduced-Metal oxides\u00a0\u226a\u00a0CNFs.As it is possible to notice, the trend of capacity growth is quite similar to the trend of anodic growth in the OER reaction (Fig. 9, leading to the conclusion that, the increase in current density followed by a lowering of the overpotential is related not only to a progressive improvement of the intrinsic mechanism of charge transfer of the catalysts (as seen from the EIS analysis in Fig. 10A\u2013B), but also due to an increase in the following characteristics: 1) electrocatalytically active surface accessible (ECSA) [63]; 2) the active sites immediately present at the electrolyte-electrode interface, and 3) possible active sites formed by activation processes that occur in the activation of carbon edges when increasing potentials [64].To better understand how the capacitive characteristics change between the reduced samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy and their counterparts encapsulated inside the carbon nanofibers, the surface area accessible to electrocatalytic reactions (EASA) was evaluated (Fig. S13 in Supplementary Informations). For this, the EASA was estimated from double layer capacitance (Cdl) via cyclic voltammetry measurements in a non-faradic region (0.95\u20131.15\u00a0V) in function of various scan rates (Fig. S13) and calculated using the formula EASA\u00a0=\u00a0Cdl/Cs, where Cs stands for specific capacitance [63,65], subsequently converted into m2/g, the values of which were reported in the summary (Table 6\n) with the name of ECSA (electrochemical catalytic surface area). In principle, it is possible to note from the summary table, the large difference in the BET surface area, compared to its electrocatalytically active component (ECSA). This is probably due to the fact that the surface area calculated for N2-physisorption includes areas, that are difficult to reach by the ions of the electrolyte, such as the inner walls of the smaller diameter micropores [66], not to mention the set of factors that can make the measurements of the double layer capacitance inaccurate, such as the chemical capacitance given by the population of trap states or forms of pseudo-capacitance due to the coordination of ions [65]. The higher content of microporous component (Table 2), would explain the lower ECSA of sample 5Fe5Ce_Ni_@C compared to 9Fe1Ce_Ni_@C and hence, the lower electro catalytic activity in OER. The ECSA values reported in Table 6, including the lowest values of the reduced metal oxides, are congruent with the benchmarking study conducted by Jung et al. [65] on similar materials and conditions, and are in agreement with the measurements and characterizations already shown in this study.To better individuate the OER rate determining step (rds), Tafel plots were examined(Fig. 12\n). The kinetics of the OER reaction on the surface of catalysts composed of mixed metal oxides and reduced metal oxides, seems to follow a similar trend. The progressive increase in the slope of the curves from \u223c60 mV/dec passing through \u223c135 mV/dec up to \u223c290 mV/dec, can be interpreted as a progressive increase in the activation overpotential [67] used by surface active sites to overcome the energy barrier, necessary to activate the various process steps (see equations (1.1), (1.2) and (1.3), 1.4) of the OER reaction.According to the theoretical approach of Shinagawa and coworkers [68], a Tafel slope close to 120 mV/dec, indicates that the rate determining step in that determined process range can be attributed to two main conditions: I) formation of hydroxides according to the reaction M\u00a0+\u00a0OH\u2212 \u2194 MOH\u00a0+\u00a0e\u2212; (where M is the active site); II) coverage of the given surface by species such as: MOH, MO, MOOH or MOO \n-\n, at high overpotential values and immediately before the respective rds. In the case of mixed metal oxides and especially for their reduced analogues, the formation of hydroxides of Ni and Fe cannot be excluded within the range of potential under examination. Furthermore, the slope values higher than 120 mV/dec, can probably be attributed to a sum of surface mechanisms with a positive and simultaneous energy demand that occur at high potential values, while the slopes that are equal or lower than 60 mV/dec (at low current values) can be attributed to a partial coverage of the surface with oxygenated species, or to the formation of catalytically active species with low activation energy [68].The reaction of OER on the surface of the CNFs (5Fe5Ce_Ni_@C and 9Fe1Ce_Ni_@C), follows a different kinetics than the previously evaluated samples. It is possible to notice how at low current density values (until 1\u00a0mA/cm2), the Tafel slope is about 60 mV/dec with an interpretation similar to what was previously described. Remarkable is the region of current density between \u223c1.5 and 20\u00a0mA/cm2, where unlike the previously evaluated samples, the CNFs produce high currents at relatively low overpotential values and with significantly lower Tafel slope (42 mV/dec), using a similar reaction mechanism for both nanofibers (given the overlapping of the curves). Such a surface reaction mechanism can be rationalized with the progressive activation and formation of the active sites through a semi-radical mechanism studied by Lin et al., of the carbon edges in the OER [64], where Tafel slope close to 60 mV/dec can be considered as an indicator of a deoxygenation/deprotonation process of the *OOH species assuming that the mechanism is single-site [69]. At higher overpotentials, the Tafel slope increases to approximately 134 mV/dec. This value, as previously discussed, could be rationalized as a super saturation of the surface with oxygenated species, or as a difference in the Gibbs free energy of the intermediates that mediate the rates of the process [68].It was previously shown through elemental analysis but also partially through TPO analysis (Table 4), as the same quantities of reduced metal oxides catalysts (200\u00a0mg), subjected to the same decomposition times of methanol (2\u00a0h), lead to a different percentage content by weight of carbon and therefore of CNFs. Specifically, 30.7%wt. of carbon acquired from the sample 9Fe1Ce_Ni_Alloy (subsequently called 9Fe1Ce_Ni_@C), was found, while a carbonaceous quantity of 49.2%wt. from the sample 5Fe5Ce_Ni_Alloy (further called 5Fe5Ce_Ni_@C) was acquired. It is therefore evident that the two catalysts composed of reduced metal oxides act differently in the decomposition reaction of methanol. In order to investigate this difference, the CNFs synthesis procedure was replicated for both catalysts, by decomposing a constant flow of methanol (flow rate of 30\u00a0ml/min), on their surface at a temperature of 773\u00a0K for a total time of 2\u00a0h. The gas phase produced was on-line analyzed by gas chromatograph every 20\u00a0min. Although both catalysts at a temperature of 773\u00a0K converted 99% of methanol, Fig. 13\n A-B, the difference in the gas phase composition, which was approximately stable during the time-on-stream, was registered. Here in the histogram, to the sample 5Fe5Ce_Ni_Alloy is attributed a selectivity to CO, higher than the one found for 9Fe1Ce_Ni_Alloy, which instead shows a greater predisposition in the formation of CH4. The percentage of CO2 remains comparable between the two samples. For both samples, C2H6, C2H4, C3H8 were also detected, but with a total selectivity percentage lower than 1%.As can be seen from the histogram in Fig. 13 A-B, the decomposition process of methanol, under the conditions of temperature and pressure in question, give rise to the main reaction: CH3OH \u2192 CO + 2H2 (eq. (2.1)) with the formation of syn-gas, and with two collateral secondary reactions: CO + 3H2 \u2194 CH4\u00a0+\u00a0H2O (eq. (2.2); methanation reaction) and CO\u00a0+\u00a0H2O \u2194 CO2\u00a0+\u00a0H2 (eq. (2.3); reaction of water-gas shift), with the following stoichiometric overall reaction: 4CH3OH \u2194 3CH4 + 2H2O\u00a0+\u00a0CO2 (eq. (2.4)). Although the overall reaction does not suggest a final production of syn-gas, the reaction described by equation (2.1) is thermodynamically irreversible under the operating conditions in question, while the methanation and water-gas shift reactions are reversible and do not proceed completely [70], leading to the formation of H2, CO, CO2, CH4 as a gaseous mixture resulting from the decomposition of methanol.It is therefore evident that one of the carbonaceous molecules previously mentioned has a main role in the formation of the carbon nanofibers synthesized in this study. The synthetic literature of this type of CNFs is very vast and uses an equally vast amount of carbon sources, such as: ethylene, acetylene, propylene, or other metal-organic frameworks (MOFs) [71], but more common are the syntheses that use the decomposition of CH4 and CO [72], employing in this regard, respectively, the methane cracking reaction: CH4 \u2192 2H2+ C(s) (eq. (2.5)) and the carbon monoxide disproportion reaction: 2CO \u2192 C(s)\u00a0+\u00a0CO2 (eq. (2.6)) [70].Given the complexity of the system under study and given that both CO and CH4 are formed on the surface of the catalysts, it is reasonable to assume that both molecules could contribute to the growth of nanofibers under the same operating conditions. The quantity of carbon acquired during the carbon-coating process will depend on the decomposition thermodynamics of the two molecules on the catalyst surface and also on the concentration of CO and CH4 molecules, respectively, formed during the methanol fragmentation. From this particular aspect, the metal ratio of the catalysts composition plays a fundamental role both in the decomposition of methanol even at temperatures below 773\u00a0K, and in the different mechanisms of its fragmentation, which justify the preponderant difference in selectivity of the gas phase between the two catalysts. Specifically, Fig. 14\n shows the methanol conversion values obtained from a second methanol decomposition experiment, developed at increasing reaction temperature values under the same operating conditions as the synthesis process. The curves in Fig. 14 show the superior activity of 5Fe5Ce_Ni_Alloy to its counterpart, reaching a complete conversion of methanol at 648\u00a0K. This substantial difference in the degree of activity of the catalyst, as well as its high selectivity for the syn-gas formation, may be closely linked to the greater presence of CeO2 in the sample. Furthermore, the doped variant of the latter is visibly more active than pure CeO2, which accentuates the importance of the synergy between the metallic Ni and Fe\u2013Ni alloy nanoparticles with the ceria support, in the mechanism of fragmentation of methanol and consequently in the nature and distribution of its products.The fragmentation mechanism generally described for the single-site decomposition reaction of methanol on the surface of a catalyst, consisting of metal nanoparticles supported on high oxygen storage capacity (OSC) metal oxides such as CeO2 [73], contemplates a preliminary dissociative adsorption of methanol on the surface of the ceria support, with the formation of methoxy groups, the following step involves a superficial diffusion to the proximal catalytic metal particles and therefore a translation of the aforementioned methoxy groups, with consequent dehydrogenation, formation of formyl groups that can lead to a release of CO, or to subsequent transverse reactions such as methanation (eq. (2.2)) or the water-gas shift reaction (eq. (2.3)) or to a deposition of carbon through the decomposition of CO or CH4 molecules (eqs. (2.5), (2.6)) with the formation of carbides and a subsequent growth of carbon nanofibers [30].In our previous study [23], using the insitu FTIR spectroscopy, we demonstrated the effect of molar ratio between Fe and Ce on the methanol fragmentation mechanism. The lack of formation of methoxy groups at low temperatures for the samples with high iron content as well as a different distribution of the formyl, formate and carbonate groups at high temperatures, compared to the molar ratios with lower iron content and high ceria content was established. In light of these considerations, the low CeO2 content of sample 9Fe1Ce_Ni_Alloy delays the propaedeutic fragmentation of methanol on the catalyst surface at low temperatures (Fig. 14) while its high content of Fe and Ni, widely recognized in literature as key promoters of Fisher-Tropsh reactions [74], at a temperature of 773\u00a0K favor the methanation of CO (eq. (2.2); Fig. 13), inhibiting the growth of carbon nanofibers.The progressive modification of the metal species constituting the catalysts brings about profound changes to the catalytic properties of the samples. Specifically, the metal oxides of iron and nickel within CeO2, initially not sufficiently electrocatalytically active for the reaction of OER in an alkaline environment, after being reduced, lead to the formation of finely dispersed nanoparticles of Ni\u2013Fe alloys and Ni0, improve the qualities intrinsically linked to the electronic transport or to the pseudo-capacitive ability of the catalysts which reflected into an increased activity in the OER reaction compared to the initial catalysts. The 5Fe5Ce_Ni_Alloy catalyst obtained an electro-catalytic response exceeding expectations, an effect attributed precisely to the acquisition of better performing textural and structural properties. The previous catalytic knowledge, in terms of active sites present and the relative products formed by the decomposition reaction of methanol, on the surface of this kind of catalysts, allowed a further modification of the active metal phase. By exploiting the greater solubility of carbon in metallic iron compared to its oxides, the main products of the decomposition reaction of methanol (CO and CH4) were partially used as carbon sources, for a preparatory carburization of the Fe\u2013Ni alloys, and a subsequent growth of carbon nanofibers through tip-growth mechanism. Although structural differences have been found between the two fibers, dictated by the different activity and selectivity of the catalysts precursors to the decomposition of methanol and its products, the electro-catalytic activities of the synthesized CNFs are almost similar, with relatively low overpotential values (280\u00a0mV at 10\u00a0mA/cm2) and the found relatively high current density values. Analyzing the results from a broader point of view, however, the samples produced in the 5Fe5Ce_Ni molar ratio (especially 5Fe5Ce_Ni_Alloy and 5Fe5Ce_Ni_@C), have triple advantage:\n\n1)\nGreater selectivity for the syn-gas production from methanol (CH3OH \u2192 CO\u00a0+\u00a02H2) with the possibility of almost total conversion of methanol even at temperatures below 773\u00a0K.\n\n\n2)\nGreater carbon acquisition for the growth of carbon nanofibers,\n\n\n3)\nLow overpotential values combined with high relative values of current density produced in the alkaline environment OER reaction.\n\n\nGreater selectivity for the syn-gas production from methanol (CH3OH \u2192 CO\u00a0+\u00a02H2) with the possibility of almost total conversion of methanol even at temperatures below 773\u00a0K.Greater carbon acquisition for the growth of carbon nanofibers,Low overpotential values combined with high relative values of current density produced in the alkaline environment OER reaction.All mentioned above, suggests a potential link between the two techniques, destined for hydrogen production, and opening the possibility of using the catalysts exhausted by the techniques of decomposition of organic molecules into catalysts useful for electro-catalytic purposes.\nConsolato Rosmini: Conceptualization, Validation, Formal analysis, Investigation, Data curation, Writing \u2013 original draft, preparation. Tanya Tsoncheva: Supervision, Project administration, Funding acquisition, Writing \u2013 review & editing. Daniela Kovatcheva: Formal analysis, Validation. Nikolay Velinov: Formal analysis, Validation. Hristo Kolev: Formal analysis, Validation. Daniela Karashanova: Formal analysis, Validation. Momtchil Dimitrov: Formal analysis, Writing \u2013 review & editing. Boyko Tsyntsarski: Formal analysis. David Sebasti\u00e1n: Writing \u2013 review & editing. Mar\u00eda Jes\u00fas L\u00e1zaro: Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the BIKE project, which received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 813748.The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2022.04.036.", "descript": "\n                  Ceria-iron oxide mesoporous materials with Fe:Ce molar ratio of 5:5 and 9:1 were synthesized by hydrothermal method using CTAB as a template and subsequently modified with NiO (molar ratio Ni:Fe\u00a0=\u00a01:2) by incipient wetness impregnation technique. In order to increase the electro-capacitive properties and reduce the intrinsic impedance of the metal oxides, the samples were consecutively modified by reduction in hydrogen to obtain highly dispersed Ni\u2013Fe alloys into ceria matrix. By exploiting the high permeability of carbon inside ferrous alloys, the metal phase has been further modified into ferrous carbides and metal alloys encapsulated within carbon nanofibers. For this purpose, a reaction, already widely studied for the production of hydrogen, was used, that is the decomposition of methanol vapors. In fact, this decomposition, in addition to producing syn-gas and methane, changes the catalysts in use through a chemical vapor deposition-carbon coating process. This fact, has been used by us to demonstrate how the newly obtained metal-carbon nanocomposites can be used for electro-catalytic purposes. The modified phases of the two\u00a0molar ratios of the Fe\u2013Ni\u2013Ce catalysts were tested in the Oxygen Evolution Reaction (OER) in an alkaline environment (1\u00a0M KOH), showing a satisfactory and progressive increase in activity and a surprising decrease in the overpotential at 10\u00a0mA/cm2 of current density. The morphological, textural and physicochemical properties of the samples were characterized in details by XRD, N2-physisorption, TG-TPO, TEM, EDX, FTIR, XPS, Raman and Moessbauer spectroscopies.\n               "}
{"full_text": "No data was used for the research described in the article.In recent years, increasing demands in energy and the awareness of the negative environmental impact of fossil fuels have accelerated the transition to green alternative energies (such as solar, wind, and tidal energy). However, the intermittent accessibility of renewable energies by daily, seasonal, and regional factors limits their adoption to a consumption-adjusted energy supply. Thus, it is imperative to develop innovative energy conversion and storage technologies, such as water-splitting devices.In particular, understanding the mechanisms and technical challenges of the sluggish oxygen evolution reaction (OER) is indispensable to boost the electrolysis market. Unfortunately, classical electro- or physico-chemical analytic methods alone cannot provide a full picture of all processes occurring during the reaction, due to missing information on the electrochemical states of the catalysts under operation conditions. Captivatingly, the combination of electrochemistry with physicochemical characterization by operando techniques provides innovative ways to circumvent this obstacle and permits investigation of material properties, of OER intermediates or actual active sites, as well as of other processes taking place on the electrocatalyst surface during the reaction. Importantly, the field of operando analysis covers a wide range across different experimental techniques and length scales, all providing complementary information on the system under investigation.Here, we want to expand beyond the discussion of a single technique, giving a concise outline of recent advances in the understanding of OER catalysts obtained by a set of operando techniques that span the range from atomistic to systemic scales and from fundamentals to actual applied conditions. The here selected techniques aiming for a fundamental understanding are electrochemical quartz crystal microbalance (eQCM), operando X-ray spectroscopy, and inductively coupled plasma\u2014optical emission spectrometry and mass spectrometry, respectively (ICP-OES and -MS), which are typically applied in half-cell configurations. Additionally, we set a second focus on catalyst analysis during its technical application, shedding light on further operando techniques that utilize single-cell setups to mimic technical conditions for water electrolysis.An electrochemical quartz crystal microbalance (eQCM) provides highly sensitive information on transient mass changes of catalysts. It consists of an electrochemical cell in which typically Au-coated oscillating quartz discs serve as support electrodes. For details about this technique, the reader is referred to a remarkably comprehensive eQCM guide written by Buttry and Ward [1].With eQCM, a variety of information can be obtained. In its earliest form, the QCM was used ex situ to measure the mass of electrodeposited films [2,3]. Later in the 1980s, it was used for the first time during electrochemistry to monitor adsorbates and surface reconstructions on Au surfaces [4\u20136]. The effect of current on Ni(OH)2 electrodeposition [7] or the redox behavior of \u03b1- and \u03b2-Nickel double hydroxides (Figure\u00a01\nI) [8] were just two further applications to follow. In the latter case, the lower relative mass changes observed in the \u03b2-phase compared to the \u03b1-phase was assigned to the shorter slab distance in the \u03b2-phase preventing water intercalation.More recently, studies on more complex processes came along, such as the investigation and identification\u2014in terms of molar mass, kinetics, and concentration variation\u2014of the reversibility of intercalated/deintercalated ions, such as K+ and OH\u2212, migrating in and out of the spacing of a Ni layered double hydroxide (LDH) structure [9]. Wu et\u00a0al. used nickel hydroxide Ni(OH)2 to study its ion intercalation-driven \u03b1/\u03b3 and \u03b2/\u03b2 phase transformations in LiOH, NaOH, and KOH electrolytes [10]. In this work, quantitative Ni(OH)2 stoichiometry performed via eQCM revealed that displacing structural water, as depicted by the higher relative mass changes during the first cycle (K+ > Na+ > Li+) inside the LDH by intercalated cations (Figure\u00a01II), accelerates catalyst degradation, a process which was found to be exacerbated with increasing size of the electrolyte cations (Li+\u00a0<\u00a0Na+\u00a0<\u00a0K+).Additionally, eQCM was used to study the effect of Fe impurities on the structural transformations between the \u03b1/\u03b3 and \u03b2/\u03b2 couples on Ni-based electrocatalysts during OER via different microgravimetric characteristics of phase transition between \u03b1/\u03b3 or \u03b2/\u03b2 couples. Such transformation processes are inherent to Ni(oxy)hydroxides, and via eQCM, it was concluded that Fe in NiFe-LDH inhibits the conversion of \u03b1/\u03b3 to \u03b2/\u03b2 which would otherwise lead to lower activity [11]. Feng et\u00a0al. reported a gradually decreasing self-healing effect of Fe impurities on the activity and stability of a Ni-based electrocatalyst due to the instability of Fe(VI)O4\n2\u2212 clusters within the electrolyte, leading to Fe precipitation [12]. The authors identified an onset potential of Fe redeposition via eQCM as well as a potential gap between OER potential and Fe redeposition, which deteriorates catalyst self-healing.Another strategy to increase the performance of LDH electrocatalysts that can be investigated by eQCM is the increase of the charge transfer ability of these otherwise insulating materials. For example, modifying an LDH structure with a conductive polypyrrole (ppy) polymer was shown to enhance the adsorption and desorption of reaction intermediates and reactants (OH\u2212 and H2O) on a NiFe-LDH electrocatalyst [13]. This was performed in a setup with dissipation measurement capability (eQCM-D), which allows following the activation and the atomic rearrangement processes on the catalyst surface in real time, using a potentio-dynamic protocol. Larger observed mass changes during a potential cycle of a ppy-modified LDH catalyst were related to a layer distance increase and ion intercalation, which in turn lead to more efficient intermediate adsorption and desorption.eQCM can also shed light on the effectiveness of material alloying for enhancing activity and stability. Escudero-Escribano demonstrated the decrease of both activity and dissolution rate of Ru during OER of surface-modified RuO2 electrocatalysts by increasing the coverage by an IrOx sub-monolayer (1, 2, and 4\u00a0\u00c5) via eQCM coupled with ICP-MS [14]. Recently, we demonstrated via eQCM the dynamic nature of IrOx structures and the manner in which OER conditions regulate their hydration degree. During potentio-dynamic conditions, a charge growth (Figure\u00a01IIIa) was found to be accompanied by deactivation, due to the deprotonation-induced dehydration (Figure\u00a01IIIb) of \u03bc2-OH(H2O)x and subsurface species like sulfates etc. (species observed in the cathodic sweep with masses between 14 and 60\u00a0g\u00a0mol\u22121) during OER [15]. Activity can be, however, fully restored upon electrochemical reduction with an accompanied mild dissolution, observed via the higher mass of the species involved (> 140\u00a0g\u00a0mol\u22121) in the anodic sweep from 0.04 to 1.4\u00a0V (Figure\u00a01IIIb), meaning that Ir species are dissolved. On the other hand, potentiostatic conditions irreversibly deactivate IrOx, due to accelerated growth of \u03bc2-O species, with simultaneous decrease of \u03bc1-O species.Although the detection of mass changes might appear of limited use at first glimpse, the proper correlation to electrochemical processes makes eQCM a highly versatile operando tool for the fundamental analysis of electrocatalysts. We believe that more interesting insights about formation-, degradation-, and sorption-processes will follow in the future when eQCM results are linked to other operando methods, such as spectroscopy techniques, of which X-ray spectroscopy is discussed in the next section.X-ray spectroscopies provide a comprehensive toolbox for the investigation of the atomic as well as electronic structures of a material in an element-specific fashion. However, its operando application on catalysts (e.g. for OER) comes along with challenges such as the design of specialized equipment, carefully prepared measurements, and cross-checks for radiation-induced damage [16\u201320]. Further, accurate interpretation of OER mechanisms usually requires assistance from computational approaches [21]. The proper application of operando hard X-ray spectroscopy was recently discussed in detail by King et\u00a0al. [22]. Further, a protocol to apply this technique in a potentio-dynamic fashion was evaluated by Pascquini et\u00a0al. [23]. For a comprehensive overview of operando X-ray techniques, the reader is referred to recent reviews and overview papers [16,18,24\u201328]. Here, we focus on a concise outline of the capabilities of operando X-ray spectroscopy underlined by recent publications.For the elucidation of the reaction and degradation mechanisms of an electrocatalyst, knowledge about its electronic and spatial structure under operative conditions is an important prerequisite. Such knowledge can be obtained by detecting the X-ray absorption near edge structure (XANES), which is e.g. sensitive to the oxidation state, and the extended X-ray absorption fine structure (EXAFS), providing information e.g. about atomic distances.Via combining operando XANES and EXAFS for investigating Ir oxide, Czioska et\u00a0al. proposed a stronger Ir\u2013Ir interaction as the main reason for the higher stability of IrO2 after calcination [29]. Further, they elucidated the effect of temperature, suggesting a stabilizing effect of elevated temperatures on the oxidation state of Ir oxide [30]. Regarding activity, an unusually shortened Ir\u2013O distance at OER potential was detected by EXAFS in highly active (Ni-leached) IrNiOx nanoparticles in comparison to conventional Ir oxide, supporting the interpretation of an increase in the oxygen ligand electrophilicity as an important factor to facilitate the OER [31]. In another study, a high structural \u201cflexibility,\u201d measured on Li-modified amorphous IrOx, was suggested to be beneficial for high OER activity (Figure\u00a02\nI) [32].The full power of X-ray spectroscopy becomes evident when investigating multimetallic systems such as 3d transition metal compounds for alkaline OER since it enables an element-specific analysis. Several studies, on NiFe-based catalysts [33\u201336], suggested the changes in metal oxidation states mostly occur at the Ni sites indicated by pronounced K-edge shifts (typically related to an oxidation state change) in the Ni spectra for applied potentials, while the K-edge shift of Fe appeared to be only minor (Figure\u00a02II). However, by dynamically tracking changes in the Ni and Fe K-edge spectra with varying potentials, a modification of the Fe coordination environment concomitantly occurring to the Ni oxidation was proposed [33].In addition to the above-mentioned hard X-ray approaches, operando spectroscopy on OER catalysts can also be performed in the soft X-ray regime, yielding intense L-edge spectra of 3d transition metals that are usually very distinct for different oxidation states and phases, therefore being interesting for the determination of material phases by comparison to simulations or for \u201cfingerprint interpretation\u201d [35\u201339]. Furthermore, in-depth analysis can be performed when applying soft X-ray spectroscopy. In an operando, soft X-ray study on MnOx, in addition to a phase transformation into \u03b4-MnO2 for oxidative potentials, an increase of oxygen-metal-charge-transfer features in resonant inelastic X-ray scattering (Figure\u00a02III) was revealed, associated with an increase in the hybridization of O 2p and Mn 3d states preceding the OER [40].Soft X-rays also cover O K-edge excitations, which is of particular interest for understanding the reaction mechanism. Recently, the O K-edge of oxygen evolving catalysts was extensively studied, identifying the formation of \u03bc1-and \u03bc2-OH species and subsequent deprotonation as an important intermediate reaction step (Figure\u00a02IV) [35,41\u201345]. It is noteworthy that the formation of electrophilic oxygen species as a prerequisite to OER is proposed for both, noble metal [18,41\u201345] as well as 3d transition metal [35,40] oxides.The capability of X-ray spectroscopy to access the local spatial and electronic structure of different types of OER catalysts has led to important insights, still, we see terra incognita to discover. For example, when pushing reaction conditions towards harsher parameters such as elevated temperatures [30] as well as the extension towards time-resolved experiments to capture the dynamics of reaction and transformation processes on a (sub)millisecond timescale [28].To elucidate catalyst degradation rates, the application of ICP-MS or -OES during transient OER has been paramount in identifying activation/stabilization factors for catalyst development [46]. One prominent example is the work of Cherevko and co-workers on the dynamic dissolution of Au, Pt, and Ir under pre-OER and OER conditions (Figure\u00a03\nI) [47,48], in which the authors hypothesized an unstable intermediate and the transition from oxide to metallic iridium during the cathodic sweep as the main contributors to Ir dissolution. Based on these findings, alloying of Ir and Ru has been established to form catalysts of similar activity but of superior stability compared to their monometallic counterparts [14,49,50]. An example is alloying of Ru with Pt [51]. By monitoring the Ru and Pt dissolution, Yi and co-workers identified the stabilizing and activating role of Pt on synthesized and thermally treated crystalline Ru0.9Pt0.1O2 via the formation of an amorphous RuxPt1-xOy on top of the crystalline Ru0.9Pt0.1O2. This surface|bulk-combination formed via surface Pt dissolution led to enhanced activity and stability.Highlighting the necessity to find alternative catalyst supports to carbon due to its inferior stability during OER, Maillard and co-workers in 2020 were able to identify the stabilizing effect of 5% Ta on a SnO2 support for IrOx electrocatalysts, by keeping a balance between stability and activity for the catalyst material [52]. In 2020 and 2021 Over, Cherevko and Grunwaldt with co-workers used a combination of X-ray techniques and ICP-MS to study the stabilizing effect of IrO2 on highly active IrxRu1-xO2 nanoparticles as long-term operating catalysts for PEMWE, due to the enhanced Ir\u2013Ir interactions in IrO2 during OER [29,53\u201355]. We want to emphasize here that these studies on the stabilizing effect caused by enhanced Ir\u2013Ir interaction based on ICP-MS and operando X-ray techniques are a very good example regarding new insights that can be obtained when successfully combining complementary operando techniques.Online ICP spectrometry has been successfully used to identify the dissolution rates of 3d transition metal catalyst materials [46,56,57] and the dynamic nature of active sites of hydroxides [58] and perovskites, respectively [59] also, in alkaline electrolytes. Additionally, the effect of Fe impurities on activity (Figure\u00a03II(a)) and, for the first time, the uptake of Fe impurities by a Ni-based electrocatalyst and its effect on stability (Figure\u00a03II(b),(c)) were quantified in 1M KOH with the aid of online ICP-OES [60], highlighting the importance for better understanding the effect of electrolyte impurities in alkaline OER.In summary, the use of ICP-OES and -MS is a very powerful for fundamental investigations of catalyst stability. Most interestingly, its operando application is easily transferable from half- to single-cell configurations, as discussed in the next section, to study catalyst dissolution on a more technical scale.Electrochemical testing of newly developed catalyst materials under industrially relevant conditions is indispensable for proving their technical applicability for water electrolysis. On top of the previously discussed atomistic analysis, here we shed light on some operando techniques applied in single-cell configuration. As an example, we focus on PEM-water-electrolyzer (PEMWE). A single cell is typically composed of a membrane electrode assembly (MEA), in which the membrane is sandwiched in between liquid/gas diffusion layers (LGDLs) and catalysts, flow fields (also acting as current collectors), and end plates (Figure\u00a04\nI). However, special designs might be required when applying operando techniques, e.g., to circumvent that thick cell elements, such as end plates and flow fields inhibit access of the chosen measurement technique to the MEA. In contrast to the very controllable conditions on half cells, in single cells, the intrinsic activity of the catalyst can be misinterpreted by many other factors. Such factors, that can contribute to the overall cell performance, are i) the accessibility of water to the catalyst interface, ii) the electrical resistance between the individual components, iii) proton conductivity of the membrane and ionomers, and iv) the transport properties of oxygen or hydrogen gas. The interplay of these factors renders their analysis a complex endeavor, and various attempts of operando investigations have been undertaken to elucidate and understand the individual contributions to the overall performance of a single cell.Classical techniques such as cyclic voltammetry (CV), current-voltage (IV) measurements, and electrochemical impedance spectroscopy (EIS), are simple yet powerful methods [61]. In particular, EIS is capable to separate kinetic, ohmic, and mass transport resistances [61\u201363] (Figure\u00a04II). However, an interpretation solely based on EIS needs attentive caution, since the measured resistances reflect interconnected phenomena occurring in the single-cell setup. For instance, trapped microbubbles can lead to an increased mass transport resistance but also to an apparent increase in the kinetic and ohmic resistance due to a decreased overall utilization of the catalyst.To overcome this obstacle, the combination of EIS with the operando observation of gas formation inside the single cell can add crucial information related to microkinetic and mass transport. In literature, different attempts can be found to visualize the track of bubbles through LGDLs or flow fields, namely via high-speed optical imaging [63\u201370], neutron imaging, and X-ray imaging [62,71\u201379] (Figure\u00a04III and IV). By applying such operando imaging techniques, for instance, an increase of gas bubble volume was observed when following the flow field path from the inlet towards the outlet, leading to non-uniform gas distribution and bubbly to slug flow [64,67,71,77]. The effects of such slug flow are debated. While Majasan et\u00a0al. and Wang et\u00a0al. related it to a decrease in performance [67,69], Dedigama et\u00a0al. suggested the tendency of the enlarged bubbles in the flowing electrolyte to combine with microbubbles on the surface of catalysts causing an increase of local current near the outlet [64]. Another observation by operando imaging was made by the F\u2013Y. Zhang group discovered that only the catalyst material directly in contact with the LGDL is catalytically active and generates bubbles. This was related to low conductivity and high in-plane resistance in the catalyst layers located in the pore region [65]. Based on this, they developed thin and well-organized gas diffusion layers for the anode [63,70,80] and cathode [65,68] of PEMWE. Further details on the analysis of mass transport using the aforementioned operando techniques are well summarized in recent reviews [61,81].As an alternative to operando approaches in assembled cells, also studies exist on the separate investigation of individual components of single cells by scanning electrochemical microscopy (SECM). SECM uses a 4-electrode configuration (comprising two working electrodes) in which an ultramicroelectrode (UME) is brought close to the electrode substrate to enable functional operando electrochemical analysis on a micrometer scale (Figure\u00a04V). By this, local information, e.g. of gas emission through catalyst layers or LGDLs, can be obtained. Kim et\u00a0al. investigated the influence of Ir nanowire alignment in the catalyst layer on oxygen emission by detecting the frequency of bubble-collision on the UME and the designed Ir catalyst layers exhibiting an over 30-fold higher mass activity compared to conventional arrangements (Figure\u00a04VI) [82]. Lim et\u00a0al. developed an amphiphilic LGDL to enhance the mass transport of both liquid and gas in PEMWE (and also PEMFC), and proved selective gas emission through hydrophobic channels by SECM [83].Due to significantly different operation conditions in half cells and single cells, analyzing the catalyst stability in a single-cell configuration requires an adapted way of evaluation and analysis, as discussed recently for PEMWE [84]. The S-number (= generated O2/dissolved Ir) is five orders of magnitude higher in single cells compared to half cells [85\u201387]. Kn\u00f6ppel et\u00a0al. figured out that the difference in pH and catalyst stabilization during long-term operation are the main factors for the discrepancy when detecting dissolution via operando ICP-MS analysis coupled with either flow cells or MEAs, respectively [87]. It is still a blue ocean of research due to the limitation of applying operando analysis to these extremely stable systems, which possess a lifetime of up to tens of thousands of hours. For an advanced grasp of affecting factors on the deterioration of technical electrolyzers, i) standardized accelerated protocols for water electrolysis [88\u201391] should be established in a similar manner as for fuel cells [92\u201394], and ii) new pathways of detecting the origins of the degradation via long-term operando techniques should be identified.Within this work, we reviewed selected techniques for the operando analysis of OER catalysts on different scales. We started with an overview of the operando application of eQCM, X-ray spectroscopy, ICP-OES and -MS for fundamental studies in half-cell reactions covering the atomistic scales of reaction and degradation mechanisms and concluded with an overview of the application of different techniques in single cells, such as high-speed optical imaging, neutron radiographs, and SECM, enabling investigations on a more systemic level of technical OER.The here summarized findings span the range from structural transformations of LDH catalysts, intercalation behavior of ions, and the formation of crucial intermediates for OER, towards the stabilizing effect of alloying on noble metal catalysts and the distribution and flow of bubbles in actual cells. We hope that by giving an insight into the range of possible operando investigations, we could point out the complexity of understanding the variety of aspects that govern catalyst performance for OER, in particular when looking at catalysts in the configuration close to technical application. While the fundamental methods are of paramount importance to elucidate reaction mechanisms and properties of the catalyst material itself, only by applying operando techniques in a more technical configuration, additional phenomena occurring in its application can be elucidated.Rational catalyst design goes far beyond pure electrochemical characterization. Many factors (fundamental and technical) along the way from developing a catalyst to its application in technical setups determine its final performance. Most importantly, these factors have to be identified and optimized by applying operando analyses.Finally, it has to be noted that the full range of operando experiments includes many more techniques than the selection presented herein. We are convinced that their rational combination to obtain complementary information, such as the interplay of different phenomena, will promote a deeper understanding of OER catalyst. Moreover, future improvements regarding spatial/time resolution and application parameters such as temperature or current density will help to further unravel both, the fundamental as well as the technical aspects that govern OER.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors are grateful for funding within the German BMBF cluster projects \u201cDERIEL\u201d (FKZ 03HY122I) and \u201cPrometH2eus\u201d (FKZ 03HY105E).", "descript": "\n                  Developing high-efficiency and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a crucial bottleneck on the way to practical applications of water electrolysis toward clean H2. Determining the OER mechanism and understanding the characteristics that affect OER activity and catalyst stability are of vital importance for this endeavor. In this aspect, operando characterization techniques performed under dynamic OER conditions are powerful tools to monitor key reaction intermediates, active sites, charge transfer, and material transformation coupled processes. This mini review covers noble and 3d transition metal-based OER electrocatalysts and their analysis by different operando techniques that span the range from the characterization in half cells to elucidate intrinsic properties to the analysis of phenomena occurring during their technical operation in single cells.\n               "}
{"full_text": "No data was used for the research described in the article.With increasing demands for fuel and deteriorating fossil fuel reserves, the primary concern over the last five decades have been exploring sustainable fuels and revamping energy efficient technologies. However, the research to achieve ambient temperature reduction of nitrogen to ammonia is far from perceivable. The N2 molecule being non-polarizable, highly inert and a strong triple bond, its dissociation energy of 940\u00a0kJ/mol is attainable only under high temperature and pressure [1]. This also lays forth the constant reliance on the highly energy intensive Haber-Bosch process of ammonia synthesis. Therefore, a scalable and viable synthetic route of N2 reduction at ambient conditions is an absolute necessity. The electrocatalytic nitrogen reduction reaction (eNRR) is one green approach to replace Haber\u2013Bosch process, as this process can be actuated from renewable sources of energy and ammonia synthesis can be regulated at ambient conditions [2]. However, electrochemical N2 reduction is laced with two major challenges: a large NRR overpotential and low NH3 faradaic efficiency (FE) caused by its competing hydrogen evolution reaction (HER) [3,4].An extensive research in the recent years have been made to improve the Faradaic efficiency of NRR by implementing noble-earth metal electrocatalysts (Pd, Au, Ru,\netc.), transition metal electrocatalysts (Mo, Fe, Co,V, etc.), and metal-free electrocatalysts (B-doped graphene, black phosphorus,\netc.) [5\u201312]. Efficient NRR performance at lower overpotentials have been reported mostly on metal based electrocatalysts; in particular, NRR with FE\u00a0>\u00a020% till date has been reported on Ru single atom catalysts anchored on N-doped porous carbon (21% FE) and active Mo/MoO2 species anchored on carbon cloth (FE\u00a0=\u00a022.3% FE) [13,14]. A major advantage of such metal centers@porous carbon electrocatalyst is the synergistic utilization of buffer electrons from the two-dimensional (2D) substrates and the catalytic metal center. These metal centers route the delocalised electrons from the 2D surfaces into the antibonding orbitals of N2 molecule leading to an activation of the N-N bond, which in turn is an essential determinant to the reduction of N2. Consequently, this has prompted computationally driven studies on several noble and earth abundant transition metal centers@2D electrocatalysts for nitrogen reduction [15,16].In this respect, the main group metals owing to their electronic arrangement show only specific oxidation number and restricted orbital states fail to exhibit purported N2 reduction, except for Li and Al clusters. Li has been reported to be used directly as a catalyst for NRR or via a lithium-mediated route as metallic Li forms the only stable nitride, Li3N in ambient conditions. The Li-mediated NRR electrocatalysts are known to exhibit high NRR FE closely approaching 100% in a high-concentration imide-based lithium salt interface [17]. However, the implementation of Li for NRR becomes unsustainable due to its limited presence in the earth\u2019s crust which correspond to only 0.002\u20130.006\u00a0wt%. On the other hand, Al being the most abundant metal in earth\u2019s crust, has been less explored for ambient nitrogen reduction. Notable reports have been made on Al-based electrocatalyst for ammonia fixation include Li-aided Al doped graphene, aluminium (III) coordination complex, Al-Co3O4/NF, MoAlB single crystals, and Al-N2 battery with an Al - ionic liquid electrolyte. Huang et al implemented Al metal as a dopant on graphene as a ligating center to the NxHy intermediates generated by Li\u00a0+\u00a0ion aided reduction of N2 to ammonia at ambient conditions [18]. A substantial advancement in NRR performance of Al-electrocatalysts had been realized by Berben and co-workers when an aluminium (III) complex with 0.3\u00a0M Bu4NPF6 THF and DMAPH\u00a0+\u00a0electrolyte exhibited ammonia production at\u00a0\u2212\u00a01.16\u00a0V (vs. SCE) with 21% FE in ambient condition [19]. However, Al as a catalytic metal center for nitrogen fixation has been reported in urchin like Al-doped Co2O3 nanospheres (Al-Co3O4/NF) with a FE of 6.25% at\u00a0\u2212\u00a00.2\u00a0V vs RHE by Yuan et. al, and in a multicomponent boride, MoAlB wherein the layered electrocatalyst reported by Ma and co-workers showed ammonia production with a FE of 30.1% at \u22120.05\u00a0V vs RHE [20,21]. The highest FE of 51.2% at \u22120.1\u00a0V for Al-based electrocatalyst in ammonia production has been reported by Zhi and co-workers in a rechargeable Al-N2 battery composed of a graphene-supported Pd (graphene/Pd) cathode and Al anode with an ionic liquid electrolyte (AlCl/1-butyl-3-methylimidazolium chloride) [22]. The study reveals a higher feasibility of AlN (\u0394G\u00a0=\u00a0-287\u00a0kJ/mol) formation as compared to Li3N (\u0394G\u00a0=\u00a0-154\u00a0kJ/mol), thereby revealing a more spontaneous nitriding reaction in Al over Li. However, AlN is extremely susceptible to air unlike its lithium counterpart, and it gets easily oxidised, thereby the catalytic activity of Al gets thwarted.Fundamentally, Al being an element of boron family possess similar electronic distribution and certain similarities in electronic properties can be expected. Boron has been reported in several metal-free electrocatalyst as a dopant or catalytic center that can hold N2 and influence ammonia production via an electron \u201cdonor-acceptor\u201d mechanism [23,24]. While the \u201cdonor-acceptor\u201d mechanism is unlikely to occur in Al atom catalyst due to a lower electronegativity of Al (1.61) as compared to B (2.01), Al clusters have been reported to chemisorb N2 and activate the N-N bond effectively. Aguado et al. reported the chemisorption of N2 and N-N bond activation upto 1.65\u00a0\u00c5 on Al44 nanoclusters with an energy barrier of 3.4\u00a0eV [25]. The N-N bond activation barrier becomes as low as 0.65\u00a0eV in smaller Al-clusters, in particular, Al5 cluster on BN-graphene, as observed by Kumar and co-workers [26]. Henceforth, it is important to probe into these smaller Al clusters and explore the plausibility of implementing them as NRR electrocatalyst. Furthermore, the experimental realization of Al-based catalysts on graphene is not far-fetched research as synthesis of pristine Al-clusters with pulsed laser vaporization can be dated back to 2007 by Neal et. al. [27] With experimental improvements brought about by electron-beam irradiation, single atom substitution on graphene has been reported by Zagler and co-workers [28]. However, the evidence of graphene-Al clusters/nanoparticle composites is as well-known as other graphene-metal composites and the synthesis route follows the conventional chemical exfoliation or powder metallurgy technique [29\u201331]. In this work, we make a radical comparison of the electronic properties of NRR active Ru and Mo single atom to Al atom and Al-clusters (Aln) supported on N-doped double vacancy graphene (N4-DVG). The study focuses on modulating the electronic and catalytic properties of atomic Al catalysts by inducing changes in their shape and size.All metal atoms and clusters supported N-doped double vacancy graphene (M@N4-DVG) systems, as shown in Fig. 1\n are optimized using Density Functional Theory (DFT) calculations with Vienna ab\u00a0\u2212\u00a0initio Simulation Package (VASP.5.4.4) [32]. The ionic-electronic interactions on all systems are sampled with a 2\u00a0\u00d7\u00a02\u00a0\u00d7\u00a01 Monkhorst- Pack kpoint grid and 520\u00a0eV energy cut-off with a generalized gradient approximation Perdew-Burke-Ernzerhof (PBE) functional [33]. All M@N4-DVG systems have been relaxed with DFT-D3 corrections to incorporate long range forces till the atomic forces and energies converge to 0.005\u00a0eV/\u00c5 and 10\u22125 eV/atom, respectively [34]. Electronic property analysis has been carried out to evaluate the density of states and Bader charges of the M@N4-DVG systems by considering a higher kpoint grid of (9\u00a0\u00d7\u00a09\u00a0\u00d7\u00a01) Monkhorst-Pack grid [35]. The thermal stability of the M@N4-DVG systems analysed through Ab initio molecular dynamics (AIMD) simulations carried out in an NVT ensemble at 298\u00a0K described with a Nose\u2013Hoover thermostat at 3\u00a0ps time step for 10\u00a0ps [36]. Furthermore, the feasibility of achieving chemically stable M@N4-DVG systems is realized via the binding energies (Eb) of atomic metal catalysts and clusters on the N4-DVG system computed using the equation,\n\n(1)\n\n\n\nE\nb\n\n=\n\n\n\n\nE\n\n\n\nM\n@\nN\n\n4\n\n-\nD\nV\nG\n\n\n\n\n\n-\n\n(\n\nE\n\n\nN\n4\n\n-\nD\nV\nG\n\n\n)\n\n-\n\nE\nM\n\n\n\n\n\nwhere, \n\n\nE\n\nM\n@\n\nN\n4\n\n\n\n\n\n-DVG is the total electronic energy of Ru, Mo, Al metal atom catalysts or Aln (n\u00a0=\u00a02\u20137) clusters supported N4-DVG systems, \n\n\nE\n\nN\n4\n\n\n\n\n-DVG is the total electronic energy of N4-DVG systems and \n\n\nE\nM\n\n\n is the electronic energy of Ru, Mo, Al single atom catalysts or Aln clusters. Following this, the N2 chemisorption efficacy on the M@N4-DVG catalysts is investigated via the end-on and side-on modes of N2 adsorption; and the adsorption energy, Eads is computed using the equation,\n\n(2)\n\n\n\nE\n\nads\n\n\n=\n\n\n\n\nE\n\n\n\nM\n@\nN\n\n4\n\n-\nD\nV\nG\n-\n\nN\n2\n\n\n\n\n\n\n-\n\n\n\n\nE\n\n\n\nM\n@\nN\n\n4\n\n-\nD\nV\nG\n\n\n\n\n\n-\n\nE\n\nN\n2\n\n\n\n\n\n\nwhere, \n\n\nE\n\nM\n@\n\nN\n4\n\n-\nD\nV\nG\n-\n\nN\n2\n\n\n\n\n is the total electronic energy of M@N4-DVG system after N2 adsorption, \n\n\nE\n\nM\n@\n\nN\n4\n\n-\nD\nV\nG\n\n\n\n and \n\n\nE\n\nN\n2\n\n\n\n are the total electronic energy of M@N4-DVG systems and free N2 molecule, respectively.The free energy of the NxHy intermediates involved in the Nitrogen Reduction Reaction (NRR) is represented by the Gibbs free energy change, \u0394G and the computational Standard Hydrogen Electrode model of N\u00f8rskov et al. has been implemented to calculate \u0394G using the following equation [37].\n\n\u0394G\u00a0=\u00a0\u0394E\u00a0+\u00a0\u0394ZPE\u00a0\u2212\u00a0T \u0394S(3)\n\n\nwhere, \u0394E and \u0394ZPE is the change in electronic energy and zero-point energy respectively, \u0394S is the change in entropy at room temperature, T is room temperature (298.15\u00a0K). The zero-point energy and entropy corrections are computed from the non-negative vibrational frequencies of the gas phase species in each intermediate. The potential rate-determining step (PDS) for the reaction is intermediate step with the highest free energy change (\u0394Gmax) and the limiting potential, UL is equal to \u2013(\u0394Gmax)/e. For an electrocatalyst under applied potential, the free energy is calculated as, \u0394GNRR\u00a0=\u00a0\u0394E\u00a0+\u00a0\u0394ZPE\u00a0\u2212\u00a0T\u0394S\u00a0+\u00a0neU\u00a0+\u00a0\u0394GpH\n, where n is the number of electrons, U is the applied electrode potential equivalent to the limiting potential, UL and \u0394GpH\n is the free energy correction to pH of the solvent. The pH correction to free energy is represented by \u0394G\npH\u00a0=\u00a02.303\u00a0\u00d7\u00a0k\nB\nT\u00a0\u00d7\u00a0pH, where\nk\nB\nis the Boltzmann constant. The pH value is assumed to be zero as the overpotential of NRR is unaffected by the change in pH.[38\u201339].The electronic stability of the M@N4-DVG systems as evaluated from the binding energy calculations show the metal single atoms (Ru, Mo and Al) to be positioned in the N tetra-coordinated vacancy in the graphene plane, while the Aln clusters are anchored with one Al atom occupying the double vacant site in graphene plane (i.e., in-plane) and the remaining Al atoms bound to the in-plane Al-atom. Mo single atom (Mo@N4-DVG) has been found to possess the highest binding energy of \u22129.78\u00a0eV followed by Ru@N4-DVG with \u22128.19\u00a0eV and Al@N4-DVG with \u22127.81\u00a0eV. The Aln clusters supported N4-DVG systems show a gradual decrease in their binding energies as the size of the clusters increase. The Aln clusters with a planar geometry and higher coordination with the in-plane Al are more stable with binding energies ranging from \u22127.1 to \u22127.5\u00a0eV as provided in Supporting Information, Table S1. On the other hand, Al7@N4-DVG system with a nearly spherical structure and two coordinated Al atoms to the in-plane Al atom show the least binding energy of \u22125.82\u00a0eV. Larger clusters are thus, not considered in this study as they tend to form spherical and symmetric structures. For ambient nitrogen fixation, the thermal stability of all M@N4-DVG systems are further analysed through AIMD simulations at 298\u00a0K and small structural distortions are observed in Al4@N4-DVG, Al5@N4-DVG and Al7@N4-DVG while both Al6 clusters showed large distortions after 10\u00a0ps, see Supporting Information\nFigure S1. Henceforth, all the M@N4-DVG systems, except Al6a@N4-DVG and Al6b@N4-DVG are eligible candidates to be implemented as stable catalysts at room temperature and further electrocatalytic studies will be carried out on the stable catalysts.The catalytic activity of a system, being an inflection of electronic properties and charge distribution or transfer efficiency, can be primitively assessed from its work function(\u03a6) and p-band center. While catalysts with a lower work-function will require a smaller energy to activate the N2 molecule, a more positive p-band center will ascertain the p-orbitals of the active centers are closer to the Fermi level and possess higher carrier density. A comparative plot of work function and p-band centers of Ru, Mo, Al metal atoms and Aln clusters in Fig. 2\n(a) shows the Ru single atom with most positive p-band center of \u22125.25\u00a0eV and work function of 4.29\u00a0eV. The p-band centers of Ru and Mo has been computed in place of d-band center to have a consistent comparison with Al which possess only p-orbitals. Ru, being the best performing metal single atom catalyst on N-doped graphene, is used as a reference for another active transition metal, Mo and our metal atom of interest Al and its clusters, Aln. Al@N4-DVG system with Al single atom shows a much lower (i.e., negative) work function than the Ru or Mo counterparts, however its p-band center is relatively more negative (-5.74\u00a0eV) thereby inferring a lower charge carrier density. Although the work function gradually increases with the increase in size of Aln clusters and a decrease in catalytic activity is expected, the p-band centers show an interesting trend with the Al5@N4-DVG system has its p-band center at \u22125.69\u00a0eV and more positive than Al@N4-DVG. This primitive screening of catalytic activity is ratified through the N2 adsorption strengths of the M@N4-DVG catalysts. While the presence of d-orbitals in Ru and Mo single atom catalyst allow both parallel and perpendicular modes of N2 adsorption on Ru@N4-DVG and Mo@N4-DVG, the most optimal adsorption mode and site of N2 on the Aln@N4-DVG catalysts are found to vary with the change in shape and size of the Al cluster. The most exothermic adsorption of N2 in perpendicular and parallel mode has been considered as N2 adsorption sites on Aln@N4-DVG catalysts and are shown in Table S2 of Supporting Information. From Fig. 2(b), it is evident that lower (or positive) p-band center in Ru@N4-DVG influences the exothermic adsorption of N2, while a lower work function is responsible for the same in Mo@N4-DVG. The higher activity and exothermic adsorption of N2 on Al5 cluster can also be ratified due to the shape and orientation of the Al atoms that are available for interaction with the incoming N2 molecule. As the Aln cluster size increases, one Al atom lies in-plane to the N-doped graphene sheet while the remaining Al atoms orient themselves with 3 or 4 as its coordination number. While most Aln clusters prefer to form 3-coordination leading to triangular facets, the stable Al5@N4-DVG catalyst prefer to form a rectangular facet with four Al-atoms exposed as catalytic sites from the ELF plots as shown in Table S3 of Supporting Information. Furthermore, the Bader charge analysis provided in Table S3 ensues these exposed Al atoms to be electron rich while the Al-atom ingrained to the graphene plane is positively charged or electron deficient. This further corroborates to the lower N-N activation of Al single atom as compared to electron rich Al-atoms lying above the graphene plane which can easily render electrons to N2 molecule.The N2 adsorption energies on Al single atom and its clusters are relatively lower than the transition metal counterparts; however, a similar trend of work-function influencing adsorption can be observed in Al@N4-DVG, Al2@N4-DVG and Al3@N4-DVG while the p-band center becomes accountable for Al5@N4-DVG. More interestingly, Al5@N4-DVG is the only Aln based catalyst that shows exothermic side-on N2 adsorption (-0.34\u00a0eV) and N-N bond activation (1.37\u00a0\u00c5) brought about with the nitrogen atoms attached to different Al centers. Fig. 3\n shows the overlap of Al p-orbitals (Al5@N4-DVG) and N p-orbitals (N2) in the Fermi region of PDOS and electron localization function (ELF) plot with localized electrons on the N2. Besides Al5@N4-DVG, the systems of interest that show exothermic N2 adsorption and N-N bond activation are Ru@N4-DVG, Mo@N4-DVG, Al@N4-DVG, Al2@N4-DVG and Al3@N4-DVG. The electron localization functions of adsorbed N2 on the above-mentioned M@N4-DVG catalysts are provided in Supporting Information, Figure S2. ELF plots with higher electron density concentration on the atoms will correspond to ionic bonding while contribution from covalent bonding can be accounted when the electron density is concentrated on the respective bond between two atoms. A prominent electron localization on N2 can be observed in Ru@N4-DVG and Mo@N4-DVG systems inferring an ionic bonding or stronger binding which can be interpreted as chemisorption led by electron transfer, whereas a more covalent bonding between Al atoms and N2 molecule can be observed in the Al-based catalysts inferring towards physisorption of N2. The presence of higher electron density in Aln clusters as compared to Al single atom can be a major contributor in enhancing the catalytic activity of Al metal for NRR. This is supported by the PDOS plot of N2 adsorbed Al@N4-DVG catalyst in Figure S3, Supporting Information shows minimal contribution from the Al p-orbitals. However, in aluminium cluster catalysts the contribution of Al p-orbitals is found to increase gradually along with a shift towards the Fermi level due to the conducting nature of Al. The distribution of electron density as seen from ELF plots and smaller orbital overlap between Al and N2 can be inferred as N2 physisorption on the Al-based catalysts and the N2 adsorption energies corroborate to this finding.Following this screening of N2 activation, the mechanisms of nitrogen reduction reaction (NRR) on all possible routes, Fig. 4\n(a), are explored on the select M@N4-DVG catalysts that show exothermic N2 adsorption. Nitrogen reduction on Ru@N4-DVG and Mo@N4-DVG catalysts with Ru and Mo single atom center have been investigated via the distal, alternating and enzymatic route. The consecutive route has been found unfeasible as the *N-*NH2 intermediate could not be realized on the single atoms. Computational calculations on Al@N4-DVG, Al2@N4-DVG and Al3@N4-DVG catalysts that show end-on N2 adsorption have been restricted only for the distal and alternating route. Finally, Al5@N4-DVG catalyst which showed exothermic side-on N2 adsorption have been investigated for enzymatic and consecutive route of NRR, with multiple Al atoms being involved in N2 adsorption, the consecutive route becomes feasible in this catalyst. The free energy diagrams of the above-mentioned routes of NRR reaction coordinates on stable M@N4-DVG catalysts are provided in Supporting Information, Figure S4-S9. The reduction of N2 to NH3 is a multistep reaction with six elementary protonation steps and release of two NH3 molecules, the usual uphill reaction steps are the first (*N2\u2192*N2H) and last protonation (*NH2\u2192*NH3) steps in all routes along with the fourth protonation (*N2H3\u2192*N2H4) step in alternating route. The uphill elementary step with the highest energy barrier becomes the potential rate determining step (PDS) of NRR and a summary of all possible routes and the PDS with \u0394Gmax values on all M@N4-DVG catalysts as shown in Fig. 4(b). The \u0394Gmax value on Ru single atom, reported as the best catalyst, has been found to be 0.53\u00a0eV in the first protonation step. However, Mo single atom which has been reported as an active NRR catalyst shows a relatively high energy barrier of 1.43\u00a0eV in the last protonation step. The \u0394Gmax values on the Al-based catalysts are 0.84\u00a0eV (*N2\u2192*N2H), 1.35\u00a0eV (*NH2\u2192*NH3) and 1.08\u00a0eV (*NH2\u2192*NH3) in Al@N4-DVG, Al2@N4-DVG and Al3@N4-DVG respectively. While Al single atom and smaller Al-clusters show a relatively higher NRR performance than Mo single atom, the NRR performance improves to 0.78\u00a0eV in Al5@N4-DVG catalyst that showed exothermic side-on N2 adsorption. Furthermore, upon application of an external potential 0.78\u00a0eV as shown in Fig. 4(c), the elementary (*NH2\u2192*NH3) protonation steps become exothermic, thereby inferring this catalyst can also be implemented as an electrocatalyst.A detailed electronic analysis of the Bader charges as shown in Fig. 5\n(a) of Ru@N4-DVG shows a correlation between the charge transfer from Ru to the N atoms. Most importantly, the PDS fourth protonation step, *NH\u2013*NH2 \u2192 *NH2\u2013*NH2 step has been found to show a large difference between Ru charge and N charges, thus signifying that the electronic barrier essential to bring about ammonia production. Al@N4-DVG catalyst that show a preference of the alternating route with better stabilized *NxHy intermediates exhibit contrastingly higher Bader charge difference in the last protonation step although the PDS is the first protonation step, *N2\u2192 *N-NH. This is in concordance to an uphill step of 0.81\u00a0eV observed in\u00b7NH3 formation as shown in the Supporting Information, Figure S6. Furthermore, in the ELF plot shown in Fig. 5(b) inset for\u00b7NH3 intermediate, a relatively higher electron density can be observed in the N atoms from the Al single atom along with Al-N covalent bond further stabilising the system. This possess a major challenge in the functionality and applicability of Al@N4-DVG catalyst, as the active Al metal site gets deactivated due to strong adsorption efficacy of NH3, which is \u22120.90\u00a0eV exergonic from N2 adsorption. Interestingly, Al5@N4-DVG catalyst behaves similar to Ru@N4-DVG catalyst and shows a large variation in Bader charge of Al and N only in its PDS step, i.e., *NH2\u2192\u00b7NH3 step, Fig. 5(c). Additionally, the dissemination of electron density in the constituting Al atoms in Al5 center leads to efficient electron transfer to N atoms, leading to formation of NH3 without the manifestation of any covalent bond between Al center and N atoms of NxHy intermediates. The corresponding NH3 adsorption on Al5@N4-DVG is exoergic by \u22120.41\u00a0eV as compared to N2 adsorption and the possibility of catalytic center deactivation or poisoning can be reduced as 5-Al metal centers are involved. Another similarity of the Al5@N4-DVG catalyst to the Ru@N4-DVG catalyst is an exclusive NRR selectivity over the competing hydrogen evolution reaction (HER), Fig. 5(d). An interesting outlook can be accounted on the NRR performance of Al5@N4-DVG catalyst in the presence of water solvent, details of implementing solvent model and calculations are discussed in Supplementary Information. N2 molecule being non-polar, its adsorption energy in water should be endothermic as compared to its value in vacuum; while the protonated NxHy intermediates possess dipoles and water as a solvent enhances the formation of NxHy intermediates. Thereby, a lower adsorption energy of N2 and higher free energies of NxHy intermediates on Al5@N4-DVG system can be expected with solvent effects and the same has been compared with the energetics in vacuum for the consecutive route of NRR on Al5@N4-DVG, as seen in Fig. 5(e). In the presence of water, the adsorption energy of N2 on Al5@N4-DVG system reduces from \u22120.06\u00a0eV to \u22120.05\u00a0eV in perpendicular mode and \u22120.37\u00a0eV to \u22120.27\u00a0eV in parallel mode. As anticipated, the following protonation steps leading to formation of NxHy intermediates are energetically favourable with more negative \u0394G values when compared to vacuum state (Figure S10). The corresponding \u0394Gmax reduces from 0.78\u00a0eV to 0.70\u00a0eV for the consecutive route and PDS shift from the last protonation step, *NH2\u2192\u00b7NH3 in vacuum to fourth protonation step, *N \u2192*NH in water solvent. This can be attributed to the solvation of the *N intermediate with open coordination sites and the water pockets hindering the transport of H+ to form the *NH intermediate. Contrastingly, the protonation of *NH2 intermediate to\u00b7NH3 intermediate which was less feasible in vacuum becomes more facile as water can enhance the transport of protons and formation of\u00b7NH3. This also substantiates that Al5@N4-DVG catalyst can be rendered for lab-scale experimentations in aqueous conditions.A comparison of the NRR performance on the Al-based catalysts has been highlighted in Table S4 of Supplementary Information. The \u0394Gmax of NRR on Al5@N4-DVG catalyst has been observed to be higher than several homoatomic or heteroatomic bimetallic transition metal catalysts, however in several cases of homoatomic catalysts, i.e., Ru2 @PC6, Cu2@NG, Ni4@Gr catalysts the NRR performance is on-par and higher in some cases. It can also be noted that Al5 cluster anchored on BN-doped graphene showed the lowest barrier for N-N bond activation in the study carried out by Kumar et al. and our studies concur to their findings.[26] Aluminium clusters on N-doped double vacancy graphene, despite a less attractive NRR performance than transition metal single atoms, perform as par the Ru single atom catalyst with a high selectivity for NRR and a trade-off can be achieved when researchers aim for scalable and sustainable catalyst for ammonia production.In this study, DFT investigation has been made to conform an earth-abundant metal Al to conform and exhibit similar catalytic properties to another rare earth transition metal, Ru for nitrogen reduction. Al-based catalysts have been modulated into a Ru-single atom like catalytic center by varying number of Al centers. A detailed study on the electronic and thermal stability of the model catalysts have been made via AIMD studies and the catalytic properties are primitively scoured through their inherent electronic properties. An analysis of the electron localization function and projected density of states plots shows a strong chemisorption in the transition metal, while a weak physisorption is observed in the Al-based catalysts. The change in shape and size of the atomic Al clusters reflects to a change in their corresponding catalytic properties, and Al5 supported on N-doped double vacancy graphene (N4-DVG) conform to Ru-single atom like catalyst. Bader charge analysis of the NRR reaction intermediates show a similarity in the large charge transfer to N atoms from Ru single atom and Al5 center, with respective \u0394Gmax of 0.53\u00a0eV and 0.78\u00a0eV in Ru@N4-DVG and Al5@N4-DVG catalysts. Despite a higher free energy change in the potential rate determining step, the high NRR selectivity of Al5@N4-DVG catalyst makes it a highly attractive catalyst for electrocatalytic ammonia production. The understanding from this work can be used to further the research on developing Al-based catalysts for nitrogen fixation and feasible ambient ammonia production with the most abundant metal, aluminium.\nAshakiran Maibam: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing \u2013 original draft. Sailaja Krishnamurty: Software, Validation, Supervision, Writing \u2013 original draft. Ravichandar Babarao: Conceptualization, Software, Validation, Supervision, Writing \u2013 original draft.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.A.M. acknowledges AcSIR-RMIT for hosting the Joint-PhD Program and RMIT University for research funding. The authors gratefully acknowledge National Computing Infrastructure (NCI), and Pawsey supercomputing centre, Australia for providing the computational resources.Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2023.157024.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Density Functional Theory (DFT) investigation on the most earth-abundant Al-based catalysts, has been conducted detailing its electronic properties and catalytic efficacy for nitrogen reduction at ambient condition. The Al-based catalysts have been modulated to perform as par a highly performing, but rare, Ru-single atom catalytic center by varying number of Al atoms, shape, and size. The coalesce of band-center, work function and electronic properties in metal atom catalysts along with N-N bond activation has been demonstrated to be responsible for an efficient nitrogen reduction reaction (NRR) with \u0394Gmax of 0.78\u00a0eV in Al5 supported on N-doped double vacancy graphene (Al5@N4-DVG) catalyst. Electron localization function analysis has shown a weak physisorption of N2 in the Al-based catalysts. Projected Density of States (PDOS) illustrates the enhancement of aluminium electron density in Al5@N4-DVG led to enhanced orbital densities overlap of Alp and Np electrons. The Bader charge analysis and electronic analysis of the intermediates show efficient electron gain on the N atoms, leading to formation of NH3 from the NxHy intermediates in Al5@N4-DVG catalyst.\n               "}
{"full_text": "Dyes are a significant part of aquatic pollution released by the paper, tanning, cosmetic, textile and paint industries. Properties like structure uniqueness, chemical stability, different functional groups, and characteristic color of methyl orange and methylene blue make them relevant in industries [1\u20135]. Methyl orange is an azoic dye, and methylene blue is a thiazine dye. Wastewater containing these two compounds causes toxic effects on humans and aquatic life by getting into the food chain. The stability of dyes and their by-products makes them more hazardous due to their mutagenic and carcinogenic nature [6,7]. Due to their non-biodegradability conventional biological degradation methods are not applicable. Today scientists are extensively trying to remove or degrade these dyes from industrial waste before their release in freshwater bodies [8\u201311].Physical and chemical methods like adsorption and degradation, respectively, are used to remove these dyes from wastewater. Photocatalytic degradation is a widely used method for degrading toxic dyes from water bodies with nanoparticles help. NPs are directly or indirectly involved in degrading more toxic dyes into less harmful by-products [12\u201314]. NPs TiO2, Ag, Zn, Au, Pd, Cu, bi-metallic NPs Ag\u2013Zn, Ag/Ni, Ag/Cu, Co/Fe, nano-composites like SnO2 decorated polystyrene, carbon-doped TiO2 and ZnO/CMC NCs are used for the catalytic degradation of dyes. Their activity in the visible region makes them photo-chemically active and efficient for dye degradation. Their small size and large surface area offer more active sites for the reaction to take place easily with less time compared to bulk material [9,15,16].Metal and non-metal NPs have ostentatious catalytic properties compared to bulk materials. High surface-to-volume ratio, surface plasmon effect, catalytic, optical, conductivity, and thermal properties make their applications in wastewater treatment by catalytic and photocatalytic reduction of organic dyes. Bimetallic nanoparticles (BMNPs) show extraordinary results in almost all fields compared to mono-metallic or non-metallic NPs in technical and scientific fields. BMNPs alter the surface plasmon band, stability, and dispersion of NPs [17\u201319].The number of metal-NPs comprising silver, zinc, TiO2, gold, platinum, and cobalt have been synthesized through physical, chemical, biological, and green routes. NPs prepared by chemical methods cause pollution and have adsorbed harmful chemicals on their surface, causing annoying adversarial effects in the treatment and diagnostic fields. Physical methods are expensive. They follow a top-down technique and produce NPs having a narrow morphological range [20]. Biological route uses microorganisms, enzymes and fungus for the stabilization of NPs. Green synthesis is the best way to synthesize the metallic, bimetallic NPs due to no hazardous waste, low cost, ease of characterization and bottom-up technique make one able to control their morphology, crystallinity, and size [21,22].This research work focusses on the use of Nicotiana tabacum (NT) dried leaves extract to synthesize Ag\u2013Zn doped TiO2 nano-catalysts (NCs). Identification techniques comprises UV VIS spectroscopy, FTIR, SEM and XRD were used for the confirmation, morphology, and crystallinity of prepared NCs. These prepared nano-catalysts were used for the catalytic and photocatalytic decolorization of methylene blue and methyl orange from industrial wastewater.Analytical grades reagents including AgNO3 (\u226599%), zinc nitrate hexahydrate Zn(NO3)2.6H2O (\u226597%) were purchased from Fisher (UK) and TiO2 (\u226597%) was purchased from Sigma Aldrich (UK). All the reaction mixtures were prepared in deionized water. pH was optimized using NaOH. Fresh leaves of N. tabacum were collected from farms at Lodhraan Uggoki Sialkot, Pakistan.Freshly collected leaves were cleaned with DI water 2\u20133 times to remove dust particles and dried in the oven at 40\u00a0\u00b0C. Fully dried leaves were crushed in fine powder using a grinder. 1\u00a0g of leaves powder in 200\u00a0mL of distilled water is heated on hotplate at 60\u00a0\u00b0C for 20\u00a0min greenish brown color solution is obtained which is filtered to separate leaves powder. The supernatant is stored in 4\u00a0\u00b0C for further use.Titanium oxide (TiO2), silver nitrate (AgNO3), and zinc nitrate hexa-hydrate (ZnNO3.6H2O) were used for starting precursors. In the synthesis of Ag\u2013Zn doped TiO2 NCs, 5\u00a0mM solution of TiO2 with 5% (w/w) AgNO3 and ZnNO3.6H2O was dissolved in 100\u00a0mL of distilled water and stirred well as done previously [23]. This solution was added to leaves extract dropwise on continuous stirring at maintained pH, color changes occurred from greenish brown to white, which conformed the synthesis of NCs. The solution was further centrifuged and washed many time to remove the un-reacted ions.The NT stabilized Ag\u2013Zn doped TiO2 NCs were characterized by recording full scan of UV-VIS spectra from 300 to 800\u00a0nm and the surface plasmon of the NPs was also observed over UV-VIS Spectrophotometer CECIL CE 7400s Aquarius. Fourier transformation infrared spectroscopy (FTIR) by ATR was adopted to determine the functional groups using (Bruker, Alpha-II, UK) in the range of 4000\u2013500\u00a0cm\u22121. SEM morphologies were evaluated over (FE-SEM NOVA 450, UK). The crystalline nature was examined using an X-ray diffractometer (Bruker D-8 with Cu K\u03b1 \u039b\u00a0=\u00a01.54\u00a0\u00c5, UK).Methylene blue (MB) and methyl orange (MO) were degraded photo catalytically using synthesized Ag\u2013Zn doped TiO2 NCs under direct sunlight. Decolorization of 0.01\u00a0mM\u00a0MB and MO dyes has been studied by varying catalyst concentration (1\u20135\u00a0mL of 0.2\u00a0mg/L) at maintained pH and 31\u00a0\u00b0C. The reaction mixtures were prepared and stirred under room light and then placed under sunlight for decolorization. Progress of the reaction was monitored by UV-VIS spectra ranging from 300 to 800\u00a0nm in specific time. Blue and orange color of dyes appeared in case of MB and MO, respectively [24\u201326].MB and MO were degraded by using NaBH4 reagent at prepared NCs catalyst and its factors were also studied to find the optimum concentrations of reagents for maximum decolorization. Dyes (MO, MB) and catalyst dose effects on decolorization were observed by varying concentration from 0.004\u00a0mM to 0.025\u00a0mM and 0.02\u00a0mg/mL to 0.1\u00a0mg/mL respectively in addition with 1.8\u00a0mM NaBH4. The catalyst (Ag\u2013Zn doped TiO2) was confirmed over UV/Vis spectrophotometer at full scan of 300\u2013800\u00a0nm. Absorbance peak of MB at 665\u00a0nm and that of MO at 460\u00a0nm was monitored for decolorization studies.In recent studies, synthesized Ag\u2013Zn doped TiO2 NCs using leaves extract without any external hazardous chemical stabilizing agent. In this study, obtained NCs were better in terms of size, which was improved than other reported methods [27]. Plant leaves extract itself acted for stabilization of doped NPs by controlling their size, prevented coalescence with controlled nucleation and ordered crystallinity of NPs. The methods which confirmed the formation of NCs are followed.FTIR analysis of NT stabilized Ag\u2013Zn doped TiO2 NCs was caries out to examine which biomolecules are responsible for their reduction and stabilization. Fig.\u00a01\n and Table 1\n represents the transmittance peak at 3357, 2919, 2851, 1248, and 1091\u00a0cm\u22121. The spectra of Ag\u2013Zn doped TiO2 NCs peak broad peak ranging from 3650 to 3100 indicating \u2013OH stretching due to the water of crystallization adsorbed on the NCs [28], peaks located at 2919 and 2851\u00a0cm\u22121 exemplifies C\u2013H stretching and peak at 1248\u00a0cm\u22121 illustrated amide (III) stretching of amino groups present in leaf extract. Peaks at 1593\u00a0cm\u22121 indicating CC stretching [29]. Strong peak at 1091\u00a0cm\u22121 of C\u2013O stretching [30]. Displacing of spectral lines from NT leave to NCs showed interconversion of functional groups for reduction and stabilization of NCs. Kumar et\u00a0al. also reported the synthesis of silver NPs using NT leaf extract with peaks at 3403, 2934, 2396, 1761, 1624, 1384, 1075, cm\u22121 with almost similar functional groups [31].For the determination of crystalline nature of prepared Ag\u2013Zn doped TiO2 NCs XRD analysis performed under the range of 10\u201380\u00b0. Fig.\u00a02\n corresponds to XRD pattern peaks appeared at 2 \n\n\u03b8\n\n having values 69.00\u00b0, 65.50\u00b0, 64.04\u00b0, 62.75\u00b0, 56.62\u00b0, 54.31\u00b0, 44.04\u00b0, 41.23\u00b0, 39.18\u00b0, 36.08\u00b0 and 27.43\u00b0 represent the TiO2 (rutile) (JCPDS 75\u20131748), and additional peaks at 41.00\u00b0, 59.38\u00b0 and 74.70\u00b0 has confirmed the presence of AgZn (JCPDS 65\u20136585) as doped material. The presence of curve at lower angle may be due to its cubic structure and narrow peaks in the spectra are due to the nano-sized structure of the particles. High intensity peaks at 27.34\u00b0 for 2 \n\n\u03b8\n\n value is preferred orientation for synthesized nano-composites with comparatively lowest surface energy than other planes.Debye Scherer equation applied for the determination of average size calculation of synthesized Ag\u2013Zn doped TiO2 NCs.\n\n\n\n\nD\n\na\nv\ng\n\n\n=\n\n\nk\n\u03bb\n\n\n\u03b2\n\ncos\n\n\u03b8\n\n\n\n\n\nWhere k is dimensionless crystalline factor with value of 0.96, \n\n\u03bb\n\n represents the wavelength of Ni\u2013Cu K\u03b1 radiations and d and \n\n\u03b8\n\n represents the full width half maximum in radian of each peak and angle is radiant, respectively. Average crystallite size of synthesized Ag\u2013Zn doped TiO2 NCs was found 5.66\u00a0nm.Field emission scanning electron microscopy (FE-SEM) was utilized for the morphological and structural determination of fabricated Ag\u2013Zn doped TiO2 NCs (Fig.\u00a03\n). It is defined that the NCs has a wide range morphology including clusters, rose petals like arrangements and nano-rods (Fig.\u00a02a). NCs mainly formed and stabilized by the NT extract. As it is evident from the XRD analysis, the crystallite size of the Ag\u2013Zn BMNPs was 5.66\u00a0nm. A variable shape (poly-morphological structure) was observed of Ag\u2013Zn doped TiO2 NCs. In earlier studies, it was shown that surface area and shape play a more significant impact in the adsorption and reduction of adsorbed molecules in catalytic processes. The wide range of morphology and large size of synthesized NCs was may be due to the doping of Ag\u2013Zn BMNPs onto the surface of TiO2 which is a well-known catalyst to enhanced the NCs catalytic properties by improving the energy bandgap and leads its vast range application in different fields [35].The UV-VIS spectroscopy was used to investigate the optical activity of Ag\u2013Zn doped TiO2 NCs. UV-VIS measurements verified the presence of co-doped NCs. Metallic NPs have some absorption maxima in the UV-VIS region; hence UV\u2013vis spectrophotometric measurement can be used as a quick preliminary test to validate the nanoparticles fabrication. Sudden colour change was the primary sign of Ag\u2013Zn doped TiO2 NCs, Ag doped TiO2 and Zn doped TiO2 synthesis, and this change was then analyzed by UV\u2013Vis spectrophotometer at wavelengths ranging 200\u2013800\u00a0nm to find the surface plasmon resonance (SPR) of Ag NPs and lamda max of Zn NPs. Fig.\u00a04\n indicated a prominent peak at 400\u00a0nm in the UV\u2013Vis spectra of greenly produced Ag doped TiO2 and Ag\u2013Zn doped TiO2 NCs. Similarly, the previously results proven our results for peaks of TiO2, ZnO and Ag NPs at 276 [36], 292 [37] and 420\u00a0nm [38] respectively. The minor displacement of the spectral peaks was observed for Ag doped TiO2, Zn doped TiO2 and Ag\u2013Zn doped TiO2 NCs, which was due to the doping and combined effect of bi-metals. These peaks were handful for the determination of bandgap.Methylene blue (MB) is the widely used dye in major industries including cosmetics, fabrics, chemical, pharmaceutical, and agriculture industries. Almost 6 tons of dyes are wasted yearly in water which effect water bodies, animals and humans. These days, scientists are producing materials to reduce these dyes by adsorption, catalytic, photocatalytic decolorization and advanced oxidation processes. In our research we mainly focused to reduce MB catalytically, photo catalytically and different factors that affect the reduction of dyes including pH, Catalyst, dye and reducing agent concentrations [2,13,39]. Catalytic decolorization means reduction of dyes using a catalyst system which provide specific area for dye and reducing agent to adsorb and complete reaction of decolorization either by reduction or breakage of the compound. It is a slurry phase reaction in which catalyst is in solid phase and reactants are in liquid phase [17,40]. Herein, Ag\u2013Zn doped TiO2 NCs as the catalyst for the reduction and decolorization of MB dye has been done by using 0.01\u00a0mM\u00a0MB, 1.8\u00a0mM NaBH4 and 0.06\u00a0mg/mL catalyst concentrations as clearly shown in Fig.\u00a05\na. While, the decolorization also monitored in the absence of catalyst at same optimum conditions (Fig.\u00a05b). The results revealed that after 90\u00a0min there was minimal effect of NaBH4 for the decrease in the absorbance of MB dye. Similarly, MO dye decolorization was also studied at optimum conditions, 0.1\u00a0mM MO dye, 0.06\u00a0mg/mL catalyst and 1.8\u00a0mM NaBH4 (Fig.\u00a05c). The results were outstanding as the decolorization was completed in just 16\u00a0min and to confirm whether it is decolorization or just NaBH4 is reducing the dye molecule, another reaction was done on same conditions but without catalyst presence (Fig.\u00a05d). The results shown no decolorization after 75\u00a0min, which proven the efficacy of NCs catalyst. Decolorization of dye in the presence of reducing agent NaBH4 and catalyst Ag\u2013Zn doped TiO2 has been explained on the basis of Langmuir Hinshelwood (LH) mechanism. Fig.\u00a05e explains the decolorization mechanism of both dyes. First of all, reducing agent ionizes in water and produce BH-\n4 ions which get adsorb on the surface of catalyst and provide H+ on it. At the same time, dye molecules get adsorb on the surface of catalyst and reaction starts in which double bonded nitrogen atom present in the dye molecule which give characteristic color and properties to dye molecules reduces by up taking of hydrogen from catalyst surface and reduction of dye occurred in almost 8\u00a0min. Different factors including effect of catalyst, effect of dye, effect of reducing agent, pH of solution and at various temperatures the rate of decolorization of dye has been studied. Previously decolorization of dye has been monitored using NiFe2O4 NPs [41].Decolorization of MB was studies under different initial dye concentrations range from 0.001 to 0.025\u00a0mM, 0.2\u00a0mg/L NT stabilized TiO2 Ag\u2013Zn doped TiO2 NCs dose, 27\u00a0\u00b0C temperature and contact time of 8\u00a0min (Fig.\u00a06\na). MB colour removal was observed 73% at 0.020\u00a0mM concentration. After this concentration the decolorization decreases due the conjugation of dye molecules in solution. They face difficulty to reach at the surface of catalyst due to hyper conjugation and less molecules reach there to degrade [42]. It was previously reported that the decolorization of MB dye at Ag\u2013Zn doped TiO2 NCs was 25% in 8\u00a0min. So it can be stated that the decolorization of dye depends upon the initial concentration of dye. It would be maximum at 0.020\u00a0mM with 1\u00a0mL of 0.2\u00a0mg/L and decreases after and before this concentration.Kinetic studies revealed the rate of the chemical reaction. Model reaction (Fig.\u00a06b) explains that the decolorization of dye at 0.01\u00a0mM completed in just 8\u00a0min. The rate of the chemical reaction and different steps included in the complete decolorization of organic pollutant MB dye were studied (Fig.\u00a06c). The reaction followed pseudo first order kinetics according to the equation\n\n(1)\n\n\nln\n\n(\n\nA\nt\n\n/\n\nA\no\n\n)\n\n=\n\u2212\n\nk\n\na\np\np\n\n\n\u00d7\nt\n\n\n\n\nIt can be seen that at first there was very slow decolorization in 2\u00a0min which is induction time for the reaction to start, from 2 to 8\u00a0min' reaction speed up and completed in 8\u201310\u00a0min. The induction time, decolorization time and reaction completion time for different dye concentrations were different and almost increases with increase in concentration of dye. Fig.\u00a06d and i explains the decolorization time of the catalytic reaction with same optimal conditions for MB and MO dyes\u2019 decolorization at which the rate of decolorization was comparatively very fast which was at 0.01\u00a0mM for both dyes [43]. The more time taken by the reaction to complete at higher dye concentration is due to the limited active sites at constant amount of catalyst in all the reaction mixtures. At higher concentration of dye, molecules of dye face difficulty of come and adsorb on the surface of catalyst due to steric hindrance and take more time for completion of reaction.Decolorization of MB and MO with respected to time was observe and recorded in the form of percentage decolorization in Fig.\u00a06e\u2013j, which was 90% and 93% respectively. MO was completely degraded in just 16\u00a0min which is efficient as already reported decolorization time, which was about 90 to 60\u00a0min [44,45]. Kinetic studies revealed that the decolorization of MO dye was almost zero in first 5\u00a0min which could be called induction time. The next step was observed the decolorization time from 5 to 13\u00a0min and finally reaction completion time 13\u201316\u00a0min at different dye concentrations (Fig.\u00a06g). Fig.\u00a06h, corresponds to the slopes of pseudo first order catalytic decolorization kinetics of MO dye for the determination of kaap. Further, Fig.\u00a06i, shows the MO decolorization in terms of kaap. This shows the remarkable effectiveness of NCs in the presence of NaBH4 to degrade anionic dye (MO) as well as for cationic MB dye.To study that which concentration of catalyst is most suitable for maximum decolorization of methylene blue dye, different amount of catalyst ranging from 0.02\u00a0mg/mL to 0.10\u00a0mg/mL were used against 0.02\u00a0mM\u00a0MB dye. Fig.\u00a07\na clearly shows that decolorization increases with increase in catalyst concentration. Fig.\u00a07b shows the percentage decolorization of dye which more clearly explains that the decolorization of dye increases rapidly with increasing catalyst dose up to 1\u00a0mL and after this decolorization rate increases with very low rate. It means that 1\u00a0mL of catalyst dose if more reliable competitively for maximum decolorization of MB dye. By increasing the catalyst dose more active sites are available for catalytic decolorization but at very high concentration catalyst molecules hinders dye molecules to get adsorb on the specific active sites of the catalyst and react with the reducing agent to complete the reaction.Kinetic of the reaction revealed the best concentration at which the rate of the reaction is maximum and give better results in decolorization of pollutants in water treatment. As shown in Fig.\u00a08\na, 0\u20132\u00a0min the reaction rate was almost zero at this state the molecules of the dye move towards the surface of Ag\u2013Zn doped TiO2 NCs, after this from 2 to 8\u00a0min the rate raised exponentially due to reaction between dye molecules and reducing agent, at the end the reaction completed in 8\u201310\u00a0min. Fig.\u00a08b shows the negative slope for the determination of kapp. It can be seen clearly that the rate of the reaction was very fast for 0.1\u00a0mg/mL catalyst dose due to the availability of large number of active sites for the decolorization of dye. The rate increases with increase of catalyst dose which reveals that the rate of concentration varies directly with amount of catalyst shows the efficiency of the catalyst. Fig.\u00a08c explains the kapp at different concentration of catalyst by keeping the concentration of dye and reducing agent constant. The value of half-life and observed rate constants are shown in Table 2\n [46\u201348].Decolorization of dyes using nano-catalysts are greatly affected by pH of solution. The effect of pH on decolorization of methylene blue (MB) and methyl orange (MO) was studied by retaining its array from 3 to 11\u00a0at 0.01\u00a0mM and 0.0045\u00a0mM primary concentration of dyes respectively, 1\u00a0mL (0.2\u00a0mg/L) Ag\u2013Zn doped TiO2 NCs dose, 0.1% NaBH4 at room temperature for 8\u00a0min and response is exposed in Fig.\u00a09\n. Maximum MB removal was attained at highly acidic pH 3, it was 58.5% which gradually reduces to 15% at pH 11. Deceptively, the decolorization manner is acid-focused for MB. On other hand the removal of MO was maximum at highly basic pH 11, it was 67% and reduce to 0% at pH 3. Liu et\u00a0al., 2017 reported that MB removal form waste water was higher at acidic medium and decreases with increase in pH of medium [49].\nTable 2 gives the complete description of apparent rate constant, regression factor and Half-life of the different concentrations of MB and Ag\u2013Zn doped TiO2 NCs dosage for decolorization of dye. It would be easy to select the optimum concentrations of reagents to get efficient decolorization of dye.The photocatalytic decolorization studies was observed on UV-VIS spectrophotometer with 300\u2013800\u00a0nm range. The spectra were observed with initial 0.01\u00a0mM\u00a0MB (Fig.\u00a010\na) and MO dye (Fig.\u00a010b) concentration at 1\u20135\u00a0mL of 0.2\u00a0mg/L. The photo-catalytic decolorization of MB and MO was studied as the function of Ag\u2013Zn doped TiO2 NCs catalyst dose (Fig.\u00a010c). Different conditions was adjusted including neutral pH and 180\u00a0min contact time at 26\u00a0\u00b0C room temperature. In case of MO the decolorization at 1\u00a0mL catalyst was 30% which increases to 45% at 5\u00a0mL catalyst dose after this decolorization remains constant this is due to the low absorption of light by the surface of catalyst to absorb and degrade the methyl orange. On other hand decolorization of MB increases with increase the catalyst dose. This is due to the increase the surface area for the MB molecules to be adsorbed by Ag\u2013Zn doped TiO2 NCs. In the presence of light, the surface of catalyst absorbs the light photon to excite electron from valence to conduction band and to generate hydroxyl (OH\u2022) free radical to decolorize dye effectively (Fig.\u00a010d).The catalytic activity and photocatalytic activity of Ag\u2013Zn Doped TiO2 increased due to the reduced bandgap of TiO2 after doping with Ag\u2013Zn nano-particles. The bandgap of TiO2 material was reported to be 3.4\u00a0eV [50]. The bandgaps for Ag, Zn and Ag\u2013Zn doped TiO2 was were calculated using UV-VIS studies (Fig.\u00a04). In our studies the prepared nano-composites bear the bandgap of over 3\u00a0eV as shown in Fig.\u00a011\n. This was the main reason for the efficient catalytic activity of Ag\u2013Zn doped TiO2 nano-composites. Table 3\n, illustrates some previously done work for decolorization of dyes.Ag\u2013Zn doped TiO2 NCs were effectively synthesized utilizing 5% AgNO3, ZnCl2 on TiO2 as a precursor and Nicotiana Tabacum leaves extract at 60\u00a0\u00b0C with continuous stirring. The Nicotiana Tabacum leaves extract acted as both a reducing and a stabilizing agent. The production of the Ag\u2013Zn doped TiO2 NCs was confirmed by UV\u2013visible spectral peaks at 274, 296 and 400\u00a0nm. Ag\u2013Zn doped TiO2 NCs had an average crystallite size of 5.66\u00a0nm and a tetragonal geometry. The catalytic decolorization of MB-dye, NaBH4, and Ag\u2013Zn doped TiO2NCs was then carried out using synthesized Ag\u2013Zn doped TiO2 NCs. Detailed analysis showed that the reaction was completed in 8\u00a0min, and kinetic analyses of the data supported the pseudo-first-order process having kapp value 0.2431 min\u22121. Hence, Nicotiana Tabacum leaves extract can be used for synthesis of Ag\u2013Zn doped TiO2 NCs. The authors suggested that Ag\u2013Zn doped TiO2 NCs can be used to reduce azo-dyes, which can be a useful tool for treating wastwater from the textile industry.This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R11), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors express their gratitude to Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R11), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.", "descript": "\n                  A facile green synthesis route was employed for the fabrication of Ag\u2013Zn doped TiO2 nano-catalyst and utilized for the remediation of dyes. Nicotiana tabacum leaves extract was used to prepare the Ag\u2013Zn doped TiO2 nano-catalyst by utilizing TiO2 as precursor and AgNO3, ZnNO3 as doping agents. A strong UV\u2013Vis spectra peak confirmed the Ag\u2013Zn doped TiO2 formation. Furior transformed infrared spectroscopy analysis revealed the role of phenolics in the N. tabacum leaves extract for the formation of Ag\u2013Zn-doped TiO2 nano-catalyst. Moreover, Ag\u2013Zn doped TiO2 nano-catalyst formation was confirmed by X-ray diffraction analysis, which reveals the cubic and tetragonal geometries with average size of 5.66\u00a0nm. The scanning electron microscopy analysis also confirmed the poly-morphological structure of prepared Ag\u2013Zn doped TiO2 nano-catalyst. The nano-catalyst was used for the remediation of dyes and conditions were optimized for maximum removal of dyes using NaBH4 reducing agent. The catalytic activity results revealed that Ag\u2013Zn doped TiO2 showed promising features versus individual counterparts, which is correlated with reduced bandgap of doped nano-catalyst. The N. tabacum leaves extract efficiently stabilized Ag\u2013Zn doped TiO2 nano-catalyst that exhibited remarkable catalytic activity and have potential to treat the dyes in wastewater.\n               "}
{"full_text": "Steadily rising CO2 emission produced by human activities results in negative environmental consequences such as global warming and the increase of global mean sea level. It is imperative to reduce the emission of CO2. Various carbon capture and storage technologies have been developed for the reduction of CO2 emission and are employed to capture CO2 from abundant industrial sources such as fossil fuel-fired power plants. However, the availability of sufficient storage capacity is still an open question. Researchers have devoted efforts to developing more efficient approaches, which could employ CO2 to produce fuels, chemicals, and hydrocarbons [1\u20133]. The reverse water-gas shift (RWGS) reaction [4\u20137] has attracted increasing attention, especially high-temperature RWGS, which offers further CO-based process to methanol as well as long-chain hydrocarbons via Fischer-Tropsch synthesis.Various metals, including Cu, Fe, Ni, Pd, Pt, Rh, and Au, are active for the RWGS reaction. It is reported that Pd, Ni, and Cu show high catalytic activity with formate groups as an intermediate by combined in-situ FT-IR experiments and first principles [7]. Dai et\u00a0al. reported that CO2 RWGS reaction catalytic activities decrease in the order Ni/CeO2 > Cu/CeO2 > Co/CeO2 > Fe/CeO2 [8]. Konsolakis et\u00a0al. found that CO2 conversion followed the order: Co/CeO2\u00a0>\u00a0Cu/CeO2\u00a0>\u00a0CeO2 [9]. The activity can be affected by reaction condition, catalyst dispersion, particle size, surface morphology, and the nature of the oxide support [2,5,10\u201312], and thus there is no consensus on the activity trend of various metals. Catalysts screening of RWGS reaction under the consistent criterion is highly desired.Identify the reaction mechanism is essential to develop a more active and selective catalyst; thus, substantial efforts have been devoted to the mechanism investigation. Different reaction mechanisms have been proposed [12\u201315], for example, direct CO2 dissociation, COOH- and HCOO-mediated mechanism. The reaction mechanism depends on the specific catalyst and the reaction condition. DFT calculation found that direct CO2 dissociation is favorable on Rh, Ni, and Cu, while the COOH-mediated route is preferred on Pt and Pd [16]. The direct dissociate barriers can correlate to the oxygen adsorption strength, where the stronger adsorption of O provides a low CO2 dissociation barrier and results in the direct dissociation as a favorable route. CO2 dissociation can be followed by CO methanation reaction. Methane reaction is thermodynamically favored at low temperature, and high pressure [5], especially on Ru, Fe, Ni, Co, and Mo based catalyst [10]. CO methanation can either by CO-direct dissociation route or H-assisted route via HCO or COH, and it is generally accepted that the H-assisted path is more energetic favorable [17\u201319]. However, the results were mainly based on the analysis of DFT calculations performed at 0\u00a0K and 0\u00a0bar. It is essential to carry out a microkinetic analysis using temperature and pressure corrected free energy to identify the reaction mechanism and the rate-control steps at the realistic reaction.Microkinetic modeling based on DFT calculations is a powerful technology for the development of new or improved catalysts without intensive empirical testing. The models enable the incorporation of the fundamental catalytic surface chemistry into a kinetic model, and they can provide a fundamental understanding of reaction mechanism in addition to the prediction of activity and selectivity. Moreover, descriptor-based microkinetic modeling can correlate the activity to two simple descriptors and thus accelerate the catalyst screening. Herein, DFT calculations serve a tool to gain the adsorption energies and activation energies for the catalytic surface reaction among eight metals including Co (0001), Ru (0001), Fe (110), Ni (111), Cu (111), Rh (111), Pd (111) and Pt (111) surfaces. Microkinetic modeling of the RWGS reaction on each surface was carried out to identify the reaction pathway and rate-relevant steps. Descriptor-based microkinetic modeling on the eight metals was performed to predict the activity trend among various metals and achieve further catalyst screening, which could substantially contribute to the discovery of the RWGS catalysts.All DFT calculations were performed with the Vienna ab initio simulation package [20\u201322], where spin polarisation was employed for Fe, Ni and Co. The exchange-correlation functional was described by using Bayesian error estimation functional with van der Waals correlation (BEEF-vdW) [23]. The interaction between ion cores and valence electrons was described by the projected augmented wave (PAW) method [24], combined with the plane-wave expansion at a kinetic energy cut-off of 400\u00a0eV. M (111), M (110), and M (0001) surfaces were modeled by a p (3\u00a0\u00d7\u00a03) unit cell with five layers, and a vacuum of 12\u00a0\u00c5 is set between two periodic repeated slabs. The bottom two layers were fixed at their corresponding bulk positions during the optimization. A 5\u00a0\u00d7\u00a05\u00a0\u00d7\u00a01 k-point grid was used to describe the Brillouin zone. The transition state was located by Dimer method [25], and the vibrational frequencies were calculated to confirm the transition states with one negative mode corresponds to the desired reaction coordinates.The adsorption energy and activation energy were calculated as \n\n\nE\n\na\nd\ns\n\n\n=\n\nE\n\nA\n+\ns\nl\na\nb\n\n\n\u2212\n\nE\nA\n\n\u2212\n\nE\n\ns\nl\na\nb\n\n\n\n and \n\n\nE\na\n\n=\n\nE\n\nT\nS\n\n\n\u2212\n\nE\n\nI\nS\n\n\n,\n\n respectively, where E\n\nA\n is the total energy of the gas phase species, E\n\nslab\n is the total energy of the slab, E\n\nA+slab\n is the minimum total energy of molecule adsorbed on the slab. E\n\nTS\n is the total energy of the transition state, and E\n\nIS\n is the total energy of the initial state.The Gibbs free energy of each specie is calculated by using the following equation. \n\nG\n=\nE\n+\n\nE\n\nZ\nP\nE\n\n\n+\n\u0394\n\nH\no\n\n\n(\n\n0\n\u2192\nT\n\n)\n\n\u2212\nT\nS\n\n , where E refers to electric energy determined by DFT, E\n\nZPE\n is zero-point energy. H, S, and T are enthalpy, entropy, and temperature, respectively. The precise calculation methods for zero-point energy, entropy, and enthalpy of adsorbed species was based on the harmonic approximation, which has been reported in the literature [26,27]. The same vibrational frequencies are employed for all the metals based on the results from Pt (111) since the variations in zero-point energies for various metal surfaces are significantly smaller compared with the adsorption energies [28]. The Gibbs free energies of the gaseous species were calculated with the Shomate equation, where the corresponding Shomate constants were reported in the NIST WebBook [29].The microkinetic modeling was carried out in Catalysis Microkinetic Analysis Package (CATMAP) [30], which can generate the catalytic trend based on the descriptor-based microkinetic modeling and is suitable for catalysts screening [31,32]. Formation energies are inputs to the model, which were calculated with the total energies of gas-phase CH4, H2O, and H2 as references. The simulation is conducted at T\u00a0=\u00a0973\u00a0K and P\u00a0=\u00a01\u00a0atm with a H2/CO2 ratio of 3. High temperature chemical reactions have attracted growing attention for the next-generation energy conversion and storage processes [7]. RWGS reaction is thermodynamically favored by higher temperatures. Besides, the carbon formation by Boudouard reaction and methanation are disfavored at high temperatures. A H2/CO2 ratio of 3 was selected for stoichiometrically conversion CO2 to synthesis gas with a H2/CO ratio of 2, a typical ratio for methanol synthesis and Fischer-Tropsch synthesis. The reaction rates are generated by solving a mean-field model under the steady-state approximation. The differential equations in the microkinetic models are the following.\n\n\n\n\nr\ni\n\n=\n\n\nk\ni\n+\n\n\n\n\u220f\n\nj\n\n\n\u03b8\n\ni\nj\n\n\n\n\n\n\u220f\n\nj\n\n\nP\n\ni\nj\n\n\n\u2212\n\n\nk\ni\n\u2212\n\n\n\n\u220f\n\nl\n\n\n\u03b8\n\ni\nl\n\n\n\n\n\n\u220f\n\nl\n\n\nP\n\ni\nl\n\n\n\n\n\n\n\n\n\n\n\n\n\u2202\n\n\u03b8\ni\n\ns\n\n\n\u2202\nt\n\n\n=\n\u2211\n\n\ns\n\ni\nj\n\n\n\nr\nj\n\n\n\n\n\nwhere \n\n\nr\ni\n\n\n is the rate of each elementary step, \n\n\nk\ni\n+\n\n\n and \n\n\nk\ni\n\u2212\n\n\n are the forward and reverse rate constant, respectively. \n\n\n\u03b8\n\ni\nj\n\n\n\n and \n\n\n\u03b8\n\ni\nl\n\n\n\n are the site coverage of surface reactants and products, respectively, while \n\n\nP\n\ni\nj\n\n\n\n and \n\n\nP\n\ni\nl\n\n\n\n are the pressure of reactants and products, respectively. \n\n\ns\n\ni\nj\n\n\n\n is stoichiometry coefficients of species i in the elementary step j. \n\n\n\n\u2202\n\n\u03b8\ni\n\n\n\n\u2202\nt\n\n\n\n equals zero at the steady-state, and the sum of site converges is constrained to 1. The pre-exponential factors of all the adsorption steps were calculated by assuming the sticking coefficient equals 1 [33].Three different reaction mechanisms were reported [16], namely, direct CO2 dissociation mechanism, COOH-mediated mechanism, and the HCOO-mediated mechanism. However, the high stability of HCOO on the surface makes it a spectator rather than a reactive intermediate [34,35] and results in high barriers for the decomposition of HCOO [36]. Therefore, we considered two reaction pathways, that is direct dissociation and COOH-mediated reaction mechanism, as shown in Scheme 1\n. Methanation reaction occurs in addition to RWGS reaction, and a better catalyst should own higher RWGS activity and lower methanation activity. Thus, two methane formation pathways are taken into account, that is, CO direct dissociation and H-assisted CO to HCO and HCOH followed by dissociation to CH.The adsorption energies of various surface species on the eight metal surfaces (i.e., Co (0001), Ru (0001), Fe (110), Ni (111), Cu (111), Rh (111), Pd (111) and Pt (111) surfaces) are summarized in Table\u00a01\n. The adsorption configurations on Co(0001) have been reported in our previous publications [17,32]. C and O are firmly bound to the surfaces, indicating the high potential of carbonization and oxidation of the metals if it is not consumed efficiently in the reactions. Oxygen prefers to stick on Co (0001), Fe (110), Ru (0001), Ni (111) surfaces, while carbon more strongly binds to Fe (110), Ru (0001), Pt (111), and Rh (111). CO2, CH4, and H2O weakly adsorbed on the surfaces. Most metal surfaces display affinity to CO molecule with adsorption energies from \u22121.48\u00a0eV to \u22121.82\u00a0eV, except for Cu. The adsorption energy of CO on Cu is much lower than others, indicating that CO is more easily desorb from the copper surface rather than participates in the following methanation reactions, which may result in a high CO selectivity.The adsorption behavior of all the surface species among different metals can be roughly divided into two groups, namely carbon-based (C, CO, CH, CH2, CH3, and COOH) and oxygen-based species (O and OH), as illustrated in Fig.\u00a01\n. The adsorption energies of C, CH, CH2, and CH3 among metal surfaces follow a similar trend, where the adsorption becomes weaker in the sequence of Fe, Ru, Rh, Pt, Pd, Co, Ni, and Cu. The pattern of adsorption energies of CO and COOH slightly deviates from the CHX species. The adsorption strength of O and OH on different metals follows a similar trend.\nFig.\u00a02\n illustrates that the adsorption energies of CH, CH2, CH3, CH4, CO, and COOH are correlated to the adsorption energy of C on the various surface since these carbon-based species bind with the metal surface via carbon. The adsorption energies of OH and H2O can be correlated to the adsorption energy of O. It indicates the adsorption energies can be represented by two descriptors, such as adsorption energies of C and O [37].\nTable\u00a02\n summarizes the forward reaction barriers of elementary steps in the RWGS reactions on the eight metal surfaces (i.e., Co (0001), Ru (0001), Fe (110), Ni (111), Cu (111), Rh (111), Pd (111) and Pt (111) surfaces). Hydrogen gas easily dissociates on the most surfaces except for Cu. CO2 direct dissociation shows barriers smaller than 1.60\u00a0eV, indicating CO2 tends to dissociate on Co (0001), Ru (0001), Fe (110), Ni (111), Cu (111), and Rh (111), compared to the hydrogenation of CO2, as illustrated in Fig.\u00a03\na. The direct CO2 dissociation barriers on various metal surfaces can be correlated to the adsorption energy of oxygen, as illustrated in Fig.\u00a04\na, which is consistent with the previous report [16]. The difference between CO2 direct dissociation and CO2 hydrogenation can also be connected to the oxygen adsorption energy. It indicates the surface with higher oxygen-binding strength tends to direct dissociation rather than hydrogenation. The hydrogenation of CO2 can activate the CO\u2013O(H) bond and decrease the CO\u2013O(H) bond dissociation barrier on Co (0001), Ni (111), Cu (111), Rh (111), Pd (111), and Pt (111) surfaces compared with the direct dissociation. It is difficult to conclude which is the dominating reaction pathway of CO2 activation based solely on the comparison of the reaction barriers. Thus, microkinetic modeling is necessary to be carried out to figure out the reaction mechanism.The barriers of elementary steps in the two CO methanation pathways are compared in Fig.\u00a03b. CO direct dissociation (the green column) exhibits high barriers on all the surfaces, and the barriers are higher than 3\u00a0eV for the precious metals Pd, Pt, and Cu. Similarly, CO direct dissociation barriers can be correlated to the oxygen adsorption energy, as displayed in Fig.\u00a04a. CO hydrogenation presents lower barriers compared to the direct CO dissociation. The barrier difference between CO hydrogenation and direct dissociation can also be correlated to the oxygen adsorption energy, as illustrated in Fig.\u00a04b. C\u2013O bond dissociation in HCO needs a lower barrier than the direct CO dissociation, and HCO hydrogenation to HCOH can further decrease the C\u2013O bond cleavage barrier. It seems that hydrogen-assisted CO dissociation to methane is energetically favorable on all the metal surfaces. However, the coverage of surface species also plays an essential role in the determination of reaction rates. Thus, the hypothesis needs further validation by microkinetic modeling.We performed microkinetic modeling of each metal surface to identify the reaction mechanism. The reaction rate of each elementary step is summarized in Table\u00a03\n. The reaction rate of CO2 direct dissociation is largely higher than COOH-mediated dissociation on Co, Fe, Ru, Rh, Cu, and Ni, while it is just one order of magnitude higher on Pt and Pd. Therefore, we can deduce that the direct CO2 dissociation is the dominating route on Co, Fe, Ru, Rh, Cu, and Ni, while the two pathways are competing on Pt and Pd.As we discussed in the last section, CO direct dissociation seems impossible to occur due to extremely high barriers. Surprisingly, their reaction rate of direct CO dissociation is at the same order of magnitude, or just one order of magnitude smaller than the H-assisted dissociation via HCO on Co, Fe, Pt, Pd, Ru, and Rh. It indicates the two reaction pathways for CO methanation compete to occur on these surfaces. Moreover, the HCOH dissociation to CH even owns a lower reaction rate than the direct CO dissociation on Co, Fe, Pt, Pd, Ru, and Rh, despite that the barrier of HCOH to CH is much lower, which may be due to the low coverage of hydrogen. The hydrogen-assisted pathway is the dominating route on Cu and Ni surfaces.Degree of rate control analysis is a powerful tool to identify the influential transition states, and thus a higher reaction rate can be achieved by adjusting their energies [38,39]. The degree of rate control of each intermediate and transition state to the CO2 consumption rate and the products (CO and CH4) generation rate were calculated based on the following equation. X\n\nij\n\u00a0is the degree of rate control matrix,\u00a0r\n\ni\n\u00a0is the rate of production for product\u00a0i,\u00a0G\n\nj\n\u00a0is the free energy of species\u00a0j,\u00a0k\u00a0is Boltzmann's constant, and\u00a0T\u00a0is the temperature.\n\n\n\n\nX\n\ni\nj\n\n\n=\n\n\nd\nl\no\ng\n\n(\n\nr\ni\n\n)\n\n\n\nd\n\n(\n\u2212\n\nG\nj\n\n/\nk\nT\n)\n\n\n\n\n\n\n\nWe summarized the most influential transition states and intermediates in Table\u00a04\n. The same rate-determining steps are observed for the CO2 conversation rate and CO production rate. It is elucidated that H\u2013OH or CO\u2013O is the rate-determining transition state, despite that the exact value varies on various metals. The eights metals can be divided into two groups based on the rate-determining step. CO\u2013O bond cleavage is rate-determining on Pt, Pd, and Cu, owing to the high barrier on the surfaces, while OH binding with H to H2O is rate-determining on Co, Fe, Ru, Ni, and Rh. A negative degree of rate control of CO2 binding energy is found on Co, Fe, and Ru, while O on Ru and Ni. The negative value indicates that decrease the adsorption stability of the surface species could increase the activity.Additional rate-determining states or intermediates can affect the methane formation rate, in addition to the same rate-determining step with CO2 consumption rate. CO desorption has a negative effect on the methane formation on Co, Pt, and Cu, while O on Co, and Fe. CH3\u2013H bond cleavage is the rate-relevant one for Co, Fe, and Ru, which is attributed to the higher barrier of the step. HC-OH bond cleavage is rate-controlling for Pt and Pd, HC-O for Rh and Cu, and CH2\u2013H for Ni. These specific rate-relevant steps for methane is also the rate-controlling steps for methane selectivity. Tuning the energies for the particular rate-relevant transition states and intermediates can modify methane selectivity.The descriptor-based microkinetic modeling is a versatile tool to predict the catalyst activity trend and achieve catalyst screening. As we discussed, the adsorption energies of different surface species can be correlated to the carbon or oxygen binding energy. Thus, we employed the formation energy of C\u2217 and O\u2217 as two descriptors to describe the reaction kinetics of RWGS. Br\u00f8nsted-Evans-Polanyi [40,41] relations were employed for transition-state scaling relations of all the steps except CO2 and CO dissociation. CO2 and CO dissociation are correlated to the final state in our setting since the barrier can be connected to the oxygen binding energy, as we discussed above.O and H are the abundant surface species at the reaction condition 973 K, as displayed in Fig.\u00a05\n. Talin et\u00a0al. reported that the most abundant surface species are CO and H at 500 K [13]. The contrary is due to the modelings were performed at different temperatures. CO desorption becomes much easier compared to the CO dissociation or hydrogenation at high temperatures. Fe, Co, Ru, and Ni are covered by oxygen, indicating these surfaces are oxidized at this reaction condition, which is attributed to the strong bonding between oxygen and metal and the high barrier of oxygen hydrogenation on these surfaces.The activity of CO formation is in the sequence of Rh\u223cNi >\u00a0Pt\u223cPd>\u00a0Cu\u00a0>\u00a0Co\u00a0>\u00a0Ru\u00a0>\u00a0Fe, which is consistent with the experimental result from Dai et\u00a0al. [8]. They reported that RWGS reaction catalytic activities are ranked as follows: Ni\u2013CeO2\u00a0>\u00a0Cu\u2013CeO2\u00a0>\u00a0Co\u2013CeO2\u00a0>\u00a0Fe\u2013CeO2. The turnover frequency of methane formation is many orders of magnitude smaller than CO formation, and the activity trend of CH4 formation is in the sequence of Rh\u223cNi > Pt\u223cPd\u223cCo\u00a0> Cu\u00a0> Ru\u00a0>\u00a0Fe.We can find that CO selectivity is almost 100% for all the metals at high temperatures, which agrees with the experimental result performed at high temperatures [7]. They reported that CO selectivity of Pd and Cu achieve 100% CO selectivity at 973 K with H2/CO\u00a0=\u00a03, while it is slightly lower than 100% on Ni. Increasing the temperature or decreasing the H2/CO ratio can achieve 100% CO selectivity on Ni. Moreover, we calculated the ratio between the CO formation rate and methane formation rate (CO/CH4) and plotted the descriptor-based ratio mapping. CO/CH4 ratio selectivity ranks as follows: Cu\u223cFe\u00a0>\u00a0Ru\u223cPt\u223cPd\u00a0>\u00a0Co\u00a0>\u00a0Ni\u00a0>\u00a0Rh, which is consistent with the experimental results. Chen et\u00a0al. found that the trend of CO/CH4 ratio is Pt\u00a0>\u00a0Co\u00a0>\u00a0Ni. [42].\nFig.\u00a06\n demonstrated that the most active catalysts own the carbon formation energy and the oxygen formation energy around 1.64 and 0.24\u00a0eV, respectively. The interpolation concept of adsorption energy was used to search for potentially interesting bimetallic catalysts [43,44]. We performed DFT calculations to get the carbon and oxygen formation energies on hundreds of A3B type bimetal terrace surfaces, where M and N represent metals. As we found above, the methane selectivity is very low for all the metals. Thus only the energies close to the predicted optimum carbon and oxygen formation energies are interesting to us, as shown in Fig.\u00a07\n. We identified potential bimetals with high activity, that is, Cu3Ni, Ir3Sn, Pd3Co, Pt3Co, Pt3Rh, Pt3Sc, and Rh3 (Sc/Ga/Ge/In/Ir/Ni/Zn). PtCo has been reported to be highly active for the RWGS in the literature [42,45]. Cu3Ni is identified as to allow cost and highly active catalysts, which has recently been demonstrated by Xiao and coworkers [46]. The other catalysts need further experimental validation.The adsorption behavior of all the surface species on different metals can be split into two groups, carbon-based and oxygen-based species. Each group follows a similar trend among metals, which indicates that two descriptors can represent the adsorption energies of various species. It is difficult to identify the dominating route for CO2 dissociation to CO by solely comparing the reaction barriers. In contrast, hydrogen-assisted CO dissociation to methane is energetically favorable on all the metal surfaces.The microkinetic modeling suggested that the direct CO2 dissociation is the favorable pathway on Co, Fe, Ru, Rh, Cu, and Ni, while it competes with the COOH-mediated route on Pt and Pd. CO direct dissociation and H-assisted pathways for CO methanation compete to occur on Co, Fe, Pt, Pd, Ru, and Rh, despite that the high barriers of CO direct dissociation. It demonstrates that it is appropriate to identify the reaction mechanism by performing microkinetic modeling rather than exclusively comparing the reaction barrier.The degree of rate control analysis demonstrates that the rate-determining step varies on different surfaces. CO\u2013O bond cleavage is the rate-determining on Pt, Pd, and Cu, while OH binding with H to H2O is rate-determining on Co, Fe, Ru, Ni, and Rh. Methane formation has an additional rate-controlling step, which is CH3\u2013H bond cleavage for Co, Fe, and Ru, HC-OH for Pt and Pd, HC-O for Rh and Cu, and CH2\u2013H for Ni. The methane selectivity can be hindered by adjusting the surface properties to increase the barrier of the rate-determining step for methanation.Two-dimensional volcano plots were constructed by coupling the scaling relations in a microkinetic model using C- and O- formation energy as descriptors. The microkinetic modeling elucidates that the activity trend of CO formation is in the sequence of Rh\u223cNi\u00a0>\u00a0Pt\u223cPd\u00a0> Cu\u00a0>\u00a0Co\u00a0>\u00a0Ru\u00a0>\u00a0Fe, which agrees with the reported experimental results. Moreover, the model suggests that Fe, Co, Ru, and Ni tend to be oxidized at the reaction condition. We also constructed volcano plots of the ratio of CO/CH4 as a function of the two descriptors. The two-dimensional volcano plots were used to search for new alloy catalysts of high activity and low selectivity to methane, based on the interpolation concept of adsorption energy. As a result, several bimetallic catalysts were identified to be potentially interesting catalyst materials, where the Cu3Ni was screened as a candidate with a low cost and high activity.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The financial support from the Centre for Industrial Catalysis Science and Innovation (iCSI), which receives financial support from the NO-237922. The Research Council of Norway, is gratefully acknowledged.", "descript": "\n                  Reverse water gas shift (RWGS) catalysis, a prominent technology for converting CO2 to CO, is emerging to meet the growing demand of global environment. However, the fundamental understanding of the reaction mechanism is hindered by the complex nature of the reaction. Herein, microkinetic modeling of RWGS on different metals (i.e., Co, Ru, Fe, Ni, Cu, Rh, Pd, and Pt) was performed based on the DFT results to provide the mechanistic insights and achieve the catalyst screening. Adsorption energies of the carbon-based species and the oxygen-based species can be correlated to the adsorption energy of carbon and oxygen, respectively. Moreover, oxygen adsorption energy is an excellent descriptor for the barrier of CO2 and CO direct dissociation and the difference in reaction barrier between CO2 (or CO) dissociation and hydrogenation. The reaction mechanism varies on various metals. Direct CO2 dissociation is the dominating route on Co, Fe, Ru, Rh, Cu, and Ni, while it competes with the COOH-mediated path on Pt and Pd surface. The eights metals can be divided into two groups based on the degree of rate control analysis for CO production, where CO\u2013O bond cleavage is rate relevant on Pt, Pd, and Cu, and OH\u2013H binding is rate-controlling on Co, Fe, Ru, Ni, and Rh. Both CO-direct dissociation and hydrogen-assisted route to CH4 contribute to the methane formation on Co, Fe, Pt, Pd, Ru, and Rh, despite the significant barrier difference between the two routes. Besides, the specific rate-relevant transition states and intermediates are suggested for methane formation, and thus, the selectivity can be tuned by adjusting the energy. The descriptor (C- and O- formation energy) based microkinetic modeling proposed that the activity trend is Rh~Ni\u00a0>\u00a0Pt~Pd\u00a0>\u00a0Cu\u00a0>\u00a0Co\u00a0>\u00a0Ru\u00a0>\u00a0Fe, where Fe, Co, Ru, and Ni tends to be oxidized. The predicted activity trend is well consistent with those obtained experimentally. The interpolation concept of adsorption energy was used to identify bimetallic materials for highly active catalysts for RWGS.\n               "}
{"full_text": "Electrochemical reduction of CO2 to generate valuable chemical feedstocks and fuels is considered as one of the most promising technologies not only to reduce the rapid increasing atmospheric CO2 concentration but also to mitigate the serious fossil fuel shortage.\n1\u20134\n Furthermore, the electrochemical processes could be powered by those renewable energies that suffer from unpredictable and intermittent supply, for example solar, wind and tidal electricity. Compared with other CO2 reduction technologies, electrochemical reduction exhibits some unique advantages such as simple operation, free H source, controllable products, compact modules, etc.\n5\u20138\n The performance and economic feasibility of CO2RR can be efficiently evaluated in terms of its overpotential, activity (current density), selectivity and stability, which highly depend on the catalysts used. Therefore, most of researches focus on designing and fabricating high-performance catalysts.Traditional heterogeneous catalysts can be used for CO2RR, including metals,\n9\u201311\n metal compounds (oxides,\n12\n\n,\n\n13\n sulfides,\n14\u201318\n etc), MOFs\n19\u201321\n and their composites.\n22\n Although much progress has been made in the aspect of low overpotential,\n21\n\n,\n\n23\n near-unity selectivity,\n24\u201327\n large current density,\n28\u201332\n high turnover frequency (TOF)\n33\n\n,\n\n34\n and long-term stability.\n22\n\n,\n\n35\u201337\n The inexplicit active sites and limited atom utilization of bulk catalysts greatly hinder the in-depth understanding of the reaction process and the fabrication of electrocatalysts with high performances. Fortunately, single atom catalysts (SACs) have been developed since Pt1/FeO\nx\n was reported by Zhang et\u00a0al., in 2011,\n38\n which rapidly attracted global attention due to their high activity and maximum atom utilization efficiency.\n39\n\n,\n\n40\n Additionally, due to the relative simple coordination configuration of SACs, their active sites for CO2RR can be clearly identified, which can be efficiently modulated through tuning coordination environment for the selective adsorption of certain reactants/intermediates. SACs could also promote the electrochemical reduction of CO2 through suppressing the competitive hydrogen evolution reaction (HER).\n41\u201344\n\n\nFig.\u00a01\n shows the elements distribution which have been used to construct SACs for CO2RR. The color filled elements represent the center active sites, including transition metals (Mn, Fe, Co, Ni, Cu, Zn \u2026), noble metals (Pd, Ag, Ir \u2026) and main-group metals (In, Sn, Sb, Bi \u2026). Although most of SACs selectively reduce CO2 to CO, other products (HCOOH, CH3OH, CH4 and C2+) can also be generated over some SACs.\n5\n The dominant CO2RR products are marked by different colors as shown in Fig.\u00a01.Many previous literatures have summarized the progress and advances of SACs in the field of CO2RR. Non-precious metal based SACs could efficiently decrease the cost of catalysts, and was reviewed by Li et\u00a0al. recently.\n45\n Xu's review article in 2021 focused on the 3d transition metal based SACs,\n46\n which are recognized as the most promising candidates for CO2RR. Among them, Ni SACs are known as the earliest and widest studied SACs for CO2RR, which was discussed by Yadav et\u00a0al.\n47\n Besides central metal sites, modulating coordination environment as an efficient strategy to achieve better performance, was summarized by Wang and co-works.\n48\n Also, dual-atom catalysts were systematically summarized and reviewed by An et\u00a0al.\n49\n In addition to the common CO produced by SACs, hydrocarbons and alcohols generated through multi-electron process were also discussed in detail.\n50\n Despite of many previous reviews on SACs for CO2RR, one that summarizes latest progress on different metal sites, coordination environments and further dual-atom catalysts is highly needed. In this review, the possible CO2RR pathways for the formation of various products were firstly summarized, followed by the recent progress of SACs for CO2RR in terms of different metal centers including transition metals, noble metals and main-group metals. Then, introducing heteroatom as the most popular coordination modulation strategy was summarized in terms of different elements (vacancies, O, S, P and halogens). The dual atom catalysts applied in CO2RR were also introduced as the case of extended SACs, which can provide additional atomic active sites for the adsorption of intermediates, realizing the formation of products beyond C1. Finally, the existing issues and possible solutions of SACs for CO2RR were discussed. Fig.\u00a02\n summarizes the main contents of this review. Table\u00a01\n lists the CO2RR performances of recent SACs.Due to the discrete active site in SACs, C1 products are preferred in CO2RR over SACs. Therefore, in this section, we mainly focus on the reaction pathways for the generation of C1 products including carbon monoxide, formic acid (formate), methanol and methane, and give a brief introduction of the formation of C2+ products.The electrochemical reduction of CO2 to CO through \u2217COOH intermediate is now widely-accepted. Due to the moderate adsorption energy of \u2217COOH on the active sites of SACs, CO is more preferred for most SACs. CO has important applications in both Fischer-Tropsch process and water-gas shift reaction.\n51\u201357\n The possible reaction pathways for CO is shown in Fig.\u00a03\na. Typically, gaseous CO2 undergoes a proton-coupled electron transfer (PCET) step to directly generate \u2217COOH intermediate. Afterwards, \u2217COOH can further convert to \u2217CO through another PCET step, simultaneously releasing one water molecule. Finally, the weak-bonded \u2217CO desorbs from the catalyst surface, forming gaseous CO product. On the other hand, \u2217COOH intermediate can also be generated through proton-decoupled electron transfer step. Specifically, gaseous CO2 firstly forms \u2217CO2\n\u2022\u2212 through one electron transfer. Then, a protonation process takes place, where a proton from self-ionization of water attacks \u2217CO2\n\u2022\u2212 to form \u2217COOH. Tafel slope can be used to study the rate-determining step (RDS), which relates to the overpotential vs. partial current density for specific product. A Tafel slope around 118\u00a0mV dec\u22121 indicates the initial CO2 activation to from \u2217CO2\n\u2022\u2212 as the RDS, while it turns to following one electron transfer step for the Tafel slope around 59\u00a0mV dec\u22121.Formic acid has wide applications in tanning, textile and pharmaceutical industries as well as the hydrogen carrier in fuel cell.\n58\u201360\n During the formation of formic acid, the \u2217CO2\n\u2022\u2212 species is firstly generated through one electron activation of gaseous CO2. It is worth noting that bidentate mode (two oxygen atoms bond on the catalyst's surface) is not applicable for most of SACs due to the lack of adjacent active sites. Instead, \u2217CO2\n\u2022\u2212 species prefers to bond with SACs through one carbon atom (monodentate mode), which undergoes a protonation process to form \u2217OCHO intermediate. Similarly, gaseous CO2 can be directly converted to the same intermediate (\u2217OCHO) through PCET step. As an alternative process, \u2217OCHO can also be generated through the attack and insertion of CO2 molecule on \u2217H species. At last, \u2217OCHO intermediate will convert to HCOOH or HCOO\u2212 through another PCET step.Methanol is considered as a promising fuel due to its high energy density (4.8\u00a0kWh L-1) and easy storage/transportation. Through above-mentioned CO2 activation and following PCET or proton-decoupled electron transfer step, \u2217CO intermediate can be formed. If the adsorption energy of \u2217CO is high enough, it will not desorb from catalyst's surface but further undergo multiple PCET steps to form CH3OH.Following the generation of methanol, \u2217OCH3 can undergo an additional PCET step instead of desorbing from the surface of catalyst to form CH3OH, where the surface adsorbed oxygen atom can be quenched through protonation process. As a result, CH4 can be generated and desorbed from the catalyst's surface. Alternatively, \u2217COH intermediate can transform to adsorbed carbon species (\u2217C) through the protonation process and simultaneous release of water. Followed by continuous four PCET steps, CH4 can also be generated.Although most C2+ products have higher market prices and more specific industrial applications, their selectivities in CO2RR are always not as high as the C1 counterparts. C\u2013C coupling or dimerization is crucial for the formation of C2+ products, which requires adjacent active sites. However, except for some dual atom catalysts, most SACs can only provide discrete active sites, making the generation of C2+ products very difficult.As one of the most important chemical feedstocks, ethylene (CH2\nCH2) can be formed through either dimerization of two neighboring \u2217CH2 species or direct PCET to ethylene oxide species (\u2217OCHCH2). Both of the two reaction pathways need to compete with the formation of ethanol (CH3CH2OH) through the insertion of adjacent \u2217CO species into catalyst-C bond of \u2217CH2 intermediate or direct protonation of oxygen sites in \u2217OCH2CH2 intermediate.Transition metals have shown impressive catalytic performances not only for CO2RR but also for many other important catalytic processes,\n61\u201376\n making them ideal candidates for replacing expensive noble metal catalysts that are applied in industry. For example, Huang et\u00a0al. reported a general method to synthesize a series of transition metal SACs (Fe, Co, Ni) for oxygen evolution reaction (OER). The identical M-N4-C4 configuration was confirmed, which offered ideal platform for quantitatively building up the relationship between metal center and catalytic properties. The theoretical and experimental results demonstrated Ni-NHGF as a high active and stable OER catalyst.\n77\n The excellent performances can be attributed to their abundant d electrons and unoccupied orbitals, which make them easy to coordinate with other species. Also, the d band can be efficiently tuned through many approaches, resulting in tunable adsorption energy towards reactants, intermediates and products. As for SACs, transitional metals can coordinate with surrounding atoms through abundant configurations due to their multiple valence states. Furthermore, other coordination elements beyond N can also be introduced into such SACs, endowing more favorable electronic structure for CO2RR.Ni SAC is one of the most widely studied SAC for CO2RR.\n78\n Ni SAC with Ni loading of 1.53\u00a0wt% can be easily synthesized through the ion exchange between adsorbed Ni2+ and Zn2+ nodes in ZIF-8, followed by pyrolysis (Fig.\u00a04\na).\n79\n Compared with Ni2+, Zn nodes are easy to evaporate during pyrolysis process, leading to defective N\u2013C support.\n80\n Subsequently, neighboring Ni2+ ions tend to occupy those sites, which is protected by surrounding N atoms from aggregation and further reduced by carbon atoms. The atomically dispersed Ni atoms were confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) (Fig.\u00a04b and c), while the dominant Ni\u2013N coordination and absent Ni\u2013Ni path were observed through Fourier transformed k3-weighted \u03c7(k) function of the EXAFS (extended X-ray absorption fine structure) spectrum (Fig.\u00a04d). The maximum Faradaic efficiency (FE) toward CO (up to 71.9%) was achieved at \u22120.9\u00a0V vs. RHE, while the partial CO current density could reach up to 7.37\u00a0mA\u00a0cm\u22122 with a high TOF of 5273 h\u22121. Xie et\u00a0al.\n81\n also successfully synthesized Ni\u2013N4 sites (1.41\u00a0wt%) via topo-chemical transformation method. Due to the coating of carbon layer, Ni\u2013N4 sties could be well-preserved from agglomeration, which showed high CO FE over 90% across a wide potential range from \u22120.5 to \u22120.9\u00a0V vs. RHE (Fig.\u00a04e and f). In 2018, Yang et\u00a0al.\n82\n prepared atomically dispersed monovalent Ni (I) on nitrogenated graphene (2.8\u00a0wt%) through facile pyrolysis of a mixture containing amino acid, melamine and nickel acetate in Ar atmosphere. The HAADF-STEM image of the as-prepared A-Ni-NG was presented in Fig.\u00a04g, indicating atomically dispersed Ni sites. As shown in Fig.\u00a04h, under a mild overpotential of 0.61\u00a0V, the A-Ni-NG displayed impressive CO2RR performance, including high specific current of 350 A gcatalyst\n\u22121, CO FE of 97% and TOF of 14800 h\u22121. Besides, after continuous reaction for 100\u00a0h, the Faradaic efficiency could still remain 98% of its initial value, showing excellent stability. The Ni(I)\u2013N4 with d9 electronic configuration was demonstrated as the real active sites for the electrochemical reduction of CO2 through operando X-ray absorption spectroscopy (XAS, Fig, 4i and j) and near ambient pressure X-ray photoelectron spectroscopy (XPS, Fig, 4k and l). Under CO2-saturated electrolyte, a positive shift toward higher energy was observed in Ni K-edge XANES spectra, which can be attributed to the delocalization of unpaired \n\n3\n\nd\n\n\nx\n2\n\n\u2212\n\ny\n2\n\n\n\n\n electrons and charge transfer from Ni(I) site to C 2p orbital of adsorbed CO2 molecule, forming \n\n\nCO\n2\n\n\u03b4\n\u2212\n\n\n\n species. This was also evidenced by the existence of \n\n\nCO\n2\n\n\u03b4\n\u2212\n\n\n\n species in deconvoluted O 1s spectrum of A-Ni-NG induced by CO2 adsorption. Additionally, as shown in Fig.\u00a04l, the decreased Ni 3d density of states and down-shift of valence band (VB) edge could also be attributed to the formation of \n\n\nCO\n2\n\n\u03b4\n\u2212\n\n\n\n species. This work not only clearly revealed the real active sites of Ni SAC and the reaction mechanism of CO2RR, but also realized impressive catalytic performance, therefore guiding the following numerous researches targeted at Ni SACs.As displayed in Fig.\u00a05\na, multiwalled carbon nanotubes (MWCNTs) confined Ni particles could be successfully converted to thermal-stable Ni\u2013N3 SAC through facile pyrolysis treatment.\n83\n In addition to use commercial MWCNT as support, the internal residual Ni particles could serve as the Ni source and be converted to Ni single atoms during the pyrolysis process, showing the extreme simplicity of such synthesis strategy. The favorable \u2217COOH formation and exothermic CO desorption were demonstrated by DFT calculations as shown in Fig.\u00a05b, which accounted for the excellent CO2RR performance with CO selectivity over 90% and TOF of 12000 h\u22121. Using ZnO as the self-sacrificial hard template and Zn source, ZnO@ZIF-NiZn could be fabricated by introducing Ni(NO3)2 and 2-methylimidazole, which could be further converted to NiSA/N-CNT after carbonization treatment. The superb CO2RR performance of NiSA/N-CNT (CO FE of 98% and TOF of 9366 h\u22121) was attributed to the reduced energy barrier.\n84\n As presented in Fig.\u00a05d, Lu et\u00a0al.\n85\n developed an unsaturated Ni\u2013N2/C SAC (0.45\u00a0wt%) for CO2RR through an interesting CO2-to-carbon strategy. Specifically, ethylenediamine (EDA) could form Ni-coordinated compounds through simultaneous Ni2+ chelation and CO2 adsorption, which were then added with excess Mg powder to produce metal-organic complex. By directly igniting such flammable Mg-containing complex in air, the violent combustion could reach 1000\u00a0\u00b0C for 30\u00a0s, during which single Ni atoms were anchored onto N-doped carbon matrix, forming NiN/C SAC. The impressive catalytic performance (100% CO selectivity @ 51\u00a0mA\u00a0cm\u22122) could be explained by the formation of unpaired 3d electrons resulting from rich vacancies as shown in Fig.\u00a05e, which effectively decreased the energy barrier for the formation of \u2217COOH (Fig.\u00a05f). Polydopamine (PDA) was also used to assist the atomic dispersion of Ni2+ through chelating effect.\n86\n After pyrolysis at 1000\u00a0\u00b0C under N2 atmosphere, PDA anchored Ni2+ was stabilized by N atoms and then reduced by the surrounding carbon, while g-C3N4 decomposed to form nitrogen doped carbon matrix. Considering that high temperature pyrolysis (above 900\u00a0\u00b0C) was always needed to prepare Ni SACs, He et\u00a0al.\n87\n developed a low temperature (450\u00a0\u00b0C) strategy to synthesize Ni nanoparticle wrapped by Ni\u2013N SA/C (Ni-NC@Ni), which could reduce the energy consumption during the preparation of SACs. NiO was firstly grown onto carbon fiber paper through hydrothermal and annealing process, which was then mixed with urea and annealed at 450\u00a0\u00b0C to finally obtain Ni-NC@Ni. Although several literatures have observed similar good performance of CO2RR to CO by nitrogen doped carbon encapsulated Ni particles,\n88\n Liang et\u00a0al. synthesized and studied several Ni/N\u2013C catalysts, which demonstrated the pristine high activity of single Ni sites during the conversion of CO2 to CO. In order to further elucidate the complicated CO2RR mechanism, Liu et\u00a0al.\n89\n designed and fabricated a model Ni SAC (0.27\u00a0wt%) through the synthesis of well-defined molecular Ni-TAPc and subsequent anchoring to CNTs by C\u2013C coupling (Fig.\u00a05g). TEM (Fig.\u00a05h) and HAADF-STEM (Fig.\u00a05i) images demonstrated the morphology and atomically dispersed Ni atoms. Such Ni-CNT-CC was composed of precise NiN4 moiety, which provided a model system for the investigation of CO2RR mechanism. In Ni K-edge XANES, the negative shift (0.3\u00a0eV) of rising edge was observed under cathodic potentials (Fig.\u00a05j and k), demonstrating the in-situ reduction of Ni2+ to Ni+, which was also confirmed by the negative shift of Ni\u2013N vibration band in operando Raman spectra (Fig.\u00a05l\u2013o). The in-situ generated low-valence Ni+\u00a0could efficiently activate CO2 molecule through donating long pair of electrons, which accounted for the high activity and selectivity (near 100% CO FE and high TOF of 100179 h\u22121) of Ni SAC in CO2RR. In addition to experimental studies, theoretical approaches were also carried out to investigate the catalytic performances of Ni SACs. Hossain et\u00a0al. predicted the onset potential, Tafel slope, FE and TOF of CO2RR over different Ni SACs (Ni\u2013N4, Ni\u2013N3C1 and Ni\u2013N2C2) under different potentials using recently developed grand canonical potential kinetics formulation (GCP-K) to calculate the kinetics step by step.\n90\n Compared with traditional Butler-Volmer kinetics, GCP-K allowed continuously changing the transition states geometry and charge transfer during the reaction coordinates, which gave more accurate and convincing prediction of the interaction between active sites and adsorbed species as well as transition states (Fig.\u00a06\na). It turned out that the most positive onset potential (\u22120.84\u00a0V vs. RHE) could be achieved on the Ni\u2013N2C2 site, while the Tafel slope, FE for CO and TOF were calculated to be 52\u00a0mV dec\u22121, 98% and 3903 h\u22121, respectively, which agreed well with the experimental results.In addition to widely-used graphene or CNT matrix, Ni single atoms could also be anchored onto commercial carbon black with much lower price.\n91\n The abundant defects and oxygen-containing functional groups (\u2013OH, \u2013COOH \u2026) on carbon black ensure its easy adsorption of Ni2+ ions. After annealing Ni2+-adsorbed carbon black (Ni2+-CB) and urea, gram scale NiSA-NCB could be successfully prepared (Fig.\u00a06b). Tested in a gas-phase reactor, a high current density over 100\u00a0mA\u00a0cm\u22122 with close to 100% CO selectivity was recorded. The large electrode with 10\u00a0\u00d7\u00a010\u00a0cm2 as shown in Fig.\u00a06c delivered an extremely high current above 8 A (Fig.\u00a06d) and CO generation rate of 3.34\u00a0L\u00a0h\u22121 (Fig.\u00a06e), which greatly promoted the practical application of electrochemical CO2RR. Membrane electrode assembly (MEA) could be fabricated by depositing the as-prepared Ni SAC between the gas diffusion layer (GDL) and ion exchange membrane (IEM),\n92\n which could deliver very high current density beyond 300\u00a0mA\u00a0cm\u22122 with nearly 100% CO selectivity. Thanks to the satisfactory mechanical strength and gas permeability, carbon paper has been widely used as the support and current collector for CO2RR, which can also be anchored with Ni single atoms to fabricate self-standing and binder-free electrode.\n93\n As presented in Fig.\u00a06f, after acid activation followed by Ni2+ adsorption and pyrolysis, Ni\u2013N3S SAC configuration with loading of 1.04\u00a0wt% could be successfully fabricated, which exhibited optimal CO selectivity of 91% at an overpotential of 660\u00a0mV (Fig.\u00a06g).As one of the most abundant elements on earth, iron has also been widely used to synthesize SACs for CO2RR. Li et\u00a0al.\n94\n synthesized the Fe-NC-S through the ion exchange between Fe2+ and ZIF-8, followed by pyrolysis process. Demonstrated by operando 57Fe M\u00f6ssbauer spectroscopy and XAS, the in-situ generation of low-spin Fe (I) coordinated with four pyrrolic nitrogen (LS FeIN4) was confirmed (Fig.\u00a07\na and b). Fig.\u00a07c displays the operando XAS results. Compared with clear negative shift under cathodic potential in Ar-saturated electrolyte, the unchanged spectra in CO2-saturated electrolyte indicated electron transfer from Fe site to C 2p orbital of CO2 to form CO2\n\u03b4\u2212 species. Further DFT calculations revealed the optimal binding energy of CO2 on such LS FeIN4 site when compared with other Fe\u2013N\u2013C sites. On the other hand, the single-occupied 3dz2 orbital of LS Fe (I) could strongly interact with single-occupied \u03c0\u2217 orbital of key intermediate \u2217COOH (Fig.\u00a07d), which accounted for the excellent CO2RR activity of such Fe\u2013N\u2013C. It was found that the final intermediate \u2217CO tended to strongly bind with Fe\u2013N4 site, which limited the desorption of CO. Such Fe2+ doped ZIF-8 followed by pyrolysis strategy was also adopted by Gu et\u00a0al.\n95\n Operando XAS results demonstrated the discrete Fe3+ ions in Fe\u2013N\u2013C, which could maintain +3 oxidation state during electrochemical reduction reaction due to electronic coupling with carbon support. The superb CO2RR performance (low overpotential of 80\u00a0mV and partial current density of 94\u00a0mA\u00a0cm\u22122) was attributed to the fast adsorption of CO2 and favorable CO desorption. Pan et\u00a0al.\n96\n introduced graphene oxide with abundant pores through H2O2 etching, which was then used as the support to anchor Fe single atoms (Fig.\u00a07e). Compared to Fe\u2013N4 sites supported on bulk graphene (FeN4-bulk), those Fe\u2013N4 sites at the pore edges showed an obvious downshift of d-band center of Fe (Fig.\u00a07f), therefore weakening the adsorption of \u2217CO intermediate and reducing the reaction barrier (Fig.\u00a07g). Consequently, a high CO FE of 94% and TOF of 1630 h\u22121 could be achieved at \u22120.58\u00a0V vs. RHE (Fig.\u00a07h). The role of defective graphite was also investigated by Qin and Ni.\n97\n\n,\n\n98\n According to in-situ FTIR and DFT calculation results, Fe\u2013N4 sites (0.71\u00a0\u200bwt%) supported on complete graphite layer showed very strong binding with \u2217CO, and they were heavily poisoned and could not be the real active sites for CO2RR. Instead, Fe\u2013N4 moieties anchored on defective graphite sites with nanopores showed moderate binding strength with \u2217COOH and \u2217CO. Ni et\u00a0al. prepared NG-SAFe and DNG-SAFe through pyrolysis at different temperatures. Thanks to the coupling between carbon defects and Fe\u2013N4 moieties, DNG-SAFe presented significantly improved CO2RR performance than NG-SAFe, which was further applied in Zn\u2013CO2 battery with good CO selectivity of 86.5% at the current density of 5\u00a0mA\u00a0cm\u22122. As shown in Fig.\u00a07i, Pan et\u00a0al.\n99\n successfully fabricated gram-scale Fe SAC (1.72\u00a0wt%) starting from commercial carbon nanotube (CNT). Highly oxidative solution containing KMnO4 and H2SO4 was used to create defects and longitudinally unzip CNT, generating carbon nanotube (CNT)@graphene nanoribbon (GNR) hieratical structure. It is worth noting that the degree of CNT unzipping could be easily controlled by tuning the ratio of CNT/KMnO4. It was figured out that the Fe\u2013N/CNT@GNR-2 (ratio of CNT/KMnO4 equals to 2) exhibited the best properties, including high surface area and fast mass transport. The residual Fe seeds during fabrication of CNT could serve as the Fe source to form Fe\u2013N4 moiety under 900\u00a0\u00b0C pyrolysis. Due to the high activity of Fe\u2013N4 sites, high surface area and efficient mass transport, the Fe\u2013N/CNT@GNR-2 achieved stable CO FE of 96% with partial current density of 22.6\u00a0mA\u00a0cm\u22122 at \u22120.76\u00a0V vs. RHE.Normally, Fe tends to coordinate with four N atoms to form Fe\u2013N4 moiety. Interestingly, as displayed in Fig.\u00a08\na, Zhang et\u00a0al.\n101\n fabricated unique Fe\u2013N5 moiety (1.2\u00a0wt%) through pyrolysis of a mixture containing hemin, melamine and graphene, while Fe nanoparticle and common Fe\u2013N4 could be synthesized by pyrolysis of hemin and the mixture of hemin and melamine, respectively. Demonstrated by Fe K-edge XANES and FT-EXAFS, the unique Fe\u2013N5 configuration was well-verified. According to DFT calculation results (Fig.\u00a08b), the additional axial N coordination could deplete the d electrons of Fe through electron transfer from d orbital of Fe to p\nx\n and p\ny\n orbitals of pyrrolic N, therefore weakening the Fe\u2013CO binding strength and facilitating the desorption of CO. As a consequence, a high CO FE around 97% at a low overpotential of\u00a00.35\u00a0V could be achieved (Fig.\u00a08c and d). Such Fe\u2013N5 active sites (2.68\u00a0wt%) with additional axial N coordination was also prepared by Li's group\n102\n through the introduction of aminated CNT. The N atom in aminated CNT could firmly anchor Fe porphyrins and prevent the agglomeration of Fe during pyrolysis at 700\u00a0\u00b0C (Fig.\u00a08e). Through efficient control of both geometric and electronic structure of Fe\u2013N5 site, the free energy for \u2217CO desorption could be significantly decreased while the energy barrier for competitive HER was increased (Fig.\u00a08f and g). As a consequence, high CO FE around 95.47% at \u22120.6\u00a0V vs. RHE could be achieved, which could maintain above 95% after 10\u00a0h of continuous reaction. A two-step annealing treatment was also developed to fabricate similar Fe\u2013N5 sites (0.47\u00a0wt%).\n103\n During annealing, FeCp powder would evaporate if the annealing temperature was higher than its sublimation point, which would be trapped by neighboring ZIF-8 with abundant cavities (Fig.\u00a08h). Following pyrolysis at 1000\u00a0\u00b0C, FeN5/N\u2013C could be successfully prepared. The downshift of d-band center of Fe 3d orbital was verified by DFT calculation (Fig.\u00a08i and j), which was attributed to the modulation of out-of-plane pyridinic N, leading to the decrease of CO adsorption energy (from \u22121.71 to \u22121.49\u00a0eV). The potential dependent free energy of CO2RR was also studied by Gao et\u00a0al. using ab initio molecular dynamics (AIMD) simulation and constrained molecular dynamics method.\n104\n Compared with traditional computational hydrogen electrode model, the slope of reaction free energy vs. potential was calculated as adsorbate-specific value instead of 1\u00a0\u200beV. Moreover, compared with electron transfer (ET) step during the formation of Fe\u2013C bond, the subsequent proton transfer (PT) step forming \u2217COOH is thermodynamically and kinetically more favorable, resulting in the decoupling of PT step from ET during the rate- or potential-determining calculations. Finally, the onset potential, potentials when maximum FECO and FECO=FEH2 were achieved can be semi-quantitatively reproduced.Among various metal catalysts, Cu-based catalysts are recognized as the most promising candidates for CO2RR due to their abilities to convert CO2 to C2+ products. Nevertheless, different from Cu-based catalysts, most Cu SACs can only reduce CO2 to C1 products. Zheng et\u00a0al.\n105\n prepared Cu\u2013N2/GN with unsaturated coordination (1.45\u00a0wt%) through a facile direct pyrolysis of graphene, chlorophyllin and dicyandiamide under inert atmosphere. The as-prepared Cu\u2013N2/GN could deliver a maximum CO FE around 81% at \u22120.5\u00a0V vs. RHE, which was further used to fabricate a rechargeable Zn\u2013CO2 battery, showing a peak power density of 0.6\u00a0mW\u00a0cm\u22122. DFT calculations demonstrated the favorable adsorption of CO2 molecule on Cu\u2013N2 sites and accelerated electron transfer from Cu\u2013N2 to \u2217CO2 due to shorter length of Cu\u2013N bond. Yang et\u00a0al.\n106\n employed Cu/ZIF-8 and poly-acrylonitrile as the precursor to electrospin polymer fiber, which was then carbonized under Ar atmosphere. The nanofiber structure is shown in Fig.\u00a09\nb with atomically dispersed Cu atoms (Fig.\u00a09c). At laboratory conditions, the self-supported and flexible single atom Cu decorated carbon nanofibers (CuSAs/TCNFs) displayed excellent bending, twisting and tensile properties (Fig.\u00a09a), which could be directly used as the cathode for the CO2RR to methanol with Faradaic efficiency of 44%. As demonstrated by DFT calculation, the binding energy of \u2217CO on Cu\u2013N4 sites was relatively high, enabling its further reduction to methanol. Gram-level Cu\u2013N4/C (0.32\u00a0wt%) with outstanding CO2 catalytic performance (CO FE of 92% at \u22120.7\u00a0V vs. RHE) could also be prepared through pyrolysis of carbonized chitosan, KOH and CuCl2 slurry, followed by acid washing.\n107\n\nIn addition to generate C1 products, some Cu SACs with well-designed configurations can also be used to reduce CO2 to C2+ products (ethylene, ethanol, acetone, etc). Zheng and coworkers\n108\n found that the concentration of Cu moiety and their configurations could be well-tuned by the pyrolysis temperature. Starting from the same Cu(BTC)(H2O)3 precursor, pyrolysis at 800\u00a0\u00b0C could retain more Cu active sites, producing Cu SACs with a high Cu content of 4.9\u00a0mol%, while pyrolysis at 900\u00a0\u00b0C generated Cu SACs with a lower Cu concentration of 2.4\u00a0mol% (Fig.\u00a09e). More importantly, the distance between neighboring Cu-N\nx\n species in Cu SACs with high Cu concentration was found to be close enough for C\u2013C coupling, resulting in the formation of C2H4. On the other hand, the more discrete Cu-N\nx\n sites in Cu SACs with low Cu concentration favored the generation of C1 products (mainly CH4). As displayed in Fig.\u00a09f, DFT calculation confirmed that two adjacent Cu\u2013N2 moieties could bind two \u2217CO intermediates to produce C2H4, while other Cu-N\nx\n configurations, including isolated Cu\u2013N2, isolated Cu\u2013N4 and neighboring Cu\u2013N4 moieties could only produce CH4. Karapinar et\u00a0al. synthesized Cu\u2013N\u2013C catalysts through two-step method: (1) low-energy ball milling of ZIF-8, CuCl2 and phenanthroline; (2) pyrolysis at 1050\u00a0\u00b0C under Ar atmosphere.\n109\n Surprisingly, the synthesized Cu\u2013N\u2013C (1.4\u00a0wt%) could efficiently reduce CO2 to ethanol with a considerable FE of 55% at \u22121.2\u00a0V vs. RHE in 0.1\u00a0M CsHCO3 electrolyte. As evidenced by the operando XAS results (Fig.\u00a09g), the appearance of Cu\u2013Cu coordination was observed under cathodic potential, which disappeared after 10\u00a0h exposure in air. Therefore, the dynamic transformation from Cu\u2013N4 moieties to metallic Cu nanoparticles was verified, which should be responsible for the unexpected generation of C2 product via CO2RR. It is also worth highlighting that such transformation of Cu\u2013N4 moieties to metallic Cu nanoparticles was reversible, thus ensuring good catalytic stability of the catalyst. Xu et\u00a0al.\n110\n also found that Cu single atomic sites could accumulate to form Cun clusters (n\u00a0=\u00a03 and 4) under CO2 reduction potential. Specifically, molten Li was used to dissolve bulk Cu, which would remain as atomic sites during the quenching of molten Li. After blending with carbon matrix (XC-72) followed by leaching process, atomically dispersed Cu could be synthesized. Owing to in-situ generation of Cun clusters evidenced by the appearance of Cu\u2013Cu moieties with coordination number of 2 or 3\u00a0at \u22120.7\u00a0V vs. RHE (Fig.\u00a09h), such Cu SAC delivered a low onsetpotential of only \u22120.4\u00a0V and an high Faradaic efficiency (91%) towards ethanol at \u22120.7\u00a0V vs. RHE. Besides ethanol, acetone could also be produced from CO2 reduction over Cu-SA/NPC (0.59\u00a0wt%) prepared by Chen's group.\n111\n Cu(OAc)2 was firstly encapsulated in the pores of ZIF-8, which was then carbonized at 1000\u00a0\u00b0C under inert atmosphere to prepare Cu-SA/NPC. During the electrochemical reduction of CO2, acetone was identified as the main product with FE of 36.7% and production rate of 336.1\u00a0\u03bcg\u00a0h\u22121. DFT calculation revealed that Cu coordinated with 4 pyrrole N was the main active site, which could reduce the energy barriers for CO2 activation and C\u2013C coupling, as well as stabilize the key intermediates for acetone formation (Fig.\u00a09i and j).During fabrication of Zn-based single atom catalysts, increasing Zn content is difficult because of the low boiling point of Zn. Zn\u2013N4 based SACs could be prepared through pyrolysis of Zn(OAc)2, carbon black and urea at 800\u20131000\u00a0\u00b0C.\n112\n The Zn\u2013N4 configuration with metal loading of 0.1\u00a0\u200bwt% was verified by EXAFS, such configuration could lower the energy barrier of \u2217COOH formation according to the DFT calculation. The highest CO FE could be recorded as 95% at \u22120.43\u00a0\u200bV vs. RHE, which showed no deterioration after 75\u00a0h continuous reaction. Low-valence \n\n\nZn\n\n\u03b4\n+\n\n\n\n SAC (1.08\u00a0wt%) could be fabricated by annealing Zn-BTC and dicyandiamide at 1000\u00a0\u00b0C.\n113\n XAS results demonstrated the existence of saturated Zn\u2013N4 and unsaturated Zn\u2013N3 moieties. The Zn\u2013N3 moieties could hold electrons, leading to lower valence states of Zn sites, which could reduce the energy barrier for \u2217COOH intermediate formation (Fig.\u00a010\na). Near-unity CO selectivity at an overpotential of 0.31\u00a0V was achieved in an H-type cell, while high current density up to 1\u00a0A\u00a0cm\u22122 along with CO selectivity over 95% was realized in a flow cell. In addition to CO, Han et\u00a0al.\n114\n also successfully applied Zn SAC to reduce CO2 to produce CH4. A high Faradaic efficiency of 85% and partial current density of 31.8\u00a0mA\u00a0cm\u22122 could be measured at \u22121.8\u00a0V vs. SCE. DFT calculations revealed high energy barrier for the formation of \u2217CO, which would promote the protonation of \u2217CO to form CH4.Co SACs for CO2RR could be easily fabricated through coordination interaction between CoPc and the as-prepared hollow N-doped porous carbon spheres (HNPCSs).\n115\n Atomically dispersed Co\u2013N5 delivered a maximum CO FE of 99% and maintained the CO FE over 90% across a wide potential window (from \u22120.57 to \u22120.88\u00a0V vs. RHE). Both current density and CO FE kept unchanged after 10\u00a0h of electrochemical CO2 reduction reaction. The fast formation of \u2217COOH intermediate and \u2217CO desorption were responsible for the excellent CO2RR performance. The coordination environment of Co single atoms with loading of 0.63\u00a0wt% could also be well-regulated by changing pyrolysis temperature.\n116\n Through pyrolyzing ZIF at 800 and 900\u00a0\u00b0C, Co1\u2013N4 and Co1\u2013N4-x\nC\nx\n could be prepared, respectively. Based on XAFS, Co\u2013C coordination was believed to partially replace the Co\u2013N coordination under high temperature pyrolysis. CoSA-based 3D free-standing membrane could also be fabricated through electrospinning of ZIF-8 NPs, Co(NO3)2 and PAN. Due to the large electrochemical surface area and favorable reactant transportation, the as-fabricated CoSA/HCNFs delivered a maximum CO FE of 91% and partial current density of 67\u00a0mA\u00a0cm\u22122 in an H-cell, while that could be further improved to 92% and 211\u00a0mA\u00a0cm\u22122 in a flow cell.Most of today's commercial catalysts applied in industry are based on noble metals because of their high intrinsic catalytic properties. Therefore, it is of great significance to design and fabricate noble metal based SACs to maximize the noble metal utilization efficiency. Pd SAC with loading of 2.95\u00a0wt% could be prepared using a classic co-pyrolysis strategy.\n117\n Compared with Pd NP counterpart, such Pd SAC showed much superior CO2RR performance (373.0 vs. 28.5\u00a0mA mg\u22121\nPd). The obvious shift toward lower energy in in-situ XAFS (Fig.\u00a010b) and the increased Pd\u2013Pd bond length in FT-EXAFS (Fig.\u00a010c) evidenced the formation of PdH species on the surface of Pd/C. In contrast, the peaks for Pd SAC (Fig.\u00a010d and e) showed negligible changes, verifying the absence of PdH species. As shown in both in- situ XAFS and DFT calculations, the Pd\u2013N4 served as the active sites without forming PdH species that was very crucial for bulk Pd catalysts. Recently, Ag single atom was successfully dispersed onto MnO2 substrate through the thermal-induced transformation of Ag NP and surface reconstruction of MnO2.\n118\n As shown in Fig.\u00a010f, in-situ environmental transmission electron microscopy (ETEM) clearly demonstrated the transformation from Ag NP to single Ag atom. The gradual decreased Ag (111) peak in Fig.\u00a010g again verified the reduction of Ag NP, while the dominant diffraction peak of MnO2 changed from (211) to (310), indicating the preferential lattice plane to stabilize Ag single atoms. Compared with Ag NPs located at surface and corner, Ag1/MnO2 displayed the lowest free energy of \u2217COOH (0.44\u00a0\u200beV). Therefore, a high CO FE up to 95.7% at \u22120.85\u00a0\u200bV vs. RHE could be achieved due to the high electronic density near Fermi level of single Ag sites (Fig.\u00a010j). Single atom Au could be well-dispersed onto tensile-strained Pd NPs through MOF-assisted adsorption strategy.\n119\n The tensile-strained Pd substrate could stabilize all intermediates, while atomically dispersed Au sites could selectively destabilize CO\u2217, thus breaking the scaling relationship between COOH\u2217 and CO\u2217. As a result, excellent CO2RR performance to generate formic acid with FE up to 99% could be achieved at \u22120.25\u00a0V vs. RHE, which is much superior to Pd/C catalyst.Main group elements including Sn, Sb, Bi, Pb and In have been extensively studied in CO2RR due to their intrinsic high overpotentials for competitive HER.\n120\n Similar to their bulk counterparts, main group metal based SACs also favor the production of formic acid and formate in CO2RR. Kilogram \n\n\nSn\n\n\u03b4\n+\n\n\n\n sites could be atomically dispersed onto N-doped graphene substrate through a so-called quick freeze-vacuum drying-calcination strategy.\n121\n XAFS and HAADF-STEM results evidenced the positively charged \n\n\nSn\n\n\u03b4\n+\n\n\n\n sites, which could stabilize CO2\n\u00b7\u2212\u2217 and HCOO\u2212\u2217 to efficiently activate CO2 molecule and promote the subsequent protonation steps. Moreover, the introduction of N could promote desorption of formate (RDS), evidenced by the reduced desorption energy (1.01 vs. 2.16\u00a0eV) and the extended Sn\u2013HCOO\u2212 bond. As a consequence, the reaction overpotential could be reduced to as low as 60\u00a0mV along with an impressive TOF of 11930 h\u22121 and long-term stability over 200\u00a0h. When anchored onto other substrates, Sn SACs could also generate reduction products beyond CO. Li et\u00a0al.\n122\n utilized CuO with oxygen vacancy to anchor Sn single atoms. The as-prepared SnI/Vo-CuO could reduce the energy barrier for \u2217COOH generation to promote the formation of \u2217CO, which was then adsorbed by CuO substrate to be further reduced to methanol. Faradaic efficiency toward methanol could reach as high as 88.6% at the current density of 67\u00a0\u200bmA\u00a0\u200bcm\u22122. Bi-N\nx\n single atomic sites could be synthesized through facile pyrolysis of Bi-MOF with assistance of dicyandiamide, which promoted the formation of \u2217COOH intermediate by lowering its reaction barrier.\n123\n The as-prepared BiSAs/NC delivered a maximum CO FE up to 97% along with TOF around 5525 h\u22121. A high TOFCO above 16500 h\u22121 was achieved on Sb SAC, which was prepared by pyrolysis of SbCl3, activated carbon black and urea.\n124\n Bi\u2013N\u2013C SAC and corresponding Bi nanosheets were used as models to theoretically investigate the preferred product on SACs.\n125\n Under cathodic potentials, CO2 tends to chemisorb on Bi\u2013N\u2013C SAC because of charge accumulation effect. Subsequently, \u2217COOH can be formed through more kinetically favorable approach when compared with the formation of \u2217OCHO. In contrast, physisorption is adopted for the CO2 activation on Bi nanosheets, resulting in the selective production of formate. Additionally, it was proposed that CO2 adsorption modes can serve as solid criterion for the prediction of preferred product. On the other hand, the similar \n\n\nSn\n\n\u03b4\n+\n\n\n\n\u2013N4 sites (2.86\u00a0wt%) could selectively reduce CO2 to formic acid instead of CO due to the favorable adsorption of HCOO\u2217.\n126\n Zeng et\u00a0al. reported an in-situ electrochemical deposition method to atomically disperse Pb onto Cu nanosheets, forming Pb1Cu SAC. However, modulated Cu sites, instead of atomic Pb sites serve as active sites. The impressive formic acid selectivity (\u223c96%) at larger current density of 1\u00a0A\u00a0cm\u22122 can be attributed to the shift from HCOO\u2217 pathway to COOH\u2217 pathway.\n127\n Atomic In sites on nitrogen-doped carbon (In\u2013N\u2013C) was prepared through the pyrolysis of In-doped ZIF-8 at 900\u00a0\u00b0C.\n128\n Demonstrated by DFT calculations, compared with \u2217COOH formation energy of +0.84\u00a0\u200beV, the mild endothermic formation of \u2217OCHO (+0.19\u00a0\u200beV) should be responsible for the TOF as high as 26771\u00a0\u200bh\u22121\u00a0at \u22120.99\u00a0V vs. RHE. Similar strategy was also reported by Li et\u00a0al.,\n129\n synthesizing isolated \n\n\nIn\n\n\u03b4\n+\n\n\n\n\u2013N4 SAC. A high TOF up to 12500 h\u22121 and maximum HCOOH Faradaic efficiency of 96% can be obtained at a potential of \u22120.65\u00a0V vs. RHE. Nevertheless, when using mixed electrolyte containing ionic liquid and MeCN, CO instead of formic acid, was selectively produced (97.2% at 39.4\u00a0mA\u00a0cm\u22122). The catalytic performances can be attributed to the high double-layer capacitance, CO2 adsorption capacity and low interfacial resistance.\n120\n\nDue to the low binding energy between C and metal atoms, it is difficult to anchor single metal sites on pure carbon support. In order to efficiently fix single metal sites, N-doped carbon substrates are widely used to form strong M\u2212N bonds. In most circumstances, the metal center in SACs is coordinated with four nitrogen atoms to form stable configurations. The electronic and geometric structures of metal sites are highly related to their coordination environments, which accounts for the different adsorption free energy of intermediates and reaction pathways. Nitrogen coordination displays different electron-donating or -withdrawing features due to their different configurations. Specifically, pyrrole and quaternary N can donate electron to metal site, resulting in n-type doping, while pyridine N will delocalize electron from metal site, leading to p-type doping. The coordination environment can be efficiently adjusted by introducing vacancies or other non-metal heteroatoms, providing more complex electronic and geometric structures resulting from different atomic sizes and electronegativities.\n130\n\nChen et\u00a0al. synthesized Ni SACs with nitrogen vacancies (SA-NiNG-NV) through microwave-induced plasma treatment.\n131\n The coordination environment could be easily tuned by changing the duration of plasma treatment (Fig.\u00a011\na). As compared with original SA-NiNG, SA-NiNG-NV displayed a higher ID/IG ratio in the Raman spectrum (Fig.\u00a011b), an increased ratio of sp3 C to sp2 C (Isp3/Isp2) in the XPS spectrum (Fig.\u00a011d) and higher spin numbers in electron paramagnetic resonance (Fig.\u00a011c), demonstrating more unpaired electrons resulted from surface defects and vacancies. AC-HAADF-STEM images and EXAFS results confirmed the atomically dispersed Ni atoms in both SA-NiNG and SA-NiNG-NV. Via fitting the FT-EXAFS, the coordination configurations of two catalysts were determined (Fig.\u00a011e). The original SA-NiNG composed of Ni-pyrrolic N3 moiety underwent a plasma-induced-reconstruction to form Ni-pyridinic N2 moiety by removing 2 neighboring N atoms. Such unsaturated coordination environment in SA-NiNG-NV offered sufficient space for CO2 adsorption, therefore reduced the energy barrier for the generation of key intermediates COOH\u2217 and CO\u2217. As displayed in Fig.\u00a011f and g, thanks to the favorable thermodynamics and kinetics of CO2RR, SA-NiNG-NV exhibited outstanding catalytic performance with an impressive CO FE of 96% at the overpotential of 590\u00a0mV, a high partial CO current density of 33\u00a0mA\u00a0cm\u22122 at the overpotential of 890\u00a0mV as well as good stability. Coordination vacancy could be introduced into Ni SAC by selective removal of O in the as-prepared Ni\u2013N3O due to the weaker Ni\u2013O bond.\n132\n According to DFT calculation, the more favorable reduction of CO2 to COOH\u2217 (regarded as the rate-determine step) contributed to the high CO selectivity (over 90%) and impressive TOF of 135000 h\u22121.M\u2013N\u2013O coordination structures also showed good CO2RR performances due to the efficient coordination modulation by the O atom. In order to prepare low-coordinated Cu active sites with O, the original Cu\u2013O\u2013C moiety in the as-prepared CuDBC was treated with high energy plasma, which was converted to low-coordinated Cu\u2013O2\u2013C moiety.\n133\n Thanks to the in-situ formed hierarchical porous matrix and low coordination environment, Cu\u2013O2\u2013C SAC could efficiently reduce CO2 to CH4 with a maximum FE of 75.3% and partial current density of 47.8\u00a0mA\u00a0cm\u22122. As shown in Fig.\u00a011h, a gas transportation strategy was adopted to synthesize atomically dispersed SnN3O sites on N-doped carbon support (Sn\u2013NOC), where SnO2 powder and N-doped carbon were separately placed in the tube furnace serving as metal source and support, respectively.\n134\n Different from the dominant formate formed over classic Sn\u2013N4 sites, Sn\u2013NOC catalyst could reduce CO2 exclusively to CO. DFT calculations revealed that the introduction of O could promote the activation of CO2 and reduce the free energy of \u2217COOH, while increase the energy barrier for HCOO\u2217. As presented in the in-situ surface-enhanced Raman spectra (Fig.\u00a011i), the peaks located at 718, 1030 and 1130\u00a0cm\u22121 could be assigned to \u2217CO2\u2212, \u2217OCO and \u2217COOH, which showed similar trend (firstly became stronger then weaker with the potential changing from \u22120.1 to \u22120.9\u00a0\u200bV vs. RHE), agreeing well with DFT calculation results, implying favorable \u2217COO\u2212 and \u2217COOH reaction pathways.Microwave-induced plasma treatment could be applied to introduce alien S into unsaturated Ni\u2013N2 moiety to tune the electronic structure by forming NiNG-S.\n135\n The larger ID/IG ratio (1.19 vs. 1.09) in the Raman spectra, the reduced pyridinic and pyrrolic N intensities as well as the increased binding energy of Ni 2p in the XPS spectra confirmed the successful introduction of S into the Ni\u2013N2 moiety. In addition, by fitting FT-EXAFS in R space, the introduced S was deduced to occupy N vacancy in NiN2 species, forming NiN2\u2013S configuration. Thanks to the S dopant, NiNG-S showed obviously superior CO2RR performance than the control NiNG catalyst, including a CO FE around 97% and CO partial current density of 40.3\u00a0mA\u00a0cm\u22122 at \u22120.8 and \u22120.9\u00a0V vs. RHE, respectively (Fig.\u00a011j and k). Such outstanding catalytic performance could be explained by the reduced energy barrier for CO2RR due to the favorable electronic structure of NiNG-S. Interestingly, at the potential of \u22120.8\u00a0V vs. RHE, a negative shift of XANES as shown in Fig.\u00a011l indicated decreased valence state of Ni site, which was further confirmed by the loss of S according to the disappearance of Ni\u2013S coordination in FT-EXAFS (Fig.\u00a011m). This observation also agreed well with the absence of Ni\u2013S bond in XPS. The unstable S dopant at high applied cathodic potentials (beyond \u22120.8\u00a0V vs. RHE) could result in S vacancies in Ni\u2013N2 structure. Based on DFT calculations, although the rate-limiting steps for NiN2, NiN2\u2013S and NiN2\u2013Vs were different, the CO2RR overpotential for NiN2\u2013S (0.55\u00a0eV) and NiN2\u2013Vs (0.76\u00a0eV) were significantly lower than that for NiN2 (1.12\u00a0eV), which agreed well with the experimental results. Substitutional S could also be successfully introduced into Cu\u2013N4/Cu\nx\n tandem catalyst through additional pyrolysis with addition of sulfur powder at 950\u00a0\u00b0C.\n136\n DFT calculations showed that the Cu\u2013S coordination could effectively reduce the Gibbs free energy for \u2217COOH formation by 0.61\u00a0\u200beV, resulting in favorable generation of key intermediate for CO. On the other hand, the neighboring Cu\nx\n cluster could accelerate the dissociation of H2O to provide sufficient H+ to promote the protonation of \u2217CO2\u2212. Thanks to the synergistic effect of S coordination and Cu\nx\n cluster, the Faradaic efficiency for CO generation could reach up to 100% at \u22120.65\u00a0V vs. RHE, and it could be maintained above 90% in a wide potential window (from \u22120.55 to \u22120.75\u00a0V vs. RHE).Li and coworkers introduced P into N-doped carbon to modify the substrate electronic structure.\n137\n Due to the absence of Fe\u2013P bond in XPS, P was evidenced to connect with carbon instead of Fe, which was further confirmed by FT-EXAFS. To investigate the role of P doping on the CO2RR performance, the free energy diagram of the catalysts with different P coordination was computed. The existence of P in the second coordination shell could increase the free energy of \u2217COOH but significantly decrease the energy barrier for HER. Nevertheless, when P was placed in higher coordination shells (\u22653), the formation of \u2217COOH became more favorable along with suppressed HER. The electron localization over Fe sites induced by P doping explained the selective formation of \u2217COOH intermediate to produce CO.In addition to replacing one or two N atoms with heteroatoms in SACs, the heteroatoms can also be introduced to form additional M-X bond, which shall efficiently tune the electronic structure of metal sites through charge polarization effect. Predicted by DFT calculation, when grafted with additional O atom on Ni along the axial direction, the electron pushing effect would result in significant charge polarization and lower energy barrier to form COOH\u2217.\n138\n O was chosen because of its slightly larger and moderate electronegativity than N, which could induce charge polarization effect but not destroy the pristine Ni\u2013N4 moiety. Wang et\u00a0al. then successfully synthesized Ni SAC with axial O atom supported by carbon matrix (Ni\u2013N4\u2013O/C) using Mn-based MOF as the host and Ni2+ as the guest ion. During the subsequent carbonization process, O was anchored onto Ni\u2013N4 moiety as the axial atom. As demonstrated in FT-EXAFS, the dominant peak of Ni\u2013N4\u2013O/C was located at 1.52\u00a0\u00c5, which is between Ni\u2013N (1.47\u00a0\u00c5) and Ni\u2013O (1.68\u00a0\u00c5) scattering paths, confirming the charging polarization effect of axial O. Thanks to the lower reaction barrier of transforming CO2\u2217 to COOH\u2217, Ni\u2013N4\u2013O/C exhibited excellent CO2RR performance to produce CO. The role of axial O on Ni\u2013N4 moiety was systematically studied by Hu et\u00a0al.,\n139\n who adopted an explicit solvent model with thermodynamic integration method to build a realistic electrochemical environment. It was elucidated that axial O could facilitate the charge transfer and stabilize \u2217CO2 species. Combining dynamic solvent molecules and applied cathodic potential, the energy barriers and free energies could be precisely calculated, which evidenced that axial O could activate the formation of both \u2217CO and \u2217COOH and simultaneously suppress the competitive HER.Through sequential pyrolysis of SnO2/PTFE and ZIF-8, Ni et\u00a0al. fabricated Sn SAC with Sn\u2013C2O2F configuration, where Sn site was not only coordinated with 2 carbon and 2 oxygen atoms in planar direction, but also axially coordinated with a F atom.\n140\n Compared with the predominant formate production on typical Sn\u2013N4 sites, the unique Sn\u2013C2O2F realized selective CO2RR to CO with a high CO FE over 90% in a potential window from \u22120.2 to \u22120.6\u00a0V vs. RHE. Comprehensive DFT calculations revealed the switching of rate-determining step (RDS) from \u2217CO desorption on Sn\u2013N4 sites to activation of \u2217CO2 on Sn\u2013C2O2F sites. The additional axial F coordination could significantly suppress the competitive HER, giving a more positive value of UL(CO2)-UL(H2). Convex inversion of carbon plane induced by firm interaction between Sn sites and CO2 molecules prohibited the possible CO2 to HCOOH conversion on Sn\u2013C2O2F. Axial chlorine could also be coordinated to Fe\u2013N4 sites through additional hydrochloric acid incubation method.\n141\n The high CO FE around 90.5% and TOF of 1566 h\u22121 could be attributed to the strengthened charge transfer between Fe site and the intermediate, which facilitated the desorption of \u2217CO and suppressed the adsorption of \u2217H.Although single atom catalysts show many advantages, the low single atom loading and discrete active sites limit their reaction activity and pathway.\n142\n In this aspect, dual-atom catalysts containing two atomically dispersed metal sites could offer additional benefits.\n143\n\n,\n\n144\n\nDifferent metal sites can be atomically dispersed onto carbon substrate. By adding CoCl2 and NiCl2 in the precursor, the dual-atom catalysts containing both Co-SA and Ni-SA sites could be prepared.\n145\n The final loading of Co and Ni could be well-tuned by changing the precursor composition. DFT calculations showed that Co-SA and Ni-SA could produce H2 and CO by stabilizing their corresponding intermediates. Therefore, syngas with controllable CO/H2 ratio could be produced with a total current density above 74\u00a0mA\u00a0cm\u22122. Similar strategy was adopted to synthesize Zn\u2013La dual atomic catalyst by annealing melamine sponges soaked with Zn(NO3)2 and La(NO3)2.\n146\n The lack of both Zn\u2013Zn bond and La\u2013La bond as shown in Fig.\u00a012\na and b evidenced atomically dispersed Zn and La atoms. DFT calculations demonstrated that the electronic structures of Zn and La sites were well-tuned, which favored the CO2RR to CO and HER, respectively. Therefore, ZnLa-1/CN catalyst could produce syngas with a CO/H2 ratio of 0.5 across a wide potential range from \u22121.6 to 1.3\u00a0V vs. RHE.The above-mentioned dual-atom catalysts can be considered as a combination of two types of SACs with different metal centers, in which two kinds of metal sites are randomly dispersed on the substrate. As a step further, with appropriate design, two kinds of metal atomic sites can be anchored on the support next to each other, where synergistic effect between the two types of active sites and tunable electronic structure can be realized. By introducing Ni(acac)2 and Cu(OAc)2 into the synthesis of ZIF-8, atomically dispersed Ni/Cu dual sites could be prepared by pyrolysis method.\n147\n The adjacent atomic Ni and Cu sites were confirmed by fitting the EXAFS spectrum as displayed in Fig.\u00a012c. By introducing neighboring Cu site, the bandgap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was narrowed from 0.64 to 0.37\u00a0eV (Fig.\u00a012d), which effectively improved the electronic conductivity and strengthened the bond between key intermediate \u2217COOH and Ni active site. Thanks to the reduced energy barrier of RDS (Fig.\u00a012e), the CO FE could be significantly enhanced to 99.2% at \u22120.79\u00a0V vs. RHE and could maintain above 95% in a wide potential range from \u22120.39 to \u22121.09\u00a0V vs. RHE (Fig.\u00a012f). Similar mechanism of lowering the d-band center of Ni 3d orbital was achieved by constructing dual atomic Ni\u2013Zn sites.\n148\n The distance between Ni\u2013Zn was estimated to be 2.5\u00a0\u00c5 (Fig.\u00a012g and h), which matched well with the cut-off distance of Ni or Zn atom. The region between Ni and Zn atoms showed an electron localization function (ELF) of 1/2, indicating electronic interaction between two adjacent atoms. Compared with Ni (Fig.\u00a012k) or Zn SAC (Fig.\u00a012l), NiZn\u2013N6\u2013C catalyst has 0.15 and 0.06 more electrons. In-situ FTIR measurement was carried out to investigate the possible CO2RR pathway (Fig.\u00a012m). The appearance of \u2217CO species on NiZn\u2013N6 at a more positive potential (\u22120.15\u00a0V vs. RHE) verified weaker adsorption and unfavorable formation of \u2217CO. DFT calculations demonstrated that the heteronuclear coordination could tune the electronic structure of Ni single atomic site, leading to reduced energy barrier in the aspect of thermodynamics and strengthened Ni\u2013C bond in the aspect of kinetics. High CO FE over 90% could be achieved across a wide potential window from \u22120.5 to \u22121\u00a0\u200bV vs. RHE. Ren et\u00a0al.\n149\n adopted a similar MOF-assisted pyrolysis method to fabricate Ni\u2013Fe dual sites anchored on nitrogenated carbon. The excellent CO2RR performance was attributed to the structural change of adsorbed CO under CO2 atmosphere, which could further decrease the energy barrier for \u2217COOH generation and favor the desorption of CO molecule. Zhang et\u00a0al.\n150\n also found that Ni\u2013Fe dual atom sites exhibited excellent CO2RR performance. In addition to favorable formation of \u2217COOH and desorption of \u2217CO as mentioned above, the competitive HER was also significantly suppressed over dual-atom catalysts.Neighboring Zn/Co monomer anchored on N-doped carbon (ZnCoNC) could also selectively reduce CO2 to CO, where CoSA moiety was considered as the active site that was modulated by the neighboring Zn site.\n151\n It is worth mentioning that NiSA is not always the active site in dual-atom catalysts despite of its high intrinsic catalytic activity. Xie et\u00a0al.\n152\n prepared an integrated electrode containing NiSn atomic pair coordinated with four nitrogen atoms (N4\u2013Ni\u2013Sn\u2013N4) through impregnation followed by subsequent pyrolysis. Instead of Ni\u2013N4, Sn\u2013N4 moiety was identified as the active site for CO2RR, resulting in the selective production of formate. Specifically, a high TOF of 4752 h\u22121, formate productivity of 36.7\u00a0mol h\u22121gSn\n\u22121 and high utilization of active sites (57.9%) could be realized, which are the highest values among reported literatures. DFT calculation demonstrated electron redistribution over the SnSA sites, thanks to the introduction of adjacent Ni\u2013N4 moieties, which effectively decreased the energy barrier for \u2217OCHO formation.Same metal atoms can also be adjacently anchored on substrate to form dual atom catalysts. Pd2 dual atom catalyst could be prepared by anion replacement deposition-precipitation (ARDP) method using binuclear Pd(II) complex as the precursor.\n153\n Compared with Pd1SAC, the electron transfer between neighboring dimeric Pd sites could weaken the adsorption of \u2217CO, thus resulting in a lower energy barrier and outstanding CO selectivity (98.2% FE at \u22120.85\u00a0V vs. RHE). In Li's recent work,\n154\n binuclear Ag complex (Ag(NO3\u2013O)(phtz-N)2(\u03bc-phtz-N,N\u2032)2) was firstly prepared by adding AgNO3 to phthalazine, followed by crystallization in a fridge, where each Ag atom was coordinated with three N atoms and two adjacent Ag atoms were weakly bonded with a length of 3.434\u00a0\u00c5. After mixing with GO suspension, binuclear Ag complex could strongly adsorb on graphene substrate through \u03c0-\u03c0 interaction. The final dual-atom Ag2-G catalyst could be fabricated after a final pyrolysis treatment of the centrifuged precipitates. Two adjacent Ag sites could interact with the C and O atom in the CO2 molecule, stabilizing \u2217CO2 and lowering the reaction barrier to form \u2217COOH. The CO2RR could be driven at \u22120.25\u00a0V vs. RHE, showing a CO FE of 93.4% with a current density of 11.87\u00a0mA\u00a0cm\u22122. In addition to two identical adjacent metal sites, one element could also form two neighboring metal sites with different states. Jiao et\u00a0al.\n155\n fabricated Cu atom-pair catalyst by introducing Cu(NO3)2 into the precursor solution to grow Pd10Te3 nanowires. XAFS results verified that the adjacent Cu species would form stable Cu1\n0\u2013Cu1\n\nx\u00a0\u200b+\u00a0pair structures, which worked together to promote the CO2RR. Specifically, Cu1\n\nx\u00a0\u200b+\u00a0favored the adsorption of H2O while the adjacent Cu1\n0 could adsorb CO2 molecule efficiently. A high CO FE over 92% could be achieved with nearly completely suppressed undesired HER. DFT calculation attributed the reduced activation energy to the unique adsorption configuration on the Cu1\n0\u2013Cu1\n\nx\u00a0\u200b+\u00a0pair structure.In this review, we firstly introduced the reaction mechanism and pathways of electrochemical reduction of CO2 to various products (CO, HCCOH, CH4, CH3OH and C2+). Then recently reported single atom catalysts were systematically summarized and discussed in aspect of different active centers (transitional metals, noble metals and main group metals). Furthermore, as the most efficient coordination modulation method in SACs, strategies of introducing hetero non-metal atoms were discussed. Then, dual atom catalysts were summarized and reviewed, which could not only keep advantages of SACs, but also provide more opportunities to manipulate the coordination environments. Although many progresses of SACs have been achieved in the field of CO2RR, there still exist many challenges and problems that retard the in-depth understanding of SACs and their application in industry.\n\n1.\nThe catalytic activity (partial current density and turnover frequency) is highly related to the metal loading. Most of current researches on SACs can only achieve relatively low metal loadings below 5\u00a0wt% in order to prevent metal aggregation and maintain atomically dispersed active sites. Therefore, it is of great significance to further increase the metal loading amount in SACs to realize industrial-level partial current density (>300\u00a0mA\u00a0cm\u22122).\n\n\n2.\nAs most strategies for preparation of SACs require high-temperature, the as-prepared SACs are actually composed of metal sites with very complex coordination environments. Many advanced characterization techniques including XAS and HAADF-STEM could be used to provide direct evidences of the existence of single atomic sites and their coordination environments. It is still worth mentioning that XAS can only give statistical results, demonstrating the overall information about the catalysts. On the other hand, despite of the very high resolution (0.06\u00a0nm) of aberration-corrected transmission electron microscopy (AC-TEM), it is still difficult to distinguish the coordination atoms (C, N, O, S \u2026) surrounding the metal sites due to their similar contrast under TEM or STEM mode. Additionally, those species with the same coordination environment but at different sites (perfect sites, defect sites or edge sites) of substrate will bring even more uncertainties for precise understanding of coordination environment of SACs, which is considered as the key factor influencing the electronic structure of metal sites, adsorption energy of intermediates and final selectivity of catalysts. It is suggested that strategies adopting low-temperature preparation or immobilized molecular catalysts can be developed to establish well-designed SACs with precise coordination environment, which can serve as the platform to investigate the catalytic mechanism of SACs.\n\n\n3.\nDue to the stable structure of CO2 molecule, the electrochemical reduction of CO2 always requires high cathodic potential when compared to HER. In this case, the chemical state of metal sites and coordination atoms are very likely to evolve under CO2RR condition, making the real active sites elusive. In-situ or time-resolved characterizations are encouraged to monitor the change of active sites during the whole reaction process,\n100\n\n,\n\n156\n which can help to figure out the real active sites catalyzing CO2RR and understand the catalytic mechanism.\n\n\n4.\nWith exception to few literatures reporting C2+ products, most predominant product over SACs in CO2RR is CO due to the lack of C\u2013C coupling sites, which hinders the application of SACs. Fortunately, some Cu SACs have been fabricated for the catalytic production of advanced C2+ products (ethylene, ethanol, and acetone) either based on in-situ formation of Cun clusters or adjacent Cu\u2013N2 moieties. The investigation of SACs to generate C2+ products can not only provide additional choice for the generation of high-value products, but also serve as an ideal platform to study the complex multiple protonation and electron transfer during the reduction process.\n\n\n5.\nWidely used wet-chemistry or ball-milling methods tend to prepare powder-based catalysts, where binders or additives are needed for the preparation of electrodes and may have side effects on the catalytic performances. Therefore, the direct fabrication of monolithic SAC electrodes or dispersing single atoms onto self-supported gas diffusion substrates is very promising and can be further applied in membrane electrode assembly (MEA) electrolyzers and flow cells to overcome the solubility and diffusion limitation of CO2 molecules in electrolyte.\n44\n\n\n\n\nThe catalytic activity (partial current density and turnover frequency) is highly related to the metal loading. Most of current researches on SACs can only achieve relatively low metal loadings below 5\u00a0wt% in order to prevent metal aggregation and maintain atomically dispersed active sites. Therefore, it is of great significance to further increase the metal loading amount in SACs to realize industrial-level partial current density (>300\u00a0mA\u00a0cm\u22122).As most strategies for preparation of SACs require high-temperature, the as-prepared SACs are actually composed of metal sites with very complex coordination environments. Many advanced characterization techniques including XAS and HAADF-STEM could be used to provide direct evidences of the existence of single atomic sites and their coordination environments. It is still worth mentioning that XAS can only give statistical results, demonstrating the overall information about the catalysts. On the other hand, despite of the very high resolution (0.06\u00a0nm) of aberration-corrected transmission electron microscopy (AC-TEM), it is still difficult to distinguish the coordination atoms (C, N, O, S \u2026) surrounding the metal sites due to their similar contrast under TEM or STEM mode. Additionally, those species with the same coordination environment but at different sites (perfect sites, defect sites or edge sites) of substrate will bring even more uncertainties for precise understanding of coordination environment of SACs, which is considered as the key factor influencing the electronic structure of metal sites, adsorption energy of intermediates and final selectivity of catalysts. It is suggested that strategies adopting low-temperature preparation or immobilized molecular catalysts can be developed to establish well-designed SACs with precise coordination environment, which can serve as the platform to investigate the catalytic mechanism of SACs.Due to the stable structure of CO2 molecule, the electrochemical reduction of CO2 always requires high cathodic potential when compared to HER. In this case, the chemical state of metal sites and coordination atoms are very likely to evolve under CO2RR condition, making the real active sites elusive. In-situ or time-resolved characterizations are encouraged to monitor the change of active sites during the whole reaction process,\n100\n\n,\n\n156\n which can help to figure out the real active sites catalyzing CO2RR and understand the catalytic mechanism.With exception to few literatures reporting C2+ products, most predominant product over SACs in CO2RR is CO due to the lack of C\u2013C coupling sites, which hinders the application of SACs. Fortunately, some Cu SACs have been fabricated for the catalytic production of advanced C2+ products (ethylene, ethanol, and acetone) either based on in-situ formation of Cun clusters or adjacent Cu\u2013N2 moieties. The investigation of SACs to generate C2+ products can not only provide additional choice for the generation of high-value products, but also serve as an ideal platform to study the complex multiple protonation and electron transfer during the reduction process.Widely used wet-chemistry or ball-milling methods tend to prepare powder-based catalysts, where binders or additives are needed for the preparation of electrodes and may have side effects on the catalytic performances. Therefore, the direct fabrication of monolithic SAC electrodes or dispersing single atoms onto self-supported gas diffusion substrates is very promising and can be further applied in membrane electrode assembly (MEA) electrolyzers and flow cells to overcome the solubility and diffusion limitation of CO2 molecules in electrolyte.\n44\n\nT. W., B. L. and S. K. conceived the topic and structure of the article. All authors reviewed and contributed to this paper.There are no conflicts of interest to declare.This work was supported by the fund from NUS Green Energy Program (WBS: A-0005323-05-00), FRC\nMOE T1 (WBS: A-0009184-00-00), A\u2217STAR LCERFI Project (Award ID: U2102d2011), Ministry of Education of Singapore (Tier 1: RG4/20, RG2/21 and Tier 2: MOET2EP10120-0002), and Agency for Science, Technology and Research (AME IRG: A20E5c0080).", "descript": "\n                  Powered by electricity from renewable energies, electrochemical reduction of CO2 could not only efficiently alleviate the excess emission of CO2, but also produce many kinds of valuable chemical feedstocks. Among various catalysts, single atom catalysts (SACs) have attracted much attention due to their high atom utilization efficiency and expressive catalytic performances. Additionally, SACs serve as an ideal platform for the investigation of complex reaction pathways and mechanisms thanks to their explicit active sites. In this review, the possible reaction pathways for the generation of various products (mainly C1 products for SACs) were firstly summarized. Then, recent progress of SACs for electrochemical reduction of CO2 was discussed in aspect of different central metal sites. As the most popular and efficient coordination modulation strategy, introducing heteroatom was then reviewed. Moreover, as an extension of SACs, the development of dual atom catalysts was also briefly discussed. At last, some issues and challenges regarding the SACs for CO2 reduction reaction (CO2RR) were listed, followed by corresponding suggestions.\n               "}
{"full_text": "Hydrogen is an ideal alternative to fossil fuels for its high energy density and abundant resources [1]. However, safe, efficient and economical hydrogen storage is still a challenge for the large-scale application of hydrogen energy. [2] In the last few decades, solid-state hydrogen storage materials, including light metal hydrides [3,4] and complex hydrides [5\u20138], have attracted considerable attention due to their high hydrogen density and safety. Among them, MgH2, with a capacity of 7.6 wt% H2, is regarded as one of the most promising candidates owing to its high reversibility, low cost and environmental friendliness [9]. Unfortunately, it suffers from high thermodynamic stability and slow sorption kinetics [10].A variety of strategies have been developed to tackle these problems. One is to thermodynamically destabilize MgH2 by alloying Mg with other metal elements [11], such as Al [12], Ni [13], and Fe [14]. A representative intermetallic hydride is Mg2NiH4, which shows a favorable dehydrogenation enthalpy of 65\u00a0kJ mol\u22121 H2, [15] lowered by 10\u00a0kJ mol\u22121 H2 compared with that of the pristine MgH2. The reduced enthalpy change results in a low dehydrogenation temperature of 255\u00a0\u00b0C at 1\u00a0bar equilibrium H2 pressure. [15] However, a main drawback of this strategy is the inevitable capacity loss, where the theoretical hydrogen storage of Mg2NiH4 is only 3.6 wt% [16].Nanostructuring is also an effective way to modify the thermodynamics and especially kinetics of MgH2. [17\u201321] Theoretical calculations suggest that there is a significant decrease in thermodynamic stability when the grain size of MgH2 is reduced to less than 2.0\u00a0nm. [22] For MgH2 with a grain size of 0.9\u00a0nm, the decomposition enthalpy is only 63\u00a0kJ mol\u22121 H2, corresponding to a desorption temperature of only 200\u00a0\u00b0C. [23] The grain refinement also improves the hydrogen de-/sorption kinetics, owing to abundant diffusion paths and shortened diffusion distance. [24] Nevertheless, it is so far difficult to experimentally synthesize MgH2 NPs smaller than 20\u00a0nm. [25,26] In this case, attempts turn to confining MgH2 in various porous scaffolds, which makes it possible to obtain MgH2 in several nanometers according to the pore size of scaffolds. However, the scaffolds commonly take up considerable content in the confined systems, and hence the available hydrogen storage capacity is significantly lowered. For instance, a confined MgH2 system using activated carbon fibers as scaffold shows a low dehydrogenation enthalpy and a low activation energy of 63.8\u00a0\u00b1\u00a00.5\u00a0kJ mol\u22121 H2 and 1438\u00a0\u00b1\u00a02\u00a0kJ mol\u22121, respectively,\u00a0 [27] indicating reduced thermal stability and improved kinetics. However, this system can only load 22 wt% MgH2, representing a theoretical hydrogen capacity of merely 1.67 wt%.Introduction of catalysts is another effective way to reduce the dehydrogenation temperature and improve the reaction kinetics of MgH2, where transition metals and their compounds are the commonly used catalysts [28\u201333]. Among them, Ni has attracted considerable attention due to its active role in the dissociation of hydrogen, where a vacant d-orbital of Ni first accepts electron of hydrogen and then the bind is stabilized by back-donation of electrons from the filled d-orbital to the anti-bonding orbital (\u03c3*) of H2, thus facilitating the break of H\u2013H bond [34,35]. The combination of H atoms to form H2 molecular is just the inverse process. Especially nanosized Ni-based catalysts are highly effective since they can provide large contact surface area and abundant active sites [36]. This helps to decrease their addition amount for effective catalysis, hence minimizing the loss of the theoretical hydrogen storage capacity. However, nanosized Ni particles are dimensionally unstable and easily agglomerate during hydrogen sorption cycling due to the high surface energy [37]. Recent studies show that the size stability and catalytic activity of Ni NPs can be further enhanced by loading them on substrates like carbon-based materials [37,38]. Moreover, carbonaceous materials are also favorable for the nucleation of Mg or/and MgH2 and provide additional channels for hydrogen diffusion, exhibiting a synergistic catalytic effect with Ni [39]. The reported carbonaceous substrates for supporting nano-sized Ni particles include graphene nanosheets [24,39,40], carbon aerogel [41], mesoporous carbon CMK-3 [42], hard carbon spheres [16,43], and so forth. Nevertheless, the Ni loading in the currently reported carbon-supported catalysts is commonly low in order to stably disperse Ni NPs and maximize their catalytic effectiveness. Challenges remain on achieving small size, high loading and uniform dispersion of nano-Ni particles on substrates simultaneously. Although the hydrogen storage properties of MgH2 are improved by these carbon-supported nickel catalysts to date, developing highly effective catalysts is still highly imperative to further improve the overall hydrogen storage properties of MgH2 at milder conditions.In the present study, a type of porous hollow carbon nanospheres (PHCNSs) that we previously reported [44] was used as the substrate to prepare the Ni-incorporated PHCNSs composites, and their catalytic effect on MgH2 was investigated. The merits of PHCNSs as the substrate are as follows. First, the PHCNSs possess high specific surface area, large pore volume and hierarchical pore structure, which can provide large amounts of dispersive sites for Ni NPs. Second, the PHCNSs have a highly amorphous structure with massive defects, brought by the pores generated during the CO2 activation process. As reported, surface defects can change the electronic structures of carbon surfaces, creating trapping centers for metal atoms [45]. Lastly, except for C, the PHCNSs also contain 18.2 at% O and minor amounts of N and H, which are originated from the surface functional groups of PHCNSs. The heteroatom-induced coordination sites in carbon substrates can anchor metal precursors via chelation [45]. It is thus believed that the abundant dispersive sites, surface defects and heteroatoms of PHCNSs favor the dispersion of metal Ni NPs. In order to optimize the catalytic effect, different amounts of Ni are incorporated to the PHCNSs substrate. Up to 90 wt% Ni NPs can be highly dispersed on to the PHCNSs without agglomeration. With an addition of only 5 wt% of the 90 wt% Ni-incorporated PHCNSs, the system shows evidently lowered dehydrogenation temperature, improved kinetics and superior cycling stability. The evolution of phase compositions and microstructures of the system during cycling is investigated for identifying the reasons for the improved hydrogen storage properties.The detailed synthesis process and structural characteristic methods of the PHCNSs were as reported in our previous work [44]. The PHCNSs have intact spherical morphology with a uniform size distribution of ca. 90\u00a0nm, and with inner cavities in size of ca. 30\u00a0nm and hence a shell thickness of 30\u00a0nm. The specific surface area and pore volume of the PHCNSs are up to 2609 m2 g\u00a0\u2212\u00a01 and 2.275 cc g\u00a0\u2212\u00a01, respectively. The volume of the inner cavities is not included in the measured value as the size is beyond the measurement range of the analyzer. The nanopores generated by CO2 activation in the carbon shell are mostly smaller than 4.5\u00a0nm. In addition, the PHCNSs contain 18.2 at% O and minor amounts of N and H.Ni(NO3)2\u20226H2O (98%, Aladdin) was used as the Ni source. Different amounts of the PHCNSs and Ni(NO3)2\u20226H2O corresponding to mass ratios of Ni: PHCNSs of 5: 5, 7: 3, 9: 1 and 9.5: 0.5, respectively, were added to a medium of ethanol and stirred for 30\u00a0min. The mixed suspension was then subjected to ultrasonication under dynamic vacuum for 6\u00a0h to ensure a full wetting of the Ni(NO3)2 solution to the outer and inner surface of the PHCNSs, including the pores in the shell. During this process, most of ethanol medium is removed by vacuum extraction, leaving a viscous mixture, which is then dried at 60\u00a0\u00b0C for 12\u00a0h in a vacuum drying oven to remove the residual ethanol. Thereafter, the completely dried mixture was heat-treated at 500\u00a0\u00b0C for 4\u00a0h under a flowing mixture gas of 10 vol% H2/Ar for the thermal reduction of Ni precursor to metal Ni. The final products were denoted as Ni\nx\n@PHCNSs, where x represents the mass fraction of Ni in the composites.MgH2 used in the present study was synthesized in-house by hydriding a commercial Mg powder (Macklin, purity 98%, 20 - 100 mesh) at 340\u00a0\u00b0C under 20\u00a0bar H2 for 12\u00a0h. The obtained Ni\nx\n@PHCNSs composites were introduced to MgH2 at an addition amount of 5 wt% by ball-milling under 50\u00a0bar H2 for 24\u00a0h using a planetary ball mill (QM-3SP4, Nanjing). A ball-to-sample weight ratio of 120: 1 and a rotating speed of 500\u00a0rpm were used. The mill rotated for 0.3\u00a0h in one direction, and paused for 0.1\u00a0h, and then revolved in the reverse direction for 0.3\u00a0h, to minimize the temperature increase during ball milling. The obtained systems were denoted as MgH2/Ni\nx\n@PHCNSs. In addition, the MgH2/PHCNSs system was also prepared under the same experimental procedure for comparison. All operations were conducted in a glove box (MBRAUN 200B, Germany) filled with pure Ar (H2O < 1\u00a0ppm; O2 < 1\u00a0ppm) to avoid the moisture and air contamination.The crystal structures of the samples were analyzed by X-ray diffraction (XRD, MiniFlex 600 X-ray diffractometer, Rigaku). The samples were sealed in a custom-designed container covered with a Scotch tape to prevent air and moisture contamination during operation. The Ni content of the Nix@PHCNSs composites was measured by thermogravimetry analysis (Netzsch, TG 209 F3). A NOVA-1000e automated surface area analyzer (Quantachrome, USA) was used to conduct N2 sorption measurement, and the specific surface area and pore size distribution of the samples were calculated based on the Brunner\u2013Emmet\u2013Teller (BET) and density functional theory (DFT) methods. Morphologies of the samples were analyzed by scanning electron microscopy (SEM, Hitachi, S-4800) and transmission electron microscopy (TEM, FEI, Tecnai G2 F20 S-TWIN). The distribution of C, Ni and Mg elements in the samples was identified with an energy-dispersive X-ray spectrometer (EDS) attached to the TEM facility. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi) was used to determine the chemical valence state of Ni in the samples. XPS spectra were recorded using monochromatic an Al K\u03b1 (1486.6\u00a0eV) X-ray source with a base pressure of 5\u00a0\u00d7\u00a010\u00a0\u2212\u00a010\u00a0Torr of air. The adventitious C 1\u00a0s peak centered at 284.8\u00a0eV was used as a reference to calibrate the obtained XPS data, and the binding energy spectra were fitted by XPSPEK software.The temperature-dependent dehydrogenation properties of the catalyzed systems were evaluated by a custom-designed temperature-programmed desorption (TPD) instrument coupled with a mass spectrometer (MS). The TPD-MS curves at different heating rates (1, 2, 4, 8\u00a0\u00b0C min\u22121) were collected to evaluate the apparent dehydrogenation activation energies (E\na) of the systems based on the Kissinger method [46]:\n\n\n\n\n\nd\n\n(\n\nln\n\n\u03b2\n\nT\nP\n2\n\n\n\n)\n\n\n\nd\n\n(\n\n1\n\nT\nP\n\n\n)\n\n\n\n=\n\u2212\n\n\nE\na\n\nR\n\n\n\n\nwhere \u03b2 is the heating rate, T\np is the absolute temperature corresponding to the maximum desorption rate and R is the gas constant. In this work, T\np is the peak temperature of the TPD-MS curves with different heating rates. Dehydrogenation and hydrogenation of the systems were quantitatively measured by a volumetric method on a custom-designed Sievert-type apparatus. For the non-isothermal testing, the systems were heated from room temperature to 400\u00a0\u00b0C at a heating rate of 2\u00a0\u00b0C min\u22121 under static vacuum for dehydrogenation and heated from room temperature to 250\u00a0\u00b0C at a heating rate of 1\u00a0\u00b0C min\u22121 under 50\u00a0bar H2 for hydrogenation. For the isothermal testing, the systems were rapidly heated to the pre-set temperature and dwelled for 1\u00a0h under static vacuum for dehydrogenation, while for hydrogenation, the systems were first heated to the pre-set temperature under static vacuum and then rapidly loaded with 50\u00a0bar H2, dwelling for 10\u00a0min. A heating rate of 10\u00a0\u00b0C min\u22121 was used for both isothermal dehydrogenation and hydrogenation. Cyclic performance of the optimized system was evaluated with a regime of dehydrogenation to 350\u00a0\u00b0C under static vacuum and hydrogenation to 250\u00a0\u00b0C under 50\u00a0bar H2.The preparation process of the MgH2/Ni\nx\n@PHCNSs systems is illustrated in Scheme\u00a01\n. The porous hollow carbon nanospheres are firstly fully impregnated with the ethanol solution of nickel nitrate under dynamic vacuum. After heat reduction, the Ni NPs-loaded PHCNSs composites are obtained, which are then introduced to MgH2 by ball-milling. Fig.\u00a01\n presents the XRD patterns of the Ni\nx\n@PHCNSs composites with different Ni contents. The diffraction peaks of Ni (JCPDS no. 04\u20130850) are clearly seen for all composites, indicating the successful formation of elemental Ni with high crystallinity. Thermogravimetric analysis (TGA) of the Ni\nx\n@PHCNSs composites (Fig. S1) conducted in air shows that there are weight gains for the composites with high Ni contents while weight losses for the composites with low Ni contents, which is ascribed to the competitive result of the oxidation of Ni to NiO and the combustion of the PHCNSs to carbon dioxide during the heating process. Based on the TGA results, the Ni contents for the composites with weight ratios of Ni: PHCNSs of 5:5, 7:3, 9:1 and 9.5:0.5 are calculated to be 54.4, 72.2, 90.7 and 96.7 wt%, respectively, very close to the designed values.\nFig.\u00a02\n shows the SEM images of the Ni\nx\n@PHCNSs composites. It is seen that the spherical structure of the PHCNSs remains stable after the incorporation of Ni NPs. For the composite with Ni contents of 50 and 70 wt%, the morphology is almost the same as that of the original PHCNSs, without other evidently different morphology, suggesting that the Ni NPs are mostly deposited in the inner cavities and nanopores of the PHCNSs. With the Ni content increasing to 90 wt%, there are numerous bright dots corresponding to Ni NPs observed, which are uniformly deposited on the surface of the PHCNSs. The corresponding EDS mapping (Fig. S2) shows that Ni element is well distributed in the C substrate without obvious aggregation, demonstrating the ultrafine size and the well dispersion of Ni NPs. However, when the Ni content reaches 95 wt%, some bulk Ni aggregates with the size of several hundred nanometers are observed, as marked by white dashed circles in Fig.\u00a02d.Further TEM observation of the Ni\nx\n@PHCNSs composites is shown in Fig.\u00a03\n, where the bright area is the PHCNSs substrate and the dark dots represent Ni NPs. The stable spherical structure of the PHCNSs is demonstrated by white dashed circles, and the Ni NPs with an average size of ca.10\u00a0nm are uniformly dispersed on the surface of the PHCNSs and also in their inner cavities. The latter is generated from the nickel nitrate solution infiltrating into the inner cavities of the PHCNSs during the vacuum impregnation process. As seen from Fig.\u00a03a \u2013 d, with increasing Ni content from 50 to 95 wt%, the density of the dispersive Ni NPs is gradually increased. What's notable is that the Ni NPs up to a high loading of 90 wt% maintain fine size and high dispersibility, with almost no agglomeration. Although there is aggregated Ni observed at a higher loading of 95 wt%, most Ni particles are still dispersive, in consistence with the SEM observation. In addition, in the case that the samples for TEM characterization are subjected to strong ultrasonic treatment during the preparation process, the Ni NPs are still immobilized on the PHCNSs, suggesting a strong bonding between the Ni NPs and the PHCNSs substrate. Such a strong metal-substrate interaction is helpful for suppressing the aggregation of metal NPs and tailoring the geometric structures and electronic configurations of catalytic active sites [45].The nitrogen sorption isotherms and pore size distributions of the Ni\nx\n@PHCNSs composites (Fig. S3a and b) show that both specific surface area and pore volume value are all extremely lowered compared with those of the PHCNSs. Besides the observed Ni NPs that deposit at the surface and cavity of the PHCNSs, there are also some ultrafine Ni particles with the size of only a few nanometers dispersed in the pore channels of the carbon shell, leading to the reduction of porosity. The unique structure of the PHCNSs not only provides large amounts of dispersive sites for Ni NPs, but also realizes their hierarchical size distribution.Fig. 4a shows the XRD patterns of the as-milled MgH2/Ni\nx\n@PHCNSs systems as well as the pristine MgH2 and the MgH2/PHCNSs system. \u03b2-MgH2 is the main phase for all the systems. Besides, a minor amount of MgO is also identified from its main diffraction peak at 42.8\u00b0 (JCPDS no. 45\u20130946) although its intensity is very weak, which is suggested from the chemical reaction between MgH2 and the oxygen-containing functional groups of the during ball milling. Notably that there are no diffraction peaks of Ni detected for all the MgH2/Ni\nx\n@PHCNSs systems, possibly due to its low relative content. Further XPS analysis of the Ni 2p spectrum of the MgH2/Ni90@PHCNSs system, Fig.\u00a04\nb, shows that there are two peaks at 852.9 and 870.0\u00a0eV, which are well assigned to the binding energy of Ni 2p3/2 and Ni 2p1/2, respectively, [47,48] demonstrating the existence of elemental Ni. Therefore, it is obtained that ball milling is only a physical mixing process.A representative SEM image of the as-milled MgH2/Ni90@PHCNSs system (Fig. S4a) shows that the ball-milled system is composed of irregular particles with an overall size distribution from tens to hundreds of nanometers. By contrast, the as-milled pristine MgH2 shows an obviously large size distribution, where nano-scale and micron-scale MgH2 particles coexist (Fig. S4b). The result demonstrates that the introduction of the Ni\nx\n@PHCNSs composites enhance the ball milling efficiency by serving as grinding agents. Further TEM characterization of the as-milled MgH2/Ni90@PHCNSs system is conducted, as shown in Fig.\u00a05\na. The original spherical morphology of the Ni-incorporated PHCNSs disappears after ball milling, which is transformed into lamellar carbon with uniformly embedded Ni NPs under stress, covered homogeneously on the surface of MgH2. Although the lamellar carbon is hardly identified due to its poor contrast relative to MgH2, the superfine Ni NPs in size of ca.10\u00a0nm are vaguely seen, as marked by the circles in Fig.\u00a05a. EDS analysis (Fig.\u00a05b \u2013 d) further shows that Ni and C elements are uniformly distributed on the MgH2 particles without aggregation. It is thus obtained that the PHCNSs can not only disperse a high loading of Ni in the initial structure, but also preserve the high dispersibility of Ni NPs even after the high energy ball milling, which is likely due to the strong metal-substrate interaction as mentioned above.\nFig.\u00a06\na shows the temperature-dependent dehydrogenation behavior of the MgH2/Ni\nx\n@PHCNSs systems analyzed by TPD-MS measurement, and that of the pristine MgH2 and the MgH2/PHCNSs system is also shown for comparison. It is seen that with the introduction of only 5 wt% of the Ni\nx\n@PHCNSs composite, both onset and peak dehydrogenation temperatures of MgH2 are significantly reduced. By contrast, the individual PHCNSs has limited catalytic effect on the dehydrogenation of MgH2, where the onset dehydrogenation temperature is almost the same as that of the pristine MgH2 and the peak temperature is only 7\u00a0\u00b0C lower than 317\u00a0\u00b0C for the pristine MgH2, suggesting that Ni is central important for the catalysis. Additionally, it should be noted that there is an additional shoulder peak at 360\u00a0\u00b0C for the pristine MgH2, while such peak disappears in the MgH2/PHCNSs system, corresponding to an evidently reduced ending dehydrogenation temperature. As reported previously [49,50], such a shoulder peak is attributed to the nonuniform size distribution of MgH2 particle. The result illustrates that the PHCNSs act as the grinding agent during ball-milling, increasing the size homogeneity of the MgH2 particles. Moreover, with the Ni loading in the Ni\nx\n@PHCNSs composites increasing from 50 to 90 wt%, the reduction of the dehydrogenation temperatures is more significant, confirming that Ni plays the main role for the catalysis. Among them, the system introduced with Ni90@PHCNSs exhibits the lowest onset and peak dehydrogenation temperatures of 195\u00a0\u00b0C and 242\u00a0\u00b0C, respectively, which are 55 and 75\u00a0\u00b0C lower than those of the pristine MgH2. Further increasing the Ni loading to 95 wt% reverses the decreasing trend of the dehydrogenation temperatures, which is supposed due to the formation of the Ni aggregates, resulting a slightly lowered catalytic effect compared with the well dispersive Ni NPs in the Ni90@PHCNSs composite.The volumetric dehydrogenation curves of the MgH2/Ni\nx\n@PHCNSs systems as well as the pristine MgH2 and the MgH2/PHCNSs system are shown in Fig.\u00a06b. For the MgH2/Ni\nx\n@PHCNSs systems, the main dehydrogenation temperature range is evidently down-shifted compared with the pristine MgH2, and dehydrogenation almost accomplishes at ca. 310\u00a0\u00b0C. The reduction on the dehydrogenation temperature is extremely small for the MgH2/PHCNSs system, but the system does not contain the two-step dehydrogenation process as in the pristine MgH2. The result further demonstrates that nano Ni particles play important role in improving the dehydrogenation kinetics while the PHCNSs contribute to the uniform size distribution of the MgH2 particles, which are in good agreement with the results from the TPD-MS measurement. The main non-isothermal dehydrogenation properties of the investigated systems are summarized in Table\u00a01\n. The dehydrogenation capacities of the MgH2/Ni\nx\n@PHCNSs systems with different Ni loadings upon heating to 300\u00a0\u00b0C are very close, which are 6.3, 6.4, 6.5 and 6.4 wt%, respectively, for Ni contents of 50, 70, 90 and 95 wt%. Further heating to 400\u00a0\u00b0C only results in less than 0.5\u00a0wt% H2 released. By taking overall consideration of the dehydrogenation temperature and capacity, the MgH2/Ni90@PHCNSs system shows the optimized performance among all the systems, which is performed further studies on isothermal kinetics and reversibility.\nFig.\u00a07\na shows the isothermal dehydrogenation curves of the MgH2/Ni90@PHCNSs system as well as the pristine MgH2 at 225, 250 and 275\u00a0\u00b0C. It is seen that the dehydrogenation rate of the catalyzed system is significantly increased compared with that of the pristine MgH2. There are 3.8 and 6.2 wt% H2 released for the MgH2/Ni90@PHCNSs system dwelling at 225 and 250\u00a0\u00b0C for 30\u00a0min, respectively, while for the pristine MgH2, less than 0.5 wt% H2 is released at the same condition. When the isothermal temperature is set to a slightly higher temperature of 275\u00a0\u00b0C, there is 6.4 wt% H2 already released when the temperature is just approached to 275\u00a0\u00b0C, which is almost the stable value of the dehydrogenation capacity at this temperature. By contrast, there is only 1.4 wt% H2 desorbed for the pristine MgH2 upon heating to 275\u00a0\u00b0C, and the dehydrogenation capacity is only 2.2 wt% H2 even after dwelling for 20\u00a0min. The dehydrogenation products performed at 275\u00a0\u00b0C is further used for isothermal hydrogenation property testing. The corresponding hydrogenation curves at 100, 150 and 200\u00a0\u00b0C under 50\u00a0bar H2 are shown in Fig.\u00a07b, where hydrogen pressure is loaded only when the pre-set temperature is reached. It is seen that the MgH2/Ni90@PHCNSs system achieves a saturated hydrogen capacity of 6.3 wt% within only 100\u00a0s at 200\u00a0\u00b0C. When the isothermal temperature is decreased to 150\u00a0\u00b0C, a capacity of 6.2 wt% H2 is also achieved within 250\u00a0s. Even at a low temperature of 100\u00a0\u00b0C, there is still 5.3 wt% H2 absorbed with a dwelling period of 600\u00a0s. Whereas for the pristine MgH2, the hydrogenation capacities are only 0.6, 1.9 and 5.0 wt% at 100, 150 and 200\u00a0\u00b0C, respectively, even for a dwelling period of 600\u00a0s. It is concluded that the superfine PHCNSs-supported Ni NPs show highly bifunctional effect on both dehydrogenation and hydrogenation kinetics of MgH2.Based on the TPD-MS curves at different heating rates and the Kissinger's plots (Fig. S5a and b), the dehydrogenation apparent activation energy (E\na) of the MgH2/Ni90@PHCNSs system is estimated to be 98\u00b16\u00a0kJ mol\u22121, corresponding to a reduction of 30% compared with 139\u00a0kJ mol\u22121 for the pristine MgH2\n[4], the value of which is also smaller than those of other recently reported MgH2-catalyst systems [51\u201357]. Therefore, a reduced dehydrogenation energy barrier is achieved with the introduction of the Ni-incorporated PHCNSs composite, which contributes to the improvement in dehydrogenation kinetics.\nFig.\u00a08\n shows the selected cyclic dehydrogenation (a) and hydrogenation (b) curves of the MgH2/Ni90@PHCNSs system with a regime of dehydrogenation to 350\u00a0\u00b0C under static vacuum and hydrogenation to 250\u00a0\u00b0C under 50\u00a0bar H2. There is 6.8 wt% H2 desorbed in the first dehydrogenation. The dehydrogenation product is highly reversible in the subsequent hydrogenation. Notable that from the second cycle, the onset and ending dehydrogenation temperatures are further decreased to ca. 200 and 290\u00a0\u00b0C, respectively, and the dehydrogenation and hydrogenation curves are almost overlapped for the subsequent cycles, demonstrating a superior cyclic stability. The practical available hydrogen capacity after 50 cycles remains to be 6.4 wt%, corresponding to a capacity retention of 94.1%. In addition, the expression of cyclic de/re-hydrogenation kinetics of time versus hydrogen sorption capacities is shown in Fig. S6a and b. It is seen from that hydrogen is rapidly released during the heating period, especially from the second cycle. The main dehydrogenation occurs in 40\u00a0min from ca. 220\u00a0\u00b0C to 300\u00a0\u00b0C, corresponding to ca. 6.2 wt% H2 releasing. The dehydrogenation kinetics maintains high from the second cycle. There is ca. 6.0 wt% H2 released in 40\u00a0min from ca. 200 to 280\u00a0\u00b0C for the subsequent cycles. It terms of the hydrogenation curves, Fig.\u00a06Sb, the curves of time versus capacity is still overlapped in a high level. There is ca. 4.0 wt% H2 absorbed in the initial 50\u00a0min, corresponding to a temperature range of ca. 50 to 100\u00a0\u00b0C, then the absorption turns to slightly lower rates, where more ca. 2.5 wt% H2 is absorbed in 150\u00a0min from 100 to 250\u00a0\u00b0C.Although the hydrogen storage properties from different laboratories cannot be quantitatively compared because of the different testing programs, it is still informative to summarize the progress reported. Table\u00a02\n lists the comparison of the hydrogen storage properties of the present MgH2/Ni90@PHCNSs system and the representative MgH2 systems added with various carbon-supported nickel catalysts. Obviously, the present system shows lower dehydrogenation temperature, better hydrogen sorption kinetics and higher reversible capacity compared with those of the reported systems. This demonstrates that the ultrafine Ni particles and their high dispersibility is highly effective in improving the reaction kinetics of MgH2. Therefore, even an addition as low as 5 wt% of Ni90@PHCNSs results in significant improvement on the hydrogen storage properties of MgH2. The abundant pores, especially the ultrafine nanopore channels, and the large surface area of the present PHCNSs offer the possibility for the high content and well dispersive distribution of ultrafine Ni NPs, which contribute to a highly active catalysis to MgH2.\nFig.\u00a09\n shows the XRD patterns of the MgH2/Ni90@PHCNSs system at different dehydrogenation and hydrogenation states. After the first dehydrogenation, the diffraction peaks of MgH2 disappear and instead, peaks of Mg appear, demonstrating the complete decomposition of MgH2. Moreover, there are three new weak peaks detected at 39.8, 40.8 and 44.9\u00b0, matching well with Mg2Ni (JCPDS no. 35\u20131225), which suggests that the Ni NPs react with MgH2 in the initial dehydrogenation process forming Mg2Ni. The diffraction peaks of MgH2 reappear after the first hydrogenation, indicating the regeneration of MgH2, and besides, there is a minor amount of residual Mg, which may be responsible for the slight decrease of cyclic capacity. Meanwhile, the Mg2Ni phase is also hydrogenated and transformed to Mg2NiH4 (JCPDS no. 35\u20131225). The formation of Mg2NiH4 is likely to be the reason for the further reduction of the onset dehydrogenation temperature of the system after the first cycle (Fig.\u00a08), as Mg2NiH4 is easier to release hydrogen than MgH2\u00a0 [39] and shows better catalytic effect on MgH2 than pure Ni\u00a0 [60]. The onset dehydrogenation temperature of the catalyzed system from the second cycle (ca. 200\u00a0\u00b0C, Fig.\u00a08a) is lower than the theoretical decomposition temperature of Mg2NiH4 reported in literature (ca. 255\u00a0\u00b0C at 1\u00a0bar equilibrium H2 pressure)\u00a0 [15]. It is supposed due to the low H2 pressure in the dehydrogenation process, the extra active effect of the carbon interaction and the ultrafine particle size of Mg2NiH4 for the present system. Moreover, Mg2NiH4 is still detected in the 50th hydrogenated product. It is clearly that the in-situ formed Mg2Ni/Mg2NiH4 during the first cycle exist stably in the subsequent cycles, acting as highly effective catalyst for the hydrogenation and dehydrogenation of MgH2. In addition, the minor MgO derived from the reaction between MgH2 and the oxygen-containing functional groups of the PHCNSs during ball milling also remains during the cycling, with no evidently change in the diffraction feature.\nFig.\u00a010\na shows a SEM morphology of the MgH2/Ni90@PHCNSs system at the 50th hydrogenation cycle. It is seen that the overall morphology is similar as that of its as-milled state, without obvious particle growth. In contrast, the pristine MgH2 of the same state shows severe particle agglomeration (Fig. S7). Further EDS analysis of the catalyzed system (Fig.\u00a010b \u2013 e) shows that Ni and C elements are still homogeneously distributed in the MgH2 matrix without aggregation. The extra noises in the carbon map is originated from the carbon support film. It is thus concluded that the ultrafine size and the high dispersibility of the in-situ formed Mg2Ni/Mg2NiH4 phases maintain during hydrogen sorption cycles. Furthermore, the lamellar carbon covered on MgH2 particles act as barrier in suppressing the growth and agglomeration of MgH2 particles, both of which contribute to the superior cycling stability of the system.Ultrafine Ni NPs supported hollow carbon nanospheres (PHCNSs) are synthesized as the catalysts for promoting the hydrogen storage performance of MgH2. With a high loading of 90 wt%, the Ni NPs are well dispersed at the outer surface, in the inner cavity and also in the pore channels of the PHCNSs. Introduced to MgH2 by ball milling, the Ni NPs are uniformly distributed on the MgH2 particles with the help of the excellent dispersive role of the PHCNSs substrate, exhibiting superior bidirectional catalytic activity towards the dehydrogenation and hydrogenation of MgH2. The MgH2 system containing only 5 wt% Ni90@PHCNSs shows onset and peak dehydrogenation temperatures as low as 190\u00a0\u00b0C and 242\u00a0\u00b0C, respectively, and desorbs 6.5 wt% H2 upon heating to 300\u00a0\u00b0C. Moreover, 6.2 wt% H2 is rapidly released within 30\u00a0min at 250\u00a0\u00b0C and the dehydrogenation product can absorb almost the same amount of hydrogen within 250\u00a0s at 150\u00a0\u00b0C under 50\u00a0bar H2. Even at a low temperature of 100\u00a0\u00b0C, the system can absorb 5.3 wt% H2 in 600\u00a0s. A reversible dehydrogenation capacity of 6.4 wt% remains after 50 cycles, corresponding to a high capacity retention of 94.1%. The in-situ formed Mg2Ni/Mg2NiH4 inherit the superfine size and uniform dispersion of Ni NPs, acting as highly-active catalysts during the dehydrogenation and hydrogenation cycles of MgH2. The present work provides new ideas for developing highly effective and durable catalysts toward enhanced hydrogen storage properties of MgH2.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is supported by the National Key Research and Development Program of the Ministry of Science and Technology of PR China (No. 2018YFB1502103), National Natural Science Foundation of PR China (Nos. 52071287, 51571175, U1601212, 51831009).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jma.2021.05.004.\n\n\n\nAppendix A. Supplementary data\n\nImage, application 1\n\n\n\n", "descript": "\n                  Magnesium hydride (MgH2) is one of the most promising hydrogen storage materials for practical application due to its favorable reversibility, low cost and environmental benign; however, it suffers from high dehydrogenation temperature and slow sorption kinetics. Exploring proper catalysts with high and sustainable activity is extremely desired for substantially improving the hydrogen storage properties of MgH2. In this work, a composite catalyst with high-loading of ultrafine Ni nanoparticles (NPs) uniformly dispersed on porous hollow carbon nanospheres is developed, which shows superior catalytic activity towards the de-/hydrogenation of MgH2. With an addition of 5 wt% of the composite, which contains 90 wt% Ni NPs, the onset and peak dehydrogenation temperatures of MgH2 are lowered to 190 and 242\u00a0\u00b0C, respectively. 6.2 wt% H2 is rapidly released within 30\u00a0min at 250\u00a0\u00b0C. The amount of H2 that the dehydrogenation product can absorb at a low temperature of 150\u00a0\u00b0C in only 250\u00a0s is very close to the initial dehydrogenation value. A dehydrogenation capacity of 6.4 wt% remains after 50 cycles at a moderate cyclic regime, corresponding to a capacity retention of 94.1%. The Ni NPs are highly active, reacting with MgH2 and forming nanosized Mg2Ni/Mg2NiH4. They act as catalysts during hydrogen sorption cycling, and maintain a high dispersibility with the help of the dispersive role of the carbon substrate, leading to sustainably catalytic activity. The present work provides new insight into designing stable and highly active catalysts for promoting the (de)hydrogenation kinetics of MgH2.\n               "}
{"full_text": "Extensive use of fossil fuels is largely contributing to CO2 emissions and global warming. The current efforts of the scientific community is to develop sustainable technologies that can reduce CO2 emissions or even reach net zero or negative emission through the capture and conversion of CO2 [1]. CO2 conversion into CO, CH4, cyclic carbenes, polymers, etc can be achieved by a variety of methods [2\u20136]. CO2 capture and utilisation to produce fine chemicals accounts for a small percentage of the emitted CO2 levels [7]. A possible way to reach net zero emissions of CO2 is to use fuels that are derived from emitted CO2 [8,9]. However, CO2 is a stable molecule that requires a great deal of energy to activate, and therefore, efficient and low-cost conversion methods and catalysts for CO2 activation and conversion are highly sought. Current reseach in this area concern the use of Cr, Fe, Ni and Cu doped with a variety of materials (i.e. Ce, Cs, Zr or Y) or using photocatalysts in a photo-assisted revese water gas shift. Although the active phase varies quite considerably among these materials, the support media largely remains the same, usually metallic oxides like Al2O3, CeO2, ZrO2 or doped/mixed combinations.Production of higher hydrocarbon fuels through processes like the Fischer-Tropsch (F\u2013T) synthesis and hydrogenation of CO2 to form methanol via reverse-water-gas-shift reaction (CAMERE process) are promising routes to utilise emitted CO2 [10,11]. F\u2013T synthesis and CAMERE processes reported better efficiencies (approximately 20%) when CO generated from RWGS reaction was used as raw material [11]. Conversion of CO2 to CO through RWGS reaction is shown in (eq. 1). The reaction is endothermic and as such, is expected to demonstrate increased efficiencies at higher temperatures. There exists a major competing exothermic methanation reaction (Sabatier process), (eq. 2) that occurs at lower temperatures producing methane. A highly unwanted material where F\u2013T or CAMERE processes are concerned.\n\n(1)\nCO2 + H2\u2194 CO + H2O; \u0394H298 = +41\u202fkJ/mol\n\n\n\n\n(2)\nCO2 + 4H2\u2194 CH4 + 2H2O; \u0394H298= \u2212165\u202fkJ/mol\n\n\nAs an additional drawbackthe Sabatier reaction consumes 4\u202fmol of hydrogen per mol of CO2 thus imposing extra process cost. In this regard, if we aim to desing an efficient reverse water gas shift unit, it is of paramount importance to control the competition CO2-Methanation/RWGS to ensure the process is selective towards carbon monoxide. In this sense, a variety of catalytic materials has been investigated for RWGS reaction [12\u201315]. Noble metals, such as Au, Pt, Pd, Rh and Ru, exhibit high activity, stability and selectivity for CO2 reduction to CO. However, due to their cost and scarcity, it is desirable to replace these materials by introducing more ecominally appealing catalysts. Transition metals-based catalysts represent an economically interesting alternative [16]. For instance, copper-based catalysts were found to be more selective for CO production favouring RWGS reaction [17\u201324]. However, they suffered stability problems [25]. Modified Ni-based catalysts were also designed to achieve better selectivity and stability for RWGS reaction [26]. Bimetallic or metal alloy catalysts of Cu, Ni and Co exhibited activities comparable to noble metals and their alloys but, had stability issues [27]. Hence, in addition to catalyst activity and selectivity for CO, stability and sustainability of the chosen catalyst at reaction conditions are also critical.The nature of the catalyst support is known to influence the coking characteristics, stability and dispersion of catalysts. Different supports interact differently with the active catalyst [28,29]. For example, CeO2 supported Au was more active than the TiO2 supported Au catalyst due to the higher oxygen mobility of CeO2 [30]. in situ generated carbon support due to decomposition of a metal organic framework precursor showed high stability of the catalyst [31]. Mixed oxide supports of CeO2/ZrO2 or CeO2/Al2O3 supporting Ni altered the activity and selectivity of the catalyst when compared to unsupported Ni [26,32]. Reduced surface acidity of Al2O3 modified with CeO2 caused lesser extent of coking and retained catalyst activity for a longer time [33]. Specialised methods such as magnetron sputtering, atomic layer epitaxy (ALE), atomic layer/chemical vapour deposition, etc have also been employed to increase catalyst activity and stability [34,35]. Avoiding sophisticated, expensive techniques and excessive use of chemicals in catalyst preparation would make the whole process commercially more viable.Accordingly, it would be advantageous to design a sustainable synthesis of transition metal-based catalysts supported on low-cost, environmentally benign supports such as clays. Saponite is a smectite clay, having the formula NaMg6(Si7Al)O20(OH)4, with magnesium substituted 2:1 aluminosilicate layers and interlayer regions occupied by Na+ ions and water molecules. A number of transition metal based saponite catalysts has been used as catalysts in reducing gaseous compounds [36]. Change in the surface acidity of the magnesio-aluminosilicate clay layers in comparison to Al2O3 could affect the overall catalyst dispersion and stability. Recently, adamantanecarboxylates of transition metals and alkaline earth metals have been synthesized by sustainable green protocols. These adamantanecarboxylates under controlled decomposition are known to generate in situ carbon that stabilize metal nanoparticles/metal oxides/ mixed metal oxides [37].In this paper, aqueously exfoliated saponite clay layers have been used to provide better dispersibility and higher thermal stability to the active catalyst. The catalyst precursors, saponite-transition metal adamantanecarboxylates have been prepared by using metal hydroxides and 1-adamantane carboxylic acid. The resultant saponite / transition metal adamantanecarboxylates were reduced under hydrogen to achieve saponite supported carbon stabilized NiCu and NiCo metal alloy nanoparticles \u2013 our nature inspired multicomponent catalysts. These catalysts were then tested for reduction of CO2 to CO through RWGS reaction. The selectivity, activity and long-term stability of the resultant catalysts were tested and compared with that of monometallic Ni catalyst supported on saponite.Ni(OH)2, Cu(OH)2 and Co(OH)2 were used as the transition metal sources, and 1-adamantanecarboxylic acid, used as the carboxylate source (All chemicals were procured from Sigma Aldrich and used as received without further purification). Deionised water (18\u202fM\u03a9.cm resistivity, Millipore water purification system) was used throughout the experiment. Na+-saponite, NaMg6(Si7Al)O20(OH)4, was synthesized hydrothermally by a procedure reported by Kawi and Yao [38].In a typical synthesis of saponite supported NiCu-adamantanecarboxylate (NiCu-Ada/Sap), 1\u202fg of Na+-saponite was stirred in 100\u202fml of water for 2 days at room temperature to produce exfoliated colloidal dispersion of saponite clay. 3.79\u202fg 1-adamantanecarboxylic acid and 0.5\u202fg Ni(OH)2 and 0.5\u202fg Cu(OH)2, (1-adamantanecarboxylic acid / M\u202f=\u202f2) were added to the 100\u202fml exfoliated saponite suspension and stirred for 1\u202fh at room temperature. This reaction mixture was then transferred into a teflon-lined vessel and hydrothermally treated at 150\u202f\u00b0C for 24\u202fh. The resultant product was washed with excess of water to remove any ionic impurities and dried at 75\u202f\u00b0C overnight. The preparation of saponite supported NiCo-adamantanecarboxylates (NiCo-Ada/Sap) and Ni-adamantanecarboxylates (Ni-Ada/Sap) followed the same procedure, except that 0.5\u202fg of Co (OH)2 was used instead of Cu (OH)2 and 3.88\u202fg 1-adamantanecarboxylic acid was used for synthesis of NiCo-Ada/Sap. Ni -Ada/Sap was synthesised by using 0.5\u202fg Ni (OH)2 with 1.75\u202fg of 1-adamantanecarboxylic acid by following the same procedure.Saponite supported metal/metal alloys were synthesized by decomposing the catalyst precursors, NiCu-Ada/Sap, NiCo-Ada/Sap and Ni-Ada/Sap under reducing atmosphere by passing H2. In a typical decomposition experiment, about 1.0\u202fg of the catalyst precursors was loaded into a quartz tube and subjected to decomposition at 600\u202f\u00b0C under H2 atmosphere (50\u202fml/min, 10\u202f\u00b0C/min, residence time 2\u202fh).Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance power diffractometer, using Ge-monochromated Cu-K\u03b11 radiation (\u03bb =1.5406\u202f\u00c5) from a sealed tube, operating at 40\u202fkV and 40\u202fmA with a Lynx Eye linear detector in reflectance mode. Data were collected over 2\u03b8 angular range of 2\u201390\u00b0 with a step size of 0.009\u02da. Fourier Transform Infrared spectra (FTIR) of samples were measured using Perkin Elmer spectrometer in ATR mode (4000\u2013600 cm\u22121). Thermal analysis of the precursor samples was performed using a thermogravimetric analyser (TA Instruments TA 500). C and H analysis of the precursor samples was carried out by placing approximately 3\u202fmg of sample in a tin capsule and combusting in a high oxygen environment at 950\u202f\u00b0C using an Exeter Analytical CE-440 elemental analyser calibrated with acetanilide. Metal loading on saponite in NiCuSap, NiCoSap and NiSap were quantified by XRF using a Pananalytical Zeitium WD-XRF with a 4\u202fkW rhodium anode tube in helium path. During XRF analysis, each sample was weighed accurately and placed in the sample cups under helium path. The cups were prepared with 50-micron prolene film.Raman spectra were recorded using Renishawin Raman Microscope with a 785\u202fnm red laser operating WiRE\u00ae version 4.2. The data was obtained using a 10\u202fs exposure time with 5\u201310% laser power.X-ray Photoelectron Spectroscpopy (XPS) was undertaken using a K-ALPHA Thermo Scientific device, utilising Al-K radiation (1486.6\u202feV) and a twin crystal monochromator to produce a focussed x-ray spot at 3\u202fmA x12\u202fkV (400\u202f\u03bcm major axis length of the elliptical shape). Prior to the spectral acquisition, the samples were pre-reduced simulation the activation treatment. The data was then processed using the Avantage software package.N2 adsorption isotherms and textural analysis was performed on a Micrometrics 3Flex at 77\u202fK over P/P0 0\u20130.99 under nitrogen. Transmission electron microscopy (TEM), high resolution TEM (HRTEM) images are acquired using a FEI Titan Themis instrument equipped with a FEI SuperX EDX detector. Surface morphology and elemental analysis of spent catalyst were done by using JEOL 7100\u202fF scanning electron microscope (SEM) microscope with an Energy Dispersive X-Ray spectroscope (EDX) to determine both active phase dispersion and the presence and type of coking. Oxidation TGA experiments were undertaken on a Q500 V6.7, ramping from room temperature to 900\u202f\u00b0C at 10\u202f\u00b0C/min in air. Cation exchange capacity (CEC) was estimated by a method reported by Chapman et al. [39] by exchanging Na+ ions in Na+-saponite with NH4+ ions for three consecutive times with 1\u202fN NH4CH3COO solution. 0.1\u202fg of saponite was stirred with 10\u202fml of NH4CH3COO solution for 30\u202fmin. The supernatant was separated from saponite and collected in a volumetric flask. The exchange reaction with NH4CH3COO solution was repeated two more times and the supernatant collected as before into the same volumetric flask. The supernatant was diluted suitably to estimate for Na+ ions by comparing with a set of standard solutions using iCE 3300 AA Thermo Fischer Scientific Atomic Absorption Spectrophotometer (AAS). Na+ ions were estimated using Na- hallow cathode lamps at 589\u202fnm under air/acetylene flame.The RWGS experiments were conducted in a tubular quartz reactor (10\u202fmm ID) at atmospheric pressure. The catalysts were supported on a layer of quartz wool acting as a bed. The reactant gas flow used for temperature screening and stability tests contained CO2 and H2 in a ratio of 1:4 balanced with N2 to maintain a WHSV\u202f=\u202f15\u202fL/gcath. The gas products were analysed using an online gas analyser (ABB AO2020, ABB Ltd., Zurich, Switzerland) equipped with both an IR and TCD detectors. All catalysts were reduced pre-reaction in the reactor by flowing 100\u202fml min\u22121, 20% H2/ 80% N2 at 850\u202f\u00b0C for 1\u202fh. Temperature screening reactions were conducted using a temperature range of 300\u2013850\u202f\u00b0C at 50\u202f\u00b0C intervals. The stability study was conducted at 500\u202f\u00b0C for 89\u202fh.Saponite supported metal-adamantanecarboxylates were prepared by hydrothermally treating the respective metal hydroxides with twice the number of moles of 1-adamantanecarboxylic acid in the presence of exfoliated saponite clay layers as described in the experimental section. Only the required stioichiometric amounts of metal hydroxides (metal ion source) and 1-adamantane carboxylic acid (carboxylate ion source) were used for the synthesis and no excess amounts of chemicals were used. Exfoliation of smectite clays in water is spontaneous and well known [40]. Exfoliated clay layers have been used as 2D starting materials for synthesis of various composites [41]. Using exfoliated clay layers as opposed to bulk clays enables homogeneous mixing in the catalyst precursors by accessing the interlayer regions that were generally inaccessible prior to exfoliation. Metal hydroxides and 1-adamantanecarboxylic acid under hydrothermal conditions were precipitated as NiCu/NiCo/Ni-adamantanecarboxylates over the clay sheets. 1:2 ratio of metal to 1-adamantanecarboxylic acid was taken as divalent metal cations would need two monovalent 1-adamantanecarboxylate ions for charge compensation. The elemental (C and H) analysis of the resultant catalyst precursors (Table S1, Supporting information) confirms the presence of expected amounts of 1-adamantanecaboxylates in the samples. Excess amount of hydrogen was observed due to OH groups of clay layers and adsorbed water molecules.PXRD patterns of the as synthesized catalyst precursors and pristine saponite are shown in Fig. 1\n(a\u2013d). Saponite (Fig. 1a) shows first basal (00\u202fl) reflection at d-spacing of 12.56\u202f\u00c5 that matches well with the values reported for sodium ion intercalated saponite [38]. Other reflections observed at higher 2\u03b8 values of about 19.6\u00b0, 28.7\u00b0, 35.5\u00b0 and 60.5\u00b0 correspond to the 2D reflections of saponite. The (060) reflection at 60.55\u00b0 with d-value of 1.53\u202f\u00c5 categorises saponite as a tri-octaherdral smectite clay [38]. Empirical formula for saponite deduced from XRF analysis was found to be Na0.97Mg5.96Si6.94Al0.93O24.20H5.38. CEC of saponite that evaluates the amount of exchangeable interlayer cations in the clay was estimated by a method reported by Chapman et al. [39]. CEC of Na+-saponite was found to be 123\u202fmeq/100\u202fg which is quite high for smectite clays. High CEC values indicates spontaneous swelling and exfoliation of clays in water [42]. High degrees of exfoliation results in homogeneous composite precursors as the exfoliated 2D alumino-silicate clay layers are available for composite formation. Surface properties such as surface area, pore size and pore volume of the saponite was calculated by N2 adsorption technique as described in the experimental section. The saponite clay shows BET surface area of 147\u202fm2/g and pore diameter and volume were 3.1\u202fnm and 0.118 cm3/g respectively. Surface area value of saponite indicate a well stacked clay layers formed due to hydrothermal method of synthesis followed as reported [43].The PXRD patterns of Ni-Ada/Sap, NiCo-Ada/Sap and NiCu-Ada/Sap catalyst precursors shown in Fig. 1b, c and d respectively match well with Ni, Cu and Co-metal adamantanecarboxylates reported in literature [37]. Ni-Ada/Sap (Fig. 1b) shows characteristic reflections at 2\u03b8 values 6.09\u00b0, 6.29\u00b0, 6.76\u00b0, 11.10\u00b0, 12.03\u00b0, 15.97\u00b0, 16.57\u00b0, 17.77\u00b0 and 24.25\u00b0 corresponding to d-values of 14.49\u202f\u00c5, 14.03\u202f\u00c5, 13.06\u202f\u00c5, 7.96\u202f\u00c5, 7.35\u202f\u00c5, 5.54\u202f\u00c5, 5.35\u202f\u00c5, 4.98\u202f\u00c5 and 3.66\u202f\u00c5 respectively. Similar reflections were seen in the PXRD pattern of NiCo-Ada/Sap in Fig. 1c. PXRD pattern of NiCu-Ada/Sap catalyst precursor (Fig. 1d) have additional reflections (at 2\u03b8 values of 7.49\u00b0 and 8.69\u00b0 corresponding to d-values of 11.78\u202f\u00c5 and 10.16\u202f\u00c5 respectively) compared to the other catalyst precursors and this observation matches well with previous reports [37]. The (00\u202fl) reflection of saponite is not observed in the catalyst precursors, due to higher intensity of the metal adamantanecarboxylates phases. However, the 2D reflections of saponite clay layers are observed in the enlarged portion of the PXRD patterns of the catalyst precursors [Fig. S1 (supporting information)]. PXRD patterns of all the catalyst precursors, thereby, indicate composite formation of metal adamantanecarboxylates with saponite.FTIR spectra of all the catalyst precursors (Fig. 2\nb\u2013d) and saponite (Fig. 2a) show a characteristic strong SiO stretching vibration at around 950\u202fcm\u22121. Similarly, the hydrogen bonded OH stretching vibration is observed in all the samples at ca. 3400\u202fcm\u22121. All - trans, CH stretching modes are observed at 2850\u202fcm\u22121 and 2900\u202fcm\u22121 in the catalyst precursors (Fig. 2b\u2013d), attributed to the adamantane moiety. COO\u00af stretching vibrations (Fig. 2b\u2013d) observed between 1350\u202fcm\u22121 and 1550\u202fcm\u22121 confirm the presence of carboxylate ions in the catalyst precursors. The absence of CH and the carboxylate vibrations in Na+-saponite is evident from Fig. 2a. PXRD and FTIR analysis of the resultant precursor catalysts show the successful composite synthesis of metal adamantanecarboxylates over saponite clays sheets, as anticipated.\nFig. 3\n shows the thermal decomposition profile of Ni-Ada/Sap (Fig. 3b), NiCo-Ada/Sap (Fig. 3c) and NiCu-Ada/Sap (Fig. 3d) and saponite (Fig. 3a) under nitrogen gas flow as described in the experimental section. The different thermal decomposition profiles of the catalyst precursors in comparison to pristine saponite clay further indicates their composite nature. The NiCo-Ada/Sap and NiCu-Ada/Sap loses mass in three steps leaving about 30\u202fwt% residues. Ni-Ada/Sap loses 50\u202fwt% of mass, whereas saponite loses about 15\u202fwt% of its mass in accordance to previous reports [38]. Lower mass loss in the case of Ni-Ada/Sap in comparison to NiCo-Ada/Sap and NiCu-Ada/Sap samples was expected due to lower amounts of metal-adamantanecarboxylate in Ni-Ada/Sap. DTG profiles of the catalyst precursors are given as supplementary information, Fig. S2. All samples show a mass loss below 200\u202f\u00b0C that could be due to loss of water molecules. Pristine saponite sample (Fig. S2a) loses mass in two more steps at 557\u202f\u00b0C and 749\u202f\u00b0C due to loss of water of hydration, dehydroxylation of outer and inner hydroxyl ions as reported in literature [41]. Mass loss of Ni-Ada/Sap (Fig. S2b), NiCo-Ada/Sap (Fig. S2c) and NiCu-Ada/Sap (Fig. S2d) in between 200\u2013550\u202f\u00b0C could be due to the degradation of the metal-adamantanecarboxylates in the catalyst precursors.The catalyst precursors, NiCu-Ada/Sap, NiCo-Ada/Sap and Ni-Ada/Sap were reduced under hydrogen, as described in the experimental section, to obtain saponite supported NiCu, NiCo and Ni metal alloys/metal nanoparticles. PXRD patterns of the freshly prepared active catalysts NiSap, NiCuSap and NiCoSap are shown in Fig. 4\na, b and c respectively. Reflections due to the Ni metal, NiCo and NiCu alloys were observed along with those due to saponite. The reflections of Ni-metal (Fig. 4a) appear at 2\u03b8 values of 44.42\u00b0, 51.84\u00b0 and 76.33\u00b0 with d-values of 2.04\u202f\u00c5, 1.76\u202f\u00c5 and 1.25\u202f\u00c5, respectively. Similarly, reflections due to NiCu-alloy (Fig. 4b) are seen at 2\u03b8 values of 43.99\u00b0, 51.26\u00b0 and 75.32\u00b0 with d-values of 2.06\u202f\u00c5, 1.78\u202f\u00c5 and 1.26\u202f\u00c5 respectively. While, reflections due to NiCo-alloy (Fig. 4c) are seen at 2\u03b8 values of 44.37\u00b0, 51.63\u00b0 and 76.06\u00b0 with d-values of 2.03\u202f\u00c5, 1.76\u202f\u00c5 and 1.49\u202f\u00c5, respectively. Reflections due to saponite in the freshly prepared catalysts were observed at 19.67\u00b0, 28.42\u00b0, 31.12\u00b0 and 35.99\u00b0. The resultant active catalyst was also characterised using FTIR as shown in supporting information, Fig. S3. All samples show vibrations due to SiO at 950\u202fcm\u22121 due to saponite and CC vibration (1600 cm\u22121) due to residual carbon. The samples also show a broad vibration at 3400\u202fcm-1 due to adsorbed water.The metal loadings on saponite were determined by XRF analysis. Table 1\n shows the amount of Cu, Co and Ni present in various samples per gram of catalyst. The resultant catalysts were further characterized with Raman spectroscopy (Fig. S4, supporting information). The Raman spectra of the freshly prepared catalysts showed two intense bands which are attributed to vibration modes of sp2-bonded carbon atoms. The G-band observed at 1594\u202fcm\u22121 is due to the sp2 carbon stretching modes in aromatic rings derived from the incomplete decomposition of 1-adamanatanecarboxylate unit. The peak at approx. 1336\u202fcm\u22121 is the graphitic D-band that becomes active in the presence of structural disorders [44].The role played by the solid surface is essential in catalysis. Herein, x-ray photoelectron spectroscopy (XPS) allows us to determine the oxidation states and electronic environment of the elements in the outermost layers of the material. The Ni 2p3/2 spectra of all samples can be found in Fig. S5a, with the associated Cu 2p3/2 and Co 2p3/2 regions for the NiCu-Sap and NiCo-Sap catalysts in Fig. S5b and S5c, respectively. Table 2\n contains a summary of the main peaks found in Fig. S5. Prior to this analysis, all samples were reduced under the same conditions used before a reaction (850\u202f\u00b0C, 1\u202fh, 20% H2:80% N2). As seen in both Fig. S5a and Table 2, there are several Ni oxidation states that exist following reduction and are seen in the deconvoluted spectra. The bands ca. 851\u2013852\u202feV are characteristic of Ni0, while the bands at 852\u2013854\u202feV are attributed to Ni2+ species interacting with the support [26,45,46]. The band centred at binding energy (BE) 857\u202feV, in the case of the NiCu-Sap catalyst, is attributed to Ni2+ as part of surface NiCu alloy species [47,48]. This assignment is corroborated when considering the Cu 2p3/2 region that details a shift towards BEs associated with NiCu alloys [49]. The remaining bands are the shake-up satellite peaks associated with the previous species. However, the BE displayed in the NiCo-Sap catalyst at 855.48\u202feV is indicative of Ni3+ cations present [51,52], characteristic of NiCo alloys [53].The Cu 2p3/2 region in the NiCu saponite, Fig. S5b, shows two significant bands. One band at 932\u202feV can be assigned to the Cu+/Cu0 species, with the higher bands at 934, 940 and 943\u202feV attributed to the Cu2+ species and two shake up satellites, respectively [45,54]. The main bands for Cu at 932 and 934\u202feV can be explained to be at higher binding energies than monometallic Cu as found in literature, due to the charge transfer from Cu to the partially empty d-band present in Ni and the oxidation of Cu [55,56]. Such electronic interaction between the two metals has been theorised to contribute to increased catalytic activity since it results in an electronically rich metal-metal interface which is ideal for reactants activation [48,49]. Furthermore, the Ni-Cu interface has been identified to be the active site for the forward and reverse water gas shift reactions by enhancing CO/CO2 adsorption and supressing methane production [50]. Additionally, it has been found that high compositional contents of Cu in a Cu-Ni alloy sufficiently increased the reducibility and mesoporosity of the structure to subsequently increase the catalytic activity [45].Finally, the Co 2p3/2 spectrum for the NiCo saponite found in Fig. S5c details two main bands at 778 and 780\u202feV, which are attributed as Co3+ and Co2+, respectively [53]. While the other two bands are the associated shake-up satellites [57,58]. Another key factor found in the data for this region, is that the splitting (spin-orbit coupling) energy between the Co2p1/2 and Co 2p3/2 orbitals (not shown) is approximately 15\u202feV, further indicating the coexistence of Co2+/Co3+ species [57].The estimated Ni/Sap atomic dispersions (Table 2) show very similar surface dispersions of the Ni over the saponite material regardless of the inclusion of Co or Cu. This is indicative of the homogeneous precipitation of the metal-adamantane carboxylates over the exfoliated clay layers during the hydrothermal synthesis of catalyst precursors. However, the slightly increased value for the Ni-Sap could indicate a slight enrichment of surface Ni on the Ni-Sap sample.The textural properties of the active catalysts were analysed using N2 adsorption. Fig. 5\n shows the adsorption isotherms of all the samples and surface properties are tabulated in Table 3\n. All the samples show type IV adsorption isotherm with the H4 hysteresis loop characteristic of mesoporous solids. Surface area of the catalysts calculated by the BET method and pore size and pore volume were calculated by BJH method by using the desorption branch of the isotherm. The NiSap catalyst shows 120\u202fm2/g of surface area whereas, NiCuSap and NiCoSap show surface area of 85 and 87\u202fm2/g, respectively, which indicates the nucleation of the second metal in the catalysts\u2019 porous structure.Ultimately, however, beyond the onset relative pressure of the loop, the isotherms confirm the presence of narrow or slit shaped mesopores within the material that are confirmed by the BJH analysis of the material (Table 3) confirming average pore diameters between 2\u201350\u202fnm.Freshly prepared catalysts were further characterised by electron microscopy and the TEM images are shown in Fig. 6\n. Clay layers of saponite are clearly identified supporting the metal/metal alloy nanoparticles in the images. Fig. 6a and d show monodispersed NiCu nanoparticles in the range of 15\u201320\u202fnm homogeneously distributed over the clay layers. Fig. 6b and e show the NiCo nanoparticles with many of them having sizes between 10\u201315\u202fnm. A small percentage of the NiCo nanoparticles are however larger size measuring about 30\u201350\u202fnm. The bigger nanoparticles in NiCo could be due to aggregation of nanoparticles on the surface of the clay sheets. Similar observation was made for NiSap catalyst (Fig. 6c and f). Different shades of the nanoparticles could indicate that they are present at varying depths in the clay matrix. The lighter shaded, smaller nanoparticles in Fig. 6 could be the ones formed due to restricted growth in the clay interlayers at greater depths. Whereas, the nanoparticles formed on the surface of the clay layers might have undergone greater extent of agglomeration resulting in a small percentage of larger particles. Excluding which, the average particle sizes of monometallic Ni-nanoparticles in the catalyst varied between 10\u201315\u202fnm. It is worth noting that, larger nanoparticles are less abundant in the NiCuSap catalyst (Fig.6a and d) in comparison to the other two catalysts.NiCuSap was further characterised using STEM to map the active sites distribution on the clay support. The electronic image of the freshly prepared NiCuSap catalyst is shown in Fig. 7\na, along with the corresponding elemental maps of Ni, Cu, Mg, Al, Si and C. The NiCu-alloy nanoparticles are bright spots and the grey hazy matrix belongs to saponite clay sheets in the STEM image (Fig. 7a). The image also depicts a homogeneous dispersion of the NiCu-alloy nanoparticles in the saponite clay matrix. Complementing Ni and Cu elemental maps indicate the presence of both Ni and Cu in each of the nanoparticles. Saponite support displays coherent distribution of Mg, Al and Si accounting for homogeneously formed saponite clay layers. in situ generated carbon in the freshly prepared catalysts was found to be distributed homogeneously over the saponite clay layers. The presence of carbon in the catalysts was also indicated in the Raman analysis as discussed earlier.All saponite based catalysts were tested using a reactant ratio favourable for both the RWGS and methanation reactions and a WHSV 15\u202fLgcat\n\u22121\u202fh\u22121 that was applied across different temperatures as mentioned in the experimental section. Fig. 8\n shows the conversion and selectivity results of this testing and clearly depicting the competitive nature of both the RWGS and CO2-methanation reactions. The monometallic NiSap catalyst favours the methanation reaction, attaining 83% CH4 selectivity at 450\u202f\u00b0C. In fact, as pointed out in the XPS section, the monometallic sample has greater exposition of Ni on the surface acting as active centres for methanation. The NiCoSap material displays an intermediate behaviour with good levels of CO2 conversion and higer selectivities to CO compared to the monometallic sample. As for the NiCuSap catalyst a highly interesting trend was obverved, specifically due to preferential formation of CO over CH4, even at lower temperatures, where typically the methanation reaction is the dominant process. This is seen in the selectivity plots, Fig. 8b and c, where there is little to no methane produced, while CO is being produced in abundance. Overall the Ni-Cu catalyst is the best material within the studied series and hence we have compared its performance with reference systems. As shown in Table 4\n the NiCuSap shows either markedly improved or comparable performance as a number of transition metal and noble metal catalysts reported recently [22,26,31,59,62\u201364] using the same temperature window and reaction mixture (CO2:H2 1:4). Only the bimetallic Fe-Ni catalysts reported in [26] outperforms our CO2 conversion levels but the selectivy of the material is much lower than that exbited by our Ni-Cu catalysts. Hence the Ni-Cu/saponite catalyst represents an excellent balance activity/CO selectivy when compared with benchmark materials. The homogenous distribution and high dispersion of the Ni-Cu active centres shown in the STEM study, along with the Ni-Cu electronic interaction discussed in the XPS section, can explain the excellent performance of this sample. Cu suppresses the methanation activity of Ni and the Ni-Cu ensemble is an advanced active phase for the RWGS reaction, leading to high levels of CO2 conversion in the whole temperature range and remarkable selectivity levels towards CO. In fact the presence of Cu opens up the possibility to conduct the RWGS reaction via redox and/or formate mechanism as previously reported elsewhere thus favouring the CO route over the CH4 pathway [65]In any case, the superior behaviour of our catalysts compared to reference materials is indeed a very encouraging result and showcases the viability of low cost nature inspired multicomponent catalysts for the reverse water gas shift process. Due to this behaviour, the NiCuSap catalyst was selected for a stability study at 500\u202f\u00b0C as this temperature indicated significant conversion at lower temperature for the RWGS reaction, while not reaching equilibrium.The results shown in Fig. 9\n clearly illustrate this catalysts\u2019 resistance to deactivation, maintaining considerable conversion (ca. 55% CO2 conversion) and high selectivity for CO (ca. 80%) for over 89\u202fh of continuous reaction. Furthermore, these results are a considerabe enhancement over recently published materials.The suppression of methanation by copper containing materials has been previously reported by our team in the forward WGS reaction [60,61]. However, the potential of this alloy for the reverse water gas shift process in still under explored. The enhanced selectivity to CO at low temperatures is an encouraging result to achieve the successful coupling of RWGS with downstream processes such as Methanol synthesis and Fischer-Tropsch which typically take place at around 250\u2013350\u202f\u00b0C. This way we could close the cycle: CO2 conversion to fuels and chemicals in a two step-process with the RWGS as front unit and the F\u2013T or methanol conversion reactors as second unit to produce the upgraded end products.The x-ray diffraction patterns of the spent catalysts from the temperature screening experiments are shown in Fig. 10\n. All the diffractograms display peaks at around 28.2\u00b0, 31.05\u00b0 and 35.5\u00b0 2\u03b8 due to saponite support. Additionally, the expected peaks for the loaded metal/metal alloys at around, 44\u00b0, 51\u00b0 and 76\u00b0 remain unaltered in the spent catalysts in comparison to the freshly prepared catalysts. No phase segregation of the metal alloys was observed after the screening tests and therefore the PXRD patterns of the spent (Fig. 10) and the freshly prepared catalysts are identical. While the results of the Scherrer equation from the respective PXRD patterns of both the fresh and the spent catalysts (after temperature screening experiment) displays no change, indicating almost no agglomeration of the active metal/metal alloy nanoparticles; due to the overlap of the crystal peaks, it is impossible to determine the level of sintering present for the individual components. This explains partially the continuous activity of NiCuSap catalyst for 89\u202fh with negligible loss in activity. Presence of crystalline carbon peaks could not be found, making any carbon formation amorphous. The stability pattern of the catalyst is characteristic of the in situ generated carbon stabilized metal nanoparticle catalysts [31]. However, in this case, saponite support and the metal alloy combination has added to the conversion levels of CO2 and improved selectivity of the catalyst for RWGS reaction.Following the temperature screening experiments, the spent sample was analysed using a JEOL 7100\u202fF Scanning Electron Microscope (SEM) with an Energy Dispersive X-Ray spectroscope (EDX) to determine both active phase dispersion and the presence and type of coking. Fig. S6, S7 and S8 present the SEM/EDX results for the post reaction NiCuSap, NiCoSap and NiSap samples, respectively. These results show clearly well dispersed active phase with some small amount of amorphous carbon present on the surface of the spent catalysts, which is in good agreement with the lack of crystalline carbon peaks in the spent materials XRD diffractograms. Fig. S8 clearly details the presence of Ni as small particles on the surface of the material, while Fig. S6 and S7 show that the Ni is highly dispersed throughout the catalyst.Combusting the spent material under air from room temperature to 900\u202f\u00b0C at 10\u202f\u00b0C/min revealed several zones (Fig. 11\n). Each sample underwent an initial loss between room temperature (RT) \u2013 160\u202f\u00b0C that is attributed to free water loss. Each sample then displayed a significant weight gain (+5-14\u202fwt %) in between 200\u2013450\u202f\u00b0C for NiCuSap and NiCoSap that is attributed to the oxidation of the metals. The same weight gain zone for the NiSap occurred at the slightly higher zone of ca. 275\u2013500\u202f\u00b0C.The spent NiCoSap catalyst displayed a two-step weight loss totalling 3.8\u202fwt% which is attributed to the loss of surface carbon and then engrained carbon. In a similar fashion, the spent NiCuSap catalyst details a one-step weight loss (1.5\u202fwt %), which is also attributed to the loss of surface carbon. These conclusions are supported by the presence of an exothermic heat flow curves for the oxidation of the metals and the associated oxidation of the amorphous carbon (not shown). The weight gain of these materials being related to metallic oxidation is further supported by the curve displayed for the NiSap material, which details a much smaller increase owing to its monometallic loading. Interestingly, however, the NiSap material did not display any weight loss at higher temperatures,. In any case the TGA profiles corroborate the absence of crystalline carbon deposits in good agreement with XRD. This observation along with the lack of metallic sintering validate that these \u201cnature inspired\u201d catalysts developed in this work are not only highly active, but also very robust for the RWGS reaction.This work demostrates the viability of nature inspired transition metal based catalyst for gas phase CO2 upgrading via RWGS. Aqueously exfoliated saponite magnesio-aluminosilicate layers have been effectively used to support the synthesis of metal adamantanecarboxylates under hydrothermal conditions. The catalyst precursors underwent controlled decomposition under hydrogen atmosphere to produce in situ generated carbon stabilised saponite supported metal alloy catalysts. All the as prepared catalysts (mono: Ni and bimetallic: Ni-Cu and Ni-Co) display excellent activity levels in the RWGS process outperforming the activity levels exhibited by reference catalysts reported in literature. Interestingly, the undesired parallel reaction \u2013 the Sabatier process \u2013 which typically is the dominant reaction in the low temperature window can be supressed using Ni-Cu alloys as active phases. Indeed, the bimetallic Ni-Cu system is the best performing material within the studied series with an outstanding balance activity/CO selectivity in addition to be a very stable catalysts for long term runs. The electronic interaction Ni-Cu evidenced by XPS contributes to this exceptional behaviour. Indeed, such a close metal-metal contact results and electronically rich Ni-Cu interface which is ideal for CO2 activation. No signs of carbon deposition due to the reaction, nor metallic sintering were observed, explaining the enhanced stability of this material.Considering a potential application where the RWGS unit is coupled to a downstream process using syngas such as Fischer-Tropsch or methanol synthesis \u2013 the obtained results are very encouraging since our Ni-Cu catalyst is very active and selective towards CO in the low temperature range minimising the temperature gap between RWGS and the Fischer-Tropsch or methanol unit. In other words, the catalysts developed in this study may facilitate the integration of a RWGS reactor with a syngas convertor \u2013 such an integrated dual system would enable the direct conversion of CO2 to added value chemicals.The authors declare that they have no competing interests.The team at Surrey acknowledges the financial support provided by the EPSRC grant EP/R512904/1 as well as the Royal Society Research Grant RSGR1180353. This work was also partially sponsored by the CO2Chem UK through the EPSRC grant EP/P026435/1. The RCCS team at Heriot-Watt University acknowledges the financial support from EPSRC through grants EP/N024540/1 and EP/N009924/1, as well as the Buchan Chair in Sustainable Energy Engineering.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118241.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Chemical CO2 upgrading via reverse water gas shift (RWGS) represents an interesting route for gas phase CO2 conversion. Herein, nature inspired clay-based catalysts are used to design highly effective materials, which could make this route viable for practical applications. Ni and transition metal promoted Ni saponite clays has been developed as highly effective catalysts for the RWGS. Saponite supported NiCu catalyst displayed a remarkable preference for the formation of CO over CH4 across the entire temperature range compared to the saponite supported NiCo and Ni catalysts. The NiCu sample is also highly stable maintaining \u223c 55% CO2 conversion and \u223c 80% selectivity for CO for long terms runs. Very importantly, when compared with reference catalysts our materials display significantly higher levels of CO2 conversion and CO selectivity. This confirmed the suitability of these catalysts to upgrade CO2-rich streams under continuous operation conditions.\n               "}
{"full_text": "\n\n\nNo data was used for the research described in the article.\n\n\nNo data was used for the research described in the article.\n\nAbbreviations\nAIL,\n\nAcidic ionic liquid;\n\nBDC,\n\nBenzene dicarboxylic acid;\n\nBTC,\n\nBenzene tricarboxylic acid;\n\nDAIL,\n\nDicationic acid ionic liquids;\n\nGua,\n\nGuanidine;\n\nHKUST,\n\nHong Kong University of Science and Technology;\n\nHPA,\n\nHeteropolyacid;\n\nIL,\n\nIonic liquid;\n\nMBAIL,\n\n2-Mercaptobenzimidazole IL;\n\nMIL,\n\nMat\u00e9riaux de l\u2032Institut Lavoisier;\n\nMSA,\n\nMethanesulfonic acid;\n\nPTSA,\n\nParatoluenesulfonic acid;\n\nPTA,\n\nPhosphotungstic acid;\n\nPMA,\n\nPhosphomolybdic acid;\n\nUiO,\n\nUniversitetet i Oslo;\n\nZIF,\n\nZeolitic imidazole framework\n\n\nAcidic ionic liquid;Benzene dicarboxylic acid;Benzene tricarboxylic acid;Dicationic acid ionic liquids;Guanidine;Hong Kong University of Science and Technology;Heteropolyacid;Ionic liquid;2-Mercaptobenzimidazole IL;Mat\u00e9riaux de l\u2032Institut Lavoisier;Methanesulfonic acid;Paratoluenesulfonic acid;Phosphotungstic acid;Phosphomolybdic acid;Universitetet i Oslo;Zeolitic imidazole frameworkThe shift from fossil fuels to renewable biofuels is being forced by rising energy use and the related environmental challenges [1]. Biodiesel [also known as fatty acid methyl esters (FAMEs)] has been known to be a greener alternative to diesel. Biodiesel has attracted a lot of interest in recent years because of its renewability and sustainability and can be used as a substitute for fossil fuels that are becoming scarce [2]. Environmental concerns have risen as a result of the massive use of these conventional resources, prompting calls for green and alternative resources [3]. The catalysts, feedstocks, and by-products are the key differences between transesterification and esterification reactions. Long-chain triglycerides are commonly used as feedstocks for transesterification, which necessitate high-quality oils that are free of free fatty acids (FFAs), water, and contaminant. Most edible oils, such as soybean, sunflower, and rapeseed, are used for biodiesel synthesis by transesterification [4,5].The classic homogeneous and heterogeneous catalysts for biodiesel generation are rapidly being phased out in favour of innovatively designed catalysts [6,7]. Homogeneous catalysts have been exploited, however, their low recyclability and high corrosivity require proper disposal procedures. Furthermore, homogenous-catalysts for biodiesel purification are supposed to consume a lot of water [8\u201310]. Heterogeneous catalysts, on the other hand, are profitable due to their relative ease of separation for reusability and limiting mass transfer. To support green chemistry and sustainable biorefining, increasing the activity and efficiency of heterogeneous catalysts are critical [10\u201312]. Several magnetized catalysts were used in this situation for easier separation from waste resources. In this context, nanocatalysts are becoming a popular study topic as a way to speed up reactions and improve catalytic efficiency [13,14].Metal-organic frameworks (MOFs) are porous materials in which molecular building components (such as organic linkers and metal ions/inorganic clusters) are linked together by strong coordination interactions [15\u201317]. MOFs have shown considerable promise in sectors like as separation, storage, delivery, and catalysis due to their high function tunability, porosity, and crystallinity [18,19]. MOF structures have been reported by Tomic and others since the 1960s [20]. MOF research has blossomed since the early 1990s, notably Yaghi and his research group developed MOF-based porous materials [21,22]. MOFs are often made via a method known as modular synthesis, where porous shapes are formed by the gradual formation of crystals from a heated solution utilizing a nucleation and growth mechanism [23].Following Yaghi and co-workers\u2019 report on MOF-5 (shown in Fig.\u00a01\n) synthesis, the chemistry of MOF synthesis sparked a lot of attention in the material research world [24\u201327]. MOF-5 is a 3D cubic network made up of tetrahedral [Zn4O]6+ clusters connected by Benzene dicarboxylic acid (BDC) ligands [28]. This innovation was further developed, with ultraporous MOFs like MIL-101 (MIL stands for Material Institute Lavoisier), ultra-stable MOFs like UiO-66 (UiO stands for University of Oslo) and flexible MOFs like MIL-88 [29\u201332]. Synthetic strategies to bring hierarchy into secondary MOF structures, such as epitaxial growth, controlled assembly, and labilization approaches, have recently been developed in MOF growth, resulting in MOF tertiary architectures with remarkable complexity and characteristics [33,34]. The current level of hierarchy and functions of artificial framework materials, on the other hand, lags significantly behind the complex hierarchical systems seen in nature [35,36]. For the improvement of cooperative catalysis in MOFs, it will be vital to optimize the capability to control these hierarchical structures on various scales, as it needs tuning of both the activity of the catalytic core as well as the selection of porous framework [37,38]. In recent years, several noteworthy review articles on MOFs have been reported. Cong et\u00a0al. [39] emphasized the mechanism of functionalized MOFs and the catalytic performance of MOF-based catalysts. Basumatary et\u00a0al. [40] discussed different types of MOFs used for a variety of feedstocks to convert into biodiesel and machine learning techniques to optimize process parameters. Depending on different MOFs, the catalyst's characteristics and activity vary quite a bit. Thus, it is crucial to outline the role of MOFs to direct the catalyst synthesis employed in biodiesel production for future research. Following this idea, Ma et\u00a0al. [6] tried to summarize the functions of different MOFs categorically and discussed the challenges in MOF synthesis. MOFs hold the possibility as an effective heterogeneous catalyst for the generation of biodiesel with intriguing features of huge porosity, uniform pore size, controllable functional groups, and structural tunability enabling itself as an ideal material for biodiesel production in the form of acidic, basic, bifunctional, and enzymatic catalysts [39,41,42]. To achieve this goal, a review on biodiesel production using MOF-based catalysts is required to strengthen their environmentally favourable uses [43,44]. Moreover, there is a need of addressing the challenges in the MOF synthesis and catalysis which causes deactivation and leads to less catalyst reusability. Although, several reviews have come up with promising objectives, none has given a detailed study on deactivation of MOFs and methods to overcome the poor catalytic activity of MOFs after a couple of cycles of reuse. The major objective of the present work is to review various MOF-based catalysts having primary application in biodiesel production. This review emphasised on MOF stability based on their reusability and the study of deactivation of MOFs catalyst due to various factors which recently published reviews [39,45\u201349] have not discussed in detail although it carries a lot of significance in the field of MOF research. Several characterization techniques also have been discussed in brief which could be used to obtain the most critical analytic results from the perspective of the future progress of MOFs.MOFs modular nature allows for an almost endless number\u00a0of structures to be imagined. Statistical data analysis in Fig.\u00a02\n depicted the increasing trend of published research papers in the field of metal-organic framework. These data were collected from \u201cScopus Database\u201d using the keyword \u201cmetal-organic framework\u201d, that signifies the tremendous scope and need of MOFs in various fields of applications. MOFs, which are constructed by combining inorganic units with organic linkers, are the most promising materials among all the novel catalysts [50], with benefits such as high surface area, high pore size, structural stability, and tunable functions [51,52]. There is a lack of study on specialized MOF-based catalysts for biodiesel synthesis [53]. Numerous studies referring to MOFs for biodiesel generation have been reported in the last decade [54] and few inspections have been done on the customized applications of MOF-based catalysts in biodiesel production.Biodiesel is produced in an energy-efficient manner, which necessitates catalysts with intense activity, selectivity, and stability. The use of pristine MOFs as catalysts is limited by their poor activity due to a lack of active sites, as well as their low mechanical, thermal, and chemical stability due to the metal-linker coordinative bond's fragility, thus making them unsuitable for direct application in biodiesel refineries despite their unprecedented surface area, tunability and porosity [55,56]. Hence, based on MOFs synthesis and significant features they are broadly classified as acidic, basic, bifunctional, and enzymatic catalysts. The list of various MOFs with their metal ion and the structure of the ligand are enlisted in Table\u00a01\n that is discussed in this review.MOFs are being used as catalysts in biodiesel production and must be precisely characterized using several analytical techniques which provide a lot of ideas to predict the catalytic activity of catalysts to produce greater quality of biodiesel. Some of the significant characterization techniques have been discussed to emphasize roles of analytical techniques in MOF synthesis.Among several used characterization techniques powder X-ray diffraction (PXRD) is commonly used to investigate the degree of crystallinity and particle size of the MOF catalyst. In the XRD pattern of CoFe2O4/MIL-88B(Fe)-NH2 reported by Xie and Wang [57], the characteristic XRD peaks of CoFe2O4 and MIL-88B (Fe)-NH2 could be easily found in the XRD patterns which were in quite good agreement, indicating the successful formation of the CoFe2O4/MIL-88B(Fe)-NH2. Further Xie and Wan reported [58] another polyoxometalate- based sulfonated ionic liquid functionalized MOF AIL/HPMo/MIL-100(Fe) where the XRD peaks (Fig.\u00a03\n\ni) were consistent with pristine MIL-100(Fe), with no distinct XRD peaks attributable to HPMo or AIL, implying that the MIL-100(Fe) framework structure remained stable during the surface modification operations and that the active species of AIL and HPMo were well-dispersed on the MOF support. Wu et\u00a0al. [59] described the confinement of Fe3O4 and ionic liquid in the NH2\u2212MIL-88B(Fe) MOF material using XRD as the tool of analysis (Fig.\u00a03. ii) which remained as crystalline material like the MOF with a slight shift in Bragg's position compared to the pristine. Such conformational predictions on catalysts could easily be reported using PXRD as an analytical technique.New MOF structures are being reported to have high specific surface areas, that require MOF characterization, this regard, accurate surface area measurements are crucial, as this is a key characteristic of microporous materials [60]. Generally, MOF surface areas are computed using Brunauer\u2013Emmett\u2013Teller (BET) theory, which obtains surface areas from gas adsorption isotherms, typically with the usage of N2 gas and other gases like CO2 and Ar.Elyazed et\u00a0al. [61] reported a series of UiO-66(Zr)-structured materials with defects where BET analysis results revealed that inclusion of electron withdrawing groups changed the physical properties of those materials as the specific surface areas of UiO-66(Zr), UiO-66(Zr)-NH2 and UiO-66(Zr)-NO2 were 1115, 823 and 649\u00a0m2/g, respectively. Rafiei et\u00a0al. [62] strategized to immobilize lipase (5.2\u00a0nm) inside ZIF-67 MOF, but the pore diameter of MOF came out to be 1.2\u00a0nm and surface area 1320\u00a0m2g\u22121 from BET analysis which allowed them to came up with another method where they incorporated a lipase solution into the initial reaction mixture forming a lipase@ZIF-67 nano bioreactor. The BET-characterization technique has a vital role in the MOF synthesis as MOFs with high surface area, porosity and stability could be better candidates for biodiesel production. Han et al. [63] synthesized MIL-100(Fe) and MIL-100(Fe)@DAILs MOFs, further BET analysis results (Fig.\u00a04\n) of which confirmed the presence of DAILs in the nanocages of MIL-100(Fe) as the surface area and total pore volume decreased from 1183 to 170\u00a0m2g\u22121 and from 0.72 to 0.20\u00a0cm3g\u22121 for MIL-100(Fe) and MIL-100(Fe)@DAILs, respectively. Therefore, routes of synthesis like encapsulation, impregnation and immobilization can be confirmed for catalysts using BET analysis as a tool of characterization.Scanning electron microscope (SEM) and Transmission electron microscope (TEM) are one of the most extensively used instruments for the investigation of micro- and nanoparticle imaging and solid object characterization. SEM and TEM both are the powerful tools in providing invaluable topography and morphology about any material under investigation.The size, shape, composition, structure, and other physical and chemical aspects of a specimen are well known from SEM analysis [64]. Hirschle et al. [65] reported a SEM analyses on a sample that was synthesised by drying an ethanol-based dispersion of Zr-fumarate MOF nanoparticles (NPs) that revealed spherical morphology (Fig.\u00a05\na) of the NPs. Wang et al. [66] synthesized 3D- hollow porous hierarchical Co/Ni@C microspheres from bimetallic Co/Ni-MOF as a precursor. They observed that Fig\u00a05(a), (b) exhibited a spherical shape and constituted of several nanosheets with thickness of 2\u00a0nm, hollow structure of the MOF microspheres could clearly be observed from the SEM micrographs in Fig.\u00a05 (c) while Fig.\u00a05 (e) and (f) revealed morphology without structural breakdown after carbonization in N2 atmosphere, indicating the great structural stability of the 3D Co/Ni@C.One of the most common applications of TEM in MOF research is to provide direct confirmation of the crystalline structure [67]. The direct imaging of both missing-linker and missing-node defects in UiO-66(Zr) was observed by Liu et\u00a0al. [68] where they used HRTEM analysis that revealed both missing-linker and missing-node defects were present corresponding to reo and scu nets (as shown in Fig.\u00a06\n). Furthermore, HR-TEM study of isoreticular series of MOF-74 [69] also revealed the DOT (2,5-dioxidoterephthalate) link as shown in Fig.\u00a07\n displaying the expanded pore aperture.X-ray photoelectron spectroscopy (XPS) analysis is highly recognized as a measurement instrument for a wide range of organic and inorganic materials [70]. The survey spectrum is obtained using an XPS spectrometer, from which the components present can be identified. Individual spectral peaks are then investigated with a higher energy resolution to reveal chemical state information [71]. The binding energies (BEs) of ejected photoelectrons from atoms at or near the surface of a catalyst are measured using the XPS analysis technique. It enables for the detection of electronic changes as a result of varying surface penetration and material composition [72,73].Ni-MOF [74] consisting of Ni, P, O in Ni-PO ((Nickel Phosphate) displayed the high-resolution Ni 2p XPS spectrum in Ni-PO, where the Ni 2p was deconvoluted into two doublets and two satellites peaks. The asymmetric peak of P 2p. spectrum in Ni-PO was deconvoluted into two signals indicating that all P atoms were in the +5-oxidation state (shown in Fig.\u00a08\n).In Co-BDC (Benzene dicarboxylic acid) MOF [75], the XPS spectrum displayed the peaks of C1s, O1s, and Co 2p (Fig.\u00a09\n). Incorporation of carboxyferrocene (Fc) led to the origin of Fe 2p peak in Co-BDC-Fc MOF [76]. O 1\u00a0s in Co-BDC\u2013Fc MOF has greater binding energy and wider peaks, showing that the inclusion of missing linkers has changed the coordination at the active centre environment. In comparison to Co-BDC (1.65\u00a0eV), the valence band maximum energy of Co-BDC\u2013Fc shifts to the vacuum level at around 0.37\u00a0eV (shown in Fig.\u00a010\n), implying that inserting missing linkers can effectively alter the electronic structure of MOFs.Thermogravimetric analysis (TGA) in which a sample specimen is submitted to a controlled temperature programme in a controlled atmosphere and the weight change of the material is monitored as a function of temperature or time [77]. For isothermal investigations, the weight of the sample is plotted against time, while for tests carried out at a constant heating rate, it is plotted as a function of temperature. When a loss of mass occurs due to thermal degradation or desolvation, this approach is commonly used to monitor thermal stability and the loss of volatile components [78].Saha and Deng [79] reported about the stability of MOF-177 (a framework consisting of a [Zn4O6]6+ cluster and linker 1,3,5-benzenetribenzoate) where TGA was performed under continuous oxygen flow and evacuated circumstances using MOF-177. When handled under oxygen flow, the weight loss of the MOF-177 sample was found to be very less (3.65\u00a0wt.%) from 25\u00a0\u00b0C to 330\u00a0\u00b0C (Fig.\u00a011\n) due to desorption of gas and solvent molecules, while there was no weight loss for the evacuated sample. At temperatures ranging from 330 to 420\u00a0\u00b0C, the oxygen-treated sample lost 74.7\u00a0wt.% of its weight, while the sample in vacuum lost 55\u00a0wt.% due to the framework breakdown. From such advantageous analysis technique, the structural change in the MOF-177 was further compared with the XRD of the heated sample at 330 and 420\u00a0\u00b0C, which confirmed the structural phase change during the heat treatment.Feng et\u00a0al. [80] compared the TG-plots (shown in Fig.\u00a012\n) of UiO-66, hemilabile-UiO-66 (HI-UiO-66) and hemilabile-UiO-66 sulphate (HI-UiO-66-SO4) and reported that the average number of defects per cluster increased from 4.4 to 6.0 after treating the samples with H2SO4 solution, and greater thermal stability was observed with growing numbers of defects, i.e., UiO-66 (450\u00a0\u00b0C), Hl-UiO-66 (480\u00a0\u00b0C), and Hl-UiO-66-SO4 (515\u00a0\u00b0C). Therefore, TGA, over the years has been proved to be one of the efficient tools in finding the defects while engineering different MOFs as based on the weight loss the number of defects can be calculated [81,82].Biodiesel, mixture of FAMEs generated through a transesterification process, is a carbon-neutral and sustainable energy source that can meet the world's growing energy demand [83]. Due to the establishment of subsidiaries and tax exemptions, this green fuel has steadily become more affordable and widely used in many parts of the world. Glycerol, the main by-product of biodiesel manufacturing plants, which accounts for about 10% of the total volume, can be valorized into combustion improvers for diesel/biodiesel, such as solketal, solketalacetin, and acetins, to further strengthen the industry's economic benefits [84]. The biodiesel production process includes different generation of feedstock such as first generation (edible oils- Soybean oil, Palm oil, Mustard oil, Coconut oil, Olive oil, etc.), second generation (non-edible oils- Neem oil, Jatropha oil, Karanja oil, Rubber seed oil, etc.), third generation (Microalgae and Waste cooking oils) and fourth generation (Photobiological solar fuels and Electro-fuels) [85]. Biodiesel production is popularly carried out by transesterification and esterification process.Transesterification-It is the most widely used biodiesel synthesis technology, that is carried out in three steps. Triglyceride interacts with alcohol in the first step, producing monomolecular Fatty acid alkyl esters (FAAE) and diglyceride. The monomolecular FAAE and monoglyceride are formed when diglyceride combines with alcohol. Finally, alcohols react with monoglycerides to produce monomolecular FAAE and glycerol (shown in Fig.\u00a013\n) [86].Esterification- The carbonyl group in carboxylic acid is first protonated using an acid catalyst during esterification. The alcohol group acts as a nucleophile and attacks on the carbonyl carbon with intermediates formation. The proton is then next transferred with the elimination of H2O, and lastly the proton is removed with the production of an ester [87] (shown in Fig.\u00a014\n).The transesterification and esterification processes depend on several factors for a productive synthesis of biodiesel. These factors/parameters include methanol to oil ratio (MTOR), catalyst loading, reaction temperature and reaction time.\n\n\n(i) Alcohol to oil ratio (A:O)\n\n\n\n\n(i) Alcohol to oil ratio (A:O)\nThe stoichiometric relation between alcohol and oil for the transesterification process to carry out is 3:1. However, to drive the transesterification process towards forward reaction, excess methanol is required [112]. The transesterification reaction is reversible in nature so after a threshold, excess methanol starts decreasing the conversion rate of FFA. Kataria et\u00a0al. [113] observed the highest yield of 98.5% biodiesel from vegetable oil using a methanol to oil ratio as 12:1. Beyond the optimum ratio they observed depletion in yield as excess methanol produced excess glycerol which could cause blocking of active sites in the catalyst. Bhatia et al. [114] also observed similar kind of results when using A:O ratio of 25:1 obtaining a conversion of 75. 3% and any further increase in A:O would decrease the conversion rate. Furthermore, excess methanol may deactivate enzymatic catalyst and affect the overall biodiesel conversion [115]. Hence, conversion of different oil feedstocks to biodiesel requires optimization of A:O ratio to obtain maximum and successful conversion.\n\n\n(ii) Catalyst loading\n\n\n\n\n(ii) Catalyst loading\nCatalyst loading is one of the vital parameters which accelerates the reaction process of transesterification/esterification. After the attainment of optimum catalyst loading, sometimes excess addition of catalyst increases the viscosity of the reaction mixture and restrict the mass transfer of reactants and products to and from the catalyst reactive sites. Santya et al. [116] reported optimum catalyst loading of 1.5\u00a0wt.% for the conversion of 96.23% biodiesel from WCO. They observed that further increase of catalyst loading to 2 and 2.5\u00a0wt.% decrease the conversion rate. Mass transfer limitation was also found to be the major reason for the downfall in the conversion rate to biodiesel by Muhtaseb et\u00a0al. [117] since upon increasing the catalyst loading from 4.5\u00a0wt.% (optimum) they observed a declining rate in conversion.\n(iii) Reaction temperature\nReaction temperature is also an important factor for biodiesel production. Generally high temperature reduces the viscosity of liquid and faster the transesterification/esterification reaction. In some reported catalysts by Guan et\u00a0al. [118], Laskar et al. [119], and Silva et al. [120] high biodiesel yield even have been obtained at room temperature. However, beyond optimum temperature there is decrease in the yield of biodiesel since the reaction process should be within the boiling temperature of alcohol to prevent evaporation of alcohol. Gunay et al. [121] also evidently claimed that mainly high temperature encourages saponification reaction that deaccelerates the biodiesel yield. Thus, the reaction temperature should be optimised to obtain a high biodiesel yield.\n\n\n(iv) Reaction time\n\n\n\n\n(iv) Reaction time\nReaction time is one of the significant parameters which is also one of the deciding factors of turnover frequency of the catalyst. An optimum reaction time period is necessary for an effective biodiesel production because once reaching the maximum conversion the catalyst would lack active sites as a result long reaction time would push for saponification which would retard the transesterification reaction [122]. During transesterification reaction if catalyst amount is higher upon increment of time could decrease the viscosity of the reaction mixture but with long period if temperature is higher than boiling point of alcohol, there is chance of methanol evaporation too. Hence, reaction parameters are interdependent and require optimisation based on the types of feedstocks and catalyst used for the production of biodiesel [123].Low-quality oils have a high concentration of FFAs and moisture, which can drastically destabilize the alkali catalyst while also causing serious separation problems. Although basic catalyst is highly efficient for transesterification of triglyceride in vegetable oil, it cannot esterify the FFA to biodiesel. In the meantime, acidic catalyst can catalyse esterification of FFAs and transesterification of triglycerides in vegetable oil simultaneously. Hence it is desirable to use acidic catalyst for the conversion of vegetable oil with high FFA to biodiesel [14,124]. Table 2 provides the list of acid functionalized MOFs employed for the preparation of biodiesel reporting the conditions, biodiesel yield, catalyst stability in terms of number of reuses and analytical/spectroscopic techniques used to prove their stability.\nLiu et al. [7] have synthesized a highly stable sulfonated catalyst using MIL-100(Fe) MOF with dilute sulfonic acid. The induction of SO3H causes a degree of obstruction, which results in reduction of surface area from 1150.7 to 6.18\u00a0cm2\ng\n\u22121. Furthermore, after being sulfonated by dilute sulphuric acid, the pore diameter of MIL-100(Fe) increases from 1.6 to 9.88\u00a0nm, which is favourable for esterification because the large pore is advantageous to boost the effective interaction between catalyst and reactants. The catalyst reusability test was investigated under the optimal conditions (MTOR of 10, reaction temperature of 70\u00a0\u00b0C, catalyst loading of 8 and reaction time of 2\u00a0h) and the conversion of oleic acid to biodiesel was found to reduce from 95.86% to 88.5% at fifth cycle. The stability of the recovered catalyst was assessed from FT-IR analysis. However, further decline in the conversion to 58.34% after the seventh cycle was attributed to the deactivation of catalyst due to leaching of -SO3H.Pangestu et al. [101] produced a MOF based heterogeneous catalyst from the coordination of benzene-1,3,5-tricarboxylic acid and divalent copper, Cu-BTC MOF by solvothermal method. The morphology of Cu-BTC is affected at a moderate temperature of 100\u2013110 \u00b0C where it exhibited rod-like structure while at a higher temperature the shape altered to round shape structure. Moreover, the reusability test of catalyst is a tool of stability measurement which most importantly discusses about the catalyst activity as it proceeds to be used in repetitive cycles of reactions. Cu-BTC under optimised conditions (see Table\u00a01) afforded biodiesel yield of 91% whereas recycled catalyst gave a yield of 86% from palm cooking oil indicating a slight decrease in its activity, attributing to blockage of active sites. Activating the catalyst through thermal treatment and series of solvent exchange can reactivate the active sites which could impart potentiality in the catalyst to provide a higher yield of biodiesel. However, no characterization data were provided to prove the recovered catalyst stability. The study of the stability of recycled catalyst is an important in heterogeneous catalysis, particularly in biodiesel synthesis since there are huge possibility of pore blocking or leaching of active sites due to the high molecular weight of reactants.Ionic liquids (ILs) incorporated MOFs have been regarded as innovative materials with tremendous potential in a variety of fields. ILs are molten salts in liquid form at low temperatures, often below 100\u00a0\u00b0C, and consist of large asymmetric organic cations and inorganic or organic anions. Wan et al. [11] proposed a new methodology to construct polyoxometalate (POM), and MIL-100 MOF composite where POM encapsulated into the cage of the MOF using direct hydrothermal method followed by sulfonation making the catalyst more acidic than the MIL-100(Fe)@DAILs catalyst reported by Han et al. [63] to obtain a higher conversion of 94.6% biodiesel from oleic acid. The phosphotungstate was evenly distributed throughout the interior of the grains and additionally the ionic liquid had no effect on the phase distribution of POM, indicating that the heteropolyanion-based IL had been successfully inserted into the MIL-100 cages. Xie and Wan [58] reported another phosphomolybdenum-sulfonated IL functionalized MIL-100(Fe) MOF which was developed using a heterogeneous MOF microreactor to combine the benefits of Lewis and Br\u00f6nsted acids by immobilising IL on the MIL-100(Fe) host matrix, which led to a substantial rise in surface acidities. The recovered catalyst after first cycle was used further under the optimised conditions of MTOR of 30:1, catalyst loading of 9\u00a0wt.%, reaction temperature of 65\u00a0\u00b0C and reaction time of 5\u00a0h exhibiting conversion of 90.3% without a significant loss attributing to the ion exchange effect that electrostatically linked the imidazolium cations to the POM anions. Thus, the active species was prevented from leaching\u00a0off the MIL-100(Fe)support. Additionally, Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) confirmed leaching of 1.8\u00a0ppm of molybdenum in the biodiesel thus reasoning out the decrease in catalytic activity in fifth cycle. Further Xie and Wan [90] synthesised Keggin-type 12-tungstophosphoric heteropolyacid (HPW) encapsulated in UiO66\u20132COOH, prepared via the in-situ synthesis method with the incorporation of AIL (acidic ionic liquid) [SO3H- (CH2)3\n\n\nHIM][HSO4], resulting in biodiesel conversion of 95.8% from soybean oil. Over the years hierarchically porous Zr based MOFs have been exploited to a great extent. Ye et al. [89] followed a strategy of approximate ligand substitution where task specific IL (TSIL) was introduced into the porous network of Zr MOF. After ligands and metal clusters were replaced with propionic acid, UiO-66 was etched to form hierarchical porous UiO-66 (H-UiO-66). The numerical optimisation of catalytic activity done using response surface methodology (RSM) approach provided optimum reaction parameters (MTOR of 10.39:1, catalyst loading of 6.28\u00a0wt.%, reaction temperature of 80\u00a0\u00b0C and reaction time of 5\u00a0h) for biodiesel yield of 93.82%.Wu et al. [59] reported IL based MOF catalyst by with the incorporation of 1,4-butanediyl-3,3\u2032-bis-(3-sulfopropyl) imidazolium dihydrogensulfate in amino-functionalized magnetic MOF composite, DAIL-Fe3O4@NH2\u2212MIL-88B(Fe) optimized through RSM with fine catalytic performance of 93.2% conversion rate of oleic acid. Xie and Wang [57] reported an IL based acidic MOF catalyst where the reaction of pyridine with 1,3-propane sultone, followed by ion exchange with POM acids, produced POM-based sulfonated ILs containing Br\u00f6nsted-Lewis acid sites, (indicated as (Py-Ps) PMo) which was then injected onto the CoFe2O4/MIL-88B(Fe)-NH2 framework. The functionalized catalyst was novel in that it could simultaneously catalysed the transesterification of soybean oil and the esterification of FFAs. Additionally, the catalyst exhibited a conversion of 82.5% even after five cycles which is not far from the initial activity of 95.6% conversion. This minor loss in activity could be attributed to the partial deactivation of the catalyst due to the possible leaching of IL and blockage of active pore sites. However, these assumptions were not proved with convincing evidence.Recently Youssef et al. have reported MOF-5 [102], synthesized from BDC linker and zinc nitrate hexahydrate which could convert WCO and Jatropha Curcas oil (JCO) without any further functionalization. The transesterification reaction of WCO using the MOF-5 resulted in a decrease of FAME conversion from 82.0 to 41.2% when investigated for reusability. There might be a loss of active sites or a deposition of intermediates in the pore of the catalyst, which could be the probable reasons for such decrease in catalytic activity of MOF-5. Catalyst stability issue should be addressed to understand the deactivation mechanism. Copper-based MOF efficiently oxidized alcohols, transforming them into corresponding products, making it a viable catalyst for improved and stable biodiesel generation [130]. Jamil et al. [40] synthesized a Cu-Ca-MOF where the Cu-MOF showed the biodiesel yield of 78.3% while Ca-MOF recorded 78% and Cu-MOF\u00a0+\u00a0Ca-MOF showcased 85% of biodiesel yield. Also, the Cu-MOF\u00a0+\u00a0Ca-MOF showed a specific gravity of 0.88 (in the range of ASTM standards, 0.86\u20130.9).Microwave-assisted technology is a promising technique that has transformed chemical synthesis and research [131]\n[132]. Salam et\u00a0al. [97] synthesized Mg3(bdc)3(H2O)2 via microwave (MW) irradiation (Fig.\u00a015\n), then in turn used microwave for the production of biodiesel from oleic acid. A high conversion of 97% was obtained with the reaction conditions of methanol to oil molar ratio of 15:1, catalyst loading of 0.15\u00a0wt.%, power of 150\u00a0W, and reaction time of 8\u00a0min. Because of the MOF's hygroscopic nature, methanol molecules were completely replaced by water molecules after heating at 220\u00a0\u00b0C under vacuum conditions and subsequently exposing to the surrounding environment.Heteropolycids have some drawbacks of low surface area and high solubility in the reaction medium which limits their industrial applications while to overcome this immobilization of HPA come up as new strategy. Nikseresht et\u00a0al. [126] reported a heterogeneous catalyst that fabricated by heteropolyacid and Fe(III) based MOF under ultrasound irradiation at ambient temperature and pressure. In reusability test of the PTA@MOF, it was recovered 5 times during oleic acid conversion and showed upto 80% conversion in the last cycle. ICP analyses showed 5\u00a0wt.% PTA leaching during the recovery process which could be due to clinging of PTA ions to the network of the MIL-53. The cages of the PTA-MIL-53 were occupied by phosphotungestic acid (PTA) molecules, so the weight loss of bare MOF was the greatest (confirmed from TGA-analysis Fig.\u00a016\n), and the weight loss of prepared PTA-MOF was the least.Liu et al. [125] reported UiO-66-supported sulfonic acid catalysts (MSA-UiO-66) which successfully converted palmitic acid into biodiesel using impregnation method of synthesis. After ten runs, elemental analysis revealed that the\u00a0MSA concentration was 209.3\u00a0mg g\n\u22121 in the solid catalyst supporting the hypothesis that low alcohol polarity prevented organosulfonic acid leaching during the esterification reaction process. Another stable catalyst MOF-801 was reported by Shaik et\u00a0al. [103] a unique microporous Zr-based structure consisting of Zr6 nodes connected by fumarate linkers converted used vegetable oil (UVO) to biodiesel confirmed from 1H NMR of biodiesel and the morphology analysed by SEM provided a dense and defect-free morphology.Rodr\u00edguez et al. [100] synthesized Co based acidic MOF by the inclusion of two organic linkers 1,2-di-(4-pyridyl)-ethylene and 5-nitroisophthalic with the inorganic node of Co(NO3)2\u00b76H2O resulting in the yield of Erythrina Mexicana oil into biodiesel by 83.7%. The biodiesel mainly consists of certain alkyl esters (as confirmed from gas chromatography) that included methyl linoleate (21.3%), methyl palmitate (15.24%), methyl oleate (4.8%), methyl stearate (37.2%) and methyl arachidate (0.98%).Although basic catalysts can cause saponification with low-quality raw oils, but the base-catalysed system is relatively favourable because of the mild reaction conditions i.e., lower temperature and shorter time (enlisted in Table\u00a03\n). Xie and Wan [104] developed a core\u2013shell structured Fe3O4@HKUST-1 composite by using layer by layer assembly method with the encapsulation of basic ionic liquid (Fig.\u00a017\n). The amino groups in the ABILs molecule can act as plausible coordination sites to the coordinative unsaturated metal sites (Cu2+) in the HKUST-1 framework and the ABILs are thought to be attached to the Fe3O4@HKUST-1 support via a coordination mode.Furthermore Xie and Wan [105] reported another basic MOF ZIF-90-Gua where the guanidine base anchored onto the porous support by covalent bonds to form the hybrid solid catalyst. Due to the firmly covalent binding of guanidine to the support, the such-formed solid catalyst was expected to possess long-terms catalytic activities. Notably the acidity and basicity were assessed by the Temperature programmed desorption (TPD) peak integration from CO2 and NH3 profiles of the ZIF-90 and ZIF-90-Gua catalyst. When Gua loading was increased in the support from 2.26 to 4.02\u00a0mmolg\u22121, the basicity, as well as the corresponding soybean oil conversion over the solid catalyst were both increased from 1.05 to 1.56\u00a0mmolg\u22121 and 81.38 to 95.42%, respectively.Saeedi et al. [106] synthesized basic solid catalyst out of Zeolitic imidazole framework doped with potassium, which was used without any prior thermal pre-treatment at high temperatures, which is commonly used to activate heterogeneous catalysts for the activation. The maximum biodiesel conversion of 98% was achieved with 0.08% of potassium loading. ICP (Inductively coupled plasma) analysis was used to analyse the leached metals in the biodiesel phase after the catalyst was removed. The authors analysed the potential leaching of potassium and sodium in the produced biodiesel and have observed a leaching of 7 and 3\u00a0ppm respectively, indicating that very little amount of metal dissolved in the reaction medium during the transesterification process.MOFs can be a better choice if they can be utilized as a support to restrict leaching of metal oxides. Li et al. [133] have developed a strontium oxide supported by MIL-100(Fe) derivate for transesterification of palm oil using a novel magnetic catalyst generated by using MIL-100(Fe) as a carrier to support strontium carbonate and calcining it in an inert atmosphere producing SrO-MIL-100(Fe) derivative. The highest conversion of 96.19% was accomplished at a methanol/oil molar ratio of 12 at 65\u00a0\u00b0C after 30\u00a0min. Further, the catalyst showed a conversion of 82.49% after 3 cycles.Yang et\u00a0al. [134] developed a basic catalyst MgO@Zn-MOF synthesised by thermally decomposing Mg precursor particles contained within Zn-MOF to MgO without disrupting the Zn-MOF structure using different rou\u00e9 as shown in Fig.\u00a018\n. MgO nanoparticles were found to be inside the Zn-MOF particles and the shape of the particles grew more irregular and the relative intensity of Mg to Zn elements rose as concentration of Mg was raised from 12.1 to 24.1\u00a0wt.%. A substantial amount of MgO nanoparticles were deposited at the outer surface of the Zn-MOF particle when the concentration of Mg was 24.1\u00a0wt.%. The catalytic activity of the catalyst towards transesterification was investigated under the optimised conditions (MTOR of 3:1, catalyst loading of 1\u00a0wt.%, reaction temperature of 210\u00a0\u00b0C and reaction time of 2\u00a0h) to obtain a biodiesel yield of 73.3\u00a0wt.%. Further, towards the recyclability the catalyst activity dropped to 5.9% but the MOF structure was not found to be much affected, and the crystallite size remained unchanged as evident from the XRD of spent catalyst (Fig.\u00a019\n).ZIFs (Zeolite Imidazolate Frameworks) have received a lot of attention since they combine the benefits of both zeolites and regular MOFs. Another basic catalyst Fazaeli and Aliyan [135] reported using ZIF-8 MOF where graphene oxide (GO) nanoparticles was encapsulated into the large sodalite cavity structure of it and further ZIF-8@GO doped with Sodium and Potassium. The KNa/ZIF-8@GO exhibited much rougher surfaces than the aggregated graphene nanosheets in a dried state which might be attributed to the adsorption of cubic ZIF-8 nanoparticles on the surface of graphene sheets. The solid basic catalyst was found to contain\u00a0K loading of 0.05%, resulting in a 98% soybean oil conversion. It was observed that with the increase of catalytic run during recyclability test (Fig.\u00a020\n) there was decrease in the biodiesel yield which was attributed to the development of multilayer on the support due to blockage of active sites.Porous magnetic materials are increasingly employed as support for preparing heterogeneous catalysts. Not only for its abundant pore structure, but also it allows for quick and simple separation by using extra magnetic field compared with the conventional catalyst support. Li et\u00a0al. [136] synthesized magnetic mesoporous Fe@C support SrO heterogeneous catalyst where MIL-Fe (100) was initially employed as precursor to obtain the mesoporous magnetic support (Fe@C) through carbonization under nitrogen atmosphere followed by loading of SrO. The morphologies of Fe@C and Fe@C-Sr depicted highly uniform virgulate shaped particles in comparison with Fe@C, Fe@C-Sr showed an obvious rise in size with cross linked rod structure and more rugged surface revealing SrO was loaded on porous Fe@C. Moreover, the saturation magnetization of Fe@C-Sr used in the fourth cycle (displayed in Fig.\u00a021\n) was 87.95\u00a0emu g-1, where the corresponding magnetization curve almost coincided with fresh Fe@C-Sr indicating the magnetic substance was stable during transesterification.Table 3 summarizes the use of base functionalized and MOF-derived basic catalysts for biodiesel production along with the yield of biodiesel and evidence for catalyst stability through number of reuses.Bifunctional MOF catalysts with both acidic and basic active sites have been considered as a suitable candidate for esterification/transesterification for low-quality raw oils. Table 4 provides the experimental conditions, biodiesel yield, number of reuses and stability evidence for these MOFs. Jeon et\u00a0al. [108] reported a bifunctional catalyst where in the HZN (HPA functionalised ZIF-8 nanoparticles) show a bi-functionality resulting from the acidity of HPA and basicity of the imidazolate in ZIF-8. The total basicity of 22.82\u00a0mmol g\n\u22121 from the pure ZIF-8 is approximately half compared to the total acidity of 41.95\u00a0mmol g\n\u22121 from the pure HPA, along with a small acidity from pure ZIF-8 (4.26\u00a0mmol g\n\u22121). The catalyst converted rapeseed oil into 98% biodiesel under the optimized conditions of MTOR 10, catalyst amount of 4\u00a0wt.%, reaction temperature of 200\u00a0\u00baC and reaction time of 2\u00a0h.Recently another bifunctional catalyst was reported by Ahmed et\u00a0al. [61], where a series of UiO-66(Zr)- structured materials with defects were used as solid catalysts for the esterification reaction of oleic acid. They observed that the catalytic activity of UiO-66-(Zr) -NH2 > UiO-66-(Zr) -NO2 > UiO-66(Zr) prior to the bifunctionality and biodiesel conversion. The electron donating groups like -NH2 group are inductively pushing electron toward zirconium sites through benzene ring and decrease the acidity of zirconium centers to increase lewis basicity of Zr-sites and Br\u00f8nsted basicity of Zr-OH, Zr-O-Zr. The BET surface areas of UiO-66(Zr), UiO-66(Zr)-NH2, and UiO-66(Zr)-NO2 were 1115, 823, and 649\u00a0m2g\u22121 respectively, indicating that the addition of electron withdrawing and donating groups to the BDC ligand can significantly alter the physical properties of UiO-66(Zr). The basic heterogeneous catalyst UiO-66-NH2 converted oleic acid into 97.3% biodiesel under the optimized conditions of MTOR 39, catalyst amount of 6\u00a0wt.%, reaction temperature of 60 \u00baC and reaction time of 4\u00a0h.Hasan et al. [138] successfully synthesized Zr (IV)-Sal Schiff base complex incorporated into amino-functionalized MIL-101(Cr) framework by salicylaldehyde condensing to amino group and coordinating Zr (IV) ion (Fig.\u00a022\n\n). The specific surface area amounts to 1691\u00a0m2g\u22121 for pristine NH2\u2212MIL-101(Cr) and decreases to 571\u00a0m2g\u22121 and 437\u00a0m2g\u22121 for NH2\u2212MIL-101(Cr)-Sal-Zr and NH2\u2212MIL-101(Cr)-Sal-Zr respectively, while the pore volume decreases from 1.18\u00a0cm3g\u22121 to 0.48 and 0.35\u00a0m2g\u22121 for the same materials. The catalyst successfully converted oleic acid to 74.1% of methyl oleate under the optimized conditions of MTOR of 10:1, catalyst amount of 4, reaction temperature of 60 and reaction time of 4\u00a0h. The stability of the recovered catalyst was for six cycles and there were no significant changes in the conversion.The alternative way to address enzyme stability and recyclability difficulties in biodiesel synthesis is to immobilize enzymes using porous materials as supports. In order to improve recycling stability, enzymes can be covalently bonded to MOF surfaces. Normally, the free amino groups on the enzyme or MOF surface bind to the carboxylate groups on the enzyme or MOF surface to form peptide linkages [139]. The cage inclusion process involves the diffusion-mediated encapsulation of small enzymes within the cages of mesoporous MOFs. Even in hostile environments or unnatural conditions, the stability of enzymes may be considerably improved by encapsulation. It also creates a protective environment that reduces the impact of denaturation [140]. Li et\u00a0al. [53] reported an immobilized lipase in metal-organic based frameworks constructed by biomimetic mineralization where zinc acetate was used as an inorganic node and adenine as an organic linker (Fig.\u00a023\n). The temperature-tolerance assay performed at 70\u00a0\u00b0C, 40% (approx.) of the initial activity of lipase@Bio-MOF was detected after the incubation for 100\u00a0min, while free lipase was completely inactivated. Moreover, lipase@Bio-MOF could still retain approximately 32% of the enzymatic activity after storage at room temperature for 4 weeks, but free lipase lost almost all the enzymatic activity which suggests that the enzyme possessed enhanced tolerance against high temperatures and long-term storage after the immobilization in MOFs, which is useful for executing the catalytic reaction under harsh conditions.Adnan et\u00a0al.\n[109] reported a zeolitic imidazolate framework (ZIF -sub class of MOF) acting as a carrier for lipase to generate a hierarchical ZIF-8 towards immobilizing Burkholderiacepacia Lipase (BCL-ZIF-8, as shown in Fig.\u00a024\n). Immobilization efficiency improved continuously between 20 and 45\u00a0\u00baC and despite the high temperature, activity recovery increased at the same time, reaching a maximum of 1196% at 25\u00a0\u00b0C. Higher operational stability was demonstrated by the immobilized BCL in the mesoporous ZIF-8. The BCL-ZIF-8 was damaged by the excess alcohol and the glycerol byproduct that formed many layers around it. for the decline in biodiesel generation as the number of cycles increased up to 8, because of the mechanical stress generated by the ongoing reaction, a portion of the carrier was damaged, causing enzyme leakage and a decline in the immobilized enzyme's catalytic activity.Enzyme immobilization onto/into appropriate carriers may improve bio-catalytic industrial biodiesel production by increasing enzyme stability, allowing for continuous reuse, and allowing easy separation [141,142]. Rafiei et\u00a0al. [62] synthesized a heterogeneous biocatalyst by encapsulating lipase into the microporous zeolite imidazolate framework, ZIF-67. The free lipase retains just 43.7% of its initial activity at 50\u00a0\u00b0C, whereas the encapsulated lipase retains 72.6%. These findings showed that encapsulating enzymes in MOFs can prevent them from changing conformation at high temperatures, hence improving their thermal stability. Moreover, the stiff scaffold of the ZIF-67 improves the pH and heat stability of the embedded enzyme, preventing it from deactivation and allowing for up to 8 cycles of reusability in biodiesel production from soybean oil.Adnan et\u00a0al. [110] also reported a one-step encapsulation method of synthesizing X-shaped ZIF-8 (as shown in Fig.\u00a025\n) and immobilizing Rhizomucor miehei lipase. They observed a 26-fold of increase in the activity recovery of the enzyme because of the encapsulation method as it inhibits direct contact with the substrate. Secondly, ZIF-8, an immobilization carrier, has a major impact on enzyme activity by establishing microenvironments that are favourable for enzyme catalysis for a limited time in mild biocompatible conditions, allowing enzymatic activity to be preserved. Investigation into the reusability of RML@ZIF-8 in an isooctane medium revealed that after a continuous run of 10 cycles, the encapsulation of ZIF-8 to RML still maintained an 84.7%. The additional ethanol and glycerol by-products that were adsorbing to the surface of RML@ZIF-8 were thought to be the cause of the declining of biodiesel yield as the number of cycles rose.\nAspergillus niger lipase (ANL) was employed by Hu et\u00a0al. [94] with a hydrophobic UiO-66, which was modified by using facile polydimethylsiloxane (PDMS)-coating using chemical vapour deposition (CVD) treatment. The contact angle of a water droplet on UiO-66 is 111\u00b0, which increases to 121\u00b0, 144\u00b0 and 157\u00b0 after PDMS coating by CVD (2\u00a0h, 6\u00a0h, 10\u00a0h) indicating that the PDMS coating significantly enhances the surface hydrophobicity of UiO-66. Another hydrophobic MOF was reported by Zhong et\u00a0al. [143] where hydrophobic ZIF-L coated with polydimethylsiloxane (PDMS) was prepared by CVD and used to immobilize lipase from Aspergillus oryzae (AOL) for biodiesel production (Fig.\u00a026\n\n). The maximum activity obtained when lipase concentration was at 0.24\u00a0mg/mL but decreased with further increase of lipase concentration due to steric hindrance effect of the enzyme molecules and the diffusional limitations. PDMS hydrophobic modification on the surface of MOF improved the activity of immobilized lipase and the strong hydrophobic interaction between the lipase and the PDMS coating stabilizes the confirmation of the lipase [144,145]. Table 5 provides the summary of enzymatic MOFs used for biodiesel synthesis with their experimental conditions, yield, number of reuses and stability evidence. These data indicate that less attentions are given to characterize the spent catalyst. This is one of the important issues that should be given higher priority to understand the reasons for activity decay.A major issue\u00a0for heterogeneous catalysis is the leaching of active species into the liquid phase, which ultimately causes\u00a0a significant deactivation of solid catalysts. ILs are the anionic-cationic salts with the ability to dissolve a wide range of compounds, carry some features of being non-volatile and possess excellent thermal, chemical, and electrochemical stability [146,147]. Acidic and basic ILs are being used for the functionalization of MOFs which none the less show high conversion of oil to biodiesel conversion. However, leaching of ILs have been a concern as it deactivates the activity of the catalyst. Wan et al. [11] reported hteropolyanion-based IL within the framework of MIL-100(Fe) where they observed instability of MOF due to leaching of Cu2+ and HPW in presence of acetic acid. Leaching test of DAIL-Fe3O4@NH2\u2212MIL-88B(Fe) was performed by Wu et al. [59] by running the reaction with catalyst for 2\u00a0h and then further without catalyst for 3 observed that there was slight conversion into biodiesel without catalyst possibly due to leaching of ionic liquid DAIL (Fig.\u00a027\n) caused by ion exchanges supposed to take place during catalytic process.AILs/UiO-66\u20132COOH [90] composite, has the synergistic effect between the Bronsted acid sites from AILs and the Lewis acid sites from the HPW that boost the catalytic activity of the catalyst. The strong interaction between the sulfonic acid groups and the HPW molecules appeared to be effective in preventing the active component loss from MOF supports. However, it was observed that there was significant reduction in the solid catalyst's catalytic activity in the second cycle of oil to biodiesel conversion. Furthermore, the major leaching of MBIAILs from the channels of MOFs was noticed by Han et\u00a0al. [50] which could be leaching of ILs on the surface of MIL-101(Cr) frameworks due to weak physical adsorption. However, to enhance the recyclability Liu et\u00a0al. [125] suggested separation of methanol firstly to reduce leaching of organosulfonic acids from UiO-66 which could amplify solid catalyst recovery effect. In most cases leaching is due to weak interaction between precursors, Abdelmigeed et\u00a0al. [107] synthesised NaOH/ Magnetised ZIF-8 catalyst where they reported that the interaction between NaOH and magnetised ZIF-8 was enhanced by calcining at 200 \u2103 under inert atmosphere.During catalytic processes, Pd nanoparticles (Pd-NPs) supported on porous materials are prone to significant leaching or aggregation, resulting in a loss of catalytic activity and cyclic\u00a0stability [148]. In ZIF-8 supported carbon-stabilized Pd nanoparticles (C@Pd/ZIF-8) MOF [149] based catalyst, TEM analysis (Fig.\u00a028\n)\u00a0confirmed\u00a0that Pd particles supported by pure ZIF-8\u00a0suffered substantial aggregation or leaching during the reactions, which accounted\u00a0for the catalyst deactivation.Enzyme immobilization technique with the use of MOF to solve the enzyme solubility and leaching issues is one of the strategies recently adopted for the synthesis of enzymatic based MOF catalyst. However, still challenges related to leaching prevails due to weak interaction between MOF and enzymes [150]. MOF based catalysts like MIL-100(Fe)/PPL (PPL-Porcine pancreatic lipase) and HKUST-1/PPL [151] were used for enzymatic esterification of cinnamic acid. ICP-AES studies of both the catalyst suggested 0.8\u00a0wt.% of Fe from MIL-100(Fe)/PPL and 10\u00a0wt.% of Cu from HKUST-1/PPL were leached after 12 catalytic cycles but significant amount of leaching was from PPL (Fig.\u00a029\na) as 53% and 34% of activity (Fig.\u00a029\nb) were retained after 12 cycles of test respectively. Recently, strategies like entrapment, crosslinking [94,152] have been adapted to prevent enzyme leaching. Chen et\u00a0al. [153] developed a new method to encapsulate enzymes in hollow MOF. They encapsulated catalase inside ZIF-67 and made ZIF-8 to overgrow on it followed by removing ZIF-67 cores by means of modest hollowing process [154] which in turn was confirmed by SEM and TEM micrographs displaying confined enzymes in the MOF core (Fig.\u00a030\na\u2013c) and enzymes in the hollow MOF cavity (Fig.\u00a028\nd\u2013f) without affecting the morphology of the MOF.MOFs structural deformation can also lead to the deactivation of catalyst. Excess amounts of modulators used in the synthetic process cause deformation in the MOF structure which deactivates the catalytic activity during the catalytic run. Recently, Conley and Gates [155] reported about the deactivation stages of UiO-66 (Zr) MOF with various proportion of acetic acid (aa) modulator and provided a quantitative approach towards methanol dehydration. The life cycle of the MOF catalyst included an activation stage (blue), during which the conversion reached its maximum peak, subsequently deactivation stage (yellow) and finally the catalyst almost completely lost (red) (as shown in Fig.\u00a031\n. a). The MOF loses its crystallinity and leads to MOF unzipping when reaction of methanol proceeded with the node linkers to form methyl esters, where the inference was confirmed by XRD (Fig.\u00a031. b) which showed missing of characteristic peaks of UiO-66 at 2\u03b8\u00a0=\u00a07.45\u00b0 and 8.6\u00b0 when allowed for overnight reaction. Furthermore, Yang et al. [156] also observed the deactivation in UiO-66 type MOFs where the carboxylate groups of the linkers reacted with ethanol to create esters, thereby unzipping the MOFs.Opanasenko et\u00a0al. [157] studied the deactivation pathways for the catalytic activity of Fe-MOFs in condensation reactions. The study was based on monitoring of byproducts which could deactivate the activity of the MOF by deteriorating its lattice. Li et al. [137] also evaluated such deactivation of catalyst and considered it as a cause of organics blockage since XRD of the MOF- derived catalyst after reuse gave similar peaks as that of the fresh catalyst. Formation of strongly adsorbed byproducts formed during the reaction interacts with the MOF lattice and cause structural disintegration in the MOF. The loss of active sites cause the catalyst to deactivate as a result of pore obstruction since intermediates or products such as diglyceride, monoglyceride, glycerol, and biodiesel masque the catalysts [158]. Thus, the problem before the MOF research is to control the disintegration of MOF structure. In this run so far, Yunan et al. [159] provided striking results with the usage of custom-built reactor developed at the Christian- Albrechts University (Kiel, Germany) virtue of which any crystallographic changes with respect to chemical reactions could be estimated efficiently. Their observation in probing of Pd(II)@MIL-101-NH2 during Heck coupling was quite successful. They reported that deactivation of catalyst was neither due to Pd leaching nor MOF decomposition rather chemical deactivation caused due to catalyst poisoning by Cl\u2212ions that masked the surface of Pd clusters and obstructed the access of starting materials from reacting with the active sites. As solutions arrive from knowing the cause of problem, the use of different precursors of Pd for encapsulation or carrying out reactions under continuous flow would solve the catalyst poisoning and deactivation of catalyst [160].The review summarises different types of MOFs as MOF derived, and novel precursor used as heterogeneous catalyst for biodiesel production. MOFs can be utilized as a template of support for metal oxides, an immobilising material for enzymes or whole new framework of sophisticated 3-D material with various organic linkers. MOFs emerging as a new class of materials with unimaginable tunability, porosity and unprecedented surface area which increases its potential to be used in industrial scale to meet the requirement of selectivity for biodiesel production. This review consists of some relevant characterization techniques like XRD, BET, SEM and TEM, XPS and TGA for MOF catalysts where important findings have been mentioned from several literatures which include the hollow MOF structure, missing linker detection, thermal stability, and crystallinity of MOFs from morphological, elemental, thermal and structural characteristic. The aim of this review is to provide a deep insight to readers about the extended coordinated network, facile tunability of different class of MOFs, and factors (leaching, blocking of active sites, unzipping of MOF structure) causing deactivation of MOF catalysts which will enhance the understanding of immense possibilities and opportunities carried by MOFs. Though this review discussed catalytic activity of MOF in biodiesel but still there are some major challenges in MOF synthesis and needs special consideration. The stability of MOFs is one of the major issues in the synthesis of acid/base functionalized MOFs as it affects the pristine structures. Therefore, different design strategies need to be adopted during the synthesis process to keep the highly coordinated network intact. More stable MOF could be an ideal candidate to immobilize types of enzymes to promote eco-friendly production of biodiesel.There are very few bifunctional MOFs which have been used to produce biodiesel, although bifunctional MOFs are strategically more efficient to show synergistic effect of both acidic and basic nature, there are still less reported number of bifunctional MOFs. Thus, bifunctional based MOFs need to be synthesised for the rapid transesterification reaction process. Designing MOFs by inserting heteroatoms in the interstitial voids of MOF in appropriate proportions by controlling its electronic dynamics and environment is still a great challenge to tackle. Engineering defects in MOFs enhance the reactivity of MOFs and such underexplored field in MOF requires ample attention from its future catalytic activity perspective in the field of biodiesel.The reticular synthesis of MOFs is also a field of synthetic strategy confined with the issues of solubility and stability which needs to be explored more in near future. We believe in the vast scope of MOF chemistry and that may be get uncovered in the coming years which not only would pave the way for well stabilised gigantic architectural framework but also would find practical applications in the present world.CaO works well as a catalyst for producing biodiesel. It can achieve conversion of more than 95% oil to biodiesel. Moreover, it can resist more than 5 cycles with good efficiency. Natural waste products like chicken eggs, snail shells, and animal bones can be used as a source of generating CaO. However, leaching of CaO is an issue which is difficult to achieve using ion exchange resin. On the other, MOF derived catalysts possess the potential to restrict leaching of CaO where MOFs can act as a support to CaO, boost up the surface area and stability of the catalyst to achieve high biodiesel yield. Above all MOF catalysts deactivation due to catalyst poisoning, leaching or self-inactivation require a detailed experimental study in order to improve the catalyst reusability that supposed to be the backbone of the heterogeneous catalyst.This research received no specific grant from any funding agency.Authors declare no conflict of interest.", "descript": "\n                  Nowadays, as there is a rapid depletion of fossil fuels, the need for alternative resources has become inevitable. Biodiesel is such an alternative that is environmentally friendly and sustainable. Over the years there has been a significant amount of research done for biofuel production by converting various feedstocks such as edible and non-edible vegetable oil, animal fats, waste cooking oil, and microalgae as the feedstock which requires a highly workable and efficient catalyst for the transesterification process. Several heterogeneous catalysts have been used for the transesterification of biodiesel feedstock to biodiesel, among which metal-organic framework (MOF) has gained popularity owing to its high surface area, high pore volume, and facile tunability of the active sites. This review focuses on different types of MOFs, characterization techniques used to identify vital structurally invoked changes, and deactivation of MOFs due to leaching of various active species, blocking of active sites, and unzipping of MOFs by covering the literature from the year 2000 to till date. Finally, a brief conclusion and the author's perspective depicting several challenges and scope for future research needs in the MOF study have been provided.\n               "}
{"full_text": "The authors do not have permission to share data.Human society is confronted with many difficulties such as climate change, environment protection and energy security issues arising from the depletion and the over-reliance on fossil fuels [1\u20138]. With the increasing demand for clean energy and medicine, it is imperative to adjust petroleum-based resources to renewable biomass for the industrial production of bulk chemicals [9\u201314]. As a promising intermediate, lignocellulose-derived furfural (FF) can be potentially utilized for many downstream organic synthesis [15\u201317]. Among these, 2-methylfuran (2-MF) has been the dominant alternative deriving from FF selective hydrogenation of CO bond [18,19]. It has been broadly used in bio-refinery and chemical manufacture [20]. In particular, 2-MF is a pivotal compound to synthesis chloroquine phosphate, which is vital for ultimate production of the 2019-nCov commercially [21].The selective hydrogenation reaction (HDO) of FF provides a route to prepare 2-MF without breaking the furanic \u201cO\u201d and the ring double bonds [22]. Actually, CuCr catalyst has been used for the industrial production of 2-MF for decades [23,24]. However, it is essential to concern at the consequent environmental pollution and potential human health threat caused by toxicity chromium in the catalyst. In recent studies, given that the natural scarcity and expensive price of noble metals might impede the large-scale commercialization, transition metals (Cu, Co and Ni) are adopted and showed high selective hydrogenation activity for 2-MF fabrication [25\u201328]. Unfortunately, most of these studies are performed with FF instead of xylose as initial feedstock. The tandem conversion [29] of xylose hydrolysis to produce FF and subsequent HDO of FF is still a huge challenge within the scale utilization of biomass.Followed this cascade strategy, Lessard et al. has employed a continuous two-liquid-phase (aqueous-toluene) plug-flow reactor with (H+) mordenite and Cu/Fe as catalysts respectively and obtain 96% yield of 2-MF from the xylose [30]. Cui et al. designed a process for the production of 2-MF on a continuous fixed-bed reactor with butyrolactone/water as solvent [31]. After the xylose was dehydrated by H\u03b2 zeolite, the obtained FF was selectively hydrogenated to furfuryl alcohol (FA) or 2-MF by changing the hydrogenation temperature over the Cu/ZnO/Al2O3 catalyst. The highest 2-MF yield of 86.8% was achieved at 190\u00a0\u00b0C. In recent research, Rafael F. et al. converted xylose to furfuryl alcohol over Zr-SBA-15 at 130\u00a0\u00b0C, 30\u00a0bar\u00a0N2 for 6\u00a0h and about 30% xylose conversion and 45% selectivity to 2-MF were obtained [32]. Deng et al. completed one-pot cascade conversion of xylose to furfuryl alcohol over a bifunctional Cu/SBA-15-SO3H catalyst at 140\u00a0\u00b0C and 4\u00a0MPa for 6\u00a0h [33]. 93.7% xylose conversion and 62.6% 2-MF yield were achieved.As a novel contribution to this challenge, xylose was employed as the staring material to produce FF and 2-MF via tandem conversion in this work, where H\u03b2 zeolite and NiCu/C were used as dehydration and HDO catalyst in isopropyl alcohol/water solution. Subsequently, the stability of NiCu/C and the effects of reaction conditions on FF conversion were evaluated comprehensively. At last, the tandem conversion of xylose was investigated with different catalyst combination. 95.1% 2-MF yield based on xylose was achieved over H\u03b2 zeolite and 0.5NiCu/C catalyst.D-xylose, nickel (II) nitrate hexahydrate (Ni(NO3)2\u22196H2O), copper (II) nitrate hydrate (Cu(NO3)2\u22193H2O), citric acid (C6H8O7), 2-methylfuran (2-MF), furfural (FF), furfuryl alcohol (FA) and 2-methyltetrahydrofuran (2-MTHF) were purchased from Shanghai Macklin Chemistry Co., Ltd. in analytical grade. HY, H\u03b2, USY and MCM-41 zeolites were purchased from Nankai University Catalyst Co., Ltd. Isopropanol and tetrahydrofurfuryl alcohol (THFA) were applied by Aladdin Chemistry Co., Ltd. All chemicals were used as received without further purification.The bimetallic catalysts were prepared according to the following procedures. 5.8\u00a0g citric acid (CA), 7.3\u00a0g Cu(NO3)2\u22193H2O and a designated amount of Ni(NO3)2\u22196H2O (the weight ratio of Ni/Cu\u00a0=\u00a00, 0.25, 0.5, 1, 2.5, 5) were added to 10\u00a0ml deionized water. According to Ni/Cu weight ratio, different bimetallic catalysts were denoted as xNiCu/C. After being kept at 90\u00a0\u00b0C for 3\u00a0h with vigorously stirred, the gelatinous mixture was transferred into a 100\u00a0\u00b0C drying oven over night. The obtained complex was denoted as CA-Ni-Cu. Finally, the bimetallic catalysts were collected by annealing the spongy compound CA-Ni-Cu at a temperature range of 500 to 800\u00a0\u00b0C for 3\u00a0h in N2.The synthesis of Cu/C catalyst was conducted according to our previous literature [34].The Brunauer-Emmett-Teller (BET) surface areas of the prepared catalysts were determined by the nitrogen physisorption at \u2212196\u00a0\u00b0C on a Tristar II 3020 volumetric adsorption analyzer. Prior to nitrogen physisorption, the samples were pretreated at 200\u00a0\u00b0C for 12\u00a0h under the high vacuum. X-ray diffraction (XRD) were carried out in the 2\u03b8 range from 10 to 80 o at the scanning rate of 0.02 degree\u22121 by an X'Pert Pro MPD equipped with CuKa radiation. The X-ray photoelectron spectroscopy (XPS) analysis were taken on a Kratos AXIS ULTRA DLD spectrometer equipped with a monochromatic Al-Ka radiation source (h\n\nv\n\u00a0=\u00a01486.6\u00a0eV). The C 1\u00a0s peak (284.8\u00a0eV) was applied as the reference for binding energy calibration of other elements. HRTEM was performed using a JEOL-2100F microscope operated at the accelerating voltage of 200\u00a0kV. The surface acidity of catalyst was conducted by NH3-temperature programmed desorption (NH3-TPD) using a Tp-5080 China Xianquan machine (TCD detector).All typical FF hydrogenation experiments were performed in a 50\u00a0ml stainless steel micro autoclave (MS-50-316\u00a0L, Anhui Kemi Machinery Technology Co., Ltd., Hefei, China). 0.1\u00a0g catalyst, 5\u00a0ml deionized water and 30\u00a0ml isopropyl alcohol were loaded to the autoclave. Afterward, the reactor was sealed and purged with hydrogen for 6 times. H2 pressure was kept at certain figure, i.e., 1, 2, 3, 4 or 5\u00a0MPa. Finally, the reactor was maintained at specific temperature for 0\u201310\u00a0h under mechanical stirring (800\u00a0rpm). After completion, the autoclave was cooled down to room temperature naturally and the solid catalyst was separated from the residual liquid through the centrifugation. For the recycling test, the collected catalyst was washed with deionized water after each cycle and dried at \u221248\u00a0\u00b0C under vacuum before the next run.For the typical tandem conversion, 1.0\u00a0g xylose and 0.3\u00a0g H\u03b2 zeolite were added to react firstly at 140\u00a0\u00b0C for 5\u00a0h and the obtained liquid product could be separated by distillation. Subsequently, NiCu/C catalyst was added for FF hydrogenation experiments at 220\u00a0\u00b0C for 5\u00a0h. As a contrast, H\u03b2 zeolite and NiCu/C catalysts were respectively used for the conversion at the segmented optimal temperatures. For one pot conversion of xylose, H\u03b2 zeolite and NiCu/C at different temperatures catalyzed the reaction for 10\u00a0h. Two-step reaction was completed at 140\u00a0\u00b0C and 220\u00a0\u00b0C for 5\u00a0h on the basis of one pot conversion.The liquid products were further analyzed using a gas chromatograph (Agilgent, USA, CP-Wax 58 capillary column). The product identification was analyzed using a GC\u2013MS (Agilgent, USA, FID, INNOWAX column) equipped with a flame ionization detector. The calibration curve was established by an external standard method. The conversion of substrate and the product yields were calculated using the following equation:\n\n(1)\n\nConversion\n\n%\n\n=\n\n\n\nn\nc\n\n\u2212\n\nn\ns\n\n\n\nn\nc\n\n\n\n\n\n\n\n(2)\n\nYield\n\n%\n\n=\n\n\nn\ni\n\n\nn\nc\n\n\n\n\nwhere n\n\nc\n represents the moles of FF or xylose before the reaction; n\n\ns\n represents the moles of remained substrate after the reaction and n\n\ni\n is the mole amount of product i.The impact of Ni content on the hydrogenation activity of xNiCu/C catalysts at 220\u00a0\u00b0C under 4\u00a0MPa H2 is shown in Table 1\n. Except for the targeted 2-MF, side products including FA, 2-methyltetrahydrofurfuryl alcohol (2-MTHF) and tetrahydrofurfuryl alcohol (THFA) were also detected. For single metal catalyst Cu/C, 84.1% FF was converted and the medium yield of 2-MF was 61.5%. With the introduction of Ni, the feedstock was consumed completely and the highest yield of 2-MF (97.5%) was achieved over 0.5NiCu/C catalyst. This could be assigned that the recommendation of Ni facilitated the quantitative growth of reactive sites, leading to the enhancement of the selectivity for 2-MF. However, superfluous Ni is not always advantageous because it is stimulative for the formation of side products. Especially, the yield of 2-MTHF also varies with the increase of Ni proportion, suggesting that the introduced Ni of xNiCu/C catalyst breaks CC bonds adequately.In order to investigate the relationship between the structure and activity of the xNiCu/C catalyst, the physicochemical properties of xNiCu/C were characterized. As shown in the Table 2\n, the specific surface area (S\nBET) of Cu/C sample is 48.8\u00a0m2/g, the pore volume (V\np) is 0.08\u00a0m3/g and the average pore diameter (D\np) is 6.7\u00a0nm. Minor but significant changes in S\nBET, V\np and D\np of the xNiCu/C catalysts are observed with the introduction of Ni metal. When the Ni loading is below 1.0%, the S\nBET is gradually ascending, indicating that, to a great extent, the slight nickel can promote the increase of the specific surface area of the catalyst. Otherwise, once too much nickel (>1%) diffuses to the catalyst pore, the S\nBET decreases apparently, thus reducing the specific surface area and average pore diameter of xNiCu/C. Because of the above variations, the decrease of catalyst activity ultimately affects the conversion of FF and the yield of 2-MF.The crystalline structures of xNiCu/C catalysts were identified by XRD (Fig. 1\n). All the samples show the characteristic diffraction peaks at 2\u03b8\u00a0=\u00a043.3o, 50.4o and 74.1o, corresponding to the (110), (200), and (220) crystal planes of Cu, respectively. The XRD results suggest that the Cu2+ is reduced to metallic Cu during the fabrication of xNiCu/C. It is worth noting that the typical peaks of Ni or NiO have not been detected in xNiCu/C samples. It might be explained by the low loading of Ni or the coverage of the characteristic diffraction peak of Cu particle. In addition, the diffraction peak of copper does not shift significantly when the content of Ni increases, verifying that the Ni or CuNi alloy has little effect on the crystal form of copper.The HRTEM images of Cu/C and 0.5NiCu/C are presented in Fig. 2\n. Virtually, it is feasible to observe that the particle size and morphology of copper and nickel particle are distinguishable. The (110), (200) and (220) crystal planes of copper can be found from the HRTEM pattern of Cu/C catalyst (Fig. 2a), as confirmed by the results of XRD. In Fig. 2b, the Ni particles with sizes of 2\u20135\u00a0nm are uniformly dispersed on the 0.5NiCu/C catalyst and the presence of nickel has a negligible effect on the morphology of copper.The valence states of metal phases in xNiCu/C with different nickel loading were determined by XPS (Fig. 3\n). For Ni 2p doublets, there is an absence of the characteristic peaks of Ni0 and Ni2+ when the Ni loading is below 1.0%. However, with the increase of Ni amount, the Ni 2p spectra are successfully fitted to Ni0 and Ni2+. Ni 2p3/2 (\u223c855.4\u00a0eV) with its associated satellite (\u223c861.3\u00a0eV) as well as Ni 2p1/2 (873.1\u00a0eV) and the related satellite (\u223c879.3\u00a0eV) are observed for NiO. The doublets at around 852.3\u00a0eV (2p3/2) and 868.4\u00a0eV (2p1/2) are ascribed to Ni0 species. Additionally, the intensity of the typical peaks of Ni 2p also enhance with the Ni amount. This suggests that more Ni amount cannot be completely reduced. Similar to the XPS spectra of Ni, the Cu 2p spectra are fitted into four peaks, representative of Cu0 (932.5\u00a0eV, 2p3/2; 952.4\u00a0eV, 2p1/2) and Cu2+ (934.9\u00a0eV, 2p3/2; 953.4\u00a0eV, 2p1/2). Contrary to the CuO characteristic peak, the intensity of the Cu0 peak has gotten an elevation with the increase of nickel loading, demonstrating the promotion of nickel in the reduction of Cu2+. According to the above results, the existence of Ni0 and Cu0 phase accounts for an efficient reduction of xNiCu/C during the reduction\u2011carbonation process.Because of the surface acidity of catalyst as a crucial factor for the hydrogenation of FF, these directional parameters of the xNiCu/C were determined by the TPD method based on the various desorption temperatures of NH3 on different acid sites. From the entire curves of NH3-TPD profiles (Fig. 4\n), three NH3 desorption peaks are observed within the ranges of 150\u2013250\u00a0\u00b0C, 300\u2013500\u00a0\u00b0C and 600\u2013800\u00a0\u00b0C respectively, which implies that all catalysts have similar acid sites [35]. The three peaks are assigned to NH3 desorbed from weak, medium and strong acid sites [36]. In comparison to the other catalysts, the highest total acid content (0.971\u00a0mmol/g) is the Ni free catalyst (Cu/C). According to the literature, Clemens et al. studied a larger number of copper loadings and the first two peaks was assigned to different unspecified types of Cu sites on this basis. As the increase of Ni, the contents of weak and medium sites decrease strikingly. Lezcano-Gonzalez et al. attributed the intermediate peak to NH3 adsorbed over Cu2+ sites [35]. For strong acid sites, its peak intensity remains constant or even slightly decreased, further proving that the acid sites of the xNiCu/C catalyst mainly originates from CuO [37].When the deionized water and isopropyl alcohol were chosen as the reaction media, FF hydrogenation experiments to 2-MF over 0.5NiCu/C were conducted at diverse reaction parameters for optimization.The effects of initial H2 pressure on FF conversion are shown in Fig. 5a (220\u00a0\u00b0C, 5\u00a0h). When H2 pressure is at 1\u00a0MPa, the conversion of FF lies at around 56% and merely 40% 2-MF could be collected eventually. The highest 2-MF yield is 97.5% at 4\u00a0MPa H2 pressure with 100% FF conversion. The result data confirms that reducing H2 pressure properly is beneficial to modulate the hydrogenation capability without causing the appearance of other by-products. When H2 pressure is 5\u00a0MPa, over\u2011hydrogenation process leads to the formation of 2-MTHF. Therefore, there is a decline about the 2-MF yield.The influence of reaction time (220\u00a0\u00b0C, 4\u00a0MPa) during the conversion was also investigated (Fig. 5b). In the range of 5\u00a0h, FF conversion and 2-MF yield grow linearly with the reaction time and the hydrogenation about FF is further developed. However, when the reaction time is prolonged persistently, 2-MF is gradually converted to 2-MTHF. The preference toward 2-MTHF is discovered, thus leading to a descending trend of 2-MF yield.As shown in Fig. 5c (5\u00a0h, 4\u00a0MPa), the hydrogenation reaction of FF is also sensitive to the reaction temperature. Around 70% of FF was converted at 140\u00a0\u00b0C with 44.1% 2-MF obtained. Meanwhile, the other liquid products include FA (18.9%), THFA (6.8%), 2-MTHF (1.1%). The conversion of FF is rapidly increased when the reaction temperature is elevated from 140\u00a0\u00b0C to 180\u00a0\u00b0C. In this case, the temperature of 220\u00a0\u00b0C is appropriate for the formation of 2-MF, resulting in the maximum yield (97.5%). However, the trend of 2-MF yield is in the opposite direction during the temperature range of 220\u00a0\u00b0C to 260\u00a0\u00b0C. The typical FF hydrogenation is an exothermic reaction and its thermodynamically unfavorable properties promotes the hydrogenation of CC bonds in the furan ring at high reaction temperature, thus generating the by-product 2-MTHF. And the yield of 2-MTHF elevates from 1.9% (220\u00a0\u00b0C) to 25.9% (260\u00a0\u00b0C).The recyclability experiments of the 0.5NiCu/C was carried out (Fig. 6\n). A slightly decline in the conversion of FF and the yield of 2-MF is observed after multiple circulation. In the fifth reaction, the yield of 2-MF is still maintained at the higher level of 85%. The structure and acid properties of the used catalyst were analyzed by XRD, XPS, HRTEM and NH3-TPD. As shown in Fig. 7a, compared with the fresh catalyst, the location of (110), (200) and (220) crystal planes of Cu remain invariant but the intensity of different diffraction peaks further strengthens after reaction. No significant change is found in Cu 2p spectra of fresh and used catalysts, showing the valence stability of surface copper species (Fig. 7b). Noticeably, the result of NH3-TPD (Fig. 7c) reveals that the total acid sites amount of the catalyst after hydrogenation apparently drop, which might be responsible for the decrease of 2-MF yield. Besides, according to the HRTEM images (Fig. 7d), the retention of immutable nickel and copper species reasonably explain the high catalytic activity of NiCu/C.The tandem conversion from xylose to 2-MF over 0.5NiCu/C catalyst and H\u03b2 zeolite was tested and the consequences were listed in Table 3\n. For one pot conversion of xylose, using H\u03b2 zeolite and 0.5NiCu/C as catalyst, the yield of 2-MF is only 45.7% with 64.2% xylose conversion at 140\u00a0\u00b0C (Entry 1,\nTable 3). Elevating the reaction temperature to 220\u00a0\u00b0C, more xylose is converted (100%). But the selectivity of 2-MF decreases and its yield is only 24.7% (Entry 2,\nTable 3). This verifies that xylose dehydration and FF hydrogenation correspond to different optimum temperatures and no single temperature is suitable for the whole catalytic process. Therefore, the conversion process following the cascade strategy is divided into two stages at their respective optimized temperature for 5\u00a0h (Entry 3,\nTable 3). In comparison to single temperature, the yield of 2-MF is elevated to 67.2%. The problem about rate-limiting step under different reaction temperature is solved. Then, the process is changed to two-step tandem reaction. Xylose is converted to FF with a yield of 99% over H\u03b2 zeolite, and the obtained liquid product can be separated by distillation. Tandem method applied xylose conversion improves the 2-MF yield rate by 41.5% and 95.1% 2-MF yield is obtained eventually (Entry 4,\nTable 3). To understand the catalysis of H\u03b2 zeolite and 0.5NiCu/C, single catalyst experiments were performed (Entry 5 and 6,\nTable 3). Using H\u03b2 zeolite or 0.5NiCu/C as catalyst alone, the yield of 2-MF is relatively low (4.2% and 1.4%). It can be inferred from these results that H\u03b2 zeolite has more acid sites compared to 0.5NiCu/C catalyst, which is beneficial for xylose conversion. But H\u03b2 zeolite lacks the hydrogenation active sites and the generated FF fails to convert to 2-MF over H\u03b2 zeolite. For 0.5NiCu/C, the opposite is true. Hence, we can conclude that, in the whole tandem conversion, xylose is converted to FF under H\u03b2 zeolite at 140\u00a0\u00b0C for 5\u00a0h firstly, followed by FF hydrogenation to FA over 0.5NiCu/C at 220\u00a0\u00b0C for 5\u00a0h. H\u03b2 zeolite and 0.5NiCu/C realize the sequential catalysis of the corresponding process through tandem strategy, bringing the obvious enhancement of the ultimate increase in 2-MF yield.A tandem strategy that combined acid dehydration catalyst and HDO catalyst to convert xylose to 2-MF was developed in a continuous reaction process. Effective HDO of furfural via NiCu/C under isopropyl alcohol/water was achieved. Compared with NiCu/C catalyst, H\u03b2 zeolite provides sufficient acid sites used for xylose hydrolysis. Therefore, 0.5NiCu/C catalysts assisted with H\u03b2 zeolite promotes tandem conversion of xylose. 2-MF yield from tandem utilization increases by 41.5% as compared with those from two-step reaction. In this work, the exploitation of tandem dehydration and HDO provides new ideas for one-step conversion of monosaccharide to 2-MF.\nHao Li: Methodology, Investigation, Writing \u2013 original draft, Writing \u2013 review & editing. Huimin Liu: Methodology, Validation. Chiliu Cai: Investigation, Validation. Haiyong Wang: Conceptualization, Methodology, Investigation, Funding acquisition, Writing \u2013 review & editing. Youwang Huang: Formal analysis, Data curation. Song Li: Resources. Bin Yang: Project administration. Chenguang Wang: Resources, Supervision. Yuhe Liao: Supervision, Validation, Formal analysis. Longlong Ma: Supervision, Project administration, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is financially supported by the National Natural Science Foundation of China (52006225, 52236010, 52006228, 52206288), and R&D Plan of Key Fields in Guangdong Province (2020B1111570001), Natural Science Foundation of Guangdong Province (2018A030310135).\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106625.", "descript": "\n                  Nowadays, 2-methylfuran is urgently needed especially in pharmaceutical manufacture. However, the direct synthesis of 2-methylfuran via the efficient dehydration and hydrogenation of xylose usually requires a multistep reaction. Herein, a route of tandem conversion xylose to 2-methylfuran was developed. The xylose was firstly converted into furfural by H\u03b2 zeolite, followed by the catalysis of furfural to 2-methylfuran over 0.5NiCu/C. Compared with two-step reaction, cascade strategy applied xylose conversion improved the 2-methylfuran yield rate by 41.5%. In general, this work expanded the application of tandem method for the enhancement of xylose effective conversion and provided a new biomass utilization technology.\n               "}
{"full_text": "With the depletion of conventional energy sources and the growing need for energy in society, coupled with the challenge to ameliorate climate change, the development and improvement of safe, renewable, and low-cost clean energy technologies are necessary. The development of fuel cells has attracted significant attention recently owing to their promising potential to provide a clean and possibly sustainable energy generation [1,2]. Currently, the Proton Exchange Membrane (PEM) fuel cell is highly developed, however, there are many inherent safety and practical problems such as the production, storage, and distribution of hydrogen that still present strong limitations for industrial-scale applications [3\u20136]. In recent years, the direct hydrazine fuel cell (DHFC) has gained significant research interest because of the high power density and therefore has promising application for the automotive industry [7\u20139]. DHFC is a carbon-free process and has the potential to output a power generation performance of 0.5\u00a0W\u00a0cm\u22122 and a cell voltage of 1.56\u00a0V, which is comparable to the PEM fuel cell [10,11]. However, its full implementation is hindered by the lack of highly efficient, stable and inexpensive catalysts. Other suggestions for using hydrazine towards an environmentally friendly economy include its potential as a hydrogen storage material, via its decomposition pathway:\n\n\n\n\n\n\nN\n2\n\nH\n\n4\n\n\u2192\n\n\n2\nH\n\n2\n\n+\n\nN\n2\n\n\n\n\n\nThis is a competitive pathway with the decomposition to ammonia \u2013 with the route depending upon whether the NN or NH bonds of hydrazine are broken first [12].Ni-based catalysts - non-noble metal alternatives to platinum have been shown to be active for each of these processes. Hydrazine decomposition has been shown on Ni-nanofibres when supported on carbon nanotubes by Ding, Lin, and Guo - achieving 100% selectivity towards hydrogen [13]. Researchers at the Dihastu Motor Company have also shown that carbon supported-Ni was active for the DHFC, showing good selectivity and low amounts of ammonia production [14]. Unfortunately, Ni-nanoparticles suffer from atom run-off, hence dramatically lowering the activity of the monometallic catalysts [15]. Bimetallic catalysts can be formed to overcome these issues, as Ni has an innate ability to produce alloyed systems with other transition metals [16]. Ni-based bimetallic heterogeneous catalysts often show new and improved activity and stability towards their respective reactions - such as Ni-Rh [17], Ni-Ir, [18,19] or Ni-Pt [20\u201322] - which show marked improved activity for hydrazine decomposition. However, avoiding expensive and less abundant noble metals is necessary if we are to achieve widespread, sustainable commercialisation of DHFC technology. Promising non-noble bimetallic catalysts for DHFC\u2019s include alloys with other first row transition metals, such as Ni-Cu, [23] Ni-Fe, [24] and particularly Ni-Zn [25,26]. Recently, Feng et al. [27] synthesised an Ni-Zn catalyst combined with a reduced graphene oxide layer, finding this gave almost 100% selectivity towards hydrazine electro-oxidation in the DHFC. The catalyst also showed good stability over time, hence improving substantially on the monometallic Ni-catalyst. Atanassov et al. [11] have also found NiZn catalysts to be active for DHFC\u2019s when supported on Ketjenblack, a carbon-based support material. As a result, the surface area of the catalyst was increased, leading to higher contact times with hydrazine, thereby increasing the activity.Hydrazine adsorption onto the bimetallic Ni-Zn catalyst surface would precede any reactions that take place inside the DHFC or any decomposition routes, as such, understanding of the strength and features of adsorption of hydrazine onto the catalyst surface is essential for the successful development of efficient bimetallic Ni-Zn catalysts in these areas. The fundamental aspects of hydrazine adsorption, including the initial adsorption geometries, adsorption energies, structural parameters, and electronic properties, are deemed vital for the rational design of improved Ni-Zn catalysts. Detailed information is, however, difficult to obtain directly from experiments and the underlying physical driving forces that control the reactivity of hydrazine with the bimetallic Ni-Zn surfaces remain not fully understood. First-principles density functional theory (DFT) calculations provide an alternative way to gain fundamental insight, as it is capable of accurately predicting lowest-energy adsorption geometries and identifying charge transfer and further electronic effects [28\u201331]. DFT-based calculations have been employed extensively to predict the adsorption geometries of hydrazine on metallic surfaces and offers good insight into catalyst activity [24,32,33]. Previous DFT work has been done on the Ni-Zn alloy, although the specific factors behind how hydrazine adsorbs are yet unknown [33].In the present study, dispersion-corrected DFT-D3 calculations are employed to comprehensively investigate the adsorption properties of hydrazine on the bimetallic \u03b21-NiZn alloy catalyst (100), (110), and (111) surfaces. Insights into the synergistic beneficial effects of Ni-Zn alloying was derived by drawing a comparison between the hydrazine adsorption energetics and mechanisms on the bimetallic NiZn surfaces to the monometallic Ni(111) and Zn(001) surfaces. The energetics and structural parameters of the lowest-energy adsorption configurations of the hydrazine are presented and a d-band model was developed to gain insight into the differences in reactivity of the bimetallic catalyst compared to the monometallic counterparts. Differential charge density iso-surface contour and projected density of states analyses were carried out to gain further atomic-level insights into the hydrazine adsorption mechanism.The optimized surface and adsorption structures were determined using the plane-wave-based DFT method, implemented in the Vienna Ab-Initio Simulation Package (VASP) [34\u201336]. The interactions between the valence electrons and the ionic core were described with the projected augmented wave (PAW) method [37,38]. The electronic exchange\u2013correlation potential was treated using the Perdew-Burke-Ernzerhof (PBE) functional [39]. A high energy cut-off of 600\u00a0eV was used for the plane-wave basis sets, with a convergence criterion set to 10\u22126 eV between two ionic steps for the self-consistency process. The Brillouin zone was sampled with a Monkhorst-Pack [40] k-point grid of 5\u00a0\u00d7\u00a05\u00a0\u00d7\u00a05 for bulk Ni, Zn, and NiZn, while for geometry optimisation of cleaved surfaces, a k-point grid of 3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a01 was used.The low miller index surfaces of \u03b21-NiZn were created from the relaxed bulk material using the METADISE code [41], which ensures the creation of surfaces with zero dipole moment perpendicular to the surface plane. The surfaces were modelled using the slab model and for each surface a slab thickness of at least 10\u00a0\u00c5 was increased until convergence of the surface energy was achieved within 1\u00a0meV per cell. In each simulation cell, a vacuum region of 15\u00a0\u00c5 was tested to be sufficient to avoid interactions between periodic slabs in the z-direction. From the full geometry relaxation of each surface we have calculated the surface energy (\u03b3), which is the energy required to cleave an infinite crystal in two along a given crystallographic plane using the relation:\n\n\n\n\u03b3\n=\n\n\n\nE\n\nsurface\n\n\n-\nn\n\nE\n\nbulk\n\n\n\n\n2\nA\n\n\n\n\n\nwhere \n\nE\n\nsurface\n\n\n is the energy of the naked surface, \nn\n is the number of repeating unit cells in the z-direction, \n\nE\n\nbulk\n\n\n is the energy of the bulk system, A is the surface area of the relaxed system - where the factor of 2 reflects the fact that there are two surfaces for each slab with identical atomic ordering at the bottom and top layers.The hydrazine adsorption calculations were carried out on a 3\u00a0\u00d7\u00a03 supercell of the bimetallic NiZn (111), (110) and (100) surfaces, which are large enough to minimize the lateral interactions between the hydrazine molecules in neighbouring image cells. The structural optimizations of Ni-Zn systems were carried out without any symmetry constraint and the hydrazine molecule was free to move away laterally and vertically from the initial binding site or reorient itself to find the minimum energy adsorption structure. To determine the optimum adsorption sites and geometries, the hydrazine molecule and the topmost three layers of each surface slab are allowed to relax unconstrainedly until residual forces on all atoms had reached 0.03\u00a0eV\u00a0\u00c5\u22121. Van der Waals dispersion forces were accounted for by utilising the Grimme DFT-D3 functional [42], which adds a semi-empirical dispersion correction to the conventional Kohn-Sham DFT method. This is important because standard DFT calculations fail to provide an accurate description of the asymptotic decreasing behaviour of the long-range vdW interactions that are ubiquitous in hybrid inorganic/organic systems [43\u201345]. Previous studies have shown that the inclusion the dispersion correction have led to proper description of the hydrazine adsorption structures and energetics on metallic Ni and Cu surfaces [32,46,47]. To quantify the hydrazine adsorption strength on the NiZn surfaces, the adsorption energy (Eads) was calculated using the following equation:\n\n\n\n\nE\n\nads\n\n\n=\n\nE\n\nSystem\n\n\nPBE\n+\nD\n3\n\n\n-\n\n(\n\nE\n\nSurface\n\n\nPBE\n+\nD\n3\n\n\n+\n\nE\n\nAdsorbate\n\n\nPBE\n+\nD\n3\n\n\n)\n\n\n\n\nwhere \n\nE\n\nSystem\n\n\nPBE\n+\nD\n3\n\n\n is the energy of the adsorbed hydrazine to the catalyst slab, \n\nE\n\nSurface\n\n\nPBE\n+\nD\n3\n\n\n is the energy of the naked surface and \n\nE\n\nAdsorbate\n\n\nPBE\n+\nD\n3\n\n\n is the energy of the adsorbate in the gas phase. Therefore, a negative \n\nE\n\nads\n\n\n value indicates an exothermic, favourable adsorption, whereas a positive value indicates an endothermic, and less favourable adsorption.The d-band centre (E\nd) was calculated for each surface using the relation:\n\n\n\n\nE\nd\n\n=\n\n\n\n\u222b\n\n-\n\u221e\n\n\u221e\n\nE\n\u00b7\nD\n\n\nE\n\n\nd\nE\n\n\n\n\u222b\n\n-\n\u221e\n\n\u221e\n\nD\n\n\nE\n\n\nd\nE\n\n\n\n\n\nwhere Ed\n is the d-band centre and D(E) are the density of states (DOS) for the surface atoms. The d-band centre is a useful descriptor as the Fermi level of transition metals originates primarily from the d-orbitals. Hybridisation between surface orbitals and adsorbates forms bonding and anti-bonding states. A higher d-band centre is therefore associated with higher energy anti-bonding states and hence, stronger adsorption.The bulk \u03b21-NiZn was modelled in the tetragonal crystal structure, as shown in Fig. 1\na. The fully optimized lattice parameters are predicted at a\u00a0=\u00a0b\u00a0=\u00a02.686\u00a0\u00c5, and c\u00a0=\u00a03.262\u00a0\u00c5 in good agreement with previous experimental results of a\u00a0=\u00a0b\u00a0=\u00a02.75\u00a0\u00c5 and c\u00a0=\u00a03.21\u00a0\u00c5 [48]. The partial density of states (PDOS) shown in Fig. 1b reveals the metallic conductivity of the alloy, with the Ni-d orbitals dominating states surrounding the fermi level, in agreement with earlier DFT results [33]. From the optimized bulk NiZn material, the (100), (110), and (111) surfaces were created and fully relaxed in order to determine their relative stabilities (Fig. 2\n). The surface energies of the (100), (110), (111) surfaces are calculated at 2.06, 1.75, and 2.53\u00a0J\u00a0m\u22122 respectively, which indicates that the order of decreasing stability is (110) < (100) < (111). The differences in the stabilities can be attributed to differences in the surface terminations and the coordination numbers of the topmost surface atoms. The (110) surface is terminated by Ni:Zn in a 1:1 ratio, with the Ni and Zn atoms in an 8-fold coordination number (CN). The (111) surface is also terminated by Ni and Zn, but in a 1:2 ratio, with the surface Ni atoms having coordination number of 6. The (100) surface on the other hand is terminated by a Zn-ad-atom having 5-fold coordination, with accessible Ni-sites below with higher coordination. Based on the calculated surface energies, we have simulated the equilibrium crystal morphology of the NiZn nanoparticle using Wulff construction [49\u201351]. As shown in Fig. 3\n, all three surfaces are expressed in the NiZn nanoparticle with the (110) covering the largest area, in line with it being the most thermodynamically stable.Prior to investigation the adsorption of hydrazine on the bimetallic NiZn surfaces, the adsorption process has been systematically characterized on the monometallic Ni(111) and Zn(001) surfaces for comparison. Hydrazine in the gas phase has been found to adopt the gauche formation [52]. This is due to the hyper conjugate mechanism, which minimises repulsion between the lone pairs of each nitrogen by rotation about the NN axis [53]. Consequently, the relative energies of the eclipsed and trans geometries are higher than gauche in the gas phase [54].Shown in Fig. 4\na-c are the lowest-energy gauche, trans, and eclipsed adsorption configurations of hydrazine on the monometallic Ni(111) surface, with the characteristic binding energies and structural parameters given in Table 1\n. The gauche, trans, and eclipsed binding configurations released adsorption energies of \u22121.62, \u22121.65, and \u22121.97\u00a0eV, respectively. This indicates that the eclipsed binding mode, wherein both N atoms interact with adjacent Ni sites, is the most stable binding geometry on the Ni(111) surface. The surface Ni-N bond distances in the most stable eclipsed geometry are calculated at 1.996 and 2.011\u00a0\u00c5, with hydrazine\u2019s NN bond converged at 1.440\u00a0\u00c5. For the monodentate trans and gauche adsorption configurations, the interacting Ni-N bond distance is calculated at 1.965 and 2.012\u00a0\u00c5, respectively, whereas the NN bonds are converged at 1.457 and 1.441\u00a0\u00c5. Bader population analysis revealed that the hydrazine molecule is oxidised to only a small extent, characterized by loss of charge to the interacting surface species. The hydrazine molecule lost 0.108, 0.171, and 0.178 e\u2212 to the surface when adsorbed in the gauche, trans, and eclipsed configurations, respectively. Although small, the loss in electron density resulted in structural modification (internal rotation) of the hydrazine molecule with the dihedral angle reduced to 37.1\u00b0 in the eclipsed adsorption geometry, compared to the gas phase gauche conformer dihedral angle of 91.0\u00b0. This disturbance of the hyper conjugate mechanism has also been observed on several other metallic surfaces, such as hydrazine adsorption to Pt(111) [53]. The NN distance of 1.440\u00a0\u00c5 also shows a minute change from the gas phase species, 1.441\u00a0\u00c5, hence presenting suitability towards the DHFC and decomposition mechanisms by reducing ammonia formation through the NN bond cleavage mechanism and favouring N2 production [18,55].Compared to the Ni(111) surface, the gauche, trans and eclipsed adsorption configurations of hydrazine on the monometallic Zn(001) surface (Fig. 4d-f) released lower adsorption energies of \u22121.15, \u22120.94, and \u22120.49\u00a0eV, respectively, indicating that the Ni(111) surface is more reactive towards hydrazine adsorption than the Zn(001) surface. In the most stable gauche geometry, the Zn-N adsorbate\u2013surface bonds and NN hydrazine bonds are predicted at 2.196 and 1.428\u00a0\u00c5, respectively. In the trans and eclipsed adsorption configurations the interacting Zn-N bond distance is predicted at 2.142 and 2.267\u00a0\u00c5, respectively, whereas the NN bond length is converged to 1.461 and 1.453\u00a0\u00c5. Similar to the Ni(111), the hydrazine is only slightly oxidized upon adsorption on the Zn(001) surface: the hydrazine molecule lost a charge of 0.047, 0.062, and 0.086 e\u2212 to the interacting surface species when adsorbed in the gauche, trans, and eclipsed configurations, respectively. The dihedral angle is predicted at 98.8, 179.9, and 26.3\u00b0 for the gauche, trans and eclipsed adsorption configurations at the Zn(001) surface. The smaller changes in the dihedral angles, along with the extremely small Bader charge transfers indicate only minor disruption in the hyper conjugate mechanism. Considering that stronger adsorption is correlated with high activity [33], the weaker hydrazine adsorption on the Zn(001) compared the Ni(111) surface suggest that the monometallic Zn(001) may be inappropriate for the DHFC applications. This is consistent with the work of Wang et al. [25], who found Zn-film to be inactive for the DHFC.As for the monometallic Ni and Zn catalysts, hydrazine has been adsorbed onto the bimetallic NiZn (100), (110), and (111) surfaces in the gauche, trans, and eclipsed conformations. The lowest-energy adsorption geometries are displayed in Fig. 5\n and the optimized structural parameters are summarized in Table 2\n. For the NiZn(111)-hydrazine interactions, the eclipsed geometry (Fig. 5a) has been found to be the most stable, with a highly exothermic adsorption energy of \u22122.71\u00a0eV. In the eclipsed structure, hydrazine is bound to adjacent Ni and Zn sites, with the NiN and ZnN bond distances calculated at 2.032 and 2.136\u00a0\u00c5, respectively. As shown by the dihedral angle, the hydrazine NH2 units are not fully aligned to 0\u00b0, as for gas-phase eclipsed, but to 35.0\u00b0, indicating that hydrazine has not been fully oxidised and some repulsion between the two N-lone pairs remains. Consistent with this, our Bader population analysis shows a lesser extent of oxidation (\u0394q(N2H4)\u00a0=\u00a00.128 e\u2212) compared to the eclipsed geometry on the monometallic Ni(111) surface (\u0394q(N2H4)\u00a0=\u00a00.178 e\u2212). Although these differences are small, it is enough to favour the eclipsed formation. The high binding energy is attributed to the low-coordinated Ni atoms present on the surface (CN\u00a0=\u00a06), strengthening the surface-hydrazine interactions. The second most stable hydrazine binding geometry on the NiZn(111) surface is a monodentate trans configuration (Fig. 5b), also releasing a high adsorption energy of \u22122.25\u00a0eV. In the adsorbed trans geometry, the dihedral angle calculated at 173.4\u00b0 is relatively close to that of the gas phase trans configuration (180.0\u00b0). The NN bond is somewhat activated, as reflected in the small increase in the NN distance from 1.468 to 1.491\u00a0\u00c5. The gauche adsorption configuration on the NiZn(111) surface (Fig. 5c) released the lowest adsorption energy, calculated at \u22121.99\u00a0eV. In the gauche configuration, hydrazine adsorbs atop a single Ni-site with the NiN, NN bond lengths, and dihedral angle predicted at 1.960\u00a0\u00c5, 1.450\u00a0\u00c5, and 106.4\u00b0, respectively. The generally stronger binding of hydrazine to the bimetallic NiZn(111) surface compared to the monometallic Ni(111) and Zn(100) surfaces indicates that the Ni-Zn alloy presents a more active site for hydrazine activation, which results from the synergistic effects between the two metals.For the NiZn(110)-hydrazine interactions (Fig. 5d-f), the preferred adsorption configuration is predicted to be the eclipsed geometry, wherein the hydrazine binds via both N atoms at adjacent Ni and Zn sites (Fig. 5d), releasing an adsorption energy of \u22122.08\u00a0eV. The NiN, ZnN, and NN distances are calculated at 2.016, 2.179, and 1.472\u00a0\u00c5, respectively. The dihedral angle is predicted to be closer to zero (\u03b8\u00a0=\u00a08.7\u00b0) than atop the NiZn(111) surface, where the dihedral angle is predicted at 35.0\u00b0. This may partly be attributed to the differences in the surface structure as the NiZn(110) surface has a flat topology, with the NN axis contorted to allow adsorption to adjacent surface sites. However, the coordination number of the surface Ni and Zn both stand at 8 on NiZn(110), compared to 6 and 7, respectively, on the NiZn(111) surface. Hence, a lower adsorption energy is observed for NiZn(110) as shown from bond order conservation theory [56]. Compared to the most stable bidentate eclipsed geometry, the monodentate trans and gauche configurations at Ni top sites on the NiZn(110) surface released adsorption energies of \u22121.89 and \u22121.71\u00a0eV, respectively. The NiN and NN distances are calculated at 2.000 and 1.464\u00a0\u00c5, for the trans geometry and at 2.022 and 1.448\u00a0\u00c5 the for gauche geometry. Minor oxidation occurs in hydrazine for both the trans and gauche conformations and leads to an adsorption structure similar to that of the respective gas-phase species. This is shown by the dihedral angle of 177.1 and 104.0\u00b0 for the trans and gauche systems.The bimetallic NiZn(100) surface is terminated in Zn ad-atoms and hence exhibits adsorption trends akin to the monometallic Zn(001) surface. Low-coordinated Zn sites dominate the NiZn(100) surface (CN\u00a0=\u00a05) and as a result the gauche adsorption has been found to be the most stable (Fig. 5g). The bidentate gauche adsorption geometry released an adsorption energy of \u22122.11\u00a0eV, with the Ni-N and Zn-N distances converged at 2.024 and 2.342\u00a0\u00c5, respectively. Although the gauche adsorption is favoured here, as for Zn(001), the adsorption energy and interaction is much stronger, owing to the synergistic effect between Ni and Zn in the alloy. The geometry has been ascribed to be gauche, however, the dihedral angle of 63.4\u00b0, shows it lies between an eclipsed and gauche formation. The top-most NH2 unit has rotated in order to align its lone pair with the low-coordinated Zn atoms, hence stabilising the system. This is consistent with the inability of Zn sites to bind to hydrazine efficiently. The trans (Fig. 5h) and eclipsed (Fig. 5i) configurations released adsorption energies of \u22122.10 and \u22122.00\u00a0eV, respectively. In both geometries, the hydrazine molecule is only marginally oxidized as reflected in the Bader charges reported in Table 2. The less stable hydrazine adsorption structures and energetics at Zn-sites on the (111) and (110) NiZn surfaces are shown in Supporting Information\nFig. S1 and Table S1.Modification of the d-band centre resulting from the synergistic effects between the two metals is another origin of the differences in reactivity of the bimetallic NiZn surfaces compared to the monometallic. Since the adsorbate interaction to the metal surface occurs via the N-lone pair (N-p) and surface metal d-states, the bonding interaction creates a set of bonding and anti-bonding states. The energetic level of these is then defined by the d-band centre as the anti-bonding states are less occupied when the d-band centre is closer to the Fermi level due to their resulting higher energy [57]. To probe the electronic effects of NiZn surface reactivity, the d-band centres (E\nd\n) for each surface Ni and Zn sites have been analysed [56\u201358]. A plot of the calculated E\nd\n values for the Ni(111), Zn(001), NiZn(111), (110) and (100) surfaces is shown schematically in Fig. 6\na. The d-band centre projected on Ni-atoms of Ni(111) and Zn-atoms of Zn(001) surface is predicted at \u22121.65\u00a0eV and \u22127.14\u00a0eV, respectively (Fig. 6b and c). The weaker binding of hydrazine on Zn(001) compared to Ni(111) can thus be attributed to the higher d-band centre and formation of higher energy antibonding orbitals for Ni(111), causing them to be less filled [59]. Compared to the monometallic surfaces, Ed\n of Ni and Zn atoms of the bimetallic NiZn surfaces has shifted closer to the Fermi level from each respective monometallic counterpart (Fig. 6d-f). The Ni d-band shifts from \u22121.65\u00a0eV for Ni(111) to \u22120.91, 1.13, and \u22120.99\u00a0eV for NiZn (111), (110) and (100), respectively. Similarly, the Zn d-band centre shifts from \u22127.14\u00a0eV of the Zn(001) to \u22126.51, \u22126.74 and \u22126.60\u00a0eV for the NiZn (111), (110) and (100) surfaces, respectively. The shift in the d-band centre closer to the Fermi level in the bimetallic NiZn surfaces is consistent with the stronger binding energy observed on the NiZn surfaces compared to the monometallic surfaces [60].These results suggest that the combination of two weakly active metals (Ni and Zn) gives a highly active bimetallic NiZn catalyst for hydrazine adsorption and activation towards direct hydrazine fuel cell applications. The improved activity of the bimetallic NiZn catalyst may, therefore, be attributed to the beneficial synergistic effects derived from the composition and electronic structure modulation. Further insights into the hydrazine adsorption process on the bimetallic NiZn catalyst was ascertained through the projected density of states (PDOS) and differential charge density isosurface analyses. As shown in Fig. 7\na-c, the chemisorption of hydrazine on the bimetallic NiZn catalyst is found to be characterized by strong hybridization between the d-orbitals of interacting surface Ni and Zn sites and on the N p-orbitals of hydrazine. Consistent with chemisorption, we observed electron density accumulation within the Ni-N and Zn-N bonding regions as shown in Fig. 7d-f. Considering that the charge transfers between hydrazine and the surface is small and that there is no clear trend between charge transfers and the calculated adsorption energies, the differences in the hydrazine adsorption strength to the different surfaces and may also be attributed to the differences in the coordination numbers of the surface Ni and Zn atoms. This invariably dictates adsorption environment at each active site, whereby the low-coordinated Ni sites on the NiZn (111) surface enables the strongest hydrazine adsorption.In summary, we have performed a comprehensive first-principles dispersion-corrected DFT investigation of hydrazine adsorption on bimetallic NiZn (111), (110) (100) surfaces and compared the results to the monometallic Ni(111) and Zn(100) surfaces. The synergistic beneficial effects derived from surface composition and electronic structure modification with Ni and Zn alloying gave rise to more reactive surface sites that bind hydrazine more strongly than the single-component nickel and zinc metal surfaces. It is found that the Ni-terminated (111) and (110) NiZn surfaces preferentially bind the hydrazine molecule in a bidentate eclipsed geometry, compared to the Zn-terminated (100) surface where binding a bidentate gauche adsorption geometry is favoured. The stronger adsorption of hydrazine on the bimetallic NiZn nanocatalyst than the monometallic is shown to be characterised by stronger hybridisation between the d-orbitals of the interacting surface sites and the N p-orbitals of the hydrazine, which is corroborated by the observed shift in the d-band centre closer to the Fermi level. These results should provide new possibilities for the design and development of Ni-Zn alloy catalysts with improved activity and selectivity of hydrazine electro-oxidation in the DHFC.R.W.C is responsible for data curation, formal analysis, and writing of original draft. S.R.R performed writing - review and editing. N.Y.D is responsible for funding acquisition, project conceptualization, administration, supervision, and writing - review and editing of manuscript.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the UK\u2019s Engineering and Physical Sciences Research Council (EPSRC), grant number EP/S001395/1. R.W.C. acknowledges the College of Physical Sciences and Engineering, Cardiff University for studentship. We also acknowledge the used of computational facilities of the Advanced Research Computing at Cardiff (ARCCA) Division, Cardiff University, and HPC Wales. Information on the data that underpins the results presented here, including how to access them, can be found in the Cardiff University data catalogue at http://doi.org/10.17035/d.2020.0115779666.Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2020.147648.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  We present a systematic first-principles density functional theory study with dispersion corrections (DFT-D3) of hydrazine adsorption on the experimentally observed (111), (110) and (100) surfaces of the binary \u03b21-NiZn alloy. A direct comparison has been drawn between the bimetallic and monometallic Ni and Zn counterparts to understand the synergistic effect of alloy formation. The hydrazine adsorption mechanism has been characterised through adsorption energies, Bader charges, the d-band centre model, and the coordination number of the active site - which is found to dictate the strength of the adsorbate\u2013surface interaction. The bimetallic \u03b21-NiZn nanocatalyst is found to exhibit higher activity towards adsorption and activation of hydrazine compared to the monometallic Ni and Zn counterparts. The Ni-sites of the bimetallic NiZn surfaces are found to be generally more reactive than Zn sites, which is suggested to be due to the higher d-band centre of \u22120.13\u00a0eV (closer to the Fermi level), forming higher energy anti-bonding states through NiN interactions. The observed synergistic effects derived from surface composition and electronic structure modification from Ni and Zn alloying should provide new possibilities for the rational design and development of low-cost bimetallic Ni-Zn alloy catalysts for direct hydrazine fuel cell (DHFC) applications.\n               "}
{"full_text": "With the wide use of fossil fuels, a substantial amount of CO2 emissions is emitted into the atmosphere, leading to severe environmental issues [1], such as sea level rise [2], ocean acidification [3] and extreme weather [4]. In order to reduce industrial carbon emissions, CO2 capture is one of the most promising techniques and has attached a lot of attention [5\u20138]. Currently, the most mature CO2 capture technologies are solvents adsorption (i.e. MEA) [9] and calcium looping [10\u201313], which are both operated by swinging temperature to regenerate the adsorbents. The obtained high concentration of CO2 could be stored by deep-sea injection or mineralisation [14], which is known as carbon capture and storage (CCS). However, the CCS requests a high capital investment and may not be a long-term sustainable solution due to the risk of the second release of stored CO2\n[15]. Therefore, there is an increasing interest in the direct utilisation of the captured CO2, which is known as integrated CO2 capture and utilisation (ICCU).The ICCU process can eliminate the CO2 enrichment, storage and transportation steps by in-situ converting the captured CO2\n[16] and isothermally producing valuable products (e.g. CH4 or CO [17\u201320]). In a typical ICCU process, CO2 is first adsorbed on adsorbents (i.e. CaO), and then a reducing agent (i.e. H2) is introduced to react with the captured CO2 with the assistance of active catalytic sites. Among the final products of the ICCU process, carbon monoxide (CO) is considered to be one of the most valuable chemicals to be used as the feedstock for the mature Fischer-Tropsch process to further produce liquid products [21\u201323]. The commonly used CO2 reduction technologies include thermal-catalysis, photo-catalysis, electro-catalysis, plasma-catalysis and etc. [24]. Thermal-catalyzed CO2 reduction is a relatively compromised choice in terms of high process efficiency and low input cost (e.g. equipment investment and materials costs). The reverse water\u2013gas shift (RWGS) reaction, as shown in Eq. (1), is a promising process for thermal catalytic conversion of CO2 that uses renewable H2 to reduce carbon emissions and produce syngas.\n\n(1)\n\n\n\n\nCO\n\n2\n\n+\n\nH\n2\n\n=\nC\nO\n+\n\nH\n2\n\nO\n\n\n\n\nThe bifunctional combined materials (BCMs), containing CO2 adsorbent and active catalytic sites, have been proven effective for the ICCU process [20,23,25\u201327]. Specifically, CaO is widely used as high temperature CO2 adsorbent [28,29], and Ni can act as catalytic sites for RWGS [30,31]. The previous study [32] has concluded that the physical mixing is a superior material preparation method by avoiding catalytic sites coverage due to CaO sintering. The supports of Ni-catalysts are widely believed to play key roles in the catalytic process owing to the metal dispersion, metal-support interaction, etc. [33\u201335]. However, as a novel process, ICCU provide CO2 in the form of carbonates, which possesses different chemical environment than traditional RWGS. Therefore, there is a gap on understanding of the effects of supports in ICCU, including the effects on CO2 adsorption and catalytic conversion.In this work, to investigate the support effects on ICCU-RWGS, two active supports (CeO2 and TiO2) and inert materials (ZrO2 and Al2O3) supported Ni are synthesised as catalysts, and a sol\u2013gel prepared CaO as the adsorbent to prepare the BCMs. The sol\u2013gel CaO has been proven a stable and excellent CO2 adsorbent in carbon capture [36]. The CeO2 and TiO2 have been widely applied to catalytic processes, including RWGS, CO2 methanation, dry reforming etc. [37,38]. Furthermore, the active TiO2 and inert Al2O3 could form spinel with Ni (strong metal-support interaction) [39,40], which provide valuable benchmarks for identifying active sites.ZrO2 (Sigma-Aldrich, 99%), TiO2 (Sigma-Aldrich, 99.5%) and Al2O3 (Sigma-Aldrich, 99.5%) were dried at 120\u202f\u00b0C before the impregnation process. CeO2 was prepared by a hydrothermal method as reported in previous work [17,19]. Briefly, 5.21\u202fg Ce(NO3)3\u00b76H2O (Sigma-Aldrich, 99%) was dissolved in deionised water (30\u202fml) to prepare a Ce source solution, followed by the dissolution of 57.6\u202fg NaOH (Sigma-Aldrich, 99%) in deionised water (210\u202fml) to prepare the precipitant. The Ce source was mixed with the precipitant dropwise for 30 mins at room temperature to obtain a slurry. The slurry was transferred into a stainless-steel autoclave and kept at 100\u202f\u00b0C for 24\u202fh. The precipitate was washed and separated by vacuum filtration using distilled water and ethanol to neutrality and dried at 120\u202f\u00b0C overnight, to produce a yellow powder, labeled as CeO2. Ni-based catalysts were prepared by the wet impregnation method, using Ni(NO3)2\u00b76H2O (Sigma-Aldrich, 97%) as the metal precursor. Typically, 3.0\u202fg support material was added into 30\u202fml Ni(NO3)2 (0.15\u202fmol L-1) aqueous solution, stirred at room temperature for 2\u202fh and evaporated under stirring to produce a sample paste. The sample paste was dried at 120\u202f\u00b0C overnight, and calcined at 800\u202f\u00b0C for 5\u202fh with a heating rate of 5\u202f\u00b0C\u202fmin\u22121, to produce NiO/ZrO2, NiO/TiO2, NiO/CeO2 and NiO/Al2O3, respectively. The NiO/supports were reduced at 550\u202f\u00b0C for 2\u202fh with a heating rate of 5\u202f\u00b0C\u202fmin\u22121 in 5% H2/N2, and labeled as Ni/ZrO2, Ni/TiO2, Ni/CeO2 and Ni/Al2O3, respectively.The sol\u2013gel derived CaO was prepared as reported in previous work [20,32]. Briefly, 23.6\u202fg Ca(NO3)2\u00b74H2O (Sigma-Aldrich, 99%) and 19.2\u202fg citric acid monohydrate (Sigma-Aldrich, 99.5%) were dissolved into 72\u202fml distilled water, stirred at room temperature at 80\u202f\u00b0C, and dried at 120\u202f\u00b0C overnight. The sample was ground and calcined at 850\u202f\u00b0C for 5\u202fh with a heating rate of 5\u202f\u00b0C\u202fmin\u22121, labeled as sol\u2013gel CaO. The bifunctional combined materials were prepared by physically mixing the Ni/support catalysts and the sol\u2013gel CaO with a mass ratio of 1:2, labeled as Ni/ZrO2-CaO, Ni/TiO2-CaO, Ni/CeO2-CaO and Ni/Al2O3-CaO, respectively.The loadings of Ni on various supports were measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The samples were digested in nitric acid and then analysed using a Perkin Elmer PE2400 CHNS. X-ray diffraction (XRD) patterns (2 Theta: 5\u00b0-75\u00b0) were measured by a PANalytical Empyrean Series 2 diffractometer with a Cu Ka X-ray source. The surface area and pore structure of the Ni/supports catalysts were characterised by an ASAP 3000 analyser at 77\u202fK. The Brunauer-Emmett-Teller (BET) method and the desorption isotherm branch were applied to calculate the surface area and the pore size distribution, respectively. The X-ray photoelectron spectrum (XPS) analysis was performed on a Thermo Fisher Scientific NEXSA spectrometer. H2 temperature-programmed reduction (H2-TPR) of the NiO/support catalysts was tested on a Hi-Res TGA 2950 thermogravimetric analyser. Typically, the samples were pre-treated under N2 at 300\u202f\u00b0C for 30 mins to remove the adsorbed H2O and gases, equilibrated to 50\u202f\u00b0C and temperature programmed reduced in 100\u202fml\u202fmin\u22121 5% H2/N2 (10\u202f\u00b0C/min to 800\u202f\u00b0C). The CO2 temperature-programmed desorption (CO2-TPD) patterns of the Ni/support catalysts were measured by a Micromeritics Autochem II 2920 analyser. The Ni/support catalysts were in-situ reduced at 550\u202f\u00b0C in H2 for 1\u202fh and then cooled down to 30\u202f\u00b0C in He. After adsorbing CO2 at 30\u202f\u00b0C in 10% CO2/He, the temperature was increased to 800\u202f\u00b0C in He with a heating rate of 10\u202f\u00b0C\u202fmin\u22121. Scanning electron microscopy coupled with an energy dispersive X-ray spectrometer (SEM-EDX, FEI Quanta FEG) was used to characterise the morphology and element dispersion. Transmission electron microscopy (TEM, FEI Titan3 Themis 300) and high-angle annular dark-field transmission electron microscopy (HAADF-TEM) were utilised to observe the morphology and Ni particle size of Ni/support catalysts. The Ni particle size distribution was calculated from TEM observation.The ICCU-RWGS evaluations of Ni/support-CaO were carried out in a fixed-bed reactor and monitored by an online gas analyser (Kane Autoplus 5). The stainless-steel reaction tube (length: 500\u202fmm, inner diameter: 64\u202fmm) was placed in the middle of the tube furnace (Elite TSH-2416CG). Briefly, 0.30\u202fg Ni/support-CaO bifunctional combined materials (BCMs) were placed in the middle of the reaction tube and fixed in place by quartz wool. Two thermocouples were placed in the reaction tube and inside the tube furnace to monitor the temperature of the BCMs and tube furnace, respectively.In a typical evaluation test, the BCMs were reduced at 550\u202f\u00b0C in 5% H2/N2 for 2\u202fh, and then the gas was switched to 100\u202fml\u202fmin\u22121 20% CO2/N2 for \u223c28 mins. 100\u202fml\u202fmin\u22121 5% H2/N2 was then introduced for \u223c28 mins for the RWGS. Then the flowing gas was switched to N2 and equilibrated the temperature for the following test. The baseline of carbonation and hydrogenation steps was monitored using 0.3\u202fg SiO2 to eliminate the analyser signal delay. The CO2 conversion, CO yield and selectivity were calculated by integrating the real-time data collected from the online gas analyser from 0\u202fs to 1700\u202fs, referring to the equations below.\n\n(2)\n\n\n\nC\n\nC\nO\n2\n\n\n=\n\n\n\n\u222b\n\n0\n\n1700\n\n\n(\nC\nO\n\n+\n\n\nCH\n\n4\n\n)\n\n\n\n\n\u222b\n\n0\n\n1700\n\n\n(\nC\nO\n\n+\n\n\nCH\n\n4\n\n+\n\n\nCO\n\n2\n\n)\n\n\n\n\n\u2217\n\n%\n\n\n\n\n\n\n(3)\n\n\n\nS\n\nCO\n\n\n=\n\n\n\n\u222b\n\n0\n\n1700\n\nC\nO\n\n\n\n\n\u222b\n\n0\n\n1700\n\n\n(\nC\nO\n\n+\n\n\nCH\n\n4\n\n)\n\n\n\n\n\u2217\n\n%\n\n\n\n\n\n\n(4)\n\n\n\nY\n\nCO\n\n\n=\n\n\n\n\u222b\n\n0\n\n1700\n\nC\nO\n\n(\n%\n\n\u2217\n\ns\n)\n\n\n\n\u2217\n\n1.667\n\n(\nm\nl\n/\ns\n)\n\n\n\n22.4\n(\nm\nl\n/\nm\nm\no\nl\n)\n\n\u2217\n\n0.30\n(\ng\n)\n\n\n\n\n\n\nCCO2, SCO and YCO represent to CO2 conversion (%), CO selectivity (%) and CO yield (mmol gmaterial\n-1).The elemental analysis of the Ni/support catalysts using inductively coupled plasma (ICP) showed that the Ni loadings on the various supports were controlled at 8\u202f\u00b1\u202f1.5\u202fwt% (Table 1\n). N2 isothermal adsorption\u2013desorption was carried out to characterise the surface area and pore structure of the Ni/support materials (Fig. 1\n). As summarised in Table 1, the Ni/ZrO2 and Ni/TiO2 showed poor porosity with low BET surface areas (< 5 m2 g\u22121). In contrast, the Ni/CeO2 and Ni/Al2O3 were more porous and possessed typical type IV isotherms with distinct hysteresis loops. As shown in Fig. 1, the Ni/CeO2 and Ni/Al2O3 exhibited type H3 and H4 hysteresis loops with various pore size distribution. The pore size of Ni/CeO2 and Ni/Al2O3 are \u223c10\u201340\u202fnm and 3\u201310\u202fnm, respectively. It can also be demonstrated from TEM observation (Fig. 2\ni) that Ni/CeO2 could accumulate slit-shaped pores with regular rod-like morphology. The higher surface area and porous structure could contribute to the dispersion of metals and the diffusion of reactants in catalytic reactions.High-angle annular dark-field transmission electron microscopy (HAADF-TEM) was carried out to observe the dispersion of Ni species and the morphologies of the Ni/support catalysts. As shown in Fig. 2, all the Ni/support catalysts possessed gaussian distribution of Ni species size, following the decreasing order: Ni/TiO2\u202f>\u202fNi/ZrO2\u202f>\u202fNi/Al2O3\u202f>\u202fNi/CeO2. The Ni species size distribution and the average Ni size are presented in Fig. 2d, h, l, p. The CeO2 and Al2O3 can disperse Ni better than other supports, which is attributed to the morphology and porosity of supports. Specifically, the Ni/CeO2 possesses the optimal Ni dispersion with a \u223c14.1\u202fnm Ni species size.The X-ray diffraction patterns of the Ni/support materials are presented in Fig. 3\n. Except for the amorphous Al2O3 supported Ni, all the other three samples showed distinct metallic Ni peaks (PDF#87-0712) and strong diffraction peaks which belong to the support materials, referring to ZrO2 (PDF#86-1451), TiO2 (PDF#75-1753) and CeO2 (PDF#78-0694). In addition, the Ni/TiO2 and Ni/Al2O3 catalysts exhibited NiTiO3 (PDF#76-0334) and NiAl2O4 (PDF#77-1877) spinel peaks, respectively. There were no distinct NiO peaks, indicating that NiO was fully reduced at 550\u202f\u00b0C during the reduction process. The reducibility of spinel is much poorer than NiO species [41,42], which could be verified by the presence of significant spinel peaks after reduction.XPS was carried out, as shown in Fig. 4\n, to further confirm the valence state and chemical environment of surface nickel species of the reduced Ni/support catalysts. The Ni 2p3/2 XPS profiles of Ni/supports exhibit common peaks at \u223c852.1\u202feV and \u223c855.8\u202feV, which are attributed to the metallic Ni and its satellite peak, respectively [43]. The air contact could quickly oxidise surface metallic Ni, resulting in the existence Ni2+ peaks on the reduced Ni/support catalysts [44]. The multi-split peaks at 853.5\u2013855.5\u202feV and 857\u2013861\u202feV could be assigned as Ni2+ signals from NiO and their satellite peaks. Notably, there is no distinct NiO peaks on XRD characterisations (Fig. 3), indicating the oxidation only limitedly occurs on the surface of Ni. It is believed that the support interacted Ni would possess higher binding energy [45], which can be located at 856.5\u202feV and 862.3\u202feV. Notably, the Ni/Al2O3 possessed the most abundant metal-support interaction, indicating that the NiAl2O4 dominates on the surface of Ni/Al2O3\n[39]. As a comparison, the surface NiTiO3 could be effectively reduced and exhibited \u223c34% Ni0 even after air oxidation. It is noted that NiTiO3 still dominate in bulk (Fig. 3), indicating that the Ni might parse out from NiTiO3 spinel and act as catalytic sites in ICCU. The Ni/CeO2 exhibits lower metallic Ni fraction (18%) on the surface, attributed to the highly dispersed Ni, which could be easily reacted with O2 in air.The reducibility of the Ni species over the prepared NiO/support catalysts was further investigated by H2-TPR, as shown in Fig. 5\na. There are two reduction peaks over Ni/ZrO2 and Ni/CeO2 at 380\u2013430\u202f\u00b0C and 450\u2013480\u202f\u00b0C, which could be assigned as weakly and strongly interacted NiO species, respectively [41,46]. It is noted that the NiO/CeO2 possessed the lowest reduction temperature, which is attributed to the smallest Ni particle size [47]. The spinel species (i.e. NiAl2O4 and NiTiO3) formed during the high temperature calcination are more difficult to reduce, which is consistent with the XRD analysis. Specifically, the NiTiO3 spinel can be reduced from \u223c600\u202f\u00b0C and showed only one reduction peak at \u223c680\u202f\u00b0C [41]. And there was no distinct reduction of Ni/Al2O3 in the TPR procedure [42]. The formation of spinel is believed to improve Ni dispersion [23],[48]; however, only the reducible Ni species are active catalytic sites.CO2-TPD profiles are presented in Fig. 5b to investigate the basicity of the Ni/support catalysts. Only Ni/CeO2 and Ni/Al2O3 possessed CO2 desorption peaks, which are attributed to their surface basic sites and superior porosity. The two desorption peaks of Ni/CeO2 and Ni/Al2O3 at temperatures of\u202f<\u202f100\u202f\u00b0C and between 150 and 250\u202f\u00b0C are attributed to weak and intermediate CO2 chemisorption, respectively [49]. Ni/CeO2 showed a higher CO2 desorption temperature indicating that CeO2 could provide stronger basic sites, which are beneficial for CO2 adsorption and activation.The bifunctional combined materials (BCMs) for integrated CO2 capture and reverse water\u2013gas shift reaction (ICCU-RWGS) in this work were prepared by physically mixing the Ni/support catalysts and the sol\u2013gel prepared CaO. As shown in Fig. 6\na, 6b and 6c, the images of SEM and TEM were applied to exhibit the morphologies of sol\u2013gel CaO and fresh Ni/CeO2-CaO BCM. The sol\u2013gel CaO adsorbent possessed a sponge-like structure with abundant large pores, consistent with the BET characterisation (Fig. 1 and Table 1). The porous structure could improve the CaO accessibility and prevent the severe sintering of CaO due to volume expansion during the carbonation process [36]. As shown in Fig. 6b, 6c and 6d, the physical mixing method could disperse Ni/CeO2 into the sponge structure CaO, providing active sites near the adsorbent. In the previous research [32], the physical mixing method has been demonstrated to avoid the coverage of the catalytic sites caused by the volume expansion of CaO during the carbonation-hydrogenation ICCU process.The ICCU-RWGS performance using Ni/support-CaO bifunctional combined materials (BCMs) with various supports (e.g. ZrO2, TiO2, CeO2 and Al2O3) are shown in Fig. 7\n. Here Ni particles were used as active catalytic sites, and CaO played as the CO2 adsorbent. A typical ICCU-RWGS process mainly included two steps, one was CO2 adsorption from a diluted CO2 source (e.g. 20% CO2/N2), and another was hydrogenation of the captured CO2. The above two steps were isothermally operated by switching the inlet gas between 20% CO2/N2 and 5% H2/N2. The SiO2/CaO was used as a blank benchmark material to demonstrate the ICCU-RWGS performance with bare inert support compared to active Ni metal over active supports. To reveal the optimal support for the ICCU-RWGS process, various Ni/support-CaO bifunctional combined materials were evaluated at 550\u2013750\u202f\u00b0C.As shown in Fig. 7a and 7c, the CO2 conversions and CO yields of various Ni/support-CaO BCMs followed the decreasing order: Ni/CeO2-CaO\u202f>\u202fNi/TiO2-CaO\u202f>\u202fNi/ZrO2-CaO\u202f>\u202fNi/Al2O3-CaO. All of the Ni/support-CaO BCMs exhibited excellent CO selectivity (\u223c100%) over the investigated temperature range. The Ni/Al2O3-CaO showed only a comparable performance with SiO2-CaO in terms of CO2 conversion and CO yield, indicating the poor catalytic activities of nonreducible NiAl2O4 spinel (Fig. 5a). However, Ni/TiO2-CaO with NiTiO3 spinel achieved much high CO2 conversion compared to Ni/Al2O3-CaO owing to the better spinel reducibility. Although there was no spinel formation, Ni/CeO2-CaO outperforms Ni/ZrO2-CaO in relation to CO2 conversion and CO yield. For example, Ni/CeO2-CaO and Ni/ZrO2-CaO achieved 56.1% and 34.0% for CO2 conversion and 2.7 and 1.1\u202fmmol\u202fg\u22121 for CO yield at 650\u202f\u00b0C, respectively.The performances of ICCU-RWGS were also significantly affected by the reaction temperature. For example, 87.4% and 25.6% of CO2 conversions were obtained over Ni/CeO2-CaO at 550\u202f\u00b0C and 750\u202f\u00b0C, respectively. As an endothermic reaction, RWGS favors a higher reaction temperature which is consistent with the decomposition of CaCO3. However, the fast decomposition of CaCO3 could increase the CO2 concentration near the catalytic sites resulting in a decrease in CO2 conversion.It is necessary to monitor the real-time ICCU-RWGS process to evaluate the CO2 adsorption performance and catalytic activity. The promotion effect of support of Ni could be clearly demonstrated in the CO2 adsorption stage, presenting as the enhanced CO2 capture rate compared to the benchmark (SiO2 line in Fig. 8\na). Specifically, Ni/CeO2-CaO and Ni/Al2O3-CaO exhibited superior CO2 capture rate and capacity (\u223c9.58 and 9.31\u202fmmol\u202fg-1 at 650\u202f\u00b0C for \u223c28\u202fmin), which was attributed to the abundant basicity of Ni/CeO2 and Ni/Al2O3, as indicated in Fig. 5b.The various Ni/support-CaO BCMs also exhibited distinct CO generation rates and real-time CO2 conversion during the hydrogenation step. As shown in Fig. 8b and 8c, the Ni/CeO2-CaO BCM could achieve an optimal real-time CO2 conversion (\u223c60%) and CO generation rate (\u223c1.7\u202f\u03bcmol\u202fs-1g\u22121) at 650\u202f\u00b0C. The excellent Ni dispersion and stronger basicity of Ni/CeO2 might contribute to the superior ICCU-RWGS performance. It is worth noting that Ni/TiO2-CaO also exhibited outstanding real-time CO2 conversion (\u223c57%), which might be attributed to the slowly released Ni from easily reducible NiTiO3 spinel species (Figs. 3 and 4) [50]. As a comparison, Ni/Al2O3 showed poorer catalytic activities, indicating that the nonreducible spinel (Figs. 3\u202fand\u202f4a) played poor catalytic performance in ICCU.In this work, we focused on the initial 1500\u202fs of CO2 conversion to evaluate the real-time gas production in ICCU-RWGS. The CO2 desorption is relatively fast in the initial stage of the hydrogenation step (0\u2013500\u202fs), especially under higher temperature (e.g. 700\u202f\u00b0C), which directly limits the CO2 conversion rate at this stage (as shown in Fig. 8e and 8f). Since only 5% H2/N2 was used in this work, excessive CO2 release would decrease the ratio of H2:CO2 and affect the equilibrium of RWGS reaction. After the rapid decomposition of the surface layer of CaCO3, the release of CO2 and the performance of RWGS is gradually stabilised until the carbonates are thoroughly consumed.The scaled-up preparation of Ni/supports-CaO BCM is critical for deploying the ICCU-RWGS in an industrial scale. The adsorbents (CaO) are expected to be obtained from limestones, representing a low-cost mineral (<30 dollars per ton (DPT)). The Ni/supports are comprised of earth-rich elements, such as CeO2 (\u223c1600 DPT), TiO2 (\u223c3300 DPT), Al2O3 (\u223c350 DPT), ZrO2 (\u223c1000 DPT) and Ni precursor (Ni(NO3)2*xH2O, \u223c4000 DPT). By impregnating Ni onto a support and physically mixing catalysts and CaO, the BCMs could be obtained with an expected cost of \u223c300\u20131000 DPT. Notably, although H2 can be obtained from renewable energy, the cost would dominate in the ICCU-RWGS process. The development of low-cost hydrogen production will also be the key to the low-cost deployment of ICCU.The stability of Ni/CeO2-CaO bifunctional combined material was presented by carrying out 20 cycles of ICCU-RWGS at 650\u202f\u00b0C (Fig. 9\n). The Ni/CeO2-CaO possessed a\u202f<\u202f5% decrease for CO and C1 yield (CO2\u202f+\u202fCO\u202f+\u202fCH4) and\u202f>\u202f99% CO selectivity after 20 cycles, indicating the excellent and stable ICCU-RWGS performance. Notably, the overall CO2 conversion slightly increased from 56.07% in the 1st cycle to 62.03% in the 20th cycle, which outperforms the state-of-art ICCU-RWGS performance using similar conditions (Table 2\n). It is believed that the effective reactant spillover onto catalytic sites is critical in the catalytic process [51]. As shown in Fig. 6e, Ni/CeO2 and CaO exhibited closer contact in spent Ni/CeO2-CaO, indicating a shorter CO2 spillover distance. The self-optimisation of Ni/CeO2-CaO bifunctional combined material in ICCU-RWGS might be attributed to the volume expansion\u2013shrinkage effect of the sol\u2013gel CaO in cyclic carbonation-hydrogenation, which partially embedded Ni/CeO2 onto the surface layer of CaO.In this work, several Ni/support-CaO bifunctional combined materials (BCMs) were prepared by physically mixing sol\u2013gel CaO and Ni catalysts with various supports (ZrO2, TiO2, CeO2 and Al2O3). The CO2 adsorption and catalytic performance of Ni/support-CaO BCMs were evaluated via integrated CO2 capture and reverse water\u2013gas shift (ICCU-RWGS) process at different temperatures from 550 to 750\u202f\u00b0C. The Ni/CeO2-CaO outperformed the other Ni/support-CaO (support\u202f=\u202fZrO2, TiO2 or Al2O3) over ICCU-RWGS performance (CO2 adsorption and catalytic activity). The enhanced CO2 adsorption performance was attributed to the stronger basicity of Ni/Al2O3 and Ni/CeO2. And the improved catalytic performance (CO2 conversion, CO yield and CO generate rate) was related to the excellent Ni dispersion and reducibility. The spinel formation contributed to the Ni species dispersion by forming strong interaction with support; however, only the easy-reducible spinel (NiTiO3) was active in the ICCU process. Furthermore, the Ni/CeO2-CaO exhibited excellent stability in 20 cycles of ICCU and showed a self-optimising trend in CO2 conversion (56.07% and 62.03% for the 1st and 20th cycles, respectively) due to the gradually close distance between CaO and Ni/CeO2 owing to the volume expansion and shrinkage of CaO during carbonation-hydrogenation cycles.\nShuzhuang Sun: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing \u2013 original draft. Chen Zhang: Methodology, Investigation, Formal analysis. Shaoliang Guan: Formal analysis, Investigation, Resources. Shaojun Xu: Formal analysis, Resources, Writing \u2013 review & editing, Supervision. Paul T. Williams: Formal analysis, Resources, Writing \u2013 review & editing, Supervision. Chunfei Wu: Conceptualization, Writing \u2013 review & editing, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge financial support from the China Scholarship Council (reference number: 201906450023). This project has received funding from the European Union\u2019s Horizon 2020 research and innovation programme under the Marie Sk\u0142odowska-Curie grant agreement No 823745. The UK Catalysis Hub is kindly thanked for the resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC grant: EP/R026939/1, EP/R026815/1, EP/R026645/1, EP/R027129/1 or EP/M013219/1(biocatalysis). The XPS data collection was performed at the EPSRC National Facility for XPS (\u2018HarwellXPS\u2019), operated by Cardiff University and UCL, under contract No. PR16195.", "descript": "\n                  Integrated CO2 capture and utilisation (ICCU) is a promising strategy for restricting carbon emissions and achieving carbon neutrality. Bifunctional combined materials (BCMs), containing adsorbents and active catalysts, are widely applied in this process. Producing syngas via reverse water\u2013gas shift reaction (RWGS) and integrating with Fischer-Tropsch (F-T) synthesis is an attractive and valuable CO2 utilisation route. This work investigated a series of Ni/support-CaO BCMs (supports\u202f=\u202fZrO2, TiO2, CeO2 and Al2O3) for the integrated CO2 capture and RWGS (ICCU-RWGS) process. The Ni/support-CaO BCMs were prepared by physically mixing various metal oxide supports loaded Ni with sol\u2013gel derived CaO. The ICCU-RWGS performance (CO2 conversion, CO yield and CO generation rate) of these BCMs followed the order during tested conditions (550\u2013750\u202f\u00b0C): Ni/CeO2-CaO\u202f>\u202fNi/TiO2-CaO\u202f>\u202fNi/ZrO2-CaO\u202f>\u202fNi/Al2O3-CaO. A comprehensive characterisation of Ni/support materials showed that Ni/CeO2 had the characteristics of stronger basicity, optimal Ni dispersion and improved NiO reducibility, which led to the outperforming ICCU-RWGS activity over Ni/CeO2-CaO (e.g. 56.1% CO2 conversion, 2.68\u202fmmol\u202fg\u22121 CO yield and \u223c100% CO selectivity at 650\u202f\u00b0C). Furthermore, the Ni/CeO2-CaO BCM showed a stable, yet, self-optimising catalytic performance during the cyclic ICCU-RWGS reaction over 20 cycles. The TEM characterisation suggested that was ascribed to the volume expansion and shrinkage of CaO in the cyclic adsorption\u2013desorption altering the distance between the adsorbent and Ni/CeO2, resulting in an enhanced CO2 conversion during the cycle.\n               "}
{"full_text": "No data was used for the research described in the article.Rhenium is sometimes described as a \u2018chameleon\u2019 among the elements due to the large variety of oxidation states it can adopt and interchange between [1]. This protean nature makes it an interesting candidate as a potential catalytically active component for myriad reactions [2,3]. The main industrial catalytic application of Re is in the form of bimetallic Re\u2013Pt/Al2O3 as a catalysts for the catalytic reforming of hydrocarbons in the petrochemical value chain, where Re benefits the catalyst stability compared to the monometallic Pt catalyst [4,5]. Re is also investigated as a promoter for Fischer-Tropsch synthesis [6,7] and can catalyze olefin metathesis reactions [8,9].In recent years, alongside generally increasing research interest into biomass utilization, a growing number of research articles reporting Re-based or Re-modified catalysts for a variety of biomass-related catalytic reactions has been published (Fig.\u00a01\n). As previously outlined, e.g. by Sadaba et\u00a0al. [10], two aspects are characteristic for biomass utilization: (1) The non-volatile nature of many biomass-derived compounds demands reactions to be conducted in liquid phase and (2) reactions are preferably conducted over solid catalysts since a heterogeneously catalyzed process makes recovery of the catalyst more facile. Solid Re catalysts were successfully applied in all types of heterogeneously catalyzed biomass upgrading reactions that rely on active metal centers. As outlined, e.g.\u00a0by Alonso et\u00a0al. [11] these are: (1) aqueous-phase reforming (APR), i.e. the production of H2 typically from carbohydrates or glycerol; (2) hydrogenation (HG), i.e. the saturation of double bonds such as C=C bonds as well as C=O bonds resulting in a selective reduction e.g. of carboxylic acids or ketones to alcohols; (3) hydrogenolysis (HL), i.e. the cleavage of bonds involving H2, which is in particular applied to glycerol and other polyols; (4) hydrodeoxygenation (HDO), i.e. a subtype of HL where C\u2013O bonds are cleaved, which can be employed to upgrade e.g. lignin-derived molecules such as guaiacol or anisole. In addition, just recently (5) deoxydehydration (DODH) was successfully implemented as a heterogeneously catalyzed process. For this last reaction, which completely removes oxygen from vicinal diol groups by converting them into unsaturated C=C bonds, Re-based catalysts are of particular importance [12,13]. A common feature of all five reaction types is the importance of redox reactions. Moreover, a common aspect of reaction types 2\u20135 in the aim of removing oxygen from organic molecules, mostly relying on H2 as reducing agent.The general importance of Re-containing catalysts for biomass transformation is evident from many recent review articles that put the performance of these catalysts into perspective [2,12\u201348]. Most notably, Re is very versatile and can fulfill several different functions. Perhaps the most common effects ascribed to ReOx species as a catalyst promoter are based on their acidity and oxophilicity, which facilitate bifunctional reaction mechanisms and additional interactions with functional groups of adsorbed substrate molecules [49\u201358]. Similarly, the presence of oxygen vacancies on low-valent Re oxide species has been suggested to be of crucial importance [59\u201362]. Furthermore, electron transfer effects are discussed [63] as well as effects on stabilization of the active phase [64,65], enhanced reducibility of non-noble metals in the presence of Re [66], and H2 spillover from a second metal to Re [67,68]. In other cases, metallic alloys between Re and a second metal were found to be beneficial [53,64,69]. While for DODH high-valent Re species are considered crucial for catalytic activity [13,70,71], HG of carboxylic acids was found to require low-valent Re species including metallic Re [72\u201375]. This influence of the Re oxidation state is lucidly described by Tomishige et\u00a0al. [26].While considerable effort has been dedicated to identifying the role Re plays in enhancing catalytic activity and how to modify catalysts to improve their efficiency, the stability of Re-containing solid catalysts has been investigated to a much lower degree. Even though many recent studies include catalyst reuse experiments, they are typically performed only for the best-performing catalyst, which can hardly reveal which factors are relevant to catalyst deactivation. On the other hand, a few studies have been dedicated to the issue of catalyst stability and phenomena like (Re) metal leaching in recent years [76,77]. The importance of this topic, however, is apparent from several reports of Re catalysts showing strong deactivation over several recycling runs or over longer time on stream [52,53,55,58,78\u201387].The purpose of this review article is to shed a light on the stability of Re-containing solid catalysts applied in various heterogeneously catalyzed reactions conducted in liquid phase that are relevant for biomass conversion and upgrading. Based on the >200 research articles identified to match these criteria, about half of them were found to contain information on catalyst stability and are included in the analysis here. Different deactivation mechanisms will be discussed and strategies to restore catalytic activity, which is paramount to establishing sustainable catalytic processes.The main focus is placed on leaching of Re, which is a phenomenon characteristic for liquid-phase reactions and can lead to irreversible loss of the valuable and rare element Re [10]. Therefore, a variety of different catalyst properties as well as reaction conditions will be analyzed regarding their role in Re leaching throughout the lifetime of the catalyst. In particular, the influence of the Re oxidation state is analyzed. Furthermore, Re leaching-related issues like catalytic activity of dissolved species and strategies to reliably detect leaching are discussed as well as approaches to turn the tide and use of leaching of Re to one's advantage.Systematic studies of catalytic deactivation are scarce in the context of liquid-phase reactions of biomass-derived compounds, probably because catalyst stability only attracts attention when it poses an apparent problem. However, many studies include tests regarding the catalyst deactivation to prove that a well-performing new catalyst can also withstand the reactions conditions over an extended period of time without significant decrease in catalytic performance, i.e.\u00a0in catalytic activity and/or selectivity. Usually only if problems with catalyst stability are apparent, their causes are investigated more thoroughly. In case of experiments conducted in a continuous-flow mode, deactivation is tested by observing changes in catalytic behavior over a certain time-on-stream (in certain cases up to several weeks [83,88]). For batch experiments the typical procedure is reuse/recycling of the catalyst for one or more consecutive experiments, where fresh reactants are supplied. In certain cases, this was repeated up to 25 times [89], but one to three recycling runs are most common. Excellent insights into the methodology of catalyst stability tests are provided by Hammond [90] and Miceli et al. [91] give an overview of conventional as well as advanced catalyst recovery methods.Assessing reusability (or performance over time-on-stream) alone is an unspecific method to investigate catalyst stability. It will only provide an overlaid overall picture of all occurring types of catalyst deactivation. Additional information can be gained from changing the treatment of the catalyst in between different recycling runs. Often, thermal treatment(s) under oxidizing (i.e. calcination) and/or reducing atmosphere are conducted to remove deposits or adsorbates on the catalyst accumulated during reaction or to (re)adjust the oxidation state of the metal phase. While these types of catalyst deactivation are reversible, i.e. catalytic performance after treatment is comparable to the fresh catalyst, others like e.g. irreversible catalyst poisoning, sintering, or metal leaching are not. Thus, limited insights into the nature of the deactivation are possible. However, reusability experiments rather serve as a general assessment whether or not catalyst deactivation takes place and to what degree it may be recoverable by certain treatment procedures. Deeper understanding of the underlying deactivation process requires additional experiments, in particular the extensive characterization of fresh and spent catalysts.A plethora of thermal, chemical, and mechanical deactivation processes have been described for heterogeneously catalyzed reactions. In one way or another, the physical and/or chemical properties of the solid catalyst are altered during the reaction, which results in a detrimental effect on the catalytic behavior, i.e. decreasing catalytic activity and/or selectivity over time. Previous articles [10,92\u201396] provide classifications of different deactivation types;\u00a0however, an unambiguous assignment can be difficult because deactivation mechanisms are often not entirely mechanical or thermal but intertwined chemical contributions play a role.In general, the deactivation of catalysts in the liquid phase follows similar mechanisms as in the gas phase. However, especially the presence of a liquid solvent leads to different ways of chemical deactivation and can be detrimental to catalyst stability. The solvent itself can react with the catalyst, and\u00a0most importantly, partial dissolution of the solid catalyst and subsequent loss of the active metal phase can be encountered. Therefore, leaching-related phenomena are characteristic for liquid-phase reactions [10,94]. The use of liquid medium is often necessitated by the instability and/or the low volatility of the highly functionalized reactants. On the other hand, many biomass-based feedstocks, in particular when biotechnological processing steps are involved, are already in the form diluted aqueous solutions [97]. Therefore, hydrothermal conditions are very typical for the catalytic conversions of biomass-derived compounds. Another important aspect when dealing with such feedstocks is the biogenic impurities they may contain either from the original feedstock or from previous processing steps. These can detrimentally effect the heterogeneously catalyzed reaction and, in the worst case, irreversibly poison the catalyst [97].The relevant deactivation types will be outlined in more detail in the subsequent sections with a focus on examples of liquid-phase biomass valorization over Re catalysts. An overview of all Re-related studies and experiments included in this review is given in Table\u00a01\n (continuous flow) and Table\u00a02\n (batch) summarizing catalyst and reaction conditions used as well as the observations of quantitative descriptors of catalyst deactivation and leaching. Furthermore, other (suspected) types of catalyst deactivation are listed as well as information on how deactivation and leaching was experimentally assessed.Mechanical alterations can be induced to a solid catalyst by mechanical, thermal, and/or chemical stress. Typical processes are crushing of shaped catalysts, attrition, and erosion and lead to breaking of catalyst particles, decrease in particle size, and formation of \u2018fines\u2019 [92]. In case of continuously operated reactors, this affects the permeability of the catalyst bed and can lead to a continuous loss of catalyst material due to the transport of fine particles. In case of batch reactions, too fine particles may not be recyclable causing lower activity in subsequent runs. According to Sadaba et\u00a0al. [10], attrition is the main pathway of mechanical catalyst degradation for heterogeneously catalyzed reactions in the liquid phase and especially problematic for fluidized-bed and slurry reactors.In many studies evaluating the reusability of a solid catalyst, the amount of catalyst that can be recovered is lower than the initial amount. Losses during separation are almost inevitable due to the small scale of catalytic experiments in most studies (typically 10\u2013103\u00a0mg). Whether there is a distinct contribution of mechanical degradation is usually not considered. However, to compensate for the expected incomparability among catalyst reuse experiments with significantly different catalyst amounts, two adjustments are commonly applied: either the lost amount of catalyst is compensated with fresh catalyst (e.g. [79]) or the batch size is adjusted to the amount of catalyst (e.g. the study by Liu et\u00a0al.[139]and Liu et\u00a0al.[140]). In general, the influence of mechanical processes resulting in catalyst deactivation are of considerably lower research interest compared to all other deactivation pathways. Several factors may play a role, e.g. the comparably low number of experiments operated under continuous flow, the predominant use of powder catalysts as well as the low relevance of mechanical stability at the current stage of catalyst development for liquid-phase biomass conversion.The vast majority of Re-containing catalysts for liquid-phase biomass applications (Table\u00a01 and Table\u00a02) are supported catalysts with Re loadings below 10\u00a0wt.-%. Thus, their mechanical stability will be primarily governed by the respective support material. Besides the chemical nature of the support material, a variety of parameters can influence the resistance of different materials toward\u00a0mechanical deactivation, such as particle shape, modification, and porosity. Moreover, the preparation method of oxidic supports can have significant influence. These aspects are discussed in detail e.g. by Argyle and Bartholomew [93].A catalyst poison is an inhibitory substance that can very strongly and selectively bind to the active sites of a catalyst. A decrease in the catalytic activity occurs already in the presence of small quantities of the poison and the effect is often irreversible under process conditions [173]. Besides the poison molecule physically occupying the catalytically active center by being chemisorbed to it, its effect on the catalytic behavior of the catalyst can also arise from e.g. electronic interactions with adjacent active sites, inhibit surface diffusion processes, or lead to surface restructuring [92]. Catalyst poisons can either be formed during the reactions, typically as undesired side products, or already be present in the feedstock. The latter is often the case for biomass-based feedstocks which can contain several types of impurities, many of which are biogenic [95,97]. These types of impurities range from inorganic compounds to functionalized small molecules, e.g. carboxylic acids, amino acids, sugars, sugar-derived molecules, to larger structures such as proteins and cell (fragments), all of which can interact with the solid catalyst. Therefore, catalyst poisoning is often characteristic for studies using raw biomass sources or hardly purified products from biomass processing such as fermentation broths rather than using pure model substrates. Due to the highly selective interaction between poison and catalytically active sites, there are different types of poisons and correspondingly different poisoning mechanisms depending on the catalyst. A typical experiment to assess the deactivation caused by a suspected poison is the deliberate addition of this compound to the feed of the reaction and the variation of its concentration. Especially insightful are studies conducted under continuous flow, which allow switching between poisoned and un-poisoned feed since the possible recovery of the catalytic activity can be investigated. On the other hand, the direct poisoning of the catalyst, e.g. by impregnation before the reaction, is another option. Moreover, adsorption studies can be used to quantify the interactions of the catalyst with different poisons and reveal competitive adsorption between reactants and other molecules.Br\u00f8nsted acid sites have been found to be susceptible to poisoning by (earth) alkali metal cations. This deactivation process was described to take place in the manner of an ion exchange process [10]. Poisoning by metal cations has been primarily reported for zeolite-based catalysts, e.g. by Metkar et\u00a0al. [174], but similar deactivation is possible for Re-containing catalyst where ReOx species are acting as Br\u00f8nsted acid sites. This was suggested by Falcone et\u00a0al. [57] for a supported Pt\u2013Re catalyst, which showed significant deactivation in the HG of crotonaldehyde after adding NaOH or NaCl to the feed. Another study by Liu et\u00a0al. [132] showed similar results for an Ir-ReOx/SiO2 catalyst applied in the heterogeneously catalyzed valorization of xylan. An increasing amount in Na+ ions had detrimental effect on the catalytic behavior probably due to ion exchange of the proton associated with the Br\u00f8nsted acidity of the Re species (Fig.\u00a02\n). It was also shown that the addition of acid could restore the activity due to the formation of new acid sites [132]. Besides their influence on acid sites, cations can also poison metal centers as described in a study by Mortensen et\u00a0al. [175] on Ni-based HG catalysts.Commonly observed is also catalyst poisoning due to heteroatom-containing organic molecules, in particular amino acids. Very instructive studies were published study by Schwartz et\u00a0al. [176], Zhang et\u00a0al. [177], and Harth et\u00a0al. [178] investigating the influence of additives comprising different biologically relevant molecules as well as structural motifs on a heterogeneously catalyzed HG reaction in liquid phase. The poisoning strength was shown to be in a direct correlation with the adsorption energy of the heteroatom-containing molecules to the Ni, Pd, or Pt center [176]. While sulfur-containing molecules like cysteine showed a highly selective and very strong poisoning effect, nitrogen-containing ones deactivated the catalysts to a lesser degree. This is in accordance with several other studies that identified sulfur-containing amino acids as irreversible catalyst poisons [173,177,179,180]. Non-sulfur containing amino acids are less problematic and were found to inhibit the supported metal catalysts either reversibly [176,177] or hardly at all [173]. The presence of other organic acids (acetic acid, pyruvic acid) on the HG of succinic acid over a Ru\u2013Re catalyst was investigated by Di et\u00a0al. [67,68]. These acids, which can be present in fermentation-derived succinic acid feedstock, resulted in a slight decrease in catalytic activity. Preferable adsorption of the \u2018impurity acids\u2019 to the catalyst's active sites was suggested, which temporarily poisons the catalyst until the acids are hydrogenated and release the active sites for succinic acid [67].Unfortunately, the number of studies conducted with realistic, biomass-derived feedstocks is very limited. Among the Re-based catalysts included in this review article, this was studied for a carbon-supported Re\u2013Pd catalyst that was used for the aqueous-phase HG of pure as well as fermentation-derived succinic acid purified by crystallization [181]. As depicted in Fig.\u00a03\n, the fermentation-derived succinic acid was converted significantly slower while the selectivity was not affected. Which impurities in the feed cause the decrease in catalytic activity was not investigated. Studies by Binczarski et\u00a0al. [180] and Zhang et al. [177] show how different purification techniques can be employed and how gradual the removal of impurities affects the HG of fermentation-derived lactic acid. In particular, when irreversible poisoning occurs, so that the catalyst cannot or only with significant effort be reactivated, it is of particular importance to remove the poisons from the feed before the reaction.On the other hand, the catalyst poison may be formed during the reaction. The most common example for this is poisoning by CO, which can be formed by the decomposition of organic compounds. This was observed when formic acid was added as a renewable hydrogen donor in HG reactions. Di et\u00a0al. [67,68] found that Re-containing bimetallic catalysts for the HG of succinic acid could not tolerate the presence of formic acid and were probably poisoned by CO from its decomposition. Similar findings are reported for other studies using formic acid as an in-situ hydrogen donor [173,182,183]. Due to the fact that the catalytic activity is recovered upon the decomposition of formic acid, it is also possible that adsorbed formic acid species cause the poisoning effect [173,184].Fouling describes a type of catalyst deactivation that arises from depositions accumulating on the surface of the catalyst that prevent reactants from accessing the active sites. Lange [95] distinguishes between two types of fouling based on the origin of the deposited species. First, the depositions can already be present in the feed, e.g. impurities like large molecules, proteins, cells, particles, and so on, which are physically depositing on the catalyst during the reaction. In this case, the deactivation is of mechanical nature and similar to membrane fouling [185]. Such impurities are typical for direct biomass-derived feedstock and their impact can be influenced by feed purification. Among the studies included in this article, typically model feedstock was used. Therefore, no example with a Re catalyst can be mentioned for this particular type of fouling. One study by Zhang et\u00a0al. [177] of a supported Ru catalyst in the aqueous-phase HG of lactic acid conducted in continuous flow showed that proteins deposited in the pores of the carbon support. This was evident from drastically deteriorating textural properties of the catalyst after exposure to protein-enriched feed (loss of specific surface area from 780 to 80\u00a0m2/g). The blockage of reactant molecules from accessing some of the catalyst's active sites was suggested to be partially responsible for a considerable decrease in catalytic activity (conversion decreased from 58 to 35%) [177].The other possible origin of physical deposits on the catalyst is their formation during the reaction by undesired side reactions. This phenomenon is a well-known problem from gas-phase reactions such as reforming of methane or naphtha, fluid catalytic cracking, or others, where it is often referred to as catalyst coking as well as deposition of carbon or carbonaceous species [186,187]. In the context of biomass valorization in liquid phase, organic depositions often originate from condensation reactions of reactants, intermediates, or products that form oligomeric species [95]. In case of solid carbonaceous species formed from sugar or hydroxymethylfurfural feedstocks, the term humins is used frequently [188]. Depending on the respective reaction system and catalyst, the underlying reactions as well as the chemical nature of the deposited species can vary strongly and it is also not unusual that the deposits interact with the catalyst in ways (e.g. by acting as catalyst poisons) beyond the mere physical blockage of active sites. In general, fouling due to the formation of carbonaceous species is (unfortunately) a very common type of deactivation when converting renewable feedstocks [95].Due to the versatile nature of Re catalysts, they can be applied in several types of reactions with different feedstock. As can be seen from Table\u00a01 and Table\u00a02, however, carbonaceous deposits resulting in significant catalyst deactivation was reported in many studies regardless of the type of catalyst, the feedstock or the reaction conditions. Typically, the proof of deactivation by carbon deposition is based on the combination of two experiments. On the one hand, the presence (and the amount) of carbon-containing species deposited during the reaction is investigated e.g. by thermogravimetric analysis\u00a0or total organic carbon\u00a0analysis of the spent catalyst. On the other hand, the influence of the deposits on the catalyst activity is assessed. Since the carbonaceous deposits can often be removed by comparatively simple thermal treatment, a calcination and/or reduction step is conducted to reactivate the catalyst by uncovering its surface. The catalytic behavior of the thus reactivated catalyst compared to the fresh catalyst as well as the non-activated spent catalyst reveals the influence of deactivation caused by fouling by carbonaceous species. It should be noted, however, that during the reactivation procedure also other properties of the catalyst can be altered, e.g. oxidation states of active species, and that processes like sintering can take place. Additional characterization (e.g. by infrared (IR) spectroscopy, elemental analysis, imaging techniques, etc.) of the spent catalyst to investigate the detailed chemical nature of the deposited species and to elucidate the formation of these species from the feedstock may provide additional insights. They are, however, beyond the scope of this review and more detailed information regarding the characterization of carbonaceous deposits is available, e.g.\u00a0in a review by Ochoa et\u00a0al. [189].A representative example was reported by Jin and Choi [69], who investigated the deoxygenation of palm oil to paraffin over a carbon nanotube-supported\u00a0bimetallic Re\u2013Pt catalyst. The results of recycling experiments are shown in Fig.\u00a04\n(b). When the recycling experiments were conducted without calcination, catalyst deactivation was pronounced very strongly. Paraffin yield dropped by nearly 50% in the second run and down to 20% of the initial yield in the third run. Thermogravimetric analysis of the spent catalyst (Fig.\u00a04(b)) revealed the presence of ca. 14\u00a0wt.-% of solid deposits formed during the reaction, probably by the undesired polymerization of reactants and/or products. Introducing catalyst calcination as a reactivation method in between consecutive catalyst recycling experiments proved that the deactivation of the catalyst is indeed mainly due to the carbon deposition. Only a slight decrease (ca. 10%) in paraffin yield was observed over 5 reuse runs, which could have been caused by Re leaching or by metal sintering [69].Many similar examples for deactivation by fouling can be found in Table\u00a01 and Table\u00a02. Often, the deactivation by carbon deposition is to a large degree reversible, which is evident from the comparison of experiments with and without the removal of the deposit [58,79,80,85,128,132,170,171]. In some cases [55,86], the overall catalyst deactivation could only slightly be mitigated by thermal reactivation, which is indicative of severe deactivation by other causes. The wide range of feedstock and reaction types as well as the different catalysts for which deactivation by fouling by carbonaceous species formed during the reaction was observed proves that it is a widespread but manageable type of deactivation. An important aspect that must not be overlooked is that even though carbonaceous deposits are formed on the catalyst this does not necessarily mean that the catalytic activity is (negatively) influenced by that. Two examples [101,118] of Re-based catalysts were reported where despite significant deposit formation (5.5 and 6.8\u00a0wt.-% of the catalyst after reaction) catalytic behavior was not significantly changed. One aspect that might explain some of these differences is that while in the ideal sense fouling is an entirely physical deactivation mechanism, in reality deposited species can additionally react with the catalyst or act as catalyst poisons. Therefore, the chemical composition and the interaction with the catalyst are of crucial importance as well.Thermal deactivation describes the decline of catalytic activity due to the changes in material properties of the catalyst upon exposure to elevated temperatures. Considering that myriad deactivation processes require thermal activation, this term may include almost all types of catalyst deactivation. On the other hand, a narrow definition of \u2018purely\u2019 thermal deactivation would exclude all pathways including chemical reactions with other compounds of the reaction system and is therefore limited to sintering of the support or the active metal phase of a catalyst as well phase transformations, thermal decomposition, and other restructuring processes.Sintering is defined as the temperature-induced coalescence and densification of porous solid particles below the melting points of their major components [190]. It is driven by the thermodynamically favored reduction of surface area and typically includes migration or diffusion processes resulting in crystal growth [191,192]. Looking at the catalyst support, sintering leads to a densification of the bulk material and a collapse of pores, which reduces the specific surface area of the catalyst. In most instances, however, sintering occurs in the form of crystal growth of the dispersed metal, which is typically the active phase. Therefore, the catalyst deactivation by metal sintering is caused by a reduction in active metal surface area. Typical techniques to investigate metal sintering are chemisorption experiments with probe molecules such as CO or H2 to quantify the amount of active metal centers as well as XRD and microscopy techniques to determine the size of the metal particles. Comparing fresh and spent catalysts reveals to what degree metal surface area is reduced during the reaction. However, for structure-sensitive reactions, not all surface metal atoms contribute to catalytic activity in the same way. Furthermore, in case of bimetallic catalysts, metal segregation has been observed, which reduces the interface between the two metals [55,83]. So far, these effects have rarely been identified as crucial mechanisms for the catalyst deactivation in liquid phase and require further research attention.In solid-gas reaction systems with supported metal catalysts, sintering occurs at high reaction temperatures (from 500\u00a0\u00b0C) [92]. The conversion of biomass-derived compounds, however, is conducted mostly in the liquid phase and at lower temperatures (mostly between 100 and 250\u00a0\u00b0C, rarely >300\u00a0\u00b0C), as the examples in Table\u00a01 and Table\u00a02 show, which makes classical sintering less likely. Especially, the presence of liquid phase has an immense influence on the stability of supported catalysts, which will be discussed in a subsequent separate section. Unfortunately, there is hardly any information available, how sintering of the metal phase as a catalyst deactivation mechanism of supported metal catalysts is influenced by solvents in general and water in specific. From liquid-phase sintering, as a materials preparation technique, it is evident that the solvent plays a crucial role since soluble species are involved that drastically accelerate sintering processes [193]. It is certainly conceivable that dissolution and redeposition of metal species, which are mobile in the solvent, can play a role, similar to the contribution of volatile species to sintering in solid-gas reaction systems. Regardless of the underlying mechanisms, the term sintering is generally used when the growth of metal particles is observed during the reaction.Among the studies on solid Re-based catalysts selected for this article, metal sintering is claimed to contribute to the overall catalyst deactivation in several cases. In all cases, this is evidenced by proving the growth of metal particles during the reaction. One instance of sintering of a monometallic Re catalyst is reported in the study by Godina et\u00a0al. [100]. A carbon-supported Re catalyst was employed for the APR of xylitol, operated under continuous flow at 225\u00a0\u00b0C for 30\u00a0h. Re particle size was <0.5\u00a0nm before the reaction but increased slightly to 0.7\u00a0nm during the time-on-stream. At the same time, the catalyst completely lost its activity for hydrogen formation. Unfortunately, the reason for deactivation was not investigated further and the contribution of other types of deactivation is therefore not known. It appears unlikely, however, that the observed difference in metal particle size can explain the full deactivation of the catalyst. In another study [60], the sintering of Re/SiO2 during catalytic HDO of guaiacol in n-dodecane at 300\u00a0\u00b0C lead to an increase in average Re particle size from 2.8 to 3.8\u00a0nm. Jeong et\u00a0al. [119] investigated different supported Re catalysts for the HDO of guaiacol in n-heptane at 280\u00a0\u00b0C. After 1\u00a0h, sintering was observed for Re/SiO2 (Re particle size increase from 0.5 to 1.1\u00a0nm), while the changes for Re/TiO2 were not significant. The corresponding transmission electron microscopy images are shown in Fig.\u00a05\n. For these catalysts, the effect of sintering as well as leaching, which was also detected in considerable amounts, on the catalytic activity was not investigated.Looking at supported bimetallic or trimetallic Re catalysts, a common finding is that the Re species are highly dispersed while the other metals form comparatively larger particles. (The factors governing Re dispersion are investigated e.g. in the study by Mine et\u00a0al. [194], while more general aspects on this topic are summarized in recent review articles [195,196].) In many cases of supported catalysts with high Re dispersion, the particle size of Re species is below the detection limit in the catalyst's XRD pattern [85,113,131,139]. Besides high dispersion this could, on the other hand, also be the result of Re being present as a variety of species or as amorphous particles. In practice, this means that sintering is typically identified only for the other metal. In several examples for all kinds of reactions, the growth of metal particles has been observed, as indicated in Table\u00a01 and Table\u00a02. The gradual sintering of Ir on a supported Ir-ReOx catalyst was e.g. shown by Luo et\u00a0al. [83]. Ir particle size increased from 2.1\u00a0nm on the freshly reduced catalyst to 3.1\u00a0nm after 150\u00a0h and to 3.2\u00a0nm after 500\u00a0h on stream in the aqueous-phase HL of glycerol. Similar increases are reported for Re-based bimetallic catalysts with Ir [85,113,132,133,140], Pt [69,84], Pd [86], Rh-Ir [52], Pt-Ir [107] as well as Ni [142]. While it is typically assumed that sintering takes place during the actual reaction time, Liu et\u00a0al. [131] suspect the calcination treatment in between reuse runs to be responsible. When metal particle growth was identified its effect on the catalytic activity was considered to be rather small compared to e.g. fouling due to carbon deposit formation and never was a clear correlation between the decrease of metal surface area proven in the literature.As mentioned before, for bimetallic and multimetallic catalysts, the interface between different metals is considered of particular importance, which complicates matters. Often it is assumed that the effect of combining different metals relies on their direct interaction, which requires special contact of the metals. Therefore, the interface between metal species is considered to be most relevant for catalytic activity. Migration and sintering of metal species can lead to different effects on such catalysts. In some studies metal segregation was suggested to cause catalyst deactivation e.g. for Pt\u2013Re APR catalysts [99,101] or other bimetallic catalysts [55,104]. Interestingly, in certain instances, the opposite effect, i.e. the accumulation of Re species on Ir particles during the reaction, was considered to be detrimental to catalytic activity [139,140]. Either way, the thermally induced changes in metal arrangement resulted in a decrease in accessible bimetallic sites. A more detailed insight into such migration processes should be possible e.g. by X-ray absorption or photoelectron spectroscopy.Thermal deactivation is not only relevant under the actual reaction conditions but can also play an important role in thermal pretreatment or recovery processes such as catalyst calcination or reduction. While sintering can also occur under these conditions, there are other types of catalyst degradation that are relevant. First, carbon supports often cannot withstand typical calcination conditions and undergo decomposition or combustion. This means that often carbonaceous deposits cannot completely be removed without damaging the catalyst support. The carbon nanotube-supported catalyst in Fig.\u00a04, however, is comparatively thermally stable, which is why the catalyst could be successfully reactivated. In other studies [170,171], this was not the case. Moreover, Jang et\u00a0al. [172] showed that thermal treatment under H2 atmosphere can be a suitable approach to remove organic deposits while preventing carbon support decomposition for a Pt-ReOx/C catalyst applied in mucic acid DODH.A related thermally induced type of catalyst deactivation is the loss of Re from the catalyst due to vaporization of volatile species. It is most commonly observed during the thermal treatment of Re-containing catalysts rather than during the catalytic reaction itself. E.g., the oxidative treatment of Re species can result in the formation of high-valent, volatile species, and subsequent removal of Re from the catalyst. She et\u00a0al. [197] observed a decrease in Re content of a silica-supported ReOx catalyst from 21.5 to 18\u00a0wt.-% after calcination in a flow of dry air at 400\u00a0\u00b0C for only 1\u00a0h. This was also observed by Liu et\u00a0al. [139] for bimetallic Re\u2013Ir catalysts with high Re loading. In general, the sublimation of Re2O7 at elevated temperature is a common phenomenon, which can be influenced by the choice of support material [198].One instance of a similar phenomenon occurring during a liquid-phase was reported by Canale et\u00a0al. [199]. During the DODH reaction of neat glycerol over unsupported Re2(CO)10\u00a0at 170\u00a0\u00b0C, it was found that the catalyst underwent (partial) sublimation. Unfortunately, neither the amount of Re loss not the effect on catalytic activity was reported. It should be noted that this reaction was conducted in a semi-continuous setup under constant purging of the liquid phase with a flow of air or H2, which is a crucial factor since this leads to the transport of Re species out of the reactor. Interestingly, under more reducing reaction conditions (100\u00a0bar H2, 160\u00a0\u00b0C), another study showed that Re2(CO)10 decomposes into Re nanoparticles [200], which indicates that for HG and HDO reactions, volatile Re species are not likely to be present and, thus, the loss of Re due to vaporization is a rather seldom phenomenon.Since most heterogeneously catalyzed transformation processes of biomass-derived feedstocks are conducted in the liquid phase, the role of the solvent is of central importance. In the context of catalyst stability and deactivation, two processes are particularly affected: Leaching of the active (metal) phase, which will be discussed in detail as the main focus of this article, and degradation of the support material in the liquid reaction medium. Especially the use of water and other polar reaction media can induce severe structural changes to solid catalysts. Being the natural process medium for biological processes and considered as a green solvent [201,202], water is the most commonly used reaction medium for biomass conversion and upgrading reactions, which is also the case for Re-based catalysts (Table\u00a01 and Table\u00a02). A series of heterogeneously catalyzed reactions can be conducted in sub-critical liquid water (Fig.\u00a06\n, left). The issue of hydrothermal stability has attracted considerable attention and a comprehensive review article was published by Xiong et\u00a0al. [203].As depicted in Fig.\u00a06 (right), silica- and alumina-based as well as zeolites show a comparatively low stability in hot liquid water. In case of zeolites, this behavior has been thoroughly investigated in recent years [204\u2013208]. Depending on the reaction conditions, in particular pH, the ordered framework structure of zeolites disintegrates via dealumination or desilication, i.e. hydrolysis of the zeolite lattice starting preferably from defect sites. During treatment in hot liquid water, the characteristic zeolite framework disintegrates and the support is gradually converted into a non-microporous, amorphous solid. However, the degree of disintegration and structural collapse also depends on the framework type and the modulus. Similar to high-surface-area alumina or silica [203,209], the drastic loss of specific surface, which is caused by the hydrothermal transformation of the support, can have severe detrimental effects on the catalytic activity of the supported metal catalyst. Loss of crystallinity is apparent from X-ray diffraction patterns of the catalyst and the loss of porosity can be quantified by physisorption of nitrogen or other gases. Structural changes can also be investigated by IR spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy. Besides the physical loss of surface area, changes e.g. in acidic properties are crucial for bifunctional catalysts involving acid-catalyzed reactions. This can be studied by temperature-programmed desorption of probe molecules in combination with IR spectroscopy.Zeolites are rarely used in combination with Re, perhaps because Re itself is often suspected to provide Br\u00f8nsted sites to enable bifunctional reaction mechanisms [51,56,57]. In general, none of the studies in Table\u00a01 or Table\u00a02 determined support degradation to be the main deactivation pathway which may, in part, be due to a lack of interest in this particular phenomenon. It is, however, noteworthy that especially for the hydrothermally demanding APR processes exclusively activated carbon or TiO2 were used as support materials [50,54,82,88,98\u2013100], which are among the most hydrothermally stable materials. Unfortunately, this does not necessarily guarantee thermal stability as well. In particular, the use of carbon-based support can be limited by thermal decomposition when thermal reactivation of the catalyst is required (see above). Thermodynamic considerations by Lange [95] further prove that TiO2 and ZrO2 are the most hydrothermally stable among commonly used oxide supports. Furthermore, high-surface area modifications, like e.g. mesoporous silica materials, are metastable phases, which make them more prone to recrystallization.There are myriad ways in which supported metal catalysts can undergo chemical reactions. Many of such reactions play a role for previously described types of catalyst deactivation, such as thermal deactivation by combustion of carbon supports, hydrothermal support degradation by hydrolysis of oxide materials, or chemical reactions resulting in the formation of carbon deposits or poisons, the latter of which cause deactivation by chemisorption to the catalytically active centers of the catalyst. Besides theses effects, that lead to reduced numbers of accessible active metal sites for supported metal catalysts, there is also the possibility of chemical transformations of the active metal species converting them into a non-active phase. In particular, the contact of supported metal catalysts with reducing or oxidizing atmosphere either inside or outside the reactor can change the oxidation state of the metal, which can have severe effects on the catalytic behavior. As mentioned in the introduction, Re catalysts in particular are very versatile in the context of heterogeneously catalyzed biomass valorization due to their characteristic redox chemistry [26]. On the other hand, this makes them susceptible to deactivation by changes in oxidation state.Of particular importance is the actual Re oxidation state under reaction conditions. Conventional characterization techniques include temperature-programmed reduction and/or oxidation experiments to reveal the redox-behavior of the catalyst under different gas atmospheres as well as X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (e.g. XANES) to give insight into the Re oxidation states(s). Furthermore, Raman spectroscopy has been used to identify different Re species [50,71,210,211]. These ex-situ methods, however, cannot account for the actual reactions conditions. In particular, the influence of the solvent, which can strongly influence Re reduction [74], and of high pressure can only be investigated by more complex in-situ methods [212].Several studies thoroughly investigated the oxidation state of Re and other metals. It is not uncommon to find the oxidation state change during the course of a catalytic reaction but it is rarely considered a factor responsible for catalyst deactivation. Such examples are also highlighted in Table\u00a01 and Table\u00a02 distinguishing between reduction and oxidation of the Re species during the reaction. In case of HG reactions, e.g. the catalytic reduction of carboxyl or carbonyl functionalities, the catalytic activity strongly depends on the oxidation state of Re for both monometallic [72,87,213] and bimetallic Re catalysts [74,150,151]. In the latter case, the ratio between different oxidation states was found to be a crucial factor governing catalytic behavior. Any changes to the Re oxidation state can therefore shift the delicate equilibrium to a less active position, i.e. the catalyst shows deactivation.Staying with the example of HG reactions, Takeda et\u00a0al. [74] found a clear correlation between the initial ratio of metallic and oxidized species and the catalytic performance of a Pd\u2013Re/SiO2 catalyst in the HG of succinic acid. Despite the catalyst being handled under N2 atmosphere during recovery in between reuse experiments, the catalytic activity decreased by ca. 60% over four consecutive runs. Metal leaching was contributing to catalyst deactivation but it was suggested that in addition changes to the active sites must have occurred. After one catalytic run, it was found that the average oxidation state of Re, determined by XANES analysis, changed from\u00a0+1.6 to\u00a0+1.2 [74]. While other effects like changes in spatial arrangements may also play a role, it appears likely that the overall reduction of Re species during the reaction is a crucial factor for the observed catalyst deactivation.For the DODH of vicinal diols it is typically suggested, e.g. by Tomishige et\u00a0al. [26], that highly oxidized Re species are required. They proposed that during the catalytic cycle the oxidation state of Re switches between\u00a0+6 and\u00a0+\u00a04. Due to the presence of a reducing agent, often H2, the overreduction of Re is a possible deactivation mechanism. Ota et\u00a0al. [71] suggested that the benefit of a CeO2 support compared to SiO2 was to maintain a higher oxidation state of Re (+5.0 vs.\u00a0+3.2; determined by XPS) throughout the reaction.Furthermore, the oxidation of Re species can also lead to catalyst deactivation. For the HDO of isoeugenol over bimetallic Ir\u2013Re catalysts, Alda-Onggar et\u00a0al. [115] suggested that the presence of Re4+ (ca. 12% of all Re species) corresponds to high catalytic activity. During the course of the reaction, however, the Re species were slightly oxidized and no Re4+ was found on the spent catalyst (Fig.\u00a07\n). The fact that the catalyst's activity could not be fully restored for a second run despite reactivation was suggested to be a result of the lack of Re4+ [115].Considering the crucial importance of the Re oxidation state for the catalytic behavior of Re-containing catalysts in redox reactions, it is imperative to control the conditions that the catalyst is exposed to at all stages of its lifetime, i.e. during pretreatment, reaction, recovery as well as reactivation. The pronounced susceptibility of Re catalysts to changes in oxidation state was also reported e.g. by Ly et\u00a0al. [214] for a TiO2-supported Re catalyst, which impressively shows the complexity of the Re redox chemistry. XPS analysis of the catalyst (Fig.\u00a08\n) after (a) gas-phase reduction at 450\u00a0\u00b0C, followed by (b) exposure to air, and (c) subsequent second reduction revealed the overall reduction and oxidation of different Re species as well as the ratios of different oxidation states. While after the first reduction, Re is primarily in the metallic state (89%) and some in oxidation state\u00a0+3, contact with air rapidly oxidizes the Re species to a mixture of higher oxidation states (17%\u00a0+\u00a04, 44%\u00a0+\u00a06, 39%\u00a0+\u00a07). A second reduction step can only restore parts of the initial metallic Re (15%) while most Re is\u00a0+3 [214].This influence is even more important considering that to reactivate the catalyst after other types of deactivation occurred (e.g. fouling by carbonaceous deposits or poisoning), it often has to be calcined or treated and/or undergo thermal treatment in reductive atmosphere. The example showed that despite nominally similar treatment (reduction at 450\u00a0\u00b0C), the obtained distribution of Re species was not comparable. On the other hand, often the recovery procedure in case of carbonaceous depositions is analogous to the initial preparation of the catalyst. Therefore, the influence of Re oxidation or reduction may often be overlooked.Deactivation by reduction can also be a result of catalyst reactivation, as a recent study showed [172]. Attempting to mildly remove carbon deposits from a Pt-ReOx/C catalyst in between recycling experiments, thermal treatment at 230\u00a0\u00b0C under H2 atmosphere was conducted. This resulted in a significant reduction of the Re species from an overall oxidation state of\u00a0+6.1 (primarily\u00a0+7, only 1% metallic Re; determined by XPS) in the fresh catalyst to\u00a0+1.82 after reduction (Fig.\u00a09\n). This caused a ca. 10-fold decrease in catalytic activity in the DODH of mucic acid. An additional oxidative treatment at 120\u00a0\u00b0C brought the Re species back to the approximately the initial state (+5.7). This resulted in a nearly complete recovery in the catalytic activity despite 9% of Re remained as metallic Re. Interestingly, after 6\u00a0h of reaction, the initial catalyst was found to be already significantly reduced to an average Re oxidation state of\u00a0+3.4. However, the contribution of this change to the observed lower activity of the untreated spent catalyst (ca. 35%) was not assessed in the study [172], even though this could further underline the previously discussed findings by Tomishige et\u00a0al. [26]. It should also be noted that besides Re also the oxidation state of Pt was slightly changed;\u00a0however, at least ca. 80% were present as metallic Pt in all catalysts, which does hint at a decisive role for catalyst deactivation.Besides the immediate effect of changes in oxidation state described so far, the solubility of Re species strongly depends on their oxidation state as will be discussed later in detail. In particular, in water as well as in polar, oxygenated solvents high-valent Re oxides are considerably more soluble than more reduced Re species [3,215]. Therefore, the oxidation of supported Re catalyst before, during, or after the reaction can also result in various leaching phenomena and corresponding effects on catalytic activity.According to Sadaba et\u00a0al. [10], leaching of the active phase is a characteristic feature of heterogeneously catalyzed reactions in the liquid phase. Leaching describes an extraction process, which results in the transfer of a compound from the solid catalyst into the liquid reaction medium [216]. In the context of supported metal catalysts (and in this review article), this typically refers to the selective loss of active (metal) phase of the catalyst but the previously described hydrothermal destruction of catalyst supports may also include leaching processes. In the simplest case, a soluble species exists on the catalyst and is dissolved when the solid catalyst comes into contact with the liquid reaction medium. On the other hand, the soluble species can also be formed during the reaction, which means that the leaching process includes e.g. a redox reaction. The details on mechanistic aspects of leaching can be found, e.g.\u00a0in the study by Eremin et\u00a0al. [217]. Sadaba et\u00a0al. [10] distinguish between two types of leaching: direct solubilization and leaching involving chemical transformations. Moreover, the solubility of a compound can be significantly increased in the presence of complexing agents, which facilitates metal leaching. Besides the leaching of single atoms or ions, there is also the possibility of multi-atom nanoparticles being solubilized or being formed in the solution [217].Among the articles included in this review article, which all contain Re, there are several bimetallic or trimetallic catalysts that can give an indication regarding the susceptibility of Re to leaching compared to other metals. A drastic example is e.g. the study by Zhang et\u00a0al. [82] on APR of glycerol over bimetallic supported Pt\u2013Re. While after one week of continuously operated reaction at 225\u00a0\u00b0C ca. 50% of Re was found to be leached from the catalyst its Pt content was not changed significantly. Similarly, a leaching test of another Pt\u2013Re catalysts revealed leaching of ca. 10% of Re without any Pt leaching after treatment in hot liquid water. For Ir-ReOx catalysts, for glycerol HL leaching of Ir was in some studies higher than Re (ca. 2% vs. 1%) [58,126,127], but in most similar studies, Ir leaching was negligible while slight Re leaching was observed, e.g. 0.9% Re leaching without detectable Ir leaching [132]. Bimetallic supported Pd\u2013Re catalysts also rather show Re leaching than Pd leaching, e.g. 3% [152] or 3.5% [74] compared to no detectable Pd leaching in both HG studies, and Ru leaching was at least 50% lower than Re leaching in a different study [150]. Similar results were obtained for different trimetallic Re-containing catalysts, where among Ir, Rh, Pt, and Re, only the latter was found to leach into the solution in quantifiable amounts (0.8\u20132.8%) [52,107]. Overall, it becomes apparent that Re has a stronger tendency to be leached from solid catalysts than commonly used noble metals like Au, Pt, Pd, Ir, Ru, or Rh barring a few exceptions mentioned here.The combination of Re with non-noble is not as common as with noble metals. Therefore, not much data are available of the leaching behavior of such catalysts. However, two studies indicate that when combining Re with non-noble transition metals, the latter are either similarly or more prone to leaching than Re. In the first study [112], Re\u2013Ni/ZrO2 was investigated in the aqueous-phase HG of lignin model compounds. After 1\u00a0h at 300\u00a0\u00b0C ca., 10% of Ni was leached while no Re leaching could be detected. In a recent study, similar amounts (ca. 10%) of Re and Ni were leached during lignin hydrotreatment [142]. In a recent study on levulinic acid HG in 1,4-dioxane at 180\u00a0\u00b0C over bimetallic Co\u2013Re/TiO2, the amounts of relative metal leaching for both metals were comparable and below 0.2% [159].Among the studies included in this article, metal leaching and, in particular, Re leaching are the most commonly found type of catalyst deactivation. Furthermore, when Re leaching occurs, a strong deactivation is typically observed (Fig.\u00a010\n). Even though the graph is a raw plot not taking into account other types of deactivation as well as ignoring the bimetallic nature of many catalysts, it becomes apparent that there are several drastic examples of catalyst deactivation where Re leaching strongly contributes to loss of catalytic activity. In addition, it shows that severe leaching of Re even beyond 10% is not uncommon.Leaching is not only problematic from the viewpoint of lowering catalytic activity; if the leached species remains in the liquid phase, the compound is irreversibly lost once the solid and liquid phase are separated. Furthermore, leached metal species can remain in and contaminate the product and/or the waste of the catalytic process, which is particularly problematic in case of toxic metals. While the latter is not the case for Re, it is on the other hand, economically fatal to lose significant amounts of such a precious element due to leaching. In particular, this non-recoverability makes Re leaching highly problematic and its prevention imperative for a successful catalytic process. Therefore, the different catalyst properties and reaction conditions will be discussed regarding their influence on Re leaching.A common approach to influence both catalytic activity as well as the stability of the catalyst is the use of different support materials. Therefore, several studies investigated support effects on leaching and/or deactivation of supported Re catalysts. This is in particular the case for the DODH reaction of vicinal diols, which was first discovered as a homogeneously catalyzed process and recent efforts have been dedicated to immobilizing Re species on different support materials to enable a heterogeneously catalyzed process. To prove the heterogeneous nature of the reaction significant effort is dedicated to identifying Re leaching and especially the contribution of dissolved species to the overall catalytic activity. This homogeneous contribution is typically investigated by removing the solid catalyst from the reaction medium (preferably at reaction temperature) and observing whether the reaction continues even without the solid catalyst, which is a strong indication for dissolved Re species. More details on the methodology of such \u2018(hot) filtration tests\u2019 will be discussed in a later chapter. In early studies [77,78,81] of supported monometallic Re catalysts for DODH reactions, both a clear contribution of dissolved species to catalytic activity and a significant loss in catalytic activity when reusing spent catalysts were observed. Therefore, Re leaching has from early on been considered a critical challenge for this reaction.Sandbrink et\u00a0al. [77] tested NH4ReO4 on different supports and found a clear trend regarding the stability of the catalysts in the DODH of 1,2-hexanediol in 3-octanol at 170\u00a0\u00b0C. While Re/SiO2 was already deactivated completely in the second recycling run, the stability of other catalysts, which were not fully deactivated in the seventh consecutive run, increased in the following order: (Re/SiO2)\u00a0<\u00a0Re/ZrO2\u00a0<\u00a0Re/C\u00a0<\u00a0Re/TiO2. After the catalysts were exposed to a reductive pretreatment at 300\u00a0\u00b0C, the influence of which will be discussed in a subsequent section, the trends of Re leaching observed during the reaction were similar. Reduced Re/TiO2 was most resistant to catalyst deactivation and showed no significant loss of catalytic activity in recycling runs. However, a hot filtration test confirmed the presence of leached Re species during the reaction even for this most stable catalyst. Interestingly, from the third consecutive catalytic run on leaching could not be observed anymore [77].In a similar study, Sharkey et\u00a0al. [78] investigated a range of different support materials on which NH4RO7 was deposited followed by calcination. While dissolved Re species were not quantified in the reaction solution after the DODH of 1,2-decanediol, hot filtration tests indicated that Re leaching was increasing in the following order: ReOx/CeO2\u00a0\u2248\u00a0ReOx/Al2O3\u00a0<\u00a0ReOx/Fe2O3\u00a0<\u00a0ReOx/SiO2. Furthermore, catalyst recycling was investigated with ReOx/Fe2O3. Product yield decreased by ca. 50% in the third run, which was suggested to be most likely due to Re leaching [78].A subsequent study by Sharkey and Jentoft [76] primarily targeted understanding the phenomenon of Re leaching observed previously. A series of supported ReOx was prepared and applied to the DODH of 1,2-dodecanediol in toluene at 150\u00a0\u00b0C. Subsequent recycling experiments (Fig.\u00a011\n, a) showed a drastic loss of catalytic activity already in the first reuse run. This was most significant for the most active catalyst Re/SiO2 (decrease by ca. 80%). Moreover, ReOx/TiO2 (which was the most stable catalyst in Sandbrink et\u00a0al.\u2018s study [77]) and ReOx/C showed considerable deactivation while ReOx/ZrO2, ReOx/Al2O3 and ReOx/Fe2O3 were considerably less active but also deactivated to a considerably lower degree. These observations were in excellent agreement with elemental analysis of the reaction medium proving Re leaching. Cumulative Re leaching (Fig.\u00a011, b) was clearly responsible for the low recyclability of the catalysts. In particular, 95% of Re was leached from ReOx/SiO2 during the initial reaction, followed by ReOx/TiO2, which lost 60% of Re in the first run and over all five runs >80%. From the other oxide supports \u2018only\u2019 40\u201350% was leached overall. The authors concluded that due to strong interactions on some types of support materials, e.g. ZrO2, Re species are less easily soluble compared to others like SiO2. In particular, a reducible support was suggested to benefit leaching resistance. This was further proven by investigating leaching at different temperatures. While Re leaching was comparable at 25\u00a0\u00b0C and 150\u00a0\u00b0C for ReOx/TiO2 indicating facile dissolution, Re leaching from ReOx/ZrO2 was negligible at 25\u00a0\u00b0C. Interactions between Re and support were suggested to be stronger in the latter case, which requires elevated temperatures to induce leaching.Recently, Meiners et\u00a0al. [168] investigated different zeolites as supports for the Re-catalyzed DODH of 1,2-hexanediol in 3-octanol. Stability was assessed for two (NH4)ReO4/H-ZSM-5 catalysts with different nSi:nAl ratio by recycling experiments. While the catalyst with higher Al amount (and lower nSi:nAl of 30 compared to 400) was initially twice as active, both were completely deactivated even in the first recycling run. At the same time, severe Re leaching was occurring with the high-Al catalyst losing 65% and the low-Al catalyst 85% of its initial Re loading (ca. 4\u00a0wt.-%) after 0.5\u00a0h reaction at 170\u00a0\u00b0C. Including additional characterization data, this was suggested to be linked considerably stronger interactions of the Re species with Al sites than Si sites, which also leads to Re being preferably located at Al sites.A very commonly used support material for DODH catalysts is CeO2, in particular, in studies by the Tomishige group [79,161,166,169\u2013171]. While other factors may also play a role (e.g. H2 atmosphere or the presence of a second metal on the catalyst, factors that will be discussed later), none of these CeO2-based catalysts is as susceptible to leaching as some of the previously described catalysts were, as is apparent from comparing the DODH catalysts in Table\u00a02. Even though no direct comparison of quantitative leaching for Re on different supports was conducted, it was concluded that Re leaching is more pronounced on carbon than on CeO2 [171], which is in agreement with Sharkey et\u00a0al. [78]. In another study [170], it was found that high-valent Re species were dissolved during the initial time in the reactor and migrated from a carbon support to a CeO2 support, which can serve as an additional indication that ReOx species are more stable on CeO2 than on carbon supports.Re leaching is not as well investigated for other catalytic reactions as it is for DODH. However, there is one study proving the susceptibility of SiO2-supported Re to leaching beyond DODH. Jeong et\u00a0al. [119] investigated monometallic Re catalysts for the HDO of guaiacol in heptane at 280\u00a0\u00b0C. While 20% of Re were lost from Re/SiO2 during the reaction Re/TiO2 was considerably more stable (0.5%).In general, the support material is a crucial factor determining the stability of supported Re catalysts towards Re leaching. In particular, the redox activity of the support appears a crucial criterion and strong interactions of Re with the support enhance its stability. At the same time, catalyst activity and selectivity also strongly depend on the support material. Unfortunately, higher stability toward Re leaching was at least in some cases found to correlate with a lower catalytic activity [76,119].Sharkey and Jentoft [76] also investigated the influence of Re content of supported catalysts on the leaching occurring during the DODH reaction. Both TiO2 and ZrO2 were selected and the Re content varied from 1 to 4\u00a0wt.-%. Independent of the support material leaching was lower for lower Re loading; however, there was hardly any difference between 1 and 2\u00a0wt.-%. For the TiO2-supported catalysts, which are more prone to leaching, ca. 60% of Re were lost in the first batch DODH reaction in toluene at 150\u00a0\u00b0C when the Re loading was 4\u00a0wt.-%. Using 2\u00a0wt.-% Re or less, only around 20\u201325% of Re were leached. As observed for different supports, there was an unfortunate correlation between leaching and catalytic activity. With increasing stability toward leaching, catalytic activity normalized to Re content\u00a0of the solid catalysts decreased. Despite >50% of Re remaining on the catalyst, catalysts with 2\u00a0wt.-% or lower initial Re loading showed almost no catalytic activity in their third run. It should be noted that, for these cases, there was a strong contribution of homogeneously catalyzed reactions by dissolved Re species to the overall activity, which also affects the observed catalytic activity in catalyst recycling experiments. However, there is also the possibility of other types of catalyst deactivation limiting the reusability of supported Re catalysts in this case [76]. Regardless its eventual impact on the catalytic activity, catalysts with higher Re loadings are more prone to Re leaching.Besides this detailed study, it is very rare to encounter information on Re leaching for similar catalysts with different loadings in the literature. Even though the variation of metal loading as well as the ratio of two metals in case of bimetallic catalysts is often investigated to enhance catalytic activity and to gain insight into the way the catalyst functions, the issue of catalyst deactivation is typically only investigated for a single selected catalysts. In a study by Liu et\u00a0al. [139], therefore, different earlier studies from the same research group [58,124,126] on similar SiO2-supported Re\u2013Ir catalysts were in included in the assessment of the influence of metal loading on Re (and Ir) leaching. Despite differences in reaction conditions, it could be concluded that, in this case, a catalyst with higher loadings (ca. 20\u00a0wt.-% Ir, 6.8\u00a0wt.-% Re) was slightly more stable toward metal leaching during the aqueous-phase hydrolysis of glycerol than one with lower metal contents (ca. 3.9\u00a0wt.-% Ir, 3.2\u00a0wt.-% Re). While for the high-loading catalyst leaching of Re was below the detection limit (<0.25%), the low-loading catalyst showed detectable leaching (up to ca. 1%). Ir leaching behaved analogously. Considering only Re loading, this appears to be a contradiction to the study by Sharkey and Jentoft [76] presented before on monometallic DODH catalysts. Leaving other aspects like the different reaction system aside, there are at least two major aspects that need to be taken into account. First, the catalysts studied by Tomishige et\u00a0al. are bimetallic. The (often inhibitory) influence of a second metal on Re leaching will be discussed in the subsequent chapter. Second, when the metal loadings were varied in Liu et\u00a0al.\u2018s study [139], the Re/Ir ratio was not kept constant, but at higher loading, the Re/Ir was significantly lower (0.34 vs. 0.83). Therefore, it is likely that, in this case, the Re/Ir ratio is a deciding factor and the effect Re loading per se may not be visible.In a study by Ly et\u00a0al. [157], supported Re\u2013Pd catalysts were investigated for the aqueous-phase HG of succinic acid. Very significant leaching was detected during the heat-up phase (to 160\u00a0\u00b0C) in the reactor, which was conducted under inert Ar atmosphere. A catalyst with a Re loading of 3.4\u00a0wt.-% lost ca. 60% of its initial Re content due to leaching into the reaction medium while 100% of Re was leached from a similar catalyst with 0.8\u00a0wt.-% Re content. The Pd content was 2\u00a0wt.-% in both cases; however, the preparation method was different (sequential impregnation vs. reductive deposition). As will be discussed in Section 3.2.4, it is likely that, in particular, the differences in Re oxidation state (confirmed in an earlier study on the same catalysts [214]) arising from the different preparation methods is mainly responsible for the different leaching behavior of the two catalysts. Furthermore, the fact that one of the materials was exposed to air could play a role, which is also mentioned by the authors [157]. Besides the initially discussed study, which varied only the parameter of Re content and therefore allows for an unambiguous assessment of the influence of Re loading, all other studies varied other parameters at the same time as Re loading. Thus, in these latter examples, it is not clear to what degree Re loading plays a role, if any.When considering the possible influence of Re content on Re leaching, differences may actually arise from increasing Re particle sizes. Therefore, for this review article also correlations between Re particle size and Re leaching as well as catalyst deactivation were investigated based on the collected literature (Table\u00a01 and Table\u00a02). While it is difficult to draw universally applicable conclusions from such an inhomogeneous set of data, there appeared to be no indication that larger or smaller Re particles are more prone to leaching. Thus, while Re loading on supported catalysts can have considerable effect on its leaching, other factors are likely more influential.Another factor that can affect the leaching of Re from supported catalysts during their use in catalytic applications in the liquid phase is the presence (or absence) of additional metals. Unfortunately, there is no direct comparison available in the literature of how quantitative leaching of Re catalysts behaves in the presence of a second metal even though the effect of different Re-metal combinations are well-investigated regarding their effects on catalytic activity and selectivity. Furthermore, monometallic Re catalysts are often used for DODH reactions under inert or oxidizing atmosphere whereas bimetallic catalysts are typically used under reducing atmosphere and involve HL and HG reactions, which makes comparisons of different studies problematic. In addition, different support materials are used throughout the literature, which is a strong influencing factor as discussed in chapter 3.1.1.When Re and a second metal are combined on a support material, this usually results in changes in the redox properties of the metal phase, which can e.g. be observed by temperature-programmed reduction. A typical finding is that the combination of Re with noble metals, such as Pt [54,218], Pd [71,75,219], Ir [52,126,134], Rh [51,52], Ru [116,211,220], or Au [161,162,170], result in a lower reduction temperature for (alloyed) Re species. Di et\u00a0al. [68] showed by XPS analysis that Pt stabilizes the metallic state of Re and lowers Re re-oxidation when exposed to air. Daniel et\u00a0al. [221] observed that while monometallic Re/C could not be reduced in H2 at 200\u00a0\u00b0C, the presence of Pt on a comparable bimetallic Pt\u2013Re catalyst facilitated Re reduction to partially reduced species. Moreover, Wang et\u00a0al. [170] concluded that the presence of metallic Au particles can provide H2 for ReOx reduction, which prevents leaching compared to monometallic catalysts.There are further indications that bimetallic interactions can influence the leaching behavior of Re. For instance, Liu et\u00a0al. [139,140] found that during reuse experiments of SiO2-supported Ir\u2013Re catalysts in the HL of glycerol Re species accumulate on and cover Ir particles. It was suggested that some high-valent Re species are leached from the support and redeposit on the metal particles. This finding suggests that Re species are stabilized in the proximity of Ir metal particles. A similar phenomenon was observed by Ly et\u00a0al. [157], who investigated the deposition of dissolved Re species on a Pd/TiO2 catalyst under HG conditions. Re was found to be primarily deposited in the vicinity of Pd, which was suggested to enable chemical reduction of Re by H2 activation. This further proves that, at least under reducing conditions, deposited Re species are stabilized in the presence of a noble metal. Re redeposition on Pd was also suggested in a different study [114]. Similarly, Pieck et\u00a0al. [222] found that Pt catalyzes and thereby enables the reductive deposition of Re on Pt/Al2O3 under H2 atmosphere. Importantly, under the same conditions, deposition on Al2O3 was more than five times lower, indicating the importance on Pt.Wei et\u00a0al. [50] investigated supported Pt\u2013Re APR catalysts. They identified two different Re species on their bimetallic catalysts: Re associated with Pt and terminal Re\u2013O. Only the latter was found to be leached from the catalyst while interaction with Pt apparently stabilized the other species against leaching. A similar behavior was also suggested in other studies [69,150]. During subsequent reuse experiments with bimetallic catalysts, stabilization was observed, which was explained by a non-stabilized Re species being completely leached in the initial stages whereas another metal-stabilized Re species remained deposited on the catalyst.Even though there is no unambiguous proof that the presence of noble metals can by reduce the tendency of supported Re species toward\u00a0leaching, several individual findings strongly indicate a positive effect. Most likely, this is linked to a redox interaction that promotes the reduction of Re. All the presented examples of catalysts were tested under reducing H2 atmosphere, therefore it could be speculated whether this is a requirement for this stabilizing effect to occur. Interestingly, it was found that Pt on a bimetallic Pt\u2013Re/C catalyst can also promote the oxidation of Re species [82]. The influence of the gas atmosphere during the reaction will be discussed in a subsequent chapter. It should be noted that, even though trimetallic and multimetallic catalysts were not discussed here due to the low number of studies on such catalysts, similar stabilization against Re leaching can be expected for those materials. It remains an open question, however, which metals are particularly suited to stabilize Re. Furthermore, for combinations of Re with non-noble transition metals, there was not sufficient data available to analyze the effect of those metals on Re leaching.The preparation method of supported Re catalysts can influence the properties of the catalyst in myriad ways. Depending on the Re precursor and the process chosen to load the support material e.g. the Re dispersion or the oxidation state of Re can vary, which in turn affects catalytic activity and stability. Among the most commonly used preparations methods are wet impregnation, incipient-wetness impregnation and reductive deposition. For the case of bimetallic or multimetallic catalysts, an important aspect is the order in which the different metals are introduced and what other treatment steps may be included in the preparation procedure. Regardless the preparation method, catalysts may be subjected to various pretreatment procedures, the influence of which is discussed in the next chapter.Regarding the influence of the preparation method on the leaching of Re during catalytic application, there is only limited information available. In the previously mentioned study by Ly et\u00a0al. [157], bimetallic TiO2-supported bimetallic Pd\u2013Re catalysts were investigated. On the one hand, one catalyst was prepared by successive impregnation, where Re was introduced onto a monometallic Pd/TiO2 (2\u00a0wt.-%) catalyst by wet impregnation. On the other hand, a (catalytic) reductive deposition procedure was used where the monometallic Pd catalyst was first activated in H2 at 300\u00a0\u00b0C. Afterward, an aqueous NH4ReO4 solution (like in the case of wet impregnation) was added and H2 gas was led through the dispersion. Both catalysts were also reduced in H2 gas at 450\u00a0\u00b0C. Interestingly, Re leaching was only detected during the heat-up phase of the catalytic experiment, which was conducted with aqueous succinic acid at 160\u00a0\u00b0C. Under Ar atmosphere, the catalyst prepared by successive impregnation lost 60% of its 3.2\u00a0wt.-% Re content due to leaching. The Re species on the catalyst prepared by reductive deposition, however, completely dissolved. Characterization revealed that chemical reduction allows for a more controlled deposition of Re at the interface between Pd and TiO2 [214]. Another major difference was the Re oxidation state for the two catalysts. Two Re species were identified by XPS, metallic Re0 and Re3+. The latter was the only Re species on the catalyst prepared by reductive deposition while successive impregnation (and subsequent reduction and passivation) resulted in a mixture of both states with a majority (70%) being Re3+. As previously discussed, higher-valent species are known to be considerably more soluble in the reaction medium. Therefore, the difference in Re oxidation state is likely the reason for the different leaching behavior. It is, however, not unambiguously clear to what degree the Re oxidation states are directly related to the preparation method itself or to differences in the catalyst pretreatment. While the catalyst from successive impregnation was passivated after reduction and stored under Ar, the catalyst prepared by reductive deposition, which was subjected to the same gas-phase reduction, was not passivated but exposed to and stored under air atmosphere. As will be discussed further in Section 3.2.2, air exposure can strongly influence the catalytic properties. In the study, it is also shown that in-situ preparation methods, which completely avoid the exposure to air and do not necessitate passivation, were a successful approach to prevent Re leaching. Unfortunately, no other studies on Re catalysts include the effect of different preparation methods on catalyst stability or Re leaching.Before a supported metal catalyst is applied in a reaction, it may be subjected to a variety of treatment procedures influencing the nature of the metal species in order to generate a specific one desired for the catalytic application. Besides an initial preparation method, like e.g. different impregnation techniques or reductive deposition to load the metal(s) on the support material, the preliminary material is in many cases thermally treated under defined gas atmospheres to generate the catalyst that is used in the reaction. Oxidative treatments using O2 or air atmosphere, aiming at decomposing metal precursors and forming a defined metal oxide species, are referred to as \u2018calcination\u2019, while reductive treatments to generate low-valent and often metallic species using (diluted) H2 gas are called \u2018reductions\u2019. In some cases, after reduction, the catalyst is passivated, i.e. mildly oxidized, to protect it against uncontrolled oxidation upon contact with air. The latter can be avoided by reduction directly in the reactor, in certain cases even in the presence of the reaction medium, directly prior to the catalytic phase of the experiment. Therefore, it can be distinguished between such \u2018in-situ\u2019 treatment compared to the for liquid-phase batch reactions more typical case of \u2018ex-situ\u2019 treatment. Using this variety of available catalyst pretreatment procedures is a common approach to tune the properties of supported metal catalysts. As discussed above, the catalytic behavior strongly depends on the oxidation state of the metal and changes during the reactions can be a reason for catalyst deactivation. Furthermore, different studies found that also the Re leaching is influenced by the pretreatment conditions a catalyst is subjected to. However, literature is mostly limited to the influence of reductive pretreatments.In the previously mentioned study by Sandbrink et\u00a0al. [77], a series of different monometallic Re catalysts was investigated for the DODH of 1,2-hexanediol. Four differently supported Re materials were both directly used after the deposition of (NH4)ReO4 on the support and each catalyst was subsequently reduced in H2 atmosphere at 300\u00a0\u00b0C. In catalyst recycling tests, all reduced catalysts were considerably more stable than their unreduced counterparts (Fig.\u00a012\n). The unreduced silica-supported Re catalyst was completely deactivated after the second run while the initially reduced catalyst still showed some activity in the fifth consecutive experiment. Similarly, unreduced Re/TiO2 (Re oxidation state most likely\u00a0+7 as in NH4ReO4) showed a decreased activity to ca. 40% of its initial activity in the seventh run while the reduced analog (Re oxidation state ca. 4.5 calculated from XAF spectroscopy) did not show signs of catalyst deactivation. Unfortunately, this study does not include quantitative information on Re leaching. Considering similar studies [76,78], it is, however, very likely that Re leaching is the main cause of catalyst deactivation under the applied reaction conditions. Due to the lower solubility of reduced Re species compared to Re+7, reduction of the catalyst was suggested to increase stability toward Re leaching [77].The influence of reductive pretreatment on quantitative Re leaching was investigated in more detail by Chia et\u00a0al. [51,53], who developed bimetallic supported Re\u2013Rh catalysts for polyol HL. Catalysts were reduced at different temperatures under H2 atmosphere (and in one case not at all) to investigate the influence of this pretreatment step on the catalytic activity and stability. A clear trend could be observed indicating that reduction at higher temperatures is beneficial to prevent Re leaching during the aqueous-phase HL of 2-(hydroxymethyl)tetrahydropyran at 120\u00a0\u00b0C. The unreduced catalyst Rh\u2013Re/C lost ca.\u00a02% of its Re content due to leaching after the 4-h reaction. Prior reduction of the catalyst at 120\u00a0\u00b0C slightly reduced the leached amount to 1.2% and when the temperature was 250 or 450\u00a0\u00b0C, no Re leaching could be detected. Despite the drastic differences regarding both reaction conditions and the fact that a bimetallic catalyst was used compared to the previously discussed study, the effect of reductive catalyst pretreatment is the same. The reduction of Re-containing catalysts mitigates leaching most likely by lowering the amount of particularly water-soluble high-valent species.However, it has to be considered that the oxidation state is crucial in governing catalytic activity, as e.g. outlined by Tomishige et al. [26]. This aspect was already discussed in Chapter 2.7 since changes in Re oxidation state during the reaction can be one type of catalyst deactivation. Manipulating the Re oxidation state by pretreatment procedures to increase stability, therefore, can have the negative side-effect of lowering catalytic activity. This was observed by Chia et\u00a0al. [51,53]. While reduction at 250\u00a0\u00b0C compared to 120\u00a0\u00b0C completely prevented Re leaching catalytic activity decreased by ca. 40%. Thus, in this case as well as in other reactions where high-valent Re species are required, there is a trade-off between catalytic activity and stability toward Re leaching. Interestingly, Sandbrink et\u00a0al. [77] encountered findings (gradual loss in DODH activity of ReOx/TiO2 over multiple recycling runs when changing reduction temperature from 300\u00a0\u00b0C to 400 or 500\u00a0\u00b0C) that can also be interpreted in this way.Another important aspect is that while pretreatment procedures can be used to control the initial state of a supported metal catalyst, it may change strongly under reaction conditions. During reactions under reducing or oxidizing atmosphere, the Re oxidation state can be altered almost immediately. In particular, for HG or HL reactions, it was found that already during the heat-up phase of the reaction the catalyst was reduced [119]. In some cases, this was even a deliberate choice and considered an (additional) in-situ catalyst reduction [74,131,139,140]. In many other cases, this effect is neglected even though, as shown in Chapter 2.7, changes in Re oxidation state in spent catalysts compared to the fresh catalyst are not a rare finding.Another type of unintentional \u2018pretreatment\u2019 is the exposure of supported metal catalyst to air. This is particularly relevant when transferring the freshly reduced catalyst from ex-situ reduction to the reactor. Reduced rhenium species are known to be sensitive to air due to their oxophilicity and prone to oxidation [51,57,68,135,151,158,214,223]. Therefore, handling a reduced Re catalyst in air prior to the catalytic experiment as well as before subjecting it to a characterization method can be problematic due to the alterations in the catalysts\u2019 properties that can occur.Considering the beneficial effect of catalyst reduction before conducting a catalytic experiment, the influence of re-oxidation due to air exposure can be expected to have a negative influence on Re leaching. In the previously mentioned study by Ly et\u00a0al. [157], one catalyst was exposed to air and showed complete leaching of all its Re during the heat-up phase of the catalytic experiment under Ar atmosphere. A second catalyst that was passivated and kept under inert atmosphere without uncontrolled air exposure showed less pronounced Re leaching (70%). For these examples, however, also Re loading and catalyst preparation were different and offer possible explanations for the differences in Re leaching. In the same study, an in-situ preparation method was developed, where Re was introduced directly in the reactor, which makes catalyst transfer (and possible air exposure) completely obsolete. In contrast to the partially oxidized catalysts, the in-situ prepared catalyst showed no significant Re leaching under inert atmosphere in aqueous medium. Even though the Re oxidation state was not determined for the latter catalyst, it is highly likely that the protection from oxygen prevented the oxidation of Re to more water-soluble high-valent species.Exposure of catalysts to air is also relevant for the recovery procedure in recycling experiments. Wang et\u00a0al. [171] found that Re leaching could only be detected during the recovery of ReOx/C and ReOx-Au/CeO2 catalysts (0.25% leaching) when the catalysts were separated from the reaction medium in air. In the absence of air, leaching was <0.01% of the overall Re amount on the catalysts. This can be explained by the oxidation of Re to soluble species. A more drastic example was reported by Sadier et\u00a0al. [137] investigating Rh-ReOx/ZrO2 catalysts for aqueous-phase HL reactions. While Re leaching was insignificant during the reaction, as analysis of the reaction medium showed, considerably leaching occurred during separation and washing of the catalyst. Analysis of the solid catalyst revealed that Re loading decreased from 3.2\u00a0wt.-% to 2.5\u00a0wt.-% during the recovery procedure.There are several other studies on Re-containing catalysts that found that exposure of the catalyst to air during recovery has detrimental effects to the catalysts' recoverability. However, the role of Re leaching is not always clear, even though the previously presented studies indicate that leaching is likely to play a decisive role. One example is studies by Liu et\u00a0al. [139,140] on supported Re\u2013Ir catalysts for polyol HL. Catalysts only retained their catalytic activity if the recovery procedure was conducted in ways that prevented the catalysts from coming into contact with air. For catalysts with low Re loading this even required degassing of water used to wash the catalyst. Similar to the study by Sadier et\u00a0al. [137], the reaction medium did not contain significant amounts of leached Re that could explain catalyst deactivation. It could be speculated whether also in the case of the catalysts by Liu et\u00a0al.\u2018\u00a0[139,140] leaching primarily occurred during the recovery procedure itself. However, other possibilities that were discussed is a migration of Re species (probably via dissolved species) that leads to an accumulation of Re on top of Ir particles that is detrimental to catalytic activity [139] or that the oxidation of Re itself is the cause of catalyst deactivation [140]. Furthermore, an earlier study by Takeda et\u00a0al. [151] showed that also for Re\u2013Pd/SiO2 catalysts for stearic acid HG, the catalyst recovery procedure has to be conducted without exposure to air to avoid catalyst deactivation.In agreement with the previous studies, Haus et\u00a0al. [224] found that their Pt\u2013Re/TiO2 HG catalyst suffered severe deactivation due to Re leaching during catalyst recycling. Also in this case, Re oxidation upon air exposure was suggested as the main cause for the catalyst's susceptibility toward leaching. In particular, a washing step with water during the recycling procedure was highly detrimental and its omission mitigated catalyst deactivation.Overall, the examples presented here conclusively show that air exposure at different stages of the catalyst lifetime can lead to severe Re leaching and subsequent catalyst deactivation. Consequently, attention should be paid to ensure suitable handling of the catalysts at all times.Like for many other chemical processes, Re leaching from solid catalysts can be expected to be strongly influenced by the temperature of the process. First, the solubility of Re species is temperature dependent. It can therefore be expected that leaching is more pronounced at higher reaction temperatures in the case that solubility is limiting the leaching process. Consequently, when the reaction mixture is allowed to cool down before separation of the solid catalyst, this can result in the redeposition of the dissolved species [76,78,81]. This also has implications for the limitations to detect Re leaching and will be discussed in more detail in a subsequent chapter. Sharkey and Jentoft [76] also found that whether or not temperature-dependent redeposition occurs is also dependent on the other parameters of the reaction system, in their particular case the concentration of diol molecules that was depending on the reaction progress.Besides the temperature effect on Re species solubility, there is also the possibility that the dissolution of the Re species is a thermally activated process. This aspect was investigated by Sharkey and Jentoft [76] for two oxide-supported ReOx catalysts (Fig.\u00a013\n). As previously discussed, the differences between the leaching behaviors of the two different catalysts arise from the different support-metal interactions. In case of the TiO2 support, Re leaching is essentially temperature-independent in the investigated range. On the other hand, ReOx/ZrO2 shows a typical behavior for thermally activated Re leaching. While at room temperature negligible Re was dissolved, Re leaching is clearly apparent at 150 \u00b0C. In general, it can be assumed that thermally activated leaching can be observed in cases of strong interactions of Re species with the support material, a second metal or a promoter. When the Re oxidation state is affected as a result of these interactions, it can also be expected to play a role in governing the temperature dependence of the Re leaching behavior.Both previously discussed effects result in increased Re leaching at higher temperature. However, in the presence of a reducing atmosphere, there is also the possibility to observe reductive deposition of dissolved Re species at elevated temperature. Ly et\u00a0al. [157] found that dissolved Re7+ species could be rapidly and almost completely (>95%) deposited in-situ onto a Pd/TiO2 catalyst under H2 atmosphere at 160\u2013180\u00a0\u00b0C. While in this case the reductive deposition is a deliberate method of catalyst preparation, the previously discussed (section 3.2.1.) in-situ reductive pretreatment of supported catalysts is a comparable process and is a measure to mitigate leaching by lowering the amount of oxidized Re species [119]. Regarding the temperature-dependence of the Re reduction as well as the deposition process, no data are reported. It can, however, be expected that, similar to gas-phase reduction processes, there is a threshold temperature required that is characteristic for different Re species. Moreover, it can be influenced by the catalyst composition and depends on the process conditions.Therefore, process temperature has influence on Re leaching and deposition processes by governing solubility, via activated leaching processes as well as by influencing temperature-dependent redox reactions between different Re species. Among the studies included in this article, there is no clear overall trend regarding the influence of reaction temperature influence on Re leaching and deactivation, which is, on the one hand, a consequence of the described different underlying processes. On the other hand, there are other parameters that play a more pronounced role in governing Re leaching behavior. While it may appear reasonable to assume that at higher reaction temperatures leaching is more prevalent, this is not supported by the compiled data. Even though DODH processes are typically conducted at comparatively mild reaction temperature between 100 and 200\u00a0\u00b0C and often <150\u00a0\u00b0C, Re leaching is very common and severe due to other leaching-promoting factors. Overall, the effect of reaction temperature has to be considered in the context of the reaction system and its characteristic process parameters.Due to the nature of the leaching process, which is a transfer of the leached species from the solid phase of the catalyst into the liquid reaction medium, the reaction medium must be expected to play a crucial role in governing the leaching process. As mentioned previously, high-valent Re oxides are known to be most prone to leaching in water and polar oxygenated solvents [3,215]. Due to the ionic nature of these species, e.g. the [ReO4]- ion, their solubility is higher in polar solvents. Reviewing leaching of various metals from solid during biomass conversion, Sadaba et\u00a0al. [10] concluded that polar media can be considered more aggressive and therefore leaching is, in general, more pronounced in polar reaction media than in less polar ones.Catalyst stability is, however, rarely a main priority when selecting a suitable solvent for a catalytic reaction. For the catalytic conversion of many polar, biomass-derived compounds like, e.g. organic acids, polyols, or sugars, water is often the solvent of choice and the significance of aqueous-phase process is apparent from Table\u00a01 and Table\u00a02. Typical catalytic processes in aqueous phase are reforming, HL or HG of biomass-derived molecules. On the other hand, less polar molecules like fatty acids or lignin-derived aromatics are typically dissolved in non-polar hydrocarbons. In general, due to the use of H2 for most of the reactions catalyzed by supported Re-containing catalysts, the use of oxygen-containing solvents, with the exception of 1,4-dioxane, is rare. Only in case of DODH, alcohols are commonly used as solvents.DODH is also the only Re-catalyzed reaction for which the influence of the solvent on Re leaching was systematically investigated. Sharkey and Jentoft [76] studied leaching of a TiO2-supported monometallic Re catalyst for the DODH of vicinal diols in different solvents (Fig.\u00a014\n). The results of detected Re leaching into the liquid phase after 15\u00a0min in the respective pure solvent at 150\u00a0\u00b0C confirms the outlined assumptions regarding the influence of solvent polarity. Re leaching was insignificant for non-polar toluene due to the low solubility of the Re species but slightly increased for more polar solvents like secondary alcohols. In pure water, however, the concentration of leached Re was more than one order of magnitude higher than in all other solvents. Therefore, Re leaching follows the trend of solubility of high-valent Re species in pure solvents. Interestingly, in case of the DODH reaction, the presence of especially the reactant can drastically enhance leaching in less polar solvents by the complexation of Re, which will be outlined in the next section.Even though water is the solvent that has the highest potential to cause Re leaching, there are myriad examples of catalysts stable in aqueous-phase reactions (see Table\u00a01 and Table\u00a02). In particular under H2 atmosphere and in combination with noble metals, Re-containing catalysts rarely show leaching >1%. Therefore, on the one hand, it appears that polar solvents and especially water can strongly enhance Re leaching for catalysts that, due to other factors, contain considerable amounts of easily leached Re species. On the other hand, there is no clear indication that polar solvents have a corroding effect that can convert deposited Re into more leachable species. Regardless, it should be noted that due to a lack of targeted investigations into the leaching phenomenon (besides the discussed study on DODH in different solvents), no definitive conclusions can be drawn.For the case of heterogeneous catalytic DODH over Re catalysts, an investigation by Sharkey and Jentoft [76] revealed that the diol reactant can play decisive role in causing Re leaching from supported Re catalysts such as ReOx/TiO2. As shown in Fig.\u00a014, in non-polar solvents, Re leaching is enhanced drastically in the presence of diol molecules. In case of the least polar solvent, toluene, this effect is most pronounced, where without diol, no significant leaching is observed, but in its presence, the concentration of leached Re is greater than the amount dissolved water. Interestingly, Re leaching was also promoted in non-polar solvents when the reaction product decene was present, but by a factor of ca. 2\u20133 lower than for the corresponding reactant 1,2-decanediol. During the course of the catalytic DODH reaction, the concentrations of both compounds change, which also has implications for Re leaching. When the DODH reaction was stopped at around 50% conversion, Re leaching was very severe and the recycled catalyst was ca. 90% less active than the fresh catalyst. However, running the reaction to full diol conversion drastically reduced Re leaching and the catalyst deactivation was significantly lower. It was concluded that primarily the diol enhances Re solubility, and after its consumption, Re redeposition onto the catalyst takes place.The key to this phenomenon lies in the interaction of the Re species with the reactant molecules. Re is suggested to be present as high-valent species on the ReOx/support catalysts, which suggests that the catalytic cycle is similar to the case of homogeneous reactions catalyzed by Re+7-based catalysts like methyltrioxorhenium or similar [225\u2013227]. It is known that the diol reactant binds to the (partially reduced) ReOx species as a bidentate chelating ligand. In case of diols with a hydrophobic alkyl chain, such as the investigated 1,2-decanediol and 1,2-hexanediol, the corresponding chelate complex can be expected to have a considerably higher solubility in non-polar reaction media, e.g.\u00a0toluene, than the oxidic Re species alone. As mentioned, also the presence of alkene product leads to enhanced leaching due since the reverse reaction with ReOx species also leads to the formation of a chelate complex, however, to a lower degree. Additional tests with a primary alcohol and 1,4-butanediol confirmed that the vicinal 1,2-diol motif is crucial for this leaching mechanism, which further suggests a bidentate chelate structure of the leached Re species [76].Further evidence for the Re-diol-complex was provided by UV\u2013Vis spectroscopy [76,172]. As shown in Fig.\u00a015\n, spectra of the leached Re species from solid Re catalysts are similar to complexes found during the homogeneously catalyzed reaction for the case of the model compound (R,R)-(+)-hydrobenzoin. Moreover, the band at 476\u00a0nm is not a characteristic of dissolved (ReO4)- ions. While additional characterization of the leached Re complexes by EPR [76] was inconclusive, 1H NMR spectroscopy [172] further indicated that a Re-diolate complex is the leached species formed.Leaching of Re via the chelate complex mechanism is characteristic for the DODH over supported Re catalysts, where it was suggested or confirmed in several studies [71,76,81,172]. It requires specific conditions, i.e. non-polar solvent, vicinal diol, and probably also high-valent Re species. When water is used as a solvent, which is common for other Re-catalyzed reactions, this phenomenon can be neglected (Fig.\u00a014).The gas atmosphere, under which a reaction is performed, can have significant effects on the oxidation state of supported Re catalysts as described in Section 2.7, and consequently, also influences Re leaching. In general, under reducing, i.e. H2-containing, atmosphere, the easily leached high-valent ReOx species can be reduced to more stable, low-valent, or metallic species, an effect that is occasionally utilized as an in-situ reduction [74,131,139,140] of the catalyst. On the other hand, under oxidizing conditions, the reverse process is possible resulting in overall increase in Re oxidation state. Oxidation processes can be further enhanced under hydrothermal conditions [82,106]. Unfortunately, there is not a single study available where the influence of the gas phase composition on the Re leaching behavior is systematically investigated. This is mainly due to the fact that many types of reactions, e.g. HG, HL or HDO, demand a H2 atmosphere. It is, however, noteworthy, that among the plethora of studies on these reactions, the number of severe cases of Re leaching in heterogeneous catalytic reactions is very small (compare data in Table\u00a01 and Table\u00a02).In contrast to that, Re-based catalysts for DODH reactions, which are typically conducted under N2 or air, are often reported to suffer pronounced Re leaching. While many other parameters need to be considered (often no reductive catalyst pretreatment, leaching via Re-reactant complexes possible, often monometallic catalysts), the non-reducing atmosphere is one of the major characteristics of the reaction and is likely playing a crucial role. At the same time, alcohol solvents, that can act as reducing agents, and/or molecular reducing agents are present, whose influence is also largely unknown. When DODH is coupled with HG and H2 is used, Re leaching also appears less relevant than in pure DODH processes (compare data in Table\u00a02). However, considering the lack of direct evidence, the influence of gas phase composition on Re leaching remains ambiguous.Additional insights can be gained by looking at the reverse process, i.e. deposition of dissolved Re species. Ly et\u00a0al. [157] found that dissolved Re7+ species could be reductively deposited onto a Pd/TiO2 catalyst in-situ (Fig.\u00a016\n). Under non-reducing Ar atmosphere, about 10% of the dissolved Re was found to deposit on the catalyst at 160\u00a0\u00b0C. This was explained by the sorption of (ReO4)- on Pd/TiO2 similar to comparable studies on Re sorption on a Pd/C catalyst [228,229]. Another possibility is that a small amount of hydrogen was adsorbed on the Pd/C catalyst, which lead to the observed amount of reductive deposition of Re, as observed by Pieck et\u00a0al. [222], for a Pt/Al2O3 catalyst under inert atmosphere. When H2 gas (150\u00a0bar) was added to the initially described Re deposition experiment onto Pd/TiO2, the amount of Re in the solution was rapidly decreased to\u00a0<\u00a05% of the initial value (Fig.\u00a016). This clearly indicates that the deposition of Re is governed by a redox reaction and the composition of the gas phase is crucial for the process. However, the study by Pieck et\u00a0al. [222] also showed that for the reductive Re deposition, the presence of Pt was crucial to facilitate Re reduction.In this context, it should also be considered that exposure of Re catalysts to air in between catalytic experiments was found to have detrimental effect on catalyst recyclability. As outlined above (Section 3.2.2.), this is in most cases probably due to Re oxidation when in contact with air and subsequent leaching of the more water-soluble high-valent species during washing steps or in the reuse experiment. Overall, there are strong indications that reducing atmosphere can mitigate Re leaching by preventing Re oxidation.The addition of mineral acids, typically H2SO4, or solid acids, such as zeolites or ion exchange resins, as co-catalysts in the catalytic HL of biomass-derived molecules over bimetallic Re-containing catalysts goes back to investigation by the Tomishige group [124,126,127]. In the aqueous-phase HL of glycerol over Ir-ReOx/SiO2, the presence of H2SO4 (pH\u00a0=\u00a03) as a co-catalyst strongly influences catalytic activity and selectivity. Moreover, the catalyst was only reusable without significant loss in catalytic activity when H2SO4 was present. This is most likely due to the reduced Re leaching of <0.3% (compared to 2%) in the acidified reaction solution [124,126]. Further investigations, including density functional theory calculations, showed that H2SO4 stabilizes protonated Re-OH sites on the catalyst [138,139]. While this can explain the differences in catalytic activity, in remains unclear how it mitigates Re leaching. The beneficial effect of H2SO4 was also proven for a supported Rh-ReOx for the aqueous-phase HG of erythritol [135]. Re leaching decreased from 6% to 2% upon acid addition, which indicates similar behavior compared to the initially discussed Ir-based catalysts.Besides aqueous mineral acids, solid acids can have a similar stabilizing effect as shown by Nakagawa et\u00a0al. [58]. Both an ion exchange resin (Amberlyst 70) and different zeolites were studied. Re leaching was only investigated for an H-ZSM-5 co-catalyst in combination with Ir-ReOx/SiO2. The amount of Re leaching (0.4%) was considerably lower than without co-catalyst (2%) but higher than with H2SO4 (<0.3%). Most notably, pH was also not as low in the presence of the zeolite (4.4) compared with H2SO4 (2.8, compared to 5.7 without co-catalyst). It remains ambiguous to what extent the H-ZSM-5 co-catalyst also mitigates the deactivation of the Re-based catalyst during recycling experiments. When calcination treatments were applied between the runs, both with and without the zeolite catalytic HL activity declined by ca. 12% over 3 runs. However, the presence of the zeolite enhances activity by ca. 75% compared to only Ir-ReOx/SiO2. It is noteworthy that the ion exchange resin had an even stronger promoting effect but its influence on the catalyst deactivation was not analyzed in detail since the catalyst recovery procedure required calcination. It was suggested that the addition of solid acids results in enhanced amounts of Re-OH on the supported Ir-ReOx catalyst [58,128], similar to H2SO4.Overall, the stabilizing effect of acids is remarkable but it is still not understood. Moreover, it is unclear, whether it is limited to the specific, bimetallic catalysts and/or the HL reaction. One speculative explanation could be that Re leaching is suppressed by the protonation of ReOx species due to alterations of the redox properties, possibly facilitating Re reduction or stabilizing (partially) reduced oxidation states or due to changes in surface charge. Those were found to play a role, e.g. during reductive Re deposition [228,230]. In general, the pH of the reaction medium can be an important factor impacting metal leaching from solid catalysts [10], but its effect has not yet been systematically studied for Re-based catalysts.The vast majority of Re-catalyzed reactions in the context of liquid-phase biomass upgrading are conducted as batch experiments, as Table\u00a01 and Table\u00a02 show. However, a few examples of mainly APR and HL reactions have also been conducted under continuous flow operation. In principle, continuous operation allows for additional insight into the time-dependent leaching behavior, similar to the observation of catalyst deactivation over time-on-stream that can be followed without additional effort and over comparatively long periods of time. Tracking Re leaching continuously may, however, be analytically challenging in case of low amounts of metal leaching and high liquid flow rates due to low concentrations. To the best of our knowledge, time-dependent Re leaching has not yet been studied even though it could be a promising tool to understanding Re leaching by revealing kinetic information of the processes. In particular, it could reveal whether leaching takes place continuously or only during specific phases of the reaction. While in some cases catalytic activity stabilizes overtime-on-stream [51,101], this is not the case for others [53,83,103]. Here, the data on the corresponding leaching behavior would provide further insights into how and Re leaching contributes to catalyst deactivation. However, in current literature on continuous, heterogeneously catalyzed reactions the investigation of Re leaching typically relies on elemental analysis of the catalyst after the reaction, from which only the overall amount of leached metal is available.Only one study with supported Re catalysts investigated differences between batch and continuous operation mode. Chia et\u00a0al. [53] compared the stability of a RhRe/C catalyst for the aqueous-phase HL of 2-(hydroxymethyl)tetrahydropyran for both operation modes. It was observed that over 100\u00a0h time-on-stream under continuous operation at 120\u00a0\u00b0C, the catalyst strongly and continuously deactivated to ca. 60% of its initial activity, which was explained by severe Re leaching. In contrast, in a batch experiment (4\u00a0h) under comparable conditions, only 1.2% of Re were leached from the catalyst (same pretreatment conditions). It was, therefore, suggested that leaching is enhanced under continuous flow operation. It remains unclear, however, whether this is merely an effect of the longer exposure time or an inherent effect of the operation mode. Moreover, there was no check for other types of deactivation. On the other hand, the fact that differently pretreated catalysts showed no detectable Re leaching in the batch experiments also did not deactivate under continuous flow can be seen as further evidence that the flow operation can exacerbate the leaching process.In cases where leaching is governed by the solubility of the Re species or by the concentration of a complexing agent, an equilibrium is reached that limits the total leaching amount during batch reactions. In contrast to that, under continuous flow the leached species is constantly removed from the reactor and leaching can continue. This is particular problematic in cases where Re species are leached at an early stage but can later redeposit on the support material [76,78,81,157]. The same phenomenon is known, e.g. in the better known case of the Pd-catalyzed Heck reaction [217,231,232], and is a critical challenge to the viability of continuous processes.In the previous sections, it was shown how different individual catalyst preparation and pretreatment conditions as well as process parameters can influence Re leaching and thereby cause catalyst deactivation. A common aspect of many of them is that they exert their influence via redox reactions. This is mainly due to the fact that the oxidation state of Re plays a crucial role for many of them due to the significantly higher (water) solubility of high-valent Re species (in particular Re7+) compared to low-valent or metallic Re [215,233].Most prominently, exposing Re-based catalysts to reducing or oxidizing conditions was shown to have often pronounced effects on Re leaching and catalyst stability. Reductive pretreatment of solid Re catalysts was shown to mitigate Re leaching and initially leached Re species were found in some cases to redeposit under reducing reaction conditions. On the other hand, exposure of catalysts to air in several cases leads to enhanced Re leaching. Besides, there are indications that e.g. the presence of an additional noble metal on bimetallic Re catalysts can stabilize deposited Re species by catalyzing their reduction.For a number of Re catalysts included in this study, the initial average Re oxidation state is known or can be estimated from the characterization data provided. In Fig.\u00a017\n the respective amount of Re leaching is plotted against initial average oxidation state. As expected, Re leaching appears to be considerably more likely when Re is predominantly in a high oxidation state, in particular\u00a0+7. At the same time, there is an indication that Re leaching is also more pronounced under non-reducing, i.e.\u00a0N2 or air, atmosphere whereas H2 appears to have a mitigating effect. However, it should be noted that several other factors, e.g. support-metal interactions, metal-metal interactions, structure of the catalyst, reaction conditions, and solvent, can significantly influence Re leaching as well. Thus, Fig.\u00a017 cannot be as conclusive as a dedicate study into these effects. A good example for this is the huge variation in Re leaching found in the study by Sharkey and Jentoft [76] even though the Re oxidation states of the catalysts is given, the Re precursor and the calcination pretreatment very likely comparable around\u00a0+7.Overall, however, it is apparent that Re oxidation state is arguably the most important material property governing Re leaching. Therefore, controlling the Re oxidation state and preventing the formation of more leachable, high-valent Re species is an important parameter in mitigating the deactivation of solid Re catalysts in liquid media by Re leaching. It is noteworthy that the Re oxidation state was also suggested to be the crucial property in governing the catalytic activity and selectivity of Re-containing catalysts [26]. Another conclusion from the correlations in Fig.\u00a017 is that due to the typically applied reductive catalyst pretreatment and the reducing atmosphere in HG, HDO, HL, and also APR processes, solid Re catalysts applied in these processes are far less subject to extensive Re leaching than in the particularly challenging DODH reaction. Importantly, in the latter reaction type, high-valent Re species are necessary to obtain catalytically active Re materials [26], and it should be highlighted again that the change in oxidation state itself is a type of catalyst deactivation that can significantly influence catalytic activity and selectivity (see also Section 2.7).One of the key insights from the systematic analysis of all available studies on Re leaching during biomass-related liquid-phase reactions it that both leaching and redeposition can happen at various stages of the reaction. Moreover, also typically neglected phases during the lifetime of supported Re catalysts, such as the transfer between pretreatment and reactor set-up or heat-up stages during the reaction, can have pronounced effects on leaching behavior. Therefore, a summarizing overview of possible leaching and redeposition effects and different influencing factors throughout the catalyst lifetime is provided in Fig.\u00a018\n.The initial steps of catalyst preparation and ex-situ pretreatment steps, e.g.\u00a0calcination or reduction, are highly relevant in regards to Re leaching since they determine the initial physical and chemical properties of the catalyst, in particular the initial oxidation state of the catalyst. Similarly, catalyst regeneration procedures can be used to regenerate the initial Re oxidation state. Even though liquid-phase Re leaching is not directly occurring during the catalyst preparation, pretreatment or regeneration, two related effects have are taking place. Many typical impregnation and deposition procedures have similarities to Re leaching and deposition processes during reactions since they are governed by the same parameters, e.g. the solubility of Re species and redox reactions. A detailed look into the preparation processes, however, is beyond the scope of this article and an overview of different preparation techniques can be found, e.g.\u00a0in the study by Gothe et\u00a0al. [2]. Second, during calcination pretreatment of Re-based catalysts, Re can be lost via sublimation of volatile Re7+ oxides. In some cases [139,197], this has led to significant decrease in Re loadings before the catalyst was even applied in the liquid-phase reaction. Re leaching observed during the reaction phase depends strongly on the initial Re oxidation state (Section 3.4.), which is a result of all preparative and pretreatment steps. Moreover, it has been shown that also the transfer process between catalyst pre- or regeneration treatment can cause significant leaching when this leads to exposure of a reduced catalyst to air due to the facile oxidation of Re species (Section 3.2.2.). Consequently, these processes need to be considered to ensure that Re leaching is avoided as best as possible during the reactions itself. Particular attention should also be paid to filtration or washing steps in between subsequent catalytic reuse experiments. Cases have been reported where conducting these processes under air atmosphere resulted in severe leaching and the reused catalyst contained significantly lower amounts of Re [137].As soon the fresh or recycled catalyst is exposed to the reaction medium, Re leaching can occur. However, it needs to be considered that during different stages of the reaction the conditions change, which can lead to a variety of possible leaching and redeposition processes. Depending on the catalyst, Re leaching may occur directly when the catalyst is exposed to the reaction solvent even at room temperature, as was shown for the case of a ReOx/TiO2 catalyst [76]. In this case, Re is leached before the catalytic reaction is beginning to taking place. In case of Pd-ReOx/TiO2 catalysts investigated by Ly et\u00a0al. [157], initially up to 100% of Re was leached into the aqueous reaction medium during the heat-up phase under inert atmosphere. However, nearly all of it was redeposited via reductive deposition as soon as the HG reaction was started by pressurizing the reactor with H2, and throughout the following reaction time, the concentration of leached Re species in the reaction medium remained low. Therefore, in this particular procedure, Re leaching is only problematic in the initial heat-up phase. On the other hand, in other studies, a reducing atmosphere of H2 is applied from the very beginning of the reaction to allow for in situ reduction of Re species and to prevent Re leaching [74,119,131,139,140].To understand the mechanism of the processes involved in Re leaching, it is required to understand the time-dependent behavior. In the previously described examples, it can be typically assumed that leaching or redeposition happen very fast, even though evidence is rarely provided, e.g.\u00a0for the reductive (re-)deposition of Re on Pd/TiO2 as soon as H2 is applied [157]. There is, however, also the possibility of gradual leaching over reaction time, which may also be related to a slow oxidation of the catalyst. Due to the lack of time-dependent leaching data in the available literature, no such case has been identified so far. Especially, under continuous flow operation when catalytic activity was found to decline gradually this could be a reasonable explanation (Section 3.3.6.).A very interesting conversion-dependent Re leaching behavior has been observed during DODH reactions [76]. As discussed in detail in Section 3.3.4, the complexation of Re species with the diol reactant results in pronounced Re leaching. As the diol concentration decreases toward\u00a0complete conversion, Re is found to redeposit on the support material. In this example, the understanding of the leaching mechanism and the corresponding conversion- and time-dependent leaching behavior is crucial to mitigating the loss of Re during recycling experiments.Finally, redeposition of leached Re species after the main reaction phase can also be a results of cooling down the reactor [78,81]. While Re leaching was considerably during the reaction in these studies, the recovered catalysts could be reused without significant loss in Re content as well as with comparable catalytic activity due to the nearly complete redeposition. In general, it could be worthwhile to consider deliberate aftertreatment procedures that promote the deposition of Re species after the actual reaction period is completed. This could significantly enhance catalyst recyclability if Re leaching cannot be avoided.Overall, Re leaching can occur at many stages during the lifetime of a catalyst, inside and outside of the reactor. Moreover, the conditions the catalyst is exposed to at any stage can have, in certain cases severe, effects on catalyst stability. Knowing the underlying processes and relevant parameters governing Re leaching is crucial in avoiding or at least mitigating Re leaching.Investigations into catalyst stability and metal leaching are typically driven by the initial observation of catalyst deactivation over time-on-stream or in recycling experiments. These experiments reveal whether or not a catalyst is stable, however, alone they do not allow to distinguish which types of deactivation are present (as outlined in Chapter 2.1). Consequently, the observation of catalyst deactivation will not reveal directly to what degree leaching may play a role in causing the decrease in catalytic performance. On the other hand, the information is crucial for assessing the consequences of Re leaching when it occurs, in particular when the contribution of other deactivation processes is known. In this context, it should be mentioned that while many other types of deactivation are reversible and the catalyst can (partially) be regenerated, this is often not the case for Re leaching.In addition to experiments revealing catalyst deactivation, other approaches are required to directly identify and quantify Re leaching (Fig.\u00a019\n). Elemental analysis of both the recovered catalyst and the reaction medium after the catalytic experiment allow for a quantitative assessment of the amount of Re leached from the solid catalyst to the liquid. Techniques that are commonly applied are primarily atomic emission spectroscopy or mass spectrometry with inductively coupled plasma (ICP-AES, ICP-MS) but also atomic absorption spectroscopy\u00a0or X-ray fluorescence spectroscopy\u00a0are used. The detection of the, in many cases very low, concentrations of leached metal as well as the precise determination of small differences in metal loading on a supported catalyst before and after the catalyst is challenging. Depending on the detection limit of the respective technique, the reliable detection of Re leaching is only possible to a certain limit, which can be in the range of several percent in case of supported catalysts as highlighted by Sadaba et\u00a0al. [10]. Nevertheless, only elemental analysis can unambiguously and quantitatively proof Re leaching.To detect metal leaching, probably the most important aspect to consider is the time in the catalyst lifecycle at which the analysis is conducted and the corresponding conditions the catalyst is exposed to. As discussed in Section 3.5, Re leaching can differ widely during different stages of the lifetime. The typically applied procedure of separating catalyst and reaction ex situ after the reaction gives valuable insight whether irreversible leaching of Re from the catalyst is occurring. However, depending on the conditions of the separation process it is possible that the Re leaching detected by this procedure is not happening during the reaction itself but, e.g.\u00a0after subsequent contact with air [137]. This can be avoided by preventing direct exposure of the catalyst to oxidizing atmosphere to enable an assessment of how much Re is lost during the catalytic application.Besides, the possible redeposition of Re species needs to be considered that can occur at the end of the reaction either due decreasing concentration of a reactant using as a complexing agent [76] or due to the lower solubility of Re species at lower temperatures [78,81]. While this does not result in an overall loss of Re metal, revealing Re leaching during the reaction is crucial to understanding the behavior of the catalyst and the mechanism of the reaction. The only possibility to reveal this hidden leaching is by separating the catalyst from the reaction medium under reaction conditions. Similar to sampling during the course of a batch reaction to gain information on the reaction kinetics, analyzing immediately separated reaction medium at different stages can reveal the time-dependent leaching behavior.Uncovering the presence of leached Re species is particularly important in revealing whether homogeneous catalysis contributes the observed overall catalytic activity. While a lower concentration of leached Re in reuse experiments in combination with decreased catalytic activity can be an indication that there is a contribution of dissolved Re species to the catalytic activity, this can and should be tested directly. This is possible by separating catalyst and reaction medium at an intermediate stage and observing the progress of the reaction of the liquid reaction medium alone under reaction conditions. Following Sheldon et\u00a0al. [234], three cases can be distinguished. In the cases where either no Re leaching occurs (1) or the leached Re species are not catalytically active (2), no further reaction progress can be observed (or only conversion comparable to the control experiment without catalyst). On the other hand, when the catalytic reaction still occurs after complete separation of the solid catalyst, this proves the catalytic activity of leached metal species (3) and allows for a quantitative assessment of the homogeneous contribution to the overall reaction. The observation of catalytic activity of homogeneous species also serves as an indirect proof of Re leaching.Due to the previously discussed problem of possible redeposition, it is crucial to separate the catalyst from the reaction medium under conditions as close to the actual reaction conditions as possible. In particular, due to the lower solubility of Re species at lower temperature, this requires separation at reaction temperature, which is typically referred to as \u2018hot filtration\u2019. Therefore, \u2018hot filtration test\u2019 is a common phrase for this type of experiment to detect possible homogeneous catalytic contributions to a process that is supposed to or thought to be heterogeneously catalyzed. As indicated in Table\u00a02, in a small number of studies, hot filtration tests were performed; in a few additional ones, separation was conducted at room temperature before heating up for the activity test of homogeneous species. Hot filtration tests have comparatively often been used in studies on the DODH reaction, probably due to increased awareness of catalytic activity of leached Re species given the origin of the reaction being a homogeneously catalyzed process. Moreover, the hot filtration tests have impressively shown that in some cases almost all catalytic activity originates from homogenous catalysis [76,78].Overall, the phenomena of Re leaching, the deactivation of solid catalysts and catalytic activity of dissolved species are interlinked, however, each should be verified directly since the observation of one does not necessarily allow conclusions on the others. E.g.\u00a0when the hot filtration test indicates the absence of catalytically active dissolved species and the solid catalyst does not show signs of deactivation, this does not mean that no leaching is occurring. Still, non-active Re species can be leached that are also not active as homogeneous catalysts, as found in the study by Wei et\u00a0al. [50]. Another example is reported in the study by Li et\u00a0al. [141], where severe Re leaching did not affect catalyst recyclability. Therefore, Re leaching should always be confirmed by elemental analysis.Finally, additional analytical methods can be applied to gain additional insight into the nature of the leached Re compounds. As discussed in Section 3.3.3, this is particularly interesting when a complexing agent is involved in the leaching mechanism. Leached Re species were successfully detected and characterized using UV\u2013Vis spectroscopy [76,172], an example is shown in Fig.\u00a015. Moreover, NMR [172] and EPR [76] spectroscopy can be applied to gain further information on the chemical composition and the structure of the leached Re complex. In these cases, identification of the leached species was also crucially important to reveal information on the reaction mechanism.In the previous sections, Re leaching was primarily viewed as an undesired process resulting in catalyst deactivation and/or loss of precious Re metal. There are, however, also opportunities arising from Re leaching when it can be applied in a controlled manner. One such strategy relying on Re leaching is the in-situ preparation of Re-containing catalysts by reductive deposition. Ly et\u00a0al. [157] observed that in the initial heat-up phase of the catalytic experiment under inert atmosphere >50% of Re was leached from their Pd-ReOx/TiO2 catalyst before it was redeposited upon the addition of H2. Instead of trying to prevent leaching from occurring, an in-situ deposition method was developed based on reductive deposition. This method relies on the metallic Pd sites of the catalyst, where the reduction and deposition of dissolved high-valent Re species occurs, and high H2 pressure (150\u00a0bar) at elevated temperature (160\u00a0\u00b0C). The in-situ prepared bimetallic catalysts were suitable for the HG of succinic acid and comparison with a catalyst prepared by conventional (ex-situ) reductive deposition revealed only slightly lower selectivity for the desired product 1,4-butanediol (18% compared to 23%). Unfortunately the influence on catalytic activity was not reported. Overall, the study showed that the catalyst preparation steps related to Re introduction can be omitted and a more efficient in-situ method can be applied.Re leaching was also utilized for a different type of catalyst system consisting of a physical mixture of two different solids by Tomishige et\u00a0al. [170,171]. In these investigations, Re-based catalysts were developed for the catalytic conversion of 1,4-anhydroerythritol to 1,4-butanediol including an initial DODH reaction as well as HG and ether hydrolysis steps. In the initial heat-up phase of the reaction conducted in aqueous phase under H2 atmosphere, partial Re leaching from one material and redeposition on the second solid occurs. In the simplest case of ReOx/C and CeO2, the migration of Re species from the carbon to the CeO2 support was observed, as shown in Fig.\u00a020\n. The process results in the formation of high-valent ReOx species on CeO2, which are considered the main active site for the DODH reaction. Importantly, two separate processes are occurring in parallel. On the one hand, high-valent Re species are leached from the carbon support and redeposited on the CeO2, where they are considerably more stable against being leached again (see also Section 3.2.1.). On the other hand, the reducing reaction conditions result in the reduction of Re species to low-valent or metallic species, which are considerably less soluble than high-valent Re. Therefore, reduction mitigates the migration of Re species from the carbon support. Since the Re species on CeO2 are not as easy to reduce as the ones on carbon, they remain in a high-valent oxidation state required for DODH. Overall, this is an example for an innovative in-situ preparation method utilizing Re migration between two materials via leaching. It was also shown that the overall migration process depends on the redox properties of the catalysts and promoting ReOx reduction, which can be achieved in the presence of noble metals, mitigates the amount of Re leached from the carbon support. It should, however, be noted that these catalyst systems were not fully recyclable and, in particular, fresh ReOx/C had to be supplied.Besides the potential benefits of the purposeful in-situ catalyst preparation methods shown above, similar Re leaching-related processes might be occurring during the heat-up or the reaction phase of catalytic experiments of many other studies without being detected. As a consequence, the chemical and/or structural composition of the catalyst can be significantly altered from the initial ex-situ state of the catalyst. This highlights the importance of in-situ characterization methods, such as in-situ liquid phase XANES [212], to characterize the properties of the catalyst as it exists under reaction conditions.A second possibility to employ Re leaching in a controlled and advantageous manner has been suggested as \u2018release and catch\u2019 catalysis by Sharkey and Jentoft [76] for the case of Re-catalyzed DODH. The concept is based on the dependence of Re leaching on the concentration of the diol reactant, which decreases during the course of the reaction (details in Section 2.3.3.). While initially large amounts of Re are leached from supported Re catalysts, it later redeposits to a large degree when the diol is completely consumed. Thus, while the reaction is occurring Re species are dissolved and act as homogeneous catalysts, which were found to significantly contribute the overall catalytic DODH activity. Afterward, the solid catalyst with the redeposited Re species can be separated and reused. It should be noted, however, that in the example discussed here catalytic activity declined in recycling experiments, and it was suspected that unreactive Re species can be formed. Since the leaching of Re from DODH catalysts is very difficult to prevent due to a combination of unfavorable conditions, \u2018release and catch\u2019 concepts could be an interesting alternative, in particular when considering that often stability toward\u00a0Re leaching comes at the cost of lower catalytic activity [76,77]. Thus, the concept allows for combining the advantages of homogeneous (high catalytic activity) with heterogeneous catalysis (facile recyclability). It is also worth mentioning that \u2018release and catch\u2019 concepts have also been employed for other catalyst systems [217].Due to its versatile chemical nature, Re has been used as a catalytically active metal in many different applications, in recent years also increasingly in the context of biomass utilization. While much attention is paid to the activity and selectivity of catalysts, catalyst stability is often neglected. The overview of stability data accumulated in this article on solid Re-containing catalysts applied in liquid-phase reactions in the context of biomass valorization reveals that several different types of catalyst deactivation need to be considered. While many severe causes of catalyst deactivation, e.g.\u00a0fouling due to carbon deposition or catalyst poisoning, can be to a large degree reversible by suitable regeneration procedures such as calcination and/or reduction, Re leaching is particularly undesirably due to the irreversible loss of this precious and rare element. The plethora of studies reporting severe deactivation by Re leaching, especially compared to noble metals, indicates that its oxophilic nature makes Re particularly susceptible.A thorough look at possible influencing factors revealed that the oxidation state of Re species on the catalyst plays arguably the main role in governing leaching behavior since high-valent Re species are considerably more prone to leaching than low-valent or metallic Re. While this property is related to the composition of the catalyst (support material, promoters, bimetallic catalyst), considerable attention needs to be paid to the conditions the Re-containing catalyst is exposed to. Even short contact of a reduced Re catalyst with air can result in partial catalyst oxidation, and also during the catalytic reaction the oxidation state of Re is often changed, which can then result in severe leaching. One of the primary conclusion is, therefore, that the Re oxidation state needs to be controlled at all times to prevent Re leaching. In this context, it is important to consider that Re leaching can not only occur during the actual phase of the reaction but throughout the lifetime of the catalyst and many factors can have significant impacts on the catalyst stability.Of particular importance is the gas atmosphere of the reaction and reducing conditions in the presence of H2 are arguably the most effective way to mitigate Re leaching. This is also the reason why there is a large discrepancy between different reaction types and Re leaching occurring under the corresponding reaction conditions. Most reactions in the context of biomass valorization involve H2, which mitigates Re leaching even though they are often conducted in aqueous-phase and Re species are considerably more soluble in water than in less polar solvents. Moreover, often bimetallic catalysts are used and a noble metals can provide H2 to promote the reduction of Re species and stabilize low oxidation states. On the other hand, DODH\u00a0reactions require high-valent Re species and are conducted under inert or oxidizing atmosphere relying on organic molecules as reducing agents. While this make DODH catalysts exceptionally susceptible to Re leaching, there is also the phenomenon of chelate complex formation between Re species and the reactant diols to be considered which was found to considerably enhance Re leaching. This explains the drastic examples of catalyst deactivation for this particular type of reaction.The particular requirements of different reactions regarding the chemical and physical properties of the catalyst as well as the reaction conditions makes the development of strategies to mitigate Re leaching particularly challenging. In general, different strategies to improve catalyst stability have been reported [10,96,235]. For the case of DODH reactions, a promising strategy could be the combination with an additional HG step to obtain saturated products. Studies on this combined process indicate that Re leaching is considerably less pronounced than in pure DODH processes, probably due to the combination of different factors.While the dark side of Re leaching, i.e. the role it plays in catalyst deactivation, is often in the focus of reports on Re leaching, it can also be applied in a controlled fashion to one's benefit. First, Re leaching and redeposition can be used for in-situ preparations as well as the modifications of catalyst systems. Second, the concept of \u2018release and catch\u2019 catalysis utilizes that in some cases, the superior catalytic activity of dissolved Re species acting as homogeneous catalysts and combines it with the recyclability of a solid catalyst. The crucial aspect is the controlled deposition of initially leached Re species after the completion of the reaction.Whether or not Re leaching is desired, its detection is crucial to fully understanding the catalytic behavior including the deactivation processes of a solid catalyst. Due to the complexity of Re leaching and, in particular, its occurrence at different stages of the catalyst lifetime as well as the possibility of Re deposition, this is not trivial. While elemental analysis of the reaction medium, if possible at different stages, unambiguously and quantitatively detects Re leaching, care must be taken to ensure that no dissolved Re can be redeposited before solid catalyst and reaction medium are separated, e.g.\u00a0by filtration under reaction conditions. An indirect way to detect Re reaching is through the catalytic activity of leached species, which is preferably conducted as a hot filtration test. However, in cases of catalytically non-active or predominant homogeneous catalytic activity, the extent of Re leaching can be over- or underestimated. In general, the phenomenon of Re leaching and its effects on catalytic activity can be interlinked in different ways, and, therefore, both aspects should be investigated independently. Even though not commonly applied in conventional studies, there is considerable benefit in experimentally tracking the time-dependent leaching behavior to gain insight into the mechanism and kinetics of Re leaching. Moreover, additional methods like, e.g.\u00a0UV\u2013Vis or NMR spectroscopy, are invaluable to identify the chemical nature of leached Re species. These techniques resulted in the discovery of the crucial role of diols as chelating agents that promote Re leaching during DODH reactions in non-polar solvent.Overall, the phenomenon of Re leaching during liquid-phase biomass valorization reactions is widespread and often complex. Given the importance of catalyst stability for industrial applications, in particular since the rare element Re is involved, it is unfortunate that little attention is paid to this side of catalyst behavior in many studies. Understanding the leaching behavior of Re (and metal leaching in general) is a necessity to be able to design stable catalysts and sustainable processes.Finally, it should be noted that the findings in this article are not limited to Re but of similar relevance also for similar types of catalysts. The deactivation mechanisms outlined in Chapter 2 are of general applicability, even though the relevance of each mechanism may vary from case to case. Similarly, the lessons learned from understanding Re leaching are transferable to metal leaching from other supported metal catalysts. Nevertheless, Re is an outstanding example due to its versatile and sensitive redox behavior, the role of redox reactions in governing catalyst deactivation is particularly prominent and the leaching phenomena considerably more pronounced than e.g. for most noble-metal catalysts.Florian M. Harth: Writing - Original Draft, Investigation, Visualization, Conceptualization; Bla\u017e Likozar: Conceptualization, Project administration, Miha Grilc: Supervision, Writing - Reviewing and Editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the Slovenian Research Agency (basic J1-3020 research project and research core funding P2\u20130152). The work was partially carried out within the RDI project Cel. Cycle: \u00bbPotential of biomass for development of advanced materials and bio-based products\u00ab, which is co-financed by the Republic of Slovenia, Ministry of Education, Science and Sport of the Republic of Slovenia, and the European Union through the European Regional Development Fund, 2016\u20132020. The authors acknowledge Dr. Brigita Ho\u010devar and Dr. Vili Resnik, pioneers on Re-catalyzed valorization of aldaric acid, for their kind support. The authors also acknowledge Mr. Ja\u0161a Bukovec for preparing the graphical abstract.", "descript": "\n                  Rhenium is a versatile element and increasingly used in solid catalysts for the conversion of biomass, where it can fulfill different roles in providing or improving catalytic activity. On the other hand, this also makes Re-based catalysts susceptible to various types of catalyst deactivation. Deactivation mechanisms, detection methods, and coping strategies are discussed for each type of deactivation using the collected literature on Re-containing catalysts for biomass utilization in liquid phase. Particular focus is placed on the correlation between catalyst deactivation and Re leaching, a commonly observed problem when using Re-containing catalyst in liquid-phase reactions that can lead to severe and irreversible loss of catalytic activity. Material properties, reaction conditions, and other factors influencing Re leaching are systematically assessed and leaching mechanisms discussed, which also opens possibilities how the problem can be mitigated. In particular, the role of the Re oxidation state is identified as a key material property influencing Re leaching and a variety of processes and parameters that alter the Re oxidation state during the lifetime of the catalyst are analyzed. Moreover, insights into the intricate procedure of detecting Re leaching and identifying the correlation between leaching and catalyst deactivation are presented. Finally, strategies to purposefully use Re leaching are shown.\n               "}
{"full_text": "Data will be made available on request.Oxygen evolution reaction (OER) is an enabling step for most of the key electrochemical applications such as hydrogen production, CO2 electrolysis, and energy storage [1]. The interface between catalyst and electrolyte (or membrane) is at the heart of the OER catalysis, and plays a pivotal role in determining the activity, selectivity, and stability of the electrochemical systems [2]. However, these catalysts are usually unstable under oxidation potentials, and are subject to dynamic surface restructuring during the reactions [3,4]. This operando surface restructuring process has a profound impact on the overall reaction system [5\u20137].Recent studies are sought to take advantage of such surface reconstruction phenomena to boost the OER activity by tuning the compositional chemistry of metal oxides [8]. The most common strategy is to incorporate electrolyte-soluble ions into the metal oxide lattice, such as alkali (e.g., Li+) [9], alkaline-earth (e.g., Sr2+ and Ba2+) [10,11], Al3+ cations [12], and halide anions (e.g., Cl\u2212) [13]. Under oxidation potentials, the dissolution of these cations or anions from the catalysts causes significant surface reconstruction to form an OER-active surface layer consists of catalytically-active phases (e.g., cobalt oxyhydroxide [3] in cobalt oxides) and lattice vacancies [14]. However, there are also reports raising concerns of the cation dissolution and anode restructuring that could also cause rapid cell failure in the application of CO2 electrolysis [15], where the local reaction environment (e.g., (bi)carbonate cross-over from the cathode to the anolyte) is drastically different from water electrolysis. This discrepancy originates from the negligence of the electrolyte\u2019s role in determining the overall reaction reactivity and stability [16]. The catalytic process and surface restructuring process should depend both on the catalyst properties and potentials and their local reaction environment, such as electrolyte compositions and local pH, which remain poorly understood by far [17].Hence, this work seeks to study the roles of electrolyte ions and catalyst compositions in the catalyst ion leaching and surface restructuring process during OER catalysis in a non-acidic medium, mostly in a pH-neutral electrolyte. We chose SrCoO3 perovskite (SC) as the model anode catalyst and phosphate buffer solutions (PBS) as the main model electrolytes. The perovskites are emerging cost-effective alternatives to precious metals for the OER catalysis [18], while the PBS-electrolytes with a neutral pH have the potential to minimize the potential corrosions in the electrolyzers [19,20]. We chose PBS as the model electrolyte mainly because of the phosphate anions are different from hydroxide ions [21,22] and allow us to explore the impacts of anions on the catalyst surface restructuring and reactivity. Our experimental results confirm that the catalyst surface restructuring process during OER in varied electrolytes takes place involving ion leaching, electrolyte cation backfilling and anion incorporation. This restructuring process is closely related to the solubility of the cations of the metal oxides, the sizes of the ions of catalyst and electrolyte, and buffering capacity of the electrolyte anions. Consequently, an amorphous surface shell structure can be formed covering the Sr-containing perovskite core in the presence of Na+-PBS electrolytes after anodic conditioning. The shell structure contains more oxygen vacancies that strengthen binding with oxygen intermediates and phosphate ions that promote proton transfer, so as to exhibit significantly enhanced OER activity in pH-neutral electrolyte and in alkaline medium.The solid-state preparation method was applied to synthesize the catalysts, including SrCoO3-\u03b4 (SC), BaCoO3-\u03b4 (BC), LaCoO3-\u03b4 (BC), La0.5Sr0.5Co0.8Fe0.2O3-\u03b4 (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3-\u03b4 (LSCF), SrNb0.1Ta0.1Co0.8O3-\u03b4 (SNTC) and SrSc0.175Nb0.025Co0.8O3-\u03b4 (SSNC). Stoichiometric mixtures of Co3O4 (Aldrich, \u226599.5\u00a0%), SrCO3 (Aldrich, \u226599.9\u00a0%), BaCoO3 (Aldrich, \u226599.98\u00a0%), La2O3 (Aldrich, \u226599.9\u00a0%), Nb2O5 (Aldrich, \u226599.99\u00a0%) and Ta2O5 (Alfa Aesar, \u226599.0\u00a0%) were weighed and ball-milled at 260\u00a0rpm for 20\u00a0h. Then the samples were dry-pressed in a die under 90\u00a0MPa and sintered at 1200\u00a0\u00b0C for 20\u00a0h. Finally, the sintered tablets were crushed into powders through ball-milling at 350\u00a0rpm for 8\u00a0h. As the SC powders were prepared via solid-state method and ground through ball-milling, their sizes may vary widely. Thus, the as-prepared SC powders were dispersed in ethanol via ultrasonication for 1\u00a0h, and centrifuged at the rotation speed of 1000\u00a0rpm. The supernatant was collected and dried under vacuum overnight to obtain the SC-origin with monodispersed particle size.La0.6Sr0.4CoO3-\u03b4 (LSC) and La0.6Sr0.4MnO3-\u03b4 (LSM) were purchased from Fuel Cell Materials, whose particles size is 0.4\u20130.8\u00a0\u03bcm and 0.4\u20131.0\u00a0\u03bcm, respectively.To prepare the 1.0\u00a0M sodium phosphate buffer solution (Na+-PBS), 15.6\u00a0g sodium phosphate monobasic dehydrate (NaH2PO4\u00b72H2O, Aldrich, \u226599.0\u00a0%) was dissolved in 100.0\u00a0mL deionized water, and denoted as solution A. Meanwhile, 28.4\u00a0g sodium phosphate dibasic heptahydrate (Na2HPO4\u00b77H2O, Aldrich, \u226598.0\u00a0%) was dissolved in 200.0\u00a0mL deionized water, and denoted as solution B. Then 97.5\u00a0mL of solution A was mixed with 152.5\u00a0mL of solution B to obtain the 250\u00a0mL of Na+-PBS. Its pH was tested to be 6.65.To prepare the 1.0\u00a0M potassium phosphate buffer solution (K+-PBS), 13.6\u00a0g potassium phosphate monobasic (KH2PO4, Aldrich, \u226599.0\u00a0%) was dissolved in 100.0\u00a0mL deionized water, and denoted as solution A. Meanwhile, 24.2\u00a0g potassium phosphate dibasic heptahydrate (K2HPO4\u00b73H2O, Aldrich, \u226598.0\u00a0%) was dissolved in 200.0\u00a0mL deionized water, and denoted as solution B. Then 97.5\u00a0mL of solution A was mixed with 152.5\u00a0mL of solution B to obtain the 250\u00a0mL of K+-PBS. Its pH was tested to be 6.62. To prepare the 1.0\u00a0M sodium sulfate solution (Na2SO4), 35.51\u00a0g anhydrous Na2SO4 was dissolved in 250\u00a0mL deionized water. Its pH was tested to be 7.18.10.0\u00a0mg active catalyst and 10.0\u00a0mg carbon black were dispersed in 1.0\u00a0mL ethanol with 100\u00a0\u00b5L 5\u00a0wt% Nafion solution through ultrasonication for 30\u00a0min. Then 400\u00a0\u00b5L of the obtained ink was loaded on the Ni foam (1\u00a0cm\u00a0\u00d7\u00a02\u00a0cm) to achieve the loading amount of 1.67\u00a0mg\u00a0cm\u22122, and dried under vacuum overnight. To carry out the catalyst reconstruction process, the continuous potentiometry V-t treatment was employed under a constant current density of 3.0\u00a0A\u00a0g\u22121 for 40000\u00a0s.The reconstructed catalysts were stripped from the Ni foam through ultrasonication with ethanol for 20\u00a0min and then dried under vacuum overnight for characterization. High-resolution transmission electron microscope (HR-TEM, Tecnai F20), with energy dispersive spectrometer (EDS) mapping details of Sr, Co, and O elements, were applied to study the morphologies of the materials at a voltage of 200\u00a0kV. The line scan spectra of as-prepared SC and reconstructed SC were collected on the HF5000 Cs-TEM at the accelerating voltage of 80\u00a0kV. X-ray diffraction (XRD) patterns (2\u03b8, 10\u201370\u00b0) were recorded on a Bruker D8-Advanced X-ray diffractometer with the nickel-filtered Cu-K\u03b1 radiation. The different electrodes were immersed in the various electrolytes, including Na+-PBS (1.0\u00a0M), K+-PBS (1.0\u00a0M), Na2CO3/NaHCO3 (1.0\u00a0M), and Na2SO4 (1.0\u00a0M), and the anodic conditioning was then conducted for a certain period. Subsequently, the electrolytes were collected and sent for ICP analyses. The concentrations of their ions were analyzed with a Varian Vista Pro ICP-OES instrument. Co K-edge XAS spectra of all samples were recorded on at BL14W1 station in Shanghai Synchrotron Radiation Facility (SSRF). The beam energy was 3.0\u00a0GeV and the maximum beam current was 400\u00a0mA. The FTIR spectra were obtained by a PerkinElmer Spectrum 100 FT-IR spectrometer.In 1.0\u00a0M Na+-PBS solution, high-resolution transmission electron micrographs (HR-TEM) of SC manifest that a surface restructuring process took place during the electrochemical anodic conditioning at 3.0\u00a0A/g for 10\u00a0min, 40\u00a0min, 6\u00a0h, and 12\u00a0h (Fig. 1\na\u20131e), respectively. The bulk core of SC could sustain its high crystallinity after conditioning treatment in neutral solution, which is confirmed by the distinguishable lattice fringes in HR-TEM images with a lattice spacing of 0.277\u00a0nm that corresponds to (0\u00a01\u00a01) lattice of cubic perovskite phase of SC [23,24]. The sustained structural integrity of SC is also confirmed by its X-ray diffraction (XRD) patterns before and after conditioning (Fig. S1) [25,26]. In contrast, the amorphous surface shell of the SC becomes thicker when the anodic conditioning duration increases, with an average thickness of the amorphous region increasing from 5.0\u00a0\u00b1\u00a00.3\u00a0nm for 10\u00a0min treatment to about 35.0\u00a0\u00b1\u00a01.0\u00a0nm for 12\u00a0h treatment. The thickness of the shell structure increases quickly at the first 6\u00a0h treatment and gradually slows down in the next 6\u00a0h (Fig. 1f). The slowing down of restructuring should be ascribed to the steric hindrance of the thick shell that prevents further penetration of electrolyte to the SC core when the shell structure is thick.Analysis of energy-dispersive X-ray spectroscopy (EDS) line scans over SC oxides before and after 12-h treatment suggests a notable decrease of Sr/Co atomic ratio but an increase of phosphorus (P) content near the surface shell (Fig. S2), meeting well with the X-ray photoelectron spectroscopy (XPS) data (Fig. S3). X-ray absorption spectroscopy (XAS) was applied to investigate the relatively thick shell structure to reveal the chemistry of the catalyst restructuring process. Cobalt K-edge of the X-ray absorption near edge structure (XANES) spectrum of the 12\u00a0h-treated SC oxide shifts to a higher energy by nearly 1.0\u00a0eV as compared to the original SC (Fig. 1g), indicating that the cobalt ions would be oxidized during treatment [22,27]. When fitting the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra against SrCoO3 model [28], we find that the coordination number is reduced by 4.0 for the first cobalt-strontium shell and by 0.5 for the first cobalt-oxygen shell in the SC lattice (Fig. 1h and Table S1). The reduced coordination number indicates loss of strontium atoms in the lattice after the anodic conditioning, which is consistent with the EDS line analysis result (Fig S2). We also observe the formation of cobalt-phosphate bonds in SC oxides after 12\u00a0h treatment from the k3\n-weighted EXAFS spectra particularly at k\u00a0=\u00a05.0\u00a0\u00c5\u22121, 8.5\u00a0\u00c5\u22121, and 11.5\u00a0\u00c5\u22121 (where k represents photoelectron wavenumber, Fig. 1i) [29]. The formation of second cobalt-oxygen shell and cobalt-phosphate shell as observed from the EXAFS analysis further confirm the incorporation of phosphate into the SC lattice from the electrolyte solution. Moreover, FTIR results reveal a stronger broad characteristic peak at 1230\u00a0cm\u22121 for the typical vibrational and bending modes of phosphate when the anodic treatment duration increases (Fig. S4) [20]. The amorphous surface shell should be the region where the restructuring takes place, involving cobalt oxidation, loss of strontium cations, and phosphate incorporation.The surface restructuring significantly improves the OER activity of SC in 1.0\u00a0M Na+-PBS. The linear sweep voltammograms presented in Fig. 2\na show that the mass-specific OER current densities increase with the conditioning durations. Specifically, the current densities at 1.75\u00a0V versus reversible hydrogen electrode (vs RHE) increased rapidly in the first 40\u00a0min treatment and then further to 31.4\u00a0A\u00a0g\u22121 after 12\u00a0h treatment (Fig. 2b and Fig. S5). From the TEM-EDX mapping images, the homogeneous distribution of the ions in SC-Re could be confirmed (Fig. S6). We further carried out the in situ EIS test to study the charge-transfer ability of SC during the anodic conditioning. From Fig. S7 it can be found that charge-transfer ability of SC can be improved with the treatment duration. The enhanced charge-transfer ability of SC-Re should also contribute to the highly improved OER activities of SC-Re. The 40000-s stability test over pristine SC catalyst also reveals that the required potential decreases from 1.764\u00a0V to 1.675\u00a0V vs RHE to drive a current density of 6.0\u00a0A\u00a0g\u22121, meaning that the OER activity of SC was greatly enhanced under anodic restructuring (Fig. 2c).Interestingly, we also studied the surface change of Co3O4, BaCoO3-\u03b4 (BC) and LaCoO3-\u03b4 (LC) catalysts during the anodic conditioning in 1.0\u00a0M Na+-PBS, and found that they experienced no significant surface restructuring (Figs. S8\u2013S10). Their TEM-EDX mapping images have also been provided (Figs. S11 and S12). Consequently, negligible OER improvements could be observed over Co3O4, LC, and BC catalysts after anodic conditioning treatment (Fig. 2d, 2e). Meanwhile, the SC exhibits no significant activity enhancement after being treated in 1.0\u00a0M\u00a0K+-PBS electrolyte (Fig. 2f), because the anodic conditioning in K+-PBS electrolyte could not initiate the surface restructuring process of SC (Fig. 2g and Fig. S13).Noticeably, the SC-Re exhibits a remarkably high activity towards OER in 1.0\u00a0M Na+-PBS, reaching a current density as high as 31.4\u00a0A\u00a0g\u22121 at 1.75\u00a0V vs RHE. This performance is 2.1 times higher than that of IrO2 (13.5\u00a0A\u00a0g\u22121), and 9.2 times that of IrO2 after 12\u00a0h anodic treatment under the same conditions (2.2\u00a0A\u00a0g\u22121) (Fig. S14). We also compared the performance (i.e., overpotentials at 5.0\u00a0A\u00a0g\u22121 and current densities at 1.75\u00a0V vs RHE) of our catalyst against the recently reported catalysts in neutral electrolytes, such as Ni0.1Co0.9P [30], RuIrCaOx\n[31], and 1-D CoHCF [32] (Fig. 2h and Table S2), and the OER activity of SC-Re is among the best-reported values. In addition, the performance of SC-Re in 1.0\u00a0M KOH is also comparable to the benchmark catalysts such as NiCo2S4 NW/NF [33] and CoSx/Ni3S2@NF (Table S3) [34]. Specifically, SC-Re could achieve the current density of 65.3\u00a0A/g at 1.65\u00a0V vs RHE, 3.0 times of SC-origin (21.7\u00a0A/g), and 1.5 times of IrO2 (43.0\u00a0A/g).To unveil the surface restructuring mechanism of SC, we compared the electrolyte compositions before and after anodic conditioning in 1.0\u00a0M Na+-PBS and K+-PBS electrolyte solutions at 3.0\u00a0A\u00a0g\u22121 for 12\u00a0h. The ICP-OES results reveal that there are significant amounts of Sr2+ ions in both electrolytes (i.e. 0.44\u00a0mM in used Na+-PBS and 0.37\u00a0mM in used K+-PBS) after the treatment (Fig. 3\na), and the Sr2+ ion concentration in 1.0\u00a0M Na+-PBS electrolyte is consistent with the loss amount of Sr in the SC surface as observed in EDS results (Fig. S2). The relatively high solubility of Sr in the PBS (solubility product (Ksp) of SrHPO4 is 1.072\u00a0\u00d7\u00a010\u22127) should be the main driving force for the Sr leaching to the electrolyte [35].More interestingly, it is found that the Na+ ion concentration in Na+-PBS electrolyte decreases significantly during anode conditioning, meaning that Na+ ions could be backfilled into the SC (Fig. 3b). In contrast, the loss of K+ ion is negligible in K+-PBS after the anodic conditioning (Fig. S15), but the concentration of Co species in used K+-PBS (ca. 0.054\u00a0mM Co) was clearly higher than that in Na+-PBS (ca. 0.018\u00a0mM Co) (Fig. 3c). It indicates that the Sr2+ leaching could lead to the slow decomposition of the reconstructed surface of SC in K+-PBS, exposing the unchanged inner SC crystal to the electrolyte. Therefore, the TEM image of SC after anodic conditioning in K+-PBS shows no amorphous outer layer (Fig. S13). Instead, the observed backfilling of Na+ in SC could help stabilize the reconstructed SC framework and restrain Co dissolution into the electrolyte and thus lead to the formation of an extended amorphous structure on the SC surface.We postulate that the easier backfilling of Na+ in SC than K+ should be attributed to the smaller ionic size of Na+. Compared to the host Sr2+ in SC with an ionic radius of 1.44\u00a0\u00c5, Na+ shows a relatively smaller radius (1.39\u00a0\u00c5) while K+ has an obviously larger ionic radius of 1.64\u00a0\u00c5 [36]. To better understand the roles of Na+ and K+ during the reconstruction of SC, we performed density functional theory (DFT) simulations to mimic the process of Na and K atoms passing a simple and respective channel caused by the leaching of Sr atom (Fig. 3d) [37,38]. We consider the passage of an atom through the [O4] neck structure from one Sr-vacancy to another as a 5-step process with six defined states (Fig. 3e): (0) reference state with the isolated atom and the supercell; (1) entering the Sr-vacancy; (2) approaching the [O4] neck structure; (3) locating at the center of the [O4] neck structure; (4) moving out of the ring; (5) entering another Sr-vacancy [39]. The DFT calculations show a substantial energy decrease when the Na atom enters the channel, indicating that the introduction of the Na atom is favorable for the stability of the reconstructed structure. At the same position, the Na atom is more likely to pass through the channel spontaneously, clearly different from the case with the K atom. Specifically, Fig. 3e shows positive relative energy for the K atom at State (3), which indicates that it is difficult for the K atom to pass through the [O4] neck structure. The high selectivity of Na+ over K+ can be also observed in the experimental measurements based on an artificial sodium-selective ionic device with sub-nanometer pores, which is attributed to the size effect and molecular recognition effect [40]. To gain insight into the influence of the Na atom on the Co-O bond, we further explored the electronic structures of a unit cell of SC as shown in Fig. 3f, and compared the initial cubic structure (Case 1) to the case when the Sr-vacancy (Case 2) is occupied by the Na atom (Case 3). Charge density differences of the [O4] neck structure for Case 1 and Case 3 are calculated by subtracting the charge density of Case 2 and corresponding atom from that of Case 1 and Case 2, respectively, as the sectional diagrams shown in Fig. 3g. We confirm that the stability of the Na-backfilled structure is much higher than that of the Sr-vacant structure, while the Co-O bond weakens (i.e., benefiting the formation of oxygen vacancies) when the Sr atom is replaced by the Na atom. The Na-backfilling is likely the reason for the formation of the amorphous shell at the SC core during the restructuring process. The weakened Co-O bond could also contribute to the observed reduced Co-O coordination number from the EXAFS results (Fig. 1i).We also studied the SC surface restructuring process at the same anodic conditioning treatment in other electrolytes with different anions such as 1.0\u00a0M Na2CO3/NaHCO3 and 1.0\u00a0M Na2SO4. We noticed that these anions failed to maintain a relatively neutral local pH close to the catalyst surface during 12\u00a0h anodic conditioning (which releases protons as product), and led to the significant dissolution of the SC catalyst and even nickel support into the electrolytes.We further studied the role of A-site cations in the surface restructuring processes over Co3O4 and BC catalysts during the anodic conditioning (Fig. 3b). The absence of the Sr in the Co3O4 led to negligible surface restructuring after 12\u00a0h anodic conditioning in Na+-PBS (Fig. S8), and no Co species was detected in the used electrolyte. In addition, the negligible surface restructuring on BC (Fig. S9) should be attributed to the low solubility of barium phosphate (Ksp\u00a0=\u00a03.40\u00a0\u00d7\u00a010\u221223) that limits the dissolution of the A-site cations. The limited Ba2+ dissolution can be confirmed by the ICP-OES results that there is a much lower concentration of Ba2+ (ca. 0.11\u00a0mM) as compared to Sr2+ for SC (ca. 0.44\u00a0mM) in the Na+-PBS. This result indicates that the Sr2+ dissolution is one of the main drivers for the surface restructuring and causes the Co2+ leaching in SC. After 12\u00a0h anode conditioning, the content of Na+ remained almost unchanged for both Co3O4 and BC samples, further confirming that Sr dissolution is essential for the Na+ backfilling in SC.The electrochemical conditions should also play a role in facilitating the SC surface restructuring. We treated the SC catalysts in Na+-PBS electrolyte using three different current densities while maintaining the same passage quantity of total charge. There was no clear trend spotted from HR-TEM images of the structural evolution over the SC particles (Fig. S16). Similarly, there are no obvious correlations between the treatment conditions and dissolution of the Sr and Co species in the electrolyte, suggesting that the Sr dissolution is not initiated by the electrochemical conditioning (Table S4). Instead, the Sr dissolution is likely driven by the Sr concentration gradient across the catalyst-electrolyte interfaces. However, the concentrations of Na+ in the electrolyte are similar for the three current densities, meaning that the Na+ backfilling is correlated to the total charges transferred in the reactions (Table S4). We also found that immersing SC in the Na+-PBS with no charge transferred could not induce noticeable surface restructuring (Fig. S17).To examine the role of the surface reconstruction on the OER intrinsic activity of the active sites, we compared the activity of SC before and after surface reconstructions in 0.0316\u00a0M, 0.1\u00a0M, 0.316\u00a0M, and 1.0\u00a0M Na+-PBS electrolytes. Fig. 4\na\u20134c presents obvious dependency of the OER activity over the Na+-PBS concentration, confirming the contribution of Na+-PBS to the OER catalysis over both original and restructured SC oxides. Interestingly, evidenced by its smaller slope, the SC-Re should have a lower OER dependency over Na+-PBS concentration than the original analogue, meaning that the effect of the Na+-PBS concentration is weakened after the development of the core\u2013shell structure. We believe the phenomenon is related to the aforementioned steric hindrance of the thick amorphous shell that limits further electrolyte penetration and SC/electrolyte interfacial interactions. The lower OER dependency on Na+-PBS could be attributed to the enhanced proton transfer process by the incorporation of phosphate, which was previously reported for (La, Sr)CoO3 with the surface-modified with phosphate [41]. Our results of the density-functional theory (DFT) calculation further confirm that phosphate could also lower the activation energy by 0.19\u00a0eV by accelerating the proton removal from H2O and HO* intermediate, where * represents the adsorption site at SC surface (Figs. S18 and S19) [42,43].Furthermore, we observe a stronger pH dependency of OER activities over SC-Re than over the SC-origin in KOH electrolytes (Fig. 4d-4f). A high pH dependency indicates the participation of the lattice oxygen in the OER catalysis, where lattice oxygen vacancy is the essential ingredient [18,44]. The surface reconstruction process can create surface oxygen vacancies to strengthen intermediate adsorption and tether the surface with phosphate to accelerate proton transfer, jointly enhancing the overall OER reactivity [45]. This proposed controllable catalyst/electrolyte cations matching-induced restructuring strategy can be a more effective pathway to achieve a high electrochemical activity compared with the conventional surface restructuring in alkaline solution. After reconstruction in 1.0\u00a0M Na+-PBS, the SC-Re even shows a remarkably higher OER activity compared with the SC-origin in 1.0\u00a0M KOH (Fig. 4g). To achieve the mass current density of 100.0\u00a0A/g, SC-Re needs only an overpotential of 385\u00a0mV, clearly lower than that of SC-origin (494\u00a0mV, Fig. 4f). In contrast, the reconstructed SC sample in 1.0\u00a0M KOH and 1.0\u00a0M NaOH can only induce minor activity enhancement, which needs 491\u00a0mV and 450\u00a0mV, respectively, to achieve 100\u00a0A/g (Fig. 4h).We tested the single-chamber full cell water electrolysis in pH-neutral 1.0\u00a0M Na+-PBS electrolyte by applying the SC-Re as the anode catalyst and Pt-loaded carbon black (Pt/C) as the catalyst to evolve hydrogen (Fig. S20a). The SC-re-based cell achieved a current density of 28.8\u00a0A\u00a0g\u22121 at an overall cell voltage of 2.2\u00a0V, outperforming the RuO2-based equivalent (4.1\u00a0A\u00a0g\u22121 at 2.2\u00a0V) by almost seven-folds (Fig. S20b, c). We also assembled a 5.0\u00a0cm2 SC-Re electrode into an AEMWE, where the Pt/C catalyst deposited on Ti foam serves as the cathode and the Sustainion X37-50 Grade T as the anion exchange membrane (Fig. S20d). This cell can achieve a current density of 90.7\u00a0A\u00a0g\u22121 at an overall cell voltage of 2.2\u00a0V (Fig. S20e), and the gas chromatograph results as shown in Fig. S21 confirmed that the products of the cell are H2 and O2. No CO and CO2 were detected (precision: ppm). This cell can produce 1.8\u00a0\u00b1\u00a00.2\u00a0mL\u00a0min\u22121 O2 gas and 3.9\u00a0\u00b1\u00a00.2\u00a0mL\u00a0min\u22121 (Fig. S20f), and therefore faradaic efficiency for OER and HER are both\u00a0\u223c100\u00a0%. The stability test of SC-Re \u2016 Pt/C at 85.0\u00a0A\u00a0g\u22121 shows negligible degradation for 24\u00a0h (Fig. S22). These results demonstrated the potential of the SC-Re to be applied in a large-scale water electrolyzer.Based on the results of the controlling experiment, we could safely conclude that the surface restructuring of SC during anodic conditioning involves the dissolution of A-site cations and backfilling of electrolyte cations (Fig. 5\na). This restructuring process mainly arises from (1) the solubility of A-site cations in the electrolyte, (2) the size of electrolyte ions, and (3) the charge transfer process. The solubility of the catalyst cation determines the concentration gradient across the catalyst-electrolyte interface, which drives the catalyst dissolution into the electrolyte. The catalyst dissolution together with anodic conditioning charge transfer enables the backfilling of Na+ in the electrolyte to the vacant Sr site and subsequently stabilizes the B-site cobalt structures at the SC surface. The slightly higher cobalt oxidation states as observed from the XAS results should be attributed to the Sr-Na swap on the SC surface that causes the reduction of A-site cation valence and the electrochemical oxidation during the anodic treatment (Fig. 1g, h). Meanwhile, the exchange between Sr and Na could be associated with the formation of oxygen vacancies (as evidenced by the reduced Co-O coordination numbers) and phosphate incorporation (as evidenced by the featured XAFS spectra of Co-phosphate (Fig. 1i) and FTIR characteristic peak for phosphate (Fig. S3). The induced charge imbalance, like other perovskite metal oxides [46], could also create oxygen vacancies with each carrying two positive charges, as depicted by the equation (inset in Fig. 5a). Therefore, the negatively charged oxygen ions from phosphate could interact with the positively charged oxygen vacancies on the surface, resulting in the adsorption of phosphate in the surface shell lattice.The surface restructuring process in Na+-PBS can be a general restructuring pathway for the ABO3 materials that meet the rules described in Fig. 5a, and could effectively enhance the OER activity. To demonstrate its generality, we prepared a few Sr-containing cubic perovskites, including La0.6Sr0.4CoO3-\u03b4 (LSC), La0.6Sr0.4MnO3-\u03b4 (LSM), La0.5Sr0.5Co0.8Fe0.2O3-\u03b4 (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3-\u03b4 (BSCF), SrSc0.175Nb0.025Co0.8O3-\u03b4 (SSNC), and SrNb0.1Ta0.1Co0.8O3-\u03b4 (SNTC) catalysts, and compared the OER reactivity in 1.0\u00a0M Na+-PBS before and after anodic treatment for 12\u00a0h at 3.0\u00a0A\u00a0g\u22121. Their XRD patterns are shown in Fig. 5b. All these materials show discernible enhancement of the OER activity after the anodic treatment. The overpotential to achieve a current density of 3.0\u00a0A\u00a0g\u22121 is reduced by 70\u00a0mV for LSCF, 125\u00a0mV for BSCF, 50\u00a0mV for SSNC, 68\u00a0mV for SNTC, 130\u00a0mV for LSM, and 86\u00a0mV for LSC. (Fig. 5c\u20135\u00a0h). This result further confirms the important role of pairing cathode cations (Sr2+) with electrolyte cations (Na+) in improving OER activity via surface restructuring.To conclude, we report a novel operando surface restructuring pathway, highlighting the important role of pairing cations in catalyst and electrolyte in the electrochemical surface restructuring process. In our study, we use the SrCoO3-\u03b4 perovskite as the model catalyst to evolve oxygen from water in the Na+-PBS electrolyte and study the effect of surface restructuring in enhancing OER catalytic activity. We find that the surface restructuring process requires the dissolution of soluble A-site cation (Sr) to the electrolyte, backfilling of small electrolyte cations (Na+) to large A-site vacancy in the catalyst lattice, and anion (phosphate) incorporation. Through both experimental and theoretical studies, we confirm that the A-site cation dissolution is driven by the concentration gradient across the catalyst-electrolyte interface, and this dissolution process together with the anodic polarization initiates electrolyte ion backfilling and incorporation. Consequently, the surface restructuring leads to the formation of an amorphous shell with a thickness of ten of nanometers at the catalyst/electrolyte interfaces. This shell structure is highly active in OER catalysis due to the created lattice oxygen vacancies that strengthen intermediate adsorption and incorporate phosphate that accelerate proton transfer. Overall, this work highlights the important roles of the cations in both the catalyst and electrolyte in determining the electrode\u2013electrolyte interactions during electrocatalysis. We anticipate this work to offer alternative strategies to advance electrochemical applications such as water and CO2 electrolysis via optimizing the catalyst compositional chemistry with properties of electrolyte.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.L. Z. and Z. L. contributed equally to this work. Z. Z. likes to thank the financial support from Australian Research Council Discovery Projects (DP190101782) and (DP200101397). The authors thank the valuable advice from Prof. Honglai Liu about the DFT calculation. The authors also thank the Shanghai Synchrotron Radiation Facility (BL14W1, SSRF) for XAS equipment access.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2022.140071.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The highly efficient and stable electrolysis needs the rational control of the catalytically active interface during the reactions. Here we report a new operando surface restructuring pathway activated by pairing catalyst and electrolyte ions. Using SrCoO3-\u03b4-based perovskites as model catalysts, we unveil the critical role of matching the catalyst properties with the electrolyte conditions in modulating catalyst ion leaching and steering surface restructuring processes toward efficient oxygen evolution reaction catalysis in both pH-neutral and alkaline electrolytes. Our results regarding multiple perovskites show that the catalyst ion leaching is controlled by catalyst ion solubility and anions of the electrolyte. Only when the electrolyte cations are smaller than catalyst's leaching cations, the formation of an outer amorphous shell can be triggered via backfilling electrolyte cations into the cationic vacancy at the catalyst surface under electrochemical polarization. Consequently, the current density of reconstructed SrCoO3-\u03b4 is increased by 21 folds compared to the pristine SrCoO3-\u03b4 at 1.75\u00a0V vs reversible hydrogen electrode and outperforms the benchmark IrO2 by 2.1 folds and most state-of-the-art electrocatalysts in the pH-neutral electrolyte. Our work could be a starting point to rationally control the electrocatalyst surface restructuring via matching the compositional chemistry of the catalyst with the electrolyte properties.\n               "}
{"full_text": "Data will be made available on request.Fossil fuels are a finite resource and cannot be replenished once they are used up. This implies that eventually, fossil fuels will run out and we will need to find new sources of energy. As per the reports about 934 million tons of diesel are consumed globally each year and approximately 97.6% of all oil resources used in transportation are derived from fossil fuels [1,2]. The consequences of all these factors contribute to the potential depletion of petroleum reserves as well as the increase in fuel prices and global warming [3\u20136]. Hence Renewable energy sources are becoming increasingly important as we strive to reduce our reliance on fossil fuels and address the negative environmental impacts associated with their use. Biodiesel, being a renewable fuel, adapts as a great alternative to fossil fuels because it is more sustainable and environmentally friendly [7\u20139]. And it possesses desirable characteristics like biodegradability, a high cetane number (which reduces ignition delay time and provides smoother engine running), a high flash point (which allows safer handling and storage), non-toxicity, renewability, a reduced sulphur and aromatic content, great lubricity, and about 10\u201312% oxygen content in the molecular structure (which reduces CO, HC, smoke, and life cycle net carbon dioxide emissions) [10\u201313]. The use of biodiesel as fuel improves air quality, create new jobs and stimulate economic growth. Additionally, without requiring any modification, it can be used straight in diesel engines [14,15]. Overall, the transition to renewable energy sources is an important step towards a more sustainable and resilient energy system.Biodiesel is produced from vegetable oils or animal fats have a chemical resemblance to that conventional diesel. To produce biodiesel (methyl or ethyl ester), a chemical reaction involves known as transesterification in which triglycerides of oil or fat with an alcohol (usually methanol or ethanol) in the presence of either homogeneous or heterogeneous catalysts [16\u201319]. The use of homogeneous catalysts has the disadvantage of being chemically manufactured, increases production cost, generates a lot of effluents, affects biodiesel yield, and limits catalyst re-use. To overcome these issues the heterogeneous catalyst plays crucial role in biodiesel production as they shows high catalytic activity could speeds up the reaction, making it occur quickly, efficiently and improve the product yield and shows reusability. For producing biodiesel there are two basic requirements for feedstocks: low manufacturing costs and large production scales [20]. Over 350 oil-producing crops have been identified as potential feedstocks for biodiesel production. Considering these factors soybean oil is a popular feedstock for biodiesel production because it is widely available, relatively inexpensive, has good cold weather performance. Soybeans are crops that can be grow each year, unlike petroleum, which is a non renewable source. Additionally, soybean oil has a lower carbon footprint than petroleum based diesel fuel because it is derived from a renewable resource and produces fewer greenhouse emissions during its production and use. Hence soybean oil is versatile and sustainable choice as feedstock for biodiesel production.There is increasing interest in using ash-based heterogeneous catalysts obtained from biomass waste for biodiesel synthesis, as they are renewable and sustainable catalysts often derived from non renewable sources, they can be produced using relatively low cost methods, eliminates the need for toxic chemicals, and help to reduce waste generation. It is also suggested that biomass-derived ash catalysts shows good catalytic activity in reaction process with low leaching and great recyclability [21] this is may be the presence of alkali and alkaline earth metals in their chemical composition. Biomass derived ash catalysts that can be used to produce biodiesel have been evaluated in numerous studies. Such as the use of coconut-husk ash [22], ripe plantain fruit peel ash [23], M. acuminata peel ash [24], snail shell ash [25], banana peel ash [26], B. nigra plant ash [27], sugarcane leaves ash [28], pineapple leaves ash [29], walnut shell ash [30], waste ginger leaves ash [31], wheat bran ash [32], moringa leaves ash [33], acai seed ash [34], and hazelnut shell ash [35]. The catalytic performance of these ash catalysts are summarized in Table 8. Owing to all of that research, the use of biomass ash catalysts with high catalytic activity represents an exciting, relatively novel approach and promising development in the field of renewable energy. Consequently, all these factors make ash-based catalysts more useful and efficient than conventional catalysts due to their abundance, simplicity of collection, and reusability. These biomass ash catalysts are typically produced by drying, burning and calcination and can be directly utilized as catalysts without any modification and showed potential catalytic activities due to their basic nature. As a result, in the current work, simple combustion was used to obtain ash from discarded karanja seed shells.Karanja seed shell (KSS) ash is a waste material that is produced from the combustion of karanja shells, which are the outer coverings of karanja seed. Karanja, also known as pongamia pinnata, abundantly available, can easily grow on the edges of roadways, rivers, and agricultural boundaries with no maintenance. From the himalayan foothills to kanyakumari, it can be found in India and many other regions [36]. The karanja tree plant has a variety of uses notably it is used in the production of soap, lamp fuel, finishing and tanning of leather, veterinary medication, and other products that are used to cure humans and animals. The oil of karanja seed is utilized as a feedstock in the production of biodiesel. However in this research we provide novel information of the use of discarded karanja seed shells as catalyst which are elliptical, 2\u20133\u00a0cm broad, 3\u20136\u00a0cm long have a thick walls and contain a single seed [36] because they presents high amount of potassium, calcium and magnesium, which are known to have catalytic properties. Thus it can be used as a low cost and environmentally friendly catalyst for the transesterification to produce biodiesel.As a consequence, the feasibility of karanja seed shells (KSS) ash as a green heterogeneous catalyst for biodiesel production utilizing soybean oil has been investigated in the present work. The catalyst was analyzed to illustrate how the structure, elemental composition, and morphology impacted the production of biodiesel. Investigations were conducted into how reaction conditions affected the transesterification reaction. In addition, four reuse cycles were performed to assess the catalyst's reusability. It is therefore a promising catalyst for biodiesel production since it is a renewable resource, rich in alkaline elements and offers a viable, long-lasting precursor.For the catalyst preparation, Karanja seed shells were collected from the Jiwaji University campus in Gwalior, Madhya Pradesh, India. A market in Gwalior, Madhya Pradesh, India, was visited to purchase soybean oil to test the proposed catalyst's catalytic performance. The methanol was acquired from Merck and was of HPLC quality (purity of 99%). Rankem's distilled water was used throughout the research. All chemicals were utilized without any purification.The Karanja seed shells were collected and sun-dried for 10 days after being rinsed many times with distilled water to remove impurities. The Karanja seed shells were then crushed and burned in the air to produce ash catalyst. Following that, the catalyst was calcined for 4\u00a0h at a range of 200\u00a0\u00b0C\u2013800\u00a0\u00b0C in a muffle furnace. To prevent the catalyst from coming into touch with the air, it was placed in a desiccator.KSS ash catalyst was studied using XRD, WD-XRF, SEM; FT-IR; BET; and TGA techniques, respectively. XRD is a non-destructive technique for examining a sample's atomic structure and determining the qualities and characteristics of the atoms' chemical bonds. X-ray diffraction patterns of a KSS ash catalyst were obtained on a 5th generation Rigaku X-ray powder diffractometer, (Model No - Mini Flex 600). Analysis of the catalyst elemental composition was carried out using X-ray fluorescence (WD-XRF, PANalytical spectrometer, AxiosMAX, The Netherlands). Scanning Electron Microscopy analysis was used to study the catalyst surface texture (SEM, Carl Zeiss Ultra Plus model). The functional groups of the KSS ash catalyst were discovered using FT-IR spectroscopy. Measurements in the 400-4000\u00a0cm\u22121 range were made with an FT-IR spectrometer (PerkinElmer, serial number 105627). Surface area, porosity, and pore diameter were determined using the BET method under N2 gas using the BELSORP max equipment. TGA (Shimadzu TGA50 series) instrument was used to determine the thermal decomposition of the catalyst (TGA). An Auto deluxe digital pH meter was used to measure the pH of the catalyst and the Hammet indicator titration method was used to determine the KSS ash catalyst basicity using benzoic acid [18].Transesterification reaction of soybean oil with methanol to produce biodiesel was performed in 250\u00a0mL three-necked round-bottom flasks with refluxing condensers and temperature-regulated magnetic stirrer. At room temperature, the magnetic stirrer was used to swirl the round bottom flask for 10\u00a0min to ensure that the prepared ash catalyst with 2\u00a0wt% amount and methanol were homogeneously mix. Then the soybean oil was poured into flask and reaction takes 60\u00a0min to complete at 65\u00a0\u00b0C. once the reaction is complete, the mixture is allowed to cool and separate into two layers - a top layer of biodiesel and a bottom layer of glycerol. The prepared biodiesel then washed to remove any residual catalyst or impurities and allow drying to eliminate any water content before storing or using. To learn about the KSS ash catalyst's effectiveness in soybean oil transesterification to biodiesel, calcinations temperatures ranging from 200\u00a0\u00b0C to 800\u00a0\u00b0C, catalyst amounts (1, 2, 3, and 4\u00a0wt% of oil) and the methanol to oil ratio (6:1, 10:1, 12:1, and 15:1) under the optimal reaction conditions were tested. The produced biodiesel was characterized using GC-MS, FT-IR, and ASTM standards. To check the conversion of triglycerides of oil into fatty acid methyl esters (FAME), gas chromatography-mass spectrometry (GC-MS) (Clarus*680\u00a0GC, Clarus*SQ8C MS) was used. Functional groups were studied using PerkinElmer's FT-IR spectrophotometer (Serial No. 105627). The prepared biodiesel physico-chemical properties such as density, kinematic viscosity, flash point, fire point, cloud point, pour point, cetane index and oxidation stability were determined by ASTM standards.X-ray diffraction spectrum of uncalcined and calcined KSS ash catalyst is given in Fig. 1\n. The crystalline phases of the prepared ash catalyst are inspected from the XRD results by comparing the 2\u03b8 values with JCPDS data (joint committee on powder diffraction standard, ICDD2003) and reported literature. The catalytic activity of the catalyst was observed to be mediated by a number of potassium carbonates, chlorides, and oxides, as well as a few other metal oxide compounds. According to the XRD results, the peaks at 2\u03b8 values of 28.298, 40.427, 50.088, 58.601, 66.297, and 73.594 are attributable to KCl (JCPDS file no 41\u20131476). Vadery et al. and Nath et al. found the similar 2\u03b8 value for KCl in ash based catalysts derived from coconut husk, B. nigra, and sesamum indicum [22,27,37]. K2CO3 (JCPDS file no 87\u20130730) is attributed to the peaks at 26.278, 29.734, 31.27, and 41.722, whereas K2O (JCPDS file no 77\u20132176) is assigned to the peaks at 27.87, 38.792, 46.78, and 48.10. Nath et al. revealed the comparable 2\u03b8 value for K2CO3 and K2O in B. nigra and Sesamum indicum catalysts [27,37]. And Gohain et al. also reported similar 2\u03b8 values for K2CO3 in M. balbisiana peel catalysts, corroborating this study's findings [38]. The presence of CaO was identified at 2\u03b8\u00a0=\u00a032.670, 37.109, and 54.05 (JCPDS file no 82\u20131691) these results are consistent with the results reported by Laskar et al. and Zhao et al. [25,39] and the peaks at 21.283 and 43.296 confirm the existence of SiO2 in the catalyst (JCPDS file no 81\u20130069). Hence, according to the XRD results, the catalyst contains a number of basic oxides and carbonates of K, Ca, and Si and potassium was observed to be the main element of the ash catalyst in the forms of KCl, K2CO3, and K2O, all of which were essential in the converting oil into biodiesel.The presence of inorganic elements in the calcined KSS ash catalyst was assessed using WD-XRF analysis, and the findings are shown in Table 1\n. Many elements notably \u201cK, Ca, Mg, Na, Al, Si, P, S, Cl\u201d, and other elements were identified as producing the phases that gave catalysts their catalytic activity for biodiesel production. Some transition metal oxides also coexisted with these metal oxides in trace amounts, as can be seen in Table 1. From the results we can conclude that \u201cK, in the form of KCl, K2CO3, and K2O components\u201d, is the primary basic metal accountable for catalytic activity in transesterification reaction to produce biodiesel [24], as proven by XRD analysis.As depicted in Fig. 2\n, the calcined KSS ash catalyst shows various adsorption bands of functional groups. OH groups are attributed to IR peak at 3139\u00a0cm\u22121, whereas peaks at 1646 and 1373\u00a0cm\u22121 are ascribed to CO stretching frequencies and peak at 1117\u00a0cm\u22121 evident to CO bending frequency in the form of K2CO3, which confirmed that the metal carbonates in the form of K2CO3 was present. These results are good agreement with the FT-IR results of the reported ash based catalysts [23,27,39\u201342]. The peak at 844\u00a0cm\u22121 may likely show the presence of the CO3\n2\u2212 group [43]. Si\u2013O\u2013Si stretching band of SiO2 is represented by peak at 1041\u00a0cm\u22121, while OH bending vibrations of water molecules adsorbing on catalyst are represented by peak at 616\u00a0cm\u22121. The 532\u00a0cm\u22121 signal is caused by bond stretching vibrations of K\u2013O and CaO, indicating the presence of these components in the catalyst. All of the peaks found in the calcined KSS ash catalyst are good agreement with the ash catalyst of banana peel [40], B. nigra [27], cocoa pod husk [41], ripe plantain peel [23], and C. papaya stem [42]. Hence as demonstrated in this study, the presence of carbonates and metal oxides in the prepared ash catalyst can improve their catalytic activity in reaction process and these results are consistent with XRD findings.\nFig. 3 (A)\n reveals surface structure and morphology of the prepared KSS catalyst at different magnifications. These micrographs display the calcined ash catalysts have porous surface morphology, large aggregation of particles, fibrous and spongy texture which are in support of the characters of porosity of the materials. Oxygenated catalyst matter, such as metal oxides, could explain the bright particles seen in the SEM images [44]. Hence the prepared ash catalyst shows better catalytic activity as the highly porous catalyst achieves higher efficiency in the process of biodiesel production [42,45].Thermal gravimetric analysis (TGA) was used to investigate the weight loss percentage of the calcined KSS ash catalyst. Fig. 4\n depicts the relationship between weight loss percentage and temperature as a whole. The presence of two weight loss is observed showing similar profile to walnut shell ash catalyst [30]. First loss observed in the range of 25\u00a0\u00b0C\u2013200\u00a0\u00b0C is about 3.1%, which was attributed to the removal of moisture and low molecular weight compounds [34]. The main weight loss happened in range 200\u00a0\u00b0C\u2013800\u00a0\u00b0C with a 22%, which may be related to degradation of carbonaceous materials, carbonates, releasing CO and CO2 [28,30]. It is reported that most of the carbonaceous materials decomposed after the calcination process [30,46]. Therefore this work uses 650\u00a0\u00b0C as the calcined temperature to enable the transition from carbonaceous materials to the materials that composed of mostly of metal oxides which is comparable calcined temperature reported by Etim et al. and Gohain et al. [23,38].Surface area and pore structure had a great impact on catalytic activity in the transesterification reaction process. KSS has a total specific surface area of 4.2\u00a0m2\u00a0g-1, pore volume 0.006 cm3g-1 and mean pore diameter 5.3\u00a0nm which are in range of mesoporous structure. The prepared ash catalyst's N2 adsorption-desorption isotherm exhibited the Type-IV isotherm, that is particularly closely followed by mesoporous material as shown in Fig. 5\n. Mesoporous catalyst assisted biodiesel synthesis reactions have been reported with good catalytic properties and this is due to their ability to accommodate the basic sites that enhance the catalytic activity [47]. A mesoporous material can increase the rate of reaction by dispersing the reactants throughout its pores. On the other hand, microporous materials have a lower reaction rate than mesoporous materials because reaction occurs at the pores entrances [48,49]. As a conclusion, this mesoporous calcined KSS ash catalyst has the potential to considerably improve reaction rate while biodiesel synthesis. Relatively low surface area of various ash catalysts such as Gasified straw slag, B. nigra, M. Acuminata, Tucuma peels are reported of 1.26, 1.45, 3.66, and 1.0 respectively with high catalytic activity, shown in Table 2\n [24,27,50,51]. This is due the highly basic nature of the materials that arises because of the dominant quantity of alkali and alkaline earth metal carbonates and oxide s that facilitate the strong basic sites on the surface of the catalyst to carry out the reaction [47,52].The pH of an aqueous solution of calcined KSS ash catalyst was determined by diluting KSS catalyst ash in distilled water in the following ratios, in Fig. 6\n, the ratios of (w/v) and pH change were shown at 1:5, 1:10, 1:15, 1:20, 1:30, and 1:40. The catalyst was observed to be a strong base, with a pH of 11.46, which could indicate the presence of high potassium amount, as demonstrated by WD-XRF, and XRD analysis. High concentrations of alkali metals, particularly potassium are suggested to be behind the catalyst's increased pH. The catalytic activity of M. paradisiacal plant ash [52], Sesamum indicum plant ash [38], B. nigra ash [24], and Eichhornia crassipes ash [53] catalysts in biodiesel synthesis was considered inadequate compared to the current calcined KSS ash catalysts, with pH values of 11.30, 12.8, 11.76 and 9.6 respectively, presented in Table 3\n.Transesterification can be carried out using KSS ash catalyst due to its high basic strength. The Hammett indicator test is used to determine its basicity and the results are based on color variations [54]. The tests shows the basic strength of the prepared catalyst in the range of 11.5 <H_ <15 which is comparable to walnut shell ash [30] and hazenut shell ash [35] catalysts, presented in Table 4\n. The calcined KSS ash resembles these shell ash catalysts in terms of constituents, with minor differences in element percentages [30]. It is reported that high basic strength of the catalysts was due to the presence of metal oxides in the ash [35,55]. Hence, the presence of several strong basic sites in ash composition is an important to improve biodiesel conversion [56].In order to evaluate the calcination effect of KSS ash catalyst, the calcination temperature range of 200\u00a0\u00b0C\u2013800\u00a0\u00b0C were used in transesterification of soybean oil with reaction conditions of methanol to oil ratio of 10:1, and catalyst amount of 2\u00a0wt% at 65\u00a0\u00b0C. The results can be seen in Fig. 7\n. The calcination of the ash catalyst at 650\u00a0\u00b0C led to the resulted in a high 96% biodiesel output. Catalytic activity increases when an uncalcined catalyst is activated at 650\u00a0\u00b0C, but it decreases when it is further calcined at 800\u00a0\u00b0C. This is due to a decrease in catalyst carbonate (CO3\n2\u2212) concentration and a corresponding decline in reaction activity when the calcination temperature was raised to 800\u00a0\u00b0C. It could be linked to the partial or complete breakdown of K2CO3 to K2O at higher temperatures (800\u00a0\u00b0C). Compared to K2O, K2CO3 is more basic because of its weak acid and strong base nature, as well as the decrease in carbonate at higher calcinations temperatures [57]. As a result, the high concentration of K2CO3, a crucial element of the catalyst's high basic nature, leads to the selection of 650\u00a0\u00b0C as the appropriate calcination temperature to activate the catalyst. This outcome is consistent with the XRD and FT-IR predictions.Transesterification reaction rate and biodiesel yields can be affected by catalyst amount. Fig. 8\n shows the results of the catalyst amounts (1, 2, 3, and 4\u00a0wt%) in a biodiesel production process using a calcined KSS catalyst with a 10:1 methanol to oil ratio at 65\u00a0\u00b0C. According to the findings, the KSS ash catalyst amount of 2\u00a0wt% resulted in the best results, yielding 96% biodiesel in a reaction time of 60\u00a0min compared to the other catalyst amounts. When the catalyst amount was increased from 1\u00a0wt% to 2\u00a0wt% the marginal rise in biodiesel yield from 75% to 96% observed. Moreover, despite increasing the catalyst loading by 4\u00a0wt% while maintaining the same reaction conditions, no noticeable improvement in biodiesel yield and reaction time was observed. This could be due to the initiation of a side-product or backward reaction. The literature has reported that increasing the amount of catalyst increases the yield of biodiesel up to a specific concentration of catalyst, after which the yield of the product either does not increase or remains intact [27,38]. It is also possible that a further increase in catalyst loading may make the three-phase solution more viscous, restricting mass transfer between the phases and saponification side reactions can be produced, lowering oil conversion for base-catalyzed reactions. Accordingly, in this work KSS catalyst amount of 2\u00a0wt % were found to be the best experimental conditions for soybean oil transesterification to biodiesel [57].Effect of different methanol to oil ratios on biodiesel production was also investigated with soybean oil at 65\u00a0\u00b0C with an optimized reaction time 60\u00a0min and catalyst amount 2\u00a0wt%. Fig. 9\n demonstrates that raising the methanol-to-oil ratio from 6:1 to 10:1 increases biodiesel output from 60% to 96%. Farooq et al. observed that increasing methonaol produced more biodiese by increasing methoxy groups on the catalyst surface [58]. But when the methanol to oil ratio is further increased to 15:1, it does not show a considerable increase in biodiesel yield. This could be because transesterification is a reversible process that can be catalyzed both forward and backward by a base; however, due to high concentrations of methanol in the reaction mixture, reversible reactions occur, formulating monoglycerides and diglycerides, resulting in low oil to biodiesel conversion [24]. The works of Nath et al. and Basumatary et al. also displayed a similar pattern of experimental results, and in their reports, it was indicated that high levels of methanol to oil ratio beyond the optimized reaction condition dilute the reaction mixtures, swamping the catalyst's active sites, which in turn reduces their interactions for effective reaction, and as a result, the rate of reaction and the product yield fall [27,38,52].Catalyst reusability research is required to establish the efficiency of utilizing a catalyst to minimize production costs. The KSS ash catalyst was tested for reusability under ideal conditions, which included a methanol to oil ratio of 10:1, catalysts amount 2\u00a0wt%, and a reaction time of 60\u00a0min at 65\u00a0\u00b0C temperatures. After the transesterification reaction product was centrifuged, cleaned, and dried overnight in an oven at 100\u00a0\u00b0C. After 4\u00a0h of calcination at 650\u00a0\u00b0C, the catalyst was reactivated. Then, using fresh reactants, a similar reaction was carried out. The experimental results, shown in Fig. 10\n, demonstrated that the yield dropped to about 70% after 4th run. As a result, biodiesel yields were found to have decreased significantly, possibly due to triglyceride contamination and pores clogged by glycerol and triglycerides on the catalyst surface. The catalyst amount was changed after each usage to achieve optimal reaction conditions. However, because of leaching during the washing phase, there was some catalyst loss in each cycle. This could explain the steady decline in yield after each catalytic cycle [59]. The SEM image of recycled catalyst revealed morphological alterations, as well as the collapse and emergence of agglomerated particles, Fig. 3 (B). These results coincide with the reusability findings reported by Gohain et al. and Basumatary et al. [42,52].Analysis of soybean oil biodiesel's chemical composition utilizing GC-MS testing is depicted in Fig. 11\n, and in Table 5\n, we summarize the composition of prepared biodiesel based on retention time which appears to contain a total of five main different fatty methyl esters (FAME). As a result soybean oil biodiesel consists primarily of methyl linoleate (Rt-21.93\u00a0min, C18:2, 43.23%) as the main FAME followed by methyl oleate (Rt-20.38\u00a0min, C18:1, 7.57%), methyl palmitoleate (Rt-22.03\u00a0min, C16:0, 6.05%), methyl stearate (Rt-20.84\u00a0min, C18:0, 5.87%), and methyl nonadecan (Rt-23.18\u00a0min C20:0, 3.72%).\nFig. 12\n represents the FT-IR spectral analysis of prepared soybean oil and soybean oil biodiesel. Soybean oil's CO stretching vibration at 1744\u00a0cm\u22121 is replaced by the methyl esters in biodiesel's 1739\u00a0cm\u22121 peak, indicating that the transesterification process has gone through an intermediate step, this results is agree with results reported by Nath et al. [27]. The result of C\u2013H stretching vibrations, soybean oil has peaks at 2923\u00a0cm\u22121 and 2854\u00a0cm\u22121, while biodiesel has peaks at 2924\u00a0cm\u22121 and 2854\u00a0cm\u22121. At 1463\u00a0cm\u22121 and 1377\u00a0cm\u22121 in soybean oil; and at 1456\u00a0cm\u22121 and 1436\u00a0cm\u22121 in biodiesel, the CH3 bending vibrations are observed. The IR peaks at 1196\u00a0cm\u22121 and 1170\u00a0cm\u22121 for biodiesel and 1160\u00a0cm\u22121 and 1098\u00a0cm\u22121 for soybean oil demonstrate triglyceride and ester molecule C\u2013O stretching bands, respectively. Methyl ester molecules emitted an infrared signal at 722\u00a0cm\u22121 and 723\u00a0cm\u22121 due to the rocking of fatty acid chains. All of the above-noted peaks are consistent with the FT-IR results reported by Nath et al. [27].Soybean oil biodiesel physicochemical properties using the KSS ash catalyst have been studied using ASTM standards, as shown in Table 6\n and comparison of physicochemical properties of produced soybean oil biodiesel with reported soybean oil biodiesel by different authors illustrated in Table 7\n\n. The results show that prepared biodiesel is almost within ASTM limits. In Table 6, the density of produced soybean oil biodiesel is 0.879\u00a0g/cm\u22123, well within the ASTM limit and in line with similar reported biodiesels [25,34]. It has been reported by Moser et al. that kinematic viscosity is the key element in selecting biodiesel as an alternative fuel compared to pure vegetable oil [60]. There is a slight increase in kinematic viscosity over the ASTM limit, but it is comparable to what Laskar et al. has reported [25]. This could be due to saturation and un-saturation, un-reacted molecules in the product, or manual errors in the separation of glycerol and biodiesel. Among other properties the produced soybean oil biodiesel exhibited flashpoint and fire points of 170\u00a0\u00b0C and 184\u00a0\u00b0C, respectively, to allow safer storage and transportation. And the cloud point and pour point were determined to be (+) 1 and (\u2212) 2, respectively, which were found to be comparable to those of the previously reported soybean oil biodiesels. The cetane index of the produced soybean oil biodiesel was determined to be 48.5, which is lower than the cetane index of soybean biodiesel observed by Nath et al. [27].In this experiment, the KSS ash biomass-based heterogeneous catalyst was revealed to be exceptionally basic and make a significant contribution to catalytic activity. This outcome is a result of the potassium content, which is found in the forms of KCl, K2CO3 and K2O as the active components, having the highest level as determined by WD-XRF, XRD and SEM analysis. Studying the best conditions for biodiesel production with KSS ash catalysts revealed that they were 2\u00a0wt% catalyst amounts, 10:1 methanol to oil ratio, and 65\u00a0\u00b0C reaction temperatures. Under these conditions, calcined KSS ash catalysts demonstrated better catalytic activities and 96% of the product yield. Table 7 compares calcined KSS ash catalyst to other biomass-derived ash heterogeneous catalysts for biodiesel production [22\u201335]. In their study, ash was calcined in order to create a catalyst and with comparable reaction conditions high catalytic activities in the transesterification reaction were discovered. Table 7 illustrates that biodiesel yields of greater than 95% were achieved in all studies that used biomass ash based heterogeneous catalyst for biodiesel production without any chemical modification. Thereby, the findings from the present research support the utility of using KSS ash as a basic heterogeneous catalyst in the production of biodiesel and exhibit a pattern that is consistent with that previously reported.To conduct a cost analysis of soybean oil biodiesel production using KSS ash as a catalyst, we need to consider various factors such as raw material cost, catalyst cost, equipment cost, utility cost, and transportation cost etc. Soybean oil's price as a raw material can change based on market prices and the quantity needed to produce biodiesel. In general, it is envisioned that using KSS ash as a catalyst will reduce the cost of producing soybean oil biodiesel compared to using traditional catalysts. This is because KSS ash is a rich in alkaline earth metals, low in cost, and readily available discarded material in many areas, and it can be used in a simple and relatively low cost process. In the present work, the produced KSS ash catalyst exhibited adequate catalytic activity under optimum reaction conditions including catalyst amounts (wt%) and methanol to oil ratios when compared to those reported literatures and it showed reusability up to a fourth cycle. However, other aspects like transportation costs or the requirement for more tools may outweigh the cost savings. To ascertain its economic viability, a thorough cost analysis would need to be done.We have concluded in this paper, the karanja seed shells (KSS) which is a widely available agricultural waste, is a potential green heterogeneous catalyst for the synthesis of biodiesel. The use of KSS ash catalyst offers several benefits over non heterogeneous catalysts including cost effectiveness, environmental friendliness, high catalytic activity, reusability and easy separation.\n\n-\nStudies demonstrated that calcined KSS ash catalyst exhibits high activity and reusability. This is because it contains high amounts of potassium and calcium which can effectively catalyze the transesterification reaction and produce high yield of soybean oil biodiesel.\n\n\n-\nBased on the results the KSS ash catalyst presented a biodiesel yield of 96% from soybean oil in 60\u00a0min at 65\u00a0\u00b0C with catalyst amount of 2\u00a0wt % and 10:1 methanol to oil ratio. The catalyst was reused up to the 4th cycle which yielded a drop to 70% after the 4th run.\n\n\n-\nCatalyst characterization was done using XRD, WD-XRF, SEM, FT-IR, BET, and TGA techniques, and results were compared to those of previously reported biomass-based ash catalysts. The produced biodiesel is confirmed by GC-MS, FTIR, and ASTM testing.\n\n\nStudies demonstrated that calcined KSS ash catalyst exhibits high activity and reusability. This is because it contains high amounts of potassium and calcium which can effectively catalyze the transesterification reaction and produce high yield of soybean oil biodiesel.Based on the results the KSS ash catalyst presented a biodiesel yield of 96% from soybean oil in 60\u00a0min at 65\u00a0\u00b0C with catalyst amount of 2\u00a0wt % and 10:1 methanol to oil ratio. The catalyst was reused up to the 4th cycle which yielded a drop to 70% after the 4th run.Catalyst characterization was done using XRD, WD-XRF, SEM, FT-IR, BET, and TGA techniques, and results were compared to those of previously reported biomass-based ash catalysts. The produced biodiesel is confirmed by GC-MS, FTIR, and ASTM testing.Consequently, utilizing KSS catalyst will result in simple reaction procedures, low-cost catalyst preparation, and minimal waste problems for biodiesel synthesis. In addition, there is scope for future studies to investigate the feasibility and economic viability of KSS ash as catalyst in large scale biodiesel production.\nPooja Prajapati: Conceptualization, Methodology, Investigation, Experiments, Formal analysis, Writing \u2013 original draft, review & editing. Sakshi Shrivastava: Conceptualization, Investigation, Collaborator, Formal analysis, Writing \u2013 review & editing. Varsha Sharma: Investigation, Writing \u2013 review & editing. Priyanka Srivastava: Conceptualization, Writing \u2013 review & editing, Resources. Virendra Shankhwar: Formal analysis, Writing \u2013 review & editing, Resources. Arun Sharma: Formal analysis, Writing \u2013 review & editing, Resources. S.K. Srivastava: Conceptualization, Supervision, Writing \u2013 review & editing. D. D. Agarwal: Conceptualization, Investigation, Supervision, Writing \u2013 review & editing, Resources.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge the CIF, Jiwaji University, Gwalior (M.P.), India for providing XRD, FT-IR, and TGA facilities; the Innovation Centre, Bundelkhand University (U.P.), India for GC-MS analysis; the SAIF, IIT Patna, India for the 1H NMR facility; the CSIF, University of Calicut, India for providing BET facility; IIT Roorkee, India for SEM analysis; SICART Anand, India for WD-XRF facility; and Hindustan Laboratories Regd., New Delhi, India for testing biodiesel properties.\n\nKSS\n\nKaranja seed shell\n\nXRD\n\nX-ray diffraction\n\nWD-XRF\n\nWavelength dispersive-X ray fluorescence\n\nFT-IR\n\nFourier transforms infrared spectroscopy\n\nSEM\n\nScanning electron microscopy\n\nTGA\n\nThermogravimetric analysis\n\nBET\n\nBrunauer \u2013 Emmett - Teller\n\nGC-MS\n\nGas chromatography-mass spectroscopy\n\nASTM\n\nAmerican standards testing methods\n\n\nKaranja seed shellX-ray diffractionWavelength dispersive-X ray fluorescenceFourier transforms infrared spectroscopyScanning electron microscopyThermogravimetric analysisBrunauer \u2013 Emmett - TellerGas chromatography-mass spectroscopyAmerican standards testing methods", "descript": "\n                  In the present study, the ash from discarded Karanja seed shell (KSS), obtained after burning dried seed shells has been identified as one of the most cost-effective and efficient green heterogeneous catalysts for biodiesel production. Soybean oil methanolsis was used to study the catalytic activity of karanja seed shell ash as solid base heterogeneous catalysts in the biodiesel production. To characterize the catalyst, XRD, WD-XRF, SEM, FT-IR, BET and TGA techniques were utilized. Soybean oil methyl ester was converted under the following experimental conditions, as determined by GC-MS, and FT-IR: a catalyst amount of 2\u00a0wt%, a methanol to oil molar ratio of 10:1, a reaction temperature of 65\u00a0\u00b0C, a reaction time of 60\u00a0min. The synthesized heterogeneous catalyst is an efficient alternative that may be utilized as an eco-friendly catalyst because it is benign, reusable, sustainable, and widely available.\n               "}
{"full_text": "No data was used for the research described in the article.Functional amines constituting primary, secondary, and tertiary amines, as well as imines are key precursors and central intermediates in the synthesis of bulk and fine chemicals and have a wide range of applications in biology [1], medicine [2], energy [3], materials, and environment [4,5]. Generally, the primary benzylic and aliphatic amines can be easily functionalized to further form highly valuable chemicals, and pharmaceuticals [6,7]. Secondary and tertiary amines are considered as precursors for life science and material applications. Their structural motifs are widely found in a large number of biological molecules, pharmaceuticals, and agrochemicals as well as natural products [8\u201310]. On this basis, great efforts for the synthesis of functional amines are developed, including but not limited to the reductive amination of carbonyl compounds [11,12], the alkylation of ammonia or amines [13,14], and the hydrogenation of nitrile [15]. The catalytic reductive aminations of aldehydes in the presence of molecular hydrogen are found to be a convenient and cost-economic process for the preparation of different kinds of amines. However, reductive amination reactions often are non-selective and easily to be over\u2011hydrogenated to form corresponding alcohols or aminated to generate secondary/tertiary amines. The development of suitable catalysts is thus crucial for selective controlling in the reductive amination of heterocycle aldehyde towards specific amines with high activity and selectivity.Regarding the potential hydrogenation and amination catalysts for the synthesis of amines, transition metal-based (Ru [16,17], Ir [18], Pt [19,20], Rh [21], etc.) heterogeneous ones are frequently applied. Gu et al. [22] reported a strategy for the direct reductive amination of aldehydes by the unsupported ultra-thin Pt-nanowires catalyst, which exhibited excellent catalytic performance towards dibenzylamines with a high yield of 96% rather than primary amines under mild reaction conditions. Moreover, the Pt-nanowires were stable and recycled 6 times without any metal leaching or decrease in catalytic activity, and 9 examples of corresponding aromatic secondary amines were obtained in 70\u201396.9% yield (Fig. 1a). In the meantime, Kawanami et al. [23] chose Rh/Al2O3 catalyst for the reductive amination of furfural to furfurylamine by using aqueous ammonia solution and molecular hydrogen with very high selectivity of furfurylamine (\u223c92%) under the reaction time of 2\u00a0h at 80\u00a0\u00b0C. Interestingly, benzaldehyde was converted to benzylamine and dibenzylamine with a selectivity of 77.1 and 22%, respectively (Fig. 1b). In 2017, Beller and co-workers [24] developed the MOF-derived cobalt nanoparticles as an expedient and advantageous catalyst for the reductive amination of carbonyl compounds (aldehydes, ketones) with ammonia, amines or nitro compounds and molecular hydrogen to produce corresponding primary, secondary, tertiary and N-methylamines (Fig. 1c). Shortly afterward, their study group [25] immobilized Ni-tartaric acid complex on silica by solvothermal synthesis and then applied pyrolysis (400\u20131000\u00a0\u00b0C). For the one pyrolyzed at 600\u00a0\u00b0C (Ni-TA@SiO2\u2013600), 2-bromobenzylamine was obtained in up to 60% yields. Some secondary amines could be also observed. However, the Ni-based catalyst obtained at 800\u00a0\u00b0C exhibited significant activities and produce 2-bromobenzylamine in 96% yield (Fig. 1d). This work provides a simple and obvious method to precisely control catalyst nanostructures for better selectivity in reductive amination process. Similarly, several groups also demonstrated that suitable support could provide interaction for the reactants, so the intrinsic catalyst structure can be responsible for the diversity of hydrogenation and ammonifying process, and thus affecting the catalytic activity and selectivity in reduction hydrogenation reaction.Very recently, covalent organic frameworks (COFs) had been used as sustainable support candidates for catalysis applications [26\u201329]. In contrast to other support to immobilize active metal nanoparticles, the COFs own diverse chemical structures, which could precisely tune the surrounding coordination environment and electronic interaction between metal nanoparticles and support materials [30,31]. Tremendous results in previous study demonstrate that the uniform porous structure in COFs can provide abundant metal active sites, thus improving the catalytic activity [32,33]. Based on this, it would be natural to expect that immobilizing the different kinds of active transition metals on the porous COFs support would be a feasible and referable route for the selectivity control-oriented modulation in the catalytic reductive aminations reactions. However, this application of precise hydrogenation and amination selectivity control has seldom been explored.In this study, we designed and synthesized COF support via a facile hydrothermal synthesis method, and noble metal (Pt, Pd, and Rh) was introduced to COF host matrix, respectively. Moreover, the influence of reaction conditions towards the reductive aminations activity and selectivity performance of three kinds of COFs supported metals catalysts were investigated. Among them, Pd and Pt based COF catalysts were more active in catalytic reductive amination towards secondary amines, while the Rh based COF catalyst was effective for the selective reductive amination of benzaldehyde to secondary imines and a primary amine, respectively. This kind of selectivity performance-responsive can be employed for the development of smart catalyst systems to rationally control the catalytic activities and functions on demand shortly.The COF support was synthesized similarly to our previous work with slight modified [34]. In brief, 0.2\u00a0mmol of 1,2,4,5-tetrakis(4-formylphenyl) benzene (TFPB) and 0.4\u00a0mmol of p-phenylenediamine (PD) were added to the mixture solution containing 5\u00a0mL of 1,4-dioxane, and then 0.3\u00a0mL of citric acid and 5\u00a0mL of 1,3,5- trimethyl-benzene were subsequently added under vigorous stirring. 1, 4-dioxane was used as a polar solvent. Then the system was purged with nitrogen gas for 5\u00a0min and transferred in an autoclave at 120\u00a0\u00b0C for 72\u00a0h. After cooling to room temperature, the obtained precipitates were washed with N, N-dimethylformamide, tetrahydrofuran, and acetone, respectively. Finally, the crystals were collected by drying under a high vacuum at 80\u00a0\u00b0C for 12\u00a0h and denoted as COF.Typically, COF supported metal catalysts were prepared by a vacuum-assisted impregnation method in this experiment. To prepare Pt/COF composite, 4\u00a0mg Pt precursor (1\u00a0g H2PtCl6\u00b76H2O dissolved in 250\u00a0mL ethanol) was added drop by drop into the slurry of COF and followed under vacuum pressure. After the solvent was evaporated, sodium borohydride methanol solutions with a volume of 10\u00a0mL were quickly mixed as a reducing agent and stirred for 2\u00a0h. Subsequently, the obtained powder was washed with methanol several times to remove the excess solvent and finally dried at 60\u00a0\u00b0C for 12\u00a0h. In the same process, Pd/COF and Rh/COF catalysts were synthesized by using PdCl2\u00b72H2O as the Pd source and RhCl3 as the Rh source, respectively. The theoretical amount of noble metal loading amount was 4\u00a0wt%.The crystalline structure of COF and COF supported metal catalysts was studied by the X-ray powder diffraction (XRD) patterns. As shown in Fig. 2a, for the COF sample, a narrow and sharp peak at 4.9 o and a broad diffraction peak at 19 o were observed, revealing a good crystallinity. When introducing metal into the COF, the diffraction peaks ascribed to the COF were still present; however, the intensity was decreased, indicating the lower crystallinity of COF after reduction with borohydride sodium borohydride. For the Pt/COF sample, another peak at around 39.7 o was detected, associated with the (111) plane of Pt [35]. In addition, the peak represented to Pd or Rh was observed in neither Pt/COF nor Rh/COF samples, possibly because of the small particle size and high dispersion of palladium and rhodium nanoparticles [36,37]. Fig. 2b showed the FTIR spectra of COF and corresponding COF supported noble metals catalysts. In the spectrum of COF, the absorption broad band at 1606\u00a0cm\u22121 representing the stretching vibration of CN was a result of schiff base formation in the COF synthesis. Moreover, the characteristic behavior corresponded to aromatic structures (vCC and vCC) were observed in the range from 1650\u00a0cm\u22121 to 1450\u00a0cm\u22121\n, the peaks located at around 1276\u00a0cm\u22121 were a counterpart of CN stretching vibration. For Pd/COF and Rh/COF catalysts, the band at 1276\u00a0cm\u22121 assigned to CN stretching vibration became broad, indicating that the metal center was successfully coordinated with the nitrogen of COF outer layer. In addition, the elemental analysis results in Table S2 shown that the amount of carbon, hydrogen and nitrogen in the COF was 86.79%, 4.39% and 8.82% in terms of the weight, respectively. To investigate the thermal stability of COF and the corresponding COF supported metal catalyst, TGA was performed with a heating rate of 10\u00b0C/min under a nitrogen atmosphere from ambient temperature to 800\u00b0C (Fig. S5). The results indicated that the COF support has thermal stability up to 300\u00b0C.As displayed in Fig. 3\n, the COF exhibited spherical morphology with an average diameter of 1.2\u00a0\u03bcm. The COF sphere consisted of different amorphous nanoflakes, revealing a highly ordered porous structure. To observe the metal nanoparticles and confirm their particle sizes on the COF, TEM images for the Pt/COF, Pd/COF, and Rh/COF TEM were shown in Fig. 4\n. The Pt NPs size in Pt/COF (average particle size: 10\u00a0nm) was mainly in the range of 3\u20134 Fig. 4(b-c). Especially, some relatively larger sizes reaching around 5\u00a0nm could be detected. For the Pd/COF sample, small Pd nanoparticles (about 3\u20135\u00a0nm) were densely and uniformly assembled on the surface of COF, as shown in Fig. 4(d-f). The Rh NP was highly dispersed and its average size in the Rh/COF was approximately 2\u20133\u00a0nm (Fig. 4i). This result suggests that the use of well-dispersed Rh particles with a smaller size exposed more catalytically active metal sites, which could be favorable to activity performances. Moreover, elemental mapping using scanning transmission electron microscopy (STEM) confirms the homogeneous distribution of metal elements throughout the whole COF matrix in Fig.S1-S3, indicating the suitable procedure during the preparation process.We further explored the chemical state's identification and quantification of the synthesized COF and M/COFs. In the C 1\u00a0s spectra of COF in Fig. 5\n, two prominent peaks were observed, where the one at around 284.8\u00a0eV was in accordance with the \u2013C\u2013(C, H)/C=C bonds in the graphitic structures, and the one located at 286.8\u00a0eV was attributed to the \u2013C\u2013N bonds. For the Pt/COF catalyst, an obvious shift of \u2013C\u2013N\u2013 XPS peak towards higher binding energy compared to that for COF was observed, suggesting that the C atoms are probably coordinated with Pt ions at the hybrid interface. Furthermore, the peak for \u2013C\u2013N\u2013 of the Rh/COF composite showed a lower energy shift, which was attributed to the interaction between the C and Rh species. The N 1\u00a0s spectra of COF could be deconvoluted into sp2 hybridized nitrogen in the form of C-N=C appeared at 398.9\u00a0eV and the C\u00a0\u2212\u00a0N bond at 401.4\u00a0eV. Meanwhile, a similar shift of C\u00a0\u2212\u00a0N 1\u00a0s XPS peaks towards higher binding energy in Pt/COF catalyst and a shift towards lower binding energy in Rh/COF catalyst could be detected as compared to that of pristine COF. This result implies a strong chemical bond forming between noble metal and N. On the other hand, the high resolution Pt 4f7/2 XPS spectra showed two characteristic peaks at 71. 7\u00a0eV and 74.2\u00a0eV, which were ascribed to the Pt0 and Pt2+ groups, respectively. Notably, the Pt 4f7/2 characteristic peaks at 71. 7\u00a0eV were slightly higher than the typical Pt0 (71.3\u00a0eV) groups, revealing partial electron migration from Pt to contiguous C or N. The Pd 3d5/2 spectrum was split into two peaks located at 335.83, and 337.63\u00a0eV (Fig. 5e), which can be attributed to Pd0 and Pd2+, respectively. The binding energies observed in Fig. 5f showed the appearance of Rh0 3d5/2 and Rh3+ 3d5/2, with their characteristic peaks at 307.4 and 309.2\u00a0eV. The Rh3+ characteristic peak at 309.2\u00a0eV was also slightly higher than the typical Rh3+3d5/2 (308.6\u00a0eV) groups, corresponding to partial electron migration between metal and surrounding C or N element.The N2 adsorption-desorption isotherms and corresponding pore size distribution of the prepared catalysts were displayed in Fig. 6\n. It can be seen that the COF exhibited a typical type IV isotherm, revealing the mesoporous structures with a small number of micropores. The corresponding BET surface area and pore size were calculated to be 297 m2/g and 4.5*10\u22121\u00a0cm3/g, respectively (Table 1\n). The Pt/COF and Pd/COF had a relatively lower BET surface area (98.7\u00a0m2*g\u22121 and 68.7\u00a0m2*g\u22121, respectively) than that of COF, which implies that the introduction of Pt and Pd species slightly blocked the pore structure. In addition, the BET surface area of Rh/COF was calculated to be 185.1m2/g, while the corresponding total pore volumes was 3.2 *10\u22121\u00a0cm3/g. In comparison, the slightly higher BET surface area of Rh/COF was detected after metal incorporation, suggesting that the void space in the pores is not occupied by the Rh particles and the small particle size of Rh dispersion on COF support, as confirmed by the TEM images. Additionally, the average pore diameter of the COF supported Pt and Pd samples all increased to more than 10\u00a0nm, indicating the partial blocking of the pore under 10\u00a0nm after metal incorporation. Additionally, the Py-IR of COF and COF supported metal catalyst at 35\u00a0\u00b0C was shown in Fig. S4, the peaks near 1550 and 1450\u00a0cm\u22121 were attributed to the pyridinium ions adsorbed on Br\u03c6nsted and Lewis acid sites, respectively, indicating the acidic sites were mainly composed by weak acidity. Since Lewis acid catalysts have been demonstrated as the profitable catalyst for reductive amination reactions.We investigated the catalytic performance for the reductive amination of benzaldehyde (BD) and active metal-dependent amines selectivity over the COF supported metal catalysts along with commercial Pt/C, Pd/C, and Rh/C catalysts as a comparison in the presence of ammonia solution and H2 gas. As shown in Fig. 7a, among them, the Pt/COF and Pd/COF exhibited excellent selectivity in the production of dibenzylamine (DA) with a yield of 81% and 91%, respectively, while the Rh-based COF tended to selectively convert benzaldehyde to benzenemethanamine (BN) with approaching 90% yield under mild reaction conditions (90\u00a0\u00b0C, 2\u00a0MPa H2) in a 1.2:1\u00a0M ratio of BD to ammonia. It could be found that the metal Pt/COF and Pd/COF catalysts were more active in catalytic reductive amination towards DA when compared to the metal Rh/COF catalyst. It seems that the main influence on the activity and selectivity in tandem reductive amination reactions is not only the Lewis acid active sites within the COF but also the hydrogenation activity of active metal nanoparticles. This phenomenon was also detected in commercial noble catalysts, in which the Pt/C owned a relatively high DA yield of 72% and Pd/C possessed a DA yield of 61%. Additionally, as proved in previous literature, the adsorption of a high concentration of ammonia improves primary amine selectivity. We further evaluate the amine selectivity of Rh/COF with an enhanced molar ratio of ammonia to BD from 1/1.2 to 60/1.2 in Fig. 7b. When the ratio of ammonia to aldehyde increased to 60:1.2, the yield of BN decreased from 90% to 2%, while the yield of primary amine BM was indeed grown from 0 to 76%. It could be concluded that not only the active catalyst but also the concentration of NH3 is critical to the amines' selectivity. It seems that the Rh/COF catalyst had high selectivity for secondary imine with a 1.2/1\u00a0M ratio of BD to ammonia, and was more active in the production of a primary amine with a 1.2/60\u00a0M ratio of BD to ammonia. Moreover, the reuse experiment under identical conditions in successive runs for the reductive amination reaction of benzaldehyde by using COF supported catalysts was added in Fig. S6\u20138, the heterogeneous catalyst remained stable after three cyclic runs. A trifling drop in the yield was obtained, which might be attributed to the inevitable loss during the recovery process and active sites leaching under the high-temperature reaction conditions. Moreover, the concentration of the metal leaching test was also added by ICP-AES. As detected in Table S1, the tiny loss rate of \u223c0.2\u00a0wt% in the content of Pd or Rh was observed in Pd/COF or Rh/COF catalysts after three runs.To test this, we further explored the substrate scope of the Rh/COF in the reductive amination of heterocycle aldehyde for the selective synthesis of target amines by adjusting the mole ratio of heterocycle aldehyde to ammonia. As listed in Table 2\n, the reaction was carried out under the same conditions optimized for benzaldehyde. In all five cases, approximately 68\u201392% yields of the corresponding secondary imines were obtained from each benzaldehyde under the 1.2/1\u00a0mol ratio of heterocycle aldehyde to ammonia. Furthermore, the Rh/COF catalyst afforded the corresponding primary amines with preferable yields (42\u201378%) under the 1.2/60\u00a0mol ratio of heterocycle aldehyde to ammonia. Accordingly, it was found that Rh/COF could selective transformation of heterocycle aldehydes towards corresponding target heterocycle secondary and primary amines via reductive amination reaction, which can easily be functionalized further and thus was diversity-oriented synthesis for fine chemicals or pharmaceutical drugs.According to our studies and controlled experiments, we expected the reaction pathways for the reductive amination of benzaldehyde with NH3 in the presence of H2 gas in Fig. 8\n. Firstly, the BD reacted with ammonia to spontaneously form the intermediate product [38]. In the following step, the HH bond was activated on the surface of metal based COF catalyst, and subsequently reacted with an intermediate product to yield BN. The secondary imine BM was unstable and it rapidly hydrogenated to reduce to DA under the catalysis of active metals such as Pt and Pd with strong hydrogenolysis ability. On the other hand, the BM could be also transformed to primary amine BM over the active catalyst with relatively weak hydrogenolysis but strong ammonolysis capacity. Meanwhile, the intermediate product could be directly heated up to generate tertiary amine. Most notably when hydrogen was excessive, the BA could be obtained as the main product. Based on the above results, we might considered that a suitable amount ratio of ammonia to hydrogen, hydrogenolysis, and ammonolysis capacity of active catalyst were responsible for the high selectivity production of target amines.In summary, we successfully designed and synthesized the Pt-based, Pd-based, and Rh-based COFs by a convenient and efficient post-treatment method and realized a selective and high-yield synthesis of target primary or secondary amines from heterocycle aldehyde via reductive amination reaction. The optimized Pd/COF catalyst displayed better catalytic performance for the reductive amination of benzaldehyde towards secondary amines with 91% yield under a 1.2:1\u00a0M ratio of aldehyde and ammonia under mild reaction conditions (2\u00a0MPa of H2 and 90\u00a0\u00b0C, 15\u00a0h), while the Rh/COF catalyst exhibited relatively high selectivity for secondary imines with 90% yield. More importantly, the Rh/COF catalyst had a relatively high selectivity towards corresponding primary amine from benzaldehyde derivatives when adjusting the molar ratio to 1.2:60. Thus, we anticipate that the present study will provide the feasible preparation of efficient active metal based catalysts for selective control in the reductive amination of heterocycle aldehyde towards functional amines.J.L. and L.M designed and supervised this research. N.W. performed the experiments data, analysis and wrote the original manuscript. J.L. reviewed and corrected the manuscript. All authors talked over the results and helped during manuscript preparation.Correspondence and requests for materials should be addressed to J. G. L. or L. L. M.The authors declare no competing financial interests.This work was supported financially by National Natural Science Foundation of China (52236010, 51976225), Science and Technology Program of Guangzhou, China (202201010014).\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106620.", "descript": "\n                  Since selective control in catalytic reductive aminations of aldehyde for the synthesis of functional amines was highly valued, we developed three kinds of COFs supported catalysts (Pt/COF, Pd/COF, and Rh/COF) for selectively catalytic reductive amination of benzaldehyde. Results indicated that Pd/COF and Pt/COF exhibited good selectivity towards secondary amines with a 1.2/1\u00a0M proportion of aldehyde and ammonia, while Rh/COF catalyst can selectively convert benzaldehyde to secondary imine with an excellent yield of 90%. Furthermore, an improved yield of primary amine could be obtained over Rh/COF catalyst with a 1.2/60\u00a0M proportion of aldehyde and ammonia.\n               "}
{"full_text": "Data will be made available on request.There is a constantly increasing interest in improving electrochemical devices for the production of energy in order to satisfy its currently high demand and sustainability requirements. However, these devices traditionally employ noble metals catalysts [1\u20134], since they have excellent properties mainly related to activity and stability, such as (i) avoiding detachment of particles, (ii) minimizing loss of electrical contact, and (iii) improved durability due to their less susceptible to degradation [5]. In the search for innovative catalysts, noble metals have been usually incorporated into different materials that act as supports and facilitate the availability of these active sites. Furthermore, even more recently, some innovative materials are being developed to provide physicochemical properties, such as porosity and high surface area, to the noble metal catalyst themselves in order to improve their performance avoiding the use of supports [6,7]. Among these new materials the use of noble metals for the manufacture of aerogels has been of great interest in recent years, however, most of the metallic aerogels investigated recently are made with an organic compound and noble metals, such as Pt [8\u201310], Au [11,12], Pd [13\u201316], Ag [17,18] and their combinations [7,19\u201323], as well as aerogels of metal oxides and noble metals [16,24,25]. These unique materials have been applied as efficient electrocatalysts for the electrooxidation of organic fuels such as ethanol, formic acid, and oxygen. These materials are highly novel due to their unique morphology of assembled nanoparticles; recent work shows that assembling sheets considerably improves the catalytic activity of aerogels towards oxidation reactions [49\u201352].The synthesis methods of aerogels have changed over the years [6,13], although literature shows that slight modifications of the synthesis steps will be traduced in notable differences in their final properties [26]. In 1846, J.J. Ebelmen, synthesized the first silica aerogel using the sol-gel method [27]. This methodology involves a gelation process, where a solution loses its fluidity and takes the appearance of an elastic solid, followed by curing and drying steps. The drying step is usually performed under supercritical conditions (i.e. temperatures higher than 300\u00a0\u00b0C and up to 219\u00a0atm) [28]. However, other drying methods have been proposed in the last decades, such as lyophilization, which avoids such as extreme operating conditions [7,29].Microwave heating can be used for chemical synthesis as a heating source besides to favour interactions between reactants [30,31]. Their dipolar polarization and conduction of ions in liquid solutions cause the collision of ions and molecules to produce uniform heating in the precursor mixture, promoting the reactions. This technology is very effective as the energy is transferred directly to the reactants, minimizing gradients and energy losses. As a consequence, synthesis time is considerably reduced. Hence has been previously used for sol-gel synthesis of carbon [32,33] and silica gels [34,35]. Moreover, Mart\u00ednez-L\u00e1zaro et\u00a0al. used microwave heating to produce Pd aerogels with high yield by using a rapid and controlled synthesis process [29].Pd has been successfully used as electrocatalyst due to its high speed of electron transfer [21,36,37], and is considered a replacement for Pt as electrocatalyst in the ethanol oxidation reaction due to the higher density current in alkaline media [13,19\u201321,38]. On the other hand, there is an increasing interest in developing new and efficient electrocatalysts but avoiding the use of noble metals due to their high cost and low availability [46\u201348]. Therefore, in this work, Pd was partially substituted by transition metals (TM). Specifically Fe, Co, and Ni were combined with Pd to produce a cost- and electrocatalytic-effective metallic aerogel by means of microwave heating. The innovative procedure and final aerogels obtained are presented as an interesting step forward in the development of innovative and efficient electrocatalysts for energy generation devices.PdCl2 (99%, Sigma-Aldrich ReagenPlus\u00ae, anhydrous powder, St Louis, MO, USA), CoCl2 \u2217 6H2O (98%, Sigma-Aldrich \u00ae, ACS reagent, St Louis, MO, USA), NiCl2 \u2217 6H2O (99.9%, Sigma-Aldrich\u00ae, St Louis, MO, USA), FeCl2 (98%, Sigma-Aldrich \u00ae, St Louis, MO, USA), Na2CO3 anhydrous (99.5%, JT. Baker R\u00ae), Glyoxilic acid (98% Sigma-Aldrich). Water was provided with the UPT-IV-10L system (18.3\u00a0M\uab65 cm).The synthesis procedure is based on previous work on Pd-aerogels [29] with some modifications. Aerogels [Pd:TM] were prepared in proportion [4:1] for each metal salt (Co, Fe, Ni). The concentration of the precursor salt solution mixture was 2\u00a0mg/ml20\u00a0ml of the mixture was taken to react with sodium carbonate and glyoxylic acid monohydrate solution (ratio 6:1) [14] in deionized water. The heating source consisted in a microwave heating (MW) at 67\u00a0\u00b0C controlled by introducing a thermocouple into the precursor mixture. All process was carried out during 7\u00a0h: the first 2\u00a0h takes place the formation of the colloids and the last 5\u00a0h the gelation and crosslinking processes are carried out. Subsequently, the samples were cooled to room temperature for washing before drying.The drying process was performed with a residual volume of deionized water. The mixture was then frozen with liquid N2 and the solvent eliminated by lyophilization (HyperCOOL, model: HC3110) at\u00a0\u2212110\u00a0\u00b0C for 24\u00a0h. A scheme of the whole synthesis process is presented in Fig.\u00a01\n.The morphology of Pd-TM aerogels was characterized using a scanning electron microscope (SEM, Quanta FEG 650 microscopes from FEI). The crystal structures were evaluated by X-ray diffraction (XRD; D8-advance diffractometer Bruker) equipped with CuK\u03b1 X-ray source (\u03bb\u00a0=\u00a00.1541\u00a0nm, 40\u00a0kV, 40\u00a0mA) using a step size of 0.02\u00b0 2\u03f4 and a scan step time of 5\u00a0s. The calculation of the lattice constant \n\n(\n\nd\n\nh\nk\nl\n\n\n\n) was made from the following expressions:\n\n(1)\n\n\n\nd\n\ns\np\na\nc\ni\nn\ng\n\n\n=\n\n\u03bb\n\n2\n\nsin\n\n\u03b8\n\n\n\n\n\n\n\n\n\n(2)\n\n\n\nd\n\nh\nk\nl\n\n\n=\n\nd\n\ns\np\na\nc\ni\nn\ng\n\n\n\n\n\nh\n2\n\n+\n\nk\n2\n\n+\n\nl\n2\n\n\n\n\n\n\nwhere \n\n\nd\n\ns\np\na\nc\ni\nn\ng\n\n\n\n is the interplanar spacing and h, k and l the miller index. The position of the 2\u03f4 peaks and the width at the half height of the peak (\n\n\n\u03b2\n\nh\nk\nl\n\n\n\n)sample were also determined. The instrumental correction was carried out with a standard pattern and the error of the instrument and \n\n\n\u03b2\n\nT\no\nt\na\nl\n\n\n\n was adjusted using the following expression:\n\n(3)\n\n\n\n\u03b2\n\nT\no\nt\na\nl\n\n\n=\n\n\u03b2\n\nE\nx\np\ne\nr\ni\nm\ne\nn\nt\na\nl\n\n\n+\n\n\u03b2\n\ns\na\nm\np\nl\ne\n\n\n\n\n\nwhere \n\n\n\u03b2\n\nE\nx\np\ne\nr\ni\nm\ne\nn\nt\na\nl\n\n\n\n is the error of the instrument and \n\n\n\u03b2\n\ns\na\nm\np\nl\ne\n\n\n\n the sample values. From the data collected in the XRD, the crystallite size and the microstrain of the crystal lattice were determined. First, the crystallite size (\n\n\nD\n111\n\n\n)\n was calculated using the Scherrer equation:\n\n(4)\n\n\n\nD\n111\n\n=\n\n\nK\n\u03bb\n\n\n\n\u03b2\n111\n\n\ncos\n\n\u019f\n\n\n\n\n\nwhere K is Scherrer constant corresponding to cubic symmetry, and \n\n(\n\n\u03b2\n111\n\n)\n\n is the line broadening of width at half maximum.The specific surface area was estimated by nitrogen adsorption-desorption isotherms at\u00a0\u2212196\u00a0\u00b0C (Micromeritics ASAP 2020), after outgassing the samples overnight at 120\u00a0\u00b0C. The chemical surface composition and the oxidation state of elements were measured by X-ray photoelectron spectroscopy (XPS, Analyzer Phoibos 100, SPECS, Germany). A monochromatic AlK\u03b1 X-ray source operating at 14\u00a0kV was employed to perform the analysis.The electrochemical evaluation of the Pd-TM aerogels was carried out in a conventional three-electrode cell in alkaline media at 50\u00a0mV/s scan rate. A glassy carbon electrode (3\u00a0mm of diameter), an Ag/AgCl electrode, and a Pt wire were used as working, reference and counter electrodes, respectively. The electrocatalyst ink was prepared using each sample in a mixture of deionized water (500\u00a0\u03bcL) and Nafion\u00ae (5%, 50\u00a0\u03bcL) per milligram of aerogel.The ink was sonicated during 15\u00a0min and then 10\u00a0\u03bcL were deposited over the working electrode surface. The electrolyte was bubbled with N2 for 40\u00a0min before the electrochemical measurements. For each sample, the electrochemical profile was obtained by cyclic voltammetry (CV) experiments in 0.5\u00a0M KOH within a potential range of\u00a0\u22120.25 to 1.6\u00a0V vs RHE. The electrochemical active surface area (ECSA) of each aerogel was estimated from the cyclic voltammograms by using the reduction charge of Pd (II) oxide according to the following equation:\n\n(5)\n\n\nE\nC\nS\nA\n=\n\n\nQ\nm\n\n\n\nm\n\nP\nd\n\n\n\n\ne\nd\n\nm\n\n\n\n\n\n\nwhere, Q\n\nm\n denotes coulombic charge (Q per \u03bcC cm-2) for the reduction of Pd (II) oxide achieved by integrating the charges related to the reduction of Pd (II) oxide for the different samples; mPd is the mass amount of Pd loaded (g cm-2) on the glassy carbon electrode surface and ed\n\nm\n is a constant (424\u00a0\u03bcC/cm), which corresponds to the reduction of a Pd (II) oxide monolayer.The electrocatalytic activity of the Pd-TM aerogels towards the electrooxidation of ethanol was tested by cyclic voltammetry (CV) in 0.5\u00a0M KOH\u00a0+\u00a00.5\u00a0M ethanol solution within a potential range between\u00a0\u22120.25 and 1.6\u00a0V vs. RHE. This solution was bubbled with N2 for 40\u00a0min before each electrochemical measurement.The stability performance was carried out by chronoamperometry in the three-electrode system at 0.6\u00a0V vs. RHE for 90\u00a0h in 0.5\u00a0M KOH\u00a0+0.5\u00a0M ethanol as electrolyte.XRD spectra of the synthesized aerogels are shown in Fig.\u00a02\na. In the XRD pattern of the Pd and Pd-TM aerogels were observed four major diffraction peaks appear at the 2\u03f4 values. The peak with the highest intensity was used for the calculation of crystallite size. For Pd aerogel, the peaks were 40.1\u00b0, 46.6\u00b0, 68.1\u00b0 and 82.1\u00b0 which are ascribed to the (111), (200), (220) and (311) reflection planes of Pd (JCPDS #46\u20131043), respectively. These peaks are according with a face-centered cubic (FCC) crystal structure. Despite the resulting aerogels revealed characteristic peaks of Pd, a shift in the position of the Pd-peaks was observed for Pd-TM aerogels. These show a change to the left suggesting the presence of alloys. The PdCo aerogel presents the peaks in 2\u03f4 were 40.1\u00b0, 46.2\u00b0 68.1\u00b0 and 81.8\u00b0, comparing with the reference JCPDS #65\u20136075 corresponding to PdCo, which has a peak with high intensity in 2\u03f4 with the value of 41.7\u00b0 corresponding to reflection plane (111), being further away from the one obtained and coinciding even more with the JCPDS #46\u20131043 of Pd [39]. In the case of PdNi aerogel, peaks were located at 39.9\u00b0, 46.6\u00b0, 67.9\u00b0 and 81.9, presenting a more significant displacement compared to the JCPDS #46\u20131043 of Pd, JCPDS #72\u20139071 corresponding to PdNi [40] with peaks in 40.2\u00b0, 47\u00b0, 68.8\u00b0, 82.9\u00b0 and 87.3\u00b0 which do not correspond to those obtained, again indicating more excellent proximity to the JCPDS of the Pd. Finally, PdFe aerogel with peaks in 35.5\u00b0, 40.1\u00b0, 46.9\u00b0, 57.1\u00b0, 62.9\u00b0, 68\u00b0, 81.9\u00b0 and 86.6, presenting again the trend observed with the previously mentioned aerogels where a shift to the left is shown. Although the closeness of the peaks is observed, corresponding to the JCPDS of the Pd, it is taken into account presents peaks at 35.5\u00b0 and 86.6\u00b0 that correspond to reflection planes of FeO(OH) (JCPDS #74\u20133080) [41].The \n\n\nd\n\ns\np\na\nc\ni\nn\ng\n\n\n\n for Pd, PdCo, PdNi and PdFe aerogels calculated at (111), (200), (220) and (311) were 2.24\u00a0\u00c5, 1.95\u00a0\u00c5, 1.37\u00a0\u00c5 and 1.17\u00a0\u00c5, respectively. These values were employed to calculate the lattice constants (\n\n\nd\n\nh\nk\nl\n\n\n\n) and evaluate if the addition of TM to the Pd structure makes any difference. Thus, the calculated \n\n\nd\n111\n\n\n values were 3.89, 3.90, 3.89 and 3.88 for Pd, PdCo, PdNi and PdFe, respectively (the error is\u00a0\u2264\u00a00.3 of the standard JCPDS #46\u20131043 with a value of 3.89).The crystallite sizes obtained using the 2\u03f4 value of 40.1\u00b0, with the Scherrer equation (Eq. (4)) were 9.46, 3.70, 3.96 and 11.04\u00a0nm for Pd, PdCo, PdNi and PdFe, respectively. There is a great difference between materials, detecting a notable decrease of crystallite size for PdCo and PdNi aerogels and a slight increase for PdFe aerogel compared to that obtained for Pd aerogel.Calculation of microstrain (\u03b5) with XRD data was also performed to evaluate if it was the cause of the broadening of the peaks, indicating dislocations within the crystal structure. For this evaluation the following equation was used:\n\n(6)\n\n\n\u03b5\n=\n\n\n\u03b2\n\nT\no\nt\na\nl\n\n\n\n4\n\ntan\n\n\u03b8\n\n\n\n\n\n\nThe data obtained could indicate a decrease in microstrain as the angle increases and the crystallite size should be consistent in a range of two theta values. However, the microstrain values obtained do not show a decreasing trend and the crystallite size decreases as the angle increases. This result clearly indicates that the crystallite size also contributes to peak broadening. Therefore, the Williamson Hall method (W\u2013H) was employed using the following expression:\n\n(7)\n\n\n\n\u03b2\n\nT\no\nt\na\nl\n\n\n\ncos\n\n\n\u03b8\n\nh\nk\nl\n\n\n=\n\u03b5\n\n(\n\n4\n\ns\ne\nn\n\n\n\u03b8\n\nh\nk\nl\n\n\n\n)\n\n+\n\n\nK\n\u03bb\n\nD\n\n\n\n\n\nThe first four reflections (111), (200), (220) and (311) were used to construct the plot of 4sen\u03f4hkl vs \u03b2Totalcos\u03f4hkl. The crystallite size and the microstrain are obtained from the intercept and slope of the fitted line, respectively. The values obtained for crystallite size were 5.5\u00a0nm and 2.53\u00a0nm for PdNi and PdFe (Fig.\u00a02b). According to the results, an increasing trend is observed, implying the presence of defects (i.e. distortions, dislocations, faulting and relaxation of the grain surface). In addition, the fitting line does not cross the ordinate axe through the origin, indicating a significant size effect. The microstrain obtained in both samples is very close, as shown by the slope of the adjustment. For PdNi a linear adjustment was obtained without taking into account the point that is too deviated (1.6, 0.041). In the case of PdCo, the W\u2013H method was not able to be applied, since the values of \n\n4\n\ns\ne\nn\n\n\n\u03b8\n\nh\nk\nl\n\n\n\n do not vary with \u03b2Totalcos\u03f4hkl. This is due to the fact that the stress in the crystal lattice since the strain is not related to the broadening of the diffraction peaks. In the case of PdNi and PdFe assuming that the broadening of the peaks is due to the contribution of microstrain.The specific surface area of the Pd-TM aerogels was determined by the Brunauer-Emmett-Teller (BET) method applied to the nitrogen adsorption-desorption isotherms (Fig.\u00a03\n) yielding values of 77, 66, 47 and 4\u00a0m2/g for Pd, PdCo, PdNi, and PdFe respectively. This indicates the presence of certain microporosity in these samples, especially for Pd, PdCo and PdNi aerogels. The helium densities of these samples are 1.4, 1.2 and 2.5\u00a0g/cm3 for PdCo, PdNi and PdFe, respectively. These values indicate a clear difference in the total porosity of the samples, being the Pd\u2013Ni the sample less dense, and therefore, with higher total porosity. By applying 2D-NLDFT to the adsorption isotherms, a pore size distribution is obtained (Fig.\u00a03b). Although it cannot be provided a mean pore size for each aerogel obtained due to the wide pore size distribution, it can be said that there is a clear trend to increase in the pore size from PdFe, PdNi, to PdCo. The lowest helium density of PdNi aerogel probably indicates the presence of macroporosity in higher extent than in PdCo and for sure in much higher extent than PdFe. This latter aerogel is the less porous of the series.BET surface areas of Pd aerogels should be in the range of 32\u2013162\u00a0m2/g, the density in the range of 0.10\u201312\u00a0g/cm3 and the pore size <50\u00a0nm according to the bibliography [6,19,20]. In fact, the surface area of the Pd aerogel is referenced as 50\u00a0m2/g and the pore size of about 11\u00a0nm [20]. Comparing these values with the values obtained in this work, except in the case of PdFe aerogel, in general, the porous properties are preserved. This is a very relevant feature because is a key factor for promoting mass transport through the 3D porous network.Further, the morphology of the aerogels was characterized by Field Emission Scanning Electron Microscopy (FESEM) and High Resolution Transmission Electron Microscopy (HRTEM). The micrographs are presented in Fig.\u00a04\n.The aerogels present a foam-like morphology, with a 3-dimensional open porous network. The FESEM images show a more open porosity structure in the case of PdCo (Fig.\u00a04a), and a more fused and less porous structure in the case of the PdFe aerogel (Fig.\u00a04c). The aerogel PdNi (Fig.\u00a04b) presents an intermediated porous morphology. These observations are in agreement with the results previously discussed from helium density and N2 adsorption-desorption isotherms. HR-TEM analysis reveals an interconnected distribution of particles in all samples obtained (Fig.\u00a04 d-f). Images at different magnifications show chains of particles defining pores of different sizes between them, typically characteristic for aerogels. PdCo aerogel shows particle sizes between 2 and 4\u00a0nm, similar to the sample PdFe (between 1 and 3\u00a0nm). It seems that Co and Fe metal transition incorporation allows obtaining smaller sizes than that obtained for PdNi particles (5\u20138\u00a0nm). At higher magnification, it can be seen that the interplanar distances are slightly higher for PdNi aerogel (0.225\u00a0nm), which can be attributed to the presence of alloy between the two metals used in the synthesis. The interplanar distance for Pd\u2013Co and PdFe is 0.22 and 0.223\u00a0nm respectively.By means of X-ray photoelectronic spectroscopy (XPS), the analysis of the Pd\u2013Ni, Pd\u2013Fe, and PdCo aerogels was carried out to know their chemical composition and the oxidation state (see Fig.\u00a05\n). The general spectrum of the samples (Fig.\u00a05a) indicates the presence of C, Pd, Co, O, Fe, and Ni, as expected. The spectra of Pd 3d are shown in Fig.\u00a05b, where a comparison is made of the oxidation states present in Pd\u2013Fe, Pd\u2013Ni, and Pd\u2013Co samples. Each spectrum was deconvoluted into the orbitals Pd 3d5/2 and Pd 3d3/2, which peaks are centered at 335.1\u00a0eV and 342.2\u00a0eV corresponding to Pd2+, 336.2\u00a0eV and 342.5 corresponding to Pd4+ while those located in 335.1\u00a0eV and 340.7\u00a0eV refer to Pd0 [22,39,40]. A higher concentration of Pd2+ is observed in the Pd\u2013Fe and Pd\u2013Ni samples, meanwhile, Pd4+ is present in a higher concentration in the Pd\u2013Fe and Pd\u2013Co samples. The high-resolution scan of Fe 2p is presented in Fig.\u00a05e. The spectra of Fe 2p3/2 and Fe 2p1/2 were deconvoluted, giving the peaks 709.8\u00a0eV and 724.09\u00a0eV that refer to Fe0, signals in 712.5\u00a0eV and 724.2\u00a0eV that corresponds to Fe2+ and those located in 715.0\u00a0eV and 732.5\u00a0eV attributed to Fe3+. These results suggest the presence of the Fe2O4 structure as already reported in the literature [41]. The quantitative evaluation of each peak was also made from the XPS results by dividing the area of the peak, which was calculated from the cross-sections and the mean depth of the exhaust electrons [42]. XPS data were interpreted using the Avantage Thermo software, with angular resolution (ARXPS) and the adjustment of the peaks by the Analyzer software. Fe0.94 Pd0.1 was the molar proportion obtained for the PdFe aerogel. Furthermore, the chemical composition was also determined by calculating the percentage of the relative weight of the sample. PdFe aerogel presents a higher percentage of Pd2+ (41.8%) in comparison with Pd4+ (11.7%), the percentage of the iron species in its metallic state (Fe0) is 40.2%, while the percentage of the oxidized species Fe2+ and Fe3+ is 38.5% and11.9%, respectively. These results suggest that PdFe aerogels can be oxidized, resulting in the oxidation of Fe0 to Fe3+. However, the oxidation is not complete, as can be seen in Fig.\u00a05e, which is consistent with the results of XRD. The spectrum of Ni 2p is observed in Fig.\u00a05c, having deconvolution orbitals in Ni 2p3/2 and Ni 2p1/2, with the peaks of 855.1\u00a0eV and 872.5\u00a0eV corresponding to Ni2+, while those located at 856.6\u00a0eV and 874.0\u00a0eV refer to Ni3+ [43,44]. The molar proportion obtained for PdNi aerogel is Ni 0.64Pd0.1. From the calculation of the percentage of the relative weight of the sample, it can be inferred that PdNi presents a higher percentage of Ni2+ (45.8%) than Ni3+ (28.7%).The high-resolution scanning of Co 2p is presented in Fig.\u00a05d. The spectra of Co 2p3/2 and Co 2p1/2 were deconvoluted, giving the peaks 784.9\u00a0eV and 797.2 that correspond to Co2+ and those located in 780.2\u00a0eV and 796.72 referred to Co3+, which correspond to the Co3O4 structure [45]. The presence of the PdCo alloy on the surface can be appreciated, having the two spectra in their oxidation state. The quantitative evaluation of each peak was obtained by dividing the area of the peak, which was calculated from the cross-sections, and the mean depth of the escape electrons. The molar proportion for PdCo aerogels was Pd0.1Co0.72. In addition, the percentage of the relative weight of the sample indicates that there is a higher percentage of Pd0 (40.3%) than the oxidized species Pd2+ (10.5%) and a higher proportion of the oxidized species Co2+ (38.5%) in comparison to Co3 (18.6%). This shows that PdCo aerogels can be oxidized, resulting in a larger proportion of Co2+ than Co3+, which is consistent with the results of XRD.The elemental composition, the oxidation states, and the relationship between their concentrations evaluated by the XPS analysis will be related to the following electrochemical analysis.Aerogels were evaluated by cyclic voltammetry (CV) to determine their electrocatalytic activity for ethanol oxidation reaction (EOR) in a potential range between\u00a0\u22120.25 and 1.6\u00a0V vs RHE. Electrochemical profiles were obtained in 0.5\u00a0M KOH as electrolyte at ambient conditions at 50\u00a0mV/s scan rate (Fig.\u00a06\na). A Pd aerogel sample obtained by the same synthesis method was also characterized for comparative purposes. The experimental CVs show the peaks attributed to Pd activity: (i) the hydrogen desorption in the range 0.1\u20130.2\u00a0V vs RHE, (ii) hydrogen adsorption at 0.3\u00a0V vs RHE, (iii) reduction of Pd (II) oxide from 0.55 to 0.8\u00a0V vs RHE and (iv) formation of Pd (II) oxide at 1\u20131.4\u00a0V vs RHE. All these phenomena are present in the cyclic voltammograms of all Pd aerogels. However, samples containing transition metals show peaks that are attributed to the oxidation and reduction of each metal species (i.e. Co, Ni and Fe). The redox processes of the noble metal species can be observed at different potentials (Fig.\u00a06a), for PdNi at 1.2\u00a0V vs RHE, marked with the legend 3 [Ni]; for PdCo oxidation and reduction processes are observed between 1.1 and 1.3\u00a0V vs RHE, which can be seen with the legend 2 [Co]; while Fe redox processes occur between 0.3 and 0.6\u00a0V vs RHE, which are labeled 1 [Fe] and 1 [Fe]'. ECSA values obtained were 3.2, 5.7, 9.4 and 26.5\u00a0m2/g for Pd, PdNi, PdFe, and PdCo respectively. The Pd-TM aerogels show a significant improvement in the electrochemical performance in comparison to bare Pd and using low loading of noble metals. The ECSA values obtained in the PdTM improve the diffusion of the reagents, in this case, ethanol, through the electrode. In addition, XRD analysis shows that the formation of crystals between Pd and transition metals is clearly an advantage for electron transfer in comparison to bare Pd.In order to compare the electrochemical performance of the Pd-TM electrocatalysts, the aerogels were evaluated for the ethanol oxidation reaction (EOR). It was performed by CV experiments in an electrolyte containing 0.5\u00a0M\u00a0C2H6O\u00a0+ 0.5\u00a0M KOH. EOR was carried out in the same range of potential than the CV tests (i.e., from\u00a0\u22120.25 to 1.6\u00a0V vs RHE). The electrochemical profiles for EOR demonstrated that Pd-TM aerogels show high performance at room temperature in the range of 0.7\u20131.1\u00a0V vs RHE, Fig.\u00a06b.It is observed that the current activity of the Pd-TM aerogels is improved, in comparison to bare Pd aerogel with a value of 48\u00a0mA/mg, following the trend PdNi\u00a0>\u00a0PdCo\u00a0>\u00a0PdFe, with 117, 96.1, and 73.35\u00a0mA/mg, respectively. PdNi aerogel shows the best electroactivity towards EOR (117\u00a0mA/mg). However, the thermodynamic reaction potential is slightly favored by the PdCo aerogel, whose highest peak current can be observed at (0.86\u00a0V vs. RHE, while thePdNi peak appears at 0.91\u00a0V vs. RHE. Since the difference in reaction potential towards the OER is very small, it can be assumed that the best catalyst is PdNi. These two values are close to the potential obtained with the Pd bare aerogel (i.e. 0.88\u00a0V vs RHE). In the case of the PdFe aerogel, this sample shows its maximum peak at 0.93\u00a0V vs RHE. The suggested mechanism for C2H5OH in an alkaline environment on electrode surfaces with Pd-TM aerogels is the next:\n\n(8)\nPd\u00a0+\u00a0OH\u2212 \u2192 Pd-OHads\u00a0+\u00a0e\u2212\n\n\n\n\n\n(9)\nPd\u2013CH3CH2OH \u2192 Pd(TM)-(CH3CH2OH)ads\n\n\n\n\n\n(10)\nPd-(CH3CH2OH)ads\u00a0+\u00a03OH\u2212 \u2192 Pd-(CH3CO)ads\u00a0+\u00a03H2O\u00a0+\u00a03e\u2212\n\n\n\n\n\n(11)\nPd-(CH3CO)ads\u00a0+\u00a0Pd-OHads\u00a0+\u00a0OH\u2212 \u2192 CH3COO\u2212\u00a0+\u00a02Pd\u00a0+\u00a0H2O\n\n\nEthanol oxidation mechanism with these Pd-TM aerogels occurs in two pairs of reactions (Eqs. (8)\u2013(11)) [51\u201353]. In the first two steps, the OH\u2212 present in the solution reach the surface of the electrode and it absorbs OH\u2212 efficiently since the transition metals accelerate this process [58]. On the other hand, the CH3CO (Eq. (3)) generated in this reaction are adsorbed on the surface of Pd catalysts, this combination between Pd-TM considerably accelerates the EOR (Fig.\u00a06b). In addition, the combination of the reaction of Pd-TM with hydroxyl groups and the CH3CO adsorbed on the catalyst surface are the rate-determining step of the EOR (Eq. (4)). The transition metals (Ni, Fe, Co) provide an opportunity to improve the adsorption of OHads onto the surface of the catalyst and thus activate the catalyst surface and finally help to enhance the ethanol oxidation process [59]. On the other hand, the C\u2013C bond in the C2H5OH molecule is very difficult to break, despite the fact that Pd-TM aerogels work efficiently, the interaction between C2H5OH and the catalyst ends up generating subproducts. Pd-TM catalysts promote the oxidation of the fuel following the trend: Co\u00a0>\u00a0Ni\u00a0>\u00a0Fe. These aerogels are highly active catalysts for EOR, and their performance is shown in Fig.\u00a06b, where peaks between 0.7 and 1.1\u00a0V vs RHE for all aerogels represent the EOR reaction and CO2 formation, and CO2 desorption is observed in the potential range between 0.55 and 0.68\u00a0V vs RHE [38,53,54]. Comparison of ECSA, potential peak value and current density for EOR for obtained catalysts in this work with electrodes already reported is shown in Table\u00a01\n. The results demonstrated that the electrocatalysts applied in this study have relative advantages, in terms of the potential value and current density.The maximum electrochemical activity value among all Pd-TM aerogels evaluated was detected in the range from 0.5 to 1.0\u00a0V vs RHE. Therefore, the stability test was carried out at 0.6\u00a0V vs RHE. Chronoamperometry was performed for 90\u00a0h as shown in Fig.\u00a07\n. Apparently, PdNi retains 25\u00a0mA\u00a0mg-1 at 10\u00a0h, and is higher than the rest of the catalysts, except PdCo, which retains 23\u00a0mAmg-1 in the same time interval. These two catalysts show the highest antitoxic capacity of the intermediate products and the high electrochemical stability [56] due to their constant performance. Although the Pd and PdFe samples also turn out to be highly stable and resistant to the corrosion of the medium, the PdNi and PdCo samples are superior. The retention capacity of the samples can be attributed to the number of active sites and their BET surface area mainly [56\u201358]. At the end of the 90-h test, the PdNi and PdCo samples showed only a 10% decrease in their charge retention. Some analysis of the materials after these electrochemical tests reveals that the degradation of PdNi and PdCo aerogels are negligible (see Supplementary Material) showing that they are very stable and effective electrocatalysts.Pd-TM aerogels were successfully synthesized using a quick and simple synthesis procedure based on microwave heating and freeze-drying. The nanostructured aerogels obtained present high specific surface areas and electrochemical surface areas, favoring the diffusion of reactants and their activity versus electrochemical reactions. On the other hand, the chemical characterization of these Pd-TM aerogels may suggest the formation of alloys, although the crystalline size and morphology are maintained.These physicochemical properties are traduced in a notable improvement of the electrochemical behavior of these aerogels. In fact, in the ethanol oxidation reaction, the Pd-TM aerogels result in 117, 96.1, and 73.35\u00a0mA/mg for PdNi, PdCo and PdFe, respectively. These values are clearly higher than the 48\u00a0mA/mg corresponding to the bare Pd aerogel.The incorporation of these transition metals clearly improves the ethanol oxidation reaction compared to Pd due to their excellent combination of physicochemical properties. This opens the possibility towards the development of highly active catalysts with a low loading of noble metals, reducing dependence and costs in electrochemical conversion energy systems as fuel cells and water electrolyzers.\nA.Mart\u00ednez-L\u00e1zaro: Writing-original draft, Methodology.\nM.H. Rodriguez-Barajas: Writing-original draft, Methodology.\nN. Rey-Raap: Investigation process, visualization, supervision.\nF.I. Espinosa: Data curation.\nJ. Ledesma-Garc\u00eda: Resources, Project administration, supervision.\nA. Arenillas: Supervision, Resources.\nL.G. Arriaga: Writing \u2013Review and editing, Project administration, supervision.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: L. G. Arriaga reports financial support was provided by Mexican Council for Science and Technology.Authors thank to Consejo Nacional de Ciencia y Tecnolog\u00eda CONACYT (Mexico) for funding through the Ciencia de Frontera 2020\u2013845132, 2019-39569 and LN-321116. Also to Ministerio de Ciencia e Innovacion (Spain), the European Union Next Generation EU/PRTR and Science, Technology and Innovation Plan 2018-2022 of the Principado de Asturias with the projects PCI2020-112039 MCIN/AEI-10.13039/501100011033, PID2020-113001RB-I00 MCIN/AEI-10.13039/501100011033 and IDI/2021/50921. N.R.-R is grateful to the Horizon-MSCA-2021-PF-01-01 call for financial support through the Metgel project 101059852. Finally, thanks to Anabel de la Cruz and Cesar Leyva for TEM-analysis.", "descript": "\n                  In the present research work, unsupported Pd-TM aerogel catalysts are prepared by an ultrafast synthesis by means of a combination of microwave heating and lyophilization. These novel aerogels are synthesized to explore the effect of transition metals on a Pd aerogel matrix in order to reduce the dependence on noble metals and increase its electrocatalytic properties for different electrochemical reactions. Physicochemical characterization of Pd-TM aerogels reveals a successful combination of high specific surface area, electrochemical surface area, and specific oxidation states. The combination of these properties in Pd-TM aerogels enhances the electrocatalytic activity towards ethanol electrooxidation compared to bare Pd aerogels. Among all the Pd-TM, PdNi aerogel presents the highest current per unit mass with 117\u00a0mA/mg, being a clear improvement compared to the Pd aerogel (48\u00a0mA/mg).\n               "}
{"full_text": "No data was used for the research described in the article.CO2 is one of the most important greenhouse gases that cause global warming, and its emissions keep increasing in recent years. Thus, it is urgent to reduce CO2 emissions to mitigate global warming. CO2 capture and utilization (CUU) enable the synthesis of a wide range of value-added products such as methane, long-chain hydrocarbons, olefins, methanol, and higher alcohols, presenting a promising way to achieve carbon neutralization. Recently, higher alcohols have drawn increasing interest because they can be used as an alternative fuel, fuel additive, solvent, and raw material. Most of the studies on higher alcohol synthesis by CO2 hydrogenation focus on analyzing the thermodynamics[1,2], designing highly efficient catalysts[3\u20136], as well as unraveling the reaction mechanism[3\u20136].As far as the catalyst design is concerned, four categories of catalysts, including Rh, Cu, Mo, and Co-based catalysts have been widely studied. Amongst, Co-based materials have been intensively investigated due to Co being non-noble metal and possessing relatively high CO2 conversion. Metallic Co generally catalyzes the conversion of a CO2/H2 mixture yielding alkanes. However, it is found that the ability of Co to break the C-O bond can be turned down by forming metal alloys or interacting with oxide support[7\u20139], which is crucial for the formation of higher alcohols. As presented in \nTable 1, Co-based catalysts such as reduced CoAlOx, Na-Co/SiO2, and Pt/Co3O4 have been studied in a fixed bed or tank reactor for higher alcohol synthesis. Accordingly, CO2 conversion ranges from 4.6% to 67.2%, and selectivity to higher alcohols ranges from 0.05% to 92.1%, with a higher alcohol yield of 0.01\u20132.16\u00a0mmol\u00a0gcat\n\u22121 h\u22121 are obtained.Generally, higher alcohols formation over Co-based catalysts involves the following steps (\nFig. 1): (i) activation of CO2 and H2 forming C1 intermediates, including CHx*, CO*, and HCOO*; (ii) formation of C-C bond through CHx*-CO* or CHx*-HCOO*coupling; (iii) hydrogenation of C2 intermediates to form ethanol. Besides, the C2 intermediates can also participate in forming higher alcohols with a longer carbon chain.In a recent review by Tang et al., they suggested that metallic Co (Co0), ionic cobalt (Co\u03b4+), cobalt oxide (CoO), and cobalt carbide (Co2C) are catalytically active species for CO2 conversion[18]. Even though the exact nature of the active Co site has not yet reached a consensus due to the interconversion of these species under reaction conditions, and more importantly the intermediates such as CHx*, CO*, and HCOO* are suggested to form on different surfaces of these species or the modified ones. Thus, the synergy between the active species on which CHx* and CO*/HCOO* occur respectively is of great importance for the coupling of C1 intermediates and higher alcohol formation. Moreover, many hetero-site Co-based catalysts such as Co0-CoO, Co0-Co\u03b4+, and Co2C-NaCo2C have been proved efficient for higher alcohols synthesis by CO2 hydrogenation; however, a review focusing on the catalytic activity of various Co species as well as their synergy in higher alcohols synthesis by CO2 hydrogenation is still absent.Thus, in this review, we first introduce the catalytic activity of Co0, CoO, Co\u03b4+, and Co2C in CO2 hydrogenation, as well as the strategies to tailor their structure. Then, the formation of hetero-site Co catalysts and the synergy of these hetero sites in promoting higher alcohols synthesis are discussed. Finally, we propose new strategies to further enhance the synergy of hetero sites in Co-based catalysts for boosting higher alcohols synthesis by CO2 hydrogenation. Hence, we hope that this review will be beneficial to those working on Co-based catalysts for higher alcohols synthesis by CO2 hydrogenation.Co0, CoO, Co2C, and Co\u03b4+ are active for CO2 hydrogenation forming various products including CO, CH4, C2+ hydrocarbons, and alcohols. The catalytic activity of Co species depends on their structure which can be tailored by rational catalysts design. In Sections 2 to 4, we discuss the roles of Co0, CoO, Co2C, and Co\u03b4+ in CO2 hydrogenation as well as the strategies to tailor their structure.Even though both Co0 and CoO catalyze CO2 hydrogenation, the activity and selectivity vary over catalysts prepared by different synthesis methods under various conditions. For example, Have et al. prepared SiO2, Al2O3, CeO2, and TiO2 supported Co0 and CoO catalysts by controlling the reduction temperature by H2 (i.e., 450 and 250\u2009\u00b0C for Co0 and CoO, respectively) and studied their catalytic performance in CO2 hydrogenation (in a fixed bed reactor, 250\u2009\u00b0C, 20\u2009bar, H2/CO2 = 3)[19]. Accordingly, SiO2, Al2O3, and CeO2 supported Co0, as well as TiO2 supported CoO, possessed a higher Co-time yield than their counterparts (\nFig. 2a). Regarding product selectivity, Co0/Al2O3 mainly produces CO while all other catalysts produce mainly methane with a minor amount of CO (Fig. 2b). The differences in the catalytic performance of these catalysts are ascribed to the differences in the oxidation state of Co and the Co-metal oxide support interaction, indicating the complex nature of Co-based catalysts in CO2 hydrogenation. More specifically, in Co0 catalysts, the incomplete reduction, the Co-metal oxide support interaction, the oxidation by CO2/H2O during the reaction, as well as the passivation during catalyst synthesis and transport may lead to the presence of CoO/Co\u03b4+ species, while in CoO catalysts, the reduction by H2 during the reaction may result in the formation Co0. Hence, all these factors make it difficult to unravel the exact nature of the catalytically active species.However, it is generally believed that Co0 is more active to dissociate H2 offering active H[20,21]. Besides, Co0 possesses a high electron density near the Fermi level (\nFig. 3), offering excited electrons for the hydrogenation steps[22]. Accordingly, the active H formation together with the excited electrons provided by Co0 enhances CO2 hydrogenation. On the other hand, the onset of the edge of the CoO valence band is 1.15\u2009eV below the Fermi level (Fig. 3), so it is difficult to thermally excite sufficient electron-hole pairs for catalytic reaction at low temperatures (e.g., <350\u2009\u00b0C)[22]. Thus, Co-based catalysts with more Co0 species generally possess higher CO2 conversion and CH4 selectivity (or CH4 production rate)[22\u201324].Moreover, Co0 and CoO offer different surfaces for CO2 adsorption and activation. Density functional theory (DFT) calculation is used to study the adsorption of CO2 on Co0 (110) and CoO (100) facets. It is found that the O\u2013C\u2013O bond angle deforms more and the C\u2013O bond elongates more on Co (110) facet than on the CoO (100) facet, suggesting a stronger activation of CO2 by Co0 (110) surface[19]. Zhao et al. compared the CO2 adsorption on (Co)0.5(CoO)0.5 and (Co)0.2(CoO)0.8 catalysts using CO2 temperature programmed desorption (CO2-TPD) experiments[22]. They found that CO2 binds stronger over (Co)0.5(CoO)0.5, suggesting the presence of a more basic surface probably due to a higher concentration of Co0. Furthermore, Yin et al. proposed that the defective unsaturated CoO can effectively accelerate CO2 adsorption and activation forming carboxylate intermediates at the Co-CoO interface. The carboxylate intermediates can be further hydrogenated to CH4 or decomposed to form CO at higher temperatures[25]. Besides, the CO2 may also directly dissociate on Co0 surface[21], which will be further discussed (next paragraphs in this section, \nFig. 4).Co0 and CoO possess different activities for carbon chain propagation. Co0 has been recognized as the active species for Fischer-Tropsch synthesis (FTS) and is active for carbon chain growth[26\u201330]. Consistently, in CO2 hydrogenation, Zhao et al. found that C2H6 generated through carbon chain growth is preferably produced on catalysts with higher Co0 concentration. This suggests that Co0 rather than the CoO is responsible for the C-C coupling in CO2 hydrogenation[22]. However, Ten Have et al. observed that the CoO catalysts produce more olefinic C2 and C3 products than Co0, indicating that the lower active H availability and less favorable hydrogenation on CoO, which may promote C-C coupling[19].Notably, the CO2 hydrogenation reaction mechanisms over Co0 and CoO are different. Ten Have et al. studied the reaction mechanism over SiO2, Al2O3, CeO2, and TiO2 supported Co0 and CoO by time-resolved infrared spectroscopy (IR)[19]. And it was found that CO* species only occur on Co0 catalysts irrespective of the support material employed, indicating a direct CO2 dissociation mechanism over the Co0 sites (Fig. 4). However, formyl, formate, and carbonate species instead of CO* species are observed over CoO-based catalysts, suggesting that CoO catalysts follow the H-assisted formate mechanism over CoO (Fig. 4)[31]. It is also proved that the direct CO2 dissociation pathway occurs at a higher rate than the H-assisted pathway. However, the H-assisted pathway is more favorable for C2+ hydrocarbons formation[31]. Besides, Wang et al. proposed the carboxylate pathway over Co0, in which the carboxylate is subsequently dissociated into *CO and then further hydrogenated to CH4, due to the stronger CO and H2 adsorption and activation on Co0 sites (Fig. 4)[31]. The carboxylate intermediate also forms on the oxygen vacancies presenting in CoO followed by hydrogenation or decomposition to form CH4 and CO respectively[23].Interestingly, there are also differences in the CO2 hydrogenation over hexagonal close-packed (hcp) and face-centered cubic (fcc) Co0 sites. DFT calculations indicate that the dissociation of CO2 into chemisorbed CO* and O* occurs on both hcp-Co0 and fcc-Co0 (\nFig. 5). Subsequently, over hcp-Co0 (10\u221210) facet, the energy barrier of CO*+H*\u2192HCO* is lower than CO* desorption, leading to high CH4 selectivity. In the contrast, over fcc-Co (111) facet, CO* is easier to desorb to form CO product due to a lower CO* desorption energy (Fig. 5) [32].Controlling the reduction of Co3O4 is a simple way to tailor the content of both CoO and Co0 species in the catalyst. Generally, the reduction of Co3O4 proceeds through two steps, i.e. Co3O4 \u2192 CoO and the subsequent CoO \u2192 Co. Lv et al. studied the reduction of spherical Co3O4 nanoparticles by temperature programmed reduction (TPR) using a H2/N2 mixture (10\u2009vol%\u2009H2)[33]. Accordingly, two distinct reduction peaks at 276\u2009\u00b0C and 350\u2009\u00b0C are observed. Besides, the amount of H2 consumed by the low-temperature reduction step is about 1/3 of that of the high-temperature reduction step, confirming the reduction through Co3O4 \u2192 CoO \u2192 Co. Moreover, Zhao et al. used a microbalance to track the weight loss during the reduction of Co3O4 in a H2/He mixture (40\u2009v% H2)[22]. As presented in \nFig. 6, the mass loss indicates that unsupported Co3O4 starts to be reduced at around 200\u2009\u00b0C and completely transformed to CoO at 300\u2009\u00b0C. The complete reduction to Co0 occurs at 370\u2009\u00b0C. Thus, it is feasible to tailor the percentage of CoO and Co0 in the catalyst by controlling the reduction temperature and the reduction duration.\nIn situ X-ray diffraction (XRD) study on the reduction of unsupported Co3O4 also proves a two-step reduction process; however, the reduction temperature differs due to different properties of the Co3O4 precursor used as well as different reduction conditions employed[34]. It is found that Co3O4 is first reduced to CoO, followed by complete transformation to hcp-Co0 at 250\u2009\u00b0C in the H2 atmosphere. Then, hcp-Co0 transforms to fcc-Co0 at 350\u2009\u00b0C, and at 450\u2009\u00b0C only fcc-Co0 is detected. Increasing the temperature to 500\u2009\u00b0C increases the crystallinity of the fcc-Co0. However, reducing unsupported Co3O4 first at low temperatures (e.g., 250\u2013300\u2009\u00b0C) for 3\u20135\u2009h before increasing the temperature enables the production of hcp/fcc-Co0mixture up to 450\u2013500\u2009\u00b0C[34].By controlling the reduction temperature, a series of Cox(CoO)1\u2212x catalytic systems were obtained for CO2 hydrogenation[22]. At 1\u2009bar and 180/200/220\u2009\u00b0C, the CO2 conversion increases with increasing content of Co0, reaching the maximum when x\u2009=\u20090.2 (\nFig. 7). Then, CO2 conversion decreases with further increasing Co0 content. Moreover, methane is the main product, whose selectivity and production rate possess a similar volcano trend.The support/promoter plays a key role in tailoring the reducibility as well as the dispersion of the Co species and in turn, affects the catalytic performance. Jacobs et al. found that the interaction between Co surface species and the support is stronger for Al2O3 and TiO2 supported catalysts than the SiO2 supported one[35]. The strong interaction on one hand decreases the reducibility of Co, and on the other hand, leads to the formation of smaller clusters. Al2O3 supported catalyst is more difficult to be reduced, but it possesses a smaller cluster size and thus resulting in higher availability of surface metal sites after reduction than the SiO2 and TiO2 supported ones. Further incorporation of noble metals (i.e., Ru, Pt, Re) increased the degree of reduction but shows a negligible change in the cluster size[35]. More specifically, Ru and Pt were found to facilitate the reduction of both Co oxides and Co species which strongly interacted with the support, while Re mainly promotes the reduction of the Co species which strongly interacted with the support. Accordingly, on the catalysts possessing weak Co clusters-support interaction (i.e., Co/SiO2), the noble metal promoter was found to only slightly increased the number of surface Co metal. However, over the catalysts with strong interaction (i.e., Co/Al2O3 and Co/TiO2), the number of active sites increases significantly. Besides, the metal oxide (i.e., B, La, K, Zr) promoter decreases the reducibility of the catalyst, thereby reducing the number of surface Co atoms[35].The change in reducibility and dispersion further influences CO2 hydrogenation activity. As a support/promoter, metal oxide shows significant influences. Wang et al. investigated the effects of silica on the property and catalytic activity of Co-based catalysts[36]. Without silica, the Co is completely reduced to Co0, which possesses a higher hydrogenation activity and produces mainly methane in CO2 hydrogenation. Incorporating silica inhibits the complete reduction of Co, forming Co2+ which weakens the hydrogenation ability. And, a balance between the Co0 and Co2+ species formed favors methanol synthesis by CO2 hydrogenation (\nFig. 8). Besides, Janlamool et al. found that incorporating Ti in mesoporous silica support facilitates the reduction of Co oxides which are strongly interacted with the support, enhancing CO2 hydrogenation and methane formation[37]. Moreover, Co supported on various oxides and reduced at 450\u2009\u00b0C in H2 shows different activity for CO2 hydrogenation to methane. The methane yield is in the order of Co/CeO2 (\u223c96%) >\u2009Co/ZnO (\u223c54%) >\u2009Co/Gd2O3 (\u223c53%) \u223cCo/ZrO2 (\u223c53%)[38] with CO as the side product. The high catalytic performance of Co/CeO2 is mainly due to improved reducibility caused by Co-ceria interaction. Co may also react with the support/promoter forming a new species possessing different reducibility. For example, reducing Co/KIT-6 at high a reduction temperature leads to the formation of Co2SiO4 and/or Co-O-Si species. Thus, the formation of Co0 is inhibited, resulting in poor CO2 adsorption and activation. Accordingly, the high reduction temperature for Co/KIT-6 leads to decreasing activity and CH4 selectivity for CO2 hydrogenation[39].(\nFig. 9).Noble metal also presents an efficient promotional effect to tailor the reducibility and the CO2 hydrogenation activity. Beaumont et al. found that Pt nanoparticles (NPs) can promote the reduction of Co, due to the transportation of H atoms dissociated on Pt NPs to the Co NPs via H atom spillover[40]. This probably also occurs under reaction conditions and facilitates the removal of surface Co oxide formed during the reaction, generating more catalytically active sites and therefore significantly promoting CO2 hydrogenation to CH4\n[40]. Alkali metals such as K can enhance metal-support interaction, inhibiting the catalyst's reducibility and shifting the reduction to higher temperatures[32]. Besides, the K promoter improves the surface basicity of the catalyst and meanwhile increases the particle size. All these contribute partly to the formation of C2+ hydrocarbons by CO2 hydrogenation which will be further discussed.In some cases, the interface between Co0/CoO and the support/promoter is also catalytically active for CO2 hydrogenation. Melaet et al. found that CoO/TiO2 outperforms Co0/TiO2 in CO2 hydrogenation and is more selective to methane, but Co0/SiO2 is more active than CoO/SiO2\n[41]. The high catalytic performance of CoO/TiO2 is ascribed to the unique interface formed between CoO and TiO2. Besides, a Co/Mn-oxide hybrid catalyst possesses high activity for CO2 hydrogenation with high selectivity even at high temperatures and ambient pressure which are thermodynamically unfavorable for CO2 hydrogenation[42]. In this catalyst, a unique Co0/Mn2+ interface is formed. At the interface, Mn2+ activates CO2 through bridge bonded formate, while Co0 could deliver H atoms to rupture the C-O bond in the formate intermediate for methane formation. Moreover, a Co-Mn hybrid oxide catalyst is found to be active for methanol synthesis by CO2 hydrogenation[43]. The MnO/CoO interface is suggested to facilitate the CO2 conversion as well as the formation of methanol. Furthermore, based on in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments, Yue et al. proposed that the reduced-Co3O4/ZnO interfaces are the active sites and facilitate the formation of HCOO* and CH3O* intermediates during CO2 hydrogenation, promoting the conversion to C1 products[44].Furthermore, the support/promoter may influence CO2 hydrogenation via tailoring the reaction route. For example, K as a promoter enhances CO2 hydrogenation over Co0 due to the K-Co0 interaction which causes increased electron density around Co0 and strengthened CO2 adsorption[32]. It is further suggested that two reaction routes occur over the K-Co0 catalysts. The major one follows the direct CO2 dissociation to CO*, which then desorbs to form CO as a product (\nFig. 10). The minor one follows the H-assisted CO2 activation through HCOO* intermediate. Subsequently, the reduction of HCOO* and C-C coupling steps results in the formation of C2+ products (Fig. 10). Besides, the CO2 hydrogenation performance and reaction route over Co-based catalysts supported on anatase (Co/a-TiO2) and rutile (Co/r-TiO2) differ[45]. CO is the main product over Co/a-TiO2, while Co/r-TiO2 selectively produces CH4. In-situ DRIFTS measurements suggest that the reaction on Co/a-TiO2 follows direct CO2 dissociation to CO*, forming gas-phase CO instead of subsequent hydrogenation. However, over Co/r-TiO2, CO2 hydrogenation proceeds via formate species, followed by hydrogenation to CH4.Further addition of K, Zr, and Cs improves the CO2, CO, and H2 adsorption capacity and strength over Co/r-TiO2 and Co/a-TiO2\n[45]. Adding Zr promoter to Co/a-TiO2 enables CO2 hydrogenation via formate intermediate, which is subsequently hydrogenated to CH4. Moreover, the promoters influence the surface C/H ratio significantly, following the order of unpromoted <\u2009Zr-promoted <\u2009K-Zr-promoted \u223c Cs-Zr-promoted. The high C/H ratio benefits C2\n+ hydrocarbons formation. Accordingly, K-Zr-Co/a-TiO2 achieves the highest C2\n+ hydrocarbons selectivity of 17% with 70% CO2 conversion (\nFig. 11).The particle size is another important factor influencing the properties and catalytic activity of Co0/CoO catalysts. Generally, the bigger Co cluster resulting from the higher Co loading decreases the Co-support interaction and increases the reducibility of Co[35]. Besides, the more Co-oxide interface as a result of low Co loading leads to more positively charged Co due to the strongly perturbated structural and electronic properties at the interface[24]. As a result, over Co/CeO2 catalysts, the high Co loading catalysts (i.e., 2 and 4\u2009wt% Co/CeO2) with a larger particle size as well as a higher percentage of Co0, while the low Co loading catalyst (1\u2009wt% Co/CeO2) is smaller with a significant amount of CoOx after reduction. Accordingly, 1\u2009wt% Co/CeO2 is less active for H2 activation and thus less efficient in CO2 hydrogenation. Moreover, in all cases, the direct dissociation of CO2 to CO* with subsequent hydrogenation is the main route for CH4 formation. However, when carbonyl production is faster than its consumption, part of it will desorb into CO gas (\nFig. 12). In the minor reaction route (i.e., CO2 associative mechanism), the HCOO* intermediate forms at the Ce3+. The subsequent hydrogenation of HCOO* to CH4 and CO is favored over 2 and 4\u2009wt% Co/CeO2 rather than over the 1\u2009wt% Co/CeO2, due to the insufficient H atoms migrating from Co0 particles to adjacent ceria over 1\u2009wt% Co/CeO2 (Fig. 12).K as a promoter can also influence the Co particle size. For example, over reduced 15\u2009wt% Co/Al2O3 catalysts, the particle size of Co increases with increasing K loading[46]. Moreover, the Co particle size directly correlates with CO2 conversion and product selectivity. Generally, very small particles favor CO formation, and methane formation increases with particle size. However, K addition also enhances Co-support interaction and decreases the reducibility, inhibiting the reduction of the catalyst. Thus, a higher K loading suppresses methane formation, increasing the selectivity to C2+ hydrocarbons.The carbon chain growth is also particle size dependent. Somorjai group found that the CO2 hydrogenation turnover frequency increases with increasing Co particle size (reduced in H2 at 450\u2009\u00b0C)[47,48]. CH4 which is most favorable and CO are the main products for all the catalysts with different particle sizes (i.e., 3\u201310\u2009nm). At low temperatures (i.e., below 250\u2009\u00b0C), the FTS reaction shows slight changes in the product selectivity for all particle sizes, but at high temperatures, the carbon chain growth is enhanced with increasing Co particle size. Besides, the crystal size of the precursor may influence the reduction process. A crystal size smaller than 200\u2009\u00c5 favors fcc-Co0 formation, while hcp-Co0 is obtained when the crystal size is larger than 400\u2009\u00c5[34], accordingly the different crystal phase may show different catalytic performance for CO2 hydrogenation as discussed in Section 2.1.Partially oxidized Co sites (Co\u03b4+) are known to be active for CO2 hydrogenation/reduction. It is generally believed Co\u03b4+ favors the formation of *CO intermediate, which may insert into *CHx intermediate promoting higher alcohols synthesis[10,49]. Jimenez et al. also found that SiO2 supported single atom Co (Co2+) as an active species for CO2 hydrogenation, reaching a CO2 conversion of 7%, with 95% selectivity to CO[50]. Besides, Co single atoms supported on carbon material in the form of Co\u2013C2N2 moieties are active sites for electrochemical reduction of CO2 to CO[51]. Furthermore, DFT calculation demonstrated that the ligand is important to tailor the catalytic activity of single Co site. For single atom Co sites supported on N doped carbon material, the adsorption strength of the CO* intermediate becomes weaker as the coordination number of the N ligand increases from 1 to 4, providing an opportunity to tailor the catalytic activity of Co site for higher alcohols synthesis by CO2 hydrogenation[52].Co\u03b4+ site generally exists at the Co-support/promoter interface. The interaction between Co and support/promoter as well as the charge transfer from Co to metal oxide at the interface is the main reason for the formation of Co\u03b4+. For example, Co\u03b4+ forms at the Co-CeO2 interface[53,54]. Moreover, in a Ga-doped Co-Al spinel-derived catalyst, the transfer of electrons to Ga3+ leads to the formation of Co\u03b4+ sites[49]. Rodrigues et al. also identified Co\u03b4+ species over Co/SiO2\n[55]. The formation of silicates and/or Co hydrosilicate that are formed by the reaction between silanol groups on the surface of silica and Co ions is the main reason for the presence of Co\u03b4+.Single-atom Co site anchor on the surface of carbon, metal oxide, and other materials also provide Co\u03b4+. The electronic properties of the supported Co single atoms depend on the host materials and the employed preparation methods[56]. Co atoms supported on N-doped carbon nanosheet derived from cocoon silk are positively charged Co\u03b4+ (2\u2009< \u03b4\u2009<\u20093)[57]. Besides, Jang et al. found that the valence state of Co single atoms on N-doped porous carbon nanotubes is between Co2+ and metallic Co0\n[58]. Details on the effect of synthesis methods and electronic properties of single atom Co catalysts can be found in the review by Kaiser et al.[56].The formation of Co carbide has traditionally been considered a possible reason for the deactivation of Co-based FTS catalysts[59\u201362]. However, more recently, van Ravenhorst et al. found that when Co carbide formation over Co/TiO2 is detected, the product formation does not noticeably change[63], indicating that Co carbide formation is not a major reason for deactivation. Besides, for higher alcohols synthesis by CO hydrogenation, Co carbide acts as an active site for CO insertion during the formation of higher alcohols[64\u201368]. Moreover, Co carbide nanoprisms are promising catalysts for lower olefin synthesis by CO hydrogenation[69], and the facet geometry and preferential exposure of Co carbide play a key role[59,61,67,69\u201371]. Thus, the knowledge of CO hydrogenation over Co carbide is an important input to rationalize the design of catalysts for CO2 hydrogenation[72].Co carbide is also active for CO2 hydrogenation, mainly leading to the formation of methane. Yu et al. compared the CO2 hydrogenation activity of Co oxide (CoO/\u03b3-Al2O3) and Co carbide (Co2C/\u03b3-Al2O3)[73]. Accordingly, Co2C/\u03b3-Al2O3 was found to be highly active for methanation, reaching a CO2 conversion up to 89% with nearly 100% selectivity to CH4. However, CoO/\u03b3-Al2O3 possesses much lower activity, producing both CH4 and significant amounts of CO. Besides, Khangale et al. investigated the deactivation of 15%Co-6%K/Al2O3 catalyst during the hydrogenation of CO2 to long-chain hydrocarbons[74]. CO2 conversion, as well as the selectivity to long-chain hydrocarbons, decreased with increasing reaction time, while the selectivity to methane increased. And, the formation of Co2C in the spent catalyst is supposed to account for the formation of methane.Furthermore, Lin et al. studied the effects of CO2 on the structure and catalytic performance of Na-promoted CoC2, which is active in the conversion of syngas to olefins[75]. In CO hydrogenation, the main products were olefins and oxygenates (88.4% selectivity) with methane as a minor product (3.1% selectivity). However, CO2 hydrogenation mainly produces paraffins. Furthermore, increasing the CO2 content in the syngas feed reduces the activity, olefin selectivity, and olefin/paraffin formation, meanwhile the carbon chain growth is inhibited (\nFig. 13a-d). Interestingly, the Na-promoted Co2C nanoprisms remained stable in syngas; however, in the presence of CO2 in the feed, competitive adsorption between CO and CO2 occurs on the catalyst\u2019s surface, reducing the CO coverage and creating a higher H2/CO surface ratio. Accordingly, Co2C nanoprisms with (101) and (020) facets change to nanospheres with (111) facets and even reduce to Co0, changing the structure of the active sites as well as their catalytic behavior (Fig. 13e).Co carbide forms under FTS conditions and has traditionally been considered a possible cause for the deactivation of Co-based FTS catalysts[59\u201362]. However, Claeys et al. proposed that the formation of Co carbide seems to be kinetically inhibited[76]. Generally, only a small percentage (e.g., 5\u201310%) of Co may be transformed into Co2C under extreme reaction conditions such as low H2/CO ratios. However, in the H2 free syngas (i.e., only CO in the feed), a significant amount of bulk Co2C may form over an extended duration.Co0 and CoO can be also transformed to Co2C under FTS conditions. For CoO \u2192 Co2C process, CoO transforms directly to Co2C, and no Co0 forms in the whole process. Thermodynamic analysis indicates that the Gibbs free energy change of the CoO \u2192 Co2C process is much smaller than that of the Co0 \u2192 Co2C process, suggesting that the CoO process is thermodynamically more favorable[77].However, Paterson et al. suggested that Co0 is the intermediate for the conversion of CoO to Co2C[78]. In this study, Co3O4 spinel was used as a precursor for the synthesis of Co2C. The reduction of Co3O4 to CoO in CO occurs rapidly at 200\u2009\u00b0C, and the transition to Co2C occurs at 250\u2009\u00b0C. Co0 is likely formed and then quickly consumed to produce the Co carbide. Furthermore, Paterson et al. treated Co0 with CO at 200\u2009\u00b0C to synthesize Co carbide with a particle size of about 9\u2009nm[78]. A direct relation between the CO partial pressure with the rate of Co2C formation was observed. The carbide formation rate in 5% CO feed at 10\u2009barg and 50% CO feed at 1 bara is similar (\nFig. 14). Besides, the crystal structure of Co0 influences the carbonization process. Under the FTS conditions, the hcp-Co0 transforms into Co2C quickly, while a fraction of the fcc-Co0 remains metallic[79], consistent with in situ XRD studies[80].Alkali metal promoter also influences the carbonization process. Pei et al. used a reduced physical mixture of Co3O4 and alkali metals (i.e., Li2O, Na2O, and K2O) as the precursor and CO as the carbonization source[81]. They found that adding Li2O accelerates Co2C formation and shortens the carbonization time significantly. The promoting effect of Li can be related to the adsorption of H2 on the reduced precursor, enhancing the precursor\u2019s ability to react with CO.It is generally believed that Co2C is unstable[75], and its decomposition into Co0 is observed under FTS conditions[61]. Kwak et al. found that the hydrogenation of Co2C starts at \u223c160\u2009\u00b0C in H2. Gnanamani et al. also observed the conversion of Co2C to Co0 for the sample without an alkali metal and adding alkali metal significantly stabilizes Co2C[82]. Furthermore, The Co carbide is stable in air up to 90\u2009\u00b0C; however, it decomposes to Co0 in H2 at 120\u2009\u00b0C. During the decomposition, fcc/hcp mixed phases form after initial reduction but finally transform to an hcp phase after the carbides are removed[78].Co0, CoO, Co2C, and Co can activate CO2 and form C1 intermediates such CHx*, CO*, and HCOO*, which are also the intermediates for higher alcohols synthesis. Thus, designing hetero-site Co catalysts and tailoring their structure and catalytic activity by the above-mentioned methods to enable CHx*-CO*/HCOO* coupling may promote higher alcohols synthesis by CO2 hydrogenation. The hetero-site Co catalysts can be created by tailoring the reduction process, incorporating alkali metals, or interacting with support/promoter. We discuss these strategies in Sections 5 to 7.Co0 possesses a strong hydrogenation ability and favors CH4 formation, while CoO favors CO formation. Thus, designing Co0-CoO hetero site catalysts and tailoring the synergy between them may facilitate CHx*-CO* coupling, thus facilitating ethanol formation. Controlling the reduction process is a facile way to create the Co0-CoO hetero sites.Wang et al. reduced a layered double hydroxide (LDH) derived CoAlOx precursor in H2 at various temperatures to create Co0-CoO hetero sites and tailor their structure[15]. At a reduction temperature of 300\u2009\u00b0C, only Co3O4 is obtained, and Co0 is almost undetectable. At 400\u2009\u00b0C, Co3O4 is reduced to CoO, while Co0 and CoO coexist after reduction at 600\u2009\u00b0C. Further increasing the reduction temperature decreased the CoO content and the Co0 content increased (\nFig. 15a-d). Compared with the catalysts reduced at other temperatures, the 600\u2009\u00b0C reduced catalyst achieved a balanced content between Co0 and CoO, promoting their synergy. Accordingly, it possesses the best performance in CO2 hydrogenation (Fig. 15e), reaching the highest ethanol selectivity (92.1%) and yield (0.444\u2009mmol\u2009g\u22121 h\u22121).\nIn-situ Fourier-transform infrared spectroscopy (FT-IR) is further used to unravel the mechanism for the enhanced ethanol formation over the hetero site Co0-CoO catalyst[15]. It is suggested that ethanol formation proceeds via CO2 adsorption and activation, formation of HCOO* and CHx* intermediate, formation of acetate through CHx*-HCOO* coupling, and subsequent hydrogenation to ethanol. The enhanced ethanol formation is ascribed to the coexistence of Co0 and CoO after the reduction treatment at 600\u2009\u00b0C, which promotes the formation of CHx* for converting HCOO* into acetate by insertion, an important intermediate for synthesizing ethanol. A poorly reduced catalyst (i.e., reduced at 400\u2009\u00b0C) possesses lower activity and ethanol selectivity but increased methanol selectivity. The reason is ascribed to the weak hydrogenation activity due to the low reducibility, inhibiting the formation of acetate intermediate.It is further suggested that CHx* formation is the rate-determining step for ethanol synthesis over reduced CoAlOx catalysts. Hence, to enhance the formation of CHx*, a Co0Ni0 alloy-CoO hetero site catalyst is further prepared by reducing Co0.52Ni0.48AlOx at 600\u2009\u00b0C[12]. The incorporated Ni facilitates the reduction of Co. More importantly, Ni0 possesses a stronger hydrogenation ability, enhancing the formation of CHx*. Furthermore, the CHx* is involved in the formation of CH3COO* and C2H5O*, which are subsequently hydrogenated to form ethanol. As a result, an ethanol yield of 15.8\u2009mmol\u00b7gcat\n\u22121 with an ethanol selectivity of 85.7% was obtained. However, the reduced Co-free NiAlOx produces mainly methanol, CH4, and CO, again indicating the importance of CoO as well as the synergy between Co0Ni0 alloy and CoO for ethanol synthesis.The morphology of the Co precursor is another important factor influencing the reduction of Co3O4 and the formation Co0-CoO hetero site. Mesoporous Co3O4 and Co3O4 nanoparticles have shown different reduction behavior as well as different catalytic performance in higher alcohols synthesis by CO2 hydrogenation[11]. Even though mesoporous Co3O4 reduced at 300\u2009\u00b0C (Co3O4-m-300) and Co3O4 nanoparticle reduced at 300\u2009\u00b0C (Co3O4-np-300) possess similar CO2 conversion, the selectivity to higher alcohols over Co3O4-m-300 was found to be significantly higher. It was observed that Co3O4-np-300 comprises hcp-Co0 and fcc-Co0, while Co3O4-m-300 consists of CoO, hcp-Co0, and fcc-Co0. Co0 provides dissociative H, while O vacancy on the surface of partially reduced Co3O4 enhances CO2 dissociation over Co3O4-m-300. The synergy between the Co0 and CoO sites is partially responsible for the high performance of higher alcohol synthesis. Besides, Co3O4-np-250 and Co3O4-m-300 are composed of similar phases; however, their space-time yield (STY) for higher alcohols differs. Interestingly, the STY of higher alcohols for Co3O4-m-300 is 40 times that over Co3O4-np-250 (i.e., 1.6 and 0.04\u2009mmol\u2009gcat\n\u22121 h\u22121, respectively). This is probably due to the confinement effect of the mesoporous structure.Besides, Ouyang et al. prepared Pt-promoted Co3O4 rods (Pt/Co3O4-r) and nano-plates (Pt/Co3O4-p) for CO2 hydrogenation to higher alcohols[13]. The incorporation of Pt promotes the reduction of Co3O4. Thus, after reduction at a low temperature (i.e., 200\u2009\u00b0C), some of Co3O4 in Pt/Co3O4-r is reduced to CoO, while in Pt/Co3O4-p, CoO and Co0 coexisted. Accordingly, the synergy between the Pt0/Co0 nanoparticles and the O vacancy over the partially reduced Co3O4 facilitates H2 and CO2 adsorption, resulting in a C2+OH yield of 0.69\u2009mmol\u2009gcat\n\u22121 h\u22121, which is higher than 0.62\u2009mmol\u2009gcat\n\u22121 h\u22121 over Pt/Co3O4-r.Alkali metals can donate electrons to the Co site and have been widely used in promoting higher alcohol synthesis by CO hydrogenation. Also, they are effective promoters in creating hetero Co sites for higher alcohol synthesis by CO2 hydrogenation. Gnanamani et al. studied the hydrogenation of CO2 over a Na/Co\u2013SiO2 catalyst[16]. Reducing Na-Co/SiO2 in H2 at 350\u2009\u00b0C results in the formation of Co0, which mainly hydrogenates CO2 to methane and C2\u20134 hydrocarbons. However, reduction of Na-Co/SiO2 in pure H2 or syngas at 250\u2009\u00b0C leads to the formation of CoO, suppressing the high hydrogenation ability and decreasing the selectivity to methane. Moreover, activating Na-Co/SiO2 in CO forms CoO and Co carbide species, significantly decreasing methane selectivity to 15.3% and reaching 73.2% selectivity for alcohols. In contrast, the Na-free Co/SiO2 catalysts pretreated in CO mainly produce methane. Furthermore, it was found that Co carbide which forms during CO activation converts to Co0 after the reaction. This suggests that the Na promoter and carbide phase formation are important to reduce methane selectivity and enhance oxygenate formation. The Co0-CoO/Co2C hetero sites, which are stabilized/promoted by Na seem to play a key role in the CO2 hydrogenation to alcohols.Besides, Zhang et al. studied the CO2 hydrogenation over a Na-promoted Co2C catalyst derived by treating a Na-Co complex in CO using both theoretical and experimental methods[83]. By DFT calculation, they found that Na-free Co2C favors the formation of CHx* intermediates. Incorporating Na as a promoter inhibits the dissociation of CO* intermediate with higher energy barriers and favors non-dissociative CO* adsorption (\nFig. 16), increasing the CO*/CHx* ratio on the surface of the catalyst. Hence, the synergy between NaCo2C and Co2C is of great importance. At the interface, CO* at Na-Co sites insert into the adjacent CHx* on Co atoms to form ethanol. Thus, CO*-CHx* coupling could be tailored by controlling the interaction between Na and Co2C. Introducing 2\u2009wt% Na results in moderate interaction reaching an ethanol STY of 1.1\u2009mmol\u2009g\u22121 h\u22121, which is 10 times higher than the catalyst without Na. However, higher Na contents (e.g., 5\u2009wt% Na) weaken the strength of CO adsorption, which is unfavorable to CO coupling and increases CO selectivity.Besides, Witoon et al. studied K-Co/In2O3 catalysts for CO2 hydrogenation to higher alcohols[84]. Over Co/In2O3 catalysts, a mixture of Co0 and CoO forms after reduction in H2. CO2 is converted to CO at the surface O defects of In2O3. The Co0 site takes part in the dissociative adsorption of C-O and C-C bond formation, as well as the hydrogenation of adsorbed carbon to form CxHy*. Then, the CO* formed on the surface of CoO migrates and inserts into the adjacent CxHy* species at Co0 sites, forming C2+OH. However, they observed a higher selectivity to hydrocarbons than the oxygenated products over Co/In2O3. The reason is ascribed to the faster hydrogenation of CxHy* species than the CO* insertion due to the presence of weakly adsorbed H*. After adding K promoter, K-O-Co species is created, which considerably reduces the weakly adsorbed H* and strengthens H* adsorption, suppressing the hydrogenation of alkyl intermediates to form hydrocarbons. Accordingly, CO* insertion and C-C bond formation are promoted, enhancing the formation of higher alcohols.Co hetero site catalysts can also be created through tailoring the Co-support/promoter interaction. Zheng et al. prepared a Co/La2O3-La4Ga2O9 catalyst by reducing the LaCo1\u2212xGaxO3 perovskite for CO2 hydrogenation to ethanol[10]. The interaction between Co and Ga leads to the existence of Co\u03b4+ at the interface after reduction. The synergy between Co0 and Co\u03b4+ moderately weakens the hydrogenation ability of Co0 and inhibits the hydrogenation of CO2 to CH4, promoting the formation of ethanol. Accordingly, the optimized Co/Ga ratio of 7:3 leads to a CO2 conversion of 9.8%, an alcohol selectivity of 74.7% with an ethanol content of 88.1% in the alcohols mixture.Besides, An et al. prepared a series of Co-based catalysts for CO2 hydrogenation to ethanol by reducing SiO2-supported CoGaxAl2\u2212xO4 precursor[49]. The reverse H spillover ability of Ga inhibits excessive reduction of Co, and the electron donation from Co0 to Ga3+ leads to the formation of Co0-Co\u03b4+ active pairs. Furthermore, Co-Ga interaction stronger than the Co-Al enables tailoring the Co0/Co\u03b4+ ratio by changing the Ga/Al ratio. It is suggested that CO2 hydrogenation over the reduced CoGaxAl2\u2212xO4 proceeds through a reverse water gas shift reaction followed by CO hydrogenation to higher alcohols. Associative CO adsorption (forming CO*) and dissociative CO adsorption (forming CH4*) occur on Co\u03b4+ and Co0 sites, respectively. The synergy between Co0 and Co\u03b4+ with an optimized Co0/Co\u03b4+ ratio promotes CHx*-CO* coupling for ethanol synthesis (\nFig. 17). On the other hand, the selectivity to ethanol reaches 20.1% over the reduced CoGa1.0Al1.0O4/SiO2 catalyst.Na-promoted Co catalysts supported on different metal oxides (Al2O3, ZnO, AC, TiO2, SiO2, and Si3N4) were also studied for CO2 hydrogenation to higher alcohols[17]. After a reduction in pure CO, Co2C was formed; however, during the reaction, Co2C only remained intact on the SiO2 and Si3N4 supports rather than on other supports. Hence, due to the strong Co\u2013support interaction through Si-O-Co bond formation, CO* as a reactive intermediate could regenerate and reconstruct the decomposed Co2C on the surface. Moreover, CO* produced over the Co2C sites can be inserted into CHx* intermediates to form ethanol. Thus, the coexistence of Co2C and Co0 seems to play a key role in enhancing ethanol synthesis by CO2 hydrogenation. Accordingly, the SiO2 and Si3N4-supported catalysts are efficient, possessing 18% CO2 conversion and 62% selectivity in the alcohol distribution, while CH4 is found to be the main product over other supported catalysts.Synthesis of higher alcohols by CO2 hydrogenation presents a promising way to reduce CO2 emission and meanwhile produce value-added fuels and chemicals. Earth-abundant and economic Co-based catalysts, which possess relatively high activity and selectivity have been intensively studied for higher alcohols synthesis by CO2 hydrogenation. Amongst, hetero Co sites and the synergy between them are reported as important sites for CO2 hydrogenation to higher alcohols. Besides, Co0, CoO, Co\u03b4+, and Co2C are the main catalytic species for CO2 hydrogenation. Until now, various strategies have been developed to tailor the properties and structure of the active species, which further influence their catalytic performance in CO2 hydrogenation. Moreover, by controlling the reduction process, incorporating alkaline metal as a promoter, and tailoring the Co-support/promoter interaction, hetero Co sites such as Co0-CoO, Co0-Co\u03b4+, and Co2C-NaCo2C could be obtained for higher alcohol synthesis; however, the CO2 conversion and selectivity to higher alcohols are still low, hindering their commercial application. Taking commercial methanol synthesis by syngas conversion as a reference, we believe that catalysts with a single-pass conversion higher than 5%[85], a C2+OH selectivity higher than 95%[86], as well as a space-time yield higher than 0.5\u2009kg\u2009L\u22121\ncat h\u22121\n[87] would be of interest for industrial application. Thus, strategies to further tailor the structure and properties of the active Co sites as well as promote their synergy are indispensable. These strategies include:\n\n(1)\n\nDesigning new hetero sites\n\nOnly a few hetero-site Co catalysts such as Co0-CoO, Co0Ni0-CoO, Co2C-NaCo2C, and Co0- Co\u03b4+/metal oxide are investigated until now. Hence, other hetero-site catalysts including Co0-Co\u03b4+/N doped carbon material, Co2C-CoO, Co2C-Co\u03b4+/metal oxide, Co2C-Co\u03b4+/N doped carbon material, etc. can be considered as potential candidates. We, therefore, believe that due to their different properties, these hetero sites potentially possess different catalytic performances compared with the already reported hetero Co-sites for the synthesis of higher alcohols by CO2 hydrogenation. The synthesis of these hetero-site structures is the main challenge. Co2C-CoO sites can be obtained by carbonization of CoO in CO. By carefully controlling the carbonization atmosphere, temperature, and duration, the nature and amount of Co2C-CoO sites can be rationally tailored. We propose two synthesis routes to prepare Co2C/Co0-Co\u03b4+ hetero sites. In the first one, during the synthesis of single-atom catalysts, both unstable metal species (i.e., metal clusters/nanoparticles) and stable single-atom metal form on the surface of N doped carbon/metal oxide support. The unstable metal species can be removed by acid leaching, thus their amount can be controlled. By subsequent treatment in CO or H2, the Co2C/Co0-Co\u03b4+ hetero sites can be obtained. The second synthesis route to synthesize Co2C/Co0-Co\u03b4+ hetero sites is based on the diffusion and aggregation of single metal atoms by treating in H2 at high temperature. By controlling the treating conditions such as H2 concentration, the content of CO in the atmosphere, temperature, and duration, the amount and nature of the Co2C/Co0 and Co\u03b4+ sites can be rationally tailored.\n\n\n(2)\n\nTailoring the structure and property of the hetero sites\n\nTailoring the adsorption strength and surface abundance of CHx* and CO*/HCOO* on Co sites as well as controlling the ratio and proximity between the hetero sites is of great importance in promoting their synergy, which further enhances CHx* and CO/*HCOO* coupling and higher alcohols formation. The formation of CHx* and CO*/HCOO* intermediates depends on the amount and structure/properties of the Co0, CoO, Co2C, and Co\u03b4+ sites. The key factors influencing the structure and properties of the active sites include the predominant exposure of the crystal facets, degree of reduction, particle size, employment of promoter (K, Zr, Ce, Pt, Ni, Ru, Re, B, La, etc.) and support (SiO2, Al2O3, CeO2, TiO2, ZnO, Gd2O3), and coordination environment of Co, offering various methods to modify the structure and properties of the hetero sites. Besides, tailoring the proximity between the hetero sites by surface and interface control may favor the coupling between C1 species, promoting the formation of higher alcohols. Moreover, understanding the nature of the hetero Co sites using advanced characterization techniques, including in situ DRIFTS, in situ X-ray absorption spectroscopy (XAS), and near ambient pressure X-ray photoelectron spectroscopy (XPS), will facilitate illustrating the structure-activity relationships and rationalizing the catalyst design.\n\n\n(3)\n\nSynthesizing Co-based alloy followed by surface segregation\n\nBy forming a Co-based alloy such as CoCu, CoPt, and CuNi[7,12,88], the structure of the active sites on the catalysts\u2019 surface can be tailored to favor the formation of higher alcohols. Moreover, it is recently found that treating the CoCu alloy with CO leads to surface segregation and accordingly influences the surface composition of the catalysts[7]. Moderate capacity for C-O bond cleavage due to a moderately CO-induced CoCu surface results in a high ethanol selectivity. Thus, by controlling the CO treatment process, the Co/Cu molar ratio on the surface of the catalysts can be rationally tailored to promote the formation of higher alcohols. Extending CO-induced surface segregation of alloy to tailor the surface structure of other Co-based alloys deserves further study.\n\n\n(4)\n\nConfining the hetero sites in a nano volume\n\nConfining the hetero sites in a nano volume can inhibit CHx* and CO*/HCOO* intermediates from fast removal, increasing their concentration on the catalyst\u2019s surface. Accordingly, the formation of CO and methane by the fast desorption of CHx* and CO* can be reduced to a limited degree, which will further incorporate in higher alcohol synthesis. The active species can be encapsulated in inorganic nanoshells (e.g., SiO2, TiO2, CeO2, and C shells) or nanopores (e.g., zeolites, MOFs, and COFs) through various synthesis methods such as sol-gel coating, coating by hydrolysis of metal ions, impregnation, templated synthesis, etc.[89] This enables tailoring the dimension and volume of the nanoreactor. Accordingly, the residence of reactants, intermediates, and products on the surface of the catalysts can be controlled to enhance the formation of higher alcohols.\n\n\n\nDesigning new hetero sites\nOnly a few hetero-site Co catalysts such as Co0-CoO, Co0Ni0-CoO, Co2C-NaCo2C, and Co0- Co\u03b4+/metal oxide are investigated until now. Hence, other hetero-site catalysts including Co0-Co\u03b4+/N doped carbon material, Co2C-CoO, Co2C-Co\u03b4+/metal oxide, Co2C-Co\u03b4+/N doped carbon material, etc. can be considered as potential candidates. We, therefore, believe that due to their different properties, these hetero sites potentially possess different catalytic performances compared with the already reported hetero Co-sites for the synthesis of higher alcohols by CO2 hydrogenation. The synthesis of these hetero-site structures is the main challenge. Co2C-CoO sites can be obtained by carbonization of CoO in CO. By carefully controlling the carbonization atmosphere, temperature, and duration, the nature and amount of Co2C-CoO sites can be rationally tailored. We propose two synthesis routes to prepare Co2C/Co0-Co\u03b4+ hetero sites. In the first one, during the synthesis of single-atom catalysts, both unstable metal species (i.e., metal clusters/nanoparticles) and stable single-atom metal form on the surface of N doped carbon/metal oxide support. The unstable metal species can be removed by acid leaching, thus their amount can be controlled. By subsequent treatment in CO or H2, the Co2C/Co0-Co\u03b4+ hetero sites can be obtained. The second synthesis route to synthesize Co2C/Co0-Co\u03b4+ hetero sites is based on the diffusion and aggregation of single metal atoms by treating in H2 at high temperature. By controlling the treating conditions such as H2 concentration, the content of CO in the atmosphere, temperature, and duration, the amount and nature of the Co2C/Co0 and Co\u03b4+ sites can be rationally tailored.\nTailoring the structure and property of the hetero sites\nTailoring the adsorption strength and surface abundance of CHx* and CO*/HCOO* on Co sites as well as controlling the ratio and proximity between the hetero sites is of great importance in promoting their synergy, which further enhances CHx* and CO/*HCOO* coupling and higher alcohols formation. The formation of CHx* and CO*/HCOO* intermediates depends on the amount and structure/properties of the Co0, CoO, Co2C, and Co\u03b4+ sites. The key factors influencing the structure and properties of the active sites include the predominant exposure of the crystal facets, degree of reduction, particle size, employment of promoter (K, Zr, Ce, Pt, Ni, Ru, Re, B, La, etc.) and support (SiO2, Al2O3, CeO2, TiO2, ZnO, Gd2O3), and coordination environment of Co, offering various methods to modify the structure and properties of the hetero sites. Besides, tailoring the proximity between the hetero sites by surface and interface control may favor the coupling between C1 species, promoting the formation of higher alcohols. Moreover, understanding the nature of the hetero Co sites using advanced characterization techniques, including in situ DRIFTS, in situ X-ray absorption spectroscopy (XAS), and near ambient pressure X-ray photoelectron spectroscopy (XPS), will facilitate illustrating the structure-activity relationships and rationalizing the catalyst design.\nSynthesizing Co-based alloy followed by surface segregation\nBy forming a Co-based alloy such as CoCu, CoPt, and CuNi[7,12,88], the structure of the active sites on the catalysts\u2019 surface can be tailored to favor the formation of higher alcohols. Moreover, it is recently found that treating the CoCu alloy with CO leads to surface segregation and accordingly influences the surface composition of the catalysts[7]. Moderate capacity for C-O bond cleavage due to a moderately CO-induced CoCu surface results in a high ethanol selectivity. Thus, by controlling the CO treatment process, the Co/Cu molar ratio on the surface of the catalysts can be rationally tailored to promote the formation of higher alcohols. Extending CO-induced surface segregation of alloy to tailor the surface structure of other Co-based alloys deserves further study.\nConfining the hetero sites in a nano volume\nConfining the hetero sites in a nano volume can inhibit CHx* and CO*/HCOO* intermediates from fast removal, increasing their concentration on the catalyst\u2019s surface. Accordingly, the formation of CO and methane by the fast desorption of CHx* and CO* can be reduced to a limited degree, which will further incorporate in higher alcohol synthesis. The active species can be encapsulated in inorganic nanoshells (e.g., SiO2, TiO2, CeO2, and C shells) or nanopores (e.g., zeolites, MOFs, and COFs) through various synthesis methods such as sol-gel coating, coating by hydrolysis of metal ions, impregnation, templated synthesis, etc.[89] This enables tailoring the dimension and volume of the nanoreactor. Accordingly, the residence of reactants, intermediates, and products on the surface of the catalysts can be controlled to enhance the formation of higher alcohols.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the financial support from National Natural Science Foundation of China (22208143), Nanjing Tech University (39801170), and the State Key Laboratory of Materials-Oriented Chemical Engineering (38901218). Longfei Liao gratefully acknowledges the financial support received from the Foundation of National Key Laboratory of Human Factors Engineering, China Astronaut, Research and Training Center (Grant number: 6142222210601).", "descript": "\n                  Synthesis of higher alcohols by CO2 hydrogenation is a promising way to mitigate CO2 emissions, meanwhile producing value-added fuels and chemicals. However, CO2 hydrogenation to higher alcohols is kinetically hindered and due to the absence of highly efficient catalysts, its industrial implementation is still limited. Among the catalysts designed for this reaction, Co-based catalysts are widely investigated due to earth-abundance and economic, possessing also relatively high activity and selectivity for this reaction. Considering the nature of the active site, the hetero sites of Co-based catalysts such as Co0-CoO, Co0-Co\u03b4+, and Co2C-NaCo2C are critical for higher alcohol synthesis by CO2 hydrogenation. Thus, in this review, we first introduce the roles of Co0, CoO, Co\u03b4+, and Co2C, as well as strategies to tailor their structure which influences the performance in CO2 hydrogenation. Then, we discuss the strategies to create highly efficient hetero-site Co-based catalysts. Finally, emerging methodologies yet to be explored and future directions to achieve highly efficient hetero-site Co catalysts for CO2 hydrogenation to higher alcohols are discussed.\n               "}
{"full_text": "Molar flow rate of gaseous products (i = H2, CO2, CO and CH4), mol\ni\n\u00b7min\u22121\nMolar flow rate of glycerol, mol\nG\n\u00b7min\u22121\nPurity of H2, %Selectivity towards carbon-containing gaseous products (i = CO2, CO and CH4), %Selectivity towards H2, %Glycerol conversion into carbon-containing gaseous products, %Total glycerol conversion, %Yield of gaseous products (i = H2, CO2, CO and CH4), \n\nm\no\n\nl\ni\n\n\u00b7\nm\no\n\n\nl\n\nG\n,\ni\nn\n\n\n\n\u2212\n1\n\n\n\n\nAnnular bright fieldBrunauer, Emmet and TellerCitrate sol-gelFlame ionization detectorGas chromatographGlycerol steam reformingHigh-angle annular dark fieldHydrogen evolution reactionHigh performance liquid chromatographyInductively coupled plasma optical emission spectroscopyIncipient wetness impregnationMass spectrometerOxygen evolution reactionSteam-to-carbon molar ratioSteam methane reformingThermal conductivity detectorTransmission electron microscopy/energy-dispersive X-rayTime-on-streamTemperature-programmed oxidationTemperature-programmed reductionWater-to-glycerol molar feed ratioWater-gas shiftWeight hourly space velocityX-ray diffractionglycerolreactor inlet conditionsreactor outlet conditionsThe environmental consequences of increased fossil fuel consumption due to energy demands, such as greenhouse gas emissions, have compelled the world to find carbon-neutral, cost-effective and especially renewable eco-friendly alternatives for a sustainable future [1]. As an alternative and eco-friendly fuel, biomass-based hydrogen has been receiving special attention worldwide. However, around 95% of the hydrogen produced nowadays comes from fossil fuels, particularly via steam methane reforming (SMR), which is a non-renewable process in terms of feedstock point of view [2]. As a result, it is urgent to start producing hydrogen from renewable sources.The biodiesel production has been on a increasing trend over the last few decades [3]. Nevertheless, the increase in the biodiesel demand has resulted in an undesirable oversupply of crude glycerol \u2013 the main by-product [4]. In 2016, it was reported that the biodiesel manufacture resulted in 3.28 megatons of crude glycerol and it is expected to increase in the near future [5]. This surplus has led to a collapse in terms of glycerol price over the last years, which jeopardizes biodiesel competitivity. The impurities present in crude glycerol limit its purification to be used in pharmaceutical or food industries [6]. However, among many potential applications for the use of crude glycerol, the steam reforming technique is emerging as one of the most suitable for solving the glycerol utilization problem, while at the same time providing a new and potentially green source of fuel [2,7,8]. In addition, the use of crude glycerol not only would promote both commercialization and further development of biodiesel production \u2013 and therefore improve the economics of this biofuel production \u2013, as it would also reduce the dependence on non-renewable sources [9].The technological advances in the fuel cell industry are expected to considerably increase in the near future, which also implies the increase in hydrogen demand. Apart from environmental advantages that glycerol steam reforming (GSR) provides, there are also processual advantages \u2013 while the SMR leads to production of only 4\u00a0mol of H2 per mole of reformed CH4, the GSR allows the formation of 7\u00a0mol of H2 per mole of reacted glycerol (Table 1 \u2013\n Eq. (1)). The global GSR reaction is a combination of glycerol decomposition (Eq. (2)) into syngas (CO and H2) and the water-gas shift (WGS) reaction (Eq. (3)). In addition, the methanation of CO (Eq. (4)) and CO2 (Eq. (5)) and the dry reforming of methane (Eq. (6)) are considered as the main secondary reactions. The formation of coke is also very common to occur during GSR [10\u201314] \u2013 carbon-containing gaseous products (CO, CH4 and CO2) can be coke precursors according to Eqs. (7)-(10) [15]. Moreover, some intermediates and by-products that are formed during the reaction may also react, further complicating the process [11,16]. In fact, Pompeo et\u00a0al. [14] noticed the formation of heavy compounds (2-methyl-2-cyclopentanone, phenol and 5-hydroxyl-2-methyl-1,3-dioxane), which were assumed that could have been formed by hydrogenolysis, dehydration and condensation reactions; these compounds are known to possibly be coke precursors.In order to achieve a good performance in the GSR process, new catalysts must be developed. Ideally, a GSR catalyst should promote the cleavage of C\u2013C, O\u2013H and C\u2013H bonds, while at the same time eliminating the metal-passivating carbon monoxide formation via reverse WGS reaction (Eq. (3)). This means that the catalyst should be able to inhibit C\u2013O cleavage and the CO2 or CO hydrogenation, which can lead to the formation of methane and/or more polar compounds [17\u201319]. In the last years, most of the works related to GSR have focused on the development of Ni- and Co-based catalysts on a variety of supports because they provide excellent intrinsic activity when well dispersed over the support, are easily available and cheaper comparatively to noble metal-based catalysts [3,20]. In the last years, the use of Co-based catalysts for GSR has been proven to be efficient since cobalt has the capacity to break C\u2013C bonds and suppress the formation of coke [21].The nature of the catalyst support also has a huge influence, affecting especially the catalyst selectivity. Several supports have been investigated in steam reforming reactions \u2013 among all, alumina is known as the best choice due to its high specific surface area and thermal stability. Nevertheless, alumina supports are also prone to suffer deactivation due to carbon deposition and metal particle sintering. Therefore, the addition of proper elements to alumina supports \u2013 e.g., zinc oxide (ZnO2), magnesium oxide (MgO), cerium oxide (CeO2) or lanthanum oxide (La2O3) \u2013 can be beneficial due to an increase in the catalyst basicity and, therefore, stability [3]. Kraleva et\u00a0al. [22\u201324] studied Ni- and Co-based catalysts supported on AlZnOx for hydrogen production from bio-ethanol partial oxidation and observed a good catalytic activity of those materials, as well as a high resistance to coke formation. In another work by Goicoechea et\u00a0al. [25], Ni- and Co-promoted catalysts supported on single La2O3 and mixed AlLaOx were prepared and tested for hydrogen production from steam reforming of acetic acid. All catalysts were observed to be active, stable and highly selective towards the production of H2 during the screening experiments.Cobalt-based catalysts have also been extensively used to produce molecular H2 from water splitting [26\u201329]. The water splitting reaction can be viewed as a combination of two reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Several reports demonstrated that Co-based catalysts are highly active electrocatalysts towards the HER under either basic, neutral or acidic media, apart from being easily coupled with the most active catalysts that are available towards the OER, which plays a huge role in the overall water splitting reaction [26].Based on this background, cobalt-based catalysts supported on La2O3, AlZnOx and AlLaOx were prepared to be used for hydrogen production from GSR for the first time to the best of our knowledge. The materials were prepared using the incipient wetness impregnation (IWI) and citrate sol-gel (CSG) methods and physico-chemically characterized in terms of inductively coupled plasma optical emission spectroscopy (ICP-OES), physical sorption-desorption of N2 at \u2212196\u00a0\u00b0C, X-ray diffraction (XRD), transmission electron microscopy/energy-dispersive X-ray (TEM/EDX) and temperature-programmed reduction (TPR). The materials were screened in terms of catalytic activity with the aim of identifying the effect of temperature on the total conversion of glycerol and on the conversion of glycerol into gaseous products, as well as on the yield towards gaseous products (H2, CH4, CO2 and CO). Additionally, time-on-stream stability experiments were conducted to assess the deactivation of the materials. For comparative purposes, an additional NiAlLaOx catalyst was prepared to be compared with the best Co-based sample: CoAlLaOx. In conclusion, the main goal was to prepare and find promising materials having high catalytic performances (i.e., high H2 yields) and stability (i.e., low coke formation and/or low sintering) during the GSR process.Cobalt nitrate hexahydrate (Co(NO3)2\u00b76H2O) (Merck, \u2265 99.0%) and nickel nitrate hexahydrate (Ni(NO3)2\u00b76H2O) (Merck, \u2265 99.0%) were used as Co and Ni precursors, respectively. Single oxide support La2O3 was obtained from the calcination of lanthanum nitrate hexahydrate (La(NO3)3\u00b76H2O) (Alfa Aesar, \u2265 99.9%), whereas AlLaOx and AlZnOx mixed oxide supports were obtained from lanthanum nitrate hexahydrate (La(NO3)3\u00b76H2O) (Alfa Aesar, \u2265 99.9%) for AlLaOx, zinc nitrate hexahydrate (Zn(NO3)2\u00b76H2O) (Merck, \u2265 98.0%) for AlZnOx, and from aluminum nitrate nonahydrate (Al(NO3)3\u00b79H2O) (Merck, \u2265 98.0%) for both AlLaOx\u00a0and AlZnOx. Anhydrous citric acid (ACS, \u2265 99.5%) was used for catalysts prepared by the CSG method.Nitrogen (Linde, \u2265 99.999%) was used as dilution gas in the catalytic experiments and makeup gas in the gas chromatograph (GC) for the flame ionization detector (FID); argon (Linde, \u2265 99.999%) was used as carrier, reference and makeup gas in the GC for the thermal conductivity detector (TCD); hydrogen (Linde, \u2265 99.999%) was used for catalysts activation and as fuel gas in the GC-FID; and reconstituted air (20\u00a0vol% of O2/N2) was used in the GC-FID and to regenerate the catalysts (as detailed in Section Catalytic experiments).An aqueous solution of glycerol (C3H8O3) (VWR Chemicals BDH, \u2265 99.6%) with a water-to-glycerol molar feed ratio (WGFR) of 15 \u2013 corresponding to 25.4\u00a0wt% of glycerol and a steam to carbon (S/C) ratio of 5 \u2013 was used in all reaction tests.Inert silicon carbide (SiC) (Alfa Aesar) was used as catalyst bed diluting agent during all experiments.Co/La2O3 single supported catalyst was synthesized by the incipient wetness impregnation (IWI) method. The single oxide support (La2O3) was obtained from the calcination of lanthanum nitrate hexahydrate at 610\u00a0\u00b0C for 2\u00a0h, as suggested by Mentus et\u00a0al. [30]. The support was thereafter impregnated with an aqueous solution containing the Co active metal precursor. The metal hydrate nitrate solution was stirred for 30\u00a0min at room temperature to obtain a homogeneous solution prior to the addition of the support. The required amount of Co metal hexahydrate nitrate was used to yield the catalyst with a Co loading of 10\u00a0wt% in comparison to La2O3 (90\u00a0wt%). Afterwards, the metal was impregnated by adding the pre-calcined powdered single oxide support; the solution was mixed continuously at 60\u00a0\u00b0C for 60\u00a0min and then concentrated in a rotary evaporator at 80\u00a0\u00b0C under vacuum. At last, the samples were dried at 120\u00a0\u00b0C for 12\u00a0h and calcined in air at 700\u00a0\u00b0C for 2\u00a0h.CoAlZnOx, CoAlLaOx and NiAlLaOx mixed oxide catalysts were synthesized by the citrate sol-gel (CSG) method. The CSG method is a successful technique to produce homogeneous mixed oxides having high specific surface area, good stability, and high porosity in the mesoporous range [31]. The required amounts of hydrate nitrates (Al, La or Zn and Ni or Co) were weighted and mixed at the same time with 15\u00a0mL of water per 5\u00a0g of catalyst in order to yield Ni or Co metal loadings of 10\u00a0wt% and theoretical Al2O3/La2O3 or Al2O3/ZnO molar ratios of 1. Subsequently, citric acid was added to an aqueous solution containing the metal salts in a proportion equal to the amount of metal cations and the solution was stirred at 60\u00a0\u00b0C for 120\u00a0min. The formed wet gel was thereafter concentrated by slowly evaporating the water in a rotary evaporator at 75\u00a0\u00b0C under vacuum until a viscous clear gel was obtained. The final gel obtained was dried at 120\u00a0\u00b0C for 12\u00a0h and calcined in air at 700\u00a0\u00b0C for 2\u00a0h. The high calcination temperature of 700\u00a0\u00b0C was selected for all prepared catalysts to ensure structure stability of the materials in accordance to Barroso et\u00a0al. [32]. Detailed information on the prepared catalyst can be found in Table 2\n.The IWI method is by far the most attractive and widely used to prepare heterogeneous catalysts due to its technical simplicity, limited amount of waste and low cost. In general, the IWI method allows the impregnation of a simple support with a precursor-containing solution [33]. On the other hand, the CSG method offers significantly better control of stoichiometry and produces samples with high homogeneity, in particular in multi-component materials, since no washing or filtering steps are needed [34]. For this reason, the Co/La2O3 catalyst was prepared by IWI, while the CoAlZnOx, CoAlLaOx and NiAlLaOx catalysts, which are considered as multi-component materials, were prepared by the CSG method.The catalysts and supports elemental composition were determined by inductively coupled plasma optical emission spectroscopy (IPC-OES) on a Varian 715-ES spectrometer.The specific surface areas of the catalysts were determined through N2 sorption-desorption isotherms recorded at \u2212196\u00a0\u00b0C using a BELSORP-mini II apparatus (BEL Japan, Inc.). Samples were degassed under vacuum at 250\u00a0\u00b0C for 2\u00a0h before analysis. The specific surface area was determined using the Brunauer, Emmett and Teller (BET) method for the N2 relative pressure range of 0.05\u00a0<\u00a0p/p0\u00a0<\u00a00.30.The powder X-ray diffraction (XRD) measurements were carried out using a STADI P automated transmission diffractometer (STOE, Darmstadt) with CuK\u03b11 radiation and a Ge monochromator. The XRD patterns were collected in the 2\u03b8 range of 5\u201360\u00b0 in 0.5\u00b0 steps with the dwell time of 100\u00a0s and recorded with a STOE position sensitive detector. The phase analysis was performed using the program suite WINXPow (STOE&CIE) with inclusion of the Powder Diffraction File PDF-2 of the International Center of Diffraction Data (ICDD).A high-resolution transmission electron microscopy (TEM) study was done with an aberration-corrected JEM-ARM200F (JEOL, Corrector: CEOS) 200\u00a0kV transmission electron microscope. The microscope was equipped with a JED-2300 energy-dispersive X-ray (EDX) spectrometer for chemical analysis. The aberration corrected STEM imaging high-angle annular dark field (HAADF) and annular bright field (ABF) were performed with a spot size of approximately 0.13\u00a0nm, a convergence angle of 30\u201360\u00b0, and collection semi-angles for HAADF and ABF of 90\u2013170\u00a0mrad, respectively. For the TEM analysis, the samples were deposited without any pretreatment onto a holey carbon supported Cu-grid (mesh 300) and then transferred into the microscope.Temperature-programmed reduction (TPR) experiments were carried out on 40\u201350\u00a0mg of each sample. The samples were loaded into a fixed-bed continuous flow quartz reactor and heated to 900\u00a0\u00b0C at 10\u00a0\u00b0C/min in a 5\u00a0vol% H2/Ar gas mixture (total flow 7 mLN/min). The hydrogen consumption and water formation were monitored with a quadrupole mass spectrometer (MS, Pfeiffer OmniStar). The total amount of hydrogen consumed by each sample was determined by integration of the profile of the MS signal.In situ temperature-programmed oxidation (TPO) was performed prior to the stability tests for all catalysts. After performing the GSR at different temperatures, the reactor was flushed with N2 for 1\u00a0h, and the reactor was cooled down to 600\u00a0\u00b0C (since the reactor was at 700\u00a0\u00b0C). Then 100 mLN\u2219min\u22121 of reconstituted air passed through the catalyst bed until no CO2 nor CO were observed at the reactor outlet. The CO2 and CO vol% concentrations in the outlet stream were continuously monitored by an infrared-based CO2/CO analyzer (Servomex, model 4210).The catalytic tests were carried out in an experimental setup as described in Fig.\u00a01\n. The catalysts were placed in a stainless steel packed-bed reactor (120\u00a0mm of height and 7.2\u00a0mm of inner diameter), which was placed inside a tubular oven (model Split from Termolab, Fornos El\u00e9tricos, Lda.) divided in a three-zone PID programmable temperature controller (model MR13 from Shimaden), connected to three type-K thermocouples in contact with the wall of the reactor. In addition, for the continuous monitoring of the bed temperature, two type-K thermocouples were inserted laterally and radially centered (40 and 80\u00a0mm of the column length) in direct contact with the catalyst bed.Nitrogen was fed to the reactor to serve as dilution gas by using a mass flow controller (model F201 from Bronkhorst High-Tech) while the glycerol aqueous solution was fed by an HPLC pump (Eldex, 1LMP model) and forced to pass through an evaporator/mixing zone at 315\u00a0\u00b0C before entering the reactor. The pressure in the setup was monitored by means of two pressure transducers (both model PMP 4010 from Druck) placed before and after the reactor. A system of two Peltier condensers, located after the reactor (cf. Fig.\u00a01), was used to collect the condensable products produced during the reaction. In addition, a coalescence filter and a filter were used between the reactor and the analysis system in order to further retain any condensable species and eventually catalyst/inert particles, respectively. The tube between the reactor outlet and the first Peltier condenser was kept at 120\u00a0\u00b0C to avoid water/glycerol condensation.During the GSR experiments, small samples of the dry outlet gas stream containing N2, H2, CO, CO2 and CH4 were analyzed using a gas chromatograph (GC, Agilent 7820A) \u2013 equipped with a thermal conductivity detector (TCD \u2013 using argon as makeup and reference gas), a flame ionization detector (FID \u2013 using nitrogen as makeup gas, reconstituted air as oxidizing gas and hydrogen as fuel gas) and a CO/CO2 methanizer at 350\u00a0\u00b0C fed with hydrogen \u2013 used to analyze and determine the concentration of these gas phase species produced during the GSR experiments. The GC is equipped with two capillary columns: Plot 5A (30\u00a0m\u00a0\u00d7\u00a00.32\u00a0mm) and Plot Q (30\u00a0m\u00a0\u00d7\u00a00.32\u00a0mm). The H2 that was formed during the reaction was analyzed by TCD (with argon as carrier gas to achieve better response owing to the higher difference in thermal conductivity). On the other hand, CO2 and CO were detected in the FID detector in the methane form through the utilization of the methanizer.The condensed species were collected periodically and analyzed in terms of glycerol concentration by high performance liquid chromatography (HPLC, Elite LaChrom HITACHI apparatus) equipped with a refractive index detector. An Alltech OA-1000 ion exclusion column (300\u00a0\u00d7\u00a06.5\u00a0mm) \u2013 using 0.005\u00a0mol\u00a0L\u22121\u00a0H2SO4 solution as mobile phase at a flow rate of 0.5\u00a0mL\u00a0min\u22121 \u2013 was used to separate the liquid products collected. The quantity of glycerol was determined based on the calibration curve of the standard compound.For each experiment, a stainless steel reactor, closed at both ends with stainless steel mesh discs (10\u201315\u00a0\u03bcm), was filled with inert SiC (241\u2013559\u00a0\u03bcm) at both ends and the middle was filled with 200\u00a0mg (315\u2013500\u00a0\u03bcm) of catalyst homogenously diluted with SiC. The dilution of catalyst with inert SiC is known to enhance the heat transfer and minimize temperature gradients, which was confirmed through the measurement of the bed temperature throughout the experiments (it remained nearly constant, i.e., \u00b11\u00a0\u00b0C). The temperature profile along the reactor length under inert atmosphere was observed to be uniform (differences <2\u00a0\u00b0C).Prior to the GSR experiments, the catalysts were activated at 600\u00a0\u00b0C for 1\u00a0h under a 20\u00a0vol% H2/N2 mixture (total flow rate of 100 mLN min\u22121) taking into account the H2-TPR results (see Section 3.1.5. H2-TPR). The setup was then purged under N2 atmosphere (100 mLN\u00a0min\u22121) for 30\u00a0min and the temperature of the reactor was lowered to 400\u00a0\u00b0C. Subsequently, the glycerol aqueous solution (WGFR of 15), which was kept under continuous stirring at room temperature, was fed into the evaporator at a constant flow rate of 0.1\u00a0mL\u00a0min\u22121, corresponding to a weight hourly space velocity (WHSV) of 8.05 h\u22121 (defined as the ratio between the mass flow rate of glycerol and the mass of catalyst; similar to other works [13,35,36]). The vaporized solution was then mixed with N2 flowing at 25 mLN\u00a0min\u22121. The screening tests were carried out in the temperature range of 400\u2013700\u00a0\u00b0C and atmospheric pressure.Prior to the catalytic screening, a blank test \u2013 herein referred to as case study #1 \u2013 was conducted at various temperatures for a time-on-stream (TOS) of approximately 5.5\u00a0h, with the reactor filled with only SiC (bed diluting agent/inert). Following that, two distinct catalytic experimental runs were performed. The case study #2 was performed for all catalysts and aimed at assessing the catalytic activity and selectivity towards the reaction products for approximately 5.5\u00a0h (TOS) at temperatures ranging from 400 to 700\u00a0\u00b0C; prior to this experiment, catalysts were reduced/activated under an H2 atmosphere. The final experiment (case study #3) was carried out for all catalysts using the samples employed during case study #2; however, prior to this, a TPO experiment was performed at 600\u00a0\u00b0C using a reconstituted air stream to regenerate the catalysts as stated by Silva et\u00a0al. [13], followed by a new reduction/activation step. Case study #3 consisted of a stability test performed at 625\u00a0\u00b0C that was run until the glycerol conversion into gaseous products reached approximately 10%. These experiments are summarized in Table 3\n.The performance of the catalysts used in this work was measured in terms of products yield, selectivity towards H2 and other gases (CO2, CO and CH4) and conversion of glycerol.The glycerol conversion obtained during GSR was determined in two different ways, namely in terms of glycerol conversion into carbon-containing gaseous products (XG,gas) and total glycerol conversion (XG,total). The glycerol conversion into carbon-containing gaseous products was calculated as follows:\n\n(11)\n\n\n\nX\n\nG\n,\ng\na\ns\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\n\nF\n\nC\n\nO\n2\n\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\nO\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\n\nH\n4\n\n\n\no\nu\nt\n\n\n\n\n3\n\u00d7\n\nF\nG\n\ni\nn\n\n\n\n\n\u00d7\n100\n\n\n\nin which \n\n\nF\n\nC\n\nO\n2\n\n\n\no\nu\nt\n\n\n\n, \n\n\nF\n\nC\nO\n\n\no\nu\nt\n\n\n\n and \n\n\nF\n\nC\n\nH\n4\n\n\n\no\nu\nt\n\n\n\n are the molar flow rates of CO2, CO and CH4 at the reactor outlet, respectively, and \n\n\nF\nG\n\ni\nn\n\n\n\n is the molar flow rate of glycerol fed to the reactor. The total glycerol conversion was calculated based on Eq. (12).\n\n(12)\n\n\nX\n\nG\n,\nt\no\nt\na\nl\n\n\n\n\n%\n\n\n=\n\n\n\n\nF\nG\n\ni\nn\n\n\n\u2212\n\nF\nG\n\no\nu\nt\n\n\n\n\nF\nG\n\ni\nn\n\n\n\n\u00d7\n100\n\n\n\nIn this equation, \n\nF\nG\n\no\nu\nt\n\n\n is the molar flow rate of unreacted glycerol at the reactor outlet. The yield of the gaseous products (Yi) and the selectivity to the gas products denoted as \n\n\nS\n\nC\n\nO\n2\n\n\n\n\n, SCO and \n\n\nS\n\nC\n\nH\n4\n\n\n\n\n (Sc,i) were defined as follows:\n\n(13)\n\n\n\nY\ni\n\n=\n\n\n\nF\ni\n\no\nu\nt\n\n\n\nF\nG\n\ni\nn\n\n\n\n\n\n\n\n\n\n(14)\n\n\n\nS\n\nC\n,\ni\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\nF\n\nC\n,\ni\n\n\no\nu\nt\n\n\n\n\nF\n\nC\n\nO\n2\n\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\nO\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\n\nH\n4\n\n\n\no\nu\nt\n\n\n\n\n\u00d7\n100\n\n\n\nwhere \n\n\nF\ni\n\no\nu\nt\n\n\n\n is the molar flow rate of species i at the reactor exit, in which i corresponds to H2, CO2, CO or CH4 in Eq. (13) and \n\n\nF\n\nC\n,\ni\n\n\no\nu\nt\n\n\n\n is the molar flow rate of gas product i at the reactor outlet, wherein i corresponds to CO2, CO or CH4 in Eq. (14). Regarding the selectivity to H2 comparatively to carbon-containing gaseous products (\n\n\nS\n\nH\n2\n\n\n\n), it was calculated according to the following equation:\n\n(15)\n\n\n\nS\n\nH\n2\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\nF\n\nH\n2\n\n\no\nu\nt\n\n\n\n\nF\n\nC\n\nO\n2\n\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\nO\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\n\nH\n4\n\n\n\no\nu\nt\n\n\n\n\n\u00d7\n\n1\n\nR\nR\n\n\n\u00d7\n100\n\n\n\nin which \n\n\nF\n\nH\n2\n\n\no\nu\nt\n\n\n\n is the molar flow rate of H2 at the reactor outlet. The RR is the H2/CO2 reforming molar ratio and is related to the stoichiometry of the global GSR reaction (Eq. (1)) in which 7\u00a0mol of H2 and 3\u00a0mol of CO2 are ideally produced from each mole of reacted glycerol. Finally, the purity of the produced H2 (\n\nP\nu\n\nr\n\nH\n2\n\n\n\n) was calculated as follows:\n\n(16)\n\n\nP\nu\n\nr\n\nH\n2\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\nF\n\nH\n2\n\n\no\nu\nt\n\n\n\n\nF\n\nH\n2\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\n\nO\n2\n\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\nO\n\n\no\nu\nt\n\n\n+\n\nF\n\nC\n\nH\n4\n\n\n\no\nu\nt\n\n\n\n\n\u00d7\n100\n\n\n\n\nThese definitions are in line with several reported works in the literature on GSR [13,37,38].Besides assessing the catalytic performance of the materials, this study aimed also to correlate physicochemical features of the materials with the catalytic behavior. Samples herein prepared were physicochemically characterized through several techniques, namely ICP-OES, physical sorption-desorption of N2 at \u2212196\u00a0\u00b0C, XRD, TEM/EDX and H2-TPR, as detailed below.The results regarding the elemental composition of catalysts and mixed supports measured through Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) are shown in Table 4\n. As it can be observed, the obtained elemental compositions of the samples are close to the theoretical ones. However, the amount of Co present in the calcined single supported catalyst (Co/La2O3) was only of 6.8\u00a0wt%, while for the mixed supported catalysts the Co amount was superior and of 7.9\u00a0wt% and 8.4\u00a0wt% for the CoAlZnOx and CoAlLaOx catalysts, respectively. This might indicate that the mixed oxide supported catalysts \u2013 prepared by the CSG method \u2013 are able to incorporate more quantities of Co into their structure. The Al/Zn and Al/La weight ratios of the mixed oxide supported catalysts were of around 0.88 and 0.18, respectively, which are close to the intended theoretical values \u2013 these correspond approximately to the expected Al2O3/ZnO and Al2O3/La2O3 molar ratios of 1 (as described in section Catalysts preparation), therefore corresponding approximately to Al/Zn and Al/La molar ratios of 2 and 1, respectively, as can be observed in Table 4.The BET specific surface area was determined for all samples through physical sorption-desorption of N2 at \u2212196\u00a0\u00b0C (Table 5\n). Although the specific surface areas of the materials are quite small, it can be clearly seen that the type of support influences the BET specific surface area following the order La2O3\u00a0>\u00a0AlLaOx\u00a0>\u00a0AlZnOx. On the other hand, the addition of Co precursors resulted in an increase in the specific surface area for the CoAlZnOx and CoAlLaOx catalysts. However, this increase by Co addition was not observed for the single oxide supported catalyst, maybe due to the precision of the measurement method. This indicates that catalysts prepared by the CSG method (mixed oxide supported catalysts) are able to achieve higher specific surface areas, suggesting a high degree of incorporation of Co precursors into the structure of the mixed oxides. Although the AlZnOx support possesses the lowest BET specific surface area, it did not hinder the deposition of high amounts of Co on the surface.The X-ray diffraction patterns of the catalysts calcined at 700\u00a0\u00b0C are shown in Fig.\u00a02\n. The diffractograms of the single oxide supported catalyst (Co/La2O3) and pure La2O3 support (Fig.\u00a02 (a)) show an identified crystal phase that corresponds to lanthanum hydroxide (La(OH)3, 2\u03b8\u00a0=\u00a015.7\u00b0, 27.3\u00b0, 27.9\u00b0, 31.7\u00b0, 35.9\u00b0, 39.5\u00b0, 48.5\u00b0, 49.9\u00b0 and 55.2\u00b0), which was expected due to the hygroscopic nature of La2O3. The diffractogram of Co/La2O3 shows also four peaks that correspond to lanthanum cobalt oxide perovskite (LaCoO3, 2\u03b8\u00a0=\u00a032.9\u00b0, 33.3\u00b0, 47.4\u00b0 and 58.6\u00b0) and two peaks of Co3O4 spinel phase (2\u03b8\u00a0=\u00a031.7\u00b0 and 36.4\u00b0).In the diffractogram of AlZnOx mixed support and CoAlZnOx catalyst (Fig.\u00a02 (b)), intense reflections corresponding to a zinc aluminate spinel phase (ZnAl2O4, 2\u03b8\u00a0=\u00a031.2\u00b0, 36.7\u00b0, 44.7\u00b0, 49.0\u00b0, 55.5\u00b0 and 59.2\u00b0) were observed. However, reflections of a ZnO phase were not found in the diffractogram of CoAlZnOx mainly because the Zn atoms were totally combined with Al to form the ZnAl2O4 spinel phase. Even though Co oxide phases, essentially in the form of Co3O4, were not observed in the CoAlZnOx diffractogram, the active metal should be present in the catalyst as confirmed by ICP, as well as through TEM/EDX analysis (see Section TEM/EDX). The reason for the non-appearance of Co must be because either its phase might be amorphous, and/or because the particle size is below the limit of detection of the XRD technique (4\u00a0nm).Finally, the diffractograms of AlLaOx mixed support and CoAlLaOx catalyst are represented in Fig.\u00a02 (c). Pure AlLaOx shows a lanthanum aluminate perovskite structure (LaAlO3, 2\u03b8\u00a0=\u00a023.4\u00b0, 33.4\u00b0, 41.2\u00b0, 47.9\u00b0, 54.0\u00b0 and 59.6\u00b0), being also present in the diffractogram of CoAlLaOx. Reflections that correspond to Co3O4 (2\u03b8\u00a0=\u00a031.0\u00b0 and 36.7\u00b0) were also found in the CoAlLaOx sample. The presence of mixed phases between Co and La2O3 or Al2O3 were excluded (i.e., CoAl2O4) since reflections of pure single oxides were not identified.As a physicochemical complementary analytical technique, the single supported catalyst (Co/La2O3) and the catalyst supported on AlZnOx were examined by TEM/EDX to obtain information regarding the phase composition of the samples as well as the Co distribution on both supports. The Co/La2O3 catalyst (Fig.\u00a03\n (a)) contains particles having different sizes and shapes, but in general it has round particles with diameters between 20\u00a0nm and 100\u00a0nm as well as some particles up to 150\u00a0nm. Resorting to EDX measurements, it was possible to observe that the biggest particles are composed mainly by LaCoO3 perovskite, being also observed long needle-like particles (size of around 100\u00a0nm) corresponding to LaO. The images having superior resolution (located on the right side of Fig.\u00a03 (a)) demonstrate the lattice planes of areas where perovskite and LaO were detected. The perovskite high resolution image displays an electron diffraction pattern in which the characteristic rings of the reflections of the nanometric polycrystalline material can be observed.Regarding the TEM images of CoAlZnOx (Fig.\u00a03 (b)), it is shown that the morphology of the sample is very similar to that of Co/La2O3 having round particles but having smaller diameters (between 20\u00a0nm and 30\u00a0nm); however, for the CoAlZnOx material, no needle-like particles were observed. The EDX chemical mapping (micrograph image placed on the fourth position in right in Fig.\u00a03 (b)) showed that Co (red), Al (green) and Zn (blue) were uniformly distributed in the sample. However, in some locations are observed characteristic crystallite rings of Co3O4 (red) and ZnAl2O4 (green) smaller than 20\u00a0nm. In addition, in the EDX chemical mapping is observed that the Co3O4 crystallite size is below of around 4\u00a0nm; this might be the reason for the non-appearance of Co in the diffractogram of the CoAlZnOx catalyst, which is below the detection limit of the XRD technique (4\u00a0nm).To establish appropriate pre-treatment temperatures for the activation (reduction) of the catalysts prior to the GSR experiments, and access their reducibility, samples were submitted to temperature-programmed reduction under H2 (H2-TPR).The H2-TRP profiles of the Co-based single oxide and mixed oxide catalysts are shown in Fig.\u00a04\n. The reducibility of Co/La2O3 depends on the potential of the cobalt present in the Co3O4 spinel structure and LaCoO3 perovskite to be reduced since they were the identified phases present in this catalyst at room temperature, as concluded by XRD. However, the reduction process of LaCoO3 perovskite is still not well defined in the literature. The H2-TPR profile of Co/La2O3 demonstrates the first event at 322\u00a0\u00b0C, being also observed a small shoulder prior to this, which might be attributed to the reduction of Co3+ species in the LaCoO3 perovskite [25]. Depending on the oxygen defects in the Co/La2O3 perovskite lattice, the Co3+ species may have different natures inside the structure, therefore resulting in different reducibilities and, consequently, in the broad H2 consumption event registered in the TPR profile. This might also be the explanation for the appearance of the shoulder before the peak at 322\u00a0\u00b0C. The H2 consumption before 322\u00a0\u00b0C might also be due to the reduction of Co3+ species to Co2+ in the Co3O4 spinel phase [25]. Subsequently, three peaks registered at 417\u00a0\u00b0C, 468\u00a0\u00b0C and 614\u00a0\u00b0C and a small plateau between 505 and 529\u00a0\u00b0C were observed, which can be attributed to the reduction of Co2+ species to metallic Co0 present in the Co3O4 spinel phase [32].In relation to the pattern of the CoAlZnOx catalyst, it can be observed multiple H2 uptake events, which might be associated to Co species, such as surface cobalt oxides having different strength in the interactions with the support. At first, a small peak is observed at 369\u00a0\u00b0C which can be attributed to the reduction of Co3+ to Co2+ in the Co3O4 species weakly attached to the support [22,24]. These species are present in low quantities based on the peak size. The second and broad peak, displayed from approximately 445\u00a0\u00b0C\u2013702\u00a0\u00b0C has an asymmetric shape suggesting that it might also be due to the reduction of Co3+ to Co2+ in the Co3O4 species having different interaction strength with the support [22,24]. The major and last peak registered at 775\u00a0\u00b0C and having a pronounced tailing on the high temperature side might be associated to the reduction of Co2+ to metallic Co0 as well as to the reduction of lattice-intercalated Co, as is the case of CoAl2O4 [22,24].Finally, as is observed from the H2-TPR pattern, the Co species present in the CoAlLaOx catalyst were more easily reduced than the Co species in the other two catalysts. Multiple major events registered at 338\u00a0\u00b0C, 565\u00a0\u00b0C and 676\u00a0\u00b0C were observed in the pattern of the CoAlLaOx catalyst. The first event was due to the reduction of Co3+ to Co2+ in the Co3O4 weakly attached to the support [25]. The second event, registered as a broad peak at 565\u00a0\u00b0C, suggests the simultaneous reduction of Co3+ to Co2+ species in the Co3O4 particles having stronger interactions with the support, as well as the reduction of Co2+ species to metallic Co0 [25]. The final event is associated to the total reduction of Co2+ species to metallic Co0 [25].Taking into account the H2-TPR results, catalysts were reduced/activated at 600\u00a0\u00b0C for 1\u00a0h under a 20\u00a0vol% H2/N2 mixture (total flow rate of 100 mLN\u00a0min\u22121). The selected temperature is enough to reduce both Co/La2O3 and CoAlLaOx catalysts as it was observed in Fig.\u00a04. On the other hand, although the used temperature may seem low to completely reduce the CoAlZnOx sample, it should be emphasized that a high H2 percentage of volume was used (20\u00a0vol% instead of only 5\u00a0vol% used during the H2-TPR experiments) to ensure a complete activation of catalysts. In fact, it was verified for the CoAlZnOx catalyst that the use of a reduction temperature of 780\u00a0\u00b0C did not demonstrate any significant changes on its performance in terms of glycerol conversion into gaseous products and H2 yield (data not shown).Prior to the catalysts screening, a blank test (case study #1) was performed in the reactor filled with only SiC particles (bed diluting agent/inert) at temperatures ranging from 400\u00a0\u00b0C to 700\u00a0\u00b0C. Additionally, the yields of H2, CO2, CO and CH4, as well as the total glycerol conversion and glycerol conversion into carbon-containing gaseous products in the thermodynamic equilibrium were determined using the equations listed in section Reaction performance indicators. All thermodynamic equilibrium values were obtained using the Aspen Plus V9 software under the same operating conditions \u2013 i.e., nitrogen was used as dilution gas and the species considered for the simulation were C3H8O3, H2O, H2, CO2, CO, CH4 and C (coke), based on the compounds formed during the catalytic experiments. Details on the methodology used for the simulations can be found elsewhere [39,40]. Results obtained are shown in Table 6\n.The maximum theoretical yield of H2 \u2013 considering a complete glycerol conversion and assuming that no secondary reactions occurred, in accordance to Eq. (1) shown in Table 1, is of 7\u00a0mol of H2 per mole of glycerol fed. An excess of H2O promotes the GSR, according to the thermodynamics, shifting the equilibrium of the WGS reaction (Eq. (3) \u2013 Table 1) in order to produce more H2, while inhibiting the methanation of CO (Eq. (4) \u2013 Table 1) and CO2 (Eq. (5) \u2013 Table 1). However, it has been reported that for very high WGFRs (i.e., >15), the production of H2 increases moderately. For this reason, and taking into account that an excess of H2O was used in the simulations (WGFR of 15), the H2 yield obtained at the thermodynamic equilibrium for temperatures above 550\u00a0\u00b0C was close to the maximum theoretical of 7 \n\nm\no\n\nl\n\nH\n2\n\n\n\u22c5\nm\no\n\n\nl\n\nG\n,\ni\nn\n\n\n\n\u2212\n1\n\n\n\n; for excessive temperatures such yield decreases due to the exothermal nature of the WGS reaction that is therefore inhibited. In addition, the use of an excess of H2O has also the advantage of minimizing coke formation (see Eqs. (9) and (10) \u2013 Table 1). On the other hand, in Table 6 is observed that the use of only inert does not catalyze the GSR as observed from the low glycerol conversion into gaseous products and, consequently, the low production of gaseous products. On the contrary, at high temperatures (>550\u00a0\u00b0C), the total glycerol conversion was not low, which can be ascribed to the thermal decomposition of glycerol. Nevertheless, taking into consideration that the glycerol conversion into gaseous products was residual \u2013 low formation of H2, CO2, CO and CH4 \u2013, it means that almost none of the converted glycerol and/or condensable products formed were steam reformed. Instead, it might be suggested that the majority of glycerol fed was converted into condensable products.To better comprehend how the catalysts perform under the GSR process, two sets of experiments were carried out. The case study #2 (see Table 3) aimed to assess the catalytic performance in terms of activity and selectivity towards the reaction products at temperatures ranging from 400\u00a0\u00b0C to 700\u00a0\u00b0C, while case study #3 (see Table 3) consisted on determining the stability of materials at 625\u00a0\u00b0C.The catalytic performance of the Co-based materials in terms of both total glycerol conversion and glycerol conversion into gaseous products at different temperatures is shown in Fig.\u00a05\n. The increase in temperature favors the total glycerol conversion as well as the glycerol conversion into gaseous products for all catalysts, as expected, due to the endothermic nature and kinetics of the reforming process, which are favored with the increase in temperature. On the other hand, according to the activity results, it can be concluded that the type of support plays an important role. At 700\u00a0\u00b0C, the total glycerol conversion obtained for Co/La2O3 and CoAlLaOx catalysts was of around 90% and the glycerol conversion into gaseous products was of 59% and 65%, respectively (values being referred to the first point obtained at that temperature). This constitutes a significant enhancement in comparison to the blank test with only inert, in which only approximately 11% of glycerol was converted into gaseous products (Table 6). In addition, it is observed that at low temperatures (400\u2013550\u00a0\u00b0C) the difference between total glycerol conversion and glycerol conversion into gaseous products is superior than for higher temperatures (625\u00a0\u00b0C and 700\u00a0\u00b0C), suggesting that at low temperatures a preferential conversion of glycerol into condensable products occurs. In fact, at low temperatures \u2013 particularly at 400\u00a0\u00b0C, 475\u00a0\u00b0C and 550\u00a0\u00b0C \u2013 was observed a change in color (see Fig.\u00a0S1 in Supplementary Information) between the fed aqueous glycerol solution and the liquid effluent collected in the Peltier condensers from transparent to yellow/light brown, respectively, contrarily to what happens at higher temperatures, meaning that conversion of glycerol into condensable products occurred mostly at lower temperatures. As found in literature, such conversion of glycerol into condensable products occurs mainly through thermal decomposition of glycerol [41,42]; also, in accordance to some works [43,44], a temperature as low as 400\u00a0\u00b0C favors substantially the formation of such condensable products comparatively to higher temperatures. Even though a detailed analysis of such compounds was not performed, condensable products such as acetaldehyde, acrolein, propanal, acetone, acetic acid, methanol, ethanol, allyl alcohol and acetol might have been formed during GSR [36,38,42\u201345]. Additionally, it is observed that the promotion of Al2O3 support with La2O3 is better in terms of catalytic activity when compared to the promotion of Al2O3 support with ZnO (cf. Fig.\u00a05). On the other hand, by comparing the results of both Co/La2O3 and CoAlLaOx catalysts, it is observed that the addition of Al2O3 to La2O3 support increases the glycerol conversion into gaseous products within all temperature range, in particular at 550\u00a0\u00b0C and 625\u00a0\u00b0C.The influence of reaction temperature on the yields of gaseous products is represented in Fig.\u00a06\n. For all samples, the yields of products followed, in general, the same trend as total glycerol conversion and conversion of glycerol into gaseous products as the temperature increased. Among all gaseous products, the H2 yield was the highest, followed by CO2 and CO, suggesting that these catalysts present capacity to convert CO to H2 and CO2 through the WGS reaction. As can be observed, in general, the CoAlLaOx catalyst demonstrates a higher H2 yield in the whole range of temperature in comparison to the other two materials. A maximum H2 yield of 3.85 \n\nm\no\n\nl\n\nH\n2\n\n\n\u22c5\nm\no\n\n\nl\n\nG\n,\ni\nn\n\n\n\n\u2212\n1\n\n\n\n was obtained for both Co/La2O3 and CoAlLaOx at 700\u00a0\u00b0C. On the other hand, residual contents of CH4 were observed for all catalysts at low temperatures (400\u00a0\u00b0C and 475\u00a0\u00b0C), while for higher temperatures (above 475\u00a0\u00b0C) a slight increase was observed, reaching a plateau between 625\u00a0\u00b0C and 700\u00a0\u00b0C. The increase in the production of CO and CH4 might be associated with the thermal decomposition of acetaldehyde (\n\nC\n\nH\n3\n\nC\nH\nO\n\n\u2192\n\nC\n\nH\n4\n\n\n+\n\nC\nO\n\n) \u2013 favored at high temperatures \u2013, which might have been formed as an intermediate product through glycerol dehydrogenation by a mechanism of radical decomposition [43]. Although the yield of CH4 increased with temperature, it is worth mentioning that only small amounts of this compound were formed within the range of temperatures studied and, in almost all the cases, was even below the thermodynamic equilibrium, which means that the methanation reactions (Eqs. (4) and (5) \u2013 Table 1) occurred in a low extent.Furthermore, the decrease observed in the total glycerol conversion (cf. Fig.\u00a05), glycerol conversion into gaseous products (cf. Fig.\u00a05) and yield of gaseous products (cf. Fig.\u00a06) with the TOS at each temperature might be attributed to the deactivation of the catalysts provoked by the deposition of carbonaceous deposits on their surfaces. This issue is further detailed in Section Catalytic stability.The influence of reaction temperature on the H2 selectivity and purity is represented in Fig.\u00a07\n. Regarding the H2 selectivity (Fig.\u00a07 (a)), the increase in temperature provides in general its increase for all catalysts. The CoAlLaOx sample provides higher hydrogen selectivities when compared to the other materials at temperatures ranging from 400\u00a0\u00b0C to 550\u00a0\u00b0C. Over 550\u00a0\u00b0C, the Co/La2O3 catalyst becomes superior in terms of hydrogen selectivity. On the other hand, the purity of hydrogen is a very important criterion when evaluating the efficiency of steam reforming processes. In Fig.\u00a07 (b) is observed that the purity of the H2 produced slightly increases with temperature for all Co-based catalysts from around 62-66% to 68%. This indicates that although the production of all gaseous products increased with temperature, the increase in H2 production was slightly superior.Catalyst deactivation is one of the main concerns in industrial reforming processes as it seems to be unavoidable, affecting the performance of the entire process over time, and for this reason, it is a subject that should be carefully addressed. Co-based catalysts have demonstrated good catalytic performance to produce H2 through GSR, being suggested as appropriate materials to be applied in catalytic systems. However, Co-based catalysts are also prone to suffer from deactivation as reported in the literature [46\u201348]. It has been suggested that the deactivation of such catalysts can be attributed to three main reasons: (i) oxidation and sintering of the metal particles, (ii) transformation of the solid state that involves the diffusion of Co into the support, therefore allowing the formation of irreducible Co support compounds (i.e., silicates and aluminates) and (iii) formation of carbonaceous species, namely through Boudouard (see Eq. (7) \u2013 Table 1) and methane cracking (see Eq. (8) \u2013 Table 1) reactions that lead to a blockage of Co active sites on the catalyst surface [49].As mentioned in Section 2.5, stability experiments were proceeded until approximately 10% of the glyerol conversion into gaseous products was reached. It should be emphasized that prior to the stability experiments, all catalysts were regenerated in situ using a TPO step (for coke gasification) followed by a new reduction/activation step, as described before. It should be mentioned that this step was carried out since the catalyst samples used during case study #3 were the same as the ones used during case study #2. The evolution of total glycerol conversion and glycerol conversion into gaseous products over time at 625\u00a0\u00b0C is represented in Fig.\u00a08\n for the Co-based catalysts. For all catalysts, it is observed that both total glycerol conversion and conversion of glycerol into gaseous products decrease drastically after a few hours, indicating a loss of catalysts\u2019 capability to break C\u2013C bonds of the glycerol molecules. During the overall time of reaction with each catalyst, XG,total decreased from 89% to 75%, 80%\u201361% and 91%\u201372%, while XG,gas decreased from 53% to 15%, 42%\u201312% and 65%\u201314% for Co/La2O3, CoAlZnOx and CoAlLaOx catalysts, respectively. This might suggest that both the formation of coke (and possibly also metal particle sintering) and the conversion of glycerol into secondary condensable products increased over time, which was visually noticed in the periodically collected liquid samples that became darker with time. After 2.9\u00a0h of the experiment at 625\u00a0\u00b0C, XG,gas decreased 71%, 65% and 44% for Co/La2O3, CoAlZnOx and CoAlLaOx catalysts, respectively, which indicates that the Co/La2O3 catalyst, followed by CoAlZnOx, is more prone to suffer from deactivation due to coke deposition (and possibly also from metal particle sintering) and also from the increase in the conversion of glycerol into secondary condensable products. This can also be observed in Fig.\u00a08 since the decline in XG,total and XG,gas over time for the CoAlLaOx catalyst was smoother comparatively to the other two. The difference in the time-dependence for the conversion of glycerol might be directly related to differences in the amount of carbon deposits formed by secondary reactions that involve polymerization and/or breakage of C\u2013H bonds of intermediate species. Additionally, as can be observed in Table 7\n, both XG,total and XG,gas increase after regeneration of the catalysts by coke gasification (through TPO) followed by Co reduction (case study #3), which indicates that during case study #2, carbon deposits were formed, therefore decreasing the catalytic activity. While the deactivation of catalysts due to carbon deposition can be reversed by performing a coke gasification with air (O2), the metal particle sintering, on the other hand, is an irreversible phenomenon that occurs due to the exposure to high temperature leading to agglomeration of crystals on the surface of the support; as a consequence, it causes an irreversible loss of catalyst activity. In this sense, the fact that all catalysts were able to recover activity after regeneration by coke oxidation, followed by a new catalyst reduction, might suggest that metal particle sintering did not occur during case study #2.The yields of H2, CO2, CO and CH4 over time for all Co-based catalysts are depicted in Fig.\u00a09\n. All catalysts demonstrate not being stable in terms of H2 production during the stability experiment. The main products were H2, CO2 and CO since low contents of CH4 were observed during the experiments for all catalysts. On the other hand, it was observed that the CO2 selectivity decreases with the TOS, while the CO selectivity increases (Fig.\u00a0S2 in the Supplementary Information). At the beginning of the reaction, the CO2 selectivity was superior to CO; however, as the reaction proceeded, the selectivities of both CO and CO2 became closer. Although both CO2 and CO yields decreased over time, as was observed in Fig.\u00a09 (b) and (c), the decrease in the CO production was less affected in comparison to CO2 production, which might suggest that the WGS reaction (Eq. (3) \u2013 Table 1) was the most affected over time \u2013 this might also be confirmed by the decrease in the H2 selectivity over time (see Fig.\u00a010\n (a)). On the other hand, although the CH4 yield decreased over time, the CH4 selectivity slightly increased (specially for the CoAlZnOx catalyst), meaning that the methanation reaction (Eq. (4) \u2013 Table 1) was probably less affected by the deactivation of the catalyst. In addition, the reason for the increase in selectivity of both CO and CH4 might also be associated to the thermal decomposition of acetaldehyde, which may have been less affected by catalyst deactivation due to coke deposition. It should be stressed that during the stability experiments at 625\u00a0\u00b0C and due to the formation of solid compounds in the catalysts\u2019 surface, the reactor was being plugged over time, which slightly increased the pressure in the system. Although the activity of the catalyst was being lost due to formation of coke, the increase in pressure favors thermodynamically the forward CH4 methanation reaction, which might be the reason for the methanation reaction be less affected overtime, therefore leading to an increase in the CH4 selectivity.The selectivity towards H2 (Fig.\u00a010 (a)) suffers a reduction over time for Co/La2O3 and CoAlZnOx catalysts. The H2 selectivity reduction was accompanied by a decrease in the CO2 selectivity and an increase in the CO selectivity, as observed above, being suggested the weakening of the WGS reaction with time-on-stream (Eq. (3) \u2013 Table 1). Contrarily, for the CoAlLaOx catalyst, during the first hours, the selectivity towards H2 remained almost constant at around 80%, decreasing to 66% in the last 2\u00a0h of experiment. Consequently, for all catalysts, the H2/CO molar ratios (Fig.\u00a0S3 (a) in Supplementary Information) decreased drastically, while the CO/CO2 molar ratios (Fig.\u00a0S3 (b) in Supplementary Information) increased. On the other hand, the purity of H2 (Fig.\u00a010 (b)) remained almost constant during all experiments for both Co/La2O3 and CoAlZnOx catalysts, while for the CoAlLaOx catalyst a slight decrease was observed in the last 2\u00a0h of the experiment.In Table 8\n is shown the stability performances of different Co-based catalysts used for GSR found in the literature. However, it should be emphasized that although being difficult to make a detailed comparison between catalysts tested in different works \u2013 as reaction conditions and catalyst pre-treatment may differ from work to work and are considered as crucial factors for catalytic efficiency \u2013 the comparison was made between the results herein obtained and the results obtained in other works found in the literature. Papageridis et\u00a0al. [48] observed that the conversion of glycerol into gaseous products obtained for the 8\u00a0wt% Co/Al2O3 catalyst decreased from around 64%\u201341%, while the H2 yield decreased from 2.5 to 1.9 \n\nm\no\n\nl\n\nH\n2\n\n\n\u22c5\nm\no\n\n\nl\n\nG\n,\ni\nn\n\n\n\n\u2212\n1\n\n\n\n in approximately 3.0\u00a0h \u2013 see Table 8. Comparing these results with the results obtained in this work for the 10\u00a0wt% Co/La2O3 catalyst (decrease of XG,gas from 53% to 15% and H2 yield from 3.23 to 0.82 \n\nm\no\n\nl\n\nH\n2\n\n\n\u22c5\nm\no\n\n\nl\n\nG\n,\ni\nn\n\n\n\n\u2212\n1\n\n\n\n in 2.6\u00a0h), it can be concluded that the Co/Al catalyst suffered a less aggressive catalyst deactivation. However, in the work of Papageridis et\u00a0al. [48], a higher WGFR was used, and it is well known that the higher the WGFR, the lower the probability of forming carbon deposits. On the other hand, the metal loadings of Co were also distinct. In another work, Menezes et\u00a0al. [46] observed that for the 20\u00a0wt% CoNbAl catalyst, the glycerol conversion into gaseous products decreased from 96% to 89%, while the H2 yield decreased from 66% to 64% in 3.0\u00a0h of experiment. Once again, it is observed that the CoNbAl catalyst suffered from a less severe deactivation in comparison to the CoAlLaOx catalyst prepared in this work, which resulted in a lower loss in the catalytic activity; however, the operating conditions used by Menezes et\u00a0al. [46] were slightly different from the ones used in this work.The catalysts used in case study #2 (section Catalytic activity) were regenerated resorting to a temperature-programmed oxidation (TPO) with air, as previously described, and were thereafter used to carry out case study #3 (section Catalytic stability). Regenerative oxidation was performed at 600\u00a0\u00b0C as this temperature is known to be sufficient to fully remove both amorphous and graphitic carbon species [50]. The CO2 and CO vol% concentrations obtained from the burning of carbon deposits were continuously monitored during the regeneration step (Fig.\u00a0S4 in Supplementary Information). The total amount of carbonaceous deposits was determined by integration of the respective TPO curves of each catalyst \u2013 see results in Table 9\n. It can be observed that the Co/La2O3 catalyst formed the highest quantity of carbon deposits during case study #2 (25.2 mgC\u00b7gcat\n\u22121), followed by the CoAlZnOx catalyst, which produced 20.0 mgC\u00b7gcat\n\u22121 of carbonaceous species. On the other hand, on the surface of the CoAlLaOx catalyst were deposited 13.2 mgC\u00b7gcat\n\u22121 of coke. It is known that the rate of carbon removal during combustion with air depends on the type of catalyst, and from the TPO profiles it is possible to observe some particular differences. The Co/La2O3 catalyst (Fig.\u00a0S4 (a)), which produces a higher quantity of carbon deposits in comparison to the other two materials prepared, required more time to be fully regenerated. On the other hand, the CoAlLaOx catalyst (Fig.\u00a0S4 (c)), which formed the lower quantity of coke, was the fastest catalyst to be fully regenerated. These results are in line with the deactivation of catalysts observed during the stability experiment. As can be seen in Fig.\u00a011\n, the higher the quantity of carbon deposits formed during case study #2, the higher the percentage of loss in XG,gas and in H2 yield obtained during case study #3. In other words, the loss in catalytic activity over time may be attributed to the formation of carbonaceous species on the catalyst surface, therefore decreasing the performance of the GSR process, namely in terms of hydrogen production.The catalyst deactivation, mostly due to coke deposition, is still one of the main concerns for the application of GSR in industrial scale. Catalysts based on noble metals, specially Ru- and Pt-based, are normally more stable and active for GSR comparatively to Co-based catalysts; however, these materials are known to be much more expensive [3]. In this sense, it is desirable to develop efficient, reliable and economic methods to regenerate catalysts. Fortunately, the catalyst deactivation due to the formation of coke deposits can easily be reverted by resorting to TPO with air as this method burns the carbon deposits formed on the surface of the catalyst, therefore allowing to recover its catalytic activity. However, hot spots can be originated during this process due to the exothermicity of coke burning, which can damage the catalyst and affect its catalytic performance [3]. Additionally, to remove all carbon deposits (i.e., all amorphous and graphitic coke), high temperatures are mandatory (\n\n\u2265\n\n 600\u00a0\u00b0C), therefore implying high operating costs. However, if the use of high temperatures is not an option to regenerate the catalyst (due to high operating costs, for example), it should be mentioned that the use of lower temperatures (350\u2013450\u00a0\u00b0C [51]) is efficient to remove at least the amorphous coke as this is considered to be more detrimental to the performance of the catalyst in comparison to graphitic carbon [50]. There are other methods to regenerate catalysts with minor consequences and that allow the removal of coke at low temperatures, as is the case of solvent extraction methods, supercritical fluid extraction and the use of ozone (O3) [50]. Other alternatives that meet the concept of \u201cgreen carbon science\u201d, such as the catalysts regeneration combined with coke gasification (resorting to CO2 and H2O), are also efficient to revert catalyst deactivation with the advantage of producing synthesis gas, instead of CO2 [49]. Nevertheless, it should be stressed that for traditional reactors, if a catalyst regeneration is necessary, a shut-down of the entire process is required, which will affect the continuous H2 production (otherwise parallel reactors are required). In addition, for most of the catalysts, after an oxidative regeneration a new activation/reduction is mandatory. With this being said, it is always preferable to have active and stable catalysts that are resistant to deactivation either due to coke deposition \u2013 therefore not being implied the shut-down of the GSR process \u2013 and/or to sintering.The catalytic behavior of Co-based catalysts supported on La2O3, AlZnOx and AlLaOx was studied for the GSR. It was concluded that the type of support is essential for both catalytic activity and stability. In addition, the findings from the catalysts physicochemical characterization contributed to better comprehend the results obtained during the GSR experiments.The CoAlLaOx mixed oxide catalyst was observed to provide the highest catalytic activity and H2 yield during GSR, which might be attributed to different physicochemical properties. This catalyst was also observed to be the most stable. By comparing the results between the Co/La2O3 and CoAlLaOx catalysts, it was observed that the addition of Al2O3 to the La2O3 support was responsible for the increase in the specific surface area of the CoAlLaOx catalyst. In addition, among the catalysts prepared, it was observed by ICP-OES that the CoAlLaOx catalyst incorporated the highest Co content. These factors might have been responsible for the good performance of this material in comparison to the others. On the other hand, although the CoAlZnOx catalyst demonstrated to have a higher specific surface area in comparison to CoAlLaOx, the Co-based catalyst supported on AlZnOx incorporated a lower Co content into its structure, which might have been responsible for the lower catalytic activity. Although the addition of ZnO to Al2O3 and La2O3 to Al2O3 lead to a decrease in the specific surface area, as observed by Kraleva et\u00a0al. [24] and Charisiou et\u00a0al. [17], respectively, it also lowered the acidity of the support in comparison to pristine Al2O3, therefore increasing the long-term stability of catalysts. The alumina acid sites are known to be responsible for the formation of coke as carbon deposits are preferably formed on those sites.In the materials prepared were identified perovskite structures, as well as some spinel phases. From XRD and TPR analyzes, a LaCoO3 perovskite structure was observed in the structure of the Co/La2O3 catalyst, which was the active metal promoter. Regarding the mixed oxide CoAlZnOx catalyst, it was observed through XRD that the active phase was mainly a ZnAl2O4 spinel. In relation to the CoAlLaOx catalyst, it was observed through XRD the formation of a LaAlO3 perovskite structure, which is known to be very stable even at high temperatures [25]. Taking into account the results obtained from the GSR experiments, it might be concluded that the ZnAl2O4 spinel phase present in the CoAlZnOx catalyst was less active in terms of glycerol conversion into carbon-containing gaseous products and H2 yield. On the other hand, the Co/La2O3 catalyst suffered from a more severe deactivation with time-on-stream, followed by CoAlZnOx. This might suggest that the LaAlO3 perovskite structure can prevent more efficiently the coke formation, while on the contrary, the LaCoO3 perovskite structure is not so efficient in preventing the deposition of carbon deposits.Additionally, in relation to the CoAlLaOx catalyst, it might be suggested that the presence of lanthanum can facilitate the dispersion of active species, strength the interactions between Co species and support, as well as increase the number of basic sites and redistribution of the acid ones in terms of strength and density, providing a catalyst with improved performance for GSR in terms of H2 production and stability, as lanthanum is known to hinder the formation of olefins and oxygenates, considered as coke precursors [17]. Nevertheless, although the addition of basic promoters to alumina supports might improve the catalyst activity, it has also been demonstrated that the use of basic materials does not necessarily ensures the stability of catalysts during GSR.The Co-based catalysts prepared in this work were concluded to not being stable during the catalytic stability experiments (cf. Section 3.2.2.2). In this sense, and taking into account that the CoAlLaOx catalyst was observed to be the most active and stable towards the GSR, comparatively to the other Co/La2O3 and CoAlZnOx catalysts, a NiAlLaOx catalyst was additionally prepared. The NiAlLaOx catalyst was evaluated in terms of catalytic stability during GSR and compared with CoAlLaOx \u2013 results are represented in Fig.\u00a012\n. Although both catalytic performances at the beginning of the experiment are similar, the NiAlLaOx catalyst demonstrated an outstanding improved stability comparatively to CoAlLaOx in terms of both glycerol conversion into gaseous products and H2 yield, making it a suitable candidate to be applied in a continuous GSR process. Additional experiments on the NiAlLaOx would be interesting; detailed analysis on this catalyst will be done in a subsequent work in conjunction with other Ni-based catalysts.Co-based catalysts supported on La2O3, AlZnOx and AlLaOx were prepared with the aim of assessing the effect of the support on the catalytic activity \u2013 at different reaction temperatures \u2013 and stability of the materials for the valorization of glycerol through steam reforming for H2 production.For all catalysts, the increase in temperature favored both total glycerol conversion and conversion of glycerol into gaseous products, and subsequently, favored the H2 production. As for the CH4 yield, although it increased with temperature, residual amounts of this compound were formed within the temperature range studied. The CoAlLaOx catalyst demonstrated higher catalytic activity than the other catalysts prepared, producing at 700\u00a0\u00b0C a H2 yield of 3.85 \n\nm\no\n\nl\n\nH\n2\n\n\n\u22c5\nm\no\n\n\nl\n\nG\n,\ni\nn\n\n\n\n\u2212\n1\n\n\n\n and a glycerol conversion into gaseous products of 65%.On the other hand, it was observed that the Co-based catalysts were not stable with time-on-stream for the GSR at 625\u00a0\u00b0C, demonstrating a drastic decrease of the catalytic activity after a few hours of reaction, suggesting a loss of catalysts\u2019 capability to break C\u2013C bonds of the glycerol molecules. The production of carbon-containing gaseous products was more affected over time than the total glycerol conversion, being this attributed to the increase of the production of liquid products and to the formation of carbon deposits on the surface of the catalysts. The CoAlLaOx catalyst demonstrated to be more carbon-resistant \u2013 followed by CoAlZnOx \u2013, being observed a clear relationship between the quantity of coke formed and the loss in H2 yield and glycerol conversion into gaseous products, i.e., the more the quantity of carbonaceous species formed, the more the loss in H2 yield and glycerol conversion into gaseous products.The combination of GSR with oxidative regeneration of catalysts is a possible way to revert the catalyst deactivation. However, this step involves the shut-down of the entire process when traditional reactors are being used (unless parallel devices are employed), and as a consequence, the H2 production decreases. In this sense, active and stable catalysts that are resistant to deactivation due to coke formation should be preferably selected.An additional NiAlLaOx catalyst was prepared \u2013 taking into account that the CoAlLaOx catalyst was the one that showed the best catalytic performance comparatively to the other catalysts prepared \u2013 and evaluated in terms of catalytic stability for GSR. The NiAlLaOx catalyst showed a remarkable stable behavior throughout all experiment, both in terms of glycerol conversion into gaseous products and H2 yield. Detailed analysis on this catalyst will be done in a subsequent work.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by i) Base Funding \u2013 LA/P/0045/2020 of the Associate Laboratory in Chemical Engineering (ALiCE) and UIDB/00511/2020 \u2013 UIDP/00511/2020 of the Laboratory for Process Engineering, Environment, Biotechnology and Energy (LEPABE) \u2013 funded by national funds through the FCT/MCTES (PIDDAC) and by ii) the German Federal Ministry of Economics and Energy (BMWi), Project KF2031911ZG2 and the Leibniz Society.M. Salom\u00e9 Macedo is grateful to the Portuguese Foundation for Science and Technology (FCT) for her Ph.D. grant (SFRH/BD/137106/2018), with financing from national funds of the Ministry of Science, Technology and Higher Education and the European Social Fund (ESF) through the Human Capital Operational Programme (POCH). M. A. Soria also thanks the FCT for the financial support of his work contract through the Scientific Employment Support Program (Norma Transit\u00f3ria DL 57/2017).The authors are indebted to Luc\u00edlia Ribeiro from LSRE-LCM for the glycerol conversion measurements.The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2022.07.236.", "descript": "\n                  A comparative study of 10\u00a0wt% Co-based catalysts supported on La2O3, AlZnOx and AlLaOx was performed for glycerol steam reforming (GSR). The catalysts physicochemical characterization was done through several techniques. All catalysts were screened in terms of catalytic activity and time-on-stream stability for GSR. The catalytic activity experiments aimed to assess the effect of temperature (400\u2013700\u00a0\u00b0C) on the glycerol conversion and yield of gaseous products (H2, CO2, CO and CH4). Additionally, catalytic stability experiments were conducted at 625\u00a0\u00b0C to investigate deactivation of the catalysts, in which a drop in the activity was observed, especially for Co/La2O3. The glycerol conversion into gaseous products as a function of the time-on-stream was more affected for all catalysts in comparison to total glycerol conversion, being this effect assigned to the increase in the formation of liquid products and to the formation of coke. CoAlLaOx was observed to be more carbon-resistant, followed by CoAlZnOx, through the measurement of the quantity of carbonaceous species formed during the GSR experiments. A NiAlLaOx catalyst was also prepared and assessed in terms of catalytic stability for GSR; a stable behavior was observed throughout all experiment in relation to glycerol conversion into gaseous products and H2 yield.\n               "}
{"full_text": "The development of innovative heterogeneous catalysts with enhanced performance and prolonged lifetimes takes a prominent place to tackle global environmental issues whilst meeting increasing demands for commodity chemicals [1,2]. Significant advances have been made in engineering well\u2013defined materials with versatile architectures at the nanoscale [3,4], thereby overcoming structural nonuniformity that hinders the identification of active sites and the correlation of relationships at the molecular level [5]. Among several examples, single\u2013atom catalysts, containing spatially isolated metal atoms on appropriate hosts, have attracted significant attention in recent years [6\u20139]. These systems have proven effective for the derivation of structure\u2013performance trends in various chemical transformations [10,11], such as hydrochlorination [12,13] or alkynes semi-hydrogenation [14,15]. Specifically, isolated atoms displayed distinct characteristics with respect to the conventional supported nanoparticles [16,17]. Therefore, a systematic investigation of the nanostructure using a platform of catalysts ranging from single atoms with defined environments up to nanoparticles with controlled size, on catalytic performance constitutes an important step for the rational design of promising systems [18].An application of potential practical relevance is the halogen\u2013mediated natural gas upgrading to chemicals and fuels through hydrodehalogenation of dihalomethanes (CH2X2, X \u00a0=\u00a0Cl, Br) [19,20]. Selectively reforming these polyhalogenated compounds is required since they contribute to halogen and carbon\u2013losses in the downstream halomethanes (CH3X) upgrading step [21,22]. Various nanoparticle\u2013based metal catalysts (Fe, Co, Ni, Cu, Ru, Rh, Ag, Ir, Pt) deposited on SiO2 were studied in CH2Br2 hydrodebromination (HDB), revealing an outstanding CH3Br selectivity over ruthenium (\u226496%), great propensity to CH4 over iridium and platinum (>50%), intermediate selectivity performance of nickel and rhodium (<60%), and inactive behavior of the other metals (Fe, Co, Cu, and Ag) [23]. Despite its selective character, Ru/SiO2 deactivated rapidly due to coking and sintering, whereas the iridium and platinum nanoparticles displayed the highest activity and stability. A global performance descriptor based on the adsorption strength of the halogen/carbon fragment on the active metal phase was presented to rationalize the observed reactivity patterns [23]. Building on these results, a recent study systematically investigated nuclearity\u2013 and host effects, using a platform of activated\u2013 (AC) and nitrogen\u2013doped (NC) carbon\u2013supported platinum nanostructures, from single atoms to nanoparticles of ca. 4\u00a0nm, in CH2Br2 hydrodebromination. The exceptional CH3Br selectivity over the NC\u2013supported single atoms (\u226498%) was disclosed, outperforming AC\u2013supported analogues and the reference catalyst Ru/SiO2. The performance was explained by the geometric effects of the single atom and the participation of nitrogen sites in the reaction by storage of H\u2013atoms [24].In contrast to HDB, only a single study targeted selective CH2Cl2 hydrodechlorination (HDC) to CH3Cl [25]. Therein, SiO2\u2013supported ruthenium, platinum, and iridium nanoparticles were investigated, showing a low selectivity to CH3Cl (\u226438% with CH4 as main product, achieved over Ir/SiO2). Attempts to increase the selectivity by supporting Ir nanoparticles (0.8\u20131.6\u00a0nm) on ZrO2, Al2O3, CeO2, anatase TiO2, and MgO did not provide the desired improvements. On the other hand, epitaxially directed iridium nanostructures on rutile TiO2 showed unprecedented activity and CH3Cl selectivity (\u226495%), though limited lifetime due to poisoning by chlorination. Other relevant iridium nanostructures, such as single atoms, were not evaluated which hampers the formulation of robust structure\u2013performance relationships. Furthermore, current CH2X2 HDH studies were confined to a single halogen, leaving ample room for further investigations [23].To systematically address the catalytic search, we synthesized a platform of Ir/NC catalysts with distinct nanostructures, ranging from single atoms to size\u2013controlled nanoparticles of ca. 3.5\u00a0nm, and assessed their performance in both HDC and HDB. By extending the scope to other NC\u2013supported metals (Pt, Ru, and Ni) prepared as single atoms and nanoparticles (Fig. 1\n), we consistently investigate active phase size\u2013 and halogen effects with the aim to advance the design of promising hydrodehalogenation catalysts.Commercially available AC (Norit ROX 0.8) was used for evaluating the catalytic response of this metal\u2013free support. NC was synthesized following the protocol reported by Kaiser et al.\n[12] Prior to its use as carrier for metal species, the NC was ground and sieved into particles of 0.4\u20130.6\u00a0mm. The metal precursors, IrCl3\u00b7xH2O (abcr, 99.9%), H2PtCl6 (abcr, 99.9%), RuCl3\u00b7xH2O (abcr, 99.9%), and Ni(NO3)2\u00b76H2O (Strem Chemicals, 99.9%) were dispersed on the support via incipient wetness impregnation. Appropriate amounts of the precursors required to obtain a nominal metal loading of 1\u00a0wt% were fully dissolved in a volume of deionized water (Pt and Ni) or aqua regia (Ir and Ru) equal to the pore volume of the carrier. The precursor solution was added dropwise to the support, and the resulting mixture was magnetically stirred for 2\u00a0h at room temperature. The impregnated solids were dried at 473\u00a0K for 16\u00a0h in static air (heating rate\u00a0=\u00a05\u00a0K\u00a0min\u22121). Subsequently, all samples were thermally activated (T\nact\u00a0=\u00a0473\u20131073\u00a0K) for 16\u00a0h in N2 atmosphere (heating rate\u00a0=\u00a05\u00a0K\u00a0min\u22121). The catalysts were referred to as M/NC\u2013T\nact, where M denotes the metal (Ir, Pt, Ru, and Ni). The Ir/NC-1073, Pt/NC\u20131073, and Ni/NC\u20131073 catalysts underwent an additional reductive treatment in 20\u00a0vol% H2/He (PanGas, purity 5.0) flow for 3\u00a0h at elevated temperatures (T\nred\u00a0=\u00a0773 or 873\u00a0K, heating rate\u00a0=\u00a010\u00a0K\u00a0min\u22121) and are denoted as Ir/NC(773), Pt/NC(873), and Ni/NC(773). Fig. 1 provides a guideline for the catalysts developed in this study.Powder X\u2013ray diffraction (XRD) was measured in a PANalytical X\u2019Pert PRO\u2013MPD diffractometer with Bragg-Brentano geometry by applying Ni\u2013filtered Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.54060\u00a0\u00c5). The data were recorded in the 10\u201370\u00b0 2\u03b8 range with an angular step size of 0.017\u00b0 and a counting time of 0.26\u00a0s per step. N2 sorption at 77\u00a0K was measured in a Micromeritics TriStar II analyzer. The samples (ca. 0.10\u00a0g) were degassed to 50\u00a0mbar at 423\u00a0K for 12\u00a0h prior to the measurement. The Brunauer\u2013Emmett\u2013Teller (BET) method was applied to calculate the total surface area, S\nBET. The pore volume, V\npore, was determined from the amount of N2 adsorbed at a relative pressure of p/p\n0\u00a0=\u00a00.98. The metal content in the catalysts was determined by inductively coupled plasma optical emission spectroscopy (ICP\u2013OES) using a Horiba Ultima 2 instrument equipped with photomultiplier tube detection. The solids were dissolved in a HNO3:H2O2\u00a0=\u00a03:1 mixture under sonication until the absence of visible solids. CO pulse chemisorption was performed on a Thermo TPDRO 1100 set-up equipped with a thermal conductivity detector. Prior to the analyses, the samples (ca. 0.15\u00a0g) were pretreated at 423\u00a0K under flowing He (20\u00a0cm3 STP min\u22121) for 30\u00a0min, and reduced at 623\u00a0K under flowing 5\u00a0vol% H2/He (20\u00a0cm3 STP min\u22121) for 30\u00a0min. Thereafter, 0.344\u00a0cm3 of 1\u00a0vol% CO/He were pulsed over the catalyst bed every 4\u00a0min at 308\u00a0K until the area of the pulses remained constant. To avoid desorption of CO, the interval between successive pulses was minimized. Scanning transmission electron micrographs with a high-angle annular dark-field detector (HAADF\u2013STEM) were acquired on FEI Talos and Hitachi HD2700CS microscopes operated at 200\u00a0kV. All samples were dispersed onto lacey carbon coated copper or nickel grids. The size distribution of the metal nanostructures was obtained by examining over 100 nanoparticles. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Physical Electronics Quanterna SXM X\u2013ray instrument using monochromatic Al K\u03b1 radiation, generated from an electron beam operated at 15\u00a0kV, and equipped with a hemispherical capacitor electron-energy analyzer. The samples were analyzed at constant analyzer pass energy of 55.00\u00a0eV. The spectrometer was calibrated for the Au 4f\n7/2 signal at 84.0\u00a0\u00b1\u00a00.1\u00a0eV. The envelopes were fitted by mixed Gaussian\u2013Lorentzian component profiles after Shirley background subtraction. The selected peak positions of the different species were based on literature reported data [26]. X-ray absorption fine structure (XAFS) measurements at the (Ir, Pt) L\n2 and L\n3- and (Ru) K\u2013edge were carried out at the SuperXAS beamline. The incident photon beam provided by a 2.9\u00a0T superbend magnet was selected by a Si(111) channel-cut Quick-EXAFS monochromator. The rejection of higher harmonics and focusing were achieved with rhodium-coated collimating and toroidal mirrors, respectively, at 2.5 mrad. The area of sample illuminated by the X-ray beam was 0.5\u00a0mm\u00a0\u00d7\u00a00.2\u00a0mm. All spectra were recorded in transmission mode at room temperature. The extended X-ray absorption fine structure (EXAFS) spectra were acquired with a 1\u00a0Hz frequency (0.5\u00a0s per spectrum) and then averaged over 15\u00a0min. The procedures for analysis and fitting of the EXAFS spectra are reported elsewhere [12,27].The hydrodechlorination of CH2Cl2 (HDC) and hydrodebromination of CH2Br2 (HDB) were conducted at ambient pressure in a home\u2013made continuous\u2013flow fixed\u2013bed reactor set-up. H2 (PanGas, purity 5.0), He (carrier gas, PanGas, purity 5.0), and Ar (internal standard, PanGas, purity 5.0) were dosed by a set of digital mass flow controllers (Bronkhorst) and the liquids, CH2Br2 (Acros Organics, 99%) or CH2Cl2 (Sigma Aldrich, >99.9%), were fed by a syringe pump (Fusion 100, Chemyx) equipped with a water-cooled syringe to a vaporizer unit operated at 393 (CH2Br2) or 353\u00a0K (CH2Cl2). A quartz reactor (internal diameter, d\ni\u00a0=\u00a012\u00a0mm) was loaded with the catalyst or metal\u2013free carrier (catalyst/carrier weight, W\ncat\u00a0=\u00a00.05\u20131\u00a0g, particle size, d\np\u00a0=\u00a00.4\u20130.6\u00a0mm) and heated to the reaction temperature (T\u00a0=\u00a0523\u00a0K) in an electrical oven under He flow. The catalyst bed was allowed to stabilize for at least 10\u00a0min before the reaction mixture was fed at a total volumetric flow of F\nT\u00a0=\u00a030 and 50\u00a0cm3 STP min\u22121 for HDB and HDC, respectively, and with a composition of CH2X2:H2:Ar:He\u00a0=\u00a06:24:5:65 (vol%, X \u00a0=\u00a0Cl, Br). The kinetic tests were performed with a variable feed composition of CH2X2:H2:Ar:He\u00a0=\u00a06:6\u201372:5:17\u201383. Downstream linings were heated at 393\u00a0K to prevent the condensation of unconverted reactants and/or products. Carbon-containing compounds (CH2X2, CH3X, CH4) and Ar were quantified online via a gas chromatograph equipped with a GS\u2013Carbon PLOT column coupled to a mass spectrometer (GC\u2013MS, Agilent GC 6890, Agilent MSD 5973\u00a0N). After GC\u2013MS analysis, the gas stream was passed through two impinging bottles in a series containing an aqueous solution of NaOH (1\u00a0M) for neutralization prior to its release in the ventilation system.The conversion of the reactant, X(CH2X2), was calculated using Eq.\n\n(1)\n,\n\n(1)\n\n\nX\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\n=\n\n\n\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nin\n\n\n-\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nout\n\n\n\n\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nin\n\n\n\n\n\n\u00d7\n\n100\n,\n\n%\n\n\n\nwhere n(CH2X2)in and n(CH2X2)out are the molar flows of CH2Br2 or CH2Cl2 at the reactor inlet and outlet, respectively. The selectivity, S(j), to product j (j: CH3X, CH4) was calculated according to Eq.\n\n(2)\n,\n\n(2)\n\n\nS\n\n(\nj\n)\n\n\n=\n\n\n\nn\n\n\n(\nj\n)\n\n\nout\n\n\n\n\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nin\n\n\n-\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nout\n\n\n\n\n\n\u00d7\n\n100\n,\n\n%\n\n\n\nwhere n(j)out is the molar flow of product j at the reactor outlet. The selectivity to coke, S\ncoke, in all tests was calculated according to eq.\n\n(3)\n, which is based on a generally applied carbon balance that determines the accumulation of carbon\u2013containing species in the catalyst.\n\n(3)\n\n\n\n\nS\n\n\ncoke\n\n\n\n=\n\n\n\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nin\n\n\n-\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nout\n\n\n-\nn\n\n\n(\nj\n)\n\n\nout\n\n\n\n\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nin\n\n\n\n\n\n\u00d7\n\n100\n,\n\n%\n\n\n\nTherein, n(CH2X2)in, n(CH2X2)out, and n(j)out stand for the molar in\u2013 and outlet flows of gaseous reactants and products. The reaction rate, r, based on the metal loading and expressed with respect to the consumption of CH2X2, was calculated using eq.\n(4),\n\n(4)\n\n\nr\n\n=\n\n\n\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nin\n\n\n\n\u00d7\n\nX\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\n\n\n\nW\n\n\ncat\n\n\n\n\u00d7\n\n\n\n\u03c9\n\n\nM\n\n\n\n\n\n,\n\n\n\nmol\n\n\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n\n\n\n\n\nh\n\n\n-\n1\n\n\n\n\n\nmol\n\n\nM\n\n\n-\n1\n\n\n\n\n\nwhere W\ncat is the weight of the catalyst and \u03c9M\n is the metal loading determined by ICP\u2013OES analysis (Table 1\n). The turnover frequency, TOF, was calculated using eq.\n\n(5)\n,\n\n(5)\n\n\nTOF\n\n=\n\n\n\nn\n\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\nin\n\n\n\n\u00d7\n\nX\n\n(\n\n\nCH\n\n\n2\n\n\n\n\nX\n\n\n2\n\n\n)\n\n\n\n\n\nW\n\n\ncat\n\n\n\n\u00d7\n\n\n\n\u03c9\n\n\nM\n\n\n\n\u00d7\n\n\nD\n\n\nM\n\n\n\n\n\n,\n\n\n\nh\n\n\n-\n1\n\n\n\n\n\nwhere DM\n is the metal dispersion, determined by CO pulse chemisorption. A metal dispersion of 100% was used for the single atom\u2013based catalysts. After the tests, the reactor was quenched to room temperature in He flow, and the catalyst was retrieved for further characterization analyses. Evaluation of the dimensionless moduli based on the criteria of Carberry, Mears, and Weisz\u2013Prater [28,29] indicated that the catalytic tests were performed in the absence of mass and heat transfer limitations.Density Functional Theory (DFT) on models of the nanoparticles and single atoms representing the different catalytic systems was employed as implemented in the Vienna ab\u2013initio Simulation Package (VASP 5.4.4) [30,31]. Generalized Gradient Approximation with the Perdew\u2013Burke\u2013Ernzerhof functional (GGA\u2013PBE) [32] was used to obtain the exchange\u2013correlation energies with dispersion contributions introduced via Grimme\u2019s DFT\u2013D3 approach [33]. Projector Augmented Wave (PAW) [34,35] and plane waves with a cut-off energy of 450\u00a0eV, with spin polarization allowed when needed, were chosen to represent the inner electrons and the valence monoelectronic states, respectively. For simulations of the single atoms, a one-layer (6\u00a0\u00d7\u00a06) slab of graphitic carbon separated by 19\u00a0\u00c5 of vacuum was used and sampled through a gamma\u2013centred grid of 3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a01\u00a0k\u2013point grid. Carbon materials are ill\u2013defined for several reasons: (i) the graphene layers constituting them can be stacked in different arrangements due to the van der Waals interactions; (ii) they store different types of structural and particularly point defects; (iii) depending on the atmosphere under which they are prepared they can have a variable (almost continuous) stoichiometry with different types of functionalities; (iv) overall if doped or created by different precursors a wider chemical versatile (and defect types) renders an almost continuous of chemical environments. This wide spread cannot be addressed efficiently by present characterization techniques to set structure\u2013activity relationships as, for instance, EXAFS is not able to distinguish C/N/O coordination and vibrational patterns can hint on the nature of N-cavities. The computational solution to this conundrum is to devise a set of models compatible with the stoichiometry and nature of the most-abundant cavities according to the available characterization. In line with this, the NC support was represented by a set of three defects containing various nitrogen functionalities, including pyrrolic and/or pyridinic moieties, and different coordination structures; (i) non-planar 3\u00a0N and square-planar 4\u00a0N arrangements, labelled 3\u00a0\u00d7\u00a0N5 (tri\u2013pyrrolic), (ii) 4\u00a0\u00d7\u00a0N6 (tetra-pyridinic), (iii) and 2\u00a0\u00d7\u00a0N5\u00a0+\u00a02\u00a0\u00d7\u00a0N6 (tetra-pyrrolic/pyridinic), whereas AC was modelled by adding an epoxide on the carbon matrix, all adopted from previous studies [12]. Single atom catalysts were modelled by placing the metal atom in the centre of each cavity. For nanoparticles, systems were modelled as a four\u2013layer p(3x3)\u2013(111) fcc (Pt, Ir, and Ni), or a p(3x3)\u2013(0001) hcp slab (Ru) interspaced along the z\u2013direction by a vacuum space of 15\u00a0\u00c5, and k-point sampling of 5\u00a0\u00d7\u00a05\u00a0\u00d7\u00a01 (Gamma centered). The two top layers and the adsorbates were allowed to relax while the bottom two were fixed to the bulk lattice. The arising dipole was corrected in all slab models [36]. Gas-phase molecules were optimized in a box of 14.0\u00a0\u00d7\u00a014.5\u00a0\u00d7\u00a015.0\u00a0\u00c53. For all investigated systems, structures were relaxed using convergence criteria of 10\u20134 eV and 10\u20135 eV for the ionic and electronic steps, respectively.To assess the stability of the single atoms in the NC cavities, formation energies were estimated using the metal species and the scaffold as reference states. Binding energies of the halogen to the halogenated single atoms were calculated using the non-halogenated single atom and Br2/Cl2 as reference states. For the Gibbs free energy on the reaction network, CH2X2, H2, and metal surfaces or pristine single atoms (defined as the isolated and non\u2013chlorinated metal species) were utilized, and the vibrational, rotational, and translational entropic contributions from gas\u2013phase reactant molecules were included.The Climbing Image Elastic Band (CI\u2013NEB) method [37,38], improved dimer method [39,40] and quasi\u2013Newton algorithms were employed to locate the transition states (TS) in the reaction profiles, where the TS were further verified by their single imaginary frequency character.All the structures presented in this work have been uploaded to the ioChem-BD database [41]\n\nhttps://iochem-bd.iciq.es/browse/review-collection/100/29816/440d6583bf1645c23bc615b4\n\n[42].To build a fully consistent platform on NC supports and since iridium has been targeted as a potential HDX active phase our work starts with the preparation of NC\u2013supported iridium catalysts, applying the reported procedures, which include dry impregnation of the chloride precursor on NC followed by thermal activation to obtain the final catalyst, designated Ir/NC\u2013T\nact (T\nact, 473\u20131073\u00a0K). This procedure follows our recent synthetic strategies presenting structures of platinum and ruthenium (1\u00a0wt% metal basis) on NC, from (chlorinated) single atoms to nanoparticles [24,27].In the low\u2013 and medium\u2013temperature catalysts, Ir/NC\u2013473, 673, and 873, the metal was mostly atomically dispersed as evident from the HAADF\u2013STEM images (Fig. 2\n) and corroborated by the absence of iridium diffraction peaks in the XRD patterns (\nFig. S1\n). The high\u2013temperature system, Ir/NC\u20131073, contains nanoparticles with an average size of 1.3\u00a0nm, although single atoms still make up a considerable fraction of the nanostructures. Aimed at further sintering the active phase, an additional reduction step in an H2\u2013rich atmosphere at 773\u00a0K was applied to Ir/NC\u20131073. The resulting Ir/NC(773) exhibited a narrow nanoparticle size distribution with an average size of 2.1\u00a0nm (Fig. 2), in agreement with its XRD pattern that shows diffraction peaks compatible with the metallic phase. Although nanoparticles are dominant in Ir/NC(773), the presence of single atoms cannot be totally discarded. Further analysis of the Ir/NC catalysts by N2\u2013sorption revealed the close similarity of the specific surface areas (S\nBET, 310\u2013392\u00a0m2 g\u22121) and pore volumes (V\npore, 0.23\u20130.32\u00a0cm3 g\u22121), whereas ICP\u2013OES confirmed that the metal content was approximately the targeted 1\u00a0wt% (Table 1). The speciation of the nitrogen content was preserved over the whole temperature range, as indicated by N 1\u00a0s XPS analysis (Table S1). Fitting the Ir 4f spectra of the Ir/NC\u2013473, 673, and 873 samples revealed dominant contributions at binding energies (BEs) of ca. 62.4 and 63.0\u00a0eV commonly assigned to oxidized species (Fig. 3\n, \nFig. S2\n) [25,43], corroborating the atomic dispersion of iridium as visualized by HAADF\u2013STEM. In contrast, the main feature of the Ir/NC\u20131073 and Ir/NC(773) samples was centered at ca. 61.0\u00a0eV, compatible with the metallic phase [43]. Notably, analysis of the Cl 2p XPS suggests that the nature of the iridium site in the single\u2013atom based catalysts is directly affected by the activation temperature (\nFig. S2\n). In particular, Ir/NC\u2013873 shows no peaks that could be ascribed to Cl\u2013species, suggesting that the iridium single atom is only coordinated to N/O\u2013related cavities in the scaffold, whereas the low\u2013temperate catalyst reveal contributions at 198.0 and 200.2\u00a0eV, indicative of the presence chlorinated species [44]. This evolution is in line with previous reports that documented the gradual change of the coordination environment of platinum single atom from predominant Cl\u2013 to N/O\u2013neighboring atoms at higher activation temperatures [12,24]. As anticipated, chlorine was not detected in the nanoparticle\u2013based (NP\u2013based) catalysts, Ir/NC\u20131073 and Ir/NC(773).The catalytic performance of the derived iridium nanostructures was evaluated in HDC and HDB, which were conducted at constant reaction temperature (523\u00a0K), feed composition (CH2X2:H2:Ar:He\u00a0=\u00a06:24:5:65), and atmospheric pressure. The initial HDC activity, expressed per surface iridium atom (TOF), decreases in the following order (Fig. 4\n\na): Ir/NC(773) \u226b Ir/NC\u20131073\u00a0>\u00a0Ir/NC\u2013873\u00a0>\u00a0Ir/NC\u2013673 \u2248 Ir/NC\u2013473. Nanoparticles display a higher activity than their chlorinated SA\u2013based analogues (>5 times), showing comparable performance to benchmark rutile TiO2\u2013supported iridium (Ir/r\u2013TiO2, Table S2). Moreover, single atoms with a N/O coordination (Ir/NC\u2013873) exhibit up to 2.5\u2013fold higher TOF than their chlorinated counterparts, possibly due to the ability of the metal center to activate the reactants. Assessment of the product distribution at ca. 20% CH2Cl2 conversion (Fig. 4\na), revealed that Ir/NC\u2013873 yields a high CH3Cl selectivity (\u226495%), matching that of Ir/r\u2013TiO2. A key feature that governs this performance is the absence of coordinating chlorine atoms, which mainly promote coking pathways (\nFig. S3\n).Upon increasing the active phase size from single atoms to nanoparticles of 2.1\u00a0nm, the selectivity to CH3Cl decreases to ca. 48% at the expense of the generation of CH4. The Ir/NC\u20131073 system, with an average nanoparticle size of 1.3\u00a0nm, exhibits a relatively high CH3Cl selectivity (ca. 70%), which is due to the considerable number of single atoms still present. The activity\u2013 and selectivity trends were complemented with stability tests, revealing that all systems deactivate over time. The stability decreases in the following order: Ir/NC(773)\u00a0>\u00a0Ir/NC-1073\u00a0>\u00a0Ir/NC\u2013873\u00a0>\u00a0Ir/NC\u2013673\u00a0>\u00a0Ir/NC\u2013473 (\nFig. S4\n), thus showing that NP\u2013based systems display improved stability compared to their SA\u2013based counterparts (up to ca. 4 times higher activity after 10\u00a0h, Table S2). Despite these results, Ir/r\u2013TiO2 remains the best performing catalyst, showing unparalleled reactivity and stability in HDC (Table S2). Nevertheless, the fixed structure directed by the epitaxial growth of iridium on rutile\u2013type carriers does not allow investigations on active phase size effects, which is one of the main aims of this study.Further, the NC\u2013supported iridium nanostructures were also tested in HDB to determine possible halogen effects (Fig. 4\nb). Notably, the activity was comparable to that in HDC, indicating that the type of halogen plays a minimal role. The selectivity to CH3Br shows a comparable volcano shape as to that of CH3Cl, with the N/O\u2013coordinated single atom system presenting the highest selectivity (>90%) regardless of the halogen. The increasing propensity to coke and CH4 in chlorinated single atoms (<43%) and NP\u2013based systems (<25%), respectively, are also observed, albeit less pronounced than in HDC (\nFig. S3\n). To provide a complete overview of halogen effects, HDB stability tests were conducted. The deactivation patterns were comparable to that in HDC (\nFig. S4\n), with nanoparticles preserving their initial activity better than single atoms.Briefly, the results identify that the initial HDH reactivity of iridium catalysts is predominantly governed by their active phase nanostructure: irrespective of the choice of halogen, nanoparticles display the highest activity and stability, and chlorine\u2013free single atoms exhibit superior selectivity to CH3X compared to their Cl\u2013coordinated counterparts and nanoparticles, which favor coke and CH4 production, respectively.This study was further expanded with nanostructures of platinum and ruthenium (Fig. 1), which were chosen as representative metals based on previous HDH studies and prepared following established synthesis procedures [12,27]. Furthermore, nickel\u2013based catalysts were also included in the evaluation, despite that their HDH performance is considered poor [23]. Previous investigations revealed that moderate changes in the adsorption energies of the CH/Br fragments could lead to a dramatic increase in the HDB selectivity of nickel, rendering it an attractive candidate for studying nuclearity\u2013 and halogen effects [23].To achieve the targeted metal speciation (one system based on single atoms and one on nanoparticles, Fig. 1), each system was prepared applying a thermal treatment step (under N2 atmosphere) at a specific temperature (T\nact), indicated as M/NC\u2013T\nact. Systems that underwent an additional reduction step under H2 at elevated temperatures (T\nred) with the aim to induce sintering of the metal were labelled M/NC(Tred). The resulting six catalysts were denoted Pt/NC\u20131073, Pt/NC(873), Ru/NC\u2013473, Ru/NC\u20131073, Ni/NC\u20131073, and Ni/NC(773) (for an overview, please consult Table 1).The data revealed that the porous properties of the catalysts are comparable with the iridium\u2013based systems, exhibiting S\nBET and V\npore in the range of 324\u2013545\u00a0m2 g\u22121 and 0.27\u20130.43\u00a0cm3 g\u22121, respectively; whereas ICP\u2013OES confirmed that the actual metal content was close to the nominal value of 1\u00a0wt% (Table 1). The HAADF\u2013STEM images clearly visualize the attainment of single atoms in Ru/NC\u2013473 and Pt/NC\u20131073 (Fig. 5\n), and corroborated by XRD analysis, where reflections assigned to the metallic phases are not observed (\nFig. S5\n). The micrographs further show that nanoparticles are the dominating nanostructure in the high\u2013temperature catalysts, Ru/NC\u20131073 and Pt/NC(873), with average particle size of 1.6 and 2.9\u00a0nm, respectively (Fig. 5). Similar to platinum, the high\u2013temperature nickel catalyst (Ni/NC\u20131073) still exhibits single atoms as main species (\nFig. S6\n), thus requiring an additional high-temperature reduction step for the evolution of nickel into nanoparticles. The micrographs of the resulting catalyst, Ni/NC(773), indicate that active phase sintering occurred, leading to an average nanoparticles size of ca. 11.3\u00a0nm, in agreement with the XRD patterns that show a sharp reflection of metallic nickel. Interestingly, the necessary synthesis conditions to obtain nanoparticles are different across the metals: at 1073\u00a0K, iridium and ruthenium have an average nanoparticle size of 1.3 and 1.6\u00a0nm, respectively. At that temperature, platinum and nickel are still atomically dispersed, suggesting that these metals are highly stabilized in the cavities of the host as demonstrated by modeling, see below.To gain insights on the chemical state of the metals, XPS analysis was conducted. In accordance with the micrographs, a metallic phase was absent in the SA-based ruthenium and platinum catalysts, Ru/NC\u2013473 and Pt/NC\u20131073 (Fig. 6\n). The Ru 3d spectrum of Ru/NC\u2013473 revealed a contribution at a binding energy of ca. 464.1\u00a0eV (Fig. 6\na), commonly assigned to RuCl3\n[27,45], suggesting that these single atoms are coordinated to chlorine. On the other hand, the dominant feature of the Pt 4f spectrum of Pt/NC\u20131073 is centred at ca. 73.0\u00a0eV, indicating the oxidized character of the platinum atoms (Fig. 6\nb) [26,46]. In line with the XPS results, the EXAFS analysis shows pronounced Cl\u2013 and N/O\u2013coordination for the ruthenium and platinum single atoms, respectively (Fig. 7\n). However, a clear N/O\u2013Ru signal is also present, implying that a fraction of ruthenium single atoms are also N/O\u2013coordinated.The performance of the platinum\u2013, ruthenium\u2013, and nickel\u2013based catalysts was evaluated in HDC as well as in HDB at 573\u00a0K. For comparative purposes, representative iridium SA\u2013 and NP\u2013based catalysts were selected (Ir/NC\u2013873 and Ir/NC(773), respectively) and included in Fig. 8\n. Analysis of the HDC results revealed the following trend of decreasing metal activity with the speciation of the dominating species indicated between brackets (Fig. 8\na): Ir (NP) \u2248 Pt (NP)\u00a0>\u00a0Ir (SA) \u2248 Pt (SA)\u00a0>\u00a0Ru (NP)\u00a0>\u00a0Ru (SA)\u00a0>\u00a0Ni (NP)\u00a0>\u00a0Ni (SA), with the nickel\u2013based systems being virtually inactive. Over each metal, nanoparticles provide higher activity than the single atom analogues, which is in line with the trend previously distinguished over iridium\u2013based catalysts (Fig. 4). A similar pattern was observed in HDB (Fig. 8\nb), implying that halogen effects on catalytic activity are minimal, irrespective of the metal.In contrast, the selective behavior of the catalyst depends on both nuclearity and the type of halogen. In HDC, SA\u2013based systems display a CH3Cl selectivity of ca. 95% (Ir), 80% (Pt), and 70% (Ru), higher than the NP\u2013based catalysts (Fig. 8\na). Nanoparticles provided a lower selectivity (\u226450%), favoring CH4 (Ir and Pt, up to 46%) and coke (Ru and particularly Ni, up to 92%).These results are comparable with the selectivity patterns obtained in HDB (Fig. 8\nb), although iridium and platinum nanoparticles are less prone to over hydrogenation (CH4 selectivity up to ca. 30%). In stark contrast, ruthenium nanoparticles display a higher propensity to CH3Br (\u226494%) than their single atom counterparts (\u226471%). In addition, whereas ruthenium nanoparticles coke significantly in HDC (up to 60%), this side reaction does not occur in HDB as evidenced by the only two products CH3Br and CH4. The performance of ruthenium breaks with the trend that single atoms are more selective to the monohalogenated product, as seen in HDC.Stability tests were also conducted to gain a complete overview of the catalytic performance. The depletion of activity was expressed with the constant k\nD to enable a direct comparison, indicating the activity loss per hour derived via linear regression of the data in the time\u2013on\u2013stream (tos) range of 0.25\u201310\u00a0h (\nFig. S7\n). The active phase nanostructure clearly affects catalyst lifetime, with nanoparticles preserving their initial activity better than single atoms in HDC and HDB (Fig. 8\nc, d). The stability decreases in the following order for both reactions: Pt (NP)\u00a0>\u00a0Ir (NP)\u00a0>\u00a0Ir (SA) \u2248 Pt (SA) \u2248 Ru (NP)\u00a0>\u00a0Ru (SA). Evolution of the products are presented in \nFig. S7\n, revealing that SA\u2013based systems, except for ruthenium which shows an opposite trend, display enhanced propensity to CH4 over time. This suggests that sintering of the active phase into nanoparticles occurred during exposure to the reaction conditions.In summary, NC-supported metal (Ir, Pt, Ru, and Ni) catalysts with single atoms or nanoparticles as dominating nanostructure were prepared, characterized, and tested in HDC and HDB. It was revealed that hydrodehalogenation activity and stability are enhanced over nanoparticles when compared to single atoms, whereas the latter limit over hydrogenation and coking pathways, leading to outstanding CH3X selectivity. Ruthenium displays an inverse trend due to halogen effects, showing a higher selectivity to CH3Br over nanoparticles. Nevertheless, all catalysts deactivate over time and further investigation of the used systems is required to gain insight on deactivation mechanisms.Kinetic experiments reveal significant differences in the partial order of H2 over the catalysts, which can be grouped as systems (i) selective to CH3X, showing a p(H2) in the range of 0.41\u20130.58, (ii) that display significant selectivity to CH4 with p(H2) of 0.78\u20130.91, and (iii) that predominantly produce coke (Ru nanoparticles in HDC), showing a partial order of 0.67 (Fig. 9\n). Particularly single atoms are in the first group, whereas platinum and iridium nanoparticles are in the second. Notably, all nanoparticles have a higher p(H2) than their single atom analogues in both HDC and HDB except for ruthenium in HDB, which exhibits a p(H2) of 0.55 for single atoms and 0.43 for nanoparticles. These fingerprints suggest that the reaction mechanism may differ over the nanostructures and depends on the type of halogen. The results are likely a direct consequence of the ability to activate H2 and store H\u2013atoms that can react with surface species, which may depend on the geometry of the active phase and participation of the basic sites of the carrier in the reaction, as was found in previous HDB studies [24].To examine the development of the iridium\u2013based catalysts during exposure to HDC and HDB conditions, selected systems were characterized after 10\u00a0h on\u2013stream using N2\u2013sorption, XRD, HAADF STEM, and XPS, revealing three main deactivation mechanisms: (i) fouling due to coking, (ii) metal sintering, and (iii) poisoning by halogenation. HAADF\u2013STEM micrographs of the used systems display the sintering of single atoms into nanoparticles with an average size in the range of 2.1\u20132.7\u00a0nm, regardless of the halogen type (Fig. 2). Even though the nanoparticle size of these catalysts is comparable after 10\u00a0h on\u2013stream, their performance is dissimilar due to the contributions of coking and surface halogenation. Analysis of the structural properties point at the remarkable instability of the NC carrier (Table 1), with the specific surface areas (S\nBET) and pore volumes (V\npore) strongly decreasing after exposure to the reaction conditions (up to 90 and 70% lower than the original values, respectively). This suggests that deposition of carbonaceous species on the active sites contributes significantly to activity losses over all catalysts, likely more pronounced over catalysts that generate coke as main product, such as Ir/NC\u2013473, Ir/NC\u2013673 (\nFig. S3\n). On the other hand, the NP\u2013based iridium catalyst, (Ir/NC(773)), was less prone to active phase agglomeration with an increase of the particle size from 2.1 to 3.5 and 2.6\u00a0nm in HDC and HDB, respectively. These results were corroborated with XRD analysis (\nFig. S1\n), showing reflections compatible with metallic iridium, and by the XPS spectra that display contributions assigned to the metallic phase (Fig. 3, \nFig. S2\n). Furthermore, the peaks at BEs of 67.4 and 70.0\u00a0eV reveal the poisoning of the surface via bromination in HDB (Fig. 3) [24], whereas chlorination occurs in HDC as evidenced by the contributions at 197.9 and 200.8\u00a0eV (\nFig. S2\n). This indicates that all iridium\u2013based catalysts, in addition to fouling due to the deposition of carbonaceous species and active phase sintering, also suffer from halogenation.Whereas all active systems, regardless of the metal, suffer from the poor stability of the carrier, used ruthenium and platinum catalysts were characterized to assess the extent of metal sintering and halogenation on their lifetime. Similar to their iridium counterparts, HAADF\u2013STEM microscopy reveals that ruthenium\u2013 and platinum single atoms undergo pronounced sintering, increasing up to 2.7 and 3.6\u00a0nm, respectively, whereas nanoparticles remain relatively stable (Fig. 5). The weak reflections assigned to the metallic phase in the XRD spectra of the used platinum catalysts and Ru/NC\u20131073 suggest that larger nanoparticles were formed. This observation was confirmed by XPS analysis, distinguishing contributions at BEs of 461.8 (Ru/NC\u2013473) and 72.1\u00a0eV (Pt/NC\u20131073), representing metallic ruthenium and platinum, respectively (Fig. 6). Further analysis of the spectra indicates that surface chlorination is limited over both metals, displaying a relatively unaltered fraction of oxidized species in used ruthenium\u2013based systems compared to the fresh one, whereas platinum catalysts are largely metallic. On the other hand, the catalysts used in HDB display contributions assigned to Br\u2013species (Fig. 6, \nFig. S8\n), suggesting bromination of the surface. However, as platinum is mostly in the zero-oxidation state, it suffers the least from halogenation, which implies that mainly the carrier was brominated.In short, the loss of catalytic performance over time\u2013on\u2013stream is caused by: (i) fouling by coking, which is expected to occur over all systems due to carrier metastability, (ii) active phase sintering, more pronounced for single atoms, and (iii) halogenation. While chlorination mainly occurs on iridium\u2013based systems, bromine poisons iridium, ruthenium, and platinum catalysts, albeit the latter to a lesser extent. Overall, these results underline that deactivation mechanisms are complex and intertwined, governed by three modes that depend on the type of halogen, the metal, and its nuclearity, thereby highlighting the need for individual optimization strategies to develop stable systems.Whereas Ir/NC activated at 1073\u00a0K results in an average nanoparticle size of 1.3\u00a0nm (Fig. 2), the platinum\u2013 and nickel\u2013based systems that underwent a similar thermal treatment (Pt/NC\u20131073 and Ni/NC\u20131073) have single atoms as predominant species (Fig. 5\n,\n\nFig. S6\n). Furthermore, the platinum single atoms are non\u2013chlorinated, in contrast to the single atoms in Ru/NC\u2013473, Ir/NC\u2013473, and Ir/NC\u2013673 (Fig. 5, Fig. 6), which are coordinated to Cl. These results suggest that the speciation, as nanoparticle or as (chlorinated) single atom, depends on various factors, including the anchoring sites in the NC carrier, the metal, and the activation temperature. Therefore, to complement the characterization of the materials, a molecular\u2013level understanding of the active phase speciation can help establishing more robust structure\u2013performance relationships. For this purpose, Density Functional Theory (DFT) studies were conducted. Therein, the NC support was represented by a set of three defects; (i) non-planar 3\u00a0N and square-planar 4\u00a0N arrangements, labelled 3\u00a0\u00d7\u00a0N5 (tri-pyrrolic), (ii) 4\u00a0\u00d7\u00a0N6 (tetra-pyridinic), (iii) and 2\u00a0\u00d7\u00a0N5\u00a0+\u00a02\u00a0\u00d7\u00a0N6 (tetra-pyrrolic/pyridinic) [12]. Formation energies were evaluated to shed light on the interaction of the single atom with the host, using metal chloride precursors MCl (M\u00a0=\u00a0Ir, Pt, Ru, or Ni), the pristine metal species (M), and the NC support as reference states.The results of the speciation analysis reveal two general trends related to the Cl\u2013ligands and cavities in the NC\u2013carrier (Fig. 10\n\n, Table S3):\n\n(i)\nstarting from the pristine single atom, iridium and ruthenium display the highest affinity toward chlorination, with binding energies ranging from \u20131.13 to \u20132.44\u00a0eV, explaining why these systems remain chlorinated at elevated temperatures, and\n\n\n(ii)\nsingle atoms in 3\u00a0\u00d7\u00a0N5 sites are expected to be more chlorinated than those in the 4\u00a0\u00d7\u00a0N6 and 2\u00a0\u00d7\u00a0N5\u00a0+\u00a02\u00a0\u00d7\u00a0N6 cavities due to the square\u2013planar arrangement in 4\u00a0N-defects, which is responsible for the superior atom stabilization. This prevents the sintering of single atoms into nanoparticles during the removal of Cl at increasingly higher activation temperatures.\n\n\nstarting from the pristine single atom, iridium and ruthenium display the highest affinity toward chlorination, with binding energies ranging from \u20131.13 to \u20132.44\u00a0eV, explaining why these systems remain chlorinated at elevated temperatures, andsingle atoms in 3\u00a0\u00d7\u00a0N5 sites are expected to be more chlorinated than those in the 4\u00a0\u00d7\u00a0N6 and 2\u00a0\u00d7\u00a0N5\u00a0+\u00a02\u00a0\u00d7\u00a0N6 cavities due to the square\u2013planar arrangement in 4\u00a0N-defects, which is responsible for the superior atom stabilization. This prevents the sintering of single atoms into nanoparticles during the removal of Cl at increasingly higher activation temperatures.To further understand the speciation trends, cohesive energies were computed. Ruthenium and iridium present the strongest propensity to leaching, with cohesive energies of \u22127.3 and \u20138.0\u00a0eV/atom, respectively (Fig. 10), whereas nickel (\u20135.3\u00a0eV/atom) and platinum (-6.2\u00a0eV/atom) display lower values. These results are in agreement with the experimentally observed metal speciation, which showed that nickel and platinum remained as single atoms while iridium and ruthenium were mainly nanoparticles at 1073\u00a0K (Figs. 2 and 5, \nFig. S6\n). Consequently, the stabilization of iridium and ruthenium single atoms in N\u2013containing defects is not sufficient to prevent their sintering at elevated temperatures, while nickel and platinum single atoms exhibit superior stability in most cavities. To further assess the effect of N\u2013functionalities on the atomic dispersion on the carrier, adsorption energies of single atoms on a N\u2013free carrier were evaluated. Previous investigations disclosed the synthesis of platinum single atoms on activated carbon (AC) [12,24]. Nonetheless, the significantly smaller adsorption energy of the atoms in AC cavities (\u20133.15\u00a0eV, Table S3) relative to the anchoring sites in NC (<-5.44) indicate that the latter provide better stability. Comparable values were found for nickel (\u20133.97 and\u00a0<\u00a0\u20137.12\u00a0eV on AC and NC, respectively). In stark contrast, the atomic dispersion of iridium and ruthenium on AC is not favored due to the full coordination and high cohesive energy of these metals. This leads to insufficient atom anchoring on AC, thereby highlighting the positive impact of N\u2013moieties on single atom stability. The potential catalytic activity of single atoms in each defect was studied by exploring the adsorption energy of the substrates, CH2Cl2 and CH2Br2. Metal single atoms in the tri\u2013pyrrolic (3\u00a0\u00d7\u00a0N5) cavity displayed superior substrate activation (Table S4), in line with previous studies [24] and in agreement with the presence of pyrrolic sites as shown in the characterization data (Table S1). Hence, a single atom in the 3\u00a0\u00d7\u00a0N5 cavity was retained as the most representative for single atom\u2013based catalysts.The reaction network leading to CH3Br, CH4 and C is described (Table S5) and the associated thermodynamic and kinetic parameters were calculated (Tables S6-S8). The Gibbs free energies were computed including entropic contributions from the molecules, using CH2X2, H2, and the corresponding catalysts as reference systems operating at 523\u00a0K (\nFig. S9\n, \nFig. S10\n).At first, the effect of a full coordination of the single atom with Cl ligands was assessed. The network starts by the dissociative adsorption of CH2X2 on the active ensembles, leading to CH2X*. The presence of Cl in the active phase hinders the activation of CH2X2 over the single atoms, with barriers ranging from 0.9 to 1.8\u00a0eV. Furthermore, subsequent adsorption of H2 on CH2X* is virtually prohibitive as the valence of the metal atom is full. Consequently, chlorinated sites are prone to coke, which is consistent with the observed selectivity performance of chlorinated iridium single atoms (\nFig. S3\n). In contrast to the chlorinated systems, the dissociation of CH2X2 on the pristine (non\u2013chlorinated) single atoms is quite exothermic and instantly leads to CH2X* fragments. Nickel\u2013based systems form an exception to that rule. The challenging activation of CH2X2 on the nickel single atoms (\u223c2 eV) can be rationalized in terms of the low affinity toward both halogens, being ca. 1\u00a0eV lower compared to the iridium, platinum, and ruthenium single atoms (Table S9), resulting in poor catalytic activity in both HDC and HDB (Fig. 8).Once the substrate is dissociated into CH2X*, one of the following steps take place: (i) heterolytic H2 dissociation promoted by the basicity of the cavity [24] or (ii) halogen elimination to form CH2*, thereby filling the valence of the metal atom in an octahedral configuration, which poisons the active site by impeding the incorporation of H2, thus promoting coke formation. Although this step is energetically feasible (<0.9\u00a0eV in all cases, except for Ni), the greater stability of the CH2X* compared to the dissociated CH2*+X* inevitably lowers the population of CH2*X* intermediates, which promotes H2 incorporation on CH2X*. Upon the heterolytic dissociation of H2, HX* and CH3X* are formed on the single atom, thereby re\u2013establishing the active site after their desorption.Among the three active single atom catalysts (Ir, Ru and Pt, Fig. 8), ruthenium displays the lowest activity and a significant selectivity to coke (\u223c30%), owing to the presence of chlorinated sites in the active phase, thus following comparable performance patterns as the chlorinated iridium single atoms. In contrast, single atoms of iridium and platinum display superior activity and CH3X selectivity performance due to their facile hydrogen dissociation (<1.10\u00a0eV, Fig. 11\n), and favourable CH2X2 adsorption (<0.6\u00a0eV).The basic N\u2013sites of the host can participate in the reaction by storing H\u2013atoms that can be transferred to other moieties. This leaves the single atom free for coordination, thereby enhancing catalytic activity [24]. To further assess the contribution of N\u2013functionalities to the reaction, dissociation energies of H2 over single atoms on the carbon supports were computed. Hydrogen undergoes barrierless homolytic dissociation over platinum and nickel on AC (Table S10). In contrast, even though the reaction is energetically feasible, activation of H2 over AC\u2013supported iridium and ruthenium is not possible, as these single atoms cannot be stabilized (Table S3). To gain further insights, experimental evaluation of bare NC and AC in CH2X2 hydrodehalogenation (reaction conditions specified in the caption of Fig. 2) was conducted, revealing the inactivity of AC. On the other hand, NC displayed low activity (Table S2), suggesting that N\u2013species can act as catalytically active sites. Nevertheless, the metal\u2013free carrier deactivates rapidly, showing deactivation constants more than 6\u2013fold higher than Pt/NC\u20131073 in both reactions. These results indicate that the catalytic response is mainly determined by the metal species in the cavity.Still, the contribution of the host is crucial in enabling the adsorption of the substrates and the full mechanism. Particularly, the evolution of CH2X* moieties depends on the cooperation of the support with the single atom species, since it enables the formation of CH3X by providing a sufficient number of protons and prevents the generation of CH2*, which likely leads to coke. High CH3X selectivity is ensured over iridium and platinum due to the participation of the NC carrier. In addition to promoting the heterolytic dissociation of H2, the diffusion of carbonaceous species and halogen fragments over the surface of the scaffold is hindered due to the non-continuous structural morphology of the host. Consequently, CH4 is not generated and only coke is formed as a side product when H2 cannot be incorporated to CH2X*. The active phase sintering occurs due to the formation of mobile fragments during the course of the reaction. Such moieties are obtained when substrates are adsorbed on single atoms with non-planar configurations. The binding energy of the single atom to the scaffold is lowered and the resulting metal complex is susceptible to diffusion.The facile homolytic dissociation of H2 over the surface of metal nanoparticles results in higher hydrogen coverages when compared to single atoms, explaining the superior activity of nanoparticle-based catalysts in the hydrogenation paths (Table S4). The reaction starts similar to that over their single atom counterparts, but the system is more prone to form CH2*, facilitated by the diffusion of halogen and methylene moieties over the surface [24], which in general (except for ruthenium in HDB) leads to CH4 and C. However, if H* is incorporated on CH2X*, CH3X* is generated and subsequently desorbed. After the formation of CH2*, one of the following steps takes place: (i) CH2* dissociation to CH* and H* promoted by the diffusion of H* and the affinity of the metal toward CH* or (ii) H* incorporation to form CH3* and subsequently, CH4.Whereas metal surfaces of the nanoparticles are continuous, multiple adsorption/reaction events take place simultaneously. Particularly, the HDC and HDB reactions lead to CH2* with almost no barrier for cleaving the second CX bond. Over hydrogenation occurs over iridium and platinum sites due to the greatly exothermic formation of H* intermediates (Fig. 11), which leads to the formation of CH4. On the other hand, the high affinity to carbonaceous fragments such as CH2*, CH*, and C* results in coke over nickel nanoparticles. Even the penetration of C into the metal lattice is exothermic (-0.8\u00a0eV, Table S11), which can contribute to rapid catalyst deactivation in both HDC and HDB.Among the nanoparticles, ruthenium shows a notable performance trend in HDB, where nanoparticles are more selective to CH3Br than the single atom analogues. In HDC, the controlled H2 coverage, due to a less favourable H2 dissociation, leads to the formation of coke. However, in HDB, the binding strength of CH2Br2 on the metal surface is optimal (\u20130.13\u00a0eV, 0.94\u00a0eV for CH2Cl2), resulting in the exceptional performance of the nanoparticles. This defines energetic regions where HDC and HDB take place with improved selectivity (Fig. 11). Optimal H2 adsorption and CH2X2 binding energies were found to be between 0.25 and 1.25, and lower than 0.6\u00a0eV, respectively.Altogether, the systematic study of the synthetic platform for two closely related reactions allowed us to widen the conceptual framework encompassing synthesis, characterization and understanding allowing us to establish robust structure (species)-performance patterns.In this study, a strategy combining catalytic evaluation complemented with extensive characterization of a platform of NC\u2013supported metal nanostructures (Ni, Ru, Ir, and Pt), from single atoms to nanoparticles of ca. 3\u00a0nm, coupled with kinetic analysis and density functional theory was adopted to systematically investigate to effects of the active phase and the halogen on activity, selectivity and stability in CH2X2 hydrodehalogenation. Substantial activity differences were observed over the nanostructures, attaining the highest reaction rates over NP\u2013based systems, ranking as Ir \u2248 Pt\u00a0>\u00a0Ru \u226b Ni independent of the halogen. Moreover, these systems preserve their initial activity better than their single atom analogues, which mainly suffer from sintering. The catalytic tests further revealed a marked impact of nuclearity, single atom coordination environment, and halogen type on the product distribution. In hydrodechlorination, CH3Cl is the main reaction product over single atoms, whereas nanoparticles exhibited significant selectivity to CH4 or coke. Among the metals, iridium\u2013based single atoms exhibit exceptional CH3Cl (\u226495%) selectivity, in stark contrast to their chlorinated analogues which favored the formation of coke (<90%). Comparable performance patterns were observed in hydrodebromination with the exception for ruthenium, which displayed an inverted selectivity\u2013structure trend with improved CH3Br selectivity over nanoparticles (\u226496%) compared to the single atoms (\u226472%). Kinetic and mechanistic studies correlate these results with the ability of the active phase to activate CH2X2 and H2, and to store H\u2013atoms. Furthermore, the intrinsic stability of the single atoms in the cavities and the potential catalytic response were computed, setting a basis for understanding the effects of synthetic protocols on speciation and coordination. The findings reported in this work are directed at elucidating hydrodehalogenation performance patterns, highlighting the impact of nanostructuring and the halogen type to advance future catalyst design.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by ETH research grant ETH-43 18-1 and NCCR Catalysis, a National Centre of Competence in Research funded by the Swiss National Science Foundation. We thank BSC\u2013RES for providing generous computational resources. The authors thank the Scientific Center for Optical and Electron Microscopy, ScopeM, the Paul Scherrer Institute, PSI, and the Swiss Federal Laboratories for Materials Science and Technology, EMPA, for access to their facilities. The authors thank Dr. Frank Krumeich for performing some of the microscopic analyses.Supplementary information associated with this article, containing additional characterization and catalytic data, can be found in the online version. The computed structures have been added to the ioChem-BD database ref. [42]. Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2021.10.008.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Nanostructuring metal catalysts has been demonstrated as an attractive strategy to enable selective hydrodehalogenation of CH2X2 (X\u00a0=\u00a0Cl, Br) to CH3X, but active phase size effects of promising metals and the role of the halogen are still poorly understood. Herein, the impact of these parameters on performance (activity, selectivity, and stability) is systematically assessed by employing a platform of N\u2013doped carbon\u2013supported metal nanostructures (Ir, Pt, Ru, and Ni), ranging from single atoms (SA) with defined coordination environment to nanoparticles (NP) of ca. 3.0\u00a0nm. Catalytic tests reveal that when compared to single atoms, highest reaction rates are attained over NP\u2013based systems, which also exhibit improved stability ranking as Ir \u2248 Pt\u00a0>\u00a0Ru \u226b Ni, independent of the halogen. The product distribution was markedly affected by the nanostructure and speciation of the active center as well as the dihalomethane type. Specifically, CH3Cl is the main reaction product over SA in hydrodechlorination, achieving an exceptional selectivity over Ir (up to 95%). In contrast, NP mainly generated CH4 or coke. Comparable patterns were observed in hydrodebromination, except over Ru, which exhibited an inverse structure\u2013selectivity trend. Density Functional Theory simulations shed light on the speciation of the active phase and identified the adsorption and dissociation energies of CH2X2 and H2 as descriptors for catalytic reactivity. These findings elucidate hydrodehalogenation performance patterns, highlighting the impact of nanostructuring and the halogen type to advance future catalyst design.\n               "}
{"full_text": "Climate change has greatly threatened the sustainable development of mankind. (Nong\u00a0et\u00a0al., 2021; Nielsen\u00a0et\u00a0al., 2021) In this regard, the net-zero technologies are of great practical importance, which can convert carbon dioxide (CO2) into reusable chemical feedstocks (e.g., CH4, CO, etc.). (Vo\u00a0et\u00a0al., 2021; Teo\u00a0et\u00a0al., 2022; Cai\u00a0et\u00a0al., 2023) Among various approaches, the feasibility of CO2 methanation has been demonstrated by many methods, like electrocatalysis, (Chen\u00a0et\u00a0al., 2021a; Zhao\u00a0et\u00a0al., 2021) photocatalysis, (Shi\u00a0et\u00a0al., 2022; Cabrero-Antonino\u00a0et\u00a0al., 2022) and thermal catalysis. (Lee\u00a0et\u00a0al., 2021; Galadima\u00a0and Muraza,\u00a02019) Due to the multiple-electron transfers occurring throughout the reaction as well as the thermodynamic equilibrium between CO2, its derivatives and by-products (such as hydroxide and bicarbonate), conducting effective hydrogenation of CO2 with long-term stability remains difficult in electrocatalysis and photocatalysis. (Ma\u00a0et\u00a0al., 2020; Ding\u00a0et\u00a0al., 2020) In comparison, thermal catalysis shows its advantages in efficiency and stability. (Tu\u00a0et\u00a0al., 2014; Wang\u00a0et\u00a0al., 2019) However, due to the carbon deposition during the thermal CO2 methanation process, the catalysts could be deactivated in long-term reactions. It remains a challenge to rationally design a catalyst that can efficiently suppress the carbon deposition.Recently, immobilizing metal nanoparticles (NPs), on supporting materials has emerged as an attractive strategy to realize efficient CO2 methanation processes, for example, Ni (Wang\u00a0et\u00a0al., 2021a) and Ru. (Chen\u00a0et\u00a0al., 2021b) Nickel-based catalysts are traditional for methanation. (Wang\u00a0et\u00a0al., 2022a) However, carbon deposition is more prone to occur on Ni-based catalysts. In contrast, platinum group metal NPs (Ru, etc.) have received great attention since they are more active than those made of Ni-based catalysts. (Zhang\u00a0et\u00a0al., 2012) However, the high price of platinum group metals greatly limits their applications. (Wang\u00a0et\u00a0al., 2022b) Therefore, developing catalysts with less platinum group metal usage is regarded as a promising method to make the CO2 methanation closer to industrialization. (Yu\u00a0et\u00a0al., 2021) In this regard, constructing bimetallic catalysts is considered as one of the most promising approaches, benefiting from the synergistic effects of the two metallic components. (Tahir\u00a0et\u00a0al., 2017; Zhang\u00a0et\u00a0al., 2021) Besides platinum group metal, metallic Cu is also widely studied due to its outstanding performance and affordability in CO2 methanation. (Zhang\u00a0et\u00a0al., 2021) Moreover, Cu NPs can also be employed as a promoter to improve catalysts\u2019 structure and surface properties, reduce deactivation, and even boost activity and selectivity during CO2 reduction reactions. (Dias\u00a0and Perez-Lopez,\u00a02020) However, the current study on Cu/Pt-group bimetallic catalyst's application in CO2 methanation is still in its infancy, which calls for more investigation.Besides the catalytic active sites, the supports also play a vital role during the methanation process. Currently, most metal NPs are deposited on oxide supports, such as silica (SiO2), (Wang\u00a0et\u00a0al., 2021a) alumina (Al2O3), (Quindimil\u00a0et\u00a0al., 2021) titania (TiO2), (Wang\u00a0et\u00a0al., 2022c) zirconia (ZrO2), (Gao\u00a0et\u00a0al., 2022) and ceria (CeO2). (Xie\u00a0et\u00a0al., 2022) However, exothermicity of CH4 methanation reactions (\u0394H=\u2212165\u00a0kJ mol\u22121) remains a challenge for all investigated systems, (Hervy\u00a0et\u00a0al., 2021) since the high surface temperatures may displace the thermodynamic equilibrium and favor competing reactions (reverse water-gas shift and steam reforming) to produce CO instead of CH4. (Fatsikostas\u00a0and Verykios,\u00a02004; Baudouin\u00a0et\u00a0al., 2013) In this regard, carbon-based supports provide us with an alternative due to their high thermal conductivity that can reduce the local temperature of the catalyst. (Sun\u00a0et\u00a0al., 2021) Additionally, porous carbon has also shown its boosted adsorption capability towards H2 and CO2 due to its relatively large surface area. (Lee\u00a0et\u00a0al., 2021) But in terms of its application in thermocatalytic methanation, it remains veiled.Herein, we developed a novel RuCu bimetallic catalyst on nitrogen-doped mesoporous carbon nanospheres (NMCN) for thermocatalytic CO2 methanation. Cu and Ru are supported onto NMCN to create bimetallic active sites for thermocatalytic CO2 methanation with different Ru/Cu ratios. In our study, it is demonstrated that, due to the formational of RuCu bimetallic catalysts, the thermocatalytic activity of RuCu-x/NMCN (x\u00a0=\u00a01, 2, 3) is obviously higher than that of pure NMCN and NMCN-support Cu NPs (Cu/NMCN). Moreover, the synergistic effect between Ru and Cu nanoparticles is found, which could efficiently prevent the carbon deposition processes. As a result, the optimal RuCu-3/NMCN catalyst exhibits remarkable thermocatalytic stability in 1380\u00a0min.3-Aminophenol (99 wt%, Sigma-Aldrich Co.), formaldehyde (36 wt% in water, Sigma-Aldrich Co.), sodium hydroxide (98 wt%, Sigma-Aldrich Co.), Pluronic F127 (Mw=12,600, PEO106PPO70PEO106, Sigma-Aldrich Co.), L-cysteine (98 wt%, Sigma-Aldrich Co.), Copper (II) chloride (99 wt%, Sigma-Aldrich Co.), Ruthenium (III) chloride hydrate (99 wt%, Sigma-Aldrich Co.), ethanol (99 wt%, Sigma-Aldrich Co.) were used as received without any further purification.NMCN is synthesized according to the reported literature. (Yang\u00a0et\u00a0al., 2014) At a temperature of 25\u00a0\u00b0C, 80\u00a0mL of ethanol and 200\u00a0mL of distilled water were mixed to obtain an aqueous-alcoholic solution. Then, while the mixture was continuously stirred, 1\u00a0g of Pluronic F127 (Mw\u00a0=\u00a012,600, PEO106PPO70PEO106), 1.3\u00a0g of CTAB, and 2\u00a0g of cysteine were added. Then, 2\u00a0g of 3-aminophenol was added, and it was thoroughly dissolved while being agitated. After 24\u00a0h of stirring, 2.85\u00a0mL of 36 wt% formaldehyde was added. The mixture was then placed in an autoclave and kept at 100\u00a0\u00b0C for 24\u00a0h. After three rounds of washing with water and ethanol, a brown-red powder was obtained. The powder was calcined in N2 flow with a heating rate of 1\u00a0\u00b0C min\u22121 up to 350\u00a0\u00b0C, dwelled for 1\u00a0h, and resumed a heating rate at 1\u00a0\u00b0C/min up to 800\u00a0\u00b0C and dwelled for 2\u00a0h to produce NMCN.Cu/NMCN was prepared by a reduction method. The NMCN was impregnated with CuCl2\u00b73H2O solution. Then dried at 110\u00a0\u00b0C in an oven overnight before calcination in H2/Ar at 400\u00a0\u00b0C for 2\u00a0h. The as-prepared catalysts were denoted as Cu/NMCN.The RuCu-x/NMCN (x\u00a0=\u00a01, 2, 3) catalysts were synthesized by a reduction method. The NMCN was impregnated with a mixture of RuCl3 and CuCl2\u00b73H2O solution with varied Ru/Cu molar ratios. All samples were subsequently dried at 110\u00a0\u00b0C overnight before calcinated in H2 /Ar at 400\u00a0\u00b0C for 2\u00a0h. The as-prepared catalysts were denoted as RuCu/NMCN.A scanning electron microscope (SEM, Zeiss Auriga) was used to capture the morphology of nanospheres. The high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission microscopy (HAADF-STEM) images of the samples were acquired from on FEI Themis Z equipped with double spherical aberration corrector operating at 300\u00a0kV. X-ray diffraction (XRD) measurements were recorded on a Rigaku D max-3C diffractometer using Cu K\u03b1 radiation (40\u00a0kV, 20\u00a0mA, \u03bb\u00a0=\u00a00.15408\u00a0nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Fisher ESCALAB 250Xi spectrometer with a focus monochromatic Al K\u03b1-rays (1486.6\u00a0eV) source. The samples were tested under a vacuum below 5.0\u00a0\u00d7\u00a010\u221210 Mbar, energy resolution spectra were collected using a pass energy of 20\u00a0eV. Peak fitting of the high-resolution data was carried out by Thermo Avantage 5.9925 surface chemical analysis software. N2 sorption isotherms were recorded on an Autsorb iQ gas sorption system at 77\u00a0K. The Brunauer-Emmett-Teller (BET) method was used to calculate to measure the specific surface areas (SBET) using adsorption at a relative pressure of P/P0\u00a0=\u00a00.05\u20130.30 and the Barrett Joyner Halenda (BJT) method was used to estimate the total pore volume and pore size distribution. The Cu and Ru content were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) on an R4 Perkin Elmer ICP-OES 8300DV. Before analysis, the powder samples were digested by aqua regia and dissolved in 70% HNO3 and diluted by deionized water.Catalytic experiments were carried out under atmospheric pressure in a continuous quartz glass flow reactor, loaded with 100\u00a0mg variable of catalyst powder. Gaseous mixtures of CO2 and H2 diluted with nitrogen were fed into the reactor at the gas hourly space velocity (GHSV) of 28,350\u00a0ml gcat\n\u22121 h\u22121 and H2/CO2 mass ratio of 4/1. The temperature was varied step by step between 300 \u00b0C and 800 \u00b0C.The pre-reduction of catalysts at 400\u00a0\u00b0C was adopted to reduce the metal oxides on the catalyst surface into metallic state. The procedure included heating the catalysts in a gas flowing of 20% H2 at 60\u00a0ml/min for 30\u00a0min at 400\u00a0\u00b0C. After termination of reduction, the catalyst was cooled in flowing H2 to room temperature.The reaction products were analyzed by Agilent 7890B gas chromatograph equipped with a ShinCarbon column and a thermal conductivity detector (TCD). The activity of the catalysts was indicated by the CO2 conversion and CH4 selectivity.NMCN is prepared based on a previous procedure utilizing a dual-soft templating approach. (Yang\u00a0et\u00a0al., 2014) Herein, 3-aminophenol and formaldehyde in ethanol aqueous are mixed with L-cysteine to generate the resin spheres. Then, the obtained precursors are carbonized at 800\u00a0\u00b0C to get the NMCN samples, as depicted in Scheme\u00a01\n. The RuCu/NMCN with different molar ratios of Ru/Cu (denoted as RuCu-x/NMCN) is synthesized according to the procedure detailed in the experimental section. For comparison, the reference Cu supported on NMCN nanoparticles (denoted as Cu/NMCN) is prepared without adding the Ru-precursor. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis shows that the molar ratios of Ru/Cu in RuCu-1/NMCN, RuCu-2/NMCN and RuCu-3/NMCN are 1:128, 1:27 and 1:10, respectively (Table S1).The scanning electron microscopy (SEM) images of as-prepared Cu/MCN and RuCu-x/NMCN (x\u00a0=\u00a01, 2, 3) catalysts are depicted in Fig.\u00a01\na and Supplementary Fig. S1, respectively. The morphology of RuCu-x/NMCN remains spherical. The obtained Cu/MCN and RuCu-x/NMCN (x\u00a0=\u00a01, 2, 3) show a narrow average size distribution of 40\u201350\u00a0nm. Taking the RuCu-3/NMCN as an example, with the loading of Cu and Ru metals, the presence of well-dispersed Cu and Ru can be observed in the elemental mapping images (Supplementary Fig. S2). The uniform spherical morphology of the catalyst is further validated by the transmission electron microscopy (TEM) images (Fig.\u00a01b and Supplementary Fig. S3a). The randomly dispersed nanoparticles on the carbon nanospheres could be evidenced with a metal distribution diameter of 2.16\u00b10.60\u00a0nm (Supplementary Fig. S3b and 1c). HAADF-STEM energy dispersive X-ray spectroscopy (HAADF-STEM-EDS) is adopted to further identify the Ru nanoparticles from the Cu nanoparticles. The corresponding elemental mapping images of RuCu-3/NMCN catalysts (Supplementary Fig. S4a-e) indicate the homogeneous distribution of each elements. Moreover, the line scanning profile (Supplementary Fig. S4f) also shows a uniform distribution of Ru and Cu on the particles. Additionally, high-resolution transmission electron microscopy (HR-TEM) is also used to analyze the detailed structure of the metal particles. As shown in Fig.\u00a01d, the clear lattice fringes with an interplanar distance of approximately 2.3\u00a0\u00c5 can be assigned to the (100) facets of the hexagonal phase of Ru-Cu alloy. (Wang\u00a0et\u00a0al., 2022d; Zheng\u00a0et\u00a0al., 2021)To further confirm the composition of the RuCu-3/NMCN, Fig.\u00a02\n shows the X-ray diffraction (XRD) patterns of RuCu-x/NMCN (x\u00a0=\u00a01, 2, 3), Cu/NMCN, and NMCN catalysts. For all catalysts, diffraction peaks are observed at 2\u03b8 values of 25\u00b0 and 43\u00b0, corresponding to the (002) and (100) planes of graphite, respectively, and the broadness of these peaks indicated the amorphous nature of the carbon-based catalysts. (Jaleel\u00a0et\u00a0al., 2022) Moreover, compared with the standard Cu (PDF#04\u20130836), the diffraction peak of Cu/NMCN, RuCu-1/NMCN and RuCu-2/NMCN catalysts could match well at 2\u03b8 values of 43.6 and 50.7 representing the (111) and (200) planes of Cu. The absence of the associated Ru diffraction peaks may due to the small size of the metal particles. In addition, the XRD pattern of the RuCu-3/NMCN catalyst reveals that the characteristic peak of Cu is positively shifted compared to the standard pattern of Cu for the formation of the RuCu alloy. (Wang\u00a0et\u00a0al., 2022d) Combined with the HR-TEM image (Fig.\u00a01d), it can be concluded that RuCu alloy is formed in RuCu-3/NMCN.The chemical state information of the above catalysts is investigated through X-ray photoelectron spectroscopy (XPS). XPS survey spectra for all catalysts confirm the presence of Cu, Ru, O, N, and C, shown inFig. S5. For Cu/NMCN, RuCu-1/NMCN and RuCu-2/NMCN catalysts, the Cu 2p core-level spectra had two sets of peaks, one set had Cu 2p3/2 and Cu 2p1/2 peaks at 931.7 and 951.6\u00a0eV, respectively, indicating the existence of metallic Cu (Fig.\u00a03\na). Meanwhile, the peaks of Cu 2p3/2 at 934.66\u00a0eV and Cu 2p1/2 at 955.89\u00a0eV in combination with the satellite peaks are the typical characteristics of Cu2+. (Wang\u00a0et\u00a0al., 2022e) From the Ru 3p doublet spectra (Fig.\u00a03b), the Ru 3p3/2 and Ru 3p1/2 peaks at 461.7 and 438.8\u00a0eV comfirm the presence of metallic Ru. Meanwhile, the Ru 3p3/2 peak at 463.8\u00a0eV and Ru2p1/2 at 485.6\u00a0eV are the typical characteristic peaks of Ru4+. (Kim\u00a0et\u00a0al., 2022) More importantly, the Ru 3p binding energy of the RuCu-3/NMCN catalyst shifted to a higher binding energy position, while the Cu 2p binding energy shifted to the lower energy position, which also indicates that the presence of RuCu alloy in RuCu-3/NMCN catalyst. The positive shift of the binding energy of metallic Ru reveals the electron transfer from Cu to Ru due to the higher electronegativity of Ru than that of Cu. (Wu\u00a0et\u00a0al., 2022) Compared to the Cu/NMCN catalyst, no obvious shifts of the binding energies of Cu 2p spectra can be observed for RuCu-1/NMCN and RuCu-2/NMCN catalysts, which could be attributed to the absence of metal alloyed phase. (Salazar\u00a0et\u00a0al., 2014)Nitrogen-sorption analysis is used to describe the porosity of catalysts. Fig.\u00a04\n displays the nitrogen adsorption and desorption isotherms for the RuCu-1/NMCN, RuCu-2/NMCN, RuCu-3/NMCN, Cu/MCN, and NMCN catalysts together with the related pore size distribution curves. The BET surface area, total pore volume and average pore diameter are shown in Table 1. All the nitrogen adsorption-desorption isotherms of RuCu-1/NMCN, RuCu-2/NMCN, RuCu-3/NMCN, Cu/MCN and NMCN exhibit pseudo-type IV curves. The average pore diameters of RuCu-1/NMCN, RuCu-2/NMCN, and RuCu-3/NMCN are similar to each other, and are calculated to be in the range from 2 to 3\u00a0nm based on the Barrett-Joyner-Halenda (BJH) method. The pure NMCN exhibits the highest surface area (282 m2 g\u22121) and pore volume (0.30 m3 g\u22121). After loading metal particles, both the surface area and pore volume of the catalysts decrease slightly. All these results indicate the successful incorporation of the metal particles within the support. Moreover, the change in pore volume is slight, indicating that the loading of metal NPs will not block the pore channels of NMCN.The CO2 conversion performance at different reaction temperatures is summarized in Fig.\u00a05\n. The results are the average values of five GC measurements at each temperature. Fig.\u00a05a demonstrates that a very small amount of CO2 conversion is observed at 300 \u00b0C. The CO2 conversion rate significantly increases from 400 to 800 \u00b0C, reaching its maximum at 800 \u00b0C. For NMCN (Fig.\u00a05b-d), the conversion of CO2 is extremely low. The low conversion of NMCN is due to the absence of metal sites, which is crucial for H2 and CO2 dissociation. (Sun\u00a0et\u00a0al., 2020) In the absence of Ru, the highest conversion of CO2 reached around 15% with 100% selectivity for CO of Cu/NMCN at 600 \u00b0C. It could be concluded that the existence of Ru had a promoting effect on the catalytic activity and selectivity. Compared to the poor catalytic performance of NMCN and Cu/NMCN, both the conversion and selectivity of CH4 are obviously enhanced when RuCu-x/NMCN (x\u00a0=\u00a01, 2, 3) is used as the catalyst, indicating that the synergy between Ru and Cu can significantly promote the catalytic CO2 methanation. In the presence of Ru, the RuCu-1/NMCN catalysts still keep a relatively low conversion when compared with Cu/NMCN catalysts, due to the minimal loading of Ru. RuCu-2/NMCN catalysts of CO2 conversion increase 2-fold to RuCu-1/NMCN catalysts at 500 \u00b0C with 100% selectivity for CH4. The increase in Ru loading leads to a higher CO2 conversion level, since more active sites are present on the RuCu-3/MCN catalysts. The CO2 conversion of RuCu-3/NMCN catalysts increases 18 folds and 9 folds to RuCu-1/NMCN and RuCu-2/NMCN catalysts at 500 \u00b0C, relatively. The conversion of RuCu-3/MCN catalysts in CO2 methanation is 53% and the major product is CH4 with a small amount of CO at 500 \u00b0C. It should be noted that, in such a readction condition,the CH4 selectivity achieves 88%, as shown in Fig.\u00a05c. At 500\u00a0 \u00b0C, the catalysts' selectivity for CH4 is in the following trends: RuCu-3/MCN >\u00a0RuCu-2/MCN >\u00a0RuCu-1/MCN. As for the RuCu-2/NMCN catalysts, the CO2 conversion decreases as the reaction temperature increases. The Cu/NMCN catalysts have a similar phenomenon at 700\u00a0 \u00b0C, starting to decrease due to reverse water gas shift reaction (rWGS) becoming the most favored reaction at high temperatures. Due to the low conversion below 300\u00a0\u00b0C, the selectivity behavior in this range is not conclusive.The long-term stability of RuCu-3/NMCN has been investigated by adopting a realistic feed composition of 15% CO2, 60% H2 and 25% N2. The space velocity (28,350\u00a0mL gcat\n\u20131\u2009h\u20131) and reaction temperature (500\u00a0\u00b0C) employed are chosen so as to keep the conversion of CO2 around 52%. The results obtained are presented in Fig.\u00a06\n. The stability of the RuCu-3/NMCN catalysts is evaluated over a 1380\u00a0min successive run at atmospheric pressure. The RuCu-3/NMCN catalyst appears to be quite stable, retaining nearly 99% of its initial activity. Furthermore, the RuCu-3/NMCN can also retain a CH4 selectivity above 85% after the 1380\u00a0min reaction. Therefore, it could be concluded that RuCu-3/NMCN is a promising candidate for CO2 methanation with H2-rich gas.XRD analysis of the RuCu-3/NMCN catalysts before and after the reaction suggests that the catalyst is stable over an extended time (Supplementary Fig. S6). According to the Brunauer-Emmett-Teller (BET) method, the specific surface areas (SBET) are calculated to be 259 m2 g\u22121 for the used RuCu-3/NMCN. The total pore volumes and pore diameter are 0.20 cm3 g\u22121 and 2\u20134\u00a0nm for the used RuCu-3/NMCN, respectively. Therefore, it could be confirmed that the texture properties of the used catalysts show no significant change. TEM image of the used catalysts is shown in Supplementary Fig. S8a, it could be seen that the particle size of RuCu-3/NMCN remains unchanged, and the metal alloy particles are away from sintering during the reaction. To further prove the component stability of the used catalyst, the bimetallic catalyst after reactions is analyzed by HADDF-STEM and EDS (Supplementary Fig. S8). The HADDF-STEM image (Supplementary Fig. S8b) further confirms the unchanged spherical morphology of the used alloy catalyst and the well dispersion of the metal particles on the NMCN nanospheres. More important, it should be pointed out that no carbon deposition could be evidenced on the catalyst's surface. From the EDS mapping images (Supplementary Fig. S8c-f), the uniform distribution of Cu and Ru could be evidenced. These findings solidly support the component stability of the used RuCu-3/NMCN catalyst. High-resolution XPS scans show the chemical states ofRuCu-3/NMCN after recycling. In the Cu 2p spectra (Fig. S9a), the peaks at around 931.5 and 951.6\u00a0eV are referred to Cu0 for Cu 2p3/2 and Cu 2p1/2, respectively. The satellite peaks observed at around 934.7\u00a0eV and 955.9\u00a0eV indicate the presence of Cu in its oxidation state. Ru on the surface exists in an oxidized state as Ru4+ (the Ru 3p3/2 peak at 463.8\u00a0eV and Ru 3p1/2 at 485.6\u00a0eV). And the surface layer revealed the existance of metallic Ru0 in Ru 3p spectra (461.7 and 438.8\u00a0eV). These results suggest that the Cu and Ru chemical states of RuCu-3/NMCN remain unchanged compared with the fresh catalyst.The results of this experiments enabled a possible thermal catalytic CO2 methanation reaction mechanism over the RuCu-3/NMCN alloy catalysts (Scheme\u00a02\n). CO2 and H2 are adsorbed on the catalysts at the beginning of the reaction. Then, CO2 prefers to form key initial intermediates (CO2\n\u03b4\u2212, bicarbonate) on Ru and Cu surface, (Kapiamba\u00a0et\u00a0al., 2022; L\u00f3pez-Rodr\u00edguez\u00a0et\u00a0al., 2021) which are then reacted with H2\u00a0dissociatively adsorbed on Ru sites to generate the H* species at the RuCu alloy. As generated H* could react with the CO adsorbed on the RuCu alloy, which is stepwise hydrogenated to CH4. The proposed reaction sequence is:\n\n(1)\n\n\nC\n\nO\n2\n\n+\n\nH\n2\n\n\u2192\nCO\n+\n\nH\n2\n\nO\n,\n\nfacilitated\n\nby\n\nsites\n\nat\n\nthe\n\nbimetal\n\nsurface\n\n\n\n\n\n\n(2)\n\n\nCO\n+\n2\n\nH\n2\n\n\u2192\nC\n\nH\n4\n\n,\n\nfacilitated\n\nby\n\nRu\n\nsites\n\n\n\n\nIt is widely acknowledged that the product selectivity can be affected by the binding strength of the generated CO on the surface of the metal nanoparticles. (Kim\u00a0et\u00a0al., 2021; Wang\u00a0et\u00a0al., 2021b) While weak interactions between CO and metal nanoparticles result in either desorption of CO as the end product or the production of alcohols and aldehydes. Strong interactions between CO and nanoparticles cause C-O bond dissociation, which leads to the formation of CH4. Cu nanoparticles undergo CO2 reduction, resulting in the formation of CO by a non-dissociative C-O bond mechanism. In contrast to Cu, Ru tends to generate CH4 due to C-O bond dissociation followed by C-H bond creation. (Wang\u00a0et\u00a0al., 2011; Ciobica\u00a0et\u00a0al., 2003; Patra\u00a0et\u00a0al., 2020)As for the reaction process, a hypothesized reaction mechanism is proposed. Due to the existence of Ru species, the formate (HCOO*) species will act as a spectator. (Xu\u00a0et\u00a0al., 2021; Eckle\u00a0et\u00a0al., 2011) The CO2 adsorbed on the RuCu alloy sites is firstly dissociated into CO* and O* to meet the goals of CO2 activation and CO production. In addition, the first step in the CH4 production pathway is the formation of the HCOO* species, which is then hydrogenated in stages to produce HCOOH* and H2COOH*. Due to the dissociation of H2COOH* and the breakage of the C-OH bond, H2CO* and OH* would be generated. Then the C-O bond in H3CO* is subsequently broken and releases H3C* and O*, which ultimately forms CH4 and H2O. (Yan\u00a0et\u00a0al., 2018) In our work, it is evidenced that Ru has exceptional CH4 selectivity in thermocatalytic CO2 methanation. (Zhuang\u00a0and Simakov,\u00a02021) However, the high cost of Ru greatly limits its practical application. In this regard, partially substituting Ru with low price Cu, which could lead to a synergistic effect to hinder the carbon deposition process, turns to be an ideal choice, (Liu\u00a0et\u00a0al., 2013) as Cu NPs can be utilized as promoters to enhance the activity of the applied catalysts. (Ndolomingo\u00a0et\u00a0al., 2020) Therefore, the optimal RuCu-3/NMCN catalyst shows remarkably enhanced product selectivity with a balanced Ru/Cu ratio.In this work, bimetallic RuCu/NMCN is successfully synthesized for thermal catalytic CO2 methanation. As a result, the bimetallic NPs achieve remarkably enhanced CO2 methanation performance and stability. It is demonstrated that the thermocatalytic activity of RuCu-x/NMCN (x\u00a0=\u00a01, 2, 3) is substantially higher than that of pure NMCN and Cu/NMCN. Especially for the RuCu-3/NMCN catalyst, the carbon deposition side reaction is suppressed, leading to a good thermal stability at 500 \u00b0C. Moreover, at this temperature, the CO2 conversion and CH4 selectivity of RuCu-3/NMCN remain at 52% and 85%, respectively, for at least 1380\u00a0min. This work demonstrates the potential for the NMCN-supported bimetallic catalysts to mitigate the anthropogenic greenhouse gas emissions from the fossil fuel combustion and encourage sustainability.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the Australian Research Council (ARC) under the Discovery Projects funding scheme (DP 220102851) and the Sydney Nano Grand Challenge, the University of Sydney.Dedicated to Professor Jianzhong Chen on the occasion of his 70th birthday.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ccst.2023.100100.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  CO2 methanation draws great attention since it could relieve the global climate change while providing important chemical feedstock simultaneously. In this work, we designed a nitrogen-doped mesoporous carbon spheres supported RuCu bimetallic nanoparticles (RuCu/NMCN) for high-performance thermal catalytic CO2 methanation. It is found that, due to its high thermal conductivity, and excellent capacity to store H2 and CO2, NMCN is an ideal supporting material. Moreover, the relative Ru/Cu ratio of the RuCu bimetallic catalysts strongly influences the reaction performance, among which the optimal reaction performance is evidenced with a Ru/Cu ratio of 1/10. More interestingly, it is proved that the presence of Cu is more conducive to inhibiting carbon deposition. At 500 \u00b0C, the optimised RuCu/NMCN-3 exhibited the highest catalytic activity for thermocatalytic CO2 methanation (53%) with 88% CH4 selectivity. This work provides insight into the rational application of bimetallic catalyst for CO2 methanation processes.\n               "}
{"full_text": "Currently, the depletion of oil reserves due to the rapid growth of its global consumption as the industry develops intensively and against the background of stricter requirements for the environmental friendliness of technology have contributed to the search for alternative sources of raw materials. One of the sources of raw materials is bioethanol, obtained by processing biomass, which can replace oil in the chemical industry, and is also used in the production of fuel. The use of biomass helps to reduce the \"greenhouse effect\" by reducing carbon dioxide emissions into the atmosphere (Bridges\u00a0et\u00a0al., 2015; Galadima\u00a0and Muraza,\u00a02015; West\u00a0et\u00a0al., 2009). Promising is the further catalytic processing of bioethanol as a feedstock for the production of various valuable chemicals, such as hydrogen (Rossetti et\u00a0al., 2020), propylene (Xue\u00a0et\u00a0al., 2019), ethylene (Dugkhuntod and Wattanakit, 2020; Sun and Wang, 2014), butanol (Dai\u00a0and Zhang,\u00a02019; Wu\u00a0et\u00a0al., 2018), 1,3-butadiene (Zhao et\u00a0al., 2020), acetaldehyde (Carotenuto et\u00a0al., 2013), aromatic compounds (Liu\u00a0et\u00a0al., 2020; Phung\u00a0et\u00a0al., 2015), etc. Among the above products obtained from bioethanol, aromatic hydrocarbons are of great importance in the chemical and petrochemical industries as a component of motor fuels, solvents and intermediate products, and also have a wide range of applications in many other industries (Kianfar,\u00a02019; Kianfar\u00a0et\u00a0al., 2018; Lu\u00a0et\u00a0al., 2018; Nezam\u00a0et\u00a0al., 2021).It should be noted that a large amount of aromatic hydrocarbons is produced mainly during oil refining and catalytic reforming of naphtha (F. Wang et\u00a0al., 2016; X. Wang et\u00a0al., 2016; Wang et\u00a0al., 2021). This process is carried out at high temperatures (up to 800\u00b0\u0421) and pressures (10\u201385\u00a0bar), as well as the use of mineral resources, the high cost of the process, and the low selectivity of aromatic hydrocarbons indicate the inefficiency of the process (Rodr\u00edguez\u00a0and Ancheyta,\u00a02011). A non-petroleum method for producing aromatic hydrocarbons from methanol is known in the literature. The main disadvantages of this method include the rapid deactivation of the catalyst and the need for sequential purification of the target products (Fu\u00a0et\u00a0al., 2021; Li\u00a0et\u00a0al., 2014). In addition, in the main products of this process, alkanes and alkenes are formed first, and not light aromatic hydrocarbons. Methanol is a toxic substance.Thus, one of the effective sustainable methods for obtaining aromatic hydrocarbons is the catalytic conversion of ethanol. Many advantages, such as low cost of raw materials, high yield of target products, and reduced dependence on oil, make this method the most promising way to obtain aromatic hydrocarbons from renewable sources (Liu\u00a0et\u00a0al., 2016; Xiang\u00a0et\u00a0al., 2022). Zeolites HZSM-5 and ZSM-5 are mainly active in the production of aromatic light hydrocarbons due to their large surface area, adsorption capacity, and controlled acid properties (Migliori\u00a0et\u00a0al., 2017; Wannapakdee\u00a0et\u00a0al., 2019; Zeng\u00a0et\u00a0al., 2022; Said\u00a0et\u00a0al., 2020; Zhang\u00a0et\u00a0al., 2011; Zhang et\u00a0al., 2013; Ramesh\u00a0et\u00a0al., 2010). However, the strong acidic properties of these zeolites contribute to the occurrence of undesirable reactions, limiting the selectivity towards aromatic compounds, and the catalyst undergoes coke formation (Almutairi\u00a0et\u00a0al., 2012; Wang\u00a0et\u00a0al., 2020). To eliminate these shortcomings, various metals/metal oxides were included in the composition of zeolites, such as Ni (Liu\u00a0et\u00a0al., 2020; Niziolek\u00a0et\u00a0al., 2016), Fe (Calsavara\u00a0et\u00a0al., 2008), Zn-Ga (Hodala\u00a0et\u00a0al., 2016), Mo (Barthos\u00a0et\u00a0al., 2006), P (Lu\u00a0and Liu,\u00a02011), Ag (Hsieh\u00a0et\u00a0al., 2017), Ce (Bi\u00a0et\u00a0al., 2011), etc. Among them, the zinc-based catalyst has been intensively studied due to its relatively low cost, availability, and low toxicity. The best results in the process of ethanol conversion are provided by mixed systems for the preparation of characteristic acid-base catalysts (Vlasenko\u00a0et\u00a0al., 2019.; Cheng\u00a0et\u00a0al., 2014.; Chistyakov\u00a0et\u00a0al., 2014). Compared to the ZnO/ZSM-5 monometallic catalyst, the modified catalysts (Zn-ZrO2/ZSM-5) show resistance to carbonization and increased catalytic activity (Ohayon\u00a0Dahan et\u00a0al., 2021).In a number of works (Ramesh\u00a0et\u00a0al., 2009; Song\u00a0et\u00a0al., 2010), it is reported that the addition of phosphorus to the ZSM-5 increases the catalytic stability of the catalyst and its resistance to coke formation.The main disadvantage of ZSM-5 zeolite is its high cost; therefore, in our work, cheaper molecular sieves of KA modified with zinc and phosphorus oxides were studied as a catalyst. In terms of pore size, KA is close to ZSM-5 zeolite, the pore size of KA is approximately 4\u20135\u00a0\u01fa; for ZSM-5 zeolite, the pore size is 5.4\u20135.6\u00a0\u01fa (Baerlocher\u00a0et\u00a0al., 2007). An analysis of the literature shows that KA as a support for zinc catalysts has not been previously studied.Thus, the purpose of this work is to study the activity of catalysts based on KA modified with zinc and phosphorus oxides and the effect of the method of catalyst preparation on its activity in the conversion of ethanol to aromatic hydrocarbons.The catalysts were synthesized by capillary impregnation of the support and by the \"solution combustion\" method. The choice of synthesis methods as capillary impregnation and \"solution combustion\" is justified by the fact that these methods have a number of advantages compared to other methods (sol-gel, deep impregnation, etc.): relative simplicity, less hazardous waste and more efficient use of low-percentage active component, there is no loss of the impregnating solution, which is especially important in the manufacture of expensive catalysts (Yergaziyeva\u00a0et\u00a0al., 2021).Materials used in the preparation of catalysts and catalytic reaction: molecular sieve KA (Shanghai JiuZhou Chemicals Co. Ltd, China), Zn(NO3)2 (Sigma Aldrich, USA 98%), phosphoric acid H3PO4 (GOST 6552\u201380, 99%), helium (LLP \u00abIkhsanTechnoGas\u00bb, 99%), argon (LLP \u00abIkhsanTechnoGas\u00bb, 99%), dispersing agent: urea (Sigma Aldrich, USA 99,5%), distilled water.\nZn\u041e/KA was obtained by the method of capillary impregnation according to the moisture capacity of the KA carrier to aqueous solutions of Zn(NO3)2\u00b76H2O (GOST TU 5106\u201377). The content of zinc oxide in the catalyst was 1\u00a0wt.%.The Zn\u041e-P2\u041e5/KA (CI) catalyst was also obtained by capillary impregnation according to the moisture capacity of the KA carrier with an aqueous solution of Zn(NO3)2\u00b76H2O salt (GOST 5106\u201377) and H3PO4 (GOST 6552\u201380, 99%).The catalyst ZnO-P2O5/KA (SC) was obtained by the \"solution combustion\" method with the addition of a dispersing agent-urea, ratio of ZnO/urea\u00a0=\u00a01:0.5. The content of oxides in the bimetallic samples was 1\u00a0wt.% ZnO and 1\u00a0wt.% P2O5.Heat treatment of all samples was carried out in air at 300\u00a0\u043e\u0421 for 2\u00a0h, then at 500\u00a0\u043e\u0421 for 3\u00a0h.The activity of synthesized catalysts in ethanol conversion was tested in a stainless steel reactor with an internal diameter of 1.7\u00a0cm in a flow mode at a temperature of 200\u2013400\u00a0\u00b0C, pressure P\u00a0=\u00a00.1\u00a0MPa, with a volumetric ethanol flow rate of 1\u00a0h\n\u22121. A catalyst in the amount of 2\u00a0ml was placed in the reactor between thin layers of glass wool. Raw material - ethanol (95%) was supplied with the help of a high-pressure pump. The reaction products were identified online on a CHROMOS GCH-1000 (GCH-1000 LLC \"Chromos\" Russia) device.Separation of the components was carried out on two columns (length 2\u00a0m, inner diameter 3\u00a0mm) filled with NaX zeolite and porapak-T, carrier gas - helium and argon. Ethanol conversion was calculated according to the following equation:\n\n(1)\n\n\nEthano\n\nl\nconversion\n\n\n(\n%\n)\n\n=\n\n\nEthano\n\nl\nin\n\n\u2212\nEthano\n\nl\nout\n\n\n\nEthano\n\nl\nin\n\n\n\n\u00b7\n100\n%\n\n\n\n\nThe physicochemical characteristics of the catalysts were studied by SEM, TEM, TPD-NH3, and TPR-H2.The surface morphology of the catalysts was studied by scanning and transmission electron microscopy (Quanta 200i 3D, FEI Company, USA). SEM micrographs of the catalysts (Zn\u041e/KA, Zn\u041e-P2\u041e5/KA (CI) and Zn\u041e-P2\u041e5/KA (SC)) are shown in Fig.\u00a01\n.The results of SEM analysis showed that the catalysts contained round particles with sizes from 300 to 2500\u00a0nm. It can be seen from the images that the preparation of catalysts by means of capillary impregnation and \"solution-combustion\" does not show any significant morphological difference, having the same particle size distribution. Similar results were observed in (Chen\u00a0et\u00a0al., 2015). SEM images show that the zeolite structure did not collapse during synthesis and morphology of zeolite crystals. The ZnO-P2O5/KA (CI) and ZnO-P2O5/KA (SC) catalysts were also studied by TEM (FEI Tecnai) (Fig.\u00a02\n). The results of TEM images show that there is a difference in the distribution of active phases on the catalysts.On the ZnO-P2O5/\u041a\u0410 (SC) catalyst, the particle distribution is more uniform than on the ZnO-P2O5/\u041a\u0410 (CI) catalyst. The ZnO-P2O5/\u041a\u0410 (CI) catalyst contains nanoparticles ranging in size from 51 to 30\u00a0nm (Fig.\u00a02a). Whereas the preparation of 1\u00a0wt.% ZnO-1\u00a0wt.% P2O5/KA catalyst by the \"solution combustion\" method (Fig.\u00a02b) leads to a decrease in the size of the catalyst particles, nanoparticles with sizes from 2\u00a0nm are observed. An increase in the dispersion of particles of the catalyst ZnO-P2O5/KA (SC) is also confirmed by the TPR-H2 (USGA-101, Russia) method.From the results of TPR-H2 shown in Fig.\u00a03\n, it can be seen that the ZnO-P2O5/\u041a\u0410 (CI) and ZnO-P2O5/KA (SC) catalysts have 3\u20134 reduction zones in the range of 250\u2013850\u00a0\u00b0\u0421.On the TPR profile of the ZnO-P2O5/KA (SC) catalyst, three reduction zones are observed with T1\nmax\u00a0=\u00a0606 \u043e\u0421, the amount of absorbed hydrogen A1\u00a0=\u00a047\u00a0\u03bcmol/g, T2\nmax\u00a0=\u00a0661\u00a0\u043e\u0421, A2\u00a0=\u00a062\u00a0\u03bcmol/g, T3\nmax\u00a0=\u00a0770\u00a0\u043e\u0421, A3\u00a0=\u00a030\u00a0\u00b5mol/g. On the TPR-H2 profile of the ZnO-P2O5/KA (CI) catalyst, there are 4 reduction zones with T1\nmax\u00a0=\u00a0500\u00a0\u043e\u0421, A1\u00a0=\u00a032\u00a0\u00b5mol/g, T2\nmax\u00a0=\u00a0579\u00a0\u043e\u0421, A2\u00a0=\u00a0114\u00a0\u00b5mol/g, T3\nmax\u00a0=\u00a0720\u00a0\u043e\u0421, A3\u00a0=\u00a0328\u00a0\u00b5mol/g and T4\nmax\u00a0=\u00a0760\u00a0\u043e\u0421, A4\u00a0=\u00a0100\u00a0\u00b5mol/g. Peaks in the region of 500\u2013720\u00a0\u00b0C refer to the reduction of zinc oxide. It is known that zinc oxide has several reduction zones in the region of 100\u20131000\u00a0\u00b0C (Ebrahimi\u00a0et\u00a0al., 2022). The presence of several zones is associated with zinc oxide with different dispersity or with different forces of interaction with the carrier. The peaks T4\nmax\u00a0=\u00a0760\u00a0\u043e\u0421 and T3\nmax\u00a0=\u00a0770\u00a0\u043e\u0421 can be attributed to the restoration of the P\u2013O bond (Huang\u00a0et\u00a0al., 2021).The results obtained show that the preparation method affects the reduction characteristics of the catalyst. The preparation of the catalyst by the \u201csolution burning\u201d method leads to a decrease in the amount of hydrogen consumed for reduction, as well as in the reduction temperature of zinc oxide from 720 to 661\u00a0\u00b0C, which indicates an increase in the dispersion of catalyst particles. (Ganiyu\u00a0et\u00a0al., al.,2017; Van\u00a0et\u00a0al., 1990). Table\u00a01\n.Acidity is one of the important factors in the catalytic performance of a zinc-based catalyst. NH3-TPD (USGA-101, Russia) analysis was performed to determine the acidity of the ZnO-P2O5/\u041a\u0410 (SC) and ZnO-P2O5/\u041a\u0410 (CI) catalysts. The TPD profiles of the carrier KA and catalysts are shown in Fig.\u00a04\n, TPD data are presented in table\u00a02\n.According to the literature data (Chen\u00a0et\u00a0al., 2015; Niu\u00a0et\u00a0al., 2014), ammonia desorption can be divided into three temperature ranges: low temperature (120\u2013200\u00a0\u00b0C), medium temperature (200\u2013300\u00a0\u00b0C), and high temperature (300\u00a0\u00b0C<), corresponding to weak, medium, and strong acid sites.It can be seen from the figure that the KA carrier has acid sites, and ammonia desorption occurs in two temperature ranges. A peak in the temperature range of 50\u2013365\u00a0\u043e\u0421 with a maximum of \u04221\nmax\u00a0=\u00a0173\u00a0\u043e\u0421 may indicate the presence of weak acid centers, and a peak in the range of 585\u2013780\u00a0\u043e\u0421 with \u04222\nmax\u00a0=\u00a0627\u00a0\u043e\u0421 may indicate the presence of strong acid centers. The amount of desorbed ammonia is 171 and 95\u00a0\u00b5mol/g, respectively (Table\u00a02).With the application of zinc oxide to the KA, the intensity of the peaks increases, a new acid center of medium strength appears with the participation of zinc oxide (\u04222\nmax\u00a0=\u00a0223\u00a0\u043e\u0421, the amount of desorbed NH3 is 558\u00a0\u03bcmol/g). The amount of desorbed ammonia from strong acid sites increases from 95 to 292\u00a0\u00b5mol/g compared to the initial KA (table\u00a02). Modification of the ZnO/KA catalyst with phosphorus leads to an increase in the intensity of ammonia peaks related to weak and strong acid sites. The amount of ammonia desorbed from weak acid sites increases from 136 to 550\u00a0\u00b5mol/g, and from strong acid sites from 292 to 351\u00a0\u00b5mol/g, respectively (Table\u00a02). The preparation of ZnO-P2O5/KA by the \"solution combustion\" method leads to an increase in the total acidity of the catalyst. Compared to ZnO-P2O5/KA (CI), the amount of ammonia desorbed from weak acid sites increases from 550 to 783\u00a0\u00b5mol/g, from strong acid sites from 351 to 423\u00a0\u00b5mol/g.The results of ammonia TPD showed that zinc-phosphorus-containing catalysts ZnO-P2O5/KA (CI) and ZnO-P2O5/KA (SC) synthesized by various methods have both weak and strong acid sites.It is known that oligomerization strongly depends on Br\u00f8nsted acid sites (Zhang\u00a0et\u00a0al., 2022). According to the literature (Kamyar\u00a0et\u00a0al., 2020), the low-temperature peak (120\u2013200\u00a0\u00b0C) is attributed to weakly adsorbed NH3 on weak Lewis acid sites, the medium-temperature peak (200\u2013300\u00a0\u00b0C) to ammonia adsorbed on strong Lewis acid sites, and the high-temperature peak (300\u00a0\u00b0C<) of NH3 desorbed from the Bronsted acid site. Therefore, an increase in Bronsted acid sites is favorable for further ethylene oligomerization into aromatic hydrocarbons. The highest concentration (23\u00a0vol.%) of aromatic hydrocarbons is observed on the ZnO-P2O5/KA (SC) catalyst. The catalyst prepared by the \"solution combustion\" method has a higher acidity than the catalyst prepared by the impregnation of the carrier in terms of moisture capacity.The thermal catalytic conversion of ethanol on a KA and a ZnO-P2O5/KA catalyst includes the parallel formation of gaseous products CH4, CO2, CO, H2, ethylene, and aromatic hydrocarbons through decomposition, dehydration of ethanol, and oligomerization of ethylene, respectively (Tretyakov et\u00a0al., 2010; Dosumov\u00a0et\u00a0al., 2014) (Scheme\u00a01\n).The application of zinc oxide leads to an increase in the decomposition products of ethanol (CH4, CO2, CO, H2), possibly due to an increase in the average acid sites. Modification of ZnO/\u041aA with phosphorus oxide leads to a decrease in the concentration of methane decomposition products and an increase in ethylene, a dehydration product (Table\u00a03\n).The concentration of ethylene in the reaction products increases symbatically with an increase in the weak acid sites of the catalysts. These data are confirmed by the data in (Kamsuwan\u00a0et\u00a0al., 2020; Xin\u00a0et\u00a0al., 2014), where it is indicated that catalysts with increased weak acidity provide a higher yield of ethylene. The highest concentration of aromatic hydrocarbons is observed on ZnO-P2O5/KA (SC), which has a high total acidity. For this ZnO-P2O5/KA (SC) catalyst, the highest ethanol conversion of 85% is observed. Therefore, an increase in acid sites is favorable for the adsorption of ethanol and its activation.In this work, we studied the activity of catalysts based on KA modified with zinc and phosphorus oxides and the effect of the method of catalyst preparation on its activity in the conversion of ethanol to aromatic hydrocarbons.It has been shown for the first time that KA-based catalysts can be used to produce aromatic hydrocarbons from ethanol. According to SEM, TEM, and TPR-H2 data, the method of preparation affects the dispersity of catalyst particles and its reduction characteristics. The preparation of the ZnO-P2O5/KA catalyst by the \"solution combustion\" method, in comparison with capillary impregnation, leads to an increase in the dispersion of particles. In the composition of the ZnO-P2O5/KA (SC) catalyst, nanoparticles with sizes from 2\u00a0nm are observed. The results of TPD-ammonia showed that catalysts based on KA modified with zinc oxides have weak, medium and strong acid sites. Modification of ZnO-P2O5/\u041aA with phosphorus oxide leads to an increase in the number of weak and strong acid sites. The highest ethanol conversion of 85% and the concentration of aromatic hydrocarbons (23\u00a0vol.%) was obtained for the ZnO-P2O5/KA (SC) catalyst. According to physicochemical methods, the particle size of the ZnO-P2O5/KA (SC) catalyst decreases to 2\u00a0nm, their uniform distribution on the catalyst surface is observed, the total number of acid sites reaches up to 1206\u00a0\u03bcmol/g, which positively affects the activity of the catalyst in conversion ethanol to aromatic hydrocarbons.The results obtained indicate that the cheaper molecular sieve KA can be used as a carrier for zinc-containing catalysts for the production of aromatic hydrocarbons from ethanol as an alternative to the expensive ZSM-5. The preparation method can control the acidity of the catalyst and the dispersion of its particles.The authors declare no conflict of interests.This research has been funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP08855936).", "descript": "\n                  The activity of catalysts based on KA modified with zinc and phosphorus oxides and the effect of the method of catalyst preparation on its activity in the non-oxidative thermocatalytic conversion of ethanol to aromatic hydrocarbons was studied in this work. The ZnO-P2O5/KA catalyst was prepared by capillary impregnation and solution-combustion methods and characterized by SEM, TEM, TPD-NH3, and TPR-H2. The results showed that the method of preparation affects both the reducibility, dispersity, as well as the acidity of the catalyst. The preparation of the ZnO-P2O5/\u041a\u0410 catalyst by the \"solution combustion\" method, in comparison with capillary impregnation, leads to an increase in the dispersion of particles. In the composition of the ZnO-P2O5/\u041a\u0410 (SC) catalyst, nanoparticles with sizes from 2\u00a0nm are observed. The results of TPD-ammonia showed that catalysts based on KA modified with zinc oxides have weak and strong Lewis acid sites, and Bronsted acid sites. Modification of ZnO-P2O5/\u041a\u0410 with phosphorus oxide leads to an increase in the number of weak and strong acid sites. The highest ethanol conversion (85%) and the concentration of aromatic hydrocarbons (23\u00a0vol.%) was obtained for the ZnO-P2O5/\u041a\u0410 (SC) catalyst, which has the highest acidity of 1206\u00a0\u00b5mol/g.\n               "}
{"full_text": "No data was used for the research described in the article.As per a report published by the Joint Monitoring Program of the World Health Organization and the United Nations International Children\u2019s Emergency Fund, 1 out of 3 people worldwide lack clean sources of drinking water (Progress on household drinking water, sanitation and hygiene I. 2000-2017, 2019). Furthermore, access to clean drinking water is expected to get consistently more difficult as pollutants continue to be released into water (Tijani et al., 2013). For the past few decades, emerging contaminants have caught the attention of scientists and government organizations alike (Montes-Grajales et al., 2017a; Tijani et al., 2013, 2016). According to the United States Environmental Protection Agency (US EPA), these emerging micropollutants (MPs) are compounds that are, at present, unregulated, and the environmental impacts of which are not very well understood (Montes-Grajales et al., 2017a). Similarly, the US Geological Society defines MPs as chemicals, microorganisms, or metabolites that may cause adverse effects to the environment or human health, but the release of which into the environment is not monitored (Tijani et al., 2016).The environmental impact of MPs was first described in Rachel Carson\u2019s 1962 book The Silent Spring (Tijani et al., 2016). While MPs have existed in the environment for decades, they have only recently been able to be detected (Richardson and Kimura, 2017). This is due to the development of highly sensitive analytical techniques and on-site sensing devices that allow quantification of MPs, which are usually present in concentrations in the order of ng L\u22121 to \n\u03bc\ng L\u22121 (Richardson and Kimura, 2017; Vodyanitskii and Yakovlev, 2016). Even at these extremely low concentrations, MPs can pose a significant threat to aquatic life and human health. Hazards include bioaccumulation (Vodyanitskii and Yakovlev, 2016), toxicity to humans and aquatic life, increased risk of cancer, increase in antibiotic-resistant bacteria, reproductive health problems (Gogoi et al., 2018a), brain damage, development of cardiovascular disease, liver damage, pulmonary defects, and feminization of fish and other aquatic life (Tijani et al., 2013).Recently, there has been great interest in using MOF composite-based catalysts for degradation of MPs in wastewater. Like the aforementioned composites, activity of conventional catalysts used for this application shows significant increase when they are paired with MOFs (Wang et al., 2020c). This review describes the different MP degradation techniques currently under study, with special focus on MOF-catalyst composites. It also discusses synthesis methods of MOF-catalyst composites and the photodegradation mechanism of MPs using these materials. Finally, current trends in research, the associated cost, and environmental considerations of these composites are presented.The remainder of the paper is organized as follows. In Section\u00a02, the different types of micropollutants ranging from pharmaceuticals and personal care products to dyes are examined along with their global presence. In Section\u00a03, the commonly used degradation techniques are discussed. Next, in Section\u00a04, MOF-composite-based catalysts are introduced along with their potential applications. We also examine the synthesis methods of MOF-composite-based catalysts and discuss their associated photodegradation mechanism. We then present the current research on the use of MOF-composite-based catalysts for MPs degradation, their associated costs along with the health and environmental considerations. Lastly, Section\u00a05 concludes the paper and proposes potential topics for future research.Anthropogenic activities are the main cause of MPs in potable water (Noguera-Oviedo and Aga, 2016). Most of the MPs come from municipal wastewater (Gogoi et al., 2018b; Noguera-Oviedo and Aga, 2016), while industrial waste (Tkaczyk et al., 2020), mining (Tijani et al., 2016), aquaculture (Noguera-Oviedo and Aga, 2016), and agricultural practices (Richardson and Kimura, 2017; Tijani et al., 2013) also contribute to MPs in drinking water. Given that the harmful effects of MPs, being recently identified and studied, current wastewater treatment practices are ineffective towards MPs (Luo et al., 2014). Therefore, MPs pass through wastewater treatment plants (WWTPs) unaltered and are released into the environment or drinking water supplies (Luo et al., 2014).Before MP degradation methods can be developed, the nature of MPs present in WWTP effluents must be identified. The most common types of MPs are human and veterinary pharmaceuticals, personal care products, endocrine disruptor compounds (Bolong et al., 2009; Gogoi et al., 2018b; Luo et al., 2014; Montes-Grajales et al., 2017a; Tijani et al., 2013), and dyes (Tkaczyk et al., 2020). Furthermore, these MPs can react with each other in wastewater, or transform during wastewater treatment process, to give transformation products that may be more harmful than the original materials (Richardson and Kimura, 2017). As such, it is imperative to understand the nature and activities of these MPs, and develop and implement efficient techniques to remove them from WWTP effluents. Table\u00a01 lists the maximum allowable concentrations of some MPs in surface waters in addition to their structures and applications. We notice that these concentrations are significantly low. For example, the maximum allowable concentration of ibuprofen is 0.011 \n\u03bc\ng L\u22121 and Atenolol is 150 \n\u03bc\ng L\u22121. This demonstrates the need for early detection and appropriate treatment of MPs even at very low concentrations.\n\nPharmaceuticals are biologically active compounds that can enter the ecosystem in a number of ways, including disposal of unwanted medicines and wastes from pharmaceutical industries, research labs, hospitals, and animal husbandry (Tiwari et al., 2017). Due to their aversion to degradation, pharmaceuticals can accumulate in the environment and have the potential to cause long-term detrimental effects (Tiwari et al., 2017). At the same time, because they are designed to be effective at very low concentrations, they can even induce toxicity in aquatic environments at concentrations in the order of ng L\u22121 (He et al., 2016). For example, the concentration of estrogen, which is known to cause the expression of feminine traits in male fish, was found to be ranging from 0.3 to 12.6 ng L\u22121 in municipal WWTP effluents (Tiwari et al., 2017). Chronic exposure to estrogen has toxic effects on fish populations (Tiwari et al., 2017). Other drugs such as antidepressants which are known to increase serotonin levels were found to be present in the aquatic environment in concentrations ranging from 0.3 \n\u03bc\ng L\u22121 to 100,000 \n\u03bc\ng L\u22121 (Brodin et al., 2014). This in turn was found to reduce the aggression levels in several fish species such as the coral reef fish and the Siamese fighting fish making them vulnerable to attacks from other species. They also cause disorders in tadpole development (Tiwari et al., 2017). Moreover, antiepileptic drugs were found to be present in wastewaters at a concentration of 6100 \n\u03bc\ng L\u22121 (Argaluza et al., 2021).Further, a study revealed that consumption of diclofenac, an anti-inflammatory drug, caused renal failure in vultures, and resulted in a decrease in population (Tiwari et al., 2017). Moreover, large amounts of antibiotics consumed pass through the body unmetabolized and enter municipal wastewater. For instance, up to 62% of the administered dose of ciprofloxacin, a broad-spectrum antibiotic, is excreted, which can lower the number of microbes used in WWTPs and decrease efficiency (Girardi et al., 2011). This could also give rise to antibiotic resistance in bacteria populations (Girardi et al., 2011).Nevertheless, some metabolites of drugs can be more toxic than the starting compounds, therefore, these must be identified and regulated as well (Tiwari et al., 2017). Some techniques, like UV irradiation and ozonation, can be used to degrade pharmaceuticals in WWTP, but their applications are limited by high capital and running costs (He et al., 2016). Hence, there is a need for more cost-effective and efficient ways to remove pharmaceuticals from wastewater as their presence is harmful to aquatic life and the overall environment.The World Health Organization and the European Union Commission define Endocrine Disruptors (EDs) as external chemicals that cause disruptions in an organism\u2019s endocrine system, and as a result, have negative impacts on the organism or its progeny (Kuckelkorn et al., 2018; Varjani and Sudha, 2020). Several studies, conducted all over the world, have found EDs in drinking water, which is a major source of exposure for humans (Kuckelkorn et al., 2018). EDs are known to be carcinogens and cause a number of endocrine-related health issues, including, pituitary and thyroid gland malfunction at concentrations of less than 1 \n\u03bc\ng L\u22121 (Kuckelkorn et al., 2018; Petrovic et al., 2002; Varjani and Sudha, 2020). For instance, wastewaters in Greece were found to contain different EDs with concentrations ranging from 64.7 to 15,320 ng L\u22121 (Pironti et al., 2021). Whereas the EDs found present in the drinking water in Serbia were at concentrations ranging from 0.4 to 6.6 ng L\u22121 (Pironti et al., 2021).EDs act by either imitating natural hormones produced by the body in order to bring about physiological changes, or by competitively binding to hormone receptors and inhibiting natural bodily functions (Varjani and Sudha, 2020). Furthermore, it is difficult to predict which synthetic chemicals will exhibit endocrine-disrupting effects (Varjani and Sudha, 2020), as EDs comprise numerous categories of chemical, from pesticides and pharmaceuticals to plastics (Kedzierski et al., 2018; Petrovic et al., 2002; Pironti et al., 2021; Varjani and Sudha, 2020).Plastics, in particular, can be dangerous to aquatic life and human health as, upon breakdown in seawater, they do not only tend to release estrogenic EDs, but also microplastics (Kedzierski et al., 2018). These microplastics (\n<\n5\u00a0mm in size) can adsorb EDs like polycyclic aromatic hydrocarbons (PAHs) and can transport them around the world through ocean currents (Kedzierski et al., 2018). PAHs are known to desorb faster in the guts of aquatic animals than in seawater and can easily enter the food chain (Kedzierski et al., 2018).Personal Care Products (PCPs) include toiletries, cosmetics, perfumes etc., and they typically get released into the ecosystem through WWTPs (Montes-Grajales et al., 2017a). This constitutes a global problem. A study, conducted in several countries, showed evidence of the presence of PCPs in municipal wastewater, soil, and drinking water supplies with Spain and the United States reporting the largest numbers of PCPs with 42 and 36 compounds, respectively (Hao et al., 2019; Montes-Grajales et al., 2017b). Table\u00a02 summarizes the maximum and minimum concentrations of different PCPs found in the surface waters of various countries. As can be seen Spain reports the presence of most of the PCPs mentioned. Moreover, it is clear that even the maximum concentration of each PCP is still considered to be low as it is in a nanoscale. While several PCPs are short-lived, their consistent release into the environment can pose serious risks to human and aquatic life (Hao et al., 2019), due to their bioactive nature and endocrine disrupting effects (Montes-Grajales et al., 2017a). For instance, compounds found in sunscreens that filter UV radiation are known to act similar to estrogen, and those are some of the most common PCP contaminants found in water (Gogoi et al., 2018b). This suggests that even at low concentrations PCPs can pose serious risks to human health and the environment. Moreover, PCPs such as galaxolide (HHCB) were found in wastewater effluents in 10 countries with concentrations ranging from 0.14 to 108,000 ng L\u22121 (Montes-Grajales et al., 2017b). HHCBs at high concentrations tend to pose toxicological risks to green algae, fish and daphnids (Montes-Grajales et al., 2017b). Tonalide (AHTN) is another PCP found in the wastewater effluents of 16 countries at concentrations ranging from 0.05 to 7555 ng L\u22121 (Montes-Grajales et al., 2017b). It also poses health risks to aquatic life.\n\nDyes are complex aromatic compounds that are used in numerous industries like, paint manufacturing, printing paper, textiles, cosmetics, pharmaceutical, food, etc. (Chiong et al., 2016). Since natural dyes tend to degrade quickly, synthetic dyes have replaced them in almost all instances (Chiong et al., 2016). Of these, azo dyes (Chiong et al., 2016) and sulfur dyes (Nguyen et al., 2016) are among the most common while sulfur dyes are also very cheap and are thus used extensively (Nguyen et al., 2016). In addition, Table\u00a03 mentions the types of dyes along with their industrial applications. It is obvious that dyes are mainly used in textile industry as shown in Table\u00a03. The wide use of dyes; however, is associated with environmental problems. The textile industry is a major source of dyes released into the environment. In fact, it is responsible for 20% of all industrial water pollution (Tkaczyk et al., 2020). Not only are synthetic dyes resistant to breakdown in WWTPs, but they are also known to be carcinogens (Rani et al., 2017), as are their transformation products (Chiong et al., 2016). Furthermore, dyes change the color of water, thereby, affecting the amount of sunlight that can penetrate the water\u2019s surface (Chiong et al., 2016). This reduces the photosynthetic ability of underwater flora, decreasing the amount of oxygen dissolved in water, which adversely affects aquatic life (Chiong et al., 2016).\n\n\nFig.\u00a01 depicts the way in which MPs may enter drinking water supplies from wastewater. Occurrence of emerging MPs in WWTP effluents is a global problem. Though, the concentrations and variety of MPs are specific to each place and dependent on the annual consumption trends of MPs by the population. A review study on the presence of MPs in 14 countries, from Europe, North America, and Far East Asia, reported that the most common source of MPs in potable water is treated sewage water that is released into surface water from WWTPs (Jiang et al., 2013). Furthermore, a study conducted on the Yangtze River Estuary in China identified high concentrations of pharmaceutical compounds in the area where a sewage treatment plant discharge treated water, indicating inefficiency of the plant at removing pharmaceuticals (Yang et al., 2011).\nAnother study examined the presence of EDs in Anzali Wetland in Iran, and found high concentrations of 4-nonylphenol, octylphenol, and bisphenol A (BPA) in the area (Mortazavi et al., 2012). The investigators attributed the presence of these EDs to industrial waste discharge, and WWTP effluents (Mortazavi et al., 2012). Similarly, a survey conducted in Canada by the Ontario Ministry of the Environment over 16 months concluded that several of the MPs considered, including pharmaceuticals and EDs, were present in drinking water, with carbamazepine, gemfibrozil, ibuprofen, and BPA being detected most frequently (Kleywegt et al., 2011). Furthermore, a comprehensive review of emerging MPs in India reported extremely high concentrations of pharmaceuticals in wastewater. The concentrations of fluconazole (a fungicide) and ciprofloxacin were noted to be 236,950 \n\u03bc\ng L\u22121 and 31,000 \n\u03bc\ng L\u22121, respectively (Philip et al., 2018). In addition, samples of drinking water from Indian rivers contained antibiotic resistant genes (Philip et al., 2018). According to the aforementioned study, inefficiency of sewage treatment plants for processing pharmaceutical waste was a major reason for the high concentrations of pharmaceuticals in drinking water (Philip et al., 2018).Studies analyzing the concentrations of MPs in WWTP influents and effluents in two different regions of Spain noted the inefficiency of WWTPs for removing MPs after observing a slight difference in the concentrations of MPs in and out of WWTPs (Fern\u00e1ndez-L\u00f3pez et al., 2016; Rodil et al., 2012). Another study, which focused on surface waters near agricultural lands in North-East Denmark found pharmaceuticals, PCPs, and EDs with concentrations of up to 1476 ng/L (Matamoros et al., 2012). Similar to the aforementioned study, concentrations of MPs in Central Greece (Papageorgiou et al., 2016) and Berlin, Germany (Pal et al., 2014) were also found to be high due to the inefficiency of WWTPs to remove them.In contrast, low, yet significant (ng L\u22121), concentrations of MPs (pharmaceutical and PCPs) were detected in drinking water in Milan (Riva et al., 2018). As per the study, the low concentrations are the result of drinking water being sourced from groundwater present deep within the earth (Riva et al., 2018). Similar to Milan, drinking water in Singapore is largely protected from MPs, as WWTP effluents are not discharged into surface water (Xu et al., 2011; You et al., 2015). Even so, MPs have been detected in urban surface waters in Singapore, albeit in low concentrations (Xu et al., 2011; You et al., 2015). Nevertheless, You et\u00a0al. found that BPA concentration exceeded its Predicted No-Effect Concentration (You et al., 2015).\nMoreover, it is clear that current treatment plants are not efficient in the removal of certain micropollutants such as pharmaceuticals, EDs, etc. as seen in Fig.\u00a02 (Joseph et al., 2019). This is mostly due to low concentrations which makes this treatment a low priority. Another reason could be the sudden emergence of such micropollutants. Therefore, conventional treatment plants are not equipped to treat and remove the micropollutants. However, as is evident, MPs are ubiquitous. As such, there is an urgent need for regulations on the concentrations of MPs that can be discharged into surface water, as well as, for development of techniques to efficiently remove MPs in WWTPs. For this reason, the EU Water Framework Directive has recently updated its Watch List, which already contained some emerging MPs, to include several pharmaceutical compounds (European Commission, 2020). Additionally, the US EPA added various pharmaceutical compounds and EDs to its Drinking Water Contaminant Candidate List 4, published in 2016 (Drinking Water Contaminants Candidate List 4, 2016). While these lists do not pose legal ramifications for release of MPs into surface waters, they help to emphasize the importance of controlling these substances in WWTP effluents.As demonstrated previously, current WWTPs lack the ability to remove emerging MPs efficiently from wastewater. Hence, new techniques are under development that can be incorporated into the wastewater treatment process in order to improve WWTPs\u2019 removal efficiency. The main techniques currently under study are discussed in the subsequent discussion. The efficiencies of some MP removal techniques applied in WWTPs are summarized in Table\u00a04. It can be observed that the efficiency of the removal technique varies depending on the micropollutant. For example, in the removal of Ciprofloxacin, membrane bioreactor has a removal efficiency of 92.1% whereas primary and secondary activated sludge treatment has an efficiency of 42.4%.\nBecause there are so many different types of MPs that need to be considered, the removal method that is applicable to a broad range of MPs would be the most suitable. Moreover, the removal technique must not require high capital and operating costs and needs to be easily integrated into current WWTPs. Furthermore, health and safety concerns must be considered before employing these techniques. Each of the techniques described in this review will be evaluated based on the aforementioned parameters.Membranes selectively restrict the flow of chemical species, and in doing so, they filter out unwanted compounds. Here, filtration is based on: (1) the pore size of the membrane, (2) the adsorption onto the surface of the membrane, or (3) the repulsion from a charged membrane (Luo et al., 2014; Silva et al., 2017). Because MPs are generally much smaller than conventional membrane pore sizes, they cannot be filtered by currently available techniques of microfiltration and ultrafiltration (Luo et al., 2014). This is because the pore size of the membrane is much bigger than the size of the MPs. Furthermore, the diversity in physicochemical properties of MPs does not allow for a single type of membrane to remove multiple MPs. For this reason, there has lately been much emphasis on combining membrane technology with other MP removal techniques in an integrated system, which, reportedly, are more efficient than the individual removal methods (Silva et al., 2017). Two of these systems are discussed below.Membrane bioreactors (MBRs) have gained traction as alternatives to conventional activated sludge treatment over the last couple of decades, with plants set up in China, the USA, and the EU (Park et al., 2017). MBRs can be used to remove various MPs but are most widely employed for pharmaceuticals and PCPs. They act as physical barriers for MPs, as well as sites for degradation of MPs through photo transformation or biodegradation (Besha et al., 2017; Goswami et al., 2018). The main advantages of MBRs over other MP removal methods are their capability to remove a wide variety of MPs from wastewater more efficiently than other biological treatment techniques and their scalability which enables them to be adapted to the size of the WWTP (Rodriguez-Narvaez et al., 2017). The ability of MBRs to degrade the microplastics makes it much easier for removal and can even reduce the toxicity of the MPs. However, it has been reported that MBRs are not well-suited for eliminating MPs that exhibit low biodegradability (Rodriguez-Narvaez et al., 2017), possess high degree of branching or saturation, and contain sulfate and halogen groups (Bui et al., 2016). For instance, in addition, the high energy requirement of MBR drives up its operating cost, which is much higher than the cost of existing wastewater treatment technologies (Besha et al., 2017; Goswami et al., 2018), even though the cost of labor associated with MBR is low due to the process being highly automated (Besha et al., 2017). Therefore, in using MBR there seems to be a tradeoff between efficiency and operating cost.As noted previously, MBRs are not adept at removing chlorinated MPs due to their low biodegradability. Therefore, other techniques are needed to eliminate these as chlorinated MPs are known to be highly toxic and have strong likelihood of environmental persistence and bioaccumulation (Nieto-Sandoval et al., 2019). For such species, degradation is generally achieved through oxidation using ozone or other strong oxidizing species, like hydroxyl (\n\n\n\n\u2022\n\n\nOH) or sulfate radicals (Lee et al., 2019). Due to the highly reactive nature of the catalysts being involved, it is important to use materials possessing high chemical and physical stability to construct the membranes, instead of the usual polymers (Lee et al., 2019; Wang et al., 2020a). For this reason, ceramic membranes are preferred as catalyst support, as they are able to withstand high mechanical strain and oxidation (Lee et al., 2019), which allows them to have a long lifetime (Wang et al., 2020a). Additionally, due to the catalysts embedded within its pores, CCMs are less susceptible to membrane fouling (Lee et al., 2019). This makes CCMs particularly advantageous over MBRs, as the latter process has a very high degree of membrane fouling especially in the presence of highly reactive species such as the hydroxyl radical. In fact, about 85% of the total energy required for MBR is used to reduce fouling (Bui et al., 2016).Despite the tremendous advancement in the membrane filtration technology for water reuse, yet the commercially available state-of-the-art membrane technologies including polyamide reverse osmosis (RO) and nanofiltration (NF) membranes suffer a critical deficit due to their insufficient rejection (below 50%) of some toxic organic micropollutants (OMPs) (Guo et al., 2022) Consequently, several studies focused on tailoring and modifying the chemistry and structure of polyamide membranes for enhanced removal and rejection of OMPs. Various parameters have been reported to influence the removal of OMPs such as operating conditions, membrane fouling, and characteristics of OMPs, media/solute and membrane (Ojajuni et al., 2015; Wu et al., 2022a,b). The most important factor in achieving an optimal removal/rejection efficiency of OMPs is through member selectivity enhancement which could be attained by one of these techniques: (1) surface modification, (2) membrane nanoarchitecture, and (3) alternative membrane chemistry. Membrane surface plays a significant role in the rejection of OMPs, therefore membrane surface modification through coating and grafting is among one of the most promising techniques to enhance OMPs rejection. Huang et al. (2021) investigated the use of hydrophilic polydopamine (PDA) as a model coating of NF90 membrane for the transmission of 34 OMPs. The PDA-coated NF90 membrane has shown a reduced transmission of MOPs up to \n>\n70%. Another study by Zhu et al. (2022) demonstrated the in situ grafting of ferric ion and tannic networks onto the polyamide membrane. Furthermore, the grafting layer was implemented into commercial NF 270 membrane. It was found that the proposed grafted membrane has achieved much superior chlorine resistance (ratio of salt rejection decline 7.4%) than pristine membrane (ratio of salt rejection decline 26.9%). In addition, the grafted membrane has shown enhanced hydrophilicity, smaller pores size and decreased negative charge. Compared to the modification performed on the surface of the membrane, controlling the interfacial polyamide (IP) reaction conditions could directly control the properties of the membrane nanoarchitecture and thus affect its structural and physiochemical properties including roughness, pore size and distribution, and interior voids/channels (Guo et al., 2022; Liu et al., 2022). For example, vaporization of the organic solvent during the exothermic interfacial polymerization process has contributed to the formation of nanovoids which in turn let to large size polyamide thin film voids and higher membrane water permeability (Peng et al., 2021). Moreover, the addition of secondary monomers such as zwitterions, bipiperidine (BP), 3,5-diaminobenzoic acid (BA), or melamine could also influence the IP reaction and enhance the OMPs rejections. For example, Guo et al. (2020) prepared a grafted zwitterionic membrane with efficient separation ability toward monovalent salt/antibiotics. The grafted zwitterionic membrane exhibited optimal permeability of 14.6 L m\u22122 h\u22121. Bar \u22121 and high rejection of organics. Lastly, due to the advancement in nanotechnology and development of novel materials for water treatment an alternative strategy to tailor and enhance membranes performance is through synthesis of next generation of high-performance membranes, by incorporating novel materials including carbon nanotube (CNT), graphene oxide (GO), covalent organic frameworks (COFs) and metal organic frameworks (MOFs), while replacing current polyamide-based membranes (Guo et al., 2022). In a recent study, Zhu et al. (2022) reported high performance GO-based membrane modified with the phytic acid which exhibited a high-water flux of 6.31 L m\u22122 h\u22121 bar\u22121 under ultralow pressure nanofiltration condition. In addition, it was found that the new GO-based membrane is also capable of rejecting difference charged dye with rejection rate higher than 99.88%. In addition, Liu et al. (2019) successfully modified a commercial cellulose acetate membrane support by integrating nanocomposite MOF-HKUST-1 reduced graphene oxide (GO) incorporating polydopamine (PDA) on the surface. The prepared PDA/RGO/HKUST-1 membrane was tested for the removal of methylene blue and Congo red with removal rate of 99.8% and 89.2%, respectively. Furthermore, the PDA/RGO/HKUST-1 membrane exhibited superior performance of 33-fold increase in the dye flux compared to PDA/RGO membrane. Despite the potential reward of the aforementioned studies considering enhance selectivity and removal of OMPs, the lack of a deep understanding of the rejection mechanism guarantees continued research in this area.Advanced oxidation processes and MBRs have significant drawbacks in the form of harmful oxidation by-products and high energy demand (Sher et al., 2021). For this reason, other methods of MP removal are being studied. One such technique is adsorption of MPs using activated carbon (AC), which is widely considered to be a highly efficient method (Ruiz-Rosas et al., 2019; Sher et al., 2021). AC has significant advantages such as high tunability of physical and chemical properties, which allows for adsorption of a wide variety of MPs, fewer by-products, and low cost (Rodriguez-Narvaez et al., 2017; Ruiz-Rosas et al., 2019; Sher et al., 2021). However, because AC is a category of highly porous carbon-based materials, the production of AC was originally heavily dependent on coal, which makes the process expensive and unsustainable (Ouyang et al., 2020). In addition, AC usually requires to be regenerated which can add to the overall cost of the process. As a result, much emphasis is currently put on generating AC using biomass from agricultural and municipal waste (Chen et al., 2020a; Ouyang et al., 2020; Ruiz-Rosas et al., 2019). This is known as biochar. While biochar is considered a cheap and renewable material that can be used to efficiently remove MPs from wastewater (Ruiz-Rosas et al., 2019), its production is energy-intensive as it requires biomass to be heated at high temperatures for a long time (Chen et al., 2020a; Ouyang et al., 2020). Another disadvantage of using AC is that it increases turbidity of WWTP effluent, so additional measures (i.e.,\u00a0membrane filtration, sedimentation, etc.) must be taken to remove AC from water (He et al., 2016; Kumar et al., 2016). Such incorporation of other techniques would decrease the turbidity of WWTPs and increase the overall cost. In addition, membrane filtration can be used to remove MPs. Therefore, the use of AC is not feasible in terms of cost and efficiency.Recently, catalysts have garnered much interest as viable materials for the degradation of MPs. Particular attention is made on catalyst-assisted photodegradation, as photocatalysts are known to make the natural photolysis of MPs and their by-products by sunlight more efficient (He et al., 2016; Kumar et al., 2016). Furthermore, among the numerous photocatalysts that have been studied for this application, titanium dioxide (TiO2) constitutes one of the most promising candidates because it is cheap, highly efficient, physically and chemically stable, and is non-corrosive (Dong et al., 2015). It can also be used to degrade a wide range of MPs (Dong et al., 2015). Nevertheless, commercialization of TiO2 is limited by its poor absorption of the full spectrum of sunlight. TiO2 absorbs light in the UV region, which makes up less than 5% of sunlight (Dong et al., 2015). Therefore, TiO2 would need to be manipulated further before it can be used to degrade MPs efficiently under sunlight. Table\u00a05 shows some examples of photocatalysts along with their structure and band gap. NiFe2O\n\n\n\n4\n\n\n possesses the smallest band gap which implies higher intrinsic conduction making it the most efficient photocatalyst mentioned.Extensive research has been conducted on enzymes for degradation of MPs. One clear advantage of these over other catalysts is that enzymes are biological materials, and hence pose little to no danger to the environment (Shakerian et al., 2020). Moreover, enzymes require ambient conditions of temperature and pressure to work efficiently, which would make the process highly energy efficient (Shakerian et al., 2020). Two enzymes that have been heavily researched are laccase and horseradish peroxidase, as these can be easily extracted and used to degrade pharmaceuticals, PCPs, Eds and dyes (Bilal et al., 2016; Kadam et al., 2018; Kashefi et al., 2019; Li et al., 2017b; Nadaroglu et al., 2019; Zhou et al., 2021). However, most enzymes target specific chemical species only, therefore, a significant work needs to be done on identifying other enzymes that are as versatile as laccase and horseradish peroxidase (Stadlmair et al., 2018). Another limitation of enzymes is that they need to be immobilized on a substrate as the activity of free enzymes is significantly lower, comparatively (Bilal et al., 2016; Kadam et al., 2018; Kashefi et al., 2019; Li et al., 2017b; Nadaroglu et al., 2019; Zhou et al., 2021). Table\u00a06 compares the different MP removal methods by listing some advantages and disadvantages of each method.\n\n\nBiochar is a carbon rich material produced by the pyrolysis of biomass (Santos et al., 2019). Biochar can be used in the removal of contaminants from water due to its outstanding properties such as high surface area, high porosity, presence of functional surface groups, excellent ion exchange ability, and high stability (Qiu et al., 2021). As a result, the main mechanism for removal is biosorption. Biochar can be incorporated with other materials to enhance adsorption capacity and visible light absorption, thus increasing the removal efficiency of materials (Qiu et al., 2021). For instance, through adsorption and photocatalytic reduction heavy metals, are removed from water using biochar-based materials (Qiu et al., 2021). Moreover, biochar-based materials can also remove organic pollutants through the same mechanism (Qiu et al., 2021).Biochar can also be used as a catalyst for the removal of micropollutants. For instance, Song et al. (2017) used wheat straw as a catalyst for the dechlorination of hexachlorobenzene (Song et al., 2017). The presence of chlorine in many compounds is the main contributor to its toxicity. The more chlorine present, the more toxic a compound is likely to be. Therefore, the dichlorination of hexachlorobenzene will result in a less toxic compound. The study by Song et al. (2017) showed a removal efficiency up to 56% and the main driving force was carbon centered plug flow reactors. This low removal efficiency suggests that the use of biochar, specifically wheat straw, or the driving force is inappropriate. Another study conducted by Yavari et al. (2019) used rice husk for the degradation of imazapic and imazapyr herbicides. The results demonstrated that the use of rice husk has enabled to decrease the half-life of imazapic from 40.7 days to 25.6 days and that of imazapyr decreased from 46.2 days to 26.5 days (Yavari et al., 2019). This demonstrates the effectiveness of rice husk in reducing the overall time over which the herbicides remain present in the environment by about 50%. On the other hand, the extensive use of biochar in water applications is limited due to the association of toxic elements such as heavy metals, metalloids, and polycyclic aromatic hydrocarbons with biochar (Tan et al., 2015).MXenes belong to a family of two-dimensional nanomaterials that have the formula \n\n\nM\n\n\nn+1\n\n\nX\n\n\n\nn\n\n\nT\n\n\n\nx\n\n\n (n \n=\n 1\u20133) (Yu et al., 2022). M denotes transition metals such as Ti, Zr, Hf, X denotes carbon and/or nitrogen, and T denotes surface terminated groups such as F, OH, O (Rafieerad et al., 2021). Titanium based MXenes are the most promising for environmental applications due to the abundance of the element and the production on non-toxic byproducts during degradation (Chen et al., 2021; Hermawan et al., 2021). Specifically, Ti3C2T\n\n\n\nx\n\n\n constitutes a good candidate because of its intrinsic properties such as abundant functional groups and large surface area, outstanding metallic conductivity, and the reactivity of the terminal metal sites (Yu et al., 2022). Moreover, MXenes can be used in the removal of heavy metals as it can effectively capture copper, lead, mercury, and chromium. This is achieved through the reaction of the heavy metal with the surface groups of the MXenes (Yu et al., 2022). MXenes can also remove organic contaminants present in water such as dyes, aromatic compounds, and pharmaceuticals through photocatalytic degradation (Kim et al., 2021).A study by Tu et al. (2022) investigated the efficiency of Carbon nitride coupled with Ti3C2-Mxene derived amorphous titanium (Ti)-peroxo heterojunction in the degradation of RhB and tetracycline (Cao et al., 2020a). The results revealed that the degradation efficiency of RhB reached 97.2% while the degradation efficiency of tetracycline was 86.3% under visible light within 60\u00a0min (Cao et al., 2020a). Moreover, Shahzad et al. (2018) studied another Maxine based catalyst (TiO2/Ti3C2T\n\n\n\nx\n\n\n) for the degradation of carbamazepine (Tu et al., 2022). The catalyst demonstrated a high removal efficiency of 98.7% under UV light. The main radicals associated with the degradation of carbamazepine were found to be \n\u2022\nHO and \n\u2022\nO2\n (Shahzad et al., 2018). These results indicated that MXene-based-catalysts are quite promising in the degradation of micropollutants. However, MXenes can produce TiO2 in water which presents a challenge in water purification. In addition, oxidized MXenes can release adsorbed contaminants causing secondary pollution (Yu et al., 2022).Nanoscale zero valent iron consists of an iron (0) core and iron oxide layers (Li et al., 2021). Due to its structure and composition, it possesses the ability to adsorb contaminants or transform them via oxidation or reduction (Li et al., 2021). For instance, NZVI was able to remove organic contaminants, nitro-aromatic compounds, inorganic contaminants, heavy metal ions, and radionuclides by adsorption or reduction techniques (Choi and Lee, 2012; Gu et al., 2010; Khalil et al., 2016; Li et al., 2016a, 2019b; Qiu et al., 2018).\nGao et al. (2020) synthesized a sulfided nanoscale zero-valent iron catalyst supported by biochar for the removal of ciprofloxacin (CIP) (Gao et al., 2020). The study showed that the catalyst had a removal efficiency of 89.8%. Furthermore, the radicals \n\u2022\nHO and SO\n\n\n\n\n4\n\n\n\n\n\n\u2212\n\n\n\n\n\n\u2022\n played a major role in the degradation of CIP (Gao et al., 2020). Jia et al. (2019) developed a graphene-like sheet supported nZVI for the degradation of atrazine (Jiang et al., 2021). The results demonstrated that the catalyst rapidly degraded atracine and achieved a removal efficiency of 97.2% within 2\u00a0min (Jiang et al., 2021). The main degradation pathways were found to be dichlorination, dealkylanation, and alkyl oxidation. SO\n\n\n\n\n4\n\n\n\n\n\n\u2212\n\n\n\n\n\n\u2022\n was also observed to play a major role in the degradation. These studies showed that when nZVI is combined with other materials such as biochar and graphene, it can achieve higher removal efficiencies. In addition, it seems that SO\n\n\n\n\n4\n\n\n\n\n\n\u2212\n\n\n\n\n\n\u2022\n plays a major role in both studies and this indicates that this radical may be a common amongst nZVI catalysts. Regardless of its promising applications in the removal of contaminants from water, NZVI can be toxic to aquatic organisms and can harm human cells (Laurier et al., 2013). Therefore, extreme care must be taken when using NZVI in the remediation of water in order to prevent human and animal exposure.MOFs are highly porous frameworks made up of inorganic clusters linked together through organic species (Jiang et al., 2018). Furthermore, their chemical properties can be easily tailored to the application requirements by selecting the organic and inorganic species most suited for the application (Jiang et al., 2018). This outstanding property of MOFs is further illustrated in Table\u00a06 and Fig.\u00a03 where several MOFs are compared based on their metallic cluster, organic linker and surface area. For example, UiO-67 and NU-100 MOFs are two water stable Zirconium-based metal organic frameworks, yet they possess different structures and surface areas due to the difference in the organic linker. In addition, Table\u00a07 shows that MOFs possess very high surface areas as the highest surface area mention is Cr-MIL-101 (3360\u00a0m2 g\u22121) and the lowest surface area mentioned is HKUST-1 (1800\u00a0m2 g\u22121). Owing to this versatility, there has lately been great interest in developing MOFs that can be used for photocatalysis of MPs (Dias and Petit, 2016). Because MOFs can be specialized for an application, they can be made to absorb light over a large range of wavelengths, thus ensuring greater performance compared to other catalysts, such as TiO2\n (Dias and Petit, 2016). Nevertheless, some leakage of the inorganic and organic species from MOFs has been noted in research conducted on MOFs for wastewater treatment (Jiang et al., 2018). Since MOFs constitute a relatively new class of materials, they need to be studied further to assess their potential environmental impact before they can be commercialized (Dias and Petit, 2016). We summarize in Table\u00a08 the main advantages and disadvantages of MOFs for photocatalytic degradation.\n\n\nAnother class of MOF catalysts is biomimetic MOFs which are inspired by photosynthesis of microorganisms and plants (Wu et al., 2022b). Biomimetic MOFs can be used to degrade micropollutants in water. For instance, Wu et al. (2022a, b) developed a three-dimensional coral zirconium-based metal organic framework (Zr-TCPP-bpydc) via a double-ligand strategy for the degradation of tetracycline (TC) and ofloxacin (OFX) (Wu et al., 2022b). The catalyst was synthesized using a solvothermal method. Characterization studies showed that Zr-TCPP-bpydc has a pore volume of 0.307\u00a0cm3 g\u22121 and a surface area of 228\u00a0m2 g\u22121 which indicates the presence of mesopores and micropores (Wu et al., 2022b). Moreover, Zr-TCPP-bpydc showed high adsorption under ultraviolet and visible light range. Regarding the degradation of TC and OFX, the catalyst demonstrated an almost complete degradation of both pollutants after 120\u00a0min in the dark. This resulted in a removal efficiency of 98% for TC and 97% for OFX. The main radicals associated with the degradation of TC and OFX are \n\u2022\nHO and \n\u2022\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n\n (Wu et al., 2022b). We report in Table\u00a09 a set of biomimetic MOFs used for degradation of micropollutants along with their main characteristics.\nEnzymes can be used as a catalyst to convert micropollutants present in water to fewer toxic substances (Zdarta et al., 2022). Properties such as high activity and selectivity as well as quick catalytic reaction make enzyme a viable option for biological treatment. Enzymes can be used as free enzymes or as immobilized biocatalysts (Zdarta et al., 2022). Metal organic framework composites can be used as carriers in the immobilization of enzymes as it shows high efficiency on enzyme binding and high stability. However, the binding of enzymes to MOFs reduces its enzyme activity (Zdarta et al., 2022).\nLi et al. (2020) conducted a study on the removal of dyes using immobilized enzymes on MOFs (Li et al., 2020). ZIF-8 was selected as the carrier for horseradish peroxidase (HRP). However, due to the micropore range of the metal organic framework, a single layer microporous (SOM-ZIF-8) was synthesized and used. SOM-ZIF-8 enhanced the mass diffusion, stability, and recyclability of the composite. Furthermore, the composite was formed by encapsulating the enzyme in SOM-ZIF-8 using post-synthetic immobilization. Characterization results revealed that the surface area of the composite was 1350\u00a0m2 g\u22121 while the pore volume was 0.90\u00a0cm3 g\u22121 (Li et al., 2020). The composite displayed a detection limit of 0.48 \n\u03bc\nM indicating that the composite can be used as a one-step indicator. Regarding dye removal, HRP@SOM-ZIF-8 was able to rapidly degrade Congo red and rhodamine blue after 2\u00a0min resulting in high removal efficiencies (Li et al., 2020).Another study by Jia et al. (2019) investigated enzyme immobilized onto MIL-53(Al) for the catalytic conversion of triclosan (TCA) (Jia et al., 2019). The enzyme used in the composite is laccase which was encapsulated within MIL-53(Al). Mesoporous MIL-53 was specifically used due to its high water and chemical stability. The surface area of Lac-MIL-53(Al) was found to be 1030\u00a0m2 g\u22121 with a pore size of 4\u00a0nm (Jia et al., 2019). Moreover, the tests, conducted to determine the efficiency of the composite in the removal of TCA, revealed a removal efficiency of 99.24% within 120\u00a0min. Of interest, the main mechanism for the removal of TCA by the composite was adsorption combined with oxidation as the surface area decreased from 1030\u00a0m2 g\u22121 to 268\u00a0m2 g\u22121 (Jia et al., 2019). Table\u00a010 reports other Enzyme immobilized on MOFs used in various applications related to micropollutants degradation and removal. Table\u00a010 shows that enzyme-immobilized MOFs can achieve up to 100% removal efficiency and can even enhance the properties of MOFs such as stability.\nThe Fenton reaction is an efficient oxidation method as it produces highly reactive hydroxyl radicals that can degrade most organic compounds present in wastewater (Gao et al., 2017a). However, Fe2\uff0b and Fe3\uff0b are hard to recover from the system. A solution to this problem is to immobilize them in porous materials such as MOFs. The typical Fenton reaction is as follows where reaction (3) shows the decomposition of the organic pollutant via hydrogen abstraction and reaction (4) shows the decomposition of the organic pollutant via hydroxyl addition (Cheng et al., 2018): \n\n\n(1)\n\n\n\n\nFe\n\n\n2\n+\n\n\n+\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n2\n\n\n\u27f9\n\n\nFe\n\n\n3\n+\n\n\n+\n\n\nHO\n\n\n\u2212\n\n\n+\n\u2022\nHO\n\n\n\n\n(2)\n\n\n\n\nFe\n\n\n3\n+\n\n\n+\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n2\n\n\n\u21c6\n\n\nFe\n\n\n2\n+\n\n\n+\n\n\nH\n\n\n+\n\n\n+\n\u2022\n\n\nHO\n\n\n2\n\n\n\n\n\n\n(3)\n\n\nRH\n+\n\u2022\nHO\n\u27f9\n\n\nH\n\n\n2\n\n\nO\n+\nR\n\u2022\n\u27f9\nfurther\n\noxidation\n\n\n\n\n(4)\n\n\nR\n+\n\u2022\nHO\n\u27f9\n\u2022\nROH\n\u27f9\nfurther\n\noxidation\n\n\n\n\n For instance, Gao et al. (2017a, b) synthesized MIL-88B-Fe for the degradation of organic pollutants. The catalyst consists of open iron sites which are filled by non-bridging ligands (Gao et al., 2017a). \n\n\nH\n\n\n2\n\n\nO2 can then displace these ligands as it is absorbed into the catalyst. Furthermore, characterization tests of MIL-88B-Fe were conducted and showed that the surface area is 165.4\u00a0m2 g\u22121 and the pore volume is 0.2\u00a0cm3 g\u22121 (Gao et al., 2017a). TGA measurements revealed good thermal stability of the catalyst up to 350\u00a0\u00b0C. Next, the efficiency of phenol removal by the catalyst was tested by first employing an \n\n\nH\n\n\n2\n\n\nO2 oxidation process without MIL-88B-Fe. The results indicated that phenol barely degraded. However, adding MIL-88B-Fe and \n\n\nH\n\n\n2\n\n\nO2 showed 99% removal efficiency (Gao et al., 2017a). Increasing the dosage of the catalyst from 0.1\u00a0g L\u22121 to 0.2\u00a0g L\u22121 reduced the removal efficiency to 98% due to the agglomeration of the catalyst. Lastly, \n\u2022\nHO was found to be the main radical involved in the oxidation of phenol (Gao et al., 2017a).A study conducted by Wang et al. (2020a, b, c, d) investigated the enhancement of Fenton catalytic performance by the addition of NH2-MIL88B(Fe) for the degradation of acetamiprid (ACTM) (Wang et al., 2020d). In this study, the MOF was enhanced to achieve photo-induced electron and Fenton-generated radicals. NH2-MIL88B(Fe) consists of various ligand defects; therefore, benzoic acid (Bac), pyrrole (Py), pyrrole-2-carboxylic acid (Pca) were in-situ engineered into the MOF (Wang et al., 2020d). The effect of each ligand and MOF was then tested. Characterization tests showed that the surface area and pore volume of the MOFs increased in the following order: MIL88(Fe) \n<\n Bac-MIL88(Fe) \n<\n\n Py-MIL88(Fe)\n<\n Pca-MIL88(Fe) (Wang et al., 2020d). Regarding ACTM adsorption, Pca-MIL88(Fe) showed the highest uptake. Pca-MIL88(Fe) also demonstrated the highest catalytic activity as it completely degraded ACTM within 40\u00a0min. \n\u2022\nHO is the main radical involved in the oxidation of ACTM for all MOFs (Wang et al., 2020d). Table\u00a011 summarizes other Fenton-like MOFs used in various applications and reports their main capabilities in terms of removal efficiencies and other characteristics.\n\nMOFs lend themselves very well to catalytic applications due to their large surface areas and uniform porous structures (Guo et al., 2017; Xiang et al., 2017). In order to take advantage of these features, other chemical species may be added to MOFs to improve their chemical and thermal stability and enhance some physicochemical properties like electrical conductivity and magnetism (Chen et al., 2019a; Xiang et al., 2017). At the same time, MOFs may help stabilize a catalyst and protect it from harsh environments (Zhong et al., 2019), promote even distribution of chemical species due to their uniform crystal structure, and provide many active sites for catalytic activity (Xiang et al., 2017). As a result, MOF-composite-based catalysts often exhibit higher efficiency than free catalysts.\nLiu et al. (2022) reported the use of some MOF composites in the adsorption of heavy metals. For example, Zr-DMBD and UiO-66-(SH)2 are employed in the adsorption of Hg\n\n\n\n2\n+\n\n\n and were found to have a removal efficiency of 99% (Liu et al., 2022). Moreover, they were also used in the removal of dyes from wastewater. To elaborate, MIL-101 was found to remove reactive black 5 with an efficiency of 99.9%, while Co-MOF removed Congo red with a removal efficiency of 99.4% (Davoodi et al., 2021; Karmakar et al., 2019). Additionally, MOF composites exhibit high antibacterial removal efficiency. In particular, PCN-124-stu (Cu) was used in the removal of NOR with a removal efficiency of 99.8% and HpZIF-8-10 in the removal of TH with a removal efficiency of 98.6% (Chen et al., 2019b; Jin et al., 2017). These studies demonstrate that MOFs are highly efficient adsorption materials which is mainly due to their surface area and porous structure.Moreover, Zhong et al. (2019) found that immobilizing the enzyme \n\u03b1\n-glucosidase in a Cu-MOF, to be used for screening antidiabetic drugs, not only helped to stabilize the enzyme, but also improved selectivity, sensitivity, and thereby, efficiency by about 4.58 times (Zhong et al., 2019). Similarly, Zhang et al. (2018) observed that Hemin-Au@MOF has higher affinity for alpha-fetoprotein \u2013 a biomarker present in very low levels in human body fluids during early stages of cancer \u2013 than free hemin catalyst (Zhang et al., 2018). This has potential application in early diagnosis and treatment of cancer (Zhang et al., 2018). As previously discussed, enzymes immobilized on MOFs are highly efficient in the degradation of micropollutants as well as sensors in the human body. Moreover, immobilizing the enzyme on MOFs does not only stabilize the MOF but stabilizes the enzyme as well.Another study examined the role of Mn-MOF-74 grown on carbon nanotubes (Mn-MOF-74@CNT) as Li-\n\n\nO\n\n\n2\n\n\n battery electrode material in limiting side reactions during operation, and improving its performance (Zhang et al., 2019a). Furthermore, Chen et al. (2017) showed that electrodes made of Cu-based MOF-199 and graphene oxide composite material can be used to detect the hazardous chemicals, catechol and hydroquinone in the environment, at concentrations as low as 1 \n\u03bc\nM (Chen et al., 2017). Moreover, Zhu et al. (2018) successfully synthesized a composite of Cu-based MOF, MoS2/rGO-MOF, to facilitate a hydrogen evolution reaction (Zhu et al., 2018). The authors attributed the effectiveness of the material to its large surface area and the combined effect of MoS2 and GO-MOF on the reaction (Zhu et al., 2018). The outstanding properties of MOFs make them suitable for a variety of applications and enable them to achieve extremely high efficiencies.One study conducted on desulfurization of oil used a polyoxometalate (POM) catalyst encapsulated in MOF-199, which was constructed in the pores of another MOF, MCM-41, to form the composite: POM@MOF-199@MCM-41 (Li et al., 2016b). The researchers found that the composite material was more effective at desulfurization than its constituents on their own (Li et al., 2016b). This is due to the fact that MOF composites incorporate the properties of the isolated materials and enhances the overall properties. Lastly, several studies have reported the effectiveness of composite materials made of quantum dots and MOFs, as well as TiO2 and MOFs, in applications ranging from hydrogen evolution, carbon dioxide reduction, environmental remediation, etc. (Aguilera-Sigalat and Bradshaw, 2016; Wang et al., 2018, 2020c; Wu et al., 2020).CuBTC@NH2 composite was synthesized by Samuel et al. (2022) for the removal of ibuprofen and acetaminophen (Samuel et al., 2022). First, they found that the composite had an increased surface area from 47\u00a0m2 g\u22121 to 64\u00a0m2 g\u22121 while other properties such as pore volume, average pore diameter, and pore size decreased compared to CuBTC (Samuel et al., 2022). Next, they observed that the degradation efficiency of both pharmaceuticals was around 96% after 300\u00a0min with a maximum adsorption capacity of 187.97 mg g\u22121 for ibuprofen and 125.45 mg g\u22121 for acetaminophen. They also conducted a secondary test to determine the presence of any leaching and found that CuBTC@NH2 composite can be used several times in a treatment process as there was no evidence of further pollution (Samuel et al., 2022).\nNikou et al. (2021) synthesized GO/ZIF-8 composite using the solvothermal method for the simultaneous removal of two pesticides diazinon and chlorpyrifos (Nikou et al., 2021). They found that the adsorption of the pesticides was higher when using GO/ZIF-8 in comparison to using ZIF-8 as seen in Fig.\u00a04. This is due to the enhanced interactions of the carboxylic and hydroxyl graphene oxide groups present in GO/ZIF-8 with the functional groups of the pesticides (Nikou et al., 2021). In addition, the optimum removal efficiency (83% of diazinon and 73% of chlorpyrifos) were found at a pH of 7, adsorbent dosage of 24 mg, and a contact time of 24\u00a0min. The composite also displayed high reusability and can be easily regenerated (Nikou et al., 2021).\nZIF-8/ZnO@ESM an MOF biocomposite was studied by Shahmoradi\u00a0Ghaheh et al. (2021). The composite was fabricated using a green method in which water is used as a solvent instead of methanol. The green method is highly advantageous as it reduces the carbon emissions which is usually associated with the synthesis of MOFs or MOF based composites using solvents other than water. A Brunauer\u2013Emmett\u2013Teller (BET) test was then conducted to determine the surface area of the composite which showed a decrease in the surface area from 1493.5\u00a0m2 g\u22121 ZnO/ZIF-8 to 1428.1\u00a0m2 g\u22121 (Shahmoradi\u00a0Ghaheh et al., 2021). A similar trend was also seen in the pore volume and mean pore diameter. This decrease in surface area, pore volume, and mean pore diameter is due to the presence of ESM in the pores of the MOF. In comparing the removal efficiency of methyl green by ESM and by the composite, it is observed that the removal efficiency increased from 10.66% to 52.11% with a contact time of 90\u00a0min (Shahmoradi\u00a0Ghaheh et al., 2021). ZIF-8/ZnO@ESM was also used in the removal of tetracyline and found a removal efficiency of 50%.\nGhorbani-Choghamarani et al. (2021) considered the use of Fe3O4@GlcA@Ni-MOF composites as a green catalyst for the synthesis of Rhodanine (Ghorbani-Choghamarani et al., 2021). The BET surface area and pore volume were found to be 97\u00a0m2 g\u22121 and 22.25\u00a0cm3 g\u22121, respectively. The composite further exhibited high catalytic activity in the purification of the desired product and demonstrated a high degree of reusability making it an optimal catalyst in the synthesis of Rhodanine (Ghorbani-Choghamarani et al., 2021). Furthermore, when comparing Fe3O4@GlcA@Ni-MOF to other catalysts such as TiO2 nanoparticles and MCM-41, it has been demonstrated that Fe3O4@GlcA@Ni-MOF is helpful from different aspects including the low cost of synthesis, the non-toxicity and stability of the catalyst, and the easy separation of the catalyst from the products (Ghorbani-Choghamarani et al., 2021).Depending on the nature of precursors, MOFs, catalysts, and reaction conditions, there are a few methods that can be applied to obtain MOF-catalyst-based composites. The synthesis technique must be carefully selected, as this can have an effect on the structure of the composite (Chen et al., 2020c) and activity of the catalyst (Xiang et al., 2017). We display in Fig.\u00a05 a schematic depiction of all the synthesis methods mentioned below.The \u201cShip-in-a-bottle\u201d method involves depositing catalyst nanoparticles, or catalyst precursors into the pores of an already formed MOF (Aguilera-Sigalat and Bradshaw, 2016; Chen et al., 2020c; Wu et al., 2020; Xiang et al., 2017) as seen in Fig.\u00a05(a). The nanoparticles may be deposited into the MOF cavities through solution deposition, vapor deposition, (Wu et al., 2020; Xiang et al., 2017), or mechanochemistry (grinding) (Xiang et al., 2017), after which the precursors may undergo transformations inside the pores to form catalysts under heat, light, or chemical stimulation (Chen et al., 2020c). Zheng. et al., demonstrated this approach by synthesizing Pt@DUT-5 (Zhang et al., 2016). It was found that the effectiveness of this method depends on the selection of MOF material. Specifically, MOFs that have big size cavities but small pores such as MIL-101 work best with this method. The main challenge of this approach, however, is to control the deposition of the nanoparticles such that they do not attach to the external surface of the MOF instead of being embedded within the pores (Chen et al., 2020c). Other aspects of interest are the stability of MOF under the conditions required to form the catalyst, the control over where the nanoparticles are deposited within the MOF, and the ability to deposit a sufficient number of nanoparticles (Aguilera-Sigalat and Bradshaw, 2016).The \u201cbottle-around-ship\u201d approach is also referred to as the template synthesis method (Aguilera-Sigalat and Bradshaw, 2016). In this approach and as observed in Fig.\u00a05(b), the catalyst is synthesized first and stabilized in a solvent using surfactants or other materials (Xiang et al., 2017). The role of the surfactant is to prevent agglomeration of catalyst nanoparticles (Wu et al., 2020). MOF precursors are then added to this dispersion, which leads to the MOF being constructed around the nanoparticles (Aguilera-Sigalat and Bradshaw, 2016; Chen et al., 2020c; Xiang et al., 2017). This approach not only ensures encapsulation of catalysts within MOFs instead of on their external surfaces (Xiang et al., 2017), but also avoids any limitations associated with diffusion of nanoparticles deep within the framework (Wu et al., 2020). This method also prevents the nanoparticles from leaching into the water when it is used in certain applications. Additionally, because the catalyst nanoparticles are formed before encapsulation, there is greater control on their size and structure, which can be tailored as per the application\u2019s requirements (Wu et al., 2020; Xiang et al., 2017). Furthermore, Xiao et al. (2016) demonstrated the difference between this approach and the \u201cship-in-a-bottle\u201d approach by synthesizing Pt/UiO-66-NH2 and Pt@UiO-66-NH2, respectively (Xiao et al., 2016). It was observed that Pt@UiO-66-NH2 shortens the electron transport distance which in turn enhances the catalytic activity compared to Pt/UiO-66-NH2. It was also found that Pt@UiO-66-NH\n\n\n\n2\n\n\n does not undergo aggregation and thus leading to better catalytic recyclability (Xiao et al., 2016). As a result, it can be concluded that while the \u201cbottle-around-ship\u201d approach has its own advantages, \u201cship-in-a-bottle\u201d enables superior performance due to better confinement of the nanoparticles.The self-sacrificed template method is similar to the \u201cbottle-around-ship\u201d approach in that the MOF forms around the nanoparticle species. However, Fig.\u00a05(c) shows that in this case the nanoparticles serve as the source of metal ions for the MOF (Chen et al., 2020c). Organic linkers and metal-containing nanoparticles are mixed together, and the metal ions form coordination bonds with organic linkers at the same rate as they are released from the nanoparticles (Chen et al., 2020c). This method allows a high degree of control over the structure and morphology of MOFs, but it limits the choice of nanoparticles that can be encapsulated into the MOFs (Chen et al., 2020c). Table\u00a012 shows the different MOF-catalyst composites synthesized using self-sacrificed template approach along with their respective applications and properties. Table\u00a012 indicates that MOFs synthesized via self-sacrificial templates can be applied in a variety of applications as it produces MOF based composites with different advantageous properties. Table\u00a012 also implies that the application of the MOF based composite relies on its properties\nNanoparticles may be sandwiched between two layers of MOFs as a solution to the problem of adherence of nanoparticles to MOF surface as seen in Fig.\u00a05(d). This can be thought of as employing \u201cship-in-a-bottle\u201d and \u201cbottle-around-ship\u201d approaches sequentially. First, nanoparticles are loaded onto MOF, and then a thin layer of MOF is constructed around the MOF-nanoparticle composite (Chen et al., 2020c). This approach was reported to improve selectivity of the catalyst, but it is very important that the outer layer of MOF be thin enough to allow diffusion of reactants and products in order to not adversely affect catalytic activity (Chen et al., 2020c). In addition, Liu et al. (2017) synthesized TiO2 nanosheets with MIL-100 using this approach and found an increased absorption ability due to an increase in the surface area. This in turn demonstrates the ability of the approach in enhancing the overall photocatalytic performance (Liu et al., 2017).In the one-pot synthesis method, the precursors of nanoparticles and MOF are mixed together for simultaneous synthesis of the two similar to Fig.\u00a05(e), followed by assembly of the composite (Chen et al., 2020c; Xiang et al., 2017). This one-step synthesis strategy is simpler than the ones mentioned previously, nevertheless, the rates of nanoparticle, MOF and composite formation must be carefully balanced in order to achieve the desired product (Chen et al., 2020c; Xiang et al., 2017). For this reason, the choice of reactants and solvents that can be used is limited, and this approach is not applicable to all MOF-catalyst composites (Chen et al., 2020c; Xiang et al., 2017). Chen et al. (2014) used this approach in the synthesis of Pd@MOF composites without the addition of any stabilizing agents. It was observed that the catalysts were stable, reusable, and its performance did not decrease with the amount of time it was used (Chen et al., 2014). Furthermore, this method seems very promising as it reduces production cost and eases scale up by reducing the synthesis method to one step.\n\nMolecular orbitals in semiconductors are arranged in the valence band and conduction band, which are separated by a band gap (Lopes Colpani et al., 2019). Electrons (e\n\n\n\n\u2212\n\n\n) in the valence band can be promoted to the conduction band via light energy if the energy is greater than or equal to the band gap; at the same time, holes (h\n\n\n\n+\n\n\n) are generated in the valence band (Candia-Onfray et al., 2021; Lopes Colpani et al., 2019; Ni and Khan, 2021). Similarly, organic species have the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) that act as the valence band and conduction band, respectively (Ni and Khan, 2021; Zhang et al., 2020a). In MOFs, the organic linkers absorb light energy, and e\n\n\n\n\u2212\n\n\n are promoted from the HOMO to the LUMO and are then transferred to the metal cluster (Zhang et al., 2020a). In this way, the organic linkers in a MOF can be thought of as the valence band and the metallic clusters as the conduction band (Ni and Khan, 2021). The mechanism is similar for MOF-catalyst composites, with the addition of valence and conduction bands of the photocatalyst (Xue et al., 2018).The mechanism can be further elaborated by considering the photocatalyst TiO2. In the presence of light, TiO2 produces electrons in the conduction band (\n\n\n\ne\n\n\ncb\n\n\n\n\n\n\u2212\n\n\n\n) and electron holes in the valence band (\n\n\n\nh\n\n\nvb\n\n\n\n\n\n+\n\n\n\n), as illustrated in Eq.\u00a0(5). The generated holes then reach the surface of TiO2 and react with either absorbed hydroxyl groups or water to produce HO\n\u2022\n radical, as shown in Eq.\u00a0(6). The generated HO\n\u2022\n radical then desorbs from the surface into the bulk of the medium to form free HO\n\u2022\n. If electron donors are present, then the reaction follows Eqs. (7) and (8), and if electron acceptors are present then the reaction follows Eqs. (9), (10), (11), and (8) (Georgaki et al., 2014). A schematic illustration of the overall mechanism can be seen in Fig.\u00a06. \n\n\n(5)\n\n\n\n\nTiO\n\n\n2\n\n\n+\nh\n\u03bd\n\u2192\n\n\nh\n\n\nvb\n\n\n\n\n\n+\n\n\n+\n\n\ne\n\n\ncb\n\n\n\n\n\n\u2212\n\n\n\n\n\n\n(6)\n\n\n\n\nh\n\n\nvb\n\n\n\n\n\n+\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n\u2192\n\n\nH\n\n\n+\n\n\n+\n\u2022\nHO\n\n\n\n\n(7)\n\n\nOxidation\n\nsite\n:\n\n\n\nh\n\n\nvb\n\n\n\n\n\n+\n\n\n+\n\n\nRed\n\n\norg\n\n\n\u2192\n\n\nOx\n\n\norg\n\n\n\n\n\n\n(8)\n\n\n\u2022\nHO\n+\n\n\nRed\n\n\norg\n\n\n\u2192\n\n\nOx\n\n\norg\n\n\n\n\n\n\n(9)\n\n\nReduction\n\nsite\n:\n\n\n\ne\n\n\ncb\n\n\n\n\n\n\u2212\n\n\n+\n\n\nO\n\n\n2ads\n\n\n\u2192\n\u2022\n\n\nO\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n\n\n\n(10)\n\n\n\u2022\n\n\nO\n\n\n2\n\n\n\u2212\n\n\n+\n\n\ne\n\n\ncb\n\n\n\n\n\n\u2212\n\n\n\n(\n+\n2\n\n\nH\n\n\n+\n\n\n)\n\n\u2192\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n2\n\n\n\n\n\n\n(11)\n\n\n\u2022\n\n\nO\n\n\n2\n\n\n\n\n\n\u2212\n\n\n+\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n2\n\n\n\u2192\n\u2022\nHO\n+\n\n\nOH\n\n\n\u2212\n\n\n+\n\n\nO\n\n\n2\n\n\n\n\n\n\n(12)\n\n\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n2\n\n\n+\nh\n\u03bd\n\u2192\n2\n\u2022\nHO\n\n\n\n\n\n\n\nSeveral research groups have investigated the applicability of MOF composite-based catalysts for degradation of MPs. For instance, Chang et al. (2017) successfully synthesized TiO2@MIL-101 through the ship-in-bottle method and studied its ability to degrade methyl orange in solution (Chang et al., 2017). The composite was synthesized from TiO2 precursor, and MIL-101 (Chang et al., 2017). The BET surface area of MIL-101 reduced from 1918.8\u00a0m2 g\u22121 to 1033\u00a0m2 g\u22121 (for TiO2@MIL-101), indicating that TiO2 was deposited in the pores of the MOF. The authors noted that TiO2 also formed a 100\u00a0nm thick shell around the MOF (Chang et al., 2017). The removal mechanism of MO was both adsorption and photocatalytic degradation. In terms of adsorption the MOF based catalyst was able to adsorb 19.23 mg g\u22121 (Chang et al., 2017). If adsorption was not employed, the photocatalytic degradation of MO reached 99% which is much higher than that of isolated TiO2. This is due to the fact that the MOF adsorbed the MO which increased its concentration in TiO2 allowing a greater portion to be degraded.\nAnother study examined the efficiency of NH2-MIL-125(Ti)/BiOCl for the removal of tetracycline hydrochloride (TC) and bisphenol A (BPA) (Hu, 2019). BiOCl was coated on the outer surface of the MOF through the solvothermal method as seen in Fig.\u00a07 (Hu, 2019). It was found that the composite removed 78% of TC in 2\u00a0h, and 65% of BPA in 4\u00a0h, which is much higher than the degradation efficiency of BiOCl on its own which is found equal to 48% (Hu, 2019). The improved activity was attributed to the incorporation of MOF, which provided a greater surface area for MP adsorption (Hu, 2019). In addition, it was noted that the main reactive species responsible for photodegradation were h\n\n\n\n+\n\n\n and \n\n\n\n\u2022\n\n\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n, which were detected via radical trapping experiments and the electron spin resonance technique (Hu, 2019).\nChen et al. (2019a, b, c) studied the effectiveness of the composite of a titanium based MOF, CdS/g-\n\n\nC\n\n\n3\n\n\nN4/MOF, in degrading Rhodamine B (RhB) dye under visible light (Chen et al., 2019c). The possible photodegradation mechanism is illustrated in Fig.\u00a08. When exposed to visible light CdS, g-\n\n\nC\n\n\n3\n\n\nN4, and RhB produce electrons. The produced electrons then reduce Ti\n\n\n\n4\n+\n\n\n in MIL-125 to Ti\n\n\n\n3\n+\n\n\n and O2 from the atmosphere is absorbed on the surface of the MOF and reduced to \n\u2022\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n. Lastly, \n\u2022\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n. Along with the electron holes in the valence band of CdS and g-\n\n\nC\n\n\n3\n\n\nN4 oxidize RhB directly (Chen et al., 2019c). Moreover, the CdS and g-\n\n\nC\n\n\n3\n\n\nN4 were deposited on the outer surface of a previously synthesized MOF, and the final BET surface area for the composite was found to be 283.43\u00a0m2 g\u22121 (Chen et al., 2019c). The research study showed that while CdS on its own could degrade 40% of the initial concentration of RhB in solution in 60\u00a0min, the composite degraded RhB by about 90% in the same amount of time (Chen et al., 2019c). The authors also reported that the composite could be reused for three cycles with minimal decline in photocatalytic activity (Chen et al., 2019c). The composite\u2019s success was achieved due to the enhanced visible light absorption, and decrease in recombination rate of e\n\n\n\n\u2212\n\n\n and h\n\n\n\n+\n\n\n, which allowed \n\n\n\n\u2022\n\n\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n and h\n\n\n\n+\n\n\n to degrade RhB (Chen et al., 2019c).\nIn another study, Abdelhameed et al. (2018) reduced the band gap of NH2-MIL-125(Ti) from 2.51 eV to 2.39 eV by coating Ag3PO4 nanoparticles on its outer surface (Abdelhameed et al., 2018). The authors claimed that this is a significant contributing factor towards Ag3PO4@NH2-MIL-125 being up to 39 times more effective at photodegradation of methylene blue and RhB than P25 TiO2, a well-known photocatalyst for the two MPs (Abdelhameed et al., 2018). Furthermore, the composite exhibited greater catalytic activity and reusability than Ag3PO4 on its own (Abdelhameed et al., 2018). Adding scavengers for ROS revealed that \n\n\n\n\u2022\n\n\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n and h\n\n\n\n+\n\n\n radicals are involved in the catalytic mechanism, while \n\n\n\n\u2022\n\n\nOH radicals are not (Abdelhameed et al., 2018).Another study investigated the loading of carbon quantum dots (CQDs) on NH2-MIL-125 for photodegradation of RhB (Wang et al., 2018). A summary of the synthesis method and reduction conditions is demonstrated in Fig.\u00a09. Furthermore, according to the findings of the study, CQDs can accept electrons from the MOF and allow for charge separation in the composite (Wang et al., 2018). CQDs may also convert NIR radiation into visible light, thereby expanding the range of wavelengths over which NH2-MIL-125 can function as a photocatalyst (Wang et al., 2018). CQDs/NH2-MIL-125 with 1% CQD loading proved to be a highly efficient catalyst under full spectrum of light, visible light, and NIR, as it was able to remove RhB by almost 10 0%, with the smallest amount of time taken for 100% removal (125\u00a0min) under full spectrum (Wang et al., 2018). Furthermore, this composite was reusable for 7 cycles, and showed negligible decrease in photocatalytic activity (Wang et al., 2018). The main ROS involved here were \n\n\n\n\u2022\n\n\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n, h\n\n\n\n+\n\n\n and \n\n\n\n\u2022\n\n\nOH (Wang et al., 2018).\n\nHan et al.\u2019s (2019) work on photodegradation of methyl orange under visible light using RhB/MIL-125 yielded promising results, with methyl orange being degraded by more than 90% in just 60\u00a0min (Han et al., 2019). RhB acted as a sensitizer and helped broaden the range of visible light absorbed by MIL-125(Ti) as seen in Fig.\u00a010 as the removal efficiency of MO increased when using RhB/Mil-125 in comparison to MIL-125 alone (Han et al., 2019). The main reactive species were found to be \n\n\n\n\u2022\n\n\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n, h\n\n\n\n+\n\n\n, and the composite was tested for reusability for three cycles and showed minimal loss of activity (Han et al., 2019).\n\nLi et al. (2016a, b, c) attached 2-anthraquinone sulfonate (AQS) to NH2-MIL-101(Fe) to enhance the MOF\u2019s ability to degrade bisphenol A (BPA) through persulfate activation as observed in Fig.\u00a011 (Li et al., 2017a). The resulting composite, AQS-NH-MIL-101(Fe), was able to remove more than 97.7% of BPA from aqueous solution in 180\u00a0min, which was much higher than removal by NH2-MIL-101(Fe), and NH2-MIL-101(Fe) in the presence of free AQS (Li et al., 2017a). This increased catalytic activity was attributed to improved electron transfer between Fe(III) and Fe(II) in the MOF, and AQS (Li et al., 2017a). Moreover, when persulfate was added to the reaction mixture, degradation of BPA became more rapid (Li et al., 2017a). The main reactive species were \n\n\n\n\u2022\n\n\nSO\n\n\n\n\n4\n\n\n\n\n\n\u2212\n\n\n\n, while \n\n\n\n\u2022\n\n\nOH and \n\n\n\n\u2022\n\n\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n also contributed to the reaction (Li et al., 2017a). Lastly, the composite was tested for three cycles, and did not show any significant losses in activity (Li et al., 2017a).\n\n\nHuo et al. (2019) demonstrated the ability of \n\u03b1\n-Fe2O3/MIL-101(Cr) to degrade carbamazepine (CBZ) under visible light (Huo et al., 2019). The composite removed 100% of CBZ in 180\u00a0min, and was successfully reused, with the removal efficiency in the fourth cycle being 91% (Huo et al., 2019). Fig.\u00a012 also demonstrates that the photodegradation efficiency of different combinations of \n\u03b1\n-Fe2O3/MIL-101(Cr) is higher than that of \n\u03b1\n-Fe2O\n\n\n\n3\n\n\n and MIL-101(Cr) alone. In addition, It was found that \n\n\n\n\u2022\n\n\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n, h\n\n\n\n+\n\n\n, \n\n\n\n\u2022\n\n\nOH were responsible for degradation of CBZ, with \n\n\n\n\u2022\n\n\nOH controlling the photocatalytic activity (Huo et al., 2019).\nMehrabadi and Faghihian (2018) compared the ability of TiO2 supported on clinoptilolite nanoparticles (TiO2/NCP) and TiO2 supported on salicylaldehyde-NH2-MIL-101(Cr) (TiO2/SN-MIL-101(Cr)) to degrade atenolol under UV irradiation and visible light (Mehrabadi and Faghihian, 2018). The conducted tests showed that TiO \n\n\n\n2\n\n\nhad a band gap of 3.20 eV, TiO2/NCP had a band gap of 2.70 eV, and TiO2/SN-MIL-101(Cr) had a band gap of 2.15 eV (Mehrabadi and Faghihian, 2018). While under UV TiO2/NCP performed better (75% removal), the TiO2/SN-MIL-101(Cr) degraded 82% of atenolol under visible light in just 60\u00a0min (Mehrabadi and Faghihian, 2018). The authors attributed this high degradation efficiency to low energy band gap in TiO2/SN-MIL-101(Cr), and its large surface area, which allowed greater interaction between atenolol and reactive species (h\n\n\n\n+\n\n\n, \n\n\n\n\u2022\n\n\nOH, e\n\n\n\n\u2212\n\n\n) (Mehrabadi and Faghihian, 2018). This indicates that the degradation efficiency of TiO2/SN-MIL-101(Cr) is much higher than that of TiO2 due to the reduction in band gap as its removal efficiency was 65% in 180\u00a0min. Several TiO2 doses were evaluated, and the composite with 3.4% TiO2 loading was found to perform most efficiently (Mehrabadi and Faghihian, 2018). It was suggested that in higher doses, TiO2 particles agglomerated, increasing the turbidity of the reaction mixture, and thereby decreasing penetration of light (Mehrabadi and Faghihian, 2018). Additionally, the reusability of TiO2/SN-MIL-101(Cr) was considered, and its degradation efficiency was observed to decrease steadily with each cycle, with only 35% of atenolol being removed in the sixth cycle (Mehrabadi and Faghihian, 2018).\nEmam et al. (2019), compared the effect of silver vanadate (Ag3VO4) and silver tungstate (Ag2WO4) on the ability of MIL-125-NH2 to degrade methylene blue and RhB under UV and visible light (Emam et al., 2019). All of the experiments were conducted over 60\u00a0min, and while both Ag3VO4@MIL-125-NH2 and Ag2WO4@MIL-125-NH2 performed well, the former was more efficient for both dyes under UV and visible light, as shown in Table\u00a013 (Emam et al., 2019). Ag3VO4@MIL-125-NH\n\n\n\n2\n\n\n achieved a removal efficiency of around 100% in 60\u00a0min while Ag2WO4@MIL-125-NH\n\n\n\n2\n\n\n achieved a removal efficiency of around 95%. The higher removal efficiency achieved for composites, in comparison to that of the MOF (75%) on its own as well as Ag3VO\n\n\n\n4\n\n\n(80%) and Ag2WO4 (70%) was due to lower band gaps of the composites (Emam et al., 2019). Adding Ag3VO4 decreased the band gap of MIL-125-NH2 from 2.65 eV to 2.27 eV, while the band gap of Ag2WO4@MIL-125-NH2 was evaluated to be 2.56 eV (Emam et al., 2019). The lower band gap of Ag3VO4@MIL-125-NH2 was also the reason for performing better than Ag2WO4@MIL-125-NH2\n (Emam et al., 2019). Upon checking for reusability under UV light for a total of five cycles, it was found that the degradation efficiency of Ag3VO4@MIL-125-NH2 decreased from 99.9% to 74% and 68% for methylene blue and RhB, respectively, while that of Ag2WO4@MIL-125-NH2 decreased from 96.2% to 72% for methylene blue and 84.2% to 45% for RhB (Emam et al., 2019). Lastly, it was found that \n\n\n\n\u2022\n\n\nOH radical was majorly responsible for the photodegradation of the dyes, and h\n\n\n\n+\n\n\n had also some effect (Emam et al., 2019).Another research compared the photodegradation efficiencies of uncoated MIL-53(Al), MIL-53(Al)@TiO2 and MIL-53(Al)@ZnO, specifically in the depredation of Naproxen, Ibuprofen, and Methyl Orange in both single and binary systems (Murtaza et al., 2022). It was found that uncoated MIL-53 was efficient in the removal of all three micropollutants and was the most efficient in the removal of naproxen (89.5%) and ibuprofen (76.1%) when compared to MIL-53(Al)@TiO2 (80%), TiO2 (55%), MIL-53(Al)@ZnO (72%), and ZnO (67%), as shown in Figs.\u00a013 and 14 (Murtaza et al., 2022). Whereas MIL-53(Al)@ZnO was the only photocatalyst able to degrade MO in both systems and in fact was able to completely degrade it under UV light in 30\u00a0min (Murtaza et al., 2022). Moreover, \n\u2022\nOH plays a crucial role in the photodegradation for all the catalysts.\n\nA different research by Roshdy et al. (2021), studied the adsorption and removal efficiency of 4-nitrophenol by P/W@ZIF-8 and P/W@UiO-66-NH2\n (Roshdy et al., 2021). Both composites were synthesized by \u2018in-situ\u2019 growth of the MOFs in the tungsten NPs. Characterization studies demonstrated that the surface area of P/W increased from 236.51\u00a0m2 g\u22121 to 424.54\u00a0m2 g\n\n\n\n\u2212\n1\n\n\n for P/W@ZIF-8 and increased to 823.01\u00a0m2 g\u22121 for P/W@UiO-66-NH2\n (Roshdy et al., 2021). Whereas the pore volume and pore size decreased in comparison to P/W. In regard to the photocatalytic degradation of 4-nitrophenol, P/W@ZIF-8 had an efficiency of 89.8% whereas P/W@UiO-66-NH2 had an efficiency of 100% after a time of 180\u00a0min under visible light and P/W had a removal efficiency of 50% as seen in Fig.\u00a015. Moreover, the main reactive species involved in the photodegradation were found to be \n\u2022\nOH and \n\u2022\nO\n\n\n\n\n2\n\n\n\n\n\n\u2212\n\n\n\n. Both composites also demonstrated incredible stability and reusability (Roshdy et al., 2021).\nSeveral studies reporting the utilization of MOF-catalyst composites for the degradation of numerous micropollutants, including dyes and pharmaceutical compounds, are summarized and their main characteristics are compared in Table\u00a013. These include the removal efficiency, reactive species and catalysis driving force. As is evident, much research is currently being carried out on different combinations of MOFs and catalysts that would be useful in removing/degrading emerging MPs from wastewater. Some of the most promising MOF-catalyst composites are stated in Table\u00a013 along with their removal efficiency. Of interest, some of the MOF-catalyst composites can remove 100% of a micropollutant such as Ag3PO4@NH2-MI in the removal of Methylene blue. However, certain driving forces must be present in order to achieve such high removal efficiency of a micropollutant such as visible light and UV.\n\nOne factor that is likely to hinder the large-scale commercialization of MOF-based technologies is the high cost of the associated materials. Due to the raw materials such as the salts and ligands, the operating conditions such as the temperature, pressure, and solvent, as well as the energy-intensive synthesis methods, the cost of MOFs can be as high as 27,500 USD per kg (Kumar et al., 2019a). Moreover, the majority of the cost comes from the organic ligand and the solvent used. Larger organic ligands are much more expensive than the smaller ones and these large organic ligands are typically used for the synthesis of MOF (Witman et al., 2017). An example of the ligands used are 2,5-dioxido-1,4-benzenedicarboxylate (DOBDC) and benzenedicarboxylic acid (BDC) which cost 3268 USD per kg and 394 USD per kg, respectively (2, 5-Dihydrox \n\ny\n\u2212\n1\n,\n4\n\u2212\nb\n\n enzoquinone 98 \n\n615\n\u2212\n94\n\u2212\n1\n\n, 2022; Terephthalic acid 98 100-21-0, 2022). Table\u00a014 presents a list of MOFs available through Sigma-Aldrich and their respective costs. Table\u00a014 indicates the high cost associated with the use of MOFs for micropollutants degradation. For example, the Basolite C300 costs approximately 28,516 USD per kg. Preparing MOF composite-based catalysts that hold the high removal efficiency while reducing the raw material cost could be the future for implementing MOFs in large scale applications.\nMoreover, when it comes to the process of synthesizing MOF catalysts, it can also be expensive depending on the approach being used: whether it is \u201cShip-in-a-bottle\u201d, \u201cBottle-around-ship\u201d, Self-sacrificed template, Sandwich-like heterostructure, or One-pot synthesis. While it is difficult to determine the direct cost of each process, the reduction conditions provide enough information to approximate whether the process would be costly or not. Table\u00a015 provides examples of different synthesis processes along with the catalyst produced and its reduction conditions/agents. Table\u00a015 shows the different synthesis methods along with the associated enthalpy of reaction. In general, the smaller (more negative) the enthalpy of reaction is, the less costly it is as it does not require the addition of a large amount of energy as the process is exothermic. As shown in Table\u00a015, the one-pot synthesis method is the most costly as it is the most endothermic reaction meaning that it requires the addition of a large amount of energy for the MOF composite-based catalyst to be produced. On the other hand, the Pd@MOF-5 formation is considered as the least costly as it has the smallest enthalpy of reaction.\n\nIn addition to their cost, another limitation that may affect the progress of MOF composite-based catalysts in the application of wastewater treatment is that most MOFs exhibit poor stability in the presence of water (Kumar et al., 2019a). In addition, the presence of chemicals such as hydroxides, amines, and alkoxides displace the organic ligand in the MOF resulting in the disintegration of the MOF in the water (Chen et al., 2020c). Therefore, there is a danger for metal ions and organic components to leach into water, which could lead to the presence of toxic chemicals in water such as iron, cadmium, terephthalic acid (TPA), and 4,4-bipyridine (BP) (Kumar et al., 2019b).Furthermore, the synthesis of MOFs and MOF catalysts employ solvents such as DMF and Dimethyl sulfoxide (DMSO) which can be toxic to humans. Since MOFs are porous materials, there is a possibility that these solvents are trapped within the structure during the synthesis procedure. As a result, once the MOF is placed in the water, it releases the solvent toxifying the water. The impact of these toxic solvents varies from mild to severe. For example, exposure to DMF, which is one of the most commonly used solvent in the synthesis of MOF, can lead to health issues such as nausea, vomiting, rashes, and even liver damage (Kumar et al., 2019b).The size of a material is also an important parameter when determining its toxicity. As the size of any material decreases, it becomes more reactive due to the high surface to volume ratio. Therefore, MOFs tend to be more toxic when their size decreases to the nanoscale (Kumar et al., 2019b; Sajid, 2016). Another problem with nanoscale MOF is that due to their small size they can penetrate the blood\u2013brain barrier and the cell membrane of living organisms causing adverse effects (Grande et al., 2017).Regarding the environmental impact of MOFs, the degree of the impact depends on the synthesis route such as the operating conditions and the used solvent. For instance, the synthesis of ZIF-8 and MOF-74 can be done using DMF as a solvent and using water as a solvent. Research studies demonstrated that in the synthesis of the two aforementioned MOFs, the use of water has less environmental impact when compared to the use of DMF. The environmental impact of using water in the synthesis of ZIF-8 is quantified as 86.60\u00a0kg CO2 eq. whereas using DMF it is increased 1571.16\u00a0kg CO2 eq. (Grande et al., 2017). In the case of MOF-74, the use of water leads to 12.3\u00a0kg CO2 eq. On the other hand, it increases to 1136.2\u00a0kg CO2 eq. when using DMF (Ntouros et al., 2021).With a rise in micropollutant concentrations in different water bodies the accessibility to clean, drinkable water is becoming more strenuous. As a result, this serious issue has recently caught the attention of various organizations such as the United States Environmental Protection Agency (Montes-Grajales et al., 2017a). While their impact and presence are not fully understood yet, it is clear that MPs pose a hazard to the environment and human health, even when present at very low concentrations in the order of ng L\u22121 and \n\u03bc\ng L\u22121. Some of these hazards include bioaccumulation (Vodyanitskii and Yakovlev, 2016), toxicity to human and animal health, increased risk in cancer, increase in antibiotic-resistant bacteria, and reproductive health problems (Gogoi et al., 2018b). This review discussed the use of MOF composite-based catalysts for the degradation of MPs ranging from the synthesis of MOF catalysts to their photodegradation mechanism, as well as current trends in research, associated cost, and environmental considerations.First, different emerging micropollutants were discussed with an emphasis on their source, their nature, and their impact. Since little attention has been given to MPs in the past, current WWTPs seem to be quite inefficient in their removal. Several studies conducted in various regions in the world from Europe such as Spain, North America, and Far East Asia such as China, revealed that the most common source of MPs in potable water was from sewage water released into surface water from WWTPs (Jiang et al., 2013). As a result, the EU Water Framework Directive and the US EPA both updated their watch list in order to emphasize the importance of controlling these compounds in WWTP effluents.Next, Metal Organic Frameworks were introduced and reviewed. Their properties can be easily tailored to their specific application through the selection of the inorganic linkers and organic species. Indeed, they are very advantageous in many applications such as the photodegradation of MPs. To compare the promising effect of MOF composite-based catalysts in the degradation of WWTPs, current methods were first discussed. Membrane technology for instance must be used alongside another removal technique in an integrated system as the diversity in the MPs properties make it difficult to remove them all using a single membrane type (Silva et al., 2017). Membrane bioreactors seem very promising in the removal of various micropollutants; however, it was found that it is not well-suited for the removal of MPs exhibiting low biodegradability, process high degree of branching and saturation, and contain sulfate and halogen group. In addition, to the high energy requirement which leads to a high operating cost. Another option is the use of CCM for advanced oxidation processes. This method was used for the removal of chlorinated MPs that have high toxicity and a strong likelihood to persist in the environment using a ceramic membrane making this method more efficient compared to a membrane bioreactor as it also has a less chance of fouling (Goswami et al., 2018). Other methods used the application of activated carbon and biochar. An advantage to this method is that it does not produce harmful by-products and does not require high amounts of energy (Sher et al., 2021). On the other hand, the production of biochar is extremely expensive as it is energy intensive (Chen et al., 2020a; Ouyang et al., 2020) and AC increases the turbidity of WWTP effluents which would require additional measures to be taken to remove it from the water (Sher et al., 2021). Recently, catalysts have gained a lot of attention as a material in the photodegradation of MPs. Specifically on catalyst assisted photodegradation in which photocatalysts are used to make the natural photolysis of MPs and their by-products more efficient using sunlight (He et al., 2016). Examples of photocatalysts include TiO2 which is considered the most promising as it is cheap, highly efficient, physically, and chemically stable, and non-corrosive (Dong et al., 2015). However, it is limited by its poor absorption of the full spectrum of sunlight (Dong et al., 2015). Enzymes have been also studied as photocatalysts and particularly two types of enzymes have been found to be particularly efficient laccase and horseradish peroxidase. A disadvantage of using enzymes is that they can only remove specific individual MPs therefore more studies should be conducted on other enzymes and the type of MPs that they can target (Stadlmair et al., 2018).Another promising option of photocatalysts is MOF catalyst composites which can be synthesized using 5 different approaches. Regardless, of the synthesis method, the photodegradation mechanism remains unchanged. Organic linkers absorb sunlight causing the electron to go from the HOMO to the LOMO and then transferred to the metallic cluster.In addition, the present review highlighted the current research being conducted using different MOF catalyst composites and their respective results. It was observed that MOF catalyst composites are quite effective in the degradation of various micropollutants, and the efficiency of MOF catalyst composites increased in comparison to free catalysts. Lastly, the cost and environmental impacts were also considered in order to determine the applicability of the MOF catalyst composites on a large scale. It was demonstrated that free MOFs are quite expensive and can reach up to 28,516 USD per kg. However, this high capital cost is balanced out by a low maintenance and operating cost. Furthermore, the synthesis of MOF catalyst composites can also increase the overall capital cost depending on the synthesis method. Another limitation to their large-scale application is the MOFs instability in water causing it to degrade leaving traces of metals and organic compounds in the water which can in turn harm humans and the environment. The synthesis method also plays a role in the environmental impact. Although MOF-composite-based catalysts have proven to be a promising hybrid catalyst with versatile applications, yet their investigation is still in its infancy and many challenges should be addressed before they are employed in real-life industrial applications. One of the main issues that should be further studied is the MOF-composite stability in aqueous solutions. It is also very crucial to develop MOF-composites that can possess high chemical and physical stable even after multiple cycles of photodegradation. Therefore, recyclability is another important property that future research should investigate in more depth. Furthermore, it is necessary to develop mechanically stable MOF-composite and be able to shape it into pellets or beads for its the final industrial applications. In addition, the mechanism of photodegradation of micropollutants still lacks in depth understanding of active species involved in the degradation steps and deactivation procedure. This aspect can be considered in future research studies.\nSana Z.M. Murtaza: Data curation, Writing \u2013 original draft. Hind Tariq Alqassem: Data curation, Writing \u2013 original draft. Rana Sabouni: Conceptualization, Investigation, Methodology, Project administration, Resources, Supervision, Writing \u2013 review & editing. Mehdi Ghommem: Funding acquisition, Project administration, Resources, Supervision, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The study presented in the paper was financially supported via the faculty research grant from the American University of Sharjah, United Arab Emirates (FRG21-M-E63 and FRG-S22-E05) and Sharjah Research Academy (SRA 223150). The work in this paper was supported, in part, by the Open Access Program from the American University of Sharjah\n (OAP23-CEN-033) This paper represents the opinions of the author(s) and does not mean to represent the position or opinions of the American University of Sharjah.", "descript": "\n                  The presence of various micropollutants in different water sources has become a major problem due to their significant impact on both humans and the environment. This review highlights the different types of micropollutants present at the global scale and the methods applied to reduce and possibly eliminate them. These methods include membranes, adsorption and photocatalysis. While membrane filtration is extremely effective, one membrane can eliminate only a few micropollutants and its deployment remains expensive. On the other hand, adsorption constitutes a very efficient and cost-effective method, but the production of adsorbents is extremely energy intensive. Lastly, the photocatalysis method is considered to be the most promising as it avoids the problems associated with the aforementioned methods. Specifically, photocatalysts make use of direct sunlight in order to degrade micropollutants. Several types of photocatalysts, including biochar, Mxenes, nanoscaled zero valent iron, and MOFs, are discussed. Unlike the first four aforementioned types, MOFs can be combined with different materials to enhance the overall property of the composite and its efficiency in the degradation of micropollutants. The MOF-catalysts discussed in this paper include biomimetic MOFs, enzyme MOFs, and Fenton-like MOFs. The obtained system is referred to as MOF-composite-based catalysts. MOFs can be synthesized by combining an appropriate organic linker with a metallic cluster that would provide the material with the required properties for photodegradation. Several metal\u2013organic framework catalyst composites synthesis approaches are reviewed and discussed. The selection of the approach depends on the requirements associated with the application of interest. To date, extensive research has been conducted on the performance analysis of metal\u2013organic framework composites to investigate their efficiency in the removal of micropollutants. Several studies demonstrated their great removal capability which may reach up to 99 %. Finally, cost, health and environmental considerations are discussed with the view of the industrial applicability of MOF-composite-based catalysts. This comprehensive review presents the current state of the art and proposed promising research directions for the implementation and advancement of MOF-composite-based catalysts for micropollutants degradation.\n               "}
{"full_text": "Any additional information required to reanalyze the data reported in this paper is available from the lead contact on reasonable request.The development of our society is full of rapid industrialization progress, but this progress comes with many serious environmental issues. In particular, we are facing global warming caused by excessive greenhouse gas (GHG) emissions, as well as increasing resource constraints, such as clean water.The amount of carbon dioxide (CO2) as one of the primary GHG emissions accelerates with growing industrial and farming activities.\n1\n\n,\n\n2\n The concentration of CO2 in our atmosphere had already surpassed 400 ppm in 2016 and is predicted to increase to 550 ppm by roughly the end of this century, which threatens ecosystem and human health.\n3\n As an emerging solution, the CO2 could be electrochemically converted into value-added chemicals.\n4\n However, sustainable efforts that consider an entire system, including catalyst preparation, CO2 conversion, and downstream product separation, are still rare despite the raised demand.\n5\n\nWater contaminated with heavy metal ions and organic micropollutants from minerals mining and industrial processes poses severe risks to aquatic ecosystems and human health.\n6\n\n,\n\n7\n Adsorption by porous composites is a commonly used strategy to remove heavy metal ions because of high adsorption efficiency with desired porous structure and surface functional groups. However, the desorption process is always required to regenerate adsorbents, and the effluent-containing metal has to be treated as hazardous waste.\n8\n Catalyst-based oxidation processes have been recognized as a techno-economic option to remove the organic micropollutants in wastewater.\n9\n However, extra energy and chemicals are necessary for catalyst recycling and treatment after its use.Although CO2 emissions and water quality issues, representing two major environmental challenges, have been tackled from standalone perspectives, a sustainable strategy is urgently required to solve these environmental issues effectively and synergistically. Herein, we propose a systematic strategy to address the environmental issues based on the upcycling of post-consumer waste to green chemicals and clean water (Figure\u00a01\n). Every year, industrial waste from seafood, such as crab, shrimp, and lobster, could reach 6\u20138 million tonnes globally, resulting in a non-negligible burden on the environment.\n10\n Chitosan (CS), a biopolymer that could be derived from these seafood residues, has become an emerging solution to treat metal-contaminated water owing to its remarkable ability of metal ion chelation.\n11\n Instead of being disposed of as hazardous waste as usual, here we directly convert post-consumer metal/CS, containing Cu, Pd, Cd, Mn, Zn, Ni, Ag, and Cr ions as a mixture, from simulated wastewater to metal-doped laser-induced graphene (M-LIG) under air via rapid laser scribing. The metal/CS composite with individual ion metal represents the post-consumer after treating wastewater with those ions as the dominant contaminant, for example, Cu in rinsing tank wastewater from an electroplating plant.\n12\n Thus, these seafood residues with wasted metal ions could be upcycled into valuable M-LIG catalysts. Most importantly, the upcycled Cu-LIG catalysts here are successfully utilized for the electroreduction of CO2 (CO2ER) to high-valued chemicals. The degradation of organic micropollutants in wastewater is also demonstrated via a Cu-based Fenton-like reaction with high efficiencies and recyclability of catalysts. A life-cycle assessment (LCA) is also used to systematically evaluate the carbon footprint of the entire system from catalyst preparation to product separation after CO2ER and organic micropollutants degradation. Therefore, producing green chemicals and clean water from industrial waste without introducing external environment and energy concerns may be realized by such an upcycling strategy.CS, as a biopolymer obtained from alkaline N-deacetylation of chitin, derived from waste seafood shells, contains abundant free amino groups. As a result, CS shows an effective adsorption ability of heavy metal ions, owing to the electrostatic attraction between them and its protonated amino groups, as well as hydroxyl groups.\n13\n In brief, CS solution was prepared by dissolving short-chain CS powder in deionized (DI) water with glycerol that could improve the flexibility of CS film by forming hydrogen bonding within CS chain as plasticizer.\n14\n After drying, the CS film was immersed in 2 wt\u00a0% NaOH solution for precipitating and washed in the DI water until neutral. Finally, the CS film was immersed in diverse heavy metal ions solution for chelation (Figure\u00a02\nA). For adsorption of Cu2+, the field emission scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (SEM-EDS) shows a homogeneous distribution of Cu2+ chelated in CS film (Figure\u00a0S1). The interaction of Cu2+ is also evidenced by Fourier transform infrared spectroscopy (FTIR) analysis (Figure\u00a0S2).In our strategy, the post-consumer metal/CS film could be directly converted to M-LIG at ambient conditions under a commercial 10.6-\u03bcm CO2 infrared laser. Eight heavy metal ions (Zn2+, Mn2+, Ni2+, Cu2+, Cd2+, Pd2+, Ag+, and Cr6+, respectively) and a simulated industry wastewater containing Cr6+ and Cu2+ were tested (see experimental procedures). With computer control, the M-LIG pattern could be customized under laser scribing (Figure\u00a02B). The resulted graphene composite contains macropores and nanopores simultaneously (Figures\u00a0S3 and S4A). The nanopore sizes are distributed between 1.3 and 1.8\u00a0nm, suggesting that the M-LIG has a porous structure at multiple scale that could enlarge accessible surface areas and facilitate the mass transfer (Figure\u00a0S5). The high-resolution transmission electron microscopy (HR-TEM) image also shows the average lattice space (about 3.4\u00a0\u00c5) between two neighboring planes (002) as multilayer graphene, agreeing with the intense X-ray diffraction (XRD) peak at 2\u03b8\u00a0= 25.9\u00b0 (Figures\u00a0S4B and S6). The D peak (\u223c1,350\u00a0cm\u22121) of graphene presents in the Raman spectra (Figure\u00a02C), reflecting the doping effect of heteroatoms (especially for N atoms introduced by CS) or the bent sp2-carbon bonds. The 2D peak at 2,700\u00a0cm\u22121 is originated from the second-order zone-boundary phonons,\n15\n and the I2D/IG ratio demonstrates the M-LIG has a multilayer structure.It is noted in the X-ray photoelectron spectroscopy (XPS) that heavy metal ions are reduced during the lasing process. For instance, Cu2+ is reduced into Cu+/Cu0 after lasing, which could enable more potential as catalysts (Figures\u00a0S7E and S7F). Meanwhile, the core-shell structure is also formed in laser printing with the Cu wrapped by graphene layer, shown in HR-TEM (Figure\u00a0S8). CS provides not only the essential carbon but also the abundant nitrogen (N), which could be doped in graphene as the heteroatom to tune its electronic properties. Four types of N atom are doped in the graphene structure. Specifically, the pyridinic N (398.9 eV) is more dominant, compared with oxidized N (403.5 eV), graphic N (401.6 eV), and pyrrolic N (400.6 eV) in N 1s peaks (Figure\u00a0S7C).Laser-induced graphitization of biopolymers is challenging. To our best knowledge, lignin from lignocellulosic biomass as a complex of phenolic compounds is the only biopolymer converted to LIG via one-step lasing scribing in air atmosphere reported so far, owing to its aromatic ring structures.\n16\n Polysaccharide chains, such as cellulose and CS, could be easily decomposed into volatile compounds under the lasing process with the intensive local heat (Figure\u00a02D).\n17\n Alternatively, the multi-step carbonization,\n18\n reduced gas protection,\n19\n and flame-retardant\n20\n methods have been applied together with the lasing process to obtain graphene from non-phenolic biopolymers. As a result, additional operations steps, chemical input, and energy consumption are essential, compromising the sustainability of lasing technology. Here, we successfully demonstrated the direct conversion of CS to LIG in the presence of wasted metal ions.These metals are expected to play important roles during the lasing process.\n21\n Take Cu-doped CS film as an example. The thermal gravimetry (TG) and derivative thermal gravimetry (DTG) curves show that the temperature corresponding to the maximum weight loss is higher for Cu-doped CS film than CS film, revealing improved thermal stability of CS film in the presence of copper ions. At 900\u00b0C, 7.6% of Cu-doped CS film remains eventually, compared with CS film that is almost decomposed (Figure\u00a0S9; Table\u00a0S1). This reservation of solid char could provide the essential carbon as graphene precursors. The finite element analysis (FEA) was applied to simulate the temperature increase on the surface of CS film and Cu-doped CS composite when they were exposed to laser irradiation (Figures\u00a02D, S10, and S11). An ultra-high temperature of over 1,500 K is expected on the surface for Cu-doped CS film with laser power of 2.5\u00a0W at 1\u00a0ms, which is high enough to melt reduced Cu nanoparticles into their liquid form. This endothermic phase change of copper potentially happened during the process and echoes the simulation results that a lower surface temperature was obtained for Cu-doped CS film (5,880 K) compared with CS film (6,340 K) after lasing for 30\u00a0ms. In addition, the liquid copper could be an ideal catalytic component for the rapid orientation of graphene structure, caused by its relatively low carbon solubility and quasi-atomic surface.\n22\n In detail, the thermal motion of liquid Cu atom together with the surface tension collection minimized the surface energy, leading to the formation of a non-defect and smooth liquid Cu surface. This surface enables the rapid migration of carbon atoms and subsequent alignment of graphene structure.\n23\n Moreover, because of the higher temperature induced by laser irradiation, the surface tension of liquid Cu tends to decrease further, leading to more rapid transport and diffusion of carbon atoms.\n24\n Different from copper, other liquid metals have higher carbon solubilities, resulting in the extra carbon atom precipitated from the liquid metal surface to form the multilayer graphene structure.\n25\n Notably, because of the inertness of Cr for catalyzed graphene formation,\n22\n more graphene defects were observed in Cr-containing LIG (Figures\u00a02C and S12). After cooling at room temperature, the M-LIG was successfully formed (Figure\u00a02E). Moreover, the characteristics of M-LIG could be tuned by controlling the lasing parameter and the concentration of chelated metal ion in CS film, revealing its potential as high-valued M-LIG catalysts for a wide range of applications (Figures\u00a0S13\u2013S15). As a demonstration, we first employed the Cu-LIG as the catalyst for CO2ER and investigated its sustainability in turning multiple waste streams into green chemicals.First, the CO2ER was performed in the H-type reaction cell. A series of electrochemical tests had been explored to demonstrate the CO2ER activity of Cu-LIG catalysts with different initial Cu contents (2, 5, and 10\u00a0mg mL\u22121), termed Cu-LIG-x (x\u00a0= 2, 5, and 10, respectively) (Figures\u00a0S16\u2013S20). The results demonstrate that the Cu-LIG-5 with a lower Tafel slope of 270\u00a0mV dec\u22121 is more beneficial for CO2ER than its counterparts. The Faraday efficiency (FE) of ethanol reaches up to about 20%, and the selectivity of ethanol exceeds 50% in the liquid products at \u22120.6\u00a0V (versus RHE) for Cu-LIG-5, whereas the selectivity of formic acid (FA) is over 90% in the liquid products at \u22121.2\u00a0V (versus RHE) (Figure\u00a03\nA). When the contents of Cu are decreased (Cu-LIG-2) or increased (Cu-LIG-10), the primary liquid product changes to FA under all overpotentials, but the FE of total carbon products is still maintained above 60% for both catalysts at even higher overpotentials. The semi-in situ XPS was employed to investigate the CO2ER process of the three Cu-LIG catalysts (Figure\u00a0S21). For the Cu-LIG-5 catalyst, the Cu+ and Cu0 could be simultaneously observed at a lower overpotential of \u22120.6\u00a0V (versus RHE), whereas almost all the Cu+ is reduced to Cu0 at a higher overpotential of \u22121.2\u00a0V (versus RHE) (Figures\u00a03B and S22). For the Cu-LIG-2 and Cu-LIG-10 catalysts, in contrast, the Cu+ can be barely observed at even lower overpotentials (Figure\u00a0S23). This indicates that the concurrence of Cu+ and Cu0 in the Cu-LIG-5 at lower overpotentials is the key to the C\u2013C coupling and the formation of C2 products (ethanol), but the Cu0 mainly leads the production of C1 product (FA), which shows a good consistency with many other reports\n26\n\n,\n\n27\n\n,\n\n28\n and can also be supported by density functional theory (DFT) calculations (see Figures\u00a0S25\u2013S29 and the supplemental information for more DFT calculation details).Moreover, in order to satisfy the higher current density required on an industrial scale, we also performed the CO2ER in the flow cell with the optimal Cu-LIG-5 catalyst. The detailed setup of flow cell is shown in Figure\u00a03C. The linear sweep voltammetry (LSV) curves demonstrate that the CO2ER effectively occurs in the flow cell (Figure\u00a03D). The CO2ER results indicate that the FE of total carbon products is higher in low current density (almost 85% at \u221250 mA cm\u22122), because of the better suppression of hydrogen evolution reaction (HER) (Figure\u00a03E). The gas products are more dominant (above 70%) in the flow cell due to the rapid diffusion of reactants and intermediates, in which FE of ethylene (C2 product) is around 25% at \u2212100 mA cm\u22122. Besides, the CO2ER could be stably conducted for at least 420\u00a0min in the flow cell, where the FE of total carbon products gradually decreases to 60%, but the C2+ products still preserve above FE of 20% (Figure\u00a03F). Similarly, the production pathway of ethylene as the main C2 product was also calculated with DFT models, where the Cu+\u00a0+ Cu0 shows a lower change of free energy (1.42 eV) in the C\u2013C coupling (\u2217CHOH-CO formation) and is more conducive to produce the ethylene, compared with Cu0 and Cu+ models (Figures\u00a03G\u20133I, S30, and S31).\n28\n Moreover, pyridinic N is conducive to enriching CO2 molecules into the carbon layer around Cu sites, and the increase of local concentration of CO2 around the Cu sites provides a favorable condition for promoting efficient CO2ER (Figure\u00a0S32; Table\u00a0S3).Apart from heavy metal ions, the organic micropollutants in the wastewater also greatly negatively affect human health and our ecosystem. In the proposed strategy, we chose the representative six kinds of organic micropollutants (2,4-dichlorophenol [2,4-DCP], 2-naphthol [2-NO], bisphenol A [BPA], bisphenol S [BPS], methylene blue [MB], and ethinyl estradiol) as the degradation models to demonstrate an application of M-LIG in producing clean water (Figure\u00a04\nA). As a demonstration, the Cu-LIG film is prepared and assembled in an in-house device containing H2O2, where the self-standing membrane form could be very attractive for instant applications (Figure\u00a04B). For the Cu-based Fenton-like reaction, the parameters, such as H2O2 concentration and catalyst dosage, were first optimized with the BPA as the model pollutant (Figures\u00a0S33 and S34). Compared with Cu powder catalysts, the consumption of membrane-based catalysts is much less (Table\u00a0S7). Neutral reaction condition (pH 7.0) was chosen because of the favorability of practical application. Under the optimized conditions, the degradation efficiencies within 180\u00a0min for BPA, 2-NO, 2,4-DCP, and ethinyl estradiol were more than 90%, whereas those for BPS and MB are 80% and 75%, respectively (Figure\u00a04C). Their total organic carbon (TOC) removal efficiencies were about 55% for MB, BPA, BPS, and 2-NO, whereas the efficiencies were 30% and 20% for 2,4-DCP and ethinyl estradiol, respectively, indicating reasonably good mineralization percentages. Furthermore, the continuous batch-type degradation of BPA was tested for self-standing membrane catalysts. The removal efficiency is up to 93.2%, achieved in the first cycle and gradually stable at 70% after the fourth cycle. According to the inductively coupled plasma mass spectrometer (ICP-MS) results, 0.45 ppm copper was leached after the first cycle, which could reduce the copper active sites, weakening the catalytic activity (Figure\u00a04D). However, the leached Cu concentration is well below the control limit for drinking water (1.3 ppm).\n9\n The stable regeneration ability of Cu-LIG could be attributed to its core-shell structure formed during lasing printing. For the degradation mechanism, the electron paramagnetic resonance (EPR) results confirm the generation of \u00b7OH radical in the presence of Cu-LIG catalyst with H2O2 reaction (Figure\u00a04E). The process of \u00b7OH generation could be explained in Figure\u00a04F, Figure S35, and Note S5 (Supplemental experimental procedures). The possible degradation pathway of BPA is also proposed in Table\u00a0S8 and Figure\u00a0S36. Overall, the \u00b7OH free radicals attack organic micropollutants and finally mineralize them to nontoxic CO2 and H2O. Combined with heavy metal ions removal, the wastewater could be synthetically purified into clean water via natural seafood waste.As emerging technology being developed in the lab, prospective LCA based on experiments and assumptions is a method to give environmental guidance to identify sustainable technologies in the early stage.\n29\n\n,\n\n30\n\n,\n\n31\n\n,\n\n32\n Herein, the carbon footprint of our proposed CO2ER was assessed according to the system boundary as defined in Figure\u00a05\nA. The baseline case (CO2ER) was constructed based on in-house experimental results and a state-of-art membrane electrode assembly (MEA) design using a cation-exchange membrane (CEM) coupled with a permeable CO2 regeneration layer (PCRL). This design enables over 85.0% single-pass CO2 conversion efficiency and results in FA product, instead of formate.\n33\n The produced gas products (ethylene, H2, and CO), as syngas after recycling CO2 were converted to olefins via the methanol (MTO) process and treated as by-products replacing the conventional fuels (see supplemental experimental procedures for more details). Global warming potential value (GWP), also as known as carbon footprint, illustrates that electricity and heat for FA distillation are the significant contributors in the baseline case reflecting the FE achieved in the lab, resulting in an overall GWP of 14.77\u00a0kg CO2 e kg\u22121 FA (Figure\u00a05B). To incorporate future improvement, we simulated a forward-looking case with over 80% FE for FA, whose GWP could be reduced to 8.62\u00a0kg CO2 e kg\u22121 FA, as a result of a reduction in electricity consumption. For both cases, the electrolyte was assumed to circulate until the FA concentration accumulated to 9.6 wt\u00a0% before being concentrated to 85 wt\u00a0% via conventional distillation.\n34\n As an alternative to this energy-intensive product separation, a hybrid extraction-distillation (HED) using 2-methyltetrahydrofuran\n35\n was incorporated in the scenario analysis whose GWP reaches 3.85\u00a0kg CO2 e kg\u22121 FA, close to the lower bound of the reported range (3.10\u20135.30\u00a0kg CO2 e kg\u22121 FA) from other CO2ER studies.\n36\n This value is comparable with the conventional FA produced via the methyl formate route (2.24\u00a0kg CO2 e kg\u22121 FA)\n37\n and much lower than that via decarboxylative cyclization of adipic acid (7.34\u00a0kg CO2 e kg\u22121 FA).\n38\n Furthermore, decarbonized electricity supplies were also incorporated from a low-carbon future perspective. According to the projection in energy supplies for the national grid, along with China\u2019s 3060 ambition, the GWP of our system could be further reduced to be 3.17\u00a0kg CO2 e kg\u22121 FA (S2), 1.38\u00a0kg CO2 e kg\u22121 FA (S3), and 1.30\u00a0kg CO2 e kg\u22121 FA (S4), respectively.\n39\n Under the most optimistic conditions, our system could achieve carbon negative (\u22120.55\u00a0kg CO2 e kg\u22121 FA) if both electricity and heat could be provided by renewable sources (i.e., solar and biomass). Overall, these results reveal that the upcycling of post-consumer waste to catalyst for CO2ER has the promising potential to convert captured CO2 to FA with a comparable or even lower GWP. Alternative to FA, if the FE is improved in the prospective scenario toward producing ethylene, a lower GWP would be expected because of the elimination of liquid product separation. To be conservative, emission credits from avoided disposal of post-consumer waste Cu-doped CS film were not included in the analysis because of its minor contribution to GHG emissions compared with energy use. However, other environmental benefits such as water- or human health-related categories from the avoided disposal could be substantial because of the appearance of heavy metal ions in the waste.\n40\n Nevertheless, our prospective LCA incorporating joint efforts in reactor design and products upgrading provides a holistic view on future directions to improve the sustainability of CO2ER. From the economics reviewed by Somoza-Tornos et\u00a0al.,\n36\n carbon monoxide and FA are the two products that are closest to being cost competitive, with electricity consumption, capital cost, and CO2 feedstock as major contributors to the cost of CO2ER. However, because of variabilities in process parameters assumption and capital cost estimate, future techno-economic assessment (TEA) is encouraged to be validated by pilot plant or industrial process. Therefore, similar conclusions from TEA are expected for our study, and further work is proposed to combine efforts in industrial development in the future.With regard to organic pollutants degradation, Table\u00a0S7 summarized GWPs per ppm BPA degraded, mainly caused by the consumption of H2O2. Results represent that GWP of BPA degradation demonstrated in our study is at the lower end of the range owing to relatively higher degradation efficiency. However, it is notable that, to be conservative, credits as a result of the significantly smaller loading of catalyst in the process were not considered.In this work, we present an upcycling strategy to tackle environmental challenges caused by multiple waste streams. After removing the heavy metal ions in wastewater, the post-consumer metal/CS film is successfully upcycled into M-LIG via one-step laser scribing in ambient air. As a demonstration, the obtained Cu-LIG with controllable contents of metal ions and N sites in the porous graphene is utilized as a catalyst for CO2ER. The FE of total carbon products in the flow cell exceeds 80%, in which the FE of ethylene as a C2 product is approximately 25% at 100 mA cm\u22122. Together with DFT calculation results, it is confirmed that the concurrence of Cu+ and Cu0 in the Cu-LIG catalyst is the key to promoting the C\u2013C coupling at low overpotentials. LCA results from the evaluation of a system design, which integrates joint efforts in reactor design and upgrading downstream products, reveal that CO2ER has the promising potential to convert captured CO2 to FA and olefins with a considerably low GWP in the decarbonized future. Furthermore, the organic micropollutants in wastewater could also be effectively degraded by a Fenton-like reaction using the self-standing Cu-LIG film. The estimated GWP per unit of BPA degraded demonstrated in our study is at the lower end of the range owing to the relatively higher degradation efficiency. Overall, our proposed strategy could be an inspiring and synergetic solution to upcycle multiple waste streams to green fuels and clean water as we strive for a sustainable future.Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Lei Wang (wang_lei@westlake.edu.cn).This study did not generate new unique reagents.Short-chain CS (degree of deacetylation >90%, viscosity 45 mPa\u00b7s for 1% (w/v) solution, molecular weight: \n\n\u2248\n\n10\u00a0kPa) was purchased from Jinhu Company, China. Glycerol, copper nitrate trihydrate (Cu(NO3)2\u00b73H2O), manganese sulfate monohydrate (MnSO4\u00b7H2O), zinc acetate dihydrate (Zn(CH3COO)2\u00b72H2O), nickel chloride hexahydrate (NiCl2\u00b76H2O), cadmium nitrate tetrahydrate (Cd(NO3)2\u00b74H2O), lead nitrate (Pb(NO3)2), silver nitrate (AgNO3), and potassium dichromate (K2Cr2O7) were of A.R. grade and purchased from Sigma-Aldrich. Pollutant model compounds were obtained from commercial sources and used as received.The glycerol (0.4 g) was dissolved in the DI water (98.0 g), before mixing with CS powder (2.0 g). Then, the CS solution was stirred at 900\u00a0rpm for 24\u00a0h until the solution became totally transparent. After that, the solution was poured into a Petri dish and dried at 40\u00b0C for 48 h. As control, each dried CS film was cut into 0.37 g, followed by immersion in NaOH solution (2 wt\u00a0%) for 24\u00a0h and rinsing with DI water until neutral. The prepared CS film was preserved in DI water before using.The prepared CS film was sunk in heavy metal ion solution with chelation process for 24 h. In detail, eight heavy metal ions (Cu2+, Pd2+, Cd2+, Mn2+, Zn2+, Ni2+, Ag+, and Cr6+, respectively) were commonly found in wastewater and chosen with initially gradient concentrations (2, 5, and 10\u00a0mg mL\u22121, respectively). Meanwhile, to mimic the real application, we tested simulated industry wastewater from an electroplating process that contains Cr6+ (42\u00a0mg mL\u22121) and Cu2+ (3\u00a0mg mL\u22121).\n12\n After removal, the metal/CS film was dried at room temperature for 24\u00a0h and put in a desiccator for transforming to M-LIG.Laser printing was performed on conversion of metal/CS into M-LIG using a CO2 laser system (Universal laser cutter platform, 10.6\u00a0\u03bcm, 50-W laser) at a scan rate of 3% (measured to be \u223c3.6\u00a0mm s\u22121), 1,000 pulses per inch, and laser powers ranging from 2.2 to 2.6\u00a0W with increments of 0.1 W. The image density was set at 4, which means the spacing between raster lines during lasing. The laser beam was focused at a z distance of 6.4\u00a0mm, to partially defocus the laser (\u223c1.4\u00a0mm).In the FEA, the photothermal model of laser scribing was suitable for demonstrating the temperature change during the laser processing. A few of the simulations have been successfully performed, based on the carbonized silk and polyimide film.\n41\n\n,\n\n42\n Followed with similar steps, we built two simple photothermal models to better understand the generation and difference of temperature with or without Cu doping in CS film.Combined with Gaussian beam used as the laser source and Beer-Lambert law, the heat source density per unit volume at a position (r, z) was\n\n(Equation\u00a01)\n\n\nq\n\n(\n\nr\n,\nz\n\n)\n\n=\n\u03b1\n\n(\n\n1\n\u2212\nR\n\n)\n\n\n\n2\nP\n\n\n\u03c0\n\nw\n0\n2\n\n\n\n\nexp\n\n[\n\n\n\u2212\n2\n\nr\n2\n\n\n\nw\n\n\n(\nz\n)\n\n2\n\n\n\n]\n\n\nexp\n\n[\n\n\u2212\n\u03b1\nz\n\n]\n\n\n\n\n\n\n\n(Equation\u00a02)\n\n\nw\nz\n=\n\nw\n0\n\n\n\n1\n+\n\n\n(\n\n\nz\n\u2212\nE\no\nF\n\n\nz\nR\n\n\n)\n\n2\n\n\n\n\n\n\n\n\n\n(Equation\u00a03)\n\n\n\nz\nR\n\n=\n\n\n\u03c0\n\nw\n0\n2\n\n\n\n\u03bb\n0\n\n\n,\n\n\n\nwhere r is the radius distance away from the center spot of laser beam and z is the vertical distance from CS film surface, P is the laser power, and w\n0 is the incident laser beam radius at focus. The extent-of-focus (EoF) is the focus distance, which could be 0 (on focus plane), negative (over focus plane), and positive (under focus plane). \u03b1 and R are the optical adsorption coefficient and reflectivity of material, respectively. For simplicity, \u03b1 and R could be calculated according to the complex refractive index:\n41\n\n\n\nn\n+\ni\nk\n=\n2.49\n+\n0.015\ni\n\n for carbon materials because of the rapid carbonization of biopolymer.With the time t evolution and spatial (r, z) distribution of the temperature T (r, z, t), the heat transfer equation during the entire process is as follows:\n\n(Equation\u00a04)\n\n\n\n\n\u03c1\n\nC\np\n\n\u2202\nT\n\n\n\u2202\nt\n\n\n\u2212\n\u2207\n\u22c5\nk\n\u2207\nT\n=\nq\n\n(\n\nr\n,\nz\n\n)\n\n.\n\n\n\n\nCorresponding to the initial condition,\n\n(Equation\u00a05)\n\n\nT\n\n(\n\nr\n,\nz\n;\nt\n=\n0\n\n)\n\n=\n\nT\n\ne\nx\nt\n\n\n=\n293\n\nK\n.\n\n\n\n\nThe boundary condition was:\n\n(Equation\u00a06)\n\n\nn\n\u22c5\nq\n=\n\nh\n1\n\n\n(\n\n\nT\n\ne\nx\nt\n\n\n\u2212\nT\n\n)\n\n\nfor\n\nsurface\n\nand\n\n\n\n\n\n\n(Equation\u00a07)\n\n\nn\n\u22c5\nq\n=\n0\n\nfor\n\nedges\n,\n\n\n\nwhere \u03c1, Cp, and k are, respectively, the density, specific heat, and thermal conductivity of CS film. These parameters were measured with pure CS film and Cu-doped CS film at room temperature (Table\u00a0S2). The photothermal model built through Equations 1\u20137 was finally solved by FEA method with commercial COMSOL Multiphysics. To be mentioned, the laser process is really complicated to accurately simulate the energy transfer, caused by chemical reaction, pyrolysis, and material phase transition. The models still need to be optimized.The Cu-LIG powders (5.0\u00a0mg) were scraped from Cu-LIG film surface using a razor blade and poured into mixture solution with 400\u00a0\u03bcL isopropanol and 75\u00a0\u03bcL DI water. After bath sonication for 10\u00a0min, the 25\u00a0\u03bcL Nafion solution (5 wt\u00a0%) was added into the above prepared suspension solution, followed by sonicating for another 60\u00a0min until the ink was homogeneous. For each electrode, a 100\u00a0\u03bcL mixture solution was repeatedly dropped onto a piece of carbon fiber paper and dried in the oven for 6\u00a0h at 80\u00b0C. Note that the carbon fiber paper was dried in a vacuum oven for 120\u00a0min at 100\u00b0C before using. The prepared electrode was directly exposed to the electrolyte with the fixed geometric area (1\u00a0cm2).The morphology of samples was observed with Zeiss Gemini 500 instrument field emission SEM. TEM and HR-TEM images were operated using a Thermo Fisher Scientific Talos F200X G2 system. FTIR spectra were recorded using a Thermo Fisher Scientific Nicolet iS50 spectrophotometer using an attenuated total reflection (ATR) mode over the range 400\u20134,000\u00a0cm\u22121. Semi-in situ XPS data were obtained on a Thermo Fisher ESCALAB Xi\u00a0+ microprobe using monochromatic Cu-K\u03b1 radiation (hv\u00a0= 1,486.6 eV) as the excitation source. The electrode used in CO2ER was protected in the N2 atmosphere to avoid oxidation of the catalytic surface. Powder XRD measurement was carried out on a Bruker D8 Advance with Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5). A Raman microscope using 532-nm laser excitation at room temperature with a laser power of 2 mW was employed to obtain Raman spectrum. The surface area of Cu-LIG was measured with a Micromeritics 3FLEX Brunauer-Emmett-Teller (BET) surface analyzer.Similar with Xu et\u00a0al.\u2019s work,\n43\n the semi-in situ XPS was performed as follows: the CO2ER was conducted in a glove box that was filled with nitrogen for 6\u00a0h in advance to ensure the catalyst is protected from oxidation. After CO2ER, the electrode was taken out and moved to a gas-tight chamber of XPS in the glove box. The gas-tight chamber was then sealed and passed to the XPS system for testing immediately (Figure\u00a0S21).The electrochemical experiments were carried out at ambient temperature and pressure in the two-compartment electrochemical (H-type) cell, separated by Nafion 117 cation exchange membrane. The measurement used a prepared carbon fiber paper with catalyst loaded as the working electrode operated by a CHI660E electrochemical workstation (Shanghai Chenhua, China). The counterelectrode was platinum, and the reference electrode was Ag/AgCl (3\u00a0M KCl), calibrated against hydrogen reference electrode (RHE; HydroFlex; Gaskatel). Before CO2ER tests, 0.1\u00a0M KHCO3 as electrolyte was saturated by CO2 (99.999%) through bubbling the gas for at least 30\u00a0min and calibrated by mass flow controller at a constant rate of 30 sccm. The gas continuously was bubbled into the electrolyte during electrolysis at a flow rate of 10 sccm. The LSV and cyclic voltammetry (CV) at a scan rate of 10\u00a0mV s\u22121 were performed at different overpotentials. The ohmic drop between the working electrode and the reference electrode was determined using potentiostatic electrochemical impedance spectroscopy at \u22120.6\u00a0V versus Ag/AgCl between 106 and 1\u00a0Hz with an amplitude of 10\u00a0mV. The current density was calculated on the basis of the total current divided by the geometric area of Cu-doped graphene. All overpotentials (V)\u00a0were converted to the RHE scale using the following formula:\n\n(Equation\u00a08)\n\n\n\nV\n\nR\nH\nE\n\n\n=\n\nV\n\nA\ng\n/\nA\ng\nC\nl\n\n\n+\n\n(\n\n0.199\n\nV\n\n)\n\n+\n\n(\n\n0.0592\n\nV\n\n)\n\n\u00d7\np\nH\n.\n\n\n\n\nFor durability analysis of the CO2ER process, the electrolyte was taken out of the electrochemical cell every 2\u00a0h to quantify the product by nuclear magnetic resonance (NMR) measurement.For the electrochemical tests in gas diffusion flow cell, three compartments, including gas diffusion chamber, cathode chamber, and anode chamber, were separated by working electrode and anion exchange membrane. The electrolyte (1.0\u00a0M KOH) with 10\u00a0mL min\u22121 is continuously pumped into the cathode and anode chambers, respectively, while CO2 gas was introduced into the gas diffusion chamber at the rate of 20\u00a0mL min\u22121. The electrochemical test process is consistent with those in the H-type electrolytic cell.The gas-phase products were collected and analyzed at 20-min intervals by gas chromatography (GC), which is equipped with thermal conductivity detector (TCD), flame ionization detector (FID), and methanizer. High-purity Ar was used as the carrier gas of TCD, and high-purity N2 was used as the carrier gas of FID. The peak areas of the products (H2, CO, C2H4, and CH4) were converted to gas volumes using calibration curves that were obtained, using a standard gas diluted to different concentrations. The liquid products were quantified using 600 MHz Solution NMR (Bruker BioSpin, AVANCE NEO) spectrometer. After electrolysis, 450\u00a0\u03bcL electrolyte was mixed with 50\u00a0\u03bcL D2O (99.9%; Sigma-Aldrich) for the 1H NMR spectroscopy analysis with water suppression. Standard curves for each product were prepared by the relative peak area ratio between product and internal standard.FE calculation, based on the definition of FE, is\n44\n\n\n\n(Equation\u00a09)\n\n\nF\n\nE\ni\n\n=\n\n\nQ\ni\n\n\nQ\n\nt\no\nt\na\nl\n\n\n\n,\n\n\n\nwhere i represents different products, such as H2, CO, C2H4, CH4, HCOOH, and ethanol, and Qi and Qtotal are the number of charges transferred to the product and the total number of charges passed into the solution, respectively. FE for formation of gas products, such as C2H4, CH4, H2, and CO, was calculated as follows:\n\n(Equation\u00a010)\n\n\nF\nE\n=\n\n\nn\nF\nV\nv\n\nP\n0\n\n\n\nR\nT\ni\n\n\n\u00d7\n100\n%\n\n\n\nwhere n is the number of electrons required for products, V (vol\u00a0%) is the volume concentration of gas product from the electrochemical cell, \u03bd (ml min\u22121 at room temperature and ambient pressure) is the gas flow rate, i (mA) is the steady-state cell current, p\u00a0= 1.01\u00a0\u00d7\u00a0105 Pa, T\u00a0= 273.15 K, F\u00a0= 96,485 C mol\u22121, and R\u00a0= 8.314\u00a0J mol\u22121 K\u22121.The FE for formation of liquid products, such as ethanol and FA, was calculated as follows:\n\n(Equation\u00a011)\n\n\nF\nE\n=\n\n\n\u03b1\nn\nF\n\nQ\n\n\u00d7\n100\n%\n,\n\n\n\nwhere \u03b1 is the number of electrons required for each product, n is the mole number of products (mol), F is the Faradaic constant (96,485 C mol\u22121), and Q is the total charge passed during the overall run.The partial current density of each product was calculated as follows:\n\n(Equation\u00a012)\n\n\nj\n\n(\n\np\na\nr\nt\ni\na\nl\n\n)\n\n=\n\n\nF\nE\n\n(\n\np\na\nr\nt\ni\na\nl\n\n)\n\n\n\n100\n%\n\n\n\u00d7\nj\n\n(\n\nt\no\nt\na\nl\n\n)\n\n,\n\n\n\nwhere FE (partial) is the FE of product and j (total) is the total current density of steady-state cell.First, three models (Cu0, Cu2O (Cu+), and Cu0\u00a0+ Cu2O (Cu0\u00a0+ Cu+), respectively) were built, followed with the results of semi-in situ XPS. We employed the Vienna Ab Initio Package (VASP)\n45\n\n,\n\n46\n to perform all the DFT calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) formulation.\n47\n We chose the projected augmented wave (PAW) pseudopotentials to describe the ionic cores and take valence electrons into account, using a plane wave basis and setting a kinetic energy cutoff of 400 eV.\n48\n\n,\n\n49\n Partial occupancies of the Kohn-Sham orbitals were allowed to use the Gaussian smearing method and a width of 0.05 eV. The electronic energy was taken self-consistently when the energy change was smaller than 10\u22124 eV. A geometry optimization was considered convergently when the force change was smaller than 0.05 eV \u00c5\u22121. The vacuum spacing perpendicular to the plane of the structure was 18\u00a0\u00c5. The Brillouin zone integral used the surfaces structures of 2\u00a0\u00d7\u00a02\u00a0\u00d7\u00a01 Monkhorst-Pack K point sampling.In a typical vial experiment, two pieces of prepared catalyst films (effective area: 0.5\u00a0cm\u00a0\u00d7\u00a01\u00a0cm for each piece) were set up in 10\u00a0mL of 23 ppm pollutants solution (specifically, 2 ppm for ethinyl estradiol). After establishment of adsorption/desorption equilibrium for 30\u00a0min, 10\u00a0mM H2O2 was added to the pollutant suspension under stirring at 150\u00a0rpm throughout the experiment. At time intervals, 500\u00a0\u03bcL of suspension was collected and centrifuged at 10,000\u00a0rpm for 20\u00a0min, then 200\u00a0\u03bcL of the supernatant was sampled and analyzed immediately. For regeneration analysis of catalyst film, the fresh BPA solution was added into the vial with the recycled catalyst film, followed by reaching balance of adsorption and desorption with 30\u00a0min. Then, 10\u00a0mM H2O2 solution was added, and removal efficiency was measured with a specific time interval.The samples were analyzed by ultra-performance liquid chromatography (UPLC) equipped with PDA detector (H-Class; Waters). The compounds were separated by C18 column (1.7\u00a0\u03bcm, 2.1\u00a0\u00d7\u00a050\u00a0mm, Acquity UPLC@BEH) with gradient elution. Mobile phases of H2O (0.05% FA) (A)\u00a0and acetonitrile (0.05% FA) (B)\u00a0were used. The flow rate was kept at 0.4\u00a0mL min\u22121. The initial gradient of 10% B was held for 0.5\u00a0min and increased to 40% B within 1\u00a0min and held for 2\u00a0min. The gradient was further increased to 100% B and held for 3\u00a0min. The chromatogram was acquired with a wavelength of 276\u00a0nm. TOC was determined by a Shimadzu TOC-L-CPH analyzer. The leached Cu during Fenton-like catalytic reaction was determined by ICP-MS (iCAP RQ; ThermoFisher). The EPR spectra were recorded on Bruker EMXPLUS EPR spectrometer at room temperature, and DMPO was used as the spin trap agent for \u00b7OH free radical. The intermediates during the degradation process were analyzed by GC-mass spectrometry (GC-MS; Trace1300-ISQ7000; ThermoFisher) with DB-5 MS capillary column. The GC oven temperature program was set as follows: initial 60\u00b0C was held for 2\u00a0min followed by linear temperature gradient to 280\u00b0C at 6\u00b0C min\u22121, held for 5\u00a0min. For liquid samples for GC-MS analysis, the catalytic film was separated with pollutant suspension after 180-min reaction, followed by freeze-drying for 2\u00a0days. Then both residues were dissolved in 2\u00a0mL of dichloromethane. After dehydration by anhydrous sodium sulfate, 0.2\u00a0mL of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) was added during stirring at 150\u00a0rpm for 2 h, and precipitates were separated by centrifugation until chromatographic analysis.A prospective LCA based on lab experiments and assumptions reflecting the recent progress in reactor and process designs was conducted in this study. GWP is for CO2ER system where products replace traditional fuels and chemicals. Contribution analysis was applied to reveal major contributors to LCA results, while scenarios analysis is supplemented to incorporate future improvements in technologies. A schematic diagram of \u201ccradle-to-gate\u201d system for CO2ER is defined in following the ISO 14040 series.\n50\n The objectives of LCA studies are (1) to assess environmental performances of CO2ER using an innovative catalyst, and (2) to compare the new systems with conventional products. The system boundary thus includes energy and materials flows associated with CO2 capture, electrochemical CO2 conversion, CO2 recycling, products separation, and the conversion of syngas to olefins. The functional unit for CO2ER system was defined as 1\u00a0kg of main product FA (>85.0 wt\u00a0%) for the ease of comparison with other studies. To deal with by-products such as O2 and olefins produced from syngas, environmental burdens from their productions are assumed to be avoided as a result of replacing the conventional, following the system expansion principle. More information about technology description, assumptions, and life-cycle inventory can be found in the supplemental experimental procedures.This work was supported by the Natural Science Foundation of China (grant 52003225), the Westlake Multidisciplinary Research Initiative Center (MRIC), the Research Centre for Industries of the Future (RCIF), and the foundation of Westlake University. The authors thank the Westlake Center for Micro/Nano Fabrication, the Instrumentation and Service Center for Molecular Sciences, and the Instrumentation and Service Center for Physical Sciences (ISCPS), Westlake University.Conceptualization: Z.L., X.H., B.Z., L. Wen, and L. Wang; methodology: Z.L., X.H., and L. Wang; investigation: Z.L., X.H., S.Z., Y.C., L. Wang, and X.D.; visualization: Z.L.; funding acquisition: B.Z., L. Wen, and L. Wang; project administration: L. Wen and L. Wang; supervision: L. Wen and L. Wang; writing\u00a0\u2013 original draft: Z.L. and X.H.; writing\u00a0\u2013 review\u00a0& editing: L. Wen and L. Wang.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2023.101256.\n\n\nDocument S1. Supplemental experimental procedures, Figures\u00a0S1\u2013S36, Tables\u00a0S1\u2013S8, and Notes S1\u2013S5\n\n\n\n\n\nData S1. Life-cycle assessment results\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  Our ecosystem is endangered by an increasing demand for resources, emission of pollutants, and wastes generated from industrial processes. Herein, we propose a sustainable upcycling strategy to tackle excessive carbon dioxide emissions and water quality issues synergistically. The crux of the strategy is to use seafood waste, easily converted to chitosan, to chelate heavy metals in contaminated water and produce metal-doped graphene composites. Graphene composites are prepared by scalable one-step laser scribing with controllable metal ion and nitrogen sites. The catalysts enable the efficient electroreduction of carbon dioxide to carbon products with over 80% Faradaic efficiency at 100 mA cm\u22122 in a flow cell reactor and can also catalyze the degradation of organic micropollutants. A prospective life-cycle assessment demonstrates a lower global warming potential, compared with conventional systems, for this system being used to produce formic acid and olefins in the future. Our sustainable upcycling strategy is expected to inspire practical techniques for producing green chemicals and clean water.\n               "}
{"full_text": "In recent years, researches on renewable and sustainable fuels have been highly prioritized around the world to create alternatives to fossil fuels. In this way, different types of biofuels have gained evidence due to their biodegradable, non-toxic, and physical\u2013chemical properties, which allow the total or partial replacement of diesel (Li et al., 2014). Biofuels also are interesting under economic viewpoint since they can be synthesized from vegetable oils, animal fats, and raw materials rich in free fatty acids, such as residual oils (Lam et al., 2010).The improper disposal of the residual oils can cause environmental problems since each liter of oil poured into the drain can pollute about 20 thousand liters of water (Georgogianni et al., 2009; Al-Hamamre and Yamin, 2014; Baskar et al., 2018). Indeed, it is possible to find in the literature several works (Widayat et al., 2019; Dai et al., 2017; Corro et al., 2016; Gan et al., 2010, Ashok et al., 2019) which the cook oils are used to synthesize biodiesel, and it is undoubtedly an eco-friendly and sustainable strategy for the next generations (Aghbashlo and Demirbas, 2016).Biodiesel is synthesized via transesterification or esterification chemical reactions. In some cases, depending on the origin of the raw material (such as the presence of triglycerides and free fatty acids), the reaction kinetics can be directed by simultaneous transesterification and esterification. On an industrial scale, biodiesel can be produced by both homogeneous and heterogeneous catalysis. Homogeneous catalysis has several disadvantages, such as the formation of soaps and their by-products, corrosion of the reactors, in addition to requiring several purification steps during the production process (Lee et al., 2014; Avhad and Marchetti, 2015; Mardhiah et al., 2017). On the other hand, the synthesis of biodiesel via heterogeneously catalyzed reactions has been the subject of promising studies being a viable solution to replace homogeneous catalysis, since it is possible to significantly reduce the number of purification steps and the possibility of separating and reusing the catalyst (Correia et al., 2014; Rashtizadeh et al., 2014; Paiva et al., 2015; Kim et al., 2016).In the heterogeneous catalysis, several types of catalysts stand out; however, ceramic compounds in the form of oxides (Baskar et al., 2018; Xie and Zhao, 2014; Gurunathan and Ravi, 2015; Sun et al., 2015; Sulaiman et al., 2019), have been extensively investigated in recent years for their high catalytic activity, excellent thermal and chemical stability, high corrosion resistance and environmentally optimized properties (Pradhan and Parida, 2012), and can be recovered and reused without significant loss efficiency in synthesis (Dantas et al., 2013).Several chemical synthesis techniques can be used to obtain ceramic oxide catalysts, among them stand out the sol\u2013gel route (Kesavamoorthi and Raja, 2016), co-precipitation (Zaharieva et al., 2015), Pechini method (Gerasimov et al., 2015) and combustion reaction (Dantas et al., 2020). The combustion reaction, which is the chemical synthesis techniques used in this work, has stood out for being a simple, effective, economical method (it uses low-cost reagents, less reaction time), it allows control of stoichiometry and morphology, besides promoting the obtaining of high crystallinity ceramic powders (Costa and Kiminami, 2012). Due to its various advantages, numerous studies have reported the use of the combustion reaction in the production of materials applied in several areas, such as photocatalysis (Das et al., 2019; Hermosilla et al., 2020), electronic materials (Vieira et al., 2014; Shanmugavani et al., 2015; Tholkappiyan et al., 2015; Diniz et al., 2017), heterogeneous catalysis (Manikandan et al., 2014; Alaei et al., 2018; Dantas et al., 2020; Kombaiah et al., 2019; Mapossa et al., 2020), and biomaterials (Ara\u00fajo et al., 2018; Khot et al., 2013; Kombaiah et al., 2018; Leal et al., 2018).Among the oxides already reported in the literature with catalytic potential, the hematite (\u03b1-Fe2O3) with binary structure type AnXp is widely used in several catalysis reactions because it is stable in ambient conditions and easy to process by different methods (Aghbashlo and Demirbas, 2016; Gurunathan and Ravi, 2015; Tholkappiyan et al., 2015; Kombaiah et al., 2019; Widayat et al., 2019). Widayat et al. (2019) used hematite (\u03b1-Fe2O3) synthesized by chemical co-precipitation in residual oil esterification/transesterification reactions and obtained 87.88% conversions in methyl esters. Studies conducted by Shi et al. (2017) showed the efficiency of hematite (Fe2O3) also as the support of oxides (CaO) in the production of biodiesel, in transesterification reactions of soybean oil and methanol, showing conversion to esters of 98.80%.Iron-based catalysts, such as ternary oxides of the type (AB2X4), have also attracted attention from the scientific community due to their properties and new technological applications, especially when the particle size approaches the nanoscale, which allows the control of properties such as magnetic characteristic and anisotropy (Dantas et al., 2020, Mapossa et al., 2020; Dai et al., 2017). In heterogeneous catalysis (Dantas et al., 2013, Dantas et al., 2017), the Ni0.5Zn0.5Fe2O4 and Ni0.7Zn0.3Fe2O4 ferrites synthesized by combustion reaction and tested the catalytic behavior in transesterification and esterification using methyl and ethyl routes, obtaining conversions in esters above 94%.Another heterogeneous catalyst that has a consolidated catalytic activity in the literature is zinc oxide (ZnO). According to Lamba et al. (2019), which synthesized the ZnO by combustion reaction and tested catalytically against methanol and madhuca oil, obtaining about 80% in conversion into esters. Baskar et al. (2018) also revealed the efficiency of the ZnO phase as support for biodiesel production, presenting conversions of 95.20% in methyl esters.In this work, the combustion reaction was used to synthesize a new magnetic catalyst with composition equal to ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3. The material was synthesized in a pilot-scale (Costa and Kiminami, 2012) and characterized in terms of its structure, morphology, magnetic and catalytic properties. Also, its catalytic capacity was investigated on the synthesis (TES reaction) of biodiesel from residual oil.In this research, the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was synthesized via a combustion reaction from the following chemical reagents, nickel nitrate hexahydrate (Ni(NO3)26H2O), hexahydrate zinc nitrate (Zn(NO3)2\u00b76H2O), iron (III) nitrate nonahydrate (Fe(NO3)3\u00b79H2O) and urea. All chemical reagents used were purchased on the Din\u00e2mica (Brazil) with purities between 98 and 99%. The performance of the catalyst was evaluated on the conversion of residual oil into biodiesel via simultaneous transesterification and esterification reactions (TES). The residual oil used was collected in pastry shops in the city of Campina Grande, located in Para\u00edba state - Brazil. The physicochemical parameters of the residual oil were accomplished in agreement with AOCS Cd 3d-63 standard, and the result showed a value of 14.8\u00a0\u00b1\u00a00.005\u00a0mg of KOH/g of sample, methyl alcohol (CH3OH)-purity 99.8% (Dynamic) and ethyl alcohol (CH3CH2OH) - purity 99.5% (Dynamic).The combustion reactions were accomplished in a pilot plant, which was built in the agreement of the patent BR 10 2012 002181\u20133 (Costa and Kiminami, 2012), see Fig. 1\n. The pilot-plant is constituted of the stainless-steel container, which is connected to a conical reactor with a capacity of 200\u00a0g/batch. The system reaches a maximum temperature equal to 350\u00a0\u00b0C after 60\u00a0min.Before the synthesis of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst, the initial composition of the precursor solution was calculated based on the total valence of oxidizing agents and reducing reagents using the propellants and explosives chemistry concepts (Jain et al., 1981). The auto-ignition (combustion) of a stoichiometric mixture of metallic nitrates and urea allocated in a stainless-steel container in a conical reactor with a production capacity of 200\u00a0g/batch (see Fig. 1). The temperature of the combustion reaction was measured every 5\u00a0s with the aid of an infrared pyrometer (Raytek, model RAYR3I\u00a0\u00b1\u00a02\u00a0\u00b0C).The performance of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was evaluated in the synthesis of biodiesel from residual oil via TES reaction. Before biodiesel synthesis, the residual oil was filtered (filter paper \u20b515,00\n\n'\n\u00b1\n\n0,15\u00a0cm) to remove the suspended particulate matter. The catalytic tests were conducted in duplicates and a pressurized stainless-steel reactor equipped with a pressure gauge, a thermocouple inlet duct, a borosilicate glass (80\u00a0mL). The conditions of the experiment were 30\u00a0g oil mass, time 1\u00a0h, and alcohol/oil ratio (15:1), see Table 1\n. The heating and agitation of the system were carried out with the aid of a plate model IKA C-MAG HS 7, external electrical resistance, and a magnetic bar of approximately 2.5\u00a0cm. After the reactions, the products of the catalytic tests were centrifuged to separate the catalyst, purified, and dried in an oven at 110\u00a0\u00b0C for 20\u00a0min with manual stirring at 5-minute intervals.For the analysis and optimization of the biodiesel synthesis from residual oil, a 23 factorial experimental design was drawn up in which it was analyzed the response surface and Pareto graph, evaluated using the Statistic 7.0 program. Table 1 describes the input levels and variables for the proposed planning.The temperature, catalyst concentration, and alcoholic route were the factors considered in the 23 factorial experimental design. The three levels for the selected factors were determined from preliminary experiments and literature published elsewhere (Dantas et al., 2020) (Table 1). The conversion of residual oil into biodiesel was performed as the answer to determine the optimized parameters. The effect of the independent factors on dependent factors was analyzed according to Eq. (1):\n\n(1)\n\n\nY\n=\n\na\n0\n\n+\n\na\n1\n\n\nX\n1\n\n+\n\na\n2\n\n\nX\n2\n\n+\n\na\n3\n\n\nX\n3\n\n+\n\na\n12\n\n\nX\n1\n\n\nX\n2\n\n+\n\na\n13\n\n\nX\n1\n\n\nX\n3\n\n+\n\na\n23\n\n\nX\n2\n\n\nX\n3\n\n+\ne\n,\n\n\n\nwhere Y is the answer (biodiesel conversion, %), a0 is the compensated term; a1, a2 e a3\n are linear coefficients; a12, a13\n, and a23\n are the interaction coefficients; and e is the error. X1, X2 e X3\n are input variables: temperature, the quantity of catalyst, and alcoholic route, respectively.The reuse tests were accomplished under the best reaction conditions established by the experiments and experimental planning. Before each reuse step, the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was removed from the synthesis products and clean with water 70\u00a0\u00b0C and hexane 99% (C6H14). After biodiesel synthesis, the catalyst was removed from the reaction medium using the following experimental procedure: application of an external magnetic field (magnet), washing with hot distilled water (~60\u00a0\u00b0C), washing the hexane solvent, centrifugation for 15\u00a0min, and oven drying at 110\u00a0\u00b0C for 24\u00a0h. This experimental procedure was adapted from the work published by Dantas et al. (2020). Finally, the reuse tests were performed under the best reaction conditions established by the experiments and experimental planning with the tested catalyst.The ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst synthesized was characterized by X-ray diffraction (XRD) using a BRUKER X-ray diffractometer (model D2 PHASER, Cu-K\u03b1 radiation), operating with 30\u00a0kV and 10\u00a0mA. The angular step and counting time used were 0.016\u00b0 and 44\u00a0min, respectively. The crystallite size was calculated with the aid of the Scherrer equation (Klug and Alexander, 1974), and from the peak of the most intense basal reflection, spinel d(311). The identification of the main crystalline phases was performed with the DiffracPlus Suite Eva software and Joint Committee on Powder Diffraction Standards (JCPDS). The quantification of each main crystalline phase was carryout by the Rietveld refinement (Rietveld, 1967; Moulton, 2019) with the aid of Diffrac. Topas software. The residual error of the Rietveld refinement was calculated from Eq. (2), which Wi\u00a0=\u00a01/Iobs\n and Iobs\n e Icalc\n are the observed and calculated intensities for each step, respectively.\n\n(2)\n\n\nSy\n=\n\n\u2211\ni\n\n\nW\ni\n\n\n\n\n\n\nI\n\nObs\n\n\n-\n\nI\n\nCalc\n\n\n\n\n\n2\n\n\n\n\n\nThe surface of the catalyst was characterized using the nitrogen gas adsorption and desorption technique. All experiments were carried out in a Quantachorme model NOVA 3200 equipment. The surface area and pore diameter were calculated using the Brunauer, Emmett, and Teller (BET) and by Brunauer, Joyner, and Halenda (BJH) methods, respectively.The morphological aspects of the catalyst sample were acquired by scanning electron microscopy (SEM), brand Tescan, model Vega3. The Laser diffraction technique was used to measure the particle size distribution using a nanoparticle analyzer SZ-100 series (HORIBA Scientific).Hysteresis plots were measured at room temperature using a vibrating sample magnetometer (VSM, Lake Shore model 7404), with a maximum applied magnetic field of 13,700 G. Saturation magnetization (Ms), remaining magnetization (Mr), and coercive field (Hc) were the properties obtained from this experiment.The acidity of the catalyst was determined through desorption analysis at the programmed ammonia temperature (TPD-NH3) in the SAMP3 multipurpose analysis system. Approximately 100\u00a0mg of sample was pretreated at 400\u00a0\u00b0C under helium atmosphere (30\u00a0mL.min\u22121). Then, the temperature was reduced to 100\u00a0\u00b0C, and the sample was subjected to ammonia current, for chemical adsorption, for 45\u00a0min. In the final step of the adsorption process, NH3 molecules were removed at 100\u00a0\u00b0C for 1\u00a0h and helium flow rate 30\u00a0mL.min\u22121. The thermograms were obtained on heating (from 100\u00a0\u00b0C to 800\u00a0\u00b0C), at 10\u00a0\u00b0C.min-1, and under a helium flow rate (30\u00a0mL.min\u22121).Thermogravimetric analysis (TG/DTG) was performed using Perkin Elmer STA 6000 TG-DTA equipment in N2 atmosphere with the flow of 20\u00a0mL.min\u22121 and heating rate of 10\u00a0\u00b0C.min\u22121, using 10\u00a0mg of sample in an alumina crucible, and a temperature range from 30 to 850\u00a0\u00b0C;The percentages of methyl or ethyl esters were determined via gas chromatography, using a chromatograph (VARIAN 450c) instrument with a flame ionization detector and a capillary column as the stationary phase (Varian Ultimetal \u201cSelect Biodiesel Glycerides RG\u201d; dimensions: 15\u00a0m\u00a0\u00d7\u00a00.32\u00a0mm\u00a0\u00d7\u00a00.45\u00a0mm). The initial injection temperature was 100\u00a0\u00b0C, the oven temperature was 180\u00a0\u00b0C, and the detector operated at a temperature of 380\u00a0\u00b0C.The acidity index (official AOCS method, Cd 3d-63) was used to characterize both the residual oil and the products resulting from the catalytic tests. It was possible to quantify the mass yield of synthesized biodiesel, considering the initial mass of the residual oil in the TES reaction, assuming that the complete reaction of a specified amount (x) of residual oil leads to the achievement of 100% yield mass (X) of biodiesel. Therefore, the percentages of mass yields were defined and calculated as the values that express the masses of the final products of the reactions after the purification processes.\nFig. 2\n shows the combustion reaction behavior measured during the synthesis of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. In summary, it was possible to identify three stages, where stage 1 was characterized by an oscillation in temperature that favored the evaporation of moisture followed by liquefaction of the reagents. In stage 2, the formation of the \u201cmushroom\u201d was observed (due to an increase in viscosity), followed by an excessive gas release. The ignition of the reagents combustion occurred in the final part of stage 2 (~1500\u00a0s). Stage 3 was instantaneous (~10\u00a0s) and reached a maximum temperature of 316\u00a0\u00b0C. In this last stage, there was the formation of an orange flame with a continuous and intense gas release. Still in step 3, it was possible to see a reaction explosion with flaking of the reaction product. The yield of the synthesis of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was 82.93%, more precisely 165.8\u00a0g of catalyst per batch.As the maximum temperature reached during the synthesis was relatively low (<500\u00a0\u00b0C), the materials synthesized have a high surface area, and a very pronounced nanometric characteristic, therefore, is suitable for its use as catalysts. This constitutes ease and versatility of the combustion reaction technique because, due to the control of the synthesis temperature, it becomes possible the morphological and structural control of the material, which is required for a given application (Dantas et al., 2017).\nFig. 3\n shows X-ray diffraction obtained from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst synthesized via a combustion reaction. The following crystalline phases were identified, inverse spinel of Ni-Zn ferrite (JCPDS 52-0278), hematite (JCPDS 89-0599), and zinc oxide (JCPDS 36-1451). The total crystallinity of the synthesized material was estimated at 43%, and the average crystallite size (calculated by Scherrer equation (Klug and Alexander, 1974; Avila et al., 2019) was equal to 25\u00a0nm. The estimated low crystallinity presented probably is related to the low-temperature of synthesis of the catalyst via combustion reaction (316\u00a0\u00b0C, see Fig. 2). This result is in agreement with other works that synthesized materials via combustion reaction and with a chemical composition similar to the one studied in this work (Dantas et al., 2020; Mapossa et al., 2020).\nFig. 4\n shows the Rietveld refinement accomplished on the X-ray diffractogram obtained from a ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. From this analysis, it was possible to see that the hematite was the major crystalline phase (55.87%); the inverse spinel of Ni-Zn ferrite was the second most abundant crystalline phase (36.96%), and zinc oxide was the crystalline phase with the lowest percentage (7.16%). Table 2\n summarizes the crystalline system, percentage of the crystalline phases, and the space groups calculated from the Rietveld refinement. In general, it is observed that the calculated parameters were very close to the theoretical values, and the values of the GOF, Rwp, and Rexp were 2.87, 0.99, and 0.35, respectively. Similar results were related by Mapossa et al. (2020).\nFig. 5\n shows the N2 adsorption/desorption isotherms measured from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. The isotherm obtained is of type III; which is an indication that the adsorption process is characteristic of non-porous or macroporous materials (Alothman, 2012). Also, the isotherms showed an inflection at a relative pressure (P/P0) of approximately 0.2\u00a0cm3/g, which is also indicative of the presence of micropores (Alothman, 2012).In agreement with IUPAC, solids containing pores diameter greater than 50\u00a0nm are called macroporous, between 2 and 50\u00a0nm are mesoporous, and those with pores smaller than 2\u00a0nm are called of microporous. The measured pore diameter from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was equal to 3.33\u00a0nm. Thus, the pore diameter and the isotherm profile corroborate with the indication that the synthesized catalyst has a mixed surface, that is, non-porous regions and other regions that have mesoporous or microporous.The specific surface area values (SBET) measured from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst was 52.9 m2g\u22121. This value is considered relatively high and is a consequence of the method used to synthesize the catalyst (combustion reaction at temperatures below 500\u00a0\u00b0C). The fact of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 has a relatively high specific surface area and nanometric characteristics make it an excellent candidate to be used as a catalyst. Some studies report that the synthesis temperature is a significant factor in obtaining materials with high surface area and nanometric characteristics. Materials synthesized at high temperatures (>1000\u00a0\u00b0C) have surface changes that are more pronounced, and in some cases, these modifications considerably reduce the surface area and active sites of catalysts, which negatively affect their catalytic activity (Tang et al., 2012).\nFig. 6\na\u2013b shows SEM images obtained from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. It is possible to observe several clusters of different sizes (Fig. 6a). This characteristic is more evident in Fig. 6b, where it was possible to detect agglomerates with high porosity and dimensions between 20\u00a0\u03bcm and 10\u00a0\u03bcm, respectively. These results are in line with the discussion as mentioned earlier about the specific surface area analysis when it is indicated that the catalyst obtained has disordered surface characteristics with non-porous regions and other regions that have mesopores or micropores with different types of shapes and sizes. The high porosity is due to the release of large quantities of gas during the synthesis process by combustion reaction (see step 3 in Fig. 2). Still in Fig. 6a\u2013b, both indicate a surface with a certain roughness, it is also possible to infer that the particles are weakly connected in an interparticular way. Similar results were observed by Tatarchuk et al. (2020) when studying the morphology of zinc spinel type ferrites.\nFig. 7\n shows the cumulative curve of the distribution range of the agglomerates (\u201cS\u201d shape) and histogram of the frequency of the distribution of agglomerate populations with the same diameter (first derivative of the distribution curve) measured from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. The distribution range of the particle diameter was between 20\u00a0nm and 100\u00a0nm, with an average diameter of 39.2\u00a0nm.From the distribution of the clusters, it was possible to observe that all samples showed a symmetrical and monomodal distribution of clusters, indicating samples with most of the total number of their clusters, as well as a finer particle size between them (values\u00a0<\u00a0100\u00a0nm). Such a result can be associated with the characteristics of particle size; smaller particle diameters necessarily imply a more remarkable ability to agglomerate by electrostatic forces.The structure, shape, and reactivity of the catalyst surface have a strong interaction with nature, the number and the intensity of the active sites available for the reaction. Thus, the reactivity of the catalyst surface is one of the inherent characteristics and its processing method. Therefore, to obtain a better precision of the surface reaction and to verify if this material is promising for catalysis, one must understand the acidity and alkalinity of the catalyst. (Dantas et al., 2017). In this way, the active acid sites of ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 were determined via TPD-NH3 analysis, see Fig. 8\n.Still in Fig. 8, it is possible to identify three NH3 desorption peaks. The first peak presented greater intensity, occurred at 208\u00a0\u00b0C, and is related to weak to moderate acidic sites. The second and third peaks occurred at 493\u00a0\u00b0C and 595\u00a0\u00b0C, respectively. These peaks are related to the strong acidic sites. The temperatures and intensity of the peaks observed in this work are in agreement with Dantas et al. (2017) and Dantas et al. (2020).\nTable 3\n lists the results obtained from the TPD analysis. The acidity of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst was calculated from the integration of the Gaussian curves observed in the TPD-NH3 analysis. The result indicated the existence of two types of NH3 desorption sites, which the first peak related to weak and moderate acid sites, represented by the temperature range between 100 and 350\u00a0\u00b0C, while the strong acidity sites are in the range temperature between 450 and 650\u00a0\u00b0C. Similar results were also reported by Masiero et al. (2009).Therefore, from TPD-NH3 analysis, the desorption events present in the samples showed concentrations corresponding to weak, moderate, strong acidic sites, and the calculated values were 169, 73, and 14\u00a0\u03bcmol/g of NH3, respectively. The sample had a total acidity of 256\u00a0\u03bcmol/g of NH3. From the highlighted results, it is possible to conclude that the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst has a strong acid character. Also, the results shown in this work corroborate with studies published by Dantas et al. (2020) and Mapossa et al. (2020), which investigated acidic sites by means TPD-NH3 analysis, and confirmed the presence of weak, moderate, and strong total acidic sites for spinel-type ferrites with composition chemistry, structure, and morphology similar to those synthesized in this work.\nFig. 9\n shows the dependence of magnetization (M) as a function of the applied magnetic field (H) for the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. From the hysteresis curve, it is possible to conclude that the studied catalyst has characteristics of soft magnetic materials. The low values of remaining magnetization (Mr\u00a0=\u00a06.12\u00a0emu/g) and coercivity (Hc\u00a0=\u00a05.3 G) support this information, since the magnetic hysteresis cycle is shown is narrow. Also, the well-defined S shape of the hysteresis curve is indicative that the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst has ferrimagnetic properties (Wang et al., 2012; Nihore et al., 2019).Also, in Fig. 9, the low value of the remaining magnetization (Mr) observed can be explained in terms of the composition of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst, since there is coexistence between crystalline ferromagnetic phases (55% Fe2O3), (36.96% Ni0.5Zn0.5Fe2O4) (Dantas et al., 2020, Shi et al., 2017), and diamagnetic phase (7.16% ZnO) (Franco et al., 2017). Similar results have been reported in the literature (Diniz et al., 2017) for the Ni-Zn system by microwave energy, where its magnetic characteristics are of a ferrimagnetic material (Hajalilou et al., 2015). Also obtained Ni-Zn ferrites synthesized by high-energy grinding and found that their magnetic hysteresis characteristics were presented in an \u201cS\u201d format with a unique coercive field. The results of this work corroborate those reported in the literature, emphasizing that the material is magnetic and its application in obtaining biodiesel because under the incidence of an external magnetic field, the catalyst will be easily removed from the reaction medium and thus reused.The fact that the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst has magnetic properties can help minimize the cost of biodiesel production since the catalyst can be easily removed from the reaction medium by applying an external magnetic field (magnet). Thus, magnetic separation is a relevant alternative to filtration and/or centrifugation since it contributes to reducing the loss of the catalyst and increases the reuse capacity, making the cost-benefit of the catalysts quite promising for industrial applications (Vieira et al., 2014).\nFig. 10\n shows the conversion of residual oil into esters and the mass yield obtained from the TES reaction through the ethyl and methyl routes catalyzed by ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3. All experiments were accomplished in agreement with the experimental planning indicated in Table 1. In general, it was possible to observe (Fig. 10) that the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst was active and satisfactory conversions were obtained in esters of fatty acids, in 96.1% ethanolysis, and 92.5% methanolysis. The best catalytic activity was obtained for the ethyl route, which is beneficial for the process since the alcohol used is less polluting and comes from the culture of sugarcane (Shikida and Bacha, 1998). The efficiency of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst may be associated with the presence of acidic and basic sites (see the results obtained in Section 3.6), which gives the system great versatility, such as the possibility of conducting simultaneous esterification and transesterification reactions.The experimental data shown in Fig. 10 also made it possible to observe that the profile of mass yield in biodiesel corroborates the profile of conversion into ethyl esters obtained from the tested reaction condition that showed the best catalytic activity, i.e., 30\u00a0g oil mass, time 1\u00a0h, and alcohol/oil ratio 15/1, 5% by weight of catalyst and 200\u00a0\u00b0C.Besides, regard to the percentage of acidity index reduction (Fig. 10), it was possible to verify that in all reactions, there were still unreacted free fatty acids. However, this occurs with greater emphasis on milder conditions of the percentage of catalyst and temperature. For example, using 3% catalyst and 180\u00a0\u00b0C, there was a reduction in the acidity of biodiesel by an average of only 44%, on the other hand, when elevated conditions reactions of the percentage of catalyst (5%) or temperature (200\u00a0\u00b0C), occurs a greater consumption of the fatty material available in the reaction, causing percentages of acidity reduction and conversions in higher esters, respectively, in the methyl route (88.4% and 92.5%) and the ethyl route (84.4% and 96.2%) using the so-called optimal conditions.\nOprime et al. (2017) emphasize in its research the importance of developing and using a formulation well resolved by process optimization methods through experimental planning, thus reporting the gain in time and amount of experiments as well as total process costs. In this context, in the present work, the statistical study was carried out, and Table 4\n describes the planning matrix used to analyze the statistical data on biodiesel production from reaction TES, using the magnetic catalyst and residual oil through the routes methanolic and ethanolic.Based on Table 4, it was possible to infer that the best catalytic condition for biodiesel\u2019s synthesis from residual oil was verified in experiment 7 since this favored a higher conversion into esters (96.16\u00a0\u00b1\u00a00.08). Still, it was possible to verify that was obtained using ethyl alcohol, which is beneficial because besides being considered less toxic, it is produced directly from sugarcane. It is relevant to highlight that Brazil in the world ranking, is among the largest producers of sugarcane, trailing only Colombia, Australia, China, and the USA.The responses of the statistical analysis carried out for the synthesis of biodiesel by TES from residual oil were evaluated using the Pareto graph, see Fig. 11\n.Analyzing the Pareto Graph (Fig. 11), it is possible to observe that the statistical analysis indicates 95% reliability (p\u00a0<\u00a00.05) (Gan et al., 2010), showing as significant variables: quantity of catalyst (%) and temperature, as well as the effects of secondary interaction, the quantity of catalyst\u00a0\u00d7\u00a0temperature (1by2), the quantity of catalyst\u00a0\u00d7\u00a0alcoholic route (1by3), and the interaction between temperature\u00a0\u00d7\u00a0alcoholic route (2by3). From this analysis, it was possible to conclude that the variable quantity of catalyst (%) and the secondary interactions (1by2) and (1by3) had a positive influence. In contrast, the variable temperature and the secondary interaction (2by3) had an influence negative. These observations are confirmed through the data in Table 4 and Fig. 10, suggesting that a mass increase in the quantity of catalyst significantly increases the conversion of residual oil into biodiesel through TES reaction.\nFig. 12\n(a) illustrates the level curves obtained as a statistical response of the independent input variables: quantity of catalyst (%) and temperature. The reliability of the analysis was p\u00a0<\u00a00.05, see Fig. 11. Fig. 12(b) illustrates the results obtained from the linear regression model (Eq. (3)), which dependence between a dependent variable or response (the content of converted esters) and a series of values (predicted results) of the independent variables describes the experimental data versus the predicted ones.The effect of secondary interaction between the percentage of catalyst and reaction temperature was better evaluated from the level curve (Fig. 12a). It was possible to observe that for high levels (+1) of temperature (200\u00a0\u00b0C), and quantity of catalyst (5%), the conversion of biodiesel was maximum (close to 100%), corroborating what was observed in the Pareto graph (Fig. 11). However, maintaining the upper level (+1) of the percentage of catalysts, at both temperatures studied, the conversion into esters remained at or above 85%. Therefore, it was possible to infer that the catalyst variable is the most significant and shows an excellent catalytic behavior of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic composite.In the investigated region, the response surface is described satisfactorily by the linear mathematical model given by Eq. (3), which presented an R2 of 94% and which defines the plane represented in perspective on the contour line (Fig. 12a), from according to the experimental planning carried out for the TES reaction of the residual oil, which best represents the data collected, analyzed and adjusting to the data in Table 4.\n\n(3)\n\n\nY\n=\n82.08375\n+\n7.07625\n\u2217\nx\n-\n2.395\n\u2217\ny\n+\n7.0475\n\u2217\nx\n\u2217\ny\n+\n\n4\n,\n2575\n\n\u2217\n0\n.\n\u2217\nx\n-\n,\n16625\n\u2217\n0\n.\n\u2217\ny\n+\n0\n.\n\n\n\n\nThe linear regression model (Fig. 12b) specifies the linear relationship between a dependent variable (or response) and a series of the predicted independent variables. This linear model governed by Eq. (3), represents a good description of the experimental data related to the content of the converted esters. It is possible to see in Fig. 12b that the results obtained experimentally are close to the values predicted by the model, considering that the modeling shows a correlation factor (R2) equal to 0.94. This figure shows that the model represents a relatively good description of the experimental data related to the methyl/ethyl ester content at 1\u00a0h reaction time and the alcohol-oil ratio of 1/15. The modeling results showed that the most significant effects were the linear effects of the quantity of catalyst, temperature, and the combined effects between temperature, catalyst concentration, and alcoholic route. The other effects showed less significance.The suitability of the linear model was also tested by analysis of variance (ANOVA) according to Table 5\n.The ANOVA results (Table 5) for biodiesel production showed the Fcal/Ftab value of 6.54, which indicates that the model was statistically significant. Therefore, the regression model is given in Eq. (3) was a reasonable prediction of the experimental results, and the factors affected were real at a 95% confidence level, as already observed in Fig. 11. Based on all the statistical planning, we can conclude that the planning gives the optimized condition at a 95% confidence level. The maximum conversions in esters would be observed: upper level (+1) for the catalyst quantity variables (5%) and temperature (200\u00a0\u00b0C) and lower level (\u22121) for the alcoholic route variable (methyl route).The recovery of magnetic particles for reuse in catalytic processes, in the most varied applications, has been highly reflected in the literature (Baskar et al., 2018; Dai et al., 2017; Guldhe et al., 2017). In the field of biodiesel production, some authors have already started to report excellent performances; for example, it is mentioned in (Ashok et al., 2019) with the use of nanoparticles of the ZnFe2O4Mn magnetic catalyst. Saxena et al. (2019), using magnetic Fe III nanocatalysts doped with ZnO, obtained high catalytic activity for the production of biodiesel, around 90\u00a0\u00b1\u00a02%, and exhibited excellent transesterification capacity after its reuse. About the reuse of heterogeneous catalysts, this is one of the advantages highlighted in the literature and in this case, those catalysts with intrinsic magnetism are considered an advantageous resource, as they can promote different results in terms of recovery and reuse (Dantas et al., 2020; Farias et al., 2020).In this context, the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was reused in the TES reaction using the optimized conditions: 30\u00a0g oil mass, time 1\u00a0h, and 15/1 alcohol/oil ratio, 5% by weight of catalyst and 200\u00a0\u00b0C. The conversion results obtained on reuse are illustrated in Fig. 13\n.Based on Fig. 13, it is possible to see that after 2 reuses, a loss of about 19.23% in efficiency in the catalytic activity was found. However, it was found that the catalyst showed an average conversion of 90.29\u00a0\u00b1\u00a00.44%. Therefore, the magnetic catalyst sample is economically viable for practical industrial applications.To assess possible structural modification on the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst during the TES reaction, thermogravimetry analyses were performed before the first catalytic test, and XRD analyses were accomplished before and after the catalytic tests.As shown in Fig. 14\n(a), there was no significant structural change in the catalyst after evaluating its useful life, when comparing the two diffractograms before and after the TES reaction. Also, it was possible to observe that the structural parameters remained unchanged, i. e., crystallinity and crystallite size showed a value of 41.2% (before 43%) and 26\u00a0nm (before 25\u00a0nm). This characteristic was confirmed with the aid of thermogravimetric analysis (Fig. 14(b)), where it was possible to observe that in the range up to 200\u00a0\u00b0C (maximum temperature used in TES reaction), it refers to the mass loss corresponding to humidity. According to Farias et al. (2020), subsequent mass loss events are attributed to decomposition processes and do not interfere with the TES reaction, since that the temperatures (180 and 200\u00a0\u00b0C) used in this work were lower. It is also possible to verify that the catalyst shows up to 800\u00a0\u00b0C, a total loss of mass of only 4.66%, where stabilization is verified. This statement is consolidated in the literature (Silva et al., 2019; Dantas et al., 2020; Farias et al., 2020) that indicates the thermal stability of spinel-type ferrites, starting at 800\u00a0\u00b0C. Based on the above, it is evident that there was no structural modification in the catalyst produced after the TES reactions, and the thermal stability of the same was also proven.These data suggest that the decrease in catalytic capacity of the catalyst after the second cycle of reuse may be related to the residual presence of triglycerides, and/or unconverted fatty acids and/or impurities arising from the frying process in the residual oil that were possibly adsorbed on the surface of the catalyst, preventing the participation of the active sites available for the reaction. Therefore, it becomes clear the need to optimize the cleaning process of the residual starting oil and the catalyst after TES reaction, for greater efficiency in subsequent reactions.The Fe2O3-Ni0.5Zn0.5Fe2O4-ZnO magnetic catalyst was synthesized on a pilot-scale using combustion reactions. The pilot-scale production was safe, reproducible, and efficient. The catalyst synthesized is ferrimagnetic (6.12\u00a0emu/g), polyphasic (Fe2O3-Ni0.5Zn0.5Fe2O4 - ZnO), nanometric (24\u00a0nm), and with high surface area (SBET\u00a0=\u00a052.9 m2g\u22121). Before, the use of factorial design made it possible to evaluate the process in a multivariate manner, leading to the identification of variables that significantly influenced the response variable (conversion of residual oil into esters). The factorial design allowed to identify the influence of the variables (percentage of catalyst, alcoholic route, and temperature) on the TES reaction, which, according to the statistical study, the quantity of catalyst and temperature followed by secondary interactions between all input variables (percentage of catalyst, temperature, and alcoholic route), were the ones that most affected the value of the response variable, with a significance level of 95%. The catalyst was effective in all conditions tested with conversions from 58% to 96%, with significantly promising results in the ethyl route. From the results obtained, it can be concluded that the studied catalyst can be successfully applied in the production of biodiesel, as the advantages have surpassed traditional methods since this polyphasic catalyst is a new product with innovative, magnetically active, and sustainable characteristics, and had a catalytically active useful life for two reuse cycles.The authors thank CAPES/CNPq for their financial support.", "descript": "\n                  A magnetic catalyst with composition ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 was synthesized by a combustion reaction on a pilot-scale and applied in the conversion of residual oil into biodiesel by simultaneous transesterification and esterification reactions (TES). For that, statistical analysis of the factors that influence the process (catalyst concentration, alcoholic route, and temperature) was evaluated by 23 factorial experimental design. The ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was characterized in terms of the structure, morphology, magnetic, TPD-NH3 acidity analysis and catalytic properties. The results indicate the formation of a catalyst with a surface area of 52.9 m2g\u22121, and density of the sample was 4.8\u00a0g/cm3 which is consisted of a mixture of the phases containing 55.87% Fe2O3, 36.96% Ni0.5Zn0.5Fe2O4, and 7.16% ZnO. The magnetic characterization indicated that the synthesized catalyst is ferromagnetic with magnetization 6.12\u00a0emu/g and coercive field of 5.3 G. In the TES reactions, the residual oil was active showing conversion to 96.16% ethyl esters and with a long useful life maintaining sustained activity after two consecutive reuse cycles with the conversion of 95.27%, 93.07% and 76.93%, respectively. The experimental design was significant and presented a 95% reliability level. The statistical analysis identified (+1) and (\u22121) as higher and lower level variables, respectively. The amount of catalyst used was equal to 5%, at 200\u00a0\u00b0C in methyl alcohol (alcoholic route). In summary, a new catalyst composed of a mixture of magnetically active phases was developed and successfully applied in biodiesel\u2019s synthesis from residual oil. Undoubtedly these results have a positive and significant impact on the environment and to society as a whole.\n               "}
{"full_text": "With accelerating climate change and a dependence on dwindling supplies of non-renewable carbonaceous energy resources, much research is focused on lowering greenhouse gas emissions as well as switching from a carbon-based to a hydrogen-based economy (Oliveira\u00a0et\u00a0al., 2021). Currently the majority of hydrogen is produced from steam-methane reforming (SMR), a non-sustainable and polluting process (Bauer\u00a0et\u00a0al., 2022). While attempts are being made to improve sustainability through the sequestration of the produced CO2 to provide so-called blue hydrogen, realistically, this is not sufficiently sustainable (Bauer\u00a0et\u00a0al., 2022). Another alternative which has been gaining increasing attention (Jang\u00a0et\u00a0al., 2019) as a potentially more environmentally friendly method of production, is the dry reforming of methane (DRM) (Lavoie,\u00a02014). The DRM process replaces the water used in SMR with second greenhouse gas, carbon dioxide (Eq.\u00a0(1)).\n\n(1)\n\n\nC\n\nH\n4\n\n+\nC\n\nO\n2\n\n\u2192\n2\nCO\n+\n2\n\nH\n2\n\n\n\u0394\n\n\n\nH\n\no\n\n=\n+\n247.3\n\nkJ\n\nmo\n\n\nl\n\n\n\u2212\n1\n\n\n\n\n\n\nCO2 and CH4 are the chief contributors to the greenhouse effect and as such, methods that convert both of these gases into other more environmentally benign products are key to reducing global greenhouse gas emissions. As a highly endothermic reaction, typical operating temperatures for DRM of over 600\u00a0\u00b0C are implemented to boost conversion, though catalysts are still required to achieve commercially viable reaction rates. Noble metal catalysts have shown high DRM activity with good stability (Edwards\u00a0and Maitra,\u00a01995), however their high cost prohibits their implementation as the sole active catalytic species (Pakhare\u00a0and Spivey,\u00a02014). More recently, focus has shifted towards cheaper, more abundant, transition-metal-based catalysts in an attempt to achieve a cost-effective process. Of the metals investigated thus far, nickel-based catalysts, supported on inorganic oxides, represent one of the most industrially relevant choices, due to their low cost, good activity, and the relatively high abundance of nickel (Kawi\u00a0et\u00a0al., 2015).Despite their high activity for DRM, nickel catalysts still present issues that must be overcome before they are utilised more widely. The main issue at present is rapid catalyst deactivation through metal sintering and degradation through coking (Kim\u00a0et\u00a0al., 2007). Coking refers to the deposition of carbon on the surface of the catalyst, mainly through unwanted side reactions such as the disproportionation of the carbon monoxide product, the Boudouard reaction (Eq.\u00a0(2)), and the cracking of methane (Eq.\u00a0(3)). The formation of coke through these reactions is more serious for DRM than SMR due to the higher CH4:CO2 ratio and lack of the oxidising steam component which helps to prevent carbon build-up.\n\n(2)\n\n\n\n2\nCO\n\n\u2192\nC\n+\nC\n\nO\n2\n\n\n\u0394\n\n\n\nH\n\no\n\n=\n\n+\n75\n\n\nkJ\n\nmo\n\n\nl\n\n\n\u2212\n1\n\n\n\n\n\n\n\n\n(3)\n\n\nC\n\nH\n4\n\n\u2192\nC\n+\n2\n\nH\n2\n\n\n\u0394\n\n\n\nH\n\no\n\n=\n\n\u2212\n172\n\n\nkJ\n\nmo\n\n\nl\n\n\n\u2212\n1\n\n\n\n\n\n\nThe formation of carbon can block catalyst pores and coat its active sites, resulting in a decrease in catalytic activity. The type and quantity of coke formed is dependant on the reaction conditions, such as the ratio of reactants, the temperature and the nature of the catalyst employed. At the relatively high temperatures employed in DRM, the formation of coke through the cracking of methane (Eq.\u00a0(3)) is reported to be the main reaction responsible for coke formation (Liu\u00a0et\u00a0al., 2011).As aforementioned, a typical supported catalyst used for this transformation is composed of metallic nickel supported on an inorganic oxide, such as alumina. Catalysts of this nature have thus far, however, proven too prone to coke formation (Liu\u00a0et\u00a0al., 2011; He\u00a0et\u00a0al., 2021; Swaan\u00a0et\u00a0al., 1994). Much current work focuses on the optimisation of this catalyst system to reduce coke formation while retaining the desired high activity (Wang\u00a0et\u00a0al., 2018). The addition of promoters such as noble metals and rare earth elements has proven successful in reducing the extent of coking during the DRM (Laosiripojana\u00a0et\u00a0al., 2005; Tsyganok\u00a0et\u00a0al., 2005). The support used can also play an important role in producing catalysts with improved resistance to coke deposition (Luisetto et\u00a0al., 2012; Gadalla\u00a0and Sommer,\u00a01988; Erdogan\u00a0et\u00a0al., 2018). In addition to the composition of the utilised catalyst, the preparation method has also been demonstrated to have an effect on the nature of coke formation(Abdollahifar\u00a0et\u00a0al., 2016).An important area of research is now the use of bimetallic catalysts for reduced carbon deposition (Sasson\u00a0Bitters et\u00a0al., 2022; Bian\u00a0et\u00a0al., 2017; Yentekakis\u00a0et\u00a0al., 2021). The chemical and physical properties of bimetallic catalysts differ from the properties of either of the metals when used in isolation and as such the combination can offer systems with higher catalyst activities and less coking than is achievable with monometallic catalysts. Co-metals studied in conjunction with nickel include cobalt, copper, iron, chromium, and bismuth, amongst many others (Sasson\u00a0Bitters et\u00a0al., 2022; Han\u00a0et\u00a0al., 2021; Sharifi\u00a0et\u00a0al., 2014; Li\u00a0et\u00a0al., 2019; Rouibah\u00a0et\u00a0al., 2017; Sutthiumporn\u00a0et\u00a0al., 2012). Of these, cobalt has generated some of the most significant interest as a potential co-metal for both improved catalyst activity and resistance to coking (Sasson\u00a0Bitters et\u00a0al., 2022).The addition of a small amount of cobalt to a nickel system is sufficient to enhance both the catalytic performance and improve resistance to coking, however, due to cobalt's lower activity relative to nickel, the addition of too much can be detrimental to performance. As such, the ratio of cobalt to nickel in a system can have a significant effect on catalytic activity (Sengupta\u00a0et\u00a0al., 2014; Li\u00a0et\u00a0al., 2022). Understanding how this nickel-to-cobalt ratio effects the composition and quantity of the coke formed is less explored, although in general it has been observed that higher quantities of cobalt result in less carbon formation (Takanabe\u00a0et\u00a0al., 2005). Despite a higher resistance to coking being observed in nickel-cobalt bimetallic catalysts, carbon formation still occurs and is unlikely to be eliminated as an issue entirely. Therefore, to develop catalyst systems that are stable for the long durations required for industrial applications, a better understanding of coke formation in this process is required to aid appropriate catalyst design.Characterisation of coke, understanding its formation and its effect on catalyst degradation is still a challenging process since carbon formation and its consequences on catalyst performance and durability occur across multiple time and length scales. Several characterisation techniques have proven vital in understanding the bulk properties of the coke formed, such as Raman spectroscopy and thermogravimetric analysis (TGA) (Sasson\u00a0Bitters et\u00a0al., 2022). While these techniques can provide important information about the bulk properties and quantity of carbon formed, they provide little insight into how this carbon formation affects individual catalyst particles and no spatial information as to where coke formation occurs.More recently, tomographic studies of catalytic systems have begun to be reported and can provide information about catalyst structure, performance and degradation that is not available through bulk techniques (Beale\u00a0et\u00a0al., 2014; Meirer\u00a0and Weckhuysen,\u00a02018). Studies have shown that these tomographic techniques are useful for understanding the porosity in fluidised catalytic cracking (FCC) catalysts (Meirer\u00a0et\u00a0al., 2015; Dasilva\u00a0et\u00a0al., 2015; Bare\u00a0et\u00a0al., 2014) as well as their degradation after use, including structural changes and the redistribution of elements (Meirer\u00a0et\u00a0al., 2015; Gambino\u00a0et\u00a0al., 2020). A limited number of studies have investigated coke formation on FCC catalysts with a focus on its position and effect on porosity (Vesel\u00fd et\u00a0al., 2021; Zhang\u00a0et\u00a0al., 2020). Similar techniques have also been applied to other catalyst systems such as those used for the Fischer-Tropsch process (Cats\u00a0et\u00a0al., 2016; Price\u00a0et\u00a0al., 2017) and in automotive applications (Schmidt\u00a0et\u00a0al., 2017). Recently hard X-ray ptychographic computed tomography was conducted using synchrotron radiation to investigate the structure of Ni/Al2O3 DRM catalysts (Weber\u00a0et\u00a0al., 2020) and the deposition of coke (Weber\u00a0et\u00a0al., 2021). Using this technique, Weber and co-workers were able to visualise the three-dimensional position of the formed coke throughout a single catalyst particle.Since catalyst properties influence the quantity, type and location of coke and have long-term implications on the catalyst performance and structural stability, we report here an investigation into the effect of the nickel-to-cobalt ratio on coke formation in Ni/Co/Al2O3 catalysts. The properties and quantities of coke formed are reported alongside easily accessible, lab-based X-ray nano-computed tomography data which gives spatially resolved information about coke formation on individual catalyst particles and new insights into catalyst degradation pathways after extensive coking, as a function of the nickel-to-cobalt ratio.A range of catalysts containing various ratios of nickel and cobalt were prepared using incipient wetness impregnation. Table\u00a01\n shows the nominal nickel, cobalt and alumina content of the prepared catalysts based on fully reduced, metallic nickel and cobalt. EDX measurements were conducted to verify these values which showed broad agreement with the trend expected from calculated loadings. Particle-to-particle variation was observed; the discrepancy between nominal and measured values is thought to be due to the small number of particles available for measuring using this approach. Various quantities of aqueous solutions of Ni(NO3)2\u20226H2O (99.98% metal basis, Sigma Aldrich) and Co(NO3)2\u20226H2O (\u226598%, Sigma Aldrich) were slowly added to \u03b3-alumina, 50\u2013200\u00a0\u00b5m, 60\u00a0\u00c5 pore size (Acros Organics). The quantity of each metal precursor was calculated to give the values shown in Table\u00a01 when fully reduced. The mixtures of alumina and metal precursors were dried in an oven at 100\u00a0\u00b0C for 4\u00a0h. The resulting dried powder was then calcined in static air at 550\u00a0\u00b0C for 16\u00a0h and allowed to slowly cool to room temperature inside a baffle furnace.Powder X-ray diffraction (pXRD) of the as-calcined, as-reduced, and coked samples was performed on a SmartLab diffractometer (Rigaku, Japan) fitted with a Mo K\u03b1 (\u03bb\u00a0=\u00a00.71\u00a0\u00c5) source. Samples were scanned from 2\u03f4\u00a0=\u00a05\u00b0 to 80\u00b0 with a step size of 0.01\u00b0 and a step time of 0.6\u00b0.min\u22121. Peak identification and data analysis were conducted using SmartLab Studio II Software v4.1.0.182 (Rigaku, Japan) and the Crystallography Open Database (Gra\u017eulis\u00a0et\u00a0al., 2012).The carbon deposited on each sample was analysed using a PerkinElmer Pyris 1 (PerkinElmer, US) thermogravimetric analyser. Approximately 4\u20138\u00a0mg of each coked catalyst was placed in a platinum sample crucible and heated to 900\u00a0\u00b0C with a ramp rate of 10\u00a0\u00b0C.min\u22121 under an air flow rate of 20\u00a0mL.min\u22121.Raman spectra of the as-reduced and coked samples was acquired using a Renishaw Invia Raman (Renishaw, UK) microscope with a RL532C class 3B continuous wave, diode-pumped solid-state laser which operates at 532\u00a0nm. Data was obtained over a period of 30\u00a0s at low laser power (< 15\u00a0mW), averaging over five acquisitions taken between 1000 and 1800 cm\u22121. Data was collected at five separate points on different catalyst particles to ensure the results were representative of the bulk sample. Data fitting was conducted using OriginPro 2021b data analysis and graphing software (OriginLab Corporation, U.S.A.) according to the method reported by Sadezky\u00a0et\u00a0al. (2005).Transmission electron microscopy (TEM) was conducted using a JEOL 2100 LaB6 transmission electron microscope (JEOL, Japan). Samples were prepared by sonicating in ethanol before dropping the suspension onto carbon-coated copper discs.Powder scanning electron microscopy (pSEM) of the as-calcined, as-reduced, and coked samples was conducted on a Zeiss SEM EVO MA10 (Carl Zeiss, Germany) by loading the respective powders on conventional sticky carbon tabs atop aluminium stubs and coating with nanometre-thick layers of gold using a Quorum SC7620 Mini Sputter Coater (Quorum, UK) to avoid charging. Micrographs were taken at various magnifications to capture general morphology, cracking extent, and the carbon formed on coked samples.Cross-sectional scanning electron microscopy (xsSEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on the same instrument as pSEM with the use of an INCA x-act SDD detector and INCA suite V5.05 software (Oxford Instruments, UK). For the 20Ni, 10Ni10Co and 20Co samples, as calcined powders were encased in an epoxy puck by overnight vacuum-curing of a mixture of epoxy resin and hardener (EpoFix kit, Struers, Denmark) before grinding with increasingly fine silicon carbide papers (Buehler, US). The epoxy pucks were subsequently carbon coated to prevent charging. This allowed for mapping of the metal distribution within the catalyst particles.Lab-based X-ray nano-computed tomography (nano-CT) was performed for both the reduced and coked variants of the 20Ni, 10Ni10Co, and 20Co samples, using a Zeiss Xradia 810 Ultra (Carl Zeiss) with a fixed X-ray energy of 5.4\u00a0keV and field-of-view of 65\u00a0\u00d7\u00a065\u00a0\u00b5m. Imaging was performed with either binning\u00a01 (voxel dimension of approximately 63\u00a0nm) or binning 2 (voxel dimension of approximately 126\u00a0nm) in both absorption and phase contrast modes. The full set of parameters are given in Table\u00a02\n.Reconstruction of all scans was performed in Zeiss XMReconstructor software utilising a standard filtered back-projection algorithm. Absorption and phase contrast tomograms were combined for all but the 20Ni coked sample (due to imperfect registration of the two tomograms) using the Zeiss DSCoVer tool (Version 16.1) before data processing and visualisation were carried out in Avizo 2022.1 (Thermo Fisher Scientific, US). All tomograms were Gaussian-filtered (3D, standard deviation of 1.1) and imported into open-source, machine-learning segmentation software (Ilastik 1.3.3 (Berg\u00a0et\u00a0al., 2019)) whereupon manual training was provided for the central slice and an initial segmentation prediction was generated. This approach (shown in Fig. S1) was taken as traditional supervised segmentation approaches in Avizo, such as watershed segmentation, did not appear to generate a reliable result. From the overlay of this prediction on the raw data, manual corrections were made by training two more slices before the final segmentation output was generated. Raw and segmented orthoslices and volume renderings were output from Avizo and equally sized sub-volumes (of ca. 6600 \u00b5m3) were extracted from each segmented tomogram before applying voxel counting to assess the phase fractions for solids and pores. The spatial resolution in each tomogram was estimated by using a sharp-edge fitting (Fig. S2). This involved drawing line plots across sharp feature boundaries and the number of voxels required to move from 10% to 90% of the voxel intensity difference between the feature and the background was combined with the nominal voxel size to provide an estimate of the true spatial resolution in each acquired tomogram (See Table\u00a03\n for estimated spatial resolution). Small particles, meso\u2011 and micropores below these resolution limits will not be detected and only features at or above will appear detectable in the acquired images.Catalyst pre-treatment and accelerated coking tests were conducted in a tube furnace (Lenton, UK). Catalysts were held within a quartz tube with gas flow of nitrogen, hydrogen and methane controlled using EL-Flow mass flow controllers (Bronkhorst, Germany). For catalyst pre-treatment, the sample under test was heated at 20\u00a0\u00b0C.min\u22121 to 700\u00a0\u00b0C under a flow of 100 sccm N2. Once at temperature, a flow of 100 sccm 4% H2 in N2 was introduced and maintained for 2\u00a0h. To understand the long-term effects of coking on catalysts for CO2 reforming of methane, accelerated coking tests were conducted. These tests involved inducing high levels of coking by using only methane as a feed gas as is commonly done for in situ experiments (Weber\u00a0et\u00a0al., 2021; Mutz\u00a0et\u00a0al., 2018). The catalyst samples under test were heated to 600\u00a0\u00b0C under a flow of 100 sccm N2. Once at temperature, a mixture of 5% CH4 in N2 was introduced at 100 sccm. The accelerated coking tests were conducted over a period of 4\u00a0h and once complete the gas flow was switched back to 100 sccm N2 and the sample allowed to cool naturally.To study the effect of nickel-to-cobalt ratio on the quantity and properties of the carbon formed during accelerated coking tests, a range of catalysts were prepared according to the procedure outlined in Section\u00a02.1 and hereafter will be referred to by the nomenclature shown in Table\u00a01. Representative SEM micrographs of each catalyst composition in the as-calcined state are shown in Fig.\u00a01\n (a-f). Each of the catalysts were observed to be similar in morphology with particle sizes remaining relatively consistent across all compositions. Some surface roughness was observed in each of the samples, potentially related to the nickel and/or cobalt metal oxide loaded onto the support. Some evidence of support particle cracking was also observed and, although minimal and difficult to observe in the 20Ni and 20Co samples, this was more clearly observed in the catalysts containing mixtures of the two metals. An SEM image of 10Ni10Co at higher magnification is shown in Fig.\u00a01(f); here a small number of cracks appeared to be running parallel to one another, suggesting a slight delamination in a particular crystallographic direction. pXRD analysis performed on the as-calcined catalysts, displayed in Fig.\u00a01(g), shows that the monometallic samples (20Ni and 20Co) consisted primarily of their respective oxide and the alumina support. A small unidentified peak (2\u03b8\u00a0=\u00a017.1\u00b0), potentially indicating a minor impurity, is present in all samples. The fact that this is observed in an XRD pattern of the as-received alumina source (see Fig. S3) suggests that this is the source; since it is present in all samples it is unlikely to affect the results of these studies. For the bimetallic cases (15Ni5Co, 10Ni10Co and 5Ni15Co), all three phases, \u03b3-Al2O3, NiO, and Co3O4 are evident, with no indication of any residual nitrates. Despite overlapping peaks, there is some evidence of the formation of some NiAl2O4, which is common for catalysts calcined at these temperatures (Siang\u00a0et\u00a0al., 2018). There is some evidence that this phase can improve nickel dispersion and reduce nanoparticle size, improving catalytic activity (He\u00a0et\u00a0al., 2021). The presence of this spinel structure is important since this phase has been shown to be more tolerant to coking and less likely to sinter (Salhi\u00a0et\u00a0al., 2011). Based on the XRD patterns, there is still a significant quantity of NiO present in the sample which will be more readily reduced and will likely contribute more significantly to the coking behaviour of the catalysts.The majority of metals present in the catalyst are expected to be present as nanoparticles throughout the internal microporous structure of the alumina support. To investigate the distribution of nickel and cobalt in the alumina support particle, xsSEM/EDX measurements were taken on 20Ni, 10Ni10Co, and 20Co. These tests involved encasing the catalyst powder in an epoxy resin before grinding with silicon carbide abrasives allowing for SEM and EDX measurements to be conducted on a smooth cross-section of each of the particles. The results from these tests are displayed in Fig.\u00a02\n. The 20Ni and 20Co particles investigated showed a uniform distribution of each of the metals throughout the particle, indicating thorough impregnation of the precursor salts throughout the alumina support. The bimetallic 10Ni10Co sample showed a similar, even distribution throughout the particle, again indicating appropriate impregnation and effective mixing of cobalt and nickel with no major agglomeration of either metal in any area.Prior to use, the catalysts analysed in Section\u00a03.1 were first reduced to form metallic nickel and cobalt. This reduction step was achieved by heating the as-calcined samples at 700\u00a0\u00b0C and flowing 4% H2 in N2 over each sample for 2\u00a0h. Since this reduced form is the active phase of the catalyst, analysis of these resulting powders was also conducted to determine any changes that may have occurred during this short, high-temperature treatment. Representative SEM micrographs of each of the reduced catalysts are shown in Fig.\u00a03\n. No significant change in catalyst morphology was observed, with the individual catalyst particles remaining similar in both size and morphology to the as-calcined samples shown in Fig.\u00a01. No significant change in cracking or crack sizes was observed, indicating that short-term exposure to a high temperature reducing environment does not result in significant catalyst particle cracking.Higher magnification images of selected catalysts are shown in Fig.\u00a03 (f-i), where slight changes in morphology of some of the smaller particles on the catalyst surface were observed, suggesting that while the majority of the metals are well dispersed throughout the support particle, some small micron-scale deposits may be present on the outer surface of each particle, somewhat agglomerated by the reduction procedure.After reduction at 700\u00a0\u00b0C for 2\u00a0h in an atmosphere of 4% H2 in N2, pXRD analysis was performed on the complete set of as-reduced catalysts, X-ray diffraction patterns of which are shown in Figs. S4-8. As expected, a characteristic new Ni0 peak appeared at 2\u03b8\u00a0=\u00a023.2\u00b0 in the reduced 20Ni sample and equally, characteristic NiO peaks at 2\u03b8\u00a0=\u00a019.6\u00b0, 27.8\u00b0 were no longer present, consistent with the full reduction of any nickel oxides present. In the reduced 20Co case, a characteristic new Co0 peak appeared at 2\u03b8\u00a0=\u00a023.1\u00b0, however, some characteristic Co3O4 peaks at 2\u03b8\u00a0=\u00a016.7\u00b0, 28.7\u00b0 persisted, albeit at a lower intensity. This indicated that some residual oxide remained present after the reduction step, although it is not clear if this residual oxide is present due to incomplete reduction of the nickel oxide, or due to a small degree of re-oxidation after reduction, prior to XRD measurements. In the bimetallic cases (15Ni5Co, 10Ni10Co and 5Ni15Co), there was evidence of \u03b3-Al2O3 as well as characteristic peaks for both metals (Ni and Co), and as with the pure Co sample, there was still some evidence of small quantities of residual Co3O4. Testing showed that while small quantities of residual Co3O4 remain at this temperature (700\u00a0\u00b0C), the majority of the oxides are reduced.Industrial catalysts are subjected to use online for thousands of hours to make processes cost effective. As such, for the bimetallic Ni/Co catalysts to be developed commercially, thorough understanding of the effects of long-term coking are required. Most papers that cover the coking of Ni/Co catalysts tend to only test catalysts for hours to occasionally tens of hours (Sasson\u00a0Bitters et\u00a0al., 2022). This is not sufficient to understand the long-term catalyst degradation that occurs after extended coking. To simulate the effect that long-term exposure to carbonaceous gases would have on varying nickel-to-cobalt ratios, accelerated coking tests were carried out. To maximise the amount of coke-induced degradation in a reasonable time, a relatively low, but still applicable, temperature of 600\u00a0\u00b0C was used. Since the presence of CO2 in the stream can oxidise and aid coke removal, the accelerated coking tests were carried out in 5% CH4 in N2. Accelerated coking tests such as these are commonly applied when running in-situ experiments (Weber\u00a0et\u00a0al., 2021; Mutz\u00a0et\u00a0al., 2018). After four hours on stream under these accelerated coking conditions, each of the catalysts was analysed in order to gain a deeper understanding of how the nickel-to-cobalt ratio affected the type and quantity of the coke formed and the influence these factors had at the catalyst-particle scale.After accelerated coking tests, the catalyst samples all appeared black, although their consistency and the apparent tap density of the spent catalyst powders appeared to vary. TGA was performed on each of the catalyst samples to determine the quantity of carbon and gain information about its type and how this varied with nickel-to-cobalt ratio. The results from the TGA testing are shown in Fig.\u00a04\n. Fig.\u00a04(a) shows the percentage weight change against the temperature for each of the samples. Based on these results it was possible to infer the quantity of carbon formed on each of the samples (see insert Fig.\u00a04(a)). As expected from previous studies (Li\u00a0et\u00a0al., 2021), the 20Ni sample contains the largest quantity of carbon with a weight loss of 35 wt%. Due to the higher quantity of nickel present in this sample it is likely that more of the NiAl2O4 spinel phase is present. Despite this, the significant quantities of metallic nickel present in the sample from the reduced NiO results in significant quantities of coke formation. Alteration of the preparation methods to produce more of the spinel phase would likely improve the resistance of this catalyst to coking. The addition of Co to give the 15Ni5Co sample had a significant impact on the quantity of carbon formed with the carbon content dropping to 15 wt%. Addition of further cobalt to give the 10Ni10Co sample further improved the coke resistance of the catalyst with carbon content dropping to 7 wt% and the 5Ni15Co sample carbon content was found to be less at ca. 6 wt%, however by this point it appears that the improvements were diminishing. Interestingly, when the nickel is removed to give the 20Co sample, there was an increase in coke formation (ca. 12 wt%), illustrating the benefits of bimetallic catalysts, whereby the synergistic effects of combining two metals results in properties not observed for either of the metals alone.The derivative weight change is shown in Fig.\u00a04(b), the positions of the peaks in this plot indicate the temperatures at which the majority of the carbon is removed from the bulk catalyst. If the type and composition of the carbon were consistent across all samples, the temperature at which the carbon burnt off would be expected to be the same, barring any small catalytic effects of the metals present. Based on the results shown in Fig.\u00a04(b), the type of carbon formed was dependant upon the nickel-to-cobalt ratio present in each sample.For the 20Ni sample, most carbon was removed at ca. 700\u00a0\u00b0C, higher than the other samples, and this relatively high temperature could be interpreted as there being more graphitic carbon present in this sample, since graphitic carbon tends to be more thermally stable than amorphous carbon. With the addition of small quantities of cobalt to produce the 15Ni5Co sample, the peak position moved to a lower temperature of ca. 600\u00a0\u00b0C, indicating that the type of carbon is different in this sample relative to the 20Ni, as the coke formed is less thermally stable. Addition of more cobalt to form the 10Ni10Co and 5Ni10Co samples further changed the peak position and shape, although these two samples did appear similar to one another. For these two catalysts, two small peaks were present at ca. 700\u00a0\u00b0C, which indicate that there was a small quantity of carbon present with a similar thermal stability to that seen in the 20Ni sample, but the majority was composed of carbon that decomposes at ca. 550\u00a0\u00b0C.Finally, the 20Co sample showed peaks similar to the high-cobalt-containing mixed catalysts with peaks present at both 700\u00a0\u00b0C, similar to 20Ni, but the 20Co also had a major peak at just below 550\u00a0\u00b0C, indicating the majority of the carbon present was in the less thermally stable form. These results show the significant effect that the nickel-to-cobalt ratio has on the quantity and type of carbon formed during operation.Raman spectroscopy was performed on each of the coked catalyst samples to provide bulk characterisation of the formed carbon. Two major peaks associated with the D and G band of carbon were observed in the 1000\u20131800 cm\u22121 range. The Raman spectra obtained in this wavenumber range for each of the catalysts is shown in Fig.\u00a04(c) \u2013 (g). According to work by Sadezky et\u00a0al., the first-order region of the Raman spectra can be deconvoluted and fitted to five bands; four Lorentzian-shaped bands at 1580, 1350, 1620 and 1200\u00a0cm\u22121 referred to G, D1, D2 and D4, respectively, and one Gaussian-shaped band at 1500 cm\u22121, referred to as D3 (Sadezky\u00a0et\u00a0al., 2005). The G band is commonly referred to as the graphitic band and corresponds to an ideal graphitic lattice vibration mode. As such, the ratio of this peak to the D1 peak is often used to give an indication of how disordered or graphitic a carbonaceous material is. While these two bands are regularly used, it is less common to investigate the D2, D3 and D4 bands. The D3 band is likely related to the amorphous sp2-bonded forms of carbon such as those present in polycyclic aromatic compounds or other organic molecules and fragments. The D4 band can likely be attributed to sp2-sp (Jang\u00a0et\u00a0al., 2019) bonds or CC and C\u00a0=\u00a0C stretching vibrations of polyene-like structure (Sadezky\u00a0et\u00a0al., 2005).From the results shown in Fig.\u00a04(c) - (g), it may appear that all catalyst samples give similar Raman spectra, however, when deconvoluted to give the five bands discussed, information on the differing nature of the carbon can be obtained. The ratio of the intensity of the D1 band to G band gives an indication of the ratio of the disordered carbon to the carbon that is graphitic in nature. Based on the acquired data, the 20Ni catalyst had the highest quantity of disordered carbon present. This apparent discrepancy with the TGA data is addressed in the next section. Addition of a small quantity of cobalt resulted in a decrease in the ID1/IG ratio, suggesting more graphitic carbon was formed for this sample. (Fig.\u00a04(h)) Further addition of cobalt to form the 10Ni10Co and 5Ni15Co samples both resulted in a decrease in ID1/IG, suggesting that the more cobalt that there is in the system, the higher the degree of graphitic carbon formation. While the addition of increasing quantities of cobalt caused ID1/IG to continuously decrease, when nickel was removed completely (20Co sample), the ID1/IG value increased. This again illustrates the beneficial nature of bimetallic catalysts possessing properties that neither metal possesses alone.For all nickel-to-cobalt ratios, bands representing graphitic carbon (G) and defects/heteroatoms present in graphitic lattices (D1 and D2) were observed, however there were no D3 and D4 bands present. The D3 and D4 bands are related to the sp (Bauer\u00a0et\u00a0al., 2022) and sp (Jang\u00a0et\u00a0al., 2019) hybridised carbons found in typical carbon chains, e.g., polyene CC and C=C type bonds, suggesting that there were few of these forms of carbon present within the coked catalyst samples, implying that the carbon formed was almost purely graphitic in nature, but with varying amounts of defects present.XRD of the coked samples (Figs. S9\u201313) indicated that after accelerated coking studies there was little change to the crystallographic nature of the catalysts observable using this technique. The majority of the metals remained present in their reduced form with small amounts of NiO reformed in the high nickel containing catalysts and a small additional peak ascribed to the formed coke was observed. The exception to this was the 20Ni system where larger NiO peaks were observed in addition to the metallic nickel peaks, it is not clear why this sample shows oxidation while others do not, all samples were treated in the same manner, but it is possible that this oxidation may be related to the significant morphological changes that occurred to this sample which are discussed in more detail below.To understand how the nickel-to-cobalt ratio influences the extent and type of coking of these catalyst systems, methods other than bulk analysis techniques are required. The ratio of metals not only affects the quantity and properties of the coke formed as analysed in Section\u00a03.4.1 but can also have a significant effect on the catalyst-support particle as whole. SEM micrographs of the coked samples are shown in Fig.\u00a05\n(a) \u2013 (f). Comparison of these images with those of the reduced catalysts shown in Fig.\u00a03 show that a significant change in morphology has occurred during the accelerated coking procedure. In addition to this, while the reduced catalyst particles all appeared relatively similar, there was a significant difference between the coked samples based on the composition of the catalyst, indicating that nickel-to-cobalt ratio has a significant influence on the resulting morphology.The 20Ni catalyst was distinctly different from its reduced form, with individual catalyst particles no longer visible - these were replaced with long elongated forms with a texture significantly different to that of the original catalyst particles. Based on the image shown in Fig.\u00a05(a), it appears as though the particles were entirely coated or consisted almost entirely of fibrous carbon. A similar, albeit less drastic, effect was observed for the 15Ni5Co catalyst sample shown in Fig.\u00a05(b). Here the elongated forms did not appear as long and some of the rough catalyst-support particle shape observed in the reduced sample was retained. This image appears to show plates of the support connected by fibrous carbon, consistent with carbon having formed within particles, cracking them apart. Interestingly, all cracks appeared parallel as if delamination had occurred along the same crystallographic plane. The 10Ni10Co and 5Ni15Co samples shown in Fig.\u00a05(c) and (d) most closely resembled that of the reduced catalysts prior to coking. The catalyst particles were clearly visible with little change in shape or size, although they did appear to have some fibrous, carbonaceous species formed on the surface.The 20Co sample was distinctly different to both the 20Ni and the bimetallic catalysts (Fig.\u00a05(e) and (f) (magnified)). The formation of fibrous carbon across the surface of catalyst particles was observed and while the particles retained their rough shape, significant cracks were evident. These cracked particles, however, appeared significantly different from the cracks observed for the other systems.\nFig.\u00a05(g) - (j) shows TEM micrographs of the coked catalysts. TEM analysis confirms the presence of filamentous carbon. This form of carbon is thought to form through the following mechanism. First, carbon is absorbed on the surface of metal particles giving Ca. Most of this carbon is gasified, first methane dissociates on the surface of metallic nickel producing reactive carbon species, often referred to as alpha carbon, Ca. Most of this form of carbon is gasified but a small proportion is converted into the beta form, Cb, a less reactive form which either begins to encapsulate the active nickel or dissolves into the nickel itself (Trimm,\u00a01997). Dissolution of carbon into the nickel structure can result in the formation of carbon whiskers. The dissolved carbon tends to dissolve through the nickel particle to the rear of the crystallite, where the carbon begins to precipitate, such that continuous build-up results in the formation of carbon whiskers which break the nickel particle from the surface of the support (Trimm,\u00a01997). While this type of carbon formation is often seen as less severe than other forms of coke which coat the active surface and deactivate the catalyst, excessive formation of this type of carbon has been shown to destroy catalyst particles or block the reactor (Liu\u00a0et\u00a0al., 2011). The dark particles observed in Fig.\u00a05(g) \u2013 (j) are thought to be metal particles and their presence in all samples supports this universal mechanism of filament/ whisker formation. The formation of carbon whiskers in this manner has previously been deemed responsible for the fragmentation of catalyst particles (Ochoa\u00a0et\u00a0al., 2020), as is clearly observed in the case of 20Ni, 15Ni5Co and 20Co samples. The extent of cracking appears to be related to the quantity of carbon formed since the 20Ni sample, which had the largest amount of carbon present after testing, resulted in the most significant particle breakdown, followed by the 15Ni5Co catalyst which has the second highest quantity of carbon present.Despite all systems showing the presence of this filamentous carbon, there are differences between the samples, with the nickel-to-cobalt ratio appearing to have an influence and effect on the morphology. One of the main differences between the 20Ni and the other samples is the presence of small particles of support intertwined with the carbon filaments. This suggests a more significant break-up of the support in this sample than all of those containing cobalt. In addition to the presence of small support fragments, there are significant differences in the thickness of the carbon filaments. For the 20Ni sample, it mainly consisted of relatively thick, multi-walled, carbon filaments, predominantly ca. 40\u00a0nm in diameter with a wall thickness of ca. 17\u00a0nm. For the 15Ni5Co sample, there were still some of these larger thicker-walled filaments but in addition there were also numerous thinner filaments with a diameter of ca.\u00a020\u00a0nm and wall thickness of ca. 5\u00a0nm. When the cobalt loading is increased (10Ni10Co), no larger, thicker filaments are present with most filaments measuring ca. 15\u00a0nm in width, with wall thickness of ca. 5\u00a0nm. The 5Ni15Co sample consisted almost entirely of filaments in the 15\u201320\u00a0nm range with a wall thickness of approximately 5\u00a0nm.The TEM micrographs support the Raman spectroscopy and TGA data presented in Fig.\u00a04. As the cobalt content is increased, the ID1/IG ratio decreases, suggesting an increase in the graphitic nature of the carbon present in the sample. Consequently, it would be expected that the temperature at which the carbon is thermally degraded and removed would increase, however for these samples the opposite is true. Bimetallic samples with increasing cobalt content combust at lower temperatures, which would typically be expected for samples with higher ID/IG ratios. This apparent contradiction is explained by the carbon morphology. Studies have shown that carbon nanotubes (CNTs) with larger diameters and thicker walls will thermally decompose at higher temperatures than narrower CNTs. (Singh\u00a0et\u00a0al., 2010) In addition to increased thermal stability, a more prominent d-band is observed in their Raman spectra due to the more defective nature of thick, multi-walled CNTs, resulting in higher ID1/IG ratios for larger, thicker-walled CNTs (Singh\u00a0et\u00a0al., 2010). There are two main peaks present in the derivate weight change TGA plot (Fig.\u00a04(b)), one at ca. 700\u00a0\u00b0C and another at ca. 550\u00a0\u00b0C. It is likely that the peak at 700\u00a0\u00b0C relates to the thicker-walled, 40-nm-diameter filaments, whereas the peak at ca. 550\u00a0\u00b0C corresponds to the narrower CNTs observed with diameters in the range 15\u201320\u00a0nm. This would be consistent with the fact that the 20Ni sample has almost exclusively thick CNTs, and has a prominent peak at 700\u00a0\u00b0C, whereas the bimetallic catalysts have peaks covering both types. The 10Ni10Co and 5Ni15Co samples, which are composed almost exclusively of the thinner type of filament possess only a very small peak at 700\u00a0\u00b0C, with a far more prominent peak present at 550\u00a0\u00b0C. The decreasing ID/IG ratio in the bimetallic catalysts with increasing cobalt supports the formation of thinner filaments.Accessible, lab-based X-ray nano-CT provided three-dimensional information about the catalyst samples both post-reduction and post-coking. Raw ortho-slices, segmented ortho-slices, full volume renderings, and volume renderings of \u201cinternal non-support\u201d phase of sub-volumes are shown for the as-reduced samples in Fig.\u00a06\n, and similarly for the samples post-coking in Fig.\u00a07\n. It should be noted that \u201cinternal non-support\u201d phase relates to the darker grayscale regions, internal to the catalyst support particles which are either macropores (in the case of as-reduced samples) or a combination of macropores and carbonaceous material (in the case of as-coked samples), to a greater or less degree. The carbonaceous filaments are not sufficiently large or X-ray attenuating to be detected here.Figs. S14-19 show several slices from various sections of each sample. It is clear from Fig.\u00a06(a)\u00a0\u2013\u00a0(c) that there were interparticle voids present within all as-reduced samples, prior to exposure to CH4 at elevated temperatures, as well as macroporosity within individual support particles; the tomograms showed that the samples prior to coking were nominally the same in this regard. It is worth noting that since the spatial resolution of this technique has been estimated to be between 320 and 740\u00a0nm, the meso\u2011 and micropores known to be present within the Al2O3 support particles are not resolved in this study, although the largest of these may be resolvable by synchrotron X-ray ptychographic techniques (Weber\u00a0et\u00a0al., 2022). Some \u2018excess\u2019 large metal particles were observed on the support particle surfaces but this is not thought to majorly influence the behaviour of samples in this study given that the size of these particles (indeed, observable by X-ray CT) is significantly larger than the catalytically active particles contained within the catalyst support particles. This demonstrates the power of lab-based X-ray CT for identification of large metal particles, something that is not easily achieved with the majority of other techniques normally employed for heterogenous catalyst characterisation. None of these larger metal particles are observed within the support particle. Based on these observations it might be expected that there may be a gradient from the centre of the support particle to the surface in terms of metal concentration, however based on the cross-sectional SEM-EDX shown in Fig.\u00a02, this gradient is not significant. Fig.\u00a06(d) \u2013 (f) represent robust segmentations of air (black), yellow (supported catalyst), blue (internal non-support) and purple (residual metal) using a machine-learning-based approach, whilst the volume renderings in Fig.\u00a06(g)\u00a0\u2013 (i) display the similar morphology of each of the samples examined, constituting multiple individual support particles adjoined to one another. Fig.\u00a06(j) \u2013 (l) illustrate a similar quantity of internal non-support phase associated with a sub-volume (ca. 6600 \u00b5m3) extracted from within each sample. A semi-quantitative approach has been taken to characterise the change in morphology associated with the extent of coking observed for each sample. The volume of \u201cinternal non-support phase\u201d, which encompasses interparticle voids and macropores, as well as undetectable carbonaceous material, has been estimated by voxel counting after segmentation. It should be reiterated that features below the resolution limit will not be detected and therefore are excluded from this analysis. Table\u00a04\n illustrates that this phase constituted approximately 0.4\u20131.4% across the as-reduced samples, supporting the assertion that these samples were nominally the same in terms of interparticle voids and macropores.As observed from the two-dimensional SEM imaging, the morphology is significantly changed following the accelerated coking procedure, and vast differences between samples are apparent, as seen in Fig.\u00a07. Firstly, the most significant morphological change can be seen to occur in the 20Ni sample, as observed in 2D in the SEM micrograph of Fig.\u00a05(a). At first viewing, the foreground material appears counterintuitively to be unconnected and therefore floating in the air, however this apparent discrepancy is explained by the low X-ray attenuation coefficient of the carbonaceous material that surrounds the broken shards of the support. It is supposed that coking has occurred to such a significant degree that the bulk of the imaged sample is in fact carbon, but the contrast versus air is insufficient to detect all three phases. It is evident that the original support structure is no longer present, and the vast majority of the remaining sample comprises carbon material that is not directly observed using the X-ray CT technique, both due to the technique's resolution and the low density of the carbon material. Fig.\u00a06(a) \u2013 (c) shows that there are some parallel cracks within particles, as inferred from the SEM micrographs shown in Fig.\u00a01 (highlighted by red arrows). It is thought that these represent weak points that help explain the morphology of the coked samples wherein parallel shards of alumina support appear interspersed within carbonaceous filament networks. A sub-volume within the sample, delimited by the visible fragments, was extracted and the blue volume shown in Fig.\u00a07(j) represents \u201cinternal non-support phase\u201d, comprising both previous and newly formed voids but mostly filamentous carbon, all ascribed to this one composite phase as the carbon phase is not distinguishable. Clearly, a vast reduction in the volume percentage attributable to supported catalyst and excess metal was observed, which is quantified in Table\u00a04. The next most significant transformation occurred in the pure cobalt sample (20Co). Fig.\u00a07(f) clearly displays that coking has led to significant breakdown of the individual support particles and their cracks can be seen to align with one another, potentially along a single crystallographic direction. Although the support particle structure has degraded, the overall morphology is intact, highlighting that the behaviour of 20Co differs from that of 20Ni, and the lesser degree of coking (corroborated by TGA) leads to less support particle degradation in the former. This is also consistent with the observations by SEM of parallel cracking within catalyst support particles (Fig.\u00a05(f)).However, the morphological change in the 10Ni10Co catalyst is clearly less dramatic still. Although not observable from two-dimensional SEM, some minor internal cracking between previously adjoined catalyst support particles was evident from the X-ray tomograms of this sample (see Fig.\u00a07(b)). On the other hand, far fewer cracks were present within each individual catalyst-support particle, indicating a different degradation pathway from that followed by 20Co (and indeed 20Ni). Consistent with the lesser extent and different nature of the coking indicated by TGA and Raman spectroscopic analysis, the bimetallic catalyst (10Ni10Co) appeared more resistant to the accelerated coking procedure used in this study than the individual metals, exhibiting greater morphological durability and therefore suitability for this important industrial process. For the 10Ni10Co and 20Co samples, wherein the support particle morphology is broadly retained, there is a significant increase in the presence of large metal particles on the exterior surface of the catalyst-support particles. When compared to the as-reduced samples in Fig.\u00a06, the significant extent of this agglomeration can be seen. This shows the power of X-ray CT for not only tracking the breakdown of catalyst particle structures but also for the study of the spatial redistribution of metals during coking, something that is not possible with the techniques regularly used for catalyst characterisation. This metal redistribution is significant since the formation of these large metal particles must result in the loss of the smaller, high-activity particles, below the resolution of the X-ray CT, meaning a loss in catalyst performance. It is thought that this phenomenon is not only related to the high temperatures that the catalysts have been subjected to but also due to the atmosphere the catalyst is exposed to. All as-reduced catalysts shown in Fig.\u00a06 were subjected to higher temperatures (750 \u00b0C) than those during the accelerated coking (600 \u00b0C), as such the agglomeration behaviour is likely related to the presence of methane. Further time-resolved 4D tomography is planned to investigate this effect.By extraction of a similarly sized sub-volume from each of the six samples investigated, simple voxel counting of the phases attributed to solids (supported catalyst and excess metal) and internal non-support phase (i.e., voids and filamentous carbon) was performed and the results are shown in Table\u00a04. The quantification is consistent with the above qualitative analysis: the largest \u201cinternal non-support phase\u201d increase, thought to be mostly carbon, was observed to occur for the 20Ni sample, and least for the bimetallic case (10Ni10Co), whilst the pure cobalt (20Co) case displays intermediate behaviour. Volume renderings of the segmented phase that does not contain support or catalyst are shown in Fig.\u00a07 (j-l), with the internal non-support phase for the 10Ni10Co case shown in black, indicating the lower degree of intra-particle cracking (blue) present in this sample, versus the 20Co sample.This sort of particle degradation can have a significant impact on catalyst performance, potentially leading to reactor blockage but just as significantly, it may impede catalyst reactivation. Coking is a common occurrence in hetero-catalytic processes where carbon is involved. In many cases, catalysts can be reactivated relatively easily by oxidising carbon deposits, removing them in the form of carbon dioxide and re-exposing the coated catalyst surfaces. Due to the lower thermal stability of the filamentous carbon, it is often seen as a less serious form of coking since it can be more easily removed at lower temperatures and is less prone to coating active layers. Here, however, the serious implications of the filamentous carbon formation are demonstrated. Should the 20Ni catalyst be reactivated by the removal of carbon the remaining support particles are so deleteriously affects that the regenerated catalyst would retain none of its mechanical strength and cause serious issues with reactor performance. The introduction of cobalt to the system (e.g., 10Ni10Co) aids the reduction of coke formation resulting in a catalyst that would retain some mechanical strength after regeneration or would have to be regenerated significantly less often, improving process efficiency.The X-ray nano-CT results shown here clearly corroborate the findings from the other characterisation performed as part of this study, but also give rise to extra insights with respect to the types and extent of cracking occurring in the samples with different nickel-to-cobalt ratios. Moreover, X-ray nano-CT has been shown to be a robust tool for quantifying the extent of support degradation by quantifying the \u201cporosity\u201d changes as a function of the nickel-to-cobalt ratio, indicating once more than the synergy between Ni and Co gives rise to a more robust supported catalyst than either of the monometallic analogues. In addition to these important insights into support particle behaviour, the technique has been used to clearly show agglomeration issues that are occurring during the accelerated coking tests that would have significant impact on the catalyst performance. The spatially resolved nature of X-ray CT means that this technique can give vital information to understand catalyst deactivation that is not clearly obtained from any of the other techniques employed.The use of bimetallic catalysts has been shown to have a significant impact on the performance of catalyst systems for the dry reforming of methane, whereby dual metal systems result in behaviour not observed in either of the single metal systems. The nickel-to-cobalt ratio in the bimetallic system has a considerable influence on both the quantity and properties of carbon formed during accelerated coking.The system containing only nickel was observed to result in the largest quantity of carbon formation with the introduction of cobalt significantly reducing the extent of coking. For the bimetallic systems, a nickel-to-cobalt ratio of 10Ni10Co or 5Ni10Co was observed to be optimal in terms of coking resistance. An increased quantity of cobalt also resulted in the formation of less thermally stable carbon fibres.Lab-based X-ray nano-computed tomography was used effectively to show that it is not only the quantity and bulk properties of the deposited carbon that are important. To gain a deeper understanding of coke formation and ensure long catalyst lifetimes, it is important to consider phenomena occurring at the scale of the supported catalyst particles. Both the nickel- and cobalt-only samples showed significant and detrimental supported catalyst particle cracking that is not observed in the mixed 10Ni10Co system. Interestingly, the technique also provides insights into agglomeration of active metal particles in to large, likely inactive, particles during coking. Further studies are planned to gain a better understanding of this phenomenon since the effect, if any, of nickel-to-cobalt ratio on agglomeration is not clear.The ease and relatively cheap nature of lab-based X-ray computed tomography measurements opens the possibility of the specially resolved tomographic techniques becoming used in the design and formulation of future catalysts. The nature of the information that can be obtained using these methods is something that is not currently easily available for catalyst designers but, as shown in this work, can give information key to understanding catalyst degradation and methods for overcoming these factors.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the Qatar National Research Fund (QNRF) from National Priority Research Program (NPRP9\u2013313\u20132\u2013135) and funding from the Faraday Institution (EP/S003053.1, grant numbers FIRG014 and FIRG015). Use of X-ray CT instruments was supported by the EPSRC (EP/K005030/1 and EP/P009050/1). The Royal Academy of Engineering is acknowledged for funding the Research Chairs of Shearing and Brett (including the National Physical Laboratory and HORIBA MIRA). Steve Hudziak and the CDT-ACM are gratefully acknowledged for their help with the Raman spectroscopy measurements.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ccst.2022.100068.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  The switch from a carbon-based to a hydrogen-based economy requires environmentally friendly methods for hydrogen production. CO2-reforming of methane promises to be a greener alternative to steam-methane reforming, which accounts for the majority of hydrogen production today. For this dry process to become industrially competitive, challenges such as catalyst deactivation and degradation through coke formation must be better understood and ultimately overcome. While bulk characterisation methods provide a wealth of useful information about the carbon formed during coking, spatially resolved techniques are required to understand the type and extent of degradation of supported catalyst particles themselves under coking conditions. Here, lab-based X-ray nano-computed tomography, in conjunction with a range of complementary techniques, is utilised to understand the effects of the nickel-to-cobalt ratio on the degradation of individual supported catalyst particles. Findings suggest that a bimetallic system greatly outperforms monometallic catalysts, with the ratio between nickel and cobalt having a significant impact on the type and quantity of the carbon formed and on the extent of supported catalyst breakdown.\n               "}
{"full_text": "\n\n\nData will be made available on request.\n\n\nData will be made available on request.Volatile organic compounds (VOCs) were considered organic compounds with high vapor pressures that evaporate easily at normal temperatures. There are many sources of VOCs emissions, such as the chemical processing industry, household activities and vehicles [1\u20134]. The emission control of VOCs is an urgent technology due to its highly toxic. Among the various technologies, catalytic oxidation is the most ideal method to eliminate large amounts of VOCs [5,6]. Therefore, the research on VOC combustion has become a key project [7].At present, metal catalysts were widely studied as highly efficient catalysts. Noble metal-based catalysts were often used in the catalytic oxidation of toluene because of their superior catalytic performance [8,9]. However, its practical application was limited. Transition metal oxides have become alternative catalysts owing to low cost, abundant resources and excellent catalytic performance [10]. Manganese-based catalysts have been used for catalytic oxidation of VOCs [11\u201313]. It is well known that the chemical valence state and oxygen species of manganese species are the key factors affecting its performance. Guo et\u00a0al. found that it could achieve completed oxidation and removal of benzene at 210 \u00b0C [14]. Liu et\u00a0al. confirmed that T-MnO2 manifested the best degradation of toluene [15]. Therefore, the MnO2 catalysts were widely investigated in the field.Generally, the catalytic efficiency of manganese-based catalysts was related to the electron transfer between manganese ions of different valence states [16]. The researchers found that the amount of manganese ions could be changed by the adjusting the proportional content of the Mn precursors. Li et\u00a0al. found that better crystallinity and the abundance of Mn4+ was related to the ratio of precursor drugs, which could improve the catalytic oxidation of toluene over MnO2-based catalyst [17]. Dong et\u00a0al. proved that MnO2 with the different the ratio of Cu/Mn exhibited the superior catalytic performance due to the highest content of Mn3+\n[18]. Qu et\u00a0al. demonstrated that Ag-Mn/SBA-15 possessed the excellent catalytic activity because of the highest ratio of Mn4+/Mn3+\n[1].Another important factor affecting toluene removal was the oxygen vacancy content, which could contribute to the superior catalytic performance. Zhang et\u00a0al. adjusted the molar ratio of Mn(NO3)2/KMnO4 to prepared a series of manganese oxides. MnO\nx\n exhibited the optimal catalytic performance with higher oxygen vacancies when the ratio was 3:7 [19]. Yang et\u00a0al. changed the ratio of MnSO4 to KMnO4 to prepare a series of MnO2-USY [20]. The catalyst with a content of 3% exhibited the best activity because of more surface-active substances. Huang et\u00a0al. explored the effect of different Ni/Mn ratios on the catalytic activity of toluene, and reported that the Ni-Mn catalyst presented the best outstanding catalytic activity due to the abundance of oxygen vacancies while the Ni loading was 10 wt.% [21]. Hence, the valence states and oxygen vacancy content of MnO\nx\n-based catalyst depended on the content of the precursor, which was closely related to the catalytic performance.In the present study, a series of \u03b4-MnO2 catalysts for catalytic oxidation of toluene were synthesized by hydrothermal method. The effects of different MnSO4 contents on the generation of oxygen vacancies and valence states were investigated. The physico-chemical properties of the obtained catalysts were investigated by characterization techniques such as SEM, XRD, XPS, BET, Raman and In situ DRIFT.A series of MnO2 were prepared by hydrothermal method. Dissolved 0.1g MnSO4 in 64 mL of water, dissolved fully through magnetic stirring, then added 1.2g KMnO4, and form a homogeneous mixture after 10 min, and conducted hydrothermal reaction at 160 \u00b0C for 24 h, which was marked as Cat-1. Subsequently, the content of MnSO4 was changed to 0.2g, 0.4g, 0.8g and 1.2g, respectively, and the catalyst was prepared by the same preparation method. And the samples were named as Cat-2, Cat-3, Cat-4 and Cat-5, respectively. After the hydrothermal reaction, the reaction product was collected, washed and dried with distilled water, and then placed in a muffle furnace to heat up to 300 \u00b0C at a rate of 3 \u00b0C/min from room temperature. After holding for two hours, the calcination temperature continued to increase to 550 \u00b0C at a heating rate of 3 \u00b0C/min, keeping for three hours and cooling to room temperature to obtained a series of MnO2 catalysts. As a comparison, Cat-0 was prepared by pure KMnO4 with the same preparation method.Power X-ray diffraction patterns (XRD) of the samples were recorded Bruker/AXS D8 Advance diffractometer with a radiation source of Cu K\u03b1 (\u03bb=0.15406 nm) and operated at 35 kV and 35 mA. Diffraction data were collected with the 2\u03b8 ranged from 10\u00b0 to 80\u00b0 with a scanning rate of 8\u00b0/min.The Brunauer\u2013Emmett\u2013Teller (BET) specific surface area and pore characteristics of the catalyst was measured via nitrogen adsorption and desorption isotherms at -196 \u00b0C on an SSA-6000 adsorption analyzer with the samples being performed under vacuum at 200 \u00b0C for 150 min before testing.Temperature-programmed reduction by hydrogen (H2-TPR) was carried out in a chemisorption analyzer(PCA-1200). 200 mg of catalyst was pretreated under hydrogen steam at 200 \u00b0C for 30 min and cooled down to room temperature. Then the samples were measured continuously by a thermal conductivity detector (TCD) from 100 to 900 \u00b0C with 10\u00b0C min-1 under 5% H2/Ar flow.XPS were obtained on a ESCALab 250Xi electron spectrometer using Al K\u03b1 as an excitation source. The experimental condition was constant at 15kV. The binding energies were calibrated via using the adventitious carbon at 284.6 eV as an internal standard.O2-TPD was conducted using the same reactor as the H2-TPR. The samples (100 mg) were first degassed at 200 \u00b0C for 60 min in highly pure He and cooled down to 50 \u00b0C. When the temperature dropped below 50 \u00b0C, O2 was introduced to absorb oxygen for 1 hour, and then He was introduced again, and the temperature was raised from 50 \u00b0C to 900 \u00b0C with a heating rate of 10\u00b0C min-1.In situ DRIFT spectra were recorded of the toluene oxidation on a Nicolet IS 10 FT-IR spectrometer, equipped with a MCT detector. The catalysts were pretreated at 350 \u00b0C for 45 min in a flow of N2, and then cooled down to room temperature naturally. Adsorption was carried out at 260 \u00b0C (flow of toluene/N2), followed by desorption or oxidation (flow of O2/N2) which were recorded at different times.The catalytic performance was analyzed and detected in a fixed-bed quartz tube reactor, a gas chromatograph with FID flame ion was connected to the reactor, and the toluene concentration was tested by FID. The sieved catalyst (40-60 mesh) was weighed to 0.1 g and placed in a quartz tube, and 500 ppm of toluene (100 mL/min) diluted with N2 was used as the gas inlet. The reaction temperature range was 180-300 \u00b0C using a thermocouple detector. Before the test, the pretreatment of the mixed gas stable gas system was carried out, and the heating experiment was carried out after 15 min of treatment.The conversion of toluene (\n\nC\nt\n\n) was calculated by the following formulas:The conversion of toluene (\n\nC\nt\n\n):\n\n(1)\n\n\n\nC\nt\n\n=\n\n\n\n\n(\n\nt\no\nl\nu\ne\nn\ne\n\n)\n\n\ni\nn\n\n\n\u2212\n\n\n(\n\nt\no\nl\nu\ne\nn\ne\n\n)\n\n\no\nu\nt\n\n\n\n\n\n(\n\nt\no\nl\nu\ne\nn\ne\n\n)\n\n\ni\nn\n\n\n\n*\n100\n%\n\n\n\n\nThe CO2 selectivity (\n\nC\n\nC\n\nO\n2\n\n\n\n):\n\n(2)\n\n\n\nC\n\nC\n\nO\n2\n\n\n\n=\n\n\n\n(\n\nC\n\nC\n\nO\n2\n\n\n\n)\n\n\no\nu\nt\n\n\n\n7\n\n\n(\n\nt\no\nl\nu\ne\nn\ne\n\n)\n\n\ni\nn\n\n\n\n\n*\n100\n%\n\n\n\n\nThe crystal structures of different samples were investigated by XRD. As could be observed in Fig.\u00a01\n, all the samples showed typical MnO2 structures (PDF#44-0141). The peaks at 12.5\u00b0, 17.9\u00b0, 28.8\u00b0, 37.4\u00b0, 41.9\u00b0, 52.8\u00b0, 56.3\u00b0 and 60.2\u00b0 corresponded to the (110), (200), (310), (211), (301), (440), (600) and (521) planes, respectively. More crystalline phases appear after the addition of MnSO4. A large number of diffraction peaks indicated a high degree of structural disorder in the crystal structure of the MnSO4-containing catalyst, and then the catalytic activity [22]. The Mn3+ ion has a strong Jahn-Teller effect, which leads to the stretching of the Mn-O bond length [23]. Therefore, a large amount of Mn3+ could be readily dissociated and activated the surrounding oxygen atoms and promoted their catalytic properties. Furthermore, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 were all weak and broad peaks, which indicated structural ordering reduction. This phenomenon represented that different MnSO4 contents could change the crystal size of the catalyst, which improved the catalytic activity. The grain size of the catalyst was calculated by Scheler's formula, and the order was ranked as: Cat-0 (36.1 nm) > Cat-1 (20.2 nm) > Cat-2 (19.5 nm) > Cat-3 (18.9 nm) > Cat- 5 (18.7 nm) > Cat-4 (18.3 nm). The smaller grain size facilitated the adsorption of toluene and accelerated the redox reaction [23]. Obviously, Cat-4 possessed a weaker peak shape and the smallest grain size, which could enhance the catalytic activity [24].The morphology of catalysts with different MnSO4 contents was studied by SEM technique. Fig.\u00a02\n was the SEM images of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5. All catalysts exhibited nanorod-like structures. In Fig.\u00a02A, the MnO2 by hydrothermal treatment of pure KMnO4 was a bulk nanorod aggregate. With the addition of MnSO4, small aggregates of MnO2 nanorods were formed in Fig.\u00a02B, Fig.\u00a02C and Fig.\u00a02D, respectively. However, no agglomerated structure was detected in Fig.\u00a02E. Interestingly, agglomeration was observed in Fig.\u00a02F. The addition of MnSO4 gradually dispersed the MnO2 nanorods, but the addition of excessive MnSO4 would cause to the re-agglomeration of MnO2. The agglomerated structure was not observed in Cat-4, which made it easier for toluene to adhere to its surface, speeding up the reaction rate.\nFig.\u00a03\n showed the N2 adsorption-desorption isotherm and pore size distributions of catalysts with different contents of MnSO4. The catalysts all showed IV-type isotherms and H3-type isotherms according to IUPAC classification, which proved that the catalysts were all mesoporous materials. Table\u00a01\n showed the specific surface area (SBET), pore volume (Vp) and pore size (Dp). The average pore size of Cat-4 was 6.8 nm. It was well known that the diameter of toluene was 0.583 nm, which was much smaller than the pore size of the prepared samples. Hence, toluene could be easily adsorbed in the pore channel of the catalyst during the degradation process of toluene catalytic combustion. The SBET of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 was 20, 19, 24, 20, 22 and 24 m2/g, respectively, which showed that the content of MnSO4 had no significant effect on the specific surface area of the catalyst.The atomic states of the outermost layer of catalysts with different contents of MnSO4 were detected by XPS test. From Fig.\u00a04\na, there were two obvious main peaks for Mn 2p3/2, which was further decomposed into four peaks. The peaks at 642.7-643.8 eV and 654.4-655.9 eV were assigned to Mn4+. The peaks at 641.9-642.4 eV and 653.5-653.8 eV corresponded to Mn3+. In general, the amount of Mn3+ would determine the catalytic efficiency of MnO2. The ratio of Mn3+/(Mn3++Mn4+) of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 was 0.53, 0.55, 0.56, 0.57, 0.59 and 0.58 in Table\u00a02\n, respectively, which was consistent with their catalytic activity. More Mn3+ meant more oxygen vacancies, which provided more active sites for MnO2\n[26]. Zhang et\u00a0al. suggested that Mn-120 exhibited a high proportion of Mn3+/Mn4+, and enhanced the catalytic efficiency [28]. Wang et\u00a0al. reported that the valence state transition between manganese ions facilitated the migration of oxygen species [26]. Santos et\u00a0al. demonstrated that the lower binding capacity between Mn3+ and O resulted in more oxygen vacancies [29]. Cat-4 exhibited the highest proportion of Mn3+, which enhanced the catalytic ability.\nFig.\u00a04b was the O 1s energy spectrum of the catalyst. 529.4-529.8 eV was adsorbed oxygen (O\nads\n) and 531.2-531.5 eV was lattice oxygen (O\nlatt\n) peaks [27]. The transport transformation between oxygen species was the key to catalytic reaction generation. Table\u00a02 listed the ratios of O\nads\n/(O\nads\n+O\nlatt\n) in Cat-0, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5, which was 0.60, 0.38, 0.53, 0.62, 0.69, 0.67, respectively. Obviously, Cat-4 exhibited the highest O\nads\n content, which accelerated the catalytic process [30]. Zhang et\u00a0al. found through LaMnO\nx\n perovskite catalyst owned the abundance of adsorbing oxygen, which improved the catalytic performance [31]. Huang et\u00a0al. demonstrated that a large amount of O\nads\n could be easily volatilized at lower temperatures, which could be contributed to the superior catalytic performance [32].Raman testing was used to test the catalyst microstructure. Fig.\u00a05\n was the Raman spectrum of the catalyst. The peak positions of Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 all appeared at 600 cm-1, which was the stretching between Mn3+ and O-connected Mn-O bonds in the MnO6 octahedron vibration [33]. According to literature reports, the weakening of Raman spectral peaks was related to the formation of structural defects or the formation of solid solutions [34]. The formation of structural defects could be induced a large number of oxygen vacancies, accelerating the oxygen migration rate. Compared with other catalysts, Cat-4 exhibited the weakest and broadest peaks, which indicated massive oxygen vacancy defects in Cat-4. The presence of oxygen vacancy defects accelerated oxygen species migration and transformation. The appearance of a peak at 400 cm-1 in the spectrum could be observed, which was attributed to the asymmetric stretching vibration of the oxygen species. Cat-0 showed a sharp peak at 1200 cm-1, which proved that it processed less oxygen vacancy defects. Through the analysis of Raman spectrum, the proportion of precursor content would affect the content of oxygen vacancy defects in the catalyst.The redox performance of the catalysts was characterized using temperature-programmed studies. The reducibility of catalyst was demonstrated using H2-TPR. Two peaks appeared for all catalysts, the first was attributed to MnO2 reduction to Mn2O3, and the second was assigned to Mn2O3 reduction to MnO. Compared with other samples, Cat-4 exhibited the lowest initial reduction temperature (265 \u00b0C). Generally, a lower reduction temperature represented higher content of surface-active sites and oxygen species, which enhanced the reduction ability of the catalyst [26]. The, Cat-4 showed the largest peak area for the second reduction peak, which represented the most Mn3+. The presence of a large number of Mn3+ and more active sites improved the degradation efficiency of toluene. The hydrogen consumption of the sample was calculated by integrating all the main peaks, compared with Cat-0 (1.27 mmol/g), Cat-1 (1.41 mmol/g), Cat-2 (1.43 mmol/g), Cat-3 (1.48 mmol/g) and Cat-5 (1.58 mmol/g), H2 consumption of Cat-4 (1.64 mmol/g) was the largest, which proved that Cat-4 exhibited more reducible oxygen content. Fig.\u00a06\nb showed the hydrogen consumption rate, and Cat-4 exhibited the highest H2 consumption rate at the same temperature. Thereby, Cat-4 possessed the excellent redox ability, which was responsible for the superior catalytic activity.In order to understand the effect of different MnSO4 contents on oxygen vacancies, O2-TPD was used for characterization analysis in Fig.\u00a07\n. All catalysts showed two main peaks. Generally, the peak below 400 \u00b0C corresponded to surface adsorption oxygen desorption, the peak between 400-700 \u00b0C was surface lattice oxygen desorption, and the peak above 700 \u00b0C was bulk lattice oxygen desorption [35]. Typically, the peak area represented the amount of oxygen desorbed. Compared with other catalysts, Cat-4 desorbed the highest and widest surface oxygen peaks in the range of 400-700 \u00b0C, which indicated that Cat-4 exhibited the best oxygen mobility. According to previous XPS studies, Cat-4 owned the most Mn3+. In the Raman spectrum, the binding force of Mn3+-O was the weakest and more oxygen species were released, which was the reason for the best oxygen mobility of Cat-4 catalyst.The toluene catalytic oxidation performance of all the catalysts with different MnSO4 loading content were evaluated, and the results were shown in Fig.\u00a08\n(a). All samples showed a positive correlation between toluene conversion and temperature. The T50 (50% toluene conversion) of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 was 308, 291, 282, 268, 251, and 257 \u00b0C, respectively. The temperature order of T90 (90% toluene conversion) followed: Cat-4 (260 \u00b0C) < Cat-5 (263 \u00b0C) < Cat-3 (273 \u00b0C) < Cat-2 (291 \u00b0C) < Cat-1 (298 \u00b0C) < Cat-0 (326 \u00b0C). The combined results of T50 and T90 showed that all catalysts with MnSO4 addition outperformed the original catalysts, indicating that the interaction of MnSO4 with KMnO4 promoted the catalytic activity. In the four comparative catalysts with MnSO4 addition, Cat-4 showed the most significant improvement in catalytic activity, the T90 decreased by a full 56 \u00b0C, indicating that the appropriate amount of MnSO4 doping contributed to the improvement of the catalytic performance of toluene. In addition, Fig.\u00a08(a) and Fig.\u00a08(b) illustrated that the carbon dioxide selectivity of the catalyst was essentially the same as the catalytic activity, indicating that few intermediates were produced during the degradation of toluene.The stable and continuous use of the catalyst was more critical in industrial applications, so the stability of the catalyst was important. The stability of Cat-4 catalyst was investigated. The catalytic reaction was continued for 100 hours at 260 \u00b0C. Cat-4 exhibited the excellent catalytic stability, and the toluene conversion rate was maintained at 90%. Moreover, the catalyst showed the superior catalytic performance for dry toluene gas. However, the emission of VOCs was often mixed with the existence of water vapor, so the water resistance of the sample was worth exploring. After water vapor was introduced, the conversion rate of toluene gradually decreased to about 81% over Cat-4, and the conversion rate of toluene was maintained at 80% after continuous introduction of water vapor for 10 hours. After the water vapor was stopped, the conversion rate of toluene quickly rose to 90%. The results showed that the Cat-4 catalyst was tolerant to water vapor, and there was reversibility between the water vapor and the sample. Noteworthy, the conversion rate of toluene was slightly higher than 90% after stopping the introduction of water vapor because that the presence of hydroxyl groups enhanced the surface-active oxygen species and increased the catalytic activity [36] (Fig.\u00a09\n).To detected the intermediates in toluene combustion, in situ DRIFTS experiments was conducted to further study the reaction mechanism on Cat-4. Fig.\u00a010\n showed the DRIFTS spectra over Cat-4. The adsorption bands at 1601 and 1487 cm-1 were ascribed to typically skeletal vibration of the aromatic ring, manifesting that toluene was adsorbed on the surface of catalyst [37\u201340]. Notably, the peaks at 1067, 1117 and 1338 cm-1 were recognized as the alkoxide species (C-O stretching vibration), indicating that the adsorbed toluene could form benzyl alcohol (C6H5-CH2O) by smashing the C-H bond of the methyl (-CH3) [40\u201342,35]. Moreover, the peaks at 1473 and 1646 cm-1 corresponded to the C-O-H and \u028b(C=O) stretching vibrations, declaring the formation of benzaldehyde (C6H5-CHO) [43,44]. The peaks at 1413, 1542 and 1582 cm-1 were belonged to typical bands carboxylate species, demonstrating the formation of benzoate species [38\u201340,45,46]. Benzoate species were present almost throughout the entire time period, suggesting that benzoic acid species were key intermediates in the oxidation of toluene. The detected peaks near 1734, 1807 and 1865 cm-1 was belonged to the acid anhydride species, manifesting the formation of maleic anhydride [40,41,47,48], which was an important intermediate for the ring opening of benzoic acid. For the Cat-4, in situ DRIFT spectrum, the formation of maleic anhydride did not increase with extension of time for adsorption during the oxidation of toluene molecules. On the contrary, the characteristic band of benzoic acid increased monotonically with the increased of reaction time, confirming that maleic anhydride was prior transformed to CO2 and H2O.To sum up, according on the in situ DRIFTS results, the proposed oxidation reaction mechanism of toluene on Cat-4 catalyst followed the MVK mechanism. Firstly, toluene was adsorbed over the Cat-4 catalyst and then reacted with lattice oxygen species to form benzyl alcohol species through one-step dehydrogenation [44]. Simultaneously, the resulting oxygen vacancies could be replenished by gas-phase O2 and reactive oxygen species (O2\u2192O2\n\u2212\u2192O2\n2\u2212\u2192O\u2212\u2192O2\u2212). The benzyl alcohol would be oxidized to benzaldehyde, which was then oxidized to benzoic acid. Benzoate was cleaved by ring opening to form maleic anhydride, which was further converted to CO2 and H2O. It was important not to mention that oxygen vacancies played an important role in the catalytic combustion process, because oxygen vacancies could quickly adsorb and activate O2 molecules, thus accelerating the degradation of reactants. There was the highest Mn3+/(Mn3++Mn4+) over Cat-4 catalyst, which accelerated the catalytic oxidation efficiency of toluene.XRD analysis showed that with the increase of MnSO4 content in the precursor, the crystal size of the catalyst gradually decreased. The addition of excess MnSO4 increased the catalyst particle size again. Liu et\u00a0al. reported that the catalyst with small particle size exhibited excellent catalytic activity, and the smaller particle size was beneficial to enhance the oxygen transport ability of the catalyst [49]. Zhu et\u00a0al. demonstrated that the small particle size of catalyst showed abundant adsorption sites, which was favorable for the adsorption and activation of toluene [25]. Therefore, Cat-4 with the smallest particle size could accelerate the adsorption of toluene and the redox capacity, leading to the best catalytic performance.To further explore the specific factors affecting the catalytic activity, the XPS results and catalytic activities were shown in Fig.\u00a011\n. The change trend of the catalytic activity at 260 \u00b0C was related to Mn3+/(Mn3++Mn4+), which proved that the presence of Mn3+ could be promoted the catalytic reaction. It was generally believed that because of the existence of the Mn4+\u2013O2\u2212-Mn4+\u00a0\u2192\u00a0Mn3+-\u25a1-Mn3++1/2O2 cycle, more Mn3+ concentration was favorable for the formation of oxygen vacancies to balance the charges over MnO\nx\n\n[50]. In addition, higher Mn3+ concentration caused lattice defects for MnO\nx\n, which promoted the formation of oxygen vacancies [51]. Genuino et\u00a0al. proposed that Mn3+ could be induced a large number of active sites, thereby promoting the catalytic oxidation of toluene [52]. The change of MnSO4 content affected the content of Mn3+ in the catalyst, which in turn changed the content of oxygen vacancies. In addition, with the increased of MnSO4 content, the content of Mn3+ increased first and then decreased, and Cat-4 possessed the highest content of low-valence Mn3+, consequently it had the best catalytic activity. O\nads\n/ (O\nads\n\u00a0+\u00a0O\nlatt\n) and Mn3+/Mn4+ showed the same trend. As MnSO4 increased from 0 to 0.8, the Oads\n concentration concurrently increased Cat-4 possessed the most amounts of O\nads\n species. With the further increase of MnSO4, the O\nads\n content decreased, which was consistent with the trend of catalytic activity of the catalyst. The phenomenon implied that surface adsorbed oxygen played a key role in the catalytic combustion process of toluene. The ratio of O\nads\n was considered as an indicator of oxygen vacancy concentration because that gas-phase O2 could be adsorbed on oxygen vacancies and activated as electrophilic reactive oxygen species (ads), which participated in the catalytic oxidation process of toluene [53]. Li et\u00a0al. demonstrated that O\nads\n accelerated the migration of O\nlatt\n, which in turn enhanced the catalytic oxidation performance [54]. Qu et\u00a0al. reported that a great quantity of O\nads\n facilitated the circulation of oxygen species, which promoted the catalytic combustion reaction of toluene [55]. The addition of appropriate amount of MnSO4 could be increased the content of Mn3+ and O\nads\n, which played a vital role in the formation of oxygen vacancies. The abundant oxygen vacancies improved the adsorption of oxygen in the gas phase and promoted the conversion of lattice oxygen, which greatly improved the oxygen mobility [56]. The H2-TPR results proved that the reduction performance of MnO\nx\n was improved by changing the content of MnSO4. Furthermore, Cat-4 showed more O\nads\n, which led to the best catalytic activity.Therefore, combined with the analysis of XPS, H2-TPR, O2-TPD and XRD, the content of MnSO4 improved the reducibility of MnO\nx\n and the content of oxygen vacancies, which increased the catalytic activity. More amounts of Mn3+ and the oxygen vacancy content could be provided more adsorption sites for O\nads\n and then accelerated the migration rate of oxygen species. The relationships among Mn3+, O\nads\n and toluene catalytic activity were shown in Fig.\u00a011. Evidently, the concentration of Mn3+ and O\nads\n had a linear relationship with the catalyst activity. The superior activity of Cat-4 should be attributed to the increased of oxygen vacancies. As shown in Fig.\u00a011, the particle size of the catalyst was negatively correlated with the catalytic activity. Obviously, the smaller grain size provided more toluene adsorption sites of toluene, which improved the catalytic activity. Hence, Cat-4 with the smallest particle size possessed the best catalytic performance.In this work, the toluene oxidation reaction by adjusting the content of MnSO4 in the precursor samples was investigated. Compared with the single KMnO4, the catalyst with the addition of MnSO4 showed better catalytic activity. When the MnSO4 content increased from 0 to 0.8, the catalytic activity simultaneously increased. With the further increased of MnSO4, the activity started to decreased, which indicated that the content of MnSO4 had a significant effect on the performance of the catalyst. Among them, Cat-4 showed the best catalytic performance and reaches 90 % conversion of 260 \u00b0C. According to the characterization of Cat-4 samples, the improved catalytic activity, better stability and water resistance could be attributed to better low-temperature reduction, more oxygen desorption, more oxygen vacancies, higher content of Mn3+/(Mn3++Mn4+) and more surface adsorbed oxygen content. In addition, Cat-4 showed excellent stability and water resistance. Furthermore, the oxidation process of toluene was inferred from the in situ DRIFT result: toluene\u00a0\u2192\u00a0benzyl alcohol\u00a0\u2192\u00a0benzaldehyde\u00a0\u2192\u00a0benzoic acid\u00a0\u2192\u00a0maleic anhydride\u00a0\u2192\u00a0CO2 and H2O. Therefore, different MnSO4 contents changed the microscopic properties of the catalysts as well as the redox capacity of the catalysts. Cat-4 exhibited the best catalytic activity because it showed the most structural defects, excellent oxidative ability and strongest oxygen transport ability.We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company.\nZhongxian Song: Writing \u2013 review & editing, Funding acquisition. Haiyang Li: Writing \u2013 original draft. Xuejun Zhang: Writing \u2013 review & editing, Project administration. Zhuofu Zhang: Investigation. Yanli Mao: Formal analysis. Wei Liu: Formal analysis. Zepeng Liu: Resources. Dujuan Mo: Software. Xinfeng Zhu: Methodology. Zhenzhen Huang: Writing \u2013 review & editing.This work is supported by the National Natural Science Foundation of China (No.21872096), Natural Science Youth Fund of Henan Province (No. 202300410034); Young Teacher Foundation of Henan University of Urban Construction (No. YCJQNGGJS201903), Academic Leader of Henan Institute of Urban Construction (No. YCJXSJSDTR202204), Science and technology major special of Pingdingshan, (No. 2021ZD03) and Doctoral Research Start-up Project of Henan University of Urban Construction (No. 990/Q2017011).", "descript": "\n                  A series of MnO\n                        x\n                      catalysts with different content of MnSO4 were prepared by hydrothermal method and used for the catalytic oxidation of toluene. The MnO\n                        x\n                      (Cat-4) showed the best catalytic activity when the MnSO4 content was 0.8 g, and the toluene conversion rate reached 90% at 260\u00b0C. Cat-4 catalyst showed more Mn3+ and O\n                        ads\n                      concentrations, which provided more oxygen vacancies and accelerated the migration rate of oxygen species, and then enhanced the redox performance, resulting in the improvement of the catalytic activity, which depended on the content of MnSO4 over Cat-4. The 100 h stability test showed that the Cat-4 catalyst presented superb stability and water resistance. Furthermore, the oxidation process of toluene was inferred from the in situ DRIFT result: toluene\u00a0\u2192\u00a0benzyl alcohol\u00a0\u2192\u00a0benzaldehyde\u00a0\u2192\u00a0benzoic acid\u00a0\u2192\u00a0maleic anhydride\u00a0\u2192\u00a0CO2 and H2O.\n               "}
{"full_text": "Data will be made available on request.Recently, hydrogen (\n\n\nH\n\n\n2\n\n\n) as a fuel has been the focus for a great research attention due to its importance in the development of clean energy technologies [1,2]. Proton exchange membrane fuel cells (PEMFCs), with their have high efficiency, low operating temperatures, and low emissions, have been considered promising to utilize \n\n\nH\n\n\n2\n\n\n or any other proton-containing fuel to directly generate electricity in electrochemical galvanic reactors [3,4].In this regard, the direct formic acid (FA) fuel cells (DFAFCs) owned a high (1750\u00a0kW h L\n\n\n\n\u2212\n1\n\n\n) energy density [5], large theoretical open-circuit potential (\n\u223c\n1.40\u00a0V) [6,7], and non-toxic fuel that has a lower crossover through the Nafion\u00ae membrane compared to other liquid fuels [8]. Although platinum (Pt) was identified as the most frequently used catalyst for FA electro-oxidation reaction (FAOR) [9], its scarcity and high cost along with the lack of a long-term durability prohibited the large-scale commercialization of DFAFCs. At Pt surfaces, FAOR adopts a dual pathway mechanism, i.e.,\u00a0dehydrogenation to CO2 and dehydration to CO that further blocks the Pt active sites and therefore retards the catalyst activity [10]. To overcome such a bad impact of a catalyst\u2019s poisoning with CO, Pt was modified previously with Pd [9], Ag [11], Au [3], Bi [12], and Ni [13] that could switch the mechanism (completely or to a great extent) toward the direct dehydrogenation pathway. We, herein, report on the fabrication of a simultaneously co-electrodeposited PtPd binary catalyst that was assembled onto a GC surface for enhanced FAOR. A combination of electrochemical and materials measurement techniques assisted in the evaluation of the morphology, activity, and stability of the proposed catalyst.The chemicals used in this investigation were of high purity and used as received from trusted suppliers as Sigma Aldrich and Alfa Aesar. The surface morphology of the proposed catalysts was investigated using the field emission scanning electron microscopy (FE-SEM, Quattro S, Thermo Fisher Scientific USA equipped with AMETEK USA Element Detector). The electrochemical experiments were performed using a Bio-Logic SAS (model SP-150) potentiostat operated with EC-Lab software. All electrochemical experiments were performed using a conventional three-electrode system including a GC working electrode (5\u00a0mm in diameter with 0.196\u00a0cm 2 geometric area), spiral Pt wire as an auxiliary electrode, and an Ag/AgCl/NaCl (3M) reference electrode. All these electrodes were purchased from ALS Japan. The PtPd catalyst was fabricated using the simultaneous co-electrodeposition technique as previously explained [3]. Briefly, this involved the potentiostatic electrodeposition of Pt and Pd onto the GC surface at \u22120.2 V permitting the passage of only 10 mC in 0.1 M Na2SO4 aqueous solutions containing 2.0\u00a0mM \n\n\nH\n\n\n2\n\n\n PtCl6. xH2O and 2.0\u00a0mM Pd (CH3COO)2.The surface composition of the catalyst\u2019s ingredients was explored using the cyclic voltammetry (CV) experiments. Fig.\u00a01 represents the CVs measured in aqueous solution of 0.5 M \n\n\nH\n\n\n2\n\n\nSO4 for (a) Pt/GC and (b) PtPd/GC catalysts in a potential range between \u22120.2 and \uff0b1.2 V at a potential scan rate of 100\u00a0mV s\n\n\n\n\u2212\n1\n\n\n. Fig.\u00a01a (Pt/GC catalyst) shows the characteristic performance of a poly-Pt electrode in an acidic conditions [14]. This displayed the Pt oxidation (Pt\n\u2192\nPtO) which extended over a wide potential range between ca. 0.6 and 1.2 V and coupled with the subsequent (PtO \n\u2192\nPt) reduction peak at ca. 0.5 V. Moreover, the peaks that appeared in the potential range between 0 and \u22120.2 V were assigned to the hydrogen adsorption/desorption (\n\n\nH\n\n\nads/des\n\n\n). The PtPd/GC catalyst (Fig.\u00a01b) retained such behavior with an increasing in the currents of the Pt\n\u2192\nPtO, PtO \n\u2192\nPt, and \n\n\nH\n\n\nads/des\n\n\n peaks due to the overlapping of Pd with Pt peaks [14].\n\n\nFig.\u00a02 shows FE-SEM micrographs of the Pt/GC (Fig.\u00a02A) and PtPd/GC (Fig.\u00a02B) catalysts. Fig.\u00a02A displayed the Pt electrodeposition onto the GC surface in a spherical well-dispersed structures with an average particle size of ca. 220\u00a0nm with some intensive aggregations reaching ca. 600\u00a0nm. On the other hand, the PtPd/GC catalyst (Fig.\u00a02B) retained the same spherical shaped structure that has an average particle size a little bit larger (ca. 238\u00a0nm) with very little aggregations (reaching ca. 250\u00a0nm) compared with the Pt/GC catalyst.\n\n\nFig.\u00a03 displays the CVs of FAOR at the (a) Pt/GC and (b) PtPd/GC catalysts in an aqueous solution of 0.3 M FA (pH\n\u223c\n3.5) at a potential scan rate of 100 mV\u00a0s\n\n\n\n\u2212\n1\n\n\n. Commonly, the mechanism of FAOR on Pt-based electro-catalysts proceeds in two different routes [15]. The direct one involves the dehydrogenation of FA to CO2. This direct route takes place at a low potential domain and this will shift the actual voltage of DFAFCs closer to its theoretical value. That is why this route is the preferred route for FAOR. In Fig.\u00a03a (Pt/GC catalyst), two oxidation peaks were observed at 0.35 and 0.82 V in the anodic-going scan. The first one (at ca. 0.35 V) was assigned to the direct (dehydrogenation pathway) oxidation of FA to CO2. The current density of this peak will be abbreviated as \n\n\n\n\nI\n\n\np\n\n\n\n\nd\n\n\n. The second one (at ca. 0.82 V) was assigned to the oxidation of the pre-adsorbed CO (CO\n\n\n\nads\n\n\n) to CO2 after the Pt surface hydroxylation (Pt-OH) at ca. 0.7 V. The current density of this peak will be abbreviated as \n\n\n\n\nI\n\n\np\n\n\n\n\nind\n\n\n.\nIn fact, the main challenge of proposing Pt-based catalyst toward FAOR is related to the adsorption of the poisoning CO which takes place spontaneously from the non-faradaic dissociation of FA at open circuit potentials. This deactivates the Pt surface and prompts a potential poisoning for a significant number of Pt active sites, which, in turns, impede the direct \u201cpreferred\u201d pathway of FAOR [4,15\u201319]. Herein, several significant parameters (\n\n\n\n\nI\n\n\np\n\n\n\n\nd\n\n\n, \n\n\n\n\nI\n\n\np\n\n\n\n\nind\n\n\n, \n\n\n\n\nI\n\n\np\n\n\n\n\nb\n\n\n and \n\n\nE\n\n\nonset\n\n\n) were calculated to quantify the degree of catalytic enhancement toward FAOR and the reduction in CO poisoning for both (Pt/GC and PtPd/GC) modified catalysts. The relative ratios of \n\n\n\n\n\nI\n\n\np\n\n\n\n\nd\n\n\n/\n\n\n\n\nI\n\n\np\n\n\n\n\nind\n\n\n\n (that evaluates the enhancement in the catalytic activity in the favorable direct oxidation pathway) and \n\n\n\n\n\nI\n\n\np\n\n\n\n\nd\n\n\n/\n\n\n\n\nI\n\n\np\n\n\n\n\nb\n\n\n\n (that estimates the catalytic tolerance of the catalyst for poisoning CO species) at the Pt/GC catalyst (Fig.\u00a03a) were 0.59 and 0.16, respectively with \n\n\nE\n\n\nonset\n\n\n (that reflects the capability of the catalyst to overcome unnecessary overpotentials (particularly of charge transfer) that normally detracts the voltage output of the cell) of ca. 96\u00a0mV measured at 0.4\u00a0mA cm \n\n\n\n\u2212\n2\n\n\n. Interestingly, as obviously observed from Fig.\u00a03b (PtPd/GC catalyst), these values reached 7.33 (i.e.,\u00a0ca. 12.4 times higher) and 0.32 (i.e.,\u00a0ca. 2 times higher) compared with the Pt/GC catalyst. This was observed concurrently with ca. -91\u00a0mV shift in the \n\n\nE\n\n\nonset\n\n\n. This indicates the superiority of the modified PtPd/GC catalyst toward FAOR.Besides the catalytic activity enhancement, it is also very important to improve the catalytic stability [7]. Herein, the stability of the Pt/GC (Fig.\u00a04a) and PtPd/GC (Fig.\u00a04b) catalysts were measured by recording the current transients (\n\ni\n\u2212\nt\n\n) curves, under continuous electrolysis for 3600 s in FA solution at 0.2 V.\nThese measured data came consistent and as expected with the data of Fig.\u00a03, the PtPd/GC catalyst acquired a lower current decay with time which means a prolonged stability during continuous electrolysis. As obviously seen from Fig.\u00a04 after 3600 s of continuous electrolysis, the current density of the PtPd/GC catalyst was ca. 2.5 times higher than that observed at the Pt/GC catalyst. This represented an additional value for Pd in boosting the catalytic tolerance of the PtPd/GC catalyst against CO poisoning during FAOR.A PtPd/GC binary catalyst prepared by the simultaneous co-electrodeposition method was endorsed for efficient FAOR. The catalyst retained the highest catalytic activity (with up to ca. 12.4 times increase in the \n\n\n\n\n\nI\n\n\np\n\n\n\n\nd\n\n\n/\n\n\n\n\nI\n\n\np\n\n\n\n\nind\n\n\n\n index, 2 times increase in the \n\n\n\n\n\nI\n\n\np\n\n\n\n\nd\n\n\n/\n\n\n\n\nI\n\n\np\n\n\n\n\nb\n\n\n\n index and \u221291\u00a0mV shift in \n\n\nE\n\n\nonset\n\n\n) toward FAOR compared to the conventional Pt/GC catalyst. This associated a critical improvement in the catalytic stability that appeared in maintaining a higher (by ca. 2.5 times) current density after prolonged electrolysis for 3600 s at 0.2 V. It was thought that minimizing the CO adsorption at the Pt surface was mostly behind the observed enhancement.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supoorted from the British University in Egypt and the Faculty of Science - Cairo University\n .", "descript": "\n                  This investigation displayed a superb catalytic performance toward the formic acid electro-oxidation reaction (FAOR) in an alkaline medium at a binary catalyst composed of Pt and Pd that were simultaneously electrodeposited onto a glassy carbon (GC) surface. Interestingly, the proposed PtPd/GC catalyst displayed a significant enhancement in the catalytic activity (by ca. 12 times higher direct (\n                        \n                           \n                              I\n                           \n                           \n                              p\n                           \n                           \n                              d\n                           \n                        \n                     ) to indirect (\n                        \n                           \n                              I\n                           \n                           \n                              p\n                           \n                           \n                              ind\n                           \n                        \n                     ) current ratio concurrently with ca. -91\u00a0mV shift in the onset potential (\n                        \n                           \n                              E\n                           \n                           \n                              onset\n                           \n                        \n                     ) and stability (ca. 2.5 times higher current density after 3600 s of continuous electrolysis) toward FAOR if compared to the Pt/GC catalyst. The catalytic enhancement was thought to arise mostly from minimizing the CO adsorption, i.e., third body effect, at the Pt surface during FAOR.\n               "}
{"full_text": "Data will be made available on request.The combustion of fossil fuels for energy production releases into the atmosphere a large amount of CO2 which is claimed as a main cause of the greenhouse effect [1\u20134]. The commitment of the International governments to mitigate the inherent issues has recently driven 195 Parties setting new targets during the 27th UN Climate Change Conference (COP27, Sharm el-Sheikh 2022) to reach zero emissions by 2050 and to keep global warming below 1.5\u00a0\u00b0C.Furthermore, the energy crisis in Central and Eastern Europe poses the need for searching safe alternatives to energy supply to give immediate and practical answers to the contingent needs. From this point of view, the technologies related to Carbon Capture and Utilization (CCU) are arousing a strong interest, for the possibility to close the carbon loop by an effective recycling of CO2 captured either directly from the air or from industrial power plants, then reusing it in presence of renewable hydrogen for the production of value-added products [5\u20138].Dimethyl ether (DME) has been getting growing attention as an alternative diesel fuel and also as a feedstock for producing versatile chemicals and fuels, such as light olefins, methyl acetate, dimethoxyethane, etc [9\u201313]. Conventionally DME is synthesized starting from syngas in two steps, involving first the formation of methanol over a multi-metallic catalyst and then the dehydration of methanol to DME over solid acid systems [14\u201320]. The final productivity is controlled by the rate of methanol synthesis, in turn limited by thermodynamic constraints, feed composition, extent of the recycling stream [21,23]. The direct synthesis of DME taking place in a single reactor from either CO/CO2/H2 or CO2/H2 mixtures can overcome these limitations, leading to higher DME productivity owing to a more favourable equilibrium conversion prompted by the continuous consumption of methanol initially formed [20\u201322,24\u201326]. However, this one-step process has not reached an industrial maturity yet, being still performed on a lab-scale in presence of a hybrid metal-oxide-acid catalyst.Typically, copper-based ternary systems, such as CuO-ZnO-Al2O3 or CuO-ZnO-ZrO2, are integrated with solid acidic phases, like \u03b3-Al2O3 or zeolite materials [23\u201331], active in the dehydration of methanol. The extent of the interface area among different phases leading to an effective interaction, the proximity of catalytic sites of different nature preventing a measurable mass diffusion control, the sample reproducibility overcoming the complexity in controlling several variables during preparation, the need for a stable lifetime under the adopted experimental conditions, represent all challenging factors requiring further R&D for an industrial process scalability.Among variously shaped solid catalysts (e.g., powders, beads, granules, pellets, scaffolds), matrix-like structures with different geometry of channels and typically prepared by impregnation or washcoating have been demonstrated to be promising systems for various catalytic processes [32\u201338], offering many benefits in terms of control of the sample architecture, increase of mass and heat transfer, decrease of pressure drops and related costs of process management. Nevertheless, their specific utilization in CCU applications results to be scarcely documented yet, now receiving a decisive boost by the recent progresses in the three-dimensional (3D) printing techniques, expected to revolutionize in the short term all the sectors of research and industry and to have implications on the concepts of production and work too, with economic and ethical consequences [39,40]. 3D Printing, also referred as \u201cadditive manufacturing\u201d (AM), designates a technology suitable to build a material directly from a virtual 3D model by overlapping layers of the same material. In general, to produce a piece by 3D printing is sufficient the choice of a 3D software, a 3D model and a starting material. Consequently, it is possible to generate parts with arbitrary geometries without the need to adopt the usual productive processes bound to mass production [41,42]. The pieces thus created are ready for use, not requiring other finishing treatments, as well as the manufacture of semi-finished components results to be economically profitable. The starting material in the process is typically used in the form of a powder, paste, ink, suspension or solid in an optimized phase for layered deposition.Among the AM methods more and more importantly impacting the sector of the 3D printing, the direct-ink-writing (DIW), or robocasting, offers the superior potential not only for the realization of purely ceramic [43,44], but also of metallic or hybrid materials [45\u201350]. Its strong point is the relatively low cost of the machine, which normally uses an ink paste with specific rheological and viscosity features due to the addition of binders or additives in the parent material. This technology uses an extrusion process through a nozzle which generally varies from 0.1 to few millimeters, from which the material comes out in the form of a continuous filament which is deposited in superimposed layers through the control of a system robot, following a path generated starting from a suitably designed 3D model. Therefore, in a robocasting procedure, a 3D model is layered similar to other additive manufacturing techniques, but the nozzle position is controlled, extrapolating the shape of each layer by a CAD model. The first part of a product made by robocasting is obtained by extruding the \u201cink\u201d threads onto the first layer. Subsequently, the working area is moved down or the formation hole rises and the next layer is applied to the required position. This is repeated until the product is completed.In this work, the effectiveness of structured catalysts, prepared via robocasting in the form of matrix-like cylinders, was evaluated as viable alternative to conventional powdered catalysts for the development of a scalable CO2 hydrogenation technology for the direct synthesis of DME. The physico-chemical and catalytic properties come out upon the robocasting procedure were compared with their powdered counterparts, in order to understand the key aspects behind a 3D-printing technique in the preparation of effective materials for CCU applications.The composition of the ink-pastes to be micro-extruded by 3D printing was based on a 85\u00a0wt% dry content basis of a previously optimized hybrid CuO-ZnO-ZrO2/zeolite formulation, well diluted (ca. 15\u00a0wt%) by an inorganic silica-based binder. In particular, the hybrid paste was preliminarily prepared via slurry coprecipitation of nitrate metal precursors, in a relative atomic ratio Cu/Zn/Zr of 60/30/10, by adding a suitable amount of oxalic acid to an ethanolic slurry solution containing a calculated amount of a commercial MFI-type zeolite (Alfa Aesar, Si/Al=25\u00a0mol/mol) so to get a final CuO-ZnO-ZrO2:zeolite weight ratio of 1:1. The hybrid coprecipitated catalyst was then dried overnight, calcined at 500\u00a0\u00b0C for 4\u00a0h, before undergoing the mixing with the binder for printing. The micro-extrusion process was carried out through a customized LUTUM\u00ae 3D clay printer, by setting specific printing parameters in relation to the ink paste viscosity, with nozzles diameters of few hundreds of micron and stack layers in controlled patterns according to the desired architecture. After printing, the cylinders were dried in a humidity chamber at 25\u00a0\u00b0C for two days until conferring a firm structure. Afterwards, the dried monoliths were calcined by applying a slow heating rate of 1\u00a0\u00b0C/min until 500\u00a0\u00b0C under a helium atmosphere (100 STP mL min\u22121).A powdered hybrid CuO-ZnO-ZrO2/MFI catalyst (Hyb-pwd), prepared by conventional coprecipitation with a Cu/Zn/Zr ratio of 60/30/10 at/at and a metal-oxide(s)-to-zeolite ratio of 1:1\u00a0wt/wt, was taken as a reference [20].The elemental composition of catalysts was determined by X-ray fluorescence analysis, using a S8 TIGER spectrometer (Bruker AXS, Germany), equipped with a rhodium anode tube (power 4\u00a0kW and 75\u00a0\u00b5m Be window and LiF 220 crystal analyze). The samples were analyzed as loose powders, considering the emission transitions of copper, zinc and zirconium (Cu-K\u03b11, Zn-K\u03b11, Zr-K\u03b11).The crystallinity of the prepared samples was analyzed upon crushing by a D8 Advance diffractometer (Bruker AXS, Germany), operating with a Ni b-filtered Cu-K\u03b1 radiation (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5) in the 2\u03b8 range 5\u201380\u00b0 at 40\u00a0kV and 40\u00a0mA and a scan step of 0.03\u00b0\u00a0s\u22121.The measurements of reducibility under hydrogen atmosphere (TPR) were performed in a linear quartz micro-reactor (i.d., 4\u00a0mm) fed with a 5.6\u00a0vol%\u00a0H2/Ar mixture at the flow rate of 60 STP mL/min. The experiments were carried out in the range 0\u2013800\u00a0\u00b0C with a heating rate of 12\u00a0\u00b0C/min. The hydrogen consumption was monitored by a thermal conductivity detector, calibrated by the peak area of a known amount of CuO. TPR data resulted very reproducible in terms both of maximum position (\u00a0\u00b1\u00a03\u00a0\u00b0C) and extent of H2 consumption (\u00a0\u00b1\u00a03%).The copper surface area (S\nCu) was obtained by \u201csingle-pulse\u201d N2O-titration measurements at 90\u00a0\u00b0C. Preliminarily the samples were reduced in situ at 300\u00a0\u00b0C in flowing H2 (100 STP mL/min) for 1\u00a0h, then \u201cflushed\u201d at 310\u00a0\u00b0C in nitrogen carrier flow (15\u00a0min) and further cooled down at 90\u00a0\u00b0C. The values of metallic area were calculated assuming a Cu:N2O=\u00a02:1 titration stoichiometry and a surface atomic density of 1.46\u00a0\u00d7\u00a01019 Cuat/m2.A stereomicroscope Nikon\u00ae SMZ1500, with a zoom ratio of 15\u20131 accounting for a total magnifying capability of 3.75x up to 540x, was coupled to a Coolpix 5400 digital camera to carry out the high-resolution study of the printed structure of the samples.Measurements of temperature-programmed desorption of carbon dioxide (CO2-TPD) and ammonia (NH3-TPD) were performed in the experimental setup used for TPR to determine the surface concentrations of base and acidic sites respectively. Before TPD experiments, the catalyst samples (\u223c100\u2013200\u00a0mg) were pre-reduced in a linear quartz micro-reactor (l., 200\u00a0mm; i.d., 4\u00a0mm) at atmospheric pressure, by flowing hydrogen (100 STP mL/min) from room temperature to 300\u00a0\u00b0C (heating rate of 10\u00a0\u00b0C/min). After an isothermal step of 60\u00a0min at 300\u00a0\u00b0C under hydrogen flow, followed by purging with helium, the samples were saturated for 60\u00a0min at 200\u00a0\u00b0C in a gas mixture (flow rate of 50\u00a0mL/min) either of 20\u00a0vol% CO2/He or 5\u00a0vol% NH3/He. Then, the samples were cooled down to 100\u00a0\u00b0C in He flow until a constant baseline level was maintained. The desorption measurements were carried out in a range from 100\u00b0 to 600\u00b0C, at a heating rate of 12\u00a0\u00b0C/min, using helium as the carrier flow (50 STP mL/min). CO2 (m/z, 44) or NH3 (m/z, 17) desorption process was monitored by a quadrupole mass spectrometer (Thermo\nStar) equipped with a heated (150\u00a0\u00b0C) fast-response inlet capillary system, quantitatively calibrated by known pulses of CO2 or NH3.In order to operate under a similar residence time without any control due to possible mass and heat diffusion resistances, two differently sized fixed bed stainless steel reactors were adopted either for the catalytic measurements with the 3D sample (i.d., 12.8\u00a0mm; l., 400\u00a0mm) or the powdered sample (i.d., 6.4\u00a0mm; l., 400\u00a0mm) respectively, being jacketed within a stainless steel rod to maintain an effective control of temperature during the run. The 3D monoliths were reduced in situ at 300\u00a0\u00b0C for 1\u00a0h under a \u201cpure\u201d hydrogen flow at atmospheric pressure. The catalytic data were achieved at 30\u00a0bar, in a range of temperature between 200 and 260\u00a0\u00b0C, under CO2-to-DME hydrogenation conditions by feeding a mixture of CO2/H2/N2 at a volumetric ratio of 3/9/1, operated at a space velocity of 1000 NL/kgcat/h. The reaction stream was analyzed by a GC equipped with a two-column separation system connected to a flame ionized detector (FID) and a thermal conductivity detector (TCD), respectively. Both internal standard and mass-balance methods were adopted for the calculation of conversion-selectivity data, with an accuracy of \u00b1\u00a03% and carbon balance close to 100%.\n\nFig. 1-A) displays the digital image of the micro-extruded matrix-like sample. It should be noted that the calcined cylinder typically presented an external diameter of 12.5\u00a0mm suitable to fit the inner diameter of the reactor. In particular, the top-view image in Fig. 1-B) reveals the uniform square channel cross-section of the structures with the wall thickness of \u223c0.65\u00a0mm and channel width of \u223c0.4\u00a0mm.Both the XRD patterns of the printed and powdered samples, after the reduction treatment, are shown in \nFig. 2. As it can be observed, not only the reflections of the MFI framework of the HZSM-5 structure (JCPDS 38\u20130246), but also the crystallinity of the metallic phase of copper at 43\u00b0 (JCPDS 01\u2013089\u20132838), were retained after the three dimensional method.Regarding other main catalyst features, \nTable 1 reports a comparison of the main physico-chemical properties determined for the printed (Hyb-3D) and the powdered sample (Hyb-pwd).As it is possible to observe, the printing procedure really allows a perfect control of the catalyst properties, considering that in terms of composition (from XRF analysis), texture (values of surface area) and metallic properties (TPR and N2O chemisorption) the Hyb-3D sample practically mirrors the features of the Hyb-pwd sample prepared by conventional coprecipitation.The only differences are visibly associated to the surface properties, as the result of a relatively lower acid-base capacity exhibited by the printed sample. Despite similar desorption profiles of the two samples both in terms of surface sites and temperatures of maximum desorption (not shown for the sake of brevity), the quantitative data reported in Table 1 show that the Hyb-3D sample exhibits not only a smaller CO2 uptake (0.089\u2009mmol/gcat), but also a comparably smaller NH3 uptake (0.362\u2009mmol/gcat), both values resulting ca. 50% than in the counterpart (0.178\u2009mmol/gcat and 0.721\u2009mmol/gcat for CO2 uptake and NH3 uptake respectively). Even without any distinction between Br\u00f8nsted or Lewis sites, however the desorption profiles clearly suggest, on one hand, the presence of sites of same nature (although quantitatively different) and, on the other hand, how in presence of a multi-site surface the acid-base capacity is significantly dependent on the preparation procedure. Evidently, the intrinsic thermofluidic properties of the catalytic ink-paste prepared for the process of micro-extrusion (as related to the use of binders and additives for proper rheological features [44\u201346]), significantly controls the surface affinity of the chemisorption sites, namely basic CO2 activation sites at the metal-oxide(s) surface and acidic dehydration sites at the zeolite surface [23,26,46,47]. Anyhow, the CO2/NH3 uptake ratio, taken as an index of relative concentration of the acid/base population, results to be practically identical on both samples (246.1\u2013247.2), suggesting the same balance of acid-base population directly affecting the CO2 activation process as well as the final step of methanol dehydration into DME.Regarding the catalytic behavior, in \nTable 2 the catalytic results obtained in the direct hydrogenation of CO2 to DME are reported, in terms of CO2 conversion (X\nCO2, %), selectivity to the various compounds (Si, %) and yield to DME (Y\nDME, %), in the temperature range 220\u2013260\u2009\u00b0C, 30\u2009bar and space velocity of 1000 NL/kgcat/h.As a rule, irrespective of the sample considered, the CO2 conversion progressively increases with temperature, the highest values (22.8\u201323.6%) being attained at 260\u2009\u00b0C, as leveled in proximity of the thermodynamic equilibrium [15]. At lower temperature, however, the sample Hyb-pwd exhibits a relatively higher activity as much higher as more determinant is operation under a pure kinetic regime (200\u2013220\u2009\u00b0C). In terms of product distribution, on each catalyst the DME selectivity regularly decreases with temperature, the trend resulting more marked on the 3D sample (51.6\u219236.0%) compared to the trend exhibited by the powdered catalyst (51.2\u219242.6%). On the other hand, despite a progressive increase on both samples, the CO selectivity more steeply rises on the Hyb-3D catalyst from 29.6% (at 200\u2009\u00b0C) up to 53.1% (at 260\u2009\u00b0C), against a less limited increase (36.3\u219243.7) exhibited by the Hyb-pwd sample in the range of temperature considered. Regardless of the reaction temperature, the MeOH selectivity remains almost stable and comparable (11.0\u201313.7%) on both systems, apart from a maximum value of 18.8% recorded for the printed sample at 200\u2009\u00b0C.It is clear that, with similar composition, texture and metallic features, the observed differences in the activity-selectivity pattern of the investigated samples have been primarily associated to their different surface properties as induced by the preparation method. Accordingly, to confirm the control of surface availability of acid-base adsorption sites on the catalytic behaviour, the rate of CO2 conversion was normalized for each catalyst at a temperature as low as 200\u2009\u00b0C (wherein the low activity allows to rule out any control exerted by possible diffusional phenomena), both with respect to the number of basic sites (from CO2 uptake) and with respect to the number of acid sites (from NH3 uptake), so determining turnover frequency values associated to the CO2 conversion (TOF CO2) and DME formation (TOF DME), respectively. As it is possible to argue from \nFig. 3, similar values of TOF CO2 and TOF DME allows a rational overview of the peculiar reactivity of the 3D-printed hybrid CuO-ZnO-ZrO2/zeolite system, clearly pointing to the chemisorption capacity as the critical factor controlling the specific activity of the investigated sample. On the whole, this finding matches the evidence that the reactivity of the 3D sample basically depends on the surface availability of the C-containing surface intermediates, in turn proportional both to CO2 desorbed from basic CO2 activation sites at the metal-oxide(s) interface [18,19,21] and to ammonia desorbed mainly from Br\u00f8nsted acidic sites, considered of primary catalytic importance in the MeOH-to-DME dehydration reaction [26,29]. Although necessary in the step of H2 activation, less decisive appears the role of metallic Cu0 sites on the catalytic reactivity, considering not only a similar copper surface exposure exhibited by the two differently prepared investigated samples (19.7\u201320.3\u2009m2/g, see Table 1), but also a stoichiometrically larger surface availability of activated hydrogen species in respect of the other intermediate species, prompted by the volumetric H2/CO2 feed ratio of 3\u20131.Finally, in \nFig. 4 the stability pattern over the 3D-printed hybrid catalyst is reported in terms of CO2 conversion and DME yield as a function of the time on stream, evidencing an almost steady state during 5 days of experimental run. Considering that, under the adopted conditions, no coke or metal sintering have been ever detected over the \u201cused\u201d catalysts [51], this meaningful result reflects the effectiveness of matrix-like materials on the management of the water formed during reaction, which is considered the main factor affecting the catalyst lifetime in powdered catalysts [28,51].Once found the proper combination among ink-paste composition, 3D model and sintering treatments, the robocasting technique shows all its effectiveness, offering an alternative, cost-effective and facile approach to fabricate structured catalysts with tunable structural, chemical and morphological properties, comprehensively mirroring the features of conventional powdered catalysts used in CO2 utilization technologies.Probing the reactivity of a 3D-printed hybrid CuO-ZnO-ZrO2/zeolite catalyst in the direct hydrogenation of CO2 to DME, the activity-selectivity pattern was put in relation with the surface availability of acid-base adsorption sites, in turn controlled by the thermofluidic properties of the catalytic ink-paste prepared for the process of micro-extrusion. Considering the multi-site nature of the process considered, the specific functionality of 3D catalysts perfectly matches the behavior of the corresponding powdered counterparts only if a suitable exposure both of basic CO2 activation sites at the metal-oxide(s) interface and acidic dehydration sites at the zeolite surface can be ensured, together determining the rate of formation/transformation of the C-containing surface intermediates.This new knowledge ultimately informs process and equipment design for extrusion-based 3D-printing using not only ceramics but also hybridized catalytic ink-pastes, suggesting the need for an optimized setup capable of realizing structured systems with tailored chemico-physical properties.\nGiuseppe Bonura: Writing \u2013 review & editing, Funding acquisition, Supervision, Project administration. Serena Todaro: Investigation (XRF, XRD, BET, TPR, TPD) and Writing \u2013 original draft preparation; Vesna Middelkoop: Conceptualization, Funding acquisition, Project administration; Yoran de Vos: Formal analysis; Hendrikus C.L. Abbenhuis: Supervision; Gijsbert Gerritsen: Methodology; Arjan J.J. Koekkoek: Data curation; Catia Cannilla: Investigation (SEM measurements), Visualization; Francesco Frusteri: Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is part of the CO2Fokus project which is supported by the European Union\u2019s Horizon 2020 Research and Innovation programme under Grant Agreement No. 838061. The authors would like to thank the EU Horizon 2020 Programme for this opportunity. This document reflects only the authors\u2019 view and the Innovation and Networks Executive Agency (INEA) and the European Commission are not responsible for any use that may be made of the information it contains.", "descript": "\n                  This work highlights the effectiveness of an unconventional synthesis of hybrid systems for the direct hydrogenation of carbon dioxide into dimethyl ether (DME), based on micro-extrusion of a ink-like catalytic paste by a robocasting procedure. Due to the possibility to exert a fine control over the structure, surface and geometric architecture, the adopted printing technique really ensures a superior management of heat and mass constraints in respect of the conventional powdered catalysts, the catalyst functionality resulting to be tightly dependent on the cooperation between metal-oxide and acidic phase. Additionally, the accessibility both of the CO2 activation and methanol (MeOH) dehydration sites over the hybrid micro-extruded catalyst most importantly affects the catalytic performance, as suggested by the values of turnover frequency of CO2 conversion and DME formation pointing out the need for a favorable exposure of chemisorption sites of different nature to enhance the specific reactivity.\n               "}
{"full_text": "\n\n\nData will be made available on request.\n\n\nData will be made available on request.Since the Industrial Revolution, one of the main concerns of industries has been to provide human beings with the energy necessary for daily activities. Nowadays, petroleum and its derivatives are the main primary energy sources worldwide. According to a recent statistics study, approximately 37% of primary energy consumed around the globe comes are derived from petroleum. The sectors that depend energetically on petroleum-derived fuels are extensive; the most critical is transportation, which obtains 93% of its energy consumption from fossil fuels, and industrial activities, which obtain around 40% of its energy from these resources [1].Additionally, oil provides a large number of raw materials for other industrial processes, such as chemicals, petrochemicals, plastics, fibers, and pharmaceuticals. Specifically, the refining industry faces two serious problems today: on the one hand, environmental regulations are increasingly demanding lower levels in the content of polluting compounds in gasoline, and on the other, the crude oil extracted worldwide is increasingly more viscous and difficult to refine; these facts have an impulse that an important part of the current research in refining processes has focused his efforts on the preparation of more effective catalysts for oil hydrotreatment, specifically for the refining of heavy crudes.In recent years, numerous studies have demonstrated the attractiveness of using porous silica materials as catalytic supports. They have specifically highlighted research focused on using mesoporous silica materials that have shown remarkable results in terms of their catalytic performance derived from the high surface area offered by these materials and the physicochemical properties of silica [2\u20139]. However, due to the small pore size, these materials have disadvantages related to the diffusion of viscous fluids (such as heavy crude oils) through the mesoporous structure. Which negatively impacts their catalytic performance and, in consequence, the application of these materials in the synthesis of industrial catalysts; therefore, it is necessary to look for alternatives of catalytic supports that combine the textural properties of mesoporous materials while presenting lower resistance to mass transfer processes involved in hydrotreating reactions.Materials with hierarchical porosity are those which contain a network of interconnected pores in the range of micro (< 2\u00a0nm), meso (2 \u2013 50\u00a0nm), and macropores (> 50\u00a0nm); due to their unique properties. They have generated substantial interest in materials science research in recent years, and numerous studies have reported their convenience in applications such as biomaterials, semiconductors, chemical separation, and catalysis [10\u201313]. These materials combine the properties of high surface area and chemical selectivity provided by the microporous and mesoporous materials and the low mass transfer limitations attributed to macroporous materials; this fact makes them especially useful in applications such as catalysis and separation of chemical compounds where both features are significant [14]. Hierarchical porous structures appear naturally in numerous biological materials such as corals, shells, rocks, and even bones, which have inspired the development of synthetic porous materials with the properties and structures of these materials.The first material with hierarchical porosity was reported in 1993 [12,15], but only a few years ago, the catalytic applications of these materials began to be considered. A lot of research has been developed in recent years related to the catalytic applications of hierarchically structured materials in areas such as photocatalysis [16,17], organic oxidation reactions [18\u201320], hydrogenation [21], alkylation [22], and isomerization [23,24]. They show that the hierarchical porous structure promotes greater accessibility to active sites and lower diffusive impediments, which results in improved catalytic activity. It has also been reported the use of silica materials with hierarchical structures as catalytic supports for biodiesel production reactions, finding that the activity depends on the degree of macroporosity of the material, which seems to demonstrate the importance of said structure for reactions with complex and viscous systems [25,26].Regarding using materials with hierarchical porosity as catalytic supports within the petrochemical and refining industry, many studies have reported the convenience of using silica materials with meso\u2011macroporous structure in the Fischer-Tropsch reactions [27\u201329], showing improved activities and selectivity concerning only mesoporous and microporous catalysts. The application of modified zeolites with pores arranged in the micro\u2011meso-macroporous range in the catalytic cracking reaction of aromatic petroleum derivatives showing higher activities than commercial zeolites has also been reported [30\u201333].In the case of hydrodesulfurization (HDS) reactions, several studies have reported using alumina and carbon materials as catalytic support. Han et\u00a0al. have used a meso\u2011macroporous alumina material impregnated with the Mo-Co system, synthesized by molding with PMMA and Pluronic F127\u00ae, which have exhibited an improved activity compared with mesoporous alumina in the HDS reaction of dibenzothiophene [34]. Liu et\u00a0al. have used alumina with disordered hierarchical porosity as support for the Co-Mo system, tested in the hydrodesulfurization of 4,6-dimethyl-dibenzothiophene, which provides an activity comparable to that of commercial mesoporous alumina catalysts [35]. Additionally, Huang et\u00a0al. prepared a catalyst with the Co-Mo-Ni system supported on meso\u2011macroporous alumina with little ordering, which is more active in the HDS of thiophene compared to its mesoporous counterpart; the increase in activity is attributable to the reduction of diffusion limitations provided by the macroporous structure [36]. Regarding using coal with hierarchical porosity as catalytic support, Hussain et\u00a0al. have synthesized catalysts with the Co-Mo system, which have shown a greater catalytic activity when compared to mesoporous carbons and activated carbons used for the same purpose [37].Finally, for the use of hierarchical porous silica materials, Zhang et\u00a0al. have synthesized micro-mesoporous silica materials used in the catalysis of HDS reactions of dibenzothiophene by the Ni-Mo system, which have given better results than type-SBA mesoporous silicas materials and commercial alumina-supported catalysts. The improved catalytic performance is attributed to a better diffusion of the reagents involved and to a more significant number of acid sites promoted by the hierarchical porous structure [38,39]. Despite the fact that in recent years numerous studies have been carried out on the synthesis of materials with hierarchical porosity and their application in catalysis, few have been applied to the catalysis of reactions involved in oil refining; specifically, there are no studies reported to date that use silicas with a hierarchical structure in the macropore to micropore range that have been applied to hydrodesulfurization reactions of complex organic molecules present in petroleum fractions. Previous studies have shown the relevance of the use of mesoporous silica materials as catalytic supports, so the incorporation of a quasi-ordered hierarchical porous structure can result in a decrease in the resistance to diffusion of reactants and products and, with it, improve the catalytic activity in hydrodesulfurization reactions.Silica materials with hierarchical porosity were synthesized using the sol-gel method in combination with a dual soft-hard templating route that has already been applied in the synthesis of carbon and aluminosilicate materials with hierarchical structure [10,15,40,41]. Macroporous structure was templated by a hard polymer stencil, while the micro-mesoporous structure was obtained by a soft colloidal frame.The polymer template was prepared from polystyrene(PS)-HEMA spheres obtained by emulsion polymerization by the \"emulsifier-free\" method, which has proved to be suitable for obtaining polymer spheres with uniform and monodisperse sizes [41,42]. Styrene has been used as the main monomer, 2-hydroxyethyl methacrylate (HEMA) as co-monomer and ammonium persulfate as initiator. Before the polymerization, the monomers were purified to remove inhibitor traces in the commercial reagent. Styrene was purified by washing with sodium hydroxide (NaOH) in 0.1\u00a0M aqueous solutions, using a volume equal to one-third of the volume of the monomer to be washed and dried with anhydrous calcium chloride for 12\u00a0h; 2-hydroxyethyl methacrylate was distilled under vacuum at 35\u00a0\u00b0C and 50\u00a0mmHgCo-polymerization reaction was performed in a batch glass reactor equipped with a condenser, paddle stirrer, nitrogen bubbling system, and thermometer. The reaction medium was deionized water (1000\u00a0mL) previously bubbled with nitrogen to remove the dissolved oxygen. 50\u00a0mg of sodium persulfate was added as a polymerization initiator and stirred until complete dissolution. Subsequently, the co-monomers were added in a weight ratio of 100:2 (Styrene:HEMA). The reaction was carried out at 70\u00a0\u00b0C and 300\u00a0rpm for eight hours. The obtained PS-HEMA copolymer emulsion was discharged and stored in polypropylene containers at 4\u00a0\u00b0C.The PS-HEMA copolymer spheres were packed by centrifugation at 2000\u00a0rpm for 2\u00a0h, the liquid phase was decanted and obtained solid phase was washed and dried at 60\u00a0\u00b0C for 4\u00a0h. The dry solid was used as a hard template for the synthesis of the silica material.Silica support monoliths with hierarchical porous structures were synthesized by the sol-gel method combined with a dual hard-soft templating route. For the synthesis, 4\u00a0g of hard polymer template, synthesized according to the method described in the previous section, was used. The soft template was prepared by an emulsion of amphoteric polymeric surfactant type PPO-PEO-PPO (Pluronic P-123\u00ae and Pluronic F-127\u00ae) according to the methodology previously described by Zhang and Cooper [43].Silica precursor solution was obtained using tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 99%) as silica precursor and HNO3 1.0\u00a0M as acid catalyst. The appropriate amounts of deionized water, TEOS, and surfactant were used to obtain a molar ratio of 1:208:6.21:0.0017 (TEOS:H2O:HNO3:surfactant). The solution was prepared by initially dissolving the necessary surfactant mass in deionized water and nitric acid solution until total dissolution at 35\u00a0\u00b0C and 300\u00a0rpm. Once a homogeneous mixture was obtained, the TEOS was added dropwise, and it was left in agitation for 15\u00a0min before being transferred into polypropylene tubes containing hard template until it was completely covered. The tubes were sealed and placed in an oven at 60\u00a0\u00b0C for 72\u00a0h to consolidate the silica structure. Synthesized monoliths were washed and dried at room temperature for seven days. Dried monoliths were pre-calcined at 110\u00a0\u00b0C for 18\u00a0h with a 1\u00a0\u00b0C per minute temperature ramp and finally calcined at 550\u00a0\u00b0C for 15\u00a0h with the same temperature ramp in order to remove the template. For comparison purposes, silica monoliths with only mesoporous structure were synthesized by a similar method to that previously described, by the remotion of the hard template. The synthesized silica materials were identified by nomenclature shown in Table\u00a01\n. As-synthesized materials are shown in Fig.\u00a01\n.Co-Mo-W trimetallic oxide-state catalysts were prepared by simultaneous impregnation via immersion method. Each support was loaded with fixed equal amounts of molybdenum (5.75 wt% as MoO3), tungsten (10.92 wt% as WO3), and cobalt (3.05 wt% as CoO. Impregnation aqueous solution containing ammonium heptamolybdate tetrahydrate [(NH4)6Mo7O24\u00b74H2O, assay: 81\u201383%, Sigma\u2013Aldrich], ammonium metatungstate hydrate[(NH4)6H2W12O40xH2O, assay: 99%, Sigma\u2013Aldrich] and cobalt nitrate hexahydrate [Co(NO3)2\u00b76H2O, assay: 98%, Sigma\u2013Aldrich] was used. The concentration of each transition-metal precursor was calculated to achieve a Mo(W)/(Mo+W) atomic ratio of 0.5 and a Co/(Mo+W) atomic ratio of 0.43. Monoliths was inmmersed for an hour in the impregnation solution with stirring.The impregnated monoliths were dried at room temperature for 24\u00a0h and then at 110\u00a0\u00b0C for 16\u00a0h in a static air muffle furnace with a 2\u00a0\u00b0C/min temperature ramp. Finally, they were calcined at 500\u00a0\u00b0C for 4\u00a0h in a static air muffle with the same temperature ramp. It is important to mention that the aqueous solution containing the precursors of the three transition metals is stable during the preparation of the catalysts.Fresh sulfided catalysts were prepared by sulfidation of the oxide-state catalysts. The sulfidation reaction was carried out in a glass tubular reactor; oxide-state catalysts were triturated and sieved between ASTM No. 100 and 120 meshes, the material retained by the latter was sulfided (particle size between 0.125 and 0.149\u00a0mm). Meshed catalysts were charged into the reactor and heated until 400\u00a0\u00b0C in N2 atmosphere at 1\u00a0\u00b0C/min temperature ramp; when the reaction temperature was raised, the sulfidation reaction was carried out at constant temperature for 4\u00a0h using a stream of 15 (v/v)% of H2S in H2 with a flow of 26 cm3 per minute. Once the sulfidation time was completed, sulfided catalysts were cooled to room temperature and charged directly to the HDS reactor in an inert atmosphere in order to avoid oxidation of the transition metal sulfides.The morphology of the supports and catalysts particles was studied through high-resolution scanning electron microscopy. The images of the silica monoliths and catalysts were obtained in a Hitachi SU-8230 Cold Field Emission Scanning Electron Microscope (FESEM) which was operated with an acceleration voltage of 1\u00a0kV, with ultra-high vacuum and using secondary and backscattered electron detectors. Images were obtained with amplifications of 5000X, 10000X, 20000X, 50000X and 100000X. The samples were sprayed prior to analysis and deposited on copper supports with graphite conductive ink. The analysis was carried out without coating the sample.Images for the hard PS-HEMA template were obtained in a JEOL JSM-6060LV Scanning Electron Microscope (SEM). The analysis was carried out under high vacuum conditions, with an acceleration voltage of 20\u00a0kV, using a secondary electron detector. Images were obtained at 2500X, 5000X, and 10000X. The samples were prepared by depositing the polymer material on copper supports with graphite conductive tape and coated with a gold film.The chemical analysis of the oxide-state and fresh sulfided catalysts (wt%) was determined by energy-dispersive X-ray spectroscopy (EDS); samples were analyzed with a Bruker X Flash 6/60 system reporting the average of five measurements in different points. Elemental distribution maps of fresh sulfided catalysts were recorded in the same system making a complete scanning of the samples. Samples were measured as granulated powder.Catalysts composition of the oxide-state and sulfided samples was also determined by energy dispersive X-ray fluorescence spectrometry (ED-XRF). A Bruker PUMA S2 X-ray fluorescence spectrometer was used, equipped with a silver X-ray source and a X-Flash\u00ae Standard detector with a resolution of 135\u00a0eV for the K\u03b1 line of the Mn and 100,000\u00a0cps. The samples were measured in powder form. The analysis was carried out in air atmosphere at 20, 40 and 50 KV.Support and catalysts textural properties (type of porosity, surface area, and pore diameter), in the ranges of mesopores and micropores were studied by nitrogen physisorption at 77\u00a0K. The analysis was carried out in an Autosorb Quantachrome 1MP. Before the analysis, the samples were milled to a fine powder and degassed under vacuum conditions at 150\u00a0torr and 270\u00a0\u00b0C for 24\u00a0h. The volume of adsorbed nitrogen was normalized to standard temperature and pressure. The specific surface area was calculated through the BET method (Brunauer-Emmett-Teller), taking the data in the range of relative pressures (P/Po) from 0.05 to 0.30. The distribution of pore diameters was calculated by means of two models: the BJH method (Barret-Joyner-Halenda) using the data of the desorption branch of the isotherms and the DFT method (Density Functional Theory), making a mechanical-statistical analysis of all the points of the isotherms. The pore accumulated volume was obtained from the nitrogen adsorption-desorption isotherms at 77\u00a0K for a relative pressure (P/Po) of 0.99.Macroporous structure of silica supports were determined by mercury intrusion porosimetry (MIP). The analysis was performed in a Micromeritics AutoPore Series IV 9500 porosimeter. Prior to the analysis, the samples were milled to a fine powder and evacuated to an initial measurement of 5\u00a0psi/min (1.8\u00a0mmHg/min), with a suction limit of 500\u00a0\u03bcm Hg and a maximum vacuum of 50\u00a0\u03bcm Hg. The pressure applied for the test was between 0.0026\u00a0MPa and 220.08\u00a0MPa, with 0.222\u00a0MPa being the dividing point between the high and low pressures. The decreasing pressure for the determination of the extrusion branch of the curve has been applied between 220.08 and 0.0634\u00a0MPa. The following parameters for mercury have been considered: density 13.5335\u00a0g/mL, surface tension of 485 dynes/cm and a contact angle of 139\u00b0. Total surface area and pore diameter were calculated by the Washburn method, assuming the presence of cylindrical and spherical pores.The ordered mesoporous structure of the monolithic supports was investigated by small-angle X-ray diffraction in the 0.05\u20135\u00b0 range for 2\u03b8. Samples were analyzed on a Rigaku Ultima IV diffractometer, using Ni-filtered Cu-K\u03b1\u00a0radiation (\u03bb\u00a0=\u00a01.54\u02daA), operated at 40\u00a0kV and 30\u00a0mA and with a step size of 0.5\u00b0 per minute and scanning every 0.02\u00a0s, with rotation of the specimen at 30 rpm. Samples were analized as fine powders.The presence of crystalline phases of oxides and sulfides of transition metals in the synthesized catalysts was determined by X-ray diffraction in the 5\u201380\u00b0 range for 2\u03b8; diffractograms were recorded in a Rigaku Ultima IV diffractometer, working with Co-filtered Cu-K\u03b1 radiation (\u03bb\u00a0=\u00a01.54\u02daA), with a step size of 5\u00b0 per minute and sampling every 0.02\u00a0s and rotating the specimen at 30 rpm. The crystalline phases and indexing were determined using the MDI -Jade\u00ae V 5.0.37 software.The structure of the crystalline phases of transition metal oxides in the oxide-state catalysts was established by Micro Raman spectroscopy. The samples were analyzed at room temperature in a Horiba-Xplora Plus micro spectrometer, using a 532\u00a0nm laser beam operated at a power of 25\u00a0mW. A holographic filter of 1800\u00a0gr/mm was used. The catalysts were analyzed as fine powders without additional preparation, in the range of 200 \u2013 2000\u00a0cm\u22121.Diffuse reflectance spectroscopy in the UV\u2013visible range was aimed to determine the coordination environment of transition metal atoms in oxide-state precursors. Spectra were recorded in the 200\u2013800\u00a0nm range at room temperature using a Varian Cary 5000UV\u2013vis spectrometer equipped with an integration sphere. Spectra were determined using an internal MgO reference material and using the MonoSBA-15 material as blank in order to avoid the appearance of the electronic transitions corresponding to the siliceous material. Samples were analyzed as monoliths.The XPS spectra of the samples were recorded using a SPECS\u00ae spectrometer with a PHOIBOS\u00ae 150 WAL hemispherical energy analyzer with angular resolution (< 0.5\u00b0), equipped with an XR 50 X-Ray Al-X-ray and \u03bc-FOCUS 500 X-ray monochromator (Al excitation line) sources. The binding energies (BE) were referenced to the C 1\u00a0s peak (284.8\u00a0eV) to account for the charging effects. Gaussian/Lorentzian functions were employed to fit the spectra after background subtraction according to the Shirley equation.Hydrodesulfurization (HDS) reaction of dibenzothiophene (DBT) was carried out in a Parr model 4842 high-pressure batch reactor at 350\u00a0\u00b0C and 33.8\u00a0bar, for 5\u00a0h and using an excess of hydrogen. Samples were thoroughly ground in a mortar to a fine powder and meshed for use materials with particle sizes between 106 and 125\u00a0\u03bcm.For the experiment, 0.5\u00a0g of sulfided catalyst was introduced into the batch reactor containing a solution of DBT in decalin with a concentration of 195\u00a0mmol/L at room temperature. The reactor was then pressurized to 33.8\u00a0bar with hydrogen and heated up to 350\u00a0\u00b0C at a rate of 10\u00a0\u00b0C per minute. The stirring rate of the reaction mixture was 900\u00a0rpm. Once the working temperature was reached, samples of the reaction medium were extracted every 30\u00a0min until reaction time was reached. Concentracion of DBT and HDS products in the samples was measured by gas chromatography to determine conversion versus time dependence and products distribution; the analysis was done in an HP 4890 gas chromatograph provided with a 10-ft packed column. This column contains 3% OV-17 as a separating phase on Chromosorb WAW 80/100. A commercial CoMo/Al2O3 catalyst (KF-752) was used as a reference.Catalytic performance was measured by taking into account two parameters: the apparent rate constant (related to the rate of reaction) and selectivity (related to the capability of the catalyst to promote the formation of desirable products). The apparent rate constant is a good approximation to the reaction rate in the sense that it involves the effect of the concentration of reactants and products, the effect of temperature, and the negative effect of the presence of the H2S in the batch reactor. Previous studies have shown that HDS reactions of simple sulfur compounds follow pseudo-first-order kinetics [44\u201349]. Therefore the apparent rate constants (kapp) were calculated by this model (Eq.\u00a0(1), where rHDS is the reaction rate for the global HDS reaction, kapp is the apparent rate constant and CDBT is the DBT concentration along the time). We assumed pseudo-zero order respect to hydrogen and excess of hydrogen. Hydrogen partial pressure could be considered as constant during the overall reaction time. Apparent reaction constants were calculated using an exponential fit of the DBT concentration data and a linearization of this fitting according to Eq.\u00a0(2), where CDBT0 represents the initial concentration of DBT and XDBT is the DBT conversion at overall reaction time.\n\n(1)\n\n\n\nr\n\nH\nD\nS\n,\na\np\np\n\n\n=\n\n\nd\n\nC\n\nD\nB\nT\n\n\n\n\nd\nt\n\n\n=\n\u2212\n\nk\n\na\np\np\n\n\n\nC\n\nD\nB\nT\n\n\n\n\n\n\n\n\n(2)\n\n\n\nC\n\nD\nB\nT\n0\n\n\n\n(\n1\n\u2212\n\nX\n\nD\nB\nT\n\n\n)\n\n=\n\nk\n\na\np\np\n\n\nt\n\n\n\n\nHydrodesulfurization of dibenzothiophene can occur by two parallel pathways shown in Fig.\u00a02\n: direct desulfurization (DDS) and hydrogenation (HYD); the first leads to the formation of biphenyl (BP) via hydrogenolysis, this is, the sulfur atom is removed from the molecule by the breaking of the carbon-sulfur bonds, while the hydrogenation of one or two aromatic rings produces cyclohexylbenzene (CHB) and bicyclohexyl (BCH), respectively. The ability of the catalyst to promote one route preferably is known as selectivity, and it was determined as the ratio between hydrogenation and direct desulfurization products at 30% conversion of dibenzothiophene according to the Eqs.\u00a0(3)\u2013(5), where the factors in brackets are the molar concentrations of the HDS products.\n\n(3)\n\n\nD\nD\nS\n=\n\n\n[\nB\nP\n]\n\u00d7\n100\n\n\n[\nC\nH\nB\n]\n+\n[\nB\nC\nH\n]\n+\n[\nB\nP\n]\n\n\n\n\n\n\n\n\n(4)\n\n\nH\nY\nD\n=\n\n\n(\n[\nC\nH\nB\n]\n+\n[\nB\nC\nH\n]\n)\n\u00d7\n100\n\n\n[\nC\nH\nB\n]\n+\n[\nB\nC\nH\n]\n+\n[\nB\nP\n]\n\n\n\n\n\n\n\n\n(5)\n\n\nS\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n=\n\n\nH\nY\nD\n\n\nD\nD\nS\n\n\n\n\n\n\nFor comparative purposes, in the reaction experiments were used equal catalyst mass, with similar particle size (100\u2013125\u00a0\u03bcm) and with the same metal charge (added during catalysts synthesis); in addition, the HDS performance of a commercial CoMo/Al2O3 catalyst was measured as a comparative model (Mo\u00a0=\u00a014.2%; Co\u00a0=\u00a03.8%; P\u00a0=\u00a00.83%; SBET\u00a0=\u00a0223 m2/g; average pore diameter\u00a0=\u00a06.6\u00a0nm).The morphology of the synthesized materials was analyzed by high-resolution scanning electron microscopy, which allowed us to obtain images of the materials in the micrometer ranges up to nanometers, being able to study the porosity in the range of macropores and mesopores.\nFig.\u00a03\n shows the images for the styrene-HEMA copolymer hard template. Image A in the Figure corresponds to the obtained copolymer spheres in the emulsion polymerization procedure; it is possible to see the formation of spheres of copolymer with a well-defined spherical shape and with little dispersion in size, with an average diameter of about 800\u00a0nm, as planned in the synthesis methodology. The image in B shows the structure of the synthesized hard template obtained after centrifugation and dying of the copolymer emulsion, a regular and uniform arrangement of the spheres was obtained, with a cubic-like packing of solid spheres, which is convenient for the synthesis of hierarchical porous silicas.Scanning electron microscopy images obtained for mesoporous silica monoliths are shown in Fig.\u00a04\n. MonoSBA-15 material (A and B) is constituted for worm-like aggregated in roller-like clusters of several micrometers of length, which have been widely documented previously [4,50\u201353]; in Fig.\u00a05\n is possible to see that mesoporous structure is constituted for cylindrical channels of few nanometers (5-10\u00a0nm) arranged in a hexagonal array as expected in the proposed method. Likewise, it is possible to notice the presence of interstices between the sintered particles whose dimensions are both in the mesoporous and macroporous range, which produces surface porosity, confirmed from the nitrogen physisorption and small-angle X-ray diffraction analyses presented in the following sections. On the other hand, images for MonoSBA-16 (shown in Fig.\u00a04C-D) reveal the presence of particles with quasi-spherical morphology, which are sintered to form large clusters (30\u00a0\u03bcm approximately); this is in agreement with previous reports of the morphology of materials SBA-16 [9,52,53], additionally is possible to appreciate the existence of interstices between the particles that generate surface porosity. At higher amplifications, the material shows the presence of quasi-spherical pores in the walls with a certain degree of ordering and whose dimensions are in the mesoporous range, with an approximate diameter of 5-10\u00a0nm.Concerning hierarchically porous silica monoliths, scanning electron microscopy images are shown in Fig.\u00a05. Images for HOPSHM material display the presence of a macroporous interconnected network with a certain degree of order and an average pore diameter of 626\u00a0nm (measured by statistical analysis on pore sizes). This pore size is slightly smaller than the diameter of the styrene-HEMA copolymer spheres shown in Fig.\u00a03, because, during the calcining process of the silica material, the spheres suffer from reduction due to the high temperature; this effect has already been previously reported [15,41]. An analysis at larger amplifications shows that particles of mesoporous silica material constitute the walls of the macropores with similar morphology to SBA-15 particles. The Figure also reveals that macropores walls possess interstices between sintered mesoporous particles, whose dimensions are in the range of 20\u2013100\u00a0nm, originating porosity in the material. Regarding HOPSCM material, scanning electron microscopy images clearly showed a macroporous structure similar to that of the HOPSHM sample, with a similar average pore diameter (621\u00a0nm). At higher amplifications, it was possible to appreciate that particles of silica constitute the walls of the material with morphology similar to SBA-16 with pores in the mesoporous range (4\u20135\u00a0nm); surface porosity is also appreciable in the walls of macropores in the range of 20\u2013100\u00a0nm.It must be emphasized that information obtained from scanning electron microscopy images is consistent with the results found by nitrogen physisorption, mercury intrusion porosimetry, and small-angle X-ray diffraction techniques described in the following sections.The nitrogen adsorption-desorption isotherms at 77\u00a0K for the supports are shown in Fig.\u00a06\n. According to the IUPAC classification, all the supports show Type IV nitrogen adsorption-desorption isotherms [54\u201357], which are characteristic of mesoporous materials which present a hysteresis cycle related to the capillary condensation and evaporation within the mesopores. For the MonoSBA-15 material, a well-defined type H1 hysteresis cycle is observed in the range of relative pressures from 0.5 to 0.8, with adsorption and desorption isotherms nearly parallel, which is indicative of the presence of pores with regular morphology, as expected for the characteristic cylindrical channels in SBA-15-type materials that have been widely reported in previous studies of this material [4,41,52,58\u201360].With respect to the MonoSBA-16 sample, it shows a type H2 hysteresis cycle in the range of relative pressures from 0.4 to 0.7 in which the desorption branch is wider and vertical than the adsorption one, which means that mesopores do not have uniform morphology. This is because this material is characterized by the presence of mesopores in the form of interconnected spheres, which generates differences between the input diameter and inside the pore. This isotherm has also been widely documented in previous studies [9,52,58,60].On the other hand, HOPSHM material presents an isotherm with a bimodal hysteresis cycle, enclosed to the H1 type in the range of relative pressures of 0.4 to 0.8, which may be indicative of the presence of mesoporous with regular morphology like those of the MonoSBA15 material but with two different pore sizes [41,54]. Regarding the HOPSCM material, it presents a type H2 hysteresis cycle type in the range of relative pressures between 0.4 and 0.7, similar to that of MonoSBA-16. This is indicative of the presence of bottle-ink pores, which have been widely reported for the SBA-16 material. It is also possible to see an increase in the amount of nitrogen adsorbed in all the synthesized samples at relative pressures above 0.95. This is indicative of the presence of larger pores in the range of 30 to 50\u00a0nm which may be due to the superficial porosity that forms in the walls of the macropores by the sintering of individual particles of mesoporous silica.The fact that the amount of adsorbed nitrogen below the occurrence of hysteresis in all isotherms is greater than the increase due to capillary condensation suggests that not only does multilayer adsorption occur on the surface of the mesopores. Also, there is the filling of the micropores present in the samples, which comes from the insertion of the PEO block terminations in the silica walls. As well as from the mesopores entrances if these are in the range of micropores; this is consistent with previous findings related to the fact that silica networks formed with oligomers or block copolymers tend to exhibit microporosity, which holes originally occupied by PEO constitutes blocks occluded in the silica matrix [60,61].\nFig.\u00a07\n shows pore diameter distributions for the synthesized supports. In all cases, it can be observed that closed distributions are generated, which points to the presence of pores of uniform sizes in all the synthesized materials. In the case of the MonoSBA-15 material, the distribution is centered at 6.57\u00a0nm, while the MonoSBA-16 material shows a bimodal distribution, in which one of the tails is wide and centered at 3.89\u00a0nm while the second is very narrow and centered at 5.81\u00a0nm, this suggests the existence of pores with two different diameters that can be attributed to pore and interconnection diameters. With respect to materials with hierarchical porosity, the HOPSHM material shows a bimodal distribution in which the two loops are closed and centered at 3.92\u00a0nm and 4.98\u00a0nm, respectively, which may also be due to the synergistic effects between the methods of colloidal molding and hard templates [15,54]; On the other hand, the HOPSCM sample shows a distribution of wide pore diameters in the range of 2 to 3.8\u00a0nm, centered at 2.82\u00a0nm.The nitrogen adsorption-desorption isotherms and pore diameter distributions for the oxide-state and sulfided catalysts (not shown here) have the same tendency and shape that those for the supports. Being indicative that the porous morphology of the materials is preserved through the synthesis process, uniquely low displacements to lower relative pressures are observed, related to the decreasing of the pore diameter associated with the deposition of the precursors and active phases over the silicas surface onto the pores.\nTable\u00a02\n summarizes the textural properties of the porous silica supports. It is possible to observe that the MonoSBA-15 and MonoSBA-16 materials have large surface areas, suitable for the use of these as catalytic supports. While the materials with hierarchical porosity (HOPSHM and HOPSCM) showed slightly smaller surface areas as previously reported for similar materials [15,41,54]. In those reports the samples showed a loss of surface area due to the sintering of the individual particles of mesoporous silica around the hard template in order to form the walls of the macroporous structure, which may be occluding part of the mesoporous structure. In the same table, it can be appreciated that a drastic reduction in surface area is obtained after oxide-state catalysts synthesis (between 50\u201360%) related to the deposition and formation of oxide precursors of the active phases; after sulfidation, the reduction is almost 70\u201380% respect to support surface area; similar tendency is seen in the reduction of pore diameter and pore volume, which is indicative of the high dispersion of the active phases onto the support\nFig.\u00a08\n shows the mercury intrusion-extrusion curves for the synthesized silica materials with hierarchical porosity. Both samples present typical curves for highly porous materials, which exhibit a considerable increase in the volume of intruded mercury in the range of 100 \u2013 5000\u00a0psi, which is indicative of the presence of large pores in the range of 500 \u2013 600\u00a0nm, fact that is consistent with the used experimental method. Also, a moderate increase in the volume of intruded mercury is observed in the range of high pressures above 10,000\u00a0psi, corresponding to pore diameters between 50 and 200\u00a0nm, corresponding to the appearance of surface porosity that is formed by the sintering of individual mesoporous silica particles around the macroporous template during the synthesis. Finally, a slight increase in the amount of mercury intruded at pressures above 100,000\u00a0psi is observed, which corresponds to pores in the mesoporous range. However, the analytical technique is not conclusive concerning this range of porosity because of the limitations of the technique.The fact that intrusion and extrusion curves do not coincide, giving rise to a hysteresis region, indicates the entrapment of mercury inside the pores during the extrusion process. This is justified by the presence of interconnected ink-bottle macropores, as expected from the use of spherical polymer particles as a template so that lower pressures are required to empty the pores during the mercury extrusion until pressure decays to that corresponding to the interconnection diameters. The connectivity and the pore size determine the extrusion that occurs when the pressure decreases. Therefore a deep analysis of the differences between both curves allows determining the dimensions of macropore diameter and the interconnection channels between them.Pore diameter distribution for the materials can be seen in Fig.\u00a08. It is evident the predominance of pores in the macroporous range, with an average diameter of 613\u00a0nm for the HOPSHM samples and 621\u00a0nm for the HOPSCM sample. This is consistent with the synthesis method in which polystyrene-HEMA copolymer spheres with average diameters of 650\u00a0nm were used as a hard template, with a slightly smaller diameter derived from the contraction of the spheres that have already been reported in previous studies [10,15,20]. Likewise, the presence of pores in the boundary between the macropores and mesopores (50\u2013200\u00a0nm) can be observed, related to the appearance of interparticle porosity generated by the sintering of SBA-type particles around the polymer spheres during the synthesis process, as previously described [10,15]. Textural properties obtained by mercury porosimetry are summarized in Table\u00a03\n, it can be seen that both materials show a surface area smaller than that measured by the nitrogen physisorption technique; also, it is observed that the main part of the porosity of the material is in the macroporous range (approximately 80% for both samples). However, this technique is not accurate for analysis of the mesoporous fraction of the supports, so it is expected that the mesoporous fraction could be more significant.Small-angle X-ray diffractograms obtained for porous silica supports are shown in Fig.\u00a09\n, in all of them is noticed the existence of defined peaks which evidence the existence of well-structured mesoporous phases within the synthesized materials. MonoSBA-15 and HOPSHM materials show peaks corresponding to (100), (110), and (200) reflections of a 2D hexagonal array of cylindrical mesopores with spatial group p6mm. They have already been widely reported for materials SBA-15-type [4,21,41,51,59,60]. On the other hand, MonoSBA-16 and HOPSCM show peaks corresponding to the (110), (211), and (220) reflections of a cubic structure Im3m, which are also already widely characterized in the literature [9,51,52,60].All synthesized materials exhibit a very wide peak of high intensity between 0.1\u00b0 and 0.5\u00b0. This peak can be attributed to the presence of pores in the range of 10 \u2013 30\u00a0nm, corresponding to the surface porosity which appears from the sintering of individual silica particles around the macroporous template, the analysis of nitrogen physisorption and mercury intrusion porosimetry also described the presence of these phases.Concerning the oxide-state and sulfided catalysts, small-angle diffractograms (not shown) still exhibit the presence of these characteristic peaks, indicating that the mesoporous structure is maintained after impregnation with the active phases and their subsequent oxidation and sulfidation, solely a shift of the peak at 0.9\u20131.0\u00b0 at slightly higher angles is observed, demonstrating the reduction of the mesoporous diameter of the catalysts, an effect that had already been observed in the nitrogen physisorption results presented previously.Wide-angle diffractograms obtained for porous silica supports (not shown) reveal, in all the cases, a very broad peak centered around 24\u00b0 corresponding to amorphous silica, which has been widely reported in previous studies [4,14,41,51,52,59,60], this peak still appears in the oxide-state and fresh sulfided catalysts as shown in the Figs. 10 and 11\n\n.\nFig.\u00a010 shows diffractograms for the oxide-state supported catalysts, in all of them the presence of low intensity peaks at angles 2\u03b8 of 19.5, 25, 28, 32.5, 38, 43, 28, 57 and 59.5\u00b0 corresponding to the \u03b2-CoMo(W)O4 (JCPDS-ICDD 21\u20130868, 15\u20130867). The presence of this phase has already been previously reported in numerous studies of hydrodesulfurization catalysts based on active species of cobalt, molybdenum, and tungsten, and that has been documented as a precursor phase of active sites for catalysis [4\u20138, 34,36,62,63]. Low intensity peaks have also been identified corresponding to the isolated molybdenum trioxides (MoO3, JCPDS-ICDD 76\u20131003) and tungsten trioxides (WO3, JCPDS-ICDD 85\u20132460), with an orthorhombic and hexagonal structure, respectively; which agrees with previous results presented by Huirache et\u00a0al. [4\u20138, 62\u201363]. Finally, diffractograms also show the presence of dispersed species of molybdenum and tungsten polyoxides with the general formula MoxW1-xO3 type (JCPDS-ICDD 28\u20130668), as well as polymolybdates and polytungstates (not shown in the Figure) corresponding to compounds containing a large number of molybdenum and tungsten atoms, in which the metal/oxygen ratio is 2.75 to 2.9, such is the case of compounds such as Mo4O11, Mo17O47, Mo9O26, W24O68, W17O47, W20O58, W19O55, which present coordination structures similar to those of Mo(W)O3. The low intensity diffraction peaks in all samples are indicative that most of the supported species are widely dispersed on the surface of the supports and the presence of not well-defined peaks is due to the supported species are amorphous or have crystallite sizes below the detection limit of the technique (<4\u00a0nm).Fresh sulfided diffractograms are shown in Fig.\u00a011; for all the synthesized materials, characteristic peaks corresponding to hexagonal molybdenum and tungsten disulfides (MoS2, JCPDS-ICDD 75\u20131539; WS2, JCPDS-ICDD 08\u20130237) were observed which have been widely documented as hydrodesulfurization catalytic species [4\u20138, 36,62\u201366]; showing differences in the intensity of the same, due to the degree of dispersion of the sulfides on the surface of the support. The presence of wide peaks in all the samples is indicative of the presence of crystalline domains of different sizes supported on the porous silica support.The presence of cobalt sulfide as Co9S8 form (JCPDS-ICDD 02\u20131459) has also been identified and documented as an active phase in previous studies of hydrodesulfurization catalysts containing molybdenum and cobalt [62], hierarchical porous silica supports exhibit very intense peaks for this phase, exposing the strong influence of the support structure over the type of formed supported sulfur species in catalysts. Finally, the presence of peaks corresponding to Co1.62Mo6S8 (JCPDS-ICDD 30\u20130450) has also been identified. Samples exhibited low intensity peaks in the range of 20 \u2013 30 \u00b0 due to the presence of oxidized transition metal species because of an incomplete sulfidation of oxide state precursor or for oxidation of sulfided catalysts before the analysis.Micro Raman spectra obtained for the oxide-state catalysts are shown in Fig.\u00a012\n; all the samples show a very intense band in the range of 900\u20131000\u00a0cm\u22121, which due to its amplitude and position, indicates the presence of various tungsten and molybdenum species with different molecular symmetries. Decomposition of such band reveals an intense peak at 940\u2013960\u00a0cm\u22121, which is usually attributed to the symmetric stretching vibration of the terminal bond Mo(W)=O in various types of polymolybdates and polytungstates with octahedral metal coordination, such as Mo7O24\n6\u2212 and W7O24\n6\u2212, whose intensity is enriched by the contribution of the Si-O stretch of the silanol groups of the silica. Additionally, a shoulder is present at 980\u2013985\u00a0cm\u22121, and a band of low intensity around 860\u00a0cm\u22121 that has been reported as corresponding to the stretches of the Mo-O-Mo bonds in irregularly shaped polymolybdates [67,69,70]. The presence of polytungstates in the catalysts is confirmed by the appearance of low intensity bands at 510\u00a0cm\u22121 and 200\u2013300\u00a0cm\u22121, corresponding to the stretching and angular deformation of the W-O-W bonds, respectively [68,71,72]. In the same region, contributions of tetrahedral isolated dioxo molybdenum and tungsten compounds are found. These appear as an intense band at 970\u2013975\u00a0cm\u22121 related to the asymmetric stretch of the bond O=Mo(W)= O [67,68,71,72].The presence of supported isolated CoMo(W)O4, with tetrahedral coordination, is confirmed by the presence of an intense band between 935 and 945\u00a0cm\u22121 (attributed to the symmetrical stretching of the W=O of the tungsten in WO4\n2\u2212), accompanied by a low intensity peak at 730\u2013740\u00a0cm\u22121 related with the asymmetric vibration of O-W-O bonds [68,71,73]. The displacement of this band is indicative of distortion in the tetrahedral structure. Isolated species of molybdenum in tetrahedral coordination is assumed by the presence of the bands in 890\u2013900\u00a0cm\u22121, corresponding to the stretch Mo-O-Co mode, in 830\u2013840\u00a0cm\u22121, associated with the asymmetric stretch Mo-O and 317\u00a0cm\u22121 related to the angular bend of O-Mo-O [67,74].Bands in 990\u2013995 and 815\u2013820\u00a0cm\u22121 appear within the two regions of more intense bands of the spectra for the whole catalysts, in addition to small peaks in 708, 666, 417, 377, 338, 290, 248, 217, 198 and 160\u00a0cm\u22121, indicating the occurrence of MoO3 supported on silica as previously reported [70]. Low intensity peaks are observed in all spectra at 715\u00a0cm\u22121 and 435\u00a0cm\u22121 associated with WO3 [68,71].Finally, all the spectra exhibit bands in 990, 970, and 910\u00a0cm\u22121, which appear as shoulders in the main band, in addition to peaks of low intensity in 635, 252, and 220\u00a0cm\u22121, which have been previously reported as corresponding to silico-molybdic anion [SiMo12O40]4\u2212. In addition, around 1020\u00a0cm\u22121, very low intensity peaks are observed associated with stretching the Mo=O bond in mono-oxo molybdenum species directly linked to the silica network (SiO2\u2212Mo=O) [70].Values for the ratio between terminal links, Mo(W)=O, and bulk links, Mo(W)-O-Mo (W), were obtained from the areas of the deconvolution peaks of the bands found at 950\u2013960\u00a0cm\u22121 and 980\u2013985\u00a0cm\u22121 associated with such vibrations. This value is of paramount importance in the catalysis of HDS reactions since previous studies have shown the influence of the presence of metal-oxygen terminal bonds in the formation of catalytic sites [75]. Results are shown and discussed in the Catalytic Performance section.Chemical analysis of the catalysts was made by energy dispersive spectroscopy (EDS) and X-ray fluorescence spectrometry. EDS spectra for oxide-state and sulfided catalyst (not presented) have shown the presence of characteristic signals corresponding to the electronic transitions of the expected elements in the samples (O, Si, Mo, Co, W and S); the absence of the signal corresponding to carbon is indicative of complete removal of the polymeric porogen agents (surfactant and styrene-HEMA copolymer spheres). By this technique, semiquantitative elemental compositions of the catalysts were determined which are reported in Table\u00a04\n. It can be noted that the values determined experimentally are close to those established theoretically in the synthesis. This fact is evidence of the homogeneity in the distribution of the metals transition on the supports and the effectiveness of the impregnation method used for the synthesis of catalysts. The table also reveals that the chemical composition is similar for all the catalysts, and the differences between samples are in the range of experimental error. Hence it can be assumed a similar metal charge in all the catalysts.In the other hand, energy dispersive X-ray fluorescence spectra (not shown) reveal the presence of characteristic bands corresponding to the lines K\u03b11 and K\u03b21 of silicon, K\u03b11 and K\u03b21 of cobalt, K\u03b11 of molybdenum and L\u03b11 and L\u03b21 of tungsten, indicating the presence of such metals in the catalysts, these results are consistent with EDS results. The semiquantitative composition of the catalysts, determined as oxides, is presented in Table\u00a04. It is possible to appreciate that all the catalysts exhibit similar quantities of each of the oxides of transition metals, which are close to the expected percentages according to the atomic proportions proposed in the experimental procedure, differences are in the level of experimental error and not significant difference is seen between samples.Oxide-state catalysts DRS-UV\u2013Visible spectra are shown in Fig.\u00a013\n, as well as the peaks deconvolution. All spectra exhibit a very intense band between 200 and 350\u00a0nm that, when deconvolved in peaks, reveals the presence of three different bands. The band between 220 and 240\u00a0nm corresponds to the transition of ligant-metal charge transfer related to the presence of Mo6+ and W6+ ions with tetrahedral coordination, as in the isolated species of WO4\n2\u2212 and MoO4\n2\u2212, whose presence in HDS catalysts supported on silica has been previously reported [4,5,36,73]. The band at 290-300\u00a0nm has been attributed by Jeziorowski et\u00a0al. to transitions of binder-metal charge transfer in Mo-O-Mo groups present in octahedral polymolybdates and polytungstates [76,77]. While that at 320-340\u00a0nm has been reported as corresponding to the ligant-metal charge transfer transition of O2\u2212 to Mo6+ or W6+ in compounds with octahedral coordination of the transition metal, such as polymolybdates and polytungstates [36,70,71,78]. Both types of compounds were identified in the X-ray diffraction analysis, and these results are consistent with the fact that the structure of the supported species is governed by the acid-base interactions between the transition metals and the acid silica surface. The presence of wide bands indicates that metals are present in aggregates of different sizes, in all the samples, a similar quantity of molybdenum and tungsten species with octahedral and tetrahedral coordination is observed. However, the electronic transition of the octahedral species is less probable, so the fact that it can be seen in the spectra indicates that a high concentration of those structures is present.Concerning the band at 500\u2013520\u00a0nm, this has been previously reported as corresponding to the transitions of charge in complexes of Co2+ with octahedral coordination [4,5,79\u201381]. The appearance of the band between 565 and 580\u00a0nm has been attributed to \nd-d electronic transitions (4T2g to 4A2g and 4T2g to 4T1g) in octahedral cobalt complexes of high spin, present in the \u03b2-CoMoO4. In which the cobalt interacts with the molybdenum and whose presence has already been identified by X-ray diffraction. Octahedral cobalt ions are important in HDS catalysis because of their easy sulfidation [4,5,81]; since the transitions of these octahedral ions are not very probable, intensity refers that they are present in a high concentration. In the same range (500\u2013520\u00a0nm), appears the band adscript to charge transitions for Co2+ species with tetrahedral coordination that has also been reported [79\u201381], indicating the presence of cobalt ions interacting directly with the support in the form of Co2SiO4, hence there could be small amounts of these species supported on the catalyst.The catalytic activity of the synthesized materials in the hydrodesulfurization reaction (HDS) of dibenzothiophene (DBT) was measured using the apparent rate constant calculated by the method descript in the Experimental section. In Fig.\u00a014\n the dibenzothiophene conversion profiles are shown, as was experimentally determined; it can be seen that all catalysts follow a quasi-linear behavior, as is expected for a pseudo-first-order chemical reaction in which kinetics is the controlling reaction step; linearity is an evidence of mass transfer limitations absence in the reaction media [44\u201348]. At high reaction times, catalysts supported on mesoporous silica monoliths, CoMoW-MonoSBA15 and CoMoW-MonoSBA-16, exhibit less linear behavior compared to those supported in hierarchically porous silicas (CoMoW-HOPSHM and CoMoW-HOPSCM) indicating that the diffusive and mass transfer effects in the reaction system and the presence of reversible collateral reactions become important for long reaction times.In the Fig.\u00a014 it is possible to note that the highest conversion is obtained for catalysts supported on hierarchically structured porous silicas, even greater than that of the commercial catalyst used as reference, nearly 20\u201330% of extra conversion is obtained by the use of hierarchical porous support in comparison with an only-mesoporous one. Catalysts supported on hierarchical porosity silicas present a conversion between 75 and 85% higher than that obtained for the commercial catalyst used for comparative purposes.The values found for the apparent rate constant (kapp) for the tested catalysts are shown in Table\u00a05\n. These values were obtained from the time versus conversion data using linear regression, calculated regression coefficient (R2) values were between 0.982 and 0.999 for the different catalysts and the standard error for the values of apparent rate constants were between 3 and 5%. The fact that an equal mass of catalyst was used for every experiment and that all of them have similar metal charges (as determined by the EDS and ED-XRF results) allows using this parameter as a comparative parameter of the catalytic activity of the synthesized materials. Catalysts supported on hierarchically structured silica monoliths exhibit higher values for the apparent reaction constant than those for the catalysts supported on an only-mesoporous media and the commertial catalyst, indicating better performance in terms of conversion of dibenzothiophene and, thus, sulfur remotion. The difference between the values (15 \u2013 20%) is higher than the experimental error (5%), indicating that the use of hierarchically porous silica as support is advantageous for preparing catalysts based on metal transition sulfides for hydrodesulfurization of dibenzothiophene.The observed behavior in catalytic activity is related to a higher density of molybdenum and tungsten sulfides deposited on the surface of the catalysts. Which is higher for those supported on hierarchical silicas, as established by the results of X-ray diffraction, where more intense peaks are obtained for these catalysts. This tendency is confirmed by the diffuse reflectance UV\u2013Visible spectroscopy results, in which more active catalysts exhibited more intense bands related with the tetrahedral and octahedral molybdenum and tungsten ions, representing a higher concentration of polymolybdates and polytungstates which are appropriate oxide-state precursors for HDS active phases. It has also been found in all samples the presence of oxide precursors with cobalt ions with octahedral coordination, which are advantageous in the formation of active sites for hydrodesulfurization catalysis [44\u201347, 75].Additionally, Micro Raman spectra reveal the presence of molybdenum and tungsten species in octahedral coordination in all samples, showing the catalysts supported on hierarchical porous silica monoliths a greater intensity of the bands related to this species. This is related to the fact that those catalysts contain the highest number of Mo(W)=O terminal bonds, which has been reported as responsible of the catalytic activity in the hydrodesulfurization of dibenzothiophene [75]. The relation between terminal and bulk bonds (obtained from Micro Raman spectra deconvolution) and apparent reaction rate constant (kapp) is shown in Fig.\u00a015\n, indicating that activity increases with a higher value of this ratio (corresponding to a higher quantity of easy-to-sulfide terminal bonds and a higher quantity of Mo(W)S2 edge sites). On the other hand, it is known that the sulfidation of isolated \u03b2-CoMo(W)O4 produces the segregation of crystals of Co9S8 and curved structures of Mo(W)S2 doped with cobalt, therefore the catalytic activity is the result of a \u201cjoint effect\u201d derived from the presence of metal centers with octahedral and tetrahedral coordination, hence more active catalysts are those with higher concentrations of both kind of chemical species.Another parameter determined for the catalytic performance was selectivity. Two parallel pathways are typically described in literature in which HDS of DBT may occur: direct desulfurization (DDS) and hydrogenation (HYD). The first one leads to the formation of biphenyl (BP) via hydrogenolysis, while the hydrogenation of one or two aromatic rings produces cyclohexylbenzene (CHB) and bicyclohexyl (BCH), respectively. Selectivity results for synthesized catalysts are shown in Table\u00a05, it can be noted that catalysts supported on mesoporous silica monoliths exhibit low selectivity values related with a strong tendency of those catalysts to promote the direct desulfurization route (DDS), as have seen in previous studies of catalysts supported in mesoporous silicas [4\u20139]. On the other hand, catalysts supported in hierarchically structured porous silicas show higher selectivity values demonstrating a better promotion of both catalytic routes, despite a tendency to DDS is still observed, just similar to the commercial catalyst used as a comparative model. Hierarchically structured HDS catalysts include a network of large and small interconnected pores which reduce diffusion limitations and allow more facile access of the active sites for DBT conversion compared with their counterparts (SBA-15 mesoporous support). Selectivity results suggest that the use of silicas with hierarchical porosity seems to improve not only the apparent reaction rate but also the selectivity, promoting almost simultaneously both reaction routes, probably because the hierarchical porous structure could promote the formation of Mo(W)S2 edge active sites for direct desulfurization (Type I and II) as well as Brim bulk sites necessary for the hydrogenation route.The XPS analysis of the CoMoW-HOPSCM and CoMoW-MonoSBA-15 are presented in oxide and sulfide states to compare the surface of the highest-lowest activity catalysts. In Fig.\u00a016\n is possible to observe the Co 2p (Fig.\u00a016A), Mo 3d (Fig.\u00a016B), W 4f (Fig.\u00a016C), and S 2p (Fig.\u00a016D) core emission-line regions for the mentioned catalysts. Spectra related to the oxide state of CoMoW-HOPSCM and CoMoW-MonoSBA-15 samples were labeled with a and b in the plots of Fig.\u00a016, while sulfided catalysts with c and d. The electrons arising from the Co 2p3/2 spin-orbit in the oxide state spectra resulted at 782.15 eV. The sulfided spectra presented a shift in the electrons from the Co 2p3/2 to lower BE (778.5\u00a0eV), indicating the change in the Co environment. A similar observation was registered for the Mo 3d core level as the oxide spectra presented the main peak of the characteristic doublet Mo 3d5/2 at 233.3\u00a0eV (\u0394\u00a0=\u00a03.13\u00a0eV) for both oxide samples. In the sulfided samples, this peak was observed at 228.3 eVIn the same way, the oxide samples presented the peak of W 4f7/2 at 36.4\u00a0eV and the sulfided samples at 32.1 eV. In the case of the S 2p sulfided samples, they presented only one peak centered at 161.7 eV. No shoulder related to the presence of sulfates species at 168\u00a0eV were observed, indicating that the experimental procedure for the sulfidation and sample transfer to the spectrometer chamber was efficient in avoiding air contact. The general surface quantification for the samples in both states is presented in Table\u00a06\n. The atomic% for Co, Mo, and W resulted in 0.35, 1.45, and 1.38 in the sulfided CoMoW-HOPSCM and 0.37, 1.53, and 1.45 in the sulfided CoMoW-MonoSBA-15. Both samples presented a global promotional ratio Co/(Co+Mo+W) equal to 0.11; this value is closer to the intended nominal ratio. The average at.% values for the O and Si atoms at the surface resulted in 57% and 37.5% for both sulfided CoMoW-HOPSCM and CoMoW-MonoSBA-15 samples. Finally, the sulfur at.% resulted in 2.91 for the CoMoW-HOPSCM and 1.67 at.% for the CoMoW-MonoSBA-15 catalysts. This difference in the sulfur content could be related to better transforming the surface oxide species into sulfide species in the CoMoW-HOPSCM catalyst. Usually, a better sulfidation degree impacts beneficially, confirming the trend observed in activity presented in Section\u00a03.5. Additionally, a careful deconvolution process was performed in the Co 2p3/2, Mo 3d, and W 4f core emission regions (see Fig.\u00a017\n) to get light on the species on the surface of the catalyst.As seen in Fig.\u00a017A in the sulfided samples, Gaussian-Lorentizian curves related to Co oxide, CoMoS, and Co9S8 species were used to fit the spectra with BE at 781.2\u00a0eV, 778.8\u00a0eV, and 777.5\u00a0eV, respectively [82,83]. For the Mo region presented in Fig.\u00a017B, the MoO3, MoOxSy, and MoS2 species at 230.7, 229.4, and 228.4\u00a0eV were used to form a perfect envelope [82,83]. In this region, the peaks of S2\u2212 and S2\n2\u2212 were also used to perform the fit at 225.8 and 227.8 eV In the case of the W 4f region observed in Fig.\u00a017C, the WO3, WOxSy, and WS2 species with BE at 36.1, 32.6, and 31.7\u00a0eV were used to deconvolute the spectra as reported in [84]. The results of the relative proportions of species in the mentioned core levels are presented in Table\u00a07\n.As seen in Table\u00a07, the CoOx species represent 25% in the CoMoW-HOPSCM; meanwhile, in the CoMoW-MonoSBA-15, the contribution resulted in 37.4%. For the CoMoS species related directly with the promotion and the catalytic activity, the% resulted in 64.2 in the CoMoW-HOPSCM and 53.4% for the CoMoW-MonoSBA-15. The third species related to the segregate cobalt sulfide (Co9S8) was around 10% for both sulfided catalysts. In the region of Mo 3d, the MoO3 resulted in 18% lower in the CoMoW-HOPSCM than in the CoMoW-MonoSBA-15. Inversely the MoOxSy species displayed a value 39% higher in the CoMoW-HOPSCM than in the CoMoW-MonoSBA-15. In the meantime, the active phase MoS2 resulted in approximately the same (4%\u00b1) in both catalysts.Likewise, the W species presented only minor differences between catalysts not larger than 10%. From the chart, it is possible to observe that the tungsten and molybdenum species containing sulfur are approximately the same in both regions and catalysts, i.e., the sum of MoOxSy and MoS2 is 90.3% in the CoMoW-HOPSCM and 88.4% in the CoMoW-MonoSBA-15. Meanwhile, the sum of WOxSy and WS2 species is 75.2% and 74.3% for CoMoW-HOPSCM and CoMoW-MonoSBA-15, respectively. Nevertheless, Mo oxide species are better sulfided than their W counterpart under the same sulfidation conditions. It has to be considered that MoOxSy and WOxSy species undergo higher sulfidation under reaction conditions, helping to continuously transform these phases into more active MoS2 and WS2 phases. This effect could explain the activity trend observed for the catalysts tested.Hierarchical structured porous silicas have shown advantages when used as support for trimetallic hydrodesulfurization catalysts, based on CoMoW sulfide system, compared with only-mesoporous materials. Improvement in catalytic performance can be attributed to the synergetic effect caused by the presence of pores in different lengths of scale, and this fact has a positive effect on the diffusion processes of the oxide-state precursors for the sulfided active phases and over the mass transfer limitations of reactants, which result in a higher catalytic activity.The presence of pores of different dimensions also improves the sulfidation processes of the oxide-state precursors; hence a greater quantity of sulfided phases are obtained in the catalysts supported on silicas with hierarchical porosity. In addition, hierarchical porous structure promotes the formation of terminal metal-oxygen bonds, which enhances the formation of Mo(W)S2 edge bonds that are more active in the hydrodesulfurization reactions. The diversity of porous sizes in the support structure also favors the appearance of cobalt species in octahedral coordination, which is known to be of great importance in the catalysis of hydrodesulfurization reactions, therefore an increase in the number of octahedral cobalt results in an improvement of the promotion effect, which is reflected in greater catalytic activity.The synergistic effect obtained by the presence of pores of different scales promotes the formation of more active sites for the two parallel reactions that occur in the hydrodesulfurization of dibenzothiophene: type I and II edge bonds related to direct desulfurization and Brim hydrogenation sites. This results in more significant catalytic activity and selectivity for hydrodesulfurization of the dibenzothiophene reaction.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank CONACYT for the financial support of this project. Dr. Antonio G\u00f3mez Cort\u00e9s (IF-UNAM) by the assistance in nitrogen physisorption analysis, Dr. Dami\u00e1n Comp\u00e9an (IPICYT) by the help with Micro-Raman Spectroscopy characterization, David Dominguez for the XPS acquisition and M. Ing. Q. Alicia del Real L\u00f3pez for her assistance in the HR-SEM and EDS studies. J.N. D\u00edaz de Le\u00f3n wants to recognize the financial support of DGAPA-PAPIIT project IN104122. Dr. R. Huirache-Acu\u00f1a thanks to CIC UMSNH 2022 and ICTI PICIR 2022\u201323 project support. The authors acknowledge the support and facilities of the Laboratorio Nacional de Caracterizaci\u00f3n de Materiales (LaNCaM).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2023.100454.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  In this work, monolithic silica materials with hierarchical porosity were synthesized by the sol-gel method combined with a dual hard-soft templating route. Silica materials were used for the synthesis of hydrodesulfurization CoMoW-S catalysts by the immersion technique using transition metal salts as precursors, followed by oxidation and sulfidation in H2S/H2 mixture. Styrene-HEMA copolymer hard template presented homogeneous well-defined spherical shape with an average diameter of about 800\u00a0nm. Samples prepared over the hard template presented similar morphology. The surface areas of all supports prepared resulted in around 800\u00a0m2.g\u22121 and decreased to 220\u00a0m2.g\u22121 on average after the sulfidation process. Small-angle X-ray diffraction confirmed the presence of the 2D hexagonal or Im3m array of mesopores in all samples. The CoMoW oxide state catalysts presented low intensity peaks assigned to the b-CoMo(W)O4 phase and minor peaks related to MoO3 and polyoxides with the general formula MoxW1-xO3. The high conversion was obtained for catalysts supported on hierarchically structured porous silicas, even greater than that of the commercial catalyst used as reference (>30\u201350%). XPS results revealed that the degree of sulfidation and CoMoWS active species resulted higher in the CoMoW-HOPSCM catalyst compared to the CoMoW-MoNoSBA-15 sample, which in turn coincides with the catalytic activity results.\n               "}
{"full_text": "Currently, the industry of ethylene derivatives presents two opposing trends. On the negative side, the storage and disposal of plastics (among which those derived from ethylene are highlighted) is environmentally complicated. On the other hand, the future limitation of oil derivatives and natural gas for their use in the automotive industry and as fuels suggests a great availability of oil and natural gas for conversion into light olefins in the mid-term. Unlike other chemicals, ethylene production has continuously increased even in the economic recession of 2008. In fact, before the corona outbreak, the expected ethylene consumption worldwide for 2021 was higher than 190 million metric tons [1,2], which is ca. 25% more than the consumption in 2011.Currently, most ethylene is produced via steam cracking of ethane, LPG and naphtha. This process presents several problems [2\u20135] among which the high energy consumption stands out. To save these drawbacks related to the energy consumption, the oxidative dehydrogenation (ODH) of ethane is a clean alternative worthy to be studied. Moreover, the ODH of ethane requires fewer separation units than steam cracking, uses catalysts which hardly deactivate (due to the in-situ regeneration by the oxygen consumed), and shows negligible coke formation (which reduces the number of by-products) [1,5,6].The two most promising catalytic systems for the ODH of ethane are multicomponent Mo-V-Te-Nb-O catalysts [7\u20139] and promoted or supported NiO catalysts [10\u201316].Bulk NiO is a p-type semiconductor [17\u201319] which, when working as a catalyst in the presence of ethane or olefins and oxygen, tends predominantly to the formation of carbon oxides [18,19]. However, by incorporating an appropriate promoter to the NiO (such as Nb5+ [11,20\u201322]) or supporting it with a suitable material such as Al2O3 [10,23\u201326], the ethylene formation can be drastically enhanced. The nature of the support and/or the promoter must be carefully selected, otherwise the amount of ethylene formed can be even lower than that using unpromoted NiO, as this is the case of potassium as promoter [21]. Then, it has been widely reported that the excess of the electrophilic oxygens in undoped NiO can be decreased by using different cations with high valences (such as W6+, Nb5+, Sn4+, Zr4+, Ti4+) and this positively affects the formation of ethylene [17\u201321,27\u201332]. Promoters with acidic characteristics [17\u201321,27\u201332] or supports like TiO2 [23], Al2O3 [10,34,35] or siliceous porous clay heterostructure [36] favour the ethylene formation. However, the presence of strong Lewis acid sites provokes the decomposition of ethylene resulting in a low selectivity to olefin [1,6,34,37,38]. Therefore, an excess of promoter usually leads to a decrease in the olefin formation. In this way, it has been demonstrated that in promoted NiO catalysts a loss in the conductivity takes place in the catalysts with an enhanced ethylene formation [18,19,39], which could be related to a decrease in the non-stoichiometric oxygen.On the other hand, the crystallite size of NiO is also a factor that has been shown as crucial in promoted NiO catalysts, those with lower size presenting the highest ethylene formation [11,28,30\u201332]. This is especially notable in the case of supported NiO catalysts, in which the desired Ni-support interaction involves a decrease in the mean NiO crystallite size [34\u201336], whereas low Ni-support interaction leads to large NiO crystallites [40,41].In most of these studies dealing with promoted NiO catalysts, the catalytic results have been explained in terms of the characteristics of NiO and the way the cation employed influences the physicochemical properties of the nickel oxide (NiO crystallite size, amount of non-stoichiometric oxygen, morphology, lattice parameters, conductivity\u2026). However, it seems that most times the possible direct role of the promoter has been underestimated. Then, in order to properly assign the catalytic properties to the nickel species it would be desirable to study pure NiO catalysts instead of supported or promoted NiO catalysts. For example, in Nb-promoted NiO catalysts the formation of a Ni\u2013Nb-O solid solution was identified as the selective sites in the ethane ODH, and maybe the amount of non-stoichiometric oxygen is not so tightly related to the selectivity to ethylene [19]. However, this behaviour can change if promoters, especially Nb5+, are absent (if niobium is not present in the catalyst formulation).Recently, Zhao et al. [41] studied undoped NiO showing that it is possible to tune the concentration of non-stoichiometric oxygen with low changes in morphology. In that work, using NiO samples calcined at different temperatures the amount of non-stoichiometric oxygen could be modified and, unlike that proposed in promoted NiO catalysts, it was observed that the selectivity to ethylene increased as the amount of that excess of oxygen increased. On the other hand, the particle size has also been proposed as an important parameter in the performance in the ODH of ethane. In this way, mesostructured NiO catalysts seem to present higher selectivity to ethylene (at isoconversion conditions) than the corresponding nanostructured NiO [42].At the present work, we have synthesized, characterized and tested in the oxidative dehydrogenation of ethane a set of pure NiO catalysts modifying both the concentration of oxalic acid in the synthesis gel and the final calcination temperature. Moreover, we have studied the influence of different parameters such as the nature of Ni-species, the amount of non-stoichiometric oxygen, the NiO crystallite size or the charge carriers\u00b4 density of the catalysts on the catalytic performance.Nickel oxides catalysts were prepared through the evaporation at 90\u00a0\u00b0C of aqueous solutions of nickel nitrate (> 99.99% Sigma-Aldrich) with different amount of oxalic acid (> 99.5% Acros Organics) in the synthesis gel. The solids were dried overnight in a furnace at 120\u00a0\u00b0C and, finally, they were calcined in static air for 2\u00a0h at 350 or 500\u00a0\u00b0C with a heating ramp of 5\u00a0\u00b0C/min from room temperature to the desired final calcination temperature. The catalysts will be named as Ni-x-Y, where x is the oxalic acid/nickel ratio in the synthesis gel (i.e. Oxalic acid/Ni molar ratio of 0, 0.8, 1.0, 1.5 or 3), and Y is the calcination temperature (350 or 500\u00a0\u00b0C). Nomenclature and physicochemical properties of these catalysts are shown in Table 1\n.X-ray diffraction patterns were collected in an Enraf Nonius FR590 diffractometer with a monochromatic CuK\u03b11 source operated at 40\u00a0keV and 30\u00a0mA. The size of the NiO domains has been calculated through XRD technique by the Scherrer\u2019 equation, D\u00a0=\u00a0k\u00b7\u03bb/w\u00b7cos\u03b8, where D is the crystallite size, k is a shape factor (0.9), \u03bb is the X-ray wavelength (0.15406\u00a0nm), w is the full width at half maximum intensity (FWHM, in radians) and \u03f4 is the Bragg angle [43].The surface area of catalysts were determined by multi-point N2 adsorption at \u2212196\u00a0\u00b0C. Estimations of surface areas were made in accordance with the BET method.Raman spectra were obtained in an inVia Renishaw spectrometer, equipped with an Olympus microscope, using a wavelength of 514\u00a0nm (visible Raman) or 325\u00a0nm (UV-Raman), generated with a Renishaw HPNIR laser with a power of approximately 15\u00a0mW.UV\u2013vis diffuse reflectance spectroscopy (DRS) measurements of the solids were carried out within the 200\u2013800\u00a0nm range using a Varian spectrometer model Cary 5000.Temperature-programmed reduction experiments (H2-TPR) were carried out in an Autochem 2910 (Micromeritics) equipped with a TCD detector, using 0.10\u00a0g of catalyst and a reducing gas of 10% H2 in Ar (total flow rate of 50\u00a0mL\u00a0min\u22121). The samples were heated from room temperature to 800\u00a0\u00b0C, with a heating rate of 10\u00a0\u00b0C/min.X-ray photoelectron spectroscopy (XPS) measurements were performed on a SPECS spectrometer equipped with a Phoibos 150 MCD-9 detector using a monochromatic Al K\u03b1 (1486.6\u00a0eV) X-ray source. Spectra were recorded using an analyzer pass energy of 50\u00a0eV, an X-ray power of 100\u00a0W, and an operating pressure of 10\u22129\u00a0mbar. Spectra treatment was performed using CASA software. Binding energies (BE) were referenced to C 1\ns at 284.5\u00a0eV.TEM (transmission electron microscopy), HRTEM (high resolution transmission electron microscopy) and SAED (Selected area electron diffraction) were conducted using a FEI Field Emission gun Tecnai G2 F20 S-TWIN microscope working at 200\u00a0kV. Structural and morphological characterizations were obtained from the TEM and HRTEM images. The lattice parameters were determined via Fourier transformation from HRTEM images. The preparation of the samples for the microscopy analyses involves the sonication in ethanol of the sample and a further deposition of the solution over a holey\u2011carbon film supported on a Cu grid where it is finally dried.Electrochemical Impedance Spectroscopy measurements (EIS) were performed at a frequency of 5000\u00a0Hz in a 0.1\u00a0M Na2SO4 electrolyte. The potential was scanned from 1 VAg/AgCl in the negative direction using an amplitude signal of 0.01\u00a0V at 0.05\u00a0V/s. For this purpose, a three-electrode electrochemical cell was used. The catalyst was the working electrode (deposited on fluorine tin oxide (FTO)), a platinum tip the counter electrode and an Ag/AgCl (3\u00a0M KCl) the reference electrode. An area of 0.5\u00a0cm2 of the catalysts was exposed to the electrolyte. From EIS results, Mott-Schottky plots were carried out to evaluate the semiconductor behaviour of the catalysts and to determine their density of dopants.The catalytic tests in the oxidative dehydrogenation of ethane were carried out at atmospheric pressure in a tubular isothermal flow quartz reactor (15\u00a0mm inner diameter) at 300\u2013340\u00a0\u00b0C, with a mixture consisting of C2H6/O2/He with a molar ratio of 3/1/29. Ethane (99.95% purity, Carburos Met\u00e1licos) and O2 (99.995% purity, Carburos met\u00e1licos) were used in these experiments. Typical reaction conditions used were 0.1-0.5\u00a0g of catalyst and 100\u00a0mL\u00a0min\u22121, although both the total flow and/or the catalyst amounts were changed in order to achieve different ethane conversions. Catalysts were introduced in the reactor diluted with silicon carbide in order to keep a constant volume in the catalytic bed. Reactants and products were analyzed by gas chromatography using two packed columns: (i) molecular sieve 5\u00a0\u00c5 (2.5\u00a0m); and (ii) Porapak Q (3\u00a0m). Blank runs were undertaken without catalyst until 450\u00a0\u00b0C and no conversion was observed in all cases [28].NiO catalysts heat-treated in air at different temperatures (350 or 500\u00a0\u00b0C) with or without the addition of oxalic acid in the synthesis gel have been prepared, tested in the oxidative dehydrogenation of ethane and characterized through several physicochemical techniques. Selected physicochemical characteristics of the prepared samples are included in Table 1.Representative catalytic results of the differently synthesized NiO catalysts in the oxidative dehydrogenation of ethane are shown in Table 2\n. The reaction temperature used in this study has been fixed at 300 and 340\u00a0\u00b0C in order to have a better comparison of the catalytic performance of the catalysts, minimizing the effect of the reaction temperature in the selectivity to ethylene vs ethane conversion plot. Moreover, by not exceeding 350\u00a0\u00b0C in the reaction temperature, the modification of the catalysts heat-treated at the lowest calcination temperature is minimized.In all cases, the only reaction products detected have been ethylene and CO2. No carbon monoxide, acetic acid nor another O-containing product have been identified. An accurate carbon balance in the 98\u2013102% range has been obtained in all the experiments.Overall, the most active catalysts were those calcined at 350\u00a0\u00b0C with a catalytic activity ca. 3\u20134 times higher than that of the catalysts calcined at 500\u00a0\u00b0C. Moreover, a clear positive effect of the addition of oxalic acid has been observed regardless of the calcination temperature. Then, the addition of oxalic acid increases the reaction rate for ethane conversion by a factor of 2 (Fig. 1a). This increase is observed regardless of the amount of oxalic acid employed, so that the key factor of the catalytic activity is the presence or the absence of oxalic acid rather than its amount. As it can be seen in Table 2, the specific activity (activity normalized per surface area) is very similar for all catalysts regardless of the presence of oxalic acid or the calcination temperature used. Therefore, the surface area (i.e. the available active sites) seems to be the governing factor determining the catalytic activity.Whenever there is not a drastic difference in reactivity, the key factor to be optimized in the catalytic performance for the oxidative dehydrogenation of ethane is the selectivity to ethylene. Then, Fig. 1b shows the values of the selectivity to ethylene at isoconversion conditions (10%). As it can be seen, the presence of oxalic acid in the synthesis gel leads to a remarkable increase in the selectivity to ethylene regardless of the calcination temperature. Then, the catalysts prepared in absence of oxalic acid present a selectivity of ca. 45\u201348% whereas that achieved on catalysts prepared in the presence of oxalic acid ranges between 63 and 75%. According to these results, the optimal catalysts were the ones prepared with an oxalic acid/nickel molar ratio of 1.0. Additionally, for a fixed oxalic acid/nickel molar ratio, the lower the calcination temperature the higher the selectivity to ethylene is.Since the reactivity of the catalysts is different, we employed different contact times in order to obtain a comparative selectivity to ethylene vs. ethane conversion curve. Fig. 2\n shows the variation of the selectivity to ethylene with the ethane conversion for selected catalysts. Noteworthy, in the range of conversions studied (until 20%) the fall in the selectivity to ethylene is not drastic. Therefore, it can be seen that the Ni-1-350 catalyst presents the highest selectivity to ethylene, whereas Ni-0-500 is the least selective.Although there are many factors that define the catalytic performance of NiO based catalysts, the crystallite size has shown to have certain effect on both the catalytic activity and the selectivity [11,21,27\u201332,41]. Fig. 3\n shows the XRD patterns of nickel oxide catalysts calcined at 350 (Fig. 3a) or 500\u00a0\u00b0C (Fig. 3b). In all cases, the main peaks appear at 2\u03b8: ~37.2, 43.2, 62.8, 75.3 and 79.3\u00ba which correspond to the (111), (200), (220), (311) and (222) planes of cubic NiO crystalline phase (JCPDS: 78\u20130643), respectively. The presence of partly (Ni2O3) or totally reduced (Ni0) phases have not been detected. As it can be observed, all the samples present five intense peaks, regardless of the addition of oxalic acid and the calcination temperature, which have been considered to apply the Scherrer\u2019 Eq. [43].However, interesting differences in terms of crystallinity are observed (Fig. S1, supporting information). Samples heat treated at 500\u00a0\u00b0C present narrow Bragg peaks which means that relatively large crystallites have been formed. Then, according to the Scherrer equation, a mean NiO crystallite size that varies from ca. 22\u00a0nm in the catalyst prepared without oxalic acid in the synthesis gel to ca. 17\u00a0nm in the catalysts prepared with oxalic acid in the synthesis gel has been determined (Table 1 and Fig. S1). As expected, the calcination at 350\u00a0\u00b0C leads to a widening of the Bragg peaks, which is related to material with lower crystallinity with tinier NiO crystallites. Then, average NiO crystallite size of ca. 12\u00a0nm in the catalyst prepared without oxalic acid decreased to 7\u20138\u00a0nm for those catalysts with moderated use of oxalic acid (oxalic acid/Ni ratio of 0.8 to 1.5). Finally, the use of higher oxalic acid loadings resulted in a further decrease of the average size since Ni-3-350 presents an average size for NiO crystallites of ca. 5\u00a0nm. The determined crystallite sizes of the catalysts follow the trend expected considering their surface areas.We must inform that the as-synthesized samples (before calcination) have been also characterized by XRD and Thermogravimetric analysis (Figs. S2 and S3, respectively, in the Supporting Information), in order to understand better the changes achieved in calcined samples, depending on the amount of oxalic acid in the synthesis gel. Thus, the XRD pattern of the as-synthesized samples indicates de presence of Nickel(II)\u00a0nitrate\u00a0hexahydrate (Ni(NO3)2\u00a0\u00b7 6 H2O, in the sample prepared without oxalic acid in the synthesis gel), whereas Nickel(II) oxalate dihydrate (NiC2O4\u00b72 H2O) is mainly observed in samples prepared with oxalic acid in the synthesis gel (Fig. S2; Supporting information). This is also confirmed by Thermogravimetric analysis (Fig. S3; Supporting Information), in which the decomposition of Ni-nitrate (broad peak, not very intense) and Ni-oxalate (narrow peak, very intense) occurs at ca. 350\u00a0\u00b0C in TG.The samples have been characterized by Visible Raman (using an excitation wavelength of 514\u00a0nm, Fig. S4) and UV Raman (using an excitation wavelength of 325\u00a0nm, Fig. 4\n), in order to differentiate the crystallinity and crystal sizes of catalysts.Raman spectra of catalysts, using a 514\u00a0nm laser (Fig. S4), present main bands at 462 and 497\u00a0cm\u22121, in addition to small broad bands at 710, 930 and 1081\u00a0cm\u22121. Bands at 462 and 497\u00a0cm\u22121 have been assigned to the NiO stretching mode related to a rhombohedral deformation of the structure or to non-stoichiometric NiO [19,20,44]. The other bands have been related to overtones of the first ones or their combinations [20]. A small band at 1060\u00a0cm\u22121 is also observed in some cases, which can be related to the \u03bd1 vibration mode of carbonate groups [20,45,46]. These results agree with previous ones on samples calcined at temperatures higher than 400\u00a0\u00b0C [19,20,44\u201346].In addition to these, it can be also seen a shoulder at 410\u00a0cm\u22121 (especially in samples prepared with an oxalate/Ni ratio of 3.0), which could be related to the non-stoichiometry of catalysts and/or a higher nickel vacancy concentration [19,20]. On the other hand, the appearance of 2-magnon band (at ca. 1402\u00a0cm\u22121) in samples calcined at 500\u00a0\u00b0C (Fig. 4a), which is absent in samples calcined at 350\u00a0\u00b0C, can be attributed to the transition from ferromagnetic to antiferromagnetic characteristics, in NiO materials calcined at temperatures higher than 400\u00a0\u00b0C [47,48].\nFig. 4 shows the UV Raman spectra of nickel oxide catalysts calcined at 350 or 500\u00a0\u00b0C, which can help to follow the spin-phonon interaction in these catalysts [48\u201350]. In general, it can be distinguished the presence of two main bands at ca. 571 and 1128\u00a0cm\u22121 (1145\u00a0cm\u22121 in samples calcined at 500\u00a0\u00b0C), which can be related to the one-phonon (1P) longitudinal optical (LO) mode and two-phonon (2P) longitudinal optical (2LO) mode, respectively, in NiO crystals [48,49]. In addition, two small bands are also observed at ca. 724 and 901\u00a0cm\u22121 related to two-phonon (2TO-transverse optical) and (TO) modes.As it can be seen, the intensities of bands at ca. 571 and 1128\u00a0cm\u22121 change depending on the catalyst preparation procedure and/or calcination temperature. In this way, it has been proposed that the intensity of these most characteristic bands (1P LO band at ~571\u00a0cm\u22121 and 2P 2LO band at ~1120\u00a0cm\u22121) could be related to the specific structural and chemical features of the catalyst. Thus, an increase of the intensity of 2P 2LO band (I1120) higher than that of 1P LO band (I571) is observed for catalysts calcined at 500\u00a0\u00b0C and for samples prepared without oxalic acid in the synthesis gel, suggesting that they present the higher NiO crystal size and/or the lower concentration of defects [51]. In an opposite trend, samples prepared in the presence of oxalic acid in the synthesis gel and calcined at 350\u00a0\u00b0C present the highest values of I571 and the lowest I1120/I571 ratios. Accordingly, low I1120/I571 ratios in the UV Raman spectra should correspond to samples presenting NiO particles with the lowest crystal size and/or the highest concentration of defects [50], being these aspects mainly related to the catalyst preparation (presence of oxalic acid in the synthesis gel) and/or lower calcination temperature. In the present work, a clear correlation between the relative intensity of the bands at 571 and 1120\u00a0cm\u22121 (ILO/I2LO) and the NiO crystallite size determined by XRD has been observed (Fig. S5). Therefore, an estimation of the concentration of over-stoichiometric oxygen in these catalysts is not straightforward to reach according to UV Raman spectroscopy.The catalysts were also characterized by Diffuse Reflectance UV\u2013vis spectroscopy (DRS). The DRS spectra of catalysts calcined at 350 or 500\u00a0\u00b0C are shown in Fig. 5\n. No distinguishable differences were observed in the UV area of the spectra, but some features can be observed in the visible area.Several absorption bands have been observed (at 380, 416, 450, 650 and 722\u00a0nm) which are typical of bulk NiO [10,35,52,53] with Ni2+ in its octahedral coordination. In fact, it is well known that bulk NiO shows bands at 715, 420 and 377\u00a0nm, which can be related to the presence of octahedrally coordinated Ni2+ species in the cubic (rock-salt) NiO lattice [10,51,52]. In addition, a band at ca. 510\u00a0nm could be also related to charge transfer in NiO crystals [53,54]. On the other hand, and although there is not a clear trend in these spectra, it can be observed a higher intensity of the absorption in the 400\u2013600\u00a0nm range for the catalysts calcined at 350\u00a0\u00b0C (Fig. 5A) than those at 500\u00a0\u00b0C (Fig. 5B). This is especially notorious in the sample prepared in the absence of oxalic acid in the synthesis gel (i.e. Ni-0-350). A high background absorbance for this area has been linked with a high concentration of non-stoichiometric oxygen. Therefore, it seems that those samples calcined at 350\u00a0\u00b0C, and especially Ni-0-350, present the highest amount of non-stoichiometric oxygen.A detailed study of these catalysts was also undertaken through TEM and High Resolution TEM (Fig. 6\n). Catalysts calcined at 350\u00a0\u00b0C show drastic differences when adding oxalic acid in the synthesis. The basic Ni-0-350 catalyst presents large agglomerations of several micrometres consisting of highly porous poorly defined particles of variable size, ranging from large (50\u00a0nm) to small (5\u00a0nm) particles. The Ni-1-350 sample presents well defined particles with sizes between 5 and 50\u00a0nm. Interestingly, small cavities in the range of 1\u20132\u00a0nm are located within these particles. In the inset of Fig. 6b, it is observed by HR-TEM some ca. 10\u201315\u00a0nm particles with tiny cavities. Finally, the sample prepared with the highest concentration of oxalic acid (sample Ni-3-350) is formed by tiny particles with a rather uniform size (ca. 4\u00a0nm) and aspect. In the inset of Fig. 6c, small 3 to 6\u00a0nm particles are clearly observed in close contact with each other.\nFig. 6 shows also selected TEM/HRTEM images of NiO catalysts calcined at 500\u00a0\u00b0C. Overall, the effect of the oxalic acid is similar to that observed for catalysts calcined at 350\u00a0\u00b0C but at 500\u00a0\u00b0C the porosity and the presence of cavities is much lower. This fits with the decreased surface area observed in the latter. Ni-0-500 catalyst has large agglomerations of rather big particles without apparent porosity and a few small nanoparticles of 10\u00a0nm and less (Fig. 6d). Ni-1-500 presents well defined particles most of them between 10 and 30\u00a0nm of diameter without apparent internal cavities (Fig. 6e). Finally, Ni-3-500 shows well defined particles and a similar morphology but with slightly smaller particles than that sample with lower oxalic acid content (Fig. 6f).SAED patterns (Fig. S6) demonstrate that all NiO catalysts are formed by crystalline Ni-containing nanoparticles, which can be indexed unambiguously to cubic NiO. No metallic Ni nor Ni2O3 were identified. A further analysis indicates that the lattice parameter ranges from 0.419\u00a0nm to 0.416\u00a0nm (see Table 1) and a clear effect of the preparation procedure on this value has not been observed. However, by taking separately both calcination temperatures, a decrease of the lattice parameter has been observed in the samples treated with oxalic acid. In this way, the decreased particle size could partially explain the decrease lattice parameter observed.The reducibility of the NiO catalysts has been also studied by temperature programmed reduction (Fig. 7\n) as it can influence the catalytic performance of this type of catalysts [11,19\u201321,27\u201333,54\u201357]. Thus, the oxidative dehydrogenation of ethane takes place by a redox mechanism [57], being the reduction part considered as the limiting step at moderate and high temperatures. Then, the catalytic activity in this reaction could be related to the reducibility of the sample. Moreover, it has been proposed for many NiO based catalysts [11,19\u201321,27\u201333,54\u201357] that the most selective sites are the ones that present the lowest reducibility.Overall, the reduction profile of these catalysts is rather similar with a broad reduction peak centred between 254 and 351\u00a0\u00b0C. This broad peak, which can contain a shoulder, has been related to the reduction of bulk NiO (lattice oxygen) [55\u2013 57]. Interestingly, the onset temperature for this band takes place at similar temperatures regardless of the catalyst, but the maximum shifts towards lower temperatures when the amount of oxalic acid used increases. It is noteworthy the presence of a low intensity reduction band at ca. 200\u00a0\u00b0C.This band at 200\u00a0\u00b0C has been reported to be related to the reduction of over-stoichiometric oxygen, i.e. Ni3+ species or even Ni2O3 [29,56]. An enlargement of the area around 200\u00a0\u00b0C (see Fig. 7) shows that the low temperature band is especially intense in the samples calcined at 350\u00a0\u00b0C and, above all, in the reference catalyst (sample Ni-0-350). However, the use of oxalic acid in the synthesis gel leads to a decrease in its height. This reduction peak, with low intensity, can be also observed in the reference sample calcined at 500\u00a0\u00b0C (sample Ni-0-500) but not in the other catalysts.The hydrogen consumption observed during the TPR experiments slightly exceeds the hydrogen necessary to completely reduce Ni2+O into metallic Ni (102\u2013105% of the theoretical NiO\u00a0+\u00a0H2 \u2794 Ni\u00a0+\u00a0H2O reaction). However, a defined trend regarding calcination temperature or presence of oxalic acid has not been detected. Nevertheless, this higher hydrogen consumption could indicate that these catalysts present an excess of oxygen over the stoichiometric NiO.The surface of the nickel oxide catalysts has been characterized by XPS of samples calcined at 350 or 500\u00a0\u00b0C. Fig. 8\n shows the Ni 2p\n\n3/2\n core level spectra for the catalysts calcined at 350\u00a0\u00b0C (Fig. 8a) and 500\u00a0\u00b0C (Fig. 8b), respectively. In all cases, the spectra show a wide band centered at ca. 854.5\u00a0eV. This band presents two maxima. The first one (usually referred as Main Peak) at binding energy ca. 853\u00a0eV is attributed to structural Ni2+ species. The second maximum corresponds to a satellite (usually referred as Sat I) that appears at 855\u00a0eV and it is related to the presence of multiple defects in the structure (i.e. Ni2+ vacancies, Ni3+ and/or Ni2+-OH species) but also to nickel atoms not coming from lattice oxygen\u2011nickel bound but from octahedral NiO6 neighbour cluster units [57,58]. A second wide satellite (Sat II) with its maximum at 860.2\u00a0eV is associated to ligand-metal charge transfer [28,36,57,58].The relative intensity of the Sat I compared to the Main peak (Sat I/Main peak ratio) has been determined by the deconvolution of the wide band into two bands (centered at 853 and 855\u00a0eV). This ratio has been roughly related to the presence of nickel defects [27]. As it can be seen, and in accordance with the XRD results, lower calcination temperatures lead to worst crystallization of the NiO regardless of the oxalic acid amount, and therefore, higher presence of nickel defects, as it can be stated for the higher Sat I / Main Peak relationship of their relative intensity values (see Table 3\n). However, the Sat I / Main Peak relationship for the catalysts treated at 500\u00a0\u00b0C leads to fairly lower values, suggesting that a severe thermal treatment favors better crystallization of the catalysts. Regrettably, due to the uncertainty of the assignment of Ni bands accurate conclusions cannot be drawn.On the other hand, Fig. 8 shows also the O 1\ns core level XPS spectra of the NiO catalysts calcined at 350\u00a0\u00b0C (Fig. 8c) and 500\u00a0\u00b0C (Fig. 8d). In our case, it can be differentiated three types of signals [41,59,60]: the first one (OI) at binding energy of ca. 528.6\u00a0eV is the majority and it is related to structural nucleophilic lattice oxygen species; the second one (OII) appears at 530.5\u00a0eV, and it is attributed to OH\u2212 species; and finally, the third signal (OIII) at 532.5\u00a0eV is ascribed to electrophilic (O2\n\u2212 and/or O-) species on the surface of the catalysts. Table 3 shows the relative proportion of these oxygen species.According to these results, those catalysts calcined at 350\u00a0\u00b0C present a similar concentration of nucleophilic species (51\u201360%), those catalysts with an oxalic acid/Ni ratio of 1 presenting the highest concentration. In addition, it is clear that the signal corresponding to OH\u2212/defects (OII) is more intense in the catalysts calcined at 350\u00a0\u00b0C to the detriment of the electrophilic oxygen (OIII) signal (Table 3), which are shown to be responsible for the deep oxidation of the paraffin [62].These results are in good agreement with all the characterization reported above and suggests that a lower presence of electrophilic species enhance the selectivity to the partial oxidation product.In order to get more information about the number of vacancies we have decided to evaluate the semiconductor behaviour of the catalysts through Mott-Schottky plots (see conditions in Experimental section). Mott-Schottky plots represent the reciprocal of the square capacitance vs the applied potential. In particular, the Mott-Schottky equation for a p-type semiconductor (such as NiO [63]) is the following:\n\n(1)\n\n\n1\n\nC\n2\n\n\n=\n\n1\n\nC\nH\n\n\n\u2212\n\n2\n\n\n\u03b5\nr\n\n\u00b7\n\n\u03b5\n0\n\n\u00b7\ne\n\u00b7\n\nN\nA\n\n\n\n\n\nE\n\u2212\n\nE\nFB\n\n\u2212\n\nkT\ne\n\n\n\n\n\nwhere C is the value of total interfacial capacitance calculated from EIS, CH is the capacitance of the Helmholtz layer, \u03b5r is the dielectric constant of the semiconductor used as a catalyst (~12 for nickel oxide) [64], \u03b50 is vacuum permittivity (8.85\u00b710\u221214\u00a0F\u00a0cm\u22121) and e is the electron charge (1.60\u00b710\u221219C), NA is the density of acceptors in the semiconductor, E is the applied potential, EEF is the flat-band potential, k is the Boltzmann constant (1.38\u00b710\u221223\u00a0J/K) and T is the absolute temperature. From the slope of the linear region of the Mott-Schottky plots, NA values for the different catalysts can be calculated.Mott-Schottky plots for the catalysts with and without oxalic acid in the synthesis gel are presented in Fig. 9\n. It is observed that, for all the catalysts, a linear region with a negative slope is displayed in the Mott-Schottky plots, which is characteristic of p-type semiconductors with an excess of cationic vacancies [64\u201367], the predominant defect type in NiO [41].\nFig. 9 also shows the value of the acceptor densities calculated for catalysts calcined at 350 or 500\u00a0\u00b0C, synthesized with or without oxalic acid in the synthesis gel. Independently of the oxalic acid content in the synthesis gel, NA values are in general higher for catalysts calcined at 350\u00a0\u00b0C but the presence of oxalic acid clearly diminished the number of NA with respect to the base NiO catalyst (i.e. Ni-0-350), regardless of the calcination temperature (Table 1). This confirms that catalysts prepared with oxalic acid in the synthesis gel present less cationic vacancies, which are related with the excess of non-stoichiometric oxygen, i.e. mainly electrophilic oxygens [11,41,57].These electrochemical results are in good agreement with the DRS and H2-TPR measurements, where the highest amount of non-stoichiometric oxygen species correspond to the catalysts synthesized without oxalic acid in the gel solution and calcined at 350\u00a0\u00b0C. Additionally, acceptor densities are consistent with XRD and XPS results, since catalysts obtained at low calcination temperatures presented the highest concentration of nickel defects.In the present article we have demonstrated that by controlling the calcination temperature and adding oxalic acid in the synthesis gel, a rather unselective material as unsupported and unpromoted NiO can turn into a selective catalyst for the oxidative dehydrogenation of ethane. A 73% selectivity to ethylene at 10% of ethane conversion as well as a highly stable behaviour has been reached with the optimal catalyst.Several factors have been shown to influence the catalytic performance (and more importantly, the selectivity to ethylene) of NiO-based catalysts. Tiny NiO crystallites seem to be desirable to obtain high selectivity to ethylene in promoted NiO catalysts [19\u201321,27\u201332] although in supported catalysts this trend is not clear [33\u201336]. In those cases, the nature of the support and the NiO-support interaction play an important role.\nFig. 10\n shows the influence of the average NiO crystallite size over the selectivity to ethylene. It can be observed that, among those samples calcined at a given temperature, the catalysts with the lowest size are the most selective ones. However, it is not a general trend.For example, the catalyst prepared in the absence of oxalic acid calcined at 350\u00a0\u00b0C presents a selectivity lower than 50% with a mean crystallite size of ca. 12\u00a0nm and those prepared with oxalic acid at 500\u00a0\u00b0C can reach 66% selectivity with a size of ca. 18\u00a0nm. Therefore, other factors are involved in the enhanced performance of the catalysts synthesized in the presence of oxalic acid.Recently, Zhao et al. [41] studied pure NiO catalysts heat-treated at different temperatures between 400 and 1000\u00a0\u00b0C and observed a direct correlation between the amount of non-stoichiometric oxygen and the selectivity to the olefin. This observation contrasts with that reported previously in different articles. Relationships between nucleophilic oxygen and the formation of partial oxidation and dehydrogenation products, as well as electrophilic oxygen and formation of total oxidation products, have been often proposed [11,21,61,68,69]. In the case of electrophilic oxidation, it proceeds through the activation of the molecular oxygen fed favouring the formation of cracking products and carbon oxides (total oxidation is highly favoured). Conversely, nucleophilic oxygen tends to attack CH bonds in a previously activated organic molecule, maintaining the same size of the hydrocarbon. In this sense, non-stoichiometric oxygen is more electrophilic than lattice oxygen, so that one could think that catalysts with high concentration of oxygen in excess tends preferentially to transform ethane into carbon oxides. In fact, several authors have found in promoted NiO catalysts that the decrease in the amount of non-stoichiometric oxygen leads to an enhanced selectivity to ethylene [19\u201321,27\u201331,70]. Similarly, it was demonstrated an inverse relationship between the selectivity to the olefin in promoted NiO catalysts with the p-conductivity [19,39,70]. Accordingly, NiO doped with high valence promoters (+4 and above) have been reported to be the most selective catalysts for the olefin formation, but they also provoke a decrease of the mean oxidation state of Ni, consequently decreasing the amount of non-stoichiometric oxygen [11,19\u201321,27\u201332]. In the same way, it has been shown for supported NiO catalysts that the selectivity to ethylene increases concomitantly when the number of Ni neighbours in the first (NiO) and the second coordination shell (NiNi) decreases [33]. The elimination of surface electrophilic species (which is somewhat related to the density of over stoichiometric oxygen) has been also linked to a higher ethylene formation in supported or promoted Nb- and Ti- doped NiO catalysts [41].Most of these studies have been focused in promoted or supported NiO since undoped bulk NiO usually presents a poor performance with a massive formation of carbon dioxide. Maybe, in those cases, the role of the promoter and/or the support has been underestimated. In the present article several pure NiO catalysts have been tested, being the catalysts calcined at 350\u00a0\u00b0C the ones that present the highest concentration of non-stoichiometric oxygen. As expected, the amount of non-stoichiometric oxygen decreased when the calcination temperature increases [71]. However, the most selective catalysts are those prepared with oxalic acid and, although the calcination temperature plays a role, it is upset by the effect of the use of oxalic acid. The presence of oxalic acid in the synthesis gel also leads to a decrease in the concentration of overstoichiometric oxygen species. This is due to the reductant behaviour of the oxalic acid, which favors the Ni3+ to Ni2+ transition. However, this elimination of non-stoichiometric oxygen when using oxalic acid takes place to a lesser extent than by increasing the calcination temperature. Therefore, no relationship has been observed between the amount of non-stoichiometric oxygen and the selectivity to ethylene.\nFig. 11\n shows the evolution of the ethane conversion and the selectivity to ethylene with the time on line for Ni-1-350 and Ni-1-500 catalysts together with the DR-UV\u2013Vis. spectra of these catalysts before and after use. It can be seen that, after 9\u00a0h of use the catalytic performance kept invariable, maintaining both the same conversion and selectivity.Interestingly, the UV\u2013Vis. spectra of the used catalysts changed with respect to the fresh catalysts since the absorbance in the 450\u2013600\u00a0nm range, which is qualitatively related to non-stoichiometric oxygen, is remarkably lower. Then, after a moderate use in an atmosphere with ethane and oxygen (9\u00a0h with an ethane/O2 ratio\u00a0=\u00a03\u00a0M), the catalysts seem to lose part of the over-stoichiometric oxygen. This is in agreement with the decrease in the oxygen excess observed in Nb-doped NiO catalysts after their use in the oxidative dehydrogenation of ethane [29,72]. In our case, the loss of oxygen in excess has not meant a drop of either catalytic activity or selectivity to ethylene.The apparent contradiction observed in different works about the need for the presence or absence of non-stoichiometric oxygen can be due to the fact that electrophilic oxygen species have been reported to be the ones that activate the ethane molecule [73]. Thus, these electrophilic species would be necessary for the activation of the ethane, but they should be isolated (and, therefore, be present in low concentration) to avoid the transformation of ethane into carbon dioxide. Accordingly, surface area should also play a role as higher surface areas would allow a higher dispersion of electrophilic species. Perhaps, due to the need for electrophilic species to activate the ethane molecule, NiO based catalysts have not been described in the literature to present consistent selectivity to ethylene above 95%. Hence, a general correlation between the concentration of electrophilic oxygen and the selectivity to ethylene is not direct and will depend on other factors such as the surface area, the presence of promoters or supports or even to the different morphologies and exposed planes.As mentioned above, it has been proposed in several articles that in supported or promoted NiO based catalysts [11,19\u201321,27\u201333,54\u201357] the most selective sites are those with the lowest reducibility. However, in the present work, the study of unpromoted bulk NiO does not show a clear correlation although the least selective catalysts (the ones without oxalic acid in the synthesis gel) are those that show the highest reduction maxima temperature, according to our H2-TPR assays.On the other hand, considering that the reduction band at 200\u00a0\u00b0C of the TPR experiments is related to non-stoichiometric oxygen (Ni3+-like species), the extent of isolation of electrophilic species could be roughly estimated by dividing that area by the specific surface area of each catalyst. Then, for a given calcination temperature the most selective catalysts are those in which non-stoichiometric oxygen concentration (normalized per surface area) is lower, i.e., non-stoichiometric oxygens are more isolated (Fig. 12a). In any case, samples calcined at 350\u00a0\u00b0C present higher selectivity than samples calcined at 500\u00a0\u00b0C.As mentioned in the Results section, cationic vacancies have been estimated through an electrochemical assay determining the NA value (acceptor density). This NA value turned out to be higher for catalysts calcined at 350\u00a0\u00b0C; although the presence of oxalic acid in the synthesis gel also leads to a decrease of the NA value. Thus, Fig. 12b shows the relationship between the selectivity to ethylene with the acceptor density values for the three set of catalysts, i.e. catalysts prepared with an oxalic acid/Ni molar ratio of 0, 1.0 or 3.0, and calcined at 350 or 500\u00a0\u00b0C. It can be clearly observed that the highest selectivity to ethylene is related to lower acceptor densities (in Fig. 12b). In particular, the catalysts synthesized with oxalic acid show lower NA values, that is, the lowest concentration of cationic vacancies. Hence, catalysts prepared with oxalic acid have less concentration of non-stochiometric oxygen which, in turn, could overoxidize ethane to non-desired carbon compounds, such as CO2.The main achievement of the present article is that the simultaneous use of low calcination temperatures and a certain amount of oxalic acid leads to the formation of highly selective undoped NiO catalysts which additionally are very stable.The characterization of as-synthesized samples (before calcination) suggests important changes depending on the absence or presence of oxalic acid in the synthesis gel. Thus, Ni-nitrate was observed in the sample prepared without oxalic acid in the synthesis gel, whereas Ni-oxalate is observed in the precursors of catalysts prepared with oxalic acid in the synthesis (Fig. S2, in the Supporting information). These differences, which determine also a different behaviour during the thermal decomposition of precursors (Fig. S2, in the Supporting information) should have an important influence on the physicochemical characteristics, and then, on the catalytic properties of the calcined catalysts [74,75], especially in samples calcined at 350\u00a0\u00b0C.In addition, these catalysts also present high catalytic reactivity, making it possible for them to operate at low reaction temperatures (ca. 300\u00a0\u00b0C) with a negligible loss of activity. This high stability contrasts with that reported in several articles in which a certain deactivation is observed in the ODH of ethane using NiO based catalysts. Deactivation has been related in former articles to formation of mixed inactive phases such as NiWO4 [72] or NiNb2O6 [20], the reduction of NiO overstoichiometric oxygen [20] and the decrease in the surface area [73]. In our catalysts, no mixed phases can be formed since the only chemical element present in these catalysts, apart from oxygen, is nickel. Interestingly, we have observed an apparent decrease in the amount of non-stoichiometric oxygen after using the optimal NiO catalyst, which, on the contrary, has had no effect on catalytic activity and ethylene selectivity.A non-promoted and non-supported NiO catalyst active and with a reasonably high selectivity to ethylene in the oxidative dehydrogenation of ethane has been prepared. The joint use of low calcination temperature (350\u00a0\u00b0C) and the inclusion of an appropriate oxalic acid loading in the synthesis gel during the preparation procedure has led to notable selectivity to ethylene (ca. 73%). Interestingly, a stable catalytic performance with the time on line has been observed in the optimal catalysts. This high stability can be related to the low reaction temperature required to undertake the reaction. The use of oxalic acid in the synthesis gel has been shown to highly improve the catalytic performance as an increase in the selectivity to ethylene by ca. 25 points and the reactivity by a factor of 1.5\u20132.0 compared to the reference sample have been obtained. Moreover, the calcination temperature has been shown as a determining factor in a way that catalysts calcined at 350\u00a0\u00b0C are more active and more selective to ethylene than analogous catalysts calcined at 500\u00a0\u00b0C. The enhanced catalytic performance of catalysts prepared in the presence of oxalic acid has not been only related to the NiO crystallite size and, more interestingly, the amount of electrophilic oxygen does not seem to play alone a determining role in the selectivity to ethylene. However, although a precise correlation has not been obtained, the most selective catalysts present high extent of isolation of non-stoichiometric oxygen and low p-type semiconductor character.\nYousra Abdelbaki: Investigation, Formal analysis. Agust\u00edn de Arriba: Investigation, Formal analysis. Rachid Issaadi: Methodology, Supervision. Rita S\u00e1nchez-Tovar: Methodology, Investigation, Formal analysis. Benjam\u00edn Solsona: Conceptualization, Supervision, Writing \u2013 review & editing. Jos\u00e9 M. L\u00f3pez Nieto: Conceptualization, Project administration, Writing \u2013 review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Authors would like to acknowledge the Ministerio de Ciencia, Innovaci\u00f3n y Universidades in Spain through projects CRTl2018-099668-B-C21 and MAT2017-84118-C2-1-R. A.A. acknowledges Severo Ochoa Excellence Program for his fellowship (BES-2017-080329). Y.A. and R.I. thank the Ministry of Higher Education and Scientific Research of Algeria for the National Exceptional Program for the fellowships.\n\n\n\nSupplementary material\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.fuproc.2022.107182.", "descript": "\n                  Highly stable and selective bulk NiO catalysts have been synthesized for the oxidative dehydrogenation (ODH) of ethane to ethylene. Interestingly, by optimizing synthesis parameters such as the amount of oxalic acid in the synthesis gel and the calcination temperature, undoped NiO catalysts have shown a consistent selectivity to ethylene of ca. 75%. The optimal catalyst requires the presence of a certain amount of oxalic acid in the synthesis gel and a final calcination at low temperatures (i.e. 350\u00a0\u00b0C). These catalysts have been deeply characterized by means of XRD, TPR, HRTEM, Raman and UV\u2013vis diffuse reflectance spectroscopies, XPS and Electrochemical Impedance Spectroscopy measurements and tested in the ethane ODH. A novel electrochemical study has been undertaken, showing the p-character of all the NiO catalysts synthesized but differing in their capacitance values and density of cationic vacancies. The catalytic performance of NiO catalysts has been explained in terms of the different physicochemical properties (including changes in the number of vacancies) of the samples and the isolation of electrophilic oxygen species.\n               "}
{"full_text": "phosphorphosphor-doped titaniaphosphor-doped iron on titaniaphosphor-doped copper on titaniaphosphor-doped cobalt on titanianickel phosphide on titaniaalkly-methoxyphenolThe growing demand for clean energy with depleting fossil resources and climate change because of increased carbon dioxide (CO2) emissions have motivated renewable energy development. Lignocellulosic biomass is an abundant renewable energy source that can replace crude oil to produce commodity chemicals and liquid transportation fuels. The biological and chemical processing of biomass can produce various chemicals and fuels [1]. However, biomass pyrolysis oil, referred to as bio-oil, has emerged as an essential feedstock to supplement petroleum oil because it can completely utilize organic components of the biomass. Moreover, replacing petroleum-based fuels with bio-oil-derived liquids reduces carbon footprints. A previous study suggested that producing petroleum-like fuels using bio-oils derived from several lignocellulose feedstocks could reduce the amount of greenhouse gas by 53\u201372% [2]. Although bio-oil exhibits physical properties similar to those of crude petroleum oil, its poor chemical properties (low heating value, high acidity, and high water content) limits its application as an energy source [3]. Furthermore, bio-oil contains labile chemical components with a high degree of oxygen functionalities and therefore requires chemical processes, including hydrotreatment [4] and water-soluble phase separation [5], before converting into chemicals and fuels.Hydrodeoxygenation (HDO) has been performed to valorize the bio-oil into a petroleum-like liquid. Noble metal catalysts, such as ruthenium (Ru) [6] and rhodium (Rh) [7], have been frequently used for hydrodeoxygenation. However, many transition metal catalysts, such as nickel (Ni) [8], molybdenum (Mo) [9], and Ni-Ru bimetals [10], modified using various promoters and supports have been developed for the inexpensive HDO reactions of bio-oil. Amongst the base transition metals, supported nickel (Ni) catalysts have potential as HDO catalysts because of the high hydrogenation activity of Ni [8]. However, the low affinity of Ni toward oxygen functionalities of lignin-derived oxygenates (phenolic monomers) suppresses the complete deoxygenation on Ni catalysts. Therefore, titania (TiO2)-supported Ni catalysts have been suggested for improving the poor deoxygenation activity of Ni by providing strong metal support interactions on the Ni\u2013titanium (Ti) interfacial sites [11].Supported phosphor (P)-modified transition metal, including Ni, molybdenum (Mo), tungsten (W), and iron (Fe) [12], catalysts have displayed high activity in HDO reactions of biomass-derived oxygenates [13] and bio-oil [14] because of their high thermal stability and synergistic effects between Br\u00f8nsted and Lewis acid sites [15]. However, HDO using supported phosphor-modified transition metal catalysts has been limited to the conversion of simple model compounds, such as alkyl esters [16], furfural [17], phenol [18], and guaiacol [19] (Table 1\n), and their application in the HDO of bio-oil has not been well discussed. In addition, it is challenging to extend the results of such model compound studies to the complex mixture of actual bio-oil. Therefore, to use phosphor-modified transition metal catalysts practically for upgrading bio-oil, it is necessary to use catalysts for the HDO of actual bio-oil and analyze the process information for determining optimum conditions.The objectives of this study are: (i) to successfully operate the HDO of bio-oil and its model compounds using phosphor-modified transition metal catalysts and (ii) to determine the optimum conditions for catalyst preparation and process operation. The lignin-derived model compounds and actual bio-oil were converted using several TiO2-supported phosphor-modified transition metal catalysts, and the optimum catalysts were selected based on their catalytic activity. Hydrothermal methods were employed for improving the interaction between transition metals and the TiO2 support. Metal nitrate salts of Ni, cobalt (Co), copper (Cu), and Fe were used as precursors of transition metals, which were mixed with a phosphor source of diammonium hydrogen phosphate ((NH4)2HPO4). The effects of metal types and phosphor doping on the HDO activity were investigated. The active sites of the catalysts were elucidated to understand the HDO of lignin-derived oxygenates and actual bio-oil. Therefore, the successful HDO of complex bio-oil using phosphor-modified transition metal catalysts can replace the HDO processes using noble metals and help reduce the production cost of biomass to fuels.Catalyst preparation, reaction procedure, chemical analysis, and catalyst characterization techniques used in this study have been described to provide insights to the preparation of deoxygenated fuels.All chemicals were used without further purification. Nickel(II) nitrate hexahydrate (Ni(NO3)2\u00b76H2O, 99.999%), copper(II) nitrate hemipentahydrate (Cu(NO3)2\u00b72.5H2O, 99.99%), cobalt(II) nitrate hexahydrate (Co(NO3)2\u00b76H2O, 99.999%), iron(III) nitrate nonahydrate (Fe(NO3)3\u00b79H2O, \u226599.95%), titanium(IV) oxide (TiO2, P25, \u226599.95%), 2-methoxy phenol (guaiacol, C7H8O2, 99%), 3-methyl phenol (m-cresol, C7H8O, 98%), 2-methoxy-4-ethylphenol (ethyl guaiacol, C9H12O2, 98%), 2-methoxy-4-propylphenol (propylguaiacol, C10H14O2, 99%), 2-methoxy-4-(2-propenyl)phenol (eugenol, C10H12O2, 98%), n-decane (C10H22), and n-dodecane (C12H26) were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Diammonium hydrogen phosphate ((NH4)2HPO4, 99%) and ammonium bicarbonate (NH4HCO3) were purchased from Daejung Chemicals and Metals (Siheung, Gyeonggi-do, Korea). 2-Methoxy-4-methylphenol (methyl guaiacol, C8H10O2, 98%) was purchased from Alfa Aesar (Haverhill, Massachusetts, USA). Hydrogen gas (H2, 99.999%), nitrogen gas (N2, 99.9%), H2 mixed with argon (5% v/v H2/Ar), oxygen gas mixed with N2 (0.5% v/v O2/N2), and carbon monoxide mixed with helium (10% v/v CO/He) were purchased from Shinyang Medicine (Seoul, Korea). The bio-oil, which was prepared as a liquid condensate of the fast pyrolysis of the pellet residue from pinewood, was purchased from Biomass Technology Group (BTG) BV (Enschede, Overijssel, The Netherlands). Deionized (DI) water was obtained from a purification system (EXL\u00ae 7S Analysis Water, Vivagen Co., Ltd., Seongnam, Gyeonggi-do, Korea) equipped with a 0.22\u00a0\u00b5m filter.A hydrothermal method was used to prepare TiO2-supported phosphor-modified transition metal catalysts. For the preparation of 20\u00a0wt% Ni and 5\u00a0wt% P on a TiO2 support, denoted as Ni\u2013P/TiO2, Ni(NO3)2\u00b76H2O (1.65\u00a0g) and (NH4)2HPO4 (0.19\u00a0g) were dissolved in DI water (20\u00a0mL) and placed in a 100\u00a0mL Teflon-lined chamber. Subsequently, TiO2 (P25, 1.50\u00a0g) was added to the prepared solution, which was ultrasonicated for 30\u00a0min to obtain a well-dispersed suspension. An aqueous solution of NH4HCO3 (1\u00a0M, 30\u00a0mL) was added dropwise to the solution and simultaneously stirred at room temperature. The Teflon-lined chamber was closed and heated to 150\u00a0\u00b0C for 6\u00a0h. The mixture was cooled to room temperature, and the prepared powder was recovered through vacuum filtration. The powder was washed with DI water to reach a neutral pH and further washed three times with ethanol (50\u00a0mL each). The washed powder was calcined in a N2 flow at 500\u00a0\u00b0C for 2\u00a0h and reduced in a H2/Ar flow (5% v/v) at 450\u00a0\u00b0C for 4\u00a0h. Additionally, Cu\u2013P/TiO2, Co\u2013P/TiO2, and Fe\u2013P/TiO2, containing 20\u00a0wt% of the corresponding transition metal and 5\u00a0wt% P, were prepared using a procedure similar to that used to prepare Ni\u2013P/TiO2. Cu(NO3)2\u00b72.5H2O, Co(NO3)2\u00b76H2O, and Fe(NO3)3\u00b79H2O were used as precursors of the corresponding transition metal catalysts.X-ray diffraction (XRD) results of the catalysts were obtained using a Dmax2500/PC diffractometer (Rigaku, Tokyo, Japan) equipped with a scintillation counter with a graphite monochromatic detector and an average Cu K\u03b1ave radiation generated at 40\u00a0kV and 200\u00a0mA. The particle size (d\nXRD) was calculated using the Scherrer equation (Equation (1)).\n\n(1)\n\n\n\n\nd\n\n\nX\nR\nD\n\n\n=\n\n\nK\n\u03bb\n\n\nB\nc\no\ns\n\u03b8\n\n\n\n\n\n\nwhere d\nXRD (nm) is the crystal domain size obtained using selected diffraction (hkl); K is the Scherrer constant (0.94 for spherical crystals with a cubic symmetry); \u03bb is the wavelength (0.15418\u00a0nm for Cu K\u03b1ave); B is the modified full width at a half maximum (FWHM) of the diffraction peak calculated as (FWHM)2 \u2212 (FWHM of the bulk crystal, 0.2\u00b0 in this study); \u03b8 is the Bragg angle of the selected diffraction (hkl). The morphology of the catalysts was observed through field-emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (FE-SEM/EDS, Teneo VS, Thermo Fisher Scientific, Waltham, Massachusetts, USA). The atomic structure and elemental distribution of the catalysts were investigated using high-resolution transmission electron microscopy (TEM, Talos F200X, Thermo Fisher Scientific, Waltham, Massachusetts, USA). The electronic structures of the catalysts were observed through high-performance X-ray photoelectron spectroscopy (XPS, ESCALAB 250 spectrometer, VG Scientific, Thermo Fisher Scientific, Waltham, Massachusetts, USA).The HDO activity of the catalysts was measured using a custom-built SUS 316 batch reactor [21]. For a typical HDO reaction with a model compound, catalyst (0.12\u00a0g), n-decane solvent (50\u00a0mL), and a reactant mixture of phenolic monomers (1.2\u00a0g) were placed into the reactor. The inner reaction system was purged three times with N2 and filled with 4\u00a0MPa H2 at room temperature. The HDO reaction was performed under three different conditions. After the reaction was completed, the reactor system was cooled to 50\u00a0\u00b0C or lower temperature using a cold-water coolant. The liquid product was the mixed with n-dodecane, an internal standard, and further diluted with methanol for gas chromatography (GC) measurements. The catalytic activity was illustrated by calculating conversion (Xfeed, %), product yield (Yproduct, %), and selectivity (Sproduct, %). The parameters were determined as follows:\n\n(2)\n\n\n\n\nX\n\n\nf\ne\ne\nd\n\n\n\n\n\n%\n\n\n=\n1\n-\n\n\n\n\nn\n\n\nf\ne\ne\nd\n\n\n\n\n\n\nn\n\n\nf\ne\ne\nd\n\n\n0\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(3)\n\n\n\n\nY\n\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\n\n%\n\n\n=\n\n\n\n\nn\n\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\n\n\nn\n\n\nf\ne\ne\nd\n\n\n0\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(4)\n\n\n\n\nS\n\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\n\n%\n\n\n=\n\n\n\n\nn\n\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\n\u2211\n\n\nn\n\n\np\nr\no\nd\nu\nc\nt\n\n\n\n\n\u00d7\n100\n\n\n\n\nwhere nfeed is the moles of the remaining reactant; n0\nfeed is the initial moles of reactant; nproduct is the moles of product; \u03a3nproduc\nt is the moles of all identified products. A two-step reaction was performed for the HDO of bio-oil reactant. In the first step, bio-oil (2\u00a0g) and n-decane (50\u00a0mL) were placed in the reactor without catalyst and heated to 300\u00a0\u00b0C under H2 atmosphere at 4\u00a0MPa (measured at room temperature) for 3\u00a0h. The product was collected, and the liquid phase was vacuum filtered from the solid residue (heavy oil). The obtained liquid filtrate is denoted as BO-S1. BO-S1 was mixed with a fresh catalyst powder (0.5\u00a0g) in the reactor and heated at 300\u00a0\u00b0C and a pressure of 4\u00a0MPa (measured at room temperature) under H2 atmosphere for 3\u00a0h. After the reaction was completed, the liquid product was vacuum filtered. The obtained liquid filtrate is denoted as BO-S2. The liquid products were identified through GC\u2013mass spectrometry (GC\u2013MS, Agilent 7890A with 5975C inert MS XLD with triple axis-detector, Agilent Technologies, Santa Clara, California, USA) equipped with an HP\u20135MS capillary column (60\u00a0m\u00a0\u00d7\u00a00.25\u00a0\u00b5m\u00a0\u00d7\u00a00.25\u00a0mm). The liquid products were also quantified using a GC-flame ionization detector (FID) (Hewlett 5890 Packard Series II) equipped with an HP-5MS capillary column (60\u00a0m\u00a0\u00d7\u00a00.25\u00a0\u00b5m\u00a0\u00d7\u00a00.25\u00a0mm). The spent catalyst powder was collected from the remaining liquid product via vacuum filtration and washed three times using acetone (25\u00a0mL). The obtained solid product was dried in air at 105\u00a0\u00b0C for 16\u00a0h. The dry solid product is referred as the spent catalyst.The observed results are described in this section and further discussed to demonstrate the effect of Ni\u2013P/TiO2 for improved HDO.According to the manufacturer, the commercially available bio-oil from BTG contained a high oxygen content (O/C\u00a0=\u00a00.74 atom/atom and H/C\u00a0=\u00a01.8 atom/atom), leading to high acidity (pH\u00a0=\u00a02.42) as reported in the literature [22]. The organic compounds present in the bio-oil were detected using GC\u2013MS. A considerable number of compounds, including carboxylic acids, esters, aldehydes, ketones, alcohols, and phenols, were observed along with sugars and minuscule amounts of other hydrocarbons (Table 2\n and Fig. 1\n). Acetic acid, acetic acid methyl ester, 1-hydroxyl-2-propanone, ethanol, guaiacol, 2-methoxy-4-methyl-phenol (4-methyl guaiacol), and D-allose were the most abundant compounds in the bio-oil based on the GC\u2013MS peak areas. Because the presence of molecules with carbonyls can produce high molecular weight polymers by condensing small molecules [23], the stabilization or promotion of bio-oil via hydrotreatment to reduce carbonyls can produce the highly stable bio-oil; however, complete deoxygenation was not easily achieved [1,8,10].The surface structures of the catalysts can highly manipulate the catalytic activity. Therefore, the crystal structures of the prepared TiO2-supported phosphor-modified transition metal catalysts (Ni\u2013P/TiO2, Co\u2013P/TiO2, Cu\u2013P/TiO2, and Fe\u2013P/TiO2) were observed through powder XRD to elucidate the modification of metal structures by adding phosphor (Fig. 2\n). For all catalysts, a mixture of anatase and rutile TiO2 phases was observed for the TiO2 support, exhibiting strong diffraction peaks at 2\u03b8\u00a0=\u00a025.2\u00b0 and 27.4\u00b0, respectively. The addition of P (without transition metal) to TiO2 did not significantly alter the TiO2 crystal structure (Fig. 2(i-v)). The mixtures of metals and metal oxides were observed for phosphor-added Ni, Co, Cu, and Fe. However, weak diffractions of metal phosphide were observed only for Ni. Distinct diffraction peaks of Ni phosphide (Ni3P) were observed at 2\u03b8\u00a0=\u00a041.7\u00b0, 43.6\u00b0, and 46.6\u00b0, corresponding to the (231), (112), and (141) planes of Ni3P (PDF#34\u20130501), respectively, when Ni precursor was mixed with the phosphor precursor and TiO2 (Fig. 2(A-i and B)). The reaction between NiO and PH3, which was prepared via the thermal decomposition of (NH4)2HPO4, formed Ni3P [24]. Phosphides of Co and Cu were not observed on Co\u2013P/TiO2 and Cu\u2013P/TiO2, respectively, indicating the poor reaction of Cu and Co with PH3. The XRD results also confirmed the higher dispersion of metal particles for Ni\u2013P/TiO2 and Co\u2013P/TiO2. Based on the crystal size (d\nXRD) of Ni and Co metal particles (excluding their oxides and phosphides) calculated using the Scherrer equation, Ni\u2013P/TiO2 and Co\u2013P/TiO2 demonstrated small metal particles with a size of 17.0 and 21.1\u00a0nm, respectively. However, Cu\u2013P/TiO2 exhibited sharp XRD peaks of Cu, indicating agglomeration of Cu particles during the high-temperature preparation (Table 3\n). Notably, the calculations using Scherrer equation considering the broad Ni3P(112) diffraction peak also suggested the existence of small particles with a diameter of 11.4\u00a0nm, indicating the formation of small particles in the range of 11.4\u201317.0\u00a0nm because of the formation of Ni metal and Ni3P domains.The structures of the catalysts were investigated through TEM (Fig. 3\n). For Ni\u2013P/TiO2 (Fig. 3(A)), Ni nanoparticles with an average diameter of 20\u00a0nm (Fig. 3(A1)) and an irregular shape were dispersed on the TiO2 support. As observed in the XRD results, Ni\u2013P/TiO2 exhibited the co-existence of Ni and P (Fig. 3(A1\u2013A3)). The boundaries between the Ni phosphide particles and the TiO2 support were also observed, confirming the formation of isolated Ni phosphide particles. The high-resolution TEM images demonstrated d-spacings of 0.210, 0.195, and 0.242\u00a0nm, which were attributed to the Ni3P phase (Fig. 3(A5\u2013A7). Significantly, Ni/TiO2 without phosphor formed isolated Ni particles on TiO2 (Fig. 3(D)). However, other catalysts prepared in this study did not form the corresponding metal phosphides compared with Ni\u2013P/TiO2. Highly dispersed non-metallic Co and P were observed for Co\u2013P/TiO2 in the EDS results, suggesting the less agglomeration of Co and P on the TiO2 support. Furthermore, Co, P, and Ti were highly dispersed without clear segregation, indicating the possible formation of the Co\u2013P\u2013Ti mixed oxide, although its formation was not confirmed from the XRD results. Large Cu particles were observed on the TiO2 support for Cu\u2013P/TiO2 (Fig. 3(C)), and P was concentrated on the Cu particles. The P atoms isolated on the surface of Cu particles indicated the segregation of Cu and P.The surface chemical oxidation states of catalysts were observed through XPS, and the electron transfer from Ni to P that formed slightly cationic Ni0 was elucidated (Fig. 4\n). The wide survey of XPS exhibited binding energy peaks of P 2s, P 2p, C 1s, Ti 2p, and O 1s (Fig. 4(A)). In addition, binding energy peaks were observed at 705\u2013950\u00a0eV, confirming the presence of transition metals (Fig. 4(i\u2013iv)). The peaks of Ni0 and Ni2+ were observed for Ni\u2013P/TiO2 at 853.3 and 857.0\u00a0eV, respectively. The Ni0 peak of Ni\u2013P/TiO2 shifted to the higher binding energy by 0.5\u00a0eV compared with that of Ni/TiO2 (852.8\u00a0eV Ni 2p for Ni/TiO2, Fig. 4(B)(iv)). The negatively charged P\u03b4\u2212 correlated with the slight cationic shift of the Ni0 peak, which was attributed to the electron transfer from Ni to P, confirming the formation of Ni3P on silica [25] or without support [26]. The formation of slightly cationic Ni0 led to the Lewis acidity of Ni0, improving the interaction between Ni0 and oxygen atoms of reactants. The peaks of Co0 and Co2+ were observed for Co\u2013P/TiO2 at 778 and 782\u00a0eV, respectively. Both metal and metal oxide peaks were observed for Ni\u2013P/TiO2 and Co\u2013P/TiO2. Furthermore, the satellite peaks of Co 2p and Ni 2p were observed at 785 and 864\u00a0eV for Co\u2013P/TiO2 and Ni\u2013P/TiO2, respectively, exhibiting their multiple oxidation states. In contrast, Cu\u2013P/TiO2 and Fe\u2013P/TiO2 exhibited less complex oxidation states. Reduced metallic Cu0 species were observed for Cu\u2013P/TiO2 with a strong peak at 932\u00a0eV, confirming the formation of large Cu particles observed in the TEM image (Fig. 3(C)). Furthermore, the absence of Cu2+, Cu+, and satellite peaks suggested the absence of Cu oxides and other Cu species. Although the peaks of Fe2+ and Fe3+ were observed for Fe\u2013P/TiO2, the Fe0 peak was absent. Further examination of phosphor at P 2p exhibited peaks at 137\u2013131\u00a0eV for the P\u2013O bonds and approximately 129\u00a0eV for the negatively charged P\u03b4\u2212 for NiP catalysts on mesoporous silica [27] or without support [26]. The strong peaks for P\u03b4\u2212 species observed for Ni\u2013P/TiO2 at 129.1\u00a0eV confirmed a strong electron transfer from Ni0 to P. Additionally, a weak peak of P\u03b4\u2212 was observed for Co\u2013P/TiO2. However, P\u03b4\u2212 was not observed for Fe\u2013P/TiO2, Cu\u2013P/TiO2, and P/TiO2 (Fig. 4(C)).The HDO of lignin-derived phenolic monomers as model compounds was investigated using TiO2-supported phosphor-modified transition metal catalysts. The catalytic activity was measured, and the process for upgrading bio-oil was suggested. Mixtures of alkyl-methoxyphenols (AMPs) containing different amounts of guaiacol, m-cresol, methyl guaiacol, ethyl guaiacol, propyl guaiacol, and eugenol were used as reactants. As illustrated in Fig. 5\n(A), the conversion of AMPs was negligible at 300\u00a0\u00b0C under an initial H2 pressure of 4\u00a0MPa for 2\u00a0h in the absence of P/TiO2. The products obtained without catalysts and using metal-free P/TiO2 were almost identical with the mixture of reactants (Fig. 5(A-i, ii, and iii)), indicating their negligible HDO activity. Before conducting HDO using metal phosphide catalysts, HDO using Ni/TiO2 and commercially available Ru/C catalysts was performed. For Ni/TiO2, the conversion of phenolic compounds yielded saturated hydrocarbons, including cyclohexyl ketones or alcohols (42%) and cycloalkanes (57%), suggesting the hydrogenation of aromatic compounds via the HDO reaction using Ni/TiO2. However, the conversion of phenolic compounds was incomplete for Ru/C, producing cyclic ketones and cyclic alcohols (43% combined yield). Additionally, several unknown compounds were observed.Based on the observed complete conversion of phenolic compounds on Ni/TiO2, the phosphor-modification of metals on TiO2 was attempted. Although HDO using Fe\u2013P/TiO2 formed methoxybenzenes and diols, significant hydrogenation did not occur (Fig. 5(B-i and D) and 6(A)). The corresponding products are illustrated in Fig. 5(C-i, ii, and iii) and 6(A) for the reaction using Ni\u2013P/TiO2, Co\u2013P/TiO2, and Cu\u2013P/TiO2 catalysts. The complete HDO of the phenolic mixture was observed for Ni\u2013P/TiO2, which selectively produced cyclic alkanes (87% yield). These observations indicate that multifunctional catalysts containing both metals and acid sites are essential for improving HDO with hydrogenation and deoxygenation [7]. Notably, Cu\u2013P/TiO2 and Co\u2013P/TiO2 exhibited lower AMP conversions (40.5 and 69.5%, respectively) and cycloalkane yields (3.8 and 30.0%, respectively) compared to those of Ni\u2013P/TiO2 (100 and 87% AMP conversion and cycloalkane yield, respectively).The catalytic activity of Ni\u2013P/TiO2 was adjusted by manipulating the reaction temperature (Fig. 6\n(C)). Both AMP conversion and cycloalkane yields increased from 90 to 100% and 45 to 87%, respectively. The product distribution at 300\u00a0\u00b0C included a small number of light compounds, including CH4, indicating that the cracking occurred at 300\u00a0\u00b0C or higher temperatures. Based on these observations, HDO at 300\u00a0\u00b0C is optimum to suppress further cracking at a higher temperature.The stability of Ni\u2013P/TiO2 was observed for different reaction times from 0 to 3\u00a0h (Fig. 6(B)). The yield of cycloalkanes increased to reach the plateau of 87% when the reaction time was increased up to 3\u00a0h. The recyclability of Ni\u2013P/TiO2 was observed by repeatedly performing the HDO of AMP at 300\u00a0\u00b0C and 4\u00a0MPa H2 (measured at room temperature) for 2\u00a0h (Fig. 6(D)). The AMP conversion and cycloalkane yield gradually decreased, which could be attributed to the deactivation of the catalyst.The high HDO activity of Ni\u2013P/TiO2 can be attributed to the highly dispersed Ni particles of Ni\u2013P/TiO2. Moreover, modifying Ni particles with phosphor to form Ni3P particles can improve HDO activity. The negatively charged P\u03b4\u2212 affects the slightly cationic Ni0, which can be an active site for the complete HDO of phenolic compounds. Furthermore, the slightly cationic Ni0 increases the Lewis acidity of the catalyst surface, which improves the hydrodeoxygenation at the catalyst surface, as observed in the previous study (Fig. 7\n) [7].Based on the catalyst characterization results and optimized conditions of Ni\u2013P/TiO2, the HDO of AMP was suggested to proceed by the adsorption of aromatic rings on the Ni surface and was promoted by the presence of cationic Ni\u03b4+ The HDO of the AMP molecule may have proceeded by the dehydration reaction because of the phosphor sites adjacent to Ni, which act as Br\u00f8nsted acid sites to remove oxygen atoms in the AMP compounds.The HDO of actual bio-oil was performed using Ni\u2013P/TiO2 at 300\u00a0\u00b0C and 4\u00a0MPa H2 (measured at room temperature) based on the observed HDO results of phenolic mixtures. A two-step HDO was performed (each step for 3\u00a0h) to achieve better HDO results because of the complex nature of the bio-oil feedstock. The first step was the thermal pretreatment of the bio-oil dissolved in n-decane without catalyst, while the second step was the HDO of bio-oil using Ni\u2013P/TiO2. A transparent liquid was obtained from the two-step HDO reaction: gasoline (C6\u2013C9: n-hexane (10)), methyl cyclopentane, cyclohexane, methyl cyclohexane, ethyl cyclohexane (11), propyl cyclohexane (12), kerosene-like (C10\u2013C13: 1,1\u2032-methylenebiscyclohexane (13)), and diesel-fractions (C14\u2013C17: 1,1\u2032-ethylenebiscyclohexane (14) and 1-(cyclohexylmethyl)-4-isopropylcyclohexane (15)) were observed in the liquid product (Fig. 8(A-i) and Table 4\n). The formation of dimeric compounds (13, 14, and 15) using Ni\u2013P/TiO2 was distinct for this process, indicating the successful production of high carbon number hydrocarbon fuels via condensation between phenolic compounds observed in BO-S1 (Fig. 8\n(A-iii)). For Ni\u2013P/TiO2, the oil yield was 85%, leading to 37.4% of total cycloalkane yield containing 37 and 7% monocycloalkane and dicycloalkane selectivities, respectively (Fig. 8(C)).Compounds with lower molecular weights, including carboxylic acids, esters, aldehydes, ketones, and alcohols derived from cellulose and hemicellulose, were not significantly observed after the HDO. This observation can be attributed to the conversion of the small molecules to deoxygenated light hydrocarbons. These light hydrocarbons may not be condensed at the condenser of the reactor and thereby are not observed in the GC results of the liquid products.Compared to the products obtained using Ni\u2013P/TiO2, the liquid product after the first step without catalyst contained (holo)cellulose-derived small oxygenates (acetone (1) and ethyl acetate (2)) and lignin-derived phenolic monomers (guaiacol (4), methyl guaiacol (5), ethyl guaiacol (6), 4-allyl-guaiacol (7), isoeugenol (8), and 4-allyl-syringol (9)) (Fig. 8(A-ii)). These observations indicate that the first step reaction removed or cracked the high carbon number products present in the bio-oil (Fig. 1). Compounds with high molecular weights, including the GC-undetectable molecules, can be cracked during the first step reaction, thereby improving the HDO activity at the second step. Because the compounds described in Fig. 8(A-ii) are close to the lignin-derived phenolic molecules, further HDO at the second step using Ni\u2013P/TiO2 is essential. A close examination of the GC peak at 11.30\u201311.40\u00a0min indicated that the predominant guaiacol monomer disappeared when the two-step HDO using Ni\u2013P/TiO2 was performed (Fig. 8(B-iii)), while the first step reaction with or without the catalyst resulted in the conversion of guaiacol (Fig. 8(B-ii and iii)).The HDO of phenolic monomers as a model mixture of lignin-derived compounds was performed and the successful deoxygenation and hydrogenation were achieved using Ni\u2013P/TiO2. Based on these observations, the HDO of the bio-oil diluted in n-decane was also attempted. Complete deoxygenation was observed with a distinct production of high carbon number hydrocarbons formed by condensing phenolic monomers. The titania-supported nickel phosphide catalyst produced completely saturated deoxygenated compounds with an 87% yield of cycloalkanes at 300\u00a0\u00b0C and 4\u00a0MPa H2 (measured at room temperature) for a reaction time of 2\u00a0h. Upgrading bio-oil using titania-supported nickel phosphide catalyst afforded a 37.4% yield of deoxygenated hydrocarbons. Although transition metals supported on TiO2 exhibited high catalytic hydrogenation activity, adding phosphor to Ni/TiO2 improved the HDO activity and induced the production of deoxygenated cyclic alkanes. The formation of Ni nanoparticles modified with highly dispersed phosphor atoms was also observed. In addition, the electron transfer from Ni0 to P formed slightly cationic Ni0. The strong interaction between the cationic Ni0 and oxygen atoms of the reactant could improve HDO. Therefore, the findings of this study provide insights for promoting the efficient conversion of lignocellulose into chemicals and fuels using non-precious transition metal catalysts.\nRizki Insyani: Investigation, Writing \u2013 original draft. Jae-Wook Choi: Methodology, Investigation. Chun-Jae Yoo: Methodology, Investigation. Dong Jin Suh: Conceptualization. Hyunjoo Lee: Methodology, Investigation. Kyeongsu Kim: Data curation. Chang Soo Kim: Methodology, Investigation. Kwang Ho Kim: Methodology, Investigation. Jeong-Myeong Ha: Conceptualization, Writing \u2013 review & editing, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was supported by the Technology Development Program to Solve Climate Change of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (NRF-2020M1A2A2079798).", "descript": "\n                  Biomass pyrolysis oil is a potentially essential renewable energy source that can serve as an alternative to petroleum-based fuels and chemicals. In this study, biomass pyrolysis oil was converted into petroleum-like deoxygenated hydrocarbons via catalytic hydrodeoxygenation using a titania-supported nickel phosphide catalyst. The phosphor precursor was added to several transition metals, including nickel, cobalt, copper, and iron, supported on titania. The formation of isolated nickel phosphide particles, which were active for complete hydrodeoxygenation, was confirmed by the characterization of prepared catalysts. As a model reactant of biomass pyrolysis oil, a mixture of alkyl-methoxyphenol compounds was hydrodeoxygenated to produce completely deoxygenated compounds, generating an 87% yield of cycloalkanes at 300\u00a0\u00b0C and 4\u00a0MPa H2 for a reaction time of 2\u00a0h. The hydrodeoxygenation of biomass pyrolysis oil also generated a 37.4% yield of hydrocarbon fuels. The high hydrodeoxygenation activity can be attributed to the synergy between the hydrogenating metals and the acid sites, which can be improved by electron transfer from a slightly cationic nickel to a slightly anionic phosphor. Furthermore, the addition of phosphor improved the formation of highly dispersed nickel particles, increasing the quantity of hydrogen-adsorbing surface metals. The observations in this study indicate that the efficient conversion of lignocellulose-derivatives into chemicals and fuels can be achieved using modified non-precious transition metal catalysts.\n               "}
{"full_text": "All data supporting this study are available in the article and supplemental information. Any additional requests for data will be handled by the lead contact upon reasonable request.As an important chemical, hydrogen peroxide (H2O2) has been widely applied in a series of processes including disinfection, wastewater purification, and oxidation reaction.\n1\n\n,\n\n2\n It has been demonstrated that H2O2 serves as an effective and environmentally benign oxidant in propylene epoxidation\n3\n and cyclohexanone ammoximation because water serves as the only byproduct.\n4\n In addition, in situ generation of H2O2 in tandem reactions for selective oxidation has recently attracted increasing attention in several challenging reactions, for example methane conversion to methanol\n5\n; hydroxylation of benzene to phenol\n6\n\n,\n\n7\n; selective oxidation of benzyl alcohol\n8\n; selective oxidation of cyclohexane\n9\n; and oxidation of propylene to propane oxide.\n10\n\n,\n\n11\n\nThe current industrial-scale production of H2O2 proceeds primarily through the anthraquinone process, which involves sequential hydrogenation and oxidation.\n12\n This process is energy intensive and environmentally unfriendly due to the substantial content of waste chemicals. Therefore, the direct H2O2 synthesis from H2 and O2 is highly desirable and has attracted considerable attention given the advantages of environmental benignity and atomic economy.\n13\n\n,\n\n14\n\n,\n\n15\n\n,\n\n16\n However, it remains a challenge due to several critical issues, especially the H2O2 degradation, in the direct H2O2 synthesis from H2 and O2.\n1\n\n,\n\n17\n\n,\n\n18\n Hutchings and co-workers\n19\n first reported that the incorporation of Pd into a supported gold (Au) catalyst could improve the direct H2O2 synthesis from H2 and O2 and confirmed the detrimental effect of H2O2 degradation on the synthesis process. It is thus crucial to improve H2O2 productivity and selectivity by inhibiting the reactivity of H2O2 degradation. Several strategies, for example optimization of reaction conditions\n20\n\n,\n\n21\n; introduction of halogen ions and acids\n22\n\n,\n\n23\n; modulation of metal-support interactions\n24\n; development of bimetallic\n2\n\n,\n\n25\n\n,\n\n26\n (e.g., Pd-Au, Pd-Ag, and Pd-Zn) and multi-metallic\n18\n\n,\n\n27\n (e.g., Pt-Pd-Au) components; and modification of supports,\n28\n\n,\n\n29\n have been explored. A supported Au-Pd catalyst prepared by Edwards et\u00a0al. via the acid pretreatment on carbon support could prevent the H2O2 hydrogenation due to a size decrease of alloy nanoparticles (NPs), giving high H2O2 productivity of 175\u00a0mol kgcat\n\u22121 h\u22121 with H2O2 selectivity of higher than 95%.\n28\n Choudhary and co-workers found that adding chloride or bromide anions to an acidic aqueous medium could enhance the H2O2 selectivity and yield as a result of the significant decrease in H2O2 hydrogenation and decomposition activities.\n30\n\nConsidering the higher safety and easier availability of CO compared with H2, the direct H2O2 synthesis from CO, O2, and H2O may turn out to be highly attractive and economic. However, this process is found to be much lower in productivity compared with the direct H2O2 synthesis from H2 and O2. Brill et\u00a0al. reported for\u00a0the first time that the H2O2 productivity over Pd/CaCO3 and Ru/graphite catalysts was only 0.1 molH2O2 kgcat\n\u22121 h\u22121 toward the H2O2 synthesis from CO, O2, and H2O,\n31\n\n,\n\n32\n which is about 3 orders of magnitude lower than that of the H2O2 synthesis from H2 and O2 on Pd-based catalysts.\n24\n Other catalysts, like Ni-La-B/Al2O3,\n31\n\n,\n\n33\n Cu/A12O3,\n34\n and Ni-P-B/A12O3,\n35\n have been explored to improve the H2O2 productivity to 0.326 molH2O2 kgcat\n\u22121 h\u22121. Recently, Cao and co-workers reported a highly efficient H2O2 productivity of \u223c74.6 molH2O2 kgcat\n\u22121 h\u22121 (9,097\u00a0mmolH2O2 gAu\n\u22121 h\u22121) employing anatase supported an Au catalyst in the solvent consisting of H2O and\u00a0acetone (1:1).\n36\n This impressive work, together with the results of Ma and co-workers,\n37\n\n,\n\n38\n demonstrated that supported Au catalysts can potentially achieve high productivity of H2O2 from CO, O2, and H2O. However, phosphorous acid or acetone was used with H2O as co-solvent in these studies,\n31\n\n,\n\n36\n which may lead to serious separation problems.Therefore, it is imperative to further improve H2O2 productivity by designing efficient catalysts and circumventing the use of environmentally unfriendly co-solvents in H2O2 synthesis from CO, O2, and H2O. In this work, we report a tetraethyl orthosilicate (TEOS)-modified ZnO supported Au catalyst (Au/ZnO-TEOS) in which Au NPs are partially encapsulated by an SiO2 overlayer. This catalyst is highly efficient for H2O2 synthesis from CO, O2, and H2O, and H2O2 productivity of 168 molH2O2 kgcat\n\u22121 h\u22121 is achieved only using H2O as the reaction solvent. Detailed characterizations including aberration-corrected scanning transmission electron microscope with high-angle annular dark-field detector (AC-HAADF-STEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) revealed the importance of structural confinement of the SiO2 overlayer on the Au/ZnO-TEOS catalyst to restrict the agglomeration of Au NPs, inhibit the degradation of H2O2, and improve the hydrophilicity of the catalyst surface in the Au/ZnO-TEOS catalyst, thus significantly enhancing the catalytic performance. A facile reaction mechanism is also suggested according to the isotopic labeling experiments and theoretical calculations. This work not only provides an efficient approach to construct a highly active supported Au catalyst toward the direct H2O2 synthesis from CO, O2, and H2O but also highlights the importance of the confinement effect in supported catalysts leading to several concurrent positive effects.The catalysts of Au/ZnO (without TEOS modification) and Au/ZnO-TEOS (with TEOS modification) series were prepared by a co-precipitation method and calcined at 200\u00b0C, 400\u00b0C, and 600\u00b0C, respectively, and which are designated as Au/ZnO-T or Au/ZnO-TEOS-T (T is the calcination temperature). The activity of direct H2O2 synthesis from CO, O2, and H2O was evaluated under the optimized conditions of 60\u00b0C, 5\u00a0mg catalyst, 10\u00a0mL H2O, 4 MPa with 75% O2:5% CO:20% He feed gas, 5\u00a0min reaction time, and 1,200 RPM stirring speed. It should be noted that both H2O2 productivity and selectivity usually need to be reported to evaluate the performance of the direct H2O2 synthesis, while in this work, the CO conversion was too low (<3%) to be accurately measured by conventional gas chromatography, leading to the poor reproducibility in H2O2 selectivity (see supplemental experimental procedures 2.3 for the associated details and discussion on H2O2 selectivity measurement).\n31\n\n,\n\n34\n\n,\n\n35\n As a result, only the data of H2O2 productivity are reported and compared in the batch autoclave reactor with 10\u00a0mL H2O. ZnO support itself was inactive in the direct H2O2 synthesis. Interestingly, it is evident from Figure\u00a01\nA that the H2O2 productivity of Au/ZnO-TEOS catalysts is remarkably higher than\u00a0that of Au/ZnO catalysts under the same calcination temperature. Notably, the H2O2 productivity of 168\u00a0mol kgcat\n\u22121 h\u22121 on Au/ZnO-TEOS-200 is achieved in the additive-free conditions, which is more than twice the current highest value obtained using the Au/anatase catalyst under the conditions of 1:1H2O and acetone as solvents by Cao and co-workers in the direct H2O2 synthesis from CO, O2, and H2O.\n36\n In addition, Figure\u00a01A shows that H2O2 productivity of the Au/ZnO series significantly decreases from 84 to 17\u00a0mol kgcat\n\u22121 h\u22121 as the calcination temperature increases from 200\u00b0C to 600\u00b0C. Meanwhile, Au/ZnO-TEOS series after TEOS modification bears resemblance to the Au/ZnO series in the decline tendency of the catalytic performance with calcination temperature. This can be attributed to the size variation of Au NPs with calcination temperature.\n39\n\n,\n\n40\n\n,\n\n41\n\n,\n\n42\n AC-HAADF-STEM was performed to analyze the Au NP size distribution of the Au/ZnO and Au/ZnO-TEOS series (Figures\u00a01B\u20131G). The mean sizes of Au NPs in Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts are nearly identical at 1.8 and 1.7\u00a0nm, respectively. This indicates that TEOS modification imposes a negligible effect on the mean size of the Au NPs after calcination at 200\u00b0C (Figures\u00a01B and 1E). As the calcination temperature elevates from 200\u00b0C to 600\u00b0C, the mean size of Au NPs in both Au/ZnO and Au/ZnO-TEOS series gradually increases. Combined with the aforementioned catalytic performance, AC-HAADF-STEM characterizations reveal obvious particle size effects of Au/ZnO and Au/ZnO-TEOS series, i.e., the larger the Au NP size, the poorer the H2O2 productivity. It is worth mentioning that the mean size of Au NPs of the Au/ZnO-TEOS series catalysts only increases to 4.7\u00a0nm, much smaller than that of the Au/ZnO series (\u223c12.9\u00a0nm) after calcination at 600\u00b0C, demonstrating that the TEOS modification could effectively restrain the agglomeration of Au NPs.Comparing the performance of the catalysts with similar Au particle sizes, such as Au/ZnO-200 and Au/ZnO-TEOS-200 (1.8 versus 1.7\u00a0nm) as well as Au/ZnO-400 and Au/ZnO-TEOS-600 (4.2 versus 4.7\u00a0nm), the H2O2 productivity of the TEOS-modified catalyst is about twice that of the non-TEOS-modified catalyst. Meanwhile, the Au loadings of Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts are also close (1.11 versus 1 wt\u00a0%) as determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Table\u00a0S1). As a result, the difference in the H2O2 productivity between them cannot be rationalized solely by the effects of Au particle size and loading content. Other critical roles of TEOS modification could be expected.To further clarify the role of TEOS modification on the improvement of the H2O2 productivity, we comprehensively compared the microstructures of Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts using energy-dispersive X-ray (EDX), FT-IR, and AC-HAADF-STEM. EDX measurements (Figure\u00a0S1) reveal a highly dispersed Si species on the surface of the Au/ZnO-TEOS-200 catalyst. IR measurements verify the existence of SiO2 on the Au/ZnO-TEOS-200 catalyst as a result of the detected peak at 1,088\u00a0cm\u22121, which is assigned to the antisymmetric stretching vibrations of Si-O-Si (Figure\u00a0S2).\n43\n\n,\n\n44\n The presence of defined edge and lattice fringes of Au NPs on the Au/ZnO-200 catalyst was testified to by AC-HAADF-STEM characterizations (Figure\u00a02\nA). In contrast, the lattice fringes of Au NPs on the Au/ZnO-TEOS-200 catalyst become fuzzy (Figures\u00a02B\u20132D). Despite the presence of fully encapsulated Au NPs by the SiO2 overlayer (Figure\u00a02B), partially encapsulated Au NPs by the SiO2 overlayer can be identified on the Au/ZnO-TEOS-200 catalyst (Figures\u00a02C and 2D; more AC-HAADF-STEM images of the Au/ZnO-TEOS-200 catalyst are presented in Figure\u00a0S3). AC-HAADF-STEM characterizations clearly demonstrate that TEOS modification results in Au NPs encapsulated by an SiO2 overlayer, diminishing the exposure of the Au-ZnO interface.It is widely recognized that H2O2 degradation can significantly affect H2O2 productivity.\n13\n\n,\n\n17\n\n,\n\n24\n\n,\n\n28\n\n,\n\n45\n H2O2 degradation experiments were thus performed to examine whether the TEOS modification influences H2O2 degradation activity of the supported Au catalysts. As shown in Figure\u00a02E, the H2O2 degradation rate is 8.6\u00a0mol kgcat\n\u22121 h\u22121 on the Au/ZnO-TEOS-200 catalyst, which is only about 1/9 of the value of the Au/ZnO-200 catalyst (78.9\u00a0mol kgcat\n\u22121 h\u22121), unambiguously demonstrating that the H2O2 degradation rate was remarkably suppressed by TEOS modification. This suggests that the inhibition of H2O2 degradation activity is indeed critical to enhance the H2O2 productivity on the Au/ZnO-TEOS-200 catalyst. Considering the structural characteristics of the Au/ZnO-TEOS-200 catalyst, it is reasonable to speculate that the Au-ZnO interface and/or the exposed Au NPs may be the active site for H2O2 degradation. In order to identify the origin of the suppression activity of H2O2 degradation over the Au/ZnO-TEOS-200 catalyst, two typical models, i.e., stepped Au(211) surface and Au/ZnO(110) interface, were taken to address the degradation as calculated using periodic density functional theory (DFT) calculations. Similar to the previous correction on the energetics of aqueous H2O2 solution,\n46\n it is obvious that the adsorption of H2O2 is endothermic on the Au(211) surface, while it is exothermic at the Au/ZnO(110) interface (see Figure\u00a02F). The subsequent decomposition of the adsorbed H2O2 proceeds readily into two OH\u2217 on both sites. It is noted that the decomposition of H2O2 into an O2 molecule is less favored. As a result, the difference in the adsorption behavior of H2O2 on the Au surface and the Au/ZnO interface rationalizes the experimental results; the Au-ZnO interface is thus the dominating active site for the H2O2 degradation, and the degradation activity is indeed weakened on Au/ZnO-TEOS-200 catalysts with a less-exposed Au-ZnO interface.Water acts as both reactant and solvent in this reaction, and its adsorption and activation on the catalyst surface would be crucial for the reaction. Figure\u00a03\nA displays the XPS O 1s spectra of Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts, which deconvolute into three peaks (shown in Figure\u00a0S4). The peaks at 530.1, 532.2, and 533.4 eV can be assigned to lattice oxygen (OI), hydroxyl groups (OII), and adsorbed molecular water (OIII), respectively.\n47\n\n,\n\n48\n It can be observed that the percentage of surface hydroxyl groups (OII) on the Au/ZnO-200 catalyst is about 30%. In contrast, it shoots up to approximately 70% on the Au/ZnO-TEOS-200 catalyst (Figure\u00a03B), revealing that TEOS modification can also modulate the surface structure of the catalyst. FT-IR characterization was employed to clarify the change of surface properties after TEOS modification as well. The hydroxyl region of IR spectra of the Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts is shown in Figure\u00a03C, and the specific group assignment of each peak is listed in Table\u00a0S2.\n49\n\n,\n\n50\n The peaks at 3,743 and 3,654\u00a0cm\u22121, which can be attributed, respectively, to isolated Si-OH and vicinal Si-OH, are observed on the Au/ZnO-TEOS-200 catalyst. This result agrees well with the percentage variation of surface oxygen species from the XPS measurements. Furthermore, the hydroxyl groups on the surface alter the hydrophilicity of the Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts as confirmed by water droplet contact-angle tests (91\u00b0 versus 68\u00b0; Figure\u00a03D).To further investigate the effect of the catalyst hydrophilicity on H2O2 productivity in the direct H2O2 synthesis from CO, O2, and H2O, we prepared three amorphous SiO2 supported Au catalysts by varying the calcination temperature of support, i.e., Au/200-SiO2, Au/500-SiO2, and Au/800-SiO2. It can be seen from Figure\u00a0S5 that the H2O2 productivity of these model catalysts toward the direct synthesis of H2O2 monotonously decreases (107, 88, and 78\u00a0mol kgcat\n\u22121 h\u22121), while the size of the Au NPs and the Au loading are almost identical in all three catalysts (Figure\u00a0S6; Table\u00a0S3). Additional effects other than the size and loading content of Au NPs may lead to such difference in the activity. 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) analyses (Figure\u00a0S7; Table\u00a0S4) show that the proportion of surface hydroxyl groups (Q3 configuration) on the supports gradually decreases from 24.2% to 15.8% when the pretreatment temperature of the supports elevates from 200\u00b0C to 800\u00b0C. The contact angles of Au/200-SiO2, Au/500-SiO2, and Au/800-SiO2 catalysts are 12\u00b0, 32\u00b0, and 38\u00b0, respectively (Figure\u00a0S8), which may be the consequence of the decrease of surface hydroxyl groups. It is therefore more reasonable to deduce that the difference of the H2O2 productivity between these amorphous SiO2 supported Au catalysts comes from the decrease in the hydrophilic ability of the catalyst surface. Likewise, the Au/ZnO-TEOS-200 catalyst is more hydrophilic compared with the Au/ZnO-200 catalyst (see Figure\u00a03D), which means that the intensification of hydrophilicity induced by TEOS modification may also benefit the enhancement of H2O2 productivity of the Au/ZnO-TEOS-200 catalyst.The stability of the Au/ZnO-200 and Au/ZnO-TEOS-400 catalysts toward the direct H2O2 synthesis from CO, O2 and H2O was tested through multiple catalyst use and\u00a0recovery cycles. An apparent deactivation was observed for the Au/ZnO-200 catalyst only after two runs (see Figure\u00a04\nA), and H2O2 productivity decreases from 86 to 58\u00a0mol kgcat\n\u22121 h\u22121. The loss of H2O2 productivity was frustratedly close to 80% after six runs. AC-HAADF-STEM displays an obvious agglomeration of Au NPs on Au/ZnO-200 catalysts with an average size increase from 1.76 to 3.55\u00a0nm and a broader particle size distribution after six runs (Figure\u00a0S9). In addition, the loading of Au apparently decreases to 0.13 wt\u00a0% as well (Table\u00a0S5). The agglomeration and\u00a0the loss of Au NPs are presumably the predominant effects upon the deactivation of the Au/ZnO-200 catalyst. In contrast, the decay of H2O2 productivity of the\u00a0Au/ZnO-TEOS-400 catalyst is negligible and holds approximately 150\u00a0mol kgcat\n\u22121 h\u22121 after 12 runs (see Figure\u00a04B). H2O2 productivity only decreases by 22% after 24 runs, and the average size of Au NPs only increases from 1.89 to 1.91\u00a0nm (see Figure\u00a0S10), which can be attributed to the confinement effect of the SiO2 overlayer. These results manifest that TEOS modifications prominently improve the stability of ZnO supported Au catalysts.To gain insights into the underlying reaction mechanism, the direct H2O2 synthesis from CO, O2 and H2\n18O was first performed. The oxygen isotope distribution in the generated H2O2 was detected using highly sensitive synchrotron-based vacuum UV\u00a0photoionization mass spectrometry (SVUV-PIMS). Only a signal of mass/charge (m/z)\u00a0= 34 assigned to H2\n16O16O was observed in the effluents at a photon energy of 12 eV, and no signal of H2\n16O18O or H2\n18O18O appears (see Figure\u00a0S11). This indicates that oxygen in molecular O2, rather than H2O, is more likely to serve as the exclusive oxygen source for the synthesis of H2O2. Furthermore, periodic DFT calculations were carried out to elucidate the detailed reaction mechanism. Two possible reaction pathways were proposed according to the origin of two oxygen atoms of H2O2 (see Figure\u00a05\n and Table\u00a0S6).\n51\n\n,\n\n52\n\n,\n\n53\n The stepped Au(211) was employed as the active surface for the reaction. In path 1, H2O2 comes from the consecutive hydrogenation of an O2 molecule, while in path 2, the coupling of two OH molecules results in the formation of H2O2. More specifically, both pathways start from the decomposition of H2O assisted by O2 to form OH\u2217 and OOH\u2217 species (TS1, 0.42 eV of the free energy barrier at 60\u00b0C). The direct decomposition of H2O without O2 assistance is energy demanding (1.91 eV). Starting from the formed intermediates including CO\u2217, OH\u2217, and OOH\u2217, the following pathways produce H2O2 and then bifurcate. The coupling between CO\u2217 and OH\u2217 (TS2, 0.69 eV) is involved in path 1, and the formed OCOH\u2217 reacts with OOH\u2217 to form CO2 and H2O2 (TS3, 0.32 eV), while in path 2, the coupling between CO\u2217 and OOH\u2217 results in the formation of peroxycarboxylic species OCOOH\u2217 (TS4, 0.08 eV), and the latter could be easily decomposed into CO2 and OH\u2217 (TS5, 0.19 eV). However, the formed two OH\u2217 species are particularly stable, and the coupling between them (TS6) needs to overcome a free-energy barrier as high as 2.12 eV at 60\u00b0C. As indicated by the calculated Gibbs free-energy profile (see Figure\u00a05A), path 1 is kinetically favorable over path 2, and the coupling between CO\u2217 and OH\u2217 (TS2) is rate determining in the reaction. In addition to OCOH\u2217, it should be noted that H2O can also proffer the proton to OOH\u2217 for subsequent hydrogenation to H2O2. CO thus acts as the eliminator of surface OH\u2217 species because the direct coupling of OH\u2217 is kinetically demanding (see Figure\u00a05B).In summary, the TEOS-modified ZnO supported Au catalyst, in which the Au NPs are encapsulated by an SiO2 overlayer, was precisely prepared. The catalyst shows excellent catalytic activity and high stability for the direct H2O2 synthesis from CO, O2, and H2O under mild and additive-free conditions. An H2O2 productivity of 168\u00a0mol kgcat\n\u22121 h\u22121 on the Au/ZnO-TEOS-200 catalyst has been achieved, which is two times higher than the Au/ZnO-200 catalyst without SiO2 encapsulation and, to the best of our knowledge, is also the highest value reported so far. Detailed analyses based on AC-HAADF-STEM, XPS, FT-IR, and EDX characterizations and DFT calculations reveal that such enhancement of H2O2 productivity of Au/ZnO-TEOS catalysts can be attributed to the functional confinement of the moderate SiO2 overlayer. This confinement leads to the inhibition of the Au NP agglomeration, the suppression of the H2O2 degradation at the Au/ZnO interface, and the improvement of the catalyst surface hydrophilicity. Oxygen isotope labeling experiments and DFT calculations further confirm that both oxygen atoms in the H2O2 product exclusively come from molecular O2 and that the reaction proceeds via the consecutive hydrogenation of O2. This work proposes an efficient strategy to construct highly active catalysts utilizing the confinement effect of an overlayer for the direct synthesis of H2O2 from CO, O2, and H2O and further broadens the concept of confinement to surface systems.Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Chuanming Wang (wangcm.sshy@sinopec.com).The materials generated in this study will be made available on request to the lead contact.We gratefully acknowledge financial support from the National Natural Science Foundation of China (U22B6011 and 92045303) and the China Postdoctoral Science Foundation (2020M681444).J.L. performed the catalyst preparation, characterization, and catalytic test. W.H. and C.W. performed the DFT calculations. J.L. and W.H. contributed equally to this work. Yu Wang performed the electron microscopy characterization. W.W., C.L., and Y.P. conducted the SVUV-PIMS experiment. S.L. and J.D. helped with the catalytic performance tests and discussed the results. J.L., Yu Wang, C.W., and W.H. wrote the manuscript. Yangdong Wang and Z.X. designed this study, analyzed the data, and supervised the project.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2022.101236.\n\n\nDocument S1. Figures\u00a0S1\u2013S14, Tables\u00a0S1\u2013S6, and supplemental experimental procedures\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  Due to the intensive energy consumption and environmental unfriendliness of the current industrial anthraquinone process for producing hydrogen peroxide, it is of interest to develop a clean and efficient alternative. Herein, we report that a zinc-oxide-supported gold catalyst encapsulated by a silica overlayer efficiently catalyzes\u00a0the direct synthesis of hydrogen peroxide from carbon monoxide, oxygen, and water. Detailed characterizations demonstrate that the confinement effect of the silica overlayer may enhance hydrogen peroxide productivity by limiting the agglomeration of gold nanoparticles, inhibiting the hydrogen peroxide degradation activity, and improving the hydrophilicity of the catalyst surface. Isotope-labeling experiments and theoretical calculations reveal that both oxygen atoms in hydrogen peroxide come from molecular oxygen, and that a consecutive hydrogenation process is followed. This work poses a facile strategy to construct highly active catalysts for the direct synthesis of hydrogen peroxide, employing the confinement effects of an overlayer.\n               "}
{"full_text": "Due to the increasing urgent concern about energy scarcity and environmental pollution, the use of alternatives to traditional fossil fuels has drawn increasingly attractive in recent years (Liang et al., 2022; Cipolletta et al., 2022). Biodiesel, as a sustainable, biodegradable, and clean fuel, can be produced from conventional lipid feedstocks (Ennaceri et al., 2022; Costa and Oliveira, 2022).\nHermetia illucens L. is a non-pest, which is found in warm temperate regions worldwide (Rehman et al., 2017). Commonly, insect larvae can feed on various kinds of organic matter in the biowastes (Lalander et al., 2019). Importantly, insect larvae have high levels of lipids after decomposing this organic waste (Deng et al., 2022). The sum of saturated and unsaturated fatty acids accounted for more than 90% of the total insect lipids. In recent years, the city of Wuhan in China has been producing roughly 365,000 tons of kitchen waste annually (Zheng et al., 2012). Meanwhile, approximately 3,000 tons of insect lipids will be produced. Due to society\u2019s environmental awareness and the considerably high production of insect lipids, the energy insect larvae as a renewable resource are expected to have great potential for biodiesel production (Li et al., 2015; Feng et al., 2019).Many investigations have been reported on biodiesel production by using solid sulfonated carbon-based biomass catalysts (Mansir et al., 2017; Ibrahim et al., 2020). Recently, various biomass wastes have been converted into sulfonated carbon supports for biodiesel production. In this study, in continuation of our interest in the development of renewable methodologies, the waste biomass carbon was derived from the shell of the energy insect (Hermetia illucens L.) as above. The life cycle of the insect consists of four stages: egg, larva, pupa, and adult (Proc et al., 2020). The process of pupation is the transformation from a pupa into a fly. The adult fly will crawl out from the part of the shell and then fly away (Raksasat et al., 2020). This process is similar to the cocooning process of butterflies. A large number of insect shells as residues were generated during pupation. Notably, the insect shell has a unique composition (called chitin) and can be used as a bio-waste carbon feedstock with cost-free and renewable (Guo et al., 2021). The utilization of the energy insect shell as a good candidate feedstock is expected to be highly promising for synthesizing sulfonated activated biochar.In addition, it should be noted that the metal oxide elements as promoters have been introduced into sulfonated carbon-based biomass catalysts to induce and accelerate the conversion of oils. Among these, zinc oxide (ZnO) is extensively used as a solid base catalyst because of its distinctive features and low cost (Gurunathan and Ravi, 2015; Liu and Zhang, 2011). Note that ZnO as a base metal oxide is a reactive metal promoter in the catalytic process of biodiesel. Baskar et al. (Baskar et al. 2018) conducted a conversion reaction on castor oil with high free fatty acid over a Ni-doped ZnO nanocatalyst under optimum conditions (biodiesel yield\u00a0=\u00a095.2%). As ZnO is a base promoter, a bifunctional acid-base catalyst may be synthesized by incorporating ZnO material on the surface of the support with acid sites. Furthermore, the presence of excellent catalytic activity of these catalysts with many strong acid-base sites is useful for promoting simultaneous esterification and transesterification.Until now, the sulfonated biochar-based heterogeneous catalyst prepared from the insect shell for biodiesel production has been rarely reported. Here, a surfactant methodology was adopted to meditate the mesopores of sulfonated biochar support from the outset, which ensures spatial compartmentalization of chemically distinct active sites. The molecular weight, polarity, and hydrophilicity of the introduction of polymer matters in the catalyst synthesis process influence the specific surface area and catalytic activity of the catalyst and result in composite catalyst with different morphologies and catalytic capacities (Du et al., 2019; Jeon et al., 2013). Based on the present studies, polyvinyl pyrrolidone (PVP) as a support mediator was introduced to improve the textural properties and catalytic capacity of the catalyst for the conversion reaction. This paper focused on the utilization of the insect shell for the development of a novel bifunctional acid-base catalyst (ZnO/PVPmediate-BC-S) for the conversion of the insect lipid into biodiesel. The synergistic catalysis of both active acidic and basic sites will be an efficient and reusable approach for the production of biodiesel from renewable insect lipid to biodiesel with environmentally friendly. The physiochemical characterization of the synthesized composite catalyst was investigated. The influences of the composite catalyst preparation conditions and the catalytic reaction conditions on the biodiesel yield were also investigated. More importantly, the possible catalytic mechanism of the prepared catalyst was comprehensively described. Moreover, the reusability of the prepared catalyst during five reaction cycles was demonstrated and studied to evaluate its stability. Finally, the physicochemical properties of biodiesel were also studied.The energy insect shells were collected from the Wuhan Institute of Technology (Fig. 1\n). The lipids extraction process is similar to that reported in our previous work (Feng et al., 2019). The fatty acid compositions of insect lipids were listed in Table 1\n. Besides, Zn(Ac)2, NaOH, PVP, sulfuric acid, and methanol were analytical reagents and purchased from a local supplier in China.Briefly, the insect shell was first washed and dried in an oven at 80 \u00b0C overnight. Then, the shell was milled and sieved to obtain uniform-sized particles (80 mesh screen, average pore size\u22480.18\u00a0mm). Subsequently, appropriate masses of resultant particles and PVP (5\u201345% by weight, occupy the weight of insect particles) were introduced into the distilled H2O. The resulting mixture was continuously stirred at 60 \u00b0C for 3\u00a0h. After treatment, the precipitate was separated and then calcined at different temperatures (300\u2013700 \u00b0C\uff09in a muffle furnace for 6\u00a0h. The carbonized product (denoted as PVPmediate-BC) was later further treated with sulphuric acid at 90 \u00b0C for 3\u00a0h. Immediately after that, the activated biochar was washed and dried at 100 \u00b0C overnight (denoted as PVPmediate-BC-S).Typically, the required amount of Zn(Ac)2 (Zn(Ac)2 concentration\u00a0=\u00a00.05 to 0.7\u00a0mol/L) and NaOH were dissolved in distilled water. Then the support (PVPmediate-BC-S) was immersed in the mixture solution. Furthermore, the mixture was heated at 90 \u00b0C for 3\u00a0h. Finally, the resulting solid precipitate (ZnO/PVPmediate-BC-S) was isolated from the solution and then dried at 80 \u00b0C for 12\u00a0h.The prepared solid catalyst was used to evaluate its catalytic performance in converting insect lipid into biodiesel. An appropriate amount of methanol (molar ratio of methanol/lipid\u00a0=\u00a03:1\u201315:1), catalyst (2\u201310% by weight), and 10\u00a0g of insect lipid are poured sequentially into a 250\u00a0mL flask. Then, the mixture was heated at 65 \u00b0C for 4\u00a0h. The reaction mixture was centrifuged to separate the solid catalyst after the reaction. The biodiesel yield was calculated based on a formula (1) (Zhao et al., 2018):\n\n(1)\n\n\n\nBiodiesel yield\n\n\n\n\n%\n\n\n\n=\n\n\nWeight\n\nof\n\nobtain\n\nbiodiesel\n\n\n\n\ng\n\n\n\n\n\nWeight\n\nof\n\ninsect\n\nlipid\n\n\n\n\ng\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\nThe morphologies of the prepared catalyst were investigated using scanning electron microscopy (SEM, JSM-7500, Japan). Transmission electron microscopy (TEM) images were recorded with a Tecnai G2 F20 S-TWIN microscope instrument. The structure and crystalline phase of the synthesized catalyst were determined using X-ray diffraction technique (XRD, Rigaku, Japan). The XRD patterns were recorded from 2\u03b8 of 10 to 60\u00b0. The XPS analyses were performed on an ultra-high-vacuum VG channel detector, using Al K\u03b1 radiation (1486.7\u00a0eV). Surface functional groups of the catalyst sample were tested by Fourier transform infrared spectroscopy (FTIR, American Thermo Electron) with spectrum of 400\u20134000\u00a0cm\u22121. The surface area of the prepared catalyst was evaluated by multi-point Brunauer-Emmett-Teller (BET) analysis with a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). The pore volume and pore diameter were determined by the Barrett-Joyner-Halenda (BJH) method. 1H NMR spectrum of insect lipid and biodiesel were recorded with CDCl3 in FT-NMR spectrometer (Bruker Avance II). The basicity of the prepared catalyst was measured using Hammett indicators.The catalytic reaction procedures were followed in duplicate. The experiment results were processed and analyzed by using a traditional editing software (origin pro 9.1, USA).\nFig. 2\n depicts the mesoscopic morphological characteristics of raw biochar support, PVPmediate-BC-S support, and ZnO/PVPmediate-BC-S catalysts. The presence of a few pores on the surface of raw biochar support is shown in Fig. 2a. Meanwhile, it is interesting to find that there are some discrete pores of various sizes on the surface of PVPmediate-BC-S support when PVP was used as a support mediator (Fig. 2b). The results of this study indicated that high molecular polymer mediation was effective in improving the porous properties of biochar support and was also a prominent technique for realizing hierarchical pore structure in support. However, it is clear that the pores on the support surface were blocked due to the dispersion of the ZnO materials from Fig. 2c. It can thus be concluded that ZnO materials have been successfully dispersed in the biochar-based support.The textural properties of the prepared catalyst were further investigated via N2 adsorption\u2013desorption isotherms (Fig. 3\n).The prepared catalyst shows H3-hysteresis and a typical IV isotherm, indicating the formation of a predominantly mesoporous structure (Fig. 3). A significant increase was observed at a relatively high P/P0 (0.8\u20131.0), indicating the presence of capillary structures (Fig. 3a). Furthermore, based on these observations, it is speculated that the synthesized catalyst had uniform pores. The primary requirement for an ideal conversion reaction in the presence of solid catalysts is the pore structure. The pore diameter fractions of raw biochar (BC-S) are 5\u201310\u00a0nm, according to BJH desorption isotherms (Fig. 3b). The average pore diameter and pore volume of raw BC-S were 8.79\u00a0nm and 0.0821\u00a0cm3/g, respectively (Table 2\n). Clearly, the pore diameter fractions and surface area are increased after PVP mediation. The pore size distribution revealed that the PVPmediate-BC-S support had more mesopores. The SEM micrographs also revealed these changes. Furthermore, after loading ZnO material on the support, there is a slight decrease in pore volume and pore diameter. This could be attributed to the fact that the surface of catalyst support was covered upon ZnO active components and the pores of the catalyst were blocked, which in turn caused the decrement in pore properties.The typical XRD pattern obtained from ZnO/PVPmediate-BC-S is displayed in Fig. 4\n. The peaks at 2\u03b8\u00a0=\u00a031.5\u00b0, 34.2\u00b0, 36.3\u00b0, 47.35\u00b0, and 56.3\u00b0, indicate (100), (002), (101), (102), and (110) crystalline phases in the ZnO structure (JCPDS 36-1451), respectively (Lokhande et al., 2009). The appearance of peaks associated with the synthesized biochar at 2\u03b8\u00a0=\u00a010\u201330\u00b0 and 2\u03b8\u00a0=\u00a040\u201350\u00b0 can be readily indexed to (002) and (101) crystalline phases, respectively (Wang et al., 2018). According to Aziz et al. (Aziz et al., 2015), the broad peaks (2\u03b8\u00a0=\u00a010\u00b0\u201330\u00b0) can be well assigned to those of amorphous carbon. Zhao et al. (Zhao et al., 2009) mentioned that the COC cleavage bonds of the biochar precursor were disrupted by the synergistic interaction between the carbonization process at high temperature and the sulfonation process. The sulfonated carbon became more rigid and amorphous due to the high degree of carbon disorder during this process. Hence, this structural modification of the sulfonated carbon catalyst is an important step that is responsible for the efficiency of the catalytic reaction in this study.The structural information of the synthesized catalyst has been further examined by TEM observation, and some representative images are presented in Fig. 5\n. According to Fig. 5a, the polymer functionalization approach used in this study can introduce a significant number of mesopores with worm-holes into BC support. In addition, the lattice fringes on the surface region correspond to the (110) plane of ZnO. This finding suggests that ZnO material is well loaded on the surface of the biochar-based support (Fig. 5b) (Malhotra and Ali, 2019; Kazmi et al., 2020).The FT-IR analysis of the functional group bond on the support surface was presented in Fig. 6\n. The broad absorption band at around 1426.89\u00a0cm\u22121 can generally be assigned to the aromatic ring CC stretching mode of carbonate species (Hsiao et al., 2011). Moreover, the bands at 1120\u00a0cm\u22121 are related to the symmetric stretch of the SO3H bond, present in the structure of the sulfonic groups (Zong et al., 2007). It should be pointed out that the sulfoxide group still exists on the synthesized carbon support, although the support was washed three times during the end of the pretreatment process. This finding suggests the successful grafting of -SO3H groups onto the carbon support after the sulfonation process for the application in catalytic reaction (Sangar et al., 2019). In addition, two characteristic peaks at 601.99 and 670.74\u00a0cm\u22121 were attributed to the stretching vibrations of ZnO (Zhang et al., 2009.The surface chemical composition of the prepared sample was further investigated by using XPS analysis (Fig. 7\n). The peak at 169\u00a0eV is ascribed to the sulfur of -SO3H groups (Song et al., 2020). These indicate that the SH, SO3H groups are present in the prepared composite catalyst. The two peaks in the spectrum of C 1s (Fig. 7) at 285.01\u00a0eV and 288.2\u00a0eV represent the binding energies of carbon of carbonates present in the catalyst (Pan et al., 2020). The predominant chemical states in ZnO materials are oxygen (O 1s) and Zinc (Zn 2p). There is one peak with a binding energy of 531.11\u00a0eV (O 1s) associated with the elemental oxygen of oxides present in the catalyst (Medeiros et al., 2020). Furthermore, the peak at 1022.1\u00a0eV (Zn 2p) is associated with Zn in the zinc oxide (Feng et al., 2016). These binding energies fit well with the chemical bonding structure of the composite catalyst.The synthesized catalyst was applied to the catalytic reaction of the insect lipid. Here, a series of experiments were performed to test the effects of different conditions on catalytic performance.The carbonization temperature was investigated to optimize the synthesis of catalysts for higher biodiesel yield. As seen in Fig. 8\na, the biodiesel yield increases with the increase in carbonization temperature from 300 to 600 \u00b0C (from 57.14% to 94.36%). The carbonization at higher temperature led to the desorption of CO2 from the insect shell support, producing more pores that catalyzed the transesterification of insect lipids. Additionally, the surface area of the catalyst could be affected by carbonization temperature, which consequently affects the efficiency of the catalyst. Moreover, it is also found that the catalytic capacity of the catalyst (86.68%) decreased with the further increase in the carbonization temperature. This can contribute to both the high sintering rate of the catalyst and the high energy consumption (Obadiah et al., 2012).A series of experiments were carried out by altering Zn(Ac)2 concentrations to investigate the influence of ZnO loading on the catalytic process (Fig. 8b). Remarkably, with this increasing Zn(Ac)2 concentration (0.05 to 0.7\u00a0mol/L), the biodiesel yield gradually increased from 52.66% to 94.36%. However, there was no noteworthy increase in conversion yield with Zn(AC)2 concentration beyond 0.3\u00a0mol/L. The reason for this result can be deduced from the crystallization or agglomeration of ZnO materials which resulted in poor dispersion of active components on the surface of the support (Tantirungrotechai et al., 2013). Meanwhile, according to the Hammett indicator method results, the catalyst with 0.3\u00a0mol/L Zn(Ac)2 concentration had the strongest basicity. With the catalytic activity of samples, it could be concluded that high basicity could contribute to a higher biodiesel yield.It is evident from this work as well as from literature that the polymer amounts can have a pronounced effect on pore textural properties of the prepared catalysts, and therefore play a key role in the catalytic performance. We can see that the pore size of the catalyst is primarily distributed at 10\u201319\u00a0nm for the synthesized catalyst with PVP as a structural meditation agent (Table 3\n).Since the pore size of the synthesized catalyst is larger than that of the triglyceride molecule (58\u00a0\u00c5), the triglyceride molecule would react with the active component on the surface or in the channel of support (Li et al., 2019). The biochar support by PVP-mediated had a significant influence on the catalyst, whereas the catalyst without PVP had a smaller specific surface area and pore volume (Fig. 9\n). Based on these observations, this is presumed to be due to the introduction of appropriate PVP intensifying the dispersion of active components on the surface of the catalyst (Margellou et al., 2018). The other possible reason is that the organic polymer with free radical structures and organic groups such as Lewis acids could withdraw or donate electrons (Figueiredo et al., 2007). It has been reported in the literature that the carbonyl/quinone groups on the surface of carbon are the strong active sites for oxidative dehydrogenation reactions (Pereira et al., 1999). Following a further increase in PVP amount (beyond 35\u00a0wt%), no appreciable improvement in biodiesel yield was seen. One possible explanation for this phenomenon is that the excessive organic calcination residues on the surfaces of support may lead to a significant decrease in the pore volume and pore size of support (Qu et al., 2021). This phenomenon in this study is similar to that reported in other previous literature (Guisnet and Magnoux, 2001; Moreno-Castilla et al., 2001). Nevertheless, the mechanism by which the organic polymer assisting improves catalytic performance remains unclear. Thus, further investigations are required to uncover this mechanism, and this is the next step in our future studies.The catalytic reactions were carried out by varying the loading of the synthesized catalyst (Fig. 10\na). It is interesting to notice that the conversion yield can reach 65.37% even at lower catalyst loading. The biodiesel yield increased progressively with increasing in the catalyst loading. Thus, the biodiesel yield of 94.36% could be achieved when the catalyst loading was 6\u00a0wt%. The increase in biodiesel yield with increasing loading of the catalyst was due to the increase of active sites of the catalyst participated in the conversion reaction. Moreover, the biodiesel yield decreased with increased catalyst loading by more than 6\u00a0wt%. There are two possible reasons caused the decrease in biodiesel yield at a higher catalyst loading: (1) The higher catalyst loading may lead to the increase of the viscosity in the mixture reaction system, which has influenced the mass transfer between the reactants and the catalyst (Negm et al., 2017; Hwa et al., 2015). (2) ZnO catalyst is an amphoteric composite, the saponification side reaction might have induced resulting in decreasing biodiesel yield (Santana et al., 2012).The catalytic reactions of the insect lipid were conducted at methanol to lipid molar ratio of 3:1\u201315:1 (Fig. 10b). The lowest conversion of lipid into biodiesel (34.22%) was obtained when the molar ratio of methanol to lipid was 3:1. It is evident from Fig. 10b that the biodiesel yield increases linearly with methanol amount. The biodiesel yield of lipid was 94.36% at a methanol/lipid molar ratio of 9:1. However, the biodiesel yield decreases slightly when the molar ratio of methanol/lipid exceeded 9:1. But at a higher molar concentration of methanol, there were two possibilities for reducing biodiesel yield: (1) Due to the reversible nature of the trans-esterification reaction, the equilibrium of reaction has shifted toward the backward reaction (Bhatia et al., 2020; Mutreja et al., 2014). (2) Additionally, further excess deluges the active sites of the catalyst that may hinder the protonation of fatty acid at the active sites, resulting in a decrease in biodiesel yield (Khan et al., 2020; Roy et al., 2020).To evaluate the effect of the catalytic reaction temperature, the catalytic reaction was conducted at different ranges of temperature (Fig. 10c). It is thus apparent that the biodiesel yield increased with increasing the reaction temperature. The reason for this result can be deduced from the mass transfer principle considering that the high temperature condition is helpful to increase the probability of effective collisions between the catalyst and the reactant molecules, resulting in a faster rate of mass transfer, hence resulting in increased conversion yield of biodiesel (Sahani et al., 2019). At the higher temperature (65 \u00b0C), a conversion yield of 94.36% was obtained. However, a slight decrease in the biodiesel yield was observed when the reaction temperature exceeded 65 \u00b0C under the same reaction conditions. This phenomenon occurred because of the vaporization of the excess methanol at the optimal temperature (Saravanan et al., 2015).The effect of alcohol types in the conversion reaction is shown in Fig. 10d. Using the prepared catalyst, the biodiesel yield decreases with increasing alcohol molecular weight. The order of the conversion capacity is methanol, ethanol, propanol, isopropanol, and tert-butanol. The reason was ascribed to the fact that the larger alcohol chains have inferior nucleophilicity. By comparing the decomposition capacity of other alcohol, methanol with more nucleophilicity of methoxide ion decomposes more quickly (Basumatary et al., 2021). In addition, larger chains have a greater sterile hindrance, which prevents fatty acids from interacting with the catalyst (Araujo et al., 2019).As regards the homogeneous catalyst, the most intriguing feature of this heterogeneous catalyst is that it still had high catalytic activity and stability after successive reaction cycles. Therefore, further recycling experiments were performed using the synthesized catalyst under the defined conditions to evaluate the reusability of the catalyst (Fig. 11\n). Upon completion of each reaction, the solid was collected and directly reused in subsequent cycles. The second catalytic reaction was carried out under the previous same conditions and a conversion yield of 88.66% was obtained. This procedure was repeated five times. It is noticeable that there appears to be no significant decrease in biodiesel yield for each repeated experiment. There are three possible reasons caused the decrease in the activity of the catalyst. (1) The loading of catalyst used in the next cycle was lower than the initial cycle, which might partly be responsible for reducing biodiesel yield during the recycling experiments. (2) The produced by-product glycerol covered the surface of the catalyst and blocked the active sites of the catalyst (Gohain et al., 2020). (3) The leaching of active components of the catalyst would lead to the decrease of catalytic activity during the recycling experiments (Khan et al., 2020).To further explore the feasibility of the synthesized catalyst, the characterizations of SEM and TEM were carried out for the fresh and reused catalyst (five cycles). The SEM (Fig. 12\na) micrograph shows that the synthesized catalyst retains its bulk-like nanostructure. Compared with Fig. 2(c) and Fig. 5(a), it is likely that the surface and porous structure of the synthesized catalyst changed slightly. The change in the surface and porous structure of the synthesized catalyst was due to the catalyst being poisoned by the irreversible adsorption of the free fatty acids on the active sites, which destroys the pore structure and increases the mass transfer resistance (Kwong and Yung, 2016; Li et al., 2022).Meanwhile, combining Fig. 13\n(a) with Fig. 13(b), there was no obvious apparent difference in the structure and the physicochemical properties of the reused catalyst after five cycles. However, there was a significant decrease in the peak intensity of ZnO after the reused reaction, implying the loss of active components during each reaction. Besides, the newly detected functional groups (2920 and 2850\u00a0cm\u22121, CH stretching vibration of glycerol molecule) on the reused catalysts, may block the active sites and result in reduced activity on the reused catalyst (Yusuff et al., 2021). Consequently, the biodiesel yield decreased as the reaction cycle increased. Further work is ongoing to optimize the composite catalyst to limit the decrease in the yield of biodiesel in the subsequent cycles of use.In comparing the catalytic activity of those previously reported catalysts, it is clear that the synthesized catalyst has better stability and reusability for biodiesel production in the current work (Table 4\n). For instance, biodiesel yield reduced from 94.36% to 47.52% after the fifth cycle when corncob biomass waste-based acid catalyst was utilized as an efficient catalyst in the transesterification of palm fatty acid (Tang et al., 2020). Macawile et al. reported that biodiesel yield decreased from 93.10% to 58.09% after the third cycle of reuse of the novel modified solid acid catalyst (Macawile et al., 2020).The ZnO materials were loaded on the sulfonated biochar-based support in this study. As both acid and base groups are present on the surface of the biochar support, both acid-base mechanisms are likely to occur. Fig. 14\n depicts the proposed mechanism simplified for the catalytic reaction.Acid catalysis: The first step involves protonation of the carbonyl group of insect lipids by the protons generated from the acidic sites. The sulfonate groups (-SO3H) on the framework of the prepared catalyst have labile hydrogen protons that can be easily migrated to attack the most nucleophilic centers of lipids (CO) to form the protonated form of lipids (Macawile et al., 2020). Then, the electron-rich hydroxyl groups of methanol would nucleophilic attack the cationic centers on protonated lipid (Betiha et al., 2020). The unstable protonated lipid-methanol ether complex formed undergoes two successive stabilization steps. The first includes the loss of water molecules to produce the protonated lipid-methanol ester intermediate. The second involves the conversion of the protonated lipid-methanol ester intermediate into the corresponding lipid-methanol ester by losing protons, which are combined with the ionized acidic catalysts to retain their chemical structures (Khan et al., 2021).Base catalysis: The two types of esterification and transesterification simultaneously took place on the surface of the biochar-based catalyst. ZnO as a Br\u00f8nsted base was loaded on the support during the catalysis process. A part of methanol and triglyceride reactants may be adsorbed at different Zn active sites on the ZnO catalyst surface. Thus, it was reasonable that the interaction of the ZnO catalyst and triglyceride would form the bond of Zn and -O-CH2. The presence of ZnO in the chemical structure of the catalyst has a high influence on its alkalinity (Kouzu et al., 2009; Putra et al., 2018). The fatty acid methyl ester is then produced and formed diglyceride. The diglyceride further reacted with methanol to produce the object product (fatty acid methyl ester) and intermediate (monoglyceride). In the meantime, the monoglyceride is further involved in the reaction to form fatty acid methyl ester and by-product (glycerol). Finally, the by-product glycerol may be desorbed from the active sites of the biochar-based catalyst. In this way, the catalyst can be reused in subsequent reactions.\nFig. 15\na shows the FTIR spectra of insect lipids and biodiesel. The absorption band at 3400\u00a0cm\u22121 for insect lipids, which indicates the occurrence of (OH) stretching vibration of carboxylic acid, alcohol, or phenol (Ayoob and Fadhil, 2019). Interestingly, the observed absorption band disappears in the FTIR spectra of biodiesel. Its absence in biodiesel indicates that esterification has been highly effective. As shown in Fig. 14a, the biodiesel spectra have two sharp and broad peaks at 2920\u00a0cm\u22121 and 2851\u00a0cm\u22121, respectively. The bands at 2920\u20132851\u00a0cm\u22121 are due to symmetric and CH asymmetric stretching vibrations (Falowo et al., 2020; Falowo and Betiku, 2022). The CO stretch vibration at 1745\u00a0cm\u22121 of insect lipid shifts to 1742\u00a0cm\u22121 in biodiesel, which represents the conversion reaction (Ayoob and Fadhil, 2020). The peak at 1461\u00a0cm\u22121 corresponds to the bending of CH2 (Li et al., 2015). A peak at 720\u00a0cm\u22121 was observed and assigned to the vibration of the CH2 bond from the long fatty acid chain (Li et al., 2019). The existence of major absorption bands proved that biodiesel was produced in this work. The results obtained in this study were in good agreement with those reported in the previous study (Tsanaktsidis et al., 2013).The synthesis of biodiesel from insect lipids was characterized by the 1H NMR spectra (Fig. 15b). The major differences in signals indicated the conversion of insect lipids to biodiesel. In Fig. 13b, the peaks present at 4.28\u00a0ppm and 5.33\u00a0ppm correspond to the glyceridic protons and olefinic protons (CHCH) in insect lipids, respectively (Fadhil et al., 2019). A new singlet observed on the spectra around 3.57\u00a0ppm indicates the appearance of methoxy protons (COOCH3) (Li et al., 2015). Instead, the glyceridic peaks near 4.28\u00a0ppm completely disappeared. The results observed here illustrated that the insect lipid was successfully converted to biodiesel. The signals at 2.30\u00a0ppm in the 1H NMR spectrum of insect lipid and 2.22\u00a0ppm in the 1H NMR spectrum of biodiesel are due to \u03b1-CH2 protons to the ester (CH2\nCO2R) groups present in the insect lipid and biodiesel, respectively (Macina et al., 2019). The 1H NMR signals of the methylene (CH2)n\nprotons of the fatty acid chain in the insect lipid and biodiesel are observed at 1.25\u00a0ppm and 1.18\u00a0ppm, respectively (Nath et al., 2019). The terminal methyl protons (CH3) of the triglycerides and biodiesel are indicated by the 1H NMR signals at 0.87\u00a0ppm and 0.81\u00a0ppm, respectively.The physicochemical properties of the biodiesel sample obtained from the conversion of the insect lipid over the prepared catalyst were determined and listed in Table 5\n. It can be noted that most of the fuel properties of biodiesel produced in this paper meet the specifications of the ASTM standard. Biodiesel with low viscosity is preferable for better combustibility in the engine. The viscosity reported in this study was well below the maximum limits of the ASTM standard. It is interesting to note that measurements of other fuel properties (acid value, density, and moisture content) from insect lipids derived from biodiesel were similar to the previously reported results.In summary, a novel biochar-based heterogeneous catalyst (ZnO/PVPmediate-BC-S) with acid-base bifunctional catalytic capacity was successfully synthesized by using PVP as a support mediator for the conversion of the insect lipid into biodiesel. The results of these experiments suggested that the biodiesel yield was 94.36% at the defined condition. As a mediator, PVP has enhanced the interaction between active components and support in the catalyst synthesis process. The appropriate PVP amount will be beneficial for the catalyst to have greater structure and crystallography of support, to maintain better catalytic activity. Meanwhile, the biodiesel yield did not decrease significantly after five cycles of reuse, indicating excellent stability and reusability of the prepared catalyst. Most of the fuel properties of the prepared biodiesel in this paper meet the ASTM standard specifications. This work demonstrates that biodiesel production with high yield catalyzed by inexpensive and facilely fabricated biochar support from the waste insect shell has tremendous potential for large-scale production and practical applications.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the Open Project of Beijing Key Laboratory for Enze Biomass and Fine Chemicals, Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing (No. 20210005), Hubei Key Laboratory of Novel Reactor and Green Chemical Technology (No. NRGC202209).", "descript": "\n                  This paper focused on the utilization of the waste insect shell for the development of a novel biochar-based heterogeneous catalyst (ZnO/PVPmediate-BC-S) with a highly acid-base bifunctional catalytic capacity for the conversion of the insect lipid into biodiesel. The introduction of polyvinyl pyrrolidone (PVP) as a support mediator was believed to improve the textural properties of support and catalytic activity of the catalyst for the conversion reaction. Meanwhile, the physicochemical properties of the synthesized composite catalyst were characterized with XRD, SEM, TEM, XPS, BET, and FT-IR analysis. The high biodiesel yield (94.36%) was obtained at the defined condition (carbonization temperature\u00a0=\u00a0600 \u00b0C, Zn(Ac)2 concentration\u00a0=\u00a00.3\u00a0mol/L, PVP amount\u00a0=\u00a035\u00a0wt%, reaction temperature\u00a0=\u00a065 \u00b0C, catalyst loading\u00a0=\u00a06\u00a0wt%, methanol/lipid molar ratio\u00a0=\u00a09:1). Moreover, the possible catalytic mechanism of the prepared catalyst was comprehensively described. In addition, the stability and reusability of the prepared catalyst during five reaction cycles were also demonstrated. Finally, the physicochemical properties of the biodiesel studied were well comparable with the ASTM standard as well as with the reported literature.\n               "}
{"full_text": "The progress of human society depends on the development of industry, and the latter has increasingly resulted in the production of several harmful and poisonous pollutants that are difficult to degrade in sewage. In recent years, pharmaceuticals, personal care products [1,2], and endocrine disruptors [3] that adversely affect human health and ecological environment have attracted increased research attention. Therefore, efficient methods for wastewater treatment are necessary to achieve the goal of clean production and promote the sustainable development of human society.Several organic pollutants are difficult to remove effectively using conventional water treatment technologies [4\u20136], and the average removal of many organic pollutants, including atrazine, in sewage treatment plants is less than 50% [7]. To resolve this problem, advanced oxidation technologies are emerging with the rapid development of wastewater treatment technologies. Advanced oxidation processes are powerful and efficient methods to degrade the pollutants in water. Among these methods, sulphate radical-based advanced oxidation processes have attracted considerable research interest owing to their high redox potential and selectivity for oxidation [8,9]. The activation of peroxymonosulfate (PMS) can be accomplished using techniques, such as thermal activation, photoactivation [10], ultrasonic irradiation, electrochemical methods, homogeneous metal-ion catalysis, and heterogeneous catalysis [11,12]. In recent years, heterogeneous catalysis has been widely studied owing to its high efficiency and less secondary pollution [8]. Currently, semiconductors, transition metals, and metal-free materials are widely used to activate PMS [13,14]. In addition, the development of magnetic heterogeneous catalytic materials resolves the problem of material separation in aqueous solutions and improves the possibility of practical use [15\u201317].The development of heterogeneous catalytic oxidation materials encounters several problems: The use of precious metals makes the materials expensive; the catalysts are difficult to separate from the aqueous environment [18], and the recycling effect is limited [19]. In general, the development of materials is a trade-off between their cost and efficiency. Although the adsorption process is simple and economical, it does not resolve the fundamental problem of pollution in wastewater treatment. In contrast, the advanced oxidation technologies combined with the adsorption process could be more economical and efficient techniques that further promote clean and sustainable development. Several studies have investigated the application aspects of the adsorption\u2212catalytic oxidation process. For example, Wang et al. [20] observed that the adsorption\u2212degradation cycle was conducive to the removal of the bisphenols. Peng et al. [21] demonstrated that the synergistic effect of the adsorption and catalysis on Fe/Fe3C@NG achieved an efficient removal of norfloxacin (Nor).Metal\u2013organic frameworks (MOFs) were selected as the potential adsorbents and heterogeneous catalytic materials owing to their large specific surface area and variable reaction sites [22,23]. MOFs are three-dimensional ordered porous materials formed by metal ions and organic ligands [24]. MOFs are also called porous coordination polymers (PCPs), and are widely used in gas storage [25], catalysis [26], adsorption [27], chemical sensing [28], drug transport [29], semiconductors [30], and biomedical imaging [31]. Moreover, many researchers have used MOFs as templates or precursors to synthesize carbonaceous materials or metal composites [32\u201335] to investigate their applications. MOFs-based carbon composites that are a combination of metal composites and carbon, exhibit superior potential in adsorption and heterogeneous catalysis [36,37].However, stability is an important factor for all heterogeneous catalysts. Therefore, the practical applications of MOFs are controlled by their recycling performance and stability. Among all the reported MOFs, MOF-5 is one of the most typically used materials that exhibits open-skeleton structure, large pore surface area, and good thermal stability [38]. However, MOFs comprising divalent metal centers and multi-carboxylate ligands, such as MOF-5, are sensitive to water and can collapse in aqueous environment [39], making them less competitive in wastewater treatment. Considering that the ligands bind to nickel ions in a more stable manner than to zinc ions, the doping of MOF-5 with nickel ions can improve its stability in aqueous environment. Thus, the nickel-doped MOF-5 can be used in wastewater treatment [40]. Moreover, the addition of nickel to MOF-5 and its subsequent calcination yields a magnetic composite that facilitates the solid\u2013liquid separation and its subsequent regeneration, as well as resolves some of the problems encountered in the development of heterogeneous materials.We prepared a magnetic heterogeneous catalyst, denoted as ZN-CS, via a previously reported hydrothermal synthesis method [41]. The removal of rhodamine B (RhB) was selected as the model process to investigate the proposed mechanism and the coupling effects. Furthermore, the removal of different target pollutants (acid orange 7 (AO7), methylene blue (MB), tetracycline hydrochloride (TC), and Nor), and the factors affecting the degradation of RhB were studied. Finally, the analysis results of scanning electron microscopy (SEM), Brunauer\u2013Emmett\u2013Teller (BET) analysis, powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and electron paramagnetic resonance (EPR) analysis and the quenching experiments demonstrated that the degradation of the absorbed pollutants enabled the regeneration of the active sites, contributing to a high recycling performance. Compared with the systems used in some previous studies, this system did not use any precious metals. Moreover, this system employed the adsorption\u2013degradation process to achieve a balance between the economic and treatment effect. Additionally, the synthesized catalyst exhibits magnetic properties, recyclability, stable structure, and good removal efficiency for a variety of organic matter. The adsorption\u2212interpretation coupling process provides a new approach for the development of catalytic materials with adequate adsorption performance.Ethylene glycol, zinc nitrate hexahydrate (Zn(NO3)2\u00b76H2O), N,N-dimethylformamide (DMF), methanol, tert-butanol (TBA), ethanol, nickel nitrate hexahydrate (Ni(NO3)2\u00b76H2O), RhB, anhydrous sodium sulphate, and potassium hydrogen phosphate (K2HPO4) were obtained from Chengdu Kelong Chemical Reagent Co., Ltd. (China). p-Phthalic acid (H2BDC), AO7, Oxone (PMS), and TC were obtained from Aladdin Chemistry Co., Ltd. (China). Nor and MB trihydrate were obtained from TCI (Shanghai) Development Co., Ltd. and Sinopharm Chemical Reagent Co., Ltd. (China), respectively. Ultrapure water was used to prepare all the aqueous solutions. All chemicals used in the experiments were of analytical grade.The core\u2013shell ZN-CS nanocomposite was prepared using a previously reported method [41] with some modifications. First, 0.75 g each of Zn(NO3)2\u00b76H2O and Ni(NO3)2\u00b76H2O were added to the solvent mixture (75 mL ethylene glycol and 120 mL DMF). The resulting sample was stirred under magnetic stirring till the solids dissolved completely. Subsequently, 0.45 g of H2BDC was dissolved in the prepared solution. The solution was placed in a Teflon-lined stainless-steel autoclave at 150 \u00b0C for 6 h. The contents were collected through centrifugation, purified with ethanol and DMF, and subsequently dried in a blast drying oven at 100 \u00b0C overnight. The sample thus obtained was calcined at 450 \u00b0C in a tube furnace under a nitrogen atmosphere for 20 min, washed with deionized water. and finally dried to obtain ZnO@Ni3ZnC0.7. The high structural stability of the synthesized catalyst (denoted as ZN-CS) was confirmed using XRD and XPS analysis.The RhB concentration was analyzed using a spectrophotometer (MAPADA UV-1800PC, China) with maximum absorption wavelength of 554 nm. The N2 adsorption/desorption isotherms were obtained using a QuadraSorb Station 2 at \u2212196 \u00b0C. The zeta potential of the ZN-CS surface was determined using a zeta potential analyzer (Nicomp Z3000, USA). The surface morphologies and atomic composition of the newly prepared and used catalysts were analyzed using a JSM-5900LV scanning electron microscope (JEOL. Ltd. Akishima, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. The XRD patterns were obtained using an X\u2019 Pert Pro MPD DY129 X-ray diffractometer. Infrared spectra were obtained using FT-IR (Nicolet 6700, Thermo Scientific, USA).The adsorption performance of the ZN-CS towards RhB was studied by an extra batch-adsorption experiment in a glass beaker at 20 \u00b0C. The catalysts were withdrawn at the pre-determined intervals and immediately separated by Whatman GF/F glass-fiber membranes to measure the residual RhB concentration. To evaluate the activation ability of the ZN-CS towards PMS, catalytic experiments were conducted with the pristine ZN-CS in a 500 mL glass beaker at room temperature. Because ZnO has been widely studied as a semiconductor photocatalyst [42], we conducted a control experiment under dark conditions to eliminate the effect of light. The results thus obtained exhibited no significant difference (Fig. S1 in Appendix A). Therefore, the subsequent experiments were conducted under indoor light conditions. Before the addition of PMS, different dosages of the catalysts were dispersed in a 200 mL RhB solution, which was stirred for approximately 15 min to achieve the adsorption equilibrium. The degradation reaction was triggered by adding the desired amount of PMS. The samples were withdrawn, and filtered at certain time intervals to determine the residual pollutant concentration. The blank test without the catalyst was conducted under the same conditions. PMS was the principal source of hydroxyl and sulphate radicals that are essential for the degradation process. Therefore, to investigate the effect of the initial PMS concentration, experiments were carried out using PMS concentrations in the range 100\u2013400 mg\u00b7L\u22121. The experimental results indicated a noticeable increase in the RhB removal with 200 mg\u00b7L\u22121 PMS. Therefore, the subsequent experiments were performed using the PMS concentration of 200 mg\u00b7L\u22121. The effect of the catalyst dosage was evaluated at 25, 50, 100, and 150 mg\u00b7L\u22121. Additionally, the effect of the initial RhB concentration was investigated.To study the contribution of the reactive species, methanol and TBA were used as the radical scavengers. To observe the effect of the reactive sites, dipotassium phosphate was used to mask them. The used catalysts were washed with ultrapure water and dried at 100 \u00b0C overnight. The recycling experiments were carried out at [RhB]0 = 3.40 mg\u00b7L\u22121, which was equal to the concentration of RhB after adsorption by the pristine ZN-CS at [RHB]0\u2019 = 7.6 mg\u00b7L\u22121; all other steps remained the same. All the experiments were carried out twice or thrice, and the average data with their standard deviations were presented.The ZN-CS exhibited a strong adsorption affinity for RhB before the addition of PMS, with over 50% removal of RhB in 15 min (Fig. 1\n). The pH change of the solution during the removal process and all the kinetic results are shown in Fig. S2, Text S1 and Tables S1 separately. Additionally, the adsorption rate of RhB increased gradually, probably owing to both the decreasing RhB concentration in the aqueous phase and the gradual exhaustion of the adsorption sites. PMS was added to the solution to initiate the reaction when the adsorption equilibrium was reached. After 30 min, approximately 90% RhB was eliminated in the ZN-CS/PMS system, while only 8% RhB was removed in the PMS system. Moreover, the ZN-CS exhibited the best removal efficiency among the precursor and the catalyst with single metal (Appendix A Fig. S3). Additionally, the PMS concentration decreased rapidly in the beginning, and the decrease became gradual with time (Appendix A Fig. S4). The rapid consumption of PMS at the beginning was probably owing to the adsorption or some binding effects with the catalyst. Subsequently, the activation of PMS became gradual because of the saturation and depletion of the active sites. Thus, the material can adsorb RhB, and activate PMS for further degradation of the substrate. The coupling effect of adsorption\u2212degradation presents certain practical application potential (Fig. S5). In the following analysis, the elimination process of RhB could be separated into two stages, adsorption and degradation. The possible mechanisms of both the stages were proposed.To confirm the crystallographic structure, phase purity, and structural stability of the ZN-CS, XRD patterns of the pristine and used samples were recorded (Fig.\u202f2\n(a)). The results demonstrated that the catalyst comprised ZnO (Joint Committee on Powder Diffraction File (JCPDF) #89-0510) and Ni3ZnC0.7 (JCPDF #28-0713). The distributions of ZnO and Ni3ZnC0.7 in the shell and core were approximately uniform (Appendix A Fig. S6 and Table S2). This indicated that the sample was of high purity, and no other crystalline impurities were detected. Additionally, the phase of the used sample was confirmed by XRD analysis. The phase of the obtained catalyst remained unchanged during the process. As can be confirmed from the wide XPS spectrum (Fig.\u202f2(b)), the ZN-CS comprised four elements\u2014Zn, Ni, C, and O. This result was consistent with those obtained from XRD analysis. The high-resolution Zn 2p spectrum (Fig.\u202f2(c)) revealed two components: ZnO with binding energies of 1024.3 and 1047.6 eV, as well as Zn\u2212Ni with two peaks positioned at 1021.7 and 1043.5 eV. The high-resolution Ni 2p spectrum (Fig.\u202f2(d)) revealed two components: Ni(0) at 852.3 and 869.5 eV, and Ni2+ at 854.8 and 872.2 eV. Two shake-up satellite peaks at 859.6 and 879.1 eV were also observed. In general, the two forms of metals corresponded to the two main components\u2014 ZnO and Ni3ZnC0.7\u2014 in the XRD analysis. The formation of Ni2+ occurred possibly because of the surface oxidation of Ni. In addition, the relative content of ZnO slightly increased from 20% to 30% after the degradation process, indicating that Zn was partially oxidised and thus acted as an electron donor. There was no remarkable change in the valence ratio of Ni. This indicated that the contribution of metal gain and loss electrons to degradation was not significant.There are some classical explanations for the adsorption mechanism, including physical and chemical adsorption. Physical adsorption mainly involves the van der Waals forces and electrostatic attraction. In contrast, chemical adsorption involves the formation of chemical bonds, either by transfer or sharing of electrons, between the adsorbent molecules and the atoms or molecules on the solid surface of absorbent [43\u201345]. To determine the adsorption mechanisms, several experiments and theoretical calculations were conducted.\n(1)\nPhysical adsorption capacity\nTo determine the physical adsorption capacity of the samples, we examined their surface morphologies and atomic composition using SEM-EDS, and calculated their specific surface area and the average pore diameter by the nitrogen adsorption/desorption experiment. The synthesized catalyst exhibited a sphere-like morphology with a core\u2013shell structure. Fig.\u202f3\n(a) presents an image of the pristine catalyst. The catalyst surface was loose and porous, with the external shape similar to that of Hydrangea macrophylla. Fig.\u202f3(b) illustrates the electron micrograph of the catalyst magnified to 2000 times. The particle size of the catalyst was uniform, and the shell structures of few particles were damaged. Agglomeration in the range of approximately 2\u22124 \u03bcm within the particles was observed. After the adsorption, the pore channels were filled, with further aggregation of the particles (Figs.\u202f3(c) and (d)). The catalyst shape did not change remarkably, and the core\u2013shell structure remained stable (Figs.\u202f3(e) and (f)) after the degradation. This was consistent with the results of XRD analysis. Owing to the continuous deposition of the surface materials, the surface pores were filled, and the particle surface was gradually passivated. This can be expected to result in a decrease in the adsorption capacity. with subsequent release of the active sites in the degradation process. Thus, the adsorption ability was regenerated for reuse. The bars in Figs.\u202f3(d) and (f) were considered to be the impurities introduced in the recycling process. Additionally, the results of EDS analysis (Table 1\n) indicated changes in the oxygen-containing functional groups, which will be explained in the collaborative analysis of the FT-IR characterization later.As illustrated in Fig. 4\n, the N2 adsorption\u2013desorption isotherms were identified as type II with a type H3 hysteresis loop [46]. This was owing to the presence of large pores formed by the accumulation of flaky particles, and was consistent with the morphology of the precursors. The specific surface area of the sample calculated using BET analysis was 55.311 m2\u00b7g\u22121. As can be observed from the pore-size distribution diagram, the average pore diameter of the ZN-CS was less than 20 nm. The large specific surface area and narrow pores may also contribute to the enrichment of RhB and potentially provide enough active sites for the heterogeneous reaction process.\n(2)\nElectrostatic attraction\nThe electrostatic factor may also play an important role in the adsorption process [47] as discussed here. We measured the zeta potential of the catalyst to determine its charge properties at different pH levels. The pH value at the point of zero charge (pHPZC) of the catalyst in the reaction system measured by zeta potential analyzer was approximately 7.5 (Appendix A Fig. S7). This result could be discussed from the following two aspects. First, the catalyst surface was negatively charged, and the negative charge increased with the pH value at pH > 7.5 for the ZN-CS. Moreover, when the pH was less than 7.5, the surface became positively charged, and the positive charge increased as the pH value decreased. The pK\na of RhB is 3.0 and its K\nOW is 190 [48]. For pH > 7.5, 90% of the carboxylic acid molecules on RhB dissociated, and the number of the amphoteric ions (those containing the carboxylate ion and quaternary ammonium cation) of RhB increased with the pH value. For pH < 7.5, the carboxylic acid dissociation of RhB decreased with decrease in the pH value.Thus, an increase in the pH value was conducive to the improvement in the electrostatic attraction between the catalyst and quaternary ammonium cation of RhB. In addition, the electrostatic repulsion between the catalyst and carboxylate ion on RhB increased with the increase in the pH value. As can be observed from Appendix A Fig. S8 an improved adsorption ability was obtained with the pH value of 3.02 or 8.96, both at severe conditions. The possible reason is that the greater charge on the catalyst led to stronger electrostatic attractions under the abovementioned conditions.In addition to the physical adsorption between RhB and the ZN-CS, the surface complexation that involved chemical bonding and contributed to the adsorption process is also discussed here. As the functional groups played a vital role in the chemical bonding between the absorbent and adsorbate, FT-IR analysis of the catalyst was conducted to determine the main functional groups involved in the adsorption process. The samples were dried at 100 \u00b0C overnight to decrease the interference of the bound water with the absorption peak. The band at 750 cm\u22121 was assigned to the bending vibration of O\u2212H (\u03b3 O\u2212H) (Fig. 5\n). The broad band observed at approximately 3425 cm\u22121 was attributed to the stretching mode of O\u2212H (\u03bd O\u2212H) owing to the presence of hydroxyl [18], and the decline in the band intensity might be attributed to the consumption and regeneration of the surface hydroxyl groups. Furthermore, when the catalyst was used to adsorb RhB, the abovementioned peak underwent a blue-shift of 5 cm\u22121 (from 3425 to 3430 cm\u22121), which indicated that RhB bonded with the catalyst by replacing the O\u2212H groups on the surface of the oxide [49,50]. Additionally, because of the vibration of the aromatic rings [51,52], a new peak at 1178 cm\u22121 was observed for the RhB-adsorbed sample. These results indicated that the adsorption mechanism involved the surface complexation between RhB and the ZN-CS. To substantiate the role of chemical adsorption, we used phosphate to mask the hydroxyl groups on the surface of the ZN-CS, as phosphate exhibits stronger affinity for this adsorption site [53]. The results revealed that the adsorption capacity decreased by approximately 10% in presence of the masking agent (Appendix A Fig. S9). This indicated that the hydroxyl groups were involved in the chemisorption.The study of adsorption kinetics is essential to elucidate the adsorption mechanism. Therefore, we calculated the kinetic data of the adsorption process using pseudo-first-order [54] and pseudo-second-order [55,56] simulations (Appendix A Text S2). The calculated kinetic data (Table 2\n) revealed that the adsorption process was better described with the second-order kinetics, indicating that the chemisorption was the rate-determining step [56].In addition, the fitting results of different adsorption models demonstrated that the adsorption process can be best described with the Freundlich model and the sorption of RhBon the ZN-CS surface was essentially chemical (Appendix A Text S3 and Tables S3). The values of thermodynamic parameters (\u0394G, \u0394S, and \u0394H) revealed that the adsorption of RhB on the ZN-CS surface was spontaneous, feasible, and exothermic (Appendix A Fig. S10, Text S4 and Tables S4). In conclusion, the adsorption process was mainly determined by the van der Waals forces, electrostatic attraction, and the surface complexation of the hydroxyl groups, with chemisorption being the rate-determining step.To determine the reactive species involved, different quenchers were used, and their contribution to the RhB degradation was investigated (Fig.\u202f6\n(a)). Methanol and TBA were used to quench the \n\nS\n\nO\n4\n\n\n-\n\u2219\n\n\n\n and \n\nH\n\nO\n\n\n\u2219\n\n\n\n radicals [57\u201359]. However, stronger inhibition effect was observed after the addition of TBA (Fig.\u202f6(a)), which was contrary to what was expected. This might have resulted from the high viscosity of TBA [60]. Therefore, additional experiments were required to be conducted to identify the active species.EPR analysis was carried out to determine the responsible radical species using dimethyl pyridine N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP) as the spin-trap agent. The characteristic peaks corresponding to the DMPO-OH adducts and feeble signals corresponding to the DMPO-SO4 adducts were observed in the PMS/ZN-CS system (Fig.\u202f6(b)). As the capture of the trace sulphate radical in the actual detection was difficult, the signal corresponding to the DMPO-SO4 adducts was weak, and had the same height as that of the noise signal. Moreover, no signal corresponding to \n\nO\n2\n\n\n\u2219\n-\n\n\n was detected. These results were consistent with some previous observations [61\u201363]. Considering the production of the singlet oxygen during the self-decomposition of PMS [64], we conducted a controlled experiment using TEMP as the capture agent to determine if the catalyst could promote the production of the singlet oxygen. Consequently, the signal strength of the PMS system was similar to that of the PMS/ZN-CS system within the error range. This indicated that the production of the singlet oxygen cannot be facilitated in the PMS/ZN-CS system. Moreover, the self-decomposition of \n\nS\n\nO\n5\n\n\u00b7\n-\n\n\n\n\nradicals can readily proceed owing to its high reaction rate (\n\u2248\n 2\u202f\u00d7\u202f108\nM\u22121\u00b7s\u22121) and low activation energy (E\na\u202f=\u202f7.4\u202f\u00b1\u202f2.4\u202fkcal\u00b7mol\u22121), resulting in the fast generation of 1O2(Appendix A \n\nT\ne\nx\nt\nS\n5\n,\nE\nq\ns\n.\n(\nS\n14\n)\na\nn\nd\n(\nS\n15\n)\n\n) [65\u201368]. Thus, it can be concluded that radicals (mainly \n\nH\n\nO\n\n\n\u2219\n\n\n\n) were generated in the ZN-CS/PMS system and that these radicals played an important role in the degradation of RhB. In the radical pathway, Zn and Ni were involved in the direct redox process of PMS; the possible reaction is given in \n\nT\ne\nx\nt\nS\n6\n,\n\n Eqs. (S1 \n\n6\n-\nS\n19\n\n) in Appendix A [69\u201371]. However, the effect on the degradation rate was small, owing to the masking of the \n\nS\n\nO\n4\n\n\n-\n\u2219\n\n\n\n and \n\nH\n\nO\n\n\n\u2219\n\n\n\n radicals by methanol. This suggests a more dominant mechanism of degradation.Ding et al. [72] summarized the methods for the estimation of the contribution rates of the radical and non-radical processes (Appendix A Text S7, Eqs. (S20\u2212S22)). The results revealed that the contribution rate of the radical process was approximately 34.1%, and that of the non-radical process was approximately 65.9%. This indicates that the non-radical process played an important role in this system.In recent years, the mechanism of the indirect oxidation of pollutants by oxidants has been proposed. Increased attention has been paid to the direct electron transfer between the pollutants and high-potential intermediates formed by the carbon materials and oxidants. Ren et al. [73] suggested that peroxydisulfate (PDS) can be catalyzed by carbon nanotubes (CNTs) to form a high-redox potential composite to degrade organic compounds directly. Based on his research, we used the catalyst as electrodes to confirm the formation of the high-potential intermediates (Appendix A Text S8), and monitored the open-circuit potential by chronopotentiometry analysis. The open-circuit potential increased remarkably after the addition of PMS (Fig.\u202f6(c)), indicating that the catalyst and PMS combined to form the high-potential intermediate (denoted as ZN-CS*PMS). The gradual decrease in the potential can be attributed to the consumption of highly potential-active substances. Subsequent supplementation with PMS can aid in the recovery of the potential (Fig.\u202f6(d)). This indicates the potential for the direct oxidation ability of ZN-CS*PMS.Additionally, it is essential to determine the active sites on the catalysts to elucidate the mechanism. To determine the active sites that activate PMS, changes in the functional groups of the catalysts after degradation were analysed. Fig. 5 illustrates the FT-IR spectra of the ZN-CS in the range 4000\u2212400 cm\u22121. Five distinct adsorption bands were identified at approximately 460, 750, 1137, 1383, 1570, and 3425 cm\u22121. As mentioned earlier, owing to the presence of the hydroxyl [18], the increased intensity of the broad band at approximately 3425 cm\u22121 indicated the regeneration of the O\u2212H in the degradation process. The red shift of this band after the addition of PMS indicated that the complexation between RhB and the ZN-CS was destroyed, and that the RhB adsorbed on the catalyst surface was partially degraded. After the reaction, the decrease in the absorption band at approximately 460 cm\u22121 (corresponding to the stretching of Zn\u2212O bond) [74\u201377], indicated that ZnO was either consumed or leached. Finally, the remaining three spectral lines were almost the same, confirming the stability of the ZN-CS. To confirm the role of the surface hydroxyl groups, a masking experiment was conducted using a phosphate with stronger affinity [53]. The RhB degradation exhibited a remarkable inhibition (Fig.\u202f6(a)), confirming the role of the surface hydroxyl groups in the PMS activation. ZnO is a semiconductor that contains numerous mobile electrons and exhibits good capacitance characteristics [78]. It can transfer and store electrons, and is conducive to the electron transfer and conduction between the ZN-CS*PMS intermediates and pollutants. In addition, Ni3ZnC0.7 exhibits good electrical conductivity [79] and electron-transfer ability. Thus, it can be assumed that Zn and Ni play an important role in the electron transfer between the ZN-CS*PMS and organic contaminants in the non-radical pathway. Therefore, Zn and Ni may act as electron donors and carriers in the free radical process, whereas in the non-radical process, both of them mainly contribute to the electron conduction. Thus, a possible degradation mechanism with the effect of organic moieties of the ZN-CS can be expressed in terms of the following equations (Eqs. (1\u20137)).\n\n(1)\n\n\n\ne\n-\n\n+\n\nH\nS\n\nO\n5\n\n\n\n\n-\n\n\n\u2192\nS\n\nO\n4\n\n\n\n\n2\n-\n\n\n+\nH\n\nO\n\u2219\n\n\n\n\n\n\n\n\n(2)\n\n\n\ne\n-\n\n+\nH\nS\n\nO\n5\n\n\n\n\n-\n\n\n\n\u2192\nO\n\nH\n-\n\n+\nS\n\nO\n4\n\n\n\n\n-\n\u2219\n\n\n\n\n\n\n\n\n(3)\n\n\n\ne\n-\n\n+\nH\nS\n\nO\n5\n\n\n\n\n-\n\n\n\n\u2192\nS\n\nO\n5\n\n\n\n\n-\n\u2219\n\n\n+\n\n\nH\n+\n\n\n\n\n\n\n\n(4)\n\n\nO\n\nH\n-\n\n+\nS\n\nO\n4\n\n\n\n\n-\n\u2219\n\n\n\u2192\nS\n\nO\n4\n\n\n\n2\n-\n\n\n+\nH\n\nO\n\u2219\n\n\n\n\n\n\n\n(5)\n\n\n\nSO\n4\n\n\n\n\n\n2\n-\n\n\n+\n\n\nHO\n\n\u2219\n\n\u2192\n\nSO\n4\n\n\n\n\n\n\n-\n\u2219\n\n\n+\n\n\nOH\n\n-\n\n\n\n\n\n\n\n(6)\n\n\nZN\n-\nC\nS\n-\nO\nH\n\n+\n\nP\nM\nS\n\u2192\nZ\nN\n-\nC\nS\n\u2217\nP\nM\nS\n\n\n\n\n\n\n(7)\n\n\nZN\n-\nC\nS\n\u2217\nP\nM\nS\n\n+\n\nP\no\nl\nl\nu\ntan\nt\ns\n\u2192\nC\n\nO\n2\n\n+\n\n\nH\n2\n\nO\n\n\n\n\nIn the recycling experiments, the removal of RhB was divided into two stages. To simplify the regeneration and reuse, we exclusively used deionized water to clean and dry the catalyst without taking special measures for the catalyst desorption. In the recycling experiments, the catalyst maintained the removal rate of over 90% as shown in Fig. 7\n. The regenerated catalyst exhibited better degradation effect on RhB. The reasons for the better recycling performance of the catalyst are as follows. First, the recycled samples exhibited a certain adsorption capacity towards RhB in the recycling experiments even without desorption. This was because some of the originally adsorbed RhB had been degraded in the batch experiments, and the recycled samples could recover a certain adsorption capacity. The second factor is that the generated free radicals or the ZN-CS*PMS mainly attacked the adsorbed dyes on the surface. The free RhB molecules in the solution were rarely attacked, resulting in a low decolorization rate in the solution during the degradation stage. In contrast, the pre-adsorption step was omitted in the recycling experiments, and the free radicals generated in the ZN-CS/PMS system attacked many free RhB molecules in the solution, thereby improving the removal efficiency. The third factor is that the main active sites in the adsorption and degradation stages were all surface hydroxyl groups. In the recycling experiments, only a fraction of the surface hydroxyl groups was occupied by the dye molecules, thereby leading to more available active sites and improved removal efficiency. This result indicated that the adsorption and degradation processes exhibited a coupling effect, and the ZN-CS maintained adequate performance in such a continuous process. The recyclability of the catalyst is conducive to promoting cleaner production techniques.Based on the phenomenon and analysis mentioned above, we can summarize the mechanism of the entire process (Fig. 8\n). First, because of the van der Waals forces, electrostatic attraction, and hydrogen bond complexation, several RhB molecules were adsorbed on the catalyst surface leading to their partial removal from the solution. Simultaneously, the RhB molecules were transformed into two forms: the adsorbed form and the free form (free in the aqueous phase). After the addition of PMS, a radical and a non-radical pathway of the degradation were observed; these pathways simultaneously attacked the RhB molecules in both the forms. Subsequently, the free RhB in the solution was almost completely removed, and so was the adsorbed RhB. This resulted in the partial regeneration of the adsorption capacity of the ZN-CS. The catalyst was now ready for reuse. Furthermore, the surface hydroxyl groups were the main active sites for both the adsorption and degradation processes. Therefore, the degradation of the adsorbed RhB was conducive to the regeneration of the active sites that promotes the degradation process. This may be a reason for the improvement in the regeneration performance. Finally, the magnetic ZN-CS could be easily separated from the solution that had been degraded.To check the wide suitability of the ZN-CS, elimination experiments on different target contaminants (AO7, MB, Nor, and TC) were carried out using the ZN-CS/PMS system. Fig. 9\n and Fig. S11 illustrate the experimental results. The basic information about the target pollutants and experimental conditions are given in Appendix A Table S5. MB is a typical cationic dye, AO7 is a typical anionic dye, and TC and Nor are representatives of pharmaceuticals and personal care products (PCPs), respectively, in water. They exhibit different sizes and structures, different electronegativities in water, and different hydrogen bond receptors and donors, leading to their possibly different removal effects. As illustrated in Fig. 9, the ZN-CS/PMS system exhibits a removal efficiency of more than 70% for the AO7 removal, while in the PMS system, the removal effects could be ignored. The increase in the AO7 concentration in the solution phase at the fifth minute may be because of the addition of PMS that leads to the desorption of the partially adsorbed AO7. For MB, the ZN-CS/PMS system demonstrates over 90% removal efficiency, while the removal efficiency under the PMS system is less than 20%. For Nor, the system exhibits a removal efficiency of more than 50%, while the efficiency is approximately 20% in the PMS system. For TC, the removal efficiency of the system can reach approximately 80%, while the removal efficiency of the PMS system is approximately 40%. Fig. S9 and Text S9 illustrate the effect of several vital parameters. Fig. S12 illustrate the elimination of RhB in real water sample. In brief, the ZN-CS offers good adsorption and degradation efficiency towards various pollutants that exhibit different electric properties and sizes. Thus, the ZN-CS system presents a wide range of application prospects.In summary, the magnetic composite ZnO/Ni3ZnC0.7 was successfully synthesized and developed as an effective adsorbent and a heterogeneous catalyst for the PMS oxidation to eliminate a variety of organic compounds. The magnetic properties of this nanocomposite led to a rapid and easy separation from the solutions. This study proposes a probable mechanism of the adsorption process that relates to the electrostatic factor and hydrogen bonding. The mechanism of the degradation process indicated that the organic compounds were mainly oxidized by the high-potential intermediate, ZN-CS*PMS, and the hydroxyl radicals generated by PMS, which were primarily activated by the surface hydroxyl groups. The adsorption capacity of the ZN-CS is regenerated owing to the maximum degradation of the adsorbed substrate, achieving the coupling effect. Compared with the systems used in some previous studies, this system used no precious metals. Moreover, this system employed the adsorption\u2013degradation process to achieve a balance between the economic and treatment effect. Furthermore, the synthesized catalyst exhibits magnetic properties, recyclability, stable structure, and good removal efficiency for a variety of organic matter. Our work provides an insight into the development of highly efficient magnetic MOF-based materials for wastewater treatment, and has potential application prospects in the treatment of printing and dyeing wastewater or medical wastewater.This work was supported by the National Natural Science Foundation of China (51878357), the National Science Foundation of Tianjin (18JCYBJC23200), the Innovation Spark Project of Sichuan University (2019SCUH0009), and the Foundation of Science & Technology Department of Sichuan Province (2020YJ0061).Youwen Shuai, Xue Huang, Benyin Zhang, Lu Xiang, Hao Xu, Qian Ye, Jinfeng Lu, and Jing Zhang declare that they have no conflicts of interest or financial conflicts to disclose.", "descript": "\n                  The heterogeneous catalytic activation of peroxymonosulfate for wastewater treatment is attracting increased research interest. Therefore, it is essential to find a sustainable, economical, and effective activated material for wastewater treatment. In this study, metal\u2013organic frameworks (MOF)-5 was used as the precursor, and a stable and recyclable material ZnO@Ni3ZnC0.7 that exhibited good adsorption and catalytic properties, was obtained by the addition of nickel and subsequent calcination. To investigate and optimize the practical application conditions, the elimination of rhodamine B (RhB) in water was selected as the model process. This study demonstrated that the degradation of organic matter in the system involved a coupling of the adsorption and degradation processes. Based on this, the mechanism of the entire process was proposed. The results of scanning electron microscopy, infrared spectrum analysis, and theoretical analysis confirmed that the van der Waals forces, electrostatic attraction, and hydrogen bonding influenced the adsorption process. Electron paramagnetic resonance analysis, masking experiments, and electrochemical tests conducted during the oxidative degradation process confirmed that the degradation mechanism of RhB included both radical and non-free radical pathways, and that the surface hydroxyl group was the key active site. The degradation of the adsorbed substrates enabled the regeneration of the active sites. The material regenerated using a simple method exhibited good efficiency for the removal of organic compounds in four-cycle tests. Moreover, this material can effectively remove a variety of organic pollutants, and can be easily recovered owing to its magnetic properties. The results demonstrated that the use of heterogeneous catalytic materials with good adsorption capacity could be an economical and beneficial strategy.\n               "}
{"full_text": "Nowadays, governments around the world are all committed to achieving carbon neutral goals. Under this background, many countries are developing new energy technology and searching for alternative fuel [1\u20133]. Hydrogen is a kind of non-carbon energy. It is helpful for solving energy depletion and environmental pollution problems [4]. Water electrolysis is a common way for hydrogen production [5,6]. The most efficient catalysts for water electrolysis are Platinum group materials [7]. But they are always rare and expensive [8]. It is particularly important to develop highly active non-noble metal catalyst. Nickel-based coatings are excellent electrode materials for HER. Various Ni-based electrode materials, typically metallic oxide composite material, have been intensively investigated as promising alternative catalysts for the HER. Ren B et. al. [9] prepared Ni-MoO2 composite electrodes for HER. The combination of Ni and MoO2 species increased the Ni-H bond strength, which accelerated the formation of Hads on Ni. The catalyst activity was enhanced by the synergistic effect between Ni and MoO2. Wang N et. al. [10] made Ni-ZnO electrode. The ZnO nanowires on Ni substrates provided high electron mobility to facilitate hydrogen evolution. Kullaiah R et. al. [11] deposited Ni-TiO2 electrode by composite electroplating method. The addition of TiO2 facilitated the formation of tiny nickel grains and increased the number of active site. These composite electrodes exhibited excellent HER activity in alkaline solutions. However, the addition of metallic oxide might reduce the electrical conductivity for electrodes, which greatly limited the further increase of electrocatalytic performance [12]. The carbon materials were always used to modify metallic oxide for their high specific surface and good electronic conductivity [13\u201315]. Shibli et. al. [16] used RGO to modify Fe2O3-TiO2-NiP coating. The synergistic effect between Fe2O3-TiO2 and RGO enhanced HER performance. Sasidharan et. al. [17] studied the effect of GO on the activity of Fe2O3-TiO2-NiCoP coatings for HER. The results showed that the Fe2O3-TiO2-GO-NiCoP exhibited much better catalytic activity than Fe2O3-TiO2-NiCoP. The addition of GO greatly decreased the electrical resistance of composite coatings. It had been proved in our pervious study that SnO2 was a good catalyst for HER [18]. However, its low conductivity limited the further increase of the catalyst activity. In this study, SnO2 combining with XC-72 carbon were used as composite particles to improve the hydrogen evolution activity. We first synthesized C-SnO2 composite particles by hydrolysis method. And then, these C-SnO2 composite particles were suspended in nickel plating solution. The Ni/C-SnO2 composite coating was prepared by composite electrodeposited method. Multiple physical and electrochemical tests were employed to investigate the effect of C-SnO2 particles embed in Ni coating on the activity of HER.Preparation of C-SnO2: The 200\u00a0mg carbon powder was dispersed in 25\u00a0mL ethylene glycol. The mixture was stirred for 3\u00a0h at room temperature. The powder of 64\u00a0mg anhydrous SnCl2 was dissolved in 25\u00a0mL ethylene glycol. The acquired solution was dispersed by ultrasonication for 30\u00a0min and then stirred for 30\u00a0min at room temperature. The above two solutions were mixed evenly and then moved into three-necked flask (100\u00a0mL). The mixture was heated and refluxed at 196\u00a0\u2103 for 8\u00a0h. After cooling to room temperature naturally, the product was separated from reaction mixture by suction filtration. The products were washed in sequence with deionized water and absolute ethanol, and then dried at 80\u00a0\u2103 for 12\u00a0h in vacuum.Preparation of Ni/C-SnO2 composite coating: The Ni/C-SnO2 composite coating was fabricated by a composite electrolytic deposition method. The composition of the bath was 350\u00a0g\u00b7L\u22121 Ni(NH2SO3)2\u00b74\u00a0H2O, 10\u00a0g\u00b7L\u22121 NiCl2\u00b76\u00a0H2O, 30\u00a0g\u00b7L\u22121 NH4Cl and different content of C-SnO2 nanoparticles. The composite electrolytic deposition process was carried out at a current density of 3\u00a0A/dm2 for 30\u00a0min at 35\u00a0\u2103 with a magnetic stirring at 850\u00a0rpm. The pH value was 3.8. These obtained composite coatings were labeled as Ni/C-SnO2-0.25, Ni/C-SnO2-0.5, Ni/C-SnO2-0.75 and Ni/C-SnO2-1, which represented 0.25\u00a0g\u00b7L\u22121, 0.5\u00a0g\u00b7L\u22121, 0.75\u00a0g\u00b7L\u22121 and 1\u00a0g\u00b7L\u22121 C-SnO2 nanoparticles applied in above nickel plating bath, respectively.The morphology of the Ni/C-SnO2 coatings was studied using SEM on FEI Quanta 400 equipment operating at 20\u00a0kV acceleration voltage. The composition of coatings were analyzed from quantitative EDX coupled with the SEM. The crystal structure of Ni/C-SnO2 coatings were determined by XRD (D8 Advance, Bruker). Fourier transform infrared (FT-IR) spectra were performed on a Nicolet iS10 with the wave number from 4000 to 400\u00a0cm\u22121.All the electrochemical measurements were performed in a standard three-electrode cell with 1\u00a0M NaOH solution by the PARSTAT 4000 electrochemical workstation. The composite electrodes were used as the working electrode. A platinum foil (1\u00a0cm\u00a0\u00d7\u00a01\u00a0cm) was used as the counter electrode and an Hg/HgO electrode was used as the reference electrode. The scan rate of steady-state polarization test was 5\u00a0mV\u00b7s\u22121. The EIS measurements were carried out at a potential of \u2212\u00a00.125\u00a0V (vs. SCE) in frequency range frm 100\u00a0kHz to 0.01\u00a0Hz with an AC voltage amplitude of 0.005\u00a0V. The chronopotential curves were continuous measured at \u2212\u00a0100\u00a0mA\u00b7cm\u22122 for 10\u00a0h.\n\nFig. 1 is the FTIR spectra of homemade C-SnO2 nanoparticles. Before the spectra, the C-SnO2 particles are dehydrated at 200\u00a0\u00b0C under vacuum condition. The band at 500\u2013750\u00a0cm\u22121 is related to Sn-O stretching vibration. The bands at 1600\u00a0cm\u22121 and 3434\u00a0cm\u22121 are due to O-H bending vibration and O-H stretching vibration [19]. As shown in Fig. 1, there are abundant OHads species on the surface of C-SnO2 nanoparticles. These OHads promote the formation of Hads and improve the activity of HER [20].The XRD pattern of C-SnO2 particles is shown in \nFig. 2. The SnO2 has a cassiterite phase structure, showing the major diffraction peaks of (110), (101), (210), (211), (301), (321), and so on. The first diffraction peaks for C-SnO2 at 2\u03b8 about 26\u00b0 can be attributed to the hexagonal graphite structures (002) of the XC-72 carbon black [21]. The SnO2 (211) peak is chosen to calculate the mean particle size of SnO2 according to Debye\u2013Scherrer formula [22]. The calculated particle size of C-SnO2 is about 150\u2009nm.\n\nFig. 3 shows the XRD patterns of Ni and Ni/C-SnO2-0.5 coatings. It can be seen that the diffraction peaks of Ni and Ni/C-SnO2-0.5 at 44.5\u00b0, 51.8\u00b0 and 76.4\u00b0 are indexed to (111), (200) and (220) crystal planes of standard Ni(JCPDS:03\u20131051). The peaks of Ni/C-SnO2-0.5 at 26.6\u00b0, 33.8\u00b0 and 38.9\u00b0 are assigned to SnO2 (JCPDS:41\u20131445). The C-SnO2 composite particles are nicely co-deposited in Ni coating. The addition of C-SnO2 composite particles increases the diffraction intensity of Ni (111) and decreases that of Ni (200). The preferred orientation has been changed to Ni (111) in composite coatings. The catalytic activity of Ni (111) is higher than Ni (200) for HER [23]. The half width values of Ni (111) for Ni and Ni/C-SnO2-0.5 are 0.194\u00b0 and 0.223\u00b0, respectively. According to Debye-Scherrer formula, the larger value of FWHM represents the smaller crystal size [24,25]. Therefore, the grain size of Ni/C-SnO2-0.5 is smaller than Ni. The tiny Ni grains of composite electrode support more active sites for HER.\n\nFig. 4 shows EDX analysis of Ni/C-SnO2-0.5 composite electrodes. The presences of all elements are confirmed, and the quality percentage of C-SnO2 in the composite coatings is about 15.94%. The content of C-SnO2 in the coatings analyzed by EDX are shown in \nFig. 5. The amount of C-SnO2 in composite electrode tend to rise with the increase of the C-SnO2 concentration in the bath.\n\nFig. 6 is the SEM images of different electrodes with low and high-magnification. As shown in Fig. 6(a), the pure Ni electrode appears a typical block shape. Fig. 6(b) shows that the spherical C-SnO2 particles are deposited uniformly on the surface of composite electrode. The particle size of C-SnO2 is about 150\u2009nm. This result is consistent with the above XRD. The addition of C-SnO2 composite particles refine the crystallization of coating, which enlarge the active surface area of composite coatings. Fig. 6(c)-(j) shows that, too much amount of C-SnO2 makes the coating compact and nonporous.The effect of C-SnO2 on catalytic activity of composite coatings for HER are investigated by several electrochemical tests. The relevant parameters of these electrochemical measurements are listed in \nTable 1. Here \u03b7\n\n10\n is the cathode overpotential at 10\u2009mA\u00b7cm\u22122. j\n\n0\n is the exchange current density. b is the Tafel slope. The cathode polarization curves of different coatings are shown in \nFig. 7(a). Fig. 7(a) reveals that the C-SnO2 addition enhance the HER performance. Too much C-SnO2 in electrodes are bad for HER. The Ni/C-SnO2-0.5 electrode shows the best catalytic activity for HER. Ni/C-SnO2-0.5 composite coating has the lowest overpotential of 304\u2009mV to reach current densities of 10\u2009mA\u00b7cm\u22122.The Tafel curves of different electrodes are shown in Fig. 7(b). As shown in Table 1, the composite electrodes exhibit higher j\n\n0\n and lower \u03b7\n\n10\n than pure Ni. The j\n\n0\n values firstly enhance and then decline with the increase of C-SnO2 content in coatings. The Ni/C-SnO2-0.5 composite coating presents the highest j\n\n0\n value of 57.44 uA\u00b7cm\u22122, which is 87.96 times higher than that of the Ni coating. The Tafel slope b is a crucial parameter. b can reflect the dominant mechanism of HER. There are three principal steps in HER in alkaline media, including Volmer, Heyrovsky and Tafel. When Volmer step dominates the reaction, the corresponding b values is about 120\u2009mV\u00b7dec\u22121\n[26]. As shown in Table 1, the b values of the Ni and those Ni/C-SnO2 coatings are 122, 144, 136, 140 and 156\u2009mV\u00b7dec\u22121 respectively. The rate determined step of HER is Volmer reaction. The facilitation of Hads formation can accelerate HER. The abundant OHads on C-SnO2 surface can accelerate the water molecular decomposition and Hads formation. The addition of C-SnO2 enhances the catalytic activity of HER [18,20].\nFig. 7(c) displays the Nyquist plots of electrodes, and the inset shows the equivalent circuit. The fitting results are listed in Table 1. R\n\ns\n is the solution resistance. R\n\nct\n is the charge transfer resistance, and C\n\ndl\n is the double layer capacitance. It can be seen that the R\n\nct\n value of Ni/C-SnO2-0.5 is the lowest. It means that the performance of HER on Ni/C-SnO2-0.5 is the highest. The R\n\nS\n value of the composite electrodes are smaller than that of Ni. The carbon in C-SnO2 increases the conductivity of composite electrode. As shown in Table 1, the C\n\ndl\n value of Ni/C-SnO2-0.5 is the highest. The value of C\n\ndl\n is proportional to the quantity of active sites [27]. The large surface area is conducive to raise the hydrogen evolution activity of composite coating [28\u201330]. The results are conform with the analysis of LSV and Tafel.\n\nFig. 8 exhibits the volume of H2 produced on different electrodes for 300\u2009s at a cathode current density of 300\u2009mA\u00b7cm\u22122. The Ni/C-SnO2-0.5 composite electrode produces the largest amount of H2 (19.2\u2009mL), The Faraday efficiency values of the different electrodes Ni, Ni/C-SnO2-0.25, Ni/C-SnO2-0.5, Ni/C-SnO2-0.75, and Ni/C-SnO2-1 are calculated to be 92.18%, 95.24%, 96.46%, 95.75%, and 94.65% according to the formula (1)\n[31], which verifies the best activity of Ni/C-SnO2-0.5 for HER.\n\n(1)\n\n\nF\nE\n=\n\n\n2\n\u22c5\n\n\nN\n\n\nA\n\n\n\u22c5\nP\n\u22c5\n\n\nV\n\n\n\n\nH\n\n\n2\n\n\n\n\n\u22c5\ne\n\n\ni\n\u22c5\nt\n\u22c5\nR\n\u22c5\nT\n\n\n\n\n\nwhere V is the volume of H2, i is the current, t is the reaction time, P is the standard atmospheric pressure, T is the reaction temperature.\n\nFig. 9(a) shows the chronopotentiometry curves of different coatings in 1\u2009M NaOH. The analysis results of the stability test are shown in \nTable 2. \u03c6\n\n0\n and \u03c6\n\n10\n are the hydrogen evolution potentials of the electrodes before and after the 10\u2009h stability test. \u0394\u03c6 is the potential difference between \u03c6\n\n0\n and \u03c6\n\n10\n. After stability test, the \u0394\u03c6 of Ni/C-SnO2-0.5 is the lowest and the decrease of potential is less than 2%. The polarization curves of Ni/C-SnO2-0.5 before and after the stability test are shown in Fig. 9(b). The Ni/C-SnO2-0.5 composite electrode shows excellent stability in alkaline solution.The Ni/C-SnO2 composite coating are compared with some composite electrode materials reported in literatures. The kinetic parameters of HER on these electrode materials are listed in \nTable 3. The Ni/C-SnO2 composite coating exhibits more excellent activity of HER.In this work, the C-SnO2 composite particles are synthesized by high temperature hydrolysis method. The Ni/C-SnO2 electrodes with high HER catalytic activity have been prepared by composite electrolytic deposition technology. Compared to the Ni coating, the Ni/C-SnO2 catalyst shows higher exchange current density and lower hydrogen evolution overpotential for HER. Moreover, the activity of Ni/C-SnO2 catalyst is better than other reported electrocatalysts for HER. The co-deposition of C-SnO2 refines the crystallization of Ni, which enlarge the active surface area of coating. The co-deposition of C-SnO2 changes the preferred orientation of Ni to (111) crystal plane which is more conducive to HER. The addition of C-SnO2 enhances the conductivity of composite coating and accelerates the rate of HER. Meanwhile, the OHads on C-SnO2 surface promotes the decomposition of water and the formation of adsorbed Hads. These results indicate that the Ni/C-SnO2 catalyst is a high-activity, low-cost and stable electrocatalysts for HER.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the Natural Science Foundation Project of Heilongjiang Provincial (No. LH2022B023, JJ2022LH0472), the Basic Scientific Research Program of Heilongjiang (No. 1452ZD008), the Youth Backbone Project of Mudanjiang Normal University (No. QN2022007), the Ideological and Political High-quality Construction Project of Mudanjiang Normal University, the Innovation Project of University Students (202210233001).", "descript": "\n                  The high performance Ni/C-SnO2 composite electrodes were successfully prepared for hydrogen evolution reaction (HER) in alkaline media by an easy and low cost composite electrolytic deposition method. The Ni/C-SnO2 composite coating was physically examined by using various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive analysis of X-ray (EDX). The XRD and EDX results showed that C-SnO2 particles prepared by high temperature hydrolysis method were successfully incorporated in Ni coatings. The preferred orientation of Ni particle in composited coatings had been altered to (111) crystal plane which was more benefit for HER. The SEM and XRD results indicated that the addition of C-SnO2 particles refined Ni crystallization in composite coatings. The catalytic activity and stability of the composite coatings for hydrogen evolution reaction were examined by steady-state polarization, Tafel plots, electrochemical impedance spectroscopy (EIS) and chronopotentiometry technique. Compared to Ni coating, the Ni/C-SnO2 composite coatings exhibited higher hydrogen evolution performance. The abundant OHads on C-SnO2 surface accelerated the water molecular decomposition and Hads formation which determined the rate of HER (Volmer step). The carbon in C-SnO2 improved the conductivity of composite electrode, which was also helpful for HER.\n               "}
{"full_text": "Much attention was paid to the fluid catalytic cracking (FCC) of heavy oil due to its greatly enhanced processing difficulty because of the large molecules (Fu et\u00a0al., 2006; Puente et\u00a0al., 2007; Talmadge et\u00a0al., 2014; Corma et\u00a0al., 2017). It was well known that FCC of large molecules of heavy oil would be carried out in a successive way: (1) the large heavy oil molecules are precracked firstly in the macropores of the matrix, such as kaolin; (2) the products obtained in step (1) cracked further in the mesopores; (3) the intermediate products of step (2) cracked more selectively to valuable products, such as gasoline, diesel, and other chemical products (Ji et\u00a0al., 2018). In addition to this, the improvement of FCC catalysts in the face of new challenges (such as new feeds and less polluting products) would be extremely urgent (Valle et\u00a0al., 2019; Palos et\u00a0al., 2021). Therefore, it is of vital importance to implant mesoporosity into FCC catalysts of heavy oil.To date, many approaches to introducing mesoporosity into FCC catalysts have been developed. A typical way is the so-called \u201ctop-to-down\u201d method, which involves the removal of Al or Si species of zeolites (Verboekend et\u00a0al., 2013; Jia et\u00a0al., 2019). Unfortunately, this process suffers from zeolitic loss due to the destructive route. Another procedure \u201cdown-to-up\u201d is a convenient approach to generating mesoporosity directly (Saxena et\u00a0al., 2014; Kerstens et\u00a0al., 2020). However, the low hydrothermal stability and high synthesis cost of the obtained materials still hinder its industrial application in severe conditions of FCC. Therefore, how to obtain hydrothermally stable MAs synthesized with low synthesis cost is still a great challenge in both FCC process and materials science.Our group (Liu et\u00a0al., 2013a; Cao et\u00a0al., 2014; Jin et\u00a0al., 2014; Mi et\u00a0al., 2018a; Chen et\u00a0al., 2021; Li et\u00a0al., 2022) has obtained MAs with excellent hydrothermal stability by a novel strategy that introduces zeolite Y precursors (the primary and secondary structural unit of zeolite Y) into the walls of mesoporosity. Just in this idea, our group obtained MAs with hydrothermal stability comparable to that of USY (Liu et\u00a0al., 2014a, 2021; Mi et\u00a0al., 2017a, 2017b, 2018b). For example, the BET surface area of MAs decreased from 595.4\u00a0m2\u00a0g\u22121 to 153.9\u00a0m2\u00a0g\u22121 after a severe hydrothermal treatment in 100% water vapor at 800\u00a0\u00b0C for 12\u00a0h. Moreover, the materials showed excellent catalytic properties when it was employed in heavy oil FCC in the MAT unit (Liu et\u00a0al., 2013b, 2014b, 2014c; Deng et\u00a0al., 2022). Although the significant progress in synthesis and application of hydrothermally stable MAs, the commercial manufacturing of materials, preparation of industrial catalysts, and industrial application in pilot FCC unit are still a great challenge for the material scientists.Herein, we report the most recent progress in commercial manufacturing and catalytic cracking performance in the FCC unit of 1.2-million tons in a refinery. To the best of our knowledge, the research progress represents the most advanced stage of commercial manufacturing and industrial application in FCC of MAs.Industrial reagent of Pluronic P123 triblock copolymer (EO20PO70EO20) was obtained from Henan Ruiyi Chemical Company. Water glass (containing 27.78\u00a0wt% SiO2 and 8.98\u00a0wt% Na2O) was purchased from Tangshan Shihe Sodium Silicate Company.The mixtures of Na2SiO3, Al2(SO4)3\u00b718H2O, and NaOH solution with a molar ratio of Al2O3/SiO2/Na2O/H2O\u00a0=\u00a01/16\u201319/15\u201320/300\u2013320 were prepared. After rapidly stirring for 30\u201380\u00a0min, the prepared mixtures undergo an ageing with stirring at 80\u2013100\u00a0\u00b0C for 4\u201320\u00a0h. The obtained sticky solution was denoted as \u201czeolite Y precursors\u201d.\n\n\n(1)\nP123 and water were added to the reactor. Zeolite Y precursors and H2SO4 (6\u00a0M) were slowly added to the above reactor to keep the pH of the system at about 1.0\u20133.0. The obtained solution was stirred at 30\u201360\u00a0\u00b0C for 10\u201340\u00a0h.\n\n\n(2)\nThe liquid product of step (1) were transferred into crystallization autoclaves of 10\u00a0m3 for crystallization under 100\u2013140\u00a0\u00b0C for 24\u00a0h. The as-synthesized products were processed by the subsequent filtering, washing, and drying at 120\u00a0\u00b0C for 2\u00a0h. Then, the resultant solid was calcined at 550\u00a0\u00b0C for 6\u00a0h in order to remove the organic template, the obtained product was denoted as \u201cIS\u201d\n\n\n(3)\nIS was hydrothermally aged under a severe condition (800\u00a0\u00b0C, and 100% water vapor) for 4\u00a0h and the product was named as \u201cHIS\u201d.\n\n\nP123 and water were added to the reactor. Zeolite Y precursors and H2SO4 (6\u00a0M) were slowly added to the above reactor to keep the pH of the system at about 1.0\u20133.0. The obtained solution was stirred at 30\u201360\u00a0\u00b0C for 10\u201340\u00a0h.The liquid product of step (1) were transferred into crystallization autoclaves of 10\u00a0m3 for crystallization under 100\u2013140\u00a0\u00b0C for 24\u00a0h. The as-synthesized products were processed by the subsequent filtering, washing, and drying at 120\u00a0\u00b0C for 2\u00a0h. Then, the resultant solid was calcined at 550\u00a0\u00b0C for 6\u00a0h in order to remove the organic template, the obtained product was denoted as \u201cIS\u201dIS was hydrothermally aged under a severe condition (800\u00a0\u00b0C, and 100% water vapor) for 4\u00a0h and the product was named as \u201cHIS\u201d.Diffractometer Rigaku D/Max 2500VB2+/PC equipped with Cu K\u03b1 radiation was used for the study of X-ray diffraction (XRD) patterns for the obtained samples. JEM 100CX instrument with 200\u00a0kV acceleration voltage was used to study the TEM images. A Micromeritics ASAP 2405N system was used to investigate the N2 ad-desorption isotherms using liquid nitrogen at 77\u00a0K. Moreover, The curves of pore-size distribution of materials was obtained by the traditional Barrett-Joyner-Halenda (BJH) method.The amount of acid and the type of acid are calculated using the following formula:\n\n\n\nC\n\np\ny\n\u2212\nB\n\n\n=\n\n\n1.88\n\nI\n\nA\n\nB\n\n\n\n\nR\n2\n\n\nW\n\n;\n\n\n1.88\n=\n\n\u03c0\n\n\u03b5\nB\n\n\n,\n\n\u03b5\nB\n\n=\n1.67\n\u00b1\n0.1\nc\nm\n/\n\u03bcm\no\nl\n\n\n\n\n\n\n\n\n\nC\n\np\ny\n\u2212\nL\n\n\n=\n\n\n1.42\n\nI\n\nA\n\nL\n\n\n\n\nR\n2\n\n\nW\n\n;\n\n\n1.42\n=\n\n\u03c0\n\n\u03b5\nL\n\n\n,\n\n\u03b5\nL\n\n=\n2.22\n\u00b1\n0.1\nc\nm\n/\n\u03bcm\no\nl\n\n\n\n\nwhere C\npy-B and C\npy-L represent the concentration of Br\u00f8nsted and Lewis acids, respectively. I\nA(B) and I\nA(L) represent the integrated absorbance of Br\u00f8nsted and Lewis acids, respectively. R is the radius of the wafer (cm). W is weight of wafers (mg).The catalytic properties of FCC catalysts were evaluated at Lanzhou Petrochemical Research Center, using the ACE unit Model R+MM from Kayser Technology. The fluidization of the reactor was achieved by a stream of nitrogen. 9 g catalyst was fluidized and stabilized at catalyst/oil ratio 5 and the reaction temperature 530\u00a0\u00b0C.The feeding heavy oil was characterized with viscosity 10.38\u201314.35\u00a0mm2/s, residual carbon 3.4\u20134.2\u00a0wt%, and density 900\u00a0kg/m3.\u223c25 tons of MAs were manufactured at a commercial zeolite manufacturing corporation by using the existing production equipments. This is the first time that MAs are manufactured at an industrial scale. Characterization results (Fig.\u00a01\n and Fig.\u00a02\n) showed that the industrial product had similar physicochemical properties with those obtained at the laboratory. With a BET surface area of 769\u00a0m2\u00a0g\u22121, the industrial product has a narrow mesopores centered around 3.5\u20134.5\u00a0nm (Fig.\u00a03\n). After the hydrothermal deactivation at 800\u00a0\u00b0C for 10\u00a0h under 100% water vapor, the total surface area decreased from 769\u00a0m2\u00a0g\u22121 to 154\u00a0m2\u00a0g\u22121, the total pore volume decreased from 0.77\u00a0cm3/g to 0.32\u00a0cm3/g, while the mesopore volume dropped from 0.57\u00a0cm3/g to 0.25\u00a0cm3/g (Table\u00a01\n). Interestingly, the diameter and the size distribution of mesopores became wider (Fig.\u00a03). All these results demonstrated the excellent hydrothermal stability of industrial products. TEM images (Fig.\u00a04\n) showed that IS after the severe hydrothermal ageing still exhibited mesoporous structure which is typically wormlike, suggesting the high hydrothermal stability of IS.The MAs was milled and then mixed with kaolin, rare earth salts, pseudo-boehmite, and alumina sol. The mixtures are spray-dried into 429 tons of FCC catalysts microspheres with the diameter of 75\u00a0\u03bcm. The final catalyst was named \u201cLPC-65\u201d. The incumbent catalyst applied in the industrial FCC unit LDO-70 was used to compare with LPC-65.To study the acidity characteristics of LDO-70 and LPC-65, Br\u00f8nsted acid sites (BAS) and Lewis acid sites (LAS) were calculated from Py-FTIR spectra in the temperature range of 473\u2013623\u00a0K, and the corresponding results were depicted in Fig.\u00a05\n, Fig.\u00a06\n and Table\u00a02\n. From Table\u00a02, we could see that both the total acid sites and the BAS of LPC-65 are much higher than those of LDO-70. In this sense, it could be reasonably deduced that the conversion of heavy oil would increase greatly.The final catalyst LPC-65 exhibited a total surface area of 258\u00a0m2\u00a0g\u22121, including micropores area of 173\u00a0m2\u00a0g\u22121 and mesopores area of 85\u00a0m2\u00a0g\u22121 (Table\u00a03\n). As for comparison, the incumbent industrial catalyst LDO-70 has a total surface area of 239\u00a0m2\u00a0g\u22121, including micropores area of 169\u00a0m2\u00a0g\u22121 and mesoporosity surface area of 70\u00a0m2\u00a0g\u22121.LPC-65 and LDO-70 were hydrothermally deactivated at 800\u00a0\u00b0C for 4\u00a0h under 100% water vapor to simulate the catalytic properties of equilibrium catalysts. The mesoporous surface area of steamed LPC-65 is 195\u00a0m2\u00a0g\u22121, in which the mesoporous surface area is 64\u00a0m2\u00a0g\u22121, much higher than that of the reference catalyst (49\u00a0m2\u00a0g\u22121), indicating that the remaining ratio of mesoporosity is high even after severe hydrothermal treatment. ACE test of the two deactivated catalysts using the same heavy oil feed suggested the clear advantage of LPC-65 in selectivity, mainly in increased conversion of 3.36%, reduced heavy oil yield of 1.39% and increased total liquid yield of 0.67% (Table\u00a04\n). The lower coke factor of LPC-65 compared with that of LDO-70 exhibited much enhanced selectivity, which was the most important factor for FCC catalysts. These interesting results could be reasonably ascribed to the presence of enhanced hydrothermally stable mesoporosity.It is well known that vanadium and nickel will cause damage to zeolites in FCC conditions (Trujillo et\u00a0al., 1997; Xu et\u00a0al., 2002; Hagiwara et\u00a0al., 2003; Cerqueira et\u00a0al., 2008). The ACE test of the contaminated LPC-65 and LDO-70 (deactivated with 3000\u00a0ppm Ni and 5000\u00a0ppm\u00a0V) suggested the clear advantage of contaminated LPC-65 in selectivity, mainly in increased conversion of 4.48%, reduced heavy oil yield of 2.82% and increased total liquid yield of 2.14% (Table\u00a05\n). These results demonstrated that the obtained MAs could withstand the severe conditions in industrial FCC units. To the best of our knowledge, this is the first time that a mesoporous aluminosilicate demonstrated excellent hydrothermal stability in FCC units. Moreover, it is the first time that a mesoporous aluminosilicate is employed in FCC catalysts with good activity and selectivity.LPC-65 was added to 1.2-million tons equipment at the same addition rate with that of the incumbent catalyst. As a result of this, a constant catalyst inventory of 429 tons was maintained in the industrial unit. The change-over from the incumbent catalyst to LPC-65 resulted in an 83.37% inventory ratio at the end of 68 days trial. Equilibrium catalyst samples in different inventory ratios were collected and characterized periodically. Interestingly, the surface area of trial equilibrium catalysts (30% inventory ratio) increased from 110\u00a0m2\u00a0g\u22121 to 120\u00a0m2\u00a0g\u22121, consistent with the higher surface area of fresh LPC-65. Surprisingly, a significant increase in the mesoporous surface area of trial equilibrium catalysts (30% inventory ratio) from 33\u00a0m2\u00a0g\u22121 to 40\u00a0m2\u00a0g\u22121 (22% increase), indicating the high hydrothermal stability of the mesoporosity in this industrial unit.\nTable\u00a06\n summarized the industrial results of equilibrium catalysts of the incumbent and LPC-65. Compared with LDO-70, the equilibrium catalyst that contain 80% LPC-65 yields significantly lower heavy oil (0.23%) and higher total liquids (0.53%). These results are very close to those obtained from laboratory ACE testing. Generally, the activity of equilibrium catalyst could be considered as the average activity of all the catalyst in FCC unit, which included fresh catalyst and deactivated catalyst. Therefore, it could be suggested that LPC-65 was an ideal FCC catalyst.Mechanical resistance is another important quality of FCC catalyst, because it reduces the flow of catalyst that must be purged. Interestingly, the abrasion index of LPC-65 is 1.6, much lower than the prerequisite value 3.0. These results indicated that LPC-65 developed in this paper had comprehensive advantages compared with the commercial catalysts.For the first time, mesoporous aluminosilicates with excellent hydrothermal stability were manufactured at the commercial scale by a unique process. FCC catalysts obtained from the MAs exhibited high stability in an industrial FCC unit. The catalyst showed improved product selectivity compared with the incumbent catalyst, at a high inventory ratio of 80%. Gasoline oil yield with 80% LPC-65 equalized catalyst enhanced by 1.22% and the total liquid yield enhanced by 0.53%. The results of synthesis and application represent the most advanced development of MAs in heavy oil FCC to date, which bring a ray of hope for the industry-scale application of MAs in heavy oil FCC.The authors acknowledge PetroChina Co. Ltd. for financial support through the research programs (Grant Nos. DQZX-KY-21-008, KYWX-21-023, and KYWX-21-022).", "descript": "\n                  Well-ordered aluminosilicates (MAs) were prepared by in-situ assembly of pre-crystallized units of zeolite Y precursors at a commercial scale, and applied in an industrial fluid catalytic cracking unit for the first time. Compared with incumbent equilibrium catalyst, the surface area of trial equilibrium catalysts (30% inventory ratio) increased from 110\u00a0m2\u00a0g\u22121 to 120\u00a0m2\u00a0g\u22121. Moreover, a significant increase of the mesoporous surface area of trial equilibrium catalysts (30% inventory ratio) from 33\u00a0m2\u00a0g\u22121 to 40\u00a0m2\u00a0g\u22121 (22% increase). Furthermore, the equilibrium catalyst that contain 80% LPC-65 yields significantly lower heavy oil (0.23%) and higher total liquids (0.53%) compared with LDO-70. The industrial results demonstrated excellent hydrothermal stability and superior catalytic cracking properties, showing the promising future in the industrial units.\n               "}
{"full_text": "Among the primary fossil energies, the natural gas composed of mainly methane is believed to be a cleaner energy carrier than coal, which has already been widely used in the industrial and transportation sectors. However, the worldwide reservoir, distribution, and the market demanding of the natural gas are greatly unbalanced. In this case, the coal to natural gas via the syngas route emerges as a viable and important technology for the region or country such as China, where characterizes in a high market demanding but greatly insufficient supply of the natural gas and relatively rich coal reserves (Kopyscinski et al., 2010a; Razzaq et al., 2013; R\u00f6nsch et al., 2016). Thus, the production of synthetic natural gas (SNG) from coal via the CO methanation reaction has drawn intensive attention in recent years.In essence, the CO methanation reaction is a part of the Fischer-Tropsch synthesis, of which the carbon-containing products are limited to methane, i.e., CO\u00a0+\u00a03H2\u00a0=\u00a0CH4\u00a0+\u00a0H2O, \u0394H\n298K\u00a0=\u00a0\u2212206.1\u00a0kJ\u00a0mol\u22121 (Kopyscinski et al., 2010a). Considering the highly exothermic and entropy-decreased nature of the methanation reaction, the industrial process is commonly composed of a series of adiabatic fixed-bed reactors, which are operated in wide ranges of temperatures from ~200\u2013350\u00a0\u00b0C to 650\u2013700\u00a0\u00b0C (Nguyen et al., 2013). To improve the process efficiency, contradictory requests are imposed on the catalyst, i.e., sufficiently active at lower temperatures vs. highly stable at higher temperatures resistant to the sintering. Thus, the development of an efficient catalyst pertinent to the opposing requirements is practically important for a more efficient CO methanation process although it has already been industrialized. Moreover, the high activity and anti-sintering property are commonly challengeable issues for designing and developing high-performance metal-supported catalysts.Generally, the supported group VIIIB metals including Pt, Ru, Rh, Fe, Ni, and Co are active catalysts for the CO methanation reaction. Among these metals, Ni is concentrated for the industrial application in considering its relatively richer reservoir and the reasonably higher activity. In the case of the support, alumina or silica are commonly employed. Thus, Ni/alumina and Ni/silica catalysts are extensively studied for the CO methanation reaction (Liu et al., 2016; Tao et al., 2016). To achieve a higher activity at lower temperatures, small Ni particles with a high dispersion are generally targeted. On the contrary, bigger Ni particles are relatively resistant to the sintering at higher temperatures although their activity is lower (Munnik et al., 2014). More importantly, the deactivation of the Ni-supported catalysts induced from either the coke deposition, the sintering of Ni particles and/or the support is a chronic process for the CO methanation reaction (Barrientos et al., 2014). Thus, great efforts have been devoted to mitigate the sintering of Ni particles and to suppress the coke deposition as summarized in our previous work (Xiao et al., 2020).As an efficient method to retard the sintering of Ni particles, the embedment or encapsulation of Ni particles within the pore wall of oxide supports such as ordered mesoporous alumina (OMA) has been quantitatively practiced in recent years (Tian et al., 2015b). Indeed, as a result of the strong Ni-support interactions, both the sintering of Ni particles and the coke deposition over the embedded or encapsulated Ni catalysts such as Ni-OMA are greatly suppressed, leading to a stable catalyst for the CO or CO2 methanation reactions at higher reaction temperatures (Aljishi et al., 2018; Tian et al., 2015a). Unfortunately, the activity of these catalysts is low due to the lower reduction extent of Ni, which is still originated from the strong Ni-support interactions (Aljishi et al., 2018; Liu et al., 2016). Very recently, we demonstrate that a high-performance Ni/Ni-OMA catalyst for the CO methanation can be obtained by balancing the free Ni via the impregnation route and the confined Ni within OMA, which shows a high space-time yield of methane and long-term stability under severe conditions (Xiao et al., 2020).Alternatively, the introduction of the second metal into the Ni-based catalysts is also practiced as an effective method to meet the contrary requirements of the CO methanation reaction. According to the generally observed orders for the specific activity of Ru\u00a0>\u00a0Fe\u00a0>\u00a0Ni\u00a0>\u00a0Co\u00a0>\u00a0Rh\u00a0>\u00a0Pd\u00a0>\u00a0Pt\u00a0>\u00a0Ir and the CH4 selectivity of Pd\u00a0>\u00a0Pt\u00a0>\u00a0Ir\u00a0>\u00a0Ni\u00a0>\u00a0Rh\u00a0>\u00a0Co\u00a0>\u00a0Fe\u00a0>\u00a0Ru, different configurations of bimetals including Ni-Co, Ni-Fe, Ni-Ru are applied for the CO methanation reaction, and an enhanced catalytic performance was commonly observed (Chen et al., 2010; Liu et al., 2020, 2017). Among these bimetal catalysts, Ni-Co was concentrated as a result of the relatively higher performance for the titled reaction, which is normally explained as the synergetic effects between Ni and Co metals or the formation of the Ni-Co alloy (Liu et al., 2020).In fact, the intrinsic kinetics has long been practiced as an effective aid for developing high-performance catalysts and understanding the reaction mechanism (Zhang et al., 2020). If the references on the CO methanation reaction are analyzed, the catalyst development and the interpretation of the reaction results are overwhelmingly dependent on the correlation between the characterization data and the catalytic results. In contrary, the kinetics study on the CO methanation is scare. Moreover, two different reaction mechanisms over Ni- and Co-based catalysts, namely \u201cdirect CO dissociation\u201d and\u201chydrogen-assisted CO dissociation\u201d are proposed for the methanation reaction (Chen et al., 2017; Lim et al., 2016). However, contributions from the kinetics results on understanding the reaction mechanism and explaining the catalytic performance are very limited. Thus, more deep understanding on the CO methanation reaction is reasonably expected if the catalyst characterizations and the intrinsic kinetics studies are integrated.From these analyses and understandings, in this work, a series of Co/Ni-OMA catalysts with varied Co/Ni ratios was synthesized by impregnating Co over Ni-OMA. We found that the Co/Ni ratio had a strong effect on the CO conversion, CH4 selectivity, and the long-term stability of the catalyst for the CO methanation. Combining the characterization results of the catalysts with the intrinsic kinetics, it is revealed that Ni confined within OMA undertake the dominant active sites for catalyzing the CO methanation, while the post-impregnated Co promotes the H-assisted CO dissociation step, resulting in an enhanced low-temperature activity over the optimal 8Co/15Ni-OMA catalyst. These findings are important for further optimizing or designing a high-performance bimetallic catalyst for the methanation reactions of CO or CO2.All chemicals with analytical grade were directly employed as received without further purifications. Nickel nitrate hexahydrate (Ni(NO3)2\u00b76H2O), cobalt nitrate hexahydrate (Co(NO3)2\u00b76H2O), nitric acid (HNO3, 67\u00a0wt%), hydrochloric acid (HCl, 37\u00a0wt%) and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Aluminum isopropoxide (Al(OPr\ni\n)3) and (EO)20(PO)70(EO)20 triblock copolymer (Pluronic P123, Mn\u00a0=\u00a05800) were bought from Sigma-Aldrich.The Ni-OMA catalyst was fabricated via the one-pot evaporation induced self-assembly (EISA) process with some modifications according to reported reference (Morris et al., 2008; Yuan et al., 2008). Typically, 2.5\u00a0g Pluronic P123 was dissolved in 40\u00a0mL anhydrous ethanol at room temperature. Next, 3.2\u00a0mL nitric acid, 4.08\u00a0g Al(OPr\ni\n)3, and 0.70\u00a0g Ni(NO3)2\u00b76H2O were successively introduced to the above ethanol solution under a vigorous stirring. After stirring vigorously for 6\u00a0h, the solvent evaporation process was conducted in a drying oven at 60\u00a0\u00b0C for 48\u00a0h to form a green xerogel. Finally, the xerogel was calcined at 600\u00a0\u00b0C for 4\u00a0h with a temperature ramp of 1\u00a0\u00b0C min\u22121. The thus reaped solid was denoted as 15Ni-OMA, where 15 represented the weight percentage of NiO. For the sake of comparison, the pristine OMA was synthesized without introducing Ni(NO3)2\u00b76H2O, and the 15Co-OMA counterpart was prepared by adding Co(NO3)2\u00b76H2O.The xCo/15Ni-OMA catalysts with varied content of CoO were synthesized by incipiently impregnating Ni-OMA with the aqueous solution of Co(NO3)2\u00b76H2O, where x indicates the weight percentage of CoO, i.e., 1, 3, 5, 8, 13\u00a0wt%, respectively, and the content of NiO is always fixed at 15\u00a0wt%. During preparation, the exact dosage of Co(NO3)2\u00b76H2O and Ni(NO3)2\u00b76H2O required were summarized in Tab. S1. After impregnation, the samples were dried at 120\u00a0\u00b0C for 12\u00a0h, and then calcined in air at 600\u00a0\u00b0C for 2\u00a0h with a temperature rate of 2\u00a0\u00b0C min\u22121. For comparison, 15Co/OMA catalyst was synthesized via impregnating OMA with the aqueous solution of Co(NO3)2\u00b76H2O.The exact NiO and CoO content over the calcined catalysts was determined by inductively coupled plasma mass spectroscopy (ICP-MS, M90, Bruker). Before each measurement, the sample was digested in a mixed solution of concentrated nitric acid (67\u00a0wt%) and hydrochloric acid (37\u00a0wt%). The calculated NiO and CoO contents over different catalysts are summarized in Tab. S2 in Supplementary materials.The N2 physisorption isotherms of the different samples were measured on a Micromeritics ASAP 2020 instrument at \u2212196\u00a0\u00b0C for calculating textual data including the BET surface area, pore volume, average pore size, and pore size distribution (PSD) curves. Prior to the measurement, all the samples were pretreated under vacuum condition at 90\u00a0\u00b0C for 1\u00a0h and then at 300\u00a0\u00b0C for 8\u00a0h.Powder X-ray diffraction (XRD) patterns of the samples were conducted on a Bruker D8 Advance X-ray diffractometer using the monochromatized Cu/K\u03b1\n radiation at 40\u00a0kV and 40\u00a0mA in the 2\u03b8 ranging from 0.5\u00b0 to 6\u00b0 and 10\u00b0 to 90\u00b0. The scanning rate of the sample was 1\u00b0 and 6\u00b0 per minute with a step size of 0.02\u00b0 (2\u03b8) for small-angle and wide-angle region, respectively.The hydrogen temperature-programmed reduction (H2-TPR), O2 titration, and H2 pulse chemisorption experiments were performed on a Micromeritics AutoChem 2920 device. For H2-TPR, all the samples were pretreated in a high-purity Ar flow at 450\u00a0\u00b0C for 1\u00a0h and then cooled down to 50\u00a0\u00b0C before measurement. Afterward, H2-TPR was performed until 1000\u00a0\u00b0C with a heating rate of 10\u00a0\u00b0C min\u22121 in a 10\u00a0vol% H2/Ar flow. The O2 titration experiment was used to estimate the degree of reduction for the different samples. Firstly, the sample was reduced in a high-purity H2 flow at 700\u00a0\u00b0C for 1\u00a0h, and then purged with a high-purity Ar for 0.5\u00a0h at 700\u00a0\u00b0C. After cooling down to 600\u00a0\u00b0C in the Ar flow, 3\u00a0vol% O2/Ar was pulsed consecutively. The extent of reduction was calculated from the total oxygen consumption, the amount of which is measured by a pre-calibrated thermal conductivity detector (TCD). The H2 pulse chemisorption experiment was used to calculate the dispersion of the metallic Ni and Co. Firstly, the fresh catalyst was reduced at the temperature of 700\u00a0\u00b0C for 1\u00a0h in a high-purity H2 flow, and then cooled down from 700\u00a0\u00b0C to 35\u00a0\u00b0C in an Ar flow. Finally, 0.5\u00a0mL 10% H2/Ar was introduced by the consecutive pulse-dosing until a constant area of the TCD peak.X-ray photoelectron spectra (XPS) were collected in a high vacuum environment (approximately 5\u00a0\u00d7\u00a010\u22129 torrs) on an X-ray photoelectron spectrometer (Kratos Analytical Ltd.) with an Al K\u03b1 radiation (1486.6\u00a0eV) at the room temperature. The C1 s peak at 284.8\u00a0eV was applied to calibrate the binding energy.To investigate the location of metallic Ni and Co species as well as their particle size distributions, transmission electron microscope (TEM) and high-angular annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were carried out on a FEI Tecnai G2 F20 transmission electron microscope with the accelerating voltage of 200\u00a0kV.The CO methanation reactions were performed under 0.1\u00a0MPa in a quartz-lined stainless-steel fixed bed reactor with an inner diameter of 8\u00a0mm. Prior to the evaluation, 100.0\u00a0\u00b1\u00a00.1\u00a0mg catalyst (40\u201360 mesh) diluted with 4.0\u00a0g quartz sands (40\u201360 mesh) was reduced under the conditions of T\u00a0=\u00a0700\u00a0\u00b0C, reducing time\u00a0=\u00a01\u00a0h, and pure H2 flow rate\u00a0=\u00a0100\u00a0mL\u00a0min\u22121. After the temperature had been cooled to the desired temperature, the syngas with a molar feed ratio of H2/CO/N2\u00a0=\u00a03/1/1 was purged into the fixed-bed reactor. The low-temperature tests of all the catalysts for the CO methanation were assessed at the temperature region of 300\u2013450\u00a0\u00b0C with the gas hourly space velocity (GHSV) of 60,000\u00a0mL\u00a0g\u2212\n1 h\u22121. A harsh reaction condition comprised of 40\u00a0h running at 180,000\u00a0mL\u00a0g\u2212\n1 h\u22121 and high temperature, i.e., 400\u00a0\u00b0C for 10\u00a0h, 700\u00a0\u00b0C for 20\u00a0h and 400\u00a0\u00b0C for 10\u00a0h again, was conducted to evaluate the high-temperature stability of the representative catalysts. The long-terms durability experiments were carried out under 600\u00a0\u00b0C with GHSV of 180000\u00a0mL\u00a0g\u2212\n1h\u22121. After remove of water with the ice-water trap, the products were analyzed by an on-line GC (GC-9560, Shanghai Huaai chromatographic analysis Co., Ltd.) equipped with a 5A molecular sieve column and a Porapak Q column and a TCD. The CO conversion (X\nCO), selectivity of methane (\n\n\nS\n\n\nC\nH\n\n4\n\n\n\n), and the yield of methane (\n\n\nY\n\n\nC\nH\n\n4\n\n\n\n) are defined as follows:\n\n\n\n\nX\nCO\n\n\n\n\n%\n\n\n\n\n=\n\n\n\n\n\n\nF\n\nCO, in\n\n\n-\n\nF\n\nCO\n,\nout\n\n\n\n\n\n\n/\n\nF\n\nCO\n,\nin\n\n\n\u00d7\n100\n\n\n\n\n\n\n\n\n\nS\n\n\nC\nH\n\n4\n\n\n\n(\n%\n)\n\n=\n\nF\n\nC\n\nH\n4\n\n,\no\nu\nt\n\n\n/\n\n(\n\nF\n\nC\nO\n,\ni\nn\n\n\n-\n\nF\n\nC\nO\n,\no\nu\nt\n\n\n)\n\n\u00d7\n100\n\n\n\n\n\n\n\n\n\nY\n\n\nC\nH\n\n4\n\n\n\n(\n%\n)\n\n=\n\nF\n\nC\n\nH\n4\n\n,\no\nu\nt\n\n\n/\n\nF\n\nC\nO\n,\ni\nn\n\n\n\u00d7\n100\n\n\n\nwhere F\ni, in and F\ni, out are the flow rates of CO or CH4 at the inlet and outlet of the reactor, respectively.The kinetics measurements of CO methanation were conducted at 0.1\u00a0MPa and 300\u00a0\u00b0C with different gas partial pressures of CO and H2 (5\u201330\u00a0kPa for CO, 10\u201360\u00a0kPa for H2, total pressure of 100\u00a0kPa balanced with N2). The formation rate of CH4 was used to represent the total reaction rate. To regulate the experimental conversion level (<10%) far below the thermodynamic equilibrium conversion of CO methanation, the relatively lower temperature range from 260 to 290\u00a0\u00b0C cooperating with varied GHSV of 60,000 and 120,000\u00a0mL\u00a0g\u2212\n1 h\u22121 were employed to assess the apparent activation energy (E\na) according to the standard Arrhenius equation. In addition, the potential mass transport and heat transfer limitations were estimated via Weisz-Prater and Mears criterions. The kinetics models were derived by quasi-equilibrium approximations and Langmuir-Hinshelwood mechanism. The unknown kinetics parameters involved in the derived kinetics expressions based on different rate-determining steps were fitted by the nonlinear least-square method according to the experimental data. The minimization problems arisen in the least square-curve fitting were solved by the trust-region-reflective algorithm. The lower bounds of the unknown parameters were set as zero to ensure that the fitted values are positive. The coefficient of determination (r\n2) is used to measure how well the experimental data replicated by different kinetics models.\n\n\n\n\n\nr\n\n2\n\n=\n1\n-\n\n\n\n\u2211\ni\n\n\n\n\n\n\nr\n\ni\n\n\nE\nx\np\n.\n\n\n-\n\nr\n\ni\n\n\nF\ni\nt\n.\n\n\n\n\n\n2\n\n\n\n\n\u2211\ni\n\n\n\n\n\n\nr\n\ni\n\n\nE\nx\np\n.\n\n\n-\n\n\nr\n\u00af\n\n\ni\n\n\nE\nx\np\n.\n\n\n\n\n\n2\n\n\n\n\n\n\n\nThe ordered mesoporous structure and crystalline phase of as-synthesized catalysts were characterized by XRD, as shown in Fig. 1\n. From the small-angle XRD patterns (Fig. 1A), 15Ni-OMA and 15Co-OMA showed the prominent peak at 2\u03b8\u00a0=\u00a01.0\u00b0 and the weak peak at 2\u03b8\u00a0=\u00a01.6\u00b0, indicative of the (100) and (110) plane of the p6mm hexagonal symmetry, respectively (Morris et al., 2008; Yuan et al., 2008). In the case of the Co-impregnated catalysts, i.e., xCo/15Ni-OMA and 15Co/OMA, the loss of characteristic peak (110) and the broadened characteristic peak (100) were observed, indicating the preserve of the ordered mesoporous structure with slightly decreased orderliness. Moreover, 15Ni-OMA and 15Co-OMA showed a type IV isotherm with a sharply steep H1 hysteresis loop at the relatively pressure of 0.7\u20130.9 (Fig. S1A), indicating the typical feature of mesoporous materials because of the capillary condensation of N2 in uniform mesochannels (Yuan et al., 2008). When the Co-impregnated catalysts were concerned, both total volume of absorbed N2 and the relative pressure range of hysteresis loop decreased, indicating the decreased pore volume and average pore size (Tab. S3), as illustrated in pore size distribution (PSD) curves (Fig. S1B).For the wide-angle XRD patterns (Fig. 1B), since no diffraction peaks of NiO and \u03b3-Al2O3 were observed in the case of 15Ni-OMA, it is determined that Ni species were highly dispersed within pore wall of amorphous alumina matrix (Liu et al., 2016). This conclusion is also the same with 15Co-OMA catalyst. However, in the cases of Co-impregnated catalysts, those characteristic diffraction peaks corresponding to \u03b3-Al2O3 phase were observed, which was probably attributed to solvent-induced the crystalline transformation during the post-impregnation process, as reported in our previous works (Xiao et al., 2020). More importantly, the peak intensity of \u03b3-Al2O3 phase decreased with increasing the cobalt content from 1 to 15\u00a0wt%, especially for 13Co/15-OMA and 15Co/OMA, which is due to the enhanced interactions between cobalt species and \u03b3-Al2O3. Given diffraction peaks overlapping of NiAl2O4, CoAl2O4 and \u03b3-Al2O3 (Gon\u00e7alves et al., 2018), these spinel phases cannot be excluded tentatively from xCo/15Ni-OMA catalysts. When cobalt content was increased from 1 to 15\u00a0wt%, a set of diffraction peaks assigning to the crystalline phase of Co3O4 were observed, and continuously intensified with increasing cobalt content. Taking account of post-impregnated 15Co/OMA and 13Co/15Ni-OMA catalysts with similar cobalt content, the diffraction peaks of Co3O4 over 15Co/OMA are weaker than that of 13Co/15Ni-OMA, from which it is proposed that (I) there is strong interaction between CoO and OMA, and (II) Ni species embedded within pore wall have steric hindrance for post-impregnated cobalt. After reduction (Fig. 1C), three new diffraction peaks at 44.6\u00b0, 51.8\u00b0, and 76.6\u00b0 corresponding to (111), (200) and (220) lattice planes of metallic Ni and/or Co were observed for all the reduced catalysts. Comparing 15Co-OMA with 15Co/OMA, it is found that the diffraction peak at 2\u03b8\u00a0=\u00a044.6\u00b0 over the former is much lower than that over the latter, indicating that the post-impregnated Co species are relatively easier to be reduced.HAADF-STEM and TEM images of all the catalysts after reduction were shown in Fig. 2\n and Fig. S2, respectively. Obviously, these catalysts clearly represented a highly ordered cylindrical pore aligned along the [110] direction, indicating the preserve of the highly ordered mesoporous structure even after impregnating as high as 15\u00a0wt% CoO and with pretreatment of high-temperature calcination and reduction. This is a strong evidence for high thermal stability. Moreover, the metal mean size distributions were determined via counting overall particles from the HAADF-STEM images, as shown in insets in Fig. 2 and Table 1\n. Specifically, in the case of 15Ni-OMA, Ni particles with small size (4.2\u00a0\u00b1\u00a01.2\u00a0nm) were uniformly distributed into mesochannels. The similar observation can be obtained for 15Co-OMA with mean size of 4.8\u00a0\u00b1\u00a01.0\u00a0nm. The 15Co/OMA catalyst synthesized via post-impregnation delivered a relatively larger size (11.5\u00a0\u00b1\u00a03.7\u00a0nm). However, for the xCo/15Ni-OMA catalysts, the metallic mean size also gradually increased (from 4.9\u00a0\u00b1\u00a01.4 to 9.3\u00a0\u00b1\u00a04.1\u00a0nm) and size distribution became widened, while a few large metal particles were partially distributed outside mesochannels, but all of them are better than 15Co/OMA. Therefore, it is assumed that the post-impregnated Co species prefers to aggregate on the outer surface of the OMA compared with Ni species confined within pore wall of OMA.\nFig. 3\n showed the H2-TPR profiles of all catalysts. The 15Ni-OMA catalyst exhibited only one broad hydrogen-consumption peak centered at around 654\u00a0\u00b0C, indicating the presence of Ni particles with varied sizes, in accordance with the HAADF-STEM images (Fig. 2). In the case of 15Co-OMA, two reduction peaks at ~810\u00a0\u00b0C and ~915\u00a0\u00b0C were observed, which were attributed to the reductions of Co species confined within pore wall of OMA and non-stoichiometric amorphous CoAl2O4, respectively. This result points out that Co species are more difficult to be reduced than Ni species if both are separately confined within pore wall of OMA (Xu et al., 2016). Moreover, there is no reduction peak at temperature blow 500\u00a0\u00b0C for 15Ni-OMA and 15Co-OMA, indicating the absence of free bulk NiO and Co3O4 (Ma et al., 2016). However, 15Co/OMA synthesized via post-impregnation showed a very broad peaks centered at around 574\u00a0\u00b0C, 763\u00a0\u00b0C, and 934\u00a0\u00b0C (Wang et al., 2018). The maximum reduction temperature (\nT\n\nmax\u00a0=\u00a0574\u00a0\u00b0C) is different with that of 15Co-OMA (\nT\n\nmax\u00a0=\u00a0810\u00a0\u00b0C), which can be attributed to the medium strong interactions between the impregnated Co species and OMA. Based on the above insights into reduction behaviors of monometallic catalysts, the assignment of the reduction peaks of xCo/15Ni-OMA can be roughly identified. Specifically, the peaks at temperature below 500\u00a0\u00b0C and at the range of 800\u20131000\u00a0\u00b0C ascribed to the reduction of surface free bulk Co3O4 and CoAl2O4, respectively (Wang et al., 2018; Xu et al., 2016). The \nT\n\nmax shifted toward higher temperature (~690\u00a0\u00b0C), which was indicative of enhanced metal-support interactions. These broad and unresolved reduction peaks were further deconvoluted into five reduction peaks to differentiate the interaction difference, as illustrated in Table. 2\n and Fig. S3. The first reduction at 340\u2013460\u00a0\u00b0C (\u03b1) is attributed to the reduction of bulk Co3O4 with large sizes via the CoO intermediate. The second reduction at around 500\u2013590\u00a0\u00b0C (\u03b2) is ascribed to the reduction of NiO and/or CoOx weakly interacted with OMA (Ma et al., 2016). The third reduction at around 650\u2013710\u00a0\u00b0C (\u03b3) is referred to the reduction of those Ni species confined within pore wall of OMA, and/or the non-stoichiometric amorphous NiAl2O4, and/or the CoOx moderately strong interacted with OMA (Ma et al., 2016). The fourth reduction at around 740\u2013810\u00a0\u00b0C (\u03b8) is associated with the reduction of the NiAl2O4 spinel bearing with very strong metal-support interactions, and/or those Co species confined within OMA framework, and/or the non-stoichiometric amorphous CoAl2O4 compound (Tao et al., 2013). The final reduction at around 850\u2013940\u00a0\u00b0C (\u03b4) is due to the reduction of CoAl2O4 spinel bearing with very strong metal-support interactions (Huang et al., 2017; Xu et al., 2016).In general, the stronger interaction between metal and OMA will lead to a worse reduction degree of the Ni and Co oxide species, vice versa. The reduction degree of Ni and Co oxide species was determined by the O2 titration, as shown in Table 1. As expected, the reduction degree of 15Ni-OMA (81.2%) is higher than that of 15Co-OMA (42.6%), in line with H2-TPR profiles. In the cases of xCo/15Ni-OMA catalysts, an increased reduction degree was observed in comparison with pristine 15Ni-OMA except 13Co/15Ni-OMA, indicating the addition of CoO could improve the reduction of Ni oxide species in the OMA. In view of the similar reduction extent between 15Co/OMA (73.7%) and 13Co/15Ni-OMA (78.9%), it is supposed that 13Co/15Ni-OMA with low surface area and hydrogen diffusion limitation probably results in a relatively lower reduction degree than other xCo/15Ni-OMA catalysts. In addition, the metal dispersion degree was measured by H2 pulse chemisorption (Table 1). Both 15Co-OMA and 15Co/OMA showed poor dispersion, but it is contrary for 15Ni-OMA. As increasing the content of Co oxide, the dispersion gradually decreased from 11.3% to 2.8%, which can be explained by the hydrogen diffusion limitation of Ni and Co oxide generated from the confinement of OMA (Wang et al., 2018; Xiao et al., 2020).To further study the intermetallic interaction of xCo/15Ni-OMA catalysts, XPS analyses of fresh and reduced catalysts were conducted, and the deconvoluted XPS spectra of Ni 2p and Co 2p were shown in Fig. S4. In the case of fresh catalysts, two characteristic spin-orbit splitting of Ni 2p3/2 peak and Ni 2p1/2 peak at binding energy (BE) of 856.2 and 873.5\u00a0eV were observed over 15Ni-OMA, corresponding to those unreduced Ni2+ species (Fig. S4A). A set of Co 2p3/2 peaks and Co 2p1/2 peaks at BE of 780.6 and 782.2\u00a0eV, 796.0 and 797.6\u00a0eV were observed over 15Co/OMA catalyst (Fig. S4B), of which BE values located at 780.6 and 796.0\u00a0eV were assigned to the presence of Co3O4 with two oxidation states (Co2+ and Co3+), and BE at 782.2 and 797.6\u00a0eV was attributed to CoAl2O4 compound (Horlyck et al., 2018; Ji et al., 2000). For bimetallic xCo/15Ni-OMA catalysts, it is shown that there is a positive shift of binding energy (0.3\u20130.4\u00a0eV) for the peaks containing Co2+ and Co3+ species, implying that post-impregnated cobalt oxide might donate electrons to unreduced Ni2+ species and/or OMA support resulting in decreased outer electron density. However, no significant alteration in the BE values of CoAl2O4 compound and unreduced Ni2+ species, which is probably because the effect of electron transfer to unreduced Ni2+ species is shielded by strong metal-support interaction or formation of NiAl2O4 compound, as evidenced by H2-TPR. After reduction, the BE values centered at 852.5 and 869.7\u00a0eV, 778.2 and 793.5\u00a0eV are attributed to metallic Ni0 and Co0 (Fig. S4C and D), respectively. As compared with pristine 15Ni-OMA and 15Co/OMA, it is noted that BE values of Co0 increases from 778.2 to 779.0\u00a0eV, and BE values of Ni0 increases from 852.5 to 853.2\u00a0eV in the cases of bimetallic xCo/15Ni-OMA catalysts. Likewise, there is still no any significant shift in the BE values of 856.2 and 782.2\u00a0eV corresponding to those compounds in oxidation state that are very difficult to be reduced (e.g., NiAl2O4 and CoAl2O4). In principle, an increase in BE values is characteristic of decreased outer electron density of atomic nucleus. Together with the difference of electronegativity (\n\n\n\u03c7\n\nC\no\n\n\n\n\u00a0=\u00a01.88, \n\n\n\u03c7\n\nN\ni\n\n\n\n = 1.91), it is thus assumed that metallic Co0 might donate electrons to metallic Ni0, but a reversed shift in BE values of metallic Ni0 occurs probably because those nickel oxides embedded inside OMA framework are partially reduced, resulting in the formation of Co-Ni-NiAl2O4 intermetallic phase and subsequent electron transfer to unreduced compounds with strong metal-support interaction. In short, it is thus confirmed that the intermetallic interaction occurs via cobalt donating electron to nickel species, and subsequent electron transfer to OMA support or hard-to-reduce metal-support compounds, which would contribute an effect on catalytic behavior of CO methanation.The as-synthesized catalysts for CO methanation were initially tested at low temperature region (300\u2013450\u00a0\u00b0C), and the resultant CO conversion, CH4, CO2 selectivity and CH4 yield as function of reaction temperature were plotted in Fig. S5. In the cases of 15Ni-OMA and xCo/15Ni-OMA catalysts, CO conversion and CH4 yield (Fig. S5a and d) increased gradually as reaction temperature rising, and eventually reached to ca.99% and ca.85%, respectively. In addition, it is shown that CH4 selectivity initially decreases as reaction temperature increasing, but it increases if reaction temperature increases further (Fig. S5b). However, the lesser CO2 selectivity is observed at a low temperature (300\u2013320\u00a0\u00b0C) and it increases rapidly when reaction temperature is higher than 320\u00a0\u00b0C, but it decreases if temperature increases continuously (Fig. S5c). Based on total Gibbs free energy minimization method and the feed gas with a molar ratio of H2/CO/N2\u00a0=\u00a03/1/1 in absence of catalyst, two parallel side-reactions, e.g., water-gas shift reaction (WGSR) and CO disproportionation reaction (Boudouard reaction) were involved during CO methanation, and the thermodynamic equilibrium composition were calculated via HSC Chemistry software. The resultant conversion, selectivity and equilibrium constant K values are shown in Fig. S6. It is found that the variation trend of methane selectivity obtained over xCo/15Ni-OMA catalysts agrees with our thermodynamic calculation (Fig. S6a) and the early reports (Liu et al., 2016, 2017; Gao et al., 2012). The equilibrium constant value calculated (Fig. S6b) of CO methanation (K\nM) is higher than that of water-gas shift reaction (K\nW), and almost same with that of Boudouard reaction (K\nB), revealing CO methanation and Boudouard reaction occurs easier than water-gas shift reaction at reaction temperature of 300\u2013450\u00a0\u00b0C, accounting for the formation of CO2 and carbon deposition (R\u00f6nsch et al., 2016). However, the carbon selectivity (Fig. S6a) obtained from thermodynamic calculation is negligible when reaction temperature is lower than 450\u00a0\u00b0C, which is in keeping with our experimental result. Interestingly, the variation trend of CO2 selectivity obtained from experimental results seems to be not in line with thermodynamic calculation, which is probably because those side-reactions involving CO2 are controlled simultaneously by both thermodynamics and kinetics in the presence of xCo/15Ni-OMA catalysts. Fig. 4\n exhibited the effect of Co loading on the reaction rate under the different reaction temperatures. Clearly, on the condition of reaction temperature of 300\u2013340\u00a0\u00b0C, a set of volcanic curves were observed when cobalt loading increased from 0 to 13\u00a0wt% as shown in Fig. 4a. However, when reaction temperature was operated at 360\u00a0\u00b0C, the consuming rate of CO remained constant and then decreased sharply as cobalt loading was beyond 8\u00a0wt%. The maximum consuming rate of CO (0.14\u00a0mol kgcat\n\u2212\n1 s\u22121) was obtained at 340\u00a0\u00b0C over 8Co/15Ni-OMA. Moreover, the similar observations for the formation rate of CH4 as a function of cobalt content were obtained under different reaction temperatures as indicated in Fig. 4b. However, both catalyst counterparts of 15Co-OMA and 15Co/OMA showed poor activity (Fig. S5) even if a higher reduction temperature of 750\u00a0\u00b0C was applied (Fig. S7) to reduce those Co oxide species confined within pore wall of OMA. Thus, it can be concluded that incorporating an appropriate amount of cobalt into 15Ni-OMA matrix favors the enhancement of CO methanation reactivity at low-temperature.Since the industrial CO methanation over Ni-based catalysts are usually operated at a wide temperature window (typically from 300 to 700\u00a0\u00b0C), the high-temperature stability of catalyst is an important index for practical application (R\u00f6nsch et al., 2016). In this regard, a harsh reaction condition comprised of 40\u00a0h running at 3-fold GHSV (180,000\u00a0mL\u00a0g\u2212\n1 h\u22121) and high temperature, i.e., 400\u00a0\u00b0C for 10\u00a0h, 700\u00a0\u00b0C for 20\u00a0h and 400\u00a0\u00b0C for 10\u00a0h again, was conducted to evaluate the high-temperature stability of the representative catalysts (8Co/15Ni-OMA and 15Ni-OMA) (Fig. S8). When CO methanation reaction was operated at 700\u00a0\u00b0C, CO conversion and CH4 selectivity over 8Co/15Ni-OMA and 15Ni-OMA were almost same and close to the thermodynamic equilibrium. After running at 700\u00a0\u00b0C for 20\u00a0h, 8Co/15Ni-OMA regained the original CO conversion level (97.7%) at 400\u00a0\u00b0C, but 15Ni-OMA showed a ca.10% loss of CO conversion. This is a clear evidence that post-impregnating a certain amount of cobalt into 15Ni-OMA matrix could facilitate the improvement of high-temperature stability. Moreover, the long-term durability of 8Co/15Ni-OMA were assessed at 600\u00a0\u00b0C and with GHSV of 180,000\u00a0mL\u00a0g\u2212\n1 h\u22121, and the results were shown in Fig. 5\n. Obviously, a very slight change on CO conversion was observed over 8Co/15Ni-OMA for TOS of 200\u00a0h, of which the initial and final CO conversions were about 69.0% and 67.9%, respectively (Fig. 5a), and the selectivity (Fig. 5b) and yield (Fig. 5c) of CH4 also remained stable. As a consequence, both stability and long-term durability of 8Co/15Ni-OMA at high temperature are reasonably good.Prior to the kinetics analysis, the potential effects of mass and heat transfer limitations were checked by Weisz-Prater criterion and Mears\u2019 criterion (Mears, 1971; Weisz and Prater, 1954). Specifically, the internal diffusion and external diffusion limitations were evaluated via Eqs. (1) and (2), respectively. The extent of the interphase heat transfer was evaluated via Eq. (3), whereas the interparticle, intraparticle and the axial conduction are negligible in the present reaction condition, as reported in our previous work (Xiao et al., 2020). All related parameters are listed in Tables 3\n and 4\n.\n\n\n\nC\n\nW\nP\n\n\n=\n\n\n-\n\nr\n\no\nb\ns\n\n\n\n\u03c1\nc\n\n\nR\n\np\n\n2\n\n\n\n\nD\ne\n\n\nC\ns\n\n\n\n<\n1\n\n (1)\n\n\n\nC\n\nM\nM\n\n\n=\n\n\n-\n\nr\n\no\nb\ns\n\n\n\n\u03c1\nb\n\n\nR\np\n\nn\n\n\n\nk\nc\n\n\nC\n\nA\nb\n\n\n\n\n<\n0.15\n\n (2)\n\n\n\nC\n\nM\nH\n\n\n=\n\n|\n\n\n-\n\n\n\u0394\nH\n\nr\n\n\n\n(\n-\n\nr\nA\n\n)\n\u03c1\n\nb\n\nR\nE\n\n\n\nh\nt\n\n\n\n\nT\n\nb\n\n2\n\nR\n\ng\n\n\n\n|\n\n<\n0.15\n\n (3)After substitution of corresponding values into C\nWP and C\nMM equations, results in C\nWP\u00a0=\u00a02.80\u00a0\u00d7\u00a010\u2212\n5\u00a0<\u00a01 and C\nMM\u00a0=\u00a01.82\u00a0\u00d7\u00a010\u2212\n4\u00a0<\u00a00.15. Therefore, it can be determined that both internal and the external diffusion limitation were negligible under the conditions applied in this work. Likewise, according to C\nMH equation, leads to C\nMH\u00a0=\u00a00.03\u00a0<\u00a00.15, indicating that the resistance of the interphase heat transfer is negligible. Moreover, according to the previous work (Mears, 1971; Xiao et al., 2020), the temperature difference between the fluid phase and catalyst particle calculated is 4.9\u00a0K when the CO methanation is carried out under reaction condition of 653\u00a0K, 0.1\u00a0MPa, and 60,000\u00a0mL\u00a0g\u2212\n1 h\u22121. Following the same method, the values of Weisz-Prater and Mears\u2019 criterions under different reaction temperatures can be obtained and collected in Tab. S4. All in all, the effect of mass transport under the conditions applied in the present work is negligible. The interphase heat transfer resistance is also insignificant based on within 5% deviation (Mears, 1971), leading to the maximum temperature difference of less than 4.9\u00a0K. Therefore, it is reasonable to conduct a kinetics analysis on CO methanation over the different catalysts.To better understand the cooperative effect of bimetallic xCo/15Ni-OMA catalysts on CO methanation at low temperature, the apparent activation energies (E\na) of different catalysts for CO methanation were determined according to Arrhenius equation. Specifically, the relationships between CO consuming rate and reaction temperature, as well as CH4 formation rate and reaction temperature were plotted, respectively (Fig. 6\n). Obviously, the E\na values obtained showed same variation trend over different catalysts no matter which reaction rate was used. In addition, E\na values derived from CH4 formation rate are higher than that from CO consuming rate due to in presence of side-reactions during CO conversion process. Herein, taking CH4 formation rate for example because of methanation purpose, it can be determined that E\na values obtained over 15Ni-OMA, 15Co-OMA, and 15Co/OMA are 124.0, 131.9, and 117.2\u00a0kJ\u00a0mol\u22121, respectively. However, when an appropriate amount of cobalt was introduced into 15Ni-OMA matrix, E\na values firstly decreased with increasing cobalt content, but it increased again for 13Co/15Ni-OMA. As expected, 8Co/15Ni-OMA showed the lowest E\na value of 100.2\u00a0kJ\u00a0mol\u22121, accounting for high activity at low temperature.The effects of CO and H2 partial pressure on reaction rate of CH4 formation were first investigated and showed in Fig. 7\n. Under the present methanation condition, the main products were CH4 and H2O, almost in absence of CO2 and C2\n+ products. With the exception of 8Co/15Ni-OMA, other catalysts showed that the CH4 formation rate decreased with increasing CO partial pressure. On the contrary, the CH4 formation rate was positively related to H2 partial pressure, in keeping with literatures (Chen et al., 2017; Yang et al., 2013). For 8Co/15Ni-OMA, a volcanic curve of reaction rate as function of CO partial pressure was observed, where the maximum CH4 formation rate was obtained at CO partial pressure of 15\u00a0kPa. This result is an indicative of different kinetics behavior proceeding on 8Co/15Ni-OMA and other catalysts.In view of the reaction mechanism of CO methanation, the hydrogen-assisted dissociation mechanism as compared with direct dissociation pathway is so far well-recognized, where the oxygenated intermediate species of COH is involved (Kopyscinski et al., 2010b; Lim et al., 2016; Miao et al., 2016; Pham et al., 2014; Yang et al., 2013). In addition, our experimental results revealed that methanation rate increased as function of H2 partial pressure increasing (Fig. 7b) when CO partial pressure was fixed, which provided a favorable evidence supporting the domination of hydrogen-assisted CO dissociation mechanism regarding the driving force of hydrogen. Therefore, a couple of potential micro-kinetics models consisted of nine elementary steps are proposed based on hydrogen-assisted dissociation and Langmuir-Hinshelwood mechanism (Scheme 1\n), where all kinds of surface adsorbed species, i.e., H*, CO*, COH*, C*, CH*, CH2*, CH3* and OH* are fully considered. Moreover, given that different effects of CO and H2 partial pressure on reaction rate, H-assisted CO dissociation, the first-step hydrogenation of surface carbon species (C*) and last-step hydrogenation of surface CH3* species are assumed as rate-determining steps (RDS), respectively, whereas other steps are counted as quasi-equilibria approximation.Model I: The H-assisted CO dissociation (Step 3) is RDS. The resultant reaction rate expression is indicated as the Eq. (I-1).\n\n\n\nr\n\n=\n\nk\n\n3\n\n+\n\n\n\u03b8\n\nC\nO\n\n\n\n\u03b8\nH\n\n-\n\nk\n\n3\n\n-\n\n\n\u03b8\n\nC\nO\nH\n\n\n\n\u03b8\n\n\u2217\n\n\n\n (I-1)Based on quasi-equilibria approximation for each of elementary steps, the coverage of various surface species can be determined as function of measurable experiment variables:\n\n\n\n\u03b8\nH\n\n=\n\n\u03b8\n\n\u2217\n\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n (I-2)\n\n\n\n\u03b8\n\nC\nO\n\n\n=\n\n\u03b8\n\n\u2217\n\n\n\nK\n2\n\n\nP\n\nC\nO\n\n\n\n (I-3)\n\n\n\n\u03b8\nC\n\n=\n\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\u03b8\n\n\u2217\n\n\n\n\n\nK\n\n1\n\n2\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nP\n\n\nH\n2\n\n\n2\n\n\n\n\n (I-4)\n\n\n\n\u03b8\n\nC\nH\n\n\n=\n\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\u03b8\n\n\u2217\n\n\n\n\n\nK\n\n1\n\n\n3\n2\n\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nP\n\n\nH\n2\n\n\n\n3\n2\n\n\n\n\n\n (I-5)\n\n\n\n\u03b8\n\nC\n\nH\n2\n\n\n\n=\n\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\u03b8\n\n\u2217\n\n\n\n\n\n\n\nK\n1\n\nK\n\n7\n\n\nK\n8\n\n\nP\n\nH\n2\n\n\n\n\n\n (I-6)\n\n\n\n\u03b8\n\nC\n\nH\n3\n\n\n\n=\n\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\u03b8\n\n\u2217\n\n\n\n\n\nK\n8\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n\n (I-7)\n\n\n\n\u03b8\n\nO\nH\n\n\n=\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\u03b8\n\n\u2217\n\n\n\n\n\nK\n9\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n\n (I-8)\n\n\n\n\u03b8\n\nC\nO\nH\n\n\n=\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\u03b8\n\n\u2217\n\n\n\n\n\nK\n\n1\n\n\n5\n2\n\n\n\nK\n4\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nK\n9\n\n\nP\n\n\nH\n2\n\n\n\n5\n2\n\n\n\n\n\n (I-9)Because of coverage normalization of all surface species (Eq. (I-10)),\n\n\n\n\u03b8\n\n\u2217\n\n\n+\n\n\u03b8\nH\n\n+\n\n\u03b8\n\nC\nO\n\n\n+\n\n\u03b8\nC\n\n+\n\n\u03b8\n\nC\nH\n\n\n+\n\n\u03b8\n\nC\n\nH\n2\n\n\n\n+\n\n\u03b8\n\nC\n\nH\n3\n\n\n\n+\n\n\u03b8\n\nO\nH\n\n\n\n\n+\n\u03b8\n\n\nC\nO\nH\n\n\n=\n1\n\n (I-10)The coverage of empty active site \n\n\n\u03b8\n\n\u2217\n\n\n\ncan be formulated as function of known variables (Eq. (I-11)):\n\n\n\n\u03b8\n\n\u2217\n\n\n=\n\n1\n\n\n\n1\n+\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n+\n\nK\n2\n\n\nP\n\nC\nO\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\nK\n\n1\n\n2\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nP\n\n\nH\n2\n\n\n2\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\nK\n\n1\n\n\n3\n2\n\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nP\n\n\nH\n2\n\n\n\n3\n2\n\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\n\n\nK\n1\n\nK\n\n7\n\n\nK\n8\n\n\nP\n\nH\n2\n\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\nK\n8\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n+\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\nK\n9\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n+\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\n\nK\n\n1\n\n\n5\n2\n\n\n\nK\n4\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nK\n9\n\n\nP\n\n\nH\n2\n\n\n\n5\n2\n\n\n\n\n\n\n\n\n\n (I-11)After substitution of corresponding values into Eq. (I-1) and grouping the constants for convenience, the kinetics model I is derived as Eq. (I-12):\n\n\nr\n=\n\n\n\n\nK\n\n'\n\n\n\n\nP\n\nC\nO\n\n\nP\n\n\n\nH\n2\n\n\n3\n\n-\n\nP\n\nC\n\nH\n4\n\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\nK\n\n\n'\n'\n\n\n\nP\n\n\nH\n2\n\n\n\n5\n2\n\n\n\n\n\n1\n+\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n+\n\nK\n2\n\n\nP\n\nC\nO\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\nK\n\n1\n\n2\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nP\n\n\nH\n2\n\n\n2\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\nK\n\n1\n\n\n3\n2\n\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nP\n\n\nH\n2\n\n\n\n3\n2\n\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\n\n\nK\n1\n\nK\n\n7\n\n\nK\n8\n\n\nP\n\nH\n2\n\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\nK\n8\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n+\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\nK\n9\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n+\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\n\nK\n\n1\n\n\n5\n2\n\n\n\nK\n4\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nK\n9\n\n\nP\n\n\nH\n2\n\n\n\n5\n2\n\n\n\n\n\n\n\n\n2\n\n\n\n\n\n\n\n\n\nK\n\n'\n\n=\n\n\n\nk\n\n3\n\n+\n\n\n\n\nK\n\n1\n\n3\n\nK\n\n2\n\n\nK\n4\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nK\n9\n\n\n\nk\n\n3\n\n-\n\n\n=\n\n\n\nK\n\n1\n\n3\n\nK\n\n2\n\n\nK\n3\n\n\nK\n4\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nK\n9\n\n,\n\n\nK\n\n\n'\n'\n\n\n=\n\n\n\nK\n\n1\n\n\n5\n2\n\n\n\nK\n4\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nK\n9\n\n\n\nk\n\n3\n\n-\n\n\n\n (I-12)Model II: The first-step hydrogenation of surface carbon species (Step 5) is RDS. The resultant rate expression is written as the Eq. (II-1).\n\n(II-1)\n\n\n\nr\n\n=\n\nk\n\n5\n\n+\n\n\n\u03b8\nC\n\n\n\u03b8\nH\n\n-\n\nk\n\n5\n\n-\n\n\n\u03b8\n\nC\nH\n\n\n\n\u03b8\n\n\u2217\n\n\n\n\n\n\nFollowing the same method as above, the coverage of various surface species can be determined as the Equation SII-1 to SII-8 (Supplementary materials). Finally, the resultant kinetics model II is formulated as Eq. (II-2):\n\n\n\nr\n=\n\n\n\n\nK\n\n'\n\n\nP\n\nC\nO\n\n\n\nP\n\n\nH\n2\n\n\n3\n\n-\n\nP\n\nC\n\nH\n4\n\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\nK\n\n\n'\n'\n\n\n\nP\n\n\nH\n2\n\n\n\n3\n2\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\n1\n+\nK\n\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n+\n\nK\n2\n\n\nP\n\nC\nO\n\n\n+\n\nK\n\n1\n\n\n1\n2\n\n\n\nK\n2\n\n\nK\n3\n\n\nP\n\nC\nO\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n+\n\n\n\nK\n1\n\n\nK\n2\n\n\nK\n3\n\n\nK\n4\n\n\nK\n9\n\n\nP\n\nC\nO\n\n\n\nP\n\nH\n2\n\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\nK\n\n1\n\n\n3\n2\n\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nP\n\n\nH\n2\n\n\n\n3\n2\n\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\n\n\nK\n1\n\nK\n\n7\n\n\nK\n8\n\n\nP\n\nH\n2\n\n\n\n\n+\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\nK\n8\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n+\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\nK\n9\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n\n\n\n\n2\n\n\n\n\n\n\n\n\n(II-2)\n\n\n\n\nK\n\n'\n\n=\n\n\n\nk\n\n5\n\n+\n\n\nK\n\n1\n\n3\n\n\nK\n2\n\n\nK\n3\n\n\nK\n4\n\n\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\nK\n\n9\n\n\n\nk\n\n5\n\n-\n\n\n=\n\n\n\nK\n\n1\n\n3\n\nK\n\n2\n\n\nK\n3\n\n\nK\n4\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nK\n9\n\n,\n\n\nK\n\n\n'\n'\n\n\n=\n\n\n\nK\n\n1\n\n\n3\n2\n\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\n\nk\n\n5\n\n-\n\n\n\n\n\n\nModel III: The last-step hydrogenation of surface CH3* species (Step 8) is RDS. The resultant rate expression is indicated as the Eq. (III-1).\n\n(III-1)\n\n\n\nr\n\n=\n\nk\n\n8\n\n+\n\n\n\u03b8\n\nC\n\nH\n3\n\n\n\n\n\u03b8\nH\n\n-\n\nk\n\n8\n\n-\n\n\nP\n\nC\n\nH\n4\n\n\n\n\n\u03b8\n\n\n\u2217\n\n\n2\n\n\n\n\n\nFollowing the same method as above, the coverage of various surface species can be determined as the Equation SIII-1 to SIII-8 (Supplementary materials). Finally, the resultant kinetics model II is formulated as Eq. (III-2):\n\n\n\n\nr\n\n=\n\n\n\n\nK\n\n'\n\n\nP\n\nC\nO\n\n\n\nP\n\n\nH\n2\n\n\n3\n\n-\n\nP\n\nC\n\nH\n4\n\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\nK\n\n\n'\n'\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\n1\n+\nK\n\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n+\n\nK\n2\n\n\nP\n\nC\nO\n\n\n+\n\nK\n\n1\n\n\n1\n2\n\n\n\nK\n2\n\n\nK\n3\n\n\nP\n\nC\nO\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n+\n\n\n\nK\n1\n\n\nK\n2\n\n\nK\n3\n\n\nK\n4\n\n\nK\n9\n\n\nP\n\nC\nO\n\n\n\nP\n\nH\n2\n\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n+\n\n\n\nK\n\n1\n\n\n3\n2\n\n\n\nK\n2\n\n\nK\n3\n\n\nK\n4\n\n\n\n\nK\n5\n\nK\n\n9\n\n\nP\n\nC\nO\n\n\n\nP\n\n\nH\n2\n\n\n\n3\n2\n\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n+\n\n\n\nK\n\n1\n\n2\n\n\nK\n2\n\n\nK\n3\n\n\nK\n4\n\n\n\n\nK\n5\n\n\nK\n6\n\nK\n\n9\n\n\nP\n\nC\nO\n\n\n\nP\n\n\nH\n2\n\n\n2\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n+\n\n\n\nK\n\n1\n\n\n5\n2\n\n\n\nK\n2\n\n\nK\n3\n\n\nK\n4\n\n\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\nK\n\n9\n\n\nP\n\nC\nO\n\n\n\nP\n\n\nH\n2\n\n\n\n5\n2\n\n\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n+\n\n\nP\n\n\nH\n2\n\nO\n\n\n\n\nK\n9\n\n\nK\n\n1\n\n\n1\n2\n\n\n\nP\n\n\nH\n2\n\n\n\n1\n2\n\n\n\n\n\n\n\n\n2\n\n\n\n\n\n\n\n\n(III-2)\n\n\n\n\nK\n\n'\n\n=\n\n\n\n\n\nk\n\n8\n\n+\n\nK\n\n\n1\n\n3\n\n\nK\n2\n\n\nK\n3\n\n\nK\n4\n\n\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\nK\n\n9\n\n\n\nk\n\n8\n\n-\n\n\n=\n\n\n\nK\n\n1\n\n3\n\nK\n\n2\n\n\nK\n3\n\n\nK\n4\n\n\nK\n5\n\n\nK\n6\n\n\nK\n7\n\n\nK\n8\n\n\nK\n9\n\n,\n\n\nK\n\n\n'\n'\n\n\n=\n\n1\n\nk\n\n8\n\n-\n\n\n\n\n\n\nHerein, a group of representative catalysts, including bimetallic and monometallic catalysts, i.e, 8Co/15Ni-OMA, 15Ni-OMA and 15Co/OMA, were selected to match with three kinetics models, respectively. The results of experimental rate fitted with estimated rate using different kinetics models were showed in Fig. 8\n. It is obvious that not all the rate expressions given agree with the experimental results. In the case of bimetallic 8Co/15Ni-OMA catalyst, the experimental rate measured matched very well with the predicted reaction rate by all of the kinetics models, as reflected by aligning on the diagonal line with a slope of unity, along with a determination coefficient of 0.927, 0.930 and 0.958 for model I, II and III, respectively. Likewise, three kinetics models were fitted well with the experimental results measured over monometallic 15Ni-OMA catalyst, as evidenced in good coefficient of determination (r\n2\u00a0=\u00a00.904, 0.920, 0.964). Thus, it is assumed that the kinetics behavior occurring on 8Co/15Ni-OMA and 15Ni-OMA is probably same. For monometallic 15Co/OMA catalyst, however, the reaction rate were predicted very well by kinetics model II and III (r\n2\u00a0=\u00a00.929 and 0.989), whereas kinetics model I could be rejected according to the dataset with poor coefficient of determination (r\n2\u00a0=\u00a00.809), indicating the reaction rate difference of stepwise hydrogenation of surface species over Co-based active sites might be very little, and namely the overall reaction rate of lumped hydrogenation steps could be considered as rate-determining step.As demonstrated in catalyst characterization (Table 2), 15Ni-OMA and 15Co/OMA showed a comparable degree of reduction (81.2% versus 73.7%), but the latter dispersion is poor mainly due to the very low surface area (Tab. S3) and hydrogen diffusion limitation of Co oxide generated from the confinement of OMA. Taking into the catalytic assessment results account (Fig. S5), it was obviously observed that the catalytic performance obtained over 15Ni-OMA and 15Co/OMA was totally different at low temperature range (300\u2013450\u00a0\u00b0C), and the latter only delivered less than 10% CO conversion and CH4 yield. On the other hand, 15Ni-OMA and xCo/15Ni-OMA catalysts showed parallel trends of CO conversion and CH4 yield as function of reaction temperature, and the corresponding activity are much higher than that of 15Co/OMA. Moreover, 13Co/15Ni-OMA also exhibited higher catalytic performance than 15Co/OMA even though both of them represent comparable physicochemical and textural properties (Table 2 and Tab. S3). More importantly, the kinetics behavior observed over 15Ni-OMA and 8Co/15Ni-OMA catalysts are well suited to all the kinetics models, indicating to follow the same elementary sequence and rate-determining step. Thus, it is reasonably proposed that Ni species confined within mesochannels of OMA undertake the dominant active sites for catalyzing CO methanation, whereas the post-impregnated Co species might serve a promotion effect. Considering the kinetics model discrimination over 15Co/OMA, it is assumed that H-assisted CO dissociation rate is faster than the rate of stepwise hydrogenation of surface species because the approving kinetics models II and III are developed under the assumption of the first-step and last-step hydrogenation as the rate-determining step, respectively. As a result, it is supposed that Co-promoted H-assisted CO dissociation results in an enhanced catalytic activity at low-temperature over the bimetallic 8Co/15Ni-OMA catalyst.In summary, we successfully synthesized a set of Co-Ni bimetallic catalysts via post-impregnating cobalt-precursor within the mesochannel of Ni-OMA. Among all of the catalysts, 8Co/15Ni-OMA showed the highest activity at a temperature of as low as 300\u00a0\u00b0C and long-term stability (TOS of 200\u00a0h) under the conditions of 600\u00a0\u00b0C and an extremely high GHSV of 180,000\u00a0mL g\u22121 h\u22121 for the CO methanation reaction. Without the potential limitation of the mass transport and the heat transfer, the apparent activation energy of 8Co/15Ni-OMA for the CO methanation was clearly lower than those of 15Ni-OMA and 15Co-OMA. Based on hydrogen-assisted CO dissociation and Langmuir-Hinshelwood mechanism, three micro-kinetics models were developed by assuming the H-assisted CO dissociation, the hydrogenation of surface carbon species (C*) and surface CH3* species as a rate-determining step, respectively. Moreover, the kinetics data over 15Ni-OMA and 8Co/15Ni-OMA were well fitted with all of the kinetics models. The CH4 formation rate over 15Co/OMA catalyst was satisfactorily fitted with the calculated rate from kinetics model II and III, whereas kinetics model I could be ruled out according to the dataset. Based on the kinetics model discrimination, Ni species confined within mesochannels of OMA was revealed to be dominant active sites for catalyzing the CO methanation, while the post-impregnated Co undertook a promotion effect in the H-assisted CO dissociation step, leading to an enhanced activity at low-temperature and lower apparent activation energy of 100.2\u00a0kJ\u00a0mol\u22121 over the bimetallic 8Co/15Ni-OMA catalyst.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The financial supports from the National Natural Science Foundation of China (U1862116 and 21706155), the National Key Research and Development Program of China (2018YFB0604600-04), and the Fundamental Research Funds for the Central Universities (GK201901001) are highly appreciated.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cesx.2021.100094.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The severe requirement of a higher activity at lower temperatures and a longer stability at higher temperatures evokes a great challenge for the development of an industrially viable catalyst for the CO methanation reaction. In this work, the Co-Ni bimetallic catalysts were synthesized via post-impregnating the cobalt precursor within the mesoporous channel of Ni-embedded ordered mesoporous alumina (Ni-OMA). The low-temperature activity and high-temperature stability of Co/Ni-OMA for the CO methanation were significantly regulated by easily tuning the ratio of free Co and confined Ni species. The optimal catalyst of 8Co/15Ni-OMA showed a high activity with the CH4 formation rate of 126\u00a0mol kgcat\n                     \u2212\n                     1 h\u22121 at a temperature of as low as 300\u00a0\u00b0C and a long-term durability for a time-on-stream of 200\u00a0h without an observable deactivation under the conditions of 600\u00a0\u00b0C and an extremely high GHSV of 180000\u00a0mL g\u22121 h\u22121. Kinetics results reveal that the apparent activation energy of the CO methanation over 8Co/15Ni-OMA (100.2\u00a0kJ mol\u22121) was clearly lower than that over 15Ni-OMA (124.0\u00a0kJ mol\u22121) or 15Co-OMA (131.8\u00a0kJ mol\u22121). In the absence of mass transport and heat transfer limitations, three microkinetics models were developed following the H-assisted CO dissociation and Langmuir-Hinshelwood mechanism, which the H-assisted CO dissociation, the hydrogenation of surface carbon species (C*) or surface CH3* species are proposed as the rate-determining step, respectively. The kinetics behaviors over 15Ni-OMA and 8Co/15Ni-OMA are matched well with all of the kinetics models, indicating the same elementary sequence and rate-determining step. In the case of 15Co/OMA, the CH4 formation rate was predicted very well by the kinetics models derived from the stepwise hydrogenation of surface carbon species as the rate-determining step, and the kinetics model based on the H-assisted CO dissociation as the rate-determining step could be ruled out, indicating that the rate for the H-assisted CO dissociation rate is faster than that of the following stepwise hydrogenation. Based on the discrimination of different kinetics models, Ni species confined within OMA matrix were proposed as the dominant active sites for catalyzing the CO methanation, while the post-impregnated Co was acted as a promoter for the H-assisted CO dissociation. As a result, an enhanced low-temperature activity was achieved over the optimal 8Co/15Ni-OMA catalyst.\n               "}
{"full_text": "Carbon dioxide is the most abundant greenhouse gas and is mainly responsible for the observed global warming. It is the main product of total combustion in power plants and its concentration in the atmosphere has risen considerably since the beginning of the industrial era. In fact, CO2 emissions related to energy grew by 1.4% in 2017, with a record level of 32.5 gigatonnes (Gt) according to the International Energy Agency [1]. Today, CO2 recycling is under increased scrutiny as an alternative to the carbon capture and storage strategy to help CO2 mitigation [2,3]. Carbon dioxide contributes to the synthesis of various higher value chemicals through organic carboxylation reactions leading to chemicals like urea, carboxylic acids, and isocyanates among others [2]. Another route to valuable chemicals is through the formation of syngas followed by further conversion processes such as the Fischer-Tropsch synthesis (FTS) that produces a variety of hydrocarbon fractions. The direct synthesis of methanol from syngas requires a H2/CO ratio of about 2 [4], where the hydroformylation process needs a H2/CO ratio of 1. On the other hand, the FTS usually uses a H2/CO syngas ratio of 2 but would benefit from a higher selectivity towards long-chain hydrocarbons with lower H2/CO ratios [5]. The most commonly used technology to produce syngas is the Steam Reforming of Methane (SRM, Eq. 1) producing a hydrogen rich syngas with a H2/CO ratio of about 3. The dry reforming of methane (DRM, Eq. 2) on the other hand, uses CO2 as oxidant and leads to a syngas H2/CO mixture ratio of a maximum of 1. DRM has the advantage of utilising two of the most abundant greenhouse gases and hence has been increasingly investigated as a CO2 recycling strategy [6,7]. Because the syngas produced by DRM is too poor in H2 to be fed to a FT unit, bi-reforming of methane (BRM, Eq. 3) combining SRM and DRM is proposed to tune the syngas composition.\n\n(1)\nSRM: CH4 + H2O \u21cc CO + 3H2\n\n\n\n\n\n(2)\nDRM: CO2 + CH4 \u21cc 2CO + 2H2\n\n\n\n\n\n(3)\nBRM: 3CH4 + CO2+ 2H2O \u21cc 4CO + 8H2\n\n\n\nBRM is also advantageous in terms of biogas upgrading [8]. The composition of biogas produced through anaerobic digestion varies depending of the source and type of waste used, but consists mainly of CH4, CO2, O2, H2O and impurities which can, after purification treatments, be used in a BRM unit.Reforming processes require high reaction temperatures to reach full reactant conversions but when exposed to such temperatures, typical metal supported on oxides catalysts are subject to deactivation due to sintering of the active phase [9,10]. Coke formation is also a major cause of deactivation due to several side reactions producing carbon such as the Boudouard reaction, CH4 decomposition, CO reduction and CO2 reduction [11\u201313]. Noble metals such as Rh, Ru, Pd and Pt have shown great catalytic activity and coke resistance, however for applications in large scale industrial processes low cost transition metals are preferred. Extensive research has been conducted in the recent years using Ni catalysts. They are low cost and exhibit good performance for reforming but suffer from severe deactivation. Stabilising Ni is essential to prevent sintering and at the same time to reducing carbon formation by preserving small Ni particles. The use of materials such as hexaaluminates, fluorites, perovskites and pyrochlores have been investigated for this purpose in reforming reactions [14\u201319]. Pyrochlores are mixed oxides of general formula A2B2O7. The A-site represents a large trivalent cation, typically a rare-earth metal such as La and the B-site is occupied by a tetravalent cation of smaller diameter, typically a transition metal such as Zr [20]. They are benefit from high thermal stability and high oxygen mobility which makes them suitable candidates for high temperature operations and coke resistance [17]. For this reason, pyrochlores have been previously investigated, in particular in the steam reforming reactions. Ma et al. demonstrated that Ni supported on La2Zr2O7 had superior activity to Ni supported on La2Sn2O7 or \u03b3-Al2O3 due to the large amount of La2O2CO3 formed, effectively suppressing coke formation [18]. Zhang et al. supported Ni on various Ln2Zr2O7 supports with different degrees of order, from pyrochlore to defective fluorites. The amount of oxygen vacancies and therefore mobility was key to mitigate carbon deposition [17]. Substitution of Ni in the B site of a pyrochlore has also shown promising activity in reforming reactions [21\u201324]. Previous work in our group showed that the substitution of 10\u2009wt.% Ni on the B site of a La2Zr2O7 pyrochlore led to a very active, stable and carbon resistant catalyst for DRM [25,26]. However the syngas obtained through DRM had a H2/CO ratio of maximum 0.8 which limits its applicability for further chemical upgrading. With the purpose of tuning the H2/CO ratio of the syngas produced by a stable DRM pyrochlore catalyst, a 10\u2009wt.% Ni doped La2Zr2O7 catalyst was tested under different sets of conditions, including BRM and compared to a supported 10\u2009wt.% Ni on La2Zr2O7 catalyst. The effect of temperature, space velocity and water content in the feed stream were studied as well as the catalyst stability and coke resistance. The studies show promising results for flexible syngas production.The pyrochlore based materials were prepared using a modified citrate method described elsewhere [25]. Lanthanum nitrate [La(NO3)3\u00b76H2O], nickel nitrate [Ni(NO3)2\u00b76H2O], and zirconium nitrate [ZrO(NO3)2\u00b76H2O] provided by Sigma-Aldrich were used as precursors. The necessary amount of each precursor was dissolved in deionized water and then mixed with a citric acid (CA) solution using a CA:metal molar ratio of 0.6:1. The solution was stirred and concentrated in a rotary evaporator. The resulting mixture was dried for 12\u2009h at 100\u2009\u00b0C prior combustion at 200\u2009\u00b0C. The final powders were calcined at 1000\u2009\u00b0C for 8\u2009h to insure phase transition to pyrochlore. Ni was impregnated on the prepared un-doped pyrochlore using an incipient wetness method. [Ni(NO3)2\u00b76H2O] was dissolved in ethanol and mixed to the support. The solvent was removed in a rotary evaporated and the resulting powder was dried for 12\u2009h at 100\u2009\u00b0C before calcination at 500\u2009\u00b0C for 4\u2009h. The doped catalyst will be referred as LNZ10 and the supported catalyst as Ni/LZ.The textural properties of the material were determined by nitrogen adsorption-desorption measurements at \u2212196\u2009\u00b0C in an AUTOSORB-6 fully automated manometric equipment. The sample was degassed under vacuum at 250\u2009\u00b0C for 4\u2009h before each measurement. The BET equation was applied to estimate the specific surface area whilst pore-size distributions were determined using the Barett\u2013Joyner\u2013Halenda (BJH) method.X-ray diffraction (XRD) analysis was conducted on fresh, reduced and used catalysts using an X\u2019Pert Pro Powder Diffractometer by PANalytical. The 2\u03b8 angle was increased by 0.05\u00b0 every 240\u2009s over a range of 20\u201380 \u00b0. Diffraction patterns were recorded at 30\u2009mA and 40\u2009kV, using Cu K\u03b1 radiation (\u03bb =0.154\u2009nm).Temperature programmed reduction with hydrogen (TPR) analysis was carried out on the calcined catalyst in a U-shaped quartz reactor. A 50\u2009mg sample was heated to 900\u2009\u00b0C at a rate of 10\u2009\u00b0C\u2009min\u22121 in a flow of 50\u2009mL min\u22121 of 5% H2 in Ar. A CO2-ethanol trap was used to condense the gaseous products, mostly water, before the on stream thermal conductivity detector (TCD). The H2 uptake was quantified by comparison with the hydrogen consumption of a CuO reference sample.Temperature programmed oxidation (TPO) was conducted in a U-shaped quartz reactor coupled to a PFEIFFER Vacuum PrismaPlus mass spectrometer. Samples were heated up to 900\u2009\u00b0C at a rate of 10\u2009\u00b0C\u2009min\u22121 in a flow of 50\u2009mL min\u22121 (5% O2, 95% He).Raman spectroscopy measurements were performed on a Thermo Scientific DXR Raman Microscope using a green laser (\u03bb =532\u2009nm, maximum power 10\u2009mW) with a spot diameter of 0.7\u2009\u03bcm and a pinhole aperture of 50\u2009\u03bcm. A diffraction grating of 900 grooves mm\u22121, a CCD detector and a 50\u00d7 objective were used.Catalytic activity tests were performed in a computerised commercial Microactivity Reference catalytic reactor (PID Eng&Tech), employing a tubular quartz reactor of 9\u2009mm internal diameter. The catalyst was sieved and the 100\u2013200\u2009\u03bcm fraction was used for testing, diluted with quartz to achieve a catalytic bed of 0.32 cm3. Water was injected into the system by an HPLC pump (Gilson) before being vaporized and mixed with the gas stream before entering the reactor. The composition of the outlet of the reactor was followed by on-line gas chromatography using a MicroGC (Varian 4900) equipped with Porapak Q and MS-5A columns. Prior to reaction, the catalyst was reduced for 1\u2009h at 650\u2009\u00b0C in H2 (10%, v/v in N2). The gas composition was set to CH4/CO2/H2O/N2: 1/1/1/1 to achieve Weight Hourly Space Velocity (WHSV) from 20 to 60\u2009L.g\u22121.\u2009h\u22121.The effect of water partial pressure variation on the catalytic activity was also studied at 700\u2009\u00b0C. In these experiments, the total flow was kept constant using N2 to maintain the WHSV at 60\u2009L.g\u22121.\u2009h\u22121. The feed composition was 25% CH4 and 25% CO2 while the water concentration was modified taking values of 15%, 25% and 35% (v/v).ChemStations\u2019 ChemCad software package was used to calculate the thermodynamic equilibrium fractions for both DRM and BRM reactions over a range of temperatures. The Soave-Redlich-Kwong equation of state was used in a Gibbs reactor. Material flows into the reactor are identical to those intended to be used for experimentation.The XRD profiles of the freshly prepared catalysts are shown in Fig. 1\n. The LNZ10 sample presents the characteristic diffraction features of two different phases. First, a La2Zr2O7 pyrochlore phase (JCPDS Card No. 01-73-0444) was identified, the superstructure peaks (331) and (551) at 36.2\u00b0 and 43.5\u00b0 respectively, indicates that the phase transition between fluorite and pyrochlore was achieved [27\u201329]. These two diffraction peaks, low in intensity, correspond to the ordering of the cations (and anions) in the pyrochlore structure. This was confirmed by Raman analysis (Figure S2) where 5 peaks attributed the pyrochlore phase can be observed. Indeed the group theory predicts six Raman active modes (A1g + Eg + 4 F2g) for the pyrochlore structure (Fd3m) and only one Raman mode (F2g) for the fluorite structure (Fm3m). Here, five peaks corresponding to the pyrochlore-type structured lanthanum zirconate are visible in agreement with the XRD results. The intense Raman peak at 280\u2009cm\u22121 is the Eg mode associated to O-Zr-O bending vibrations and two F2g modes at 492 and 391\u2009cm\u22121 are associated to Zr-O and La-O bond stretching with bending vibrations. The A1g mode at 530\u2009cm\u22121 corresponds to Zr-O6 bending vibrations and the peak at 680\u2009cm\u22121 is assigned to the dopant \u2013O6 symmetrical stretch in the pyrochlore phase [30]. Second, two diffraction peaks at 31.5 and 45.1\u00b0 indicate the presence of a La2NiZrO6 rhombohedral double perovskite oxide phase (JCPDS Card No. 00-044-0624). The Ni loading used here is above of the maximum substitution limit of the pyrochlore structure in agreement with Haynes et al. findings [31], leading to the formation of this additional phase. No characteristic diffraction peaks of individual La2O3 or ZrO2 oxides are observed suggesting a complete incorporation of La2O3 and ZrO2 into the pyrochlore and double perovskite structures. Since no peaks attributed to Ni or NiOx species are detected, Ni is either fully incorporated into the mixed structures or some individual Ni particles are formed outside of the bulk inorganic lattice but are sufficiently small and well dispersed not to be detected by XRD. The Ni/LZ sample on the other hand presents the typical diffraction peaks of NiO additionally to the La2Zr2O7 pyrochlore phase. The La2NiZrO6 rhombohedral double perovskite oxide phase was not formed on the supported catalyst since this phase requires calcination temperatures \u2265 800\u2009\u00b0C to be formed [31].In order to obtain information about the reduction behaviour and interactions among the active species of the as-prepared catalysts, temperature-programmed reduction treatment were performed and the resulting H2 consumption profile are shown in Fig. 2\n. The La2Zr2O7 pyrochlore alone is not reducible [18,26] therefore NiOx species are responsible for any H2 consumption in the catalysts [17,18]. Four reduction processes can be distinguished in the doped catalyst. First, easily accessible NiOx particles located on the outer layer of the catalyst are reduced at 330\u2009\u00b0C. Their weak interactions with the bulk of the catalyst facilitate their reducibility [17,31]. The second reduction process at 370\u2009\u00b0C is attributed to NiOx exsolved from the pyrochlore structure. Those particles are interacting with the La2Zr2O7 pyrochlore and are therefore reduced at higher temperature [26]. The temperature peak at 485\u2009\u00b0C corresponds to most of the H2 uptake and is probably due to the reduction of La2NiZrO6 as observed by Haynes et al. [31]. The fourth reduction process at 600\u2009\u00b0C could correspond to Ni exsolved from the pyrochlore structure and still strongly interacting with the pyrochlore. Overall the hydrogen uptake of the doped catalyst was 1.49\u2009mmol/gcat which corresponds to the reduction of 69% of the Ni content of the catalyst. This suggests that some Ni2+ remains under the double perovskite phase and inside the pyrochlore structure in fair agreement with the X-Ray diffraction results. Indeed, the XRD profile of the reduced catalyst shown in Fig. 7 still presents the characteristic pattern of the double perovskite phase. The supported catalyst on the other hand only presents two reduction processes. The peak at 345\u2009\u00b0C corresponds to the reduction of large NiO particles in loose contact with the support and the second process at 400\u2009\u00b0C is attributed to the reduction of NiO clusters in intimate contact with the pyrochlore [17].The performance of the catalysts were tested under DRM and BRM conditions at 700\u2009\u00b0C for a period of 24\u2009h. The CH4 and CO2 conversions under both reaction conditions are shown in Fig. 3\n. Under DRM conditions, the doped pyrochlore exhibits excellent catalytic activity with CH4 and CO2 conversions of 87% and 90% respectively, reaching thermodynamics equilibrium. However, when 25% steam is introduced into the system, the conversions decrease to 54% for CH4 and 39% for CO2. The latter is a consequence of the thermodynamic constraints when DRM and BRM are coupled and also reflects the increased competition of both reactants with the new reactant (water) to reach the active sites of the catalysts. Thermodynamics predict a methane conversion of 92% and a carbon dioxide conversion of 47%. The lower performance of the doped catalyst, in particular in terms of methane conversion may be attributed to the reduced activation of H2O on the pyrochlore. In both scenarios, the doped catalyst stabilises very rapidly and shows no deactivation over the time frame of the experiments. On the other hand, the supported catalyst deactivates rapidly, emphasizing the Ni stabilisation induced by the doping strategy. In dry conditions the conversion of CO2 is slightly larger than CH4 likely due to the occurrence of the reverse water gas shift reaction (RWGS), consuming some of the carbon dioxide as reported elsewhere [32]. On the other hand, under BRM conditions, CH4 conversion is largely above the one of CO2. When steam is introduced, RWGS is no longer favoured and in turn SMR and forward WGS occur, therefore consuming more methane and increasing the H2/CO ratio.The Ni-doped pyrochlore catalyst showed great performance in terms of activity and stability for both DRM and BRM. In order to tune the H2/CO ratio for downstream processes, the effect of steam addition in the feed stream was studied. The catalytic activity of the catalyst in terms of CH4 and CO2 conversions and H2/CO ratio as a function of water content is shown in Fig. 4\n. The performance of the catalyst was tested at relatively high space velocity (60\u2009L.g\u22121.\u2009h\u22121) due to equipment limitations. As expected, the H2/CO ratio of the products increases greatly as the water content increases. DRM produces a syngas of H2/CO\u2009=\u20090.7 but, by introducing 35% steam, this ratio can be increased to 2.5. For an FT unit or methanol production, a quantity of 30% water would be necessary to obtain a H2 rich syngas or metgas of H2/CO\u2009=\u20092. It seems however that the improvement in selectivity is made at the expense of conversion. Indeed, as more water is introduced the reactant conversion decreases. Water may promote the SMR reaction but overall, the catalytic activity of the pyrochlore catalyst decreases. Thermodynamically, the addition of water should lead to larger CH4 conversion but a reduced CO2 conversion. The observed decrease in both conversions is possibly due to a change in the kinetics of the reaction induced by water introduction. To the best of our knowledge, no kinetic or mechanistic study has been conducted on BRM to date using a comparable reactants mixture. However, SMR kinetics have been studied. Various studies in the literature have claimed a negative order of steam for SMR [33,34]. The dependence of steam on the rate of reaction can be due to the competition between CH4 and H2O on the catalyst active sites as previously reported elsewhere and in good agreement with our trends [35].Space velocity is a major parameter to consider for scaling up. It determines the volume of the reforming unit and the amount of catalyst needed. The space velocity effect was investigated under BRM conditions with 25% of steam and the results are shown in Fig. 5\n. Overall conversions of CH4 and CO2 decrease by increasing the space velocity, although the selectivity remains unchanged. At high space velocity, conversions are far from equilibrium values. However, when the space velocity is decreased to 20\u2009L.g\u22121.\u2009h\u22121, CO2 conversion nearly reaches the thermodynamic value. CH4 on the other hand seems to be more affected by WHSV as a more significant decrease in conversion is observed when WHSV is increased. This observation actually reflects the fact that methane activation is the rate limiting step for this reaction [33] and therefore the conversion of this reactant is very sensitive to the operation conditions and the catalysts choice. In any case the fact that our catalyst can maintain a H2/CO ratio of over 1.5 (very close to the equilibrium limit) is a commendable achievement for the pyrochlore-perovskite material which reflects the potential of this advanced catalyst for hydrogen-rich syngas production.The effect of temperature in BRM conditions was studied using a feed containing 25% of water and results are shown in Fig. 6\n\n. An increase in conversion is observed as the temperature increases as the thermodynamics predicts. At low temperature methane conversion is low and far away from equilibrium but as the temperature increases it gets closer to the equilibrium values and reaches a conversion of 80% at 750\u2009\u00b0C. The low methane conversion at low temperature can be related to the high activation energy of CH4. Methane needs high temperature to overcome the energy barrier necessary for its activation. Similarly to methane, CO2 conversion is lower than the equilibrium at low temperature but gets closer to it as the temperature increases. At 750\u2009\u00b0C, CO2 conversion is only 2% below equilibrium reaching 51% conversion. In terms of selectivity, the H2/CO ratio follows the equilibrium trend and decreases slightly with the temperature.The development of stable catalysts is one of the bottleneck for the implementation of combined reforming in commercial CO2 conversion units. In this scenario, post reaction analysis is necessary to ascertain the robustness of our multicomponent catalyst under the studied reaction conditions. XRD was performed on the catalyst after reduction pre-treatment and after reaction at 700\u2009\u00b0C under DRM and BRM conditions to detect any structural changes induced to the catalyst. The resulting profiles are shown in Fig. 7. No structural changes were detected between the fresh catalyst (Fig. 1) and the reduced catalyst. The characteristic diffraction features of La2Zr2O7 are still present attesting of the thermal stability of this material. No trace of metallic Ni was detected showing that either the reduced Ni particles are small and well dispersed or that Ni remains in the pyrochlore and double perovskite phases. Moreover the double perovskite phase La2NiZrO6 is still present and did not completely reduce to Ni, La2O3 and ZrO2. After DRM and BRM reactions, a shoulder is detected at 44.4\u00b0 and a small peak appears at 51.7\u00b0, corresponding to the main diffraction peaks of metallic Ni. The appearance of these peaks could be due to a certain degree of Ni sintering but also to the exsolution of Ni from the pyrochlore structure during reaction [26]. In view of the excellent performance with no deactivation observed during 24\u2009h of continuous run the exsolution of Ni is plausible in agreement with previous reports [36]. In fact, the low surface area of the catalyst (Table S1, Supporting information) supports this claim as Ni is very small (around 21\u2009nm after reaction, according to the Scherrer equation) and therefore must be well dispersed on the surface and possibly released from the structure as the reaction takes place. Traces of La2O3 (JCPDS Card No. 01-074-2430) are detected after reaction, resulting either from the partial decomposition of the pyrochlore or from the partial reduction of the double perovskite.Carbon is a side product of CO2-reforming reactions and is the main cause of catalyst deactivation. Carbonaceous species potentially formed on the catalyst were quantified and identified by temperature programmed oxidation. The CO2 production profiles of samples that have undergone DRM and BRM with different amount of steam are shown in Fig. 8\n. This analysis shows that no carbon was formed on the catalyst when water was introduced into the system. Indeed under BRM conditions, carbon formation is minimised due to the reverse CO reduction reaction (C(s) + H2O \u2192 CO + H2). The presence of steam prevents the deactivation of the catalyst by carbon formation. However under DRM conditions (i.e. 0% water) a significant amount of carbon was formed (0.01gC/gcat). Three different oxidation processes can be distinguished corresponding to different carbonaceous species. The low temperature peak at 270\u2009\u00b0C is attributed to the gasification of C\u03b1 amorphous carbon. C\u03b1 are believed to be active species in reforming, originating from nickel carbide produced during methane decomposition [37]. The second peak at 445\u2009\u00b0C corresponds to C\u03b2 filament carbon. This type of carbon can be eliminated at relatively low temperature [38]. Finally, the third peak, at 705\u2009\u00b0C, corresponds to the oxidation of more graphitic carbon, inert and requiring high temperature to remove. The later indicates that coking will be a factor to consider if our pyrochlore-perovskite catalysts are going to be used in a DRM unit. Nonetheless, the addition of water (BRM mode) heavily mitigates the impact of carbon deposition resulting in a stable catalyst to produce H2-rich syngas streams. The Raman analysis presented in Figure S2 supports this claim. Experiments were conducted on the samples after reaction. No evidence of carbon species were found on the samples after BRM reaction. However the typical D and G bands of multiwall carbon nanotubes were observed at 1342 and 1576 cm-1 respectively on the sample after DRM.This work provides evidence of the excellent performance of a Nickel-doped pyrochlore catalyst for chemical CO2 recycling via DRM and BRM. Structural analysis revealed the presence of the pyrochlore and a secondary double perovskite phase which constitutes the basis of this novel catalyst. After the reaction, small Ni clusters are present on the surface of the catalyst as suggested by XRD and TPR. In fact, it is very likely that active Ni clusters are exsolved from the pyrochlore during BRM and DRM leading to highly dispersed active ensembles which account for the high activity and stability of the catalyst during both reactions. Very importantly, the H2/CO ratio produced by the catalyst can be fine-tuned by introducing steam into the system, enabling a flexible syngas production for a variety of applications. Our engineered catalyst also allows adjustment of the syngas ratio under different reactions conditions such as temperature and space time, thus making it very versatile when process integration is considered. As an additional advantage, carbon deposition over the pyrochlore-perovskite catalyst is fully eliminated when steam is added to the reforming mixtures. Overall, this work showcases a strategy to design highly effective heterogeneous catalysts for gas-phase CO2 valorisation \u2013 the stabilisation of Ni particles on a complex mixed oxide structure resulting in a powerful dry and bi-reforming catalyst able to deliver customised syngas for chemical synthesis.This work was supported by the Department of Chemical and Process Engineering at the University of Surrey and the EPSRC grant EP/R512904/1 as well as the Royal Society Research Grant\nRSGR1180353. This work was also partially sponsored by the CO2Chem through the EPSRC grant EP/P026435/1. The Ministerio de Econom\u00eda, Industria y Competitividad of Spain (Project ENE2015-66975-C3-2-R) co-financed by FEDER funds from the European Union supported the work done in Spain. Finally, E. le Sach\u00e9 would like to acknowledge the Armourers & Brasiers Gauntlet Trust for their travel grant award.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.05.039.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  The bi-reforming of methane (BRM) has the advantage of utilising greenhouse gases and producing H2 rich syngas. In this work Ni stabilised in a pyrochlore-double perovskite structure is reported as a viable catalyst for both Dry Reforming of Methane (DRM) and BRM. A 10\u2009wt.% Ni-doped La2Zr2O7 pyrochlore catalyst was synthesised, characterised and tested under both reaction conditions and its performance was compared to a supported Ni/La2Zr2O7. In particular the effect of steam addition is investigated revealing that steam increases the H2 content in the syngas but limits reactants conversions. The effect of temperature, space velocity and time on stream was studied under BRM conditions and brought out the performance of the material in terms of activity and stability. No deactivation was observed, in fact the addition of steam helped to mitigate carbon deposition. Small and well dispersed Ni clusters, possibly resulting from the progressive exsolution of Ni from the mixed oxide structure could explain the enhanced performance of the catalyst.\n               "}
{"full_text": "Oxygen-involved electrocatalytic reactions, including 4e\u2212/2e\u2212oxygen reduction reaction (ORR), 4e\u2212oxygen evolution reaction (OER), and 2e\u2212water oxidation reaction (WOR), are key reactions for new-generation energy technologies utilizing renewable clean fuels [1\u20133]. Up to now, tremendous effort has been devoted to the discovery and design of advanced electrocatalysts, such as transition metal/alloys [4], sulfides [5], phosphides [6], oxides [7,8], (Oxy)hydroxides [9], and carbon-based materials [10]. Among them, precious metal materials, mainly platinum-group metals [11,12], are the only commercially available catalysts, but their high costs and scarcity severely inhibit their large-scale commercial applications. Designing non-precious metal catalysts with high oxygen electrocatalytic performance to replace noble-metal-based catalysts is essential for practical applications.Single-atom catalysts (SACs) can be obtained by anchoring the single transition metal (TM) atom on nitrogen-doped carbon materials [13\u201317], and have been employed in oxygen-involving reactions. The active centers of SACs are mainly tetracoordinate planar TM-N4 moieties, and some SACs (e.g., Mn, Fe, Co, Ni, and Cu) can efficiently catalyze oxygen reactions [18\u201323]. For example, Fe-based SACs favor ORR, while Co-based SACs tend to catalyze O2 into H2O2 [24,25]. Nevertheless, other SACs, especially those that originated from early TMs, generally exhibit extremely poor performance due to their relatively excessive adsorption strengths [26].In this work, to effectively reduce the adsorption strength of single-atom catalysts, we tune strong axial coordination (-OH,\u00a0=O, and \u2261N) to SACs for tailoring the adsorption ability of TM-derived SACs. Using density functional theory (DFT) calculations, we systematically investigated the role of axial coordination in regulating O2 adsorption and catalytic performance on various SACs. Our results revealed that many SACs, not reported yet, exhibited exceptionally good activity for 4e\u2212/2e\u2212 oxygen reactions, including V-SAC-OH with an overpotential of 0.61\u00a0V for 4e\u2212ORR, Mo-SAC-OH with an overpotential of 0.05\u00a0V for 2e\u2212ORR, Mo-SAC-O with an overpotential of 0.52\u00a0V for 4e\u2212OER, and Nb-SAC-OH with an overpotential of 0.14\u00a0V for 2e\u2212WOR. Herein, we demonstrated that weakening the adsorption ability of SACs towards oxygenated species (\u2217OOH, \u2217OH, and \u2217O) can promote the catalytic activity toward different oxygen-involved reactions. Furthermore, we proposed a theoretical framework that integrates the SAC configuration, TM species, and TM charges to describe the catalytic ability of SACs. Our results built a full profile to understand the catalytic behavior of SACs and provided a new approach for developing highly active SACs in oxygen-involved reactions.All spin-polarized [27] DFT calculations were conducted with the Vienna Ab initio simulation package (VASP) [28,29]. The exchange-correlation functional was described by the popular Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) [30]. The frozen-core projector-augmented wave (PAW) method with a cutoff energy of 520\u00a0eV was used to describe the interaction between core electrons and valence electrons [31]. In addition, Grimme's DFT-D3 scheme was used to describe the long-range vdW interactions [32]. A 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a01 supercell of graphene layer embedded with The TM-N4 moiety was embedded in a 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a01 supercell of graphene layer to simulate the TM-SAC catalyst. A 15\u00a0\u00c5 vacuum layer was set to eliminate the interactions with the periodic images along the Z axial direction. \u0393-centred Monkhorst\u2013Pack k-point mesh grid of 3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a01 was employed for all supercells [33]. Moreover, the criteria of energy and force convergence were set to 1.0\u00a0\u00d7\u00a010\u22125\u00a0eV per atom and 0.02\u00a0eV\u00a0\u00c5\u22121 for geometry optimization, respectively. Bader charge analysis was used to study the atomic charge changes. The VESTA program [34] was employed to construct all models and to plot charge density differences.The elementary steps for 4e\u2212ORR in an acidic medium are shown in Eqs (1)\u2013(4), whereas the elementary steps for 2e\u2212ORR to produce H2O2 are the combination of Eqs (1) and (5).\n\n(1)\n\n\n\u2217\n\n+\n\n\nO\n2\n\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\u2192\n\u2217\nO\nO\nH\n\n\n\n\n\n\n(2)\n\n\n\u2217\nO\nO\nH\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\u2192\n\n\u2217\nO\n\n+\n\n\nH\n2\n\nO\n\n\n\n\n\n\n(3)\n\n\n\u2217\nO\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\u2192\n\n\u2217\nO\nH\n\n\n\n\n\n\n(4)\n\n\n\u2217\nO\nH\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\u2192\n\n\u2217\n\n+\n\n\nH\n2\n\nO\n\n\n\n\n\n\n(5)\n\n\n\u2217\nO\nO\nH\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\u2192\n\n\u2217\n\n+\n\n\nH\n2\n\n\nO\n2\n\n\n\n\n\nThe 4e\u2212OER pathway is shown in Eqs (6)\u2013(9), and the 2e\u2212WOR pathway to form H2O2 is the combination of Eqs (6) and (10).\n\n(6)\n\n\n\u2217\n\n+\n\n\nH\n2\n\nO\n\n\u2192\n\n\u2217\nO\nH\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\n\n\n\n\n(7)\n\n\n\u2217\nO\nH\n\n\u2192\n\n\u2217\nO\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\n\n\n\n\n(8)\n\n\n\u2217\nO\n+\n\n\nH\n2\n\nO\n\n\u2192\n\n\u2217\nO\nO\nH\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\n\n\n\n\n(9)\n\n\n\u2217\nO\nO\nH\n\n\u2192\n\n\u2217\n\n+\n\n\nO\n2\n\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\n\n\n\n\n(10)\n\n\n\u2217\nO\nH\n+\n\n\nH\n2\n\nO\n\n\u2192\n\n\u2217\n\n+\n\n\nH\n2\n\n\nO\n2\n\n+\n\n\nH\n+\n\n\n+\n\n\ne\n-\n\n\n\n\nwhere \u2217 denote adsorbed sites on SACs, adsorbed intermediates \u2217OOH, \u2217O, and \u2217OH are adsorbed intermediates, respectively.For each elemental step, the Gibbs free energy \n\n\u0394\n\nG\ni\n\n\n can be calculated using Eq (11).\n\n(11)\n\n\n\u0394\nG\n\n=\n\u0394\nE\n\n+\n\n\u0394\n\nE\n\nZ\nP\nE\n\n\n\n-\n\nT\n\u0394\nS\n\n+\n\n\u0394\n\nG\n\np\nH\n\n\n\n+\n\n\u0394\n\nG\nU\n\n\n\n\nwhere \n\n\u0394\nE\n\n is the total energy of reactions obtained from DFT calculations, \n\n\u0394\n\nE\n\nZ\nP\nE\n\n\n\n and \n\n\u0394\nS\n\n represent the changes of zero-point energy and entropy, respectively. T denotes the temperature (298.15\u00a0K). The zero-point energy and entropy are calculated using the vibrational frequencies of the oxygenated intermediate species based on the harmonic normal mode approximation while fixing the catalyst slab. \u0394G\npH\u00a0=\u00a0\u2212k\nB\nTln[H+]\u00a0=\u00a0k\nB\u00a0\u00d7\u00a0pH\u00a0\u00d7\u00a0ln10 is the contribution of H+ concentration change for Gibbs free energy during the ORR process, where k\nB is the Boltzmann constant, and the value of pH is assumed to be zero. \n\n\u0394\n\nG\nU\n\n\n=\n\n-\nn\ne\nU\n\n is the contribution of applied electrode potential, where n is the number of electrons transferred in each elemental reaction and U is the applied electrode potential. Besides, according to the computational hydrogen electrode (CHE) model suggested by N\u00f8rskov et\u00a0al. [35,36], the chemical potential of a proton/electron pair is equal to half of the energy of H2. Due to the difficulties in the DFT calculations of open-shell triplet O2, the free energy of the O2(g) molecule is calculated by \n\n\nG\n\n\nO\n2\n\n\n(\ng\n)\n\n\n\n\n=\n\n2\n\nG\n\n\nH\n2\n\nO\n\n\n\nt\n\n2\n\nG\n\nH\n\n2\n\n\n\n+\n\n4.92\n\n eV [35].Based on the free energies of elemental steps, the thermodynamic overpotential of ORR/OER/WOR on SACs can be obtained via Eqs (12)\u2013(15). The elementary step with the maximum overpotential is considered the potential-determining step (PDS), which limits the ORR/OER/WOR processes.\n\n(12)\n\n\n\n\u03b7\n\n4\n\ne\n\u2212\n\n\nO\nR\nR\n\n\n\n=\nm\na\nx\n\n{\n\n\u0394\n\nG\n1\n\n,\n\n\u0394\n\nG\n2\n\n,\n\n\u0394\n\nG\n3\n\n,\n\n\u0394\n\nG\n4\n\n\n}\n\n/\ne\n\n+\n\n1.23\n\nV\n\n\n\n\n\n\n(13)\n\n\n\n\u03b7\n\n2\n\ne\n\u2212\n\n\nO\nR\nR\n\n\n\n=\nm\na\nx\n\n{\n\n\u0394\n\nG\n1\n\n,\n\n\u0394\n\nG\n5\n\n\n}\n\n/\ne\n\n+\n\n0.68\n\nV\n\n\n\n\n\n\n(14)\n\n\n\n\u03b7\n\n4\n\ne\n\u2212\n\nO\nE\nR\n\n\n\n=\nm\na\nx\n\n{\n\n\u0394\n\nG\n6\n\n,\n\u0394\n\nG\n7\n\n,\n\n\u0394\n\nG\n8\n\n,\n\n\u0394\n\nG\n9\n\n\n}\n\n/\ne\n\u2212\n\n1.23\n\nV\n\n\n\n\n\n\n(15)\n\n\n\n\u03b7\n\n2\n\ne\n\u2212\n\nW\nO\nR\n\n\n\n=\nm\na\nx\n\n{\n\n\u0394\n\nG\n6\n\n,\n\u0394\n\nG\n10\n\n\n}\n\n/\ne\n\n-\n\n1.78\n\nV\n\n\n\n\nEqs (16)\u2013(18) are the reactions for the formations of key intermediates on SACs.\n\n(16)\n\n\n\u2217\n\n+\n\n2\n\nH\n2\n\nO\n\n\u2192\n\n\u2217\nO\nO\nH\n\n+\n\n3\n/\n2\n\nH\n2\n\n\n\n\n\n\n\n(17)\n\n\n\u2217\n\n+\n\n\nH\n2\n\nO\n\n\u2192\n\n\u2217\nO\n\n+\n\n\nH\n2\n\n\n\n\n\n\n\n(18)\n\n\n\u2217\n\n+\n\n\nH\n2\n\nO\n\n\u2192\n\n\u2217\nO\nH\n\n+\n\n1\n/\n2\n\nH\n2\n\n\n\n\n\nThe calculated free energy of formation of each key ORR-intermediate can be obtained by using Eqs (19)\u2013(21):\n\n(19)\n\n\n\u0394\n\nG\n\n\u2217\nO\nO\nH\n\n\n\n=\n\n\nG\n\n\u2217\nO\nO\nH\n\n\n\n+\n\n3\n/\n2\n\nG\n\nH\n\n2\n\n\n\n\n\u2212\n\nG\n\n\u2217\n\n\n\n\u2212\n\n2\n\nG\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\n\n(20)\n\n\n\u0394\n\nG\n\n\u2217\nO\n\n\n\n=\n\n\nG\n\n\u2217\nO\n\n\n\n+\n\n\nG\n\nH\n\n2\n\n\n\n\n-\n\n\nG\n\n\u2217\n\n\n\n-\n\n\nG\n\n\nH\n2\n\nO\n\n\n\n\n\n\n\n\n(21)\n\n\n\u0394\n\nG\n\n\u2217\nO\nH\n\n\n\n=\n\n\nG\n\n\u2217\nO\nH\n\n\n\n+\n1\n/\n2\n\n\nG\n\nH\n\n2\n\n\n\n\n-\n\n\nG\n\u2217\n\n\n-\n\n\nG\n\n\nH\n2\n\nO\n\n\n\n\n\n\nExperimentally, TMs can be embedded into the bulk vacancies such as single vacancy, double vacancies, and Stone-Wales defects [37], to form single-atomic active sites. It is observed that most of the metals are trapped at the nitrogenated double vacancies based on the advanced characterization including transmission electron microscopy and X-ray absorption near edge structure [23,38], displayed in Fig.\u00a01\n(a), and it is the SAC model in this study. A total of 18 metals, including all 3d TMs except Sc, six 4d TMs, and three 5d TMs can be anchored on N-doped graphene via chemically binding with four pyridinic-nitrogen atoms (Figs.\u00a0S1\u20132). Due to their lone-pair orbitals, we choose axial ligands (e.g., \u2013OH,\u00a0=O, and \u2261N) to strongly bound to the central metals of these SACs (Fig.\u00a0S3) via single-, double-, and triple-bonds, respectively, in Fig.\u00a01(b). Increasing the axially coordinated orbitals with d orbitals can gradually increase the interaction with central metals of SACs. In addition, the dissolution potential in Fig.\u00a0S4 shows that SACs with axial ligands exhibit higher stability in an electro-chemical environment. Finally, 72 SACs were screened for further investigation.Theoretically, O2 adsorption/desorption is a key process for ORR/OER, reflecting the reactivity of SACs. We thus estimated the O2 adsorption ability on SACs through O2 adsorption energy and configuration. Fig.\u00a01(c) demonstrated four adsorption configurations of O2 molecules, including dissociation adsorption O\n\n2ad\n\n(1), side-on adsorption O\n\n2ad\n\n(2), top-on adsorption O\n\n2ad\n\n(3), and physisorption O\n\n2ad\n\n(4). These configurations of O2 adsorption highly correlate with the interaction between oxygen molecules and active sites [39], revealing the change in the adsorption ability of SACs.We then optimized O2 adsorption on SACs. Our computations in Fig.\u00a01(d) demonstrated that SACs derived from early TMs strongly adsorb O2 and form the O\n\n2ad\n\n(2) configuration with stretched O2 bonding length. This indicates that these early TMs delivered a high adsorption ability towards O2. SACs derived from late TMs show relatively weak adsorption ability with the O\n\n2ad\n\n(3) or O\n\n2ad\n\n(4) configuration. With the axial ligands being stronger, the O2 adsorption configuration gradually changes from O\n\n2ad\n\n(2) to O\n\n2ad\n\n(4), especially for Ti, V, Nb, and Mo. Besides, the corresponding O\u2013O binding length and adsorption energy decrease with stronger ligand bonding as displayed in Figs.\u00a0S5 and S6. Hence, axial coordination was confirmed to effectively regulate the adsorption of SACs for further investigation in oxygen reactions.The intrinsic catalytic activity of SACs with axial ligands towards oxygen reaction was further investigated. As exhibited in Fig.\u00a02\n(a), the 4-electron ORR can be divided into four steps based on the number of electron transfers. Depending on the O2 adsorption configuration, the adsorbed O2 interacted with the first H+/e\u2212 pair and then generate three different species 1, including \u2217O+\u2217OH, \u2217O\u2217OH, and \u2217OOH. Besides, all these species 1 can expose two oxygen sites for the subsequent hydrogenation. The second H+/e\u2212 pair can accordingly attack either of the oxygen atoms of species 1 to form species 2, including the intermediates (\u2217OH+\u2217OH or \u2217O) or the production of H2O2. The generation of H2O2 implies the completion of 2e\u2212ORR and the start of the next reaction cycle [2]. If the third electron/proton transfer occurs, the above intermediates can only convert into species 3, namely \u2217OH, and then reduced to H2O, followed by desorption. With 4-electron transferred, O2 can be reduced to H2O, which finalizes the 4e\u2212ORR process [40]. Along the reverse direction of electron transfer, the H2O molecule couples two or four electrons to form H2O2 and O2, corresponding to 2e\u2212WOR and 4e\u2212OER, respectively [8,41].Within the theoretical frame of such four oxygen-involving reactions, we calculated the free energy change for each elementary reaction in Eqs (1)\u2013(10) (Tables\u00a0S3\u20134). Based on Eqs (12)\u2013(15), we estimated the catalytic activity of SACs (with/without axial coordination) by using the DFT calculated overpotential. Fig.\u00a02(b-e) summarized the heatmaps of the overpotentials on SACs. Taking the 4e\u2212ORR overpotential of 0.43\u00a0V on Pt (111) as a golden standard [35], a few SACs even without axial coordination exhibit high catalytic performances such as Cr- and Mn-SAC for 2e\u2212ORR, Co- and Rh-SAC for 4e\u2212ORR, 2e\u2212ORR, and 4e\u2212OER, Ni-, Cu-, Ag-, and Au-SAC for 2e\u2212WOR, Zn-SAC for 4e\u2212OER. None of the SACs derived from the other 11\u00a0TMs, especially the early TMs, exhibit low overpotentials for the oxygen-involving reactions, revealing that the late TM metal-derived SACs have a superior catalytic activity compared to the early TM-derived SAC in catalyzing O2/H2O conversion [42]. With axial coordination, a large number of newly active SACs arise including 6 SACs for 4e\u2212ORR, 16 SACs for 2e\u2212ORR, and 5 SACs for 4e\u2212OER, and 20 SACs for 2e\u2212WOR. Meanwhile, some SACs possess higher or comparable catalytic performance to the conventional tetra-coordinated SACs: 4 SACs for 4e\u2212ORR, 10 SACs for 2e\u2212ORR, and 6 SAC for 2e\u2212WOR. Especially, Mo-SAC-OH possesses the lowest overpotential of 0.05\u00a0V towards 2e\u2212ORR, revealing that the Mo-SAC-OH is a potential excellent catalyst for 2e\u2212ORR. Nb-SAC-OH exhibited the best catalytic activity towards 2e\u2212WOR with the lowest overpotential of 0.01\u00a0V. Therefore, it proved again that moderate axial coordination can be an efficient strategy to regulate the catalytic activity of SACs. Furthermore, it is worth noting that early TM-derived SACs display great potential in the development of new catalysts for catalyzing oxygen reactions via axial coordination.The above activity data motivated us to find an underlying mechanism to shed light on the regulation rule for SAC systems. First, we constructed the scaling relation between the adsorption energy of three key intermediates (species 1, 2, and 3) to describe the adsorption behavior of SACs. Apart from \u2217O+\u2217OH on Ti-SAC, V-SAC, Nb-SAC, and Mo-SAC, the adsorption energy of other species 1 (\u2217O\u2217OH and \u2217OOH) linearly correlated with the adsorption energy of \u2217OH with R-square of 0.93 in Fig.\u00a03\n(a).The linear relation between \u0394G\n\u2217OOH and \u0394G\n\u2217OH is \u0394G\n\u2217OOH\u00a0=\u00a00.87\u0394G\n\u2217OH\u00a0+ 3.2. This is similar to the scaling relation: \u0394G\n\u2217OOH\u00a0=\u00a0\u0394G\n\u2217OH\u00a0+ 3.2 in metal oxides [43]. Meanwhile, due to the different bonding patterns in \u2217O/\u2217OH (double-bond vs single-bond) and the high sensitivity of double-bonded \u2217O to the adsorption site, the linear relationship for \u2217O/\u2217OH (R-square\u00a0=\u00a00.84) is not as good as that for \u2217OH/\u2217OOH (R-square\u00a0=\u00a00.93), and the fitted linear relationship for \u2217O/\u2217OH is: \u0394G\n\u2217O\u00a0=\u00a01.57\u0394G\n\u2217OH\u00a0+\u00a00.9. Accordingly, these linear fitting showed that the adsorption energy of \u2217OH can be employed as a descriptor to describe the influence of the change of adsorption in the catalytic activity. Furthermore, Fig.\u00a03(b) exhibits a volcano-shaped relationship between free energy change along oxygen reactions and the adsorption energy of \u2217OH. The commercial Pt/C has an overpotential of 0.43\u00a0V for ORR, and we have used this value to screen the electrocatalysts. It is worth noting in Fig.\u00a0S7 that there are four volcano peaks corresponding to 4e\u2212ORR [44], 4e\u2212OER [45], 2e\u2212ORR [46], and 2e\u2212WOR [41], respectively. The optimal \u0394G\n\u2217OH is corresponding to 0.92, 1.10, 1.20, and 1.93\u00a0eV. It indicates that the high-performance 4e\u2212ORR catalysts require relatively strong adsorption, whereas 4e\u2212OER, 2e\u2212ORR, and 2e\u2212WOR require weak \u0394G\n\u2217OH. Fig.\u00a0S8 demonstrates that weakening \u2217OH adsorption can gradually increase the catalytic activity toward 4e\u2212ORR, 4e\u2212OER, 2e\u2212ORR, and 2e\u2212WOR. Fig.\u00a03(c) presents the \u0394G\n\u2217OH values of conventional SACs, and Ti-, V-, Cr-, Mo-, Nb-, Ru-, and Os-SACs possess strong adsorption with \u0394G\n\u2217OH below or close to 0\u00a0V and are located in area I in Fig.\u00a03(b). This indicated that they have poor catalytic activity towards all oxygen reactions, which is consistent with the previous discussion. Mn-, Fe-, Co-, Ni-, and Rh-SACs exhibited a feasible \u0394G\n\u2217OH near 1\u00a0eV in area II. These SACs deliver good ORR performance [21]. Among them, with the larger value of the \u0394G\n\u2217OH, the SACs tend to be closer to area II, implying an improved activity of 2e\u2212ORR, and 2e\u2212WOR. With high \u0394G\n\u2217OH above 1.90\u00a0eV, Ni-, Cu-, Zn-, Pd-, Pt-SACs exhibit weak adsorption in area IV, which indicates the high activity of 2e\u2212WOR. Besides, it can be found that axial coordination can decrease \u0394G\n\u2217OH. Furthermore, early TMs exhibit a sharp decrease of \u0394G\n\u2217OH compared with late TMs and a moderate \u0394G\n\u2217OH approaching or slightly exceeding 1\u00a0eV. This breaks the limitation of SACs derived from early TMs to catalyze oxygen reactions. Consequently, as shown in Fig.\u00a03(d), there are a growing number of SACs with axial coordination exhibiting high catalytic activity towards either 4e\u2212ORR, 4e\u2212OER, 2e\u2212ORR, or 2e\u2212WOR. Therefore, the axial coordination strategy can effectively tune the catalytic activity of SACs towards oxygen-involved reactions and extend the SACs to the early TMs.In this section, we discuss the correlation between the reduced adsorption energy of SACs and the improved catalytic activity towards oxygen-involved reactions. Electron transfer processes on \u2217OOH, \u2217OH, and \u2217O adsorption on Mo-SAC with/without axial coordination were analyzed, and displayed in Fig.\u00a04\n(a-d). Mo-SAC displays strong adsorption of \u2217OOH (\u0394G\n\u2217OOH\u00a0=\u00a0\u22121.55\u00a0eV), \u2217OH (\u0394G\n\u2217OH\u00a0=\u00a0\u22121.52\u00a0eV), and \u2217O (\u0394G\n\u2217O\u00a0=\u00a0\u22122.56\u00a0eV), phenomenally corresponding to the prolonged O\u2013O the bond length of 2.63\u00a0\u00c5 in \u2217OOH, short Mo\u2013O bond lengths of 1.86 and 1.69\u00a0\u00c5 in \u2217OH and \u2217O. Comparably, with the additional \u2013OH ligand in an axial direction, the adsorption ability is weakened (e.g., \u0394G\n\u2217OOH\u00a0=\u00a04.18\u00a0eV, \u0394G\n\u2217OH\u00a0=\u00a01.04\u00a0eV, and \u0394G\n\u2217O\u00a0=\u00a00.70\u00a0eV), which attributed to a shorter O\u2013O the bond length of 1.46\u00a0\u00c5. Additionally, the Mo\u2013O bond lengths in \u2217OH and \u2217O were stretched to1.91 and 1.72\u00a0\u00c5, respectively.As shown in Fig.\u00a04(b) and (d), the presence of axial coordination can reduce the electrons transferring from the central Mo site into oxygenated intermediates, which is in good agreement with the observation of the change of adsorption. In addition, considering the effect of potential by using the potential-fixed method, the axial coordination still promotes the catalytic activity (Fig.\u00a0S11) [47]. Therefore, weakening the electron-donating ability of a central metal atom in SACs can tailor the adsorption ability. The Bader charge of central metals is shown in Fig.\u00a04(e), and the plot of \u0394G\n\u2217OH vs Bader charge is given in Fig.\u00a05\n, which demonstrates that most conventional SAC configuration without axial coordination locates at the lower bound of the triangle framework. This certifies SAC configuration plays a significant role in tailoring the \u2217OH adsorption energy of SACs. Besides, increasing the Bader charge of the TM center via axial coordination boosts the \u2217OH adsorption energy of SACs. Increasing axial bond endows TM the highest positive charge, accordingly resulting in the weaker \u2217OH adsorption. Furthermore, it can be observed that the SACs, located farther to the left in the periodic table, exhibit a higher charge state, indicating the role of TM species in influencing adsorption. Therefore, SAC configuration, charge state, and transition metal species were integrated into the theoretical triangular framework (SI) to describe the catalytic behavior of SACs.To explore the role of TM species in tailoring the adsorption of SACs, we further investigated the frontier d-orbital distribution. As displayed in Fig.\u00a0S12, the coefficients of determination R-squares between spin-up (UP) d-band center and spin-down (DW) d-band center, including the projected \n\n\nd\n\ny\n\nz\n\u2212\n\n\n\n\n, \n\n\nd\n\nx\n\nz\n\u2212\n\n\n\n\n, \n\n\nd\n\nx\n\ny\n\u2212\n\n\n\n\n, \n\n\nd\n\n\nx\n2\n\n\u2212\n\n\ny\n2\n\n\u2212\n\n\n\n\n, \n\n\nd\n\n\nz\n2\n\n\u2212\n\n\n\n band and the full d-band center, are 0.78, 0.88, 0.88, 0.84, 0.88, and 0.88, respectively. Besides, their corresponding slopes are close to 1, indicating the high correlation between the spin-up (UP) d-band center and the spin-dw (DW) d-band center. It is thus reasonable to apply the single spin-up (UP) d-band center to describe the variation of the total d-band center. Furthermore, the linear fitting of \n\n\nd\n\ny\n\nz\n\u2212\n\n\n\n\n, \n\n\nd\n\nx\n\nz\n\u2212\n\n\n\n\n, \n\n\nd\n\nx\n\ny\n\u2212\n\n\n\n\n, \n\n\nd\n\n\nx\n2\n\n\u2212\n\n\ny\n2\n\n\u2212\n\n\n\n\n, \n\n\nd\n\n\nz\n2\n\n\u2212\n\n\n\n band center (UP) exhibited in Fig.\u00a0S13 showed their R-squares of 0.88, 0.91,0.93, 0.81, and 0.94, respectively, and their corresponding formulas are shown in Table\u00a01\n. These fitting results manifest that the energy level of all d-band centers (UP) can be expressed as a function of the energy level of \n\n\nd\n\n\nz\n2\n\n\u2212\n\n\n\n band center (UP). As can be seen in Fig.\u00a06\n(a), with the increase of the \n\n\nd\n\n\nz\n2\n\n\u2212\n\n\n\n band center (UP), the order of the projected d-band is changing. Based on the points of intersection listed in Table\u00a01, we can divide Fig.\u00a06(a) into four areas, and each area corresponds to a d-orbital splitting shown in Fig.\u00a06(b).As shown in Fig.\u00a06(a) and (b), with the increase of the \n\n\nd\n\n\nz\n2\n\n\u2212\n\n\n\n band center (UP), the relative order of \n\n\nd\n\n\nz\n2\n\n\u2212\n\n\n\n orbitals downshift, while \n\n\nd\n\nx\n\nz\n\u2212\n\n\n\n\n and \n\n\nd\n\ny\n\nz\n\u2212\n\n\n\n\n orbitals upshift to a relatively higher order. As reported previously, decreasing d-electrons can induce the upshift of the d-band center [48\u201350]. Such upshift will lead to the reordering of the dz2-orbital (downshift) and \n\n\nd\n\nx\n\nz\n\u2212\n\n\n\n\n and \n\n\nd\n\ny\n\nz\n\u2212\n\n\n\n\n orbitals (upshift) in Fig.\u00a06(b). It drives the \n\n\nd\n\n\nz\n2\n\n\u2212\n\n\n\n orbital approaching the Fermi level and becoming a good electron donor, whereas the \n\n\nd\n\nx\n\nz\n\u2212\n\n\n\n\n, \n\n\nd\n\ny\n\nz\n\u2212\n\n\n\n\n orbitals becoming good electron acceptors. Such change can strengthen the interaction between SACs and the adsorbates since the three d-orbitals are mainly involved in the bonding with adsorbates [51]. Therefore, the early TM-derived SACs, with fewer electrons in d-orbitals, have the weak adsorption ability of SACs. We then divided the framework in Fig.\u00a06(c) into 4 areas. There are various cases of 3d-orbital splitting in each area. In conventional SACs, Cu, Ag, and Au belong to area I, and most Mn, Fe-, Co-, Ni-, Ru-, Pd-, and Pt-derived SACs are in area II, while Ti-, V\u2013 Cr-, Nb-, Mo-, Rh-, and Os-derived SACs located at area III. It is worth mentioning that part of SACs, such as Ni, exists in two areas because axial coordination can regulate not only the charge state of metal sites but also the energy level distribution of 3d-orbitals through orbital interaction [52\u201354]. Therefore, the number of d-electrons in the 3d-orbital of SACs, determining the 3d-orbital splitting, further affects the adsorption behavior of SACs. Therefore, a theoretical framework plotted in Fig.\u00a06, combining TM species, charge states, and SAC configurations can be employed to describe the adsorption of SACs and catalytic activity toward oxygen-involved reaction.In this work, we investigated the catalytic activity of a single atomic catalyst (SAC) with axial coordination towards oxygen-involving reaction, including 4e\u2212/2e\u2212ORR, 4e\u2212OER, and 2e\u2212WOR by density functional theory calculations. With the presence of axial coordination (-OH,\u00a0=O, and \u2261N), SACs, even early TMs-derived SACs, can exhibit high catalytic activity towards four oxygen-involved reactions. The accurate scaling relation confirmed weakening the adsorption ability can improve the catalytic activity towards different oxygen reactions. More importantly, a theoretical framework of SAC configuration, transition metal (TM) species, and TM charge states have been established to describe the adsorption ability of SACs. This work offers an intrinsic landscape to explore the catalytic activity of SACs, providing rational guidelines for designing high-performance SACs.\nZhang Chengyi: Conceptualization, Software, Writing- Original draft preparation. Dai Yuhang: Conceptualization. Sun Qi: Supervision, Data Curation. Ye Chumei: Visualization. Lu Ruihu: Conceptualization, Software, Formal analysis. Zhou Yazhou: Validation. Zhao Yan: Funding acquisition, Project administration, SupervisionThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The work in this paper was supported in part by the Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory (XHT2020-003) and by the Key R&D program of Hubei (2021BAA173).The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.mtadv.2022.100280.", "descript": "\n                  Single-atom catalysts (SACs) are promising for 4e\u2212oxygen reduction reaction (4e\u2212ORR). However, they are rarely utilized in other oxygen-involved reactions, e.g. 2e\u2212ORR to produce H2O2, 4e\u2212oxygen evolution reaction (4e\u2212OER), and 2e\u2212water oxidation reaction (2e\u2212WOR). Herein, we applied density functional theory (DFT) calculations to investigate the applicabilities of SACs, including Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Ru, Rh, Pd, Pt, and Au with axial coordination (e,g, \u2013OH,\u00a0=O, and \u2261N) for all 4e\u2212/2e\u2212 oxygen reactions. With axial coordination, SACs derived from early transition metals exhibit high catalytic performance, including V-SAC-OH with an overpotential of 0.61\u00a0V for 4e\u2212ORR, Mo-SAC-OH with an overpotential of 0.05\u00a0V for 2e\u2212ORR, Mo-SAC-O with an overpotential of 0.52\u00a0V for 4e\u2212OER, and Nb-SAC-OH with an overpotential of 0.14\u00a0V for 2e\u2212WOR. Among them, most SACs deliver a trend of adsorption-energy decreasing with the increase of axial bond, which successively meets various adsorption requirements of all 4e\u2212/2e\u2212 oxygen reactions. This finding has led to the discovery of highly active SACs adapted to different oxygen reactions. Importantly, an intrinsic framework that combines SAC configuration, transition metal (TM) species, and TM charges was established to describe the adsorption ability of SACs. This work offers an intrinsic landscape to understand the correlation of the adsorption ability of SACs with the tendentiousness of oxygen-involved reactions and guides the rational design of SACs.\n                  \n               "}
{"full_text": "Global warming and environmental protection concerns triggered by fossil fuel combustion have accelerated our demand for sustainable and clean energy resources [1,2]. As one of the perfect clean energy sources, hydrogen is oxidized into water and release high energy, which has no pollution to the environment and has a promising application prospect [3,4]. So far, many methods for producing sustainable hydrogen have been developed [5]. Considering the safety and stability of commercial products, the conversion of electric energy to hydrogen energy by electrolysis of water to hydrogen has caused wide attention. And hydrogen evolution reaction (HER) is the fundamental step in the process of water electrolysis [6,7]. Currently, precious metal Pt is believed to be the most effective and stable catalyst for HER, and Pt containing catalysts have been used to catalyze the decomposition of water to make hydrogen [8,9]. Unfortunately, scarce reserves and high prices of Pt have greatly hindered the practical application of Pt-based catalysts.As economical and efficient replacements to Pt-based catalysts, nickel-based electrocatalysts have showed potential electrocatalytic activity and kept stable for HER, such as nitride, alloys, phosphides, chalcogenides, metal organic frameworks (MOFs) [10\u201314]. Despite the low price and good conductivity, Ni-based electrocatalysts have much higher overpotential than that of Pt-based, so Ni-based electrocatalysts still face challenges [15,16]. Hence, in order to further improve HER performance of Ni-based catalysts, one of the strategy is to synthesize multi-metal catalysts containing Ni. For example, Alinezhad et\u00a0al. achieved the goal of improving HER activity by growing Pt islands on branched Ni nanoparticles [17]. Moreover, Pd and Pt belong to the same family in the Periodic Table and show very similar catalytic properties in many cases. Pd-based catalysts could be the ideal substitute for Pt-based catalysts in multiple applications. Furthermore, Pd/Ni bimetallic catalyst may improve electrocatalytic effect. It has been reported that ternary Pd-Ni-P nanoparticles are superior in HER performance [18,19]. Due to the influence of Ni and P, the hydrogen adsorption energy of Pd is weakened and hydrogen is more easily released [20]. Therefore, the combination of Pd with Ni will be a promising method for the outstanding electrocatalytic performance.However, bare transition metal alloy catalysts are not sufficiently stable, especially when the durability test is carried out in the electrolyte solution. In recent years, metals@carbon composites are recognized as effective catalysts for HER, due to the protection of carbon shell structure. This kind of materials has good corrosion resistance and facilitate electron transfer [21\u201324]. To implement this kind of structure, MOFs are considered as appropriate precursors, which are synthesized by automatic combination of metal ions and organic ligands. The conversion from MOFs to carbon shell usually only requires a simple carbonization process. The carbon shell derived from MOFs can achieve huge surface and significantly improve the dispersity of the active ingredients, while protecting the transition metal by means of fending off direct contact with electrolytes [25\u201328]. In addition, N contained in the organic component of MOFs will create more reactive sites at the interfaces to activate transition metal after calcination. In summary, MOFs containing N element are appropriate precursors for the construction of N-doped porous carbon containing metal nanoparticles [29].Although Pd/Ni binary materials could be used as potential efficient catalysts for HER, little research has been done in this field [30]. Herein, we synthesized Pd/Ni bimetallic nitrogenous carbon (Pd/Ni-NC) material with a unique structure by calcination for high efficient HER, which showed better performance than Pt/C in electrocatalytic hydrogen evolution. In this work, the precursor was obtained in two steps, including the synthesis of Ni-MOF and the realization of cation exchange between Pd and Ni. The product was calcined into Pd/Ni-NC with a setaria-shaped morphology. Pd and Ni in the material have a good synergistic effect. Moreover, the precursor contains a large of N element, which can be used to prepare nitrogen-containing carbon materials without additional doping of nitrogen, thus improving the electronic conductivity and electrocatalytic activity [31,32].All chemicals used in the experiment were not further purified. Nickel(II) acetate tetrahydrate (Ni(CH3COO)2\u20224H2O) and palladium(II) chloride (PdCl2) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dimethlylglyoxime (DMG) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Hydrochloric acid was purchased from Chinasun Specialty Products Co., Ltd.Dimethylglyoxime (DMG, 0.1161 g) was dissolved in ethanol (25 mL) at 65\u00b0C. Then, 25 mL of ethanol dissolved Ni(CH3COO)2\u20224H2O (0.1244g) was added to the above DMG solution. The mixture was allowed to react under stirring for 3 h. The red precipitation (Ni-MOF) was collected by centrifugation and ultrasonic washing for several times. Pd-MOF was prepared through the same method except replacing Ni(CH3COO)2\u20224H2O with PdCl2.Ni-MOF (0.03 g) was added to an ethanol-water mixture (15 mL, VEtOH: VH2O= 2:1), followed by addition of 5 mL of PdCl2 solution (5 mg/mL). After 20 min, 0.5 mL dilute hydrochloric acid was added to the above mixture under stirring at 65\u00b0C and keep it for 6 h. The black precipitation (Pd/Ni-MOF) was collected by centrifugation and ultrasonic washing for several times.In order to highlight the advantages of the above two-step method for synthesis of Pd/Ni-MOF, one-step method was adopted to prepare MOFs containing Pd and Ni ions named as Pd-Ni-MOF. The procedure of one-step method is the same as synthesis of Ni-MOF only except replacing Ni(CH3COO)2\u20224H2O with the mixture of different Pd/Ni proportions. The obtained precursor is marked as Pd-Ni-MOF-x (x= 1, 2, 3, 4 and 5, which is corresponding to the molar ratios of Pd/Ni are 4:1, 2:1, 1:1, 0.5:1 and 0.25:1, respectively).Ni-MOF, Pd-MOF, Pd/Ni-MOF and Pd-Ni-MOF-x were sintered from room temperature to 450\u00b0C at a heating rate of 2\u00b0C/min. The samples were kept warm in Ar atmosphere to form Ni-NC, Pd-NC, Pd/Ni-NC and Pd-Ni-NC-x, respectively.Scanning electron microscopy (SEM Hitachi, S-4700), transmission electron microscopy (TEM, TecnaiG220, FEI) and high-resolution TEM (Tecani G2 F20 S-TWIN) were used to characterize the morphologies. X-ray diffraction (XRD, X'Pert-Pro MPD diffractometer, Cu K\u03b1 radiation of 1.540598 \u00c5) and X-ray photoelectron spectroscopy (XPS, Escalab250Xi) measured the composition and structure. Inductively coupled plasma (ICP, Varian 710-ES) analysed the elements.A traditional 3-electrode system (CHI660E workstation, CH Instruments, China) was used to conduct the electrocatalytic properties of the materials at room temperature. Glassy carbon (GC) disk electrode (5 mm in diameter), Platinum electrode and Ag/AgCl (KCl saturated) electrode were used as working electrode, counter electrode and reference electrode respectively. The catalyst suspension was prepared by dispersing 5 mg sample and 5 mg carbon powder in 1 mL of solution containing 970 \u03bcL isopropanol and 30 \u03bcL 0.5 wt. % Nafion solution. Then, the combination was processed by ultrasonication for 0.5 h. At the last, 21 \u03bcL reagent were dropped on the working electrode in 7 times. After calculation, the area of the working electrode was 0.19635 cm\u22122 and the loading of catalyst on the working electrode was 0.53476 mg/cm\u22122. According to the potential conversion equation, all potential values were referenced to the reversible hydrogen electrode (RHE) [33]. All electrochemical-related tests were performed in 1 M KOH solution. Linear sweep voltammetry (LSV) was measured with a scan rate of 10 mV\u2022s\u22121. The differences of current density (\u0394J) in different scan rates of cyclic voltammetry (CV) further determined the double layer capacitance (Cdl) values. Cdl is equal to the liner slopes of curves of \u0394J/2 vs. scan rate. Most electrochemical tests were modified with IR correction except for the chronopotentiometry. Electrochemical impedance spectra (EIS) experiments were performed in a frequency range from 10\u22122 to 105 Hz with 5 mV amplitude.\nScheme\u00a01\n demonstrates the complete synthesis process of Pd/Ni-NC. Firstly, Nickel ions in solution react with DMG to form a rod-like material (Ni-MOF). Afterwards, under weak acidic conditions, Ni2+ will slowly dissociate with DMG, while Pd2+ in solution will quickly react with the dissociated DMG to generate particles on the surface of Ni-MOF to form the Pd-doped Ni-MOF (Pd/Ni-MOF) subsequently. Finally, Pd/Ni-NC is obtained by calcining Pd/Ni-MOF in argon atmosphere.In this work, the obtained Ni-MOF exhibits a cuboid shaped microrod (Fig.\u00a01\na and 1d) with a common diameter of about 1 \u03bcm and the size of about 10 \u00b5m (Fig. S1a and S1b). In the process of synthesizing the Pd-doped Ni-MOF precursor, the acid will slowly dissociate part of the coordination between Ni2+ and DMG, in the meantime, Pd2+ will rapidly coordinate with the dissociated DMG. The coordination field theory gives the explanation that the energy level difference (\u03940) of d orbital of Pd2+ caused by coordination field is much larger than that of Ni2+, and the corresponding coordination bond strength formed by Pd2+ with DMG must be stronger than that by Ni2+ with DMG [34]. This inference has been confirmed by the fact that the stretching vibration constant of Pd-N (2.84 * 10\u22128 N/\u00c5) is greater than that of Ni-N (1.88 * 10\u22128 N/\u00c5) [35]. Resulting from Pd ions displacing Ni ions on the surface of Ni-MOF, Pd complexes form particles, and the resulting Pd/Ni-MOF are showed in Fig.\u00a01b and 1e. Particles of uneven size are distributed on Ni-MOF and there is no great change for the microrod. After subsequent calcination, the obtained Pd/Ni-NC catalyst has a porous interior with particles on the surface and needle-like carbon tubes growing also on it, as shown in Fig.\u00a01c and 1f. The tubes are extremely thin, just a few nanometers (Fig.\u00a01g and 1h). This special structure with nanotubes on the surface is like the shape of setaria which is considered a serious weed of crops. The unique morphology of Pd/Ni-NC will provide large contact areas with electrolytes and abundant active sites for rapid generation and release of hydrogen [36,37]. Furthermore, the amount atomic ratio of C, N, O, Ni elements in Ni-MOF showed by energy dispersive X-ray spectroscopy (EDS) is 51.11: 18.35: 25.94: 4.6 (Fig. S2a) and that of C, N, O, Pd, Ni elements in Pd/Ni-MOF is 50.82: 17.20: 22.83: 4.37: 4.79 (Fig. S2b). After calcination by Ar, Pd/Ni-NC is composed of C, N, O, Pd, Ni elements with the atomic percentages of 59.25, 10.71, 7.68, 10.96, 11.41 (Fig. S2c). By comparing the element content, we find that carbonization greatly reduced the amount of O element and makes Pd/Ni-NC stable. N is also partially lost during carbonization, which is beneficial to produce more defects. The content of Pd and Ni in Pd/Ni-NC is approximately equal. The content of Pd and Ni content in Pd/Ni-NC was 37.30 wt.% and 20.56 wt.%, respectively, which is consistent with EDS results (Table. S1).In order to evaluate the superiority of Pd/Ni-NC, a series of contrast materials were synthesized. Firstly, Ni-MOF and Pd-MOF were annealed into Ni-NC and Pd-NC as single metal catalyst, respectively. The surface morphology and internal structure of Ni-NC and Pd-NC are showed in Fig. S3a - S3d. Both Ni-NC and Pd-NC have similar rod-like structures and rough porous surfaces. Then, the one-step method was used to mix Pd2+ and Ni2+ in different proportions and react with DMG simultaneously to synthesize the corresponding materials, which were labelled as Pd-Ni-MOF-x (x= 1, 2, 3, 4 and 5). These precursors were calcined to obtain five kinds of Pd-Ni-nitrogenous carbon materials labelled with Pd-Ni-NC-x (x= 1, 2, 3, 4 and 5), respectively. The metal contents of Pd-Ni-NC-x measured by ICP are listed in Table S2. Fig. S4a - S4j show SEM and TEM morphologies of the above materials and the amount ratios of Pd to Ni. The metal ratios of Pd to Ni in Pd-Ni-NC-x decrease with the decrease of the feed ratio, but do not equal to the feed ratio. There are no Pd particles and carbon tubes on the surface of Pd-Ni-NC-x, which indicates that carbon tubes are formed only in the case of calcining Pd/Ni-MOF.XRD analysis depicted in Fig.\u00a02\na shows the characteristic peaks of metal elemental Pd and Ni in both Pd/Ni-NC and Ni-NC. Diffraction peaks appear at 40.23\u00b0, 46.77\u00b0 and 68.36\u00b0, which correspond to (111), (200) and (220) planes of elemental Pd (JCPDS#87-0639), respectively. The x-ray diffraction signals at 44.35\u00b0, 51.74\u00b0 and 76.05\u00b0 correspond to the diffraction peaks of (111), (200) and (220) planes of elemental Ni (JCPDS#89-7128). For Pd-Ni-NC-x, their X-ray powder diffraction peaks are between the corresponding diffraction peaks of elemental Pd and elemental Ni, and approach the peak of Pd with the increase of the relative content of Pd to Ni (Fig. S5). The surface compositions and chemical states of Pd/Ni-NC was analyzed by XPS. It can be seen from the survey spectrum that C, N, Ni and Pd elements exist in Pd/Ni-C (Fig.\u00a02b). In Fig.\u00a02c, Ni 2p has two main peaks at 854.6 eV and 871.98 eV, as well as a satellite peak of each, matching with Ni 2p3/2 and Ni 2p1/2 of elemental Ni [38]. In addition, Ni 2p also have peaks at around 852.1 eV and 869.27 eV. These peaks correspond to the characteristic of Ni2+ 2p3/2 and Ni2+ 2p1/2, indicating the presence of oxidized Ni in the sample (Fig.\u00a02c) [39,40]. The existence of such peaks is related to the incomplete reduction from Pd/Ni-MOF calcination and the oxidation of the metal Ni in the air. Fig.\u00a02d shows the two characteristic peaks of the XPS spectra of N 1s at 398.07 eV and 399.05 eV, which are attributed to pyridinic-type N and pyrrolic-type N, respectively [41,42]. For C 1s, sp3C, C=N, C-N bonds can be used to explain peaks at 284.21 eV, 285.08 eV and 287.97 eV, respectively (Fig.\u00a02e) [41,43]. These results indicate that N atoms partly remain in the carbon materials and perform an important function in promoting electrical conductivity and reactivity. Concerning the XPS spectrum of Pd 3d (Fig.\u00a02f), the peaks of metallic Pd 3d5/2 and Pd 3d3/2 are positioned at 334.75 eV and 340.75 eV [44]. Meanwhile, there are still existing weak Pd metal oxidation peaks due to air exposure [45]. Similarly, after calcination, the elements contained in Pd-Ni-NC-x remain unchanged (Fig. S6a) and the metal elements are reduced to the zero valence stats. In the Ni 3p spectrum (Fig. S6b), the peaks located at 66.17 and 68 eV are assigned to Ni 3p3/2 and Ni 3p1/2. It is worth noting that the peaks of Ni 3p weaken with the decrease of Ni content. The XPS spectrum of Pd 3d can be assigned to two peaks shown in Fig. S6c. There is also a very small amount of oxidized Pd in Pd-Ni-NC-x. Fig S7 shows the element distribution of Pd/Ni-MOF (the precursor of Pd/Ni-NC), which proves that Pd replaces Ni and forms many Pd particles on the surface of Ni rods. From the element distribution, Fig.\u00a03\na-3f show that an amount of Ni and Pd uniformly disperse in the materials and heterostructured Pd particles are scatted on the surface. Regarding the mechanism of carbon tube formation, the Ni compounds may be influenced by Pd particles to release the carbon of ligands at high temperature, and then elemental Ni play a role of catalyst for the growth of carbon tubes. Meanwhile, the elemental Pd will be loaded on the carbon tube during the growth process. This can be verified by the following tests. On the one hand, Fig.\u00a03g shows the HRTEM image of the carbon tubes on the surface of Pd/Ni-NC. Good crystallinity is further demonstrated by the wide and clear rings in selected area electron diffraction (SAED) shown in Fig.\u00a03h. After calculation, these diffraction fringes are caused by the (111), (200), (220) lattice planes of Pd, respectively. On the other hand, the lattice fringes of the carbon tubes have been photographed (Fig.\u00a03i). The unique lattice spacing of about 0.22 nm on carbon tube is corresponding to the (111) plane of metallic Pd. Any other lattice spacing could not find. It proves the formation of carbon tubes and the loading of Pd on carbon tubes.The electrocatalytic hydrogen evolution properties of different materials were analyzed by liner sweep voltammetry (LSV). In general, the catalytic capacity of electrode materials is determined by comparing the magnitude of overpotential under a certain density. At the first, Fig.\u00a04\na shows the polarization curves (I-V plots) of Ni-MOF, Ni-NC, Pd/Ni-MOF, Pd/Ni-NC and Pd-NC, respectively. The comparison of corresponding overpotential (\u03b7) vs. RHE at 10 mA cm\u22122 was exhibited in Fig.\u00a04b. It is found that Pd/Ni-NC only needs a low overpotential of 16 mV to drive current density of 10 mA cm\u22122, which is the lowest \u03b7 value among these materials. Ni-NC and Pd-NC as single metal catalysts need 84 mV and 61 mV, respectively. One important reason why the catalytic activity of Pd/Ni-NC is higher than that of Ni-NC or Pd-NC is that Ni atoms speed up the water dissociation by means of adsorbing OH\u2212 and promote the formation of H2 by the recombination of H+ at Pd sites [46\u201348]. At the same time, as shown in the Fig.\u00a04e and Fig.\u00a04f, the \u03b7 value of Pd-Ni-NC-x are 84.5, 82.6, 104.5, 276.6 and 296.6 mV, respectively. The HER performance of Pd/Ni-NC is evidently still the best. These results explain the advantages of Pd/Ni-NC with setaria-shaped structure. Compared with previous literatures, such HER activity of Pd/Ni-NC catalyst is superior to that of most Pd/Ni-based catalysts (Table S2). To understand the whole catalytic process, the dynamic process is usually analyzed by calculating Tafel slope [48,49]. Fig.\u00a04c shows the Tafel slope of Pd/Ni-NC is 130 mV dec\u22121, which is lower than that of Ni-MOF (144.9 mV dec\u22121), Ni-NC (155.35 mV dec\u22121), Pd/Ni-MOF (205.04 mV dec\u22121), Pd-NC (136 mV dec\u22121). This further proves that a synergistic impact of Pd and Ni exists in Pd/Ni-NC, which explains the better HER activity of Pd/Ni-NC than that of Ni-NC or Pd-NC. In an alkaline medium, HER occurs at the cathode as a multistep reaction, including the first step for Volmer reaction, the second step for Heyrovsky reaction or Tafel reaction [50,51]. According to the Tafel slope values, all materials follow a Volmer-Heyrovsky pathway, involving a relatively slow hydrogen adsorption (Volmer step) and a fast electrochemical desorption process (Heyrovsky step) [52\u201354]. This means the Volmer step is the rate-limiting step. In order to further evaluate its electrocatalytic performance, the effective sites of Pd/Ni-NC were evaluated by electrochemically active surface area (ECSA). And ECSA is linearly proportional to double layer capacitance (Cdl), so the active sites can also be evaluated by comparing the Cdl values calculated at different scanning speeds [55,56]. As shown in Fig. S8, CV curves at various scanning speeds (10-50 mV s\u22121) were tested in the region of 0.88-0.98 V (vs. RHE). In Fig.\u00a04d, the Cdl value of Pd/Ni-NC is 16.02 mF cm\u22122, which is greater than that of Ni-NC (12.53 mF cm\u22122) but lower than that of Pd-NC (18.9 mF cm\u22122). This result directly proves that the introduction of metal Pd into Ni carbon material can increase ECSA and also directly indicates that Pd/Ni-NC has a larger surface roughness than that of Ni-NC and exposes more active sites, which is conducive to the adsorption of water and electron conduction. Notably, even if the Cdl of Pd/Ni-NC is not as great as that of pure Pd-NC, HER performance of Pd/Ni-NC is the best. This result indicates that Pd/Ni-NC can better utilize the synergistic effect of Pd and Ni and highlight the importance of the setaria-shaped structure.To further investigate the kinetic mechanism of hydrogen evolution, the electrochemical impedance spectroscopy (EIS) of above materials was tested in alkaline solution. The upper right corner of Fig.\u00a05\na is the equivalent circuit diagram of the fitting curve, where R1 represents solution resistant (Rs) and R2 represents charge transfer resistance (Rct). After fitting, the R1 value of each material are close to each other, which indicates that the difference in activity caused by the electrode manufacturing process can be ignored. Fig.\u00a05a shows that the Rct of Pd/Ni-NC (10.69 \u03a9) is smaller than that of Pd-NC (14.7 \u03a9), Ni-NC (29.93 \u03a9) and much less than Pd/Ni-MOF (244.6 \u03a9) and Ni-MOF (647.4 \u03a9). This means that the transfer rate of electrons in the electrode-electrolyte interface is the fastest, when Pd/Ni-NC is used as electrocatalyst. The reason could be related to the composition and the setaria-shaped structure of Pd/Ni-NC. Fig. S9 shows the EIS Nyquist plots of Pd-Ni-NC-x. The Rct values of Pd-Ni-NC-x (x=1, 2, 3, 4 and 5) are 1.538, 4.298, 3.396, 3.355 and 3.917 \u03a9, respectively. Although the impedance values of Pd-Ni-NC-x are smaller than that of Pd/Ni-NC, their electrochemical catalytic effects are still lower. This proves that the setaria-shaped structure of Pd/Ni-NC has the great promotion to the catalytic performance. Moreover, stability is also an important index for a high efficient catalyst, which was investigated by testing overpotential at constant current density. Fig.\u00a05b shows the overpotential under the catalytic function of Pd/Ni-NC at 10 mA cm\u22122, and it could be stabled well even over 25 h. Then polarization curves before and after 25h constant current test is shown in Fig. S10, which further highlights the excellent stability and practical applications potential.According to the above characterization results and HER performance for the Pd/Ni-NC catalyst, the good HER activity and long-term stability can be ascribed to the cooperative effect from the following facts: (1) Ni atoms are widely regarded as excellent hydrolytic off-center, while Pd atoms have excellent adsorption properties to hydrogen. The corresponding overpotential of Pd/Ni-NC at 10 mA cm\u22122 is 68 mV and 45 mV less than that of Ni-NC and Pd-NC, respectively. The synergistic effect of Pd and Ni in the Pd/Ni-NC makes the materials exhibit excellent electrocatalytic activity, which is much better than their respective single metal catalysts. (2) The Pd/Ni-NC material derived from the corresponding MOF has a fluffy, porous and special structure with carbon tubes on the surface. This bimetallic carbon material offers abundant active sites. In addition, the carbon tubes on the surface of Pd/Ni-NC increase the contact area with electrolyte, which is more favorable for hydrogen release and mass transport. (3) As the reactant of MOF precursors, one molecule of DMG ligand has 2 nitrogen atoms. The presence of rich nitrogen causes more defects in the calcination process of Pd/Ni-MOF, exposing more active sites. Meanwhile, the nitrogenous carbon structure of Pd/Ni-NC can be conductive to optimize the interfacial electronic structure, so as to improve the overall electrocatalytic activities.In conclusion, we presented a setaria-shaped micron rod with carbon tubes and Pd particles grown on the surface as the electrocatalyst. With the addition of metal Pd, the nitrogenous Pd/Ni-NC catalyst has many more active sites and better synergies among its components. This catalyst exhibits an excellent activity for the HER with small overpotential of 16 mV, low resistive impedance and high stability. Although the Tafel slope of Pd/Ni-NC is 130 mV dec\u22121, it is lower than that of the corresponding single metal catalysts, Ni-NC or Pd-NC. Volmer step is the rate-limiting step for the catalyzed HER in alkaline medium. This work provides an idea for the synthesis of Pd/Ni bimetallic nitrogenous carbon material with a special structure. And the catalyst exhibits Pt-like catalytic properties, which maybe act as an alternative non-platinum electrocatalyst.There are no conflicts to declare.H.W.G. and X.Q.C acknowledges financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the project of Scientific and Technologic Infrastructure of Suzhou (SZS201708) and the Research Fund Program of Key Laboratory of Rare Mineral, MNR (No. KLRM-KF202004).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2021.100101.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Developing stable, efficient and economical electrocatalytic materials is still challenging for hydrogen evolution reaction (HER). Hence, we develop a Pd/Ni bimetallic carbon electrocatalyst (Pd/Ni-NC) with outstanding electrocatalytic performance. The catalyst derived from Pd-doped Ni-MOF (Pd/Ni-MOF) has particles and needle-like carbon tubes on its surface and is similar in shape to setaria. Benefiting from the composition and the unique structure, Pd/Ni-NC shows excellent HER catalytic performance with 16 mV at 10 mA cm\u22122, superior to Pd or Ni single metal-carbon catalyst. Furthermore, it maintains stable catalytic activity under constant current for 25h. These results show the strategy that obtaining Pd and Ni bimetallic MOF by cation exchange and its corresponding bimetallic carbon material with setaria-shaped structure by calcination is powerful for high efficient HER performance.\n               "}
{"full_text": "Excessive carbon dioxide (CO2) emissions caused by human activities, for example, the consumption of fossil fuels, deforestation, and forest degradation, have been considered the major culprit of climate change and ocean acidification [1\u20133]. In the past decade, shale gas has set off a global energy revolution. Especially in the United States, shale gas has become the most important source of natural gas. With CO2 as a soft oxidant, ethane (C2H6), the second most abundant component (approximately 10% of the content) of shale gas, can be converted into important raw materials [4]. Generally, the reaction between C2H6 and CO2 occurs via two distinct pathways: \u2460 dry reforming of ethane (DRE) into syngas through cleavage of the C\u2013C bond (C2H6 + 2CO2 \u2192 4CO + 3H2); and \u2461 oxidative dehydrogenation of ethane (ODHE) into ethylene (C2H4) by blocking cleavage of the C\u2013C bond (C2H6 + CO2 \u2192 C2H4 + CO + H2O) [5\u20137]. Syngas, a mixture of hydrogen (H2) and carbon monoxide (CO), is an important feedstock for fuels and chemicals and is conventionally produced by steam reforming or partial oxidation of natural gas, liquefied gas, naphtha, and so on [8\u201310] or dry reforming of methane (DRM) [11\u201314]. However, the above processes are highly endothermic, with high energy consumption, and most DRM catalysts suffer deactivation due to coke formation and active site sintering at high operation temperatures above 1000\u202fK [6,15\u201317]. A reaction temperature at least 100\u202fK lower for DRE enables the production of syngas and the reduction of catalyst deactivation under milder conditions [5,6].Ni-based catalysts, especially supported Ni catalysts, are widely used in DRM because of their high catalytic activity [12,18\u201322]. However, Ni-based catalysts suffer from deactivation due to poor coke resistance and particle sintering. Thus, alloying Ni with transition metals (Co, Ru, Pd, Pt) [21,23\u201325], developing advanced supports [26\u201328], and using alkali cations as promoters [29\u201331] have been investigated to overcome the drawbacks of traditional Ni-based catalysts. Because of the broad application prospects in producing chemicals and fuels at operation temperatures below 900\u202fK, DRE has drawn much attention, and a series of catalysts have been developed, including trimetallic perovskites [32,33], supported Pt-based bimetallic catalysts [5,6,17], and supported Ni composite catalysts [34,35]. The strong metal\u2013support interaction (SMSI) effect has been proven to have a significant impact on the catalytic performance of Ni-based catalysts for DRM and DRE. Ceria (CeO2) supports with more oxygen vacancies show a stronger SMSI effect, which not only improves the dispersion of Ni species but also enhances the bonding between Ni species and the CeO2 supports [36,37]. Liu et al. [38] reported the SMSI effect between small Ni nanoparticles (NPs) and partially reduced CeO2. CO2 adsorbs and dissociates at oxygen vacancies to generate CO and active oxygen. The synergy between Ni and active oxygen reduces the activation barrier of CH4 bond dissociation and generates CH\nx\n (x = 2, 3) species on the surface of the Ni/CeO2 catalyst, making the dissociation temperature of CH4 as low as 700\u202fK. Lustemberg et al. [39] further proved the important role of Ce3+ in the dissociation of C\u2013H bonds. Smaller Ni particles on the CeO2 support experience larger electronic perturbations, resulting in a more significant binding energy and a lower activation barrier for the first C\u2013H bond cleavage. Recently, Xie et al. [40] investigated the effects of oxide supports for DRE over both reducible and irreducible oxide-supported Pt\u2013Ni bimetallic catalysts. The DRE performance over the PtNi/CeO2 catalyst was greatly improved because the reducible CeO2 effectively activated CO2 and promoted the dissociation of C2H6 through a bifunctional Mars\u2013van Krevelen redox mechanism.Moreover, the dry reforming process is always accomplished by the reverse water\u2013gas shift (RWGS) reaction with a low activation barrier [13,17,40,41]. It has been proven that the introduction of Al into CeO2 promotes the formation of oxygen vacancies, which inhibits the RWGS reaction because of the powerful activation and dissociation capacity of CO2 [13,42]. FeO\nx\n has also been reported to be a promoter of supported Ni catalysts for both the RWGS reaction [43] and DRM [44] due to the enhancement of Ni dispersion and the formation of Ni-rich NiFe alloys. Recently, Yan et al. [45] reported the adjustment of active interfacial sites by changing the composition of FeNi active components, which was shown to greatly influence the selectivity toward ODHE or DRE. Herein, we consider an FeNi/CeO2 catalyst with high ethylene selectivity as the initial system and adjust the distribution of surface active components by introducing Al into the CeO2 support to obtain high-performance catalysts for DRE.In this article, we synthesized a series of FeNi/Al\u2013Ce\u2013O catalysts with an enhanced SMSI effect via a facile sol\u2013gel and impregnation method and investigated the catalytic properties for DRE over supported FeNi catalysts under steady-state reaction conditions. The enhanced SMSI effect between the surface active components and Al\u2013Ce\u2013O supports and its influences were confirmed via X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (TPR), and energy dispersive X-ray spectroscopy (EDS) element mapping. This work also proposes a possible reaction mechanism and the corresponding adsorbed intermediates over supported FeNi catalysts via in situ Fourier transform infrared (FTIR) spectra under the reaction conditions. We established a relationship between catalytic properties and surface structure over supported FeNi catalysts, which will greatly contribute to the research on related catalytic systems.Pluronic\u00ae F127 (EO106PO70EO106, M\nw\u2009=\u200912600) was purchased from Sigma-Aldrich Chemical Inc. (USA). Ce(NO3)3\u00b76H2O (analytical reagent (AR), 99.0%), Al(NO3)3\u00b79H2O (AR, 99.0%), Ni(NO3)2\u00b76H2O (AR, 98.0%), quartz sand (25\u201350 mesh, AR, 95.0%), and absolute ethanol (AR, 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Fe(NO3)3\u00b79H2O (AR, 98.5%) was purchased from Nanjing Chemical Reagent Co., Ltd. (China). All the chemicals were used as received.Al\u2013Ce\u2013O supports were synthesized via a facile sol\u2013gel process combined with evaporation-induced self-assembly (EISA) in ethanol using F127 as the template, which provides a large specific surface area and superior catalytic performance [13,46\u201349]. In a typical synthesis, 1.6 g of F127 was dissolved in 40\u202fmL of ethanol at room temperature (RT). A total of 10.0\u202fmmol of metal precursors (Ce(NO3)3\u00b76H2O and Al(NO3)3\u00b79H2O) with an Al molar ratio between 10% and 90% (10% increment of each sample) were added into the above solution with vigorous stirring. The mixture was covered with polyethylene (PE) film and stirred for at least 5\u202fh at RT. The homogeneous sol was then transferred into an oven and underwent solvent evaporation. After aging at 313 and 333 K for 24\u202fh successively, the gel product was dried in an oven at 373 K for another 24 h. Calcination was performed by slowly increasing the temperature from RT to 923 K (2 K\u00b7min\u22121 ramping rate) and then heating at 923 K for 4 h in air. The CeO2 and Al2O3 supports were synthesized via the same procedure except the mixed metal precursors were replaced with 10.0\u202fmmol of Ce(NO3)3\u00b76H2O (4.34\u202fg) or Al(NO3)3\u00b79H2O (3.75\u202fg), respectively. The yellow or white products were then ground into a powder.Supported FeNi bimetallic catalysts were synthesized via an incipient wetness impregnation method over as-synthesized CeO2, Al2O3, and Al\u2013Ce\u2013O supports [6,40,45]. In a typical synthesis, a bimetallic co-impregnation procedure was used to maximize the interaction between the two metals. Precursor solutions were prepared by dissolving 101\u202fmg of Fe(NO3)3\u00b79H2O and 83\u202fmg of Ni(NO3)2\u00b76H2O in a specific amount of deionized water sufficient to fill the pores of 0.981\u202fg of corresponding metal oxide support, the pore volume of which was determined by means of nitrogen adsorption measurements. The solution was added dropwise to the support with thorough stirring. The loadings of bimetallic active components were 1.15 wt% for Fe and 0.40 wt% for Ni to obtain a 3:1 molar ratio of Fe/Ni. The catalyst was then dried at 353\u202fK for 12\u202fh and calcined at 723\u202fK for 4\u202fh with a ramping rate of 2 K\u00b7min\u22121 from RT to 723\u202fK. Since the FeNi/CeO2 catalyst with an Fe/Ni molar ratio of 3:1 shows high ethylene selectivity due to its special Ni\u2013FeO\nx\n interfacial sites [45], such active metal components would better reflect the enhanced SMSI effect of Al\u2013Ce\u2013O supports on the catalytic properties.Powder X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance SS diffractometer (Bruker Corporation, USA) operated at 40\u202fkV and 40\u202fmA with a slit of 0.5\u00b0 at a 2\u03b8 scanning speed of 2\u00b0\u00b7min\u22121 under a Cu-K\u03b1 source (0.15432\u202fnm). Nitrogen adsorption\u2013desorption isotherms were measured at 77\u202fK using a Micromeritics ASAP 2010 analyzer (Micromeritics Instrument Corporation, USA). The Brunauer\u2013Emmett\u2013Teller (BET) specific surface area was measured after degassing the samples at 373 and 623\u202fK for 3\u202fh successively under vacuum. The elemental composition of the supported FeNi catalysts was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; Varian Vista AX, Varian Inc., USA). 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was conducted with a Varian VNMRS-400WB nuclear magnetic resonance instrument (Varian Inc., USA) with a frequency of 104.18 MHz, a spinning speed of 10 000\u202fHz, and a relaxation delay of 4\u202fs. Chemical shift values are reported with respect to KAl(SO4)2\u00b712H2O as the standard. XPS was performed on a Thermo ESCALAB 250 spectrometer (Thermo Fisher Scientific Inc., USA) with a monochromatic Al-K\u03b1 X-ray source (1486.6\u202feV, 1\u202feV = 1.602176\u202f\u00d7\u202f10\u221219 J) and an analyzer pass energy of 20\u202feV. The C 1s line at 284.6\u202feV was used to calibrate the binding energies (BEs) of the measured elements.H2-TPR experiments were conducted on a Micromeritics AutoChem II 2920 chemisorption analyzer (Micromeritics Instrument Corporation, USA). In a typical experiment, 50\u202fmg of the as-synthesized catalyst was put into a U-shaped quartz tube and pretreated in a 50\u202fmL\u00b7min\u22121 He flow at RT. Then, the TPR experiment was performed under a 10 vol% H2/Ar mixture at a space velocity of 50\u202fmL\u00b7min\u22121 with a ramping rate of 10 K\u00b7min\u22121 from RT to 1173\u202fK. Pulse CO chemisorption experiments were also performed on a Micromeritics AutoChem II 2920 chemisorption analyzer to measure the CO uptake value of the catalyst. Approximately 150\u202fmg of the catalyst was first reduced at 873\u202fK in a 10 vol% H2/Ar flow for 30\u202fmin. Then, the reduced catalyst was purged in a He flow until the temperature was decreased to 313\u202fK. The loop gas of 10% CO/He (590\u202f\u03bcL) was pulsed with a He stream over the catalyst until the peak area of CO became constant. The CO uptake values of the catalysts could provide an approach to estimate the turnover frequency (TOF), as reported previously [17,33,45].Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and EDS element mapping were conducted on an FEI Talos F200X TEM (Thermo Fisher Scientific Inc., USA) with a probe aberration corrector operating at 300\u202fkV. The TEM samples were prepared by drying a drop of the sample dispersion in ethanol on carbon-coated copper grids.\nIn situ FTIR spectra were obtained on a Bruker Vertex 70\u202fV FTIR spectrometer (Bruker Corporation, USA) with a stainless steel high-vacuum transmission infrared cell. The samples were pressed on a tungsten mesh support and heated to 623\u2013673\u202fK at a ramping rate of 10\u202fK\u00b7min\u22121 under vacuum for 2\u202fh to remove the surface adsorbed water. The background spectra were collected after the tungsten mesh support was cooled to RT. In a typical in situ infrared (IR) experiment under the reaction conditions, 1.0\u202fmbar (1\u202fmbar\u202f=\u202f100\u202fPa) of C2H6 and 1.0\u202fmbar of CO2 were introduced into the cell, and IR spectra of each sample were collected in the temperature range of 373\u2013873\u202fK. In situ CO adsorption IR spectra of the FeNi/Ce\u2013Al0.5 catalyst were collected under a CO pressure of 5.0\u202fmbar from 373 to 573\u202fK and 1.0\u202fmbar from 673 to 873\u202fK.The catalytic performance of supported FeNi bimetallic catalysts was evaluated in a continuous-flow fixed-bed quartz tabular reactor (7.5\u202fmm inner diameter) under atmospheric pressure, utilizing a mixture of 100\u202fmg of catalyst (20\u201340 mesh) and 100\u202fmg of quartz sand loading between two quartz wool plugs at the center of the reactor. The catalyst was pretreated in situ with 40\u202fmL\u00b7min\u22121 H2 at 673\u202fK for 1\u202fh and then heated to 873\u202fK at a ramping rate of 5\u202fK\u00b7min\u22121 with a constant total flow rate of C2H6, CO2, and N2 of 40\u202fmL\u00b7min\u22121. The volume ratio of the C2H6, CO2, and N2 mixed gas was 1:1:2. The temperature of the catalyst bed was held at 873\u202fK for 8\u202fh, and the outlet stream was analyzed online using a gas chromatograph (Agilent 6820B, Agilent Technologies, Inc., USA) with a thermal conductivity detector (TCD). Water within the outlet stream was removed by a condenser. N2 was used as an internal standard to account for the volume effects due to the high temperature during the reaction. Blank experiments without supported FeNi catalysts were conducted at 873\u202fK to evaluate the contribution of the gas-phase reaction and system, and the results showed a negligible effect on DRE performance. In this article, the steady-state conversion (X), TOF, C2H6-based selectivity (S), and yield (Y) of species i were defined using the following equations:\n\n(1)\n\n\n\nX\ni\n\n=\n\n\n\nF\n\ni\n,\ni\nn\n\n\n-\n\nF\n\ni\n,\no\nu\nt\n\n\n\n\nF\n\ni\n,\ni\nn\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(2)\n\n\n\n\nT\nO\nF\n\ni\n\n=\n\n\n\nF\n\ni\n,\ni\nn\n\n\n\u2219\n\nX\ni\n\n\n\n\nU\n\nC\nO\n\n\n\u2219\n\nm\n\nc\na\nt\n\n\n\n\n\n\n\n\n\n\n(3)\n\n\n\nS\ni\n\n=\n\n\nF\n\ni\n,\no\nu\nt\n\n\n\n\nF\n\n\nC\n2\n\n\nH\n6\n\n,\ni\nn\n\n\n-\n\nF\n\n\nC\n2\n\n\nH\n6\n\n,\no\nu\nt\n\n\n\n\n\u2219\n\n\nN\n\ni\n,\nC\n\n\n\nN\n\n\nC\n2\n\n\nH\n6\n\n,\nC\n\n\n\n\u00d7\n100\n%\n\n\n(\ni\n\u2260\nC\nO\n)\n\n\n\n\n\n\n\n(4)\n\n\n\nS\n\nC\nO\n\n\n=\n1\n-\n\n\u2211\n\ni\n\u2260\nC\nO\n\n\n\nS\ni\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(5)\n\n\n\nY\ni\n\n=\n\nX\n\n\nC\n2\n\n\nH\n6\n\n\n\n\u2219\n\nS\ni\n\n\n\n\nwhere F\nin and F\nout are the inlet and outlet flow rates of the reactant (mol\u00b7min\u22121), U\nCO is the CO uptake value (mol\u00b7g\u22121), m\ncat is the mass of the catalyst (g), and N\nC is the carbon atom number of the products.Powder XRD patterns of the as-synthesized catalysts with standard Joint Committee on Powder Diffraction Standards (JCPDS) cards are shown in Fig. 1\n. The diffraction peaks of FeNi/Ce\u2013Al\nx\n (x \u2264 70%) and FeNi/CeO2 confirm the fluorite cubic structure of CeO2 (Fm\n\n\n\n\n3\n\n\n\u00af\n\n\n\n\nm, JCPDS 75-0120), while the pattern of FeNi/Al2O3 is consistent with the structure of \u03b7-Al2O3 (Fd\n\n\n\n\n3\n\n\n\u00af\n\n\n\n\nmS, JCPDS 77-0396). FeNi/Ce\u2013Al0.9 shows significant phase separation into CeO2, \u03b7-Al2O3, and Al(OH)3 (P1(1), JCPDS 24\u20130006). No peaks attributed to Fe and Ni are observed in these patterns. As shown in Table 1\n, the average crystallite size of the catalysts calculated by the Scherrer equation [50] significantly decreases with increasing Al content, which indicates that Al improves the sintering resistance of the CeO2 supports [47]. The size distributions of supported FeNi catalysts were further estimated by the size statistics of NPs from TEM images. The TEM and HRTEM images of supported FeNi catalysts are shown in Figs. S1 and S2 in Appendix A. The size distributions of the FeNi/CeO2 and FeNi/Ce\u2013Al\nx\n (10% \u2264 x\u202f\u2264\u202f50%) catalysts in Table 1 are close to the corresponding average crystallite sizes. As shown in Table 1 and Fig. S3 in Appendix A, the BET specific surface area of supported FeNi catalysts is positively related to the Al content. The average crystallite size of the FeNi/Ce\u2013Al\nx\n (70% \u2264 x\u202f\u2264\u202f90%) catalysts becomes even smaller upon formation of a mesoporous structure, as shown in Fig. S2, which reveals the relationship between the size distribution and BET specific surface area of supported FeNi catalysts. In Figs. S1 and S2, lattice spacings of 3.12, 2.71, 1.91, and 2.38\u202f\u00c5 correspond to the (111), (200), and (220) facets of CeO2 and (311) facet of \u03b7-Al2O3, respectively. The lattice spacing of FeNi/Ce\u2013Al0.9 is indistinguishable owing to the poor crystallinity and severe phase separation.To further discuss the microstructure of the catalysts, the microstrains and lattice parameters were calculated, and the results are listed in Table 1. The microstrain in the lattice (lattice strain) of the samples was estimated via the single line method for analysis of XRD line broadening using a pseudo-Voigt profile function [51,52]. As shown in Fig. 1, for the FeNi/CeO2 and FeNi/Ce\u2013Al\nx\n (10% \u2264 x\u202f\u2264\u202f70%) catalysts with fluorite cubic structures, the microstrain in the crystal lattice of the oxide supports increases as a function of Al content to maintain the original crystal structure. The lattice distortion is relieved by a decrease in the microstrain and phase transition after the introduction of a high content of Al. The lattice parameters determined via Bragg\u2019s law from the (111) diffraction peak of CeO2 (Fm\n\n\n\n\n3\n\n\n\u00af\n\n\n\n\nm, JCPDS 75-0120) for FeNi/CeO2 and FeNi/Ce\u2013Al\nx\n (10% \u2264 x\u202f\u2264\u202f70%) as well as the (440) peak of \u03b7-Al2O3 (Fd\n\n\n\n\n3\n\n\n\u00af\n\n\n\n\nmS, JCPDS 77-0396) for FeNi/Ce-Al0.9 and FeNi/Al2O3 are also listed in Table 1. Fig. S4 in Appendix A shows the 27Al MAS NMR spectra of FeNi/Ce\u2013Al\nx\n (10% \u2264 x\u202f\u2264\u202f50%) and of FeNi/Al2O3. The peaks at approximately 8 and 66\u202fppm are assigned to octahedrally (Aloct) and tetrahedrally (Altet) coordinated Al3+, while the Al species with a chemical shift of 38\u202fppm is a CeO2 lattice occupied by Al3+ [53]. The increased peak intensity at 38\u202fppm for FeNi/Ce\u2013Al0.1 and FeNi/Ce\u2013Al0.3 is a result of Al3+ ions present in the CeO2 lattice. The similarity of the spectra of FeNi/Ce\u2013Al0.5 and FeNi/Al2O3 indicates a stable octahedral coordination of Al3+ species with a high content of Al in FeNi/Ce\u2013Al\nx\n (50% \u2264 x\u202f\u2264\u202f90%). Regardless of the phase transition, the change in the lattice parameter of the supports is mainly related to two factors: \u2460 the formation of oxygen vacancies by replacing Ce4+ with Al3+, which leads to crystal lattice shrinkage, and \u2461 the transition from Ce4+ to Ce3+ with a corresponding reduction in the ionic radius, which is essential to balance the electric charge of the unit cell. The change in the lattice parameter is thought to be a result of the synergistic effect of the two factors: the formation of surface oxygen vacancies and Ce3+ species.The steady-state catalytic performance of the FeNi/Al\u2013Ce\u2013O catalysts at 873\u202fK is shown in Figs. 2 and 3\n\n, where the FeNi/CeO2 and FeNi/Al2O3 catalyst data were also plotted as a reference. The experimental data indicate that the composition of oxide supports plays an important role in the catalytic properties for DRE over supported FeNi catalysts. As shown in Figs. 2(a) and (b), C2H6 and CO2 conversion is positively correlated with the Al content (0\u202f\u2264\u202fx\u202f\u2264\u202f50%), whereas completely opposite trends are observed in Figs. 3(a) and (b) when the Al content is above 50%. In Fig. S5 in Appendix A, the CO selectivity from C2H6 also significantly increases with increasing Al content (0\u202f\u2264\u202fx\u202f\u2264\u202f30%), while the ethylene selectivity decreases correspondingly. As the Al content changes between 30% and 90%, the CO selectivity of the FeNi/Al\u2013Ce\u2013O catalysts remains stable at 96%\u201398%. In Figs. 2(c) and 3(c), the CO yield from C2H6 over the FeNi/Al\u2013Ce\u2013O catalysts follows the same trend of first increasing and then decreasing with increasing Al content. The FeNi/Ce\u2013Al0.5 catalyst provides the best DRE performance, with the highest C2H6 and CO2 conversions and CO selectivity and yield. The introduction of Al into CeO2 possibly enhances the interaction between the surface active components and the Al\u2013Ce\u2013O support, which further affects the catalytic properties over supported FeNi catalysts.The average catalytic performance data of the supported FeNi catalysts between 420 and 480\u202fmin are summarized in Table 2\n. After several hours of steady-state reaction, the C2H6 and CO2 conversion, CO selectivity, and CO yield over the supported FeNi catalysts maintain the same relative order. The FeNi/Ce\u2013Al0.5 catalyst exhibits the best DRE performance with the highest C2H6 conversion (11.7%), CO2 conversion (33.1%), and CO yield (11.5%). The TOF values based on CO uptake also indicate the outstanding catalytic activity of the FeNi/Ce\u2013Al0.5 catalyst for both C2H6 (47.1\u202fmin\u22121) and CO2 (133.1\u202fmin\u22121). As a comparison, the catalytic performance data of recently reported DRE catalysts are listed in Table S1 in Appendix A. The FeNi/Ce\u2013Al0.5 catalyst shows high TOFs and CO selectivity similar to other high-performance DRE catalysts, whereas the conversions are possibly restricted by the low loading of bimetallic active components. Catalysts with Al contents above 50% show lower TOF values for both C2H6 and CO2 in this reaction, which demonstrates that the enhancement of DRE performance over the FeNi/Ce\u2013Al0.5 catalyst should be attributed not only to the increasing Al content but also to the interaction between surface active components and the Al\u2013Ce\u2013O support. H2/CO molar ratios lower than 0.75 due to the side reaction of the RWGS are also shown in Table 2 [33]. The highest H2/CO ratio of the FeNi/Ce\u2013Al0.5 catalyst indicates that the SMSI effect enhanced by the introduction of Al partially inhibits the RWGS process in this reaction, which is also related to the surface oxygen vacancy over the FeNi/Al\u2013Ce\u2013O catalysts.The elementary composition and chemical valence on the surface of hydrogen-reduced catalysts were detected via XPS. Typical Ce 3d and Al 2p core level spectra of the FeNi/CeO2, FeNi/Al\u2013Ce\u2013O, and FeNi/Al2O3 catalysts are shown in Appendix A Fig. S6. During typical data processing, the complex spectra of the samples in the Ce 3d region were deconvoluted into ten components via the generally accepted approach of extracting the ratio of Ce3+ and Ce4+ [54,55]. The ten peaks contained five spin-orbital split pairs of Ce 3d5/2 (vi\n: v\n0, v, v\u2032, v\u2033, and v\u2034) and Ce 3d3/2 (ui\n: u\n0, u, u\u2032, u\u2033, and u\u2034), of which the area intensities, the full widths at half-maximum (FWHM), and the position distances were fixed as constants during the deconvolution. Herein, the peak positions are marked in Fig. S6(a); the relative contents of Ce3+ to the total Ce content (c\nCe)in the samples were calculated via the following equation, and the results are listed in Table 3\n:\n\n(6)\n\n\n\n\nc\n\n\nC\ne\n\n\n3\n+\n\n\n\n\nc\n\nC\ne\n\n\n\n=\n\n\n\nI\n\nv\n0\n\n\n+\n\nI\n\nv\n\u2032\n\n\n+\n\nI\n\nu\n0\n\n\n+\n\nI\n\nu\n\u2032\n\n\n\n\n\n\u2211\ni\n\n\n(\n\nI\n\n\nv\n\ni\n\n\n+\n\nI\n\n\nu\n\ni\n\n\n)\n\n\n\n\u00d7\n100\n%\n\n\n\nwhere c\nCe3+\n is the content of Ce3+, and I is the area intensity of the given component. As shown in Table 3, the surface relative content of Ce3+ increases significantly with Al content. Shyu et al. [56] reported that the area under the u\u2034 peak in the total Ce 3d region could be used to describe the relative content of Ce4+ in the samples. In Table 3, it is observed that the area under the u\u2034 peak shows a negative relationship with the Al content, which also confirms the correlation above. Herein, the theoretical effective surface area (TESA, S\neff) of supported FeNi catalysts is defined by the following equation:\n\n(7)\n\n\n\nS\n\ne\nf\nf\n\n\n=\n\nS\n\nB\nE\nT\n\n\n\u2219\n\nP\n\nr\n,\n\n\n\nC\ne\n\n\n3\n+\n\n\n\n\n\u2219\n\nP\n\nC\ne\n\n\n\n\n\nwhere S\nBET is the BET specific surface area of the catalyst, \n\n\nP\n\nr\n,\n\n\nC\ne\n\n\n3\n+\n\n\n\n\n\n is the relative surface content of Ce3+, and P\nCe is the total surface content of Ce of the catalyst. According to the data in Table 3, as the Al content increases, the TESA shows the same trend as the conversions and TOFs of C2H6 and CO2, indicating that the reactivity of C2H6 with CO2 is closely related to the content of surface Ce3+ species over supported FeNi catalysts. Moreover, the dispersion of surface active components should be another important factor to be discussed later. In Fig. S6, the binding energy of the Ce 3d core level of the FeNi/Al\u2013Ce\u2013O catalysts decreases slightly with increasing Al content compared with that of FeNi/CeO2. In addition, the peaks of the Al 2p core level of the FeNi/Al\u2013Ce\u2013O catalysts shift to lower binding energies than that of FeNi/Al2O3 with increasing Ce content, which demonstrates electron transfer between Ce or Al and adjacent atoms.\nFig. S7 in Appendix A shows the O 1s and Fe 2p XPS spectra of the reduced FeNi catalysts. In Fig. S7(a), the binding energies at approximately 529.2 and 531.7\u202feV are ascribed to the lattice oxygen of Ce-based oxides (OI) and the adsorbed oxygen or hydroxyl groups (OII) on the surface, respectively [41,57,58]. The O 1s core level binding energy of Al2O3 is located at 530.9\u202feV [57]. The OII/OI ratios in Table 3 increase gradually with increasing Al content, which indicates an increase in surface oxygen vacancies over the FeNi/Al\u2013Ce\u2013O catalysts [13,41]. In Fig. S7(b), the Fe 2p core level binding energies of the reduced FeNi catalysts at approximately 710.9 and 724.0\u202feV are ascribed to Fe2O3, which means that the surface Fe species should be highly oxidized during the reaction [57]. Raman spectroscopy was also conducted to determine the surface oxygen vacancy of the catalysts. Fig. S8 in Appendix A shows the Raman spectra of the supported FeNi catalysts excited by a 532\u202fnm laser. The strong band at approximately 462\u202fcm\u22121 is ascribed to the F2g vibration mode of the Ce\u20138O vibrational unit of the fluorite structure, while the weak bands at approximately 254 and 596\u202fcm\u22121 are attributed to the second-order transverse acoustic (2TA) mode and the defect-induced (D) mode of oxygen vacancies, respectively. The relative intensity ratio I\nD/I\nF2g reflects the content of oxygen vacancies [59,60]. As seen in Fig. S8, the intensity ratio of I\nD/I\nF2g increases slightly with Al content, which also indicates that the introduction of Al improves the content of surface oxygen vacancies and the inhibition of the RWGS reaction over the FeNi/Al\u2013Ce\u2013O catalysts. According to the results above, the introduction of Al into the CeO2 support leads to a higher density of surface Ce3+ species and oxygen vacancies, which further improves the catalytic performance for DRE over FeNi/Al\u2013Ce\u2013O catalysts.The H2-TPR profiles of the as-synthesized FeNi catalysts are shown in Fig. 4\n. To assign the peaks in the pattern, profiles of the as-synthesized Ce\u2013Al0.5 supported monometallic catalysts and pure CeO2 support are also shown in Fig. S9 in Appendix A as a comparison. As seen in Fig. S9, CeO2 reduction can be divided into two stages. The first short and wide peak located between 600 and 800\u202fK is assigned to the reduction of surface active oxygen of CeO2, which leads to the formation of surface oxygen vacancies and nonstoichiometric CeO\nx\n, and the second peak above 800\u202fK is attributed to bulk CeO2 reduction [61\u201363]. In Fig. 4, after adding surface active components to the support, a strong peak at approximately 550\u202fK appears for the FeNi/CeO2 catalyst. This peak can be attributed to the reduction of both surface active components and the CeO2 support by surface hydrogen spillover [64]. Compared with the patterns of monometallic catalysts in Fig. S9, for the Ni/Ce\u2013Al0.5 catalyst, the peak below 673\u202fK shows a lower reduction cutoff temperature than that of Fe/Ce\u2013Al0.5, which indicates that surface Ni species are much easier to reduce than Fe on Ce-based composite oxides. For the FeNi/Al\u2013Ce\u2013O catalysts, compared with the pattern for FeNi/CeO2 in Fig. 4, the peak broadening and cutoff temperature rise also indicate that the introduction of Al significantly enhances the SMSI effect between surface active components and oxide supports.To identify the dispersion of surface active components on the composite support, EDS elemental mapping measurements were conducted on three representative samples: FeNi/Ce\u2013Al0.1, FeNi/Ce\u2013Al0.5, and FeNi/Ce\u2013Al0.9. The element mapping images in this article are representatively chosen from many different regions of the samples. As shown in the EDS mapping images of Ce and Al in Figs. 5\u20137\n\n\n, Ce and Al are well distributed over the FeNi/Al\u2013Ce\u2013O catalysts. Nevertheless, the elemental distributions of Fe and Ni are quite different. Small bimetallic FeNi NPs are observed on the surface of FeNi/Ce\u2013Al0.1, as confirmed by the EDS mapping images shown in Fig. 5. Bimetallic FeNi NPs with similar structures have been proven to have high selectivity for ethylene [45]. As demonstrated by the gradual FeNi distribution changes shown in Figs. 6 and 7, as the Al content increases, the surface Fe and Ni species become well dispersed on the Al\u2013Ce\u2013O supports. The introduction of Al greatly increases the interaction between the surface active components and the composite support. The surface Fe and Ni species are dispersed randomly and independently throughout the support because of the enhanced SMSI effect, leading to peak broadening and an increase in the reaction cutoff temperature of H2-TPR over the supported FeNi catalysts, as shown in Fig. 4.To further investigate the surface active species during the DRE reaction, in situ IR spectroscopy studies were carried out at temperatures ranging from 373 to 873\u202fK and a total pressure of 2.0\u202fmbar (C2H6:CO2\u202f=\u202f1:1). In situ IR spectra of the FeNi/Ce\u2013Al0.1, FeNi/Ce\u2013Al0.5, and FeNi/Ce\u2013Al0.9 catalysts are shown in Figs. S10\u2013S12 in Appendix A. All the spectra were normalized by subtracting the corresponding IR spectrum under vacuum at RT. A typical IR spectrum is divided into three different characteristic vibrational regions that will be discussed individually.The in situ IR spectra in the region of 3900\u20133500\u202fcm\u22121 in Fig. 8\n provide information on surface hydroxyl and carbonate species. The wide band at 3770\u20133790\u202fcm\u22121 and the strong band at approximately 3706\u202fcm\u22121 are ascribed to the monocoordinated OH groups (Type I OH) of Al and Ce, respectively [65\u201368]. The band at approximately 3732\u202fcm\u22121 is mainly ascribed to the Type II-A OH species of Al (hydroxyl groups bibridged across Al\u2013Al ion pairs) with a possible contribution of terminal OH groups bound to surface Ce4+ cations [65]. The band located at approximately 3625\u202fcm\u22121 is thought to be a combination of two bands: \u2460 the Type II-B OH species of Ce with adjacent oxygen vacancies (O\u2013Ce\u2013OH\u2013Ce\u2013\u25a1) at 3630\u202fcm\u22121; and \u2461 the surface bicarbonate (HCO3\n\u2212) species at 3619\u202fcm\u22121, as confirmed by the delay of the band at approximately 3706\u202fcm\u22121 [67,68]. The presence of Type II-B OH species and the absence of Type II-A OH species of Ce indicate that the surface of the FeNi/Al\u2013Ce\u2013O catalysts is highly active with oxygen vacancies under the reaction atmosphere. The band at approximately 3598\u202fcm\u22121 can also be separated into two bands: the tribridged OH species (Type III OH) of Ce at approximately 3600\u202fcm\u22121 and surface protonated carboxylate species (\u2013COOH) at approximately 3593\u202fcm\u22121 [67\u201370]. The formation of surface carbonate species will be discussed in detail below. As seen in Figs. 2(d) and 3(d), as the reaction temperature increases, the decreased intensity of the OH band over the FeNi/Ce\u2013Al0.5 catalyst implies a reduction in H2O production, which further leads to lower RWGS activity and a higher H2/CO ratio than those of other supported FeNi catalysts.The bands in the region of 3150\u20132750\u202fcm\u22121 correspond to the CH stretching bands of adsorbed species. The strong wide bands at approximately 3005 and 2931\u202fcm\u22121 in Fig. 9\n can be ascribed to the antisymmetrical (\u03bdas) and symmetric (\u03bds) CH stretching vibrations of a series of methyl species in the gas phase [71]. The CH vibration bands in this region indicate the presence of adsorbed ethyl (\n\n\n\u03bd\n\nas,C\n\nH\n3\n\n\n\n\n at 2970\u202fcm\u22121, \n\n\n\u03bd\n\nas,C\n\nH\n2\n\n\n\n\n at 2931\u202fcm\u22121, and \n\n\n\u03bd\n\ns,C\n\nH\n3\n\n\n\n\n at 2880\u202fcm\u22121) [72,73] and ethanol (\n\n\n\u03bd\n\nas,C\n\nH\n3\n\n\n\n\n at 2977\u202fcm\u22121, \n\n\n\u03bd\n\nas,C\n\nH\n2\n\n\n\n\n at 2933\u202fcm\u22121, and \n\n\n\u03bd\n\ns,C\n\nH\n3\n\n\n\n\n at 2878\u202fcm\u22121) [73\u201375]. The sharp band at 2953\u202fcm\u22121 is ascribed to the CH vibration band of bridged formate species, while the other sharp band at 2895\u202fcm\u22121 is attributed to the CH stretching band of bidentate formate [67,76,77]. Little difference in the spectra of the three catalysts is observed in the temperature range from 373 to 873\u202fK in the region of 3150\u20132750\u202fcm\u22121, which means that the FeNi/Al\u2013Ce\u2013O catalysts have the same kind of surface CH-containing species during the reaction, independent of the Al content and reaction temperature.\nFig. S13 in Appendix A shows the in situ IR spectra in the region of 1800\u20131000\u202fcm\u22121, where the peaks are mainly ascribed to the carbonate-like (OCO) species adsorbed on the samples [67,68,77\u201379]. The complex band assignments of different carbonate, carboxylate, and formate species adsorbed on the supported FeNi catalysts are summarized in Table S2 in Appendix A. The peaks attributed to the corresponding carbonate-like species are marked in the original spectra of FeNi/Al\u2013Ce\u2013O in Fig. 10\n. The band distribution in the IR spectra over the FeNi/Ce\u2013Al0.1 catalyst in Fig. 10(a) is quite similar to those of the other two samples in Figs. 10(b) and (c), except for the bands at 1430\u20131425, 1236\u20131217, and 1057\u20131050\u202fcm\u22121. The CO adsorption IR spectra of the FeNi/Ce\u2013Al0.5 catalysts are shown in Fig. 10(d) for comparison. The missing bands at 1430\u20131425\u202fcm\u22121 in both Figs. 10(a) and (d) are attributed to the intermediate adsorbed species of C2H6. Since it has been reported that the bands at 1580\u202fcm\u22121 (\u03bdas,OCO), 1429\u202fcm\u22121 (\u03bds,OCO), 1306\u202fcm\u22121 (\n\n\n\u03b4\n\nC\n\nH\n3\n\n\n\n\n), and 1026\u202fcm\u22121 (\n\n\n\u03c1\n\nC\n\nH\n3\n\n\n\n\n) are the characteristic peaks of acetate species, an oxidation product of C2H6 on CeO2 at 355\u202fK [74], the increased IR spectral intensity for the FeNi/Ce\u2013Al0.5 and FeNi/Ce\u2013Al0.9 catalysts at 1430\u20131425\u202fcm\u22121 can be attributed to the formation of surface acetate species. The bands at 1660\u20131640\u202fcm\u22121 and approximately 1230\u202fcm\u22121 arising from the olefinic C=C and CH stretching vibrations imply the formation and adsorption of ethylene on the FeNi/Ce\u2013Al0.1 catalyst, rather than surface acetate species [80]. Thus, it can be inferred that the different product selectivity of the FeNi/Al\u2013Ce\u2013O catalysts results from the changes in the surface adsorbed species. Moreover, the bands at 1057\u20131050\u202fcm\u22121 in Fig. 10(a) below 673\u202fK are attributed to the CO stretching vibration of bidentate ethoxide and methoxide species [74,75]. Since the formation of the *C2H\ny\nO intermediate has proven to be essential for C\u2013C bond cleavage and syngas production [7], the reduced band intensity at high reaction temperatures indicates that the weak adsorption of surface bidentate ethoxide species on the FeNi/Ce\u2013Al0.1 catalyst is beneficial to the formation of ethylene.On the basis of the discussion above, a typical catalytic cycle of CO2 over FeNi/Al\u2013Ce\u2013O catalysts involves the following process, as shown in Fig. 11\n. CO2 in the gas phase first adsorbs onto the surface hydroxyl species or oxygen vacancies and generates adsorbed carbonate-like species, such as bicarbonate and carboxylate. In addition, C2H6 adsorbs on metallic or oxidized FeNi active sites and dissociates into ethyl or ethoxy groups and a hydrogen atom. As a result of surface hydrogen spillover, adsorbed bicarbonate or carboxylate species are reduced to carboxylic acid or formate species, which further decompose into surface hydroxyl and carbonyl species through a possible formyl transition intermediate [77]. Surface hydroxyl species or oxygen vacancies regenerate after the release of CO to the gas phase. Nevertheless, the oxidation of C2H6 involves two different paths determined by the dispersion of FeNi active components. The impregnation of Fe and Ni precursors on pure CeO2 tends to generate bimetallic FeNi NPs, which prevents the excessive oxidation of adsorbed ethyl or ethoxy species and improves the selectivity of ethylene [45]. The introduction of Al into the lattice of CeO2 greatly improves not only the content of surface Ce3+ species and oxygen vacancies but also the dispersion of surface active components through the enhanced SMSI effect. The strong interaction between FeNi and the Al\u2013Ce\u2013O support stabilizes the adsorbed ethoxy moiety and its further oxidation products, which are essential for C\u2013C bond cleavage and syngas generation.FeNi/Al\u2013Ce\u2013O catalysts synthesized via a facile sol\u2013gel and impregnation method exhibit a composition-induced SMSI effect for DRE. The Al content in the Al\u2013Ce\u2013O supports significantly influences the metal\u2013support interface structure of the catalysts and further determines the catalytic properties during the reaction. As the Al content increases, the C2H6 and CO2 conversion, CO selectivity and yield, and TOF first increase and then decrease according to the same trend as the TESA. The FeNi/Ce\u2013Al0.5 catalyst exhibits the best DRE performance with the highest C2H6 conversion (11.7%), CO2 conversion (33.1%), and CO yield (11.5%). The increased surface oxygen vacancy partially inhibits the RWGS reaction over FeNi/Ce\u2013Al0.5 catalysts, which leads to a higher H2/CO ratio than that of other FeNi/Al\u2013Ce\u2013O catalysts. The selectivity over the supported FeNi catalysts is determined by the dispersion of the surface active components. As the Al content in the Al\u2013Ce\u2013O supports increases, the dispersion of surface active components is promoted by the enhanced SMSI effect over the supported FeNi catalysts. The enhanced SMSI effect stabilizes the adsorbed *C2H\ny\nO intermediate and produces excessive oxidation products, leading to C\u2013C bond cleavage and syngas generation. In summary, the introduction of Al into the CeO2 support not only increases the content of surface Ce3+ and oxygen vacancies but also promotes the dispersion of surface active components, which further adjusts the catalytic properties for DRE over supported FeNi catalysts.The authors gratefully acknowledge the support from the National Key Research and Development Program of China (2017YFB0702800), the China Petrochemical Corporation (Sinopec Group), and the National Natural Science Foundation of China (91434102 and U1663221).Tao Zhang, Zhi-Cheng Liu, Ying-Chun Ye, Yu Wang, He-Qin Yang, Huan-Xin Gao, and Wei-Min Yang declare that they have no conflicts of interest or financial conflicts to disclose.Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2021.11.027.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Dry reforming of ethane (DRE) has received significant attention because of its potential to produce chemical raw materials and reduce carbon emissions. Herein, a composition-induced strong metal\u2013support interaction (SMSI) effect over FeNi/Al\u2013Ce\u2013O catalysts is revealed via X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (TPR), and energy dispersive X-ray spectroscopy (EDS) elemental mapping. The introduction of Al into Al\u2013Ce\u2013O supports significantly influences the dispersion of surface active components and improves the catalytic performance for DRE over supported FeNi catalysts due to enhancement of the SMSI effect. The catalytic properties, for example, C2H6 and CO2 conversion, CO selectivity and yield, and turnover frequencies (TOFs), of supported FeNi catalysts first increase and then decrease with increasing Al content, following the same trend as the theoretical effective surface area (TESA) of the corresponding catalysts. The FeNi/Ce\u2013Al0.5 catalyst, with 50% Al content, exhibits the best DRE performance under steady-state conditions at 873\u202fK. As observed by with in situ Fourier transform infrared spectroscopy (FTIR) analysis, the introduction of Al not only increases the content of surface Ce3+ and oxygen vacancies but also promotes the dispersion of surface active components, which further alters the catalytic properties for DRE over supported FeNi catalysts.\n               "}
{"full_text": "No data was used for the research described in the article.With the rapid development of the world economy and industry, CO2 produced by human activities has had a significant impact on the ecological environment of the earth, such as the sea level rise, glacier melting, and earth temperature rise, so it is urgent to reduce CO2 emissions [1,2]. 70% of global CO2 emissions comes from the burning of fossil fuels in which converts them into energy needed for human life [3\u20135]. Under the current conditions of aggressively developing renewable energy, it is not possible to limit the use of fossil fuels in order to reduce CO2 emissions in a short period of time. Therefore, the most beneficial method to reduce CO2 emissions is CO2 capture and utilization (CCU) technology, which attracts more and more attention from all walks of life [6]. CCU not only has great environmental and economic benefits, but also has a great impact on the future energy structure of the world [7]. Pre-combustion capture, post-combustion capture, and oxygen-rich combustion are the three basic types of carbon capture\u00a0[3,8]. The method of removing carbon from fuel before it is consumed is known as pre-combustion capture [9]. Post-combustion capture is the capture of fuel after combustion, such as coal-fired power plants in the process of flue gas emissions set up adsorption devices to capture CO2\n[10]. Oxygen-rich combustion refers to combustion in a medium with a higher oxygen content than the air, so that the fuel is fully burned so as to generate a high concentration of CO2 for compression and storage [11,12]. CO2 is a low-cost, non-toxic, plentiful carbon-one material that can be utilized in food packaging, carbonated drinks, as a refrigerant to make dry ice, and injected into geological formations to enhance oil recovery (\nFig. 1) and more chemical conversion utilization routes [13]. There are two methods for CO2 chemical conversion: reduction and non-reduction [14,15]. Under certain catalysts and other circumstances, reduction conversion is the conversion of CO2 to carbon monoxide, methane, methanol, formic acid, and so on. A non-reduction reaction is the reaction of CO2 and other molecules under certain conditions to form esters, urea, carboxylic acids, and so on. From the perspective of inorganic chemistry, the standard heat of formation of CO2 is 394\u00a0kJ/mol and the standard Gibbs free energy is 395\u00a0kJ/mol. Because of its great thermodynamic stability, the conversion of CO2 demands a lot of energy, whereas the non-reduction process requires less. So, the non-reduction reaction conversion is easier than the reduction reaction conversion.In the non-reduction reaction conversion, as early as 1969, Inoue et al. discovered the copolymerization of CO2 and epoxide, which made the non-reducing transformation of CO2 develop rapidly [16]. CO2 and epoxides react to generate cyclic carbonate, this route is atom economy reaction process by 100%. Compared with the traditional toxic phosgene to participate in the preparation of cyclic carbonate, CO2 and epoxide bonus to prepare cyclic carbonates is environmentally friendly, which is in line with the concept of modern green chemistry, therefore become one of the high-profile CO2 conversion path [17]. Cyclic carbonates is widely used in medicine and the chemical industry as it is shown in \nFig. 2. It is not only a good organic solvent, but also in the preparation of other chemical materials and intermediates, such as CO2 and ethylene oxide cycloaddition after ethylene glycol synthesis, ethylene carbonate reproduction; additionally, cyclic carbonates have a wide range of applications in the production of lithium batteries, etc [18\u201320]. Similarly, CO2 and epoxides can be copolymerized to produce polycarbonate, which also has a wide range of uses. Charlotte Williams et al. have reported a number of studies on this route in recent years [21,22].Cyclic Carbonates are synthesized when CO2 combines with epoxides. Due to its high kinetic stability, catalysts and solvents are frequently utilized to speed up the chemical conversion of CO2, as indicated in \nFig. 3(a). Epoxides are ethers containing oxygen ternary rings, but they are more reactive than other ethers, especially with nucleophiles. The epoxides selected as substrates are generally propylene oxide (PO), styrene oxide (SO), epichlorohydrin (ECH), etc. The reaction generates ring carbonic acid esters such as propylene carbonate (PC), chloropropyl carbonate (CPC), and so on [23].As illustrated in Fig. 3(b), the overall mechanism of the cycloaddition reaction between CO2 and epoxide may be separated into three parts. The ring-opening reaction of the epoxide is the first step [24]. First, lewis acids activate epoxides. Generally, transition metals (such as Cu2+ and Zn2+) or hydrogen bond donor groups (such as OH-, COOH-, and NH2\n-) are regarded as the active components of lewis acids [25]. They are bonded with oxygen atoms of epoxides through hydrogen bonds to achieve epoxide activation. The ring is subsequently opened by the X- molecule's nucleophilic assault on the epoxy carbon atom, which has a low steric barrier. Following the opening of the epoxide, CO2 combines with the oxygen anion on the ring to generate the carbonate intermediate. Here, the adsorption and activation of CO2 by the catalyst is particularly important. The final step is intramolecular cyclization to obtain cyclic carbonates and regenerate the catalyst simultaneously. Catalysts serve a crucial role in decreasing reaction time and decreasing reaction pressure during the initial activation of epoxides, ring-opening, and carbonate intermediate stabilization [26].In this review, we focus on the reaction path of cyclic carbonates through the reaction of CO2 and epoxide. The importance of CO2 activation in the reaction process is emphasized, and typical modes of CO2 activation are summarized and compared. Then, the research progress in various typical catalysts in recent years are reviewed, and the catalytic properties and principles of the promising catalysts are introduced in detail. Finally, the development trend and characteristics of this pathway are summarized and prospected. It is hoped that this review can provide a reference for future researchers in the synthesis of cyclic carbonates from CO2 and epoxides.Among the many CO2 chemical conversion routes, the activation of CO2 has always been a crucial step or first step in the conversion reaction, and the conversion of CO2 to cyclic carbonates is also the case. In 2018, Moya et al. [27] synthesized aprotic heterocyclic anion ionic liquids (AHA-ILs) to convert cyclic carbonates from CO2. This ILs first activated CO2 to form IL-CO2. The epoxide is then attacked to open the ring for intramolecular cyclization, and finally the cyclic carbonate is formed. They performed DFT calculations and operando FTIR analysis of the conversion process and found that the reason why AHA-IL was better at converting CO2 to cyclic carbonates than other catalysts was that the ILs could first activate CO2 molecules to form intermediates, which was beneficial for the whole reaction process [27]. Therefore, it is important to consider the activation of CO2 by various catalysts. The key to CO2 activation is its high stability, which is closely related to its structure. From the standpoint of CO2 structure, CO2 is a normal linear molecule with zero dipole moment [28]. The carbon atom in the CO2 molecule forms a bond with oxygen in the form of a sp hybrid orbital. The remaining 2\u2009Py and 2\u2009Pz orbitals of carbon and oxygen atoms and their electrons form two perpendicular three-center four-electron delocalized bonds. Compared with the carbon-oxygen double bond and the carbon-oxygen triple bond, the C-O bond in the CO2 molecule has the characteristics of shorter bond length and higher bond energy [29]. Therefore, the CO2 molecule is relatively stable, so in order to achieve large-scale transformation of CO2, its activation is a key link. Generally speaking, the electronegativity of oxygen in the CO2 molecule is 3.44, and that of carbon is 2.55. The electron cloud obviously favors oxygen, making carbon more positively energetic [30]. From a coordination chemistry point of view, CO2 usually complexes with some transition metals and organic molecules in a variety of ways. A CO2 molecule has two active sites, one of which is its lowest unoccupied molecular orbital (LUMO), that is, its carbon atom, which has lewis acidity and usually acts as an electrophilic. The other active site is its highest occupied molecular orbital (HOMO), which contains its two oxygen atoms, which have a weak lewis base and usually act as nucleophiles [31]. In most cases, the chemical transformation of CO2 requires at least one form of coordination activation of CO2, that is, nucleophilic coordination with carbon, or electrophilic coordination with oxygen, or both [32]. Seven major CO2 activation methods are amine activation, frustrated Lweis pairs (FLPs) activation, ILs activation, N-heterocyclic carbenes (NHCs) activation, transition metal coordination activation, photocatalytic reduction activation and electrocatalytic reduction activation. They can form complexes with CO2, which react with substrates such as epoxides activated by organic or metal-based catalysts to form cyclic carbonates for conversion purposes.The basic group can react acid-base with CO2 to generate acid-base adducts, which can then react with epoxides as key intermediates. Woolee Cho et al. [33] exploited tertiary amines as green organocatalysts to activate CO2 and produce cyclic carbonates. \nFig. 4 illustrates the possible mechanism of cycloaddition reaction catalyzed by tertiary amines. The combination of amine and CO2 induces the CO2 activation, producing carbamate salt. The reaction of amine and epoxide generates quaternary ammonium salt and therefore achieve epoxide activation. The carbamate salt and quaternary ammonium salt are considered are pivotal intermediates corresponding to Cycle I and Cycle II.One of the most common and effective CO2 activation methods is the coordination reaction of CO2 and transition metal to form transition metal CO2 complexes, which reduces the activation energy required for further conversion reactions and makes CO2 conversion reactions easier. Mascetti et al. [34] summarized four binding modes of coordination between CO2 and transition metals (\nFig. 5). The \u03b71(C) and \u03b72(C, O) coordination modes are both affected by electrostatic and orbital overlap. Under the \u03b71(C) coordination mode, there is a strong electron transfer phenomenon between the dz\n2 orbital of transition metal space and the \u03c0\u2009*\u2009orbital of CO2. In the \u03b72(C, O) coordination mode, the dz\n2 orbital of the transition metal space forms \u03c3 bonds with the \u03c0 orbital of CO2, while the occupied dxy orbital of the transition metal forms feedback bonds with the \u03c0\u2009*\u2009orbital of CO2. The repulsive electrostatic interaction is decreased when the metal is in a low oxidation state, making it simpler to adopt a \u03b71(C) coordination mode, such as [Rh(Diars)2(Cl)], [Co(Salen)], and so on. d\u03c0 with high-energy orbitals more inclined to eta \u03b72(C, O) coordination, such as [Ni(PR3)2], [Mo(PMe3)4], [Fe(PMe3)4], [Cp2Mo], etc. Under certain conditions, CO2 can be inserted into the M-C, M-H, M-O, M-N, and other chemical bonds of transition metal complexes to form carboxylic esters and carboxylic acid complexes containing new M-C, M-H bonds [28,35].Michael North et al. [36] reviewed six-class heterogeneous catalysts used in the cycloaddition reaction of epoxide and CO2, and pointed out that some catalysts have little metal loading or do not require cocatalysts, but it is difficult to obtain conversion rates comparable to those of metal catalysts. CO2 coordinates with the metal center to form a complex, which is \u03b72-O, C-side coordination to the metal center. There are two possible forms of CO2 bonding, one is CO2 binding to metal strong \u03c0-donation, \u03b72(O, C) [1], and the other is reduction to a \u03b72(O, C) [2] when the bond between carbon and oxygen in CO2 is formed simultaneously by metal center \u03c0-back and M-C, M-O bonds.According to the classical lewis acid-base theory, the reaction of lewis base (LB) and lewis acid (LA) generates a classic lewis adduct (CLA). The essence of the acid-base reaction is the formation of coordination covalent bonds between electron docking receptors and electron pair donors [37]. However, not all LA and LB could generate stable CLA. It was not until 2006 that Douglas W.S. Tephan proposed a definition of \"frustrated Lweis pairs\" (FLPs), which refers to electron donor and electron acceptor pairs that do not bind to stable CLA due to steric inhibitors or that can be dissociated by CLA [38]. Lewis acids and lewis bases, which cannot form coordination bonds due to steric hindrance, form an active region in which the bonding orbitals of reactant molecules interact with the vacant orbitals of lewis acids. At the same time, the antibonding orbital of lewis base interacts with the nonbonding orbital of lewis group, the electrons in the molecular bond orbital move to the unoccupied orbital of lewis acid, while the lone pair electrons of lewis base transfer to the antibonding orbital of the molecule, leading to the polarization of the reactant molecules, the elongation of molecular bonds, and finally the isomerization. Therefore, FLPs have the characteristics of an acid-base double active center, which makes them have an important application in CO2 transformation [39]. Through the interactions between Lewis basic sites/C and Lewis acidic sites/one O atom of CO2, FLPs can activate CO2. Zhang et al. [40] addressed interfacial FLPs constructed on defect-enriched CeO2 (110). The oxygen vacancy clusters and isolated Ce3+ ions are generated when two adjacent oxygen atoms are removed from CeO2 (110). At a proper distance, adjacent Lewis acidic Ce3+ ions in surface and Lewis basic lattice O2\u2013 produce the interfacial FLPs. Although the defective CeO2 can achieve the activation of CO2, FLPs constructed on defective CeO2 can enhance furtherly CO2 adsorption. The C atom and two O atoms of CO2 are bound at a Lewis basic lattice O2\u2013 and two Lewis acidic Ce3+ ions, respectively. The isolated Ce3+ ions can catalyze olefin epoxidation and interfacial FLPs can effectively activate CO2. A catalytically tandem conversion of olefins and CO2 into cyclic carbonates is proposed, as shown in \nFig. 6. Increasing the surface defects could create more FLPs and weaker interactions between epoxide and catalyst. The more FLPs can benefit CO2 activation, while the subdued interactions can improve the selectivity of cyclic carbonates.Ionic liquids (ILs) have attracted a lot of attention in the last decade. Compared with traditional organic solvents, ILs have the characteristics of low volatility, low flammability, excellent thermal stability, and strong solubility [41,42]. The ILs contains large organic cations and anions with different structures, and its melting point is less than 100\u2009\u00b0C [43]. Generally, the structural design of cations and anions in ILs is carried out to fine-tune the physical and chemical properties of ILs to meet specific needs [44]. Therefore, ILs are generally known as \"design solvents\" [45]. The most common cations in ILs include imidazole, pyridine, piperidine, quinoline, morpholine, pyrrolidine and its monoalkyl or polyalkyl derivatives, as well as tetraalkyl ammonium or phosphonate and trialkyl sulfonic acid [46]. Most cations are formed by proton or alkyl substitution of heteroatoms in the molecular structure of ILs. By adding R+\u2009and H+\u2009groups to the cations, \"aprotic\" and \"proton\" ILs were created. Protic ILs vary from aprotic ILs in which they have proton acceptors and donor atoms, as well as the ability to build large hydrogen bond networks. Functionalized side chain cations, such as polar, fluorinated, or chiral cations, have sparked a lot of attention in recent years, and they are typically created for specific uses. \"Mission-specific ILs\" is a term used to describe certain ILs. \nFig. 7 depicts the various cations and anions that are often employed in the synthesis of ILs.ILs are widely used in the conversion of CO2 into products such as methanol, formic acid, and cycloaddition reactions to form cyclic carbonates and other more complex organic compounds [48]. The functional design of ILs can effectively reduce the energy required for the conversion reaction and plays an important role in the activation of CO2 and epoxides [49]. Lian et al. [50] introduced amino-based ILs, imidazolium-based ILs, pyrazolium-based ILs, others ILs and ILs-modified catalyst. ILs and ILs-modified catalysts have inherently high affinity to activate CO2. CO2 molecules can be easily activated and the cycloaddition reaction can occur at mild conditions in the presence of catalysts to obtain cyclic carbonates. The mechanism of CO2 activation can be ascribed to the hydrogen bond in ILs. ILs have biological toxicity, which could not be ignored [51]. Indeed, ILs have greatly promoted the conversion of CO2 and will have far-reaching effects.Generally speaking, under certain conditions, the catalytic material is irradiated with light of appropriate energy [52]. Driven by the incident light energy, the electrons in the valence band (VB) are excited, and the excited electrons jump across the band gap to the conduction band (CB) with higher energy [53]. When an electron leaves the valence band (VB), an equal number of holes will be created simultaneously in the valence band, forming an electron-hole pair [54]. The electron-hole pair then moves together to the active site on the surface of the catalytic material to participate in the reaction [55]. The lifetime of the excited electron-hole pair is only a few nanoseconds, but this is enough to facilitate redox reactions [56]. Common reduction products are carbon monoxide, methane, methanol, ethanol, acetic acid, and so on [57]. The main reason for the different products is the different numbers of electrons involved in the reduction reaction [58]. One problem is that electrons and holes may recombine during the journey of the electron-hole pair to the surface of the catalytic material. Catalytic materials are a key factor in the process of photoactivation reduction of CO2, and there are many classical semiconductor materials such as TiO2, CdS, G-C3N4, ZnO, BiVO4, etc [59\u201363]. However, the band gap of traditional semiconductor materials is generally wide, resulting in a high electron-hole pair recombination rate, and the activity is greatly affected by the wavelength of incident light. For example, TiO2 only has Ultraviolet (UV) light activity (wavelength less than 380\u2009nm), its adsorption capacity for CO2 is weak, and its structural tunability is not satisfactory [64]. With the development of energy band engineering, many materials have been designed, such as core shells, egg yolk shells, multi-shells, hollow structures, etc [65\u201367]. Not only do they have a band gap that matches the thermodynamic reduction capacity requirements, but also they have a structure that facilitates electron transport [68]. Anjan et al.\u00a0[69] reported the successful activation of CO2 and reaction with epoxides to synthesize cyclic carbonate by using covalent organic framework (COF) as a photocatalytic material under atmospheric pressure and visible light. The experiment shows excellent yield under visible light irradiation and CO2 (1\u2009atm), and the reaction can be easily controlled by changing the illumination.Electroactivation of CO2 is also a promising method of activation, highlighting many advantages such as controlled reaction rates, mild reaction conditions, and product selectivity through the potential [70]. Electrochemical activation of CO2 is achieved either directly at the electrode or indirectly by heterogeneous or homogeneous catalysis [71]. It has become a focus to search for catalysts that can reduce the relatively high evidence and improve the selectivity of the reduction process. Many transition metal complexes have been shown to be effective in the electrolytic reduction of CO2\n[72]. For example, Fisher and Eisenberg\u00a0[73] reported for the first time the catalytic activity of the Ni tetraaza macroring for CO2 reduction and found that it reduced the CO2 reduction potential by about 0.5\u2009V in non-aqueous media. Cycloaddition reactions of CO2 and cycloalkyl oxides with cyclocarbonates have been studied by some groups under the electrolysis of transition metal complexes. Electrocatalysis is actually a superposition of electrochemistry and catalysis. Electrochemistry converts electrical energy into chemical energy to catalyze the reaction [74]. Khoshro et al. [75] reported that a nickel complex, 2,4,10,12-tetramethyl-1,5,9,13-(14-nitrobenzene) tetra-cyclopentalkyl (2-) nickel (II), exhibited good electrocatalytic activity in acetonitrile (ACN) solution at room temperature. The intermediate is then oxidized by a nickel (II) complex to obtain a cyclopropane product. However, although electrocatalysis has been developing in recent decades, research groups around the world have not yet been able to design efficient electrostatic CO2 reduction solutions for industrial applications [76].N-heterocyclic carbenes (NHCs) are heterocyclic compounds containing carbon with at least one nitrogen atom in the ring structure, including a large number of substituents [77]. At the same time, it is a new strong \u03c3 coordination ligand, especially with the formation of excessive metal complexes, which has important applications in catalysis, materials, and other aspects [78]. In 1968, Ofele and Wanzlick et al. [79] synthesized the metal complex of NHCs for the first time, but they did not separate the free NHCs. In 1991, Arduengo et al. [80] successfully separated the free carbines. Common NHCs are shown in \nFig. 8. These free carbines have strong electron-giving ability, show strong nucleophile and lewis alkalinity, and form NHC-CO2 admixtures with CO2 molecules, thereby activating CO2\n[81]. It can also directly coordinate with many transition metals to form compounds with specific structures and functions [82,83].\n\nTable 1 summarizes the research progress and characteristics of the seven activation methods. Among them, amines, transition metals, and FLPs have been widely studied in the activation of CO2, which are often used in catalysts such as metallic, ILs, Metal Organic Frameworks (MOFs), and NHCs. ILs are very promising catalysts, because their functions can be designed, and different types of ILs can activate CO2 in different ways. The key to photocatalytic CO2 activation is the selection of materials and the control of the process. There are few reports on the non-reductive transformation of CO2. In recent years, NHCs is one of the most promising organic catalysts, which is often used as an efficient nucleophile to activate CO2. The synergistic catalytic activation of CO2 by NHCs and other catalysts is a hot research topic.Catalysts and epoxides have different effects on the rate and product of the CO2 cycloaddition reaction [84]. Specifically, the catalyst to reaction rate adjustments to the appropriate range, different epoxy compound cycloadditions of CO2 have different reactivity, and the result of CO2 and epoxide reaction selectivity (i.e., cyclic carbonates and polycarbonates) at the same time under the influence of catalyst, epoxide, and reaction conditions [85]. As a result, the major subject in the field of CO2 conversion to cyclic carbonate is the design and development of the entire catalytic system [86]. At present, the trend of catalytic system development is under mild conditions, requiring both high efficiency and selectivity, but also good stability and recyclability, and the catalytic process does not need cocatalyst and volatile organic solvents for the catalyst itself to be cheap and easy to obtain and easy to synthesize [87\u201390].Over the past few decades, many catalytic systems have been developed for coupling reactions of CO2 and epoxy compounds. B\u00fcttner et al. [91] reviewed recent catalysts, which can be classified into three types: organic catalysts, special metal complex catalysts, and miscellaneous catalysts based on transition metals and main group elements. Ammonium, phosphonate, imidazolium, amide-based catalysts, and carbenyl catalysts were introduced as organic catalysts. Among transition metal and main group element-based catalysts, alkali metal and alkali earth metal base catalysts, boron and carbon base catalysts, and transition metal catalytic systems are introduced. Catalysts containing halogen salts are poisonous, but halogen-free catalysts exhibit great attraction. Zhang et al. [19] reviewed the research progress of halogen-free catalysts, including metal catalysts, metal-free catalysts, and other catalysts. Metal oxides, metal complexes, metal salts, molecular sieves, MOFs, zeolitic imidazolate frameworks, and metal-porous materials were introduced in metal catalyst materials. Metal free catalysts include nitrogen-rich catalysts, CO2 adducts, and HBDs catalysts (e.g., alkyl amines, salophens, amino acids, biological acids). There are other catalysts, such as ammonium salts, organic bases, and so on. Liang et al. [92] reviewed many porous catalytic materials, such as MOFs, COF, nanoporous ionic organic networks (NION) and amorphous porous organic polymers. ILs, metal complexes, nitrogenous polymers, organic catalysts, and metal-Salen complexes, metal-organic frameworks, and molecular sieves are examples of homogeneous catalysts. The homogeneous catalyst has the advantage of high efficiency and high activity. For example, the earliest dual-functional or binary catalytic systems composed of homogeneous metal complexes (such as Al, Zn, Mg, etc.) usually convert CO2 into cyclic carbonate at room temperature and pressure [93\u201396]. However, such catalysts are generally poorly selective, expensive to use on a large scale, and even toxic [97]. The shortcomings of these metal complex catalysts have promoted the development of metal-free organic catalysts such as the organic halide [98,99]. In recent years, several functional organic catalysts have been reported to be as competitive as binary or ternary catalysts [100]. Some metal-free catalysts have also performed well in combination with organic halides [101]. At present, such metal-free organic catalysts are not as active as metal complex catalysts, but they have obvious advantages such as low price, simple synthesis, and non-toxicity. Recently, some researchers have synthesized some metal polyphase catalysts by combining metal with MOFs and Porous Organic Polymers (POPs). Such catalysts not only increase the active center of the catalyst and improve the catalytic activity, but also significantly improve the inherent difficulty in dissolution and separation of metal catalysts. Therefore, heterogeneous catalysts are more suitable for future chemical industry applications [102].Metallic catalyst, ILs, MOFs, NHC catalyst for CO2 or epoxide activation effect is better, which has a higher catalytic production rate, and provide the future development direction of CO2 conversion catalyst of cyclic carbonate.Catalysts with metallic elements are continuously developed. The crust contains a lot of metal elements, so the raw materials for the synthesis of metal catalysts are easy to obtain [103]. Metal catalysts involved in the catalytic conversion of CO2 and epoxides into cyclic carbonate can be divided into transition metals, main group metals and Rare-Earth (RE) metals. B\u00fcttner et al. [104] found that the binary catalytic system composed of FeCl3 and [Oct4P] had the best catalytic effect after screening a variety of combinations of ferric salts and phosphine salts on the premise of ensuring non-toxicity of the catalyst. Metal-amide combined catalysts are also commonly used to catalyze the reaction of CO2 with epoxides. Rios Yepes et al. [105] prepared a series of single, double, and trimethylamide-aluminum complexes that exhibit high catalytic activity with the help of tetrabutylammonium iodide (TBAI) as a catalyst. The activity of this type of aluminum complex catalyst is closely related to the coordination mode, showing the great potential. RE-based complex catalysts always have strict requirements on reaction conditions, usually 10\u201320\u2009bar CO2 or high temperature [106]. Xin et al. [107] synthesized for the first time a lanthanum complex catalyst stabilized by a polydentate N-methylethylenediamine bridged triphenol ligand. Under the conditions of room temperature, 1\u2009bar CO2, TBAI as cocatalyst and 1,2-epoxyhexane as substrate, the cyclic carbonate yield reached 99% and TOF reached 18.3\u2009h-1, and the catalyst reacted with 1,2-epoxyhexane and CO2 for 5 consecutive cycles, and the yield was maintained at 96%. For RE based catalysts, their reported results are unprecedented. At present, NHCs linked RE metal catalysts are still in the development stage, which is also very exciting [108].\n\nTable 2 shows the metal catalysts reported in the last decade. These metals combine with porphyrins, such as Entry 1, 2 and 3 to form catalysts with multiple active centers, and halogen ions also appear in the porphyrins framework. Or with molecules or group ligands form binary bifunctional catalysts. Adding cocatalyst is also a good strategy. Ema et al. [109] developed a catalytic system by combining metal porphyrins with nucleophiles, and examined the effects of Mg, Co, Ni, Cu, Zn and Tetraphenylphosphonium Bromide (TPPB), TetrabutylAmmonium Bromide (TBAB), phenyltrimethylammonium Tribromide (PTAT), and 4-dimethylaminopyridine (DMAP). The combination of Mg porphyrin and TBAB was found to have the best catalytic effect by comparison.Catalytic mechanism of metal catalyst is roughly same, usually by metal center as a lewis acid, activate the epoxide, formation of intermediates, add the nucleophilic reagent, attack epoxide steric lower side, promote and realize the epoxide ring opening, this step is largely affect the reaction rate and time. The nucleophilic reagent is commonly halogen ions. They are present in porphyrin frameworks, in ligands or provided by cocatalysts. The catalyst is renewed when the ring-opening epoxides react with CO2 molecules to create cyclic carbonates [119].Compared with traditional ILs, ILs containing hydroxyl, carboxyl and amino functional groups show very good advantages [120]. Qu et al. [121] reported the synthesis of new AAILs ([HTMG] [AA] [X], AA=(Histidine [His]2-, Lysine [Lys]\u2212), X\u2009=\u2009Cl, Br, or I) consisting of superbase 1,1,3,3-tetramethylguanidine ([HTMG]+), which has the ability to well catalyze CO2 and epoxide reactions at room temperature. In particular, the reaction of CO2 and propylene oxide at room temperature and atmospheric pressure for 20\u2009h can obtain 99% of the yield of propylene carbonate. In previous reports of amino acid ILs being difficult to reuse under the reaction conditions of ideal, Yue et al. [122] reported the synthesis of dual amino functional imidazole ILs under the conditions of solvent-free helpless catalyst as CO2 and epoxide catalyst for the synthesis of cyclic carbonate reaction. After repeated use for five cycles, the production rate did not decline. Excitingly, the reaction of CO2 with epachlorohydrin for 13\u2009h at 105\u2009\u00b0C and 0.5MPa resulted in a 98.3% yield of allyl chloride carbonate. Yang et al. [123] have synthesized multifunctional monocomponent zinc halide-based ionic liquids, amidinothiourea-ZnI2 (ATUI) and N-phenylthiourea-ZnI2 (NTUI), which are used as homogeneous catalysts for the conversion of CO2 with epoxides to cyclic carbonates. Owing to the synergistic effects of multifunctional components in ATUI, ATUI has the better catalytic activity and more superior reusability for CO2 cycloaddition than NTUI. Under 110 \u00b0C, 1.0 MPa, 4.0 h or even mild conditions, the yield of propylene carbonate can reach 95%. Only a slight decrease in the yield is caused by the loss of ATUI during the reuse process. As shown in \nFig. 9(a), Dai et al. [124] produced a variety of new functionalized phosphonium-based ionic liquids (FPBILs) constituted of hydroxyl, carboxyl, and amino functionalized phosphonium-based ILs. For the first time, it was utilized as a catalyst in the cycloaddition reaction of CO2 with epoxides to produce cyclic carbonate. By comparison, [Ph3PC2H4COOH]Br with carboxyl functional group had a high catalytic activity with a TOF of 64.9\u2009h-1. In the absence of solvent or cocatalyst, the reaction with 7 epoxides showed a conversion rate of more than 80%, among which the conversion rate of ethylene oxide as the reaction substrate was up to 100% within 1\u2009h. The advantages of FPBILs lie in the synergistic effect of hydrogen bonding between functional groups and epoxides and the nucleophilic attack of halide anions on epoxides [124]. The hydrogen bond connection between the functional group H atom and the epoxide O atom may lead to the polarization of the C-O bond of the epoxide and the generation of intermediate I. Br then performs a less steric impeding nucleophilic assault on the carbon atoms of the epoxide complex, encouraging the opening of the epoxide ring and the production of intermediate II. After CO2 is inserted into intermediate II, halogenated carbonate III is formed, which is converted to cyclic carbonate by intermolecular displacement, as shown in Fig. 9(b).Hydrogen bonding has been known to efficiently activate epoxide or CO2 and significantly promote cycloaddition reactions. Therefore, many hydroxyl-functionalized ILs have been developed and designed [125]. Peng et al. [126] reported a series of novel polyhydroxyl bisquaternary ammonium ILs and designed six novel bifocal polyhydroxyl ILs as shown in the \nFig. 10. The design principle is that halide ions act as lewis bases to carry out nucleophilic attacks, and hydroxyl groups act as lewis acids to activate epoxides within a molecule. At 120\u2009\u00b0C, 2\u00a0MPa, 3\u2009h, 99% yield of propylene carbonate can be obtained.The application of supported ILs in the fixation of CO2 was first proposed by Xiao et al. [127] in 2006. They used immobilized ILs-zinc chloride polyphase catalysts to chemically immobilize CO2 to cyclocarbonate effectively under mild conditions at high TOF without any additional cosolvent, and they maintained selectivity above 98%, which can be reused twice. Liu et al. [128] reported that at 80\u2009\u00b0C atmospheric pressure, a novel polystyrene supported ILs (PS-IMPCOOHTMGBR) demonstrated high catalytic activity for the cycloaddition reaction of CO2 and epichlorohydrin (ECH), with a product yield of 97.2%. The optimal reaction conditions were determined as follows: 0.1\u2009MPa CO2 pressure, 3\u2009mol% catalyst dose, 80\u2009\u00b0C temperature, 5\u2009h reaction duration, and it may be reused nine times. It has several active centers and a good capacity to absorb CO2 when compared to other PS loaded ILs. \nFig. 11 depicts a putative reaction process. CO2 is initially adsorbed by the \u2013COO- group in PS-IMPCOOHTMGBr in S1. The hydrogen atoms in the imidazole ring work as electrophiles to activate ECH, which is then polarized in the nucleophilic area by the anion Br-. Electrophilic and nucleophilic activation work together to open the ring and produce a complex (I), while hydrogen bonds form between ILs and ECH. The collected CO2 is transferred to ECH to generate complex (II) beginning with complex (II) (III). Finally, ring closure converts the intermediate (III) to chloropropylene carbonate (CPC). PS-IMPCOOHTMGBr retains a strong electrophilic activity center and a nucleophilic activity center after trapping CO2, which is another major explanation for the high activity of PS-IMPCOOHTMGBr. The proton in the [TMGH+] cation is attracted to the \u2013COO- group in Scheme 2. The TMG group then adsorbs the CO2, and the next stages differ from scheme one in that hydrogen is utilized as an electrophile. The proton transfer from the [TMGH+] cation to the \u2013COO- group, on the other hand, is difficult. And because the TMG group will quit after many runs, the chance of a response based on S2 is extremely low. PS-IMPCOOHTMGBr has a lower adsorption capability than porous materials. However, it plays a vital role in improving catalytic performance [129].Organic amines or alcohols (e.g., urea, ethylene glycol) combined with metal halides in appropriate molar ratios can form eutectic ILs, such as urea and zinc chloride, simply mixed in a molar ratio of 3:1, heated to about 100\u2009\u00b0C, and stirred in air until a transparent, homogeneous liquid is formed. The formation and structure of eutectic ILs are shown in \nFig. 12(a). This ILs has the advantages of low synthesis cost, environmental friendliness, simple operation, and excellent performance [130]. It was not until 2016 that Liu et al. [131] reported the [urea-Zn]I2 eutectic based ILs, which catalyzed the reaction of CO2 with propylene oxide through a cationic structure and halogen anion to generate propylene carbonate (PC). It has a 95% yield, 98% selectivity, and does not require cocatalysts and solvents. This avoids the need for additional cocatalysts and nucleophiles in the catalytic cycle.The mechanism is the oxygen coordination of NH2 groups and lewis acid zinc sites with epoxides. Polarization of the C-O bond in epoxides occurs through hydrogen bonding and the formation of zinc adducts to epoxides (Step 1) [132]. Then, during the ring-opening process of the epoxide, the I- ion nucleophilic attacks the carbon atoms in the epoxide with low steric resistance, forming the oxygen anion intermediate (Step 2). It is stabilized by synergy between cations and I- anions. Thereafter, the oxy-anion intermediate further interacts with the active CO2 via the amino group of the catalyst, leaving a catalyst behind (Step 3). Finally, I- is released simultaneously during intramolecular cyclization to generate cyclic carbonate products for catalyst regeneration (Step 4) as shown in Fig. 12(b). Thus, eutectic ILs provides lewis acid-base bifunctional groups that activate both CO2 and epoxides in the catalytic cycle [133].In summary, as shown in \nTable 3, ILs can be divided into three categories: functionalized, supported, and common. Functionalized ILs contain a variety of functional groups, which are key to the activation of CO2 or epoxide and accelerate the reaction. Supported ILs are grafted onto solid materials, and the activation sites on these materials can assist ILs to activate CO2 and epoxide, and this kind of ILs is also convenient for separation. Common ILs consists of basic anions and cations with nucleophilic ability, and halogen ions are usually involved in the catalytic reaction of CO2 and epoxide.MOFs have received a lot of attentions since they were first reported in 1995. MOFs used as heterogeneous catalysts have unique properties such as high specific surface area, high stability, open channels, and permanent porosity, giving MOFs an advantage over other adsorbents or catalysts in CO2 chemistry [134\u2013137]. In recent years, it has become a promising stationary CO2 heterogeneous catalyst [138]. In the past, the addition of lewis acid active sites to MOFs, such as coordination unsaturated metals, has been very beneficial in catalyzing cycloaddition reactions of CO2 and epoxides [139]. In recent years, some researchers have prepared catalysts by combining one or two metals with inorganic nodes in MOFs, as shown in \nTable 4.The reaction conditions are relatively mild, even at room temperature, but also have a high yield. Compared with mono-metal organic complexes, catalysts with MOFs generally have the advantage of easy separation and recovery [144]. Gao et al. [142] innovatively introduced zinc into Mg-MOF-74 inorganic nodes and added catalytic sites in MOFs to enhance catalytic performance. They used a facile One-Pot method to synthesize and characterize catalytic examples with different Zn-Mg ratios, as shown in \nFig. 13(a).The reaction mechanism is shown in Fig. 13(b). Firstly, unsaturated coordination of Zn and Mg serves as lewis acid centers to cooperatively activate oxygen atoms in epoxides to form an admixture of metal epoxides. At the same time, metal oxygen pairs (Zn-O, Mg-O) adsorbed and activated CO2 as lewis alkaline sites, which enhances the charge of oxygen atoms in CO2 and the nucleophilic ability. Then Br- in Bu4NBr acts as a nucleophile to attack the \u03b2-C atom with low steric hindrance in the epoxide, realizing the ring-opening of the epoxide and obtaining an oxygen anion intermediate. This oxygen anion intermediate reacts with CO2 to form an alkyl carbonate intermediate. Finally, the inner ring is closed to form cyclic carbonate, and the catalyst is regenerated (\nFig. 14).It is of great significance to graft ILs onto the carrier for the development of an efficient heterogeneous CO2 conversion catalyst [145]. Xu et al. [146] recently reported that MIL-101-N(Bnme2)Br functionalized by quaternary ammonium salts was employed as a bifocal catalyst for the cycloaddition of CO2 and epoxides for the synthesis of cyclic carbonate at 1.4\u2009MPa CO2 pressure, 17\u2009mmol epoxide, 100\u2009\u00b0C, 5\u2009h. At 0.93\u2009mol percent catalyst, the yield and selectivity of PC were 93% and 99%, respectively. Fig. 13 (a) shows the preparation process and structural model of MIL-101-NH2. As illustrated in Fig. 13 (b), MIL-101-N(Bnme2)Br polyphase catalyst was produced by an aldehyde-amine condensation process employing MIL-101-N(Bnme2)Br ILs (N(Bnme2)Br). \nFig. 15 describes the activation mechanism. The epoxide is first activated by the interaction of the chromium in the lewis acid core of MIL-101-N(Bnme2)Br with the oxygen atom of propylene oxide (PO). At the same time, the \u03b2-carbon atom of the epoxides with moderate steric hindrance is attacked by the nucleophilic bromide ions of the ILs, promoting ring opening of the epoxides. Subsequently, CO2 enters the ring-opening intermediate and eventually forms the corresponding ring carbonate, which is also released by the intramolecular Br- sealing to regenerate the catalyst [147].Puthiaraj et al. [148] employed Friedel-Crafts polymer to manufacture porous ZnBr2 grafted with N-heterocyclic carbene-based aromatics for CO2 adsorption and conversion to cyclic carbonate. Using 1, 3-dibenzylbenzimidazolium bromide (DBBIBr), triphenylbenzene, formaldehyde dimethyl acetal, and 1,2-dichloroethane as raw ingredients, a porous N-heterocyclic carbinyl crosslinked aromatic ionic polymer (NHC-CAP-1) was produced at 80\u2009\u00b0C for 18\u2009h. Following that, NHC-CAP-1 was suspended in a DMF:\u00a0tetrahydrofuran mixture, tert-butyl potassium was added and agitated at 50\u2009\u00b0C under nitrogen blowing conditions, ZnBr2 was added, and the solution was heated to 80\u2009\u00b0C for 12\u2009h. To remove and metallize the solid, it is filtered and washed with water, resulting in strong NHC-Zn linkages in the polymer network. \nFig. 16 (a) shows the synthesis process. When selecting different epoxides as substrates for cycloaddition processes, this catalyst exhibits outstanding performance and a high TOF, and the reaction condition was 100 \u00b0C, 2\u00a0MPa, and no cocatalyst and solvent was required in particular, the yield of propylene carbonate was 95%, and the TOF was as high as 2202\u2009h-1 as illustrated in Fig. 16(b).Multiphase and multifunctional charged polymers have many advantages, such as graded pore structure, high thermal stability, abundant active metal sites, and high specific surface area, which have broad application prospects in the formation of cyclic carbonate from CO2 and epoxide. Bai et al. [149] synthesized zinc(II)porphyrin-based porous ionic polymers [PIP-ZnTIPP/DVB (1:20)], which had high yields in catalyzing the reaction of CO2 with a variety of epoxides. Surprisingly, at 120\u2009\u00b0C, 1\u00a0MPa of CO2, 6\u2009h, 99% yield of propylene carbonate was obtained, the selectivity was greater than 99%, and TOF reached 759\u2009h-1.Zeolitic imidazolate frameworks (ZIFs) are highly adsorbable and selective for CO2, and have been used to catalyze the formation of cyclic carbonates from CO2 and epoxide [150]. Li et al. [151] reported a new type of zeolitic tetrazolate\u2212imidazolate frameworks (ZTIFs), which can be regarded as N-functionalized ZIF-8, called ZTIF-8. In fact, ZTIF-8 contains a basic N atom active site that is the key to CO2 conversion [152]. The yield of chloropropene carbonate catalyzed by ZTIF-8 was more than 99% (the first cycle) and 81.2% (the third cycle) at room temperature of 1\u2009atm for 48\u2009h with cocatalyst tetra-n-tert-butylammonium bromide (TBAB). Therefore, the recovery and recycling of catalysts still needs to be improved.Directly reducing the concentration of CO2 in the atmosphere is an effective way to improve the greenhouse effect, and the conversion and utilization of CO2 resources has been a huge and promising industry. There are many ways to convert CO2 in the past, but all of them are insufficient and do not conform to the development strategy of green chemistry. The reaction of CO2 with epoxides to form cyclic carbonates is a promising conversion method, and many types of catalysts have been reported with good results for these reactions. All kinds of catalysts, which still adopt effective classical methods for the activation of CO2 or epoxide, this paper summarizes seven CO2 activation methods, explains their respective research progress, and points out that the commonly used methods for the activation of CO2 by various catalysts, these other reviews are not described in such detail. At present, metal-based catalysts ILs and MOFs have obvious advantages in CO2 conversion reactions. The yield of some products can reach more than 95%, and the catalyst can also be reused. In addition, catalysts such as MOFs also have a good effect on CO2 adsorption. Dozens of organic catalysts have been reported, and NHC is a very promising one. There are few experiments related to NHC catalysts, but it also has a good application prospect. The design trend of future catalysts is easy synthesis, cheap, high yield, multiple active sites, designable performance, mild reaction conditions, and environment-friendly. However, more researches are needed to be made to achieve industrialization.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank the support from Shanghai Non-carbon energy conversion and utilization institute.", "descript": "\n                  With the continuous emission of greenhouse gases, the rational transformation and utilization of CO2 is particularly important. Cyclic carbonates are a kind of versatile compounds and have wide applications in Li-ion batteries, pharmaceutical manufacturing and many fine chemicals. Cycloaddition of CO2 and epoxide to synthesize cyclic carbonates is considered one of the most promising CO2 conversion routes because of its 100% atomic economy, non-toxicity, as well as a more economic technical route for the utilization of CO2. In this paper, this review surveys the synthesis of cyclic carbonates employing CO2 as a building block. The mechanisms of CO2 activation have been described in detail due to the thermodynamic stability of CO2 molecule. The reaction mechanism of CO2 and epoxide is expounded, and seven CO2 activation methods are summarized and compared, deeply analyzing the research progress of recent years. To reduce the activation energy of the CO2 conversion, the utilization of catalysts is very crucial. Various types of catalysts suitable for the synthesis of cyclic carbonates derived from CO2 have been expounded in depth. Finally, the development trend of catalysts is prospected. The development of improved catalysts is strongly demanded for successful commercialization of CO2 transformation technologies. This review enables researchers to timely seize the current advancements and thus may provide some rewarding insights for future investigations on the synthesis of cyclic carbonates employing CO2 as the feedstock. It will provide a good reference and guide for scholars to achieve the better improvements.\n               "}
{"full_text": "Perovskite catalyst general formula ABO3; typically the A elements are rare earth alkaline (Ce, La, Pr etc.), alkaline earth metals (Ca, Cs, Sr, Ba etc.) and the B sites are usually occupied by transition metals (Fe, Co, Cu, Mn, Ni, and Cr). Perovskite shows high activity for CO oxidation and high thermal resistance [1]. The performance of perovskite catalysts in CO oxidation processes can be increased by fractional replacement of metal in position A and/or position B with metal cations varying in their valence number [2]. The catalytic activity of perovskite catalysts can be enhanced by the incomplete substitution of metal in position A or B with cations of noble metals like Ag, Au, Pt and Pd etc [3]. When deposited of noble metals into perovskite is main factor in the catalyst's activity, which, however, is also highly effected by the type of perovskite was used. The activity of perovskite catalyst is strongly influenced by their preparation methods [4]. The main benefit of perovskite catalysts lies in the fact that they are posses' higher activity and thermal stability compared to pure oxides [5]. The addition of noble metals into perovskite reduces sintering and reduction in mass as a consequence of volatilization at a high temperature in oxidizing conditions [6].Increasing the number of vehicles on roads, CO concentration has reached an alarming level in urban areas. In the CO oxidation process, the oxygen is first adsorbed on the perovskite catalyst surface with the energy of activation [7]. By substituting the A and B cations, one can control the total amount of substitution and apply for suitable cations that will get important structural changes, such as lattice distortions, stabilization of multiple oxidation states or generation of cationic and anionic vacancies, all have a direct effect vary in catalytic activity [8,9]. The dispersion of perovskites on a support, one can select the most excellent matrix to contain the oxide particles and expose the major amount of active sites in order to get the better catalytic activity for CO oxidation [10]. Perovskite-type (ABO3) catalysts can be well modified by the partial substitution of atoms at A and/or B-sites, producing iso-structural, which may stabilize unusual oxidation states of B component, induce structural distortions and create cationic or anionic vacancies [11,12]. The best catalyst for CO oxidation could able to maintain the oxide structure throughout the process. The fast changes in temperature of wash coat and active layer on the support may break due to their thermal extension [13,14]. Partial substitution of lanthanum perovskite increases with increases of noble metals into the catalyst. It also influenced by the time and temperature of perovskite calcination and increase with the rise in calcination temperature [15]. The size of perovskites has a smaller influence on the activity of catalysts. This catalyst exhibits good adhesion to the monolithic metal support. In geometric factors, the perovskites shows that lanthanum, which is major lanthanide ion in the series, leads to the most steady perovskite structure [16]. The substituting cations increase the activity and stability of perovskite oxide structure. Manganite and cobaltate perovskite catalysts have been reported to be highly active for CO oxidations [17,18]. In comparison to manganese-based perovskite, the cobalt-based oxides are difficult to support directly on alumina because cobalt ions easily diffuse into the bulk of support to form cobalt aluminum perovskite structure [19]. The catalytic activity of various perovskite catalysts having different compositions in CO oxidation reactions involving at various temperatures has been discussed in this review paper.Perovskite-catalyst (ABO3) can crystallize in cubic structure in space group Pmm or indistinct rhombohedral, tetragonal, orthorhombic and triclinic symmetry as represent in Fig.\u00a01\n. The presence of oxygen and vacancy can be change depending on the composition due to a great stability range of structure [20]. The larger A-site cation is frequently rare earth, alkaline earth or an alkali metal cation coordinated to 12 oxygen anions. The B-site cation is usually a minor transition metal cation covering octahedral interstices in oxygen structure. Several combinations of A and B site cations can form a stable perovskite-like structure. These cations with oxygen anions can be partially substituted by other suitable elements. Electronic structure descriptions in Fig.\u00a02\n are sum of active quantities used to generate qualitative correlations for a wide range of properties. In particular, the oxygen p-band center has been used to direct material finding and basic considerate of a range of perovskite compounds for utilize in catalyzing the oxygen reduction and advancement reactions [21,22].Partial chemical substitutions of A and/or B sites for ABO3 type perovskite and structures drive structural and electronic transformations, foremost to functional properties such as large magneto-resistance and high-temperature superconductivity [23]. A couple of various metal ions covering equal crystallographic sites create spatial ordering of atoms, crystallizing in ordered perovskite structures with chemical formulae such as A2BB'O6, AA'B2O6 and AA'3B4O12 [24]. Combinations of A, A\u2032 and B site ions provide surprising properties, some of which are functional. The charge-disproportionate or transfer transitions are abruptly switched by bond strains on rare-earth metals. Crystal structure analysis shown in Fig.\u00a02 suggests that metal oxygen bonds make neighboring adsorbates close enough to interact, probably facilitate two active site reaction mechanisms [25]. The conventional single-active-site reactions for plain perovskite catalysts and expected to keep away from rate-determining steps of usual mechanisms. Perovskites may be arranged in layers, with the ABO3 structure separated by thin sheets of interfere material [26]. The various forms of intrusions, based on the chemical structure of groups are defined as:\n\n\u2022\n\nAurivillius phase: The major layer is contained a [Bi2O2]2+ ion, covering every nABO3 layers, foremost to an generally chemical formula of [Bi2O2]-A(n\u22121)B2O7.\n\n\n\u2022\n\nDion\u2212Jacobson phase: The main layer is collected of an alkali metal (M) each nABO3 layers, giving the in general formula as M+A(n\u22121)B\nn\nO(3n+1).\n\n\n\u2022\n\nRuddlesden-Popper phase: The major layer occurs between everyone (n\u00a0=\u00a01) or two (n\u00a0=\u00a02) layers of the ABO3 lattice.\n\n\n\nAurivillius phase: The major layer is contained a [Bi2O2]2+ ion, covering every nABO3 layers, foremost to an generally chemical formula of [Bi2O2]-A(n\u22121)B2O7.\nDion\u2212Jacobson phase: The main layer is collected of an alkali metal (M) each nABO3 layers, giving the in general formula as M+A(n\u22121)B\nn\nO(3n+1).\nRuddlesden-Popper phase: The major layer occurs between everyone (n\u00a0=\u00a01) or two (n\u00a0=\u00a02) layers of the ABO3 lattice.In the cubic unit cell, the \u2018A' atom sits at cube corner positions (0 0 0), type \u2018B' atom sits at body-center position (\u00bd \u00bd \u00bd) and oxygen atoms meet at face-centered positions (\u00bd \u00bd 0). The comparative ion size for steadiness of cubic structure are moderately rigid, so small buckling and warp can generate many lower-symmetry indistinguishable versions, in which the coordination numbers of A cations and B cations [27,28]. The most active oxygen reaction catalysis for quadruple perovskite oxides containing of earth-abundant elements exposed that exploitation of ultra-high-pressure preparations facilitates the increasing of novel functional materials. A large amount of perovskites compounds prepared in high pressure also shows best candidates for functional materials. Several crystal structures are closely related to perovskite structure is called hexagonal perovskites. The perovskite structure (shown in Fig.\u00a03\n) contains two A-site cations with robustly various sizes are used, further complication increases from ordering of A-sites and oxygen vacancies as in the double perovskite (AA\u2019B2O5+\u03b4) [29]. The natural of cation in ABX3 act as a major role in the formation of structure of perovskites structure moreover a large outcome on stability and electronic property of the materials. The sharing of various cation and anions in different perovskite catalysts is shown in the Fig.\u00a03.A cation exchange should be based on BX6 octahedral allocation with respect to a Goldschmidt tolerance factor. A cation substitution is intentional to get more-stable and suitable dynamic position of transmission band of perovskite film. The octahedral deformation increases by increase in an ionic radius of organic cation [30]. The traditional position of perovskite lattice is discussed in Table\u00a01\n. It consists of small B cations within oxygen octahedral and larger A cations which are XII fold coordinated by oxygen. The A3+B3+O3 perovskites are most symmetric structure observed in rhombohedra structure. It involves a rotation of BO6 octahedral with respect to cubic structure [31]. The A cations are present in corners of cube and B cation in center with oxygen ions in the face-centered positions. The decrease of A cation size will be reach where the cations will be very small to stay behind in make contact with anions in the cubic structure. The lowest lattice energy was recognized for all compounds so that an energy value was assigned to all the composition.The A cation is huge and B cation is lower as a result of the lowest energy structure is rhombohedral in nature. As the A cation radius decreases and B cation radius increases, the lower energy structure changes to be orthorhombic. The reducing of A cation radius and raising in B cation radius result in the creation of hexagonal structure. The lattice or internal energy does not change significantly with the changes in crystallographic structure [32]. The structures of perovskites compounds have been studied by many workers. The actual perovskite compounds with few binary oxides have simple cubic in structure as shown in Fig.\u00a04\n at room temperature and this structure maintained at higher temperatures. The X-ray patterns of many compounds can be indexed on the basis of distortion of perovskite structure [33]. In addition to various types of disturbances that involve a multiplication of pseudo cubic cell resultant in tetragonal, orthorhombic and rhombohedral symmetries. The most interests study of ferroelectric forms of perovskite structure, especially in two groups of mixed oxides A+2B+4O3 and A+3B+3O3. A classification of perovskite-type structures was done on the basis of radii of their metallic ions [34].Perovskites showed excellent catalytic activity and high chemical stability; therefore, they were studied in a wide range in the catalysis of different reactions. Perovskites can be described as a model of active sites and as an oxidation or oxygen-activated catalyst. The stability of the perovskite structure allowed the compounds preparation from elements with unusual valence states or a high extent of oxygen deficiency. In Fig.\u00a05\n shows a unit cell of perovskite structure. Perovskites exhibited high catalytic activity, which is partially associated with the high surface activity to oxygen reduction ratio or oxygen activation that resulted from the large number of oxygen vacancies. Perovskites can act as automobile exhaust gas catalyst, intelligent automobile catalyst and cleaning catalyst, etc., for various catalytic environmental reactions. It was reported in the literature that perovskites containing Cu, Co, Mn, or Fe showed excellent catalytic activity toward the direct decomposition of NO at high temperature, which is considered one of the difficult reactions in the catalysis (2NO \u2192 N2+O2). Perovskites showed superior activity for this reaction at high temperatures because of the presence of oxygen deficiency and the simple elimination of the surface oxygen in the form of a reaction product. NO decomposition activity was enhanced upon doping. Also, under an atmosphere that is rich with oxygen up to 5%, Ba(La)Mn(Mg)O3 perovskite exhibited superior activity toward the decomposition of NO.Perovskite showed a great impact as an automobile catalyst; intelligent catalyst. Pd\u2013Rh\u2013Pt catalysts was utilized as an effective catalyst for the removal of NO, CO and uncombusted hydrocarbons. There is another catalyst that consists of fine particles, with high surface-to-volume ratio, and can be utilized to reduce the amount of precious metals used. However, these fine particles exhibited very bad stability under the operation conditions leading to catalyst deactivation. Therefore, the perovskite oxides can be used showing redox properties to preserve a great dispersion state. The crystalline structure of various perovskite catalysts and their formation is shown in the Fig.\u00a06\n. Upon oxidation, Pd is oxidized in the form of LaFe0.57Co0.38Pd0.05O3 and upon reduction; fine metallic particles of Pd were produced with radius of 1\u20133\u00a0nm. This cycle resulted in partial replacement of Pd into and sedimentation from the framework of the perovskite under oxidizing and reducing conditions, respectively, displaying a great dispersion state of Pd. Also, this cycle improved the excellent long-term stability of Pd during the pollutants removal from the exhaust gas. Exposing the catalyst to oxidizing and reducing atmosphere resulted in the recovery of the high dispersion state of Pd. This catalyst is known as intelligent catalyst because of the great dispersion state of Pd and the excellent stability of the perovskite structure.One of the important characteristic of perovskites is ferroelectric behavior, which is obvious in BaTiO3, PdZrO3, and their doped compounds. The ferroelectric behavior of BaTiO3 was strongly related to its crystal structure. BaTiO3 was subjected to three phase transitions; as the temperature increases, it was converted from monoclinic to tetragonal then to cubic. One of the major properties of perovskites is superconductivity. The halide perovskite catalyst crystalline structure is shows in the Fig.\u00a07\n. Cu-based perovskites act as high-temperature superconductors, and La\u2013Ba\u2013Cu\u2013O perovskite was first reported. The presence of Cu in B-site is essential for the superconductivity and various superconducting oxides can be manufactured with different A-site ions. Furthermore, some perovskites exhibited great electronic conductivity similar to that of metals like Cu. LaCoO3 and LaMnO3 are examples of perovskites exhibiting high electronic conductivity, and therefore they are utilized as cathodes in solid oxide fuel cells displaying superior hole conductivity of 100\u00a0S/cm. The electronic conductivity of the perovskites can be improved by doping the A-site with another cation, which resulted in increasing the amount of the mobile charge carriers created by the reparations of charges.In the ABO3 form, B is a transition metal ion with small radius, larger A ion is an alkali earth metals or lanthanides with larger radius, and O is the oxygen ion with the ratio of 1:1:3. In the cubic unit cell of ABO3 perovskite, atom A is located at the body center, atom B is located at the cube corner position, and oxygen atoms are located at face-centered positions. The 6-fold coordination of B cation (octahedron) and the 12-fold coordination of the A cation resulted in the stabilization of the perovskite structure. The perfect perovskite structure was a corner linked BO6 octahedra with interstitial A cations. Some distortions may exist in the ideal cubic form of perovskite resulted in orthorhombic, rhombohedral, hexagonal and tetragonal forms. In general, all the perovskite distortions maintaining the A and B site oxygen coordination was achieved by the tilting of the BO6 octahedra and an associated displacement of the A cation. The different perovskite catalyst unit cell structure is shown in the Fig.\u00a08\n.Goldschmidt presented much of the early work on the synthetic perovskites and developed the principle of the tolerance factor\u00a0t, which is applicable to the empirical ionic radii at room temperature. Goldschmidt presented much of the early work on the synthetic perovskites and developed the principle of the tolerance factor\u00a0t, which is applicable to the empirical ionic radii at room temperature. Where\u00a0rA\u00a0is the radius of the A-site cation,\u00a0rB\u00a0is the radius of the B-site cation, and\u00a0rO\u00a0is the radius of oxygen ion O2\u2212. The tolerance factor can be used to estimate the suitability of the combination of cations for the perovskite structure. It is a real measure of the degree of distortion of perovskite from the ideal cubic structure so that the value of\u00a0t\u00a0tends to unity as the structure approaches the cubic form. From the equation, the tolerance factor will decrease when\u00a0rA\u00a0decreases and/or\u00a0rB\u00a0increases. Based on the analysis of tolerance factor value, Hines et\u00a0al. solely suggested that the perovskite structure can be estimated. For 1.00 <\u00a0t\u00a0<\u00a01.13, 0.9 <\u00a0t\u00a0<\u00a01.0, and 0.75 <\u00a0t\u00a0<\u00a00.9, the perovskite structure is hexagonal, cubic and orthorhombic, respectively. For\u00a0t\u00a0<\u00a00.75, the structure was adopted to hexagonal ilmenite structure (FeTiO3).\n\n\n\nt\n=\n\n\n(\n\n\nr\nA\n\n+\n\nr\no\n\n\n)\n\n\n[\n\n\u221a\n2\n\n(\n\n\nr\nB\n\n+\n\nr\no\n\n\n)\n\n\n]\n\n\n\nBB\n\u2081\n\n\n\n\nElectro neutrality; the perovskite formula must have neutral balanced charge therefore the product of the addition of the charges of A and B ions should be equivalent to the whole charge of the oxygen ions. An appropriate charge distribution should be attained in the forms of A1+B5+O3, A4+B2+O3 or A3+B3+O3. Ionic radii requirements; r\nA\u00a0>\u00a00.090\u00a0nm and r\nB\u00a0>\u00a00.051\u00a0nm, and the tolerance factor must have values within the range 0.8\u00a0<\u00a0t\u00a0<\u00a01.0. Perovskite exhibited a variety of fascinating properties like ferro electricity as in case of BaTiO3 and super conductivity as in case of Ba2YCu3O7. They exhibited good electrical conductivity close to metals, ionic conductivity and mixed ionic and electronic conductivity. In addition, several perovskites exhibited high catalytic activity toward various reactions. There are some properties inherent to dielectric materials like ferroelectricity, piezoelectricity, electrostriction and pyroelectricity.In solid-state reactions, the raw materials and the final products are in the solid-state therefore nitrates, carbonates, oxides and others can be mixed with the stoichiometric ratios. Perovskites can be synthesized via solid-state reactions by mixing carbonates or oxides of the A- and B-site metal ions corresponding to the perovskite formula ABO3 in the required proportion to obtain the final product with the desired composition. They are ball milling effectively in an appropriate milling media of acetone or isopropanol. Then the obtained product is dried at 100\u00a0\u00b0C and calcined in air at 600\u00a0\u00b0C for 4\u20138\u00a0h under heating/cooling rates of 2\u00a0\u00b0C/min. After that, the calcined samples are grinded well and sieved. Then it was calcined again at 1300\u20131600\u00a0\u00b0C for 5\u201315\u00a0h under the heating/cooling rate of 2\u00a0\u00b0C/min to confirm the formation of single phase of perovskite. Again grinding and sieving was carried out for the calcined samples. The synthesis of BaCeO3-based proton conductor perovskites and BaCe0.95Yb0.05O3\u2212\n\u03b4 was achieved through the previous methodology using BaCO3, CeO2 and Yb2O3 as the starting materials and isopropanol as the milling media.These methods included the sol-gel preparation, co-precipitation of metal ions using precipitating agents like cyanide, oxalate, carbonate, citrate, hydroxide ions, etc., and thermal treatment, which resulted in a single-phase material with large surface area and high homogeneity. These methods presented good advantages such as lower temperature compared to the solid-state reactions, better homogeneity, greater flexibility in forming thin films, improved reactivity and new compositions and better control of stoichiometry, particle size, and purity. Therefore, they opened new directions for molecular architecture in the synthesis of perovskites. Solution methods were classified based on the means used for solvent removal. Two classes were identified: (i) precipitation followed by filtration, centrifugation, etc., for the separation of the solid and liquid phases and (ii) thermal treatment such as evaporation, sublimation, combustion, etc., for solvent removal. There are several factors must be taken in consideration in solution methods like solubility, solvent compatibility, cost, purity, toxicity, and choice of presumably inert anions.This method is built on the assimilation of oxalic acid with carbonates, hydroxides, or oxides producing metal oxalates, water and carbon dioxide as products. The solubility problem is minimized as the pH of the resulting solution is close to 7. An oxidizing atmosphere like oxygen was used during calcination to avoid the formation of carbide and carbon residues. It utilized an aqueous chloride solution with oxalic acid to obtain unique and novel complex compound of BaTiO(C2O4)2.4H2O as a precursor for the preparation of finely divided and stoichiometric BaTiO3.This method is often used due to its low solubility and the possible variety of precipitation schemes. The sol-gel process can be used to produce a wide range of new materials and improve their properties. It presented some advantages over the other traditional methods like chemical homogeneity, low calcination temperature, room temperature deposition, and controlled hydrolysis for thin film formation. BaZrO3 powders in its pure crystalline form can be prepared by the precipitation in aqueous solution of high basicite. LaCoO3 was prepared by the simultaneous oxidation and coprecipitation of a mixture containing equimolar amounts of La(III) and Co(II) nitrates producing a gel containing hydroxide then calcination at 600\u00a0\u00b0C.Different perovskites were prepared by mixing acetate ions alone or together with nitrate ions with the metal ions salts. La1-xSrxCoO3 with x\u00a0=\u00a00, 0.2, 0.4, 0.6 was prepared using acetate precursors then calcination at 1123\u00a0K in air for 5\u00a0h La1-\nxSrxCo1-yFeyO3 was prepared using iron nitrate and strontium, cobalt and lanthanum acetates then calcination at 1123\u00a0K in air between 5 and 10\u00a0h.Citrate precursors can be used and undergo several decomposition steps in the synthesis of perovskite. These steps included the decomposition of citrate complexes and removal of CO3\n2\u2212 and NO3\n\u00af ions. LaCo0.4Fe0.6O3 can be prepared by this method, and the mechanism was investigated by thermo-gravimetry, XRD, and IR spectroscopy.The freeze-drying method can be achieved through the following steps: (i) dissolution of the starting salts in the suitable solvent, water in most cases; (ii) freezing the solution very fast to keep its chemical homogeneity; (iii) freeze-drying the frozen solution to get the dehydrated salts without passing through the liquid phase; and (iv) decomposition of the dehydrated salts to give the desired perovskite powder. The rate of heat loss from the solution is the most important characteristic for the freezing step. This rate should be as high as possible to decrease the segregation of ice-salt. Also, in case of multi-component solutions, the heat loss rate should be high to prevent the large-scale segregation of the cation components.This method was applicable to various precursors, including gaseous, liquid, and solid materials. It was applied for the preparation of various ceramic, electronic, and catalytic materials. It presented many advantages in terms of economy, purity, particle size distribution, and reactivity. This method was achieved through two steps: (i) injection of the reactants and (ii) generation and interaction of the molten droplets (with substrate or with the previously generated droplets). The thick film of YBa2Cu3Ox covering large areas was prepared via this approach, and the optimum superconducting oxide phase was obtained by varying the preparation conditions like plasma parameters, substrate temperatures, and film post deposition treatment.A redox reaction, which is thermally induced, occurs between the oxidant and fuel. A homogenous, highly reactive, and nanosized powder was prepared by this method. When compared with the other traditional methods, a single-phase perovskite powder can be obtained at lower calcination temperatures or shorter reaction times. One of the most popular solution combustion methods is citrate/nitrate combustion, where citric acid is the fuel and metal nitrates are used as the source of metal and oxidant. It is similar to the Pechini process \u201csol-gel combustion method\u201d to a large extent, but in citrate/nitrate combustion, ethylene glycol or other polyhydroxy alcohols are not used. In addition, in citrate/nitrate combustion, the nitrates are not eliminated in the form of NOx, but they remain in the mixture with the metal-citrate complex facilitating the auto-combustion. Iron, cobalt, and cerium-perovskite can be prepared via citrate/nitrate combustion synthesis. In addition, uniform nanopowder of La0.6Sr0.4CoO3\u2212\n\u03b4 was prepared by the combined citrate\u2013EDTA method, where the precursor solution was made of metal nitrates, citric acid and EDTA under controlled pH with ammonia. La0.8Sr0.2Co0.2Fe0.8O3\u2212\n\u03b4 and Sr or Ce-doped La1\u2212\nxMxCrO3 catalysts were prepared by citrate/nitrate combustion method. Furthermore, the Pechini \u201ccitrate gel\u201d process includes two stages: (i) a complex was formed between the metal ions and citric acid, then (ii) the produced complex was polyesterified with ethylene glycol to maintain the metal salt solution in a gel at a homogenous state. This approach presented some advantages like high purity, minimized segregation and good monitoring of the resulting perovskite composition. LaMnO3, LaCoO3, and LaNiO3 were prepared by citric acid gel process producing nanophasic thin films.The microwave irradiation process (MIP), evolving from microwave sintering, was applied widely in food drying, inorganic/organic synthesis, plasma chemistry, and microwave-induced catalysis. MIP showed fascinating advantages: (i) fast reaction rate, (ii) regular heating, and (iii) efficient and clean energy. The microwave preparations were achieved in domestic microwave oven at frequency of 2.45\u00a0GHz with 1\u00a0kW as the maximum output power. Dielectric materials absorbed microwave energy converted directly into heat energy through the polarization and dielectric loss in the interior of materials. The energy efficiency reached 80\u201390% which is much higher than the conventional routes. MIP was recently utilized to prepare perovskites nanomaterials reducing both the high temperature of calcination (higher than 700\u00a0\u00b0C) and long time (greater than 3\u00a0h) required for pretreatment or sintering. GaAlO3 and LaCrO3 perovskites with ferroelectric, superconductive, high-temperature ionic conductive and magnetic ordering properties, faster lattice diffusion, and grain size with smaller size were prepared in MIP. The CaTiO3 powders prepared in MIP presented a fast structural ordering than powders dealt in ordinary furnace. Hydrothermal conventional and dielectric heating were utilized to prepare La\u2013Ce\u2013Mn\u2013O catalysts. Hydrothermal MIP leads to formation of La1\u2212\nxCexMnO3+\n\u03b5CeO2 (x\u00a0+\u00a0\u03b5\u00a0=\u00a00.2) with enhanced catalytic activity while using the conventional heating methods lead to formation of LaMnO3\u00a0+\u00a0CeO2. Moreover, nanosized single-phase perovskite-type LaFeO3, SmFeO3, NdFeO3, GdFeO3, barium iron niobate powders, KNbO3, PbWO4, CaMoO4 and MWO4 (M: Ca, Ni), strontium hexaferrite and SrRuO3 were prepared in MIP showing finer particles, higher specific surface areas and shorter time for synthesis of single crystalline powders.Perovskites showed a good catalytic activity, which is moderately associated with more surface activity to oxygen decline ratio or oxygen activation that creates from huge amount of oxygen vacancies. It can act like a catalytic converter and cleaning catalyst, etc., for different catalytic environmental reactions [35]. Perovskites containing Cu, Co, Mn or Fe showed excellent catalytic activity toward direct oxidation of CO at high temperature. The LaCoO3, LaMnO3 and BaCuO3 perovskite catalysts showed great catalytic activity for CO oxidation at higher temperatures. The perovskites represents best activity for reaction at high temperatures because the presence of oxygen shortage and easy removal of surface oxygen in the form of reaction product. The addition of smaller amounts of element in perovskite catalyst improved their performances. The Cu0.15Ce(La)0.85Ox catalyst synthesized by wet impregnation method showed that the best activity toward CO oxidation [36]. It fine particles with high surface-to-volume ratio be capable of utilized to decrease the amount of noble metals used. However, the fine particles bad stability in operation conditions mostly to catalyst deactivation. So that it can be used to the showing redox properties to maintain a more dispersion state [37]. Lanthanum (La) is oxidized in the form of various La oxide catalysts with fine metallic particles of La were produced in a radius of 1\u20133\u00a0nm. The LnCoO3 catalyst is known as an intelligent catalyst because of great dispersion state of Ln and excellent stability of perovskite structure. The different properties of perovskites and their catalytic activity are highly affected by the method of preparation, calcinations conditions and A- and/or B-site substitutions [38,39].The doping in perovskite catalysts the catalytic activity, ionic radius, electronic conductivity, physical and chemical properties can be changed for exploitation in different applications. Different cations with various sizes and charges can be hosted in perovskites; thus, many studies can be performed to utilize doped perovskites in CO oxidation [40,41]. The chemisorptions of CO and CH4 over perovskite catalysts are shown in the Fig.\u00a09\n\n. The material characteristics of perovskite oxides mainly related with structural characters were very much affected by structural changes from perfect cubic structure of perovskite catalysts. The synergism effect between the crystal lattice of perovskite and metal ions dissolved in lattice upon doping [42]. It results in an improved redox reaction and best catalytic activity of synthesized perovskite was obtained. A remarkable modifies in transportation and magnetic properties of ABO3 perovskite can be done by doping in the B-site due to an ionic valence effect and/or anionic size effect. The doping in B-site of ABO3 perovskites with transition metals mainly noble metals, the strength of perovskite was enhanced and catalytic activity was improved considerably [43]. In LaMnO3\u00a0+\u00a0CeO2 perovskites with a low surface area (<15\u00a0m2/g) the Ce4+ replacement into La3+ sites reduces both cell parameters of rhombohedral unit cell and crystalline domain sizes, since the ionic radius of Ce4+ is lesser than La3+. The selective of CO oxidation in which CO and O2 were totally converted into CO2 and presence of cerium affected the reaction kinetics shifting CO conversion to higher temperatures [44]. The Ce4+ distorted some Co3+ to Co2+ to maintain the charge neutrality within the LaxCe1-XCoO3 structure, as a result decreasing the amount of active Co3+ sites on the LaCoO3 surface and declining the activity for CO oxidation. The charge neutrality would stabilize the total Co3+/O2 on the surface, ensuring high CO2 selectivity for cerium substituted perovskites [45]. The effect of strontium insertion into La0.5Sr0.5CoO3-d on the catalytic performance of CO oxidation was discussed in Table\u00a02\n (see Fig.\u00a010).Differently, from Ceria the Sr2+ as a cationic dopant is probable to raise cobalt oxidation state and/or produce oxygen vacancies inside the crystal lattice, it was the oxygen mobility and supply lattice oxygen for CO oxidation on the surface [46]. The complete Sr2+ substitution into the rhombohedral crystalline structure of perovskite which led to declined and extension of unit cell volume, since the ionic radius of Sr2+ (0.132\u00a0nm) is superior than La3+ ion. In LaFeO3 molecular oxygen chemisorbs on Fe+\u00a0cations as an O2\u2212 anion, dissociating to form atomic oxygen (O\u2212) on the iron sites. The CO adsorbs on the surface oxide ions formed a labile species that interacts with adsorbed atomic oxygen, producing carbonates which decompose towards CO2 and oxygen [47]. The manganese promoted La0.7Sr0.3Mn1-XCoXO3 perovskites were investigated as a catalyst in the CO oxidation reactions. Increasing the amount of Mn atoms on the La0.7Sr0.3Mn1-XCoXO3 catalyst surfaces affecting the catalytic behavior: the greater Coo\u00a0+\u00a0Mn0 exposition. The higher extension of La0.7Sr0.3Mn1-XCoXO3 phase derived from the perovskite structure, higher the activity and stability of the catalysts. In Zn1\u2212XNiXMnO3 catalyst the complete conversion of CO was obtained at 300\u00a0\u00b0C [48].This catalyst is highest resistance to carbon deposition among all the catalysts. The rhombohedral structure of perovskite towards metallic Ni0 and hexagonal Mn2O3 phases (for high Zn content, ZnO and NiXMnO3 phases also emerged). The fractional replacement of Ba by Zn in La0.9Ba0.1CoO3 raising the oxidation temperature of perovskite, signifying a more constant structure in reaction conditions which might be stay away from Ba sintering [49]. In LaMn1-xCuxO3+\u00a7 perovskites the Cu replacement could enhance the amount of chemisorbed oxygen species over the perovskite, improving the catalytic activity for CO oxidation. The addition of iron (Fe) in LaFe0.8Co0.2O3 lattice the iron valence changed from Fe3+ to Fe4+ improving the catalytic performance [50]. The A series of B site replacements over LaCoO3 perovskite showed that Mn2+, Fe2+, Ni2+ and Cu2+ dopants could get better CO conversion. To modify the characteristics of supported metal catalysts obtained from precursor perovskite under oxidation conditions. The important catalytic reactions made to better comprehend the role of active sites on the perovskite-type oxides [51].The efficiency of perovskite catalysts for reactions with CO molecules is strongly depending upon the chemisorptions process. The discrete reaction mechanisms are steady with the observed kinetics [51,52]. A better device for measuring the activity of perovskite catalysts for CO oxidation is reported the activation energy of the process. Early study represented that the catalyst starting oxidized CO before its oxidized by air, and this is an investigation of a Mars-van Krevelen-type mechanism which has consequently found support [53]. The perovskite oxides frequently exhibit strong electronic and/or magnetic correlations, band gaps and bending, which may affect the mechanism. Various synthesis methods have been presented in Table\u00a02 intended at increasing the surface area mainly mixed oxide and fast synthesis; still the surface areas between 5 and 50\u00a0m2/g at most are achieved [54,55]. The macro-porous perovskite catalysts illustrate better catalytic activities for CO oxidation than consequent nanometric sample. Calcination temperature highly affects the crystallization and particle size of perovskite catalyst [56].In the calcination of perovskite at higher temperature raise the crystalline and particle size. Carbon monoxide can be adsorbed either in a linear or bridged form covering respectively over perovskite catalysts. Which structure is formed depends on the chemisorptions conditions and nature of support. The CO adsorbed on perovskite could react with oxygen held by these species. This catalyst able to absorb oxygen at low temperature suggests that the CO oxidation should be done at low temperatures. The activation of surface oxygen vacancy in the perovskite catalysts performance for CO oxidation is properly represents in the Fig.\u00a011\n.However, the catalytic activity of perovskite catalyst is quite low in spite of fact that catalysts contain adsorption sites both for CO and O2 adsorption. It causes a result of no dissociative adsorption of oxygen. The reaction mechanism of perovskite catalysts is represents in the Fig.\u00a012\n. In stoichiometry of CO oxidation reaction needs the dissociation of oxygen molecules followed by reaction between adsorbed oxygen atom and CO to CO2 is one of the accepted mechanisms for CO oxidation. In this condition, the reaction rate is limited by the dissociation of O2. The molecular adsorption of CO occurs at higher temperatures, which ensures that the appearance of reactive oxygen forms [57,58].The oxygen adsorption occurs mainly in the form of O2\n\u2212, while above the calcination temperature of 350\u00a0\u00b0C the O\u2212 species is predominate. The O\u2212 ions are highly active and reactivity of superoxide is also high, though much lower as compared to O\u2212. In oxygen species, the CO molecules from gas phase can be directly oxidized [59]. The marsvan krevelen mechanism for conversion of CO over perovskite catalysts is shown in the Fig.\u00a013\n. The conversion of CO by the Mars-van Krevelen mechanism would give details the relationship between easiness of catalyst activity and reducibility. Different mechanisms have been suggested for the oxidation of CO over metals and metal oxides. The CO oxidation over metals is thought to follow a Langmuir-Hinshelwood mechanism [60]. The CO2 produced is poorly adsorbed and does not influence the rate substantially, since it's rapidly desorbed to the gas phase. The rate of reaction will be proportional to the total coverage of Oads and COads [61].\n\n(1)\n\n\n\nO\n\n2\n\n(\ng\n)\n\n\n\n\u2192\n\nO\n\n2\n\n(\n\na\nd\n\n)\n\n\n-\n\n\u2192\n2\n\nO\n\n(\n\na\nd\n\n)\n\n-\n\n\n\n\n\n\n\n(2)\n\n\nC\n\nO\n\n(\ng\n)\n\n\n\u2192\nC\n\nO\n\n(\n\na\nd\n\n)\n\n\n\n\n\n\n\n\n(3)\n\n\nC\n\nO\n\n(\n\na\nd\n\n)\n\n\n+\n2\n\nO\n\n(\n\na\nd\n\n)\n\n-\n\n\u2192\nC\n\nO\n\n3\n\n(\n\na\nd\n\n)\n\n\n\n2\n-\n\n\n\n\n\n\n\n\n(4)\n\n\nC\n\nO\n\n3\n\n(\n\na\nd\n\n)\n\n\n\n2\n-\n\n\n\u2192\nC\n\nO\n\n2\n\n(\n\na\nd\n\n)\n\n\n\n+\n\nO\n\n(\n\na\nd\n\n)\n\n\n2\n-\n\n\n\u2192\nC\n\nO\n\n2\n\n(\ng\n)\n\n\n\n+\n\nO\n\n2\n(\n\na\nd\n\n)\n\n\n-\n\n\n\n\n\n\n\n\n(5)\nO2 + 2\u2217\u21922Oads\n\n\n\n\n\n(6)\nCO +\u2217 \u2192 COads\n\n\n\n\n\n(7)\nCOads + Oads \u2192 CO2 + 2\u2217\n\n\n\nThe procedure of CO oxidation does not take place as long as the adsorbed molecules of O2 change to the reactive form of oxygen. The variation of activity and the binding energy of perovskite catalysts as a function of tolerance factor for the series of catalysts. The high spin state of perovskite catalysts at the surface may be favorable for the strong chemisorptions of oxygen which accounts for increased activity [62]. As far as catalyst development is concerned, it is critical to discover the structure\u2013activity correlation of catalysts. A Langmuir-hinshelwood mechanism predicts the reactivity of perovskite catalysts in CO oxidation. Low lattice oxygen mobility and kinetic effect of O2 rule out the MvK redox mechanism [63]. Under reaction conditions, the rate was proportional to the O2 pressure and independent of CO pressure. The rate of CO oxidation was done by following either rate of formation of CO2 or, when the CO was intent and rate of losing of CO [64]. The mechanism for CO oxidation over perovskite catalysts shows in the Fig.\u00a014\n. The reaction rate was found to be reduced sharply when CO was introduced into the gas phase during the oxidation. In the presence of CO, the reaction was first order in CO and zero order in O2. The CO molecule retained its integrity during the oxidation reaction [65].The number of CO2 molecules adsorbed corresponded to the number of oxygen atoms pre-adsorbed on the surfaces of catalyst. The equivalent concentration of oxygen atoms in the gas phase and on the surface, therefore, heterogeneous exchange reaction was taking places. The mechanisms of CO oxidation on the surfaces of catalysts are a top tactic in nature, therefore the reversible failure and uptake of huge oxygen or for the production and destruction of vacancies reported these systems as attractive oxidation catalysts. The inconsistent oxygen in the perovskite structure is accountable for the unusual performance of these materials. The removal of oxygen from frame works of perovskite structure and possibility of deriving different structural in ideal perovskites catalysts [67\u201370]. The various mechanisms of CO oxidation and formation of CO2 over perovskite catalysts are shown in the Fig.\u00a014.The mixing of the pollutants gases are constantly measured by an oxygen sensor and the air-to-fuel ratio is tuned consequently by the fuel-control reaction conditions. The kinetics study of CO oxidation over perovskite catalysts at the adsorption and desorption cycle is shown in the Fig.\u00a015\n. The performance of perovskite-type oxides convincingly increases with increasing the concentration of available active phase. Therefore the higher performance was obtained on extruded and layered on the structured catalysts containing a more adding of energetic component [71,72].The activity and selectivity of perovskites catalysts in catalytic converter are crucial for CO oxidation reaction. The catalyst deactivation can be divided into six different types: (i) poisoning, (ii) thermal degradation, (iii) fouling, (iv) vapor compound formation (v) vapor-solid reactions and (vi) crushing/abrasion. The lead, sulphur poisoning, carbon formation and sintering is the main cause of catalyst deactivation. The dispersion of active phase rapidly decreases, which is one of the main reason for catalyst deactivation. The catalytic activity of metal support (La2O3) is susceptible to sulphur poisoning, which is one of the most contaminants in catalytic converter exhaust emissions. The substitutions of materials in perovskite catalysts should not influence their activity in reforming reaction; but the changes in structure should remain their resistance to carbon deposition as well as to sulphur poisoning [66]. The promising stability of this catalyst could be attributed to the high mobility of oxygen on the interface between the MnCeOx solid solution and MnOx, which is critical for removing the Cl species produced during CB decomposition. The Ce\u2013Pr mixed oxides, specifically Ce0.5Pr0.5O2, have been reported to exhibit higher stability for the catalytic combustion of 1,2-dichloroethane. Conspicuous catalytic deactivation was, however, induced through the formation of by-products such as C\u2013C coupling products, higher chlorinated compounds and cracking compounds. The sulphur poisoning over perovskite catalysts are shown in the Fig.\u00a016\n. The LaMnO3 perovskite oxide catalyst synthesized by co-precipitation was found to exhibit significant activity for the catalytic oxidation of CO emissions. Moreover, its activity was enhanced by A or B site substitution. Because a promising catalyst for industrial applications should present not only high catalytic activity, but also good stability and durability, further study relative to stability and deactivation issues for LaMnO3 is now of the utmost urgency and significance [73,74].The stability of perovskite catalyst could be well recognized to the high mobility of oxygen on the interface of mixed oxides. The dispersion of active phase rapidly decreases, which is one of the main reasons for catalyst deactivation. Chemical poisoning and coke formation are one of the main reasons for catalytic deactivation. The deactivation of perovskite catalyst reduced the surface area of catalysts if available to the surroundings [75,76].The poisoning is due to strong adsorption of feed impurities; therefore, the poisoned catalysts are generally difficult to regenerate. Catalyst restoration is the least desirable approach to defeat catalyst deactivation and restore their activity and selectivity. The catalyst regeneration and reforming processes are mostly classified into three types: semi-regenerative, cyclic and constant regenerative process. Catalyst regeneration is mainly to recover activity defeat due to fast coking with failure of active metal diffusion. The regeneration of perovskite catalysts by various processes is shown in the Fig.\u00a017\n. The small amounts of noble metals added in perovskite due to their regenerating mechanism. The Pd in LaFe0.95Pd0.05O3 exists as a solid solution dispersed all over perovskite lattice. In the perovskite catalyst, the oxidizing/reducing cycle maintains the catalytic activity by regenerating the nano-particles and preventing metal nano-particle growth [77,78].Perovskite oxides are versatile materials due to their wide variety of compositions offering promising catalytic properties, especially in oxidation reactions. Perovskites ABO3 are exciting materials for oxidation catalysis as they provide considerable flexibility regarding their compositions and the possibility to implement oxygen vacancies with a selective modification of the cationic sublattice An interesting property of perovskite nanocrystals is their ability to undergo reversible exchange of halide ions (I, Br and Cl). While this property is useful in preparing nanocrystals of different halide composition, it also hinders the use of different nanocrystals or films. Upon contact, two different nanocrystals (e.g., CsPbBr3 and CsPbI3) form mixed halide perovskites. Another interesting approach is to couple perovskite nanocrystals with a different semiconductor to create a hetero structure with Type I or Type II configurations, based on their bandgap alignment. Furthermore, the perovskite structure is tolerant to the formation of anionic and cationic vacancies, which can tune the catalytic properties of the materials. The oxygen activation and dissociation capabilities at perovskite surfaces are strongly correlated to the composition and number of oxygen vacancies. These vacancies can promote the formation of monoatomic oxygen (O\u2212), which would act as the primary type of oxygen in the system. One straightforward approach to determine catalytic activity and oxygen activation capability is the CO oxidation reaction as a prototypical reaction for heterogeneous processes. The reaction only has a single gaseous product, which interacts with metal oxides either strongly or weakly. For Co3O4, no adsorption of CO2 on the surface was found, whereas an adsorption capacity has been reported for Al2O3. Furthermore, this reaction pathway is involved in the total oxidation mechanism of hydrocarbons and oxygenated molecules, which leads to a decrease in selectivity towards valuable intermediates. The effect of Co incorporation into oxides and LaFeO3 perovskite on CO oxidation catalysis has also attracted attention. For example, including only 1% Co has been shown to increase the CO oxidation activity of NiO significantly, but no steady conversion increase with Co incorporation has been observed. On Sr and Co-doped LaFeO3, highly at intermediate Co level were observed for transition metal surface content, oxygen storage capacity, reducibility and methanol oxidation activity. In our upcoming research work, we will perform further mechanistic studies on the CO oxidation on different perovskites catalysts and also test the performances in different oxidation reactions.In perovskite catalysts the partial substitution of cations, which stabilizes unusual oxidation states of metal components and creates anionic or cationic vacancies within the perovskite lattice. The partial substitution of cations can increase the reducibility and metal dispersion of catalyst. The support perovskites on porous materials like a monolith or a usual high surface area material to raising the amount of uncovered perovskite active sites. The reaction intermediates and a mechanistic condition are paramount for fundamental insight into the origin of activity and product selectivity. Structure-function relationships are crucial concept to develop basic guidelines for the design of more active catalysts after the mechanism is sufficiently understood. The surface area of perovskite oxides falls behind into simple metals. In addition, organometallic halide perovskites exhibited efficient intrinsic properties to be utilized as a photovoltaic solar cell with good stability and high efficiency. The reducing of particle size, making perovskites with hierarchical porosity is a promising approach to enhance mass activity and control applications. Nano-perovskites have been utilized as catalysts in oxygen reduction and hydrogen evolution reactions exhibiting high electro-catalytic activity, lower activation energy and high electron transfer kinetics. In addition, some perovskites are promising candidates for the development of effective anodic catalysts for direct fuel cells showing better catalytic performance. The relative ease of preparation, thermal and chemical stability and good catalytic activities of perovskites catalysts offer good performances for environmental pollution.This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.The statements in the paper are properly cited in the manuscript and no additional data is available.The authors declare no conflict of interest.The authors are thankful for the support from all the faculty members and lab in charges of Environmental Engineering Department, Rajiv Gandhi Proudyogiki Vishwavidyalaya, Bhopal, India.", "descript": "\n                  Automobile exhaust contributes the largest sources of carbon monoxide (CO) into the environment. To control this CO pollution, the catalytic converters have been discovered. The catalytic converters have been invented for regulating the CO discharge. There are many types of catalysts have been investigated for CO emission control purposes. Inorganic perovskite-type oxides are fascinating nanomaterials for wide applications in catalysis, fuel cells, and electrochemical sensing. Perovskites prepared in the nanoscale have recently received more attention due to their catalytic nature when used as electrode modifiers. Perovskite catalysts show great potential for CO oxidation catalyst in a catalytic converter for their low cost, high thermal stability and tailoring flexibility. It is active for CO oxidation at a lower temperature. The catalytic activity of these oxides is higher than that of many transition metals compounds and even some precious metal oxides. They represents attractive physical and chemical characteristics such as electronic conductivity, electrically active structure, the oxide ions mobility through the crystal lattice, variations on the content of the oxygen, thermal and chemical stability, and supermagnetic, photocatalytic, thermoelectric and dielectric properties. The surface sites and lattice oxygen species present in perovskite catalysts play an important role in chemical transformations. The partial replacement of cations A and B by different elements, which changes the atomic distance, causes unit cell disturbances, stabilizes various oxidation states or added cationic or anionic vacancies inside the lattice. The novel things disturb the solid reactivity by varying the reaction mechanism on the catalyst surface. Thus, the better cations replacement may represent more activity. There are lots of papers available to CO oxidation over perovskite catalysts but no review paper available in the literature that is represented to CO oxidation.\n               "}
{"full_text": "Currently most energy sources used by our society are based on fossil fuels. Their combustion (coal, oil, and gas), together with large-scale deforestation, is causing massive emissions of greenhouse gases. Given the destructive environmental impact of these gases, effort has focused on the production, storage and transport of renewable energy (wind or sunlight) [1]. A promising technology to address this issue uses renewable energy to produce chemical energy through the splitting of water into hydrogen and oxygen (water electrolysis) [2]. However, the efficiency of the electrolysis process is hampered by the sluggish kinetics of water oxidation to O2, also known as oxygen evolution reaction (OER). This reaction has been described as the bottleneck of the water splitting and understanding its mechanism at the atomic scale could be a first step in addressing this challenge [2]. Many catalysts have been proposed to reduce the overpotential losses for OER and investigated in different pH conditions [3], from acidic (2H2O\u00a0\u2192\u00a04H+\u00a0+\u00a0O2\u00a0+\u00a04e\u2212) to alkaline (4OH\u2212\u00a0\u2192\u00a02H2O\u00a0+\u00a0O2\u00a0+\u00a04e\u2212) media. In acidic media, noble metals such as Ru or Ir show promising OER stability and activity. However, due to their limited availability and high price many researchers are seeking alternative catalysts based on earth-abundant elements [3,4,5,6,7,8]. In an alkaline environment, oxides and hydroxides of late first-row transition metals (Mn, Fe, Co, Ni) have been found to have comparable performances to noble metals [3]. In particular, NiFe-based (oxy)hydroxide catalysts are reported to show the lowest overpotential for OER in alkaline conditions (pH 13 and 14) [9], but the synergistic role of Fe and Ni is still under debate.Comparing OER catalysts is complicated by many underlying factors, including differences in electrochemically active surface area, catalyst electrical conductivity, surface chemical stability, surface composition, and reaction mechanism. In this work, we describe our efforts to circumvent these issues by using a combined surface science/electrochemistry approach to develop an atomically controlled model system for the OER on FeNi-based catalysts. Having previously solved the surface structural model for the Fe3O4(001) surface [10] and learned how to judiciously alter this surface by doping with nickel atoms [11], we have prepared well-defined Ni-modified Fe3O4(001) surfaces in ultra-high-vacuum (UHV) with different Fe:Ni ratios and, after characterization with surface science techniques, we have studied their electrochemical performances towards OER using cyclic voltammetry and electrochemical impedance spectroscopy. A significant increase in the OER activity is observed as the Ni content increases, and the optimum composition has an iron fraction among the cations in the top surface layer in the range of 20-40%. These results are in good agreement with literature for the best OER powder catalysts [9]. Furthermore, based on the analysis of the surface morphological changes before and after reaction, together with adsorption capacitance measurements, we propose that the active sites responsible for the formation of the OER precursor are the same on the clean and on the Ni-modified magnetite. Nevertheless, the presence of the Ni on the surface shifts the formation of this precursor to lower overpotential.Our study provides a well-defined model catalyst that is at the same time simple, highly active, and stable under operation conditions, and therefore ideal to be used as model system to gain atomic-scale insights into the complicated OER mechanism.The experiments were performed on a natural Fe3O4(001) single crystal (SurfaceNet GmbH) prepared in UHV by cycles of 1\u00a0keV\u00a0Ar+ sputtering and 900\u00a0K annealing. Every other annealing cycle was performed in an O2 environment (\n\np\n\nO\n2\n\n\n\u00a0=\u00a05\u00a0\u00d7\u00a010\u22127\u00a0mbar, 20\u00a0min) to maintain the stoichiometry of the crystal selvedge. Surface analysis was performed in a UHV system with a base pressure <10\u221210\u00a0mbar, furnished with a commercial Omicron SPECTALEED rear-view optics and an Omicron UHV STM-1. XPS data were acquired using non-monochromated Al K\u03b1 x-rays and a SPECS PHOIBOS 100 electron analyser at grazing emission (70\u00b0 from the surface normal). The same analyser was used to carry out the low-energy He+ ion scattering (LEIS) experiments (1.225 keV He+, scattering angle 137\u00b0), an exquisitely surface-sensitive technique. For quantification of LEIS data, we assumed that the concentrations are proportional to the peak areas, which is justified due to the very similar cross sections, electronic structure, and work function of these metals. Ni was deposited using a Focus electron-beam evaporator, for which the deposition rates were calibrated using a temperature-stabilized quartz crystal microbalances (QCM). One monolayer (ML) is defined as one atom per (\u221a2\u00a0\u00d7\u00a0\u221a2)R45\u00b0 unit cell, which corresponds to 1.42\u00a0\u00d7\u00a01014 atom/cm2. Ni depositions higher than 2 ML were prepared by first depositing 2 ML Ni on the surface at room temperature, followed by mild annealing at 200 \u00b0C for 10 min. This causes a transition from Ni being present as 2-fold coordinated adatoms to 6-fold coordinated \u201cincorporated\u201d cations [11], see Fig.\u00a01\n; the procedure was then repeated as many times as necessary to reach the desired coverage.After UHV-preparation and characterization as well as after the electrochemical measurements, the samples were brought to air and imaged using an Agilent 5500 ambient AFM in intermittent contact mode with Si tips on Si cantilevers.Cyclic voltammetry and impedance spectroscopy were performed using a Metrohm-Autolab PGSTAT32 potentiostat and a custom-made electrochemical flow cell (made from perfluoroalkoxy alkane, PFA), mounted to the vacuum chamber. Prior to experiments, the chamber was filled with Ar (99.999%, Air Liquide, additionally purified with Micro-Torr point-of-use purifiers, SAES MC50\u2212902 FV) to ambient pressure. The contact between sample and flow cell was sealed with Kalrez O-rings. Prior to measurements, the electrolyte reservoir was evacuated and ultrasonicated to remove dissolved CO2. The flow cell was filled with electrolyte by increasing the pressure in the electrolyte compartment with Ar to slight overpressure. A glassy carbon counter electrode and a leak-free Ag/AgCl reference electrode (Innovative Instruments Inc.) were used. For impedance measurements, the latter was coupled to a glassy carbon quasi-reference electrode through a 100 nF capacitor. All electrochemical data were corrected for iR\nu drop; the uncompensated solution resistance R\nu was determined from impedance Nyquist plots by extrapolating the minimum total impedance in the linear regime between 10 kHz and 100 kHz. All electrochemical potentials are referred to either the measured Ag/AgCl reference electrode E\nAg/AgCl or given as the overpotential \u03b7, which was determined via the equation \u03b7\u00a0=\u00a0E\nAg/AgCl+E\nRHE\u22121.229 V \u2212iR\nu. E\nRHE is the potential of the reversible hydrogen electrode (RHE) vs a Ag/AgCl electrode. The potential of the RHE (Hydroflex) was measured before and after the electrochemical measurements to improve consistency of the results. The electrolyte was prepared from level-1 water (Merck Milli-Q, \u03c1= 18.2 M\u03a9 cm, 3 ppb total organic carbon), and reagent-grade NaOH (50 mass % in water, Sigma-Aldrich). Prior to use, all glassware and PFA parts where cleaned by boiling in 20% nitric acid and copious rinsing with Milli-Q water.\nFig.\u00a01a shows a schematic model of the UHV-prepared Fe3O4(001) surface. The surface is oxidized with respect to the bulk Fe3O4 and is not a simple bulk truncation. Specifically, an interstitial tetrahedrally coordinated iron in the second layer (Fetet, light blue in the model) replaces two octahedrally coordinated iron atoms (Feoct, dark blue) in the third layer [10], giving rise to a (\u221a2\u00a0\u00d7\u00a0\u221a2)R45\u00b0 periodicity. All surface Fe is in the 3+ state in the so-called subsurface cation vacancy (SCV) reconstruction, and it is the most stable termination of Fe3O4(001) over the range of oxygen chemical potentials encountered in UHV-based experiments [10].In the lower part of Fig.\u00a01a, a typical STM image of the UHV-prepared Fe3O4(001) surface is shown. Undulating rows of surface Fe atoms appearing as protrusions run in the [110] direction. It is common to observe surface hydroxyl groups OsH (i.e. hydrogen atoms bonding to surface oxygen atoms, which are themselves not imaged) as bright protrusions on the Fe rows. This occurs because the hydroxyl modifies the density of states of the nearby Fe cations, causing them to appear brighter in empty-states STM images [12,13]. Fig.\u00a01a also displays other common defects visible on the clean surface, such as antiphase domain boundaries, which are imaged as meandering line defects, and unreconstructed unit cells, which appear similar to two neighboring hydroxyl groups. These are caused by two additional Fe atoms in the subsurface layer (instead of one interstitial Fe), which again modifies the density of states of the surface atoms [12,14]. It is not possible to image the surface oxygen atoms in STM as they have no density of states in the vicinity of the Fermi level. However, their positions are exactly known from density functional theory calculations and quantitative low-energy electron diffraction (red in model in Fig.\u00a01a) [10].The surface reconstruction makes it possible to progressively modify the magnetite surface and accommodate foreign metal atoms (such as nickel) in specific positions. [11] Following Ni evaporation under the appropriate temperature conditions, it is possible to obtain two different Ni geometries: Ni adatoms 2-fold coordinated to surface oxygen atoms (model in Fig.\u00a01b, green) and incorporated Ni occupying octahedrally coordinated sites below the surface (model in Fig.\u00a01c) [11,15]. Ni deposition at room temperature leads to Ni adatoms in the 2-fold coordination, which are imaged in STM as isolated, bright protrusions appearing between the Fe rows (light blue circles in Fig.\u00a01b). The transition from 2-fold to 6-fold coordination is achieved by annealing the surface at 200 \u00b0C for 10 minutes. As the incorporated Ni atoms are in the subsurface, they cannot be imaged directly in STM, but they modify the electronic structure of the nearby Fe cations, making them to appear brighter in empty-state images (red circles in Fig.\u00a01c) [11,15]. Their appearance is similar to the unreconstructed cell discussed earlier (Fig.\u00a01a). Furthermore, the STM image in Fig.\u00a01c shows additional protrusions within the Fe rows (highlighted with yellow circles), which we previously assigned to Ni replacing Fe atoms in the 5-fold-coordinated position in the top surface layer [15].The incorporation of Ni in the vacant subsurface octahedral site is only possible if the interstitial Fetet moves back into the other subsurface octahedral site of the unit cell. The resulting cation rearrangement closely resembles a bulk-truncated Fe3O4(001) surface [11,16], and a (1\u00a0\u00d7\u00a01) periodicity is observed in LEED. It is possible to recover the clean (\u221a2\u00a0\u00d7\u00a0\u221a2)R45\u00b0 reconstructed surface by annealing to high temperatures, which causes the Ni atoms to diffuse into deep bulk layers.Hereafter, we deal exclusively with the incorporated Ni-doped magnetite shown in Fig.\u00a01c, which resembles the structure of mixed spinel ferrite, i.e., a NixFe3-xO4-like system, suggested to be one of the most active phases in OER [17,18].The XPS spectra in Fig.\u00a02\na shows the Ni 2p region for different coverages after Ni was deposited onto the Fe3O4(001) surface at room temperature and annealed at 200 \u00b0C. Five different total Ni depositions are considered: 1 ML (green), 10 ML (purple), 50 ML (blue), 120 ML (pink), and 180 ML (light blue). Corresponding fits for the Ni 2p peaks are shown in Figure S1 in the supporting information.After deposition of 1 ML, a small signal is observed in XPS at 855.5 eV, corresponding to the Ni 2p\n3/2 peak [19,9]. This is a higher binding energy than metallic Ni [19], which, together with the strong satellite at \u2248862 eV, indicates that the nickel is oxidized. Earlier DFT calculations predicted that incorporated Ni atoms are Ni(II) [11], as in NiFe2O4.As the Ni deposition increases to 10 ML, the Ni 2p3/2 at 855.5 eV increases in intensity, together with the 861.9 eV satellite and the 2p1/2 peak at 873 eV, which are harder to see at lower Ni coverage. These features increase in intensity as the Ni deposition increases up to 50\u00a0ML. At even higher Ni load (120\u00a0ML), two new signals at 853.1 eV and 870.2 eV emerge, indicating that metallic Ni is present on the surface [19]. At 180 ML Ni doping, , the XPS spectrum changes shape to a peak with only two main features at 853.1 eV and 870.2 eV, indicating that the surface is fully covered with metallic Ni.We imaged the Fe3O4(001) surface before and after Ni-doping using ambient AFM right after removing the crystal from the UHV chamber (Fig.\u00a03\na-d). The corresponding LEED patterns acquired in UHV are shown as insets in each AFM image.The clean Fe3O4(001) surface appears overall flat in ambient AFM, with micrometer-wide terraces separated by step bunches [20] (Fig.\u00a03a). The corresponding LEED pattern exhibits the (\u221a2\u00a0\u00d7\u00a0\u221a2)R45\u00b0 periodicity of the SCV reconstruction [10] (yellow square in the inset).\nFig.\u00a03b shows the AFM image of a magnetite surface doped with 50 ML Ni. The large terraces as well as the step bunches observed earlier [20] on the clean magnetite remain visible, suggesting that the doping did not affect the overall surface morphology. Isolated (white) features 0.4-0.6\u00a0nm high are visible on the surface. Based on the corresponding line profile (Fig.\u00a03a\u00b4\u00b4, blue), which shows step heights similar to what is observed in Fig.\u00a03a, we suspect these to be residues originating from dust or carbonaceous species. The LEED pattern in the inset shows that the reconstruction spots are now absent and a (1\u00a0\u00d7\u00a01) symmetry is observed (blue square), which is known to occur above 1 ML Ni atoms incorporated in the subsurface [11].\nFig.\u00a03c-d show AFM images of magnetite surfaces following doping with 120 ML and 180 ML Ni, respectively. The surface in (c) exhibits a rougher morphology than observed in (a) and (b), with a corrugation of \u22480.5 nm (Fig.\u00a03c\u00b4\u00b4, lilac). Accordingly, the corresponding LEED pattern shows weaker (1\u00a0\u00d7\u00a01) spots. Following higher Ni doping, the surface morphology changes considerably (d). Although the step bunches are still visible underneath, the surface appears covered in round features having height of ~2nm (Fig.\u00a03d\u00b4\u00b4, lilac). Based on the XPS data showed in Fig.\u00a02a, we assign these features to metallic Ni clusters. The corresponding LEED pattern shows very weak (1\u00a0\u00d7\u00a01) spots with a high background, indicating an increasing fraction of the surface covered by structures with no well-defined crystallographic relationship to the substrate, in agreement with the presence of metallic agglomerates on the surface.A quantitative measurement of the surface composition, given as the Fe:Ni ratio for each Ni modified surface can be obtained with LEIS measurements (Fig.\u00a02b-e). The clean surface exhibits a LEIS peak centered at 910 eV (Fig.\u00a02b), corresponding to the surface Fe atoms. Following 10 ML Ni doping, the LEIS signal is broader and shifts to higher kinetic energy KE (Fig.\u00a02c, purple). This peak can be well fitted by a (slightly shifted) peak from the surface Fe and an additional component at 931 eV corresponding to the Ni (Fig.\u00a02c, green and blue respectively). By comparing the area of the Fe and Ni contributions we can estimate an Fe:Ni top surface ratio on the 10 ML Ni-doped surface of 55:45. Similarly, we calculate that the surfaces following 50 ML and 120 ML Ni-doping show Fe:Ni ratios of 40:60 and 15:85, respectively. At higher Ni-doping (180 ML) the whole surface is covered in metallic Ni particles, which makes it difficult to use LEIS to quantify the Fe:Ni surface ratio. Therefore, we restrict ourselves to the coverage regime prior to the formation of metallic Ni clusters.\nFig.\u00a02b-e also shows how the surface oxygen peak (centered at ~470 eV) evolves as a function of the Ni doping. The intensity of the surface oxygen peak seems to remain constant as the Fe:Ni ratio decreases down to 40:60. Differently, a clear decrease in the oxygen intensity is observed for the surface with lower Fe:Ni ratio (15:85). We can speculate that this behavior correlates with the presence of some metallic Ni on top, as observed in the XPS in Fig.\u00a02a, pink.Importantly, no systematic change in consecutive scans was observed, which rules out substantial damage to the surface by He+ sputtering during LEIS measurements. In what follows, we will use the LEIS-determined Fe:Ni ratio to refer to our model catalysts.The electrochemical performance of the clean and Ni-doped Fe3O4(001) surfaces was investigated using cyclic voltammetry. The overpotential required to reach a given current density is a key catalytic parameter to compare several catalysts and to estimate the energetic efficiency of integrated (photo-) electrochemical water splitting devices [3]. The cyclic voltammograms (Fig.\u00a04\na) were acquired in 1 M NaOH under Ar with a scan rate of 10 mV s\u22121 after cycling the electrode until a stable OER current could be observed on two subsequent CVs. Data corresponding to the surfaces imaged in Fig.\u00a03a-d are shown, as well as for surfaces with an Fe:Ni ratio of 98:2 and 55:45. Furthermore, CVs collected before and after electrochemical impedance spectroscopy (EIS) measurements - described later in section 3.4 - up to 1mAcm\u22122 (see Figure S2) showed that our catalysts are stable over the time range of our experiments (typically 5-9 hours).The clean Fe3O4(001) surface shows an overpotential of 597 mV at a current density of 5 mAcm\u22122 (Fig.\u00a04a, black), and the surface with an Fe:Ni ratio of 98:2 (green) exhibits similar performance. As the Ni content in the subsurface increases, higher activity towards OER is observed. The OER overpotential decreases by ~110 mV when the Fe:Ni ratio is 55:45 (purple), and reaches ~340 mV vs RHE when the Fe:Ni ratio is 40:60. A higher Ni load (Fe:Ni\u00a0=\u00a015:85, pink) results in a similar activity as the surface with Fe:Ni ratio of 40:60. Additionally, the surface with an Fe:Ni ratio of 15:85 exhibits a pair of anodic and cathodic peaks at 1.369 and 1.311 V vs RHE respectively (pink, inset in Fig.\u00a04a), consistent with the reversible oxidation of Ni(II) to a higher oxidation state (III), as it is reported for the case of the nickel hydroxide/oxyhydroxide couple (Ni(OH)2/NiOOH) [21]. It can also be observed that the charge (peak area) of this peak increases with cycling, indicating the growth of a thicker Ni oxide film on top of the Fe3O4(001) surface. These observations suggest a change in the Fe:Ni ratio at the surface following electrode cycling. When only metallic Ni is present on the as-prepared sample, an increase in the overpotential of ~88 mV is observed (Fig.\u00a04a, light blue). A corresponding increase in the charge of the Ni(OH)2/NiOOH peak is observed, as well as anodic shifts of 170 mV and 130 mV for the anodic and cathodic peaks respectively. A similar anodic shift of the Ni peak has been observed with increasing Fe:Ni ratio in the NiOOH phase either by co-deposition of Fe during the film synthesis [22,23] or by incorporation of Fe impurities from the electrolyte into NiOOH electrodes [24]. Moreover, the charge of the Ni(OH)2/NiOOH peak remains constant with cycling, indicating a saturation of the surface with nickel (oxy)hydroxide.As a comparative metric of activity, Tafel plots are also shown (Fig.\u00a04b). The determination of Tafel slopes can help elucidating the rate-limiting step of a mechanism, but their analysis is particularly difficult in the case of multiple electron-proton transfer reactions such as OER [3]. The clean and low Ni-doped (Fe:Ni\u00a0=\u00a098:2) surfaces display values of 92 mV/dec and 88 mV/dec respectively, whereas the Ni-doped Fe3O4(001) surfaces with a Ni load of 50-85% all show similar values in the range 50-61 mV/dec.In Fig.\u00a04c we plot the overpotential values and the Tafel slopes showed in Fig.\u00a04a-b as a function of the surface Fe:Ni ratios. Interestingly, the lowest OER overpotential values are obtained for the catalysts with a surface Fe:Ni ratio between 15-40 %, in agreement with what is reported in literature for the best OER powder catalysts [9]. Furthermore, the Tafel slopes fall in the same range as observed for NiFe (oxy)hydroxide catalysts, which typically vary between 25 and 60 mV/dec [9], which could point towards a similar OER reaction mechanism [21].To check whether catalyst aging in electrolyte affects the activity, we performed cyclic voltammetry on the same surfaces after leaving the Ni-doped electrodes for three days in electrolyte. Fig.\u00a05\na-b show CVs of the surfaces with an Fe:Ni ratio of 40:60 (blue), 15:85 (pink), and a sample with metallic Ni clusters (light blue); the dashed curves show the performance after aging. The aged samples show a decrease of the OER overpotential by ~20-100 mV, in good agreement with what has been observed for powder catalysts prepared by wet chemistry [9,25]. Interestingly, the surface with an Fe:Ni ratio of 15:85 is similarly active to the one with Fe:Ni ratio of 40:60 when freshly prepared, but shows a much lower onset of the overpotential after aging. This observation indicates a profound structural difference in the two catalysts, despite the similar performance at first. Fig.\u00a05b shows a magnification of the capacitive regions of the CVs. The Ni-doped magnetite with metallic Ni at the surface (light blue) shows an anodic oxidation peak before OER onset and subsequent cathodic reduction in the backward scan direction. On the surface with an Fe:Ni ratio of 15:85 (pink), these peaks evolve upon cycling and aging, both in terms of charge as well as shift in overpotential. However, this effect is not so marked in the case where the whole surface is covered with metallic Ni clusters, where only a (slight) shift in potential is observed (blue). The interpretation of the redox behavior is in general very difficult due to possible formation of electrically disconnected domains upon cycling because of the different conductivity of the oxidized and reduced phase [26].\nFig.\u00a05c shows the comparison of the Tafel plots for the surfaces in (a). The aged surfaces show Tafel slopes values in the range 43-62 mV/dec range, similarly to the freshly prepared catalysts (Fig.\u00a04b).\nFig.\u00a03a\u00b4-d\u00b4 shows the AFM characterization of the surfaces imaged in Fig.\u00a03a-d after OER and three days aging in electrolyte. Before imaging, each surface was rinsed in milli-Q water several times, for several minutes and blow-dried using a gentle Ar flow to minimize the presence of salt residue from the electrolyte.The morphology of the clean Fe3O4(001) remains unchanged after OER (a\u00b4), and shows a smooth appearance with the wide terraces and step bunches still visible, in agreement with earlier stability tests[20]. The presence of small particles (white) is associated with residue from the electrolyte.\nFig.\u00a03b\u00b4 shows the AFM image of the surface imaged with initially 40:60 Fe:Ni (Fig.\u00a03b) after resting in electrolyte for three days, followed by cycling the electrode until a stable current was observed (Fig.\u00a05). The terraces and step bunches remain visible underneath, but white features of irregular shape and height between 1-2 nm are now common on the surface (blue line profile in Fig.\u00a03b\u00b4\u00b4).\nFig.\u00a03c\u00b4-d\u00b4 show AFM images of the 15:85 and metallic Ni surfaces, respectively, after the electrode has been exposed to the electrolyte for three days and cycled until a stable OER current was observed (Fig.\u00a05). Their morphologies appear similar in AFM. Due to the appearance of protrusions with 3-7 nm (line profile in Figure 3 c\u00b4\u00b4, blue) and 4-8 nm high (line profile in panel d\u00b4\u00b4, blue), it is almost impossible to discern remainders of the original surface morphology consisting of flat and wide terraces. Since the density and height of the protrusions increases with Ni content, they likely consist of a Ni-(oxy)-hydroxide phase, grown from pre-existent metallic Ni upon electrochemical cycling [25], in agreement with equilibrium potential\u2212pH diagrams (i.e. Pourbaix diagrams) that show NiOOH as the predominant species in neutral-to-basic aqueous solutions at OER potentials [25].Electrochemical impedance spectroscopy (EIS) measurements were performed on the Ni-doped model catalysts electrochemically investigated in Fig.\u00a04. In the OER region the EIS Nyquist plots (Figure S3a) exhibit two relaxation processes characterized by two semi-circles that can be assigned to two capacitances while the phase in Bode plots (Figure S3b) exhibits two maxima eventually merging into a broad peak. This impedance behavior is consistent with previous measurements on metal transition oxides and perovskites during the OER. [27,28,29] The EIS response can be modelled by the equivalent circuit (EC) shown as an inset in Fig.\u00a06\na with a double-layer capacitance (Cdl) in parallel with the combination of a polarization resistance (Rp) and an adsorption pseudo-capacitance (Cads) in parallel with a resistor Rs. The Cdl element accounts for the charging of the electrified interface. Cads models the accumulation of an adsorbed intermediate involved in the rate-limiting step of the OER. The sum of the resistive elements Rs and Rp bear a physical meaning as the zero-frequency electron transfer resistance defined as Rf\u00a0=\u00a0Rp\u00a0+\u00a0Rs, i.e., the slope of the steady-state polarization curve after Ohmic-drop compensation. R\u03a9 represents the electrolyte resistance. It has to be noted that both capacitors were modeled as constant phase elements (CPEs), defined as \n\nZ\n=\n\nC\n\nn\n=\n1\n\n\n\u2212\n1\n\n\n\n\n(\n\nj\n\u03c9\n\n)\n\n\n\u2212\nn\n\n\n\n\n, where \n\nC\n\nn\n=\n1\n\n\n\u2212\n1\n\n\n is the impedance of the capacitor without frequency dispersion, i.e., if the coefficient n\u00a0=\u00a01 which is the case for a perfect capacitor. The interpretation of the CPEs dispersion coefficient n is varied and complicated; its origin has been attributed to surface roughness, inhomogeneities, or inhomogeneous adsorption of ions [30]. In the double-layer region, prior to the onset of the OER, we will show in a separate work that the impedance response of the single-crystal magnetite electrode has to be modified by adding a Warburg element in series with Cads corresponding to a diffusion impedance that we attribute to electrolyte cations intercalating into the oxide (Figure S4a). Of interest in this work is the impedance response in the OER region.All the surfaces investigated in this work, with the exception of the one fully covered by metallic Ni clusters (light blue), show a roughly constant double layer capacitance values in the 10-25 \u03bcFcm\u22122 range prior to the OER onset (Fig.\u00a06a). The exponent of the CPE element used for the fitting was equal to 1 in the double-layer region (Figure S4d) and diverged from 1 at high current densities or when Ni is exposed such that Ni(OH)2 is oxidized to NiOOH. These values are comparable to a Cdl observed on metallic single crystals, suggesting that our catalysts have a perfect capacitor-like behavior. Fig.\u00a06c shows the value of this capacitance as a function of the Ni content: Cdl slightly increases from 10 to 15 \u03bcF cm\u22122 as the Ni loading increases, but a higher value is observed in the case of the surface fully covered with Ni metallic clusters (180\u00a0ML). The higher Cdl values observed for this surface may be explained by the formation of an irregular Ni(OH)2 layer upon oxidation of the metallic Ni by contact with the electrolyte. In this way, more active surface area is exposed to the electrolyte and polarized, leading to a higher Cdl.The adsorption capacitance plot in Fig.\u00a06c shows that the Ni-doped Fe3O4(001) surfaces display a peak with similar Cads values independent of the Ni doping level, which however shifts to lower overpotential as the Ni load increases (Fig.\u00a06d). The surface fully covered with metallic Ni clusters appears to develop two additional capacitance peaks (Fig.\u00a06c). The overlay of the corresponding CV and Cads in Figure S3e, shows that the additional initial (pre-)peak is observed at the same potential as the Ni(OH)2 oxidation peak.The group of Bandarenka [31,32] associated the observation of peaks or increase in Cads to the adsorption of OER reaction intermediates and reported them for various transition metal oxides. These observations suggest that the formation of the intermediate species before the onset of the OER involves similar mechanisms for pure and Ni-modified magnetite. This is also supported by the fact that value of Cads retains similar values at the maximum of the peak. From the capacitance data in Fig.\u00a06a and c we can draw the following conclusions: (i) given that the initial Cdl values hardly vary with Ni loading, there is no significant increase in electrochemically active surface area, and the catalytic effect of Ni shown in Fig.\u00a04 cannot be ascribed to an effective enhancement of the surface area; (ii) the fact that a similar peak in Cads is observed for all surfaces, also the one where Fe is expected to be the only active site (98:2), would be in agreement with the commonly held view that Fe is the active site in NiFe catalysts, but that it becomes more active in an Ni environment. The presence of two peaks in the EIS of the metallic Ni-decorated surface if not a noise effect can be interpreted as two types of adsorbates on Ni (and perhaps Fe) sites that are accessible due to the porosity and layered structure of Ni films, providing access to active sites down to 5 nm in depth [33].The experimental data acquired on clean and Ni-doped Fe3O4(001) surfaces show that Ni doping enhances the OER activity of magnetite. Electrochemical voltammetric responses, in combination with surface sensitive techniques, suggest a strong dependence of the OER activity on the atomic structure of the surface exposed to electrolyte. In particular, LEIS measurements indicate that the catalyst with the best OER performances, with an overpotential of 340 mV vs RHE at 5 mAcm\u22122, exhibits a surface Fe:Ni ratio of 40:60.In order to shed light on how the presence of Ni affects the magnetite atomic surface structure-activity relationship, the following observations have to be considered:We have previously shown that following 1 ML Ni-doping and subsequent mild annealing at 200\u00b0C, the Ni atoms fill all the vacant sites in the Fe3O4(001) subsurface, resulting in neighboring Fe and Ni in the second surface layer [8]. The voltammetric response of this surface (green, Fig.\u00a04a) shows no improvement in the OER activity compared to the clean magnetite. Corresponding LEIS measurements (see supplemental material, Figure S6) suggest that this surface exhibits a surface Fe:Ni ratio of 98:2, confirming that almost no Ni is present in the outermost surface layer. These results suggest that the presence of subsurface Ni is insufficient to improve the OER activity. Based on our XPS and LEED data, we propose that modification of the Fe3O4(001) surface with a Ni load > 1 ML leads to the formation of a multilayer mixed ferrite spinel oxide with a structure similar to NixFe3-xO4-like systems. Now the model catalyst exposes both, Fe and Ni atoms in the outermost surface layer as seen from LEIS. In these conditions, the OER activity increases, reaching a maximum when the surface exposes an optimum Ni content of 60-85%. XPS and LEED suggest that the structure of this surface stays characteristic of mixed spinel up to a Ni doping corresponding to a surface Fe:Ni ratio of 40:60. Higher Ni doping results in the formation of metallic Ni clusters, which compromise the spinel long-range order, leading to a loss of atomic control without substantial further enhancement of the activity and, eventually, a decrease in the OER activity when Fe is no longer accessible.Our AFM results suggest that the surface prepared with an Fe:Ni ratio of 40:60 appears stable after OER, albeit with some new features, 1-2 nm high, scattered all over the surface. In contrast, the surfaces with higher Ni loads show the growth of a new phase, which increases in volume and roughness (effective surface area) as the metallic Ni concentration increases. This suggests the growth of a new phase on top of the doped magnetite surface. On the basis of our XPS results, as well as earlier studies [25,34,35], we interpret this phase as the growth of Ni-(oxy)-hydroxide. Similar phases have been also observed on powder Fe-Ni based catalysts, and have reported in literature to affect the catalytic activity towards OER [9]. In particular, Burke et al. [25] observed that electrochemical cycling leads to a transformation from nano-crystalline NiOx films to a layered (oxy)-hydroxide that correlates with an increase in OER activity. Similarly, Deng et al. [35] monitored the dynamic changes of single layered Ni(OH)2 using in situ electrochemical-AFM, and observed dramatic morphology changes already after one linear voltammetry sweep, as well as a direct relation between increase in OER activity and increase in volume and redox capacity of the layered oxy-hydroxide phase. Our results are, however, are not entirely in agreement with these observations. The increase in volume and surface area of the hydroxide phase does not correlate directly with our catalysts\u2019 activity: the surface with the highest amount of the Ni-(oxy)-hydroxide phase and redox capacity is \u2248 200 mV less active than the (almost) flat surface with Fe:Ni ratio 40:60. At this Ni surface concentration, we do not observe in AFM (Figure\u00a03b\u00b4) the growth of the new phase covering a large fraction of the surface, but only some scattered features. Nevertheless, the activity of this surface (expressed by the overpotential, Fig.\u00a05a) is close to the optimum. This clearly indicates that the layered Ni-(oxy)-hydroxide is not the active phase in our catalyst.It is also important to mention that the activity exhibited by the surface prepared with a Fe:Ni ratio of 40:60, with an overpotential of 340 mV vs RHE is comparable to values reported for OER on (Fe)Ni based catalysts [34-40]. For comparison, the overview in Table\u00a01\n shows a selection of some of the best OER catalysts based on Ni-Fe oxides reported in literature. The lowest overpotential values measured on these catalysts at 5 mAcm\u20132 vary typically in the 210 - 347 mV vs RHE range (in 1 M KOH or NaOH electrolyte). Similar overpotential values were also obtained from our surface prepared with a higher Ni load (Fe:Ni\u00a0=\u00a015:85).When comparing the latter surface with the one having an Fe:Ni ratio of 40:60 after electrochemical cycling and subsequent aging for three days in electrolyte, a different activity trend is observed (dashed lines in Fig.\u00a05a). On the one hand, both surfaces show a significant increase in activity following voltammetric cycling and aging, in agreement with previous studies [3,35]. On the other hand, their activity does not increase in the same way. Surprisingly, the surface with metallic Ni shows a much lower onset of the OER overpotential than the one with an Fe:Ni of 40:60, despite the similar performances when freshly prepared. This surface is by far the most active with an overpotential of 247 mV vs RHE. However, it has to be taken into account that this surface, being characterized by the presence of a large fraction of metallic Ni in the as-prepared state, shows neither a well-defined spinel structure nor any other ordered structure over most of the surface and, therefore, cannot serve as a model system. Since one of the scopes of this work is to propose a working model system for the understanding of the OER mechanism, a compromise between activity and the ability to preserve atomic control has to be made. In this regard, the surface with a Fe:Ni ratio of 40:60, very well defined, stable and highly active, fits the criteria to be used as model catalyst.Finally, the analysis of the Tafel plots and adsorption capacitance measurements can help extracting information to identify the OER active sites. Our Ni-modified magnetite surfaces show similar absolute Tafel slopes values (Figs.\u00a05b and 6c) in the 43-62 mV/dec range, independent of the degree of Ni doping for Fe:Ni ratios down to 15:85. Furthermore, the clean and the Ni-modified surfaces show similar maximum values of the adsorption capacitance before the OER onset. These values are associated to the appearance of the OER precursors [31,32] and the shift to lower overpotentials as the Ni doping increases and finally reaches a steady value with the optimal Ni content (Ni content \u2248 60-80%).To explain these observations, we propose the following scenario: the intermediate species that forms on the surface before the onset of the OER might be the same on the clean surface as well as on the Ni- modified one, indicating Fe as the active sites. Accordingly, the right amount of Ni in the spinel surface does not cause the formation of intermediates but facilitates it. Similar conclusions have been proposed by Bell and co-workers who used DFT to compare the OER activity of pure and Fe-doped \u03b3-NiOOH and of pure and Ni-doped \u03b3-FeOOH catalysts [43]. They showed that pure \u03b3-NiOOH adsorbs the OER intermediates too weakly and pure \u03b3-FeOOH too strongly. They found a considerable increase in activity for Fe sites that are surrounded by Ni next-nearest neighbours in both \u03b3-NiOOH and \u03b3-FeOOH. Similar results have also been obtained by Klaus et\u00a0al. who, on the basis of turnover frequency (TOF) calculations, proposed Fe atoms as the OER active sites in Fe-doped NiOOH catalysts [24].Nevertheless, despite the fact that the OER mechanism on NiFe-based catalysts is still unclear together with several fundamental open questions such as the clear identification of the rate limiting step, our results tend to confirm that the OER intermediates are located on Fe sites, the surrounding Ni having a promoting effect on the latter. Furthermore, a deep understanding of the observed electrode aging effect on the OER activity, following long exposure of the material to the electrolyte, remains open, but reveals the importance of the nature of the electrolyte and its interaction with the material.Moreover, it should be pointed out that the use of a single crystal enables an accurate determination of the electrochemically active surface area (ECSA) of these materials and provides reference values for the double-layer capacitance and adsorption capacitance on Fe-Ni based catalysts. The double-layer capacitance values are slightly affected by the Fe:Ni ratio and this should be taken into consideration for further determination of the ECSA of such electrodes [31,32]. Additionally, our results point out that, beyond the Fe:Ni ratio, the nature of the interface (spinel or separated NiOOH/Fe-Ni spinel) significantly affects the capacitance of the interface and its use as a reference for ECSA determination could be compromised.The high intrinsic OER activity of mixed Fe-Ni oxides motivated our efforts to make further steps in the understanding of the fundamental roles of Fe and Ni in OER catalysis.In this work, we show a combined surface science/electrochemistry approach for the preparation of well-defined Ni-modified Fe3O4(001) surfaces and the investigation of their electrochemical performances with respect to OER. We have found that the surface prepared with an Fe:Ni ratio of 40:60 shows a performance comparable to those of the best powder catalysts reported in literature, and still maintains a well-defined structure. Being at the same time simple, highly active, and stable under operation conditions, this surface is an ideal candidate to serve as a working model system to gain atomic-scale insights into the complicated OER mechanism. Whereas a Ni-based phase, probably a Ni (oxy)hydroxide covers all of the surface at high Ni coverage, the highest activity is observed when the Ni-modified Fe3O4(001) surface is still accessible, indicating that this surface is essential for the reaction. Electrochemical impedance spectroscopy suggests that on our Fe-Ni catalyst, the active site for the OER is located on Fe atoms at the surface regardless of the Ni:Fe ratio in the structure, suggesting that the Ni does not cause the formation of intermediates but facilitates it.Putting our results in the context of future perspective, a well-defined model system such as the Ni-modified Fe3O4(001) presented in this work is desirable to address the fundamental aspects that are still controversial. With a limited variety of possible adsorption sites and being accessible to methods benefitting from on single-crystal surfaces, this model surface could thus be used for further investigations on the exact nature of the adsorbates involved in the rate limiting step, using in-situ surface science techniques, to shed more light on key parameters to improve the stability and activity of amorphous catalysts used in water splitting devices. We also believe that the good agreement of our results with what is reported in the literature for powder or amorphous catalyst makes our model surface worthwhile to be used as a model to guide future computational studies.Francesca Mirabella: Conceptualization, Methodology, Data Curation, Writing- Original draft preparation; Matthias M\u00fcllner: Conceptualization, Methodology, Data Curation; Thomas Touzalin: Conceptualization, Methodology, Data Curation; Michele Riva: Reviewing and Editing; Zdenek Jakub: Reviewing and Editing; Florian Kraushofer: Reviewing and Editing; Michael Schmid: Reviewing and Editing; Marc T.M. Koper: Supervision, Reviewing and Editing; Gareth S. Parkinson: Supervision, Reviewing and Editing, Ulrike Diebold: Supervision, Reviewing and Editing.None.This work was supported by the European Union under the A-LEAF project (732840-A-LEAF), by the Austrian Science Fund FWF (Project \u2018Wittgenstein Prize, Z250-N27), and by the European Research Council (ERC) under the European Union's HORIZON2020 Research and Innovation program (ERC Grant Agreement No. [864628]).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2021.138638.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4(001) single crystal, using a combination of surface science techniques in ultra-high vacuum such as low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an iron fraction among the cations in the range of 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. We propose that the well-defined Fe3O4(001) surface can serve as a model system for understanding the OER mechanism and establishing the structure-reactivity relation on mixed Fe-Ni oxides.\n               "}
{"full_text": "Recently, the study of transition metal oxide (TMO) nanostructures, specifically ZnO nanostructures on solid metallic substrates has taken center stage due to advantages, such as high electrical conductivity, better adsorption of reactant on substrates, reusability, and easy retrievability [1\u20133]. Various solid substrates such as silicon [4,5], glass [6], polyethylene terephthalate-indium tin oxide (PET-ITO) [7,8], brass [9], aluminum [10], and stainless-steel [11] mesh have been utilized for the growth of ZnO nanostructures by different techniques. Graphene has also been used for the precipitation of TiO2 -based photocatalyst [12]. Moreover, there has been a recent reports of growth on tubular substrate having higher surface-area-to-volume ratio, making it suitable for various applications [13,14]. The broad band gap of ZnO (3.2\u00a0eV) makes it efficient only in a narrow range of solar spectrum; however, the narrow-band-gap semiconductors like CuO (2.0\u00a0eV) demonstrate superior photo response in a wide solar spectrum. However, their photocatalytic efficiency is hampered by the recombination of electron-hole pairs on the semiconductor. Hence, in the past several years, coupling of a broad band gap semiconductor like ZnO with narrow band gap semiconductors, like CuO, Cu2O, Fe2O3, WO3 has taken center stage [15\u201317].Although these nanocomposite structures have shown great potential in the field of photocatalytic degradation as well as antibacterial activity, it has been a challenge to stabilize these nanostructures on metallic substrates. Therefore, the development of composite metal oxide nanostructures anchored over thin metallic films is strongly desired. Besides, the corrosion and wear resistance of metallic films along with their utilities in electronic industries, pipeline, heat conductors, and heat exchangers make them useful substrates in tubular form.The mixed oxide system with n-type ZnO and p-type CuO offers some interesting properties. It has been developed through various fabrication processes, such as hydrothermal-thermal oxidation [18], hydrothermal/sonochemical [19], plasma assisted synthesis [20], SILAR (successive ionic layer adsorption and reaction) [21,22], and chemical bath deposition [23,24]. Hydrothermal technique has also been utilized to manufacture other heterostructures, such as ZnO-TiO2\n[25,26] and novel composite, of BiSbO4 and BiOBr [27]. However, most of these processes require high processing or annealing temperatures and repeated cycles. In contrast, electrodeposition is garnering attention for the synthesis of metal oxide thin films because of its ability to engineer the size and morphology of the nanostructures through control of its parameters like electrolyte concentration, current density, electrode distance [28] etc. In addition, electrodeposition as a low-cost, simple and low-temperature process makes large scale deposition possible on a range of substrates. The electrochemical route has also been reported to enhance the acceptor level in nanostructures because of the externally applied potential [29]. A great number of studies have involved electrochemical deposition for the development of unary oxides such as TiO2, ZnO, WO3,Cu2O etc. [30\u201332]. However, little literature is available for the co-electrodeposition of binary oxides on metal substrates, though substrate-based growth of oxide nanostructures addresses the challenge of retrieving photocatalyst existing in powdered-form from the water medium. Moreover, template or substrate-based growth renders larger surface area for catalytic reaction and enhances the dispersion of metal-oxide nanostructures. Metal-based substrates like Ni are particularly useful owing to their magnetic and anti-corrosion properties. Some recent research have developed ZnO/CuO nanostructures on Cu substrates through a two step process [33,34]. Jung \n\netal\n\n fabricated ZnO-CuO nanowires on a stainless steel mesh, but the mesh structure posses the drawback of transmitting light thereby resulting in a loss of net photon conversion for a given intensity of light [35]. Also, the photodegradation time for various dyes has been invariably long with the use of these substrates [36,37]. Therefore, the present research has presented the fabrication of ZnO/CuO nanocomposite both on nickel and copper tubular thin film substrates through the economic and environment-friendly technique of electrodeposition. Moreover, metal oxide nanoparticles such as CuO and ZnO have piqued the interest of researchers due to their potential use in antibacterial applications. Some evident benefits include low costs and low toxicity, high extraction performance, long-acting antibacterial capabilities, and quick adsorption rates for contaminants [38,39]. Both co-electrodeposition of ZnO and CuO and step-by-step deposition of ZnO decorated CuO nanostructures have been undertaken for a comparison of their morphologies, properties, photocatalytic and anti-bacterial performances. The challenges of improvement in surface area for the catalytic substrates and reduction in the degradation time are addressed. Furthermore, the developed tubular photocatalytic substrates are characterized and utilized for dye degradation and reduction in bacterial growth.Nickel sulphate (NiSO4\u00b76H2O),Copper sulphate (CuSO4\u00b75H2O), Nickel chloride (NiCl2\u00b76H2O), Boric acid (H3BO3), zinc nitrate hexahydrate (Zn (NO3)2\u00b76H2O), hexamethylenetetramine (HMTA, (CH2)6N4), potassium hydroxide (KOH), sodium nitrate (NaNO3) and acetone are among the chemical reagents employed to fabricate the metallic tubular films and the subsequent development of nanostructures. The above-mentioned chemicals were obtained from Central Drug House (CDH), Delhi, India while Isopropyl alcohol (IPA) was purchased from Merck Limited, Mumbai. The compounds were utilized without additional purification. All the aqueous solutions were made using deionized water. For dispersion and solution preparation, ultrasonication and mechanical stirring were utilised. The experimental work employed a hot plate with a temperature control unit and a magnetic stirrer (IKA RCT Basic model, range: 0/50\u20131700\u00a0rpm, 50\u2013380\u00a0\u00b0C). The electroforming process for the manufacturing of thin walled tubes as well as the pulse deposition of ZnO nanostructures employed a pulse generator (Scientific SM5035, range: 20\u00a0mHz- 20\u00a0MHz). The following steps were followed for the preparation of metallic film and ZnO nanostructures.The fabrication of the nickel and copper tubes was materialized through electroforming using a set-up designed and developed by retrofitting of a micro-drilling equipment. The novel arrangement made the micro-fabrication of hollow tubular structures possible by providing controlled rotation to the cathodic mandrel. Being an additive manufacturing technique, electroforming involves the building up of material atom by atom on a pre-shaped cathode, that is a negative replica of the component to be manufactured. Ni and Cu wires were used as anodes and aluminum mandrels of diameter 3\u00a0mm as the cathodic mandrels. The deposition process for nickel tubes was carried out in an electrolyte cell containing 600\u00a0ml of Watts bath having a composition of 300\u00a0g/L of NiSO4\u00b76H2O, 30\u00a0g/L of NiCl2 \u00b76H2O and 35\u00a0g/L of H3BO3. The bath temperature and pH value were kept at 4.5 and 54\u00a0\u00b0C, respectively. The hot plate was utilized to both regulate the temperature and agitate the bath. The results of a previous set of experiment [40] were used to optimize and choose the parameters (duty cycle of pulse current waveform and deposition time) that were most appropriate for the fabrication of the tubular films. Similarly for the fabrication of copper electroformed microtubes, an electrolyte containing 0.5\u00a0M CuSO4\u00b75H2O and 0.5\u00a0M H2SO4 was utilized. Following the deposition of Ni and Cu upon the mandrel, the elctroformed section was chemically etched from aluminum in a 2\u00a0M solution of KOH.Bare ZnO as well as CuO/ZnO nanorods were synthesized on the electroformed nickel substrates using co-electrodeposition technique. The fabrication of bare ZnO nanorods (NRs) was materialized using the electrolyte containing 5\u00a0mM of zinc nitrate hexahydrate (Zn (NO3)2\u00b76H2O) and 5\u00a0mM hexamethylenetetramine (HMTA, (CH2)6N4). The growth of Cu doped ZnO NRs were carried out by incorporating copper sulphate pentahydrate (CuSO4\u00b75H2O), into the electrolyte in different concentrations (25\u00a0\u03bcM, 50\u00a0\u03bcM). The nickel tubular substrates were cleansed in running water followed by acetone and ethanol before deposition. The electrodeposition was performed in a three-electrode electrochemical cell (Metrohm Autolab PGSTAT302N). The nickel tube of surface area 3.77\u00a0cm2 was treated as the working electrode while Pt and Ag/AgCl electrodes were taken as the counter and reference electrode respectively. The electrodes were set at a distance of 6\u00a0cm from each other and a potential of \u22121.1\u00a0V was applied with respect to the Ag/AgCl electrode. A pulse current waveform with 40% duty cycle was used to carry out the electrodeposition. The temperature was maintained at 80\u00a0\u00b0C and deposition carried out for 3600 s. The deposited samples were rinsed with deionized water afterwards and dried.The fabricated Cu tubes in size of \n\n2\ncm\n\u00d7\n\n \u00f83\u00a0mm were used for the synthesis of CuO nanorods. The thermal oxidation route was utilized by keeping the substrates at 400\u00a0\u00b0C for 3\u00a0h.The samples were then cooled inside the furnace. ZnO electrodeposition was then carried out by utilizing Cu tubes with CuO nanorods on them as cathodes. The electrolyte contained 5\u00a0mM of zinc nitrate hexahydrate (Zn (NO3)2\u00b76H2O) and 5\u00a0mM hexamethylenetetramine (HMTA, (CH2)6N4) with 4\u00a0h of deposition time. The temperature was maintained at 80\u00a0\u00b0C with constant stirring rate of 100\u00a0rpm.The surface morphologies and composition analysis of bare and CuO/ZnO NRs were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) using a ZEISS MERLIN compact) instrument. The crystalline phases and compound formation in the samples were analysed by X-ray diffraction (XRD) (PANalytical X\u2019Pert ProMPD) with a monochromatizer Cu K\n\n\u03b1\n\n irradiation (l 1/4 1.5406 \u00c5).For the calculation of band gap energy of the various photocatalytic substrates the UV\u2013vis spectra in diffuse reflectance mode has been recorded using Shimadzu UV-2450 (Japan) spectrophotometer. The photocatalytic degradation experiments for Methylene Blue (MB) dye were carried out in the presence of three tubular nickel and copper tubular films (2\u00a0cm length and 3\u00a0mm diameter) immobilized with ZnO or CuO/ZnO photocatalyst. The photo-reactor containing 60\u00a0mL of aqueous MB dye solution was illuminated in sunlight (Month of July, 2021, Bhubaneswar, Odisha, India). The concentration of dye solution was varied to study their effect on the degradation efficiency with pH remaining constant at 10. Before the experiments, the solution along with the catalysts were magnetically stirred for 30\u00a0min to achieve the adsorption/desorption equilibrium. The dye samples of about 1.5\u00a0mL were taken out at regular intervals and recorded using UV spectrophotometer. The adsorption intensity peaks (664\u00a0nm) were monitored at certain time intervals. All the degradation experiments were continued till complete decoloration of the dye solution have been achieved. The degradation efficiency (\n\n\u03b7\n\n) was calculated using the following formula:\n\n(1)\n\n\n\u03b7\n=\n(\n1\n-\n\n\nC\n\n\n\n\nC\n\n\n0\n\n\n\n\n)\n\u00d7\n100\n\n\n\nwhere C is the equilibrium concentration of MB dye after exposure to sunlight while \n\n\n\nC\n\n\n0\n\n\n\n is the initial concentration of the dye in the solution. The antibacterial activity of ZnO/CuO nanocomposite immobilized metallic tubular structures were assessed by evaluating their performance against gram negative E.Coli bacteria. The bacteria E.coli were grown initially in LB medium and the culture was cultivated at 37\u00a0\u00b0C for 8\u201312\u00a0h. The growth of bacteria was confirmed by turbidity visualization. 50\u00a0\u03bcL of the isolated bacteria were then added to each media containing different samples of ZnO/CuO nanocomposite. The antibacterial activity of these nanocomposite grown substrates were supported by measurement of optical density at 600\u00a0nm using UV\u2013visible spectrophotometer (Hitachi U-2900).Nickel tubes were fabricated at an optimum parameter of 50% duty cycle and 6\u00a0h of deposition time. Only peaks pertaining to nickel were detected indicating the purity of deposition. The narrow peak width and intensity as shown in Fig. 1\n(a) confirm high crystallinity and fine grain size of the deposition. The high intensity of the (111) plane as compared to the other peaks indicate it to be the preferred orientation of the microcrystalline. The crystallite size was calculated from the well-known Scherrer formula [41] and found to be 55\u00a0nm.The tube films were characterized using FESEM for their structural integrity. The thicknesses of the tubes were found to be around 90\u00a0\u03bcm as shown in Fig. 1. The above-mentioned process conditions resulted in manufactured tubes with a satisfactory hardness value of 158 HV and a surface roughness of \n\n2.02\n\n\u03bc\nm\n\n. As hardness values do not play any role in the current application of nanostructure growth, the parameters were chosen to fabricate rigid tubes having minimum wall thickness in order to render higher surface area films. The copper tube shown in Fig. 2\n(a) was fabricated at 50% duty cycle and 4\u00a0h of deposition time to further reduce the thickness and thereby enhance the exposed area within a limited space for growth of photocatalyst. The Fig. 2b demonstrates the EDS analysis of the tube with strong presence of Cu peaks. This confirms the fabrication of high purity Cu microtubes through the process of electroforming.The electrochemical synthesis of ZnO and CuO-ZnO nanocomposite underwent with the electro-reduction of nitrate ions generating hydroxide ions. HMTA was used as a chemical agent to control the growth direction and morphology of the NRs when nitrate precursor is utilized. HMTA, being water soluble decomposes into ammonia and formaldehyde. Thereafter the ammonia acts as an additional source of hydroxide ions [42]. HMTA also contributes towards regulating the pH of the electrolyte. The NaNO3 as a supporting electrolyte plays the role of supplying the OH\u2212 ions through reduction of nitrate ions. The high solubility of NaNO3 makes it possible to maintain the nitrate concentration in excess of the Zn+2 concentration. The step-by-step reaction mechanism can be summarized in the following equations:\n\n(2)\n\n\nZn\n\n\n(\n\n\nNO\n\n\n3\n\n\n)\n\n\n2\n\n\n\u00b7\n6\n\n\nH\n\n\n2\n\n\nO\n\u2192\n\n\nZn\n\n\n+\n2\n\n\n+\n2\n\n\nNO\n\n\n3\n\n\n+\n6\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n(3)\n\n\n\n\nC\n\n\n6\n\n\n\n\nH\n\n\n12\n\n\n\n\nN\n\n\n4\n\n\n+\n6\n\n\nH\n\n\n2\n\n\nO\n\u2192\n6\nHCHO\n+\n4\n\n\nNH\n\n\n3\n\n\n\n\n\n\n\n\n(4)\n\n\n\n\nC\n\n\n6\n\n\n\n\nH\n\n\n12\n\n\n\n\nN\n\n\n4\n\n\n+\n4\n\n\nH\n\n\n2\n\n\nO\n\u2192\n\n\n(\n\n\nCH\n\n\n2\n\n\n)\n\n\n6\n\n\n\n\n(\nNH\n)\n\n\n4\n\n\n+\n4\n\n\nOH\n\n\n-\n\n\n\n\n\n\n\n\n(5)\n\n\n\n\nNH\n\n\n3\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n\u2192\n\n\nOH\n\n\n-\n\n\n+\n\n\nNH\n\n\n4\n\n\n+\n\n\n\n\n\n\n\n\n(6)\n\n\n\n\nZn\n\n\n+\n2\n\n\n+\n2\n\n\nOH\n\n\n-\n\n\n\u2192\nZn\n(\n\n\nOH\n\n\n2\n\n\n)\n\n\n\n\n\n\n(7)\n\n\nZn\n(\n\n\nOH\n\n\n2\n\n\n)\n\u2192\nZnO\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n(8)\n\n\n\n\nZn\n\n\n+\n2\n\n\n+\n2\n\n\nOH\n\n\n-\n\n\n\u2192\nZnO\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\nWhile the involvement of an intermediary phase (Zn (OH)2) has been established by literature [43,44], a direct synthesis of ZnO from \n\n\n\nZn\n\n\n+\n2\n\n\n\n and \n\n\n\nOH\n\n\n-\n\n\n\n ions as given in the step of eqn. (8) has been supported by the investigation carried out by Mcpeak \n\netal\n\n. [45]. The ZnO formation, although not a Faradic process, can be termed as an electrochemically activated precipitation process that can be controlled by experimental parameters like potential, concentration and pH. The precipitation reaction of CuO is similar to that of ZnO on the electrode surface through the process of co-electrodeposition (Eqn. (9)).\n\n(9)\n\n\n\n\nCu\n\n\n+\n2\n\n\n+\n2\n\n\nOH\n\n\n-\n\n\n\u2192\nCu\n\n\n(\nOH\n)\n\n\n2\n\n\n\u2192\nCuO\n+\n\n\nH\n\n\n2\n\n\nO\n\n\n\nThe present electrolyte containing Zn(NO3) and NaNO3 as supporting electrolyte facilitates the deposition of CuO instead of Cu2O as the nitrate system has a more positive potential (E0 = 0.93\u00a0V vs. SHE) compared to Cu(II)/Cu(I) redox couple (E0 = 0.16\u00a0V vs. SHE) [46].Based on the above reaction steps, it is clear that the formation of \n\n\n\nZn\n\n\n+\n2\n\n\n\n ions come from the source material, hence constant and predictable. But the source of OH\u2212 ions are the hydrolysis of HMTA as well as reduction of nitrate ions. The reduction of nitrate ions is catalyzed by metal ions present in the solution like \n\n\n\nZn\n\n\n+\n2\n\n\n\n though it gets consumed subsequently [44]. Hence, it can be hypothesized that the presence of \n\n\n\nCu\n\n\n+\n2\n\n\n\n ions play the same role as that of \n\n\n\nZn\n\n\n+\n2\n\n\n\n. This is supported by the cyclic voltammetry curves obtained in the growth solutions without or with copper sulphate in Fig. 3\n.In the CV studies, it can be noticed that the cathodic current begins at \u22120.9\u00a0V (vs. Ag/AgCl electrode) and a steep increase of current up to 6\u00a0mA can be seen at a potential of \u22121.1\u00a0V corresponding to deposition of ZnO film. As the potential becomes more negative, the current increases because of increased rate of nitrate reduction. Hence a more negative potential leads to faster growth of nanorods. The hydrothermal process of growth is dependent on parameters like growth temperature and concentration. But the electrodeposition process has a significant parameter like applied potential as well. The effect is enhanced with an increase in the current density as \n\n\n\nCu\n\n\n+\n2\n\n\n\n is added to the electrolyte. The cathodic current also becomes slightly positive as the concentration of copper ions becomes higher and higher. Hence, the overpotential decreases leading to ready electrochemical reaction. Two successive cathodic waves can be noticed which can be attributed to the presence of copper, but further investigation is necessary.The preparation process of the ZnO decorated CuO nanorods on Cu tubular film substrate materialized in a two step process as demonstrated in Fig. 4\n. In the first step, the CuO nanorods were developed as a result of the accumulation and relaxation of the stress during the thermal heating process. The Cu tubes were annealed at 400\u00a0\u00b0C for 180\u00a0min. During the process of annealing, a compressive stress is induced because of an increase in molecular volume (Cu\n\n<\n\nCu2O\n\n<\n\nCuO). The protrusion on the oxide surface is brought on to release the stress. The diffusion of atoms is driven by this stress besides the thermal diffusion [47]. Meanwhile, accumulating larger stress needs higher temperature and long time. But, if the growth temperature was too high, like in the case of 600\u00a0\u00b0C or the growth time too long, CuO crystallites formed rather than nanorods owing to surface diffusion. To develop the ZnO shell on the CuO core, electrodeposition was utilized with the electrolyte mentioned above. Hence the mechanism for deposition of ZnO was similar to the development of ZnO on Ni substrate.SEM micrographs were utilized to analyze the surface morphologies of the ZnO as well as CuO/ZnO nanorod composite developed on the Ni tubular thin film. The undoped ZnO nanorods in Fig. 5\n(a) show flower shaped arrangement of hexagonal ZnO nanorods and some porous film in the background. The agglomeration of nanorods and nanocrystals may have led to this film formation. The undoped ZnO nanorods have a diameter in the range of 250\u2013300\u00a0nm and length of 1\u00a0\u03bcM. Likewise, the CuO-doped ZnO nanorods in Figs. 5(b\u2013c) have been aligned in the form of flower. The CuO/ZnO nanorods with 25\u00a0\u03bcmol \n\n\n\nl\n\n\n-\n1\n\n\n\n\n\n\n\n\nCu\n\n\n+\n2\n\n\n\n concentration (Ni_ZnO/CuO_25) in the electrolyte have an average diameter of 100\u2013150\u00a0nm while the average length is 2.5\u00a0\u03bcm. The incorporation of CuO has led to the fragmentation of ZnO nanorods as well as its reduction of size. These rods have pencil like sharp tip instead of the blunt tips of undoped ZnO. When the Cu concentration was further increased to 50\u00a0\u03bc\n\n\n\nmol l\n\n\n-\n1\n\n\n\n (Ni_ZnO/CuO_50), a further reduction in the diameter of nanorods was observed, as shown in Fig. 6\n. The nanorods possessed an average diameter of 90\u00a0nm. Higher concentration of copper has led to the arrangement of nanorods in the form of petals which was also noticed by Wei \n\netal\n\n. [48].The atomic Cu/Zn ratios for the composite films fabricated with different concentrations of \n\n\n\nCu\n\n\n+\n2\n\n\n\n in the electrolyte (0, 25\u00a0\u03bcmol \n\n\n\nl\n\n\n-\n1\n\n\n\n, and 50\u00a0\u03bcmol \n\n\n\nl\n\n\n-\n1\n\n\n\n) were calculated from the EDS element distribution given in the Fig. S1 and S2 of the Supplementary information. Those were found to be 0, 1.11, and 1.52 respectively. This indicates that the concentration variation in the electrodeposition process can easily engineer the composite film. However, the Cu/Zn atomic ratios in the CuO/ZnO composite film is much higher than the actual concentration ratio in the electrolyte solution (0.005 and 0.01 corresponding to 25\u00a0\u03bcmol \n\n\n\nl\n\n\n-\n1\n\n\n\n and 50\u00a0\u03bcmol \n\n\n\nl\n\n\n-\n1\n\n\n\n, respectively).The reason behind this can be attributed to the difference in solubility of Cu(OH)2 and Zn(OH)2 as both act as precursors for the reduction of CuO and ZnO, respectively. While several literature have reported the formation of flakes and grains at higher Cu/Zn atomic ratios (\n\n>\n\n1.1), the current research demonstrates an economical fabrication technique with superior nanorod morphology, rendering higher surface area. Moreover, the strong peaks of Cu, Zn and O reveal the composition of the composite film, without the presence of any impurity element.SEM images of the CuO nanorods grown on the Cu tubes through thermal oxidation as well as the ZnO decorated CuO nanostructures were studied to reveal information regarding the external morphology and topography. It can be observed clearly in Fig. 7\n (a & b) that the surface is covered with CuO nanorods along with some nanoparticles. The nanorods have an average size of 250\u00a0nm.At a temperature of 600\u00a0\u00b0C the samples prepared demonstrated agglomerated CuO plates, thereby not enhancing the surface area of the composite photocatalyst for performance improvement.The higher temperature may have led to fusion in the crystal growth phase. The EDS analysis in the Supplementary Information (Fig. S3) presents the composition of the nanorods and the atomic ratio of Cu and O confirms the synthesis of CuO.\nFig. 7 (c & d) depicts the ZnO decorated CuO nano-structure arrangement with ZnO being wrapped around the CuO nanorods in the form of nanobulges. The diameter and morphology of the CuO/ZnO nanorods are different from that of CuO nanorods. The deposition of ZnO on the CuO nanorods has resulted in an average diameter increase from 250\u00a0nm to 700\u00a0nm. The step like formation on the nanorods has led to a further improvement in the active surface area for the nanocomposite films. Elemental mapping and distribution of atoms in the formation of CuO/ZnO heterostructured nanorods are illustrated in Fig. \u2014 of the Supplementary information. The figure indicates the distribution and interfacial contact between the two oxides. It can be noticed that there is uniform distribution of Cu and Zn all over the surface with abundance of oxygen.\nFig. 8\n(a) shows the XRD spectra for ZnO and ZnO/CuO composite nanostructures on Ni tubular substrate while Fig. 8(b) shows the spectra for CuO and ZnO decorated CuO nanostructures on the Cu tubular substrate. For comparison the XRD pattern of Ni and Cu tubes were also taken. The diffraction peaks of all samples are well-defined revealing the crystalline structure of the nanocomposites.In the XRD data of Cu_CuO sample, the diffraction peaks clearly indicate the monoclinic CuO phase, which is consistent with the published findings (JCPDS Card. No. 89\u20135899) [49]. The hexagonal wurtzite structure of ZnO has also appeared in the sample Ni_ZnO (JCPDS card No.36\u20131451) similar to data reported earlier [50]. The XRD pattern of the Cu_CuO/ZnO sample shows diffraction peaks of both ZnO and CuO. The nickel tubular samples having ZnO/CuO composite nanorods show peaks of ZnO and CuO owing to their co-electrodeposition. While, in the case of nanocomposite grown on the Cu tubular substrates, the CuO peaks remain dominant. The uncontrolled growth during thermal oxidation can also lead to larger crystallite as observed in the crystallite size calculation. The observed lattice constant values were found to be a \u00a0=\u00a04.653 \u00c5, b \u00a0=\u00a03.410 \u00c5, c \u00a0=\u00a05.108 \u00c5for CuO and a \u00a0=\u00a03.22 \u00c5, c \u00a0=\u00a05.20 \u00c5for ZnO respectively. The average crystallite sizes were calculated using the Debye\u2013Scherrer equation which is expressed as d\u00a0=\u00a00.9\n\n\u03bb\n\n/FWHM\n\ncos\n\n\n\n\n\u03b8\n\n where d is the average crystallite size, \n\n\u03bb\n\n is the wavelength of incident X-ray beam (1.541 \u00c5), FWHM is the full width at half maxima of the most dominant peak and \n\n\u03b8\n\n is the Bragg\u2019s diffraction angle. The estimated crystallite size for ZnO at 14.15\u00a0nm and CuO at 15.3\u00a0nm were minimum for the sample Ni_ZnO/CuO_25. The Cu tubular substrate having nanocomposite (Cu_CuO/ZnO) has slightly larger crystallite size at 18.9\u00a0nm for ZnO and 15.58\u00a0nm for CuO. The low intensity CuO peaks can be due to the ZnO decorated CuO nanorods. Hence, the XRD patterns confirmed that the synthesis has been successful, and the separate peaks indicate that ZnO and CuO exist as two individual phases instead of forming an alloy.The FTIR study of the electrodeposited nanocomposite structures were carried out to determine the functional groups present. The FTIR spectra of CuO, ZnO and ZnO/CuO heterostructures have been depicted in Fig. 9\n in the range of 400\u20134000 \n\n\n\ncm\n\n\n-\n1\n\n\n\n. The absorption bands of metal-oxides owing to the inter-atomic vibrations lie in the fingerprint region, i.e, below 1000 \n\n\n\ncm\n\n\n-\n1\n\n\n\n. The strong bands in the range of 400\u2013600 \n\n\n\ncm\n\n\n-\n1\n\n\n\n in the current study is attributed to the combined presence of ZnO and CuO. It has been reported that ZnO demonstrates absorption band around 464 \n\n\n\ncm\n\n\n-\n1\n\n\n\n\n[51] but depends on the particle morphology and chemical composition. The spectra for electrodeposited ZnO shows a peak at 466 \n\n\n\ncm\n\n\n-\n1\n\n\n\n while the CuO sample demonstrates a peak at 473 \n\n\n\ncm\n\n\n-\n1\n\n\n\n. The ZnO decorated CuO nanocomposite from the Cu tube shows additional peaks at 475 \n\n\n\ncm\n\n\n-\n1\n\n\n\n and 492 \n\n\n\ncm\n\n\n-\n1\n\n\n\n signifying the inclusion of CuO which represents Cu-O stretching. CuO absorption peaks around 500 \n\n\n\ncm\n\n\n-\n1\n\n\n\n have also been noticed for samples having CuO/ZnO heterostructures. The broad peaks with higher intensity in case of Ni_ZnO/CuO_25 and Ni_ZnO/CuO_50 confirms good presence of CuO. The absorption peak at 1380 \n\n\n\ncm\n\n\n-\n1\n\n\n\n can be assigned to the stretching vibration of left over nitrate ions. The peaks are prominent for ZnO/CuO samples deposited on the Ni tubes. The absorption peak at 1635 \n\n\n\ncm\n\n\n-\n1\n\n\n\n and the broad absorption band centered at 3435 \n\n\n\ncm\n\n\n-\n1\n\n\n\n corresponds to the \n\nO\n-\nH\n\n bending and stretching vibration respectively. The FTIR results are in accordance with those obtained from EDS analysis.The optical properties of the functional substrates with ZnO nanorods and modulation in their properties with CuO addition were estimated through the reflectance spectra obtained with respect to wavelength. The range of wavelength considered was 300\u2013800\u00a0nm. The UV\u2013vis spectra in Fig. 10\n (a) demonstrates a trailing edge to the spectrum in the UV region followed by a sharp increase for the sample Ni_ZnO while the spectrum slightly shifts towards higher wavelength for the Ni_ZnO/CuO_25 sample. Higher reflectance in the visible region was noticed for Ni_ZnO/CuO_25 as well as Cu_CuO/ZnO sample as shown in Fig. 10 (c).The band gap energies (\n\n\n\nE\n\n\ng\n\n\n\n) of the bare semiconductor nanorods and their composites were determined using the theory proposed by P. Kubelka and F. Munk in 1931 [52]. The theory proposed a method for transformation of reflectance spectra to absorption spectra using the Kubelka\u2013Munk function F(R) which is given by the following equation.\n\n(10)\n\n\nF\n(\nR\n)\n=\n\n\nK\n\n\nS\n\n\n=\n\n\n\n\n(\n1\n-\nR\n)\n\n\n2\n\n\n\n\n2\nR\n\n\n\n\n\nThe reflectance is denoted by R, while the absorption and scattering coefficients are K and S, respectively. The approach developed by Tauc [53] for determination of band gap using absorption spectraF(R) can then be employed and F(R) can be used in the place of \n\n\u03b1\n\n. The resulting equation is given by:\n\n(11)\n\n\n\n\n(\nF\n(\nR\n)\nh\n\u03bd\n)\n\n\n1\n/\n\u03b3\n\n\n=\nB\n(\nh\n\u03bd\n-\n\n\nE\n\n\ng\n\n\n)\n\n\n\nwhere h is the Planck constant; \n\n\u03bd\n\n is the photon\u2019s frequency; \n\n\n\nE\n\n\ng\n\n\n\n is the band gap energy, and B is a constant. The value of \n\n\u03b3\n\n depends the nature of semiconductor band transition and is 1/2 and 2 for direct and indirect band gap transitions respectively.\nFig. 10 (b) and (d) demonstrates the Tauc plot for ZnO/CuO nanocomposite with varying percentages of CuO on Ni and Cu tubular film substrates respectively. The x-axis linear interpolation of the Tauc plot gives the band gap energy. For CuO semiconductor oxide, it has been reported to have direct band gap [54,55]; therefore, both ZnO and CuO have been considered as direct band gap semiconductor oxides. The band gap energy of CuO nanorods on the Cu tubes was found to be 1.38\u00a0eV which is in good agreement with previously reported literature [56]. The bare ZnO nanorods on the Ni substrate has revealed a band gap energy of 2.87\u00a0eV. The lower \n\n\n\nE\n\n\ng\n\n\n\n can be attributed to the higher surface area of nanostructured morphology. Further narrowing of \n\n\n\nE\n\n\ng\n\n\n\n to 2.48\u00a0eV has been observed for Ni_ZnO/CuO_25 samples due to the incorporation of CuO. This can be attributed to the interfacial contact between the ZnO and CuO nanorods. But further increase in CuO incorporation in Ni_ZnO/CuO_50 sample has led to bare CuO like behavior with band gap energy of 1.25\u00a0eV. The improvement in optical properties because of the ZnO/CuO heterostructure has been lost here which is reflected in the photocatalytic study as well. In the literature, the band gap of ZnO/CuO, developed through solution-based co-precipitation method has been observed to be in the range of 2.34\u20133.25\u00a0eV [57,58]. The optical band gap of Ni_ZnO/CuO_25 and Cu_CuO/ZnO nanocomposite, developed in the current study is 2.48\u00a0eV and 2.95\u00a0eV, respectively. The optimum band gap suggests that electron transfer is inevitable. Moreover, the variation in band gap energy arises because of the synergistic effect of ZnO and CuO nanostructures. The lower band gap energy of CuO is the reason behind a decrease in the band gap of the ZnO/CuO composite.The photocatalytic activity of CuO, ZnO and CuO/ZnO heterostructure fabricated both on Ni and Cu tubes under solar irradiation were examined till complete decoloration. Significant decomposition of MB dye was observed for all the catalyst substrates used with varying efficiency and degradation time. Initially, a 50\u00a0ppm aqueous solution of MB dye was utilized for Ni_ZnO, Ni_ZnO/CuO_25 and Ni_ZnO/CuO_50 samples in order to distinguish clearly the degradation time and efficiency. Afterwards 20\u00a0ppm aqueous solution of MB dye was used to demonstrate a comparison between different CuO/ZnO nanostructures on Ni and Cu substrates.\nFig. 11\n (a) and (b) demonstrates the diminishing peak for characteristic peak at 664\u00a0nm with gradual increment in time. The complete decoloration of the dye samples were achieved within 105\u00a0min and 95\u00a0min of time for Ni_ZnO and Ni_ZnO/CuO_25 respectively. The complete decoloration thereby the decomposition of the dye sample was not achieved in the presence of Ni_ZnO/CuO_50 samples. The Cu content in the solution and the morphology of the nanocomposite structure play a significant role in the degradation process. Higher amount of CuO results in the coverage of active sites on ZnO surface thereby reducing the photocatalytic efficiency. The Ni_ZnO/CuO_50 samples presented an efficiency of 74% while the most significant efficiency of 95.3% was demonstrated by Ni_ZnO/CuO_25 samples. The Ni_ZnO samples also rendered good efficiency of 94% though it consumed higher amount of time for complete decoloration. Fig. 11 (c) shows the comparison between degradation results of different samples. The improved activity of the mixed nanocomposite at an optimum concentration is due to the formation of a heterojunction system between the p-CuO and n-ZnO which prevents the recombination of charge carriers. The appropriate position of the band edges provides sufficient and irreversible charge transfer thereby reducing the photocatalytic degradation time. Besides, the hierarchical structure and morphology play a significant role in the superior photodegradation efficacy. The petal like morphology in sample Ni_ZnO/CuO_50 has reduced its exposed area for degradation thereby restricting the activity. Other literature have also reported such optimal concentration of CuO for superior photodegradation of various dyes [59\u201361]. The data obtained from the degradation experiments were further examined to find out whether they follow the Langmuir\u2013Hinshelwood kinetics model as given below:\n\n(12)\n\n\nln\n(\n\n\n\n\nC\n\n\n0\n\n\n\n\nC\n\n\n)\n=\nkt\n+\nconstant\n\n\n\nThe plots between ln (C0/C) and irradiation time were constructed for the samples that are depicted in Fig. 11 (d). The rate constants for Ni_ZnO, Ni_ZnO/CuO_25 and Ni_ZnO/CuO_50 are 0.024\u00a0min\u22121, 0.026\u00a0min\u22121 and 0.009\u00a0min\u22121 respectively. Thus, it is clearly evident from the rate constants that the Ni_ZnO/CuO_25 samples possess the highest degradation rate, hence the most suitable for MB dye degradation.\nFig. 12\n (a) and (b) demonstrates the gradual decoloration and thereby the reduction in the characteristic peak absorbance at 664\u00a0nm for Cu_CuO and Cu_CuO/ZnO samples respectively. The degradation experiments were performed on a 20\u00a0ppm aqueous solution of MB dye and the ZnO decorated CuO nanostructures on Cu tubes (Cu_CuO/ZnO) exhibited superior performance by decomposing the dye into non-toxic products within 60\u00a0min of time. The Cu_CuO substrates took longer time with lower efficiency of 87%, but it has been found to be superior compared to other literature [34,62,63].This can be attributed to the nanorod like morphology and substrate based growth which has been rarely reported in literature. The interface between the metal substrate and the semiconductor oxide nanostructures is also expected to separate the charge and enhance the photocatalytic activity. The enhanced efficacy of the Cu_CuO/ZnO samples is due to the presence of CuO as an efficient photocatalyst in the visible range of sunlight along with the broad band gap of ZnO preventing the recombination of electrons and holes. Fig. 12 (c) and (d) show the degradation efficiency and rate constants for samples on Cu tubular substrates. The rate constants for Cu_CuO and Cu_CuO/ZnO samples are 0.029\u00a0min\u22121 and 0.035\u00a0min\u22121 respectively.Here, an interesting aspect of the research work is to compare the samples prepared by two different techniques. While the first process involved the fabrication CuO/ZnO heterostructure by a one step electrodeposition method, the second process involved both thermal annealing and electrodeposition. Hence, the later samples have resulted in ZnO decorated CuO nanostructures. Therefore, for comparison, the photocatalytic degradation of 20\u00a0ppm MB dye solution was carried out using Ni_ZnO/CuO_25 samples. The rapid decoloration can be noticed in Fig. 13\n (a) with the decreasing trend in characteristic peak at 664\u00a0nm. In merely 40\u00a0min, the characteristic absorbance spectrum of MB dye ranging from 550\u00a0nm to 725\u00a0nm nearly vanished with 93.57% efficiency. It is evident from Fig. 13 (b) that an efficiency of 91.58% has been achieved within 20\u00a0min of time. A higher rate constant of 0.074\u00a0min\u22121 which is more than double that of the degradation rate constant in the presence of Cu_CuO/ZnO validates the superior photocatalytic performance of Ni_ZnO/CuO_25 samples under solar irradiation. The activity followed the trend Ni_ZnO/CuO_25 \n\n>\n\n Ni_ZnO \n\n>\n\n Cu_CuO/ZnO \n\n>\n\n Cu_CuO \n\n>\n\n Ni_ZnO/CuO_50. The following Table 1\n shows a comparison of the photocatalytic results of present work with earlier reported literature.The reusability of the prepared photocatalytic samples were examined through repeated use of the Ni_ZnO/CuO_25 samples for the degradation of MB dye. The photocatalytic substrates retained their photocatalytic degradation capability even after 3rd cycle with an efficiency of 91.5%. Fig. 14\n shows the degradation kinetics and photocatalytic efficiency for repeated cycles. (see Fig. 15\n).The enhancement of photocatalytic activity of ZnO/CuO nanocomposite is attributed to two main factors that are the separation of electrons and holes and the utilization of a large portion of sunlight, i.e, the visible light because of the incorporation of CuO. The special flower like morphological feature of the nanorods has enhanced the adsorption of the dye molecules on the semiconductor oxide nanocomposite structures. The use of organic coloured dye has facilitated the process as they get excited by the impingement of sunlight and may transfer electron to the conduction band (CB) of CuO [74]. And the nanojunctions formed at the interfaces of the n-type-ZnO and p-type-CuO makes the migration of those electrons from the CB of ZnO to CB of CuO possible. The holes also migrate from the valence band (VB) of CuO to VB of ZnO as that is thermodynamically advantageous. While the electrons lead to the generation of superoxide radical anions(\n\n\u00b7\n\n\nO\n\n\n2\n\n\n-\n\n\n\n), the holes at the surfaces are captured by the H2O molecules to produce the hydroxyl radicals (\u00b7OH). These play a dominant role in the redox degradation of the organic dye compound. Moreover, the coloured dye used in the present research acts as a medium for the transfer of electrons from the excited dye to the electron acceptors (e.g\n\n\n\n\nO\n\n\n2\n\n\n\n). The excited dye molecules absorbed on the semiconductor oxide surface inject the electrons to the conduction band of CuO. The possible decomposition mechanism has been depicted by the following equations:\n\n(13)\n\n\nMB\n+\nh\n\u03bd\n\u2192\n\n\nMB\n\n\n\u2217\n\n\n\n\n\n\n\n\n(14)\n\n\n\n\nMB\n\n\n\u2217\n\n\n+\nCuO\n\u2192\n\n\nMB\n\n\n+\n\u00b7\n\n\n+\nCuO\n(\n\n\ne\n\n\n-\n\n\n)\n\n\n\n\n\n\n(15)\n\n\n{\np\n-\nCuO\n}\n{\nn\n-\nZnO\n}\n+\nh\n\u03bd\n\u2192\n{\np\n-\nCuO\n}\n{\nn\n-\nZnO\n}\n(\n\n\ne\n\n\n-\n\n\n(\nC\n.\nB\n)\n+\n\n\nh\n\n\n+\n\n\n(\nV\n.\nB\n)\n)\n\n\n\n\n\n\n(16)\n\n\nZnO\n(\n\n\ne\n\n\n-\n\n\n)\n+\n\n\nO\n\n\n2\n\n\n\u2192\n\u00b7\n\n\nO\n\n\n2\n\n\n\n\n\n\n\n\n(17)\n\n\nCuO\n(\n\n\nh\n\n\n+\n\n\n)\n+\n\n\nH\n\n\n2\n\n\nO\n\u2192\n\n\nH\n\n\n+\n\n\n+\n\u00b7\nOH\n\n\n\n\n\n\n(18)\n\n\n\u00b7\n\n\nO\n\n\n2\n\n\n/\n\u00b7\nOH\n+\nDye\n\u2192\nDegradationproducts\n\n\n\nIt has been observed in the present work that the photocatalytic activity increases with incorporation of CuO but then decreases with higher concentration of CuO. The CuO crystallites facilitate the movement of photo-generated electrons and holes to the surface, but a higher growth can make the process time-consuming. This can lead to recombination and absorption of the charge carriers within the crystalline resulting in a delayed and incomplete degradation as in the case of Ni_ZnO/CuO_50. Besides, the shadowing effect of CuO on ZnO can lead to reduction in number of photo-generated electrons and holes thereby diminishing the absorption capacity of ZnO. The band gap energy values of different samples present a similar picture with lowest band gap energy of 2.48\u00a0eV for Ni_ZnO/CuO_25. It implies that the concentration ratio of ZnO/CuO as well as their morphology greatly affects their photocatalytic performance.The antibacterial activities of Cu and Ni tubular substrates carrying ZnO/CuO nanocomposites were determined by assessing their resistance towards bacteria E.coli. The optical densities measured at 600\u00a0nm were analyzed and Fig. 16\n (a) shows the ability of various samples to inhibit bacterial growth with respect to the positive control.It has been found that the sample Ni_ZnO/CuO_25 demonstrates highest inhibition towards the growth of E.coli with 92% reduction in bacterial growth. The Ni_ZnO substrate also showed good activity against E.coli with 88% reduction. The Cu tubes with ZnO decorated CuO nanostructures and Ni_ZnO/CuO_50 sample were able to inhibit the growth only up to 55% of the positive control. It has been shown in previous research that the cell wall of E.coli possesses negative charge, hence has an electrostatic attraction towards positively charged metal ions [75]. As higher percentage of metal ions are released from nanocomposites, the ZnO/CuO nanostructures have performed well as compared to other samples. It can be said that the \n\n\n\nZn\n\n\n+\n2\n\n\n\n and \n\n\n\nCu\n\n\n+\n2\n\n\n\n ions were able to interact with the cell wall of the bacteria thus causing surface irregularities and destabilization [76]. The antibacterial activity of metal oxide nanostructures depends on the detachment of metal ions, nanostructure morphology and size and generation of reactive oxygen species (ROS). The good morphology with nanorod diameter of around 100\u00a0nm in case of Ni_ZnO and Ni_ZnO/CuO_25 samples may have contributed towards their superior antibacterial activity. Besides, direct contact between the nanorods and bacterial cells obstruct the electron transport to and from the bacterium cell thereby triggering cell death. A similar reduction in bacterial growth of approximately 90% has been achieved in literature, through a multi-step process of layer-by-layer casting [38]. Also it was demonstrated in the photocatalytic studies that the Ni_ZnO/CuO_25 samples posses the capability of producing higher ROS (Hydroxyl, Superoxide radical), thus leading to faster degradation of MB dye. The same can be attributed to its ability of preventing bacterial growth. The presence of Ni tube as substrate has prevented the agglomeration of the nanostructures thereby enhancing their reactive surface area and retaining their morphology.In summary, ZnO/CuO heterostructures possessing different morphology and composition were successfully anchored over electroformed Ni tubular thin films, through pulse electrodeposition. ZnO decorated CuO nanostructures were developed on electroformed Cu tubular substrates by means of a two step process of thermal oxidation and pulse electrodeposition. The FE-SEM analysis and FTIR study confirmed the mixed structures based on simple oxide constituents. The CuO content was optimized for enhanced photocatalytic and antibacterial performance of the ZnO/CuO nanocomposite immobilized catalytic tubes. ZnO/CuO nanocomposite structures on Ni tubes having 25\u00a0\u03bcM \n\n\n\nCu\n\n\n+\n2\n\n\n\n concentration in electrolyte demonstrated superior photocatalytic and antibacterial activity.The catalytic tubes decolorized high concentration of MB dye within 40\u00a0min of time with a degradation efficiency of 95.3%. This could be attributed to the high surface area of ZnO/CuO nanostructures attached to the tubular substrates, efficient utilization of the visible light, and enhanced separation of electron-hole pairs. The results were corroborated by the band gap energy study as well. Moreover, it was economical and easy to retrieve, recycle, and reuse the metallic catalytic tubes. The same sample was found to have prevented bacterial growth by 92% with respect to positive control. The Ni tubes being corrosion resistant could be retrieved and reused with good photocatalytic efficiency. Therefore, the Ni_ZnO/CuO_25 samples fabricated through electrodeposition route with optimum concentration of CuO demonstrated superior photocatalytic and antibacterial performance. Hence, this study introduced a new route of fabricating highly efficient metallic-tube-based photocatalysts having antibacterial activity.HJB performed the experimental work; HJB, KM, and AG analyzed the results; HJB wrote the manuscript; and AG, KM, NPS and PRV did the review and editing.\nHrudaya Jyoti Biswal and Ankur Gupta: Conceptualization, Methodology, Writing. Pandu R. Vundavilli, Kunal Mondal and Nagraraj Shetty: Reviewing and Editing. Ankur Gupta: Supervision, Resources, Funding Acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Ankur Gupta wishes to acknowledge the affiliating institute (IIT Jodhpur) for providing the research seed grant (I/SEED/AKG/20190022), which was instrumental in completing the work. Kunal Mondal gratefully acknowledges Department of Energy and Environment Science and Technology at the Idaho National Laboratory, USA, for their support.", "descript": "\n                  In this work, we report the fabrication of Ni and Cu tubular substrates and the synthesis of ZnO/CuO nanocomposite on them through the process of pulse electrodeposition. The systematic variation in CuO incorporation in the ZnO matrix and the processing technique were noticed to affect the structural, optical, photocatalytic, and anti-bacterial properties, which are well in accordance with the Field Emission-Scanning Electron Microscope, X-ray Diffraction, Fourier transform Infrared Spectroscopy and UV-Differential reflectance spectroscopy results. The remediation capabilities of the photocatalytic substrates were assessed through the degradation of methylene blue (MB) dye under solar irradiation. Optimized CuO incorporation within the ZnO nanorods resulted in the degradation of a 20\u00a0ppm of MB dye solution within 40\u00a0min and a higher concentration of 50\u00a0ppm within 95\u00a0min. The Ni and Cu electroformed tubes as substrates provided not only a reusable supporting frame but also a large surface-area for the growth of ZnO/CuO nanocomposite. The current study also dealt with the anti-bacterial efficacy of the above-mentioned substrates against E.coli. Hence, the Ni and Cu tubular thin film substrates with nanorods of ZnO/CuO composite were explored for the removal of organic as well as biological contaminants from waste water.\n               "}
{"full_text": "Greenhouse gas emissions need to be reduced drastically to meet the Paris Agreement's climate objectives of limiting global average temperature increase below 2\u00a0\u00b0C and pursuing efforts limiting it to 1.5\u00a0\u00b0C [1,2]. This will require energy systems that differ much from today. Since industrial practices will depend on non-renewable sources for a relatively long time before there is a drastic shift to renewable energy, capture of produced CO2 emerges as transit solution up to that date [3]. Once the CO2 is captured, it can be stored (CCS) or used (CCU). At this point, creating a sustainable market demanding recovered CO2 may provide a better option than CCS, while also helping the economy [4,5]. Specifically, research is shifting towards CO2 capture and utilization (CCU). So that, the use of CO2 in the synthesis of value-added products is increasing the attention of several industrial companies [6].Recently, to avoid energy penalties associated with the regeneration and compression steps required for transportation and storage prior to conversion, researchers have attempted to integrate the CO2 capture and utilization (ICCU) [7]. In this context, the CO2 is captured and converted at the same place using dual function materials (DFMs). The DFMs consist of CO2 adsorbents and catalytic phases. First, DFMs capture CO2 from flue gas (4\u201314\u00a0vol% CO2) to effectively reduce carbon emissions. When the carbon capture process is completed, the feed gas is switched to a reducing renewable agent for the conversion of the adsorbed CO2 to synthetic fuels. An interesting option is the conversion of the adsorbed CO2 into CH4 through the Sabatier reaction (Eq. 1).\n\n(1)\nCO2 + 4\u2009H2\u21c6CH4 + 2\u2009H2O\n\n\nDuyar et al. [8] in 2015 published the first work of the operation in cycles of CO2 adsorption and hydrogenation to CH4. The authors used a DFM based on CaO as adsorbent, Ru as metal phase and Al2O3 as support. They demonstrated the possibility of producing CH4 from CO2 adsorbed in an earlier step. From then, publications on cycles of CO2 adsorption and hydrogenation to CH4 is growing exponentially [7,9\u201311]. In general, DFMs are a combination of a compound based on Na [12,13], Ca [8,14], Mg [15,16] or K [15,17], as adsorbent, and a Ru- [12,18], Ni- [17,19] or Rh-based [20] as catalytic phase. Both phases are commonly supported on a high surface area carrier. Specifically, \u03b3-Al2O3 is proposed as the best support [21].One of the problems of the CO2 methanation reaction is its highly exothermal character [22]. Therefore, it leads to a demand for highly thermostable catalysts to resist deactivation phenomena caused by hotspot formation in industrial fixed-bed application [23]. This problem is highly relevant in methanation with continuous feeding of CO2 and H2. In the operation in cycles of CO2 adsorption and hydrogenation to CH4, the temperature control is easier. Nevertheless, the main deactivation phenomenon in the dual operation is the presence of oxygen and steam in the feed stream of the adsorption period, which influence has been analysed by several authors [21,24\u201326]. The presence of O2 partially oxidizes the metal phase, which is reduced again during the hydrogenation period. On the other hand, the presence of H2O reduces the CO2 adsorption capacity due to competitive adsorption of both compounds over the same basic sites.Another way to analyse the resistance of the DFMs to the presence of O2 and steam is to age the DFMs at high temperature in the presence of O2 and steam. This strategy is commonly used for NSR or SCR catalysts for NOx removal in diesel vehicle engines [27\u201329]. In this way, the state of the catalyst at the end of the life of the vehicle can be simulated at a laboratory scale. In order to simulate long periods of DFM operation in cycles, with the presence of oxygen and steam during the adsorption period, the analysis of aged DFMs can be of great interest. The evaluation of the activity of the DFMs after being subjected to the aging protocol, as well as their physicochemical properties, will provide valuable information on their resistance to aging. However, to the best of our knowledge, the study of the resistance to hydrothermal aging in the presence of oxygen of DFMs has not yet been reported.In this work, the effect of hydrothermal aging in the presence of oxygen on DFMs is analysed. Changes that are caused in their physicochemical properties are studied, as well as its influence in the activity in cycles of CO2 adsorption and hydrogenation to CH4. To have a broad view of the effect of aging, DFMs with different formulations are studied. With that aim, DFMs based on ruthenium, nickel or both as metals and on sodium, calcium or both as adsorbents are selected.Five DFMs have been selected based on ruthenium, nickel or the combination of both as active metals, whereas sodium, calcium or the combination of both are used as adsorbents. \nTable 1 lists the complete formulation of DFMs prepared and their nomenclature used in this work, also classified into Ru-DFMs or Ni-DFMs group. Ru-DFMs with single-Na and single-Ca have been chosen, both with comparable activity [12]. In addition, a Ru-DFM with both adsorbents jointly (Na and Ca) is also studied, as it has recently been shown that this combination improves activity [30]. On the other hand, in the case of Ni-DFMs only Na is used as adsorbent, due to its significant higher activity as Ni-DFMs with Ca [14].The DFMs were prepared by wetness impregnation. Firstly, appropriated amount of Ca(NO3)2.4\u00a0H2O (Merk) and/or Na2CO3 (Riedel de-Ha\u00ebn) was impregnated over \u03b3-Al2O3 (Saint Gobain). The impregnated powder was dried at 120\u00a0\u00b0C overnight. Then the powder was calcined at 400\u00a0\u00b0C (Ru-DFMs) or 550\u00a0\u00b0C (Ni-DFMs) for 4\u00a0h (1\u00a0\u00b0C\u00a0min\u22121). Afterwards, Ru(NO)(NO3)2 (Sigma Aldrich) and/or Ni(NO3)2.6\u00a0H2O (Fluka) was impregnated over the previous calcined powder. After drying at 120\u00a0\u00b0C, the samples were stabilized by calcining again at the same conditions.The calcined DFMs were placed in their granulated form (0.3\u20130.5\u00a0mm) in a quartz tube reactor and were heated rom RT to 400\u00a0\u00b0C (Ru-DFMs) or 500\u00a0\u00b0C (Ni-DFMs) at 10\u00a0\u00b0C\u00a0min\u22121 during 1\u00a0h under 10% H2/Ar (50\u00a0cm3 min\u22121).For hydrothermal aging studies in the presence of oxygen, the DFMs were placed in their granulated form (0.3\u20130.5\u00a0mm) in a quartz tube reactor placed in a horizontal furnace. The DFMs were aged under 5% H2O and 5% O2 in Ar for 3\u00a0h, at a total flow rate of 550\u00a0ml\u00a0min\u22121. The temperature for the aging procedure was 400\u00a0\u00b0C for Ru-DFMs and 550\u00a0\u00b0C for Ni-DFMs.X-ray diffraction spectra were obtained in a Philips PW1710 diffractometer. The DFMs were finely ground and were subjected to Cu K\u03b1 radiation in a continuous scan mode from 5\u00b0 to 70\u00b0 2\u03b8 with 0.02 per second sampling interval.Textural properties of the DFMs were determined from N2 adsorption-desorption isotherms measured at \u2212\u00a0196\u00a0\u00b0C using a Micromeritics TRISTAR II 3020 instrument. Pore volumes were calculated by t-plot method while pore size distribution of mesoporous solids was determined using BJH method. The samples were pre-purged with nitrogen for 10\u00a0h at 300\u00a0\u00b0C using SmartPrep degas system (Micromeritics).The dispersion of active metal sites was measured by H2 chemisorption using a Micromeritics ASAP 2020 instrument. Prior to the experiments, DFMs were reduced with pure H2 for 1\u00a0h at 400\u00a0\u00b0C (Ru-DFMs) or 500\u00a0\u00b0C (Ni-DFMs) in order to obtain a material with similar reduction degree than in the catalytic activity test. After that, the samples were degasified at the same temperature for 90\u00a0min. For both groups of DFMs, the adsorption isotherms were recorded at 35\u00a0\u00b0C varying the pressure between 50 and 450\u00a0mmHg. Adsorption stoichiometries of Ni/H\u00a0=\u00a01 and Ru/H\u00a0=\u00a01 were assumed [31].The morphology of the DFMs was analysed by transmission electron microscopy (TEM) in a JEM-1400 Plus instrument using a voltage of 100\u00a0kV. The reduced DFMs were dispersed in distilled water ultrasonically, and the solutions were then dropped on copper grids coated with lacey carbon film.The CO2-TPD experiments were carried out on a Micromeritics AutoChem 2920 instrument coupled to a HIDEN ANALYTICAL HPR-20 EGA mass spectrometer. The DFMs (0.1\u00a0g) were pre-reduced at 400\u00a0\u00b0C (Ru-DFMs) or 500\u00a0\u00b0C (Ni-DFMs) under 5% H2/Ar flow (1\u00a0h) and then cooled down to 50\u00a0\u00b0C. The adsorption of CO2 was performed at 50\u00a0\u00b0C in a flow of 5% CO2/He (50\u00a0cm3 min\u22121) for 60\u00a0min. After CO2 adsorption, the samples were treated with He for 90\u00a0min and heated at 10\u00a0\u00b0C\u00a0min\u22121 up to 1000\u00a0\u00b0C in flowing He (50\u00a0cm3 min\u22121).The catalytic activity, of the synthesized DFMs, in the cyclic operation of CO2 adsorption and hydrogenation to CH4 was evaluated in a vertical tubular stainless steel reactor. In each experiment, the reactor was loaded with 1\u00a0g of DFM with a particle size between 0.3 and 0.5\u00a0mm. Prior to the analysis, the DFMs were reduced with a stream composed of 10% H2/Ar, progressively increasing the temperature from RT to 400\u00a0\u00b0C (Ru-DFMs) or 500\u00a0\u00b0C (Ni-DFMs) and maintaining the final temperature for 60\u00a0min. During the adsorption period, a stream composed of 10% CO2/Ar was fed for 1\u00a0min, followed by a purge with Ar for 2\u00a0min to remove weakly adsorbed CO2. Next, during the hydrogenation period, a stream consisting of 10% H2/Ar was fed for 2\u00a0min, followed by an Ar purge for 1\u00a0min before starting the adsorption period again. The total flow rate was 1200\u00a0cm3 min\u22121, which corresponds to a space velocity of 45000\u00a0h\u22121. The flue gas composition was continuously measured using the MultiGas 2030 FT-IR analyzer for quantitative analysis of CO2, CH4, CO and H2O. The experiments were carried out in the 280\u2013400\u00a0\u00b0C (Ru-DFMs) or 280\u2013520\u00a0\u00b0C (Ni-DFMs) temperature ranges. At this point, it is important to note that nickel has a lower intrinsic activity than ruthenium [32]. Therefore to favour kinetics Ni-DFMs operate at higher temperatures compared to those based on ruthenium [12,14]. Hence, as detailed in the aging procedure section, the temperatures at which DFMs were aged, were also different.The CH4 and CO productions were calculated by the following expressions:\n\n(2)\n\n\n\n\nY\n\n\n\n\nCH\n\n\n4\n\n\n\n\n\n(\n\n\n\n\u03bc\nmol\n\ng\n\n\n\u2212\n1\n\n\n\n)\n\n=\n\n1\n\nW\n\n\n\n\u222b\n0\nt\n\n\n\n\nF\n\n\n\n\nCH\n\n\n4\n\n\n\n\nout\n\n\n\n(\nt\n)\n\nd\nt\n\n\n\n\n\n\n\n(3)\n\n\n\n\nY\n\n\nCO\n\n\n\n(\n\n\n\n\u03bc\nmol\n\ng\n\n\n\u2212\n1\n\n\n\n)\n\n=\n\n1\nW\n\n\n\u222b\n0\nt\n\n\n\n\nF\n\n\nCO\n\n\nout\n\n\n\n(\nt\n)\n\nd\nt\n\n\n\n\nwere W is the catalyst weight loaded in the reactor. On the other hand, CH4 selectivity is determined by relating the CH4 and CO productions since they were the only detected carbon based products:\n\n(4)\n\n\n\n\nS\n\n\nCH\n4\n\n\n\n(\n%\n)\n\n=\n\n\n\n\nY\n\n\n\n\nCH\n\n\n4\n\n\n\n\n\n\n\n\nY\n\n\n\n\nCH\n\n\n4\n\n\n\n\n+\n\n\nY\n\n\nCO\n\n\n\n\n\u00d7\n100\n\n\n\n\nThe error in the carbon balance was deduced by the following expression:\n\n(5)\n\n\n\n\ns\n\n\nCB\n\n\n\n(\n%\n)\n\n=\n\n(\n\n\n\n\nY\n\n\n\nCH\n\n\n4\n\n\n\n\n+\n\nY\n\nCO\n\n\n\n\ns\nt\no\nr\ne\nd\n\nC\n\n\nO\n\n\n2\n\n\n\n\n\u2212\n1\n\n)\n\n\u00d7\n100\n\n\n\nwhere the amount of CO2 stored was calculated from Eq. (6). For that, the amount that leaves the reactor was subtracted from the amount fed. To determine the amount of CO2 fed, the stream from the feed system was led directly to the analyser. This profile corresponds to the actual CO2 input that was fed to the reactor.\n\n(6)\n\n\ns\nt\no\nr\ne\nd\n\nC\n\n\nO\n\n\n2\n\n\n\n(\n\n\u03bc\nm\no\nl\n\n\n\ng\n\n\n\u2212\n1\n\n\n\n)\n\n=\n\n1\nW\n\n\n\u222b\n0\nt\n\n\n[\n\n\n\nF\n\n\nC\n\n\nO\n\n\n2\n\n\n\n\ni\nn\n\n\n\n(\nt\n)\n\n\u2212\n\n\nF\n\n\nC\n\n\nO\n\n\n2\n\n\n\n\no\nu\nt\n\n\n\n(\nt\n)\n\n\n]\n\n\nd\nt\n\n\n\n\n\n\nFig. 1 shows the X-ray diffraction spectra of the DFMs after the calcination step, the reduction protocol and the aging process. In general, in all spectra a background belonging to alumina can be seen on which different peaks stand out. Fig. 1a shows the spectra of the calcined DFMs. In all the Ru-DFMs there are three peaks at 28.0, 35.1 and 54.2\u00b0 2\u03b8 belonging to RuO2 and in the Ni-DFMs another three peaks at 37.3, 43.4 and 63.0\u00b0 2\u03b8 belonging to NiO. Furthermore, a peak belonging to NaNO3 appears at 31.9\u00b0 2\u03b8 (marked with \u201co\u201d) for Na-based DFMs, whereas for the 4Ru-16Ca two peaks belonging to Ca6Al2O6(NO3)6\u00b7xH2O appear at 11.1 and 18.9\u00b0 2\u03b8 (marked with \u201c+\u201d). The appearance of NaNO3 peak evidences that this is an intermediate formed from nitrates coming from Ru, Ni and Ca precursors (nitrates) during the calcination step, which in the subsequent reduction step is finally reduced into the Na2O active sites for adsorption. The detection of peaks assignable to nitrogenous species indicates the presence of residual nitrates that have not been completely decomposed during the calcination step. In general, the intensity of the peaks of the nitrogen species is higher for the Ru-DFMs compared to the Ni-DFMs. Note that calcination temperatures are different for Ru- (400\u2009\u00b0C) and Ni-DFMs (550\u2009\u00b0C). At this point, a higher calcination temperature achieves a deeper decomposition of the nitrates. Echegoyen et al. [33] obtained similar conclusions in their study of the calcination temperature with Ni-Al and Ni-Cu-Al catalysts.\nFig. 1b shows the diffraction spectra of the reduced DFMs. As expected, there is only one peak at 44.0\u00b0 2\u03b8, belonging to metallic ruthenium, in the Ru-DFMs and two peaks at 44.6 and 51.8\u00b0 2\u03b8, belonging to metallic nickel, in the Ni-DFMs. No nitrogenous compound or any peak assignable to the adsorbent phases is detected. Therefore, it can be concluded that after the calcination step and the reduction protocol all elements of the DFMs are in the desired oxidation state. In previous work [19], we concluded that, as the calcination temperature increases, the nitrates decompose largely. However, too high calcination temperature, despite achieving complete decomposition of nitrates, penalizes notably the physicochemical properties of DFMs and consequently their activity. In addition, as demonstrated by XRD, even though the calcination step does not decomposed nitrates completely, the reduction protocol does.The aging process does not modify significantly the X-ray diffraction spectra (Fig. 1c). In all cases the spectra are very similar to those of the reduced DFMs, with the only exception of two peaks belonging to glass wool (17.0 and 26.5\u00b0 2\u03b8), which cannot be completely removed after the aging process, since it is used to fix the catalytic bed in the reactor.\n\nTable 2 summarises the values of specific surface area, pore diameter and pore volume for the different DFMs after the calcination step, the reduction protocol and the aging process. Ru-DFMs increase their specific surface after reduction pretreatment (104.5\u2013110.6 vs. 124.6\u2013130.6\u2009m2 g\u22121) while Ni-DFMs do not (115.2\u2013115.6 vs. 113.0\u2013118.7\u2009m2 g\u22121). This fact is assigned to the different calcination temperatures of the DFMs. A higher calcination temperature decomposes nitrates to a greater extent, as deduced from XRD. In this context, in the calcined Ni-DFMs there is a lower proportion of residual nitrates that are partially or totally blocking the smaller pores. After the reduction protocol, the nitrates decompose completely. At this point, comparing the two families, the reduced Ru-DFMs present a greater specific surface area (121.6\u2013130.6\u2009m2 g\u22121) compared to the reduced Ni-DFMs (113.0\u2013118.7\u2009m2 g\u22121). This fact is assigned to a minor proportion of alumina in the Ni-DFMs formulation (73\u201374 vs. 80%) and also due to a certain pore blockage by larger nickel particles, as will be verified in the next section. On the other hand, the pore diameter and pore volume increase in reduced DFMs, confirming the decomposition of residual nitrates observed by XRD. Specifically, the increase in pore volume is significantly greater in the Ru-DFMs, confirming the presence of a greater quantity of residual nitrates.The aging process causes a reduction in specific surface area and pore volume and an increase in pore size (Table 2). It is suggested that continued exposure of DFMs to temperature in the presence of O2 and H2O leads to sintering of the catalytic phase and agglomeration of the adsorbent, which causes irreversible blocking of smaller pores. In order to confirm this aspect, \nFig. 2 shows the pore size distributions for the different DFMs after the calcination step (black line), the reduction protocol (red line) and the aging process (blue line). All distributions are unimodal centered around 80\u2013120\u2009\u00c5. As can be observed, the distribution shifts towards higher values with the reduction pretreatment. This fact is ascribed to the elimination of the residual nitrates which partially block the pores. On the other hand, with the aging process, only the beginning of the curve moves towards higher values, while the declines are almost coincident. This fact confirms the total blocking of the small-size pores. Burger et al. [23] observed a progressive decrease in the specific surface area of NiAl2Ox and NiFeAl2Ox catalysts as time increased in the operation with continuous feeding of CO2 and H2. The authors assigned the decrease to the growth of Ni particles and to sintering of the mixed oxide phase. De-La-Torre et al. [27] also observed a reduction in the specific surface area after hydrothermal aging for Pt-Ba/Al2O3 and Pt-Ce-Ba/Al2O3 NSR catalysts. The authors assigned the decrease to the formation of barium aluminate and the blocking of the pores of the alumina by platinum and cerium.The dispersion of the active phase/s in the DFMs is determined by H2 chemisorption considering a stoichiometry H/X\u2009=\u20091 (X = Ru or Ni) [31]. \nTable 3 shows the dispersion values (D\nm) of the reduced DFMs. Very different dispersion values are obtained, comprised in the range 2.2\u201324.8%. The choice of DFMs with such disparate dispersion values allows us deeping into the influence of the aging process on DFMs. In general, different dispersions are obtained depending on whether the DFMs are based on Ru or Ni and depending on the adsorption phases. The discussion about the different dispersion values can be found in previous works [12,14,19,30].The aging process causes a drastic reduction in the metallic dispersion of DFMs (Table 3). The dispersion values of aged DFMs are comprised in the range 0.9\u201311.3%, which corresponded to a reduction of 54.1\u201364.4% compared to the values of the reduced DFMs. Based on these results, it can be confirmed that the continued exposure of DFMs to high temperatures in the presence of O2 and H2O leads to a sintering of the catalytic phase.Transmission electron microscopy (TEM) is used to corroborate the sintering of metallic particles after the aging process. \nFig. 3 shows the TEM micrographs of the reduced (left column) and aged (right column) DFMs. The darkest areas with circular-shaped in the micrographs correspond to metallic particles. In the case of DFM 1Ru/10Ni-16Na they can correspond to metallic or bimetallic particles of Ni and Ru [19,34,35]. At this point, the average particle size of the metallic or bimetallic particles is estimated by measuring at least 100 particles and the results are collected in Table 3. The histograms of the distribution of the metallic particles of the reduced and aged DFMs are been shown in the supplementary material (Fig. S1). In general, the particle sizes obtained are similar to that determined by H2-chemisorption. De-La-Torre et al. [27] also observed a considerable reduction in platinum dispersion after hydrothermal aging process for Pt-Ba/Al2O3 and Pt-Ce-Ba/Al2O3 NSR catalysts. The reduction in dispersion was assigned to the sintering of the metallic phase.The basicity of DFMs is determined by CO2 desorption experiments at programmed temperature. Samples were first saturated with a 5% CO2/Ar mixture, and then purged in an inert atmosphere, and finally a temperature controlled ramp was applied in He. During the temperature ramp, the intensity of signal 44 is monitored with a mass spectrometer. \nFig. 4 shows the evolution of the CO2 signal as a function of temperature during the CO2-TPD experiments for the reduced (solid line) and aged (dotted line) DFMs. Depending on the desorption temperature, weak, medium and strong basic sites are distinguished. Weak basic sites are unstable and easily decompose below 250\u2009\u00b0C. Medium strength basic sites decompose between 250 and 700\u2009\u00b0C and strong basic sites are highly stable and do not decompose until 700\u2009\u00b0C. All DFMs studied show weak and medium basicity, while only DFM 4Ru-16Ca shows strong basicity. This fact indicates that the strength of calcium carbonates is higher compared to sodium carbonates. A more in-depth analysis of the different types of basicity depending on the phase or phases of the adsorbent can be found in own previous works [12,14,19,30].If the profiles of the reduced samples and the aged samples are compared, both follow a similar evolution. On the one hand, the CO2 desorption for aged Ru-DFMs shifts towards lower temperatures compared to reduced counterparts. On the other hand, the desorption profiles of aged Ni-DFMs present a lower intensity than the reduced counterparts. At this point, it must be taken into account that the aging temperature is different, 400\u2009\u00b0C for Ru-DFMs and 550\u2009\u00b0C for Ni-DFMs. Furthermore, also the metallic contents are different, 4% for the Ru-DFMs and 10\u201311% for the Ni-DFMs. The aging process for the Ni-DFMs causes greater coverage of the adsorbent by larger nickel loading. On the other hand, given the lower metallic loading of Ru a lower proportion of the adsorbent phases are covered. Therefore, aging causes agglomeration of the adsorbent and consequently desorption of CO2 at lower temperatures.The catalytic activity is evaluated in cycles of CO2 adsorption and hydrogenation to CH4. \nFig. 5 shows a complete cycle for the reduced and aged DFM 4Ru-8Na/8Ca at 400\u2009\u00b0C. The cycles have a total duration of six minutes. First, it begins with the adsorption period by introducing a stream of 10% CO2/Ar for one minute, followed by a two-minute purge. Then, the hydrogenation period begins by introducing a stream of 10% H2/Ar for two minutes. Finally, the global cycle ends with an additional one-minute purge. The detailed description of the temporal evolution of reagents and products, as well as the mechanism, can be found in own previous works [12,14]. \nTable 4 summarizes the chemical reactions proposed in each period for DFMs based on sodium, calcium or both.In the adsorption period, the CO2 is adsorbed, forming carbonates. CO2 can be adsorbed on oxide sites (Eq. 7 and Eq. 8) or on hydroxide sites (Eq. 9 and Eq. 10). On the other hand, in the hydrogenation period, the carbonates decompose due to the presence of hydrogen (Eq. 11 and Eq. 12), the desorbed CO2 is hydrogenated to CH4 (Eq. 1) and part of the produced water remains adsorbed, forming hydroxides (Eq. 13 and Eq. 14). If the evolution of the reagents and products between the reduced and aged DFM is compared, it can be seen that both follow a very similar evolution with small differences in intensity. Therefore, it is concluded that the aging process does not modify the previously proposed mechanism. Fig. S2 shows a complete cycle for the reduced and aged DFM 10Ni-16Na at 400\u2009\u00b0C. The previously proposed mechanism is also valid for reduced and aged Ni-DFMs.The CO2 adsorption and hydrogenation cycles to CH4 have been carried out at different temperature ranges for Ru-DFMs and Ni-DFMs. Note again that, given the lower intrinsic activity of nickel compared to ruthenium, Ni-DFMs commonly operate at higher temperatures. Therefore, the samples have been aged at different temperatures, 400\u2009\u00b0C (Ru-DFMs) and 550\u2009\u00b0C (Ni-DFMs). Hence, the influence of the aging protocol is studied independently for Ru- and Ni-DFMs in the following sections.\n\nFig. 6 shows the evolution of CH4 concentrations profiles during the hydrogenation period for the reduced (solid line) and aged (dotted line) Ru-DFMs in the temperature range 280\u2013400\u2009\u00b0C. In all cases, CH4 production begins at minute 3 of the cycle that is immediately after the H2 admission, in correlation with cycle timings shown in Fig. 5. Consequently, the decomposition of carbonates (Eq. 11 and Eq. 12) and their subsequent hydrogenation (Eq. 1) is an instantaneous process with the change of the feed to 10% H2/Ar. Comparing the DFMs with each other, different evolutions and different trends are observed with the increase in operating temperature. In general, the DFMs 4Ru-16Na (Fig. 6a-e) and 4Ru-8Na/8Ca (Fig. 6f-j) show little variability in CH4 concentration evolution with increasing operating temperature. On the other hand, DFM 4Ru-16Ca (Fig. 6k-o) clearly modifies its CH4 concentration evolution with operating temperature. These results are consistent with the different strength of basic sites. The DFM 4Ru-16Ca is the only formulation that presents strong basic sites (Fig. 4), therefore more quantity of CO2 is available to hydrogenate at high temperature.Another important aspect to take into account is the CH4 formation rate. The faster CH4 forms, the greater the utilization of the H2 fed. In own previous works [36,37], the CO2 adsorption and hydrogenation to CH4 operation has been modeled, simulated and optimized. It was concluded that adsorption times close to DFM saturation and moderate hydrogenation times are optimal, in which there is a compromise between the amount of CH4 produced and the conversion of H2 fed. At this point, among the reduced DFMs, the maximum CH4 concentration for DFM 4Ru-16Na is 4700\u2009ppm at 340\u2009\u00b0C, while for DFM 4Ru-16Ca is 7300\u2009ppm at 400\u2009\u00b0C. Furthermore, the maximum production for DFM 4Ru-16Ca is detected at earlier hydrogenation times. Consequently, Ca-based DFMs has more favourable CH4 formation rate than Na-base one for cyclic operation. On the other hand, DFM 4Ru-8Na/8Ca reaches a concentration of 8000\u2009ppm, also in the first moments of the hydrogenation period. At this point, the simultaneous presence of both adsorbents increases the CH4 formation rate which contributes to improve the compromise between the amount of CH4 produced and the H2 conversion.Comparing the evolutions of the reduced and aged DFMs, in general, they follow the same trend, the concentration of CH4 for the aged DFMs being slightly lower. However, for DFM 4Ru-16Ca when operating at low temperatures (280\u2013340\u2009\u00b0C) the CH4 concentration of the aged DFM is higher in the early seconds of the hydrogenation period. This fact is due to the adsorbent agglomeration caused by the aging process. As observed in CO2-TPD experiments (Fig. 4) this favours the desorption of higher amount of CO2 at lower temperature for aged 4Ru-16Ca.\n\nFig. 7a shows the evolution of CH4 production for the reduced and aged Ru-DFMs. The CH4 productions are obtained from the direct integration of the profiles shown in Fig. 6 aplying Eq. (2). To check the reliability of the data, the error, with which the carbon balance is closed is determined (Eq. 5). In all cases, it is possible to close the carbon balance with an error below 5%. The DFM 4Ru-16Ca shows an upward trend with operating temperature and the DFMs 4Ru-16Na and 4Ru-8Na/8Ca show less variability. At this point, the DFM composed of both adsorbents (4Ru-8Na/8Ca) presents the highest CH4 production in the entire temperature range studied. In agreement with that reported in a previous work [30], the modification of the Na2CO3/CaO ratio modulates the basicity of DFM (Fig. 4) and improves the dispersion of the metallic phase (Table 3). Consequently, these aspects promote the CO2 adsorption and hydrogenation to CH4.The CH4 productions of the aged DFMs (hollow symbols linked by dotted lines) decrease with respect to reduced DFMs. Therefore, it is confirmed that exposure to temperature with a stream composed of O2 and H2O limits the activity of DFMs. This limitation, as mentioned above, is caused by the decrease in dispersion due to sintering of the active phase (Table 3) and the agglomeration of the adsorbent. Even so, DFMs composed of a single adsorbent, 4Ru-16Na and 4Ru-16Ca, produce 256\u2009\u00b5mol\u2009g\u22121 at 340\u2009\u00b0C and 271\u2009\u00b5mol\u2009g\u22121 at 400\u2009\u00b0C, respectively. On the other hand, DFM containing both adsorbents (4Ru-8Na/8Ca) produces 286\u2009\u00b5mol\u2009g\u22121 at 400\u2009\u00b0C and in the temperature range 340\u2013400\u2009\u00b0C the production does not fall below 275\u2009\u00b5mol\u2009g\u22121. At this point, DFM 4Ru-8Na/8Ca is proposed as the most active after the hydrothermal aging in the presence of oxygen. However, the higher activity is not due to a better resistance to aging, but to the higher activity of the reduced DFM. The three Ru-DFMs present a similar aging resistance. For an easier interpretation. Fig. 7b shows the percentages of decrease in the CH4 production of the aged Ru-DFMs with respect to the reduced at the different operating temperatures. Remarkably, no decrease of methane production higher than 25% is observed. In general, the reduction values are between 17% and 25%, with the exception of DFM 4Ru-16Ca at 280\u2009\u00b0C which reduction in only 7%. As previously suggested, the agglomeration of the adsorbent leads to desorption of CO2 at lower temperatures, which contributes to maintain the production at similar level for this DFM.\nFig. 7c shows the evolution of CO production for reduced and aged Ru-DFMs. In all cases, an upward trend is obtained with temperature and the aging process does not modify the quantity produced. All Ru-DFMs are highly selective to CH4, in all cases, the selectivity is above 90% (Eq. 4). Furthermore, in DFMs with Ca, the selectivity is above 95%. Previous studies carried out by other authors [38,39], or by ourselves [12,14], reported that the presence of Ca favours the selectivity to CH4 while the presence of Na favours the formation of CO.Based on the results of Ru-DFMs, it can be concluded that although the aging process limits the textural properties; considerably high CH4 productions are still obtained. The fact that all Ru-DFMs studied are highly resistant to aging indicates the possibility of operating for long periods.\n\nFig. 8 shows the evolutions of the CH4 concentrations of the reduced and aged Ni-DFMs at different operating temperatures. For both reduced DFMs a strong dependence on the operating temperature is appreciated. At low temperatures (280\u2009\u00b0C) the maximum CH4 concentration is limited. Subsequently, it increases markedly at moderate temperatures (360\u2013440\u2009\u00b0C), and the maximum CH4 concentration decreases again at higher temperatures (520\u2009\u00b0C). On the other hand, comparing the DFMs with each other, it is clearly observed that the promotion of Ni-DFM with small amount of Ru (1% wt.) boosts the CH4 production. Furthermore, Ru-promoted Ni-DFM (1Ru/10Ni-16Na) exhibits significantly faster CH4 formation rate. In the previous sections of characterization, the Ru-promotion of a Ni-DFM boosted the metallic dispersion. In agreement with higher melting point of Ru relative to Ni, it is suggested that ruthenium acts as a shell protecting from sintering nickel in the nucleus during the calcination step [34]. Besides, the strong interaction between Ni and Ru also prevents nickel nanoparticles from sintering [19,40]. Tsiotsias et al. [41] in their review analysed bimetallic Ni-Based catalysts for CO2 methanation. They conclude that the insufficient low-temperature activity, low dispersion and reducibility, as well as nanoparticle sintering of Ni-based catalysts can be partly overcome via the introduction of a second transition metal (e.g., Fe, Co) or a noble metal (e.g., Ru, Rh, Pt, Pd and Re). Through Ni-M alloy formation, or the intricate synergy between two adjacent metallic phases, new high-performing and low-cost methanation catalysts can be obtained. Renda et al. [42] and Zeng et al. [43] in their studies also obtained similar conclusions.Regarding to aged samples, a noticeable decrease in CH4 concentration can be observed with respect to reduced sample. This limitation is accentuated in the second minute of hydrogenation (minute 4 of the cycle). It is proposed that with the higher metallic loading (10\u201311%) the aging process modifies the proximity between the adsorbed carbonates and the available metallic sites. In fact, a significant decrease in the dispersion of the metallic phase and the coverage of the adsorbent phase by the metal has been observed for aged DFMs by characterization techniques. Consequently, a significant proportion of carbonates do not have nearby metal sites available for decomposition and hydrogenation.For a more in-depth interpretation, the profiles in Fig. 8 were integrated (Eq. 2) and the evolution of CH4 production per cycle with reaction temperature is shown in \nFig. 9a. In all cases, it is possible to close the carbon balance with an error below 5% (Eq. 5). Both DFMs present a similar trend with a maximum of CH4 production at intermediate temperatures (400\u2009\u00b0C) as has also been observed in Fig. 8. The reduced DFMs 10Ni-16Na and 1Ru/10Ni-16Na yield 172 and 250\u2009\u00b5mol\u2009g\u22121, respectively. On the other hand, analysing the productions of the aged DFMs, a significant decrease is clearly appreciated. For an easier interpretation, Fig. 9b shows the percentages of decrease in the CH4 production of Ni-DFMs at the different operating temperatures. On this occasion, compared to the Ru-DFMs (Fig. 7b) the decrease is significantly greater. In fact, production is reduced by up to 60% for the DFM 1Ru/10Ni-16Na operating at 280\u2009\u00b0C. At this point, keep in mind that nickel-based catalysts or DFMs commonly operate at higher temperatures than ruthenium-based ones. Therefore, to emulate a long period of operation at a higher temperature, the aging process of Ni-DFMs has been carried out at 550\u2009\u00b0C compared to 400\u2009\u00b0C of Ru-DFMs. Consequently, the aging process is carried out under more severe conditions in the case of Ni-DFMs.Comparing both Ni-DFMs with each other, 1Ru/10Ni-16Na shows a greater decrease in CH4 production at all operating temperatures (Fig. 9b). It is suggested that improvements in the textural properties due to synergistic effects between ruthenium and nickel in the reduced DFM are limited after the aging process. Even so, in general, 1Ru/10Ni-16Na presents a higher CH4 production in the studied operating temperature range. A production of 137\u2009\u00b5mol\u2009g\u22121 is obtained at 360\u2009\u00b0C for DFM 1Ru/10Ni-16Na and 108\u2009\u00b5mol\u2009g\u22121 at 400\u2009\u00b0C for DFM 10Ni-16Na.\nFig. 9c shows the evolution of CO production of reduced and aged Ni-DFMs. Again, an upward trend is obtained with temperature, however, this time the aging process also decreases the amount of CO produced. DFM 10Ni-16Na exhibits low selectivity to CH4. In fact, at 400\u2009\u00b0C, being the temperature at which the CH4 production and its selectivity is maximized, both for the reduced and for the aged DFMs, the selectivity is 88%. On the other hand, 1Ru/10Ni-16Na presents a selectivity above 95% in the temperature range 320\u2013400\u2009\u00b0C, both for reduced and aged DFM.The ability to produce CH4 from DFM 1Ru/10Ni-16Na with high selectivity after the aging process stands out. Despite the fact that the textural properties are remarkably diminished and the CH4 production is remarkably decrease, it is possible to produce 137\u2009\u00b5mol\u2009g\u22121 at 360\u2009\u00b0C with fast CH4 formation rate. \nFig. 10 shows the accumulated methane production with the duration of the hydrogenation period. At 0.7\u2009min (3.7\u2009min of the complete cycle), 100\u2009\u00b5mol\u2009g\u22121 was produced, confirming the adequate CH4 formation rate of the aged DFM for the dual process of CO2 adsorption and hydrogenation to CH4.Based on the results of the Ni-DFMs, it can be concluded that aged DFM 1Ru/10Ni-16Na still exhibits acceptable CH4 production with high selectivity and fast CH4 formation rate, allowing proper use of hydrogen. However, the DFM 10Ni-16Na has poor catalytic activity after the hydrothermal aging in the presence of oxygen and is therefore not suitable for long periods of operation.Ru/Ni-Na/Ca-Al2O3 DFMs with different formulation have been aged, characterised and evaluated in cycles of CO2 adsorption and hydrogenation to CH4. The aging process causes a decrease in the textural properties of all DFMs. Furthermore, the dispersion of the metallic phase is also reduced. The continuous exposure of DFMs to temperature in the presence of oxygen and steam causes the sintering of the metallic phase, the agglomeration of the adsorbent phase and the blocking of smaller alumina pores. In Ru-DFMs, adsorbent agglomeration shifts CO2 desorption at lower temperatures. However, in Ni-DFMs CO2 desorption is decreased after the aging process. This difference is assigned to the greater coverage of the adsorbent phase due to a higher nickel loading (10\u201311 vs. 4%) and to the higher temperature of the aging process for the Ni-DFMs (550 vs. 400\u2009\u00b0C).In the activity in cycles of CO2 adsorption and hydrogenation to CH4, all the aged DFMs show, in the entire temperature range, a lower CH4 production compared to the reduced DFMs. Consequently, continued exposure of DFMs to temperature in the presence of oxygen and steam also limits activity. However, the decrease in the CH4 production does not exceed the 25% for Ru-DFMs subjected to aging process. Therefore, despite the fact that the aging process limits the physicochemical properties of Ru-DFMs, considerable CH4 productions are obtained with adequate CH4 formation rate. From that it can be concluded, that all Ru-DFMs studied are suitable for long operation periods. Specifically, the aged DFM 4Ru-8Na/8Ca produces 286\u2009\u00b5mol\u2009g\u22121 at 400\u2009\u00b0C. Furthermore, in the range 340\u2013400\u2009\u00b0C, the production is above 275\u2009\u00b5mol\u2009g\u22121 with a selectivity greater than 95%. As the decrease in CH4 production after the aging process is comparable for all Ru-DFMs, 4Ru-8Na/Ca is proposed as the best alternative given the higher activity of the reduced DFM.The catalytic activity in Ni-DFMs is considerably reduced after the aging process. CH4 production is limited, especially in the second minute of the hydrogenation period. It is proposed that given the higher metallic loading (10\u201311%), the aging process limits the proximity between the carbonates and the metallic sites. The aged DFM 1Ru/10Ni-16Na produces 137\u2009\u00b5mol\u2009g\u22121 at 360\u2009\u00b0C with a selectivity greater than 95%. Furthermore, in the first 0.7\u2009min of the hydrogenation period, 100\u2009\u00b5mol\u2009g\u22121 are produced. Therefore, it can be concluded that despite the fact that the aging process considerably decreases the physicochemical properties and the activity, the DFM 1Ru/10Ni-16Na continues to present acceptable CH4 production with adequate CH4 formation rate for the dual operation of CO2 adsorption and hydrogenation to CH4. The higher CH4 production of the aged DFM is due to the higher activity of the reduced DFM. Nevertheless, 10Ni-16Na DFM is not suitable for long periods of operation due to its poor catalytic activity after the hydrothermal aging in the presence of oxygen.\nAlejandro Bermejo-L\u00f3pez: Validation, Methodology, Investigation, Writing \u2013 original draft. Be\u00f1at Pereda-Ayo: Conceptualization, Methodology, Visualization, Writing \u2013 review & editing. Jon A. Onrubia-Calvo: Methodology, Visualization, Writing \u2013 review & editing. Jos\u00e9 A. Gonz\u00e1lez- Marcos: Methodology, Data curation, Supervision, Funding acquisition. Juan R. Gonz\u00e1lez-Velasco: Conceptualization, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The financial support from the Science and Innovation Spanish Ministry (PID2019-105960RB-C21) and the Basque Government (IT1297-19) is acknowledged. The authors thank for technical and human support provided by SGIker (UPV/EHU Advanced Research Facilities/ ERDF, EU). One of the authors (JAOC) acknowledges the post-doctoral research grant (DOCREC20/49) provided by the University of the Basque Country.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2022.107951.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Integrated CO2 capture and utilization (ICCU) is a promising alternative to revalue CO2. In this work, the influence of the aging process on dual function materials (DFMs) Ru/Ni-Na/Ca-Al2O3, for the conversion of CO2 into CH4 is studied. DFMs are characterized by N2 adsorption-desorption, XRD, H2 chemisorption, TEM and CO2-TPD. The catalytic behavior of the prepared DFMs is analyzed in consecutive cycles of CO2 adsorption and hydrogenation to CH4. The aging process notably limits the physicochemical properties, especially the metallic dispersion. However, the CH4 production decrease is less than 25% for aged Ru-DFMs, which makes them suitable for long-term operation. The aged DFM 4Ru-8Na2CO3/8CaO-Al2O3, presents a CH4 production greater than 275\u00a0\u00b5mol\u00a0g\u22121 with high selectivity in the range 340\u2013400\u00a0\u00b0C. On the other hand, the aging process is more noticeable for Ni-DFMs; in fact, it limits the CH4 production to half compared to reduced Ni-DFMs.\n               "}
{"full_text": "The world consumption of vast quantities of fossil fuels and its influence on climate change and environmental issues have sparked a lot of interest in developing clean, environmentally friendly, and renewable energy systems. Due to its greater specific gravity density than other fuel cells, hydrogen (H2) is an excellent and clean energy carrier with zero carbon dioxide emissions and a higher gravimetric energy density than other fuels that may be utilized in fuel cells.\n1\n H2 has also been employed as a reducing agent in a variety of sectors, including ammonia synthesis, hydrocarbons hydrogenation, and metal manufacturing. Unfortunately, coal steam gasification or methane reforming accounts for more than 95% of the hydrogen manufacturing business, which uses non-renewable fossil fuels and generates significant volumes of carbon dioxide. As a result, it is critical to produce green hydrogen using non-polluting and eco-friendly methods. Electrochemical water splitting is a crucial energy conversion reaction that converts plentiful and simple H2O molecule into valuable H2 and O2 molecules.\n2\n Specifically, alkaline electrochemical water splitting is one of the most exciting approaches for generating hydrogen via cathodic hydrogen evolution reaction (HER) with high conversion efficiency and a broad potential range.\n3\u20137\n It plays a great role to treat the excreted alkaline media such as chlor-alkali and water-alkali electrolyzers.\n8\n Theoretically, the thermodynamic equilibrium potential for the overall water electrolysis is approximately 1.23\u202fV at standard conditions.\n9\n However, experimentally, the operating potential is far beyond the thermodynamic equilibrium potential due to the sluggish kinetics, especially for the alkaline HER, where the difference value is called overpotential (\u03b7) applied to overcome the reaction energy barrier.\n10\u201312\n In this context, it is crucial to design electrocatalysts that can give high performance and long-term stability at lower overpotential and minor parasitic reactions.\n13\u201315\n Even though platinum-based catalysts are the most effective for HER, their widespread utilization is hampered by their high cost and insufficient reserve.\n16\u201320\n Consequently, it is highly appealing to explore alternative electrocatalysts based on transition-abundant elements, such as metal sulfides, oxides, phosphides, borides and carbides to replace the noble metal-based catalytic materials.\n21\u201323\n Besides, there are many strategies to improve the catalyst performance for HER, such as exposing high-active facets,\n24\n\n,\n\n25\n constructing nanostructures,\n26\n modifying electronic structures\n27\n and doping with other elements.\n20\n Nonetheless, low activity still plagues non-precious metal-based catalysts. Furthermore, the chemical composition and density of the active sites impact the catalytic activity of HER.\n28\u201330\n\nSingle-atom catalysts (SACs) have been demonstrated to be indispensable materials in electrochemical energy conversion and storage applications.\n31\u201333\n They are well-defined mononuclear active sites in which all active metal species are isolated and maintained by the support of or alloying with another material. Compared with nanoparticles, SACs do not have metal-metal bonds and have positive charges. The reduction of the metal atoms generates plenty of unsaturated coordination centers which stimulate high surface energy of the metal components resulting from the high chemical interaction affinity of the metal center with the support and adsorbates. The significant electronic and geometric properties of SACs have displayed a potential change in the reaction pathways.\n34\n\n,\n\n35\n The simplicity and homogeneity of SACs facilitate the identification of active sites and correlation of the structure-activity relationships. Moreover, SACs are considered the bridge between the homogenous and heterogeneous catalysts.\n36\n\nSAC structures range from single metal atoms supported on a mesoporous oxide surface to those supported on single-layer materials to isolated surface metal atoms as part of an alloy, and beyond.\n37\n There are generally two strategies to improve the SAC performance, namely tuning the intrinsic properties of SACs\n38\n and increasing the metal amount on the support to increase the active site concentration.\n39\n More importantly, SACs allow the use of a tiny amount of noble metals, resulting in a decrease in cost while achieving increased mass activity for HER, which is highly promising for practical applications.In this review, we provide a timely overview of the very recent development of SACs in the area of alkaline HER. While several review papers have covered the topic of SACs for HER,\n40\u201346\n a comprehensive review dedicated especially to the alkaline HER application of SACs is still lacking in the literature and is in need of time given the large interest of researchers. We begin by introducing the fundamentals of alkaline HER and basics of SACs. A special focus is given on the rational design of SACs categorized into four aspects, including regulating the inherent element properties, adjusting the coordination environment, tuning the SAC morphology, and increasing the mass loading of single atoms on the substrate surface. Finally, major challenges and prospects for further research on SACs for alkaline HER are highlighted. With the present review, we hope to feature the outlines of designing efficient SACs to be used for hydrogen production in alkaline media.HER is a two-electron transfer and a multi-step electrochemical reaction that occurs on the electrode surface. It takes place in a wide range of pH; acidic, neutral, and alkaline media. Proton or molecule H2O adsorption, quick proton union (combination), and rapid electron transfer to the active centers all play a role in this process. Volmer-Heyrovsky and Volmer-Tafel pathways are the two mechanisms that govern HER. More accessible protons created by a simple reduction of the hydronium ion promote HER production in acidic environments. In alkaline media, on the other hand, it takes more energy to break the covalent link H\u2013O\u2013H of H2O molecules to form protons, which governs the total reaction kinetics.\n47\n\n,\n\n48\n Alkaline HER is regulated by three stages, according to theoretical and experimental studies: water adsorption, water dissociation, and adsorbed hydrogen intermediate.\n45\n\nIn alkaline electrolytes: \u2217 indicates the single-atom catalyst surface.Adsorption of water molecules on catalyst surface\n45\n\n\n\n\n\n\nH\n2\n\nO\u00a0\u200b\n\n+\n\n\n\u2217\n\n\n\u2192\n\n\u00a0\u200b\u00a0\u200b\u00a0\u200bH\n2\n\n\nO\n\u2217\n\n\n\n\n\n\n\ni)\nProton adsorption in Volmer reaction: Coupling of H2O molecule and an electron to form adsorbed hydrogen intermediate on the electrode surface (H\u2217).\n\n\n\n\n\n\n\n\u00a0\u200b\u00a0\u200bH\n2\n\nO\u00a0\u200b\n+\n\ne\n\u2212\n\n\u2192\n\nH\n\u2217\n\n+\n\nOH\n\u2212\n\n\nVolmer\u00a0\u200breaction\u00a0\u200b\u00a0\u200b\u00a0\u200b\n\n(\n\n120\n\n\u00a0\u200b\u00a0\u200bmV\u00a0\u200bdec\n\n\u2212\n1\n\n\n\n)\n\n\n\n\n\n\n\n\n\n\nT\n\n\nS\n1\n\n,\n\u00a0V\n\n\n=\n\n\n2.3\nR\nT\n\n\n\u03b1\nF\n\n\n\n\n\nwhere T\ns is the Tafel slope, R is the universal gas constant, T is the absolute temperature, \n\n\u03b1\n\n is the symmetry coefficient (0.5), and F is the Faraday constant.\n\nii)\nElectrochemical desorption: H\u2217 combines with a water molecule.\n\n\n\n\n\n\n\nH\n2\n\nO\u00a0\u200b\n+\n\ne\n\u2212\n\n+\n\n\u00a0\u200bH\n\u2217\n\n\u2192\n\nH\n2\n\n+\n\nOH\n\u2212\n\n\nHeyrovsky\u00a0\u200breaction\u00a0\u200b\u00a0\u200b\u00a0\u200b\n\n(\n40\n\n\u00a0\u200b\u00a0\u200bmV\u00a0\u200bdec\n\n\u2212\n1\n\n\n)\n\n\n\n\n\n\n\n\n\n\n\nT\nS\n\n\n2\n,\n\u00a0H\n\n\n=\n\n\n2.3\nR\nT\n\n\n\n(\n1\n+\n\u03b1\n)\n\nF\n\n\n\n\n\n\n\n\niii)\nChemical desorption: two H\u2217 couple together on the electrode surface to form H2 molecule.\n\n\n\n\n\n\n\nH\n\u2217\n\n+\n\nH\n\u2217\n\n\u2192\n\nH\n2\n\n\nTafel\u00a0\u200breaction\u00a0\u200b\u00a0\u200b\n\n(\n30\n\n\u00a0\u200b\u00a0\u200bmV\u00a0\u200bdec\n\n\u2212\n1\n\n\n)\n\n\n\n\n\n\n\n\n\n\n\nT\nS\n\n\n2\n,\n\u200bT\n\n\n=\n\n\n2.3\nR\nT\n\n\n2\nF\n\n\n\n\n\n\nProton adsorption in Volmer reaction: Coupling of H2O molecule and an electron to form adsorbed hydrogen intermediate on the electrode surface (H\u2217).Electrochemical desorption: H\u2217 combines with a water molecule.Chemical desorption: two H\u2217 couple together on the electrode surface to form H2 molecule.Experimentally, Tafel analysis is a great method for gaining experimental insight into the mechanism of HER happening at the catalyst surface. In Tafel analysis, the logarithm of current or current density is plotted against the (over)potential, the linear portion of which is fitted to the Tafel equation to extract the Tafel slope. According to Butler-Volmer kinetics, the theoretical values for the Tafel slope are 120, 40, and 30\u202fmV dec\u22121, corresponding to the Volmer, Heyrovsky, and Tafel reaction, respectively, being the rate-determining step. Theoretically, a promising HER catalyst should have a d band that can extend across the Fermi level and have a strong coupling to hydrogen.\n45\n Hydrogen adsorption Gibbs free energy (\u0394G\nH) is considered the major descriptor to evaluate the catalyst performance. Additionally, overpotential, Tafel slope value, exchange current density, stability, Faradaic efficiency and turnover frequency are also good measures of the catalytic HER activity.There is a great effort to develop HER catalysts in alkaline system to substitute the acidic system to overcome the dissolution of some catalysts and the difficulties of finding inexpensive anodes stable in acidic solutions in addition to the cost and safety concerns.\n49\n\n,\n\n50\n Due to the large activation energy of the water dissociation, the benchmark Pt-based catalyst performance for alkaline HER is roughly 2\u20133 orders of magnitude lower than in acidic media. As a result, the poor hydrogen kinetics associated with H2 generation in alkaline media is a significant obstacle.\n51\u201353\n The features of binding hydrogen species and dissociating water are required for the rational design of electrocatalysts with excellent alkaline HER performance.Atomically dispersed active sites have attracted great attention as a new frontier in the catalysis field.\n54\n\n,\n\n55\n Supported metal nanoparticles maximize the efficiency of atom utilization and provide the opportunity to alter the reaction pathway. With the supported nanometal catalysts being mostly surface atoms, SACs display an isolated atom from each other and high tunability owing to the strong interaction between the metal center and the substrate. While the intrinsic activity of nanoparticles is determined by the accessibility of the exposed edges, defects or corners and the interfaces between two phases, the down-scaling of the catalysts to single atoms could extremely improve the activity and durability. SACs could maximize the atom-utilization efficiency, leading to cost-effectiveness, in particular for the noble metal-based catalysts such as Pt, Ru, Ir and Pd.\n56\u201358\n Surprisingly, SACs not only have homogenous active centers for the reactions due to the unsaturated coordination atoms as homogenous catalysts but also possess the advantages of reusability and stability similar to the heterogeneous catalysts.\n31\n Moreover, the potential metal-support interactions could modulate the electronic structure of the metal atoms because of the electron transfer between the substrate and metal centers.\n16\n\nTheoretically, SACs have a unique HOMO (highest occupied molecular orbital)-LOMO (lowest occupied molecular orbital) gap due to the quantum size effect reflecting distinct energy level distribution.\n49\n\n,\n\n59\n The hybridization with atoms could generate asymmetrical spin and charge density.\n55\n As a result, synthesizing SACs and sustaining atomic dispersion of single metal atoms in the face of particle agglomeration under realistic synthesis and reaction circumstances are significant challenges. Currently, various synthetic methodologies have been employed to prepare SACs aiming to obtain high SACs capacity on the support surface and enhance the stability of dispersed atoms on the host framework. The high-temperature pyrolysis technique, wet-chemistry approach, and physical and chemical deposition method are the most prevalent synthesis procedures for SACs.\n53\u201356\n The structure and properties of SACs, as well as the chemical state of the metal center and metal-support interactions, are often studied and verified using advanced characterization techniques such as atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) and synchrotron-based X-ray absorption spectroscopy (XAS).\n60\u201362\n Furthermore, because SACs have essentially homogenous single dispersed active sites, the catalytic process may be identified using rational design and computation techniques like density functional theory (DFT).The performance of HER catalysts is related to the chemical nature and density of active centers. Furthermore, the development of alkaline HER electrocatalysts is critical to addressing the reaction kinetics and stability difficulties. Tuning the surface chemistry by altering the electronic structure, composition, morphology, and porosity/active surface area can solve these problems. Downscaling the particle size results in a greater volume-to-surface-area ratio of SACs, which allows them to tune the atomic distribution and electronic structure. For alkaline HER catalysis, noble metal-based and non-noble metal-based catalysts (e.g., Co, Ni, Fe, V) have recently been explored. Pt-based catalysts are the most efficient electrocatalysts for HER, with higher mass activity than nanoparticle catalysts. Non-noble metal-based SACs, on the other hand, have been widely adopted due to their significantly reduced cost.\n44\u201350\n\nSACs are different from cluster catalysts and nanoparticle catalysts.\n31\n\n,\n\n32\n The surface of catalysts including certain atoms featuring unsaturated coordination, such as the atoms at defect sites, edges, and vertices influence the catalytic reactions. Thus, downsizing the nanoparticles and more undercoordinated surface atoms on the surface have been used to increase the catalyst efficiency. This smaller particle often has size, structural effect as well as coordination environment effect that grant the promising physicochemical property of the catalysts. SACs have gained momentum since Zhang et\u00a0al. originally introduced them in 2011,\n63\n particularly in the electrocatalytic sector. Numerous SACs have demonstrated exceptional catalytic properties for electrochemical water splitting. According to the different types of support materials, SACs can be divided into many different types, such as alloy-based SACs, carbon-based SACs, and SACs supported on other compounds.\n42\n\nThe excellent atom utilization of SACs and one-of-a-kind size quantum impact have aroused interest in catalysis and chemical transformation applications. The development of a flexible and simple synthesis approach to modify the interaction between the metal centers and supports can be used to tune the inherent features of SACs.\n64\n\n,\n\n65\n In the alkaline HER electrocatalysis, key challenges for SACs include the exact control over the local structure of single-atom sites and the increase of the active-site density. The intrinsic activity of atomic structures is determined by their rational design, which affects the activation and adsorption of reactants across single sites.\n49\n\n,\n\n66\n Increasing the metal loading of SACs, on the other hand, would greatly increase the density of active sites and the related mass activity.It is known that a SAC made of a certain element should have its unique chemical properties. Tailoring the electronic properties and structures can effectively control the adsorption behaviour during the HER and consequently the catalytic activities.\n67\n\n,\n\n68\n By employing DFT calculations, Chen et\u00a0al. performed a systematic investigation on the HER performance of more than 20 different single transition-metal (TM) atoms implanted in phosphorus carbide monolayer (\u03b1-PC) (denoted TM@\u03b1-PC).\n69\n It was found that all the TM doped \u03b1-PC monolayers are energetically stable. The Gibbs free energy for hydrogen adsorption, which concerns adsorption sites of either TM sites or reversed P site, was found to show a volcano-shaped relationship with respect to the HER activity (characterized by the overpotential) (Fig.\u00a01\na).\n69\n The absolute adsorption energies of Ir-\u03b1PC (TM site), Fe-\u03b1PC (TM site), Cu-\u03b1PC (P site), and Rh-\u03b1PC (P site) are all lower than that of Pt (111), with Ir-\u03b1PC (TM site) attaining the ideal value of 0.008\u202feV. Computational screenings have also been conducted on other SACs, such as TM atoms supported on a C9N4 monolayer (TM@C9N4) and on a graphdiyne monolayer (TM@GDY) (Fig.\u00a01b and c).\n49\n\n,\n\n70\n In a similar fashion, volcano relationships were obtained between the Gibbs free energy for hydrogen binding and the HER activity (characterized by exchange current), from which optimal SACs can be determined. These theoretical studies may provide useful guidelines for the experimental development of advanced SACs toward the alkaline HER.While the catalytic efficacy of SACs can be modified by selecting a proper active center, the simplicity of a single-atom center sometimes makes it difficult for further materials tuning to achieve improved catalytic performance. Incorporating a secondary metal atom to fabricate a metal-metal dual atom site (single-atom dimer: SAD) has the potential to further alter the electronic structure of SACs and increase their intrinsic activity, which is likely due to the distinctive atomic interface and synergistic effect of the dual-metal site.\n71\u201373\n Inspired by this approach, Kumar et\u00a0al. theoretically and experimentally reported the synergistic interaction between Ni\u2013Co at the atomic level in the SAD configuration for the alkaline HER (Fig.\u00a02\n).\n74\n Firstly, different transition metal-based SADs (TM-SADs) anchored on N-doped carbon (NC) were studied by DFT calculations for the HER catalysis in alkaline media (Fig.\u00a02a). The d-band center of TM-SADs was found to correlate linearly with the kinetic barrier for water dissociation (Fig.\u00a02b). Of note, the d-band centers of Co and Ni atoms in the NiCo-SAD-N6C sample were the nearest to the Fermi level (\u22120.87\u202feV), showcasing its excellent capability to bring fast water dissociation and favorable proton adsorption, both beneficial to the alkaline HER kinetics.\n75\n Following this theoretical understanding, the authors experimentally prepared NiCo-SAD supported on NC (NiCo-SAD-NC) by trapping Ni/Co ions in the polydopamine spheres followed by annealing. Combined aberration-corrected high-angle annular dark-field STEM (HAADF-STEM) and electron energy loss spectroscopy (EELS) studies verified the emergence of Ni\u2013Co dual sites in the NiCo-SAD-NC sample with an average dimer distance of 0.241\u202fnm (Fig.\u00a02c\u2013f). Due to the strong electronic coupling between the Ni\u2013Co dual sites at the atomic level, the NiCo-SAD-NC catalyst exhibited outstanding HER activity in 1\u202fM KOH (overpotential of only 61\u202fmV at \u221210\u202fmA\u202fcm\u22122), much superior to the monoatomic Ni-SAC or Co-SAC, the NiCo nanoparticle counterparts, and the commercial Pt/C benchmark (Fig.\u00a02g and h). This work thus offers a viable approach to leverage the dual-metal atom synergism for the design of highly efficient SAC-based HER electrocatalysts. Considering the ratio of dimer structure in NiCo-SAD-NC to be about 78%, developing methodologies that can achieve pure SADs may further improve the HER performance.The coordination environment is another factor with a profound effect on the catalytic performance of SACs.\n76\n It is clear that depending on the coordination environment the single atom is embedded in, the corresponding SAC should exhibit different HER activities. Very often, the great diversity of SACs and supports can present a challenge to determine an optimum coordination scenario. Ma and colleagues employed DFT simulations to screen a series of TM single atoms (from Ti to Zn and from Zr to Cd) embedded at the different vacancy sites of the MoSSe monolayer as HER electrocatalysts.\n77\n The stability of the formed SACs was first assessed by calculating the binding energy (E\nb) of the 18 different TM atoms anchored on Mo, S, and Se vacancy (TM@MovaSe, TM@MoSvaSe, TM@MoSSeva, respectively) (Fig.\u00a03\na and b). The negative value of E\nb means strong bonding between the TM atoms and the defective MoSSe substrate, while the positive value indicates difficulty in the adsorption of TM atoms. Based on the results of E\nb, 23 SAC structures could be excluded due to the positive E\nb values. In addition, a general repeated trend for E\nb of TM@MovaSSe in the same row of the periodic table was found, namely, E\nb increases as elementary metallicity decreases. The hydrogen adsorption process on various TM@MoSSe surfaces was further studied (Fig.\u00a03c and d). Three candidates, i.e., Zn@MoSvaSe, Cd@MoSvaSe and Co@MovaSSe were found with near-zero Gibbs free energy for HER, with values of around \u22120.095, \u22120.098, and \u22120.050\u202feV, respectively, comparable to the ideal Pt-SACs (about \u22120.070\u202feV). These theoretical investigations highlight the importance of anchoring single-atoms to the appropriate site of a support material in bringing about optimized HER performance.As mentioned, surface defects on the supports can be used to stabilize the single atoms,\n77\n sometimes via an increased charge\u2013transfer process, preventing the isolated atoms from aggregating.\n59\n In addition, the defect-induced stabilization effect could also originate from the intimate interaction of the resulting SAC structure.\n78\n Recently, Zhang et\u00a0al. rationally constructed a single atom ruthenium SAC anchored on defective NiFe layered double hydroxide nanosheets (Ru1/D-NiFe LDH) for use in the alkaline HER (Fig.\u00a04\n).\n79\n Ru1/D-NiFe LDH was fabricated using a simple electrodeposition and etching method (Fig.\u00a04a and b). The combined aberration-corrected TEM and X-ray absorption spectroscopy measurements including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) confirmed the existence of Ru single atoms with good homogeneous dispersion on the surface of defective NiFe LDH (Fig.\u00a04c\u2013e). The presence of the defects and Ru single atoms was found to have a synergistic effect in increasing the active site density and promoting the charge transfer process. As a consequence, Ru1/D-NiFe LDH displayed exceptional HER performance in alkaline solutions. The optimum HER activity for the Ru1/D-NiFe LDH system was observed on the sample with a moderate Ru loading of 1.2\u202fwt%, requiring an ultralow overpotential of 18\u202fmV to reach a HER current density of 10\u202fmA\u202fcm\u22122 in a 1\u202fM KOH electrolyte while also giving a stable operation for 100\u202fh at current densities of about 10 and 100\u202fmA\u202fcm\u22122 (Fig.\u00a04f\u2013g). The quantity of surface defects present is likely to limit the loading amount of single atoms and eventually the HER activity of the SACs. It is intriguing to study the influence of defect concentration/amount on the stabilization and HER performance of SACs.The homogeneous atomic coordination environment of SACs makes them a suitable and simplified model system for studying catalytic processes both mechanistically and experimentally. Rodriguez and colleagues proposed the electronic metal-support interaction (EMSI) as one of the ways to improve the electronic characteristics and to regulate the interaction between the single atom and the support.\n80\n Due to orbital rehybridization and charge transfer for the metal-support interface, the EMSI can result in the formation of new chemical bonds and the realignment of molecular energy levels.\n81\n\n,\n\n82\n Shi et\u00a0al. recently found that the EMSI can be leveraged to realize the fine tailoring of the oxidation state of single-atom Pt.\n17\n By applying a site-specific electrodeposition method,\n83\n they loaded Pt single atoms onto four types of different 2D transition metal dichalcogenides (TMDs) (MoS2, WS2, MoSe2, and WSe2) (Fig.\u00a05\na). The core anchoring chalcogen (S, Se) and the neighboring transition metal (Mo, W) were found to synergistically regulate the electronic structure of SAMC through EMSI, giving rise to Pt SACs with fine control over the Pt oxidation state ranging from +1.24 to +2.61 (Fig.\u00a05b). Notably, the Pt SACs supported on MoSe2 (Pt\u2013SAs/MoSe2) with a suitable Pt oxidation state (+2.11) exhibited the optimal HER activity in 1\u00a0\u200bM KOH (Fig.\u00a05c), due to its neither too weak catalyst\u2013OH interaction for water dissociation nor too strong catalyst\u2013H interaction for hydrogen release. As schematically illustrated in Fig.\u00a05d, the kinetic rate of water dissociation increases with the increase in oxidation state of single-atom Pt while the kinetic rate of H2 desorption decreases, hence optimal performance is achieved at a moderate Pt oxidation state. This work highlights how engineering the interaction between single atoms and supports by modifying the nature of support materials can result in different electronic and catalytic effects of the SACs. One question remains open as to if the Pt oxidation state can be tailored to a near-zero value and its effect on the alkaline HER catalysis.The potential cohesive energy between the atoms induces aggregation when the metal is split into single atoms.\n84\n Anchoring the single atom on the supporting molecule via a strong chemical interaction is an excellent technique for addressing the agglomeration challenge. This interaction not only affects the stability of metal atoms, but also influences the catalytic activity by modulating the electronic structure of single atoms.\n85\u201389\n The support has a similar role as the ligand in homogenous catalysts to stabilize the metal site. The type of support influences the coordination number, strain environment, as well as the chemical interaction of metal sites that is reflected in obtaining different electronic and geometric structures of SACs.\n90\n Metals, metal (hydro)oxides/nitrides/carbides, and carbon-based nanomaterials have all been employed as supports.\n63\n\n,\n\n87\n\n,\n\n91\n\n,\n\n92\n However, the conventional support materials may not be sufficient for use in the alkaline HER which involves adsorption toward both OH\u2217 and H\u2217 species. Interestingly, Zhou et\u00a0al. reported the adoption of a 2D NiO/Ni heterostructure as a novel support for Pt single-atoms (PtSA-NiO/Ni), which could provide dual active sites to independently regulate the binding strength of OH\u2217 and H\u2217 (Fig.\u00a06\n).\n16\n By using a facile electrodeposition method, Pt single-atoms were successfully immobilized in NiO/Ni nanosheets, as evidenced by HAADF-STEM image (Fig.\u00a06a). The as-obtained PtSA-NiO/Ni catalyst exhibited excellent performance for hydrogen production in alkaline media, showing a mass activity much greater than Pt single-atoms supported on NiO or Ni alone (Fig.\u00a06b\u2013c). DFT computations suggest that the dual active sites comprising metallic Ni sites and oxygen vacancies-modified NiO sites adjacent to the interfaces of the NiO/Ni heterostructure are responsible for the high alkaline HER activity (Fig.\u00a06d\u2013g). The former efficiently promotes water adsorption, reaching a barrier-free water dissociation step with a lower energy barrier of 0.31\u202feV in the Volmer step compared with that of PtSA-Ni (0.47\u202feV) and PtSA-NiO (1.42\u202feV), while the latter offers more suitable hydrogen binding (\u22120.07\u202feV) than that of PtSA-Ni (\u22120.38\u202feV) and PtSA-NiO (0.74\u202feV), together accelerating the overall alkaline HER kinetics. This work paves the way for the advancement of alkaline HER SACs by coordinating single atoms with heterostructure supports. Introducing a silver nanowire network into the 2D PtSA-NiO/Ni resulted in a seamlessly conductive 3D nanostructure, which brought further enhancement of the alkaline HER performance.\n16\n\nThe geometric structure of electrocatalysts plays an important role in the distribution of active sites that also controls their adsorption nature and catalytic performance. Theoretically, the utilization of SAC active sites could be 100%, however, the practical efficiency of the active sites is still less than 15% with mass loading >2\u202fwt% due to the aggregation within the support and/or encapsulation of the active sites.\n93\n\n,\n\n94\n Finding strategies to optimize the SAC structure by constructing more accessible surfaces or interfaces is of high importance. Fei et\u00a0al.\n95\n used a two-step technique to build a single cobalt atom on nitrogen-doped graphene architecture (Co-NG) via sonication, followed by freeze-drying and pyrolysis in ammonia atmosphere. Co-NG exhibited similar morphologic features to graphene, showing nanosheet-like structures with ripples observed on the surface (Fig.\u00a07\na and b). Thanks to the single-atom nature and the favorable morphological character, the Co-NG catalyst showed appreciable catalytic activity in alkaline solutions (Fig.\u00a07c). Jiang and coworkers employed nanoporous MoS2 with a bicontinous structure as a template to fabricate a single ruthenium atom catalyst (Ru/np-MoS2) (Fig.\u00a07d).\n96\n Changing the ligament size of the nanoporous MoS2 can be used to precisely tailor the strain of the catalyst as induced by the sample's curvature. As shown in Fig.\u00a07e, such a bending strain could effectively regulate the electronic structure of single-atom Ru, hence effectively catalyzing the water dissociation and H\u2013H coupling. As a result, Ru/np-MoS2 demonstrated a low overpotential of 30\u202fmV to afford a current density of 10\u202fmA\u202fcm\u22122 and a Tafel slope of 31\u202fmV dec\u22121 toward the HER in 1\u202fM KOH. While effective, morphology tuning in the design of SACs for alkaline HER is relatively less explored. It is suggested that more design strategies should be developed to tune the morphology of SACs for the HER in basic media.SACs are a rising star in catalysis due to the unique features of combining the merits of both heterogeneous and homogeneous catalysts. Although tailoring the morphology of SACs influences the available active sites for the reaction, the number of active sites is also important to the catalysis, which is mainly dependent on the SAC mass loading on the support surface. In other words, a higher mass loading of SAC would give more exposed active sites for the reactions of interest that could enhance the catalytic activity. Li and coworkers reported Pt single atoms incorporated in a nitrogen-doped porous carbon (Pt1/NPC) with Pt loading of up to 3.8\u202fwt% relative to the carbon.\n97\n Because of the large specific surface energy of SACs, the primary hurdles to obtaining the high mass loading of SACs are migration and agglomeration tendency of the active atoms during synthesis or applications.\n64\n\n,\n\n94\n\n,\n\n97\u2013104\n When a large number of metal atoms are fixed to a support surface, they invariably aggregate into nanocluster particles rather than disperse as single metal atoms. In this regard, effective synthesis strategies for constructing SACs with high mass loading content are critical. Wei and coworkers developed an iced-photochemical reduction approach to synthesize Pt SACs on different substrates including carbon nanotubes, graphene, mesoporous carbon, zinc oxide nanowires and titanium dioxide nanoparticles.\n105\n The atomically dispersed Pt SACs were formed via the exposure of the frozen chloroplatinic acid solution to UV light to prevent agglomeration and obtain high mass loading (approximately 13%).More recently, Zhang et\u00a0al. reported that electrochemical deposition (either cathodic or anodic deposition) can be utilized as a generic strategy to fabricate SACs (Fig.\u00a08\na), as demonstrated by the successful synthesis of SACs consisting of a wide variety of metals (metal: Ru, Rh, Pd, Ag, Pt, and Au) and supports (support: MnO2, MoS2, Co0.8Fe0.2Se2, and NC).\n106\n Importantly, the loading amount of single atoms can be facilely controlled by varying the concentration of metal precursors, the number of scanning cycles, or the scanning rate. For example, during the cathodic deposition of Ir in 1\u202fM KOH electrolytes, the mass loading of Ir constantly increased with the increasing Ir concentration. At an Ir concentration of 150\u202f\u03bcM, Ir single atoms were still obtained with a mass loading of 3.5% while Ir clusters emerged with a mass loading of 4.7% when Ir concentration further increased to 200\u202f\u03bcM, suggesting that for the formation of SACs, there exists an upper limit of mass loading between 3.5% and 4.7% (Fig.\u00a08b). Notably, SACs obtained from cathodic deposition show great potential for catalyzing the alkaline HER, with Ir single atoms on Co0.8Fe0.2Se2 nanosheets delivering the lowest overpotential of 8\u202fmV to achieve a current density of 10\u202fmA\u202fcm\u22122 in 1\u202fM KOH (Fig.\u00a08c). Similarly, Liu et\u00a0al. established an electrochemical pulse voltammetry method for the preparation of U single atoms on MoS2 nanosheets (U/MoS2) from radioactive wastewater (Fig.\u00a08d\u2013e).\n107\n The mass loading of U single atoms was controlled by increasing the pulse cycle, and the sample obtained at a pulse cycle of 100 had an appropriate U loading of 5.2% giving the optimum HER activity in 1\u202fM KOH (overpotential of 72\u202fmV at 10\u202fmA\u202fcm\u22122) (Fig.\u00a08f).To summarize, SACs with the lowest size, highest volume-surface ratio, and optimal atom utilization efficiency of any metal-based catalysts provide exciting opportunities for the alkaline HER application including high activity and stability. During the last few years, a wide range of metal-based SACs (e.g., Pt, Pd, Ru, Fe, Co, Ni, Mo, and W) have been systematically explored for the alkaline HER. Table\u00a01\n gives a performance summary of the state-of-the-art SACs for the HER in alkaline media. Despite the enormous progress made in the field of SACs for electrochemical HER, there are still obstacles to overcome in this fascinating field of research. For example, the structure-activity relationship of SACs and their catalytic mechanism at the atomic scale remain elusive. To address this, well-defined SAC model systems should be established by advanced synthesis methodologies. In addition, the combined use of operando characterization tools and theoretical calculations (e.g., DFT) should be applied. Due to their intrinsic activity advantage, SACs are very appealing for industrial electrochemical hydrogen generation. One major difficulty, however, lies in the unsatisfactory stability of SACs, especially at high single atomic metal loadings. Furthermore, catalyst evaluation in commercial applications usually involves testing under more harsh conditions (e.g., high temperature and high-concentration electrolytes) than what is currently seen in fundamental lab-based research. Therefore, developing SACs for real water splitting devices represents another research topic worthy to be investigated. With efforts from both research and industry communities, we anticipate to see breakthroughs in SACs for HER catalysis in both fundamental understanding and practical applications.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.F.A. and X.X. contributed equally to this work. This work was supported by the Australian Research Council Discovery Projects (Grant Nos. ARC DP200103332 and ARC DP200103315) F.A. acknowledges the Egyptian Ministry of Higher Education and Scientific Research, Cultural Affairs and Missions Sector, Egypt, for a PhD scholarship.", "descript": "\n                  Electrochemical water splitting powered by renewables-generated electricity represents a promising approach for green hydrogen production. However, the sluggish kinetics for the hydrogen evolution reaction (HER) under an alkaline medium causes a massive amount of energy losses, hindering large-scale production. Exploring efficient and low-cost catalyst candidates for large-scale H2 generation becomes a crucial demand. Single-atom catalysts (SACs) demonstrate great promise for enabling efficient alkaline HER catalysis at maximum atom utilization efficiency. In this review, we provide a comprehensive overview of the recent progress in SACs for the HER application in alkaline environments. The fundamentals of alkaline HER are first introduced, followed by a justification of the need to develop SACs. The rational design of the SACs including the inherent element property, coordination environment, SAC morphology, and SAC mass loading are highlighted. To facilitate the development of SACs for alkaline HER, we further propose the remaining challenges and perspectives in this research field.\n               "}
{"full_text": "CNT fiber has made it possible to significantly translate the properties of individual CNTs, trapped in nano-length scale, to macro-length scale. This has enabled CNT fiber to obtain extraordinary properties like tensile strength values of above 9\u00a0GPa, making it one of the strongest synthetic fibers, elapsing conventional materials by a huge margin [1]. CNT fiber is also a good conductor of electricity with achievable electrical conductivity of 6.7\u00a0\u00d7\u00a0104 S cm\u22121\n[2]. Among the methods available for the synthesis of CNT fiber, floating catalyst chemical vapor deposition (FC-CVD) has produced the fiber with the best mechanical properties [3]. The FC-CVD process also has the added advantage of being continuous and scalable. One of the major problems in the synthesis of CNT fiber by the FC-CVD method is its low carbon conversion that hinders in commercial deployment of this process. The carbon conversion reported by most of the researchers is in the range of 3\u20135% [4]. In the case of traditional CNT powder synthesis conversion above 50% is achieved by many researchers [4, 5]. Also, the catalyst utilization in CNT fiber synthesis is less than 1% [4, 6]. This restricts the production rate of CNT fiber to be in few grams per day for most research reactors. Researchers have so far employed five strategies to improve the conversion of CNT fibers, namely, improving catalyst carbon diffusivity, reducing catalyst agglomeration, removing the amorphous coating from the catalyst, improving hydrocarbon cracking rate, and reuse of precursor. The diffusivity of carbon in the catalyst has been improved by using Group 16 elements and also by using bi and trimetallic catalysts. The most commonly used group 16 element is S and it is used by almost all the researchers. Other than S, CNT fiber from Se and Te has been spun by Mas et\u00a0al. [7]. Bi-metallic catalyst (Fe\u2013Ni) was first used by Moon et\u00a0al. [8] and trimetallic catalyst (Fe\u2013Ni\u2013Co) was first used by Karaeva et\u00a0al. [9] for enhancing the diffusivity of C in the catalyst. Reducing catalyst agglomeration was done either by the use of S or by deep injection method. S forms FeS with Fe catalyst, which reduces agglomeration due to its low surface energy [10]. The agglomeration of catalyst can also be reduced by deep injection by introducing the catalyst deep into the reactor, away from a vortex at the inlet, where the catalyst is susceptible to getting trapped and getting agglomerated [11]. The removal of amorphous carbon coating from the catalyst is done mainly by the use of hydrogen or water. Hydrogen gas is used as the carrier gas by most of the researchers. Hydrogen can etch the amorphous coating from the catalyst [12, 13]. Water can also be used for the removal of amorphous coating as it can oxidize the amorphous carbon coating [14]. Improving hydrocarbon cracking is generally not done by many researchers as it could lead to the formation of amorphous soot. Rodiles et\u00a0al. [15]. has improved carbon conversion by enhancing hydrocarbon cracking by using mullite reactor tube. The reuse of precursor has been carried out by Zhang et\u00a0al. [16] were the tail gas from the reactor was reused by sending into another reactor as the precursor for CNT fiber synthesis. An improvised method of FC-CVD called blown aerosol FC-CVD was used by Zhang et\u00a0al. [17] to produce transparent CNT film with higher conversion. However, this method has been utilized for the production of film. Apart from the discussed strategies, researchers have also used the statistical method like design of experiments to obtain optimized process conditions for improved conversion [18].Flow pattern in a CNT fiber reactor was first determined by Conroy et\u00a0al. [19] in a vertical reactor by computation fluid dynamics (CFD) and found a re-circulation vortex in the inlet. Hou et\u00a0al. [20] were the first to determine the flow pattern in a horizontal CNT fiber reactor and found recirculation vortex in both the inlet and outlet. The recirculation vortex in the outlet was shown to produce a 50-fold increase in the velocity which helps in CNT assembly and alignment. Lee et\u00a0al. [11, 21] have shown that the recirculation in the inlet can lead to a reduction in the yield due to catalyst agglomeration. Lee et\u00a0al. [11, 21] have overcome this by deep injection method as discussed earlier. The attempt of reducing recirculation in the inlet was carried out by Oh et\u00a0al. [22] by lowering the Grashof number by optimizing the tube diameter and tube material. Oh et\u00a0al. [22] was able to reduce the recirculation at the inlet by reducing tube diameter and using an alumina tube. The reduction in recirculation in the inlet resulted in an increase in specific strength. In the present work based on the analysis of the flow pattern in the reactor, we came up with the idea of bi-directional injection of catalyst into the CVD reactor for enhancing the carbon conversion. So far, nobody has reported such a technique in FC-CVD method for CNT fiber synthesis. We also propose a mechanism based on the yield and structural characterization.The synthesis of CNT fiber was carried out in a CVD furnace, fabricated in-house, with an attached glove box. The furnace retort was made of alumina tube of 45\u00a0mm inner diameter and 1\u00a0m length. It was heated to 1200\u00a0\u00b0C by resistance heating of SiC rods. The synthesis of CNT aerogel was achieved with a precursor composition consisting of 0.5\u20134.5\u00a0wt% ferrocene (catalyst and source of Fe, purity \u226599.98%, Sigma Aldrich, USA), thiophene (promoter and source of S, purity \u226599.98%, Sigma Aldrich, USA adjusted to maintain a Fe/S molar ratio of 2.65), and remaining ethanol (hydrocarbon and source of C, purity \u226599.9%, Hayman Ltd, England). The precursor mixture was injected at a flow rate of 0.2\u00a0ml/min using a syringe pump (Model \u2013 Legato \u00ae 270 / 270P Syringe Pump, KD Scientific\u2122, USA). Ferrocene beyond 2\u00a0wt% was sent into the reactor by sublimating in a preheater as ferrocene start precipitating beyond 2%. The flow rate of the ferrocene was controlled based on Eq (1-3) [4].\n\n(1)\n\n\nln\n\n(\n\nP\nv\n\n)\n\n=\n\n\n273.6\n\nR\n\n\u2212\n\n\n,\n815\n,\n35\n.\n,\n7\n\n\nR\n,\n.\nT\n\n\n\u2212\n\n\n29.6\n\nR\n\nln\n\n(\n\nT\n\n298.15\n\n\n)\n\n\n\n\n\n\n\n(2)\n\n\n\n\nn\n\u02d9\n\n\ni\nn\n\n\n=\n\n\nP\nv\n\n\nP\no\n\n\n\n\nT\no\n\nT\n\n\n\n\nV\n\u02d9\n\n\ni\nn\n\n\n\n0.0224\n\n\n\n\n\n\n\n\n(3)\n\n\n\n\nm\n\u02d9\n\n\ni\nn\n\n\n=\n\n\nn\n\u02d9\n\n\ni\nn\n\n\n\nM\nm\n\n\n\n\nWhere,\nPv\n\u00a0=\u00a0Parital pressure of ferrocene (Pa)\nR\u00a0=\u00a08.3145\u00a0J/mol K (Gas Constant)\nT= Temperature (K)Po\u00a0=\u00a0Standard atmospheric pressure (101,325\u00a0Pa)To\u00a0=\u00a0Standard temperature (298.15\u00a0K)\u1e45in\u00a0=\u00a0Molar flow rate (mol/s)\n\n\nV\n\u02d9\n\n\nin\u00a0=\u00a0Volumetric flow rateMm\u00a0=\u00a0Molar mass (g)\u1e41in\u00a0=\u00a0Mass flow rate (g/s)Argon and hydrogen gases (1 lpm each) were used as the carrier gases and were introduced into the system through mass flow controllers (Model - MC-10SLPM-TFT, ALICAT, USA). Argon gas provided an inert condition in the reactor. Hydrogen controlled the cracking of hydrocarbon and maintained a reducing atmosphere in the reactor. The aerogel produced in the reactor was taken out of the outlet and was passed through a water bath inside the glove box to condensate and form fiber (see supporting video SV1). A rotating drum was used to collect the fiber in a continuous manner by synchronizing the drum speed with the formation rate of aerogel. Unidirectional injection of all the precursors (hydrocarbon, catalyst and promoter) through inlet is the conventional method of FC-CVD, which has been followed by almost all researchers in literature. The bi-directional catalyst injection was achieved by sublimating ferrocene separately in a preheater and introducing the ferrocene vapor at the outlet which was taken into the heating zone by the backflow gas. Additional details and snapshot are given in section 3.1.3. The mass flow rate of the ferrocene was varied using Eq (1-3) as discussed earlier. Schematic of bi-directional catalyst injection for CNT fiber synthesis are shown in Fig.\u00a01\n.The Raman spectra of the CNT fiber were recorded using an alpha 300R confocal Raman spectrometer (WITec GmbH, Germany). 514\u00a0nm diode laser with a max power of 80\u00a0mW was used as the light source for recording spectra in a CCD-based ultra-high-throughput efficiency spectrometer which was cooled to \u221260\u00a0\u00b0C. Thermogravimetric analysis (TG) was carried out on CNT fiber samples using Labsys EVO, SETRAM, France, in an oxygen atmosphere with a gas flow rate of 20\u00a0ml/min and a heating rate of 10\u00a0\u00b0C /min up to 1000\u00a0\u00b0C. The microstructural imaging of the samples was carried out using GEMINI SEM 300 field emission scanning electron microscope (Carl Zeiss, Germany). Imaging was carried out at an acceleration potential of 2\u00a0kV and a working distance of 4.6\u00a0mm. The electrical conductivity of CNT fibers was measured using a four point probe technique on the Ossila Four-Point Probe System (Ossilla Ltd., United Kingdom).Conventional unidirectional catalyst injection in FC-CVD was carried out to obtain a benchmark or reference for comparing it with the bi-directional catalyst injection. The conversion of carbon into CNT fiber in unidirectional injection for different catalyst concentrations was calculated Eq\u00a0(4).\n\n(4)\n\n\nC\na\nr\nb\no\nn\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n\n(\n%\n)\n\n=\n\n\nM\na\ns\ns\n\no\nf\n\nC\nN\nT\n\nf\ni\nb\ne\nr\n\n\n(\n\na\nf\nt\ne\nr\n\na\nc\ni\nd\n\np\nu\nr\ni\nf\ni\nc\na\nt\ni\no\nn\n\n)\n\n\n\nM\na\ns\ns\n\no\nf\n\nc\na\nr\nb\no\nn\n\ni\nn\n\np\nr\ne\nc\nu\nr\ns\no\nr\n\n\n\u00d7\n100\n\n\n\n\nThe carbon conversion values of CNT fiber were based on CNT fiber weight after acid purification for 10\u00a0h in concentrated HCl to prevent error to weight by iron followed by partial oxidation at 400 \u00b0C to remove the amorphous carbon. Fig.\u00a02\n\n(a) depicts the variation in carbon conversion with ferrocene wt% in unidirectional injection mode. The major chemical reaction involved in the formation of CNT is given in Eq (5-9) [23\u201325].\n\n(5)\n\n\nFe\n\n\n(\n\n\nC\n5\n\n\nH\n5\n\n\n)\n\n2\n\n\u2192\nFe\n+\n\nH\n2\n\n+\nC\n\nH\n4\n\n+\n\nC\n5\n\n\nH\n6\n\n\u2026\n\n(\n\nother\n\nhydrocarbon\n\n)\n\n\n\n\n\n\n\n(6)\n\n\n\nC\n4\n\n\nH\n4\n\nS\n\u2192\nS\n+\nHCC\n\u2212\nCH\n=\nC\n\nH\n2\n\n\n\n\n\n\n\n(7)\n\n\nC\n\nH\n3\n\nC\n\nH\n2\n\nOH\n\u2192\nC\n\nH\n4\n\n+\nCO\n+\n\nH\n2\n\n\n\n\n\n\n\n(8)\n\n\n2\nCO\n\u2192\nC\n\nO\n2\n\n+\nC\n\n\n\n\n\n\n(9)\n\n\nC\n\nH\n4\n\n\u2192\nC\n+\n2\n\nH\n2\n\n\n\n\n\nIn the CNT formation, initially ferrocene starts decomposing to iron Eq\u00a0(5)) followed by the decomposition of thiophene to S (Eq\u00a0(6)). The S prevents the agglomeration of iron and also forms FeS, FexS, Ls as per the temperature and S concentration according to phase diagram as shown in Fig.\u00a02\n(b). Lee et\u00a0al. [10] has shown FeS formed on catalyst surface acts as nucleation site for CNT formation and Weller et\u00a0al. [4] has shown S rich liquid (Ls) phase acts as nucleation site. The ethanol cracks to carbon (Eq\u00a0(7)-(9) at higher temperatures after the formation of catalyst and diffuses into catalyst to form CNT. The catalyst also lowers the barrier for hydrocarbon cracking and makes cracking on catalyst preferable [26].Unidirectional catalyst injection was carried out for different catalyst precursor concentrations from 0.5\u00a0wt% to 4.5\u00a0wt%. Experiments were not continued below 0.5\u00a0wt% due to excess amorphous carbon (soot) generation. Ferrocene beyond 4.5\u00a0wt% could not produce spinnable CNT aerogel as higher ferrocene causes an decrease in C/Fe ratio which leads to the inability of the CNT to form sufficient bundles for aerogel formation. Weller et\u00a0al. [4] have reported a minimum C/Fe requirement of 300 for the production of spinnable CNT aerogel. In the present study ferrocene beyond 4.5\u00a0wt% causes C/Fe to go below 160 which is much lower the threshold proposed by Weller et\u00a0al. [4]. The conversion of CNT fiber is extremely low in comparison to conventional CNT powder where the conversion is above 60% [4]. This low conversion is a result of extremely dilute carbon concentration in the reactor along with the presence of hydrogen, which lowers the cracking rate. Amorphous soot formation is highly susceptible in CNT fiber synthesis as the process is carried out at a high temperature in comparison to conventional powder synthesis. Hence, increasing the carbon concentration in the reactor is not a good option but increasing catalyst concentration is possible which can result in increased CNT nucleation resulting in enhanced conversion. The unconverted hydrocarbon leaves the reactor along with carrier gas as lower hydrocarbon [27] and also as carbonaceous particles [6]. In literature, it can also be seen that catalyst utilization has been reported less than 1%. More than 99% of the catalysts do not take part in CNT formation and get either absorbed in aerogel as impurity [6] or deposit at reactor wall [4]. Catalyst particles also leave the reactor along with the carrier gas as particulate [6]. The low utilization also suggests that increased catalyst concentration may be required. But as observed in the present study, for unidirectional injection, beyond 1\u00a0wt% of catalyst precursor, the conversion starts to drop. It can be seen from Fig.\u00a02\n(c and d) that catalyst particles tend to form large agglomerate in a higher concentration of ferrocene (encircled in Fig.\u00a02d), which is incapable of nucleating CNT. Asli et\u00a0al. [28] have observed a similar drop in conversion at higher concentrations of catalyst which was attributed to the catalyst agglomeration. The formation of catalyst particles and their agglomeration occur at two places in the reactor as per Hoecker et\u00a0al. [29]. Initially it occurs towards inlet side before the highest temperature zone and also towards the outlet after the highest temp zone, in between the catalysts undergo evaporation. The schematic of the agglomeration process is depicted in Fig.\u00a03\n\n(a).\nLow catalyst concentration results in low conversion due to a smaller number of interactions between the catalyst and the carbon particles. Increased catalyst concentration results in an increase in conversion due to increase in total numbers of interaction between catalyst and carbon. But beyond a certain concentration catalyst also tends to agglomerate resulting in reduced conversion.The increase in catalyst size with catalyst concentration can be mathematically determined by Eq\u00a0(10)\n[30].\n\n(10)\n\n\n\nd\np\n\n=\n\n\n\n(\n\n\n3\n\u03b2\n\nV\n\ne\nf\nf\n\n\n\nm\n\u02d9\n\n\n\n\u03c1\n\u03c0\n\n\nQ\n\n2\n\n\n\n)\n\n\n\n1\n3\n\n\n\n\n\nWhere,\ndp\n =particle size (m)\n\u03b2\u00a0=\u00a0coagulation kernel (m3 s\u22121)\nVeff\n =effective volume (m3)\n\u1e41\u00a0=\u00a0Flow rate (kg/s)\n\u03c1\u00a0=\u00a0density (kg/m3)\nQ\n2\u00a0=\u00a0Volumetric flow rate (m3/s)Feng et\u00a0al. [30] have utilized the above Eq\u00a0(10) for determining the Au nanoparticle size in a flow system accurately. It can be seen that the catalyst size is directly proportional to the catalyst flow rate (\u1e41). This direct relation shows the reason for agglomeration coupled with a loss of conversion in higher catalyst precursor flow. Hence, it can be inferred that increased concentration of catalyst without agglomeration is required for improved conversion. Even though the effect of temperature on catalyst agglomeration is not given directly in Eq\u00a0(10) it affects the coagulation kernel term (\u03b2). The coagulation kernel increases with temperature due increase in Brownian diffusion and thermophoretic convection [31]. The coagulation kernel for the Fe\u2013S catalyst system as a function of temperature is not available in the literature. The approximate trend of catalyst size along the tube is calculated by replacing Veff\n with A*L where A is the cross-section area and L is the distance along the tube. Coagulation kernel of pure metal at room temperature condition given by Feng et\u00a0al. [30] was used for calculation and the calculated catalyst size as a function of distance along reactor tube is given in Fig.\u00a03\n(b). The effect of temperature on carbon conversion is shown in Fig.\u00a03\n(c). It can be seen that carbon conversion increases with temperature up to 1200\u00a0\u00b0C which can be attributed to increase in reaction rates and diffusion. At temperatures beyond 1200\u00a0\u00b0C the carbon conversion starts dropping which can be due to increase in non-catalytic cracking of hydrocarbon and also increase in agglomeration.In order to achieve a higher concentration of catalysts without agglomeration leading to higher carbon conversion, bi-directional catalyst injection was thought of based on CFD results.In bi-directional catalyst injection, catalyst precursor was also introduced through the outlet of the reactor tube along with the inlet. To know whether the catalyst precursor introduced in the outlet can reach up to the heating zone of the reactor, CFD studies were carried out. COMSOL multi-physics software was used for carrying out CFD analysis. A 3D model comprising both the reactor tube and glove box was generated as flow from the glove box could enter the reactor tube. In addition, the glove box was maintained at a slight positive pressure with argon purging to prevent ingress of atmospheric gases which could also alter the flow behavior. The flow was determined by coupling a weakly compressive fluid flow condition model with heat transfer. Radiation in the model was incorporated by the Hemicube method in-build in COMSOL software. The flow of catalyst and CNT in the reactor were neglected and only the flow of carrier gas was analyzed. The governing equations utilized in the model are given in Eq(11\u201314):\n\n(11)\n\n\n\n\nd\n\u03c1\n\n\nd\nt\n\n\n+\n\u2207\n.\n\n(\n\n\u03c1\nu\n\n)\n\n=\n0\n\n\n\n\n\n\n(12)\n\n\n\u03c1\n\n\nd\nu\n\n\nd\nt\n\n\n\n+\n\u03c1\nu\n.\n\u2207\nu\n=\n\u2212\n\u2207\np\n+\n\u2207\n.\n(\n\u03bc\n\n\n(\n\n\u2207\nu\n+\n\n\n\n(\n\n\u2207\nu\n\n)\n\n\nT\n\n\u2212\n\n2\n3\n\n\u03bc\n\n(\n\n\u2207\n.\nu\n\n)\n\nI\n\n)\n\n+\n\u03c1\ng\n\n\n\n\n\n\n(13)\n\n\n\u03c1\n\nC\np\n\nu\n.\n\u2207\nT\n+\n\u2207\n.\n\nq\nh\n\n=\n\nQ\nh\n\n\n\n\n\n\n\n(14)\n\n\n\nq\nh\n\n=\n\u2212\nk\n\u2207\nT\n\n\n\nWhere,\n\u03c1\u00a0=\u00a0Density (kg/m3)\n\u03bc\u00a0=\u00a0Dynamic viscosity (Pa\u00b7s)\nu\u00a0=\u00a0Velocity vector (m/s)\np\u00a0=\u00a0Pressure (Pa)\ng\u00a0=\u00a0Gravitation acceleration (m2/s)\nCp\n\u00a0=\u00a0Specific heat capacity at constant pressure (J/(kg\u00b7K))\nT\u00a0=\u00a0Absolute temperature (K)\n\n\nq\nh\n\n= Heat flux vector (W/m2)\nk\u00a0=\u00a0thermal conductivity (W/(m\u22c5K))\nQh\n\u00a0=\u00a0heat sources (W/m3)The buoyancy which is essential for generating backflow in the reactor is taken into account by the addition of a body force \u03c1g in the Navier\u2013Stokes equation (Eq\u00a0(12)). The density (\u03c1) is given as a function of temperature (from the COMSOL database) and gravitational force is given with its directional as per the horizontal configuration of the furnace. The change in density with temperature generates a force in the gravitational direction and generates buoyancy.The temperature profile and the flow pattern in the FC-CVD system are shown in Fig.\u00a04\n\n(a & b). It can be observed that backflow from the bottom side of the reactor tube in the outlet is possible. The gas from the outlet is capable of reaching high temperature region of the reactor. The maximum temperature that backflow gas can attain is above 1000\u00a0\u00b0C, which is a sufficiently high temperature for CNT nucleation.The temperature distribution along the centerline of the reactor is given in Fig.\u00a05\n\n(a). The maximum distance traveled by the backflow gases can be determined by finding the distance where the axial component of velocity becomes negative to positive which signifies a direction change and hence the end of the backflow. The axial component of velocity is given in Fig.\u00a05\n(b). It can be seen that the backflow gases can traverse 320\u00a0mm into the reactor which enables the gases to reach temperature above 1000 \u00b0C. As the minimum nucleation temperature of CNTs is 600 \u00b0C which is achieved at 276\u00a0mm [32], the backflow gases can nucleate CNTs for 88\u00a0mm (combining inward and backward travel of gases). The flow of gases has a residence time of \u223c1\u00a0s in the CNT nucleation temperature zone. The tendency of a system to generate backflow can be quantified by the density variation of gasses in the system. In the present study, a maximum density variation (Fig.\u00a05\n(c)) of 0.682\u00a0kg/m3 was seen.Many researchers use vertical FC-CVD for the production of CNT fibers, so we have explored the possibility of bi-directional catalyst injection in a vertical FC-CVD reactor as well. CFD of a vertical reactor was carried out with the same carrier flow and temperature as the present study. The CFD results (Fig.\u00a06\n) indicate that the backflow from the glove box to the heating zone is absent which makes bi-directional catalyst injection impossible. The temperature gradient and the resulting buoyancy are only able to generate a recirculation near the inlet of the reactor in a vertical reactor. Presence of convective backflow is not necessary for the formation of CNT aerogel, as the CFD analysis indicates the absence of convective backflow in a vertical reactor and many researchers were able to successfully produce CNT fiber in a vertical reactor [21, 33]. The formation of CNT aerogel primarily depends on the CNT length, the number density of CNT, and the impurity level. The CNT does not stick to the reactor wall due to thermophoresis which causes the migration of particles from the hot reactor wall to the gas stream [19].The photograph of the reactor outlet shown in Fig.\u00a07\n\n(a) indicates that the aerogel comes out only from the top region of the reactor tube when the aerogel is left undisturbed (without fiber production). The bottom half of the reactor is empty and has a clear path for the backflow gas to reach the heating zone without interference from aerogel. The snapshots of CNT fiber synthesis by unidirectional catalyst injections and bi-directional catalyst injections are shown in Fig.\u00a07\n(b). The snapshots show that the CNT aerogel is darker and the resulting fiber is thicker for bi-directional catalyst injection compared to unidirectional injection, which indicates higher conversion of carbon in the former case. The bulk densities of the aerogel are \u223c2.3\u00a0kg/m3 and \u223c3.4\u00a0kg/m3 for uni and bi-directional catalyst injections, respectively. The video of CNT fiber synthesis by bi-directional catalyst injection is given in the supplementary video (SV1). The maximum conversion in bi-directional catalyst injection in FC-CVD is shown in Fig.\u00a07\n(c). It can be observed that a 56% increase (corresponding to value 6.49) in conversion could be achieved when bi-directional injection was employed. The carbon conversion in bi-directional catalyst injection for different ferrocene flow rates at the outlet is given in Fig.\u00a07\n(d). The aerogel comes out of the reactor as a hollow tube filled with hot carrier gas which makes the aerogel go upwards by buoyancy. This upward movement of aerogel gives us some gap at the outlet for inserting the ferrocene vapor injection tube as shown in Fig.\u00a07\n(b). The gap can be increased by reducing the winding speed.The mechanism of improvement of conversion in bi-directional catalyst injection is proposed schematically in Fig.\u00a08\n. In the case of unidirectional injection, the additional catalyst beyond 1% from inlet resulted in a drop in carbon conversion, whereas, for bi-directional injection additional catalyst from the outlet side resulted in improvement in carbon conversion. In the previous section, it was concluded that additional catalyst concentration makes CNT nucleation even worsen by increasing catalyst agglomeration for unidirectional injection. This indicates that catalysts injected from outlet nucleate CNTs independently without getting affected by the agglomeration of catalyst from the inlet. The catalyst vapor can reach the reaction zone with the help of the backflow from the outlet as predicted in the CFD studies. After reaching the region in the reactor with a temperature more than required, the catalyst particles nucleate CNTs. This enables the bi-directional catalyst injection to have two separate CNT nucleation zones enabling higher conversion in the reactor in comparison to unidirectional catalyst injection which has only one CNT nucleation zone.Raman spectra of the CNT fibers (with highest conversion) synthesized by unidirectional and bi-directional catalyst injection are shown in Fig.\u00a09\n\n(a). The Raman spectrum of the CNT fiber from unidirectional catalyst injection shows the D, G, and 2D peaks, whereas, the Raman spectrum of the CNT fiber from bi-directional catalyst injection shows additional radial breathing mode (RBM) peak. The RBM peak is the signature of SWCNT and this peak is absent in MWCNT as the large number of walls restrict the radial vibration [34]. The G peak is due to the vibration of sp2 hybridized structure, D peak is due to disorder in the sp2 hybridized structure, and 2D is the overtone of D peak. The CNTs from the unidirectional catalyst injection are multi-walled CNT (MWCNT) that can be inferred from the lack of RBM peak. For the bi-directional injection, the CNTs are a mixture of MWCNT and SWCNT. The presence of RBM peak and splitting of G peak into G\n\u2212 and G\n+ confirms the presence of SWCNTs [35]. The Fit of G\n\u2212 peak can determine whether the SWCNTs are metallic or semi-conducting. A Lorentz Fit was observed for G- peak which indicates the semi-conducting nature of SWCNTs [35, 36]. The distribution of SWCNTs and MWCNTS in the CNT fiber was obtained by performing Raman mapping and is shown in Fig.\u00a09\n(b & c)). This indicates that SWCNTs are formed from the catalysts injected from the outlet that back flowed into the reactor. The CFD derived flow pattern (Fig.\u00a04\nb) indicates that backflow gas from the outlet only penetrated the heating zone for a short distance hence spending less time in the reaction zone. The SWCNT is produced when small patches of S-rich liquid is formed on the catalyst surface which enables nucleation of SWCNT [4]. Generally, a low concentration of S causes SWCNT since at a high S concentration S rich liquid is high enough to cover entire catalyst promoting MWCNT. The amount of S-rich liquid depends on the temperature and S concentration according to the phase diagram (Fig.\u00a03(a)). The low residence can reduce the catalyst agglomeration and also S intake into the catalyst which can reduce S rich liquid on the catalyst, hence promoting SWCNT formation. The distribution of ID/IG ratios for the unidirectional and the bi-directional injections is shown in Fig.\u00a09\n(d) as comparative histograms. ID/IG ratios of 0.4\u00a0\u00b1\u00a00.2 and 0.4\u00a0\u00b1\u00a00.3 were obtained for unidirectional and bi-directional flows, respectively. Bi-directional catalyst flow has a larger standard deviation of ID/IG ratios due to multiple types of CNTs.\nFig.\u00a010\n\n(a) shows the mass loss of CNT fiber samples performed by TG under oxygen atmosphere. The oxidation behavior of various structures of carbon can be utilized for determining the phase composition of CNT fiber. Amorphous carbon generally oxidizes below 400\u00a0\u00b0C and CNT oxidize at a higher temperature [37]. Fe does not convert in gas after oxidation hence the mass remaining after TG analysis is considered as the Fe2O3 which can be used to calculate the fraction of Fe [38]. The mass fraction of amorphous carbon and CNT can be determined by finding the Differential TG (DTG). The DTG of the samples from unidirectional and bi-directional catalyst injection are shown in Fig.\u00a010\n(b & c). The area under the peak of DTG plots below 400\u00a0\u00b0C gives the amorphous carbon fraction and peaks at higher temperature corresponds to CNT mass fraction [39]. CNT shows multiple peaks in DTG. The peaks above 600\u00a0\u00b0C are considered highly crystalline and peaks at lower temperatures have lower crystallinity [40]. The unidirectional catalyst injection sample has one CNT peak in the 500\u00a0\u00b0C range and another peak in the 600\u00a0\u00b0C range, whereas the bi-directional catalyst injection sample has two peaks in the 500\u00a0\u00b0C range and 2 peaks in the 600\u00a0\u00b0C range. More amount of highly crystalline CNTs in the case of bi-directional injection is because of the presence of SWCNT in this sample. The calculated phase fraction of various phases in CNT fiber is given in Fig.\u00a010\n(d). It can be seen that bi-directional catalyst injection results in lower amorphous content. The residual iron in the CNT fiber is also higher for bi-directional catalyst injection. These can be attributed to the higher catalyst utilization in the bi-directional injection.The SEM images (Fig.\u00a011\n\n(a-b)) were taken for the CNT aerogel samples (unidirectional and bi-directional) that were not condensed to make fiber. This was done as the condensation in the bath could change the macro-structure of the CNT aerogel making capture of the structural changes due to catalyst injection difficult. SEM micrographs indicate that the content of iron was more for the sample obtained by bi-directional injection though the CNT bundle diameter was similar to that of unidirectional injection. Bundle diameters of 41.75\u00b110.67\u00a0nm and 44.04\u00b111.01\u00a0nm were obtained for unidirectional and bi-directional flow respectively and their distribution is shown in Fig.\u00a011\n(c) (ImageJ software was utilized for determining bundle size from SEM image [41, 42]). The high resolution (HR) TEM images of the CNT aerogel samples (unidirectional and bi-directional) are shown in Fig.\u00a011\n(d-e). The HRTEM shows the multi-walled nature of the synthesized CNT aerogel. SWCNT was additionally detected (inset Fig.\u00a011(e)) for bi-directional injection.\nTable\u00a01\n shows the electrical conductivities of CNT fibers grown by unidirectional and bi-directional catalyst injections. It can be seen that the electrical conductivities of the fibers from unidirectional catalyst injection was slightly higher than that of the fibers from bi-directional injection. The electrical conductivity of CNT fiber is generally controlled by the impurities, alignment of CNT, and conductivity of individual CNT [43]. The changes in conductivity for the present case comes from the type of CNTs. The CNT fiber produced by bi-directional catalyst injection consists of MWCNTs and SWCNTs having semi-conductive nature. On the other hand, unidirectional catalyst injection produces only MWCNTs which are exclusively metallic making CNT fiber produced by unidirectional catalyst injection more conductive.The SEM micrographs of the CNT fiber produced by bi-directional catalyst injection after purification are shown in Fig.\u00a012\n (a and b). The SEM micrograph (Fig.\u00a012 (a and b)) indicates the absence of catalyst particles and amorphous particles after purification. The CNTs are also aligned in the rolling / gas flow direction which can enhance the electrical and mechanical properties. Issman et\u00a0al. [44] reported improvement in the electrical conductivity and tensile strength with improvement in CNT alignment.CNT in the form of powder was recovered when produced by only adding catalyst in the outlet. The SEM and Raman spectra of CNT powder formed are given in Fig.\u00a012 (c and d). CNT produced contained a significantly large amount of amorphous carbon and was single-walled in nature. Even though some CNT powder could be recovered from the reactor tube, aerogel formation did not occur as the hydrocarbon introduced from the inlet would crack and form soot in absence of the catalyst in the inlet region. The excess soot formed in the inlet region would mix with the limited CNT formed in the outlet making self-assembly into fiber impossible. The soot thus formed can also deactivate the catalysts introduced in the outlet by coating them. Hence, due to the soot interference, it is not possible to make fiber by sending catalyst only from the outlet.A novel concept of bi-directional injection of catalysts in FC-CVD for the synthesis of CNT fiber demonstrates a 56% improvement in carbon conversion with respect to unidirectional injection. This method does not require any changes in the hardware of the synthesis system. CFD analysis predicts that the convection vortex naturally present in the system can be utilized to implement bi-directional injection. CNT fiber synthesized by bi-directional catalyst injection has lower amorphous carbon content and is composed of both SWCNT and MWCNT. The ID/IG ratio and the CNT dimensions are similar for both unidirectional and bi-directional catalyst injection. The electrical conductivity for the CNT fiber obtained by bi-directional injection drops slightly due to semiconducting nature of the SWCNTs present in the fiber.The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing studyThis research was funded by Bhabha Atomic Research centre, Mumbai, IndiaThere are no conflicts to declareSupplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cartre.2022.100211.\n\n\nImage, video 1\n\n\n\n", "descript": "\n                  Carbon nanotube fiber (CNT fiber) synthesized through floating catalyst chemical vapor deposition (FC-CVD) is one of the strongest man-made fibers ever synthesized. The poor carbon conversion in the FC-CVD process is one of the major hurdles in its commercial deployment. In this work, we have employed a novel method of bi-directional catalyst injection where catalysts were injected from both inlet and outlet sides of the reactor. The injection of the catalyst from the outlet into the reactor reaction zone was possible by a backflow caused by the convection vortex as predicted by the computational fluid dynamics (CFD) analysis. Bi-directional catalyst injection was able to enhance the carbon conversion by 56% compared to conventional unidirectional injection. CNT fibers obtained in bi-directional catalyst injection are a mixture of multi-walled (MW) and single-walled (SW) CNTs whereas unidirectional catalyst injection resulted in MWCNTs only. The average CNT bundle diameter dimensions were similar in both unidirectional and bi-directional catalyst injection. The amorphous carbon content was lower for bi-directional catalyst injection. A mechanism for the improvement of carbon conversion in bi-direction catalyst injection has been proposed.\n               "}
{"full_text": "The substantial reserves of gas resources worldwide necessitate technological advances to obtain cost-effective clean energy and chemical feedstocks [1]. Methane, the most abundant component of natural gas, typically produces energy by forming CO2 or by itself as a greenhouse gas. Therefore, methane requires careful management from an environmental perspective. The synthesis of transportable liquids can effectively control the methane reserves diverging in depopulated areas. A key challenge is to determine a conversion process that can decrease the stability of methane and cleave its C\u2013H \u03c3 bond, which requires high energy (438.3\u00a0kJ\u00a0mol\u22121). Therefore, for several decades, the industry has adopted methane reforming-based multistage processes that co-feed steam or oxygen for producing hydrogen and chemicals such as methanol and fertilizers [2]. Contrary to expectations, only 7.8% of methane (in the United States of America) is currently used as a non-combustion source in the industry, because the existing process requires a substantial capital investment owing to its low energy efficiency (up to 60%) [3,4]. Therefore, energy-efficient conversion processes must be developed to use methane as an alternative to petroleum [5,6].Non-oxidative conversion of methane is the desirable chemical route because it potentially enhances the carbon efficiency through a simplified process for hydrocarbon commodity chemicals [7,8]. Methane exhibits high selectivity toward olefins and aromatics through catalytic conversion owing to the functionality of metallic [9] and metal\u2013acid [10] sites for selective C\u2013H activation and consecutive C\u2013C coupling. However, solely optimizing the catalytic active sites does not ensure an increase in hydrocarbon yields, because this approach is thermodynamically unfavorable. Engineering approaches for improving the aerodynamics [11,12] or equilibrium yields of hydrocarbons [13,14] can increase the economic viability of the process. The efficient control of methyl radicals, with a lifetime of microseconds during the reaction, is a driving force that determines the productivity [15]. Excessive carbon formation is inevitable unless the partial pressure of unstable methyl radicals is lowered during short reaction times [15]. Metallic nanoclusters catalyze methane conversion; however, coke is formed from the radicals on less sophisticated surfaces [16].Guo et al. [17] were the first to demonstrate rigorously designed lattice-confined single iron sites that can non-oxidatively convert methane to olefins, aromatics, and hydrogen (MTOAH). In their study, the catalytic reactor was stable at\u00a0>\u00a0950\u00a0\u00b0C, which thermodynamically favored coke formation; an ethylene selectivity of 48.4% was achieved at a maximum methane conversion of 48.1% at 1090\u00a0\u00b0C [17]. This unprecedented selective behavior for C\u2013H activation signifies the importance of site isolation and neutralization of surface defects [17,18]. Based on techno-economic analysis, the conversion of MTOAH is economically viable if the coke formation is less than 20% and the minimum conversion of the product is 25% [19]. \u0160ot et al. [20] used high-surface-area silica with Fe2+ single sites for the MTOAH process at 1000\u00a0\u00b0C and found that iron sites selectively inhibit the surface reactivity and enhance the hydrocarbon selectivity. According to detailed theoretical analyses of the surface reaction mechanism, the Fe2+ single sites and adjacent carbon sites can be sensitively involved in C2 formation through the activation of methane, and can subsequently serve as dual sites for C\u2013C coupling and hydrogen transfer [21\u201323]. The design of In- [24\u201326], Pb- [27], Pt- [28\u201332], Pd- [33,34], and Ni-based [35,36] active sites has been further studied to increase the hydrocarbon selectivity while decreasing the coke selectivity. Li et al. [32] designed atomically thin Pt nanolayers anchored on two-dimensional molybdenum titanium carbide (MXene) and found that the altered adsorption properties of these Pt active sites promote methane coupling to ethane/ethylene with 98% selectivity at 750\u00a0\u00b0C.Subsequent studies were conducted to demonstrate that the modulation of gaseous free-radical chemistry for iron-based catalytic surfaces is a prerequisite for increasing the hydrocarbon yield. Hydrogen removal from hydrogen-permeable tubular membrane reactors during the MTOAH process can favorably drive the gas-phase reaction and consequently increase methane conversion [37\u201339]. The millisecond catalytic wall reactor provides an appropriate Fe/SiO2\n[40] or Fe/SiC [41,42] surface on the reactor wall to effectively activate methane in short contact times and enhance gas-phase reactions within the reactor compartment. The methane activation rate can be uniformly improved by hydrogen radicals, which are typically formed in the presence of hydrocarbons that can generate H-radicals in the gas-phase reaction [41]. Dong et al. [43] recently designed a Joule-heating-based programmable heating and quenching system involving rapid switching between low (815\u00a0\u00b0C) and high temperatures (2000\u00a0\u00b0C). Consequently, the temperature-dependent reaction pathway between gas-phase reactants and the adsorbed surface species could be precisely controlled. A C2 product selectivity exceeding 75% was obtained at a methane conversion of approximately 13%. Postma and Lefferts [44] proposed a practical approach to separate the catalytic and gas-phase reaction zones of a tubular reactor, wherein the axial temperature profile and residence times upstream and downstream of the catalyst bed were varied to increase the methane performance. However, the difference in reactivity for complex catalysts and gas-phase reactions in the MTOAH process has not been experimentally verified and elucidated thus far.In this study, we aimed to determine the effect of the catalyst surface on the selective production of hydrocarbons from methane based on systematically controlled parametric studies of the MTOAH process. We further investigated the selective conversion of C2 species (ethane, ethylene, and acetylene) from 500 to 1020\u00a0\u00b0C to elucidate the secondary reactivity of the primary product of methane conversion. Finally, we attempted to maximize the reactivity by optimizing the ratio of catalytic to gas-phase reactions in the reactor.Fe(NO3)3\u00b79H2O (Sigma Aldrich, 216828) and SiC (Alpha Aesar, A14470) were used to prepare SiC catalysts doped with 0.17, 0.26, 0.32, 0.59, and 1.25 wt% Fe via wet impregnation; the samples are denoted 0.1Fe, 0.2Fe, 0.3Fe, 0.5Fe, and 1Fe, respectively. Fe(NO3)3\u00b79H2O was dissolved in de-ionized water and mixed with SiC, whose amount was 50 times lower than that of the aqueous Fe solution. The mixture was stirred at 60\u00a0\u00b0C for 6\u00a0h at 120\u00a0rpm and evaporated using a rotary evaporator (RV 10 digital V, IKA). The final solid was dried overnight at 110\u00a0\u00b0C and calcined at 550\u00a0\u00b0C for 4\u00a0h at a ramping rate of 4\u00a0\u00b0C\u00a0min\u22121. The actual Fe loading in the catalyst was analyzed through inductively coupled plasma\u2013optical emission spectroscopy (ICP-OES; iCAP 6300 Duo, Thermo Fisher Scientific), and more than four batches of catalyst were prepared to confirm reproducibility.Solid density was determined using an AccuPyc II 1340 pycnometer (Micromeritics) with He as a gas displacement medium. Void space in the catalytic reactor was calculated by subtracting the apparent density of the sample from the reactor space in the heating zone of the furnace.All samples were subjected to nitrogen physisorption at\u00a0\u2212\u00a0196\u00a0\u00b0C in an ASAP-2420 system (Micromeritics) for a relative pressure (P/P\n0) range of 0.01\u20130.30, and the Brunauer\u2013Emmett\u2013Teller (BET) equation was used to determine the specific surface areas (S\nBET). Before measurement, each sample was degassed at 90\u00a0\u00b0C for 30\u00a0min and subsequently heated at 150\u00a0\u00b0C for 6\u00a0h under a vacuum. The catalyst amounts were adjusted; consequently, the relative error was minimized when constant C in the BET equation exceeded 100.To analyze the crystallinity of each sample, powder X-ray diffraction (XRD) patterns were obtained using an Ultima IV diffractometer (Rigaku) with Cu K\n\u03b1 radiation (\u03bb\u00a0=\u00a00.154\u00a0nm) at 40\u00a0kV and 40\u00a0mA. The crystal structures were assigned according to the Inorganic Crystal Structure Database (ICSD).To confirm the surface species of each sample, X-ray photoelectron spectroscopy (XPS) was performed using an AXIS Nova (Kratos) instrument equipped with a monochromatic Al-K\n\u03b1 X-ray source. The acceleration voltage and the pass energy were 15\u00a0keV and 40\u00a0eV, respectively. The binding energies were calibrated to the C1s peak at 284.8\u00a0eV.The coke deposition on the spent samples was determined via thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) using an SDT Q600 instrument (TA Instruments) in the temperature range of 30\u2013900\u00a0\u00b0C at a heating rate of 5\u00a0\u00b0C\u00a0min\u22121 under a constant airflow of 100\u00a0mL\u00a0min\u22121. The total weight loss after 200\u00a0\u00b0C was regarded as the amount of coke.The morphologies of the spent catalysts were investigated through transmission electron microscopy (TEM) using a FEG S/TEM instrument (Talos F200S) operated at 200\u00a0kV. Scanning transmission electron microscopy (STEM) was performed using a TalosTM F200S device (FEI) containing a 200\u00a0kV field-emission gun. For energy-dispersive X-ray spectroscopy (EDS) analysis, a high-angle annular dark-field (HAADF) detector (Super-X EDS) with 0.16\u00a0nm beam resolution was selected.To analyze the nature of coke in the spent catalysts, Raman spectroscopy was performed (DXR3 Raman microscope, Thermo Fisher Scientific) under 30\u00a0\u00b0C with 532\u00a0nm laser excitation. For all measurements, the excitation power and exposure time were 2 mW and 120\u00a0s, respectively. Measurements from at least five runs were averaged at different positions.Reaction measurements of methane were performed in a vertical fixed-bed quartz reactor (inner diameter\u00a0=\u00a04\u00a0mm) at ambient pressure (1\u00a0bar). To ensure plug flow behavior in the reactor, sieved catalyst particles (425\u2013850\u00a0\u03bcm) were charged into a reaction zone supported by a minimal amount of quartz wool. Three R-type thermocouples were placed in direct contact with the outer surface of the reactor to ensure a uniform temperature profile in the catalytic reactor in the furnace. The void space between the catalyst beds was varied to modify the gas-phase reaction. To minimize the secondary reaction, a quartz rod (inner diameter\u00a0=\u00a03\u00a0mm) was placed at the bottom of the reactor. The downstream lines were heated to 150\u00a0\u00b0C to prevent partial condensation of hydrocarbons. The catalytic reactor was ramped up to the reaction temperatures at a rate of 6\u00a0\u00b0C\u00a0min\u22121 in a He flow. Methane gas containing 10% Ar was subsequently allowed to flow into the reactor, and mass flow controllers (5850E, Brooks Instrument) were used to achieve the desired gas hourly space velocity (GHSV). Non-oxidative conditions were established for the system, with all gases except C2 (ethane, ethylene, and acetylene) passing through oxygen/moisture traps (OT3-4, Agilent). Reaction measurement was performed at 1020\u00a0\u00b0C for 10.2\u00a0h. The reaction rate was measured as a function of temperature from 965 to 1020\u00a0\u00b0C at a rate of 0.10\u00a0\u00b0C\u00a0min\u22121 after the catalyst was pretreated with 90% CH4/10% Ar at 1020\u00a0\u00b0C for 1\u00a0h to stabilize the catalytic surface. Similarly, the reaction measurement of C2 (ethane, ethylene, acetylene) was performed from 500 to 1020\u00a0\u00b0C at a rate of 0.70\u00a0\u00b0C\u00a0min\u22121 after the catalyst was pretreated with 90% CH4/10% Ar at 1020\u00a0\u00b0C for 1\u00a0h. The reactants used in the study were: 5% C2H6/5% H2/10% Ar in He balance, 5% C2H4/5% H2/10% Ar in He balance, and 1% C2H2/1% H2/4% Ar in He balance. Additionally, the reactor configuration was modified to optimize the catalytic reactor. The gas effluent from the reactor was analyzed through online gas chromatography (GC, 6500GC, Youngin) using a thermal conductivity detector coupled with a ShinCarbon ST column (Restek Corp., Catalog No. 80486\u2013800) and two flame ionization detectors coupled with an RT-Alumina BOND column (Restek Corp., Catalog No. 19756) and an Rtx-VMS column (Restek Corp., Catalog No. 49915) column. Detailed online GC analysis conditions and calculation formulae to obtain the conversion, product selectivity, and product distribution based on moles of carbon have been described in our previous report [18]. The product yield was calculated by multiplying the methane conversion by the product selectivity and dividing by 100. Herein, products of C2 or higher are classified as hydrocarbons.The catalytic activities of Fe/SiC catalysts are shown in Fig. 1\n. The reactivities of the blank reactor (B) and pure SiC were also measured at the same GHSV (637\u00a0h\u22121) for comparison. In this experiment, the particles were packed in all the reactor spaces in the heating zone of the furnace. We measured the amount of Fe in the catalyst-packed reactor to distinguish the surface reaction between SiC and Fe species. SiC, 0.1Fe, 0.2Fe, 0.3Fe, 0.5Fe, and 1Fe resulted in the presence of 0, 97, 151, 186, 327, and 695\u00a0\u00b5mol of Fe in the reactor, respectively. The internal surface of the quartz reactor could function as a catalytic surface; therefore, we minimized its reactivity by adjusting the reaction parameters. As shown in Fig. 2\n, the spent catalysts maintain the high crystallinity of SiC (ICSD #015325). The Fe species include Fe3C (ICSD #044354), which is weakly confirmed at 37.6\u00b0; the formation of Fe3C is thermodynamically favorable under the reaction conditions considered in the present study (Fig. S1). The catalysts contain crystalline SiO2 (ICSD #039830) as an impurity. SiC has a low S\nBET of 0.72\u00a0m2 g\u22121, which approximately doubles with increasing Fe loading (Table 1\n). These changes in physical properties are possibly induced by the Fe particles adhering to the SiC surface. The Fe species are dispersed on the external surface of SiC, minimizing the chemical interactions (metal\u2013support interaction) between them.The Fe/SiC catalysts are stabilized in the reaction condition before 0.2\u00a0h, whereupon CO and CO2 are observed. These gases can be generated during the carbonization of metal oxides. According to theoretical calculations using HSC Chemistry 9 (Fig. S2), 57.8% of methane can be converted to C2, benzene, and naphthalene at 1020\u00a0\u00b0C. These values are considerably higher than those presented in Fig. 1, implying that the reaction in the catalytic reactor proceeds selectively in a nonequilibrium state. Over 10.2\u00a0h, the blank reactor achieves methane conversions in the range of 2.4%\u20133.0%, which are lower than those of the catalytic reactors (Fig. 1a). The increase in the methane conversion primarily depends on the amount of Fe in the reactor, which predominantly induces coke formation in the initial 0.2\u00a0h of the reaction (Fig. 1b). The blank reactor, SiC, 0.1Fe, and 0.2Fe result in hydrocarbons with minimal coke formation during the initial 0.2\u00a0h of the reaction, wherein the highest hydrocarbon yield of 4.4% is obtained using SiC. The activities of the Fe/SiC catalysts are stabilized after 0.2\u00a0h, and the results are averaged over 1.2\u201310.2\u00a0h, except for catalysts wherein the amount of Fe in the reactor exceeds 186\u00a0\u00b5mol (0.3 Fe, 0.5 Fe, and 1 Fe), for which the results of the last 3\u00a0h are averaged (Fig. 1c). At this stage, each catalyst can begin to minimize the coke yield. The hydrocarbon yield is proportional to the coke yield, and is higher than the coke yield at a steady state. Compared with the results at 0.2\u00a0h, the hydrocarbon yield is increased by 12% and 16% with coke formation in the blank reactor and on SiC, respectively. The hydrocarbon yield is maximized when the amount of Fe in the reactor is 327\u00a0\u00b5mol, and this value slightly decreases when the Fe content is increased by 2.1 times. Based on the hydrocarbon product distribution (Fig. 1d), compared with the blank reactor, the SiC surface increases the selectivity of acetylene and ethylene. Therefore, the initial activation of methane on the surface is accompanied by C2 formation. The catalytically improved hydrocarbon formation through the decreased selectivity of C3\u2013C5, benzene, toluene, naphthalene, and alkyl-aromatics is accompanied by coke formation.The Arrhenius plot (Fig. 3\na) and apparent activation energies (E\na) (Fig. 3b) for methane consumption between 965 and 1020\u00a0\u00b0C indicate that E\na is decreased owing to the higher reactivities of the SiC surfaces compared with that of the blank reactor. The blank reactor exhibits an E\na of 388.5\u00a0kJ\u00a0mol\u22121, which is similar to the values for the thermal decomposition of methane (362\u2013422\u00a0kJ\u00a0mol\u22121) [45]. Packing the blank reactor with pure SiC results in a 29.1% decrease in Ea\n, which further decreases when SiC is doped with Fe. The pre-exponential factor (A) of the blank reactor (Fig. 3b) is comparable to that of the gas-phase unimolecular reaction (1013 s\u22121) [46]. The value of A decreases in the presence of the SiC surface, and further decreases with the increasing amount of Fe in the reactor. The experimentally obtained A values are strongly influenced by surface elementary steps including adsorption, surface diffusion, surface reactions, or desorption [46]. The gradual decrease in E\na and A along the surface of Fe/SiC catalysts indicates that gas\u2013solid reactions substantially contribute to the reactivity of methane. However, this decrease entails coke formation via higher-order surface reactions (Fig. 1), and a catalyst surface that can maximize the ratio of hydrocarbon to coke is essential. Therefore, selective hydrocarbon production requires optimization of the catalyst surface for bimolecular reactions in the gas phase. Although SiC alone can induce the surface reaction, the Fe species must be loaded to improve the surface reactivity within the reactor.\nFig. 4\n shows the effect of the GHSV on the product distribution in the catalytic reactor at 1020\u00a0\u00b0C, based on the methane conversion. More methane is converted by the SiC surfaces than by the blank reactor at GHSV values exceeding 319\u00a0h\u22121 (Fig. 4a). In this region, the methane conversion further increases as additional Fe (above 151\u00a0\u00b5mol) is included in the reactor. The effectiveness of catalytic surfaces compared with the gas phase is observed when the methane conversion is below 15%. Above this value, the reactor used in this study exhibits minimal differences in activity with and without a catalyst. Both the blank and catalyst-packed reactors exhibit similar trends in product selectivity as a function of hydrocarbon yield (Fig. 4b\u2013f). The ethane selectivity decreases, whereas the acetylene selectivity reaches the maximum value with increasing hydrocarbon yield. In this region, the C3\u2013C5 selectivity gradually decreases. With the decreasing selectivity of C2 (ethane, ethylene, and acetylene) and C3\u2013C5, the aromatic (benzene, toluene, naphthalene, and alkyl-aromatics) selectivity increases when the hydrocarbon yield is increased further. This phenomenon is consistent with the typical methane pyrolysis mechanism comprising a series of reactions involving dehydrogenation and C\u2013C coupling. In this study, ethane is the primary product of the conversion of methyl radicals that are produced during the rate-determining step of methane conversion. If ethylene and acetylene are sequentially produced from ethane through endothermic reactions, aromatics and coke can be produced spontaneously through exothermic reactions. According to Puente-Urbina et al. [47], C2\u2013C5 radical species, including propargyl radicals, are produced under non-oxidative conditions at 945\u20131,400\u00a0\u00b0C during the chain growth of hydrocarbons in methane conversion, thus leading to aromatic formation. In the present study, a hydrocarbon yield of approximately 7% is considered a marginal range for stabilizing C2 species. The formation of C2\u2013C5 radical species appears to be minimized in this range, whereas ethane, ethylene, and acetylene are stably balanced. At this stage, the acetylene selectivity is maximized in the blank and catalytic reactors. At similar hydrocarbon yields, the catalytic surfaces demonstrate higher C2 selectivity but lower aromatic selectivity than that of the blank reactor, implying that the aromatics are partially converted to coke at the catalytic surfaces. At similar C2 yields, the catalytic surfaces induce more ethane and ethylene, but less acetylene than that induced by the blank reactor (Fig. 4g\u2013i). The low selectivity of acetylene and aromatics in hydrocarbons in the catalysts indicates that they are further subjected to C\u2013C coupling. These results suggest that the catalyst surfaces contribute to changes in the composition of C2 species and their conversion.The methyl radicals initially couple to form ethane, which undergoes a series of dehydrogenation reactions to form ethylene and acetylene, which can further compete with each other as reactants for subsequent reactions. To elucidate the C2 reactivity on the catalytic surfaces, reactions were performed at 500\u20131020\u00a0\u00b0C, wherein each C2 species was used as a model compound (Fig. 5\n). The C2 conversion was measured by regarding ethane, ethylene, and acetylene present in the stream as lumped reactants. For comparison, the equilibrium of C2 species with temperature for each feedstock composition was calculated using HSC Chemistry 9. At temperatures above 800\u00a0\u00b0C, the C2 conversion in the blank reactor increases in the order of acetylene\u00a0<\u00a0ethylene\u00a0<\u00a0ethane feed but increases in the order of ethane\u00a0<\u00a0ethylene\u00a0<\u00a0acetylene feed at the catalytic surface (Fig. 5a\u2013c). When ethane is added to the reactor with hydrogen, the catalytic surfaces are less reactive than the blank reactor. This is probably because homolytic cleavage of the C\u2013C bond of ethane is selectively promoted on the catalyst surfaces, resulting in methane formation [48,49]. In contrast, for ethylene and acetylene, the reactivity on catalytic surfaces tends to improve compared with that in the blank reactor after 850\u00a0\u00b0C. The C2 conversion tends to increase with the increasing Fe content in the reactor, and the maximum increase is observed for acetylene conversion. Furthermore, considering the acetylene conversion, the blank reactor exhibits a different trend for C2 conversion starting at 800\u00a0\u00b0C. However, at the same point, the catalytic reactor does not demonstrate a change in the trend, indicating that it further promotes the C\u2013C coupling reaction of acetylene.The lumped C2 species can be balanced with respect to temperature through dehydrogenation and hydrogenation at non-equilibrium conditions (Fig. 5d\u2013f). With increasing temperature, the blank reactor exhibits higher ratios of acetylene to ethylene, when ethylene is used as a reactant instead of ethane. Under the non-equilibrium conditions, acetylene is partially converted to ethylene through hydrogenation in the blank reactor. For ethane and ethylene reactions, the catalytic surfaces shift the ratio of acetylene to ethylene toward equilibrium with increasing temperature, which allows additional ethylene to be present in the stream. Although this tendency increases with increasing Fe content, the difference is not as noticeable as that observed between the ratios for the blank reactor and the reactor containing SiC. However, for the acetylene reaction occurring below 717\u00a0\u00b0C, the ratios of acetylene to ethylene are higher for the catalytic surfaces than for the blank reactor. This difference decreases with increasing temperature and decreasing Fe content. Therefore, in contrast to the gas-phase reaction, the Fe catalyst favors the C\u2013C coupling of acetylene over selective hydrogenation. Using excess hydrogen as a reactant may accelerate the conversion of acetylene to ethylene at the catalyst surface.The lumped C2 is subsequently converted to aromatics through C\u2013C coupling reactions, and the maximum selectivity at 933\u00a0\u00b0C in the blank reactor increases in the order of acetylene\u00a0<\u00a0ethane\u00a0<\u00a0ethylene feed (Fig. 5g\u2013i). When acetylene is used as a reactant, the aromatic selectivity is at least 2.7 times higher than that of ethane and ethylene below 803\u00a0\u00b0C. Here, the catalytic surfaces decrease the aromatic selectivity irrespective of the C2 feedstock, depending on the amount of Fe in the reactor. Aromatics are found to be sensitive to partial pressure [10], and they are converted to coke above a certain partial pressure; coke formation is further promoted by the Fe/SiC surface. Compared with ethane and ethylene, acetylene in the reactor favors the conversion of aromatics to heavier hydrocarbon products (coke) via hydrogen abstraction\u2013acetylene addition routes [47]. This is consistent with the finding that the coke yield increases simultaneously with the hydrocarbon yield as the amount of Fe is increased in the reactor (Fig. 1). Thus, the catalytic surface is suitable for the C\u2013H activation of methane. However, optimization with the gas phase is necessary to decrease coke formation.\nFig. 6\n shows the results of methane conversion according to the ratio of void space to 0.2Fe catalyst-packed space in the quartz tube reactor. Here, only the post-catalyst zone is considered for the void space, which is controlled using a quartz rod with an outer diameter of 3\u00a0mm (Fig. 6a). Here, the surface reaction is considered to occur in the catalyst-packed space, whereas the gas-phase reaction is considered to occur in the void space in the blank reactor or post-catalyst zone. A gas-phase reaction can occur in the interparticle space; however, it has not been considered in this study because the interparticle space is substantially smaller than the void space. Moreover, we defined the interfacial space as the space between the reactor and the quartz rod. The void space includes the interfacial space. If the inner space of the reactor is completely occupied by the quartz rod, the interfacial space is 0.89\u00a0mL; additionally, the methane conversion, which is the sum of hydrocarbon (1.0%) and coke (0.2%) yields in this space, is as low as 1.2% (Fig. 6b). The methane conversion in the interfacial space is at least twice as low as that in the blank reactor (Fig. 1). However, when comparing the methane reactivity on a per-unit volume basis, the values obtained for the interfacial space and the blank reactor are almost identical (1.3% mL\u22121). The yields of hydrocarbons and coke increase rapidly as the catalyst space increases from 0 to 0.63\u00a0mL; the yields steadily increase as the space is further increased to 1.88\u00a0mL (Fig. 6b). When the ratio of interfacial space to catalyst space is less than 0.95, the ratio of hydrocarbon to coke yields converges between 4.1 and 5.4. When the catalyst space exceeds the interfacial space, a hydrocarbon yield of approximately 6% is achieved with a coke yield of 1.1%\u20131.4%. The molar carbon selectivity values for C2, C3\u2013C5, and aromatics converge when the ratio of interfacial space to catalyst space is less than 0.95 (Fig. 6c). The selectivity of the C2 species is in the range of 52.0%\u201366.9%, which is higher than that of C3\u2013C5 and aromatics (11.0%\u201322.6% and 13.0%\u201320.1%). The C3\u2013C5 selectivity is higher than the aromatic selectivity in the catalyst space ranging from 0 to 0.12\u00a0mL, wherein coke formation is minimized. With the decreasing ratio of hydrocarbon to coke yields, the preceding trend is reversed. Therefore, the catalytic surfaces are probably involved in improving the methane conversion and inducing the C\u2013C coupling of aromatics, leading to coke formation. In the present study, increasing the catalyst space favors the formation of aromatics and coke from methane, indicating that additional catalytic surfaces serve as active sites for the C\u2013C coupling reaction in the axial direction of the reactor. The C\u2013C coupling and coke formation reactions are kinetically faster than methane activation even in autocatalysis [50]; therefore, the possibility of excessive catalytic surface reactions should be reduced.Increasing the void space in the post-catalyst zone increases the hydrocarbon yield relative to coke over catalyst spaces of 0.37 and 0.63\u00a0mL (Fig. 6d). The post-catalyst zone can induce gas-phase reactions, and a 2.1-fold increase in this space increases the hydrocarbon yield of 0.37\u00a0mL and 0.63\u00a0mL catalyst-packed reactors by 1.6 and 1.8 times, respectively. A catalyst space of 0.63\u00a0mL converges to a hydrocarbon yield of 7.1% depending on the void space, whereas a catalyst space of 0.37\u00a0mL converges to a hydrocarbon yield of 6.6%. The coke selectivity at the maximum hydrocarbon yield is less than 2% in both catalytic reactors. Compared with the catalyst surface, the void space in the post-catalyst zone is less reactive to C\u2013C coupling reactions. Similar results were observed by Van Der Zwet et al. [51], who investigated the effect of surface area on methane conversion at 1125\u00a0\u00b0C; the high surface area was found to considerably affect the termination of chain reactions involving free radicals, and graphitic coke and hydrogen were primarily produced. The 0.63\u00a0mL catalyst-packed reactor is more active than the 0.37\u00a0mL catalyst-packed reactor in a similar post-catalyst zone. Similar trends are observed for the selectivity of C2, C3\u2013C5, and aromatics in the two catalytic reactors. However, at similar hydrocarbon yields, the 0.67\u00a0mL catalyst-packed reactor demonstrates higher aromatic selectivity and lower C3\u2013C5 selectivity than that of the 0.37\u00a0mL catalyst-packed reactor (Fig. 6e).In Fig. 6, the 0.2Fe catalyst (0.63\u00a0mL) indicates that 50.3\u00a0\u00b5mol of Fe is present in the reactor; this Fe amount is 6.5 times lower than that (327\u00a0\u00b5mol) in the reactor exhibiting the maximum hydrocarbon yield in Fig. 1. Although less catalyst surface (6.5 times lower) is used in the reactor containing the 0.2Fe catalyst, the hydrocarbon yield is 0.45% higher than that in the reactor containing 327\u00a0\u00b5mol of Fe, and the coke selectivity is also 24.5% lower. Considering methane conversion, the gas-phase reaction in the void space is less active than that on the 0.2Fe catalyst surface, with the same volume (Fig. 1). The significant increase in coke selectivity implies that hydrocarbons stably present in the gas phase are converted to coke through adsorption on the 0.2Fe surface.If the catalyst selectively converts methane to hydrocarbons under non-oxidative conditions, these hydrocarbons can act as radical donors for further methane conversion in the void space located below the catalyst [49]. Postma and Lefferts [52] reported similar results, confirming that methane activation in the gas phase could be promoted by co-feeding ethane or ethylene with methane to the reactor. In the present study, when an additional 0.63\u00a0mL of void space is added, the hydrocarbon yield increases by 1.6 times, compared with that when the interfacial space below the 0.2Fe catalyst is only 0.60\u00a0mL (Fig. 6b and d). Simultaneously, the aromatic selectivity sharply increases, compared with the C2 selectivity in the void space (Fig. 6e). In gas-phase reactions, aromatics are expected to considerably influence the methane conversion. Hao et al. [41] experimentally demonstrated that the methane conversion could be enhanced by co-feeding aromatics such as 1,2,3,4-tetrahydronaphthalene or benzene into a Fe-coated catalytic quartz reactor. Furthermore, they [42] used the H-atom Rydberg tagging time-of-flight technique to demonstrate the formation of hydrogen radicals that were only decomposed from the aromatic structure during the MTOAH reaction over a catalytic quartz wall reactor containing embedded iron species. These results imply that aromatics, which are hydrogen radical donors, can promote methyl radical formation in the gas-phase reaction even under the reaction conditions considered in the present study.In contrast to the results depicted in Fig. 4, if the reactivities of aromatics and acetylene on the catalyst surface are minimized by controlling the space velocity of the catalyst, acetylene can be stably present in the post-catalyst space. Similarly, Toraman et al. [23] and Postma and Lefferts [44] reported that reactor configurations with sufficient void space in the post-catalyst zone after catalyst overhead packing could decrease coke formation. In this study, an optimal hydrocarbon range provided by the catalyst exerts a synergistic effect on hydrocarbon formation in the post-catalyst space; the effect is more pronounced when the product contains more aromatics than C3\u2013C4 in the catalyst space (Fig. 6c and e). This is also considered the reason for the minimized coke yield when the catalyst space is less than the void space, in contrast to the maximum hydrocarbon yield that is obtained when the catalyst is completely packed in the reactor space. These results suggest that increasing the ratio of hydrocarbon to coke while increasing the methane conversion requires a balance between the catalyst surface and the gas-phase reaction. Toraman et al. [23] presented a first-principles-based microkinetic model consisting of catalytic and gas-phase reactions over iron atoms anchored on silica, and the results revealed that increasing the influence of the gas-phase reaction on the overall reaction increases the selectivity of aromatics rather than that of ethylene. Nevertheless, the balance between the catalyst and the gas-phase reaction is crucial for increasing the hydrocarbon yield. The present study reveals that the hydrocarbon selectivity-to-methane conversion trend is unaffected by the rate-determining or catalyst initiation steps, whereas a given level of methane conversion can be attained rapidly with minimal coke formation.\nFig. 7\n shows the XPS profiles of the spent SiC and Fe/SiC catalysts. In the C 1s spectrum (Fig. 7a), the spent SiC surface is characterized by deconvoluted peaks with binding energies of 282.5, 284.4, 285.3, and 286.5\u00a0eV associated with C\u2013Si, C\u2013C, C\u2013O, and C\u2013N bonds, respectively, implying a typical SiC surface [53]. The Fe loading on SiC increases the ratio of C\u2013C to C\u2013Si bonds in the spent catalysts, indicating that carbon is developed on the catalytic surface during the reaction. Note that solid carbon in the interparticle space, which is physically separated from the catalyst, appears to be included in the above results. In the Fe 2p spectrum (Fig. 7b), the spent Fe/SiC catalysts exhibit weak broad peaks in the range of 705 and 740\u00a0eV, associated with Fe carbides (707.7\u00a0eV), Fe3+ (711.4\u00a0eV), and Fe 2p\n1/2 (724.9\u00a0eV) [54]. A substantial proportion of the Fe particles is possibly obscured from the surface by carbon layers. For methane decomposition with iron ores, Zhou et al. [55] report that graphite layers are developed on the Fe particle surface to form carbon nano onions, which are responsible for encapsulation. Therefore, the difference in Fe species exposed to the surface between each catalyst is negligible. As the Fe and carbon species are not directly comparable when considering the low intensities and high heterogeneity, respectively, it can be seen that the spent catalysts exist together with the deposited carbon.\nFig. 8\n shows the TG and DSC profiles of the spent SiC and Fe/SiC catalysts. In the temperature range of 400\u2013800\u00a0\u00b0C, the amount of carbon deposited in the spent SiC is 1.5 wt%, which is increased to 8.5 wt% with increasing Fe loading on SiC (Fig. 8a). The spent SiC has a broad exothermic peak, which gradually becomes stronger as the Fe loading on SiC increases (Fig. 8b). In contrast to the 5.7-fold increase in the amount of carbon deposited, the DSC peak position shows a maximum of 712\u00a0\u00b0C at 0.3Fe and then decreases to 683\u00a0\u00b0C at 1Fe (Fig. 8c). This trend is attributed to the nature of carbon and the thickness of the carbon layer [56]. Similarly, we observed an increase in the amount of carbon deposited in the spent 0.2Fe catalyst as the catalyst space increased, probably because the partial pressure of C2\u00a0+\u00a0hydrocarbons produced from methane in the stream increases along the axial direction of the reactor (Fig. S3). There was less than 1.5 wt% of coke in 0.63\u00a0mL of the catalyst space, most likely formed during the initial activation stage of the 0.2Fe catalyst. This observation regarding the carbon deposited on the spent catalysts is consistent with the trend depicted in Fig. 6, wherein only a small amount of catalyst surface is required at the top of the reactor to minimize the coke selectivity in the MTOAH reaction. Consequently, the rate of the C\u2013C coupling reaction accelerates, and carbon formation on the catalyst surface increases with the increase in the Fe content.\nFig. 9\n shows the morphological results that can be used to qualitatively analyze carbon deposition on the spent 0.2Fe catalyst. The spent 0.2Fe catalyst has non-uniform Fe particle sizes due to low metal\u2013support interaction between the Fe species and SiC. The results of STEM with EDS analysis (Fig. 9a) show that carbon is deposited differently on the Fe particles and SiC in the 0.2Fe catalyst. Fig. S4 depicts the lattice structure with a lattice parameter of 0.26\u00a0nm, which is identical to that of \u03b2-SiC. A thin carbon layer (1.5\u00a0nm) is observed on the SiC surface (Fig. 9b and S4), whereas the Fe particles are covered by a thick carbon layer (2.3\u2013114.6\u00a0nm), as shown in the TEM image (Fig. 9c). Upon measuring the carbon layers surrounding the Fe particles, the thickness of the graphitic layer is found to increase with the size of Fe particles on the SiC surface for the spent 0.2Fe catalyst (Fig. 9d\u2013e). Here, we note that the Fe particle size of the spent catalysts with different Fe content differs slightly (Fig. S5), indicating that the carbon deposition depends on the concentration of Fe species on SiC. Catalysts with different Fe loadings undergo particle stabilization (sintering), and the reaction proceeds under the MTOAH reaction condition of 1020\u00a0\u00b0C; this reaction temperature is higher than the Tamman temperature, which is the absolute melting temperature of Fe nanoparticles (Fe\u00a0=\u00a0631\u00a0\u00b0C, FeO\u00a0=\u00a0585\u00a0\u00b0C, and Fe3O4\u00a0=\u00a0699\u00a0\u00b0C). If the Fe particles are considered spherical, the volume of the graphite layer can converge to become equal to that of the Fe particles. This statement implies that the amount of carbon deposited over the Fe particles is more than four times smaller than the Fe content, which is lower than the amount of bulk carbon based on TG profiles in Fig. 8. The physical desorption of carbon occurs during the reaction, and it appears to proceed after carbon sufficiently grows from the surface [55]. The predominant presence of solid carbon in the interparticle space is probably derived from the active Fe species, and carbon layers on the Fe particles do not considerably decrease the activity. Because the hydrocarbons can permeate the internal structure of the crystalline carbon layer, the surface reaction of the catalyst is still considered to be dominated by the Fe species. This finding is consistent with the results in Fig. 1, indicating that Fe species on the SiC surface can enhance the methane reactivity but further induce coke formation through consecutive C\u2013C coupling reactions.\nFig. 10\n shows the Raman spectra, which can be used to distinguish the type of carbon in the spent catalysts. The Raman spectra of each catalyst exhibit two distinct peaks, which can be deconvoluted into five Lorentz peaks, as reported by Sadezky et al. [57]. The spent catalysts in Fig. 10(a) exhibit the morphology of typical graphite carbons based on a subdivision of peaks: the range 1570\u20131590\u00a0cm\u22121 represents the sp\n2-hybridized carbon bond of the ideal graphitic lattice (G); 1346\u20131353\u00a0cm\u22121 represents vibrations of disordered carbon such as graphene layer edges (D); 1602\u20131619\u00a0cm\u22121 represents disordered aromatic structures such as surface graphene layers (D\u2019); 1492\u20131521\u00a0cm\u22121 represents vibrations of carbon defects such as amorphous carbon (D\u2019\u2019), and 1171\u20131235\u00a0cm\u22121 represents C\u2013H vibrations of disordered graphitic lattice (I) [57\u201359]. Here, we obtained the characteristic coke fraction as the area fraction of deconvoluted peaks (Fig. 10b). In characteristic fractions, the ideal graphitic lattice-induced (G)-band becomes stronger as the Fe content on SiC increases. These structural differences are likely attributed to the increased probability that hydrocarbons with different carbon numbers expose the catalytic surface.We predicted the crystallite dimension of carbon materials according to the equation established by Tuinstra and Koenig [60]: I\nD/I\nG = (2.4\u00a0\u00d7\u00a010\u221210)\u03bb\n4/L\n\u03b1, where \u03bb represents the wavelength of the laser, and L\na represents the nano-sized graphite crystallite. Here, the crystallite size of the carbon is inversely proportional to the area ratio of the D and G bands (I\nD/I\nG). Ishii et al. [61] characterized typical non-graphitizable and graphitizable carbons and found that the crystallite size of carbon was inversely related to the number of carbon edge sites. We measured the distribution of graphite crystallite by analyzing at least five points, as shown in Fig. 10(b). The crystallite size of carbon is almost constant until the Fe content on SiC is 0.26 wt%, but further increase leads to its non-uniformity. The minimum values of the crystal size of carbon in the spent catalysts range from 8.7 to 10.0\u00a0nm, which are similar to the results obtained after increasing the space of the 0.2Fe catalyst along the axial direction of the reactor (Fig. S6). The 0.2Fe catalyst space in the reactor is directly proportional to the amount of carbon deposited in the spent 0.2Fe catalyst. However, the characteristic fraction of carbon differs only slightly during the methane conversion from 1.2 to 7.2% (Fig. 6 and S6). However, the spent 0.3Fe, 0.5Fe, and 1Fe catalysts include carbon deposits with high crystallinity ranging from 13.6 to 29.0\u00a0nm, which is attributed to the Fe particles.Compared to pure SiC, adding up to 97\u00a0\u00b5mol of Fe to the reactor (0.1Fe) induces a smaller crystallite size of carbon, which improves the methane reactivity by minimizing the coke selectivity (Fig. 1a and Fig. 10b). However, providing excess Fe sites up to 186\u00a0\u00b5mol (0.3Fe) appears to increase the surface reactivity of aromatics, resulting in more disordered carbon, which is probably responsible for the increase in the coke yield. When more than 327\u00a0\u00b5mol of Fe is present in the reactor (0.5Fe), the smaller carbon crystallites partially accompany the formation of larger carbon crystallites. At this time, high carbon deposits reduce the interparticle space in the catalyst zone, which appears to be the reason for the slight decrease in the reaction activity, as shown in Fig. 1(c). The carbon deposition in this range appears to be governed by graphite formation from Fe particles [55]. Fe particles appear to induce different types of coke precursors (i.e., acetylene, benzene, and polyaromatic compounds) with methane activation depending on their concentration and size. These coke precursors may terminate in solid carbon on the catalyst, with different properties [62]. Under our experimental conditions, the catalyst appears to provide sufficient surface area to randomly act as a radical terminator for coke precursors.Solid carbons with different structures, such as activated carbon, carbon black, mesoporous carbon, and carbon nanofiber, have different reactivities in methane decomposition at 900\u00a0\u00b0C; the activity is affected by the surface area [64]. This interpretation and the results presented in Fig. 10 suggest that the carbon deposited with smaller crystallite sizes on the catalyst may provide additional carbon edge sites, which can participate in the reaction as additional active sites. According to Muradov et al. [65], who used varied surfaces including carbon materials at 850\u00a0\u00b0C for the catalytic decomposition of methane, the hydrocarbon formation rate increased in the order: methane\u00a0<\u00a0ethylene\u00a0<\u00a0acetylene\u00a0<\u00a0benzene as the crystallite size of carbon materials decreased. Solid carbon is considered to behave similarly to SiC surfaces by promoting the desorption of hydrocarbons rather than Fe species. However, further studies are required to obtain insights into more atomically precise active sites.With reference to catalysis, one possibility to consider is that the proximity between the Fe particles on the SiC surface affects methane reactivity. In the thermal decomposition of methane, Ea\n for carbon nuclei formation was determined to be considerably higher (316.8\u00a0kJ\u00a0mol\u22121) than that required for carbon crystallite growth (227.1\u00a0kJ\u00a0mol\u22121) [63]. This range of values is similar to that of the experimentally obtained Ea\n between SiC and 0.3Fe catalysts (Fig. 3b), emphasizing the effect of the surface Fe concentration in reducing coke selectivity during the MTOAH reaction. To confirm this observation, we performed a reaction by physically mixing the catalyst with different Fe contents and SiC so that 97\u00a0\u00b5mol of Fe was present in the reactor, and the results of hydrocarbon and coke yield are shown in Fig. 11\n. The proximity of Fe particles on the SiC surface increases with the Fe loading of each catalyst (Fig. S5). The hydrocarbon and coke yields increase along with the Fe content in the Fe/SiC catalyst. Coke formation is favored over hydrocarbon formation with the increase in the proximity of Fe particles. The proximity of the Fe particles increases the amount of carbon surrounding the Fe particles and that is present in the interparticle space. This indirectly indicates that the catalytic surfaces are active as radical terminators to increase the coke selectivity. The solid carbon in the interparticle space is probably derived from the active Fe species, and carbon layers on the Fe particles do not appreciably decrease the activity. Consequently, the coke selectivity increases through consecutive C\u2013C coupling reactions, implying that the presence of highly concentrated Fe on the SiC surface does not favorably decrease the coke selectivity in the product. This finding is consistent with that reported by Han et al. [18], who confirmed through electronic structure calculations on methane activation that Fe3C clusters favor coke formation, whereas confined Fe sites favor methyl radical formation. To maximize hydrocarbon yield margins while minimizing coke selectivity, further studies should be conducted to optimize the surface dispersion and nature of metal cations, thus preventing carbon formation on the surface or ensuring crystalline coke formation with extremely few defects.\nFig. 12\n shows the space\u2013time yield (STY) optimized by the reactant flow rate in enlarged reactors packed with 0.63\u00a0mL of the 0.2Fe catalyst on top. In this case, only the post-catalyst zone was considered for the void space, and a tube with an inner diameter of 7\u00a0mm was connected so that the ratio of the void space to the catalyst space exceeded 2. A quartz rod (inner diameter\u00a0=\u00a03\u00a0mm) was used to minimize the effect of the overall reactivity of the unwanted sections of the reactor and the void space of this part was also included in the total void space. As the ratio of void space to catalyst space increases by 3.1 times, the space velocity in the total volume of the reactor decreases by 0.4 times (Fig. 12a). For the same space velocity in the catalyst (20\u00a0mL\u00a0min\u22121), as the ratio of void space to catalyst space increases, the STY of coke increases, whereas that of C2, C3\u2013C5, and aromatics decreases. The STY of methane evidently does not differ with the reactor configuration, indicating that hydrocarbons favor being converted to coke in the excess void space. However, each catalytic reactor has an appropriate methane flow rate, which reduces the coke selectivity. In this case, the STY of methane is almost unchanged. In a previous study, we used artificial intelligence to optimize the reaction parameters of a gas-phase-dominated reactor for methane conversion and found that partial pressure was the most important factor affecting C2 and coke selectivity [66]. This indicates that the stability of the hydrocarbons (such as acetylene and aromatics) in the gas phase depends on the partial pressure. Thus, the role of the 0.2Fe catalyst is possible to produce hydrocarbons from methane without coke formation, and the hydrocarbons promote the formation of methyl radicals in the gas phase of the post-catalyst zone, thus improving the reactivity.When the ratio of void space to catalyst space is 6.13, the 0.2Fe catalytic reactor exhibits a stable STY of hydrocarbons with minimal coke formation during 40.2\u00a0h of the reaction at 1020\u00a0\u00b0C (Fig. 12b). The 0.2Fe catalytic reactor converts methane to 43.8% C2, 5.2% C3\u2013C5, and 51.0% aromatics on average, comparable to the selectivity of Fe\u00a9SiO2, as reported by Guo et. al. [17]. From a practical engineering perspective, catalytic reactor optimization is considered to minimize coke selectivity, thereby potentially increasing the chemical process stability. To further increase the STY of hydrocarbons in a packed bed reactor, a reactor may need to be designed with a gas-phase zone that facilitates local control of the partial pressure of the product. In addition, based on Fig. 5, when a surface reaction is induced in the thermal gradient reactor, acetylene can be more selectively hydrogenated to ethylene at a lower temperature than that required for methane activation. In the future, the design of catalysts that selectively produce radical donor molecules capable of reversibly donating CH3\u2013 and H-radicals will be instrumental in obtaining higher olefin yields.In this study, the effect of the catalytic surface in a quartz tube reactor on the MTOAH process was investigated using SiC catalysts impregnated with 0\u20131.25 wt% Fe. The SiC surface decreased E\na and A for methane consumption, compared with the values obtained for the blank reactor. The presence of 97\u2013695\u00a0\u00b5mol of Fe in the reactor further decreased E\na and A, thus increasing hydrocarbon yield, which reached a maximum of 6.7% when the amount of Fe in the reactor was 327\u00a0\u00b5mol at 1020\u00a0\u00b0C. The hydrocarbon formation from methane in the Fe/SiC packed reactor was 2.7\u20138.4 times higher than the accompanying coke formation, for each catalyst. The Fe/SiC surfaces induced more ethane and ethylene than the blank reactor, but less acetylene at similar C2 yields. According to selective C2 conversion studies of ethane, ethylene, and acetylene mixed with hydrogen, an excess amount of Fe in the reactor favors the C\u2013C coupling reaction over the selective hydrogenation of acetylene, resulting in coke formation. The closer contact between Fe particles with increasing Fe loading on SiC at 97\u00a0\u00b5mol of Fe in the reactor increased the coke yield. By optimizing the ratio of the void space of the post-catalyst zone to the 0.2Fe catalyst-packed space, the obtained hydrocarbon yield was 7.1% with a coke selectivity of less than 2% at a catalyst space of 0.63\u00a0mL and void space of 1.26\u00a0mL. Although the ratio of void space to catalyst space was further increased to 6.13, no significant difference was observed in the STY of hydrocarbons in the 0.2Fe catalytic reactor. Moreover, at this ratio, the STY of the hydrocarbons in the 0.2Fe catalytic reactor was maintained at 1784\u00a0\u00b5mol C mL\u22121h\u22121 during 40.2\u00a0h of the reaction. Therefore, these findings can provide guidelines to optimize the design of catalytic reactors, thereby facilitating the scale-up from the laboratory to the commercial scale.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was supported by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017M3D3A1A01037001). This research was supported by the Ministry of Trade, Industry and Energy (MOTIE), Korea Institute for Advancement of Technology (KIAT) through the Virtual Engineering Platform Program (P0022334).Supplementary data to this article can be found online at https://doi.org/10.1016/j.jechem.2023.03.019.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The conversion of methane to olefins, aromatics, and hydrogen (MTOAH) can be used to stably obtain hydrocarbons when the effect of the catalytic surface is optimized from the reaction engineering perspective. In this study, Fe/SiC catalysts were packed into a quartz tube reactor. The catalytic surfaces of SiC and the impregnated Fe species decreased the apparent activation energies (E\n                     a) of methane consumption in the blank reactor between 965 and 1020\u00a0\u00b0C. Consequently, the hydrocarbon yield increased by 2.4 times at 1020\u00a0\u00b0C. Based on the model reactions of ethane, ethylene, and acetylene mixed with hydrogen in the range of 500\u20131020\u00a0\u00b0C, an excess amount of Fe in the reactor favored the C\u2013C coupling reaction over the selective hydrogenation of acetylene; consequently, coke formation was favored over the hydrogenation reaction. The gas-phase reactions and catalyst properties were optimized to increase hydrocarbon yields while reducing coke selectivity. The 0.2Fe catalyst-packed reactor (0.26 wt% Fe) resulted in a hydrocarbon yield of 7.1% and a coke selectivity of\u00a0<\u00a02% when the ratio of the void space of the post-catalyst zone to the catalyst space was adjusted to be\u00a0\u2265\u00a02. Based on these findings, the facile approach of decoupling the reaction zone between the catalyst surface and the gas-phase reaction can provide insights into catalytic reactor design, thereby facilitating the scale-up from the laboratory to the commercial scale.\n               "}
{"full_text": "1,2-propanediol (1,2-PDO), also called propylene glycol (PG) is widely known as valuable chemicals used as monomer or additive in production of pharmaceuticals, cosmetics, solvent in food, as engine coolant, de-icing agent, and raw material for polyester resins (Gallegos-Suarez et al., 2015; Mauriello et al., 2015). Therefore, it has been regarded as a major commodity chemical with an estimated global production of about 1.4 million tons yearly at a 4% of annual market growth rate (Vasiliadou et al., 2011). The conventional production of 1,2-PDO is from petroleum derivatives via hydration process of hazardous propylene oxide (Bagheri et al., 2015; Rajkhowa et al., 2017). However, due to concern of petroleum shortage in the long-term, as well as the environment pollution issue, it is highly desirable to produce 1,2-PDO from a renewable source which may also substantially alters the price of 1,2-PDO. The surplus of glycerol as by-product from the rapid development of biodiesel (1\u00a0kg glycerol for every 9\u00a0kg biodiesel produced) could serve as an advantage and ideal solution for converting it into 1,2-PDO (Pandhare et al., 2016; Zhao et al., 2020). Due to above, the conversion of glycerol into 1,2-PDO via catalytic hydrogenolysis reaction has generated research interest. Generally, hydrogenolysis reaction require molecule bond dissociation and insertion of hydrogen into generated fragments which involve the cleavage of C-O bond of glycerol molecule while the C-C bond cleavage is undesired as it would lead to side products (Zheng et al., 2015). The evolution of 1,2-PDO from glycerol hydrogenolysis was described to proceed via dehydration of glycerol molecule to form acetol on acid site and further hydrogenation of acetol intermediate to 1,2-PDO on metal site (Balaraju et al., 2009, Mallesham et al., 2016, and Gandarias et al., 2012). The general reaction route for 1,2-PDO production is shown in Scheme 1\n.Various heterogeneous catalysts have been well studied in glycerol hydrogenolysis, yielding different product compositions. In particular, the use of noble and lanthanide metals such as Pd, Pt, Ru and Ce have been reported with high selectivity to 1,2-PDO and high conversion of glycerol (Soares et al., 2016, Xia et al., 2011, and Yu et al., 2010). Alternatively, the use of transition metal-based catalysts such as Cr, Co, Ni, Cu, Zn and Zr, have also been associated with high catalytic activity. The transition metal-based catalysts were often preferred as a choice due to their efficiency towards C-O bond cleavage in contrast to C-C bond cleavage reaction (Freitas et al., 2018, Putrakumar et al., 2015, Zhao et al., 2020, and Mauriello et al., 2015). In addition, the lower price of transition metals in comparison to noble metals, emerge as the promising cost-effective substitute for noble metal as catalysts for hydrogenolysis of glycerol. However, the use of transition metal alone is of a big concern since metal leaching and sintering commonly occur. This is because the metal particles tend to aggregate under elevated reaction temperature due to a weak interaction among the metal species and thus easier to deactivate especially for a long reaction time thereby decreasing the catalytic performance (Wen et al., 2013). The presence of a support is highly desired in order to raise the activity and stability of the metal catalyst. The support behaves as a reservoir of spill over hydrogen that helps to hydrogenate surface species in which the available hydrogen from the support surface can pass to the surface metal and generate interfacial active reaction sites on metal-support surface and thus promote higher catalytic activity.Substantial studies have been performed to probe the relationship between the catalytic reaction performance and the interaction of supports with catalyst metals of different nature. It is generally accepted that a combination of support and metal catalyst exhibit more attractive properties such as chemical and thermal strength, metallic phase stability, high metal dispersion and high metal reducibility. All these properties promote higher glycerol conversion and 1,2-PDO selectivity. Specifically, during glycerol hydrogenolysis, a catalyst support is supposed to be defined as a good reduction agent by the interaction of support and metal oxide. In this way, the reduction property of oxide species is enhanced. It has been reported that when a metal oxide is supported, the electrons from the support were directly transferred to the metal oxide species which then promote the formation of metallic species acting as active reaction sites for hydrogenation of C-O bond (Gallegos-Suarez et al., 2015). Due to the concern of good electronic properties, a metal catalyst is suggested to preferably interact with the oxide supports than the non-oxide supports such as carbon and polymeric resin.In view of the above facts, thus supporting metal on a good support will no doubt improve catalyst efficiency in glycerol hydrogenolysis. In this study, dolomite with a mixture of mainly calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) with several appreciable amount of SiO2, Fe2O3, Al2O3 at (<5%) has been selected to be used as support in this work. Apart from its lower price, this support material has gained attention due to its acidic characteristics which are important in glycerol hydrogenolysis but the characteristic is rarely reported in literature. The presence of calcium and magnesium in dolomite may help in reducing metal oxides into metallic species since both metals have been identified as good reducing agents in electrochemical series. Incidentally, dolomite is abundantly available in Perlis, Malaysia and therefore can easily be accessible. Therefore, the objective of this research is to develop bifunctional supported metal catalysts comprising of acid and metal sites that possess good metal-support interaction species for high hydrogenolysis efficiency.In this study, the dolomite used as catalyst support was supplied by quarries in Chuping, Perlis Dolomite Industries, Malaysia. The metal precursors of copper nitrate hexahydrate (Cu(NO3)2\u00b76H2O) (\u226599%), nickel nitrate hexahydrate (Ni(NO3)2\u00b76H2O) (\u226599%) and cobalt nitrate hexahydrate (Co(NO3)2\u00b76H2O) (\u226599%) were purchased from R&M Chemical Company, Malaysia. For iron nitrate nanohydrate (Fe(NO3)3\u00b79H2O) (99%) and zinc nitrate hexahydrate (Zn(NO3)2\u00b76H2O) (98%), the precursors were supplied from Bendosen Company. Glycerol (\u226599.5%) was acquired from Sigma-Aldrich. All chemicals in this study were used as provided.Impregnation method was used in the production of all catalysts with a metal loading of 20\u00a0wt%. In a typical synthesis, 3.8\u00a0g of metal nitrate precursor was separately dissolved in 10\u00a0ml distilled water and was then poured into 4\u00a0g dolomite powder and was referred as supported metal catalyst denoted as M*/Dol (M\u00a0=\u00a0Cu/Ni/Co/Fe/Zn). The mixture was then stirred using magnetic stirrer at 300\u00a0rpm and further dried for 3\u00a0h with heating at 90\u00a0\u00b0C on a hot plate. The dried mixture was then aged in drying oven for 24\u00a0h at 120\u00a0\u00b0C. After that, the synthesized catalyst was charged for calcination at 500\u00a0\u00b0C for 3\u00a0h in a tube furnace under static air with ramping of 10\u00a0\u00b0C/min in order to remove all the nitrate salt present in the catalysts. The calcined catalysts were then reduced by 5% H2/Ar at 600\u00a0\u00b0C for 3\u00a0h at a heating rate of 2\u00a0\u00b0C/min in the same tube furnace. All the synthesized catalysts were used as such for glycerol hydrogenolysis reaction.The textural properties of catalysts were determined from the adsorption\u2013desorption isotherms of nitrogen using Gemini apparatus (Micromeritics 2010 Instrument Corporation). Prior to measurements, the catalyst sample was degassed at 150\u00a0\u00b0C for 24\u00a0h in order to remove all the moisture and foreign gases deposited on the catalyst surface. Then the adsorption and desorption processes of N2 was then analyzed in a vacuum chamber at \u2212196\u00a0\u00b0C. The catalyst surface area was determined by Brunauer-Emmett-Teller (BET) method while the pore size distribution was calculated using the method of Barrett, Joyner and Halenda (BJH).The X-ray diffraction (XRD) analysis was performed in order to analyse the phase composition structure of the crystalline catalysts and its crystallite size. It was conducted using a Shimadzu diffractometer model XRD-6000 by employing CuK\u03b1 radiation source with wavelength of \u03bb\u00a0=\u00a00.1541\u00a0nm, generator current of 30\u00a0mA and voltage of 40\u00a0kV. The finely ground samples were scanned at a speed of 2\u00b0/min using a Siemens D-500 diffractometer and the corresponding diffractogram data were collected from scattering angles at range 2\u03b8\u00a0=\u00a010\u201380\u00b0 while phase identification was determined by matching experimental patterns with the JCPDS diffraction file. The crystallite size (nm) of the catalyst particle was calculated using Debye-Scherrer equation corresponding to full width of half maximum (FWHM) of respective peak.The characteristic of metal reducibility was measured by temperature-programmed reduction (H2-TPR), using Thermo-Finnigan TPD/R/O 1100 SERIES equipped with a TCD (thermal conductivity detector). In a typical experiment, the amount of hydrogen consumption was initially calibrated using known amount of CuO powder as reference standard by pulse chemisorption technique in order to ensure the sensitivity of thermal conductivity detector (TCD) signal. The H2 consumption generated from the calibration of CuO powder was calculated and the value was set as a calibration factor to calculate the H2 uptake for the next analysis. Prior to sample analysis, catalyst sample (~0.05\u00a0g) was pre-treated for removing moisture content using N2 flow at a heating of 120\u00a0\u00b0C for 30 mins (at a rate of 20\u00a0cm3/min) before cooling down to room temperature. After the catalyst pretreatment, an in-situ H2 chemisorption analysis was performed from 50 to 1000\u00a0\u00b0C for 1\u00a0h (10\u00a0\u00b0C/min) in 5% H2/Ar (25\u00a0cm3/min). Thereafter, the data from reduction of chemisorbed sample was measured from the generated peak area of hydrogen consumption.The acid sites distribution and total acidity amount of catalyst were studied by temperature programmed desorption of ammonia (NH3-TPD) (Thermo-Finnigan TPD/R/O 1100 SERIES). Before sample analysis, the TCD signal was initially calibrated using known amount of CuO powder as reference standard. The generated NH3 concentration was then referred as calibration factor value for the next sample analysis. As for sample analysis, catalyst sample (~0.05\u00a0g) was initially carried out with ammonia adsorption in ammonia flow at room temperature for 1\u00a0h. Thereafter, the adsorbed ammonia was desorbed at 50\u20131000\u00a0\u00b0C in helium flow (30\u00a0cm3/min) and with heating rate of 10\u00a0\u00b0C/min. The total acidity amount of the catalyst was determined by the integration of peak area (area under graph) of the analyzed sample.The morphological characteristic of all the catalysts were acquired using scanning electron microscopy (SEM) using an apparatus from Rayny EDX-720. During the analysis, the surface images of a catalyst were spotted through LEO 1455 VP electron microscope in a high-vacuum condition at 20\u00a0kV.The catalytic tests were conducted in a 150\u00a0ml stainless steel autoclave reactor (SS316L series) equipped with Teflon lining cup, an electrical heating jacket and a magnetic stirrer. In a typical experiment, the autoclave reactor was charged with 4\u00a0g glycerol solution, 16\u00a0g distilled water, and 1\u00a0g synthesized catalyst. The reactor then was purged and pressurized with H2 to the desired pressure. Afterwards, the reactor was heated in a defined reaction time for hydrogenolysis reaction. During the catalytic reaction, the reactor was set at maximum H2 pressure, temperature and time of 4\u00a0MPa, 200\u00a0\u00b0C and 10\u00a0h, respectively. For all catalytic reactions, the reactor was left stirred at 400\u00a0rpm. The reaction starting time was defined once the reactor temperature reached the desired reaction temperature. After completion of the reaction, the reactor was cooled down to room temperature, and the obtained liquid product was collected and separated from the catalyst by centrifugation process at 3000\u00a0rpm for 15\u00a0min. For comparison study, a blank reaction (reaction being conducted without the presence of any catalyst powder and/or support) was also performed under similar reaction parameters.The obtained liquid product from glycerol hydrogenolysis reaction was analyzed using gas chromatography-flame ionization detector (GC-FID) equipped with HP-5 capillary column (length: 30\u00a0m \u2a2f inner diameter: 0.32\u00a0mm \u2a2f film thickness: 0.25\u00a0\u00b5m). It was operated at 300\u00a0\u00b0C with splitless inlet mode. Prior to analysis, the liquid product was extracted using ethyl acetate in a 1:1 ratio. The extraction was carried out three times. Subsequently, the product solution was dried in oven at 70\u00a0\u00b0C for 15 mins in order to concentrate the solution. Lastly, a derivatization process was charged to the liquid sample before it is being analyzed by GC analysis. Typically, N-O-bis(trimethylsilyl)trifluroacetamide (BSTFA) was used as the silyl agent and was mixed with pyridine (C5H5N) as binding solvent in a 1:1 ratio and was then left dried in oven for 20 mins at 60\u201370\u00a0\u00b0C so as to achieve complete silylation process. 1\u00a0\u00b5L amount of the derivatized product was directly injected to GC. The initial temperature was determined at 40\u00a0\u00b0C and held for 6\u00a0min with rate of 7\u00a0\u00b0C\u00a0min\u22121 towards reaching the final temperature of 270\u00a0\u00b0C. The temperature for injection was set at 250\u00a0\u00b0C. The glycerol conversion and the selectivity of product were acquired by comparing the retention time of standard with the obtained experimental-based products on GC chromatogram peak. The equations for calculation of glycerol conversion and 1,2-PDO selectivity are depicted in Equation (1.1) and Equation (1.2), respectively.\n\n(1.1)\n\n\n\nGlycerol conversion\n\n,\n\n%\n\n=\n\n\n\nC\n\nglycero\n\nl\n\n,\ni\nn\n\n\n\n\n-\n\nC\n\nglycero\n\nl\n\n,\no\nu\nt\n\n\n\n\n\n\n\u2211\n\nC\n\nglycerol\n,\ni\nn\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(1.2)\n\n\n1\n,\n2\n-\nP\nD\nO\n\ns\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n,\n\n%\n\n=\n\n\nC\n\n1\n,\n2\n-\nP\nD\nO\n\n\n\n\n\n\nC\n\nTotal\n\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\nWhere, Cglycerol,in is described as the initial concentration of glycerol and Cglycerol,out as the final concentration of glycerol. And Ctotal is the sum of the product detected in the liquid product. (All peaks regarded to the product in this study were confirmed by the peak of standard solution)The textural properties of all catalysts derived from N2 adsorption\u2013desorption isotherms are presented in Table 1\n. The BET specific surface area of dolomite, Cu/Dol, Ni/Dol, Co/Dol, Fe/Dol and Zn/Dol, were found to be 13.3, 9.7, 3.5, 7.8, 2.1 and 2.9 m2g1, respectively. The decreasing surface area of supported metal oxide samples as opposed to dolomite was due to the filling of metal oxide in the support pores. This finding was similar to Thirupathi et al. (2012), who stated that the reduction in the surface area of Mn\u2013Ni(0.4)/TiO2 catalyst was due to the blocking effect of the loaded nickel oxide on the support material. The decrease of catalyst\u2019s surface area was consistent with the catalyst pore volume which shows decrease from 0.276 cm3g\u22121 (dolomite) to 0.096, 0.071, 0.145, 0.037, 0.073 cm3g\u22121 for Cu/Dol, Zn/Dol, Co/Dol, Fe/Dol and Ni/Dol catalyst, respectively. This behavior was due to the blockage and destruction of the catalyst structure similar to the report of Zhao et al. (2013).Similarly, different supported metal catalysts exhibited different pore diameter ranging from 19.04\u00a0\u00c5 to 156.34\u00a0\u00c5. Cu/Dol, Fe/Dol and Zn/Dol catalysts showed smaller pore diameter in the range 19.07\u201327.82\u00a0\u00c5 compared to that of dolomite support (152.02\u00a0\u00c5) while Ni/Dol and Co/Dol catalyst presented bigger pore diameter of 155.90\u00a0\u00c5 and 156.34\u00a0\u00c5, respectively. It is worth mentioning that the Cu/Dol, Fe/Dol and Zn/Dol catalysts exhibited both smaller in pore diameter and pore volume of (0.098 cm3g\u22121 and 19.07\u00a0\u00c5), (0.037 cm3g\u22121 and 19.04\u00a0\u00c5) and (0.071 cm3g\u22121 and 27.82\u00a0\u00c5), respectively than dolomite support (0.276 cm3g\u22121 and 152.02\u00a0\u00c5). The small pore diameter and pore volume of those catalysts could be related to the occurrence of new active sites (new pore) formed on the catalyst surface. This characteristic is somehow an advantage because it would reduce the metal species from being easily leached. This consequently may lead to the stronger adsorption\u2013desorption of the active sites during catalytic reaction. Also, the presence of active sites inside the small pore was assumed to help in the reusability and stability for the next reaction cycle.Meanwhile, the N2 adsorption\u2013desorption isotherms and pore size distribution curves of all catalysts are compiled in supplementary material (Fig. S-1 and Fig. S-2). It shows the type III isotherm for dolomite support and all supported metal catalysts. This isotherm was assigned to the weak interaction characteristic of multilayer adsorption of typically clustered catalyst material. Furthermore, the similar isotherm of dolomite support and supported metal catalysts suggesting that the dolomite structure was not significantly modified even with addition of metals. The formation of hysteresis p/p\u00b0 > 0.8 was observed, which showed a characteristic of typical non-rigid aggregates of plate-like particles (slit pore shape) with non-uniform size of the catalyst (Luna et al., 2018). Meanwhile, the isotherms for dolomite support exhibited lower N2 adsorbed volume (~1 cm3g\u22121) than supported metal catalysts, indicating the macroporous characteristic in the catalyst material. The higher N2 adsorbed volume exhibited by supported metal catalysts could be corresponded to the presence of some mesopores. The distribution of pore size curve of all catalysts obtained by Barrett, Joyner, and Halenda (BJH) method shown in Fig. S-2 revealed that the pore size distributions of dolomite, Cu/Dol and Ni/Dol catalysts were in range 2 to 50\u00a0nm. On contrary to the Co/Dol, Fe/Dol and Zn/Dol catalysts, the pore sizes were in range 1\u201390\u00a0nm.The XRD diffractograms of calcined and reduced samples are presented in Fig. 1\n(A) and Fig. 1(B), respectively. The XRD pattern of dolomite support was observed with mixed crystalline phases. The diffraction peaks at 2\u03b8\u00a0=\u00a018.1\u00b0, 28.3\u00b0, and 33.8\u00b0 were assigned to CaMg2 (JCPDS; 01\u20131070). The peaks at 2\u03b8\u00a0=\u00a037.51\u00b0, 50.76\u00b0, and 62.20\u00b0 were due to dolomite phase (JCPDS; 02\u20130767) while two peaks at 2\u03b8\u00a0=\u00a044.2\u00b0 and 47.4\u00b0 were denoted as MgCO3 phase ((JCPDS; 02\u20130871). The presence of CaO phase shows peak at 2\u03b8\u00a0=\u00a032.4\u00b0 and 54.2\u00b0 (JCPDS; 01\u20131160) and the less intense peak for MgAl2O4 phase was detected at 2\u03b8\u00a0=\u00a078.7\u00b0 (JCPDS; 03\u20131160). These phases are also present in all supported metal catalysts but with reduced peak intensities as a consequence of the embedment of metal oxides in the dolomite matrix. In return new phases corresponded to the respective metal oxide were seen such as CuO at 2\u03b8\u00a0=\u00a035.5\u00b0 and 38.8\u00b0 (JCPDS; 44\u20130706), NiO at 2\u03b8\u00a0=\u00a037.2\u00b0 (JCPDS; 01\u20131239), Fe2O3 at 2\u03b8\u00a0=\u00a023.7\u00b0, 34.1\u00b0, 50.1\u00b0 and 58.2\u00b0 (JCPDS; 02\u20130915) and ZnO at 2\u03b8\u00a0=\u00a032.5\u00b0, 36.5\u00b0, 56.8\u00b0, 68.3\u00b0 and 69.7\u00b0 (JCPDS; 03\u20130888).Nevertheless, in the case of Co/Dol catalyst, no characteristic peak of CoO phase was detected, instead cobalt carbonate (CoCO3) was formed at 2\u03b8\u00a0=\u00a034.1\u00b0 (JCPDS; 01\u20131020). Apart from that, an alloy phases were also detected with the presence of MgNiO3 spinel (JCPDS; 03\u20130999) at 2\u03b8\u00a0=\u00a075.2\u00b0, Ca2Fe2O5 spinel (JCPDS; 02\u20130936) at 2\u03b8\u00a0=\u00a024.8\u00b0, 34\u00b0, 45.5\u00b0 and 60.1\u00b0, MgZn (JCPDS; 08\u20130206) and CaZn3 (JCPDS; 35\u20131159) at 2\u03b8\u00a0=\u00a050.2\u00b0 and 2\u03b8\u00a0=\u00a053.5\u00b0, respectively. On the other hand, it was noted that there was no characteristic peak related to any metallic species was observed in all calcined samples.The XRD patterns of the reduced catalysts in Fig. 1(B) shows that the intensity of diffraction peak at 2\u03b8\u00a0=\u00a043.5\u00b0 corresponded to MgCO3 phase of all supported metal catalysts became more intense and higher upon the addition of respective metal to dolomite support. This is due to the interaction of metal species with dolomite support. For reduced dolomite, apart from peaks presented in calcined dolomite, new peaks were also detected such as CaCO3 at 2\u03b8\u00a0=\u00a029.3\u00b0 and 72.2\u00b0 (JCPDS; 01\u20131032). For supported metal catalysts, it was displayed that upon reduction by H2 at 600\u00a0\u00b0C, the diffraction peaks of CuO (2\u03b8\u00a0=\u00a035.5\u00b0, 38.8\u00b0) and NiO (2\u03b8\u00a0=\u00a037.2\u00b0) of Cu/Dol and Ni/Dol catalysts disappeared, while the characteristic peaks attributed to metallic Cu species (2\u03b8\u00a0=\u00a043.5\u00b0 and 74.1\u00b0) (JCPDS; 085\u20131326) and metallic Ni species (2\u03b8\u00a0=\u00a044.2\u00b0, 52.1\u00b0 and 76.1\u00b0) (JCPDS; 001\u20131260) were emerged. Similar diffraction peaks of metallic copper and nickel was also reported by Wen et al. (2013) and Srivastava et al. (2017). Additionally, it was found that no characteristic peak attributed to any Cu2O and Ni2O phases was detected in Cu/Dol and Ni/Dol catalysts, indicating the reduction of Cu2+ and Ni2+ species was complete (Zhu et al., 2013, Zhao et al., 2013and Gandarias et al., 2012). The presence of metallic Cu and Ni species are regarded as active reaction site for the catalytic reaction and thus could increase the glycerol hydrogenolysis reaction.The presence of Cu0 and Ni0 species was attributed to their high reduction ability from metal oxides-dolomite interaction. It could be suggested that the migration of electron (oxidation and reduction) happened on metal oxide-support surface via electrons lone pair would cause the destabilization of metal oxide bond and thus promote the reducibility of oxides (Nagaraja et al. 2007). In this present work, the CaO, CaMg(CO3)2 and MgCO3 species were suggested to be the one involved for the copper and nickel oxide reduction since calcium and magnesium has been identified as good reducing agent (Tasyurek et al., 2018).Accordingly, it has been revealed that metal oxide species was prone to generate spinel when it was supported with clay or limestone material containing Mg and Ca (Kovanda et al., 2001; Pardeshi et al., 2010). Apparently, the Cu2MgO3 (2\u03b8\u00a0=\u00a035.3\u00b0, 37.5\u00b0, 38.2\u00b0 and 48\u00b0), MgNiO2 (2\u03b8\u00a0=\u00a075.3\u00b0 and 79.2\u00b0) and (Ca2Fe2O5) (2\u03b8\u00a0=\u00a023\u00b0, 24\u00b0, 32\u00b0, 33\u00b0, 34\u00b0, 44\u00b0, 47\u00b0 and 49\u00b0) phases were detected for Cu/Dol, Ni/Dol and Fe/Dol catalysts, respectively, thereby confirming the formation of metal species in spinel. For Co/Dol and Zn/Dol catalysts, alloy phases of Co2Mg and (CaZn3, MgZn) were detected. Notably, no characteristic peak of any metallic Co, Fe, and Zn species was observed in Co/Dol, Fe/Dol, and Zn/Dol catalysts possibly due to the incomplete H2-reduction of the catalysts. In particular for Zn/Dol catalyst, ZnO phases was obviously seen, indicating higher reduction temperature is required to transform the oxide phase into metallic species.The dolomite\u2019s crystallite size was estimated from the XRD peak by choosing 2\u03b8\u00a0=\u00a062.45 and the results are summarized in Table 1. The dolomite\u2019s crystallite size was found increases when Ni and Cu were supported on it. The trend of crystallite size ranks as Cu/Dol\u00a0>\u00a0Ni/Dol\u00a0>\u00a0Dol\u00a0>\u00a0Co/Dol \u2248 Zn/Dol\u00a0>\u00a0Fe/Dol. The larger crystallite size of Cu/Dol and Ni/Dol than dolomite probably attributed to the metal species which occupied in the interstitial support bulk thus increased the catalyst crystal size. This could also be correlated to the non shifted peak of MgCO3 phase at 2\u03b8\u00a0=\u00a043.5\u00b0, with respect to dolomite support. The non shifted peak reflected to the presumption that metal promoter was incorporated well on the support (Asikin et al., 2017). However in the case of Co/Dol, Fe/Dol and Zn/Dol catalysts, the dolomite\u2019s crystallite size were decreased from 27.4\u00a0nm to 22.9\u00a0nm, 19.6\u00a0nm, and 22.9\u00a0nm, respectively, indicating Co, Fe and Zn species were prone to dissolve in the support lattice as substitutional metal rather than occupied in the interstitial support lattice (Liu et al., 2014). The presence of substitutional metal could be also corresponded to the shifted peak of MgCO3 phase at 2\u03b8\u00a0=\u00a043.5\u00b0 to slightly lower degree than dolomite peak.In the case of Fe/Dol and Zn/Dol catalysts, a higher reduction temperature of 900\u00a0\u00b0C was applied to reduce both oxide species into their metallic form and the XRD patterns are depicted in Fig. 2\n. The results obtained were compared with the previous catalysts reduced at 600\u00a0\u00b0C. It can be seen that the characteristic peak of metallic Fe (JCPDS; 01\u20131267) was clearly appeared at 2\u03b8\u00a0=\u00a044.8\u00b0 indicating that the reduction temperature of 900\u00a0\u00b0C successfully reduced iron oxide to its metallic species. Meanwhile the formation of Ca2Fe2O5 spinel was also noticeable. Nevertheless, it was observed that the presence of Ca2Fe2O5 spinel became gradually invisible as compared to 600\u00a0\u00b0C reduced sample which suggested that the spinel species was also reduced at higher temperature.In the case of Zn/Dol catalyst, no characteristic peak attributed to metallic Zn species was detected even after reduction at 900\u00a0\u00b0C, rather the presence of alloy phase (CaZn3 and MgZn). This indicates that reduction at 900\u00a0\u00b0C was still not able to transform Zn oxide into its metallic form. However, it was noticed that the diffraction peaks of ZnO phase at 2\u03b8\u00a0=\u00a032.5\u00b0, 36.5\u00b0, 56.8\u00b0, 68.3\u00b0 and 69.7\u00b0 disappeared, while CaZn3 and MgZn phases at (2\u03b8\u00a0=\u00a032.5\u00b0 and 54.5\u00b0) and (2\u03b8\u00a0=\u00a068.5\u00b0 and 75.5\u00b0), respectively became more intense peak. This finding is in good agreement with the work of Consonni et al. (1999), who investigated the reduction property of Pt/ZnO catalyst and found that the reduced ZnO catalyst had resulted to the formation of PtZn alloy instead of metallic Zn species.The H2-TPR profiles of dolomite and supported metal catalysts (Cu/Dol, Co/Dol, Zn/Dol, Ni/Dol and Fe/Dol) are depicted in Fig. 3\n(A) while the corresponding hydrogen consumption data is tabulated in Table 2\n. From TPR profiles, it was discovered that Cu/Dol and Co/Dol catalysts gave a lower reduction peaks as opposed to dolomite support at 689\u00a0\u00b0C. The reduction of Cu/Dol and Co/Dol was assigned at (291, 455 and 630\u00a0\u00b0C) and (435 and 638\u00a0\u00b0C), respectively. Apart from that, it is worthy to note that, the reduction peak of all supported metal catalysts was found to emerge broader and higher than dolomite due to the species reduction from metal alloy phases thereby consumed higher hydrogen adsorption and hence enlarged the reduction peak (Li et al., 2009). Similar behavior was outlined by Soares et al. (2016a,b), the authors indicated that the broader reduction range was detected after addition of Cu to Ru/ZrO2 catalyst which caused interphase hydrogen adsorption of the metals (slow adsorption) due to metal cluster formation from Ru and Cu alloys.According to Zhao et al. (2017), the reduction of dispersed copper oxide species to metallic copper (Cu0) was effective at\u00a0<\u00a0250\u00a0\u00b0C. Smaller catalyst particles reduce faster when compared with that of CuO in bulk (Zhu et al., 2013). Correspondingly, the reduction of bulk CuO phase took place at temperature higher than 250\u00a0\u00b0C (Wen et al., 2013). According to Vargas-Hernandez et al. (2014), reduction at\u00a0>\u00a0400\u00a0\u00b0C was due to the metal-support species or copper in spinel phase. Tanasoi et al. (2009) reported that the Cu-containing mixed oxide reduced at range 400\u2013750\u00a0\u00b0C due to the presence of complex copper phases of CuAl2O4 and CuxMgxAl2O4. Therefore, in this study, the first two reduction profiles of Cu/Dol catalyst were assigned for reduction of CuO to metallic Cu. Peak at 291\u00a0\u00b0C corresponded to the reduction of small and big clusters of CuO to metallic copper (Cu0) while peak at 455\u00a0\u00b0C attributed to the reduction of copper oxide in interstitial defects in dolomite crystalline phase since Cu2MgO3 was previously detected by XRD peak. Reduction of CuO corresponded to two reduction steps of Cu2+ ions to Cu+ ions (CuO\u00a0\u2192\u00a0Cu2O), followed by reduction of Cu+ ions to metallic copper (Cu2O\u00a0\u2192\u00a0Cu0). Peak maximum at 630\u00a0\u00b0C was ascribed to reduction of dolomite because the peak profile was close to that of bulk dolomite (639\u00a0\u00b0C).As for Co/Dol catalyst, it was reported that the reduction temperature of CoO to Co was occurred below 400\u00a0\u00b0C (Yan et al., 2011). Thus, peak at 435\u00a0\u00b0C could be referred to reduction of cobalt species from CoCO3 phase as presented in XRD profile in Fig. 1(B). The presence of metallic cobalt species was not noticed in XRD profile probably due to well dispersed metallic Co species or with minor proportion, rather the formation of CoMg2 phase was observed. Apart from that, the formation of CoCO3 peak was still detected in the reduced catalyst. This shows that cobalt species in the form of carbonate was not easily reduced at 600\u00a0\u00b0C. As stated in literature, the reduction cobalt oxide depends on the cobalt particle size and the properties of the support used (Yan et al., 2011). The presence of broad peak at 638\u00a0\u00b0C could be due to the reduction of cobalt species which strongly interacted with support. From this study, the high metal reducibility and lower reduction temperature of Cu/Dol and Co/Dol catalysts could be due to the good electronic interaction of Cu and Co oxide with calcium and magnesium species from dolomite.For Ni/Dol, Fe/Dol and Zn/Dol catalysts, higher reduction temperature was observed at (690 and 962\u00a0\u00b0C), (646 and 946\u00a0\u00b0C) and 700\u00a0\u00b0C, respectively. In the case of Ni/Dol catalyst, the broader and higher peaks at 690\u00a0\u00b0C and 962\u00a0\u00b0C than that of dolomite peak could be attributed to the reduction of nickel and dolomite species which had stronger metal-support interaction or attributed to the reduction MgNiO2 phase. Similar results were proposed by Srivastava et al. (2017), who stated that the broad peak and high reduction temperature of Ni/Al2O3 catalyst was due to the reduction of NiO species which was in intimate contact with Al2O3 support and/or attributed to the reduction of NiAl2O4 phase. For Fe/Dol catalyst, peak at 646\u00a0\u00b0C was attributed to the reduction of dolomite. Peak at 946\u00a0\u00b0C was ascribed to the reduction of iron species in Ca2Fe2O5 spinel phase which strongly interacted with dolomite. In the case of Zn/Dol catalyst, peak at 700\u00a0\u00b0C could most likely be related to the reduction of dolomite with zinc species from CaZn3 and MgZn alloys. On a general note, Cu/Dol catalyst could be proposed to predominantly exhibit higher metal reducibility than Co/Dol due to its lower reduction temperature. The presence of metallic Cu in XRD peak agreed well with its high reduction character. This observation provided the bases for conducting the hydrogenolysis of glycerol reaction at 200\u00a0\u00b0C since the presence of active reaction sites (metallic copper species) would be preserved and thus stable during the catalytic reaction.From Table 2, it was observed that the total hydrogen consumption of all supported metal catalysts was higher than that of dolomite following this trend Co/Dol\u00a0>\u00a0Ni/Dol\u00a0>\u00a0Fe/Dol\u00a0>\u00a0Cu/Dol\u00a0>\u00a0Zn/Dol\u00a0>\u00a0Dol. This finding could be correlated to a study reported by Zhao et al. (2019) who stated that the total amount of H2 consumed for CuO/CeO2 catalyst was far exceeded than that necessary for the complete reduction of pure CuO, specifying that some ceria support would be involved during the reduction process. In this study, the addition of respective metal to dolomite support influenced catalyst reducibility due to higher species exposure area and thus elevates the hydrogen consumption amount. Mallesham et al. (2016) and Gandarias et al. (2012) proposed that when a support was promoted by reducible metal oxide, the availability of hydrogen to be consumed was enhanced as well as the catalytic hydrogenation of alcohol. The presence of CaCO3, MgCO3, CaO and MgAl2O4 phases from dolomite could be considered to provide a source of chemisorbed hydrogen atoms and increased the amount of hydrogen uptake.Additional analysis of metal oxides reduction was carried out as shown in Fig. 3(B) while the corresponding hydrogen consumption data is given in Table 2. It can be seen that copper oxide shows the lowest reduction temperature at 338 and 428\u00a0\u00b0C, while the reduction peak for nickel oxide was appeared at 392\u00a0\u00b0C. The peak for cobalt oxide and iron oxide was at 498\u00a0\u00b0C and (416\u00a0\u00b0C and 821\u00a0\u00b0C), respectively. In the case of zinc oxide, the reduction profile was rather flat and the blow-up image shows peaks at 411\u00a0\u00b0C and 702\u00a0\u00b0C. Comparing the reduction profiles of both Cu and Co oxides with that of the supported metal catalysts, the later gave a lower reduction peaks. This confirms that the reducibility of those oxide species in supported catalysts was enhanced which due to the promotional effect of metal oxide-dolomite interaction. On the other hand, the reduction of nickel oxide, iron oxide and zinc oxide was noticed far from the reduction region of their supported metal catalysts. This confirms the poor metal reducibility of their oxides. From Table 2, it was demonstrated that the H2 consumption needed for the complete reduction of metal oxides was in range 47\u2013929\u00a0\u00b5mol/g which was obviously far less than the amount required for supported metal catalysts (14955\u201373962\u00a0\u00b5mol/g). Thus, it was suggested that the capability for hydrogen consumption was enhanced in the case of metal oxide supported on dolomite.The available acid sites of dolomite and all supported metal catalysts were performed via NH3-TPD. The classification of acid strength was interpreted as weak (<250\u00a0\u00b0C), medium (250\u2013500\u00a0\u00b0C) and strong (>500\u00a0\u00b0C) (Srivastava et al., 2017). The desorption profiles of all catalysts are indicated in Fig. 4\n while the corresponding acidity (amount of ammonia uptakes) is tabulated in Table 3\n. All catalysts exhibited desorption peaks above 500\u00a0\u00b0C, indicating the presence of strong acid sites on the catalyst surface. In all supported metal catalysts, the presence of desorption peaks showed rather smaller and lower desorption profile than dolomite support. Cu/Dol catalyst appeared with higher desorption temperature (at 948\u00a0\u00b0C) than dolomite (at 874\u00a0\u00b0C) and other supported catalysts. Dolomite showed two desorption peaks at 805\u00a0\u00b0C and a shoulder at 874\u00a0\u00b0C. For Cu/Dol and Co/Dol catalysts, two desorption peaks emerged at (718 and 948\u00a0\u00b0C) and (712\u00a0\u00b0C and 815\u00a0\u00b0C), respectively. In the case of Ni/Dol, Fe/Dol and Zn/Dol, only one desorption peak appeared which at 722\u00a0\u00b0C, 672\u00a0\u00b0C and 755\u00a0\u00b0C, respectively.Notably, the desorption peak of supported metal catalysts with the exception of Cu/Dol shifted towards lower desorption temperature than dolomite from 805 to 874\u00a0\u00b0C (dolomite) to 672\u2013815\u00a0\u00b0C (supported metal catalysts). In the case of Cu/Dol, the second desorption peak was shifted to 948\u00a0\u00b0C (Cu/Dol), indicating the presence of much stronger acid sites in the catalysts. This attributable to the strong interaction of copper on the dolomite support and a sign that copper was well incorporated and dispersed over dolomite surface than other catalysts. The acidity data in Table 3 shows that the addition of respective metal promoter to dolomite support contributed to different acid amount. Incorporation of nickel, cobalt and iron, gave a decrease in the acidity from 16149\u00a0\u00b5mol/g (dolomite) to 14305, 11172, 6075\u00a0\u00b5mol/g for Ni/Dol, Co/Dol and Zn/Dol catalyst, respectively. This could be assigned to the coverage of the metal species (Ni, Co and Fe) on dolomite surface, therefore allowed limited accessibility of NH3 gas to be bonded with the catalyst pore. This finding was in agreement with the work of Priya et al. (2017), who proposed the decrease of acid amount in metal-promoted mordenite catalyst was attributed to the blockage caused by the metal species. In another study reported by Vasilidiaou et al. (2009), it claimed that the decrease in catalyst acidity of Ru-supported catalyst was due to the Ru species was possibly reside (occupied on the alumina support surface).On the contrary, when copper and zinc was added to dolomite support, the acidity was increased from 16149\u00a0\u00b5mol/g (dolomite) to 19528\u00a0\u00b5mol/g (Cu/Dol) and 17085\u00a0\u00b5mol/g (Zn/Dol), respectively. The similar behavior was also reported by Thirupathi et al. (2012), where the addition of nickel species on Mn/TiO2 catalyst improved and broadened the acid site distribution of the catalyst. The order of catalyst acidity ranks as Cu/Dol\u00a0>\u00a0Zn/Dol\u00a0>\u00a0Dol\u00a0>\u00a0Ni/Dol\u00a0>\u00a0Co/Dol\u00a0>\u00a0Fe/Dol. It should be noted that the high acid sites of Cu/Dol and Zn/Dol catalysts could act as active reaction sites which contribute to high reactivity of C-O bond cleavage of glycerol molecule during dehydration step and consequently enhance the catalytic reaction of glycerol hydrogenolysis to a higher level. The presence of carbonate phases (CaCO3, MgCO3) in dolomite could be the one responsible to provide a source of chemisorbed NH3 gas and increase catalyst acidity of Cu/Dol and Zn/Dol catalysts.\nFig. 5\n shows the surface morphology of all catalysts. As seen in the figures, all samples present agglomerated structure with an irregular shape of an average size of 10\u00a0mm (from scale bar), emphasizing the formation of a macroporous solid in a cluster of closely spaced crystals.The glycerol hydrogenolysis reaction was carried out and the results of catalytic activity are summarized in Table 4\n.A reaction without the presence of catalyst and/or support was also conducted and referred to as blank experiment. For the blank experiment, a very low glycerol conversion (8.7%) and no selectivity towards 1,2-PDO were observed. When dolomite was added, a little increase of glycerol conversion of 10.6% was observed but still no selectivity to the desired product (1,2-PDO). These results indicate that the support by itself cannot catalyze the hydrogenolysis of glycerol. A significant catalytic activity was observed on supported metal dolomite samples. Cu/Dol exhibited the highest activity in both glycerol conversion (78.5%) and 1,2-PDO selectivity (79%) among all supported metal catalysts. In contrast, Ni/Dol, Co/Dol, Fe/Dol and Zn/Dol catalysts exhibited lower glycerol conversion and 1,2-PDO selectivity of (69.5%, 52.7%), (60.9%, 58.1%), (44.8%, 5%) and (20.4%, 2.7%), respectively.Notably, Zn/Dol showed the lowest activity of both glycerol conversion (20.4%) and 1,2-PDO selectivity (2.7%) while Ni/Dol and Co/Dol catalysts presented moderate activity in both glycerol conversion and 1,2-PDO selectivity. The trend of catalytic activity ranks as Cu/Dol\u00a0>\u00a0Ni/Dol \u2248 Co/Dol\u00a0>\u00a0Fe/Dol\u00a0>\u00a0Zn/Dol\u00a0>\u00a0dolomite. A high glycerol conversion over Cu/Dol catalyst was due to its high surface acid sites. The reaction was initiated by the adsorption of glycerol on the acid sites, dehydrated and then converted to give acetol as intermediate product, and consequently yielded to 1,2-PDO. Therefore, a higher acidity provides a greater number of acid sites, hence a greater number of glycerol molecules can be adsorbed on the catalyst surface (Cu/Dol) than other catalysts. This result is consistent with the literature reported that the acid site was a great influencing factor in hydrogenolysis reaction (Putrakumar et al., 2015; Yuan et al., 2009).Apart from that, metallic site is also important for the hydrogenation of acetol intermediate to 1,2-PDO. In the case of Cu/Dol catalyst, based on the H2-TPR profile presented in Fig. 3(A), it showed that copper species of Cu/Dol catalyst was essentially reduced at the lowest reduction temperature (291\u00a0\u00b0C) than other supported metal catalysts. According to that, the catalytic reaction conducted at 200\u00a0\u00b0C in this study is within the reduction temperature region of copper species (\u2265~200\u00a0\u00b0C). While for Ni/Dol, Co/Dol, Fe/Dol and Zn/Dol, the presence of respective metallic species was poor and incomplete since the reduction temperature of the catalysts occurs at higher temperature of 690\u00a0\u00b0C, 435\u00a0\u00b0C, 646\u00a0\u00b0C, and 700\u00a0\u00b0C, respectively and thus corresponded to the lower 1,2-PDO selectivity obtained by those catalysts.For Fe/Dol and Zn/Dol catalysts, the absence of the metallic species when catalysts were reduced at 600\u00a0\u00b0C (Fig. 2(B)) was consistent with the poor activity attained by the catalysts. However, when both catalysts were reduced at 900\u00a0\u00b0C, improved activity result was obtained. This was attributed to their metal reduction behavior, of which iron and zinc oxide species was low reducible compared to copper, nickel and cobalt oxides. Apart from the formation of high 1,2-PDO product, another essential character for a good hydrogenolysis catalyst is the ability to promote high dehydration of glycerol to form acetol intermediate product and in return suppress the excess hydrogenation reaction towards side product, methanol (C-C bond cleavage). In this study, it was revealed that no selectivity towards acetol intermediate was detected for Fe/Dol and Zn/Dol catalysts. Thus, the poor selectivity towards 1,2-PDO obtained by both catalysts could be related to their incapability for producing acetol as intermediate product which subsequently hydrogenate to desired 1,2-PDO product. In addition, both catalysts also prone to catalyse C\u2013C cleavage reaction as the selectivity towards methanol were significantly higher with 95% and 97.2%, respectively than other supported metal catalysts. Cu/Dol catalyst exhibited the lowest selectivity towards methanol (2.1%), indicating the addition of Cu to dolomite support seemed to hinder C-C cleavage reaction.In this study, various metals supported on dolomite (Cu/Dol, Ni/Dol, Co/Dol, Fe/Dol and Zn/Dol) were synthesized for glycerol hydrogenolysis reaction and Cu/Dol catalyst was found to exhibit a promising activity when compared to other catalysts with highest glycerol conversion and 1,2-PDO selectivity of 78.5% and 79%, respectively at 200\u00a0\u00b0C of reaction temperature, 4\u00a0MPa of reaction pressure and 10\u00a0h of reaction time. The performance was attributed to its high acidity and high metal reducibility. Additionally, the reduction profile of Cu/Dol which occurred within the range of catalytic reaction temperature (at 200\u00a0\u00b0C) was important to preserve the stability of metallic Cu during catalytic reaction. The finding from this study is a valuable step in search of precious metal free and environmentally benign catalysts for the development of biomass valorization.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors thank the Universiti Putra Malaysia for the Research Grant under Geran Inisiatif Putra Siswazah, GP-IPS/2018/9619500 in support of the project.All authors contributed to the success of this paper from the conception to the methodology, analysis of the results, writing proofreading, and review of the paper.Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2021.103047.The following are the Supplementary data to this article:\n\nSupplementary Data 1\n\n\n\n", "descript": "\n                  Hydrogenolysis of biomass-derived glycerol is an alternative route to produce 1,2-propanediol. A series of transition metals supported on dolomite catalysts (Cu/Dol, Ni/Dol, Co/Dol, Fe/Dol, Zn/Dol) were prepared via impregnation, calcined at 500\u00a0\u00b0C and reduced at 600\u00a0\u00b0C. The synthesized catalysts were then characterized by BET, BJH, XRD, H2-TPR, NH3\u2013TPD, and SEM, and subsequently evaluated in glycerol hydrogenolysis reaction to produce 1,2-propanediol (1,2-PDO). The nature of transition metals was found to influence the activation of the catalysts. Among the tested catalysts, copper supported on dolomite (Cu/Dol) exhibited appreciable hydrogenolysis performance due to the mutual interaction between the copper species and the dolomite. The findings from the various characterization results showed the addition of copper to dolomite ameliorates the chemical and reduction of the catalyst. It appears that the copper species were essentially enriched on the grain surfaces of the dolomite, the reduction properties, and the acidity of the catalyst enhanced. All the features of Cu/Dol catalyst contributed to the high glycerol conversion (78.5%) and high 1,2-PDO selectivity (79%) with low methanol production as the by-product at 200\u00a0\u00b0C of reaction temperature, 4\u00a0MPa of reaction pressure and 10\u00a0h of reaction time.\n               "}
{"full_text": "With the increasing demand for energy, it is expected that the world's total energy consumption will increase from 30% to 50% in the next 20 years. The use of traditional fossil fuels will accelerate the increase in greenhouse gases and change the climate environment. As shown in Fig.\u00a01\n, the total apparent CO2 emissions in China have continued to increase over the past few decades. At the General Debate of the 75th United Nations General Assembly, China made a solemn commitment to achieve a \"carbon peak\" by 2030 and a \"carbon neutral\" by 2060, which also shows the determination of mankind to reduce carbon dioxide content. This has accelerated the development of a range of technologies for the degradation/reduction of CO2, including chemical, photocatalytic, electrocatalytic and photoelectrochemical methods (Chang\u00a0et\u00a0al., 2019; Guan\u00a0et\u00a0al., 2021; Prabhu\u00a0et\u00a0al., 2020; Zhang\u00a0et\u00a0al., 2020c; Zhang\u00a0et\u00a0al., 2019b), in the hope of converting CO2 into useful chemical products/fuels in dynamic equilibrium. Among these technologies, the electrocatalytic reduction of CO2 (CO2RR) to CO and other chemical products using renewable electricity serves the twofold purpose of CO2 storage and clean energy conversion, the possible reaction mechanism of which is indicated in Fig.\u00a01. The CO2RR is appealing for both industry and academia with the following advantages: (1) it is easy to combine with other renewable energy sources, such as solar energy and wind energy, and will not generate additional CO2; (2) it has a low cost, safe operating conditions and mild reaction conditions, i.e., room temperature and normal pressure; (3) the reaction pathway can be controlled by electrochemical parameters; (4) by optimizing the electrocatalyst, CO2 reduction byproducts can be minimized; (5) the electrolyte can be reused to reduce its overall chemical consumption; and (6) the electrochemical system can be applied in practice, with the characteristics of compact, modular and on-demand expansion (Clark\u00a0et\u00a0al., 2018; Feng\u00a0et\u00a0al., 2021; Koshy\u00a0et\u00a0al., 2021; Lee\u00a0et\u00a0al., 2021; Morimoto et\u00a0al., 2018). Therefore, the electrochemical reduction of CO2 has received extensive attention and has become an effective method with potential.However, there are still many problems and challenges in the electrocatalytic CO2RR, including the following: (1) As CO2 with linear chemical bonds is a chemically inert molecule, it is difficult to activate. The electrocatalytic process of converting CO2 into desired products is challenging. (2) Electrocatalytic reduction of CO2 often faces problems such as high reaction potential, poor product selectivity and inability to maintain high catalytic activity for a long time. (3) The side reaction hydrogen evolution reaction (HER) must be suppressed. Although H2 is a good clean energy source, it has the disadvantage of low energy density, as well as H2 transportation, storage and safety issues. That is why our goal is to suppress H2 production while maximizing the production of energy-intensive carbon-based fuels. Therefore, it is very important to select suitable electrocatalysts to improve the selectivity stability and reduce the overpotential.As the only known monometal that can reduce CO2 to a number of hydrocarbons, aldehydes, and alcohols, metallic Cu has been extensively studied (Peterson\u00a0et\u00a0al., 2010; Schouten\u00a0et\u00a0al., 2012; Zhang\u00a0et\u00a0al., 2022c). This has also led to an upsurge in CO2RR research on different metal and alloy catalysts. Second-phase metals were introduced to provide active sites (Andrews\u00a0et\u00a0al., 2018; Barasa\u00a0et\u00a0al., 2019; Li\u00a0et\u00a0al., 2021d; Zhang\u00a0et\u00a0al., 2018c), thereby improving the activity and selectivity of the reaction. As a representative of traditional electrocatalysts, metal-based electrocatalysts have an irreplaceable position in the field of electrocatalysis. However, metal-based electrocatalysts themselves are always expensive pure noble metals (Ag, Au, Pt, etc.) or need to be doped with noble metals (Ag, Au, etc.), which leads to an increase in the cost and disturbs the HER side-reaction. Porphyrin-based frameworks, as a special class of metal-organic frameworks and covalent organic frameworks, have also been widely used in the field of energy conversion. Their unique and tunable structures greatly reduce the design difficulty for highly selective electrocatalysts (Liang\u00a0et\u00a0al., 2021). On the other hand, with the increasing demand for the environment, nonmetallic nitrogen-doped carbon electrocatalysts using biomass as carbon precursors have received extensive attention. Such biomass-derived catalysts could achieve comparable activity to the first three types of catalysts without the introduction of expensive noble metals and exhibit a lower Faradaic efficiency for the HER, making them inexpensive and environmentally friendly electrocatalysts. We therefore systematically reviewed the recent important progress of CO2RR electrocatalysts, including metal-based electrocatalysts, single-atom catalysts (SACs), porphyrin-based framework complexes and metal-free nitrogen-doped carbon electrocatalysts, in Section\u00a02. Challenges and future research directions are also proposed.The reaction thermodynamics and kinetics determine the activity and selectivity of electrocatalysts (Fig.\u00a02\n). It is important to focus on the thermodynamic and kinetic details of various CO2RR catalysts. Thermodynamically, reactants can be driven only if the free energy of the system is reduced after the reaction. For the electrocatalytic reaction, the energy of electrons can be tuned by the biased potential and thus launch the reaction. Generally, the thermodynamics of the electrocatalytic reactions are relatively simple to obtain with the computational hydrogen electrode (CHE) model (Norskov\u00a0et\u00a0al., 2004). Dynamically, reaction rates depend exponentially on the reaction energy barrier, i.e., r \u223c exp (-Ea/kT). To calculate the energy barrier, the transition state calculation is usually performed by DFT NEB (Kildgaard\u00a0et\u00a0al., 2020) or the Br\u00f8nsted-Evans-Polanyi (BEP) relation (\n\n\nE\na\n\n=\n\u03b1\n\n\u0394\n\nG\n+\n\u03b2\n\n) (Darby\u00a0et\u00a0al., 2018; Liu\u00a0et\u00a0al., 2019a). Although the numerical relationship between the CO2RR thermodynamics and the kinetics has not been established, we still know that the reactive thermodynamics determines the energy barrier. As in the Sabatier principle, a volcano relationship could be obtained when correlating the electrocatalyst activity with the adsorption energies of key intermediates, and the optical performance comes with a moderate adsorption strength that is neither too strong nor too weak. (Valdes\u00a0et\u00a0al., 2012) The most widely used thermodynamic model, the CHE model, the improved models involving kinetics such as the implicit/explicit solvation model, the H-shuttling model, the water-solvated model, and the possibility of thermokinetic models are reviewed and discussed in Section\u00a03.In terms of the current development of CO2RR, catalyst design and theoretical investigation are equally important and mutually promoted. Obtaining a highly effective catalyst is important, and the structure-dependent mechanism and the reactive thermodynamics and kinetics behind the performance are even more important. We will review the preparation, performance, and reactive mechanism as well as the theoretical insights of four groups of efficient CO2RR catalysts in recent years. Aside from the traditional theoretical techniques either thermodynamically or kinetically, a new concept of thermodynamic-kinetic synergy is also highlighted, which could probably facilitate the design and selection of CO2RR electrocatalysts. Finally, the issues of current catalyst and theory as well as the outlooks for future work are offered.Metal-based catalysts are one of the most popular electrocatalytic materials for the CO2RR, which not only provide abundant binding sites for reaction intermediates but also produce different valuable chemical products, such as carbon monoxide, methane, formic acid, methanol, and ethylene. In addition, it is interesting that unstable metal-based compounds can also be used as catalysts, which would be in situ reduced to the metallic state during the CO2RR, leading to the reconstruction of surface structures with higher CO2RR activity (He\u00a0et\u00a0al., 2021). The early work of pure metal catalysts was performed by Hori in the 1980s, where various metal electrodes were tested for the CO2 electroreduction reaction (Hori\u00a0et\u00a0al., 1994; Ju\u00a0et\u00a0al., 2019). This work opened the door for research on CO2RR. Based on multiple reaction routes and products, as shown in Fig.\u00a03\n, pure metals can be divided into three main groups (Hori\u00a0et\u00a0al., 1985; Wu\u00a0et\u00a0al., 2020a): (1) transition metals generating CO as the main product, such as Ag, Au and Pd, etc. (2) metals generating HCOOH as the predominant product, such as Sn, Pd and Bi, etc. (3) Cu is the only metal that is capable of producing a considerable yield of hydrocarbons and multi-carbon products (Weng\u00a0et\u00a0al., 2018). The representative pure metals and metal alloys developed recently as CO2RR electrocatalysts are reviewed as follows, focusing on their preparation, performance and reaction mechanism. Considering the existence of a mass of reviews focused on metal-based catalysts in recent years (Hoang\u00a0et\u00a0al., 2020; Li\u00a0et\u00a0al., 2021b; Wu\u00a0et\u00a0al., 2020b),\nNanostructured Cu catalysts. Cu, as a kind of metal catalyst, often exhibits limited selectivity and activity toward a specific product, leading to low productivity and substantial postreaction purification. In recent year, nanostructured Cu have attracted much attention for the CO2RR (Jeon\u00a0et\u00a0al., 2017; Kim\u00a0et\u00a0al., 2017a; Zhang\u00a0et\u00a0al., 2022b). For example, Cu nanoparticles, nanocubes and nanoclusters have shown the Faradaic efficiency (FE) of the C2 product (mainly ethanol and acetic acid) to be 60%, while plasma-activated Cu has shown 60% FE toward ethylene (Xu\u00a0et\u00a0al., 2020; Yin\u00a0et\u00a0al., 2019; Zhu\u00a0et\u00a0al., 2019b). Nanostructured Cu can be easily prepared via appropriate synthetic processes, allowing for a deep understanding of catalyst performance through precise control of the active sites. For example, Cu nanoparticles. Dong and his coworkers developed a strategy to improve the selectivity of CH4 by increasing the adsorption energy difference between CO* and CHO* intermediates (Dong\u00a0et\u00a0al., 2018). They prepared Cu79 NPs and compared it with Cu (211) and Cu (111) for the reduction of CO2 to CH4. According to the first principles calculations, Cu79 NPs exhibited a higher negative onset potential for the formation of CH4 than Cu (211), which induced better selectivity toward CH4 for Cu79 NPs. Dongare et\u00a0al. synthesized a highly stable metallic copper nanoparticles (Cu NPs) (Dongare\u00a0et\u00a0al., 2021). The total faradic efficiency for the liquid products reached to 58% at -0.8\u00a0V vs. RHE using prepared Cu NPs as electrocatalysts and the maximum FE of formic acid is over 45% (Fig.\u00a04a\n). Nanocrystals Cu may have a better catalytic performance than pure bulk Cu. Pranit Iyengar at el. synthesized octahedral Cu nanocrystals (Oh-NCs) in the range of 75-310\u00a0nm as a promising platform to study the electrochemical performance of CO2 to CH4 conversion (Iyengar\u00a0et\u00a0al., 2019). The best performance was achieved by the 75\u00a0nm Oh-NCs with 77% FE towards the CO2RR and 55% FE for CH4 at -1.25\u00a0V vs. RHE (Fig.\u00a04b). Compared with the bulk Cu electrodes, Cu nanowires (NWs) possess advantages, including larger surface-to-mass ratio, more low-coordinated adsorption sites, and capability of resisting much higher elastic strains. Vijayakumar et\u00a0al. synthesized an aligned copper nanowires catalysts with tunable selectivity for producing CO or formate in aqueous media (Vijayakumar\u00a0et\u00a0al., 2021). All these Cu nanowires were demonstrated with excellent catalytic activity regarding the total faradic efficiency of carbonaceous products (FEC, sum of FECO and FEHCOO-) over 70% from -0.5\u00a0V to -0.9\u00a0V vs. RHE (Fig.\u00a04c), peaked at -0.8\u00a0V vs. RHE and Cu-8 showed the highest EFC of 88%. In addition, for nanoporous Cu, Yang et\u00a0al. constructed a vanadium oxide integrated on hierarchically nanoporous Cu electrocatalysts (Yang\u00a0et\u00a0al., 2021a). As a CO2RR catalyst, the nanoporous copper integrated with vanadium oxide reached a 30.1% faradic efficiency for CO2 to ethanol production and an ethanol partial current density of -16\u00a0mA/cm2 at -0.62\u00a0V vs. RHE (Fig.\u00a04d).\nOther Cu catalysts. External factors also greatly impact Cu catalysts (Chen\u00a0et\u00a0al., 2022c). First, functional groups can be used to tune the selectivity and activity of Cu for the CO2RR. Chang and his coworkers investigated the role of functional groups (-COOH and -CF2) on the CO2RR of Cu catalysts (Chang\u00a0et\u00a0al., 2022). As seen from the reaction pathways in Fig.\u00a05a\n, the formation of *HCOOH was the rate-determining step (RDS), the formation energy of which on COOH-Cu (111) was more energetically favorable than that on Cu (111) and CF-Cu (111). This work revealed that functional groups influenced the binding energies of key intermediates involved in both the CO2RR and the competing hydrogen evolution reaction. Second, the electrochemical pulse also influences the selectivity. Tang used pulsed overpotential to improve the selectivity of ethylene on Cu (100) foil (Fig.\u00a05b) (Tang\u00a0et\u00a0al., 2021b). C2H4 was the major product with 50% \u223c 67% selectivity, while the selectivity for CH4, H2, and CO was less than 20%, 15% and 10%, respectively. The enhancement of C2 (ethanol and ethylene) selectivity was attributed to the improved CO dimerization kinetics on the Cu (100) surface resulting from the reduced hydrogen adsorption coverage during the pulsed process. Third, engineering the surface strain is a powerful method to improve the electrocatalytic performance. For example, tensile and compressive strains arising from the two-way shape memory effect of a NiTi substrate were used to control the activity and selectivity of CO2RR on Cu nanofilms (Du\u00a0et\u00a0al., 2021a). It was found that tensile strain could improve the CO2RR activity of the 32\u00a0nm-thick Cu nanofilm and favored CH4 generation from 42.76% to 50.64% (Fig.\u00a05c). The strain effects on the C2 products were relatively weak, with a little FE increase. Compared with the pristine Cu, tensile strain contributed to a total CO2RR faradaic efficiency from 65.02% to 76.48% at -1.2\u00a0V, while compressive strain had an opposite effect. According to DFT calculations and the derived positive correlation between the thermodynamic free energy change and the kinetic energy barrier, the mechanism of strain-controlled CO2RR performance was revealed. An upshifting of the d-band center of Cu and a larger adsorption strength of key intermediates induced by tensile strain were proved. As a result, the reduced free energy change in the potential-limiting step and the corresponding positive-correlated smaller energy barrier contributed to a higher CO2RR performance for the tensile-strained Cu nanofilm.\nCu-Ag alloy. Compared with pure Cu, Cu-based alloys have advantages including low coordination (Andrews\u00a0et\u00a0al., 2018; Qin\u00a0et\u00a0al., 2022), reduced overpotential (Lu\u00a0et\u00a0al., 2018), and long-term durability (Barasa\u00a0et\u00a0al., 2019). Due to the excellent CO formation ability of Ag, coupling Ag with Cu was considered to be an efficient way to improve the selectivity toward C2 products (Zhang\u00a0et\u00a0al., 2021a). Dutta et\u00a0al. synthesized bimetallic Cu-Ag metal foams by means of an additive-assisted electrodeposition process using the dynamic hydrogen bubble template approach (Dutta\u00a0et\u00a0al., 2020). They denoted this Cu-Ag alloy as oxide-derived Ag15Cu85, which showed high selectivity toward alcohol formation. The FE of C2H5OH was 33.7% at -1.0\u00a0V vs. RHE, and the FE of n-propanol was 6.9% at -0.9\u00a0V vs. RHE (Fig.\u00a06a\n). The reason for the high selectivity of alcohol was that the oxide-derived bimetallic catalyst exhibited the ability to effectively suppress the C1 hydrocarbon reaction. In the above-mentioned report, the ratio of Cu to Ag was fixed at 15 to 85. The ratio was changed by Xu et\u00a0al., who used E-beam evaporation to synthesize a series of Cu-Ag films with uniform distribution and controllable stoichiometry (Xu\u00a0et\u00a0al., 2022). They confirmed that when the Ag dopant was 20%, the Cu1-xAgx (x=0.05-0.2) alloy showed an apparent suppression of HCOOH, and the Faradic efficiency of the C2 product (mainly C2H5OH) increased (Fig.\u00a06b). In addition, the change of core-shell structures also affects the C2 product selectivity. Zhang, et\u00a0al., synthesized a series of Cu@Ag core-shell nanoparticles by tunning different silver layer thicknesses to improve the selectivity of C2 products (Zhang\u00a0et\u00a0al., 2021a). Notably, Cu@Ag-2 (core-shell with a thickness of 11.2\u00a0nm) exhibited excellent selectivity and activity for C2 products (Fig.\u00a06c-d). Specifically, the total FE of C2 products for Cu@Ag-2 reached a maximum of 67.7% at -1.1\u00a0V vs. RHE, and the C2 partial current density for Cu@Ag-2 presented the highest value of -22.7\u00a0mA/cm2 at -1.1\u00a0V vs. RHE.\nAu-based catalysts. Au is one of the most widely studied noble metal catalysts for electrochemical conversion of CO2 to CO with a high selectivity at a low overpotential. In order to improve the electrocatalytic performance, Au-based materials with specific structure are developed. As it has been proved that Au-based nano-catalysts showed nanostructure dependent performance for CO2RR, indicating that we can enhance the electrocatalytic properties by controlled the synthesis of structure. In earlier research, a variety of Au-based nanostructure, such as nanoparticles (Andrews\u00a0et\u00a0al., 2015; Feng\u00a0et\u00a0al., 2015), nanoclusters (Kauffman\u00a0et\u00a0al., 2012), nanowires (Cho\u00a0et\u00a0al., 2019), nanoporous structure (Kwok\u00a0et\u00a0al., 2019) and so on, has been proposed to improve catalytic performance. There is no end to the quest to improve the catalytic performance of Au-based catalysts.Doping is a facile way to improve the property for Au-based catalysts. Sun et al. synthesized an Mo-doped Au nanoparticles (MDA NPs) (Sun\u00a0et\u00a0al., 2020a). MDA NPs was reported that effective CO2 reduction by the synergies between electronic and geomatic effects. A 97.5% CO faradic efficiency and 75-fold higher current density than pure Au nanoparticles were achieved at -0.4\u00a0V vs. RHE for MDA NPs with durability of over 50 h (Fig.\u00a07a\n). DFT calculation results revealed that the increased electron density of Mo on Au surface could effectively enhance CO2 activation and *COOH may be further stabilized by the local Mo atom through additional Mo-O binding to decrease the energy barrier. Additionally, the electrocatalytic performance of Au-based may improve because of the formation of some special active interface. Chen et al. successfully synthesized heterogeneous Ag2S-Au nanoparticles (NPs) as effective catalysts for CO production (Chen\u00a0et\u00a0al., 2022a). The Ag2S-Au showed a high selectivity for CO2 to CO (FE=94.5%) and an appreciable CO particle current density of 9.17\u00a0mA/cm2. Moreover, the Ag2S-Au exhibited good stability of over 30 h. DFT result revealed the formation of the heterogeneous Ag2S-Au interface was beneficial to the generation of COOH* intermediate, and the charge density proved a good number of electrons was concentrated on the Ag2S-Au interface (Fig.\u00a07b), indicating the interface was the active site for CO2RR. Besides, designing bimetallic electrocatalysts is also an attractive strategy to enhance the catalytic performance of Au-based material. An AuNi bimetallic catalyst was prepared, which supported on electrospum carbon nanofibers (NCFs) (Hao\u00a0et\u00a0al., 2021a). The AuNi bimetallic catalyst exhibited high CO selectivity with CO faradic efficiency of 92% at -0.92\u00a0V vs. RHE and good durability of over 16 h. DFT results indicated that the incorporation of Ni into Au made the d-band center more positive (Fig.\u00a07c) and reduced the free energy barrier of *COOH. In addition, whether the presence of the support may affect the catalytic property of Au-based material? Zhang et al. reported carbon nitride (C3N4) supported Au nanoparticles (Au/C3N4) exhibited a better electrocatalytic performance (Zhang\u00a0et\u00a0al., 2018b). Notably, compared with Au/C, Au/C3N4 exhibited a higher current density and FECO (Fig.\u00a07d). Au/C3N4 reached FECO of over 90% at a wide potential window of -0.45\u00a0V to -0.85\u00a0V vs. RHE, demonstrating C3N4 could significantly enhance the CO2RR activity. The key of the excellent catalytic performance was Au-C3N4 interaction induces the formation of negatively charged Au surface, which could stabilize *COOH intermediate. Last but not least, strain is a new facile strategy to easily enhance the catalytic property for Au-based material. Zhang et al. synthesized gold nanoparticles (Au NPs) with rich compressive strain (Au-LAL) for electrocatalytic CO2 reduction (Zhang\u00a0et\u00a0al., 2022a). Au-LAL achieved a CO partial current density of 24.9\u00a0mA/cm2 and a maximum CO faradic efficiency of 97% at -0.9\u00a0V vs. RHE, which demonstrated that the rich compressive strain could greatly promote the CO2RR performance. The presence of the compressive strain could induce a unique electronic structure change in Au NPs, significantly up-shifting the d-band center of Au, and greatly enhance the adsorption strength of Au NPs toward the key *COOH intermediate (Fig.\u00a07e).\nAg-based catalysts. Compared with Au, Ag catalysts are more popular due to the lower price and high selectivity. Ag is a candidate catalyst for converting CO2 to CO, the faradaic efficiency of which still needs to increase at a relatively lower overpotential (Kuhl\u00a0et\u00a0al., 2014). Changing the morphology of Ag nanoparticles is a facile strategy. Liu et\u00a0al. prepared triangular Ag nanoplates (Tri-Ag-NPs) (Liu\u00a0et\u00a0al., 2017). Compared with similar sized Ag nanoparticles (SS-Ag-NPs) and bulk Ag, Tri-Ag-NPs exhibited excellent CO2RR performance (Fig.\u00a08a\n). The maximum current density was over 5.5\u00a0mA/cm2, and the maximum FE of CO was 96.8% at a lower overpotential of 0.746\u00a0V. Liu, et\u00a0al., further explored Ag nanocubes (NCs) with lengths below 25\u00a0nm and 70\u00a0nm (L25- and L70-Ag-NCs), respectively (Liu\u00a0et\u00a0al., 2020b). The L25-Ag-NCs exhibited a larger current density, a significant FECO and a better stability of 18\u00a0h compared to L70-Ag-NCs, Ag bulk and Ag nanoparticles (Fig.\u00a08b). Dutta et\u00a0al. synthesized Ag-foam catalysts based on a concerted additive- and template-assisted metal-deposited process (Dutta\u00a0et\u00a0al., 2018). Such Ag foams showed high activity, selectivity and stability toward CO production at both low and moderate overpotentials. The FE for CO never fell 90% from -0.3\u00a0V to -1.2\u00a0V vs. RHE, and the stability was more than 70 h. They proposed the possible mechanistic pathway of CO2 conversion on the Ag-foam catalyst (Fig.\u00a08c); CO was the only and final CO2RR product. In addition to the morphology engineering of Ag, size control is also feasible. Liu et\u00a0al. synthesized 5-fold twinned Ag nanowires (NWs) with diameters less than 25\u00a0nm (D-25) and 100\u00a0nm (D-100) (Fig.\u00a08d) by a facile bromide-mediate polyol method (Liu\u00a0et\u00a0al., 2018). Compared with D-100\u00a0nm Ag NWs and Ag nanoparticles, D-25 Ag NWs had markedly enhanced current density, together with significant Faradic efficiency. The maximum values of the current density and FECO are 6.65\u00a0mA/cm2 and 99.3%, respectively. In addition, the low onset overpotential and the stability at 24 h further verified the superior performance of D-25 Ag NWs for the CO2RR.In view of the poor absorption capacity of CO2 on Ag, significant efforts have been dedicated to preparing Ag-based alloys to create new binding sites and improve the CO2RR activity. For example, an Au-Ag bimetallic alloy was synthesized by the thermal evaporation method (Li\u00a0et\u00a0al., 2021a). It showed the best performance with a CO FE of 89% at -0.7\u00a0V vs. RHE with good stability (Fig.\u00a09a\n). The well-dispersed Ag atoms at the grain boundaries were inferred to be the contribution of such good performance. Interestingly, it was found that composition changes of Ag-based alloys could break the inherent scaling relationship of the binding energies of various intermediates (Zeng\u00a0et\u00a0al., 2019). A series of Pd1-xAgx bimetallic alloys were prepared, and the optimal Pd0.75Ag0.25/C provided a higher CO FE of 95.3% at -0.6\u00a0V vs. RHE. This was attributed to the Pd0.75Ag0.25 alloy gaining obviously weakened *CO and *H bindings but retaining the binding with *COOH well, thus facilitating the kinetics toward CO generation (Fig.\u00a09b). An Ag-Zn bimetallic alloy was designed by pulse deposition of Zn dendrites onto Ag foams (PD-Zn/Ag foam) (Low\u00a0et\u00a0al., 2019). The nanostructure PD-Zn/Ag foam reduced CO2 to methanol with an FE of 8.1% and a current density of -2.1\u00a0mA/cm2 at -1.38\u00a0V vs. RHE (Fig.\u00a09c). An Ag-Co surface alloy electrocatalyst was obtained by the cold H2-plasma activation method (Fig.\u00a09d) (Zhang\u00a0et\u00a0al., 2020d). It exhibited high activity for the CO2RR to ethanol with an FE of ethanol (72.3%) and a current density of 7.4\u00a0mA/cm2 at -0.80\u00a0V vs. RHE. The formation of the (Zhao et\u00a0al.) Ag-Co surface alloy was believed to distort the Ag lattice and reduce the energy barrier for *CO2\n\u03b4\u2212 formation.\nSn-based catalyst. Sn is abundant, nontoxic and quite suitable for large-scale applications but also has a low cost compared to other noble metals. More importantly, Sn exhibits high selectivity and catalytic activity for C1 (mainly formate) products (Fig.\u00a010a\n). Therefore, Sn has attracted much attention in the field of CO2RR. Zhong, et\u00a0al., developed ultrasmall Sn nanoparticles inlaid on N/P-doped carbon (Warnan et\u00a0al.) using Sn electroplating sludge (Zhong\u00a0et\u00a0al., 2022). Sn@NPC exhibited excellent selectivity and activity for HCOOH (Fig.\u00a010b). The FE of HCOOH reached 87.93% together with a stable j\nHCOOH of -8.05\u00a0mA/cm2 at -1.05\u00a0V vs. RHE. Moreover, the Sn@NPC electrode achieved excellent long-term stability up to 105 h. In addition, Li, et\u00a0al., reported a unique Sn-doped Bi2O3 nanosheet (NS) electrocatalyst by a simple solvothermal method for the highly efficient electrochemical reduction of CO2 to formate (Li\u00a0et\u00a0al., 2021c). By synthesizing different atomic percentages of Sn-doped Bi2O3 NS (1.2%, 2.5%, 3.8%), they found that the 2.5% Sn-doped Bi2O3 NS exhibited the highest FE of 93.4% with a current density of 24.3\u00a0mA/cm2 for formate at -0.97\u00a0V vs. RHE and achieved long-term stability over 8 h with formate FE maintained at 90%. DFT calculations revealed the strong synergistic effect between Sn and Bi contributing to the larger adsorption capacity of the OCHO* intermediate, thus improving the activity toward formate generation (Fig.\u00a010c). Rahman Daiyan et\u00a0al. obtained Sn electrode (referred to as An-Sn) by anodizing Sn foil in organic solvents (Daiyan\u00a0et\u00a0al., 2017). This as-prepared An-Sn electrode reduced CO2 to formate with a maximum FEHCOO\n\u2212 of 77.40% and a stable current density of 4.80\u00a0mA/cm2 at -1.09\u00a0V vs. RHE (Fig.\u00a010d). Ivan Merino-Garcia et\u00a0al. synthesized high surface area SnO2 nanoparticles (NPs) for continuous formate generation at high current density in a flowing electrolyzer (Merino-Garcia\u00a0et\u00a0al., 2021). The manufactured SnO2-based gas diffusion electrodes (SnO2-GDEs) exhibited a maximum formate generation concentration of 27 g/L with 44.9% FE at 300\u00a0mA/cm2 and could be sustained for up to 10 h (Fig.\u00a010e).Sn-based alloys, in contrast to pure Sn, accurately control the surface electronic state as well as the binding energy of intermediates via the addition of foreign atoms (Shao\u00a0et\u00a0al., 2019). Thus, various Sn-based alloy systems have been reported (He\u00a0et\u00a0al., 2017; Ren\u00a0et\u00a0al., 2016). For example, a bimetallic Sn-Sb alloy film was obtained by electrodeposition on different substrates (Lucas\u00a0and Lima,\u00a02020). Compared with the pure Sn film, the Sn-Sb alloy film exhibited excellent electrocatalytic performance, and the faradic efficiency reached 96.2% at -1.25\u00a0V vs. RHE (Fig.\u00a011a\n). This was ascribed to the balance between Sn atoms and the induced morphologic effect brought by Sb, generating cube-shape crystallites. These crystallites have a large number of undercoordinated surface atoms and grain boundaries to generate more reactive Sn atoms as CO2RR active sites, increasing the overall activity and FE for formate production. In addition, Sn-Ni alloy was also proven to be an efficient catalyst. Xie et\u00a0al. reported an efficient NiSn atomic pair electrocatalyst (NiSn-APC) on a hierarchical integrated carbon nanosheet array electrode, which boosted the activity and selectivity of formate (Xie\u00a0et\u00a0al., 2021b). As seen from Fig.\u00a011b-c, the maximum current density was -43.7\u00a0mA/cm2, and the maximum FE of formate was 86.1%. The electron redistribution of Sn imposed by adjacent Ni was ascribed to the activity improvement, which reduced the energy barrier of the *OCHO intermediate and made the potential-limiting step thermodynamically spontaneous. Moreover, a bimetallic Sn-Bi aerogel with a 3D porous structure was reported (Wu\u00a0et\u00a0al., 2021b). Compared with Sn, Bi and bulk Sn-Bi, Sn-Bi, the Sn-Bi aerogel exhibited better catalytic activity and higher selectivity for formate (Fig.\u00a011d-e). The Sn-Bi aerogel exposed more active sites and had favorable mass transfer properties, endowing it with a high FEHCOOH of 93.9%. Sn-Bi also possessed good stability for up to 10 h when 90% FEHCOOH was maintained. DFT results revealed that the coexistence of Sn and Bi degraded the energy for the production of HCOOH, thereby improving the catalytic activity (Fig.\u00a011f).\nBi-based catalyst. Due to it possesses some advantages, such as eco-friendly, cost-effective, inhibit H2 production and highly active for CO2 reduction (Guan\u00a0et\u00a0al., 2021; Jiang\u00a0et\u00a0al., 2021a), Bi-based is a promising catalyst for CO2RR. For example, it is reported that Bi-based catalyst can selectively reduce CO2 to formic acid in aqueous solution (Duan\u00a0et\u00a0al., 2020). As it has proved that the CO2RR is more attractive in aqueous solution, most of the existing studies focused on improving the formate production efficiency on Bi-based catalysts. The scholar has found the selectivity and activity to formate production on Bi-based catalysts were greatly improved by tunning the catalyst structure. Bi-based catalysts can be flexibly synthesized into a variety of different structures, such as dendrites, nanosheets, nanoparticles, etc. by various techniques including Electrodeposition, in situ electrochemical transformation of Bi precursors, wet chemical reduction and so on.Electrodeposition is one of the various techniques, which is a facile approach to tune the nanostructure to improve the catalytic efficiency of Bi-based catalysts (Wang\u00a0et\u00a0al., 2019b). Guo et al. synthesized oxide-containing Bi (Bi-PMo) nanosheets by electrodeposition in the presence of phosphomolybdic acid. (Guo\u00a0et\u00a0al., 2019). These Bi nanosheets catalyzed CO2 to formate with a faradic efficiency of 93\u00b12% at -0.86\u00a0V vs. RHE with a formate particle current density as high as 30\u00a0mA/cm2 and the stability over 10 h (Fig.\u00a012a\n). In addition, in situ electrochemical transformation of Bi-precursors like oxide, oxychloride and so on were widely used to prepare Bi-based catalysts. Wei et al. dispersed Bi dendrites on 3D carbon cloth and then used in situ chemically oxidized Bi dendrites to Bi nanoparticles (Bi NPs/CC) (Wei\u00a0et\u00a0al., 2022). The Bi NPs/CC exhibited a current density of 6.8\u00a0mA/cm2 at -0.87\u00a0V vs. RHE with a CO2-to-formate faradic efficiency of 97.4% and excellent durability of 72 h. DFT calculation revealed that the exposed specific facet was the key to stabilize the OCHO* intermediate contributed to high activity, selectivity and durability of Bi NPs/CC (Fig.\u00a012b). Liu et al. obtained a 2D Bi nanosheets electrocatalyst via in situ transformations from optimized thickness and sizes of the bismuth oxychloride precursors (Liu\u00a0et\u00a0al., 2021a). The 2D Bi nanosheets (EG/H2O,1:1) showed high selectivity of formate at -0.9\u00a0V vs. RHE and high stability of 15 h with a current density of 10.5\u00a0mA/cm2 (Fig.\u00a012c). Wet chemical reduction is also used to prepare nanostructure of Bi-based catalysts. Yang et al. synthesized stable free-standing hexagonal Bi-based nanosheets catalyst (Bi NSs) with different thickness via wet chemical reduction and demonstrated its high electrocatalytic performance for formate formation from CO2RR (Yang\u00a0et\u00a0al., 2020a). The prepared 0.65\u00a0nm Bi-based catalyst exhibited high CO2RR electrocatalytic activity (Fig.\u00a012d), which offered a superhigh FECHOO\n\u2212 of 99% at -0.58\u00a0V vs. RHE and durability of over 75 h. The reason was the structures-sensitivity of the CO2RR over Bi-based nanosheets, leading the unique compressive strain to have a high selectivity of formate. Use the same preparation way, Xie et al. synthesized a single-crystalline Bi-based rhombic dodecahedrons (Bi RDs) exposed with (104) and (110) facets via wet chemical reduction (Xie\u00a0et\u00a0al., 2021a). The Bi RDs reached high selectivity for formate production of over 92.2% at a low overpotential and an excellent electrocatalytic active (partial current density range from 9.8 to 290.1\u00a0mA/cm2). The significantly reduced overpotential was caused by the enhanced adsorption of *OCHO on the Bi RDs. The key of the high selectivity of formate was ascribed to the topological surface states and the trivial surface states opening small gaps in the bulk gap on Bi RDs. Due to this change, the adsorption of *OCHO had been strengthened and stabilized, while the competing adsorption of *H had been mitigated (Fig.\u00a012e).In this section, the representative metal-based CO2RR electrocatalysts including Cu and Cu-Ag alloys for high-vale hydrocarbons, Au-based and Ag-based catalysts for CO product, Sn-based and Bi-based catalysts for formic acid/formate products were overviewed.For Au-based electrocatalysts, we summarized the CO partial current density of different Au-based electrocatalysts via different strategies in Section\u00a02.1.2. As seen from Fig.\u00a013a\n, MDA NPs show the best activity among the all Au-based electrocatalysts. The activities of Au/C3N4 and Au-LAL are similar and second-best. It is obvious that the active of Au film is the worst. For Ag-based electrocatalysts, the shape and morphology effects are further discussed based on the comparison of CO partial current density (jCO) among the abovementioned Ag catalysts in Section\u00a02.1.2. As seen from Fig.\u00a013b, Ag foam exhibits the best activity with the largest jCO of -17.658\u00a0mA/cm2, which is obviously superior to other catalysts. However, the largest jCO values for Tri-Ag-NPs, L25-Ag-NCs and D25 Ag NWs were slightly different, indicating weak morphology effects on them. It is also interesting to find that jCO of bulk Ag, Tri-Ag-NPs, Ag foil and Ag foam all first increased and then decreased along with the biased potential negatively swept. For Sn and Sn-based alloy electrocatalysts, as indicated in Fig.\u00a013c, the HCOO\n\u2212\n partial current densities (jHCOO\n\u2212) for all modified Sn-based electrocatalysts are larger than that for Sn foil. Among the modified Sn-based electrocatalysts, Sn-Bi2O3 exhibited the highest activity for formate generation.The comparison of Bi-based catalysts is listed in Table\u00a01\n. Several strategies have been proven to be effective for the improvement of formate selectivity, for example, wet chemical reduction, in situ electrochemical transformations, and so on. It is clear that the electrocatalytic performance of Bi-based catalyst for formate generation has been improved a lot. Most of the reports show a quite high FE (over 90%) toward formate, but the long-term stability of CO2RR is still insufficient (less than 100 h), indicating these catalysts to be far from the industrial application requirements.The comparison of Cu and Cu-based catalysts is listed in Table\u00a02\n, the product selectivity of which varied from sample to sample. Several strategies have been proven to be effective for the improvement of C1 or C2 product selectivity, e.g., alloying, facet tuning, morphology engineering, defect engineering, strain effects, etc. However, the active sites, reaction pathway and reactive thermodynamics and kinetics are still not clearly understood. To date, most of the mechanistic insights have been obtained by DFT calculations. Profound theoretical insights and operando characterizations are highly necessary to decode the dynamic CO2RR process and reveal the composition/structure-performance relationship of electrocatalysts. It is noteworthy that the long-term stability of CO2RR catalysts and cells has been far from the industrial application requirement. Most of the reports show a very limited test time, usually less than 100 h. For reference, industrial water electrolyzers have demonstrated stable performance over 80,000 h (Kibria\u00a0et\u00a0al., 2019).Single-atom catalysts (SACs) are defined as catalysts consisting of only uniformly separated isolated single active sites on the surface of the substrate (Qiao\u00a0et\u00a0al., 2011). Individual active sites of SACs consist of an isolated metal atom bonded to adjacent atoms in the host material (Sun\u00a0et\u00a0al., 2021). In principle, SACs can expose all metal atoms on the surface, thus achieving 100% atomic utilization, which is particularly attractive for reducing the cost of precious-metal-based catalytic materials. In addition, SACs also feature uniformly distributed uncoordinated active sites, providing a bridge to combine the advantages of heterogeneous catalysts and homogeneous catalysts for efficient chemical conversion and energy conversion (Chen\u00a0et\u00a0al., 2022b; Wang\u00a0et\u00a0al., 2018a). Moreover, compared with nanoparticles or clusters, SACs have a greater number of active sites per mass and prominent size effects (Chen\u00a0et\u00a0al., 2020c; Liu\u00a0et\u00a0al., 2022), giving rise to distinctive behaviors of reactants/intermediates and outstanding performance. To distinguish these catalysts from other single-atom catalysts, we denote single-atom catalysts used in the electrocatalysis field as single-atom electrocatalysts (SAECs), whose active sites are usually transition metals in cationic or metallic states. In the territory of electrocatalysis, the footprint of SAECs has gradually extended to the hydrogen evolution reaction (HER), oxygen evolution reaction (Wu et\u00a0al.), CO2 reduction reaction (CO2RR) and N2 reduction (N2RR) from the initial focus on the oxygen reduction reaction (ORR). In this review, the latest and representative work of SAECs in CO2RR are summarized.M-N-C-type SAECs, as emerging metal-nitrogen-doped carbon materials wherein dispersive metal atoms are coordinated to nitrogen atoms doped in carbon nanomaterials, have presented a high expectation to be substitutes for metallic electrodes. Because only the single atom position can be used for intermediate adsorption, it can stabilize carbonaceous reaction intermediates (e.g., CO2\n\n\u00b7\u2212\n) and restrict the configurations of the adsorbate, giving rise to enhanced catalytic activity and selectivity (Yang\u00a0et\u00a0al., 2013). In addition, M-N-C-type SAECs have an appealing nitrogen-doped carbon (NC) substrate (Shi\u00a0et\u00a0al., 2020). The NC substrate possesses prominent advantages, including good mechanical properties, large specific surface area, excellent electronic conductivity, large specific surface area, excellent electronic conductivity, structural flexibility beyond the atomic scale, low cost, and ideal stability under acidic/alkaline conditions. There are three main reasons (Jin\u00a0et\u00a0al., 2019) why M-N-C-type SAECs can be widely used: 1) the carbon-based structure, such as graphene, shows superior ductility and electroconductivity; 2) N shows better coordination ability with metals; and 3) the exposed metal atoms, as the extra active sites, enhance their electrocatalytic performance. However, M-N-C SEACs also have some problems, including (Wang\u00a0et\u00a0al., 2020b): 1) the synthesis is complex, which makes industrial production difficult to realize; 2) the carbon substrate and the heteroatomic dopants are usually active for the HER, reducing the FE of CO2 reduction. How to adjust the atomic structure of the atomic metal center is the key for solving the above problems. Next, we will further discuss the M-N-C type of SAECs for the CO2RR based on the different metals involved as the active site to provide suggestions for the design of efficient M-N-C SAECs.Fe-based SAECs are embedding nonnoble metal Fe in a nitrogen-doped carbon support electrocatalyst, efficiently converting CO2 to CO in aqueous solution. As early as 1985, Fe metal electrodes were studied as CO2RR catalysts but were found to mainly produce hydrogen, with FEH2 being > 95%. Follow-up research by the same period confirmed the inactive nature of metallic Fe in CO2 reduction, so Fe as an electrocatalyst for the CO2RR was stagnated. In 2015, Strasser and coworkers demonstrated the structure of Fe\u03b4+\u2212Nx centers being active for the CO2RR (Varela\u00a0et\u00a0al., 2015), and the research status of Fe-based electrocatalysts changed. They found that single atoms of Fe coordinated on N-doped carbon (Fe\u2212N\u2212C, Fig.\u00a014a\n) could achieve >80% FECO at -0.5\u00a0V vs. RHE, which is comparable to the CO2RR activity of Au (FECO is 87%), one of the most active metal catalysts for the electroreduction of CO2 to CO. Moreover, compared to Au, the onset potential of Fe-N-C was also found to be reduced by 100 mV. In Fe-N-C-type SAECs, the structure of Fe-N4 is proposed. Huan and his coworkers found that the structure of Fe-N4 was the key catalytic substance for CO2 to CO by studying a series of Fe-based catalyst pyrolyzes (Huan\u00a0et\u00a0al., 2017). Materials containing only Fe-N4 sites are able to selectively reduce CO2 to CO in an aqueous solution with a Faraday efficiency yield of over 90% and at low overpotential (Fig.\u00a014b). Although the structure of Fe-N4 is the critical catalytic substance, we do not know who acts as the catalytic active site. To explore this question, Tour and his coworkers dispersed single Fe atoms on N-doped graphene to prepare Fe/NG (Zhang\u00a0et\u00a0al., 2018a). The oxidation status of Fe/NG was analyzed by XPS (Fig.\u00a014c), and they identified a +2-oxidation state for Fe within Fe-N4, which was believed to be the active site for the CO2RR. The +2 oxidation state for Fe can be the active sites for the CO2RR in Fe-N-C, and the +3 oxidation state for Fe can also be the active sites for the CO2RR in Fe-N-C. Gu and his coworkers reported a catalyst of dispersed single-atom Fe sites that produces CO at an overpotential as low as 80 mV (Gu\u00a0et\u00a0al., 2019). X-ray absorption spectroscopy revealed the active sites to be discrete Fe3+ ions (Fig.\u00a014d), and their electrochemical data suggested that the superior activity resulted from faster CO2 adsorption and weaker CO absorption on Fe3+ sites. The difference in oxidation state was suggested to be caused by different ligand environments, particularly with regard to the N atoms. The spectroscopic data indicated that Fe3+-N-C comprises pyrrolic N ligands, whereas Fe2+-N-C comprises pyridinic N ligands. The above study discusses the CO2RR activity of Fe-N4, and some scholars proposed a new Fe\u03b4+-Nx group, namely, Fe-N5. They think the catalytic activity of Fe-N5 surpasses that of Fe-N4. Zhang and his coworkers reported a novel synthesis approach involving thermal pyrolysis of hemin and melamine molecules on graphene to prepare Fe-N5-C SAECs (Zhang\u00a0et\u00a0al., 2019a). These SAECs exhibited a high Faradaic efficiency (\u223c97.0%) and CO production at a low overpotential of 0.35\u00a0V, outperforming all the Fe-N-C-based catalysts. The DFT calculations revealed that the axial pyrrolic-nitrogen ligand of the FeN5 site further depleted the electron density of the Fe 3d orbitals and thus reduced the Fe-CO \u03c0 back-donation (Fig.\u00a014e), which was responsible for the rapid desorption of CO with a high selectivity toward CO production.Co-based SAECs have also attracted much attention for their CO2RR activity in the past decade. In early studies, it was proposed that cobalt phthalocyanine and cobalt porphyrin are active for the CO2RR. For instance, cobalt porphyrin was used as a building unit to prepare a heterogeneous catalyst for the aqueous electrochemical reduction of CO2 to CO. These atomic Co-N4 sites exhibit a high FE (90%) at an overpotential of 0.55 V Lin\u00a0et\u00a0al., 2015). Similarly, carbon nanotube-supported cobalt phthalocyanine with four Co-N bonds has been reported to fulfill CO2-to-CO conversion with 90% CO selectivity in bicarbonate electrolyte (Zheng\u00a0et\u00a0al., 2018b). Zhang and his coworkers used metal phthalocyanines (MePcs) as a model to synthesize FePc, NiPc and CoPc catalysts (Zhang\u00a0et\u00a0al., 2018e). All these catalysts had a clear metal-N4 coordination structure similar to the structure of the M-N-C SAECs active site. They established the linear relations of the reaction energies of COOH* formation and CO* desorption as functions of the *CO adsorption energy, as shown in Fig.\u00a015a\n. This figure reveals an inverted volcano curve in the activity trend of MePcs for CO2 reduction to CO, and CoPc is located at the position closest to the volcano peak. This means that the activity of CoPc is best. CoPc and Co-N4 moieties have similar coordination structures, but their catalytic activities are different, which provides more opportunities for the study of Co SAECs in the electrocatalysis of CO2 reduction reactions. In addition, the coordination number also affects the electronic structures of the metal centers in SAECs. To strengthen the molecular understanding of the reaction intermediates and the reactive sites, a series of atomically dispersed Co catalysts with different N coordination numbers were prepared, and their catalytic performance toward CO2 reduction was studied. Based on the fact that the coordination number of single Co atoms was controlled by varying the pyrolysis temperature, Wang et\u00a0al. prepared different coordination numbers Co-N-C SAECs by pyrolyzing bimetallic Co/Zn zeolitic imidazolate frameworks (ZIFs) (Wang\u00a0et\u00a0al., 2018c). These electrocatalysts were Co-N4, Co-N3 and Co-N2, respectively. To compare their electrocatalytic activity, they also prepared Co NPs. As a result, they found that the catalytic activity for CO production followed the trend Co-N2> Co-N3> Co-N4. The central current density of Co-N2 moieties reached 18.1\u00a0mA\u00b7cm\u22122, and FECO achieved 94%. They investigated the relationship between the coordination number of Co centers and the activity of the CO2RR. It was generally regarded that the first electron transfer largely determined the overall reduction process rate, during which the adsorbed CO2 would be reduced into a CO2\n\u2022\u2212 intermediate (Lei\u00a0et\u00a0al., 2016). Electrochemical impedance spectroscopy (EIS) revealed that Co-N2 exhibited the lowest charge transfer resistance from the catalyst surface to the reactant. This meant that Co-N2 was faster to transfer to the CO2\n\u2022\u2212 intermediate. For further understanding, they employed OH\u2212 to evaluate the binding affinity of CO2\n\u2022\u2212 through oxidative LSV scans in N2-saturated 0.5 M NaOH electrolyte (Fig.\u00a015b). As a result, the stronger adsorption of CO2\n\u2022\u2212 on Co-N2 relative to the Co-N4 surface benefits the overall CO2 reduction. Moreover, compared with Co-N4, DFT showed a lower energy required to form CO2\n\u2022\u2212* on Co-N2, thus explaining the higher CO2 electroreduction catalytic activity. This result demonstrated that a lower coordination number facilitates the activation of CO2 into the CO2\n\u2022\u2212 intermediate and hence enhances CO2 electroreduction activity. In addition to Co-N2 moieties, Co-N3 moieties, Co-N4 moieties and Co-N5 moieties are also active sites. Pan and his coworkers prepared a type of SAECs with an atomically dispersed Co-N5 site anchored on polymer-derived hollow N-doped porous carbon spheres (Pan\u00a0et\u00a0al., 2018a). For CO2 to CO conversion, Co-N5 was an excellent active center, exhibiting high selectivity for CO with a Faradaic efficiency above 90% over a wide potential range from -0.57\u00a0V to -0.88\u00a0V. The catalyst exhibited a higher FECO of -0.73\u00a0V and -0.79\u00a0V, 99.2% and 99.4%, respectively, which was equivalent to a 15.5-fold increase in cobalt phthalocyanine activity. The CO2RR pathway was investigated via a computational hydrogen electrode model (Fig.\u00a015c, left), which revealed that the free energy difference for Co-based SAECs from CO2 to COOH* was close to zero (0.02\u00a0eV), lower than other catalysts (Fig.\u00a015c, right), indicating a higher CO2RR activity.The single atoms of Ni coordinated on the N-doped carbon substrate material to form Ni-N-C-type SAECs. This SAECs shows high selectivity for CO, and its EFCO can achieve over 90%, making it a highly efficient and durable electric catalyst for CO2 reduction. To explore highly efficient Ni single-atom catalysts, He and coworkers designed Ni single-atom catalysts that consisted of isolated Ni single atoms anchored on nitrogen-doped winged carbon nanofibers (NiSA-NWC, Fig.\u00a016a\n) (He\u00a0et\u00a0al., 2020). This catalyst exhibited high intrinsic selectivity for CO2 to CO. The single-atom Ni catalyst possessed a maximum CO FE of over 95% in 0.1 M NaHCO3 solution, -1.6\u00a0V vs. Ag/AgCl. Similar to Fe-Nx moieties and Co-Nx moieties, Ni-Nx moieties are the most widely studied and are generally considered to be the active sites for the CO2RR. Mou and his coworkers obtained a Ni single-atom catalyst with 2.6 wt% Ni loading, denoted NiSA-NGA, through one-step pyrolysis of graphene oxide aerogel (Mou\u00a0et\u00a0al., 2019). This catalyst showed excellent electrochemical reduction of CO2 to CO. They found that the high selectivity of CO was caused by coordinatively unsaturated Ni-Nx sites (Fig.\u00a016b). In addition, they simulated the Gibbs free energy of the reaction pathway of Ni-Nx sites and found the reaction mechanism of NiSA-NGA for CO formation. Among the various coordination structures of Ni-Nx, Ni-N4 moieties are widely considered active sites for the CO2RR. Li and his coworkers used a topochemical transformation strategy to synthesize Ni-N4-type SAECs (Li\u00a0et\u00a0al., 2017c). This strategy successfully ensured preservation of the Ni-N4 structure to the maximum extent and avoided the agglomeration of Ni atoms to particles, providing abundant active sites for the catalytic reaction. The Ni-N4 structure exhibited excellent activity for the electrochemical reduction of CO2 with particularly high selectivity for CO, achieving a high faradaic efficiency over 90% for CO in the potential range from -0.5\u00a0V to -0.9\u00a0V and giving a maximum faradaic efficiency of 99% at -0.81\u00a0V with a current density of 28.6\u00a0mA/cm2. To explain the high selectivity of Ni-N4 moieties, they performed density functional theory calculations. Comparing Ni-N4-C with N-C, the reduction free energy is indicated in the left subfigure of Fig.\u00a016c. The formation of the adsorbed intermediate COOH* was the potential limiting step for both Ni-N4-C and N-C. From a thermodynamic point of view, the reaction free energy can be linked to the reaction energy barrier, so the trend of free energy can be associated with the activity of CO2 reduction. It can be clearly seen from the left subfigure of Fig.\u00a016c that the introduction of Ni-N4 sites lowered the formation energy of COOH* compared with that for N-C, facilitating the subsequent formation of CO and thus showing higher activity. As seen in the right subfigure of Fig.\u00a016c, Ni-N4-C showed a significantly more positive value for UL(CO2)-UL(H2) (the difference between thermodynamic limiting potentials for CO2 reduction and H2 evolution denoted as UL(CO2)-UL(H2), which can reflect the selectivity in CO2 reduction) than that of N-C. This meant that Ni-N4-C possessed high selectivity for CO2 reduction to CO. Ni-N4-C showed highly efficient activity for the CO2RR, as did Ni-N3-C. Zhang and his coworkers used a postsynthetic metal substitution (PSMS) strategy to prepare single-atom Ni catalysts with different N coordination numbers (denoted Ni-Nx-C, Fig.\u00a016d) on predesigned N-doped carbon derived from metal-organic frameworks (Zhang\u00a0et\u00a0al., 2021b). At -0.65\u00a0V, the current density (JCO) was 6.64\u00a0mA/cm2, and the obtained Ni-N3-C catalyst achieved a CO Faradaic efficiency up to 95.6%, much higher than those of Ni-N4-C and N-C. To explain this, DFT calculations were performed. The lower Ni coordination number in Ni-N3-C greatly reduced the formation energy of the rate determining step, thereby promoting the CO2 reduction process. In addition, a framework other than MOFs was used to build Ni-based SAECs. Su et\u00a0al. prepared covalent triazine frameworks (CTFs) modified with coordinatively unsaturated 3D Ni metal atoms (Su\u00a0et\u00a0al., 2018). Such Ni-CTF catalysts effectively reduced CO2 to CO at -0.5\u00a0V vs. RHE, and the Faradaic efficiency reached 90% at -0.8\u00a0V vs. RHE (Fig.\u00a016e).Due to its unique capability of catalyzing C-C coupling, Cu is unique as the only metal known for the production of hydrocarbons from the electroreduction of CO2 and is also the most studied metal for CO2 reduction. It can be used to produce value-added hydrocarbons, such as ethylene, acetate, ethanol, etc. Many studies have reported different mechanisms, but it is generally accepted that the C-C coupling pathway requires a high coverage of CO* intermediates on continuous Cu surfaces, which suggests the unlikely formation of C2+ hydrocarbons on single-atom Cu sites (Jiao\u00a0et\u00a0al., 2019; Liu\u00a0et\u00a0al., 2019b). Therefore, research on Cu SAECs has enhanced the selectivity of C1 hydrocarbons, such as methanol and methane. In addition to C1 hydrocarbons, the effect of Cu-Nx on CO2 reduction to CO was also studied. Zheng and his coworkers prepared a highly efficient CO2 electrocatalyst (Cu-N2/GN) composed of unsaturated single-atom copper coordinated with nitrogen sites anchored into a graphene matrix (Zheng\u00a0et\u00a0al., 2019). Benefitting from the unsaturated coordination environment and the atomic dispersion, Cu-N2/GN exhibited a high CO2RR activity and selectivity for CO production with an onset potential of -0.33\u00a0V and a maximum Faradaic efficiency of 81% at a low potential of -0.50\u00a0V (Fig.\u00a017a\n). For comparison, they also prepared Cu-N4/GN. Compared with the activity of Cu-N4 sites, the current density and Faradaic efficiency of Cu-N2 sites were better. DFT calculations revealed that the catalytic activity of Cu-N2 surpassed that of Cu-N4, and Cu-N2 was the true active site. The effect of Cu-N3 active sites can not be ignored. Chen and his coworkers prepared a Cu SAECs with Cu-N3 coordination, which achieved a high CO faradic efficiency of 98% at -0.67\u00a0V (vs RHE) as well as an excellent stability over 20 h of successive electrolysis (Fig.\u00a017b) (Chen\u00a0et\u00a0al., 2021).For comparison, they prepared Cu-N4 active sites. DFT showed the catalytic activity of Cu-N3 was better than Cu-N4. However, not all the activity of Cu-N4 sites is weak. Yang and his coworkers prepared a Cu single atom (Cu SAs/NC) with exceptional CO production performances (Yang\u00a0et\u00a0al., 2020b). The as-prepared Cu SAs/NC catalyst delivered a high CO faradaic efficiency of 92% at -0.7\u00a0V vs RHE as well as the excellent durability over 30 h of successive electrolysis. DFT calculations revealed that Cu-N4 was the true activity site, which gave rise to the high selectivity and the high production of CO (Fig.\u00a017c). Cheng and his coworkers prepared a Cu-N4-C SAECs through facile one step thermal activation (Cheng\u00a0et\u00a0al., 2021). The as-prepared Cu-N4-C catalyst exhibited high CO faradic efficiency with a maximum value of 98% at -0.9\u00a0V vs RHE (Fig.\u00a017d) and an excellent durability over 40 h. DFT results revealed that Cu-N4 active sites substantially lowered the energy barrier for the information of COOH*, thus enhancing catalytic performance. In addition, Cu+ can act as active sites to affect the reduction of CO2 to CO. Zhang and his coworkers designed a strategy of single-atom Sn anchored on Cu2O nanosheets to stabilize the key Cu+ species for electroreduction of CO2(Zhang\u00a0et\u00a0al., 2020e). Infrared spectroscopy suggested that the survival of Cu+ species on the catalyst surface promotes the adsorption of CO* during the CO2RR, leading to the obvious improvement of CO2-to-CO conversion. As a result, the catalyst possesses 30% remarkably stable Cu+ species during the reaction and was able to selectively convert CO2 to CO with a CO Faradaic efficiency of 87.9% at -1.3\u00a0V (Fig.\u00a017e). In CO2RR research, Cu SAECs could also turn CO2 into other hydrocarbons. Guan and his coworkers reported that the CO2RR products depend on the distance between neighboring Cu-Nx moieties (Guan\u00a0et\u00a0al., 2020). The Cu doping concentrations and Cu-Nx configurations were well-tuned by the pyrolysis temperature. All the prepared Cu-Nx configurations had efficient catalytic activity to reduce CO2 to hydrocarbons (methane and ethylene). At a high Cu concentration of 4.9%mol, the distance between neighboring Cu-Nx species was close enough to enable C-C coupling and produce C2H4. In contrast, at Cu concentrations lower than 2.4%mol, the distance between Cu-Nx species was large so that the electrocatalysis favored the formation of CH4 as C1 products. DFT calculations confirmed the capability of producing C2H4 by two CO intermediates binding to two adjacent Cu-N2 sites, while the isolated Cu-N4, the neighboring Cu-N4, and the isolated Cu-N2 sites led to the formation of CH4 (Fig.\u00a017f). It is also possible to reduce CO2 to methane without other byproducts. Cai and his coworkers reported a carbon dot-based SAC with unique CuN2O2 sites for the first time (Cai\u00a0et\u00a0al., 2021b). The catalyst exhibited extraordinary selectivity (99% ECR) for the electrochemical reduction of CO2 to CH4 over a wide potential range from -1.14\u00a0V to -1.64\u00a0V vs. RHE. In addition, for methanol generation, Yang and his coworkers synthesized isolated Cu-decorated through-hole carbon nanofibers (CuSAs/TCNFs,) on a large scale by a facile strategy (Yang\u00a0et\u00a0al., 2019). These CuSAs/TCNFs could generate nearly pure methanol with 44% Faradaic efficiency in the liquid phase at a current density of 93\u00a0mA/cm2 (Fig.\u00a017g). DFT calculations indicated that Cu single atoms possessed a relatively higher binding energy for the CO* intermediate. Therefore, CO* could be further reduced to products such as methanol instead of being easily released from the catalyst surface as a CO product. Moreover, synergistic effects between the carbon substrate and the Cu single atoms were claimed to be the key for the reduction of CO2 to methanol. The same CO2RR products, but different preparation. Yang and his coworkers synthesized C3N4 supported Cu-N4 SAC via the thermal pyrolysis of melamine for low temperature CO2 hydrogenation (Yang\u00a0et\u00a0al., 2021b). The Cu-N4 catalyst favored CO2 hydrogenation to from CH3OH, the productivity and selectivity of CH3OH reached 4.2\u00a0mmol/(g\u2022h) and 95% (Fig.\u00a017h), respectively. It is demonstrated that the first H easily preferred to bond with C atom of CO2 to from *HCOO instead of forming the *HOCO intermediate, and thus generated CH3OH.We discussed the M-Nx-C type of SAECs for the CO2RR based on the different metals involved as the active site (Table\u00a03\n). As seen from Table\u00a03, when at the same overpotential, these SAECs exhibit excellent catalytic activity and selectivity, and their FECO could be over 80%. The rate-determining step of these SAECs is the 1st step as CO2 +*+H++e\u2212\u2192COOH*, and the reaction energy of this uphill step varies from 0.25\u00a0eV to 1.78\u00a0eV. Some problems indeed exist for the development of SAECs. For instance, the stability. In all M-Nx-C-type SAECs, the stabilities were weak, the majority of which could not be sustained for 50\u00a0h. In addition, a comprehensive understanding of the thermodynamic adsorption energies, surface electronic configuration, reactive-interface structure, and reaction kinetics of M-Nx-C-type SAECs in real electrolytes during the reaction is urgently needed. In the Sabatier principle, a volcano relationship is reached when correlating the electrocatalyst performance with the adsorption energies of intermediates. First-principles quantum chemical calculations, a typical DFT formalism, enabled calculation of the adsorption energies of the presumed intermediates. However, the DFT method can only provide a qualitative result to account for the experimental trend. An improved model or a new theoretical method with higher precision is highly desired, which will be discussed in detail in the next section.Porphyrin is a highly heterocyclic macromolecule with \u03c0-electron conjugation. As a planar macrocyclic molecule, porphyrin consists of four pyrroles connected in a ring fashion through four methine carbons at their \u03b1-positions (Jiang\u00a0and Sun,\u00a02019). Due to their chemical properties, porphyrin can exist in the form of derivatives of the basic molecules, the peripheral eight pyrrole \u03b2-carbon atoms and the four meso-carbon centers of which can be replaced. This leads to its extensive natural existence in plants and animals and plays an important role in life activities. For example, chlorophyll and heme (or hemoglobin) are important participants in photosynthesis and oxygen transport. Owing to its special structures, porphyrin has many advantages, such as strong visible absorption, excellent emission intensity, convenient structural functionalization, good redox performance and relatively high stability. Due to these features, different kinds of porphyrin-based complexes have been synthesized as optical catalysis and electrochemical catalysts.Metalloporphyrin, as one of the various porphyrin-based complexes, is widely used to activate and reduce small molecules, particularly O2 and CO2 (Zhang\u00a0and Warren,\u00a02021). The porphyrin ring has a vacant site at its center, which is suited for a metal atom to cooperate with it to form a metalloporphyrin complex. Because of the structure of metalloporphyrin, the common coordination number of the central metal ion of metalloporphyrin is four, and the number of coordinates can change through ligation of suitable moieties, either neutral or anionic. The advantages of metalloporphyrin as a CO2RR electrocatalyst are as follows (Liang\u00a0et\u00a0al., 2021). First, porphyrin can provide a stable and rigid coordination environment for the incorporated metal ion. This ensures that the obtained metalloporphyrin is stable in acidic and alkaline solutions. Second, porphyrins are redox-active, which can enhance the redox chemistry of the obtained metalloporphyrin, benefiting multielectron catalytic processes. Third, the meso- and \u03b2-positions of porphyrin can be modified by different functional groups, endowing metalloporphyrin with different properties. Fourth, catalytic efficiency can be improved by tuning the second coordinate spheres of metalloporphyrin in a stable coordinate environment (Nam\u00a0et\u00a0al., 2020).Iron porphyrin and cobalt porphyrin are the most extensively studied metalloporphyrins. Two types of iron-porphyrins (Davethu\u00a0and de Visser,\u00a02019)were used to explore why iron metalloporphyrin is active for the CO2RR: tetraphenyl porphyrin (TPP) and meso-(ortho-2-amide-phenyl) (triphenyl)porphyrin ligands. Their calculations showed that CO2, as an \n\n\n[\n\nF\n\n\ne\n\nIII\n\n\n(\nC\n\nO\n2\n\n2\n\u2212\n\n\n)\n\n\n(\nT\nP\n\n\nP\n\n\n\u2212\n\u2022\n\n\n)\n\n\n]\n\n\n2\n\u2212\n\n\n complex, existed stably in the electron transfer processes during the CO2 reduction cycle (Fig.\u00a018a\n). Step III in the CO2 reduction cycle should be considered as a proton-coupled electron transfer, while the second proton transfer does not change the electronic configuration of the metal complex. In addition, electron transfer mechanisms and second-coordination sphere effects on the reaction mechanism have also been studied. Iron porphyrins are efficient CO2 reducing systems that can reduce CO2 into CO efficiently. Modifying the constituents or the structure, the electrocatalytic performance of iron porphyrins will be further improved. For example, an iron hangman porphyrin interacting with phenol, guanidinium and sulfonic acid proton donor groups on the iron porphyrin platform, as shown in Fig.\u00a018b (Margarit\u00a0et\u00a0al., 2018), could reduce CO2 to CO at above 93% Faraday efficiency. Fe-1, Fe-2, and Fe-3 porphyrins (Liu\u00a0et\u00a0al., 2020a) were synthesized by simple aminophenyl substitution. Compared with Fe-2 and Fe-3, Fe-1 porphyrin exhibited an improved turn over frequency and high CO selectivity (88% FE, Fig.\u00a018c), confirming that amino groups in the secondary sphere of iron could enhance the catalytic activity of the CO2RR.Compared with iron porphyrin complexes, the activity and selectivity of cobalt porphyrin complexes for CO2RR can be easily adjusted by structure. For example, a bimetallic central electrocatalyst denoted CoCoPCP/CNT (Wang\u00a0et\u00a0al., 2021) exhibited high CO2RR activity and CO selectivity in aqueous solution benefiting from the regulation of the central site of the cobalt porphyrin conjugated polymer (PCP) (Fig.\u00a019a\n). The FECO of this catalyst reached 94% at an extremely low overpotential of 0.44 V(Valero-Romero et\u00a0al.). Enlarging the \u03c0-conjugation of porphyrin by appending more aromatic subunits on the periphery of 5,10,15,20-(tetraphenyl) porphyrin (TPP), the 5,10,15,20-tetrakis(4-(pyren-1-yl) phenyl) porphyrin (TPyPP) molecule was generated, which coordinated with cobalt ions and noncovalently immobilized on carbon nanotubes (CNTs) as CO2RR electrocatalysts (denoted CoTPyPP/CNT) (Dou\u00a0et\u00a0al., 2021). CoTPyPP/CNT exhibited a large CO2 reduction current density and high Faraday efficiency (Fig.\u00a019b), indicating that a larger \u03c0-conjugation on the porphyrin could lead to higher electrocatalytic CO2RR activity and selectivity. In addition, a grafted catalyst (denoted as CoPP@CNT) was synthesized by protoporphyrin IX cobalt chloride (CoPPCI) and hydroxyl-functionalized carbon nanotubes (Fig.\u00a019c) (Zhu\u00a0et\u00a0al., 2019a). The TOF for CO formation of the catalyst was significantly improved. The Faradic efficiency toward CO generation reached 98.3%, and the current density was 25.1\u00a0mA/cm2 at an overpotential of 490 mV with excellent stability, indicating that covalent grafting is an effective way to improve CO2RR electrocatalysis.Metalloporphyrin, as a homogeneous molecular catalyst, exhibits high activity and selectivity for the CO2RR but is difficult to use in electrocatalytic devices. First, it suffers from poor electrical conductivity and weak interactions with electrodes, leading to low electron transfer efficiency (Hu\u00a0et\u00a0al., 2019). Second, it is difficult to recycle and recover molecular catalysts in homogeneous catalysis Sun\u00a0et\u00a0al., 2020b). Third, in the electrocatalytic process, only molecules near the electrode surface can be reduced or oxidized, while other molecules are not active (Sun\u00a0et\u00a0al., 2020b). To solve these problems, some strategies must be implemented. It was found that porphyrin could be integrated into frameworks to avoid these problems, including metal organic frameworks (Kung\u00a0et\u00a0al., 2017; Wang\u00a0et\u00a0al., 2017; Yi\u00a0et\u00a0al., 2021) and covalent organic frameworks (Huang\u00a0et\u00a0al., 2020b; Huang\u00a0et\u00a0al., 2019b; Yao\u00a0et\u00a0al., 2018). The superiorities of porphyrin-based frameworks are as follows (Liang\u00a0et\u00a0al., 2021). First, porphyrin backbones can be directly used to design and synthesize porphyrin-based frameworks. Second, porphyrin backbones can provide many sites to install functional groups. These functional groups can not only improve catalysis efficiency but also control the structure and morphology of the framework. Third, in porphyrin-based frameworks, porphyrins act as catalytic sites and structural units. Bimetallic and polymetallic porphyrin-based frameworks can be readily constructed for synergistic catalysis. Therefore, the application of porphyrin-based frameworks builds a bridge from homogeneous molecular catalysts to multiphase electrocatalysts and provides a larger and more attractive platform for electrocatalysis.Metal organic frameworks (MOFs) are a class of crystalline porous materials with periodic network structures that consist of metal ions or clusters and organic linkers (Khan\u00a0et\u00a0al., 2022; Lu\u00a0et\u00a0al., 2020a). Due to their unique structure, MOFs have some special and appealing features. First, a regular and adjustable pore size (Lei\u00a0et\u00a0al., 2018) is suitable for capturing small molecules such as CO2. The frame structure facilitates the adsorption, diffusion and reaction of carbon dioxide. Second, controllable active metal sites (Hinogami\u00a0et\u00a0al., 2012) and organic ligands (Kirchon\u00a0et\u00a0al., 2018). Due to the presence of nanopores, the metal sites in MOFs have very limited space. The metal sites can be controlled to enhance the reactive selectivity. Third, the large specific surface area (Yang\u00a0et\u00a0al., 2017) benefits gas adsorption. These features make MOFs an ideal electrocatalytic platform and provide desired catalytic environments for the electrocatalytic of CO2. A Co metal-porphyrin MOF nanofilm was prepared (Fig.\u00a020a\n) (Kornienko\u00a0et\u00a0al., 2015) and showed efficient and selective reduction of CO2 to CO in aqueous electrolytes. Implanted polypyrrole in Co metal-porphyrin MOF, denoted PPy@MOF-545-Co (Xin\u00a0et\u00a0al., 2021). It exhibited better CO2RR performance, with 98% FECO at -0.8\u00a0V vs. RHE and the largest current density of 32\u00a0mA\u00a0cm\u22122 at -1.1\u00a0V vs. RHE (Fig.\u00a020b). In addition, implanting polyoxometalates in MOFs, denoted as M-PMOF (M=Fe, Co, Ni, Zn)(Xie\u00a0et\u00a0al., 2018a), resulted in the best performance. The Faradic efficiency remained larger than 94% over a wide potential range (-0.8\u00a0V to -1.0 V), and the stability was also impressive (>36 h). The largest Faradic efficiency was 99%, and the turnover frequency reached 1656 h\u22121. DFT calculations indicated that the favorable active site is the cobalt in Co-TCPP instead of POM and the efficient synergistic electron modulation between POM and the porphyrin metal center (Fig.\u00a020c). In addition to reducing CO2 to CO, other reduction products are possible for porphyrin MOFs. Take a copper(Valero-Romero et\u00a0al.) paddle wheel cluster-based porphyrinic MOF nanosheet as an example (Wu\u00a0et\u00a0al., 2019b). It exhibited activity for formate and acetate production (Fig.\u00a020d). The highest faradaic efficiencies of formate and acetate were 68.4% and 16.8%, respectively, at -1.55\u00a0V vs. Ag/Ag+. The maximum current densities for formate and acetate were 3.5\u00a0mA\u00a0cm\u22122 and 1.0\u00a0mA\u00a0cm\u22122 at -1.6\u00a0V vs. Ag/Ag+, respectively.Covalent organic frameworks (COFs) are porous materials consisting of organic molecules through covalent bonds with ordered crystal and periodic structures (Kandambeth\u00a0et\u00a0al., 2019). COFs have some features, such as large specific surface areas and permanent porosity. These features could improve the local CO2 concentration near the active sites and provide efficient transport channels for carriers (Duan\u00a0et\u00a0al., 2019; Ozdemir\u00a0et\u00a0al., 2019; Veldhuizen\u00a0et\u00a0al., 2019). Compared with MOFs, COFs have three additional features (Abednatanzi\u00a0et\u00a0al., 2022). First, COFs have a variety of structural units, and these units are all organic small molecules. Second, the periodic network structure of COFs is formed by covalent bonds (Fig.\u00a021a\n). Strong covalent bonds are more stable than the coordinate bonds of MOFs. Third, the density of COFs is low since they are composed of light elements, such as C, H, O, and N. COFs have become a promising platform for fabricating efficient electrocatalysts for the CO2RR. In 2015, Co porphyrin-based COF (Lin\u00a0et\u00a0al., 2015) showed high catalytic activity for the CO2RR in water, denoted COF-366-Co (Fig.\u00a021b). It exhibited high Faradaic efficiency for CO (90%) and turnover numbers (up to 290,000, with an initial turnover frequency of 9400\u00a0h\u22121) at pH 7 under an overpotential of -0.55\u00a0V. Furthermore, a Co porphyrin-based COF containing donor-acceptor (D-A) heterojunctions, termed TT-Por (Co)-COF, was prepared (Fig.\u00a021c) (Wu\u00a0et\u00a0al., 2021a). It was able to selectively convert CO2 to CO with a high FECO of 91.4% at -0.6\u00a0V vs. RHE and exhibited a large partial current density of 7.28\u00a0mA/cm2 at -0.7\u00a0V vs. RHE in aqueous solution. In addition, there was a Co porphyrin-based COF based on amino-functionalized carbon nanotubes (Lu\u00a0et\u00a0al., 2020b) capable of an efficient electrocatalytic CO2 reduction reaction, denoted COF-366-(OMe)2-Co@CNT. This Co porphyrin-based COF exhibited high activity and selectivity for the CO2RR, exhibiting the highest FE of up to 93.6% at -0.68\u00a0V and delivered a total current density up to 40\u00a0mA/cm2 at -1.05\u00a0V (Fig.\u00a021d).Recently, reported electrocatalytic CO2RR performances of different porphyrin-based complexes were compared. As shown in Table\u00a04\n, porphyrin-based complexes exhibit outstanding catalytic performance and higher selectivity toward CO. At the same bias potential, the FECO of most porphyrin-based complexes is over 85%. The rate-determining step of porphyrin-based complexes is CO2 +*+H++e\u2212\u2192COOH*, the reaction energy of which varies from 0.21\u00a0eV to 1.86\u00a0eV. Compared with SAECs (Table\u00a01), the stabilities of porphyrin-based complexes are better, the majority of which could be sustained longer than 10\u00a0h. However, 10\u00a0h is still obviously far from the industrial requirements. The phase transition of porphyrin building units, the decomposition of frameworks and the corrosion of byproducts need to be suppressed to further improve the stability of porphyrin-based complexes. For example, Cu and Co were introduced into COF-367. Such COF-367-Co (1%) showed superior activity compared to COF-367-Co (10%), COF-367-Co and COF-367-Cu, along with a highly stable operation for 136\u00a0h (Lin et\u00a0al., 2015). In addition, a comprehensive understanding of the correlation between the thermodynamic adsorption energetics and the reaction kinetics of porphyrin-based complexes in a real electrolyte during the reaction is urgently needed, which will be discussed in detail in the next section.Biomass is a broad concept that includes the lignocellulosic material starch/oilseed/sugar aquaculture and bioderived waste. From an environmental point of view, the use of biomass for catalysis is very friendly and inexpensive, so biomass-derived catalysts have received extensive attention. However, biomass-derived materials contain a large number of unwanted compounds, making it difficult to control the morphology, porosity and surface chemistry. Thus, their application in industry is restricted (Rodr\u00edguez\u2010Padr\u00f3n et\u00a0al., 2018). Appropriate synthesis for controlling the chemical and physical properties of biomass-derived materials is important for the development and application of biomass-derived catalysts.Carbon is often used as the substrate of metal catalysts (Pt, Ir, Ru, etc.) to improve the specific active area and conductivity of the catalyst (Wang\u00a0et\u00a0al., 2020a). However, these catalysts often suffer from high cost and sensitivity to CO poisoning (Paul\u00a0et\u00a0al., 2019). Incorporating heteroatoms into carbon, the physicochemical and electronic properties of the catalyst can be specifically tuned, a strategy that has become quite popular. At present, modified carbon-based materials with different heteroatoms (B, P, N, S, and F) have been reported (Hu\u00a0et\u00a0al., 2020; Jiang\u00a0et\u00a0al., 2021b; Li\u00a0et\u00a0al., 2019; Sui\u00a0et\u00a0al., 2018; Tang\u00a0et\u00a0al., 2021a; Xie\u00a0et\u00a0al., 2018b; Yu\u00a0et\u00a0al., 2018). Among them, N-doped carbon-based nonmetallic catalysts exhibit high electrocatalytic activity, mainly attributed to three configurations of N (graphite-type N, pyridine-type N, and pyrrolic-type N) (Hao\u00a0et\u00a0al., 2021b; Zhang\u00a0et\u00a0al., 2020b). It is generally believed that the pyridine nitrogen is the main active site for the electroreduction of CO2 because the pyridine nitrogen retains a lone pair of electrons that can bind CO2 and could greatly reduce the free energy required for *COOH intermediate formation (Sharma\u00a0et\u00a0al., 2015). The large electronegativity difference between N and C also leads to a lower work function of graphitic carbon, high surface energy, increased n-type carrier concentration and tunable polarization of graphitic carbons. In addition, since the dopant atoms are often covalently bound to the C atoms in carbon materials, they exhibit extremely strong durability, even comparable to noble metal and transition metal-based catalysts.In this section, the development and application of biomass-derived metal-free nitrogen-doped carbon electrocatalysts reported in recent years are summarized.Wood exhibits a porous hierarchical structure and good electrical conductivity, the efficient gas and ion transport capacities of which have been demonstrated in lithium-CO2 or lithium-O2 batteries (Song\u00a0et\u00a0al., 2018; Xu\u00a0et\u00a0al., 2018; Zhu\u00a0et\u00a0al., 2018). Wood-derived electrocatalytic materials have become one of the important branches of novel electrocatalysts (Chen\u00a0and Hu,\u00a02018; Huang\u00a0et\u00a0al., 2019a). There are two categories of wood-derived electrocatalytic materials: Wood-derived materials used as the catalysts support, where the electrocatalysts were loaded in (Huang\u00a0et\u00a0al., 2020c; Min\u00a0et\u00a0al., 2020; Sekhon\u00a0et\u00a0al., 2022). The other is the modified wood, which is doped with N, S, P, Fe and other elements to improve the electrocatalytic performance (Chen\u00a0et\u00a0al., 2020a; Meng\u00a0et\u00a0al., 2019; Zhang\u00a0et\u00a0al., 2020b). As we all know, catalytic performance is closely related to the structure, especially the specific surface area. Thus, most of these catalysts are produced by hydrothermal carbonization (HTC) (Lei\u00a0et\u00a0al., 2021; Zhang\u00a0et\u00a0al., 2020f), a process that could increase solubility and the speed of carbonaceous structure formation (Hu\u00a0et\u00a0al., 2010).N-doped carbon materials are effective for the reduction of CO2 to CO, HCOO\u2212 and CxHyOz (Wanninayake\u00a0et\u00a0al., 2020), and N atoms can change the charge and spin density of carbon atoms and serve as the active sites (Yao\u00a0et\u00a0al., 2019). Nicolas Brun et\u00a0al. (Brun\u00a0et\u00a0al., 2014) reported for the first time in 2014 a microporous nitrogen-doped carbon material (N-doped carboHIPE) (Fig.\u00a022\n), which used the dehydration product of N-acetylglucosamine and hexose, 5-hydroxymethyl-2-furaldehyde and phloroglucinol to design functional carbon-based monomers. The conductivity of the resulting monolithic N-doped carbon-based foam reached 300 S m\u22121, and the specific surface area and porosity were 568 m2 g\u22121 and 0.26\u00a0cm\u22123 g\u22121, respectively. It was believed that the continuous monolithic structure provides higher electrical conductivity (avoiding the electrical resistance associated with particle contact) and better mechanical integrity compared to powder. Treatment of nitrogen- or oxidation-modified wood-based activated carbon with melamine showed good CO2 reduction activity (Li\u00a0et\u00a0al., 2017b), with 40% FE for CO and 1.2% FE for CH4. The positively charged carbon close to the pyridine nitrogen could stabilize the \n\nC\n\nO\n2\n\n\u2022\n\u2212\n\n\n\nintermediate. In addition, N-oxide (C-N+-O\u2212) is also the active site of the CO2RR. The contents of pyridine-N and FE in the process of CO2 production are linearly correlated. These results showed that these substances play a leading role in the CO2RR. The graphitization degree of the porous structure and N doping type of wood-derived carbon can be controlled by the ratio of the initial raw materials (Hao\u00a0et\u00a0al., 2021b). What is exciting is that the activated wood also exhibits favorable CO2RR activity and stability (Fig.\u00a023\n). Huang et\u00a0al. (Huang\u00a0et\u00a0al., 2019a) The poplar diameter section was heated at 240\u00a0\u00b0C for 6 h, placed in an Ar atmosphere for carbonization at 1000\u00a0\u00b0C for 6 h, and finally heated at 750\u00a0\u00b0C in a CO2 atmosphere for 6 h to obtain the activated wood. The highest formic acid Faradaic efficiency was 70.8% at -1.8\u00a0V, and the high current density in aqueous solution was 53.8\u00a0mA/cm2, with a minimum of 24 h electrolytic activity. Moreover, it was also applied to the electrode matrix for the electroanalysis and detection of drugs for the first time. Compared with the glass carbon electrode (GCE) and its derivative-modified electrodes, it has a wider linear detection range and a lower detection limit.Metal-CO2 fuel cells are capable of converting CO2 into valuable chemicals and generating electricity at the same time. However, metal-CO2 fuel cells are currently in the preliminary stage, and the development of more efficient cathode catalysts is particularly important. The use of woods\u2019 excellent ion and gas transport capacity to improve the efficiency of metal-CO2 cathode catalysts is of significance. Xu et\u00a0al.(Xu\u00a0et\u00a0al., 2018) reported a high-capacity, mechanically flexible and highly rechargeable lithium-carbon dioxide battery. The flexible cathode of the battery utilized the natural structure of wood, and the battery exhibited a stable cycle period of more than 200, with low overpotential and ultrahigh discharge capacity (11\u00a0mAh\u00a0cm\u22122). Cedar was used as the biomass carbon precursor (Hao\u00a0et\u00a0al., 2021b), together with melamine as the nitrogen source and FeCl3 (activator) for the preparation of the Zn-CO2 battery. The cedar biomass-derived N-doped graphitized carbon (CB-NGC)-2) exhibited a three-dimensional (3D) structure, the specific surface area of which was as high as 1673.6 m2 g\u22121. It selectively converted CO2 to CO at a high Faraday efficiency of 91% at 0.56\u00a0V (vs. RHE).Mesoporous carbon can be directly synthesized from various biomass polysaccharides, such as wheat flour, sodium alginate, and chitosan. This family of carbonaceous materials, also known as Starbons (Rodr\u00edguez\u2010Padr\u00f3n et\u00a0al., 2018), have achieved remarkable texture characteristics and adjustable surface function, making them good candidates for catalysts and catalyst supports. These materials can be designed and prepared by a simple method, primarily hydrogel formation followed by solvent exchange and thermal carbonation (Fig.\u00a024\n). Li et\u00a0al. (Li\u00a0et\u00a0al., 2017a) used wheat flour to obtain a high specific surface area and hierarchical porous nitrogen-doped carbon material by one-step thermal carbonization. The nitrogen content and functional species were controlled by the pyrolysis temperature. The catalyst had a high Faraday efficiency FECO of 83.7%, the current density of which reached 6.6\u00a0mA/cm2 with an overpotential of 0.71 However, the stability was only 72 h, far from the requirement of industrial application. Chen et\u00a0al. (Chen\u00a0et\u00a0al., 2020b) started from silk cocoon, an animal N-rich product, to synthesize intrinsic defect-rich biocarbon by the activation of the pre-carbonized precursor by mixing with ZnCl2 and then pyrolyzing (Fig.\u00a025a\n). The activation produced vacancy defects by removing part of the N-containing part of the graphitic carbon substrate, which greatly improved the electrocatalytic efficiency. Therefore, at the optimal pre-carbonization and carbonization temperatures (350\u00a0\u00b0C and 1000\u00a0\u00b0C, respectively), the catalyst exhibited the highest current density jCO \u2248 1.3\u00a0mA\u00a0cm\u22122, FECO \u2248 89%, and maintained good selectivity for 10 days (Fig.\u00a025. b-e). The rate-determining step was considered to be a direct electron transfer step to CO2 rather than proton coupling electron transfer. Theoretical calculations also showed that the inherent defects (especially pentagonal defects) of the carbon matrix were the main active sites for direct electron transfer during CO2 reduction (Fig.\u00a025.f-g).The performance of biomass-derived nitrogen-doped carbon electrocatalysts is listed in Table\u00a05\n. Compared with Bi-based metallic electrocatalysts (Table\u00a01), Cu-based metallic electrocatalysts (Table 2), SAECs (Table\u00a03) and porphyrin-based complexes (Table\u00a04), biomass-derived nitrogen-doped carbons show low activity for the HER owing to the pyrrolic nitrogen (Wu\u00a0et\u00a0al., 2016). Nitrogen plays an important role in the electrocatalytic performance; both pyridine N atoms and graphite N atoms show high activity for CO2 reduction and H2O reduction and greatly reduce the free energy difference of the rate-determination step. Adjusting N atoms in nonmetallic carbon-based materials greatly improved the Faraday efficiency of the catalysts, comparable to that of metallic catalysts. However, the current density of biomass-derived nonmetallic nitrogen-doped carbon is generally smaller than that of metallic catalysts. It\u00a0should also be\u00a0stressed that increasing the ratio of pyrrolic N in nitrogen-doped carbon is an effective way to improve CO2 reduction selectivity. Although it was demonstrated that pyridine N could be converted to pyrrolic N via thermal nitro-nitro rearrangement (Cui\u00a0et\u00a0al., 2017), controlling the structure after nitrogen doping remains a challenge.The partial current densities toward the CO2RR (jCO2RR) as the kinetic descriptor along with the thermodynamic free energy changes of the rate-determining step (\u0394GRDS) for the four groups of electrocatalysts are shown in Fig.\u00a026\n. It is interesting to see a negative correlation between the reactive thermodynamics and kinetics for all these electrocatalysts, i.e., a larger partial current density is obtained by reducing the \u0394GRDS of the uphill rate-determining step. It should be emphasized here that the \u0394GRDS and jCO2RR of the metal-free nitrogen-doped carbon catalyst in Fig.\u00a026 is not strictly negatively correlated, mainly because the thermodynamic data of such a catalyst is calculated for a specific form of N, and therefore does not show a typical negative correlation mode. However, from the overall perspective of Fig.\u00a026, \u0394GRDS and jCO2RR of catalysts still show a negative correlation. Accordingly, a positive correlation between the thermodynamic \u0394GRDS and the kinetic activation energy could be inferred. Such correlation is largely neglected and underestimated by electrocatalyst researchers. The thermodynamic-kinetic synergistic relationship was proposed by Liu et\u00a0al. in structural metallic materials in 2020 (Huang\u00a0et\u00a0al., 2020a).To the best of our knowledge, there is no review focusing on the reactive thermodynamics, kinetics and the correlation between the two, as well as their application in the design and screening of CO2RR catalysts. Thus, the electrochemical thermodynamic framework, kinetics, and thermodynamic-kinetic correlation in the CO2RR are discussed in detail in this section.The design of selective and efficient catalysts for CO2 electroreduction requires deep insights into the relationship between the composition, structure and catalytic performance. Reactive thermodynamics determines the energy barrier of a chemical reaction. In 1920, Sabatier firstly proposed the Sabatier principle (van Santen et\u00a0al., 2010) to sketchily describe general correlation. In the Sabatier principle, Sabatier assumes that a \u2018\u2018volcano relationship\u2019\u2019 exists between the binding energy of key intermediates and catalytic activity. The best catalyst is located on the top of the volcano, which indicates adsorbates binding on the catalyst neither too strongly nor too weakly. Why does the volcano relationship exist, and how can such a relationship be established theoretically? N\u00f8rskov and coworkers proposed the famous electrochemical thermodynamic framework (ETF) to provide an answer (Norskov\u00a0et\u00a0al., 2004). ETF is also called the computational hydrogen electrode (CHE) method, which relies on the calculation of the adsorption energy and the H2 gas reference to account for the free energy of the proton-electron pair, \n\n\u03bc\n\n\nH\n\n+\n\n+\n\u03bc\n\n(\n\n\ne\n\n\u2212\n\n)\n\n=\n\n1\n2\n\n\u03bc\n\n(\n\nH\n2\n\n)\n\n\u2212\ne\nU\n\n, where U is the potential of the relative reversible hydrogen electrode (RHE). In this way, the explicit handling of solvated protons and electrons in each proton-coupled electron transfer (PCET) process is elegantly avoided, greatly reducing the computational cost. In other words, the CHE model can be used to generate the thermodynamic free energy diagram (without explicit kinetic barriers) of a series of PCET steps under an applied potential and avoid the explicit treatment of solvated protons and electrons in the reaction (Alfonso\u00a0et\u00a0al., 2018).With ETF, the Sabatier principle can be quantified rather than a conceptual statement. First, for the horizontal axis in volcano plots, linear correlations exist between any two of the absorbents, which is called the \u201cscaling relationship\u201d. The scaling relationship is used to analyze and predict catalyst reactivity and efficiency since it is easy to obtain other absorbents\u2019 absorption energy in the case of a known adsorbent's adsorption energy. Second, for the vertical axis in volcano plots, due to the low proton transfer barriers, the kinetic aspects in the proton transfer can be ignored. The ETF considers the maximum standard Gibbs free energy differences (\n\n\n\u0394\n\n\nG\n\nm\na\nx\n\n0\n\n\n) among all the elemental steps to be the potential-determining step (PDS). The overpotential is generally calculated by \n\n\u03b7\n=\n\nU\n0\n\n\u2212\n\nU\n\nl\ni\nm\n\n\n=\n\nU\n0\n\n\u2212\n\n\u0394\n\n\nG\n\nm\na\nx\n\n0\n\n/\ne\n\n, where \n\nU\n0\n\n is the equilibrium potential and \n\nU\n\nl\ni\nm\n\n\n is the limiting potential of the reaction. A positive overpotential is required for the oxidation process, whereas a negative overpotential is necessary for the reduction reaction.The CHE model and the thermodynamic overpotential \n\u03b7\n are by far the most popular way to evaluate the electrocatalytic activity. The application of the electrochemical thermodynamic framework in the research of SAECs toward CO2 electroreduction reactions, as an example, will be summarized subsequently. For the M-N-C type of SAECs, ETF has been applied to understand the reaction mechanism of M-Nx (M=Fe, Co, Ni, Cu) moieties embedded in graphene (N-doped). The generation of CO on Fe-N-C SAECs, Co-N-C SAECs, Ni-N-C SAECs and Cu-N-C SAECs consisted of the following electron/proton transfer steps, \n\nC\n\nO\n2\n\n+\n*\n+\n2\n\n\nH\n\n+\n\n+\n2\n\n\ne\n\n\u2212\n\n\u2192\nC\nO\nO\n\n\nH\n\n*\n\n+\n\n\nH\n\n+\n\n+\n\n\ne\n\n\u2212\n\n\u2192\nC\n\n\nO\n\n*\n\n+\n\nH\n2\n\nO\n\u2192\nC\nO\n+\n*\n+\n\nH\n2\n\nO\n\n. The calculated free energy profiles are shown in Fig.\u00a027\n. The rate-determining step for the four M-N-C types of SAECs is \n\nC\n\nO\n2\n\n+\n\n\nH\n\n+\n\n+\n\n\ne\n\n\u2212\n\n\u2192\nC\nO\nO\n\n\nH\n\n*\n\n\n. The adsorption of CO2 molecules initially occurred on the Fe-N4 catalytic site with concerted protonation and electron transfer, leading to the formation of COOH* with an uphill energy of 0.63\u00a0eV (Fig.\u00a027a). With nitrogen doping in graphene, the energy difference for the generation of *COOH intermediates decreased from 0.63\u00a0eV (only Fe\u2013N4 motif in graphene) to 0.29\u00a0eV (Fe\u2013N4 motif with two graphitic N), indicating that nitrogen doping into the graphene matrix could facilitate the catalytic pathway (Zhang\u00a0et\u00a0al., 2018a). Wang et\u00a0al. calculated the Gibbs free energy of the Co-Nx moiety from CO2 to CO and found that the formation of *CO2\n\u2022\u2212 was key for the high electrocatalytic activity (Wang\u00a0et\u00a0al., 2018c). The less endergonic formation of *CO2\n\u2022\u2212 on the Co-Nx moiety, the more beneficial it is for CO2 to form COOH* and CO (Fig.\u00a027b). Mou et\u00a0al. calculated the Gibbs free energy of the Ni-Nx moiety from CO2 to CO, and the free energy differences of the rate-determining steps on Ni-N2, Ni-N3, and Ni-N4 were 0.17\u00a0eV, 0.84\u00a0eV, and 1.34\u00a0eV, respectively (Fig.\u00a027c). They suggested that the high CO2RR activity originated from coordinatively unsaturated Ni-N sites (Mou\u00a0et\u00a0al., 2019). To identify the high activity of Cu-Nx for CO generation, Zheng et\u00a0al. calculated the free energy profiles of the whole process according to ETF (Zheng\u00a0et\u00a0al., 2019). The free energy difference between CO2* and COOH* was 1.599\u00a0eV in the Cu-N4 moiety, larger than that in the Cu-N2 moiety (0.96\u00a0eV), as shown in Fig.\u00a027d. It was suggested that the length of the Cu-N bond in Cu-N2 (1.825 \u00c5) was shorter than that in Cu-N4 (1.939 \u00c5), benefiting electron transfer to CO2* and thus boosting COOH* generation.Although the CHE model could generate the thermodynamic energy landscape of a series of PCET steps and identify the PDS, it should be emphasized that the CHE model ignores activation energies for all PCET steps and is therefore unreliable for kinetic analysis. The main reason is that CHE model does not take into account the effect of solvation on reaction kinetics. Therefore, a series of models have been developed to explain the effects of solvent molecules on the kinetics of catalytic reactions. In this section, we first summarized the influence of electrolyte on reaction kinetics, and then summarized the various theoretical models involved in kinetics.\nInfluence of electrolyte. Electrolyte composition and pH are important factors in studying CO2RR performance. In the current research process, KHCO3 and KCl solutions are the most common aqueous electrolyte solutions. The biggest difference between KHCO3 and KCl is that KHCO3 is a buffer solution, which can compensate for the hydroxide ions (OH\u2212) generated by CO2RR and HER during the reaction process, so that the pH value of the electrode surface does not change much, so as to maximize the reduce the loss caused by electrode polarization. Adam Z. Weber et\u00a0al. (Hashiba\u00a0et\u00a0al., 2018) found that the methane yield of polycrystalline copper increased significantly after replacing 0.5M KCl with 0.5M KHCO3. Therefore, the effect of buffer capacity of electrolyte on CO2RR was investigated. Through experiments and the establishment of a 1D model, the limiting reaction rate and limiting current density (Jlim) of CO2 were obtained under different CO2 partial pressure and electrolyte conditions. It was finally found that KHCO3 provided a higher CO2 flux than KCl and resulted in a slower local pH increase and a slower uniform consumption of CO2 by OH\u2212. This is important under high current density conditions where large amounts of OH\u2212 are generated. The partial pressure of CO2 was changed under the condition of constant current density, and it was found that under low pressure conditions (<\u223c2.5 atm), increasing the concentration of bicarbonate was beneficial to improve CO2 transport, and under high pressure conditions (>\u223c2.5 atm), with the increase of CO2 partial pressure, the CO2 transport is improved faster.Besides the influence of the buffer capacity of the electrolyte, the cations and anions in the electrolyte also have an important influence on the selectivity of the electrode. The addition of halides to the bicarbonate electrolyte can stabilize the carboxyl intermediate to further enhance CO2RR. Larger alkali metal cations in the electrolyte lead to improved formation of C2+ products on copper foil, Cu (1 0 0) and (1 1 1) surfaces (Perez-Gallent\u00a0et\u00a0al., 2017; Singh\u00a0et\u00a0al., 2016). The dependence of the product on cation type is mainly due to its obvious specific adsorption, preferential hydrolysis or electrostatic interactions between solvated cations and adsorbed species in the outer Helmholtz plane (OHP) (Gao\u00a0et\u00a0al., 2018; Perez-Gallent\u00a0et\u00a0al., 2017; Singh\u00a0et\u00a0al., 2016). Gao et\u00a0al. (Gao\u00a0et\u00a0al., 2018)studied the effect of cation size on the activity and selectivity of copper oxide catalysts and found that with the increase of cation size, the selectivity for C2+ products increased significantly (FE \u223c 69%), and the addition of I\u2212 to CsHCO3 further increased the formation of C2+, which may be due to the formation of a large amount of species with Cu+ during the reaction. The adsorbed cations, calculated by DFT, promoted the formation of C2+ intermediates (*CO, *OCCO, *OCCOH), of which Cs showed the greatest promotion. Besides cations, anions have also received extensive attention to influence catalyst selectivity by adsorbing on surfaces and affecting active sites. Huang et\u00a0al. found that Cu (1 0 0) and Cu (1 1 1) surfaces undergo CO2RR in non-buffered solution electrolytes (KClO4, KCl, KBr, and KI), the yields of ethylene and ethanol increase sequentially with ClO4\n\u2212\u2192Cl\u2212\u2192Br\u2212\u2192I\u2212, which is attributed to the anion promoting the adsorption of more *CO, thereby promoting C\u2014C coupling into the C2 product.In addition, pH plays an important role in the CO2RR mechanism, from a computational point of view, successive proton-coupled electron transfer steps are usually assumed at each step, so that the CHE model can be used. Note that this model cannot capture pH effects, as the adsorption energies of all intermediates shift proportionally. Therefore, it's necessary to study and explain the influence of electrolyte pH value. It is currently known that pH has a critical effect on the selectivity of copper-based catalysts (Rendon-Calle\u00a0et\u00a0al., 2018; Varela,\u00a02020). For example, the formation of methane is pH-sensitive, while the formation of ethylene is not pH-sensitive, which is mainly related to the synthesis pathway of these hydrocarbons (Fig. 28\n). The green path shows a pH-dependent path that primarily produces methane and forms ethylene by dimerization of intermediates, and the orange path shows a pH-independent path that produces ethylene via the formation of a CO dimer intermediate. It should be noted that the pH value here refers to the local pH rather than the overall pH. This is because during the reaction process, the associated proton consumption of CO2RR and HER or the release of OH\u2212 can lead to a concentration gradient change of OH\u2212, resulting in a gradient change of pH. This effect of local pH can be critical when using high-performance electrocatalysts, as the local pH and the pH of the bulk solution system can vary greatly. According to this feature, the selectivity of the catalyst can be regulated by adjusting the local pH. For example, the yield of hydrogen and methane increases with the increase of bicarbonate concentration, but the yield of ethylene is not affected. This can improve the selectivity of ethylene by diluting the buffer capacity of bicarbonate and allowing alkaline local pH to inhibit CH4 formation (Hashiba\u00a0et\u00a0al., 2018).Therefore, we know that the electrolyte plays an important role in the selectivity of CO2RR products, that is, plays a decisive role in the kinetic energy barrier of the reaction. In order to fully consider the role of these solvated molecules in the kinetic process, a series of kinetic models have been derived to explain, which will be introduced in detail below.\nExplicit Solvation Model. Few layers of water molecules are placed above the catalyst surface to reproduce the dielectric response of the liquid environment. The presence of explicit solvent molecules enables the location of the transition states as well as the calculation of kinetic activation barriers. Nie et\u00a0al. proposed a water solvation model and H-shuttling model to explore the kinetic barriers of CO2 electroreduction on various copper facets (Luo\u00a0et\u00a0al., 2016; Nie\u00a0et\u00a0al., 2013; Nie\u00a0et\u00a0al., 2014). In these two models, an adsorbed H* on the electrode was used as the proton-electron pair instead of the hydrate proton. In the water-solvated model, surface protons are added to the adsorbed intermediate directly, whereas in the H-shuttling model, H* is transferred to neighboring water, and then the hydrogen of the water reacts with the intermediate. The calculated activation energy is recorded as \n\n\nE\n\na\nc\nt\n\n0\n\n\n(\n\nU\n0\n\n)\n\n\n, and then according to the Butler-Volmer equation, \n\n\nE\n\na\nc\nt\n\n\n\n(\nU\n)\n\n=\n\nE\n\na\nc\nt\n\n0\n\n\n(\n\nU\n0\n\n)\n\n+\n\u03b2\n\n(\n\nU\n\u2212\n\nU\n0\n\n\n)\n\n\n, the activation energy at any potential can be calculated, where \u03b2 is considered to be 0.5. Nie et\u00a0al. (Nie\u00a0et\u00a0al., 2014) found a completely different route from that obtained by Peterson et\u00a0al.(Peterson\u00a0et\u00a0al., 2010) in the reduction of CO2 to CH4 on the Cu(111) surface (Fig.\u00a029a\n). They believe that the formation of COH* via the H-shuttling mechanism was kinetically easier than the formation of CHO*, due to its lower activation barrier (0.21\u00a0eV vs 0.39\u00a0eV). Subsequently, COH* was reduced to CHx*(x=0-3) and eventually converted to methane or ethylene (Fig.\u00a029b, c). In addition, other metal surfaces covered with explicit water molecules have been explored, such as Pt (Hussain\u00a0et\u00a0al., 2016), Pb (Zhao\u00a0et\u00a0al., 2017) and a series of transition metals (Akhade\u00a0et\u00a0al., 2016), indicating the validity of the explicit solvation model.\nImplicit Solvation Model. The implicit solvation model represents a solvent as a polarizable medium represented by a dielectric constant (\u025b), thereby reducing the degrees of freedom and computational cost brought by solvent molecules and ions. The charge distribution in the solvent appears as an electric field that is polarized by the solute and responds to the presence of the solute. Therefore, one foundation of the implicit solvation model is the coarse-grained electrolyte. The discussion of electrolytes should start with a fully ab initio quantum mechanical treatment. Considering the mobility of molecules in the liquid phase, the evaluation of the equilibrium state requires averaging or sampling of the nuclear degrees of freedom, generally using ab initio molecular mechanics (AIMD) accomplish (Hassanali\u00a0et\u00a0al., 2014). Hybrid DFT with advanced dispersion corrections (Ambrosio\u00a0et\u00a0al., 2016), strongly constrained and appropriately normed (SCAN) meta-GGA functional(Pestana\u00a0et\u00a0al., 2018; Zheng\u00a0et\u00a0al., 2018a) and the revised version of the Perdew-Burke-Ernzerhof (RPBE) functional(Grimme\u00a0et\u00a0al., 2016) are generally recommended for use. Chan et\u00a0al. proposed an approach to relate the constant-charge result to the constant-potential condition by the equation \n\n\nE\n2\n\n\n(\n\n\u03c6\n1\n\n)\n\n\u2212\n\nE\n1\n\n\n(\n\n\u03c6\n1\n\n)\n\n=\n\nE\n2\n\n\n(\n\n\u03c6\n2\n\n)\n\n\u2212\n\nE\n1\n\n\n(\n\n\u03c6\n1\n\n)\n\n+\n\n(\n\n\nq\n2\n\n\u2212\n\nq\n1\n\n\n)\n\n\n(\n\n\n\u03c6\n2\n\n\u2212\n\n\u03c6\n1\n\n\n)\n\n/\n2\n\n\n, where \n\n\u03c6\n1\n\n is the work function and q is estimated from Bader charge analysis. Thus, the kinetic energy barrier at constant \n\n\u03c6\n1\n\n could be obtained.\nJDFT Method. Another approach to handling solvent effects is joint DFT (JDFT), which is an ab initio description of an electronic system in equilibrium with a liquid environment. In particular, the CANDLE implicit solvation model is powerful with the consideration of solvation and external potential effects, which are implemented within the framework of joint DFT (Sundararaman\u00a0and Goddard,\u00a02015). Xiao et\u00a0al. studied the initial steps of pH-dependent C1/C2 selectivity on Cu(111) surfaces and determined the rate-determining steps of the whole path (Xiao\u00a0et\u00a0al., 2016). Hossain et\u00a0al. applied the grand canonical potential kinetics (GCP-K) formulation of quantum mechanics to predict the kinetics as a function of applied potential and determined the faradic efficiency, TOF, and Tafel slope for CO2 electrochemical reduction to CO on graphene-supported Ni-single atom catalysts (Hossain\u00a0et\u00a0al., 2020).\nMicrokinetic Model. The microkinetic model makes a direct obtain of reactive thermodynamics and kinetics through experimental C-V curves. Although this method is generally used for multiple input parameters, it means that the model and the data are consistent. The idea of this method is as follows: First, each step in the whole reaction process is determined, and then the rate constant from intermediate a to intermediate b is calculated using transition state theory: \n\n\nk\n\na\n\u2192\nb\n\n\n=\n\n(\n\n\nk\nb\n\nT\n/\nh\n\n)\n\n\u00d7\nexp\n\n(\n\u2212\n\n\u0394\n\n\n\nG\n\n\u2260\n\n/\n\nk\nb\n\nT\n)\n\n\n, where \n\nk\n\na\n\u2192\nb\n\n\n is the rate constant of a\u2192b and \n\n\n\u0394\n\n\n\nG\n\n\u2260\n\n\nis the energy barrier (Motagamwala\u00a0and Dumesic,\u00a02021; Singh\u00a0et\u00a0al., 2017). The reaction rate is \n\n\nr\n\na\n\u2192\nb\n\n\n=\n\nk\n\na\n\u2192\nb\n\n\n\u00d7\n\n\u03b8\na\n\n\n, where \n\n\u03b8\na\n\n is the surface coverage of intermediate species a. Finally, all reaction rates are coupled to obtain the total reaction rate J. The details of the solution steps can be found in this article (Motagamwala\u00a0and Dumesic,\u00a02021). For the CO2RR, the energy barrier \n\n\n\u0394\n\n\n\nG\n\n\u2260\n\n\n depends on the applied potential U, so the predicted J fits the U-dependent function. The experimentally measured C-V curves for J(U) can then be compared to verify the rate-limiting step and corresponding activation energy with a function of U. However, the microkinetic model can only represent the intrinsic rate of the multistep reaction and does not consider factors such as the spatial variation of the concentration, e.g., the local pH change on the electrode surface and the local CO2 concentration changes under the condition of limited transport. This affects the comparison between theoretical predictions and measured values. To this end, Singh et\u00a0al. (Singh\u00a0et\u00a0al., 2017) combined a microscopic kinetic model with a continuum model describing mass transport in electrochemical cells. The model significantly improves the comparison with the experimental C-V curves after accounting for local pH and CO2 concentration changes at the electrode surface. Such fully coupled multiscale simulations of electrochemical interfaces are of great significance and prospects for evaluating the mechanism of CO2RR.\nMarcus theory-based methods. Akhade et\u00a0al. reported a simple and transferable DFT approach to estimate the potential-dependent activation energy (Akhade\u00a0et\u00a0al., 2017). The challenge of finding the transition state for an electrochemical reaction step A*+H++e\u2212\u2192AH* was met by using an equivalent analogous non-electrochemical reaction of A*+H*\u2192AH*. The transition state of such non-electrochemical reaction step was referenced to an equilibrium potential U0, and the analogous non-electrochemical state \u03bc(H*) was considered to be in equilibrium with its equivalent electrochemical state \u03bc(H++e\u2212), allowing for the kinetic barrier to be referenced to the chemical potential of the ion in the bulk electrolyte. Then, the potential-dependence could be incorporated by extrapolating the activation energy using Marcus theory. Later, a kinetic model based on Marcus theory was developed to calculate the potential-dependent reaction barrier of the elementary concerted proton-electron transfer (CPET) steps of the CO2RR (Gao\u00a0et\u00a0al., 2020). The rate-determining steps for CO and CH4 formation, the influence of binding energy, electrode potential and the reorganization energy on the reaction barrier were also discussed.It is emphasized that the activity and product selectivity of an electrochemical reaction are determined by the cooperation and competition of reactive thermodynamic and kinetic factors. For the CO2RR, the thermodynamic controlling product is formed by a pathway with the lowest onset potential (the least-negative potential at which the pathway to each product becomes exergonic), while the kinetic controlling product is formed by a pathway with the smallest energy barrier. One question is raised here: is the impact of thermodynamic and kinetic factors on one electrochemical reaction consistent? The answer could be sought by revealing the synergy of the thermodynamics and kinetics of the electrochemical/electrocatalytic reactions.It is not uncommon to link material properties by revealing thermodynamic and kinetic correlations. Our team has previously proposed thermodynamic-kinetic synergies in the phase transition and processing of structural materials. In these articles (Du\u00a0et\u00a0al., 2020; Gou\u00a0et\u00a0al., 2021; Huang\u00a0et\u00a0al., 2020a; Liu\u00a0et\u00a0al., 2016; Song\u00a0et\u00a0al., 2021; Wang\u00a0et\u00a0al., 2018b; Wang\u00a0et\u00a0al., 2019a), we have shown how to design high-performance structural materials based on thermodynamic-kinetic synergies, which may inspire researchers in other areas. The negative correlation between the reactive thermodynamics and kinetics for the four groups of CO2RR catalysts, as indicated in Fig.\u00a026, enlightens the similar synergies in the electrochemical/electrocatalytic reactions.The thermodynamic-kinetic correlation in electrochemistry could be firstly traced back to the Br\u00f8nsted-Evans-Polanyi (BEP) relationship, which states a linear correlation of the kinetic activation energy/transition state energy with the reaction free energy for essential bond breaking and forming reactions, including C-H, C-C, N-H, O-H, C-O, C-N,N-O, O-O, and N\u2012N (Bligaard\u00a0et\u00a0al., 2004; Cheng\u00a0et\u00a0al., 2008). The linear factor \u03b2 is the so-called BEP coefficient (0<\u03b2<1). Recently, extended to the broader category, the thermodynamic-kinetic correlation for a single electron transfer reaction and an electrocatalytic reaction were determined by our team (Du\u00a0et\u00a0al., 2021a). The correlation between the free energy change \u0394G and energy barrier Q for a single electron-transferred oxidation reaction is derived to be \n\n\n\u0394\n\nG\n=\n\nQ\n\n1\n\u2212\n\u03b1\n\n\n\u2212\n\n\u0394\n\n\nG\n\n0\nc\n\n\u2260\n\n\u2212\n\n\u03b1\n\n1\n\u2212\n\u03b1\n\n\n\n\u0394\n\n\nG\n\n0\na\n\n\u2260\n\n\n, and similarly, for a single electron-transferred reduction reaction, \n\n\n\u0394\n\nG\n=\n\nQ\n\u03b1\n\n\u2212\n\n\u0394\n\n\nG\n\n0\na\n\n\u2260\n\n+\n\n(\n\n1\n\u2212\n\n1\n\u03b1\n\n\n)\n\n\n\u0394\n\n\nG\n\n0\nc\n\n\u2260\n\n\n, where \n\u03b1\n is a transfer coefficient (0 \u2264\n\n\n\u03b1\n\n \u2264 1) that represents the symmetry of the energy profiles and \n\n\n\u0394\n\n\nG\n\n0\nc\n\n\u2260\n\n\n\nand\n\n\n\n\u0394\n\n\nG\n\n0\na\n\n\u2260\n\n\n are cathodic and anodic activation energies at the equilibrium potential\n\n\n\n\nE\n\n0\n\n\n. The free energy change \u0394G and energy barrier Q for a proton-coupled electron-transferred electrocatalytic reaction are also proven to be linearly and positively correlated. Based on this correlation, a tensile-strained Cu catalyst with improved CO2RR activity and CH4 selectivity was designed. As shown in Fig.\u00a030-a\n, b, tensile strain (TS) contributed to a higher faradaic efficiency for the CO2RR of 76.48% at -1.2\u00a0V together with a 38% enhancement in the partial current density toward CH4 generation (iCH4) compared to that of pristine unstrained Cu. Based on the derived positive correlation between the free energy change and energy barrier of the electrocatalytic reaction, the reaction mechanism and the strain effects were also revealed (Fig.\u00a030c). Tensile strain lowered the initial state free energy of CO*+H2O from the IS of the pristine Cu to the IS\u2019 of the TS Cu and moved the final state free energy of CHO* from FS to the lower FS\u2019. The free energy change \u0394G expressed by FS-IS for pristine Cu is shown in black, and the corresponding \u0394G\u2019 for tensile-strained Cu is illustrated in red. Since CHO* was found to be strain sensitive and CO* was strain insensitive, \u0394G\u2019 was smaller than \u0394G. Q was positively correlated with \u0394G, thus resulting in a smaller Q\u2019 for TS Cu. As indicated by iCH4 as the kinetic descriptor, iCH4 and \u0394G were negatively correlated, which was reasonable since I and Q were in a negative exponential relationship according to the Butler-Volmer equation.The above linear thermodynamic-kinetic correlation seems to directly contradict the Marcus theory of electron transfer, since Marcus states a quadratic relationship as \n\n\nE\na\n\n=\n\n\u03bb\n4\n\n\n\n(\n\n1\n+\n\n\n\n\u0394\n\nG\n\n\u03bb\n\n\n)\n\n2\n\n\n, where \u03bb is the reorganization energy, Ea is the activation energy and \u0394G is the Gibbs free energy change. However, this is a direct consequence of how these theories modeled the free-energy landscape: while BEP and Du et\u00a0al. assumed the free energy to be a linear function of the reaction coordinate, Marcus assumed it to be quadratic. It is stressed that the application of these correlations depends on the validity of these presumptions and should ideally be tested on a system-by-system basis. Generally, the discrepancy between the two is less apparent near equilibrium, and the predictions diverge considerably when the overpotential is increased. The most iconic example is the Marcus inverted region, where the activation energy starts to increase if the overpotential increases beyond a certain value (Hammes-Schiffer,\u00a02009).Now back to the question raised at the beginning of Section\u00a03.3, our answer is as follows: Generally, the impact of thermodynamic and kinetic factors on the electrochemical reaction should be consistent according to the positive correlation between the thermodynamic free energy change and kinetic energy barrier, i.e., the larger the driving force is, the lower the reaction barrier is, and the higher the activity is. In addition, the Sabatier principle and its thermodynamic interpretation have been successfully applied for the design and screening of catalysts. The Sabatier principle is actually based on thermodynamics and leaves the kinetics out of consideration, rendering a discrepancy between the theoretical framework and experimental performance of real catalysts. The establishment of thermodynamic-kinetic correlation makes up for the deficiency of the Sabatier principle and may play a significant role in the development of electrocatalysts. Taking the design of a highly active catalyst as an example, the thermodynamic factors and correlated coefficient could be designed or modulated to obtain a kinetic energy barrier as small as possible.Last but not least, the thermodynamic-kinetic correlation is useful to predict and design the intrinsic activity of the catalyst, but it needs to be noted that the intrinsic activity of the material may not be the limiting factor of the catalysis in some cases. Mass transport, the electrolyte (particularly the pH of the electrolyte) and the electrochemical active surface area (ECSA) are all possible limiting factors for catalytic performance. For example, Seifitokaldani et\u00a0al. reported hydronium(H3O+)-induced switching between CO2 electroreduction pathways, where the product selectivity of a silver catalyst switched from entirely CO under neutral conditions to over 50% formate in the alkaline environment (Seifitokaldani\u00a0et\u00a0al., 2018). This study provides new insights into the role of hydronium in CO2 electroreduction processes and the ability for electrolyte manipulation to influence transition state kinetics, altering favored CO2 reaction pathways. The catalytic performance is suggested to be considered less of an intrinsic catalytic property and rather a combined result of the catalyst and reaction environment.Using electrocatalytic technology to convert CO2 into multi-carbon products with high energy value is one of the most ideal solutions to alleviate global environmental problems and energy shortages. In this review, the recent developments of the most popular CER catalysts, including metal-based catalysts, single-atom catalysts (SAECs), porphyrin-based complexes and biomass-derived nitrogen-doped carbon catalysts, are summarized. Focusing on the activity, selectivity and stability of catalysts, metal-based electrocatalysts have achieved tremendous achievements in the past few years, the performance of which has been largely improved. SAECs exhibit excellent catalytic activity and selectivity, and their FECO could be over 80%. Porphyrin-based complexes exhibit outstanding catalytic performance and selectivity toward CO (>90% FECO). Biomass-derived metal-free nitrogen-doped carbon catalysts, as emerging and environmentally friendly catalysts, exhibit excellent activity and selectivity, particularly with ultralow HER activity. However, the current density of nitrogen-doped carbon catalysts during the reaction process is generally smaller than that of other catalysts. There is a common problem for the current CER catalysts, the long-term stability. Most of the reported data show a very limited test time, usually less than 100 h, which is definitely far from the industrial application requirement. For reference, industrial water electrolyzers have demonstrated stable performance over 80,000 h.The design of electrocatalysts can be facilitated by accurate computational simulations and theoretical understanding of the mechanism. Thus, the development and application of the electrochemical thermodynamic framework (CHE model) and the improved models involved with reactive kinetics are reviewed. The CHE model simply and directly generates the thermodynamic energy landscape of a series of PCET steps under an applied potential. Although the CHE model and the thermodynamic overpotential \n\u03b7\n are by far the most popular way to evaluate electrocatalytic activity, they ignore activation energies for all PCET steps and are therefore unreliable for kinetic analysis. The improved explicit solvation model, implicit solvation model, JDFT model, microkinetic model and Marcus theory-based method involving reactive kinetics, together with the electrolyte influence on the kinetics are all summarized. Furthermore, inspired by the negative correlation between the thermodynamic free energy difference (\u0394GRDS) and the kinetic partial current density for the CO2RR (jCO2RR) of various electrocatalysts (Fig.\u00a026), the thermodynamic-kinetic correlation and the thermodynamic-kinetic synergy during the electrocatalytic reactions are discussed. The linear or quadratic thermodynamic-kinetic correlations are both reasonable causes of the difference in how the theories model the free-energy landscape. The consistency between thermodynamic and kinetic factors on the electrochemical reaction could be predicted according to the positive thermodynamic-kinetic correlation. More importantly, the thermodynamic-kinetic synergy may play a significant role in the design and screening of the electrocatalyst. In other words, aiming at the larger thermodynamic driving force and smaller kinetic energy barrier bridged by a more suitable correlated coefficient, it is more efficient to modulate or design the catalysts\u2019 composition, structure, strain state, defects, etc.Identifying catalytic mechanisms of CO2 electroreduction could accelerate design of highly active and selective catalysts. In recent decades, numerous theoretical studies have contributed a lot for understanding reaction pathways, identifying rate-limiting steps, and revealing reactive thermodynamics and kinetics. In addition, the rapid development of in situ/operando characterization techniques, e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), Raman, infrared (IR) spectroscopy, have proved to be quite powerful in tracking the structure reconstruction of the heterogeneous catalysts, identifying real active sites and recording intermediates formed during the reaction.(Cao\u00a0et\u00a0al., 2021; Handoko\u00a0et\u00a0al., 2018)There is a trend of combining theoretical calculations with in situ/operando experimental analysis to provide a plausible mechanism for CO2RR. Profound theoretical insights and operando characterizations with higher resolution and higher signal-to-noise ratio are highly required. The efficient electrocatalysts combined with advanced electrochemical flow reactors, the facile and clean recycling of carbon resources for renewable fuels and high-value chemicals is expected to be realized in the future.\nFeihan Yu: Conceptualization, Data curation, Formal analysis, Writing \u2013 review & editing. Kang Deng: Conceptualization, Data curation, Funding acquisition, Writing \u2013 review & editing. Minshu Du: Writing \u2013 original draft, Supervision, Project administration. Wenxuan Wang: Funding acquisition. Feng Liu: Writing \u2013 original draft, Supervision, Project administration. Daxin Liang: Writing \u2013 original draft, Supervision, Project administration.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was supported by the National Natural Science Foundation of China (52130110). We also appreciate the support of the Fundamental Research Funds for the Central Universities (2572021BB02) and the Natural Science Foundation of Chongqing (cstc2021jcyj-msxmX1162).", "descript": "\n                  The carbon dioxide electroreduction reaction (CO2RR) can convert CO2 into value-added fuels or chemicals, thus becoming a promising approach for balancing the carbon cycle and realizing a low-carbon economy. Here, we describe in detail the preparation, performance, reaction mechanism and theoretical research progress of four representative CO2RR catalysts, including metal-based electrocatalysts, single-atom electrocatalysts, porphyrin-based complexes and biomass-derived nonmetallic carbon-based materials. In particular, the electrochemical thermodynamic framework, kinetics, and thermodynamic-kinetic correlation in the CO2RR are discussed. The concept of thermodynamic-kinetic synergy and some perspectives on the future design of electrocatalysts are also presented, with the aim of facilitating research and development in this area.\n               "}
{"full_text": "Data will be made available on request.Palladium-catalyzed cross-coupling reactions (PCCCRs) are one of the greatest milestones in organic chemistry. Particularly the procedures described by Suzuki-Miyaura, Sonogashira, Stille or Heck are good examples of this paradigm [1,2], although they all follow different reaction mechanisms. Rapid progress in heterogeneous Pd catalysts for cross-coupling reactions have been realized for the synthesis of compounds for pharmaceutical and chemical industries [3]. The three pillars of heterogeneous catalysis are activity, selectivity and stability. Consequently, it is well established that an ideal heterogeneous catalyst should has high geometric surface area that allow incorporation of high active catalytic sites, mechanical robustness, thermal stability, control loading, negligible leaching, recyclability and any loss of activity by poisoning or deactivation. All these limitations often result in a significant decrease in catalytic activity. Efficient catalyst will clearly require the combination of diverse strategies considering different aspects, such as thermal, mechanical and chemical related with the stability and morphology of the catalyst.Monolithic catalysts are structures comprising functional interconnected microchannels with a regular three-dimensional structure. They can replace conventional catalysts (homogeneous catalysts) and chemical reactors as well as helping to overcome different problems posed by traditional systems. Monolith reactors were initially developed in mid 1970s for the automotive industry to remove NO, CO and hydrocarbons through gas-solid reactions in engine emission converters. In solution phase chemistry, the monoliths have many advantages over heterogeneous powdery catalytic systems (polymeric reagents, nanoparticles, in general, supported reagents) and traditional packed-bed reactors, such as high transport rates of heat and mass per unit pressure drop, small transverse temperature gradients ease of scale-up and work-up (avoiding filtration processes) [4,5]. When a monolithic catalyst is considered as an alternative to a fixed-bed reactor packed with commercially available catalyst particles, a straightforward prototyping and development program is needed to produce the monolithic catalyst.Beyond the importance of the catalyst itself in organic reactions, microwave heating has several applications in almost every field of chemistry, due to the advantages that this technology offers compared to traditional heating methods. Microwave assisted organic synthesis (MAOS) has rapidly gained acceptance as a valuable tool for accelerating drug discovery and development processes [6]. Advantages of microwave heating over conventional heating stem mainly from its ability to interact with the material at the molecular level. Heat losses (conductive and convective) associated with traditional heating methods are negligible with microwave heating [7,8]. However, this synthetic methodology is not without risks. In the case of catalysts with metal loadings, microwave-metal discharge could trigger hot-spot formation, local microplasmas and arcing at metal sites, generating hazardous conditions in the presence of flammable solvents and/or gas [9]. These phenomena can cause sparkling or flame inside the microwave reactor. Explosions could occur, for example, in the case of homogeneous PCCCRs using common palladium reagents. These processes are particularly critical at points in the reactor where metal palladium deposits become embedded in the reactor wall in the absence of solvent and therefore under the direct action of microwave radiation [10]. The sparkling phenomena in metal-solvent mixtures has been reviewed by Kappe [11] and coworkers supporting the data reported by Hulshof [12] who observes that arcing phenomena is basically linked to large metal particles. Consequently, an effective immobilization of palladium species on a monolithic catalyst must be taken to avoid metal leaching, hot-spots formation, sparkling and electric discharges in PCCCRs under MAOS.Cermet is a composite material made up of ceramic materials and metals. Cermets are used to combine the high temperature and abrasion tolerance qualities of ceramics with the malleability of metals. The combination of a heterogeneous cermet-type catalyst with the microwave heating tool has many advantages. Most of the solid catalysts highly absorb microwave irradiation thus they can be considered as an internal heat source. In fact, a way in which microwaves can be selective is by heating a certain part of the catalyst better than the rest. For instance, low-dielectric loss materials such as alumina loaded with microwave active metal (Fe, Pt, Mo or Pd). The microwave active material will heat to very high temperatures leading to selective overheating of metallic sites that cause increased reaction rates [13]. For Mars van Krevelen mechanism [14], where the catalyst surface itself is an active form of the reaction, forming a thin layer of metal-reactant (metal-oxide, metal-sulfide etc.) on the surface, microwave heating causes selective acceleration of primary reaction step, resulting in increased overall rate of reaction [15].Most cermet manufacturing processes are based on powder metallurgy techniques. Metal and ceramic powders are mixed and ground together in a ball mill or an attrition mill. A lubricant or humectant is often added to facilitate shaping operations. In many cases, after grinding, a suspension is prepared with the raw materials, which is atomized to obtain fine, homogeneous and spherical particles. The pieces are formed by compacting the powder by cold pressing, cold isostatic pressing, or hot isostatic pressing. Except in the latter case, the pieces already formed are thermally processed for sintering at high temperatures in continuous or discontinuous furnaces, with or without controlled atmospheres, depending on the case. Numerous studies have been performed using a support material where the active metal phase is incorporated by impregnation [16\u201318], absorption [19], deposition-precipitation [20], ion exchange [21] and encapsulation [22,23] process. Some of these methods are usually followed by a thermal or chemical treatment to activate the metal. However, these strategies have their own limitations in terms of design, loading and mechanical/thermal stability.A highly promising technique for the fabrication of structured catalyst is the 3D printing, an additive manufacturing technique [24,25]. It has been widely used to generate complex-shaped structures with controlled composition and architecture. This approach is based on the extrusion of ink through a nozzle, which can be deposited over a substrate in a layer-by-layer sequence. Owing to the spatial resolution that can be achieved, this technique can be extended to a broad range of technological applications ranging from optoelectronics [26\u201329], catalysis [30\u201333], to biomaterials [34\u201336]. These functional structures can be fabricated using different materials such as metals, ceramics, metal oxides, hydrogels, and composites. The ability to fabricate 3D structures with micrometer resolutions at both meso- and micro-scale depends basically on the rheological behaviour inks and printing parameters. In this respect, the control over these features is fundamental to allow the development of 3D structures with controlled composition, architecture, and specific properties.In previous works of our research group, we have carried out the manufacture of monoliths using 3D-printing technology, following different strategies such as robocasting and sintering for the manufacture of composite materials based on alumina and copper oxide [30]. In other works, the surface functionalization of SiO2 monoliths was carried out by silanization and metalation [37], coating with polyimide-palladium composite on the surface [38] as well as incorporation of metallic catalytic species on the surface using the strong electrostatic adsorption technique [39]. To our knowledge, there are no examples of monolithic cermet-type catalysts, manufactured by 3D-printing, tailored to a microwave reactor-vessel. In this work, we present the design and fabrication of the first Pd0/Al2O3 cermet monolithic catalyst [40] specifically designed to fit in a microwave reactor. The catalyst can be rapidly prepared via combination of 3D printing (direct ink writing technique) and subsequent thermal treatment, without any other type of surface treatment after sintering. The catalyst, with 3D structured morphology and long-term stability, was used as an efficient, extremely robust and safe catalyst for Suzuki, Stille, Sonogashira and Heck palladium catalyzed cross-coupling reactions assisted by microwave heating.The ink is synthetized using a protocol similar to that previously described [30]. Briefly, 5\u00a0g of PdCl2 (on a stoichiometric basics, 99.9%, Alfa Aesar) was dissolved in 14\u00a0mL of deionized (DI) water. Then 50\u00a0g of Al2O3 powder (mean particle size 0.5\u00a0\u03bcm and real density of 3.96\u00a0g/mL, Almatis GmbH, Germany) was added into the PdCl2 solution, and mixing in a planetary mixer (ARE-250, Thinky, USA) at 2000\u00a0rpm for 2\u00a0min. After, 0.065\u00a0g of (hydroxypropyl)methyl cellulose (HPMC, viscosity 2600\u20135600\u00a0cP, Sigma-Aldrich) was added to the suspension, followed by through mixing at 2000\u00a0rpm for 2\u00a0min. After 1\u00a0h equilibrium, the resulting suspension was gelled by adding 0.13\u00a0mL of polyethylenimine (PEI, Mw\u00a0=\u00a02000, Sigma-Aldrich), and was again homogenized in the planetary mixer at 2000\u00a0rpm for 2\u00a0min. This mixing process is repeated until to obtain the desired homogeneity. The final printable ink was composed of 5.6\u00a0wt% of Pd (relative to the ceramic content) and a ceramic concentration of about 45\u00a0vol%.A robotic deposition apparatus (Model A3200, Aerotech Inc., USA) was used to fabricate the catalyst sample. The ink was loaded into the syringe (3\u00a0mL, Nordson EFD Inc., Japan), which is attached by a nozzle tip (410\u00a0\u03bcm diameter size, Nordson EFD Inc.). An air dispenser (Performus VII attached to HP7x, Nordson EFD Inc.) is used to control the ink flow rate. The ink was extruded (Direct Ink Writing) under pressures ranging from of 7\u201310\u00a0bar at a speed of 3\u20135\u00a0mm/s. Catalytic structures were designed using CAD software (Robocad 3.4, 3D Inks, USA), with cylindrical shape (diameter 10\u00a0mm and height 5\u00a0mm), and open square pores of 590\u00a0\u00d7\u00a0590\u00a0\u03bcm. After drying at room temperature, the samples were sintered in air at 1500\u00a0\u00b0C for 2\u00a0h with a ramp rate of 10\u00a0\u00b0C/min.The surface morphology and microstructure of the samples were characterized using scanning electron microscopy (SEM, JEOL 6400, JEOL Corporation, Japan) and stereomicroscope (OlympusSZX12, Olympus, Japan). The surface elemental analysis of sintered samples was measured using energy dispersive X-ray spectrometer (EDS, AZTEC/Xact, Oxford, UK). The crystal structure of samples was monitored by a Siemens D5000 diffractometer (Siemens, Germany) with CuK\u03b1 radiation (\u03bb\u00a0=\u00a00.15418\u00a0nm). Data was collected in the range of 20\u201390\u00b0 (2\u03b8) with a step size of 0.05\u00b0. The palladium content in the monolith catalyst was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Varian Liberty 200). Temperature programmed reduction (TPR): The reducible species formed during the calcination step were determined by this technique using an Autosorb 1C-TCD equipped with a thermal conductivity detector. The monolith was loaded in U shaped quartz tube and heated from room temperature to 473\u00a0K for 1\u00a0h in Ar stream (30 mLN/min). The sample was then cooled down to 313\u00a0K and the Ar was replaced by a 5\u00a0vol % H2/Ar gas stream (45 mLN/min). The sample was heated from 313\u00a0K to 673\u00a0K, at a ramp rate of 10\u00a0K/min. CO chemisorption (CO\u2013C): The metal dispersion was measured by CO pulse chemisorption using an AutoChem II 2920 apparatus equipped with a TCD detector. The analysis started by heating the catalysts sample from room temperature to 673\u00a0K at 10\u00a0K/min under 40 mLN/min of 5\u00a0vol % H2/Ar flow. Then, the sample was kept under 50\u00a0mL/min of helium for 30\u00a0min and cooled down to 308\u00a0K. Finally, when the detection baseline was stable, CO pulse chemisorption started. The dosage was repeated every 2\u00a0min until equal peaks were detected or 20 dosages were carried out. Metal dispersion was determined by assuming a stoichiometric ratio of Pd/CO\u00a0=\u00a01. For surface analysis, X-ray photoelectron spectroscopy (XPS) was carried out using a Physical Electronics PHI 5700 spectrometer with nonmonochromatic Mg K\u03b1 radiation (300\u00a0W, 15\u00a0kV, 1253.6\u00a0eV) as the excitation source. High-resolution XPS spectra were recorded at a given take-off angle of 45\u00b0 by a concentric hemispherical energy electron analyzer, operating in the constant pass energy mode at 29.35\u00a0eV, using a 720\u00a0\u03bcm diameter analysis area. Adventitious carbon at 284.8\u00a0eV has been used for charge referencing. Images of Pd nanoparticles dispersed in the ceramic matrix were acquired using a Gemini-500 Field-Emission Scanning Electron Microscope (FESEM) operating at 20\u00a0kV using a back-scattering AsB detector with a size resolution of \u00b10.5\u00a0nm. Selected images were analyzed by counting more than 100 particles using ImageJ software.All reactions were performed in a CEM/DISCOVER SP-D-Closed Vessel Microwave apparatus. Iodoarenes, 5-haloisatins, alkenes, alkynes, boronic acids and organotin reagents were purchased from Aldrich and Alfa Aesar [(4-methoxicarbonyl)phenylboronic acid)]. All reactions were monitored by TLC with 2.5\u00a0mm Merck silica gel GF 254\u00a0strips and the purified compounds showed a single spot. Detection of compounds was performed by UV light and/or iodine vapor. Purification of isolated products was carried out by preparative TLC using silica gel plates. The synthesized compounds were characterized by spectroscopic and analytical data. The NMR spectra were recorded on Bruker AM 300\u00a0MHz (1H) and XM500 spectrometers. Chemical shifts are given as \u03b4 values against tetramethylsilane as internal standard and J values are given in Hz. Proton and carbon nuclear magnetic resonance spectra (1H NMR) were recorded in CDCl3. Melting points were determined on a Gallenkamp melting point apparatus and are uncorrected. Mass spectra were obtained on a Varian MAT-711 instrument. High resolution mass spectra (HR-MS) were obtained on an Autospec Micromass spectrometer.General procedures for PCCCRs: All reactions were performed using the 3D printed Pd0/Al2O3 cermet monolithic catalyst [total content of Pd on monolithic surface: 1.1\u00a0mg; it means 1.1% mmol Pd on the reaction]. No magnetic bar was used in these experiments although they are compatible in these procedures.In a capped microwave reactor vessel (13\u00a0mm internal diameter), provided with the 3D-printed Pd0/Al2O3 cermet catalyst were dissolved the corresponding haloarene (0.98\u00a0mmol), Na2CO3 (2.94\u00a0mmol) and the boronic acid (1.07\u00a0mmol) in a mixture of iPrOH/H2O (2:1 ratio, 5\u00a0mL). The mixture was heated at 120\u00a0\u00b0C under microwave irradiation (200\u00a0W) for 20\u00a0min. Once the reaction finished, the catalyst was removed from the vial, sonicated, washed with water (5\u00a0mL), MeOH (5\u00a0mL) and acetone (5\u00a0mL) and dried under vacuum for 30\u00a0min for reuse. The mixture was washed with water and extracted in AcOEt. The organic phase was dried with anhydrous Na2SO4 and evaporated at room temperature on a rotary evaporator. The resulting solid was recrystalized by isopropanol to give the final products compounds 2a-e (entries 2a-d)].In a microwave reactor vessel (13\u00a0mm internal diameter), provided with the 3D-printed Pd0/Al2O3 cermet catalyst were dissolved the corresponding iodoarene (0.98\u00a0mmol), TEA (2.94\u00a0mmol) and the alkyne (1.07\u00a0mmol) in iPrOH (5\u00a0mL). The mixture was heated at 120\u00a0\u00b0C in a microwave reactor (200\u00a0W) for 30\u00a0min. Once the reaction was finalized, the catalyst was removed from the vial, sonicated, washed with water (5\u00a0mL), MeOH (5\u00a0mL) and acetone (5\u00a0mL) and dried under vacuum for 30\u00a0min for reuse. The mixture was washed with water and extracted in AcOEt. The organic phase was dried with anhydrous Na2SO4 and evaporated at room temperature on a rotary evaporator. The mixture was purified by preparative TLC (AcOEt/Hexane) to give compounds 2f-i.In a microwave reactor vessel (13\u00a0mm internal diameter), provided with the 3D-printed Pd0/Al2O3 cermet catalyst were dissolved the corresponding iodoarene (0.98\u00a0mmol), TEA (2.94\u00a0mmol) and the alkyne (1.07\u00a0mmol) in MeCN (5\u00a0mL). The mixture was heated at 120\u00a0\u00b0C in a microwave reactor (300\u00a0W) for 20\u00a0min. Once the reaction was finalized, the catalyst was removed from the vial, sonicated, washed with water (5\u00a0mL), MeOH (5\u00a0mL) and acetone (5\u00a0mL) and dried under vacuum for 30\u00a0min for reuse. The mixture was washed with water and extracted in AcOEt. The organic phase was dried with anhydrous Na2SO4 and evaporated at room temperature on a rotary evaporator. The mixture was purified by preparative TLC (AcOEt/Hexane) to give compounds 2j, d, k, l.In a microwave reactor vessel (13\u00a0mm internal diameter), provided with the 3D-printed Pd0/Al2O3 cermet catalyst were dissolved the corresponding iodoarene (0.98\u00a0mmol) and the corresponding organostannane (1.07\u00a0mmol), in MeCN (5\u00a0mL). The mixture was heated at 120\u00a0\u00b0C in a microwave reactor (200\u00a0W) for 20\u00a0min. Once the reaction was finalized, the catalyst was removed from the vial, sonicated, washed with water (5\u00a0mL), MeOH (5\u00a0mL) and acetone (5\u00a0mL) and dried under vacuum for 30\u00a0min for reuse. The mixture was washed with water and extracted in AcOEt. The organic phase was dried with anhydrous Na2SO4 and evaporated at room temperature on a rotary evaporator. The mixture was purified by preparative TLC (AcOEt/Hexane) to give compounds 2a, e, f, m, n.Activity, selectivity and stability of a monolithic catalyst could be positively affected during microwave-assisted heterogeneous catalysis. Consequently, in this work we set as specific objectives: the design of a cermet type monolith catalyst provided with an appropriate design (presence of interconnected channels) (1), simple and direct manufacturing (2), catalytic efficiency (3), chemical and mechanical robustness (4), as well as almost unlimited reusability and safety in MAOS (5). We present here a direct manufacturing process based on 3D-printing (direct ink writing, DIW) of catalytic inks based on \u03b1-alumina and palladium species and subsequent sintering of the monolith, to obtain a cermet type monolith Pd0/Al2O3.Monolithic catalyst design: The shape and size of the monolith, to adapt it to a certain reactor, can be modulated by 3D printing, so this technology is ideal in prototyping processes. As can be seen in Figs.\u00a01b,d\n\n and 3b,c\n, the shape of the catalyst is adjusted to the shape and dimensions of the microwave vessel. For this reason, a slightly flattened cylindrical shape structure was designed, adapted to the bottom of the vessel.Regarding the composition of the final product, it is important to highlight that the material described here contains metallic species both inside the filaments of the structure and on the surface as Pd0 and not as PdO (oxide). Therefore, the material obtained in this work is a true cermet: metal (0)-ceramic composite, so this monolith is particularly efficient in palladium-catalyzed cross-coupling reactions (Suzuki, Stille, Sonogashira, Heck).Alumina-based ceramics (Al2O3) have excellent physical and chemical properties. In addition, they have good mechanical resistance and thermal stability [41]. However, their high Young's modulus values make the applications somewhat limited due to their high brittleness, as they are not easily deformed. Consequently, alumina ceramics are very sensitive to minimal defects in their microstructure, which acts as a crack initiation point [42,43]. Nevertheless, ceramic materials can improve their fracture toughness by homogeneous incorporation of fine particles of ductile metals in the matrix. Different reinforcement metal amounts of Al2O3/Al [44], Al2O3/Cr [45], Al2O3/Cu [46], Al2O3/Ni [47], Al2O3/Mo [48], Al2O3/Ti aluminide [49] and Al2O3/Ni3Al [50] have been reported. Many of these composites are synthesized using powder techniques. These techniques start from a mixture of powders from a high-energy mechanical grinding, and later they are subjected to a pressing process, and finally they are sintered at a certain time and temperature, giving rise to the composite material [51]. Therefore, the robustness of the monolithic material improves with the incorporation of the metallic component in the composition of the internal matrix of the filaments. It is well known that cermet experiences a decrease in their hardness and elasticity module in comparison with the material base. However, his fracture toughness increases. Consequently, these factors contribute to the new composite better tolerating the generation of cracks when the material is working under conditions of high loads and friction [51].The synthesis of the monolithic catalyst is very direct and simple since it is a process of 3D-printing and sintering, which is carried out without any other type of surface treatment. Specifically, we first developed an aqueous colloidal Pd/Al2O3 ink with tailored rheological properties for 3D printing that contains moderate Pd loading (5.6\u00a0wt%). Al2O3 has been used as matrix due to its excellent thermal and mechanical properties [52], which leads to obtain a catalyst with remarkably stability. After an appropriate ink synthesis, a 3D structure with high geometric surface area and open square pores was fabricated, followed by thermal annealing at 1500\u00a0\u00b0C for 2\u00a0h to obtain a catalyst with thermal/mechanical integrity. A detailed physico-chemical characterization for the catalyst was carried out by optical, scanning electron microscopy (SEM), EDS, and X-ray diffraction (XRD) techniques, and it was evaluated as catalyst in Suzuki, copper-free Sonogashira, Stille and Heck cross-coupling reactions under microwave heating conditions. The proposed strategy can be extended to other metals, opening new opportunities in the design and synthesis of efficient metal catalyst. Fig.\u00a01 shows the physical parameters and different images of the 3D Pd0/Al2O3 catalyst. This structure was prepared using colloidal Pd/Al2O3 ink, where the stabilization of Pd in the sample combines chemical and physical strategies. In the first step, concentrated colloidal ink was prepared by using a Pd+2 precursor (PdCl2) that was absorbed into the Al2O3 particles through electrostatic interactions in an aqueous solution under stirring. Content of Pd could be adjusted in this step by varying the amount of the metal precursor. Particularly, we created ink with a weight percentage of 5.6\u00a0wt% Pd to achieve sufficient reinforcement in the internal structure, as well as a minimum palladium content on the surface, necessary for catalysis to occur. Then, a non-ionic (hydroxypropyl)methyl cellulose (HPMC) and a cationic polyethylenimine (PEI) were added to impart the desired rheological properties to the ink. Subsequently, the obtained ink was used to fabricate the Pd/Al2O3 catalyst by 3D printing technique. This catalyst was designed with well-controlled morphology using CAD software, where the size and shape is proportional to the microwave reaction vessel. Finally, as discussed above, the resulting sample was sintered at 1500\u00a0\u00b0C for 2\u00a0h to form a catalyst with remarkable mechanical and structural properties, which are strongly related with the catalytic activity. Note that the thermal treatment was selected, in order to achieve a catalyst with excellent mechanical properties and a specific oxidation state (0) of Pd. At a lower temperature, the catalyst could be easily cracked when the reactions will be carried out under shaking and microwave heating conditions. During the sintering process, no reducing atmosphere was used to get Pd0 in the cermet. As shown in Fig.\u00a01b\u2013i, the sintered sample present a uniform shape, network structure, and a homogeneous surface without cracks. Fig.\u00a01c shows the sintered structure with interconnected square pores. The cross-sectional image in Fig.\u00a01e confirms the multilayer and interconnected morphology of the final catalyst. The advantages of choosing this morphology are the formation of high surface area to volume ratio, which enhances the number of active sites for catalytic reactions. Noticeably, the sintered structure is mechanically robust to tolerate long-term and repeated reaction cycles. As expected, the colour of catalyst changed after sintering from light-grey (dried sample) to more black (sintered sample), suggesting an efficient reduction of Pd+2 into Pd0, probably as a consequence of the presence of polyethylenimine (PEI), in the catalytic ink, acting as a palladium reducing agent. In addition, moderate diffusion out of the Pd metal through the substrate was observed after sintering, suggesting a significant metal immobilization in the sample. Particularly, the results of the chemical analysis obtained by ICP-OES indicate that metallic content (Pd wt%) is around 1.66\u00a0wt% for the sample after sintering. Furthermore, the optimum sintering temperature results in a catalyst with a geometric surface area (surface area-to-volume ratio) of 33.2\u00a0cm2/cm3, according to the dimensions the external dimensions of structure.A smooth surface was further confirmed by the magnified SEM image (Fig.\u00a01g and h). The cross section of the monolith (view of the inner cross section of the metal-ceramic composite) is shown in Figs.\u00a01f, i and 2a, where the two materials (metal and ceramic) are observed. In addition, from this image it cannot be observed any segregation or precipitation of the Pd at the grain boundary region, demonstrating that Pd metal was well integrated within the Al2O3 matrix.The qualitative and quantitative distribution of palladium on the monolith surface were experimentally confirmed by the SEM, EDS and mapping, XRD, TPR and CO\u2013C. Fig.\u00a02a shows the surface SEM image of the Pd\n0\n/Al2O3 filament, and Fig.\u00a02b represents its spectrum in two different regions. The results, together with the EDS-mapping results (Fig.\u00a02c), reveal that the Pd element is evenly distributed throughout the monolith structure. This outer Pd content is available for the catalytic reactions. The presence of Pd is confirmed in the two regions (inner and outer sections) by the EDS.These results demonstrated that Pd could be efficiently loaded on the ceramic network with an excellent confinement into the Al2O3 matrix after the sintering process. In addition, FESEM images at higher resolution allow to observe that Pd is dispersed along the ceramic matrix surface but also inside their pores in the form of nanoparticles. These appear in the form of aggregates/clusters with a mean size of ca. 185\u00a0\u00b1\u00a050\u00a0nm (Fig.\u00a02f\u2013h). This agglomeration can result from the sintering of adjacent Pd ions distributed along the matrix during the thermal reduction process due to the absence of any stabilizer to allow control of nucleation and growth. In addition, the clusters seem to be formed by particles/crystallites of lower size. Inspection of selected areas in the acquired images provides approximate values of the crystallites composing the particle agglomerates of ca. 40\u00a0\u00b1\u00a05\u00a0nm.X-ray diffraction analysis was performed to verify the presence of the Pd on the sample (Fig.\u00a02d). The XRD pattern of the sample shows characteristic peaks of two different phases corresponding to \u03b1-Al2O3 (JCPDS No. 05\u20130712) and Pd (JCPDS No. 05\u20130681). In particular, the peaks observed at 2\u03b8 values of 40.11\u00b0, 46.64\u00b0, 68.19\u00b0, 82.05\u00b0 and 86.4\u00b0 correspond to (111), (200), (220), (311) and (222) planes of the Pd metal, respectively. These peaks are sharp and well defined, which indicates that the metallic Pd has a high degree of crystallinity. Clearly, there are not obvious diffraction peaks that could be associated to crystalline palladium oxide species or Pd\u2013Al alloys, indicating that the introduction of Pd into the Al2O3 not generate Pd complexes during the chosen thermal treatment. Finally, the reduction properties of the monolith were measured by TPR. The obtained TPR profile was flat suggesting that most of the palladium species incorporated was already reduced. Results of the CO chemisorption showed a metal (Pd) dispersion of 0.192% on the surface, which is sufficient and beneficial to carry out MAOS safely. To evaluate the oxidation state of the palladium in the Pd/Al2O3 catalyst, XPS was utilized. The high-resolution XPS spectrum (Fig.\u00a02e) reveals the existence of Pd0, where it showed double peaks with bending energies at 335.5 and 340.8\u00a0eV, which correspond to Pd0, Pd 3d5/2 and Pd 3d3/2, respectively. These results combined with the XRD spectra (Fig.\u00a02d) confirm the presence of Pd0 on the outer surface of the catalyst.These four reactions follow different mechanisms for the formation of carbon-carbon bonds and have been extensively used in organic chemistry. However, the reaction conditions of homogeneous catalysis are not always extrapolated to heterogeneous catalysis. This fact makes it necessary to carry out an exhaustive screening of the reaction conditions for a new catalytic material. The first studies of the catalytic activity of the monolithic catalyst adapted to the microwave vial (Fig.\u00a03b and c) focused on the following issues: performing PCCCRs in secure MAOS, obtaining selectivity and high yields for the four transformations studied (Suzuki, Sonogashira, Stille and Heck); check for possible palladium leaching on the monolith surface under different reaction conditions (alkenes, alkynes, boronic acids, stannanes, bases and solvents) at high temperature and the reusability of the monolithic catalyst in MAOS. The results of catalytic activity for the four protocols are shown in Table\u00a01\n. In all reactions, taking into account the total amount of palladium detected on the monolith surface [(0.192% Pd on surface, it means 1,1\u00a0mg on the surface (standard monolith weight: 600\u00a0mg)], the reactions worked well using 0.98\u00a0mmol of starting substrates [4-iodobenzene (1a), 4-iodotoluene (1b), 4-iodoanisole (1c) as well as 5-haloisatins such as 1-benzyl-5-iodoindoline-2,3-dione (1d) and 1-benzyl-5-bromoindoline-2,3-dione (1e)] and the corresponding coupling partner (boronic acid, alkyne, alkene or stannane) in four different protocols. For the Suzuki reaction, standard conditions were explored using inorganic bases such as sodium carbonate in hydroalcoholic mixtures. The best results were achieved using 3 equiv of base (Na2CO3) and iPrOH/H2O (2:1 ratio) as a solvent mixture, under microwave heating, 200\u00a0W at 120\u00a0\u00b0C, for 20\u00a0min, rendering almost quantitative yields. No collateral products were detected during the reactions. Therefore, the selectivity towards the desired products 2a-e was excellent. As can be observed in Fig.\u00a03d, reaction products frequently crystallize in the microwave vial when the mixture has cooled. The monolith is easily removable from the microwave reactor for reuse after washing and sonication. It is important to point out that during the optimization process of the Suzuki reaction conditions, the presence of two phases is a critical issue for the efficiency of the catalyst. The correct dissolution of all the reagents in a suitable hydroalcoholic phase is required. In this sense, the iPrOH/H2O mixture proved to be effective for these transformations. iPrOH is stable under normal conditions of use and is completely soluble in water.The TOF values (Table\u00a01) were calculated using the formula:\n\nTOF\u00a0=\u00a0mol product / [time (min)\u00a0\u00d7 mol catalyst]\n\n\nTOF values range from 100 to 450h-1, suitable for a sintered catalyst and comparable to other types of palladium-based powdery catalysts. The reactivity of the CERMET catalyst under conventional heating conditions is quite similar, although the final yield of the reactions decreases somewhat, especially the bromo-derivatives. Reaction times are longer too. Other \"pseudo heterogeneous\u201d powdery catalysts such as palladium on charcoal give a similar result under conventional heating conditions but are effective for a limited number of cycles. Therefore, under microwave heating conditions, the cermet behaved with good performance and, what is not less important: safe reaction conditions, without risk of explosion due to the presence of palladium inside the reactor. Therefore, this catalyst could be used in even more challenging reactions in which MAOS is especially required.In Sonogashira reactions, the best conditions for the copper-free coupling between iodoarene or haloisatin derivatives with simple terminal alkynes were studied. The best results were obtained using triethylamine (TEA) as base and iPrOH/H2O as solvents (200\u00a0W, 120\u00a0\u00b0C). As shown in Table\u00a01, the reactions were performed in 20\u00a0min and with high yields. Finally, Heck and Stille reactions were carried out using acetonitrile (MeCN) as a suitable solvent for these transformations, with short reaction times, at 120\u00a0\u00b0C. Organostannanes react efficiently using standard conditions (20W, 120\u00a0\u00b0C), in 10\u201330\u00a0min. In the case of Heck reactions, an increase in power (300W) was necessary to complete the reactions. The catalyst can carry out every reaction described here without the need to incorporate the magnetic stirrer into the microwave reactor. In this way, the reactants flow through the catalyst channels through the simple action of microwave energy (Fig.\u00a03b). The work-up is very simple since no filtration process is necessary. The monolithic catalysts prepared and used in this work were reused, indistinctly in the four procedures, for hundreds of times without appreciable loss of catalytic activity. In addition, the catalyst was completely safe under microwave heating conditions. No sparkling or arcing phenomena were detected during these experiments. Fig.\u00a03a shows the recyclability diagram for the first six cycles for each transformation, taking as model reactions those aimed at obtaining compounds 2a (Suzuki or Stille), 2e (Sonogashira), or 2h (Heck). This fact, together with the strong mechanical resistance offered by the catalyst (without fractures, scratches, breaks or surface poisoning) define this cermet system as a true \u201clong-life catalytic material\u201d.To determine if the monolithic catalyst works through a true heterogeneous catalysis, several studies were carried out. On the one hand, the Inductive Coupling Plasma (ICP/OES) results, measured in the reaction crudes (Suzuki, Sonogashira, Heck or Stille), showed that the Pd concentration in the reaction solution was less than the detection limit (i.e., 50\u00a0ppb) which corresponds to less than 0.02% of the starting Pd-amount (see S4 supplementary material). Secondly, the overwhelming reusability discussed above (each monolithic catalyst can work for hundreds of times in different reaction solvents) shown by the catalyst demonstrates the excellent possible applicability of this device in parallel drug synthesis (in which the presence of traces of metal is absolutely avoided). Third, the graph corresponding to the CO chemisorption made after carrying out numerous catalysis experiments was identical to the initial one (see S3 supplementary information), so it can be considered that the dispersion of the metal on the surface remains constant after numerous reaction cycles. Finally, hot filtration tests (HFT) (see S4, supplementary material) were performed for the four methods. Every experiment showed the absence of catalysis when the monolith is removed from the reaction mixture. The results of these tests indicated that the metal leaching is negligible or undetectable under the applied reaction conditions.In addition to the factors related to chemical resistance discussed above, it is worth highlighting the great mechanical robustness, demonstrated by the total absence of fractures on the surface after 200 reaction cycles (see tables S3, supplementary material). It is important to point out that, although the exact determination of the mechanical properties of the monolith is not an objective for this work, we have compared the robustness of the Pd0/Al2O3 cermet catalyst with other previously reported catalysts38-40 that contain palladium on the monolith surface but do not present palladium content throughout its internal structure. This comparison has been made based on the presence of fractures on the catalyst surface and the maximum number of times each monolith can be reused in solution phase cross coupling reactions. These studies indicate that cermet is much more resistant, robust and durable than catalysts that do not have internal metal content, due to a much lower degree of internal crystallinity than monoliths internally composed of pure Al2O3 or SiO2. (see table S3 in supplementary material).In summary, we have developed a strategy for the fabrication of an efficient and robust Pd0/Al2O3 cermet catalyst for use in safe Suzuki, Sonogashira, Stille and Heck protocols in microwave assisted cross-coupling reactions. Combining 3D printing technology and thermal annealing, it is possible to obtain a monolithic catalyst with open square pores, high geometric surface area of 33.2\u00a0cm2/cm3, uniform Pd loading after sintering of 1.66\u00a0wt% and increased robustness related to pure ceramic monoliths. EDS and mapping analysis confirmed the homogeneous distribution of Pd metal on the structure surface. Due to the synergistic effects of the metal-oxide interfaces, the monolithic catalyst is highly efficient and extremely robust in organic mixtures with aqueous solvents and high temperatures and Pd leaching was completely undetected by HFT or ICP/OES. The versatility of our fabrication approach provides an efficient strategy for the development of safe microwave-assisted cross-coupling reactions in the presence of a metal catalyst. In addition, the immobilized Pd catalyst could be easily washed, reused and applied safely as a \u201clong-life catalytic device\u201d in 200 different cross coupling reaction experiments without arcing phenomena, catalyst deactivation, surface poisoning or structural damage.The corresponding author is responsible for ensuring that the descriptions are accurate and agreed by all authors.Carmen R. Tubio: Investigation, Writing- Original draft preparation. Camilla Malatini: Investigation. Laura Barrio: Investigation. Christian F. Masaguer: Supervision. Manuel Amor\u00edn: Data Curation Management. Walid Nabgan: Data Curation Management. Pablo Taboada: Resources, Data Curation Management. Francisco Guiti\u00e1n: Funding acquisition, Resources. Alvaro Gil: Conceptualization Ideas, Supervision. Alberto Coelho: Conceptualization, Management and coordination responsibility for the research activity planning and execution, Writing- Original draft preparation, Writing- Reviewing and Editing, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the Conseller\u00eda de Cultura, Educaci\u00f3n e Ordenaci\u00f3n Universitaria of the Galician Government: EM2014/022 to A.C., ED431B2016/028 to F.G. The Strategic Grouping AEMAT grant No. ED431E2018/08 and the Spanish Ministry of Science, Innovation and Universities with grant No: MAT2017-90100-C2-1-P \"MA thanks Xunta de Galicia and the ERDF (ED431C 2021/21)\".The following is the Supplementary data to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.mtchem.2022.101355.", "descript": "\n                  A straightforward manufacture strategy is proposed to obtain an efficient and robust palladium-alumina (Pd0/Al2O3) cermet monolithic catalyst, specifically designed to perform safe microwave assisted organic synthesis (MAOS). In this approach, a cermet catalyst with high surface area, controlled composition and adapted shape and dimensions to a microwave reactor vessel is generated via 3D printing technology and sintering. The resulting catalyst has been explored in heterogeneous Suzuki, Sonogashira, Stille and Heck cross-coupling reactions, in MAOS. The Pd0 catalyst is permanently active, stable, without leaching and can be recycled and reused at least 200 reaction cycles. The generation of hot spots, sparking or hazardous discharges is controlled by the effective immobilization of the palladium in the monolithic structure during the reaction. The palladium content is forming part of both the internal and external structure, providing greater mechanical resistance and catalytic activity with respect to the basic ceramic material (alumina).\n               "}
{"full_text": "Niobia (Nb2O5) has attracted a great deal of attention for catalytic applications in aqueous media owing to its water-tolerant Lewis acidity (Barrios et\u00a0al., 2017; Brayner and Bozon-Verduraz, 2003; Chan et\u00a0al., 2017; Chary et\u00a0al., 2003; Francisco et\u00a0al., 2004; Gra\u00e7a et\u00a0al., 2013; Guan et\u00a0al., 2017; Herval et\u00a0al., 2015; Holtzberg et\u00a0al., 1957; Jasik et\u00a0al., 2005; Ko et\u00a0al., 1984; Lopes et\u00a0al., 2014; Nakajima et\u00a0al., 2013; Rojas et\u00a0al., 2013; Solcova et\u00a0al., 1993; Tanabe and Okazaki, 1995; Valencia-Balv\u00edn et\u00a0al., 2014; Wojcieszak et\u00a0al., 2006). Generally, amorphous Nb2O5 shows high surface acidity, which is related to its high specific surface area and number of surface defects (do Prado and Oliveira, 2017; Ziolek and Sobczak, 2017). However, amorphous Nb2O5 is a fragile material, susceptible to changes by temperature and pressure (Pinto et\u00a0al., 2017; Wojcieszak et\u00a0al., 2006). Considering Nb2O5 crystalline phases, they are formed by distorted octahedra (NbO6), connected by edges and corners. The distortion degree of NbO6 octahedra depends on the polymorph structure (Nico et\u00a0al., 2016; Pinto et\u00a0al., 2017; Valencia-Balv\u00edn et\u00a0al., 2014). This distortion leads to varied textural and structural stabilities as well as different surface acid properties and, therefore, impacts on catalytic properties. H-Nb2O5 (monoclinic structure) and T-Nb2O5 (orthorhombic structure) are the most common crystalline phases, whereas the TT-Nb2O5 (pseudohexagonal structure) is the least thermodynamically stable phase and is often considered as a less ordered form of the T-phase. By increasing the temperature and pressure in hydrothermal synthesis, the conversion of Nb2O5 phases takes place following the sequence: amorphous Nb2O5 \u2192 TT-Nb2O5 \u2192 T-Nb2O5 \u2192 H-Nb2O5 (Nowak and Ziolek, 1999; Pinto et\u00a0al., 2017; Valencia-Balv\u00edn et\u00a0al., 2014). Nb2O5 phase transitions are typically followed by a progressive decrease in the surface area, porosity, and acidity (Ali et\u00a0al., 2017; Gra\u00e7a et\u00a0al., 2013; Kreissl et\u00a0al., 2017; Pinto et\u00a0al., 2017; Raba et\u00a0al., 2016; Valencia-Balv\u00edn et\u00a0al., 2014). Among the crystalline phases, the TT-Nb2O5 phase is the one presenting the highest number of oxygen vacancies in the structure, and so the greatest degree of polyhedral distortion (Pinto et\u00a0al., 2017; Rani et\u00a0al., 2014). TT-Nb2O5 is characterized by the presence of distorted octahedra and pentagonal and hexagonal bipyramids, i.e., NbO6, NbO7, and NbO8 polyhedra, which are the structural units also present in amorphous Nb2O5 (Nakajima et\u00a0al., 2011; Nico et\u00a0al., 2016). Notably, TT-Nb2O5 structural features translate into a highly polarized and disordered surface with high levels of Lewis and Br\u00f8nsted acid sites, which are essential to the high performance of hydrodeoxygenation (HDO) catalysts.Metal-based (mainly Pt, Pd, Ru, Ni) catalysts supported on acidic materials have been widely examined in the HDO of lignin model compounds (Cui et\u00a0al., 2017; Shao et\u00a0al., 2017; Teles et\u00a0al., 2018; Wang and Rinaldi, 2016; Zhao et\u00a0al., 2009). For the HDO of lignin model compounds and lignin streams, niobium oxides have\u00a0been studied as supports for noble metals (Shao et\u00a0al., 2017). Studies on multifunctional Fe3O4/Nb2O5/Co/Re catalysts have also been reported (Parvulescu et\u00a0al., 2017). Pd catalysts supported on niobia revealed promising results for the dehydroxylation of phenol to benzene, presenting a reaction rate 90-fold higher than that observed for a Pd/SiO2 catalyst (Barrios et\u00a0al., 2017). Importantly, Pt/Nb2O5-Al2O3 has been reported as an active catalyst for the hydrotreating of diphenyl ether, showing stability higher than that of Pt/Al2O3 owing to the water-tolerant nature of niobium(V) Lewis acid sites (Jeon et\u00a0al., 2018). Subjecting lignin-derived dimers to a Ni0.92Nb0.08 catalyst resulted in full conversion of the substrates into liquid alkanes at 200\u00b0C after 2 h, demonstrating the outstanding ability of this material for C\u2013O cleavage and HDO (Jin et\u00a0al., 2017). For the selective production of arenes from lignin, it was reported that Ru-Nb2O5 catalysts present unique catalytic properties, compared with Ru supported on traditional oxide supports (Shao et\u00a0al., 2017).In an approach for lignin-to-liquid fuels, one of the challenges is to design inexpensive catalysts with high activity, selectivity, and stability under process conditions. Since the hydrotreating of lignin streams releases water, the solid catalyst must be stable in the presence of water under high-severity conditions. Commercial niobia (Nb2O5\u22c5xH2O) is a bulk amorphous material that lacks stability under hydrothermal conditions, thus losing surface area and leading to the sintering of supported metallic particles (Pham et\u00a0al., 2011). To overcome the poor structural stability of commercial Nb2O5, various synthesis methods have been a subject of research in producing highly stable nanostructured materials (Zhao et\u00a0al., 2012b). Nb2O5 nanoparticles with no defined shape can be obtained by precipitation and sol-gel synthesis methods followed by calcination. These routes have extensively been studied in the preparation of the Nb2O5 supports applied to the HDO of lignin and lignin-derived molecules with good results (Shao et\u00a0al., 2017). Nb2O5 crystallization under low-severity solvothermal conditions constitutes another progress in this field. This synthetic route produces single TT-Nb2O5 nanorods with controlled size and morphology, high surface area, and improved acid properties (Ali et\u00a0al., 2017; Leite et\u00a0al., 2006; Zhou et\u00a0al., 2008). TT-Nb2O5 nanorods exhibit shape-dependent acidic sites (Zhao et\u00a0al., 2012a). On (001) TT-Nb2O5 surface of the nanorods, Lewis acid sites are much stronger than those of spherical Nb2O5 particles. Despite the interesting acidic properties, the production of Nb2O5 nanorods employs oleic acid and trioctylamine as structure-directing agents in the solvothermal synthesis. Especially for catalytic applications, the use of such structure-directing agents surfactants in the synthesis of Nb2O5 presents disadvantages owing to their high costs, low volume of material production limitation, and the need to remove the agents via calcination, which may modify the morphology, particle size, and surface chemistry of Nb2O5 (Ali et\u00a0al., 2017; Zhao et\u00a0al., 2012a).Hydrothermal synthesis of TT-Nb2O5 nanorods in the presence of H2O2 represents a route receiving far less attention, but with the most promising results regarding the textural and acidic properties of niobia (Leal et\u00a0al., 2019). Despite the improved chemical and physical properties, such Nb2O5 nanorods have not yet been explored in the chemistry of lignin hydrotreating. Therefore, this knowledge gap brought us to the study of nickel supported on hydrothermally stable TT-Nb2O5 nanorods as a potential catalyst for HDO of lignin streams. As about 80% of the primary interunit linkages of lignin are ether bonds (Rinaldi et\u00a0al., 2016), and a considerable number of other oxygenated functionalities are present in lignin-derived phenolics, a highly stable and highly acidic niobia could well hold the key to produce efficient catalysts for lignin-to-liquid fuels, owing to an expected synergism between metal phase and support toward lignin depolymerization and acid-catalyzed deoxygenation of intermediates formed throughout the HDO course (Cao et\u00a0al., 2018; Wang and Rinaldi, 2016, 2013).In this report, we examine the catalytic properties of Ni-supported on TT-Nb2O5 nanorods for the hydrotreating of a model compound (diphenyl ether) and lignin oil produced by a lignin-first biorefining process based on H-transfer reductive processes, the so-called catalytic upstream biorefining (CUB), which is also denoted as \u2018reductive catalytic fractionation\u2019 (RCF) by several research groups. CUB constitutes a class of methods for deconstruction of lignocellulose that renders high-quality pulps together with depolymerized and passivated lignin streams (Ferrini and Rinaldi, 2014; Galkin and Samec, 2016; Gra\u00e7a et\u00a0al., 2018; Renders et\u00a0al., 2017; Rinaldi, 2017; Schutyser et\u00a0al., 2018; Sultan et\u00a0al., 2019; Rinaldi et al., 2019). TT-Nb2O5 nanorods were prepared via hydrothermal synthesis by employing ammonium niobium oxalate and H2O2 as the structure-directing agent (Leal et\u00a0al., 2019; Leite et\u00a0al., 2006; Pavia et\u00a0al., 2010). TT-Nb2O5 nanorods were then loaded with several Ni contents. In this report, the results and discussion are organized as follows. First, the characterizations of the as-synthesized TT-Nb2O5 nanorods and Ni/Nb2O5 catalysts are briefly presented. Ni/Nb2O5 catalysts are subsequently applied to the HDO of diphenyl ether at 160\u00b0C and 200\u00b0C under 4 MPa H2. The catalyst performance and stability in the HDO of diphenyl ether at 200\u00b0C under 4 MPa H2 were assessed. Finally, under more severe conditions (300\u00b0C and 7 MPa H2), the 15%Ni/Nb2O5 catalyst was applied to the hydrotreating of the lignin oil.\nFigure\u00a01\n shows the X-ray diffraction (XRD) patterns obtained from both the hydrothermally as-synthesized Nb2O5 after calcination at 380\u00b0C and Ni/Nb2O5 catalysts reduced at 320\u00b0C. XRD pattern of the Nb2O5 support exhibits peaks characteristic of the pseudohexagonal TT-Nb2O5 phase (Ko and Weissman, 1990). Notably, a high-intensity signal is observed at 22.8\u00b0, which is associated with (001) reflection of TT-Nb2O5. In addition, a low-intensity and broad signal related to the (100) plane appears at 28.0\u00b0. As next confirmed by scanning transmission electron microscopic (STEM) imaging, a preferred growth of TT-Nb2O5 along the (001) direction creates the preferential orientation feature in the XRD pattern, indicating the formation of TT-Nb2O5 as nanorods (Ali et\u00a0al., 2017). For the Ni/Nb2O5 materials, the XRD patterns demonstrated that the structural features of the TT-Nb2O5 phase were preserved after the reduction procedure. Hence, for simplicity, when referring to the materials produced in this study, the TT-Nb2O5 phase will be denoted as \u201cNb2O5\u201d henceforth. As expected, Ni(111) and Ni (200) reflections are observed at 44.6\u00b0 and 52.1\u00b0. These reflections become more intense and sharper with an increase in Ni content from 5 to 25\u00a0wt %. Considering the Ni(111) reflection, the average Ni crystallite sizes estimated by the Scherrer equation grow from 7 to 15\u00a0nm with the rise in Ni content from 5 to 25 wt % (Table 1\n).To verify whether the synthesis rendered Nb2O5 nanorods, the Nb2O5 material was examined by using high-angle annular dark-field (HAADF)-STEM (Figure\u00a02\n). The HAADF-STEM images show that the hydrothermal synthesis produced Nb2O5 nanorods with approximately 8\u201325\u00a0nm length and 3\u20134\u00a0nm width (Leal et\u00a0al., 2019). The nanorod dimensions are in line with those of previous studies showing that the crystal growth in the hydrothermal method follows an oriented attachment mechanism (Leite et\u00a0al., 2006), producing nanorods smaller than those synthesized in the presence of a surfactant as a shape-directing agent. In fact, surfactant-based syntheses yield large particles, owing to a decrease in the rate of crystal growth (Zhao et\u00a0al., 2012a). In turn, longer (200\u2013500\u00a0nm) and thinner (5\u201320\u00a0nm) TT-Nb2O2 nanorods were produced by a synthesis employing oleic acid as a structure-directing agent and ammonium niobium oxalate hydrate as the starting material (Zhao et\u00a0al., 2012b).\nTable 1 summarizes the textural properties of the support and Ni/Nb2O5 materials. N2 adsorption-desorption isotherms are presented in Figure\u00a0S1. Nb2O5 and Ni/Nb2O5 materials exhibit a type II isotherm with an H3 hysteresis (Thommes et\u00a0al., 2015), corroborating the non-structural porosity created by the packing of Nb2O5 nanorods. Niobium oxide nanoparticles can be obtained by various synthesis methods, which leads to the preparation of materials of different shapes with Brunauer-Emmett-Teller (BET) specific surface areas that can range from about 20 m2 g\u22121 to 530 m2 g\u22121(Luisa Marin et\u00a0al., 2014; Morais et\u00a0al., 2017; Shao et\u00a0al., 2017). Table 1 shows the as-synthesized Nb2O5 support to possess a relatively high specific surface area (196 m2 g\u22121). Notably, no significant decrease in the surface area of Ni/Nb2O5 materials, with 5\u201315 wt % Ni loading on Nb2O5 was observed. Likewise, as the porosity of the Nb2O5 support is non-structural, the deposition of Ni phase onto the nest of nanorods does not significantly decrease the specific surface area. However, for 25%Ni/Nb2O5, the specific surface area slightly decreased (from 196 m2 g\u22121 to 141 m2 g\u22121).Temperature-programmed reduction (TPR) profiles of the Nb2O5 nanorods and Ni/Nb2O5 precursors (catalysts before reduction) are shown in Figure\u00a03\n. The TPR profiles of Ni/Nb2O5 precursors exhibit two reduction events. The first occurs at around 335\u00b0C. This event is assigned to the reduction of Ni(II) to Ni(0). The reduction temperature of the Ni(II) species supported on Nb2O5 nanorods is much lower than that of bulk NiO (450\u00b0C) (Gra\u00e7a et\u00a0al., 2014). Nevertheless, the range of Ni reduction temperatures between 320\u00b0C and 345\u00b0C is in line with previous studies on Ni/Nb2O5 materials containing high nickel loadings (Jankovi\u0107 et\u00a0al., 2008; Liu et\u00a0al., 2016). Interestingly, and contrary to what has been previously observed by other research groups (Chary et\u00a0al., 2003; Wojcieszak et\u00a0al., 2006), no shift of the Ni reduction peak to higher temperatures with an increase in Ni loading was observed. As will be presented later, in the 15%Ni/Nb2O5 material, the Ni nanoparticles are embedded in a nest formed by Nb2O5 nanorods (Figure\u00a011). Thereby, such an entanglement of metal phase and oxidic support may result in few (but strong) connecting points between these phases so that an effect of the increase in the Ni loading on the reduction temperature of NiO species is not apparently observed. However, in the second reduction (at about 800\u00b0C), which is related to the partial reduction of Nb2O5 to NbO2 (Wojcieszak et\u00a0al., 2006), the temperature required for the reduction of Nb2O5 progressively decreases (from 870\u00b0C to 816\u00b0C) with increasing Ni content (Table S1). This observation indicates that there is an interaction between Ni and NbO5 phases in which the hydrogen spillover appears to be more prevalent for the samples containing higher Ni loadings.Nb2O5 polymorph crystals are formed by distorted octahedra (NbO6) connected by edges and corners, the degree of distortion depending on the polymorph structure (Nico et\u00a0al., 2016; Pinto et\u00a0al., 2017; Valencia-Balv\u00edn et\u00a0al., 2014). In the TT-Nb2O phase, the highly distorted octahedra (NbO6) units exhibit Nb=O bonds, enabling the Nb(V) center to act as a Lewis acid site. In turn, the slighted distorted NbO6, as well as NbO7 and NbO8 groups, only present Nb-O bonds, which provide scaffolding for the [Nb(V)---OH2\n2+] Br\u00f8nsted acid sites (Chan et\u00a0al., 2017). In this study, we found the quantity of Lewis acid sites on Nb2O5 (210\u00a0\u03bcmol g\u22121) to be higher than that of Br\u00f8nsted acid sites (143\u00a0\u03bcmol g\u22121, Figure\u00a0S2 and Table S2).To assess the nature of the acidic sites of Nb2O5 support and Ni/Nb2O5 catalysts (after the reduction procedure), attenuated total reflection (ATR)-Fourier transform infrared (FTIR) spectra of adsorbed pyridine (Py) were collected (Figure\u00a04\n). We chose to assess the nature of acid sites by ATR technique because of the dark color of the activated Ni/Nb2O5, which hinders FTIR transmission experiments (as those performed on the Nb2O5 support). Pyridine adsorbed on the materials exhibits infrared (IR) bands at around 1,446\u00a0cm\u22121 and 1,606\u00a0cm\u22121. These bands are related to the Py coordinated to Lewis acid sites. In addition, the IR spectra show bands at 1,639\u00a0cm\u22121 and 1,540\u00a0cm\u22121, which are assigned to the formation of the pyridinium ion (PyH+) on Br\u00f8nsted acid sites (Figure\u00a04) (Datka, 1992; Dollish et\u00a0al., 1974; Iizuka et\u00a0al., 1983; Parry, 1963). An IR band of adsorbed Py common to both Lewis and Br\u00f8nsted acid sites is also visible at 1,489\u00a0cm\u22121. These observations indicate that both Lewis and Br\u00f8nsted acid sites are present in the Nb2O5 nanorods. From the relative intensities of Py adsorbed on Lewis and Br\u00f8nsted acid sites, it can be inferred that the Br\u00f8nsted acidity decreases as the Ni loading increases, as indicated by the reduction in the intensities of the bands at 1,639\u00a0cm\u22121 and 1,540\u00a0cm\u22121. This finding is explained by the ion exchange of Br\u00f8nsted acid sites by the positively charged Ni species. However, in the 25%Ni/Nb2O5 catalyst, the support still presents some residual Br\u00f8nsted acidity. Notably, the Lewis acidity (bands at around 1,606\u00a0cm\u22121 and 1,446\u00a0cm\u22121) appears to be mostly preserved even at such a high loading of Ni on Nb2O5 nanorods.Acid supports are active in the dehydration of cyclohexanol to cyclohexene and, therefore, are vital to the hydrotreating of lignin to alkanes and arenes (Wang and Rinaldi, 2016; Zhao et\u00a0al., 2010, 2009). Hence, to further assess the effect of Ni loading on the acidic properties of the catalysts, cyclohexanol dehydration was carried out at 200\u00b0C. As will be presented in the next section, this reaction is key to produce cyclohexane from the conversion of diphenyl ether, as well as to obtain cycloalkanes from lignin. Table 2\n summarizes the cyclohexanol conversion values obtained after 30\u00a0min of reaction at 200\u00b0C.As expected, for the Nb2O5 nanorods, cyclohexanol conversion is considerably high. However, cyclohexanol conversion significantly decreases in the presence of 5%Ni/Ni2O5 catalyst. By increasing Ni loadings, cyclohexanol conversion continually drops, plateauing at 31% for 25%Ni/Nb2O5. These results show that the activity of the catalysts is partially affected by Ni deposition. Overall, the results presented in Table\u00a02 indicate that the decrease in Br\u00f8nsted acidity is detrimental to the dehydration performance. These results confirm that, at promoting the dehydration of cyclohexanol, Br\u00f8nsted acid sites are more active for alcohol dehydration than the Lewis acid sites (Foo et\u00a0al., 2014).The catalytic performance of the Ni/Nb2O5 catalysts was evaluated for the conversion of diphenyl ether as a model reaction. The cleavage of diphenyl ether serves as a model reaction for the breakdown of 4-O-5 ether linkages occurring in lignins. Owing to its high bond dissociation enthalpy (BDE: 330\u00a0kJ mol\u22121), the 4-O-5 linkages are resistant against cleavage via non-catalytic thermal processes, compared with \u03b1-O-4 and \u03b2-O-4 ether linkages occurring both in native and technical lignins (BDE: 215\u00a0kJ mol\u22121 for \u03b1-O-4 in phenylcoumaran subunits, and 290\u2013305\u00a0kJ mol\u22121 for \u03b2-O-4 in lignin's aryl alkyl ether-bonding motifs) (Dorrestijn et\u00a0al., 2000; Parthasarathi et\u00a0al., 2011; Rinaldi et\u00a0al., 2016; Wang and Rinaldi, 2012; Younker et\u00a0al., 2011). Therefore, the ability of a Ni catalyst for hydrogenolysis can be evaluated with little contribution of thermolysis to the overall reaction results. In this instance, diphenyl ether is also a useful model compound for another reason. It allows for the evaluation of the activity of the Ni phase toward hydrogenation of phenol and benzene, the intermediates formed by the hydrogenolysis of diphenyl ether. In the presence of acid sites, the intermediate mixture is ultimately funneled to cyclohexane, as schematically represented by the reaction network presented in Figure\u00a05\n.To investigate the different catalyst functionalities, the hydrotreating of diphenyl ether was carried out at two temperatures, 160\u00b0C and 200\u00b0C. These two conditions were chosen because dehydration of alcohols has significant enthalpic barriers for the formation of carbocations (Liu et\u00a0al., 2017), meaning that relatively high temperatures are required for the alcohol dehydration. By this choice, the hydrogenolysis and hydrogenation extents can be better discerned in the experiments carried out at 160\u00b0C, whereas the performance for the full HDO of diphenyl ether is better addressed by the experiments performed at 200\u00b0C.When targeting cycloalkanes, the results of a model compound reaction can be more conveniently compared by computing the HDO extent and degree of deoxygenation (DOD), as given by Equations 1 and 2, respectively (Rinaldi, 2015).\n\n(Equation\u00a01)\n\n\nH\nD\nO\n\ne\nx\nt\ne\nn\nt\n=\n\n\n\nH\n2\n\n\ni\nn\nc\no\nr\np\no\nr\na\nt\ne\nd\n\ni\nn\n\nt\nh\ne\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n\nH\n2\n\n\nf\no\nr\n\nc\no\nm\np\nl\ne\nt\ne\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\nt\no\n\nc\ny\nc\nl\no\nh\ne\nx\na\nn\ne\n\n\n\u00d7\n100\n\n\n\n\n\n\n(Equation\u00a02)\n\n\nD\nO\nD\n=\n\n(\n\n1\n\u2212\n\n\nw\nt\n\n%\n\nO\n\ni\nn\n\np\nr\no\nd\nu\nc\nt\n\n\nw\nt\n\n%\n\nO\n\ni\nn\n\nf\ne\ne\nd\n\n\n\n)\n\n\u00d7\n100\n\n\n\n\n\nFigure\u00a06A compares the performance of the Ni/Nb2O5 by evaluating the HDO extent achieved by the reaction network as a function of time for the conversion of diphenyl ether at 160\u00b0C. As expected, an increase in HDO extent with time for all tested catalysts was observed. By analyzing the results obtained at 180\u00a0min (Figure\u00a06B), the HDO extent increased linearly from 28% to 46% with the rise in the Ni content (from 5 to 25 wt %). On the other hand, as expected at this temperature, low DOD was obtained for all catalysts at 180\u00a0min, with a decrease being in general noticed with the increase in Ni content, owing to the decline in the Br\u00f8nsted acidity, as previously discussed. In these experiments, conversions of diphenyl ether in the range of 55%\u201398% at 180\u00a0min were achieved (Figure\u00a06B). A blank test and a catalytic run with the pure Nb2O5 were also carried out (Table S3). By stark contrast, in these control experiments, only very low conversion of diphenyl ether (8% and 15%, respectively) was achieved at 180\u00a0min, with no selectivity to a specific product. Furthermore, to verify whether there is a contribution of the leached species to the reactions, the catalyst 15%Ni/Nb2O5 was contacted with the solvent under the same conditions of the reaction. After this, the catalyst was separated from the liquid product, and then diphenyl ether and the internal standard were added to the reaction media for reaction run. The results were similar to those of the blank reaction, confirming that the catalytic process is exclusively taking place on the catalyst surface.To examine in more detail the results of the experiments conducted at 160\u00b0C, the product distribution at a similar conversion level of about 50%\u201360% was analyzed (Table 3\n). Two critical ratios of products' groups were considered. The ratio \u03a3(4\u20137)/\u03a3(2,3) was used to define the selectivity to monocyclic products produced by the cleavage of the C\u2013O ether bond. The ratio \u03a3(5,7)/\u03a3(4,6) indicates the selectivity to HDO after ether bond cleavage, which reflects the ability of the catalyst to execute the following reaction sequence: phenol \u2192 cyclohexanol \u2192 cyclohexene \u2192 cyclohexane. Evolution of the product selectivity with time at 160\u00b0C is given in Figure\u00a0S3.\nTable 3 shows that diphenyl ether was converted into three main products: cyclohexyl phenyl ether (22%\u201325%), cyclohexanol (29%\u201336%), and cyclohexane (27%\u201338%). Small quantities of dicyclohexyl ether, phenol, and benzene were also found in the reaction mixture (individual selectivity values lower than 11%). \u03a3(4\u20137)/\u03a3(2,3) ratio higher than 1 was observed for all the Ni/Nb2O5 catalysts. This observation indicates the formation of monocyclic products to prevail over the partial or full saturation of diphenyl ether. The latter renders the bicyclic products 2 and 3, respectively. With the rise in Ni content in Ni/Nb2O5 catalysts (from 5 to 25 wt %), a gradual reduction in the \u03a3(4\u20137)/\u03a3(2,3) ratio (from 2.57 to 2.03) was observed. Taking the results from the experiment carried out in the presence of 25 wt % Ni/Nb2O5 catalyst into account, the reduction in the \u03a3(4\u20137)/\u03a3(2,3) ratio is related to the accumulation of dicyclohexyl ether in the reaction mixture. As previously reported, dialkyl ethers are not prone to undergo hydrogenolysis in the presence of Ni catalysts under relatively mild reaction conditions (Wang and Rinaldi, 2016; Zhao et\u00a0al., 2012a). Should a dialkyl ether be cleaved, the reaction pathway would begin with a hydrolysis step instead (Figure\u00a05). However, in this study, the formation of dicyclohexyl ether constitutes a dead end, as its conversion was not observed. Confirming this, we could successfully employ n-dibutyl ether in the reaction mixtures as an internal standard for gas chromatography (GC) analysis. Likewise, no decomposition of the internal standard was detected.The results in Table 3 also shows a rise in the selectivity to cyclohexanol (from 6% to 11%) for the experiment carried out in the presence of 25 wt % Ni/Nb2O5 catalyst. This outcome agrees with the decrease in Br\u00f8nsted acidity at a high Ni content supported on Nb2O5, as verified by ATR-IR spectra of pyridine adsorbed on the reduced Ni/Nb2O5 catalysts (Figure\u00a04) and model reaction experiments (dehydration of cyclohexanol, Table 2). Therefore, the increase in Ni loading on Nb2O5 has implications for both the accumulation of dicyclohexyl ether (i.e., raises the likelihood of full saturation of diphenyl ether to dicyclohexyl ether) and of cyclohexanol (i.e., lessens the extent of dehydration of cyclohexanol).The product distributions in Table 3 show similar values of selectivity to cyclohexanol and cyclohexane, revealing that Nb2O5 plays a marginal role in the HDO extent at 160\u00b0C. Under these conditions, low DOD values (5%\u201312%) were achieved. The catalyst's ability to dehydrate cyclohexanol significantly reduces with the rise in Ni content, as indicated by the decrease in the \u03a3(5,7)/\u03a3(4,6) ratio from 1.40 to 0.81. The decrease in the dehydration capability is correlated with the decrease in the number of Br\u00f8nsted acid sites with the increase in Ni content, as discussed in the previous section.According to the results from Table 3, 10%Ni/Nb2O5 and 15%Ni/Nb2O5 catalysts present the best balance between HDO extent and selectivity to monocyclic deoxygenated products. These catalysts were thus chosen for the conversion of diphenyl ether carried out at 200\u00b0C. Figure\u00a07\n shows the monitoring of the reaction mixture components over time. For both experiments, full conversion was achieved at 180\u00a0min. Cyclohexane was the main product obtained, with selectivity values of 81% and 88% for the 10%Ni/Nb2O5 and 15%Ni/Nb2O catalysts, respectively. These results confirm that the dehydration of cyclohexanol is encouraged at 200\u00b0C. At 180\u00a0min, the HDO extent and DOD were both greater for 15%Ni/Nb2O5 (HDO extent: 91%; DOD: 85%) than for the 10%Ni/Nb2O5 catalyst (HDO extent: 82%; DOD: 72%). Based on these results, 15%Ni/Nb2O5 catalyst was considered as the most efficient. Thus the 15%Ni/Nb2O5 catalyst was selected for the recycling experiments and studies on the conversion of lignin oil.The catalytic performance after five reaction cycles was investigated for the HDO of diphenyl ether at 200\u00b0C for 240\u00a0min using the 15%Ni/Nb2O5 catalyst. Again, diphenyl ether serves as a model compound because, when targeting the full HDO of lignin streams, the accumulation of cyclohexanol (derived from hydrogenation of phenol) indicates a decay of the initial acidic properties of a bifunctional catalyst. After each reaction run, the catalyst was washed with solvent and reused in the following reaction run. Figure\u00a08\n displays the conversion and product distribution after each reaction run.\nFigure\u00a08 shows that the catalyst presents a sustained performance, still producing a 91% yield of cyclohexane after five reaction runs. A slight decrease in the cyclohexane selectivity is, however, observed from the second to fourth reaction runs, with the formation of cyclohexanol and dicyclohexyl ether (around 4%\u20135% each) from the third cycle on. These results translate into a slight decrease in both HDO extent (from 100% to 92%) and DOD (from 100% to 94%) throughout the recycling experiments.To examine surface, structural, and morphological alterations occurring in the 15%Ni/Nb2O5 catalyst, the fresh and spent catalysts were analyzed by using a set of techniques (pyridine adsorption, XRD, and HAADF-STEM). Figure\u00a09\n shows the ATR spectra of adsorbed pyridine on the fresh catalysts and spent samples after five reaction runs. The data indicates that the population of Br\u00f8nsted acid sites dramatically decreased after five successive reuses of the catalyst. This means that even though water is generated during the reaction, no regeneration of Br\u00f8nsted acidity takes place in the process. In this context, the accumulation of cyclohexanol appears to be related to a decrease in the population of Br\u00f8nsted acid sites. On the other hand, Lewis acidity is preserved, which explains the sustained high selectivity to cyclohexane at 200\u00b0C, demonstrating that Nb2O5 Lewis acid sites are stable under the reaction conditions. Moreover, a modest decrease in BET surface area was observed (fresh catalyst: 180 m2 g\u22121 versus spent catalyst: 144\u00a0m2 g\u22121). The XRD pattern of the used catalyst (Figure\u00a010\n) also shows that the crystalline structure of the catalyst is maintained after five reaction runs. Also, no significant change in the Ni average crystallite size occurred after five reaction runs (fresh catalyst: 14\u00a0nm versus spent catalyst: 15\u00a0nm). Finally, the comparison of STEM-HAADF images (Figure\u00a011\n) indicates that the spent catalyst after the fifth reaction run maintains the original features of the fresh catalyst, that is, the arrangement and size distribution of Ni nanoparticles entangled in the Nb2O5 nanorod nest remained, to a great extent, unaltered. Overall, these observations together with the sustained catalytic performance of the 15%Ni/Nb2O5 indicated that this material holds potential as a robust and active catalyst for the conversion of phenolic streams derived from lignin.To explore the potential of 15%Ni/Nb2O5 catalyst in the conversion of lignin oil, lignin oil was subjected to hydrotreatment under an H2 pressure of 7 MPa (measured at room temperature) at 300\u00b0C for 16 h. We chose to increase the reaction temperature from 200\u00b0C (as for the model compound experiments) to 300\u00b0C to encourage extensive HDO of lignin oil to cycloalkanes, leading to full conversion of lignin into products soluble in n-pentane (reaction solvent), thus avoiding the accumulation of lignin residues throughout the catalyst recycling experiments (Wang and Rinaldi, 2012). However, even under harsh conditions, an appreciable amount of a residue insoluble in n-pentane or even methanol (a good solvent for lignin oil species) was formed and, thus, accumulated with the catalyst. Thereby, in this study, the \u201cconversion of lignin oil\u201d is estimated as a \u201cnet conversion,\u201d which takes into account the weight of residue insoluble in either n-pentane or methanol formed in each reaction run (in conjunction with the initial weight of fresh catalyst) and the amount of lignin oil added in each reaction run. For the catalyst recycling experiments, the spent catalyst was washed with methanol to extract soluble residue species. The spent catalyst containing lignin-derived residues insoluble in methanol was then recovered by filtration and dried at 40\u00b0C in a vacuum oven. The liquid products and the fraction of lignin residues soluble in methanol were characterized by elemental analysis, gas chromatography (GC)-flame ionization detector (FID)/mass spectrometry (MS), and gel permeation chromatography (GPC).Throughout the catalyst recycling experiment, which processed in total ca. 6.0\u00a0g of lignin oil, the amount of lignin-derived residue increased from ca. 0.23\u00a0g (first run) to 0.30\u20130.31\u00a0g (second or third run, Table 4\n). Logically, the accumulation of the lignin-derived residue impedes the precise determination of the initial quantity of substrate present in the second and third reaction runs, as it is not possible to discern whether a part of the lignin-derived residue was also consumed throughout the recycling experiment and replaced with a fresh, more oxygenated carbonaceous residue derived from the fresh substrate. Further exploration of the data listed in Table 4, that is, the determination of weight ratio of liquid-product-to-residue, should be carried out with caution. A mass ratio of liquid-product-to-residue is only meaningful if both liquid product and residue present similar values of O/C and H/C ratios, which is not the case when the catalyst loses part of its performance in the recycling experiments.\nFigure\u00a012\n summarizes in a van Krevelen diagram the results obtained from the control experiment and catalyst recycling in the hydrotreating of lignin oil. In the absence of the catalyst, a 68% conversion of the lignin oil was achieved by thermal processes (Table 4), increasing the H/C ratio from 1.51\u00a0\u00b1 0.01, for the lignin oil, to 1.74\u00a0\u00b1 0.02 for the liquid fraction. Conversely, the O/C ratio decreased from 0.46\u00a0\u00b1 0.01, for the lignin oil, to 0.23\u00a0\u00b1 0.01 for the produced liquid fraction. This decrease in O/C ratio is associated with the elimination of the \u03b3-OH group of p-dihydrolignols, among other thermal processes, leading to deoxygenation (Table S4). The solid residue exhibited an H/C ratio of 1.45\u00a0\u00b1 0.03 and an O/C ratio of 0.34\u00a0\u00b1 0.01. These results indicate that the residue no longer corresponds to the initial lignin stream.In the catalytic experiments, an 89% conversion of lignin in the first reaction run was achieved. For the liquid product obtained from the first reaction run, a substantial increase in the H/C ratio from 1.51\u00a0\u00b1 0.01, for the lignin oil, to 1.80\u00a0\u00b1 0.01 was achieved. In parallel, the O/C molar ratio decreased from 0.46\u00a0\u00b1 0.01 to 0.006\u00a0\u00b1 0.004 for the liquid product. These results demonstrate the extensive removal of oxygen and incorporation of hydrogen in the liquid product. In the subsequent catalyst reuse, the net conversion of lignin oil slightly decreased from 89% to 85%, for both the second and third reaction runs (Table 4). For the liquid products, H/C ratios of 1.76\u00a0\u00b1 0.01 and 1.75\u00a0\u00b1 0.03 for the second and third reaction runs, respectively, were obtained. These values are slightly lower than those of the liquid products from the first reaction run (H/C:1.80\u00a0\u00b1 0.01). On the other hand, O/C ratios substantially increased from 0.006\u00a0\u00b1 0.004, for the first reaction run, to 0.14\u00a0\u00b1 0.01 and 0.16\u00a0\u00b1 0.01, for the second and third reactions runs, respectively. For the residue fraction soluble in MeOH, which corresponds to ca. 10% of the lignin-derived residues, the H/C ratio decreased from 1.73\u00a0\u00b1 0.01 (first reaction run) to 1.62\u00a0\u00b1 0.01 and 1.62\u00a0\u00b1 0.06, for the second and third reaction runs, respectively. In parallel, O/C ratios rose from 0.29\u00a0\u00b1 0.04 (first reaction run) to 0.33\u00a0\u00b1 0.01 and 0.36\u00a0\u00b1 0.01 for the second and third reaction runs, respectively. Altogether, the O/C and H/C ratios found for the liquid products and residues indicate that the catalyst's hydrogenation activity deteriorated to an extent lesser than that of the deoxygenation ability.To gain an in-depth insight into the composition of the volatile fraction of the liquid products, GC-FID/MS analysis was carried out (Figure\u00a013\n). In the volatile fraction of the lignin oil (corresponding to 28% at an injector temperature of 300\u00b0C), the main components were p-dihydrolignols [4-(3-hydroxypropyl)-2-methoxyphenol and 4-(3-hydroxypropyl)-2,6-dimethoxyphenol, Table S4] followed by other alkylphenol compounds. In the control experiment, thermolytic processes on the p-dihydrolignols caused the elimination of \u03b3-OH group, rendering 4-propylguaiacol and 4-propylsyringol. Other products from the cracking of the propyl side chain were formed (Table S4). At a much lesser extent, cyclohexanols (6.6%) and cycloalkanes (1.3%) were also formed. In the presence of 15%Ni/Nb2O5 catalyst, the primary volatile products were cycloalkanes (47%) and cyclohexanols (2%). Half of the cycloalkanes' fraction content corresponded to bicyclic aliphatic compounds. In the catalyst recycling, the content of cycloalkanes in the liquid product significantly decreased from 47%, for the fresh catalyst, to 8% and 5%, for the second and third reaction runs, respectively. As a result, the dominant species in the liquid products became cyclohexanols (31%\u201335%). The high content of monophenolic species (18\u201319%) reveals that the catalyst's hydrogenation ability was also impaired after the first use of the catalyst. However, the catalyst hydrogenation ability was affected to an extent lesser than that for the dehydration of cyclohexanol intermediates.To assess the extent of decrease in the acidity of the Nb2O5 support, ATR-IR measurements of pyridine adsorbed on the spent 15%Ni/Nb2O5 catalyst were performed. After the third reaction run, Figure\u00a014\n reveals that the spent catalyst no longer presents either Br\u00f8nsted or Lewis acid sites accessible to pyridine adsorption. These results demonstrate the decrease in the deoxygenation activity of 15%Ni/Nb2O5 to be caused by the blocking of acid sites on the Nb2O5 support. Surprisingly, despite the loss of acidity, the structural properties of the Nb2O5 nanorods were not affected upon recycling (as shown by XRD pattern features, Figure\u00a0S4). By stark contrast, the size of the Ni particles increased from 14 to 80\u00a0nm after three reaction runs (Figure\u00a0S4). Thereby, the decrease in the hydrogenating activity of the catalyst appears to be related to the decrease in metal surface area due to Ni particle growth.From the data presented in Figure\u00a013, the sum of compounds visible by the GC technique corresponds to approximately half of the content of species occurring in the liquid products. To expand our analysis toward the heavy species, GPC was performed on the hydrotreated liquid products and residue fractions soluble in methanol. Noteworthy, when applied to product mixtures obtained from lignin, direct information regarding the content of species cannot be retrieved from an ultraviolet-visible (UV-vis) detector (in this study, a photodiode array [PDA] detector), as the detector response is not universal. Furthermore, in samples containing aliphatic hydrocarbons, these compounds will be invisible to the UV-vis detector. Despite these limitations, the GPC technique coupled with UV-vis spectroscopy provides useful information on the apparent distribution of M\nw and spectral signature of the eluting species. Figure\u00a015\n displays the chromatogram traces at a wavelength of 280\u00a0nm.\nFigure\u00a015 shows that the lignin oil substrate encompasses species of apparent M\nw from 100 to 66,000 Da. In the absence of the 15%Ni/Nb2O5 catalyst (control experiment), thermal processes on lignin generate soluble species of M\nw lower than 1,200\u00a0Da for both the product oil and solid residues, at the expense of the heavy species. In the presence of the fresh 15%Ni/Nb2O5 catalyst, both the liquid product and the residue fraction soluble in methanol still contain UV-absorbing species heavier and of much broader apparent M\nw distributions, compared with those from the control experiment. Surprisingly, the subsequent reaction runs yielded liquid products and residues of an apparent M\nw distribution comparable to the apparent M\nw range of products formed in the control experiment.To gain further information about the chemical nature of the UV-absorbing species, the spectral data collected by the PDA detector in the GPC analysis was examined in detail (Figure\u00a016\n). In the samples from the first reaction run, a key feature distinguishing the PDA images is the presence of species absorbing at wavelengths higher than 300\u00a0nm for the residue fraction soluble in methanol (indicated in Figure\u00a016 by a yellow-coded dotted line). In lignin chemistry, this spectral feature is often related to the presence of quinone methide intermediates, stilbene species, and other conjugated unsaturated species associated with lignin condensation processes (Lin, 1992; Schmidt, 2010). Overall these observations suggest that, in the presence of the fresh 15%Ni/Nb2O5 catalyst, the condensation of lignin species could not entirely be suppressed by the reductive processes, as the former process appears to take place at a rate faster than the latter. Interestingly, similar PDA images are found for both liquid products and the residues soluble in methanol from the second and third reaction runs. These images show no strong absorption spot at wavelengths higher than 300\u00a0nm. Altogether, these observations support the hypothesis that Lewis acid sites of Nb2O5 play a role in the condensation of lignin species. As these sites become largely blocked in the first reaction run, the condensation of lignin species should occur to a lesser extent in the subsequent reaction runs. This hypothesis appears to be plausible also considering that the weight of lignin residue accumulated with the catalyst plateaued after the second reaction run (Table 4).This study provided a beginning-to-end analysis of the multifaceted picture of the design of water-tolerant catalysts for the hydrotreating of lignin streams. From the observations of this study, the following conclusions and recommendations for future research are given:\n\n1.\nIn the design of bifunctional Ni/Nb2O5, the incorporation of the Ni phase reduces the population of Br\u00f8nsted acid sites. However, the population of Lewis acid sites remained almost unaltered. The dehydration of cyclohexanol over Br\u00f8nsted acid sites takes place at temperatures lower than those required for the reaction catalyzed by Lewis acid sites. By employing the hydrotreating of diphenyl ether to cyclohexane as a model reaction, it was possible to find a compromise between hydrogenation and dehydration catalyst's capabilities, thus taking the benefit from the catalyst Lewis acidity for the hydrotreatment. The 15%Ni/Nb2O5 catalyst showed sustained results in the recycling experiments. As a result of the high stability of the water-resistant Lewis acid sites, a 91% yield of cyclohexane could be achieved even after five reaction runs.\n\n\n2.\nDespite the promising results achieved in the hydrotreatment of diphenyl ether, the 15%Ni/Nb2O5 catalyst lost its activity toward dehydration of the cyclohexanol species already after the first reaction run performed on the lignin oil stream. This disappointing outcome is associated with the blocking of the Lewis acid sites.\n\n\n3.\nIn the current literature on lignin hydroprocessing, little attention has been given to the fact that the acid sites, needed for the dehydration of cyclohexanol species, can also catalyze the condensation of lignin oligomeric species. In this study, we demonstrated that lignin condensation occurs even under reductive conditions and when beginning the process with passivated streams from the lignin-first biorefining based on reductive processes.\n\n\n4.\nThe condensation of lignin catalyzed by Nb2O5 nanorods' Lewis acid sites appears to be a chemical process faster than the saturation or HDO of lignin species. As a result, in the presence of Ni/Nb2O5 catalysts, lignin condensation is not entirely suppressed by reductive processes. Consequently, carbonaceous matter is formed, blocking the Lewis acid sites.\n\n\n5.\nPrevious studies on hydrotreating of lignin oils in the presence of phosphided Ni/SiO2 catalysts demonstrated that recyclable hydrotreating catalysts could be produced (Cao et\u00a0al., 2018; Samec, 2018). Confronting those results with the current ones, it is concluded that the control of the surface acidity is mandatory for the success of lignin oil hydrotreating. Further research is required to define the type of acidity and a threshold of acidity required for the hydrotreating of lignin while not encouraging acid-catalyzed condensation processes on the lignin oligomeric species. Surprisingly, such a research line has not been receiving much attention from the community. Indeed, often studied model compounds (e.g., diphenyl ether, benzyl phenyl ether, (alkyl)guaiacols, and several others) cannot undergo condensation reactions. Therefore, the crucial role of lignin condensation in hydrotreating processes cannot be mimicked by the current set of model compounds employed in this research field. This fact clearly limits the translation of technologies designed for the HDO of model compounds to the hydrotreating of real-world lignin streams.\n\n\n6.\nThe balance of dehydration and hydrogenation abilities of a heterogeneous catalyst becomes a very complicated issue when considering the significant impact of lignin condensation throughout the hydrotreating process. Considering this, a tentative solution could be the utilization of two solid catalysts, one for hydrogenation and another one for dehydration, so that the balance of these specific tasks could be then more easily adjusted by the weight ratio of each catalyst component in the mixture of catalysts. This idea has been already exploited with success (Wang and Rinaldi, 2013). However, the conversion of a batch reaction process to a continuous flow process based on a mixture of catalysts constitutes a challenging task. A more practical approach appears to be the combination of flow-through reactors operating in series but at different temperatures. This approach could circumvent catalyst deactivation by the formation of coke via lignin condensation, by gradually saturating the lignin stream under conditions of gradual increase in process severity. A similar approach was demonstrated to be very fruitful for the hydrotreatment of pyrolysis oil in the presence of Ni-Cu catalysts (Yin et\u00a0al., 2016).\n\n\nIn the design of bifunctional Ni/Nb2O5, the incorporation of the Ni phase reduces the population of Br\u00f8nsted acid sites. However, the population of Lewis acid sites remained almost unaltered. The dehydration of cyclohexanol over Br\u00f8nsted acid sites takes place at temperatures lower than those required for the reaction catalyzed by Lewis acid sites. By employing the hydrotreating of diphenyl ether to cyclohexane as a model reaction, it was possible to find a compromise between hydrogenation and dehydration catalyst's capabilities, thus taking the benefit from the catalyst Lewis acidity for the hydrotreatment. The 15%Ni/Nb2O5 catalyst showed sustained results in the recycling experiments. As a result of the high stability of the water-resistant Lewis acid sites, a 91% yield of cyclohexane could be achieved even after five reaction runs.Despite the promising results achieved in the hydrotreatment of diphenyl ether, the 15%Ni/Nb2O5 catalyst lost its activity toward dehydration of the cyclohexanol species already after the first reaction run performed on the lignin oil stream. This disappointing outcome is associated with the blocking of the Lewis acid sites.In the current literature on lignin hydroprocessing, little attention has been given to the fact that the acid sites, needed for the dehydration of cyclohexanol species, can also catalyze the condensation of lignin oligomeric species. In this study, we demonstrated that lignin condensation occurs even under reductive conditions and when beginning the process with passivated streams from the lignin-first biorefining based on reductive processes.The condensation of lignin catalyzed by Nb2O5 nanorods' Lewis acid sites appears to be a chemical process faster than the saturation or HDO of lignin species. As a result, in the presence of Ni/Nb2O5 catalysts, lignin condensation is not entirely suppressed by reductive processes. Consequently, carbonaceous matter is formed, blocking the Lewis acid sites.Previous studies on hydrotreating of lignin oils in the presence of phosphided Ni/SiO2 catalysts demonstrated that recyclable hydrotreating catalysts could be produced (Cao et\u00a0al., 2018; Samec, 2018). Confronting those results with the current ones, it is concluded that the control of the surface acidity is mandatory for the success of lignin oil hydrotreating. Further research is required to define the type of acidity and a threshold of acidity required for the hydrotreating of lignin while not encouraging acid-catalyzed condensation processes on the lignin oligomeric species. Surprisingly, such a research line has not been receiving much attention from the community. Indeed, often studied model compounds (e.g., diphenyl ether, benzyl phenyl ether, (alkyl)guaiacols, and several others) cannot undergo condensation reactions. Therefore, the crucial role of lignin condensation in hydrotreating processes cannot be mimicked by the current set of model compounds employed in this research field. This fact clearly limits the translation of technologies designed for the HDO of model compounds to the hydrotreating of real-world lignin streams.The balance of dehydration and hydrogenation abilities of a heterogeneous catalyst becomes a very complicated issue when considering the significant impact of lignin condensation throughout the hydrotreating process. Considering this, a tentative solution could be the utilization of two solid catalysts, one for hydrogenation and another one for dehydration, so that the balance of these specific tasks could be then more easily adjusted by the weight ratio of each catalyst component in the mixture of catalysts. This idea has been already exploited with success (Wang and Rinaldi, 2013). However, the conversion of a batch reaction process to a continuous flow process based on a mixture of catalysts constitutes a challenging task. A more practical approach appears to be the combination of flow-through reactors operating in series but at different temperatures. This approach could circumvent catalyst deactivation by the formation of coke via lignin condensation, by gradually saturating the lignin stream under conditions of gradual increase in process severity. A similar approach was demonstrated to be very fruitful for the hydrotreatment of pyrolysis oil in the presence of Ni-Cu catalysts (Yin et\u00a0al., 2016).In a broader context, this work provides substantial evidence that the use of model reactions has severe limitations for the design of catalysts for the hydroprocessing of lignin streams. Accordingly, the catalyst screening carried out on real-world lignin streams is a more productive enterprise to pursue, regardless of the complexity of the product mixtures obtained. In this quest, the evaluation of H/C and O/C ratios as the response variables (either for the catalyst discovery or in recycling experiments) constitutes a strategy effective in the simplification of characterization procedures applied to the lignin products. Such a strategy should become a gold standard in the high-throughput screening catalysts for the hydroprocessing of lignin streams to produce drop-in lignin biofuels, as it allows for the direct comparison of catalyst performance without the need of scrutinizing the lignin product compositions at an early stage of technology-readiness levels (TRL), thus contributing to accelerating catalyst discovery.The hydrothermal synthesis of Nb2O5 is based on the decomposition of niobium peroxo species formed by the reaction of ammonium niobium(V) oxalate and hydrogen peroxide. CAUTION: As the hydrothermal synthesis is performed in a closed stainless-steel vessel, it is mandatory to check if the vessel is rated to operate under the pressure built by the decomposition of the full content of hydrogen peroxide employed in the synthesis.All methods can be found in the accompanying Transparent Methods supplemental file.R.R. acknowledges the financial support provided by the ERC Consolidator Grant LIGNINFIRST (Project Number: 725762). R.R. and A.A.S.C. thank FAPESP for the support provided (Process Number: 2016/50423-3). The authors are grateful to LNLS/CNPEM for the infrastructure (XPD beamline and chemistry laboratory), LNNano for the STEM infrastructure, the GPMMM laboratory (IQ-UNICAMP) for the quantitative FTIR of adsorbed pyridine analysis, CNPq for the PhD scholarship (Process Number: 165106/2014-0), and CAPES for the PDSE scholarship (Process Number: 88881.132245/2016-01). This study was financed in part by the Coordena\u00e7\u00e3o de Aperfei\u00e7oamento de Pessoal de N\u00edvel Superior - Brasil (CAPES) - Finance Code 001. Finally, the authors are thankful to CBMM for the ammonium niobium oxalate hydrate samples.Conceptualization, G.F.L, A.A.S.C., and R.R.; Methodology, G.F.L. and R.R.; Investigation, G.F.L., I.G., S.L., H.C., D.H.B., A.A.S.C., C.B.R., E.T.-N., and R.R.; Writing \u2013 Original Draft, G.F.L.; Writing \u2013 Review & Editing, G.F.L., C.B.R, A.A.S.C., and R.R.; Funding Acquisition, C.B.R., A.A.S.C., and R.R.; Resources, C.B.R., A.A.S.C., and R.R; Supervision, C.B.R., A.A.S.C., and R.R.The authors have no conflict of interests to declare.Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2019.05.007.\n\n\nDocument S1. Transparent Methods, Figures S1\u2013S4, and Tables S1\u2013S4\n\n\n\n", "descript": "\n                  In biomass conversion, Nb2O5 has attracted increasing attention as a catalyst support presenting water-tolerant Lewis acid sites. Herein, we address the design of Ni/Nb2O5 catalysts for hydrotreating of lignin to hydrocarbons. To optimize the balance between acidic and hydrogenating properties, the catalysts were first evaluated in the hydrotreating of diphenyl ether. The best catalyst candidate was further explored in the conversion of lignin oil obtained by catalytic upstream biorefining of poplar. As primary products, cycloalkanes were obtained, demonstrating the potential of Ni/Nb2O5 catalysts for the lignin-to-fuels route. However, the Lewis acidity of Nb2O5 also catalyzes coke formation via lignin species condensation. Thereby, an acidity threshold should be found so that dehydration reactions essential to the hydrotreatment are not affected, but the condensation of lignin species prevented. This article provides a critical \u201cbeginning-to-end\u201d analysis of aspects crucial to the catalyst design to produce lignin biofuels.\n               "}
{"full_text": "The increasing estimates of CO2 emissions, at a rate of 33 GT/year, a concentration forecast of 570\u00a0ppm by the end of the 21st century, and the serious consequences of climate change, as numerous natural disasters (heat waves, hurricanes, wildfires, droughts, sea level rise), are some of the most pressing problems for humanity. In this scenario, a deep transition period towards a zero-emissions energy model, based on the increasing utilization of renewable energy sources, may be expected. The taxes to the countries for CO2 emissions [1] and the economic consequences of climate change (valued at a loss of 31 billion-dollar in 2017 [2]) are also an incentive to take measures aimed at reducing the net emissions of CO2.The technologies for CO2 capture and storage/sequestration (CCS) have received extensive attention [3]. The physical absorption (where CO2 is scrubbed from the flue gas) is common for high CO2 partial pressure. It is carried out at low temperature, with low energy requirement and it is favored using commercial solvents. Using chemical absorption, CO2 present in low concentration can be separated reacting with alkanolamines or dissolved alkaline salts. Among the former, MEA (monoethanolamine), DEA (diethanolamine), TEA (triethanolamine), MDEA (methyldiethanolamine), DIPA (diisopropanolamine) and DGA (diglycolamine) are used. KOH is commonly used as alkaline reactant. Separating CO2 using membranes requires lower capital cost and the equipment occupies smaller space. The membranes used are prepared with different materials: zeolites, carbon nanotubes (CNT), polyamides, polyether sulfone or polydimethyl phenylene oxide, among others [4].Adsorption is effective for low CO2 concentrations using zeolites, silica based-materials (microporous as SAPO-34 or mesoporous as MCM-41 or SBA-15), activated carbon, graphene, metal-organic frameworks (MOFs), lithium orthosilizate (Li4SiO4), lithium zirconate (Li2ZrO3) and other porous materials as adsorbents. For chemical adsorption, materials (mainly carbons) functionalized by polymeric amines (polyethylenimine, polypropylenimine, polyallylamine, polyaniline, amino dendrimers, and hyperbranched polyamines) are used [5,6]. In general, the capture capacity is higher for adsorption than for absorption (88\u2013176\u00a0kg of CO2 per kg of adsorbent, and 0.4\u20131.2\u00a0kg of CO2 per kg of absorbent, respectively). Cryogenic distillation produces high purity liquid CO2, and is an interesting method to treat gas at high pressure. However, the cost of this technology is high, due to the energy requirement for refrigeration. Electrochemical technology is another emerging alternative for CO2 capture from mediums of different concentration. The characteristics of this technology and the development state have been explained by Sharifian et al. [7]. Using the \u201cpH swing\u201d concept, CO2 can be captured and recovered, which facilitates its subsequent online valorization. Given the higher cost of this technology over others commonly used (and in particular with respect to absorption with amines), its economic viability requires using renewable energies and developing low-cost membranes.CCS technologies for stationary sources contemplate the [8]: i) Direct air capture (DAC) for CO2 removal from small sources and from the transport sector, responsible of 1/3 to 1/2 of total emissions, and; ii) moving to remote sites for large-scale CO2 sequestration. To finance the expensive investments required by these technologies, it is essential to promote CO2 upgrading generating an economic benefit. Among the CO2 utilization technologies [9\u201324], two objectives are distinguished, the direct use (pure or in solution), and its use as feedstock for the production of chemicals and fuels (use after transformation). The direct use of CO2 for carbonated drinks is associated to the origin of the commercialization of soft drinks. It is also used as fire extinguisher, refrigerant, anesthetic gas, dry ice, solvent, process fluid and welding medium. Other routes directly using CO2 on a larger scale comprise methods for the extraction of mineral sources: EOR (enhanced oil recovery), ECBM (enhanced coal-bed methane) recovery, and EGR (enhanced gas recovery). The use of CO2 in micro-algae cultivation along with free sunlight has the advantage of operating at mild conditions, but requires controlling the pH (in the 6.6\u201310.5 range) and a sealed reactor.The transformation of CO2 into chemicals and fuels is difficult, given the thermodynamic stability of the molecule due to its structure, constituted by a carbon atom with its four electrons bonded to oxygen atoms through covalent double bonds (O=C=O). Moreover, the Gibbs free energy of CO2 (\u2206G0\u00a0=\u00a0\u2212394\u00a0kJ\u00a0mol\u22121) is much lower than that of the possible products of its transformation. Consequently, the challenges of the processes for this transformation are very demanding. Among them [11]: i) Great energy supply from renewable and carbon-neutral sources; ii) the use of high temperature and/or pressure, or; iii) the intervention of catalysts active sites, organisms or biological species capable for activating the reactions involved. In Fig. 1\n different routes for the transformation of CO2 are gathered.The processes for CO2 transformation through chemical and electrochemical reactions have multiple technological alternatives. As to the electrochemical reduction regards, two possible routes are distinguished [25], with CO2 as intermediate to produce formic acid, or CO and hydrocarbons (mainly methane). Jiang et al. [26] have summarized the recent advances in understanding the reaction mechanism and exploring cathode materials. The external energy source in these processes can be thermal, electrocatalytic or photocatalytic, providing the opportunity to these processes to be integrated with renewable energy production (solar, wind and marine). In the artificial photosynthesis strategy, semiconductor catalysts convert CO2 into hydrocarbons with solar energy through a multielectron transfer mechanism. In this mechanism, TiO2 (commonly used as catalyst) absorbs light upon illumination and generates a pair of photo-excited electrons and holes. These initiators interact with H2O and CO2 molecules to produce methane and other products by selecting appropriate catalyst (usually prepared by doping TiO2) and conditions [27].However, the chemical reactions occur at a high rate and are carried out in an easier-to-scale reactor. Some authors classify the chemical transformation pathways according to their energy requirements [13]. Kamkeng et al. [11] make a comparison of the CO2 transformation routes according to different criteria (technological maturity, cost considerations, market analysis and amount of CO2 used). Taking into account the technological readiness level (TRL) tool (Fig. 2\n), synthesis of methane and methanol have high TRL values (7\u20139). The main advantages of hydrogenation processes focus on the market interest of CO2-derived fuels and raw materials (gasoline, methanol, DME, methane, olefins, aromatics), and on the amount of CO2 used in their production (2.6\u00a0t fuel/t CO2 in Fischer-Tropsch synthesis). Nonetheless, in terms of cost per ton of product, the interest of these processes is conditioned by the price of H2.The different catalytic and electrocatalytic processes for CO2 conversion into fuels and chemical products have been reviewed several times [10,13,28\u201331], and these are schematized in Fig. 3\n. It can be observed that some products are, at the same time, raw materials for other processes. That is, oxygenates (methanol and DME) with interest as fuels, are converted into olefins (MTO and DTO processes, respectively) [32,33], into hydrocarbons in the gasoline range (MTG and DTG processes, respectively) [34,35], or in BTX aromatics [36]. These reactions proceed according to the dual cycle mechanism, with arenes and olefins as intermediates [37], and the product distribution is dependent on the acidity and shape selectivity of the catalyst (based on SAPO-34 in the MTO process and based on HZSM-5 zeolite in the other processes). Besides, methanol and DME are hydrogen vectors (through reforming) [38,39]. Methanol (MeOH) can also be selectively dehydrogenated towards formaldehyde [40], which will be used in polymers and resin production.Furthermore, CO2 allows for the production of synthesis gas (H2/CO) through the reverse Water-Gas-Shift (rWGS) reaction (where CO2 takes the role of H2 acceptor) [41] or by dry reforming of methane, hydrocarbons or oxygenates (where CO2 acts as oxidant agent) [42]. In addition, synthesis gas or CO2 directly can be converted into a hydrocarbons mixture, either through the Fischer-Tropsch (FT) route [43] or with MeOH/DME as intermediates, over bifunctional catalysts [44,45]. These reactions can be controlled by choosing selective acidic functions for the production of C2+ alcohols, isoparaffinic gasoline or aromatics. From the energy requirement point of view, the reactions in which the second reactant has a higher Gibbs free energy have lower energy requirement and so, are more favorable. However, CO2 hydrogenation reactions require a large amount of external energy and the use of a catalyst to overcome the activation barrier. According to this classification, already suggested by De et al. [46], the characteristics of the CO2 conversion processes are described in the next sections, distinguishing those not requiring H2 as reactant (Section 2.1) and hydrogenation processes (Section 2.2).These reactions are of greater interest in an energy transition state like the current one, prior to the availability of H2 produced from sustainable sources and using renewable energies.The direct conversion of methane into ethane (Eq. (1)) or into ethylene (Eq. (2)), through oxidative coupling (OCM) forming C\u2010C bonds, has a growing interest in valorizing burgeoning natural gas reserves, in which CO2 content may reach 10%.\n\n(1)\n\n2\n\nCH\n4\n\n+\n\nCO\n2\n\n\u2192\n\nCH\n3\n\n\nCH\n3\n\n+\nCO\n+\n\nH\n2\n\nO\n\n\n\n\n\n(2)\n\n2\n\nCH\n4\n\n+\n2\n\nCO\n2\n\n\u2192\n\nCH\n2\n\n\nCH\n2\n\n+\n2\nCO\n+\n2\n\nH\n2\n\nO\n\n\n\nThese reactions occur through the following mechanism [47]: 1) Cleavage of methane C\u2010H bonds in the active sites of the catalyst, forming CH3* and CH2* radicals; 2) dissociation of CO2 towards CO and O* active oxygen; 3) coupling of these radicals; 4) recombination of CH3* and CH2* radicals; 5) dehydrogenation, either oxidative or radical, of ethane to ethylene. The catalysts must be selective, avoiding the formation of syngas by dry reforming. The strong basic metallic oxide catalysts used can be grouped into [19,48]: 1) Pure oxides of the lanthanide series, of which La2O3 shows the greatest performance; 2) basic oxides loaded with Group 1 or 2 cations (Li/MgO, Ba/MgO, and Sr/La2O3); 3) transition metal oxides containing Group 1 cations, and; 4) redox catalysts, like CeO2 modified by Group 1 and 2 cations. Over ZrO2/TiO2 catalysts acetic acid is formed by the insertion of the adsorbed CO2 into the CH3* species, followed by the hydrogenation with H* in the adsorption of methane [49].The production of light olefins through oxidative dehydrogenation of their corresponding paraffins (ODP) (Eq. (3)) is an upgrade. In this manner, raw materials are obtained for the production of polyolefins and, at the same time, the high-energy requirement of steam cracking, as well as the rapid deactivation of the catalyst due to coke deposition (attenuated by the gasification capacity of CO2) are avoided.\n\n(3)\n\n\nC\nn\n\n\nH\n\n2\nn\n+\n2\n\n\n+\n\nCO\n2\n\n\u2192\n\nC\nn\n\n\nH\n\n2\nn\n\n\n+\nCO\n+\n\nH\n2\n\nO\n\n\n\nThe most studied catalysts for ODP are based on redox properties, principally MoO3, Cr2O3 and V2O5. CeO2 (with well-established redox properties), ZrO2, TiO2, SiO2 and zeolites (HZSM-5, MCM-41) have been used as supports, since the mesoporosity of the latter is known to favor the dispersion of metallic oxides [50]. The basic character of these catalysts favors CO2 adsorption and olefins desorption, while paraffins dehydrogenation is activated by the presence of the acidic sites.ODP mechanism [51] considers the rWGS (Eq. (5)) reaction, where H2, product of the dehydrogenation, is oxidized by CO2. Furthermore, paraffin dry reforming (Eq. (6)) and coke deposits oxidation through reverse Boudouard reaction (Eq. (7)) are considered.\n\n(4)\n\n\nC\nn\n\n\nH\n\n2\nn\n+\n2\n\n\n\u21cc\n\nC\nn\n\n\nH\n\n2\nn\n\n\n\n+\n\n\nH\n2\n\n\n\n\n\n\n(5)\n\n\nH\n2\n\n+\n\nCO\n2\n\n\u21cc\nCO\n+\n\nH\n2\n\nO\n\n\n\n\n\n(6)\n\n\nC\nn\n\n\nH\n\n2\nn\n+\n2\n\n\n+\n\nnCO\n2\n\n\u2192\n2\nnCO\n+\n\n\nn\n+\n1\n\n\n\nH\n2\n\n\n\n\n\n\n(7)\n\n\nCO\n2\n\n+\nC\n\u21cc\n2\nCO\n\n\n\nThe thermodynamic analysis of the CO2 assisted dehydrogenation of ethane (ODE) shows the need for reaction temperatures above 550\u00a0\u00b0C for a good compromise between the extent of the hydrogenation and rWGS reactions according to Najari et al. [52]. These authors review the advances in this reaction, comparing the behavior of the most used catalysts. The catalysts are based on Ni, Ni\u2010Fe, Cr2O3, Ga2O3, or CoOx, and use acidic supports as \u03b3-Al2O3, SiO2, CeO2, ZrO2, TiO2, SBA and zeolites (being HZSM-5 the most common).Jiang et al. [53] classify the catalysts for CO2 assisted dehydrogenation of propane (ODP) according to the nature of their metallic function, distinguishing: i) Redox-type catalysts (those based on CrOx are the most used ones). The redox cycle is described in Eqs. (8)\u2013(10) [54]; ii) non-redox type catalysts (Ga2O3 polymorphs, Ga2O3-Al2O3 solid solutions and mixed GaO2-ZrO2), and iii) other transition metal catalysts (Fe2O3, Fe\u2010Ni, Mo2C). As supports, the afore-mentioned ones for ODE, mesoporous zeolites (such as MCM-41) and activate carbons have been tested.\n\n(8)\n\n\nC\n3\n\n\nH\n8\n\n+\n\nCrO\nx\n\n\u21cc\n\nCrO\n\nx\n\u2212\n1\n\n\n+\n\nC\n3\n\n\nH\n6\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n(9)\n\n\nCO\n2\n\n+\n\nCrO\n\nx\n\u2212\n1\n\n\n\u21cc\nCO\n+\n\nCrO\nx\n\n\n\n\n\n\n(10)\n\n\nH\n2\n\n+\n\nCrO\nx\n\n\u21cc\n\nCr\n\nx\n\u2212\n1\n\n\n+\n\nH\n2\n\nO\n\n\n\nIt should be pointed out that the dehydrogenation of C5+ paraffins is not viable due to the fast catalyst deactivation by coke deposition.The oxidative dehydrogenation of ethylbenzene (ODE) to styrene is of great interest to avoid selectivity limitations and catalyst deactivation by coke in the conventional industrial process without oxidant agent, which require an excess of vapor. ODE with CO2 as dehydrogenating agent, with the steps described in Eqs. (11), (12) and (5), results in a styrene selectivity of 97% and its energy demand is of approximately a tenth of that of the conventional process. Therefore, it offers an attractive option for satisfying the growing demand of styrene (yearly production of 14.6 Mt) in the production of synthetic rubber, polystyrene and styrene-acrylonitrile copolymers.\n\n(11)\n\n\nC\n6\n\n\nH\n5\n\n\nCH\n2\n\n\nCH\n3\n\n+\n\nCO\n2\n\n\u2192\n\nC\n6\n\n\nH\n5\n\nCH\n=\n\nCH\n2\n\n+\n\nH\n2\n\nO\n+\nCO\n\n\n\n\n\n(12)\n\n\nC\n6\n\n\nH\n5\n\n\nCH\n2\n\n\nCH\n3\n\n\u2192\n\nC\n6\n\n\nH\n5\n\nCH\n=\n\nCH\n2\n\n+\n\nH\n2\n\n\n\n\nFe2O3/CeO2 catalyst has a high activity attributable to the redox activity of the Ce sites (changing Ce4+ and Ce3+), promoted by Fe3+ and whose presence improves the oxygen storage capacity of Ce [55]. The relevance of both the redox efficiency and the mesoporous structure of the support has been proven by Burri et al. [56] using CeO2-ZrO2 supported on SBA-15. VOx, MoOx, WOx, CrOx-based catalysts have also been studied, either supported on SiO2, mesoporous zeolite (MCM-41) or active carbon incorporated in hydrotalcite (Mg-V-Al structures) [57]. With the latter, Sakurai et al. [58] obtained an ethylbenzene (EB) conversion of 67.1% and styrene selectivity of 80%. Different mechanisms have been proposed for ODE from EB, considering the three step mechanism the most favorable thermodynamically [59]:\n\n(13)\n\n\nC\n8\n\n\nH\n10\n\n+\nos\n\u21cc\n\nC\n8\n\n\nH\n10\n\n\u2212\nos\n\n\n\n\n\n(14)\n\n\nC\n8\n\n\nH\n10\n\n\u2212\nos\n+\nrs\n\u2192\n\nC\n8\n\n\nH\n8\n\u2219\n\n+\n2\nH\n\u2212\nos\n\n\n\n\n\n(15)\n\nH\n\u2212\nos\n+\nH\n\u2212\nos\n\u2192\n\nH\n2\n\n+\n2\nos\n\n\n\n\n\n(16)\n\n\nCO\n\n2\n\n\ng\n\n\n\n+\nrs\n\u21cc\n\nCO\n\ng\n\n\n+\nO\n\u2212\nrs\n\n\n\n\n\n(17)\n\n\nH\n\n2\n\n\ng\n\n\n\n+\nO\n\u2212\nrs\n\u21cc\n\nH\n2\n\n\nO\n\ng\n\n\n+\nrs\n\n\n\n\n\n(18)\n\n\nC\n8\n\n\nH\n8\n\n\u2212\nrs\n\u2192\n\nC\n8\n\n\nH\n8\n\n+\nrs\n\n\nwhere \u201cos\u201d refers to the oxidizing sites and \u201crs\u201d to the reducing sites.Beyond CO2 transformation, the lower energy requirement of dry reforming than that of steam reforming is a remarkable advantage, although H2 yield and the resulting H2/CO ratio are lower. Its application has extended to the conversion of fossil sources (methane) and sources derived from biomass (as ethanol, glycerol and bio-oil).Methane dry reforming (MDR, Eq. (19)) is the principal route for the current production of H2. Although CH4 is a fossil source, the process has good future prospects for biogas feedstocks (with CH4 and CO2 as major components) derived from the anaerobic digestion of organic waste materials [60].\n\n(19)\n\nC\n\nH\n4\n\n+\nC\n\nO\n2\n\n\u2192\n2\n\nH\n2\n\n+\n2\nCO\n\n\n\nThe reaction steps of MDR on the catalyst surface involve:\n\n1.\nMethane adsorption and abstraction of hydrogen:\n\n\n\n\n(20)\n\nC\n\nH\n4\n\n\u2192\n\nCH\n4\n\u2217\n\n\u2192\n\nCH\n\n4\n\u2212\nx\n\n\u2217\n\n+\n\u03c7\n\nH\n\u2217\n\n\n\n\n\n\n2.\nCO2 adsorption and abstraction of an oxygen atom:\n\n\n\n\n(21)\n\nC\n\nO\n2\n\n\u2192\n\nCO\n2\n\u2217\n\n\u2192\n\nCO\n\u2217\n\n+\n\nO\n\u2217\n\n\n\n\n\n\n3.\nThe formation of CO and hydrogen on the surface:\n\n\n\n\n(22)\n\n\nCH\n\u2217\n\n+\n\nO\n\u2217\n\n\u2192\n\nCO\n\u2217\n\n+\n\nH\n\u2217\n\n\n\n\n\n\n4.\nThe formation of H2O:\n\n\n\n\n(23)\n\nO\n+\n\nH\n\u2217\n\n\u2192\n\nOH\n\u2217\n\n+\n\nH\n\u2217\n\n\u2192\n\nH\n2\n\n\nO\n\u2217\n\n\u2192\n\nH\n2\n\nO\n\n\n\n\n\n5.\nThe recombination of hydrogen on the surface and desorption:\n\n\n\n\n(24)\n\n\nH\n\u2217\n\n+\n\nH\n\u2217\n\n\u2192\n\nH\n2\n\u2217\n\n\u2192\n\nH\n2\n\n\n\n\nMethane adsorption and abstraction of hydrogen:CO2 adsorption and abstraction of an oxygen atom:The formation of CO and hydrogen on the surface:The formation of H2O:The recombination of hydrogen on the surface and desorption:In addition, the WGS reaction (Eq. (5)), the coke formation reactions by decomposition of CH4 (Eq. (25)) and formation/gasification of coke by the Boudouard reaction (Eq. (7)) take place.\n\n(25)\n\n\nCH\n4\n\n\u2192\nC\n+\n\n2H\n2\n\n\n\n\nThe main limitations of MDR are the high-energy requirement (even being lower than for steam reforming, MSR) (heats of 247\u00a0kJ\u00a0mol\u22121 and 228\u00a0kJ\u00a0mol\u22121, respectively), since temperature above 800\u00a0\u00b0C is required; and catalyst stability, affected by sintering and coke formation. The energy demand is reduced and coke formation is attenuated by combining MDR with MSR and POM (partial oxidation of methane). For that purpose, according to the tri-reforming concept, methane is co-fed with H2O and O2 [61]. Li et al. [62] have made a review on the advances on the technologies for heat supply, alternative to fossil fuels, including photochemical and electrochemical, plasma-assisted, solar energy, operating in solid oxide fuel cells, coupled with inorganic membranes and chemical looping reforming.Noble metal and transition metal based-catalysts have been exhaustively studied [63\u201367]. According to activity they can be ordered as follows [68]: Ru\u00a0\u2248\u00a0Rh\u00a0>\u00a0Ni\u00a0\u2248\u00a0Ir\u00a0>\u00a0Pt\u00a0>\u00a0Pd\u00a0>\u00a0Co. Ni catalysts are generally used regarding their high activity and low cost. Anyhow, sintering and coke formation is quite fast for these catalysts. A great deal of effort has been addressed to improve the stability of Ni catalysts in MDR. Thus, various strategies have been used for attenuating sintering through strengthening the metal-support interactions: forming bimetallic catalysts, where metal is dispersed in nanoparticles [69], incorporated within perovskites [68] or with spinel and core-shell configurations [62]. The stability of Ni catalysts has also been upgraded incorporating in the support (Al2O3, SiO2) basic promoters as alkaline metals (Li, Na, K), rare earth metal oxides (La2O3, CeO2, Y2O3, Sm2O3) and reducible transition metal oxides (ZrO2, TiO2, MnO2, MoO3). These materials promote Ni dispersion, metal-support interaction, oxygen mobility and CO2 and H2O adsorption, attenuating coke formation [70\u201372]. A strategy to avoid catalyst deactivation by coke in the dry reforming of methane is to carry out the reaction without catalyst, with acetylene as intermediate. However, very high temperature is required for this approach (1400\u20131800\u00a0\u00b0C) [73].Although the stoichiometry of ethanol dry reforming (EDR) corresponds to Eq. (26), in parallel, the steam reforming (ESR) reaction will also take place, because the H2O content in the ethanol (bio-ethanol) obtained from hydrolysis/fermentation of biomass is remarkable. This coexistence of dry and steam reforming also occurs for other bio-alcohols (such as butanol) and biomass-derived oxygenates, such as glycerol and oxygenates in bio-oil (product of the fast pyrolysis of biomass), whose stoichiometry of dry reforming ideally corresponds to Eqs. (27) and (28) respectively. Furthermore, all these bio\u2011oxygenates undergo decomposition and dehydrogenation reactions, which require a catalyst and suitable reaction conditions to reform the by-products (CH4, olefins and aldehydes) and to minimize the formation of coke.\n\n(26)\n\n\nC\n2\n\n\nH\n5\n\nOH\n+\n\nCO\n2\n\n\u2192\n3\nCO\n+\n3\n\nH\n2\n\n\n\n\n\n\n(27)\n\n\nC\n3\n\n\nH\n8\n\n\nO\n3\n\n+\n\nCO\n2\n\n\u2192\n4\nCO\n+\n3\n\nH\n2\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n(28)\n\n\nC\nx\n\n\nH\ny\n\n\nO\nz\n\n+\n\nCO\n2\n\n\u2192\nCO\n+\n\nH\n2\n\n\n\n\nThese reactions have received less attention than bio\u2011oxygenates steam reforming and the main challenge has been achieving catalyst stability [74]. In EDR catalysts based on noble and transition metals have been studied. Da Silva et al. [75] propose a mechanism for the Rh/CeO2 catalyst involving the role of oxygen vacancies in the CeO2. As to attenuate coke deactivation high values of temperature (around 1073\u00a0K) and an ethanol/CO2 ratio (around 3) are required [76]. The combination of SBA-15 (with high specific surface) with CeO2 (redox capacity) in the support improves the activity of the catalyst [77]. Comparing different supports, Drif et al. [78] determined the following activity: Rh/NiO-Al2O3 >\u00a0>\u00a0Rh/Al2O3\u00a0\u2248\u00a0Rh/MgO-Al2O3\u00a0\u2248\u00a0Rh/CeO2-Al2O3\u00a0>\u00a0Rh/ZrO2-Al2O3\u00a0\u2248\u00a0Rh/La2O3-Al2O3 at 1073\u00a0K. The high activity of Rh/NiO-Al2O3 was attributed to the smaller Rh particle size and to the presence of NiAl2O4 spinel phase, which limited the migration of Rh in Al2O3. Ir/CeO2 catalyst has also shown a good behavior in the EDR reaction at 973\u00a0K, with the complete elimination of coke formation on the catalyst [79].Ni-based catalysts are also very active, according to CO2 conversions following the order: Ni/CeO2\u00a0\u2248\u00a0Ni/Al2O3\u00a0>\u00a0Ni/MgO\u00a0\u2248\u00a0Ni/ZrO2. To attenuate sintering and coke deactivation, the interest of incorporating Co and promoters with redox capacity has been assessed [80,81]. The activity of Cu and Co as primary catalysts and the effect of promoters with redox capacity for enhancing their stability has also been studied [82,83].The studies on glycerol dry reforming (GDR) are focused on Ni-based catalysts, with particular emphasis on the influence of types of supports and promoters. As CO2 and glycerol are adsorbed at different sites of the bifunctional catalyst, the reaction is controlled by the glycerol adsorption step. The complex mechanism of glycerol conversion explains the fast deactivation by coke, whose precursors are the by-products of the reaction (CO, CH4, aldehydes, hydrocarbons). To attenuate coke deactivation, limiting the acidity of the support is essential. Thus, \u03b3-Al2O3 catalyst is very active, but undergoes fast deactivation mainly due to the deposition of whisker type of carbon on the catalyst surface [84]. The deposition of La2O3 on the Al2O3 support prior to Ni, increases Ni dispersion and attenuates coke formation [85]. Several attempts have been made to optimize the Ni-based catalysts for higher activity and stability. Among these the use of CaO [86] or SiO2 [87] or ternary oxides (Al2O3\u2013ZrO2\u2013TiO2) [74] as supports, the addition of Re to the catalyst [88] or Ag as promoter [87].Precious metal (Rh, Ru, Ir, Pd and Pt) catalysts with MgO stabilized Al2O3 as support were also tested for their activity towards GDR by Tavanarad et al. [89]. It should be noted, that after the fast initial deactivation due to whisker carbon, these catalysts maintain a pseudosteady state.Acetic acid production is an example of an opportunity to valorize low cost reactants like CO2 and CH4. The production of benzoic acid from CO2 and benzene is equally interesting. Furthermore, acrylic acid production through the direct carboxylation of ethylene with CO2 on Ni catalysts (Eq. (29)) is of great interest. This reaction is particularly interesting for valorizing CO2 generated in the ethylene production units by steam cracking of naphthas [90].\n\n(29)\n\n\n\nHere, CO2 is a raw material for the production of linear and cyclic carbonates. Among the first ones, dimethyl carbonate (DMC) (CH3O)2CO, with low toxicity, is used as solvent, gasoline additive and reactant in alkylation and acylation reactions. It is produced by reacting with methanol (Eq. (30)). Several catalysts have been reported for this reaction (based on Cu and Cu\u2010Ni, and on CeO2) [91\u201396].\n\n(30)\n\n\n\nCyclic carbonates (of ethylene, propylene, cyclohexane, styrene and others) are produced by the addition of CO2 to an epoxy (Eq. (31)). They are used as solvents, electrolytes and raw material in the production of poly\u2011carbonates, other polymeric materials and fine chemicals (dialkyl carbonates, glycols, carbamates, pyrimidines, etc.). The formation reactions are catalyzed by alkali metal halides, metal oxides, zeolites and organic bases [97].\n\n(31)\n\n\n\nAcetylsalicylic synthesis (CH3COOC6H4COOH) is an example of the insertion capacity of CO2 in the C\u2010H bonds of alkenes, aromatics or olefins. The products of greatest interest are carbonic acids, esters, lactones, and heterocyclic; in other words, compounds with functional groups potentially applicable as solvents, plasticizers, detergents, antioxidants, sun-protection agents, etc. [98].CO2 is valorized in the NH3 production industry itself for the synthesis of urea (carbamide, (NH2)2CO). This consists of the carbamate (H2N-COONH4) (Eq. (32)) formation reaction and further dehydration towards urea (Eq. (33)). Xiang et al. [99] reach a CO2 conversion up to 82.16% at atmospheric pressure and 20\u00a0\u00b0C. According to the stoichiometry, to obtain 1\u00a0t of urea 0.75\u00a0t of CO2 are required. Nevertheless, urea is principally used as fertilizer, with the role of releasing NH3 (adsorbed by plants) and CO2. Therefore, this route would not diminish CO2 emissions. Urea production at room temperature has been studied by means of electrochemical synthesis by coupling CO2 and N2 in H2O using PdCu/TiO2 electrocatalyst [100].\n\n(32)\n\n2\n\nNH\n3\n\n+\n\nCO\n2\n\n\u21cc\n\nH\n2\n\nN\n\u2212\nCOO\n\nNH\n4\n\n\n\n\n\n\n(33)\n\n\nH\n2\n\nN\n\u2212\nCOO\n\nNH\n4\n\n\u21cc\n\n\n\nNH\n2\n\n\n2\n\nCO\n+\n\nH\n2\n\nO\n\n\n\nOther polymers, like aliphatic polycarbonates, are produced by the reaction of CO2 with epoxides or through transesterification of diols with DMC. They are substitutes of polyethers for the fabrication of polyurethane (formed by urethane bonds, \u2212N\u00a0\u2212\u00a0(C\u00a0=\u00a0O)\u00a0\u2212\u00a0O\u2212) [101]. In the same way, by the reaction of CO2 with epoxides, aromatic polycarbonates based on bisphenol can be synthesized. Polyoxymethylene is another polycondensation polymer that can be produced from CO2 and 1,3,5-trioxane (in this case with formic acid as intermediate). Although polyoxymethylene incurs a higher cost than poly- ethylene and propylene, it provides a higher mechanical resistance. Moreover, using another intermediate (such as methanol) CO2 can be applied in the large scale production of polymethyl-methacrylate (PMMA).In different reviews the main advances conducted in these routes are collected [102\u2013106]. The scheme in Fig. 4\n (reproduced from [106], adapted from [107,108]) includes the main routes, which according to the products can be classified as: routes with C1 compounds as products (methane, carbon monoxide, methanol, formaldehyde); and those that form compounds with 2 or more carbon atoms (hydrocarbons and oxygenates). The mechanisms for these routes are significantly different, and, consequently, have been studied under different process conditions and with different catalysts.As aforementioned, the hydrogenation routes in Fig. 4 require external energy supply and the use of catalysts, due to unfavorable thermodynamics. In Table 1\n the standard enthalpy and Gibbs free energies values of different CO2 hydrogenation reactions are listed (values taken from [46,109]). The role of the conditions (pressure, temperature, H2/CO2 ratio) on thermodynamics is important to achieve an acceptable extent of the reaction and adequate products distribution, but the use of active, selective and stable catalysts is also necessary.Even if alternative routes for CO2 methanation are studied, including photosynthesis and photocatalysis [110], electrochemical reduction [111] and biological conversion [112], the main attention is focused on the thermal catalytic process [113,114]. It proceeds with the following stoichiometry:\n\n(34)\n\n\nCO\n2\n\n+\n4\n\nH\n2\n\n\u2192\n\nCH\n4\n\n+\n2\n\nH\n2\n\nO\n\n\n\nAdditionally, the CO formed through the rWGS reaction (Eq. (35)) also leads to CH4 formation:\n\n(35)\n\nCO\n+\n3\n\nH\n2\n\n\u2192\n\nCH\n4\n\n+\n\nH\n2\n\nO\n\n\n\nIn addition, the side reactions of methane dry reforming (MDR), (Eq. (19)), Boudouard (reverse of Eq. (7)), decomposition of CH4 (Eq. (25)) and gasification of the coke formed by the two previous reactions (Eq. (7)) also take place.According to thermodynamics, CO2 conversion and CH4 selectivity are favored at high pressure and low temperature [115,116], and the results are good (almost complete conversion and selectivity close to 100%) with the appropriate catalyst even at atmospheric pressure if temperature is low enough (< 350\u00a0\u00b0C). Catalysts based on noble and non-noble metals are used [117], according to activity ordered as: Ru\u00a0>\u00a0Fe\u00a0>\u00a0Ni\u00a0>\u00a0Co\u00a0>\u00a0Rh\u00a0>\u00a0Pd\u00a0>\u00a0Pt\u00a0>\u00a0Ir, and according to selectivity: Pd\u00a0>\u00a0Pt\u00a0>\u00a0Ir\u00a0>\u00a0Ni\u00a0>\u00a0Rh\u00a0>\u00a0Co\u00a0>\u00a0Fe\u00a0>\u00a0Ru. The general use of Ni catalysts (due to the good compromise between their performance and cost), instead of Ru-based catalysts, requires working at temperatures for which catalyst stability problems arise, especially due to the formation of coke.The improvements of Ni catalysts are aimed at increasing surface defects, to facilitate the generation of surface-dissociated hydrogen, active for the removal of surface nickel carbonyls [118]. The role of the supports, aside from increasing surface defects, is to improve the dispersion of the metal and facilitate the storage and release of oxygen (redox properties). For these purposes, Al2O3, SiO2, ZrO2, TiO2, CeO2, perovskite, structured metal oxides, carbon materials and zeolites have been used as supports. Among the interesting properties of the supports, the following are to be mentioned, providing: i) Mechanical resistance; ii) metallic sites dispersion capacity (minimizing their aggregation); iii) hydrophilicity (the presence of H2O favors the sintering of the metallic sites); iv) thermal conductivity (avoiding the generation of \u201chot spots\u201d), and: v) reduced presence of acidic sites capable for coke formation. Some of these properties are improved incorporating promoters, including ZrO2, CeO2, La2O3, Mn2O3, MgO and alkali metals [113,114].CO2 methanation mechanism takes place with three pathways, the relative importance of which depends on the catalyst and reaction conditions [116]: i) Direct CO2 dissociation and hydrogenation of CO (intermediate) to CH4; ii) through the reaction of formate (HCOO\u2212) (intermediate formed from the adsorption of CO2) with chemisorbed hydrogen, and; iii) with formyl species as intermediates. These species result from the reaction of adsorbed CO (product of CO2 dissociation) with atomic hydrogen. Miguel et al. [116] have compared the LHHW kinetic equations of these mechanisms for a commercial Ni catalyst, proving that the best fit to their experimental results corresponds to the kinetic model for the pathway with formil species as intermediates, developed by Koschany et al. [119], assuming hydroxylic groups as the most abundant species.From the operational point of view, it is important to highlight the relevance of separating the H2O from the reaction medium to favor the extent of the reaction. This objective has led to the proposal using reactors with hydrophilic, steam-selective sodalite membranes [120,121] to replace conventional packed or fluidized bed reactors.CO is more reactive than CO2 and a key intermediate for the production of methane, methanol, DME and hydrocarbons from CO2, which explains why synthesis gas is used as feedstock in commercial processes for the production of these compounds. However, these reactions are carried out under unfavorable conditions for CO production. The conversion of CO2 by the rWGS (Eq. (5)) is an endothermic reaction, and temperatures above 700\u00a0\u00b0C are required in order to obtain considerable CO2 conversion. Under these conditions, CO2 and CO methanation (Eqs. (34) and (35), respectively) and Boudouard (Eq. (7)) side reactions also take place.The reaction mechanisms for the rWGS reaction is a topic of intensive debate [122], being redox and dissociative mechanisms the most widely accepted. In the redox mechanism, H2 does not participate as reactant, but reduces the surface of the catalyst. Metallic crystals are the active sites for CO2 dissociation, and the oxidized metallic sites are reduced releasing H2O and being therefore the metallic sites regenerated. Thus, the redox stages for Cu catalysts are:\n\n(36)\n\n\nCO\n\n2\n\ng\n\n\n\n+\n2\n\nCu\n\ns\n\n0\n\n\u2192\n\nCO\n\ng\n\n\n+\n\nCu\n2\n\n\nO\n\ns\n\n\n\n\n\n\n\n(37)\n\n\nH\n\n2\n\ng\n\n\n\n+\n\nCu\n2\n\n\nO\n\ns\n\n\n\u2192\n\nH\n2\n\n\nO\n\ng\n\n\n+\n\nCu\n\ns\n\n0\n\n\n\n\nIn the dissociative mechanism H2 reacts with CO2, leading to the subsequent formation of formate species (HCO2-M), which will release CO right away. These formate species are formed by the attack of OH\u2212 groups on M-CO species and MO2H species, formed through intermediates CO2-metal protonation. According to this mechanism, the significant effect of the presence of surface hydroxyl groups to facilitate CO2 adsorption and hydrogenation has been verified [123].The activity of the catalysts for rWGS is associated with the presence of oxygen vacancies and the capability for adsorbing CO2 and generating formate active species. These are formed in the vicinity of the H supply (metal-support interface) [124]. However, the selection of the catalyst is conditioned by stability and selectivity requirements, due to the high reaction temperature. A key property for CO selectivity is to achieve a weak binding energy of CO. Cu catalysts (with low CO adsorption energy) are commercially used for the WGS reaction with CuO-ZnO/Al2O3 (CZA) configuration, but undergo notable sintering in the rWGS reaction. The stability of the Cu sites improves using different supports (\u03b2-Mo2C, In2O3 [125,126]); with Cu\u2010Al spinel [122]; or generating particular configurations as Cu/CeO2 hollow spheres [127] or an inverse metal-oxide/metal structure of CeOx/CuOx [128].Promising CO selectivity has also been achieved with other non-noble metal catalysts using carbide structures prepared with Ti, V or W [129,130] and with bimetallic catalysts (Ni\u2010Fe, Ni\u2010Co) [131]. Although noble metals have high CO adsorption energy, high CO selectivity is achieved with strategies such as the preparation of bimetallic catalysts (Pd\u2010Ni) [132] and the atomic dispersion of Rh or Ru nanoparticles on the support [133].Olah [134] reflected the relevance of the \u201cmethanol economy\u201d as a complement to the established \u201coil economy\u201d. Fulfilling his forecasts, the production of methanol is a key reaction in the development of the GTL (Gas to Liquid) concept, with synthesis gas (produced from biomass, carbon or natural gas) as feedstock (Fig. 5\n). Methanol is an energy vector according to its utilization as fuel, whether pure or mixed with gasoline and the production of H2 by reforming. Additionally, it is an important raw material for the production of other fuels, solvents and base-chemical products, such as light olefins (MTO process), BTX aromatics, formaldehyde, acetic acid, methyl methacrylate, dimethyl terephthalate, methylamines, chloromethane, dimethyl carbonate, methyl tertbutyl ether (MTBE) and others.Albeit methanol production is carried out from synthesis gas (with a small concentration of CO2) (Eq. (38)), its potential capacity for valorizing CO2 on a large scale led Goeppert et al. [136] to highlight the strategic interest of the reaction for this objective. The plant in Reykjavik (Iceland), with an annual capacity of 4000 metric tons and valorization of 5600 tons of CO2, is the main industrial reference for renewable methanol synthesis from CO2 and H2 using geothermal energy [137].\n\n(38)\n\nCO\n+\n2\n\nH\n2\n\n\u21cc\n\nCH\n3\n\nOH\n\n\n\nThe exothermic synthesis of methanol from CO2 (Eq. (39)) requires 3 H2 molecules per CO2 molecule. Thermodynamically, low temperature and high pressure are required to facilitate the extent of the reaction. However, given the low reactivity of CO2, temperature above 240\u00a0\u00b0C is necessary to achieve an acceptable reaction rate. Thus, under the reaction conditions, the side reactions of rWGS (Eq. (5)) and synthesis from CO (Eq. (38)) take place. The rWGS generates a high content of H2O, which limits the equilibrium conversion of CO2, attenuates the activity of the catalysts and favors deactivation.\n\n(39)\n\nC\n\nO\n2\n\n+\n3\n\nH\n2\n\n\u21cc\n\nCH\n3\n\nOH\n+\n\nH\n2\n\nO\n\n\n\nTo overcome the limitations of the reaction, the action routes are focused on developing new, active, selective and stable catalysts [138\u2013141], reactors and operating strategies [142]. The knowledge of the mechanism for the conversion of CO2 into methanol is necessary to progress in the improvement of catalysts, and so, has received great attention due to its relevance in the synthesis of methanol from syngas, where CO2, of greater apparent reactivity than CO at low conversion conditions [143] is co-fed in a concentration within the 2\u20138% range. In Fig. 6\n, the three routes proposed for CO2 conversion are outlined [142], that is, with formate and hydrocarboxyl species as intermediates or through the rWGS reaction.Cu/ZnO/Al2O3 is the most commonly used catalyst for the synthesis of methanol from CO2, given its commercial use for the same purpose from synthesis gas feedstock, based on the proposal of Imperial Chemical Industries in 1960. In this catalyst, Al2O3 acts as structural promoter favoring the distribution of Cu and providing surface area and mechanical resistance to the catalyst. ZnO also acts as a structural promoter, separating the Cu crystals, and modulates the electronic properties owing to the metal/support interactions between Cu and ZnO. The presence of ZnO reduces the sintering of Cu [139]. The use of ZrO2 in Cu/ZrO2 or Cu/ZnO/ZrO2 catalysts leads to good results, due to the lower hydrophilicity of ZrO2 with respect to Al2O3. Furthermore, the presence of Lewis acidic sites, non-active for the conversion of methanol into hydrocarbons, contributes to attenuate the formation of coke [144]. The incorporation of metallic oxides (SiO2, MgO, Ga2O3, La2O3, TiO2, Y2O3) and noble metals (Pd, Au) as promoters favors Cu dispersion and modifies acid-base and redox properties of the catalyst, improving the selectivity and stability of the catalyst [138]. As an alternative to Cu catalysts, more stable Pd and PdZn alloys on different supports have been proposed, including metal oxides (ZnO, CeO2, In2O3) mesoporous silica (SBA-15, MCM-41) and carbon materials [140].Alternatively to the direct synthesis of methanol from CO2, a two-step process (rWGS-syngas hydrogenation) has been adopted. The advantages over the direct methanol synthesis process rely on the ease for removing the H2O generated in the rWGS. With this approach, its entry to the hydrogenation reactor is avoided and the temperature in each reactor can be optimized. With this technology the Korea Institute of Science and Technology installed the CAMERE (carbon dioxide hydrogenation to methanol via reverse water gas shift) process on a pilot plant scale, with a capacity of 100\u00a0kg of methanol per day [142].The mechanism for ethanol synthesis from CO2 (Eq. (40)) is more complex than that for methanol, because comprises more elementary reactions involving C\u2010C coupling and accurate stages of carbon chain growth and termination. The most accepted mechanism is the so-called CO2-Fischer Tropsch (CO2-FTS). CO generated through the rWGS reaction inserts into *CH3 or *CH3-(CH2)n species produced by CO-FTS to form ethanol or superiors alcohols (C3+OH) [108].\n\n(40)\n\n2\nC\n\nO\n2\n\n+\n6\n\nH\n2\n\n\u2192\n\nC\n2\n\n\nH\n5\n\nOH\n+\n3\n\nH\n2\n\nO\n\n\n\nThe selection of the composition of the selective multifunctional catalyst is also complex. So far, good results have been obtained with Rh-based catalysts with SiO2 and TiO2 as supports and Fe, Li and Se as promoters [145]. Other catalysts also selective towards ethanol production are prepared with Pt, Au, Mo, Co, and Cu as metallic function [140].The direct production of hydrocarbons from CO2 is a paradigm of catalytic processes integration, with the attraction of lowering equipment cost. However, this route implies important challenges to select the catalyst and establish the appropriate reaction conditions for a good compliance between the thermodynamic requirements and the mechanism of the involved reaction stages [105]. The reaction is carried out in tandem catalysts in the same reactor, through two alternative routes [104,146]: i) Modified Fischer-Tropsch synthesis (MFTS), incorporating a zeolite together with the FTS catalyst. In this manner, hydrocarbons are formed according to the Anderson-Schulz-Flory mechanism [43] and selectively converted on the zeolite, and; ii) with methanol/DME as intermediates (Eq. (41)), using OX/ZEO (metal oxide/zeolite) catalysts, suitable for the reactions of methanol/DME synthesis and the in situ conversion of these oxygenates into hydrocarbons [44].\n\n(41)\n\nCO\n+\nC\n\nO\n2\n\n+\n\nH\n2\n\n\u21d2\n\nCH\n3\n\nOH\n/\nDME\n+\n\nH\n2\n\nO\n\u21d2\nLight olefins\n\u21d2\nLight paraffins\n\n\n\nThe development of the MFTS route has been carried out mainly using Fe-based catalysts. CO2 hydrogenation proceeds through a mechanism with two stages. The formation of CO by the rWGS reaction followed by the chain growth in FT reactions. The selection of the zeolite allows the selective formation of light olefins, aromatics or isoparaffinic gasoline (Fig. 7\n) [147,148]. The addition of other metals (Co, Cu or Ni) to Fe, modifies the adsorption of H2 and CO, improving conversion and selectivity. Thus, with Fe\u2010Cu the selectivity of C2-C7 hydrocarbons is four times that obtained with Fe, decreasing the formation of CH4 [149]. In this case, as Fe support \u03b3-Al2O3 (followed by SiO2 and TiO2) shows a better behavior than other supports to avoid sintering, thanks to the good dispersion of Fe obtained, based on the strong metal-support interaction [150].In the route with methanol/DME as intermediates, the limitations of the Anderson-Schulz-Flory mechanism are avoided, and as a result, achieving higher selectivities of a family of hydrocarbons is feasible. Carrying out the second reaction (oxygenates conversion) in the same reactor displaces the thermodynamic equilibrium of methanol/DME synthesis, favoring the further conversion of CO2 and CO. Consequently, the reaction can be performed at lower pressure and lower H2/CO2 ratio than for methanol/DME synthesis, easing the supply of H2 from commercial PEM electrolyzers, which supply hydrogen at 15\u201330\u00a0bar [151]. The reaction conditions must be intermediate to those suitable for the two reaction steps. Thus, the conversion of methanol/DME into hydrocarbons occurs through the dual cycle mechanism (Fig. 8\n) [152], requiring temperature above 325\u00a0\u00b0C for a significant extent [153]. However, this temperature is excessive for the synthesis of methanol/DME, which occurs through a mechanism with formate ions as intermediates [154].The presence of oxygen vacancies in the metallic function is a key feature for the adsorption of CO2 [139]. In addition, this function must have a limited capacity for over\u2011hydrogenating the double C=C bonds, as to avoid the formation of methane [155]. Besides, the distribution of hydrocarbons depends on the acidic strength and pore size of the zeolite [156]. According to these conditions, In2O3-ZrO2/SAPO-34 tandem catalyst shows good prospects for the selective production of light olefins from CO2 [157], given the capacity of the superficial oxygen vacancies of the In2O3-ZrO2 system for CO2 adsorption and the high light-olefin selectivity achieved in the conversion of methanol/DME over SAPO-34 (CHA topology). Similarly, the use of HZSM-5 zeolites (MFI topology) together with the ZnO/ZrO2 system allows obtaining high aromatics selectivity [152].Wang et al. [158,159] obtained high gasoline yield with a Fe/Zn/Zr@HZSM-5 core-shell catalyst, with isoalkanes as main components and with low aromatics concentration. However, as a drawback, CO selectivity of 40% resulted from the RWGS reaction. This reaction was later suppressed by treating the Fe/Zn/Zr catalyst with tetrapropylammonium bromide (TPAR) [160]. These authors also determine that the treatment affects the hydrocarbon formation mechanism, which proceeds through the two routes (FT and oxygenates as intermediates) with the Fe/Zn/Zr catalyst and mainly with oxygenates as intermediates with Fe/Zn/Zr-Treated catalyst, due to the enhanced adsorption strength of the HCOO* species and desorption rate of CH3O* species. The Fe/Zn/Zr-Treated@HZSM-5 core-shell catalyst is stable for 120\u00a0h on stream, with 76% hydrocarbons selectivity and C5+ isoalkane content of 93% in the gasoline, with a CO selectivity of 24% and a CO2 conversion of 18%.The interest in the production of DME is based on its usefulness as fuel and intermediate raw material for the production of hydrocarbon fuels and chemicals, and on the capacity of the process for valorizing synthesis gas derived from renewable sources (biomass) and CO2. The cost and energy- and exergy- efficiencies of DME production from syngas depend on the syngas source and the reactants used in gasification or reforming. These factors determine the H2/CO ratio of the resulting syngas. The interest of valorizing low rank coal to DME via gasification has received continued attention [161] and this attention has extended to the valorization of natural gas and biomass [162]. The urgency for mitigating the effects of climate change by reducing CO2 emission rates has reoriented DME production technologies to make CO2 co-feeding together with syngas or CO2 hydrogenation feasible. The joint valorization of CO and CO2 as carbon sources is an initiative applicable to different industrial emissions and to bio-gas (product of the anaerobic fermentation of biomass composed of CH4 (50\u201370%) and CO2 (30\u201350%) [163]). In this line, recycling in the synthesis of DME the CO2 used as biomass gasifying agent reduces up to 20% the environmental impact of the process [164].Dieterich et al. [165] gather the pathways for transforming renewable energy into sustainable energy vectors (DME, methanol and hydrocarbons) in the diagram in Fig. 9\n.DME (CH3-O-CH3) is an environmentally benign, non-toxic, non-teratogenic, and non-carcinogenic species, with a slight ethereal odour, which has multiple applications due to its properties (Table 2\n) [166\u2013168]. Among others, it is used as aerosol, propellant (substituting chlorofluorocarbons), pesticide and ecological refrigerant [169]. It is of great interest also as organic solvent, due to the low dielectric constant of liquid DME (5.34 at 30.5\u00a0\u00b0C and 6.3\u00a0MPa), medium polarity, partial miscibility with water, no reactivity, chemical inertness, and affinity for oily compounds (given its capacity for developing one-way hydrogen bonds with hydrogen bonding solutes). These properties along with the easy removal by pressure reduction make it suitable for the extraction of products in food and pharmaceutical industry (lipids, essential oil, flavonoids), of contaminants (as phenols) from mixtures with water [170,171] and in solvent injection processes for heavy oil recovery [172].The large-scale implementation of DME production is based on its properties as fuel, either for domestic use, in the automotive industry or for electrical energy generation. According to Semelsberger et al. [166] a transition from petroleum to DME to hydrocarbons is more cost-effective than a direct change to hydrogen, considered as the \u201cend-game\u201d fuel, since the existing LPG and NG transport and storage infrastructure can be used. The main advantages as fuel are: [173]: i) High oxygen content, lack of C\u2010C bonds, N, and S compounds, reasons for the soot-, SOx- and NOx-free combustion; ii) low boiling point (\u221224.9\u00a0\u00b0C) and consequently, small energy requirement for vaporization, which facilitates its use as fuel gas, alone or blended with liquefied petroleum gases (LPG: propane and butane) given its similar vapor pressure and the same storage and transport characteristics [174]. In addition to domestic use, gas DME is used as fuel in homogeneous charge compression ignition (HCCI) engines, in mixtures with natural gas and hydrogen [175]. iii) High cetane number (> 55) that results in very low auto-ignition temperature. In spite of its low heating value (LHV) of 27.6\u00a0MJ/kg, inferior to that of diesel fuel (42.5\u00a0MJ/kg), the high cetane number and the short delay-time in the injection, make DME suitable for compression ignition (CI) engines. Using the existing technology, the well-to-wheel efficiency is DME\u00a0>\u00a0LPG\u00a0>\u00a0Gasoline > CNG (compressed natural gas) and the associated greenhouse gas emissions are significantly lower (DME\u00a0<\u00a0CNG\u00a0<\u00a0LPG\u00a0<\u00a0Gasoline) [167]. Tomatis et al. [164] estimate that replacing diesel by pure DME results in a decrease in greenhouse gases (GHG) of 72%, while limiting the emission of particulates (diesel soot). This emissions decrease has an impact on human health and ecosystem of 55% and 68%, respectively. However, due its high vapor pressure, very low boiling point, high compressibility, low density, low viscosity and the capacity of dissolving some elastomers and plastics, different modifications in diesel engines and in the selection of the materials are required for using DME. The main modifications consist of incorporating a pressurized DME tank, and a high-pressure fuel pump.The evolution towards a DME economy is based not only on its use as fuel, but also on its future as intermediate sustainable raw material. Thus, DTO (dimethyl ether-to-olefins) process may replace or complement MTO (methanol-to-olefins) process, developed by UOP/Mobil and successively improved [32]; and MTP, developed by Lurgi (to selectively obtain propylene) [176]. The implementation of both processes is growing as to satisfy the burgeoning demand of light olefins, which is currently covered through naphtha steam cracking [177] and fluid catalytic cracking (FCC) [178] processes with high energy requirements and high CO2 emissions. The DTO process offers advantages over MTO: i) DME is more reactive than methanol, which allows carrying out the reaction at lower temperature [179]; ii) the lower reaction heat favors temperature control. The DTO process has been mainly studied using SAPO-34 [180,181] and HZSM-5 zeolites [182] as catalysts. For the selective production of olefins, the use of HZSM-5 zeolites of moderate acidity (SiO2/Al2O3 ratio around 180) is suitable. Indeed, the rate of coke deposition is also reduced. Whereas higher acidity (SiO2/Al2O3 of 30) boosts (C5-C11) gasoline yield [35]. Using pseudo-boehmite as a binder, HZSM-5 zeolite is embedded in a mesoporous matrix of \u03b3-Al2O3, providing mechanical resistance to the catalyst particles and attenuating the blockage of the micropores of the zeolite by coke [183,184].DME conversion into hydrocarbons proceeds, like methanol conversion, through the dual cycle mechanism [37], with polyalkylbenzenes as intermediates for the formation of light olefins as primary products. The mechanism occurs along with different side reactions (isomerization, cyclization and hydrogen transfer) forming together with olefins: light paraffins, BTX aromatics, C5\n+ aliphatics and coke. On the basis of this mechanism, kinetic models for HZSM-5 based catalysts have been established, which allow quantifying the evolution of products distribution with time on stream [185]. With these models, evaluating the effect of various operating strategies on the deactivation and on products distribution is possible, such as the effect of co-feeding H2O or feedstock dilution. Indeed, the models have been used in the design of alternative reactors (packed bed, captive fluidized and fluidized with catalyst circulation) and of a reactor-regenerator system with circulation of the catalyst between both units, based on the technology implemented for the MTO process [186].Another application for DME with development potential is as H2 vector, because its characteristics (high hydrogen content, absence of C\u2010C bonds and low toxicity) facilitates the reforming at low temperature (< 300\u00a0\u00b0C) and results in high H2 yield. This can be applied for proton exchange membrane fuel cells (PEMFC) [190] and solid oxide fuel cells (SOFC) [191], as well as to cover, on a large-scale, the growing demand of H2 in the petrochemical industry. Catizzone et al. [39] propose DME as a good candidate for energy storage through the cycle comprising the synthesis of DME from CO2 (exothermic) and the reforming of DME to H2 (endothermic). In this way, the energy demand of the reforming is covered by the energy generated intermittently from renewable sources.Steam reforming takes place on bifunctional catalysts, through DME hydrolysis in the acidic function (Eq. (42)) followed by methanol reforming in the metallic function (reverse of Eq. (39)). Additionally, the secondary reactions (rWGS (Eq. (5)), DME partial decomposition, methanation (Eqs. (34) and (35)), Boudouard (reverse of Eq. (7)) and hydrocarbons formation) contribute to products distribution,\n\n(42)\n\n\nCH\n3\n\nO\n\nCH\n3\n\n+\n\nH\n2\n\nO\n\u21cc\n2\n\nCH\n3\n\nOH\n\n\n\nThe most used catalysts in a lab-scale have been prepared with CuO-ZnO-Al2O3 (CZA) metallic function, based on the commercial catalyst for methanol synthesis and methane reforming. The main innovations have mainly consisted of the utilization of CuM2O4 spinels (M\u00a0=\u00a0Fe, Mn, Cr, Ga, Al, etc). Among these, CuFe2O4 spinel has received a great attention due to its thermal stability [192,193], which recovers its activity in reaction-regeneration cycles [194,195]. \u03b3-Al2O3 has been the most used acid function for DME hydroxylation [196,197], but has been progressively substituted by HZSM-5 (more active). HZSM-5 needs to be adequately treated (as desilicated by alkaline treatment) in order to avoid the formation of hydrocarbons and the consequent formation of coke [198,199]. Oar-Arteta et al. [194,200] have improved the properties of \u03b3-Al2O3, obtaining it by calcination of pseudo-boehmite. This treatment provides the catalyst with high mechanical resistance (a deficiency of the CuFe2O4 spinel) and also with moderate acidity, limiting the formation of hydrocarbons. Therefore, it allows for stably operating in reaction-regeneration cycles at 350\u00a0\u00b0C achieving a yield of 82%. Filling the gap in the kinetic modeling for oxygenates reforming, Oar-Arteta et al. [195] have proposed a kinetic model based on LHHW expressions for each step, establishing as optimal reforming conditions: 360\u2013380\u00a0\u00b0C and a steam/DME ratio of around 6. The use of microreactors with ceramic channels eases H2 generation for portable fuel cell applications [201].Zhan et al. [202] have conducted a review of the studies of ethanol production from DME through carbonylation. This reaction is a key stage in the valorization of synthesis gas. The reaction, as the formation of methyl acetate (MA), takes place through the Koch-type CO insertion into DME, with zeolites (typically HMOR and HZSM-5) as catalysts. The MA is later converted into ethanol on Cu-based catalysts.DME production (10 Million tons per year) is carried out in a two step process, in separate units (indirect synthesis) using syngas feedstocks [203]. Methanol is synthesized in the first unit (under reaction conditions described in Section 2.3.3) and dehydrated towards DME in the second unit (MTD process). Methanol dehydration is a reversible exothermic reaction on acid catalysts, whose thermodynamics is not favored increasing pressure, but rather decreasing temperature. The process has been reoriented towards valorizing CO2. In Fig. 10\n the routes for CO2 upgrading to DME are plotted [165]. Michailos et al. [204] estimate within the 1.83\u20132.32 \u20ac kg\u22121 range the cost of DME production from captured CO2. Schemme et al. [205] determine that the production of DME (equaling its technical maturity to that of methanol synthesis) is a cheaper route for valorizing CO2 than the production of alcohols (methanol, ethanol, butanol, octanol), polyoxy dimethyl ether, and hydrocarbons (synthetic gasoline, paraffinic diesel, and paraffinic kerosene), emphasizing the relevance of H2 production costs (58\u201383% of the total manufacturing costs). Uddin et al. [206] make a techno-economic analysis of the two stage DME synthesis via the birreforming of landfill gas (with steam and CO2 from an ammonia plant). These authors estimate a price of 0.87\u20130.91 $ gal\u22121, competitive with the price of diesel fuel. Furthermore, using landfill gas sourced CO2, the process achieved negative emissions.The industrial process has different licenses and an extensive implementation in asiatic countries since the beginning of the 21st century with carbon as raw material [174]. It is performed under moderate pressure (below 20\u00a0bar) and within 150\u2013300\u00a0\u00b0C temperature range. \u03b3-Al2O3, of low manufacturing cost, is generally used as catalyst [207\u2013209]. The weakly acidic nature of the Lewis sites of \u03b3-Al2O3 is appropriate to achieve a high DME selectivity, inhibiting the formation of hydrocarbons as by-products. Nonetheless, its activity is moderate and temperatures above 250\u00a0\u00b0C are required, besides the activity may be improved by modifying \u03b3-Al2O3 with P, Ti, Nb, B, etc. [210]. In addition, due to its hydrophilic character, it has a great capacity for adsorbing H2O (product of dehydration), reducing thereby its activity and causing dealumination, particularly when aqueous methanol is fed [211]. Catalysts with higher acidity than \u03b3-Al2O3 have also been studied, which allows the reaction to take place at lower temperature, avoiding the formation of hydrocarbons. For this purpose, the optimal performance of heteropolyacids (HPAs) (more active than HZSM-5 catalyst) has been proven, and enhanced by incorporating W and P [212] and supporting HPAs on TiO2 [213].The greatest research effort in the design of catalysts for methanol dehydration has focused on zeolites, whose performance (activity, DME selectivity and stability) is influenced by the configuration of the channels of their crystalline structures and the quantity and strength of the acidic sites [214]. HZSM-5 zeolite (MFI topology), which is less hydrophilic than \u03b3-Al2O3 has received a special attention. In particular, for the valorization of CO2 together with syngas, in order to avoid the separation of the high content of H2O in the aqueous methanol produced in the first stage. This zeolite contains pores with moderate severity of shape selectivity and the acidity is dependent on the SiO2/Al2O3 ratio, with sites of moderate acidic strength mainly. Besides, the behavior of hybrid catalysts composed of HZSM-5 zeolite impregnated with \u03b3-Al2O3 is also of interest, being it more active and selective than each separate catalyst, due to the dilution of the strong sites of the zeolite [215,216]. The desilication of HZSM-5 by means of an aqueous solution of NaOH is effective to attenuate the deactivation by coke, because the treatment decreases the acidic strength of the sites. In addition, the coke is deposited in the generated mesopores, reducing the blockage of the micropores of the zeolite [217].Catizzone et al. [218] have proposed ferrierite (FE) as ideal catalyst, since its crystalline structure with two dimension channels make it highly selective and, additionally, coke deposition is reduced. This zeolite, prepared with a high Al content, allows achieving DME selectivity close to 100% at 200\u00a0\u00b0C and high methanol conversion (up to 82%), in contrast to \u03b3-Al2O3 (conversion of 25%). Moreover, methanol conversion and DME selectivity of FE can be improved by increasing the density of Lewis sites and reducing the crystal size [219]. Comparing the features of FMI and FE zeolites, Catizzone et al. [220] achieve similar DME selectivity with nano-sized MFI and FER, whereas for the former higher reaction rate and lower coke deposition are reported.Methanol dehydration to DME (reverse of Eq. (42)) proceeds through two competitive reaction pathways: Associative (or direct) and dissociative (or sequential) (Fig. 11\n). In the first, two methanol molecules are adsorbed on an acidic site and react to form DME and H2O. The reaction can occur by splitting of protonated methanol dimer into the methyl carboxonium ion and carbenium ion at the same time, or into two methyl carboxonium ions, which are further combined to form DME molecule [221]. In the second, one adsorbed methanol molecule reacts to form H2O and a CH3 species bound to the deprotonated zeolite, and then, a second methanol molecule adsorbs to react with the CH3 group to form DME. Park et al. [222] highlight the discrepancies in the literature on the predominant mechanism, which depends on the catalyst and the operating conditions. These authors, using computational chemistry and microkinetic modeling, determine that the dissociative pathway is the dominant for the reaction with an H-zeolite, being DME formation reaction the rate-controlling step. However, these theoretical results differ from those obtained by Trypolskyi et al. [223]. Adjusting the experimental results of methanol dehydration on a HZSM-5 zeolite these authors propose methanol adsorption as the rate-limiting stage; being equally valid the kinetic expressions of LHHW deduced for the associative and dissociative pathways to adjust the experimental results.The reactions involved in the process are:Methanol synthesis (Eqs. (38) and (39)); Reverse Water Gas Shift (rWGS) (Eq. (5)); Methanol dehydration towards DME (Eq. (43)):\n\n(43)\n\n2\n\nCH\n3\n\nOH\n\u21cc\n\nCH\n3\n\nO\n\nCH\n3\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n\n\u0394H\n0\n\n=\n\u2212\n23.4\n\nkJ\n\n\nmol\n\n\u2212\n1\n\n\n\n\n\n\n\n\u0394G\n0\n\n=\n\u2212\n16.8\n\nkJ\n\nm\no\n\nl\n\n\u2212\n1\n\n\n\n\n\n\n\n\n\nand paraffins formation secondary reaction (mainly methane):\n\n(44)\n\nnCO\n+\n\n\n2\nn\n+\n1\n\n\n\nH\n2\n\n\u21cc\n\nC\nn\n\n\nH\n\n2\nn\n+\n2\n\n\n+\n\nnH\n2\n\nO\n\n\n\nn\n=\n1\n\u2212\n3\n\n\n\n\n\nThe interest in the direct route for DME synthesis is based on different factors: i) Thermodynamic advantages. Conducting methanol dehydration (Eq. (43)) in situ in the same reactor displaces the equilibrium of methanol formation reactions (Eqs. (38) and (39)). ii) lower cost of production in comparison to the synthesis of DME in two steps and to the synthesis of methanol [224]. Thus, the energy efficiency is around 64\u201368% for a 2500 equivalent t/day, higher than methanol synthesis, with an energy requirement 5% lower and a lower capital cost (8% lower) [225,226]; iii) possibility of using synthesis gas generated from various hydrocarbonated raw materials as carbon, natural gas, biomass or residues of the consumer society (Fig. 12\n), and from a steel-making plant (mixture of coke oven gas and tail gas) [227]; iv) boost of gasification and anaerobic digestion of biomass [228] in order to contribute to neutral carbon balance. A comparative exergo-economic analysis of the indirect and direct routes for DME synthesis, based on air-steam biomass gasification with CO2, has evidenced the lower cost of DME production through the direct route (1.66 $ kg\u22121, whilst 2.26 $ Kg\u22121 for the indirect route), and also, the lower energy consumption and net CO2 emission [229]. In addition, given the higher price of the product, the gasification-DME process from biomass was approximately 7% more economically feasible than the gasification-MeOH process [230]; v) opportunity to maximize the natural gas operating profit, integrating its valorization with DME synthesis.Taking into account these advantages, Olah et al. [231] considered the one step synthesis of DME (Fig. 13\n) a key route for the catalytic valorization of CO2 on a large-scale. Furthermore, these authors have placed great emphasis on the sustainability of the process when CO2 is co-fed with synthesis gas produced from lignocellulosic biomass.In the literature regarding methanol synthesis thermodynamics [232\u2013234] and one step DME synthesis [235\u2013237], synthesis gas has been studied as feestock, whereas little attention has been given to CO2 conversion capacity, whose role has been restricted to secondary product of the reaction. The interest in CO2 conversion processes on a large-scale requires new studies regarding the thermodynamics and kinetics, aimed at establishing the appropriate conditions and the reactor design. Chen et al. [238] have compared the DME synthesis thermodynamics in two steps and in a single step, co-feeding CO2 with synthesis gas. The results support that with both strategies CO2 co-feeding decreases DME yield, and also that the direct synthesis of DME has lower thermodynamic limitations and allows achieving higher CO2 conversion.Ateka et al. [153] have compared in depth the thermodynamics of both methanol synthesis (MS) and the direct synthesis of DME (DS), from the perspective of the capacity of these processes for valorizing CO2. The effect of the reaction conditions (temperature, pressure and feed composition) in regard to CO2 conversion, oxygenates yield and selectivity (MeOH and DME) and heat generated in each process were determined. Being CO and CO2 hydrogenation exothermic reactions with reduction of mole number, oxygenates production is favored with increasing reaction pressure, while penalized upon increasing temperature. The study ascertained that valorizing CO2 is feasible in MS and DS processes for CO2 rich feedstocks (CO2/COx\u00a0>\u00a050%) at 250\u2013300\u00a0\u00b0C (suitable range to obtain good catalytic performance [239] and avoid sintering [240]) (Fig. 14\n). Nonetheless, higher CO2 conversion values can be achieved in DS than in MS (for CO2/COx\u00a0>\u00a075%), greater upon further increasing CO2 concentration in the feedstock (Fig. 15\n).The study of Ateka et al. [153] highlighted the relevance of the CO2 content in the feedstock, and that the DS is more thermodynamically favorable than MS for oxygenates production under suitable operating conditions. For its interest for simplifying reactor design, the possibility for operating at thermo-neutral conditions was tested, combining the aforementioned exothermic nature of CO and CO2 hydrogenation reactions and the endothermic nature of the involved rWGS reaction (of special relevance for CO2 containing feedstocks). Clearly, CO2 co-feeding positively contributes to reduce the heat released in the reaction and helps avoiding hot spot formation (Fig. 15). Heat production diminishes from 80 to 45\u00a0kJ\u00a0mol\u22121 for MS and from 90 to 60\u00a0kJ\u00a0mol\u22121 for DS for CO2/COX\u00a0=\u00a00.5 feedstocks. Anyhow, the study reveals the impossibility of working with Cu based traditional catalysts at thermo neutral conditions, since temperatures above 340\u00a0\u00b0C are required for this purpose in any case and Cu catalysts undergo sintering at temperatures above ~300\u00a0\u00b0C.Furthermore, it should be noted that the effect of the reaction conditions on DME yield is opposite to the effect on CO2 conversion and so, that optimizing of each of these objectives requires different reaction conditions. Thus, CO2/COx ratios below 0.25 are suitable for enhancing DME production, whereas ratios above 0.5 improve the conversion of CO2. Consequently, to combine the economic objective associated with the production of DME and the economic/environmental target of reducing CO2 emission rates, intermediate conditions are necessary.For this process, bifunctional catalysts comprising metallic catalysts for methanol synthesis (as introduced in Section 2.2.3) and acidic catalysts for methanol dehydration into DME are required. In addition, by feeding CO2 various differences from the syngas-to-DME process arouse. One the one hand, as introduced in Section 3.3, according to thermodynamics, lower DME yield is obtained. On the other, the role of the rWGS (Eq. (5)) reaction is more relevant, giving way to higher H2O content in the medium. H2O inhibits the production of methanol (reduces the reaction rates of methanol formation by CO and CO2 hydrogenation and of WGS reactions) since H2O molecules tend to strongly adsorb on the surface active sites of the catalyst [241\u2013243]. Moreover, deactivation problem assumes greater relevance [244]. Thus, the higher CO2 and H2O concentrations in the reaction medium favor CuO oxidation and its sintering, which is an important feature due to its irreversibility. However, these unfavorable effects should not fade the main advantage, that is, the attenuation of coke deposition due to the aforementioned role of H2O in the reaction medium for controlling the concentration of superficial methoxy species, as well as the ability of H2O to diffuse coke precursors [245]. This said, within the research works to improve the catalyst, two pathways can be distinguished: 1) focused on improving each function of the catalyst, and; 2) oriented towards optimizing the contact between both functions of the catalyst by changing the structure of the bifunctional catalyst particle. Besides, given its importance in the viability of the process, the deactivation of the catalyst is also worth of study. These features of the bifunctional catalysts for the direct synthesis of DME from CO2 are studied separately in the following sections.In the 1960s Imperial Chemical Industries proposed CuO-ZnO-Al2O3 (CZA) metallic function a suitable option for methanol synthesis under mild conditions and has been widely used since [246]. Cu (Cu0 and Cu+) is the active species for CO and CO2 hydrogenation, whereas ZnO is used as geometric spacer for enhancing its dispersion and for stabilizing it [247,248], helping to hider sintering and poisoning. Nevertheless, it has been substituted by La2O3 [249], MgO [250], Fe2O3 and CeO2 [251,252] for promoting CuO dispersion, catalyst stability and COx conversion.Al2O3 in the CZA catalyst has also been replaced, partially or totally, by other metal and non-metal materials. Among others, MnO has been reported to enhance CuO and ZnO dispersion and reduce the temperature required for CuO reduction, giving way to a larger specific surface area of active Cu0 [253,254], and so, boosting DME yield. Moreover, the Cu\u2010Mn spinel formed resulted very active in the WGS reaction [253,254]. Likewise, the addition of ZrO2 is widely reported [255,256] to improve the performance of the catalysts as a result of the stabilization of the Cu\u03b4+ sites under reducing and oxidizing conditions [257] and higher H2O tolerance [258\u2013264]. On the one hand, the weak hydrophilicity of ZrO2 hinders the adsorption of H2O (competing with the adsorption of the reactants), and on the other, its basicity favors CO2 adsorption, improving therefore methanol production. Given the promising results of Cu/Zn/Zr catalysts, various authors have deepened in broadening the knowledge on their activity. As to tailoring the catalyst, S\u00e1nchez-Contador et al. [144] have further studied the effect of ZrO2 loading into the CuO-ZnO metallic function, synthesizing MeOH from CO2/CO/H2 mixtures under the reaction conditions required for the direct synthesis of DME. Cu/Zn/Zr\u00a0=\u00a02:1:1 was determined to be the most suitable ratio for achieving an optimal agreement between COx conversion (8.14%), methanol yield and selectivity (over 98%) and catalyst stability. Singh et al. [265] attribute the high activity of the Cu/Zn/Zr catalysts to the interactions between Cu and ZnO and ZrO2 oxides, generating oxygen vacancies and stabilizing the methoxy species intermediates in the formation of methanol. Moreover, ZrO2 tunes the acidity of the bifunctional Cu/ZnO/ZrO2, adapting it to the selective production of DME. Through steam-treatment of Cu/Zn/Zr catalysts using tetrapropylammonium bromide (TPABr) Chen et al. [266] manage to suppress the formation of CO via the RWGS reaction, in addition to increasing the activity, selectivity and stability of the catalysts, due to the increase in the concentration of oxygen vacancies. The same goal is achieved by ultrasonic-assisted impregnation of TPABr to stabilize the CuBr phase on the catalyst surface [267].As to the reaction mechanism regards, Frusteri et al. [268] hypothesize that ZrO2 could also have the capability for activating the adsorbed CO2 giving way to CO2* species. These CO2* species are assumed to react with H2* species to give intermediate species (formate, dioxomethylene, methoxy), which will further evolve to methanol. According to Witoon et al. [269,270] bicarbonate species formed from CO2* are considered to be the ones reacting with H2* to give way to methanol. Both CuO-ZnO-MnO and CuO-ZnO-ZrO2 catalysts outperform the results obtained with CuO-ZnO-Al2O3 in a similar manner for H2\u00a0+\u00a0CO\u00a0+\u00a0CO2 feedstocks. The cost of the former is lower, and so its use for CO/CO2 mixtures hydrogenation is suitable, while for pure CO2 hydrogenation the latter outstands [255]. Li and Chen [271] studied in detail the synergyes induced by ZrO2 (Fig. 16\n) and summarized the approaches to improve the catalytic performance of ZrO2-containing catalysts for CO2 hydrogenation to methanol.Ga2O3 promoter (with lower capability for adsorbing H2O than ZrO2) [272] has been reported to facilitate the reducibility of the catalyst [273\u2013275], improve Cu stability [276,277] and dispersion [278]. Moreover, enhances ZnO conductivity and favors the creation of redox-active defect sites as structural promoters [273]. Also, high methanol yields have been achieved by the addition of Ga2O3 to Cu-ZrO2 catalysts [279,280]. Furthermore, in this line, quaternary catalysts have also been proposed, like Cu-ZnO-ZrO2-TiO2 [259] given the addition of TiO2 leads to the creation of oxygen vacancies for the adsorption of CO2 [281], and Cu-ZnO-Al2O3-CeO2 [282,283].Pursuing the increase of methanol formation reaction rate by favoring the adsorption of the reactants (H2 and CO2), the addition of small amounts of noble metals to Cu-ZnO based catalysts has been suggested [284]. The promoting effects of this addition have been mainly attributed to the hydrogen spill-over mechanism [285]. Among these metals: Au [286\u2013288], Pd [289\u2013291], Pt, Rh [292].As an alternative approach, the use of SBA as support for the confinement of Cu-ZnO actives sites within its mesoporous structure has been studied by Prieto et al. [293]. This configuration enhanced the contact of the active sites with the reactants, resulting in higher activity and thus, methanol production. Carbon nanotubes [294], graphene oxides [281], and carbonaceous coordination polymers have also been reported as supports to boost the activity and stability of Cu-ZnO catalysts. These supports reduce the size of the active sites and favor distribution, facilitating the reduction, and hampering the strong adsorption of H2O, giving way to more stable and active catalysts for methanol production.Nevertheless, as to overcome the limitations of Cu based catalysts (sintering, low CO2 activation capacity) non Cu-based oxide catalysts are being tested for methanol production, especially seeking for stable catalysts for CO2 hydrogenation. In this regard, Wang et al. [295] studied binary ZnO-ZrO2 catalyst obtaining high per-pass CO2 conversion and resistance to poisoning by SO2 and H2S. The -Zn-O region for dissociating H2 is also the active site for the direct hydrogenation of CO2 to methanol with HCOO, H2COO and H2CO as intermediates. These authors reported outstanding stability during 500\u00a0h TOS, and Wang et al. [296] doubled (1000\u00a0h TOS) the stability with In2O3 catalyst. In these catalysts, defective oxygen vacancies are considered the active sites for the direct hydrogenation of CO2 to methanol with HCOO, H2COO and H2CO as intermediates [297\u2013299]. With this catalyst the rWGS reaction is inhibited [300], thus, the CO2-CO-methanol pathway of Cu based catalysts is avoided. The addition of ZrO2 as structural promoter prevents In2O3 sintering and, considering that In and Zr metals have different valence number, within the In2O3 structure additional surface oxygen vacancies are created due to the replacement of In by Zr atoms [301], helping CO2 adsorption [299,302] and so, the selective formation of methanol [299,303]. Similar effect has been demonstrated for Ga insertion into the In2O3 lattice [304], and in both cases, controlling the ratio between the metals is a key feature to be optimized for maximizing the performance of the catalyst.Co containing catalysts have also exhibited high activity for selectively producing methanol from CO2, inhibiting the rWGS reaction [305]. With Mn\u2010Co catalysts a synergy between the metals results in increasing surface basicity and improving methanol selectivity [305,306]. According to Wang et al. [307], for Co based catalysts, the addition of SiO2 leads to the formation of Co-O-Si species, favoring the formation of methanol by increasing *CH3O species reactivity and hydrogenation over methane production by C\u2010O dissociation.For their excellent stability and resistance to poisoning, noble-metal based catalysts such as Pd/ZnO [308], Pd/In2O3 [309] and Au/ZrO2 [310], with different supports (i.e. Ga2O3 [311], CeO2 [312] or In2O3 [309], MOF, SBA-15, CNT, SiC\u2026), promoters (e.g. K2O, MgO, CaO) and preparation methods are also being tested with good results despite its higher cost. Ca-promoted Pd nanoparticles (2\u20136\u00a0nm) over mesoporous CeO2 are active for metanol synthesis and dehydration to DME [313]. To be highlighted, the stability of Pd0 nanoparticles, the induction of structural defects by Ca in CeO2 that favor the absorption of CO2 and the balance between the amount of basic and acidic sites. It is claimed that Pd\u2010Zn alloys stabilize the formate intermediates and ease the direct formation of methanol from CO2, inhibiting the CO formation by the rWGS reaction [314,315].For Au-based catalysts, the relevance of the support on the overall activity and product selectivity is highlighted [142]. For these catalysts, Hartadi et al. [316] explain the selectivity order: Au/ZnO\u00a0>\u00a0Au/ZrO2\u00a0>\u00a0Au/TiO2\u00a0>\u00a0Au/Al2O3 by the larger size of Au particles, although it is accompanied by a decrease in activity. These authors determine for the Au/ZnO catalyst that CO2 is directly hydrogenated to methanol and that this reaction proceeds via an independent reaction pathway (presumably with adsorbed formate and methoxy species as intermediates) [317]. This independence of the mechanisms explains the shift in the main carbon source for methanol from CO2 to CO as the temperature increases from 240 to 300\u00a0\u00b0C [318]. Wu et al. [310] confirmed the higher activity and selectivity of Au/ZrO2 catalysts prepared with sub-nanometric particles (1.6\u00a0nm) was due to the appropriate coupling between the Au and the support.For methanol dehydration to DME solid-acid catalysts are required. Desirably hydrophobic, stable, active and selective under the required reaction conditions. In the vast majority of studies, \u03b3-Al2O3 is used, given its reported high selectivity within the temperature range required in the process (200\u2013300\u00a0\u00b0C) and relatively low manufacturing cost [208,319]. Ghorbanpour et al. [320] made a computational assessment of the reaction mechanism and determined that depending on the reaction conditions (temperature and pressure) methanol dehydration could proceed through: i) A dissociative route, that is, methanol adsorbed in an acidic site would lose a water molecule and transfer into a surface methoxy group to react to with another methanol molecule leading to the formation of DME; or ii) an associative route, where two methanol molecules co-adsorb on an acidic site to give DME. Nonetheless, given the hydrophilic nature of \u03b3-Al2O3, its activity decays significantly due to the ability for adsorbing the H2O formed in the process leading to dealumination. Moreover, H2O has multiple roles in the conversion of methanol to DME: i) shifts the thermodynamic equilibrium of methanol dehydration to DME; ii) decreases the acidity of the catalyst by adsorbing on the acid sites (competing with methanol [321,322]), and iii) inhibits the formation of methoxy ions by shifting the equilibrium [245]:\n\n(45)\n\nAl\n\u2212\nOH\n+\n\nCH\n3\n\nOH\n\u21cc\nAl\n\u2212\nO\n\u2212\n\nCH\n3\n\n+\n\nH\n2\n\nO\n\n\n\nThis feature is way more relevant for the direct CO2-to-DME process, where hydrothermal conditions are more severe than with syngas as reactant. Therefore, the research on the acid catalysts has focused on mitigating the activity decay due to H2O adsorption by progressively diminishing hydrophilicity and facilitating its desorption from the acid sites, bearing in mind the acid catalyst for the process requires limited acid strength, as to avoid the formation of hydrocarbons [323]. MCM-41 supported tungstophosphoric acid (TPA) has also been used, based on the high turnover frequencies for methanol dehydration to DME [324]. On the basis of the above premises, besides modifications of \u03b3-Al2O3 [210,325], various alternatives have aroused among which zeolites (framework types as BEA, EUO, FER, MOR, MTW, TON [326,327]) and in a wider extent MFI type (HZSM-5 [328,329] and silicoaluminophosphates (SAPO-11, \u221218, \u221234)) outstand [330]. Catizzone et al. conducted a screening among different framework type zeolites for methanol to DME dehydration and studied the effect of crystal size, Si/Al ratio and acidity. These authors claimed the better performance of FER- and MFI-type zeolites among others, especially in terms of selectivity, stability and limited formation of carbon species [326,327]. In the literature HZSM-5 is the most studied zeolite since it exhibits good hydrothermal stability and activity due to its topology and acidic properties. Anyhow, the strong Br\u00f6nsted nature of the sites makes it prone to coke deposition [331]. To overcome this a great deal of effort has been placed on tailoring HZSM-5 [332] and numerous modifications have been widely studied [333\u2013335], most of them oriented towards the passivation of the acid strength, to attenuate coke deposition [218,336]. Zeng et al. [216] determined that with the partial desilication and dealumination of ZSM-5 the strength of the surface acidic sites diminishes and the mesoporous presence increases. As a consequence, not only the catalytic performance, but also the hydrothermal stability and deactivation resistance improved. According to Ordomsky et al. [337] silication also resulted effective for stabilizing the HZSM-5 based catalyst, minimizing the progress of the hydrocarbon pool mechanism, while Wei et al. [338] used alkaline treatment passivation and partial activation for the same purpose. Aboul-Fotouh et al. [339] tuned the acidity (more active catalysts achieved) by chlorination or fluorination methods. Aloise et al. [217] reported that the increase of mesopore diameter, obtained by desilication, allows the formation of larger amount of accessible acidic sites, minimizing therefore the formation of coke deposits and upgrading DME production. Krim et al. [340] attained a DME selectivity of 74% with hollow nano-HZSM-5 with mesoporous shell synthesized by alkaline treatment.Sanchez-Contador et al. [330] compared the performance of HZSM-5 zeolite with SiO2/Al2O3 ratios of 80 and 280, subjected to thermal and dry steaming treatments for acidity passivation, and SAPO-18 and -11 [330]. This study claims that under the conditions required for the CO2-to-DME process (250\u2013325\u00a0\u00b0C, ~20\u00a0bar), the performance of SAPO-11 is slightly better than that of the thermally treated HZSM-5(280) zeolite, and this, better than for SAPO-18 [255]. The better behavior of SAPO-11 molecular sieve is attributed to the properties of the acidic sites (high density of weak strength acidic sites) and the AEL topology of its porous structure) [341,342]. These properties minimize the adsorption and retention of hydrocarbon molecules, as well as their condensation to form polyaromatic components of coke [330]. Chen et al. [342] demonstrated that the acidity of SAPO-11 could be diminished and specific surface and mesoporosity increased by synthesizing nano-sized particles (~200\u00a0nm), resulting in a better activity for methanol dehydration. On the other hand, even if high methanol conversion and DME selectivity is accomplished with SAPO-34, given the large channels and narrow openings of its structure, suffers severe deactivation since large hydrocarbon molecules are retained blocking the pores [343,344].To a lesser extent, other materials have also been tested. For example, HY zeolites or HMCM-22, Witoon et al. studied the use of sulfated zirconia, Frusteri et al. [345] and Catizzone et al. [214,326] justified the optimal performance of ferrierite by its porous structure and moderate acidic strength.For the preparation of the catalyst, the metallic functions presented in Section 4.1 and the acid functions presented in Section 4.2 have to be combined. The typical strategy is to provide an excess of acid function. In this way, the displacement of methanol synthesis equilibrium is ensured (see Section 3.3) and the overall reaction is controlled by methanol formation, which is the slowest step. Given the relevance of the intimacy of the contact between the metallic and the acidic functions on the overall performance of the catalyst, the configuration of the catalytic bed has been largely addressed. Yao et al. [346] ascertain that with a close contact between the functions DME could be generated through a shortcut methoxy-DME pathway, with no need for methanol formation as intermediate (typical methoxy-methanol-DME route), resulting in a more efficient production of DME. In the literature the following arrangements are studied: 1) Dual bed configuration, placing first the metallic function for CO2 hydrogenation to methanol, and subsequently the acidic function for its dehydration to DME; 2) physical mixture of metallic function and acidic function particles; 3) hybrid configuration, the most common configuration where both functions are mixed conforming bifunctional catalyst particles; 4) core-shell configuration, where one function is encapsulated by the other, and; 5) structured catalyst. Regarding thermodynamic basis, in the first strategy a two-set process would be taking place, at the same reaction conditions. Therefore, the lower activity of this system over other configurations reported by several authors is to be expected [258,346\u2013349].Ateka et al. [347] conducted the comparison of the strategies 1\u20133 for the combination of CuO-ZnO-MnO (CZMn) metallic and SAPO-18 acidic functions, for valorizing CO2 co-fed with synthesis gas, emphasizing the low cost of CZMn metallic catalyst among other options [153,255]. In all cases, both functions where mixed at the optimal 2/1 mass ratio (metallic function/acid function). In the dual bed strategy (strategy 1), DME selectivity did not surpass 85%, evidencing the suitability of combining the proposed functions. The conversion of the CO2\u00a0+\u00a0CO mixture fed (50% each) with the dual bed strategy resulted 50% lower than when particles of both functions where mixed conforming a single catalytic bed (strategy 2). Moreover, combining CZMn and SAPO-18 in a single hybrid catalyst particle (strategy 3), the closer contact between the functions led to improve DME selectivity (~95%) and boost CO2\u00a0+\u00a0CO conversion, doubling that obtained in the dual bed strategy (22% vs 10%). Yao et al. [346] performed a similar study for the combination of Cu-In-Zr-O (CIZO) and SAPO-34. They reported that the adjacency of both functions facilitates the migration of intermediate methoxy ions from CIZO to SAPO-34, so that DME could form directly. That is, CO2 conversion improved from <3% to ~4.5% when changing from the dual bed strategy to hybrid catalyst, whereas DME selectivity remained around 60% in all cases. In other cases, like for Bonura et al. [348], the performance of the catalyst is lower for the hybrid catalyst configuration than for the catalytic bed composed of pre-pelletized individual functions (strategy 2) of Cu-ZnO-ZrO2 (CZZr) and HZSM-5 zeolite in a 1/1 mass ratio [348]. This decay is related to the blockage of the zeolite pores inlet by the metallic function on the mortar treatment and pelletizing steps. Later, these authors studied the influence of the precipitating agent on the generation of the metallic function directly in a solution containing the zeolite (HZSM-5 [350], MOR or FER [349]) as to \u201cenglobe\u201d the latter. The procedure improved the activity of the system, presumably by the enhanced hydrogenation functionality related to the \u201cmultisite\u201d reaction path; primary adsorption of H2 on the metallic sites reacting with the CO2 adsorbed on the strong basic sites to form methanol, and the subsequent dehydration on the acidic sites of the zeolite.The core-shell structure (strategy 4) is being explored as an alternative to hybrid catalysts [342,351]. Unlike the hybrid catalysts prepared by extrusion of the metallic and acidic functions configuring each catalyst particle, the core-shell structure consists of depositing the one function on a previously prepared nucleus of the other. Typically, the acid function covering the metallic nucleus (Fig. 17\n). This structure can be prepared by either hydrothermal synthesis, single-crystal crystallization, dual-layer method or physically adhesive method. The general objective of the core-shell structure in catalytic processes is to preserve the catalyst from poisons adsorption, attenuating the sintering of the metallic particles and controlling DME selectivity by space confining the reactions. Thus, in multiple step reactions (cascade reactions), a more favorable reaction medium is achieved for each step. There are contributions in the literature for this initiative in the direct synthesis of DME, with core-shell catalysts prepared with the conventional CuO-ZnO-Al2O3 metallic function and using as acidic function HZSM-5 zeolite [352], \u03b3-Al2O3 [353,354], SiO2-Al2O3 [355] or SAPO-11 [356]. Guffanti et al. have conducted model analyses for evaluating the effect of the active phase distribution [357] and of the kinetics, adsorption capacity and mass and heat transfer [354] in the performance of hybrid, mechanically mixed and acidic-function@metallic-function and metallic-function@acidic-function structured core-shell catalysts. These works highlight the influence of the internal diffusion on productivity, pointing out metallic-function@acidic-function as the most suitable configuration, and that the small particle diameters and limited contact between phases avoids hot spots generation, favoring DME formation.S\u00e1nchez-Contador et al. [144,330,351] have prepared a CuO-ZnO-ZrO2@SAPO-11 core-shell catalyst by physical adhesive methodology (in a mass ratio of 1/2) with SiO2 solution as adhesive [356,358]. With this configuration, methanol synthesis occurs in the CuO-ZnO-ZrO2 core and diffuses for later being dehydrated in the surrounding SAPO-11 acidic shell. These authors have corroborated that the preparation method of core-shell particles prevents the partial blockage of SAPO-11 mesopores by CuO-ZnO-ZrO2 particles in the pelletizing step used for preparing hybrid catalysts. For CO2\u00a0+\u00a0CO mixture hydrogenation (50% each) a DME yield of 8.7% and selectivity of 81% are achieved with this core-shell catalyst, whereas 7% and 77%, respectively, for the hybrid system (325\u00a0\u00b0C, 30\u00a0bar, 7.6 gcat h molC\n\u22121). Fig. 18\n compares the COX conversion and products yields obtained with the core-shell configuration with those obtained with the conventional hybrid configuration. Moreover, the core-shell configuration prevents catalysts deactivation. After 24\u00a0h TOS, ~ 37% of DME yield decrease has been reported for conventional hybrid catalysts (from 7.4 to 4.7%), whereas the lessening is contained (to 21%) for the core-shell configuration (from 8.67 to 6.8%) [351].Among the causes for the better performance of the core-shell over hybrid catalysts, the above mentioned works emphasize the creation of a favorable reaction medium by separating the methanol synthesis and its dehydration reactions in different regions, providing a higher availability of acidic sites on the catalyst particle surroundings for the conversion of the methanol formed in the nucleus. On this manner, limiting the presence of H2O in the metallic nucleus leads to a greater resistance towards sintering of the Cu species in the nucleus [359]. Moreover, with a core-shell structure the adverse effects derived from the interaction between phases can be minimized. Thus, Nie et al. [360] have highlighted the advantage of the confinement of Cu species in the nucleus, avoiding their migration towards the acidic function. Garc\u00eda-Trenco and Mart\u00ednez [361] have proven through XPS analysis and 27 Al MAS-NMR spectra the migration of Al3+ species from HZSM-5 zeolite towards the CuO-ZnO-Al2O3 metallic function, resulting in catalyst deactivation by Cu sintering.An important challenge for the scale-up of the CO2-derived DME synthesis is to prepare catalysts with appropriate particle size and mechanical strength for industrial fixed-bed reactors. This requires addressing the agglomeration (using binders) of the catalysts configured with optimal structure according to laboratory scale results (as shown in this section) to build catalysts of several mm of particle size, high mechanical resistance and minimal performance loss (activity and selectivity) due to limitations of mass and heat transport. An overall view of the stages to progress towards the scale-up in the preparation of catalysts has been described in the literature [362,363].To overcome the heat transfer limitations of the commonly used packed bed reactors with catalyst particles, the use of monolithic reactors (strategy 5) has been proposed and experimentally studied for syngas feedstocks [364,365]. For such configuration, the conductivity of the materials, cell density of corrugated monoliths and tortuosity of open cell foams are relevant parameters. Magzob et al. [364] compared the performance of HZSM-5 powder and monolith-structured (HZSM-5 and HZSM-5@SAPO-34) catalysts within 180\u2013320\u00a0\u00b0C temperature range. With the HZSM-5 monolith configuration, a reduction on Br\u00f6nsted acidic sites (and increase of Lewis acidic site density) and improvement of mesoporosity was reported. With this characteristics, better catalytic performance than for the powder zeolite was achieved, thus, methanol conversion ~70%, with high DME selectivity (96%) yet at 180\u00a0\u00b0C. P\u00e9rez-Miqueo et al. [365] investigated the use of metallic structured reactors for the direct DME synthesis process. These authors prepared the monoliths by wash coating the substrates with CZA and HZSM-5, and concluded that working at almost isothermal conditions is feasible with a volumetric productivity up to 0.20 LDME h\u22121\u00a0m\u22123 at 300\u00a0\u00b0C and 4\u00a0MPa, with a catalyst hold-up of 0.33 gcat cm\u22123 in a brass monolith (for syngas feedstocks).Given its importance in the viability of the process, the attenuation of catalyst deactivation is a priority challenge. Understanding the problem is hampered by the coexistence of different causes and by the synergy between the deactivation mechanisms of the metallic and acid functions. The main causes of deactivation are [366]: i) partial blockage of the metallic sites by coke (being considered as the fastest step in the deactivation); ii) coke deposition on the micro and mesopores of the acid function; iii) sintering of the metallic function; and iv) the detrimental interactions between the metallic and the acidic sites.Coke characterization studies through Temperature Programmed Oxidation (TPO) have determined its presence both on the metallic and acidic sites, as well as on the interphase between them (corresponding to the inert Al2O3 in the CuO-ZnO-Al2O3/\u03b3-Al2O3 catalyst [254,367\u2013369]). However, coke is present on the metallic function since the initial stages of the reaction, achieving a limit value in a short period of time. This dynamic can be explained because the hydrogenation of coke precursors slows down its evolution [370,371]. The amount of coke deposited on the acidic function increases with time on stream, tending to a maximum value, resulting from the equilibrium between its formation and its diffusion to the exterior of the catalyst particles. Consequently, the properties of the acidic function are also important both for attenuating coke formation and for favoring the circulation of the intermediates towards the exterior of the catalyst particles.It is worth mentioning the contribution of promoters like MgO [250,372], CeO2 [252], and ZrO2 [373] for preventing the sintering of CuO-ZnO metallic functions. The incorporation of these promoters pursues enhancing CuO crystallites dispersion and stabilizing its interaction with the support.The presence of H2O in the reaction medium (higher in the conversion of CO2 than of syngas) has different effects on the activity of the catalyst. In first place, decreases the initial activity of the catalyst due to the competitive adsorption with the reactants in the metallic and acidic sites of the catalyst. The effect is very important for \u03b3-Al2O3, due to the affinity for H2O of its Lewis sites [211,370]. Furthermore, it favors the sintering of the metallic function, which has been proven for Cu catalysts as their oxidation is favored [337,374,375] and generates the disruption of the Cu\u2010Zn synergy [240]. Fan et al. [376] have verified the increased stability of a Cu-ZnO-ZrO2-Al2O3 catalyst used together with HZSM-5 catalyst, when modified with Fe, which is attributed to oxygen spillover between deficient iron oxide and Cu, mitigating oxidation (by CO2 and H2O) and Cu sintering.On the other hand, it is well established that the presence of H2O decreases the rate of coke formation [328]. This effect has been explained by the key role of methoxy ions as coke precursors on the metallic and acidic sites, whose formation is thermodynamically limited with the increase of H2O concentration [245]. In addition, H2O is competitively adsorbed with coke-forming intermediates, which are identified as monocyclic arenes, and whose formation takes place from hydrocarbons formed from methanol and DME [35]. Besides, the acidity and porous structure of the acid function have a great effect on the rate of coke deposition and on its nature and deactivating effect. Thus, Br\u00f8nsted sites with high acidic strength are active in the reactions of coke precursors condensation towards polyaromatic structures and, their confinement is favored in acid functions with cavities in the porous structure [366].Fan et al. [377] compare the individual deactivation of the two catalysts, CuO-ZnO-ZrO2-Al2O3 (CZZA) and HZSM-5 zeolite, when mixed or separated in cascade (first CZZA and zeolite in line). Among the conclusions, the convenience of the proximity of both catalysts stands out, but avoiding an excessive concentration of H2O on the surface of the CZZA catalyst (to attenuate the sintering of Cu) and also the excessive concentration of methanol (precursor of coke deposition in the HZSM-5 zeolite).The configuration of the catalyst particle receives great attention for avoiding deactivation due to the close contact between the metallic and acid functions. Garc\u00eda-Trenco and Mart\u00ednez [361] have verified the migration of extra-framework Al3+species of the HZSM-5 zeolite to the metallic function (CuO-ZnO-Al2O3) through a mechanism assisted by H2O, causing the disruption of the Cu\u2010Zn synergy, and facilitating the sintering of Cu. Likewise, the migration of Cu2+ ions is facilitated by the presence of H2O and hydroxyls (Br\u00f8nsted) sites [337,378,379]. These problems advise avoiding intimate contact between the metallic and acid functions in the preparation of the catalyst, being the pre-pelletization of each function separately more suitable than the joint pelletization of a fine powder of both functions in this case [380].Ateka et al. [254] have studied the regeneration of a CuO-ZnO-MnO/SAPO-18 hybrid catalyst, on which coke deposition is reported to be the main responsible for deactivation. Working at reaction-regeneration cycles, these authors have determined that it is possible to regenerate the bifunctional catalyst by coke combustion with air at 300\u00a0\u00b0C for 48\u00a0h. Even if at these conditions the catalyst undergoes a slight sintering of Cu in the first cycle, in the succeeding cycles it demonstrated to reach a pseudo-steady state, completely recovering the activity. Being therefore coke deactivation reversible, this study pointed out sintering as the limiting factor for using these type of catalysts. The small activity loss observed in the first reaction-regeneration cycle was attributed to the sintering of a certain fraction of unstable metallic sites either due to the high water content in the reaction medium or by the generation of hot spots in the regeneration step [254]. Consequently, enhancing the stability of the metallic function also favors the regeneration of the catalyst by allowing to perform coke combustion at higher temperature. In addition, the porous structure and acidity of the catalyst, besides being important for the attenuation of coke condensation [366], are also relevant factors to facilitate its combustion.The interest of the direct synthesis of DME for valorizing CO2 on a large scale is based on the capacity for the conversion of CO2 and syngas and on the good prospects of the applications of DME as \u201cgreen\u201d fuel and as raw material for the sustainable production of chemicals and H2.Carrying out the methanol dehydration reaction in situ, in the same reactor as methanol synthesis, shifts the thermodynamic equilibrium, upgrading oxygenates formation. Moreover, with this strategy co-feeding of CO2 together with syngas is more favorable than in the synthesis of methanol, which is interesting to valorize (via gasification) lignocellulosic biomass and wastes from the consumer society (as plastics and used tires). The conversion of CO2 attained in the direct synthesis of DME is higher than that in the synthesis of methanol and in the conventional production of DME in two stages.The reaction conditions (pressure and temperature) in the direct synthesis of DME are different to the optimal conditions for each of the individual reactions. Furthermore, CO2 is less reactive than CO and its hydrogenation generates a higher concentration of H2O. These differences in the operating conditions and concentration have required studying the suitable composition and properties of the metallic and acid functions of the catalyst. As consequence, a reasonable understanding of the performance of some suitable compositions has been reached, in particular for conventional configurations (hybrid catalysts prepared by mixing and pelletizing/extrusion of both functions). As in most catalytic processes, the main challenges correspond to the attenuation of the deactivation of the catalyst, being the sintering of the metallic function and coke deposition on both functions the main causes.It is well established that the contact of the metallic and acid functions favors deactivation, due to the development of species (as Cu2+ and Al3+) transport mechanisms, and also that favors the synergy of coke formation mechanisms in both functions. This knowledge has opened a wide research field pursuing to establish the ideal core-shell configuration to minimize the negative effects derived from the contact between the two functions of the catalyst, and in particular, to achieve the stability of the catalyst.The level of knowledge achieved in the fundamental aspects (collected in this review) allows considering that the CO2 to DME synthesis process can effectively contribute to the mitigation of climate change. Achieving the necessary challenges for this objective requires a multidisciplinary work at different scales (catalyst, kinetic modeling, reactor design and scaling).The scaling-up of the CO2-derived DME synthesis process requires catalysts prepared based on the important advances carried out in the design of catalysts for the reactions of CO2-to-methanol and methanol dehydration to DME. To meet this objective, the advances must be adapted to the different conditions and the different composition of the reaction medium of the integrated process. In this sense, the co-feeding of CO2 together with syngas has good perspectives to favor the viability of the process, but requires adequate catalysts, and the resolution of the unknowns regarding the different mechanism for the formation of methanol from CO and CO2 and the synergy between both mechanisms. Likewise, the stability of the catalyst is a challenge requiring more attention.The adaptation of catalysts optimized at nanometric scale to the needs of the industrial reactors is an important challenge. This requires studying composites with the appropriate size and with high mechanical resistance, without deterioration of the performance of the catalyst particles.The viability of the process on an industrial scale also requires adapting the design of the catalysts to the innovations in the design of the reactors, which, like for the hydrophilic membrane reactor, require increasing the per pass conversion. With a different composition in the reaction medium, a different thermodynamic situation is created in these reactors. Accordingly, an adaptation of the catalysts to the optimal conditions and composition in these reactors will also be required.Furthermore, the important development of CO2 valorization initiatives to mitigate climate change, advise expanding the field of study of the CO2-derived DME synthesis process, also considering it as preceding stage to the subsequent synthesis (online stage, or in an integrated process) of fuels and chemicals (olefins or aromatics). In the latter case, the direct DME synthesis catalyst will be used in a tandem catalyst together with an acid catalyst for the selective conversion of DME.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work has been carried out with the financial support of the Ministry of Science, Innovation and Universities of the Spanish Government (PID2019-108448RB-100); the Basque Government (Project IT1645-22); the European Regional Development Funds (ERDF); and the European Commission (HORIZON H2020-MSCA RISE-2018. Contract No. 823745).", "descript": "\n                  The direct synthesis of dimethyl ether (DME) on bifunctional catalysts is highly attractive for valorizing CO2 and syngas derived from biomass gasification and is a key process to reduce greenhouse gas emissions. DME economy (conventionally based on its use as fuel) arouses growing interest, in parallel with the development of different routes for its conversion into hydrocarbons (fuels and chemicals) and H2 production. This review, after analyzing different routes and catalytic processes for the valorization of CO2, focuses on studies regarding the thermodynamics of the direct synthesis of DME and the advances in the development of new catalysts. Compared to the synthesis of methanol and the synthesis of DME in two stages, carrying out the reactions of methanol synthesis and its dehydration to DME in the same reactor favors the formation of DME from CO2 and from CO2 co-fed with syngas. Starting from the experience for syngas feedstocks, numerous catalysts have been studied. The first catalysts were physical mixtures or composites prepared by extrusion of methanol synthesis catalysts (CuO-ZnO with different carriers and promoters) and dehydration catalysts (mainly \u03b3-Al2O3 and HZSM-5 zeolite). The performance of the catalysts has been progressively improved with different modifications of the composition and properties of the components to upturn the activity (lower for the hydrogenation of CO2 than for CO) and selectivity, and to minimize the deactivation by coke and by sintering of the metallic function. The core-shell configuration of the bifunctional catalyst allows physically separating the environments of the reactions of methanol synthesis and its conversion into DME. The confinement facilitates the extent of both reactions and improves the stability of the catalyst, since the synergies of the deactivation mechanisms are eliminated.\n               "}